588 10 13585 https://digitalcollections.lakeheadu.ca/files/original/d31e5e66626ffb77a8d6cff650edfda2.pdf f889347321a92c3ecf21c0564230434d PDF Text Text Program and and Abstracts 94th ANNUAL MEETING Minneapolis, Minnesota May 1998 May 6-10, 6-10,1998 Organized by MINNESOTA MINNESOTA GEOLOGICAL GEOLOGICAL SURVEY UNIVERSITY OF MINNESOTA UNIVERSITY MINNESOTA �44th Annual Meeting INSTITUTE ON LAKE LAKE SUPERIOR SUPERIOR GEOLOGY GEOLOGY INSTITUTE parts: Volume 44 contains the following parts: Part Part 1: Program Program and and Abstracts Part 2: Field Part 2: Field Trip Trip Guidebook Guidebook i—Early 1-Early Proterozoic Proterozoicintrusive intrusive rocks of east-central Minnesota 2—Geology 2-Geology of the southeastern southeastern portion of the Midcontinent Midcontinent Rift Rift System, eastern Minnesota and western Wisconsin 3—Glacial 3-Glacial exotica exotica of the Twin Cities Cities area 4—Stratigraphy and hydrogeology 4ÑStratigraph hydrogeology of Paleozoic rocks of southeastern southeastern Minnesota 5—Minnesota 5-Minnesota River River Valley and vicinity, vicinity, southwestern southwestern Minnesota Minnesota Reference to the material in Reference in this volume volume should should follow follow the example example below: below: P.K., Neymark, L.A., Sims, P.K., L.A., Peterman, Z.E., Z.E., and and Kotov, Kotov, A.B., 1998, 1998, Nd isotope isotope evidence evidence for for Middle Middle and and Early Early Archean Archean crust crust in in the the Wawa Wawa subprovince subprovinceof of the the Superior SuperiorProvince, Province, U.S.A., [abstract]: Institute on Lake Superior logy Proceedings, Proceedings, 44th Michigan, U.S.A., Superior Geology MN, 1998; v. v. 44, 44, Part Part 1, 1,p. p. .: Annual Meeting, Minneapolis, MN, Volume Volume 44 is is published by the Institute on Lake Superior Geology and distributed by the Institute Institute Secretary-Treasurer: Secretary-Treasurer: Mark Mark Jima Jirsa (term (term 1994-2000) 1994-2000) Minnesota Minnesota Geological Geological Survey Survey 2642 University Avenue MN USA 55114-1057 St. Paul, MN 55114-1057 (612)-627-4780 (6 12)-627-4780 email: jirsa001@tc.umn.edu jirsa001 @tc.umn.edu http://www.geo.mtu.edu/great_lakes/ilsgl ILSG website http://www.geo.mtu.eddgreat_lakes/ilsg/ ISSN 1042-9964 1042-9964 �INSTITUTE ON LA SUPERIOR GEOLOGY 4th Annual Annual Meeting Meeting 44th May 6-10, 1998 Minneapolis, Minnesota Sponsored Sponsored by: by: Minnesota MinnesotaGeological GeologicalSurvey Survey University University of Minnesota Minnesota PROCEEDINGS PROCEEDINGS Volume 44 44 Part Program and Part 11-Program and Abstracts �CONTENTS CONTENTS Proceedings Volume Volume 44 44 Proceedings Part1—Program 1-Program and andAbstracts Abstracts Part Instituteson onLake LakeSuperior SuperiorGeology, Geology,1955-1998 1955-1998 ...............................................iviv Institutes Constitution Constitution of of the theInstitute Instituteon onLake LakeSuperior SuperiorGeology Geology..........................................vv .. vu By-Laws By-Lawsofofthe theInstitute Instituteon onLake LakeSuperior SuperiorGeology Geology.. .........................................vii ... GoldichMedal MedalGuidelines Guidelines ........................................................................ viii vm Goldich PastGoldich GoldichMedalists Medalists. ............................................................................. x.x Past x GoldichMedal MedalCommittee Committee ...........................................................................x Goldich xi Citationfor for1998 1998Goldich GoldichMedal MedalRecipient Recipient.. .....................................................xi Citation .. ... Student Student Paper Paper Awards Awards ............................................................................. xiii xm ... StudentPaper PaperCommittee Committee ......................................................................... xiii xm Student StudentTravel TravelAwards Awards.. ........................................................................... xii xu Student Boardof ofDirectors Directors ................................................................................. xiv xiv Board LocalPlanning PlanningCommittee Committee.. ......................................................................xiv xiv Local SessionChairs Chairs ...................................................................................... xv xv Session BanquetSpeaker Speaker.. .................................................................................. xv xv Banquet xvi Reportof ofthe theChair Chairofofthe the43rd 43rdAnnual AnnualInstitute InstituteMeeting Meeting.. ...................................xvi Report ... Program.. .......................................................................................... xviii xvm Program Abstractsfor forSpecial SpecialSession: Session: Abstracts 1 Geolo~ical Overviewof ofthe theLake Lake Superior SuperiorRegion Region ..........................................1 Geolo2ical Overview Card,Ken Ken D. D. Card, Archean Archean geology geology of of the the Great Great Lakes Lakes region region of of North NorthAmerica America ................................33 Ojakangas,Richard RichardW. W. Ojakangas, Generalized GeneralizedEarly EarlyProterozoic Proterozoichistory, history,Lake LakeSuperior Superiorregion region ................................ .55 Cannon,William WilliamF. F. Cannon, Understanding Understandingthe theMiddle MiddleProterozoic Proterozoichistory history of of the theLake LakeSuperior Superiorregion: region: What's What's new? new? What's What'snext? next?. ...............................................................19 19 Runkel, Anthony Anthony C. C. Runkel, Paleozoic Paleozoicrocks rocksin inthe thenorthern northernpart part of of the thecentral centralmidcontinent midcontinentof of North North America America Patterson, Carrie CarrieJ.J. Patterson, Models Models for for interpreting interpreting the the Quaternary Quaternary history of the the Lake Lake Superior Superior region region L. Southwick, David David L. Southwick, 25 .........25 .................33 33 37 ...................................... .37 39 Abstracts for forGeneral GeneralTechnical TechnicalSessions Sessions ...................................................... .39 Abstracts What's What'snext nextfor forgeology geologyin inthe theLake LakeSuperior Superiorarea9 area? Ill �INSTITUTES ON INSTITUTES ON LAKE LAKE SUPERIOR SUPERIORGEOLOGY, GEOLOGY,1955-1998 1955-1998 # DATE DATE PLACE PLACE C HAIRS CHAIRS 1 1955 2 1956 3 1957 4 1958 5 6 1959 1960 7 1961 8 10 1962 1963 1964 11 1965 12 13 14 1966 15 16 1969 1970 17 1971 18 1972 9 1967 1968 19 1973 20 1974 21 1975 22 23 24 25 26 27 28 29 30 1976 1977 1978 1979 31 1985 32 33 34 35 36 37 38 39 40 1986 1987 1988 41 42 43 44 1980 1981 1982 1983 1984 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 Minneapolis, Minnesota Minneapolis, Houghton, Michigan East Lansing, Lansing, Michigan Duluth, Duluth, Minnesota Minnesota Minneapolis, Minneapolis, Minnesota Madison, Wisconsin Port Arthur, Ontario Ontario Houghton, Houghton, Michigan Duluth, Duluth, Minnesota Ishpeming, Ishpeming, Michigan Michigan St. Paul, Minnesota Sault Ste. Marie, Michigan Michigan East Lansing, Michigan Superior, Wisconsin Superior, Oshkosh, Wisconsin Thunder Thunder Bay, Bay, Ontario Ontario Duluth, Duluth, Minnesota Houghton, Houghton, Michigan Madison, Wisconsin Sault Ste. Marie, Ontario Ontario Marquette, Marquette, Michigan Michigan St. Paul, Minnesota Thunder Thunder Bay, Ontario Ontario Milwaukee, Wisconsin Duluth, Duluth, Minnesota Minnesota Eau Claire, Claire, Wisconsin Wisconsin East Lansing, Lansing, Michigan International Falls, Minnesota International Houghton, Michigan Michigan Wausau, Wausau, Wisconsin Wisconsin Kenora, Ontario Wisconsin Rapids, Wisconsin Wisconsin Wawa, Ontario Ontario Marquette, Marquette, Michigan Michigan Duluth, Minnesota Thunder Bay, Ontario Eau Claire, Wisconsin Hurley, Wisconsin Eveleth, Minnesota Houghton, Michigan Marathon, Ontario Ontario Cable, Wisconsin Sudbury, Ontario Minneapolis, Minneapolis, Minnesota C.E. Dutton C.E. A.K. Sneigrove A.K. Snelgrove Sandefur B.T. Sandefur R.W. Marsden Marsden G.M. Schwartz & C. Craddock E.N. Cameron Cameron E.G. Pye Pye A.K. Sneigrove Snelgrove H. H- Lepp L~PP A.T. Broderick Broderick P.K. Sims & R.K. R.K. Hogberg Hogberg R.W. White White WJ. W.J. Hinze Hinze A.B. Dickas Dickas G.L. LaBerge LaBerge Bartley & E. Mercy M.W. Bartley D.M. Davidson J. Kalliokoski Kalliokoski M.E. Ostrom Ostrom P.E. Giblin Giblin J.D. Hughes Hughes M. Walton Walton Kehlenbeck M.M. Kehienbeck G. Mursky D.M. Davidson P.E. Myers Myers W.C. Cambray Cambray D.L. Southwick Southwick T.J. Bornhorst G.L. LaBerge LaBerge C.E. Blackburn Blackburn J.K. Greenberg Greenberg E.D. & R.P. Sage Sage E.D. Frey & J. S. S. Klasner Klasner J.C. Green Green M.M. Kehlenbeck M.M. P.E. Myers A.B. Dickas D.L. Southwick D.L. Southwick T.J. Bornhorst M.C. Smyk M.C. L.G. Woodruff R.P. Sage, W. Meyer R.P. J.D. Miller, M.A. Jirsa J.D. iv �_________(some CONSTITUTION OF THE INSTITUTE ON LAKE SUPERIOR GEOLOGY Board—May 8, 1997) (Last amended by the Board-May 8,1997) Article Article II Name The name of the organization organization shall be the "Institute "Institute on Lake Superior Superior Geology". Geology". Article II II Objectives Objectives are: The objectives objectives of this this organization organization are: A. To Toprovide provideaameans meanswhereby whereby geologists geologists in in the the Great Great Lakes Lakes region region may may exchange exchange ideas ideas and and scientific scientificdata. data. Topromote promotebetter betterunderstanding understandingof of the the geology geology of of the the Lake Lake Superior Superior B. To region. region. C. To Toplan planand andconduct conductgeological geologicalfield field trips. trips. Article III ifi Status Status to the the benefit benefit of any No part of the income of the organization shall insure to member or individual. In the event of dissolution, the assets of the organization organization shall be distributed distributed to free organization). organization). (some tax free To avoid Federal and State income taxes, the organization should be not only "scientific" "scientific" or or "educational, "educational,but also also "non-profit". "non-profit". Minn. Stat. Anno. 290.01, subd. 4 Minn. Stat. Anno. 290.05(9) Mim. 1954 Internal Revenue Code Code s.501(c)(3) s.501(c)(3) Article IV Article Membership Membership The membership of the organization organization shall shall consist consist of persons who have have registered for an annual meeting within the past three years, and those who approved by the indicate interest in being a member according to guidelines approved Board of Directors. Directors. Article V Meetings organization shall meet once a year, preferably preferably during during the the month month of The organization April. The Theplace placeand andexact exactdate date of of each each meeting will be designated designated by the Board of Directors. Directors. Article VI Directors The Board of Directors shall consist of the Chair, Secretary-Treasurer, and the last three past Chairs; but if the board should at any time consist of fewer than five persons, by reason of unwillingness or inability of any of the above persons to serve serve as directors, directors, the vacancies vacancies on the board may be be filled filled by the Chair Chair so so as to bring the membership membership of the board board to to five five members. members. V �Article Article VII Officers Officers The The officers officers of of this this organization organizationshall shallbe be aa Chair Chairand and Secretary-Treasurer. Secretary-Treasurer. A. A. The TheChair Chairshall shallbe beelected electedeach eachyear year by by the the Board Board of Directors, Directors, who shall give due consideration consideration to the wishes of any group that may be promoting promoting the next annual meeting. His/her Hisher term of office office as Chair will terminate he/she presides, or terminate at at the close close of the annual meeting over which helshe hisher successor shall have been Helshe will then serve serve when his/her been appointed. appointed. He/she for as aa member member of the the Board Board of Directors. Directors. for aa period period of of three three years as B. The the annual annual meeting. meeting. Hisher His/her TheSecretary-Treasurer Secretary-Treasurer shall shall be elected at the term term of of office office shall shall be be four four years, years, or or until his/her hislher successor successor shall shall have have been been appointed. appointed. Article Article Vifi VIXI Amendments Amendments This vote of of the the membership membership of This constitution constitution may be amended by a majority vote the the organization. organization. vi �BY-LAWS BY-LAWS OF THE INSTITUTE INSTITUTE ON ON LAKE LAKE SUPERIOR SUPERIOR GEOLOGY GEOLOGY I. Duties Dutiesof of the the Officers Officers and and Directors A. ItItshall shallbe bethe theduty dutyof ofthe theAnnual AnnualChairman Chairmanto: to: 1. Preside Preside at at the the annual annual meeting. meeting. 1. Appointall allcommittees committeesneeded neededfor forthe theorganization organizationof of the the annual annualmeeting. meeting. 2. Appoint 3. Assume Assumecomplete completeresponsibility responsibility for for the organization and financing of the annual meeting over over which he/she helshe presides. B. ItItshall shallbe bethe theduty dutyof of the theSecretary-Treasurer Secretary-Treasurerto: 1. Keep Keepaccurate accurateattendance attendancerecords recordsof of all allannual annualmeetings. meetings. 2. Keep Keepaccurate accuraterecords recordsof of all all meetings meetings of, of, and and correspondence correspondence between, the Board of Directors. Directors. 3. Hold Holdall allfunds fundsthat that may may accrue accrue as as profits profits from from annual annual meetings meetings or or field trips for the the organization organization and and operation operation of of and to make these funds available for future meetings as required. C. ItItshall shallbe bethe theduty dutyof of the the Board Board of of Directors Directors to to plan locations locations of annual meetings and and to to advise advise on on the the organization organizationand and fmancing financingof of all all meetings. meetings. II. 11. Duties Dutiesand and Exoenses Expenses A. There Thereshall shallbe beno noregular regular membership membership dues. dues. B. Registration Registrationfees feesfor forthe the annual annual meetings meetings shall shall be determined by the Chair in The registration fees can include consultation with the Board of Directors. Directors. The expenses to cover cover operations operations outside of the annual meeting as determined by expenses the Board of Directors. ItIt isis strongly strongly recommended recommended that registration fees be kept at a minimum minimum to encourage encourage attendance attendance of graduate graduate students. students. ifi. Rules In. Rules of of Order Order The The rules rules contained contained in in Robert's Robert's Rules Rules of of Order Order shall shall govern govern this this organization organizationin in all all are applicable. applicable. cases to which they are IV. Amendments Amendments These These by-laws may be be amended amended by a majority vote of the membership of the organization; organization; provided that such modifications shall not conflict conifict with the constitution presently adopted adopted or or subsequently subsequently amended. amended. constitution as presently vu vii �GOLDICH MEDAL GUIDELINES GOLDICH MEDAL GUIDELINES (Adopted by the Board of Directors, 1981; 1981;amended 1997) 1997) Preamble Institute on Lake Superior Geology was born in 1955, as as documented documented by by the the fact fact that that The Institute the 27th annual meeting was held in 1981. The The Institute's Institute's continuing continuing objectives are to deal are related geographically to Lake Superior; Superior; to encourage encourage with those aspects aspects of geology that are discussion of subjects subjects and and sponsoring sponsoring field trips that will bring together geologists from the discussion academia, government government surveys, surveys, and industiy; industry; and and to maintain an informal informal but highly effective mode mode of operation. operation. During the course course of its existence, existence, the membership of the Institute (that (that is, those geologists geologists by attending) attending) has has become become aware aware of of who indicate an interest in the objectives of the ILSG by noteworthy and and meritorious meritorious the fact that certain of their colleagues have made particularly noteworthy contributions to the understanding understanding of Lake Lake Superior Superior geology and mineral mineral deposits. deposits. The first award was made by ILSG to Sam Goldich in 1979 for his many contributions to years. Subsequent the geology of the region extending over about 50 years. Subsequentmedalists medalists and and this this year's recipient recipient are are listed listed in in the the table table below. below. Award Guidelines Guidelines 1) The Themedal medal shall shallbe be awarded awardedannually annually by by the the ILSG ILSG Board Board of of Directors Directors to to aa geologist geologist with aa substantial substantial interest interestin, in,and and contribution contribution to, to, the thegeology geologyof of whose name is associated with the Lake Superior Superiorregion. Medal Committee. Committee. The initial 2) The TheBoard Boardof of Directors Directors shall shall appoint the Goldich Medal appointment will be of three members, one to serve for three years, one for two years, and appointment one for one year. The Themember memberwith withthe thebriefest briefestincumbency incumbencyshall shallbe be chair chair of of the the Nominating Committee. After Afterthe thefirst firstyear, year,the the Board Board of of Directors Directors shall shall appoint appoint at at each each years. In hisfher third third year year this this spring meeting one new member who will serve for three years. In his/her member shall be the chair. The TheCommittee Committeemembership membership should should reflect the main fields of interest and geographic interest geographic distribution distribution of ILSG ILSG membership. membership. November , the Goldich Medal Committee shall make its recommendation of November, 3) By By the the end of to the the Chair Chair of of the the Board Board of of Directors, Directors,who who will will then then inform inform the the Board Board of of the the nominee. nominee. 4) The TheBoard BoardofofDirectors Directorsnormally normallywill willaccept acceptthe thenominee nominee of of the the Committee, Committee, will will inform inform the the medalist medalist immediately, immediately, and and will will have have one one medal medal engraved engraved appropriately appropriately for for presentation presentation at the next meeting of the Institute. 5) ItItisisrecommended recommendedthat thatthe theInstitute Instituteset set aside aside annually annually from whatever sources, such funds as will be required to support support the continuing continuing costs of this award. Nominating Procedures Nominatin~ 1) Nominations Nominations shall shallbe be taken taken at at any time by the Goldich Medal Committee. Committee Committee members may themselves nominate candidates. The The deadline deadline for for nominations nominations is November 1. 1. 2) Nominations Nominationsmust mustbe beininwriting writingand andsupported supportedby by appropriate appropriatedocumentation documentationsuch such as as letters letters of recommendation, recommendation, lists of publications, curriculum vita's, and evidence of contributions contributions to Lake Lake Superior Superior geology geology and and to to the the Institute. vifi �Nominationsare arenot notrestricted restrictedto toInstitute Instituteattendees, attendees,but but are arc open open to anyone anyone who has has 3) Nominations worked on and contributed contributed to the understanding understanding of Lake Superior Superior geology. Selection Selection Guidelines Guidelines 1) Nominees Nominees are are to to be be evaluated evaluated on on the the basis of their contributions to Lake Superior geology (sensu lato) including: a) importance importanceof ofrelevant relevant publications; publications; b) promotion promotionof ofdiscovery discoveryand andutilization utilization of of natural natural resources; c) contributions contributionsto tounderstanding understandingof of the the natural natural history and environment environment of the region; region; d) generation generationof of new new ideas ideas and and concepts; concepts; and e) contributions contributionsto tothe thetraining training and and education education of geoscientists geoscientists and the public. Nomineesare aretotobe beevaluated evaluatedon ontheir their contributions contributionsto the Institute as demonstrated by 2) Nominees attendance at Institute meetings, meetings, presentation service on Institute Institute presentation of talks and posters, and service boards, boards, committees, committees, and andfield fieldtrips. trips. 3) The Therelative relativeweights weightsgiven givento to each each of of the the foregoing foregoing criteria criteria must remain flexible and at the the discretion discretion of of the the Committee Committeemembers. members. 4) There Thereare areseveral severalpoints pointsto tobe beconsidered consideredby by the theGoldich Goldich Medal Medal Committee: Committee: a) An Anattempt attemptshould shouldbe be made made to to maintain maintain a balance of medal recipients from each of the three three estates—industry, estates-industry, academia, academia, and and government. government. that much much of b) ItItmust mustbe benoted notedthat that industry industry geoscientists geoscientists are arc at a disadvantage in that their work in not not published. published. U.S., and the the other Canada. Canada. This 5) 5) Lake Lake Superior Superior has two sides, one the U.S., This is is undoubtedly undoubtedly Institute's great strengths and should be nurtured by by equitable equitable recognition recognition of one of the Institute's excellence excellence in in both both countries. countries. ix �GOLDICH MEDALISTS 1979 Samuel SamuelS. S. Goldich Goldich not awarded 1980 not 1989 Jorma Jorrna Kalliokoski Kalliokoski 1981 Dutton, Jr. 1981 Carl E. Dutton, 1990 Kenneth Kenneth C. Card 1982 Ralph RalphW. W. Marsden Marsden 1991 William William Hinze 1983 Burton Burton Boyum Boyum F. Cannon 1992 William William F. 1984 Richard RichardW. W. Ojakangas Ojakangas W. Davis 1993 Donald DonaldW. 1985 Paul PaulK. K. Sims Sims 1994 Cedric Cedric Iverson 1986 G.B. G.B. Morey Morey LaBerge 1995 Gene GeneLaBerge H. Halls Halls 1987 Hemy HenryH. L. Southwick Southwick 1996 David DavidL. 1988 Walter WalterS. S. White White 1997 Ronald Ronald P. P. Sage Sage 1998 1998 Zell Peterman Peterman GOLDICH MEDAL GOLDICH MEDAL COMMITTEE—1997-1998 COMMITTEE-1997-1998 (Committee membership membership through the meeting year shown) shown) Dan England England (1998) (1998) Eveleth Fee Office, Office, Eveleth, Minnesota John Kiasner Klasner (1999) (1999) Western Illinois University, University, Macomb, Macomb, Illinois Mark Mark Smyk Smyk (2000) (2000) Ontario Ontario Geological Geological Survey, Survey, Thunder Thunder Bay x �CITATION CITATION Zell Zell E. E. Peterman Peterman 1998 Goldich Medal Medal Recipient Recipient my personal personal pleasure pleasure to to introduce introduce Zell E. Peterman, this years Goldich medalist. Zell Zell It's my deserves this this highest highest honor honor of the Institute Institute on Lake Superior Superior Geology because of his richly deserves many outstanding outstandingcontributions contributions to to Precambrian Precambrian geology geology and and geochronology geochronology of of the the Lake Lake Superior Superiorregion. region. Zell received received aa Geologic GeologicEngineering Engineeringdegree degreefrom fromthe the Colorado ColoradoSchool Schoolof of Mines, Mines,aamasters masters Zell Geology from from the the University University of Minnesota, Minnesota, Minneapolis, and a Ph.D. in Geology Ph.D. in Geology in 1962 from the University of Alberta. At AtMinnesota, Minnesota,Zell Zellwas wasaastudent studentof of Sam SamGoldich. Goldich. 1962 Zell has 1994Ñwit the theGeologic Geologic has spent spent all all of ofhis hiscareer—except career-except for forthe theyears yearssince since1994—with Division Division of of the the U.S. U.S. Geological Geological Survey, Survey, and it was in this capacity that he carried out most of of his his geochronologic geochronologicresearch researchthat thatwe weacknowledge acknowledgetoday. today. Over aa period period of of several several years, years, working working with with field field geologists, geologists, Zell Zell has has been been mainly mainly responsible for for the the isotopic isotopictime time framework framework for for the the pre-Keweenawan pre-Keweenawan that that we we accept accepttoday. today. responsible A A major majorcontribution contributionwas wasaastudy studyof ofArchean Archeanrocks rocksacross acrossthe theGreat GreatLakes Lakestectonic tectoniczone zoneinin the the Marenisco-Watersmeet Marenisco-Watersmeet area in Michigan. This Thisstudy study established established that that gneisses gneisses in in the the Minnesota Minnesota River RiverValley Valley subprovince subprovinceare are as asold oldas as3,550 3,550 Ma Ma and and flanking flankingmetavolcanicmetavolcanicmetasedimentaryrocks rocksininthe theWawa Wawasubprovince subprovince(to (tothe thenorth) north)are arein inthe therange range2.6-2.8 2.6-2.8 Ga. Ga. metasedimentary This Thispattern pattern has hasproved proved to tobe becharacteristic characteristicof of Michigan-Wisconsin. Michigan-Wisconsin. In 1980's,in in an aninnovative innovativestudy study utilizing utilizing Rb-Sr Rb-Sr ages of biotite, Zell outlined the In the the late late 1980's, uplift uplift history history of of the theGoodwin GoodwinSwell Swellininnortheastern northeasternWisconsin—a Wisconsin-a lithospheric lithosphericflexure flexure caused by by crustal crustal loading loading along along the Midcontinent Rift System. This This isis the the sort sort of of thing thing that caused has been been characteristic characteristic of Zell. More Morethan than most most people, people, he he has has the the ability ability to use use various has geochronologic geochronologictechniques techniquesto tosolve solveproblems problemsmost mostof ofus usmortals mortalsdon't don'tknow knoweven evenexist. exist. Since Since 1994, 1994,Zell Zell has has been been involved involved in in DOE's DOE'SYucca Yucca Mountain Mountain Project, and some of the the successful successfultechniques techniquesused usedto tosolve solveprojects projectsjust just blow blowyour yourmind. mind. II should duties and and skills. skills. From should mention mention Zell's Zell's administrative administrative duties From 1971 1971to to 1976, 1976,he hewas was branch branch chief chief of of the theUSGS's USGS's Branch Branchof ofIsotope IsotopeGeology, Geology, aatime timewhen whenthe theIsotope IsotopeBranch Branch was Since 1994, 1994, he he has has assembled assembled an an isotope isotope hydrology hydrology team team under under the the was flourishing. flourishing. Since Yucca Yucca Mountain Mountain Project Project that that isisdoing doingmany many marvelous marvelousthings, things, particularly particularly using using Rb-Sr Rb-Sr tracers. tracers. P.K. Sims Sims P.K. xi �STUDENT TRAVEL AWARDS STUDENT AWARDS The 1986 Awards to support 1986 Board Board of of Directors Directors established established the ILSG Student Travel Awards the annual annual meeting meeting of ofthe theInstitute. Institute. The awards will will be be made made from from aa student participation at the fund set up for this purpose. purpose. These special fond These awards are intended to help defray some of the direct travel costs of attending Institute Institute meetings, meetings, and include a waiver of registration fees, fees, but exclude exclude expenses expenses for for meals, lodging, and field trip registration. The The number number of of awards awards and value are determined determined by the annual annual Chair Chair in consultation consultation with the Secretary-Treasurer. Secretary-Treasurer. Recipients Recipients will will be be announced announced at at the the annual annualbanquet. banquet. The following following general general criteria will be considered by the annual Chair, who is responsible for the selection: selection: 1) The The applicants applicantsmust must have have active active resident (undergraduate or graduate) student status at the the time time of of the the annual annual meeting meeting of of the the Institute, Institute, certified certified by the department department head. 2) Students Studentswho whoare arcthe thesenior seniorauthor authoron on either either an an oral oral or or poster paper will be given favored consideration. consideration. 3) ItItisisdesirable desirablefor fortwo twoor ormore morestudents studentsto to jointly request request travel assistance. assistance. 4) In Ingeneral, general,priority prioritywill willbe begiven givento tothose thosein in the the Institute Institute region region who are are farthest farthest away from the meeting location. 5) award request request shall shall be made in writing to the annual Chair, and should 5) Each travel award explain need, student student and and author author status, status, and and other other significant significant details. details. Successful applicants Successful applicants will will receive receive their their awards awards at at the time time of registration registration for for the meeting. meeting. xli �STUDENT STUDENTPAPER AWARDS AWARDS Each year, the Institute selects the best of the student presentations and honors presenters with a monetary award. award. Funding with Funding for for the the award award isis generated generated from from registrations registrations of the the annual meeting. meeting. The TheStudent StudentPaper PaperCommittee Committeeisisappointed appointedby by the the annual annual meeting meeting Chair Chairin in annual such a manner as to to represent represent aa broad broad range range of of professional professional and and geologic geologic expertise. expertise. Criteria for for best best student student paper—last paper-last modified modified by the the Board Board in in 1997—follow: 1997-follow: Criteria 1) The Thecontribution contributionmust mustbe bedemonstrably demonstrablythe thework work of of the the student. student. 2) The Thestudent studentmust mustpresent presentthe thecontribution contributionin-person. in-person. to grant, grant, and and whether whether or or 3) The TheStudent StudentPaper Paper Committee Committee shall shall decide how many awards to not to give give separate separateawards awards for for poster poster vs. vs. oral oral presentations. presentations. 4) In Incases casesof ofmultiple multiplestudent studentauthors, authors,the the award award will be made to the senior author, or the award award will will be be shared shared equally equally by by all all authors authors of of the the contribution. contribution. amount of the awards 5) The The total amount awards is left left to the discretion discretion of the meeting Chair Chair and and 5) Secretary-Treasurer, but in the the amount amount of of about about $300 $300US. US. Secretary-Treasurer, but typically typically is is in 6) 6 ) The TheSecretary-Treasurer Secretary-Treasurermaintains, maintains,and andwill willsupply supply to to the the Committee, Committee,aa form form for for the the numerical ranking of presentations. This by Student Paper Paper This form form was created and modified by Committees an effort effort to to reduce reduce the the difficulties difficultiesthat that may may arise arisefrom from Committeesover over several severalyears years in in an background. The selection by raters of diverse background. The use of the form is not required, but is left to the discretion of the Committee. discretion the Committee. 7) The Thenames namesofofaward awardrecipients recipientsshall shallbe beincluded includedas aspart part of of the the annual annual Chair's Chair's report reportthat that appears appears in in the the next next volume volume of of the the Institute. Institute. 1998 1998 STUDENT STUDENT PAPER COMMITTEE COMMITTEE Nancy Nelson—Committee Nelson-Committee Chair Chair North Shore Shore Technical Technical Communications Randy Van Schmus Schmus University University of Kansas Kansas Peter Whelan Whelan University University of Minnesota—Morris Minnesota-Morris ll �1998 BOARD OF DIRECTORS BOARD OF DIRECTORS (Board membership membership through the close of the meeting year shown) shown) James Co-chair (2001) (200 1) James D. Miller, Jr., Co-chair Mark A. A. Jirsa, Jirsa, Co-chair Co-chair and and Institute InstituteSecretary-Treasurer Secretary-Treasurer (2000) (2000) Minnesota Minnesota Geological Geological Survey Survey Ronald P. Sage Sage (2000) (2000) Ronald P. Ontario Ontario Geological Geological Survey—Sudbury, Survey-Sudbury, Ontario Ontario Laurel G. G. Woodruff Woodruff (1999) (1999) U.S. Geological Geological Survey—Mounds Survey-Mounds View, View, Minnesota Minnesota Mark Mark C. C. Smyk Smyk (1998) (1998) Ontario Ontario Geological Geological Survey—Thunder Survey-Thunder Bay, Bay, Ontario Ontario 1998 1998 LOCAL LOCAL PLANNING PLANNING COMMITTEE COMMITTEE James D. D. Miller, Miller, Jr.—Co-chair Jr.-Co-chair Mark MarkA. A. Jirsa—Co-chair Jirsa-Co-chair and andmeeting meeting Treasurer Treasurer Terrence TerrenceJ.J.Boerboom—Field Boerboom-Field Trip TripCoordinator Coordinator Lori LoriDay—Meeting Day-Meeting Coordinator Coordinator Assistance Assistanceto to the thelocal localCommittee Committeewas wasprovided providedby by the thefollowing followingindividuals individualsfrom fromthe the Minnesota MinnesotaGeological GeologicalSurvey: Survey: David David L. L. Southwick Southwick Director Directorof of host host organization organization G.B. G.B. Morey Morey Manuscript Manuscriptreview review Alan Alan Knaeble Knaeble Field Field and and meeting meeting preparation preparation Barb Lusardi Lusardi Manuscript Manuscriptpreparation preparation xiv �1998 SESSION SESSION CHAIRS CHAIRS (In (In order of appearance) appearance) P.K. P.K. Sims Sims U.S. Geological Geological Survey—Denver Survey-Denver G.B. G.B. Morey Morey Minnesota Geological Minnesota Geological Survey Survey Lundy Jim Lundy Minnesota Minnesota Pollution Pollution Control Control Agency Agency Laurel Woodruff Woodruff U.S. Geological GeologicalSurvey—Mounds Survey-Mounds View View Johnson Rod Johnson Bitterroot Resources Resources Ltd. Mark Smyk Smyk Ontario Geological Geological Survey—Thunder Survey-Thunder Bay John Green Green University University of Minnesota—Duluth Minnesota-Duluth Paul Weiblen Weiblen University University of Minnesota—Twin Minnesota-Twin Cities Cities 1998 BANQUET BANQUET SPEAKER Bevan French Smithsonian Institution Smithsonian "When the thesky skyfalls: falls: Large Large meteorite impacts impacts and and 'When of earth earth and other worlds" the history of worlds" xv �43rd ANNUAL INSTITUTE 43rd ANNUAL INSTITUTE ON ON LAKE LAKE SUPERIOR SUPERIOR GEOLOGY GEOLOGY MEETING SUDBURY, SUDBURY, ONTARIO The 43rd annual meeting of the the Institute Institute on on Lake Lake Superior Superior Geology Geology was held on on the the Laurentian University University in Sudbury, Sudbury, Ontario from May 6, through May 11, 11, 1997. 1997. campus of Laurentian The meeting was organized organized by the Precambrian Section and the Resident Geologists Office of the Ontario Ontario Geological Geological Survey. Ron Ron Sage, Sage,Precambrian Precambrian Section, Section, and Wilf Wilf Meyer, Meyer, co-chaired the the event. event. Mrs. Resident Geologist, Sudbury, co-chaired Mrs. Tracy Tracy Livingstone, Livingstone, Resident Resident Geologist's Office, helped organize and coordinate all all activities activitiesrelating relatingto tothe the meeting. meeting. All the guidebooks and individual abstracts were reviewed by W. Meyer, R.P. Sage, and T. Livingstone. The Proceedings Proceedings of of the the 43rd 43rd ILSG ILSGwere were published publishedin in 77 parts: parts: Part 1: Part 1: Program ProgramandAbstracts and Abstracts(edited (editedby byRon RonSage Sageand and Wilf Wilf Meyer); Parti: TheHuronian HuronianSupergroup Supergroupbetween betweenSault Sault Ste. Ste. Marie and and Elliot Lake: Evidence Part2: The for the the Early Proterozoic atmosphere, atmosphere, climate, and tectonics (G. (G. Bennett, K.D. Card, and K.Y. Tomlinson); K.D. Tomlinson); Part Part 3: 3:New Newinformation informationon onthe the Grenville Grenville Front Front near nearSudbury Sudbury (A. (A.Davidson); Davidson); Part Group (S.F.M. Part 4: 4: The TheSudbury SudburyStructure, Structure, with with emphasis emphasis on the Whitewater Group Gibbins); Gibbins); Part 5: Part 5:Magmatic Magmaticore oredeposits depositsofofthe theSudbury Sudbury Igneous Complex (S.A. (S.A. Prevec); Part 6: Part 6: Alkalic Alkalicrocks rocksof of the the Sudbury region (R.P. (R.P. Sage); and Part 7: Part 7:The Thegreening greeningof of Sudbury Sudbury (K. (K.Winterhalder). Winterhalder). A total of 101 101 people registered registered for the meeting in Sudbury and most of the field trips were well attended. There Therewere were13 13paid paid participants participantsfor for the the Huronian Supergroup Supergroup field trip, 22 for the Grenville Grenville Front, 15 15 for for the the Sudbury Sudbury Structure, 36 for the Magmatic ore deposits, deposits, 6 the greening greening of of Sudbury Sudburyfield fieldtrip. trip. Access to local mining for the Alkalic rocks, and 3 for the sites was granted granted by by INCO INCO Limited Limited and and Falconbridge Falconbridge Limited. Limited. Coffee breaks were supported by Laurentian University, University, Falconbridge, Falconbridge, and Ministry of of Northern Northern supported by Laurentian and the the Ministry Development and Development and Mines. Mines. The annual banquet banquet was attended attended by 83 83 diners and held in the Cavern of Science Science North on Ramsey Lake. The TheGoldich GoldichMedal Medal was was awarded awarded to to Ron Ron Sage Sage of of the the Ontario Ontario Geological Geological Survey geology in in the the Lake Superior Superior region. region. The Survey for his contributions contributions to geology The award award was was presented by by Wilf presented Wilf Meyer, Meyer, Resident Resident Geologist, Geologist, Sudbury, Sudbury, Ministry Ministry of of Northern Northern Development and Mines. The Thebanquet banquet address address was was given given by by Dr. Dr. Peter Peter Lightfoot, Lightfoot, INCO INCO talkwas was"Origin "Originof ofthe theSudbury SudburyStructure Structureand andits itsmineral mineral wealth". wealth". Limited. The Thetitle titleof ofthe thetalk The best Student Student Paper Paper Award, Award, consisting consisting of $270 $270 (Can), (Can), was won by Craig Craig Mancuso Mancuso for for his paper "Initial "Initial results of Ar/Ar ArIAr mineral dating from the Peevy node area of northern Michigan and Dunbar dome area of northeastern Wisconsin". Zachary Zachary Naiman Naiman won won $150 $150 for his poster "Petrogenesis of Chengwatana volcanics, volcanics,Minnesota Minnesotaand andWisconsin". Wisconsin". Three student travel awards of $150 $150 each each were were granted granted to to Terry Terry Arcuri, Arcuri, Zachary Zachary Naiman, Naiman, and and Dean Peterson. The Board Board of Directors Directors of of the the Institute Institute on on Lake Lake Superior SuperiorGeology Geology met met in in Sudbury Sudbury on on May May 8, 1997. Members Membersof of the the Board Board in in attendance attendancewere were Mark Mark Smyk, Smyk, Jim Miller (representing (representing Laurel Woodruff), Mark Jirsa (Secretary-Treasurer), Ted Ted Bornhorst, Bornhorst, Ron Ron Sage, Sage, and and Wilf Wilf Meyer. Invited LaBerge. Actions Actions taken were: Invitedguests guestswere were Tracy Tracy Livingstone Livingstone and Gene LaBerge. xvi �1) Accepted Acceptedthe thereport report of of the the Chair, Chair,42nd 42nd ILSG, ILSG, Laurel Laurel Woodruff, including the minutes of the last board meeting. the 1998 meeting. meeting. Jim 2) Approved Approvedan anoffer offerby by the the Minnesota Minnesota Geological Survey to host the Jim the meeting. meeting. The offer by Charles Blackburn to to chair chair Miller and Mark Jirsa will co-chair the the meeting meeting in Kenora Kenora in 1998 1998had to be withdrawn due due to circumstances circumstances beyond beyond his his control. The The1998 1998meeting meetingwill willbe be held held in in Minneapolis, Minneapolis,Minnesota Minnesota on on May 6-10. 6-10. 3) Approved Approvedthe the1996-1997 1996-1997financial financialreport reportpresented presentedby by Mark MarkJirsa. Jirsa. 4) Approved ApprovedRon RonSage Sageas asthe theGoldich GoldichMedal Medal winner winner for for 1997. 1997. 5) Goldich Medal guidelines guidelines prepared by Ken Card. 5) Approved the Goldich 6) Approved Approvedthe thereplacement replacementof of Ken Ken Card Card by Mark Mark Smyk Smyk on the Goldich Medal Committee. Committee. 7) Discussed Discussedthe theILSG ILSGnewsletter newsletter and and web site. The Thenewsletter newsletterhas has been been well well received received and and will be continued. We Wemay mayeliminate eliminatethe the annual annual publication of the Constitution Constitution and place (http://www.geo.mtu.edu/great_lakes/ilsg). The web a general preamble on the web site (http://www.geo.mtu.edu/great-lakeslilsg). site includes information information about about the organization organization and its constitution, constitution, award guidelines, guidelines, publications by the list of publications the Institute Institute and and Goldich Goldich Medal Medal recipients. recipients. 8) Approved Approvedthe themembership membershipguidelines guidelinesfor forthe the ILSG ILSG prepared prepared by Mark Mark Jirsa Jirsa and and Ted Ted in an an effort effort to reduce its size and lower mailing Bornhorst. Mailing Mailinglist listwill will be be revised revised in costs. 9) Discussed was tabled tabledfor forfurther furtherreview. review. A Discussed the the Eisenbrey Eisenbrey Fund and action on this fund was committee consisting committee consisting of Gene LaBerge, Mark Jirsa, and Mark Smyk was formed to study and make make recommendations recommendationson on how how the the monies monies in in the the Eisenbrey Eisenbrey Fund Fund are are to to be be used. Interest Interestearned earnedfrom fromthis thisfund fundmay maybe beused usedto to support supportstudent studenttravel travel to to the the annual annual Institute meeting, and corporations and individuals may may donate donate to to the the fund. fund. Criteria to Institute receive funding receive funding from from the the Eisenbrey Eisenbrey Fund Fund need need to to be be established. established. 10) Discussed Discussed the the ILSG ILSG mailing mailing list. list. Mark MarkJirsa Jirsaisisto tobe be in in charge charge of of the the list. list. 11) Discussed DiscussedUS-Canada US-Canada currency currency exchange. exchange. ItItwas wasagreed agreedthat that prices prices for for Canadian Canadian meetings should should always always list list aa US currency currency price price because many US US participants participants have have difficulty difficulty obtaining obtaining Canadian Canadian funds. funds. Budgeting for the 43rd ISLG was done with an attempt to keep cost down and and not not lose lose money. This Thiswas wassuccessful, successful, in in part, part, due due to to logistical logistical support support provided by the Ontario Ontario Geological Geological Survey. Survey. We enjoyed serving as Co-chairs of the 43rd Institute on Lake Superior Geology meeting. Organizing the meeting was a challenge, Organizing challenge, as the the Ontario Ontario Geological Geological Survey was going going through a difficult period of reorganization, reorganization, and two of the three individuals planning the height of of the the planning planningprocess. process. A special thanks meeting received surplusing notices at the height is due to Wilf Meyer Meyer and and Tracy Tracy Livingstone Livingstone for for staying staying with with the the planning planning and and organization organization of the meeting while their lives were were being being upset upset due due to to reorganization. reorganization. Without Without their their efforts, the meeting meeting could could not not have have been been organized. organized. Ronald P. Sage Wilf Meyer Co-chairs, 43rd ILSG Sudbury, Sudbury, Ontario Ontario xvii �PROGRAM �CALENDAR OF EVENTS EVENTS AND PROGRAM CALENDAR AND PROGRAM WEDNESDAY. MAY 6 0730-0800 0730-0800 0800-1800 0800- 1800 REGISTRATIONIPRE-REGISTRATTONPACKET PACKET PICK-UP PICK-UP REGISTRATIONIPRE-REGISTRATION -Function Area Holiday Inn Pre Pre-Function PRE-MEETING FIELD TRIPS TRIPS Pre-Function Area Depart from: Holiday Inn Pre-Function EARLY PROTEROZOIC PROTEROZOIC INTRUSIVE INTRUSIVE ROCKS ROCKS OF OF EAST-CENTRAL EAST-CENTRAL 11)) EARLY MINNESOTA MINNESOTA Leaders: Terry Boerboom, J i m , and Daniel Daniel Hoim Holm Boerboom, Mark Mark Jima, 2) 2 ) GEOLOGY GEOLOGY OF OF THE THE SOUTHEASTERN SOUTHEASTERNPORTION PORTION OF OF THE THE MINNESOTA AND MIDCONTINENT RJFF RIFT SYSTEM, SYSTEM, EASTERN MINNESOTA AND WESTERN WISCONSIN Leaders: Karl Wirth, Bill Cordua, Bill Kean, Mike Middleton, and Zach Naiman Naiman GLACIAL EXOTICA 3) GLACIAL EXOTICA OF OF THE THE TWIN TWIN CITIES CITIES AREA AREA Leaders: Howard Hobbs, A1 Al Knaeble, and Gary Meyer 1600-2200 REGISTRATION/PRE-REGISTRATION PACKET REGISTRATIONPRE-REGISTRATION PACKET PICK-UP PICK-UP PUBLICATIONS FOR SALE PUBLICATIONS Pre-Function Area Holiday Inn Pre-Function 1700-2200 POSTER SET-UP Pre-Function Area Holiday Inn Pre-Function 1800-2200 WELCOMING WELCOMING RECEPTION/POSTER RECEPTIONPOSTER SESSION SESSION Free Beer and Hors d'oeuvres; Cash Bar /Aragon Ballroom Holiday Inn Pre-Function Area /Aragon Ballroom A-B xd XXI �THURSDAY, MAY 7 0730-1630 0730-1630 REGISTRATIONIPRE-REGISTRATIONPACKET PICK-UP PICK-UP REGISTRATION/PRE-REGISTRATION PUBLICATIONS PUBLICATIONS FOR SALE SALE Holiday Inn Pre-Function Area 0730-0820 0730-0820 COFFEE COFFEE AND TEA TEA Pre-Function Area Holiday Inn Pre-Function SESSION I: 1: GEOLOGICAL SESSION GEOLOGICALOVERVIEW OVERVIEW OF OFTHE THELAKE LAKE SUPERIOR SUPERIOR REGION REGION Aragon Ballroom D-E-F Session Session Chairs: Chairs: P.K. Sims Sims (U.S. (U.S. Geological Geological Survey) Survey) G.B. Morey (Minnesota (Minnesota Geological Geological Survey) Survey) 0820-0830 0820-0830 WELCOME (Jim Miller—Meeting WELCOME Miller-Meeting Co-chair, Minnesota Geological Survey) Survey) 0830-0900 0830-0900 Card,Ken KenD. D.(Card (Cardand andAssociates AssociatesGeosearch) Geosearch) Card, America Archean geology of the Great Lakes region of North America 0900-0930 0900-0930 Ojakangas, Ojakangas,Richard RichardW. W.(University (UniversityofofMinnesota—Duluth) Minnesota-Duluth) Generalized Generalized Early Proterozoic history, Lake Superior region 0930-1000 Cannon, Cannon, William William F. F.(U.S. (U.S.Geological Geological Survey) Survey) Understanding the Middle Middle Proterozoic Proterozoic history of the the Lake Lake Superior Superiorregion: region: What's new? What's What's next? next? Pre-Function Area) AND POSTER POSTER SESSION SESSION (Holiday (Holiday Inn Pre-Function Area) 1000-1030 COFFEE COFFEEBREAK BREAK AND 1030-1100 1030-1 100 Runkel, Anthony Runkel, AnthonyC. C.(Minnesota (MinnesotaGeological Geological Survey) Survey) Paleozoic rocks in the northern part of the central midcontinent midcontinent of North Paleozoic America 1100-1130 1100-1 130 Patterson, Carrie Patterson, CarrieJ.J.(Minnesota (MinnesotaGeological Geological Survey) Survey) Models for interpreting the Quaternary Quatemary history of the Lake Superior region. 11130-1200 130-1200 Southwick, David L. (Minnesota (MinnesotaGeological GeologicalSurvey) Survey) What's next for geology in the Lake Superior Superior area? 1200-1400: LUNCH LUNCH BREAK ILSG BOARD MEETING (Grill (Grill Room Restaurant, Holiday Inn) Inn) POSTERS (Holiday -Function Area) Area) POSTERS (HolidayInn Inn Pre Pre-Function xdi �SESSION II SESSION I1 Aragon Ballroom D-E-F D-E-F Session Chairs: Session Jim Lundy (Minnesota (Minnesota Pollution Control Control Agency) Agency) Laurel Woodruff Woodruff (U.S. Geological Geological Survey) Survey) 1400-1420 1400-1420 Pfannkuch, H.0, H.O, and Paulson, Pfannkuch, Paulson, Richard A. A. assessments of groundwater Improved geologic geologic sensitivity sensitivity and vulnerability assessments groundwater pollution potential potential through fuzzy logic logic pollution through application application of fuzzy 1420-1440 1420-1440 Campion, Moira Ground water recharge, recharge, discharge discharge and and residence residence time time in Rice Rice County, County, Minnesota: Implications Implications for land use planning 1440-1500 1440-1500 Cannon, Cannon, W.F., W.F., Kolker, Alan, and and Westjohn, Westjohn, D.B. D.B. water in southeastern Michigan Michigan The geological source of arsenic in ground water -Function Area) AND POSTER POSTER SESSION SESSION (Holiday (Holiday Inn Pre Pre-Function 1500-1520 COFFEE COFFEEBREAK BREAK AND 1520-1540 1520-1540 Fralick, P.W., Fralick, P.W., and and Kissin, Kissin, S.A. S.A. The age and provenance of the Gunflint Gunflint lapilli tuff 1540-1600 1540-1600 Medaris, L.G., L.G., Jr., and Medaris, and Fournelle, Fournelle, J.H. J.H. Svanbergite Significance for diagenetic diagenetic processes Svanbergite in the Bamboo Baraboo Quartzite: Significance and phosphorus phosphorus flux flux in Precambrian Precambrian oceans oceans 1600-1620 1600-1620 Medaris, L.G., L.G., Jr., Brown, Medaris, Brown, P.B., P.B., and and Bunge, Bunge, R.J. R.J. PostT 1.76low-grade low-grademetamorphism metamorphismof ofthe theBaraboo Bamboo Quartzite Quartzite Post-1.76 1620-1640 1620-1640 Mudrey, M.G., M.G., Jr. Mudrey, aeromagnetic data for regional geology Use of high-resolution aeromagnetic investigations, investigations, southeastern southeastern Wisconsin Wisconsin (Where's (Where's the the kimberlite!) 1830-1900 1830-1 900 MJXER(Holiday Inn Pre-Function Pre-Function Area) MSXER(Ho1iday Area) Cash Bar 1900-2200 1900-2200 ANNUAL BANQUET BANQUET AND AND AWARDS AWARDS PRESENTATION PRESENTATION Aragon Ballroom D-E-F D-E-F • Announcement Announcement of 45th Annual Meeting Meeting location location • 1996 1996 Goldich Goldich Award Award presentation to Zell Peterman Peterman • Banquet speaker: Bevan French, French,Smithsonian SmithsonianInstitution Institution falls: Large meteorite impacts "When the sky sky/alls: impacts and the history of Earth and other other worlds" worlds" xll �FRIDAY, MAY 8 0730-1200 07301200 REGISTRATION/PRE-REGISTRATION PACKET PICK-UP REGISTRATION/PRE-REGISTRATION PICK-UP PUBLICATIONS FOR SALE (coffee (coffeeand and tea tea 0730-0800) 0730-0800) Holiday -Function Area Holiday Inn Inn Pre Pre-Function SESSION 111 III SESSION Aragon Ballroom D-E-F Session Session Chairs: Chairs: 0820-0840 0840-0900 0900-0920 0920-0940 0940-1000 Rod Johnson (Bitterroot (Bitterroot Resources Ltd.) Mark Smyk Smyk (Ontario (Ontario Geological Survey) Survey) Jirsa, Mark MarkA., A., Boerboom, Boerboom, Terrence Terrence J., and Chandler, Chandler, Val Val W. W. ursa, J., and Geologic setting of subeconomic in the Virginia Horn, subeconomic gold deposits in Minnesota: A horn of plenty or a or hornswoggle? hornswoggle? northeastern Chandler, Val W., W., Jirsa, Mark A., Chandler, Val A., and and Lively, Lively, Richard L. Gravity and magnetic studies studies in the Virginia Virginia Horn area, area, northeastern northeastern Minnesota Saini-Eidukat, Bernhardt, and Saini-Eidukat, and Bernatchez, Bernatchez, Raymond Raymond Herontrack silver-zinc-copper Preliminary ore mineralogy of the Herontrack occurrence, occurrence, Lumby Lake area, Ontario, Canada *Salo, *Salo, R.W., R.W., and and Kissin, Kissin, S.A. S.A. Kremzar Alteration and metamorphism metamorphism in an Archean lode gold deposit, deposit, Kremzar mine, Goudreau-Lochalsh gold camp, Ontario Ontario Hudak, George George J., J., Morton, Ron L., L., and Franklin, James M. Hudak, Morton, Ron M. to the Archean The recognition of a lava dome complex and its relationship to Sturgeon Sturgeon Lake caldera, caldera, northwestern northwestern Ontario Ontario 1000-1020 1000-1020 COFFEE COFFEEBREAK/FINAL BREAKfFINALPOSTER POSTER SESSION SESSION Pre-Function Area Holiday Inn Pre-Function 1020-1040 Sims, P.K., Neymark, L.A., and Peterman, Sims, P.K., Neymark, L.A., Peterman, Z.E. Z.E. Nd isotope isotope evidence evidence for Middle and Early Archean crust in the Wawa Subprovince Subprovince of the Superior Superior Province, Michigan, USA 1040-1100 Schulz, Klaus J., and Schulz, Klaus and Ayuso, Ayuso, Robert Robert A. A. Crustal Crustal recycling recycling in the evolution evolution of the Penokean Orogen: Isotopic Isotopic evidence evidence for for Archean Archean contributions contributions to crustal crustal growth growth in the the PembinePembineWausau terrane, northern northern Wisconsin Wisconsin Wausau terrane, 1100-1 120 Hoim, Daniel K., K., Schneider, David, and and Coath, Chris Holm, Daniel Schneider, David, Chris D. D. Age Age and and deformation deformation of Early Proterozoic Proterozoic quartzites quartzites in the southern southern Lake Lake Superior region: Implications Implications for extent of foreland deformation deformation during Superior during fmal final assembly of Laurentia *Czeck, Dyanna Dyanna M., M., and and Hudleston, Peter J. *Czeck, Hudleston, Peter Evidence for for aa modified modified Kinematic fabrics near Mine Centre, Ontario: Evidence transpression model Corfu, Corfu, F., and and Easton, Easton, R.M. R.M. 1140-1200 Extension of the Huronian magmatic magmatic suite suite inside the Grenville Grenville Province: New zircon U-Pb evidence from the Grenville Front Front Tectonic Tectonic Zone Zone in Street Township, Sudbury region, Ontario Ontario 1200-1400 LUNCH LUNCHBREAK BREAK(posters (postersto tobe beremoved; removed;publication publicationsales sales end) end) 1120-1 140 xxiv XXIv �SESSION IV SESSIONIV Aragon Aragon Ballroom Ballroom D-E-F D-E-F Session Session Chairs: Chairs: 1400-1420 (University of Minnesota—Duluth) John C. Green (University Minnesota-Duluth) ~ e i b l e (University n(university of Minnesota—Twin ~innesota- win Cities) Cities) Paul W. Weiblen Nicholson, Suzanne W., W., and Woodruff, Nicholson, Suzanne Woodruff, Laurel Laurel G. G. The Powder Powder Mill Mill Group Group revisited: revisited: Basal volcanic rocks of the Midcontinent The Midcontinent Rift Rift System System on on the the south south shore shore of Lake Lake Superior Superior 1420-1440 *Najman, Zachary J., J., and Wirth, Karl R. *Naiman, Zachary R. Composition Compositionand and source(s) source(s)of of Midcontinent MidcontinentRift Rift lavas lavas (Chengwatana (Chengwatana Volcanics) Volcanics) near near Clam Clam Falls, Falls, Wisconsin Wisconsin 1440-1500 *Vislova, *Vislova, Tatiana Tatiana The The relevance relevanceof of the the geology geology of of the the mid-ocean mid-ocean ridges ridgesand andophiolites ophiolitesto tothe the understanding understanding of layered layered intrusions in the Midcontinent Rift: Part Part I.I. Geometry; Geometry;Part Part II. 11.Internal Internal structure structureand and petrology petrology 1500-1520 *Maki, *Maki, John C., C., and and Bornhorst, Bornhorst, Theodore Theodore J. J. New New field field observations observations of the Clarksburg Volcanics, Upper Peninsula of Michigan Michigan 1520-1540 re-Function Area) 1520- 1540 COFFEE COFFEEBREAK(Holiday BREAK(Ho1idayInn InnPPre-Function Area) 1540-1600 Tikoff, Tikoff, Basil, Bauer, Bauer, Bob, Bob, Vigneresse, Vigneresse, Jean-Louis, Jean-Louis, and and Haggeman, Nick Nick A A gravity, gravity, magnetic, magnetic, and and structural structuralstudy study of of the the Wakemup Wakemup Bay Bay Tonalite, Tonalite, Minnesota Minnesota 1600-1620 Myers, Myers, Paul Paul E. E. Xenolithologies Xenolithologiesas asindicators indicatorsof of intrusion intrusionmechanisms mechanisms in in the theWausau Wausau Syenite SyeniteComplex, Complex,Wisconsin Wisconsin 1620-1640 BEST BEST STUDENT STUDENTPAPER PAPER AWARDS AWARDS CLOSING CLOSING 1905 BASEBALL: (HHH Metrodome) BASEBALL: TWINS TWINS vs. YANKEES (HHHMetrodome) xxv XXV �SATURDAY. SATURDAY. MAY MAY 9 0800-1800 0800-1 800 POST-MEETING FIELD TRIPS TRIPS Depart from: Holiday -Function Area Depart Holiday Inn Inn Pre Pre-Function 4) STRATIGRAPHY STRATIGRAPHY AND HYDROGEOLOGY OF PALEOZOIC ROCKS OF SOUTHEASTERN MINNESOTA Leaders: Tony Tony Runkel Runkel and and Bob Bob Tipping Tipping MINNESOTA RIVER VALLEY AND VICINITY, SOUTHWESTERN 5) MINNESOTA MINNESOTA MINNESOTA Leaders: Dave Dave Southwick Southwick and and Carrie Carrie Patterson Patterson SUNDAY, MAY MAY 10 SUNDAY, 10 0800-1800 08001800 FIELD TRIP TRIP 5, 5, DAY 2 FIELD Returns Returns to to Holiday Inn Inn Metrodome Metrodome xxvi �PRESENTATIONS POSTER PRESENTATIONS 1700 hrs. Wednesday, May May 66 through through 1400 1400 hrs. hrs. Friday, Friday, May 8 *Abbott, Kathleen *Abbott, Kathleen M., M., Thole, Thole,Jeffrey JeffreyT., T., and and Wirth, Wirth, Karl R. Petrography and and geochemistry of Midcontinent Rift rhyolite (Chengwatana Petrography (Chengwatana Volcanics) Volcanics) near near Clam Clam Falls, Falls, Wisconsin Bernatchez, Raymond Bernatchez, Raymond A. A. detailed geological geological description description of the new high grade grade silver-base silver-base A preliminary detailed metal discovery discovery in the the Lumby Lake Lake metavolcanic metavolcanic belt northeast of Atikokan, metal Ontario, Ontario, Canada Canada Boerboom, Terry, Terry, and Oberhelman, Boerboom, Oberhelman, Matt Dimension Dimension stone stone products products of Minnesota Chandler, V.W., V.W., Jirsa, M.A., Morey, Morey, G.B., G.B., and Lawler, Lawler, T. T. Chandler, Jirsa, M.A., Mineral potential assesment assesment of northern St. Louis County, southeastern Koochiching Koochiching County, County, and northeastern northeastern Itasca County, Minnesota Cannon, W.F., W.F., Laberge, Laberge, G.L., G.L., Klasner, Kiasner, J.S., J.S., and Schulz, Cannon, Schulz, K.J. K.J. Reinterpretation of the Penokean continental margin in part Reinterpretation part of of northern northern Wisconsin Wisconsin and Michigan Michigan Cannon, W.F., Cannon, W.F., McRae, McRae, M.E., M.E., and and Nicholson, Nicholson, S.W. S.W. Geographic information information system system on the geology and and copper copper deposits deposits of the the Geographic Keweenaw Keweenaw Peninsula Peninsula Daniels, D. L., Daniels, D. L., Snyder, Snyder, S. S. L., L., Nicholson, Nicholson, S. W., W., and and Cannon, Cannon, W.F. W.F. New aeromagnetic aeromagneticsurveys surveys in m Wisconsin Wisconsin by the the U.S. Geological Geological Survey Survey *Deangelis,M.T., M.T., Peck, Peck, W.H., W.H., and and Valley, *Deangelis, Valley, J.W. J.W. Polymetamorphism of skarns Polymetamorphism skarns related to the Morin Anorthosite Complex, Grenville Province, Quebec Quebec *Gramstad, Sally *Gramstad, Sally D. D. Minnesota River Valley Pre-Wisconsinan gray till in the Mankato area of the Minnesota Han, Han, T.M. T.M. A mineralographic mineralographic study of magnetite in the Biwabik Iron lion Formation, Mesabi Range, Minnesota Minnesota *Hensel, Erin, *Joslin, *Hensel, Erin, *Joslin, Rick, Rick, and Lehrmann, Lehrmann, Dan Dan Facies Ironwood Iron lion Facies and and depositional depositional environments environments of the Early Proterozoic Ironwood Formation, Formation, Mt. Whittlesey, Whittlesey, Wisconsin Wisconsin Kjerland, Kjerland, Dean Dean W., W., and Kjerland, Kjerland, Marc Marc Effects Effects of of electric-pulse electric-pulsedisaggregation disaggregation on on microfossil-bearing microfossil-bearing argillaceous argillaceous limestone of the Middle Ordovician limestone Ordovician Decorah Decorah Shale Shale xxvii �*Loofboro, Jeff, and Holm, *Loofboro, Jeff, Holm, Daniel Daniel Results of modeling Proterozoic thermal histories: Evaluating the the possible possible effects effects of of Wolf River River Batholith Batholith reheating reheating on on thermochronologic therrnochronologic data data from from northern northern Wisconsin Wisconsin Luther, Frank R. Luther, R. An Archean subaqueous subaqueous heterolithic debris flow, Irwin, Pifher, and Meader Townships, Lake Townships, Lake Nipigon Nipigon region, region, Ontario Ontario McRae, M.E., M.E., Cannon, Cannon, W.F., W.F., and and Woodruff, Woodruff, L.G. L.G. McRae, Post-glacial shorelines shorelines of Isle Isle Royale Royale -- Where are are they now? now? McSwiggen, Peter L. McSwiggen, Peter L. Electron microprobe Electron microprobe study of the Pt-Pd and related mineralization in the Minnamax/Babbitt Cu-Ni deposit MinnamaxIBabbitt *peterson, Dean Dean M., M., and Morton, *Peterson, Morton, Ronald Ronald L. GIS-based GIs-based mineral potential potential analysis analysis for lode-gold and massive sulfide sulfide deposits deposits in an Archean terrane terrane of northern Minnesota *Rausch, Deborah E., E., and and Wattrus, Nigel *Rausch, Deborah Nigel J. Seismic Seismic evidence evidence of pre-Nickerson Quaternary sediments in western Lake Superior *Schaper, D., *Suess, W., *Katzer, *Katzer, L., L., and Kean, *Schaper, D., *Suess, W., Kean, W. W. Additional paleomagnetic results for a 1500 Wisconsin 1500 Ma mafic dike at Waterloo, Wisconsin Schmidt, Susanne Susanne Th., Th., and Seifert, Schmidt, Seifert, Karl Karl Metamorphism, hydrothermal Metamorphism, hydrothermal alteration, and lateritic weathering of drilled MRS volcanic rocks in Iowa Schulz, Klaus Klaus J., J., and Schulz, and Ayuso, Ayuso, Robert Robert A. A. Crustal Crustal recycling in the evolution of the Penokean Orogen: isotopic isotopic evidence evidence for for Archean contributions contributions to crustal growth in the Pembine-Wausau terrane, northern Wisconsin *Soofi, M.A., M.A., and King, *Soofi, King, S.D. S.D. A thin viscous viscous sheet sheet approach approach to investigate investigate the post-rift post-rift evolution evolution of the the Midcontinent Rift System Grenville Orogeny Midcontinent System under the influence of Grenville *Thomas, C., C., Kean, Kean, W., W., and and Luther, F. *Thomas, F. Paleomagnetic studies Pither and Paleomagnetic studies of a Proterozoic Proterozoic porphyritic diabase dike, Pifher and Irwin Townships, Lake Townships, Lake Nipigon district, district, Ontario Wirth, Karl R., Wirth, R., and and Gehrels, Gehrels, George George E. E. Precise U-Pb zircon ages of Midcontinent Rift rhyolite (Chengwatana Volcanics), Clam Clam Falls, Falls, WI xxviii �ABSTRACTS SPECIAL SESSION: SPECIAL SESSION: Geological Geological Overview Overview of the Lake Superior Region The The 1998 1998ILSG ILSG planning planning committee committeeasked asked six six leading leading geologists geologists for for aa summary summary of of their their special specialsubfields subfieldsof of Lake Lake Superior geology, geology, including including what's new and what's next. The following following is is their their response. response. 1 �ARCHEAN ARCHEAN GEOLOGY OF THE THE GREAT GREAT LAKES LAKES REGION REGION OF NORTH NORTH AMERICA AMERICA CARD, CARD, K.D., Card Card and and Associates' Associates' Geosearch, Geosearch,Kanata, Kanata, ON, ON, K2K K2K 1M1, 1M1,email email culau2ag3@aol.com cu 1au2ag3@aol.com Archean Archean rocks rocks of of the the southern southern Canadian Canadian Shield Shield in in the the Great Great Lakes Lakes region form form the the world's world's largest km.) Archean craton, the Superior Province. These These ancient ancientrocks rocks largest (2 (2 million million sq. sq. km.) were were formed formed during during several several major major cycles cycles of volcanism, volcanism, sedimentation, sedimentation, plutonism, and and tectonism—mainly Archean(2.5-2.8 (2.5-2.8 Ga), Ga), but but also alsoin in the the Middle Middle(2.8-3.4 (2.8-3.4 Ga) Ga) tectonism-mainly in in the the Lake LakeArchean and and Early Archean (>3.4 (>3.4 Ga). Superior SuperiorProvince Province consists consists of northern and southern highgrade grade gneiss gneiss subprovinces subprovincesand and aa broad broad central central region region of of alternating alternatinggranite-greenstone granite-greenstoneand and sedimentary sedimentary belts belts (Figure (Figure X). Some Someof ofthe thehigh-grade high-grade rocks rocksrepresent represent exposures exposuresof of lower lower crust crust of of Superior SuperiorProvince Provinceuplifted uplifted in in the the Early Early Proterozoic; Proterozoic;others, others, notably notably the the Minnesota Minnesota River Archean crust. crust. The The volcanic and related River Valley Valley gneisses, gneisses, are exotic blocks of Early Archean plutonic plutonic rocks rocks are are similar similar to to those those of modern modem island island and and continental continental arcs formed formed along along convergent oceans. Middle convergent margins. The Themetasedimentaiy metasedimentary belts may mark Archean oceans. Middle Archean Archean rocks, rocks, including includingplutonic plutonicgneisses gneissesapproximately approximately3.0 3.0 Ga Gaold oldare areoverlain overlain unconformably unconformablyby by ca ca 2.8 2.8 Ga Ga platformal platformalsequences sequencesincluding includingquartzite, quartzite,quartz quartzpebble pebble conglomerate Incontrast, contrast,the theLate LateArchean Archean volcanic volcanicand and conglomerateand and komatiitic komatiitic volcanic rocks. In sedimentary sedimentarysequences sequencesare aremainly mainlyjuvenile, juvenile, derived deriveddirectly directlyfrom fromthe themantle mantleor orfrom fromcrustal crustal sources sourcesless less than than 200 200million millionyears yearsolder, older,and anddo donot nothave havestratigraphic stratigraphicbases; bases;many many greenstone greenstonebelts belts may may be beallochthonous. allochthonous. Superior SuperiorProvince Provinceisisrich rich in in minerals mineralsincluding includingcopper, copper,zinc, zinc,and andnickel nickelmassive massivesulfide sulfide and and lode lode gold gold deposits. deposits. Most Mostofofthe themineral mineraldeposits depositsoccur occurin in the the granite-greenstone granite-greenstonebelts belts where 100million million years years or orless less by by processes processesrelated relatedto tothe the where they they were were formed formedin in aa period period of of 100 major the Archean. Archean. These major volcanism, volcanism, plutonism, plutonism, and tectonism that marked the end of the These processes processes also alsoled ledto to the the formation formationof of aa large, large, stable stable craton craton called Kenorland. Kenorland Kenorland may supercontinent~oneof the ffirst i r t of Earth's supercontinents. ItIt was was may have have been been part part of aa supercontinent—one broken broken up upin inthe theEarly EarlyProterozoic, Proterozoic,parts parts of of itit migrating migrating westward westward to to Wyoming, Wyoming, and and other other parts parts going goingback backto tothe theold oldcountry countryto toform formthe theBaltic BalticShield. Shield. Archean Earthwas was significantly significantlydifferent different from modern modem Earth in many ways. The The ArcheanEarth Archean Archeanatmosphere, atmosphere,for forexample, example, lacked lacked free free oxygen oxygen and and was rich in ammonia, ammonia, carbon carbon dioxide, dioxide,and and methane. methane.Meteorite Meteoriteimpacts impactswere weremuch muchmore morefrequent, frequent,and anduntil untilabout about4.0 4.0Ga Ga probably Earthhistory history isiscalled calledthe the probably kept kept the the crust crust well-stirred. well-stirred. This Thisearly earlyperiod periodof ofEarth Hadean, Hadean, probably probably with with justification. Heat Heatflow flowwas wasalso alsohigher, higher,resulting resulting in in aa thicker thicker mantle Archean, the the rocks, rocks, mineral mineral mantle and and aa volcanically volcanically active active crust. crust. However, However,by bythe theLate LateArchean, deposits, are remarkably remarkably similar similarto to those those of of deposits,and andstructures structuresbeing being formed formedand and preserved preserved are modern modemaccretionary accretionaryorogens, orogens,notably notably those those around around the the Pacific Pacific Rim. Although Althoughcurrently currently the ArcheanSuperior SuperiorProvince Provincewas wasformed formedby by the subject subjectof ofdebate, debate,ititwould wouldappear appearthat thatthe theArchean subduction-driven subduction-drivenorogenic orogenicprocesses processessimilar similarininmost mostrespects respectstotothose thoseoperating operatingtoday. today. Additional Additional Reviews: Reviews: Card, Card,K.D., K.D.,AAreview reviewofofthe theSuperior SuperiorProvince Provinceofofthe theCanadian CanadianShield, Shield,aaproduct productofofArchean Archeanaccretion: accretion: Precambrian 48,p.p.99-156. 99-156. PrecambrianResearch, Research,v.v.48, Hamilton, Hamilton,W.B., W.B.,Evolution Evolution of of Archean Archeanmantle mantle and and crust, crust,p. p. 597-614 597-614in in Reed, Reed,J.C. J.C.Jr., Jr.,Ball, Ball,T.T., T.T.,Farmer, Farmer, G.L., G.L.,and andHamilton, Hamilton,W.B., W.B.,1993, 1993,AA broader broader view, view, in in Reed, Reed,J.C.Jr., J.C.Jr.,Bickford, Bickford,M.E., M.E.,Houston, Houston,R.S., R.S., Link, Link,P.K., P.K.,Ranking, Ranking,D.W., D.W.,Sims, Sims,P.K., P.K., and andVan VanSchmus, Schmus,W.R., W.R., eds., eds.,Precambrian: Precambrian:Conterminous Conterminous U.S, U.S, Geological GeologicalSociety SocietyofofAmerica, America,The TheGeology Geologyof of North North America, America,v.v.C-2, C-2,p.597-636. p.597-636. 3 �Figure Figurex. x. Subprovinces Subprovincesof of the the Superior Superior Province Province 4 �GENERALIZED GENERALIZED EARLY EARLY PROTEROZOIC PROTEROZOIC HISTORY, HISTORY, LAKE LAKE SUPERIOR SUPERIOR REGION REGION Department RichardW., W , Departmentof ofGeology, Geologyf Ojakangas, 0j akangas , Richard University Duluth, MN MN55812 55812 Minnesota-Duluthf Duluthf UniversityofofMinnesota-Duluth, rojakang@d*m*edu rojakang@d.umn.edu INTRODUCTION INTRODUCTION Over the the'last last century, centuryf countless countless geologists ueolouists have have worked worked to to Over decipher ~r&eroGoic geological geological history history of of decipher the the fascinating fascinating Early Early Proterozoic to this region. region* Unfortunately, Unfortunatelyf it it is is impossible impossible in in aa short s h o review review ~ to this acknowledge all all the the important important works, worksf but but each each cited cited reference reference acknowledge contains aa bibliography. bibliography* Two Two compendiums compendiums are are especially especially valuable valuable contains and and include include more more than than 500 500 and and 700 700 Precambrian Precambrian references, referencesf respectively respectively (Sims (Simset et al, alf1993; 1993; Sims Situs and and Carter, Carterf1996). 1996). As As in in any any limited limited review, reviewf the the bias bias of of the the reviewer reviewer will will be be present. present* The The integration history* integration of of plate plate tectonic tectonic theory theory permeates permeates this this history. HURONIAN HURONIAN After the the supercontinent supercontinent of of Kenorland Kenorland was was assembled assembled about about 2.7 2.7 Ga Ga After (Card, (Cardf 1990; 1990; Williams Williams et et al, alf 1991) 1991) during during the the Algoman Algoman (Keewatin) (Keewatin) orogeny, erosionf and and sedimentation sedimentation orogenyf aa long long period period of of weathering, weatheringf erosion, occurred on on this this stable stableand and thick thick crustal crustalplatform. platform* The The oldest oldest occurred sedimentary sequence, sequencef the the Huronian Huronian Supergroup, Supergroupf was deposited deposited sedimentary between 2-45Ga Ga and and 2.2 2.2 Ga, Gaf mainly mainly north north of of Lake Lake Huron. Huron* As between 2.45 As thick thick as km and and thinning thinning northward, northwardf this this is is aa sequence sequence quite quite unlike unlike 12 km as 12 in that "uncommon" glacial deposits, Archean sequences sequences in thatititcontains contains ffuncommon'fglacial depositsf Archean quartzose paleosolsf carbonates, carbonatesf and and abundant arkosic to to quartzose paleosols, sandstones. from sandstones* An An upper upper part part of of this this supergroup supergroup is is aa sequence, sequencef from the the bottom bottom up, upf of of glaciogenic glaciogenic rocks, rocksf paleosol, paleosolf quartz quartz sandstone, sandstonef and (evaporites). AA similar similar and carbonate carbonate rocks rocks with with sabkha sabkha minerals minerals (evaporites). sequence sequence is is present present as as erosional erosional remnants remnants in in the the Upper Upper Peninsula Peninsula Group, Marquette Marquette Range of Michigan, Michiganf 200 200 km km to to the the west west (Chocolay (Chocolay Groupf Range of An even even more more complete complete similar similar sequence sequence is is found found in in Supergroup)* An Supergroup). southeastern southeastern Wyoming, Wyomingf the the Snowy Snowy Pass Pass Supergroup. Supergroup* The The glacial glacial units units of of these these three three sequences sequences were were first first interpreted interpreted by by Young Young (1970) (1970)as as remnants remnants of of aa major major continental continental glaciation, glaciationf and and additional additional work work (e.g., ( e - g O fOjakangas, Ojakangasf 1984, 1984f 1985, l98Sf 1988) 1988) further further strengthens strengthens this this idea. idea* As an an interesting interesting aside, asidef Williams Williams and and Schmidt Schmidt (1997) (1997)stated stated that that paleomagnetic paleomagnetic data data indicate indicate that that this this Huronian Huronian glaciation glaciation occurred occurred 4-110 of of the the equator. equator* Further E'urther evidence evidence of the the correlation correlation within 4-11° within of these these three three sequences sequences was was provided provided by by Bekker Bekker and and Karhu Karhu (1996) (1996) and and Bekker Bekker (1998), (1998)fwho who found found similar similar high high values values of of 13C_enrichment 13C-enrichment and 'BO—depletion 180-depletion in in carbonate carbonate units. units* and Supercontinent Supercontinent Kenorland Kenorland began began to to undergo undergo extension extension at at 2.45 2.45 Ga, Gat as as indicated indicated by by mafic mafic igneous igneous rock rock units units at at the the base of of the the Huronian Huronian Supergroup Supergroup (e.g., ( e * g O Heaman, fHeamant 1997; 1997; Cheney, Cheneyf 1998; 1998; Balls, Hallsf 1998). 1998). Actual Actual breakup breakup occurred occurred at at 2.2 2.2 to to 2.1 2.1 Ga Ga with with emplacement emplacement of of Nipissing Nipissing diabase diabase sills sills and and major major dike dike swarms s w a m (Roscoe (Roscoe and and Card, Cardf 1993). 1993)- The The Kenora-Kabetogama Kenora-Kabetogama mafic mafic dikes dikes (2145 (2145 +1+/- 45 45 Ma) Ma) in in northern northern Minnesota 1983) that that show show up up so so well well on on Minnesota (Southwick (Southwickand and Day, Dayf 1983) aeromagnetic aeromagnetic maps maps (Chandler, (Chandlerf1991; 1991; Chandler Chandler et et al, alf 1984) 1984) appear appear to to be products products of of this this final final extensional extensional event. event* The The breakup breakup of of 5 �Kenorland has has an an even even broader broader intercontinental intercontinental significance; significance; aa Kenorland similar similar sedimentary sedimentary sequence sequence with with similar similar igneous igneous activity activity is is also also present in in Finland Finlandand and adjacent adjacentRussia Russia (Marmo (Mannoand and Ojakangas, Ojakangasf present 1984)fand and correlation correlation seems seems likely likely (e.g., (e.gOfOjakangas, Ojakangasf1984, 1984f1988; 1988; 1984), Ojakangasfet et al, alf1991). 1991). In In summary, summaryf Kenorland, Kenorland# which which formed formed in in Ojakangas, Archean, apparently apparentlybroke broke up up in inthe theEarly Early Proterozoic, Proterozoicf the Late Late Archean, the with with the the Wyoming Wyoming craton craton heading heading west west and and the the Svecofennian Svecofennian craton craton east* heading east. heading CORRELATION CORRELATION Correlation Correlation of of Early Early Proterozoic Proterozoic rock rock units units of of Michigan Michigan and and Ontario has has long long been been aaproblem. problem* Glacial Glacial units units have have been been used used as as Ontario marker marker units units (e.g., ( e - g oYoung, fYoungf1983; 1983; Ojakangas, Ojakangasf1988); 1988);on on this thisbasis, basisf the the lower lower portion portion of of the the Marquette Marquette Range Range Supergroup Supergroup (the (the Chocolay Chocolay Group) Group) is is correlated correlated with with the the upper upper part part of of the the Huronian Huronian Supergroup, Supergroupfincluding includingthe theGowganda Gowganda Formation Formationand and overlying overlyingunits. units. Correlation Correlation within within the the Lake Lake Superior Superior region region itself itself has has also also long long been been controversial controversial(Morey (Moreyand and Van Van Schmus, Schmusf1988; 1988;Morey, Moreyf1996). 1996). Correlationshave have long longbeen been made made on on lithostratigraphic lithostratigraphicgrounds; grounds; Correlations for for example, example! it it had had been been assumed assumed that that the the various various iron-formations iron-formations of the the Lake Lake Superior Superior region region were were correlative, correlativef but but this this is is now now of questioned* Correlations Correlations are are complicated complicated by by deformation deformation (both (both questioned. thin-and thick-skinned thick-skinned tectonics) tectonics)and and the thelack lackofofexposures exposures thin-and approaches, suchas C— because of extensive extensive glacial glacialcover. cover* New New approachesf such-as Cbecause isotopic studies studies of of carbonate carbonate units, unitsfhave have been been interpreted interpreted as as isotopic evidence evidence that that not not all all of of the the carbonate carbonate units units are are correlative, correlativef as as Unless 1998). long assumed assumed (Bekker (Bekkerand andKarhu, Karhuf1996; 1996; Bekker, Bekkerf1998). Unless the the long C-ratios have have been been affected affected by by diagenesis/metamorphism, diagenesis/metmrphismf they C-ratios they indicate indicate that that the the Kona Kona Dolomite Dolomite may may be be younger younger than than the the Bad Bad River River Dolomite Dolomite (WI (WIand and MI), MI)! the the Saunders Saunders Formation Formation (MI), (MI)fthe the Randville Randville Dolomite (MN) units units that that have have all all been been (WI)#and and the the Rabbit Rabbit Lake Lake (MN) Dolomite (WI), "correlative "correlative units". unitsw*An An excellent excellent summary summary of of the the sedimentation sedimentation and and correlation correlation of of the the continental continental margin margin assemblage assemblage is is by by Morey Morey (1996). (1996). IRON-FORMATION IRON-FORMATION Naturally Naturally enriched enriched iron iron ores ores of of the the Lake Lake Superior Superior region region made made the the U.S.A. U-S.A. an an industrial industrial giant, giantf but but only only two two of of the the many many ranges, rangesf the the are still still producing iron ore Mesabi and and the the Marquette, Marquettef are producing iron ore (taconite (taconite Mesabi Superior-type" siliceous known known pellets). "Lake "Lake Superior-typew siliceousiron-formation iron-formationisis pellets). world-wide, world-widef and and similar similar units unitshave havebeen been described described from from other other continents* For For nearly nearly aa century, centuryf geologists geologists had had thought thought they they continents. inrecent recent years years it it has were all allabout about2.0 2.0 Ga Ga old. old. However, Howeverf in has been been were deposition* In In suggested that that there there was was more more than than one one period period of of deposition. suggested Minnesotaf there thereappear appear to tobe bethree threeages agesranging rangingfrom from about about 2.2 2.2 Ga Ga Minnesota, to1 1.9 to •9 Ga Ga (Southwick (Southwickand and Morey, Moreyf 1991; 1991; Morey Morey and and Southwick, Southwickf1995); 1995); the oldest oldest two two are are laminated laminated and and fine-grained fine-grained (deposited (deposited below below the wave—base basin^?)^ in in contrast contrast to to the the younger younger wave-base in in extensional extensional basins?), Biwabik Biwabik Iron-formation Iron-formation that that contains contains sand-sized sand-sized grains grains of of chert chert and iron iron minerals, mineralsf is is cross-bedded, cross-beddedf and and contains contains two two stromatolite stromatolite and horizons shelf* In In horizons indicative indicativeof of deposition deposition on on aa shallow shallow shelf. Michigan-Wisconsinf there theremay may be be two two major major periods periods of of deposition deposition Michigan-Wisconsin, (e.g., Ojakangas, (e*go fOjakangas!1994; 1994;Morey, Moreyf1996). 1996)-The The older older Negaunee Negaunee IronIron- 6 �formation of of the t h eMarquette Marquette district d i s t r i c tappears appearstot ohave havebeen beendeposited deposited formation w a t e r s of i ndeeper deeper waters ofa adown-faulted down-faulted graben graben (with (with the t h e addition a d d i t i o n of of in t e r r i g e n o u s material material by by turbidity t u r b i d i t y currents c u r r e n t s from from the t h e south s o u t h side side as as terrigenous noted by by LaRue, LaRuef 1981), 1981)! whereas whereas the t h e Ironwood Ironwood Iron-formation Iron-formation of of the the noted Gogebic range range (Schmidt, (Schmidtf 1980) 1980) and and the t h e Bijiki B i j i k i Iron-formation Iron-formation member member Gogebic of the t h e Michigamme Michigamme Formation Formation appear appear to t ohave have been been deposited d e p o s i t e don on aa of stable marine marine shelf. s h e l f - Interesting I n t e r e s t i n g spirally s p i r a l l y coiled, coiled! megascopic megascopic stable e u k a r y o t i c algae a l g a e have have been been found found in i n the t h e Negaunee Negaunee (Han (Han and and eukaryotic Runnegar, 1992). 1992)Runnegar, The The Biwabik Biwabik Iron-formation Iron-formation (as (asthick t h i c k as as 225 225 m) m) on on the t h e Mesabi M e s a b i range range (Morey, t h e Gogebic Gogebic range range (Morey! 1992) 1992) and and the t h e Ironwood Ironwood Iron-formation Iron-formation on on the appear to t o have have aa common common origin; o r i g i n ; part p a r t of of the t h e evidence evidence is i s found found in in appear t h e underlying underlyingPokeama Pokegamaand andPalms Palms formations formationswhich which have have been been the i n t e r p r e t e dasashaving havingformed formed in i na atidal t i d aenvironment l environment(Ojakangas, (Ojakangas! interpreted 1996)- These These studies s t u d i e s indicate i n d i c a t e that t h a t the the G o W., W O f1996). 1983; Ojakangas, Ojakangas! G. 1983; marine tidally—influenced tidally-influenced iron-formations were were deposited deposited on on aa marine iron-formations shelf, s h e l f ! seaward seaward of of the t h e land-derived land-derived sand sand and and muds muds of of the t h e Pokegama Pokegama Palms formations. formations- Lougheed Lougheed (1983) (1983) also a l s o proposed proposed aa tidal tidal and Palms and environment, environment! with w i t h siderite-rich s i d e r i t e - r i c h limestone limestone the t h e primary primary preprediagenetic type- Deposition Deposition appears appears to t o have have been been related related to t o aa d i a g e n e t i c rock rock type. major marine marine transgression t r a n s g r e s s i o n (Simonson ( S h o n s o nand and Hassler, Hassler! 1996). 1996). major There There are are two two oft-cited o f t - c i t e d sources sources for f o r the t h e iron i r o n and and the t h e silica—silica-hydrothermal hydrothermal activity a c t i v i t yassociated a s s o c i a t e dwith withvolcanism volcanismiii ih the t h e basin basin ( e - g e, t Carrigan Carrigan and and Cameron, Cameron! 1991; 1991; Isley, I s l e y f 11995) 9 9 5 ) and and weathering weathering of of t h e Archean Archean craton c r a t o n ( e m g, oLepp, tLepp, 1987). 1987)- The The Ironwood Ironwood Iron-formation Iron-formation the on the t h e Gogebic Gogebic is i s interbedded interbedded with with volcanics volcanics (Klasner (Klasner et et al, alf on 1998)! the t h e Gunf G u n flint l i n t Iron-formation (once continuous with w i t h the the 1998), Iron-formation (once Biwabik Biwabik Iron-formation Iron-formation prior p r i o r to t o intrusion i n t r u s i o n of of the t h e 1.1 1.1 Ga G a Duluth Duluth Complex) i s aa prominent prominent ashy ashy bed bed in in Complex) contains c o n t a i n s tuff t u f f beds, beds, and and there t h e r e is t h e Biwabik. Biwabik- Hoffman Hoffman (1987) (1987) tied t i e d the t h e iron-formations iron-formations of of the t h e Lake Lake the and 00 isotopes i s o t o p e s of of the the t o foredeep foredeepvolcanism. volcanism- CC and S u p e r i o r region r e g i o n to Superior hydrothermal source source for f o r the t h e Fe+2 Fe+2 and and the the G u n f l i n t indicate i n d i c a t e aa hydrothermal Gunf silica! with w i t h siderite siderite being being the t h e primary primary iron i r o n mineral mineral in i n aa silica, basinward facies f a c i e s (Winter (Winter and and Knauth, Knauth! 1992). 1 9 9 2 ) - The The lower lower part p a r t of of basinward the beds t h e overlying o v e r l y i n g Virginia V i r g i n i a Formation Formation contains c o n t a i n s numerous numerous ash a s h beds (Lucente and and Morey, Morey! 1983). 1983)- Thus Thus there t h e r e is i s aa growing growing body body of of (Lucente of the evidence w a s the t h e main main source source of t h e iron iron evidence indicating i n d i c a t i n g that t h a t volcanism volcanism was silica! but b u t continental c o n t i n e n t a l weathering weathering may may indeed indeed have have been been and silica, and Drevet! 1974). 1974). Biota Biota have have long long been been touted touted a n o t h e r source source (e.g., ( e - g o ,Drever, another as as important important in i n the t h e precipitation p r e c i p i t a t i o n of of the t h e iron i r o n minerals (e.g., ( e - g O fLa La Berge! 1967; 1967; Lougheed, Lougheed! 1983; 1983; LaBerge LaBerge et e t al, a l f 1987). 1987)Berge, lint When When was w a s the t h e Biwabik Biwabik Iron-formation, Iron-formationf the t h e largest l a r g e s t and and probably probably the the P a r t of of the the youngest youngest iron-formation iron-formation in i n the t h e region, region! deposited? deposited? Part evidence comes from from the t h e less-metamorphosed less-metamorphosed and and correlative correlative evidence comes Gunf Iron-formation tthat G u n flint l i n t Iron-formation h a t iis s on sstrike t r i k e with and was once once continuous continuous with w i t h the t h e Biwabik. Biwabik. Faure Faure and and Kovach Kovach (1969) (1969) reported r e p o r t e d aa whole-rock whole-rock Rb-Sr Rb-Sr isochron isochron age age of of 1.64 1-64 +1+/- 0.2 0.2 Ga G a for f o r the the AA Sm-Nd isochron aageon Sm-Nd isochron ge-on deposition lint. ordiagenesis d i a g e n e s i sofofthe t h eGunf Gunflintd e p o s i t i o n or argillites was determined at a t 2.08 2-08 +1+/- 0.25 0.25 Ga Ga a r g i l l i t e s in i n the t h e Gunf G u n flint l i n t was (Stille whole-rock isochron o of ( S t i l l e and Clauer, Clauer! 1986). 1986)- A Sm-Nd Sm-Nd whole-rock f 2100 +1+/52 52 Ma M a was w a s reported r e p o r t e d for f o r "slaty" 88slatywportions p o r t i o n s of of the t h e Biwabik, Biwabik! but b u t this this 7 �probablya amixing mixingage ageofof Archeanand andProterozoic Proterozoic components isisprobably Archean components (Gerlach et et al, alf 1988). 1988). Quartz Quartz veins veins in in the the underlying underlying Pokegama Pokegama (Gerlach +/- 25 25 Ma Ma (Hemming (Hemminget et al, alf1990). 1990). Probably Probably most most were dated dated at at 1930 1930 +1were +/- 22 Ma Ma on on indicative of of the the age age is is aa new new U-Pb U-Pb date date of of 1876 1876 +1indicative euhedral lint euhedral zircons zirconsfrom fromaareworked reworkedtuff tuffbed bedininthe thelower lowerGunf Gunflint (Fralick and and Kissen,1998). Kissenf1998)*This This date date has has major major tectonic tectonic (Fralick implications, implicationsffor for it it indicates indicates that that these these two two iron-formations iron-formations were were deposited deposited on on the the peripheral peripheral foreland foreland bulge bulge of of the the Animikie AnMkie foreland basin, basinfrather rather than than on on the the continental continental margin margin prior prior to tothe the foreland development of of the the foreland foreland basin. basin. In development In this this scenario scenario of of aa northward-migrating northward-migrating foreland foreland basin, basinfthe the terrigenous terrigenous clastic clastic Palms Palms Formation Gogebic range rangeof of Formation and and the the Ironwood Ironwood Iron-formation Iron-formation on on the the Gogebic Wisconsin-Michigan Wisconsin-Michigan may may be be continuous continuous with with but but older older (i.e., (i.eOf diachronous) diachronous) than than the the more more northerly northerly (by (by100 100 km) km) terrigenous terrigenous clastic Pokegama Pokegama Formation Formation and and the the Biwabik Biwabfi Iron-formation Iron-formation on on the the clastic Mesabi range range of of Minnesota Minnesota (Ojakangas, (Ojakangasf1994). 1994). Mesabi PENOKEAN OROGEN OROGEN PENOKEAN of aa big big supercontinent, supercontinentf The Archean Archean ended ended with with the the formation formation of The Kenorland, Kenorlandf the the product product of of the the amalgamation amalgamation of of volcanic arcs arcs and and granitic granitic intrusive intrusivebodies, bodies! and and history history repeats repeats itself. itself* The The Penokean Penokean (Hudsonian) (Hudsonian)orogeny orogeny occurred occurred on on the the southern southern edge edge of of the the Superior Superior craton, cratonIextending extending over over aa distance distance of of 1100 1100 km km from from the the Grenville ONf westward westward to to the the area area just just west west Grenville Front Front near near Sudbury, Sudburyf ON, of of Lake Lake Superior, Superior! deforming deforming the the sedimentary sedimentary units units deposited deposited on on the the passive passive continental continental margin margin when when volcanic volcanic arcs arcs (the (the Wisconsin Wisconsin magmatic terranes) terranes) and and microcontinents microcontinents (that (that included included Archean Archean magmatic rocks) collided collided from from the the south south with with northward-directed northward-directed thrusting rocks) thrusting over over aa southward-dipping southward-dipping subduction subduction zone zone (e.g., (e.gOfMorey Morey and and Southwickf 1995; 1995; Sims, S h I1996). 1996). Southwick, The The Penokean Penokean has has long long been been an an enigma enigma for for it it seems seems to to have have been been long-lived, Ma to to 1770 1770 Maf Ma, with withthe long-lived! lasting lasting from from at least 1982 Ma the earliest earliest Proterozoic Proterozoic sedimentary sedimentary units units in in Minnesota Minnesota deposited deposited Southwickf1995). 1995). Volcanic Volcanic rocks rocks of of the the about 2200 2200 Ma Ma (Morey (Moreyand and Southwick, about orogen range range in in age age from from 1880-1840 1880-1840 Ma, MaI plutonic plutonic rocks rocks from from 19801980orogen 1770 1770 Ma, Maf and and deformation deformation from from 1982-1770 1982-1770 Ma Ma (e.g., (e.gOfSouthwick Southwick and and Moreyf 1991; 1991; Sims S h s et et al, alf1993). 1993). Volcanogenic Volcanogenic massive massive sulfide sulfide Morey, deposits CutZn, ZnIand and Pb, Pbf about about 1860-1840 1860-1840 Ma, Maf deposits and and occurrences occurrences of of Cu, are are abundant abundant based based on on extensive extensive exploration exploration drilling drilling of of geophysical geophysical anomalies; anomalies; more more than than 13 13 bodies have been identified identified (DeMatties, (Flambeau) has has been been mined mined to to ( M a t t i e s f 1994), 1994)f although although only only one one (Flambeau) date. Post-Penokean Post-Penokean rhyolites rhyolites and and granites granites occur occur in in Wisconsin Wisconsin date. ( S d et al, alf 1989) 1989) and and post-orogenic post-orogenic metamorphism metamorphism and and coeval coeval (Sims plutons plutons in in east-central east-central Minnesota Minnesota have have been dated at 1770-1760 1770-1760 Ma with with rapid rapid cooling cooling at at 1760-1750 1760-1750 Ma Ma (Holm (Holmet et al, alf 1998). 1998). Two !lWo deformations Formation have deformations in in the the southern southern portion portion of the the Thomson Formation been described, describedfwith with F1 Flisoclinal isoclinal recumbent recumbent folds folds and and F2 F2 upright upright been folds (Holstf1982, 1982f 1984). 1984)folds that that refolded refolded the the earlier earlier folds folds (Holst, Excellent Excellent interpretations interpretations of of Penokean Penokean deformation deformation can can be be found, found, S h e.gOf in in Holst Holst (1991), (1991)fKlasner Klasner et et al a1 (1991), (1991)fKiasner Klasner and and Simse.g., (1993), (1993)f Gregg Gregg (1993), (1993)! and and Sims S h and and Carter Carter (1996). (1996). 8 �By removing removing the t h e 60 60km-wide kxu-wide Middle Middle Proterozoic Midcontinent Rift Rift Proterozoic Midcontinent By System from from aa regional r e g i o n a l geological geological map, map, thereby thereby bringing b r i n g i n g Minnesota Minnesota system and Wisconsin-Michigan Wisconsin-Michigan into i n t o juxtaposition, juxtaposition, the t h e fold-and-thrust fold-and-thrust and belts of of the t h e two two areas areas become become one one continuous continuous zone. zone- Southwick Southwick and and belts Morey (1991) (1991) correlated c o r r e l a t e d the t h e Niagara ( W I ) and and Niagara fault f a u l t zone/suture zone/suture (WI) Morey t h e Malmo Malmo ddiscontinuity i s c o n t i n u i t y(MN). (MN). Similarly, S i m i l a r l y , the t h e various v a r i o u s turbiditeturbiditethe shale s h a l e units u n i t s (Thomson, (Thomson, Rove, Rove, Virginia, Virginia, Tyler, Tyler, Copps, Copps, and and Michigamme Michigamme onecontiguous contiguousuunit become one n i t ffilling i l l i n gthe t h eAnimikie Animikie Formations) then t h e n become Formations) Basin- This, This, however, however, does does not n o t imply imply that t h a t all a l l of of these t h e s e formations formations Basin. are the t h e same same age, age, for was migrating migrating northward northward during during f o r the t h e basin basinwas are d e p o s i t i o n of of these t h e s e units; u n i t s ; some some sediment sediment deposited in i n the t h e earlier earlier deposition s t a g e s of of basin b a s i n development development was was likely l i k e l y cannibalized cannibalized and and stages redepositedredeposited. asa anorthward-migrating northward-migrating The Animikie Animikie bbasin a s i n has been iinterpreted n t e r p r e t e d as The has been f r o n t of of (on (on the t h e north n o r t h side) s i d e ) of of f o r e l a n d basin b a s i n that t h a t developed developed in foreland in front of tthe t h e fold-and-thrust fold-and-thrust belt beltdue dueto t othe t h eweighting weighting down down of h e crust crust by by the t h i sfolded foldedand andthrusted t h r u s t e mountain d mountainrange rangethat t h a then t t h e nbecame became aamajor major this s o u r c e of of sediment sediment for f o r the t h e basin basin (e.g., ( e - g oHoffman, , Hoffman, 1987; 1987; Southwick Southwick source e t al, al, et 1988; Barovich Barovich et e t al, a l , 1989; 1989; Southwick Southwick and and Morey, Morey, 1991; 1991; 1988; Ojakangas, 1994; 1994; Morey Morey and and Southwick, Southwick, 1995). 1995)Ojakangas, N d isotope isotope data data determined determined by by Barovich Barovich et e t al a1 (1989) (1989) indicate i n d i c a t e that that Nd sampleshad hadananArchean Archeanprovenance provenancet ototthe some Michigainme Michigamme samples h e north n o r t h and and some some had had an a n Early E a r l y Proterozoic Proterozoic provenance provenance to t o the t h e south. southsome P a l e m u r r e n t data data for f o rthe t h Michigamme e Michigamme and h e Tyler n d i c a t e aa andt the Tyler iindicate Paleocurrent northward transport t r a n s p o r t of of sediment sediment from from the t h e south south side side of of the the northward southward transport t r a n s p o r t of of sediment sediment from from the the Animikie Basin Basin and and aa southward Animikie n o r t h side side of of the t h e basin b a s i n as as summarized summarized in i n Ojakangas Ojakangas (1994). north (1994). Hemming Hedng e t al a1 (1995), (19951, on on gochemical geochemicaland and isotopic i s o t o p i c evidence, evidence, described d e s c r i b e d the the et provenance of of the t h e Virginia V i r g i n i a Formation Formation as as aa young young differentiated differentiated provenance v o l c a n i c arc arc to t o the t h e south south of of the t h e basin. basinvolcanic The magmatic terranes t e r r a n e s were were located l o c a t e d just j u s t to t o the t h e south s o u t h of of The Wisconsin Wisconsin magmatic t h e fold-and-thrust fold-and-thrust belt. belt- The The Pembine-Wausau Pembine-Wausau terrane t e r r a n e is is comprised comprised the of older older (1889-1860 (1889-1860 Ma) M a ) magmatic magmatic rocks rocks that t h a t formed formed in i n an a n island island of arc and/or and/or back-arc back-arc basins b a s i n s above above aa south—dipping south-dipping subduction subduction zone zone arc magmatic rocks rocks ( t h e Niagara Niagara fault f a u l t zone zone or o r suture) s u t u r e ) and and younger younger magmatic (the (1845-1835 Ma) M a ) that t h a t formed formed above above aa north-dipping north-dipping subduction subduction zone, zone, (1845-1835 t h e Eau Eau Pleine P l e i n e shear s h e a r zone zone (Sims ( S h et e t al, a l , 1989, 1989, 1993). 1993)- Deep Deep seismic seismic the p r o f i l i n g indicates i n d i c a t e s that t h a t both both the t h e Niagara Niagara fault f a u l t and and the t h e Eau Eau Pleine Pleine profiling shear e t al, a l , 1991). 1991)- An ~n s h e a r zone zone may may penetrate p e n e t r a t e the t h e entire e n t i r e crust c r u s t (Cannon (Cannon et o p h i o l i t e sequence sequence in i n the t h e Quinnesec Quinnesec Formation Formation of of northeastern northeastern ophiolite Wisconsin basinWisconsin (Schulz, (Schulz, 1987) 1987) indicates i n d i c a t e s the t h e closure c l o s u r e of of an an ocean ocean basin. In I n the t h e southerly s o u t h e r l y Marshfield Marshfield terrane, t e r r a n e , 1860 1860 Ma M a rocks rocks were were deposited deposited on Archean Archean crust crust and and amalgamation amalgamation of of the t h e two two terranes t e r r a n e s along along the the on Eau Eau Pleine P l e i n e suture s u t u r e occurred occurred at a t about about 1840 1840Ma M a (Sims ( S h et e t al, a l , 1993). 1993)Three episodes episodes of of post-orogenic post-orogenic magmatism m a g m a t i s m followed, followed, dated dated at a t 1835 1835 Three Ma, M a , 1760 1760 Ma, ~ aand and , the t h e 1469+/1469+/- 28 28 Ma Ma Wolf Wolf River M v e r batholith b a t h o l i t h (Sims ( S h et et al, a l , 1989). 1989)- Nd N d isotopic i s o t o p i c data data of of Barovich Barovich et e t al a1 (1989) (1989) indicated i n d i c a t e d the the major growth growth of of new new crust crust from fromthe t h e mantle. mantlemajor The Penokean Penokean (Hudsonian) (Hudsonian) Orogen Orogen was was part p a r t of of the t h e reassembly reassembly The (Hoffman, new supercontinent supercontinent that t h a t Williams W i l l i a m s et et al a1 (1991) (1991) (Hoffman, 1988) 1988) of of aa new 9 �This supercontinent supercontinent also a l s o included included the the dubbed Hudsonland. Hudsonland. This dubbed (as did d i d the t h e Archean Archean supercontinent supercontinent of of Fennoscandian Shield S h i e l d (as Fennoscandian Kenorland). i s comparable comparable Kenorland). The The Svecokarelian Svecokarelian orogeny orogeny of of that t h a t shield s h i e l d is t o the t h e Penokean Penokean Orogen Orogen in i n many many aspects. aspects. to Q U A R T ZITES ITES QUARTZ The Penokean Penokean Orogen Orogen and and adjacent a d j a c e n t rock rock units u n i t s continued to t o be be The weathered weathered (Southwick (Southwick and and Mossler, Mossier, 1984) 1984) and and eroded for f o r about 100 100 Ma following the mountain-building culmination at about 1850 (?) M a following t h e mountain-building culmination a t about 1850 ( ? ) M a . Several S e v e r a l quartzite q u a r t z i t e units u n i t s were w e r e deposited deposited to t o the t h e south south of of the the Ma. orogen, orogen, including i n c l u d i n g the t h e Sioux Sioux Quartzite Q u a r t z i t e of of Minnesota, Minnesota, South South Dakota, Dakota, Weber, and Iowa Iowa (e.g., , Morey, Morey, 1984; 1984; Ojakangas and W e b e r , 1984; 1984; Southwick Southwick and e t al, a l , 1986) 1986) and and the t h e Baraboo Baraboo (Dott, (Dott, 1983), 1983), Barron Barron (Rozacky, (Rozacky, 1987), 1987), et Flambeau (Campbell, (Campbell, 1986), 1986), and and McCaslin McCaslin (Olson, (Olson, 1984) 1984) Quartzites Quartzites Flambeau of problem of Wisconsin. Wisconsin. Their Their ages ages and and correlation c o r r e l a t i o n have long been a problem e t al., a l e , 1991; 1991; Chandler Chandler and and Morey, Morey, (e.g., Brown, 1986; 1986; LaBerge LaBerge et (e.g., 1992; 1992; Ojakangas, Ojakangas, 1993), 1993), especially e s p e c i a l l y because because the t h e Baraboo, Baraboo, Flambeau, Flambeau, and and McCaslin McCaslin are are deformed deformed whereas whereas the t h e Sioux Sioux and and the t h e Barron Barron are are relatively r e l a t i v e l y horizontal. h o r i z o n t a l . These These "Baraboo "Baraboo Interval" I n t e r v a l w quartzites q u a r t z i t e s pose pose the t h e "Proterozoic "Proterozoic red r e d quartzite q u a r t z i t e enigma" enigmawof of Dott Dott (1983). (1983). Dott Dott (1983) (1983) e t al a1 (1993) (1993) suggested suggested that t h a t aa plate p l a t e suture s u t u r e exists exists and Van Van Schmus Schmus et and beneath the t h e Paleozoic Paleozoic cover cover to t o the t h e south, south, and tthat beneath h a t a southerly southerly t e r r a n e collided c o l l i d e d with with the t h e then-passive then-passive margin of h e continent continent terrane of tthe s o u t h e r l y dipping dipping subduction subduction zone. zone. Chandler over aa southerly Chandler and and Morey Morey over (1992) interpreted (1992) i n t e r p r e t e d paleomagnetic paleomagnetic data d a t a to t o indicate i n d i c a t e that t h a t there t h e r e are are two ages ages of of quartzite q u a r t z i t e bodies, bodies, one one late l a t e Penokean Penokean and and the t h e other other two post-1760 Ma. M a . Recent Recent U-Pb detrital d e t r i t a l zircon z i r c o n ages of 1730—1710 1730-1710 Ma M a for for post-1760 t h e Baraboo Baraboo and and the t h e McCaslin McCaslin and and Pb/Pb Pb/Pb ages on d e t r i t a l zircons z i r c o n s of the detrital 1730-1850 for f o r the t h e Sioux, Sioux, 1745—1880 1745-1880 for f o rthe t h eFlainbeau, Flambeau, and and 1750—1880 1750-1880 1730—1850 et al, a l , 1998) 1998) and and similar similar ages ages for for (Schneider et f o r the t h e Barron Barron (Schneider for d e t r i t a l zircons z i r c o n s in i n the t h e Baraboo Baraboo and and McCaslin McCaslin quartzites q u a r t z i t e s (Van detrital (Van Wyck, Wyck, 1995; Dott D o t t et e t al, a l , 1997) 1997) can can be be interpreted i n t e r p r e t e d to t o indicate i n d i c a t e that t h a t these these 1995; "Baraboo Interval" I n t e r v a l f fquartzites q u a r t z i t e s may may be be correlative. c o r r e l a t i v e . Holm "Baraboo H o l m et et al a1 (1997) have have located l o c a t e d aa 1630 1630 Ma M a thermal thermal front f r o n t in i n northwest northwest Wisconsin Wisconsin (1997) based on on cooling c o o l i n g ages ages in i n Ar/Ar A r / A r dates d a t e s on on mica; mica; this t h i s front f r o n t coincides coincides based with the t h e deformational deformationalboundary boundary between between tthe h e Barron Barron and the the with . Flambeau Flambeau. If I f one one assumes assumes tthat h a t the t h e quartzites q u a r t z i t e s are areerosional e r o s i o n a l remnants remnants of of aa l a r g e r sheet s h e e t of of quartz q u a r t z sand sand locally l o c a l l y >1500 >I500 m m thick t h i c k and deposited deposited larger on on the t h e continental c o n t i n e n t a l margin, margin, aa tremendously tremendously large l a r g e and and deeply deeply weathered must be be assumed. assumed. even weathered quartz-bearing quartz-bearing source source terrane t e r r a n e must Even without without the t h e assumption assumption of of aa single s i n g l e large l a r g e basin, basin, the t h e volume of quartz q u a r t z is i s enormous. enormous. Wheras Wheras aa fluvial f l u v i a l origin o r i g i n is i s most likely l i k e l y for for most most of of the t h e quartzites q u a r t z i t e s (e.g., (e.g., Southwick Southwick et e t al, a l , 1986), 1986), aa marine marine origin o r i g i n for f o r the t h e upper upper Sioux Sioux (Ojakangas (Ojakangas and and Weber, Weber, 1984) 1984) and and for for Baraboo (Dott, ( D o t t , 1983) 1983) has has been been proposed. proposed. p a r t of of the t h e Baraboo part REFEREHCES REFERENCES Patchett, P. P. J., J., Peterman, Petennan, Z. Z. E., E., and and Sims, Sims, P. P. K., K., 1989, 1989, Nd Nd Patchett, isotopes and and the the origin origin of of 1.9-1.7 1.9-1.7 Ga Ga Penokean Penokean continental continental crust crust of of the the Lake Lake isotopes Superior region: region: Geological Geological Society Society of of america America Bulletin, Bulletin, v. v. 101, Superior 101, p. p. 333—338. 333-338. Barovich,K. 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M., M., 1970, 1970, An An extensive extensive early early Proterozoic Proterozoic glaciation glaciation in in North North america:? Palaeography, v. 7, p. America:? Palaeography, Palaeoclimatology, Palaeoclimatology, Palaeoecology, v. p. 85—101. 85-101. , 1983, 1983, Tectono—aedimentary Tectono-sedimentary history of early Proterozoic rocks rocks of of the the northern Great northern Great Lakes Lakes region: region: in in Medaris, Medaris, L.G., L.G., Jr., Jr., (ed.), (ed.), Early Early Proterozoic Proterozoic Geology of of the the Great Great Lakes Lakes Region: Region: Geological Geological Society Society of of inerica America Memoir Memoir 160, 160, p. 15—32 p.15-32. 17 �UNDERSTANDING THE UNDERSTANDING THEMIDDLE MIDDLEPROTEROZOIC PROTEROZOIC HISTORY HISTORY OF OF THE THE LAKE LAKE SUPERIOR SUPERIOR REGION: WHAT'S REGION: WHAT'S NEW? NEW? WHAT'S WHAT'S NEXT? NEXT? W.F. Cannon, Cannon, US Geological Geological Survey, Survey, Reston, Reston, VA VA Middle Proterozoic Proterozoic time encompasses 700 million Middle million years years (1600-900 (1600-900Ma) Ma)of ofgeologic geologichistory. history. In the Lake Superior Superior region only a small part of that 1470 Ma Lake that time time is is recorded recorded in inthe therock rockrecord. record. At about 1470 large anorogenic granitic plutons plutons were were emplaced emplaced in innorthern northernWisconsin. Wisconsin. Between 1108 and about about 1108 and 1060 1060 Ma the Midcontinent rift, a 2200 km-long volcanic and subsequent sedimentary basin, formed and was was structurally structurallymodified. modified. AAbrief briefsummary summaryof ofthe thecurrent currentunderstanding understandingof ofthese thesetwo twoevents eventsisis and emphasizing findings during the presented below emphasizing the past past 15 years. years. An extended list of references for the Midcontinent Midcontinent Rift Rift presents presents some some of of the principal publications since the detailed papers in the 1982 1982 Geological Society Societyof of America America Memoir Memoir 156, 156, "Geology and Tectonics Tectonics of the Lake Superior Superior Basin". Geological For anorogenic anorogenic plutons, plutons, references references include include original original research from the 1970's 1970's and and more more recent recent For summary summary papers. papers. Anorogenic Anorogenic plutons AAsuite suiteof oflarge largegranitic graniticplutons plutons was emplaced in a broad belt spanning most of the North American continent to Arizona Arizona between betweenabout about 1500 1500and and 1450 1450Ma. Ma. The continent from Labrador to plutons plutons intruded intruded older older continental continentalcrust crust during a period generally devoid of major orogeny, orogeny, hence the name anorogenic anorogenic plutons. That Thatsuite suiteof of plutons plutons is is represented in the Lake Superior region by the Wolf River River batholith and Waussau and Stettin plutons plutons in in Wisconsin. These Theseplutons plutonswere werestudied studied extensively with the the exception exception of of extensively in the 1970's 1970's but but have have received received relatively relatively little attention since, with being discussed discussed in regional summary papers. The The Wolf Wolf River batholith, with an exposed area of about were emplaced emplaced at at about about 1470 Ma. Ma. The about 10,000 10,000km2, km2, and the smaller plutons were The Wisconsin Wisconsinplutons plutons share share characteristics characteristics with the other members of the suite in being emplaced at shallow depths in Early Proterozoic Proterozoic crust crust and and consisting consistingof of large large volumes volumes of granite granite with rapakivi affinities, affinities, and lesser lesser volumes volumes of of more mafic rocks, largely mangerite mangerite and and anorthosite. anorthosite. The Thegranites granitesof ofthe thesuite suiteare are interpreted (20±10%) of Proterozoic interpreted to have formed by relatively relatively low percentage of partial melting (20±10% granitic granitic rocks rocks in in the the lower lower crust. crust. Anorthosite Anorthosite may be derived from basaltic basaltic magma produced, produced, in part, by partial partial melting melting of mantle lithosphere. Studies Studiesof of the the Wolf Wolf River batholith indicate that mangerite magma C. Crystallization magma was was derived derived at at aa depth depth of of 21-32 21-32 km and and a temperature temperature of 950±90° 950±90C. Crystallizationof of the the granite 0-840°C and 650-840° and at at shallow shallow depths depths of of 2-4 2-4 km. km. The The granite now now exposed exposedoccurred occurredatattemperatures temperaturesof of65 cause cause of of widespread widespread melting melting of of lower lower crust crust in in a belt of continental dimensions dimensions remains obscure. obscure. Midcontinent Midcontinent rift The TheMidcontinent Midcontinentrift rift extends extendsin in an an arc arc from Kansas, through the Lake Superior basin, basin, into into southern southernMichigan. Michigan. The The physical physical character character of the rift (rock types, geometry, structure, structure, approximate decades. Research in the past 15 approximate age) age) have been well known for many decades. 15 years has added critical critical information informationto to allow allow the thekinematic kinematic and and dynamic dynamic aspects aspects of evolution evolution of the rift to be deciphered. Deep crustal seismic surveys, precise age determinations, deciphered. Deep crustal seismic surveys, precise age determinations, petrochemical petrochemical research, and experimental experimentaland and theoretical theoretical studies studiesof of mantle mantle and and lithospheric lithospheric processes can be integrated to understand understandthe the rift rift in in the the broad broad context context of of the the evolution of the North American continent continent and and globalglobalscale scale processes. processes. Seismic comprising the entire crustal Seismic surveys surveys have have shown shown that that the rift is is very deep, in places comprising thickness. thickness. Subaerial Subaerialbasalt basaltflows flowsform formthe thelower lowerpart part of of the the rift-fill rift-fill and are as as much as 20 km thick along km along the rift axis. axis. Continental Continentalclastic clasticsedimentary sedimentaryrocks rocksoverlie overliethe thebasalt basaltand andare areas asmuch muchas as10 10km thick. thick. Basalt Basalteruption eruptionand andcoeval coevalrift riftsubsidence subsidencewas was rapid rapid and and of of relatively relatively short short duration. duration. The The volcanic volcanic phase of of the the rift rift lasted lasted about about 14 14 my (1108-1094 (1 108-1094 Ma). Rates of volcanism and subsidence subsidence were were not entirely entirely uniform uniform and and appear appear to have have been greatest near the close of the volcanic phase. Subsidence Subsidenceof of volcanic volcanic basins, basins, as as inferred inferred from seismic seismic profiles, appears appears to have been by a combination combination of of normal normal faulting faulting and and broad flexure. Large Largevolumes volumesof ofmafic maficmagma magmawere werealso also 19 �emplaced both within the crust and as crustal underplating during during this this same same period. period. Basalt Basalt eruption eruption ceased rather abruptly at about 1094 Ma and subsidence rates rates declined correspondingly. correspondingly. Further filling of the rift was with continental sedimentary rocks, mostly mostly red red sandstone, sandstone, and and minor minor reduced reduced lacustrine rocks. The Theduration durationof ofsedimentation sedimentationisis not not well well constrained constrained but but most most likely likely continued, continued, lacustrine During along with subsidence, at declining rates for at least several tens tens of of millions millions of of years. years. During sedimentation, sedimentation,inversion inversion of of the the central central portions portions of of the the rift rift took took place place along along aa set set of of major majorreverse reverse faults. These Theseevents eventscombined combinedto toform formthe thepresent present geometry geometry of of the the rift. rift. A A central central horst horst contains containsthe the originally of volcanic volcanic rocks. rocks. Flanking originally deeper central parts of the rift, including thick sections of Flanking basins basins contain contain wedges of of thinner thinner volcanic volcanic sequences sequences and and overlying overlying sedimentary sedimentary rocks. rocks. The great volume of basaltic rocks and related intrusive rocks in in the the rift, rift, combined combined with with limited amounts hot during during rifting; rifling; probably amounts of lithoshperic lithoshperic extension imply that the mantle was anomalously hot about 200°C 200° hotter than present day asthenosphere. Petrochemical Petrochemical studies studies indicate indicate that much of of the the magma was generated by partial melting melting of of primitive, primitive,enriched enrichedmantle mantle.. Together, Together, these these features features point to the existence that the the rifting rifling existence of a mantle plume beneath the rift, and more specifically, suggest that the initiation of of aa new new plume plume and andthe the arrival arrival of of aa plume plume head head at at and volcanism was a consequence of the the base of the lithosphere. Modelling and numeric simulation of plume behavior shows that that when aa large mushroom-shaped mushroom-shaped head head of of hot hot new plume forms and rises from depth, it generates aa large asthenosphere. The head grows asthenosphere. grows as as it rises rises slowly slowly and is fed by its its own own narrower narrower but but faster faster rising risingstem. stem. The arrival arrival of a plume head at at the base of the lithosphere lithosphere and at at depths depths where where adiabatic adiabaticpartial partial magma generation. generation. Under suitable melting occurs creates a short period of intense magma suitable stress stress conditions conditions in the lithosphere, the plume can also initiate extension and and rifting. rifting. The The arrival arrival of of aa new new mantle mantle plume at the base of the lithosphere lithosphere at at about about 1108 1108 Ma provides the most most coherent coherentexplanation explanationfor forthe the origin of the rift and its its great great volume volume of of igneous igneous rocks. rocks. than the the volcanic volcanic basin. basin. The Rift sedimentation occurred in basins somewhat broader than The sedimentary sedimentary basins do not appear to be strongly controlled by faulting. Rather, Rather, they they appear appear as as smooth smooth flexural flexural depressions depressions on seismic seismic reflection sections. Subsidence Subsidencewas was most most likely likely aa result result of of cooling cooling of of the the head spread and and cooled cooled conductively. conductively. Sediments lithosphere as the initial plume head Sediments were initially initially derived dominantly progressed, however, however, more more older dominantly from volcanic rocks in the rift. As sedimentation progressed, with continued continued erosion erosion of of rift rift terranes outside outside of the rift were tapped as a sediment source along with volcanic volcanic rocks. Inversion of the central portion of of the the rift rift occurred occurredduring duringthe thelater laterstages stagesofofsedimentation. sedimentation. A A set set of of high angle produced displacements displacements of as angle reverse reverse faults and thrusts, in part reactivated normal faults, produced much as several tens of kilometers. The Thetime timeof offaulting faultingisis imprecisely imprecisely known, known, but but most most movement movement must have been in in the the interval interval 1080-1040 1080-1040Ma. Ma. Viewed on a more more regional scale, the Midcontinent rift has been historically somewhat of an enigma in that it is a major extensional feature feature which formed formed immediately immediately west of of the the seemingly seeminglycoeval coeval Grenville orogen. In recent years, however, precise radiometric dating shows that the short In recent years, however, precise radiometric dating shows that short interval interval (1108-1094 Ma) of extension in the Midcontinent rift appears to coincide with an interval of tectonic tectonic rift appears to coincide with an interval of (1 108-1094 quiescence, orogeny. Thus, quiescence, or possibly extension, in the multi-phase Grenvillian orogeny. Thus, aa mantle mantle plume plume arriving arriving at 1108 1108Ma Ma could could have have begun aa period period of of extension extension during during this this Grenvillian Grenvillianquiescent quiescentperiod. period. was renewed renewed and and aa period period of of northwestnorthwestAt about 1090 1090 Ma compression in the Grenville Province was directed thrusting, the Ottawan orogeny, orogeny, continued continuedfor fortens tensof ofmillions millionsofofyears. years. The The onset onset of of thrusting ininthe theGrenville GrenvilleProvince Provincecoincides coincidesremarkably remarkably closely closelywith with the the end end of of extension extensionin in the the Mideontinent Midcontinent rift rift and and the the onset onset of of compressive compressive deformation. deformation. rift marks marks the the arrival arrival of of aa new new plume plume head head at at the the It appears then that development of the Midcontinent rift at about about 1108 Ma. Ma. Melting base in the lithosphere of the Lake Superior region at Melting of the plume plume head head and and 20 �stresses generated by lateral spread of the plume caused the intensely volcanic rift. Under Undersome some stresses circumstances this event might have led circumstances led to to continental continentalbreakup. breakup. In this case, however, however, initiation of the Ottawan orogeny in the adjacent Grenville Province, transmitted northwest-southeast northwest-southeast directed stresses into the Lake Superior region. This Thisnewly newlyapplied appliedstress stressfield field not not only only terminated terminatedrift rift extension, initiated aa period period of of compressive compressive deformation deformation and and rift rift inversion. inversion. extension, but initiated The processes that formed the rift were also important in forming the major mineral deposits for which the rift is well known. The The vast vast new igneous rocks were fertile starting material for for metalmetalprocesses. Some crystallization of of concentrating processes. Someconcentration concentration occurred occurred early, during fractional crystallization magma. The Thevery verylarge large Cu-Ni Cu-Ni sulfide sulfide deposits deposits of the theDuluth Duluthcomplex complexand and other other intrusions intrusions formed formed at at this stage, as did concentrations of of platinum platinum group groupelements. elements. The rifting and related related processes processes in in the the mantle also provided a large heat engine that drove hydrothermal circulation. Those Thosehydrothermal hydrothermal systems sediment-hosted systems formed the Keweenaw native copper district, the White Pine and related sediment-hosted copper deposits, and polyrnetallic polymetallic vein vein deposits. deposits. The heat generated by the the rift rift also also caused regional regional metamorphism that converted Early Proterozoic iron-formations from non-magnetic to magnetic taconite in places such such as as the the Mesabi Mesabi Range Range and and Gogebic Gogebic Range. Range. Much progress has been made in the past 15 years in understanding fundamental causative factors of refinements the Midcontinent rift. The The data data gathered gathered in in that period lay the foundation for continued refinements of knowledge knowledge of this important structure. Research continues on refining knowledge of the age of rifting events, events, internal stratigraphy stratigraphy of rift units, the nature of plume-lithosphere interaction, and and alteration. alteration. Advances in the detail and precision precision of patterns of regional metamorphism and information information on the geometry, age, and composition of the rift from recent and continuing research, combined quantitative numerical combined with modem modem computational computational techniques, allows allows a new phase of quantitative undertaken. Examples of this new phase of research research are current research on rift evolution to be undertaken. studies studies of the transient transient thermal history of the rift, and the paleohydrology of the rift as it related to formation formation of of mineral mineral deposits. deposits. 21 �References on on the the Midcontinent Midcontinent Rift (post Selected References (post 1982) 1982) Allen, Wi., Dickas, &en, D.J., Hinze, W.J., Dickas,A.B., A.B.,and andMudrey, Mudrey,M.G., M.G., Jr., Jr., 1997, Integrated geophysical modeling of the North American Midcontinent Midcontinent Rift System: System: new interpretations interpretationsfor for western Lake Lake Superior, Superior, in northwestern Wisconsin, Wisconsin, and eastern eastern Minnesota, Minnesota, in Ojakangas, R.W., and others, others, eds., Middle Middle Proterozoic to Cambrian Proterozoic Cambrian rifting, central central North America: Geological Society Society of of America America Special Special Paper 312, p. 47-72. Cannon, Cannon, W.F., and and Hinze, Hinze, W.J., W.J., 1992, 1992,Speculations Speculationson onthe the of the the North North American American Midcontinent Midcontinentrift: rift: origin of Tectonophysics, v. 213, p. 49-55. Hinze, W.J., W.J., and Mariano, Mariano, J., J., 1995, 1995, Allen, D.J., Braile. L.W., Hinze, The Midcontinent Midcontinent rift rift system, system, U.S.A.-a major major Proterozoic in Olsen, Olsen, K.H., K.H., (ed.), (ed.), Proterozoiccontinental continental rift: in Continental rifts: evolution, evolution, structure, structure, tectonics, tectonics, International Lithosphere Lithosphere Program Program Publication Publication no. no. 264, 373-407. 264, p. p. 373-407. Cannon, W.F., W.F., and ten ten others, others, 1990, The The Midcontinent Midcontinent rift beneath Lake Superior from GLIMPCE seismic reflection profiling: Tectonics, Tectonics, v. v. 8, 8, p.305-332. p.305-332. Cannon, showing mineral Cannon, W.F., W.F., and McGervy, T.A., 1991, Map showing Midcontinent rift. rift, Lake Superior deposits of the Midcontinent region, United States and Canada: U.S. Geological Miscellaneous Field Studies Survey Miscellaneous StudiesMap Map MF-2153, MF-2153, 1:500,000. scale 1:500,000. Z.E., and and Sirns, Sims, P.K., P.K., 1993, CrustalCrustalCannon, W.F., Peterman, Z.E., scale thrusting and origin of the Montreal River monocline—a 35-km-thick cross section of the monocline-a Midcontinent rift in northern Michigan and Wisconsin: Tectonics, Tectonics, v. 12, 12, p. p. 728-744. 728-744. Behrendt, J.C., and seven Behrendt, seven others, others, 1988, 1988,Crustal Crustal structure structureof of the the Midcontinent Midcontinent rift rift system: system: results results from from GLIMPCE GLIMPCE seismic reflection profiles: Geology, v. 16, deep seismic 16, p. 81-85. 81-85. P.L., Morey, G.B., Hinze, W.J., W.J., Chandler, V.W., McSwiggen, P.L., and Anderson, R.L., 1989, Interpretation Interpretation of seismic seismic reflection, gravity and magnetic magnetic data across the Middle Proterozoic Proterozoic Midcontinent Rift System System in western Wisconsin, eastern Minnesota, and central Iowa: Iowa: American Association of Petroleum Geologists 261-275. GeologistsBulletin, v. v. 73, 73, p. p. 261-275. Behrendt, Behrendt, J.C., J.C., and seven seven others, others, 1990, 1990,Seismic Seismicreflection reflection evidence of deep (GLIMPCE) evidence deep crustal and upper mantle intrusions and magmatic magmatic underplating underplating associated with the Midcontinent rift system of North America: America: Tectonophysics, Tectonophysics,v. v. 173, 173,p.617-626. p.617-626. Davis, D.W., and Green, J.C., 1997, Geochronology Geochronology of the North American Midcontinent Midcontinent rift in western Lake Superior Superior and implications for its geodynamic geodynamic evolution: Canadian Journal of Earth Sciences, Sciences, v. 34, 476-488. p. 476-488. Berg, J.H., J.H., and and Klewin, Klewin, K.W., K.W., 1988, 1988,High-MgO High-MgOlavas lavasfrom fromthe the Keweenawan Midcontinent rift near Mamainse Mamainse Point, Ontario: Ontario: Geology, Geology, v. 16, 16, p. 1003-1006. 1003-1006. Bomhorst, T.J., 1997, Tectonic context of native copper copper Bornhorst, deposits deposits of the North American American Midcontinent Midcontinent Rift Rift system, Ojakangas, R.W., and and others, others, eds., Middle Middle system, in Ojakangas, Proterozoic Proterozoicto Cambrian Cambrian rifting, central central North America: Geological America Special Special America: Geological Society Society of of America Paper Paper 312, p. 127-136. 127-136. Davis, D.W., and and Sutdliffe, Sutcliffe, R.H., 1985, 1985,U-Pb U-Pb ages agesfrom fromthe the Nipigon plate plate and and northern northernLake LakeSuperior: Superior: Geological Society Society of America America Bulletin, Bulletin,v. v. 96, 96, p. p. 1572-1579. 1572-1579. Implications of of Campbell, I.H., and Griffiths, R.W., 1990, Implications mantle plume structure structurefor for the evolution evolution of of flood flood basalts: Earth and and Planetaiy PlanetaryScience ScienceLetters, Letters, v. v. 99, 99, p. p. 79-93. 79-93. Paces, J.B., J.B., 1990, Time Time resolution resolution of of geologic geologic Davis, D.W., and Paces, events on the Keweenaw Peninsula and implications development of the Midcontinent rift system: system: for the development Midcontinentrift Earth and Planetary Science Science Letters, Letters, v. v. 97, 97, p. p. 54-64. 54-64. Campbell McCuaig, T., T., and and Kissin, Kissin, S.A., S.A., 1997, 1997,The The Port Port Coidwell Coldwell veins, veins, northern northern Ontario: Ontario: Pb-Zn-Ag Pb-Zn-Ag deposits deposits in a rift setting, in Ojakangas. R.W., and others, eds., Middle Proterozoic Proterozoicto to Cambrian Cambrian rifting, rifting, central central North America: America: Geological Geological Society Society of of America America Paper 312, 312, p. 187-196. Special Paper 187-196. Dickas, A.B., 1986, 1986, Comparative Comparative Precambrian Precambrian stratigraphy stratigraphyand and structure along the Mid-Continent Rift: American structure Petroleum Geologists, v. 70, p. 225Association of petroleum 238. 238. Dickas, A.B., and Mudrey, M.G., Jr., Jr., 1997, 1997,Segmented Segmentedstructure structure of the Middle Proterozoic Midcontinent Rift System, Proterozoic Midcontinent North America, in Ojakangas, R.W., and others, eds., Middle Proterozoic Proterozoicto to Cambrian Cambrianrifting, rifting, central central America North America: Geological Society Society of America Paper 312, 312, p. p.37-46. Special Paper 37-46. Cannon, Cannon, W.F., 1992, 1992,The The Midcontinent Midcontinent rift rift in in the the Lake Lake Superior Superior region with emphasis emphasis on its geodynamic geodynamic evolution: Tectonophysics, v. 213, p. 41-48. Tectonophysics, 41-48. Cannon, - aafarfarCannon,W.F., W.F., 1994, 1994,Closing Closingofofthe theMidcontinent Midcontinentrift rift— field effect of Grenvillian Grenvilliancompression: compression: Geology, Geology, v. Green, A.G., and nine others, 1989, 1989, A GLIMPCE of the deep crust beneath the Great Lakes: American Geophysical Union Monograph 51, p. 65-80. 22,p. 22, p. 155-158. 155-158. Cannon, W.F., and Nicholson, S.W., S.W., 1996, 1996,Middle Middle Proterozoic Proterozoic Midcontinent MidcontinentRift Rift System: System: in in Sims, Sims, P.K., P.K., and and Carter, Carter, L.H.M., eds. eds. Archean Archean and and Proterozoic Proterozoic geology geology of of the the Lake Superior Superior region, region, U.S.A. U.S.A. U.S. U.S. Geological Geological Survey Professional Paper 1556, 1556, p.60-67. J.C., 1983, Geologic and and geochemical geochemical evidence evidence for the the Green, J.C., nature and development development of the middle Proterozoic Proterozoic (Keweenawan) (Keweenawan) Midcontinent Midcontinent Rift Rift of of North North America: America: 413-437. Tectonophysics, v. 91, p. 413-437. 22 ' �IC., 1989, Green, J.C., 1989,Physical Physicalvolcanology volcanologyof ofmid-Proterozoic mid-Proterozoic plateau lavas: lavas: the Keweenawan KeweenawanNorth North Shore Shore Volcanic Volcanic Group, Minnesota: Geological Society of America Bulletin, 486-500, America Bulletin, v 101, 101, p. p. 486-500. Manson, M.L., and Halls, H.C., 1997, Proterozoic Proterozoic reactivation Superior Province and its role in the of the southern Superior evolution evolution of the Midcontinent Midcontinent rift: Canadian Canadian Journal Earth Sciences, Sciences,v. v.34, 34,p.p.562-5 562-575. of Earth 75. Green, J.C., and six others, others, 1987, 1987,Keweenawan Keweenawandykes dykes of of the the Lake Superior region: evidence evidence for the evolution of of the Middle Proterozoic ProterozoicMidcontinent Midcontinent Rift Rift of of North North America, in Halls, Halls, H.C. and Fahrig, Fahrig, W.F., eds., America, Mafic dyke swarms, Geological Association of Canada Canada Special Special Paper Paper 34, 34, p. p. 289-302. 289-302. Mariano, J., and Hinze, W.J., 1994, Structural Structural interpretation interpretation of Suverior from the Midcontinent Rift in eastern Lake Superior seismic reflection and potential field studies: studies: seismic Canadian v.31, 619Canadian Journal of Earth Sciences, v. 3 1, p. 619628. 628. Mariano, J., and Hinze, W.J., 1994, Gravity and magnetic magnetic models models of the Midcontinent Midcontinent Rift in eastern Lake Superior: Canadian Journal of Earth Sciences, v. 31, 661-674. p. 661-674. Griffiths, R.W., and Campbell, Griffiths, Campbell,I.H., 1991, 1991,Interaction Interaction of of mantle mantle plume heads with the Earth's surface surfaceand and the onset onset of small-scale small-scaleconvection: convection: Journal Journal of of Geophysical Geophysical Research, 10. Research, v.96, v. 96,p.p.18295-183 18295-18310. P.L., Morey, G.B., 1987, McSwiggen, P.L., G.B., and Chandler, V.W., 1987, New model of Midcontinent Midcontinent rift in eastern eastern Minnesota and western Wisconsin: Wisconsin: Tectonics, Tectonics, v. 6, 677-685. p. 677-685. Hauk, S.A., and seven seven others, others, 1997, 1997,An An overview overviewof of the the geology geology and oxide, oxide, sulfide, sulfide,and and platinum-group platinum-group element element mineralization mineralizationalong along the the western western and and northern northern Ojakangas, R.W., R.W., contacts in Ojakangas, contacts of of the the Duluth Duluth complex, complex, in and others, others, eds., eds., Middle Middle Proterozoic Proterozoic to to Cambrian Cambrian rifting, of rifting, central North America: Geological Society of America 137-186. America Special SpecialPaper Paper312, 312,p.p.137-186. J.D., Jr., and Chandler, Miller, J.D., Chandler, V.W., 1997, Geology, Geology, petrology, tectonic significance of the the Beaver Bay and tectonic Complex, northeastern Minnesota, in Ojakangas, Ojakangas, R.W., and others, others, eds., Middle Middle Proterozoic Proterozoic to Cambrian rifting, central North North America: Geological Society of America Special Paper 312, p. 73-96. Heamon, L.M., and Machado, N., 1992, Timing and origin origin of of the Midcontinent Midcontinent rift rift alkaline alkaline magmatism, magmatism,North North America: America: evidence evidencefrom from the the CoIdwell Coldwell Complex: Complex: Contributions to Mineralogy and Petrology, Contributions Petrology, v. 110, 110, p. 289-303. Nicholson, SW., S.W., Shirey, Shirey, S.B., S.B., Schulz, Schulz, K.J., and Green, J.C., 1997, Rift-wide correlation of 1.1 Ga Midcontinent Midcontinent implications for multiple mantle rift system basalts: implications development: Canadian Journal of sources during rift development: Sciences, v. 34, p. 504-520. Earth Sciences, 504-520. Hieshima, G.B., G.B., and and Pratt, Pratt, L.M., L.M., 1991, 1991,Sulfur/carbon Sulfurlcarbonratios ratios and and extractable organic organic matter of the Middle extractable Middle Proterozoic Proterozoic Nonesuch Nonesuch formation, North American Midcontinent 65-79. Rift: Precambrian Research, v. 54, p. 65-79. Nicholson, SW., S.W., Cannon, Cannon, W.F., W.F., and and Schulz, Schulz, K.J., 1992, 1992, Metallogeny of the Midcontinent Rift System of North America: Precambrian Research, v., 58, 58, p. 355-386. 355-386. Hinze, Hinze, W.J., Allen, Allen, D.J., D.J., Braile, Braile, L.W., L.W., and and Mariano, Mariano,J,J,1997, 1997, Midcontinent Rift System: The Midcontinent System: aa major major Proterozoic Proterozoic continental rift, in Ojakangas, R.W., and others, eds., continental Middle Proterozoic Proterozoicto Cambrian Cambrian rifting, central central North North America: America: Geological Geological Society of America Special Paper 312, p. 7-36. Nicholson, SW., S.W., and and Shirey, Shirey,S.B., S.B., 1990, 1990,Evidence for a Precambrian mantle plume: a Sr, Nd, and Pb isotopic isotopic Midcontinent Rift System in the Lake study of the Midcontinent Superior Geophysical Research, Research, v. Superior region: Journal of Geophysical 95, p. 10851-10868. 10851-10868. Hinze, W.J., W.J., Braile, L.W., and Chandler, V.W., 1990, A geophysical profile profile of the southern southern margin margin of of the Midcontinent Rift System Midcontinent System in in western western Lake Lake Superior: Superior: Tectonics, v. v.9,9,p.p.303-3 10. 303-310. Tectonics, Non-depleted subcontinental subeontinental Paces, J.B., and Bell, K., 1989, Non-depleted mantle beneath the Superior Superior Province of the Canadian Shield: Nd-Sr isotopic and trace element element evidence Midcontinent rift basalts: Geochimica evidence from Midcontinent Geochimica et Cosmochimica Cosmochimica Acta, v. 53, p. 2023-2035. Hutchinson, Hutchinson, D.R., D.R., White, White, R.S., R.S., Cannon, Cannon,W.F., W.F.,and andSchulz, Schulz,K.J., K.J., evidence for 1990, Keweenaw hot spot: Geophysical evidence a 1.1 1.1 Ga mantle plume beneath beneath the the Midcontinent Midcontinent Rift Rift System: Journal of Geophysical Geophysical Research, Research, v. v. 95, 95, p. p. 10869-10884. 10869-1 0884. Paces, J.B., and Miller, J.D., Jr., Jr., 1993, Precise U-Pb ages of Duluth Complex and related mafic intrusions, intrusions, northeastern Minnesota: Geochronological Geochronological insights insights to physical, petrogenetic, petrogenetic, paleomagnetic paleomagnetic and tectonomagmatic Ga magmatic processes associated associated with the 1.1 1.1 Ga Midcontinent Rift system: Journal of Geophysical Geophysical Research, 13. Research, v. v. 98, 98, p. p. 13997-140 13997-14013. 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Kiewin, K.W., and Klewin, and Shirey, Shirey,S.B., S.B., 1992, 1992,The Theigneous igneouspetrology petrology and magmatic evolution evolution of the Midcontinent Midcontinent Rift System: Tectonophysics, v. 213, 213, p. 33-40. 33-40. System: 23 �Peterman, Peterman, Z.E., and and Sims, Sims, P.K., P.K., 1988, 1988,The The Goodman Goodman Swell: Swell:aa lithospheric lithosphericflexure flexure caused caused by by crustal crustal loading loading along along the Midcontinent MidcontinentRift Rift System: System:Tectonics, Tectonics,v. v. 7, 7, p. p. 1077-1090. 1077-1090. Vervoort, J.D., J.D., and Green, Green, J.C., 1997, 1997, Origin Origin of evolved evolved Vervoort, magmas in in the the Midcontinent Midcontinent rift rift system, system, northeast northeast magmas Minnesota: Nd-isotopic evidence for melting of Earth Sciences, Sciences, Archean crust: crust: Canadian Canadian Journal Journal of of Earth Archean v. V. 34, 34, p. p. 521-535. 521-535. Samson, Samson, C., and West, West, G.F., G.F., 1992, 1992,Crustal Crustal structure structureof of the the Midcontinent MidcontinentRift Rift system system in in eastern eastern Lake Lake Superior Superior from controlled-amplitude controlled-amplitudeanalysis analysisof of GLIMPCE GLIMPCE deep deep reflection seismic seismicdata: data: Canadian CanadianJournal Journal of of Earth Earth Sciences, Sciences,v. v. 29, 29, p. p. 636-649. 636-649. Weiblin, P.W., P.W., 1993, 1993,Midcontinent Midcontinentrift riftsystem, system,the theLake Lake Weiblin, Superior region region and and Trans-Hudson Trans-Hudsonorogen, orogen,in inReed, Reed, Superior J.C., J.C., Jr., Jr., and and others, others, eds., eds., Precambrian-Conterminous Precambrian-Conterminous United States: States: Boulder, Colo., GeologicalSociety Societyof of United Cob., Geological v. C2, C2, p. p. America, The The Geology Geology of of North America, America, v. America, 72-8 1. 72-81. Shirey, Shirey, SB., S.B.,1997, 1997,Re-Os Re-0s isotopic isotopiccompositions compositions of Midcontinent Midcontinent rift rift system systempicrites: picrites: implications implicationsfor for plume-lithosphere plume-lithosphereinteraction interactionand and enriched enrichedmantle mantle sources: sources: Canadian Canadian Journal Journal of of Earth Earth Sciences, Sciences,v. v. 34, 34, 489-503. p. 489-503. White, R.S., R.S., 1997, 1997, Mantle temperature and lithospheric lithospheric thinning thinning White, beneath the Midcontinent Midcontinent rift system: system: evidence evidence from from beneath magmatism and and subsidence: subsidence: Canadian ~ h a d i a Journal nJournal of of magmatism Earth Earth Sciences, Sciences,v. v. 34, 34, p. p. 464-475. 464-475. Suszek, Suszek,T., T., 1997, 1997,Petrography Petrographyand andsedimentation sedimentationof ofthe theMiddle Middle Proterozoic Proterozoic (Keweenawan) (Keweenawan)Nonesuch Nonesuch Formation, Formation, western western Lake Lake Superior Superiorregion, region, Midcontinent MidcontinentRift Rift system, in Ojakangas, Ojakangas,R.W., R.W., and and others, others, eds., eds., Middle Middle system, in Proterozoic Proterozoic to to Cambrian Cambrian rifting, rifting,central central North America: America: Geological GeologicalSociety Societyof of America AmericaSpecial Special Paper 195-210. Paper 312, 312, p. p. 195-210. White, R., R., and and McKenzie, McKenzie,D., D., 1989, 1989,Magmatism Magmatismatatrift riftzones: zones: White, The The generation generation of of volcanic volcanic continental continental margins margins and and flood basalts: basalts:Journal Journalof ofGeophysical GeophysicalResearch, Research,v.v. flood 94, 7685-7729. 94, p. p. 7685-7729. Wirth, K.R., K.R., Vervoort, Vervoort,J.D., J.D., and and Naiman, Naiman,Z.J., Z.J., 1997, 1997,The The Wirth, ChengwatanaVolcanics, Volcanics,Wisconsin Wisconsinand andMinnesota: Minnesota: Chengwatana petrogenesis of the southernmost southernmost volcanic volcanic rocks rocks petrogenesis exposedin in the the Midcontinent Midcontinentrift: rift:Canadian CanadianJournal Journal exposed 536-548. of Earth Earth Sciences, Sciences,v. v. 34, 34,p. p.536-548. of Van Van Schmus, Schmus,W.R., W.R., 1992, 1992,Tectonic Tectonicsetting settingof ofthe theMidcontinent Midcontinent Rift Rift System: System: Tectonophysics, Tectonophysics,v. v. 213, 213, p. p. 1-15. 1-15. Van Van Schmus, Schmus,W.R., W.R., and andHinze, Hinze,W.J., W.J., 1985, 1985,The TheMidcontinent Midcontinent Rift System: System: Annual Annual Reviews Reviews of of Earth Earth And And Planetary Planetary Science, Science,v. 13, 13, p. p. 345-383. 345-383. Zartman, R.E., Nisholson, Nisholson, SW., S.W.,Cannon, Cannon,W.F., W.F., and andMorey, Morey, Zartman, G.B., 1997, 1997, U-Th-Pb zircon zircon ages ages of of some some G.B., Keweenawan Supergroup Supergrouprocks rocks from from the southern southern Keweenawan shore Earth shore of of Lake Lake Superior: Superior: Canadian Canadian Journal Journal of of Earth Sciences,v. v. 34, 34,p. p. 549-561. 549-561. Sciences, Zhu, Zhu, T., T., and and Brown, Brown, L., 1986, 1986, Consortium Consortium for Continental Continental Reflection ReflectionProfiling Profiling Michigan Michigan surveys: surveys: reprocessing reprocessing and results: results: Journal Journal of of Geophysical Geophysical Research, Research, v. v. 91, 91, and p. 11477-11495. 11477-11495. p. Selected Selected references references on on anorogenic anorogenicplutons plutons Anderson, Anderson,J.L., J.L.,1993, 1993,The TheWolf WolfRiver Riverbatholith, batholith,the theLake Lake Superior in Reed, Reed, Superior region region and and Trans-Hudson Trans-Hudson orogen, orogen,in J.C., J.C.,Jr., Jr., and andothers, others,eds., eds.,Precambrian-Conterminous Precambrian-Conterminous United States: Boulder, Colo., Cob., Geological Geological Society of America, America,The TheGeology Geologyof ofNorth NorthAmerica, America,v.v.C2, C2,p.p. 69-7 69-71.1. Anderson, J.L., Cullers, Cullers, R.L., and Van Schmus, Schmus, W.R., 1980, 1980, Anderson, metaluminous and and peraluminous peraluminous granite granite Anorogenic metaluminous Anorogenic plutonism plutonism in in the the mid-Proterozoic mid-Proterozoic of of Wisconsin, Wisconsin, USA: Contributions Contributionsto to Mineralogy Mineralogyand andPetrology, Petrology,v.v. USA: 74, 74, p. p. 311-328. 3 11-328. Nelson, B.K., B.K., and and DePaobo, DePaolo, D.J., 1985, 1985, Rapid production production of of Nelson, continentalcrust crust 1.7-1.9 1.7-1.9 b.y. b.y. agoago- Nd and and Sr Sr isotopic isotopic continental evidence from from the basement basement of of the North American American evidence midcontinent: midcontinent:Geological Geological Society Society of of America America Bulletin, Bulletin,v. v. 96, 96, p. p. 746-754. 746-754. Anderson, Anderson, J.L., J.L., 1983, 1983,Proterozoic Proterozoicanorogenic anorogenic granite granite plutonism plutonism of of North North America, America, in in Medaris, Medaris,L.G., L.G., Jr., Jr., and and others, others, eds., eds., Proterozoic Proterozoicgeology: geology:selected selectedpapers papersfrom from an an International International Proterozoic ProterozoicSymposium: Symposium:Geological Geological Society Society of of America AmericaMemoir, Memoir, 161, 161,p. p. 133-154. 133-154. Sims,P.K., P.K., 1996, 1996,Early Earlyand andMiddle MiddleProterozoic Proterozoicintracratonic intracratonic Sims, rocks, rocks,in in Sims, Sims,P.K. P.K. and and Carter, Carter, L.M.H., L.M.H., eds., eds., Archeanand and Proterozoic Proterozoicgeology geology of of the the Lake Lake Archean Superior region, U.S.A.: U.S.A.: U.S. Geological Geological Survey Survey Superior ProfessionalPaper Paper1556, 1556,p.p.57-60. 57-60. Professional Anderson, Anderson,.LL., J.L., 1980, 1980, Mineral equilibria equilibria and and crystallization crystallization conditions conditionsin in the the late latePrecambrian PrecambrianWolf WolfRiver River rapakivi rapakivi massif, massif, Wisconsin: Wisconsin: American Journal of Science, Science,v. v. 280, 280, p. p. 289-332. 289-332. Van Medaris, L.G., L.G., and andBanks, P.O., 1975, 1975, Van Schmus, Schmus,W.R., W.R., ,,Medaris, Banks, P.O., Chronologyof ofPrecambrian Precambrianrocks rocksininWisconsin, Wisconsin,I:I: Chronology theWolf WolfRiver Riverbatholith, batholith,aarapikivi rapikivimassif massif the approximately1500 1500 m.y. my. old: Geological Geological Society Society of of approximately AmericaBulletin, Bulletin,v.v.86, 86,p.p.907-9 907-914. America 14. Anderson, Anderson,J.L., J.L., and andCullers, Cullers,R.L., R.L.,1978, 1978,Geochemistry Geochemistryand and evolution evolutionof of the the Wolf WolfRiver Riverbatholith, batholith,aalate late Precambrianrapakivi rapakivimassif massifininnorthern northernWisconsin, Wisconsin, Precambrian USA: v.7,7, p.p. 287-324. 287-324. USA: Precambrian PrecambrianResearch, Research,v. 24 �ROCKS IN THE THE NORTHERN NORTHERN PART OF THE THE CENTRAL CENTRAL PALEOZOIC ROCKS MIDCONTINENT OF NORTH NORTH AMERICA AMERICA RUNKEL, RUNKEL, Anthony Anthony C., C., Minnesota Minnesota Geological Geological Survey, Survey,2642 2642University University Ave AveSE, SE,St St Paul, Paul, MN, MN, 55114-1057 55 114-1057 INTRODUCTION Paleozoic Paleozoic strata strata in the central central midcontinent region of North North American American consist consistof of thin thin units of carbonate, carbonate, sandstone, sandstone, and and shale shale distributed across tens of thousands of square square kilometers. kilometers. This This overview overview focuses focuses on the northernmost extent of lower Paleozoic strata which were were deposited deposited on on the the stable stable cratonic cratonic shelf northwest northwest of the Illinois and Michigan Michigan basins sinuous belt of outcrops outcrops in southern southern basins (Figs. (Figs. 11and and 2). 2). They They are are exposed exposed in in a sinuous Minnesota, Minnesota, Wisconsin Wisconsin and and northern northern Michigan, Michigan, on the southern southern flanks flanks of the Transcontinental Transcontinental arch arch and and the the Wisconsin Wisconsin dome dome and and arch. arch. Deposition Late Cambrian Cambriantime. time. Coarse Coarse Depositionof of Paleozoic Paleozoicsediments sedimentsbegan began in in Middle Middleto to Late siliciclastic siliciclastic sediments sediments of the the Mt Mt Simon Simon Sandstone Sandstone covered covered the eroded eroded Precambrian Precambrian surface. surface. Overlying Overlying strata stratawere were deposited deposited in in two broad, laterally equivalent equivalent facies belts across the central inset) (Palmer, (Palmer, 1960). 1960).An An inner innerdetrital detritalbelt belt was was central midcontinent midcontinentregion region (Fig. (Fig.11inset) composed composed of shallow shallow marine marine siiciclastics siliciclasticsderived derived from from subaerially subaerially exposed exposed Precambrian Precambrian shield shield areas areas on on and and totothe thenorth northof ofthe theWisconsin WisconsinDome. Dome. Thin, Thin, laterally laterally extensive extensive units units dominated quartzose sandstone, sandstone, very fme fine sandstone, or dominated by either either fmefine-to tocoarse-grained coarse-grainedquartzose shale shalewere were deposited depositedin in the the inner innerdetrital detritalbelt. belt. The The middle middle carbonate carbonatebelt belt consisted consistedof ofsubtidal subtidal to to intertidal intertidal carbonate carbonate and and shale shale deposits deposits that accumulated accumulated to the south. The boundary between belts shifted, belt sedimentation shifted, with inner detrital belt sedimentation dominant during Cambrian and part part of of Middle MiddleOrdovician Ordovician time, time, and and middle middle carbonate carbonate belt sedimentation sedimentationdominant dominantduring during the as in in Silurian Silurianand and Devonian Devoniantime time (Fig. (Fig. 2). 2). the Early Early and and Late LateOrdovician, Ordovician,as as well well as Early Early Paleozoic Paleozoicdeposition depositionoccurred occurred on on aavirtually virtuallyhorizontal, horizontal,cratonic cratonicshelf shelfwith withaavery very low lOrn1m.y. (Sloss, (Sloss, low subsidence subsidence rate. Late LateCambrian Cambriansubsidence subsidenceaveraged averaged less lessthan thanlOrn/m.y. 1988), 1988),about about one-fifth one-fifth to to one-tenth one-tenth the the rate rate in in the the contemporaneous contemporaneousIllinois IllinoisBasin Basin (Sargent, (Sargent, 1991) 1991) and orders orders of magnitude magnitude slower slower than than that that of better known, known, younger younger basins basins in in North North America. (e.g. Byers Byersand and Dott, Dott, America. Maximum Maximumpaleobathymetry paleobathymetrywas was typically typically less lessthan than100 100m m(e.g. 1995; 1995; Ludvigson Ludvigson and and others, others, 1996) 1996)and and the shelf had a low gradient gradient slope slope of about about 0.1 rn/km. O.lm/km. The The Wisconsin Wisconsinarch arch and and dome dome and and Transcontinental Transcontinentalarch arch were were positive structural structural features featuresthat that influenced influencedearly early Paleozoic Paleozoic sedimentation sedimentation patterns, patterns, and and eventually eventually controlled controlledthe the distribution distribution and and configuration configurationof of gently gently folded folded and and faulted faulted Paleozoic Paleozoic strata strata today. In In northern are northern Iowa, Iowa,southern southernMinnesota, Minnesota,and and southwestern southwesternWisconsin Wisconsin Paleozoic Paleozoic strata strataare preserved preserved in in what what is is known known as as the the Hollandale Hollandale Embayment, which is a broad syncline syncline that lies lies between between these these two two positive positive features. features. South Southand andeast eastof ofthe theWisconsin Wisconsinarch archand anddome dome Paleozoic Paleozoic rocks rocks dip dipless lessthan than 11degree degreeinto into the the Illinois Illinois and and Michigan Michigan basins. basins. DEPOSITIONAL MODELS MODELS The The depositional depositionalhistory history of of lower lowerPaleozoic Paleozoicsiliciclastic siliciclasticstrata strata of of this this region regionremains remains poorly understood despite an despite over 100 100 years of study. Many Many workers workers have have lamented lamented an apparent apparent absence absenceof of modern modem or or ancient ancientdepositional depositionalanalogues. analogues.In In addition, addition,dozens dozensof of local local studies studies have have been conducted conducted (e.g. (e.g. Nelson, Nelson, 1954; 1954; Haddox and Dott, 1990) 1990) but there are few regional-scale regional-scaleinvestigations investigationsthat that incoiporate incorporate several several lithofacies lithofaciesinto into aa temporally temporally constrained constrained stratigraphic stratigraphic framework. framework.Present-day Present-day understanding understanding of early Paleozoic Paleozoic siliciclastic siliciclastic deposition deposition is is based based mostly mostly on on work work conducted conducted since since 1950, 1950,beginning beginning with with the the predominantly (1954), Nelson Nelson (1956), (1956), Berg Berg and and others others predominantly stratigraphic stratigraphic investigations investigations of of Berg Berg (1954), (1956), Bell and others (1956), and Ostrom (1964, 1970). Subsequent work has been (1964,1970). chiefly chiefly sedimentologic, sedimentologic, focusing focusing on the interpretations of near-shore marine facies (Fraser, (Fraser, 1976; 1976; Driese Driese and and others, others, 1981; 1981;Dott Dott and and others, others, 1986; 1986; Haddox Haddox and and Dott, Dott, 1990; 1990;Barnes Barnes and and 25 �Figure1.1.Location Locationmap mapshowing showing Figure earlyPaleozoic Paleozoictectonic tectonicfeatures, features, early major paleotopographic paleotopographichighs highs of of major Precambrian rocks (stippled) and the Precambrian rocks (stippled) and the approximate distribution of approximate distribution of (C),Ordovician Ordovician (0), (0), Cambrian (C), Cambrian Silurian (S), and Devonian (D) Silurian (S), and Devonian (D) stratain in the thenorthern northernpart partof ofthe the strata centralmidcontinent midcontinentregion. region. The The central inset shows the study area relative inset shows the study area relative toto theLate LateCambrian Cambrianinner inner(stipple) (stipple) the detritalbelts belts and and and outer outer(dashes) (dashes)detrital and the middle carbonate belt (block the middle carbonate belt (block pattern)of ofPalmer Palmer(1960). (1960).Modified Modified pattern) from Runkel Runkeland andothers others(1998) (1998) from GROUP! FORMATION LITHOLOGY ; i. Cedar VaHey J — I — —L —I -— I——I Manustique Burnt Bluff Cataract I—I- J —— — . Maquoketa Dubuque Galena Decorah Platteville ood !:!. — •• E •. -: • Figure2.2.Generalized Generalizedstratigraphic stratigraphiccolumn column(no (no Figure scale)for for lower lowerPaleozoic Paleozoicrocks rocksininthe thenorthern northernpart part scale) of the central midcontinent region. The Cambrian of the central midcontinent region. The Cambrian andOrdovician Ordoviciannomenclature nomenclatureisisthat thatused usedinin and southeastern Minnesota Minnesota and and Wisconsin, Wisconsin, the the southeastern Silurianfrom from eastern easternWisconsin Wisconsinand and northern northern Silurian Michigan,and and the the Devonian Devonianfrom from southernmost southernmost Michigan, Minnesotaand andnorthern northernIowa. Iowa. Minnesota Engadune Jde Group Jordan St Lawrence Franconia Ga (Wonewoc) Eau Claire and Bonneterre — . Mt. Simon E l (uatzose -sandstone sandstone sdtstone, shale (] Very Very fine fine s.s.,s.s., siltstone, shale (LoneRock) — (dashedwhere whereshaly) shaly) Carbonate(dashed Carbonate Norecord record No Unconfonnity Unconformity �others, others,1992; 1992;Mossier, Mossler,1992; 1992;Runkel, Runkel,1994). 1994).In Inaavery veryimportant importantand andwidely widelycited citedstudy study Dott Dottand andothers others(1986) (1986)examined examinedthe thetransition transitionfrom fromnonmarine nonmarinetotomarine marineenvironments environments recorded recordedin inquartzose quartzosesandstones, sandstones,and andprovided providedthe thefirst fmt set setof of criteria criteriafor foridentification identificationof of nonmarine facies.They described described an an early early Paleozoic Paleozoic terrestrial terrestrial setting setting with broad braided braided nonmarinefacies.They streams streamsand andeolian eoliansheet sheetand anderg ergsystems systemsthat that delivered delivered sand sand to to aa shallow shallowmarine marine environment environmentdominated dominatedby by small smallthree threedimensional dimensionaldunes. dunes.Dott Dottand andothers others(1986) (1986)stressed stressed the as the the lack lack of of vegetation, vegetation, between between early early Paleozoic Paleozoic depositional depositional the differences, dzfferences, such such as environments A environmentsand andthose those inferred inferredfor forboth both younger younger rocks rocks and and modem modemsettings. settings.A depositional depositional model model ofofaasimple, simple,storm-dominated, storm-dominated,texturally texturallygraded gradedshelf shelfwas wasproposed proposedby by Runkel Runkel and and others others(1998), (1998),and and isis noteworthy noteworthy in in its its similarity similari~to to models modelsbased basedon onboth both younger youngerrocks rocks and andmodem modemsystems. systems.Their Theirstudy studywas was essentially essentiallyaaregional-scale regional-scaleoutgrowth outgrowth of of the the excellent excellentoutcrop outcropwork workof of Charlie CharlieBell Belland andhis his students studentsfrom fromthe the University University of of Minnesota Minnesotain in the the 1950's. 1950's.Runkel Runkeland and others others(1998) (1998)suggested suggestedthat that early early Paleozoic Paleozoic depositional areanalogous analogousto to those thosedepicted depictedin inwell-known well-known depositionalprocesses processesand andfacies faciesdistribution distributionare models modelsof of the the Cretaceous Cretaceousinterior interior seaway and the modem Bering Bering Shelf. Shelf. The The thin widespread widespread siliciclastic chamcteristic of lower lower Paleozoic strata strata result from large largelateral lateralfacies facies siliciclasticunits units characteristic migrations relativesea sealevel. level. migrationswhich whichoccurred o c c m din inresponse responseto tocontinental-scale continental-scalechanges changesininrelative Taylors Falls Precambrian lava flow rrdges, (J,/ Mayor sand 0 — SILIcICLASTIC OFFSHORE SHELF — CARBONATEI — — — —(M0StIV — — SHELF — — — - — — — — — — o — — — Storm—t,anspolt of !° sand. suit. shale !rom shoretace — — 0 — — — — —— = — — - — — 0 — — — 0 — — — — — — Rudge : — — — — - — — — 'mmodks - Tempestltes- 01 0 0 00 — Depth 5Pm = -o — — — — 0 4OUI — -o 2O4O= I •.: 0 00 00 00 : 0 —— — .._- on Arch 0 —0 — — — — source sandstone Tudal - — — FLUVIAL& o . EOLIAN 0 SANDPLAIN sand transport by storms s.. — - Dome • — SHOREFACE 0 — ORE Figure Figure3.3.Conceptual Conceptual source from — — — — 0000 0000 0 0 0 0 (East) 0 depositional depositional model model of Late Late Cambrian Cambrianstorm-dominated, storm-dominated, texturally textxrailygraded gradedshelf. shelf. Quartzose Quartzosefmefme- to to coarsecoarsegrained gained sand sandaccumulated accumulatedin in nearshore nearshoreterresirial terrestrial and and shoreface shorefaceenvironments. environments.Finer Finer grained feldspathic grained,,feldspathic siiciclastic siliciclasticsediments sedimentsand and subtidal subtidalcarbonates carbonates accumulated accumulated in in deeper deeperwater water of the the offshoreshelf. offshoreshelf. Carbonate Carbonaterocks rocks within within the the lower lower Paleozoic Paleozoic sequence sequence have have historically historically been regarded regarded as as more more amenable amenableto to comparison comparisonwith with both both modem modem and and ancient ancientfacies facies models. models. Deposition Deposition occurred occurred in in environments environmentsthat thatrange m g efrom fromevaporitic, evaporitic,supratidal supratidalconditions conditionsfor forparts parts of of the the Prairie du Chien Chien Group Group(Smith (Smithand andothers, others,1993) 1993)to to subtidal subtidalsettings settingsin indepths depthsof of 100 100m mor or Prairie du more more for for parts parts of of units units such suchas asthe theDecorah DecorahShale Shale(Ludvigson (Ludvigsonand andothers, others,1996). 1996).Most Most investigations in scope scoperelative relativeto to the the great g m t lateral lateral extent extent of of these these units units and and investigationshave havebeen been local localin there aretoo too many many to tocite cite them them individually. individually. Ordovician Ordovician carbonate carbonatestrata strata of of the the Hollandale Hollandale thereare embayment embayment are are the the focus focus of of aa monograph monograph edited edited by by Sloan Sloan (1986). (1986). Silurian Siluriancarbonates carbonateson onthe the westem westem flank flankof of the the Michigan Michigan Basin Basin have have recently been studied studied by Harris Harris and and Waldhuetter Waldhuetter (1996). There are many excellent excellent studies studies of Ordovician through Devonian age strata strata in There are northern northem Iowa, Iowa, largely largely conducted conducted by the Iowa Geological Survey; the results can often be extrapolated several papers in Witzke and others, 1996) 1996) extrapolated to to Minnesota Minnesota and and Wisconsin Wisconsin (e.g. several Some Some regional regional scale scalestudies studies have have recently recently been been completed; completed;aa stratigraphic stmtigxaphicframework frameworkhas has been been developed developedfor for the the Prairie Fhkie du du Chien Chien Group Group from the Michigan basin to the Hollandale Hollandale embayment 1996; Barnes embayment (Smith (Smith and others, 1993, 1993,1996; Barnesand and others, others, 1996). 1996).Simo Simoand and others others (1997) are currently currently constructing constructingaa large large scale scale stratigraphic stratigraphic framework framework for for Ordovician Ordovician rocks rocks (1997) are of as identification identification of the the central centralmidcontinent midcontinentin in order order to to address address long-standing long-standing problems problems such such as of the controls responsible for the the apparently synchronous regional-scale changes from from "tropical" to %opicalV1 to "temperate" "temperate1'styles stylesof of carbonate carbonatedeposition. deposition. . 27 �NOTABLE FEATURES FEATURES AND AND LONGSTANDING LONGSTANDING PROBLEMS PROBLEMS NOTABLE LowerPaleozoic Paleozoicstrata strataininthe thecentral centralmidcontinent midcontinentregion regionare areamong amongthe thelongest longeststudied studied Lower sedimentary sedimentaryrocks rocksin in North North America. America. They Theyare arewell wellknown knownto togeologists geologistsoutside outsideof ofthe thearea area forseveral severalenigmatic enigmaticfeatures features(Dott (Dottand andByers, Byers,1980). 1980).Most Mostnotable notableisisthe theextreme extremetextural textural for andmineralogical mineralogicalmaturity maturityofofthe thefme fineto tocoarse-grained coarse-grained sandstones, sandstones,and andthe the sheet-like sheet-like and geometryofofthese theseand andother othersiliciclastic siliciclasticunits. units. The Theoverall overalldearth dearthofofshale shalehas haspuzzled puled geometry sedimentologists, sedimentologists,and andthe thefundamental fundamentalcontrols controlson onthe theepisodic episodicchange changefrom fromsiliciclasticsiliciclasticdominated dominatedto tocarbonate-dominated carbonate-dominatedsedimentation sedimentationremain remain poorly poorly understood. understood. Lastly, Lastly, the the presence, presence,position positionand andmagnitude magnitudeofofunconformities unconformitieshas hasbeen beendebated debatedfor fordecades. decades.The The remainderof ofthis thisabstract abstractsummarizes summarizesthe theprogress progressmade madeininunderstanding understandingthese theseenigmatic enigmatic remainder features featuressince sincethe theoverview overviewpublication publicationby byDott Dottand andByers Byers(1980) (1980)on onlower lowerPaleozoic Paleozoicstrata. strata. Sheet Sheetgeometry geometryAAlong-standing long-standingquestion questionhas hasbeen beenhow howsheet-like sheet-likesiliciclastic siliciclasticlayers layerswere were initially initiallydeposited depositedand andthen thenpreserved preservedacross acrosstens tensof ofthousands thousandsof ofsquare squarekilometers kilometersof ofthe the cratonic cratonicshelf. shelf.Most Mostworkers workershave haveattributed attributedthe theformation formationof ofsuch suchsheets sheetsto todepositional depositional processes aremarkedly markedlydifferent differentfrom fromthose thosedescribed describedfor formost most processesand andenvironments environmentsthat thatare younger youngerrocks rocksand andmodern modernsettings. settings.Lochman-Balk Lochman-Balk(1970) (1970)inferred inferred the the presence presence of of enormous kilometersperpendicular perpendiculartotothe the enormoustidal tidalflats flatsextending extendingwell wellover overtwo twohundred hundredkilometers shoreline. shoreline.Dott Dottand andByers Byers(1980) (1980)suggested suggestedthat that quartzose quartzosesand sandaggraded aggradedmore more or or less less vertically verticallyinto intoaasheet sheetunder underhigh-energy high-energy conditions conditions across across a vast shallow shallow sea. sea. The Themost most widely widelycited citedhypothesis hypothesisisisone onewhich whichattributes attributesthe theorigin originof ofquartzose quartzosesandstone sandstonesheets sheetstoto fluvial fluvialand andeolian eolianprocesses processesthat thatdispersed dispersedsand sandacross acrossaa vegetation vegetation free freelandscape landscapeprior priorto to marine marinereworking reworkingduring duringaatransgression bansgression(Dott (Dottand andothers others1986). 1986).Recent Recentstudies, studies,however, however, have havedemonstrated demonstratedthat thatsome somequartzose quartzosesheets sheetswere weredeposited depositedentirely entirelywithin withinthe themarine marine realm realmunder underregressive regressiveconditions conditions(e.g. (e.g.Runkel, Runkel, 1994, 1994,Hughes Hughes and and Hesselbo, Hesselbo,1997, 1997,Runkel Runkel and and others others1998), 1998),and andthat thatthe thedepositional depositionalprocesses processes responsible responsible for for the the dispersal dispersalof of siliciclastic siliciclasticsediments sedimentswere were similar similar totothose thoseoperating operatingduring duringthe the deposition deposition of of ancient ancient sandstones that are not sheet-like. sheet-like. sandstones The Thelateral lateralpersistence persistenceand andsheet-like sheet-likegeometry geometryof oflower lowerPaleozoic Paleozoicsiliciclastic siliciclasticunits units probably probablyreflect reflectthe theroles rolesof ofbasin basinphysiography physiographyand andtectonics tectonicsin in controlling controllingthe the lithostratigraphic lithostratigmphicarchitecture, architecture,rather ratherthan thanthe theexistence existenceof ofatypical atypicalsedimentary sedimentary environments. environments.Deposition Depositionwas was characterized characterized by a continuous continuous and abundant supply of sediment sedimenttotoaarelatively relativelystable, stable,nearly nearlyfiat flatbasin basin with with aa slow, slow,uniform uniformrate rateof ofsubsidence. subsidence. Individual (e.g.Fig. Fig.3) Individual sheets sheetsof of siliciclastic siliciclasticsediment sediment were were deposited when whendiscrete discretefacies facies(e.g. 3) migrated migratedgreat greatdistances distancesduring duringchanges changesininsea sealevel. level.Deep Deepincision incisionof ofthe theindividual individualsheets sheets during duringepisodes episodesof of subaerial subaerialexposure exposuredid did not not occur, occur, resulting resulting in in more more or or less less uniform uniform preservation. preservation. Textural Texturalmaturity maturitvof ofsandstones sandstonesThe Theextreme extremetextural textural and and mineralogical mineralogical maturity maturity of of the the finefineto to coarsecoarse-grained grainedsandstone sandstoneunits, units, such such as as the the St St Peter Sandstone, Sandstone, may maybe bethe thebest bestknown known and andlongest longeststudied studiedfeature featurein in the the lower lowerPaleozoic Paleozoicstrata strataof ofthe thecentral centralmidcontinent. midcontinent.The The sandstones are moderately moderately to to well wellrounded. rounded. sandstonescontain containmore morethan than 98% 98%quartz quartzand andmost most grains grainsare Such Suchtextural texturaland andcompositional compositionalmaturity maturitycould couldnot not have have been been achieved achievedsolely solelyby by fluvial fluvialand and marine marine abrasion abrasioneven evenover overtransport transportdistances distancesof of hundreds hundreds of of kilometers kilometersifif the the grains grainswere were derived derived directly directly from from crystalline source rocks. Rare Rare grains grains with abraded overgrowths are are reworked reworkedfrom fromolder oldersedimentary sedimentaryrocks, rocks,but butthe thevolumetric volumetricsignificance significanceof of such suchrecycling recyclingisis uncertain uncertain and and aa source source of of suitable suitable sedimentary rock has never been identified. Odom Odom (1975, (1975, 1978) 1978)conducted conductedthe the most mostcomprehensive comprehensivemineralogical mineralogicalstudy studyof of lower lowerPaleozoic Paleozoic sandstones; sandstones;he henoted noted that thatthe thevery very fme finegrained grainedfraction fractionis is feldspathic, feldspathic,whereas whereas the the fine fine to to coarse-grained coarse-grainedfraction fractioncontains containsmore more than than 98% 98%quartz quartz (with (with the exception exception of some some lower lower Mt Simon Simon Sandstone Sandstonebeds). beds). He He suggested suggested that that aa long history of abrasion in a marine setting 28 �could account account for for both the maturity of the quartzose grains and the reduction in size of feldspar first to to clearly clearly identi@ identify the thepresence presence of of feldspar grains. Dott and others (1986) were the fmt substantial eolian deposits quartzose sandstones, and they suggested that deposits within some quarkose eolian abrasion abrasion could contribute contribute to the textural and mineralogic features features described described by Odom Odom (1975, 1978). 1978). Lastly, Lastly,chemical chemicalweathering weatheringin inthe thesource sourcearea area must mustbe be taken taken into into consideration (Morey, consideration (Morey, 1972). 1972). Odom Odom (1975, (1975, 1978) 1978)noted that the the very very fine fine grained, grained, feldspathic sandstones are arerich rich in in K-feldspar K-feldspar but have have only a "trace" "trace" amount of plagioclase grains, even though though plagioclase plagioclase was presumably dominant dominant in in the the source source area area of of the the Precambrian shield. Selective plagioclase grains grains from from the the sandstone Precambrian Selective diagenetic leaching of plagioclase chemical weathering weathering probably preferentially dissolved was discounted by Odom. Thus, chemical plagioclase crystals and reduced reduced the thesize sizeand and amount amount of of other other relatively unstable unstable minerals minerals prior prior to to mechanical mechanicalabrasion abrasionduring during transport transport to to the the shoreline. shoreline. While each of the processes processes described above above could have been responsible responsible in part for for the the mineralogic and textural attributes of lower Paleozoic sandstones, they do not entirely mineralogic that virtually virtually all all fmeaccount for the compositional record that appears to indicate that fine- to coarse-grained sand sand was was exceptionally exceptionally mature mature when itit arrived arrived to to the the early early Paleozoic Paleozoic shoreline. shoreline. Fine- to coarse-grained coarse-grained immature immature sand would have almost certainly been abrasion model modelof ofOdom Odom(1975) (1975)were wereaccurate. accurate. It It is difficult difficult deposited locally if the marine abmion in which whichall all sand abrasion sand is subjected to prolonged eolian abrasion to reconstruct a terrestrial setting in (Dott and others others 1986) 1986) prior to deposition deposition in a marine marine environment. centmlmidcontinent midcontinent region region are are noted noted for for aa Dearth of Shale Shale Lower LowerPaleozoic Paleozoicrocks rocksof of the thecentral dearth of shale shale compared compared to other shallow marine deposits. Two Twovery very different d i f f e ~ nhypotheses hypotheses t have been proposed to account account for this dearth of shale. In a "marine bypassing" model, were delivered to the Pettijohn and others (1973) suggested that clay- and silt-sized particles were shoreline, shoreline, but subsequently subsequently carried carried in suspension suspension by repeated storms across the shallow shallow shelf before final burial in basinward areas. In contrast, Dalrymple Dairymple and others (1985) suggested that strong kilometers strongwinds windsblew blewclay clayand andsilt siltparticles particleshundreds hundredsto to thousands thousands of of kilometers offshore, to be ultimately ultimately deposited deposited in the outer detrital belt (Fig 1). Recent investigations investigations of Upper Cambrian siliciclastic siliciclastic strata support the marine bypassing well known known dearth of shale isis model of Pettijohn and others (1973), demonstrating that the well of the the outcrop outcrop belt on the flanks of the Wisconsin dome and arch characteristic only of (McKay, 1988, Wisconsin arch, subsurface 1988, Runkel and others, 1998). Basinward of the Wisconsin records from Minnesota Minnesota and and Iowa Iowa show show that facies facies dominated by shale shale and and siltstone siltstoneare are equivalents to the shale-poor offshore equivalents shale-poor common. The The shale shale and and siltstone siltstone dominated facies are offshore sandstone facies of the sandstone theoutcrop outcropbelt, belt, and and their their geographic geographic position changed through time in response outcrop belt has response to to changes changes in in the the position position of the storm-fairweather storm-fairweather wave base. The outcrop a dearth of shale shale simply because the depositional depositional record is dominated by highstand shoreface shoreface base. This facies deposited above fairweather wave base. This stratigraphic stratigraphic and sedimentologic sedimentologic attribute is not unique attribute unique to the the lower lower Paleozoic Paleozoic rocks of the central central midcontinent midcontinentregion-it region-it is is landward margins margins of of common in many younger wave-dominated wave-dominated sequences along the landward shale-free early earlyPaleozoic Paleozoicepeiric epeiricsea seaisis a deeply individual basins. The notion of a shale-free entrenched idea ideathat thatsimply simply reflects reflectsthe thebias biasof ofthe theoutcrop outcropbelt. belt. The The dearth dearth of of shale shale in in lower Paleozoic Paleozoic rocks should should be removed from the list of enigmatic enigmatic features unless the scarcity scarcity can can be be demonstrated demonstratedto to exist exist on on aa scale scale larger larger than than that that of of the the outcrop outcropbelt. belt. Siliciclastic-Carbonate Thefundamental fundamentalcontrols controlson on the the transition transition from from nearshore nearshore Siliciclastic-Carbonatecycles The siliciclasticsiliciclastic- to carbonate-dominated carbonate-dominated deposition deposition have have never never been been satisfactorily satisfactorilyunderstood. understood. the For example, example, in Early Ordovician time carbonate deposition apparently dominated across the entire region even in the shallowest water water conditions conditions (Smith and others 1993). Both older Sandstone) and younger (St Peter Sandstone) shallow marine environments were (Jordan Sandstone) dominated dominated by siliciclastics. siliciclastics. Only speculative speculative and vague hypotheses have been suggested suggested to to account account for for such such fundamental fundamental changes. For example, Adams (1978) suggested that the Early Ordovician Ordovician change change in in depositional depositionalstyles styles may have have been in response response to drowning drowning of of the the 29 �siliciclastic siliciclastic source source area by a shallow sea or the result of siliciclastics being "deflected" "deflected" to another l y Paleozoic carbonate-dominated systems another region. b Early systems developed developed following following extended extended highsea sealevel level(Smith (Smithand andothers, others,1993) 1993)which which suggests suggeststhat that the the siliciclastic siliciclastic periods of high source area may have been covered with a blanket of carbonate rocks. rocks. The The spread of certain source forms of terrestrial life such as bacterial encrustation, and later the development development of vascular vascular plants, may may have have also also reduced reduced siliciclastic siliciclasticinput. input. Crvntic of lower lower Paleozoic Paleozoic strata strata of of the the central central C w t i c unconformities Previous investigations of midcontinent region have also been hindered by an inability to recognize midcontinent recognize unconformities. unconformities. The are fairly fairly well well The unconformities unconformities bounding bounding the the major major sequences sequencesof of Sloss Sloss (1963) (1963) are established within the strata of the outcrop belt. The presence and position of "lesser" "lesser" unconformities, within siliciclastic siliciclasticunits, have have been aa matter matter of of considerable considerable unconformities,especially especially within relations and interpreted depositional histories indicate that such debate. Stratigraphic Stratigraphic relations unconformities "should" "should" be present. However, However, little or no evidence evidence for for regionally regionally extensive extensive subaerial erosion has been documented. Such sequence-bounding unconformities unconformities are are they are are contained containedwithin withincoarse coarsesiliciclastics siiciclastics because because they they difficult to recognize where they are texturally and mineralogically similar, similar, and because because separate quartzose quartzose sandstones sandstones that are that occurred occurred on they are relatively flat-reflecting erosion that on aa loose, loose, sandy sandy substrate substrate along along aa low, uniform gradient, gradient, and in a nonvegetated terrestrial terrestrial environment. environment. Furthermore, Furthermore, the the ultra ultra development of a distinctive mature mineral composition composition of the exposed substrate substrate inhibits inhibits development distinctive weathering profile. Recent work has shown some promise for identifying subtle subaerial surfaces of of regional regional erosion. Recognition Recognition of cryptic unconformities requires interpretation of stratigraphic at individual outcrops stratigraphic relations in conjunction with physical evidence collected at and high resolution resolution biostratigraphic biostratigraphic data. Smith and others (1993) interpreted local deposits of silica cement cement in uppermost Cambrian Cambrian and Lower Ordovician Ordovician strata strata as subaerially subaerially formed formed silcrete. Runkel fronton Runkeland andothers others(1998) (1998)suggested suggestedthat that cryptic cryptic unconformities unconformities within the Ironton and at at the the top of the Jordan Jordan Sandstone Sandstoneand andcan canbe be identified only and Galesville Sandstones and the coarsest coarsest bed bed within within nearshore nearshore by overlying lag deposits. 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A., and and Day, Day, J., 1991, 1991, eds., eds., Paleozoic Paleozoic Sequence Sequence Stratigraphy:Views from the North American Craton: Geological Society of America 446p. Special Paper 306, 306,446~. 32 �MODELS FOR INTERPRETING INTERPRETING THE THE QUATERNARY QUATERNARY HISTORY OF THE THE LAKE SUPERIOR SUPERIOR REGION PATTERSON, PATTERSON, Carrie Carrie J., Minnesota Minnesota Geological GeologicalSurvey Survey The interpretation interpretation of the Quaternaiy Quaternary glacial and interglacial history for the Lake Superior as aa result result of of two twonew newmodels. models. Firstly, oxygen isotope region is in the midst of change as data have provided evidence leading to the development of a model that indicates a large during the the last lasttwo twomillion millionyears. years. Secondly, a new number of glaciations occurred during mechanical model for the dynamics of ice flow has been developed. In In order orderto to integrate integrate the new models into into our our interpretations, interpretations,aa broad broad geographic geographicperspective perspectiveisis required. required. Rather than provide a summary of the history of the investigations in the Lake Superior in the the context contextof ofthese thesenew newmodels. models. Even ifif region, I will present relevant observations in to look beyond the confines the models are eventually eventually replaced, they will have forced us to of the Upper Midwest Midwest and and the the traditional traditional glacial glacial interpretations. interpretations. Number of Number of glaciations glaciations Evidence indicates indicates that the number of Quaternary glaciations is significantly greater than the turn turn of the century the four-fold glacial chronology devised for North America around the which included the Wisconsinan, Illinoian, Kansan and Nebraskan glaciations (Flint, 1957). The The new new precision precisionisis based based on on nonterrestrial nonterrestrial records records of global global ice ice volume, record of of ocean ocean carbonates. carbonates. During glaciation, 160 is is including the oxygen-isotope record the ice ice sheets, enriching enriching the preferentially evaporated evaporated from the oceans and is stored in the Fluctuationsin in the the amount amount of 160 160 and 180 180 can be measured measured in glacial oceans in 180. Fluctuations used to to determine global ocean sediments sediments and ice cores (Shakleton and others, 1984) and used of oxygenice volume. Based Based on on these these calculations calculations for for global ice volumes, a series of represent isotope stages has been identified. Even Even numbered oxygen-isotopes stages represent periods. There glaciations, and odd numbers represent interglacial periods. There were were 40 40 glaciallinterglacial oscillations volumes between between 2.4 2.4 Ma Ma glacialhterglacial oscillations involving moderate global ice volumes and .9 Ma, and andthere therewere were 22 22 oscillations oscillations involving involvinggreater greaterice icevolumes volumes between between .9 .9 Ma Ma al., 1984). For the most recent 11 glaciations, oxygenand the present (Shackleton et al., 6, 12 2,6, 12 and 16 16 show the greatest ice volumes, possibly possibly corresponding correspondingto to isotope stages 2, traditional four-fold four-fold record record of terrestrial terrestrial glaciations glaciations in in North America. America. that the the most most extensive Another feature of the traditional four-fold record of glaciations is that glaciation formerly formerly recognized, recognized, (the (the Nebraskan) Nebraskan) was was also also interpreted interpreted to to be be the the oldest. oldest. Successively Wisconsinan) were Successively less extensive extensive glaciations, (the Kansan, Illinoian and Wisconsinan) younger (Flint, (Flint, 1957). 1957). This chronology is probably probably an interpreted to be successively younger of terrestrial terrestrial glacial glacial sediments. sediments. While there were clearly more artifact of the preservation of than four glacial periods during the past 0.9 my, each glacial advance obscures, either by of previous, previous, less less extensive extensiveadvances. advances. If, for example, there there erosion or burial, the record of random distance, probability probability were ten successive successive glacial advances, each advancing some random the surface and the oldest anaylsis suggests suggests that only three glacial limits will survive at the surviving advance advance will be the most extensive (Gibbons and others, 1984). ItIt isis therefore therefore 11 most recent glaciations we have historically recognized only not surprising surprising that of the 11 four glacial limits limits at at the the surface. surface. i r t half of the Pleistocene did The oxygen isotope record shows that glaciations in the ffirst (Shaldeton and others, not have as great great a global ice volume as those in the latter half (Shakleton 1984). However, However, these theseearly early advances advances of of ice ice were were apparently apparently extensive extensive enough enough in in North North 33 �America to to reach reach Iowa Iowa and and Nebraska Nebraska (Hallberg, (Hallberg, 1986) 1986)and and the the foothills foothillsof of the the Canadian Canadian America Rockies (Kiassen, (Klassen, 1989). 1989). The Thesediment sedimentof ofthese theseearly earlyadvances advancesmay may be be preserved preserved in in Rockies Minnesota (e.g. (e.g. Meyer, Meyer, 1997; 1997;Patterson, Patterson, 1997) 1997)and and in in Canada Canadain in the the subsurface subsurface(e.g. (e.g. Minnesota Klassen, 1989) 1989)in in areas areasof of net net glacial glacialdeposition, deposition, but but the the record record is is discontinuous discontinuousand and Kiassen, localized localized in in deep, deep, preglacial preglacialvalleys. valleys. Discontinuous,subsurface subsurfaceglacial glacial sediment sediment is is difficult difficultto to place place in in aa stratigraphic stratigraphic Discontinuous, framework because: (1) (1)absolute absolutedating datingof of the theunits units isis difficult; difficult;(2) (2) till till of of different differentglacial glacial framework terranetraversed traversedby by the the periods isis lithologically lithologicallyand andtexturally texturally similar similarifif the the source sourceterrane periods glacier (3)although althoughtill till may may change changecomposition compositiongradually graduallywith withdistance distanceinin glacierisisthe thesame; same;(3) the the ice-flow ice-flow direction, direction,isolated isolatedexposures exposuresof of till till may may be be dissimilar dissimilarenough enoughto toprevent prevent correlation. correlation. Radiocarbon Radiocarbondating datingof oforganic organicremains remainsisis only only useful useful for for Wisconsinan-age Wisconsinan-age deposits scattered predeposits because because of the limitations of the half life of 14C. A few dates from scattered Wisconsinan Wisconsinansamples sampleshave havebeen been secured securedusing using vertebrate vertebratepaleontology paleontology (e.g. (e.g.Klassen, Klassen, 1989), 1989), dating of volcanic ash present in the glacial glacial sediment (summarized (summarized in in Richmond Richmond and 1986),studies studiesof of remnant remnant magnetization magnetizationin in till till and andlake lakesediment sediment(e.g. (e.g. and Fullerton, Fullerton,1986), Baker Baker and and others, others, 1983), 1983),and and cosmosgenic cosmosgenic isotope isotope age age estimates estimates of of striated striatedrock rock surfaces surfaces (Bierman (Bierman and and others, others,1998). 1998). Owing Owingto to the thedifferent differentsubstrates substratesover overwhich whichthe theice iceflowed, flowed,ititisispossible possibleto todistinguish distinguish tills tills using using matrix matrix color, color,texture texture and and mineralogy, and clast lithology. During Duringthe theLate Late Wisconsinan, Wisconsinan,Minnesota Minnesotawas wasnearly nearlyequidistant equidistantfrom from the the two twomajor majorice iceaccumulation accumulation centers centersof of the theLaurentide Laurentideice icesheet sheetand andreceived received ice icefrom fromboth both (Dyke (Dykeand andPrest, Prest,1986). 1986). Minnesota's Minnesota'sunique uniqueposition position has hasfacilitated facilitatedprovenance provenancestudies studiesto to determine determineice-flow ice-flow paths paths and and source sourceareas. areas. The Theglacial glacialstratigraphy stratigraphyof ofCanada Canadaand andMinnesota Minnesota(Fulton, (Fulton, 1989; 1989; Meyer, Meyer,1997) 1997)indicate indicatethat thatice icesheets sheetshad had similar similargeometries geometriesthroughout throughoutthe theQuaternary. Quaternary. At At least leastfive fiveice icelobes lobesadvanced advancedinto intoMinnesota Minnesotaduring duringoxygen-isotope oxygen-isotopestage stage22(Late (Late Wisconsinan). Wisconsinan). Many Manyof ofthese theselobes lobeshad had multiple multiple readvances. During Duringadvance advanceeach eachlobe lobe had had the the potential potentialto toerode erodeolder oldersediment sedimentand androck, rock, and andto to deposit depositits itsown ownglacial glacial sediment. sediment.Therefore, Therefore,aasingle singleglacial glacialperiod periodmay maybe berepresented representedby by ten's ten'sof of compositionally -each each compositionallyand andtexturally texturallydifferent, different,incompletely incompletelypreserved preserved glacial glacialunits units— having having several severaldepositional depositionalfacies. facies. Interpretation Interpretationof of the themost mostrecent recentadvances advancesisismost most straightforward straightforwardbecause becausethe thedeposits deposits are are continuous, continuous,and and the the associated associated landforms are well preserved. For Forthis thisreason, reason, research research in in Minnesota Minnesota and and neighboring neighboring states states has historically focused of the activity of ice ice lobes lobesduring during the the Late LateWisconsinan Wisconsinan(for (forregional regional summaries summariessee seeWright, Wright,1972; 1972;Clayton Clayton and and Moran, Moran, 1982). 1982). Dynamics Dynamicsof ofice iceflow flow During 12 and Duringoxygen-isotope oxygen-isotopestage stage22(Late (LateWisconsinan), Wisconsinan),and andprobably probably during duringstages stages6,6,12 and 16 an ice ice sheet sheet that that extended extended 16 (all (all pre-Late pre-Late Wisconsinan), Wisconsinan),Minnesota Minnesota was was marginal marginal to to an across across most most of of Canada. Canada. Minnesota Minnesotaand andthe theGreat GreatLakes Lakesregion regionwere wereaffected affectedmainly mainlyby by thin, thin, dynamic dynamicice ice lobes lobes that that advanced advanced beyond the main body of the ice sheet. Much Much effort efforthas has been been made madeto tocorrelate correlatethe theadvances advancesof of these theselobes lobes(Clayton (Claytonand andMoran, Moran,1982; 1982; Mickelson Mickelsonand andothers, others,1983) 1983)but butitithas hasbecome becomeapparent apparentthat thatthey they were werenot notsynchronous synchronous on on aa time time scale scaleof of hundreds hundreds to to thousands thousands of of years. Movement Movementof ofthe theice icelobes lobesisis ultimately ultimatelydriven drivenby by the theoverall overallmass massbalance balanceof of the theice icesheet, sheet,which whichisiscontrolled controlledby by climate. climate.Ice Icelobes, lobes,however, however,respond respondquickly quicklytotochanges changesininmass massbalance balanceand andbed bed 34 �conditions of their their respective respective icesheds and may therefore appear to be out of phase with conditions nearby nearby lobes lobesand andregional regionalclimate. climate. Because km Because the the ice icelobes lobeswere werecomparatively comparativelythin—decreasing thin-decreasing from fromapproximately approximately11km where they they left leftthe the ice ice sheet— sheet- exisiting exisiting structural structural and and alluvial alluvial lowlands lowlands controlled controlled the where direction of of ice ice flow. flow. These Theselowlands lowlandswere weredeepened deepened even even more more owing to the enhanced enhanced direction glacial erosion erosion resulting resulting from fast, focused flow of the The Great Great Lakes are well glacial the ice. ice. The known examples examplesof of basins basins that that controlled controlled the flow flow of lobes; lobes; others others include include Great Great Bear known MacKenzie, Lake Lake Lake in in the the Northwest Northwest Territories, Territories, Great Great Slave Slave Lake in the District of MacKenzie, Lake Athabasca, on on the the Alberta-Saskatchewan border, Reindeer Reindeer Lake Lake in Saskatchewan, Saskatchewan, and and Athabasca, Lakes Winnipegosisin inManitoba. Manitoba. Lakes Winnipeg Winnipeg and andWinnipegosis Evidenceindicates indicatesthat that the the development development of of ice ice lobes (or (or 'outlet lobes') may be the result result Evidence of of ice ice streams streams(Patterson, (Patterson,1997). 1997).Ice Iceflow flowhundreds hundredsof ofkm kmup-ice up-icefrom fromthe theoutlet outletlobes lobes may may begin begin to toconverge convergeinto intoaanarrow, narrow,fast-moving fast-movingstreams streamsof ofice ice(Dyke (Dykeand andPrest, Prest,1986; 1986; Fulton, Fulton,1989; 1989;Thorliefsson Thorliefssonand and Kristjansson Kristjansson 1993; 1993; Fulton, 1995). The The nature nature and and distributionof of glacial glacialsediment sedimentand andlandforms landformsimplies impliesthat thatice iceflow flowwas wasmuch muchdifferent differentinin distribution thesenarrow narrowconvergent convergentzones. zones.Till Tillassociated associatedwith withice icestreams streamsisismore moredistally distallyderived, derived, these homogenous, homogenous, traceable traceablefor for hundreds hundreds of of km, and and typically has a level-to-streamlined level-to-streamlined surface surfaceexpression. expression.Tills Tillsdeposited depositedoutside outsidethese thesenarrow narrowconvergent convergentzones zonesare areusually usually thin thinand andlocally locallyderived. derived. are based based on on the the study study of of modem modem ice ice Modelsfor forthe themechanics mechanicsof of ice icemovement movementthat that are Models sheets (Fowler and Johnson, 1995) predict that slow, uniform flow of ice is unstable and sheets (Fowler and Johnson, 1995) thereforeunlikely. unlikely.What Whatisispredicted predictedand andobserved, observed,by bycontrast, contrast,isisaabimodal bimodalstyle styleofofice ice therefore flow, (km/yr)moving movingwithin within slower slower ice ice (cm/yr). (cdyr). The Thelocation locationofofice ice flow,with withfast fastice icestreams streams(km/yr) streams streamsappears appearstotochange changeover overtime, time,but butisisinfluenced influencedby bybed bedtopography topographyand and hydrogeology. Ice Icestreams streamsand andtheir theiroutlet outletlobes lobesare arethe themajor majordischarge dischargeareas areasfor foran anice ice hydrogeology. sheet, sheet,and andtheir theirdevelopment developmentisislargely largely aa function function of of subglacial subglacialwater. Ice Icestreams streamsdraw draw down downthe theice icemass massin inthe theice iceaccumulation accumulationcenters. centers. Ice-lobe Ice-lobeadvances advancestherefore thereforerepresent represent aa redistribution redistributionof ofice, ice,and andmay may actually actually signal signalthe the overall overall decay decay of the the ice ice sheet sheet rather rather than thanan anincrease increaseininvolume volumeofofthe theice icesheet. sheet. The LateWisconsinan Wisconsinan Laurentide Laurentideice ice sheet sheet indicates indicates that that in in the the Thedeglacial deglacialrecord recordof ofthe theLate areas areasof of continuous continuousice icecover coverin inCanada, Canada,ice ice streams streamsdeveloped developed within within the the ice ice sheet sheet and and fed fedthe theice icelobes lobesthat thatmoved movedthrough through the the Great Great Lakes Lakes basins basins into into the the upper upper Midwest Midwest (Dyke Hicockand andDreimanis, Dreimanis,1992; 1992;Thorleifson Thorleifsonand andKristjansson, Kristjansson, (Dykeand andPrest, Prest,1986; 1986;Hicock 1993). 1993).The Theice icestreams streamswere wereactive activethroughout throughoutthe theretreat retreatof ofthe theice iceacross acrossCanada Canadaas as documented documentedin inthe thepattern patternof of moraines moraines(Dyke (Dykeand and Prest, Prest, 1986; 1986;Fulton, Fulton, 1995). 1995).ItItisis unclear growthbecause because the theeffects effectsof of unclearififice icestreams streamswere werealso alsoimportant importantduring duringice-sheet ice-sheetgrowth subglacial subglacialwater waterare aredifficult difficultto toascertain. ascertain.This Thismodel modelofofice iceflow flowcan canbe becautiously cautiously extended extendedto toearlier earlierglaciations, glaciations,bearing bearingininmind mindthat thatbed bedconditions conditionscould couldhave havebeen beenvery very different differentfrom fromthose thoseof ofmore morerecent recentevents. events. The Theadvantages advantagesofofstudying studyingthe thegeological geologicalrecord recordofofQuaternary Quaternaryglaciations glaciationsinclude: include:(1) (1) the (2) the the therelatively relativelyexcellent excellentpreservation, preservation, continuity continuity and and accessibility accessibilityof of the the deposits; deposits; (2) location locationof ofthe thedeposits depositswithin withinaawell wellestablished established paleogeographic, paleogeographic, paleomagnetic, paleomagnetic,and and tectonic (4)records recordsfor forfloral floraland andfaunal faunalsuccession successionand andpluvial pluviallake-level lake-level tectonicsetting; setting;(4) histories (4) direct direct historiesthat thatallow allowthe theconstruction constructionof of predictive predictiveglobal globalcirculation circulationmodels; models; (4) evidence evidenceof ofglobal globalice icevolume volumefrom fromthe theocean oceanoxygen oxygen istope istoperecord record as as well well as as the the record record of ofcrustal crustalrebound; rebound;and and(5) (5)fairly fairlyeasy easyaccess accessto to modem modemanalogs analogs(for (forexample, example,the thestudy study 35 �of Antarctic chamber). Antarctic ice is relatively easy compared to the study of a deep magma chamber). These advantages advantages also provide the greatest greatest challenge challenge to Quaternary researchers, researchers, who are are required to integrate data across a wide range range of of disciplines disciplines and and geographic geographic areas. areas. The ultimate goal is to provide provide a integrated picture of the geosphere, geosphere, hydrosphere and biosphere for a specific specific period of earth's history, and to extend this understanding understanding to more more obscure glacial periods in the geologic record. REFERENCES REFERENCES Baker, R. W., Diehi, Diehl, J. F., Simpson, Simpson, T. W., W., Zelazny, Zelazny, L. L. W., W., and and Beske-Diehl, Beske-Diehl, S., S., 1983, 1983,Pre-Wisconsinan Re-Wisconsinan paleomagnetics of of west-central west-central Wisconsin: Wisconsin: Geological glacial stratigraphy, chronology, palmmagnetics Geological Society Society of America Bulletin, Bulletin, v. 94, p. 1442-1449. 1442-1449. Bierman, P. R., Marsella, K. A., Patterson, Patterson, C., Davis, P. T. Caffee, M., 1998, 1998, Mid-Pleistocene cosmogenic Mid-Pleistocene cosmogenic minimum-age pre-Wisconsinan glacial surfaces in southwestern minimum-age limits for pre-Wisconsinan southwestern Minnesota Minnesota and southern southern Island --- aa multiple nuclide approach. Accepted Accepted for for publication publication in Geomorphology. Geomorphology. Baffin Island Clayton, L., and Moran, S. R., 1982, 1982, Chronology Chronology of late Wisconsin glaciation in middle North America. America. Quaternary Quaternary Science Science Reviews, v. 11 p. 55-82. Dyke Dyke A. S. S. and and Prest, Prest, V. V. K., K., 1986, 1986,Paleogeography Paleogeographyof northern northern North America, America, 18,000-5000 18,000-5000 years years ago. ago. Geological Survey of Canada, Map Map 1703A, scale scale 11::12,500,000. 12,500,000. Hint, R. F., 1957. 1957. Glacial Glacialand andPleistocene PleistoceneGeology. Geology.New NewYork, York,John JohnWiley Wileyand andSons, Sons,553 553p.p. Flint, 1995, Hydraulic run-away: A mechanism mechanism for thermally regulated surges of Fowler, A. C., and Johnson, C., 1995, run-away: A ice sheets: 1. sheets: Journal JournalofofGlaciology, Glaciology,v.v.41, 41,p.p.554-56 554-561. Fulton, R. J., 1989, 1989, Foreward in Fulton, Fulton,R. R. J., J., ed., ed., Quaternary Quaternary geology geology of Canada Canada and Greenland: Greenland: Ottawa, Ottawa, Geological Survey Survey of Canada, Canada, Geology Geology of Canada series, v. 1;(Geological l;(Geological Society Society of America, America, Geological Geology of North America, v. v. K-1), K-i), p. 1-11. 1-11. Fulton, R. J., 1995, 1995, Surficial Surficial map of Canada: Geological GeologicalSurvey Survey of Canada Canada Map 1880A, 1880A, scale 1:5,000,000. 1:5,000,000. Gibbons, A. B., Megeath, Megeath, J. D., and Pierce, K. L., 1984, 1984, Probability of moraine survival in a succession succession of glacial advances. Geology, Geology,v. v. 12, 12,p. p. 327-330. 327-330. Hallberg, Hallberg, G. G. R., 1986, 1986,Pre-Wisconsin Pre-Wisconsin glacial glacial stratigraphy stratigraphyof the the central central plains plains region region in in Iowa, Iowa, Nebraksa, Nebraksa, Kansas, and Missouri. Quaternary Quaternary Science Science Reviews, Reviews, v. 5, p. 11-15. Dreimanis, A., 1992, Deformation till implications for rapid Hicock, S. T., and Dreimanis, till in in the the Great Great Lakes Lakesregion: region: implications of the the Laurentide Laurentide Ice Ice Sheet. Sheet. Canadian Journal of Earth flow along the south-central margin of Sciences, Sciences, v. 29, 29, p. 1565-1579. 1565-1579. Kiassen, Klassen, R. W., W., 1989, 1989,Quaternary Quaternarygeology geology of of the the southern southern Canadian Canadian interior interior plains, plains, in in Fulton, Fulton, R. R. J., J., ed., ed., and Greenland: Greenland: Ottawa, Quaternary geology of Canada and Ottawa, Geological Survey of Canada, Geology of of America, America,Geology Geologyof ofNorth NorthAmerica), America),v.v.K-1, K-i, p. p. 138Canada series, v. 1; (Geological Society of 174. 174. Mickelson, D. M., Clayton, L., Fullerton, D. S., S., and Boms, Borns, H. H. W. W. Jr., Jr., 1983. 1983. The late Wisconsin glacial S, C., LateLaterecord of the Laurentide ice sheet in the United States, in Wright, H. E., Jr., and Porter, S. Quaternary Environments of of the the United United States, States, Vol. Vol. 1, 1,The TheLate LatePleistocene. Pleistocene. Univeristy of Minneapolis, p. 3-37. Minnesota Press, Minneapolis, Meyer, G. N., 1997, of north-central north-centralMinnesota. Minnesota. Minnesota 1997, Pre-late Wisconsinan till stratigraphy of 48, 67 p. Geological Survey Survey Report of Investigations Investigations no. 48,67 Patterson, in Patterson, Patterson,C. C. J., J., Contributions Contributionsto to Patterson, C. C. J., 1997, 1997,Quaternary Quaternarygeology geology of of southwestern southwesternMinnesota, in the Geology Geology of Southwestern Southwestern Minnesota, Minnesota, Minnesota Geological Geological Survey Report of Investigations Investigations 47, p. 1-45. 1-45. Richmond, Richmond, G. G. M., M., and and Fullerton, Fullerton, D. D. 5., S., 1986, 1986,An An introduction introductionto to Quaternary Quaternaryglaciations glaciationsin in the theUnited UnitedStates States of America, Quaternary Quaternary Science Science Reviews, Reviews, v. 5, p. 3-10. Shackelton, N. J. and others, Shackelton, others, 1984, 1984, Oxygen isotope isotope calibration calibration of the onset of ice-rafting ice-rafting and history history of glaciation in the North Atlantic region: region: Nature, Nature, v. 307, p. 216-219. Thorleifson, L. H., and Kristjansson, Kristjansson, F. J., 1993, 1993, Quaternary Quaternary geology and drift prospecting, prospecting, BeardmoreBeardmoreGeraldton area, Ontario: Geological GeologicalSurvey Survey of of Canada Canada Memoir 435, 146 146 p. of Minnesota, Minnesota, in in Sims, Sims,P. P.K. K.and andMorey, Morey,G. G.B, B,eds., eds.,Geology Geologyof of Wright, H. E. Jr, 1972, 1972, Quaternary History of Minnesota: AAcentennial centennialvolume. volume. Minnesota MinnesotaGeological GeologicalSurvey, Survey,p.p.5515-547. 36 �WHAT'S NEXT FOR GEOLOGY GEOLOGY IN IN THE THE LAKE LAKE SUPERIOR SUPERIOR AREA? WHAT'S NEXT SOUTHWICK,David David L., L., Minnesota Minnesota Geological Geological Survey, Survey, 2642 2642University University Ave. Ave. W., W., St. St. Paul, Paul, MN MN SOUTHWICK, 55114-1057 55114-1057 Marvelous Marvelous progress progress has has been been made madein indeciphering decipheringthe thecomplex, complex,diverse diversegeological geological framework frameworkof of the the Lake Superior Superior region over the past twenty years. In particular, our comprehension comprehension of the Precambrian rock record has progressed progressed remarkably remarkablythrough throughexcellent excellent programs programs of of geologic mapping, regional geochronology, and and related related topical research research supported supported largely by government government and geophysics, geochronology, and justified understanding of on the premise that that geologic understanding of Precambrian terranes terranes will will lead to to discovery discovery of of economic mineral be sustained sustained and and that intellectual and mineral deposits. One hopes that this momentum can be progress toward the goal of finding and developing technological progress developing viable ore deposits deposits will continue. continue. Sustaining it will not not be be easy easy in in the theprevailing prevailingpolitical politicalclimate, climate, however. however. of democratic governments governments are determined Ultimately, albeit indirectly, the policies of determined by by public public opinion that is transmitted transmitted to to elected elected representatives. At the present time the public is not particularly p&ticularly concerned about mineral commodities commodities and therefore minerals issues do not have aa high high political political profile. There are no pressing shortages, prices are moderate moderate and stable, stable, unemployment unemploymentis is generally generally issues greatly greatly outweigh outweigh concerns concerns for for the the long-term long-term low, and concerns concerns for present-day "quality "quality of life" issues of mineral and energy availability of energy resources resources in in the the minds mindsof of most most people. people.As As aa consequence, consequence, elected elected representatives are representatives are being urged by their constituents to preserve and promote promote environmental environmentalquality, quality, provide provide recreational recreational opportunities opportunitiesin inwilderness wildernessareas, areas, improve improveurban urbaninfrastructure, infrastructure,combat combatcrime, crime, opportunites, and reduce taxes. They are are not not being being urged urged to improve health care, extend educational opportunites, of aa mining mining industry industry that we geologists geologists know know is is vital vital to to the the long-term long-term provide for the well-being of sustainability of because there there is is aa powerful powerful human human tendency not to worry about sustainability of the quality quality of life, because about future difficulties until circumstances absolutely demand demand it. Instead of of mining, mining, the the plexus plexus of of future affect human human health health and and happiness will be the primary environmental issues that immediately affect primary driver driver of public resource policy and derivative of derivative geological investigations in the Lake Superior region over the the next twenty twenty years. Although Although metal metal mining and and related related industries industries surely surelywifi will remain remain significant significant to local and will receive passive passive political politicalsupport, support, they they will wifinot notin in themselves themselvesprovide provide the the rationale rationale economies and for increasing public expenditures expenditureson ongeological geologicalinvestigations. investigations. be growth areas for applied geologic thinking thinking and research in the Lake The following will be Lake Superior Superior Detailed characterization of of unconsolidated unconsolidated materials within the uppermost region. (1) Detailed uppermost100 100m m of of the the This work work will involve collaboration among among glacial glacial geologists, geologists, stratigraphers, stratigraphers, geologic section. This hydrogeologists,and and soil soil scientists scientistsand and will will have have direct direct application to a geophysicists, geochemists, hydrogeologists, number number of of societal societalconcerns. concerns. It will will also also benefit mineral exploration, exploration, but itit wifi will not be justified on on that that basis. (2) The The development development and deployment deployment of improved geophysical geophysical tools for obtaining information mapping of about the the shallow shallow subsurface. subsurface.(3) (3) Detailed geologic geologic mapping of surficial deposits, deposits, especially especially in in urban, urban, mapping to to provide provide the stratigraphic framework of of the the urbanizing, and agricultural agricultural areas. Regional mapping Quaternary materials Quaternary materials will will be necessary as well, but the more pressing need will will be for detail. detail. (4) (4) Hydrogeologic studies studies on on aa variety variety of of scales, scales, with particular particular emphasis emphasison onthe thegeological geologicalcontrols controlsof of ground-water flow. be a developing area of of research research in in which which ground-water flow. Fractured-rock Fractured-rock hydrogeology will be sedimentologists, petrologists, sedimentologists, petrologists, and and structural structuralgeologists geologistswill willbecome become increasingly increasinglyinvolved. involved.(5) (5)LowLowtemperature rock-water of surface-water surface-water and and groundgroundtemperature rock-water geochemistry, geochemistry, with applications to problems of The mapping mapping and (6)The water composition composition and and the the transport transportof of dissolved dissolvedchemical chemical species. species. (6) of mined mined land and mine waste, characterization characterizationof of aggregate aggregateresources. resources.(7) (7) Characterization Characterization and control of in which the brought to the full full spectrum spectrumof of hydrogeological hydrogeological and and geochemical geochemical methodologies are brought to bear. bear. (8) (8) Geological and geophysical studies within Lake Superior Superior itself itself that that will will be be predicated predicated on calibration Geological and of the paleoclimate record but but will will extend extend to to investigations investigations of of lake-basin lake-basin evolution. evolution. As the geological profession profession becomes becomes ever evermore moredependent dependent on on government government policies, policies, our our near-term near-term research opportunities inevitably will reflect public priorities. This need not and will not mean opportunities inevitably This need not mean the the total demise of economic economic geology geologyand andthe thehard-rock hard-rockdisciplines disciplinesthat thatsupport supportit.it. However, traditional hard-rock work work will will diminish diminish in in favor favor of of non-traditional non-traditional investigations investigationsin in which which hard-rock hard-rock thinking thinking and skills skills can can be applied. applied.Flexibility Flexibility of training training and andoutlook outlookwifi willbe be critical criticalto to professional professionalsuccess. success. 37 �ACTS ABSTRACTS GENERAL TECHNICAL SESSIONS 39 �PETROGRAPHY AND GEOCHEMISTRY O OF F MIDCONTINENT RIFT RIFT RHYORHYOVOLCANICS) NEAR NEAR CLAM CLAM FALLS, FALLS, WISCONSIN LITE (CHENGWATANA (CHENGWATANA VOLCANICS) R.,Geology Geology Department, Department, ABBOTT, Kathleen M., THOLE, THOLE, Jeffrey Jeffrey T., T., WIRTH, WIRTH, Karl KarlR., Macalester College, St. Paul Paul MN, MN, 55105; 55105; kabbott@macalester.edu; kabbott@macalester.edu; thole@macalester.edu; wirth@macalester.edu The southwest southwest limb of the Midcontinent Rift (MCR) includes includes the the poorly-exposed poorly-exposed Chengwatana Chengwatana Volcanics (CV) (CV) which which are comprised of predominantly tholeiitic basalts (Wirth et al., Volcanics al., 1997, 1997, Naiman et al., this volume), volume), with minor rhyolites which are believed to be the the southernmost southernmost exposed rocks of this type within the the MCR. MCR. Recent analysis of these rhyolites found near Clam exposed Falls, Wisconsin, provides physical, chemical, and isotopic data data which which has has helped helped constrain constrain the the processes (Naiman (Naiman and Wirth, this timing (Wirth and Gehrels, this volume), and magmatic processes and the the southern southernMCR. MCR. These data are also used volume) involved in the generation of the CV and . rhyolites of northern Michigan to compare compare the CV rhyolites rhyolites with the Portage Lake rhyolites Michigan (Nicholson, (Nicholson, 1992, Nicholson Nicholson and Shirey, 1990), and the felsic rocks of the the North North Shore ShoreVolcanic Volcanic Group Group (Green (Green and and Fitz, Fitz, 1993, 1993,Vervoort Vervoort and and Green, Green, 1997). 1997). Two exposures exposures of rhyolite rhyolite are are found just west of Clam Clam Falls Falls including including about about 55 meters of rhyolite (KC-302) which is approximately 640 meters above the base of the exposed volcanic volcanic section. The The only only other otheroutcrop outcropof of rhyolite rhyolite (KC-310) (KC-310) is exposed exposed near the base of of the the section section approximately 3 kilometers south of KC-302 and is is less less than than 10 10meters metersthick. thick. No flow contacts contacts approximately or foliations are are present present but but the the outcrops outcrops appear appear to to be beconcordant concordantwith withsurrounding surroundingbasalts basalts which trend northeast and dip approximately 15 degrees to the northwest in this area. The northeast and dip approximately 15 degrees Thebase base with the underlying basalt basalt and of the outcrop at KC-302 appears to be an intrusive contact with the rhyolite contains contains subangular subangular inclusions inclusionsof ofophitic ophiticbasalt. basalt. The nature of emplacelocally, the ment ment of these these rhyolite rhyolite bodies bodies isis unclear. unclear. porphyritic small phenocrysts phenocrysts(-(—.1mm) The rhyolites rhyolites are generally weakly (<5%) (4%) porphyritic containing small lmm) The euhedral to and trace trace amounts of subhedral of euhedral to subhedral subhedralplagioclase plagioclase(avg. (avg.Ab94 Abo4An2 An2 Or4) and subhedralFe-Ti Fe-Ti locally. Amphiboles, oxides. Quartz Quartzphenocrysts phenocrysts and and albite albite glomerocrysts glomerocrysts are present only locally. Amphiboles, partly replaced replaced by chlorite/epidote/carbonate, chlorite/epidote/carbonate,are are rare. rare. The Thegroundmass groundmassisiscommonly commonlyspheruspherucontaining tabular quartz (tridymite (tridymite paramorphs), paramorphs), with phenocryst phenocryst nuclei, nuclei, containing litic (up to 22-- 3 mm) with Or95),very veryfine-grained fine-grained disseminated disseminated Fe-oxides, Fe-oxides, Ani Org5), dusty, anhedral anhedral alkali alkalifeldspar feldspar(avg. (avg.Ab4 Ab4An1 and local poikilitic anhedral quartz. quartz. Accessory zircon cessory zircon is present present as as minute minute I I I a euhedral prisms prisms in in most most samples. samples. The euhedral The C1fl Falls Clam Falls (CV) (CV) presence of minor epidote epidote and and carbonate carbonate BSa1t a Basalt D within the groundmass groundmass is is ubiquitous. ubiquitous. -3 . . Rhyolite Rhyolite samples contain minor diktytaxitic Some samples contain minor diktytaxitic Some a cavities. The presence of of tridymite tridymite g cavities. paramorphs pararnorphsindicate indicatethese theserocks rocks are are simisimi-Do 22 -lar to some rhyolites of the NSVG which a & erupted at athigh highterntern- 0 C are believed to have erupted I L7 f-4 peratures (Green and Fitz, 1993). No ap00° o na ° parent flow foliation or pyroclastic pyroclastic texn o 1 00 observed in the exposures exposures near tures were observed Falls. Clam Falls. Only basalt and rhyolite are present I I I 0 in the Clam Clam Falls' section; section; no rocks rocks of 50 60 40 50 60 70 70 80 intermediate intermediate composition compositionhave have been been idenidenS.1 2 wt /0 SiO, % tifled. tified. Rocks of of intermediate intermediate composicomposiI I I ^ 1 I 41 - �_______________________________ DI 0 I I 0 NSVG Granophyres \ ocJJ om 0 0a Portage Lake Portage Lak  0a -4 — -8 NSVG Icelandites Icelandi -12 NSVG Rhyolites Clam Clam Falls Falls (CV) (CV) -16 0 Basalt Basalt I Rhyolite Portage Lake Type II -20 40 50 60 70 80 Si02 wt % both the the NSVG NSVG and and the thePortage PortageLake LakeVolcanics. Volcanics. Chengwatana Chengwatana basalt basalt geochemgeochemtions are found in both Naiman et al., a!.,this thisvolume. volume. The The felsic felsic Chengwatana Chengwatana rocks rocks can can be be classified classified istry is discussed by Naiman as tholeiitic rhyolites. The Therocks rocks are are metaluminous metaluminous and and only only one one sample sample has normative normative coruncorundum (0.16%). In In general, general,the therhyolite rhyolite chemistry chemistry is is similar similar to to both both NSVG NSVG and and Type Type I Portage Portage Lake rhyolites. Si02 (7 (71-72%), rhyolites.Notable Notablevariations variationsfrom fromthese thesetwo two groups groups include include slightly less Si02 1-72%), higher 13higher total total Fe Fe (4.87 (4.87 to to 5.78 5.78Fe203%), Fe203%),higher Th (25-27 ppm), and significantly significantlyhigher higherZr Zr(7 (713986 986 ppm). Initial ENd (1100Ma) Ma)values valueson ontwo twoChengwatana Chengwatanarhyolites rhyolitesare are0.95 0.95and and-0.06. -0.06. These ENd (1100 Chengwatana basalts of of the the Clam ClamFalls Fallsregion region(-2 (-2toto+3.4). +3.4).The TheChengwatana Chengwatana values are similar to Chengwatana rhyolite similar to Portage Lake (Type I) rhyolites rhyolites which which range rhyolite ENd ENd values are most similar range from from-0.3 -0.3to to -4.7 (Nicholson (Nicholson and andShirey, Shirey, 1990) 1990)and and are are distinct distinct from rhyolites of the NSVG which range from -2.3 to ENd to -14.8 -14.8 (Vervoort (Vervoort and Green, Green, 1997). 1997). NSVG NSVGgranophyres granophyresand andicelandites icelanditeshave haveENd values with a smaller and these these values values are also lower than smaller range range than NSVG rhyolites rhyolites and rhyolites. Large degrees of crustal melting melting are are invoked invoked as as a major process in in Chengwatana rhyolites. forming forming the the NSVG NSVG rhyolites rhyolitesas asindicated indicatedby bythe thelow lowENd ENd values values (Vervoort (Vervoort and and Green, Green,1997). 1997). The Chengwatana rhyolites at Clam Falls lack the pronounced crustal Nd isotopic isotopic signature signature evident in the NSVG rhyolites rhyolites and are are believed to be fractionates fractionates of a primary basaltic basaltic magma magma derived from an enriched enriched mantle mantle source. References Cited Green, v. 54, 54, p. p. 177-196. Green, J.C. J.C. and and Fitz, Fitz, T.J., T.J., 1993, 1993, Journal of Volcanology and Geothermal Research, v. Nicholson, S.W., S.W., and Shirey, S.B., 1990, Journal of Geophysical Research, v. 95, p. 10,851-10868. Nicholson, S.W., U.S.G.S. Bulletin 57 pp. S.W., 1992, U.S.G.S. Bulletin 1970, 1970,57 K.R., Vervoort, J.D., J.D., Naiman, Z.J., 1997, Canadian Journal of Earth Sciences, v. 34, no. 4, p. 536-548. Wirth, K.R., 536-548. J.C., 1997, Vervoort, J.D., and Green, Green, J.C., 1997, Canadian Journal of Earth Sciences, Sciences, v. 34, p. 521-535. 42 �A PRELIMINARY DETAILED GEOLOGICAL GEOLOGICAL DESCRIPTION DESCRIPTION OF OF THE NEW A PRELIMINARY DETAILED NEW IN THE LUMBY HIGH GRADE SILVER-BASE SILVER-BASE METAL DISCOVERY DISCOVERY IN LUMBY LAKE METAVOLCANIC BELT BELT NORTHEAST METAVOLCANIC NORTHEAST OF ATIKOKAN, ATIKOKAN, ONTARIO, ONTARIO, CANADA BERNATCHEZ, Raymond A., President & Consulting Geologist to Atikokan Resources Inc., P.O. Box Box 1376, 1376,126 126Willow Willow Rd., Rd., Atikokan, Atikokan,Ontario, Ontario,Canada, Canada,POT POT1CO. 1CO. Tel. 807-597-4526, Fax 807-597-4636 (rbernatc@atikokan.lakeheadu.ca) (rbernatc @ atikokan.1akeheadu.ca) A high mineraloccurrence occurrencehas hasbeen beendiscovered discoveredwithin within an an east-west east-west high grade grade silver—zinc—lead silver-zinc-lead mineral stratabound felsic volcaniclastic horizon at the southern edge of the Lumby Lake Metavolcarnc Metavolcanic Belt. Located Locatednorth northof ofthe thecreek creekconnecting connectingLumby Lumbyand and Herontrack HerontrackLakes, Lakes,the thediscovery discovery occurs at the southern margin of a 1.5 1.5 km thick east-west trending felsic volcanic volcanic package package near Marmion Lake tonalite the southern contact of the Lumby Lake Metavolcanic Belt with the Marmion of batholith to the south. The The Lumby Lumby Lake Lake volcanic belt is located 23 air miles northeast of Atikokan, Ontario, Canada and forms the northeasterly extension extension of of the the Steep Rock and Finlayson Lake volcanic stratigraphy. This This discovery discoveryisis contained containedwithin within highly highly (hydrothermally) (hydrothermally) altered felsic volcaniclastic rocks having geological and geochemical geochemicalfeatures features similar to other Archean massive sulphide deposits. However, the felsic volcanic rocks hosting metaloccurrence occurrence have have been been dated dated at 3.0 Ga (Mesoarchean; this high grade grade silver—base silver-base metal (Mesoarchean;Davis Davis and Jackson, 1988), 1988), whereas all other base metal deposits found in the Archean in Ontario have been dated at 2.7 2.7 Ga. The The occurrence occurrence contains contains unusually high silver content with some random grab samples assaying as high as 416 ounces/ton (14,265 (14,265 gm/tonne) gdtonne) or or270 270ozfton ozlton samples assaying as high as 416 ounces/ton over 1.1 1.1 meter (9292 (9292 gm/tonne). gdtonne). The Lumby Lake greenstone greenstone belt is located within the Wabigoon subprovince, 30 km north of the Wabigoon-Quetico Wabigoon—Queticosubprovince subprovinceboundary boundaryand and 37 37 air airkm km northeast northeast of Atikokan, Ontario. Ontario. The Lumby Lake km xx 20 20 km km synclinal synclinalsupracrustal supracrustalsequence sequence Lake greenstone greenstonebelt belt consists consists of of aa 60 60 km of rocks composed of mafic (pillowed and massive), ultramafic (komatiitic), and felsic (tuff, lapili lapillituff, tuff,tuff tuffbreccia, breccia,massive), massive),metavolcanic metavolcanicrocks rockswith with lesser lesseramounts amountsof of metasedimentary metasedimentary rocks (chert, (chert, argillite, argillite, arkose, arkose, conglomerate conglomerateand and iron iron formations). formations).The Themetavolcanic— metavolcanicmetasedimentary assemblage Mannion Lake Batholith to the south and the assemblage is in contact with the Marmion White Otter Batholith to the northwest. The The belt belt has has been been intruded intruded by two larger oval oval and monzodiorite-granite plutons, the Norway Lake and van Nostrand Lake Stock heterogeneous monzodiorite-granite and smaller ones (the Bar Lake and Viking Lake Lake stocks). The Belt has been subjected subjected to four major structural structural events: at least two folding events and two faulting-shearing events. The TheBelt Belthas has had had aa major major synclinal synclinal folding folding event event with with its its axis axis trending east-west east-west through through the central central portion of the the belt through through Seahorse-Garnet Seahorse-GarnetBayBayPinecone-Hematite Lakes system. The The van van Nostrand Lake stock intrudes this axis in the central part of the belt. AAsecond secondfolding foldingevent event occurs occurs within within the the interfiow interflow sedimentary sedimentaryunits. units. belt to to the the west. west. Two The Redpaint Lake Fault truncates the Lumby Lake belt Two east-west east-west Spoon Lake Deformation Deformation Zone and the deformation zones have been noted: the Bufo Lake - Spoon Garnet Garnet Bay-Viking Lake-Hematite Lake-Hematite Lake Lake Deformation Deformation Zone. Zone. The silver—base metaldiscovery discoveryisislocated locatedin inthe thewestern westernblock block of of claims claims known known as the silver-base metal Herontrack Herontrack Lake Lake Block Block and and within within the the southwest southwestportion portion of of the the Lumby LumbyLake Lakegreenstone greenstonebelt. belt. A 130 krn grid has been established established in the the central central portion of this block from Lumby Lumby Lake Lake to to 130 km Hutt Lake. AAdetailed detailedexploration explorationprogram programover overthis this grid grid consisting consistingof of geological geologicalmapping, mapping, lithogeochemical soil geochemical sampling, magnetic and partial VLF EM and IP surveys, lithogeochemical 43 �sampling sampling and overburden overburden mechanical stripping has identified and exposed exposed numerous numerous suiphide sulphide bearing felsic volcaniclastic horizons horizons containing containing economic economic to to subeconomic subeconomicvalues valuesin in zinc, zinc, copper, silver, lead and gold. gold. The detail detail geological geological mapping mapping has identified identified five five rock rock types: mafic mafic volcanic, volcanic, massive massive and and pilowed; pillowed;felsic felsicvolcanic, volcanic,tuff, tuff, lapili lapillituff, tuff,tuff tuffbreccia brecciaand andcoarse coarse breccia, breccia, massive massive rhyolite rhyolite as feldspar feldspar and/or andfor quartz quartz porphyries porphyries and and cherty cherty rhyolite; rhyolite; mafic mafic to to intermediate intermediateintrusive intrusiverocks rocks (gabbro and diorite) as dykes and sills intruding the above; Marmion Lake batholith (tonalite) Lake batholith (tonalite) and finally felsic dikes intruding intruding the the felsic felsic volcanic volcanic rocks. rocks. The bulk of the mineralization is contained within stratabound stratabound interflow interflow sediments. sediments. Pyrite, Pyrite, and in some some interfiow interflow units, units, pyrrhotite pyrrhotitemake makeup upthe thebulk bulkofofthe thesulfides. sulfides.The Thehigh highgrade gradesilver— silverzinc—lead discovery zone contains significant amounts of pyrite, sphalerite, galena, acanthite zinc-lead discovery zone contains significant amounts of pyrite, sphalerite, galena, acanthite and native silver with minor chalcopyrite. This This mineralization mineralization is is contained contained within within highly highly altered felsic volcanic rocks. The The felsic felsic rocks rocks have have been been altered altered to sericite sericite and chlorite chlorite in some some locations. The Thesericite sericitewas waspreviously previouslyinterpreted interpreted as as an an altered altered and and sheared shearedquartz quartz diorite; diorite; itit now appears that this alteration was caused as the result of the footwall alteration associated with massive sulphide deposits. This This has has been been partially partially confirmed confirmed by some some limited limited lithogeochemical lithogeochemicalrock rock sampling. sampling. The exploration exploration program carried out out to date has identified three highly altered base metal mineralized horizons. The The most most economically economically significant significanthorizon identified to date is the Lake—SpoonLake LakeHorizon Horizonwhich whichhosts hosts the the high high grade grade silver silver discovery. Several Lumby Lake-Spoon Several other other been discovered discovered along along this 250 to to significant silver and base metal-bearing occurrences have been 300 meter wide sulphide horizon that has now been traced for a strike length of about about 66 km by surveys. Two additional suiphide base metal mechanical stripping and geophysical surveys. additional altered sulphide bearing felsic volcanic horizons (Delos Lake and Pond Lake Horizon) have been located a few hundred meters north of the silver discovery horizon. Geophysical Survey The Questor Airborne Geophysical Survey carried out by the Ontario Geological Survey in Lake mineralized mineralized 1980 did not identify any input input anomalies anomalies along along the the Lumby Lumby Lake—Spoon Lake-Spoon Lake horizon. However, a broad magnetic low covers the area over the newly identified However, a broad magnetic low covers the area over the newly identified felsic felsic strata. This Thismagnetic magneticlow lowcan canbebetraced tracedeast—west east-west along alongthe thesouthern southernvolcanic—granite volcanic-granite contact contact for over 12 km km from from Bufo Bufo Lake Laketo toHutt Hutt Lake. Lake. Other similar of the Lumby Lake greenstone belt for of over 13 km. km. Similar Similarfelsic felsic magnetic lows can be traced eastward to Old Man Lake a distance of volcanic rocks have been identified in these areas. A new copper—zinc—silver occurrence has rocks have been identified in these areas. A new copper-zinc-silver occurrence been found along this magnetic low response response near a series of airborne input anomalies anomalies at the west end of Old Man Lake. The TheOGS OGSairborne airbornegeophysical geophysicalsurvey survey has has detected detected numerous numerous other anomalies anomalies in the Lumby Lake greenstone greenstone belt, of which most have never been examined or tested. What Whatother otherbase basemetal metaldiscoveries discoverieswill will the the Belt Belt reveal? reveal? References References cited: Davis, D.W., and Jackson, M.C., 1988, 1988, Geochronology of the Lumby Lake greenstone belt: a 3 Ga complex within the Wabigoon Subprovince, northwest Ontario: Bull. G.S.A., G.S.A., v. v. Ontario: Bull. 100, p. 818-824. 44 �L5 r I Ii 0 z 0 -' Ic ii I 45 �DIMENSION STONE PRODUCTS DIMENSION PRODUCTS OF OF MINNESOTA MINNESOTA BOERBOOM, BOERBOOM, Terrence Terrence J., Minnesota Minnesota Geological Geological Survey, Survey, 2642 University University Avenue, Avenue, St. St. Paul, MN 55114; 55114;boerbO01@tc.umn.edu; boerbOOl@tc.umn.edu; and and OBERHELMAN, OBERHELMAN, Matt, Matt, Minnesota Minnesota Division of Minerals, 1525 E. 3rd Street, Hibbing, Department Department of Natural Resources Resources - Division matt.oberhelman@dm.state.mn.us 55746-1461;matt.oberhelman@dnr.state.mn.us MN 55746-1461; The first fmt dimension dimension stone stone quarry was opened in 1868, 1868, to supply gray granite granite from the St. St. Paul Customs Customs House House and and Post Post Office. Office. Since Cloud District for construction of the St. Paul Since then, grown considerably. considerably. At present there are the dimension stone industry in Minnesota has grown are fourteen active fourteen active dimension dimension stone stone quarries quarries in in the the state, state, and and several several others others that that are are active activeon on an an intermittent basis. Dimension Dimension stone stoneproducts products from from Minnesota are marketed worldwide, and are used for for a variety of purposes purposes that that include include interior interior tiling, counter counter tops, monuments monuments and and memorials, "surface memorials, "surface plates" platest' for for precision machine mounts, acid-resistant acid-resistant industrial applications, decorative decorative and structural structural exterior panels for major buildings, and grinding mill liners liners and and balls. balls. The Resources' dimension stone The results of a recent Minnesota Department of Natural Resources' stone inventory potential for for krther further development development of of inventory show show that northern Minnesota offers excellent potential high quality dimension dimension stone products. Field Field investigations investigations have identified twenty-two for quarry quarry development. development. Prospects prospect sites that exhibit potential for Prospectshave havebeen been identified identified in in Middle Proterozoic Proterozoic rocks (ca. 1100 1100 m.y.) of the Duluth Complex and Archean rocks (ca. 2700 m.y.) rock units. units. Prospects may.)of the Vermilion Vermilion Granitic Complex and other granitoid rock Prospects include include black, green, with aa variety variety of of textures. textures. Prospect green, pink, beige, and multi-colored stone with Prospect evaluation evaluation included included outcrop outcrop observations observationsthat that considered consideredjoint spacing, spacing, color, color, texture, texture, and and deleterious deleterious materials. Results Resultsof ofthe theinventory inventoryare aredescribed described in in the the following followingthree three MDNR MDNR reports: Dimension Dimension Stone StoneInventory Inventory of of Northern Northern Minnesota Minnesota 1991, 1991,Report Report 289; 289; Dimension Dimension and Dimension Dimension Stone Inventory Inventory of Stone Inventory of Northern Minnesota 1993, Report 298; and Northern Minnesota Minnesota 1995, 1995, Report Report 298-2. Cold Spring Spring Granite Granite Co. is currently currently leasing three of the prospect sites identified by the have opened opened quarries quarries on ontwo twoof ofthese theseleases. leases. Both dimension stone inventory study, and have utilize gabbroic h m these these sites sites is is marketed gabbroic anorthosite anorthosite of the Duluth Complex; the the stone stone from under Lake Superior SuperiorGreen. Green. under the the names names Mesabi Mesabi Black Black and and Lake This poster display shows shows products from all of the currently active quarries in the state state of Minnesota, along with samples of dimension stone prospects that were collected and prepared Minnesota, along with samples of dimension stone collected and prepared by the Minnesota Minnesota DNR-Minerals Division (see below). The The numbers numbers on on the the samples samples refer refer to locations on the index map. The table on the following page summarizes the geologic The table on the following page summarizes the geologic aspects aspects of the various various dimension stone dimension stoneproducts products of of Minnesota. Minnesota. ACKNOWLEDGMENTS AcmowLEDGMENTs All of the quarry companies listed on companies listed on the the following following table table kindly contributed contributed samples samples of of their their products to the Minnesota Geological Survey for this display. products to the Minnesota Geological Survey for this display. 46 �Sample Sample Company Company Trade Trade Name Name Geologic Geologic Unit 1 1 CSG Mesabi Black Complex Duluth Complex 2 CSG Lake Superior Superior Green Green 3 CSG Indian Iridian 4 CSG 5 Texture -1090 Ma —1090 Dominant Mineralogy P, cpx, urn ilm Duluth Complex Complex -1090 Ma —1090 P, cpx, gp, ilm p, very cgr, porph Isle Granite Isle Granite 18121770 Ma Ma 1812 - 1770 K, P, Q, Q, bi m-cgr, porph porph Charcoal Charcoal Black Reformatory Reformatory granodiorite granodiorite 1812±9 Ma 11 18 1Z?HMa P, K, Q, P, Q, bi, hbl, sph m-cgr, si. sl. porph. porph. CSG Diamond Diamond Pink Rockville Rockville Granite Granite 11812±9 8 1 2 s Ma Ma 11 K, P, Q, Q, bi, hbl, sph m-cgr, porph, rkv, 6 CSG Rockville Rockville Beige Beige Rockville Rockville Granite Granite Ma 11 1812±9 1812&9Ma K, P, Q, Q, bi, hbl, sph cgr, porph, rkv, 7 CSG Rockville Rockville White White Rockville Rockville Granite Granite Ma 11 11812±9 8 1 2 s Ma K, P, Q, Q, bi, hbl, sph cgr, porph, porph, rkv, rkv, 8 CSG Agate Agate Unnamed 2618±1 Ma 22 261833 Ma K, P, Q, Q, bi cgr ClY 9 DGC Bellingham Granite Granite Unnamed 2618±lMa? 2618klMa ? bi K, Q, Q, P, K, P, bi cgr C lY 10 JSC Quartzite Sioux Quartzite 1700Ma 1700 - 1750 Ma Q recrystallized recrystallized quartzite quartzite 11 CSG Rainbow Rainbow Morton Gneiss Gneiss 2600 2600- K, P, Q, bi, hbl gneissic banded, gbl 12 MKS Varieties Varieties of pink-gray Oneota Dolomite Dolomite Earliest Ordovician Earliest Ordovician dolomite dolomite bioturbated bioturbated micro-xln. micro-xln. 13 VSC vsc Varieties pink-gray Varieties of pink-gray Oneota Dolomite Dolomite Earliest Earliest Ordovician dolomite dolomite bioturbated bioturbated micro-xln. micro-xln. 14 BSC Winona Dolomite Dolomite Oneota Dolomite Dolomite Earliest Ordovician Earliest Ordovician dolomite dolomite bioturbated bioturbated micro-xln. micro-xln. Age - - 3600 Ma mgr, lam, lin, un, oph Company abbreviations: abbreviations: CSG -- Cold Spring Granite, JSC JSC -- Jasper Stone Stone Company, DGC - Dakota Granite Company, Company, MKS MKS -- Mankato-Kasota Stone, Inc., Company VSC Co., BSC -- Biesanz Stone Co. VSC -- Vetter Stone Co., Mineral abbreviations: abbreviations: P - plagioclase, plagioclase, K -- potassic feldspars, Q jim - ilmenite, ilmenite, sph sphene, Q - quartz, quartz, cpx - clinopyroxene, hbl -- hornblende, hornblende, bi -- biotite, ilm sph -- sphene, Mineral granophynic quartz/feldspar, gp - granophyric quartdfeldspar, xln xln -- crystalline crystalline Texture abbreviations: abbreviations: mgr mgr - medium-grained, Texture medium-grained, cgr cgr -- coarse-grained, coarse-grained, lam - igneous igneous lamination (cumulus or trachytoid), trachytoid), tin lin -- lineated, lineated, rkv rkv -- rapakivi, gbl-rapakivi, gbl granoblastic, oph oph -- ophitic, ophitic, porph porph -- porphyritic 1U-Pb I u - P ~zircon date (Goldich pers. commun., as reported in Horan and and others, others, 1987) 1987) MassachussettesInstitite Instititeof ofTechnology, Technology, pers. pers. comm., comm., 1998) 22U.Pb ~ - zircon ~ b date (Mark Schmitz, Massachussettes �GROUND GROUND WATER WATER RECHARGE, RECHARGE,DISCHARGE DISCHARGE AND AND RESIDENCE RESIDENCETIME TIMEIN INRICE RICE FOR LAND USE PLANNING IMPLICATIONS COUNTY, COUNTY,MINNESOTA MINNESOTA---- IMPLICATIONS FOR LAND USE PLANNING CAMPION, CAMPION?Moira Moira---- Minnesota Minnesota Department Department of of Natural Natural Resources Resources e-mail moira.campion@d~.state.mn.us e-mail--- moira.campion@dnr.state.mn.us Rice Rice county county is is located located 40 miles miles south south of of the Minneapolis-St. Paul metropolitan metropolitan area. The The county county contains containsurban, urban, suburban, suburban,and and rural rural areas areasand and faces facesincreasing increasingdevelopment developmentpressure pressure for for housing housing as as well as as for for recreational and high intensity agricultural agricultural purposes. Water Watersupply supply in in the the county county isis primarily primarily from fromground groundwater waterand andmost mostof ofthe the1600 1600wells wellsininthe thecounty countydata data base base are are completed completed in in the St. St. Peter-Prairie du Chien-Jordan aquifer. This Thisbedrock bedrockaquifer aquiferisis confined throughout the western and southeastern portions of the county and is unconfined confined throughout the western and southeastern portions of the county and is unconfinedinin the the east-central east-central and and northeast. northeast. The Thepotentiometric potentiometricsurface surfaceshows showsaaregional regionalhigh highin insouthwest southwest Rice Rice County Countyand and lows lowsin inthe the central centraland andnortheast northeastparts partsof ofthe thecounty countywhere wherethe theaquifer aquifer discharges dischargesinto intolocal localstreams. streams. Throughout Throughout most most of of the the county county the the aquifer aquifer isis protected protected from fromdirect directsurface surfacerecharge rechargeby by various consolidated and unconsolidated low permeability materials, except in the northeast various consolidated and unconsolidated low permeability materials, except in the northeast where where the the aquifer aquifer is is exposed exposed at at or or near near the surface. surface. The Thevarious variouslow lowpermeability permeabilitydeposits deposits and and their their distribution distribution result in different different recharge conditions conditions to the bedrock aquifer. In In the the northeast, northeast, where where the the aquifer aquifer is is not not protected protected by by overlying overlying material, material,recharge recharge to to the the aquifer aquifer occurs occurs directly through percolation from the land surface. In Inwestern westernRice Rice County, County,there there are are abundant lakes and wetlands in a hummocky terrain of thick supraglacial tills. Recharge to abundant lakes wetlands supraglacial Recharge to the the bedrock bedrock aquifer occurs occurs in complicated flowpaths flowpaths through the lakes and tills in this part of the the county. county. InInthe thesoutheast, southeast,the theDecorah-Platteville-Glenwood Decorah-Platteville-Glenwoodconfining confiningunit unitand andsome some older older tills tills cover cover the the aquifer. aquifer.Where Where this this relatively relatively thin thin package package of ofshales shalesand andlimestones limestonesisis continuous, continuous7the the St. St. Peter-Prairie Peter-Prairie du du Chien-Jordan Chien-Jordanaquifer aquiferreceives receivesvery verylittle littlevertical verticalrecharge recharge from the surface. Conversely, near the edge of this confining unit, recharge may be as from the surface. Conversely, near the edge of this confining unit, recharge may be asmuch much as fiomthe theaquifer aquifer as 30 30 times times greater greater than underneath this unit (Delin, (Delin, 1991). 1991). Discharge Dischargefrom occurs occurs where where ground ground water water drains drainsinto into the the Cannon CannonRiver, River, the the Straight StraightRiver, River,and andPrairie Prairie Creek. Creek. Residence Residencetime timeofofground groundwater waterwas wasestimated estimatedusing usingtritium tritiumand and14C 1% age-dating. age-dating. These These isotopic isotopic age age estimates estimatesconfirm confirmthe the conceptual conceptualmodel model of ofrecharge rechargein inthe thecounty. county. Ground water is youngest in the northeast where low permeability cover is absent youngest in the northeast where low permeability cover is absent and and oldest oldest in in the the southeast southeast under the Decorah-Platteville-Glenwood confining unit. Isotopic Isotopicresults resultsin in western western Rice Rice County County show showages agesgreater greaterthan thantritium tritium dating datinglimits limitsbut butless lessthan thanaafew few centuriesold. old. centuries Knowledge Knowledge of of these these differing differing recharge recharge conditions, conditions, coupled coupled with with geologic geologic and and hydrologic hydrologic information, provides county planners with an important water resource management information, provides county planners with an important water resource managementtool. tool. This This information information allows allows county county staff staff to to decide decidewhere where modification modificationto to current currentland land use use practices would minimize potential negative impacts on ground water quality. Basing Basing land land use planning decisions on resource management concepts rather than on politically based planning decisions on resource management concepts rather than on politically based situations situationswill will increase increasethe the chance chanceof of resource resource protection. protection. 48 �RICE RICE COUNTY, COUNTY, MINNESOTA Hydrogeology of the Hydrogeology St. Peter-Prairie Peter-Prairie du du Chien-Jordan Chien-Jordan Aquifer Aquifer -5m 5) 5 (0 ln - *- 64 - 0 0 0 2 4 I 1 I 6 Miles I Cross Section Section Legend Cross Legend Map Legend /V Potentiometric Potentiometric contour Cross section section Cross A A Well sampled Well sampled for chemistry + + Well measured for water level, level, but not sampled sampled Saturated thickness in feet LIII 0-100 R E 0 Aquifer absent '1 Aquifer ix Aquiferunconfined; unconfined; aquifer confined to west + 100-200 200-300 300-400 400-500 500-600 From: Rice Counly County Geologic Atlas, Atlas. Part 6.1997 6,1997 Original Scale l1:100,000. : l C J O , O ~ . Fmm: Copyright: Minnesota Minnesota Dept. Dept. of of Natural Natural Resources Cop*ght: 49 Unsaturated Unsaturated Recent waters; waters; tritium tritium >10 sf0TU TU Mixed waters; waters 1< 7c tntium tritium <10 c70 TU TU Vintage waters; tritium c <1 Vintage 7 TU and between 50 50 to 5,000 5,000years old Vintage waters; tritium e <1 l TU and Vintage 5,000to to 10,000 70,000years old more than 5,000 Decorah-Platteville-Glennwood Decorah-Plaffeville-Glennwmd Confining Confining unit unit St. Peter-Prairie Peter-Prairie du Chien-Jordan ChienJordan aquifer — Direction ofofgroundwater Direction groundwater flow flow �THE THE GEOLOGICAL GEOLOGICALSOURCE SOURCE OF OF ARSENIC ARSENIC IN IN GROUND WATER IN SOUTHEASTERN MICHIGAN MICHIGAN W.F. W.F. Cannon Cannon and and Alan Alan Kolker, Kolker, U.S. U.S. Geological Geological Survey, SurveyyReston, Reston?VA VA D.B. Westjohn, Westjohn?U.S. U.S. Geological Geological Survey, SurveyyLansing, Lansing?MI MI The ground ground water water in in parts parts of of nine counties in southeastern Michigan contains anomalous quantities of The dissolved arsenic, arsenic?presumably presumably derived from a natural source. Many Manywells wellstested tested by bythe theMichigan Michigan dissolved Department U.S.Environmental Protection Department of of Community Community Health and by the USGS exceed the U.S.Environmenta1 Agency's maximum maximum contaminant contaminant level level (MCL) of 50 pg/L Extreme values values are are as as Agency's .tg/L for drinking water. water. Extreme high as as 350 350 jtg/L. pgL. In In one one county county (Huron) (Huron) about about 30% of the tested wells exceeded the MCL for arsenic. high Most Sandstone, but other Most wells wells with with elevated elevated arsenic arsenic were completed in the Mississippian Marshall Sandstone? units, units?particularly particularlyglacial glacial aquifers, aquifers?can can also alsoyield yield arsenic-contaminated arsenic-contaminatedwater. water. The nine nine affected affected counties countieshave have aa combined combinedpopulation populationof of more more than than 22 million millionpeople. people.The TheMarshall Marshall The Sandstone Sandstone is is the principal bedrock bedrock aquifer aquifer in in most most of of the the affected affected counties countiesand and in in some someareas areasisisthe the sole-source aquifer. aquifer. These Theseconditions conditionscombine combineto to make make knowledge knowledge of of the the natural natural source source for for arsenic arsenic aa sole-source critical critical aspect aspectin in designing designingaa strategy strategyto to provide provideaa continued continuedsupply supplyof ofsafe safedrinking drinkingwater waterfor forthis this region. region. As As part part of of aa larger larger study study of of this this problem, we have been examining the lithology, mineralogy, and chemistry stratigraphic, and mineralogic chemistry of of the the Marshall Marshall Sandstone Sandstoneto better define the geographic, geographic?stratigraphic? distribution distribution of arsenic arsenic in bedrock. In InOctober October1997, 1997?aa well well was was drilled drilled and and cored cored to to collect collect aa complete complete stratigraphic overlying and and underlying underlying units. units. The stratigraphic section of the Marshall Sandstone and immediately overlying The hole County?near nearone oneof ofthe themost mostcontaminated contaminatedwells wellsknown knownininthe thestudy studyarea. area. hole was was drilled drilledin in Huron HuronCounty, Core Core from from that that hole hole and and cuttings cuttings and and core core from other producing wells have formed the basis for this study. study. The The Marshall Marshall Sandstone Sandstoneis is aa fluvial .fluvialto marginal marine sequence that is present at subcrop in a belt encircling the central part of the encircling the central part of the Michigan Michigan Basin. Arsenic-contaminated Arsenic-contaminatedground groundwater waterisisknown knownonly only along along the the eastern eastern flank flank of of the the basin. basin. As Asseen seenininthe theHuron HuronCounty Countycore, core,the theMarshall Marshallisisaavery very heterogeneous interbedded with massive heterogeneousunit unit of of mediummedium- to to coarse-grained, coarse-grained, gray to brown sandstone sandstone interbedded to to laminated laminated gray gray and and red red siltstone siltstoneand and thin units of black to gray shale. Fossil Fossilplant plantdebris debrisisispresent present and and pyrite pyrite isis common commonin in accessory accessoryamounts amountsin inmany manyunits. units. Arsenic Arsenic isis very very unevenly unevenly distributed distributed in in the Marshall. Highest Highestvalues valuesin inthe theHuron HuronCounty Countycore, core,up upto to 255 mg/kg, are in black shale units. Most sandstone and siltstone have an arsenic content below the 255 m a g yare in black shale units. Most sandstone and siltstone have an arsenic content below the analytically mglkgarsenic. arsenic. analyticallydetectable detectablelimit limit (5 (5 mg/kg), mgkg) but butsome somesandstone sandstonebeds bedscontain containas asmuch muchas as25 25mg/kg Average Average sandstones sandstonescontain contain about about 22 mg/kg mgkg arsenic, arsenic?and so even these relatively low values are anomalous. adjacentLapeer LapeerCounty Countycontain contain as as much much as as 350 350 mg/kg mgkg arsenic. arsenic.The The anomalous. Cuttings Cuttingsfrom fromwells wellsininadjacent strongly strongly anomalous anomalousarsenic arseniccontent content of of the the Marshall Marshall Sandstone Sandstonein in the the area area of of arsenic arseniccontaminated contaminated ground ground water water lends lendsstrong strongsupport supportto to the the earlier earlierbelief belief that that the the source sourceof of arsenic arsenicisisnatural naturaland andwithin within the the Marshall Marshallaquifer. aquifer. Electron Electron microprobe microprobe studies studies of of well cuttings cuttings from Lapeer and Tuscola Counties show that essentially all all arsenic arsenic is is in in pyrite. pyrite. Pyrite Pyriteisisubiquitous ubiquitousininthe theMarshall MarshallSandstone, Sandstone?typically typicallyin in trace traceconcentrations, concentrationsy but locally rock, mostly as porelocally constitutes constitutesfrom from aa few few percent to as much as 20 percent of the rock? occluding occluding cement. cement. Pyrite Pyrite formed formed during during several stages of diagenesis. Early Earlydiagenetic diageneticpyrite pyriteformed formed coatings on detrital grains. Framboidal pyrite formed in intermediate stages of diagenesis and coatings on detrital grains. Framboidal pyrite formed in intermediate stages of diagenesis and precipitated overgrowths. Late precipitated on on authigenic authigeniccarbonate carbonate and chlorite chlorite or on authigenic quartz overgrowths. Latestage stage pyrite pyrite encapsulated encapsulatedframboids framboidsand andin inplaces placesformed formeddisplacive displacivepyrite pyritemasses. masses. 50 �Arsenic Arsenic is very unevenly distributed in pyrite at virtually all scales, even to the limit of resolution of the microprobe. Many Manypyrite pyritegrains grains have have little little or no arsenic arsenic at the limit of detection (about 0.01 mass percent), whereas enrichment. whereas nearby grains grains (within the same rock chip) may have extreme arsenic enrichment. Individual 6.5 mass percent arsenic. Arsenic Arsenic shows a variety of Individual analyzed analyzed points points contain contain as as much much as as 6.5 modes of occurrence, but the most common is as strong enrichments enrichments in rims on individual framboids framboids (see figure). ItItappears appearsthat that aa majority majorityof of arsenic arsenic was introduced during an intermediate stage of pyrite diagenetic growth, at the end of framboidal pyrite formation. Both Both framboid framboid cores cores and and later laterovergrown overgrown pyrite are not enriched in arsenic. Some Someother othertrace traceelements elementsalso alsoshow showenrichment. enrichment.Some Somearsenicarsenicenriched enriched pyrite contains contains as much as 0.7 mass % Ni Ni and and0.5 0.5 mass mass% %Co. Co. A critical critical remaining remaining question question is where and how arsenic is released from pyrite to contaminate ground water. In Ingeneral, general,the theoxidation oxidationof ofarsenic-bearing arsenic-bearingpyrite pyritemust mustbe bethe thefundamental fundamentalcontrol controlof ofarsenic arsenic release. Most Mostarsenic arsenicseems seemsto to be be contained contained in in black shale units, which are not aquifers and have not been observed to be undergoing undergoing oxidation. oxidation. It seems likely, therefore, that arsenic is introduced to concentrations in the coarser sandstones that that form form the the aquifers aquifers within the ground water from smaller concentrations 25 mg/kg mglkg over several Marshall Sandstone. Observed Observedarsenic arseniccontent contentof of coarse coarse sandstone sandstoneisis as as high high as as 25 feet, or tens of feet, of section. The Thesandstones, sandstones,therefore, therefore,have have concentrations concentrationsseveral severalhundred hundredtimes times in quantitative terms terms to to provide provide enough arsenic to greater than the MCL for drinking water and are able in contaminate contaminate a large volume of ground water. Results Results to to date dateindicate indicate that that the the potential potentialfor for natural natural contamination of ground water is controlled by a variety variety of of factors factors including total total arsenic arsenic contamination characteristics of the the rock, rock, and by by poorly poorly know factors such as content of the rock and hydrologic characteristics biological mediation of pyrite solution and variable redox conditions of ground water in both time and place. in arsenic-rich pyrite, in Electron microprobe microprobe map showing showing framboids of arsenic-poor pyrite encased in turn overgrown overgrown by arsenic-poor pyrite. Map of iron distribution (left) outlines large pyrite grains (lightest shade) shade) that have grown in pore space between quartz clasts (black). Authigenic clay (intermediate gray) also fills pore space. space. Arsenic distribution is shown on the the right, right, the the same same field field of of framboids now completely enclosed view as the iron map. Arsenic is concentrated on rims of pyrite frarnboids within late late stage stage pyrite. pyrite. 51 �REiNTERPRETATION REINTERPRETATIONOF OF THE THE PENOKEAN PENOKEAN CONTINENTAL CONTINENTAL MARGIN MARGIN IN IN PART PART OF NORTHERN NORTHERN WISCONSIN WISCONSIN AND AND MICHIGAN MICHIGAN W.F. W.F. Cannon, Cannon, U.S. U.S. Geological GeologicalSurvey, Survey,Reston, Reston,VA VA G.L. G.L. Laberge, Laberge, UW-Oshkosh, UW-Oshkosh, Oshkosh, Oshkosh, WI WI J.J. S. Klasner, Kiasner, Western Illinois Illinois University, University, Macomb, Macomb, IL IL K.J. Schulz, U.S. Geological Survey, Reston, K J. Schulz, U. S. Geological Survey, Reston,VA VA The Thedepositional depositionaland and tectonic tectonic history history of of part part of of the the Penokean Penokean continental continental margin margin has has been clarified clarifiedby by field field studies, studies, and and examination examination of of previously previously proprietary' proprietary1magnetic, magnetic,electroelectromagnetic, drillcore coredata. data. AAcontinental continentalmargin marginterrane, terrane,informally informallycalled calledthe theMarquette Marquette magnetic,and anddrill terrane, terrene, inasmuch inasmuchas asitit isis composed composed largely largely of of rocks rocks of of the Marquette Marquette Range Range Supergroup Supergroup and and its Archeanbasement, basement,consists consistsof ofthree threesubterranes, subterranes,each eachwith withits itsown owncharacteristic characteristicdepodepoitsArchean sitional sitional and and tectonic tectonic history (see figure). The Thesubterranes subterranesare areseparated separatedby bymajor majorPenokean Penokean faults, faults,each eachprobably probablyaanorthward-directed northward-directedthrust. thrust. POWELL POWELLSUBTERRANE SUBTERRANE LAKESUBTERRANE SUBTERRANE PINE LAKE Marquette Vflyj metasedimentary MarquetteRange RangeSupergroup Supergroup metasedirnentary & metavolcanic metavolcanicrocks rocks [i] Graywacke. metavolcanic rocks, Graywacke, metavolcanic rocks,schist schist & Archeangranitic graniticrocks rocks Archean Basal Basalferrugenous ferrugenousunit unit Archean Archeangreenstones greenstones Archean Archean gneiss gneiss WISCONSIN TERRANES WISCONSINMAGMATIC MAGMATICTERRANES PARK PARK FALLS FALLS SUBTERRANE SUBTERRANE ±Xg Granite Graywacke, Graywacke,schist, schist, metavolcanic metavolcanicrocks rocks gi Archeangneiss - Metavolcanic rocks - Keweenwan thrustfaults faults — A.. — Keweenwan thrust Penokean 7 Penokeanthrust thrust faults faults y 1 'We Corporation for permission penuission to We thank thank Cominco ComincoAmerican American Incorporated and and Kerr-McGee Corporation to publish publish detailed detailed aeromagnetic area aeromagneticand and electromagnetic electromagnetic data data for for parts of our study a rea 52 52 �The Marquette terrane includes those areas where the Marquette Range Supergroup was deposited in a tectonically unstable environment for at least part of its depositional history and was subsequently significantly deformed and metamorphosed during the Penokean orogeny. The Theterrane terrane isis bounded bounded on on the north by areas where the Marquette Range Supergroup Supergroup was deposited in a stable cratonic or foreland setting and was not appreciably deformed during the Penokean orogeny (central Gogebic Iron Range for instance), and on the south by the volcanic arcs of the the Pembine-Wausau Pembine-Wausau terrane. terrane. The essential characteristics characteristics of each subterrane subterrane are outlined below. Pine Lake subterrane subterrane 0 Archean granite, granite, greenstone, greenstone, and and metagraywacke. metagraywacke. • Basement is Late Archean 0 Early Proterozoic has quartzite and dolomite of Proterozoic has quartzite and dolomite Chocolay Group at base. • Formation, iron-formation iron-formation including including shallow-water shallow-water oolitic ooliticjasper, jasper, and and • Menominee Group consists of Palms Formation, abundant volcanic volcanic rocks. rocks. Group abundant Group was was deposited deposited in in tectonically tectonically active region, probably in syndepositional grabens. Early Proterozoic rocks were strongly deformed, generally with increasing intensity of deformation deformation to the • south. south. basement rocks rocks were were not not deformed deformedpenetratively penetratively during duringthe thePenokean Penokeanorogeny. orogeny. • Archean basement Penokean granitic rocks are rare. • Penokean granitic rocks are rare. grade ranges ranges from from chlorite chloriteto to garnet garnet (locally (locally staurolite). staurolite). • Metamorphic grade Powell subterrane subterrane Powell • Basement is is Early and and Late Late Archean Archean gneiss. gneiss. 0 Proterozoic rocks widespread basal ferruginous slate slate and and lean leaniron-formation, iron-formation, Early Proterozoic rocks include include a widespread basal unit of ferruginous • Early quartzite, and black suffidic sulfidic slate and an overling succession of metapelitic metapelitic and and lesser lessermetavolcanic metavolcanicrocks. rocks. correlative with the Marquette Range Supergroup, The rocks are probably broadly correlative Supergroup, but details detailsof of correlation correlation are not known. known. basement was penetratively penetratively deformed deformed along along with with Early Early Proterozoic cover during the Penokean Penokean • Archean basement orogeny. orogeny. • Dikes and segregations segregations of of Penokean Penokean granitic granitic rocks rocks are are widespread. widespread. pressure metamorphism metamorphism occurs throughout throughout. Pelitic assemblages are biotitebiotite• High temperature and high pressure gamet-staurolite-kyarnte. garnet-staurolite-kyanite. Park Falls subterrane • Basement is is Archean Archean gneiss. gneiss. 0 politicschist, schist,carbonaceous carbonaceoussulliclic sulfidicslate, slate, and and subordinate subordinatemetavolcanic metavolcanicrocks. rocks. • Early Proterozoic rocks are peitic equivalent to the Marquette Range Supergroup but details of correlation The rocks are probably broadly equivalent correlation are are unknown. unknown. 0 are intensely folded folded in in multiple multiple folding foldingevents. events. • Rocks are 0 segregationsof of Penokean Penokean granitic graniticrocks rocks are are abundant. abundant. • Dikes and segregations Peliticassemblages assemblagesare arebiotite-garnet-sillimanite. biotite-garnet-sillimanite. • Metamorphism is moderate temperature and pressure. Pelitic The assemblage assemblage of subterranes subterranes hereby defined in Wisconsin and Michigan is similar to the assemblage known in analogous parts of the Penokean orogen in Minnesota. In Inparticular, particular, we suggest suggest that the Flambeau Flowage Fault in Wisconsin and Michigan is the eastward extension of the Malmo discontinuity discontinuity in in Minnesota. Minnesota. Both structures structures thrust Archean gneiss and highly metamorphosed Early Proterozoic strata northward over less deformed and metamorphosed Early Proterozoic Proterozoic strata. strata. 53 �GEOGRAPHICINFORMATION INFORMATIONSYSTEM SYSTEMON ONTHE THE GEOLOGY GEOLOGYAND AND COPPER COPPER GEOGRAPHIC DEPOSITS DEPOSITSOF OFTHE THEKEWEENAW KEWEENAW PEMNSULA PENINSULA W.F. W.F.Cannon, Cannon,U.S. U.S.Geological GeologicalSurvey, Survey,Reston, Reston,VA VA Michele MicheleE. E.McRae, McRae,Oak OakRidge RidgeAssociated AssociatedUniversities, Universities,Reston, Reston,VA VA Suzanne Reston, VA VA Suzanne W. W. Nicholson, Nicholson, U.S. U.S. Geological Geological Survey, Survey,Reston, Twonew newproducts productson onthe thegeology geologyand andmineral mineraldeposits depositsof ofthe theKeweenaw KeweenawPeninsula Peninsulaand and Two surroundingarea areahave haverecently recentlybeen beendeveloped: developed:aatraditional traditional1:100,000 1:100,000scale scaleU.S. U. S.Geological Geological surrounding Surveygeologic geologicmap map(now (nowininpress) press)and andaageographic geographicinformation informationsystem system(GIS) (GIs)database. database. Survey These Theseproducts productsare arebased based largely largelyon ondetailed detailedgeologic geologicmaps, maps, many manyatat aascale scaleof of1:24,000, 1:24,000, published during during the the past past 50 50 years. years. They Theypresent presentnew newdata dataand andinterpretations interpretationsininparts partsofofthe the published area.The Thepaper paperproduct productwas wascompiled compileddigitally digitallyand anddigital digitalproduction productiontechniques techniquesare arebeing being area. used usedtotostreamline streamlinethe theprinting printingprocess. process. /1 \ Formations Paleozoic undivided J Jacobsville Sandstone Freda Sandstone Nonesuch Formation Copper Harbor Conglomerate Indiana Felsite Portage Lake Volcanics Diabase dikes Siemens Creek Volcanics Michigamme Formation Granitic gneiss Geology Geologyofofthe dieKeweenaw KeweenawPeninsula Peninsulaand andvicinity vicinitygeneralized generalizedbybyformation. formation. 54 �The The data data were were developed developed using using the Environmental System Resource Institute's Institute's ARC/INFO ARCIINFO softwareand and exported exported to to ArcView. ArcView. ArcView ArcViewisisaadesktop desktopmapping mappingsoftware softwarepackage packagethat that software provides provides aa variety variety of of tools tools for for the display, display, query, analysis, and output of geographically geographically referenced data sets. sets. This Thisparticular particular GIS GIs consists consistsof of coverages coverages and tabular data on on the the geology,structure, structure,mines, mines, mineral mineral deposits, deposits, hydrography, hydrography, and and transportation transportation networks networks of of the the geology, area. Most Mostofofthe thedetail detailfrom fromthe thesource sourcemaps maps has has been incorporated incorporated into the database. database. Map Map area. views showing showingfull fall detail detail can can be be constructed; constructed; however, however, the the data data have have been structured structuredto to allow allow views generalizationby by age, age, rock rock type, type, tectonic tectonic setting, setting, or or stratigraphic stratigraphicrank. rank. The Thefigure figureabove above was was generalization generated in inArcView, ArcView,by by generalizing generalizingthe thegeologic geologicunits unitsbased basedon onstratigraphic stratigraphicformation. formation. generated ArcView ArcView also alsoallows allows the the user user to to access accessinformation information either either by interactively interactivelyselecting selecting features features on on aa map map view, view, or or by by performing performing logical logical selection on the data tables. For Forexample, example,clicking clicking on tool will will open open aa window window that that provides provides tabular on aa mineral mineral deposit deposit with the 'identify' tool information informationon onmines minesthat thatworked workedthe thedeposit, deposit,amount amountof ofproduction, production,years yearsofofproduction, production, and and an an estimate estimate of of the the amount amount and and grade of remaining identified resources. resources. AAlogical logicalsearch search could could be be used to to identify identify native native copper copper lodes lodes with greater than 1% 1% Cu. Finally, Finally,this thisproduct product will will include includegeologic geologiccross crosssections, sections,aa correlation correlation chart, chart, aa description description of of map map units, units, and and interpretative interpretativetext. text. The Thecross crosssections sectionsare aredynamically dynamicallylinked linkedto tothe thecross crosssections sectionslines lineson on the themap mapview. view. 55 �GRAVITY VIRGINIA HORN HORN AREA, AREA, GRAVITY AND AND MAGNETIC MAGNETIC STUDIES STUDIES IN IN THE VIRGINIA MINNESOTA NORTHEASTERN MINNESOTA CHANDLER, Val W Minnesota CHANDLER, Va! W.,.,JIRSA, Mark A., and LIVELY, Richard L. Minnesota Geological Survey, 2642 2642 University Ave., St. St. Paul, Paul, MN MN 55114 551 14 Geological Survey, (chandOO4@maroon.tc.umn.edu) (chand004@maroon.tc.umn.edu) and magnetic magnetic data data were were used used to to supplement supplement geologic geologic field field mapping mapping of of Archean Archean rocks rocks Gravity and of the Virginia Horn area of northeastern Mimesota. Minnesota. Although Although outcrop outcropcontrol controlisis locally locally excellent, gravity gravity and and magnetic magnetic data data assist assist to a minor minor degree degree in mapping mapping areas areas covered covered by by excellent, glacial deposits and mine dumps. More More importantly, importantly, they provide additional information on geologic structure at depth. Preexisting Paleoproterozoic Preexisting subdivisions subdivisionsof the Archean and Paleoproterozoic rocks were used to guide mapping and geophysical work. Although the gross lithologic divisions divisions have have remained remained essentially essentiallyunchanged, unchanged, many many details detailsof of their their stratigraphic stratigraphicand and structural relations are now much better better known. known. Gravity data used in this study consist of of together with with 203 203 new new stations. stations. The new preexisting stations in the state-wide database together gravity stations development of a 50 km stations fill fill gaps in the previous coverage, and allow development northeast-southwest Virginia Horn area, area, together together with northeast-southwest profile profile across across the the Archean rocks of the Virginia (5-15 km) northwest-southeast profiles. The The magnetic magnetic data data are are from from the high shorter (5-15 five shorter resolution aeromagnetic resolution aeromagneticsurvey survey of of Minnesota; Minnesota; line line spacing spacing in in the the Virginia Virginia Horn Horn area area is is 400 400 Subsurfacestructure structurewas wasinvestigated investigatedusing using gravity gravity and and magnetic modeling on the long m. Subsurface profile. The gravity gravity and and magnetic data data were interpreted using grid images of processed data. Unfiltered and Unfiltered and derivative-enhanced derivative-enhanced versions versions of the the gravity and and aeromagnetic aeromagneticgrids grids reveal reveal many details of the bedrock geology. The Thenorthern northern part part of the the study area is underlain by Archean granitic rocks of the Giants Range Batholith. ItIt isis characterized characterizedby by aa strongly strongly negative Bouguer negative Bouguer gravity gravity anomaly anomaly and and aa busy, busy, moderatemoderate- to to high-amplitude high-amplitude magnetic magnetic mafic phases phases within within the the batholith is indicated indicated by signature. The Thepresence presenceof ofintermediate intermediateto to mafic signature. more more magnetic areas areas that that are are associated associated with highs in the derivative-enhanced derivative-enhanced gravity gravity data. Derivative-enhanced magnetic magnetic data data also delineate several previously unidentified northweststriking faults that extend across the batholith. A discontinuous discontinuous sliver sliver of high-grade strildng batholith. A metavolcanic metavolcanic and metasedimentary rocks, the Mimtac Minntac sequence, occurs along the the southern margin of the batholith. AAnegative negativeBouguer Bouguergravity gravity anomaly anomaly implies implies that the supracrustal supracrustal rocks of the Mimtac Minntac sequence are under-plated under-plated by by granitic graniticrocks rocksatatshallow shallowdepths. depths. The Minntac sequence sequence is separated separated from Archean rocks to the south by the east-west striking Laurentian fault. South Laurentian Southof ofthe theLaurentian Laurentianfault faultsubsub- to to low-greenschist low-greenschist grade grade metavolcanic metavolcanic and metasedimentary metasedimentary rocks rocks of the the Archean Archean Mud Lake Lake and and Midway sequences sequences are are characterized by aa positive characterized positive gravity gravity anomaly anomaly and and an an extremely extremely subdued subduedmagnetic magnetic signature. signature. Rocks of the Mud Lake sequence form aa broad broad southwest-plunging southwest-plungingsyncline. syncline. To the south are and west, the supracrustal supracrustal rocks rocks of the the Minntac, Midway and and Mud Lake Lake sequences sequences are Group. Regional covered by the Paleoproterozoic Animikie Group. Regional gravity data indicate that the major belt beltbeneath beneaththe theAnimikie Animikiebasin. basin. The The Biwabik Biwabik Archean rocks extend southwest as aa major Formation of the high-amplitude Iron Formation the lower lower Animilde Animikie Group Group is associated with complex, high-amplitude magnetic signatures; signatures; magnetic magnetichighs highsdelineate delineateoxide-rich oxide-richtaconites taconitesand andmagnetic magneticlows lows delineate natural ores and faults. delineate natural ores and faults. Gravity constmined by surface geologic mapping and rock Gravity and and magnetic modeling was constrained the properties determined from surface samples. The The models indicate that most structures in the Archean rocks have near-vertical dips. The Giants Range Batholith and the low-grade supracrustal extend to to about about 55 km km depth. depth. The supracrustal rocks of the Midway and Mud Lake sequences extend The composition Minntac sequence composition of the the crust crust below 5 km remains uncertain, although the Mimtac (which we infer to have been uplifted along the Laurentian fault) may yield clues because it has been extensively extensively invaded invaded by tonalitic and other early phases of the Giants Range Batholith. Models sedimentaryrocks rocksof of the theMud Mud Lake Lake Modelsclearly clearlyshow showthat thatthe thelow-grade low-gradesedimentary as thick thick as as 1 km km are are underlain underlain sequence lie within a structural structural trough; low-density sediments as 56 �by by moderately moderately high-density high-density rocks rocks inferred inferred to be largely basaltic. Thickness Thicknessof of the the sedimentary rocks rocks increases increases in in stepwise stepwise fashion fashion northwestward northwestward towards the center center of the sedimentary basin against againstnortheast-striking northeast-strikingfaults. faults. Sediment Sedimentthickness thicknessisisalso alsointerpreted interpretedto to increase increase basin appreciably to to the the northeast. northeast. These Theseresults resultsdemonstrate demonstratethat that gravity gravity and and magnetic studies are appreciably valuable valuableto to aa geologic geologicmapping mappingprogram, program,even evenin inareas areasof ofabundant abundantoutcrop outcropcontrol. control. ACKNOWLEDGMENTS ACKNOWLEDGMENTS This This study studywas wassupported supportedby by the theMinnesota MinnesotaLegislature Legislaturethrough throughthe theState StateSpecial Special Appropriation an appropriation appropriationrecommended recommended by the Minnesota Minnesota Minerals Coordinating Coordinating Appropriationand and an Committee. U.S.S.Steel SteelCorporation, Corporation,Inland InlandSteel SteelMining MiningCompany, Company, Committee. The Theauthors authorsthank thankU. Cliffs LTV Mining Mining Company, Company, American American Shield Company, the Cliffs Mining Mining Services ServicesCompany, Company, LTV Duluth Duluth Mesabi Mesabi and and Iron Iron Range Range Railway Railway Company, Company, and and the Minnesota Minnesota Power Power Company Company for for access access to their their properties properties and and for for providing providing elevation data. 57 �MINERAL POTENTIAL POTENTIAL ASSESSMENT ASSESSMENT OF NORTHERN ST.LOUIS ST.LOU1S COUNTY, COUNTY, SOUTHEASTERN KOOCHICHING COUNTY, AND NORTHEASTERN ITASCA COUNTY, SOUTHEASTERN MINNESOTA Va! Chandler, M.A. Jirsa, and G.B. Morey, Minnesota Geological Survey Survey Val W. Chandler, Presented Presented by Tom Tom Lawler, Lawler, Department Department of Natural Resources, Division Division of Minerals Minerals Minnesota Geological Survey The Minnesota Survey in a contract with the Department of Natural Resources, Resources, Division of Minerals Minerals produced a geologic map and mineral-potential assessment assessment of a contiguous contiguous township area twenty-six township area in northern St. Louis, southeastern southeastern Koochiching, and northeastern northeastern Itasca Counties, Counties, Minnesota. Six Six tholeiitic to caic-alkaline calc-alkaline volcanic sequences sequences of the Archean Archean Wawa subprovince subprovince are resolved that are usually separated by faults or metasedimentary belts, and are intruded intruded by a variety of syn- to late tectonic granitoid plutons. Eight criteria criteria are identified identified which indicate indicate potential for twenty-two lode gold deposits; Six Six criteria criteria identify identifj potential potential for for two two iron-formation hosted replacement gold deposits; Seven criteria identify potential for four iron-formation Seven criteria identifj potential four volcanic associated massive sulfide deposits; Seven Seven criteria criteria identify identi@ potential for maficultramafic intrusion intrusion hosted Cu-Ni-PGE deposits; Six Six criteria criteria identify identifj potential potential for for komatiite komatiite associated Ni-Cu-PGE deposits deposits (although (although the criteria criteria were developed only one area with PGE potential was identified); identified); and Two Two criteria identify identifj potential for two kimberlite kimberlite hosted diamond diamond deposits. All of these W h e r evaluation evaluation these areas areas are to be regarded with appropriate caution and further would require require detailed detailed exploration explorationincluding including drilling. The compilation map (Plate compilation of the bedrock geologic map (Plate 6), 61, the magnetic and gravity gravity model crosssections (Plate 7) and the mineral mineralpotential potential assessment assessment map map (Plate (Plate 9) used available available geologic geologic data combined combined with gravity gravity and and airborne airborne magnetic data. The interpretation interpretation used gridded gridded forms forms of of geophysical phenomena. Using geophysical data data that have have been enhanced enhanced to emphasize near-surface geologic phenomena. Using polarization the UTM based grid grid the aeromagnetic aeromagnetic data were enhanced by reduction to vertical polarization and calculation calculation of the second second vertical derivative. These procedures shift anomalies anomalies more directly directly over their sources anomalies to help clarify fiom regional scale anomalies clarifj the sources and eliminate eliminate interference from short wavelength wavelength signatures signatures of shallow shallow sources sources that lie at or near the Precambrian Precambrian surface. surface. With With aa similar similar procedure the gravity data data were enhanced by the calculation of the second second vertical derivative after smoothing by continuation to a level of two kilometers above derivative smoothing continuation kilometers above surface surface to eliminate eliminate "noise" caused caused by by variations variationsin in overburden overburden thickness. thickness. Much Much of of the the quantitative quantitativeanalysis analysisof of this this study are based on the Werner deconvolution method of inverse modeling using the approach approach proprietary software and proprietary software developed developed by by R.J. R.J. Ferderer Ferderer (1988). (1988). The contract resulted in a twenty-seven page open-file report 97-5: Chandler, V.W., Jirsa, M.A. and Morey, G.B., (1997) Mineral potential assessment of northern St. Louis County, southeastern southeastern Koochiching Koochiching County, County, and northeastern Itasca County, Minnesota. The report report includes includes a detailed detailed account of analytical procedures and results, rock property data, six cross cross sectional studies using gravity and magnetic modeling, five tables and nine plates displaying displaying results. This report and the plates plates are available in hard copy and digital digital format format at the Minnesota Minnesota Geological Survey, 2642 University Avenue, St. Paul, Minnesota, 55114-1057, 551 14-1057, Phone (612) (612) 6276274780 also the Minnesota Department of Natural Resources, 1525 Third Avenue East, Hibbing, Minnesota Department of Resources, 1525 Hibbing, Minnesota Minnesota 55746-1461, 55746-1461, Phone Phone (218) (218) 262-6767. 262-6767. 58 �EXTENSION OF THE HURONIAN MAGMATIC SUITE INSIDE INSIDE THE THE GRENVILLE PROVINCE: PROVINCE: NEW ZIRCON ZIRCON U-Pb EVIDENCE FROM THE THE GRENVILLE FRONT TECTONIC TECTONIC ZONE ZONE IN IN STREET STREETTOWNSHIP, TOWNSHIP,SUDBURY SUDBURY REGION, ONTARIO. F. Corfu, Museum, 100 100Queen's Queen'sPark, Park,Toronto, Toronto,ON ON M5S 2C6 and C o f i , Royal Ontario Museum, R.M. Easton, Precambrian Precambrian Geoscience Geoscience Section, Section, Ontario Ontario Geological GeologicalSurvey, Survey,933 933 Ramsey Lake Road, Sudbury, Sudbury, ON ON P3E P3E 6B5 6B5 The Sudbury region is characterized by very complex geological relationships relationships due to to the confluence and overlapping of multiple geological domains ranging in age from Archean to rifling of the Archean crust led to the the deposition of Neoproterozic. Early Paleoproterozoic rifting extensive ian Supergroup and was initially extensive clastic clastic sedimentary sedimentaryassemblages assemblagesof of the the Huron Huronian accompanied by the emplacement emplacement of bimodal mafic mafic and felsic, felsic, intrusive intrusiveand and extrusive extrusiverocks. rocks. Although the distribution distribution and petrogenetic features of these magmatic rocks is well understood in the region west of Sudbury, Sudburyythe fate of the Huronian remains a matter of speculation farther to the Frontythe only only exception exception being being the the previously previously dated dated River RiverValley Valley east and south of the Grenville Grenville Front, gabbro-anorthosite. In conjunction program gabbro-anorthosite. conjunction with an Ontario Ontario Geological Geological Survey S u ~ e mapping mapping y program along alongthe the Southern-Grenville Southern-GrenvilleProvince Province boundary, we have examined examined two examples examples of of metamorphosed metamorphosed granitic bodies and a metapyroxenite plug straddling the Grenville Front Front tectonic tectonic zone in Street 15 km east of Sudbury. Sudbury. Township, roughly 15 The metapyroxenite is part of a suite of small bodiesy bodies, generally less than 500m in size, that occur Coniston and and River River Valley. Valley. Mineralogically, Mineralogically, within the Grenville Grenville Front Front tectonic tectonic zone zone between between Coniston these bodies consist of roughly equal amounts of orthopyroxene phenocrysts (0.5-5 cm in size) in locally olivine phenocrysts are preserved. Metamorphic crystallization in an amphibole matrix; locally these bodies increases with increasing increasing distance from the Grenville Front, consequently, sampling was conducted on a body located only 250m southeast of the Grenville Front. The metapyroxenite contains highly resorbed, prismatic zircon yielding a U-Pb U-Pb data data array array that that points points toward toward an an upper upper intercept age of about 2490 Ma. The age corresponds corresponds approximately approximately to to that that of of the the East-Bull East-BullLake Lake and Shakespeare-Dunlop intrusionsyemplaced emplaced in in Archean crust crust at at the the margin margin Shakespeare-Dunlop (a.k.a. Agnew Lake) intrusions, Huronian basin basin east east of of Sudbury. Sudbury.On On the the basis basis of of SEM SEM examination, examinationymost mostof ofthe thezircon zirconfound found of the Huronian in this sample is located within the orthopyroxene orthopyroxene phenocrysts, in in conjunction conjunctionwith with Cr-spinel Cr-spineland and chromite. In addition, the matrix of the metaproxenite is chemically differentiated. Thus, we are confident that the zircon fraction yielding the upper intercept age of 2490 Ma dates emplacement metapyroxenite was metamorphosed and developed metamorphic zircon during of these rocks. The metapyroxenite an event at about about 1700-1600 1700-1600Ma, Ma, and and was was subsequently subsequentlyoverprinted overprintedby by Grenvillian-age Grenvillian-age metamorphism. metamorphism. Two granitic bodies were sampled. The first was a foliated monzogranite located in an area south of the Ess Creek fault and north of the Grenville Front boundary fault as mapped by Lumbers 59 �(1973). This fault-bounded fault-bounded region has been variously variously assigned assigned to the Southern Southern or or Grenville Grenville provinces, and has been subjected subjected to middle to upper amphibolite amphibolite facies facies metamorphism. metamorphism.The The second body is located about about 15 15 km south of the first, and is is one one of several several similar similar bodies bodies located located within the Grenville Grenville Province. Province. Both sampled granites, granites, as as well as as other other bodies bodies south south of of the the Grenville Grenville plutonic body or a suite of Front, are chemically similar, suggesting they form part of a larger plutonic FeOt0 >35% intrusions. intrusions. In particular, particular, they they are are characterized characterizedby by Si02 Si0266-72%, 66-72%, Al203 A120311.9-12.6%, 11.9-12.6%, FeotO&' >3.5%, CaO >1.5%, La/Yb >IS%, EufEu* Eu/Eu* .55-.70, -55--70,L a b 5-8, and Gd/Yb GdNb 1.2-1.65, 1.2-1-65, and are identical identical in major major and and REE geochemistry with Stobie Formation Formation dacites from both the Sudbury Sudbury and Street Street Township Township areas. areas. On On they plot plot in in the the Within-Plate Within-Plate granite granite field. field. In In various geochemical discrimination diagrams, they contrast, felsic rocks of the Murray and Creighton granites granites and the Copper Copper Cliff Cliff rhyolite rhyolite have have lower Feotoh' FeOt0 contents contentsand andhigher higherSi02 Si02and andalkali alkalicontents. contents.In Inaddition, addition,REE REEdata, data,only onlyavailable available from the Copper Cliff rhyolites, have Eu/Eu* EufEu* c.5, <.5, LLa/Yb <5.5and andGdffb Gd/YbC.75. <.75. The foliated a b -3.5 monzogranite monzogranite defines defines a U-Pb zircon age age of about about 2460-2450 Ma, which overlaps overlaps the the age age of of some some of the youngest magmatic expressions of the Huronian Huronian rifting rifting event event such such as as the the Copper Copper Cliff Cliff rhyolite rhyolite and the Hearst Hearst dyke dyke swarm. swarm. The The granite granite was was overprinted overprinted by by Grenvillian Grenvillianmetamorphism metamorphism that strongly, strongly, but not not totally totally reset reset the the titanite titanite ages. ages. The The zircon zircon data data for for the the second secondgranitic graniticbody body display more pronounced effects of both a 1700-1600 1700-1600 Ma event and the Grenvillian Grenvillian orogeny orogeny at at about 990-980 Ma, but are nevertheless nevertheless consistent consistent with a Huronian Huronian age and with the the chemical chemical similarities similarities between these granitic granitic intrusions. intrusions. are consistent consistent with field field relations relations which which show show aa close close spatial spatial The ages on the bodies reported here are relationship between the metapyroxenites, metapyroxenites,the the granites, granites, and and mesocratic mesocratic to to anorthositic anorthositicgabbros gabbros relationship likely correlative with the ca. 2475 Ma River Valley gabbro-anorthosite. gabbro-anorthosite. In addition, addition, the foliated foliated monzogranite is spatially associated with a thin sliver of mafic and felsic metavolcanic rocks that monzogranite that correlated with the the Huronian Huronian Stobie Stobie Formation. Formation. The The latter latter are are the the only only Huronian Huronian have been correlated volcanic rocks so so far far identified identified east east of of Sudbury. Sudbury. These These field field relationships, relationships,in in conjunction conjunctionwith withthe the geochronological results results reported reported herein, herein, suggest suggest that that the the Huronian Huronian magmatic magmatic province provinceisismore more geochronological extensive extensive east of of Sudbury Sudbury than previously previously recognized. In In addition, addition, felsic felsic magmatism magmatismof ofHuronian Huronian longer confined confined to to the the vicinity vicinity of of Copper Copper Cliff, Cliff, and and indeed, indeed, could could be be more more extensive extensivethan than age is no longer previously thought, thought, particularly particularly in in the the area area of of the the little littlestudied studiedeastern easternCobalt Cobaltplate. plate. previously Reference: 1973. River Valley area; Ontario Ontario Division of Mines, Preliminary Map, P.844. Lumbers, S.B. 1973. P.844. 60 �KINEMATIC KINEMATIC FABRICS FABRICS NEAR MINE MINE CENTRE, CENTRE, ONTARIO: EVIDENCE EVIDENCE FOR A MODIFIED MODIFIED TRANSPRESSION TRANSPRESSION MODEL. MODEL. Czeck, and Hudleston, Hudleston, P. J., Department Department of Geology and Czeck, D. M. and Geophysics, Geophysics, University of Minnesota, Minnesota, 310 3 10 Pillsbury Pillsbury Dr. Dr. SE, SE, Minneapolis, MN 55455 55455 The The boundary boundary between between the the Quetico Quetico and and Wabigoon Wabigoon subprovinces subprovinces of the the Archean Archean Superior Superior Province is characterized by ductily deformed GreenschistGreenschist- Arnphibolite Amphibolitefacies faciesrocks. rocks. The two subprovinces continental accretion accretion processes. processes. The subprovinces have been amalgamated through continental working working kinematic kinematic model model to to describe describe this this area area is is transpression, transpression, which which explains explainsthe the dominant dominant flattening flattening fabric fabric in the vertical vertical plane and the evidence for noncoaxial strain noted in the horizontal plane. Theoretical Theoreticalwork workon onclassical classicaltranspression transpression has has shown shown that that the the long long axis of the the strain strain ellipsoid,' ellipsoid,and either vertical or or axis and thus the mineral lineation, are necessarily either horizontal. horizontal. Structural Structuralfield fieldwork workin inthis thisstudy studyhas hasshown shownthat that the the lineations lineationsalong along the the Wabigoon-Quetico boundary plunge between 0-90' 0-90° within within the the foliation foliationplane. plane. Therefore, a modification modification to to the the three-dimensional three-dimensionaltranspression transpressionmodel model isis necessary necessaryto toadequately adequately describe describe the deformation deformation process. One Onepossible possiblemodification modificationto to transpression transpression which which explains explains the the oblique oblique lineations lineations is is heterogeneous heterogeneous extrusion extrusion due due to anastomosing anastomosing shear shear zones. zones. 61 �New New Aeromagnetic AeromagneticSurveys SurveysininWisconsin Wisconsinby bythe theU.S. U.S.Geological GeologicalSurvey Survey David DavidL. L.Daniels, Daniels,Stephen StephenL. L.Snyder, Snyder,Suzanne SuzanneW. W.Nicholson, Nicholson,William William F. F. Cannon, Cannon,U.S. U.S.Geological Geological Survey, Survey, MS MS 954 954National National Center, Center,Reston, Reston, VA VA20192 20192 Aeromagnetic Geological Aeromagneticsurveying surveyingin in Wisconsin Wisconsin over over the the past past 10 10years yearsby by the theU.S. U. S.Geological Survey Survey(USGS) (USGS)has has added addedconsiderably considerably to to the the coverage coveragein in the the state stateand and to tothe thedigital digital aeromagnetic database of the USGS Mineral Resources Program. Initial efforts aeromagnetic database of the USGS Mineral Resources Program. Initial effortswere weredirected directed toward towardcompleting completingthe thecoverage coverageininthe thenorthern northernpart partofofthe thestate stateininwhich whichPrecambrian Precambrianbedrock bedrockisis atatthe thesurface surfaceor orcovered coveredby byglacial glacialdeposits. deposits.The Theflight-lines flight-lineswere wereflown flownin in aa N-S N-S direction direction because because of of the the predominant predominanteasterly easterly or or northeasterly northeasterly grain grain of of the the geology, geology, were were spaced spaced '/2-mile %-mile apart, apart,and andflown flown at at 500 500or or1000 1000ftftabove aboveterrain. terrain. The Thesurveys surveyswere weredesigned designedto toextend extendthe thelarge large survey 1990), surveyconducted conductedby byJohn JohnKarl Karlofofthe theUniversity UniversityofofWisconsin, Wisconsin,Oshkosh Oshkosh(Karl, (Karl, 1986; 1986;King, King,1990), priorto to1977. 1977. prior In In1988, 1988,the theUSGS USGSflew flewaa3900 3900line-mile line-mileaeromagnetic aeromagneticsurvey surveyin in the thenorthwestern northwesterncorner comer of ofWisconsin Wisconsin south south of of Lake Lake Superior. Superior.In In 1996, 1996,USGS USGSsurveying surveyingwas was continued continued in in two two areas areas adjacent to the 1988 survey (see index map) for an additional 6700 line-miles. These two adjacent to the 1988 survey (see index map) for an additional 6700 line-miles. These twosurveys surveys completed completed the the coverage coverageof ofthe thevolcanic volcanicand andsedimentary sedimentaryrocks rocksin in the theMidcontinent Midcontinentrift riftsystem. system. These Thesenew new data datahave haveled led to tonew newinterpretations interpretationsofofstructures structuresininthe thehighly highlymagnetic magneticKeweenawan Keweenawan basalt basalt flows flows in in the the St. St.Croix Croixhorst horst (Cannon (Cannonand and others, others,1997). 1997).In Inthe themost mostrecent recent(10/97-3/98) (10197-3198) survey, survey, aeromagnetic aeromagneticdata datahave havebeen beenacquired acquiredininthree threeblocks blocksin in aabroad broadswath swaththrough throughthe the central central part part of ofthe the state, state,ininthe theMarinette-Green Marinette-GreenBay Bay area, area, the theWisconsin Wisconsin Rapids Rapids area, area, and and the the Mississippi Mississippi River River area area (about (about16,000 16,000line-miles). line-miles). In In addition, addition,two two small small areas areas along along the the Wisconsin-Michiganborder borderwere werefilled filledin. in. Wisconsin-Michigan In 50m) veneer (<150m) veneer of of In much much of ofthe thearea areacovered coveredby bythe themost mostrecent recentsurvey, survey,aathin thin(<1 magnetically magnetically transparent transparentCambrian Cambriansandstone sandstoneoverlies overliesthe themore morehighly highly magnetic magneticEarly Earlyand andMiddle Middle Proterozoic Proterozoicrocks; rocks;ininthe thewestern westernpart partnon-magnetic non-magneticMiddle MiddleProterozoic Proterozoicsedimentary sedimentaryrocks rocksform form part part of of the the basement. basement. Thus, Thus,although althoughthis thisisisan anarea areaof ofvery very little little outcrop, outcrop, the the aeromagnetic aeromagnetic data data effectively effectively delineate the structure structureand and character characterof ofthe theburied buried basement basement rocks. rocks.The Theenhanced enhanced understanding understandingof of the the basement basement geology geologyprovided providedby by the theaeromagnetic aeromagneticsurvey surveywill will allow allow better better evaluation evaluationof of the the mineral mineral potential potential of of this this region region and and aid aid in in regional correlations correlationsof of basement terranes terranesin in the the region. region. Maps Maps of of the the new new aeromagnetic aeromagneticsurvey survey and the current current aeromagnetic aeromagnetic compilationof ofWisconsin Wisconsinwill will be be shown. shown. compilation References Cited Cited References Cannon, Cannon, W.F., W.F.,Daniels, Daniels, D.L., D.L., Snyder, Snyder,S.L., S.L.,1997, 1997,New Newaeromagnetic aeromagneticmap mapofofthe theMidcontinent Midcontinent Rift Rift in in Northwestern NorthwesternWisconsin Wisconsin and and adjacent adjacent Minnesota; Minnesota;Geological Geological Society Society of of America America 1997 1997 abstractswith with programs, programs, v. v. 29, 29, no. no. 4, 4, p. p. 9.9. abstracts Karl, Karl, J.H., J.H.,1986, 1986,Total Totalmagnetic magneticintensity intensitymap mapof ofnorthern northernWisconsin; Wisconsin;Wisconsin Wisconsin Geological Geological and and Natural History Survey, Map 86-7, scale 1:250,000. Natural History Survey, Map 86-7, scale 1:250,000. King, King, E.R., E.R.,1990, 1990,Precambrian Precambrianterrane terraneofofnorth-central north-centralWisconsin: Wisconsin:an anaeromagnetic aeromagneticperspective; perspective; Canadian Journal Journal of of Earth Earth Science, Science, v. v. 27, 27, pp. pp.1472-1477. 1472-1477. Canadian 62 �Index of Recent Aeromagnetic Aeromagnetic Surveys Surveys in in Wisconsin Wisconsin 93 93- 91 91' 9T 92' 88 88- 89 89' 9O 90' 8T 87' 46 45 44 43 92 0 50 88° 89° 90° 91° 150 100 63 200 KM �POLYMETAMORPHISMOF OF SKARNS SKARNS RELATED RELATED TO TO THE MORIN POLYMETAMORPHISM MORIN ANORTHOSITE ANORTHOSITE COMPLEX, GRENVILLE COMPLEX, GRENVILLE PROVINCE, PROVINCE, QUEBEC. QUEBEC. DEANGELIS1, DEANGELIS l , M. T., PECK, W. H., and VALLEY, VALLEY, J.J. W., W., Department Department of Geology Geology and Geophysics, University of Wisconsin, Wisconsin, Madison, Madison, Madison, Madison, WI, WI, 53706, 53706, william@geology.wisc.edu. william@geology.wisc.edu.1student ^student author caic-silicate assemblages Skarn calc-silicate assemblagesassociated associated with with the the Mona MorinAnorthosite AnorthositeComplex Complexare areexposed exposed—2 -2 km west of the town of St. St. Jovite, Jovite, Quebec, Quebec,and anddisplay displayfield fieldrelations relationsand andreaction reaction textures textureswhich whichindicate indicateaa polymetamorphic origin. small dioritic dioritic and and monzonitic monzonitic intrusive intrusive bodies, bodies, which which are are located located -25 small polymetamorphic origin. A set of —25 between two mapped bodies bodiesofofmangerite mangerite(Martignole (Martignoleand andConiveau, Corriveau,1993) 1993)outcrop outcropinina a—300 -300 meter meter long long These bodies bodies are are rimmed rimmed by by 10-20 cm cm thick thick series of four roadcuts in calcite marble along Highway 117. These skarns containing containing metasomatic metasomatic banding of garnet, diopside, and wollastonite. wollastonite. This This locality locality occurs occurs adjacent adjacent ± 3 Ma (Doig, 1991). to the western margin of the Morin Anorthosite Anorthosite Massif which was emplaced at 1155  bodies that that were were emplaced emplaced 1135 1135 ±3 The anorthosite is associated with mangerite bodies 3 Ma (Doig, 1991). The The area was metamorphosed anorthosite massif metamorphosedunder granulite granulite facies facies conditions conditions following emplacement of the anorthosite of 650-775 650-775 O °C Martignole, 1989). 1989). This to pressures of 6-8 kb and temperatures of C (Indares and Martignole, metamorphism metamorphism has been been dated dated at at 1070-1100 1070-1100Ma Ma by Rb-Sr Rb-Sr whole whole rock isochrons isochrons (see (see Doig, Doig, 1991). 1991). Descriptions of skarns Descriptions of skarns Well developed developedskarns skarns appear appear on both diorite diorite and monzonite bodies. We Weinterpret interpretthese thesebodies bodies as as intrusive relict igneous igneous textures. textures. On intrusive because because of cross-cutting cross-cutting relationships relationships between different intrusions and relict exposed with diameters diameters exposed surfaces, surfaces, diorite diorite bodies bodies vary vary in in both both size size and and shape, shape,but but are are typically typically circular circular to to oval oval with ranging from 11 to 10 10 meters. meters. The Thediorite dioriteintrusives intrusivesare arecomposed composedprimarily primarily of of plagioclase plagioclase with with minor minor amounts of clinopyroxene, amphibole, biotite, magnetite, pyrite, and sphene. Metamorphic Metamorphicminerals minerals and clinozoisite. which are are made made up upof of include calcite, garnet and clinozoisite. Surrounding Surroundingthe the diorite diorite intrusives are skarns which concentric garnet-, clinopyroxene-, cinopyroxene-, and and wollastonite-rich wollastonite-rich bands. Next Next to to the the diorite, diorite, garnet-rich bands are —10cm cmthick. thick.Wollastonite-rich Wollastonite-richbands bands(in (inthe themiddle middleof ofthe theskarn) skarn)are arethinner thinner(51 (1 cm) cm)and andsometimes sometimesare are -10 cmthick thick and gradually not present. Clinopyroxene-rich Clinopyroxene-richbands bands are are —15 -15 cm gradually grade grade into the marble. The Thegarnetgarnetplagioclase, quartz, calcite, sphene, and small amounts of pyrite. pyrite. rich bands contain garnet, clinopyroxene, clinopyroxene, plagioclase, plagioclase, garnet, The wollastonite-nich wollastonite-rich bands are composed of wollastonite, clinopyroxene, calcite, plagioclase, and pyrite. pyrite. The clinopyroxene-rich bands are are made made up up of of quartz, sphene, and small amounts of graphite and clinopyroxene, calcite, garnet, clinopyroxene, garnet, plagioclase, plagioclase, quartz, quartz, sphene, sphene, with minor amounts amounts of graphite and pyrite. monzomte intrusives The monzonite intrusives are composed of potassium feldspar feldspar and plagioclase plagioclase with minor amounts of clinopyroxene, and sphene. Metamorphic quartz, clinopyroxene, Metamorphic minerals minerals include include calcite, calcite, garnet, garnet, and Fe-vesuvianite (Fe/Fe+Mg=0.65 determined determined by electron microprobe). FeFevesuvianite is visibly zoned in hand sample, with Fe-rich rims (Fe/Fe+Mg=0.67) (FeIFe+Mg0.67) and andless less Fe-rich Fe-rich cores (Fe/Fe+Mg=0.61). Fe-vesuviamte Fe-vesuvianitecrystals crystalscan can reach reach lengths cmand andare are found found within within the the monzonite monzonite and lengths of —6 -6 cm the inner most skarn band. Within Withinone onemonzonite monzonitebody, body, an unusual unusual texture texture consisting consisting of of aa cylinder cylinder of of matrix matrix minerals minerals (potassium (potassiumfeldspar, feldspar,plagioclase, plagioclase,garnet, garnet,and and quartz) quartz)encased encasedby byaasingle singlecrystal crystalofofelongate elongate(—2-6 (-2-6 cm cm long, width50 mm)clinopyroxene clinopyroxene(En26Fs23Wo50Ac1) (En2(;Fs23W050Acl) width90mm) occurs. This gives these crystals a "hollow" appearance in thin section and on the outcrop. Monzonite Monzonite occurs occurs both Fragmented diorite dioriteintrusion. intrusion. Figure 1. Fragmented semi-circular bodies (1-10 as semi-circular (1-10 m in diameter) diameter) as as well as Skam is developed Skam developed where where hot hot diorite diorite contact with with marble. marble. Brittle came into contact Brittle dikes which cross cut diorite. Skarns Skarns around the deformation of of diorite and skarn postdeformation monzonite consist of small (3-5 cm) garnet- and intrusion. dates intrusion. clinopyroxene-rich bands. Garnet-rich clinopyroxene-rich Garnet-rich bands contain clinopyroxene, calcite, garnet, clinopyroxene, calcite, potassium potassium feldspar, feldspar, Fe-vesuvianite, and sphene. Clinopyroxene-rich plagioclase, Fe-vesuvianite, Clinopyroxene-rich bands bands contain contain clinopyroxene, clinopyroxene, calcite, calcite, garnet, potassium feldspar, primarily feldspar, plagioclase, plagioclase, and sphene. sphene. Away from the igneous bodies, marble is composed of primarily equigranular calcite with small amounts of clinopyroxene, garnet, sphene, sphene, and and feldspar. feldspar. Trace amounts of equigranular graphite and pyrite are also observed. Bogoch (1997)report reportsimilar similarmineralogies mineralogiesof of calcite, calcite, potassium potassium Bogochetetala1(1997) feldspar, vesuvianite, quartz, clinopyroxene, cinopyroxene, plagioclase, spbene in a body body 2.5 2.5 m m plagioclase,wollastonite, wollastonite, graphite, and sphene long and 50 cm wide in calcite krn west of the field area. calcite marble marble approximately 12 12 km 64 �Deformation Deformation and and mineral mineral reactions reactions The majority of the exposed skarns skarns parallel the edge of the igneous igneous bodies bodies and and maintain uniform uniform thickness. However, However,some someskarns skarnsand and igneous igneous rock are Someigneous igneousbodies bodies and and their their skarns skarns locally deformed. Some are folded and boudined, boudined,2-25 2-25 cm cm size size skarn skarn fragments fragmentsare are common away from from igneous igneous rock; rock; some some of which are are variably folded. Skarn Skarnfragments fragments and and skarn skarn minerals are observed being transported by ductile flow of the marble (e.g. into boudin necks and out of fold hinges). Some Some igneous bodies and their skarns skarns show brittle deformation, breaking both skarn and intrusion intrusion into fragments fragments (see Fig. 1). The intrusionThe lack lack of of skarn skarndevelopment development on on some someintrusionmarble contacts contacts of dismembered dismemberedigneous igneous bodies bodies indicates indicates that brittle deformation deformation (of (of igneous igneous rock rock and and skarn) skarn) Figure 2. Calcite rimmed with garnet occurred after after igneous crystallization and skam skarn formation. formation. (Gç)). This texture (G texture and and garnet garnet rimming nmnung Mineral wolliastonite is Mineral textures textures show show that that original original skarn skarn wopastonite is evidence for the the reaction calcite+ quartz+ quartz+ anorthite+ anorthite+ mineralogies have mineralogies have been been modified modifiedby by aa later later granulite-facies granulite-facies wollastonite-> grossular+ wollastonite-> grossular+CO2. COT metamorphic metamorphic event event (see (see also also Martignole Martignole and and Schriver Schriver 1970). Garnet Garnet(Gr92) (Gr92)rims rimssurrounding surroundingcalcite, calcite, wollastonite, wollastonite,and and plagioclase plagioclaseindicate indicatethe the reaction reaction calcite calcite ++ quartz ++ anorthite anorthite ++ wollastonite wollastonite -> -> grossular + + + wollastonite C 0 2 (see Fig. 2). Garnet-quartz Garnet-quartz intergrowths intergrowthsindicate indicate the the reaction anorthite + ->grossular ++ CO2 wollastomte ->grossular quartz. quartz. The The univariant univariant assemblage assemblageof of garnet garnet(Gr95), (Gr95),plagioclase plagioclase(An19), (An19),quartz, quartz,wollastomte wollastoniteisisobserved observedas as fine-grained intergrowths. Dilution Dilutionof ofthe theanorthite anorthitecomponent componentof of plagioclase plagioclase by albite albite moves moves the fine-grained univariant univariant assemblage assemblage to to lower lower pressures pressures and and temperatures temperatures(Windom (Windom and and Boettcher Boettcher 1976), 1976),consistent consistent with the estimates estimates of regional regional metamorphism metamorphism (Indares (Indares and Martignole 1990). 1990). The Thepresence presenceof of wollastomte+ wollastonite+ vesuvianite, garnet+ plagioclase+ quartz+ wollastonite, wollastomte, clinozoisite+ plagioclase+ plagioclase+ quartz, quartz, and and sphene sphene vesuvianite, indicate metamorphism was was fluid fluid saturated. saturated. These Xu20 ifif metamorphism These assemblages assemblages also also consistent consistent indicate conditions conditions of high high XH2O with estimates of low H2O activityfrom from granulite granulitefacies facies terrains terrains if conditions are fluid absent or if non CH20 activity 0-H fluid 0-H fluid species speciesare are important important(Valley (Valley et et al., al., 1990). 1990). Comparison to the Comparison the Adirondacks Adirondacks Economic wollastonite deposits depositsin in the the Adirondack AdirondackHighlands Highlands(—250 (-250 km Economic wollastonite kmto tothe the south) south) are are skarns skarns associated with the the Marcy Marcy Anorthosite Anorthosite Massif Massifand andrelated relatedgranitic graniticrocks rocks.. Skarn formation adjacent to to the the anorthositewas was due due to to the the infiltration infiltration of of oxidizing oxidizing meteoric meteoric water water into the high temperature contact Marcy anorthosite zone of a shallow (<10km) anorthosite body. The Adirondacks Adirondacks underwent regional granulite facies body. The facies metamorphismwith withhigh highpressure, pressure,fluid-absent fluid-absentconditions conditions—100 -100 Ma later later (McLelland (McLellandand andChiarenzelli, Chiarenzelli, metamorphism 1990), wollastonite and 1990), but wollastonite and other other skarn skarn minerals minerals remained remained stable stable through through this this later later metamorphism metamorphism due due to to fco2 "fluid-absent" "fluid-absent"conditions conditions(Valley (Valley et et al. al. 1990). 1990). Post-intrusion Post-intrusiondeformation deformation and and mineral mineral textures in low fc02 Morin Complex Complexskarns skarnssuggest suggestpolymetamorphism, polymetamorphism,but butpublished publishedgeochronology geochronology(i.e. (i.e. dating datingof ofhigh high Monn temperature temperature metamorphic metamorphicminerals) minerals)atatpresent presentdoes doesnot notallow allowaadistinction distinctionbetween between two twoseparate separate metamorphicevents, events, or or aa continuum continuumof of metamorphism metamorphismunder underchanging changingconditions conditionsfollowing followinganorthosite anorthosite metamorphic intrusion or depth of intrusion. intrusion. References Bogoch, R., Kumarapeli, S., Can Mm Min 35:1269-1275. 351269-1275. S., and Matthews, A. A. (1997) Can Geology 99: 99: 729-738. 729-738. Doig R. (1991) J Geology J., Corriveau L. (1993) Geol Geol Surv Surv Can Can Open Open File 2640. Martignole J., Martignole J., Schrijver Schrijver K. K. (1970) (1970)Geol Geol Soc SocFinland Finland Bull Bull 42: 42: 165-209. 165-209. Chiarenzelli, J. J. (1990) (1990)JJ Geology Geology 98:19-41. 98:19-41. McLelland, J., Chiarenzelli, 1-386. (1989)Can CanJJEarth EarthSci Sci27: 27:37 371-386. Indares A., Martignole MartignoleJ.J. (1989) 1:555-596. R., Essene, Essene, E. E. J., J., and and Lamb, Lamb, W. W. (1990) (1990) J JPetrology Petrology331555-596. Valley, J., Bohlen, S. R., Am Mm Min61:889-896. 61:889-896. Windom, K. E., Boettcher, A. L. (1976) Am 65 �THE OF THE THE GUNFLINT LAPILLI LAPILLI TUFF THE AGE AND PROVENANCE OF TUFF and KISSIN, KISSIN, S.A., S.A., Department Department of of Geology, Geology, Lakehead Lakehead FRALICK, P.W. and University, Thunder Bay, Ontario, P7B University, P7B 5E1, 5E1, and and DAVIS, DAVIS, D.W., Royal Ontario Ontario Museum, Museum, 100 100 Queen's Park, Park, Toronto, Toronto,Ontario, Ontario,M5S M5S 2C6. 2C6. forms the the middle middleunit unit of ofthe the Animikie Animikie Group Group in northwestern The Gunflint Formation forms proximally to to the western end of Ontario, outcropping proximally of Lake Lake Superior. Superior. It consists of an assemblage assemblage of chemical and fine-grained elastic sediments deposited in the strand-proximal zone of a south facing (1996). facing shelf shelf as recently interpreted by Pufahl Pufahl(1996). Previous attempts at assigning an age to the Gunflint Formation (or the conformably conformably overlying Rove Formation) and equivalents equivalentscan can be be classified classified into into three three groups: groups: (1) ages from (Hurley et et a!., al., 1962; 1962;Peterman Peterman 1.556 to 1.63 Ga based on whole-rock Rb/Sr or KIAr K/Ar techniques techniques (Hurley 1969; and Franklin, 1978); 1978); (2) ages from 2.08 to 2.111 2.1 11 Ga Ga based on on 1966; Faure Faure & Kovach, 1969; a!, 1988); and (3) 1.86 1985; Gerlach et al, 1.86 and whole-rock NdJSm NdlSm techniques (Stille & Plauer, 1985; Hemmings et al; 1995). 1995). 1.99 Ga based on whole-rock PbIPb Pb/Pb from the the Virginia Virginia Formation Formation ((Hemmings Petrographic the upper upper Gunflint at Kakabeka Kakabeka Petrographicexamination examination of of aa lapilli lapilli tuff unit present in the Falls identified euhedral zircons forming a small portion of the silt population together with large, monomineralic clasts of of quartz quartz and and sanidine likely from an explosive, volcanic source. source. stratigraphic section section present present here has aa large large algal algal bioherm bioherm complex complex at at its its lowest lowest level level The stratigraphic (Figure sporadic (Figure 1). 1). This Thisisis overlain overlainby by aa thick thick sequence sequence of parallel-laminated black shale with sporadic development of layer rip-ups rip-ups caused causedby bycurrent currentactivity. activity. A A series of what what has been described as lapilli tuff beds beds occurs midway midway through through this thissequence sequence(Shegelski, (Shegelski, 1982). 1982). These are massive to disorganized disorganizedbedded. bedded. Mudstone rip-ups rip-ups are are common. common. The to cross-stratified and graded to The lapilli consist of Fe-rich Fe-rich chlorite chlorite and and are areinternally internallymassive. massive. The The above above strongly suggests suggests that the lapilli are not accretionary volcanic-rainout debris, but but rounded rounded volcaniclastic volcaniclastic mud mud rip-ups, and transported transportedby bystorm-induced storm-inducedcurrents. currents. The The black black shale unit which were eroded, abraded and is correlated with a find-grained, fmd-grained, resedimented, volcaniclastic shale, which which extends throughout upper Gunflint equivalents equivalents in the U.S.A. U.S.A. It may also be correlated correlated with aa small small region region containing containing basaltic basaltic flow flow rocks. rocks. Approximately 100 tuffs, including including altered, 100 zircons were recovered from the reworked tuffs, altered, abraided and euhedral euhedral brown brown populations. populations. Although the majority of the population analysed concordant age of 1878± gave reset Archaean ages, a euhedral zircon gave aa concordant 1878Â2Ma BP (Figure (Figure 2). 2). This most likely represents the age of of volcanism volcanism that was penecontemporaneous penecontemporaneous with sedimentation. sedimentation. volcaniclastic sediment Preliminary geochemical geochemical data data for Gunflint basalts and volcaniclastic sediment indicates indicates possibility of deep mantle mantle source for the melts melts and rules out out any any involvement involvement of the possibility of a deep Similaritieswith with the the Emperor Emperor and and Hemlock Hemlock volcanics volcanics suggest suggest a subducting lithosphere. lithosphere. Similarities eruptive event, event, which which may may in in part part overlap overlap northward time-transgressive, extensional, intraplate eruptive chronologically with the commencement commencement of arc volcanism to the south. south. 66 �References References Faure?G., G.?& & Kovach? SOy 1725-1736. Faure, Kovach,J.J.(1969). (1969). Geol. Geol.SOC. Soc.Amer. Amer.Bull. Bull. , 1725-1736. Franklin, J.M. (1978). Geol. Sum. Can. Pap. 77-14,35-39. Franklin, J.M. (1978). Geol. Surv. Can. Pap. 77-14, 35-39. D.C.? Shirey, 1515. GerlachyD.C., Gerlach, Shirey, S.B. S.B.& &Carlson, Carlson,R.W. R.W.(1988). (1988).EOS EOS@, , 1515. Hemming,S.R., S.R.?McLennan, McLennan,S.M. S.M.& & Hanson, Hanson, G.N. G.N. (1995). (1995). Jour. Jour.Geol. Geol.jQ., 147-168. 147-168. Hemming, Hurley, P.M., Fairbairn, H.W., Pinson? W.HSy & Hower, J. (1962). Jour. Geol. 489-492, Hurley, P.M., Fairbaim, H.W., Pinson, W.H., & Hower, J. (1962). Jour. Geol. 489-492. PetermanyZ.E. Z.E. (1966). (1966). Geol. Geol.Soc. SOC.Amer. Amer. Bull. Bull. fl, 103 lo31-1044. 1-1044. Peterman, Pufahl,P.K. P.K.(1996). (1996).M.Sc. M.Sc.Thesis, Thesis,Lakehead LakeheadUniv., Univ.,167 167 p. Pufahl, p. R.J. (1982). Geol. Assoc. CanJMineral. Assoc. Can.Field FieldTrip TripGuidebook Guidebook4, 4?15-31. 15-31. Shegelski, Shegeiski, R.J. (1982). Geol. Assoc. Can./Mineral. Assoc. Can. my a, , a, Breccia Breccia Stromatolites Stromatojites Wavy Beddin( '' Wavy Beddinc . .34 Cross- '..-.. Cross Stratification Stratification Brown euheclral .338 1878 +I-2 Ma .336 .334 20'Pb / 236U .332 5.2 5.25 5.3 5.35 5.4 5.45 10-01-1997 Fig.2. 2.Pb/U Pb/Uplot plotfor foreuhedral, euhedralybrown brown Fig. zircons from fiomthe thelapilli lapillituff tuffunit. unit. zircons a Granulesto to / Granules .smallPebbles Pebbles small Sandstone Sandstone siltstone .--. Siltstone .-. .:. .:... .. .. .. .. .. .. . .............. . . . . . . Shale Chert Fig. 1.1.Stratigraphic Stratigraphicsection sectionof ofthe theupper upper Fig. GunflintFormation Formationcontaining containinglapilli lapilli Gunflint tuff units in the Kakabeka Gorge. tuff units in the Kakabeka Gorge. 67 �PRE-WISCONSINAN AREA OF THE MINNESOTA PRE-WISCONSINANGRAY GRAY TILL IN THE MANKATO AIZEA MINNESOTA RIVER RIVER VALLEY GRAMSTAD, Sally D., student, Department of Geological Geological and Atmospheric Atmospheric Sciences, Sciences, Iowa Iowa University,Ames, Ames,IA IA5001 50011, sgramstaiastate.edu State University, 1, sgramsta@iastate.edu Exposures of pre-Wisconsinan gray till in the Mankato area of the the Minnesota Minnesota River River Valley Valley show that that continuous. Low shale content, moderate moderate carbonate carbonate this unit is up to 30 m thick and is somewhat continuous. content, and NW-SE trending fabrics fabrics indicate indicate aaWinnipeg Winnipegprovenance. provenance. The The texture texture and and lithology lithology of of this unit are similar to those of other pre-Wisconsinan tills in the Midwest, including the Browerville Browewille and Elmdale tills in north-central north-central and central Minnesota, and unit 1 at Salisbury Hill, which is twenty miles northeast of the study study area. The topography topography of of the bedrock bedrock valley valley that is is present present in in the the Mankato Mankato area area may may have have affected affected ice flow of the glacier that deposited the pre-Wisconsinan gray till unit. Influence Influenceof of this thisbedrock bedrock that the the thick thick till till unit is interrupted valley may explain several several anomalous anomalous fabrics, as well as the fact that by sand and gravel gravel layers layers at at some some sites sites and and is is completely completelyundisturbed undisturbedat at others. others. 68 �A A MINERALOGRAPHIC MINERALOGRAPHICSTUDY STUDYOF OFMAGNETITE MAGNETITEIN IN THE THE BIWABIK BIWABIK IRON-FORMATION, IRON-FORMATION,MESABI MESABI RANGE.M[NNESOTA MrNNESOTA RANGE, T. T.M. M.Han, Han,Senior SeniorResearch ResearchScientist Scientist(Retired), (Retired),Cleveland-Cliffs' Cleveland-Cliffs'Research ResearchLaboratory Laboratory A A substantial substantial number number of ofpolished polishedsections sectionsprepared preparedfrom from specimens specimenscollected collected from from the the Biwabik BiwabikIronIronFormation Formation were were microscopically microscopically examined examined before before and after an induced oxidation procedure. The TheBiwabik BiwabikIronIronFormation Formation is is simply simply classified classified here as cherty and slaty members. ItIt isisconsidered considered aa regionally regionally metamorphosed metamorphosed sediment sediment before before the the the the intrusion intrusion of of the the Duluth Duluth complex. complex. Based Basedon onthe the nature nature and and magnitude magnitude of of the thechanges, changes, the the existing existing iron-formation iron-formationmay may be be divided dividedinto intothree three metamorphosed metamorphosed sectors sectors progressing from west to to east, east, i.e. i.e. hematite functioned functioned as aa regionally, thermally, thermally, and andpyrometasomatically. pyrometasomatically. During During regional regional metamorphism, metamorphism, hematite regionally, "starting much of of the the magnetite magnetitethrough through~Fe Thermal "starting point" point" for for the development development of much e +diffusion. difhsion. + Thermalmetamorphism metamorphism involves involves mineralogical mineralogical transformations transformations of an isochemical isochemical nature, which may be be subdivided subdivided into into transitional, transitional, process involved amphibole, amphibole, and and pyroxene pyroxene zones. zones. Pyrometasomatism Pyrometasomatism process involved advanced advanced recrystallization recrystallization and and replacement replacementby bygabbro gabbroadjacent adjacentto, to,and andwithin, within,the theintrusion. intrusion. This This poster poster focuses focuses on on the the genesis genesisof ofmagnetite magnetiteinin the theregionally regionallymetamorphosed metamorphosed iron-formation iron-formation and and the metamorphismand andpyrometasomatism. pyrometasomatism. Most Most of the the textural textural and and mineralogical mineralogical changes caused by thermal metamorphism features features observed observed during during the the course course of of study study are are photographically and photomicrographically shown. ItIt covers: covers: A. fiomdifferent differentmetamorphic metamorphiczones. zones. A. Lithological Lithologicalcharacteristics characteristicsfrom The The irregularly irregularlyand andevenly evenlybanded bandedmacrostructures macrostructuresofofthe theregionally regionallymetamorphosed metamorphosediron-formation iron-formation show show little little change, change, whereas whereasthe themineralogical mineralogicalcomposition composition and and microstructures microstructures have have experienced experienced significant significant progressive progressive elimination eliminationand andtransformation transformationdue duetotothe thethermal thermalmetamorphism metamorphismand andpyrometasomatism. pyrometasomatism. B. B. Hematite-Magnetite Hematite-MagnetiteRelations Relationsininthe theRegional RegionalMetamorphosed MetamorphosedIron-Formation Iron-Formation occur either Before Induced Induced Oxidation. Oxidation. The hematite hematite and magnetite magnetite occur either in separate separate layers layers or or inin 1. Before 1. intimate intimate relations in in the same same layer. layer. Hematite Hematiteisisalmost almostalways alwaysmuch muchfmer fmerand andless lessabundant abundantthan thanmagnetite. magnetite. The The replacement of of hematite by magnetite is readily evident. The The magnetite magnetite differs differs in in external external morphology, morphology, i.e. i.e. octahedral octahedral and and pseudomorphs pseudbmorphs after afterother otherminerals. minerals. After Induced Induced Oxidation. Oxidation. The Themorphologies morphologiesand andarrangement arrangement of of precursor precursor hematite hematite in in existing existing magnetite magnetite 2. 2. After significantly. In the the evenly evenly banded banded slaty slaty lithology, lithology, the the precursor precursor hematite hematite isis almost almost exclusively exclusively lathlathvary significantly. shaped shaped and exhibits exhibits aa decussate decussate microstructure. In Inthe the irregularly irregularlybanded banded lithology lithology of both the slaty and cherty cherty members, the the precursor precursor hematite hematite exhibits exhibits aawide widevariety varietyofofmorphologies. morphologies. It is is either either randomly randomly arranged arranged or or displays displays vuggy vuggy or or microgeodic microgeodic structures structures within within magnetite magnetite that isis present present as asirregular, irregular,coalesced coalescedcrystals, crystals, granules, or or pinch-and-swell pinch-and-swell layers. layers. The The external external morphology morphology of the present magnetite magnetite is governed governed by by the the granules, hematite. Some Some of the the precursor precursor hematite hematite might might have have been been enlarged enlarged by by authigenesis authigenesis before before precursor hematite. inclusions isis present present in nearly all magnetite development. development. Magnetite Magnetite with precursor precursor hematite hematite inclusions all the the mineral mineral magnetite assemblages fiom assemblages of of the the iron-formation. iron-formation. The Thesize sizeand andmorphologies morphologiesof ofprecursor precursorhematite hematitein inmagnetite magnetite differs differsfrom the the existing existing hematite, hematiteyeither either coexisting coexisting with or or playing host hostto tothe themagnetite. magnetite. C. C. Effect EffectofofThermal ThermalMetamorphism Metamorphismon onHematite Hematiteand andMagnetite Magnetite In In the the transitional transitional zone, zoneymore more than than one one stage stage of of overgrowth overgrowth on euhedral magnetite (with or without the the precursor hematite inclusions) inclusions) is frequently frequently observed. Two Twoto tothree threeexternal externalmorphologies morphologiesof ofmagnetite magnetitewere were also seen in in coexistence. coexistence. The Theinduced inducedoxidation oxidationpattern patternprogressively progressively changes changes from from grain grain boundary boundary oxidation oxidation along planes of of the existing along the the precursor precursor hematite hematite to cleavage cleavage oxidation oxidation along octahedral octahedral planes existing magnetite magnetite as the metamorphic grade grade increases. increases. The Thehematite hematiteofofthe theregionally regionallymetamorphosed metamorphosediron-formation iron-formationisisprogressively progressively replaced by magnetite. magnetite. The Theeuhedral euhedralmagnetite magnetiteisisprogressively progressively changed changed to subangular subangular to nearly round grains. However, the outlines granules are still preserved. Magnetite outlines of of the the magnetite magnetite pseudomorphs pseudomorphs and magnetite granules Magnetite porphyroblasts appear in the fine-grained coalesced subangular magnetite. fine-grained magnetite. Some Some siliceous siliceous magnetite magnetite blades blades or or 69 �cubic cavities are are occasionally occasionally observed observed in in magnetite magnetite distributed distributed in the the pyroxene pyroxene zone. zone. However, However, some some hematite coexisting coexisting with with magnetite magnetitehas has also alsobeen been found foundinin the the pyroxene pyroxenezone. zone. D. Effect Effectof ofPyrometasomatism Pyrometasomatismon on Hematite Hematite and Magnetite Advanced Recrystallized Recrystallized Zone. The Themagnetite magnetite here here is is substantially substantially coarser than 1. Advanced than in the regionally and thermally metamorphosed metamorphosed zones. zones. The hematite, hematite, magnetite magnetite pseudomorphs, and magnetite magnetite granules granules no no longer exist. Two Twogenerations generationsof ofmagnetite magnetiteand andthe theintroduction introductionof ofhercyanite hercyanitealong alongmagnetite magnetitecleavage cleavageplanes planes fmeparallel parallelto tosubparallel subparallelfractures, fractures,whereas whereas the the host hostdoes does are evident. The Themagnetite magnetitegrains grainscommonly commonlyhave havefme not. The Thefractured fracturedmagnetite magnetitesometimes sometimesresembles resembles specular specular hematite. Minute Minuteeuhedral euhedral gangue gangue inclusions inclusions are are commonly present. Cleavage Cleavageoxidation oxidationisistypical. typical. Magnetite Magnetiteinclusions inclusionswithin withinmagnetite magnetiteare areeither eitheroutlined outlinedby by the effect of differential differential oxidation oxidation or or by their their grain grain boundaries. boundaries. The advanced Gabbroized Zone. 2. Gabbroized Zone. The advanced recrystallized recrystallized magnetite magnetite gradationally gradationally transforms transforms to to a 2. titanomagnetite approaching the gabbroized zone. The The latter latter contains contains minute inclusions of hercyanite, ilmenite, and ulvospinel, mineral. Most phases are are products products of of exsolution. exsolution. Both ulvospinel, and an unidentified unidentified mineral. Most of these mineral phases Both magnetite and titanomagnetite may be observed observed in in adjacent adjacent layers layers of of the thesame samespecimen. specimen. Small octahedral octahedral the same composition and texture as inclusions of titanomagnetite are occasionally seen and exhibit essentially the as titanomagnetite contains contains apatite, apatite, pyroxene, pyroxene, and and other other gangue grains; some titanomagnetite host. host. Some titanomagnetite some their titanomagnetite exhibits aa myrmekitic intergrowthtexture textureinin pyroxene; pyroxene;and and some some appears appearstoto be be replaced replaced by by ilmenite and exhibits myrmekitic intergrowth ilmenite and sulfides. sulfides. Economically, regional regional metamorphism metamorphism isis the the key key contributor in in generating generating magnetite magnetite for for the present Economically, Thermal metamorphism metamorphismhas has aa negative negative impact impact on on magnetite magnetite size size liberation liberation but but a positive mining industry. industry. Thermal positive recovery. Advanced effect on magnetite weight recovery. Advanced recrystallization recrystallization has a positive effect on both both size size liberation liberation ore. and weight recovery, whereas whereas the gabbroized gabbroized iron-formation can no longer be considered iron ore, The details details relative relative to to this this study study were were reported reported in in the the following followingpapers papersand andabstracts. abstracts. T. M. Han (1978) "Microstructures "Microstructures of of Magnetite Magnetite as as Guides Guides to Its Origin in Some Precambrian IronFormations." Fortschr. Fortschr. Mineral. Mineral.56, 56,105-142 105-142 (1982) "Iron-Formation of of Precambrian PrecambrianAge: Age: Hematite-Magnetite Hematite-MagnetiteRelationships RelationshipsininSome SomeProterozoic Proterozoic (1 982) "Iron-Formation MicroscopicObservation." Observation."Ore Ore Genesis - State of the Art, Springer-Verlag, Deposits -- A Microscopic Genesis - The The Springer-Verlag, Berlin, Berlin, Iron Deposits Heidelberg, New York. (1988) "Origin of of Magnetite Magnetite in Precambrian Iron-Formations of Low Metamorphic Grade." Proceedings of Dof the Seventh Seventh Quadrennial Quadrennial IAGOD IAGOD Symposium, Symposium, E Schweizerbart'sche Schweizerbart'sche Verlagsbuchhandlung, Verlagsbuchhandlung,D7000 Stuttgart Stuttgart II (1996) "Mineralogical Evolution Evolution of of Precambrian Precambrian Iron-Formation of Low Metamorphic Grade and Its Contribution to the U.S. Steel Steel Industry" (Abstract), (Abstract), 30th IGC, Beijing, China G . Graber, and Ruth Kramer (1992) "Effect of Duluth Gabbro Intrusion on Ore Mineralogy of T. M. Han, R. G. the Biwabik Iron-Formation, Iron-Formation, Lake Superior Superior District," District," USA International International Symposium on Mineralization Related to Mafic and Ultramafic Rocks, Rocks, 8th 8th Quadrennial Quadrennial IAGOD Symposium, Symposium, Orleans, France (Abstract) 70 �FACIES DEPOSITIONAL ENVIRONMENTS ENVIRONMENTSOF OFTHE THEEEARLY PROTEROZOIC FACIES AND DEPOSITIONAL A m Y PROTEROZOIC IRONWOOD IRONWOOD IRON FORMATION, MT. WHITTLESEY WISCONSIN. *EJ Joslin, Dan *EMHensel, Hensely*ck *Rick J o s hand , and DaLehrmann; n LehrmaqDept. Dept.ofofGeology, Geoloa,University Universityof of WisconsinWisconsinOshkosh, .54901 Kl54901 Oshkosh7Oshkosh, Oshkosh, Wi There are numerous models of depositional depositional environments environments of ofEarly EarlyProterozoic Proterozoiciiron fonnation. The k n formation. The puipose of the study was to evaluate which which of of these these models modelsbest bestfit fitthe thesedimentary sedimentaiyfacies faciesand and stratigraphic stratigraphic purpose patterns in the the Ironwood Ironwood iron iron formation formation exposed e x p o d at the Whittlesey, in the patterns in the Berkeshire Mine Ruins, at Mt. Whitfiesey, Gogebic were stripped stripped from from this this area by the the abandoned Gogebic District, near Mellen Wisconsin. Soil and vegetation were mining miningeffort. &ort. Approximately Approximately 100 100 meters meters of of stratigraphic stratigraphicsection sectionwas was described describedbed bed by by bed bed at at aa decimeter decimeter scale. bed in reconnaissance cut and and d e . An An additional additional150 150 meters was descr described reconnaissance sections. Samples were collected, cut described described in in detail detail to to help help understand understanddepositional depositionalenvironments. environments. fades were recognized from outcrop observation observation and and sample sample descriptions. descriptions. These These are are Five different facies summarized facies consists s u m a r i d as as follows. 1) 1) The The horizontal horizontallaminated hmin'atedfiia consistsof ofcontinuous continuousand anddiscontinuous discontinuouslaminae laminae of alternating alternating inagnetite structures include includethin, thin, graded graded laminae, laminae, scour scour marks, marks, vmagnetite and and chert. chert. Sedimentary structures shaped ?), and and discontinuous shaped cracks cracksthat that probably probably resulted resulted from from differential Merentid compaction compaction(j)ossibly (possibly desiccation ?), discontinuous lenses thick and commonly pinch out within a few lenses of granular granularfacies. The lenses lenses range range from fiom a few to 15 cm thick meters laterally. 2) The wavy wavy hminatedfacies consists of wavy wavy laminae laminaeof of magnetite magnetiteand and chert chertwith with few few 2) The laminated facies consists intraclasts of cheit are horizontal horizontal intraclasts chert.Sedimentary Sedimentarystructures structuresinclude includecrinidey crinkleylaminations laminationsthat thatprobably probably are stromatolites, convolute convolutelaminae, laminae,and andlenticular lenticularlayers. layers.3)3)The Theinfracladcfacies intraclasticfacies consists consistsof of intraclasts of of stromatolites, magnetite magnetiteand andchert chertderived derivedfrom fromthe thewavy wavyand andgranular granularfacies. facies.The Theclasts clastswere werecemented cementedearly earlyby bysilica. silica. include, planar cross beds, beds, imbricated clasts, clasts, differential differential syn-sedimentary syn-sedimentaiy compaction, compaction, Sedimentary structures include7 autoclastic penetrating the margins autoclastic breccia, breccia, and abundant abundant v-shaped cracks penetrating margins of of intraclasts. intraclasts. These Thesemay may be be desiccation of the the intraclasts intraclasts are are rounded roundedand andare areconcentrically concentricallycoated coatedwith withchert. cheit 4) 4) The The desiccation cracks. Some of stromatolitefacies contains ~omatolifefacies containsdomal domalstromatolites stromatolitescomposed composed of of chert chert and and magnetite. magnetite. Intraclastic Intraclastic material, containing occursbetween betweenthe the containingstromatolite stromatolite fragments hgments and and clasts clasts of of granular granular fades, facies*isisinterbedded interbeddedwith withand andoccurs stromatolites. 5) facies consists 5)The Thegranular granuhrfacies consistsof ofcoarse coarse to fine fine sand and minor silt sized chert and magnetite grains. ooids. The The granular granular facies grains. Most Mostofofthese thesegrains grainsare arepeloids peloids(featureless (featureless rounded grains), some are mids. appears appears to to be winnowed with little fine grained matrix and locally contains ripple ripple cross-lamination. cross-lamination. The stratigraphic pattern at Mt. Whittlesey is a meter- to decimeter-scale intercalation intercalation The most obvious stratigraphic of the wavy, granular, granular, and and intraclastic intraclasticfacies facies (fig. (fig. 1). 1). Careful Carefid observation obsemtion reveals, however, that that there there is is also a larger-scale are lithologic cycles. These cycles are larger-scale change change in in facies facies associations associations in the the outcrop outcrop that defines 3 lithologic approximately50 m thick. thick Within Within the the cycles cycles there there is a preferential association association in which which the the following following facies facies approximately more frequently granular and and wavy laminated facies, 2) the intraclastic intraclastic and and stromatolitic stromatolitic frequently occur together: 1) the granular three facies associations 3) the the horizontal horizontal laminated laminatedfacies. facies. The The cycles cycles consist of a repetition of the three facies, and and 3) stmmatolitic facies facies above. Interestingly, the cycles are are asymmetric in in the sense that that the intraclastic and stromatolitic associationalways always occurs beneath the the horizontally horizontally laminated laminatedfacies faciesand andnot notabove aboveitit(fig. (fig.2). 2). association Our interpretation that in aa shallow-marine, intertidal Our data data supports supports the interpretation that the the iron formation was deposited in to subtidal subtidal shelf environment. This Thisisissimilar similarto tothe theenvironments environments that that have have been been most most widely interpreted for iron Lougheed, 1983; M LaBerge, (cf. Simonson, 1985; hugheed, e r g e , 1987; Simonson Simonson and and Hassler, Hassler, 1996). 1996). iron formations formations(CL Sedimentary fatures interpretation include the stromatolites, ooids, mi&, ripple ripple cross lamination, lamination, features that that support this interpretation and and cross cross bedding. We interpret interpret that that the theiron ironformation formationformed formed as as aa biochemical biochemical precipitate precipitateof of iron iron and and silica silica minerals marine carbonate supports the minerals from from sea-water. sea-water. The The similarity similarityto to Proterozoic Proterozoic or Early Paleozoic marine carbonate facies supports idea to support the interpretation interpretation that are biochemical biochemical rocks. We found no evidence to that these these were idea that that these these are originally carbonate carbonate sediments sediments that that were were diagenetically diagenetically replaced by h w a t e r deposition that has originally by silica or for hfreshwater been postulated by 1973). by other other authors authors (Hough, (Hough, 1958; 1958;Eugster Eugsterand andChou; Chou;and and Trendall, 1973). In our laminated facies facies formed formed on on the the tidal tidal flats, the granular, granular, our preferred p r e f e d depositional model, the wavy laminated intraclastic, further seaward in shallow, wave agitated subtidal intraclastic, stromatolite, stromatolite7facies facies formed formed progressively progressively fiuther environments environmentsand and the the horizontally horizontallylaminated laminatedfacies faciesformed formed in in deeper deeper environments environmentsbetween nonnal normalwave wavebase base and and storm storm wave wave base base (fig. 3). The The horizontal horizontallaminations laminationsare are interpreted interpretedto tohave haveformed formedin inquiet quietwaters watersbelow below granular facies found found in in the the The abundant abundantscour scour structures structuresand discontinuous lenses of granular normal wave base. The horizontally laminated horizontaIly laminated facies are interpreted interpreted to to represent represent agitation agitation and and "spill "spill over over lobes" lobesnof of granular granularmaterial material 71 �___ Originally we entertained the idea that the horizontally laminated washed seaward during storms (fig. 3). 3). Originally facies formed formed landward landward of of the the oolite oolite shoals shoals in in tidal tidal flat flat ponds. ponds. If If the the v-shaped v-shaped cracks crackswere were desiccation desiccationfeatures, features, facies that that would support support such such an an interpretation. interpretation. The Thev-shaped v-shaped cracks cracks are more commonly associated with syndepositional depositional deformation deformationstructures structureshowever, however, and and the the horizontally horizontallylaminated laminatedfacies faciesisisrarely rarelyassociated associatedwith with laminatedfacies. facies. This, This, along alongwith withthe theasymmetrical asymmetricalpattern pattern of of the thelarge-scale large-scaledepositional depositionalcycles cycles the wavy laminated facies associations associations would represent supports the deeper deeper offshore offshore environment. In this this model, model, the the cycles in facies represent long long term (—million (--million year) intraclastic term year)asymmetric asynunetrictransgressions transgressions and and regressions regressions of ofthe the shelf. shelf. The The stromatolites stromatolites and intraclastic facies formed preferentially during initial transgressions, due to increased wave energy energy and and increasing increasing water Maximum deepening deepening is is represented represented by the horizontally laminated facies and regression is represented depths. Maximum by the shift facies association. association. shift back to to the the wavy laminated laminated - granular facies studies have commonly interpreted the horizontally laminated laminated facies Previous studies facies as as being being aa deep deep water water In some some cases cases itit has has been been interpreted interpretedto tobe be aa vezy very deep, pelagic and turbiditic Our data datasuggests suggests deposit. In turbiditic facies. Our that, at units interbedded interbedded in in the the horizontal horizontal facies are are at Mt. ML Whittlesey Whittlesey at least, this is not the case. The granular units far too discontinuous discontinuousto to represent represent distal distalturbidites turbiditesand andthey they lack lack Bouma Bouma sequences. sequences. - Eugster, H. P., Chou, Chou, I.M., I.M., 1973, 1973,The The depositional depositional environments environments Precambrian banded iron iron formations: formations: Economic Geology and the Bulletin Bulletin of the Society for for Economic Economic Geology, Geology,v. v. 68, 68, n. n. 7, 7, p. p. 1144-1168. 1144-1168. Economic Hough, J. L., L., 1958, of Precambrian Precambrian banded banded iron iron formation: formation: Journal Journal of of 1958, Fresh Freshwater waterenvironment environmentof of deposition of Sedimentary Sedimentary Petrology, v. 28, n. 4, p. p. 414-430. 414-430. LaBerge, G. L., L., Robbins, E. E. I. Schmidt, R K 0., G.,1987, 1987,AAmodel model for for the the biological precipitation of Precambrian iron Pubi., Athens, p. 69-96. iron formations: formations: Precambrian Precambrian Iron Iron Formations, Formations, Theophrastus Theophrastus Publ., 1983, Origin Origin of of Precambrian Precambrian iron iron formations formations in the lake superior superior region: region: Geological Geological Society Lougheed, M. S., 1983, of America Bulletin, Bulletin, v. 94, 94, ii. n. 3, 3, p. 325-340. 325-340. M., 1985, the origins of of Precambrian Precambrian iron formations: Simonson, B. M, 1985, Sedimentological Sedimentological constraints on the Geological Geological Society of America Bulletin, v. 96, n. 2, p. 244-252. Simonson, B. M M. and Hassler, S. W., of large large Precambrian Precambrian iron formations linked to W., 1996, 1996, Was the deposition of major marine marinetransgressions?: transgressions?:Journal Journalof ofGeology, Geology, v. 104, 104,n. n. 6, 6, p. p. 665-676. 665-676. Trendall, A. F.,1973, 1973,Iron Ironformation formationof ofthe theHamersley HamersleyGroup Groupof of Western Western Australia; Australia; type typeexamples examplesof ofvarved varved Trendall, A. F., Earth arth Science Science (Paris), v. 9, p. 257-270. 257-270.evaporites: E Fades Fades TO- ,"- nfradasc Fades Associations 1 HorizontallyE HorizontallyLaminated Intraclastic I Intraclastic Stromatolitic Strornatolitic Wavy Laminated /I Granular 3 I1 urn FIom Figure Figure11 ."-- Figure 2 Fades Wavy Iamânated Wavy laminated Granu [S Intradastic t.nbad&c a HorizontallyLaminated Figure Figure 3: Depositional Depositional Model Model 72 �AGE DEFORMATION OF EARLY PROTEROZOIC PROTEROZOIC QUARTZITES AGE AND DEFORMATION QUARTZITESIN IN THE THE SOUTHERN SOUTHERN LAKE SUPERIOR SUPERIOR REGION: REGION: IMPLICATIONS IMPLICATIONSFOR FOR EXTENT EXTENTOF OF FORELAND FORELAND DEFORMATION DEFORMATION DURING DURING FINAL FINAL ASSEMBLY OF LAURENTIA Daniel Daniel Hoim, Holm,Department DepartmentofofGeology, Geology,Kent KentState StateUniversity, University,Kent, Kent,OH OH44242 44242 David David Schneider, Schneider,Dept. Dept.of ofEarth Earth and and Environmental Sciences, Sciences, Lehigh Lehigh University, University, Bethlehem, PA PA Christopher ChristopherD.D.Coath, Coath,Department DepartmentofofEarth Earthand andSpace SpaceSciences, Sciences,UCLA, UCLA,Los LosAngeles, Angeles,CA CA90095 90095 Post-accretion stabilization stabilization in the Lake Superior region at 1770-1760 1770-1760 Ma resulted in deposition of locally thick successions successions of Early Early Proterozoic Proterozoic mature mature quartzites quartzites in Wisconsin Wisconsin and and southern southern Minnesota. Minnesota. Their Their age age of of deposition deposition and and the theage ageof of the thedeformation deformationwhich which caused causedwidespread widespreadfolding foldingof ofmany manyof ofthe the quartzite units has long been aa matter importance and and controversy. controversy. We matter of of considerable considerable importance We present present new new evidence these quartzites. quartzites. Also, we document evidence for both the maximum and minimum age of these document the spatial coexistence of aa thermal with a deformational front in the thermal front front in in Precambrian Precambrian crystalline crystalline basement basement with deformational front overlying quartzite units. The Theage ageofofthe thefront frontsuggests suggeststhat thatpost-Penokean post-Penokeanshortening shortening of of the thePenokean Penokean province province is is likely likely related related to to the the final final stages stages of formation formation of the Laurentian Laurentian supercontinent supercontinent at at -1650 Ma. 1650 Ma. The The bulk bulk of Laurentia Laurentia formed formed by rapid aggregation aggregation of Archean continents at 1900-1800 1900-1800 Ma during the Trans-Hudson and Penokean orogenies (Hoffman, 1989). Subsequent Subsequent accretion accretion of of juvenile crust crust along along the the southern southern margin margin of of pre-1800 pre-1800Ma Ma Laurentia Laurentiaformed formedthe theTranscontinental TranscontinentalProterozoic Proterozoicprovinces provinceswhich which consist broadly of aa northern northern 1800-1700 1800-1700 Ma inner accretionary belt and a southern 1700-1600 1700-1600 Ma outer tectonic belt. The Thetransition transition zone zone between between the the two two tectonic tectonic belts belts of the the TPP TPP represents represents the region of known Ma during during formation formation of of the outer known pre-1700 pre-1700 Ma Ma rocks rocks metamorphosed metamorphosed and and deformed deformed at at —1650 -1650 Ma tectonic belt. The Theeastward eastwardcontinuation continuation of of this this transition transition zone zone and and of the the outer outer tectonic tectonic belt from the central central plains plains is is problematic problematicas as no no 1800-1600 1800-1600Ma Majuvenile juvenile crust crustexists existsin inthe theentire entiresouthern southernGreat GreatLakes Lakes region region (Van (Van Schmus Schmus et et al., al., 1993). 1993).However, However,on onthe thebasis basisof of1680-1640 1680-1640Ma Maorogenic orogenicdeformation deformationand andthe the existence of 1650 1650 Ma Ma batholithic batholithic intrusive rocks in Labrador, Labrador, Van Schmus et al. al. (1993) (1993) have have proposed proposed that that the the outer outer tectonic tectonic belt belt was was aa single single coherent coherent continental arc extending extending from California California to Labrador. Labrador. The arc/microcontinentcollision collisionalong alongthe the southern southernpassive passivemargin margin The Penokean Penokean orogeny orogeny involved involvedisland island arc/microcontinent of of the the Superior Superior Province Province at at 1870-1830 1870-1830Ma. Ma. InInnorthern northernWisconsin Wisconsinthe thesouth-dipping south-dippingNiagara Niagarafault faultzone zone represents represents the the main main suture suturewhich whichseparates separatesuniformly uniformlymetamorphosed metamorphosed (upper (uppergreenschistllower greenschist/lower amphibolite facies) island arc arc rocks rocks of of the thePembine-Wausau Pembine-Wausauterrane terrane from fromdeformed deformedcontinental continentalmargin margin rocks rocks which whichexhibit exhibitaanodal nodalmetamorphic metamorphicpattern patternimposed imposedduring duringcollapse collapseof ofthe theorogen orogen(Schneider (SchneideretetaL, al., 1996; 1996; Marshak Marshak et et al., al., 1997). 1997). Abundant Abundant mica mica Ar/Ar ArIAr cooling cooling dates dates from from central central Minnesota Minnesota and and northernmost northernmost Wisconsin Wisconsin and and Michigan Michigan indicate indicate that that collapse collapseand andorogenic orogenicunroofing unroofingoccurred occurredatat175017501700 1700 Ma shortly shortly after after an anepisode episodeof ofwidespread widespreadmagmatism magmatismatat1770-1760 1770-1760Ma Ma(Holm (Holmand andLux, Lux,1996; 1996; Schneider Schneider et et al., al., 1996). 1996).InIncontrast, contrast,Rb-Sr Rb-Srwhole wholerock rockisocbron isochronand andbiotite biotitemineral mineraldates datesininnorthern northernand and central central Wisconsin Wisconsin (Peterman (Petermanand and Sims, Sims,1988) 1988)are are mostly mostly Middle Middle Proterozoic Proterozoic (1600-1100 (1600-1100Ma) Ma) and and reflect reflect variable variable thermal thermal resetting resettingassociated associatedwith withaalow-grade low-grade—1630 -1630 Ma metamorphic event (Van Scbmus Schmus and and Woolsey, Woolsey, 1975; Van Van Schmus Schmus et al., al., 1975), 1975), intrusion intrusion of the the 1470 1470 Ma Ma Wolf Wolf River River batholith, batholith, and and Keweenawan Keweenawan activity. activity. Post-accretion Post-accretion rapid rapid stabilization stabilization resulted resulted in in the the accumulation accumulation of of post-tectonic post-tectonic quartz quartz arenites arenites in in the the southern southern Lake Lake Superior Superior region region (Van (Van Schmus Schmus et al., 1993). 1993). Deformed Deformedquartzite quartziteunits units in in central central and and southern southernWisconsin Wisconsin yield yield post-Penokean post-Penokean detrital detrital zircon Pb-Pb ages (Van (Van Wyck, 1995). 1995). The The folded folded Baraboo Baraboo quartzite quartziteis is depositional depositionalon on aa 1752 1752Ma Ma granite granite(Medaris (Medariset et a!., al., 1996) 1996)and and contains contains detrital detrital zircons zircons as as young young as as 1712 1712Ma Ma (Dott (Dott et et a!., al., 1997). 1997). The Theminimum minimumage ageof ofthese thesequartzite quartziteunits unitsisisconstrained constrained only only by by the the fact factthat thatsome someare areintruded intrudedby bythe the1470 1470Ma MaWolf WolfRiver Riverbatholith, batholith,although althoughDott Dott(1983) (1983)and and Van Van Schmus Schmuset et al. al. (1993) (1993)speculated speculatedthat that they they may may have have been been deformed deformed during during the the 1630 1630Ma Ma event. event. The The predominantly predominantly Middle Middle Proterozoic Proterozoic mica mica mineral mineral dates of central central Wisconsin contrast sharply with the the well-grouped well-grouped 1750-1700 1750-1700Ma Ma mica mica dates dates obtained obtainedfrom from central centralMinnesota, Minnesota,northernmost northernmost Wisconsin, Wisconsin, and and northern northern Michigan. Michigan. The The1750-1700 1750-1700Ma Madates datesare arethe theoldest oldestmica micadates datesobtained obtainedfrom fromthe theinternal internal portions portions of of the thePenokean Penokeanorogen orogenand andthus thusalmost almostcertainly certainlyreflect reflect primary primary cooling cooling through through mica mica closure closure temperatures temperatures following following Penokean Penokean metamorphism. metamorphism. Considering Considering that that primary primary cooling cooling atat1750-1700 1750-1700Ma Ma was An—1630 -1630 Ma Maage age was orogen orogen wide, wide, ititisislikely likelythat thatthe theyounger youngermica micadates datesrepresent representthermal thermalresetting. resetting.An contour contour thus thus separates separates basement basement with with typical typical post-Penokean post-Penokean cooling cooling ages to the the north north (and (and west, west, in in Minnesota) Minnesota)from frombasement basement with with thermally thermally reset reset ages ages to the south. The Theeastward eastwardextent extentof ofthe thechrontour chrontourisis not not precisely preciselylocated, located,however, however,north northof ofthe theFlambeau Flambeauquartzite quartziteininnorthwest northwestWisconsin Wisconsinthe thechrontour chrontourisis sharply sharplydefined definedby byRb-Sr, Rb-Sr,K-Ar, K-Ar,and andAr-Ar Ar-Armica micadates. dates. Importantly, Importantly,the theFlambeau Flambeauthermal thermalfront frontin innorthwest northwestWisconsin Wisconsincoincides coincidesspatially spatiallywith withan anapparent apparent deformational deformational front front ininoverlying overlyingpost-Penokean post-Penokeanquartzites. quartzites. In In Minnesota, Minnesota, the thesubhorizontal subhorizontal Sioux Sioux 73 �quartzite quartzitelies liesjust justsouthwest southwestofofPenokean Penokeaninternal internalzone zonerocks rockswhich whichcooled cooledrapidly rapidlythrough throughmica micaclosure closure temperatures Ma. In temperaturesatat—1750 -1750 Ma. Innorthwest northwest Wisconsin, Wisconsin,the theBarron Ban-onquartzite quartziteisisessentially essentiallyflat-lying flat-lyingand andisis depositional depositional on on Precambrian Precambrian basement basement with mica ages between 1730-1700 1730-1700 Ma. South Southof ofthe thethermal thermal front, front,and andonly only25 25km kmsouth southofofthe theBarron Barronquartzite, quartzite,are areexposures exposuresofofsteeply-dipping steeply-dippingFlambeau Flambeauquartzite. quartzite. Here Herethe thequartzite quartziteisisfolded foldedinto intoaamoderately moderatelywest-plunging west-plungingsynform synform(Myers, (Myers,1974) 1974)similar similarto to the thestyle styleof of deformation McCaslin quartzites. quartzites. The Therelatively relativelyundeformed undeformedBarron Barronand and deformationexhibited exhibitedby by the theBaraboo Barabooand andMcCaslin Sioux SiouxProterozoic Proterozoicquartzites quartzitesmust musteither eitherlie lieoutside outsidethe theregion regionofofsignificant significantpost-Penokean post-Penokeandeformation deformation (as (asseems seemssuggested suggested by by the thespatial spatialcoincidence coincidence of of aathermal thermalfront frontwith withaadeformational deformationalfront) front)ororbebe younger younger quartzite quartzitepackages packagesdeposited depositedafter aftersignificant significantpost-Penokean post-Penokean deformation. deformation. To Totest testwhether whetherthe the subhorizontal subhorizontalquartzites quartzitesare arecorrelatable correlatablewith withthe thedeformed deformedquartzites quartziteswe weobtained obtainedsingle-grain, single-grain,singlesinglespot ages spot207Pb/206Pb 2 0 7 ~ b / 2 0 6ages ~ b on detrital zircons separated from from the the Sioux, Sioux,Barron, Barron,and andFlambeau Flambeauquartzites. quartzites. Because Becausewe weare aremostly mostlyinterested interestedininconstraining constrainingthe themaximum maximumage ageofofeach eachquartzite quartzitewe weconcentrated concentratedour our efforts possible. All efforts on ondating dating euhedral euhedral or or subhedral subhedral crystals crystals wherever wherever possible. All three three quartzite quartzite bodies bodies yielded yielded Early Early Proterozoic Proterozoic and and Late Late Archean Archean detrital detrital zircon zircon dates dates comparable comparable to to dates dates obtained obtained by by Van Van Wyck Wyck (1995) (<2000Ma) Ma)dates dates (1995)for fordeformed deformedquartzite quartzitebodies bodiesin in Wisconsin. Wisconsin. Many Manyofofthe thelate lateEarly EarlyProterozoic Proterozoic(<2000 fall fallalong alongaadiscordia discordiawith withaalower lowerintercept interceptatator ornear nearthe theorigin originindicating indicatingrecent recent lead lead loss lossattributed attributedto to upper Proterozoic uppercrustal crustalfluid fluidcirculation. circulation.Reliable Reliable207Pb/206Pb 207Pb/2w~b Proterozoicages agesare arebetween between1730 1730and and1850 1850Ma Mafor for the theSioux Sioux(9 (9grains), grains),between between1714 1714and and1880 1880Ma Mafor forthe theFlambeau Flambeau (9 (9 grains), grains),and and between between 1750 1750and and 1880 1880 Ma Mafor forthe theBarron Barron(6 (6grains). grains).These Thesedata dataattest attesttotothe thefact factthat thatall allthree threequartzite quartzitebodies bodiespost-date post-datethe the1760 1760 Ma Mamagmatic magmaticevent eventin inthe theLake LakeSuperior Superiorregion. region. Thus Thusfar farthere thereisisno noevidence evidencetotosuggest suggestthat thatthe theBarron Ban-on and andSioux Siouxquartzites quartzitesare areyounger youngerthan thanthe thedeformed deformedquartzites quartzitesfound foundthroughout throughoutmost mostofofWisconsin. Wisconsin. Abundant Abundant new new thermocbronologic thermochronologic data in the Lake Superior Superior region allow us to make make aa simple simple but but important importantfirst-order first-orderobservation observationthat thatprovides providesthe thefirst firstdirect directstructural structuralevidence evidencethat thatthe thequartzites quartziteswere were deformed deformedduring duringthe thelow-grade low-grade—1630 -1630 Ma event in the Lake Superior region. We Wenote notethat thatsubhorizontal subhorizontal post-Penokean post-Penokean quartzites quartzites consistently consistently overlie overlie crystalline crystalline basement basement with with primary primary post-Penokean post-Penokean cooling cooling ages, ages, whereas whereas highly highly deformed deformed quartzites quartzites everywhere everywhereoverlie overliecrystalline crystallinebasement basementwith withsecondary secondary (thermally (thermally reset) reset) cooling coolingages. ages.We Weare arefortunate fortunatethat thatthe theproximity proximityofofthe thedeformed deformedFlambeau Flambeauquartzite quartzitetoto the the subhorizontal subhorizontal Barron Barron quartzite quartzite in in northwest northwest Wisconsin Wisconsin allows allows us to to precisely precisely locate locatethis this deformational/thermal deformationalhhermal front. front. An Anage ageofof1650-1630 1650-1630Ma Mafor forthe thedeformation deformationseems seemsreasonable reasonablegiven giventhat that coolingages agessouth southof ofthe thefront frontprobably probablypost-date post-datethe thedeformation deformationsomewhat. somewhat. cooling The The thermal thermal front front extends extends eastward eastward into into northern northern Michigan, Michigan, the the geology geology of of which which isisdominated dominated by by gneiss gneissdomes domesand and classic classic nodal nodal metamorphic metamorphic isograds. No NoEarly EarlyProterozoic Proterozoicpost-Penokean post-Penokeanquartzites quartzitesare are known known north north of of the the McCaslin McCaslin quartzite. quartzite.However, However,it's it'sinteresting interestingtotonote notethat thatthe theRepublic Republicmetamorphic metamorphic node node located locatednorth north of of the thethermal thermalfront frontisisconcentric concentricwhereas whereasthe thePeavy Peavymetamorphic metamorphicnode nodelocated locatedsouth south of the the thermal thermal front frontisiselongate elongateeast-west. east-west. ItIt isis tempting tempting to to speculate speculate that that the the Penokean Penokean isograds isograds of the the of Peavy Peavy metamorphic metamorphic node node have have been been shortened shortenednorth-south north-south and and may may therefore therefore be be yet yetanother anotherstructural structural manifestation of of the the1630 1630Ma Madeformational deformationalevent. event. If If our our interpretations interpretations are are correct, correct, then thenthe thesimilar similar manifestation orientation orientation of of post-Penokean post-Penokean folds folds and and Penokean-age Penokean-age folds (north (north of the the front) front) requires requires that that caution caution be be used usedwhen whenattributing attributingbasement basementstructures structuressouth southofofthe thefront fronttotoPenokean Penokeandeformation. deformation. The Ma metamorphism metamorphism has has been one of the most Thelow-grade low-grade—1630 -1630 Ma most poorly poorly understood understood events events in inthe the Lake Lake Superior Superior region. region. Although Althoughitithas haslong longbeen beenspeculated speculatedupon uponthat thatthe thequartzites quartzitesmay mayhave havebeen been deformed deformed during during this this event event (Dott, (Dott, 1983; 1983; Van Van Schmus Schmus et al., al., 1993), 1993), until now there has been no no direct direct evidence evidence of of any any intrusive intrusive or or deformational deformational event of that age. We Webelieve believethat thatthe thedata datasummarized summarizedhere here provides provides the the missing missing structural structural link link to to that that event. event. The Thetiming timingand andthe thestrong strongapproximately approximatelynorth-south north-south shortening shorteningstyle styleof of post-Penokean post-Penokeandeformation deformationtogether together are are consistent consistentwith with itit being being the theresult resultof offoreland foreland deformation deformation associated associated with emplacement emplacement of the outer outer tectonic tectonic belt belt onto onto the the southernmost southernmost margin margin of of Laurentia Laurentia during the the Mazatzal Mazatzal orogeny orogeny (Van (Van Schmus et al., 1993). 1993). The Therocks rocksofofthe theTranscontinental Transcontinental Proterozoic Proterozoic provinces provinces were were subjected subjected to to aa major major magmatic magmatic event event during during the theMiddle MiddleProterozoic Proterozoicfrom from 1500-1300 1500-1300 Ma. The Theundeformed undeformedWolf WolfRiver Riverbatholith batholithinincentral centralWisconsin Wisconsinisissurrounded surroundedby byand andlocally locally intrudes intrudesthe the deformed deformedquartzite quartzitebodies, bodies,leading leadingsome someto tosuggest suggestthat thatquartzite quartzitedeformation deformationwas wascaused causedby by Middle Middle Proterozoic Proterozoic epeirogenic epeirogenic doming doming and and igneous intrusion (Greenberg and Brown, 1984). However, However, the the existence existenceof of the theFlambeau Rambeau deformational deformationalfront front(located (locatedover over 100 100km km from fromexposures exposuresof of the thebatholith) batholith) shows showsthat that the the deformation deformationdoes does not not wane wane away away from from the the batholith. Rather, Rather,the theabrupt abruptnature natureof ofthe thefront front is is characteristic characteristic of of tectonic, tectonic, not not intrusion intrusion related, deformation. This This study studysupports supportsDott's Dott's (1983) (1983) model model which which attributed attributed post-Penokean post-Penokean deformation deformation in in Wisconsin Wisconsin to to Early Early Proterozoic Proterozoic plate plate collision collisionfrom fromthe the south 1700-1600Ma Ma outer outer tectonic tectonic belt belt south and and supports supports the the hypothesis hypothesis of of Van Van Schmus Schmus et et al. al. (1993) (1993) that that the the 1700-1600 was was aa single single coherent coherentbelt belt extending extendingfrom fromCalifornia Californiato to Labrador. Labrador. References Referenceswill will be be made madeavailable availableatatthe themeeting meetingor orupon uponrequest request(dholm@kent.edu). (dholm@kent.edu). 74 �THE THERECOGNITION RECOGNITIONOF OFAALAVA LAVADOME DOME COMPLEX COMPLEXAND AND ITS ITS RELATIONSHIP RELATIONSHIP TO TO THE THEARCHEAN ARCHEANSTURGEON STURGEONLAKE LAKE CALDERA, CALDERA,NORTHWESTERN NORTHWESTERNONTARIO ONTARIO GEORGE RONALD L. MORTON GEORGEJ.J.HUDAK HUDAKAND AND RONALD MORTON Economic EconomicVolcanology VolcanologyResearch ResearchLab, Lab,Geology GeologyDepartment, Department,University Universityof ofMinnesota Minnesota —-Duluth, Duluth, Duluth, Duluth,Minnesota, Minnesota, USA USA 55812 55812 JAMES JAMES M. M. FRANKLIN FRANKLIN Geological GeologicalSurvey Survey of of Canada, Canada,601 601Booth Booth Street, Street, Ottawa, Ottawa,Ontario, Ontario,Canada CanadaK1A KIA0E8 OE8 Detailed has led to the Detailed volcanic facies mapping mapping has the recognition recognition of of the the Archean Archean Sturgeon Sturgeon Lake Caldera Caldera Complex Complex in northwestern Ontario (Morton This complex complex is isup uptoto Lake (Morton et et al., al., 1991). This 25 kilometers kilometers in in strike strike length, length, contains contains up up to to 4500 4500 meters meters of of caldera calderafill fillmaterial, material,and andhosts hosts 25 six six known known volcanogenic volcanogenic massive massive sulfide sulfide (VMS) (VMS)orebodies orebodiesand andnumerous numeroussubeconomic subeconomic massive sulfide sulfide lenses. The Thevolcanic volcanicrocks rockswithin withinthe thecomplex complexhave havebeen beendivided dividedinto intoten ten massive distinct based on the types distinct stratigraphic stratigraphic successions successions based types of ofvolcanic volcanicand andsedimentary sedimentaryrocks rocks present present (Hudak, (Hudak,1996). 1996). The TheLyon LyonCreek CreekSuccession Successioncomprises comprisesthe theuppermost uppermostof ofthese thesesuccessions. successions.Historically, Historically, these these rocks rocks were were called calledthe theNBU NBUrhyolite rhyolite(Harvey (Harvey and andHinzer, Hinzer, 1981; 1981;Severin, Severin,1982), 1982),and and were were interpreted interpreted as as rhyolite rhyolite tuffs, tuffs,lapilli lapillituffs, tuffs,agglomerates, agglomerates,and andgraphite-rich graphite-richsediments. sediments. However, However, our our detailed detailedfield fieldmapping, mapping,core corelogging, logging,and andpetrographic petrographicinvestigations investigationsindicate indicate that that little littleor ornone noneof ofthese theserocks rocksare arepyroclastic pyroclasticininorigin. origin. Instead, Instead,this thissuccession successionhas hasbeen been interpreted interpretedto torepresent representaasubaqueous subaqueouslava lavadome domecomplex. complex. krn in strike strike length length and and up up to to 540 540 A plagioclase-phyric plagioclase-phyric dacite A dacite lava lava dome dome that that is is 2-3 2-3 km meters meters thick thick forms formsthe thebase baseofofthe thesuccession. succession. This Thisdome domeoccurs occurswithin withinthe thecaldera calderaedifice edifice krn west west of of aa major major caldera caldera margin margin fault. fault. An Anabsence absenceofofflow flowcontacts, contacts, approximately 1-2 1-2 km approximately and andan anincrease increaseininthe thesize sizeofoffeldspar feldsparphenocrysts phenocrystsfrom fromthe themargins marginstoward towardthe the core coreof ofthe the dome, suggests suggests that thatthe thedome's dome'sgrowth growthwas wasendogenous. endogenous.AAseries seriesofofpredominantly predominantlydomedomedome, derived matrix-supportedbreccia breccia derived clastic clastic sediments sediments overlies overliesthe thedome. dome. These Theseinclude: include:a)a)matrix-supported deposits depositswhich which contain contain up up to to75% 75%dome domefragments; fragments;b) b) arkosic arkosicgreywacke greywackedeposits; deposits;c) c) veryveryfine finegrained grainedtuffaceous tuffaceoussandstone sandstoneand andsiltstone siltstonedeposits; deposits;and andd) d)graphite-rich graphite-richshale shaledeposits. deposits. The Theclastic clasticsediments sedimentslack lackan anabundance abundanceofofpumice pumice(rarely (rarely>3%) >3%)and andcross-bedding cross-beddingisisabsent. absent. Silica-, carbonate-, carbonate-, oxide-, oxide-, and/or and/or sulfide-facies sulfide-facies iron iron formations formations and and associated associated chert chert are are Silica-, interlayered interlayeredwith withthe theclastic clasticsediments sedimentsand, and,locally, locally,directly directlyoverlie overliethe thelava lavadome. dome. Detailed Detailedfacies faciesmapping mappingindicates indicatesthat thatthe thebreccia brecciadeposits, deposits,the thegraphite-rich graphite-richshales, shales,and and theiron ironformation formationwere wereformed formed ininrestricted, restricted,fault-bounded fault-bounded basins within the top top of ofthe the the dome. dome.The Theiron ironformation formationresulted resultedfrom fromrelatively relativelylow lowtemperature temperaturehydrothermal hydrothermalactivity activitythat that occurredproximal proximal to to the the basin basin marginal marginal faults. faults. The Thelack lackofofpumice, pumice,the theabsence absenceofofevidence evidence occurred suggestive suggestive of of reworking reworking by by surface surfacecurrents currents(e.g. (e.g.cross-bedding), cross-bedding), and andthe the presence presence of of graphitic graphitic shales shales suggests suggests aarelatively relativelydeep deepwater water(>500 (>500meters), meters),anoxic anoxicenvironment, environment, although althoughno noabsolute absolutewater waterdepth depthestimate estimateisispossible. possible. Detailed studies studies on onfelsic felsicvolcanic volcanic centers centers indicate indicate that that the the final finaltwo twostages stagesofofcaldera caldera Detailed development development involve involve major major ring ring fracture fracture volcanism volcanism and and terminal terminal hot hot spring springand andfumarolic harolic activity activity (Smith (Smith and andBailey, Bailey,1968). 1968). The The strata stratacomprising comprisingthe theLyon LyonCreek CreekSuccession Succession represent these these final final two two evolutionary evolutionary stages stages in in the the development development of of the theSturgeon SturgeonLake Lake represent Caldera CalderaComplex. Complex. 75 �References References Harvey, J. D., and Hinzer, J. J. B., 1981, 1981,Geology Geology of of the the Lyon Lyon Lake Lake ore ore deposits, deposits, Noranda Noranda Mines Mines Ltd., Ltd., Sturgeon SturgeonLake, Lake, Ontario: Ontario:Canadian CanadianInstitute Instituteof ofMining Miningand and Metallurgy Bulletin, Bulletin, v. 74, 74, no. 833, 833, p. 77-84. 77-84. Hudak, G. J., 1996, 1996, The The physical physical volcanology volcanology and and hydrothermal hydrothermal alteration alteration associated with with late late caldera caldera volcanic volcanic and and volcaniclastic volcaniclastic rocks rocks and and volcanogenic volcanogenicmassive massive sulfide sulfide deposits deposits in in the the Sturgeon SturgeonLake Lake region region of of northwestern northwestern Ontario, Ontario, Canada: Canada: unpublished unpublished Ph. Ph. D. D. dissertation, dissertation, University University of of Minnesota, Minnesota, Minneapolis, Minneapolis, MN, MN, 463 463 pages. pages. Morton, Morton, R. L., Walker, Walker, J. J. S., S., Hudak, Hudak, G. G. J., J., and and Franklin, Franklin, J.J. M., M., 1991, 1991,The The early early development developmentof of an an Archean Archean submarine submarine caldera calderacomplex complex with with emphasis emphasison onthe the Mattabi Mattabiash ash flow and its its relationship relationshipto to the the Mattabi Mattabi massive massive sulfide sulfidedeposit: deposit:Economic EconomicGeology, Geology, flow tuff and v. 86,p. 86, p.1002-1011. 1002-1011. Severin, P. W. A., 1981, 1981, Geology Geology of the Sturgeon Sturgeon Lake Lake Cu-Zn-Pb-Ag-Au deposit: deposit: Canadian Canadian Institute Instituteof of Mining Mining and and Metallurgy MetallurgyBulletin, Bulletin, v. v. 75, 75, no. no. 846, 846,p. p. 107-123. 107-123. Smith, R. L., and Bailey, R. A., 1968, of 1968, Resurgent Cauldrons: Geological Society of America 13-662. AmericaMemoir Memoir116, 116,p.p.661 3-662. 76 �GEOLOGIC SETTING OF SUBECONOMIC GOLD GEOLOGIC GOLD DEPOSITS DEPOSITSIN INTHE ThE VIRGINIA HORN, NORTHEASTERN MINNESOTA: MINNESOTA.A AHORN HORNOF OFPLENTY PLENTY OR A HORNSWOGGLE? NORTHEASTERN HORNSWOGGLE? JIRSA, Geological JIRSA, Mark Mark A., BOERBOOM, BOERBOOM,Terrence Terrence J., J.,and and CHANDLER, Val Val W.-Minnesota W.—Minnesota Geological Survey(MGS), (MGS),2642 University Avenue,St. St.Paul, Paul,MN, MN,55114-1057 55114-1057(jirsa001@tc.umn.edu) (nrsaOOl@tc.urnn.edu) Survey University Avenue, Several significant-though, significant—though, to to date date subeconomic-gold subeconomic—gold deposits deposits occur occur within within Archean Archean bedrock bedrock in the area known as the Virginia Horn. Three Three prospects prospects were were worked worked to to varying varying degrees degrees by exploration exploration companies in the late late 1980's, 1980'~ and ~ one of of these, the "Viking prospect", The and prospect",was was extensively extensivelydrilled. drilled. The exploration focused focused on pervasively pervasively altered altered felsic felsicporphyry porphyry intrusions intrusions having having variably variably well well developed deformation envelopes and and associated associated carbonate-sericite alteration. Recent mapping mapping by by the theMGS MGS (Jirsa (Jirsa deformation envelopes alteration. Recent and others, others, 1998), 1998), together together with with geochemical work work by by the the Natural Natural Resources Research Research Institute Institute (Englebert and Hauck, 1991; 1991; and information on on the the lithological lithological and work work in in progress) progress) has has provided provided further further information and structural structuralframework, framework, potential potential sources sources of of Au, and relative timing of mineralization mineralization in these prospects and the surrounding surroundingarea. area. à 'I ''''''I, ,,,,,,,,, ,f/f,, ,,,,,,,,,,, MINNTAC SEQUENCE \- %, 'S .. , .' .' \ \ '. 'S S S 'S ,1 , S.''' / / /1 / /, /1 •5%.'.'\\'SI / / / / / / /1 '''''S.''''' '''''S'S'S'''' '''''''S'S''', ,,/,//,/,,, 'S'S5'S'S'S'S'S 'S 'S 'S 'S'S.' 'S 'Ss 'S 'S '''''S ''''S'S'S' '''"'S /, MUD LAKE SEQUENCE Graywacke (stippled) and volcanic rocks (white) Figure 1—Schematicblock blockdiagram diagramofofthe theVirginia VirginiaHorn. Horn.Map Mapsurface surfacedimensions dimensions approximately approximately Figure 1-Schematic 30X30 km. km. 30x30 The Archean rocks of the Virginia Horn lie within the Wawa subprovince subprovinceof of Superior SuperiorProvince. Province. The rocks are are subdivided subdivided into northern and southern of metamorphic metamorphic grade grade and and southern panels on the basis of deformation style. The The northern northernpanel, panel,immediately immediatelysouth southofofthe theGiants GiantsRange Rangebatholith, batholith,contains contains intensely lineated, schists having having volcanic volcanicand and clastic clasticprotoliths. protoliths. The southern heated, amphibolite-grade schists southern panel that were metamorphosed to much lower grades, panel contains contains lithologically similar rocks that grades, ranging ranging from prehnite-pumpellyite to low greenschist. The two panels are separated by the east-trending, post-metamorphic, Laurentian Laurentianfault fault (Figure (Figure1). 1).The Themetamorphic metamorphiccleavage-forming cleavage-formingevent event in in both both panels of 33major majordeformations-the deformations—theother othertwo twodeformation deformationevents eventsproduced produced no no panels was was the the second second (1)2) (D2) of metamorphic affects. affects. The first (Dl) (Di) involved upright discernible metamorphic upright folding, folding, soft-sediment soft-sediment deformation, deformation, and complex faulting; the third third (D3) (D3) produced produced localized localized semi-brittle semi-brittle crenulation crenulation of of D1 Dl and and D2 D2 structures, brittle fractures, of earlier-formed earlier-formed faults. faults. Amphibolite structures, fractures, and and selective selective reactivation of Amphibolite grade grade sequence; the the low low and and sub-greenschist sub-greenschist grade grade rocks north of the Laurentian fault comprise the Minntac sequence; 77 �strata south of the Laurentian fault are subdivided The Minntac subdivided into Mud Lake and Midway sequences. The sequence contains strongly banded banded schists schists having geochemical geochemical and outcrop-scale outcrop-scale characteristics characteristics of mafic to intermediate volcanic and volcaniclastic volcaniclastic strata, strata, subvolcanic subvolcanicmafic maficsills, sills,and andgraywacke. graywacke. Although of the Minntac sequence, sequence, relict relict grading grading and bedding the possibility of tight folding is great in rocks of Incontrast, contrast,the the Mud Mud Lake Lake sequence sequence forms forms a broad, southward stratigraphic stratigraphic facing. facing. In consistently indicate southward southwest-plunging, D1 D l syncline synclinethat that isis cored cored by by graywacke, graywacke, slate, slate, and and minor minor felsic felsic tuff; and and has has outer outer southwest-plunging, of calc-alkalic calc-alkalic and limbs of and tholeiitic tholeiiticstrata. strata. The Mud Lake strata are cut by several variably The Mud Mud Lake Lake sequence and the intrusions are porphyritic, quartzofeldspathic intrusions. The and are are locally locally in fault contact with, fluvial conglomerate and and subaerially subaerially unconformably overlain by, and deposited trachyandesite trachyandesite flows flows and and pyroclastic pyroclastic rocks that comprise the Midway sequence. seauence. The The deposited Tirniskaming -like strata in the Kirkland Lake and Shebandowan gold attributes of of Timiskaming Midway has many attributes Like the the Timiskaming Timiskaming rocks of mining districts of Ontario. Like of Ontario, those those in in the Virginia Virginia Horn Horn are have been deposited deposited in in aa fault-bounded fault-boundedpull-apart pull-apartbasin basinthat thatformed formedbefore beforethe theonset onsetofof1)2 D2 inferred to have deformation and metamorphism. datingback back to tothe thedays daysof of J.W. J.W. Gruner Gruner and and The Virginia Horn has a long history of of gold "shows", 'shows', dating F.F. Grout Grout (Grout, and some some visible visible gold gold can can still still be be found found locally in in altered altered rocks in and and adjacent (Grout, 1937), and adjacent to quartz quartz veins. Sampling Samplingby byexploration explorationcompanies companiesfocused focused largely largely on the quartzofeldspathic intrusions, and the country rocks were rarely rarely analyzed. analyzed. From those data and the current study, the generalizations can be made about the distribution distribution of of gold gold in in the the region: region: following generalizations 1. Gold 1. Gold is is most abundant abundant in in quartzofeldspathic quartzofeldspathicdikes. dikes. Thegreatest greatestgold goldcontents contents(as (aslarge large as as 50,000 50,000 ppb) 2. The ppb) occur occur in in rocks rocks that that are are pervasively pervasively altered and cut by quartz-rich veins. Anomalousquantities quantitiesofofgold gold(50(50-500 500 ppb) also also exist in sedimentary sedimentary and and volcanic volcanic wall-rocks wall-rocks of of 3. Anomalous the porphyritic intrusions. intrusions. 4. Gold Goldconcentrations concentrationsgreater greaterthan thana a"mineable" "mineable"cut-off cut-offof ofabout about1000 1000ppb ppb (0.029 (0.029 OPT) are recorded exclusively from samples of quartzofeldspathic quartzofeldspathic dikes. dikes. Inrocks rocks uniformly uniformly affected affected by carbonate-sericite alteration, 5. In alteration, gold gold is is most most abundant abundant in zones of of anomalous arsenic content. anomalous arsenic content. carbonate-sericite alteration and The carbonate-sericite alteration associated associated with with gold mineralization mineralization varies from pervasive and having textures that imply strong involvement in shearing. not obviously related to to the the rock rock fabric, to having Pyrite typically is associated with carbonate and sericite; sericite; and arsenopyrite and and chalcopyrite chalcopyrite occur occur locally. Alteration is inferred to have taken place during or just after D2 deformation, as the Alteration is inferred to have taken during or are variably variably affected affected by 82. That alteration is best developed along major fault alteration products are S2. That zones, lithologic contacts, contacts, and and adjacent adjacentto to and and within within the the quartzofeldspathic quartzofeldspathic intrusions. intrusions. Although Although the the shallow subsurface subsurface have been evaluated in in some some detail, detail, comparisons with analog analog deposits deposits surface and shallow imply that that unexplored unexplored potential potential exists; exists; 1) 1) at depth in in the the low-grade low-grade rocks rocks of of the the Midway Midway in Canada imply and and Mud Mud Lake Lake sequences; sequences; and and 2) 2) throughout throughout the thehigh-grade high-gradeMirintac Minntac sequence. sequence. ACKNOWLEDGMENTS Mapping Mapping and and geochemical geochemical study study was was supported supportedby by the the Minnesota Minnesota Legislature Legislatureon on recommendation recommendationof of the Minerals Coordinating CoordinatingCommittee. Committee. REFERENCES REFERENCES Englebert, and Hauck, 1991,Bedrock Bedrockgeochemis geochemistryofofArchean Archeanrocks rocksin innorthern northern Minnesota: Natural Natural Englebert, J.A., J.A., and Hauck, S.A., 1991, Institute Technical Report NRRI/TR-91/12, 200 p. NRRI TR-91/12,200 Resources Research Institute 7 Grout, Petrographic study F.F., 1937, Petrographic studyof of gold goldprospects prospectsofofMinnesota: Minnesota:Economic EconomicGeology, Geology,v.37, v.37,P. p.56-68. 56-68. Grout,F.F., Jirsa, Bedrockgeolo geologic map of of the the Virginia Horn, Horn, Mesabi Ran e,St. St. Jirsa,M.A., Boerboom, Boerboom, T.J., and Morey, Morey, G.B., 1998, Bedrock 'c map Mesabi Iron IronRange, Louis County, County,Minnesota: Minnesota:Minnesota MinnesotaGeological GeologicalSurvey SurveyMiscellaneous MiscellaneousMap MapM-85 M-85(digitial), (digitial),scale scale1:48,000. 1:48,0%. 78 �EFFECT EFFECI' OF OFELECTRIC ELECI'RICPULSE PULSEDISAGGREGATION DISAGGREGATION ON ON MICROFOSSIL-BEARING MICROFOSSIL-BEARING ARGILLACEOUS ARGILLACEOUSLIMESTONE LIMESTOM OF OF THE THE MIDDLE MIDDLE ORDOVICIAN ORDOVICIAN DECORAH DECORAH SHALE SHALE KJERLAND, KJERLAND, Dean Dean W., W., and andKJERLAND, KJERLAND, Marc MarcP., P., amateur amateurpaleontologists, paleontologists, 73563,3520@compuserv.com E-mail: 73563,352O@compuse~.com 8007 MN 55431 S, Bloomington, Bloomington, MN 8007 Xerxes Ave S, 55431 E—mail: The The Middle Middle Ordovician Ordovician Decorah Decorah Shale is a fossiliferous fossiliferous shale with lenses and thin beds beds of of coquinoidal coquinoidal limestone limestone and and calcareous calcareous shale (Rice). The Thecalcitic calciticmacrofossils macrofossilsand andphosphatic phosphatic conodont conodont microfossils microfossils have been been studied studied (Sloan, (Sloan, Webers). Webers). Though Though known known from from acid acid processing processingresidues, residues,other othernon—calcitic non-calcitic microfossils have have not not been beendocumented. documented. We samples of of limestone from Decorah We collected collectedover over250 250typical typicalhand—sized hand-sized samples Decorah Shale Shale beds beds disturbed disturbed during during the the reconstruction reconstruction of Hwys 110, 55 & 13 13 at at Mendota, Mendota, Minnesota Minnesota in in 1993. 1993. Our Our selection selection criteria criteria was was the the presence presence of of phosphatic phosphatic microfossils or fragments fragments visible visible with with Our photographic photographic documentation of 200 of of these these aa hand hand lens lens on oneither eitherororboth bothsurfaces. surfaces, Our surface surface microfossils microfossilsusing using optical optical microscopy reveals a wide variety of microfossils including including lithified lithifiedmud mud internal internalcasts castsof of bryozoans bryozoans(Cuffey), (Cuffey), laminated laminated plates, plates, and and gastropod gastropodsteinkerns. steinkerns. Separation Separation of microfossils microfossils from the the matrix matrix isis necessary necessary for for more more efficient efficientcollection collectionand and documentation. documentation. Full Fullviewing viewingof of specimens specimensisis useful useful for for classification. classification. Mechanical Mechanicalcrushing crushing is is not not an aneffective effectivemethod methodfor forcleanly cleanlyseparating separatingeither eithermacrofossils macrofossils or ormicrofossils microfossilsfrom from matrix. matrix. Acid Acidprocessing processingyields yieldsclean cleanmicrofossil microfossilspecimens specimensbut but artificially artificially selects selects for for either either calcitic or phosphatic material. material. Both Both Ca Ca and P, as well as Fe, 5, S, Al, A,Si, Si, and and Mg, Mg, have have been been confinned confirmed from from our our S.E.M. S.E.M. analyses analyses of of aa sample sample of of several severaldozen dozen typical typical microfossils. microfossils. Saini—Eidukat andWeiblen Weiblenreported reportedthe theuse use of of an an Electric Pulse Pulse Disaggregator (EPD) for Saini-Eidukat and fossil extraction extraction (see (see References). References). In In our our study study we we evaluated evaluated the potential potential of the the EPD EPD for for separating visible microfossils from the surface 'fossil hash' of Decorah Shale limestone and separating microfossils Shale limestone and also for sampling matrix of of this this rock. rock. We sampling for microfossils microfossils buried in the matrix We used the the specialized, specialized, upgraded version EPD EPD at the University of Minnesota which is upgradedlaboratory—scale laboratory-scale version is based based on on the the electric—pulse facility in St. Petersburg, Russia (Saini—Eidukat and Weiblen). electric-pulse facility Petersburg, Russia (Saini-Eidukat Weiblen). A cm long long xx 12.5 12.5 cm cm wide weighing about 1.2 1.2 kg, was quartered quartered A lenticular lenticularslab slab fragment, fragment,16 16cm with aa rock hammer and further fragmented/delaminated with a cold chisel to rock hammer and further fragmentedldelaminated with cold chisel toproduce produce walnut—sized fragments for electric walnut-sized fragments electric pulse pulsedisaggregation. disaggregation. The rock rock consisted consistedofofnon— nonweathered, crystalline calcite calcite with with typical weathered, compact, compact, dark dark blue—gray blue-gray crystalline typical cemented cemented calcitic calcitic macrofossil clay on both sides and macrofossil fragment fragment debris and gray interstitial surface clay and phosphatic phosphatic microfossils microfossils and and fragments fragmentsvisible visible on ononly onlyone onesurface. surface. The The slab slab pieces pieces were were hand hand fed fed in in two two batches batches into into the hollow cathode of the EPD over over aa 66 mm pulses at approximately 100 kV. kV. All mm integral integral sieve sieve and and subjected subjected to approximately approximately 200 pulses All material except except the the suspended suspended fines fines was was air air dried driedand andmechanically mechanicallysorted sorted through through brass brass sieves sieves (Table (Table 1.) 1.) The Thematerial materialwhich whichpassed passedthrough throughthe the0.25 0.25 mm mm sieve sievewas wasprocessed processeddry dry through through aa series series of of teflon teflonmeshes meshes(Table (Table2.). 2.). The aggregate calcite and yellow clay and aggregate produced produced by the the EPD EPD isis fragmented fragmented crystalline calcite and mud-sized fines. fines. Only Only aa small small fraction fraction consists consists of whole whole calcite calcite fossils fossils or or fragments fragments and and mud—sized non—calcitic microfossils and orpartly partlyenclosed enclosed in in calcite calcite matrix). matrix). non-calciticmicrofossils and fragments fragments(either (eithermatrix—free matrix-free or 79 �Table 1: 1:Grading Grading criteria criteria and and results: results: Table 2. Grading of of Class VIII Material Weight(in g) and % % Class Sieve(in mm) Weight(in 31.2 >6.0 31.2 2.5% ~6.0->4.0 297.8 24.0% 24.0% I1 II <6.0—>4.0 297.8 I11 ~4.0->2.0 397.3 32.1% 32.1% III <4.0—>2.0 397.3 IV 207.6 207.6 16.8% 16.8% Iv c2.0->1.0 <2.0—> 1.0 65.6 5.3% ~1.0->.701 vV <1.0—>.701 <.701—>.495 46.7 <.701->.495 46.7 3.8% VI c.495->.250 62.9 5.1% <.495—>.250 62.9 VII <.250 129.1 10.4% 10.4% 129.1 VIII e.250 Total Total 1238.2 1238.2 100.0% 100.0% Note: The material which passed through the Note: 0.25 mm brass brass sieve sieve was wasre—graded re-graded using using meshes. Results teflon meshes. Results are from top of mesh. II Class Class VIlla VIIIa VIIIb VIIIb VIlic VIIIc VIlId VIIId Mesh# (mm) Weight(m Weight(ing) g) and and % 6.4 #60 (>.250) 5.0% 15.9 12.3%; #80 (>.177) (>.I771 15.9 12.3%; #100(>.149) 13.0 13.0 10.1%; 10.1%; thru #100 #lo0 93.8 72.6%. Total 129.1 100.0% Total 100.0% materialsfrom fromthe theteflon teflonmeshes mesheswere werenot notexamined examinedatatthis thistime. time. The VIIIa-d materials The Class VIIIa—d Class ClassII—- VII materials were were examined examined with with an an optical opticalmicroscope microscopeand andsampled sampledfor fora).a).non— nonfossils, fragments fragments and fossil crystalline crystallinecalcite, calcite,b). b).fossil—bearing fossil-bearing calcite, and c). non—calcitic non-calcitic fossils, and unknowns. Based Basedon onsubjective subjectivecriteria criteria (i.e. typical, interesting, unusual or trophy categories) 654 fossils, fragments and unknowns were were collected. collected. Specimen Specimen counts counts are: are: 11.8% 11.8%calcitic calcitic fossils, crystalline calcite, fossils, 9.8% 9.8% phosphatic fossils fossils in crystalline calcite, 74.9% 74.9% loose loose phosphatic phosphatic fossils fossils and fragments, and 3.5% fossils and fragments, 3.5% unknown. unknown. Virtually all of of the thematrix—free matrix-free non—calcitic non-calcitic fossils recovered from from the top of the sieves fragments are from that portion of the aggregate (31%) recovered sieves .250 mm sieve in the the 2.0 2.0mm mm—- -250 sieveset set(Class (ClassIV—VII). IV-VII). Conclusion: provides aa new new method method for for disaggregating disaggregating rocks for for isolating isolating Conclusion:The The EPD EPD provides microfossils mm to to 0.25 0.25 mm. mm. EPD processing microfossils in the size range from 2.0 mm processing effectively effectively separates separates microfossils from allows efficient efficient phosphatic, calcitic, and pyritic microfossils from their calcitic matrix and allows testing of rocks for the the presence presence of buried specimens. Although Although some some specimens specimens may may break break in processing or during preserved in shale are during sample sample preparation, preparation, many fossils preserved are commonly commonly fractured naturally in the matrix. We Webelieve believethat that EPD EPDprocessing processingsimply simply separates separatesalready already damaged fragments or whole whole fossils fossils along along existing existing fractures. fractures. References: References: Cuffey, Roger J., Professor of Paleontology, Department of of Geosciences, Pemsylvania Pennsylvania State correspondence 1997 1997 and and 1998. 1998. University; personal correspondence Rice, William F.; "TheSystematics Systematicsand and Biostratigraphy Biostratigraphy of the Brachiopoda of the Decorah F.; "The St.Pau1, Minnesota", Mimesota", 1985, 1985,Unpublished M.S. thesis, University of Minnesota, Mimesota, Mpls. Shale at St.Paul, Saini-Eidukat, B., Saini—Eidukat, B.,and andWeiblen, Weiblen,P.W., P.W., 1996; 1996; "A "ANew New Method Method of of Fossil Fossil Reparation, Preparation, Using High-Voltage Pulses", references therein. therein. High—Voltage Pulses", Curator Curator 3912, 39/2, and references Sloan, Robert E, E, editor, editor, Middle Middle and and Late LateOrdovician Ordovician Lithostratigraphy Lithostratigraphy and Biostratigraphy Biostratigraphy of the Upper #35, Mimesota Minnesota Geological Survey, Upper Mississippi Mississippi Valley, Report of Investigations #35, University of Minnesota, university Mimesota, 1987. 1987. Webers, Gerald F.; "The Middle Mimesotat': Middle and Upper Ordovician Conodont Faunas of Minnesota": Mimesota Geological Geological Survey Survey Special SpecialPublication PublicationSeries SeriesSP—4, SP-4, 1966 1966 Minnesota 80 �RESULTS OF MODELLING MODELLING PROTEROZOIC PROTEROZOIC THERMAL THERMAL HISTORIES: HISTORIES: THE POSSIBLE POSSIBLEEFFECTS EFFECTS OF OF WOLF WOLF FUVER RIVER BATHOLITH BATHOLITH EVALUATING THE REHEATING THERMOCHRONOLOGIC DATA =HEATING ON THEMOCHRONOLOGIC DATA FROM FROM NORTHERN NORTHERN WISCONSIN WISCONSIN Jeff Jeff Loofboro Loofboro(student) (student) and and Daniel DanielHoim, Holm,Department Departmentof of Geology, Geology, Kent Kent State StateUniversity, University, Kent, OH 44242 Introduction. Mica MicaRb/Sr RblSrand andAr/Ar ArlArthermochronologic thermochronologicresults results across across northern northern Wisconsin Wisconsin and northern Michigan (Peterman and Sims, 1988, 1988, Tectonics; Schneider and and others, others, 1996, 1996, CJES; CJE!S;Hoim Holm and and others, 1997, 1997, GSAA) reveal a 1630-1600 1630-1600 Ma chrontour chrontour which separates separates basement basement rock with with primary primary cooling ages of 1760-1750 Ma Ma to the north from 4<1630 6 3 0 Ma ages to the south. The Thechrontour chrontourcoincides coincides physically with an apparent deformational front in overlying, Early Proterozoic, post-Penokean quartzites Hoim and others (1998, ILSG; and in review) to interpret complete leading Holm interpret the the chrontour chrontour to to represent representcomplete thermal resetting resetting of of micas associated with foreland deformation deformation related related to to accretion accretion from the south. thermal associated with However, we note that the chrontour also appears appears to surround surround the known known subsurface subsurface extent extent of of the the Middle Middle Wolf Mver River batholith batholith(Fig. (Fig. 1) 1) raising raising the the possibility possibilitythat that itit might might be be an an artifact of partial partial Proterozoic Wolf resetting. In Inthis this case, case, the the1630-1600 1630-1600Ma Ma chrontour chrontour may may represent represent aa collection collection of meaningless "mixed" ?nixed'' resulting from from partial partial resetting of of the older primary 1760-1750 Ma dates at 1470 dates resulting 1760-1750 Ma 1470 Ma Ma when when the the batholith intruded. Using Using the the MacArgon MacArgon program (Uster (Listerand andBaldwin, Baldwin,1996, 1996,Tectonophysics) Tectonophysics) we we have have various Proterozoic Proterozoic thermal thermal histories histories in in an attempt to evaluate modelled various evaluate the possible effects effects of of Middle Middle Proterozoic Proterozoic reheating on thermochronologic data data from from northern Wisconsin. Initial Initial conditions conditions and andmodel modelparameters. parameters.We Weinitially initiallyconsider consideraarock rockcontaining containing muscovite muscovite with a plateau ArIAr Ar/Ar date of 1765 Ma and biotite biotite with with a plateau plateau date date of of 1755 Ma. Ma. Precambrian Precambrian basement rock north of the chrontour chrontour in northwest northwest Wisconsin are are near near the the Early Early Proterozoic Proterozoic nonconformity nonconformity suggesting suggesting these rocks were at shallow Ma. Considering this, we chose three shallow crustal depths by —1700 -1700 Ma. three different different ambient temperatures temperatures(loo0, (100°, 150° 150°,and and200') 200°) and and imposed imposedaa thermal thermal pulse pulse at at 1470 Ma Ma to simulate ambient intrusion of the batholith. batholith. We We varied varied the the peak peak temperature temperature of of the the pulse pulse between between 200° 200O and and 450°C 450° (using (using 50°C to 22 my my (using (using 0.5 0.5 my my increments). In In 50° increments) and the duration of heating from instantaneous to all cases the duration of cooling back to ambient temperatures lasted 2 my and hence the total duration duration of the pulse varied in our models models from from 2-4 2-4 my. my. The plutonism on country rock The duration duration of thermal effects effects of plutonism is normally normally shorter than this (Carslaw (Carslaw and Jaeger, Jaeger, 1959, 1959, Clarendon Press), but we chose chose such such long long durations in in order order to to maximize maximize the the effects effects of of partial partial resetting resettingby bythe the batholith. batholith. We assume no other affects after after intrusion intrusion of of the the Wolf Wolf River batholith, batholith, ending ending the the thermal thermal history with thermal overprinting affects slow cooling cooling from from the the chosen chosen ambient ambienttemperature temperaturetotoO° 0°Catat600 600Ma Ma(Fig. (Fig.2). 2). The variables in the modelling thus include the peak temperature obtained, the duration of the thermal pulse, and the the initial initial temperature. The ambient temperature. The affect affect of each each of these these parameters on argon diffusion in muscovite and biotite is is described described below. Results thermal modeling. modeling.AsAsmight mightbebeexpected, expected,the thedominant dominantfactor factorinfluencing inlluencingpartial partial resetting Results of thermal is the peak temperature obtained by by the the rock rock during during the the imposed imposedthermal thermalpulse. pulse. Peak thermal pulses at or below the closure temperature of biotite (300°C) (3W°Cor muscovite (350°C) (350°Chad little affect on on the the initial initial cooling age regardless of the duration of heating. Peak Peakthermal thermalpulses pulsesof of50°C 50°above aboveclosure closuretemperature temperature resulted in considerable considerable partial partial resetting. resetting. In this case the duration of the heating interval did have a moderate degree of resetting total gas gasages ageswhich whichare are moderate affect on the degree resetting with 2 my my heating intervals intervals resulting resulting in in total —50 myyounger youngerthan than in in the case for Because of of the difference in closure -50 my for instantaneous instantaneous heating. heating. Because closure temperature between biotite and muscovite, the modelling reveals that large differences in the degree of partial resetting resetting (and hence apparent ages obtained) are expected for 1470 1470 Ma Ma peak peak thermal thermal pulses pulses between 300 and 450°C 450°C. Temperatures Temperatures100°C 100OCabove above the the mineral's mineral's nominal nominal closure closure temperature temperature resulted in nearly complete resetting resetting of of the the isotopic isotopic systematics systematics regardless regardlessof ofthe theduration durationof ofheating. heating. Varying the initial ambient temperature between 100-200°C 100-200OC had little little affect affect on the apparent apparent ages ages obtained. obtained. Implications. Existing Existingthermochronologic therm6chronologicdata data from from northern northern Wisconsin Wisconsin show nearly concordant muscovite and biotite dates near the chrontour (with muscovite around 1620 1620 Ma and biotite around around 1600 1600 GSAA). We Ma; Romano and others, 1997, GSAA). Weare areunable unableto toobtain obtainnearly nearlyconcordant concordantpartially partiallyreset resetages ages with any of our simulations and conclude that Middle Proterozoic intrusion of of the the Wolf River batholith was probably probably not responsible for generating the 1630-1600 1630-1600 Ma chrontour by partial resetting. This is indirectly indirectly supported supported by by two two independent independent lines lines of ofevidence. evidence. First, the degree of deformation conclusion is deformation Proterozoic quartzites in in Wisconsin Wisconsin does does not not wane away from the the Wolf Wolf River batholith as of the Early Roterozoic deformation. The would be expected for intrusion related deformation. The sharp sharp deformational deformational front front (which (which coincides coincides Second, with the 1630-1600 1630-1600 Ma clirontour) chrontour) is more more characteristic characteristic of tectonic-related tectonic-related deformation. Second, 81 �thennal thermaleffects effectsof ofthe theDuluth DuluthComplex Complexon onArchean Archeancountry countryrock rockininnortheastern northeasternMinnesota Minnesotaexist existonly onlytotoaa map-view distance distance of 10 10 km away from the intrusion (Hanson (Hanwn and others, 1975, GCA). By By comparison, comparison, the 40km kmaway awayfrom fromthe theknown known the 1630-1600 1630-1600Ma Ma chrontour chrontour in in northern northern Wisconsin Wisconsin isis located locatedmore more than than 40 subsurface extent of the Wolf River batholith (l3g. (Fig. 1). Our Ourresults results support supportthe the conclusion conclusion of of Hoim Holm and and others U G )that that the the1630-1600 1630-1600Ma Machrontour chrontourrepresents represents complete completethermal thermal resetting resetting of of micas micas others (1998, (1998, ILSG) tectonism. associated with latest Early Proterozoic tectonism. Lister, Lister, G.S., G.S., and andBaldwin, Baldwin,S.L., S.L., 1996, 1996,Modelling Modellingthe theeffect effectof of arbitrary arbitraryP-T-t P-T-t histories historieson onargon argondiffusion diffbsion in in minerals minerals using using the the MacArgon MacArgonprogram program for for the the Apple Apple Macintosh: Macintosh: Tectonophysics, Tectonophysics, v. 253, p. 83-109. #%. ../ %t %/% %/%/ &S:S:S:::S 3:S::S / . F. /. /. / / / FS. %S.F SaS. —:-xc-t-:.:-: %'..—.. % . KEWEENAWAN S. subhoilzontal, , Ba ffonF % ,, S ,,,- .. .. % % % .. - S. S. S. S. S. S. S. S. S. CM%S.S.S.S.S.S.S.S.1 S. S. S. ' —46 - :Penoke ambeau ightly fold I, fit ,f' a 50km Edge of Late Prol/ Phanerozoic cover / Baraboo qtzite I 1(tightly folded) / Approximate sub . -! surface extent of WRB I 91 I I 90 89 I 88 Figure I.Simplified Simplifiedtectonic tectonicmap mapof of the the central central Penokean Penokeanorogen, northern northern Figure 1. Wisconsin and Michigan, U.S.A. (after Sims, 1992; Holm and others, Wisconsin and Michigan, U.S.A. (after Sims, 1992; Holm and others, in in review). review). CM = continental margin; EPSZ = Eau Pleine shear zone; NSZ = Niagara suture CM = continental margin; EPSZ = Eau Pleine shear zone; NSZ = Niagara suture zone; MT Archean Marshfield Marshfieldterrane; terrane; WRB WRB == Wolf Wolf River River batholith. batholith. MT == Archean E 92 PeakT Peak T 3 + 2 tzE \ a 1 1800 1800 tt == duration duration of thermal thermal pulse pulse AmbientT Ambient T ,t 1470 Age(Ma) Age (Ma) 1470 600 600 Figure Figure 2. Temperature-time Temperature-timegraph graphshowing showing modelled modelledparameters. parameters. 82 �An Archean subaqueous heterolithic debris flow, Irwin, Irwinl Pifher, Pifher! and Meader Townships, Townshipsl Lake Nipigon Nipigon Region, Ontario ft Luther, Whitewater, Wl WI 53190 Frank R. Luther!Geology GeologyDepartment, Departmentl UW-Whitewater, UW-Whitewaterl Whitewatery 53190 lutherf@mail.uww.edu lutherf@mail.uww.edu This extensive body body of volcanic breccia breccia is is located located in in northern northern Irwin, Irwinl southeastern southeastern Meader, Meader! and and aa large large part part of of southern southernand andeastern easternPifher PifherTownships. Townships. It is 10-25 10-25 km east of Lake Lake Nipigon Nipigon and and 17-28 17-28km km north-northeast north-northeastof of Beardmore. Beardmore. Mapping by Mackasey (1975) and Kresz and Zayachivsky (1989) shows this rock to be enclosed in a thick sequence of typical greenstone greenstone assemblage rocks -flows, intermediate to felsic pyroclastics pillowed and massive basalt flowsl pyroclastics and and flows, flowsl and lesser amount amount of of spatially-associated spatially-associatedvolcanogenic volcanogenic sediments. sediments. It is cut by small granitic intrusions through most of its its exposure exposure area and and by by larger larger granitic granitic intrusions in Pifher Twp to to the the northeast. northeast. Metamorphism Increases to to the the northeast, perhaps caused by these these intrusions. Keweenawan northeast! Keweenawanintrusions intrusions which which cut cut the breccia breccia include a large diabase sill which dips gently to the south and a small dike, locally termed greenspar (Luther, Thomas, Kean, Keanl and and Luther, Luther! diabase dike! (Lutherl 1997; Thomasl in preparation). preparation). fragments ranging from from c <1l mm up to over The breccia contains rounded rock fragments 1 Im m in in maximum maximum dimension. dimension. Most fragments are disk-shaped disk-shaped to cigar-shaped cigar-shaped in in form although although irregular irregular on on smaller smaller scale; some some smaller smaller (5-20 (5-20 cm) cm) fragments fragments are are of fragments is, more competent more competent and and nearly nearly spherical. spherical. Sorting and alignment of isl in in some locations, locationsl poorly-developed poorly-developed primary primary bedding while, whilel in other locations, locationsl the sorting and alignment is a result of later strain. The smaller fragments fragments vary in in Larger composition from that of the composition the matrix matrix to to mafic mafic or or ultramafic ultramaficto to carbonate-rich. carbonate-rich. Larger fragments tend to be be similar to to the the matrix matrix in in composition composition although although minor minor differences differences in composition or consolidation consolidation cause the fragments to to weather weather high high or or low. low. The matrix of fine mm), matrix of the breccia is composed of fine quartz quartz grains grains (<0.05 (~0.05 mm)l euhedral to broken 4 . 1 to to euhedrai broken crystals of plagioclase (now albite) ranging in in size size from <0.1 1 mm, a minor pelitic fraction (now muscovite), and, locally, quartz, I muscovite)l and! locallyl fragments of quartzl and/or hornblende hornblende(now (nowchlorite) chlorite)up uptoto0.5 0.5mm. mm. The primary pyroxene andlor primary texture is epidotel actinolite, actinolitel over-printed by a metamorphic metamorphic assemblages of chlorite, chloritel epidote, Representative whole rock analyses of the matrix muscovite, muscovitel and and biotite. biotite. Representative matrix and and fragments are presented Theseanalyses analysesshow showthe theaverage average(igneous (igneous presented in Table 1. These rock equivalent) composition of of the matrix to be dacitic while the fragments vary from from dacitic dacitic to to mafic. mafic. fragments, (2) the relative relative In summary (1) (I) the heterogeneity of the fragments! homogeneity of the matrix, (3) the shape, and rounding of the fragments, matrix?(3) the shape! and rounding the fragmentsl (4) (4) the the 83 �great variability variability in in size size of of the the fragments, fragments, (5) (5) the the poorly poorlysorted sortedcharacter characterof of the thewhole whole great body of of rock, rock, and and(6) (6)the thespatial spatialassociation associationwith withpillowed pillowedvolcanics volcanicsand andbedded bedded body of aasubaqueous subaqueous tuffs lead leadthe the author author to to conclude conclude that that this this rock rock isisthe the product productof tufts debris flow flow or or flows flows off offthe the side side of of aavolcanic volcanic edifice. edifice. The Theslopes slopes presumably presumably debris consisted of of poorly-consolidated poorly-consolidatedvolcanogenic volcanogenic debris debrisincluding includingfinely-crystalline finely-crystalline consisted quartz and and clay clay which which was was transported transported from from nearby nearbyweathered weathered felsic-tofelsic-toquartz intermediate volcanics volcanics and and an an admixed admixedquantity quantityof of pyroclastic pyroclasticdebris. debris. intermediate Table 1. 1.Whole Wholerock rockanalyses analysesofofthe thematrix matrixand andselected selectedfragments fragmentsof of the the Table volcanic breccia. (XRAL) (XRAL) volcanic fraaments matrix matrix fraaments LN97-6-1BB LN97-6-2A ~~97-7-9~* LN97-6-2 LN97-7-9-2 LN97-7-9-2 LN97-6-1 LN97-6-2 LN97-6-2A LN9779A* Si02 Si02 Ti02 Ti02 A1203 A12 0 3 Fe203 Fe203 Feo FeO MnO MnO MgO MgO CaO CaO Na20 Na20 K20 K20 p205 LO1 LOI TOTAL 643 0.49 15.8 1.85 3.2 0.08 2.92 4.87 4.54 1.05 0.10 0.55 99.75 60.5 0.53 63.2 0.51 0.51 16.0 1.38 4.9 0.09 4.63 3.53 4.18 1.15 0.12 2.4 16.5 1.85 16.0 1.73 5.4 0.09 4.36 4.42 3.47 2.18 99.41 3.1 0.07 2.15 6.03 3.53 1.49 0.11 0.85 99.39 57.8 1.45 45.4 0.36 22.2 7.34 0.8 0.12 0.88 17.7 0.88 0.30 0.08 2.0 97.52 98.06 0.11 95%epidote epidote ** 95% REFERENCES REFERENCES Kresz, D.U. D.U. and and Zayachivsky, Zayachivsky, B., B.,1989, 1989,Precambrian Precambriangeology, geology, Barbara, Barbara, Meader Meader Kresz, and Pifher Pifher Townships; Townships;Ontario Ontario Geological Geological Survey, Survey, rpt rpt270 270with withmaps maps2536-2537, 2536-2537, and 91p.p. 91 Mackasey, W.O., W.O., 1975, 1975, Geology Geology of of Dorothea, Dorothea, Sandra, Sandra, and and Irwin IrwinTownships, Townships, District District Mackasey, of Thunder Thunder Bay; Bay;Ontario OntarioDivision Divisionof of Mines, Mines,rpt rpt122 122with withmap map2294, 2294, 83 83 p. p. of F., 1997, 1997, The The Petrology Petrologyof ofgreenspar: greenspar: aaProterozoic Proterozoicporphyritic porphyriticdiabase diabase Luther, F., Luther, dike; Pifher Pifherand andIrwin IrwinTownships, Townships, Lake LakeNipigon NipigonDistrict, District,Ontario Ontario (extended (extended dike; abstract):43rd 43rdAnnual Annual Institute Instituteon onLake LakeSuperior SuperiorGeology Geologymeeting, meeting,Sudbury, Sudbury, abstract): 43. Ontario,v.v.43. Ontario, 84 �UPPER PENINSULA PENINSULA OF NEW FIELD OBSERVATIONS OBSERVATIONS OF THE CLARKSBURG VOLCANICS, UPPER MICHIGAN MICHIGAN MAKI, John C. and Bornhorst, Theodore J., J., Department of Geological Engineering and MAIU, MI 49931 4993 1 Sciences, Michigan Technological TechnologicalUniversity, University,Houghton, Houghton, MI The Clarksburg Volcanics are a member of the the Early Early Proterozoic Proterozoic Michigamme Michigamme Formation, Baraga Baraga Group, Marquette Supergroup and crop crop out near US-4 Group, Marquette Supergroup and US-41I between between Champion Champion and west Ishpeming, Ishpeming, Marquette County, County, Upper Upper Peninsula, Michigan (Cannon, (Cannon, 1974, 1975; Cannon and Klasner, Marquette Peninsula, Michigan Klasner, 1977; 1977; Simmons, 1974). 1974). The TheClarksburg ClarksburgVolcanics Volcanicsare are stratigraphically stratigraphically above above the the Greenwood Greenwood Iron-formation Iron-formation member of of the the Michigamme Michigamme Formation Formation and and are are laterally Member and below the Lower graywacke member restricted over a 19 19 km strike length and only crop out on the southern southern limb limb of of the Marquette Marquette trough. trough. These only provides provides data on on stratigraphic These rocks rocks dip dip about about 60 60 degrees degrees or more, hence the exposed section only and strike parallel variation. Despite metamorphism to the amphibolite amphibolite facies in most most exposures exposures and and fades in greenschist facies in aa few, few, primary primary textures textures are are well preserved in the outcrops. Clarksburg Volcanics Volcanics are composed mostly of volcanic rocks with lesser amounts The Clarksburg amounts of of clastic clastic sedimentary rocks and iron-formation. Field Field observations and available chemical data suggest that the volcanic rocks are dominantly basalt with less amounts amounts of of andesite. andesite. In general, general, the volcanic rocks are tuffs and agglomerates agglomerates containing containing fragments 0.5 to 1.5 1.5 cm in diameter. Near Near the thetowns townsof ofHumboldt Humboldtand and clast size is considerably larger larger than than elsewhere, elsewhere,up upto to 30 30 cm cm in in diameter. diameter. Laterally, Clarksburg, clast Laterally, the the typical clast clast size size is is considerably considerably finer finer in both the extreme eastern and western exposures with argillite argillite being common in the west Ishpeming exposures. These These observations observations suggest suggest a proximal proximal or or near near vent vent environment environment in the central central exposures exposures and a distal environment towards the east and west. Since Sincemafic mafic pyroclastic pyroclastic rocks rocks often often do do not not travel travel large large distances distances from from the volcanic volcanic vent, itit is is likely likely that that the the volcano volcano was quite near the central exposures of the Clarksburg Volcanics. central exposures Clarksburg Within the Clarksburg Volcanics there there are are several areas with Within Clarksburg Volcanics with interbedded interbedded banded banded ironironformation. The layers (I (1 to to44 mm mm Theiron-formation iron-formationisiscomposed composed of of mixed magnetitemagnetite- and detrital-bearing layers (< 1 mm thick). Metamorphism thick) and poorly defined chert layers (< Metamorphism has has produced produced quartzite quartzite textures textures in the iron-fonnation. iron-formation. As Asthe theexposures exposuresof ofiron-formation iron-formationare arein in the the proximal proximal section section of of the the Clarksburg Clarksburg Volcanics, we suggest suggest that the interbedded iron-formation has a volcanogenic volcanogenic origin. origin. Throughout the immediate immediate region, diabase sills, dikes, and other shaped bodies bodies are are common. common. Cannon (1974) (1974) suggested suggested that that the the diabase was was approximately approximately equivalent equivalentinin age age to to the Clarksburg Cannon Volcanics. The Themajor majorelement elementchemical chemical composition composition of the the diabase diabase and and Clarksburg Clarksburg Volcanics Volcanics are are large body body of of diabase diabase with with a surface similar. Just south of the town of Clarksburg Clarksburg is a relatively large exposure of approximately km. This approximately 1.5 1.5 by 1.5 1.5 krn. Thisbody body of of diabase diabasecuts cuts rocks rocks stratigraphically stratigraphicallyolder olderthan than the Clarksburg Volcanics and is spatially adjacent to to the the proximal proximal section. section. We suggest this diabase is part of the subvolcanic subvolcanic roots roots of of the the Clarksburg Clarksburg volcanic volcanic system. system. References Marquette County, County, Michigan: Michigan: U.S. U.S. Cannon, W. F., 1974, 1974, Bedrock geologic map of the Greenwood quadrangle, Marquette Geological 168. Geological Survey Survey Geologic GeologicQuadrangle QuadrangleMap MapGQ-l GQ-1168. Michigan: U.S. U.S. Cannon W. F., 1975, 1975, Bedrock Bedrock geologic geologic map of the the Republic Republic quadrangle, quadrangle, Marquette County, Michigan: Geological Survey Geological Survey Miscellaneous MiscellaneousInvestigation InvestigationSeries SeriesMap Map 1-862. 1-862. Klasner, J. S., 1977, 1977, Bedrock geologic map of the southern part of the Diorite and Champion Cannon, W. F., and Kiasner, minute quadrangles, quadrangles, Marquette Marquette County, County, Michigan: Michigan: U.S. U.S. Geological Geological Survey Survey Miscellaneous Miscellaneous Investigation Investigation Series 7-112 minute Map 1-1058. 1-1058. map of of the the Ishpeming Ishpemingquadrangle, quadrangle,Marquette MarquetteCounty, County,Michigan: Michigan: U.S. Simmons, G. C., 1974, 1974, Bedrock geological map Geological Survey Geological SurveyGeologic GeologicQuadrangle QuadrangleMap MapGQ-1 GQ- 1130. 130. 85 �ROYALE -- WHERE ARE THEY POST-GLACIAL SHORELINES OF ISLE ROYALE THEY NOW? NOW? M.E. McRae, Oak Ridge Associated Universities, Universities, Reston, VA W.F. Cannon, Cannon, U.S. Geological Survey, Reston, VA Woodruff, U.S. Geological Survey, Mounds View, MN L.G. Woodruff, Well-developed Well-developed shoreline shoreline features features at at elevations elevations higher higher than the the present day day lake lake level level are are basin. These well documented in the Lake Superior basin. These shorelines shorelines formed approximately approximately 10,000 10,000 to 4,500 years years ago as the last glacier to occupy western Lake Superior Superior receded to the northeast. Further, Further,mapping mapping of of stranded stranded shoreline shoreline features features has demonstrated demonstrated that these once flat-lying lake planes are now tilted from northeast to southwest as a consequence consequence of differential isostatic rebound. Previous Previousresearchers researchers have been able able to construct construct isobases of better-developed lake lake levels. levels. These Washbum and rebound for several of the better-developed These include include Lakes Washburn Lake Minong B.P.), Beaver Bay Minong(—9500 (-9500 B.P.), Lake Houghton Houghton(—8000 (-8000 B.P.), Beaver Bay(—9800 (-9800 - 9700 B.P.), Lake and Lake Lake Nipissing Nipissing(—5500 (-5500 -4700 - 4700 B.P.). Beach ridges are common features features on Isle Royale, particularly particularly on the lower lower elevations elevations of of the the southwest portion of the island where glacial debris debris is is widespread. widespread. A few wave-cut features features in bedrock have also been mapped in the the northeastern northeastern half half of of the the island. island. However, However, all of and discontinuous. discontinuous. Consequently, these features are scattered and Consequently, the exact exact positions of former shorelines shorelines are not precisely known from observable observable features features for most of the island. island. (GIs) software, we constructed constructed a series series of gridded gridded Using geographic geographic information information systems systems (GIS) surfaces surfaces representing representing lake level level planes from from the isobases of glacioisostatic glacioisostatic rebound described described above. Subtracting Subtractingeach eachgridded gridded surface surfacefrom from the the present-day digital digital elevation elevation model (DEM) (DEM) new DEM7s DEM's showing the island's morphology morphology at at several several of Isle Royale yields a sequence of new different different lake lake stages. stages. We then displayed the modeled shorelines shorelines with the known beach and wave-cut features. features. The The Beaver Bay do not correlate correlate with with two highest modeled shorelines, shorelines, Lake Washburn and Lake Beaver any of the mapped features. This This may may indicate indicatethat that the the island island was not yet free free of ice at these Minong shoreline. shoreline. Lengthy times. All All mapped mapped features features fall fall either either on or below the Lake Minong on the the modeled modeled Minong Minong shore. shore. Evidence sections of mapped beach ridges lie on Evidence also seems seems to shore. No features support the position of the modeled Nipissing shore. features seem seem to correlate correlate with the Houghton Houghton shoreline. shoreline. By overlaying overlaying the the modeled modeled shorelines shorelineson on the the modern modem digital digital elevation elevation model, model, itit isis possible possibleto to the post-glacial post-glacial shores shores around around the the island. island. This determine the approximate elevation of the knowledge could serve serve as a guide to future future mapping particularly if combined with information such as vegetation type, surficial surficial material, abundance of outcrop, outcrop, and and information accessibility accessibility to trails. 5,000 years ago, and possibly possibly earlier. earlier. Lastly, prehistoric mining of native copper copper began about 5,000 the prehistoric prehistoric discovery discovery and and mining mining of of We hypothesize that former lake levels influenced the copper. Copper would have been easily accessible and recognizable in wave-washed Copper would have been easily accessible and recognizable prehistoric shorelines shorelines seems seems probable. probable. Because shoreline rock exposures, so discovery along prehistoric Because wave-washed exposures exposures remain largely barren of vegetative material for for prolonged prolonged periods periods 86 �after afterthey theyare areabandoned, abandoned,they theyremained remainedfavorable favorablepoints pointsfor fordiscovery discoverylong longafter afterthe thelake lake levels levelshad hadreceded. receded.Major Majorprehistoric prehistoricworkings workingsatatthe theMinong MinongMine Mineare areininextensive extensivebedrock bedrock exposures exposuresatatororjust justabove abovethe theprojected projectedMinong Minongshore. shore.During Duringa abrief briefreconnaissance reconnaissanceinin 1997 1997we weobserved observedprehistoric prehistoricworking workinginintwo twoother otherareas areasof ofextensive extensiveexposure exposureatatthe the Minong Minongshore. shore.Did Didearly earlyinhabitants inhabitantsofofthe theLake LakeSuperior Superiorregion, region,such suchasasthe thePlano PianoIndians Indians who whoinhabited inhabitedthe theMinong Minongshore shorenear nearThunder ThunderBay, Bay,visit visitthe theisland islandand anddiscover discovercopper copper duringthe theLake LakeMinong Minongstage, stage,well wellbefore before the the generally generally accepted accepted date of first mining? Or Or during did didbarren barrenrock rockexposures exposuresfrom fromthe theformer formerMinong Minongshore shorefacilitate facilitaterecognition recognitionofofnative native copper eithercase, case,we wesuggest suggestthat thatthe theprojected projectedlocation locationof ofthe the copperthousands thousandsofofyears yearslater? later?InIneither Minong Minongand andyounger youngershorelines shorelinescan canprovide providea aguide guidetotofuture futurearchaeological archaeologicalstudies. studies. Projected Projecteddigital digitalelevation elevationmodel modelof ofIsle IsleRoyale Royaleduring duringthe thetime timeof ofLake LakeMinong. Minong. Shoreline Shorelineof ofthe the modem modemisland islandisisshown shownininwhite. white. 87 �ELECTRON MICROPROBE STUDY STUDY OF THE Pt-Pd Pt-PdAND AND RELATED RELATED MINERALIZATION IN THE Cu-Ni DEPOSIT MINERALIZATION THE MINNAMAX/BABBITT MINNAMAXIBABBITT Cu-Ni McSWIGGEN, Peter L., Minnesota Geological Survey, 2642 University Avenue, St. Paul, Minnesota 55114 551 14 The Minnamax deposit deposit is is one one of several several copper-nickel copper-nickel sulfide sulfide deposits deposits that occur occur along along the the base of the the Duluth Duluth Complex, Complex, which which is is aa series series of mafic intrusions intrusions that are part of the Keweenawan (1100 and Miller, Miller, 1993). The Keweenawan (1 100 Ma) Midcontinent Rift system (Paces and The ore ore body consists inclusions derived derived from from consists of troctolitic and ultramafic rocks, as well as hornfelsic homfelsic inclusions Paleoproterozoic metasedimentary rocks, and unconforrnably unconformably overlying Mesoproterozoic Mesoproterozoic Paleoproterozoic volcanic rocks. The Thecopper copperand andnickel nickel are are contained contained predominantly predominantly in disseminated disseminated sulfides that make rock. The make up between 11 and 5 percent of the rock. The sulfides sulfides consist largely of chalcopyrite, cubanite, pyrrhotite and pentlandite (Severson, 1991; chalcopyrite, 1991; Severson and Barnes, 1991). In In addition additionto tothe thecopper copperand andnickel, nickel,the the deposit depositalso alsocontains containssignificant significant concentrations of platinum concentrations platinum (Pt), (Pt), palladium palladium (Pd), (Pd), gold gold (Au), (Au), silver silver (Ag) (Ag)and and cobalt cobalt(Co). (Co). Sections Sections of core core ranging ranging from from 55 to 10 ft long have as much as 7.0 ppm Pd, 3.1 ppm Pt, and 13.1 13.1 ppm Au. As Aspointed pointed out outby by Severson Severson(1991), (1991), the high values for the platinum group elements other precious precious metals, and Co have been shown by others to be elements (PGE's), other associated associated with the the high-grade high-grade copper copper zones zones (Kuhns (Kuhns and and others, others, 1990). 1990). This This electron electron microprobe microprobe study study has has shown shown that that rocks rocks at at the the Minnamax Minnamax site sitecontain contain at least two platinum group minerals (PGM's) -froodite (PdBi2) and cabriite -- froodite cabriite (Pd2SnCu). (PdzSnCu). They are are commonly commonly associated associated with massive massive sulfide sulfide mineralization and are less common common in in samples samples consisting consisting mostly of silicates. silicates. Of Ofthe the14 14samples samplesinvestigated investigatedin indetail, detail,55 were were found to contain either froodite or cabriite. Although Although the number of PGM grains is small, small, just aa few few of of the the 1-2 1-2micron micron size sizegrains grains could could account account for for the the reported reported whole whole rock rock values. values. Therefore there is no need to invoke invoke PGE-sulfide phase solid solution solution to account for the Therefore there PGE whole-rock whole-rock values values reported reported from from these these rocks. Numerous found in in these these rocks. rocks. The Numerous examples examples of silver mineralization were found The silver silver is typically typically present either either as as solid solid solution solution in maucherite maucherite or as discrete discrete grains grains of native native silver. Significant Significantgold goldwas was found found in in aa few few of the native silver grains. Values Values as as high high as as 16 wt. percent Au were measured in 55 to 100 micron size grains of silver, but values were more typically typically in the 11 percent range. Cobalt Cobaltwas was found foundat at significant significantlevels levels only only in in maucherite maucherite and pentlandite. Detailed Detailedinspections inspections of of these these samples samples shows shows that they contain contain numerous altaite (Ni,As), shadlunite shadlunite[(Pb,Cd)(Fe,Cu)8S8], [(Pb,Cd)(Fe,Cu)$J, altaite numerous rare rare phases phases including includingdienerite dienerite(Ni3As), (PbTe), laurionite (PbTe), laurionite[PbCl(OH)} [PbCl(OH)]and and cotunnite cotunnite(PbCl2). (PbClJ. Kuhns, M.P., Hauck, S.A. and Barnes, R.J., 1990, Origin and occurrence of platinum group elements, copper-nickel mineral group elements, gold and silver, silver, in the South Filson Creek copper-nickel mineral deposit, Lake County, Minnesota. Natural Natural Resources Resources Research Institute, NRRUGMIN-TR-89-15,6Op. 60p. Univ. Minn., Duluth, Duluth, Tech. Tech.Report, Report,NRRI/GMIN-TR-89-15, Paces, J.B. and and Miller, Miller, J.D., J.D., Jr., Jr., 1993, 1993,Precise Precise U-Pb U-Pb ages ages of Duluth Complex and and related related mafic intrusions, intrusions, northeastern Minnesota: Geochronological insights to physical, petrogenetic, paleomagnetic, petrogenetic, paleomagnetic, and tectonomagmatic tectonomagmatic processes associated with with the the 1.1 Midcontinent Rift System. Journal Journal Geophysical Geophysical Research, Research, v. 98, no. B8, 13,997-14,013. p. 13,997-14,013. Severson, MinnamaxlBabbitt Severson, M., 1991, 1991, Geology, Geology, mineralization, mineralization, and geostatitics of the Minnarnax/Babbitt Cu-Ni Deposit (Local Boy Area), Minnesota. Natural Resources Resources Research Research Institute Institute Technical Report NRRIITR-91/13a, 96 96p. p. Severson, Severson, M.J. and Barnes, Barnes, R.J., 1991, 1991, Geology, mineralization, and geostatistics geostatistics of the the Minnamax/Babbitt MinnamaxBabbitt Cu-Ni Cu-Ni deposit deposit (Local (Local Boy Area), Area), Minnesota; Minnesota; Part II: II: Mineralization Mineralization and geostatistics. Natural Natural Resources Resources Research Research Institute Institute Technical Report NRRI/TR-91/13b, NRRIITR-9 1/13b, 216 p. 88 �POST- 1.76 Ga Ga LOW-GRADE LOW-GRADE METAMORPHISM METAMORPHISM OF THE BARABOO QUARTZITE POST-1.76 G., Jr., J., Dept. of Geology Geology & & Geophysics, MEDARIS, MEDA1US,L. L.G., Jr., BROWN, BROWN, P.B. P.B. and BUNGE, BUNGE, R R.J., Wisconsin-Madison,Madison, Madison,WI WI53706; 53706;medaris@geology.wisc.edu medarisgeology.wisc.edu Univ. of Wisconsin-Madison, It has long long been been recognized recognized that the the Baraboo Baraboo Quartzite Quartzite is is folded folded and and metamorphosed, metamorphosed, and recent U-Pb dating of of detrital detrital zircons zircons in the quartzite quartzite requires requires that deformation deformation and and recrystalrecrystallization were were post-1.76 Ga events. Although Although the the structure structureof ofthe theBaraboo Baraboosyncline syncline has been well studied, little attention has been devoted to to metamorphism metamorphism in the Baraboo Baraboo Range, Range, other other identifyingpyrophyllite pyrophyllite in inmetap metapelite and recognizing recognizing that that metamorphism metamorphism was was than identii'ing elite layers and low-grade. low-grade. We Wehave haveundertaken undertakenaapetrologic petrologicinvestigation investigation of of the theBaraboo BarabooQuartzite Quartziteand and post-1.76Ga Gametamorphism. metamorphism. underlying granite and rhyolite to to determine the the conditions of ofpost-1.76 Rock types, chemical compositions, well-developed paleosol paleosol compositions. and and mineral assemblages AAwell-developed occurs in granite and rhyolite at the unconformable base of the Baraboo Quartzite. Intense rhyolite the unconformable the Baraboo Quartzite. Intense chemical weathering and sedimentary chemical weathering sedimentary processes processes Table I1 produce marked geochemical combined to a combined a geochemical Chemical Compositions, Compositions, Baraboo Baraboo Rocks Rocks differentiation among basement basement rocks, paleosol, and avg avg avg pelitic layers in the Baraboo 1). pelitic layers Baraboo Quartzite Quartzite (Table (Table 1). rhyolite granite granite saproliie soil pelite saprolite soil pelite (1) (2) (10) (5) (1) Ca, (1) (2) (10) ( 5) (1 Ca, Na, Na, and and KKwere wereleached leached from from granite granite and and wt% wt% rhyolite, and and A1 Al and Fe were were concentrated concentrated in the the Si02 rhyolite, 5102 70.51 59.70 70.51 56.41 56.41 59.70 72.80 69.25 69.25 72.80 0.26 0.25 0.36 0.87 Ti02 0.26 0.25 0.36 paleosol (saprolite and soil) 0.87 1.03 1.a3 soil) and and pelitic layers in the Ti02 1AJ203 13.30 15.15 17.18 20.89 25.40 A1203 13.30 15.15 17.18 20.89 25.40 sedimentary section, which which consisted of sedimentary section, consisted originally originally of Fe203 2.99 3.46 2.82 10.18 8.04 2.99 3.46 2.82 10.18 8.04 kaolinite and variable amounts of silt-size quartz kaolinite and variable amounts salt-size quartz Fe203 MnO 0.07 0.09 0.00 0.00 0.00 0.01 MnO 0.07 0.09 0.00 0.01 elites MgO aluminousofofthree threeanalysed analysedppelites grains. The Themost mostaluminous MgO 0.14 0.89 0.07 0.38 0.38 0.04 0.14 0.89 0.07 0.04 is listed listed in Table Table 1, 1, and and two two other othersamples samples are are CaO 1.16 1.55 0.14 0.09 0.09 0.03 1.16 1.56 0.14 0.03 CaO 4.93 4.40 0.34 0.32 0.32 0.04 0.34 0.04 Na20 4.93 4.40 equally lowinin Ca, Ca, Na, Na, and meta- Na20 equally low and K During meta3.17 3.14 4.75 5.72 0.21 4.75 5.72 0.21 K20 K20 3.17 3.14 morphism, K K was was reintroduced re-introduced into the paleosol by P205 0.03 0.09 morphism, 0.13 0.10 0.10 0.12 P205 0.03 0.09 0.13 0.12 1120-tich fluids that were H,0-rich fluids that were channeled channeled along along the the LLOl 1.25 1.53 2.51 3.65 4.70 4.70 O1 1.25 1.53 2.51 3.65 sub-B araboo unconformity; unconformity; this K-metasomatism sub-Baraboo K-metasomatism SSum 100.10 99.78 98.80 98.62 um 100.10 99.78 98.80 98.62 99.32 99.32 was orethindiasp diasporewas accompanied accompaniedby by formation formationof ofthin quartz pyrophyffite-white pyrophyllite-white mica veins in basal quartzites. quartzites. The compositions compositions of Baraboo rocks are projected projected into in which which the the positions positions into the the system, system, KASH, KASH, in in Fig. Fig. 1,1,in microcline of selected selected minerals minerals are also also plotted. plotted. The KASH KASH equilibrium metamorphic metamorphic assemblages assemblagesfor for these these rocks equilibrium + quartz + microcline are: granite and and rhyolite, microcline + are: granite rhyolite, quartz muscovite; metapelite, muscovite; paleosol, paleosol, quartz ++ muscovite; metapelite, hydrothermal veins, veins quartz pyrophyllite; and hydrothermal quartz ++ pyrophyllite; + diaspore. diaspore. Kaolinite is pyrophyllite Kaolhute pyrophyllite ++ muscovite + present in elite and hydrothermal in metap metapelite hydrothermal veins, but only only as a retrograde retrograde product. Inhiaddition additiontotoKASH KASHminerals, minerals, granite and rhyolite contain albite, epidote, and chlorite. chlorite. Hematite isis abundant abundant in these these rocks, rocks, especially especially in Hematite K20 A1203 diaspore paleosol common in in paleosol and metapelite, metapelite, and and rutile rutile is common Fig. 1 Compositions Compositions of of Baraboo Baraboo metapelite. metapelite. plane rocks projected projected into into the KAS plane system, KASH KASH of the system, 89 �Metamorphic Metamorphic conditions conditions Mineral reactions and and chemographic chemographic relations relations in m the thesystem, system, KASH for unit unit activity activity of of H20), KO), are aresummarized summarized in in Figure Figure 1.1. Minimum KASH (calculated (calculated for temperatures temperatures of of metamorphism metamorphism are are defined defined by by the themetapelite metapeliteassemblage, assemblage, quartz ++ pyrophyllite, pyrophyllite, which is stable stable above above the reaction, reaction, Kin K h ++ Qtz Qtz == Pr! Prl ++ V, V, and and by the the absence absence of stable kaolinite from all observed T,, assemblages, indicating that temperatures were T- assemblages, indicating that temperatures were above above stable fiom the kaolinite. Maximum the stability stability limit of kaohite. Maximum temperatures temperatures are are defined defined by the the absence absence of of kyanite kyanite ore, in from pyrophyllite ++ diasp diaspore, in hydrothermal hydrothermal veins, vems, which which isis fiom the the assemblage, assemblage, muscovite muscovite ++ pyrophyffite stable below the reaction, Pr! Prl ++ Dsp Dsp == Ky Ky ++ V, V, and and by by the theabsence absencekyanite lqmniteininmetapelite, metapelite, which limitof of pyrophyllite. pyrophyllite. which indicates mdicates that temperatures temperaturesdid didnot not exceed exceedthe thestability stabilitylimit Fluid were analysed from quartz m in a folded vein in inmetapelite. metapelite. The The a d inclusions inclusions were analysed fiom folded quartz quartz vein quartz contains contains aa single single population population of ofabundant abundantaqueous aqueousinclusions mclusions that thathave havefinal finalmelting meltmg points corresponding points of-15 of -15toto-18°C, -lS°C corresponding to to 18 18to to20 20equivalent equivalent wt% wt% NaCL NaCL First Firstmelting meltmg below below -35OC divalent cations. H o m o g ~ t i o temperatures ntemperatureslie lie between between -3 5°C suggests suggests the the presence presence of divalent cations. Homogenization 165 the 175-185°C 175-lW°range. range. 165 and 215°C 215OC with a peak in the Intersection of with the thelimiting limiting mineral mineral reactions reactions constrains constrains of the the fluid fluid inclusion mclusion isochore isochore with temperature-pressure conditions conditions for for the the Baraboo BarabooQuartzite Quartzitetotolieliebetween between—320°C, -320°C2.7 kbar kbar and —385°C, 4.0kbar. kbar. Such values correspond correspond to aa thermal -385OC, 4.0 thermal gradient gradient of of25-30°C/km. 25-30°C/kmwhich is is typical terranes and is notably notably elevated elevated over that for typical for Barrovian-type Barrovian-type metamorphic terranes for stable stable cratons cratons(--17°C/km (-17OCh at comparable comparable depths). depths). 8 6 4 2 0 250 300 350 T,°C 400 450 Fig. Fig. 2 Mineral Mineralassemblages assemblagesand andreactions reactionsin inthe the system, systeml KASH, KASH, aa fluid fluid inclusion inclusionisochore isochorefor for quartz quartzin inaa folded foldedquartz quattzvein, vein, and and metamorphic metamorphicconditions conditions for for the the Baraboo Barabm Quartzite Quarfziie Conclusions Folding and metamorphism metamorphism of the Baraboo Baraboo Quartzite Quartzite mark mark an animportant important Conclusions the age of this post-l.76 post- 1-76Ga Gatectonothermal tectonothermal event event in in the theLake LakeSuperior Superiorregion. region. Although Although the event remains remains uncertain, uncertain, itit could could be be related related to an an eastern eastern extension extension of the the Mazatzal Mazatzal belt at at -1.63 Ga. However, However,andalusite-bearing andahsite-bearingassemblages assemblagesin the the Waterloo Waterloo Quartzite Quartzite are are probably probably -1.63 Ga. due due to to contact contactmetamorphism metamorphismassociated associatedwith withWolf WolfRiver Rivermagmatism magmatismatat—4.43 -1.43 Ga. 90 �SVANBERGITE SVANBERGITEiN INTHE THEBARABOO BAMBOOQUARTZITE: QUARTZITE:SIGNIFICANCE SIGNIFICANCEFOR FORDIAGENETIC DIAGENETIC PROCESSES ANDPHOSPHORUS PHOSPHORUSFLUX FLUXIN INPRECAMBRIAN PRJXAMBRIANOCEANS OCEANS PROCESSESAND MEDARIS, Geophysics, MEDARIS,L.G., L.G.?Jr. Jr.and andFOURNELLE, FOURNELLE?J.H., J.H.,Department DepartmentofofGeology Geology&&Geophysics, University Madison,WWI 53706;medaris@geology.wisc.edu medarisgeology.wisc.edu I 53706; University of of Wisconsin-Madison, Wisconsin-Madison, Madison? Authigenic Authigenicminerals mineralsof ofthe thebeudantite beudantiteand andcrandallite crandallitegroups groupshave haverecently recentlybeen beenrecognized recognizedasas widespread widespreadconstituents constituentsofofArchean ArcheantotoCretaceous Cretaceoussedimentary sedimentaryrocks rocks(Rasmussen, (Rasmussen71996). 1996). Although Althoughthe theabundance abundanceofofthese thesealuininophosphate-sulfate alumhophosphate-dte minerals minerals is is small, small, on the the order order of of wt%ininaagiven givensample, sample7they theyappear appearto tobe bemore moreabundant abundantthan thanauthigenic authigenicapatite apatite 0.2to to0.01 0.01wt% 0.2 and phosphorusbalance balanceininthe theoceans. oceans. andplay playan animportant importantrole roleininphosphorus The general formula i%e beudantite beudantite and andcrandaiite cranddlitemineral mineralgroups w m pThe The general formulafor forthe thebeudantite beudantite groupisisAB3(XO4XSO4XOH)6, AB3(X04)(S04)(OH)6, whereAA ==Ba, Ba7Ca, Ca7Ce, Ce7Pb, Pb7Sr; Sr;BB==Al, Al,Few; Few;and andXX==As, As7P; P; group where and where AB3(X04)2(O13,F)547 whereAA ==Ba, Ba7Bi, Bi,Ca, Ca7Ce, Ce,La, La7Nd, Nd,Sr, Sr7Th; Th; andfor forthe thecrandallite crandallitegroup groupisisAB3(XO4)2(OH,F)5, P, Si. Electron microprobe analysis of Baraboo svanbergite B = Al, Fe3+; and X = As7 P7 S i Electron microprobe analysis of Baraboo svanbergite Fe3+; and X = As, B = Al, demonstrates which demonstratesthat thatits itscomposition compositioncan canbe beportrayed portrayedby byaadistorted distortedpyramid pyramid(Fig. (Fig.1)1)ininwhich members of the beudantite group lie along the front edge of the pyramid and members members of the beudantite group lie along the fiont edge of the pyramid and membersof ofthe the crandallite crandallite group group lie lie in m the the fir fiuface face(note (notethat thatCa Caand andBa Bahave havebeen beencombined combmedfor forprojection projection purposes). purposes). florencite florencite CeAJ3(P04)2(OH)6 ce%p04)2(0H) Crandallite group SrsAJ3Q'Q4 )2(H)5 . HO goyazite (Ca,Ba)A13(P04)(S04)(OH)6 SrAI3(P04)(S04 )(OH)6 svanbergite Fig. Fig. IICompositional Compositionalspace spaceofofBeudantite Beudantiteand andCrandallite Crandallite groupminerals mineralsfrom fromBaraboo Baraboometapelite metapelite group svanbergite Small Occurrenceand andchemical chemical composition compmition of o fBaraboo svanberdte S d amounts amounts of Occurrence svanbergite elite layers layers m in the the Baraboo Baraboo Quartzite7 Quartzite, where where it is inmetap metapelite is svanbergite are are widespread widespreadin associated with pyrophyllite, quartz, hematite, and rutile. Svanbergite grains are equant associated with pyrophylbe7 hematite7 Svanbergite grains are equantand and have smallsize, size7are arereadily readilyvisible vkiile have diameters diametersof of10-20 10-20microns, microns7but butdespite despitetheir theirsmall microscopically microscopically because because of of their their marked marked difference &erence in relief compared to associated associated pyrophyllite. <1 pyrophyllite. Baraboo Baraboosvanbergite svanbergitecontains contains<2 < 2wt% wt%Fe203, Fe203? 4wt% wt%As205, 4 O s 7and andhas hasnegligible negligiile amounts ofAAl3(PO4XSO4XOH)6 solidsolution solution of ALUJPO~)(SO~)(OH)~ amountsof ofBi, Bi,Pb, Pb7Th, Th7Si, Si,and andF;F;itsitscomposition compositionisisa asolid Ca + Ba + REE, which lies near the base of the + Ba + W E 7 which lies near the base of the Ca Sr> where A = AAl&P04)2(OH)5*q07 where A = Sr AAI3(P04)2(OH)5'H20, pyramid in m Fig. 11 between between svanbergite svanbergiteand and goyazite goyazite(Fig. (Fig. 2). 2). - 91 �"crandallite" "crandallite" goyazite goyazite Ba Ba REE REE 0 0 + 0 U) 0 U) Ca Ca 0 0.2 0.4 0.6 0.8 Ba Sr - Fig. 33 Proportions Promtions of of cations cations in in the A-site of Baraboo Bamboo svanbergite-goyazite svanbergite-goyazite 1.0 "woodhouseite" "woodhouseite"Sr/(Sr+Ca+Ba) Srl(Sr+Ca+Ba) svanbergite svanbergite Fig. 2 Projection Projectionof ofmineral mineralcompositions compsitions onto onto the the base of the Beudantite-Crandallite Beudantite-Crandallite prism prism Baraboo svanbergite The The Baraboo svanbergite solid solid solution solution ranges ranges l m m from fiom almost pure svanbergite (beudantite group) to —70% goyazite(crandallite (crandallite group) group) with with a concomitant concomitant -70% goyazite in Sr/(Sr+Ca+Ba) (Fig. 2), decrease m 2)?and and A-site 10000 is dominated dominatedby by Sr Sr(Fig. (Fig.3). 3). Many occupancy is grains are svanbergite grains svanbergite are strongly strongly zoned, zoned, with with rims rimsbeing bemg in crandallite component7 component, m in which whichthe the sum sum of of enriched in reaches 5.3 wt %, La203, Ce203, %? L%O3? Ce203?and and Nd203 N&03reaches amounting to --27% -27% occupancy occupancy of of the the A-site (Fig. (Fig. 3). 3). present, they Although other RE RE elements elements may be present? Nd can't readily re&ly be determined by electron microprobe Fig. 44 Representative Representativechondrite-normalized chondritenormalized techmques due to their techniques their low low abundances abundances and spectral spectral La, contentsof of Baraboo Baraboo La, Ce, and Nd contents svanbergite-goyazite interferences. mterferences. The light REE contents contents in m svanbergite svanbergite 5000 toto40,000 times are 407000 timesgreater greaterthan thanin mchondrites, chondrites7 are -5000 Nd by by aa fhctor factor of 2 to to 3 (Fig. 4). There nomarked markedCe Ceanomaly, anomaly7 and La is enriched over Nd Thereisisno with respect respect to to La and Nd. although two samples samples may have a slight Ce enrichment with Geological simificance significance of svanbergite Precipitation Geolokcal ofsvanberkte Precipitation of ofauthigenic authigenic aluminophosphatesulfate minerals has been ascribed to bacterial decomposition of P-beakg P-bearing organic matter m in an d t e minerals has been ascriied to bacterial decomposition of within the zone of d sulfate methanogenesis ithin the a t e reduction and methanogenesis Al-rich environment, such as shale, shale?w (Rasmussen, so7the theoccurrence occurrenceof ofsvanbergite-goyazite svanbergite-goyazite in m Baraboo metapelites (Rasmussen71996). IfIfso, that such a process process may have been operative m mid-Proterozoic mid-Proterozoictime. time. signifies that operativein The widespread occurrence occurrence of of authigenic authigenic aluniinophosphate-sulfate alumhophosphate-dte minerals represents a unrecognized repository for phosphorus m in the the oceans. Rasmussen previously unrecognized Rasmussen has has estimated estimated that fluxof of7.6 7.6xx1010 10" moles that aluminophosphate alumhophosphateprecipitation precipitation accounts accountsfor for aaphosphorus phosphorusburial burialflux yr, which is comparable to that resulting from authigenic phosphates and P-bearing which is comparable to that resultmg fiom authigenic phosphates and (2.2-9.1 xx 10" moles yfl). yf'). However, However7Rasmussen Rasmussenused used the the composition composition of carbonates (2.2-9.1 florencite florencite (2 (2 moles of of P phi) pfb) in m his his calculation, calculation, and using the compositional compositional range range of of Baraboo Baraboo svanbergite-goyazite (1.0(1.0-1.7 estimate by 15 15 to 50%. 50%. 1.7 moles moles of of P pfli) pfi) reduces reduces the the burial flux estimate Regardless, factor in m oceanic oceanic P-flux. P-flux. Regardless?aluniinophosphate-sulfate aluminophosphate-dte minerals are an important fhctor REFERENCE CITED 296?601-632. 601-632. Rasmussen, B. (1996) (1996) American Journal Journal of of Science, Science?v. 296, . 92 �HIGH-RESOLUTION AEROMAGNETIC DATA DATA FOR USE OF HIGH-RESOLUTION FOR REGIONAL REGIONAL (WHERE'S THE GEOLOGY INVESTIGATIONS, INVESTIGATIONS, SOUThEASTERN SOUTHEASTERN WISCONSIN (WHERE'S KIMBERLITE!) MIJDREY, M.G., Jr., Jr.?Wisconsin WisconsinGeological Geologicaland and Natural NaturalHistory History Survey, Survey73817 3817Mineral Mineral MUDREY, Road, Madison, Point Road? Madison?WU 53705-5 100, 100, mgmudreyfacstaff.wisc.edu mgmudrey@facstaff.wisc.edu Between 1876 Between 1876 and and 1913, 1913, diamonds diamonds were found in at least seven localities in southern and central Wisconsin. All All were were found found in in Pleistocene Pleistocene gravel gravel deposits depositsor orHolocene Holoceneriver rivergravel. gravel. The bedrock unknown, but was presumed to be in bedrock kimberlite source for these diamonds is unknown7 northern northern Canada, Canada?the the only only area area north north of ofWisconsin Wisconsin previously previously known known to to contain containkimberlite. kimberlite. County, Michigan? Michigan, Cannon and With the discovery discovery of the Lake Lake Ellen kimberlite in Iron County? Mudrey (1981) (1981) suggested suggestedthe thedrift driftdiamonds diamondsin in Wisconsin Wisconsin may have come from a more more local source. source. Carison southeastern Wisconsin about 280 m Carlson and Adams (1997) described a kimberlite in southeastern across. The Thepreliminary preliminaryidentification identificationwas wasbased basedon ondrilling drillingsmall, small,highly highly magnetic magnetic anomaly anomaly identified from a little known aeromagnetic survey from the 1980s 1980s (800-meter (800-meter flight-line flight-line have aeromagnetic spacing). Only recently have aeromagneticsurveys surveysbeen been sufficiently sufficiently detailed to determine determine the presence of absence of kimberlite kimberlitein insoutheastern southeasternWisconsin. Wisconsin. Prior Prior to to the the most most recent survey? strong, small small survey, limited flight-line spacing of 110-krn 0-km precluded precluded the the identification identification of strong, magnetic bodies at the the shallow shallow bedrock bedrock surface. surface. Analysis of the aeromagnetic aeromagnetic survey survey indicates indicates that that flight-line flight-line spacing less than 800 800 m m will be ineffective in the identification identification of of small, small?highly magnetic kimberlite at at the the bedrock/surficial bedrock/surficial material material surface surface in southeastern southeasternWisconsin. Wisconsin. The identification identification of the kimberlite, and analysis of available aeromagnetic maps, indicate that other other kimberlitic kimberlitic bodies may occur occur in southeastern southeasternWisconsin and possibly northeastern Illinois and may be the source source for for the the diamond diamond discoveries discoveriesin in Wisconsin Wisconsin and and Illinois. Illinois. However, urbanization However? urbanization in in the the Milwaukee-Chicago Mlwaukee-Chicago corridor corridor may may discourage discourage further hrther geologic geologic and geophysical analysis and competing competing land use may make further hrther exploration explorationand and ultimate ultimate development difficult. development difficult. REFERENCES: REFERENCES: Cannon, W.F., and Mudrey, Cannon? Mudrey, M.G., M.G., Jr., Jr., 1981, 1981,The Thepotential potentialfor fordiamond-bearing diamond-bearingkimberlite kimberlite in northern Michigan and Wisconsin: U.S. Geological Survey Circular 842, 15 15 p. p. Carison, S.M. Carlson, S.M. and Adams, G.W., G.W., 1997, 1997,The Thediamondiferous diamondiferousSix-Pak Six-Pak Ultramafic Ultramafic U.S.A. (abs.): Institute Institute on Lake Superior Lamprophyre Diatreme, Kenosha, Wisconsin, U.S.A. Superior Geology, Proceedings, Part 1-Program Geology?Proceedings? 1-Programand and Abstracts, Abstracts?v. v. 43, 43, p. p. 11 11 Wisconsin, kimberlite, Wisconsin, kimberlite, aeromagnetic data 93 �_____________ AS INDICATORS INDICATORS OF OF INTRUSION INTRUSIONMECHANISMS MECHANISMS XENOLITHOLOGIES AS IN THE THEWAUSAU WAUSAU SYENITE SYENITE COMPLEX, COMPLEX, WISCONSIN WISCONSIN MYERS?Paul E., Geology Department, Department?University of Wisconsin, Eau 54701 MYERS, Eau Claire? Claire, WI WI 54701 area west of Wausauy (2) Wausau, Wausau, In the area Wausau, four alkaline subvolcanic subvolcanicplutons: plutons: (1) Stettin, (2) (Figure)?were intruded in a southeastward southeastward sequence sequence (3) Rib Mountain, and (4) Ninemile (Figure), into Lower Lower Proterozoic Proterozoic metavolcanic, metavolcanic, metasedimentary, and and granitic granitic intrusive intrusive rocks rocks into (Myers and others, 1984). 1984). Whereas Whereas the first three syenite plutons are concentrically concentrically (Myers plutons are zoned and and pipelike in structure, structure, the theyoungest, youngest,Ninemile Ninemilepluton, pluton, although althoughpossessing possessing an an zoned stopedits itsway wayto toaashallow shallowdepth depthunder underthe the aplitic core core rim, rim, isisaastock-like stock-likebody bodywhich whichstoped aplitic volcanoes. This paper shows the connection between magma now-eroded volcanoes. between xenolithologiesy xenolithologies, magma emplacement, emplacement?and andwalirock wallrockalteration. alteration. EXPLANATION EXPLANATION qp qp quartz quartzmonzonite monzoniteporphyry porphyry ng ng Ninemile Ninemile granite granite ga ga granite granite aplite aplite sy sy syenites syenites vv volcanic rocks rocks qq quartzite quartzite bs bs biotite biotiteschist schist fault fault 4 ', , -1 contact, infened contact, dashed where inferred 1 1 0 Scale: Scale:1 - 1 -, 1 2 . Zmiles mles 012-3. 7 -? kilometers kilometers Figure --- Map Map of of Wausau Wausau syenite syeniteplutons plutons The The Stettin Stettin pluton, plutony(Figure) (Figure) which is the oldest, oldest, most alkaline alkaline intrusion of the Wausau of 8.8 and Wausau syenite syenite complex, 8.8 xx 6.4 6.4 km. by and has has aa complexyis oval in plan with dimensions of concordant concordant NE NE elongation. elongation. Its Its outer outer wall zone zone consists of strongly strongly syenitized volcanic volcanic rocks rocks and and syenite syeniteaplite, aplite,which which grade grade inward inward into into concentric, concentricysheet-like sheet-like masses masses of gneissic gneissic nepheline nepheline and and tabular tabular syenites. syenites. The The indistinctly indistinctly bounded bounded intermediate intermediate zone zone consists consists mainly mainly of of flow-lineated flow-lineatedamphibole amphibolesyenite, syenite,and andthe the cylindrical cylindricalcore, core?with with aa diameter diameterof of 22 km kmhas hasaa rim rim of of lineated heated nepheline nephelinesyenite syeniteand and an an inner inner core core of coarse, coarse, pyroxene pyroxene syenite syenite identical identical with Stettin pluton is is separated separated from from the the syenite syenite bodies bodies to to with that that in in the the intermediate intermediatezone. zone. The The Stettin the the SE SE by by aa fault. fault. In the the Wausau Wausau pluton, pluton, coarse, coarse?massive massive pyroxene pyroxene and and amphibole amphibole syenites syenitesform form aa partial partial outer outer rim rim on on the the north and and east sides. AAbroad broad intermediate intermediatezone zone of of lensoidal lensoidal syenite syenite and and quartz quartz syenite syenite are are crowded crowded with lenticular lenticular xenoliths xenoliths of mica schist, schist?sillimanite-bearing sillimanite-bearing 94 �syenitized volcanic rocks. rocks.Xenolith Xenolith volume volume locally locally exceeds exceeds the the volume volume of of quartzite, and syenitized the the quartz quartzsyenite. syenite.As Asthe thecontact contactbetween betweenthe theWausau Wausauand andRib RibMountain Mountainplutons plutonsisis obscured by by aa broad strip strip of of Rib Rib River alluvium, its itscore coremay maynot notbe bevisible visiblein in outcrop. outcrop. obscured The Rib Rib Mountain Mountainpluton pluton produces produces aa crescentic crescenticmap map pattern pattern with with an an opening openingto to the the The south The most most striking strikingfeature feature of of this this south where where itit is engulfed engulfed by younger Ninemile granite. The concentric concentric pluton is the 8km 8km ring of very large, lenticular lenticularquartzite quartzite and and mica mica schist schist xenoliths xenoliths embedded embedded in in foliated foliated quartz quartz syenites syenites in in its its intermediate intermediate zone. zone. The The Ninemile NinemilePluton, Pluton,isisan anelliptical, elliptical, stock-like stock-likebody body which whichwas was intruded intrudedat at 1500 1500 Ma. (Van (Van Schmus Schmusand and Bickford, Bickford, 1981) 1981) into into the core core and south rim of the Wausau Wausau pluton. pluton. According According to to Anderson Anderson (1983) (1983)itit isis comagmatic comagmaticwith with rapakivi rapakivi granites granitesof of the the Wolf Wolf River River batholith. Although Althoughclassed classed as as aa granite, the theNinemile Ninemile pluton is mainly mainlycoarse coarsebiotitebiotiteamphibole amphibolemonzonite monzonitecontaining containingup up to to 30 30percent percent strained, strained,polycrystalline polycrystallinequartzite quartzitegrains grains usually accompanied accompanied by occasional occasional mica schist and quartzite xenoliths. A A crescentic crescentic mass mass of of granite graniteaplite aplite defines defines the the core corerim rim of of the the larger larger southern southernlobe lobe of of the the Ninemile Ninemilepluton. pluton. The The Wausau Wausau plutons plutons thus thus show show aa distinctive distinctive magmatic magmatic differentiation differentiationsequence, sequence, beginning Stettin pluton and and ending ending with with pegmatite pegmatite dikes dikesin in beginning with with nepheline nephelinesyenite syenitein in the the Stettin the the Ninemile Ninemile pluton. pluton. As As these these dikes dikes are are shallow-dipping shallow-dippingand and contain contain miarolitic miarolitic cavities cavitieswith with autoclasts autoclasts of of early early crystalline crystallinephases, phases,Falster Falster (1985) (1985)concluded concludedthat thatthey theycrystallized crystallized shallow shallow enough enough for for hydrothermal boiling. Several small small porphyritic quartz monzonite monzonite plugs plugs cut cut post-syenite post-syenite faults faults outside outside the Wausau complex complex and probably represent the "last gasp" Ninemile magma. magma. gasp" of of the the differentiated differentiatedNinemile Xenoliths Xenoliths in in the the Wausau, Wausau, Rib Rib Mountain Mountain and and Ninemile plutons are unassimilated wallrock wallrock fragments fragmentscarried carriedup up and and down down in in the the magmas magmas during during intrusion. intrusion. Where Wherethere therehas has been been considerable considerablevertical vertical transport, as asfor forinstance instancealong along caldera calderawalls, walls, xenoliths xenoliths show show great great diversity diversity of of protolithology protolithologyand and metamorphic metamorphicgrade. grade. They They include include upper upper amphiboliteamphibolitegrade, amphibolite, metadiorite, andesitic grade, sillimanite-bearing sillimanite-bearing quartzite, quartzite, mica schist, and arnphibolite, andesitic metavolcanics metavolcanicsand andmetasediments, metasediments,and andeven evencognate cognateinclusions inclusionsof ofearlier-phase earlier-phasesyenites. syenites. Their Their shapes shapes are are most most commonly commonly lenticular lenticular and and sheet-like with a preferential orientation orientation parallel Stettinpluton. pluton. parallel to to cylindrical cylindricalwalls. walls. Xenoliths Xenolithsare areuncommon uncommonininthe theStettin In In the the Wisonsin Wisonsin River River channel channel east east of of the the power power dam dam at at Wausau, Wausau, the the relationships relationships between betweenxenoliths xenolithsand andflow flowstructures structuresin in enclosing enclosing felsic felsic and and mafic mafic quartz quartz syenites syenites are are well displayed. with swirled swirledlineation lineation,, give displayed. Biotite amphibolite xenoliths with give way way southward southward in the outcrop outcrop to to sheet-like sheet-like masses masses of of felsic felsic and and mafic mafic rocks. The folded and fragmented mafic mafic xenoliths advanedmetasomatic metasomaticreplacement replacementand anddeformation. deformation. xenolithsare arebiotite-rimmed, biotite-rimmed, showing showingadvaned The Theenclosing enclosingamphibole amphibolequartz quartzsyenite syenitehas has highly highly discordant discordant flow flow lineation lineation with with swirls swirls and and eddies eddiessuggesting suggestingconsiderable considerableviscosity viscosity and andturbulence. turbulence. Late-stage Late-stagesyenite syenitepegmatite pegmatite veins veins with with quartz quartzcores corespobably pobably represent representresidual residual liquid liquid segregations segregations along along incipient incipient thermal thermalcontraction contractionfractures fracturesininquartz quartzsyenites syenitescontaining containingincompletely incompletelyassimilated assimilatedquartzite quartzite xenoliths. xenoliths.The Theoccurrence occurrenceof of several severalsmall smallxenoliths xenoliths of of unaltered, unaltered, porphyritic porphyritic trachyte trachyte in in the the north northend endof ofthe theoutcrop outcropsuggest suggestaadownward downwardmovement movementof ofsome someof ofthe theclasts clastsfrom fromthe the overlying overlyingvolcanic volcanicpile. pile.As Aselsewhere elsewherein inthe theWausau Wausausyenite syenitecomplex, complex,the theconsiderable considerable concentric concentricheterogeneity heterogeneityof ofxenoliths xenolithssuggests suggestsconsiderable considerablevertical, vertical,somewhat somewhatlaminar laminar transport transportof of xenoliths xenolithsalong alongcaldera calderawalls walls as as aa consequence consequence changes changes in the level of magma magma in in the the conduit. conduit. Future Future field fieldstudies studiesshould shouldinclude include detailed, detailed, comprehensive comprehensive mapping mapping of of xenolithologies. xenolithologies . References: References: Anderson, Anderson,J.J.L., L.,1983, 1983,Proterozoic Proterozoicanorogenic anorogenicgranite graniteplutonism plutonismofofNorth NorthAmerica, America,Geological Geological Society Society of of America AmericaMemoir Memoir161, 161, P.p.133-154. 133-154. Myers, Myers,P.E., P.E.,Sood, Sood,M. M. K., K.,Berlin, Berlin,L.A., L.A.,and andFaister, Falster,A.A.U., U.,1984, 1984,The TheWausau Wausausyenite syenitecomplex, complex, central central Wisconsin, Wisconsin,30th 30thAnnual AnnualInstitute Instituteon onLake LakeSuperior SuperiorGeology, Geology,58 58 pages. pages. Van VanSchmus, Schmus,W.R., W.R.,Medaris, P.O.,1975, 1975,Chronology Chronology of of Precambrian Precambrian rocks rocksin in Medaris,L.G., L.G.,and andBanks, Banks,P.O., Wisconsin, TheWolf WolfRiver Riverbatholith, batholith,aarapakivi rapakivimassif massifapproximately approximately1500 1500m.y. m.y.old, old,Geological Geological Wisconsin,I:I:The Society 14. Societyof of America, America,Bulletin, Bulletin,v.v.86, 86,p.p.907-9 907-914. 95 �COMPOSITION COMPOSITIONAND AND SOURCE(S) SOURCE(S)OF O FMIDCONTINENT MIDCONTINENTRIFT RIFTLAVAS LAVAS (CHENGWATANA VOLCANICS) NEAR NEAR CLAM FALLS, WISCONSIN (CHENGWATANA VOLCANICS) NAIMAN, NAIMAN,Zachary ZacharyJ., J.,Department Departmentof ofGeosciences, Geosciences,University Universityof ofArizona, Arizona,Tucson, Tucson,AriArizona, WIRTH, Karl znaiman@geo.arizona.edu; WIRTH, Karl R., R., Geology Geology Department, Department, zona, 85716, 85716, znaiman@geo.arizona.edu; MacalesterCollege, College,St. St.Paul, Paul, Minnesota, Minnesota, 55105, 55 105,wirth@macalester.edu. wirth@macalester.edu. Macalester Most well-exposed volcanics Moststudies studiesof of the the1.1 1.1Ga Ga Midcontinent Midcontinentrift rift (MCR) (MCR) have have focused focused on the well-exposed volcanics in the Lake Superior region from the central part of the rift. Studies of the Chengwatana Volcanics in the Lake Superior region from the central part of the rift. Studies of the Chengwatana Volcanics (CV), (CV),the the southernmost southernmostexposed exposedvolcanics volcanicsof of the the MCR, MCR, provide provide information informationabout about variations variationsin in rift rift processes processesalong alongthe theaxis axisof of the the MCR. MCR. Here Herewe wereport reportmajor majorelement, element,trace traceelement, element,and andNd Nd isotopic isotopicdata datafrom frommafic maficand andfelsic felsicCV CVflows flowsnear near Clam Clam Falls, Falls, Wisconsin. Wisconsin. Approximately Approximately3000 3000 meters of mafic volcanics, minor interfiow sediment, and rare rhyolite are exposed in the Clam meters of mafic volcanics, minor interflow sediment, and rare rhyolite are exposed in the Clam Falls Falls area. area.The Thepetrography petrographyand andgeochemistry geochemistryof ofthe therhyolite rhyoliteare arediscussed discussedby by Abbott Abbott et et al. al. (this (this volume). volume). New, New,precise preciseU-Pb U-Pbzircon zirconages ages(1,102±5 (1,102±Ma) Ma)of of rhyolite rhyolitenear near Clam ClamFalls Falls (Wirth (Wirth and and Gehrels; Gehrels; this this volume) volume) provide provide aa means means for forcorrelation correlation of of the theChengwatana ChengwatanaVolcanics Volcanics from from Clam Clam Falls Falls and and Taylors Taylors Falls Falls with volcanics from other parts of the rift. Recent Recentaeromagnetic aeromagnetic data data (USGS) (USGS)suggest suggestthat that Clam ClamFalls Fallsflows flowsare arestratigraphically stratigraphicallylower lowerand andtherefore thereforeolder olderthan than CV flows flowsexposed exposedin inthe theTaylors TaylorsFalls Fallsregion. region. CV The The mafic mafic volcanics volcanics from from the the Clam Clam Falls Falls region region are are classified classified as as basalt basalt based based on onmajor major and and trace trace element elementabundances. abundances.The Thebasalts basaltsare aremostly mostlyolivine olivinenormative, normative,but butsome someflows flowsconcontain (Mg# = = tain small smallamounts amountsof of normative normative quartz. Most Mostflows flows have have moderately moderately low Mg-numbers Mg-numbers (Mg# and Si02 contents (45-53 wt. %) indicating that they have under0.58) M ~ / [ M ~ + F = ~ 0.40 + ~ ] 0.58) and Si02 contents (45-53 wt. %) indicating that they have under= 0.40 Mg/[Mg+Fe2I gonesignificant significantfractionation, fractionation,similar similarto toflows flowsfrom fromthe the Taylors Taylors Falls Falls section section of the the Chengwatana Chengwatana gone Volcanics1. Volcanicsl. Clam ClamFalls Fallsbasalts basaltsexhibit exhibitincreasing increasingincompatible incompatibleelement elementabundances abundances(e.g., (e.g.,P,P,Ti, Ti, Y) Y) with with decreasing decreasingcompatible compatibleelement element abundances abundances (e.g., (e.g., Mg, Mg, Ni, Ni, Cr) Cr) similar similarto tothose thoseof ofthe the Taylors Taylors Falls Falls region, region, and and can can be be modelled modelled by fractional fractional crystallization processes2. processes2. Clam ClamFalls Falls flows have Ti02 concentrations which are similar to the low-Ti group (<2.3 wt. %) of basalts flows have Ti02 concentrations which are similar to the low-Ti group (42.3 wt. %) of basalts recognizedininthe theTaylors TaylorsFalls Fallssection1. section1. recognized Neodymium Neodymiumisotopic isotopicanalysis analysisof ofeight eightbasalts basaltsand andtwo tworhyolites rhyolitesfrom fromthe theClam ClamFalls Fallsarea area values between -2.0 and 3.4. Initial ENd values decrease with stratiyield initial ENd loo Ma values between -2.0 ""3.4. Initial ENd values decrease with stratiyield initial ENd(1 iOO Ma') graphic ( igure 1), 1 ,and and all all of of the the values values are are in in general general displaced displaced toward toward more more positive positive graphicheight height (1!gure values values relative relative to to those those of of the the overlying overlying flows flows of of the the Taylors Taylors Falls section1 section1 (Figure 2). AlAlthough similarly similarlyhigh highNd Ndvalues values(initial (initialENd ENd = 2) +2)have havebeen beenobserved observedininother othervolcanic volcanicsesethough quences in the Midcontinent rift quences in the Midcontinent rift 3000 3000 I rn I I I (e.g., (e.g., Group Group 66 and and 77 flows flows of of Clam Falls Region Mamainse Mamainse Point3, ~ o i n t 3Portage ,Portage Lake Lake • basalt basalt Volcanics Volcanicsand and "late "late basalts" basalts" of of northnorth- 'a i   '.' Q r yo ite em Wisconsin4), ~isconsin4),flows with with initial initial w em Â¥ . U ENd values > +2 2 are ENd values arerare rareininthe theearly early 2000 2000 .stagesof of rift riftevolution evolution(time (timeequivaequivastages lent to to upper upper Kallander Kallander Creek Creek ForForlent 0 5 lavas). Furthermation or Group mation or Group 5 Further- 3 more, Taylors Taylors Falls Falls flows flows which which 2 more, rn 1000 -overlie the the Clam Clam Falls Falls basalts basalts (and (and ^. 1000 overlie U  maybe becorrelative correlativewith withPortage PortageLake Lake .may cd 0 j=!  Volcanics Volcanics and Group Group 66 flows) flows)have have w epsilonNd Nd values valuesthat thatare aregenerally generally epsilon  45 -ENd == -4.5 more negative negative(initial (initialENd more I I n I 00 44 22 33 11 -1 00 O1) than ""0.1) thanhas hasbeen beenobserved observedinincorcor-2 -2 -1 relativemain mainstage stagelavas lavas(initial (initialENd ENd relative Initial Ed &srd Initial ^ \ s - 6 a 4-à - 96 - �12 -1 -- 4) +4)elsewhere elsewherein in the therift. rift. == -l Most models models of of the the magmagmatic evolution evolution of the the matic Midcontinent rift assume that the Midcontinent rift assume that the large volume volume of of magmas magmas were were produced primarily from from an an enenriched riched mantle mantle plume plume with with initial initial eNdnear zero3'4. zero3'4. Lavas with with ENd negative ENd values have have been been negative ENd modelled by contamination contamination of of mantle mantle plume plume melts melts with with conticontinental nental lithospheric lithosphericmantle mantle(ENd (ENd near near 9) -9)or orwith withcontinental continentalcrust crust -12 -10 -8 -6 -4 (ENd < -10). -10). In In contrast, contrast, lavas lavas (ENd with with positive positiveENd ENd values values has has been been Initial ENd £\ Initial modelled modelled by aa combination combination of of enriched enriched mantle mantle plume plume and and dedepleted pleted asthenospheric asthenosphericmantle mantle(ENd (ENd > 6) that may have been entrained in the plume head5. FolFollowing valuesof ofChengwatana Chengwatana basalts basalts suggest suggest aa progression progression of mantle lowingthis this model, model, the the initial initial ENd ENd values sources sources from from enriched enrichedmantle mantle plume+depleted plume+depletedasthenospheric asthenosphericmantle mantle(flows (flowswith withENd>>11near near the base of the the Clam Clam Falls Falls section) section) to to enriched enriched mantle plume that has been variably variably contamicontaminated nated with with lithosphere lithosphere(the (theremainder remainderof of the theClam ClamFalls Fallssection sectionand andthe the overlying overlyingTaylors TaylorsFalls Falls section). The inferred progression of mantle sources observed in the Chengwatana Volcanics section). The inferred progression of mantle sources observed in the Chengwatana Volcanics (plume+asthenosphere (plume+asthenosphere—> -> plume plume+1+I- continental continental lithosphere) occurs at a time when when mantle mantle sources sources elsewhere elsewherein in the the rift rift are are inferred inferred to to be changing changing from from plume+continental plume+continental lithosphere lithosphereto to plume+asthenosphere. IfIfthe thedepleted depletedmantle mantlecomponent component of of early early Clam Clam Falls Falls flows flows originated originated from from material material that that was was entrained entrainedin in the the plume plume head, head, as as has been suggested suggested by several several for for other other parts the proportion proportion of melts melts produced produced from entrained entrained asthenospheric asthenospheric mantle rift3'4y5 then the mantle parts of the the rift3'4'5 in the plume plume varied varied in in space space and and time time throughout throughout the rift. rift. Alternatively, Alternatively,the thedepleted depletedmantle mantle component component may may have have been differentially incorporated into plume-generated melts as they traveled continental lithosphere. lithosphere. The eled through the the asthenosphere asthenospherebefore reaching the continental The relatively relativelylow low initial values observed observed in flows of the Taylors Taylors Falls Falls and and upper upper Clam Clam Falls Falls sections indicate initial ENd EM values indicate greater greater involvement involvementof of aa continental continentallithosphere lithosphere(crust (crust and/or andlor mantle) mantle) component componentthan than has hasprepreviously been recognized during the early part part of of "Main "Main Stage" Stage" volcanism volcanism in inthe the rift. rift. In all all of these cases, cases, the the observed observed isotopic isotopic differences differences between between the the Chengwatana ChengwatanaVolcanics Volcanics and and other other volcanic "off-axis" position of the Chengwatana volcanic sequences sequencesof of the the MCR MCR might be related to the "off-axis" Volcanics relative to the location of the inferred inferred plume plume head. head. References Cited Cited (1) Jeffrey D., D., and and Naiman, Naiman, Zachary ZacharyJ,J, 1997. (1) Wirth, Karl R., Vervoort, Jeffrey 1997. The TheChengwatana ChengwatanaVolcanics, Volcanics,Wisconsin Wisconsin and Minnesota: Petrogenesis Petrogenesisof ofthe thesouthernmost southernmostvolcanic volcanic rocks rocks exposed exposed in in the the Midcontinent Midcontinent rift. rift. Canadian Canadian Journal 536-548. (2) (2)Naiman, Naiman, Zachary Zachary J., 1997. 1997. Petrogenesis Petrogenesisof of the the ChengwatanaVolcanics: ChengwatanaVolcanics: Journal of Earth Science, Science, 34: 536-548. 1.1 Wisconsin. Bachelors Thesis, Macalester College. (3) 1.1 Ga Midcontinent rift lavas in Minnesota and Wisconsin. (3)Shirey, Shirey, S.B., Klewin, K.W., Berg, J.H., J.H., and and Carlson, Carlson, R.W.. R.W.. 1994. K.W., Berg, 1994. Temporal Temporal changes changes in the the sources sources of of flood flood basalts: basalts: isotopic and 1100Ma Ma old old Keweenawan Keweenawan Mamainse Marnainse Point Formation, Formation, Ontario, Ontario, and trace trace element element evidence evidence from from the the 1100 Canada: Geochimica Geochimica et et Cosmochimica Cosmochimica Acta, Acta, 58: 58: 4475-4490. 4475-4490. (4) Nicholson Nicholson Suzanne Suzanne W., W., Shirey, Shirey, Steven Steven B., B., Shultz, Klaus J., and and Green, Green, John John C., C., 1997. 1997.Rift-wide Rift-widecorrelation correlationofof1.1 1.1Ga GaMidcontinent Midcontinentrift riftsystem systembasalts: basalts: implications for multiple mantle sources during rift development. Canadian Journal of Earth Science, 34: 504520. 520. (5) (5)White, White,Robert RobertS., S.,1997. 1997.Mantle Mantletemperatures temperaturesand andlithospheric lithosphericthinning thinning beneath beneath the the Midcontinent Midcontinent rift system: evidence from magmatism and subsidence. Canadian Journal of of Earth Earth Science, 34: 34: 464-475. 97 �MILLGROUP GROUPREVISITED: REVISITED: BASAL BASALVOLCANIC VOLCANIC ROCKS ROCKS OF OF THE ThE THE POWDER MILL MIDCONTINENT RIFT MIDCONTINENT RIFTSYSTEM SYSTEM ON ON THE THE SOUTH SOUTH SHORE SHOREOF OF LAKE LAKE SUPERIOR SUPERIOR NICHOLSON, SuzanneW. W.USGS, USGS, National NationalCenter, Center,MS MS954, 954,Reston, Reston,VA VA20192 20192 NICHOLSON, Suzanne and WOODRUFF, and WOODRUFF,Laurel LaurelG., G.,USGS, USGS, 2280 2280 Woodale Woodale Drive, Mounds View MN 55112 55112 In northern Wisconsin and Michigan the Powder Mill Group comprises the oldest volcanic Michigan Group oldest volcanic rocks related to the 1.1 Ga Ga Midcontinent Midcontinentrift rift system system (MRS). (MRS). In In the the more more than than twenty years years since Hubbard (1975) (1975) first separated separated the thePowder PowderMill MillGroup Groupfrom fromthe theoverlying overlyingPortage Portage Lake isotopic, and Lake Volcanics, Volcanics, an abundance abundance of of geophysical, geophysical, structural, structural, geochemical, geochemical, isotopic, chronological data data for for Midcontinent Midcontinentrift riftrocks rockshas has become becomeavailable. available. This report integrates integrates new and existing information to to provide provide a better existing information better understanding understanding of the the early earlymagmatic magmatic history historyof of the theMRS. MRS. Powder Mill MillGroup Groupconsists consistsofof the the basal basal Bessemer BessemerQuartzite, Quartzite, and and the reversely The Powder reversely magnetized igneous rocks of the the Siemens Siemens Creek Creek Volcanics Volcanics and and overlying overlying Kallander Kallander Creek Creek Volcanics. The outcrop outcrop distribution distributionextends extendsfor formore morethan than 180 180 km km along strike, from just Volcanics. The west of the Lake WI discontinuously eastward to toSilver Silver Mountain, Mountain, Lake Owen Owen fault fault near near Cable, Cable, WI ML The Thetrue trueextent extent of of the thegroup group is is unknown unknown because or Ml. because itit is is either structurally truncated or unconformably overlain by younger rocks. However, seismic reflection proffles suggest that unconformably by younger rocks. profiles that the Powder Powder Mill Mill volcanic volcanic rocks rocks extend extend laterally laterally beneath beneath Lake Lake Superior. Superior. The Siemens CreekVolcanics Volcanicsisissubdivided subdividedinformally informallyinto intoupper upper and and lower lower The Siemens Creek members and isotopic isotopiccharacteristics. characteristics. The The lower lower member member of of the members based based on on field, field, geochemical, geochemical, and Siemens Creek Creek Volcanics Volcanicsisisdominantly dominantlybasalt, basalt, but but includes indudes recently recently recognized basal highhighMgO has limited limited regional extent, and and MgO picritic flows. The Thelower lower member memberis is thin thin (<50 ( ~ 5m 0 thick), has is distinctive distinctive because because some someflows flowscontain containaugite augitephenocrysts, phenocrysts,uncommon uncommonamong amongyounger younger MRS basalts. basalts. Pillows Pillows formed formed in the the basal few basalt basalt flows flows that that directly overlie the MRS the Bessemer Bessemer Quartzite. Basalt Basalt with minor basaltic andesite and and andesite andesite dominates dominates the the upper uppermember, member, which is more widespread and 1.5km) km)than thanthe thelower lowermember. member. No No ages have and thicker (up to 1.5 for the Siemens Creek of been determined determined for CreekVolcanics, Volcanics,but butby bycorrelation correlationwith withthe the basal basal units of Osler Group in Ontario and the sills, volcanism volcanismprobably probablywas was initiated initiated about the Nipigon Nipigon sills, the Osier 1108 Ma. Ma. High-MgO High-MgO(picritic) (picritic)rocks rocksrelated relatedtoto the the Midcontinent Midcontinentrift rift magmatism magmatism are are now known of the section sectionin inthree threeareas: areas:1) 1)in inthe theNipigon Nipigonarea area in in Ontario known to to occur occur near the base of associated with with dikes dikes and andsills sills(Sutdiffe, (Sutcliffe,1987); 1987); 2) associated 2) at at Mamainse MamainsePoint Point in in the the basal 500 m of the MRS section (Berg (Bergand andKlewin, Kiewin,1988); 1988);and and3)3)ininthe the Club Club Lake, Lake, WI WI area area west of the MRS section the Mineral Lake Lake intrusion intrusion as as recently identified picritic picriticflows flowsnear near the the base base of the recently identified the Siemens Siemens Creek The high-MgO flows in in the Siemens Creek Volcanics Volcanicsare are strongly strongly altered altered to high-MgO flows Siemens Creek Creek. The serpentine, chlorite chlorite and and talc: no primary serpentine, primary mineralogy is is present. present. An average of of three three analyses of these of these high-MgO high-MgO flows (Ti02 (Ti02= = 1.95 1.95 wt wt %; %; A1203 A1203= 8.89 8.89 wt %; %; MgO MgO == 16.09 16.09 wt %) %) is is chemically chemically most most similar similar to to an an analysis analysisof of a Nipigon Nipigon picritic picritic dike dike (Ti02 (Ti02== 1.88 1.88 wt %; A1203 A&03== wt %; 7.72 higher Ti02 Ti02 and and lower A1203and and MgO MgOthan than a 7.72 wt %; %; MgO MgO == 17.1 17.1 wt %) in having higher picritic average wt %; %; A1203 A l a == 9.79 wt %; %; MgO MgO average reported reportedfrom fromMamainse MamainsePoint Point(Ti02 (Ti02== 0.90 wt = 19.89 wt %; and Klewin, = 19.89 %; Berg and Klewin, 1988). 1988). Picritic flows in the the Siemens Siemens Creek Volcanics show with derivation derivation from steep 18.6), consistent consistent with from partial partial steep REE-element REE-element patterns patterns(CeN/YbN (CeN/YbN= 18.6), melting melting of an an enriched enriched mantle mantle source source at at a depth depth of of more more than than about about 80 80 km. Basalts Basalts from fromthe thelower lowerSiemens SiemensCreek Creek Volcanics Volcanics show show moderate moderateTi02, Ti02,lower lowerA1203 higher MgO MgO than and higher than basalts basalts from the the upper upper Siemens SiemensCreek. Creek. REE REEpatterns patterns are steeper Creekthan thanfor foroverlying overlyingunits. units. In In addition, addition, the two = 14.2), two (CeN/YbN 14.2), for the lower Siemens Creek (CeN/YbN = of the Siemens Siemens Creek CreekVolcanics Volcanicscan canbebedistinguished distinguishedon onthe thebasis basisofof Nd Nd isotopic members of composition: lower lower Siemens SiemensCreek Creekbasalts basalts have have initial initial £v Ed == -1 -1 whereas the upper upper Siemens Siemens Creek EM,, == -3.6. Creek basalts basaltshave haveinitial initialENd Volcanics overlies overliesthe theSiemens SiemensCreek CreekVolcanics, Volcanics,and and also also has The Kallander Creek Volcanics has been informally divided into upper and lower members. members. Both Both members membersrange range from from basalt basalt to to andesite and andesite and rhyolite. rhyolite. The Thelowermost lowermost1.5 1.5km km of of the the Kallander Kallander Creek Creek consists consists of flood basalt phenocrysts,which whichlocally, locally,form formlarge largeradiating radiatingclusters. clusters. A typically containing plagioclase phenocrysts, 98 �rhyolite flow near near the the top of the lower Kallander Kallander Creek Creek gives gives an an age age of of 1107.3 rhyolite flow 1107.3 ± Â1.6 1.6 Ma Ma and Green, Green, 1997). 1997). The upper upper member (about 22 krn km thick) thick) is is dominated dominated by andesite (Davis and member (about andesite and is is considered considered to to represent representthe the extrusive extrusive products of of aa localized localized magmatic magmatic system, the the Mellen complex complex(Cannon (Cannonetetal. al.1993). 1993). A A thick thick laterally laterally extensive extensiverhyolite rhyoliteatat the the top of the the upper member upper memberofofthe theKallander KallanderCreek Creekgives givesan anage ageofof1099.0 1099.0±Â2.6 2.6 Ma (Zartman (Zartmanetetal. al.1997). 1997). Basalts of the Volcanics are are characterized characterized by the lower lower member member of the Kallander Creek Volcanics and low MgO, MgO,Cr Crand andNi, Ni,suggesting suggestingstrong strongfractionation. fractionation. Although Although the REE TiODand their high Ti02, patterns for this Creek basalts, basalts, the patterns this group group are are similar similar to those those for for the the lower lower Siemens Siemens Creek elemental REE REEabundances abundancesare aresubstantially substantiallyhigher higherfor forthis thisgroup. group.InInaddition, addition, not not only are incompatible trace trace element elementratios ratiossuch suchas as C CeN/YbN andTa/Th Ta/Th similar similarfor for both both lower incompatible e N / m and lower members of of the their Nd members the Kallander Kallander Creek Creek and andSiemens Siemens Creek CreekVolcanics, Volcanics, their Nd isotopic isotopic compositionsare are similar similar as as well well (initial (initial ENd Nd == -1). In basalts of of the In contrast, contrast, basalts the upper upper compositions Creek show show lower lower incompatible incompatible trace trace element elementratios ratios and and much less steep Kallander Creek steep REE REE patterns than despite the fact that patterns than the thelower lowerKallander Kallander Creek Creek member, member, despite that their theirmajor majorelement element of the the upper upper Kallander Creek yields compositions are similar. The The Nd isotopic composition of yields slightly lower than the an initial initialEyy d == -1.5, slightly the lower lower member. member. The Siemens Creek and and Kallander Creek Volcanics Volcanicsrepresent representthe the initiation initiation of of aa The Siemens Creek Kallander Creek continental upweiling of of aa mantle mantle plume beneath continental rifting event that has been attributed to the upwelling beneath the Lake Superior Superior region. region. The diversity diversity of of basalt basalt compositions, compositions, including induding picrites, picrites, that that are representative the first years ofof rifting rifting are are characteristic of other representative ofof the first several several million million years characteristic of other continental rift settings mantle plumes plumes play play a dominant which mantle dominant role. role. Based Based on the the continental rift settings in which geochemical and isotopic both the the lower lowerSiemens SiemensCreek Creekand and lower Kallander isotopic characteristics, both Kallander Creek units are probably the of partial melting the products products of of varying varying degrees of melting of an an enriched enriched source (mantle plume) at great great depth depth with withthe thelower lowerKallander Kallander Creek Creekhaving havingundergone undergone significant fractionation. fractionation. The upper Siemens Siemens Creek Creek member member probably represents represents larger larger degrees of of partial partial melting meltingof ofaaplume plumesource sourceatatshallower shallowerdepths, depths,producing producingmelts meltsthat that may have interacted with with lithosphere lithosphereand andundergone undergonelimited limitedfractionation. fractionation. The upper Kailander Kallander Creek probably in in shallow crustal Creek appears to to have haveundergone undergone significant significant fractionation fractionation probably crustal magma magma chambers chambersnow now seen seen as as layered layered intrusions. intrusions. References: References: Berg, J.H., J.H, and 1988, High-MgO High-MgOlavas lavasfrom fromthe theKeweenawan Keweenawanmidcontinent midcontinent rift rift near Berg, and Kiewin, Klewin, K.W., K.W., 1988, near Mamainse MarnainsePoint, Point, Ontario: Ontario:Geology, Geology,v.v.16, 16,pp. pp.1003-1006. 1003-1006. Cannon,W.F., W.F., Nicholson, Nicholson, S.W., S.W., Zartman, Zartman, R.E., R.E., Peterman, Peterman,Z.E., Z.E.,and and Davis, Davis, D.W., D.W., 1993, Cannon, 1993, The Kallander remnantof ofaa Keweenawan Keweenawan central central volcano centered near Mellen, Kallander Creek Creek volcanics—a volcanics-a remnant Mellen, Wisconsin: Institute Institute on Lake Superior Superior Geology GeologyProceedings, Proceedings,Program Programand and Abstracts, Abstracts, v. 20v. 39, 39, p. p. 2021. Geochronologyofofthe theNorth North American American Midcontinent Midcontinentrift rift in Davis, D.W., D.W., and andGreen, Green, J.C., J.C., 1997, 1997, Geochronology western Lake Superior Superior and and implications implications for for its geodynamic evolution: Canadian Journal of western Lake geodynamic evolution: Earth Earth Sciences, Sciences,v.v.34, 34, pp. pp.476-488. 476-488. 1975, Lower Lower Keweenawan Keweenawanvolcanic volcanicrocks rocksofofMichigan Michigan and and Wisconsin: Wisconsin: U.S. U.S. Hubbard, H.A., H.A., 1975, Geological Journal of Research, v. v.3, 3, no.5, no. 5,pp. pp.529-541. 529-541. Geological Survey, Journal Sutcliffe, R.H., of Middle Middle Proterozoic Proterozoicdiabases diabasesand andpicrites picrites from from Lake Lake Nipigon, Nipigon, Sutcliffe, R.H., 1987, 1987, Petrology Petrology of Canada: Canada: Contributions ContributionstotoMineralogy Mineralogyand andPetrology, Petrology,v.v.96, 96,pp. pp.201-211. 201-211. Zartman, R. 1997, U-Th-Pb zircon zircon ages ages of Zartman, R. E., E., Nicholson, Nicholson, S.W., S.W., Cannon, W.F., W.F., and Morey, G.B., G.B., 1997, of some some Keweenawan Supergroup Supergrouprocks rocksfrom fromthe thesouth south shore shore of of Lake Lake Superior: Superior: Canadian Canadian Journal Journal of of Earth Earth Sciences, Sciences,v.v. 34, 34, pp. pp.549-561. 549-561. 99 �GIS MINERAL POTENTIAL GIs BASED MINERAL POTENTIALANALYSIS ANALYSIS FOR FOR LODE-GOLD LODE-GOLDAND AND MASSIVE MASSIVE SULFIDE TERRANE OF NORTHERN SULFIDE DEPOSITS DEPOSITS IN IN AN AN ARCHEAN TERRANE NORTHERN MINNESOTA MINNESOTA DEAN M. PETERSON PETERSONand and DR. DR. RONALD RONALD L. L. MORTON DEANM. MORTON Economic Economic Volcanology Volcanology Research Lab, Geology Geology Department, University University of Minnesota - Duluth, Duluth, Duluth, 55812 Duluth, Minnesota, Minnesota,USA USA55812 The last decade change. The decade has been a period of worldwide political and economic change. The globalization globalization of of the world's world's economy economy has has created creatednew new challenges challengesand and opportunities opportunities for the mining and mineral exploration exploration industry. industry. Many Manyareas, areas,which whichpreviously previouslywere were strictly strictly the the domain domain of of nationalist nationalistenterprises enterprises (Southeast Asia, South America, and the former Soviet Union), are now open to mineral exploration (Southeast Asia, South America, and the former Soviet Union), are now open to mineral exploration by companies companies from North America and Europe. Many Many exploration exploration companies companiesare are focusing focusinglarge large portions of their resources resources away from North America and into these recently opened countries. This This switch in regional area selection selection has had aa negative negative impact impact on on the the amount amount of mineral mineral exploration exploration conducted conducted in the Archean terranes terranes of Minnesota. Aside Asidefrom fromthe the changes changesin in global global economics, economics, other other specific specific reasons reasons for the lack of recent mineral mineral exploration exploration in the the Archean Archean terranes terranes of of Minnesota Minnesota include the following: following: > no no local local prospectors are are "beating "beatingthe the bush" bush"and anddiscovering discoveringprospects prospects in in Minnesota Minnesota >>misconception that mine misconception that minepermitting permittingwould wouldbebeasascumbersome cumbersomeasaswith withWisconsin WisconsinVMS VMSdeposits deposits >>absence ofofsuccess absence successininprevious previousgold goldand andmassive massivesulfide sulfideexploration exploration programs programs > most most mineral industry industry geologists geologists have have little little knowledge knowledge of of the the geology geology of of Minnesota, Minnesota,and and > the identification identificationof of specific specificexploration exploration target target areas areas are are normally normally not not included includedin in geological geological ' reports reports and and maps maps The The discovery discovery of economic economiclode-gold lode-gold and and massive massive sulfide sulfidedeposits deposits will will only only occur occurfollowing following prolonged exploration by the mineral mineral industry. industry. To exploration in the Archean terranes of Minnesota by To increase increase exploration exploration activity, activity, state state agencies agenciesin in Minnesota Minnesota have have completed completed extensive extensivegeological geologicaland and geophysical mapping, mapping, completed completed several several mineral mineral potential potential studies, studies, and and currently currentlyare arepromoting promoting geophysical Minnesota's Minnesota's geology geology and and mineral mineralpotential potential to to the the mineral mineral industry. However, However,because becauseof ofextensive extensive glacial glacial cover, cover, an an absence absence of of producing producing mines, mines, and and the the aforementioned aforementionedreasons, reasons, the the mineral mineralindustry industry requires additional Archean additional geological geologicalincentives incentives in in order order to to start start exploration exploration programs programs in in the the Archean terranes terranes of Minnesota. Minnesota. These Theseincentives incentivescould couldinclude includedetailed detailed descriptions descriptionsof of known known prospects, prospects, detailed mineral mineral potential potential studies, studies, and and specific specifictargets targetsand and geologic geologic criteria criteriaupon upon which whichto tobase base detailed exploration exploration programs. programs. This This study study is is an attempt attempt to to generate generatespecific specific lode-gold lode-gold and and massive massive sulfide sulfidetarget targetareas areasthat thatthe the mineral the state. Current mineral industry industry can use as a premise to begin exploration programs in the Currentore ore deposit deposit models GIs databases databasesof of important important Canadian Canadian mining mining camps camps(Timmins, (Tirnrnins, models have have been been integrated integratedinto into GIS Kirkland Lake, Lake, Hemlo, Hernlo, and and Sturgeon Sturgeon Lake Lake(Figure (Figure1)), I)), and and lode-gold lode-gold and and massive massive sulfide sulfideexploration exploration Kirkland models developed from detailed spatial analysis. Exploration target areas in the Minnesota developed detailed Exploration target areas in the Minnesotastudy study area area have have been generated generated from fromthe the integration integrationof of these these models modelsinto intoaa detailed detailedGIS GIs of of aalarge largeArchean Archean tenane 2). terraneof ofnorthern northernMinnesota Minnesota(Figure (Figure2). The The methods methods developed developedto to complete completethis thisstudy studycomprise comprisefive fiveseparate, separate,but but related relatedthemes: themes: >9Thorough research Thorough researchononthethetheory theoryand andmethods methodsofofore oredeposit depositmodeling, modeling,and andthe thegeological geologicalsetting setting of of Archean Archean lode-gold lode-gold and andmassive massivesulfide sulfidedeposits. deposits. 100 �_____________________________________ Standardized from the Superior Province of of Standardized geological compilations compilations of four analog mining camps from Ontario, Canada. These Thesecompilations compilationsare areintegrated integratedwith with Ontario Ontario Geologic GeologicSurvey Survey(OGS) (OGS) drill drill hole, geochemical geochemical and and mineral mineral deposit deposit inventory inventory databases databasesand and have have been been converted convertedinto intoGIS GIs format. format. I ) Standardized geological, Standardized geological,geophysical, geophysical,and geochemical geochemical compilation compilation of the Minnesota study area, area, and and the the conversion conversionof of this this compilation compilation into into GIS GIs format. format. Development of mineral mineral exploration models and targeting targeting criteria generated from knowledge-based queries queries of of the the analog analogGIS GIs datasets. These Thesemodels modelsintegrate integrateArchean Archean massive sulfide lode-gold and massive sulfide deposit deposit models models with spatial features in the analog GIS analog GIs databases. Specific Specificgeological geological criteria criteria have been developed developed that define define the location of the lode-gold and massive sulfide deposits within the analog GIS GIs datasets. datasets. I Figure 11 Location map of the the study study areas. areas. Figure I Thoroughmineral mineralpotential potentialevaluation evaluationfor forlode-gold lode-goldand andmassive massivesulfide sulfidedeposits deposits in in the > Thorough Minnesota study area. This Thisevaluation evaluationisisbased based upon upon spatial spatialanalysis analysisof of the the Minnesota MinnesotaGIS GIs of database using targeting criteria developed from the exploration models generated from queries of the analog analog GIS GIs databases. databases. Simvlified Simolified Geology Geoloey Ely* Ely' Towns ....— 10 Figure Figure 22 0 Major MajorShear S k a Zones Zones ~ 10 20 Kilometers = Proterozoic Protemzaic Rocks Rocks Wawa Subprovince Subpronnce Vennilion Vemilion Granhtic G r a ~ t i cComplex Complex Greenstone Terrane GreenstoneTemne Internal Plutons Plutolw Giants Range Range Granitic Gramtic Complex Complex Mammnt& Simplified geology and location of USGS 1:24,000 Simplified 1 : 2 4 , 0 scale scale quadrangle quadrangle maps maps of of the the Minnesota Minnesotastudy study area. area. 101 �IMPROVED VULNERABILITY ASSESSMENTS ASSESSMENTS OF IMPROVEDGEOLOGIC GEOLOGIC SENSITIVITY SENSITIVITY AND VULNERABILITY OF GROUNDWATER GROUNDWATER POLLUTION POLLUTION POTENTIAL POTENTIALTHROUGH THROUGHAPPLICATION APPLICATIONOF OFFUZZY FUZZYLOGIC LOGIC PFANNKUCH, PFANNKUCH, H.O., H.O., and and PAULSON, PAULSON, Richard, Richard, A., Department Departmentof of Geology Geology and andGeophysics, Geophysics, University of of Minnesota, 310 Pillsbury Drive S.E. Minneapolis, MN University Minnesota, 310 Pillsbury Minneapolis, MN 55455 55455 Vulnerability/sensitivity assessments assessmentsbelong belongto to the the general general problem class of Vulnerability/sensitivity of suitability suitability assessments. In general, these these are management tools to to evaluate evaluate the feasibility, reliability assessments. management tools or risk of of using using aa given given physical physical entity entity or or process. process. In In the the environmental environmental or geologic geologic context context it is is aa decision decision aid aid ininaaresource resourceprotection, protection,or ormanagement managementplan. plan. In generall general, the procedure has three components: components: aa phvsical physical model that describes the process process in a risk analysis analvsisframework, framework, and andaamethodology methodology to to aggregate aggregate individual individual partial partial vulnerability indices indices to to aa global vulnerability alobal vulnerability vulnerabilitv index. index. In In the the context context of of aa vulnerability vulnerability assessment to groundwater contamination contamination the physical physical model model simulates simulates contaminant contaminant transport along pathways from the land surface to the groundwater groundwater body. The pathway pathway is is subdivided into a sequence sequence of of zones zones in in which which different differenttransport transport processes processesand andparameters parameters predominate. These compartments compartments are are surface surface parcell parcel, soil soil zone, zone, vadose zone, capillary fringe, and the groundwater flow zone zone which which may may be be further further subdivided subdivided ifif more more detail detail is fringe, groundwater flow wanted. wanted. In a perfectly perfectly deterministic deterministic world where all transport transport processes processes can be be described described as as mathematically exact, where flow boundary physicoboundary geometry, geometry, boundary boundary conditions conditions and and physicochemical parameters are known everywhere in the domain domain of of interest, interest, the the transport transport process process can be be modeled modeled exactly exactly along along continuous continuous pathways, pathwaysl given given appropriate appropriate computational computational power. power. Then, concentration of the contaminant Its residence residence contaminant everywhere everywhere in space and time is known. known. Its time in in the the system system and and its its exposure exposure to to and and impact impact of of exposure exposure on on the the target target can canbe bequantified. quantified, From this, choices about protection protection of the the resource, resource, the the environment, environment, or or human human targets targets can can be made. This is all that is is needed needed for solving the problem, and no further steps are necessary. necessary. Alas, the world is is not notisotropic, isotropic, homogeneous homogeneousand and perfect, perfect, and and mathematical mathematicalsimulation simulation models even less models less so. so. Therefore, Therefore, severely severely restrictive restrictive approximations approximations and and simplifying simplifying assumptions have have to to be introduced to represent the transport transport process process in in an half-way assumptions represent the adequate manner. The approximations concern the mathematical description of the process, process, limited by insufficient insufficient and and infrequent infrequent information, information, and by inadequate inadequate knowledge of impacts. impacts. the greater greater the the uncertainty that the the model The greater the simplifications, simplificationsl the uncertainty that model results results reflect reflect reality and produce reliable predictions or vulnerability designations. and produce reliable predictions or vulnerability designations. This uncertainty traditionally has been handled by a risk risk analysis analysis approach. approach. The The likelihood likelihood of aa contaminant contaminant particle particle reaching reaching the the groundwater groundwater target and its its likelihood likelihood to to produce produce unwanted unwanted consequences has been expressed in terms of conditional conditional probabilities probabilities and and statistical estimates as confidence limits or ranges. Inadequacies in the geologic database estimates as confidence limits or ranges. Inadequacies in the geologic database have been handled handled by by statistical statistical methods methods such such as as Monte-Carlo Monte-Carloapproaches approaches or or kriging. kriging. A groundwater groundwater system system is is extensive, extensive, three three dimensional, dimensional, dynamic dynamic and and non-homogeneous. non-homogeneous.Its Its properties and states must be be spatially spatially referenced referenced for most most efficient efficient analysis analysis and and map map representation. Georeferencing is best carried out by by aa Geographic Geographic Information InformationSystem System (GIS) (GIs) which also also performs performs aggregation aggregation functions functions able able to to handle handlemap mapoverlay-index overlay-index procedures using built in algorithms. procedures using algorithms. The information information for each hydrogeologic hydrogeologic flow as an an information layer layer in in the the GIsl GIS, and and can can be combined into an compartment is represented as aggregate or monothematic monothematic layer such as aa global global vulnerability vulnerability map map layer. layer. Traditional methods of groundwater translate the the protection afforded Traditional groundwater sensitivity assessment assessment translate by various elements elements of the the geological geological conditions conditions and materials between the surface and the groundwater body body into into vulnerability vulnerability indices, indices, from from which, which, pollution pollution potential potential can can be groundwater 102 �estimated. estimated.One Onesuch suchmethodology methodologyisisUSEPA's USEPA's DRASTIC. DRASTIC.Many Manyof of these theseprocedures procedureslack lack internal consistency, and give no explicit rationale for assigning rating indices, internal consistency, and give no explicit rationale for assigning rating indices, range range intervals and and thresholds, thresholds, weights to the various various factors, and leave leave the choice choice of of an an additive additive intervals aggregation aggregationmodel modelunexplained. unexplained. The Minnesota MinnesotaDepartment Department of of Natural Natural Resources' Resources' (MN (MN DNR) DNR) guidelines guidelines stand stand on on aamuch much The firmer firmer conceptual conceptual basis. basis. They They link link geologic geologic sensitivity sensitivity to to travel travel time time of ofaaconservative conservative tracer tracer from from the the surface surface to to the thegroundwater. groundwater. Long Long travel travel times times are are equated equated to low low sensitivities, and and short short travel travel times times to to high high sensitivity. sensitivity. In In order order to to take take into intoaccount account the the sensitivities, availability of of information information for for mappable mappable factors factors ititmakes makessome somesweeping sweepingsimplifications. simplifications. ItIt availability uses three three factors: factors: depth depth to to water, water, geologic geologic material material in the vadose vadose zone and material material at at the the uses water water table table and andobtains obtainsthe theaggregate aggregateindex indexthrough throughaabinary binarydecision decisiontree treeprocedure. procedure. Because Because of of these these simplifications simplificationsthe the degree degreeof of uncertainty uncertaintyand andthe theconfidence confidencelimits limits are are large. large. Introduction Introduction of of aa fuzzy fuzzy set set methodology methodology will will lead lead to to aa greater greater internal internalconsistency consistency of of the the rating ratingprocess processand andfacilitate facilitate the the use use of of vague vague and and descriptive descriptive data data in in aa more moreappropriate appropriate way. way. Fuzzy Fuzzy logic logic isisaabranch branchof ofset settheory theorythat thatallows allowsobjects objectsdegrees degreesof ofbelonging belongingto toaaset setrather rather than than aabinary binaryyes yesororno nodescription. description.Fuzzy Fuzzynumbers numbersare aredefined definedby bymembership membershipfunctions functions that thatdescribe describetheir their degree degreeof of belonging belongingto to one one set set or or another, another, and and thereby thereby are are able able to to deal deal with with verbal verbal descriptors descriptors or or linguistic linguistic variables, variables, and and to to take take into intoaccount accountthe thenatural natural vagueness vaguenessand anduncertainty uncertaintyinherent inherentiningeologic geologicdata. data.Fuzzy Fuzzymethodology methodologyprovides providesthe the algorithms algorithmsto tooperate operateon onvague vaguerating ratingclasses classesininaaconsistent consistentway. way.Fuzzy Fuzzymethodology methodologywill will also alsogive givemore morerealistic realisticboundary boundarydefinitions definitionsbetween betweenrating ratingcategories categoriesand andclasses. classes. This This research researchcasts caststhe thedefinition definitionof ofthe theMN MNDNR DNRrating ratingfactors factorsinto intoaafuzzy fuzzycontext. context.This This approach more (I) moreadequate adequaterepresentation representation of of linguistic linguistic variables variables such such as as approach isiscapable capableof: of:(1) lithologic lithologic descriptions descriptions from from well well log loginformation informationinto intomanipulable manipulableindex indexnumbers, numbers, (2) (2) combining combining of layer layer information information in in aa GIS GIsthrough throughfuzzy fuzzyrule rulebased basedinstructions, instructions,and and (3) (3) to to extend extendthe thenumber numberof of factors factorsthat thatgo gointo intothe theoverall overallevaluation evaluationto to geohydrologic geohydrologic and and land landuse uselayers. layers. Fuzzy Fuzzy methodology methodology application: application: The The technique technique for for performing performing aa Level Level 22geologic geologicsensitivity sensitivity analysis analysis as as set set forth forthby bythe theMN MNDNR DNRisisrelatively relativelysimple simple and and straightforward. straightforward. The The techniques techniques were were intentionally intentionally designed designed so so that that water water resource resource managers managers (or (or others) others) who who may maynot nothave havetraining trainingon, on,ororaccess accessto, to,a ageographic geographicinformation informationsystem system(GIS), (GIs),would wouldstill still be beable abletotoimplement implementaahand-performed hand-performedoverlay overlayanalysis. analysis. This This can canbe beaavery verytime timeconsuming consuming and andtedious tedioustask taskfor forthe theanalyst. analyst. InInan aneffort efforttotospeed speedup upand andsimplify simplifyespecially especiallythe the preliminary preliminaryassessment assessmentor orproject projectstage, stage,an anExcel Excelspreadsheet spreadsheetbased basedprogram programisisbeing being developed MicrosoffVisual VisualBasic. Basic.ItItaccepts acceptsthe thevarious variousMN MNDNR DNRspecified specifiedvadose vadosezone zone developedusing usingMicrosoft parameters parametersunder underconsideration considerationas as input. input. The Theinput inputdata datalayers layersinclude: include:depth depthtotowater watertable table below belowthe theland landsurface, surface,aquifer aquifermatrix matrixmaterial materialtype typeatatthe thewater watertable, table,cumulative cumulative thickness thicknessofofany anylow lowand andmoderate moderatepermeability permeabilityunits unitsininthe thevadose vadosezone. zone.AAsecond secondversion versionofof the theprogram programincorporates incorporates the the principles principles of of fuzzy fuzzy methodology methodology into the analysis. The The sensitivity sensitivityratings ratingsproduced producedby bythe the"fuzzy" "fuzzy"version versionare arecompared comparedtotothose thoseproduced producedusing using the thepresent presentMN MNDNR DNRtechnique. technique. 103 �SEISMIC EVIDENCE OF Pm-NICKERSON PRE-NICKERSON SEDIMENTS IN SEISMIC WESTERN LAKE SUPERIOR SUPERIOR Deborah E. Rausch and and Nigel J Wattrus Wattrus Large Lakes Observatory, University Universiiy of Minnesota Duluth, MN 55812 558I2 email:nwattrus@d. umn.edu email:nwattrus@dumn.edu The Superior Superior lobe of the Laurentide Laurentide Ice Sheet advanced and retreated out of the western basin of Lake Superior Superior several several times during during the Wisconsin glaciation. glaciation. Wright et al. (1973) recognized recognized four four they are (in order of decreasing age): separate phases of ice advance out of the Lake Superior basin, they the St. Croix; Automba; Split Rock and Nickerson Nickerson phases. phases. The tills of the last two phases show an which represents represents the the incorporation incorporationof of proglacial lacustrine sediments with each enrichment in clay, which advance. As As the the ice ice retreated retreated during during the Nickerson phase, a series of proglacial lakes formed against the edge of the retreating ice. The position of ice margins bordering these lakes have been determined by correlating correlating subaerial subaerial moraines on either side of western Lake Superior Superior (Farrand, (Farrand, 1969; 1969; Isle Royale (Huber, 1973). A Saarnisto, 1974) and Isle A series series of of subaqueous subaqueous recessional recessional moraines, moraines, presumably associated with this sequence of proglacial lakes, have been described in the western arm of Lake Superior Superior near near Isle Isle Royale Royale (Landmesser (Landmesseret et al., al., 1982). 1982). The sediment sediment record contained contained in western Lake Superior Superior is intimately related to the retreat of the An idealized idealized soft soft sediment sediment section section for western Lake Superior based upon Laurentide Ice Sheet. An coring (Farrand, (Farrand, 1963; 1963; Dell, 1971; 1971; Thomas and Dell, 1978) 1978) and high resolution seismic surveying surveying consists of of till till overlain overlain by by aa glaciolacustrine clay (Johnson, 1980; Scholz, 1985; Anderson, 1997), consists sequence followed by a thin post-glacial Holocene clay. Recent Recentdata data collected collectedby by the the Large LargeLake Lake from Observatory (LLO) suggests the presence of a relict sediment section below the till, presumably fiom advances into earlier advances into the Lake Lake Superior Superiorbasin. basin. Since 1996, the Large Lakes Observatory (LLO) has collected over 2500 km km of high resolution single channel seismic reflection data in western Lake Superior. The surveyed area extends from Duluth to Isle Royale and southeast to Houghton, Michigan. Most of this this new data was collected with a ORE Geopulse system. The firing rate was 0.5 seconds and the the average vessel vessel speed was 6.5 milliseconds/sample for later post-survey knots. The data were digitally recorded at rate of 0.5 milliseconds/sample processing. Positioning Positioninginformation information was was derived from fiom the ships GPS navigation system. Images of the recently acquired data show most of the soft sediment section and bedrock-soft sediment interface (acoustic basement). In In water water depths depths of less than 100 100 meters, the development/preservation of the g1aciaVpost-glacial and Holocene clays above the till is negligible negligible developmentlpreservation glacial/post-glacial due to current action on the lake floor. floor. In In the deeper deeper portions of the lake, the near surface reflections are associated with the post-glacial Holocene clays, glaciolacustrine glaciolacustrine clays and a thin basal till (less than six meters thick). Presumably, glaciallpost-glacial sequence is associated with the last Presumably, this glacial/post-glacial advance/retreat in the Lake Superior (Nickerson/post-Nickerson deposition). Many of recent advancehetreat Superior basin (Nickersodpost-Nickerson seismic lines, especially those in the western portion of the survey area, exhibit buried channels containing thick sequences sequences of pre-Nickerson deposits (Figure 1). There is often a well defined unconformity separating the pre-Nickerson sediments sediments from the overlying younger section. The unconformity clearly represents represents a period of erosion, perhaps associated with the last advance advance of ice across across the basin. basin. 104 �Seismic facies analysis suggests that the pre-Nickerson pre-Nickerson material material is is composed composed of of aa sequence sequence of of glacial glacial tills and glaciolacustrine sediments. Presumably Presumably these represent represent relict relict sediments sediments preserved preserved from from earlier advances of the Superior Lobe in in western Lake Lake Superior. Superior. Very few of of the the cores cores collected collected in in Lake Superior penetrated penetrated sediment sediment below below the the glaciolacustrine glaciolacustrine vvarved clays. One a n e d clays. One long long core collected Minnesota North North Shore, Shore, however, however, recorded over 190 of Split Rock on the Minnesota 190 meters of glacial and glaciolacustrine sediments glacio1acustrine sediments (Zumberge and Gast, 1961). 1961). References. Anderson, K.A., K.A., 1997, Lake Superior Superior (M.S. (M.S. Thesis): Thesis): University University of of Anderson? 1997, A seismic stratigraphic study of western Lake Minnesota, Mimesota, Duluth, Minnesota, Minnesota, 82 82 p. p. C.!., 1971, (Ph.D. Dissertation): Dissertation):University Universityof ofMichigan, Michigan, Dell, C.I., 1971, Late Quaternary Quaternary sedimentation sedimentation in in Lake Lake Superior Superior(Ph.D. Ann Arbor, Michigan, Michigan, 184 184 p. W.R., 1963, Preliminary report report of Lake Superior Coring Program, Program, Core Core Study, 33 p. p. Farrand, W.R., W.R., 1969, The Quaternary history of Lake Superior: International Farrand, W.R., InternationalAssociation Association of of Great Great Lakes Lakes Research, p. 181-197. 181-197. Huber, N.K., 1973, 1973, Glacial and postglacial history of Isle Royale National Park, Michigan: U.S. U.S. Geological Geological Survey Professional Professional Paper 754-A. Johnson, T.C., 1980, 1980, Late Late glacial and postglacial sedimentation sedimentation in Lake Superior based on seismic-reflection profiles: Quaternary QuaternaryResearch, Research, p. 380-391. 380-391. Landmesser, Landmesser, C.W., Johnson, Johnson, T.C., T.C., and and Wold, Wold, R.J., R.J., 1982, 1982,Seismic Seismicreflection reflectionstudy studyof ofrecessional recessionalmoraines moraines 173beneath Lake Superior and their relationship to regional deglaciation: Quaternary QuaternaryResearch, Research, p. p. 173190. 190. C.A., 1985, Scholz, C.A., 1985, Sediment Sediment distribution distribution and sedimentation sedimentationrates rates in the the western western arm arm of of Lake LakeSuperior Superiorusing using 3.5 kHz kHzseismic seismicreflection reflectionprofiles profilesand and210Pb ''qb geochronology University of of Minnesota, Minnesota, geochronology (M.S. (M.S. Thesis): Thesis): University Duluth, Minnesota, Minnesota, 129 129 p. Thomas, Thomas, R.L. and and Dell, Dell, C.I., C.I., 1978, 1978,Sediments Sedimentsof of Lake LakeSuperior: Superior: Journal Journal of Great Great Lakes Lakes Research, Research, p. p. 264-275. 264-275. Wright, H.E., H.E., Matsch, C.L., E.J., 1973, Superior and Des Moines Moines Lobes: Lobes: Geological GeologicalSociety Society of of C.L., and Cushing, E.J., America Memoir 136, 136, p. 153-185. 153-185. J.H., and Gast, P.G., P.G., 1961, Geological investigations in Lake Lake Superior: Geotimes, Zumberge, J.H.? Geotimes,p. p. 10-13. 10-13. sP p 0 C" I •1 C I -I -r'" . — -- Figure 1. North Shore southwest southwest of of 1. Geopulse seismic reflection data collected off the Minnesota North Grand Marais, Minnesota. Relict Relict sediments sediments are are clearly clearly preserved p r e s e ~ e in din aa broad broad channel channel and and are are unconformably overlain by by younger till and and glaciolacustrine glaciolacustrine sediments sediments of of the the Nickerson Nickerson phase. phase. 105 �____ PRELIMINARY ORE OREi MINERALOGY OF THE HERONTRACK SILVER-ZINC-COPPER OCCURRENCE, OCCURRENCE, LUMBY LAKE LAKE AREA, AREA?ONTARIO, ONTARIO, CANADA CANADA SAINI-EIDUKAT, Geosciences, North Dakota State SAINI-EIDUKAT?Bernhardt, Bemhardt, Department of Geoscien~es~ University, Fargo, Fargo?ND 58105-5517 USA (sainieid@badlands.nodak.edu). (s~fieid@badlands.nod&.edu). Inc., Box 1376, BERNATCHEZ, Raymond, Atikokan Resources, Inc.? 1376?126 126 Willow Road, Atikokan, Ontario, Canada (rbernatc@atikokan.lakeheadu.ca) Road? (rbernatc @atikokan.lakeheadu.ca) The Herontrack silver-zinc-copper silver-zinc-copper occurrence occurrence occurs near Herontrack Lake Lake in the the Lumby Lumby Lake greenstone belt. It lies in the southwestern part of the Archean Superior Province, near Atikokan, Atikokan?Ontario. Recently discovered in the region of the famous Steep Rock iron mine, the Herontrack occurrence contains numerous stratiform mineralized horizonsy horizons, some of which contain contain significant significant Ag Ag mineralization. mineralization. The Lumby Lake greenstone near its border border greenstone belt occurs occurs in the Wabigoon subprovince, subpro~ince~ near its with the Quetico subprovince (Fig. 1, 1 from Davis and Jackson, 1988). It comprises a 60 km x mafic, ultramafic (komatiitic) and felsic 20 km synclinal supracrustal assemblage of mafic? metavolcanic packages with subordinate metasedimentary units (Jackson and Chevalier, Chevalier? contacts the the older Marmion Lake tonalite batholith to the the southy south, and is intruded by 1985). ItIt contacts north, and by mafic dikes. Felsic units include lapilli the younger Norway Lake granite to the north? tuffs, quartz porphyriesy porphyries, and rhyolite flow breccias. tuffs? breccia. To the west of the Lumby Lake area, area?the northeasterly trending Red Paint Lake fault zone truncates the assemblage. '-..-_.4+.' ..L—r..JJ I *+T —: I ! ,./ \ • * * + + * * Finlays0 Lake '* Belt. • * ++ A * + e't * — . Marmion Lake Batholith * —---—-——- : + * * : --+ kilornetres Figure 1. Geologic map showing the location map of the the western western Wabigoon Wabigoon subprovince, showing location of the Lumby Lumby Lake area. From Davis and Jackson, 1988. 106 �I The The lack lackof of aametamorphic metamorphicaureole aureolein inthe thesupracrustal supracrustalsequence sequence against againstthe theMarmion Marmion Lake Laketonalite tonaliteindicates indicatesthat thatthe thebatholith batholithwas wasaabasement basementunit unit upon upon which whichthe theLumby LumbyLake, Lake, and and possibly possibly the the Steep Steep Rock Rock Lake Lake group, group,were were deposited deposited (Davis (Davis and and Jackson, Jackson, 1988). 1988).Davis Davis and and Jackson Jackson (1988) (1988) dated dated felsic felsic samples samplesfrom from the the Lumby Lumby Lake Lake area area and and the the Marmion Mannion Lake Lake batholith batholith to to approximately approximately 33 Ga, Ga, an an age age older older than than most most greenstone greenstone belts. belts. During 1990's,Atikokan AtikokanResources, Resources,Inc. Inc. prospected prospected the the Lumby LumbyLake Lakearea areaand and During the the mid mid 1990's, discovered discovered aa high-grade high-gradesilver silveroccurrence occurrenceon onthe thewest westside sideof of Herontrack Herontracklake. lake.The The occurrence km thick thick east-west east-westtrending trending intermediate intermediateto to felsic felsicvolcanic volcanic occurrenceisis hosted hosted in inaa1.5 1.5km sequence sequencein in the the area areabetween betweenHerontrack Herontrackand andLumby LumbyLakes Lakes(Staargaard, (Staargaard,1997). 1997).This This sequence sequence thins thinsto to the theeast eastand andwest, west,indicating indicatingititmight mightbe bean aneruptive eruptivecenter. center.The Thefelsic felsic units units are are fragmental, fragmental, often often containing containing quartz quartz eyes, eyes, and and can can contain contain significant significant cherty chertyunits. units. Chip Chip samples samples (over (over one onemeter) meter)indicate indicategreater greaterthan than40 40g/tonne ghonneAg. Ag. Thin Thin and and polished polished block block sections sections of of ore ore horizon horizon samples samples are are being being investigated investigated using using transmitted transmitted and and reflected reflected light lightmicroscopy microscopyanalysis analysis to to determine determine the the host host rock rock and and ore ore mineralogy mineralogy and and genesis. genesis. The Therelationship relationshipbetween between fold fold structures structures recognizable recognizable on onboth both outcrop outcrop and and thin-section thin-section scales scalesand and mineralization mineralizationindicates indicatesthat that the the silver silvermineralization mineralization may may be be locally locally remobilized. remobilized. These Thesefold fold structures, structures,however, however, may may be be related related to to mafic mafic dike dike intrusion intrusion and and not not to to regional regional shear. shear. Mineralization Mineralizationoccurs occurs in in chert chert units units as as disseminated disseminated sphalerite, sphalerite, galena, galena, native native silver, silver,acanthite, acanthite,chalcopyrite chalcopyriteand and pyrite. pyrite. Native Native silver silverand andacanthite acanthite occur pm in diameter diameter and and as as veinlets veinlets 10—20 10 - 20 p.m pm wide. No Au Au was was occur as as isolated isolated grains grains 10 10to to 30 30p.m detected detected in in the the Ag Agor orthe theacanthite acanthiteusing using EDS EDSanalysis. analysis.Electron Electronmicroprobe microprobe analyses analysesare are being being undertaken undertaken to to determine determinethe theamount amountof of Ag Ag contained containedin in galena galena and and trace traceelement element composition composition of of sphalerite. sphalerite.Continued Continuedmapping mappingmay may show showthat that the the Herontrack Herontracksilver silver occurrence occurrence represents represents aa locally locally remobilized remobilized silver-rich silver-rich distal distal portion portion of of aa larger larger massive massive sulfide system. system. sulfide Referencescited: cited: References Jackson, Jackson, M.C. M.C. and and Chevalier, Chevalier, P., P., 1985, 1985,Precambrian Precambrian Geology Geology of of the the Lumby Lumby Lake Lake Area, Area, Western Preliminary Western Part, Part, Kenora Kenora District; District; Ontario OntarioGeological Geological Survey, Survey, Geological Geological Series Series-- Preliminary Map P.2828. P.2828. Map Davis, D.W., and Jackson, M.C., M.C., 1988, 1988,Geochronology Geochronology of of the Lumby Lake Lake greenstone greenstone belt: belt: aa Bull. G.S.A., G.S.A., v. v. 100, 100,p. p. 3 Ga complex within the Wabigoon Subprovince, northwest Ontario: Bull. 8 18-824. 818-824. Staargaard, C.F., 1997, 1997,Private Private report report to to Atikokan Atikokan Resources, Resources, Inc. Staargaard, 107 �ALTERATION AND METAMORPHISM METAMORPHISM IN AN ALTERATION AND AN ARCHEAN ARCHEAN LODE GOLD GOLDDEPOSIT, DEPOSIT, KItEMZAR KRJZMZARMINE, MINE,GOUDREAU-LOCHALSH GOUDREAU-LOCHALSHGOLD GOLDCAMP, CAMP,ONTARIO ONTARIO SALO, R.W. and Lakehead University, andKISSIN, KISSIN,S.A., S.A., Department Departmentof ofGeology, Geology, Lakehead University, SALO, RW.* Thunder P7B SE! SEl ThunderBay, Bay, Ontario, Ontario,P7B The TheKremzar KremzarMine h4heisislocated located45km 45km northeast northeast of of Wawa, Wawa, Ontario Ontarioin in the theMichipicoten Michipicoten Greenstone GreenstoneBelt. Belt.The TheKremzar Kremzarisisone oneofofseveral severalformer formergold-producing gold-producingmines minesin inthe thevicinity vicinity including well as as aa number of smaller properties. The including the Magino and Edwards Mines, as well The mine mine was was most most recently recently operated operatedby by Canamax CanamaxResources ResourcesInc. hc. in in the thelate late1980s, 1 9 8 0with with ~ ~ aa total total production productionof of 46798 46 798oz ozAu Au(Domville, (Domville,1998). 1998). The Thepresent presentstudy studywas was undertaken undertakenwhile whilethe the property is under redevelopment as the Island Gold Project of Patricia Mines Inc. property is under redevelopment as the Island Gold Project of Patricia Mines Inc. The TheNo.2 N0.2 Zone, Zone,aagold-bearing gold-bearingvein veinsystem systemwell well exposed exposed on on the the surface, surface,was was channelchannelsampled sampled in order order to tostudy studythe thealteration alterationassociated associated with with the thequartz-carbonate quartz-carbonate vein. vein. Petrographic Petrographicstudies studieswere werecarried carriedout outon on25 25samples samplescollected collectedatatO.5m OSm intervals; intervals; 13 13of these these were analyses of of major, major, minor minor and and trace elements by XRF and were selected selected for for whole-rock whole-rock analyses and NAA, respectively. respectively. NAA, The Thevein vein system system cuts cuts metavolcanic metavolcanic rocks of intermediate intermediatecomposition, composition,which which contain contain the chlorite chloritezone zonemineral mineralassemblage assemblagechiorite-sericite-albite. chlorite-sericite-albite. Alteration Alteration is is marked marked by by the intense carbonate carbonate and quartz quartz flooding, flooding, as well well as aa number number ofofchanges changesininaccessory accessory mineralogy. Primary Primaryilmenite ilmeniteisisprogressively progressively replaced by sphene, and allanite allanite occurs occurs only only within the the alteration alterationzone. zone. Zoisite, Zoisite, not present in in the the country country rock, rock, is abundant abundant in the the within altered zone. zone. Weakly Weakly pleochroic, fine-grained fie-grained biotite is is closely closely associated associated with with quartz quartz altered veining and andflooding. flooding. veining Among major components, CaO increases increases inward inward toward the the vein vein system, system, whereas components, CaO normally mobile components components Na20 Na20and and K20 K20show show little little variation. Ti02 Ti02decreases decreasesslightly, slightly, Among trace elements, W shows a marked increase. although itit usually usuallyimmobile. immobile. Among elements, shows a marked increase. although Although REE REE are areconsidered consideredtotobe beimmobile immobileiningold goldvein veinsystems systems(McCuaig (McCuaig & & Kerrich, Kemch, 1994), the the light light REEs, LLaa and Ce, are markedly markedly enriched in the alteration alteration zone. zone. The presence and prehnite and abundant presence of of euhedral euhedral zoisite, zoisite, minor minor wollastonite wollastonite and abundant The skarnoid calcite produce an an assemblage assemblage resembling resembling that of of aalow-grade low-gradeskarn. skarn. The skarnoid assemblage is the result result of of regional regional metamorphism metamorphism subsequent to formation formation of of the the vein vein and carbonate-enriched system, which has operated system, which operated on onthe thecompositionally compositionallymodified modified carbonate-enriched alteration zone. zone. Multiple Multiple metamorphic events are consistent consistent with three three deformational deformational alteration regimes &Helmstaedt Helmstaedt(1989). (1989). regimes documented documented by by Arias Arias & References References Arias, Z.G. & of central and east& Helmstaedt, Helmstaedt, H., H., 1989. 1989. Grant 343 343 Structural Structural evolution of eastcentral Michipicoten (Wawa) Greenstone Greenstone Belt, Belt, Superior Superior Province, Province, p. p. 210-226 210-226hj Michipicoten (Wawa) Milne, V.G., Research Grant V.G., ed., Geoscience Research Grant Program, Program,Summary Summary of Research Research 198819881989, 237 p. p. 1989, Ont. Geol. Geol. Surv. Sum. Misc. Misc. Pap. Pap. 143, 143,237 Domville, 1-32. J., 1998. 1998. Patricia Patricia Mines. Mines. Ont. Ont.Prospector Prospector1998, 1998,p.p.331-32. Domville, J., McCuaig, T.C. & Kerrich, of lode gold McCuaig, T.C. Kemch, R., R., 1994. 1994. P-T-t-deformation-fluid P-T-t-deformation-fluid characteristics of gold deposits: Evidence from from alteration alteration systematics, systematics,p.p.339-380 339-380 j Lentz, D.R., ed., ed., deposits: Evidence Lentz, D.R., Alteration AlterationProcesses ProcessesAssociated Associatedwith with Ore-forming Ore-formingProcesses. Processes. Geol. Geol.Assoc. Assoc. Can. Can.Short Short Course Course Notes, Notes, v. v. 11, 11,467p. 467p. *student *studentauthor author 108 �— ADDITIONAL PALEOMAGNETIC RESULTS FOR A 1500 Ma MAFIC DIKE AT WATERLOO WISCONSIN Schaper*, D. , Suess*, W. , Katzer*, L. ,and Kean, W. Department of Geosciences,University of Wisconsin—Milwaukee P.O. Box 413, Milwaukee, Wi. 53201 (* student authors) Recent expansion of the Michels Materials Quartzite Quarry (Gillen Quarry) at Waterloo Wi. has exposed additional outcrops of a near vertical mafic dike which cuts the quartzite in a North-South direction. The new exposure is approximately 1.5 meters wide and about 50 meters long located on the NE top surface of the quarry. The other exposure previously reported by Kean(1994) is on a vertical wall in the Southwest corner of the quarry (Luther,1997). The two exposures are on line and appear to be the same dike. The age of the dike is assumed to be 1500 Ma, which is the age of mafic material from a drill core in the nearby Portland Quarry (Aldrich et.al., 1959) and similar to the age of a pegmatite (1440 Ma, Aldrich 1959) in the area. Luther(personal communication, 1994) identified the dike as basalt which has been metamorphosed to possibly greenshist fades. Thirty six cores were collected from both locations, as well as from the surrounding quartzite. The edges of the dike show evidence of weathering and mineral alteration, so the majority of the mafic samples were collected from the center of the dike. Cores from all sites were subjected to detailed thermal demagnetization to either 600° or 750° C. Several dike samples were also A.F. demagnetized to lOOmT. The demagnetization characteristics and the Saturation Isothermal Remanent Magnetization characteristics (SIRN) indicate that magnetite is the primarily carrier of the magnetism. There is one primary magnetic direction for the dike which is removed with thermal demagnetization to 600°C. The magnetism of the quartzite is carried by hematite, as is evident from thermal demagnetization to 750°C., and SIRM studies.The magnetic directions of all samples including those reported by Kean, 1994, are presented in Figure 1. The dike shows negative inclinations (-30° to -50°)and declinations in the NNW-NNE direction. The quartzites have positive inclinations (30° to 40) and westerly declinations, which is similar to the earlier results of Mercer(1984). The magnetic directions for the dike show streaking, which may represent an alteration component in some of the samples. Nonetheless, the overall direction provides a pole position consistent with other 1400—1600 Ma. rocks in the Lake Superior region,and significantly different than that of the quartzite. 109 �References: References: Aldrich,L.T., Wetherill,G.W., ~etheri1l~G.W.~ Bass, MN.., M.N., Compston, Compston, W., Davis, Aldrich,L.T., Bass, W,Davis, G.L., and and Tilton,G.R.,1959, Tilton,GÈR.,1959Mineral Mineral age age measurements: measurements: G.L., 58, p.245—247. p.245-247. Carnegie Institute Washington Year Book Carnegie Institute Washington Year Book. 58, Kean, WF., W.F., 1994, 1994,Paleomagnetism Paleomagnetismofofa a1500 1500Ma. Ma. mafic maficdike dikeat at Kean, Institute on Lake Superior Geology abstracts Waterloo Wi. Waterloo Wi. Institute on Lake Superior Geology abstracts Vol.40, p.17. p.17. and proceedings, proceedings, Vol.40, and Luther, F., F., 1997, 1997, The The precambrian precambrian Waterloo waterloo quartzite, quartzite, Dodge Dodge and and Luther, Counties, Wisconsin-petrology, structure, and Jefferson Jefferson Counties, Wisconsin—petrology, structure, and Field Trip Trip No.5, No.5, in in Guide Guide to to Field Field Trips Trips in industrial uses, uses, Field industrial Wisconsin and Adjacent Areas of Minnesota. 31st annual Wisconsin and Adjacent Areas of Minnesota. 31st annual meeting of of the the NCGSA, NCGSA, Madison Madison , Wi. Wi. Ed. Ed. M.G. MçG Mudrey, Mudrey, Jr. Jr. meeting , Mercer, D., Paleomagnetism of of the theBaraboo BarabooQuartzite, Quartzite,tJW— UWMercer,. D., 1984, 1984, Paleomagnetism Milwaukee M.S. thesis, 294 p. Milwaukee M.S. thesis, 294 p. FIGURE 11 FIGURE FOR WATERLOO WATERLOO BASALTIC BASALTIC DIKE DIKEAND AND WATERLOO WATERLOO MAGNETIC DIRECTIONS DIRECTIONS FOR MAGNETIC HAS NEGATIVE INCLINATIONS AND NORTHERLY QUARTZITE. DIKE MATERIAL QUARTZITE. DIKE MATERIAL HAS NEGATIVE INCLINATIONS AND NORTHERLY DECLINATIONS. THE QUARTZITE HAS POSITIVE INCLINATIONS AND DECLINATIONS. THE QUARTZITE HAS POSITIVE INCLINATIONS AND WESTERLY DECLINATIONS. THE DATA REPRESENTS TWO DIFFERENT WESTERLY DECLINATIONS. THE DATA REPRESENTS TWO DIFFERENT EXPOSURES IN IN THE THE SAME SAME QUARRY QUARRY EXPOSURES N o EXPOSIJ( H(Z NES DC smir. £ EflU( 1(6 IPC RA.T s. HORIZ EXPO$UI( P06 DC OUNflZITE * WRT OPOSU( P06 DC IW1TZITE 110 I �I METAMORPHISM,HYDROTHERMAL HYDROTHERMALALTERATION ALTERATION AND AND LATERITIC LATERITIC METAMORPHISM, WEATHERING OF OF DRILLED MRS VOLCANIC ROCKS IN IOWA WEATHERING SCHMIDT, Susanne Th.! Th., Mineralogisch-Petrographisches Mineralogisch-Petrographisches Institut, SCHMIDT/ Susanne Institut/ Bernoullistr. Bernoullistr. 30, 30/ CH CH 4056 4056 Basel, Basel/ Switzerland, Switzerland/ schmidts~ubac1u.unibas.ch;SEIFERT, SEIFERTt Karl! schmidts@ubaclu.unibas.ch; Karl, Departement Departement of Geological & Atmospheric AtmosphericSciences, Sciences/ Iowa Iowa State StateUniversity, University/Ames, Ames/IA IA50011, 500111 kseifert@pop-2.iastate.edu. kseifertapop-2.iastate.edu. and cuttings from deep wells into the Precambrian igneous rocks Well cores and of the buried buried Midcontinent MidcontinentRift RiftSystem System in in Iowa Iowa (Iowa (Iowahorst, horst/Anderson, Anderson/1990) 1990) were studied to to identify identify primary primary and and secondary secondary mineral mineral assemblages assemblages and the compositionwas was determined determined by by instrumental instrumental neutron neutron bulk geochemical composition activation analysis (INAA) for trace elements and and by inductively inductively coupled coupled for major major elements. elements. The results indicate a complex plasma (ICP) analysis for alteration history with metamorphic and hydrothermal hydrothermal stages, stages! followed followed by lateritic weathering on the Precambrian erosion surface. surface. Minerals Minerals were analyzed analysis/clay clay minerals minerals by by X-ray X-ray diffraction diffraction analyzed by by electron electron microprobe microprobe analysis, (air-dried and gly~olated)~ and the theisotopic isotopic composition compositionof of calcite calcite was was (air-dried and and glycolated), determined. The large majority of these Midcontinent Rift samples were originally & Anderson, Anderson/ 1996) 1996) compositionally similar to basalts or diabases diabases(Seifert (Seifert & intermediate olivine olivine tholeiites tholeiites as as described described by by Brannon Brannon (1984) (1984) from the exposed north north of Midcontinent Rift or North North Shore Shore Volcanic Volcanic Goup (NSVG) (NSVG) exposed Duluth in Minnesota. Minnesota. Although Although alteration alteration has greatly modified modified the primary samples/ relicts relicts of the original original magmatic magmatic composition and texture texture of of most most samples, mineral assemblage can still be be observed observed in in some some samples. samples. Based on amygdule frequency frequency and and degree degreeof of alteration alteration itit is is possible possible to to differentiate differentiate morphological flow flowunits. units. Flow tops with a large between the various morphological Flow tops number of amygdules and and highest highest degree degree of of alteration alteration can be distinguished distinguished without any amygdules and a less from massive flow interiors without less intensive Massive alteration. In some flow tops no primary igneous texture is visible. Massive of alteration but but the primary texture is flow interiors show various degrees of often preserved. A relativly homogenous regional metamorphic alteration alteration pattern is observed based on the highly amygdaloidal amygdaloidal flow tops and amygdule amygdule minerals. In addition, some massive flow interiors have preserved part of addition/ some of the the early early alteration history. history. The Theassemblage assemblageepidote-Fe-rich-chlorite-albiteepidote-Fe-rich-chlorite-albitequartz±pumpellyite±sericite is characteristic characteristic for for all all studied studied drill sites of of the quartz~umpellyite~ericite facies. The Iowa horst and and indicates indicates conditions conditions of the beginning greenschist facies. The same facies is also observed in the lowermost part of the ca 8 km thick NSVG the lowermost part of the ca 8 k m thick NSVG in Minnesota near Duluth which is interpreted to be the result of of burial metamorphism (Schmidt, 1990; 1993). metamorphism (Schmidt/ 1990; 1993). 111 �The Sharp #1 core documents the complex alteration history that affected at least part of the Midcontinent Rift System in Iowa. A sequence of secondary alteration stages can be established. In the massive flow interior of Sharp #1 (sample 2206.4), an early Fe-poor, Si-rich phyllosilicate replaces pyroxene along grain boundaries. Fe-poor, Si-rich phyllosilicates (smectites) are also reported from massive flow interiors of the NSVG indicating an earlier alteration stage under lower temperatures (Schmidt & Robinson, 1997). The Fe-poor, Si-rich phyllosilicate is in turn replaced by a late Fe-rich chlorite which also occurs in veinlets. This Fe-rich chlorite also forms part of the epidote-Fe-rich-chlorite-albite-quartz ± pumpellyite ± sericite assemblage in the amygdaloidal flow tops. In samples, a late potassic alteration is present with K-feldspar as a frequent overgrowth not only of silicates (albite and pyroxene), but also of chalcopyrite. All samples which were derived directly from the original Precambrian erosion surface are partly intensively altered to kaolinite which is attributed to a tropical lateritic weathering (Seifert & Anderson, 1996). This correlates well with the suggested position of Iowa near the equator at the end of the Precambrian time. No kaolinite is present deeper in the drill cores. Anderson, R.R. (1992) The Midcontinent Rift of Iowa. Ph. D. thesis, University of Iowa, Iowa City, 324p. Brannon, J.C. (1984) Geochemistry of successive lava flows of the Keweenawan North Shore Volcanic Group. Ph. D. Thesis, Washington University, St.Louis, 3l2p. Schmidt, S.Th. (1990) Alteration under conditons of burial metamorphism in the North Shore Volcanic Group, Minnesota - Mineralogical and geochemical zonation. Heidelberger Geowisscnschaftliche Abhandlungen, Band 41, 309 p. Schmidt, S.Th. (1993) Regional and local patterns of low-grade metamorphism in the North Shore Volcanic Group, Minnesota, USA. Journal of metamorphic Geology, 11, 401-414. Schmidt, S.Th. & Robinson, D. (1997) Metamorphic grade and porosity and permeability controls on mafic phyllosilicate distributions in a regional metamorphic zeolite to greenschist facies transition of the North Shore Volcanic Group. Geological Society of America, Bulletin, 109, 683-697. Seifert, K. & Anderson, R.R. (1996) Geochemistry of buried Midcontintent Rift volcanic rocks in Iowa: Data from well samples. Jour. Iowa Acad. Sci. 103 (3-4), 63-7. 112 �CRUSTAL CRUSTALRECYCLING RECYCLINGIN INTHE THEEVOLUTION EVOLUTIONOF OFTHE THEPENOKEAN PENOKEANOROGEN: OROGEN:ISOTOPIC ISOTOPIC EVIDENCE EVIDENCEFOR FORARCHEAN ARCHEAN CONTRIBUTIONS CONTRIBUTIONS TO TO CRUSTAL CRUSTAL GROWTH GROWTH IN IN THE THE PEMBINE-WAUSAU TERRANE, TERRANE, NORTHERN NORTHERN WISCONSIN WISCONSIN PEMBINE-WAUSAU Schulz,Klaus KlausJ.J.and andAyuso, Ayuso,Robert RobertA., A.,U.S. U.S.Geological GeologicalSurvey, Survey,954 954National NationalCenter, Center,Reston, Reston, Schulz, VA20192 20 192(kschulz@usgs.gov) (kschulz@usgs.gov) VA Continental Continentalgrowth growthmodels modelsdepend dependon oninformation informationregarding regardingthe theage ageofofbasement basementininorogenic orogenic belts beltsand andon onestimates estimatesof ofthe theamount amountof ofcrustal crustalrecycling recycling involved involvedin in their theirevolution. evolution. The Thedistribution distribution and androle roleof ofArchean Archeanbasement basementininthe theEarly EarlyProterozoic ProterozoicPenokean Penokeanorogen orogenininthe theLake LakeSuperior Superiorregion region has haslong longbeen been aa topic topic of of debate debate and and speculation. speculation. Early Earlyworkers workersviewed viewed the the Early Early Proterozoic Proterozoic history history of theregion regionin interms termsof ofintracratonic intracratonicdeposition depositionand andreactivation reactivation of of Archean Archean crust crust of of the the Superior Superior the Province Province(e.g., (e.g.,Sims, Sims,1976). 1976).More Morerecently recentlythere therehas hasbeen beenaageneral generalconsensus consensusthat thatthe theEarly Early Proterozoic Proterozoicrocks rocksevolved evolvedthrough throughplate platetectonic tectonicprocesses processesincluding includingcontinental continentalrifling riftingand andsubduction, subduction, with wasaccreted accretedtotothe theArchean Archeancraton craton withformation formationofofsignificant significantvolumes volumesofofjuvenile juvenilecrust crustthat thatwas (Hoffman, (Hoffinan,1988; 1988;Sims Sirns and and others, others, 1989). 1989).However, However,isotopic isotopicstudies studieson onrocks rocksfrom fromthe theWisconsin Wisconsin maginatic magmaticterranes terraneshave haveagain againraised raisedquestions questionsabout aboutthe thedistribution distributionand androle roleofofArchean Archeancrust crustininthe the Penokean Penokeanorogen orogen (Barovich (Barovichand and others, others, 1989; 1989;Van Wyck and Johnson, 1997). 1997). In Inparticular, particular,Van VanWyck Wyck and and Johnson Johnson(1997) (1997)proposed proposedthat thatthe thePenokean Penokeanorogen orogenevolved evolvedthrough throughback-arc back-arcrifling riftingofofthe thesouthern southern Superior SuperiorProvince, Province,followed followedby bycollision collisionof ofaacontinental continentalarc arcterrane terrane(the (theMarshfield Marshfieldterrane) terrane)from fromthe the south. south. One Oneof ofthe theprimary primarylines linesof ofevidence evidenceused usedby byVan VanWyck Wyckand andJohnson Johnson(1997) (1997)totosupport supportaamodel model of of continental continentalback-arc back-arcrifling riftingfor forthe thePembine-Wausau Pembine-Wausauterrane terranewas wasan aninferred inferredcorrelation correlationbetween betweenthe the degree degree of of crustal crustal contamination contamination and and distance distance from the northern margin of the terrane, the Niagara fault zone, (i.e.,€Nd(T) eNd(T)values valuesbecome becomemore morepositive positive zone, in in which whichcontamination contaminationdecreased decreasedsouthward southward(i.e., southwards). southwards). To Totest testthis thismodel modelfor forthe thePembine-Wausau Pembine-Wausauterrane terraneand andfurther furtherrefme refineestimates estimatesfor forthe the involvementof of older oldercrust, crust,14 14volcanic volcanicrocks rocksand and14 14granitic graniticrocks rocksfrom fromacross acrossthe thePembine-Wausau Pembine-Wausau involvement terranein innorthern northernWisconsin Wisconsinwere wereanalyzed analyzedfor forNd Ndand/or andlorPb Pbisotopes. isotopes. These Thesesamples, samples,collected collectedfrom from terrane thethree three principal principal outcrop outcropareas areasin innorthern northern Wisconsin—the Wisconsin-the Dunbar-Pembine Dunbar-Pembine area area located located in in the northeastern northeasternWisconsin Wisconsinjust justsouth southofofthe theNiagara Niagarafault faultzone, zone,the theMonico Monicoarea arealocated locatedabout about45 45 km krn south south of ofthe theNiagara Niagarafault faultzone zoneininnorthcentral northcentralWisconsin, Wisconsin,and andthe theMarathon MarathonCounty Countyarea arealocated locatedatatthe the southern southernmargin marginof ofthe thePembine-Wausau Pembine-Wausauterrane terraneinincentral centralWisconsin—significantly Wisconsin-significantly expand expandthe the sampling sampling density density in in these thesethree threeareas areasand andfor forthe thefirst firsttime timeprovide provideisotope isotopedata datafor forseveral severalvolcanic volcanic units units within within the the Pembine-Wausau Pembine-Wausau terrane. terrane. Results Resultsare aresummarized summarizedininthe thetable tablebelow. below. The Theanalyzed analyzedmafic maficvolcanic volcanicrocks rocks from from the the Monico Monicoand and Marathon Marathon County County areas areasare are light light REEREEenriched from +1.3 +1.3 to to +3 +3 and and relatively relatively primitive primitive p values. These enrichedbut but have havepositive positiveENd(T) ENd(T)from TheseNd Ndisotope isotope data dataare are slightly slightlymore more enriched enriched than than the the Nd isotope isotope results of of Beck and and Murthy Murthy (1991) (199 1) for for the the +4.2 )from fromthe thePembine Pembinearea areaand andsuggest suggestderivation derivationfrom fromdepleted depletedEarly Early Quinnesecbasalts basalts(€Nd(T) (eNd(T) +4.2) Quinnesec Proterozoic (10%) addition contrasttoto Proterozoic mantle mantle with possibly a small (510%) additionofofolder oldercrustal crustalcomponents. components.InIncontrast the the mafic mafic volcanic volcanic rocks, rocks, the the felsic felsic volcanic volcanic rocks rocks from throughout throughout the Pembine-Wausau Pembine-Wausau terrane terrane have eNd(T) values ranging ranging from -0 —0toto-4 -4and andrelatively relativelyhigh highppvalues values >10 >10 suggesting suggesting variable but significant eNd(T) values significant input inputof of older, older, probably probablyArchean Archean crustal crustalcomponents. components. Surprisingly, Surprisingly,the thefelsic felsicvolcanic volcanicrocks rocksfrom fromthe the Monico Monico area area show show the the greatest greatest crustal crustalcontamination. contamination. The Thegranitic graniticrocks, rocks,like likethe thefelsic felsicvolcanic volcanicrocks, rocks, also alsohave havemostly mostlynegative negativeENd(T) eNd(T)values valuesbut butshow showaagreater greaterrange rangefrom from—0 -0 to to—7.4, -7.4, and relatively high high pp values values >10; >lo; younger youngergranites granitestend tendto tohave havethe themost mostnegative negative€Nd(T) e ~ d ( Tand ) highest p values values in in both both the the Dunbar-Pembine Dunbar-Pembine and and Marathon Marathon County County areas. areas. The Theisotope isotopedata datalie liealong alongmixing mixinglines linesbetween betweendepleted depleted Early Proterozoic mantle and Archean Superior Province crust; mixing models suggest from 20 Early Proterozoic mantle and Archean Superior crust; mixing models suggest from to to >70% >70%Archean Archeancrustal crustalcontamination contaminationfor forfelsic felsicvolcanic volcanicand andgranitic graniticrocks. rocks. - 113 I �I Areas Dunbar-Pembine (North) Dunbar-Pembine (North) • Pemene Pemene Rhyolite Rhyolite (2) (2) • Dunbar Dunbar Dome Dome (5) (5) • Bush Bush Lake Lake Granite Granite (1) (1) • Twelve Foot Falls Falls (1) Quartz Quartz Diorite Diorite Monico Monico Basalt (2) (2) • Basalt • Dacite/Rhyolite DaciteRhyolite (3) (3) • Quartz Quartz Porphyry Porphyry (1) (1) Marathon Marathon County County (South) (South) Basalt/Andesite (4) • BasaltIAndesite • DaciteIRhyolite Dacite/Rhyolite (3) (3) • Pre/Syn-Tectonic PrefSyn-TectonicGranite Granite (3) (3) Syn/Post-Tectonic Granite Granite (3) • SynPost-Tectonic TDM TDM(in Ga) Ga) ENd(T) e~d(T.1 - — M.35 +0.35 p (8U/204Pb) (*8~/20"~b) 12 --10.4 11.5 .7.4 -7.4 -3.2 2.3 2.3 2.2 to 2.9 2.9 2.8 +2.1 +2.1 -3.1 -3.1 •1 -4.7 2.1 —2.6 2.6 2.5 --9.811 9.4 +3 to +1.3 +1.3 to -2.0 2.0 -0.8 to 4.0 -1.0 42 -4.2 2.0 to 2.2 2.2 to 2.3 2.3 2.8 2.8 9.8 ---10.6 - 10.9 13.2 -0.9 to -4.2 - -- -- 12 10.4 11.5 - — 9.8 11 9.4 9.8 10.6 10.9 .-. 13.2 The new new data datapresented presented here here do donot notsupport supportaacorrelation correlationof ofincreasing increasing6Nd eNd with distance from Niagara fault zone. zone. The the Niagarafault The felsic felsic volcanic rocks from throughout the terrane show isotopic evidence greatest crustal crustal component component in in rhyolites rhyolites from fromthe theMonico Monicoarea. area. The for crustal contamination with the greatest granitic rocks from throughout the terrane also show isotopic evidence for significant significant crustal contamination. However, However, the the isotope isotope data datado do suggest suggest that that crustal contamination was greatest for syn- to post-tectonic granites emplaced near the northern and southern margins of the Pembine-Wausau terrane. post-tectonic isotope data data for for the Pembine-Wausau terrane suggest suggest that: (1) (1) input input of of Archean crustal crustal The new isotope greater and more widely distributed than previously components to Penokean crust crust formation was greater (2) there there isisno noclear clearcorrelation correlationbetween betweenENd EN^ and and distance from the northern (Niagara) recognized; (2) suture zone, although contamination appears to have been greatest for syn- to post-tectonic granites shown by the emplaced near the margins of the terrane; and (3) the high levels of crustal components shown felsic igneous rocks probably result from crustal assimilation and mixing and not from contamination contamination of a depleted mantle source by subduction of Archean derived sediments. References cited Barovich, KM. Cia KM.,Patchett, Patchett,P.J., P.J.,Peterman, Petennan,Z.E., Z.E.,and andSims, Sims,P.K., P.K.,1989, 1989,Nd Ndisotopes isotopesand andthe theorigin originof of1.9-1.7 1.9-1.7 Ga Penokean Penokean continental continental crust crustof of the the Lake Lake Superior Superiorregion: region: Geological GeologicalSociety Society of of America AmericaBulletin, Bulletin,v. v. 101, 101, p. 333-338. Beck, W. and and Murthy, V.R., V.R, 1991, 1991,Evidence Evidence for for continental continental crustal crustal assimilation assimilation in in the the Hemlock Hemlock Formation Formation flood flood basalts basalts of of the the Early Early Proterozoic ProterozoicPenokean Penokean Orogen, Orogen, Lake Lake Superior Superiorregion: region: U.S. U.S. Geological Geological Survey Survey Bulletin 25p. Bulletin 1904-I, 1904-1,25p. Hoffman, P.F., 1988, Hoffinan, 1988,United United plates plates of of America, America, the the birth birth of craton: craton: Early Proterozoic Proterozoic assembly assembly and and growth growthof of Laurentia: Laurentia:Annual AnnualReviews Reviews of of Earth Earth and and Planetary Planetary Sciences, Sciences, v. 16, 16,p. p. 543-603. 543-603. Sims, Sims, P.K., 1976, 1976, Precambrian Precambrian tectonics tectonics and and mineral deposits, deposits, Lake Lake Superior Superior region: Economic Economic Geology, Geology, v. 71, 71, p. 1092-1177. 1092-1177. P.K., Van Schrnus, W.R., Schulz, KJ., K.J., and Peterman, Petennan, Z.E., 1989, Sims, P.K., Schmus, W.R, 1989, Tectono-stratigraphic Tectono-stratigraphic evolution evolution of the Early Early Proterozoic ProterozoicWisconsin Wisconsin magmatic magmatic terranes terranes of the the Penokean Penokean Orogen: Orogen: Canadian CanadianJournal Journalof of Earth Earth Sciences, v. 26, p. 2145-2158. Van Wyck, N. and Johnson, Johnson, C.M., C.M., 1997, 1997, Common Common lead, lead, Sm-Nd, Sm-Nd, and and U-Pb constraints constraintson on petrogenesis, petrogenesis, crustal crustal architecture, architecture,and and tectonic tectonic setting setting of of the the Penokean Penokean orogeny orogeny (Paleoproterozoic) (Paleoproterozoic)in in Wisconsin: Wisconsin: Geological Geological Society of America Bulletin, Bulletin, v. 109, 109, p. 799-808. , 114 " - II I �I ND ISOTOPE EVIDENCE EVIDENCE FOR FOR MIDDLE MIDDLE AND AND EARLY EARLY ARCHEAN ARCHEAN CRUST CRUST IN THE WAWA SUBPROVINCE OF THE SUPERIOR PROVINCE, MICHIGAN, U.S.A. IN Sims, P.K., Neymark, L.A., L.A., and and Peterman, Petennan, Z.E., Z.E., U.S. U.S. Geological Geological Survey; Survey; Kotov, A.B., A.B., Institute Institute of Precambrian Precambrian Geology Geology and and Geochronology, Geochronology, St. St. Petersburg, Petersburg, Russia Russia Kotov, New Sm-Nd New Sm-Nd isotopic isotopic data on Late Late Archean Archean granitic granitic rocks rocks from from the theWawa Wawasubprovince subprovince(a(agranite-greenstone granite-greenstone terrane) indicate aa Middle to Early Archean crustal source for the rocks. rocks. terrane) of the Superior province, northern Michigan, Michigan, indicate Early Archean model ages between between 2.7 and 3.6 Ga At 2.7 Ga pNd values vary from -6.4 to +3.3, which correspond to depleted mantle mantle model Previous Nd-isotope Nd-isotope studies studies in in granite-greenstone terranes in in the the southern part of the (see table). Previous granite-greenstone terranes the Superior Superior province province indicated that that most most granitoid rocks are juvenile, juvenile, i.e. i.e. mantle derived, with little identifiable participation from older have indicated crust or recycled material. Samples were taken from the Puritan batholith (Fig. I), I), which which lies lies astride astride the Michigan Michigan -- Wisconsin Wisconsin border, batholith (Fig. border, and the northern complex of the Marquette district district (Fig. (Fig. 2); both igneous igneous bodies bodies have have crystallization crystallization ages agesof of—2.7 -2.7 Ga. Ga. Candidatesfor for the the source source of of the Middle Candidates Middle and Early Early Archean Archean rocks rocks in the the Michigan Michigan segment segment of the the Wawa Wawa subprovince have not been been identified, identified, but these rocks mainly constitute ancient ancient continental continental crust, crust, perhaps a proto-craton proto-craton block like that in the Sachigo and Minto subprovinces subprovinces (Percival (Percival and and other, other, 1994) 1994)to to the the north. north. Not unexpectedly, high-grade gneisses from the adjacent Minnesota River Valley subprovince, to to the south, which which contains the oldest rocks in the Superior province, have have comparable comparable old old Nd-depleted Nd-depleted mantle mantlemodel modelages ages(see (seetable), table), ranging from 2.9 to 3.3 Ga. d Table. Sm-Nd Sm-Ndisotope isotopedata datafor forgranites granitesfrom from Wawa WawaSubprovince Subprovinceand andfelsic gneisses and and granites granites from from Minnesota felsic gneisses Subprovince. River Valley Suburovince. '47Sm/ 143Ndt T(Ga) Nd Sample Sm d(t) d(O) TDM TDM '44Nd '44Nd (ppm) (ppm) Subprovince (Granite-Greenstone (Granite-Greenstone Terrane) Wawa Subprovince Terrane) 182B 183B 198 215B 0919-1-89 0919-3-89 0920-4-89 l36R M169 Ml70 177 179 D1044 D1731 D2493 D2494 D2495 2.7 2.7 2.7 2.7 2.7 2.7 2.7 2.7 2.7 2.7 2.7 2.7 2.7 2.7 2.7 2.7 2.7 8.88 19.7 5.70 2.76 4.06 8.08 3.56 0.79 1.60 2.98 40.9 98.2 32.2 17.0 30.2 57.7 17.9 3.11 11.5 17.4 2.53 13.8 1.01 4.67 4.10 16.7 2.08 10.5 1.26 6.48 5.06 33.6 5.60 33.6 0.0980 0.1214 0.1074 0.0982 0.0815 0.0925 0.1205 0.1545 0.0848 0.1041 0.1107 0.1315 0.1486 0.1200 0.1176 0.0913 0.1012 0.510695 (13) 0.511072 (5) 0.510842 (8) 0.510672 (13) 0.510594 (12) 0.510707 (6) 0.511397(10) 0.511781 (9) 0.5 10708 (10) 0.511040(10) 0.510912 (10) 0.511415(9) 0.511572(7) 0.511070(7) 0.511266(7) -3.7 -4.5 0.510928 (7) 0.510792(4) 3.3 -37.9 -30.5 -35.0 -38.4 -39.9 -37.7 -24.2 -16.7 -37.6 -31.2 -33.7 3202 3394 3276 3237 2922 3044 2840 3478 2863 2908 3277 -23.9 -20.8 -30.6 -26.8 -33.4 -36.0 3182 3646 3347 2959 2744 3163 3334 3123 3358 3347 2966 2758 3256 -39.8 -32.1 2971 3037 -31.4 -31.6 3192 3309 3072 3226 3325 3104 -6.4 -36.0 3358 3456 -2.3 -2.3 -31.8 3068 3127 -4.1 -4.2 0.1 -1.5 2.3 -2.1 1.2 1.0 -3.8 -1.2 -4.1 -4.0 0.7 -2.9 213 2.8 2.8 2.8 Subprovince (Gneiss Terrane) Terrane) Minnesota River Valley Valley Subprovince 44.9 0.0841 0.510600(8) 0.8 6.22 44.9 0.0841 0.5 10600 (8) 0.8 0.510993(11) 0.510993 (1 1 ) 2.35 12.8 0.1117 -1.5 12.8 0.11 17 2.35 0.1170 0.511027(10) 1 1027 (10) 0.5 38.2 -2.7 38.2 0.1 170 7.36 0.511018(8) 21.2 0.1089 0.0 0.1089 0.0 3.80 3.80 21.2 3-90 2.6 10.5 10.5 34-90 161-B 211 2.8 3319 3384 3352 3361 3012 3144 2839 3193 2924 2943 Tilden Granite Granite 59.2 0.510791 0.5 1079 1 (8) 0.1078 0.1078 Gwinn Granite 226-89 2.6 5.65 5.65 31.6 31.6 0.511006(10) 0.511006(10) 0.1082 0.1082 2 within-run **)) Data Data corrected to La La Jolla Jolla '43Nd/'44Nd=0.5 '"Nd/'"Nd=0.5 1 11860, 1860, 2,-r within-run errors errors are are given given inin parentheses. parentheses. 12ininthe the source source of of rocks before T Ga ** *) * ) Two-stage Two-stage Nd-model age age assuming assuming47Sm/'41Nd=0. '"Sm/'"Nd=O. 12 Ga ago. ago. 115 �________________________________ W" 89' 88O I 1 LAKE SUPERIOR 47% Figure 1. 1.Geologic Geologicmap mapofofPrecambrian Precambrianrocks rocksininnorthern northernMichigan Michiganand and adjacent adjacent Figure Wisconsin, showing showing localities Sm-Nd samples. Figure 2 is an enlargement of area outWisconsin, - localities of Sm-Nd samples. Figure 2 is an enlargement of area outlined lined south south of of Marquette. Marquette. n EXPLANA11ON EXPLANATION EXPLANATION —Contact Paleozoic Paleozoicrocks, rocks,undivided uncfwidd (1,600-900Ma) Ma) MIDDLEPROTEROZOIC PROTEROZOIC(1.600-900 MIDDLE ROCKS subprovinces '—Mylonde (ca. 1 ,I 00Ma) Ma) Rocksof ofmidcontinent midcontinentrift riftsystem system(ca. Rocks 1,100 Sm-Nd age locality . Wawa / Sub- Palmer EARLY (2,500—I .600 Ma) EARLYPROTEROZOIC PROTEROZOIC (2,500-1,600 Ma) province '.k)lcanic Volcanic.and andgranitoid granitoidrocks rocksofofWisconsin Wisconsin 1618 magmatic zone (—1.880—1,860 (-1.880-1.860Ma) Ma) magmatlc zone Sedimentary Sedimentaryand andvolcanic volcanicrocks rocksofofMarquette Marquette Range RangeSupergroup Supergroup •3.90 Puritanbatholith batholiih (-2,700 Ma) Puritan (—2,700 Ma) Granitoid Granitoidrocks rocksofofnorthern northerncomplex complexofof Marquette district (—2,700 Ma) Marquette district (-2,700 Ma) ' SAND COVER B 828 34-90 ' Minnesota A subprovince ARCHEAN ARCHEAN(2,500 (2,500Ma Maand andolder) older) ':2 PRO TEROZOIC . MinnesotaRivervalley Jacobsville JacobsvilleSandstone Sandstone n 1361 Approximate boundary between Wawa and Valley 21 21 '\\ 5 MILES 5 KILOMETERS Metavolcajijc Metavolcanicrocks rocks "3 7- 1 Gneiss (2,750—2,640 Ma) Gneissand andamphibolite amphiboliie (2.750-2.640 Ma) mat3icgneiss gneissofofMinnesota MinnesotaRiver RiverValley Valley Migmatitic prwlnce(3,550—2,800 (3,550-2,800 Ma) subprovince Ma) GLTZ, GLTZ, Ma, Ma, Mar, Mar, Me, Me, Great GreatLakes Lakestectonic tectoniczone zone Marquette, Marquette,Michigan Michigan Marenisco, Marenism,Michigan Michigan Mellen, Mellen,Wisconsin Wisconsin Sm—Nd Sm-Ndage agelocality locality Figure2.2.Geologic Geologicsketch sketchmap mapofofPrecambrian Precambrian Figure rocksininan anarea areasouth southof ofMarquette, Marquette,showing showing rocks localities of Sm-Nd samples. The Great hkes localities of Sm-Nd samples. The Great Lakes tectoniczone zoneseparates separatesgranite-greenstone granite-greenstonetertertectonic rane(Wawa (Wawasubprovince) subprovince)from fromgneiss gneissterrane terrane rane (MinnesotaRiver RiverValley Valley subprovince). subprovince). (Minnesota 116 I �I A THIN SHEET APPROACH TO A T H I N VISCOUS VISCOUS S HEET A PPROACH T O INVESTIGATE INVESTIGATE THE POST POST OF THE RIFT EVOLUTION O FT H E MIDCONTINENT M I D C O N T I N E N T RIFT RIFT SYSTEM SYSTEM UNDER UNDER THE INFLUENCE OF O F GRENVILLE GRENVILLE OROGENY OROGENY Soofi, M. A., A., and and King, King, S. S.D., D., Department Department of of Earth Earth and Atmospheric Sciences, Purdue Purdue Soofi, M. West Lafayette, Indiana University, West Indiana 47907 47907 The cause of the termination termination of of rifting rifting along along the the Midcontinent Midcontinent Rift Rift system system(MCR) (MCR) and the the presence presence of reverse faults along along the rift margins margins are are still still troublesome troublesome for the the investigators of the MCR. Suggestions for the cause of rifting investigators of Suggestions for rifting range range from fiom aapassive passive mechanism, where where processes processesatat plate plate boundaries responsible (e.g. (e.g. Hinze Hinze et al., mechanism, boundaries were were responsible al., 1982), to to an active mechanism, where where aa mantle mantle plume plume ascended ascended and and initiated initiated rifting (e.g. 19821, (e.g. the formation of reverse reverse faults faults has been suggested Cannon and Hinze, 1992). Similarly, the to be under the loading due to rift be aaconsequence consequence of plate flexure under rift related related rocks rocks (e.g. (e.g. stresses from from the the plate boundaries, which Nyquist, 1986), 19861, or an effect effect of compressive compressive stresses which thrusting along the original rift bounding not only terminated the the rifting but also also caused thrusting normal faults (e.g. (eg. Cannon, Cannon, 1994). 1994). study focuses on the evolution of of the the MCR under the the influence of plate plate boundThis study influence of forces from from the the Grenville Orogeny. We are ary forces, forces, particularly particularly the compressive compressive forces Grenville Orogeny. using the thin using thin viscous viscous sheet sheet model model (TVS) (TVS) of of England England and and McKenzie McKenzie (1983) (1983) which which is implemented in in aa finite element element code code by by Houseman Houseman and and England England (1986). (1986). This model implemented model has been used in the the study studyof of India-Asia India-Asia collision collision (England and Houseman, Houseman, 1986) 1986) and (Sobouti and and Arkani-Hamed, 1996), where where it has successArabia-Eurasia convergence convergence (Sobouti successfully reproduced reproduced the topography and deformation in the overriding Eurasian Eurasian deformation observed observed in plate. The Themodel modeluses usesincompressible, incompressible,vertically vertically averaged, averaged,power-law power-law rheology rheology for the the lithosphere. lithosphere. For a given geometry,boundary boundary conditions, conditions,stress-strain stress-strainexponent exponent(n) (n) and and Argand For given geometry, Number (Ar) (Ar) values, stresses, strain rates and Number values, the the model model calculates calculates crustal thickness, stresses, and rotation. rotation. These These can can then thenbe becorrelated correlatedwith withtopography topography and and deformation deformation in in the theover over riding plate. A assumed such such that that there is no discontinuity in the A continuous continuous medium is assumed velocity field. Fault Fault planes cannot be defined defined explicitly explicitlybut, but, information information on on the the type of of velocity field. faulting can be inferred from from the the stress and strain rate rate distribution. distribution. results indicate indicate that even (i.e. n=10) Our results even in the case case of aa very very weak weak lithosphere lithosphere (i.e. n=lO) stresses from the the Grenille GrenilleEont Front (GF) (GF) can can be be transmitted transmitted inland inland to interact with stresses from with the the processes that that were were occuring occuringalong along the the MCR. MCR. Also, Also, depending dependingon on the thesize sizeof of the the colliding colliding microcontinents (i.e. indenters) indenters) and and their their position position along alongthe the GF GF itit is is possible possiblethat that the microcontinents (i.e. MCR was subjected to different magnitude of of stresses stresses along along its its length length and itit evolved was subjected difFerent magnitude evolved through the the superposition superposition of of these stresses. The Thevarying varying degree degree of thrust thrust faulting faulting obobserved along along the the MCR could style. The models could be a manifestation manifestation of such a collision collision style. models also predict significant of crust crust next to the significant thickening of the indenter, indenter,the thethickening thickeningdecreases decreases away from the collision boundary. For For a comparatively rigid lithosphere lithosphere (n=l,Ar=l) (n=1,Ar=1) away fiom collision boundary. comparatively rigid crustal thickening is as much much as as 2 km where for a weak lithosphere (n=lO,Ar=l) (n=10,Ar=1) thickening is where as for weak lithosphere crustal thickening is as much as 19 km. Assuming kg/m3 and Assuming crustal density density of 2700 2700 kg/m3 mantle density density of of 3300 3300 kg/m3 kg/rn3 and and assuming assumingthe the surface surfaceof ofthe the 35 35 km km thick thick crust to mantle to be at at the thesea sealevel level the thechange changein in crustal crustalelevation elevation is is 0.36 0.36 km for for rigid lithosphere and 3.4 km km for for the relatively weak lithosphere. lithosphere. The high elevations elevations predicted predicted by by the the TVS 3.4 relatively weak for the the Tican initiate local local extension, as suggested by England and McKenzie McKenzie (1982) (1982) for betan betan plateau. plateau. Such Suchaamechanism mechanismmay may had hadbeen beenresponsible responsiblefor for the the late latestage stageextension extension reported for the the Grenville Grenville Orogeny (e.g. (e.g. Easton, Easton, 1992). 1992). 117 �We are also studying along the the GF studying the the effect effect of of oblique oblique convergence convergence along GJ? on on the theevoluevolution of the the MCR. MCR. For For fixed fixed boundary boundary conditions conditions and and rheology, rheology, the length length scale scale of of dede formation decreases with increasing obliquity. As discussed for the normal convergence, formation decreases increasing As discussed the normal convergence, microcontinents convergingobliquely obliquelyalong alongthe the GF GF can produce stresses microcontinents of various sizes converging magnitudes along the the MCR. Oblique convergence convergenceproduces producessame samepattern pattern of of of different magnitudes crustal thickening as observed for normal convergence except that it is not symmetric. observed for normal convergence except that it One between oblique oblique and and normal convergence is the the significant area area One significant significant difference between of (indenter). This thinning repof crustal thinning next to to the the colliding colliding microcontinent microcontinent (indenter). resent extension in the over over riding plate. For For normal normal convergence convergence the thinning is is very very localised and produced produced in a very localised and very small small area area that thatdoes doesnot notchanges changessignificantly significantly with with rheology. this area of rheology. For oblique convergence this of extension extension increases increases for for weaker weaker rheology (increasing nn value). We together with a weak We hypothesize hypothesize that that oblique oblique convergence, convergence, together weak lithosphere (due subduction), may had helped, (due to thermal thermal anomaly anomaly resulting resulting from fiom subduction), helped, if if not initiated, the the rifting rifting process. process. REFERENCES CITED CITED of Grenvillian Cannon, Cannon, W. F., F., 1994, 1994,Closing Closing of the Midcontinent Midcontinent rift rift -- A A far-field effect of compression: Geology, Geology, v. 22, 22, p. p. 155-158. 155-158. on the the origin of of the the North AmerCannon, W. F., and Hinze, Hinze, W. J., 1992, 1992, Speculations on ican Midcontinent rift, in Ziegler, P. A., ed., Geodynaniics of rifting, an Ziegler, P. A., ed., Geodynamics of rifting, Volume Volume 2. 2. Case History History Studies Studies on on Rifts: Rifts: North and and South South America America and and Africa: Africa: TectonoTectonophysics, v. 213, 213, p. 49-55. 49-55. Easton, R. R. M., M., 1992, 1992, The The Grenville Grenville Province and the the Proterozoic Proterozoic history history of of Central Central and Southern Ontario, in an Thurston, Thurston, P. P. C., C., Williams, Williams, H. R., Sutcliffe, Sutcliffe, R. H., H., and Stott, Stott,G. G.M., M.,eds., eds.,Geology Geologyof of Ontario, Ontario,Ontario OntarioGeological GeologicalSurvey, Survey,Special SpecialVolume Volume 4, Part 2, p. 715-904. Part 2, p. 715-904. England, P., and and Houseman, Houseman, G., G., 1986, 1986, Finite strain straincalculations calculationsof of continental continental defordeformation 2. 2. Comparison Comparisonwith with the theIndia-Asia India-Asiacollision collision zone: zone: Journal Journalof ofGeophysical Geophysical Research, v. 91, p.3664-3676. 3664-3676. 91, p. England, P., and and McKenzie, McKenzie, D., D., 1983, 1983, Correction to,: to: A thin thin viscous viscous sheet sheet model model for for continental continental deformation: deformation: Geophysical Geophysical Journal Journalof ofthe theRoyal RoyalAstromomical AstromomicalSociety, Society, v. 73, 73, p. p. 523-532. 523-532. Hinze, W. J., Wold, R. J., J., and and O'Hara, O'Hara, N. N. W., W., 1982, 1982,Gravity Gravity and and magnetic magnetic anomaly anomaly Hinze, W. studies of of Lake Lake Superior, Superior, an in Wold, Wold, R. R. J., J., and and Hinze, Hinze, W. J., J., eds., eds., Geology Geology and and tectonics of the Lake Superior basin: Geological Society of America Memoir 156, the Lake Superior basin: Geological Society of America Memoir 156, 203-222. p. 203-222. Houseman, G., and and England, England, P., 1986, 1986, Finite strain strain calculations calculations of continental deformation 1. Method and general results for convergent 1. Method and general results for convergent zones: zones: Journal Journalof of Geophysical Geophysical Research, v. v. 91, 91,p.p.3651-3663. 3651-3663. of the the Mid-Continent Mid-Continent rift rift [Ph.D. Nyquist, J. E., 1986, 1986, Thermal and mechanical models of thesis]: University Universityof of Wisconsin-Madison, Wisconsin-Madison,193p. 193p. Sobouti, F., and of the the deformation of of and Arkani-Hamed, Arkani-Hamed, J., 1996, 1996, Numerical Numerical modelling of the Iranian plateau: plateau: Geophysical Geophysical Journal Journal International, International, v. v. 126, 126,p. p.805-818. 805-818. 118 I �U PALEOMAGNETIC STUDIES OF AA PROTEROZOIC PORPHYRITIC PORPHYRITIC DIABASE DIABASE DIKE, PIFHER AND IRWIN DIKEf IRWIN TOWNSHIPS, TOWNSHIPS LAKE LAKE NIPIGON NIPIGON DISTRICT, DISTRICT ONTARIO ONTARIO T h ~ m a s *C., C~S fKean, Keanf W., Department Department of of Geosciences, Geosciencesf Thomas*, University of Wisconsin-Milwaukeef 53201 Wisconsin—Milwaukee, Milwaukeef Milwaukee, WI 53201 and Luther, Lutherf F., F e fGeology Geology Department, Departmentf UW—Whitewater, UW-Whitewaterf Whitewater, student author) Whitewaterf WI WI 53190 53190 (* ( * student author) This approximately This approximately 50 50 m m wide wide porphyritic porphyritic diabase diabase dike, dikef outcropping outcropping in western Irwin Irwin and Pifher Pifher Townships Townships in in the the Nipigon district, districtf is is locally locally known known as as greenspar. The The dike dike has has a characteristic characteristic greenish greenish mottled appearance appearance in in outcrop outcrop as as aa result of saussaritized saussaritized plagioclase glomerophenocrysts glomerophenocrysts in in aa diabase (Luther,1997). The The dike dike strikes strikes due due north north diabase groundinass groundmass (Lutherf1997). and dips verticallyf vertically, and is off-set by by is found found as as segments, segmentsf off—set east—west east-west faulting(Mackasey,1975). faulting(Mackasey,l975). There There are are no no radiometric radiometric dates on on this this rock rock or or associated associated rocks, rocks, although, althoughf it it is is cut cut by by middle or late Proterozoic sill a large middle sill in Irwin Irwin Township 1975).. Township(Mackasey, (Mackaseyf1975) Paleomagnetic studies were completed on 2—3 2-3 cores cores from from each of 5 locations. Each location represents represents one one dike dike segment. Samples Samples from each location location were subjected subjected to to both alternating field alternating field (A.F.) (A.F.) and and thermal thermal demagnetization demagnetization studies. studies. The is The samples samples show show one one primary primary magnetic magnetic direction direction which which is removed at demagnetization demagnetization temperatures temperatures of of 5700 570 C. or or by by A.F. fields of 60 mT. All but one location normal polarity fields 60 mT. location shows shows normal with inclinations of with northwesterly northwesterly declinations(200°-300°) declinations(200~-300') and inclinations of 40°-80°. The northern northern most most section section of of the the dike dike which which we we sampled 40'-80° The is reversally reversally magnetized magnetized with with declinations declinations of of about about 90°—100° 90'-100~ and inclinations inclinations of of —75°. -75'- These magnetic directions directions are are consistent with Keweenawan age age paleomagnetic directions directions in in the the Lake Superior Superior Region. Region. References References Luther, Proterozoic Lutherf F.,1997, F.f1997f The The Petrology Petrology of of Greenspar: Greenspar: A Proterozoic Porphyritic Porphyritic Diabase Diabase Dike; Dike; Pifher Pifher and Irwin Irwin Townships, Townshipsf Lake Lake Nipigon District, Districtf Ontario. Institute Institute on on Lake Lake Superior Superior Geology Geology Vol.43. abstracts abstracts and and proceedings proceedings Vo1.43. , Mackasey, Mackaseyf W.O.,1975, W.0.f1975f Geology Geology of of Dorothea, Dorothear Sandra, Sandra, and and Irwin Irwin Townships, Townshipsf District District of of Thunder Thunder Bay; Bay; Ontario Ontario division division of of Mines, Minesf 83 p. p. rpt 122 122 with with map map 2294, 2294# 83 119 �., GRAVITY, MAGNETIC, AND STRUCTURAL STUDY OF TJ3E THE WAKEMUP WAKEMUP BAY A GMVITY, TONALITE, MINNESOTA TIKOFF, Basil, Department of Geology and Geophysics, Rice University, Houston, TX 77005-1892, 770051892, USA, BAUER, Robert Department of Geological Sciences, University of Missouri, Columbia, Columbia, MO, 65211, 652 11, USA, VIGNERESSE, VIGNERESSE, Jean-Louis Jean-Louis Vigneresse, Vigneresse, CREGU, 54501 54501 Vandoeuvre, Vandoeuvre, Nancy Cedex, France, and HAGGEMAN, HAGGEMAN, Nick, Nick, of Mimesota, Minnesota, Minneapolis, Minneapolis, MN MN Department of Geology and Geophysics, University of 55455, USA Wakemup Bay The wakemuP Bay tonalite tonalite is is aa small small intrusive intrusive body that intrudes intrudes amphibolite-grade amphibolite-gradebiotite biotite schist on the extreme extreme southern southern edge of the Quetico sub-province sub-province in the western part of Lake Lake Minnesota. The tonalite is part of of the the Wakemup Wakemup Bay Bay block, block, which which is is bounded bounded by by Vermilion in Minnesota. the Vermilion fault to the north and the Haley fault to the south. The Haley fault has a dip-slip component that separates separates amphibolite-grade amphibolite-grade rocks of the Quetico sub-province to the north greenschist-grade rocks of the Wawa sub-province to to the the south. south. The The primary primary purpose purpose of of from greenschist-grade our investigation was to evaluate the mechanism of emplacement of the Wakemup Bay pluton and its relationship to local doming by determining its 3D shape and internal magnetic fabric. conducted aa gravity gravity survey survey over over the the pluton, and use the gravity gnvity inversion inversion to to evaluate evaluatethe the We conducted pluton shape. shape. We Wecollected collectedanisotropy anisotropyof of magnetic magnetic susceptibility susceptibility(AMS) (AMS) data data to to evaluate eval~latethe the magnetic fabric mdgnetic fabric of the pluton. pluton. Bauer Bauer (1985, (1985, 1986) 1986)reports reports previous previous structural structural analysis analysis and mapping mapping of the the Wakemup Wakemup Bay pluton. The pluton comprises a medium-grained biotite tonalite, with sphene, apatite, magnetite, appearance, magnetite, and and zircon zircon as as accessory accessory minerals. minerals. The tonalite typically has a layered appearance, indicate solidand the microstructures throughout throughout the pluton, particularly at the pluton margin, indicate soliddeformation. Foliation dips outward (20-60°) (20-60') from the middle of the pluton, in a state deformation. plagioclase, quartz, and biotite concentric concentric pattern. pattern. Biotite Biotite schist schist -- consisting dominantly of p1agioclase7 surrounds the tonalite tonalite on on all all sides, sides, and and has has foliation foliation that that also also dips dips away away from from the the center centerof of the the pluton on all sides. On On aa more more regional scale, the foliations of the schist defines a doubly plunging, EW-trending anticline with moderately moderately dipping dipping limbs. limbs. The tonalite is spatially B auer(1985, (1985, 1986) 1986)interpreted interpreted as as an an F3 F3 fold fold associated with the hinge area of this fold, which Bauer in the local deformation biotite schist deformation sequence. A hornblende hornblende diorite unit that intruded the biotite The center of of the pluton pluton is is capped by by outcrops on both the north and south sides of the tonalite. The abovethe thetonalite tonalitecontact. contact.The The roof roof consists consists of both the wallrock that that extends extends—40 -40 mmabove a roof of walirock biotite schist schist and and aa relatively relatively flat-lying flat-lyinglayer layer of of the the hornblende hornblende diorite. diorite. The Anisotropy Anisotropy of Magnetic Susceptibility Susceptibility (AMS) is a technique that is widely used in the study = K x H, where M is defmed by M = study of granitic granitic fabrics. fabrics. The The magnetic magnetic susceptibility is defined the induced magnetization and H is the inducing magnetic field. The variation induced magnetization and H is the inducing magnetic field. The variationof of this thismagnetic magnetic susceptibility with the sample placed in different orientations produces an AMS ellipsoid, sample ellipsoid, kmin, which similar to the finite strain ellipsoid. The principle AMS axes are kmax > > kint > > kmin7 plane) or magnetic lineation kint lineation may be interpreted interpreted in terms terms of a magnetic foliation (kmax (kmax orientation). orientation). The bulk susceptibility values varied widely widely in in the the Wakemup Wakemup Bay tonalite 10-1 SI), SI), interpreted as representing the presence or absence of magnetite. The (5 x 10-4 to 10-1 AMS foliation essentially essentially parallels parallels the measured field foliation. foliation. The The major major insight insight from from the the AMS study comes comes from from the orientation orientation of the magnetic lineation. Throughout the Wakemup Bay tonalite, tonalite, the the magnetic magneticlineation lineationisis oriented oriented dominantly dominantly EW, EW, plunges plunges shallowly, shallowly,and andisis generally parallel to the long long direction direction of the Wakemup Bay pluton and parallel to the trace trace of the F3 fold hinge. This This observation observation has has significance significance with respect respect to to emplacement emplacementmechanism. mechanism. We selected gravity study study because: because: 1) 1)ItIt had had selectedthe the Wakemup Wakemuptonalite tonalite for for our our detailed detailed gravity with aa previous structural structural mapping mapping (Bauer, (Bauer, 1985); 1985); 2) It has a single surrounding surrounding lithology lithology with significant and consistent ± 0.04 for the tonalite and 2.75 k ± 0.04 for the consistent density density contrast contrast (2.67 k surrounding 3) A A walirock wallrock roof exists over the center of the pluton, despite despite the surrounding schists); schists); and and 3) (<40 40 m). m). Thus, Thus, the depth recorded by the gravity inversion relatively low relief of the area (< represents represents the the true true depth depth of of the the pluton, within the limitations limitations of the gravity inversion inversion technique technique (e.g., method of Vigneresse, Vigneresse, 1995). 1995). To achieve resolution obtained in structural measurements, Wakemup pluton and immediate measurements,142 142gravity gravity stations stations were collected collected on the Wakemup immediate 120 I ' �I surroundings surroundingsusing using aa Lacoste Lacoste & & Romberg, Romberg, model model G G gravitimeter. gravitimeter. Laser theodolite theodolite from from benchmarks and the controlled-elevation shoreline Of Lake Lake Vermilion provided the existing benchmarks Bouguer anomalies is is provided provided (Fig. (Fig. la). Using a critical elevation controls. A map of the Bouguer gravimetric three-dimensional gravirnetric three-dimensional iterative technique, resulted in a good first-order picture of the (Fig. ib). lb).Most Mostofofthe thepluton plutonisisvery verythin, thin,less lessthan than0.5 0.5 km km thick. thick. There There are aretwo two root root pluton (Fig. zones, both of which which indicate depths of up to 4.0 km, using the calculated densities. The depth krn anomaly in the southwest southwest part of the pluton has a linear, NW trend and is approximately 5 km thus adjacent to the the Haley Haley fault. fault. The The long. It lies adjacent to the SW part of the pluton and is thus contains aa NW depth anomaly anomalyin in the thecenter centerof ofthe thepluton plutonisisslightly slightlyless lessdeep deep(—3 (-3 km). It also contains NW NW-SE oriented trend. trend, although the depth depth is clearly at a maximum in the SE part of the NW-SE interesting to note that the deepest portion of the tonalite does not sit under the present It is interesting exposure of the pluton. As a result of the structural geology, magnetic analysis, and gravity measurements, the Wakemup Bay pluton is interpreted as intruding a broad anticlinal hinge, hinge, during during the the folding folding Wakemup event (F3). This was one of the possibilities suggested by Bauer (1986) and consistent with the very thin nature of of the the pluton pluton (<0.5 ( ~ 0 . km). 5km).The The walirock wallrock preservation preservation in in the the roof represents representsthe the anticlinal anticlinal hinge. hinge. The The hornblende diorite unit on the north side, south side, and above the pluton is a good indication indication of of this this situation. situation. The The two NW-root zones zones were presumably presumably were were feeder feeder zones for the magma, magma, although although given the very high exposure level this claim is impossible to support by geological may have have initiated initiated on on NW NW geological or or magnetic measurements. The feeder zones may oriented fractures. oriented fractures. This This type type of of cross-faulting cross-faulting is commonly commonly seen seen in in folds, folds, particularly particularly those those with a component component of of hinge-parallel hinge-parallel extension. extension. The The large large southern southern root root zone zone may may have have affected affected the subsequent subsequent movement movement of the Wakemup Wakemup block, between the Haley and Vermilion faults. occurs just south of the deep southern root zone, and the root of the the The Haley fault occurs tonalite may have fault shows tonalite have acted acted as as a strong strong heterogeneity in the schist. The Haley fault indications of some inferredto some dextral dextral strike-slip faulting, which is inferred to have have occurred occurred after afterits its earlier normal normal movement movement (e.g., (e.g., Bauer, 1986). 1986). This type of small rigid block moving between major fault fault systems systems is is inferred inferred in other other orogenic orogenic belts. REFERENCES REFERENCES Minnesota. Minnesota Bauer, R.L., 1985, 1985, Norwegian Bay Quadrangle, Quadrangle, St. Louis county, Minnesota. Minnesota Geological Geological Survey, Survey, Miscellaneous Map Series, Map MapM-59, M-59, 1:24,000. 1:24,000. Bauer, R.L., 1986, 1986,Multiple Multiple folding folding and pluton emplacement emplacement in Archean migmatites rnigmatites of the the southern complex,- northeastern northeasternMinnesota. Minnesota. Canadian Journal of Earth Earth southern Vermilion granitic eranitic com~lex, Sciences, v. 23, 23, p. p. 1753-1764. 1753-1764. I I I I I I I I I I I I 5314.00 5312.00 5310.00 5308.00 5306.0 5304.00 5302.00 5300.00-' I I b ! t I I 53002.00 522.00 524.00 524.00 526.00 526.00 528.00 528.00 530.00 530.00 532.00 532.00 534.00 534.00 536.00 536.00 538.00 538.00 540.00 540.00 542.00 542.00 I 544.00 544.00 Fig. Fig. 1. Gravity Gravityinversion inversion model model for for Wakemup Wakemup Bay tonalite. Heavy Heavy dashed dashed line line represents represents current outcropping outcropping of the pluton. Light Lightlines linesare are inferred inferred depth depth of of the pluton, pluton, contoured contoured for for km. The 0.5 km. Thetonalite tonaliteisisgenerally generallyaathin thinsheet sheet(<0.5 ( ~ 0 . km), 5km), with with two two deep deep root root zones. zones. 121 �THE RELEVANCE OF THE GEOLOGY AND OPHIOLITES TO GEOLOGY OF OF MID-OCEAN MID-OCEAN RIDGES AND UNDERSTANDING OF MIDCONTINENT RIFT: RIFT: THE UNDERSTANDING OF LAYERED LAYERED INTRUSIONS INTRUSIONS IN IN THE MIDCONTINENT AND PETROLOGY PETROLOGY PART I. GEOMETRY; GEOMETRY; PART PART II. 11. INTERNAL STRUCTURE STRUCTURE AND Tatiana TatianaVislova, Vislova,Department DepartmentofofGeology Geologyand andGeophysics, Geophysics,University Universityof of Minnesota, Minnesota, USA USA Part Insightsfrom fromcurrent current research research on on mid-ocean mid-ocean ridges and ophiolites remain to be applied Part I.I.Insights to studies studies of ofintracontinental intracontinental rift rift systems. systems. Dynamic Dynamic magma magma chambers chambers where where fractional fractional crystallization and and formation formation of of layered with spreading are crystallization layered cumulates occur simultaneously simultaneously with being ocean island island and and mid-ocean mid-ocean ridge ridge systems. systems. But But layered layered being defined defined by by seismic data for ocean intrusions in in continental continentalcrust crustare are still still generally generallystudied studiedfrom fromthe the point point of of view view of fixed intrusions fixed geometry. conclusion that the Duluth Duluth Complex Complex and and associated associated geometry. However, However, ifif we we accept the conclusion intrusions such as the Sonju Lake and the Bald Eagle intrusions formed in an an intracontinental intracontinental rift system, the concept concept of of dynamic dynamic magma magma chambers chambers during during crystallization crystallization of of layered layered gabbro gabbro must be considered. considered. There is a growing database on the geometry geometry and dynamic dynamic processes processes of of crystallization crystallization and the formation of layered gabbro based based on seismic studies along along the the fast spreading (>-lo (>-10 cm/yr.) cmlyr.) East Pacific Rise (Sinton, 1992; 1992; Wang, 1996; 1996; Barth, Barth, 1996) 1996) and and the theslow-spreading slow-spreading (<-5 crnlyr.)Mohns Mohns Ridge Ridge (Geli, (Geli, 1994) 1994)and and Mid-Atlantic Mid-Atlantic Ridge Ridge (Rommevaux, (Rommevaux, 1994). 1994).These These (<-5 cm/yr.) studies demonstrate studies demonstrate that active active magma magma chambers: chambers: 1) are are segmented segmented and and centered centered under under and active vents along the ridge axes (McKenzie, 1997 topographic highs and 1997 and and Wright, Wright, 1995), 1995), 2) pinch and swell 15 km (Geli, swell in in plan plan view view with withtypical typicalwidths widthsand andalong-axis along-axislengths lengthsofof5 5—- 15 2.5 km below the seafloor. These features are best 1994), and 3) are are found found at atdepths depthsofof1.5 1.5— - 2.5 developed along slow spreading rifts such as the Mohns Ridge (Geli, 1994). 1994). Because ophiolites are fragments of oceanic crust, they provide an opportunity opportunity to to study study the processes that occur along along ridge axes. axes. Recent Recent detailed detailed mapping mapping and interpretation interpretation of of the the Oman generally confirm confirm the the segmental segmental structure structure of of ridge ridge magma Oman ophiolite ophiolite generally magma chambers chambers as described above above (Nicolas, (Nicolas, 1996 1996and and references referencestherein). therein). The geophysical expressions expressions of present-day oceanic oceanic ridges and the 1.1 1.1 Ga Ga Midcontinent Midcontinent bulls-eye structure on on a scale of 100-1000 Rift in North America America show the same same segmented segmented bulls-eye 100-1000 km. On aa scale scaleof ofI 1—- 100 100 km, the geophysical geophysical data on active active magma magma chambers chambers along along ridge ridge axes and the first first derivative derivative of the the aeromagnetic aeromagnetic data on the the Bald Bald Eagle Eagle Intrusion Intrusion have have aa pinch and and swell geometry. geometry. These These similarities similarities on on large large and and small small scales scales imply imply that that our similar pinch current understanding of the processes processes responsible responsible for for the the structure and and petrologic petrologic features features of of understanding of ridge magma chambers and and layered layered gabbro in ophiolites ophiolites can can be be used to explain the origin of similar features features in in layered rifts. The geometries similar layered intrusions intrusions in intracontinental intracontinental rifts. geometries of intrusions intrusions formed in dynamic or static magma chambers are related to spreading rates and continuity of static With fast, fast, continuous continuous spreading spreadingrates ratesititisis possible possibleto to produce produce layered layered gabbro gabbro at at aa spreading. With lateral scale of many kilometers from a relatively narrow, growing magma chamber which is never more than 1-2 km wide at any p.17,842). On the other any given time (e.g., Nicolas, Nicolas, 1996, p.17,842). hand, rates the the lateral lateral extent extent of layered hand, with with slow, slow, episodic episodic spreading spreading rates layered gabbro gabbro could could be restricted to the dimensions dimensions of the active active magma magma chambers. chambers. Thus Thus the the Sonju Sonju Lake Lakeintrusion, intrusion, with its sill-like could have have formed sill-like geometry geometry (Miller, (Miller, 1996), 1996), could formed in a static static magma magma chamber; chamber; whereas the Bald Eagle intrusion, with its funnel shape, could have formed in a more dynamic dynamic environment. environment. Part II. Part It.The Thefunnel-shaped funnel-shaped layering layering in in the theBald BaldEagle Eagleintrusion intrusioncontrasts contrastsremarkably remarkably with near-horizontallayering layeringofofthe the other other intrusions intrusionsinin the the Duluth Duluth Complex Complex and and in the near-horizontal the Oman Oman ophiolite. Unfortunately the current interpretations of the layering in ophiolites do not provide Unfortunately the current interpretations layering do provide 122 �readyexplanation explanation for for the the layering layering in in the the Bald Bald Eagle Eagle intrusion. intrusion. There aaready Thereisisananactive activedebate debateinin the literature literature concerning concerning the the processes processes that would would lead lead from from the the isotropic isotropic crystal crystal mushes mushes of the of active active ocean-ridge ocean-ridge magma magma chambers chambers to the the near-horizontal near-horizontal layering layering in gabbro gabbro of ofthe theocean ocean floorand andininophiolites ophiolites(Quick, (Quick,1993, 1993,and and Nicolas, Nicolas, 1996). 1996).In Inessence essenceQuick's Quick'sanalysis analysisimplies implies floor that the thelayering layeringand andthe theigneous igneouslamination laminationare arepost-crystallization post-crystallizationphenomena, phenomena,produced produced that whencrystal crystalmush mush isiscarried carried away away from from the the active activemagma magma chamber. chamber. IfIfthis thisidea ideaisisapplicable applicableto to when theDuluth Duluth Complex, Complex, the the Bald Bald Eagle Eagle intrusion intrusion could be aa frozen frozen active active chamber chamber thus thus far far not not the observed in in ophiolites. ophiolites. Layered Layered gabbro gabbro of ofadjacent adjacentSouth SouthKawishiwi Kawishiwiintrusion intrusioncould couldbe bethe the observed equivalent equivalentof oflayered layeredgabbro gabbroininophiolite ophioliteproduced producedduring duringspreading spreadingaway awayfrom fromaaridge ridgeaxis. axis. One One might might expect expect that thatthe thecryptic, cryptic,compositional compositionalvariations variations ofofcumulus cumulusminerals minerals formed formedininstatic staticchambers chambersmight mightdiffer differfrom fromthose thoseformed formedinindynamic dynamicchambers. chambers.AAsuggestion suggestion of ofsuch suchdifferences differencescan can be be seen seenin inFigure Figure1.1.All Allthree threedata datasets setslie lieon onaacommon commontrend. trend.Gabbro Gabbro inthe theOman Omanophiolite ophiolite and and rocks rocks in inthe theBald BaldEagle Eagleintrusion intrusion show show more more restricted restricted ranges ranges of of in differentiation than rocks in the Sonju Lake intrusion, and the Oman magma is the most differentiation than rocks Sonju intrusion, Oman magma is the most primitive. primitive. Additional Additional electron electronmicroprobe microprobe mineral mineral and and minorminor- and and tracetrace-element elementrock rockanalyses analyses ofthe theBald Bald Eagle Eagleintrusion intrusionare areplanned planned to toevaluate evaluatethe thesuggestion suggestionthat thatthe thechemical chemicalvariations variations of produced produced in in dynamic dynamic chambers chambers are are 100 100 definitively definitively distinct distinct from from those those L-..-.-.---L-.-..-----^---.....--i^<---.. . 90 jp. established establishedfor forstatic staticchambers. chambers.  + .ac ::80 ^n- ; - 1..-.....---- ---.--.....L--~-.~.---L-..-......L-------.-.~---------- -3 1 70 70 = 0, 0 5 " 3 - - - - -1 ; ; ; + +--+wt*^.+ Hi"++--?--.: ---------- ; L . -------..... - ------- -. Fig. Fig. 1.1.Mg/(Mg+Fe) Mgl(Mg+Fe) variation variationin inpyroxene pyroxeneand and olivine. All the data are electron microprobe olivine. All the data are electron microprobe ..--.-------------___t+ 50 analyses. analyses. Sonju SonjuLake Lakeintrusion intrusion(Miller, (Miller,1996); 1996); + Sonju Lakelnbusion Lake intrusion d.+f? - - - - - - - - - [ - - - - - - - - +SOOjU 40 --..-.40 points are averages Bald Eagle intrusion data Bald Eagle intrusion data points are averages ++ OBaldEagle Eaglelnfrusion Intrusion OBald So --------.---: ---------.; ----xman . - -ophiolite gabbro SIIISof ofover over200 200analyses analysesof ofrepresentative representativesamples samples I I 20 of oftroctolite, tructolite,olivine olivineand andoxide oxidegabbros; gabbros; Oman Oman 20 40 60 80 loo 40 20 0o ophiolite ophiolite—- data data from from gabbro gabbrosills sills(Korenaga, (Korenaga, fitom% %Mg!(Mg+Fe) Ma/(Mg+Fe)in in olivine olivine Pom 1997). 1997). , 60.-----------~----------p.++ --------~.---------~---------60  - ¥  ¥  ¥  I l D References: References:Barth, Barth,G. G.A., A ,and andJ.J.C.C.Mutter, Mutter,Variability Variabilityininoceanic oceaniccrustal crustalthickness thicknessand andstructure: structure:Multichannel Multichannel seismic resultsfrom fromthe thenorthern northernEast EastPacific Pacific Rise, J. Geophys. Res., 17,951-17,975, seismic reflection results Rise, J. Geophys. Res., 10101, 1 17,95 1 17-975,1996. 1996. Geli, Geli, L., L., and andV. V. Renard, Renard, Ocean Ocean crust crust formation formation process at very slow slow spreading spreading centers: centers: A A model model for for the the Mohns Mohns ridge, 2995-3013, 1994. basedon onmagnetic, magnetic,gravity, gravity,and andseismic seismicdata, data,J.J.Geophys. Geophys.Res., Res.,99, 99,2995-3013, 1994. ridge,near near72 72N, N,based Korenaga, Korenaga, J., J., and and P. P. B. B. Kelemen, Kelemen, Origin Origin of of gabbro gabbro sills sills in in the the Moho Moho transition transition zone zone of of the theOman Omanophiolite: ophiolite: Implications magma transport transportin in the the oceanic oceanic lower lower crust, cmst, J. Geophys. Res., 102,27,729-27,749, 102,27,729-27,749,1997. 1997. Implicationsfor formagma McKenzie, McKenzie, D., and and D. D. Fairhead, Fairhead, estimates estimates of of the the effective effective elastic elastic thickness thickness of of the the continental continental lithosphere lithosphere from from Bouguer 27,523-27,552, 1997. Bouguerand andfree freeair airgravity gravityanomalies, anomalies,J.J.Geophys. Geophys.Res., Res.,102, 102,27,523-27,552, 1997. Miller, Ripley, Layered LayeredIntrusions Intrusionsofofthe theDuluth DuluthComplex, Complex, Minnesota, USA, RG.Cawthorn Cahom Miller, J. J. D., and E. M. Ripley, Minnesota, USA, hi:In: RG. (ed) Layered Layered Intrusions, Intrusions,Elsevier ElsevierScience, Science,1996 1996 (ed.) Nicolas, crustalthickness thickness in inthe theOman Omanophiolite: ophiolite: Implication Implication for for Nicolas, A., A , F. F. Boudier, Boudier, and andB. B.ildefonse, Ildefonse,Variable Variable crustal oceaniccrust, Geophys. Res., Res., 101, 101,17,941-17,950, 17,941-17,950,1996. 1996. oceanic crust, J.J. Geophys. Quick, Quick, J.J. E., E., and and R. R P. P. Denlinger, Denlinger, Ductile Ductile deformation deformation and and the origin origin of layered layered gabbro gabbro in in ophiolites, ophiolites, J. Geophys. Geophys. Res., 98, 98, 14,015-14,027, 14,015-14,027,1993. 1993. Res., Rommevaux, study of of the theMid-Atlantic Mid-AtlanticRidge: Ridge:evolution evolution Ronunevaux,C., C., C. C. Deplus, Deplus, and andP. P. Patriat, Patriat,Three-dimensional Three-dimensional gravity gravity study of 3015-3029, 1994. of the thesegmentation segmentationbetween between 28° 28Oand and29°N 2g0Nduring duringthe thelast last10 10my, my, J.J.Geophys. Geophys.Res., Res., 99, 99,3015-3029,1994. Sinton, and R it S. 197-216, 1992. M yand S.Detrick, Detrick,Mid-ocean Mid-oceanridge ridgemagma magmachambers, chambers,J.J.Geophys. Geophys.Res., Res., 97, 97,197-216,1992. Sinton,J.J. IvL, Cochranand andG.G.A.ABarth, Barth,Gravity Gravityanomalies, anomalies,crustal crustalthickness, thickness,and andthe thepattern pattern of of mantle mantle flow flow at at Wane, X., J.J. itRCochran Wan& Res., the EastPacific PacificRise, Rise,9-10 9-10N: N:evidence evidencefor forthree-dimensional three-dimensionalupwelling, upwelling, J.J.Geophys. Geophys. Res., 101, 101, the fast fastspreading spreadingEast 17,927-17,940,1996. 1996. 17,927-17,940, Wright, D. J., Fomari, Wright, D. J., itR.MM.Haymon, Haymon,and andD.D.J. J. Fornari,Crustal Crustalfissuring fissuringand anditsitsrelationship relationship to tomagmatic magmatic and and hydrothermal processes on the East Pacific Rise crest (9°12' to 54'N), J. Geophys. Res., 100, 6097-6120, 1995. hydrothermal (9O12' to 5479, J. Geophys. Res., 100,6097-6120, 1995. - 123 �PRECISE PRECISE U-Pb U-Pb ZIRCON ZIRCON AGES AGES OF O F MIDCONTINENT RIFT RHYOLITE RHYOLITE (CHENGWATANA VOLCANICS), CLAM FALLS, WI CLAM FALLS, WI WIRTH, Karl Karl R., R.,Geology Geology Department, Department, Macalester Macalester College, College, St. St. Paul, Paul, MN MN55105, 55105, WIRTH, wirth@macalester.edu; E.,Department Departmentof ofGeosciences, Geosciences,UniUniwirth @macalester.edu;and and GEHRELS, GEHRELS,George, George,E., versity versity of of Arizona, Arizona, Tucson, Tucson, AZ 85721, 85721, gehrels@geo.arizona.edu. gehrels@geo.arizona.edu. Recent Recent studies studiesof of the the southwestern southwesternsegment segmentof of the the Keweenawan KeweenawanMidcontinent Midcontinentrift riftalong alongthe theSt. St. Croix River River have have focused focused on on the the structural structural (Leslie (Leslie et et al., al., 1994), 1994),paleomagnetic paleomagnetic(Kean (Keaneteta!., al., 1997), evolution of the poorly-exposed poorly-exposed Chengwatana 1997),and and magmatic magmatic (Wirth (Wirthet et al., 1997) 1997) evolution ChengwatanaVolcanics Volcanics (Minnesota (Minnesotaand and Wisconsin). Wisconsin). Comparison Comparisonof ofChengwatana ChengwatanaVolcanics Volcanicsfrom from this this segment segmentof of the the rift with the those of the better-studied portions of the rift in the Lake Superior region (e.g., the those of the better-studied Superior (e.g., Nicholson Nicholson et al., 1997) 1997) have have been been hindered hindered by the the lack lack of of well-constrained well-constrained ages for for the the Chengwatana ChengwatanaVolcanics. Volcanics. The Theonly onlyprevious previousgeochronologic geochronologicstudy studyof of these thesevolcanics volcanics(Zartman (Zartman et exposed in the Ashland syncline et al., al., 1997) 1997)resulted resulted in in an an age age of 1094.6 1094.6± zk 2.1 Ma for rhyolite exposed syncline in northern Wisconsin; rhyolite from drill core in Hudson-Afton Horst (SE of Minneapolis) in northern Wisconsin; rhyolite drill core in Hudson-Afton Horst (SE of Minneapolis) yielded yielded an an anomalously anomalouslyold old(1130 (1130Ma) Ma) age. age. Here Herewe wereport reportprecise preciseU-Pb U-Pbzircon zirconages ages(1102 (1102±k55 Ma) Ma) for for rhyolites rhyolitesexposed exposedin in the the lower lower part part of of the the volcanic volcanic section section along along the the Lake Lake Owen Owen Fault, Fault, which rhywhich bounds bounds the theeastern easternmargin marginof of the therift. rift. Petrographic Petrographic and and geochemical geochemical studies studies of of the the rhyolitic and basaltic flows of this segment of the rift are described by Abbott et al. and Naiman olitic basaltic flows segment Abbott Naiman et a!. al. (both this volume). volume). Rhyolite exposed one kilometer east of Clam Falls, WI contains phenocrysts of plagioplagioclase spherulitic groundmass of tabular tabular quartz quartz and and potassium potassium feldspar. feldspar. The clase and rare quartz quartz in a spherulitic The rhyolite is typically typically finefine- to to medium-grained medium-grained and massive massive (sample (sample KC-302a), but becomes becomes more coarse-grained coarse-grained near near its its base base (sample (sample KC-302d) KC-302d) and and contains containssubangular subangularclasts clastsof ofbasalt. basalt. Flow structures the large large rhyolites rhyolites of of structuresare are not apparent, apparent, but the rhyolites are otherwise similar to the the the North Shore Shore Volcanic Group. Rhyolite Rhyoliteis is rare rare in in this this portion portion of of the Midcontinent Midcontinentrift rift and and is is estimated estimated to to compose compose less less than than one one percent percent of the the exposed volcanic volcanic and and sedimentary sedimentary sections. sections. Zircons (1ength:width ratios ratios == 2:1 2: 1 to to Zircons from from both samples samples have have relatively simple simple prismatic (length:width 8:1) 8: 1) forms, forms, are are translucent, translucent, and and vary vary in in color color from from reddish-brown, reddish-brown, to to honey honey brown, brown, and andto to pale pale pink. Dark Darkgrains grainstypically typicallycontain containabundant abundant inclusions inclusions and were not selected selected for analysis. analysis. Analyses Analyses were were conducted conducted by conventional conventional isotope isotope dilution-thermal dilution-thermal ionization ionization mass mass spectromspectrometry, as described by Gehrels et al. a!. (1991). Six Six euhedral crystals (175-250 jim pm in length) from KC-302a analyzed as as individual individual grains. grains. Three KC-302awere were abraded abraded to to 75% 75% their original size and were analyzed Three ages of the grains grains yielded yielded concordant concordant ages ages with mean 2o6Pb*P38U 2 0 6 ~ b * / 2 3and and 8 ~2o7Pb*/2o6Pb* 207~b*/206~b* ages of 1102 1102 Ma; an uncertainty of k5 ±5 Ma Ma (2a) uncertainty of (20) is is assigned assigned to this age on the basis of the errors errors of the the individual individual determinations, determinations, rather than the error of the mean of the three concordant concordant analyses. analyses. Three Three additional additional grains grains are are slightly slightly discordant, presumably due to small amounts of lead loss. A discordia through these points, projected from from 1102Ma, Ma,yields yieldsaalower lowerintercept interceptofof196 196±205 5 205 A discordia Ma (MSWD (MSWD == 0.15). 0.15). Analyses of KC-302d were conducted on two multigrain fractions, one Ma unabraded unabraded single singlegrain, grain,and andfive fiveabraded abradedsingle singlegrains. grains.The Thesingle singlegrains grainswere wereallall—200 -200 .tm pminin length jim 125 pm length originally, originally, and and the the multigrain multigrainfractions fractionsconsisted consistedof: of:1)1)seven sevengrains grainsmeasuring measuring—125 in length, and 2) thirty grains —80 jim in 1enth. The three abraded grains are analytically 2) thirty grains -80 pm in length. The three abraded grains are analytically Ma k± 5 Ma. Ma. Three concordant concordant and yield mean mean 2o6Pb*/238U 2 0 6 ~ b * / 2 3and and 8 ~ZO7pb*/2O6pb* 2O7pb*/2O6pb*ages of 1101 Ma Three additional grains grains and and the the two two multigrain multigrain fractions fractions are are slightly slightly discordant; discordant;aa discordia discordiathrough through these ±217 217 Ma Ma (MSWD (MSWD==0.45). 0.45). these points, points, projected projectedfrom from 1101 1101Ma, Ma, yields a lower intercept of 63 k Given the uncertainties of ±5 Ma on each of the ages, the 1 Ma age difference between the two Given the uncertainties of k5 Ma on each 1 difference samples samples is is probably probably not not significant. significant. Analyses of the Chengwatana rhyolites near Clam Falls indicate indicate that magmatism magmatism began began by at least least 1102 1102 Ma in in this this portion portion of the rift, considerably considerably earlier than the age age reported reported by by Zartman et al. (1997) for rhyolite (1094.6 ± 2.1 Ma) exposed near the base of the Chengwatana Zartman et al. (1997) for rhyolite (1094.6 k base of the Chengwatana - 124 - I �U 0.190 0.186 00 en 0.182 0.178 Lower Intercept = Lower Intercept = 196 ±205 Ma (MS WD=0.15) I 0.174 1.82 1.90 I I 2.06 1.98 / 63±217Ma(MSWD=0.45) 1.82 1.90 1.98 2.06 207Pb / 235 u 207pb / 235 u section section in the southeastern limb of the Ashland syncline. This This implies implies that flows flows exposed exposed near near Clam Falls likely formed formed contemporaneously contemporaneously with the Upper Kallander Creek Volcanics and the Mellen Conglomerate of Mellen Complex Complex of of Northern Northern Wisconsin, Wisconsin, and the Group 5 flows and Great Conglomerate Mamainse Point. The Maage ageofofChengwatana ChengwatanaVolcanics Volcanics in in the the Clam ClamFalls Fallsregion region The1102 1102±Â55Ma implies implies that that the the Clam Clam Falls flows flows may have "reversed" paleomagnetic paleomagnetic directions directionssince sincethey they are are coeval with the time of the reverse-to-normal magnetic polarity change (1105-1100 Ma) that the time of the reverse-to-normal magnetic change (1 105-1100 has throughout the Lake Superior region. region. New aeromagnetic has been widely observed observed throughout aeromagnetic data data (USGS) (USGS) suggest Falls, MN, MN, are younger than suggest that Chengwatana Chengwatana Volcanic flows exposed near Taylors Falls, those those near Clam Clam Falls. Flows Flowsnear nearTaylors Taylors Falls Falls have predominantly "normal" paleomagnetic paleomagnetic directions (Kean et al., 1997) and are likely younger than the regionally documented reversedirections (Kean et al., 1997) and are likely younger than the regionally documentedreverseto-normal to-normal polarity polarity shift; shift;these theseflows flowsmay may be be correlative correlativewith with Portage PortageLake LakeVolcanics Volcanics (northern (northern WI) and Group Group 66 flows flows (Mamainse (Mamainse Point). A A few few Taylors Taylors Falls flows have "reversed" paleomagnetic directions (Kean reversal event that that is younger than the mean et al., 1997) 1997) and record a reversal regionally observed reverse-to-normal magnetic regionally observed reverse-to-normal magnetic Upper Mich Mich Michipicoten Is. Is. Minnesota Minnesota polarity change. NW Wisconsin NW Wisc Wisc Mamainse Mamainse Point Point polarity change. ' References Cited Cited Gehrels, G.E., W.C., Samson, Samson, S.D., and G.E., McClelland, McClelland, W.C., Patchett, SciPatchett, P.J., P.J., 1991, Canadian Journal of Earth Sciences, p. 1285-1300. 1285-1300. ences, v. 28, P. Green, J.C. and and Fitz, Fitz, T.J., T.J., 1993, 1993, Journal Journalof ofVolcanology Volcanology and Geothermal 177-196. Geothermal Research, Research, v. 54, p. 177-196. Kean, W.F., W.F., Williams, I., Chan, L., and and Feeney, Feeney, J., J., 1997, 1997, Geophysical Geophysical Research Research Letters, Letters, v. 24, p. 1523-1526. 1523-1526. Leslie, M., Wetzel, Wetzel, T., Wirth, K.R., and and Craddock, Craddock,J.P., J.P., 1994, 40thAnnual Meeting 1994,40thAnnual Meeting of of the the Institute Institute on on Lake Lake Superior Houghton, MI, May 11-14, 11-14, Part Superior Geology, Houghton, II -- Program Abstracts, v. 40, p. 35-36. Program with Abstracts, 35-36. Miller, J.D., Nicholson, Nicholson, S.W., S.W., and and Cannon, Cannon,W.F., W.F., 1995, 1995, Minnesota MinnesotaGeological GeologicalSurvey SurveyGuidebook GuidebookSeries, Series,p.p. 1-22. 1-22. Nicholson, Nicholson, S.W., S.W., Shirey, S.B., S.B., Schulz, K.J., Green, Green, J.C., J.C., 1997, Canadian Journal Journal of Earth Earth Sciences, Sciences,v.v. 34, 34, p. p. 504-520. 504-520. Wirth, K.R., Vervoort, J.D., J.D., Naiman, Z.J., Z.J., 1997, Canadian Canadian Journal of Earth Sciences, Sciences, v. 34, no. 4, p. 536-548. Zartman, Zartman, R.E., Nicholson, Nicholson,S.W., S.W., Cannon, Cannon,W.F., W.F., and Morey, G.B., 1997, 1997, Canadian Canadian Journal Journal of of Earth EarthSciences, Sciences,v.v. 34, p. p. 549-561. 549-561. 125 � https://digitalcollections.lakeheadu.ca/files/original/ef817c5c00f1dd4aa45ca2f7d4b1ff20.pdf e9633852122cc432ccfd00cdfe5f1152 PDF Text Text Minneapolis, Minnesota May 6-10,1998 Organized by MINNESOTA GEOLOGICAL SURVEY OF MINNESOTA UNIVERSITY �44th 44th Annual AnnualMeeting Meeting INSTITUTE ON GEOLOGY INSTITUTE ONLAKE LAKESUPERIOR SUPERIOR GEOLOGY Volume Volume44 44 contains contains the the following followingparts: parts: Part1: 1:Program Programand andAbstracts Abstracts Part 2: Field FieldTrip TripGuidebook Guidebook Part2: Part 1—Early 1-Early Proterozoic Proterozoic intrusive intrusiverocks rocks of of east-central east-centralMinnesota Minnesota 2—Geology 2ÑGeolog of of the the southeastern southeasternportion portionof of the theMidcontinent MidcontinentRift Rift System, stern,eastern easternMinnesota Minnesotaand andwestern westernWisconsin Wisconsin 3—Glacial 3-Glacial exotica exoticaof of the theTwin TwinCities Citiesarea area 4—Stratigraphy &Stratigraphy and andhydrogeology hydrogeologyofofPaleozoic Paleozoicrbcks rocksof of southeastern southeastern Minnesota Minnesota 5—Minnesota 5-Minnesota River River Valley Valley and vicinity, southwestern southwesternMinnesota Minnesota Reference Referenceto to the thematerial materialin inthis thisvolume volumeshould shouldfollow followthe theexample examplebelow: below: Sims, Sims,P.K., P.K., Neymark, Neymark,L.A., L.A., Peterman, Peterman,Z.E., Z.E., and andKotov, Kotov,A.B., A.B., 1998, 1998, Nd isotope isotope evidence evidence for forMiddle Middleand andEarly EarlyArchean Archeancrust crustin inthe theWawa Wawasubprovince subprovinceof of the the Superior SuperiorProvince, Province, Michigan, 44th Michigan,U.S.A., U.S.A., [abstract]: [abstract]:Institute Institute on onLake Lake Superior SuperiorGeology GeologyProceedings, Proceedings,44th Annual MN, AnnualMeeting, Meeting,Minneapolis, Minneapolis, MN,1998; 1998;v.v.44, 44,Part Part1,1,p.p.10-11. 10-11. Volume Volume44 44 is is published publishedby by the the Institute Instituteon onLake LakeSuperior SuperiorGeology Geologyand and distributed distributedby bythe theInstitute InstituteSecretary-Treasurer: Secretary-Treasurer: MarkJirsa Jirsa Mark MinnesotaGeological Geological Survey Survey Minnesota 2642 University University Avenue Avenue 2642 MNUSA USA55114-1057 55114-1057 St.Paul, Paul,MN St. (612)-627-4780 (612)-627-4780 email:jirsa001@tc.umn.edu jirsaOOl@tc.urnn.edu email: /www.geo.mtu.edu/great_lakes great-lakes /ilsg /ilsg // ILSG website http: // /www.geo.mtu.edu/ ILSG website http: ISSN ISSN1042-9964 1042-9964 �INSTITUTE ON LAKE SUPERIOR GEOLOGY 44th 4th Annual Meeting May 6-10, 1998 Minneapolis, Minnesota Sponsored by: Sponsored by: Minnesota Minnesota Geological Survey University University of Minnesota Minnesota Field Trip Guidebook Volume 44 Volume 44 2—FieldTrip Trip Guidebook Guidebook Part 2-Field EDITORS: EDITORS: Terrence J. J. Boerboom Boerboom and and Barbara A. Lusardi Lusardi Terrence Minnesota Minnesota Geological Geological Survey Survey �PROCEEDINGS VOLUME 44, 44, PART 2—FIELD %FIELD TRIPS CONTENTS CONTENTS 1: Early EarlyProterozoic ProterozoicIntrusive IntrusiveRocks Rocks of of East-Central Minnesota Trip 1: Introduction Field Trip Stops 3 18 Leaders Terry Boerboom, Boerboom, Minnesota Minnesota Geological Survey Mark Jirsa, Minnesota Minnesota Geological Geological Survey Daniel K. Hoim, Holm, Kent State State University Trip Trip 2: The TheGeology Geologyof of the the Southeastern Southeastern Portion Portionof of the the Midcontinent MidcontinentRift Rift System, System, Eastern Eastern Minnesota Minnesota and andWestern WesternWisconsin Wisconsin Introduction .................................................................................................. 33 Introduction.. 33 44 Field Trip Stops Stops............................................................................................. 44 Leaders: Karl Wirth, Wirth, Macalester Macalester College College William S. Cordua, Cordua, University University of of Wisconsin-River Wisconsin-River Falls Falls William F. Kean, Kean, University University of of Wisconsin-Milwaukee Wisconsin-Milwaukee Mike Middleton, Middleton, University of Wisconsin-River Falls Zachary J. Naiman, University of Arizona Trip 3: Glacial Trip GlacialExotica Exoticaof of the the Twin Twin Cities Introduction Field Trip Stops Leaders: Howard Hobbs, Hobbs, Minnesota Minnesota Geological Geological Survey Survey Alan Knaeble, Minnesota Minnesota Geological Geological Survey Survey Minnesota Geological Survey Gary Meyer, Minnesota 79 82 Trip 4: 4: Stratigraphy Stratigraphyand andHydrogeology HydrogeologyofofPaleozoic PaleozoicRocks Rocks of of Southeastern Southeastern Minnesota Minnesota Trip Introduction Field Trip Stops 103 113 Leaders: Anthony C. C. Runkel, Runkel, Minnesota Minnesota Geological Geological Survey Robert G. Tipping, Minnesota Geological Robert G. Tipping, Minnesota GeologicalSurvey Survey Trip Trip 5: 5: Archean Archeanand andQuaternary QuaternaryGeology Geologyofofthe theMinnesota MinnesotaRiver RiverValley Valley Introduction to Precambrian Geology 133 Geology.......................................................... 133 Field Trip 137 Trip Stops, Stops, Day Day 11................................................................................ 137 Introduction 148 Introduction to to Quaternary Quaternary Geology Geology............................................................ 148 Field Trip Stops, Day 2 154 Stops, ................................................................................ 154 Leaders: D. L. Southwick, Southwick, Minnesota Minnesota Geological Survey Carrie J. Patterson, Minnesota Minnesota Geological Geological Survey Survey ��FIELD FIELD TRIP TRIP#1 #1 EARLY PROTEROZOIC PROTEROZOIC INTRUSIVE INTRUSIVE ROCKS ROCKS OF OF EAST-CENTRAL EAST-CENTRALMINNESOTA Leaders: Terry Boerboom, Mark Mark Jirsa, Jirsa, and Leaders: and Daniel Hoim Holm INTRODUCTION rocks of the St. Cloud District The Early Proterozoic (Paleoproterozoic) granitoid rocks District in in east-central east-central Minnesota Minnesota(Fig. (Fig.1) 1)have have long long been been aa source source of both building building materials materials and and geologic geologic controversy. Quarrying Quarrying for for dimension dimension stone stone began in the St. Cloud District in the late 1800's 1800's 1930-1940, were some some 18 different differentquarry quarrycompanies companies operating operating and peaked at about 19301940, when there were in the district (Thiel (Thiel and Dutton, 1935). 1935). Currently, Currently, Cold Cold Spring Spring Granite Granite is is the the only only dimension dimension operating, with 5 active quarries stone company operating, quarries here, and and 4 others others in different different parts parts of of the the state. state. Meridian Aggregates Company also operates a large crushed-stone operation in the St. Cloud Cloud TheSt. St.Cloud Cloudarea areacontains containsabundant abundant active active and inactive quarries, as well as natural District. The outcrops. Despite Despitethese theseexposures, exposures,many many of of the the critical critical relationships relationships between various intrusive units are concealed—leaving concealed-leaving details of the magmatic history unknown and speculative. Until Until recently, geologic geologic maps reconnaissance-style (Morey recently, maps of the district were reconnaissance-style (Morey and and others, others, 1981), 1981),and and little geochronologic work had had been been conducted. conducted. This This field field trip highlights the results of more current and thorough thorough mapping mapping endeavors endeavors that utilized quarry and natural exposures, drill core, These include include the the Stearns Steams County Geologic Atlas (Boerboom and geophysical interpretation. These and others, 1995; 1995; Boerboom, Boerboom, 1996a), 1996a),and and mapping mapping north north and east of St. Cloud (Jirsa and others, 1995a; 1995b; Jirsa and Chandler, 1997). In In addition, addition,we we review review both pre-existing pre-existing and and newly newly acquired geochronologic data (e.g., Hoim Holm 1997, 1997, 1998) 1998) in light of the field, field, petrographic, petrographic, and and geophysical observations. observations. Intrusions Intrusions of the the St. St. Cloud Cloud District District were were emplaced emplaced relatively relatively late late in in development developmentof of the the Penokean orogen—a Penokean orogen-a belt belt of of Early Early Proterozoic Proterozoic volcanic, volcanic, sedimentary, sedimentary,and and plutonic plutonic rocks rocks affected affected the Penokean Penokean orogeny, orogeny, which which culminated culminated at approximately approximately 1870 Ma (Van (Van Schmus and by the 1981). Rocks Rocksofofthe theorogen orogenare aredivided dividedfrom fromnorth north to to south south(Fig. (Fig. 1) 1)into intoaavariably variably Bickford, 1981). deformed foreland basin; a fold and thrust belt; and what what has has been been inferred inferred to to be be an accreted broadly equivalent to the Wisconsin Wisconsin magmatic magmatic terranes terranes (Jirsa (Jirsa and others, island-arc terrane, broadly 1995b, Boerboom, 1996b). All All of of these these terranes terranes were were deposited deposited upon or or accreted accreted against against the the southern margin of the Archean Superior Province. Province. Using Using the terminology of Southwick Southwick and into distinct distinct segments on on the the basis basis of of others (1988), (1988), the fold fold and thrust belt is further subdivided into of the Animikie Group and lithologic and structural attributes: 1) an external zone comprised of related outliers; 2) a medial medial zone zone characterized characterized by fault slices of interlayered interlayered volcanic and related clastic including the Cuyuna north internal zone, zone, clastic strata including north and south (iron) ranges; ranges; and 3) an internal containing fault-bounded fault-bounded blocks blocks of of various metamorphic and and plutonic plutonic rocks. rocks. The The Early Proterozoic Proterozoic granitoid granitoid rocks rocks of the St. Cloud District occur within the internal zone of the western Penokean orogen, where they form a physically continuous mass of lateand post-orogenic post-orogenic intrusions. intrusions. They Theyrepresent represent some some of the most deeply emplaced magmas of the 3 �orogenicbelt. belt. Regionally, Regionally, the the Early Early Proterozoic Proterozoic supracrustal supracrustalcountry country rocks—including rocks-including the the orogenic Little Falls Formation to the north and west, and unnamed volcanic and clastic strata to the east Little Falls Formation to the north and west, and unnamed volcanic and clastic strata to the east andsoutheast—were southeast-were metamorphosed metamorphosedatatdepths depthsofof22-25 22-25km krnduring duringthe the Penokean Penokeanorogeny orogeny(Holm (Holm and andSelverstone, Selverstone,1990). 1990).Reconstruction ReconstructionofofP-T-t P-T-thistories historiesindicate indicatethat thatthe therocks rocks currently currentlyexposed exposed and remained at those depths until well after the orogeny, then were rapidly exhumed at about 1770 remained at those depths until well after the orogeny, then were rapidly exhumed at about 1770 Ma during during emplacement emplacement of of post-tectonic post-tectonic magmas magmas (Hoim (Holm and and others, others, 1998). 1998). The Thesignificant significant Ma depth of emplacement and the great volume of igneous rock provide an excellent opportunity to depth of emplacement and the great volume of igneous rock provide an excellent opportunity to midcrustal magmatic magmatic processes processes that that occurred occurred immediately immediately after after the the orogenic orogenic belt belt investigate midcrustal investigate was constructed, constructed,and andlater lateras asthe theorogen orogencollapsed. collapsed. was 950 940 930 92° b IIArchean Archean (A) (A) I I 1 Superior Province 47O 46' sland arc rocks (Equivalent to Wisconsin agma 45O thePenokean Penokeanorogen, orogen,showing showingmajor major Figu 1. Simplified geologic map of)fthe Figure tectcLLL.w.-iLLuLLLo. bciLbuuaL-Lbb~nic Penokeanintrusions intrusionsshown shownininblack. black. tectonic elements. LatetoLu post-tectonic Penokean GEOLOGYOF OFINTRUSIVE INTRUSIVEROCKS ROCKS GEOLOGY Intrusionsof ofthe theSt. St.Cloud CloudDistrict District(Fig. (Fig.2) 2)are arecomprised comprisedof of several several discrete discrete units units that that Intrusions includethe theReformatory Reformatorygranodiorite, granodiorite,the thecharnockitic charnockiticRichmond Richmondgranite, granite,the theRockville RockvilleGranite, Granite, include the St. St.Cloud CloudGranite, Granite,the theFoley Foleybatholith, batholith,and andseveral several types types of of granitic graniticand anddiabasic diabasicdikes. dikes. the Only untry rocks are recognized within the intrusions, Only scattered xenoliths of metamorphic country rocks are recognized within the intrusions, 4 �but numerous numerous mafic maficenclaves enclavesinterpreted interpretedto to be be comagmatic comagmaticwith withthe theenclosing enclosing granitoid granitoidrocks rocks but are present in some units. The intrusions can be divided on the basis of composition and relative are present in some units. The intrusions can be divided on the basis of composition and relative 1) gray gray to to pinkish-gray pinkish-gray to to greenish greenish gray gray granitic, granitic, temporal setting setting into into three three categories: categories: 1) temporal granodioritic,and and dioritic dioriticrocks—Reformatory rocks-Reformatory granodiorite, granodiorite,Richmond Rchmond granite, granite,and and Rockville Rockville granodioritic, Granite; 2) red and pink granitic intrusions-St. Cloud Granite, Foley batholith, and associated Granite; 2) red and pink granitic intrusions—St. Cloud Granite, Foley batholith, and associated aplite; and and 3) 3) diabasic diabasic and and porphyritic porphyritic microgranite microgranite dikes. Colorisis not not always always an an adequate adequate aplite; dikes. Color distinguishing characteristic between various intrusions because components of the "gray" distinguishing characteristic between various intrusions because components of the "gray" intrusionscan canbe be quite quite pink. pink. Other Otherattributes attributesthat that distinguish distinguish "gray" "gray" from from"red" "red" varieties varietiesinclude include intrusions the observation observationthat that "gray" "gray" intrusions intrusionscontain containmafic maficenclaves, enclaves,and andhave have aa better better developed developedand and the more pervasive magmatic foliation than do the "red" ones. These differing attributes have more pervasive magmatic foliation than do the "red" ones. These differing attributes have A description description importantimplications implicationsabout about the the magmatic magmatic history history that that will will be important be discussed discussed below. below. A of individual individualplutonic plutonic units unitsfollows. follows. of EARLYVPROTEROZOIC PARI PRCTFPfl7flI ', NW-striking diabase dike ..NW-strikingdiabasedike Maficto toultramafic ultramaficintrusions intrusions Mafic D St.Cloud CloudGranite Granite St. Foleybatholith batholith Foley RockvilleGranite Granite Rockville Richmondgranite granite Richmond Reformatotygranodionte granodiorite Reformatory Gabbroicintrusions intrusions Gabbroic = 0 Granite,undivided undivided Granite, AnimikieGroup Group(slate) (slate) Animikie \ LittleFalls FallsFormation Formation Little (garnet-stauroliteschist) schist) (gamet-staurolite Volcanicand andclastic clasticrocks, rocks, Volcanic iron-formation; largely large1 of of iron-formation; Mille Lacs Lacs Group, Group, undivided unchided Mule Strike- or or dipdipStrikeslip fault. slip fault. Thrustfault. fault. Thrust El ARCHEAN ARCHEAN Sartell Gneiss(may (maybe beEarly Early Sartell Gneiss Proterozoicininpart) part) Proterozoic 'NS Lithologic Lithologic contact. contact. 0 Gneissterrane terrane 0Gneiss Figure 2. 2. An An extremely extremely busy busy and and largely largely illegible illegibleregional regional geologic geologic map map of of the the Figure area around Steams County (outlined). Inset map references location within area around Steams County (outlined). Inset map references location within 1). Penokeanorogen orogen(Fig. (Fig.1). Penokean 55 �Reformatory Reformatory granodiorite granodiorite Reformatory granodiorite granodiorite (Reformatory (Reformatory Granite 1978) is presently presently The Reformatory Granite of Morey, 1978) (trade name Charcoal Granite), just city of of St. St. extracted from only one quarry (trade just south of the city Cloud. However, However,there thereare aredozens dozensof of inactive inactive pits pits from from which which this this rock rock has been quarried quarried in the the Cloud. past, including those that supplied stone to the the unit's unit's namesake-the namesake—the State State Reformatory! Reformatory! The rock is mediummedium- to dark-gray, mediummedium- to to coarse-grained, coarse-grained, weakly wealdyporphyritic porphyriticbiotite-hornblende biotite-hornblende granodiorite characterized by small small tabular tabular plagioclase plagioclase phenocrysts (oligoclase to andesine; andesine; Johnson, Johnson, 1978) 1978)in in aa groundmass groundmassof of plagioclase, plagioclase, microcline, microcline, homblende, hornblende, biotite, biotite, and and accessory accessory pyroxene, pyroxene, sphene, and apatite. apatite. AAweak, weak,generally generally subhorizontal subhorizontal trachytoid trachytoid fabric in the the granodiorite granodioritecan canbe be observed observed in in many places. Small Smallovoid ovoid mafic enclaves are scattered throughout the the intrusion. intrusion. Richmond granite granite The charnockitic Richmond granite is well exposed between the towns of Richmond and Cold Spring, but has never been significantly exploited as as a dimension stone. Although Although generally generallydark dark greenish-pink, greenish-pink,ititlocally locallyretains retainsaa unique unique deep deep green green color color where where the rock has not been been subjected subjectedto to oxidation oxidationalong alongtight tight brittle brittle fractures. fractures. The Thegranite granitedisplays displaysaa prominent prominentrapakivi rapakivi 2-6 cmlong longK-feldspar K-feldsparmegacrysts, megacrysts,as as well well as as a consistent texture defined defined by plagioclase-mantled, texture plagioclase-mantled, 2-6cm consistent Hypersthene, redeast-northeast magmatic trachytoid fabric defined by by aligned aligned phenocrysts. phenocrysts. Hypersthene, brown biotite, and and green green homblende hornblende are are the the dominant dominant mafic mafic minerals; minerals; apatite, apatite, ilmenite, ilmenite, and and zircon mafic minerals. minerals. Quartz zircon form form abundant abundant accessory accessoryphases phases concentrated concentrated with the late-formed mafic Quartz typically occurs occurs as as more-or-less more-or-less equant, equant, but anhedral-interstitial, anhedral-interstitial,monocrystalline monocrystalline clots clots as as large large typically as 11 cm. cm. Although Althoughelongate elongategrains grainsofofquartz quartzoccur occurin inlocalized, localized,meter-scale meter-scale shear shear bands, bands, the the of the the intrusion intrusion contains contains aawell-developed well-developed magmatic magmatic foliation foliation and no evidence evidence of bulk of metamorphic recrystallization. recrystallization. Scattered Scatteredoutcrops outcropsand and geophysical geophysical maps (Fig. 3) indicate that metamorphic the Richmond charnockitic intrusion intrusion Richmond granite may be be one one component component of of aa large large composite composite charnockitic composed largely of gabbroic and noritic intrusive rocks. composed largely of gabbroic and noritic intrusive Rockville Granite Granite The Rockville Rockville Granite is presently presently taken from from three separate separate quarries and marketed marketed under under the the trade trade names namesof ofDiamond DiamondPink, Pink,Rockville RockvilleBeige, Beige,and andRockville RockvilleWhite. White. It is is aa very very coarse-grained, coarse-grained, pinkish- to grayish-white grayish-white rapakivi granite characterized characterized by zoned 1-3 1-3 cm cm microcline microcline phenocrysts phenocrysts and and smaller smaller plagioclase (oligoclase; Johnson, Johnson, 1978) 1978)phenocrysts phenocrysts in in aa groundmass groundmass of of coarse coarsequartz, quartz,feldspar, feldspar, biotite, biotite, and and hornblende, hornblende, with with accessory accessoryzircon, zircon,apatite, apatite, sphene, epidote, and allanite. The Rockville locally shows a weak trachytoid fabric, defined sphene, epidote, and allanite. The Rockville locally shows a trachytoid fabric, defined by aligned generally N-S N-S and and dips dips variably variablyto to the the west. west. This aligned feldspar phenocrysts, that strikes generally This granite granite contains containsscattered scatteredenclaves enclavesthat that vary vary from from coarse-grained coarse-grainedblack black diorite, diorite, to to fine-grained, fine-grained, gray gray quartz quartz monzonite; monzonite; both both having having abundant abundant homblende hornblende and biotite. The Thedioritic dioriticenclaves enclaves commonly commonlycontain containmicrocline microclinephenocrysts phenocrystsor orxenocrysts; xenocrysts;whereas whereasthe the monzonitic monzoniticones ones contain contain scattered microcline, scatteredplagioclase plagioclasephenocrysts phenocrystsin in aa pristine pristine igneous-textured, igneous-textured, trachytoid trachytoid matrix matrix of microcline, quartz, biotite, and homblende. hornblende. Dikes Dikes5-30 5-30cm cmwide wideof of rock rocklike likethe thecoarse-grained coarse-grained dioritic dioritic phase phaselocally locallycut cut the the Rockville RockvilleGranite. Granite. These Thesetwo twoconflicting conflictingoccurrences occurrencesof of the the coarse-grained coarse-grained diorite (as both rphenocrysts' in in the thecoarsecoarseboth enclaves enclaves and and dikes), dikes), the the presence presence of of microcline microcline 'phenocrysts' grained grained enclaves, enclaves, the pristine pristine trachytoid trachytoid igneous texture of the finer grained enclaves, and the rapakivi mafk and and felsic felsic magmas magmas during during rapakivi texture, texture, all all imply imply an an origin origin by by the the commingling comminglingof of mafic magma magma ascent ascent(see (seeDISCUSSION). DISCUSSION). 66 �Figure anomaly map of Stearns Figure 3. 3. First vertical derivative, derivative, reduced to pole aeromagnetic anomaly S t e m s County (white dashed line, also also see Figure 2) and the fringing area. Outlines Outlinesonly only shown shown for for rock rock units units discussed discussed in in text. text. St. Cloud Granite Granite The The St. St. Cloud Cloud Granite Granite occurs occurs as as aa zone of variably wide dikes and sills that permeated into the Reformatory Reformatory granodiorite, granodiorite,resulting resulting in a complex geometry not easily depicted at small into map map scales. scales.At Atlarger largerscale, scale,dikes dikesof ofSt. St.Cloud CloudGranite Graniteare are irregular irregular in in shape, shape, and and contacts contacts most most commonly trend trend to to the the north north and and northeast. Small Smalldikes dikesand and irregular irregular pods pods of of pink pink aplite aplite that that commonly are associated associated with with the the St. St. Cloud CloudGranite Granite also also cut cut Reformatory Reformatory granodiorite. granodiorite. The St. extensively in in the the past, past, but but no no longer. longer. It is a St. Cloud Cloud Granite has been quarried extensively coarse-grained, coarse-grained,deep-colored, deep-colored,grayish-pink grayish-pinkto tored, red, hornblende hornblendegranite granite having having a weak porphyritic texture texture defined defined by by pink pink microcline microcline phenocrysts phenocrysts only slightly larger than the matrix. The Themain main 19781, quartz, hornblende, microcline, plagioclase (albite to oligoclase; minerals are microcline, oligoclase; Johnson, 1978), and biotite, apatite, zircon, zircon, sphene, sphene, and and rare rare epidote. epidote. Using biotite, with with accessory accessory Fe-Ti oxides, apatite, Using Maitre (1979), (19791, the the St. St. Cloud Cloud varies varies in in modal modalmineralogy mineralogy Streckeisen and and Le Le Maitre terminology of Streckeisen from alkali-feldspar (Fig. 4). 4). A alkali-feldspar granite to syenogranite (Fig. A vague vague trachytoid trachytoid fabric fabric defined defined by by aligned feldspar and hornblende crystals is common and is parallel to the margins of the dikes aligned feldspar and hornblende crystals is common and is parallel to the margins of the dikes and and sills sills in in which which the the St. St. Cloud Cloud Granite Granite occurs. Narrow Narrowdikes dikesof of pink pink aplitic apliticgranite granitelocally locallycut cut the St. St. Cloud Cloud and and Reformatory, Reformatory, and and are are inferred inferred to to be be late late differentiates differentiatesof of the the St. St.Cloud Cloudon onthe the the basis of of lithologic lithologic similarity. similarity. Coarse Coarsepegmatite pegmatiteis is exceedingly exceedingly (and (and conspicuously) rare. The The St. St. Cloud Cloud commonly commonlycontains containsangular angularxenoliths xenoliths of of Reformatory Reformatorygranodiorite, granodiorite,along alongwith withrare rare xenoliths xenoliths of of metamorphic metamorphiccountry countryrock. rock. 77 �Foley batholith batholith Foley The salmon-colored, coarse-grained, coarse-grained, hornblende-biotite homblende-biotite granite TheFoley Foley batholith batholith is is a red to salmon-colored, granite that that is is cut cut by by minor minoramounts amountsof ofgranitic graniticporphyry porphyry and andaplite—similar aplite-similar in in nearly nearly all allrespects respectsto to the km),ovoid ovoidgravity gravitylow lowthat thatextends extendsfrom fromthe the the St. St. Cloud Cloud Granite. Granite. ItItforms formsaalarge large(60 (60xx20 20km), east east side sideof of St. St.Cloud, Cloud, northeastward northeastward to to near Mule Mille Lacs Lake (Jirsa and Chandler, 1997). The The batholith batholith is is moderately moderately well well exposed exposed along along its its western western terminus, terminus, where where its its intrusive intrusive contact contact against againstReformatory Reformatorygranodiorite granodioritecan canbe beseen, seen,and andin inseveral severalplaces placeseast eastof ofthe themap maparea areashown shown on on Fig. Fig. 2. 2. Small Smallquarries quarriesininthe thewestern westernpart partofofthe theunit unitoperated operatedduring duringthe theperiod period1896-1909. 1896-1909. The The Foley Foley isismassive massiveto tovaguely vaguelyfoliated, foliated,having havingaapoorly poorlydeveloped developedand andlocalized localizedtrachytoid trachytoid fabric. fabric. Although Althoughititwill willnot notbe bevisited visitedduring duringthis thisfield fieldtrip, trip,the theFoley Foleyisis described describedhere herebecause because itit isisaalarge largeand anddistinctive distinctiveunit unitthat thatappears appearsto tobe bemagmatically magmatically related related (parental?) (parental?) to to the the St. St. Cloud Granite. Granite. Cloud Porphyritic (AKAQFP) QFP)dikes dikes Porphyriticmicrogranite microgranite(AKA Thin Thin northeast-trending northeast-trendingdikes dikesof of distinctively distinctively textured porphyritic porphyritic microgranite microgranite cut cut the Reformatory Foley batholith. batholith. None of Reformatory granodiorite, granodiorite, Rockville RockvilleGranite, Granite, St. St. Cloud Granite, and Foley these these dikes dikes were were noted noted in in the the Richmond Richmond granite, granite, but but they they would would logically logically post-date that unit. unit. Most 2-4 m m wide, wide, but but one one is is at at least least 30 30 m wide, and and another another may may be as as much much Most of of these these dikes dikesare are 2-4 as as 100 100m m wide, wide, although although correlation correlation of of the the latter latter with this dike set is tenuous. The Themargins marginsof of these dikes dikes are are chilled chilled and and locally locally flow-banded, flow-banded, texturally similar to volcanic rhyolite. The The porphyries porphyries are are characterized characterized by by 3-10 3-10 percent percent quartz quartz phenocrysts as large as 4 mm across across that have primary euhedral have aa primary euhedralshape shapemodified modified by by resorption, resorption,and and 3-10 3-10 percent percent microperthite microperthiteor or sanidine sanidine phenocrysts phenocrysts as as large large as as 10 10mm. mm. The Thefine-grained fine-grainedgroundmass groundmassvaries variesin intexture texturefrom fromanhedralanhedralgranular to spherulitic. spherulitic. InInaddition additiontotothe themain mainquartz quartzand andfeldspar, feldspar,the the rock rock contains contains minor minor amounts of of fine-grained fine-grained biotite, biotite, homblende, hornblende, actinolite, actinolite, and and chlorite, chlorite, and and accessory accessory fluorite, fluorite, apatite, apatite, Fe-Ti Fe-Ti oxides, oxides, epidote, epidote, sphene, sphene, zircon, zircon, and and allanite. allanite. Diabase dikes dikes Diabase There There are are two two main main groups groupsof of diabase diabase dikes dikes recognized recognized in in the the area: a northeast-trending northeast-trending set, and a northwest-trending northwest-trending set. Both Both sets setsare are steeply steeply dipping dipping and are interpreted to be the youngest rocks in the St. youngest Precambrian rocks St. Cloud Cloud area, area, as as they they cut cut nearly nearly all all other other rock rock types. types. Northeast-trending Northeast-trendingdiabase diabasedikes dikesare aretypically typically non-magnetic non-magnetic and andlack lackgeophysical geophysicalexpression, expression, but throughout the area. They but are are exposed exposedin in several several outcrops outcropsand and old old quarries quarries throughout They are are most most commonly commonly 1-3 m thick, but some m,and anddike dikemargins margins are aresharply sharplychilled chilled against against the the some are are as as wide wide as as 88m, neighboring metamorphism neighboringgranitic graniticrocks. rocks. These Thesedikes dikescontain containfresh fresh plagioclase, plagioclase, but low-grade metamorphism or actinolite, chlorite, or deuteric deuteric alteration alteration(?) (?)has has altered alteredthe the primary primary mafic minerals minerals to actinolite, chlorite, and and dusty dusty oxides. oxides. Northwest-trending Northwest-trendingdiabase diabasedikes dikesare arerarely rarely exposed, exposed,but are are inferred inferred from geophysical geophysical data 100m wide. They They are are geophysically geophysicallytraceable traceableby by their their distinctive, distinctive,generally generally data to to be be as as much much as as 100 reversely polarized, aeromagnetic signature (Fig. (Fig. 3). 3). One exposed northwest dike (Fig. F6 of One exposed stop descriptions) has has sharply sharply chilled chilled margins, margins, and and compared compared to to the the northeast-trending northeast-trendingset setisis strongly magnetic magnetic (pyhrrotitic) (pyhrrotitic) and and is is less less altered/metamorphosed, alteredmetamorphosed, having having substantially substantiallymore more fresh clinopyroxene. This Thisdike dikeisisnormally normally polarized polarized and and only 6 meters thick—attributes thick-attributes that contrast somewhat somewhat with the the geophysically-defined geophysically-defined northwest northwest swarm, making correlation correlation 8 �uncertain. Another Anothersmall smalloutcrop outcropof of apatitic apatiticferrodiorite, ferrodiorite,in in northwestern northwestern Stearns Steams County, County, lies over one of the reverse-dike reverse-dike anomalies and is probably part of this swarm, swam, but no no magnetic magnetic polarity tests have been performed. performed. GEOCHEMISTRY A standard standard alkali-silica alkali-silica chemical chemical classification classificationplot characterizes characterizesthe the granitoid granitoidrocks rocksas as subalkaline to alkaline composition. As As shown shown in in Figure Figure 4, 4, both both the the St. St. Cloud Cloud and and Rockville Rockville alkalic Granites plot as subalkalic subalkalic using the dividing line of Irvine and Baragar (1971), but as alkalic Miyashiro (1978). (1978). This alkalic tendency reflects the perthite-dominated perthite-dominated using the dividing line of Miyashiro mineralogy of the St. Cloud Granite. In In addition, addition, the all the granitoid rocks in Stearns Steams County are peraluminous, and I-type, I-type, consistent consistent with with the abundance of biotite peraluminous, calc-alkaline, calc-alkaline, and biotite and and hornblende and lack of muscovite. DISCUSSION DISCUSSION Temporal Relationships—Field Temporal Relationships-Field Evidence Evidence Based on field and petrographic petrographic observations, observations, geophysical geophysical maps and measurements, measurements, and and limited geochronologic geochronologic data, the following following sequence—from sequence-from oldest to youngest—summarizes youngest-summarizes the emplacement history of igneous igneous rocks in the St. St. Cloud Cloud District: 1) Reformatory granodiorite, and Rockville and Richmond granites—emplaced granites+mplaced at at about about 1812 Ma; Granite and associated associated aplite, and Foley batholith+mplaced batholith—emplaced at approximately 2) St. Cloud Granite approximately 1770 Ma; northeast-trending dikes of porphyritic 3) northeast-trending porphyritic microgranite microgranite and diabase; diabase; and 4) northwest-trending northwest-trendingdiabase diabase dikes. dikes. 10 + 0 9 z 5 SO feldspar 70 75 VDiabua dike 8W w a i n R v G ~ P w p b @ c ~ ~ARichond+te t e SL CI-4 w e 0 Refemwary pnodiaite Figure 4. Modal rocks from from the the St. St. Cloud Cloud area. area. (a) Modaland andchemical chemicalvariation variation diagrams of intrusive rocks selected selected modal modal analyses analyses compiled compiled from from Dittman (1971), (1971), Morey (1972), (19721, Johnson (1978), (19781, Skiliman Skillman (1946), and Boerboom (in prep.); (b) (19461, (b)alkali-silica alkali-silicaclassification, classification,with with dividing dividing lines lines of of Miyashiro Miyashiro (1978, solid line) and Irvine k i n e and Baragar (1971, (1971, dashed line). 9 �The TheSt. St.Cloud CloudGranite Graniteintrudes intrudesthe theReformatory Reformatory granodiorite, granodiorite,as as established establishedabove above and and by by earlier earlierworkers workers(Morey, (Morey,1978 1978and and references references therein). therein). The Theintrusive intrusiverelationships relationshipsbetween between the the Rockville Rockville Granite Granite and and other other granitoid granitoid rocks cannot be seen in outcrop. outcrop. Although Although geochronological geochronologicaldata dataimply implythat thatthe theRockville Rockvilleand and Reformatory Reformatory are are generally generally synchronous synchronous(as (as discussed discussedbelow), below),some someoutcrops outcropsof ofRockville RockvilleGranite Granitecontain containinclusions inclusionsof ofrock rockequivocally equivocally similar to to the theReformatory Reformatorygranodiorite. granodiorite. Geophysical Geophysical maps maps imply imply that that the thenon-magnetic non-magnetic similar Rockville Rockville Granite Granite forms forms an an eastward-thinning eastward-thinning wedge wedge that that overlies overlies the the more moremagnetic magnetic Reformatory 3);however, however, the the observations observationscontribute contributelittle little to to their theirrelative relative Reformatorygranodiorite granodiorite(Fig. (Fig.3); emplacementhistory. history. emplacement Porphyritic dikes are inferred inferred to to be either cut by or Porphyritic microgranite microgranite dikes or synchronous synchronous with with northeast-trending northeast-trendingdiabase diabasedikes. dikes.ItItisisinteresting interestingto tonote notethat thatthe the microgranite microgranitedikes dikescommonly commonly have have aa similar similar northeast-trending northeast-trendingdiabase diabase dike dike near them. At At one one locality locality aa thin thin diabase diabase dike dike is is chilled chilledagainst againstthe theRockville RockvilleGranite Graniteon on one one side, side, but but retains retains felty-textured felty-textured plagioclase plagioclase against against the the contact contactwith withaaporphyritic porphyriticmicrogranite microgranitedike dikeon onthe theother, other,implying implyingthat thatthe themicrogranite microgranite was porphyrywas still stillhot hot when when diabase diabase was was emplaced. emplaced. The Thediabase diabasecontains containsabundant abundantxenocrysts xenocrystsof ofporphyryderived derived quartz and feldspar grains adjacent to this contact. These Thesetextures textures indicate indicate that that the the diabase diabasemay may be be comagmatic comagmaticwith with the the microgranite, microgranite, or or that that diabase diabase post-dates post-dates microgranite micrograniteonly only slightly. A northwest-trending diabase dike exposed in a quarry in Waite Park (Fig. F6 of slightly. A northwest-trending diabase dike exposed in a quarry in Waite (Fig. F6 stop stop descriptions) descriptions)unequivocally unequivocally cuts cutsand andisis chilled chilledagainst against aa small smallnortheast-trending northeast-trendingdiabase diabasedike. dike. If If this this northwest-trending northwest-trendingdike dikeisisrepresentative representative of of the the aeromagnetically aeromagneticallydefined defined dikes dikes having having the the same same orientation, orientation,they they are are the the youngest youngest intrusive intrusive rocks in the area. Temporal TemporalRelationships—Geochronologic Relationships~GeochronologicData Data All All the the intrusions intrusionsdescribed describedabove aboveare areinferred inferredto to be be Early Early Proterozoic, Proterozoic,with withthe theexception exception of somewhat equivocal dates. Unfortunately, of diabasic diabasicdikes dikesthat that cut cut the the granitoid granitoid rocks rocks and and yield somewhat Unfortunately, U-Pb data data are are rare, rare, and andfield fieldobservations observations together together with with Ar-Ar Ar-Ar and and K-Ar K-Ar dates dates make make much much of of the the emplacement emplacementhistory historyinferential. inferential.Reported ReportedU-Pb U-Pbdates datesof of1,770 1,770Ma Ma for for the the St. St. Cloud Granite, Granite, and 1,812 Â9Ma Mafor forboth boththe theRockville Rockvilleand andReformatory Reformatory(Goldich, (Goldich,pers. pers.comm., comm.,as asreported reportedin in and 1,812±9 Horan Horan and and others, others, 1987) 1987)are are consistent consistentwith with the the field field observations observationsthat that show show Reformatory Reformatory cut cut by St. St. Cloud Cloud Granite. Granite. Although Althoughnot notdated datedby by the the U-Pb U-Pb method, method, the Richmond granite has Nd and Pb isotope and trace element systematics consistent with and Pb isotope and trace element systematics consistent with it being Early Proterozoic (Spencer, (Spencer, 1987). 1987). Most Most controversial controversial are are the dates dates for diabasic dikes. Hanson Hanson(1968) (1968)gives gives K-Ar K-Ar wholewholerock ages northeast-trending diabase diabase dikes. dikes. Of these, the ages of 1280, 1280, 1460, 1460, and 1570 1570 Ma for the northeast-trending 1570 considered aa minimum consistent 1570Ma Ma date date is considered minimum age, age, and and some some of the dikes yield Pb isotope data consistent with northeastwith an an age age of approximately approximately1800 1800Ma Ma (Horan (Horan and others, 1987). 1987). Using Using these these data, data, the northeasttrending with enclosing enclosing granitic granitic rocks. rocks. By trending diabasic dikes may be more or less comagmatic with By contrast, contrast, Holm Holm and andLux Lux (1997) (1997)report reportAr-Ar Ar-Ar whole-rock ages of 1164±12 1164k12 Ma and 1190±9 1190±Ma two of of the thenortheast-trending northeast-trendingdiabase diabasedikes. dikes. Field relationships relationships do not permit us to to from two However, some field, geophysical, and unequivocally unequivocally resolve resolve these these contrasting contrasting age age dates. dates. However, and petrographic petrographicobservations observationsraise raisequestions questionsabout aboutthe theyounger younger(Keweenawan) (Keweenawan)emplacement emplacementdate. date. For example, example, nearly nearly all all of the diabase diabase dikes—including dikes-including those those dated dated by Holm and Lux (1997)— (1997)show alteration of mafic minerals to actinolite. The The northeast-trending northeast-trending dikes dikes in in the the St. St. Cloud Cloud District dikes exposed elsewhere elsewhere Districtare are only only weakly weakly to to non-magnetic, non-magnetic,in in contrast contrast with Keweenawan Keweenawan dikes that are strongly magnetic. Furthermore, paleomagnetic measurements on a limited suite of are strongly magnetic. Furthermore, paleomagnetic 10 10 �samples samples failed failed to to isolate isolate any any magnetization magnetization direction direction that that is is consistent consistent with with either either the the reversed reversed or normal normal directions directions typical typical of of Keweenawan Keweenawan dikes dikes (Val (Val Chandler, Chandler, unpub. data, Minnesota Minnesota Geological GeologicalSurvey). Survey).The Thegranites granitessurrounding surroundingthe thediabase diabasedikes dikesare are not metamorphosed, metamorphosed,implying implying that the the alteration alteration of of mafic mafic minerals minerals to to actinolite actinolite in the diabase diabase may be of of deuteric deuteric rather rather than than metamorphic origin. In Ar-Ar ages ages in in the the range range of of 1152-1199 1152-1199 Ma Ma may may indeed indeed Inthis thiscase casethe theAr-Ar reflect emplacement, related to the onset emplacement, or alternatively alternatively a weak metamorphic metamorphic overprint related onset of of Keweenawan different ages of northeastnortheastKeweenawanigneous igneousactivity activityto to the the east. A A third third scenario scenariois is that two different trending trending dikes dikes exist; exist; however, however, no systematic systematic differences differences between dikes of this trend have been observed observed in the field field or during during subsequent subsequent petrographic and geochemical study. The The apparent apparent conflict conflict of of dates dates and and observations observationsfrom from diabasic diabasic and and quartzofeldspathic quartzofeldspathicdikes, dikes,and and uncertainties uncertainties about the age age of of other other plutonic plutonic elements elements in in east-central east-central Minnesota, Minnesota, underscores underscores the need need for for modern modem U-Pb U-Pb zircon zircon or or baddelyite baddelyitedates. dates. Mafic microgranular microgranularenclaves-evidence enclaves-evidence for for magma magma mingling mingling Mafic Mafic microgranular microgranular enclaves that occur in many intrusions of the St. Cloud area are interpreted interpreted as as the the product product of of commingled, commingled, coeval felsic and mafic magmas. The Thetextural texturaland and mineralogical attributes of mafic mafic microgranular microgranular enclaves enclaves in the Rockville Rockville Granite Granite and and Reformatory rapakivitexture textureof of the the Rockville Rockvilleand and Richmond Richmondgranites, granites,are are Reformatorygranodiorite, granodiorite,and andthe therapakivi similar commingling of mafic and felsic magmas. Although similarto to features featuresattributed attributedelsewhere elsewhereto to the commingling Although some some of of the the enclaves enclavesmay may have have originated originatedby by the the reincorporation reincorporationof of chilled chilled intrusion intrusionmargins margins by upwelling upwelling magma, magma, textural texturalevidence evidenceclearly clearly shows shows that that some some of the the enclaves enclaves coexisted coexisted as as aa melt melt with with the the host host granite granite magma. magma. Textures Texturesimplying implyingcommingling comminglingof ofmafic maficand andfelsic felsicmagmas magmas beg the question: Under what conditions are these coeval yet disparate magmas generated? beg question: Under conditions coeval yet disparate magmas generated? The mingling mingling of of mafic mafic and and felsic felsic magmas magmas is is most likely likely to occur in regions where crustal melts are generated generatedby by underplating underplatingof of continental continentalcrust crust by mafic mantle-derived magma. Melt Meltgenerated generated by this process process and and in in aa quantity quantitysufficient sufficient to to accumulate accumulate at at the the base base of of the the crust crust can can become become fully (Cruden fully mobilized mobilized and and ascend ascend rapidly, rapidly, entraining entraining the underlying underlying mafic magma as it rises (Cruden and and others, others, 1995, 1995,and and references referencestherein). therein). We suggest suggest that that the the numerous numerous mafic mafic enclaves enclavespresent present in in the the 1812 1812 Ma intrusive intrusive suite suite give give evidence evidence that that melting meltingof of the the lower lower crust crust by mafic mafic mantle mantle underplating underplatinggave gave rise rise to to the the latelatetectonic tectonicsuite suiteof of granitic graniticintrusions, intrusions,during duringthe the waning waning compressive compressivestages stagesof of Penokean Penokeanorogeny. orogeny. Furthermore, Furthermore, the numerous numerous gabbroic to noritic intrusions such as that that associated associated with with the the Richmond Richmond granite granite (Fig. (Fig. 2) 2) may may represent represent mafic mafic end-members end-members of this same same late-Penokean late-Penokean suite suite of intrusions. intrusions. of In the case Masuite suite of ofpost-tectonic post-tectonic granites, granites, mantle mantle underplating underplating as the case of of the the —1770 -1770 Ma result of mantle (e.g., Holm Hoim and and Dahl, Dahl, 1996; Cruden mantle lithospheric lithospheric delamination (e.g., Cruden and andothers, others, 1995; Turner and others, 1992) may have generated the melts. In this model, delamination 1995; Turner and others, 1992) may generated In this model, delaminationand and thinning of the the orogenically orogenically overthickened overthickened crust allows hot asthenosphere to well up, up, raising raising the asthenosphere-lithosphere asthenosphere-lithosphereboundary and increasing the thermal input into into the the base base of of the the crust. This Thisprocess processisisattractive attractivebecause because granitic granitic melt melt generation is delayed until some 40-50 m.y. after the peak of orogenesis, orogenesis, coincident coincident with the time necessary for orogenic collapse and thinning of the mantle lithosphere thinning of the mantle lithosphere(Turner (Turner and and others, others, 1992). 1992). There are post-orogenic granites of the are several several lines of evidence which indicate that the post-orogenic St. Cloud area may have formed from crustal crustal underplating underplating by by high-temperature high-temperature mantle material material following following crustal crustal collapse collapse and and mantle mantle lithospheric lithosphericdelamination. delamination. - 11 �high temperatures temperatures 1. The The granite granite melts are are inferred by Spencer (1987) to have formed at high (800-1000°C), and and are nearly coeval with the (800-1000°C) the isotopically isotopically similar similar mantle-derived mantle-derived diabasic diabasic rocks rocks (Horan (Horan and and others, others, 1987). 1987). 2. The Theclose closeand andprobable probablecogenetic cogeneticassociation associationof ofthe theporphyritic porphyritic microgranite microgranitedikes dikes with diabase dikes. This This may may reflect reflect bimodal bimodal magma generation, a common feature of post-orogenic post-orogenic plutonic plutonic complexes complexeselsewhere elsewhere (Turner (Turner and and others, others,1992). 1992). 3. The Theemplacement emplacementofofgranite granitelags lagsbehind behindpeak peakorogenesis orogenesisby by—50 -50 Ma. 4. Holm 4. Holm and and others others (1998) (1998) concluded concluded from garnet garnet and and staurolite staurolite core-rim core-rim thermobarometric, geobarometric, geobarometric, and and thermochronologic thermochronologic studies studies (see following thermobarometric, section) that crust crust thickened thickened by the the Penokean Penokean orogeny orogeny remained remained so so for for50-60 50-60m.y. m.y. before collapse, collapse, which then may then have triggered sudden lower crustal melting. 5. Spencer Spencer(1987, (1987,p. p.174, 174,e.g.) e.g.) interpreted interpreted from from isotopic isotopic evidence evidence that the granite melts were generated at aa convergent convergent continental margin that recycled a source source material material with a short-lived crustal history. However, he allows for an extensional extensional environment environment superimposed superimposedon on an an earlier earlier orogenic orogenic event, event, to produce produce basic mantle mantle melts melts beneath beneaththe the crust to melt the crustal rocks. the crustal rocks. Thermobarometric data The significant significant proportion of igneous rocks, together with the regional staurolite grade of metamorphism of of the country rocks, implies implies that that outcrops outcrops in this area represent one of metamorphism of the the deepest exposed levels of the entire entire Penokean Penokean orogenic orogenic belt. belt. Holm Hoim and others (1998) recognized recognized deepest that Early Proterozoic country rock within the internal internal zone zone (Little (Little Falls Falls Formation) Formation) were were metamorphosed twice. Thermobarometric Therrnobarometricestimates estimatesindicate indicate that early, Penokean-age, garnetgrade (Ml) metamorphism occurred at pressures of 5-6 kbar and temperatures of 450-500°C 450-500°C. (Ml) metamorphism occurred at pressures Thermobarometric estimates metamorphism yield similar pressures, pressures, and and only only slightly slightly Thermobarometric estimatesof the M2 metamorphism temperatures (500-550°C) (500-550°C). This indicates plutons exposed exposed in in the St. higher temperatures indicates that Penokean plutons District were Cloud District were emplaced emplaced during during the the interim interim between between the the two metamorphic metamorphic events events at at 19-23 19-23 depths. The km midcrustal orogenic depths. The age age of of M2 M2 metamorphism must be older than the 1750Ma mica mica Ar/Ar Ar/Ar cooling cooling ages obtained obtained throughout throughout the the region region because because of of the the 300° 300°C closure closure 1760 Ma temperature of biotite Hoim and Lux, 1997;Holm Holm and and others, others, 1998). These These ages indicate indicate temperature biotite (Fig. (Fig. 5; Holm been reheated reheated to temperatures temperatures above 300° 300°C since since that that time. time. Five hornblende hornblende that the area has not been Ar/Ar dates obtained from central Minnesota are concordant with the 1760-1750 Ar/Ar 1760-1750 Ma mica mica temperatures dates. Hornblende Hornblendehas hasaa typical typical closure closure temperature temperature of 500°C, 500°Cwhich matches the temperatures obtained for M2 metamorphism (500-550°C). This is strong evidence that the rocks of the strong the metamorphism (500-550°C) remetamorphosed to to staurolite internal zone of the orogen in central central Minnesota were widely remetamorphosed grade during the same same event event responsible for for generating generating the post-tectonic magmas at 1770 1770Ma. Ma. 5-6 kbars kbars indicates indicates that little or or no no unroofing unroofing of of the the Penokean Penokean The M2 pressure estimate of 5-6 midcrustal root occurred occurred between between about about 1820 1820and 1770 1770 Ma. Emplacement Emplacement pressure estimates estimates using an igneous barometer indicate that some of the 1770 Ma post-Penokean plutons were intruded at 4.6 kbar pressures (17 km depth); depth); whereas, whereas, others were emplaced at lower pressures of 3.5 kbar, or about 13 km depth (Darrah, lower kbar, about 13 km depth (Darrah, 1996). 1996). These contrasting contrasting pressure estimates—derived estimatesÑderive from suites of rock tentatively inferred to be be comagmatic—indicate comagmatic-indicate that post-tectonic magma emplacement was synchronous with crustal uplift (Fig. (Fig. 6). 6). 12 �________ _________________________________________ Early Proterozoic Proterozoic tectonomagmatic tectonomagmaticmodel Our mapping observations and analytical results of the last five years indicate that there significant differences in the overall character of the & vs. -Penokean plutons are significant m-Penokean plutons(Table (Table that must be explained in any tectonic model. model. We believe these differences differences reflect two very 1) that tectonomagmatic settings; different tectonomagmatic settings; one that occurred at the end of orogenic orogenic belt construction, construction, and one during its demise demise approximately approximately 40-50 40-50 my later. (I)- ci) 4-- 0 0 "0- S I- a) E z 1800 Ml metamor hism Late- Penokean plutonism ) ) ) I I I II I I '1 11 j Ar/Ar, ArIAr,hornblende hornblende H Ar/Ar, mica McGrath gneiss dome dates S Ii' 1.1.1.1 ——l I 1700 I 1600 I 1500 AGE (Ma) Post-tectonic plutonism, M2 metamorphism metamorphism Figure Figure 5. Histogram Histogramof ofAr/Ar ArlAr dates dates from from the the EarlyProterozoic EarlyProterozoic of central Minnesota Minnesota (after Holm and Lux, 1996; 1996; Holm Holm and and others, others,1998). 1998). Figure 7 presents a Temperature (*C) (C) schematic synopsis of of our schematic synopsis our interpretation of pluton 300 400 500 600 300 400 the emplacement after compressive phase phase (Fig. (Fig. 7A) 7A) of compressive semi-brittle shears Penokean deformation in central n Rapid cooling Minnesota. The Richmond, Richmond, Reformatory, and Rockville-the Rockville—the Reformatory, 1770-1760 Ma "gray granites"—represent lower granites9'-represent \ 1 post-tectonic crustal crustal melts intruded into the the midcrust (20-22 km) km) of the midcrust (20-22 the Penokean orogen (Fig 7B). These These Penokean magmas were generated during generated during metamorphism Ml the waning stages stages of of Penokean Penokean compression hence, compression and 1 late-tectonic plutons developed a moderate magmatic fabric. fabric. Figure 6. Summary Summaryof of P-T-t P-T-t diagram diagram showing showing the relation The The evidence for (M 1and andM2), M2),plutonism, plutonism,and and forforbetween metamorphism metamorphism (Ml commingled commingled mafic mafic and felsic mation of semi-brittle shears in the internal zone of the these rocks indicates magmas in Penokena orogen Penokena orogen (modified (modifiedafter after Hoim Holm and and others, others, 1998). 1998). 13 13 �TABLE 1—Comparison post-Penokean intrusions intrusions 1-Comparison of late and post-Penokean TYPEIAGE TYPE/AGE Composition/Color Composition/Color LATE LATE PENOKEAN/—1812 PENOKEXNI- 1812 Ma Ma Granodiorite-granite——typically Granodiorite-granite-typically gray—with gray-with mafic maficenclaves enclaves POST-PENOKEAN/--1770 POST-PENOKEANI- 1770 Ma Granite Granite (alkali-feldspar (alkali-feldspargranite granite to to syenogranite)—typically syenogranitel-typically red— redwith with few few mafic mafic enclaves enclaves Magmatic Fabric Fabric Magmatic Moderately Moderatelydeveloped, developed, NE NE orientation, orientation,commonly commonlyflat flat Very weak to absent absent Pluton Pluton Shapes Shapes Broad Broad and and irregular irregular Dikes, Dikes, sills, sills, and and larger larger plutons plutons with a tendency for for NE orientation orientation Geophysical Characteristics Characteristics Variable Variable -- moderate moderate and busy busy magnetic magnetic signature; signature;dense dense Typically non-magnetic, non-magnetic, pronounced pronounced gravity gravitylows lows Depth Depth Emplaced Emplaced 20-22 20-22 km km(5-6 (5-6 kbar) kbar) 13-17 13-17 km (3.5-4.5 (3.5-4.5 kbar) kbar) Associated Metamorphism Metamorphism Garnet Garnet grade grade Ml M 1(5-6 (5-6 kbar) kbar) Staurolite M2 (5-6 (5-6 kbar) kbar) Staurolitegrade gradeM2 Solid-state Solid-state Fabric Fabric Cut Cut by by semi-brittle semi-brittleshears shears Cut Cut by semi-brittle semi-brittle shears shears that that crustal crustal underplating underplatingmay may have have played played an an important important role in their generation. However, However, these these crustal melts were not able to penetrate the stronger upper crust of the orogen, but instead not able to penetrate the stronger the orogen, but instead formed formed irregular irregularshaped shaped plutons plutons at at depths depths of of 20-22 20-22 km km where where they remained for approximately approximately 40 Penokean orogeny 40 m.y. The Theperiod period of of tectonic tectonic quiescence quiescenceand and amagmatism amagmatismafter after the Penokean orogeny (during (during the the period period approximately approximately1812-1770 1812-1770Ma) Ma)was wasterminated terminatedby byaashort-lived short-livedperiod periodof of post-tectonic post-tectonic magma magma generation, generation,metamorphism, metamorphism,and and rapid rapid midcrustal midcrustal uplift uplift and and cooling coolingat at about about 1770 1770Ma. Ma. This This abrupt abrupt episode episode may may have been triggered by mantle-lithospheric mantle-lithospheric delamination delaminationthat that would would have midcrustto to temperatures temperatures have caused causedboth bothmelting meltingof ofthe thelower lowercrust crust(Fig. (Fig. 7C) 7C)and and heating heatingof of the the midcrust above above 500°C 5OO0Cduring during M2 M2 metamorphism. metamorphism. This Thisthermal thermalheating heatingsoftened softenedthe the overthickened overthickenedcrust, crust, causing At this this time time significant significant unroofing occurred and shears shears causing itit to collapse collapse and and thin (Fig. 7D). At formed as as the the rocks rocks cooled cooled and and became more brittle. The TheSt. St.Cloud CloudGranite, Granite,Foley Foley batholith, batholith, and associated associated plutons—the plutons-the "red, "red, massive massive granites"—were granites9'-were advected advected during during this this event event into into uplifting orogenic shortening uplifting crust crustthat that was was extending extendingNW-SE, NW-SE, parallel parallel to to the the previous orogenic shorteningdirection. direction. Early Proterozoic Proterozoic diabasic diabasic and and quartzofeldspathic quartzofeldspathic dikes apparently lagged behind a bit bit and and were emplaced just after after brittle brittle shears shearsformed, formed, using using the the shears shearsas as conduits conduits(Fig. (Fig. 7E). 7E). were emplaced ACKNOWLEDGMENTS ACKNOWLEDGMENTS This This field field guide guideis is an an outgrowth outgrowth of studies studies by the Minnesota Geological Survey Survey (MGS) (MGS) that were were supported supportedby by several several programs programs of the State Legislature. The The Stearns StearnsCounty County segment segment (Boerboom and others, 1995, 1996a) was funded through the Legislative Commission 1995, 1996a) was funded through the Legislative Commission on on Minnesota Resources; funding for drilling drilling and and mapping mapping east of of St. St. Cloud Cloud (Jirsa (Jirsaand andothers, others, 1995a) was made available available on on recommendation recommendation of the the Minnesota Minnesota Minerals Minerals Coordinating Coordinating Committee. On-going On-goingmapping mapping and and analytical analytical work work in in both areas areas has been supported supported by the MGS. was supported supported thermobarometricand andgeochronologic geochronologicwork work of of Daniel Daniel Holm and others was MGS. The Thethermobarometric by a National Science Foundation grant (EAR-9304070). We are grateful to Mr. Frank Lyons Science We and and the the Meridian MeridianAggregates AggregatesCompany Companyfor foraccess accessand andguidance guidancein in the theaggregate aggregatequarry, quarry,and andfor for permission to to visit visit their their developing developingsouthern southern property property (before (before the the natural natural outcrops outcropsare aregone!). gone!). A very special special thanks thanks to to Charles Charles Muehlbauer Muehlbauer and and other other employees employees of the the Cold Cold Spring SpringGranite Granite Company Company for for assistance, assistance,advice, advice, samples, samples, permission, permission, and and guidance guidance (on their their day day off!). off!). 14 �___ Schematic Schematic N ++ + — Timing Timing Penokean orogeny Penokean orogeny Mj Mi metamorphism metamorphism \ crust cmst mantle- B s* s Penokean Penokean highlands Orogenic Orogenic features features —1850Ma Ma -1850 strong upper ccrust wt —_ Late-Penokean magma emplacement emplacement _..______S2022 datum 20-22 km depth depth datum mantle km manti —1812Ma Ma -18l.2 basaltic underplating? ---- ———, I --—-—- M2 metamorpinsin metamorph Generation Generation of post-tectonic post-tectonic magmas in the lower crust 1775-1770 Ma 1775-1770 Ma Mantle delamination, basaltic underplating? underplating? D E —- H — - - a- a- magma emplacement emplacement Post-tectonic magma Crustal unroofing/cooling crustal ~ o f ~ / c o o ~ Semi-brittle shears shears developed Semi-brittle Ma km depth depth datum datum 1770-1750 \20-22 20-22 km 1770-1750 Ma Diahase and QFP dikes <1750 MaKeweenawan? —. post orogenic plutons, plutons, Figure 7. Proposed Proposed tectonomagmatic tectonomagmaticmode mode for for generation generation of late- and post central Minnesota. Minnesota. Heavy dashed lin un represents 20internal zone of the Penokean orogen, central 22 km depth datum prior to crustal crustal unroofing unroofing (B (B and andC). C). Crustal Crustal unroofing unroofing and and exhumation exhumation causes upwarping upwarping of this this datum datum livel live1 in (D (D and E). 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Minnesota,October October 5-8, 5-8, 1997: 1997:American AmericanSociety SocietyofofCivil CivilEngineers, Engineers,Reston, Southwick, D.L., Morey, G.B., and McSwiggen, P.L., 1988, Geologic Geologic map map (scale 1:250,000) Southwick, 1:250,000) of the Penokean orogen, accompanying text: text: Minnesota orogen, central and eastern Minnesota, and accompanying Minnesota Geological 25 p. Geological Survey Survey Report Report of of Investigations Investigations37, 37,25 p. Spencer, K.J., 1987, constraints on the sources of granites in 1987, Isotopic, Isotopic, major, and trace element constraints an 1800 near St. Cloud, Minnesota: 1800 Ma-old Ma-old igneous igneous complex complex near Minnesota: Unpublished Unpublished Ph.D. Ph.D. dissertation, dissertation, SUNY-Stony SUNY-Stony Brook. Brook. Streckeisen, A chemical chemical approximation to to the modal QAPF Streckeisen, A.L., and Le Maitre, Maitre, R.W., R.W., 1979, A classification classificationof of the the igneous igneousrocks: rocks:N. N. Jb. Jb. Miner. Miner. Abh., Abh., v. v. 136, 136, p. 169-206. 169-206. Thiel, G.A., and Dutton, C.E., 1935, monumental stones of of 1935, The architectural, structural, and monumental Minnesota: Minnesota MinnesotaGeological GeologicalSurvey Survey Bulletin Bulletin 25, 25, p. 63-87. Turner, Sandiford, M, and Foden, J., 1992, Some geodynamic geodynamic and compositional compositional constraints constraints Turner, S., Sandiford, on "postorogenic" magmatism: magmatism: Geology, Geology,v.v.20, 20,p.p.93 931-934. 1-934. Van Schmus, W.R., W.R., and Bickford, M.E., M.E., 1981, Van 1981, Proterozoic chronology and evolution of the the Midcontinent region, Precambrian plate in Kroner, Kroner, A., ed., Precambrian plate tectonics: tectonics: New New Midcontinent region,North North America, America,in York, Elsevier, p. 261-296. 261-296. 17 �FIELD FIELD TRIP TRIP STOPS STOPS The shown on on figure figureFFl. The general general locations locations of all stops are shown l. A A more more detailed detailed location and geologic geologic map map is is included included with each each stop stop description. description. The Thevarious various rock rock units units are are fully fully described in in the the field field trip trip introduction, introduction,and and only only the the details details pertinent pertinent to to the the individual individualsites sites described are given given below. below. are I A KILOMETERS MILES 1 ------- 1 0 0 2 1 1 3 4 2 8 6 - 3 4 7 8 5 0 10 6 Figure Fl. Fl. Locations Locationsof offield fieldtrip trip stops. stops. STOP STOP 1. 1. STEARNS STEARNSCOUNTY COUNTYQUARRY QUARRY PARK • Location: Location:T.T.124 124N., N.,R.R.2828W., W.,Section Section19 19DDA. DDA. • St. St.Cloud Cloud(red) (red)Granite, Granite,diabase diabasedikes, dikes,semi-brittle semi-brittleshears shearsin in granite, granite, outcrops. glacially polished outcrops. This This area area contains containsseveral severalinactive inactivequarries quarries(Figs. (Figs. F2 F2 and and F3), F3), and and was was recently recently made made into into a unique unique county county park. The Thepark parkprovides providesexcellent excellent exposures exposures in which which one can can view view the the relationship between the Reformatory granodiorite and the St. Cloud Granite, as well as several relationship between the Reformatory granodiorite and the St. Cloud Granite, as well as several diabase dikes. Although Althoughthe thepark parkhas hasexposures exposuresof ofboth bothnortheastnortheast- and andnorthwest-trending northwest-trending diabase dikes, we will will only only see seenortheast-trending northeast-trending ones and St. St. Cloud Cloud Granite, Granite, due due to to time time constraints. constraints. 18 18 �________-- Diabase dike (single line for thin dike; dashed where inferred) Porphyritic microgranite Outcrop IA Ii —450 32 30 .4 QUirri - I DM7 4 0 092 -. / C J -- — z I /Y kI -'zr? )( (JU :-- Q'II. rI 0 -'S 1094 - '':i • t074 p - ç— — I c..j Â¥ Mhi =;, ON jl FÑ 1 3.',, 3 1,- ' 6282 kilLS MILS 15 15MIS MgLS I ,' i 00 ^ ^ ^ - - . 2 - - , 0 1000 1000 , È I - 1 -0 I1 - - 1000 1 2000 200C 3000 3000 4SOO 4001) 5 00 .55 - - - - - , 1 MILC MILE - - 5000 5000 6000 7000FEfT 701.51 FEri 1 I KILOMETRE 11KILOMETRE i CONTOUR CONTOUR INTERVAL INTERVAL 10 10FEET FEET DOTTED LINES RPR.SNT 5.FOO1 CONTOURS DOTTED LINES REPRrSFNT 5-FOOT CONTOURS Figure FigureF2. F2. Map MapofofWaite WaitePark, Park,southwest southwestof ofSt St.Cloud, Cloud,showing showinglocations locationsof ofoutcrops outcropsand anddikes dikes in 1:24,000. in parts parts of of the theSt. St.Joseph Joseph and andSt. St.Cloud Cloud 7.5-minute 7.5-minute quadrangles. quadrangles. Scale Scalereduced reduced from from 1:24,000. 19 �2, and 3. Scale Figure F3. Location Locationmap map for for stops stops 1, 1,2, Scale1:24,000; 1:24,000;St. St. Cloud Cloud 7.5-minute quadrangle. quadrangle. At least seven different different quarry companies were active in what is now the park area—all area-all utilizing the St. Cloud Granite. Production Production started in the early 1900's, peaked in about 1940, variety of of "red" "red" and ceased in the mid-1950's. The Thestone stonethey they produced was marketed under a variety "Mahogany Red," Red," "Indian "Indian Red," Red," and and"Rose "Rose Red." Red." Most of names, including "North Star Red," Red," "Mahogany the pits were (and (and still still are!) 60 60 to to more more than 100 100 feet feet deep deep (Thiel (Thiel and Dutton, Dutton, 1935). 1935). PLEASE PILES ARE ARE NOT NOT PLEASE EXERCISE EXTREME CAUTION! THE THEQUARRY QUARRY PILES STABLE, AND AND THE THE PITS PITS ARE ARE DEEP! DEEP! parking area. From Follow the park road south from the park entrance to the parking From here here walk walk trail on on the the right right (west) (west)that thatleads leadsup upto toquarried quarriedarea. area. Proceed 400 feet south on a dirt road to a trail up this trail to the edge edge of of aa small small quarry quarry and and Stop Stop 1A 1A (Fig. (Fig. F4). F4). 20 �Diabase Dike St. Cloud (Red) Granite Reformatory (Grey) Granite Quarry Figure F4. Outcrop Outcrop map map of Stop 1A and lB. IB. STOP lA—Three-dimensional 1A-Three-dimensional view view of diabase and quartz-epidoteshears shearsin inSt. St.Cloud Cloud STOP and quartz-epidote Granite. Granite. Directly Directlywest, west, across across the the rubble rubble in the pit, is an excellent exposure in pit, is an excellent exposure of a northeast-striking, northeast-striking, steeply steeply southsouthdipping, dipping,meter-thick meter-thickdiabase diabasedike dikethat that cuts cuts St. St. Cloud Cloud Granite. Granite. The Thedike dikewas was sampled sampledfor for both both paleomagnetic paleomagneticstudy study and and Ar/Ar ArIAr dating dating (see (seeintroduction). introduction). This This is is typical typical of of the the diabases diabases in in the the area, area, having having chilled chilled margins margins and and Diabase dikes ~iab&e dikes Quartz-epidote Quartz-epidoteshears shears scattered cm-scalexenoliths xenolithsof ofgranite granite scatteredcm-scale wallrock. wallrock. The Thegranite graniteadjacent adjacentto tothe the Figure F5. F5. Stereographic Stereographicprojections projectionsof oforientations orientations Figure dike dike contains contains numerous numerous thin, thin, diabase dikes dikes (A) and and quartz-epidote-filled quartz-epidote-filledsemiserniof diabase northeast-trending, northeast-trending, quartzquartz- and and brittle brittle shears shears(B). (B). epidote-lined, epidote-lined,semi-ductile semi-ductileshears shearsthat that 21 �truncated by by and are probably probably in in part part occupied occupied by by the the diabase diabase dike. dike. This are clearly truncated This nearly nearly coincident trend typical of the regional pattern shown on the stereoplots stereoplots in coincident trend of dikes dikes and and shears shears is typical figure F5. Two Two other other diabase diabasedikes, dikes, up up to to 22 meters meters thick, thick, can can be be seen seen north north of of this this dike. dike. Although the majority of blocks in the the waste waste rubble rubble before you are are indigenous, indigenous, several several exotic rock types are present, including including blocks of Reformatory granodiorite granodiorite (gray) (gray) that we we will will see in place at Stop 3. The Reformatory granodiorite is presently extracted from a quarry The Reformatory granodiorite is presently extracted quarry just by the Cold Spring Granite south of the city of of St. St. Cloud, Cloud, marketed marketed as "Charcoal "Charcoal Granite" Granite" by Company. The Company. The maroon-colored maroon-colored granite granite blocks are probably probably imported from quarries near South Dakota. Dakota. Blocks of grayish-pink, complexly banded Ortonville, Minnesota and Milbank, South Morton gneiss are are from from the the Minnesota Minnesota River River Valley. Valley. Several Several other blocks of pink granite granite and and pale pink gneiss are from unknown localities, but are not indigenous. Return to the dirt road, Return road, and and walk walk south south (to (to the the right) right) approximately approximately 250 feet to to the the switchback right. Note the spheroidally-weathered granite along along the the switchback trail on the right. spheroidally-weatheredoutcrops of red granite west (right) edge of the road between the two trails. The The aggregate aggregatecovering covering the the switch-back switch-back trail illustrates illustrates one of the many uses of crushed stone from the Meridian Aggregates one stone Aggregates Quarry Quarry that that polished outcrop on on the the north north we will visit at Stop 2. At At the the next quarry proceed to a glacially polished (right) side of the pit. pit. STOP Granite. STOP 1B—Diabase 1B-Diabase dike in St. Cloud Granite. The diabase dike splits and rejoins along its length, and has small, 4-10 cm, north-south 'jogs' at at its its northeast northeasttrend. trend.There Thereare areno noobvious obviouscross-fractures cross-fracturesin in the the granite granite that that might might have have controlled the these small small offsets, offsets, making them somewhat enigmatic. xenoliths, as well as This stop stop affords affords a better look look at the chilled dike margins and granite xenoliths, of granite wallrock walirock torn torn partly partly loose loose during during dike dike emplacement. emplacement. Granite xenoliths in slivers of various stages of assimilation assimilation can be seen, seen, and xenoliths xenoliths are concentrated concentrated in the the center center of of the the ±0.5 main dike, presumably by flow entrainment. Also Also visible here, with some searching, are ±0. cm amygdules filled with fibrous fibrous actinolitic actinolitic amphibole. side, where where two two subparallel, subparallel, northeast-trending northeast-trending diabase Walk around the quarry to its south side, diabase approximately 3 m thick—the this thick-the maximum thickness of dikes in this dikes occur. These These dikes dikes are approximately set. They They are are cut cut by by thin, thin, white, white,silicic, silicic,and and sugary-textured sugary-textured veinlets veinletsthat that emanate emanatefrom from the the dike dike margins and pinch out in the dike interiors. The Theveinlets—interpreted veinlets-interpreted as partial melts of of the the presumably because because narrower wallrock granite—are granite-are rarely rarely present present in in dikes dikes less less than 2.5 m thick, presumably narrower dikes were not capable of heating the wallrock to temperatures sufficient for melt generation. polished outcrop mineralogy and The polished outcropsurface surfaceprovides provides aa good good look look at at the mineralogy and coarse coarse texture texture of black and white white mottled, mottled, red red color. color. The the St. Cloud Granite, characterized characterized by its distinctive black The imparted by microcline, red color is imparted microcline,the the black black by hornblende hornblendeand and minor minor biotite, biotite, and and the the white white by by saussuritized plagioclase. The The granite granite has has aa very very weak weak trachytoid trachytoid foliation, foliation, via alignment alignment of blocky feldspar and hornblende crystals. Where Where discernible, discernible, this fabric parallels the granite granite coarse-grained phase) phase) dike or sill margins. This This granite granite has has an amazing amazing lack of pegmatite (very coarse-grained and a preponderance preponderance of homblende hornblendeas as the the mafic mafic phase. Glacial features are are well well displayed displayed here here and and include include polished polished outcrops, outcrops, striations, striations, and and differential diabase/granite contacts. contacts. Most Mostof of the the subsurface subsurfacebedrock bedrock in in Minnesota Minnesota differential scouring scouring of the diabaselgranite is covered by variable variable thicknesses thicknesses of of weathered weathered bedrock bedrock residuum (saprolite) (saprolite) that has has been been highs by by glacial scouring. scouring. Saprolite stripped away from bedrock topographic highs Saprolite developed developed on on 22 22 �granite is typically pale gray-green. Many Many of of the the active active quarries in this area have exposures of granite of this saprolite saprolite along along the the margins margins of of the the pits. pits. interesting hydrologic hydrologic observation is the approximately 10-foot 10-foot difference in water An interesting level level between this pit and the small one to the south. The The two two quarries quarries are are separated separated by little little more than than the the diabase diabase dike's dike's width, width,and and the the hydrologic hydrologic gradient gradient exists exists despite despite the the abundance abundanceof of more cross-fractures cross-fractures in in the the diabase. diabase. Addendum Addendumto toStop Stop11 The The sketch sketch map shown shown on figure figure F6 is included for those who wish to visit the park to Of particular particularinterest interestisis aa pit pitatatthe theeast eastside sideof ofthe thepark park view other exposed geologic features. Of northwest-trending diabase to the the where aa northwest-trending diabase dike dike cuts cuts aa northeast-trending northeast-trendingone. one. The next pit to northwest of that thatshows showsSt. St.Cloud CloudGranite Granitehaving havingmany manyangular, angular,meter-scale meter-scale xenoliths xenoliths of of Reformatory Reformatory granodiorite—typical granodiorite-typical of of regional regional intrusive intrusiverelationships relationships between between the the two twounits. units. Diabase Dike St.Cioud(red) Granite ' Reformatory(gray) granodiorite j Mixed St. Cloud Granite and Reformatory granodiorite .# Cross—country ski trails Figure Figure F6. F6. Sketch Sketchmap mapofofthe theQuarry QuarryPark Parkshowing showingoutcrop outcropdistribution distributionand andsimplified simplifiedbedrock bedrock geology. geology. Modified Modifiedfrom fromStearns SteamsCounty CountyPark Parkwinter wintertrail trailmap. map. 23 23 �STOP STOP 2. 2. MERIDIAN MERIDIANAGGREGATES AGGREGATESQUARRY QUARRY • Location: Location:T.T.124 124N., N.,R.R.28 28W., W.,south southhalf halfofofSection Section18. 18. • Quarry Quarryoverlook, overlook,crushing crushingfacilities, facilities,restrooms. restrooms. Due to constraints constraints of time, time, safety, safety, and group size, we will will not not go go into into the the Meridian Meridian Aggregates Aggregates quarry, quarry, but we we will will get get an an excellent excellentview view of of the the pit pit from froman anoverlook overlookpoint. point. 1.2 million million tons tons of of crushed crushed Meridian Aggregates Aggregates Company produces an average average of 1.2 The Meridian rock aggregate aggregate per year. Mining Miningand andcrushing crushingoperations operationsrun run 77 months months of of the the year, year, although although stockpiled stockpiledproduct productisis shipped shippedyear-around. year-around. This Thisquarry quarryutilizes utilizes the the complex complex mixture mixture of igneous igneous rock rock types types present present in in the the pit, pit, mainly mainly the the Reformatory Reformatory granodiorite granodiorite and St. Cloud Red Granite, and and lesser lesser proportions proportions of of diabase diabaseand andquartz-feldspar quartz-feldsparporphyry porphyry dikes, dikes, such such as as will will be be seen seenatat Stop Stop 3. 3. Unlike Unlikedimension dimensionstone stonequarries, quarries,joints jointsand andfractures fracturesin inthe therock rock are are not not detrimental— detrimentalblast blast away! away! Quarrying Quarryingmethods methodsinvolve involvedrilling drillingand andblasting, blasting,primary primary crushing crushingin inaagyratory gyratory crusher, crusher, and and secondary secondary crushing crushing to various sizes. The The primary primary use use of of the the aggregate aggregate is is railroad railroad ballast, ballast, followed followed by by road road construction construction and and other other uses, uses, including including breakwater breakwater walls, decorative decorative stone, stone, and and driveway drivewaymaterial. material. A A substantial substantialfraction fractionof of the therock rockin in this this quarry quarry consists consistsof of fine-grained fine-grainedgray gray granodiorite granodiorite interpreted interpreted by by earlier earlierworkers workersas asinclusions inclusionsofofmetamorphosed metamorphosedand andmetasomatically-altered metasomatically-altered country rock in in the the Reformatory Reformatory granodiorite. granodiorite. Although Although there there are are undoubtedly undoubtedly some some small small xenoliths, finexenoliths,textural texturaland andgeochemical geochemicalevidence evidenceindicates indicatesthat thatthe thebulk bulkof ofthis thisgray graymaterial materialisisfinegrained grained border border phase phase of of the the Reformatory Reformatory granodiorite. granodiorite. If If the the light lightconditions conditionsare areproper, proper,aacross-section cross-sectionisisvisible visibleon onthe thefar farwall wallof of the the pit pit showing showing fresh fresh rock, rock, transitionally transitionallyoverlain overlainby weathered weathered bedrock residuum residuum (saprolite), (saprolite),overlain by glacial deposits. deposits. STOP STOP3. 3. MERIDIAN MERIDIANAGGREGATES AGGREGATESCOMPANY—SOUTH COMPANYÑSOUT PROPERTY PROPERTY • Location: Location:T.T.124 124N., N.,R.R.2828W., W.,Section Section19 19AB. AB. • St. quartzSt.Cloud Cloud(red) (red) Granite, Granite,aplite, aplite, Reformatory Reformatory granodiorite, granodiorite,diabase diabase dikes, quartzfeldspar feldsparporphyry porphyry(aka (akaporphyritic porphyriticmicrogranite) microgranite)dikes. dikes. This This stop stopisislocated locatedin inan anarea areaof of planned planned large-scale large-scalemining mining like like that that in in Meridian's Meridian's main main pit pit (Stop (Stop2) 2) just across acrossHighway Highway 23 23 to to the the north. north. There Thereare areaafew fewsmall smallabandoned abandonedquarries quarrieson on the the property, property, but but most most of of the the outcrops outcrops are natural (Fig. F7). Permission Permission must must be be obtained obtainedfrom from Meridian Meridian Aggregate Aggregate Company, Company, which has kindly kindly granted granted access access for for this this trip, trip, before before entering entering their their property, property,but but these these outcrops outcropswill will soon soonbe be mined mined out. out. STOP Cloud (red) (red)Granite Granite intrusive intrusive into into Reformatory Reformatory STOP3A—NE-trending 3A-NE-trending dikes dikes of St. Cloud (gray) (gray)granodiorite. granodiorite. The TheSt. St.Cloud CloudGranite Graniteisisnot notaadistinct distinctsingle singlebody, body, but but rather rather an an intricate intricate series series of of vertical vertical and and horizontal horizontal dikes dikes emplaced emplaced into into the Reformatory granodiorite. As As represented represented in this series series outcrops, outcrops,the theintrusions intrusionshave havecomplex complexgeometries geometriesand andvaried variedthicknesses thicknessesfrom fromfew fewcentimeters centimeters to to more more than than 50 50meters. meters. 24 �3B-A variety units, including including (oldest (oldest to toyoungest) youngest) STOP 3B—A variety of of intrusive rock units, St. Cloud Cloud Reformatory granodiorite, St. Cloud Granite, aplite related to the St. Granite, Granite,and anddiabase. diabase. The aplite aplite occurs occurs as as small, small,irregular irregularpod-shaped pod-shaped masses masses that that apparently apparentlycut cutboth boththe thered red The and gray gray intrusions. intrusions. Although Althoughthe thetiming timingof of emplacement emplacement of of the the aplites aplites relative to that of the and quartz-feldspar porphyry porphyry and and diabase diabase dikes dikes is is uncertain, uncertain, the the irregular irregular shapes shapes of of the the former former vs. vs. quartz-feldspar the straight straight and andcontinuous continuous(fracture-controlled (fracture-controlled emplacement) of the latter two two leads leads to to the the the inference that that the the aplites aplites are are a late late differentiate differentiate of the St. Cloud Granite. At this this site site and and the the inference Granite. At next (stop (stop 3C), 3C),note note the the contrast contrast in in jointing jointing pattern pattern of the aplite aplite and porphyry porphyry compared compared to to the the next surrounding surroundingplutonic plutonicrocks. rocks. Early Proterozoic Intrusive Rocks --------e Diabase dikes QFP (Porphyritic microgranite) dikes Aplite pods related to St. Cloud Granite (?) St. Cloud Granite Ref ormatory granodiorite Outcrop Geolo ical contacts, \ - - dashe where inferrec Small or discontinues diabase dike 0 8 ' 200 feet I 50meten I . Trails I Figure FigureF7. F7.Outcrop Outcropand andgeologic geologicmap mapofofStop Stop3.3. 25 % �STOP 3C-Porphyritic 3C—Porphyritic microgranite (aka quartz-feldspar quartz-feldspar porphyry). porphyry). This dike strikes strikes N45E, is 4-5 4-5 m thick, and has 4-6 percent percent glassy glassy subhedral subhedral quartz quartz phenocrysts and 3-4 percent subhedral phenocrysts in in an aphanitic, siliceous, phenocrysts subhedral K-feldspar K-feldspar phenocrysts groundmass. Dikes Dikes such suchas asthis thisare arescattered scattered around around the the St. St. Cloud Cloud area, area, and like the diabase the St. St. Cloud Cloud Granite. Granite. The dikes, they cut the Reformatory granodiorite, Rockville Granite and the phenocrysts vary from the 2-4 mm cm elsewhere. elsewhere. The margins phenocrysts rnm size here, to as large as 22 cm margins of the quartz-feldspar dikes are chilled, and in some cases flow-banded. A similar dike in quartz-feldspar dikes are chilled, and in some cases flow-banded. A similar dike in the the aggregate aggregate quarry north of here is clearly clearly comagmatic comagmatic and commingled with a diabase dike that contains fresh ophitic clinopyroxene, olivine, and chromite. However, However, that diabase dike contrasts with not altered/metamorphosed altered/metamorphosed all the other diabase diabase dikes dikes in this area in that the mafic minerals are not to actinolitic actinolitic amphibole. amphibole. STOP xenolith of of garnet garnet amphibolite amphibolite in St. Cloud Granite. Granite. STOP 3D—Small 3DÑSmal xenolith True xenoliths of external origin (vs. cogenetic mafic enclaves) are are rare rare in in all of the St. Cloud District intrusions. Xenolith types noted in the St. Cloud area include garnet-magnetiteCloud District intrusions. Xenolith types noted in the St. Cloud area include garnet-magnetiteamphibole schist (probably (probably metamorphosed metamorphosed iron-formation), biotite-quartz-feldspar schist of graywacke magnetite, and abundant graywacke protolith, protolith, granulitic granuliticgneiss gneiss that that contains contains garnet, sillimanite, sillimanite, magnetite, abundant cordierite, charnockitic garnet-biotite-orthopyroxene garnet-biotite-orthopyroxenegranofels. The The general general metamorphic metamorphic cordierite, and chamockitic mineral assemblages indicate that the country rock was metamorphosed to amphibolite and higher retrograde metamorphism metamorphism caused higher grade grade prior prior to to incorporation incorporationinto into the granites, granites, with minor retrograde caused by contact with with granite granite magma. magma. On the way to Stop 4 watch for the the derrick derrick in in the the deep deep quarry quarry north north (right) (right) of of the the highway highway on the east edge of the town of Rockville. Rockville. This This is the "Rockville Beige" quarry operated operated by the Cold Spring Spring Granite Granite Company. Company. STOP STOP 4. COLD COLDSPRING SPRINGGRANITE GRANITEQUARRY QUARRYAND AND PROCESSING PROCESSINGPLANT PLANT • Location: T.T.123 Location:(quarry) (quarry) 123N., N.,R.R.29 29W., W.,Section Section19 19BDAB. BDAB. • Rockville Rockville Granite, Granite,quarry quarrymethods, methods,production productionof offinished finisheddimension dimensionstone. stone. -- '—, —..__/ .1 -/ J/4 ((Gr.v • — u)j Highway 23')c-StOp 4A Figure F8. Location Location of of Stop Stop 4A. Other Otheroutoutcrops of Rockville Granite shown in black, Granite shown in black, except over the two quarries. STOP 4A—Cold 4A-Cold Spring Granite Granite Rockville Rockville White quarry quarry(Fig. (Fig. F8), F8), Rockville Rockville Granite, Granite, mafic mafic enclaves. enclaves. This is the "Rockville L'RockvilleWhite" quarry, quarry, one one of the three Cold of Cold Spring Spring Granite Granite quarries quarries developed granite developed in in Rockville Rockville Granite. Granite. The granite from the other Rockville quarries is marketed Beige" and and "Diamond "Diamond Pink." Pink." Due as "Rockville Beige" we will not be able to enter to safety concerns we the quarry, but will have ample opportunity to view the Rockville Granite Granite up close. close. This quarry uses the "drive-in This "drive-in quarry quarry method," method," a system that replaces the traditional cable-operated cable-operated derrick derrick method method of stone stone removal. Surprisingly, Surprisingly, the method was developed only in 26 �the mid-1970's mid-1970's (Muehlbauer (Muehlbauerand andFuchs, Fuchs,1997). 1997).The Thedrive-in drive-inmethod method utilizes utilizes aa series series of benches, the from which which the the stone stone is is removed removed as a series series in large "loaves" "loaves" that vary in size—15-25 size-15-25 feet (5from 8m) high high xx 20 20 feet feet (6m) (6m) wide wide xx 30-150 30-150feet feet (lO-50m) (10-50m) long. long. These Theseloaves loavesare aresubsequently subsequently 5.5 x 5.5 5.5 x 10.5 drilled off into smaller blocks that that are optimally optimally 5.5 10.5 feet feet in in size, size, weighing in at approximately 25,000 kg, or nearly 28 tons. This Thissize sizeisissmall smallenough enoughto totransport transportlegally legallyby by approximately flatbed trailer trailer without without aa special specialpermit, permit, and and allows allows for for minimum minimum wastage wastage during during processing. processing. flatbed Charles Charles Muehlbauer of Cold Cold Spring Spring Granite Granite will give a more extensive overview of the the quarry, quarry, and will will also also answer answer questions questions about about the the quarry quarry and andstone stone quarrying process at the products products in in general. general. There are many near the quarry viewing area to many blocks blocks of of Rockville Rockville Granite stacked near examine. examine. The Thefollowing followingfeatures featurescan canbe beseen seenin inthe theblocks: blocks: 1. Porphyritic Porphyritictexture, texture,rapakivi rapakivifeldspars. feldspars. 2. Mafic Maficenclaves, enclaves,some somewith with feldspar feldspar phenocrysts phenocrysts in them. The Theenclaves enclavesare areinterpreted interpreted as as the the product product of of the themingling minglingof ofmafic maficand andfelsic felsicmagmas magmas(see (seeINTRODUCTION). INTRODUCTION). Most Most occur occur as as ovoid ovoid inclusions, inclusions, although although discontinuous discontinuousand and irregular irregular dike-like dike-like masses masses of of the the same same composition compositionhave have been been noted noted in in another another quany. quarry. 3. Weak phenocrysts. This Weak trachytoid trachytoidtexture texturedefined defined by aligned aligned feldspar feldspar phenocrysts. Thisisistypical typicalof ofthe the "gray g r a y granites." granites." Stop Granite Company Processing and and Fabrication Fabrication Plant. Stop 4B—Cold 4BÑCol Spring Granite Plant. Cold two different different buildings. buildings. One Cold Spring Springpersonnel personnelwill will guide guide us in small groups through two One is is the the facing facing plant, plant, where where rough rough quarry quarry blocks blocks are are cut, cut, finished finished (polished (polished vs. vs. the the rougher rougher "fire"firefinish"), product specifications. The finish"), and and made made into into facing facing panels of varied size, depending on product The other polished, and and engraved. engraved. Notice other is the the monument monument plant, where the stone is cut, shaped, polished, surprisingly surprisinglyornate ornateworks! works! ****PLEASE * *PLEASE OBSERVE OBSERVESAFETY SAFETY REGULATIONS— REGULATIONS- HARD HARDHATAND HAT ANDSAFETY SAFETYGLASSES GLASSESMANDATORY*** MANDATORY*** STOP STOP5. 5. RICHMOND RICHMONDGRANITE GRANITE • Location: Location:T.T.123 123N., N.,R.R.3030W., W.,Section Section29 29BB BB center. center. • Charnockitic, prominent trachytoid trachytoid K-feldspar Charnockitic,rapakivi rapakivi granite granitewith with prominent K-feldspar phenocrysts. phenocrysts. ***PRIVATE PERMISSION OF *** ***PRIVATEPROPERTY—MUST PROPERTY-MUST HAVE HAVE PERMISSION OFHOMEOWNER HOMEOWNERTO TOENTER ENTER*** —HAMMERING -HAMMERING AND AND SAMPLING SAMPLJNG PROHIBITED— PROHIBITED- The TheRichmond Richmondgranite granitewas wasinterpreted interpretedby byearlier earlierworkers workersas asaahigh-grade high-grademetamorphic metamorphic gneiss gneissof ofArchean Archeanage. age. By Bycontrast, contrast,this thisstop stopdemonstrates demonstratesthat thatthe thegranite granitehas hasaa primary primary igneous igneous texture, texture, which which isis typical typical of of the the unit unit overall. overall. ItItcontains containsundeformed undeformedunit unitquartz quartzgrains grainsas aslarge large as 1 cm and euhedral feldspar phenocrysts as large as 5 cm. Because quartz is one of the as 1 cm and euhedral feldspar phenocrysts Because quartz is one of the first first minerals mineralsto toexhibit exhibitstrain strainand and recrystallization recrystallizationduring during deformation deformationand and metamorphism, metamorphism,the the rock rock clearly semiclearly has has not notexperienced experienced substantial substantial amounts amounts of of either either process, process, and and only only localized localized semibrittle brittle shearing shearing exists. exists. We Weinterpret interpretthis thisand andother otherlithologic lithologic evidence evidence to to indicate indicate that that the the Richmond emplacedafter aftermost mostof of the the regional regional deformation deformationceased, ceased, and and infer infer that that itit Richmondgranite granitewas wasemplaced probably probablyhad hadan anemplacement emplacementhistory historylike likethat thatof of the theReformatory Reformatorygranodiorite granodioriteand andthe theRockville Rockville Granite. Granite. 27 �Key features features at at stop stop 5: 5: 1. Dark green charnockitic charnockitic granite. This Thiscolor colorphase phasegrades gradesinto intothe the more moretypical typicalpink-tinted pink-tinted 1. Richmond granite, Richmond granite, due due to oxidation oxidation introduced introduced along along fracture fracture planes. At At first first glance, glance, the the green green phase looks like gabbro, gabbro, but if you trace trace across the gradation gradation to the pinker pinker phase, phase, you will will see see that it actually actually contains contains abundant abundant quartz quartzand and K-feldspar. K-feldspar. 2. Euhedral, Euhedral, carlsbad-twinned carlsbad-twinnedK-feldspar K-feldspar (microperthite) (microperthite) phenocrysts. Some Someof these are mantled by a thin rim of plagioclase, plagioclase, giving giving rise rise to rapakivi texture—best texture-best observed observed on on vertical, vertical, slightly slightly weathered faces, where plagioclase plagioclase is bleached white. 3. Magmatic-trachytoid Magmatic-trachytoidalignment alignmentof of feldspar feldsparphenocrysts phenocrysts (see (see Fig. F9). dark-colored mafic mafic enclaves. enclaves. From From aa petrographic petrographic standpoint, standpoint, these enclaves are 4. Small dark-colored mineralogically identical to aa fine-grained, mineralogically fine-grained, trachytoid, trachytoid, apatitic, apatitic,quartz-orthopyroxenequartz-orthopyroxeneclinopyroxene ferrodionte outcrops clinopyroxene ferrodiorite(orjotunite, (orjotunite,in incharnockite charnockiteterminology) terminology)that that occurs occurs in several outcrops 1/4 mile to the the west west of of this this location. location. This diorite diorite contains contains abundant abundant zircon zircon and small phenocrysts 114mile small phenocrysts phase that is of both plagioclase plagioclase and K-feldspar, K-feldspar, and locally grades into a coarser-grained coarser-grained phase compositionally and compositionally and texturally texturally similar similar to the Richmond granite as exposed here. The Thediorite dioriteto to the west is inferred inferred to to be be aaborder border phase phase of of the theRichmond, Richmond,and andthe the small smallenclaves enclavesscattered scattered throughout the Richmond throughout Richmond are are interpreted interpreted as as cognate cognate xenoliths derived from from this dioritic dioritic border border phase. •END FiELD TRIP. TRIP. THANKYOUFORATITENDING! END OF FIELD THANK YOU FOR ATTENDING!---------------You are are encouraged encouraged to to visit the poster titled You "DIMENSION "DIMENSION STONE STONE PRODUCTS PRODUCTS OF OF MINNESOTA" MINNESOTA" on display at the poster session. 28 �A ——- Strike and dip of ductile shear fabric Strike and dip of brittle shear fabric Strike and dip of trachytoid foliation vertical, inclined Figure F9. Map of central part of the Cold Spring 7.5-minute quadrangle, showing outcrops of Richmond granite. Structure symbols touch the outcrop from which they were taken. OAD �PHOTOGRAPHS OF QUARRIES QUARRIESININTHE THEST. ST.CLOUD CLOUDAREA AREAFROM FROMTHE THE EARLY 1900's. Top, QUARRIES PHOTOGRAPHS OF EARLY 1900's. TOP, QUARRIESOPERATED OPERATEDBY BY NORTH STAR GRANITE COMPANY, NOW OVERRUN AGGREGATES QUARRY?; NORTH STAR GRANITE COMPANY, NOW OVERRUNBY BYTHE THEPRESENT-DAY PRESENT-DAY MERIDIAN MERIDIAN AGGREGATES QUARRY?; BOTBOTTOM,AAQUARRY QUARRY OPERATED OPERATED BY GRANITE COMPANY, LOCATED IN FIELD TOM, BY THE MELROSE MELROSE GRANITE COMPANY, LOCATED IN THE THE VICINITY VICINITYOF OFSTOP STOP 1 1 OF FIELD TRIP 1.1. REPRODUCED FROM JOURNAL COLORED GRANITES FOUND IN TRIP REPRODUCED FROM JOURNALARTICLE ARTICLE OF OFTHE THEBRILLIANT BRILLIANT COLORED GRANITES FOUND IN AMERICA'S AMERICA'S GREAT NORTHWEST AMERICAN STONE STONE TRADE, PUBLISHED GREAT NORTHWEST ININ AMERICAN TRADE, PUBLISHEDMAY, MAY, 1931. 1931. 30 30 ��FIELD TRIP TRIP #2 FIELD THE SOUTHEASTERN FIELD GUIDE GUIDE TO THE GEOLOGY OF THE RIFT SYSTEM, PORTION OF OF THE THE MIDCONTINENT RIFT SYSTEM, EASTERN AND WESTERN WESTERN WISCONSIN WISCONSIN MINNESOTA AND Leaders: Leaders: Karl KarlR.R.Wirth, Wirth,William WilliamS.S.Cordua, Cordua,William WilliamF. F. Kean, Kean, Mike Middleton, Zachary Zachary J. J. Naiman Naiman INTRODUCTION The Middle Proterozoic Midcontinent rift rift system system extends extends over over 2,000 Proterozoic (Keweenawan) (Keweenawan) Midcontinent kilometers in an arc across central North America America from from Kansas Kansas to tocentral centralMichigan. Michigan. Magmatic kilometers Magmatic continental lithosphere lithosphere and sedimentary sedimentary rocks rocks in in this this major major tectonic tectonic feature feature record the evolution of continental million years years ago. ago. Rifting as it interacted with a mantle plume 1100 million Rifting was was aborted aborted before the production of oceanic lithosphere lithosphere began. Field, production Field, geochemical, geochemical, geochronologic, geochronologic, and and geophysical geophysical studies during the past decade have made significant contributions contributions to our understanding understanding of the evolution of of the Midcontinent rift rift system. system. The interested reader is referred to several recent evolution reviews (Van Schmus Schmus and Hinze, 1985), volumes (Tectonophysics, 213, 1992; Canadian Canadian reviews 1985), special volumes (Tectonophysics,v. 213,1992; Journal of Earth Science, Science, v. v.31, 1994; Canadian 34, 1997; Ojakangas Ojakangas Journal 3 1,1994; Canadian Journal Journal of Earth Science, Science, v. 34,1997; et al., 1997), 1997), and and guidebooks guidebooks (Miller (Miller et al., al., 1995) 1995) on on the the Midcontinent Midcontinent rift rift for for additional additional information. information. Geophysical Geophysical studies studies have provided much information about the nature of the rift, which lies buried beneath younger deposits along much much of of its its length. length. Regional gravity and magnetic surveys were important in establishing the extent and character of the rift (e.g., Hinze Hinze et et al., al., 1992). Our Our knowledge knowledge of of the the detailed detailed structure structure and and stratigraphy stratigraphy of the rift has has been been greatly greatly of the COCORP (Serpa et al., 1984; enhanced by several seismic surveys, most notably those of 1984; Zhu and and Brown, 1986) 1986)and and GLIMPCE GLIMPCE (Behrendt (Behrendt et al., 1988; 1988; Cannon et al., 1989) 1989)programs. programs. These surveys basins filled filled with with thick sequences of surveys revealed the presence of fault-bounded basins sedimentary rock. Detailed Superior volcanic and sedimentary Detailedseismic seismicimages imagesof the eastern portion of Lake Superior indicate the percent of oforiginal original thickness) thickness) in in some portions portions of the indicate the extent extentof of crustal crustalthinning thinning(—30 (-30 percent kilometers deep deep and and consists consists of of roughly roughly 20 kilometers kilometers rift; the rift in this region is more than 30 kilometers of basalt overlain by 10 10 kilometers of Keweenawan sedimentary rock. The voluminous tholeiitic basalt flows voluminous volcanic volcanic rocks of the rift consist predominantly of tholeiitic flows and minor rhyolite (Green, (Green, 1977; 1977; 1983) 1983) that were erupted during a relatively short short time time span span (1110-1090 Ma; Davis and Green, 1997). Trace element and isotopic studies indicate that the (1 110-1090 Trace element and isotopic studies indicate the basalts were derived from mantle melts that basalts that were were variably variably contaminated contaminated by continental continental lithospheric mantle Nicholson et lithospheric mantle and and crust crust (Nicholson (Nicholsonand and Shirey, Shirey, 1990; 1990;Shirey Shirey et al., 1994; 1994; Nicholson et al., 1997). The Thecombined combinedpetrologic, petrologic,structural, structural, and and geophysical geophysical evidence are consistent with a "hot-spot" "hot-spot" origin originfor for the the Midcontinent Midcontinentrift rift system system(Hutchison (Hutchisonet et al., al., 1990). 1990). 33 �SOUTHWESTERN ARM OF THE MIDCONTINENT RIFT SYSTEM SOUTHWESTERN THE MIDCONTINENT RIFT SYSTEM Most Most ground-based ground-based studies studies of the the Midcontinent Midcontinent rift have have been been conducted conducted in in the the Lake Lake Superior Superior region region where where Keweenawan Keweenawan volcanic and sedimentary rocks are well-exposed. Along Along much much of of the the southeastern southeastern (Michigan) (Michigan) and and southwestern southwestern (southern (southern Minnesota Minnesota to Kansas) Kansas) extensions extensions of the rift, rift, these these rocks rocks are are largely largely buried beneath thick Phanerozoic deposits. The The objective objectiveof of this this field fieldtrip trip is is to to provide provide an an overview overview of of recent recent studies studiesof of the the Midcontinent Midcontinentrift riftin in east-central east-centralMinnesota Minnesotaand andwest-central west-centralWisconsin. Wisconsin. Despite Despitethe therelatively relativelypoor poorexposure exposurein inthis this region, region, there there are are several several compelling compelling reasons reasons for for studying studying this portion of the rift. Most Most studies studies of of the rift in of the rift formed near the in the the Lake Lake Superior Superior region region have focused on the portion of location Peterman and Sims, Sims, locationof of the the inferred inferred Keweenaw Keweenaw plume plume beneath northern northern Wisconsin (e.g., Peterman 1988). New (e.g., Campbell Campbell and and Giffiths, Giffiths, 1990) 1990) have have New models modelsof ofmantle mantleplume plumeprocesses processes (e.g., contributed contributed significantly significantly to our understanding understanding of large igneous provinces and suggest that rift processes processes may vary with distance distance from the plume center. Unfortunately, Unfortunately, the distal portions of the Midcontinent rift system are buried, making geologic study difficult and costly. The Midcontinent rift system are geologic The rocks rocks of the the Taylors Taylors Falls Falls-- Clam Clam Falls Falls region region are are the southernmost southernmostexposures exposures of the Midcontinent Midcontinentrift rift of system system and and provide provide an an opportunity opportunity to to examine examine variations variations in rift geology geology and and processes processes along along the length length of of the the rift. rift. AAsecond secondreason reasonfor forstudying studyingthis thissegment segment of of the the rift rift relates relates to to regional regional geologic geologic provinces. provinces. The TheMidcontinent Midcontinentrift riftsystem systemcrosses crossesseveral severalmajor majorgeologic geologicprovinces provincesas as itit traverses In northern northern Minnesota Minnesotaand and Ontario Ontario traverses from from Minnesota Minnesotato to Kansas Kansas (Van Schmus, 1992). In the the rift rift is superimposed superimposedon on Late Late Archean Archean granite-greenstone granite-greenstoneterranes terranes (2.6-2.8 (2.6-2.8 Ga) Ga) of of the the Superior Superior Province. Province.South SouthofofLake LakeSuperior, Superior,the therift riftcrosses crossesaa younger younger east-west east-west belt belt of of immature immaturevolcanic volcanic and southward-dipping subduction and sedimentary sedimentaryrocks rocks (Southwick (Southwicket et al., al., 1989) 1989)that formed formed in a southward-dipping subduction zone Ga). Subduction Subduction processes processes during zone during duringthe the Penokean Penokeanorogeny orogeny(—1.85 (-1.85 Ga). during the Penokean Penokean may may have modified the mantle beneath these terranes, and thus might have had an effect on mantle an effect on the the Keweenawan magmas erupted through them. Studies Studiesof of Keweenawan Keweenawan volcanics south of the Malmo Structural Structural Discontinuity Discontinuity (Minnesota) (Minnesota) and and Niagara Niagara Fault Fault Zone Zone (Wisconsin) (Wisconsin) provide provide an an opportunity opportunity to to study study these these effects. effects. PREVIOUS PREVIOUS STUDIES STUDIES Studies of the the Taylors Taylors Falls - Clam Clam Falls region date back to the time before Minnesota became became aa state state in in 1858. 1858.AAthorough thoroughreview reviewof ofthe thehistory historyof ofinvestigations investigationsisis beyond beyondthe thescope scope of this guide, and the interested reader is referred to the review by Craddock (1972). Since this guide, and the interested reader review by Craddock (1972). Since the early 1900'sitithas hasbeen beenrecognized recognizedthat thatthe therocks rocksof ofthe theTaylors Taylors Falls Falls - Pine early 1900's Pine City City region region constitute constitute a thick, fault-bounded fault-bounded volcanic and sedimentary sedimentary sequence that has been broadly folded into a synform Hall, 1901). The synforrn (Strong, 1880; 1880; Berkey, 1897, 1898; Hall, The term term "Chengwatana" "Chengwatana" was was first first assigned by Hall in 1901 1901 to to refer refer to to aa sequence sequence of flows flows and interfiow interflow sediments of "unusual extent" that are exposed along the Snake River east east of of Pine Pine City City (Fig. (Fig. 1). As As stated stated by by Hall Hall (1901) (1901) "The flood floodof of 1898, 1898,which whichtore toreaway away the the dam dam at at the the foot foot of Cross Cross lake lake and and poured poured down down the river gorge gorge a vast volume volume of water, cleaned cleaned out in an admirable admirable manner manner for for examination examinationthe the channel channel of the the stream stream for for several several miles." At At the the time, time, Hall Hall (1901) (1901) was was able able to to describe describe aa nearly nearly continuous continuous sequence sequence consisting consistingof of 65 65lava lava flows flows and and 55 interfiow intefflow sedimentary sedimentaryunits, units, as aswell wellas as the relationships relationships of the bounding Douglas fault. He also recognized the typical three-fold flow He also morphology and described the igneous and metamorphic minerals. minerals. Unfortunately, Unfortunately, the rocks rocks along the Snake Snake River River are not not nearly nearly so so well-exposed well-exposed today. today. Later, Later, the the term term "Chengwatana "Chengwatana volcanics" volcanics" was wasused usedto torefer referto to Keweenawan Keweenawanvolcanic volcanicrocks rocks exposed exposed over over aa much much broader broader region region 34 �_______ ______ _______ ______ 2: Croix Falls 1982) (Modified from Morey Morey eta], eta!, 1982) 0 I I kilometers kilometers I I I 50 50 I Q .:. .:. .:. .;. ........... +;,:>:..:: .... :..:... .......... * * * * Ordovician Ordovician 0 N 0 Lower-Middle Lower-Middle Proterozoic Quartzite Proterozoic Cambrian Metavolcanic Rocks Metavolcanic Rocks U. Animikie Animikie Group Group Slate Metagreywacke Slate & Metagreywacke Bayfield Baytield Group Group Copper Copper Harbor Harbor Iron Iron Formation Formation Conglomerate Conglomerate Group L. Animikie Group Quartzite & wacke Duluth Duluth Complex Complex Gabbro & Anorthosite Gabbro Anorthosite F,,, - 5) Prot. Archean-L. h ot. Granite & Gneiss Mellen Complex, Complex, Basalt & Sediment Sediment Basalt Basalt (NSVG and and Chengwatana ChengwatanaVolcanics) Volcanics) 1. Generalized Generalizedbedrock bedrockgeology geologyof ofthe thesouthern southernLake Lake Superior Superiorregion region showing showing Figure 1. location of of the the field fieldtrip tripin inthe theTaylors Taylors Falls Falls -- Clam Clam Falls region. (Modified (Modified from from the location Morey et al., 1982). al., 1982). 35 35 �between the Douglas fault (west) and Lake Owen fault (east) in east-central Minnesota and between west-central Wisconsin (Morey west-central (Morey and Mudrey, 1972; 1972; Hassan and Mudrey, 1980). 1980). The distribution distribution and stratigraphy stratigraphy of the Chengwatana volcanics in the Taylors Falls - St. St. Cordua (1989a and b; b; unpublished unpublished mapping). mapping). He Croix Falls region was established by Cordua He divided divided the nearly 3,000 meter thick thick volcanic volcanic section section into into seven seven flow units based on on field field and and textural textural relationships (Fig. 2). The by Themapping matmin~ bvCordua Corduaalso alsodelineated delineated several several east-west east-west faults faults in in the the region. Nearly 3,000 meters of volcanic and sedimentary rock are also dso exposed in the Clam Nearly 3,O region, however corresponding mapping and stratigraphic been ~ h i cstudies have not been Falls Frederic regi Falls -- Frederic conducted conducted there there yet. ye1 meters above base Figure 2. Lithostratigraphic column of Chengwatana volcanics in the Taylors Falls region (Modified from Wirth et al., 1997). Top and bottom 2500 of the section are unexposed. Geochemical sample locations are shown with tick marks along the right side of the column. Flow unit abbreviations are from Cordua 2000 (1989): K!, Lotus lake; Kd, Dresser; Kt, Traprock Alley; Kep, Eagle Peak; Ks, Skitrail; Kp, Pipeline; Ky, unnamed. 1500 1000 Unexposed Interfiow Sediment Interflow Sediment Intersertal to Ophitic Ophitic Plag. phyric, intersertal 500 ....... ....... ....... Intersertal Intersertd ... Ophitic Ophitic [ Sub-ophitic Sub-ophitic 0 36 GEOPHYSICAL STUDIES Gravity, magnetic, ~agnetic,and and seismic studies lies were essential to developing developing detailed models of the buried the buried zontinent parts of the Midcontinent rift system in Minnesota mesota and and Wisconsin (see:review by Craddock, 1972). 172). The regional magnetic tic character of the Keweenawan iwan Rift E f t in eastern Minnesota lesota and and western Wisconsin sin is given given in Chandler et In Lnanuler eL a!. al. (1989) and Allen et al. al. (1997). Allen et (1997). The The rocks of the Taylors Falls Clam Clam Falls Falls region are part of the St. Croix horst, which is is bounded by by the Douglas bounded Douglas (northwest) and Hastings Hastings (northwest) (southeast) faults (Fig. 3). (southeast) 3). On On the the basis basis of of threethreedimensional dimensional gravity gravity models, models, the thickness ooff Chengwatana volcanics volcanics in Chengwatana this region ranges from 4 kilometers at Clam Falls to to 14 kilometers north of 14 kilometers north Frederic, and vertical uplift of the horst ranges from 3 kilometers in the northwest northwest to 8 kilometers in �Emerald Basin + + 0 20 kilometers strike/dip of flows and bedding I trend of aeromagnetic anomalies inferred structuraJ/strañgraphic I discontinuity 450 15' + + 920 15' 92° 00' Falls - Clam Falls region Figure 3. Sketch Sketchmap mapof ofaeromagnetic aeromagneticanomalies anomalies in the Taylors Falls interpreted from interpreted from new new aeromagnetic aeromagneticmaps mapsof of western western Wisconsin Wisconsin (Cannon (Cannonet et al., al., 1997; 1997;in in prep). prep). the southeast southeast(Allen (Allenetetal., al.,1997). 1997).Thick Thick(—5 (-5 km), wedge-shaped wedge-shaped accumulations accumulationsof of Keweenawan Keweenawan sediment (Oronto (Oronto and Bayfield Bayfield Groups) Groups) occur occur in in basins basins on the margins of St. Croix sediment Croix Horst Horst (Allen et a!., al., 1997). 1997). The magnetic character of Polk County is provided by Reich (1989), (19891, who conducted aa primarily N-S ground magnetic magnetic survey survey at 0.1 mile intervals, transecting the county along seven primarily Time-corrected raw data and 3-point smoothed data for line one are and E-W lines (Fig. 4). Time-corrected presented in Fig. 5. This This line covers some of the outcrops seen on this this field trip, trip, and shows the strong magnetic signature of the Chengwatana flows that is about 1000 nanotesla (nT) above the regional value. The Theminimum minimumin in the the magnetic magnetic line line at approximately approximately 15 15 kilometers kilometers from from the the 37 �___________ in the the bedrock bedrock caused caused west end of the line is interpreted interpreted by Reich (1989) as the result of offset in by the Cottage Grove fault. The Thehigh-frequency high-frequency nature nature of of the the magnetic magnetic signal signal on on the the west west end end of the flows. flows. These of the line is common to all the lines that cross outcrops of These high-frequency high-frequency by lightening. lightening. Lightening magnetic "spikes" may in part represent locations hit by Lightening strikes strikes on highly magnetic rock can produce produce large large localized localized anomalies. Many Many outcrops outcropsin in this thisregion regionare aremagnetically magnetically significantly deflect a compass! compass! Reich variations in the strong enough to significantly Reich (1989) interpreted interpreted the variations half-graben faults faults that thatparallel parallelthe theCottage CottageGrove Grovefault. fault. Recent Recent filtered data as resulting from half-graben work by Cannon et al. (1997; in prep.) suggest that these features are the result of folding work Cannon et (1997; suggest these features folding within within in the the magnetic susceptibility of of the the different different flows flows may may also be aa the flows. flows. Variations Variations in magnetic susceptibility contributing factor. Recent Recentstudies studiesby by Kean Kean (1992, (1992, 1993, 1993,1995) 1995)and and Kean Kean et et al. al. (1997) (1997) on on the the paleomagnetic paleomagnetic character characterof of Chengwatana Chengwatanaflows flows in in the the Taylors Taylors Falls region are described described in in the the 2. section for Stop Stop 2. et al., al., 1997; in prep.) provide New aeromagnetic data from western Wisconsin (Cannon et additional constraints on the stratigraphy stratigraphy and structure of the Chengwatana Chengwatana volcanics in the the Falls region. Previously, the relationship between the stratigraphic sections Taylors Falls Falls -- Clam Clam Previously, relationshipbetween the stratigraphic sections Taylors Taylors Falls Falls and and Clam Clam Falls Falls regions regions was wasnot notwell wellunderstood. understood. Preliminary interpretation interpretation in the Taylors suggest that the stratigraphy stratigraphy continues trending of the data suggest continuesnorth north of of the the Taylors Falls region before trending to the east east around around aa northeastnortheastsouthwest southwest antiformal antiformalstructure structure(Fig. (Fig. 3). No Nomajor majorstructural structuralbreaks breaks are are STUDY AREA POW COUNTT apparent. apparent. If this this interpretation interpretation is correct, then then the correct, the flows flows in the the Frederic - Clam Falls region are Frederic are stratigraphically below those of the stratigraphically the Falls section. section. Also apparent apparent Taylors Falls aeromagnetic map on the aeromagnetic map are are several several stratigraphic structural or stratigraphic discontinuities surrounding surrounding the the Ashland syncline, syncline, and northwest of Frederic. These relationships, relationships, combined with the results of recent geochemical geochemical and geochronologic geochronologic studies, suggest that that there may be significant differences between the flow flow sequences sequences and that further further stratigraphic subdivision of the stratigraphic subdivision the ICALI I US.SC CMLII • Chengwatana volcanics volcanics may be • C S ISKI.OMITICS warranted. warranted. U U SA8R STATION Location map map for Figure 4. 4. Location magnetic profiles of Polk magnetic profiles Polk Co., Co., 1989). Wisconsin (after (after Reich, Reich, 1989). 38 �WI., LVIE I A. A GAMMAS I .J'jii*w DATA 61500.0 •:". U.TIRCD DATA 60500.0 MM.I$ Plot of raw and filtered data for line 1 (A—A'I. - Figure Figure 5. 5. -- Magnetic Magnetic profile profile of Line Line 11 in in Figure Figure 44 (after (after Reich, Reich, 1989). 1989). METAMORPHISM IVETAMORPHISMAND AND MINERALIZATION MINEXALIZATION The The secondary s e c o n d q minerals minerals in in the the Keweenawan Keweenawan volcanic rocks form by aa combination combination of of burial metamorphism, activity and and contact contact metamorphism. metamorphism. They metamorphism, hydrothermal hydrothermal activity They form form 20-80 20-80 percent percent of of aa typical typical sample. sample. They Theyreplace replacephenocrysts phenocrystsand andgroundmass, groundmass,fill fillamygdules, amygdules,form form veins and line line joint and and fault fault planes. planes. The Theminerals mineralsreplacing replacing ground ground mass mass and and phenocrysts phenocrysts typically albite epidote+actino1ite~hlorite+qu~z+ albite+1+I- calcite calcite +1+Itypically consist consist of ofthe theassemblage assemblageepidote+actinolite+chlorite+quartz+ muscovite +/-titanite +1Fe-Ti muscovite +/-titanite +I- Fe-Ti oxides. oxides. The Thecommon commonminerals mineralsfilling filling amygdules amygdules are are quartz, quartz, chlorite, crystalline material chlorite,epidote, epidote,K-feldspar K-feldsparand andcalcite. calcite.The Thequartz quartz may may be present as coarse crystalline material or, or, more rarely, as agate. Joint Jointsurfaces surfacesand and fault fault planes planes may have veins or coatings of any of the and chrysotile. chrysotile. Minor the above above mentioned mentioned minerals, minerals, but may also show tremolite, tremolite, and Minor copper copper mineralization mineralizationin inthe theform formof of native native copper, copper, chalcopyrite, chalcopyrite, bornite, bornite, chalcocite chalcocite and and malachite malachite and and azurite occurs in amygdules, flow-top breccias or later gash veins (Grant, 1901; Dutton, 1972; azurite occurs in amygdules, flow-top breccias or later gash veins (Grant, 1901; Dutton, 1972; White, White, 1978; 1978;Cordua CorduaetetaL, al., 1979). 1979). Epidote Epidote occurs occurs as as both both granular granularand and crystalline crystalline grains. grains. Granular Granularepidote epidoteisiscommonly commonly observed observed in in the the groundmass groundmassor orasasalteration alterationofofthe theplagioclase, plagioclase,whereas whereascrystalline crystallineepidote epidote occurs occurswithin withinthe theamygdules. amygdules.Actinolite Actinoliteisispresent presentmainly mainlyininthe thegroundmass, groundmass,but butalso alsoreplaces replaces clinopyroxene clinopyroxeneand andoccurs occursin inamygdules. amygdules.Chlorite Chloriteisisubiquitous ubiquitousininall allof of the the flows flows and and is is present present in intwo two forms: forms: ripidolite ripidolite(brown (brownbirefringence) birefringence)and andpycnochlorite pycnochlorite(anomalous (anomalousblue bluebirefringence). birefringence). Although both types are present throughout the sequence of Chengwatana flows Although both types are present throughout the sequence of Chengwatana flowsthe theFe-poor Fe-poor variety variety (pycnochlorite) (pycnochlorite) is is most most abundant abundant in in the theuppermost uppermostflows, flows, and andthe thehigh-iron high-ironvariety variety (ripidolite) (ripidolite)isis more morecommon commonin inamygdules amygdules(Wirth (Wirthet. et. al., al., 1996). 1996). The The pressure pressure and andtemperature temperature conditions conditions during during metamorphism metamorphism in in these these rocks rocksare are constrained the actinolite. actinolite. The constrainedby bymineral mineralreactions reactions(Liou (Liouet etal., al., 1987) 1987)and and by the Na content of the The minimum minimum temperature temperatureof ofmetamorphism metamorphismof of the theChengwatana Chengwatanavolcanics volcanics isis constrained constrainedby bythe the 39 39 �presence of actinolite and epidote (Fig. 6) and presence and by by the the absence absence of of coexisting coexisting prehnite prehnite and and pumpellyite. The upper temperature temperature boundary of metamorphism is constrained by the lack of with epidote, chlorite, actinolite actinolite and and quartz. quartz. The hornblende coexisting with The maximum maximum pressure pressure conditions of metamorphism metamorphism (4-6 kbar) are constrained constrainedby bythe theabsence absenceofofglaucophane. glaucophane. Further Further conditions constraints on the pressure constraints pressure conditions conditions during during metamorphism metamorphism can can be estimated estimated from from the Na and and Al contents of the calcic amphiboles in the presence of of quartz + + albite + + chlorite + + magnetite A1 (Brown, 1977). Analyses (SEM-EDS) of actinolite indicate that it contains Na (M-4 site) 1977). Analyses (SEM-EDS) of actinolite indicate site) and tetrahedral A1 Al (AlIV). (Ala'). The found in in the tetrahedral Theresults resultsare are similar similar to to the the composition composition of amphiboles found Batholith and suggest suggest low low pressure pressure burial burialmetamorphism metamorphism contact aureoles aureoles of the Sierra Nevada Batholith Group. This of the Chengwatana Volcanic Volcanic Group. This suggests suggests that the the presently presently exposed exposed Chengwatana Chengwatana sediment. This flows were overlain by an additional 6-7 kilometers of volcanic flows or sediment. This estimate estimate is greater than the estimated burial depths made by Cordua (1980; 1989b) and Cavaleri (1987) by analogy Group in the Keweenaw Peninsula, Peninsula, but are consistent consistent analogy with with the the Portage PortageLake LakeVolcanic Group with the amount of uplift (<8 km) (<8 km) estimated from geophysical geophysical models models al., 1997). Assuming Assuming a peak (Allen et al., metamorphic temperature of approximately350' 350° C and a burial approximately km., the depth of approximately 7.5 km., metamorphic data from the Chengwatana flows suggest Chengwatana flows suggest aa geothermal gradient of approximately geothermal approximately 0.6 45-50° 45-50' CCwithin within this this portion portion of the midcontinental midcontinental rift. rift. Similar results were were obtained obtained by Livnat (1983) (1983) using using oxygen isotopes in the Portage Lake volcanic sequence sequence on on the the Keweenaw Keweenaw Peninsula. Peninsula. The metamorphic minerals in Polk County are are distinctly distinctly different different from those those seen seen further from further north in content of aci Wisconsin. Wisconsin. In the volcanic volcanic rocks rocks pr-s exposed in northern Douglas County cz-i-tr+H20 (south shore of Lake (south Lake Superior), Superior), \X, prehnite, laumontite and other prehnite, laumontite and other X #' \X zeolites are common common secondary secondary 9 .1. minerals (Grant, 1901, 1972, minerals (Grant, 1901, Dutton, 1972, 0 400 500 300 200 300 100 Cordua, 1990,1991). 1990,1991). In White, 1978; Cordua, southern Douglas County, southern Douglas County, in the Temperature Temperature (°C) (OC) region between Dairyland to Solon region Solon Figure 6. P-T P-Tdiagram diagramfor forunivariant univariant reactions reactionsrelevant relevant Springs, prehnite is found, but the Springs, prehnite the to low-grade low-grade metamorphism metamorphism of of mafic mafic volcanic volcanic rocks rocks zeolites zeolites are rare to absent absent (Grant, (Grant, (modified from from Wirth Wirth et al., (modified al., 1997). 1997). Estimated 1901). 1901). To To the the east, east, in in the the Minong Minong conditions conditionsof of metamorphism metamorphismof of Chengwatana Chengwatanavolcanics volcanics Copper Range, which stretches from are shown in shaded regions. Minong Minong through through Grandview Grandview in in 40 �Washburn, Washburn, Bayfield and Douglas Douglas Counties, Counties, prehnite is is common common and and pumpellyite pumpellyite has hasbeen been reported. Mi, 1982). Zeolites reported. (Grant, (Grant, 1901; 1901;Smith, Smith, 1947; 1947; Dutton, 1972; White, 1978; AH, Zeolites such such as as natrolite natroliteoccur occuralong alonglater laterjoint joint surfaces surfacesnear near Chittamo Chittamo and and heulandite heulandite and and natrolite natroliteare are reported reported occurring occurring in in amygdules amygdules with with quartz, quartz, epidote epidote and and chlorite chlorite near nearGrandview Grandview (LaPoint, (Lapoint, 1976). 1976). South South of of Grandview, Grandview, southeast southeast of of the the Lake Lake Owen Owenfault, fault,epidote, epidote,chlorite, chlorite,actinolite, actinolite,quartz, quartz, biotite zeolites (LePoint, biotiteand and ferrostilpnomelane ferrostilpnomelaneoccur, occur,with with no no prehnite, pumpellyite or zeolites (LePoint, 1976). 1976). A hornblende-epidote-biotiteassemblage assemblageisislikely likelydue duetotothe thepresence presenceof ofnearby nearbyMellen MellenIntrusive Intrusive A hornblende-epidote-biotite Complex plutons (Cannon et al., 1996). Toward the Michigan border, at Saxon Falls Complex al., 1996). Toward the Michigan border, at Saxon Fallsnear near Hurley, Hurley, prehnite prehnite and and zeolites, zeolites, such such as as thomsonite thomsonite and and laumontite, laumontite,are areagain againreported reported(Irving, (Irving, 1883; 1883;Cannon, Cannon,1996). 1996). The The higher higher metamorphic metamorphic grade grade of of the the Chengwatana Chengwatana basalts could could reflect reflect either either deeper deeper burial burialin inthe thevolcanic volcanicpile pilethan thanrocks rocksto tothe thenorth, north,or or the the presence presence of of aa higher higher geothermal geothermalgradient gradient in in that that part part of of the the Keweenawan Keweenawan terrain. Allen Allen et etal. al. (1997) (1997)estimate estimatethat that the the St. St. Croix CroixHorst Horst has has undergone undergone 3-8 3-8kilometers kilometers of of uplift uplift in in the the Taylors Taylors Falls-Clam Falls region. This This contrasts contrastswith with estimated estimateduplift upliftofof2-4 2-4kilometers kilometersininthe theAshland Ashlandsyncline synclineand and1-2 1-2kilometers kilometersininthe theTwin TwinCities Cities Basin Basin(SE (SEMinnesota). Minnesota). GEOCHEMISTRY GEOCHEMISTRY Petrographic Petrographicand andgeochemical geochemicalstudies studiesof ofthe theChengwatana Chengwatanavolcanics volcanicsin inthe theTaylors TaylorsFalls Falls -- Clam Clam Falls Falls region region have have been been conducted conducted by by Cordua Cordua(1980; (1980;1989b), 1989b),Cavaleri Cavaleri(1987), (1987),Leslie Leslieetet al. (1994), (1994),Wirth Wirthet et al. al. (1995; (1995;1996; 1996;1997), 1997),Naiman Naiman et et al. al. (1996; (1996;1997; 1997;in in press), press), and and Abbott Abbott et et al. al. al.(in (in press). press). These Thesestudies studiesestablished establishedthe thetholeiitic, tholeiitic,high-alumina high-aluminaand andhigh-iron high-iron character characterof of the the basalts basaltsin inthe theregion region(Table (Table1;1;Fig. Fig. 7); 7);intermediate intermediate and and felsic felsicrocks, rocks, with with the the exception exceptionof of rare rarerhyolite rhyolitenear nearClam ClamFalls, Falls, are are not observed. observed. Most Mostflows flows are are weakly weakly olivine-normative olivine-normative (0-15 (0-15 percent percent norm norm ol) 01) and and the theremaining remaining flows flows have have trace trace amounts amounts of quartz quartz in in their their norms norms (0-5 (0-5 percent percentnorm normqz). qz). Nickel Nickel abundances abundances (36 (36 -185 185ppm) ppm)and andMg#'s MgWs(Mg# (Mg#==Mg! Mg/ [Mg+Fe2] [Mg+Fe+2]== 0.37 0.37 -- 0.58) 0.58) are are Fe2 F ~ ++ Fe3 + Ti generally generallylow, low,suggesting suggestingthat thatthe the Chengwatana Chengwatana Volcanic Volcanic Group Group flows flowshave haveundergone undergonesignificant significant fractionation. fractionation. Chengwatana Chengwatana basalts can be subdivided basalts can be subdividedon onthe the basis basis of of TiO2 T i 0 2 (less (less than than and and greater greater than than 2.3 2.3 weight weight %), %), aa distinction distinction that that has has been been recognized recognized elsewhere elsewhere in in the the Midcontinent Midcontinentrift. rift.InIngeneral, general,the the high-Ti02 high-Ti02 basalts basalts are aremore more abundant in the upper part of abundant in the upper part ofthe the St. Croix Croix Falls Fallssection sectionand andare are St. characterized characterizedby byhaving havinggenerally generally low low Mg# Mg# (<0.5) (<0.5) and and high high A Al A1 Figure Figure7.7.Plot PlotofofChengwatana Chengwatanavolcanics volcanicsfrom fromthe theTaylors Taylors Falls region on a Jensen cation plot (Jensen, 1976) (after Falls region on a Jensen cation plot (Jensen, 1976) (after Wirth et al., 1997). Wirthetal., 1997). 41 �Table 1. Representative Representative analyses analyses of basalt and sediment from the Taylors Taylors Falls and Clam Clam Falls Falls region. region. Si02 Si02 Ti02 A1203 *l2O3 Fe203 Fe203 MnO MnO MgO MgO CaO CaO Na20 Na20 K20 K20 P2° LOT LO1 basalt KC-lb basalt KC-7 basalt KC-70 KC-70 47.60 48.30 1.69 15.57 13.34 1.71 48.13 2.64 14.07 16.40 0.21 5.17 15.65 13.61 0.19 6.69 9.56 0.19 7.48 8.14 2.38 0.16 3.09 64.75 0.93 59.64 0.84 11.11 13.55 8.75 1.63 8.41 2.45 1.43 0.31 1.25 7.37 0.08 0.97 11.92 0.21 0.73 0.10 2.02 2.60 0.55 0.19 1.13 sediment sediment sediment KC-331f KC-330g KC-330g KC-33 1f 0.10 2.66 11.55 1.28 0.61 0.13 1.59 Total Total 100.77 100.68 100.47 100.19 100.70 Mg# Mg# 0.57 0.43 0.42 0.23 0.48 39.54 0.57 — qz qz — — — — — — hy hy ol 01 17.73 17.01 20.38 9.24 6.41 2.66 ne Rb Ba Th Nb La Sm Sm Yb Zr Y 42 206 Cr Cr Ni — 9 26.68 — 16 15 128 — 206 — 13 9 11 — — — — — — — — — 108 23 121 209 42 122 23 142 165 185 125 124 128 79 67 105 248 254 169 161 1.03 9.58 3.50 2.15 Initial ENd Initial CNd+0.101 Locations: Sample Sample Locations: -KC-lb KC-1b KC-7 KC-7 KC-70 KC-70 KC-330g KC-310 48 546 14 11 V v 277 — -- -- -- 6.94 26 82 318 -1.7— -1.7 - — 19 -1.86 -1.86 NW1/4, section 5, T T 33 33 N, N, R R 18 18W NE114, NW114, W NE1/4, 1/4, NE NE 114, 1/4, section 6, T T 33 33 N, N, R R 18 18W SE W (Stop (Stop 1) 1) SE 114, NW 114, 1/4, section 14, T T 33 33 N, N, R R 19 19W NW 1/4, 114, NW W T 33 N, N, R R 18 18W NEll4, NEl14, section 6, T W NE1/4, NE1/4, T 37 37 N, N, R R 17 17W NE114, SW114 of section 11, T W (Stop 5) NE1/4, SW1/4 42 — 50 �the Taylors Taylors Falls basalts are are subsubincompatible trace-element trace-element abundances. abundances. REE patterns of the and variably variably enriched enriched in in the the light light REE, REE, with with abundance abundance levels levels 40-100 40- 100times times chondritic chondritic parallel and values for La, and from Most samples samples have have prominent prominentnegative negative from 12-22 12-22 times chondrites chondrites for Yb. Most Eu anomalies, feature that is consistent with the widespread petrographic evidence of plagioclase anomalies, aa feature petrographic evidence plagioclase Normalized trace trace element element abundances abundances (Fig. 8) of the flows are enriched in the fractionation. Normalized highly incompatible elements (e.g., Th, Nb, La) relative to the the less less incompatible incompatible trace elements incompatible elements (e.g., Th, I.) a -4 10 ID I I I I I Nd La Ce 5o I I I I I I I I I YbLu Tb Sm Eu I b : 10 I I I I I I I I I I I I I I I I I Th Nb Ta La Ce Sr P Nd Sm Zr Hf Eu Ti Th Y Yb Lu Figure 8. (a) (a)Chondrite-normalized Chondrite-normalizedplot plotof of REE REE abundances abundancesin in representative Figure 8. volcanics from from the Taylors Taylors Falls Falls region region (after Wirth Wirth et al., Chengwatana volcanics trace-element abundances normalized to primitive mantle. (b) Plot Plot of trace-element abundances normalized 1997). (b) (e.g., Zr, Y, Y,Yb); Yb);Ta, Ta,Nb, Nb,and andSr Srare arerelatively relativelydepleted. depleted. These These relationships relationships suggest that at least some of the major major and and trace trace element element variations variations exhibited by the Chengwatana Chengwatana volcanics volcanics are are aa fractional crystallization contamination. Basaltic result of fractional crystallizationand variable contamination. Basalticflows flows of the the Chengwatana Chengwatana volcanics exhibit a relatively narrow narrow range range of of Nd Nd isotopic isotopic compositions compositions (Fig. (Fig. 9). 9). Although Although flows range range from from+3.4 3.4 to initial ENd ENd values of the flows to 4.5, -4.5,the themajority majority of of samples samples plot within a initial more limited range (-1.5 and -3.0). -3.0). The The combined combined geochemical geochemical data suggest that the basalts originated contaminated by by continental continental lithospheric lithospheric originated from from plume-derived plume-derived melts that were variably contaminated mantle EM values mantle or or crust crust (Wirth (Wirth et et al., 1997). 1997).Rhyolites Rhyolitesfrom fromthe theClam ClamFalls Fallsregion regionhas hasinitial initialENd ENd of ++1.0 Thesevalues valuesare areclose closeto tothe thecomposition compositionof ofthe theinferred inferredplume plume(initial (initialENd 1.0 and -0.1. These value = = 0) 0) implying that the rhyolites formed by crystal fractionation fractionation (Abbott (Abbott et al., al., in in press). press). ' 43 43 �12 Clam Falls Rhyolite • Clam Falls Basalt 10 Falls Basalt 8 6 L 4 2 0 -12 -10 -8 -4 -6 0 6 Initial Nd £\ Initial Figure Figure 9. 9. Histogram Histogramofofinitial initialENd EM values values (1100 (1100Ma) Ma) of Chengwatana Chengwatana volcanics volcanics from from the the Taylors TaylorsFalls Falls and and Clam ClamFalls Falls regions regions(after (afterNaiman Naiman etetal., al.,in inpress). press). ITINERARY ITINERARY This in Minneapolis, Minneapolis, Minnesota. Minnesota. A This guide guide describes describes aa one-day one-day trip beginning and ending in regional regionalmap mapof ofthe theroute routeisisgiven givenin inFigure Figure10, 10,and andmore moredetailed detailedmaps mapsof ofthe thestops stopsare aregiven giveninin Figures Figures11 11and and12. 12.AAbrief briefdescription descriptionofofthe theroute routeisisgiven givenbelow: below: Proceed from from Minneapolis Minneapolis about 45 miles to Osceola Wisconsin. One One route route to to Osceola Osceola Proceed would wouldbe beas asfollows: follows:Take Take 1-35 1-35 north to the exit for Minnesota 97 east bound. Follow FollowRoute Route97 97 for 11 11miles miles to to its its intersection intersection with with Minnesota 95. Turn Turn left left (north) (north) on on Minnesota Minnesota 95 for 66 for miles. Turn Turnright righton onroute route243, 243,cross crossthe theSt. St.Croix CroixRiver Riverand andfollow followititto toits itsintersection intersectionwith with miles. Wisconsin Route 35. Wisconsin Route 35. Go Go north north on on Route Route35. 35. InInabout about4.5 4.5miles miles(about (about11mile milenorth northof ofDresser DresserWisconsin) Wisconsin)turn turnright right on Ravine Drive. Follow Ravine Drive a short distance to the entrance of the Dresser Trap Rock on Ravine Drive. Follow Ravine Drive a short distance to the entrance of the Dresser Trap Rock Company CompanyQuarry Quarry(Stop (Stop1;1;Fig. Fig.13). 13). Return Return to to Route Route 35. 35. Turn Turnleft left(south). (south). Go Go1 1mile miletotoState StateStreet. Street.Turn Turnright right(west) (west)on onState State Street. milestotothe theintersection intersection Street. This Thissoon soonbecomes becomes100th 100thAvenue. Avenue. Follow Followthis thisroad roadabout about33miles with with County County S. S.Turn Turnnorth northand andgo goabout about1/2 112mile miletotothe thepipeline pipelinecrossing crossingofofCount CountSS(Stop (Stop2;2; Fig. Fig. 15). 15). Return Return to to Route Route 35. 35. Turn Turnleft left(north). (north).Go Gothree threemiles milestotothe thecity citylimits limitsof ofSt. St.Croix CroixFalls. Falls. Turn Turn left leftinto intoInterstate InterstatePark, Park,Wisconsin Wisconsin side side (fee (fee area). area). Follow Followthe thepark parkroad road to toEagle EaglePeak Peakand andthe the group groupcamp camparea area(Stops (Stops3a 3aand and3b; 3b;Fig. Fig.17). 17). 44 �Figure Figure10. 10.Scanned Scannedimage image(not (notshown showntotoscale) scale)ofofUSGS USGSStiliwater, Stillwater,Minnesota Minnesota- -Wisconsin Wisconsin topographic topographicquadrangle quadrangle(1 (1xx22degree degreeseries; series;1:250,000 1:250,000scale) scale)showing showingthe thelocations locationsofofthe the field fieldtrip tripstops. stops. Return ReturntotoRoute Route35. 35.Turn Turnleft. left.Follow Followthe thesigns signstotomerge mergeonto ontoU.S. U.S.Highway Highway8 8- -WEST bound. WEST bound. Cross Crossthe theSt. St.Croix CroixRiver Riverinto intoMinnesota. Minnesota.Immediately Immediatelyafter afterthe thebridge bridgeturn turnleft leftatatthe thetraffic traffic light lightinto intoInterstate InterstatePark Park- -Minnesota Minnesotaside side(a(adifferent differentfee feearea) area)(Stop (Stop4;4;Fig. Fig.17). 17). Return Turnright right(east). (east).Recross Recrossthe theSt. St.Croix CroixRiver. River.Follow FollowU.S. U.S.88for for55miles miles ReturntotoHighway Highway8.8.Turn totothe theexit exitfor forRoute Route35 35north. north.Follow FollowRoute Route35 35north northabout about25 25miles. miles.About About2.5 2.5miles milesnorth northof of Frederic, Frederic,turn turnleft leftonto onto130th 130thStreet. Street.InInabout about0.5 0.5miles milesturn turnleft lefton on345th 345thStreet. Street.InIn0.7 0.7miles, miles, look anoutcrop outcropininthe thefield fieldtotothe thenorth north(Stop (Stop5;5;Fig. Fig.19). 19). lookfor foran Return for Returneast easton on345th 345thStreet StreettotoLewis. Lewis.Cross CrossRoute Route3535and andcontinue continueeast eastononCounty CountyRoad RoadEEfor 55miles; turn south on 65th Street to the town of Clam Falls. Turn west at the junction with miles; turn south on 65th Street to the town of Clam Falls. Turn west at the junction with County CountyRoad RoadI.I.Stop Stop55isisNNE NNEofofthe thejunctioin junctioinwith with73rd 73rdStreet. Street.(Stop (Stop6;6;Fig. Fig.20). 20). 45 �Figure 11. 11. Scanned Scannedimage imageof of aaportion portion of of the the USGS USGS Stiliwater, Stillwater, MinnesotaMinnesotaWisconsin topographic quadrangle (30 x 60-minute series; 1:100,000 1:100,000 scale) scale) image (not (not shown shown to to scale) Figure 12. 12. Scanned Scanned image scale) of a portion portion of of the the USGS USGS topographic quadrangle (30 x 60-minute series; Grantsburg, Wisconsin Minnesota Grantsburg, topographic quadrangle (30 60-minute series; . 1: 100,000 scale) scale) showing showing the locations locations of the the field field trip trip stops stops 5-7. 5-7. 1:100,000 46 �AlternateStop Stop66isisreached reachedfrom fromstop stop66by by continuing continuingsouth southon on73rd 73rdto tothe thejunction junction with with3315th Alternate 15th Ave. Continue Mckenzie Trail south and around Continuewest weston on315th 3 15thAve Ave to Mckenzie Trail. Follow Mckenzie the bend to to the the east. east. The Theroad roadveers veerssouth southaround around an an outcrop outcropon on the the north north side side of the the road road (Stop (Stop the 6b; 6b; Fig. Fig. 23). 23). 3 15th Ave and continue west to 90th Street along the east side side of of Sommers Sommers Lake. Lake. Return to 315th Turn north north and and continue continueon on 90th 90th Street Street to to the junction with 320th Ave. Continuewest west on on 320th 320th Turn Ave. Continue 15thAve. and on County Road I until the junction junction with with 3315th Ave. Continue Continue west west on on 315thAve. 315th Ave. Outcrops Outcrops on on north north side sideof of road road near neartop topof ofhill hill(Stop (Stop7;7;Fig. Fig.24). 24). Return 15th Ave., Return to to Frederic Fredericand andRoute Route35 35by bytravelling travellingwest weston on3315th Ave., south south on on120th 120thAve., Ave., and and west west on on 310th 3 10thAve. Ave. A Asimple simpleway way to to return return to to Minneapolis Minneapolis isis to to follow follow Route Route 35 35 south southto to U.S. U.S. Highway Highway 8. 8. Follow FollowU.S. U.S.88east easttotoits itsintersection intersectionwith with1-35. 1-35. 1-35 1-35southbound southboundleads leads direcily directly to to the the Twin Twin Cities Cities of of Minneapolis Minneapolisand andSt. St.Paul. Paul. Stop Trap Rock Quarries. Stop 1—Chengwatana 1-Chengwatana Lava Lava Flows Flows in the Dresser Trap Quarries. Location: NW 1/4 114Sec. Sec.55 Location: Approximately Approximatelyone onemile mileNNE NNE of of Dresser, Dresser, Wisconsin. North North Quarry Quarry -- NW andNEl/4Sec.6,T33N,R18W. and NE 114 Sec. 6, T 33 N, R 18W.EastQuarriesEast Quarries - SW1/4Sec.5,T33N,R18W. SW 114 Sec. 5, T 33 N, R 18 W. The The lava lava flows flows at at Dresser Dresserhave havebeen been mined mined for "trap rock" since "trap rock" since 1855. The Thelarge largequarry quarry complex, complex, owned owned by by Dresser Dresser Trap Trap Rock Rock Inc., Inc., currently produces currently produces 750,000 750,000 tons tons per per year year in in crushed crushedstone stoneof ofall allsizes. sizes. The rock rock isis extremely extremely tough an4 durable, duein in tough and durable,due large large part part to to the the interlocking of of microscopic microscopic unoriented unoriented crystals crystals of of chlorite chlorite and and actinolite. actinolite.All Allvisitors visitorsare are urged urg6d to to seek seekpermission permission to to enter, enter, and and to to use use Figure WIFigure 13. 13.Scanned Scannedimage imageof. of portions portions of of the the USGS USGS Osceola, Osceola,WIfundamental rules of fundamental rules of MN MN and and St. St.Croix CroixDalles, Dalles,WI-MN WI-MN topographic topographicquadrangles quadrangles(7.5(7.5caution. caution. The Therock rockwalls walls minute minuteseries; series;1:24,000 1:24,000scale) scale)showing showing the the location locationof of Stop Stop1.1. are are shear, shear, blasting blasting isis frequent, frequent,rock rockpiles pilesare areunstable, unstable,and andquarry quarrytruck truckdrivers driversexpect expectyou you to to yield yield right-of-way. right-of-way. We We will will visit visitseveral severalspots spotsininthe thequarry, quarry,the theexact exact localities localitiesdependent dependenton onthe theareas areasaccessible accessibleatatthe the time of the field trip. time of the field trip. Both Both quarries quarries expose expose basaltic basaltic lava lava flows flows that that have have been been recrystallized recrystallized incipiently incipiently to to completely completelyto togreenschist greenschistfacies ficiesminerals mineralsby bystatic staticburial burialmetamorphism, metamorphism,due duetotoburial burialwithin within the thevolcanic volcanicpile. pile. 47 �Geology of of the the north north quarry quarry The north north quarry quarry is is aa large largequarry quarry cut cut into into aa single singlelava lava flow flow over over 35 35 meters meters thick, thick, which which The is informally informally named the Trap Rock Alley flow. The flow is a generally subophitic subophitic to diabasic diabasic porphyritic basalt. The Themajority majority of of the the phenocrysts are plagioclase. Augite Augite shows shows up as lighter porphyritic patches on on weathered weathered surfaces, surfaces, giving giving the rock aa distinctly distinctly mottled appearance appearance (Fig. (Fig. 14 14aa gray patches Otherprimary primary igneous igneous minerals minerals are are plagioclase and ilmenite. In In places places these minerals minerals and b). Other are relatively fresh, but usually they are incipiently to completely replaced replaced by by a mixture of chlorite, titanite. The groundmass usually consists entirely chlorite, epidote, epidote, actinolite, actinolite, albite, albite, zoisite and titanite. of this mix of secondary secondary minerals minerals .Locally, bodies of coarse gabbro are present as sill-like sill-like masses. masses. Particularly Particularly common common on on the lowest lowest level level of of the the quarry, quarry, they they average average 0.5-1.0 0.5-1.0 meters meters in in thickness, thickness, and and consist consistof ofcoarse coarse augite, plagioclase and magnetite. Elongated to 44cm cmlong. long. Secondary augite, plagioclase Elongated augite augitecrystals crystals may be up to minerals, commonly developed minerals, such such as as those those found found in the surrounding surroundingbasalt, basalt, are are commonly developed at at the expense expense of the primary minerals. Angular Angularvesicles vesicles occur occur in in these these gabbros, gabbros, and are filled with euhedral quartz, quartz, epidote epidote and and felted felted masses of chlorite. chlorite. These Thesegabbros gabbrosare areinterpreted interpretedas as lenses lensesof of slowly slowly cooled intrusions. The cooled magma magma trapped trapped within within this this exceptionally exceptionallythick thick flow, flow, rather than as later intrusions. The lack lack of of chilled chilled margins margins in in the the gabbro gabbro against against the the basalt basalt support support this this conclusion. conclusion. Flow-top quarry. Here an interfiow Flow-top features features are are common common along the west wall of the quarry. interflow zone is found, basalt. The found, containing containing clasts clasts of of fine-grained fine-grained and extremely amygdaloidal basalt. The clasts clasts are are angular consisting angularand and up up to to 20 20 cm cm in diameter. Interstitial Interstitialto to these these clasts clasts in a fine-grained matrix consisting of of angular angular quartz quartz and and feldspar. feldspar. This Thismay maybe bean aninterfiow interflowsediment sedimentconsisting consistingof ofwind-blown wind-blown scoriaceous scoriaceous flow-top flow-top material material or or perhaps perhaps pyroclastic pyroclastic debris debris from a nearby cinder cone. Burial Burial metamorphism has resulted in the recrystallization of the matrix and growth of secondary the recrystallization matrix and growth of secondary minerals, minerals,especially especiallyquartz, quartz,epidote, epidote,and and chlorite. chlorite. Pay Payclose closeattention attentionto to the the sedimentary sedimentaryfeatures features at at this this stop stop for for later later comparison comparisonwith with sediments sedimentsexposed exposed at at Stop Stop 5. 5. Amygdules Amygdules are are ubiquitous ubiquitous throughout throughout the the flow, flow, but but increase increase in in size size and andnumber numbertoward toward the top of the the flow. flow. The Theminerals mineralscommonly commonly found found filling filling the the amygdules amygdules are are quartz, chlorite, epidote, K-feldspar, calcite and actinolite. The Theamygdules amygdulesare are usually usually zoned, zoned, with with chlorite chlorite forming forming the the rinds. rinds. Growing Growingon onthese theserinds rindsare areinward inward projecting projecting crystals crystals of of quartz, quartz, epidote epidote and and pink pink perthitic perthitic K-feldspar. K-feldspar. Calcite, Calcite,or oraasecond secondgeneration generation of of chlorite, chlorite, occupies occupiesthe the centers centers of the amygdules. Actinolite Actinoliteoccurs occursas asmicroscopic microscopic fibers fibers projecting from the groundmass into the amygdules amygdules and and intergrown intergrown with with the the other other amygdule amygdule minerals. Another joint surfaces surfaceswithin withinthe thebasalt. basalt. The Another group group of minerals minerals form form veins or occur along joint most prominent veins are a series reverse faults. faults. These series of of quartz quartz veins veins formed along reverse These veins veins range from 10-40 10-40 cm cm in in thickness thickness and andare aresurrounded surrounded by by zones zonesof ofparticularly particularly extensive extensive chioritization chloritization and and epidotization epidotization of the basalt. The Thequartz quartzisis coarsely coarsely crystalline crystalline and and milky. milky. At At times crystals up to 15 cm long. long. Locally Locally times the the veins veins open open up up into into vugs vugs containing containing euhedral euhedral quartz quartz crystals calcite occurs occurs on on the the quartz quartz as asyellow-green yellow-green flattened flattened rhombohedral rhombohedral or globular clusters of radiating crystals. Pink Pinktotowhite whiteK-feldspar K-feldspardruses, druses,radial radialgroups groups of of dark-green dark-green chlorite, chlorite, and and copper minerals such such as as chalcocite, chalcocite, cuprite and malachite also occur from time to time in the veins. veins. Joint chlorite, hematite, hematite, tremolite Joint surfaces surfacesmay may be be coated coated by by aa variety of minerals, including chlorite, tremolite (growing (growing in in fibers fibers up up to to 33cm cmlong), long),white whitecleavable cleavablecalcite, calcite,chrysotile, chrysotile,chalcopyrite, chalcopyrite,bornite, bornite, malachite, malachite,and andnative native copper. copper. 48 �Figure Figure 14. 14.(a) (a)Photomicrograph Photomicrograph(crossed (crossedpolars) polars)of of "ophimottled" "ophimottled"basalt basalt(scale (scalebar bar along alongleft left mrn long). long). (b) Photomicrograph Photomicrograph (uncrossed polars) of augite augite exhibiting exhibiting ophitic ophitic side is 55 mm mm long). Groundmass in (a) and (b) contains plagioclase texture (scale bar along left side is 2.5 texture (scale bar along left side is 2.5 rnrn long). Groundmass in (a) and (b) contains plagioclase microcrysts, microcrysts, Fe-Ti oxides, oxides, chlorite, chlorite, quartz, quartz, epidote, epidote, actinolite, actinolite, and and calcite. calcite. 49 �Nuggets Nuggets of native copper copper massing up to several kilograms have been found in the quarry, but the author has not seen any in place. place. These Thesereportedly reportedly are are most most common common when when quarry quarry corner of the quarry. These operations are active in the flow-top area in the northwestern comer These can can effects on the crusher, and are thus not viewed as a resource have disastrous effects resource by the the quarry quarry owner. quarry are arecut cutby bynumerous numerousfaults. faults. Numerous Numerouseast-dipping east-dipping(45O (45°)) The flows of the north quarry cross-cut by a west-dipping (35° north-south across thrust faults are cross-cut (35O ) thrust fault that trends north-south across the center of the quarry (Leslie et al., 1994). Along most of the west-dipping fault surface is (Leslie 1994). Along most of west-dipping fault surface is massive massive and milky-white milky-white quartz. epidote-filled en-echelon en-echelon fractures, fractures, quartz. Also Alsofound found within within the the gouge gouge zone are epidote-filled striated and stepped chlorite-quartz-epidote chlorite-quartz-epidote surfaces, and late-stage syntaxial calcite fillings. kinematic indicators indicators indicate indicate that the hanging wall moved up and and east east relative relative to to All of these kinematic the footwall. Motion Motionalong alongthis thissurface surfaceoffsets offsets other other thrust faults in the quarry that dip to the east; the magnitude of the motion is not known known but but is is likely likely minor. minor. The age of faulting is not known, but the similar metamorphic minerals found in both the host rock and the gouge zone suggest that motion occurred occurred while the rocks were still relatively deeply buried. Geology of of the the east east quarries quarries across the the railroad railroad tracks, tracks, is another Approximately 0.5 0.5 km km SSE of the north quarry, across complex. These quarry complex. Thesequarries quarriesexpose exposetwo twoflows flowsunderlying underlyingthe theTrap TrapRock RockAlley Alley flow flow seen seen in in the north quarry. These These flows flows are are informally informally named the Dresser flows. Ten to twelve meters of each flow are exposed. The flows The flows dip dip westerly at about 10 10 degrees. The The contact contact between between the the two flows is well exposed. exposed. been converted Both flows flows are are subophitic subophiticdiabase, diabase,and andlike likethe the Trap Trap Rock Rock Alley flow, have been to greenstones greenstones during burial metamorphism. metamorphism. The The lower lower flow flow contains contains aa primary primary igneous igneous ilmenite. The rock is microporphyritic. microporphyritic. The assemblage of augite, plagioclase and ilmenite. The groundmass groundmass phenocrysts are titanite, and phenocrysts are incipiently incipiently to completely completely replaced replaced by a mixture of chlorite, epidote, titanite, actinolite, albite albite and and zoisite. zoisite. The plus white white quartz quartz and and pink pink K actinolite, The same same mineral mineral assemblage, plus Feldspar occur in the amygdules. The top of this flow is well exposed, and has been virtually amygdules. The top of this flow is well exposed, and has been virtually entirely entirely recrystallized recrystallizedto to aa metamorphic metamorphic greenstone. greenstone. Large Largeamygdules amygdulesare areabundant, abundant,and and contain contain coarse-grained assemblage yellowan attractive coarse-grained assemblageof white white quartz, quartz, pink K-feldspar, K-feldspar, red hematite, hematite, yellowgreen epidote, and dark green chlorite. Only Onlyminor minor flow-top flow-top breccia breccia can be seen. seen. subophitic The upper flow has a fine-grained chilled base grading upward into a diabasic subophitic interior. The Theprimary primaryigneous igneousand andsecondary secondary minerals minerals are are the the same same as as that that of of the the lower lower flow. flow. The The upper flow flow contains contains fewer fewer amygdules, amygdules, and and these these are are dominated dominated by nearly isotropic isotropic chlorite chlorite with minor intergrown epidote and quartz. The Thetop topof of this thisflow flow isis not not exposed. exposed. 50 �Stop %Magnetic Characteristics of the the Chengwatana ChengwatanaLava LavaFlows. Flows. Stop 2—Magnetic Characteristics of Location: Outcrops are are located located on on both both sides sides of of the the road road near near the the intersection intersection between between the the Location: Outcrops NE 1/4, 114, NE 1/4, 114, Sec. 11, 11, T 33 N, R 19W. 19 W. pipeline and County Road S. NE Figure 15. 15. Scanned Scannedimage imageof ofaaportion portionof ofthe theUSGS USGS Osceola, Osceola,WI-MN WI-MN topographic topographic quadrangle (7.5-minute (7.5-minute series; series;1:24,000 1:24,000 scale) scale) showing showing the the location location of of Stop Stop 2. 2. quadrangle examine and discuss the magnetic characteristics characteristics of the Chengwatana Chengwatana At this stop we will examine lava flows as exposed in Polk County, Wisconsin. Wisconsin. Paleomagnetic studies have been been conducted conducted in this region over the past 10 relationship of these 10 years to better understand the stratigraphic relationship flows within within the the context context of of the the regional regional Keweenawan Keweenawan magmatic sequence. paleomagnetic studies studies for for this this region region are arereported reportedby byKean Keanetetal. al.(1997). (1997). Here Detailed paleomagnetic only on on the the general general interpretation interpretation of of the the paleomagnetic paleomagnetic results results that that help helpto to place place we concentrate concentrate only the Chengwatana flows in the context context of of the the other other Keweenawan Keweenawan rocks in the Lake Lake Superior Superior region. Keweenawan Keweenawan paleopole paleopole positions positions for the craton are well known and a major polarity reverse to normal transition from fromreverse normal occurred occurred during during the the middle middle Keweenawan Keweenawan (1105-1102 (1 105-1102 Ma) has been documented by Palmer (1970) and Halls and Pesonen (1982). Table Table 22 shows shows this this Lake Superior polarity change change as as determined determined in some some of the major basalt flows of the southern Lake predominantly region. Feeney Feeney(1990) (1990)determined determinedthat that the the lava lava flows throughout Polk Co. have predominantly a-95=15.5° ;;which is consistent D=27S0 ,,1=41.5° 1=41S0,, a-95=15.5O normal polarities, polarities, with with aa mean direction direction of D=275° consistent Pesonen, 1982) 1982) such as the the North North Shore with other other Keweenawan Keweenawan directions directions(Table (Table 2) (Halls and Pesonen, Volcanic Group Group (Books, (Books, 1972). 1972). He Volcanic He also also noted noted several several locations locations with with reversed reversed directions. directions. St. Croix CroixFalls Fallsrendered renderedamore amore precise correlation correlation Insufficient geologic geologic control controlnorth north of the city of St. impossible impossible for for those those locations. locations.However, However,the thevolcanics volcanicssouth southof of St. St. Croix Croix Falls Falls are are better exposed exposed and have been mapped in detail detail by Cordua Cordua (1989a and b) which allowed for a more detailed study. Sixty-five Sixty-fiveoriented orientedcores coreswere were collected collectedat at the 66 sites noted in ~FIgure i g u r e16. Sampling Samplingwas was 51 �Table 2. Magnetic Magnetic polarity polarity and anddirections directionsfor formany many Keweenawan Keweenawan rocks rocks in in the the Lake Lake Superior region (from Superior (from Halls Halls and and Pesonen, Pesonen, 1982). 1982). Formation DeclinDeclination Jacobsville Jacobsville Sandstone Sandstone 263 263 Freda Sandstone Sandstone Apha 95 Pole Position Polarity -11 6 183°E, -0.9°S N 271.3 271.3 0.7 2.8 179.5°E, 1.2°N N Nonesuch Shale Shale 279.8 279.8 9.8 6 176.5°E, 10.3°N N Portage Volc (Avg) (Avg) Portage Lake Lake VoIc 288.6 34.9 2.5 181.2°E, 26.5°N N Upper North Shore Shore Vol. Vol. (Avg) (Avg) 290.3 290.3 44.5 4.7 183.8°E, 32.4°N N Lower Lower North Shore Shore Vol. Vol. (Avg) (Avg) 120.9 120.9 -63 7.8 197.8°E, 9.6°N N Logan Logan Sills Sills 1111 11 -72 2.5 219°E, 49.4°N N 74 74 -71.5 4.8 232.4°E, 28.9°N N Upper Powder Mill Gr. Gr. InclinInclination and 2), 2), and and concentrated in the areas areas where where Feeney Feeney (1990) (1990) found found reversed reversed directions directions (sites (sites 11 and equivalent. Most which are considered stratigraphically equivalent. Most of of the the samples samples (Table (Table 3) 3) have have high high intensities of natural remanent magnetism (NRM) (1.0-2,0 AIm) and volume magnetic natural remanent magnetism (NRM) (1.0-2.0 Alm) and volume magnetic susceptibility (4 x 10-2 SI). SI). The Themagnetic magneticproperties propertiesdiffer differ somewhat somewhat between between flows.. flows. The The remanent intensities middle flows flows (Pothole (Potholetrail, trail, Trap Trap Rock Rock Alley and Dresser flows) have remanent intensities and and which are are at least an order of magnitude less than the other magnetic susceptibilities which other flows flows (Table 3). For Forsamples sampleswith withvery very strong strong remanent remanent values, values, the intensities intensities are typically reduced A.F. demagnetization to to 20 20 millitesla millitesla (mT). (mT). Petrographic studies 100 fold with A.F..demagnetization studies of the the opaque opaque minerals in samples from each each site site also also show show differences differences which are consistent with the bulk magnetic parameters of susceptibility susceptibility and intensity. intensity. The The upper upper lava lava flows flows (sites (sites 1-4; 1-4; Fig. 16) 16) have a high percentage percentage of coarse-grained magnetic minerals (25 percent), which are generally coarse-grained magnetic minerals (25 percent), which are generally more altered than the older flows (sites 5 and 6), 6),and and which appear to have been produced during metamorphism. metamorphism. Samples during Samplesfrom fromsites sites55and and66have haveonly only about about 10 10percent percent magnetite magnetitegrains grains and the grains are smaller smaller than the upper flows. Principal Principalcomponent componentanalysis analysisof of the the magnetic magnetic directions directions (Kirschvink, (Kirschvink,1980) 1980)for for samples samplesfrom fromthe the various various sites sitesreveals reveals two two directions directionsthat that are are approximately antiparallel to to each other. other. The most prominent direction, after removal of The most prominent direction, after removal of aa viscous component is a northwesterly with inclinations of of about 430 43O (normal (normal northwesterly declination, with magnetization). A approximately -50 500 A second seconddirection direction with with aa southeasterly southeasterly declination and approximately inclination (reverse in some some samples samples at at sites sites 11and and2.2. There are (reverse magnetization) is present only in no significant significant differences petrographic characteristics characteristics or in the the rock rock magnetic magnetic properties properties differences either either in petrographic between the normal magnetized samples and the reversely magnetized samples at both sites 11 normal samples and 2. Wirth the entire entire sequence sequence of of Wirth et et al. al. (1997) (1997)report report only minor geochemical differences for the flows. The Thedetailed detailedmagnetic magneticinformation information for for these these sites sites is is given given in Table 4. Combined Combined data data from both components yield yield a Virtual Virtual Geomagnetic GeomagneticPole Pole(VGP) (VGP)atat30.9' 30.9° N, N, 186' 186° E, which which 52 �92 42 3O Oc 1 Ouarternary Alluvium. Colluviurn. Drift d 45 25 ST CROIX FALLS 8 PRECAMBRIAN PRECAMBRIAN KEWEENAWAN KEWEENAWANVOLCANIC VOLCANIC SERIES SERIES Ky Kv 0Kp Kp 0KsKs 0Kep Kep tO Kt Kd Kd Kpt Kpt --0 Q Unnamed Unnamed Flows Flows Pipeline P~pehneFlows Flows Skitrail Skltra!lFlows Flows Eagle Eagle Peak Peak Flows Flows Traprock TraprockAlley Alley Flows Flows Dresser Dresser Flows Flows Pothole Pothole Trail Tra~lFlows Flows Exposed Exposed Contact Contact Unexposed Contact Contact Unexposed Sample Location LocatlonNormal Normal Sample Location Locat!onReversed Reversed Sample Location LocatlonMixed Mlxed 1 MILE G 0 iKM t 45 20 92 $2'42 42'30 3W 92 37 30 Figure Figure 16. 16. Map Mapshowing showingpaleomagnetic paleomagneticsampling samplingsites sites(after (after Kean Kean et et al., 1997). 1997). 53 �Table 3. NRM NRM intensities intensitiesand and susceptibility susceptibility values for the Chengwatana volcanics in the St. St. Croix Croix Falls Falls area. area. Flow Name Number of samples samples Ky Kv NRM Intensti Intenstity ty AIM A514 Magnetic Susceptibility x10i5 Volume S.I. units xlO-5 9 2.3 3591 Pipeline Pipeline 14 2.0 5118 SkiTrail Ski Trail 16 2.1 5326 Eagle Peak Peak Eagle 14 0.15 1061 6 0.14 23 0.057 299 742 Trap Rock Alley Dresser 11 Pothole Trail 6 0.05 LotusLake h t u s Lake 6 2.1 3647 Paleomagnetic data for the sites indicated in Figure 16 Paleomagnetic 16 (after Kean et a!., al., 1997). 1997). Table 4. N DecinDeclination Inclination K 288.9 49.5 5.2 6.9 29.2 36.7 17 Apha -95 (R) (3) 7 7 114.8 55.0 -57.8 Site 2 (N) (N) 7 274.0 51.1 13 -47.5 33.2 14 23.1 20.8 31 16.3 14.1 45.5 42.6 43.4 45.8 11.2 18.8 11.2 Site I1 (N) (N) (R) (3) Site 33 Site 4 Site 55 Site 66 (N) Avg (N) (N&R) Avg @&R) 10 10 88 106.7 5 280.0 88 77 88 285.1 306 6 6 88 287.9 286.8 287.5 6.7 25.4 43.9 48.5 10.2 8.04 VGP Lat(N) L atO Long(E) Long@) SITE SITE L ato L ong0 Lat(N) Long(W) 45.33 92.65 24.6 31.2 193.4 193.4 198.4 187.9 45.37 92.67 19.9 183 22 43 178.9 29.6 29.7 30.9 183.6 184.9 186.1 45.39 45.39 45.36 45.37 92.66 92.66 92.64 92.63 41 173 agrees well with published published Keweenawan Keweenawan paleopoles for stable North American (Halls and and Pesonen, the area, area, because because Pesonen,1982). 1982).ItItisispossible possiblethere thereare areother other exposures exposuresof of the reversed flows in the detailed magnetic studies have not been conducted over the full length of the two prominent ridges in the area. area. Paleomagnetic Correlation Correlation The appear wimn within aa thick thick series series of of The reversed flows in this area are important because they appear flows that are normally polarized and considered to be equivalent in age, or younger than, the 54 �NorthShore Shorevolcanics. volcanics.Past Pastinterpretations interpretationssuggested suggestedthat thatallallthe theKeweenawan-age Keweenawan-ageflows flowsequivaequivaNorth lentororyounger youngerthan thanthe theNorth NorthShore Shorevolcanics volcanicswere werenormally normallypolarized polarized(Green, (Green,1977; 1977;Halls Halls lent and andPesonen, Pesonen,1982), 19821,and andthose thoseflows flowsolder olderthan thanthe theNorth NorthShore Shorevolcanics volcanicswere werereversely reverselypolarpolarized. ized.Similar Similarshort shortreversals reversalsatatapparent apparentequivalent equivalentstratigraphic stratigraphiclevels levelshave havebeen been documented documented onlyaafew fewother otherlocations locationsin inthe theLake LakeSuperior Superiorregion. region. The Thebasalt basaltflows flowsatatMamainse MamainsePoint Point atatonly theeastern easternLake LakeSuperior Superiorregion regionshow showa asimilar similarshort shortreversal reversal(Palmer, (Palmer,1970; 1970;Robertson, Robertson, ininthe 1973). 1973).The Thedikes dikesininthe thealkali alkalisyenite-carbonatite syenite-carbonatitecomplexes complexesininthe theKapuskasing Kapuskasingstructural structural zone zoneininCanada Canadadated datedatat1097 1097Ma Mayears yearsshow showreversals reversalsin in aanormal normal polarity polarity sequence sequence(Symons (Symons etetal. al.1994). 1994).The Thegeochemical geochemicaldata dataofofWirth Wirthetetal. al.(1997) (1997)are areconsistent consistentwith withthis thisequivalence equivalenceinin age agebetween betweenthe theChengwatana Chengwatanaflows flowsand andthe themiddle middlesection sectionat atMamainse MamainsePoint. Point.AAshort shortreverreversal salatatthis thistime timeisisalso alsoconsistent consistentwith withrecent recentages agesby by Wirth Wirth and and Gehrels Gehrels (in (in press). press). They Theyreport report Mafor forslightly slightlyolder olderChengwatana Chengwatanarhyolites rhyolitesnear near preciseU-Pb U-Pb zircon zircon ages ages of of 1102 1102Ma Ma ± 55Ma precise Clam ClamFalls, Falls,which whichisison onthe theeastern easternextreme extremeofofthe theChengwatana Chengwatanaexposures. exposures. Field FieldIdentification IdentificationofofReversed ReversedFlows Flows Unfortunately, Unfortunately,ititisisnot notpossible possibleto tovisually visuallyrecognize recognizenormalnormal-from fromreverse-polarity reverse-polarityrocks rocks ininthe thefield. field.Occasionally, Occasionally,the thereversed reversedflows flowsininthis thisarea areawill willcause causeaacompass compassneedle needleto todeflect deflect by but bymore morethan than90°, 90° butthen thenso sowill will areas areasstruck struck by lightening. lightening. Only Onlydetailed detaileddemagnetization demagnetizationof of samplesin inthe thelaboratory laboratorycan canresolve resolvethe thepolarity polarityquestion. question. samples Stop Interstate Park, Stop3a—Chengwatana 3a-Chengwatana Volcanic Volcanic Flows Flows of Interstate Park,WI. WI. Location: Location: Access AccesstotoEagle EaglePeak Peakisisfrom froma ahiking hikingtrail trailleading leadingwest westfrom fromthe thegroup groupcamping camping area, (WI).NW NW1/4, 114,SE SE1/4, 114,Sec. Sec.36, 36,TT3434N,N,RR1919W. W.(Fig. (Fig.17). 17). area,Interstate InterstatePark Park(WI). NOTE: NOTE: Specimen Specimencollecting collectingisisnot notallowed allowedininthe thepark! park! Thecrest crestof ofEagle EaglePeak Peakisison ontop topof of the thedipping dippingEagle EaglePeak Peakflow, flow, aa porphyritic porphyritic basalt basalt with with The large largeplagioclase plagioclase phenocrysts. phenocrysts. The Thepanoramic panoramicview viewfrom fromthe thecrest crestallows allowsvisitors visitorstotosee seethe the contrasting contrastingtopography topographyon oneast-facing east-facingflow flowfronts frontsversus versuswest-sloping west-slopingflow flowtops, tops,the thegeneral general distribution distributionof ofCambrian Cambriansedimentary sedimentaryrocks rocksdeposited depositedaround aroundbasaltic basalticislands, islands,and and the the Quaternary Quaternary featuresof ofthe theDalles Dallesofofthe theSt. St.Croix CroixRiver. River. features TheEagle EaglePeak Peakflow flowmap mapunit unitconsists consistsof ofseveral severalflow flow units units containing containingconspicuous conspicuouspink pink The plagioclasephenocrysts phenocrystsup uptoto66cm cmlong longin inaafine fine crystalline crystallinematrix. matrix. The Thematrix matrixand andphenocrysts phenocrysts plagioclase minerals such as epidote, arepartly partlyto tocompletely completelyreplaced replacedby bysecondary secondarymetamorphic metamorphic minerals such as epidote,chlorite, chlorite, are actinolite Kactinolite and and albite. albite. The Theflow flowtop topcontains containsmany manyamygdules amygdulesfilled filledwith withquartz, quartz,epidote, epidote,Kfeldspar feldsparand andchlorite. chlorite.The Theflows flowsdip dipgently gentlywestward westwardatatan anangle angleofofabout about15 15degrees. degrees. This Thisflow flowisisan animportant importantstratigraphic stratigraphicmarker, marker,traceable traceablein inthe thearea areafor forover over99kilometers kilometers along alongstrike. strike. InInplaces placesthe theunit unitconsists consistsofofonly onlyone oneflow; flow;in inothers othersititthickens thickensto to as asmuch muchas as60 60 meters meters and and includes includesat atleast least33 flow flow units, units, one one on on top top of the other. In Inthe theEagle EaglePeak Peakarea, area,only only oneflow flowisisevident. evident. one The Theslope slopecontinuing continuingdown downto tothe thewest westisisthe thedip dipslope slopemarking markingthe thetop topof ofthe theEagle EaglePeak Peak flow. flow. This Thisisisinincontrast contrasttotothe thesteep steepeast-facing east-facingescarpment escarpmentone oneascends ascendstotoreach reachthe thesummit summit from the group camping area. A small escarpment in the trees marks a quarry exposing from the group camping area. A small escarpment in the trees marks a quarry exposing the the overlying overlyingflow. flow. This Thisemphasizes emphasizesthe thestair-step stair-steptopography topographyof of the the terrain terrain formed formed by the dipping dipping flows. flows. 55 �Figure 17. 17. Scanned Scannedimage imageof of aaportion portion of of the the USGS USGS St. St. Croix Croix Dalles, Dalles, WItopographic quadrangle MN topographic quadrangle(7.5-minute (7.5-minute series; series; 1:24,000 1:24,000 scale) scale) showing showing the locations of Stops Stops 3a, 3a, 3b, and 4. One can see see outcrops outcrops of of Cambrian Cambrian sediments sediments deposited unconformably on the complex in the Chengwatana Chengwatana flows. flows. Looking slightly south of west, across the landscape developed in sandstones exposed in road cuts along U.S. river, one sees buff-colored Cambrian Mazomanie sandstones Highway 8. Cambrian Cambrianstrata strata continue continue down down to river level on the west side of the St. Croix Croix Highway River. The fiver. The sandstone sandstoneisistraceable traceablethrough through the the trees trees along along the the bluffs north nearly to the town of Taylors Falls. Falls. Here Taylors Hereaaridge ridgeofofbasalts basalts(predominantly (predominantlythe the Eagle EaglePeak Peak and andTrap TrapRock RockAlley Alley flows) rise up from which the the town town rests. rests. The scenic gorge called from the river to form the hills on which the St. Croix Dalles is cut cut through through these flows. Locally Locally along along the the contact contact of of the the Cambrian Cambrian sediment, is aa coarse coarse basalt basalt boulder boulder conglomerate conglomerate facies called the Mill Street Street conglomerate. conglomerate. the next next stop. stop. Another ridge of basalt is found near This is not visible here, but will be visited at the Northern States Falls is is built. built. East the Northern States Power Power Plant Plant and forms the highlands on which St. Croix Falls East of the river are many outcrops A few few exposures exposuresof of Cambrian Cambrian sandstone, sandstone,particularly particularly outcrops of basalt. A along along the bluffs bluffs to to the the east, east, can can also also be be found. found. 56 �Reconstruction Reconstructionof of the the paleogeography paleogeography of of the the Cambrian-Precambrian Cambrian-Precambrian unconformity unconformity was was done done using using outcrop outcrop and and water-well water-well data. The Theunconformity unconformity is is complex complex and and shows shows aa relief relief of of over over 100 100meters meters (Scott (Scott and and Anderson, Anderson, 1983). 1983). In Insome someareas, areas,the the relief relief occurs occurs along along buried buried shear shear cliffs, Cambrian sea rose. The as the Cambrian The Mill Mill Street Street conglomerate conglomerate cliffs?which whichwere wereprogressively progressively covered covered as marks these environments. In other areas, calmer embayments and lagoons formed marks these environments. In other areas, calmer embayments and lagoons formed in in areas areas of lower lowerrelief, relief?sometimes sometimesisolated isolated behind behind basaltic basaltic islands. islands. Fine Finesandstones sandstonesand andshales shalesof ofthe theEau Eau Claire Claireand and Mazomanie MazomanieFormations Formations were were deposited deposited in in these these areas. areas. The Theshapes shapesofofthese theseislands islands were of the the flows? flows,joint joint sets, sets, and and small small faults. faults. The were controlled controlled by the the stair-step stair-step topography topography of The continuity continuity of of the the Eagle Eagle Peak Peak flow flow from from island island to toisland island suggests suggests that thatfaults faultswith withmajor major displacement displacementdid didnot notcontrol controlthe theshapes shapesof ofthe theislands. islands. Knowledge Knowledge of the the distribution distribution of of basalt basalt in inthe thesubsurface subsurfacehas hasconsiderable considerablepractical practical importance. A company building a pipeline through the area in the 1970's went bankrupt importance. A company building a pipeline through the area in the 1970's went bankruptbecause because they they failed failedto totake takeinto intoaccount accountthe theexpenses expensesinvolved involved in in blasting blastingthrough throughthese theseburied buriedbasalt basalt ridges ridgeswhen when submitting submittingtheir theirbid. bid. ItItisisalso alsonot notuncommon uncommonfor forone oneparcel parcel of of land land to to have have basalt near near the the surface surfaceto to have have little little or or no no ground ground water, while a neighboring parcel a few 100 100 meters away away will will have have aa plentiful plentifulsupply supply of of water water from fromaathick thicksection sectionof ofPleistocene Pleistoceneand andCambrian Cambrian sediments. sediments. The The scenic scenic dalles dalles of of the the St. St.Croix CroixRiver River are arewhere wherethe theriver, river,after afterexcavating excavating the the soft soft Cambrian sediments in a gap between paleo-islands, has finally cut down to the hard basaltic Cambrian sediments in a gap between paleo-islands, has finally cut down to the hard basaltic shoal shoal separating separatingdeeper deeperCambrian Cambrianembayments. embayments. Immediately Dalles, aa possible possible whirlpool whirlpool or Immediately to the north, one can see see the Lake Lake of the the Dalles, plunge more easterly easterly course. course. Finally, numerous plungepool pool formed formedwhen whenthe the St. St. Croix Croix flowed along a more glacial glacial striations striationsand andchatter chattermarks marks can canbe be found foundon on the the bedrock bedrock surface surfaceat atEagle EaglePeak. Peak. Stop Interstate Park, Stop 3b—Cambrian 3b-cambrian Rocky Rocky Shoreline Shoreline Deposits of Interstate Park,WI. WI. See Figure Figure 17 17for forlocation locationmap. map. See Location: Lmation: Ravine Ravineimmediately immediatelysouth southofofSkyline SkylineTrail, Trail,approximately approximately 400 400 m m from from trailhead trailhead at at group Park (WI). (WI). SE1/4, SE1/4, Sec. 36, 36, TT 34.N7 34N, R 19W. group campground in Interstate Park SEl14, SElI4, 19W. NOTE: Specimen Specimencollecting collectingisisnot notallowed allowedin inthe the park! park! The meters. ItIt then The Skyline SkylineTrail Trail skirts skirts aa low, marshy area for its first 150 meters. then begins begins its its slow slow ascent ascent onto onto the the Trap Trap Rock Rock Alley flow of the Precambrian Chengwatana Chengwatana basalt, basalt, with with outcrops outcropsof of basalt basalt prominent prominent on on both both sides sides of of the the trail. After Afterapproximately approximately400 400 m, m, aa steep-walled, steep-walled,narrow narrow ravine can be be seen seen by by looking lookingdown down to to the the right right (south) (south) of of the the trail. trail. This ravine ravine marks marks the the approximate approximatecontact contactbetween between Precambrian Precambrian basalts basalts and Cambrian sedimentary deposits. deposits. Basalt Basalt outcrops outcropsoccur occuron on the the floor floor of of the the ravine ravine and along the north wall. Along Along the the south south wall, wall, 88 m of Upper Mazomanie Formation Formation UpperCambrian Cambrianconglomerates conglomeratesand andsandstones sandstonestentatively tentatively assigned assigned to the Mazomanie of of the the Tunnel Tunnel City City Group Group (Mazomanie (Mazomanie Member Member of the Franconia Franconia Formation in Minnesota) Minnesota) lie lie directly directlyabove aboveand andlateral lateralto to the the basalts. basalts. The TheCambrian Cambriansediments sedimentscan canbe betraced tracedfor forsome some240 240m m to to the the east east up up the theravine, ravine,but but the the best best exposures exposuresare are in in the the first first 70 70 m. m. On the south south wail wall of of the the ravine, ravine, aa basalt basalt boulder boulder conglomerate conglomerate facies ("the Mill Mill Street Street conglomerate" of of Berkey, Berkey, 1897) 1897)lies lies against against the the lava lava flows and grades grades vertically vertically and and laterally laterally into in this this area area (Fig. (Fig. 18). In into the sandstone sandstone facies facies more typical of the Cambrian section in In sharp sharp contrast contrastto to the the gray-green gray-greenbasalt basaltflow flowoutcrops outcropson onthe the north north side sideof of the the ravine, ravine, the the basalt basalt boulders boulders in the conglomerate are heavily weathered and stained orange-brown by iron oxides. The conglomerate are heavily weathered and stained Thelargest largest 57 57 �- - - - - I 10 meters meters Figure 18. Sketch Sketch of south south wall wall of of ravine ravine along along Skyline Skyline Trail (Stop 3b) showing vertical and and boulder conglomerate conglomerate and and sandstone sandstonefacies. facies. Covered areas lateral relationships between basalt boulder indicated by lack of pattern. No No vertical vertical exaggeration. exaggeration. boulders boulders near the the base base of the exposure exposure are over 1 m in diameter. The boulders are rounded but perhaps due to to original originaljointing jointing in in the thebasalt. basalt. A quartz sandstone matrix with of low sphericity, perhaps inarticulate brachiopods brachiopods fills boulders. No abundant inarticulate fills the spaces between the boulders. No trace trace of bedding bedding is is present within the main body of the conglomerate, although a few thin conglomerate beds present the conglomerate, extend out into the sandstones. These These beds beds contain contain rip-up clasts of sandstone as well as basalt cobbles and pebbles, pebbles, in basalt conglomerate conglomerate in which sandstone sandstone cobbles in contrast contrast to to the main body of the basalt clasts have not been seen. They Theyalso alsoshow showweak weakimbrication imbricationsuggesting suggesting current current flow flow to to the the west. conglomerate and and sandstone sandstonefacies faciesisissurprisingly surprisinglysharp. sharp. The The contact between the conglomerate The sandstone sandstone beds beds thin- to medium-bedded quartz arenites. arenites. The sandstones are orange-brown, thinmedium-bedded quartz vary in texture from fine- to coarse-grained, coarse-grained, from sub-rounded to rounded, rounded, and from moderately moderately to well-sorted. Some Somefinefine-to to medium-grained medium-grained sandstone sandstoneunits units contain contain scattered scattered coarse, coarse, frosted frosted quartz grains. A A few few beds beds are aregraded, graded, and and some some contain contain quartz pebbles and rip-up clasts of sandstone with foresets foresets dipping dipping sandstone and siltstone. Many Many units units show show high-angle planar cross-bedding, with to the west. fron-oxide Iron-oxidestaining stainingand andconcretions concretions are are common, common, as are inarticulate brachiopod shell- hash layers. AAlarge trough cross-bed set (or possibly a slump) near the center of the large trough cross-bed set (or exposure exposure is indicated indicated by aa curved curved erosional erosional surface surface marked by basalt cobbles near its base, and sandstone beds which dip slightly to the east, in contrast to the slight westward westward dip dip of of most of the sandstone sandstone beds in in the the outcrop. outcrop. Interpretation Interpretation The Taylors Falls-St. Falls-St. Croix Croix Falls Falls area of Minnesota and and Wisconsin Wisconsin isis one one of the few localities in North North America America where where Upper Upper Cambrian Cambrian conglomerates conglomerates lie lie unconformably unconformably on on Precambrian rocks rocks (Webers (Webers and and Austin, Austin, 1972; 1972;Cavaleri Cavaleri et. al., 1987). 1987). 58 �Apparently, Apparently, the the Cambrian Cambrian Sauk Sauksea seatransgressed transgressedonto ontoaaland landsurface surfaceof ofvery verylow lowrelief reliefas as itit flooded advanced. At a few localities localities flooded the the North North American American craton, craton, leaving leaving a blanket of sand as itit advanced. such such as as Taylors TaylorsFalls-St. Falls-St.Croix CroixFalls Fallsand andBaraboo, Baraboo,Wisconsin Wisconsin (Dalziel (Dalzieland andDott, Dott,1970; 1970;Dott, Dott, 1974), 1974), resistant bedrock bedrock hills hills stood stood above above the the subdued subdued land land surface surface and and acted actedas aslocal localsources sources for for coarse coarse conglomerate conglomeratedeposits depositsas as the sea advanced. In In Baraboo, Baraboo, the conglomerate conglomeratewas was formed formed of quartzite quartzite boulders boulders from from local local outcrops; outcrops; in in the the Taylors Taylors Falls-St. Falls-St. Croix Croix Falls Falls area, area, the the conglomerate conglomerate was was formed formed of of basalt basalt boulders boulders from from the the local local lava lava flows. flows. The basalt boulder boulder conglomerates, conglomerates, informally called the "Mill Street conglomerate" are are exposed exposed at at only only aa handful handful of of sites sites on on both both sides sidesof of the the St. St. Croix Croix River River south southof of Taylors Taylors Falls-St. Falls-St. Croix Croix Falls. Falls. Of Ofthese thesesites, sites,the theSkyline SkylineTrail Traillocality, locality,shows showsmost most clearly clearlythe the vertical verticaland andlateral lateral relationships relationships between between the Precambrian Precambrian basalts and Upper Upper Cambrian Cambrian conglomerates conglomerates and sandstones. sandstones. As Ascan canbe beseen seenin in the the ravine, ravine, the the basalt boulder conglomerate conglomerate was deposited deposited on on top of and and against against aa steep steep face face cut cut into into the the basalt basalt lava lava flows. Very Very high high energy energy conditions conditionsmust musthave have been been present present to to round round the the corners comers of the basalt boulders. Dott Dott(1974) (1974)estimated estimatedthe theheight heightof ofthe the waves required to round boulders of similar size in Baraboo. Baraboo. His His conservative conservative estimates estimates of m high suggest suggest storms storms were important in producing producing these conglomerates. conglomerates. A A waves 55 m paleogeographic paleogeographic reconstruction reconstructionof of the the Taylors Taylors Falls-St. Croix Falls area as the Sauk Sauk sea sea began began to submerge submerge the the basalt basalt highlands highlands shows shows that the the lava flows along the Skyline Trail would have been fully exposed to waves from the west or southwest. ItIt isis tempting tempting to to envision envision basalt basalt sea sea cliffs on the windward windward side side of of islands islands or or headlands, headlands,with with storm-generated storm-generatedwaves waves crashing crashinginto into and the base base being being pounded pounded and jostled jostled and undercutting undercuttingthe the cliffs, cliffs, and and the the rubble rubble in the surf zone at the to produce rounded boulders. Such an environment evokes images of the Hawaiian Islands or rounded Such an environment evokes images the Hawaiian or perhaps perhaps the the Pacific Pacific Northwest Northwest coast coast today. today. However, However, very very little little has been written written in the geologic geologic literature literature on on the characteristics characteristics of of rocky shoreline shoreline deposits (Johnson, 1988) 1988) and care should be taken in reconstructing reconstructing the the exact exact depositional depositionalenvironment. environment. The The sharp sharp contact contact between between the conglomerate conglomerateand and sandstone sandstone and the difference in lithology between between the the two two facies facies suggest suggestthat that the the sands sands came came from from a more distant distant source source and and were were washed washed along basalt boulders. boulders. The along the shoreline shoreline and and deposited deposited against against and between the basalt The graded graded bedding, bedding, rip-up clasts and shell-hash layers show that periodic storms may have been more important and shell-hash layers been more important in deposition deposition of of the the sandstone sandstone beds beds than normal day-to-day events. Storms Stormscould couldalso also account accountfor for the two conglomerate beds extending out into the sandstone; each each bed bed with a mix of basalt basalt cobbles and sandstone rip-up clasts from the ocean floor. and sandstone rip-up clasts from the ocean floor. The The sandstones sandstones are are tentatively tentatively assigned assigned to to the the Mazomanie MazomanieFormation Formationof of the theTunnel TunnelCity City Group Group by their their similarity similarity to to outcrops outcrops of of that formation formation across across the St. Croix River in Minnesota. However, they they may represent a near-shore Ironton Member Member of ofthe theWonewoc Wonewoc near-shore facies facies of of the the fronton Formation as well. The Mill Street conglomerate varies in age from Dresbachian to Franconian Formation as well. The Mill Street conglomerate varies (Webers and Austin, 1972) 1972) and and thus thus may may grade grade laterally laterally into into sandstones sandstonesof of either eitherformation. formation. Detailed Detailed study of of the the fossil fossil assemblages, assemblages, particularly particularly the trilobites, trilobites, in the the sandstone sandstone and and conglomerate would be the best method method to tell the age age and and formation, formation, but as as yet yet this this has has not not been attempted. attempted. Such Suchaastudy studywould would be be especially especially valuable valuable since the Mill Street conglomerate is known known to to contain contain an an unusual unusual assemblage assemblage of of trilobites, trilobites, inarticulate inarticulate brachiopods brachiopods and and monoplacophoran monoplacophoran molluscs molluscs representing representing an an intertidal, intertidal, shoreline shoreline environment environment (Webers, (Webers, 1972). 1972). According to Johnson (1988), the oldest known known rocky-shore rocky-shorefaunas faunasare areOrdovician Ordovicianininage. age. It may well be be that that the theunusual unusualUpper Upper Cambrian Cambrian conglomerate conglomerate deposits deposits around around St. St. Croix Croix Falls Falls and Taylors Falls contain contain the the oldest oldest rocky rocky shoreline shorelinefaunas faunasin in the the world. world. 59 �Stop Stop4—Potholes &Potholes of ofInterstate InterstatePark, Park,MN. MN. SeeFigure Figure17 17for forlocation locationmap. map. See Location: Paths Pathsleading leadingfrom fromthe theparking parkinglot lotbybythe theinterpretative interpretativecenter, center,Interstate InterstatePark Park Location: (Minnesota). SW 1/4, SW 1/4, Sec. 30, T 34 N, R 18 W. (Minnesota). SW 114, SW 114, Sec. 30, T 34 N, R 18 W. NOTE: NOTE:Specimen Specimencollecting collectingisisnot notallowed allowedininthe thepark! park! This Thisstop stopprovides providesan anopportunity opportunity totoexamine examineoutcrops outcropsof ofbasaltic basalticlava lavaflows flowswith with spectacular spectacular potholes. potholes. From Fromthe theparking parkinglot, lot,follow followthe theservice serviceroad road(also (alsocalled calledTrap TrapRock Rock Alley) Alley)aashort shortdistance. distance.Many Manyshort, short,well-marked well-markedtrails trailstotothe theeast eastimaginatively imaginativelyand andconveniently conveniently lead over, around and through world-class potholes cut in the top of the Dresser basaltic lead over, around and through world-class potholes cut in the top of the Dresser basalticlava lava flow. flow. The Thebase baseofofthe theTrap TrapRock RockAlley Alleyflow, flow,here here at at its its type typelocality, locality, is is well well exposed exposedalong alongthe the west westside sideof ofTrap TrapRock Rock Alley. Alley. The Thetop topof of the the Dresser Dresser flows flows is exposed to the east. This Thisflow flow dips dips gently gently west westunder underthe theTrap TrapRock Rock Alley. Alley. The Theflow flowtop topisistypically typicallyamygdaloidal amygdaloidaland and extensively extensivelyepidotized. epidotized.ItItisiswell wellexposed exposedininabundant abundantoutcrops, outcrops,asaswell wellasasalong alongthe thesides sidesof ofthe the manyenormous enormouspotholes. potholes. many The Thepotholes potholesformed formedon onthe theDresser Dresserflows flows are arethought thought to to be be some someof of the thedeepest deepestin inthe the world world(Crawford, (Crawford,undated). undated).They Theyrange rangefrom fromfist-size fist-sizedepressions depressionsto to"giant's "giant's kettles" kettles"that thatcan can be beover over66meters metersacross acrossand andmore morethan than 20 20 meters meters deep. deep. The Thepotholes potholesare aregenerally generallypartly partly filled filled with withaamixture mixtureof ofsilt, silt,mud, mud,peat peatand andcoarse coarsegrind grindstones stonesranging rangingfrom frompebble pebbletotoboulder bouldersize. size. The Thesediments, sediments,swirled swirledby bythe theturbulence turbulenceof ofthe theriver riveras asititraced racedover overthe the basalt, basalt,were werethe thetools tools cuttingthese thesepotholes. potholes. cutting These Thesepotholes potholesare arepart partofofaalarger largerregion regionof of river-scoured river-scouredterrain. terrain. InInaddition additiontotothem, them,the the bedrock bedrocksurface surfaceisisscoured, scoured,smoothed smoothedand andpolished polished in in many areas. The Thepotholes potholes were were produced produced when when localized localized areas, areas, facilitated facilitated by by joint joint intersections, intersections, wore wore down down slightly slightly faster faster than than the the surrounding rock. Eddies formed over these low spots, trapping sediment. Large boulders surrounding rock. Eddies formed over these low spots, trapping sediment. Large boulders occasionally occasionally were were stranded strandedin in these these depressions. depressions. The Thestrong strongcurrent currentswirled swirledthem themabout, about,but but lacked lackedthe theforce forceto to free free them. them. This Thisaction actioncut cutthe thepotholes. potholes. Notice Noticeininplaces placesthe thegrooves groovescut cutin in the the sides sidesof ofthe thepotholes potholesas aslarge largeboulders boulders ground ground their their way way down. down. The Theboulders bouldersthemselves themselves were were shaped shaped by by the theprocess. process. Grind Grindstones stonesrecovered recoveredfrom fromthe thepotholes potholesare areasassmooth smooth and and roundedas ascannon cannonballs. balls. rounded The Thepotholes potholes here hereare are7.5-18 7.5-18 meters metersabove above the the current current river river level. level. Some Somepotholes potholeshave have beenfound foundasashigh highasas34 34meters metersabove aboveriver riverlevel. level. been The Thepotholes potholesformed formedwhen whenwater waterfrom fromthe themelting meltingglaciers glaciersfilled filledGlacial GlacialLake LakeDuluth Duluthto to the thenorth north(Cahow, (Cahow,1975; 1975;Ojakangas Ojakangasand andMatsch, Matsch,1982). 1982).The Thewater waterovertopping overtoppingthis thislake lakeflowed flowed south southdown downthe the ancestral ancestralSt. St.Croix CroixRiver. River. At AtInterstate InterstatePark, Park,these thesewaters waterswere werefurther furtherfunneled funneled down down the the narrow narrow channel channel now now seen seen as as the Dalles. Dalles. Below Belowthe theDalles, Dalles,the the river river encountered encounteredsoft soft Cambrian Cambriansandstones sandstonesand andshales, shales,which whichcut cutaway awayfaster fasterthan than the the basalt. This Thisresulted resultedininaamajor major knickpoint in in the the St. St.Croix Croixatatthe the Dalles. Dalles. This Thishigh highgradient gradientsped spedup upthe thewater, water, increasing increasing its its knickpoint ability abilityto to gouge gougeout outthe thepotholes potholesas asititexcavated excavatedits its valley valley into into the the resistant resistant basaltic basaltic bedrock. bedrock. Many Many of of the thepotholes potholeshere herehave have been been given given names names by the park service. service. Be Besure suretotoenjoy enjoy the the Lily Lily Pond, Pond,Devil's Devil'sParlor, Parlor,Cauldron, Cauldron,Bottomless BottomlessPit Pit and and Bake Bake Oven Oven as as time time permits. permits. Also Also note note the the west-dipping west-dipping thrust thrust fault fault exposed exposed in in the the cliff cliff by by the the riverboat riverboat dock dock just west west of of the the potholesarea. area. potholes 60 60 �Stop Stop 5—Volcanic 5-Volcanic Flows and Interfiow Interflow Sediments Sedimentswest west of of Lewis, WI. Location: Outcrops Outcropsare arelocated locatedon on345th 345thAve AveOne-half one-half mile west of the intersection intersection with with 130th 130th Location: Street. Street. NN1/2, 112,SE SE1/4 114and and5S1/2, 112,NE NE1/4, 114,Sec. Sec.11, 11,TT37 37N, N, RR 17 17W; W, Lat. Lat. 450 45' 42' 42' 07.6" 07.6" N, N, Long. Long. 092° 092' 26' 26'25.8" 25.8"W. W. Figure 19. Scanned Scanned image image of of a portion of the Figure 19. the USGS USGS Frederic, Frederic, Wisconsin Wisconsin topographic topographicquadrangle quadrangle(7.5-minute (7.5-minuteSeries; Series;1:24,000 1:24,000scale) scale)showing showing the the location location of of StopS. Stop 5. These permission before before entering. entering. Basalt Theseoutcrops outcropsare areon on private private land land and you should seek permission Basalt and and interfiow intefflow sediments sedimentsare areexposed exposedin inaaseries seriesof ofoutcrops outcrops20-100 20-100meters metersnorth northof ofthe theroad. road. Near Near the the road, road,fine-grained fine-grainedbasalt basalt isis exposed exposed in in aa flow that is at least 4 meters thick. Near Near the amygdules. the top top of of the theflow, flow, the the basalt basalt contains containsrare rare large large quartz-filled quartz-tilledamygdules. Interflow Intefflow sediment sedimentisis exposed exposed further furthernorth north and andisisparticularly particularly well-exposed well-exposed in inan aneasteastwest west trending trendingcliff cliff face. face. Please Pleasedo donot nothammer hammeron onthis this face. face. Excellent Excellentexamples examplesofofthe theinterfiow interflow sediment sediment can be found found in in the the many many loose loose blocks along the base of the cliff. The Thebasalt basaltisis overlain Thesediment sediment overlain by aa mixed mixed basalt basalt and and sediment sediment zone zone that is roughly 3 meters thick. The weathers weatherspale palereddish-orange reddish-orangeand andconsists consistsof offine-grained, fine-grained,poorly poorly sorted, sorted,and andthinly thinlylaminated laminated quartz, quartz, feldspar, feldspar, and and volcanic volcanic fragments. fragments. The Thefine-grained fine-grainedmatrix matrixmaterial materialisiscompletely completely recrystallized minerals. Beds recrystallized to epidote epidote and and other other greenschist greenschist facies minerals. Beds typically typically show show planar planar bedding bedding structures, structures,although although some somecross-beds cross-beds(ripple (ripplestructures), structures),channel-fill channel-fillstructures, structures,and and convoluted convoluted beds are are also also visible. Bedding Beddingstructures structuresoften often trend trend uninterrupted uninterrupted right up to the margins margins of of clasts, clasts, though though in in some someplaces places bedding bedding surfaces surfaces appear appear to to be be deflected deflected by by clasts. clasts. There There are are also also patches patches of of sediment sedimentthat that are are surrounded surrounded by basalt and appear "isolated" in in the the two-dimensional two-dimensionaloutcrop outcropface. face.'These, These,asaswell wellas asmany many narrow narrow sub-vertical sub-vertical"veins" of of sediment, sediment, exhibit exhibitwell-defined, well-defined,sub-horizontal sub-horizontallayering layeringthat thatisissharply sharplytruncated truncatedalong alongits itsmargins. margins. 61 , �Fragments of basalt range from less than than aa millimeter millimeter to to more morethan than 15 15centimeters. centimeters. The microcrystalline plagioclase, plagioclase, and are are highly highly vesicular vesicular clasts are typically sub-angular, consist of microcrystalline (1-5 mm mrn diameter); the outer outer surfaces surfaces of many many clasts are are highly highly embayed embayed and and exhibit exhibit very very "sponge-like" textures. textures. Some delicate "sponge-like" Some clasts clasts are are sub-rounded sub-rounded and exhibit decreasing vesicle sizes near their outer edges; the long long axes axes of of highly highly elliptical elliptical vesicles also parallel the clast clast surfaces. Many Many of of these thesefeatures featuresare aresuggestive suggestiveof of chilled chilled margins margins surrounding surrounding some of the clasts. clasts. Many Manylarger largerclasts clastsalso also have have numerous numerous small small "floating" fragments fragments around around their margins. margins. Viewed closely, many of these these fragments fragments appear appear to to be bedisrupted disrupted"glassy" "glassy" walls walls between between adjacent adjacent vesicles. fragments are typically filled with with coarse coarse epidote. epidote. Near the vesicles. Vesicles Vesicles in in the interiors interiors of fragments continuous layer of vesicular basalt has a highly irregular top of the outcrop, a nearly continuous irregular base base that that in some places cross-cuts layering in the underlying sediment. All All of of these these features features point point to to aa highly immature sediment. Elsewhere Elsewherein in the the rift, rift, interflow sediments are interpreted to record deposition in predominantly environments (e.g., (e.g., Jirsa, Jirsa, 1984). However, predominantly sedimentary environments However, some some features features at at this this stop stop are are suggestive suggestiveof hyaloclastites, hyaloclastites,in in which which basaltic basaltic melt melt interacts interactswith with water, water, or water-saturated water-saturated sediment. sediment.Preliminary Preliminaryinitial initialCNd EM^values valuesof ofthese thesesediments sediments(—-2.0; (--2.0; Naiman, unpublished data) are are similar similar to to those those of of the the surrounding surrounding basalts (Table 5) suggesting that the source of the sediments sediments is primarily immature rift volcanics. Stop Stop 6a—Rhyolite 6a-Rhyolite and Basalt Basalt near near Clam ClamFalls. Falls. Location: 435 435m mNNE NNE of of the the intersection intersection between County Rd I and 73rd St. SE SE 1/4, 114,SE SE 1/4, 114, 09.8" N, N, Long. Long. 092' 092° 18' 27.4"W. Sec. 14, T 37 N, R 16W; 16W,Lat. Lat. 450 45O 41' 41'09.8" 18'27.4" W. image of of a portion Figure 20. 20. Scanned image portion of the the USGS USGS Clam Clam Falls, Falls, Wisconsin topographic topographic quadrangle quadrangle (7.5 minute minute series; series; 1:24,000 1:24,000 scale) scale) showing the location of Stop Stop 6a. 6a. 62 62 �Table Table 5. 5. Representative Representativeanalyses analysesof ofbasalt basaltand andrhyolite rhyolitefrom fromthe theClam ClamFalls Fallsregion. region. Si02 Si02 Ti02 Ti02 Al203 *l2O3 Fe203 Fe203 MnO MnO MgO MgO CaO CaO Na20 Na20 K20 K20 basalt basalt KC-301b KC-301b basalt basalt KC-303 KC-303 rhyolite rhyolite KC-302a KC-302a rhyolite rhyolite KC-302d KC-302d rhyolite rhyolite KC-310 KC-3 10 47.08 47.08 49.89 49.89 1.60 15.66 14.10 0.18 1.84 14.91 13.61 72.46 72.46 0.49 71.34 7 1.34 0.49 11.52 5.30 0.06 72.18 72.18 0.48 11.93 5.78 0.12 0.40 '25 P2° 0.56 0.15 LOl LO1 1.72 0.17 5.10 8.61 3.17 0.55 0.20 2.31 Total Total 100.20 100.36 Mg# Mg# 0.53 6.84 10.00 2.31 qz qz or ab ab an an hy hy 01 ol 3.41 20.08 31.54 14.07 9.23 4.87 0.09 0.48 1.36 2.32 2.63 4.11 1.01 1.32 3.52 4.21 0.09 0.40 99.69 100.22 100.43 0.47 0.19 0.37 0.14 0.56 3.36 27.66 25.60 21.60 31.39 30.56 24.75 3.65 6.43 31.85 24.62 22.51 7.55 29.75 25.02 29.90 4.30 7.24 1.08 2.89 5.10 0.08 0.54 0.08 8.21 13 151 91 82 216 1036 80 889 123 831 108 90 12 178 68 794 62 713 38 141 118 67 965 155 205 117 24 133 281 76 299 14 6 21 Rb Rb Ba Ba Sr Sr 19 115 Nb Zr Y Y 8 Cr Cr Ni Ni v V InitialENd EN^ Initial 11.61 164 111 31 +34 "'"3.4 — -0.10 Sample Sample Locations: Locations: T 37 N, R 16W. 16 W. SW114, SW1/4, SW114, section section 13 13 KC-301b KC-301b -- SW1/4, SE 1/4, 114, SE 1/4, 114, section section 14 14 (Stop (Stop 6) 6) KC-302a -- SE KC-302a KC-302d -- SE SE 1/4, 114, SE 1/4, 114, section 14 14 (Stop (Stop 6) 6) KC-302d SE114, NW1/4, NW114, section section 13 13 KC-303 -- SE1/4, KC-303 SE114, SE1/4, SE114,section section 26 26 (Stop (Stop 66 Alt.) Alt.) KC-310 KC-310 -- SE1/4, 63 14 26 19 9 17 095 �These outcrops outcrops are are on on private private land and permission to access access them them should should be sought sought before before the intersection intersection of of Country Country entering. The The outcrops outcropscan can be be accessed accessed from from the property (north of the 73rd St. approximately 1 mile mile west west of of Clam ClamFalls. Falls. Direct access to the the outcrops outcrops from Rd I with 73rd County Rd Rd I requires landowner. An alternate County requires permission permission from another another landowner. alternate stop (6b) (6b) near near the the McKenzie McKenzie Creek Creek State State Hunting HuntingGrounds Groundsisisgiven given below. below. The purpose of this stop is to examine an example example of ofrhyolite. rhyolite. Rhyolite is extremely rare in this part of the rift rift and and only only two two localities localitiesare are known known in in the the area area extending extendingfrom fromTaylors TaylorsFalls Falls to Clam Falls. This Thisisisin instark starkcontrast contrastto tothe the relatively relatively abundant abundant rhyolites rhyolites (up (up to 25 25 percent percent of of the volcanic volcanic section; section;Green Greenand andFitz, Fitz, 1993) 1993)that that are are exposed exposed on on the North Shore Shore of Lake Lake Superior. Superior. This locality was also also sampled sampled for for U-Pb zircon studies studies (see (see below). At this exposure, that measures measures approximately approximately 150 x exposure, the rhyolite is exposed over an area that 75 meters. The the exposure exposure trends trends N N 50 50° E and Theupper uppersurface surfaceof of the the outcrop outcrop at at the north end of the dips 15° 15O NW, NW, sub-parallel sub-parallelto to the the trend of flow layering in the area. At the southern southern end of the outcrop, exposures outcrop, the the basal basal contact contact with with the underlying underlying basalt basalt flow can be seen. In a few isolated exposures approximately 3-4 3-4 meters meters below below the the top top of of the knoll one can observe isolated exposures exposures in in approximately which basalt and rhyolite are complexly inter-fingered. Sub-angular Sub-angularto to rounded rounded clasts clasts consist consist of fine-grained basalt, up to 15 cm in diameter, diameter, can can be befound foundin inrhyolite. rhyolite. The upper upper contact of of the rhyolite rhyolite with with the the overlying overlying flows flows isis not not exposed. exposed. Weakly porphyritic (An2Abo40r4)and porphyritic rhyolite rhyolite consists consistsof of phenocrysts phenocrystsof of euhedral euhedralalbite albite(An2Ab94Or4) rare quartz quartz occur occur in in aa groundmass groundmassof ofintergrown intergrownquartz quartzand andpotassium potassiumfeldspar feldspar(An0Ab5Or95); textures spherulitic (Fig. 21). textures range from from fine-grained fine-grained intergranular intergranular to medium-grained medium-grained spherulitic Spherulitic Spheruliticintergrowths intergrowths are are commonly commonly nucleated nucleated on on phenocrysts. phenocrysts. Minor Minoranhedral anhedralFe-Ti Fe-Tioxides, oxides, rare calcic amphibole and accessory apatite and zircon are amphibole (ferro-edenite-hornblende), (ferro-edenite-hornblende), and are also also commonly commonly present. present. Groundmass Groundmassquartz quartzisistypically typicallyeuhedral euhedralto to subhedral subhedral and tabular to elongate (length:width (1ength:width ratios == 10:1 10:1 to 50:1) 50: 1) indicating crystallization as tridymite, the orthorhombic, high temperature (>867O (>867° C) C) form form of of quartz. quartz. The Thepreservation preservation of of these these delicate delicate devitrification textures is remarkable remarkable given given the metamorphic metamorphic alteration alteration and and recrystallization recrystallizationthat that has has occurred occurred in this portion of the rift. The rhyolites of the Clam Clam Falls Falls region region are classified as rhyolite or rhyodacite on most diagrams (Abbott (Abbottetetal., al.,ininpress). press). The major and trace major and trace element discriminant diagrams element compositions icelandites and granophyres granophyres from the North element compositions(Table (Table 5) 5) are are similar similar to some icelandites Shore Green, 1997) to type type IIrhyolites rhyolites from from the the Portage Portage Lake Lake Shore Volcanic Group (Vervoort and Green, 1997) and to volcanics (Nicholson, (Nicholson, 1992; 1992; Nicholson and Shirey, 1990). Nd Nd isotopic isotopic studies studiesof of the the rhyolites rhyolites (Abbott ENd values = (Abbott et et al., al., in in press; press; Naiman Naiman and andWirth, Wirth, in in press) press) indicate indicatethat that they they have have initial initialENd This evidence, combined with their relatively minor abundance in the volcanic 0. their relatively abundance the volcanic section, section, 0. by magmatic fractionation. fractionation. This suggests that they originated as residual melts by This is is in contrast contrast to to many values and are interpreted interpreted many of of the the rhyolites rhyoliteson onthe theNorth NorthShore Shorethat thathave havenegative negativeinitial initialENd ENd values to be the and Green, Green, 1997). 1997). Basalt from this part of the the result result of of crustal crustal melting melting (Vervoort (Vervoort and volcanic volcanic section section (e.g., (e.g., KC-301b KC-301band andKC-303; KC-303;Table Table5) 5) isis similar similarto to low-Ti low-Ti basalt basalt of of the the Taylors Taylors Falls region and has initial ENd valuesranging rangingfrom from00toto+3.4, 3.4, consistent ENd values consistent with with derivation derivation from multiple asthenosphere (Naiman multiple mantle mantle sources, sources, including includingenriched enriched mantle mantle plume plume and depleted asthenosphere (Naiman and Wirth, in press). 64 �Figure21. 21.(a) (a) Photomicrograph Photomicrograph (plane-polarized (plane-polarized light) light) of of spherulitic Figure intergrowths of of tridymite pararnorphs paramorphs and and feldspar feldspar in Clam Falls rhyolite intergrowths bar along (sample KC-302a). KC-302a). Scale bar along left side is 0.25 0.25 mm mm long. long. (b) (b) Photomicrograph showing plagioclase (albite) phenocryst with spherulitic spherulitic mrn long. sideis 1.25 1.25 mm intergrowths around the margin. Scale Scale bar bar along along left sideis 65 �One of the major difficulties in integrating geologic information from this portion of the rift with models of of the the rift rift has has been been the thelack lackof ofgeochronologic geochronologic control control in in the the Chengwatana Chengwatana volcanic s. With this objective collected from both Stop volcanics. objective in mind, large samples of rhyolite were collected Stop 10) has not 6a and Stop 6b for U-Pb zircon dating. The Thesample samplematerial materialfrom fromStop Stop6b 6b(KC-3 (KC-310) been processed processed at at this this time, time, but two, two, nearly identical identical ages ages were determined determined for for different different rhyolite rhyolite phases (KC-302a and and KC-302d) KC-302d) from from Stop Stop 6a. 6a. Zircons from both samples samples have have relatively relatively simple reddish-brown, to honey brown, simple prismatic prismatic forms, forms,are are translucent, translucent,and and vary vary in in color color from reddish-brown, brown, and pale pink. Three Threezircon zircongrains grainsfrom fromKC-302a KC-302a yielded yielded concordant concordant ages ages (Fig. (Fig. 22) 22) with mean mean 2O6pb*/238U and 2o7Pb*PO6Pb* of 1102 k ± 5 Ma (2a). 206Pb*/238U and 207~b*/206Pb* ages of (20). Three Threeadditional additionalgrains grainsare are slightly slightly discordant, discordant,presumably presumablydue dueto to small smallamounts amountsof of lead lead loss, loss, and and aa discordia discordiathrough through these thesepoints, points, projected from ± 205 Analyses 205 Ma Ma (MSWD (MSWD== 0.15). 0.15). Analyses from 1102 1102Ma, Ma, yields yields aa lower lower intercept of 196 196 k of three and yield mean 2O6Pb*/238U three abraded abradedgrains grainsfrom fromKC-302d KC-302dare areanalytically analyticallyconcordant concordant and yield mean ^Pb*/238U ages 1101Ma Mak ±55Ma Ma(Fig. (Fig.22). 22). Three additional grains and the two and 2o7Pb*PooPb* 207~b*/206Pb*ages ofof1101 multigrain fractions are slightly discordant; a discordia through these points, projected from uncertainties o of± 1101 Ma, Ma, yields yields a lower intercept intercept of 63 63 k± 217 Ma (MSWD = 0.45). 0.45). Given the uncertainties 1101 ft 5 Ma on each of of the the ages, ages, the the 11Ma Ma age agedifference difference between between the the two two samples samples is is probably probably not not significant. significant. 0.190 Rhyolite Rhyolite 0.186 00 I Age = Age = 1080 1102 ±5Ma 1080 1101 ±5Ma 0.182 0.178 Lower Intercept 196 ±205 Ma (MSWD=0.15) 0.174 I I Lower Intercept = 63 ±217Ma (MSWD=0.45) • 1.98 2.06 1.98 1.82 1.90 1.90 1.90 1.98 2.06 1.82 1.90 1.98 2.06 2O7pb 235 u 2O7pb 235 u 207Pb / 235 U 207Pb / U Figure 22. Concordia Concordiadiagrams diagramsfor forChengwatana Chengwatanavolcanics volcanics from from the the Clam Clam Falls Falls area area (Stop (Stop6a). 6a). Error ellipses ellipses and and regression regression age age uncertainties uncertainties represent represent 95 95 percent percent confidence confidencelimits limits(after (afterWirth Wirth and Gehrels, in press). 1.82 1.82 Analyses of the the Chengwatana Chengwatana rhyolites near Clam Clam Falls indicate indicate that magmatism magmatism began began by at least 1102 1102 Ma Ma in in this this portion portion of of the the rift, rift, considerably considerably earlier than the age reported by Zartman et ± 2.1 Zartman et al. (1997) (1997) for for rhyolite rhyolite (1094.6 2 2.1Ma) Ma)exposed exposednear near the the base base of the the Chengwatana Chengwatana section section in the southeastern southeasternlimb limb of the Ashland syncline. This This implies implies that that lower lower flows flows exposed exposed near Clam Clam Falls Falls likely likely formed formed contemporaneous contemporaneouswith with the the Upper Upper Kallander Kallander Creek Creek volcanics volcanicsand and Conglomerate of the Mellen Complex Complex of of Northern Northern Wisconsin, Wisconsin, and the Group 5 flows and Great Conglomerate Mamainse Point (Fig. 23). The The 1102± 1102 k55Ma Maage ageofofChengwatana Chengwatanavolcanics volcanics in in the Clam Falls 66 �Age (Ma) Minnesota Wisconsin Wisconsin Upper Mich Michipicoten Is. Is. NW Wise Wisc Mamainse Point Point 1085 1090 1095 1100 1105 1110 Figure23. 23. Diagram Diagramillustrating illustratingthe thecorrelation correlationof ofMidcontinent Midcontinentrift riftrocks rocks from from Figure Taylors Falls-Clam Falls region with those those of of northern northern Wisconsin Wisconsin and and the Taylors Superior. The The upper upper limit limit of of the theChengwatana Chengwatana volcanics volcanics is eastern Lake Superior. Zartman et et al. al. (1997) (1997) in in the the estimated on on the the basis basis of of U-Pb U-Pb zircon zircon studies studies by by Zartman estimated Ashland syncline; syncline; the the lower lower limit limit is is based on on the the age age of of Clam Clam Falls Falls rhyolite. rhyolite. Ashland The The age age of of Taylors TaylorsFalls Falls flows flows isisbased basedon ontheir theirpredominantly predominantly "normal" "normal" paleomagnetic paleomagneticsignature signature(Kean (Keanetetal., al.,1997) 1997)(after (afterWirth Wirthand andGehrels, Gehrels,in inpress). press). 67 �region implies that the lower Clam Falls flows may may have have "reversed" "reversed" paleomagnetic paleomagnetic directions directions since they are approximately 105approximately coeval with the the reverse-to-normal reverse-to-normal magnetic magnetic polarity polarity change (1 (11051100 Ma) zk 55 Ma) that that has has been been widely widely observed observedthroughout throughoutthe theLake LakeSuperior Superiorregion. region.The The 1102 1102± age of the the Clam Clam Falls Falls rhyolites rhyolitesalso also implies impliesthat that this this section section of of the the Chengwatana ChengwatanavOlcanics volcanics Ma age during a period regarded as the was erupted erupted during the transition transition between between the the"latent" "latent" and and "main-stage" "main-stage" of rift magmatism (Millerand 1996). magmatism (Miller andVervoort, Vervoort, 1996). Stop 6b -(Alternate) —(Alternate) Rhyolite Rhyolite near near the McKenzie McKenzie Creek Creek State State Hunting Hunting Grounds. Outcrops Outcrops are located on on 310th 3 10th Ave approximately 0.8 miles south and east of the intersection intersection Ave. SE with 315th Ave. SE 1/4, 114,SE SE 1/4, 114, Sec. 26, T 37 N, R 16W; 16 W; Lat. Lat. 450 45O 39' 15.1" N, 092° 18' Long. 092O 18'28.4" 28.4" W. W. Scannedimage imageof of aaportion portion of of the the USGS USGS Clam Clam Falls, Falls, Wisconsin Wisconsin topographic topographic Figure 24. Scanned quadrangle (7.5 minute series; quadrangle series; 1:24,000 1:24,000 scale) scale) showing showing the location of Stop 6b. This stop is an region. Although an alternate alternate stop stop in the the rhyolites rhyolites of the Clam Falls region. Although the the contact stop does does permit permit examination examination contact with with surrounding surroundingbasalts basaltsisisnot not exposed exposed at at this locality, the stop of a large exposure exposure of rhyolite. The Theoutcrops outcropsare arelocated locatedalong alongthe thenorth northside sideof of 310th 3 10thAve Ave after after the end of the pavement near near where where the the road curves south south around around outcrop. outcrop. This outcrop is the the eastemmost easternmostexposure exposure of of Chengwatana Chengwatana volcanics in the Clam Falls Falls region and defines the base of the region the exposed exposed section. At At this thislocation, location,aaseveral-meter-thick several-meter-thick section of rhyolite is exposed over an area area several severalhundred hundredsquare squaremeters. meters. The The rock rock consists consists of of rhyolite. No layering is is observed. observed. The pale purple and aphanitic rhyolite. No compositional compositional or textural textural layering outcrop outcrop is cut by several sets of closely-spaced sub-vertical sub-verticalfractures fracturesthat that have have coatings coatings of iron iron oxides and other alteration alteration minerals. Quartz-rich Quartz-richveins veinsup up to to several several millimeters millimetersthick thick are are also also commonly observed. commonly 68 �Stop near Godfrey Stop 7—Porphyritic 7-Porphyritic flows near Godfrey Lake, Lake, WI. Outcrops are located 0.72 miles west of of County County Rd RdIIon on 315 St. SE SE1/4, 1/4,NW NW1/4, 1/4,Sec. Sec.29, 29,TT 37 37 N, R 16 W; Lat. Lat. 45O 45° 39' 43.0" N, Long. 092O 092° 22' 16 W, 22' 43.4" 43.4" W. W. purpose of this stop is to examine some of the strongly plagioclase-porphyritic plagioclase-porphyritic flows The purpose flows features can be observed in the exposures of the Clam Falls region. Several Severalinteresting, interesting-features exposures along along the north north side side of of the road road at this stop. The the Theeastern eastern end end of of the the exposure exposure consists consists of coarse basalt with porphyritic basalt* large (<3 cm cm long) long) large plagioclase phenocrysts phenocrysts plagioclase (40-60 percent) in aa (40-60 percent) groundmass groundmass consisting consisting of microcrysts of and plagioclase secondary minerals; secondary minerals; no minerals primary mafic minerals are present. Possible zoning structures are zoning still apparent is some of the feldspar phenocrysts. The The rock has been extensively extensively altered as evidenced evidenced by altered 25. Scanned image image of a portion of the strongly Figure 25. the USGS USGS Clam Clam Falls, Falls, the tpographic quadrangle and 1:24,000scale) scale) saussuritized Wisconsin topographic quadrangle(7.5-minute (7.5-minuteseries; series;1:24,000 showing the location epidotized plagioclase, plagioclase, location of Stop Stop 7. abundant chlorite, disseminated iron-oxides, quartz, and secondary feldspar (pink). Near the top of the the porphyritic porphyritic disseminated iron-oxides, quartz, centimeter thick "epidosite" "epidosite" that is is sub-parallel sub-parallel to toflow flowlayering. layering. This This epidosite epidosite is basalt is a ten centimeter similar interfiow sediments sediments seen at other localities. localities. Further similar to many intefflow Further west is a sparsely porphyritic basalt and vesicular basalt that basalt, which is overlain overlain by repeating layers of fine-grained basalt might represent several thin flow units. Some Somecross-cutting cross-cuttingvesicular vesicular basalt is also also observed. observed. Other coarse coarse porphyritic porphyritic basalts basalts can be found found several hundred meters south south of this this stop. stop. plagioclase phenocrysts phenocrysts typically comprise 15-25 percent percent of ofthe therock. rock. Intefflow Interfiow sediment sediment There, plagioclase is also found. Other exposures of coarse porphyritic basalt are also found further south and Other exposures of coarse porphyritic west of this stop (one mile NW of Straight Lake). Lake). The The orientations orientations of flows in this area are NW On poorly known, known, but but they theygenerally generallytrend trendNNE NNEand anddip dipgently gently(—15° (-15' ) to the NW. On the the basis of these considered to be these general general trends, trends, and and the the similarity similarityof rock types, these units are tentatively considered the same same stratigraphic stratigraphicunit. Considering Consideringthat thatflows flowswith with coarse coarse porphyritic porphyritic textures textures are are relatively relatively rare in the Chengwatana volcanics (Eagle Peak flows near St. Croix Falls and stop in the Clam Clam Chengwatana Falls in the region), it is also Falls region), also tempting tempting to suggest suggest that these geographically disparate disparate flow units units might might event. A be a roughly roughly coeval coeval eruptive event. A large large sample sample from this outcrop outcrop was processed processed for baddeleyite and zircon, but did not baddeleyite and not yield yield either either in in sufficient sufficient quantity quantity for for U-Pb U-Pb analysis. analysis. Preliminary Preliminaryinterpretation interpretationof of new new USGS USGS aeromagnetic aeromagneticdata data suggests suggeststhat that the the porphyritic porphyritic basalts basalts at Stop 7 are are stratigraphically stratigraphically below those of Stop Stop 3a. 3a. Additional geochemical geochemical and stratigraphic stratigraphic studies are needed to confirm confirm this hypothesis. 69 ' �REFERENCES CITED CITED Abbott, Abbott, K.M., Thole, J.R., and and Wirth, Wirth, K.R., K.R., (in (in press), press), Petrography Petrography and and geochemistry geochemistry of Midcontinent rift rhyolite (Chengwatana volcanics) volcanics) near near Clam Clam Falls, Falls, Wisconsin: Wisconsin: 44th Annual Annual Institute on Lake Geology,Proceedings. Instituteon LakeSuperior SuperiorGeology, Proceedings. 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Zhu, Consortium for continental continental reflection reflection profiling Michigan Michigan surveys: Zhu, T., and Brown, L., 1986, 1986, Consortium surveys: reprocessing and results: results: Journal of Geophysical Geophysical Research, Research, v. 91, p. 11477-11495. 11477-11495. 75 ��FIELD FIELD TRIP TRIP#3 #3 GLACIAL GLACIAL EXOTICA OF OF THE THE TWIN TWIN CITIES CITIES AREA Leaders: Leaders: Howard HowardHobbs, Hobbs, Alan Alan Knaeble, Knaeble, Gary Gary Meyer Meyer OVERVIEW OF OF LATE QUATERNARY QUATERNARY GEOLOGIC OVERVIEW GEOLOGIC HISTORY Howard Howard Hobbs Hobbs The The landforms landformsand andsurface surfacesediments sedimentsof of this this area area formed formed during during the the most most recent recent glaciation— glaciation- the late Wisconsinan. The Thearea areawas was covered covered by by previous previous glaciations some 10 10 to 20 times, but sediments sediments from from these these earlier earlier glaciations glaciationshave have been been partially partially eroded eroded by by subsequent subsequentglaciations glaciations and are are now now deeply deeply buried buried by sediments sediments deposited during the last glaciation. Although Although the the volume of late Wisconsinan drift in Minnesota is less than half the volume of all the older drifts volume of late Wisconsinan drift in Minnesota is less than the volume all older drifts put put together, together, the the amount amountremaining remainingper per glaciation glaciationisisso somuch muchmore morethat that itit can can be be explained explainedonly only by recycling new drift drift sheet. In recycling large large amounts amounts of older glacial sediment into each new In other other words, words, only aa portion portion of of the the sediment sediment that that we we will will see see was was brought brought from from its bedrock bedrock source source to its its present present position position in in one one glaciation. glaciation. The The Twin Twin Cities-St. Cities-St. Cloud Cloud area area is is dominated dominated by by the the deposits deposits of of two two ice ice lobes, lobes, each each from fromaa different direction (Fig. 1). 1). The TheSuperior Superiorlobe lobeadvanced advancedinto intocentral centralMinnesota Minnesotathrough throughthe the Lake Superior Superior basin from from northeast. Its Itstill tillisisreddish-brown, reddish-brown,sandy sandyto toloamy, loamy,and andrich rich in inred red and and black black rocks rocks from from the the Superior Superiorbasin and and the north shore of Lake Superior. Typical Typical rocks are red sandstone basalt and and gabbro. gabbro. Within the Superior sandstone and siltstone, rhyolite and granophyre, granophyre, basalt basin, basin, the the Superior Superiorlobe lobedrift drift contains containspractically practically no no carbonate carbonaterocks rocks or or matrix matrixcarbonate, carbonate,but butin in the field field trip trip area area ititcontains containsaamodest modestamount, amount,presumably presumably recycled recycledfrom fromolder, older,calcareous calcareous drifts. drifts. The St. St. Croix Croix moraine moraine roughly roughly marks marks the the late late Wisconsinan Wisconsinan maximum of the Superior Superior lobe. lobe. There Thereisisevidence evidencefrom frombeyond beyondthe themoraine moraineof of aa somewhat somewhat more more extensive extensive lobe that may have preceded moraine-building, but but this this zone zone is is not not very very wide. wide. On the southeast southeast side of the the lobe, the moraine moraine is is wide wide and andhigh. high. It ranges lobe, from from south south of Minneapolis Minneapolis to the St. Croix River, the from 5-10 5-10 miles miles wide, wide, and and the the thickness thicknessof of morainic morainicmaterial material ranges ranges from from100-300 100-300feet. feet. Its Its surface development surface morphology morphology is is highly highly collapsed, collapsed, indicating indicating that the final event of moraine development was was the the melting melting of of stagnant stagnantice icefrom frombemeath bemeath thick thickdebris. debris. At the west west end end of this this same same segment, segment, the moraine moraine turns sharply sharply and and runs northwest northwest to to the the St. Cloud area. Most Mostof ofthis thissegment segmentisisburied buried by by glacial glacial sediment sediment of of the the Des Des Moines Moines lobe, lobe, so so is not known in detail. Only the highest parts (generally kames) are exposed at the surface (for Only the highest parts (generally kames) are exposed surface example, Stop 1). 1). These Thesekames kamesmay may represent represent fans fans of of sand sand and and gravel built at the edge of the ice by subglacial subglacial tunnel tunnel valleys valleys (Fig. (Fig. 1.1). 1.1). The Thetop topof ofthe themoraine morainein inthis thissegment segmentisislower, lower,due due to a combination combination of the following following factors: the thebase baseon on which whichitit was was built built seems seems to to be lower; lower; the the average thickness of moraine material is less; and some of the material was eroded off by the overriding overridingDes Des Moines Moines lobe. lobe. Inasmuch Inasmuchas asthis this segment segmentrepresents represents the frontal frontal part of the moraine, the debris debris may may have have been been spread spread over over aa wider wider zone zone by minor minor fluctuations fluctuations of of the the ice ice front. front. 79 �EII1 Park 1/ 4 - do . '- :' 4Bodu I, ok A pr1 pers C 0O 0 EXPLANATION hp td do Terraces of the Mississippi River Des Moines lobe outwash (includes sandy lake sediment in eastern part) dag, dpg, Des Moines lobe ground moraine dbg Des Moines lobe stagnation moraine das dpe, dbe Des Moines lobe end moraine Superior lobe outwash so Superior lobe ground moraine ssg Superior lobe end moraine sse Figure Figure 1. 1.Simplified SimplifiedQuaternary Quaternarygeology geologyofofthe thefield fieldtrip triparea, area,showing showingmajor majorroads roadsand andlocation locationofofstops. stops. �About 15 15 miles miles south south of of St. St. Cloud, Cloud, the the moraine moraine begins begins to to emerge emerge from from under under its its Des Des About Moines Moines lobe lobe cover. cover. The Thehigher higherparts partsof ofthe themoraine moraineform formisolated isolated bumpy bumpy knobs, knobs, while while the the lower lowerparts partsare arecovered coveredwith withoutwash outwashof of the the Des Des Moines Moines lobe. lobe. Many Manyof ofthe thehigher higherparts parts appear appear to to be be ice-thrust ice-thrustfeatures, features,such suchas asthe theone oneatatStop Stop2.2. The The Des Des Moines Moineslobe lobeadvanced advanced from from the the northwest, northwest,following following the the broad broad lowland lowland now now occupied occupiedby bythe theMinnesota MinnesotaRiver RiverValley. Valley. Its Its till till isisgray gray(oxidizing (oxidizingto toolive olivebrown), brown), loamy loamy to to clayey, clayey, and and rich rich in in sedimentary sedimentaryrock rock fragments—mainly fragments-mainly carbonate carbonate and and gray shale. shale. Granite Graniteand and Anoffshoot offshootof ofthe theDes DesMoines Moineslobe, lobe,called calledthe the greenstonefragments fragmentsare are also alsofairly fairlycommon. common. An greenstone Grantsburg Grantsburgsublobe, sublobe,extended extendednortheast northeastfrom fromthe themain mainlobe, lobe,burying buryingthe theSt. St.Croix Croixmoraine, moraine,and and advancing over land that had recently been vacated by the Superior lobe. advancing over land that had recently been vacated by the Superior lobe. ItIt appears appearsthat thatthe theGrantsburg Grantsburg sublobe sublobe was was aa late late advance, advance, during a time when when the the main main Des Moines lobe was retreating. The retreat was interrupted by many minor re-advances, each Des Moines lobe was retreating. The retreat was interrupted by many minor re-advances, each one lobeApparently the the Bemis Bemis advance advance of of the the Des DesMoines Moineslobe— one less less extensive extensive than than the thelast. last. Apparently 14,000 14,000years years ago— ago- was was blocked blocked and and diverted diverted by the St. Croix moraine. How Howwas wasthis thismoraine moraine able ableto toblock blockthe theearliest, earliest,most mostextensive extensiveadvance, advance,and andyet yet was was overridden overriddenby by aa later, later, less less powerful powerful pulse? pulse? The Themost mostreasonable reasonableexplanation explanationisisthat thatearly earlyon onstagnant stagnantice iceburied buriedininthe themoraine moraine made madeititeffectively effectivelyhigher. higher.As Asthe theice icemelted, melted,the thebarrier barrierbecame becamelow lowenough enoughto tobe bebreached breachedby by the theadvancing advancingice. ice. One One could could postulate postulate that that at at the the time time of of the theBemis Bemisadvance, advance, the the St. St. Croix Croixmoraine moraine was was backed sublobeto to backedby byactive activeglacier glacierice, ice,which whichlater laterretreated retreatedenough enoughto toallow allowthe theGrantsburg Grantsburgsublobe cross crossthe themoraine. moraine.However, However,glacial glaciallake lakedeposits depositshave havebeen beendiscovered discoveredwell well back back from fromthe the St. St. Croix moraine, underneath sediments of the Grantsburg sublobe. Johnson and Hemstad (in Croix moraine, underneath sediments of the Grantsburg sublobe. Johnson and Hemstad (in prep.) varve couplets couplets from from the the thickest thickest part part of of the the lake lake sediments sediments as as prep.) estimate estimate at at least least 900 900varve extrapolated extrapolatedfrom fromthinner thinnercounted countedsections. sections.This Thissuggests suggeststhat thatactive activeice ice had had melted melted back back from from the themoraine morainelong longbefore beforethe theGrantsburg Grantsburgadvance. advance. The TheDes DesMoines Moineslobe lobeadvance advanceover overthe theSt. St.Croix Croixmoraine morainewas was in in two two phases: phases:first firsttotocross cross was wasthe theDuelm DuelmBulge Bulge(Stops (Stops44and and5). 5). The Thenorthern northern limit limit of of this thisadvance advanceisisdefined definedby by the the Duelm Duelmchannel, channel,which whichcarried carriedthe thedischarge dischargeof ofthe theupper upperMississippi MississippiRiver Riveraround aroundthe thebulge bulgeof of ice. The eastern limit of the bulge is less certain, but it may have been pinned by the high ice. The eastern limit of the bulge is less certain, but it may have been pinned by the high ground groundnorth northofofElk ElkRiver River(Stop (Stop7). 7).This Thisadvance advanceappears appearstotohave havebeen beenshort-lived, short-lived,and andquickly quickly succeeded succeededby bythe theadvance advanceofofthe theGrantsburg Grantsburgsublobe. sublobe. The TheGrantsburg Grantsburgsublobe sublobecrossed crossedthe theSt. St.Croix Croixmoraine moraineover overaabroad broadfront, front,and andadvanced advancedtoto the thenortheast, northeast,past pastthe theSt. St.Croix Croixriver rivertotothe thetown townof ofGrantsburg, Grantsburg,Wisconsin. Wisconsin. ItItwas washemmed hemmedinin by bythe theback backside sideofofthe theSt. St.Croix Croixmoraine moraineon onthe thesoutheast southeastside, side,and andby byhigh highground groundtotothe thenorth. north. The TheGrantsburg Grantsburgappears appearstotohave havebeen beenone onemain mainpulse, pulse,followed followedby bywidespread widespreadstagnation stagnationand and melting. melting. Till Tillof ofthe theGrantsburg Grantsburgadvance advanceisisquite quitevariable variablein in composition; composition;ininplaces, places,ititisisessentially essentially the thesame sameasastill tillofofthe theDes DesMoines Moineslobe. lobe.InInothers, others,ititisisbanded bandedred redand andgray, gray,on onthe thescale scaleofoffeet feet totoinches. inches.InInother otherplaces, places,ititappears appearsthe thesame sameasastill tillofofthe theSuperior Superiorlobe, lobe,except exceptfor foraatrace traceof of shale. sublobeincorporated incorporatedaalarge largeamount amountof ofSuperior Superior shale.Apparently, Apparently,the theice iceofofthe theGrantsburg Grantsburgsublobe lobe lobedebris debrisasasititwas waspassing passingover overthe theSt. St.Croix Croixmoraine. moraine. This Thisdebris debriswas waspartially, partially,but butnot not completely, completely,mixed mixedwith withsediment sedimentalready already in in the the ice. ice. InInplaces, places,especially especiallyininthe thecenter centerofofthe the lobe, lobe,Grantsburg Grantsburgtill tillisisthin, thin,orormissing missingaltogether. altogether. 81 �The ice melted melted more rapidly in the center center than along along the margins, presumably presumably because it GlacialLake Lake Anoka Anoka formed formed in the center center of the sublobe, sublobe, fed by large large had less of a debris cover. Glacial amounts Of of meltwater amounts meltwater from the upper Mississippi (a continuation of the same flow that eroded the Duelm Channel). Channel). The Thelake lakedeclined declinedin instages stagesas asice icemelted melted out out of of the the margins, margins,uncovering uncovering successively lower outlets into the St. Croix Croix River, River, and finally finally drained drained altogether altogether down down the the Mississippi River. This This drainage drainageevent event connected connectedthe the upper upper and and lower lower parts parts of the the Mississippi Mississippi for the first time in its modern course. course. of two two parts: parts: the The Anoka Sand Plain is composed of the lakebottom lakebottom sediment sediment (mostly (mostly fine fine plain-delta which extends from the Mississippi valley train train into the outwash plain-delta sand), and a sandy outwash lake. The Thelake lakewas wasshallow, shallow,and and apparently apparentlymost most of of the the silt silt and and clay clay from from the the meitwater meltwater stayed stayed until it was carried downstream. between the the delta and the in suspension suspension until downstream. The The boundary boundary between lakebottom sediment lakebottom sedimentisisdifficult difficultto to pin pin down, down, because becausethe the texture textureof of the the sand sand is is not much much different, different, and because the boundary boundary shifted shifted as as the lake filled in. STOP 1—Superior ValleyFan Fan Sand Sand and and Gravel Buried by Superior 1ÑSuperio Lobe Tunnel Valley SuperiorLobe Lobe and and Grantsburg GrantsburgSublobe SublobeTill. Till.(Gary (GaryMeyer) Meyer) A radiocarbon radiocarbon date 32,300±2000 obtained from from wood fragments fragments date of 32,300 k2000years yearsB.P. B.P. (Beta-40098) (Beta-40098) obtained in a core core sample sample taken taken below below 130 130 feet feet of of Superior Superior lobe lobe sand sand and and gravel gravel in Ramsey Ramsey County County (Meyer, 1992), 1992), gives a possible maximum age for the advance advance of the Superior Superior lobe lobe into into easteastcentral Minnesota. As Asthe thefront frontof of the the Superior Superiorlobe lobe became stabilized, stabilized, it formed the St. Croix moraine—a complex the east, east, south, south, and and moraine-a complexlinear linear series seriesof of high high hills encircling encircling the Twin Cities on the paralleling the Mississippi Cloud area area (Fig. (Fig. 1.1). 1.1). During the eventual west, and paralleling Mississippi River to the St. Cloud retreat of the ice front, front, meltwater meltwater issuing issuing from the Superior lobe became channelized through the moraine. The TheSuperior-provenance Superior-provenance(Table (Table 1.1) 1.1) sand sand and gravel deposit at Maple Grove in northeast Hennepin Hennepin County County (Fig. (Fig. 1.1), 1.l), extensively exposed in gravel pits (Fig. 1.2), was likely deposited during formation of the North 1992),which which mark mark a retreatal retreatal deposited during formation of the North Ramsey Ramsey mounds mounds (Patterson, (Patterson, 1992), position of the Superior Superior lobe lobe behind the the St. St. Croix Croix moraine. Table 1.1. General Generalcharacteristics characteristicsof of mapped mapped glacial glacial deposits of ofthe theTwin Twin Cities Cities area. area. texture Till texture Mountain Riding Mountain Provenance Provenance loamy Superior Superior Provenance Provenance sandy sandy Color oxidized oxidized unoxidized unoxidized yellow brown to olive olive brown gray gray red brown gray to to red red gray gray gray common common uncommon to common common rare to uncommon common to abundant abundant rare to uncommon uncommon uncommon to abundant abundant common absent absent Feature Pebble type white to to buff carbonate carbonate dark gray to gray-green rocks red felsite and sandstone sandstone gray shale shale 82 �u lMTED STATES Figure moraine in east-central east-central Minnesota Figure 1.1. 1.1. Tunnel Tunnelvalleys, valleys,valley-mouth valley-mouthfans, fans, and and the St. Croix moraine and adjoining counties in Wisconsin. Wisconsin. Lettered features are landforms landforms discussed discussed in inthe thetext: text: M, Maple Grove fan; E, Elk River fan complex. complex. Modified Modified from Patterson (1994). The The Maple Maple Grove Grove sand sand and and gravel gravel deposit depositwas was laid laid down down as as aa fan fan at at the the mouth mouth of of aa tunnel tunnel valley—a drainageway (Patterson, (Patterson, 1994). 1994). The trend valley-a deeply incised, broad, subglacial drainageway trend of of the the tunnel valley that fed the fan is is marked marked to to the the northeast northeast in in Anoka County by aa chain chain of of lakes lakes (Fig. 1.1) 1.1) formed formed when thick thick ice filling filling the valley later melted. The The sand sand and and gravel gravel laid laid down down at the mouth of the valley evidently buried stagnant ice, as the fan deposit is deeply pitted (Fig. 1.2). The The buried buried ice ice was was well-insulated, well-insulated, as as it apparently was still melting during a readvance of the Superior gravel below reddish reddish sandy till, and then then during during an Superior lobe, lobe, which which buried buried the the sand sand and grave1 which further buried buried the the sand and gravel with yellowish advance of the Grantsburg sublobe, which clayey till. The Thesand sandand andgravel gravelisis thickest thickest where where the the buried ice was thin, forming ridges in the postglacial landscape. landscape. Areas Areasunderlain underlain by by the the thickest thickest ice became depressions into which first Superior lobe till from Superior from the the readvance readvance (Automba?) (Automba?) phase flowed, flowed, and and then, then, following following final final deglaciation, both finally completely completely deglaciation, both Superior Superiorlobe lobe and and Grantsburg Grantsburg sublobe sublobe tills flowed flowed as the ice finally melted, deposited. A log at the base of collapsed melted, apparently apparently thousands thousandsof of years years after after it was first deposited. lake sediments and Superior Superior lobe lobe tills, tills, exposed in a Maple sediments overlying overlying collapsed Grantsburg Grantsburg and Maple Grove fan pit, yielded a date date of of 11,930±60 11,93e60radiocarbon radiocarbon years years B.P. B.P. (Beta-23746; Meyer and Hobbs, 1989), 1989), indicating indicating that that buried buried St. St. Croix Croix phase ice was still melting less than 12,000 12,000 years years ago. ago. 83 83 �9/ yr- jj ci o a Q J)I Ik0 TH II 0 0 / a 0riggI lemT I t U— - - r Y.°'— .'c 4r'/t9r( _.; . .. . . . Figure 1.2. 1.2. Topographic Topographicmap mapwith with Maple Maple Grove Grove fan deposit outlined. The The landform landform likely likely extended extended to to the Figure sublobe meltwater. meltwater. Area shown is is about about 2.8 2.8 miles miles wide; wide; northeast prior to being planed off by Grantsburg sublobe contour contour interval interval 10 10 feet. feet. (U.S. (U.S. Geological Geological Survey Survey Osseo Osseo quadrangle, quadrangle, 7.5-minute series, series, 1967.) 1967.) The movement depressions during during differential differential ice-block ice-block movement of the bulk of overlying tills into depressions melt-out melt-out reduced reducedthe the"overburden" "overburden7'thickness thicknessabove abovethe thesand sandand andgravel, gravel,allowing allowingitit to to be exploited exploited for construction construction aggregate. aggregate. Gravel Gravelmining mininghas hasinverted inverted the the topography once once again, again, turning the gravel ridges into deep pits and the till-filled depressions into into hills. hills. Gravel from the Maple till-filled depressions Grove Cities area area as as ititconsists consistsmostly mostly of ofPrecambrian Precambrian Grove fan fan deposit deposit is is the most durable durable in the Twin Cities rock clasts (Meyer and Jirsa7 ursa, 1984). 1984). Superior Superiorlobe lobe deposits deposits to to the the east east and and southeast southeast contain greater greater amounts amountsof of locally-derived, locally-derived,relatively relativelysoft softPaleozoic Paleozoicclasts, clasts,eroded erodedas asthe thelobe lobemoved movedup up the escarpment others, 1986). Des Moines lobe and escarpment formed formed by the Oneota Dolomite (Jirsa and others, Grantsburg sublobe deposits contain clasts of both Paleozoic carbonate and Cretaceous shale, as they are of Riding Riding Mountain Mountainprovenance provenance (Table (Table 1.1). 1.1). 84 �%Powder Ridge Site-Glacial Thrusting Cretaceous STOP 2—Powder Ridge Borrow Borrow Pit Site—Glacial Thrusting of of Drift and Cretaceous (Alan Knaeble) Knaeble) Bedrock. (Alan A located in the St. Croix moraine (Fig. 2.1). The moraine was deposited A large large borrow borrow pit pit is located deposited prior prior to to 16,150 16,150years years BP BP (Birks, (Birks,1976) 1976)when when the the Superior Superior lobe was at its maximum. This Thissite siteisis perched Moines lobe lobe outwash outwash plain plain (Fig. (Fig. 2.2). 2.2). The perched about about 225 ftft above above the surrounding Des Moines borrow to0build up the summit of the ski hill and and regrade borrow pit pit was was excavated excavated to to supply supply material 1 existing existing ski-run slopes. The The pit pit isis 1/4 114mile mile long long and and oriented oriented north-south. north-south. There There are are good good exposures wails (Fig. 2.3). 2.3). Six texturally and lithologically distinct exposureson on the the north, north, west, and east walls (K); pre pre Wisconsin Wisconsin sand sand (S); (S); are recognized recognized in in the the pit pit including: including: Cretaceous Cretaceous shale shale (K); units are unoxidized, dark dark gray, gray, clayey clayey pre-Wisconsin pre-Wisconsin till (Elmdale (Elmdale till) till) (wt-w); (wt-w); unoxidized, unoxidized,dark darkredredunoxidized, brown pre-Wisconsin till ("Old ("Old Red Red till") till") (st-x); (st-x);oxidized, oxidized,yellow-brown yellow-brown or orunoxidized, unoxidized,gray gray pre-Wisconsin pre-Wisconsin till (wt-x); (wt-x); and and reddish-brown, reddish-brown, sandy sandy late late Wisconsin Wisconsin till, till, lake lake sediments, sediments,and and sandlgravel identification sand/gravel (Superior (Superior lobe deposits) (st). Fossil Fossil and and mineral mineral content content have helped identification of of the Cretaceous Cretaceousdeposits, deposits,and and texture texture and and grain grain count count analyses analysesindicate indicate that that at at least least four four glacial glacial advances advances are are represented represented(Table (Table2.1; 2.1; Fig. Fig. 2.4) 2.4) (Knaeble, (Knaeble,1996). 1996). - E St. Croix moraine Payn.wC. Thrusthills IOMILES o • Thrust site I Fo a 10 - --- 15 KMOMETERS . . Figure 2.1. Simplified Simplifiedmap mapof ofStearns steamsCounty Countyshowing showingthe thelocation locationof of thrust thrust masses masses Figure (hills) (hills) within the St. St. Croix Croix Moraine and 22 thrust sites. From FromKnaeble Knaeble(1996). (1996). Exposures in the the pit pit walls walls show show that that these these sediments sediments form slabs slabs 50-250 50-250 ft thick, thick, which which appear appear to extend extend laterally for more than 400 ft. The The slab slab contacts contacts dip dip 30°-60° 30°-60north-northeast, north-northeast, in for the the Superior Superiorlobe. lobe. The The internal internal stratification stratification in what would would have been the up-glacier direction for ft 50-100 ft within these slabs slabs largely largely subparailels subparallels slab slab boundaries, although although two zones, each 50-100 withiI these thick, are intensely deformed; the sediments are sheared, streaked, mixed and, in the intensely eaked, mixed and, in the intensely thick, are intensely deformed; the sediments are sheared, str rned zone, 7nne dicnlmr distorted dictnrtpd overturned nvprhirnpd fnldc Rnqpd on deformed display folds. Based ----on comparisons comparisons with with undeformed undeformed defonAawu uALw, exposures exposures in in Stearns SteamsCounty, County, the the sequence sequence of sediments sediments in in this pit can be divided into at least 6 thrust blocks. Each Eachthrust thrustblock blockcontains containstwo twoor ormore moredistinct distinctunits unitsdifferentiated differentiatedby by color, color, texture, and lithology. block. Superior lithology. The The youngest youngest till (unit st) caps each thrust block. Superior lobe lobe ice, ice, associated with deposition of this youngest till, is interpreted to have caused the thrusting associated deposition of youngest (Knaeble, (Knaeble, 1996). 1996). 85 85 uawy*aJ -au..-A.-- �I I glaciotectonic thrusting Figure 2.2. 2.2. Topographic Topographicmap map showing showing the powder ridge ski pit, a site of glaciotectonic Figure on N., R. R. 29 29W.). on the the St. St. Croix Croix Moraine Moraine (sec. (sec. 27, 27, T. 22 N., W.). Hachured Hachured lines lines indicate indicate scarps; scarps; hachures hachures on on downslope downslope side. Area Areashown shownisis2.3 2.3 miles miles wide; wide; contour contourinterval interval10 10feet feet(U.S. (U.S.Geological Geological Survey Kimball quadrangle, 7.5-minute series, 1967). From Knaeble (1996). From Knaeble (1996). 86 �EXPLANATION Fill Unexpose area or Slope wash Superior lobe lull (Pierz). in places Containing inlerbegs of Superior lobe lacuustrine sedumenf (.1) and Superior lobe sand and gravel deposits (so) Winnipeg provenance n till )Sauk Centre or Meyer Lake) v till (Old Red) Winnipeg provenance w till (Elmdale) Sand (pre-lafe Wisconsin) Cretaceous marine Shale Thrust fault; sawteeth in upper plate Fault, Showing relative horizontal motion; approximate location where dashed —— — Unit confacl, approximate location where dashed Highly deformed mined malerial Superior provenance I. 0 tOO 300 feet Vertical exaggera(l Figure thrust blocks. blocks. Quaternary sediments sediments and Cretaceous Figure 2.3. Powder PowderRidge Ridgeski ski pit showing showing interpreted thrust clays are exposed on the north, east, and west west walls walls of of the the pit; pit; the exposure exposure on the west wall has been extended through to the outside face face of ofthe theblock blockdiagram. diagram. See Seefigures figures 2.1 2.1 and.2.2 and 2.2 for forpit pitlocation. location. From Knaeble Knaeble (1996). (1996). Table 2.1. Matrix Matrixtexture textureof of glacial glacialtills tills in in the the Powder Powder Ridge Ridge ski ski pit. Unit Percent of Sand Sand Silt No. of Samples Samples Clay Clay wt-w 4 29 29 40 31 31 st-x st-x 6 41 41 34 34 25 25 Meyer Lake 5 28 57 57 15 15 Sauk Sauk Centre Centre 8 8 41 41 41 41 18 18 20 58 58 29 29 wt-x wt-x St st 87 . 13 13 �Clay Clay 100% 100% UMTS • wt-w + st-x • wt-x o wt-x St Figure 2.4. Triangular Triangular plot of matrix texture of glacial till units in the Powder Ridge ski ski units Powder Ridge pit. From FromKnaeble Knaeble (in (in prep.). U.. U ••S• c&o • 100% Sand Sand 100% 50% Silt The The exposure exposure on on the west west wall at at the south south end of the pit, shows Cretaceous marine shale shale 10-foot-thick packet of deformed deformed Superior lobe till, which contains contains streaks streaks of of overlying a 10-foot-thick incorporated older till. The deformed Superior lobe till overlies 25-30 ft of very well sorted, The deformed Superior finely bedded, finepre-late Wisconsin, Wisconsin, which in turn finely bedded, fine- to medium-grained medium-grained sand sand interpreted interpreted to to be pre-late overlies overlies more Cretaceous Cretaceous shale. shale. The Thecontacts contactsbetween betweenthese theseunits units dip dip 30°-60° 30°-60north and and indicate indicate south-directed thrust block block overlying overlying younger south-directedthrusting thrustingwith withaa Cretaceous Cretaceousunit unit at at the base of one thrust units in another another thrust thrust block. Drill Drillcore coreof of the the Cretaceous Cretaceousshale shale at at the south south end of the pit shows them to extend to a depth of at least 56 ft, where the drill core ended. The elevation at the the top of extend ft, where the drill core ended. the Cretaceous shale is about about 1300 1300 ft; ft; yet well well logs logs show show the highest undisturbed Cretaceous level. Some Some of the thrust blocks contain contain bedrock in the region to be less than 1100 1100 ft above sea level. Cretaceous top of of other other thrust thrust Cretaceous shale, shale, which which is is interpreted interpreted to have been exhumed and stacked on top blocks to form form the moraine moraine during during ice ice advance advance (Knaeble, (Knaeble, 1996). 1996). wide at the the Powder PowderRidge Ridge ski skipit. pit. Water-well Water-well The St. Croix end moraine is about one mile wide records for mile northwest northwest records for three three wells wells located located on on the flanks flanks of the end moraine (two about a half mile of the pit, one one located located about about aa quarter quarter mile northeast northeast of the pit) also describe materials that appear appear to be thrusted thrusted (i.e., (i.e., Cretaceous Cretaceousdeposits depositsover over till, till, and and repetitive repetitive sequential sequentialcolor color changes changes in the till units). units). There is evidence evidence that that the the presence presence of permafrost (Mackay (Mackay and Mathews, Mathews, 1964; 1964; Clayton Clayton increase in in pore-water pore-water pressure pressure and Moran, 1974; 1974; Bluemle and Clayton, Clayton, 1984; 1984;Aber, 1988), an increase in the underlying underlying sediment sediment(Clayton (Claytonand andMoran, Moran,1974; 1974;Bluemle Bluemleand andClayton, Clayton,1984; 1984;Aber, Aber,1988), 1988), Aber, 1988) are conditions that and sufficient ice-flow velocities (Clayton and Moran, 1974; Aber, contribute to thrusting thrusting of substrata substrata at ice margins. When Whenthe theSuperior Superiorlobe lobeadvanced advancedover over soft soft bedrock, such as Cretaceous shale, or older glacial deposits, these conditions may have been as Cretaceous shale, or older glacial deposits, these conditions have been present and appear appear to to have have fostered fostered thrusting thrusting (Fig. (Fig. 2.5) during during the formation formation of the the St. St. Croix Croix moraine at Powder Powder Ridge. Ridge. 88 �Zuul—. 'I // \—_;.u__ \'_ — — —— 7, .. ,, -u '' 7,s_... • /\ 7/._.. . , _'•- —' — — - #•%S _ /' - — _% 'ui — , %_ \ 'S - 1s __u_u— — —I —S — -. _ .. — F I Is/i ,%7.,I, — 55 .2s I5./ ,./ — .2 .. — — '.. I ..S_I_\ — sI —,.;.. * — -' — %s '%S '\ ,%—;_ S_ — .. ',, \S S Ss1 — ' 7 — —-' ,• .'' _'L. — .2 •S__4•. I S , / 5, '5 — _.g_l' — :L L''-'.7' '•'_i r''——— '—''——— '—'r5 — Bedrock Surface Cretaceous Precambrian — — 5,5 J ———'5' 7,5,,,—5—— — —— i, — I_,,\ \_ 7, I_5—%_'S_ 5, SJ 1_ — — . .2 ': %___ ,: — — -__S... S — • _''_—— —' s._ ''SA \__ '_/t _.ez.' 'ui SS .— . ,','_',/_'. _,'..'/_!.I — __ _SS — __ — — I !. '—S . _,— L .2 .. ' —— '\ '11ui'' ' S/*'S'' I_ \__ '_'—, — '—— ,./ —— I, 5 __/% —,';— •__ us/u / z _;'u•_ ,— — .b —— — — ;_ : I—I / .j. ---I... _'5. Ice-Thrust Sites Figure 2.5. 2.5. Geologic Geologicmap mapofofSteams StemsCounty Countyshowing showingthe theSt. St.Croix Croix Figure moraine and the association of ice-thrusts with the underlying moraine and the association of ice-thrusts with the underlyingbedrock. bedrock. Powder POW^& Ridge Ridge Borrow Borrow Pit Pit STOP Thrusting of Cretaceous STOP3—Merden 3-Merden Lake Lake Esker EskerGravel GravelPit—Small-scale Pitamall-scale Thrusting Cretaceous Bedrock Bedrock and and Glacial Glacial Drift. Drift. (Alan (Alan Knaeble) Knaeble) The gravel gravel pit cuts through a section section of the Merden Lake esker and provides evidence for small-scale thrusting that is not associated with with end-moraine end-moraine formation formation(Fig. (Fig.2.1). 2.1). The esker formed formed beneath beneath the the Superior Superiorlobe lobeas asthe the ice ice advanced advanced towards towards the the southwest. southwest. Typical esker esker deposits deposits are are exposed exposedalong alongthe theeast eastwall wallof ofthe the pit. pit. Here, Here, about about 20 20 ftft of of Superior Superiorlobe lobe sand sand and and gravel gravelare are overlain overlain by aa thin 5-foot 5-foot layer layer of sandy sandy Superior Superiorlobe lobe till till (Fig. (Fig. 3.1). In represent Inthe the northwest northwestwall, wall,aa Superior-lobe-source Superior-lobe-sourcesand sand and and gravel gravel is also interpreted to represent the Superior lobe lobe till that has incorporated incorporated the core core of an an esker. Mantling Mantlingthis thiscore coreisis aa 5-10-foot 5- 10-footcap cap of Superior aa 3-5-foot Cretaceous shale 3-5-foot slab slab of old 'Elmdale 'Elmdale till' till' of of Meyer (1986). Thin Thin layers layers of dark gray Cretaceous are present present at the base of the Superior Superior lobe till cap. The The Cretaceous Cretaceous shale shaleand and the the older older Elmdale Elmdale till till are are interpreted interpretedas as substrata substratathat that were were picked up up by Superior-lobe Superior-lobe ice, transported transported at the base of the glacier, glacier, and and then deposited deposited on top top of the esker esker as as the ice ice wasted wasted (Knaeble, (Knaeble, 1996). 1996). Scattered in the sand and gravel on the pit floor you can find pebbles and cobbles of pisolitic lateriticclay. clay. These Theseclasts clastsare areinterpreted interpretedto tobe be derived derived from from the the uppermost uppermost weathering weathering pisolitic lateritic profile Precambrian igneous saprolith, and transported transported here by by Superior Superior profile of the Precambrian igneous and metamorphic saprolith, lobe landscape lobe ice ice and and meitwater. meltwater. Setterholm Setterholm(1990) (1990)states, states,"The "The pisolitic pisolitic layer layer marks marks aa soil soil of the landscape upon which which the the bulk of of the the Upper Upper Cretaceous Cretaceous rocks rocks in in Steams S t e m sCounty Countywere weredeposited." deposited.'' 89 �andravel east ____— west Cretaceous Cretaceous shale shale iiorlobetil1_ Superior lobe till sand and gravel . . . . . . .. Figure 3.1. Merden MerdenLake Lake Esker Esker gravel gravel pit: profile profile of of exposed exposed east and west walls. Brothers Gravel STOP STOP 4— 4- Bauerly Brothers Gravel Pit. Pit. (Howard (Howard Hobbs) lobe drumlin drumlin (Fig. (Fig.4.1). 4.1). The near side of the This pit is dug into the side of a Superior lobe drumlin is truncated by the the edge edge of of the theMississippi Mississippivalley valley train. train. There is about about 25 25 feet feet of Superior outwash overlain by by as much as 25 feet of reddish-brown till. The till cover reddish-brown till. cover gets thinner on the lower flanks of the drumlin. ItItisisnot not unusual unusual for for drumlins drumlins of of the the Pierz Pierz drumlin drumlin field to contain meltwater meltwater sediment, sediment, presumably deposited deposited in an earlier phase of glaciation, glaciation, as as well as till. are typical typical of ofSuperior-provenance Superior-provenancedeposits. deposits. They are reddish reddish Both the till and the gravel are brown, and contain many many red and black pebbles; pebbles; carbonate carbonatepebbles pebblesare aresparse. sparse. The till is aa brown, rocky gravelly gravelly sandy sandy loam. In Insharp sharpcontrast, contrast,this thissequence sequenceisis overlain overlain in in places places by aa thin layer layer of buff-colored buff-colored loamy till, which which contains contains only a few rocks and pebbles. This material material is is interpreted interpreted as as till deposited deposited by the Duelm Duelm bulge of the Des Moines lobe. This This site site is is only only aa few few miles from Benton and Sherburne Counties, from the the edge edge of the bulge, bulge, judged by the soil surveys of Benton and by the position of the Duelm channel. The interpreted as Thethin thin and and patchy patchy nature nature of the till is interpreted as a sign that the the ice ice did did not not persist persist long. long. 90 �I- 4 ) H .7 / Wa?j% P.... h- t: 6 ____arr t \1C4Ie 309\ t-- / _ LJ c 31 0 0w . — ?—4 - . (: UIN1 cl.1BENTON rr hl: '' M& c-o. . ii// Figure 4.1. Duelm Channel and Mississippi valley train. ________ ._/ ). r y BovJ sr c j, o.i �_ STOP STOP 5— 5- Duelm Channel Channel (Howard (Howard Hobbs) Hobbs) vantage point, on on an an erosional erosional remnant of till, till, the the essential essential elements elements of of the the From this vantage 4. I). The Thechannel channelstretches stretchesas asfar faras asthe the eye eye can can see see to to the the east east Duelm channel channel can can be be seen seen (Fig. (Fig. 4.1). Duelm Thenorthern northernbank bank of ofthe thechannel channel isis about about 1/2 I12 mile mile away, away, rising from 30-60 feet and west. The floor. The The land landsurface surface to to the the north north isismade madeup upofofdrumlinized drumlinizedtill, till, above the flat channel floor. underlain in places by outwash, outwash, of the Superior lobe. To To the the south, south, the edge edge of the channel channel is is underlain not so so well well defined. defined. In Inplaces, places,the thechannel channelcuts cutsseveral severaltens tens of of feet feet into into a drumlinized drumlinized topography topography not similar similar to the north north side, side, but but in in other other places places there there is is scarcely scarcely any any elevation elevation difference difference between between the channel channel bottom bottomand and the the land land outside outside the the channel. channel. In Insome someplaces, places,the the land land actually actuallydrops dropsoff off the to dificult to to see see how how the the channel channel could could have have held in any any water water unless unless itit was was to the the south. south. ItIt is is difficult fronted fronted by by glacial glacial ice ice on on the the south southside. side. This This channel channel defines defines Duelm channel ,Duelm channel the limit limit of of what what II have have Princeton0 called called the "Duelm bulge" bulge'' (Fig. 5.1). 5.1). ItItisislobate, lobate,but but (Fig. too small small to to be be an an ice ice lobe lobe too or even even aa sublobe, sublobe, in in the the usual usual sense. sense. All the the Duelm meltwater flowing down meltwater flowing down 0 /1 the the Mississippi Mississippi valley valley 0 0 bulge train was diverted around train was diverted around this this bulge bulge in in the the ice ice front, front, on on its its way way to to the the Mississippi Mississippi south south of of St. St. Paul. Paul. The The thin thin skim skim of of -. gray, gray, shale-bearing shale-bearing till till atat Stop Stop 44 indicates indicates that that this this was was aa bulge bulge of of the the Des Des Figure Figure 5.1. 5.1. Duelm Duelmchannel channeland andDuelm Duelmbulge. bulge. Moines Moines lobe, lobe, which which was was otherwiseheld heldback backby by the the otherwise St. St. Croix Croixmoraine. moraine. Because the till till is is thin, thin, and and has has not notaltered alteredthe theunderlying underlyingdrumlinized drumlinized topography topographyinherited inheritedfrom fromthe theSuperior Superiorlobe, lobe,IIinterpret interpretthe theDuelm Duelmbulge bulgeas asaashort-lived short-livedevent. event. ItIt was was probably probably aasingle singlepush pushfollowed followedby by stagnation. stagnation. \ The The Duelm Duelm channel channel originates originates just east east of of St. St.Cloud. Cloud. Its Its original original junction junction with with the the Mississippi Mississippi valley valley train train cannot cannot be beobserved, observed,because becauseof of subsequent subsequenterosion erosionalong alongthe thevalley valley train. train.At Atthe theupper upperend, end,the thewatercourse watercourseisisbroad broadand andits itsedge edgeisispoorly poorly defined, defined,suggesting suggestingthat that itit was Inthe themiddle middlesection, section, itit isis as aswe we see seeitithere—wellhere-wellwas not not very very erosive erosive in this this section. section. In defined definedand anderosive erosiveon on the the north northside, side,variable variable on on the the south south side. side. At Atits itslower lowerend, end,the thechannel channel gradually Anoka Sand Sand Plain. Plain. South graduallyloses losesits itsidentity identityas asitit is is buried buried by younger deposits deposits of the Anoka South of the the Duelm Duelm channel channelisisaa set setof of smaller, smaller,anastamosing anastamosingchannels channels at lower elevations. They They range range from fromwell-defined well-definedto toobscure, obscure,and andcontain containmany manyshallow shallowdepressions, depressions,apparently apparentlyaaresult resultof ofice ice collapse. Like the main channel, they trend east to southeast, and disappear beneath the Anoka collapse. Like the main channel, they trend east to southeast, and disappear beneath the Anoka Sand SandPlain. Plain.These Thesechannels channelsrepresent representshort-lived short-liveddiversions diversionsaround aroundthe theice iceas asitit was was advancing, advancing, and andpossibly possiblyalso alsoas asititwas wasmelting meltingback. back. 92 �northwestern part part of the the Duelm Duelm bulge advanced over relatively high drumlinized drurnlinized till The northwestern subsequently covered after the ice melted off. By By contrast, contrast, of the Superior lobe, which was not subsequently the southern and eastern eastern parts parts of of the the bulge bulge passed passed over over lower lower ground, ground, which which was was covered coveredby by Here, itit is is impossible impossible to to tell tell where where the the sand of the Anoka Sand Plain after the ice melted off. off. Here, boundary of the bulge was, or where the Duelm channel went. Clearly, Clearly, the the water water must must have have found its way eventually eventually to to the the Mississippi MississippiRiver River below below the the Twin Twin Cities. Cities. Stop &The 6—The Lake LakeAnn AnnDunes, Dunes,East-Central East-Central Sherburne Sherburne County, Minnesota. Minnesota. (Gary (Gary Meyer) Meyer) see figure figure 7.1 for location]. [From Stop 2 by Kerry Keen in Goldstein (1987); see Sand dunes are an uncommon uncommon feature feature of of Minnesota's Minnesota's landscape. landscape. They typically occur as small distinct areas on much much larger larger glacial outwash plains or on terraces terraces along along rivers rivers (Cooper, (Cooper, Grigal and and others, others, 1976). 1976). Dunes also occur adjacent to to lakes lakes and and on beach beach ridges ridges of of 1935; Grigal Glacial Lake Agassiz in northwestern Minnesota. Some Some of of the the largest largest tracts tracts of well developed developed dunes are scattered scattered across across the the Anoka Anoka Sand Sand Plain, Plain, just north north of of the theTwin Twin Cities CitiesMetropolitan Metropolitan Area. Of Ofthese, these,the thelargest largestareas areasof of dunes dunesare are north north of of Elk Elk River River in eastern eastern Sherburne Sherburne County. These dune areas are best best described described as parabolic parabolic dune blankets (David, 1977) that contain narrow, elongate, parabolic parabolic dunes dunes and and en en echelon echelon dune dune ridges. ridges. the typical typical geometry geometry of of dune dune blankets blankets on on the the Anoka Anoka The Lake Ann dunes (Fig. 6.1) display the Sand Plain (Fig. 6.2A). This Thisdune duneblanket's blanket's distinct distinctsoutheast southeastlobe lobeappears appearsto tohave have advanced advanced Ann. The dune blanket is highly asymmetric, with an indistinct into the southern area of Lake Ann. wing. ItIt isis composed southeastsouthwest margin and a well-developed northeast wing. composedof of several severalsoutheasttrending dune dune ridges, ridges, which with northeast northeast slopes slopes steeper steeper than than trending which also are very asymmetric, asymmetric, with outwash plain, plain, southwest slopes. The Thedune duneblanket blanket grades gradesnorthwestward northwestward into into wind-deflated wind-deflated outwash containing a number number of northwest-southeast elongate elongate deflation deflation depressions. depressions. The eolian features of the Anoka Sand Plain were originally attributed to formation by Cooper, 1935). 1935). However, study of air photos southwest winds (Leverett and Sardeson, 1932; Cooper, — the dune blankets and other data indicates that the major eolian features of the sand plain — were were formed by northwest winds (Keen, 1985). Modification Modification by by west, west, southwest, southwest, and and south south winds created the asymmetry asymmetry of of the the dune dune blankets blankets and and dune dune ridges. ridges. Sand was eroded and transported transported to the the northeast northeast over over the the dune dune ridges, ridges, steepening steepening their their northeast northeast slopes. slopes. This modification was so parabolic dune shapes were nearly obliterated modification so severe severe that the original parabolic obliterated (Fig. (Fig. 6.2B). The Thepresent presentwind windregime regime of of the thesand sandplain plain isisdominated dominated by by northwest northwest winds winds that that are are strongest in spring and fall. During Duringthe the summer, summer, winds winds from from the west and south would result in of sand. sand. The present wind appears to be similar to that a smaller net northeastward movement of responsible for eolian activity on the Anoka Sand Plain, although components of this regime regime probably varied in importance importance during during the Holocene. Holocene. A number of vegetational vegetational communities occur on the dunes of the Anoka Sand Plain and are well displayed in the Lake Ann Ann area. area. Prairie of Prairie grows on the dry sunny southwest sides of dune ridges, whereas oak woods occupy the moister northeast slopes. An An area area of of low low dunes dunes in in Ann dunes dunes supports an oak oak savanna savannacommunity. community. Various Variouswetland wetlandcommunities communities occupy occupy the Lake Ann dune depressions. depressions. Sediments from from Lake Lake Ann were studied studied with the the goal goal of of interpreting interpreting the the vegetation vegetationand and eolian history of the adjacent Lake Ann Ann dunes dunes (Keen (Keen and and Shane, Shane, 1990). 1990). As determined determined by 93 �Ann dune field. field. Area Figure 6.1. Topogaphic Area Topogaphic map map showing showing part of the Lake Ann shown (U.S. Geological GeologicalSurvey Survey shown is about 2.8 miles wide; contour contour interval interval 10 10 feet. (U.S. Orrock Orrock quadrangle, quadrangle, 7.5-minute 7.5-minute series, series, 1991). 1991). 94 �I\. Deflation area limb Obscure southwest southwasl margin parabolic dunes 4i 1 . B. WENDS WINDS 1km m .k MAP MAPVIEW VIEW CROSSSECTION SECTION CROSS Figure 6.2. 6.2. A. A.Typical Typicalasymmetic asymmeticdune dune blanket on the Anoka Proposed stages stages Figure Anoka Sand Sand Plain. Plain. B. Proposed in in the thedevelopment developmentof ofmodified modifiedparabolic parabolicdunes: dunes: A. A. Initial Initial development developmentof of symmetric symmetricparabolic parabolic dunes dunes by by strong strong northwest northwestwinds. winds. B. Modification Modificationby bycross-winds. cross-winds.Northwest Northwestdunes dunesare arestill stillstrong, strong,but butan animportant importantsouthsouthB. west wind component exists. west wind component exists. C. Stabilization Stabilizationand andrevegetation revegetation of of the the dunes. dunes. Minor Minoreolian eolianerosion erosion under under variable variable wind wind C. directions. directions. Steep Steep slope slope (slipfaces) (slipfaces)represented represented by by lined linedpattern; pattern; less less steep, steep,eroding eroding slopes slopes(backslopes) (backslopes)represented representedby by dotted dottedpattern. pattern. (Stop (Stop 22 by by Keen, Keen,from fromGoldstein, Goldstein,1987) 1987) 95 �O\ "S [H [TLTi. p.,..., •i •i•i ... —.-. I. ,:$ ,P s •' lI0 'be' !. - 'il-::j - --F— -, Istiflid b t C S $54.5. IRS, '1.! — I- ;Q' , 0.s (or rsd Din.) forest Birch-Aider forest Spruce woodland — Jack pine Prairie Oak savanna Oak I VEGETATION TYPES and sample of the spruce pollen decline). From Keen (1985). The estimated time scale was constructed by linear extrapolation between dated materials (surface lacustrine sediment, radiocarbon dated samples, Figure 6.3. A. Abbreviated pollen diagram for Lake Ann, Sherburne County, Minnesota: B. Eolian component of magnetic susceptibillity of core F, Lake Ann. Depth is estimated from the frozen lake surface. "1 4..— — 'S.— 'S..— '405— '7,.— — 6 Shi'burns Coirnuy . UInn.,oI. •5SS— CoreA lw's.. aOl ? A. CorSF C-' D??S LAKE ANN 1400- 1300. 1200. 1100. 900. SOD' 700 I 10 I 9 II I 20 I -3000 -2000 -bOO (yr. B.P.) Estkn.t.d 'C B?) (WIS-I4S2 7420*90 (WIS-1453) 43910 (yr. —8000 —4000 W — 10.000 — 9000 $000 -7000 -.000 0 w z U I- .. Radiocarbon dates Dipm AdusI.d Inagnstic auscepIty (ciTu) (.muIO.) a 1O bsss of B EOLIAN COMPONENT OF MAGNETIC SUSCEPTIBILITY CORE F. LAKE ANN, SHERBURNE COUNTY. MINNESOTA ___________ �pollen analysis (Fig. 6.3A), the postglacial vegetation vegetation history history of of spruce spruce woodland, woodland, birch-alder birch-alder forest, pine forest, and finally oak woodland other studies studies forest, prairie, and woodland is similar similar to that inferred from from other Minnesota. The in east-central Minnesota. The degree degree of of representation representation of prairie-indicator prairie-indicator species implies conditions on on the the Anoka Sand Plain during the mid-Holocene. mid-Holocene. very dry conditions The chronology of eolian eolian activity activity for the dunes dunes near Lake Ann, derived from analyzing lake sediment for eolian coarse silt and and very very fine fine sand, sand, correlates correlates extremely well with the vegetation history determined from pollen analysis. Because sediment is found history determined Because no wind-derived sediment in early Holocene Holocene sediments, present on the the sandplain sandplain in late late sediments, it appears appears unlikely that dunes were present glacial or early Holocene time, when forest covered the land. land. Eolian Eolian activity activity appears appears to to have have begun around 8000 8000 years ago, ago, when the percentage of tree pollen falls to its lowest postglacial 6.3B), Majoreolian eolianactivity, activity,as as shown shown best best by by the the magnetic magnetic susceptibility susceptibility curve curve (Fig. (Fig. 6.3B), value. Major occurs during prairie conditions occurs conditions of the mid-Holocene, mid-Holocene, until until about about4000 4000years yearsago. ago. Minor eolian activity activity follows follows throughout throughout the the late late Holocene. Holocene. Stop 7. The The "Feature" "Feature"(Howard (HowardHobbs) Hobbs) North of Elk River, an extensive extensive highland rises above the Anoka Sand Plain (Fig. 7.1). 7.1). Its largest dimensions east-west direction and a little less than 15 dimensions are are aa little little over 15 15 miles in the east-west miles north-south. Its about 100 feet; feet; local relief ranges Itsaverage average height height above the sandplain is about up to 100 100 feet, feet, but but many many parts parts are areflatter. flatter. highland has been interpreted interpreted as a tunnel-valley fan (Fig. 1). However, The origin origin of this highland However, it has been mantled mantled and and modified modified since since its its formation, formation,so so its surficial surficial sediments sedimentsare are quite quite variable. variable. Much of it has been mantled by reddish-brown sandy till, similar to till till of of the the Superior Superior lobe. lobe. In In places, places, banded banded red red and and gray gray till has been observed. The The till-covered till-covered part part of of the the feature feature is is crossed crossed by eskers, eskers, some from the Superior some of which which appear appearto to be Grantsburg, Grantsburg, some some of which which are are left left over over from Superior lobe. On gravel. They are similar Onthe thewest westside sideof of the the highland highland is is aa set of north-south ridges of gravel. to eskers, but are straighter. Stop Stop77isisaalarge largegravel gravelpit pit in in one oneof of them. them. in most most of of the the pit pit at The gravel is clean and rather coarse in at first first glance, glance, itit looks looks just like like Superior gravel. gravel. It has lots lots of of black black and and red red pebbles pebbles from from the the Superior Superior basin basin and and the the North North Shore. But Butlook lookclosely, closely,and andyou you will will find find aa few few gray gray shale shale pebbles, pebbles, which are are never found in pure Superior Superior gravels. ItItappears appearsthat thatmost most ifif not not all all the shale shale is coming out of the upper part of the exposure, but this is difficult to confirm, because the lower parts slump over so quickly. Our best interpretation interpretation of these ridges is that they were formed as a sort of ice-marginal meitwater by the the highland. highland. Most meltwater deposit where the Duelm bulge was hemmed in by Most of of the the gravel gravel is reworked from the existing existing fan, but aa little little gravel gravel was was derived derived from from the the Des Des Moines Moines lobe. lobe. This is where where the the shale shale came came from. from. 97 �Figure 7.1. Location Locationmap mapfor forstops stops66 and and 7. 7. 98 98 �REFERENCES CITED Aber, J. S., 1988, with glaciotectonic glaciotectonic examples: examples: Winston-Salem, Winston-Salem, 1988, Structural Structural geology exercises with N.C., Hunter HunterTextbooks, Textbooks,p.p.881-84. 1-84. Birks, H.J.B., 1976, 1976, Late LateWisconsin Wisconsinvegetational vegetational history history at at Wolf Wolf Creek, central Minnesota: Minnesota: Ecological Ecological Monographs, Monographs, v. v. 46, p. 495-529. glacial thrusting and related processes in Bluemle, J.P., J.P., and Clayton, L., 1984, 1984, Large-scale glacial North Dakota: Boreas, Boreas,v.v. 13, 13,p. p. 279-299. 279-299. Clayton, process-form model, Clayton, L., and and Moran, Moran, S.R., S.R., 1974, 1974,A glacial process-form model, in in Coates, Coates,D. D. R., ed., ed., Glacial Glacial geomorphology—Geomorphology Binghamton, N.Y., N.Y., 1974: 1974: Binghamton, geomorphology-Geomorphology Symposium, Symposium,5th, 5th, Binghamton, 89-119. State University University of New York, p. 89119. Cooper, W.S., 1935, Mississippi River in late Wisconsin Wisconsin and postglacial postglacial Cooper, W.S., 1935,The history of the upper Mississippi time: Minnesota MinnesotaGeological GeologicalSurvey SurveyBulletin Bulletin 26, 26, 116 116p. p. David, P.P., Sand dune dune occurrence occurrencein inCanada: Canada: A theme and resource resource inventory study study of of P.P., 1977, Sand eolian David, Dept. Dept. of Geology, Geology, University University eolian landforms landformsof of Canada: Canada: Available Availablefrom fromDr. Dr. Peter Peter P. David, of Montreal, 183 183 p. Goldstein, Goldstein, B., 1987, 1987, Geomorphology Geomorphology and and Pleistocene Pleistoceneglacial glacial geology geology of of central central Minnesota, Minnesota,in in Balaban, N.H., N.H., ed., Field trip guidebook for Quaternary and Cretaceous geology of westcentral Minnesota Minnesota and adjoining South Dakota: Minnesota Minnesota Geological GeologicalSurvey Survey Guidebook Guidebook Series No. 16, 16, p. 16-19 16-19 (Stop (Stop 2). Grigal, Grigal, D.F., Severson, Severson, R.C., and Goltz, G.E., 1976, 1976, Evidence of eolian activity in north-central Minnesota 8,000 to 5,000 years years ago: ago: Geological Geological Society Society of of America America Bulletin, v. v. 87, p. 125 1-1254. 1251-1254. Bedrock geologic geologic and topographic topographic maps Jirsa, M.A., Olsen, Olsen, B.M., and Bloomgren, B.A., 1986, Bedrock of the seven-county seven-county Twin Cities Cities Metropolitan MetropolitanArea, Area,Minnesota: Minnesota: Minnesota Minnesota Geological Geological Survey Survey Miscellaneous Miscellaneous Map Map M-55, scale scale 1:125,000. 1:125,000. Johnson, M.D., M.D., and Hemstad, Chris, in prep., Glacial Lake Grantsburg: a short-lived lake Johnson, recording recording the advance and retreat of the the Grantsburg Grantsburg sublobe, sublobe, in in Patterson, Patterson, C.J., C.J., ed., ed., Contributions to the Quaternary of Minnesota: Minnesota: Minnesota Quaternary of Minnesota Geological Survey Report of Investigations Investigations49. 49. Keen, K.L., 1985, Plain: Unpublished 1985, Sand dunes on the Anoka Sand Plain: Unpublished M.S. M.S. thesis, thesis, University University of Minnesota, Minneapolis, 191 p. Minnesota, Minneapolis, 191 Keen, K.L., and Shane, Shane, L.C.K., L.C.K., 1990, 1990,A A continuous continuous record record of of Holocene Holocene eolian eolian activity activity and and vegetation change at Lake Ann, east-central Minnesota: Minnesota: Geological Geological Society Society of of America America Bulletin, Bulletin, v. v. 102, 102,p. p. 1646-1657. 1646-1657. A.R., 1996, Knaeble, A.R., 1996, Glaciotectonic thrusting along the St. St. Croix Croix moraine, moraine, Stearns StearnsCounty, County, Minnesota, Geologic Atlas, in Meyer, Meyer, G. G. N., and and Swanson, Swanson, L., eds., Text supplement to the Geologic Minnesota, in Stearns S t e m s County, Minnesota: Minnesota MinnesotaGeological GeologicalSurvey SurveyCounty CountyAtlas Atlas Series SeriesC-1O, C-10, Part C, p. 40-47. Knaeble, A.R., in prep., Superior Knaeble, Superior lobe glacial thrusting of drift and bedrock along the St. Croix Croix moraine, Stearns County, Minnesota, in in Patterson, Patterson, C. J., County, Minnesota, moraine, J., editor, Contributions Contributions to the the Quatemary Quaternary geology of Minnesota: Minnesota MinnesotaGeological Geological Survey Survey Report Report of Investigations Investigations 49. 99 �Leverett, F., and Sardeson, Sardeson, F.W., Minnesota and parts Leverett, F.W., 1932, Quaternary geology of Minnesota parts of adjacent adjacent states: U.S. U.S. Geological GeologicalSurvey Survey Professional Professional Paper Paper 161, 161, 149 149 p. Mackay, K.R., and Mathews, W. W. H., H., 1964, The The role role of of permafrost permafrost in in ice ice thrusting: thrusting: Journal Journal of of Geology, v. 72, p. 378-380. p. 378-380. Meyer, G.N., 1986, 1986, Subsurface Subsurface till till stratigraphy stratigraphy of of the the Todd Todd County County area, area, central central Minnesota: Minnesota: Minnesota 40 p. Minnesota Geological GeologicalSurvey SurveyReportof ReportofInvestigation Investigation34, 34,40 p. Meyer, G.N., 1992, plate 55 in in Meyer, 1992, Quaternary stratigraphy, stratigraphy, plate Meyer, G.N., G.N., and and Swanson, Swanson, L., L., eds., eds., Geologic atlas of Ramsey County, Minnesota: Minnesota MinnesotaGeological GeologicalSurvey Survey County CountyAtlas Atlas Series C-7. Meyer, G.N., in Balaban, G.N., and Hobbs, Hobbs, H.C., 1989, 1989, Surficial Suficial geology, plate 3 in Balaban, N.H., ed., Geologic Geologic atlas of Hennepin County, Minnesota: Minnesota MinnesotaGeological GeologicalSurvey Survey County County Atlas Atlas Series Series C-4, scale 1:100,000. 1:100,000. Twin Cities Metropolitan Meyer, G.N., and Jirsa, M.A., 1984, 1984,Aggregate resources inventory, Twin Area, Minnesota: Minnesota MinnesotaGeological GeologicalSurvey Survey Information Information Circular 20, 16 16 p. Patterson, C.J., 1992, in Meyer, G.N., G.N., and and Swanson, Swanson, L., L., eds., eds., Geologic Geologic Patterson, 1992, Surficial Surficial geology, plate 33 in atlas of Ramsey County, Minnesota: Minnesota MinnesotaGeological GeologicalSurvey SurveyCounty CountyAtlas Atlas Series Series CC7, scale 1:48,000. 1:48,000. Patterson, C.J., 1994, Tunnel-valley fans of the the St. St.Croix Croixmoraine, moraine,east-central east-centralMinnesota, Minnesota, U.S.A., U.S.A., Patterson, 1994,Tmnel-valley in Warren, W.P., W.P., and and Croot, D.G., D.G., eds., Formation and deformation of glacial glacial deposits: deposits: Rotterdam, Balkema, Rotterdam, Balkema,p.p.69—87. 69-87. Setterhoim, D. R., 1996, Setterholm, 1996, Geology Geology of of the the Late Late Cretaceous Cretaceous Strata Strata and and Pre-Late Pre-Late Cretaceous Cretaceous Weathering Profile, Steams Stearns County, County, Minnesota, Minnesota, in in Meyer, Meyer,G. G. N., N., and and Swanson, Swanson, L., L., eds., Weathering Atlas, SStearns County,Minnesota: Minnesota: Minnesota Minnesota Geological Geological Text supplement to the Geologic Atlas, t e m s County, Survey County Atlas Atlas Series Series CC-b, 10,Part PartC, C,p. p. 7-15. 7- 15. 100 ��FIELD FIELD TRIP TRIP #4 #4 STRATIGRAPHY STRATIGRAPHYAND AND HYDROGEOLOGY HYDROGEOLOGY OF OF PALEOZOIC PALEOZOICROCKS ROCKS OF OF SOUTHEASTERN SOUTHEASTERNMINNESOTA MINNESOTA Leaders: Leaders: Anthony AnthonyC. C.Runkel Runkeland andRobert RobertG. G. Tipping Tipping Paleozoic Paleozoicstrata strataof of the thecentral centralmidcontinent midcontinentregion regionare areamong amongthe thelongest longeststudied studiedand andbest best known America. This known sedimentary sedimentary rocks in North America. This field field trip tripwill willprovide provide an anopportunity opportunityto to examine examinethe thediverse diversesuite suiteof of lower lowerPaleozoic Paleozoicrocks rocks exposed exposedalong alongthe the Mississippi MississippiRiver Riverand andits its 1) the depositional tributaries in southeastern Minnesota, focusing on two interrelated topics: tributaries in southeastern Minnesota, focusing on two interrelated topics: 1) the depositional history history of of these thesestrata, strata,highlighting highlightingadvances advancesmade madeover overthe thepast pastfifteen fifteenyears yearsin inunderstanding understanding the the origin originof of their their well well documented documented enigmatic enigmatic features, features, and 2) 2)the thehydrogeologic hydrogeologicproperties propertiesof of these these strata, strata,with withparticular particularemphasis emphasison oncharacterization characterizationof ofhydrostratigraphic hydrostratigraphiccomponents. components. INTRODUCTION INTRODUCTION Paleozoic Paleozoicstrata stratain inthe thecentral centralmidcontinent midcontinentregion regionof ofNorth NorthAmerica Americaconsist consistof ofthin thinunits units of of carbonate, carbonate, sandstone, sandstone,and and shale shaledistributed distributed across across tens tens of of thousands thousands of of square squarekilometers. kilometers. This Thisoverview overviewfocuses focuseson onthe thenorthernmost northernmostextent extentof of lower lowerPaleozoic Paleozoicstrata stratathat thatwere weredeposited deposited on the stable cratonic shelf northwest of the illinois and Michigan basins (Figs. 1 and 2). The on the stable cratonic shelf Illinois basins (Figs. and 2). The rocks rocks are areexposed exposedin in aa sinuous sinuousbelt beltof of outcrops outcropsalong alongthe the southern southernflank flankof of the theTranscontinental Transcontinental arch Minnesota, Wisconsin and northern Michigan. Michigan. archand and the the Wisconsin Wisconsindome dome and and arch in southern Minnesota, Deposition began in in the the Middle Middle to Late Cambrian. Deposition of Paleozoic Paleozoic sediments sediments began Cambrian. Coarse Coarse siliciclastic Sandstone covered the eroded Precambrian Precambrian siliciclasticsediments sedimentsof of what whatisis now now the Mt. Simon Sandstone surface. surface. Overlying Overlyingstrata stratawere weredeposited depositedinintwo twobroad, broad,laterally laterally equivalent equivalent facies facies belts belts across across the composed the central central midcontinent midcontinentregion region (Fig. (Fig.11 inset) inset) (Palmer, 1960). An inner detrital belt was composed of of shallow shallowmarine marinesiliciclastics siliciclasticsderived derived from from subaerially subaerially exposed exposed Precambrian Precambrian Shield Shieldareas areason on and to to the the north north of of the theWisconsin Wisconsin Dome. Dome. Thin, Thin,laterally laterallyextensive extensiveunits unitsdominated dominatedby by either either and fineto coarse-grained quartzose sandstone, very fine gralned sandstone, or shale were deposited fine- to coarse-grained quartzose sandstone, very fine grained sandstone, in intertidal carbonate carbonate subtidal to intertidal in this this inner inner detrital detrital belt. The Themiddle middlecarbonate carbonatebelt belt consisted consisted of subtidal and and shale shale that that accumulated accumulated to the south. south. Over Overtime, time,the theboundary boundary between between these thesedepositional depositional belts shifted: shifted: sedimentation sedimentationininthe theinner innerdetrital detritalbelt beltwas wasdominant dominantduring during the theCambrian Cambrianand and part of of the the Middle Middle Ordovician; Ordovician; and and sedimentation sedimentation in in the the middle middle carbonate carbonate belt belt was was dominant dominant during during the the Early Early and and Late LateOrdovician, Ordovician,as aswell well as asin in the theSilurian Silurianand andDevonian Devonian(Fig. (Fig.2). 2). Early Early Paleozoic Paleozoicdeposition depositionoccurred occurredon on aa virtually virtually horizontal, horizontal, cratonic cratonic shelf shelf that that subsided subsided very slowly. slowly. Late LateCambrian Cambriansubsidence subsidenceaveraged averaged less less than than 10 10m/m.y. m/m.y. (Sloss, (Sloss,1988)—about 1988)-about one-fifthto to one-tenth one-tenth the the rate rate in the contemporaneous Illinois 199I), and orders one-fifth illinois basin (Sargent, (Sargent, 1991), magnitude slower slower than that that of of better known, younger basins in North America. Maximum Maximum of magnitude 1995; Ludvigson and typically less than 100 m (e.g., Byers and Dott, 1995; paleobathymetry was typically others, 1996) 1996)and and the theshelf shelfhad hadaalow low gradient gradientslope slopeof of about about0.1 0.1 rn/km. mlkrn. others, The Wisconsin Wisconsin dome dome and and arch arch and Transcontinental Transcontinental arch were positive structural The structural features features influenced patterns patterns of of early early Paleozoic Paleozoic sedimentation, sedimentation, and and eventually eventually controlled controlled the the that influenced distributionand and configuration configurationof the gently folded and faulted Paleozoic distribution Paleozoic strata we we see see today. today. In 103 �____ Figure 1.1.Location Location map map showing showing Figure early Paleozoic Paleozoic tectonic tectonic features, features, early major paleotopographic pdeotopographic highs of major Precambrian rocks rocks (stippled) (stippled) and and the the Precambrian approximate distribution dismbution of approximate Cambrian (C) Ordovician (0), Cambrian (C),, Ordovician (0), ( S ) ,and and Devonian Devonian (D) (D) Silurian (S), strata in the northern part of the central midcontinent midcontinent region. region. The The central area relative relative to to shows the study study area inset shows the Late Late Cambrian Cambrianinner inner (stipple) (stipple) the detrital belts belts and and and outer outer (dashes) (dashes)detrital and the the middle middle carbonate carbonatebelt (block (block Palmer (1960). (1960). Modified Modified pattern) of Palmer others (1998). from Runkel and others GROUP! FORMATION UTHOLOGY .e. < Cedar Valley J ::::" — —l -— Burnt Bluff Cataract -——— —— Maquoketa Dubuque J —— Figure Figure2.2.Generalized Generalizedstratigraphic stratigraphiccolumn column(no (no scale) scale) for for lower lower Paleozoic Paleozoicrocks rocksininthe thenorthern northernpart part of the the central central midcontinent midcontinentregion. region. The The Cambrian Cambrian and and Ordovician Ordoviciannomenclature nomenclatureisis that that used used in in southeastern southeastern Minnesota and Wisconsin, the the Silurian from eastern eastern Wisconsin Wisconsinand and northern northern Silurian from Michigan, Michigan, and and the the Devonian Devonianfrom fromsouthernmost southernmost Minnesota Minnesota and and northern northern Iowa. Iowa. ngadine Manistique ——— Galena Decorah PttevilIe - St Peter ShakoPeejPrane O' I j-L--. — 2 - \_ ..: . —. —. . . Group Jordan St Lawrence Franconia (Lone Koch) (Worwwoc) EauClaireand Bonneterre • Mt. Simon Quwtzose sandstone @atzosesandstone Very shale Very fine fines.s., s.s., siltstone. s~lmone, shale - Carbonate (dashedwhere whereshahy) sbl~) Carbonate(dashed No Norecord record Unconformity Unconfonn~ty 104 �northern Iowa, southern Minnesota, Minnesota, and southwestern southwestern Wisconsin, Wisconsin, Paleozoic Paleozoic strata are are preserved preserved northern in what is known known as as the the Hollandale Hollandale embayment, embayment, which is aa broad broad syncline syncline that that lies lies between between these two positive features. South Southand andeast eastof of the theWisconsin Wisconsin dome dome and and arch, arch, Paleozoic Paleozoic rocks rocks dip less than 11 degree degree into into the Illinois Illinois and and Michigan Michigan basins. DEPOSITIONAL MODELS DEPOSITIONAL MODELS Despite over 100 100 years of study, study, the depositional depositional history of lower lower Paleozoic Paleozoic siliciclastic siliciclastic strata of this region remains poorly understood. Many poorly understood. Many workers workers have lamented an apparent apparent absence of modern or ancient depositional analogues. In addition, addition, while dozens of local studies have been conducted (e.g., Nelson, 1956; 1956; Haddox and Dott, Dott, 1990), 1990), few few are are regional-scale regional-scale investigations that incorporate several lithofacies into a temporally constrained stratigraphic framework. framework. Present-day Present-dayunderstanding understandingof of early early Paleozoic Paleozoic siliciclastic siliciclasticdeposition depositionis is based mostly mostly on work conducted since since 1950, 1950,beginning beginning with the predominantly predominantly stratigraphic stratigraphic investigations investigations of Berg (1954), (1956), Bell Bell and others (19561, (1956), and Ostrom (19541, Nelson (1956), (19561, Berg and others (19561, (1964, 1970). Subsequent sedimentologic—focusing on the interpretations interpretations (1964,1970). Subsequentwork work has has been chiefly sedimentologic-focusing of near-shore marine facies (Fraser, (Fraser, 1976; 1976; Driese and others, 1981; 1981; Dott Dott and and others, others, 1986; 1986; Haddox and Dott, 1990; 1990;Barnes Barnes and and others, others, 1992; 1992;Mossler, 1992; 1992; Runkel, Runkel, 1994). 1994). In a very important important and widely cited examined the transition cited study, study, Dott and others (1986) examined transition from nonmarine to marine environments recorded recorded in quartzose sandstones, and provided the of nonmarine nonmarine facies. facies. They described an early Paleozoic first set of criteria for identification of terrestrial setting with broad braided streams and eolian sheet and erg systems that delivered sand to a shallow marine environment dominated dominated by by small smallthree-dimensional three-dimensionaldunes. dunes. Dott and of vegetation, between between early early Paleozoic others (1986) stressed the differences, difherences, such as the lack of depositional environments environments and those inferred inferred for both younger rocks and and modern modern settings. settings. depositional model A depositional model of a simple, simple, storm-dominated, storm-dominated, texturally texturally graded graded shelf shelf was was proposed proposed by Runkel and others (1998) (Fig. 31, 3), and is noteworthy noteworthyin inits itssimilarity similarity to models based on both younger younger rocks regional-scale outgrowth rocks and modern systems. Their Their study study was essentially a regional-scale of the excellent students from the University University of Minnesota Minnesota excellent outcrop outcrop work work of Charlie Charlie Bell Bell and his students in the 1950's. Runkel Runkeland andothers others(1998) (1998)suggested suggestedthat that early early Paleozoic Paleozoic depositional depositional processes distribution are analogous analogous to those depicted in well-known and facies distribution well-known models models of of the the Cretaceous Cretaceous interior seaway and the modern Bering Shelf. Shelf. The thin, widespread siliciclastic seaway and modern Bering The thin, widespread siliciclastic units units characteristic migrations that occurred characteristic of lower lower Paleozoic Paleozoic strata strata result from large lateral facies migrations in response to to continental-scale continental-scalechanges changes in in relative relative sea sea level. Carbonate rocks within within the lower lower Paleozoic sequence sequence historically historically have have been regarded regarded as as more amenable comparison with both modern and ancient ancient facies facies models. models. Deposition occurred amenableto comparison occurred in environments supratidal conditions conditionsfor for parts parts of of the the Prairie Prairie du environments that range from evaporitic, evaporitic, supratidal Chien Group Group (Smith (Smith and and others, others,1993) 1993)to to subtidal subtidalsettings settingsin in depths depths of of 100 100m m or or more more for for parts parts of the Decorah Shale (Ludvigson and others, 1996). Most Most investigations investigations have have been local local in in scope relative relative to to the great lateral extent extent of of these these units, units, and and there there are are too too many many to cite them scope individually. Ordovician individually. Ordovician carbonate carbonate strata strata of of the the Hollandale Hollandale embayment embayment are are the the focus focus of a monograph edited by Sloan (1986). Silurian Siluriancarbonates carbonates on the western flank of the Michigan basin have recently been studied by Harris and Waldhuetter Waldhuetter (1996). (1996). There are many excellent studies of OrdovicianOrdovician- through through Devonian-age Devonian-age strata strata in northern northern Iowa, Iowa, largely largely conducted conducted by by the the Iowa Geological extrapolated to Minnesota and Wisconsin (for Geological Survey; Survey; the the results results can often be extrapolated 105 �Taylors FaNs Precambrran ava flow ndq SILICICLASTIC (WSMOPE SHELF - — — MO3ft — — — — — — sand — — — -4,:;— — - — — — — 0 trom ahotataco — — — —— ' —o Q\AILW%.PUI Iranspolt by storms Sand 0 — — — '' Storm — naport ot fn sand. Ol. srral. — — — — .'.,, 0 0 - Ternp,sIiI.I — D Wasfi Figure3.3.Conceptual Conceptualdepositional depositionalmodel modelofofLate LateCambrian Cambrianstorm-dominated, storm-dominated, texturally Figure texturally graded shelf. Quartzose fineto coarse-grained sand accumulated nearshore terrestrial graded shelf. Quartzose fine- to coarse-grained sand accumulated ininnearshore terrestrial andshoreface shorefaceenvironments. environments.Finer Finergrained, grained,feldspathic feldspathicsiliciclastic siliciclasticsediments sedimentsand and subtidal and subtidal carbonatesaccumulated accumulatedinindeeper deeperwater waterofofthe theoffshore offshoreshelf. shelf. carbonates example,several severalpapers papersininWitzke Witzkeand andothers, others,1996). 1996).Some Someregional-scale regional-scalestudies studieshave have recently example, recently been completed; a stratigraphic framework has been developed for the Prairie du Chien Group been completed; a stratigraphic framework has been developed for the Prairie du Chien Group fromthe theMichigan Michiganbasin basintotothe theHollandale Hollandaleembayment embayment(Smith (Smithand andothers, others,1993, 1993, 1996; Barnes from 1996; Barnes andothers, others,1996). 1996).Simo Simoand andothers others(1997) (1997)are arecurrently currentlyconstructing constructingaalarge largescale scalestratigraphic stratigraphic and frameworkfor forOrdovician Ordovicianrocks rocksofofthe thecentral centralmidcontinent midcontinentininorder orderto toaddress addresslong-standing long-standing framework problemssuch suchasasidentification identificationof ofthe thecontrols controlsresponsible responsible for forthe theapparently apparently synchronous problems synchronous regional-scale changes from "tropical" to "temperate" styles of carbonate deposition. regional-scale changes from "tropical" to "temperate" styles of carbonate deposition. NOTABLEFEATURES FEATURESAND ANDLONGSTANDING LONGSTANDINGPROBLEMS PROBLEMS NOTABLE LowerPaleozoic Paleozoicstrata strataininthe thecentral centralmidcontinent midcontinentregion regionare arewell wellknown knowntotogeologists geologists Lower outsideof ofthe thearea areafor forseveral severalenigmatic enigmatic features features(Dott (Dott and and Byers, Byers, 1980). 1980). Most Most notable is the outside notable is the extreme textural and mineralogical maturity of the fineto coarse-grained sandstones, and the the extreme textural and mineralogical maturity of the fine- to coarse-grained sandstones, and sheet-likegeometry geometryofofthese theseand andother othersiliciclastic siliciclasticunits. units.The Theoverall overalldearth dearthofofshale shale has puzzled sheet-like has puzzled sedimentologistsyand and the thefundamental fundamentalcontrols controls on onthe theepisodic episodicchange changefrom from siliciclasticsedimentologists, siliciclasticdominatedtotocarbonate-dominated carbonate-dominatedsedimentation sedimentationremain remainpoorly poorlyunderstood. understood.Lastly, LastIy, the presence, dominated the presence, position,and andmagnitude magnitudeofofunconformities unconfonnitieshas hasbeen been debated debatedfor for decades. decades. The Theremainder remainder of this position, of this discussion summarizes the progress made since Dott and Byers (1980) published a similar discussion summarizes the progress made since Dott and Byers (1980) published a similar overviewyand andoffers offersaaguide guideto totopics topicsthat thatcan canbe bediscussed discussedatatthe theindividual individualfield fieldtrip trip stops. overview, stops. Texturalmaturity maturityofofsandstones sandstones Textural Theextreme extremetextural texturaland andmineralogical mineralogicalmaturity maturityofofthe thefinefine-totocoarsecoarse-grained grainedsandstone sandstone The units,such suchasasthe theSt. St.Peter PeterSandstone, Sandstone,may maybe bethe thebest bestknown knownand andlongest longeststudied studied feature in the units, feature in the lower Paleozoic strata of the central midcontinent. The sandstones contain more than 98 percent lower Paleozoic strata of the central midcontinent. The sandstones contain more than 98 percent quartzand andmost mostgrains grainsare aremoderately moderatelyto towell wellrounded. rounded. Such Suchtextural texturaland andcompositional compositionalmaturity maturity quartz could not have been achieved solely by fluvial and marine abrasion, even over transport distances could not have been achieved solely by fluvial and marine abrasion, even over transport distances 106 �of hundreds of kilometers, if the grains grains were were derived derived directly from crystalline source rocks. Rare grains with abraded abraded overgrowths overgrowths are reworked reworked from older sedimentary rocks, but the the volumetric significance of suitable suitable sedimentq sedimentary rock volumetric significanceof such such recycling is uncertain and a source of been identified. identified. Odom 1978) conducted the most comprehensive comprehensive mineralogical mineralogical has never been Odom (1975, (1975,1978) Paleozoic sandstones. He fine grained grained fraction fraction is is feldspathic, feldspathic, study of lower Paleozoic He noted that the very fine whereas the fine- to coarse-grained contains more more than than 98 percent percent quartz (with the coarse-grained fraction contains exception of some lower Mt. Mt. Simon Simon Sandstone Sandstonebeds). beds). He suggested exception of suggested that a long long history history of of abrasion abrasion in aa marine marine setting setting could could account account for for both both the maturity of the quartzose quartzose grains grains and and the the reduction in size of feldspar grains. Dott Dott and and others others (1986) (1986) were the first to clearly identify the presence of substantial substantial eolian eolian deposits deposits within within some some quartzose quartzose sandstones, sandstones, and and they they suggested suggested that eolian mineralogic features features described described by Odom eolian abrasion abrasion could could contribute contributeto the textural and mineralogic Odom be taken taken into into consideration consideration (1975, 1978). Lastly, Lastly, chemical chemical weathering weathering in the source area must be 1978) noted that the very fine grained, feldspathic sandstones sandstones are (Morey, 1972). Odom Odom(1975, (1975,1978) rich in K-feldspar "trace" amount of of plagioclase plagioclase grains, grains, even even though though plagioclase plagioclase K-feldspar but have only a "trace" of the Precambrian Precambrian Shield. Shield. Selective was presumably dominant in the source area of Selective diagenetic diagenetic leaching of of plagioclase plagioclase grains grains from from the the sandstone sandstone was was discounted discountedby by Odom Odom (1978). (1978). Thus, leaching Thus, chemical weathering probably preferentially dissolved plagioclase crystals and reduced reduced the the amount of other relatively unstable unstable minerals prior to mechanical mechanical abrasion abrasion during during transport size and amount transport to the shoreline. shoreline. been responsible responsible in part for the While each of the processes described above could have been mineralogic mineralogic and and textural textural attributes attributes of lower Paleozoic sandstones, they do not entirely account for the compositional record that appears to indicate that virtually all fine- to coarse-grained coarse-grained exceptionally mature the early early Paleozoic Paleozoic shoreline. shoreline. Finesand was exceptionally mature when it arrived to the Fine- to to coarsecoarsegrained immature sand would have almost certainly been deposited locally if the marine abrasion grained immature sand would have almost certainly been deposited locally marine abrasion model accurate. Similarly, terrestrial setting model of Odom Odom (1975) (1975) were accurate. Similarly,itit is difficult to reconstruct a terrestrial in which all all sand sand isissubjected subjectedto toprolonged prolonged eolian eolian abrasion abrasion prior prior to to deposition deposition in in aa marine marine environment environment as suggested suggested by Dott Dott and and others others (1986). (1986). Sheet geometry geometry layers were were initially initially deposited deposited Another long-standing long-standing question question is how sheet-like siliciclastic layers of thousands of of square kilometers kilometers of of the the cratonic cratonic shelf. shelf. Most and then preserved across tens of workers have attributed the formation of such sheets to depositional processes and environments workers have attributed the formation such sheets processes and environments that are markedly markedly different different from those described for most younger rocks and modern settings. Lochman-Balk (1970) inferred the presence of enormous tidal flats extending well over over two two the shoreline. shoreline. Dott and Byers (1980) (1980) suggested suggested that that hundred kilometers perpendicular to the quartzose sand under high-energy high-energy conditions conditions across quartzose sand aggraded aggradedmore more or or less less vertically into a sheet under across a vast shallow sea. The most widely cited hypothesis is one that attributes the origin of quartzose The widely cited that attributes the origin of quartzose sandstone sheets to fluvial fluvial and and eolian eolian processes processes that that dispersed dispersed sand sand across across aavegetation-free vegetation-free reworking during during aa transgression transgression(Dott (Dottand andothers, others, 1986). 1986). Recent landscape prior to marine reworking studies, however, have demonstrated demonstrated that that some some quartzose quartzose sheets were deposited entirely within the marine realm under under regressive conditions conditions (e.g., Runkel, Runkel, 1994; 1994; Hughes and Hesselbo, Hesselbo, 1997; 1997; Runkel and others, 1998), and that the depositional processes responsible for the dispersal of 1998), depositional siliciclastic ancient sandstones sandstones siliciclasticsediments sedimentswere were similar similarto to those those operating operating during during the deposition of ancient sheet-like. that are not sheet-like. 107 �The lateral lateral persistence persistence and and sheet-like sheet-like geometry geometry of of lower lower Paleozoic Paleozoic siliciclastic siliciclastic units units probablyreflect reflectthe theroles rolesof of basin basin physiography physiographyand and tectonics tectonics in controlling controlling the lithostratigraphic probably lithostratigraphic architecture, rather rather than than the the existence existence of atypical atypical sedimentary environments. Deposition Deposition was was architecture, characterizedby byaacontinuous continuousand andabundant abundantsupply supplyof of sediment sedimentto to aa relatively stable, stable, nearly flat flat characterized basin that that had had aa slow, slow, uniform uniform rate rate of of subsidence. subsidence. Individual basin Individualsheets sheetsof ofsiliciclastic siliciclasticsediment sediment (e.g., Fig. Fig. 3) 3) migrated migrated great great distances distances during duringchanges changesin in were deposited deposited when when discrete discrete facies facies (e.g., were sea level. level. Deep Deepincision incisionofofthe theindividual individualsheets sheetsduring duringepisodes episodesof ofsubaerial subaerialexposure exposuredid didnot not sea occur, resulting in more or less uniform preservation. occur, resulting in more or less uniform preservation. Dearth Dearthof ofShale Shale LowerPaleozoic Paleozoicrocks rocksof ofthe thecentral centralmidcontinent midcontinentregion regionare arenoted notedfor foraadearth dearthofofshale shale Lower compared comparedto toother othershallow shallowmarine marinedeposits. deposits.Two Twovery verydifferent differenthypotheses hypotheseshave havebeen beenproposed proposed to "marinebypassing" bypassing" model, model,Pettijohn Pettijohnand and others others (1973) (1973) to account accountfor for this this dearth dearth of of shale. shale. In Inaa"marine suggested that that clayclay-and andsilt-sized silt-sizedparticles particleswere weredelivered delivered to to the the shoreline, shoreline,but but subsequently subsequently suggested carriedin insuspension suspensionby byrepeated repeatedstorms stormsacross acrossthe theshallow shallowshelf shelfbefore beforefinal finalburial burial in in basinward basinward carried areas.InIncontrast, contrast,Dairymple Dalrympleand andothers others(1985) (1985)suggested suggestedthat thatstrong strongwinds windsblew blewclay clayand andsilt silt areas. particles hundreds hundredsto to thousands thousandsof of kilometers kilometersoffshore, offshore, to to be ultimately ultimately deposited deposited in in the the outer outer particles detrital detrital belt belt (Fig. (Fig.1). 1). Recentinvestigations investigationsof of Upper Upper Cambrian Cambrian siliciclastic siIiciclastic strata strata support support the marine bypassing Recent model model of of Pettijohn Pettijohnand andothers others(1973), (1973),demonstrating demonstratingthat thatthe theabsence absenceof ofshale shaleisischaracteristic characteristic only only of of the the outcrop outcropbelt belt on on the the flanks flanksof of the the Wisconsin Wisconsin dome dome and and arch arch (McKay, (McKay, 1988; 1988; Runkel Runkel and and others, others, 1998). 1998).Basinward Basinwardofofthe theWisconsin Wisconsinarch, arch,subsurface subsurfacerecords records from from Minnesota Minnesota and and Theseshale shaleand and siltstone siltstone Iowashow showthat that facies faciesdominated dominatedby by shale shale and and siltstone siltstone are common. These Iowa facies facies are are the the offshore offshoreequivalent equivalent of of the the shale-poor shale-poor sandstone sandstone facies facies of of the the outcrop outcrop belt, belt, and and their geographic geographicposition position changed changed through through time time in in response response to to changes changes in in the the position position of of the the their storm-fairweather wave storm-fairweather wave base. base. The The outcrop outcrop belt belt has has aadearth dearth of of shale shalesimply simply because because the the depositional depositional record record isisdominated dominatedby byhighstand highstandshoreface shorefacefacies faciesdeposited depositedabove abovefairweather fairweather wavebase. base.This Thisstratigraphic stratigraphicand andsedimentologic sedimentologicattribute attributeisisnot notunique uniquetotothe thelower lowerPaleozoic Paleozoic wave rocks rocks of of the thecentral centralmidcontinent midcontinentregion—it region-it is is common common in many many younger wave-dominated wave-dominated sequences along along the the landward landward margins margins of of individual individual basins. The Thenotion notionof ofaashale-free shale-freeearly early sequences Paleozoic epeiric epeiric sea seaisisaadeeply deeplyentrenched entrenchedidea ideathat thatsimply simplyreflects reflectsthe thebias biasof ofthe theoutcrop outcrop Paleozoic belt. belt.The Thedearth dearthofofshale shaleininlower lowerPaleozoic Paleozoicrocks rocksshould shouldbe beremoved removedfrom fromthe thelist listofofenigmatic enigmatic featuresunless unlessthe thescarcity scarcitycan canbe bedemonstrated demonstratedtotoexist existon onaa scale scalelarger largerthan than that that of the outcrop features outcrop belt. belt. Siliciclastic-Carbonate Siliciclastic-Carbonatecycles cycles The The fundamental fundamental controls controls on onthe thetransition transitionfrom fromnearshore nearshoresiliciclasticsiliciclastic-to tocarbonatecarbonatedominated understood. For dominated deposition deposition have have never never been satisfactorily satisfactorily understood. For example, example, in in the the Early Early Ordovician, carbonate carbonate deposition deposition apparently apparently dominated dominated across across the the entire entire region, region, even even in in the the Ordovician, shallowestwater waterconditions conditions(Smith (Smithand andothers, others,1993). 1993).Both Botholder older(Jordan (JordanSandstone) Sandstone)and andyounger younger shallowest (St. (St. Peter PeterSandstone) Sandstone)shallow-marine shallow-marineenvironments environmentswere weredominated dominatedby bysiliciclastics. siliciclastics. Only Only speculative kndamentalchanges. changes. speculativeand andvague vaguehypotheses hypotheseshave havebeen beensuggested suggestedto toaccount accountfor forsuch suchfundamental For Forexample, example,Adams Adams(1978) (1978)suggested suggestedthat thatthe theEarly EarlyOrdovician Ordovicianchange changeinindepositional depositionalstyles styles may mayhave havebeen beenin inresponse responsetotodrowning drowningof of the thesiliciclastic siliciclasticsource sourcearea areaby by aa shallow shallow sea sea or orthe the result resultof ofsiliciclastics siliciclasticsbeing being"deflected" "deflected"to toanother anotherregion. region. Early EarlyPaleozoic Paleozoiccarbonate-dominated carbonate-dominated 108 �systems developed following level (Smith (Smith and and others, others, 1993), which which systems following extended periods of high sea level siliciclastic source area may have been covered by a blanket of of carbonate carbonate suggests that the siliciclastic rocks. The Thespread spreadof of certain certainforms formsof of terrestrial terrestriallife life such such as as bacterial bacterial encrustation, encrustation, and and later later the the development development of vascular vascular plants, may have also reduced siliciclastic siliciclastic input. Cryptic Cryptic unconformities unconformities Previous investigations investigations of lower Paleozoic strata of the central midcontinent region have also been hindered by an inability to recognize unconformities. unconformities. The unconformities bounding the major sequences sequences of Sloss Sloss (1963) (1963) are are fairly well established established within the strata strata of of the the outcrop outcrop belt. The The presence presence and and position position of "lesser" "lesser" unconformities, unconformities, especially especially within siliciclastic units, have been a matter of considerable debate. Stratigraphic Stratigraphicrelations relations and and interpreted interpreted depositional depositional histories indicate that such unconformities "should" be present. However, However, little little or or no evidence evidence for regionally extensive subaerial subaerial erosion erosion has has been been documented. documented. Such Such sequence-bounding sequence-bounding they are contained within coarse siliciclastics unconformities are difficult to recognize where they texturally and and mineralogically mineralogically similar, and and sandstones that are texturally because they separate quartzose sandstones because they are relatively flat—reflecting erosion that occurred on a loose, sandy substrate relatively flat-reflecting sandy substrate along a low, uniform uniform gradient, gradient, and and in in aa nonvegetated nonvegetatedterrestrial terrestrialenvironment. environment. Furthermore, Furthermore, the ultra-mature mineral ultra-mature mineral composition compositionof of the the exposed exposed substrate substrateinhibits inhibits development developmentof of aa distinctive distinctive weathering profile. Recent work has shown some promise for identifying subtle subaerial erosion surfaces. Recognition of cryptic unconformities interpretation of regional stratigraphic stratigraphic relations Recognition unconformities requires requires interpretation outcrops and high resolution in conjunction with physical evidence collected at individual outcrops biostratigraphic data. data. Smith deposits of of silica cement cement in biostratigraphic Smith and and others others (1993) (1993) interpreted local deposits formed silcrete. Runkel uppermost Cambrian and Lower Ordovician strata as subaerially formed Runkel and and others (1998) suggested that cryptic unconformities within the fronton and Galesville Sandstones Ironton Galesville Sandstones others (1998) suggested that cryptic unconformities within and at the top top of of the the Jordan Jordan Sandstone Sandstone and and can can be be identified identified only by overlying overlying lag deposits. deposits. These lags deposits are the coarsest bed within nearshore sandstone successions; they separate a regionally traceable, traceable, decameter-scale, decameter-scale, coarsening-upward interval below from a decameterdecameterscale, fining-upward sequence above. above. Preliminary Preliminary results of of high high resolution resolution biostratigraphic biostratigraphic verified the the presence presence of of an an erosion erosion surface surface on on top top of of the Jordan dating using conodonts has verified Sandstone, and magnitude of such cryptic cryptic unconformities unconformities Sandstone, and demonstrates demonstratespromise promise for for measuring the magnitude with detailed paleontologic paleontologic work. HYDROGEOLOGY The Paleozoic Minnesota are are the the most most widely widely used used source of Paleozoic strata of southeastern southeastern Minnesota groundwater in the state. state. Groundwater Groundwatermanagement management and andscientific scientificinvestigations, investigations, including including ground water models, models, estimates estimates of of aquifer yields and predictions of contaminant transport, ground water well construction codes have, in the past, relied on a generally accepted hydrogeologic framework that we believe believe is inaccurate inaccurate in in many many important important respects. respects. This commonly framework that commonly used classification of aquifers and confining beds beds (e.g., (e.g., Kanivetsky Kanivetskyand andWalton, Walton,1979; 1979; Delin and Woodward, 1985) assumption that Paleozoic lithostratigraphic lithostratigraphic units in Minnesota Minnesota Woodward, 1985)is based on the assumption are more or or less less internally internally homogeneous homogeneous in in hydrogeologic hydrogeologic character, character, and that they they have have boundaries. Furthermore, this paradigm is boundaries that correspond to hydrostratigraphic boundaries. based chiefly on the the hydrogeologic hydrogeologic characteristics of the Paleozoic strata in the Twin Cities Metropolitan from equivalent equivalent strata stratain insoutheastern southeasternMinnesota. Minnesota. A Metropolitan Area, which differ markedly from 109 �more accurate accurate hydrogeologic hydrogeologic framework framework is necessary to advance our understanding of groundwater flow in this region. region. We have have recently changes in the methods We recently made fundamental fundamental changes methods used used to to characterize characterize hydrogeologic attributes focusingon onhydrostratigraphic hydrostratigraphic components components rather rather hydrogeologic attributes of Paleozoic strata, focusing than lithostratigraphic lithostratigraphic units. Fig. 4 shows our preliminary results in delineating hydrostratigraphic Fig. 4 shows in delineating hydrostratigraphic components, permeability. The Paleozoic components, distinguished distinguishedand and characterized characterized by their porosity and permeability. bedrock hydrostratigraphic components components that bedrock of southeastern southeastern Minnesota Minnesota consists consists of four distinct hydrostratigraphic have been described described in in studies studies at at both both the regional regional and local scale scale (e.g., Setterholm Setterholm and and others, others, Miller and and Delin, Delin, 1993; Runkel, Runkel, 1996a, 1996b). 1996b). They are: 1) fine clastic; 2) coarse 1991; Miller coarse clastic, 3) carbonate rock, rock, and 4) mixed mixed carbonate carbonate and and clastic. clastic. The values for porosity and permeability permeability of these these components, components,discussed discussed below, below, have been determined determined through through laboratory laboratory tests of plug samples samples and and through through hydraulic hydraulic testing testing of of boreholes boreholes in in southeastern southeasternMinnesota. Minnesota. grained, feldspathic feldspathic sandstone, sandstone, siltstone The fine-clastic component consists of very fine grained, strongly to to moderately moderatelycemented. cemented. This component and shale in thin to medium beds that are strongly orders of of magnitude less than that of the has low to very very low low relative relative permeability—several permeability-several orders coarser-grained sandstone coarser-grained sandstone of the quartzose quartzose component component described described below. Vertical Vertical conductivity conductivity based on on pumping pumpingtests testscommonly commonlyranges ranges from i0 to i0 ft/day for interbedded from 1 0 to 10'~ft/day for interbedded very very fine sandstone and sandstone and shale shale (Miller (Millerand and Delin, Delin, 1993), 1993),to to as low as iO7 10'~ft/day ft/day for units composed composed almost almost entirely entirely of shale shale (Freeze (Freeze and and Cherry, 1979). In In the the former, former, horizontal horizontal permeability permeability is is typically typically more than 100 100times times greater greater than than vertical vertical permeability permeability (Miller, (Miller, 1984; 1984;Setterholm Setterholm and and others, others, 1991; 1993). 199 1;Miller and Delin, 1993). coarse-clastic component The coarse-clastic componentis is aa mostly uncemented, uncemented, moderately moderately to to well-sorted, well-sorted,finefine-to to coarse-grained sandstone composed of about 98 percent quartz. Plug-sample Plug-sampletests tests indicate indicate itit has aa high high to tovery veryhigh highpermeability permeability and andporosity porosity due due to torelatively relatively large, large, well-connected well-connected intergranular intergranular pore spaces. spaces. Pump Pumptests testsof ofwells wellsopen openmostly mostlyto tothis thiscomponent componenttypically typicallyrange range from from about about 2-20 2 -20ft/day, ft/day, indicating indicatingaamoderate moderateto to high high conductivity, conductivity, corroborating corroborating the results results of the plug tests 1 996a). tests (Miller, (Miller, 1984; 1984;Setterholm Setterholmand and others, others, 1991; 1991;Miller Miller and and Delin, Delin,1993; 1993;Runkel Runkel1996a). The carbonate-rock component consists of limestone or dolostone with minor shale and sandstone. Porosity features, and karst features. sandstone. Porosityand andpermeability permeabilityisisdue dueto to fractures fracturesand solution features, The permeability permeability varies varies substantially substantiallyfrom from place place to to place place depending dependingon on the the size, size, extent, extent, degree degree and interconnectivity interconnectivityof of fractures fracturesand andsolution solutionfeatures features(Libra (Libraand andHallberg, Hallberg,1985; 1985;Visocky Visockyand and others, 1985). Pumping Pumping tests tests indicate indicate that that this this component component in most places is moderately moderately to highly permeable permeable where where such such features features are are well developed—typically developed-typically ranging from 1-40 1-40 ft/day Woodward, 1985). 1985). However, (Delin and Woodward, However, in places where fractures and solution features are minimally developed, developed, such such as as where where deeply deeply buried by younger bedrock, the component component has has aa very low conductivity, even at a large scale (Libra and Hallberg, 1985; Visocky and others, large scale (Libra and Hallberg, 1985; Visocky and others, 1985). 1985). The mixed carbonate and clastic component is composed of interbedded siltstone, very sandstone, sandy dolostone, dolostone, and shale (Setterholm (Setterholm and others, 1991; fine to very coarse grained sandstone, 1991; Cementationisisvariable; variable; beds beds of of mediummedium- to to coarse-grained coarse-grained sandstone sandstone are are Runkel, 1996b). Cementation typically friable, but very fine to fine-grained sandstone and siltstone are commonly strongly permeability from from bed bed to tobed. bed. Coarsecemented. As As aa heterolithic heterolithic unit, unit, it varies markedly in permeability grained grained sandstone sandstone beds beds that that are are too too weakly weakly cemented cemented to to provide provide coherent coherent samples samplesare are likely likelyto to be highly permeable. Laboratory Laboratorytests testsof ofthe themore morestrongly stronglycemented, cemented,finer-grained, finer-grained, clastic clastic 110 �0 Group, Formation. Member - ,.-% 5 - Natural Gamma Log D Lithology Increasingcount LIMESTONE LIMESTONE 100 API-G units DOLOSTONE DOLOSTONE SANDY SANDY SANDSTONE SANDSTONE TO FINE VERY FINE FINETO FINE Prosser Limestone FINE TO MEDIUM FI NE TO MEDIUM M EDIUMTO COARSE MEDIUM SHALY Cummingsville Formation SILTSTONE SILTSTONE SHALE Vugs Vugs (commonly (commonlytilled filledwith with coarse coarse calcite) calcite) Decorah Shale Chert Chert K-bentonile K-bentonitebed bed(altered (altered volcanic volcanic ash ash bed bed Oolites Oolites Glaucorrite Glauconite Iron Ironstain stain Goethite-hematite ore Goethite-hematite St. Peter Sandstone Phosphate pellets Phosphate pellets AaI mats Abal mats atromatolites Algal domes; stromatolltes Fossiliferous; fossils (symbols Fossiliferous; (symbolsnot not limestone and dotostone dolostone used in limestone units) units) bored Worm bored Pebbles (gravel Pebbles (gravelIn in unconsolidated units) unconsolidated Shakopee Formation Flat-pebble conglomerate Flat-pebble Cross-bedded Cross-bedded(testoon) (festoon) Cross-bedded (planar to tangential) - Ripple cross-laiThnatlons "TT^- Ripple cross-laminations ^S- 7 J- -r 7 J. J. Dolomitic Dolomitic Calcereous Calcareous Contact Contact marks marks aa major majorerosional erosional surface surface ,,'\ .f': .-"ÂFades Fadeschange change Low permeability permeability and porosity; Low hodzontal horizontal bar shortened shortened where lateral lateralextent extent is is limited limited high permeability Moderate to high permeability end porosity porosity and Oneota Dolomite l a Outcrop Intervals. intervals. Number Number refers refersto stop number number In the the field field guide guide in Coon valley Member column showing showing units units from from the the Prosser Prosser Limestone Limestone of of the Figure Figure 4. 4. Bedrock stratigraphic column Upper Ordovician Ordovician Galena Galena Group Group to the Upper Upper Upper Cambrian CambrianEau EauClaire ClaireFormation. Formation. Column Colunm includes hydrostratigraphic hydrostratigraphicunits units based based on on estimated estimated permeability permeability and and porosity. porosity. 111 �Group, Formation, Member - Natural Gamma Log Increasingcount - O 100 API-G units Coon Valle) Member Jordan Sandstone St. Lawrence Formation Birkmose Member Ironton and Galesville Sandstones Eau Claire Formation Figure 4. Continued Continuedfrom fromprevious previous page. beds, indicate aa low beds, low permeability permeability (Setterholm and and others, others, 1991). 1991). These These latter beds beds make make up up the the bulk latter bulk of this heterolithic meager heterolithic component, and meager pumping data data indicates a horizontal pumping horizontal conductivity conductivity of of about one-fifth that of the coarse coarse clastic clastic facies facies (Runkel, (Runkel, 1996a). 1996a). Lateral and vertical variability variability in abundance and interconnectivity of abundance and fractures can markedly markedly affect affect the the hydrologic behavior of each of the four components described above. above. Such features features are most abundant abundant and best best interconnected where the bedrock is at or near (within (within 100 100 feet) feet) the the surface. surface. Siliciclastic components componentssuch such as as the fine Siliciclastic which has low to clastic component, component, which very low intergranular intergranular permeability, may be orders orders of of magnitude magnitude higher higher in in conductivity where they they lie near the conductivity where the surface because because there there is a substantial surface substantial amount of flow along fractures. amount of flow along fractures. Conversely, some carbonate units may may have a relatively relatively low low conductivity conductivity and and act as confining confining beds where they they are are covered by younger covered by younger bedrock bedrock and and secondary porosity is not well developed secondary porosity is developed (Libra (Libra and Hallberg, Hallberg, 1985; 1985;Visocky and others, 1985). 1985). Additionally, Additionally, fracture flow may be be dominant even in the flow may dominant even coarse-clastic component, which which has a high intergranular intergranular permeability permeability where it lies near the surface. surface. aquifers and Delineation of aquifers and confining confining beds Given the hydrostratigraphic hydrostratigraphiccharacter character of the components components described described above, above, the Paleozoic Paleozoic section can be divided and low low permeability permeabilityunits units(Fig. (Fig.4). 4). The The coarse coarse divided into moderate/high moderatehigh and clastic that contribute contribute most most clastic and carbonate carbonate rock rock component component (where (where karstic) karstic) are likely the sources that of the yield yield to to water water wells wells developed developed in in Paleozoic Paleozoic strata. The Thefine-clastic fine-clasticcomponent component can can potentially yield particular where it is fractured near the surface, potentially yield moderate moderatequantities quantitiesof of water, water, in particular but more importantly itit acts acts as as aa confining confiningunit unit separating separatingcoarse-clastic coarse-clastic and and karstic karstic aquifers aquifers (e.g., Wenck and Associates, Associates, Inc., 1997). The component contains The mixed carbonate carbonate and clastic component contains individual beds that can yield moderate quantities of water, but where greater than 10 feet individual beds that can yield moderate quantities of water, but where greater than 10 feetthick, thick, this unit apparently apparently also (Setterhoim and and others, others, 1991). Unfractured Unfractured carbonate also acts as an aquitard (Setterholm carbonate rock can also Visockyand andothers, others, 1985; 1985;Donahue Donahueand andAssociates, also serve serve as a confining bed (e.g., Visocky Associates, 1991; Barr Engineering, Engineering, 1996). 199 1;Ban1996). 112 �Of particular particular importance this more more rigorous rigorous hydrogeologic hydrogeologic approach importanceto to this this field field trip is that this approach indicates that that two two widely widely used used aquifers, aquifers,the the Prairie Prairie du du Chien-Jordan Chien-JordanAquifer Aquifer and and the the FranconiaFranconiaindicates Ironton-GalesvilleAquifer, Aquifer, are are not single, hydraulically connected aquifer systems as commonly Ironton-Galesville commonly believed believed (e.g., (e.g., Kanivetsky Kanivetsky and and Walton, 1979). Pumping Pumping tests (Miller, (Miller, 1984; 1984;Delta Environmental Consultants, Inc., Inc., 1992; 1992;Miller Miller and and Delin, Delin, 1993) 1993)and and carefully carefullycollected collectedlocal localstatic staticwater waterlevel level Consultants, measurements measurements(Delta (DeltaEnvironmental EnvironmentalConsultants, Consultants,Inc., Inc.,1992; 1992;Wenck Wenckand andAssociates, Associates,Inc., Inc.,1997) 1997) demonstrate that that the the fine fineclastics clasticsof ofthe theFranconia FranconiaFormation Formationhydraulically hydraulicallyseparate separate clearly demonstrate ground ground water water in in the the fractured, fractured, upper upper part part of of the the Franconia Franconia Formation Formation from from ground ground water water in in coarse below. Some fractured, fractured, carbonate-cemented carbonate-cemented coarseclastics clastics of the Ironton-Galesville Sandstones below. rock rock in in the the lower lower part part of of the the Franconia, Franconia, where it occurs at or near the surface, is the source of many many springs springs in in the the area area and andisisperhaps perhapsthe theonly onlyhighly highlypermeable permeableconduit conduitfor forgroundwater groundwater within the the Franconia Franconia Formation. Formation. The ThePrairie Prairiedu duChien-Jordan Chien-JordanAquifer Aquiferisis likewise likewisetwo two distinct distinct within and coarse clastic clastic aquifer-separated aquifer—separated and separate separateaquifers—an aquifers-an upper upper carbonate carbonateaquifer aquiferand and a lower, coarse by clastic component. component. Hydraulic by an an intervening intervening aquitard aquitard composed composed of the mixed carbonate and clastic Hydraulic separation separationof of the the carbonate carbonateand and coarse-clastic coarse-clasticaquifers aquifersisis indicated indicatedby by several severallines lines of hydrologic hydrologic evidence including: Donahue and andAssociates, Associates,1991; 1991; including: potentiometric potentiometric data data (Kanivetsky, (Kanivetsky, 1988; 1988; Donahue N.O. pumping tests (Barr N.O. Janick Janick ,,Winona Winona State University, University, oral communication communication 1994), pumping (Barr Engineering, Engineering, 1996; 1996;N.O. N.O. Janick, Janick,Winona Winona State StateUniversity, University, oral oral communication, communication,1994), 1994),and andby by ground-water ground-waterchemistry chemistry(Alexander, (Alexander,1990; 1990;Setterholm Setterholmand andothers, others,1991, 1991,Wall Wall and and Regan, Regan, 1994). 1994). FIELD FIELDTRIP TRIPSTOPS STOPS Introduction Introduction As enigmatic features As we we examine examinethese these rock rock exposures, exposures,think think about about the enigmatic features and and long-standing long-standing sedimentologic sedimentologic problems associated with these units. For For example, example,what what created created the the extreme extreme maturity of the fine to coarse-grained sandstones? sandstones? Is there really a dearth dearth of of shale? shale? Why is it difficult to place your hand on an unconformity in these rocks? In addition, difficult unconformity in these rocks? In addition, think think about about the water-bearing water-bearing characteristics that are are determined determined by grain grain size, size, cementation, cementation, fractures, fractures, and and dissolution dissolutionfeatures. features.With Withthis thisininmind, mind,judge judgefor foryourself yourselfthe thevalidity validityof of some someof of the the commonly commonly accepted accepted depictions depictions of of the the hydrogeologic hydrogeologiccharacteristics characteristics of the Paleozoic Paleozoic rocks. For Forexample, example, does doesthe thePlatteville PlattevilleFormation Formationlook looklike likeaaconfining confiningunit unit that that will will impede impedethe the downward downwardtransport transport of of contaminants, contaminants,as asititisis commonly commonlyshown shownto to be on maps? maps? Compare Comparethe theFranconia FranconiaFormation Formationtoto the try to to determine determine why the former is most often often regarded as an an the Eau Eau Claire Claire Formation, Formation, and and try aquifer aquifer and and the the latter latteras asaaconfining confiningbed. bed. Examine Examinethe thedramatic dramaticvariability variabilityininthe theporosity porosityand and permeability ChienGroup Group and and Jordan Jordan Sandstone Sandstone and ask yourself whether permeabilitywithin withinthe thePrairie Prairiedu du Chien the the commonly commonly accepted accepted hydrogeologic hydrogeologic depiction depiction of of these these units units as asinternally internallyhomogenous, homogenous, hydraulically connected aquifers is accurate. hydraulically connected aquifers is accurate. The 8, 10, Thelocation locationof of individual individualstops stops is is shown shown on Figures 5, 7, 7,8, 10,and and 11. 11. Directions Directionsfrom from major major road road intersections, intersections,or orstreet streetaddresses, addresses,are arealso alsoprovided providedfor foreach eachstop. stop. STOP STOP1—Rochester I-Rochester Public PublicUtilities, Utilities,4000 4000E.E.River RiverRoad RoadNE, NE,Rochester, Rochester, Minnesota. Minnesota. An An overview overview of of the thedepositional depositionalhistory history and andhydrogeologic hydrogeologicattributes attributes of of the thePaleozoic Paleozoic strata strataof of southeastern southeasternMinnesota Minnesotawill willbe bepresented presentedatatthis this stop. stop. Paleozoic Paleozoicbedrock bedrockininthe theRochester Rochester area areaare arenearly nearly flat flat lying, lying, and and have have been eroded into an upper and lower plateau. The The resistant resistant limestone and dolomite formations of the Galena Group form an upper plateau that skirts limestone and dolomite formations of the Galena Group form an upper plateau that skirtsthe the 113 �8, 10, Figure 5. 5. Index Index map map of of field field stops. Black Blackboxes boxes refer refer to to maps maps in in Figures Figures 7, 7,8, 10,and and 11. 11. Figure perimeter perimeter of of Rochester. Rochester. The Themostly mostlycarbonate carbonaterocks rocksof of the the Prairie Prairie du du Chien Chien Group Group form form a lower plateau along the South Fork of the Zumbro River and the lower ends of its tributaries, in and along the South Fork of the Zumbro around around the the city city of Rochester. These Thesetwo twoplateaus plateausare areseparated separatedby by an an escarpment, escarpment, composed composed of of Decorah Sandstone, which exposed DecorahShale, Shale,Platteville PlattevilleFormation, Formation,Glenwood GlenwoodShale, Shale,and and St. St. Peter Peter Sandstone, which is exposed on steep city. We will examine most steep bluffs within and around the city. most of of these these units units on on the the trip trip today, as well as as several several older older units, units, such such as as the Jordan Sandstone. Sandstone. The hydrostratigraphic attributes of of many many of the Paleozoic units that we will will examine examine today have have aa direct direct or orindirect indirectimpact impacton onthe theavailable availablewater watersupply supplyfor forthe thecity cityof ofRochester. Rochester. The understanding the The city city has has devoted devoted aa considerable considerable amount amount of of effort effort and resources to understanding the geologic geologic controls controlson on ground-water ground-waterquality qualityand andquantity quantityin in their their region. region. An Aninvestigation investigationof of variable variableyield yield from Dolomite from city city wells wells that that draw draw water water from from the Jordan Sandstone Sandstone and and lower part of the Oneota Dolomite (Runkel, 1996a) 1996a) provides provides an an example example of of where where lithostratigraphic lithostratigraphic and and hydrostratigraphic hydrostratigraphic boundaries do not coincide. coincide. The Theopen-hole open-holeinterval interval of of these these city city wells wells cross cross three three or or more more distinct hydrostratigraphic hydrostratigraphic components—the components-the fine-clastic, fine-clastic, coarse-clastic, coarse-clastic, and mixed carbonate carbonate 114 �and clastic components (Fig. 6). Permeability Permeability values values of of these these components components vary by as much as four orders of magnitude. magnitude. Grain permeability Grainsize, size,sorting sortingand and degree degree of cementation cementation affect affect the permeability of these rocks, all within boundaries of what is currently designated as a single aquifer unit— unitthe Prairie du Chien-Jordan Aquifer. Pumping records of municipal wells finished in these Chien-Jordan Aquifer. Pumping municipal components, all three components, all considered considered "Jordan" wells, wells, show show that the productivity productivity of individual individual wells wells is directly proportional to the thickness of the coarse-clastic component exposed in the openhole interval (Runkel, (Runkel, 1996a). 1996a). This stop Formation in the the large large quarry quarry west west of of stop shows shows the sandy dolomite of the Shakopee Formation the road. Notable Notablefeatures featuresinclude includethe thethin thincover coverof of the the bedrock bedrock and and the the highly highly fractured fractured nature nature rock hydrostratigraphic hydrostratigraphic component. component. The high density density of of fractures fractures isis typical of of the carbonate rock all bedrock units near the land surface surface in this region. region. Also visible visible to the southeast southeast is the quartzose quartzose facies of the St. Peter Sandstone Sandstone and the overlying Glenwood Shale and Platteville Limestone. recharge to to the the Prairie Prairie du du Chien Group and Jordan Delin (1991) showed that ground-water ground-water recharge Sandstone is focused along along the the edge edge of of the the plateau plateau above abovethe the St. St. Peter Peter Sandstone. Sandstone. Ongoing investigations into into the the phenomena phenomena of of focused focused recharge recharge by by the the USGS USGS based based on earlier work investigations (Delin, 1991; 1991; Delin and Almendinger, 1991) 1991) will provide better data on the hydrologic budget of the region by quantifying quantifying the amount amount of water recharging recharging from from upper plateau. plateau. STOP 2—Bailey Quarry near STOP %Bailey Quarry nearSt. St.Charles, Charles,Minnesota. Minnesota.West Westside sideof ofMinnesota Minnesota Highway 74, about 1.2 1.2 miles south south of Interstate Interstate 90. Winona County, Minnesota (Fig. 7). This quarry has one one of of the the most most accessible, accessible, continuous continuous exposures across the upper upper St. St. Peter Sandstone, Glenwood Shale, and Platteville Formation in Minnesota. Our work on this part of the Paleozoic reconnaissance nature at this time. Most Paleozoic section section is of a reconnaissance Most workers workerswho whohave have studied these units have suggested suggested that they are part of an overall transgressive sequence (e.g., Dott and others 1986; Mossler, 1985). A 1986; Mossier, 1985). A rise rise in in sea sea level level is is recorded recorded by the the facies facies change change upsection from fine- to coarse-grained coarse-grained quartzose sandstone deposited in a shoreface (St. Peter deposited in deeper offshore shelf areas (Glenwood (Glenwood Shale), culminating culminating in rnicritic micritic SS.), to shale deposited carbonates offshoreconditions conditions (Platteville (PlattevilleFormation). Formation). While While carbonatesthat that accumulated accumulated in the most distal offshore this scenario scenario is plausible, plausible, some some features features suggest that the depositional depositional history may be more more complex. For example the upper half to two-thirds of the St. Peter Sandstone coarsens upward For example the upper half to two-thirds of the St. Peter Sandstone coarsens upward in southeastern wh,ich in in other other sheet sheet sandstones sandstones is is indicative indicative of shallowing shallowing rather rather southeastern Minnesota, Minnesota, wh,ich than deepening. Workers Workersin inother otherstates states(e.g., (e.g., Simo Simoand and others, others,1997) 1997)have have suggested suggestedthat that the the St. Peter Sandstone Sandstone is capped by an unconformity, which also suggests that its deposition was culminated by a regressive event. Additionally, Additionally, the Glenwood Glenwood Shale contains discrete beds of medium- to coarse-grained sandstone, some with with a channel-shape, channel-shape, and and the the lower lower part part of the Platteville Formation Formation contains contains matrix-supported, matrix-supported,coarse coarsequartz quartzgrains. grains. These attributes are incompatible incompatible with with deposition depositionin in aa relatively relatively deep, deep, distal distal offshore offshore shelf shelf setting. setting. here. The There are three hydrostratigraphic hydrostratigraphic components exposed here. The Platteville Platteville Formation Formation consists fractured carbonate carbonate rock component, the Glenwood Glenwood Shale Shale consists consists mostly mostly consists chiefly chiefly of the fractured of the fine clastic clastic component, component, and Sandstone consists coarse clastic and the the St. St. Peter Sandstone consists of the coarse clastic component. component. Hence, across this part of of the the Paleozoic Paleozoic section section lithostratigraphic lithostratigraphic boundaries nearly (within a meter) coincide with the principle hydrostratigraphic boundaries. We We will see examples at principle hydrostratigraphic boundaries. subsequent stops where the lithostratigraphic lithostratigraphic boundaries do not correspond to the principal principal hydrostratigraphic hydrostratigraphic boundaries. boundaries. 115 �____ ____ _____ NORTH /—/ f • - - . . . ___ .. . . . . - S . a — . ••' * _z___7__. . . . .-r in - . ,— . ci - •—---— • s . . . coarse clastic . ._.— . .—- •'. ._._—e—t'-—-':; . .. s. — — ———— — • / . . . . . • coarse clastic . . - . mixed carbonate and clastic.,- ' ' ——-k SOUTI+ . ., __ - coarse clastic ——— .--fineclastj,. - — — . St. Lawrence Formation St. Lawrence Formation L Lines on section dashed where control is poor 434041 161425 mcc rncct 220166 1 mcc mcc cc Ic fc I I Ic xl 2C64 21ca11 222525 2226S - - mcc cc cc E)QLANATION mcc mixed mixedciastic clasticand andcarbonate carbonatecomponent component Ic mixed clastic ciastic and and carbonate carbonatecomponent component fc cc cc mixed clastic clastic and and carbonate carbonate component component mcc SI St. Lawrence Formation sl St. Lawrence Formation - - Ic fC — cc cc Ic fc Ic cc Ic SI Figure 6. West-east West-eaststratigraphic stratigraphiccross-section cross-sectionand andcorresponding correspondinggamma gammalogs logsfor forthe theJordan JordanSandstone Sandstoneand andthe thelower lowerpart partof ofthe theOneota Oneota Figure Dolomite in the Rochester Metropolitan area, showing the variable thickness of the coarse-clastic and fine-clastic fine-clastic components of the Jordan beneath the mixed carbonate and clastic Coon Valley Valley Member Member of of the the Oneota. Oneota. (Modified from from Runkel, Runkel, 1996a.) 1996a.) �Figure 7. Map Map11on onindex indexmap map(Fig. (Fig. 5) 5) showing showing the the location location of of stop stop 2. 2. quarry is a good example example of the geologic geologic controls on ground-water ground-water recharge recharge discussed This quarry discussed carbonate Platteville Formation is through through horizontal at Stop 1. Ground-water Ground-waterflow flow through through the carbonate vertical joints. joints. Matrix fractures along bedding planes and vertical Matrix porosity and permeability does not contribute substantially Platteville bench contribute substantiallyto to ground-water ground-water flow. flow. Water Water pools pools on on top of the lower Platteville because the underlying underlying Glenwood Glenwood Shale Shale restricts restricts the downward downward movement of ground ground water. water. conductivity of of the the Glenwood Shale Shale is is estimated estimated to to be be similar to values for shale Hydraulic conductivity reported in in the the literature literatureof of iO to toiO rn/day &day(Freeze (Freezeand andCherry, Cherry,1979). 1979). The Thesurface surfacewater water Platteville bench bench and and onto onto the the top top of of the Glenwood Shale entering the quarry runs across the Platteville before spilling quarry. This stream does not account for all the spilling into the St. Peter section of the quarry. recharging the St. Sandstone. Joints water recharging St. Peter Sandstone. Jointsare arevisible visiblein in the the Platteville Plattevillethat that extend extend into into the the Glenwood Shale. The ThePlatteville PlattevilleFormation Formation and and Glenwood Glenwood Shale form aa flat flat plateau plateau across across much of the Twin Cities area. The Glenwood Shale is an important confining bed in Twin area. The Glenwood an important bed that it restricts contaminant contaminant transport transport to to lower lower bedrock aquifers aquifers where where it is is not not crossed crossed by by fractures. fractures. STOP 3—Road >Road Cut Cut near nearHomer HomerMinnesota. Minnesota.From Fromintersection intersectionofofState StateHighway Highway 43 43 and and U.S. 61 61 inWinona, in Winona,take takeState StateHighway Highway 61 61 south south for 3.5 miles. Turn right on County Road 15 (at Homer) and travel 2.2 miles to large large road cut (Fig. (Fig. 8). 8). This exposures of the Jordan Sandstone This is one one of the most complete, complete, readily accessible accessible exposures Sandstone in the entire central mid-continent region. The mid-continent region. The section section is is continuously continuously exposed from the upper part of the St. Lawrence Lawrence Formation, Formation, through the entire Jordan Sandstone, Sandstone, and most of the the Oneota Dolomite. cycleisisrecorded. recorded. The very fine grained complete regressive-transgressive regressive-transgressive cycle grained Dolomite. AA complete sandstone, shale and dolostone dolostone of the St. Lawrence Lawrence Formation Formation was deposited deposited in relatively relatively deep deep water during a high high stand stand of of sea sealevel level (Hughes (Hughes and and Hesslbo, Hesslbo, 1997), 19971,which which was was followed followed by by 117 �KILoMmERs MILES 1 -1 - 0 - 1 0 - 2 I 3 4 2 5 3 6 1 4 7 8 5 9 $0 6 Map22on onindex indexmap map(Figure (Figure5) 5) showing showing the the location location of of Stop Stop3. 3. Figure 8. Map progressive shallowing recorded in the coarsening coarsening upward Jordan Sandstone, culminated by subaerial erosion marked by aa pebbly pebbly sandstone sandstone lag. lag. The Oneota Oneota Dolomite Dolomite above above the the unconformity unconformityrecords recordsthe thereturn returnof ofthe theshoreline shorelineand andgradual gradualdeepening deepeningin inthe theEarly EarlyOrdovician. Ordovician. Lowermost Oneota strata (Coon Valley Member) are mixed clastic carbonate deposits that Oneota strata (Coon Valley accumulated in supratidal to shallow subtidal conditions. conditions. Upper Upper Oneota Oneota strata strata (Hagar (Hagar City City Member) dolostonedeposited depositedin in deeper, deeper, entirely entirely subtidal subtidal conditions. conditions. Member) are are mostly mostly sand-free sand-freedolostone Particularly include large-scale, large-scale, high-angle high-angle Particularly interesting interestingsedimentologic sedimentologicfeatures features at this stop include cross cross strata stratawith with reactivation reactivationsurfaces surfacesand and shale shaledrapes drapesin in the the Jordan Jordan Sandstone Sandstonethat that may may record record a strong influence of tidal currents, and bbcabbage-head" "cabbage-head" stromatolites stromatolites in the lower Oneota. Oneota. The The depositional can account account for for the the sedimentologic sedimentologic and stratigraphic stratigraphic depositional model depicted depicted in in Figure Figure 33 can attributes of the St. Lawrence Formation and Jordan Sandstone, but does not readily explain the attributes Lawrence Formation and Jordan Sandstone, attributes attributes of the overlying overlying Oneota Oneota Dolomite Dolomite This exposure example,whereaa major major hydrostratigraphic hydrostratigraphicboundary boundary does does not not exposure includes includes an an example.where correspond hydrostratigraphic variability correspondto to aa lithostratigraphic lithostratigraphicboundary, boundq, and and also also shows shows the great hydrostratigraphic variability within internally homogenous homogenous in in hydrogeologic hydrogeologic within lithostratigraphic lithostratigraphicunits units that that are are widely regarded as internally properties. properties. The TheSt. St.Lawrence Lawrence Formation Formation and and lower lower part of the the Jordan Jordan Sandstone Sandstone are both composed confining composedof of the the fine-clastic fine-clastic component. component. The TheSt. St.Lawrence Lawrence is is commonly commonly regarded regarded as as a confining bed and the the Jordan Jordan as as an an aquifer, aquifer, yet yet there there is is no no distinct distinct hydrogeologic hydrogeologic boundary between the two lithostratigraphic units as is commonly believed. believed. As As you you walk walk up up the the outcrop, outcrop, consider consider where where you would would define define the the bottom bottom and and the the top top of the "Jordan Aquifer." Aquifer.'' The The mixed mixed carbonate carbonate and and clastic clastic component componentthat that composes composesthe the Coon Coon Valley Member of the Oneota Dolomite (Prairie (Prairie du du Chien Chien Group) Group) isis markedly markedly lower lower in in porosity porosity and and permeability permeability than than the the coarse coarseclastics clasticsof ofthe the uppermost Jordan Sandstone. The Theoverlying overlying carbonate carbonate rock rock component component of of the the Hagar Hagar City City Member fractures. Fracture Member is tightly cemented, cemented, with primarily horizontal fractures. Fracture tops show evidence 118 �of dissolution. Larger scale scale vertical vertical dissolution along dissolution. Larger along aa vertical verticaljoint joint isis also also visible visible here. here. If such fractures and dissolution dissolution features features were not developed in the subsurface subsurface this unit would be be a confining bed. BIOSTRATIGRAPHY OF OF CONODONTS CONODONTS FROM FROM JORDAN JORDAN SANDSTONE SANDSTONEAND AND LOWERMOST ONEOTA LOWERMOST ONEOTA DOLOMITE DOLOMITENEAR NEAR HOMER, HOMER,MINNESOTA MINNESOTA J.F. Miller* and and Anthony C. C. Runkel Runkel *Southwest *Southwest Missouri Missouri State State University, University, Department Departmentof of Geography, Geography,Geology, Geology, and and Planning, 901 South South National National Avenue, Springfield, Missouri 65804-0089 The physical evidence for the presence presence of aa large large magnitude, magnitude, regional regional unconformity unconformity associated with the Jordan Sandstone has been been debated for decades. Smith Smith and and others others (1993) (1993) described subaerially formed silcrete that occurred at or or below below the the Jordan Jordan Sandstone-Oneota Sandstone-Oneota Dolomite contact at at a few outcrops Wisconsin. Runkel that a pebbly Dolomite contact outcrops in Wisconsin. Runkel (1994) suggested suggested that sandstone that caps the Jordan Jordan Sandstone Sandstone at many outcrops in Wisconsin and Minnesota is an erosional lag of regional extent. ItIt everywhere everywhere separates separates a major, decameter-scale coarsening decameter-scale fining upward upward sequence sequence above, above, and and was was formed formed upward sequence sequence below from a decameter-scale extreme basinward basinward positions positions in inlatest latestCambrian Cambriantime. time. The when the regressive shoreline was in extreme to be be more more deeply deeply truncated beneath this lag on the margins of of Jordan Sandstone was shown to the Hollandale Embayment Embayment (Runkel, (Runkel, 1994). 1994). The work described above suggests that that an unconformity unconformity is is present present near near or or at at the the top top of of evidence is subtle and and cannot cannot be be recognized recognized at at all all outcrops outcrops the Jordan Sandstone Sandstone but the physical evidence (Fig. 9). In unconformity and and determine determine its magnitude, magnitude, In an an effort effort to substantiate substantiate the presence of an unconformity Jordan Sandstone and from the conodont samples samples were were collected collected at the Homer section from the Jordan overlying Coon Valley Member of the Oneota Dolomite overlying Coon Dolomite by A. C. C. Runkel Runkel and were processed processed by J.F. Miller. Samples J.F. Samples from the Jordan Sandstone Sandstone contain contain only two conodont conodont species, species, Proconodontus muelleri muelleri (Miller) Proconodontus (Miller) and and Eoconodontus Eoconodontus notchpeakensis notchpeakensis (Miller), (Miller), which which range range America and and are are diagnostic diagnostic of of throughout the Jordan. These These species species are widespread in North America of the theEoconodontus Eoconodontus Zone, Zone, approximately approximately of middle middle the Eoconodontus notchpeakensis Subzone of Age. This from samples samples 1 through through 10. 10. Sample 11 Trempealeauan Age. This fauna fauna has been recovered from 11 is from the Coon Coon Valley Valley Member of the Oneota Oneota Dolomite Dolomite and and yielded yielded aa completely completely different different conodont fauna. This Thissample sampleincludes includesAcanthodus uncinatus uncinatus (Furnish), (Furnish),Aloxoconus Aloxoconus iowensis iowensis (Furnish), Aloxoconuspropinquus (Furnish), Cordylodus intermedius (Furnish), (Furnish), Aloxoconuspropinquus (Furnish), Aloxoconus sp., Cordylodus (Furnish), Cordylodus lindstromi lindstromi (Druce (Druce & & Jones), Jones), aa new species of Cordylodus known known from from Utah Utah and and Polycostatus oneotensis oneotensis (Furnish). Many Texas, and Polycostatus Many of of these these species species were named by Furnish (1938) (1938) from from the Oneota Oneota in the Iowa-Minnesota area. The The presence presence of of these these species species in in the the Coon Coon Valley Member indicates a close age relationship between sandstones of the Coon Valley and Valley overlying dolomite the assignment assignment of these these sandstones sandstones the overlying dolomite member of the Oneota, and supports the to the Oneota rather than to the Jordan. This This assemblage assemblage of species species is at least as young as the Cordylodus angulatus angulatus Zone Rossodus manitouensis manitouensis Zone. Cordylodus Zone but but could could be from from the the overlying overlying Rossodus These two zones are are from from the the middle middle to to upper upper part part of of the the Skullrockian Skullrockian Stage Stage of of the the Ibexian Ibexian Series (Lower (Lower Ordovician) Ordovician) in the Ibex Ibex area area of western Utah (Ross (Ross and and others, others, 1997). 1997). Sandstone and Coon Valley Valley Member Member are are separated separated These faunas indicate indicate that the Jordan Sandstone by a large hiatus hiatus compared compared with with the the continuous continuous succession successionin in the the Ibex Ibex area area of of Utah, Utah, which which was was 119 �ZONATION CONODONT CONODONT ZONATION MIller MIIler (1988) ( 1 988) one I oneI z e = conodonts conodonts LU <I-. 0 wO a z-J 0 Rossodusmanitouensis Ii z 0 I— (1, 0 z Cordylodus lindstromi . 20 15 I.—. 'Jb z 0 0 . . , .5 .4 '3 C,, 7 I,,,,, It 1 1 7 .1 & 7' T1 0• liii VF FMCVC pJg Burrows B Intraclasts gpJ l r l ~ l a s t s Hunmocky cross-strata @Dolostone O ~ o r l e Ripple cross-strata R i i cross-strala =EEzL Z hlntzei HirsutodontUS simplex ... Clavohamulus elongatus d1 -. )< LU Fryxellodontus inornatus Hirsutodontus 10 z< Clavohamulus hirsutus Cambroistodus minutus 5. • LU lapetog,wthus w C-) UJ Cordylodusangulatus 25 z Subzone Subzone 1t•D• 5 < I- EoconodontUS notchpeakensis — z= < < <x WQ ILl o '- Proconodontus mueller! Trou mTang8nMl o w cross-strata cross-stram Tangentlel cross-strata cross-stmm thick thick hnes h e s are are shale shale drapes drapes MedP.sn to ccoarse-graied M d b m to o a r s w n e dS.S. S.S. m .•: nFimt+gmmd S.S. Fine-gravied S.S. Pebbles Po- Figure9. 9.Measured Measuredsection sectionofofthe theJordan JordanSandstone Sandstoneand andadjacent adjacentunits unitsatatHomer, Homer, Minnesota Minnesota Figure outcropyhighlighting highlightingbiostratigraphic biostratigraphicinterpretation interpretationbased basedon onconodonts conodontsasasreported reportedherein. herein. outcrop, Thevertical verticallines linesshow showthe therelative relativemagnitude magnitudeof of the the hiatus hiatus represented The represented by the unconformity unconformity on top of the Jordan Sandstone. on top of the Jordan Sandstone. deposited rapidly subsiding subsidingmiogeoclinal miogeoclinal carbonate platform. The Thefollowing followingconodont conodont deposited on on aa rapidly units known known in in Utah Utahare aremissing missingbetween betweenthe theJordan Jordanand andCoon CoonValley ValleyatatHomer: Homer: biozonal units Cambrooistodus Cambrooistodus minutus Subzone Subzoneof ofthe theEoconodontus Eoconodontus Zone Zone(uppermost (uppermostTrempealeauan, Trempealeauan, Upper Cordylodusproavus Zone Zone(basal (basalSkullrockian, Skullrockian,Lower Lower Upper Cambrian), Cambrian),three three subzones subzonesof of the Cordylodusproavus Ordovician), Ordovician), two two subzones subzones of the Cordylodus intermedius Zone, Zone, the the Cordylodusprolindstromi Cordylodusprolindstromi Zone, Zone, the Cordylodus Cordylodus lindstromi lindstromi Zone, Zone,and andthe thelapetognathus IapetognathusZone Zone(approximately (approximatelymiddle middle Skulirockian). Skullrockian). InInthe theIbex Ibexarea areaofofUtah, UtahyUpper UpperCambrian Cambriancarbonate carbonatestrata strataare areapproximately approximately 2850 2850ftft (870 (870m) m)thick thickand andare areoverlain overlainby by approximately approximately5000 5000ftft(1525 (1525m) m)of ofOrdovician Ordovicianstrata. strata. The The missing missinginterval intervalat atHomer Homerisisequivalent equivalentto to more more than than 500 500 ftft (150 (150 m) m) of of limestone limestonein in Utah. Utah. The The interval intervalincludes includesseveral severalstratigraphic stratigraphicsequences sequencesthat that include include at at least least two two major major lowstands lowstands of A similar similarlarge large hiatus hiatus is is of sea sealevel, level, as as well well as as several several transgressive transgressive and highstand sequences. A known known on on the the crest crestof ofthe theTranscontinental Transcontinentalarch archin inwestern westernColorado Colorado(Myrow, (Myrow, Ethington, Ethington,& & Miller, Miller,1995), 19951,which whichisisaawestern western extension extensionof of the the Minnesota Minnesota craton. craton. 120 �STOP 4—Road &Road cut cut along alongnorth northside sideofofCounty CountyRoad Road3030(time (timepermitting). permitting).From From County County STOP road 14, 14,travel travel east east on on County County road road 88 for for one one mile mile (becomes (becomesCounty Countyroad road 30 30in inWinona Winona road Continueeast easton onCounty Countyroad road 30 30for for about about 0.7 0.7 miles miles to to road road cut cut (Fig. (Fig. 10). 10). County). Continue The upper upper part part of of Oneota OneotaDolomite Dolomiteand and lowermost lowermost Shakopee ShakopeeFormation Formationof of the the Prairie Prairiedu du The Chien Group are exposed at this stop. The TheOneota OneotaDolomite Dolomite consists consists chiefly chiefly of of tan tan to to gray gray microcrystalline dolostone dolostone in thick to medium irregular beds. The Shakopee Shakopee consists of sandy, microcrystalline beds. The The New Richmond Sandstone Sandstone oolitic and intraclastic dolostone, with intercalated sandstone. The of the Shakopee uppermost part part of of this this exposure. exposure. Deposition Shakopee Formation forms the uppermost Deposition of of these these strata occurred occurred on a carbonate-dominated, laterally extensive strata carbonate-dominated, laterally extensive platform in environments ranging from from supratidal supratidal to to shallow shallow subtidal subtidal (Smith and others, 1993). The The Oneota Oneota Dolomite Dolomite and and Shakopee Shakopee Formation are separated by an unconformity unconformity less meters below below the the New New Richmond Richmond less than than 22meters Sandstone. Features Featuresdiagnostic diagnosticof ofdepositional depositionalorigin, origin,as aswell wellas asthe theunconformity, unconformity, are arelargely largely Sandstone. Stop99will willprovide provide aa better better opportunity opportunity to to more more closely closely inaccessible in this steep road cut. Stop examine the Prairie Prairie du Chien Chien Group. Group. The primary primary purpose purpose of this this stop stop is is to to discuss discuss the the karst karst features features exposed exposed in in this this road roadcut. cut. A well developed developed cave in the Oneota exists at the solution solution horizon near the Oneota-Shakopee Oneota-Shakopee contact. The Theunconformable unconformablecontact contactshows showssubaerial subaerialexposure, exposure,as as evidenced evidenced by by considerable considerable karst development development below below it, and and is is characteristic characteristic of of this this contact contact throughout throughout southeastern southeastern Minnesota and and western western Wisconsin (Smith and others, 1993, 1996). Sinkholes Sinkholes occur occur near near this this contact Maki, 1988; 1988;Magdeline Magdeline and and Alexander, Alexander, contact (Dalgleish (Dalgleishand andAlexander, Alexander,1984; 1984;Alexander Alexander and and Maki, 1995) catastrophic collapse treatment lagoons 1995) and have resulted in catastrophic collapse of several several sewage treatment lagoons (Alexander (Alexander and and Book, Book, 1984; 1984;Alexander Alexander and and others, others, 1993; 1993;Jannik Jannik and and others, others, 1991). 1991). The SW. Evidence for the The cave cave at this outcrop apparently formed in aajoint joint trending trending NE to SW. development development of this solution feature is visible. Ground Groundwater water originally originally flowed flowed horizontally horizontally along along aa permeable permeable bedding beddingplain, plain,widening wideningitself itself through throughdissolution dissolutionas as itit became became the the dominant dominant hydraulic conduit at this horizon. At the intersection of a vertical joint, water moved downward, hydraulic conduit at this horizon. At the intersection of joint, downward, dissolving dissolving the the bedrock bedrock and and increasing increasing the size of the cavity. This This feature feature widened widened from from top top to to bottom, ground-water flow. Characteristic bottom, as as the the conduit conduit captured captured increasing increasing amounts amounts of ground-water Characteristicfeatures features include fracture—and sinuous include aa flat-topped flat-topped ceiling—top ceiling-top of of the original original hydraulic fracture-and sinuous walls formed by the water water dissolving dissolving beds beds of of variable variable permeability. permeability. A A large largecollapse collapse adjacent adjacent to to the thecave, cave, presumably into a void extending into the outcrop, and the alignment of this feature with void extending with aa creek dissolution creek bed on on south south side side of of the the highway, highway, indicate indicate the extensive extensive nature of this type of dissolution within within the the Prairie Prairie du du Chien. Chien. The regional regional direction direction of ground-water ground-water flow with the Prairie du Chien Group is is to to the the southeast—perpendicular southeast-perpendicular to to the the trend of this large dissolution feature. Monitoring Monitoringof of groundgroundwater quality quality within within the the Prairie Prairie du du Chien Chien is complicated complicated by these types of features throughout southeastern southeastern Minnesota. Dye-trace Dye-trace studies studies have have documented documented cases where local flow is in aa direction perpendicular to the regional gradient (Donohue 1991). The The presence presence direction perpendicular to the regional gradient (Donohue and Associates, 1991). of of these these large-scale large-scaledissolution dissolutionfeatures featuresleads leads to to extreme extreme variability in transmissivity, and preclude the isotropic porous media in ground-water ground-water monitoring the treatment treatmentof of the the Prairie Prairie du du Chien Chien Group Group as as isotropic monitoring and and modeling. modeling. 121 �KILOMETERS MILES 1 1 - -0 0 2 - 1 1 3 4 2 6 .-7 8 3 ' 4 8 5 0 10 6 Figure 10. 10. Map Map33on onindex indexmap map (Figure (Figure 5) 5) showing showingthe the locations locationsof of stops stops44 through through 8. 8. STOP overlook of of Mississippi Mississippi River River Valley Valleynear nearWabasha. Wabasha. On STOP 5—Scenic 5ÑSceni overlook On north side of State State Highway Highway 60 60 about about 1.3 1.3miles miles east east of of U.S. U.S. 61 61 (Fig. (Fig. 10). 10). This This overlook overlook provides provides aa visual visual estimate estimate of of the the thickness thickness of of the the lower lower Jordan Jordan Sandstone, Sandstone, St. that lie lie below below our our feet. feet. The St. Lawrence, and Franconia Formations that The upper upper 50 50 ftftof ofJordan Jordan Sandstone, Sandstone,and andlower lower20+ 20+ftftof ofOneota OneotaDolomite Dolomite(Coon (CoonValley Valley Member) Member)are areexposed exposedhere. here.This This outcrop provides an excellent opportunity to examine the sequence bounding unconformity provides an excellent opportunity to examine the sequence bounding unconformity that quartzose Jordan Jordan Sandstone Sandstone below from from the that separates separates well-sorted, well-sorted,finefine- to to coarse-grained, coarse-grained,quartzose overlying overlyingheterolithic heterolithicCoon CoonValley Valley Member. Member. The TheCoon CoonValley Valley Member Member consists consists of very finefine- to coarse-grained coarse-grainedsandstone, sandstone,siltstone, siltstone,shale, shale, and and sandy dolostone. dolostone. The The erosional erosional lag lag marking marking the the unconformity Minnesota (Stop (Stop 3) 3) but but here consists of an unconformity was poorly developed developed at Homer, Minnesota an 2 m thick—that contains intraclastic, poorly sorted, complexly cross-bedded sandstone—i - 2 thick-that contains cross-bedded sandstone-1 abundant abundant pebbles pebbles of of Precambrian Precambrianrocks rocksas aslarge largeas as22cm cminindiameter. diameter. 122 �Note the the considerable considerable difference difference in grain grain size size and and degree degree of of cementation cementation between between the the Note Jordan JordanSandstone Sandstoneand andthe theCoon CoonValley Valley Member. The Theformer formerisisdominated dominatedby bythe the coarse coarseclastic clastic hydrostratigraphic hydrostratigraphic component, the latter by the the mixed mixed carbonate carbonate and and clastic clastic component. component. Permeability of the the mixed mixed carbonate carbonate and clastic component component is as much as three three orders orders of of Permeability magnitude magnitude less less than than that that in in the the underlying underlying coarse clastics. In Inunfractured unfracturedsubsurface subsurfacesettings, settings, this this unit unit and and overlying overlyingunfractured unfractured carbonate carbonate rocks rocks of of the the Hagar Hagar City City Member Member of of the theOneota Oneota Dolomite Dolomite can can act actas asaaconfining confiningbeds, beds, hydraulically hydraulically isolating isolating ground ground water water in in the the Jordan Jordan from from ground groundwater waterininthe theupper upperPrairie Prairiedu duChien. Chien.This Thisseparation separationcan canbe beseen seenininchemical chemical(Alexander, (Alexander, 1990; Setterholm and others, 1991; Smith and Nemitz, 1996; Wall and Regan, 1990; Setterholm and others, 1991; Smith and Nemitz, 1996; Wall and Regan, 1995) 1995) and and hydraulic hydraulicdata data(Barr (Ban-Engineering, Engineering,1996). 1996). STOP STOP6—Lower &Lower Franconia FranconiaFormation Formationnear nearWabasha, Wabasha,Minnesota. Minnesota.Outcrop Outcropisisalong alongwest west side sideof of U.S. U.S. Highway Highway61, 61,1.2 1.2miles milesnorth northof ofState StateHighway Highway60 60(Fig. (Fig.10). 10). The Thelower lowerFranconia FranconiaFormation Formationatat this thisstop stopisisan anexcellent excellentexample exampleof ofthe theheterolithic, heterolithic, very very fine finegrained grainedsandstone sandstoneand andshale shalefacies faciescharacteristic characteristicof of Cambrian Cambrian storm storm and and fairweather fairweather deposits that accumulated on the offshore shelf seaward of the quartzose shoreface sands. deposits that accumulated on the offshore shelf seaward of the quartzose shoreface sands. Thin Thin to tomedium mediumbeds bedsof offeldspathic, feldspathic,variably variablyglauconitic glauconiticsandstone, sandstone,siltstone, siltstone,and andshale shalecommonly commonly occur occurin innormally normallygraded gradedsequences sequencesas asmuch muchas as30 30cm cm thick. thick. Sandstone Sandstonebeds bedspinch pinchand andswell swellasas form form sets setsofofsmall-scale small-scalehummocky hummockycross-stratification, cross-stratification, which which in in aacomplete complete depositional depositional sequence, sequence,pass passtransitionally transitionallyupward upwardinto intoripple ripplecross-stratified cross-stratifiedor orstructureless, structureless,very veryfine finegrained grained sandstone sandstone and andsiltstone, siltstone,which which in in turn turn passes passes upward upward to to shale. shale. Bioturbation Bioturbationisiscommon, common,and and ranges rangesfrom fromisolated isolatedburrows burrows(Planolites (Planolitesand andSkolithos) Skolithos)totonearly nearlyhomogenized homogenizedsediment. sediment. The Thefine-clastic fine-clasticcomponent componentofofthe thelower lowerFranconia FranconiaFormation Formationisishydrostratigraphically hydrostratigraphically similar similarto tothe thesame samecomponent componentthat thatmakes makesup upthe thebulk bulk of of the theSt. St.Lawrence LawrenceFormation Formationseen seenatat Stop Stop3.3.The TheSt. St.Lawrence Lawrenceisiswidely widelyregarded regardedasasaaconfining confiningbed, bed,yet yetthe theFranconia Franconiaisiscommonly commonly designated designatedasasan anaquifer aquiferalong alongwith withthe theunderlying underlyingcoarse coarseclastics clasticsofofthe thefronton Irontonand andGalesville Galesville Sandstones. Sandstones.We Webelieve believethat thatthis thischaracterization characterizationof of the the Franconia Franconia is inaccurate. This Thispart partof of the onia has theFranc Franconia has permeability permeabilityvalues valuesseveral severalorders ordersof of magnitude magnitudelower lowerthan than underlying underlyingcoarse coarse sandstones sandstones(Miller (Millerand and Delin, Delin, 1993). 1993).Core Coresamples samplestaken takenfrom fromoutcrop outcrophad had permeabilites permeabilites of of 0.01 0.006md. md.(MGS (MGSunpublished unpublisheddata, data,1997). 1997).InInaddition, addition,wells wellsfinished finishedabove aboveand andbelow below 0.01--0.006 this thispart partofofthe theFranconia Franconiahave haveover overaa40-foot 40-footdifference differenceininhydraulic hydraulichead head(Wenck (Wenckand andAssoc., Assoc., 1997). 1997). STOP STOP7—Road 7-Road cut cuton onU.S. U.S.Highway Highway61 61near nearWabasha. Wabasha.From FromState StateHighway Highway60, 60,take takeState State Highway Highway61 61south south1.8 1.8miles milestotoroad roadcut cuton onwest westside sideofofroad road(Fig. (Fig.10). 10). The metersofoffronton IrontonSandstone Sandstoneand andabout about44meters metersof ofthe theoverlying overlyingFranconia Franconia Theuppermost uppermost55meters Formation are exposed here in a steep road cut. The fronton consists of poorly sorted sandstone Formation are exposed here in a steep road cut. The Ironton consists of poorly sorted sandstone that thathas hasaapervasive pervasivebioturbated bioturbatedtexture, texture,with withburrows burrowsasasmuch muchasasa acentimeter centimeterinindiameter. diameter. Shale Shaleand andsiltstone siltstoneoccur occuras asaamatrix matrixbetween between sand sandgrains grains and and as as lenticles. lenticles. Moderately Moderatelytotowellwellsorted sortedsandstone sandstonebeds bedsare areinternally internallystructureless structurelessororcontain containill-defined ill-definedtrough troughand andtangential tangential cross-strata cross-stratawith withrare rareshale shaledrapes drapesand andreactivation reactivation surfaces. surfaces. These Thesefeatures featuresare areindicative indicativeofof deposition deposition in inaashoreface shorefaceand andtidal tidalflat flatenvironment. environment. The Theoverlying overlyingstrata strataofofthe theFranconia Franconia Formation Formationconsists consistsmostly mostlyofoffeldspathic, feldspathic,glauconitic, glauconitic,finefine-totovery veryfine finegrained grainedsandstone sandstonewith with hummocky hummockycross-strata cross-strataand andtempestites tempestitesthat thatwere weredeposited depositedon onthe theoffshore offshoreshelf. shelf.These Theselower lower 123 �Franconia Franconia Formation Formation strata strata are are better better exposed exposed at at Stop Stop 6. 6. The The contact contact between between the the Ironton Ironton Sandstoneand andFranconia FranconiaFormation Formationisistransitional transitionaland and records records gradual gradual flooding flooding during duringan an overall overall Sandstone rise rise in in sea sealevel. level. The quartzose quartzosesandstone sandstoneof of the the fronton Ironton exposed exposed includes includesaa pebbly pebbly sandstone sandstonebed bed that that may may The mark aa regional regional unconformity. unconformity. The Theupper upper part part of ofthe theunderlying underlyingEau Eau Claire ClaireFormation Formation and and mark lower lower approximately approximately two-thirds two-thirds of of the the fronton/Galesville Ironton/Galesville Sandstones Sandstones (below (below level level of of this this exposure)form formaacoarsening-upward coarsening-upwardsequence sequencethat thatrecords recordsdeposition depositionininprogressively progressivelyshallower shallower exposure) water Ironton/GalesvilleSandstones Sandstones fines fines upward, upward, water conditions. conditions. The Theupper upperone-third one-thirdofofthe thefrontonlGalesville recordingdeeper deeperwater waterconditions. conditions.The Thenearly nearlystructureless, structureless,resistant resistantbed bedabout about22mmabove abovethe the recording base base of of this thisoutcrop outcropisisaa1.5-2 1.5-2mmpebbly pebblysandstone sandstonelag lagthat thatseparates separatesthese thesedecameter-scale decameter-scale sequences. sequences.This Thispebbly pebblylag lagcan canbe bephysically physicallytraced tracedfrom fromthe theTwin TwinCities Citiesarea areasouth southinto intoIowa, Iowa, and andfrom fromsouthwestern southwesternWisconsin Wisconsinwestward westwardatatleast least80 80kilometers kilometersinto intothe thesubsurface subsurfaceof ofsouthsouthcentral centralMinnesota. Minnesota.ItItmay mayrepresent representaatransgressively transgressivelyre-worked re-worked lag lag that that developed developed subaerially subaerially when when the the "Ironton-Galesville "Ironton-Galesville shoreline" shoreline"was wasininextreme extremebasinward basinwardpositions. positions. Compare Compare the the hydrostratigraphic hydrostratigraphic properties of the the Franconia Franconia Formation Formation and and Ironton Ironton Sandstone.The Thefronton Irontonisiscomposed composedmostly mostlyof ofthe thecoarse-clastic coarse-clasticcomponent, component,specifically specificallyof of aa Sandstone. poorly poorlycemented cementedmostly mostlyfine fine-- very verycoarse coarsegrained, grained,shaly shalysandstone sandstonewith withmoderate moderateporosity porosityand and permeability. The TheFranconia Franconiaabove aboveisisdominated dominatedby by the thefine fineclastic clastic component, component, which which is is finer finer permeability. grained Ironton,and and contains contains discrete, discrete, continuous continuous laminations laminations of of grainedand andbetter bettercemented cementedthan than the the fronton, shale. fronton. The shale.ItItisismarkedly markedlylower lowerin in porosity porosity and and permeability than the Ironton. The common common practice practice of of lumping lumpingthese thesetwo twounits unitstogether togetheras asaasingle singleaquifer aquiferis is not not reasonable, reasonable,and and gives gives the the erroneous erroneous impressionthat thatthey theyshare sharesimilar similar hydrogeologic hydrogeologicattributes attributes (see (see Stop 6 for impression for additional additional discussion). discussion) STOP STOP8—Road %Road cut cuton onU.S. U.S.Highway Highway61 61near nearWabasha. Wabasha.From FromState StateHighway Highway60, 60,take takeU.S. U.S. 61 61south south2.3 2.3miles milesto toroad roadcut cuton onwest westside sideof ofroad road(Fig. (Fig.10). 10). Intethngering InterfingeringEau EauClaire ClaireFormation Formationand andIronton-Galesville Ironton-Galesville(Wonewoc) (Wonewoc)Sandstones Sandstonesare are exposed in this long road cut. There are several interesting sedimentologic features, including exposed in this long road cut. There are several interesting sedimentologic features, including hummocky cross-strata, cross-strata, tempestites, tempestites, trough trough cross cross stratified stratified sandstone, sandstone, and and coarse coarse sand sanddune dune hummocky forms formsdraped drapedby byshale. shale.Trilobite Trilobitemolds moldsare areabundant abundantatatthis thisstop stopininthe thehummocky hummockystratified stratified sandstone sandstonefacies. facies.This Thisoutcrop outcropisisone oneofofonly onlytwo twoknown knownexposures exposuresininthis thisarea areathat thatrecord recordthe the boundary boundarybetween betweenthe theMarjumiid Marjumiidand andPterocephaliid Pterocephaliidbiomeres, biomeres,aamajor majorisochronous isochronousextinction extinction event correlatable across all of North America. event correlatable across all of North America. This Thisexposure exposureisisaagood goodexample exampleofofthe thesame samefine-clastic fine-clastichydrostratigraphic hydrostratigraphiccomponent component that thatdominates dominatesthe thelower lowerFranconia Franconia and and St. St. Lawrence Lawrence Formations seen at stops 3 and 6. Core Core samples samples from fromall allthree threeunits unitsshow showaaconsiderably considerablylower lowerdensity densityofoffractures fracturesalong alongbedding bedding planes. planes. STOP west STOP9—Vern 9-Vern Keller KellerQuarry. Quarry.Along Along westside sideofofCounty CountyRoad Road6,6,1.15 1.15miles milessouth southof ofState State Highway Highway19. 19.Goodhue GoodhueCounty County(Fig. (Fig.11). 11). About About30 30mmofofthe themiddle middlepart partofofthe thePrairie Prairiedu duChien ChienGroup Groupisisexposed exposedininthis thisquarry. quarry. Relatively Relativelypure puredolostone dolostonebeds bedsof ofthe theuppermost uppermostOneota OneotaDolomite Dolomiteare areunconformably unconformablyoverlain overlain by ShakopeeFormation Formationabout about22meters bysandy sandydolostone dolostoneand andsandstone sandstoneofofthe theShakopee metersabove abovethe thefloor floorofof the thequarry. quarry. This Thisunconformity unconformityisisregional regionalininextent, extent,traced tracedfrom fromthis thisarea areainto intothe theMichigan Michigan basin basin by bySmith Smithand andothers others(1993). (1993).Deposition Depositionofofthe thePrairie Prairiedu duChien ChienGroup Groupoccurred occurredon onaa shallow shallowcarbonate carbonateplatform platformduring duringaarelatively relativelyhighstand highstandof ofsea sealevel levelthat thatflooded floodedmost mostof of North North 124 �Map 4 KILOMETERS MILES 11 1 00 00 I1 — 22 1 1 33 — 22 44 = 55 33 6B = It 17 88 55 90 —10$0 6 Figure Figure 11. 11.Map Map44on onindex indexmap map(Figure (Figure5) 5) showing showingthe the locations locationsof of stops stops99and and10. 10. Americaduring duringthe the Early Early Ordovician Ordovician (Smith (Smith and and others, others, 1993; 1993; Smith Smith and Clark, 1997). 1997). These These America strataare are typical typical of of "tropical" "tropical" carbonate carbonatesystems, systems,dominated dominatedby oolitic oolitic and peloidal grainstones grainstones strata subtidal andpackstones, packstones,and andstromatolitic stromatoliticand andwavy-laminated wavy-laminatedboundstones boundstonesdeposited depositedininshallow shallowsubtidal and tosupratidal supratidalsettings. settings.Stromatolites Stromatolitesare areabundant abundantininthis thisquarry: quarry:digitate digitateforms formscan can be be discerned discerned to inaafew fewplaces placesininthe theuppermost uppermostOneota, Oneota,and andcabbage-head cabbage-headstromatolites stromatolitesare arecommon commonininthe the in Shakopee.Evidence Evidencefor forsubaerial subaerialexposure exposureincludes includesabundant abundantmud mudcracks, cracks,intraclastic intraclasticbreccias, breccias, Shakopee. and andsilicified silicifiedand andmoldic moldicanhydrite. anhydrite. Thereare aresome someinteresting interestingquestions questionsthat thathave haveyet yetto tobe be answered answeredabout aboutthe thedeposition depositionof of There the ChienGroup: Group: thePrairie Prairiedu duChien 1) 1)What What isis the the fundamental fundamentalcontrol control that that blocked blocked detrital detrital input input when when most most of of the the Oneota Oneota was deposited? deposited?Much Muchofofthe theOneota Oneotaconsists consistsofofstrata stratathat thatwere wereapparently apparentlydeposited depositedalong alongaa was regional shoreline in which siliciclastics were nearly absent. regional shoreline in which siliciclastics were nearly absent. 2)What What are arethe thefundamental fundamental controls controls that that led led to to an an arid, arid, tropical tropical carbonate carbonate style style of of 2) deposition,when whenboth bothyounger youngerand andolder oldercarbonate carbonatedeposition depositionwas wasreminiscent reminiscentof of modern modemcoolcooldeposition, water, water,temperate temperateconditions? conditions? This Thispart partof ofthe thePrairie Prairiedu duChien ChienGroup Groupconsists consistschiefly chieflyof of the the carbonate-rock carbonate-rockcomponent. component. ItItisisan anexcellent excellentexample exampleof of the the importance importancein in delineating delineatingthe the presence presence and and position position of of fractures fractures that that are arethe theprincipal principalgroundwater groundwaterconduits. conduits. AAprominent prominenthorizontal horizontalinterval intervalofofdissolution dissolution features, features,some somewith withstalactites, stalactites,occurs occursalong alongthe theOneota-Shakopee Oneota-Shakopeeunconformity unconformityininthis thisquarry. quarry. AAsecond seconddissolution dissolutionlayer layerisisvisible visiblehigh highon onthe thequarry's quarry's south southwall, wall,just just below belowaa1-2 1-2m mbed bedof of coarse coarsesandstone. sandstone.Work Workby byCalvin CalvinAlexander Alexanderfrom fromthe theUniversity University of of Minnesota Minnesota suggests suggests that that these these dissolution dissolution horizons horizons have have an animportant importantcontrol control on on the thehydrogeologic hydrogeologicattributes attributes of of the the Prairie Theyare arelikely likelymajor majorconduits conduitsfor forground-water ground-waterflow, flow,and andenhance enhancethe the Chien. They Prairie du du Chien. development developmentof ofsinkholes sinkholeswhere wherethey they are are nearer nearer the surface. surface. Other Othernotable notablefeatures featuresinclude includethe the 125 �the carbonate carbonate rock rock exposed exposed on the the south south wall wall of of the thequarry—in quarry-in tight, unfractured nature of the contrast with with the the carbonate carbonaterock rock of of the the Shakopee Shakopeeseen seen at at Stop Stop 11 where it is closer to the surface, contrast surface, and and secondary secondary porosity porosity isisbetter betterdeveloped. developed. Stop 10—Spring 10ÑSprin near nearVern VernKeller KellerQuarry. Quarry.From Fromquarry quarrycontinue continuesouth south0.7 0.7 miles miles on County Stop Road Road 6. 6. Turn Turneast easton onto togravel gravel road road and and travel travel 0.1 miles (Fig. 11). 11). Spring Springisison on north northside side of road on PRIVATE PROPERTY. of road on PRIVATE PROPERTY. Elevation Elevationof of this this resurgent resurgent spring spring approximately approximatelythe the same same as as the Oneota-Shakopee Oneota-Shakopeecontact contact seen seen at at the the previous previous stop. stop. This supports supports the hypothesis hypothesis that dissolution dissolution features along along the the unconformity unconforrnitymay maybe beaamajor majorconduit conduitfor forgroundwater groundwaterflow. flow. REFERENCES REFERENCES CITED CITED Adams, R. R. L., L., 1978, 1978,Stratigraphy Stratigraphy and and petrology petrology of of the the lower lower Oneota Oneota Dolomite Dolomite(Ordovician), (Ordovician), south-central Wisconsin: Wisconsin Wisconsin Geological Geological and and Natural Natural History Survey Field Trip Guidebook Guidebook3, 3, p. p. 82-90. 82-90. 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L.L., 1988, evolution of of the craton in Phanerozoic time, in Sloss, L.L., ed., Sloss, L.L., 1988, Tectonic evolution ed., Sedimentary cover—North Sedimentary cover-North American Americancraton, craton,United UnitedStates: States:The The Geology Geology of of North North America, America, DNAG Volume Volume D-2, D-2, p. p. 25-5 25-51. 1. Smith, G.L., G.L., Byers, C.W., of the Lower Smith, C.W., and Dott, R.H., R.H., Jr., 1993, 1993, Sequence stratigraphy stratigraphy of Ordovician Prairie du Chien Group on the the Wisconsin Arch and in the Michigan Michigan Basin: Basin: Association of Petroleum American Association Petroleum Geologists GeologistsBulletin, Bulletin, v. v. 77, 77, p. 49-67. 49-67. Smith, G.L., Byers, C.W., C.W., and Dott, R.H., Jr., 1996, 1996, Sequence stratigraphy of the Prairie du du Chien Group, Lower Ordovician, midcontinent, USA, in Witzke, B.J., Ludvigson, G.A., Ordovician, midcontinent, Paleozoic Sequence Sequence Stratigraphy:=ews Stratigraphy:Views from from the North American Craton: and Day, J., eds., Paleozoic Craton: Geological Society Society of of America America Special Special Paper Paper 306, 306, p. p. 23-34. 23-34. selected flow flow paths paths in in the Prairie du Smith, S.E., and Nemitz, D., 1996, 1996, Water quality along selected Chien-Jordan aquifer, Chien-Jordan aquifer, southeastern southeastern Minnesota: U.S. Geological Geological Survey Survey Water-Resources Investigations Report 95-4115, 76 p. Investigations 95-41 15,76 Visocky, A.P., A.P., Sherrill, Sherrill, M.G., M.G., and Cartwright, Cartwright, K., 1985, 1985, Geology, hydrology, and water quality quality of the Cambrian Cambrian and and Ordovician Ordovician Systems in northen northen Illinois: illinois Illinois State State Geological Geological Survey and Illinois Illinois State State Water Survey Cooperative Groundwater Groundwater Report Report 10, 10, 136 136p. p. Wall, D.B., and Regan, C.P., C.P., 1994, 1994, Water quality and sensitivy of the Prairie du Chien Chien /Jordan /Jordan and Regan, Aquifer ininwestern westernWinona WinonaCounty, County,Minnesota: Minnesota: Unpublished Unpublished report of the the Minnesota Minnesota Pollution Control MN. Pollution ControlAgency, Agency, Division Division of of Water Water Quality, Quality, St St Paul, Paul, MN. Wenck and Associates, Associates, Inc., Inc., 1997, Phase 11: II: Detailed site investigation report and Phase II Wenck investigation report I1 workplan for the hydrogeologic of the proposed proposed Red Wing Wing ash disposal hydrogeologic investigation investigation of facility facility expansion, expansion, 36 36 p. p. Witzke, B.J., Ludvigson, Ludvigson, G.A., G.A., and and Day, Day, J., eds., 1996, 1996,Paleozoic Paleozoic sequence sequencestratigraphy: stratigraphy:Views Views from from the North North American American craton: Geological Geological Society Society of America Special Paper 306, 446 p. 130 ��FIELD FIELD TRIP TRIP#5 #5 MINNESOTA MINNESOTA RIVER RIVERVALLEY VALLEYAND AND VICINITY, VICINITY, SOUTHWESTERNMINNESOTA MINNESOTA SOUTHWESTERN Leaders: D.L. D.L.Southwick Southwick and and Carrie CarrieJ. J. Patterson Patterson Leaders: INTRODUCTIONTO TOTHE THEGEOLOGY GEOLOGYOF OFPRECAMBRIAN PRECAMBRIANROCKS ROCKSIN INTHE THE INTRODUCTION MINNESOTA RIVER VALLEY MINNESOTA RIVER VALLEY D. D.L. L.Southwick Southwick The Precambrian Precambrian rocks rocks exposed exposed in inthe theMinnesota MinnesotaRiver RiverValley Valley are are within within an anArchean Archean The gneiss terrane terranethat thatconstitutes constitutesthe thesouthernmost southernmostsubprovince subprovince of of the the Superior SuperiorProvince Provinceof ofthe the gneiss CanadianShield. Shield.The TheMinnesota MinnesotaRiver RiverValley Valley(MRV) (MRV) subprovince subprovince is is further further subdivided subdivided into into Canadian fourtectonic tectonicblocks blocks by byregional regionalENE-trending ENE-trending shear shear zones zones (Fig. (Fig.1;1;Southwick Southwickand andChandler, Chandler, four 1996).This Thistrip tripwill willfocus focuson onexposed exposedrocks rocksof ofthe theMorton Mortonand andMontevideo Montevideoblocks, blocks,which whichare are 1996). EXPLANATION' EXPLANATION TECTONIC TECTONICELEMENT ELEMENT Sioux Quartzlto P.nok.an Ponokoanoreg.n orogon (undlvld.d) (undivided) ROCKTYPES TYPES ROCK AGE AGE Red sandstone and shale Middle Middle Proterozotc Proterozoic a (51,100Ma) Ma) (1,100 Quartzite Quartzite Early Early Pmterozok Proterozoic (1,630-1,770Ma) Ma) (1,630.1,770 'fl /1 Supracrustal SuPracrustal endIntrusive Intrusiverocks rocks and manytypes; types: ofofmany alsoArchean Archeangnelss gneiss also Penokean rewwked reworked InInPenokean events events Early Early Proterozoic Proterozoic (depositional (depositional andemplacement emplacement and 80681.770-2.200 1,770-2.200 ages Ma) Ma) Greenstone-belt Greenstone-belt 1tNcassociation. association, Nthlc granitoidIntrusions Intrusions granitold LateArchean Archean Late Ma) ('2.700Ma) (2,700 Greenatone-beft NtNc association, Uncertain;Late Late Uncertain; Archeanand/or and/or Arctiean EarlyProterozoic Proterozoic Early SuperiorProvince Province Superior Wawa W a w subprovlno. aubprovlnco (undivided) (undlvldod) ' VaMoy mibprovlnce ffffl'Jffl) Ilifirnillil lowtotomoderate moderate low orade rnetarnor~hic metamorphic . grade 4-; Location Map Regionalshear shearzone zoneorortectonic tectoniccontact; contact;barbs barbs Regional indicatedirection directionofofdip dipasasInterpreted interpreted from Indicate from geophysical models. geophysical models. IOWA 0 30 - Quartzofeldspathic Quartzofeldspathic MiddletotoLate Late Middle gneiss,granitold granitoid gnelsa, Archean(3,600(3,600Archean ~ ~ ~ I U S I O ~ S Intrusions 2,600Ma) Ma) 2,600 60 KNOm.tws Figure Figure1.1.Tectonic Tectonicsketch sketchmap mapof ofPrecambrian Precambrianterranes terranesin insouthwestern southwesternMinnesota. Minnesota.The TheMinnesota Minnesota River RiverValley Valley subprovince subprovinceconsists consistsof of the the Benson, Benson, Montevideo, Montevideo, Morton, Morton, and and Jeffers Jeffers blocks. The The Taunton Tauntonbelt beltisisa anarrow narrowseptum septumofofrelatively relativelylow lowgrade grademetavolcanic metavolcanicrocks rocksalong alongthe theYellow Yellow Medicine shear zone, which marks the boundary between the Montevideo and Morton blocks. Medicine shear zone, which marks the boundary between the Montevideo and Morton blocks. The Theline lineindicates indicatesthe thelocation locationofofgeophysical geophysicalprofiles. profiles.From FromSouthwick Southwickand andChandler Chandler(1996). (1996). 133 �central two two blocks blocks of the the subprovince. subprovince. The Theboundary boundarybetween betweenthese theseisisthe theYellow Yellow Medicine Medicine the central shear zone (YMSZ), (YMSZ),aamajor majorcrustal crustal feature feature that that is is well well delineated delineated aeromagnetically aeromagnetically but does does not crop crop out. The TheYMSZ YMSZcrosses crossesthe the Minnesota MinnesotaRiver River between betweentrip trip stops stops66 and and 77 (Figs. (Figs. 11 and and 2). 2). At stops stops 11 through through 6, 6, south south of the the YMSZ, we will see features of the Morton block and at Stops 6 through ii11we wewill willsee seefeatures featuresof ofthe theMontevideo Montevideoblock. block. The characteristic characteristicrock rock type type of of both blocks blocks is a modally layered gneiss gneiss that falls falls generally generally tonalite to granodiorite granodiorite range in bulk composition (Goldich and others, 1980a 1980a and b). within the tonalite irregular blocks of mafic gneiss, gneiss, This rock type, together together with less less abundant abundant layers, layers, lenses, lenses, and irregular amphibolite, constitutes that Morton Gneiss amphibolite,and and various various types types of aluminous alurninous gneiss, constitutes Gneiss south south of the YMSZ and the Montevideo Montevideo Gneiss north of it. Alert readers will note note the the correspondence correspondence of the the principal principal map mapunit unitwithin withinit. it. Mineral-assemblage between the block name and the name of data indicate that the Morton and Montevideo gneisses were metamorphosed metamorphosed under under conditions conditions of the upper upper amphibolite amphibolite and and granulitic granulitic facies (Grant, (Grant, 1973; 1973; Perkins and Chipera, Chipera, 1985). 1985). Retrograde processes Montevideo Gneiss Retrograde processeshave have affected affectedaa considerable considerablearea area of the Montevideo Gneissbut are are evident evident only locally in the Morton. The Morton and by granite (adamellite, and Montevideo Montevideo gneisses were invaded by (adamellite, in the the terminology favored favored by by Goldich Goldich and and others, others, 1980a and and b) b) at two different different times during the terminology Archean. 1 of 980a) Archean. The Thefirst firstepisode episodeof ofgranite graniteemplacement emplacement(adamellite(adamellite-1 of Goldich Goldichand and others, others,11980a) fabric in in the gneissic host rocks; preceded the development of the predominant metamorphic fabric that fabric passes passes through the the granite granite bodies, bodies, most most of of which which are are aplitic aplitic or or pegmatitic, pegmatitic,without without discernable change in in orientation. orientation. In addition, addition, much granite of this this generation generation possesses possesses of Goldich metamorphic texture. The The second second episode episode of of granite granite emplacement emplacement (adamellite-2 of and others, 11980a) 980a) occurred occurred after after the the main main fabric-forming fabric-forming events. events. Most intrusions intrusions of of this this generation generationlack lack internal internalfabric, fabric,although although some someshow show aa weak foliation foliation and therefore therefore may be latelateAdamellite-2 intrusions intrusions range from to post-tectonic. post-tectonic. Good Good igneous igneous textures textures are the norm. Adamellite-2 aplitic to medium-grained granitic in grain size and from thin dikes and sills to batholiths in form and scale. The TheSacred SacredHeart HeartGranite Granite(Stop (Stop6) 6)forms formsaa regional-scale, regional-scale, sheet-like sheet-like intrusion intrusion several kilometers kilometers in in thickness thickness that that is is interpreted interpreted to to be be of of the adamellite-2 generation. generation. The Ortonville and Fort Ridgeley granites are late- to post-tectonic post-tectonic masses that also are of of the the adamellite-2 adamellite-2 type and and are are assumed assumed to to be approximately approximately coeval with the Sacred Sacred Heart. Between colleagues devoted huge efforts efforts to to pioneering pioneering Between 1950 1950 and 1980, 1980, S.S. Goldich and colleagues geochronologic studies geochronologic studies of the the Morton Morton and and Montevideo Montevideo gneisses gneisses and and the various various granitic granitic rocks rocks that are associated associated with with them them (Goldich (Goldich and and Wooden, Wooden, 1980; 1980; Goldich and others, others, 1980a 1980a and and references therein). components of both the Morton references therein). The Thegneisses gneissesproved provedto tobe be (1) (1)very very old, old, with components and Montevideo as old as about 3,600 Ma, and (2) geochronologically as about 3,600 Ma, and (2) geochronologically complex, complex, with with several several Archean events events recorded recorded isotopically. isotopically. Moreover, Moreover, the the Goldich Goldich group group found found the discernable Archean "adamellite 1" the geologically geologically straightforwardstraightforward"adamellite 1" rocks rocks to to be be isotopically isotopically complex complexas as well, and even the "adamellite 2" rocks rocks to to yield yield less less than thandefinitive definitive crystallization crystallization ages. ages. Goldich's appearing "adamellite interpretation of interpretation of the the geologic geologichistory historyof of the theMRV MRV subprovince subprovinceisis presented presentedin inTable Table1.1. Goldich emphatically type represented represented genuine Goldich emphaticallybelieved believedthat that the the granite graniteof the the adamelliteadamellite- 1 type igneous i.e., true true intrusion. intrusion. Others contended Others have contended igneous importation importationof of magma magma from from a distant source; i.e., that some, some, if not not all, all, of the the early early granite granite was derived more or less locally through partial partial melting melting of the gneis sichost. host. A major question persists as to to whether whether the the pink pink leucosomatic leucosomatic components components gneissic of the Morton 1 Morton Uneiss Gneissin inthe thevicinity vicinityof ofMorton Mortoncrystallized crystallizedfrom frominjected injectedmelt meltofofadamelliteadamellite-1 134 �Figure 2. Locations of field-trip stops for Day 1. More detailed location maps accompany the individual stop descriptions. �Table1.1.Summary Summaryofofprincipal principalPrecambrian Precambrianevents eventsininthe theMinnesota MinnesotaRiver RiverValley Valley subprovince. subprovince. Table Modified and and interpreted interpreted from from Goldich Goldich and and Wooden Wooden (1980); Goldich and others others (1980a). (1980a). Modified 1800 Ma Ma event event(Proterozoic): (Proterozoic): Emplacement Emplacement of of small small intrusions; intrusions; local local 1800 retrogade retrogademetamorphism, metamorphism,shearing shearing 2600 2600Ma Maevent eventEmplacement Emplacementofofpost-kinematic post-kinematicaplite apliteand andpegmatite pegmatitedikes dikes Emp1acemen.tof of Sacred SacredHeart Heart Granite, Granite, other other granite granitebatholiths, batholiths,and and Emplacemen.t associated associated minor minor intrusions intrusions ("adamellite-2"; ("adamellite-2"; late-kinematic) late-kinematic) High-grade High-grade metamorphism, metamorphism, deformation deformation of of gneiss gneiss 3100 Ma event 3100Maevent High-grade High-grademetamorphism, metamorphism,deformation deformationof of gneiss gneiss Emplacement Emplacementof ofminor minorgranitoid granitoidintrusions intrusions("adamellite("adamellite-1") 1") 3600 3600 Ma Ma event event Intrusion Intrusionof oftonalite, tonalite,granodiorite granodioriteinto intoheterolithic heterolithicprecursor precursorof of layered layered gneiss gneiss typeor orfrom frommelts meltsdeveloped developedthrough throughanatexis anatexisof ofpre-existing pre-existinggranodioritic granodioritictototonalitic tonaliticgneiss. gneiss. type We and2A. 2A. We will will have have an anopportunity opportunityto toform formour ourown ownconclusions conclusionsatatstops stops22and Although the theprincipal principalrock-forming rock-forming events events responsible responsible for for the theformation formationof ofthe theMRV MRV Although subprovinceoccurred occurredover overaabillion-year billion-yearinterval intervalof ofArchean Archeantime, time,there thereare areclear clearindications indicationsof of subprovince further furthergeologic geologicactivity activityin in the the Early Early Proterozoic. Proterozoic. The Thesubprovince subprovincelay lay in in the the cratonic cratonicforeland foreland (Superiorcraton) craton)with with respect respectto toPenokean Penokeantectonism tectonism in in the the interval interval between between 2.1 2.1 and and 1.7 1.7Ga. Ga. (Superior We are arejust just beginning beginningto torecognize recognizeand andquantify quantifyevents eventsand and responses responses recorded recorded in in the the foreland foreland We rocks rocksthat thatwere wereengendered engenderedby bytectonic tectonicprocesses processeswithin withinthe thePenokean Penokeanorogen orogen (Southwick (Southwickand and Chandler,1996; 1996;Holm Holmand andothers, others,1998), 1998),the the axis axis of of which which lies lies only a few 10's 10's of kilometers kilometers to to Chandler, theeast eastof ofpresent presentMRV MRV exposures. exposures. Proterozoic Proterozoicevents eventsinclude include(1) (1)the theemplacement emplacementof of at at least least the twoand andpossibly possiblyasasmany manyas asfour fourswarms swarmsof ofdiabase diabasedikes; dikes;(2) (2)the the emplacement emplacementof of small smallintrusive intrusive two plugsthat thatrange rangein incomposition compositionfrom fromperidotite peridotiteto togranite; granite;and and(3) (3) differential differential vertical vertical movement movement plugs on onblock-bounding block-boundingshear shearzones, zones,with withconcomitant concomitantdevelopment developmentof of brittle brittlestructures structuresand and retrograde retrograde metamorphic metamorphiceffects. effects. No No discussion discussionofofthe theMRV MRVsubprovince, subprovince,regardless regardlessofofbrevity,, brevity,.is is complete completewithout withoutan an acknowledgment 1956) over acknowledgmentof ofthe theexcellent excellentgeologic geologicmapping mappingdone doneby by Ernest ErnestLund Lund (1950, (1950,1956) over45 45 years Lund'swork workwas wasthe thestarting startingpoint pointfor forsubsequent subsequentgenerations generationsof offield fieldinvestigations investigations yearsago. ago.Lund's (Himmelberg, (Hirnmelberg, 1968; 1968; Grant, Grant, 1972; 1972; Bauer, Bauer, 1980) 1980) and and was was the the principal principal framework framework for for the the geochronologicinvestigations investigationsofofGoldich Goldichand andcolleagues. colleagues.Those Thoseofofus usworking workingtoday todayfind findLund's Lund7s geochronologic maps mapstotobe beproof proofofofthe themantra mantrathat thatcareful carefulgeologic geologicobservations observationsand anddescriptions descriptionsare aretimeless; timeless; their theirvalue valuewill willoutlast outlastthe theinterpretational interpretationalframework frameworkwithin withinwhich whichthey theywere weremade. made. 136 �FIELD TRIP FIELD TRIPSTOPS—DAY STOPS-DAY 11 Stop 1—MRV 1-MRV geomorphology; geomorphology; Proterozoic diabase dikes and and walirock wallrock of of Archean Archean Morton Morton Stop Morganquadrangle, quadrangle,T. T.112 112N.-R. N.-R. 34 34W.-Section W.-Section 23AAC. 23AAC. Gneiss. Morgan At this locality locality south Franklin, newcomers can readily comprehend comprehend several south of Franklin, several elements elements of post-Precambrian geologicalhistory history that that bear bear on the condition and distribution post-Precambrian geological distribution of the the Precambrian Precambrian rock outcrops on the valley Point (1): (1): The ThePrecambrian Precambrianrocks rocks were were deeply deeply weathered weathered valley floor. floor. Point prior to continental glaciation in the Quaternary. Quaternary. Actually, Actually, most of the weathering weathering occurred prior to the Late Cretaceous. ManyMRV MRV outcrops, outcrops, such such as these, are incipiently substantially Cretaceous. Many incipiently to substantially weathered; they represent material that lay just pre-Cretaceous weathering just at the base of the pre-Cretaceous profile. Upper the Upper parts parts of of the weathering profile, remnants of of weathering Late Cretaceous marine strata, and the overlying glacial deposits deposits were were stripped away as the Glacial River Warren (a high-discharge, Warren high-discharge, lowlowgradient gradient meltwater meltwater river river ancestral ancestralto to the Minnesota) eroded downward. downward. River River downcutting downcutting slowed slowed or or perhaps ceased perhaps ceased where where hard rock was encountered at the the base of encountered at of the the (2): The The weathered section. section. Point (2'): present valley-floor outcrops were rapids and islands rapids islands in the Glacial Glacial River Warren. Warren. Their morphology morphology is due primarily due primarily to river river erosion. erosion. (31: Because Because of of the the Scoured cuts (former shoots in rapids) and and potholes potholes are are common. common. Point (3): history knobs of approximately history discussed discussedin inpoints points(1) (1)and and(2), (21, outcrops outcropstend tend to to be clusters clustersof low knobs approximately the same height. InIngeneral generalthey theydo donot notoccur occuron onthe thevalley valley walls, walls,Grand-Canyon Grand-Canyon style, style, but instead are located toward the middle middle of the the valley valley floor floor where where the the stream stream velocity velocity and and downcutting greatest. The commonly downcuttingpower power of of the the River River Warren were greatest. The sides sides of the valley floor are commonly occupied by constructional terraces (as here) and the valley walls are chiefly the Quaternary sediments sediments through through which whichthe the River RiverWarren Warren cut. cut. Diabase dikes of of Proterozoic Proterozoic age age form form prominent prominent valley-floor valley-floor ridges ridges at at this thislocality. locality. Compositionally, tholeiites. They are very similar in in composition composition Compositionally,the dikes dikes are good continental tholeiites. and petrographic characteristics characteristics to to tholeiitic tholeiitic dikes dikes near Granite Granite Falls, Falls, some some 40 40 miles miles up-river, up-river, that yield K-Ar dates of about 2,080 Ma (Hanson and Himmelberg, 1967). Several of of the the larger larger az. 095-100, approximately parallel to the dikes dikes here tholeiite dikes dikes at at Granite Granite Falls Falls strike strike about about az. tholeiite approximately parallel at Franklin. Franklin. Some Somebetter bettergeochronology geochronologyand andaalittle littlepaleomagnetic paleomagneticwork workwould wouldbe bevery very helpful helpful in deciding Falls tholeiitic tholeiitic diabase diabase dikes dikes are are or or are not deciding whether whether the Franklin Franklin and Granite Granite Falls genetically genetically related. related. that is held up up by by the the Cedar About a mile northwest northwest of Stop 11 is a prominent brushy knob that Mountain Complex, a compositionally zoned zoned igneous igneous plug plugof ofProterozoic Proterozoicage. age. The outer part of the plug plug isis melanocratic melanocratic hornblende hornblende diorite that has aa chilled chilled external external rind rind of of olivine olivine trachybasalt homblende trachybasalt porphyry. The Thecore core of of the the plug is granite and granodiorite. Biotite and hornblende from the core core yield yield essentially essentially identical identicalK-Ar K-Ar ages ages of of 1,750 1,750Ma Ma (Hanson, (Hanson,1968). 1968). from :&, 137 �1.5 miles to the east of Stop 11 is a small plug of highly silicified and carbonatized About 1.5 Thisbody body isispost-Archean post-Archean but but is is unconstrained unconstrained otherwise otherwise as to age. serpentinite. This wallrock to the dikes and other intrusions is a granodioritic granodioritic variant The walirock variant of the middle Archean Morton Gneiss. Gneiss. Because Becausethe thegneiss gneisshere hereisis in in such such poor condition (incipient (incipient weathering), Archean Morton weathering), it has not been studied in any great detail. We We will see much better outcrops of the Morton at 4, and Stops 2, 2,4, and 5. 5. Features to see: see: • Grain Grain size sizeand andfabric fabricvariations variationswithin within diabase diabasedikes; dikes; chilled chilled margins. margins. • Paleomagnetic Paleomagneticdrill drillholes: holes: Where Where are are the the paleomagnetic paliomagnetic data?? data?? owned and operated operated by by the Cold Spring Stop 2—Morton %Morton Gneiss, Morton Quarry owned Spring Granite Granite Company. Morton W.-section 3lDAA. 31DAA. Hard hats must be Morton quadrangle, quadrangle, T. 113 113 N.-R. 34 W.-section worn everywhere everywhere on company company property. quarried here here isis aa variant variantof ofthe theMorton MortonGneiss. Gneiss. It is a The flamboyant dimension stone quarried classic migmatite migmatite in which a pink, granitic of the neosome classic granitic to pegmatitic pegmatitic component component of neosome (the "adamellite-1" rock suite of Goldich and others, 1980b) is unusually unusually abundant. More "adamellite-1" More typical typical Morton Gneiss from the standpoint standpoint of areal areal abundance abundance is aa predominantly predominantly gray rock of of granodioritic to to tonalitic tonalitic composition composition that that is is more more uniform uniform in in texture texture than than the the gneiss gneiss in the granodioritic Morton complex metamorphic metamorphic Morton area. area. This Thisisisanother anothercase casewhere where the the concept concept of "type locality" for a complex rock turns out to be flawed. flawed. The TheMorton Mortonarea areasupplied suppliedthe the name name for for the the gneiss gneiss because because the the many quarries provided accessible, unweathered, unweathered, clean exposures to study, study, not not because because the rock exposed is particularly exposed is particularly typical of aa mappable mappable geologic geologic unit. gneiss from u q The gneiss from this qquarry is marketed marketed under under the the trade trade name name granite." ItIt can "rainbow granite.'' can be seen ornamental stone as a facing and ornamental stone on buildings throughout buildings throughout the the Nation, Nation, and and in in recent recent years years itit has become a favorite favorite material material for for up-scale up-scale kitchen counters counters and and tabletops. The patterns, plastic, suggestive of inhomogeneous bulk inhomogeneous bulk deformation deformationunder under conditions conditionsat at which which the the rock rock was was in in aa partially partially molten molten state, attractive and intriguing intriguing to architects architects and interior designersas designers as they are to to geologists. geologists. state, are as attractive Features to to see: see: • Paleosomatic Paleosomatic"enclaves" "enclaves" of of several several types. types. • Evidence Evidenceof of rheological rheologicalcontrasts contrastsamong amongdifferent differentrock rock types during partial melting and deformatio.n. deformati0.n. • Gently dipping, tight folds folds reoriented reoriented into the dipping, broadly undulating gneissosity; small, tight undulating undulating "regional" "regional" fabric. fabric. 138 �• Zones Zones of of extensional extensional ductile ductile shear, shear, many many of of which which contain contain granitoid granitoid segregations segregations interpreted interpreted to to have have formed formed as as partial partial melts. melts. Stop Stop 2A 2A (Alternative (Alternative to to Stop Stop 2)—Morton 2)-Morton Gneiss, Gneiss, small small quarry quarry behind behind former former school school building in in town building town of Morton. Morton. Morton Morton quadrangle, quadrangle, T. T. 113 113 N.-R. N.-R. 34 34 W.-section W.-section 3 1BCBA. 1BCBA. Because of insurance insurance liability liability considerations, considerations, the Cold Spring Granite Company does not quarry. As of press time we were not normally permit field-trip field-trip groups groups into the active Rainbow quarry. many of of the cleared for quarry access. The The small small quarry quarry behind the old Morton school shows many described under Stop 2 (above), but but the the rock rock surfaces surfaces are are smaller smaller and and less less features of the gneiss described spectacular than those in the Rainbow quarry. Please spectacular Please read read the the discussion discussion of of Stop Stop 22 and and transfer transfer it to the Stop 2A exposures. exposures. move about about Stop Stop 2A. 2A. Rock surfaces that are shaded most of the Be very careful as you move time acquire a slimy slimy coating coating that can be treacherously treacherously slick. slick. Stop 3—Weathering Morton Gneiss Gneissprior priorto tothe theLate LateCretaceous. Cretaceous. Redwood Redwood Stop &Weathering profile developed on Morton Falls Falls quadrangle, quadrangle,T. T. 113 113N.-R. N.-R.35 35W.-section W.-section3ODAB. 30DAB. Clay-rich saprolith of the general type exposed here here occurs occurs widely widely in in the the subsurface of of southwestern Minnesota. southwestern Minnesota. Where WhereLate LateCretaceous Cretaceousmarine marinestrata stratahave havenot notbeen been removed removedby by postpostCretaceous erosion they overlie the saprolith saprolith directly, confirming that most of the weathering weathering occurred before Late Cretaceous Cretaceous marine transgression transgression and the the deposition deposition of of marine marine sedimentary sedimentary occurred rocks. In places where where the Cretaceous Cretaceous rocks rocks were eroded away prior to glaciation, remnants of the weathering weathering profile profile directly underlie Quaternary glacial deposits. The The clay, clay, grus, and corestones from from the the weathered Precambrian rocks undoubtedly contributed contributed substantially substantially to the mass of subsequent Cretaceous shales and and Quaternary tills. Quaternary Reworked and inReworked deposits of Late situ clay deposits Cretaceous and postCretaceous Cretaceous age are mined at several sites mined sites along the Minnesota along Minnesota River, including one River, one about a mile southeast of 139 �these exposures. Theclay clayisisused used mainly mainly in in the the manufacture manufacture of portland portland cement cement and and for for various various exposures. The agricultural purposes. The weathering profile at this stop was was studied studiedby bySam SamGoldich Goldichininthe the1930's; 1930's; his classic the weathering weathering stability stability of of rock-forming rock-forming minerals minerals (Goldich, (Goldich, 1938) 1938) was was based based on on paper on the Thatfundamental fundamentalpaper paper on onweathering weathering has has probably probably achieved achieved wider wider materials collected here. That all of of Sam's Sam's geochronological geochronologicalcontributions contributionscombined. combined. notoriety than all Features Features to to see: see: Upwardloss lossof ofsource-rock source-rockminerals mineralsand andfabric fabricin in the the bottom bottom two-thirds two-thirds of the weathering • Upward weathering profile. profile. • Pisolitic Pisoliticclay claylayer layertoward towardthe thetop top of of the the section. section. Beddedclay clay(reworked (reworkedby by streams?) streams?)above abovethe the pisolitic pisolitic layer layer and and below Quaternary Quaternarytill. till. • Bedded Stop Stop4—Morton &Morton Gneiss Gneisscut cut by by adamellite adamellitedikes. dikes. Vicksburg quadrangle, quadrangle,T. T. 114 114N.-R. 36 36 W.-section W.-section 33DDD. 33DDD. The gray tonalitic gneiss exposed in these outcrops is a common, widespread variant of of the Morton. Much Muchof ofthe therock rock mapped mapped as as Morton Morton Gneiss Gneiss is basically basically this lithology lithology with varying included mafic "enclaves" "enclaves7'and and varying degrees degrees of intrusion intrusion by younger younger granitoid granitoid quantities of included rock types. The The gray gray gneiss gneiss inclusionhere is about as inclusionas it free gets. Furthermore,this this is is a Furthermore7 locality where where there there is a locality intrusive clear-cut relationship between between .late late relationship granite of the Sacred Heart type (sharp-walled dikes, dikes, including an easily accessible one about two accessible meters thick) thick) and meters and the the can tonalitic Morton Morton Gneiss. Gneiss. The Therelative relativeage ageof of the the two two is is not a matter of great debate, in that it can be reliably reliably inferred inferred from from textural textural evidence evidence in the rocks and from field relationships observed at a number of places in the Valley. Valley. Accessible outcrops that show the critical field relations are rather rare7however. however. rather rare, Samples of tonalitic Morton Gneiss from this this site site form form part part of of the the dataset dataset for for the the "old" Rb-Sr isochron age of about about 3,500 3,500 Ma (Goldich (Goldich and Wooden, 1980) that is widely cited in the literature. Data the Rb-Sr Rb-Sr diagram diagram (Goldich (Goldich literature. Datafrom fromhere hereanchor anchorthe the lower lower or less radiogenic end of the and Wooden? Wooden, 1980; 1980; their their fig. fig. 2 and and data data from from locality locality 5). 5). In addition, addition, zircons from from the gneiss at this locality yield an interpreted interpreted U-Pb age age of 3,662 3,662+1+I-42 42 Ma (Goldich (Goldich and and Fischer, Fischer, 1986). 1986). details is beyond beyond the scope and purpose of this Lengthy discussion of geochronologic details this out7however, however, that that healthy disagreements exist as to the exact field guide. ItItmust mustbe bepointed pointedout, antiquity of the Morton Morton Gneiss Gneiss and and the the significance significanceto to be be attached attached to to the the dates dates determined. determined.A A derives from the the complexity complexity of of the the neosomatic neosomatic components components of of major source of controversy derives migmatitic rocks rocks (as seen in the quarries migmatitic quarries at at Stops Stops 22 and and 2A, 2A, for forexample) example) and andconflicting conflicting sequence of ofprocesses processesinvolved involvedinin neosome neosome genesis. genesis. The interpretations of the processes and sequence 140 �more straightforward straightforward sequence sequence of of cross-cutting cross-cutting relationships relationships displayed at the the present present stop stop isis more informative informativebecause becauseitit eliminates eliminatessome someof of the theinterpretational interpretationalambiguities ambiguitiesassociated associatedwith withthe the more morecomplex complexphases phasesof ofthe theMorton. Morton. Featuresto tosee: see: Features • Fabric Fabricand andfabric fabricvariations variationsinintonalite tonalitegneiss. gneiss. • Folded Foldedmafic maficlayer layerinterpreted interpretedas asaadeformed deformeddiabase diabasedike. dike.(Do (Doyou youagree?) agree?) • Textural Texturaland andfabric fabricvariations variationswithin withinpostpost-Morton Mortonadamellite adamelliteintrusions. intrusions. Stop5—Morton 5-Morton Gneiss Gneissthat thatdisplays displaysaavariety varietyofofenclaves enclavesand andsmall-scale small-scalestructures. structures. Stop Iverson IversonLake Lakequadrangle, quadrangle,T. T.114 114N.-R. N.-R. 37 37W.W.- section section21CB. 21CB. These 0.5mile mile south southof of the the the the largest largestexposed exposedmass massof of Sacred Sacred Theseoutcrops outcropsare arelocated locatedabout about0.5 Heart Heart Granite Granite in in the the Minnesota Minnesota River River Valley. Valley. The The gneiss gneiss here here isis characterized characterized by by abundant abundant amphibolitic amphibolitic inclusions inclusions of of tholeiitic tholeiitic and andkomatiitic komatiiticcomposition, composition, complex complex fold foldstructures structures indicative indicative of of multiple multiple folding folding events, events,deformed deformed(boudinaged, (boudinaged,folded) folded) aplitic aplitic dikes dikes presumably presumably of the "adamellite 1" suite, and straight, undeformed granite dikes of Sacred Heart the "adamellite 1" suite, and straight, undeformed granite dikes of Sacred HeartGranite. Granite. The The quartzofeldspathic quartzofeldspathic gneiss gneiss that that encloses encloses the various various mafic rock types is variable variable in modal modal composition composition and and texture. texture. Its Itsoverall overallcomposition compositionisisapproximately approximatelyin in the the granodiorite granodioriterange. range. Wander Wander around among the cedars cedars and and thombushes thornbushes to to find find examples examples of of the thefeatures features mentioned above. Look before you kneel or sit! Little cacti with big spikes grow all over these mentioned above. Look before vou kneel or sit! Little cacti with big spikes grow all knobs. knobs. Stop Stop6—Sacred &Sacred Heart Heart Granite Granite and and associated associated rock types. Iverson Iverson Lake Lake quadrangle, quadrangle, T. 114 114 N.-R. 37 37 W.-section 7DD, 7DD, near prominent prominent knob knob named named Middag MiddagKnutea. Knutea. The The Sacred SacredHeart HeartGranite Graniteisisone oneof of several severallarge large granite granite masses masses that that intruded intrudedMRV MRV gneisses gneisses 2.55 -- 2.7 2.7 Ga Ga time time interval. interval. Available Availableradiometric radiometricage age determinations determinationson on these these intrusions intrusions in the the 2.55 in are rather imprecise, owing to the scarcity and poor poor quality quality of of the the contained contained zircons. zircons. Further efforts to obtain high-precision dates would be most welcome. efforts obtain high-precision dates would be most welcome. 141 �I / :::: Most of of the the Sacred Sacred Heart Heart is is aa Most rather ordinary ordinary medium-grained medium-grained rather granite that that consists consists of of subequal subequal granite amountsof of quartz, quartz,oligoclase, oligoclase,and andKKamounts I '7 M —p \) c !'sA - - -- 8 - N , —_.) ))c ,J) - feldspar. It was quarried as dimension stone in an earlier era but is not being worked now. Although much of the rock mapped mappedas asSacred SacredHeart Heartisisquite quite rock uniform in in texture texture and andcomposition, composition, uniform thereare areparts partsof ofthe themass massthat thatdisplay display there '-'c.-zz C' quartzstronglygneissose gneissosefabric fabricand andquartzstrongly ° poor compositions compositions trending trending toward toward poor ) " ' monzonite. Some Somefeatures featuresindicative indicative monzonite of aa complex complex intrusive intrusive history history are are of displayedin in the the road roadcuts cutsatatthis thisstop. stop. displayed Featuresto tosee: see: Features • Massive, Massive,uniform uniformSacred SacredHeart HeartGranite Granite(road (roadcuts cutsabout about1,000 1,000ftftsouth southof of Middag MiddagKnutea). Knutea). • Gneissose Gneissosefabric fabriciningranite; granite;modal modaland andtextural texturallayering layering(SE (SEflank flankof ofMiddag MiddagKnutea). Knutea). • Monzonite Monzonitewith withpyroxene pyroxenegranulite granuliteinclusions. inclusions. ;Q I 'ci, tfl i U — I Drive DrivetotoStop Stop7—The 7-The bedrock bedrocksurface surfaceexposed exposedhere herewas wasexhumed exhumedby byGlacial GlacialRiver RiverWarren, Warren, however, however, significant erosion of fresh rock was not required to form form this smoothly smoothly undulating undulating bedrock bedrocksurface surfacewith with rounded rounded bedrock bedrock knobs and boulders. Such Suchsurfaces surfacescan can be formed simply by by the the removal removal of of deeply deeply weathered weathered bedrock. bedrock. Large Largerounded roundedboulders boulderswere werenot notshaped shapedduring during fluvial fluvial and and glacial glacial transport; transport; they they represent represent chemically chemically rounded corestones resting at or near their source. source. their Pre-Cretaceous weathering penetrated to an average depth of 30 m, but the rock, the fresh fresh rock/ of relief. Joints m weathered rock interface is irregular and undulates with as much as 45 Joints and and weathered rock interface is irregular and undulates with as much as 45 m fractures isolating fracturesare are preferentially preferentiallyexploited exploitedand andpenetrated penetratedto to depths depthsof of 60 60 m in places, locally isolating meter-sized volumes volumes of fresh fresh rock. The Therounded, rounded,fresh fresh rock rock remnants remnants are are known as corestones corestones and are are aa common common feature feature of of the theweathered weathered bedrock bedrock residuum residuum in in Minnesota Minnesota and and when when incorporated incorporated into into glacial glacialsediment, sediment,may may be be misinterpreted. misinterpreted. On Homme-Kollin Unit, Scientific Scientificand and Natural Natural On the south south side side of the road is Swedes Forest, Homme-Kollin Area. quarries that that are are hibernating hibernating Area. Its Itsmain mainscientific scientificsignificance significanceisisthat thatititisisthe the site site of several old quarries areas for the rare, blue-tailed lizards lizards and and "blue "blue devils"). devils"). They rare, five-lined five-lined skink (also called blue-tailed They prefer sunny woodlands. The sunny bedrock exposures near oak woodlands. The main main range range of of this this species species is is the the eastern eastern United United States Statesand andthese theseare arean anisolated isolatedpopulation. population. 142 �Stop7—Garnet-biotite-quartz-feldspar 7-Garnet-biotite-quartz-feldspar gneiss gneiss (Montevideo Gneiss). Granite Granite Falls quadrangle, quadrangle, Stop T. 115 N.-R. 39 W.-section W.-section 10AA. bAA. 115 N.-R. The Yellow Medicine Minnesota The Medicine shear zone (YMSZ), a major crustal structure structure within the Minnesota River Valley subprovince subprovince (Southwick (Southwick and and Chandler, Chandler, 1996), 1996),crosses crosses the the Minnesota MinnesotaRiver Riverabout about River two miles miles southeast southeast of here. The The shear shear zone zone rocks rocks do do not two crop out; out; the the zone zone was first recognized from geophysical geophysical crop recognized from mapping and and the existence existence of sheared sheared rock within within itit was mapping later verified verified through core drilling. The TheYMSZ YMSZ isisnow now later informally defined defined as as the the boundary between between the the Morton Morton informally Gneiss Gneiss (to (to the the south) south) and and the the Montevideo Montevideo Gneiss Gneiss (to (tothe the north). north). The The rock rock in the rubbly rubbly road road cuts cuts at at this this locality locality is is aa garnet-biotite-quartz-plagioclasegneiss, gneiss,aamappable mappablesubsubgarnet-biotite-quartz-plagioclase unit unit of of the the Montevideo Montevideo Gneiss Gneiss that that is is fairly fairly abundant abundant in in the (Himmelberg, 1968). the Granite Granite Falls area (Hirnmelberg, 1968). Lund Lund (1950) (1950) mapped mappeditit as as the Granite Granite Falls Falls garnetiferous garnetiferousquartz quartzdiorite diorite gneiss, aa name name that never caught on. The The protolith protolithof of the the gneiss, unit as a whole is not immediately apparent. Parts Parts of of itit unit are are quite quite aluminous, aluminous, suggestive suggestive of a sedimentary sedimentary origin, origin, and and other other parts parts are are very very feldspathic, feldspathic,at at least least locally locally suggestive suggestiveof of igneous igneous derivation. derivation. This This and and other other rock rock units units in in the the Granite Granite Falls Falls area area are are folded folded broadly broadly about about east-trending, east-trending, gently F2 structures. structures. gently east-plunging east-plungingaxes axes(Himmelberg, (Himmelberg,1968) 1968)that that Bauer Bauer (1980) (1980) interpreted interpreted as as F2 The Thecomplete completedeformational deformationalhistory historyinvolves involvesfour four folding foldingevents, events, which which Bauer Bauer (1980) (1980) discusses discusses in in detail. detail. Faults Faultsand andbrittle-ductile brittle-ductileshears shearsatatand andnear nearthe thesouth southedge edgeof of the the garnet garnet gneiss gneiss outcrop outcrop belt belt are are probably probably related related to to late late deformation deformationalong along the the nearby nearby YMSZ. There Therealso alsoisisscattered scattered evidence evidence of of earlier, earlier,more more deep-seated deep-seated shear sheardeformation deformationthat that may may have have been been related relatedto toearlier earlier YMSZ activity. YMSZ activity. Features Featuresto tosee: see: • Lithologic Lithologicvariations variationsin in the the gneiss. gneiss. • Late Lateshear shearzones. zones. • Rotational Rotationalstructures structuresindicative indicativeof ofductile ductileshear. shear. Stop Stop8—Granulite-facies 8-Granulite-facies mafic mafic and and pelitic pelitic gneiss gneiss (Montevideo (Montevideo Gneiss). Gneiss). Granite GraniteFalls Fallsquadrangle, quadrangle, T. 3BBD. T. 115 115N.-R. N.-R. 39 39W.-section W.-section 3BBD. Garnet-bearing Garnet-bearinggneiss gneisssimilar similarto tothat thatatatStop Stop77 is is in in contact contactwith withgranulite-grade granulite-grademafic maficgneiss gneiss(cpx(cpxopx-plag) opx-plag)in inthese theseroad road cuts. cuts.Small Smallamounts amountsof ofbiotite biotite in in the the granulite granulite may may record record retrograde retrograde rereequilibration. The garnet gneiss is quite micaceous equilibration. The garnet gneiss is quite micaceous and and well well layered, layered, and and has has been been regarded regarded as as aa metamorphosed metamorphosedsedimentary sedimentaryrock. rock.The Themafic maficgneiss gneiss may mayhave havebeen beenaabasalt basaltflow floworiginally, originally,although althoughother other mafic maficprotoliths protolithsare arecertainly certainlypossible. possible. 143 �Stop 9—Mafic 9-Mafic gneiss gneiss (Montevideo Gneiss). Shear Shear zones zones and pseudotachylite veins. Granite Granite Stop 4AAB. Falls quadrangle, quadrangle, T. T. 115 115 N.-R. 39 39 W.-section 4AAB. Shear structures in these outcrops have been studied by John Craddock and several of his Thefollowing followingsummary summary is is taken taken from from material material provided provided by by students at Macalester College. The John. Twenty-two discrete discrete shear zones have been Twenty-two found here that contain pseudotachylite seams. The The subparallel, trend 070 on on average, average, zones are subparallel, trend about about az. az. 070 and dip and dip subvertically. subvertically. This This orientation orientation is is approximately that of approximately of the the regional-scale regional-scale Yellow Yellow Medicine shear zone and several of the Proterozoic diabase dikes dikes in in the the area. Within diabase Within aa given given zone the individual pseudotachylite pseudotachylite seams seams (black) (black) are of individual diverse orientations. orientations. pseudotachylite matrix is compositionally compositionally The pseudotachylite layered and preserves preserves flow-foliation layered flow-foliation swirls swirlsand and relics relics of glass. Dextral Dextralwinged wingedporphyroclasts porphyroclastsin in the the pseudotachylite pseudotachylite preserve dextral and sinistral kinematic kinematic displacement, displacement, primarily primarily opposite opposite the the sinistral sinistral sense sense of of S-C S-C structures structuresin in the the country country rock and and of sinistral rock sinistral fault fault drag drag indicators indicators along the strike-slip strike-slip zones that contain the pseudotachylite seams. Seam-margin pseudotachylite seams. Seam-margin forking forking directions directions are are dominantly dominantly toward toward the the west; west; poryhyroclast imbrication imbrication within the pseudotachylite is dominantly dominantly up and easterly. pseudotachylite seams yield Calcite in veins that appear to be contemporaneous with the pseudotachylite twinning strains indicative indicative of zone-parallel and zone-normal horizontal shortening. Magnetic Magnetic pseudotachylite seams give Krnax Kmax subvertical, Krnin Kmin subhorizontal fabric meaurements on the pseudotachylite and zone-normal. zone-normal. IfIfthe themagnetic magneticfabric fabriccorrelates correlateswith withflow, flow, these these results results indicate indicate that the flow of pseudotachylite pseudotachylite glass glass was was upward upward within withinthe thezone zoneof ofshear. shear. Features Features to to see: see: • Discrete Discrete shear shearzones zones (many (many are are reddened). reddened). • Pseudotachylite Pseudotachyliteseams. seams. • Sense-of-shear Sense-of-shearindicators indicatorsin in shear shear zones. zones. Stop lO—Proterozoic Gneiss. Granite Stop 10ÑProterozoi "Section "Section 28" intrusion and wallrocks of Montevideo Gneiss. Granite Falls quadrangle, quadrangle,T. T. 116 116N.-R. 39 39 W.-section 28BCA. At this locality a small plug of Proterozoic granite is in complex contact contact with with a body body of hornblende hornblende andesite andesite (also (also Proterozoic). Proterozoic). Contacts Contactswith withthe the Montevideo Montevideogneiss gneiss are are not not exposed, exposed, but they can be mapped rather closely. closely. The The granite granite yields a zircon Pb-Pb age of 1825 1825 Ma (Catanzaro, 1963). 1963). The standard standard explanation explanation of of the the relationships relationships seen seen here is as follows: andesite. Dikes (1) The body of dark rock is a dike of hornblende andesite. Dikes of of this this composition composition occur elsewhere elsewhere in the the Granite Granite Falls area area (e.g., (e.g., Stop Stop 11). 11). (2) The granite intrudes the dike and thus is the younger rock, by classical geological reasoning. reasoning. contact relations relations necessarily necessarily imply imply the the interpretation interpretation outlined outlined Do we agree that the observed contact above? above? 144 �______ _______ Stop Gneiss,Proterozoic Proterozoic dikes dikes in inthe theYellow Yellow Medicine Medicine quarry quarry owned owned and an( Stop11—Montvideo 11- -MontvideoGneiss, 115N.-R. N.-R.39 39 operatedby bythe theMeridian MeridianAggregates AggregatesCompany. Company.Granite GraniteFalls Fallsquadrangle, quadrangle,T.T.115 operated W.-section W.-section 33BB. 33BB.Hard Hardhats hatsmust mustbebeworn worneverywhere everywhereon oncompany companyproperty. property. TheYellow Yellow Medicine Medicine quarry produces crushed stone, much of which which is is used used as as railroad railroad The ballast ballast for for the theBurlington Burlington Northern Northern Railroad. Railroad. Aggregate Aggregatefrom fromhere here isisused used on onrailroad railroadtracks tracks across acrossthe theDakotas Dakotasand andMontana Montanatotothe theRocky RockyMountain Mountainfront. front. The principal principal rock rock in in the the quarry quarry isis aa The granitic granitic gneiss gneiss composed composed of of quartz, quartz, plagioclase, microcline, microcline, and and biotite biotite that that isis plagioclase, modally layered layered on on various various scales. scales. Scattered Scattered modally withinititare areconformable conformablelenses lensesand and layers layersof of within hornblende-pyroxenegneiss. gneiss.This Thisgeneral generalrock rock homblende-pyroxene '' type is is the thecommonest commonest variant variant of of the the type MontevideoGneiss Gneissin in terms termsof ofmap maparea areaand and Montevideo D / essentiallythe therock rockexposed exposedup-river up-riveratatthe the isisessentially 33L4F "type locality" locality" outcrops outcrops near near the thetown town of of "type Montevideo. Samples Samplesof ofgneiss gneissfrom fromthe the Montevideo. quarry were were used used by by Goldich Goldich and andothers others quarry = (1980a, locality 4) 4) to to identify identifyaasequence sequence (1 980a, their locality ofRb-Sr Rb-Srisotopic isotopicevents eventsthat thatrange rangefrom from3,530 3,530 of toto2,265 2,265Ma. Ma.Various Variousinterpretations interpretationsofofthese these v' )\ ranité11s rriS I \ ) iriYfA 1ru • I I :11: E5!T 145 �I isotopic results results are are discussed discussed at at length length by the original authors. U-Pb zircon data reported for an isotopic "older" adamellite adarnelliteintrusion intrusioninto into the the layered layered gneiss gneiss yield an interpreted age of 3,230 3,230 Ma Ma (Goldich (Goldich and others, 1980a). others, 1980a). Dikes of of Proterozoic Proterozoic hornblende hornblende andesite andesite are are prominently prominently displayed displayed in in the the quarry quarrywalls. walls. Dikes all other other rocks rocks in in the the quarry quarry are are shot shot through through with chioritized, chloritized, slickensided, slickensided, brittle These and all shears on all all orientations. orientations.This Thistype typebrittle brittleshearing shearingand andassociated associatedevidence evidenceof ofhydrothermal hydrothermal shears activity (chlorite (chlorite films; films; calcite-quartz calcite-quartz veins) veins) are are associated associated with a pronounced reddening of the activity disseminated hematite. The Thereddening reddeningphenomenon phenomenon affects affects many many rocks, presumably due to disseminated MRV exposures exposures north north of of the the YMSZ YMSZ but is uncommon uncommon south south of it. it. END OF OF DAY DAY ONE. REFERENCES REFERENCES CITED CITED Bauer, R.L., 1980, 1980, Multiphase deformation in the Granite Granite Falls-Montevideo area, Minnesota River in Morey, Morey,G.B., G.B.,and andHanson, Hanson,G.N., G.N.,eds., eds.,Selected Selected studies studies ofkchean ofArchean gneisses River Valley, in gneisses and lower Proterozoic rocks in the southern Canadian Shield: Geological Society lower Proterozoic the southern Canadian Society of America America Special SpecialPaper Paper 182, 182,p. p. 1-17. 1-17. Catanzaro, southwestern Minnesota: Journal of Geophysical Geophysical Research, Catanzaro,E.J, E.J, 1963, 1963,Zircon Zircon ages ages in southwestern Research, v. 68, p. 2045-2048. 2045-2048. Goldich, Goldich, S.S., S.S., 1938, 1938,A A study study of of rock rock weathering: weathering:Journal Journalof of Geology, Geology,v. v. 46, 46, p. p. 17-56. 17-56. Goldich, S.S., and and Fischer, Fischer, L.B., L.B., 1986, 1986,Air-abraison Air-abraison experiments experiments in U-Pb U-Pb dating dating of ofzircon: zircon: Chemical ChemicalGeology, Geology, v. v. 58, 58, p. 195-215. 195-215. Goldich, J.B., and North, R.M., 1980a, Goldich, S.S., S.S., Hedge, Hedge, C.E., C.E., Stern, Stem,T.W., T.W., Wooden, J.L., Bodkin, J.B., 1980a, Archean rocks of of the the Granite GraniteFalls Falls area, area, southwestern southwestern Minnesota, Minnesota, in Morey, Morey, G.B., and Hanson, G.N., G.N., eds., eds., Selected Selected studies studies of of Archean genisses and lower Proterozoic rocks, southern southern Canadian Canadian Shield: Geological GeologicalSociety Societyof of America America Special Special Paper 182, 182, p. 19-43. 19-43. Goldich, Goldich, S.S., and and Wooden, J.L., 1980, 1980, Origin of the Morton Gneiss, southwestern southwestern Minnesota: Minnesota: Part Archean Part 3. Geochronology, Geochronology,ininMorey, Morey,G.B., G.B., and andHanson, Hanson,G.N., G.N., eds., eds., Selected Selectedstudies studiesof ofkchean gneisses and lower Proterozoic rocks, southern Canadian Shield: Geological Society of gneisses Canadian Shield: America America Special Special Paper Paper 182, 182,p. 77-94. 77-94. Goldich, Goldich,S.S., S.S., Wooden, Wooden,J.L., J.L.,Ankenbauer, Ankenbauer,G.A., G.A.,Jr., Jr., Levey, Levey, T.M., and Suda, Suda, R.U., 1980b, 1980b,Origin Origin of the Morton Morey, G.B., G.B., and and Morton Gneiss, Gneiss, southwestern Minnesota: Part Part 1. 1. Lithology, Lithology,ininMorey, Hanson, G.N., eds., eds., Selected Selected studies studies of of Archean Archean gneiss and lower lower Proterozoic Proterozoic rocks, southern southern Canadian Canadian Shield: Shield: Geological GeologicalSociety SocietyofofAmerica, America,Special SpecialPaper Paper182, 182,p.p.45-56. 45-56. Grant, Grant, J.A., J.A., 1972, 1972,Minnesota MinnesotaRiver RiverValley, Valley, southwestern southwestern Minnesota, Minnesota,in in Sims, Sims,P.K., P.K., and and Morey, Morey, G.B., eds., Geology of Minnesota: A A centennial centennial volume: Minnesota MinnesotaGeological GeologicalSurvey, Survey, 177-196. p. 177-196. Grant, Grant, J.A., 1973, 1973,Phase Phase equilibria equilibriain in high-grade high-grade metamorphism metamorphism and partial melting of pelitic pelitic rocks: rocks: American American Journal Journalof of Science, Science,v. v. 273, 273, p. p. 289-317. 289-3 17. Hanson, G.N., 1968, 1968, K-Ar K-Ar ages ages for for hornblende hornblende from from granites granites and gneisses gneisses and and for for basaltic basaltic intrusives 20 p.p. intrusives in in Minnesota: Minnesota: Minnesota MinnesotaGeological GeologicalSurvey SurveyReport ReportofofInvestigations Investigations8,8,20 Hanson, Granite Falls, Minnesota: Minnesota: Hanson,G.N., G.N., and andHimmelberg, Himmelberg,G.R., G.R., 1967, 1967,Ages Ages of mafic mafic dikes near Granite Geological GeologicalSociety Societyof ofAmerica AmericaBulletin, Bulletin,v.v. 78, 78,p. p. 1429-1432. 1429-1432. 146 �G.R., 1968, Geology Geology of Precambrian rocks, Granite Granite Falls - Montevideo Himmelberg, G.R., Precambrian rocks, Montevideo area, area, southwestern Minnesota: Minnesota Geological Survey Special Publication Series Series SP-5, SP-5, 33 p. 33 Hoim, D.K., Holm, D.K., Darrah,K.S., Darrah,K.S., and and Lux, Lux, D.R., D.R., 1998, 1998,Evidence Evidencefor forwidespread widespread—1760 -1760 Ma metamorphism and rapid crustal stabilization of the Early Proterozoic (1870-1820 Proterozoic (1870-1820 Ma) Ma) Penokean orogen, orogen, Minnesota: Minnesota: American American Journal Journal of of Science, Science,v. v. 298. 298. p. p. 60-8 60-81. 1. Igneous and metamorphic rocks of the Minnesota Minnesota River River Valley: Valley: Unpublished Unpublished Lund, E.H., 1950, 1950, Igneous Minneapolis. Ph.D. dissertation, dissertation, University of Minnesota, Minneapolis. Valley: Geological Lund, E.H., 1956, 1956, Igneous Igneous and metamorphic rocks of the Minnesota River Valley: Geological Society Society of America Bulletin, Bulletin, v. 67, p. 1475-1490. 1475-1490. Moecher, D.P., ffi, Leier-Englehardt, Leier-Englehardt, P.J., and and Medaris, Medaris, L.G., L.G., Jr., Jr., 1986, Metamorphic Metamorphic Moecher, D.P., Perkins, D. in, conditions of late high-grade gneisses, conditions late Archean high-grade gneisses, Minnesota Minnesota River River valley, U.S.A.: Canadian Canadian Journal of Earth Journal Earth Sciences, Sciences,v. v. 23, 23, p. 633-645. 633-645. Perkins, D., ifi, in,and and Chipera, Chipera, S.J., S.J., 1985, 1985,Garnet-orthopyroxene-plagioclase-quartz Gmet-orthopyroxene-plagioclase-quartz barometry: Refinement and application to the English River subprovince and the Minnesota River Valley: Contributions to Mineralogy and Petrology, Petrology, v. 89, p. 69-80. 69-80. Southwick, D.L., and architecture of the Minnesota Minnesota Southwick, and Chandler, Chandler,V.C., V.C., 1996, Block and shear-zone architecture Valley subprovince: subprovince: Implications for Late Archean Archean accretionary accretionary tectonics: Canadian Canadian River Valley Journal of Earth v. 33, 33, p. 831-847. 831-847. Journal Earth Sciences, Sciences,v. 147 �I INTRODUCTION TO THE INTRODUCTION THE GLACIAL GLACIAL HISTORY OF OF THE THE MINNESOTA RIVER VALLEY Carrie Carrie J. Patterson Patterson Contrary to the four-fold devised around around the the turn of the century for four-fold glacial chronology devised terrestrial glacial sediments (Wisconsin, Illinoian, Kansan Kansan and and Nebraskan Nebraskan (Flint, 1957)), 195711, the current model for the Quaternary Quaternary record of glaciations is much more elaborate (Ruddiman and more elaborate Wright, 1987). 1987). The Theoxygen oxygenisotope isotoperecord record of of ocean ocean carbonate carbonate sediments sediments records records the the effects effects of glaciations. glaciations.During Duringglaciations, glaciations,160, 160,which whichisispreferentially preferentiallyevaporated evaporatedfrom from the the oceans oceans because because of it lesser mass, is stored enriching the glacial glacial oceans stored on land in the ice sheets, enriching oceansin in 180. 180. According to this record record of of global globalice icevolume, volume,there therewere were40 40glacial/interglacial glacialhnterglacialoscillations oscillationsinvolving involving moderate global Ma and and there therewere were22 22oscillations oscillations involving involving moderate global ice ice volumes volumes between 2.4 2.4 Ma and 0.9 Ma greater ice volumes from .9 Ma to the present present (Fig. (Fig. 3) 3) (Shackleton (Shackleton et et al., al., 1984). 1984). Counting back from our current oxygen isotope stage, stage 1, odd-numbered stages represent interglaciations interglaciations stages, glaciations. glaciations.Oxygen-isotope Oxygen-isotopestages stages2,2,6,12 and16 16show show the the greatest greatest and even-numbered even-numbered stages, 6, 12 and ice volumes, perhaps perhapsexplaining explainingthe the historical historical recognition of a four-fold record of terrestrial glaciation. glaciation. SPECMAP SPECMAP COMPOS!TE. COMPOSITE. 4 0.0 6180 6"0 (cr) (a) 0 2 —2 6"=0 ö'o (c) (a) 4 —4 0.0 • $_ 7 , $ —2 --4 7 10 0.5 0.5 a 2$-uz 1.0 0 2 25 1.0 1 .o - • 1.5 - 52 ' • 60 • 52 63 - Figure 3. Oxygen-isotope Oxygen-isotope profile derived from four cores (2 Atlantic Atlantic and 2 Pacific), Pacific), showing showing the the dominance dominance cores of the 41,000-year 41,000-year frequency before about 700,000 the 100,000-year 100,000-year frequency frequency after. after. Note years ago, and the the higher values values (and thus larger ice volumes) volumes) for 12, and and 16, 16,perhaps perhaps representing representing times times stages 2, 6, 12, when the Laurentide Laurentide ice sheet extended extended into the Great Lakes region and and beyond (from (from Williams Williams et et a!; al; 1988). 1988). Lakes data are compared compared with the the SPECMAP SPECMAP These data compilation compilation (Imbrie et al., 1984). 1984). . 2.0 148 . �oxygen-isotopestage stage 2 (the late Wisconsin7 During oxygen-isotope Wisconsin, retained retained from from the the former former chronology) chronology) and possibly possibly during during stages stages6, 6?12 12and and 16, 167Minnesota Minnesota was at the edge of an ice sheet that covered and most most of of Canada Canada (Fig. (Fig. 4). 4). thin7dynamic dynamic ice ice lobes lobes that that advanced, advanced?stagnated, stagnated?and and Minnesota was was affected affectedmainly mainly by by thin, Minnesota retreated independently independently of one another another (Wright, (Wright?1972). 1972). Ice-lobe Ice-lobedynamics dynamics were were controlled controlled by by retreated the mass mass balance balance and and bed conditions conditions of their respective icesheds. Owingto to the the different different substrates substrates the icesheds. Owing the ice ice flowed flowed over, over7it is possible possible to to distinguish distinguish the till of of the the different different ice ice lobes lobes using using matrix matrix color?texture texture and and mineralogy, mineralogy?and and clast clast lithology, lithology7and and thus thus reconstruct reconstructthe the activity activityof of the the lobes. lobes. color, few advances and aid in the reconstruction of the nature of ice ice Landforms remain for the last few advance advance and and retreat. retreat. Figure 4. Laurentide Laurentideice icesheet sheetatat14 14Ka. Ka. (Modified (Modifiedfrom fromDyke Dykeand andPrest, Prest?1986). 1986). Figure 4. Absolute dating dating of of the the late lateWisconsin Wisconsin glacial glacial sediments sediments is is possible possible using using radiocarbonradiocarbondating methods ifif organic organic remains remains are are found. found. Pre-late Pre-late Wisconsin units are difficult difficult to to date date absolutely Wright?1987). 1987). AAfew fewscattered scattereddates dateshave have been been secured securedin in the the area area absolutely (Ruddiman (Ruddiman and and Wright, using usingthe the presence presenceof of volcanic volcanicash ash (Richmond (Richmondand and Fullerton, Fullerton71986), 1986)?magnetic magneticreversals reversals preserved preserved in till and lake sediment (Patterson, 1993), amino-acid dating of shells (Gilbertson, 1990) and lake sediment (Patterson?1993)?amino-acid (Gilbertson71990) and cosmosgenic cosmosgenic isotope isotope age age estimates estimatesof of striated striatedrock rock surfaces surfaces(Bierman (Bierman et et al., al.?in in press). press). 149 �The basic stratigraphy stratigraphy was was described described by Matsch Matsch (1972). The history history of of work work in this area and the basic oldest glacial glacial sediment sediment in in southwestern southwestern Minnesota Minnesota is at least of mid-Pleistocene age age (Fig. (Fig. The oldest (Patterson,1997) 1997)and and may may even even be be of of Late Late PliocenefEarly Pliocene/Early Pleistocene Pleistoceneage age(2.1-1.2 (2.1-1.2 55 A and B) (Patterson, Ma) based based on on correlations correlations with glacial sediment in Ma) in Iowa Iowaand andNebraska Nebraska(Hallberg, (Hallberg,1986). 1986). These depositsare are attributed attributedto to northwest-sourced northwest-sourcedice iceadvances, advances,which which deposited depositedgray, gray, loam-to-clayloam-to-claydeposits loam tills tills containing containingclasts clasts of of Paleozoic Paleozoic carbonate carbonate and Cretaceous Cretaceous shale. There Thereare areatatleast leasttwo two loam tills tills of this age age that are are difficult diflicult to differentiate. Similar Similartills tills in South South Dakota are thought to be .6 Ma to 2.1 Ma (Gilbertson, 1990). (Gilbertson, 1990). / Figure 5. Ice Iceadvances advancesinto intothe the region. region. A. A.Early EarlyPleistocene Pleistoceneadvances, advances,greater greater that 0.6 Ma; B. B. Ice Ice Figure5. advance advance from from the northwest; C. C. Ice Ice advance advance from the northeast; D. D. Ice Iceadvance advancefrom fromthe thenorth, north, deposition 0-20 depositionof ofGranite GraniteFalls Fallstill till>40 >40ka; ka; E. E.Late LateWisconsin Wisconsinadvance advanceofofearly earlyDes DesMoine Moinelobe, lobe,330-20 ka; F. Late Wisconsin advance of the Des Moine lobe, 14 ka. (From Patterson, 1993). ka; F. Late Wisconsin advance of the Des Moine lobe, 14 ka. (From Patterson, 1993). 150 �Stratigraphicallyabove above these these gray gray tills tills is is aa northeast-sourced northeast-sourced glacial glacial sediment sediment(Fig. (Fig.5C) 5C) Stratigraphically that can can be be distinguished distinguishedby by its itsred-brown red-brown color, color, sandier sandier texture, texture, and and clasts clasts originating originating in in the the that Lake Superior Superiorarea area including including banded banded iron iron formation, formation, agate, agate, red Thisunit, unit, Lake red clastics clastics and and basalt. basalt. This the Hawk Creek till (Matsch, 1972) is only locally preserved and its southwestern extent is not the Hawk Creek till (Matsch, 1972) is only locally preserved and its southwestern extent is not known. ItItisisthought thoughtto tobe be >50 >50 ka ka (Gilbertson, (Gilbertson,1990). 1990). known. Thelast lastpre-late pre-lateWisconsin Wisconsinglacial glacial advance advancewas was from from the the north-northwest (Fig. 5D) 5D) and and is is The north-northwest (Fig. similarto to the theolder oldergray grayunits unitsbut but has has less less Cretaceous Cretaceousshale. shale.Wood Woodininthis, this,the theGranite GraniteFalls Fallstill, till, similar is A0,OOO years old (Matsch, 1972). is >40,000 years old (Matsch, 1972). The surficial surficial glacial glacial sediment sediment in in the the Minnesota Minnesota River River basin basin was was deposited deposited by by the the late late The Wisconsin Des Moines lobe, which advanced, stagnated and retreated repeatedly. Its first advance Wisconsin Des Moines lobe, which advanced, stagnated and retreated repeatedly. Its first advance may have have been been as as early early as as 30-20 30-20 ka ka (Fig. (Fig. 5E) 5E) (Gilbertson, (Gilbertson, 1990) 1990) The Theadvance advanceto to Des Des Moines, Moines, may Iowa is is dated dated at at 14 14ka ka (Fig. (Fig. SF) 5F) (Clayton (Clayton and 1982; Hallberg The Iowa and Moran, Moran, 1982; Hallberg and and Kemmis, Kenimis, 1986). 1986). The loam-textured till is gray when unoxidized and has Cretaceous shale and Paleozoic carbonate loam-textured till is gray when unoxidized and has Cretaceous shale and Paleozoic carbonate clasts.ItItgenerally generallycontains containsPierre PierreShale, Shale,whereas whereasthe theolder oldergray graytills tillscontain containother otherCretaceous Cretaceous clasts. formations such as the Greenhorn and Carlisle. The base of the Des Moines lobe till is locally locally formations such as the Greenhorn and Carlisle. The base of the Des Moines lobe till is marked by by aa planar planar concentration concentrationof offaceted, faceted,striated striatedboulders. boulders. marked GlacialLake LakeAgassiz Agassiz and and Glacial Glacial River River Warren Warren Glacial commonfor forlakes lakes to toform formin infront frontof ofaaretreating retreatingglacier, glacier, with with meltwater meltwater ponding ponding ItIt isiscommon between aa moraine moraine and and the the ice ice front. front. AAproglacial proglaciallake lakemay maynot notbe bevery verylarge large or or long-lived long-lived between because of the instability of the moraine dam and ice margin. However, an unusually stableand and because of the instability of the moraine dam and ice margin. However, an unusually stable large proglacial proglaciallake lakebegan began to to form form in in west-central west-central Minnesota Minnesota at approximately approximately12 12ka ka as as the theice ice large retreatednorth north of of the the Big Big Stone StoneMoraine Moraine and and the the nonspectacular nonspectacular continental continental divide divide at at Brown's Brown's retreated Valley (Fig. 6). Valley (Fig. 6). For much muchof ofits itsearly earlyhistory, history,Glacial GlacialLake LakeAgassiz Agassizwas wasdrained drainedby byGlacial GlacialRiver RiverWarren, Warren, For which flowed southeast from the southern tip of the lake (Upham, 1883, 1895; Matsch and which flowed southeast from the southern tip of the lake (Upham, 1883, 1895; Matsch and Wright, 1967). 1967). Initially Initially Glacial Glacial River Rver Warren Warren cut series of of anastomosing anastomosing channels channels with with Wright, cut aa series streamlined residual bars and boulder lags (Matsch, 1983). The style of channel formation streamlined residual bars and boulder lags (Matsch, 1983). The style of channel formation isis characteristicof ofsudden suddenand andlarge largedischarge dischargefrom fromaa lake lake that that overwhelms overwhelms the the existing existing drainage drainage characteristic system (Kehew (Kehew and and Lord, Lord, 1986). 1986). Flow Flowwas waseventually eventuallyfocused focused in inone onemain mainspillway spillwaywhich which system became more deeply incised, stranding earlier channels (Fig. 7). became more deeply incised, stranding earlier channels (Fig. 7). Continuedretreat retreatof ofthe theice icefront fronteventually eventuallyopened openedlower lower outlets outletsfor for Glacial GlacialLake Lake Agassiz Agassiz Continued to the northeast (Clayton, 1983) and possibly the northwest (Smith and Fisher, 1993) and to the northeast (Clayton, 1983) and possibly the northwest (Smith and Fisher, 1993) and Glacial Glacial River Warren was abandoned abandoned as as an an outlet. outlet. Tributaries Tributaries to to Glacial Glacial River River Warren, Warren, which which had had River steepened their their gradients gradients in in response response to tothe thedowncutting, downcutting,were were now now contributing contributingtoo toomuch much steepened sediment to be carried away by the reduced flow of the river. Fans spread across the valley floor sediment to be carried away by the reduced flow of the river. Fans spread across the valley floor partiallydamming dammingit,it,forming formingaaseries seriesof oflong longlakes lakes(Wright, (Wright,1972; 1972;1990) 1990)including includingLake LakeTraverse, Traverse, partially Big Stone Lake, Marsh Lake and Lac qui Parle. The levels of these lakes are now controlled by Big Stone Lake, Marsh Lake and Lac qui Parle. The levels of these lakes are now controlled by damsfor for recreation recreationand andflood floodabatement. abatement. dams Thefield fieldstops stopswill willfirst firstaddress addressthe the most most recent recent event event related related to to glaciation, glaciation,the the formation formation The of the the Minnesota MinnesotaRiver RiverValley, Valley, and then move move to to progressively progressively older older events events that that are are exposed exposed of because of of river river downcutting. downcutting. See Seeplate plate11(page (page168) 168)for forstop stoplocations. locations. because 151 �KM Figure Figure6. 6.Early Earlystages stagesofofGlacial Glaciallake lakeAgassiz. Agassiz.(Adapted (Adaptedfrom fromTeller, Teller,1987). 1987). 152 �pPpLrmw hf Hermanstage stagedownstream downstreamfrom fromOrtonville. Ortonville. Herman Thewide wideshallow shallowRiver RiverWarren Warren(in (inblack) black) The flowsacross acrossaaflat flatstretch stretchofoftopography, topography, flows "'i" around branching aroundislands islandsofofdrift. drift. r-d i—J 1 I I0 ost-Tintah stage downstream from rtonville. The auxiliary channels are bandoned as River Warren (in black) enches more deeply. A few islands, ome of bedrock, appear in the channel ownstream from Ortonville. 7 r-l- .-A resent valley. Alluviation by tributarie as segmented the valley abandoned by Warren into a series of long, shallow lakes, connected by the Minnesota River. r - Figure Figure 7.7.A.A.Initial Initialstages stagesofofdevelopment developmentof ofGlacial Glacial River Warren. Warren. B. B. Formation Formationofofmain mainriver rivertrenches. trenches. Abandoned Abandoned channels channels may may be be marked marked by river lags. C. C. Present-day Present-day Minnesota MinnesotaRiver River Valley. Reproduced Reproduced from from Matsch, Matsch, 1983. 1983. 153 �FIELD TRIP FIELD TRIPSTOPS—DAY STOPS-DAY 2 Stop 1—Ingebretsen's 1-Ingebretsen's Landing, Landing, or "the clay banks" on Lac qui Parle. Milanquadrangle, quadrangle, Stop Pane. Milan T. 119 42 W.-section 32 32 119 N.-R. 42 and low bank are formed in 2-3 m of reddish matrix has 43The beach and reddish brown brown till. till. The till matrix 48 percent sand and and the the clasts clasts have have aa northeastern northeastern provenance; provenance; you may find find Lake Lake Superior Superior clastic rocks on the beach. This Thiscolor, color,texture textureand and agates, banded iron formation, formation,basalt basalt and and red clastic agates, banded iron grain type are unusual for tills in this general area. area. This is not the surface till in the region but but is normally found deeper deeper in the glacial glacial section section and was exhumed by river downcutting. In places there are are remnants remnants of of another anothertill till overlying overlying this this red red till. till. It can be distinguished distinguished by its siltier northwest, including siltier texture texture and and clasts clasts typical typical of of an an ice ice advance advance from from the northwest, includingPaleozoic Paleozoic carbonate and Cretaceous Cretaceous shale. shale. discontinuous boulder A discontinuous boulder lag lag represents represents the erosional discontinuity created as Glacial River Warren, Agassiz, cut into the the glacial glacial sediments, sediments, selectively removing removing Warren, the spillway spillwayfor for Glacial Glacial Lake Lake Agassiz, the finer grains. The Thefollowing followingsections sectionsfrom from aa paper paper by Kehew Kehew and Lord (1986) describe the general formation of glacial-lake glacial-lake spiliways spillways (Fig. 8). The instability of the the spillway spillway rivers rivers was was exceptional; exceptional; torrential torrential discharges discharges no sediment from glacial-lake glacial-lake outlets, following following dam failure, had high energy and no load. Consequently, Consequently,flows flowswhich whichincised incisedspillways spillwayswere were highly highly erosive, erosive, and and channel channel sediment from these events is scarce. The The flood flood deposits deposits which do exist consist of homogeneous masses of very poorly sorted, pebbly homogeneous masses pebbly cobble cobble gravel, gravel, which indicates water. p. 165 rapid aggradation in very turbulent water. 165 154 �The morphological morphologicd characteristics characteristics of of spiliways. spillways....indicate indicate that the spiliways spillways were channels that that contained contained flow flow at at bankfull bankfull stage stage during during one one or more highly Thesize sizeof of the thespillways spillwaysqualitatively qualitatively demonstrates demonstrates that the the flood flood erosive floods. The discharges were enormous, and the estimated volumes of most proglacial proglacial lakes lakes (Kehew and Clayton, 1983) require that that bankfbll banklull discharges couldn't have been (Kehew sustained for more than aa few few weeks. weeks. In short, spiliways spillways in the the mid-continent mid-continent region geologically instantaneous instantaneous events of colossal colossal magnitude magnitude region were were incised incised by a few, geologically in comparison comparison to modern modern river discharges. p. 165 165 . At this location in Glacial Glacial River River Warren, two subparallel channels were incised rather than one main channel: channel: Lac Parleand andWatson Watson Sag. Sag. They Theyare areseparated separated by the erosional Lac qui qui Pale residual is located located on. on. According to Kehew and and Lord, Lord, (1986) (1986) erosional erosional residua1that the the town of Watson is residuals are key to identifying floods that overwhelmed existing drainage systems. residuals are identifying floods that overwhelmed existing drainage systems. organic horizon horizon associated associated with with the theboulder boulder lag. lag. A A silty silty lake lake sediment sediment In places there is an organic caps the section. The organic horizon and lake sediment may represent Glacial River section. The organic horizon and lake sediment may represent Glacial River Warren overbank deposits deposits or more recent overbank E X PP L AANAT N A T IION ON Holocene deposits. An examination examination of the organics of organics for pollen pollen to to determine their approximate determine approximate age age fihallow Channel before radiocarbon before radiocarbon dating was was considered yielded no identifiable considered yielded identifiable 9 Erosional Erosional Residual Residual grains. The bank The bank has has retreated significantly in recent years and the DNR-fisheries is planning to riprap Longitudinal it to prevent prevent further further retreat. retreat. Their Groove concern is that sediment loading of lake is negatively the negatively affecting affecting fish fish Boulder Lag habitat. habitat. Features to see: Features see: • Lake LakeSuperior Superioragates, agates, banded iron formation, basalt basalt and red elastic formation7 clastic rocks on the beach indicative indicative of a northeastern provenance provenance for the northeastern till. • Joints Joints in in till till with with narrow zones of reduced indicating reduced till indicating water water Figure 8. There Thereare are two two basic spillway levels: (1) (1) the movement along the joints. trench-like, channel; and (2) the trench-like, low sinuosity, inner channel; • Boulder lag and associated associated organic organic broad outer zone with: (a) slighity of slighlty scoured areas of honzon. horizon. channels; and anastamosing channels; and (b) (b) the the deeply deeply scoured scoured v / displays longitudinal longitudinal grooves, zone which displays grooves7streamlined erosional residuals residuals and and boulder boulder lags. lags. From erosional From Kehew Kehew and Lord (1986). (1986). 155 �Stop2—Glacial %Glacial Lake Lake Benson Benson sediment sediment exposed exposed along the Chippewa River. Bend City City Stop River. Big Bend quadrangle, T. T. 120 120N.-R. N.-R. 41 41 W-section W-section 36 36 Glacial Lake Lake Benson, a moraineGlacial morainedammed, 2800 sq km, proglacial lake in dammed, 2800 west-central Minnesota, formed formed during during the the west-central Minnesota, late Wisconsin Wisconsin retreat retreat of of the Des Moines lobe (Fig. 9). It was the last in a series of of retreated lakes before the Des Moines lobe retreated north of the divide to form form Glacial Glacial Lake Lake Agassiz (Diedrick and and Rust, Rust, 1975; 1975; Agassiz (Diedrick Patterson, Cotter Cotter and Knaeble, Patterson, Knaeble, 1998; 1998; Rittenour, Geiger and Cotter, Cotter, in prep.) Two-rn-thick exposures exposures of of lake Two-m-thick lake sediment Chippewa River in Swift sediment along along the Chippewa and Chippewa counties counties show show as many as 23 23 rhythmically bedded couplets interpreted as annual interpreted annual layers layers (varves) (varves) by Des Moines lobe till. In a underlain by series of undergraduate undergraduate and and NSF-sponsored NSF-sponsored for REU (Reseach (Reseach Experience supervised by Jim Undergraduates) studies studies supervised Cotter of University of Minnesota, Minnesota, Morris, Morris, students students have described described and counted counted varves at five five sites; 44 4 4 varves were were correlated correlated (Rittenour, (Rittenour, 1994; 1994;1995). 1995). exposure you will note that in some areas the beds beds are are overturned. overturned. We interpret interpret this In this exposure this as a result of the lake forming forming in an area where there was stagnant glacial ice which melted out, out, been deposited deposited in in a disrupting some of the beds. The Theupper, upper, massive massive silt and clay may have been shallower lake that was was unfavorable varve to development because the wave base base disturbed disturbed the the bottom bottom sediment. sediment. Features to see: see: • Difference in grain size of the winter and summer layers. • Upward decrease in thickness of summer layer indicating lessening sediment load and aid retreating glacier. retreating glacier. • Overturned beds. • Dropstones. Dropstones. • Nature of of contact contact with underlying Des Moines lobe lobe Figure 9. Approximate Approximateextent extent of of Glacial Glacial Lake Lake Benson till. till. (from Rittenour Rittenour et al., al., in in prep). prep). 1 156 �Stop 3—Ice-confined &Ice-confined streams. Montivideo Montivideo landfill landfill and nearby pits. Montevideo Montevideoquadrangle, quadrangle, Stop 117 N.-R. 40 W.-section 12; Asbury W.- section section 13 13and and T. T. T. 117 Asbury quadrangle, quadrangle, T. 1. 117 N.-R. N.-R. 40 40W.117 117 N.- R. R. 39 39 W.-section W.-section 33. 33. These These three three pits pits have have been been developed developed in in southeast-oriented, southeast-oriented?discontinuous discontinuousridges ridgesthat thatare are generally less less than than 77 m high. We We may may only only visit visit one one or two of the pits depending generally depending on the quality of the exposures exposures and time constraints. These transverse to them form These ridges ridges and others oriented transverse a rectilinear rectilinear pattern pattern that that is is common common in in the the interior interiorof of the the Des Des Moines Moines lobe. lobe. Ridges Ridges parallel to to ice ice flow, flow, and therefore to former ice surface surface slope, slope?are are predominantly predominantly composed of sorted sorted sediments. The Thetransverse transverse ridges ridges are are composed composed of sediment sediment ranging ranging from from diamicton to to sand sand and and gravel. gravel. Most Mostof of the thesampled sampledtransverse transverse ridges ridges were were only only slightly slightlycoarser coarser diamicton and more more sorted sorted than than the the underlying underlying and and surrounding surrounding till. till. ridges are result of The ridges are intepreted intepreted to to be be a result the pervasively pervasively crevassed crevassed deposition localized in the stagnant stagnant ice of the Des Des Moines Moines lobe lobe (Fig. (Fig. 10) 10) (Matsch, (Matsch, 1972; 1972; Patterson, Patterson?1997). 1997). Crevasses oriented oriented paralled paralled to to the the ice ice surface surface Crevasses slope were preferentially used by by supraglacial supraglacial streams. streams. Final melting melting of the ice ice walls walls of of the the streams inverted the the topography topography and left sorted sorted stream sediment sediment as a positive positive element element in in the the landscape. landscape. iceBenson outlets were were iceGlacial Lake Benson confined and the pits east and north of the landfill landfill are interpreted as part part of a late-stage interpreted as late-stage outlet of Glacial Lake northern pit, pit, well well Glacial Lake Benson. Benson. In the northern sorted sand is unconformably unconformably overlain by poorly sorted sorted pebbly pebbly sand which which has an an indistinct, indistinct, conformable contact with with the the overlying diamicton. This is is overlying diamicton. This interpretedas as steady steady discharge discharge interpreted followedby by aa larger, larger?short-lived followed short-lived discharge causing causing the the collapse collapse discharge of of stagnant stagnantice ice resulting resultingin inflow flow of diamicton into the channel. channel. Features Featuresto to see: see: • Climbing Climbingripples. ripples. • Uncollapsed Uncollapsedbeds beds indicating indicating no ice ice beneath beneath the the stream stream when when the sediments sediments were were deposited. deposited. 157 157 �Figure 10. 10. Heavily crevassed, postsurge Bering Glacier, surge ice surface surface of the Bering Glacier, Alaska. Photograph reproduced Photograph reproduced with with the the permission permission of of B. B. Molnia. Molnia. 158 �Stop4—Glacial &Glacial Stratigraphy, Stratigraphy,Upper UpperSioux SiouxAgency Agency State StatePark, Park, exposure exposureon on the theyellow Stop Yellow Medicine Medicine LoneTree TreeLake Lakequadrangle, quadrangle,T. T. 115 115 N.-R. 38 38 W.-section 30 River. Lone WARNING This is an unstable Be aware awareof of the the position position of others on the slope. Do not unstable outcrop. Be another person before digging digging or descending. descending. follow another personup upaagully. gully. Look Lookfor forpeople peoplebelow below you before Climbing up up is is easier easier than than getting getting down. down. Hardhats Hardhats are are recommended. If you chose not to climb around on this outcrop, samples of the till and and washed washed pebbles from the different different units will be provided for for viewing viewingon ontop topof ofthe thebluff. bluff. Do not step too close There are are rotational rotational slump slump blocks blocks waiting to descend; avoid close to to the the edge. edge. There becoming colluvium. colluvium. Nearly of glacial glacial sediment sediment is is exposured exposured along this meader bend of of the the Yellow Yellow Nearly 100 ft of Medicine River (Fig. 11). This This is almost almost the entire thickness of glacial glacial sediment at this this location. location. constructed from from waterwaterCross sections constructed well data show show the regolith regolith surface surface at at approximately immediate approximately 930 930 ft ft in the immediate just a few feet below the river area; this is just bed. bed. uppermost unit is a 2-3 m layer The uppermost of Des Moines lobe till. It accessible It is not accessible from this outcrop. Please do not Please do not try! try! There is aa discontinuous There discontinuous boulder boulder pavement pavement at the base of the Des Des Moines Moines lobe till. till. The boulders are faceted and and striated and and form form aa planar planar surface. surface. The The boulder pavement, although not continuous, is found at this stratigraphic stratigraphic position the position over over most most of of the interior of the I/i ,—ih,Des Moines lobe in Minnesota (Wright (Wright et et 1973). The Thefaceting facetingand andstriating striatinghad had to todevelop develop beneath beneath aa moving moving glacier glacier that did did not, not, al., 1973). however, however, have the shear stress to move move the theboulders boulders very very far farbecause becauseunderlying underlying beds beds are are generally generally not disturbed. Theboulders bouldersmay mayhave havebeen been concentrated concentratedat at the the surface surfacebefore before the glacier glacier overrode overrode disturbed. The them by water or periglacial processes. processes. Alternatively, some have proposed that the Alternatively, some have proposed that the boulder boulder concentration settling through an incompetent incompetent till matrix concentrationdeveloped developedsubglacially, subglacially, by large large clasts settling (Clark, 1991). 1991). The Theissue issueisisstill stillunresolved. unresolved. The exposure of the The next next unit unit is exposed exposed is is the the Granite Granite Falls Falls till. till. This may be an atypical exposure Granite Falls till; it is siltier than normal and layered layered with with sorted sorted sediment. sediment. Toward the base base of this unit, near near the the contact contact with with the the underlying underlying fine fine sand, sand, discrete discrete lobes of of diamict diamict can can be be recognized. These Thesefeatures featuresare areinterpreted interpretedas as the the result result of of flow flow of of the the till till into into standing standing water. water. This is a very plausible scenario given the unit immediately below which has been interpreted plausible scenario given unit which has been interpreted as a lake sediment. sediment. is mainly mainly aa fine fine sand sand with with silt silt and and some some clay. clay. In The unit interpreted as lake sediment is In places it is finely finely bedded. bedded. The Theextent extentof of the the lake lake sediment sediment beyond beyond this outcrop is not well 159 �Figure 11. 11. Yellow Yellow Medicine Medicine River River exposure exposure in Upper Sioux Sioux Agency State Park looking downriver. Photograph Photographby by Star Star Mulder known. From Fromthis thisexposure exposureitit appears appearsto to be be aarelatively relatively deep deep lake and therefore could have covered a large large area. area. Beneath the the lake lake sediment sedimentis a thick, gray loamy Beneath loamy till. till. Note the many well-striated Paleozoic Paleozoic carbonate clasts. clasts. The Thelower lowertill tillhas hasnot not been been well well exposed exposed here here but at other cuts along this river carbonate there are are two two gray tills tills separated separatedby aa glacial glacial stream stream sediment sediment at at this stratigraphic stratigraphicposition. Features Features to to see: see: • Striated, Striated,bullet-shaped bullet-shaped clasts clastsof of Paleozoic Paleozoic limestone limestonefrom from lower lower till. till. • Path Path of of groundwater groundwater flow through glacial units. Note Note areas areas of of lateral lateral flow. flow. • Contact of the Granite Falls till with underlying lake sediment. Contact of the Granite Falls till with underlying lake sediment. • Evidence Evidence for for depositional depositional environment environmentof of Granite GraniteFalls Falls till. • Boulder Boulder pavement pavement (from (from aa distance, distance,please). please). 160 �Stop5—Inge 5-Inge Kea's Kea's climbing climbingripples. ripples. Iverson IversonLake Lakequadrangle, quadrangle,T. T. 114 114 N.-R. 37 W.-section 23 Stop (this stop is out of stratigraphic stratigraphic order along the field trip route) located in an an This sand pit is located upper terrace along the Minnesota Minnesota The deep deep channel channel below us is River. The the Glacial Glacial River River Warren Warren part of the spillway. spillway. Glacial outwash outwash streams streams flowed from from the retreating retreating Des Des flowed Moines general area of Moines lobe lobe in the general Glacial Warren prior prior to its Glacial River River Warren formation. formation. This exposure is most most likely likely a remnant of an earlier earlier outwash stream. stream. The The depositional depositionalenvironment environment is distinctly different different from the first stop of the the day. day. Features to see: Variation in migration and migration and aggradationrates rates (angle (angle of climb). climb). aggradation Stop6—Rush &Rush River River Wayside; Deformation of Henderson quadrangle quadrangle T. 112 Stop of glacial glacial sediment. sediment. Henderson N.-R. 26W-section 15 The downcutting of this east-flowing east-flowing Minnesota River has exposed exposed tributary to the Minnesota glacial units glacial units that display display deformation deformation induced by the advance a glacier. induced by the advance a glacier. Mark Mark Johnson Johnson of Gustavus Gustavus Adolphus College has directed undergraduates from his school directed undergraduates from school and from an NSF-sponsored NSF-sponsored field program program in in work work on the details details of this outcrop outcrop (Fig. (Fig. 12). 12). The lowest lowest unit is a gray The gray loamy loamy till till and it is conformably conformably overlain by lake sediment, sediment, that, judging judging by the color, formed in front of the retreating, gray-till-bearing gray-till-bearing ice lobe. lobe. The gray lake sediment sediment grades into a red red layers. This sandy unit records sand with silt layers. the northeast. the advance advance of an ice lobe from the northeast. The next unit is a mixture of the underlying sand sand with with aa red red sandy sandyloam loamtill. till. There is a sharp contact between this and the final unit exposed, exposed, a homogenous red sandy till. Folds: The lower two units are folded into three anticlines anticlines that span the outcrop. outcrop. sand displays displays a subparallel subparallel series series of high-angle high-angle normal faults. faults. Faults: The red sand Faults: Fabric: The long axis of prolate pebbles in a till till are are commonly commonly measured measured and plotted plotted on Fabric: a Rose diagram. Pebbles Pebblesin in aa subglacial subglacialtill till may be oriented oriented by the the shear shear stress stress of the overriding overriding glacier; glacier; pebbles pebbles are are generally generally parallel parallel to to flow, flow, with a slight slight upsuggesting ice flow from from the the north-northeast. north-northeast. glacier dip. The The red red till till has a fabric suggesting 161 - ll rn �Si .5Z $2 .38 53 .10 SI .4 53 .44 53 .o Elgen Values Si .55 51 .5S 52 .38 SZ .31 53 .13 53 .0? * F,t Ta., • zs Mlyi Siiuple T.n w 0000 OlyL1y MSfldOnud RhvwO (rscxit) DdR.d Old Grey Grey EJ Hs$d LayW Leysre eI 1*. Sam.nts iiwougl.on — S Faà. Tp-.* sew Figure Figure12. 12.Structural Structuraldata datafor forRush RushRiver Riveroutcrop. outcrop.Unpublished Unpublishedwork workofofAmy AmyMoe, Moe,Gustavus Gustavus Adolphus AdolphusCollege Collegestudent, student,1996. 1996. 162 �Ouestion structural data consistent with the simple interpretation of proglacial deformation Is the structural and eventual overriding overriding by the advance of the the red-till-bearing red-till-bearing ice ice lobe lobe from from the the north-northeast north-northeast (Fig. (Fig. 13)? 13)? - p_se. — -.p mmmbwlI@ll .I—_ =-- V v - —_ V v Vv 1401 wt-mmma A. Advacing Advacing glacial glacial ice with attached frozen frozen-unfrozen substrate. Shear Shear forces forces along the frozen-unfrozen boundaryat boundaryat the base base of the slab slab initiate i ~ t i a t thrustthmste block movement. movement. —— \ • B. Thrusted Tbrustedsubglacial subglacialslabs slabs are are deformed deformed and attacked Snout. attacked at the the glaciers glaciers~ snout. C. Glacial Glacialice ice ovemdes ovenides the the thrust thrust slabs, causing more more deformation. deformation. 13. Sequence Sequenceof ofproglacial proglacialdeformation deformationA A and and B B with with eventual eventual overriding by Figure 13. advancing glacier glacier (from (fromAber, Aber, 1988). 1988). the advancing TERRACES THE MINNESOTA RIVER NEAR SHAKOPEE, SHAKOPEE,MINNESOTA MINNESOTA TERRACES ALONG THE Barbara Lusardi Lusardi The Minnesota Minnesota River River Valley near near Shakopee is almost almost 200 200 feet feet deep deep and and over over 33 miles miles wide wide in places. highlands on on either either side side of of the valley rise to an elevation places. Till highlands valley rise elevation of over 900 feet. Distinct terraces terraces step step down into into the valley where the undefiit across underfit Minnesota River meanders across a broad alluvial alluvial plain. based on on elevation. elevation. The Three terraces are distinguished based The uppermost uppermost terrace, terrace 3 (Qft3), is visible just to the the south south of of highway highway 169 169 between between Marschall Marschall Road and Marystown Marystown Terrace33 is is at at an an elevation elevation of about 850 feet-150 Road. Terrace feet—iSO feet feet above abovethe themodern modernfloodplain. floodplain. A distinct scarp separates the terrace from the till upland to the south, and smaller channel scars crisscross the surface "islands" above terrace above the level of terrace crisscross surface breaking breaking the the terrace terrace up into three small ''islands" thick over overbedrock. bedrock. Data from water 2 (Qft2). Alluvial Alluvial sediments sediments of of terrace terrace 3 are up to 40 feet thick well logs indicate that sand and gravel extends suggests that extends south beneath beneath the the till till upland. upland. This suggests the terrace sediments outwash deposits deposits that that have have been eroded eroded to form form the the terrace. terrace. sediments may may be be outwash follows the middle terrace, terrace, terrace terrace 2, 2, at atan anelevation elevationof ofapproximately approximately The new highway 169 169 follows feet—i 10 feet above the modern modern floodplain. Numerous 810 feet-1 Numerous shallow shallow channels crisscross the terrace surface. Alluvial Paleozoic bedrock. bedrock. Just south of Alluvial sediments sediments are are 10-50 10-50 feet thick over Paleozoic Shakopee, terrace miles wide. wide. It narrows to the southwest southwest and and eventually eventually disappears disappears Shakopee, terrace 2 is about 1.5 miles (Qftl) near Meniam. Many Manygravel gravel pit pit operations operations occur occur within sediments of against terrace 1 (Qftl) near Merriam. terrace 2 and and into into the the bedrock bedrock below. below. 163 �_ ___ 14. Terraces Terracesof ofthe theMinnesota MinnesotaRiver River Valley Valley near Shakopee and Chaska. Figure 14. ; \G1ajidepsits \i . ' ;7i / 2 / I :/ / ' Qf / ( / /_ /I * wa f -., — . SCALE 1:62 162500 SCALE 500 2 1/2 1 1 1 ~ (sand and gravel and till) i .. 3/4 — : I . }/' 1 / ... _,$7 I' - •— 1.1 — ikt j $ r — . L 1 . / . Lower terraces cut into glacial p- sediments and bedrock I .. : I 1 KILOMmER KILOMETER preserved as terraces above the floodplain of the modern Minnesota River Highest terrace cut into glacial sediments Z / I 0 : ALLUVIAL TERRACES—Cilacial River Warren sediments - / -.. U 11MILE MILE 0 5 .5 H - 1 Qft3 Alluvium of Terrace 3—Elevation —850 ft 150 ft (46 m) above present floodplain Silty loam to sandy loam (fine grained in shallow channels) grading into loamy shale; increasing percentage of gravel with depth. Qft2 above present floodplain Loam to sandy loam (fine grained in shallow channels) 1 2mm coarse sand than 1 percent percent shale; shale increasing fraction contains less than percentage of gravel with depth Qftl Alluvium of Terrace 1—Elevation 750 ft 50 ft (15 m) above present floodplain Variable thickness of predominantly medium to coarse coarse sand. sand. 1 / c I 1 / GLACIAL DEPOSITS—Sediment deposited by the Des / Moines lobe Contains abundant shale fragments .- 1 PALEOZOIC BEDROCK—Prairie du Chien dolostone and Jordan Sandstone mapped within 10 feet of surface. L This map consists of portions of the Shakopee and Jordan East 7.5-minute quadrangles that have been modified and reduced in scale Linework and descriptions are taken from Lusardi (1997) and Lusardi (in prep). Hennepin I Carver ---I)ak ota Scott L -Dig ram Location Digram I@ 164 �The lowest lowest terrace terrace is is at at an elevation elevation of approximately 750 feet—50 feet-50 feet feet above above the the modern modern The approximately 750 floodplain. In Inmany manyplaces places the the underlying underlying bedrock is at or near the Ordovician Prairie Prairie floodplain. the surface. surface. Ordovician du Chien dolostone is the principal source for crushed stone. Cambrian Cambrian Jordan Jordan Sandstone Sandstoneisis exposed limited areas just west exposed in limited areas near Merriam and Baden. The The only only known sandstone sandstone quarry, just of Baden, Baden, is is now now abandoned. abandoned. On the opposite terraces are narrow narrow or or obscured. obscured. Just northeast opposite side of the river, the terraces northeast of Chaska, sediments sediments of terrace terrace 11 are are buried by colluvial sediments eroded from from the the valley valley wall. Chaska, small remnants remnants of this this lowest lowest terrace terrace are visible above the floodplain. Terrace22 is is present present Only small floodplain. Terrace 114 mile as a very narrow strip, less than 1/4 mile wide, wide, between between Chaska Chaska and and Carver. Carver. Terrace 3 is is present present south of Chaska dramatically south south of Carver. Sand Sandat at the the same same elevation elevation of of terrace terrace south Chaska and widens dramatically 3 continues to the south south in aa band band almost almost 2.5 2.5 miles miles wide wide along along the the river, river, although although no no distinct distinct scarp is obvious. Unlikethe the other other terrace terracesurfaces, surfaces, this this area is pitted and channeled, scarp obvious. Unlike channeled, and may be related to to an an earlier earlier flood flood event. Similarly, Similarly,thick thick sand sand buried buried by 50-100 50-100feet feetof of glacial glacialtill till on on the the related north side side of of the the valley, valley, north north of of Shakopee, Shakopee, suggests suggests that that the the river river valley valley at at one one time timeextended extended north further further to the the north. The Thesand sandwas wascovered coveredby by glacial glacial sediment sediment during during the the last last glacial glacial advance advance and was never re-excavated re-excavated by subsequent subsequent floods. floods. REFERENCES WFEWNCES CITED CITED Aber J.S., 1988, 1988, Structural Structural geology exercises with glaciotetonic examples: WinstonSalem, 1-84. Salem, North North Carolina, Carolina,Hunter HunterTextbooks, Textbooks,p.p.881-84. 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Minnesota: fi A centennial volume: Minnesota Geological LCLILCIIIIW V U ~ U I I I G . 1~111111esota Geological Survey, Survey?p. 548-560. 548-560. Matsch, C.L., 1983, 1983,River River Warren, the southern southern outlet outlet of Glacial Glacial Lake Lake Agassiz, Agassiz, in in Teller, Teller, J.T., and Clayton, Lee, eds.? eds., Glacial Lake Lake Agassiz: Agassiz: Geological Association Association of of Canada Canada J.T., Special Special Paper 26, p. 231-244. C.J., 1993. Mapping Patterson, C.J., Mappingglacial glacialterrain, terrain, southwestern southwestern Minnesota, Minnesota, U.S.A., U.S.A.7 in J.S., ed., Glaciotectonics Aber, J.S., Glaciotectonicsand and mapping mapping glacial glacial deposits: deposits: Regina, Saskatchewan, Saskaichewan, University of Regina, Regina, Canadian Canadian Plains Plains Research Research Center, Center, p. p. 155-176. 155-176. Patterson, C.J., 1997, 1997, Quaternary Quaternary geology geology of southwestern Minnesota, in Patterson, C.J., Contributions to the geology Contributions geology of southwestern southwestern Minnesota: Minnesota Geological Geological Survey Report Report of Investigations Investigations 47, 47?p. 1-45. 1-45. Patterson, Patterson, C.J., C.J., Cotter, Cotter,J.F.P., J.F.P., and Knaeble, Knaeble?A.R., 1998, 1998, Des Moines lobe advanced advanced into into Glacial Lake Benson to form the Big Stone Stone Moraine, Moraine?west-central Minnesota: Geological Society Geological Society of of America America Abstracts Abstracts with Program, Program, v. v. 30, 30, no. 2, p. 66-67. 66-67. Richmond, Richmond, G.M., and and Fullerton, Fullerton,D.S., D.S., 1986, 1986,An introduction introduction to Quaternary Quaternary glaciations glaciations in in the the United States States of America: America: Quaternary Quaternary Science Science Reviews, Reviews, v. v. 5, 5, p. 3-10. 3-10. Rittenour, T.M., 1994, 1994, Lacustrine Lacustrinedeposits depositsin in the the Minnesota MinnesotaRiver River Valley, Valley, west-central Minnesota; evidence for Glacial Lake Benson? Abstracts Abstractswith with Program, Program, 5th 5thAnnual Annual Minnesota; evidence Argonne Symposium Symposium for Undergraduates Undergraduates in Science, Science, p. 102. 102. 1995, Strand lines and lacustrine lacustrine deposits deposits in in the the Minnesota Minnesota River River Valley, Valley? Rittenour, T.M, 1995, Benson exist? exist? Geological Society west central Minnesota; Minnesota: Did Glacial Lake Benson Society of America no. 3, 3, p. 82. 82. An 2l7?no. ierica Abstracts Abstracts with with Programs, Programs, v. v. 27, Rittenour, Geiger, K.L., K.L., and Cotter, J.J.F.P., prep., Evidence Evidence for Glacial Lake Lake Benson, F.P., ininprep., r, T.M., Geiger? Rittenou -+ ..-I XK:....--..+,. L. Dn++a..nAwest-central in W ~ ~ L - L G I I Minnesota, ~ LVLU ~ U EJ W L ~ , w Patterson, =UUWU,C. C. J., ed., Contributions to the Quaternary geology of Minnesota: Minnesota MinnesotaGeological Geological Survey Survey Report of Investigations 49. d d 166 , �Ruddiman, W.F., and Wright, 1, in Ruddiman, W.F., and Wright, H.E., H.E., Jr., 1987, Introduction, Chap. 1, Ruddiman, W.F., H.E., Jr., Jr., eds., North America and the adjacent oceans during the last Wright, H.E., Wright, TheGeology Geologyof ofNorth NorthAmerica, America, volume volume K-3, Geological Geological Society Society of deglaciation: The America, America, p. p. 1-12. 1-12. Shackelton, N.J., and and others, others, 1984, 1984,Oxygen Oxygen isotope isotope calibration calibrationof of the the onset onset of of ice-rafting ice-raftingand and Shackelton, history of glaciation region: Nature, glaciation in the North Atlantic region: Nature, v. v. 307, p. 216-219. 216-219. Smith, D.G., and and Fisher, T.G., 1993, 1993, Glacial Lake Agassiz: The The northwestern northwestern outlet outletand and Smith, paleoflood: paleoflood: Geology, Geology,v. v. 21, 21, p. p. 9-12. 9-12. Teller, J.T., J.T., 1987, Proglacial lakes and the southern margin of of the Laurentide Laurentide Ice Sheet. Ch. Ch. 3, in Ruddiman, Ruddiman,W.F., W.F., and Wright, H.E., Jr., eds, North America and the adjacent adjacent 3, in oceans oceans during during the the last last deglaciation: deglaciation: The TheGeology GeologyofofNorth NorthAmerica, America,volume volumeK-3, K-3, Geological GeologicalSociety Societyof of America, America, p. p. 39-69. 39-69. Upham, Upham, Warren, 1883, 1883, The The Minnesota Minnesota valley valley in in the the ice ice age: age: American American Association Association for forthe the Advancement 1. Advancementof of Science ScienceProceedings, Proceedings,v.v.32, 32,p.p.213-23 213-231. Upham, Agassiz: U.S. Upharn, Warren, 1895, 1895, The Glacial Lake Agassiz: U.S. Geological Geological Survey Survey Monograph Monograph25, 25, 658 p. Williams, Thunell, R.C., Tappa, E., Rio, D., and Raffi, I., 1988, 1988, Chronology Chronology of of the the Williams, F.F., F.F., Thunell, Pleistocene Pleistocene oxygen isotope record: 0-1.88 0-1.88 my. my. B.P.: B.P.: Paleogeography, Paleogeography, Paleoclimatology, Paleoclimatology, Paleoecology, Paleoecology, v. 64, no. 3-4, p. 221-240. Wright, in Sims, Sims,P.K., P.K., and Morey, G.B., Wright, H.E., H.E., Jr, Jr, 1972, 1972,Quaternary Quaternary history history of of Minnesota, Minnesota, in eds., Geology of Minnesota: A centennial volume: Minnesota eds., Geology Minnesota: A centennial MinnesotaGeological GeologicalSurvey, Survey, p.515-547. p.515-547. Wright, Jr. 1990, 1990,Geologic Geologichistory history of of Minnesota Minnesota rivers: rivers: Minnesota Minnesota Geological GeologicalSurvey Survey Wright, H.E., H.E., Jr. Educational 20 p. EducationalSeries SeriesNo. No. 7, 7,20 p. Wright, E.J., 1973. Superior in Wright, H.E., Jr., Matsch, Matsch, C.L., and Cushing, E.J., Superiorand and Des Des Moines Moines lobes, lobes, in Black, H.B., eds., The Wisconsinan Black, R.F., R.F., Goldthwait, Goldthwait,R.P., R.P., and Whillman, H.B., Wisconsinanstage: stage: Geological GeologicalSociety Societyof ofAmerica AmericaMemoir Memoir136, 136,p.p.153-185. 153-185. 167 i m- �..:. '(1 \'' \\\(\ Ir Plate Plate 1. 1. Preliminary Preliminarysurficial surficialgeologic geologicmap mapof of the theupper upper Minnesota Minnesota River River basin showing showing stop locations. locations. 168 � Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology: Proceedings, 1998 Description An account of the resource Institute on Lake Superior Geology. Minneapolis, Minnesota. May 6-10, 1998. Creator An entity primarily responsible for making the resource Institute on Lake Superior Geology Date A point or period of time associated with an event in the lifecycle of the resource 1998 Format The file format, physical medium, or dimensions of the resource PDF Language A language of the resource English https://digitalcollections.lakeheadu.ca/files/original/9660caf940f66fd29e7fe32eae6eae00.pdf 6d73e78d6c61bec84aba951e925d06e6 PDF Text Text ~ake Institute on Lake Superior Geology 45th 45th Annual Annual Meeting Meeting Ramada mi Ramada IInn Marquette, Maquette, Michigan Michigan 1999 May 4-8, 4-8, 1999 Sponsored Sponsoredby Northern Northern Michigan Michigan University and University and Michigan Michigan Technological Technological University Proceedings Volume 45: Program and Abstracts Robert S. Regis and Theodore J. Bornhorst, editors �45TH ANNUAL MEETING INSTITUTE ON LAKE SUPERIOR GEOLOGY Volume Volume 45 consists of Pan 1: Program Partl: Programand andAbstracts Abstracts Pan Part2: 2: Field FieldTrip TripGuidebook Guidebook Reference to material material in this this volume should should follow follow the the example example below: Ams, Arns, D. and Hoim, Holm, D., 1999, 1999, Characterization Characterization and timing constraints constraints of post-Penokean post-Penokean meso-scale structures in the Watersmeet and Republic gneiss domes structures domes of northern northern Michigan (abst.): Institute Instituteon on Lake Lake Superior Superior Geology Proceedings, 45th Annual Meeting, Marquette, Marquette,MI, MLv.v.45, part 1,I, p. p.2. 2. 45, pan distributed by Volume Volume 45 45 Published Published and distributed by Institute Institute on Lake Lake Superior SuperiorGeology Geology Mark Jirsa, Jirsa, Secretary-Treasurer, Secretary-Treasurer,I.L.S.G. I.L.S.G. Minneosta Minneosta Geological Geological Survey Survey 2642 University University Avenue Avenue St. Paul, MN USA USA 55114-1057 551 14-1057 (612)-627-4780 (61 2)-627-4780 email: jirsaOOl jirsaOOl@tc.umn.edu @tc.umn.edu ILSG websitehttp://www.eeo.mtu.edu/ereat http://www.geo.mtu.edu/great lakes/ilsg/ ILSG website lakes/Us$ ISSN 1042-9964 1042-9964 All volumes are are available available for photocopying costs All costs from Michigan Technological TechnologicalUniversity UniversityLibrary Library Archives �INSTITUTE ON LAKE SUPERIOR GEOLOGY PROCEEDINGS Volume 45 AND ABSTRACTS ABSTRACTS Part 1: 1 :PROGRAM PROGRAM AND *~ ~ ~ .- .> . . .. . -.. . ... ~, . .. .. .~ , , 1 !; ! I Editors: Robert S. Regis and Theodore Theodore J. J. Bornhorst Bornhorst Editors: �CONTENTS CONTENTS PROCEEDINGS VOLUME VOLUME 45 45 PROCEEDINGS ABSTRACTS PART 1 1 -- PROGRAM PART PROGRAM AND ABSTRACTS Editors: Robert S. Regis and Theodore Theodore J. J. Bornhorst Bomhorst Institutes Institutes on on Lake Lake Superior SuperiorGeology Geology to to 1955-99 1955-99......................................... Constitution Constitution of of the the Institute Institute on on Lake Lake Superior Superior Geology Geology...................................... ii iii By-Laws BY-Lawsof of the the Institute Institute on on Lake Lake Superior Superior Geology Geology ..........................................................iii iv Goldich Goldich Medal Medal Guidelines Guidelines ................................................................................................. iv Goldich Goldich Medal Medal Committee Committee.................................................................. .................v Past Goldich Past Goldich Medalists Medalists ......................................................... vi ................vi 1999 Goldich Goldich Medal Medal Recipient 1999 Recipient ............................. ....................... VIvi .. vii ............vll Citation Citation for for 1999 1999Goldich Goldich Medal Medal Recipient Recipient ............. Student Paper Paper Awards Student Awards .................................................................. ........ ... Viii .............vlll ... Student Student Paper Paper Awards Awards Committee Committee ...................................................................................~Viff 111 Student Travel Travel Award Student Award ......................................................................................... Student Travel Travel Award Student Award Application Application Form Form ............................................................ ........ ix IX. . ix ix Board of of Directors Directors ............................................................................................................... xx Board x Local Local Committees Committees ...............................................................................................................x xi Banquet Banquet Speaker Speaker................................................................................................................. xi xii Report of the Chair Chair of the the 44th 44th Annual Annual Institute Institute ...............................................................xii Program Program xv ............................................................................................................................ xv Abstracts ....................................................................................................................... 1 Abstracts �INSTITUTE ON LAKE SUPERIOR GEOLOGY DATE PLACE CHAIRS 4 1955 1956 1957 1958 5 1959 6 1960 7 8 1961 Minneapolis, Minnesota Minneapolis, Minnesota Houghton. Michigan Houghton, East Lansing, Lansing, Michigan Michigan East Duluth, Minnesota Minneapolis, Minnesota Madison, Wisconsin Madison. Port Arthur, Arthur. Ontario Houghton. Michigan Houghton, Duluth, Minnesota Ishpeming, Michigan Ishpeming, Michigan St. Paul, Minnesota Marie. Michigan Sault Ste. Marie, East Lansing. Michigan Superior. Wisconsin Superior, Wisconsin Oshkosh, Wisconsin Oshkosh. Thunder Bay. Ontario Thunder Duluth, Minnesota Duluth. Houghton. Michigan Madison, Wisconsin Madison. Marie, Ontario Sault Ste. Marie. Marquette. Michigan Paul. Minnesota St. Paul, Thunder Bay. Bay. Ontario Ontario Milwaukee. Wisconsin Milwaukee. Duluth. Minnesota Duluth, Eau Claire. Wisconsin East Lansing. Michigan Falls, Minnesota International Falls. Houghton. Michigan Wausau. Wisconsin Wausau. Kenora. Ontario Ontario Kenora, Wisconsin Rapids. Wisconsin Wisconsin Wawa, Ontario Marquette, Michigan Marquette. Duluth, Minnesota Thunder Bay, Bay. Ontario Thunder Eau Claire. Claire, Wisconsin Eau Hurley, Wisconsin Eveleth. Minnesota Eveleth, Houghton, Michigan Houghton. Michigan Marathon, Ontario Marathon. Cable, Wisconsin Sudbury. Ontario Minneapolis, Minnesota Marquette, Michigan C.E. Dutton . . . AK. Snelgrove A.K. Snelgrove B.T. Sandeflir Sandefur R.W. Marsden R.W. E.N. Cameron & & R.A. Hoppin E.N. E.N. Cameron E.N. E.G. Pye E.G. A.K. Snelerove Snelgrove *,. . A.K. H. L Lepp H. ~PP A.T. Broderick A.T. P.K. Sims & & R.K. R.K. Hogberg P.K. R.W. White R.W. W.J. Hinze A.B. A.B. Dickas G.L. G.L. LaBerge M.W. Bartley & E. Mercy Bardey & D.M. Davidson D.M. J. Kafliokoski Kalliokoski M.E. Ostrom Osuom P.E. Giblin P.E. J.D. Hughes M. Walton M.M. Kehlenbeck M.M. C. Mursky G. D.M. Davidson i-'fr ' D.M. P.E. Myers P.E. W.C. Cambray W.C. DL. Southwick D.L. Southwick T.J. Bornhorst T.J. Bomhorst -' -st CL. LaBerge G.L. LaBerge CE. Blackburn C.E. Blackburn J.K. Greenberg J.K. E.D. Prey Frey & R.P. R.P. Sage E.D. J. S. Klasner .J.C. Green Green J.C. M.M. Kehlenbeck M.M. P.E. Meyers P.E. A.B. Dickas Dickas A.B. DL. Southwick D.L. Southwick T.J. Bornhorst Bomhorst M.C. Smyk M.C. L.G. Woodruff L.G. R.P. Sage & W. Meyer R.P.Sage&W.Meyer J.D. Miller Miller & M.A. M.A. Jirsa ursa J.D. T.J. Bornhorst Bomhorst & & R.S. Regis Regis INSTITUTE NUMBER INSTITUTE NUMBER 1 2 3 9 10 11 12 13 14 15 16 17 18 19 20 2] 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 1962 1963 1964 1965 1966 1967 1968 1969 1970 1971 1972 1973 1974 1975 1976 1977 1978 1979 1980 1981 t982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 ~--., �CONSTITUTION OF OF INSTITUTE INSTITUTE ON LAKE SUPERIOR GEOLOGY CONSTITUTION Article Article II Name Nams The be the the "Institute "Institute on onLake LakeSuperior SuperiorGeology". Geology. The name of the organization shall be Article I IIl Article Article IU III Article Objectives Obiectlves The objectives objectives of of this organization organization are: are: geologists in in the the Great Great Lakes Lakes region region may may exchange exchange ideas ideas and and A. To provide a means whereby geologists scientific data. scientific data. B. Superior region. B. To promote better understanding understanding of the geology of the Lake Superior C. To plan plan and conduct C. To conduct geological geological field trips. Status Sam? No part of to the benefit benefit of any member or individual. of the the income income of the organization shall insure to In the event of dissolution the assets of the organization shall be distributed to ______________ tax free organization). organization). (some (some tax (To avoid Federal and State State income taxes, the organization should be not only only "scientific "scientific" or also "non-profit".) "educational, but also "non-profit".) .. ,2.. Minn. Stat. Slat. Anno. 290.01, subd. subd. 44 Minn. Stat. 290.05(9) S t a Anno. ~ Internal Revenue 1(c)(3) 1954 Internal RevenueCode Code5.50 s.501(~)(3) Article Article IV IV ,. Membership rasmkmm The membership of the organization shall consist of persons who have registered for an an annual annual meeting or who have indicated interest in being a member according according to to membership membership guidelines guidelines approved by the board approved board of directors directors and have paid any applicable applicable dues. Article V Meetin2s &!dlws The organization shall shall meet meet once once aa year, year, preferably preferablyduring duringthe themonth monthof ofApril. April. The The place place and and exact directors. exact date date of of each each meeting meeting will be designated by the board of directors. Article VI Article VI Directors Directors The board and the the last last three three past board of of directors directors shall shall consist consist of the the Chair, Chair, Secretary-Treasurer, Secretary-Treasurer, and Chair; but if the by reason reason of Chair, but the board board should should at any any time time consist consist of fewer fewer than five persons, persons, by unwillingness or inability of any of the above persons to serve as as directors, directo& the the vacancies vacakies on on the the board may board up to five members. may be be filled filled by by Chair Chair so as to 10 bring the the membership of the board Article VH Officers smcsls The The officers officersof this this organization organization shall be a Chair and Secretary-Treasurer. A. be elected of directors, A. The Chair Chair shall shallbe elected each each year year by by the the board boardof directors, who who shall give due due consideration to the wishes of any group DUUD that that may may be be promoting promoting the the next next annual annualmeeting. meetine. His/her terminate at at the cl&c close of theannual the annual meeting over which Hisher term of office office as Chair Chair will $lt&nate he/she presides or when hisher his/her successor shall have have been been appointed. appointed. He/she hdshe presides Hdshe will willthen thenserve serve for for a period period of of three three years years as as aa member memberof of the theboard boardof of directors. directors. B. shallbe beelected electedby by aa majority majority vote of of members B. The Secretary-Treasurer Secretary-Treasurer shall members at the the annual annual . . vote meeting. His/her of years or ~ i s / h eterm term r ofoffice officeshall shall be be four fouryears or until until his/her hisher successor successorshall shall have have been been appointed. appointed. Article VIII Amendments This may be be amended amended by by aa majority of those This constitution constitution may majority vote vote (majority (majority of those voting) voting) of of the the membership membership of of the the organization. organization. �BY-LAWS OF OF INSTITUTE INSTITUTE ON LAKE LAKE SUPERIOR SUPERIOR GEOLOGY "9.. !!i !',vp..,. Duties of the the Officers and Directors Directors I. Duties A. It shall be the duty of the the Annual Annual Chair Chair to: to: >, .,,, , . 1 -"-< I.. Preside at the annual meeting. - ,, Appoint all committees committees needed for the organization organization of the annual meeting. . . ..,.&;, 2. Appoint .. 3. Assume and of the the annual *. . ! 3. Assumecomplete completeresponsibility responsibility for for the the organization organization and financing financine annual meeting meetine - of over which he/she h&he presides. B. It shall be the duty of the B. the Secretary-Treasurer Secretary-Treasurerto: to: 1. 1. Keep accurate accurke attendance attendance records &cords of all annual meetings. 2. Keep Keep accurate accurate records records of of all meetings of, and correspondence between, the the board board of of 2. meetings of, correspondence between, directors. directors. 3. Hold all funds that may may accrue accrue as as profits profits from from annual annual meetings meetings or or field field trips trips and and to make 3. these funds available available for the organization and operation operation of future future meetings meetings as as required. required. C. It shall be the duty of the board of directors to plan locations locations of annual meetings and to advise on the organization organization and and financing financingof of all all meetings. meetings. 8 ,, Expenses and E IH. I. Duties and xuee s - I. Regular lesson 1. Regularmembership membership dues dues of $5.00 $5.00 or or less on an an annual annual basis basis shall shall be be assessed assessed each member member as determined by the board board of directors. directors. 2. Registration Registration fees for the annual meetings shall shall be be determined determined by by the Chair in consultation with the board of The registration fees can can include expenses to cover the of directors. directors. The registration fees expenses to cover operations operations outside of of the annual outside annual meeting meeting as as determined determined by the the board hoard of of directors. directors. It is strongly strongly recommendedthat thatregistration registrationfees feesbebekept keptatat aa minimum to encourage attendance of of recommended minimum to encourage attendance graduate students. ILL. Rulesof ofOrder Order ;z F .lQW The rules contained in Robert's Rules of Order shall govern this organization in all cases to which The rules contained in Robert's Rules of Order shall govern this organization in all cases to which they are are applicable. applicable. LV. Anw>nflnw*nfc Amendments N. . I , These by-laws may be amended by by aa majority majority vote vote (majority (majority of of those those voting) voting) of of the the membership membership of the organization; provided provided that that such such modifications shall shall not not conflict conflict with with the the constitution as as ;;-of adopted or subsequently amended. 5 ~ i i ~ .presently presently is adopted or subsequently amended. . . , . ..;. Last Amended May, 1996 Amended -- May, 1996 .'t . �GOLDICH GOLDICH MEDAL GUIDELINES GUIDELINES Preamble Preamble Institute on on Lake The Institute Lake Superior Superior Geology Geology was was born born on on or or around around 1955, 1955, as as documented documented by by the the fact fact that that the the 27th 27th The annual meeting annual meeting was was held held in in 1981. 1981. The TheInstitutes Institutesare areexemplary exemplaryin intheir theircontinuing continuingobjectives objectivesof ofdealing dealingwith withthose those aspects of aspects of geology geology that that are are related related geographically geographically to to Lake ~ a k eSuperior; ~ u p e r i oof ofr ;encouraging encouraging the the discussion discussion of of subjects subjects and and sponsoring field trips which will bring surveys, and industry; and of swnsoring brine together together geologists eeoloeists from academia, government surveys. maintaining an an exceedingly exceedingly informal but highly effective mode mode of of operation. operation. maintaining i n f o d buthighiyeffective During the course of of its existence, existence. the membership membership of the Institute (that is, those geologists who indicate an interest colleagues have made in the objectives of the ILSG by by attending) attending) has has become aware aware of the fact that certain of their colleagues made particularly noteworthy noteworthy and of Lake Lake Superior and particulaily and meritorious meritorious contributions contributions to to the the understanding understanding of Superior fgeology $eolo&' and mineral mineral deposits. deposits. The toSam SamGoldich Goldichin in 1979 1979for forhis hismany many contributions contributionsto to the the geology geologyof ofthe theregion region The first award award was was made made by by ILSG ILSG to extending below. extending over about 50 50 years. Subsequent Subsequentmedalists medalistsand and this this year's year's recipient are listed in the table below. Award Award Guidelines Guidelines I) The medal shall to a geologist whose whose name name is is associated associated 1) shall be awarded awarded annually by the ILSG Board of Directors to contribution to, the geology of the Lake Lake Superior region. with a substantial interest in, and contribution 2) The Board Committee. The three Board of of Directors Directors shall shall appoint appoint the Goldich Medal Committee. The initial initial appointment appointment will be of three members, members, one to serve for three three years, years, one for for two two years, years, and and one one for for one one year. year. The member with the the briefest briefest incumbency shall be chair of the firstyear, year,the the Board Boardof ofDirectors Directorsshall shallappoint appoint the Nominating Committee. After Afterthe the first at each spring three years. In his/her hidher third year year this member member shall shall be be spring meeting meeting one one new new member member who who will will serve serve for for three the chair. The Committee of interest interest and and geographic geographic distribution distribution of of ILSG ILSG Committee membership membership should reflect the main fields of membership. membership. 3) By to the the Chair Chair of of the 3) By the the end end of of November, November, the the Goldich Goldich Medal Medal Committee Committee shall shall make make its its recommendation recommendation to the Board of Directors, Directors, who who will then inform inform the Board of the nominee. 4) The 4) The Board Board of ofDirectors Directorsnormally normally will will accept accept the thenominee nominee of ofthe theCommittee, Committee. will will inform inform the themedalist medalist immediately, and will have one medal engraved appropriately appropriately for presentation at the next meeting of the Institute. 5) It is recommended recommended that that the the Institute Institute set set aside aside annually annually from from whatever whatever sources, such funds as will be be required required to to continuing costs of this award. support the continuing Nomination Procedures Procedures Nomination Nominations shall be taken 1) Nominations taken at at any anytime timeby bythe theGoldich GoldichMedal MedalCon-imittee. Committee. Committee Committeemembers membersmay may themselves themselves candidates. The nominate candidates. The deadline deadline for for nominations nominations is is November 1. 1. 2) Nominations Nominations must be in writing and supported by appropriate appropriate documentation such as letters of recommendation, recommendation, publications. curriculum contributions to Lake Lake Superior geology and to to the Institute. lists of publications, curriculum vita's, and evidence evidence of contributions are not not restricted restricted to to Institute Institute attendees, attendees, but are are open open to to anyone anyone who who has has worked worked on on and and contributed contributed 3) Nominations Nominations are to the understanding understanding of of Lake Lake Superior Superiorgeology. geology. iv �Selection Guidelines Nominees are to be evaluated 1) Nominees evaluated on the basis of their contributions contributions to Lake Superior Superior geology geology (sensu lato) lato) including: including: a) importance importance of relevant relevant publications; publications; b) b) promotion promotionof of discovery discoveryand andutilization utilizationof ofnatural natural resources; resources; c) contributions contributions to to understanding understandingof of the the natural natural history history and and environment environmentof of the the region; d) generation generation of new ideas ideas and concepts; concepts; and e) contributions contributions to to the the training training and education educationof of geoscientists and the the public. nublic. - and -geoscientistsand # ., ...,:,{ m, . P,:~:, ,y 2 : 2) 2) Nominees Nominees are are to tobe beevaluated evaluatedon ontheir theircontributions contributionsto tothe theInstitute Instituteas asdemonstrated demonstratedby byattendance attendanceatatInstitute Institute meetings, meetings, presentation presentationof of talks talksand andposters, posters. and andservice serviceon onInstitute Instituteboards, boards, committees, committees,and andfield fieldtrips. trips. 3) 3) The The relative relative weights weights given given to to each each of of the theforegoing foregoing criteria criteria must must remain remain flexible and at the discretion discretion of of the the Cornniittee Committee members. members. - . ..q .* ? ,' 4) 4) There There are are several several points points to to considered considered by by the the Goldich Goldich Medal Committee: . "...., * . . . a) of the a) An An attempt attempt should should be be made madeto to maintain maintain aa balance balanceof of medal medal recipients recipientsfrom from each each'of thethree threeestates-industry, estates-industry, academia, academia, and government. government. b) It It must must be be noted noted that thatindustry industry geoscientists geoscientistsare areat at aa disadvantage disadvantagein in that that much much of of their their work work is is not not published. published. .# 5) 5 )Lake Lake Superior Superiorhas hastwo twosides, sides,one onethe theU.S., US., and andthe theother otherCanada. Canada. This Thisisisundoubtedly undoubtedlyone one of ofthe theInstitute's Institute's great great strengths strengths and should should be be nurtured nurtured by by equitable equitablerecognition recognition of of excellence excellencein in both both countries. countries. Last Last Amended Amended 1997 1997 GOLDICH GOLDICHMEDAL MEDAL COMMITTEE COMMITTEE1998-99 1998-99 John John Klasner Klasner (1999) (1999) Western Western Illinois IllinoisUniversity, University, Macomb, Macomb, Illinois Illinois Mark Smyk MarkSmyk (2000) (2000) Ontario Ontario Geological Geological Survey, Survey, Thunder Thunder Bay, Bay, Ontario Ontario Rod Johnson Johnson (2001) (2001) Rod Rod Johnson Johnson and and Associates, Associates, Negaunee, Negaunee, Michigan ~ichi'n V *' k' * �,; :G?L GOLDICH GOLDICH MEDALISTS MEDALISTS '' 1991 991 William WilliamJ. Hinze 1992 William F. Cannon Cannon 992 F. 1993 993 Donald W. W. Davis 1994 994 Cedric Cedric Iverson 1995 995 Gene Gene LaBerge 1996 David L Southwick 996 L. Southwick 1997 997 Ronald P. Sage 1998 Zell Peterman Petennan 998 Zell 1999 Tsu-Ming Han 999 Tsu-Ming Hun 1979 Samuel S. £ Goldich Goldich 1980 not awarded E. Dutton, Jr. Jr. 1981 Carl Carl IL 1982 Ralph W. Ma Marsden 1983 Burton 1983 Burton Boyum Boyu 1984 Richard Richard W. W Ojakangas Ojakangas 1985 Paul Paul K. 1985 K. Sims Sims 1986 1986 G.B. Morey 1987 Henry H. Halls 1988 Walter £ White Walter S. White 1989 JormaKalliokoski Kalliokoski 1989 Jonna 1990 1990 Kenneth C. Card Card '' 1999 19999 Goldich Medal Recipient -.. Tsu-Ming Han vi !:- �- I I : ,.. . . ,. Citation Citation Tsu-Ming HHan an 1999 Recipient 9 Goldich Medal Recipien The 1999 1999 c3oldich Goldich Medal recipient epitomizes the characteristics characteristics required to receive receive this unique and prestigious award. He life to to solving nature's nature's this prestigious award. He has has devoted devoted his professional professional life myriad challenges challenges in in the field myriad field of of geology geology and and sharing sharing his his findings findings with with colleagues colleagues both both verbally and through published and unpublished unpublished mineral mineral papers. r He born during the 1920's of China. China. His He was was born during the 1920's in in the the Hunan Hunan Province Province of His family familystressed stressed the importance importance of of education. education. He the He pursued pursued his his education education during during a series series of of difficult difficult periods periods that included being kidnapped kidnapped and and held held by by bandits, pursued by by Japanese Japanese forces, forces, and and the that included being bandits, pursued imposition of of Nationalist Nationalist control control followed followed by by the the cultural cultural upheaval upheaval caused caused by by the the ascendancy ascendancy imposition of the Communists from Northwest Northwest University in Sian of Communists after after World War War II. U. After graduating from Province in in 1945 Province 1945 and a brief stint stint with with the the Bureau Bureau of of Mineral Mineral Exploration, he immigrated immigrated to to the U.S. the U.S. in in 1947 1947 to tocomplete complete graduate graduate work work at at the the University University of of Cincinnati Cincinnati and and the the University of Minnesota. Minnesota. His Drs. Goldich, His mentors mentors at the University of Minnesota included Drs. Gruner, and Schwartz. Gmner, Schwartz. ;, ,. ,,, After summer employment with Clevcland-Cliffs in 1952, After Cleveland-Cliffs in 1952, he he accepted accepted permanent permanent employment at at the the Ishpeming Ishpeming Research Researchfacility. facility. Soon Soon after, after, he he met met and and married married Joy. Joy. They They employment have have advanced have three three children children who who,, have also alsobenefited beqegted,professionally professionally by b yachieving .achiev&g *-* advanced .,.~ , ,. educational educational degrees. degrees. , t He has published numerous articles on the genesis of iron formations with emphasis on textural textural relations relations during duringdiagenesis diagenesisand and metamorphism. metamorphism. What is not generally known known is that he is an expert on all facets of the beneficiation and pelletizing pelletizing process process that that has has dominated dominated the the North North American Americaniron ironore ore industry industry for for half half aa and His studies have assisted in the pelletizing process to century. century. His assisted in continual continual improvement improvement in industry competitive. keep the industry competitive. He in several He has has aa special special interest interest in in the the preserved preserved algae algae present present in several areas areas of of the the Negaunee Iron-formation Iron-formation at at the the Empire Empire Mine Mine and and has has published his his findings. findings. It should be noted noted that he delivered delivered a paper at the first Institute Institute meeting in 1955 1955 and and continues his scientific inquiry as evidenced by his current presentation at the Institute. continues his scientific inquiry evidenced by current presentation the Institute. his formal formal retirement retirement in in 1992 is is that that he he now now wears Indeed, the only change noticeable since his tennis shoes shoes to the office. office. It is my pleasure to to introduce introduce to the Institute Institute the the 1999 1999 recipient recipient of the the Goldich Goldich Medal, Medal, friend and our my friend our colleague, colleague, Tsu-Ming Tsu-Ming Han. Han. Tom Wagonner vii �STUI)ENT STUDENT PAPER PAPER AWARDS AWARDS Each year, the Institute selects the best of the and honors presenters presenters with with aa monetary award. Each the student student presentations and Funding for for the Funding the award award is is generated generated from from registrations registrations of of the the annual annual meeting. meeting. The The Student Student Paper Paper Committee Committee isis appointed by appointed by the the annual annual meeting meeting Chair Chair in in such such aa manner manner as as to to represent represent aa broad broad range range of of professional professionaland andgeologic geologic expertise. Criteria expertise. Criteria for for best best student student paper-last paper-last modified modified by the the Board in 1997-follow: 1997-follow: 1) The The contribution contribution must 1) must be be demonstrably demonstrablythe the work work of of the thestudent. student, 2) The The student student must present the contribution 2) contribution in-person. 3) The Student shall decide decide how how many many awards awardsto to grant, grant, and and whether whether or or not not to give separate 3) Student Paper Committee shall separate awards awards for for poster poster vs. vs. oral oral presentations. presentations. 4) In will be be made made to to the the senior author, author, or or the the award will be 4) In cases cases of of multiple multiple student student authors, the award will be shared shared equally equally by all authors authors of of the the contribution. contribution. 5) The but The total total amount amount of of the the awards awards isisleft lefttotothe thediscretion discretionof ofthe themeeting meetingChair Chairand andSecretary-Treasurer, Secretary-Treasurer, but typically is in the amount of about $300 $300 US. 6) The maintains, and and will will supply supply to to the 6) The Secretary-Treasurer Secretary-Treasurer maintains, the Conmiittee. Committee, aa form form for for the the numerical numerical ranking ranking of of presentations. This form form was was created and modified modified by Student Student Paper Paper Committees Committees over several years in in an an effort effort to to reduce the the difficulties that that may may arise arise from selection by by raters raters of of diverse background. background. The The me "Se of of the the form form is not reduce required, but but is is left left to to the the discretion discretion of of the the Committee. Committee. 7) The 7) The names names of of award award recipients recipients shall shall be be included included as a s part part of of the the annual annual Chair's Chair's report that that appears appears in in the the next next volume volume of of the the Institute. Institute. STUDENT PAPER PAPER COMMITTEE COMMITTEE 1999 STUDENT .~ .. ~,,. Allan Blaske, STS STS Consultants, Consultants, LTD. LTD,Lansing, Lansing, MI MI Woodruff, U.S. Laurel Woodruff, U.S.Geological Geological Survey, Survey, St. Paul, MN vii . ' . B; .. , . .. . , . . .: ,, .. �I 1 STUT)ENT TRAVEL TRAVEL AWARD STUDENT AWARD The Travel Award Award to to support student participation participation at at the the The 1986 1986Board Board of of Directors Directorsestablished established the the LL.S.G. I.L.S.G. Student Student Travel support student annual find set annual Institutes. Institutes. The The awards awardswill will be be made made from from aa special special fund set up up for for this this purpose. purpose. This Thisaward awardisisintended intendedtoto help help defray defray some some of of the the direct direct travel travel costs costs to to the the Institute Institute and and includes includes aa waiver waiver of of registration registration fees, fees, but but excludes excludes expenses and field field trip trip registration. registration. The and value by the the expenses for for meals, meals, lodging, lodging, and The number number of of awards awards and value are are determined determined by annual annual Chairman Chairman in in consultation consultationwith with the the Secretary-Treasurer Secretary-Treasurerand and will will be be announced announcedat at the the annual annualbanquet. banquet. The beconsidered consideredby bythe theannual annualChairman, Chairman,who whoisisresponsible responsiblefor forthe theselection: selection: Thefollowing followinggeneral generalcriteria criteriawill willbe 1) 1) The or graduate) graduate) student student status status at at the the time time of of the the The applicants applicants must must have have active active resident resident (undergraduate (undergraduate or Institute, Institute, certified certified by by the the department department head. head. who are are the the senior author on on either either an an oral 2) Students who senior author oral or or poster poster paper paper will will be be given given favored favored 2 ) Students consideration. consideration. 3) is desirable desirablefor fortwo two or or more more students students tojointly to jointly request requesttravel travel assistance. assistance. 3) ItIt is In general, general,priority priority will will be be given given to to those thosein in the the Institute Institute region region who who are are farthest farthest away. 4) 4) In with an an explanation of 5) Each travel travel award award request request shall shall be be made made in in writing, writing, to to the the annual annual Chairman, Chairman, with explanation of 5 ) Each need, need, possible possibleauthor authorstatus statusor orother othersignificant significantdetails. details. Successful Successfulapplicants applicantswill willreceive receivetheir theirawards awardsat atthe thetime timeof of registration registration for forthe the Meeting. Meeting. INSTtTUTh ON SUPERIOR GEOLOGY ONLAKE -SUPERIOR GEOLOGY Student A want Application Student Travel TravetAwardAppUcdon Please Pleaseprint: print: Student Student Name: Name: _______________________________ Date: Date: _______________ Address: Address: _______________________________________________________ :. Icertify 1certifythat thatthe theabove abovenamed namedperson personisIsan anactive activeresident residentstudent. student. .; ;; .,>'. , .. .,." v.., Department DepartmentHead Head--Typed Typed .. - ..>i.; ;.( <: ... , ,, Date Date Department DepartmentHead Head- Signature Signature L Educational Educational Status: Status:___________________ Are an oral reyou you the theSenior Senior Author Author of ofan oral or orposter poster paper? paper? Yes Yes Will Willany anyother otherstudents studentswill willbe betraveliag (revelingwith withyou? you? c . ~,.. : ___ ___ No No-_____ How Howmany? many? - Statement (If:(Ifyou statementofofNeed: ~eed youneed needmole moreloom, room,please pleaseuse usethe theback backof ofthe thepage.) Page.) Other OtherSignificant SignificantDetails: Details: Please Please return returnto: to: ix : �BOARD BOARD OF OF DIRECTORS DIRECTORS fheodoreJ. J. Bornhorst, Bornhorst, Chair Chair(2002) (2002) Theodore Michigan MichiganTechnological TechnologicalUniversity, University,Houghton, Houghton,Michigan Michigan James JamesD. D.Miller Miller(2001) (2001) Minnesota Minnesota Geological GeologicalSurvey, Survey,St. St.Paul, Paul,Minnesota Minnesota Ronald RonaldP. P.Sage Sage(2000) (2000) Ontario OntarioGeological GeologicalSurvey, Survey,Sudbury, Sudbury,Ontario Ontario <. Laurel LaurelG. G. Woodruff Woodruff(1999) (1999) U.S. US. Geological GeologicalSurvey, Survey,Mounds MoundsView, View,Minneapolis Minneapolis Mark MarkA. A.Jirsa J i m(2000) (2000)Institute InstituteSecretary-Treasurer Secretary-Treasurer Minnesota MinnesotaGeological GeologicalSurvey, Survey,St. St.Paul, Paul,Minnesota Minnesota LOCAL LOCAL COMMITTEES COMMITTEES GENERAL Co-CHAIRS Robert RobertS.S.Regis Regis Department Northern Michigan Departmentof ofGeography/Earth Gwgraphy/EarthScience, Science,Northern MichiganUniversity, University, Marquette, Marquette,Michigan Michigan49855 49855 Theodore TheodoreJ. J. Bornhorst Bornhorst Department Department of of Geological GeologicalEngineering Engineeringand andSciences, Sciences,Michigan MichiganTechnological TechnologicalUniversity, University.Houghton, Houghton, Michigan Michigan 49931 49931 PROGRAM AND EDITORS PROGRAM AND ABSTRACTS EDITORS ,* . Robert RobertS. S.Regis Regis Department Departmentof ofGeography/Earth Geography/EarthScience, Science,Northern Northern Michigan MichiganUniversity, University, Marquette, Marquette,Michigan Michigan 49855 49855 Theodore TheodoreJ. J. Bornhorst Bornhorst Department of of Geological GeologicalEngineering Engineering and andSciences, Sciences,Michigan MichiganTechnological TechnologicalUniversity. University, Houghton, Houghton, Michigan Michigan 49931 49931 FIELD TRIP GUIDEBOOK EDITOR Theodore Theodore J. J. Bornhorst Bornhocst Department of of Geological Geological Engineering Engineering and andSciences, Sciences,Michigan MichiganTechnological TechnologicalUniversity, University,Houghton, Houghton, Michigan 49931 49931 x �SESSION CHAiRS SESSION CHAIKS Dave DaveBaxter, Baxter,PayDay PayDayResources, Resources,Lansing, Lansing,MI MI Shawn Shawn Carison, Carlson, Ashton AshtonMining, Mining,North North Vancouver, Vancouver,BC, BC, Canada Canada Ron Ron Graber, Graber, Cliffs CliffsMining Mining Services ServicesCompany, Company, Ishpeming, Ishpeming, MI MI John John Green, Green,University Universityof of Minnesota, Minnesota, Duluth, Duluth, MN MN Rod Rod Johnson, Johnson, Rod RodJohnson Johnson& &Associates, Associates,Inc., Inc.,Negaunee, Negaunee, MI MI Gene Gene Laberge, Laberge, University Universityof of Wisconsin, Wisconsin,Oshkosh, Oshkosh, WI WI Jim Jim Small, Small, Edward Edward Kraemer Kraemer & &Sons, Sons, Burnsville, Burnsville, MI MI Tom Tom Waggoner, Waggoner, retired retired Cliffs CliffsMining MiningServices ServicesCompany, Company,Ishpeming, Ishpeming, MI MI " a, 1999 999 BANQUET BANQUET SPEAKER SPEAKER ' Nelson R. R. Ham Ham Department Department of Geology Geology St. Norbert College St. College De De Pere, Pere, Wisconsin Wisconsin , . . , Ice-Flow Dynamics and Landform Landform Development Development Along the Southern Southern Margin Margin of of the the Laurentide LaurentideIce Ice Sheet Sheet xi , �REPORT REPORTON ONTHE THE44th 44thANNUAL ANNUAL MEETING MEETINGOF OFTHE THE INSTITUTE INSTITUTEON ONLAKE LAKESUPERIOR SUPERIORGEOLOGY GEOLOGY MINNEAPOLIS, MINNEAPOLIS,MINNESOTA MINNESOTA The The44th 44thAnnual AnnualMeeting Meetingof ofthe theInstitute Instituteon onLake LakeSuperior SuperiorGeology Geologywas washeld heldinin Minneapolis, Minneapolis,Minnesota Minnesotaon on May May 6-10, 6-10, 1998. 1998. The Themeeting meeting was was sponsored sponsored by by the the Minnesota Minnesota Geological GeologicalSurvey Surveyand andwas washeld held at atthe the Holiday Holiday Inn Inn on on the the West West Bank Bank of of the the University University of of Minnesota Minnesotacampus. campus. Proceedings Volume44 44of ofthe themeeting meetingwas waspublished publishedinintwo twoparts: parts: ProceedingsVolume Pan Abstracts Part1:1:Program Programand and Abstracts(including (includingextended extendedabstracts abstractsofofthe thespecial specialoverview overview session) session) Part Part2:2:Field FieldTrip TripGuidebook Guidebook 1—Early 1-Early Proterozoic Proterozoicintrusive intrusiverocks rocksof of east-central east-centralMinnesota Minnesota 2—Geology 2-Geology of of the the southeastern southeasternportion portionofofthe theMidcontinent MidcontinentRift RiftSystem. System,eastern eastern Minnesota Minnesotaand andwestern westernWisconsin Wisconsin 3—Glacial ofthe theTwin TwinCities Cities area area 3ÑGlacia exotica exoticaof 4—Stratigraphy hydrogeology of M t r a t i g r a p h y and andhydrogeology of the the Paleozoic Paleozoic rocks rocks of of southeastern southeastern Minnesota Minnesota 5—Minnesota River Valley and vicinity, southwestern Minnesota (Precambrian and 5-Minnesota River Valley and vicinity, southwestern Minnesota (Precambrian and Quaternary Quaternarygeology) geology) By By most most measures, measures,the themeeting meetingwas wasaagreat great success. success. We Wesaw sawaaconsiderable considerablejump jumpinin attendance attendancethis thisyear yearto to200 200participants, participants.compared comparedwith withrecent recentaverage averageattendance attendanceon onthe the order of 125. Fifty-six participants were first-time attendees to the ISLG; split about order of 125. Fifty-six participants were first-time attendees to the ISLG. split aboutequally equally between betweenstudents studentsand andrelated relatedfields fieldspeople—that people-that is. is, those those involved involved in in Quaternary, Quaternary, Paleozoic, Paleozoic, environmental, are not not traditional traditional topics topics of the the Institute. Institute. We We environmental,and andengineering engineeringgeology geology that that are attribute attributesome someof of this thisincrease increasesimply simplyto to the the metropolitan metropolitan location location of the the meeting meeting and and our our fairly fairly aggressive aggressiveappeal appealto to the thelocal localgeoscience geosciencecommunity. community. To Toattract attractlocal localand andmore more broadly broadlybased basedinterest, interest,we webroke brokesomewhat somewhatwith withtradition traditionby byconducting conductingaaspecial specialhalf-day half-day session region. To session overviewing overviewingthe geology of the Lake Superior region. Tomake makethe themeeting meetingeven even more moreattractive attractiveto tonew-comers, new-comers,we weoffered offeredaafirst-day-only first-day-onlyregistration registrationatataareduced reducedrate ratefor for those thoseinterested interestedin inattending attendingjust justthe thespecial specialoverview overviewsession; session;however, however,few fewtook tookadvantage advantage of of the the offer. offer. The talksthat thatoutlined outlinedcurrent currentideas ideasand and The special specialoverview overviewsession sessionconsisted consistedof ofsix six30 30minute minutetalks new new research researchdirections directionson onthe themain maingeologic geologicprovinces provincesof of the theLake LakeSuperior Superiorregion. region. Presentations (Archean),Dick DickOjakangas Ojakangas(Early (EarlyProterozoic), Proterozoic),Bill Bill Presentationswere weregiven givenby by Ken KenCard Card(Archean), Cannon Cannon(Middle (MiddleProterozoic), Proterozoic),Tony TonyRunkel Runkel(Paleozoic), (Paleozoic),Carrie CamePatterson Patterson(Quaternary), (Quaternary),and and Dave talkswere werewell wellpresented presented Dave Southwick Southwick(New (Newdirections directionsin in Lake Lake Superior Superior geology). The Thetalks and and well well received; received; comments commentsfrom fromthe the participants participants were overwhelmingly overwhelmingly favorable. The The general 23oral oralpresentations presentationscovering coveringnot not only only the the typical typical fare fareof of general sessions sessionsincluded included23 Precambrian to groundwater. groundwater. In Inaddition, addition,24 24 Precambriantopics, topics,but but also alsoincluded included several severaltalks talks related related to poster of the the meeting. meeting. The poster presentation presentation were were on on display display during during the course of Thebanquet banquet speaker speaker was was Dr. Dr. Bevan Bevan French Frenchof ofthe theSmithsonian SmithsonianInstitution, Institution,who whogave gavean anexcellent excellenttalk talkon onthe therole role of of meteorite meteoriteimpacts impactson on the the geologic geologic history history of earth earth and other planets. At At the thebanquet, banquet,Zell Zell Peterman Petennan of of the theUSGS USGSwas wasawarded awardedthe theGoldich GoldichMedal Medalfor forhis his important importantgeochronologic geochronologicand and xii �geologic studies studies of of Precambrian Precambrian geology geology in in the the Lake LakeSuperior Superiorregion. region. Another Another highlight highlight of of geologic the meeting for 40 40 participants participants was was aa night night of of baseball baseball at at the the Metrodome. Metrodome. This meeting excursions that covered a meeting also also broke broke the the mold mold a bit by offering five field excursions similarly wide breadth similarly breadth of geology gwlogy as as was was presented in the the special special overview overview session. session. Although the emphasis highlighting the Paleozoic Although-the emphasis remained remained with Precambrian geology, trips highlighting and Quaternary geology of central and southern Minnesota Minnesota were were also also included. included. Three Threeprepremeeting the Early Early Proterozoic Proterozoic gwlogy geology of meeting field field trips trips were were run run on on Wednesday, Wednesday,May May 6th; 6th. 1) the east-central Minnesota Minnesota led by Terry Boerboom, Mark Jirsa, and Daniel Holm (26 participants); Wirth, Bill participants); 2) the Keweenawan geology of the Taylor's Falls area led by Karl Wirth, Cordua, Cordua, Bill Kean, Kean, Mike Mike Middleton, Middleton, and Zach Naiman Nairnan (46 (46 participants); and 3) Glacial exotica exotica of the the Twin Twin Cites Citesarea area led led by by Howard Howard Hobbs, Hobbs, Alan Alan Knaeble Knaebleand and Gary GaryMeyer Meyer(21 (21 participants). Two the Paleozoic Paleozoic stratigraphy, stratigraphy, Twopost-meeting post-meetingtrips trips included included aa one-day trip on the sedimentology, sedimentology,and hydrogeology hydrogeology of southeastern southeastern Minnesota led by Tony Runkel and Bob Tipping Tipping (11 (1 1 participants), and a two-day excursion investigating the Quaternary and Archean geology of the Minnesota Minnesota River Valley led by Carrie Patterson and David Southwick (33 participants). participants). The meeting was also is to to at least also a financial success; that is, if your measure of success is break even (which has been the traditionally held held view). view). We actually turned turned aa small small profit profit of of several hundred dollars. dollars. Although Althoughregistration registrationwas wasmore morecostly costlythan than we we had had hoped, hoped, the thecost cost was unavoidable unavoidable given the metropolitan metropolitan location. Part Partof of the theregistration registrationwe we all all pay pay is is applied to encouraging Institute by by granting granting awards awards for for student student encouraging student student participation in the Institute travel and best student student papers. Eight Eighttravel travelawards awardstotaling totaling$750, $750, and and two two best best paper paperawards awards of $150 each were granted. granted. Best Bestpaper paperawards awardswent went to to Kathleen Kathleen Abbott Abbott (Macalester (MacalesterCollege) College) for her undergraduate undergraduate work work on on geochemistry geochemistryand and petrography petrography of of Keweenawan Keweenawan Chengwatana Chengwatana volcanic rocks of Minnesota and Wisconsin, and to Dean Peterson (University of MinnesotaMinnesota Duluth) Duluth) for for his his graduate graduate presentation presentationon on (3IS-based GIs-based mineral potential analyses analyses of Archean Archean rocks in Minnesota. Congratulations Congratulationsto to you you both! both!. The Institute's Institute's Board Board of Directors Directors met on May 7th, 7th. and the following is generalized from minutes of that meeting: 1. Accepted Acceptedthe the Report Report of of the the Chair Chair 43rd 43rd ILSG ILSG 2. Accepted Acceptedthe the1997-1998 1997-1998JLSG ILSG Financial Financial Report. Report. 3. Approved ro rovedZell ZellE. E.Peterman Petermanas as1998 1998Goldich Goldich Medal Medal recipient. recipient. 4. Modified 4. Modifiedthe thelanguage languagefor forGoldich GoldichAwards Awards regarding regarding dates dates for for receipt receipt of nominations nominations (now November 1) 1) and final decision (now the end of November). 5. Approved Approvedthe theend-of-term end-of-termreplacement replacement of of Dan Dan England England (Eveleth Fee Office) Office) as committee member member hy y Rodney Goldich Medal committee RodneyJohnson Johnson (Trans (Trans Superior Superior Resources). Rod begins a 3-year term in Novemuer Novemner of 1998. 1998. title to to "Eisenbrey "EisenbreyStudent StudentTravel TravelAwards". Awards'. This 6. Changed Changedthe the student student travel award's title satisfies a request from satisfies from individual individual and corporate corporate contributors contributors to the 1996 1996 meeting meeting that that such a fund be established in honor of Ned Eisenbrey, and permits the use of & l established =senbrey, investment income income for for encouraging encouraging student student participation participation in the Institute. ILSG meeting. meeting. Co-chairs will 7. Approved Approvedthe the host host location—Marquette—for location-Marquette~for the 1999 ILSG will Bomhorst (Michigan Bob Regis Regis (Northern be Ted Bomhorst (Michigan Technological University) and Bob Michigan University). University). xiii xiii �8.8.Discussed Discusseda apotential potentialY2K Y2Kmeeting meetinglocation—Thunder location-Thunder Bay Bayororother otheraccess accesspoint pointtoto the theAtikoken Atikokenarea. area. 9. 9.Discussed Discusseda apotential potential2001 2001meeting meetinglocation—Sault 1ocationÑSaulSte. Ste.Marie, Marie,MI. MI. 10. the 10. Other Otherbusiness—contact busines-ontact thesecretary-treasurer secretary-treasurerfor fordetails details We Wehope hopethat thatwith withthe thesuccess successof ofthe the1998 1998meeting, meeting, the the ILSO ILSGtook took an an important important step steptoward toward achieving achievingthe thegoal goalof ofexpanding expandingits itsreach, reach,both both in in terms termsof of the the range range of of geological geological topics topics and and the thebackground backgroundof of participants. participants. While Whilewe wemay maynot notbe beable ableto torepeat repeat the the attendance attendancesuccess success of of this thismeeting meetingas aswe wemove moveto tothe themore moretypically typicallyremote remoteconference conferencesites, sites, future futureorganizers organizersand and those thoseof ofus uswho whoare arelong-time long-timeparticipants participantsmust mustdo doall allwe we can can to to actively actively promote promote the the ]LSG ILSG to the broadest possible audience. If we had to pinpoint one reason for the success to the broadest possible audience. If we had to pinpoint one reason for the successofofthe the 1998 1998 meeting, meeting, itit would would be be our ourrelentless relentlesspromotion promotionof ofthe themeeting meetingto tolocal localgeoscience geoscience organizations, organizations,colleges, colleges,and andindividuals. individuals.For Forour ourpart, part,we wewill willencourage encourageTwin TwinCities Cities geoscientists geoscientiststo tocontinue continuetheir theirinvolvement, involvement,now nowthat thatthey they have have been been exposed exposed to to what what the the ILSO ILSGhas hasto tooffer. offer. We Wewish wishto tothank thankall allof ofthe thepresenters, presenters,in inparticular particularthe the special special session session speakers speakerswho who wrote wrote extended extendedabstracts abstractsthat thatwill willserve serveas assummaries summariesof of their their fields fieldsfor for years years to to come. come. This This institute institutewas was built built on onits itstradition traditionof of fine finefield field trips, trips, and and this this year's year's trips trips maintained maintained that that tradition, tradition,thanks thanksin inlarge largepart partto toefforts effortsof of the theleaders. leaders. Acknowledgment Acknowledgmentisisalso alsodue duetotothe the session sessionchairs, chairs,the thebest beststudent studentpaper paperaward awardcommittee, committee,the thelocal localplanning planningcommittee, committee,and andthe the LLSG board members members for for making jobs much easier by by doing theirs well. ILSG board making our our jobs much easier doing theirs well. AAspecial specialthanks thanks to to Terry Terry Boerboom Boerboomfor forhis hisflawless flawlessorganization organizationof ofthe thefield fieldtrips tripsand andguidebook, guidebook,and andtotoLori Lori Day, it takes. takes. Finally Day, the the meeting meeting coordinator, coordinator, for for doing doing all all that that it Finally we we want want to to thank thank our our director directorof ofthe theMinnesota MinnesotaGeological GeologicalSurvey, Survey,David David Southwick, Southwick, for for giving giving us us the the freeboard freeboard and andstaff stafftime timeneeded neededto topull pullthe themeeting meetingtogether. together. Jim JimMiller Millerand andMark MarkJirsa Jirsa Co-chairs Co-chairsof ofthe the44th 44thAnnual AnnualILSG ILSG xiv xiv �45TH 4 5ANNUAL ANNUAL ~ ~ MEETING MEETING INSTITUTE GEOLOGY INSTITUTEON ON LAKE LAKE SUPERIOR SUPERIOR GEOLOGY Program Program �Program Program of of Events Tuesday, Tuesday, May May 4 4 0800-1730 0800-1730 Field Trip 2 Field Trip 2: Archean Ishpeming Greenstone Belt and Gold Mineralization, D.J. Duskin, R.C. Mineralization, Michigan - Leader T.J. Bornhorst, D.J. Johnson, R.A. Mahin, T.O. Quigley, and G.W. Scott Mahin, T.O. Quigley, Scott Wednesday, May Wednesday, May 5 5 ,:-3. ,.. :. .;:, ;+?, 0800-1730 0800- 1730 Field Trip 1I and 2 Field Trip Trip 1: 1: Early Early Proterozoic Proterozoic Strata Strata of the Marquette Marquette Iron Iron Range, Range, Michigan Leader: W.F. Cannon Michigan - Greenstone Belt and Gold Field Trip 2: Archean Ishpeming Greenstone Leaders: T.J. T.J. Bornhorst, D.J. Duskin, R.C. Mineralization, Michigan Michigan - Leaders: Johnson, R.A. Mahin, T.O. Quigley, and G.W. Scott - Registration -- Ramada Inn, Inn, Marquette, Marquette, Michigan Michigan 1700-2000 Registration 1700-2000 1900-2100 Poster Session and cash bar in Ramada Inn 1900-2100 (Authors at posters 1930 1930 to 2100) xv �1 J Thursday, Thursday,May Mav 66 All All technical technical sessions sessions are arein inRamada RamadaInn, Inn,Marquette, Marquette,Michigan Michigan 0730-1600 0730-1600 J Registration Registration-- Ramada Ramada Inn Inn Technical Technical Session SessionII Session Session Chair: Chair: Gene Gene LaBerge LaBerge (University (University of Wisconsin, Wisconsin, Oshkosh) Oshkosh) Shawn Shawn Carison Carlson (Ashton (AshtonMining) Mining) 0820-0830 Opening Opening-- 45 45thAnnual Annual Institute Instituteon on Lake Lake Superior Superior Geology Geology R. R. S. S. Regis Regis and and T.J. T.J. Bornhorst, Bornhorst, Co-Chairs Co-Chairs J J 0830-0930 0830-0930 Invited Invited Presentation Presentation Sarah Sarah A. A. Green Green and and Judy Judy Wells-Budd Wells-Budd Cross Cross Margin Margin Transport Transportin in Lake LakeSuperior Superior . !' .,: . ' . ... . 0930-1000 0930- 1000 Invited InvitedPresentation Presentation Charles Kerfoot, W. : ~. W. Charles Kerfoot,John John A. A. Green, Green, and and Lawrence Lawrence J. J. Weider Weider , A New Approach to Historical Reconstruction: Combining A Approach to Historical Reconstruction: Combining Descriptive Descriptiveand and Experimental ExperimentalPaleolimnology Paleolimnology J 1000-1030 1000-1030 Coffee CoffeeBreak Break J " . 1030-1050 1030-1050 Ojakangas, Ojakangas, R.W. R.W. Sedimentology Sedimentology of of two two Deep Deep Wells Wells in in the the Keweenawan Keweenawan Midcontinent Midcontinent Rift Rift System System Near Munising, Munising, Upper Peninsula, Peninsula, Michigan Michigan 3'. 1050-1110 Puschner,U., U., Schmidt, Schmidt,S.Th., S.Th., and and Bornhorst, Bornhorst,T.J. T.J. 1050-1110 Puschner, Low Grade Metamorphism and Hydrothermal Low Grade Metamorphism and Hydrothermal Alteration Alteration of the the Upper Upper Keweenawan Keweenawan Portage Portage Lake Lake Volcanics, Michigan J 1110-1130 1110-1130 Maki, Maki,J.C. J.C.and andBornhorst, Bornhorst,T.J. T.J. The Gratiot Chalcocite Chalcocite Deposit, Keweenaw Peninsula, Michigan 1200-1340 LunchBreak Break 1200-1340 Lunch 1200-1340 1200-1340 Board BoardofofDirectors DirectorsMeeting Meeting xvH xvi i J �Technical II Technical Session Session H Session Chairs: Rod Johnson (Rod (Rod Johnson & & Associates, Associates, Inc.) Inc.) Jim Small (Edward Jim (Edward Kraemer Kraemer & & Sons) Sons) 1340-1400 Easton, 1340-1400 Easton, R.M. R.M. Metamorphic Map Map of the Canadian Canadian Shield Shield of Ontario, Ontario, Michigan, Michigan, Minnesota, Minnesota, and and Wisconsin Wisconsin 1400-1420 Vitton, 1400-1426 itt ton, S.J. and and Subhash, Subhash. G. G. Dynamic and Static Strength of of Aggregates: Aggregates: An Estimate Estimate of of Static Strength Strength Rate Sensitivity of Geological Materials Rate Sensitivity Geological Materials . ,. . .. . . , : .: :>: , , i.#&: ..' , 3B ?:'$ .A, '3 1420-1440 1420-1440 Yeo, Yeo, G.M. Is the Janice Lake Unconformity a Link between the Hearne Craton and the La Ronge Ronge -- Lynn Lynn Lake Volcanic Arc? . .. . . .. . ! I . - . .... : 1440-1510 1440- 15 10 Coffee CoffeeBreak Break 15 10-1530 Czeck, 1510-1530 Czeck, D.M. and and Hudleston, Hudleston, P.J. P.J. Structural Structural Fabric Fabric and and Strain Strain for for the the Seine Seine River River Conglomerates Conglomeratesat at the the Wabigoon-Quetico Wabigoon-QueticoSubprovince Subprovince Boundary Boundary Near Mine Mine Centre, Centre, Ontario Ontario 1530-1550 Cannon, Cannon,W.F. W.F.and andWoodruff, Woodruff,L.G. L.G. Isle Mercury Distribution in Bedrock, Distribution in Bedrock, Native Copper Copper Ore Ore and and Soils Soils -- Isle Royale National Park, Michigan Michigan 1830-1930 Social Cash Bar 1830-1930 Social - Cash . . . . . . . ?:. 1930-2130 Annual Banquet at Ramada Inn xvii i xvii .:i"> ,. : . . .S t . + . . . ., . . , . . ,. . . .. . . .,. , . "' �j Friday, Friday, May 7 Technical Session ffl III Technical r-Â¥f Session Chair. Chair: Ron Graber (Cliffs Mining Services Co.) Tom Waggoner (Cliffs (Cliffs Mining Services Services Co.) 0830-0850 LaBerge, 0830-0850 LaBerge,G.L. G.L. Regional Patterns on the Peno Penokean Continental Margin in the ~e~ional Southern Lake Superior Superior Region Regio 0850-0910 0850-0910 Ottke, D. Ottke,D. i.g,. , ,, , .~*: Early Descriptions of the Natural Features on the Marquette 1Iron r o Range n ~ a n ~- e ,: --. J J U j L 0910-0930 Webster, Webster,C., C.,Cambray, Cambray,W., W., Scott, Scott,0., G.,Nordstrom, Nordstrom,P., P., Wilson, Wilson, E. E. Palmer Gneiss: A Large, Low-grade Low-grade Shear Shear Zone Zone ., , '.!, , 0930-0950 Diedrich, Diedrich,T.R., T.R., and andMorton, Morton,P. P. Investigation of CaO at Thunderbird Mine, Mesabi Range: Range: Mineral and Stratigraphic StratigraphicRelationships Relationships j 0950-1020 0950- 1020 Coffee CoffeeBreak Break 1020-1040 Medaris, Medaris,L.G., L.G., Fournelle, Foumelle,J., J., Boszhardt, Boszhardt, R., R., and and Broihahn, Broihahn, J. T* Chemical and Mineralogical Comparison Comparison of Baraboo, Barron, Ban-on, and Sioux Sioux Argillite, Metapelite, Metapelite, and Pipestone Pipestone 1040-1100 1040-1100 Graber, Graber,R.G. R.G.and andStrandlie, Strandlie,A.J. A.J. Where are the Metamorphosed Natural Orebodies of the Mesabi Range? Range? j 1100-1120 1100-1120 Scott, G.W. and Lukey, Lukey, H.M. Geology of the Tilden Mine, Marquette iron range, Michigan J 1120-1140 1120-1140 Meier, Meier,J.G. J.G. Republic Wetland Preserve 1140-1300 1 140-1300 Lunch Lunch U J xix j �I Technical TechnicalSession SessionIV IV Session Session Chair: Chair: John John Green Green (University (University of of Minnesota, Minnesota,Duluth) Duluth) Dave DaveBaxter Baxter(PayDay (PayDayResources) Resources) 1300-1320 1300-1320 Boerboom, Boerboom,T.J. T.J.And AndSeverson, Severson,M. M. Geologic Geologic Map Map of of the the Western Western Penokean Penokean Orogen, Orogen, East-Central East-Central Minnesota Minnesota :. 4 8 -?-# 1320-1340 1320- 1340 Daniels, Daniels,D.L., D.L.,Nicholson, Nicholson,S.W., S.W.,Cannon, Cannon,W.F., W.F., and andBracken, Bracken,R.E. R.E. Preliminary Aeromagnetic Map of Wisconsin Preliminary Aeromagnetic Map of Wisconsin 1340-1400 1400 Johnson, Johnson,R.C. R.C.and andBornhorst, Bomhorst, T.J. T.J. Ishpeming Ishpeming greenstone greenstone belt belt -- Evidence Evidence for for Archean Archean tectonic tectonic evolution evolution of the southern edge of the Superior Province in Michigan of the southern edge of the Superior Province in Michigan 1400-1420 1420 Yeo, Yeo,G.M. G.M. Low-Resolution Low-Resolution Sequence Sequence Stratigraphy Stratigraphyof of Early Early Proterozoic Proterozoic Paragneisses, Paragneisses, Wollaston Wollaston Domain, Domain, Saskatchewan Saskatchewan I I I I I I 1420-1450 1420- 1450 Coffee CoffeeBreak Break 1450-15 10 Bickford, 1450-1510 Bickford, M.E. M.E. and and Steinhart, Steinhart,W.E. W.E. Distribution Distribution of of the the Archean Archean to to earliest earliest Paleoproterozoic Paleoproterozoic Sask Saskcraton craton beneath beneath deformed deformed orogenic orogenic rocks rocks in in the the Trans-Hudson Trans-Hudson orogen orogen and andits its tectonic implications: Evidence from common Pb and Sm-Nd isotopic tectonic implications: Evidence from common Pb and Sm-Nd isotopic data data 1510-1530 Miller, Miller, J.D. J.D. Jr. Jr. 1510-1530 Potential Potential for for stratiform stratiformPGE PGE mineralization mineralization in in mafic mafic layered layered intrusions intrusions of of the the Duluth Duluth Complex Complex - 1530-1550 Peterson, Peterson.D.M. D.M.and andMorton, Morton,R.L. R.L. Development Development of of Volcanogenic Volcanogenic Massive Massive Sulfide Sulfide Deposit Deposit Exploration Exploration Targets Targets in in Northern Northern Minnesota Minnesotafrom from GIS GIs Spatial SpatialAnalysis Analysis of of Geological, Geological, Geochemical, Geochemical, and Geophysical Geophysical Criteria 1550-1610 1550-16 10 Loughry, Loughry, J.E., Johnson, Johnson, Matthew Matthew M., and Cotter, J.F.P A Geochemical Geochemical and and Petrologic Petrologic Study Study to Determine the Origin of the the Crowduck Crowduck Lake Lake Group, Group, Kenora, Kenora, Ontario: Ontario: A A Problematic Problematic Metaconglomerate Metaconglomerate xx �—I Poster Session Poster Session J Posters will be Friday 1200 Posters be up up from from Wednesday Wednesday 1900 1900 to to Friday 1200 Authors will will be be at at posters posterson onWednesday Wednesdayfrom from1930 1930to to 2100 Authors Ams, D. and Holm, D., Characterization Characterization and Timing Timing Constraints Constraints of Post-Penokean Post-Penokean Meso-Scale Structures in the Watersmeet and Republic Gneiss Domes of Northern Structures Michigan Barber-Delach, R.D. and and Cannon, Cannon, W.F., Re-examining Stream Stream Sediment Sediment Geochemical Data from the National Uranium Resource Evaluation Program Program Geochemical (NURE), Wisconsin and Northern Michigan (NURE), and Michigan Han, T.M., T.M., Megascopic Megascopic Fossils and Their Their Possible Possible Contribution Contribution to to the the Han, Development of the Silicate Silicate Unit Unit of of the the Negaunee Negaunee Iron Iron Formation, Formation, Empire Empire Mine, Mine, Marquette Range, North Michigan Marquette Michigan Holm, D., Romano, D., and Mancusco, C., Comparison of Mica Ar/Ar AdAr and Rb/Sr Thennochronology Results from Northern Wisconsin and Northern Michigan j J J lUasner, J.S., Klasner, J.S., Cannon, Cannon, W.F., Schulz, Schulz, K.J., and LaBerge, G.L., G.L., The Iron River Syncline: An Allochthonous Structural in the the Penokean Penokean Foreland Foreland of of Structural Panel in Syncline: Northern Michigan Miller, J.D. Jr, and Miller, and Chandler, Chandler, V.W., V.W., New Geologic Geologic Map of the Central Central Duluth Duluth Complex Complex Rausch, D.E., and Wattrus, N.J., Rausch, N.J., Lake Superior's Rings: Clues to the Origin of Polygonal Fault Systems Polygo J J j xxi j �Saturday. May Saturday. Mav 8 8 0800-1730 0800-1730 Field and 4 Field Trip Trip 3 and Field of the the Marquette Marquette Iron Iron Range, Range, Field Trip Trip 3: 3: Tilden Tilden and and Empire Mines of Michigan G.W. Scott, Scott, P.M. P.M. Nordstrom Nordstrom and and H.M. H.M. Lukey Lukey Leaders: G.W. Michigan -- Leaders: Field Trip 4: Paleozoic and and Glacial Geology from Au Train to Grand Trip 4: Marais, R.S. Regis Regis and and J.B. LB. Anderton Anderton Leaders: R.S. Marais, Michigan Michigan -- Leaders: xxi xxi i �45TH ANNUAL MEETING INSTITUTE ON LAKE SUPERIOR GEOLOGY Abstracts Abstracts �CHARACTERIZATION AND AND TIMING TIMING CONSTRAINTS CHARACTERIZATION CONSTRAINTS OF POST-PENOKEAN POST-PENOKEAN MESO-SCALE STRUCTURES STRUCTURES IN IN T THE MESO-SCALE H EWATERSMEET WATERSMEET AND REPUBLIC REPUBLI~GNEISS GNEISS DOMES OF OF NORTHERN MICHIGAN MICHIGAN David Ams (undergraduate student) and Daniel Hoim, Holm, Department Department of Geology, Kent State Ams (undergraduate University, Kent OH 44242 OH 44242 We have measured late, meso-scale, structures present present in in Early Proterozoic Proterozoic gneiss gneiss meso-scale, ductile and brittle structures in order to to assess assess their their origin. origin. The Watersmeet and Republic gneiss gneiss domes domes domes of northern Michigan in lie north north of of the the region regionofofpervasive pervasivepost-Penokean post-Penokean 1600-1650 1600-1650 Ma shortening shortening (Hoim (Holm et a!., al., 1998, 1998, Geology) and southeast which closed the Keweenawan Keweenawan rift rift at ca. 1080-1860 southeast of thick-skinned thrusting which whether meso-scale meso-scalebrittle brittle and andductile ductile 1994, Geology). The The challenge challenge then is to determine whether Ma (Cannon, 1994, shear zones formed during during the late stages stages of Early Proterozoic gneiss dome formation or if they represent deformation associated defamation associated with these these younger younger events. events. Watersmeet gneiss Watersmeet gneiss dome. observations were made on the north-east side of the dome, south of its contact with the overlying Our observations metasedimentary rocks rocks (Hwy (Hwy 45 45 exposures). exposures). The dominant steep east-west fabric fabric in in Archean Archean rocks rocks of of metasedimentary brittle shear shear zones. zones. The ductile the dome are cross-cut by steep ductile, semi-brittle, and brittle ductile and larger semiand dip dip steeply steeply both both north north and and brittle to brittle shear zones show a dominant N70-80E strike orientation and etc.), the apparent sense sense south (Fig. 1). Where Where relative relativemotion motion indicators indicators exist (offset dikes, pegmatites, etc.), up. Small to be be oriented oriented NS NS (with (with steep steep east east of motion is south-side up. Small scale scale brittle brittle faults and fractures tend to dips) dips) or NW (with steep steep southwest dips). Slickenlines Slickenlineson on the the NW NW oriented oriented set set plunge plunge gently gently to to the the NW NW Pie. 1 (Fig. 1). .--o- *1 The coarse, more gentle gentle to to the the north north toward toward the the contact contact coarse, steep, steep, EW fabric fabric of the dome interior becomes more rocks and and shows shows aareduction reductioniningrain grainsize sizedue duetotoductile ductileshearing. shearing.We Weattribute attribute with the metasedimentary rocks this to ductile ductile overprinting shear zone. zone. The overprinting of the steep fabric along a basement-cover shear The timing timing of shearing shearing during gneiss gneiss dome dome formation formation is is constrained constrained by aa 1765 1765 Ma Ma hornblende hornblende Ar/Ar ArIAr plateau date date obtained obtained from from the core rocksjust just from aa concordant concordant muscovite plateau date obtained from metasedimentary rocks core rocks and from north of the see also also Schneider Schneider et et al., al., 1996, 1996, CJES). CJES). the contact contact (Fig. (Fig. 2; 2; see We propose that ductile and semi-brittle shear zones zones Conned formed during during the thelate latestages stagesof ofunroofing unroofingofof thal the theductile of the the footwall footwall In our our model model they thev represent remesent internal deformation of the gneiss dome (at ca. 1765-1755 1765-1755 Ma). In dome rocks as they were unroofed from beneath the metasedimentary cover rocks along a north-dipping detachment fault fault at at the basement-cover contact. The brittle shear zones have a completely different detachment basement-cover contact. ones. Their orientation from the ductile and semi-brittle ones. Their orientations and subhorizontal lineations suggest rift. they may be related to closure of the Keweenawan rift. Republic gneiss dome. Republic fracture zones zones which which occur occur in in the the Republic We focused on the numerous chlorite mineralized brittle fracture Ma post-tectonic post-tectonic alkali alkali red red granite granite gneiss dome north of Republic (Fig. 3). They Theycross-cut cross-cutthe the—1735 -1735 Ma be younger younger than than mica mica cooling cooling ages ages within within the the dome dome (1680-1720 (1680-1720 Ma). Ma). (Xgaf of Sims. Sims, 1992) 1992) and must must be They have a dominant north-northwest north-northwest strike strikeand anddip dip moderately moderatelytotosteeply steeplytotothe thewest. west. Most of the They lineations plunge gently gently to to the the northwest. northwest. Kinematic fiber steps) steps) on on a few few of these Kinematic indicators (crystal fiber show left-lateral, reverse motion. Ductile Ductileshear shear zones zones within within the dome dome strike dominantly northwest and dip steeply and must be older older than the mica cooling coo1i.i; ages. ages. related to to the older ductile shear zones along The brittle fractures do not appear to be kinc:iiatically kincniatically related uplifted. The which the gneiss dome block was probably uplifted. Thedominant dominant left-lateral left-lateral strike-slip strike-slip motion and be defonnational deformational features features associated associated with withclosure closureof of north-northwest attitude indicates they might be also be the Keweenawan rift. This Thisinterpretation interpretationisisconsistent consistentwith with prior prior thermochronologic thennochronologic evidence evidence for a low temperature Keweenawan CJES). temperature Keweenawan age age disturbance disturbance of this region (Schneider (Schneider et al., 1996, 1996, CJES). 2 �Ductile shear zones Smaller britile fracture zones Fig. Fig. 1. Stereonet Stereonet plots of of ductile, ductile, semi-brittle, semi-brittle, and and brittle brittle shear shear zones zones within the Watersmeet the Watersmeet gneiss gneiss dome. 1 -1 Schist outside of the dome — 2000J 2000: Amphibolite imnhibolite within within the the dome AGE 1 Ma 1500 - - 1000-0_ 94-MI-3-H 94-MI-3-H Hornblende Hornblende Tg=1751±l3Ma T g = 1751k13Ma - Tp=1765±l3Ma %39Ar 100 0 UP-i OA-M UP-1 0A-M Muscovite Muscovite Tg=1760±i4Ma Tp=1767±l7Ma 100 %39Ar Fig. 2. Ar/Ar mineral Fig. mineral age age spectra from from the the Watersmeet Watersmeet gneiss gneiss dome region. region. date. Tg Tg Is is total gas Tp is plateau date. gas date. Fig. 3. 3. Stereonet plots plots of ofrepresentative representativeductile ductileshear shear zones zones and and mineralized mineralized Fig. brittle fracture brittle fracturezones, zones. Republic Republic gneiss gneiss dome dome region. region. 3 1 �Re-examining stream sediment data ffrom National U Uranium Re-examining sediment geochemical geochemical data r o m tthe h e National r a n i u m Resource Resource Evaluation Program and nnorthern o r t h e r n Michigan Michigan Evaluation P r o g r a m (NURE), (NURE), Wisconsin Wisconsin and Robert D. Barber-Delach, Oak Oak Ridge Ridge Associated Associated Universities, Universities, at at U.S. U.S. Geological Geological Survey. Survey, Mail Mail Stop Stop 954, Reston, VA VA 20192 20192 W.F. W.F. Cannon, U.S. Geological Geological Survey, Survey, Mail Mail Stop Stop 954, 954, Reston, Reston, VA VA 20192 20192 In the In the latter latter 1970's 1970's a astream streamsediment sedimentgeochemical geochemical survey survey was was conducted conducted in in parts parts of ofnorthern northern the United United States Department Department of of Energy. Energy. A A Wisconsin and northern Michigan by the NURE program of the data set consisting I °x2° data consisting of of multi-element multi-element analyses analyses of of 3185 3 185 samples samples was was developed developed spanning spanning eight eight l0x2' quadrangles. The but also quadrangles. The data datawere wereintended intended for foruse use ininevaluating evaluatingundiscovered undiscovered uranium uranium resources, resources, but have application geochemical background background and and baseline have application for for establishing establishing regional regional variations variations in geochemical baseline concentrations. The The U.S. is concen&itions. U.S. Geological ~ e o l o ~ i c Survey Survey al is ____________________________________________ expanding research research on on regional regional geochemical geochemical Figure I. 1. Comparison of unadjusted (a) (a) and patterns, and and the the NURE data set patterns, MURE data set is is being being rerenormalized (b) (b) cobalt cobalt concentrations concentrations for for use use in in normalized examined as as part of of that that effort. effort. displaying NURE stream sediment sediment data. data. In many places places the the NURE NURE data datashows showsobvious obvious biases in in values values between between adjacent adjacent quadrangles, quadrangles, visual in some some cases cases apparent only by visual examination of element concentration maps. For instance, Figure llaa shows shows significant discontinuities of cobalt values values between quadrangles in the original original NURE NURE data. data. Biases were probably introduced introduced by by somewhat somewhat different sampling, processing, sampling, processing, and analytic analytic techniques techniques between various quadrangles, between quadrangles, and probably also changes in analytical by time-dependent time-dependent changes instrument calibration that that appear appear not not to have instrument calibration have been fully accounted for over the duration of the NURE program. program. These Thesequadrangle-based quadrangle-based biases biases render use of render of the thedata dataproblematical problematical without without careful assessment careful assessment and correction correction of of analytical analytical biases. Here Here we present present two two approaches approaches by by which the NURE NURE data data set set can can be be corrected corrected for for which the quadrangle biases. in analytical data First, systematic biases in between can be normalized between quadrangles quadrangles can normalized by by quadrangle as as a displaying data for each each quadrangle function of the statistical function statistical distribution distribution of values values for only that quadrangle. Figure lb for quadrangle. Figure I b shows shows the the same data for cobalt as shown in Figure Ia, same la, but but each of with the values within within each of the the eight separately into quadrangles quadrangles classified separately percentiles of the data data set set from from that that quadrangle. quadrangle. The between-quadrangle bias bias is is largely largely removed removed and natural patterns patterns of of cobalt cobalt variation variation are are more more 4 a. Unadjusted data . . ,do.- adom. b. Data normalized within quadrangles �evident. that standard standard desktop desktop G GIS software evident. This Thistechnique technique has has the the advantage advantage of of ease ease and and simplicity simplicity in in that I s software can perform statistical manipulation manipulation of of data data and and time time required required for for perform and display the results with minimal statistical processing. Also, Also, the technique However, two two technique does not require acquisition acquisition of new analytical analytical data. However, important shortcomings shortcomingsof of the the technique only relative of element are: 1) presents only relative variations variations of element important technique are: I) itit presents concentrations and and does does not not improve the the overall overall accuracy accuracy of of the the NURE NURE data, data, 2) 2) itit masks masks true true variations variations concentrations quadrangle. that occur at the approximate scale of a l°x2° l0x2"quadrangle. A second In the the 1980's, A second technique technique requires requires new new analytical analytical data. data. In 1980's, the the USGS USGS became became the the official official archiving agency agency for for NURE NIJRE data and maintains repository in in Denver, Denver, Colorado archiving data and and samples, samples, and maintains aa sample sample repository Colorado in which all N'URE samples samples are are accessible accessiblefor for restudy. restudy. We re-analyzed aa subset subset of of those those samples samples We have have re-analyzed in which all NURE for the Ashland and Rice Rice Lake Lake l°x2° Iox2' quadrangles quadrangles in northern Wisconsin and compared the results of our re-analyses to to the the original original data data reported reported by by the the NURE NUREprogram. program. Our Our analyses analyses were were done done using using routine routine re-analyses 40-element inductively 40-element inductively coupled coupled plasmaplasmaatomic emission spectroscopy Figure 2. 2. Scatterplot Scatterplot of of magnesium magnesium concentration concentration spectroscopy (ICP-AES) (ICP-AES) Figure technique supplemented supplemented by by more (ppm) demonstrating demonstrating the the quad quad bias bias of of original original NURE NURE technique more precise precise (ppm) single-element techniques for arsenic and data, with with fitted fined regression regression lines lines and and correlation correlation single-element techniques and data, selenium. Analytical precision and and Analytical indices (R2). indices (R'). accuracy were monitored accuracy were monitored by by replicate replicate 2.1 2.1 analyses of standard samples. ,, 1 , -, — 1 Comparison of our new new values values to to 1.8 •0 concentrations reported reported by by NURE NURE show show $b —, quadrangle-based quadrangle-based biases in virtually all 1.5 elements elements for which which aa significant significant number number —— 000 of samples exceed exceed detection limits (about (about ,. elements). Figure 20 elements). Figure 2 shows shows an extreme 0 case in which NURE magnesium analyses 09 are strongly and and consistently consistently quadranglequadrangle0.9 dependent and in which which both both quadrangles quadrangles vary significantly from our new analyses. j, vary significantly from analyses. of bias bias within within aa 0.6 This consistency of quadrangle allows derivation of equations •Rice Lake Lake Quad Quad Â¥Ri that can adjust the original original NURE data to 03 correct for previous analytical biases and and oAshland Quad Quad OAshland , new produce a better accord with new 0 data. Although Although this technique analytical analytical data. technique 1.8 2.1 0.6 0.9 1,2 1.5 0.3 has 00 0.3 0 .6 0.9 1.2 1.5 1.8 2.1 has inherent inherent statistical statistical uncertainty, uncertainty, that that the and the uncertainty is quantifiable, and Re-Analyzed Mg MgConcentration Concentration Re-Analyzed element maps are resulting element are significantly significantly L more accurate accurate than than those made from more from the the Although this this technique techniquedoes doesrequire requirethe the cost cost and and time time of original NURE data. of re-analyzing re-analyzing aa original data. Although significant number number of of NURE NURE samples, samples, the the result result produces producesaamuch much improved improveddata dataset setwithout withoutthe thecost costof of re-analyzing the entire sample set. We plan to proceed with this technique during the coming year. re-analyzing the entire set. We to the year. , . 7 • , 7 i. • — - 7 5 I �Distribution Distributionof ofthe theArchean Archean to to Earliest Earliest Paleoproterozoic PaleoproterozoicSask Sask Craton Craton Beneath Beneath Deformed Deformed Orogenic OrogenicRocks Rocksin inthe theTrans-Hudson Trans-HudsonOrogen Orogenand andits itsTectonic TectonicImplications: Implications:Evidence Evidencefrom from Common Common Pb Pb and and Sm-Nd Sm-Nd Isotopic IsotopicData Data M. E. Steinhart, III, ifi, Department M. E. E. Bickford Bickford and and William William E. Departmentof of Earth Earth Sciences, Sciences, Heroy Heroy Geology Geology Laboratory, Syracuse University, Syracuse, NY 13244-1070 Laboratory, Syracuse University, Syracuse, NY 13244-1070 Late, Late, undeformed, undefonned ,ca. ca.1.8 1.8Ga Gapegmatites, pegmatites,leucogranitic leucograniticsheets, sheets, and and post-orogenic post-orogenic plutons plutons in in the Glennie, Hanson Lake, and La Ronge Domains, and parts of the Hearne Province, were studied Glennie, Hanson Lake, and La Ronge Domains, and parts of the Hearne Province, were studied to melting ofjuvenile ofjuvenile orogenic orogenic crust crust or or of Archean todetermine determinewhether whetherthey they were werederived derived by melting Archean lower structuralwindows windowsin inthe theGlennie Glennieand andHanson Hanson lower crustal crustalrocks rocksequivalent equivalentto tothose thoseexposed exposedin instructural Lake isotopic composition of of PbPb from feldspars (206Pb/2°4Pb == 14.3 14.3-- 15.2; 15.2; LakeDomains. Domains.The The isotopic composition from feldspars (20Èpb/== 15.1 15.1-- 15.25; 15.25;208Pbf°4Pb ^"Pb/'^Pb ==34.4-36.4) 34.4 - 36.4) and andSm-Nd Sm-NdTDM TDM ages of of ca. ca. 3.2 3.2 Ga Ga m7~b/2d4pb ages for forca. ca.1.8 1.8Ga Gapost-orogenic post-orogenicintrusions intrusionsreveal revealthat thatArchean Archean rocks rocks are arenot not only onlyexposed exposedin inthe the windows, windows,but butalso alsoextensively extensivelyunderlie underlieparts partsof ofthe theGlennie Glennieand andHanson HansonLake LakeDomains Domainsininwhich which only Incontrast, contrast,feldspar feld arPb Pbfrom frompegmatites pegmatitesin inthe the only juvenile Paleoproterozoic Paleoproterozoicrocks rocksare areexposed. exposed. In La similar == 15.1 b/-b 15.1-- 15.2; 15.2; 15.4 - 15.6; 15.6;207Pb/2"Pb LaRonge Ron eDomain Domainisisisotopically isotopically similar(2°6Pb/2°'Pb ('"'Pb/^'Pb ==15.4208Pb/204Pb plutons with TDM "'PW b =34.8 34.8-- 35.2) 35.2)to tothat thatofjuvenile, ofjuvenile,ca. ca.1.85 1.85Ga GaPaleoproterozoic Paleoproterozoic plutons with TDM ages oceanic materials. materials. Sm-Nd data agesof ofca. ca.1.80-2.0 1.80-2.0Ga, Ga,indicating indicatingderivation derivation from similar oceanic data from from two yield TDM ages twoLa LaRonge RongeDomain Domainaplites, aplites,ininwhich whichSm Smand andNd Ndare areapparently apparentlyunfractionated, unfl-actionated, yield TDM ages of 19 Ga. Surprisingly, feldspar Pb data (2°'Pb/2°4Pb 207Pb/204Pb = 15.1 Surprisingly,feldsparPbdata (""Pb/^Pb== 15.515.5 - 15.8; 15.8;^Pb/^'Pb= 15.1-- 15.2; 15.2; of1.8-. 1.8-.19Ga. 35.0--35.2) 35.2) from frompegmatites pegmatitesthat that intrude intrudeArchean Archean rocks rocksof ofthe theHearne HeameProvince, Province,in in m'~b/-b ==35.0 the thehinterland hinterlandwest westofofthe theNeedle NeedleFalls FallsShear ShearZone, Zone,also alsoindicate indicatederivation derivationfrom fromjuvenile juvenilesources. sources. Thus, viewthat thatan anArchean Archeanmicrocontinent microcontinent("Sask ("SaskCraton") Craton") Thus,while whilethese thesedata datasupport supportthe theview underlies underliesparts partsof ofGlennie Glennieand andHanson HansonLake LakeDomains, Domains, they they suggest suggest that that crust crustbeneath beneathLa LaRonge Ronge Domain, Domain,and andalso alsobeneath beneathatatleast leastparts partsofofHearne HeameProvince, Province,isisofjuvenile ofjuvenilemantle mantlederivation. derivation. Alternatively, LaRonge RongeDomain Domainand andHearne HearneProvince, Province, Alternatively,Archean Archeancrust crustmay maybe bepresent presentbeneath beneaththe theLa but ca.1.8 1.8Ga Gapegmatites pegmatiteswere wereformed. formed.These These butwas wasbelow belowthe thezone zoneofofmelting meltingwhen whenthe theca. observations observationsare areconsistent consistentwith withinterpretation interpretationofofnorthwesterly-dipping northwesterly-dippingreflectors reflectorsbeneath beneath Hearne revealedby byCanadian CanadianLithoprobe Lithoprobeseismic seismic data, data, as asrepresenting representingsubducted subducted H e m eProvince, Province,asasrevealed oceanic oceaniclithosphere. lithosphere.The Thepresence presenceofofoceanic oceaniclithosphere lithospherebetween betweenHearne HeameProvince Provinceand andGlennie Glennie Domain Archeanmicrocontinent microcontinentbeneath beneaththe theGlennie GlennieDomain Domainisisprobably probablynot not Domainindicates indicatesthat thatthe theArchean subducted subductedHearne H e m ecrust, crust,but butmore morelikely likelyan anexotic exoticcrustal crustalfragment. fragment. =% 4 6 �GEOLOGIC MAP GEOLOGIC MAP OF OF THE THEWESTERN WESTERN PENOKEAN PENOKEAN OROGEN, OROGEN, EAST-CENTRAL EAST-CENTRAL MINNESOTA BOERBOOM, Terrence Terrence J., J., Minnesota Minnesota Geological Geological Survey, Survey,2642 2642 University UniversityAve., Ave.,St. St.Paul, Paul,MN MN 55114; BOERBOOM, 55114; boerboOl @maroon.tc.umn.edu; @maroon.tc.umn.edu; and boerb00l and SEVERSON. SEVERSON, Mark, Mark, Natural Resources Resources Research Institute, Institute, University University of Minnesota, Duluth. Duluth, MN 55811; 5581 1;mseverso@sage.nrri.unm.edu mseverso@sage.nrri.umn.edu The Natural Resources Resources Research Research Institute, Institute, has has recently recently The Minnesota MinnesotaGeological Geological Survey, Survey, in cooperation with the Natural completed Orogen in east-central east-central Minnesota. Minnesota. comnleted aa revised revised geologic eeoloeicmap man of the the western portion nortion of the Penokean Oroeen The geologic framework western Penokean framework for for the thewestern ~ e n o k e Orogen kOrogen has has been been previously previously interpreted interpreted by by Southwick Southwick and others (1988) using plate-tectonic theory theory (see discussion discussion below). below). This This map map builds builds on on that that base base by by data. Although utilizing new drilling information and better geophysical geophysical data. Although thousands thousands of of exploratory exploratory holes holes were drilled in this area area from the late l800s 1800sto tothe the 1950s, 1950s.nearly nearly all all focused focused on locating locating iron, iron, manganese, manganese, or or sulfur ore. As Asaaresult, result, the thedistribution distributionand and form form of of the the major major iron-formations both within and outside the iron-mining districts are fairly well established, but but little little is is known known about aboutthe the areas areas in in between. between. Thus, any geologic compilation compilation necessarily relies relies heavily on geophysical interpretations. interpretations. GEOLOGIC FRAMEWORK Southwick (1988) interpreted interpreted the Penokean Orogen to consist of several structural panels that range from sedimentary sedimentary rocks rocks filling filling aa foreland foreland basin, basin, through through aa main main fold-and-thrust fold-and-thrust belt belt of sedimentary sedimentaryand and volcanic volcanic rocks, to a complex terrane. The major terranes, referred referred to as panels in the list rocks, complex metamorphic-plutonic metamorphic-plutonic terrane. list below, are separated by linear, linear, northeast-trending northeast-trending aeromagnetic aeromagnetic anomalies, which are are interpreted interpreted as as thrust thrust faults that truncate stratigraphy-related stratigraphy-relatedaeromagnetic a e r ~ r n a ~ e tanomalies ainc o d i e s within the panels. The Thefollowing followingsummary summary of the structural I), and modified accordingly accordingly structural panels panels is is condensed condensed from from Southwick Southwick and others others (1988) (1988) (Fig. (Fig. 1), for this study study based based on on new new information. information. 1. The Animikie basin is filled with essentially essentially unmetamorphosed unmetamorphosed sedimentary 1. The Animikie sedimentary rocks rocks (Pokegama (Pokegama Quartzite, Quartzite, Biwabik Iron Formation, Formation,and and Virginia Formation). The TheAnimikie Animikie strata strata generally generally have have aa shallow shallow south south dip, but along along the the southern southern basin basin margin, margin, and and hence hence this this map map area, area, are are deformed deformed into into fairly fairly tight tight upright upright folds. folds. In Inthis thisarea areaofofthe themap, map,these thesestrata strataunconformably unconformablyoverlie overlierocks rocks of of the the foldfold- and and thrust-belt. thrust-belt. 2. The North Range 2. Range panel panel isisdominated dominatedby byweakly weaklymetamorphosed metamorphosedargillite, argillite, siltstone, siltstone, quartzite, quartzite,ironironformation, graphitic slate, graywacke, and minor limestone and and chert. chert. Volcanic and hypabyssal mafic rocks rocks are are a minor component componentof the the North Range. Structurally, Structurally,the the North North Range Range is is dominated dominatedby by aa large large northwest-inclined, northwest-inclined, doubly plunging synclinorium. synclinorium. Geophysical Geophysicalmodels models (Carlson, (Carison, 1985) 1985) show that the folded Animikie Basin. The folded Trommald Trommaldiron-formation iron-formationcontinues continuesto tothe theeast eastbeneath beneath rocks rocks of of the thehimikie Thestructural structural of those deposited in an external and stratigraphic attributes of North Range rocks are characteristic of tectonic zone. zone. 3. The 3. The South South Range Rangepanel panel also alsocontains containsan an abundance abundanceof of slaty slaty and and argillaceous argillaceousrocks, rocks, but but unlike unlike the the North North Range, rocks, all of which Ranee. it has aa substantial substantial component comoonent of mafic rnafic volcanic volcanic and hypabyssal hvpabyssal intrusive rocks, have undergone low-grade inthe theSouth SouthRange Rangeresembles resemblesAlgomaAlgomahaveundergone low-grade metamorphism. metakorphism. The Theiron-formation iron-formationin type iron-formations iron-formations in in that that itit occurs occurs as as aa series seriesof of Jenses lenses closely associated with mafic volcanic rocks and graphitic anomalies. The graphitic slate. slate. This Thispanel panel isis characterized characterizedby narrow linear aeromagnetic anomalies. The structure structure of the the South South Range Range isis dominated dominatedby by elongate elongateeast-northeast-trending east-northeast-trending overturned overturnedfolds. folds. The South Township panel panel to to the the south south by byaa South Range Range panel panel is is separated separated from from the Moose Lake-Glen Township narrow zone of graphite-rich, graphite-rich, crenulated crenulated phyllitic schist that curves from northeast to east-west. This This graphitic schist schist may have absorbed absorbed aa great great deal deal of strain strain during during thrust thrust faulting. faulting. 4. The TheMoose MooseLake-Glen Lake-GlenTownship Townshippanel panel isis dominated dominated by by quartz-rich quartz-rich metaclastic rocks and by mafic mafic volcanic graphitic argillite argillite and and schist; schist; all were subjected volcanic and hypabyssal intrusive rocks, along with graphitic subjected to lower greenschist-facies greensehist-facies metamorphism. This Thispanel panel includes includes the the Glen Glen Township Formation, a sulfiderich graphitic schist schist grading grading into into aa sulfide sulfideiron-formation iron-formation that that contains contains pyrrhotite, pyrrhotite, pyrite, pyrite, lesser lesser chalcopyrite, and consists of ofamygdaloidal amygdaloidal chalcopyrite, and minor minor sphalerite. sphalerite. The Thestratigraphic stratigraphicsection section at at Glen Township consists basalt overlying overlying the the sulfidic-graphitic sulfidic-graphiticschist, which in turn overlies graywacke. In In places places the the latter lattertwo two are separated by slaty iron-formation. iron-formation. The Thebase base of of the thesequence sequencecomprises comprises quartz-rich quartz-rich sedimentary sedimentary moderate magnetic magnetic anomaly. rocks that typically contain contain up up to several several percent magnetite, which causes a moderate Several prominent linear linear aeromagnetic anomalies anomalies apparently reflect some combination of thrust faults faults with relatively minor displacement, displacement, and stratigraphic and lithologic variation. variation. The GlenTownship Glen Township strata strata have undergone undergone at least least two two generations generationsof of folding folding that that produce produce complex complex aeromagnetic aeromagneticpatterns. patterns. �A previously unrecognized, synsyn- to to late-orogenic late-orogenic monzonitic intrusion intrusion with with aa well-defined well-defined circular circular aeromagnetic anomaly core drilline drilling iust just northwest northwest of ofMille Mule Lacs Lacs Lake. Lake. Another Another core core aeromaenetic anomalv was intersected by bv core hole hole drilled drilled on on this this anomaly anomaly intersected intersected ten ten feet feet of o faabrick-red, brick-red,unmetamorphosed unmetamorphosed quartz-pebble quartz-pebble conglomerate. This Thisunit unitisisgeophysically geophysicallyinvisible invisibleand and its its extent extent is is thus thus unknown, unknown, but but itit may may represent represent conglomerate that was deposited durine during Meso~roterozoic Mesoproterozoic time time in in aa shallow shallow basin basin on on the the backside backside of of aa conelomerate the Midcontinent Midcontinent rift, and the and thus thus is is correlative correlative with with Keweenawan Keweenawan rocks rocks to to the the east. east 5. The McGrath-Little 5. The McGrath-Little Falls Falls panel, panel, the the southernsouthern- and and internal-most internal-most component component of of the the foldfold- and and thrust-belt, thrust-belt, comprises comnrises granitic eranitic to to tonalitic tonalitic gneiss, eneiss. amphibolite-facies amnhibolite-facies pelitic nelitic schist, schist. and and latelate- to post-tectonic nost-tectonic granitic eranitic intrusions. An uncommon dike that Penokean granites of fresh, comnosed fresh. intr6sions.~n uncommon that nost-dates ~enokekn =m~ i t e s is composed -~~ ~- ~ ~ ~lamprophyre -la&ronhvredike -~~~~~ r~ ~ - r ~ ~- , ~ ~~~-~~ ~ post-dates - -~~ -~~~~-~~~ ~ euhedral be traced across across the entire entire map area area euhedral clinopyroxene clinopyroxenein in aa groundmass groundmass of of poikilitic feldspar; feldspar; it can be by its its pronounced pronounced reverse reverse aeromagnetic aeromagnetic anomaly. Abundant Abundant small small circular circular to to elongate elongate positive positive magmagnetic anomalies anomalies are areindicative indicative of of mafic-ultramafic mafic-ultramafic plugs plugs similar similar to to those that have been drilled at several locations to to the the south locations south and and east. east. Based Basedon onaeromagnetic aerom&$etic data, data, these these plugs plugs post-date post-date the the lamprophyre lamprophyre dike. The Thedike dikeisisconjectured conjecturedtotobe berelated relatedto tothe thewaning waningstages stagesof ofPenokean Penokean granite granitemagmatism. magmatism. ~ ~ ~~~~ -~ ~~~~ ~ ~ is ~ ~ References: References: Carlson, Carlson, K.E., K.E.,1985, 1985, A A combined combined analysis analysis of of gravity gravity and and magnetic magnetic anomalies anomalies in in east-central east-central Minnesota: Minnesota: M.S. 138p. p. M S . thesis, thesis. University University of of Minnesota, Minnesota, Minneapolis, Minneapolis, 138 Southwick, Geologic map map (scale (scale 11:250,000) of the the Penokean Penokean Southwick, D.L., Morey, Morey, G.B., and McSwiggen, McSwiggen, P.L., P.L., 1988, Geologic :250,000) of Orogen, and accompanying accompanying text: text: Minnesota Report Orogen, central central and and eastern eastern Minnesota, Minnesota, and Minnesota Geological Geological Survey Survey Report of Investigations 37, 25 p. 37.25 p. Figure 1. 1. Regional Regional tectonic tectonicinterpretation interpretation elements discussed in in showing the tectonic elements the abstract- From Southwick and others (1998). This map does not reflect changes made on the map accompanying this poster. Area of revised geologic map 8 �Mercury distribution National Park, Park, Mercury distributionin inbedrock, bedrock,native nativecopper copperore, ore, and andsoils—Isle soils-Isle Royale National Michigan Michigan W.F. Cannon, U.S. W.F. U.S. Geological Geological Survey, Survey, Reston, Va L.G. Woodruff, U.S. Geological Geological Survey, Survey, St. Paul, MN In response response to recent recent findings findings of elevated elevated mercury in fish in some inland lakes on Isle Royale, we have begun a study of mercury concentration study of concentration in natural geologic materials of the island. Initially Initially we aimed to determine determine if elevated elevated mercury from native copper deposits could be contributing mercury to the impacted lakes. Small Smallnative nativecopper copperdeposits depositson onthe the island island were were mined mined the 1800's. Those sporadically in the Thosedeposits, deposits,waste waste dumps dumps at at abandoned abandoned mines, and wastes from small processing operations, do contain anomalous amounts of of mercury. mercury. Our Our analyses analyses have have found individual as much as 14 14 parts per million (ppm) mercury, and stamp sands individual samples samples with as were found to contain contain from 0.6 to 0.7 ppm. In Incontrast, contrast,typical typicalbedrock bedrockof of the the island, island, aa sequence sequence of volcanic Sampleswe we have have volcanic basalt flows, appears appears to be uniformly very low in mercury. Samples analyzed all had no detectable parts per billion (ppb). detectable mercury at at aa detection detection limit limit of of 55 parts (ppb). determine if mercury contained To determine contained in in mineral mineral deposits deposits could wuld be be aa source source of of mercury mercury to to impacted impacted lakes we sampled and analyzed analyzed soil soil and and glacial glacial deposits deposits on on which which the the soils soils formed formed in in two two test test wextaken takenaround aroundthe theMinong Minong mine, mine, the the largest largest mine mine on on the the island, island, to to determine determine areas. Samples Sampleswere the extent extent to which which anomalous anomalous metal content content of the ore and surrounding altered rocks affected the drainage drainage basins basins of of the trace element content of soils. Also, Also, samples samples were taken within the mercury in in fish. fish. No copper Sargent Lake and Lake Wagejo, two of the lakes with elevated mercury copper basins, but but deposits depositscould wuld be concealed concealed beneath beneath extensive extensive deposits are known known within within those those basins, mercury. In glacial deposits, and, if present, could be a contributor of mercury. In total, total, samples samples were were taken taken was taken from from the theAAsoil soilhorizon, horizon, aa dark dark organic organic mineral soil, soil, at 65 sites. At Ateach eachsite siteaa sample samplewas 1-6 inched below the surface. A second sample was taken from a depth of about 2 feet, A second sample was taken from depth consisting of either consisting either reddish reddish sandy sandy glacial glacial till till or or till till slightly slightly modified by soil-forming soil-forming processes processes (referred to here as BC horizon). as BC (referred Results show Results show that that the the copper copper deposit depositat at Minong Minong mine mine produces produces aa very very distinct distinct trace trace element element anomaly in soil, soil, especially especially the copper copper content of the BC horizon. Mercury Mercury also also is is somewhat somewhat elevated in samples closest to tothe thecopper copperdeposit. deposit. Mean Mean copper copper content content of of BC samples of the BC horizon closest ppm and that of of mercury mercury is is 44 44ppb. ppb. Highest values are 1160 1160 soils in the Minong mine area is 342 pprn ppm copper and 110 ppb mercury. The A soil horizon near the Minong mine averages 290 ppm pprn 110 The A soil horizon near the Minong averages pprn Cu and 70 thedrainage drainagebasins basinsfeeding feedingLake LakeWagejo Wagejoand and Sargent Sargent Lake, BC soils 70 ppb ppb Hg. Hg .InInthe ppm Cu, nearly five times lower than near Minong Minong mine, mine, and and 38 ppb Hg, essentially essentially average 60 pprn A-horizon soils soilsin inthe the Sargent-Wagejo Sargent-Wagejo drainage drainage average average 63 63 ppm pprn the same as at Minong mine. A-horizon Cu and 133 133 ppb ppb Hg. Hg. The The maximum maximum value value for for Hg Hg is is 370 370 ppb, ppb, an an exceptionally exceptionally high high value value for for soil soil For comparison, similar soil geochemical studies that we are conducting in the in this this region. region. conducting Chequamegon National Forest in Chequamegon in northern northern Wisconsin show A-horizon soils soils to have a mean mean Hg content of about 54 ppb with a maximum detected concentration of 260 ppb. C-horizon content about 54 of 260 ppb. C-horizon soils soils in in that area have a mean of 19 ppb. With 19ppb ppbHg Hg and and aa maximum maximum of 80 ppb. With respect respect to to soils, soils, Isle Isle 9 �than northern northern Wisconsin. Both Royale appears to be a more Hg-rich environment than Both A A and and BC BC horizon soils soils are are at least least twice twice as as high high on on the the island island as as in in northern northern Wisconsin. Wisconsin. The results results suggest suggest that the very elevated mercury content of soils in the Sargent Sargent Lake-Lake Wagejo area is not caused by concealed concealed native native copper copper deposits depositsbecause because there there is is little littleor orno no elevated elevated copper copper content of those soils. soils. Results from the Minong mine area show that concealed deposits, if present, would produce readily detectable elevated copper content of the soils. Neither is there reason reason to conclude conclude that local bedrock is the source for mercury because all tested chemically very uniform bedrock have very low mercury mercury content. content. Because samples of the chemically Becauseof of the the general lack of mercury in the most common occurrence common rocks of the island and the very limited occurrence of native copper copper deposits deposits that that do do contain some mercury, the mercury content of the soils on Isle Royale seems very very likely likely to to result result largely largely from from airborne airborne deposition. deposition. The The processes processes causing causing the the great variability variability of mercury content content of similar similar appearing soils over short distances that thatwe wehave have observed in our survey remain only speculative. Nevertheless, it appears certain that mercury must be very mobile in the soil soil zone zone in order to redistribute what must be a relatively uniform mobile in amount of deposited mercury into the highly variable observed concentrations. 10 10 �STRUCTURAL STRUCTURALFABRIC FABRICAND AND STRAIN STRAINFOR FOR THE THE SEINE SEINERIVER RIVER CONGLOMERATES CONGLOMERATESAT AT TIlE THE WABI000NQUETICO WABIGOONQUETICOSUBPROVINCE SUBPROVINCEBOUNDARY BOUNDARYNEAR NEARMINE MINECENTRE, CENTRE,ONTARIO. ONTARIO. CZECK, Peter I., J., Department Department of of Geology Geology and M. and and HTJDLESTON, HUDLESTON, Peter and Geophysics, Geophysics,University University of of CZECK, Dyanna Dyanna M. Minnesota, Minnesota, 310 310 Pillsbury Dr. SE, Minneapolis, MN 55455, 55455, dyanna@geolab.geo.umn.edu dyanna@geolab.geo.unm.edu INTRODUCTION The The Superior Superior Province Province consists consistsof of approximately approximatelyeast-west east-west trending trending subprovinces subprovinces defined defined by by lithological lithological differences, & Ciesielski Ciesielski 1986). This This study study differences,metamorphic metamorphic grade, and structural boundaries (Card & concentrates concentrates on on the the boundary between between the the Quetico Quetico metasedimentary metasedimentary subprovince subprovince and and the the Wabigoon Wabigoon metavolcanic subprovince. The Seine River Metasedimentary Group, including the Seine River metavolcanic The Seine River Metasedimentary Group, conglomerates, are distinctive rocks found within a wedge along this boundary. along this conglomerates, are distinctive rocks found within a Structural Structural work along the Quetico Quetico and Wabigoon describes subvertical foliation overprinting overprinting large Hudieston 1991). 1991). These These structures structures indicate a strain large lithological folds (Poulsen 1986, 1986, Tabor & Hudleston history history consisting consisting of an early early recumbent recumbent nappe nappe deformation deformation followed by a more more ubiquitous ubiquitous overprinting overprinting of a subvertical flattening fabric and shear zones attributed to dextral transpression. Strain is pervasive flattening attributed to dextral transpression. throughout throughout the the boundary, boundary, but but also also localized localized into into more more discrete discrete shear zones zones including including two two main, main, connecting connecting shear shear zones zones coincident coincidentwith with the the Rainy Rainy Lake-Seine Lake-Seine River River and Quetico Quetico Faults. These Theselarge large shear shear zones zones are are interconnected interconnected with with smaller smaller shear shear zones zones forming an anastomosing anastomosing pattern. The TheSeine Seine River conglomerates conglomerates are are located located in in aa wedge wedge between between these two shear shear zones zones west west of their intersection. intersection. Poulsen (1986) (1986) interpreted interpreted the the Seine Seine River River conglomerates conglomeratesto to have have formed formed syn-kinematically syn-kinematically during during transpression, transpression, possibly possibly in in structures structuressimilar similarto topull-apart pull-apanbasins. basins. This This preliminary preliminary structural structuralwork work made made itit clear clear that that the the WabigoonWabigoon- Quetico Quetico subprovince subprovince boundary is is not not merely aa discrete discrete suture suturebetween between two two tectonic tectonic microplates, microplates, but is instead instead a complex complex network of shear zones which together accommodated oblique collision. The Thegoal goalof ofthis thisstudy studyisisto todescribe describehow how this complex shear zone geometry is reflected in spatially variable strain magnitudes and structural complex shear zone geometry is reflected in spatially variable magnitudes structural fabrics. fabrics. - STRUCTURAL FABRIC Foliations Foliations in in the the field field show show aa consistent consistent regional regional orientation orientation with strike strike of approximately approximately 080 080 and and subvertical dips. This Thisstrike strikeisisconsistent consistent with with the the dextral dextral transpression model. One Oneexception exceptionto to this this is is in the area of Shoal 045,creating creatingan anoverall overallS-like S-likegeometry geometry Shoal Lake Lake where where the the strike strikechanges changesto toapproximately approximately045, with a map pattern analogous analogous to to a dextral shear sense indicator. The Thesame sameshear shearsense senseisisobserved observedatataa smaller scale within the conglomerate clasts that often form sigma and delta dextral shear smaller scale within the conglomerate clasts that often form sigma and delta dextral shearsense senseindicators indicators on the sub-horizontal sub-horizontalplane. plane. Mineral chlorite and amphiboles. Mineral lineations lineationsin in the field field are are predominantly defined by chlorite amphiboles. General General transpression 1993). However, However, transpression theory predicts either vertical or horizontal lineations (Fossen & Tikoff 1993). lineation orientations orientations in the field field vary remarkably within the foliation plane. Typically, Typically,the theSeine SeineGroup Group lineations are spatially similar on the scale of 0.5-1 km. Therefore, the lineation variations are not Therefore, the lineation variations are not caused caused similar by small-scale-phenomena by some regional regional phenomenon. phenomenon. Rather small-scalephenomena such as clast interaction, but by Rather than than reflecting different different bulk kinematic kinematic conditions conditions along the boundary (such as partitioned triclinic transpression). transpression), the lineation lineation variations variations may reflect reflect anastomosing anastomosing shear zone patterns and variable pressure gradients gradients within within the therock. rock. 11 �Taken together, the finite strain ellipsoid Taken together. foliation and lineation match the variations in the orientation. The Thefoliation foliationpole polealigns alignswith withthe theminimum minimumstretching stretchingdirection, direction,and andthe thelineations lineationsparallel parallel the maximum stretching direction. direction. STRAIN ANALYSIS The Seine River conglomerates are useful strain recorders. recorders. Qualitative observations and quantitative strain analyses indicate heterogeneous heterogeneous strain distributions within the Seine River conglomerates. Analysis Analysis confinns confirmsaaflattening flatteningstyle stylestrain strainthat that could could only only be aa result result of of aa threethreedimensional deformational phenomenon such as transpression rather than the two-dimensional strain strain produced by wrench or thrust movements. Results to date show that the longest axis corresponds to the mineral lineation longest lineation measured . principal in the field and the shortest principal axis corresponds well with the foliation pole. This Thissupports supportsthe theidea idea that structural fabric measured in the field may reoresent the orientation of the finite finite strain mav be used to represent ellipsoid. ellipsoid. The variations in strain ellipsoid orientations orientations and magnitudes could be used to argue argue for multiple multiple deformation events or complex and variable kinematic kinematic boundary boundaryconditions. conditions. However, the geometry of of the anastomosing shear zone pattern can be used to explain the variations in the orientation of the strain ellipsoid and and the the heterogeneous heterogeneous strain magnitudes within the Seine Seine Group Group within a simple dextral ellipsoid transpressional setting. Poles to to foliation foliation 138 Measurements Measurements (equal (equal area) area) Mineral lineations 113 Measurements Measurements (eaual (equal area) Card. K. IC.D. D.& &Ciesielski, Ciesielski, A. A. 1986. 1986. DNAG DNAG Subdivisions Subdivisions of of the the Superior Superior Province Province of the Canadian Card, Geoscience Canada Canada 13, 5-13. Shield. Geoscience 13.5-13. Fossen, H. & Tikoff, B. 1993. 1993. The deformation matrix for simultaneous simple shearing, pure shearing and volume change, and its application to transpression- transtension tectonics. tectonics Journal 15, 413-422. Journal of o fStructural Structural Geology Geoloxv IS. 4 13-422. -. Poulsen. example of of an an Archean Archean Subprovince Subprovince boundary boundary Poulsen, K. H. 1986. 1986. Rainy Lake Wrench Zone: An example in Northwestern Ontario. Tectonic evolution evolution of greenstone belts Technical Technical Report Report (edited (edited Ontario. In: In: Tectonic Ashwal, L. L. D.) D.) 86-10. Houston TX, TX, Lunar and planetary Inst., 177-179. 1. Sc & Ashwal, 177-179. by de Wit, M. J. Tabor. J. J. R. & & Hudleston, P. 1 J.. 1991. Deformation at an an Archean subprovince boundary, boundary, northern Tabor, Minnesota. Canadian Canadian Journal 292-307. Journal of of Earth EarthSciences Sciences 28, 28,292-307. 12 �Preliminary PreliminaryAeromagnetic AeromagneticMap Mapof ofWisconsin Wisconsin David DavidL. L. Daniels, Daniels,Suzanne SuzanneW. W.Nicholson, Nicholson,William WilliamF. F.Cannon, Cannon,U.S. US. Geological GeologicalSurvey. Survey,MS MS 954 954National National Center, E.Bracken, Bracken,U.S. U.S. Geological GeologicalSurvey, Survey,MS MS 964, 964, DFC DFCBox Box 25046, 25046, Center,Reston, Reston,VA VA20192, 20192,Robert RobertE. Denver, Denver,CO CO80225 80225 The The Mineral MineralResources ResourcesProgram Program of of the the U.S. U.S. Geological GeologicalSurvey Survey (USGS) (USGS) has has conducted conducted aeromagnetic the magnetic coverage coverage of of the the aeromagneticsurveys surveys in in Wisconsin Wisconsin during during the past three years to improve the state. state. State Statecoverage coveragewas was completed completed with with aa survey survey carried out in 1998/1999. 199811999. The The new new aeromagnetic aeromagnetic survey survey fills fills the the southern southernthird third of of the the state statenot not covered covered by existing existing good quality surveys (see figure). Right are:north-south north-southflight flightlines linesatat½Vimile mile(800 (800m) m) separation separation draped draped at at 1000 1000ftft (305 (305 m) m) Flightspecifications specificationsare: mean mean terrain terrain clearance. clearance. Right Flightlines linesare areeast-west east-westfor foraapart partof ofsoutheast southeast Wisconsin. Wisconsin. The 4' meeting of the ILSG in 1998 1998 The results resultsof ofthe thefirst firsttwo twoyears yearsofofflying flyingwere wereshown shownatat44th (Daniels and others, 1998). These data have been released since that meeting as paper contour (Daniels and others, 1998). These data have been released since that meeting as paper contourmaps maps (Snyder, (Snyder,1998; 1998;USGS USGS Open-File Open-File Reports Reports 98-431 through 439). Digital Digital data data for for the the 1997/1998 199711998survey survey have have now now been been released released(Open-File (Open-FileReport Report99-28) 99-28)on on CD-ROM CD-ROM (Daniels, (Daniels, Nicholson, Nicholson, and and Cannon, Cannon, 1999). 1999). The accompanying figure shows a shaded image of the aeromagnetic field for current data in The accompanying figure shows a shaded image the aeromagnetic field for current data in Wisconsin years data data are are blocked and labeled. Three Threeaeromagnetic aeromagneticsurveys surveys Wisconsin and and the the areas areas of of the the first first22 years obtained (M.Mudrey, Mudrey, obtainedfrom fromindustry industrysources sourcesby by the theWisconsin WisconsinGeological Geologicaland andNatural NaturalHistory HistorySurvey Survey(M. personal for the the new new survey survey are are still still personal communication, communication, 1997) 1997)are are shown shown in southern Wisconsin. The data for preliminary. Blocks 1-5 and A-D represent the areas of the new survey that will be shown and the will Blocks 1-5 and A-D represent the areas of the new survey contour be available available later later in in 1999. 1999. contour maps maps that that will will be The been to to provide provide higher higher resolution resolution data data The rationale rationale for for continued continued aeromagnetic aeromagnetic surveying has been with with which which to to interpret interpretbasement basementgeology geologyin inWisconsin. Wisconsin.The Theaeromagnetic aeromagneticdata data image image basement basement structures by glacial glacial structures in in detail detail and and give give clues clues to to the the structural structural evolution of the Precambrian crust, covered by and and Paleozoic Paleozoiccover. cover. References References D.L., Nicholson, Nicholson,S.W., S.W., Cannon, Cannon,W.F., W.F., 1999, 1999,Aeromagnetic ~eromahetic surveying in Wisconsin Wisconsin 1997-98: 1997-98: Daniels, D.L., Daniels, surveying Digital Digital data datafiles: files:U.S. U.S. Geological GeologicalSurvey SurveyOpen-File Open-FileReport Report99-28, 99-28, CD-ROM. CD-ROM. Daniels,D.L., D.L., Snyder, Snyder,S.L., S.L., Nicholson, Nicholson,S.W., S.W., Cannon, Cannon,W.F., W.F., 1998, 1998,New New aeromagnetic aeromagnetic surveys surveysin in Daniels, Wisconsin part 1, 1, Wisconsin by by the theU.S. U.S. Geological GeologicalSurvey: Survey:Institute Instituteon on Lake Lake Superior SuperiorGeology Geology Proceedings Proceedings v. 44, part p. 62-63. 62-63. p. Snyder, S.L., 1998, areas: 1998, Aeromagnetic Aeromagnetic map map of of part part of of northwestern Wisconsin and and adjacent areas: U.S. U S . Geological Survey Survey Open-File Open-File Report Report 98-228, 98-228, Scale Scale 1:125,000, 1:125,000,22 sheets. sheets. 13 �Index Index Map Mapof of'Wisconsin Wisconsin Aeromagnetic Data Data 93E 936 92E 926 91E 916 89 9oE cct SEE 87E 46E 46E 45E 45E 44E 43E 43E 92E me. 9oE 90fi 91E 916 ssE 'O'5°20 KM 50 50 0 89E 100 • 14 150 200 KM siE 876 �INVESTIGATION INVESTIGATIONOF OFCaO CaOAT ATTHUNDERBIRD THUNDERBIRDMINE, MINE,MESABI MESABIRANGE: RANGE:MINERAL MINERALAND AND STRATIGRAPHIC STRATIGRAPHICRELATIONSHIPS RELATIONSHIPS DIEDRICH, DIEDRICH,Tamara TamaraR., R.,and andMORTON, MORTON,Penelope, Penelope,Dept. Dept.of ofGeology, Geology,University Universityof ofMinnesota Minnesota Duluth,Duluth, Duluth,MN, MN,55812, 55812, tdiedric@unnd.edu tdiedric@umn.d.eduand andpmorton@unmA.edu pmorton@umn.d.edu Duluth, Mine Minegeologists oeolomstsatatEVTAC's EVTAC'sThunderbird ThunderbirdMine, Mine,aataconite taconitemine mineininEveleth, Eveleth,Minnesota, Minnesota,have havebeen been tracking variation ofofnaturally trackingthe thevariation naturallyoccurring occurringCaO CaOinmiron ironore oreconcentrate concentratefor foratatleast leastnine ninemonths. months. Information Informationabout aboutthe theCaO CaOisishelpful helnfulininformulating formulatino thepartial fluxpellets nelleis that that EVTAC EVTAC produces. produces. The The -the .vartialflux CaO-bearing CaO-bearinephase phasewas wasunidentified, unidentified,but butpresumed presumedto to be be aa carbonate. carbonate. The Theobjective objectiveof ofthis thisstudy studywas wasto to investigate investigatethe thenature natureof ofthis thisCaO CaOininterms termsof: of:1.) I .)The The identification identificationof of its its phase or phases; 2.) Location of major majorsiratigraphic stratigraphiccontributors; contributors;and and3.) 3.)The Theininsitu situtextural texturalrelationship relationshipbetween betweenthe theCaO-bearing CaO-beanng phase(s) phase(s)and andthe themagnetite. magnetite. Manual Manualinterpretation interpretationof ofX-ray X-raydiffraction diffractionresults resultsidentified identifiedthe theCaO-bearing CaO-bearingphase phasein inthe the concentrate concentrateto tobe beankerite. ankerite.Other Otherconstituent constituentminerals mineralsinclude includequartz, quartz.hematite, hematite,siderite, siderite.greenalite, greenalite, stilpnomelane, stilpnomelane,talc, talc,and andgoethite. goethite. Thunderbird ThunderbirdMine Mineexcavates excavatesore orefrom fromthe theBiwabik BiwabikIron IronFormation, Formation,aaPrecambrian Precambrianunit unitcomposed composed of areUpper UpperUpper Upper ofseveral severalhorizons horizonsof ofbanded bandediron ironformation. formation.From Fromtop toptotobottom, bottom,these thesehorizons horizonsare Cherty, Bottom Lower Lower Cherty,Middle MiddleUpper UpperCherty, Cherty,Lower LowerUpper UpperCherty, Cherty, Lower Lower Slaty, Slaty, Top Top Lower Cherty, Cherty,and Bottom Cherty. Cherty.Horizons Horizonsdiffer differininphysical physicalcharacteristics, characteristics,such suchasasthickness thicknessand andtexture, texture,and andchemical chemical composition. areblasted blasted and and composition.When Whencollecting collectingore orefor forthe theinitial initialore oremix, mix,aacombination combinationof ofhorizons horizonsare blended achieveaadesirable desirablemagnetic magneticiron ironconcentration. concentration.Statistical Statisticalanalysis analysisdemonstrated demonstratedaastrong strong blendedtotoachieve positive the amount amount of of positiverelationship relationshipbetween betweenthe thepercentage percentageof ofore orederived derived from from the the Top Top Lower Lower Cherty Cherty and and the ankerite ankeritein inthe theresulting resuitingconcentrate. ~oncen~te. Textural Texturalrelationships relationshiosbetween betweenthe theankerite ankeriteand andmagnetite mametitewere wereobserved observedand andphotographed. photographed. Ankerite ankerite Ankexiteoccurs occursininatatleast leastfour fourdistinct distincthabits, habits,relative relativethe themagnetite: magnetite:1.) 1.)Rhombohedral ~hombohedrd ankeritecrystals crystals are granoblastic,recrystallized recrystallizedquartz, aresurrounded surroundedby bymuch muchsmaller, smaller,granoblastic, quartz,and andminor minormagnetite; magnetite;2.) 2.) FineFinegrained, areintergrown intergrownwith withmagnetite magnetiteof ofapproximately approximatelythe thesame samesize; size;3.) 3.)An An grained,irregular irregularankerite ankeritecrystals, crystals,are ankerite ankeritecrystal crystalcan canbe beentirely entirelyenclosed enclosedwithin withinaasingle singleor orcluster clusterof ofmagnetite magnetitecrystals; crystals;and and4.) 4.)A Asingle single or orcluster clusterof i fankerite ankeritecrystals crystalsencapsulates &apsulates aamagnetite mapetite crystal. crystal. The Theiron ironore oreat atEVTAC EVTACis isprocessed processed by by magnetic mameticseparation. senaration.Examining Examininothe thetextural texturalrelationship relationshipbetween betweenthese thesetwo twominerals mineralscan can produce produce information informationrelevant relevanttotomake makethe theisolation isolationof ofmagnetite magnetitemore moreefficient. efficient. ~~ ---- ~~~ - - - .~ ~ ~ ~ ~ 15 �METAMORPHIC THE CANADIAN SHIELD SHIELD OF OF ONTARIO, METAMORPHIC MAP MAP OF THE ONTARIO, MICHIGAN, MICHIGAN, MINNESOTA MINNESOTA AND AND WISCONSIN WISCONSIN R.M Easton, Ontario R.M. Easton, Ontario OntarioGeological GeologicalSurvey, Survey,933 933Ramsey RamseyLake Lake Road, Road, Sue/bury, Sudbuty, Ontario J'3E 6B5, and R. G. Berrnan. Berman, Geological Geological Survey P3E 6B5, eastonrm@vianet.Ofl.Ca eastonrm@yianet.on.ca and R G. Survey of of Canada, Canada, 601 Booth Booth Street. Street, Ottawa, OE8 607 Ottawa. Ontario Ontario K1A KIA 0E8 The Geological The Geological Survey Surveyof ofCanada's Canada's undertook undertookaaproject projectseveral severalyears yearsago ago to toproduce produce an an updated updated metamorphic map for for the Canadian Canadian Shield Shield (GSC (GSC Map Map 1475A), l475A), which which would wouldalso alsobe be used used to produce aa metamorphic map map of of the the world world (in (in conjunction conjunction with with the for the Geologic M Map World). ap of of the the World). metamorphic the Commission Commission for the Geologic At of the Geological Survey Ontario was was asked askedto to compile compile aa metamorphic metamorphic map map of of At the the request request of the Geological Survey of of Canada, Canada, Ontario the Canadian Canadian Shield Shield in in Ontario. the Ontario. In addition addition to to simply simply compiling this project project with with In compiling metamorphic metamorphic information, information, Ontario Ontarioapproached approached this the goal of the goal of attempting attempting to to integrate integrate metamorphic metamorphic information informationwith withthe thetectonic tectonicsynthesis synthesisproduced produced as a s part part of the Ontario project, with the aim of of developing mineral exploration exploration models. of the Geology Geology o f Ontario project, with the aim developing improved improved mineral models. As As aa of consequence,the the metamorphic metamorphic map map of of Ontario as to to complement complement the existing existing set set of of consequence, Ontario was was designed so as 1: 1.000,000 1.000,000bedrock, bedrock, suficial surficial geology, geology, geophysical, geophysical,and andtectonic tectonicassemblage assemblagemaps mapsand andtime-space time-space charts charts released in in 1991-92 part of of the the Geology Geology o Ontario series. released 1991-92 aas s part f Ontario series. A draft draft of of this this map, map, at roughly roughly 1:2,500,000 1:2,500,000 of scale, will will be scale, be presented presented here. here. Also at of the Geological Survey to produce produce aa Also at the the request request of the Geological Survey of ofCanada, Canada, Ontario Ontario has has undertaken undertaken to preliminary in Wisconsin, Wisconsin, Michigan, preliminary metamorphic metamorphicmap mapof ofthose those portions portions of ofthe theCanadian Canadian Shield Shield exposed exposed in Michigan, and Minnesota. One One of ofthe thereasons reasons for requesting reauestine the involvement of of the the Ontario Ontario Geological Geological Survey Survey in in this compilation, wasso soas as to to best best integrate integrate the metamorphic met-orphic pattern compilation,as asopposed opposed to toother otherorganizations, or&izations, was present in the the shield shield in in the the United United States with that that oresent present across acrossthe theborder borderininOntario. Ontario. In In addition. addition, fast fast present in States with approaching deadlines for completion of deadlines for of the the project project factored factored into into this this decision. decision. Thus. the main focus of this presentation will be preliminary draft Thus, presentation will be on on presentation presentation of a preliminary draft of ofthe the adjoining Ontario, metamorphic map for for the the shield shield rocks rocks in in the the areas areas adjoining Ontario, for forthe thepurposes purposes of of discussion discussion regarding: regarding: the style style of presentation, 1) the ,,,:~ i. . , , 2) the accuracy accuracy of ofthe the data data presented, presented, and 3) solicitation bringing this solicitation of ofpossible possible assistance assistance in bringing this project project to to aa conclusion. concl&on. The metamorphic map of the Shield is being being produced produceddigitally. digitally. In Inthe the case case of Ontario, the Tectonic used as the geologicalbase. geological base. Key aspects of of the project include as the Key digital digitalaspects the project include Tectonic Map Map of ofOntario 0ntariowas wasused digitization of representing different metamorphic facies (grade) and of linework (e.g. dieitization ofpolygons nolveons reoresentine different metamomhic facies (grade) and of linework (e.e. isoerads. . . . - isograds, facies boundary boundary information), information), into a-CAD a CAD program. facies program. In In addition, addition, point-source point-source information information (e.g. (e.g. assemblage data.P-T P-Tdeterminations) determinations) is is beinginput being input into a database. database. The final final map product will willbe bean an assemblage data. attributed map. releasedon onCD-ROM. CD-ROM.containinc containingall allthe thecom~iled compiledinformation. information. The artnbuted attributed map map will will mao. released accompany aa standard standardcoloured, coloured,hard hardcopy copymap, map,which whichwill will mainly mainly display display metamorphic metamorphic facies facies distribution. will be distribution. This Thissame sameapproach approach will be taken taken in compiling compiling the the information information for forWisconsin Wisconsin Michigan Michiganand and ofofCanada, based Minnesota, except that here, the digital digital base base is being being supplied supplied by by the the Geological GeologicalSurvey Survey Canada, based , . on a previously previously released released US geological compilation compilationmap mapof ofthe theregion. region. -- - in the Two Canadian Mineralogist on "Tectonometamorphic Twospecial special issues issues of of The The CanadianMineralogist "~ectonom&&phic Studies Studiesin the Canadian Shield Shield" are are being being produced producedaas partofthis of thisproject. project. The first (vol. (vol. 34, 34, part pan 5, 5, October October 1997), 1997). Canadian s part includes results results obtained obtained from from aa variety variety of of ancillary to the the compilation compilation effort. includes ancillary metamorphic metamorphic studies studies related related to effort The second issue, issue, which which is is in in the the process process of of being being assembled, assembled, includes regional overviews overviewsrelated related to tothe the metamorphic map well as map of of the the Canadian Canadian Shield, Shield, as as well a s additional, additional, detailed detailed metamorphic metamorphic studies. studies. 16 16 �WHERE ARE OREBODIES WHERE ARE THE THE METAMORPHOSED METAMORPHOSED NATURAL NATURAL OREBODIES OF THE MESABI RANGE? OF THE MESABI RANGE? RONALD G. 0. GRABER RONALD GRABER AND AND ALAN ALAN J. J. STRANDLJE STRANDLIE CLIFFS COMPANY,ISHPEMING, ISIIPEMING, MI. ML 49849 49849 C U F F S MINING MINING SERVICES SERVICES COMPANY. (graber@ponup.com) (graber@portup.corn) The nearly nearly century-old century-old debate regarding The regarding the genesis genesis of the hematite-goethite hematite-goethite iron iron ore deposits the Mesabi Mesabi Range Range has has recently recently been been revisited revisitedby byMorey Morey(1999). (1999). The The ore deposits of of the historic debate for the the combined combined oxidation oxidation historic debate centers centers on on the the origin origin of of the the fluids fluids responsible responsible for and leaching of the Biwabik iron-formation to form the deposits — they upward were and leaching of the Biwabik iron-formation to form the deposits moving hydrothermal moving hydrothermal fluids fluids (hypogene) (hypogene)or or downward downward moving movingmeteoric meteoricwaters waters (supergene)? (supergene)? Morey presents conceptual model model for for the the genesis genesis of of the the ores ores that that Morey presents an an interesting interesting conceptual utilizes Early Proterozoic, Proterozoic, driven driven by by uplift utilizes continental-scale continental-scale groundwater groundwater flow flow during during the the Early uplift in fold-thrust belt belt to to the the south south of of the the Mesabi Mesabi Range. Range. This This model model relies relies on on in the the Penokean Penokean fold-thrust the as aa regional regional the Pokegama Pokegama Quartzite, Quartzite, which which underlies underlies the the Biwabik Biwabik iron-formation, iron-formation, as aquifer for for the aquifer the movement movement of of basinal basinal waters waters to to the the northern northern margin margin of of the the Animike Animike basin. basin. A Ma when when loss loss of the A minimum minimum age age for for ore ore genesis genesis in in this this model model is is believed believed to to be be 1650 1650 Ma of the hydraulic gradient for the groundwater flow system occurred due to erosion of the hydraulic gradient for the groundwater flow system recharge area. This ThisEarly EarlyProterozoic Proterozoicage age for for ore ore genesis genesis is significantly significantly older than the Jurassic-Cretaceous over the Jurassic-Cretaceous age age that has been generally, generally, albeit not universally, accepted over last 30 plus plus years years (Sloan, (Sloan, 1964; 1964;Morey. Morey, 1972). 1972). last Constraints Constraints on on the the timing timing of of the the hematite-goethite hematite-goethiteores ores are are few few due due to to the the scarcity scarcity of geologic geologic events events impacting the the Mesabi Mesabi Range Range between between the the Early Early Proterozoic Proterozoic and and the the of Late Cretaceous. ItItisison onthe theeast eastend endof of the theMesabi Mesabi Range Range where where intrusive intrusiveevents eventsand and contact metamorphism associated with the Middle Proterozoic Duluth Duluth Complex provide a potential opportunity to put additional time time constraints constraints on onthe thegenesis genesisof ofthe theores. ores. Hence Hence the title of this paper. IfIf ore oreformation formation occurred occurred prior to the Middle Proterozoic, the opportunity equivalents of of the the soft soft hematite-goethite hematite-goethite opportunity should exist to find metamorphosed equivalents ores. Detailed Detaileddrilling drillingboth bothalong alongthe theiron-formation iron-formation outcrop outcropbelt belt and and downclip downdip as part of exploration to find these metamorphosed metamorphosed equivalents. equivalents. In In exploration for for Cu-Ni Cu-Ni deposits deposits has has failed failed to find these fact the distinct distinct absence absence of of enriched enriched orebodies orebodies of any sort east of the Duluth Complex's post-Middle Proterozoic Proterozoic age age metamorphic aureole aureole has long been utilized as support for aa post-Middle for ore ore genesis. genesis. Alternatively, Alternatively, thinning thinningof of the the Pokegama Pokegamaquartzite quartzitein in this thissame samegeneral general area is cited by Morey Morey to to explain explain the the absence absence of enriched enriched ores ores on on the the East East Mesabi. Mesabi. The The relative relative abrupt abrupt absence absence of of enriched enriched ores ores in in the the Biwabik Biwabik iron-formation iron-formationoccurs occurs within the mining operations Mining Company, Company, managed managed by by aa subsidiary subsidiary of of operations of LTV Steel Mining Cleveland-Cliffs, which has been active 1). Detailed Detaileddata data active for over 40 years (Figure 1). acquired acquired through through both both field field observations observationsand and thousands thousands of of drill drill holes holes provide provideinsight insighton on the controls controls of the the ore-forming ore-forming process. The Thefollowing followingfeatures featuresof of the the geology geology at at LTV LTV Steel Mining Company are are in in conflict conflict with with an an Early Proterozoic Proterozoic genesis genesis of the the hematitehematitegoethite ores: goethite ores: 17 �1. 1. Due to the economic economic significance significance that metamorphism has on the processing of the magnetic taconites, considerable effort is placed on mapping the metamorphic isograds at this operation. Further Further refinement refinement of the original zonation of the Duluth Complex's contact resulted. The Complex's contact metamorphic metamorphic aureole (French, 1968) has resulted. The relationship of metamorphic metamorphic grade with the the eastern eastern limit limit of of iron-formation iron-formation enrichment enrichment relationship stronger than the correlation with either the thickness or distribution of the appears stronger Pokegama Pokegama Quartzite. This Thisisismanifested manifestedby: by: the abrupt termination of enrichment enrichment and oxidation oxidation along along the the Wentworth Wentworth a) the abrupt termination a) structure structure due due to increasing increasing metamorphic metamorphic grade; grade; and, and, b) the b) the thepresence presenceof ofenrichment enrichmenteast eastof ofthe the Siphon Siphon structure structure which which is is the controlling structure for aa dramatic dramatic decrease decrease in in Pokegama Pokegama Quartzite Quartzite thickness. thickness. controlling structure for 2. the Biwabik Biwabik iron-formation in 2. The Aurora sill, sill, aa sizeable sizeable syenitic syenitic body that intrudes the the western portion of the mining operation, appears to have contributed to the localization James and and the the Miller-Mohawk Miller-Mohawk mines. mines. This sill localization of ore formation in the St. James has visual similarities to granophyres associated elsewhere with the Duluth Complex similarities granophyres elsewhere and as such has long been considered Keweenawan (White. 1954). Jf ore genesis in long (White, 1954). If ore genesisin these mines constrain ore mines were Early Early Proterozoic, how could a Keweenawan sill constrain formation? formation? Likewise, Likewise,no nometamorphic metamorphic overprint overprint has has been reported from these % orebodies. Definitive Definitiveage agedating dating though though is is lacking lacking for for this intrusive. ri 3. AAlarge largenumber numberofofthe theenriched enrichedorebodies orebodiesat at the the LTV LTV Steel Steel Mining Company operation are very shallow. Drilling of a Drilling has has been been conducted conducted through the bottom of number of the natural ore magnetic taconite taconite ore pits successfully successfully finding fresh, unaltered magnetic reserves below. The Thelack lackof of alteration alteration between between the the Pokegama Pokegama Quartzite Quartzite and the orebodies orebodies is is problematical problematical for for ascending ascendinggroundwater groundwaterflow flowmodels. models. French, B.M. 1968, of the the Biwabik Biwabik iron-formation, iron-formation, 1968, Progressive Progressive contact metamorphism of Mesabi Range, Minnesota, p.103. Minnesota, Minnesota Minnesota Geological Survey Bulletin 45, p. 103. Morey. Morey, eds.. eds., Geology Geology of of Morey, G.B., G.B., 1972, 1972, Mesabi Range; Range; in P.K. Sims Sims and G.B. Morey, Minnesota: Minnesota: A A Centennial Centennial Volume, Volume, Minnesota MinnesotaGeological GeologicalSurvey. Survey,pp. pp. 204-217. 204-217. Morey, Mesabi Range, Range, MinnesotaMinnesotaMorey, G.B., G.B., 1999, 1999, High-grade High-grade iron iron ore ore deposits deposits of of the the Mesabi Product of a continental-scale ground-water system. Econ. Geology, vol. 94, pp 133-142. continental-scale ground-water system, Geological Survey Sloan, R.E., R.E., 1964, 1964, The Cretaceous system in Minnesota: Minnesota Geological 5 . 64p. 64~. Report of Investigations Investigations 5, White, D.A., White, D.A., 1954, 1954,The The stratigraphy stratigraphyand and structure structureof of the the Mesabi Mesabi Range, Range, Minnesota: Minnesota: Minnesota 92p. Minnesota Geological Geological Survey Survey Bulletin Bulletin 38, 38,92p. 18 �Figure 1. Map of of LTV Steel Steel Mining Mining Company Company Figure 1. Geologic Geologic Map 19 �Cross Cross Margin Margin Transport Transport in in Lake Lake Superior Superior GREEN, GREEN,Sarah Sarah A. A. and and BUDD, BUDD, Judith Judith W., W., Dept. Dept. of of Chemistry Chemistry and Dept. of Geological Geological Engineering Engineeringand andSciences, Sciences,Michigan MichiganTechnological TechnologicalUniversity, University, Houghton, Houghton,MI MI49931 4993 1 The The Keweenaw Keweenaw Interdisciplinary InterdisciplinaryTransport Transport Experiment Experiment in Superior Superior (KITES) (KITES) is focused focused on on cross-margin cross-margintransport transportprocesses processesalong alongthe thewestern westernshore shoreof ofLake LakeSuperior's Superior's Keweenaw Keweenaw Peninsula. Peninsula. The Thedominant dominantfeature featureofofthis thisregion regionisisthe theshore-parallel shore-parallel Keweenaw Keweenaw Current, Current,which whichflows flowsto tothe thenortheast northeastand andisisstongest stongestnear nearEagle EagleHarbor. Harbor. Water, Water, sediment, sediment,and andhydrographic hydrographicdata dataare arebeing beingcorrelated correlatedwith with satellite satelliteimagery imageryto to obtain obtain an an integrated integrated view view of of transport transport of of suspended suspended sediments, sediments, nutrients, and organic organic material material near near and and across acrossthe thecurrent. current. Time show cross cross margin margin transport Time series series SeaWiFS SeaWiFS images from May to September, 1998 show events in Lake Superior. These chlorophyll and turbidity maps reveal a highly events in Lake Superior. These chlorophyll and turbidity maps reveal a highly productive productive southern of the the Keweenaw. southern corridor corridor from Duluth Harbor to the tip of Keweenaw. Distinctive Distinctivesediment sediment and and chlorophyll chlorophyll plumes plumes can can be be seen seenoffshore offshorewest west of of the the Ontonagon Ontonagon River Riverand andnear nearthe the tip of the the Keweenaw Keweenaw Peninsula. Movement Movementofofthe theplumes plumesoffshore offshorewas washighly highlyvariable variable depending dependingupon upon the thedirection directionofofprevailing prevailingwinds windsprior priortotothe thesatellite satelliteband bandpass. pass. Simultaneously Simultaneouslyacquired acquiredSeaWiPS SeaWiFS(Sea-viewing (Sea-viewing Wide Wide Field-of-View Sensor) Sensor) and AVHRR 23,1998 1998were were AVHRR(Advanced (AdvancedVery VeryHigh HighResolution ResolutionRadiometer) Radiometer)images imagesfrom fromMay May23, used used to to verify verify cross cross margin margin transport transport events events in in Lake Lake Superior. Superior. The TheAVHRR AVHRRlake lakesurface surface temperature temperatureimage imagereveals reveals aa dramatic dramatic northward northward flowing flowing offshore offshore eddy eddycurrent currentat atthe thetip tip of of the the Keweenaw Keweenaw Peninsula. Peninsula. AASeaWiFS SeaWiFSturbidity turbidity map mapacquired acquiredwithin withinhours hoursof ofthe the AVHRR AVHRR SST SSTimage imagesuggests suggeststhat that materials materials were weretransported transportedoffshore offshorenorth northof ofEagle Eagle Harbor, Harbor, providing providingevidence evidenceof of crosscross- margin margin transport. transport. These Thesepatterns patternswere werenot notpresent presentin in AVHRR 23rdevent event(on (onMay May20 20 AVHRR and and SeaWiFS SeaWiFSimages imagesobtained obtainedbefore beforeor orafter afterthe theMay May23rd and and May May 27). 27). Sediment Sedimentpatterns, patterns,in-water in-wateroptical opticalmeasurements, measurements,and andphysical physicalmodels modelsare arehelping helping clarify in the the KITES KITESstudy study region. region. clarify transport transport mechanisms mechanismsand andpathways pathwaysin 20 �Megascopic Fossils and and Their Megaswpic Fossils Their Possible Possible Contribution Contribution to to the theDevelopment Development of of the theSilicate SilicateUnit Unit of Iron-Formation, Empire of the the Negaunee Negaunee Iron-Formation, Empire Mine, Mine, Marquette MarquetteRange, Range, North NorthMichigan Michigan T. M. Han, Senior Research Scientist (Emeritus), Cleveland-Cliffs Inc. Abstract The fossils discovered discoveredin in the the Early Early Proterozoic Negaunee Iron-Formation Iron-Formation are The megascopic megascopic fossils Proterozoic Negaunee are the the world's have been been reported reported as as eukaryotic eukaryotic algae algae similar similar to to world's oldest oldest known known fossils. fossils. These These fossils fossils have Grypania may have have played Grypania spiralis spiralis (Han (Han and and Runnegar Runnegar 1992). 1992). Their Their existence existence may played a a key key role role in in the the development of of the the thin thin banded banded "silicate unit" unit" of the the Negaunee Negaunee Iron-Formation Iron-Formation at at the the Empire Empire Mine. Mine. This of magnetite-rich magnetite-rich layers layers alternating alternating with layers having This unit is is composed wmposed of having a mineral mineralassemblage assemblage of of chert, chert, siderite, siderite, ankerite, ankerite,minnesotaite ininnesotaiteand and stilpnomelane stilpnomelane in various various proportions. proportions. Two fossil types typesare are recognized. recognized. One filaments that that are long One type type is is fine fine filaments long and and uniform uniform in in width width Two fossil and "springs". The The other other type type is is short, short, coarse, coarse, in inwhole whole and andsectional sectional forms, forms, and and and coiled wiled like like "springs". resembles "coiled garden garden worms". worms". These were apparently apparently transported transported by resembles "coiled Thesefossils fossils were by direct direct current current and and deposited at at distances from their origin. Based on the relationship between the current marks distances between the marks and and the orientation coils on on the the outcrops outcrops measured, measured, the direction orientation of of the the fossil fossil patches patches and and oval-shaped oval-shaped coils of the paleocurrent where Archean rocks paleocurrent at at the the time timeof of sedimentation sedimentation was from from the the southwest southwest where rocks are are distributed. distributed. • 'a-".: z"& ,. Fossil remains resembling resembhg "coiled "coiledgarden garden worms" worms" 21 �An An induced induced oxidation oxidation study study reveals reveals that that much much of of the themagnetite magnetitewas wasdeveloped developed and andenriched enriched through Fe+ nuclei. These These laths laths are are randomly randomly arranged, arranged, through Fe+ + diffusion, diffusion, using using lath-shaped lath-shaped hematite hematite as as nuclei. either scattered, either scattered, or or in in irregular irregularclusters clustersand andexisted existedbefore beforethe themagnetite magnetitedevelopment. development. The gangue layers are believed to be derived from a highly sfficeous gel with with carbonaceous carbonaceous matter The gangue layers are believed to be derived from a highly siliceous gel matter and other and replaced and other impurities. impurities. They Theywere weresubsequently subsequently recrystallized recrystallized and replaced by by siderite, siderite, ankerite, ankerite, minnesotaite and and stilpnomelane and low-grade regional metamorphism. minnesotaite stilpnomelane during during diagenesis diagenesis and low-grade regional metamorphism. + --f -. Magn eliteand and magnetite magn elitegranule granule layers, white; Microstructrure of Magnetite white; 250x Micmstructrure of aa magnetite magnetite layer. layer. 250x gangue layers, dark. Magnetite, gray; gray; hematite grayish white dark. and gangue, gangue, black. black. Magnetite, hematite grayish white and According to LaBarge LaBarge et et al is reasonable According to a1(1987), (1989, it it is reasonable to to assume assume that that the theheavily heavilypopulated populated megascopic eukaryotic eukaryotic algae algae were were probably probably one the principal megascopic one of of the principal oxygen oxygen suppliers suppliers for for precipitating precipitating Ee+ + ± as as Fe(OH)3. Fe(OH)3. The Theclusters clustersof ofthe thelath-shaped lath-shapedmagnetite magnetite pseudorph pseudorph in in magnetite magnetitelaininae laminae and and Fe+ granules hematite . The effect of granules probably probably represent represent dehydrated dehydratedand andcrystallized crystallized Fe(OH)3 Fe(OH)3 flocculi, flocculi, hematite flocculation, if any, any, would flocculation. if would cause cause the the segregation segregation between between the the siliceous siliceous gel gel and and the theEe(OH)3. Fe(Om3. Furthermore, the gravity of of the theFe(OH)3 Fe(Om3 is is significantly significantly greater greater than thanthe thesificeous siliceousgel. gel. the specific specific gravity Consequently,the the interbanded interbanded structure structure of of the unit is Consequently, the lithological lithologic111 unit is believed believed to to be be deposited deposited simultaneously through processes of selective flocculation and and differential differential settling between simultaneously through processes of selective flocculation between the Fe(OH)3 and siliceous gelunder under aa rhymatic rhymatic and and cyclic cyclic manner maimer regulated regulated by by the the life life and and death Fe(OH)3 and siliceous gel death of of the organism, i.e. the off-season is aa period period of growing season season is aa period period of of deposition deposition while the off-season is the i.e. the the growing solution. Furthermore, Furthermore, the solution. the carbonaceous carbonaceous matter associated associated with the precipitates precipitates may may generate generate Fe+ ++during duringdiagenesis diagenesisand andmetamorphism metamorphismfor forthe thedevelopment development of of various various types types of hon iron minerals, minerals, particularly, particularly, the the development development and and enrichment enrichment of of magnetite. magnetite. . References ,. , . 1. Han, T. M. 1. M. and andRunnegar, Runnegar,B. B.(1992) (1%) Megascopic MegascopicEukaryotic EukaryotiiAlgae &ae from fromthe the2.1 2.1 Billion-YearBillion-YearAmerican Association for the Advancement Oldd Negaunee Iron-Formation, Iron-Formation.Michigan. Michigan. American Association for the Advancement of Science, science, Vol. Vol. 237, 237, pp. pp..232-235 .232-235 2. 2. Han, T. M. M. (1978) (1978) Microstructures Microstructuresof ofMaguetite Magnetite as as Guides Guides to to Its Its Origin Origin in in Some Some Precambrian Precambrian Iron-Formations. Fortschr.Miner. Miner.Vol. Vol.56 56pp. on.105-142 105-142 Iron-Formations. Fortschr. (1988) Origin of Magnetite Magnetite in inPrecambrian Precambrian Iron-Formations Iron-Formations of Low Low Metamorphic Metamorphic Grade Grade (1988) Proceedings of the Seventh Symposium. pp.641-656 Seventh Quadrennial JAGOD IAGOD Symposium. pp.641-656 3. Laflerge, LaBerge, C. G. L., L., Robbins, Robbins, E. E. I., I.,and andHan, Han,T.T.M. M.(1987) (1987)AAModel Modelfor forthe theBiological Biological Precipitation Precambrian Iron-Formations Evidence. Precambrian Precambrian Iron of Precambrian Iron-Formations- A: GeologicalEvidence. IronFormations. Formatt.ons. Publication, SA. Athens, Greece. Editors: Peter and Gene Gene LaBarge. LaBarge. Theophrastus Publication, SA. Athens, Peter Appel Appel and pp.69-96 pp. 69-96 ~ - 22 �COMPARISON OF O F MICA Ar/Ar ArIAr AND AND Rb/Sr RblSr THERMOCHRONOLOGY THERMOCHRONOLOGY RESULTS RESULTS FROM NORTHERN WISCONSIN AND NORTHERN MICHIGAN WISCONSIN AND Daniel HoIm, Holm, Denise Romano, Romano, and andCraig CraigManeuso, Mancuso,Dept. Dept.ofofGeology, Geology,Kent KentState StateUniversity, University, Kent, OH 44242; Ken Foland, Dept. of Geological Sciences, The Ohio State Poland, Dept. of Geological Sciences, The Ohio State University, University, Columbus, OH OH 43210. 43210. Columbus, Thermochronologic data from from the the southern Lake Superior Thermochronologic data Superior region has historically historically been been dominated by the RblSr Rb/Sr method method on biotite (Fig. (Fig. 1; Peterman Peterman and and Sims, Sims. 1988, 1988, ~&Q&J. Tectonics). We We summarize the results of 45 45 new new mica mica Ar/Ar ArlAr age age dates dates from from northern northern Wisconsin Wisconsin and and northern northern Michigan (Fig. Rb/Sr data. data. Because (Fig. 2) and compare these with the RbISr Because the Rb/Sr RbISr and K/Ar systems in biotite thissummary summaryprovides providesan an biotite are are known known to to have have similar similarclosure closuretemperatures temperatures(300°±50°C), (300¡±50°this excellent opportunity opportunity to to compare how susceptible these these systems systems are are to to partial partial or or complete complete excellent resetting. Northwest to 1730 M ML Northwest Wisconsin. Wisconsin. Rb/Sr RbISrbiotite biotitedates datesfrom fromthis thisregion region vary vary from from 1309 1309 to a In In contrast, Ar/Ar ArIAr mica mica dates dates fall fall tightly tightlyinto intotwo twogroups, groups,an anolder older—1755 -1755 Ma group and a younger —1600Ma Magroup. group. The two groups of Ar/Ar -1600 Ar/Ar dates dates are are separated separated by by aa sharp sharppost-Penokean post-Penokean deforinational front in the overlying defonnationallthennal defonnational front overlying Early Proterozoic quartzites. The ThedeformationalJthermal in the front here probably probably represents represents the northern northern limit of of Mazatzal Mazatzal foreland foreland deformation deformation in southern Lake Superior Superior region. Northern 1630-1650 Ma ArIArchrontours, chrontours,which whichcoincide coincidein in Northern Michigan. Michigan.The The 1630-1650 MaRb/Sr RbISrand andAr/Ar northwest Wisconsin, diverge diverge eastward eastward into into northern northern Michigan. Michigan. Given northwest Wisconsin, Given the the absence absence of of postpostPenokean quartzites quartzites in in this region, region, we suggest that the Ar/Ar proxy for for Penokean ArIAr ages can serve as aa proxy determining the northern limit of Mazatzal deformation. We suggest that the Republic area determining the northern limit of Mazatzal deformation. area escaped significant Mazatzal related deformation deformation and higher-temperature higher-temperature metamorphic affects but did undergo some form of lower-temperature, possibly hydrothermal, metamorphism metamorphism at undergo some form of lower-temperature, possibly hydrothermal, —1630 1630 Ma. - Northeast Wisconsin. largearea areaofof<1200 el200Ma MaRb/Sr Rb/Srbiotite biotitedates dates(the (theGoodman GoodmanSwell) Swell) Northeast Wisconsin. AAlarge north of the Wolf River batholith has been been interpreted as an uplifted flexural bulge created by rapid loading along Fig. 1). In In contrast, contrast, along the mid-continent rift axis to the north (dashed ellipse of Fig. Ar/Ar dates mica ArIAr dates from the the same same region region define define aa much much smaller smaller region region of <1200 c1200 Ma Ma dates dates (dashed (dashed ellipse of Fig. 2). An isolated 1170 Ma Ar/Ar biotite date has also been obtained from basement An isolated 1170 Ma ArlAr biotite date has also swell. We over 100 100 km southwest of the swell. Weinterpret interpretthe the <1200 c1200 Ma dates for both systems to reflect shallow intrusion-related resetting. The larger locus of The larger locus of anomalously anomalously young young Rb/Sr RbISr dates dates probably reflects reflects the the fact fact that that the the Rb/Sr RWSrsystem systemisismore moresusceptible susceptibleto tolower-temperature lower-temperatureresetting resettingthan thanthe the Ar/Ar system. This Thisinterpretation interpretationalso alsoprovides providesan anexplanation explanation for for the the problematic problematic Rb/Sr RbISr and and AdAr Ar/Ar dates that fall between between 1200 and and 1400 Ma. Ma. Because AdAr Because much of the 1470 1470 Ma Wolf River batholith and associated intruded into into shallow shallow crust and batholith associated plutons (Wausau (Wausau Syenite complex) complex) intruded cooled through Ma (Fig. (Fig. 1 and 2). 2), it is likely through 300°C 300OCby by —1400 -1400 Ma likely that that the the surrounding surroundingPrecambrian Precambrian country rock to the north and west was also shallow. We We suggest, suggest, therefore, therefore, that the 1200-1400 1200-1400 Ma country rock mineral dates for for both systems systems indicate indicate incomplete mineral resetting related to shallow Keweenawan intrusions. 23 �Fig. Fig.1 1Rb/Sr RbISrBIOTITE BIOTITEAGE AGEDATA DATA Republic iis 4'— KEWEENAWAN 4 - ,,____,__,__ S'S.. — - — 1686. Peavy -1018=-— W:.1537+4+!.&ioa.thJ. ifs8l c*rtJL:i 4. +.ff rj¼l63:.llss.. '..5%& 1l03j.j..j tmbeau quartz'te 1249, r ' tfQightly foldedrL!:.', .'i'!! 1516 'us-i 620 ti's 1354S 1739 IZ3Sbj 1354 1360 1384"74// a, g 408 4/ .:.LLJ.:L1.J.J.:.Lth1455 'IV. ::15e2 1385 Edge of Late Proterozol Phanerozoic cover' After Peterman and Sims (1988) but excluding data of Aldrich et al. (1965) • Biotite Fig. 2 Ar/Ar MICA AGE DATA <4 ff011 '111 4', % — —.-- KEWEENAWAN "p ,',',',','.\.'.','l 696-,' 0 MuscovIte ___ ept I R7A "'%"' 10,natIonah11ft0i , , '..'' - IA! — tTrrrrrrrw1576;n.:.E.:n cm067'r*101 r.trhTrTnT.Tr:!Tr:,Tr:tT:-r-'.r' - 1358t -. Qua 1605-— _._.._j_ _,ø.__; -- -— r,-n11366rrtt1456 -'--1 HFlambeau quarizute :1-1372t:h-htr t1753I'r!." — 4 (tughtlyfolded) 1581 - S LJ.:1461 -! - 1A7 1616 415 Edge of Late Proti Phanerozoic covei Compiled Compiledfrom fromRomano Romano(1999, (1999.Kent KentState StateM.S. M.S.thesis), thesis),Mancuso Mancuso(1999. (1099.Kent Kent State CJES), StateM.S. M.S.thesis), thesis).Hoim Holmand andLux Lux(1998, (1998. CJES).Hoim Holmetelal. al.(1996, (1998.GSAA). GSAA),and and . Schneideretetal. al.(1996, (1996,CJES). CJES). Schneider 24 24 �ISHPEMING ISHPEMING GREENSTONE GREENSTONE BELT BELT --EVIDENCE EVIDENCEFOR FOR ARCHEAN ARCHEANTECTONIC TECTONIC EVOLUTION OF THE SOUTHERN EDGE OF THE SUPERIOR EVOLUTION OF THE SOUTHERN EDGE OF THE SUPERIOR PROVINCE PROVINCE IN IN MICHIGAN MICHIGAN JOHNSON, JOHNSON,Rodney RodneyC., C., Rod Rod Johnson Johnson& &Associates, Associates,Inc., Inc., Negaunee, Negaunee, MI MI 49866, 49866, MinerDoc@aol.com; MinerDoc@aol.com;and andBORNHORST, BORNHORST, Theodore Theodore J., J., Department Department of of Geological Geological Engineering Engineeringand and Sciences, Sciences,Michigan MichiganTechnological Technological University, University, Houghton, Houghton, MI MI 49931, 4993 1, tjbomho@mtu.edu tjbomho@mtu.edu The The Ishpeming Ishpeminggreenstone greenstonebelt belt is is the the southernmost southernmostgreenstone greenstone belt of of the the Superior Superior Province. The southern boundary of the Superior Province and the Ishpeming Province. The southern boundary of the Superior Province and the Ishpeming greenstone greenstone belt zone. Late k m long long Great Great Lakes Lakes tectonic tectonic zone. LateArchean Archean greenstone greenstone and i d belt is is the the 1,000 1,000km granite graniteterrane terraneto tothe thenorth north(Ishpeming (Ishpeminggreenstone greenstone belt) belt) are are juxtaposed against against Early Early to to Middle migmatite-gneiss-amphiboliteand and Late Late Archean Archean granite granite terrane terrane MiddleArchean Archeanmigmatite-gneiss-amphibolite (southern (southern complex). complex). The TheIshpeming Ishpeminggreenstone greenstonebelt belt consists consistsof of subaqueous subaqueous tholeiitic tholeiitic basalt thickness. New calc-alkalic rhyolite, rhyolite, 10-15 10-15km in total thickness. basalt to to calc-alkalic New U-Pb U-Pb zircon zircon data data establish establishthe the age ageof of volcanism volcanism at at 2705.8+/-1.6 2705.8+/-1.6 Ma. (Table (Table 1) Recent Recent field field studies studiesdefine define aa sequence sequenceof of multiple multipledeformation deformationand and intrusive intrusiveevents events following followingvolcanism. volcanism. The The volcanic volcanic rocks rockswere wereenveloped envelopedby by aatonalite tonalitesuite suite just before before and and during during the the northnorthdirected directed thrusting thrustingand and nappe nappe formation. formation. This Thiswas wasfollowed followedby bydevelopment developmentof ofstrike-slip strike-slip shear zones and the deposition of clastic sediments into pull-apart basins. A second second shear zones and the deposition pull-apart basins. phase suite intruded the belt phase of of folding folding produced upright folds. A Atrondhjemite trondhjemiteto to granite suite and and deformed deformedearlier earlierfabrics. fabrics. The Thetiming timingof ofdeformation deformationisisconstrained constrainedwith withnew newU-Pb U-Pb zircon zircon data data yielding yielding an an age age of 2668.4÷2.l/-l.8 2668.4+2.1/-1.8 Ma Mafor foran anintrusion intrusionin inthis thissuite. suite. Intrusions Intrusionsof of hornblendite homblenditeto to syenite syenite (appinite (appinite suite) suite) and and continued movement along along shear shear zones zones represent represent the the end end of of deformation. deformation.An Anundeformed undeformedpost-tectonic post-tectonicgranite graniteintrusion intrusion has has an an age ageof of 2585 2585(Sims (Simsand andPeterman, Peteman, 1992). 1992). This are readily readily interpreted in a This sequence sequenceof of deformation deformationmagmatic/volcanic magmatic/volcanicevents events are modern modem plate plate tectonic tectonic context. context. The Thevolcanic volcanicarc arcrocks rocksare arethe theresult resultof of north-directed north-directed subduction. Tonalite-suite plutons intruded the arc just prior to collision subduction. Tonalite-suite plutons intruded the arc just prior to collisionof ofaasmall small continent from the south (southern complex) along the Great Lakes tectonic continent from the south (southern complex) along the Great Lakes tectoniczone zone (suture). (suture). Collision Collisionoccurred occurredover overan anextended extendedperiod periodof oftime timewith withmultiple multipledeformation deformation and and magmatic magmatic events. events. The Theappinite appinitesuite suiteisischaracteristic characteristicof ofmodern modemcollision-related collision-related magmatism. magmatism. The Thefield-based field-basedsequence sequenceof ofevents eventsand andnew new age agedates datesare areimportant importantdata datain in continuing continuing to to unravel unravel the the tectonic tectonic evolution evolution of the southern southern edge of the Superior Superior Province. sProvince. References: Sims, Sims, P.K. P.K. and and Peterman, Peterman, Z., Z., 1992, 1992, Guide Guideto tothe thegeology geologyof of the theGreat GreatLakes Lakestectonic tectonic paleosuture: Institute zone zone in in the the Marquette Marquettearea, area,Michigan Michigan -- A A late late Archean Archean paleosuture: Instituteon on Lake LakeSuperior SuperiorGeology GeologyProceedings, Proceedings,38th 38"' Annual Annual Meeting, Meeting, Hurley, Hurley, WI, WI, v. v. 38. 38, part part 2, 2, p. p. 105-135. 105-135. 25 �1 Table 1. and constraining constraining age age dates dates for for the the Table 1.Summary Summaryof ofdeformation deformation sequence, sequence, volcanic/magmatic volcanic/magmatic activity activity and Jshepming greenstone Ishepming greenstone belt. belt. Event Characteristic ., VolcanicfMagmatic Vnlcanic~MaematicActivity Activity Age Dates Undeformed Undeformed post-tectonic post-tectonic granite granite 2585 Ma Kink bands Kink bands with with north-easterly north-easterly striking kink kink planes planes and striking and steeply steeply plunging hinges :,'',,->$ '.f03'] D5 Ds Dip slip motion along shear zones. U4 D4 Folding associated associated with Folding with intrusion of plutons. D3 D3 Upright folding. Appinite suite Di U2 Strike-slip shearing. Trondjhemite-granite suite Tmndjhemite-granite suite U1 Di Recumbent folding. Recumbent folding. Tonalite suite Tonalite suite : yw; .8fel. Extrusion of mafic ma ic and an felsic sic volcanic rocks. 26 2668.4+2.11-1.8 2668.4+2.1/- 1.8 Ma Ma ,". ,,. , . . , : ,. :* : : 2705.8+/-1.6 Ma ; , , b �A NEW APPROACH APPROACH TO TO HISTORICAL HISTORICAL RECONSTRUCTION: COMBINING DESCRIPTIVE AND DESCRIPTIVE AND EXPERIMENTAL EXPERIMENTAL PALEOLIMNOLOGY W. W. Charles CharlesKerfoot, Kerfoot,Lake LakeSuperior SuperiorEcosystem EcosystemResearch Research Center Center and and Department Department of of Biological BiologicalSciences, Sciences,Michigan MichiganTechnological TechnologicalUniversity, University, Houghton, Houghton, MI MI 49931 4993 1 John John A. A. Robbins, Robbins,NOAA NOAAGreat GreatLakes LakesEnvironmental EnvironmentalResearch Research Laboratory, Laboratory, 2205 2205 Commonwealth CommonwealthBlvd., Blvd., Ann Ann Arbor, Arbor,MI MI 48105 48105 Lawrence LawrenceJ. 1.Welder, Weider,Max-Planck-Institut Max-Planck-Institutfür f i rLimnologie, Limnologie,Postfach Postfach165, 165,D-24302 D-24302PlOn, PlOn, Germany* Germany* Here Here we we introduce introduce aa combined combined experimental experimental and and descriptive descriptive approach (termed "Resurrection Ecology") Ecology") to toreconstructing reconstructinghistorical historicalperturbations, perturbations,pointing pointing out out how how direct direct tests tests with with sediments sediments and and hatched hatched resting resting eggs eggs complement complement the traditional descriptive calculation of microfossil microfossil fluxes. fluxes. This Thisapproach approachisisbeing beingstudied studiedin in both both NSFINOAA NSF/NOAA Cooperative sites, the KITES Project in in Lake LakeMichigan. Michigan. Additional core KITES Project in Lake Superior Superior and the EGLEE Project studies studies involve involveBaltic Baltic German German lakes lakes and and the Caspian Sea. In the the Keweenaw Keweenaw Waterway, Waterway, aa freshwater freshwater estuary estuary off Lake Superior, turn-of-the-century copper coppermining mining impacted impactedthe the resident residentbiota. biota. Remain Remain fluxes fluxes document document that that diatom, diatom, rhizopod. rhizopod, and production all declined declined during stamp sand discharges, but recovered rapidly and Bosmina production after moving above background levels due to developing developing eutrophication. eutrophication. In addition addition after WWLI, WWII, moving to biogenic biogenic silica, silica, we we discovered discovered that that bromine bromine flux holds promise as an indicator of diatom production production and and confirmed confirmed that that this this element element is present in several genera. Fluxes of Daphnia resting eggs also the 1940's. 1940's, dominated by by aa hybrid hybrid apparently apparently also increased dramatically since the produced produced from from crosses crosses between offshore offshore and interior interior Waterway species, after channeling promoted promoted greater greater mixing mixing of of water water masses. masses. Toxicity Toxicity studies studieswith with sediments sediments and and Daphnia Daphnia clones clones directly directly tested tested recovery recovery of environments environments after concentrations and after cessation cessation of of mining mining activities. activities. The The studies studies document document that increased concentrations fluxes fluxes of of copper copper in in the the Waterway Waterway during during mining mining discharges discharges were toxic to invertebrates. Once stamp stamp sand sand discharges discharges ceased, ceased, the the biota biota recovered recovered rapidly due to a combination of decreased copper copper cycling cycling and and organic organiccomplexation. complexation. Although Although sedimentation sedimentation has returned to nearbackground background conditions conditionsand and surficial surficial sediments sediments in much of Portage Lake are no longer toxic, eutrophication eutrophicationand and faunal faunal exchange exchange with with Lake Lake Superior Superior make it unlikely that the original zooplankton zooplankton community communitycomposition composition will will return return to to the Waterway system. 27 �THE IRON IRON RIVER RIVER SYNCLINE: AN AN ALLOCHTHONOUS ALLOCHTHONOUSSTRUCTURAL STRUCTURALPANEL PANELIN INTHE THE PENOKEAN PENOKEANFORELAND FORELANDOF OF NORTHERN NORTHERN MICHIGAN MICHIGAN KLASNER, John S., Survey Western S., Department Department of of Geology Geology and U. S. Geological Survey, Western Illinois Illinois University, University. Macomb, Macomb, IL IL61455, 61455. jklasner@macomb.com; jklasner@macomb.com; CANNON, CANNON, William William F. F. and and SCHULZ, SCHULZ, Klaus Klaus J., U. U. S. Geological Geological Survey, Survey, National NationalCenter, Center, MS. MS. 954, 954, Reston, Reston, VA VA 20192; 20192; and and LABERGE, S. Geological Geological Survey, Survey, University Universityof of LABERGE, Gene Gene L., L., Department Department of of Geology Geology and and U. U. S. Wisconsin 54901 Oshkosh, Oshkosh, Oshkosh,WI Wl54901 Wisconsin -- Oshkosh, The The Iron IronRiver Riversyncline syncline(James (Jamesand andothers, others, 1968) 1968)lies liesnear nearthe the southern southernmargin marginof ofthe the Penokean strata of of the Penokeanfold fold and and thrust thrust belt beltin innorthern northernMichigan. Michigan. It It contains sedimentary strata the Paint Paint River River Group Group and and underlying Badwater BadwaterGreenstone. Greenstone. The Thesyncline synclinehas has aa triangular triangular map map pattern pattern and and is is flanked flankedby by strata strataof ofthe theMichigamme MichigammeFormation Formationon onits itsnorth northand andeast eastsides sidesand andby by intensely comolex. intensely deformed deformed Early Earlv Proterozoic Proterozoic strata on its southwest side. An An unusually unusuallvcomplex, multiply-folded structure within the multiply~folded structurewithin the syncline synclinehas hasbeen beenrecognized recognizedsince since the the detailed detailedstudies studiesby by James James and and others others (1965). (1968). Traditional Traditionalstratigraphic stratigraphicinterpretation interpretationhas hasbeen beenthat that the the strata stratain inthe the syncline syncline are are stratigraphically stratigraphicallyabove abovethe the Baraga BaragaGroup Groupand and that that the the Badwater BadwaterGreenstone Greenstoneand and Paint Paint River River Group Group are are the the youngest youngest parts parts of the Marquette Marquette Range Supergroup. More Morerecently, recently, an an allochthonous allochthonousorigin originfor forthe thesyncline syncline has hasbeen beenadvocated advocated(Sims, (Sims, 1996) 1996)and andthe the exact exact correlation correlationof of units unitsin inthe thesyncline synclinewith with autochthonous autochthonousunits unitselsewhere elsewhere in inthe the foreland foreland is is uncertain. uncertain. Our Ourrecent recentre-examination re-examinationof of the the region region lends lends support to tothe theview view that that the the syncline syncline is is an anallochthon allochthonemplaced emplacedby bygenerally generallynorthward-directed northward-directed thrusting. A A thrust thrust of of considerable considerable magnitude This thrust thrust forms forms the the magnitude is is interpreted interpretedto to lie lie at at the the base base of the Badwater Greenstone. This base base of of the the allocthon allocthonand andseparates separatescomplexly complexlydeformed deformedstrata strataabove abovefrom from more moresimply simply deformed deformed strata strata below. below. Several Severalsets setsof ofstructural structuralobservations observationsargue arguefor for the the allochthonous allochthonous nature nature of of the the Iron IronRiver Riversyncline, syncline,the theprincipal principalargument argumentbeing beingthat that the the syncline synclinehas hasaafolding folding history history and and fold fold geometry geometrymore morecomplex complexthan thansurrounding, surrounding,and andpresumably presumablyunderlying underlyingrocks. rocks. ItIt must must therefore thereforehave haveacquired acquiredsome someaspect aspectof ofits itsstructure structureeither eitheras asaageographically geographicallyseparated separated terrane terrane or or during duringthin-skinned thin-skinnedemplacement emplacementover overthe thefootwall footwallrocks. rocks. ~~~~ Structures Structures in in the the allocthon allocthon Studies Studies of of outcrops outcropsof ofthe thePaint PaintRiver RiverGroup Groupshow showthat that at atleast leasttwo twoand andprobably probablythree threeor ormore more generations generations of of fold fold axes axes occur occur in in the the Iron Iron River River syncline. Most Mostfolds folds are are tight tight to to isoclinal isoclinalwith with steeply-plunging steeply-plungingaxes. axes. Similar Similarsteeply-plunging steeply-plunging fold fold axes axes are are common common in in fault fault panels panelswithin within the the Niagara Niagara fault fault complex, complex, part partof of which which bounds bounds the the syncline syncline on on the the south, south, but but are are rare rare elsewhere elsewhere in in the the foreland. Bedding Beddingand andaxial axialplanar planarfoliation foliationare are variably variably oriented, oriented, but but generally generally dip dip steeply. steeply. Remnants Remnantsof of sub-horizontal sub-horizontal foliation foliation occur occur in in aa few places places suggesting suggesting that that recumbent recumbent folds of folding. folding. AAcross folds may may have have been been significant significant during early stages of crosssection section(James (Jamesand and others, synclinenear near others,1968) 1968)from fromiron ironmine mineworkings workingsat atthe thewest westend endof ofthe theIron IronRiver Riversyncline Staumbaugh, steeplyStaumbaugh, Michigan Michigan shows shows a a recumbent recumbentfold foldthat thatisisonly only moderately moderately overprinted overprinted by by steeplydipping dipping structures. structures. The The Badwater BadwaterGreenstone, Greenstone,although althoughlargely largelymassive, massive,does doescontain containstructures structuressimilar similarto tothose those shown history. The shown in in the the Paint Paint River River Group Group and and appears appears to have the same structural history. The Badwater Badwater Greenstone Greenstoneis is therefore thereforeconsidered consideredpart partof ofthe theallochthon. allochthon. 28 �syncline Structures iinn rocks surrounding the Iron Iron River syncline Rocks within the Niagara fault fault complex, south south of of the the syncline, syncline, have havenorthwest-striking, northwest-striking,steeply steeply south-dipping foliation and tight tight to to isoclinal folds folds that that are are steeply steeply plunging, plunging,like likethose thosewithin withinthe the Structures in Baraga Group rocks (primarily (primarily Michigamme Michigamme Formation), Formation),which whichbound boundthe the syncline. Structures the north and east, are much different and and generally generally have havewest-northwest-striking west-northwest-striking syncline on the foliation to sub-horizontal sub-horizontal fold fold axes axes that that foliation that dips moderately to steeply south, and horizontal to trend fold axes axes are are rare rare in in the the Iron Iron River River trend west-northwest. Such Such sub-horizontal, west-trending fold first generation recumbent folding foldingwith with sub-horizontal sub-horizontalaxial axialplanar planar syncline. Several areas of first foliation foliation have have been been found found over over the the past past several several years years within within the the Baraga Baraga Group Group rocks, rocks, small recumbent recumbentfolds foldswith with sub-horizontal sub-horizontal suggesting the formation of nappes. Most recently, small foliation of the the Iron Iron River River foliation have have been been documented documented near Watersmeet, Michigan, northwest of structures north north and andeast eastof ofthe the Iron Iron syncline. Together Together with with previous previous observations observations of similar structures River syncline, there is now widespread evidence evidence of of an early phase of recumbent folding and an early phase foliation, now now variably variably overprinted overprintedby bymore moreupright upright generation of low angle axial planar foliation, structures. structures. Interpretation Interpretation Recumbent Recumbent folds documented documentedby by underground underground mapping mapping in in the Iron Iron River River syncline syncline (James (James and and others, 1968) and remnants of flat-lying foliation found in outcrops of the Paint River Group remnants found in outcrops of the Paint River Group Baraga Group Group had had aa similar similarearly early suggest that both both the Iron Iron River syncline and adjacent Baraga structural history history of recumbent folding and development of of low low angle anglefoliation. foliation. However, structural However, multiple generations generations of of steeply-plunging steeply-plungingfold axes axes and and steeply-dipping, variably-oriented variably-oriented axial axial planar foliations of the Iron River syncline, are distinct from the gently west-plunging fold axes Iron distinct from the gently west-plunging fold axes found in the the Michigamme Michigamme Formation Formation north northand andeast eastof of with south-dipping south-dipping axial planar foliation found the syncline. Similar Similarsteeply steeplyplunging plungingfold foldaxes axes are are common common in in very highly highly strained rocks rocks along to events events related related to to suturing suturing of ofthe thevolcanic volcanic the Niagara Niagara Fault Fault and seem clearly to be integral to terranes south south of the fault. This Thissuggests suggeststhat thatthe theIron IronRiver Riversyncline syncline was was detached detached from from folding history. history. The base interpreted to underlying rocks during much of its folding base of the allocthon is interpreted be a thrust fault along the the base base of of the the Badwater Badwater Greenstone, Greenstone, along which the allochthon allochthon has has been thrust thrust northward over foreland basin turbidite turbidite deposits deposits of ofthe the Michigamme MichigammeFormation. Formation. We folding within within the the Iron Iron River Riversyncline synclineisisaaresult resultof of suggest that the unusual unusual complexity of folding zone. During thrusting of earlier earlierdeformed deformedrocks rocks northward northward from the suture zone. During this thrusting, an earlier set of recumbent folds was intensely redeformed. In In contrast, contrast, rocks rocks in in the structural structural the fact fact that that the the Iron Iron River Riversyncline synclinecross-cuts cross-cutsthe the footwall were little little deformed as shown by the west-trending, sub-horizontal west-trending, sub-horizontal fold fold axes axes in in the the adjacent adjacent Michigamme Michigamme Formation. Formation. REFERENCES REFERENCES J., and Wier, Wier, K. K. L., L., 1968, 1968, Geology Geology and andOre Ore Deposits Depositsof of James, H. L., L., Dutton, C. E., E., Pettijohn, F. J., the Iron Iron River-Crystal River-Crystal Falls Falls District, District, Iron Iron county, county, Michigan: Michigan: U. S. Geological Geological Survey Professional Paper 570, 134 134 p. P.K., 1996, L.M.H., eds., Sims, P.K., 1996,Structure Structureof of continental continentalmargin, margin, in in Sims, P.K., P.K., and Carter, L.M.H., Archean and Proterozoic geology of the Lake Superior region, U.S.A., 1993: Proterozoic of the Lake Superior region, U.S.A., 1993: U.S. Geological Professional Paper Paper 1556, 1556, p. p.44-51. Survey Professional 44-51. 29 �REGIONAL PATTERNS PAYIERNS ON THE ON THE THE PENOKEAN PENOKEAN CONTINENTAL MARGIN IN THE SOUTHERN SOUTHERN LAKE SUPERIOR REGION Gene , Oshkosh, Oshkosh, WI L. LaBerge, M e r g e , UW UW Oshkosh Oshkosh , W I 54901 and U. U.S. S. Geological Geological Survey Survey Gene L. Studies of outcrops outcrops and exploration drill cores reveal several cores during the past decade decade reveal several regional patterns in the deposition deposition and subsequent deformation deformation and metamorphism metamorphism of of the the Early Proterozoic Earl Proterozoic Penokean continental continental margin margin rocks rocks in in northern northern Wisconsin Wisconsin and and northern northern Michigan. These datahelp helpconstrain constrain models models of of the the depositional depositional and tectonic history of the ~ichigan. Thesedata region. . region. I. Regional depositional depositional patterns. patterns. A. Thinning westward of the basal arenite of of the the Chocolav Chocolay Grouo. Group. The A. The Mesnard and Sturgeon Formations are are prominent arenite arenite units in the Marquette ~ a r ~ b e tand te Sturge& Formations Menominee districts, Menominee districts. respectively, resuectivelv. and the the Sunday Sundav Quartzite Ouartzite forms forms a thin basal unit on the eastern Gogebic &nee. range. However, basal arenite &&te is is absent absent on on the thewestern rfowever, aabasal Gogebic range, where the the the dolomite dolomite unit characteristic characteristic of the upper part of the Gogebic range, Group rests directly on on Archean basement. basement. Drill Chocolay Group Drill core coredata data from from the the Park Park Falls -- Mercer area of northern Falls northern Wisconsin suggests suggests that this this pattern pattern extends extends southward toward toward the the proposed continental continental margin. southward increase in politic pelitic rocks rocks southward toward the proposed continental B. An increase margin. Drill pelitic ParkFalls Falls -- Mercer area intersect abundant ~elitic marein. Drillcores coresfrom from the the Park schists, some of which contain interbedded carbonate carbonate units. units. Bedded Bedded sulfides sulfides are are present locally within the politic pelitic rocks. rocks. The Thestratigraphic stratigraphicposition positionof of the thepelitic politicunits units has not been ascertained, but they may be broadly correlative correlative with the Chocolay (Cannon and others, 1998). 1998). Group (Cannon C. A consistent pattern of litholoeies lithologies within the Palms formation of the Menominee Menominee Penokean platform. platform. The Group throughout the Penokean The thin-bedded argillaceous lower member and the thick-bedded thick-bedded orthoquartzite are recognizable orthoauartzite upper uooer member membea are r e c o b b l e from from member the Marenisco are also ~amekaion.These Thesemembers membekare also Marenisco area westward to Lake Namekagon. recognizable recoenizable in drill drill cores cores from from the the Pine PineLake LakeSubterrane Subterrane(Cannon (Cannonand andothers, others. 1998) from the Lake Gogebic Gogebic area southwestward WI area, area, southwestward at least to the Butternut, WI suggesting a uniform platformal depositional environment. platf-1 suggesting D. Rift-related ironRift-relatedvolcanic volcanicrocks rocks(Sims (Sims and and others, 1989) 1989) are are associated with ironformation of the Menoninee ~Group G u on p the proposed shelf. The The Emperor Emperor Volcanics Volcanics are interbedded interbedded with iron-formation on the the eastern eastern Gogebic Gogebic (LaBerge (LaBereeand and Klasner, Klasner, 1994), and volcanics are present within the iron-formation iron-formation on & the western w-&tern end end of the Gogebic ((LaBerge M e r g e and others, 1995). Volcanic Volcanic rocks rocks as as well as carbonaceous sediments are associated with iron-formation south of the Gogebic range in a belt that extends extends from from near near Lake Lake Gogebic Gogebic southwestward southwestward at at least least to to the the Butternut, Butternut, Wisconsin area, (LaBerge, (JaBer e, 1997). This Thissuggests suggeststhat that rifting, rifting, with with associated volcanism and stagnant stagnant basin development, development, occurred occurredafter after deposition deposition of of the the Palms Palms Formation on the continental margin. On Onthe themain mainGogebic Gogebicrange, range,then, then, the the ironironformation accumulated accumulated on a shallow shallow platform bounded bounded on on the the east, east, south south and and west west basins with active active volcanism. by rift basins II. deformation/metamorhpic 11. Regional Regional defonnation/metamorhmcpatterns. " A. Metamorphism ~ & o r p h i s m of ofcontinent &tin& margin margin sediments sedimentsthat that increases increasesin in intensity intensity associated with with intense intense folding foldingand andnorth-directed north-directedthrusting. thrusting. A A zone zone of of southward associated high pressure pressure metamorphism metamorphism characterized characterized by kyanite and a high temperature zone 30 �sillimanite(the Powell Subtenane Subterrane and the Park Falls Subterrane, Subterrane, characterized by sillimanitefthe respectively of Cannon and others, 1998). 1998). pegmatitic, granites, some of of B. Emplacement of post-tectonic potassic, commonly pegmatitic, two-mica granites, or garnet, garnet,typical typicalof of which are two-mica granites, or which contain sillimanite or anatectic granites. later deformational deformational event eventwith withassociated associatedretrograde retrogrademetamorphism. metamorphism.This Thisisis C. A later widespread alteration alteration of of high grade minerals such such as as sillimanite sillimaniteand and expressed as widespread rocks)to topyrophyllite pyrophylliteand andserpentine, serpentine,respectively. respectively. This This olivine (in carbonate rocks) the post-tectonic post-tectonic granites granitesassociated associatedwith withthe theearlier earlier deformation has locally affected the deformation. References: Cannon, W.F., W.F., LaBerge, G.L., G.L., Klasner, Kiasner, J.S., J.S., and Schulz, K.J., K.J., 1998, Reinterpretation Reinterpretation of the Penokean continental margin in part of northern Wisconsin and Michigan (Abstract): 44th Annual Institute on Lake Superior Geology, vol. vol. 44, 44, part part 1, p. p. 52-53. J.S., and M LaBerge, G.L., 1994, Klasner, J.S., e r g e , G.L., 1994, Structural evolution of the eastern Gogebic Michigan (Abstract): (Abstract): 40th 40th Annual AnnualInstitute Instituteon onLake LakeSuperior Superiot range, northern Michigan vol. 40, part part 1, p. 23-24. Geology, vol. LaBerge, G.L., G.L., 1997, break-up of of the the Superior SuperiorCraton: Craton:Implications Implicationsofof Merge, 1997, Early Proterozoic break-up drill core and geophysical data south of of the the Gogebic range, range, northern northern Wisconsin Wisconsin (Abstract), 43rd Annual Annual Institute Instituteon onLake LakeSuperior SuperiorGeology, Geology,vol. vol.43, 43,part part1,1,p.p. 3 1-32. 31-32. LaBerge, G.L., G.L., Cannon, Cannon, W.F., and andKiasner, Klasner,J.S., J.S., 1995, 1995,New New observations observations on the geology of the western Gogebic range (Abstract): Institute on Lake (Abstract): 41st Annual Institute Superior Geology, vol. 41, part I,1,p.p.331-32. 1-32. LaBerge, G.L. G.L. and Klasner, J.S., J.S., 1994, Tectonic Tectonic implications implications of the Early Proterozoic lithostratigraphy on the eastern Gogebic range, northern Michigan (Abstract): 41st on the eastern Gogebic Annual Institute on Lake Superior Geology, vol. vol. 41, part part 1, p. p. 33-34. Sims, P.K., P.K., Van Schmus, W.R., W.R., Schulz, K.J., K.J., and Peterman, Z.E., Z.E., 1989, TectonoTectonostratigraphic evolution evolution of of the the Early Early Proterozoic Proterozoic Wisconsin Wisconsin magmatic magmatictenanes terranesof ofthe the Penokean Orogen: Canadian Journal of Earth Sciences, v. 26, p. 2 145-2158. Orogen: Canadian Journal of Earth Sciences, v. 26, p. 2145-2158. 31 �1 I DETERMINE THE THE ORIGIN ORIGIN OF A GEOCHEMICAL AND PETROLOGIC STUDY TO DETERMINE CROWDUCK LAKE THE CROWDUCK LAKE GROUP, GROUP. KENORA, ONTARIO: A PROBLEMATIC METACONGLOMERATE. METACONGLOMERATE. JOHNSON, Matthew M. M.and and COTTER, COTTER, J.F.P., J.F.P., LOUGHRY, Joy LOUGHRY, Joy E., JOHNSON, Geology Geology Discipline, Discipline, University University of of Minnesota, Minnesota, Morris, Morris, Morris 56267 Morris MN M N 56267 The The Crowduck Crowduck LakeLake Group is is a sequence sequence of of argillite argillite and aa poorly poorly sorted sorted conglomerate (Archean (Archean ? age) age) exposed exposed near near Kenora, Kenora, Ontario. Ontario. Smith Smith (1988) may be be a debris debris flow flow (1988) suggested suggested that that the the Crowduck Crowduck Lake Lake Group Group may deposit, deposit, however however lithology lithology and and geographic geographic location location suggest suggest that that it may may be be aa correlative correlative of of the the"Gowganda "Gowganda tillites' tillites" (polymictic (polymictic conglomerates conglomerates deposited deposited during during the the Gowganda Gowganda glaciation glaciation 1.8 1.8 billion billion years years ago). ago). The goal goal of Crowduck Lake Lake Group Group is is glacial glacial in in of this this study study is is to todetermine determine ifif the theCrowduck origin. origin. Extensive research has has been been done done on Extensive research on the the deposits deposits of of the the Gowganda Gowganda Glaciation of the Glaciation (Summarized (Summarized by by Young Young 1969, 1969, 1973. 1973, 1999). 1999). Outcrops Outcrops of the Gowganda Gowganda are are located located in in Chibougamau, Chibougamau, Quebec, Quebec, Wyoming. Wyoming, and and Northern Northern Michigan. Geochemical analysis analysis indicate indicate that that there Geochemical there are are broad broad similarities similarities in diamictite constituents. Additionally. Chemical Index in constituents. Additionally, Chemical Index Alteration Alteration studies studies provide provide evidence evidence of of regionally regionally cold cold climatic climatic conditions conditions (Young, (Young, 1999). 1999). Young on the Young (1969), (1969). on the basis basis of of outcrop outcrop distribution, distribution, suggests suggests that that elsewhere in Canada there elsewhere in Canada there are arepolymictic polymictic conglomerates conglomerates which which on on detailed detailed analysis may well turn out to be tillites. analysis may well turn out to be tillites. For For this this study study an an outcrop outcrop at at the the type type locality locality of of the theCrowduck Crowduck Lake Lake Group Group was was described described and and sampled. sampled. Hand Hand samples samples were were analyzed analyzed for for sedimentary structures, Geochemistry structures, clast provenance provenance and and clast clast shape. shape. Geochemistry of matrix matrix samples samples was was determined determined using using ICP. ICP. Thin Thin sections sections of of matrix matrix were were used used for for petrographic petrographic analyis. analyis. Geochemistry of the Crowduck Lake Lake Group Group is is similar similar to to that that of of the the Geochemistry of the Crowduck type of these type Gowganda Gowganda Fotmation Fofmation of of Ontario. Ontario. However, However, interpretation interpretation of these results is results is restricted restricted because because little little is is known known about about the the geochemistry geochemistry of of the the bedrock at at the Additionally, the age age of of the the Crowduck bedrock the time time of ofdeposition. deposition. Crowduck Additionally, the Lake Group Lake Group is is poorly poorly constrained. constrained. Thus, Thus, several several possible possible origins origins of the the Crowduck Crowduck Lake Lake Group Group remain remain plausible: plausible: glacial glacial tillite, tillite, turbidite/debris turbiditeldebris flow, and flow, and basal basal conglomerate. conglomerate. Although Although the sedimentology. sedimentology, location location and and age of of the the Crowduck Crowduck Lake Lake Group Group make make aaglacial glacial origin originattractive, attractive, evidence evidence is Additional is not conclusive. conclusive. Additional petrographic petrographic analysis analysis are are underway. underway. 32 �THE GRATIOT CHALCOCITE DEPOSIT, KEWEENAW PENINSULA, MICHIGAN MAKI, John C., Department of of Geological Geological Engineering Engineeringand andSciences, Sciences,Michigan Michigan Technological Technological University. Houghton, MI 49931, jcmaki@mtu.edu; and BORNHORST, Theodore J., Department Department of of University, 4993 1Jcmaki@mtu.edu; and BORNHORST, Theodore J., Geological University, Houghton, Houghton, MI 49931, Geological Engineering and Sciences, Sciences, Michigan Technological University, tjbomho@mtu.edu tjbomho@mtu.edu ABSTRACT ABSTRACT Michigans Michigan's Keweenaw Keweenaw Peninsula Peninsula is is well well known for world-class world-class native copper deposits and the lack of significant amounts of copper sulfides. Twelve copper Twelve copper sulfide sulfide (chalcocite-dominated) (chalcocite-dominated) deposits depositshave have Peninsula. Robertson (1975) described one small deposit near Mt. been discovered in the Keweenaw Peninsula. Bohemia. Chalcocite-dominated deposits are are concentrated concentratedininthe the lower lowerquarter quarter of of the the Portage Portage Lake Lake Bohemia. Chalcocite-dominated deposits to Mt. Mt. Bohemia Bohemia (Figure (Figure 1). 1). The Gratiot chalcocite Volcanics from Suffolk to chalcocite deposit (previously termed 543-S) is the largest of the deposits and and contains about about 4.5 4.5 million metric 5434) the copper copper sulfide sulfide dominated dominated deposits tons of ore with an average average grade grade of of 2.3% 2.3% copper. copper. The The Gratiot chalcocite chalcocite deposit deposit is is hosted hosted by the Portage Lake Volcanics and is about 1.4 1.4 km from from the In vicinity of the ore, basalt lava flows are Keweenaw fault, a major reverse fault of regional extent. reverse fault of regional In vicinity of dikes. These to the the orientation orientation of of the the lava lava flows. flows. cut by two dacite-andesite dikes. These dikes are nearly subparallel to flow tops, although lesser amounts are Ore grade intercepts occur mostly in brecciated brecciated amygdaloidal flow found in flow interiors and and dikes. dikes. The both dikes are present present The highest highest grade and tonnage exists where both both dikes dikes are are thick. thick. Faulting and both dikes dikes and where both and fracturing fracturing are most prevalent in rocks where both exist. The deposit zone, the the Gratiot-Suffolk Gratiot-Suffolkfault, fault, that has a stratigraphic exist. deposit is within a 12 12 km long fault zone, displacement of about 7 meters and is sub-parallel to to the the attitude attitude of of the the basalt basalt flows flows and and sub-parallel to to fault. Two deposit are are the the Cross-Gratiot Cross-Gratiot fault fault that that is is the Keweenaw fault. Two other other significant significant faults that cut the deposit basalt flows flows with with aa stratigraphic stratigraphic displacement displacement of of about roughly perpendicular to the attitude of the basalt about 15 15 meters and and the the Gratiot fault that that is sub-parallel to the the attitude attitude of of the the basalt basalt flows flows with with very meters Gratiot fault sub-parallel to very little little The highest grades are associated with the intersection of the Gratiotstratigraphic displacement. displacement. highest grades associated with the intersection the GratiotSuffolk fault, the Gratiot Gratiot fault fault and and the the Cross-Gratiot Cross-Gratiot fault. fault. fill amygdules amygdules and and fractures fractures within within the the basalt lava A suite suite of of secondary secondary hydrothermal hydrothermal minerals minerals fill lava flows and dikes. Chlorite was the earliest secondary mineral, followed by epidote, potassium feldspar, Chlorite was the earliest secondary by epidote, potassium feldspar, Deposition of of native copper and native silver, prehnite prehnite and quartz and calcite (approximate order). Deposition pyrite clearly follows native copper and was itself followed by by chalcopyrite, bomite, bornite, chalcocite chalcocite and and hematite. The Thelast lastphase phaseof ofsecondary secondarymineral mineral deposition deposition was adularia, laumontite and more calcite. Chalcocite Chalcocite is by far far the the major majorcopper-bearing copper-bearing mineral mineral in the Gratiot Gratiot deposit, deposit, other other copper-bearing copper-bearing minerals are trace to rare in occurrence. The deposit The relative relative age of mineral deposition in the Gratiot deposit matches the relative age of mineral mineral deposition deposition in the large native copper deposits in the the Keweenaw Keweenaw Peninsula. Butler an early early suite of of minerals associated associated with with native native Butler and and Burbank Burbank (1929) (1929) documented documented an Native copper copper in in the copper followed by late adularia, adularia, laumontite and calcite. Native by chalcocite and then late Keweenaw Peninsula is interpreted to be temporally and and genetically genetically related related to to compressional tectonics The close temporal produced reverse motion motion along along the the Keweenaw Keweenaw fault (Bornhorst, (Bomhorst, 1997). 1997). The temporal that produced also chalcocite is also relationship of native copper and chalcocite relationship of chalcocite in the Gratiot deposit suggests that chalcocite are interpreted interpreted as as very Native copper copper ore-depositing ore-depositing fluids are very tectonics. Native related to to compressional compressional tectonics. related Chalcocite ore must Chalcocite ore fluids must sulfur poor and derived from basalts within the rift (Bomhorst, 1997). sulfur poor and derived from within the rift (Bomhorst, 1997). 33 �have have carried carried more moresulfur sulfurthan thannative nativecopper copperfluids. fluids. However, However, the the genesis genesis of offluids fluidsgenerating generatingthe the chalcocite chalcocitedeposits depositsisiscurrently currentlyspeculative. speculative. Gratiot-Suffolk Fault Gratiot Lake Mt Bohemia Deposit Gratiot GratiotDeposit Deposit Suffolk Deposit 1111111 Jacobsville Sandstone KXI Oronto GrOLQ I Portage Lake Volcanics I 20 0 km FIGURE FIGURE 1: 1: LOCATION LOCATIONOF OFCOPPER COPPERSULFIDE SULFIDEDEPOSITS DEPOSITS References References - ,< . Bornhorst, Bomhorst, T. T. J., J., 1997, 1997,Tectonic Tectoniccontext contextof ofnative nativecopper copperdeposits deposits of of the the North North American American Midcontinent Midcontinent Rift Rift System: System: Geological GeologicalSociety Societyof ofAmerica AmericaSpecial SpecialPaper Paper 312, 312, p. p. 127-136. 127-136 Butler, deposits of of Michigan: Michigan: U. S., 1929, 1929, The copper deposits U. S. S. Geol. Geol. Survey Survey Prof. Prof. Butler, B. S. and Burbank, W. 5., Paper 238 p.p. Paper 144, 144,238 Robertson, Robertson, J. M., M., 1975, 1975, Geology Geologyand andmineralogy mineralogy of of some somecopper coppersulfide sulfidedeposits depositsnear near Mount Mount Bohemia, Bohemia, Keweenaw Keweenaw County. County, Michigan: Economic Economic Geology, Geology, v. v. 70, 70, p.p.1202-1224. 1202-1224. 34 �CHEMICAL AND AND MINERALOGICAL MINERALOGICAL COMPARISON COMPARISON OF OF BARABOO, BARABOO, BARRON, BARRON, AND AND SIOUX SIOUX CHEMICAL ARGILLITE, METAPELITE AND PIPESTONE ARGILLITE, METAPELITE AND PIPESTONE MEDARJS, L.G., Jr., of MEDARIS, L.G., Jr., and and FOURNELLE, FOURNELLE, J.H., J.H., Dept. Dept. of of Geology Geology & & Geophysics,Univ. Geophysics,Univ. of Wisconsin-Madison,53706, 53706,medaris@geology.wisc.edu, inedarisgeology.wisc.edu, johnf@geology.wisc.edu; johnfgeo1ogy.wisc.edu; Wisconsin-Madison, BOSZHARDT, BOSZHARDT, RI., RF.,Mississippi MississippiValley ValleyArchaeology ArchaeologyCenter, Center,Univ. Univ.ofofWisconsin-La Wisconsin-LaCrosse, Crosse, 54601, rboszhar@uwlax.edu; J.H., State rboszhar@uwlax.edu, BROII{AHM, BROBHAHN, J.H., State Historical Society Society of of Wisconsin-Madison, 53706, John.Broihahn@ccmail.adp.wisc.edu 53706, John.Broihahn@ccmail.adp.wisc.edu Early Early Proterozoic Proterozoic (1760-1630 (1760-1630 Ma) Ma) red red quartzites quartzitesin in the the southern southern Lake Lake Superior Superiorregion region place place important important constraints on the climatic, sedimentary, and tectonic history of North America, and fine-grained constraints on the climatic, sedimentary, and tectonic history of North America, and fine-grained sedimentary and hydrothemially hydrothermally altered rocks in sedimentaly in the quartzite sequences sequences have provided material for for Native Native artihcts. Although American artifacts. Althoughargillites argilhtesand andpipestones pipestonesfrom fromseveral severallocalities localities have have been been investigated investigated freviously by previously by X-ray X-ray diffiaction, diffraction, little littleattention attentionhas hasbeen beenpaid paid so sofar firto totheir theirchemical chemical compositions compositions and and textural details. Representative samples of Baraboo, Barron, and Sioux argillite, metapelite, and pipestone Representative samples of Barabw, Ban-on, and Sioux have been investigated by by petrographic petrographic microscope, XRF, XRD, and EMP to determine textural characteristics to determine their textural characteristics and and chemical and and mineralogical mineralogical compositions. compositions. Such chemical Such information information provides provides aa basis basis ibr for evaluating evaluatingpossible possibleregional regional variations variations in in chemical compositions and metamorphic metamorphic conditions among conditions among the artifact determining artifact the quartzite quartzite localities localities and and for for determining provenances. provenances. Bamboo: fine-grained metasedimentary metasedimentary layers, Barabw: layers, several several to several several feet feet in inthickness, thckness, are interbedded interbedded with inches to quartzite and vary varyfrom frompink pink argillite argillite(metasiltstone) (metasiltstone) to gray phyllonite (metapelite), depending on on the the proportions proportions of quartz and pyrophyllite. Pipestone occurs Pipestone occursin in several several small pipestone quarries and one outcrop near the base of the quartzite sequence sequence in the south south limb of the the Bamboo Baraboo syncline. syncline. The pipestone is a dark purplish-red argillite, aigillite, consisting of quartz, quartz, pyrophyllite, and and hematite, hematite, which which is is cut cut by by thin thin white white hydrothermal veins containing pyrophyffite, muscovite, and hydrothermal containing pyrophyllite, and diaspore. In several pipestone samples, argillite itself has In several pipestone samples, argillite been altered to an assemblage assemblage of of muscovite-pyrophyllitemuscovite-pyrophyllitediasp ore-hematite, in in which which quartz quartz has been replaced diaspore-hematite, replaced largely largely by pyrophyllite (Fig. 1). Small amounts of kaolinite in 1). Small amounts of kaolinite inthe the metasedimentary rocks and and pipestone pipestone are are retrograde retrogradein in origin, origin, replacing replacing pyrophyllite pyrophyllite or or muscovite. muscovite. Barron: four samples fow samples of of discarded discardedpipestone pipestone were collected in the vicinity of a pipestone pipestone quarry quarryin in the the Doyle Doyle Barron County. The Forest Unit, Barren Thepipestone pipestoneisis aamassive massive to laminated red argillite, which contains quark, quartz, hematite, hematite, and and large vermicular grains of vermicular-grains of kaolinite kaolmite (Fig. 2). 2). Sioux: two samples M: samples of of pipestone pipestone from from Pipestone Pipestone National Monument and one of flaggy aigillite argillite from the Jasper quadquadrangle (sec. 32, Tl 04N, R4W) T104N, R4W) were were provided by David and Tony Tony Runkel. Runkel. The Southwick and Thepipestone pipestone samples samples are are typical catlinite, catlinite, being being fine-grained fine-grainedand and deep deepred red in in color, color, m: 35 �I locally with small pale orange-red reduction spots and pale gray elliptical domains. The Thepipestone pipestone mineral mineral assemblage assemblage is pyrophyllite-diaspore-muscovite-hematite, pyrophyllite-diaspore-muscovite-hematite,and the pale pale gray gray elliptical elliptical domains domains are are largely largely pyrophyllite with minor diaspore and muscovite (Fig. 3). Small Smallamounts amountsof ofkaolinite kaoliniteare are retrograde retrogradeafter after argillitecontains containsquartz, quartz,hematite, hematite,pyrophyllite, pyrophyllite,and and traces tracesof of detrital detrital pyrophyllite and muscovite. The Theargillite muscovite. muscovite. Chemical Comoosition Composition Six are Six samples samples of Baraboo, Barron, Barron, and Sioux Sioux argillite and metapelite are of Na,0 Na20 and CaO CaO and and contain wntain less than than essentially devoid of 0.39 wt% wt% K20, K,0, attesting attestingto totheir theirextreme extreme chemical chemical Rock Compositions in the q Baraboo Baron ICASH System matunty and maturity and contrasting contrastingmarkedly markedly with with the the composition composition of Projected onto tle KAS Plane Sioux average shale (Fig. (Fig. 4). Relative argillite and average Relative to to argiuite shale, the K-rich and Si-poor Si-poor compositions wmpositions of Bamboo Baraboo and and 19 Si Sioux pipestones (0.66-5.62 wt% lC,O Sioux K,0 and 40.90-54.20 wt% SiOJ 5i02) indicate wt% indicate that they may have have formed from ppestone argillite by addition of K and removal of Si through argilhte hydrothermal activity. Such Suchaa process processcould could account account for kin estone and replacement of quartz quartzin inBaraboo Baraboopip pipestone and the replacement suggests suggests that that the the elliptical ellipticalpyrophyllite pyrophyllite domains domains in in Sioux Sioux K pipestone may also be the result of quartz pipestone replacement. replacement. 8 Metamorphic Metamomhic Conditions: the Barron Ban-on argillite argiUite assemblage, quartz-kaolinite, assemblage, quartz-kaolimte, is limited to 6 at 22 kbar kbar (Fig. (Fig. 5). 5). The temperatures below 3300°C 0 0 T at The Bamboo and Baraboo and Sioux Siouxargilhite argillite assemblage, quartzquartzpyrophyllite, is stable stable between 300 and 390°C 3 9 0 T at 2 -. kbar, and and the the pipestone pipestone assemblage, assemblage, pyrophyllitepyropbyllite22 muscovite-diaspore, is is more more narrowly narrowlyconfined confined between 3315 and 360° 360°C (Fig. (Fig. 5). 5). The Baraboo and hetween 15 and Sioux assemblages assemblages cannot cannotbe be due due to to burial burial alone, alone, o o 450 350 400 250 300 because depths on the order of 18-19 18-19 km k m would be 250 300 350 400 450 T C T, "C required to reach reach 300°C 'raton quiredm 3000c for a typical stable craton 5 Reactions Fig. 5 Fig. Reactions in in the the KASH KASH system system and and assemblages assemblages conductive g geotherm. conductive ~ t h e n n The .Theage ageand andsource source of the u Bamboo, Barron, in Ban-on, and and Sioux Sioux rocks rocks thermal pulse responsible responsible for for Baraboo Baraboo and and Sioux Sioux metamorphism remains uncertain. In Inaddition, addition,kaolinite kaolinite is is widely reported in the Sioux Quartzite, and it is was ubiquitous in the the Sioux Quartzite (as it was was in the Baraboo quartzite), unclear whether metamorphism was or restricted restricted to to zones zones of of hydrothermal hydrothermal activity, activity, perhaps perhapsrelated relatedto tofaulting. faulting. Conclusions: Conclusions: the thechemical chemicalcompositions wmpositionsof ofargillite argilliteand andmetapelite metapeliteconfirm confirmthe the chemical chemicalmaturity maturity of of all all sedimentary members of the Bamboo, Barron, and Sioux sequences, consistent with derivation from a Baraboo, Barren, and Sioux sequences, consistent with derivation from a deeply weathered, weathered, low-relief low-relief source source in in an an Early Early Proterozoic Proterozoic warm, warm, humid humid climate. climate. Comparable Comparable mineral assemblages in Bamboo assemblages Baraboo and and Sioux Siouxpipestones pipestonesdemonstrate demonstratethat thatthe thepromotmg promotinghydrothennal hydrothermalactivity activitydoes does not coincide with the 1630 1630 Ma thermal front in the southern Lake Lake Superior Superiorregion, region, postulated postulatedby by Hoim Holm et et al. (1998); (1998); whether whether they are are contemporaneous remains remains to be seen. Regarding Regarding artifact artifactprovenance, provenance, Barron Barron pipestone is easily easily distinguished distinguished from Bamboo Baraboo and and Sioux Siouxpipestone pipestone by by the the presence presence of of vermicular vermicular kaolinite. kaolinite. Comparable Comparable mineral mineral assemblages assemblagesand and textures texturesmake makediscrimination discriminationbetween between Baraboo Baraboo and and Sioux Siouxpipestones pipestones more difficult. Baraboo Baraboopipestone pipestonemay maycontain wntainquartz, quartz,isislocally locallycut cutby bythin, thin,white white veins, veins, and and is is dark dark purplish purplish red red in in color; color; Sioux Siouxpipestone pipestoneis isdevoid devoid of of quartz quartzand andveins, veins, commonly commonly has has reduction reduction spots spotsand and conspicuous, conspicuous, small, small, pale pale gray, gray,elliptical ellipticalpyrophyllite-rich pyropbylhte-rich domains, domains, and and isis deep deep red red to to orange orangein in color. color. 36 �REPUBLIC REPUBLIC WETLAND PRESERVE John G. G. Meier - District DistrictManager Manager— Environmental Environmental Affairs Affairs Cliffs Cliffs Mining Milling Services Company Iron Iron ore ore mining, mining, by by its its very very nature, nature, causes causes large large potential impacts to natural resources. Development Developmentof of mine mine pits, pits, rock rock stockpiles stockpiles and and tailings tailings basins basins impacts impacts wetlands wetlands from time to to time. time. Permits Permitstotoimpact impactwetlands wetlandsare areissued issuedby by the the Michigan Michigan Department Department of Environmental EnvironmentalQuality Quality ifif the the impacts impacts are are "unavoidable" "unavoidable" and and are are "minimized "minimized to to the the greatest greatest extent extent possible". possible". Each Eachpermit permitrequires requiresthat that"compensatory "compensatory wetland wetland mitigation" mitigation" be be provided. provided. The The Republic RepublicWetland WetlandPreserve Preserveisis being being created created at at the the former former Republic Republic Mine Mine tailings tailings basin basin to to provide providecompensatory compensatorywetland wetland mitigation mitigation for for those unavoidable unavoidable impacts to wetlands wetlands at at other other mining operations. operations. Instead Insteadof of merely merely draining draining the water from the various various tailings tailings and and water water clarification clarification ponds and converting the land to upland at Republic, Cliffs, with MDEQ oversight, is in these these areas. areas. Several is constructing wetlands in hundred hundred acres acres of creditable creditable wetland will be created. This This wetland area, area, along along with adjacent upland and external external preservation area, will be placed in a Conservation Easement Easement for for the the benefit benefit of the the State State for long term protection. 37 �POTENTIAL PGE IN POTENTIALFOR FORSTRATIFORM STRATIFORM PGEMINERALIZATION MINERALIZATION IN MAFIC LAYERED INTRUSIONS COMPLEX MAFIC LAYERED INTRUSIONSOF OFTHE THEDULUTh DULUTH COMPLEX MILLER, 55114 MILLER,James JamesD., D.,Jr., Jr.,Minnesota MinnesotaGeological GeologicalSurvey, Survey,2642 2642University UniversityAve., Ave.,St. St.Paul, Paul,MN MN 55114 (mille066@maroon.tc.unm.edu) (mille066@maroon.tc.umn.edu) Economic Economicconcentrations concentrationsof ofplatinum platinumgroup groupelements elements(PGEs) (PGEs)ininmeter-thick, meter-thick,stratiform, stratiform,sulfide-bearing sulfide-bearing horizons horizons(reefs) (reefs)have havelong lonebeen beenknown knowntotobebeassociated associatedwith withultramafic-mafic ultramafic-maficlayered layeredintrusions intrusionssuch suchasas the theBushveld ~ u s h i e land dA dStillwater Stillwatercomplexes. complexes. Recent Recentdiscoveries discoveriesofofPGE PGEmineralization minerali&on ininthe theSkaergaard Skaergaard Intrusion Intrusionand andrelated relatedbodies bodiesininGreenland Greenland(Bird (Birdetetal., al.,1991, 1991,1995; 1995;Neilsen Neilsenand andBrooks, Brooks,1995; 1995;Aranson, Aranson,etet - al., al., 1997; 1997;Andersen Andersenetetal., al.,1998) 1998)have haveshown shownthat thatstratiform stratiformPGE PGEmineralization mineralization can can also alsooccur occurinin tholeiitic tholeiiticlayered layeredintrusions. intrusions.The Thepresence presenceofofmany manytholeiitic tholeiiticmafic maficlayered layeredintrusions intrusionsassociated associatedwith withthe the 1.1 1.1 Ga GaMidcontinent MidcontinentRift Riftsuggests suggeststhat thatthe theLake LakeSuperior Superiorregion region isisfertile fertileground groundfor forexploration explorationof ofthis this newly newlyrecognized recognizedtype typeof ofPGE PGEmineralization. mineralization. With Withthe thesupport supportofofthe theState StateofofMinnesota's Minnesota'sMinerals Minerals Coordinating the been assessing coordinatingCommittee ~ommi'ttee theMinnesota MinnesotaGeological GeologicalSurvey Surveyhas hasbeen assessingthe thepotential potentialfor forstratiform stratiform PGE PGE mineralization mineralization in in two twointrusions intrusionsof ofthe theDuluth DuluthComplex—the Complex-the Layered Layered Series Series at at Duluth Duluth and andthe the Sonju SonjuLake Lakeintrusion intrusion(Miller (Millerand andRipley, Ripley.1996). 1996). Based Skaergaardand andother othertholeiitic tholeiitic Based on onempirical empiricalobservations observations of ofPGE PGEmineralization mineralizationin inthe theSkaergaard intrusions, intrusions, and andon ontheoretical theoreticalconsiderations considerationsof ofsulfur sulfurand andPGE PGEbehavior behaviorinindynamic, dynamic,differentiating differentiating intrusions, intrusions, the the conditions conditions that that favor favorthe theformation formationofofPGE-enriched PGE-enrichedhorizons horizonsinintholeiitic tholeiiticlayered layered intrusions intrusionsare: are: 1.1.The undersaturated Theparent parentmagma magmaisisinitially initiallysulfide sulfideundersaturated. 2.2.The parent magma has a high initial PGE experiences &h experiencesaaconsiderable considerableamount amount The parent magma has a high initial PGEconcentration concentrationand/or of offractional fractionalcrystallization crystallizationtotobuild buildup upnoble noblemetal metalconcentrations concentrationsprior priorto tosulfide sulfidesaturation. saturation. of sulfide 3.3. The Theinitial initial segregation segregation of sulfide melt melt is is triggered triggered by by aaprocess processsuch suchasasmagma magmamixing, mixing, decompression, decompression, assimilation, assimilation, or or simple simple differentiation differentiation that that promotes promotes aa large large R-factor R-factor (silicatdsulfidemelt meltratio). ratio). (silicate/sulfide These Theseconditions conditionsimply implythat thatan anintrusion intrusionthat thatformed formedfrom fromananinitially initiallysulfide-undersaturated, sulfide-undersaturated,tholeiitic tholeiitic parent parentmagma magmahas hasthe thebest bestchance chancefor forsignificant significantPGE PGEmineralization mineralizationin in the themidmid- to toupper-level upper-level horizon horizon that that marks marksthe thefirst first"cumulus "cumulusarrival" arrival"ofofimmiscible immisciblesulfide sulfidemelt. melt. Theoretically, Theoretically, aa gabbro gabbrocumulate cumulate crystallized crystallizedfrom fromaamagma magmasaturated saturatedininsulfide sulfideshould shouldcontain containgreater greaterthan than400 400ppm ppmsulfur sulfur(Boudreau (Boudreauand and McCallum, McCallum, 1992). 1992).Regardless Regardlessofofhow howsulfide sulfidesaturation saturationisistriggered, triggered,ororthe theway wayininwhich whichsulfide sulfide melt melt arrives arrives at atthe themagma magmachamber chamberfloor, floor,ititisislikely likelythat thatthe thefirst firstsignificant significantsulfide-saturated sulfide-saturated melt melt has has the the greatest greatestchance chanceof ofencountering encounteringthe themost mostPGE-enriched PGE-enriched silicate silicate magma. magma. Thus, Thus, exploration exploration for foraaPGEPGEenriched enrichedhorizon horizonshould shouldattempt attempttotoseek seekout outthe thestratigraphically stratigraphicallylowest lowestgabbroic gabbroiccumulates cumulatesthat thatcontain contain 400 ppm ppm SSor ormore. more. The The Cu/Pd Cfld ratio ratioindicates indicatesthe theeffectiveness effectiveness of of the thesulfide-saturated suicide-saturated melt melt inin 400 scavenging of magnitude scavengingchalcophile chalcophilemetals metalsfrom fromthe thesilicate silicatemagma. magma. Because BecausePd Pdisisseveral severalorders. ordersof magnitude more more compatible Cu,an anefficiently efficientlyscavenged scavengedsilicate silicatemelt meltshould shouldrecord recorda asignficant signficant compatiblewith with sulfide sulfidemelt meltthan thanCu, increase increasein inCu/Pd CuIPdafter afteraasulfide-segregation sulfide-segregationevent. event. These (DLS,Miller, Miller,1998). 1998). Thesegeochemical geochemicalcriteria criteriawere werefirst firstapplied appliedtotothe theLayered LayeredSeries SeriesatatDuluth Duluth(DLS, The TheDLS DLSisisaa3.53.5-toto4-km-thick, 4-km-thick,well-differentiated, well-differentiated,layered layeredintrusion intrusionemplaced emplaced into into the the basal basal section section of of comagmatic Keweenawan Keweenawan volcanics volcanics in in the the vicinity vicinity of of Duluth. Duluth. The The DLS DLS isis composed composed of of aa comagmatic unidirectionally troctolite(P1+01) (Pl+01) to gabbro gabbro unidirectionally differentiated differentiatedsuite suiteof of mafic maficcumulates cumulatesthat that grades grades up up from from troctolite and (Pl+Aug+Fem±O1±Ap and displays displays aa complementary complementary cryptic cryptic variation variation inin cumulus cumulus mineral mineral (Pl+Aug+Fe0±0l±Ap), compositions. 1-km-thickinterval interval compositions. The Thetransition transitionfrom fromtroctolitic troctolitictotogabbroic gabbroiccumulates cumulatesoccurs occurs in inan an1-km-thick called called the thecyclic cycliczone zonewherein whereinP1+01 Pl+01cumulates cumulatesgrade gradeupward upwardto toPl+Aug+Fe0,,+0l Pl+Aug+Fe-+01 cumlates, cumlates, and and regress regress abruptly abruptly back back to to P1+01 Pl+01 cumulates. cumulates. The Thecyclic cyclic zone zone isismade made up upof ofasasmany manyasassix sixsuch suchmacrocycles, macrocycles, 38 �which implies that despite despite being being well-differentiated, well-differentiated, the DLS magma system was open open to to recharge rechargeand and venting (Miller and Ripley, 1996). (Miller and Ripley, 1996). Sulfur concentrations indicate that that the DLS was initially sulfide undersaturated undersaturated and and did did not reach Cu/Pd ratios begin to show sulfide saturation until the onset begin show irregular irregular onset of of cyclic cyclic zone zonecrystallization. crystallization. CuRd upsection from from the the base base of of the the cyclic cyclic zone. zone. The though significant increases upsection The irregular irregular increase in Cu/Pd CuIPd ratios probably reflects the open nature of the DLS system. Interestingly, Interestingly,the theanomolous anomolous concentrations concentrations of sulfur sulfur and PGEs occur occur at macrocycle macrocycleboundaries, which I interpret interpret to to indicate indicate venting venting and/or and/or recharge recharge events. These observations suggest that a PGE-enriched sulfide reef may exist near the base of cyclic These observations suggest that a PGE-enriched reef may zone, and may correspond correspond to to aa horizon horizon indicative indicativeof of open-system open-system perturbations perturbations to to the the DLS DLSmagma. magma. the stratiform PGE PCE potential potential of of the Sonju Lake Geochemical studies were recently initiated to study the intrusion (SLI) near Finland, Minnesota. The The SLI SLI isis aa 1-km 1-km thick, sheet-like sheet-like intrusion that was emplaced beneath beneath a granophyre granophyre body. body. It is is aa completely completely differentiated differentiated intrusion intrusion that displays a very very regular, regular, unidirectional cumulus paragenesis: Ol—>Pl+Ol—>Pl÷Aug+Ol—>Pl+Aug+Fe—>Pl+Aug+Fe+Ol+ unidirectional paragenesis: 01->P1+01->Pl+Aug+01->Pl+Aug+Fe_->Pl+Ang+Fe_+Ol+ Ap. This Thisphase phaseprogression progressionand andaasmooth smoothcryptic crypticvariation variationin inmineral mineral composition composition indicate indicate that that the the SLI SLI crystallized as an almost The SLI is the intrusion most most similar to the crystallized almost closed closed system. system. The the Keweenawan Keweenawan intrusion the Skaergaard. Geochemical analyses of sulfur and PGEs have not yet been completed for the SLI. Skaergaard. Geochemical analyses of sulfur and PGEs have not yet been completed for SLI. Previous analyses included Cu which provides indirect evidence for for the the abundance abundance of of sulfide in in the SLI cumulates. The TheCu Cuconcentration concentrationshows shows aa dramatic dramatic 5-fold Sfold increase increase (from cl0Oppm e100ppm to >500 >500 ppm) ppm) at at aa level equal to 2/3 2.13 of of its itsthickness. thickness. This change is level above above the base base of of the theintrusion intrusion approximately approximately equal approximately midway midway through cumulate interval; interval; notably, notably, it is at approximately through the Pl+Aug+Fe0, Pl+Aug+Fe cumulate at the theequivalent equivalent stratigraphic level, and within the same cumulate type as the Platinova reef of the Skaergaard stratigraphic the same cumulate as the Platinova reef of the SkaergaardIntrusion Intrusion ILSGmeeting. meeting. (Andersen et al., 1998). 1998). I anticipate anticipate reporting reporting geochemical analyses for the SLI SLI at at the theILSG horizon in They will provide more direct evidence of the potential potential for aa PGE-enriched PGE-enriched sulfide-bearing horizon the upper part of SLI. may exist in the In conclusion, these studies studies suggest suggest that that significant significant PGE mineralization mineralization may the medial medial orupper parts of the DLS and SLI intrusions, as well as in other tholeiitic layered intrusions of the the orupper parts of DLS and SLI intrusions, as well as in other tholeiitic layered intrusions of Midcontinent Rift. Moreover, demonstrate that that geochemical geochemical criteria criteria can can be be used (1) to Moreover, these studies demonstrate to evaluate the overall potential for stratiform stratiform PGE mineralization in tholeiitic mafic intrusions, and (2) to Because potential PGEassist in assist in identification identification of of favorable favorableintervals intervalswithin within individual individual intrusions. intrusions. Because PGEmineralized horizons horizons are likely likely to be meter-scale and contain contain only only a few modal percent meter-scale in thickness, thickness, and sulfide, core drilling drilling and and systematic systematicgeochemical geochemical sampling sampling are are necessary necessary to to confirm confirmtheir their existence. existence. References :.,!, J.C.ø., Rasmussen, J.G., 1998, The Triple Group and the Platinova gold and palladium Andersen, J.C.0.. Rasmussen, H., H., Neilsen, Neilsen, T.F.D., T.F.D., Rønsbo, R~nsbo.J.G., reefs in relations: ECONOMIC ECONOMIC GEOLOGY, 488-509. reefs in the the Skaergaard SkaergaardIntusion—stratigraphic Intusion-stratigraphicand andpetrographic petrographic relations: GEOLOGY, v.93, v.93, p. 488—509. DX., Bernstein, PB., 1997, J.G., Bird, D.K., Bernstein, S. and and Kelemen, Kelemen, P.B., 1997, Gold Goldand andplatinum-group platinum-groupelement element mineralization mineralization in the Aranson, J.G., Kruuse ECONOMIC Kmuse Fjord FjordGabbro Gabbrocomplex. complex.East EastGreenland: Greenland: ECONOMICGEOLOGY, 490-501. GEOLOGY,v.92, v.92, p. 490—501. Bird, D.K., Bird, D.K., Brooks, C.K., C.K., Gannicott, Gannicott, R.A., R.A., Turner, P.A., P.A., 1991, 1991, A A gold-bearing gold-bearing horizon horizon in the the Skaergaard Skaergaard Intrusion, Intrusion, East East Greenland: GEOLOGY, v. 092. ECONOMIC GEOLOGY, v. 86, 86,p.p.1083—1 1083-1092. Greenland:ECONOMIC Brandriss. ME., M.E., Nevle, Nevle, R.J., Kelerotn, P.R., P.B., 1995, 1995, Bird, Bird, D.K., D.K., Aranson, J.G., IC., Brandriss, It)., Radford, G., C., Bernstein, S., Gannicott, R.A.. R.A., and Kelemen, A gold-bearing gold-bearing horizon East Greenland: ECONOMIC ECONOMICGEOLOGY, 90. p. 1288—1300. 1288-1300. A horizonininthe theKap KapEdvard EdvardHoIm HolmComplex, Complex, East Greenland: GEOLOGY,v.v.90, by magmatic fluids in layered intrusions: Boudreau, A.E. A.E. and McCallum, McCallum, 1.8., IS., 1992, 1992, Concentration of platinum-group elements by intrusions: ECONOMIC GEOLOGY, v. 87, ECONOMICGEOLOGY, 87.p.p.1830—1848. 1830-1848. J.D., Jr., 1998, 1998,Potential Potential for for stratiform stratiform POE PGE deposits deposits in sulfide-undersaturated, sulfide-undersaturated, tholeiitic layered intrusions of the Duluth Miller, J.D.. Complex. Complex, Minnesota, Minnesota, USA. USA. 8th 8th International International Platinum Platinum Symposium Symposium Abstracts, Abstracts, Rustenburg, Rustenburg, South South Africa, Africa, GSSA GSSA and and SAIMM Symposium Symposium Series Series 518. S18,p.263—266. p. 263-266. J.D.. Jr.. Jr., and Ripley, Ripley, E.M., E.M., 1996, 1996,Layered Layeredintrusions intrusions of of the the Duluth Duluth Complex, Complex, Minnesota, Minnesota, USA, USA, in in Cawthorne, R .G.,ed., Miller, J.D., R.G., Layered Layered Intrusions: Intrusions:Amsterdam, Amsterdam,Elsevier ElsevierSci., Sci..p.p.257—301. 257-301. T.F.D. and Brooks, East Greenland: Greenland: Factors Factors important important to tothe themineralization mineralization Neilsen, T.F.D. Brooks, C.K., C.K., 1995, 1995,Precious Preciousmetals metals in in magmas magmas of of East in ECONOMIC GEOLOGY, v. 90, ECONOMICGEOLOGY, 90,p.p.1911—1917. 1911-1917. in the theSkaergaard SkaergaardIntrusion: Intrusion: 39 �NEW GEOLOGIC MAP OF THE CENTRAL DULUTH COMPLEX MILLER, James D., D., Jr., Jr., and and CHANDLER, CHANDLER, Val. Val. W., Minnesota Minnesota Geological Geological Survey, Survey, University of of Minnesota, 2642 University Ave., Ave., St. Paul, MN MN 55114 orchand004@tc.umn.edu) 551 14(milleO66 (mille066 or chand004@tc.umn.edu) The Minnesota The Minnesota Geological Geological Survey Survey is is preparing preparing aa new new interpretation interpretation of of the the geology geology of of the the central central part pan of of the Duluth Complex, 00,000-scale man mapthis thissummer. summer. The map 1: 100.000-scale mao will cover cover Comnlex. which which will be published nublished as a 1:1 and lat. rectangular area and 9101510011 aa rectangular areaof ofabout about1,100 1,100square squaremiles milesbounded boundedbybylong. long.92°07'30" 92007'30 and 91°15'00 andlat. 47°37'30" and Both the the southeastern southeastern comer corner of of the the map map area, area, which which is is anchored anchored to to the the Lake Lake 47'37'30 and 47°15'OO". 47°15'00" Both Superior shoreline contact of Superior shoreline near near Silver Silver Bay, Bay, and and the the northwestern northwestern corner, comer, which which covers the the basal basal contact of the the Duluth Complex, Duluth Complex, have have adequate adequatebedrock bedrock exposure exposure that that has has been been mapped mapped at at the the scale scale of of 1:24,000 1:24,000 (Miller, (Miller, Miller and others, 1993; 1988; Miller 1993; Severson and Hauck, 1990; Severson and Miller, 1999). However, However, the the intervening area, area, roughly roughly 90% 90% of of the the map map area, area, contains contains little little or orno nobedrock bedrockexposure. exposure. Geologic Geologic intervening evaluation of of aeromagnetic aeromagneticand andgravity gravitydata. data. Geologic interpretations in in this area largely depend on evaluation control for the the geophysical geophysical data data is is limited limited to to widely widely and and unevenly unevenly dispersed dispersed drill cores as as well well as as scattered scattered outcrop (Severson, 1971). areas of outcrop (Severson, 1995; 1995; Chandler Chandler and others, others, 1991; 1991;Bonnichsen, 1971). The bedrock geology geology of the the map area area is is dominated by intrusive intrusive rocks that were emplaced emplaced into into the the basaltic North North Shore Shore Volcanic Volcanic Group Group (NSVG) (NSVG) during during the the main main lower and medial portionS portions of the largely basaltic stages of the Midcontinent Rift magmatism (Paces and and Miller, Miller, 1993). 1993). The intrusive rocks can be grossly subdivided into those belonging to the more plutonic Duluth Complex and those more subdivided into those belonging to the more plutonic Duluth Complex and those composing composing the the more intrusions of the Beaver Bay Complex. The complexes hypabyssal intrusions The boundary boundary between between two intrusive complexes corresponds to a semi-continuous semi-continuous septa septa of strongly hornfelsed homfelsed volcanic rocks, locally containing minor corresponds the middle middle of of the the map maparea area(Fig. (Fig. 1). 1). Large-scale intrusions of of intrusions, that trends northeasterly through the differentiated mafic cumulates, cumulates, gabbroic gabbroic anorthosites, &orthosites. gabbros, gabbros, and &d granophyres pnophyres compose the Duluth Complex northwest of the volcanic septa. Southeast Southeast of of the the volcanic volcanic septa, septa, smaller scale, sheet-like bodies and dikes dikes of mafic mafic cumulate cumulate rocks, rocks, gabbro, gabbro,diabase. diabase, diorite dionte and granophyre granophyre constitute constitute multiple multiple intrusions intrusions of the Beaver Beaver Bay Bay Complex Complex that that were were emplaced emplaced into into higher higher levels levels of the the volcanic volcanic edifice. edifice. A number of intrusive components can be be distinguished distinguished within within the theDuluth DuluthComplex. Complex. The oldest is a structurally structurally complex complex assemblage assemblageof of mostly mostly leucogabbroic leucogabbroic to to anorthositic anorthositic rocks, small bodies of olivine olivine oxide hornfels (collectively (collectivelytermed termedAGV AGVin inFigure Figure IA). 1A). This oxide gabbro, and scattered inclusions of volcanic homfels assemblage assemblage corresponds correspondsto to aa very very heterogeneous heterogeneous aeromagnetic aeromagneticsignature signature of variable variable amplitude amplitude in the to aa more more subdued subdued signature signaturetotothe theeast east(upsection). (upsection). The The western part of the complex but gives rise to consistently anorthositic anorthositic compositions. compositions. Irregular masses of change may indicate a transition to more consistently granophyre granophyre occur occur within the the eastern eastern part of the the AGV, AGV, but contact contact relationships relationships are not exposed in the study area. Aeromagnetic Aeromagneticpatterns patternssuggest suggestthat thatthe thesubcircular subcircularto tolensoidal lensoidalgranophyre granophyrebodies bodieswere were emplaced emplaced into into the the AGV, AGV, but but exposures exposures outside outside the map map area (Boerboom (Boerboom and Miller, 1994) 1994) indicate indicate that large granophyre granophyrebodies bodies can can be be older olderthan than the the anorthositic anorthositic rocks. More than half half of of the the Duluth Duluth Complex Complex in in the the map map area area is composed of six discrete mafic layered layered intrusions intrusions which were were emplaced emplaced beneath beneath and and within the AGV and, in one case, beneath a body of granophyre. granophyre. The Thestratiform stratiforminternal internalstructure structureofofthese theselayered layeredintrusions intrusionsgives givesrise riseto toaacrudely crudelybanded banded aeromagnetic aeromagnetic pattern. pattern. Truncations Truncationsofofthe thepatterns patternsimply implyaageneral generalsequence sequenceof ofemplacement emplacementand andsuggest suggest that successive successiveintrusions intrusions overplated overplated previous intrusions. intrusions. Although Althoughthe theigneous igneousstratigraphy stratigraphyof ofeach each intrusion is generally generally poorly known, known, some some notable differences differences are evident. Apparently, Apparently, the theoldest oldestlayered layered mafic intrusion intrusion is is the the Partridge Partridge River River intrusion intrusion (PRI), which forms the basal contact in the northwestern by frequent frequent PRI suggests suggests that it formed by part of the map area. The Thepreponderance preponderanceof of troctolite troctolitein in the the FRI truncated by by the impulses of magma and little internal differentiation. The The northeast end of the PRI is truncated South Kawishiwi South Kawishiwi intrusion intrusion (SKI). (SKI). Where Whereititisismore morecompletely completelyexposed exposedin inthe theGabbro GabbmLake Lakequadrangle quadrangle dominatedby bytroctolite. troctolite. At its southern end, end, the the to the north (Green and others, 1966), 1966). the SKI is also dominated PRI is overplated overplated and and ultimately cut cut out out by another another intrusive intrusive body that is almost entirely unexposed except for a few drill except drill holes holes along along its its basal basal contact contact and some some troctolite emplaced into the AGV hanging wall of the PRI. This Thisintrusion, intrusion,which whichwe wetentatively tentativelycall callthe theWest-central West-centralmargin marginintrusion, intrusion,has hasaa subdued subdued aeromagnetic aeromagnetic signature signature indicative indicative of troctolitic troctolitic cumulates. cumulates. The Thesubdued subduedpattern pattern passes passesupsection upsection into a busy, high-amplitude high-amplitude signature signature that that shows shows some some banding and thus may indicate the presence of roof zone. The upper gabbroic cumulates beneath an AGV roof The Greenwood Lake intrusion (GLI) is a somewhat isolated somewhat isolated intrusion intrusion that that truncates truncates the the SKI SKI but otherwise otherwise intrudes AGV and granophyre rocks. troctolite The GLI is unique in in that that itit contains contains aa complete complete differentiation differentiation sequence sequence that grades from lower troctolite 40 �cumulates cumulatesto to upper upper oxide oxide gabbro gabbro cumulates. The Thenorthern northern extent extent of of the the GLI GLI is is truncated by the southern margin margin of of the the troctolitic troctolitic Bald BaldEagle Eagleintrusion, intrusion,apparently apparentlythe the youngest youngest major major mafic mafic layered layered intrusion intrusion in in the the map map area. area. The TheOsier OsierLake Lakeintrusion intrusionisisaasmall, small,plug-like plug-likebody body of of troctolite troctolite cored by gabbro gabbro cumulates that that is is known known from from two two drill drill core core and a bull's-eye aeromagnetic signature. It It was was emplaced entirely entirely into anorthositic anorthositicrocks rocks of of the the AGV, AGV, and andthus, thus,its itsage agerelative relative to to other other layered layered intrusions intrusions is is unknown. unknown. Intrusions Intrusionsof of the the Beaver BeaverBay Bay Complex Complexoccur occuras asseveral severalsuites suites of of east-dipping, east-dipping, half-saucer-shaped half-saucer-shaped sheets, anomaly patterns. patterns. The largest and sheets, which which produce produce very very distinctive, concentric concentric aeromagnetic anomaly earliest to be be aa nested nested set set of of earliestintrusive intrusivesuite suiteis is the the Cloquet Cloquet Lake Lake layered layered series (CLLS), which appears to sheet-like sheet-like intrusions. intrusions.ItItisisdifficult difficulttotoaccurately accuratelydetermine determinethe thecompositional compositionalvariability variability of of the the CLLS CLLS because dozen or so drill drill core. core. In In general, general, the the lower lower becauseitit is is completely completely unexposed unexposed and and penetrated penetrated by only a dozenor part part of of the the series seriesappears appearsto tobe becomposed composedof ofatatleast leasttwo twodifferentiated differentiatedsequences sequencesof of troctolitic troctolitic to to gabbroic gabbroic cumulates. cumulates.The Theupper upperpart partofofthe theseries seriesappears appearstotocontain containmore moreevolved evolvedintrusive intrusiverocks, rocks, including including gabbro, ferrodiorite,monzodiorite, monzodiorite,and andgranophyre, granophyre,as aswell wellas gabbro, ferrodiorite, asan an unknown unknown amount amountof of volcanic volcanic hornfels. hornfels. A A complex complexaeromagnetic aeromagneticpattern pattern emanating emanatingfrom fromthe thesouthern southernmargin margin of of the the CLLS CLLS may may be be caused caused by by numerous numeroussatellite satellitedikes dikesand and sheets sheetsemplaced emplaced into into volcanic rocks. The Theeastern easternside side of of the the CLLS CLLS is is cut cut by another anothersheet-like sheet-likebody—the body-the Sonju Sonju Lake Lake intrusion intrusion (SLI). This Thisstrongly stronglydifferentiated differentiated mafic mafii layered layered intrusion in the theFinland Finland quadrangle quadrangle(Miller (Millerand and others, others, intrusion isiswell wellexposed exposedjust just to tothe theeast eastof ofthe themap maparea areain 1993) of the the Finland Finland granite. granite. Good exposure 1993) and is is capped capped by ferromonzonitic to granophyric rocks of exposure in the the southern southernDoyle DoyleLake Lakeand andSilver SilverBay Bayquadrangles quadrangles(Miller, (Miller,1988; 1988;Miller Millerand and others. others, 1993) 1993)shows showsthat that the of the the Lax Lax Lake Lake gabbro. gabbro. Both the Finland Finlandgranite granitecuts cutsan an early early suite suiteof of gabbroic gabbroic to ferromonzonitic rocks of the the Finland Finlandgranite graniteand andthe theLax LaxLake Lakegabbro gabbromay mayoriginally originallyhave havebeen been part part of of the the CLLS CLLS but but were were disassociated are the disassociatedfrom fromitit by by the the intrusion intrusion of of the the SLI. SLI. The Theyoungest youngestintrusions intrusionsin in the the map map area area are anorthosite-inclusion-bearing is anorthosite-inclusion-bearing dikes dikes and and sills sills of the Beaver River diabase. This This intrusive intrusive swarm, swarm, which which is well exposed exposed in in the the southeastern southeastern corner comer of of the the map map area, area, is is bounded on the west by a prominent dike dike that feeds feeds into into southeast-dipping southeast-dipping sheets. sheets. The Theupper upperparts partsof ofthese thesesheets sheetscommonly commonly contain contain ferrogabbroic ferrogabbroic cumulate cumulatebodies, bodies, collectively collectivelytermed termedthe theSilver SilverBay Bay intrusions. intrusions. The 00,000-scale map of of the central Duluth Duluth Complex Complex will will represent represent aa significant significant improvement improvement on on The 1:1 1:100,000-scale previous previous regional regional geologic geologic maps maps of of the the complex. complex. Because Becausethis this map map will will be be digitally digitally compiled, compiled, we intend to to revise revise itit periodically periodically as as new new data data become become available. An Animportant importantcontribution contribution of this this new geologic map map isis that that itit will willidentify identifymany manypreviously previouslyunrecognized unrecognizedintrusive intrusivebodies bodies that that should should provide provide new new exploration explorationtargets targetsfor forstratiform stratiformPGE PGEdeposits. deposits. References References Bonnichsen, Bonnichsen, 8., B.,1971, 1971,Outcrop Outcropmap mapofofsouthern southernpart partofofDuluth DuluthComplex Complexand andassociated associatedKeweenawan Keweenawan rocks, rocks,St. St. Louis 1, scale Louis and and Lake LakeCounties, Counties,Minnesota: Minnesota:Minn. Minn. Geol. Geol. Surv. Surv. Misc. Misc. Map Map M-l M-11, scale 1:24,000. 1:24,000. Boerboom, Boerboom, T.J., T.J., and and Miller, Miller,J.D., J.D., Jr., Jr., 1994, 1994,Geologic Geologicmap map of of the the Wilson Lake Lake quadrangle and parts of the Silver Silver Island Lake and Toohey Lake quadrangles. quadrangles, Lake and and Cook Cook Counties, Counties,Minnesota: Minnesota: Minn. Minn. Geol. Geol. SUN. Surv. Misc. Misc. M-81, scale scale 1:24,000. 1:24,000. Map M-81, Map Chandler, V.W.,Miller, Miller,J.D., J.D., Jr., Jr.,and and Venzke, Venzke,E. E. A., A,,1991, 1991,Central Central Duluith Dululth Complex drilling drilling project; Minn. Geol. Chandler,V.W., S w . OFR 91-4,2 sheets,accompanying accompanyingtext. text. Surv. 91-4,2 sheets, Green, County, Minnesota: Minn. W.C., and and Weiblen, Weiblen,P.W., P.W., 1966, 1966,Gabbro GabbroLake Lake quadrangle, quadrangle. Lake County, Green, J.C., J.C., Phinney, Phinney,W.C., Surv. Misc. Misc. Map Map M-2, M-2, scale scale 1:24,000. 1:24,000. Geol. Surv. Miller, County, quadrangles. Lake County, Miller, J.D., J.D., Jr., Jr., 1988, 1988,Geologic Geologicmap mapof of the the Silver SilverBay Bay and and Split Split Rock Rock Point NE quadrangles, Minnesota: Minn. Minn. Geol. Geol. Surv. SUN.Misc. Misc. Map Map M-65, M-65, scale scale 1:24,000. 1:24,000. Minnesota: Miller, J.D., J.D., Jr., Jr., Green, Green,J.C., J.C., Chandler, Chandler,V.W., V.W., and and Boerboom, Boerboom, T.J., T.J., 1993, 1993,Geologic Geologic map of the Finland and Doyle Miller, Lake Lake quadrangles, quadrangles, Lake Lake County, County,Minnesota: Minnesota: Minn. Minn. Geol. Surv. Misc. Map M-72, scale 1:24,000. Paces, intrusions, Paces, JR., J.B., and andMiller, Miller,J.D., J.D.,Jr., Jr.,1993, 1993,Precise PreciseU-Pb U-Pbages agesof of Duluth Duluth Complex and related mafic intrusions, northeastern, northeastern, Minnesota: Minnesota: new new insights insightsfor for physical, physical, petrogenetic, paleomagnetic and tectono-magmatic 13997-14013. processes rifting: J. J. Geophys. Geophys.Res., Res.,v.v.98, p. 13 processes associated associated with 1.1 1.1 Ga Midcontinent rifling: 98, p. 997-14 013. Severson, Severson, M.J., 1995, 1995,Geology Geology of of the the southern southern portion portion of of the Duluth Complex: Univ. of Minn. Natural Resources Res. Inst., Inst.,Tech. Tech.Rept. Rept.NRRITI'R-95126, NRRI/TR-95/26, 185 185 p. Res. Severson, Severson, M.J., MJ., and and Hauck, Hauck, S.A., S.A., 1990, 1990,Geology, Geology, geochemistry, geochemistry, and stratigraphy of a portion of the Partridge River NRRIIGMIN-TR-89-l 1,236 p. intrusion: Univ. of of Minn. Minn. Natural Resources Resources Res. Inst., Tech. Rept. NRRIIGMIN-TR-89-11.236 Severson, quadrangle, Minnesota: Minn. Geol. M.J.. and and Miller, Miller, J.D., J.D., Jr., Jr., 1999, 1999,Bedrock Bedrock geologic geologic map of the Allen quadrangle, Severson, M.J., Surv. Misc. Map M-91, M-91, scale scale 1:24,000. 1:24,000. 41 �A. —fSBl Footwall Beaver Bay complex Miscellaneous Volcanic Shale Iron-formation Grariute Duluth Complex NSVG —-" — "BRD Mafic Granophyre Anohositic Rocks ftGranophyre Rocks Gabbroic Rocks Gabbroic Cumulates Gabbro, Ferrodiotite, Monzodiorite Fault Volcanic Homlels LilTroctolite Cumulates Diabase, 01 Gabbro, Troctolite • Oxide Ultramatic Metavolcanic Rocks 10 Miles =20 Kilometers FIGURE 1. A) Preliminary generalized geologic map of the central Duluth Complex. Specific intrusive units are: AGV-Anorthositic-gabbroicvolcanic assemblage (rock types distinguished in the northwest area), PRI-Partidge River intrusion, WCMI-West-central margin intrusion, SKI-South Kawishiwi intrusion,OLl-Oreenwood Lake intrusion. BEt-Bald Eagle intrusion, OLI-Osier Lake intrusion, CLLS-Cloquet Lake layered series. SLISonju Lake intrusion, FO-Finland granite, LLG-Lax Lake gabbro, BRD-Beaver River diabase, SBI-Silver Bay intrusions. Crosses mark 7.5' quadrangle corners. B) First-vertical-derivative, reduced-to-pole image of data, which forms basis for the geologic interpretation shown in A. aeromaic �SEDIMENTOLOGY OF TWO DEEP MIDGONTINENT SEDIMENTOLOGY DEEP WELLS IN THE KEEWANAWAN MIDCONTINENT RIFT SYSTEM NEAR MUNISING, UPPER RIFT UPPERPENINSULA, PENINSULA,MICHIGAN MICHIGAN OJAKANGAS. Richard Richard W., W., Department Department of of Geology, OJAKANGAS, Geology, University of Minnesota-Duluth, Duluth, MN 55812, rojakang@d.umn.edu Minnesota-Duluth, MN 55812, rojakang@d.umn.edu Amoco Production Production Company Companydrilled drilled the the St. St. Amour Amoco Amour stratigraphic stratigraphic test well well in 1987. six miles south-southeast of Munising, Ml. The well bottomed well bottomed at at 1987, miles south-southeast Munising, MI. Eight mites to the southeast 7,241 ft. ft. Eight miles to southeast of the the St. St. Amour Amour welt, well, at at Mickey Hickey 7,241 Creek, Cliffs Cliffs Mining Mining Services Services Company Companyhad haddrilled drilledaa 5,345 5.345 ftft deep Creek, deep well well in in 1969. 1969. These These wells wells are located located on on the the southwest southwest flank flank of of the theMidcontinent Midcontinent Rift System where the the rift Rift System (MRS) (MRS) where rift starts starts bending bending towards towards the the south. south. Except for for the the Pleistocene (110ftft thick thick in in the the St. Except Pleistocene (110 St. Amour Amour and and 88 ft thick thick in in the Mickey Creek), both both wells wells were the Hickey Creek), were cored from from top to to bottom. bottom. The The youngest youngest bedrock is is the the Ordovician Au Train Train Formation, Formation,about about280 280ftft thick thick in in the St. bedrock Ordovician Au St. Amour well well and and 235 Amour 235 ft thick thick in inthe theMickey Hickey Creek Creek well. well. The The Au Au Train Train overties the the Upper (?) Munising Munising Formation, Formation, a quartz quartz arenite arenite unit unit overlies Upper Cambrian Cambrian (?) that is 135 ft thick in the St. Amour and 310 ft thick in the Hickey Creek. that 135 thick in the St. Amour and 310 ft thick in the Hickey Creek. The Keweenawan rocks in in the St. Keweenawan rocks St. Amour Amour well well from from the thebase basedownward downward include QIF/L= =2200 ft of feldspatholithic sandstone (N=8: QlF/L= 72.2/20.7/ 72.2120.71 include -2200 feldspatholithic sandstone (N=8: interpreted as as Jacobsville Jacobsville Fm.; Fm.; =3000 -3000 ft of of lithofeldspathic lithofeldspathic 7.2) interpreted 725 ft sandstone (N=7: (N=7: Q/F/L=58.1/19.3/22.6) interpreted as Freda Freda Fm.; Fm.; 725 ft sandstone QIF/L=58.1/19.3/22.6) interpreted of basalt flows 400 ftft of basalt flows of quartz .4/5.0): 400 of quartz sandstone sandstone(N=4: (N=4:Q/F/L=93.6/1 QlFIL=93.611.4/5.0); with interflow with interflow black black shales shales and and red red to tobrown browncoarse coarseclastics elasticsincluding including conglomerateand and lithic lithic sandstone sandstone (N=2: (N=2: QlFlL=65.010.5134.5); QIFIL65.0I0.5/34.5); and and 300 300 ftft conglomerate of of dacitic dacitic (?) (?) ignimbritic flows. flows. - The Keweenawan rocks in in the The Keweenawan rocks the Hickey Hickey Creek Creek welt well from from the the base basedownward downward include include =1720 -1720 ft ft of of dominantly dominantly feldspatholithic feldspatholithic sandstone sandstone (N=6: (N=6: Q/F/L=78.2/16.7/5.1) interpreted interpreted as as Jacobsville Q/F/L=78.2116.715.1) Jacobsville Fm. Fm. and and —2845 52845 ft ft of dominantly lithofefdspathic sandstone (N=9: QIF/L= 54.8/19.0/26.3) dominantly lithofeldspathic sandstone (N=9: Q/FlL= 54.8119.0126.3) interpreted as Freda main two two types types of of sand-sized sand-sized rock rock interpreted as Freda Fm. Fm. The main In the fragments in the two fragments in two wells wells are aremetamorphic metamorphic and and fetsic felsic volcanic. volcanic. In the St. St. Amour well, metamorphic rock fragments (schist and greenstone) are on Amour well, metamorphic rock fragments (schist and greenstone) are on average twice as abundant average twice abundant as as felsic felsic volcanic volcanic fragments, fragments, whereas whereas in in the the Mickey Creek Creek well, well, felsic fetsic volcanics volcanics are are three three times times as Hickey as abundant abundant as as metamorphic rock fragments. metamorphic rock fragments. Porosity Porosity decreases decreases downward downward in both both wells, wells, with with virtually virtually no no porosity porosity present present below below 2750 2750 ft. ft. 43 �I I The in the the two The mineralogy mineralogy in two wells wells is is quite quite similar similar for for the the upper upper 5300 5300 ftftand and the formations, not unexpectedly, correlate well. They also correlate well the formations, not unexpectedly, correlate well. They also correlate well with with Keweenawan Keweenawan outcrops outcrops in the the Lake LakeSuperior Superior region region(Ojakangas, (Ojakangas, 1986; 1986; However, the lower portion of the St. Amour does not Adamson, 1997). However, the lower portion of the St. Amour does not Adamson, 1997). correlate correlate well well with with the theexposed exposedBayfield Bayfield and andOronto OrontoGroups. Groups. Quartz Quartz sandstones occur low low in the sandstones occur the well, well, in in direct direct contrast contrast to toexposed exposedsections sections where deep in in the rift where they they are are near near the the top. top. Quartzose Quartzose sediments sediments deep rift are arealso also present present in in the the Eischeid Eischeid ## 11 well well in in Iowa Iowa(Ludvigson (Ludvigson et et at, al, 1990). 1990).Clearly, Clearly, the the search search for for uniformity uniformity and andcorrelation correlationwhere wheredeposition deposition likely occurred occurred in in aaseries seriesofofsub-basins sub-basins(haif-grabens) (half-grabens) is is aa tenuous tenuous pursuit, pursuit, as asnoted noted earlier earlier by by Mauk Mauk(1991). (1991). Soreghan Soreghan and and Cohen Cohen (1993) (1993) interpreted interpreted relatively relatively quartzose quartzose sands sands (Q (Q to to 79%) 79%) on onhinged hingedmargins marginsofofmodern modernLake LakeTanganyika Tanganyika to to be be the theresult resultofofreworking reworkingininshallow shallowwater. water. This This may may be be aa partial partial analogue for these but eolian analogue for these quartz quartz sandstones, sandstones, but eolian activity activity in in the the vegetationless vegetationless Middle Middle Proterozoic Proterozoic terrestrial terrestrial environment environment is is also also likely. likely. Cliffs Cliffs Mining Mining Services Services Company Company kindly kindly purchased purchased the the thin thin sections. sections. REFERENCES REFERENCES Adamson, k.F, 1997, Adamson, K.F., 1997,Petrology, Petrology, stratigraphy, stratigraphy, and and sedimentation sedimentation of of the the Middle Middle Proterozoic Proterozoic Bayfield Bayfield Group. Group, Northwestern NorthwesternWisconsin: Wisconsin: Unpublished M.S. thesis. thesis, University of Minnesota, Unpublished M.S. University of Minnesota, Duluth, Duluth, 203 p. p. Ludvigson, Ludvigson, G.A., G.A., Mckay, McKay, H.M., P.M., and andAnderson, Anderson, R.R., R.R., 1990, 1990,Pekrology Petrology of of Keweenawan sedimentaryrocks rocksinin the the MG. drillhole: in in Keweenawan sedimentary M.G. Eischeid Eischeid ## 1 drillhole: Anderson, DeepPetroleum PetroleumTest, Test, Anderson, R.R., R.R., ed., ed.. The The Amoco Amoco M.G. M.G. Eischeid Eischeid ## 11Deep Carroll Carroll County, County, Iowa, Iowa, Iowa Iowa Department Department of of Natural NaturalResources Resources Special Special Report Report Series Series No.. No., 2, p. p. 77-112. 77-112. Mauk, J. J. L., L., 1991, 1991, Sub-basins Sub-basins in in the theProterozoic Proterozoic Midcontinent Midcontinent Rift: Rift: Mauk, Stratigraphic Stratigraphic evidence evidence from from Upper Upper Michigan Michigan (Abs.): (Abs.): Geological Geological Society Society of of America America Abstracts Abstracts with with Programs, Programs, San San Diego, Diego, CA, CA, p. p. A59. A59. Ojakangas, R.W., 1986, Reservoir characteristics of the keweenawan Ojakangas, R.W., 1986, Reservoir characteristics the Keweenawan MG. Jr., Supergroup, in Mudrey, M.G. Jr., ed., ed., Supergroup, Lake Superior Superior region: region: in Precambrian Precambrian Petroleum Petroleum Potential, Potential, Wisconsin Wisconsin and and Michigan. Michigan, Geoscience Wisconsin, V. V. 11, 11. p. p. 25-31. 25-31. Geoscience Wisconsin. Soreghan, M.J.. and Cohen, Soreghan, M.J., Cohen, A.S., A.S., 1993, 1993, The The effects effects of of basin basinasymmetry asymmetry on on in Examples from from Lake Lake Tanganyika. Tanganyika, Africa: Africa: in sand sand composition: composition: Examples Johnsson, Johnsson, M.J., M.J., and and Basu, Basu, A.. A., eds.. eds., Processes ProcessesControlling Controlling the the Geological Composition Composition of Clastic Clastic Sediments: Sediments: Geological Society Society of of America America Special Special Paper 284. 284, p. p. 285-301. 285-301. 1 44 �DESCRIPTIONS OF MARQUETTE EARLY DESCRIPTIONS OF THE THE NATURAL NATURAL FEATURES ON THE MARQUETTE IRON RANGE Doug Ottke, U.S. Geological Survey, 12201 Sunrise SunriseValley ValleyDr. Dr. Reston, Reston, Virginia Virginia 20192 -, -- -- r The peninsula of of Michigan Michigan is is integrally integrally linked linked to to The history history of the Marquette Iron Range in the upper peninsula the environment. The the mode mode of of production production on Thenatural naturalfeatures features of of the the Marquette Marquette Range defined the on the the Range, Range, and and conversely, conversely, as as human human systems systems became more adept in these modes of production they they became became the the most most important importantmodifiers modifiersof of the the natural natural systems. systems. Settlement natural Settlement on the the Marquette Marquette Iron Iron Range was closely related to two factors of natural environment: environment: the the iron iron ore ore formations formationsand and the the hardwood hardwood forest. The distribution of three iron ore ore formations--principally formations-principally the the Negaunee, Negaunee, but but also also the the Bijiki Biiiki and other other Iron-formations Iron-formations within the Michigamme --determined the the locations locations of of the sites sites of of extraction of iron ore Michigamme Formation Formation --determined ore in the the area. area. Stands Standsof of hardwood hardwood forest, forest, particularly particularly maple maple and yellow birch were the major factor determining determiningthe the location location for for processing pro&ssing operations operations that converted iron ore into pig pig iron. iron. The location of beehive kilns where the hardwood was converted into charcoal, the early forges, and and beehive kilns charcoal, the early forges, the of hardwood. hardwood. The iron ore formations the blast blast furnaces furnaces were reliant foremost on the availability of formations and and the the hardwood hardwood forest forest of of the the Range Range defined defined the physical subdivision of the land into tracts owned the mines, mines, forges, forges, owned by individuals. individuals. Because Becauseaareadily readilyavailable available workforce workforce was needed at the kilns, kilns, and and furnaces furnaces of of the the area, area, locations locations of of settlement settlement on the Range were within very close proximity proximity to to the the sites sites of of extraction and processing operations. Transportation, from the earliest trails to the plank road, trails to the plank road, strap straprailroad railroad and and the the Iron Iron Mountain Railroad completed in the 1857 1857 were were defined defined by by the the natural natural location location of of the the ore ore and and the processes processes involved with extracting it. In 1850's, with withadvances advancesinintransportation transportationassociated associatedwith with the the completion completion of of the the railroad railroad In the the mid mid 1850's, to to the the mining mining areas areasnear near Negaunee Negaunee and and lshpeming Ishpeming and the completion of the Sault Locks near Sault By 1868 that that Sault Saint SaintMarie, Marie, the the production production of of iron iron ore ore reached reached a level of 11,343 tons per year. By number 508,000 tons mined. The Thedepression depression of of 1873 I873 caused aa sharp sharp drop drop number had had reached reached aa level level of of 508,000 in in production. production. However, However,for forthe theyear yearbefore beforethe thedrop dropone onemillion million tons tonshad had been been withdrawn withdrawn from from 40 rose to to 40 pits pits carved carved into into the ore bodies. In In 1880 1880two two million million tons were mined and that number rose seven seven million million tons tonsmined mined during duringthe the year year of of 1890. 1890. The Thequarrying quarryingoperations operationsprogressed progressed deeper deeper into into the earth earth until until by by the the 1880's 1880's shaft shaftmining miningbecame became aa necessity, necessity, calling calling for for advancement advancement in in technological applications and forcing many of the less wealthy operations out of business. The technological applications wealthy operations out of business. The large large amounts amounts of of capital capital involved involved in in diamond drilling drilling exploration, and the operations underground underground forced forced industrial industrial consolidation consolidation of of the many small operations into a few large companies companiescommanding commandinglarge largeresources. resources. As Asmines mineswent wentunderground, underground,the the need need for for timber timber also also increased became aa necessity. necessity. Between .72 and .92 increased as as supports supports for for the underground workings became cubic cubicfeet feetof oftimber timberwas wasneeded neededper perton tonof ofore oreextracted. extracted. In In 1868 1868eleven elevenblast blast furnaces furnaceswere were in in operation operation on the Marquette Range. Twenty Twentyfive fivefurnaces furnaces total total were were built built ininMarquette MarquetteCounty. County.Numerous Numerous beehive beehive kilns kilns dotted the countryside. These These kilns pine, and and other softwoods softwoods kilns converted converted maple, maple, yellow yellow birch birch and, and, in in some instances, hemlock, pine, into iron produced produced in in furnaces furnaces varied varied from from 110 110 into charcoal. charcoal. The Thecharcoal charcoalconsumed consumedper per ton ton of of iron bushels to 140 bushels, with a bushel consisting of 20 pounds of charcoal. The Pioneer Furnace, bushels to 140 bushels, with a bushel consisting of 20 pounds of charcoal. The Pioneer Furnace, the theoriginal originalblast blastfurnace furnaceininthe thearea arealocated locatedin in Negaunee, Negaunee, used used the timber from 1,500 1,500 acres in the year year of of 1869 1869to to produce produce 9,500 9,500 tons tons of pig iron. One Oneacre acreof ofwood wood could could produce produce 14 14 tons of iron. iron. ItIt isis estimated the kilns of the the estimatedthat thatby by 1903 1903thirty thirty acres acresof of hardwood hardwood per day was needed to supply the Marquette MarquetteRange. Range.InInthe thefirst firsthalf halfcentury centuryofofiron ironproduction production330,000 330,000acres acreswere were cut. cut. In In order orderto to understand understand more more thoroughly thoroughly how natural and human systems have modified each other on pre-major European European settlement settlement natural environment must be on the the Marquette MarquetteRange, Range, the the pre-major 45 �I delineated. delineated. Beginning Beginninginin1838, 1838,the theMichigan MichiganGeological GeologicalSurvey Surveyand andthe the Linear LinearSurvey Surveyof ofthe the U.S. U.S.Land Land Office Officecreated createddescriptions descriptions of ofthe thenatural naturalenvironment environmentof ofthe theMarquette MarquetteRange. Range. These These descriptions descriptionsbetter betterenable enablethe theunderstanding understanding of ofthe thepre-major pre-majorsettlement settlementnatural naturalenvironment. environment. The Thedescriptions descriptions contained contained within withinthese these reports reports of of the location location of of the the original originaloutcrops outcropsof ofiron ironore ore and andthe thestands stands of of trees trees shows shows how how important importantthese these factors factors were were to to the the early early settlement settlement of ofthe the Marquette MarquetteRange. Range. The Thedescriptions descriptionsalso alsoallow allowus usto tosee see how how influential influentialthat thatsettlement settlementwas wasin in modi1'ing modifyingthe thenatural naturalsystems systemsofofthe theRange. Range. 46 �DEVELOPMENT OF VOLCANOGENIC VOLCANOGENIC MASSIVE MASSIVE SULFIDE SULFIDEDEPOSIT DEPOSITEXPLORATION EXPLORATION OF TARGETS IN NORTHERN MINNESOTA FROM GIS GIs SPATIAL SPATIAL ANALYSIS OF GEOLOGICAL, GEOCHEMICAL, GEOPHYSICALCRITERIA CRITERIA AND GEOPHYSICAL GEOLOGICAL, GEOCHEMICAL, AND DEANM. M. PETERSON PETERSONand andDR. DR. RONALD RONALDL. L. MORTON MORTON DEAN Economic Volcanology Research Research Lab, Lab, Geology Department, Department, University ofMinnesota of Minnesota --Duluth, Duluth, Economic Duluth, Minnesota, Minnesota, USA USA 55812 massive sulfide (VMS) deposits are are syngenetic syngenetic stratifm stratiformiron ironand andbase-metal base-metal(Cu-Zn-Pb) (Cu-Zn-Pb) Volcanogenic massive systems in in subaqueous subaqueousvolcanic volcanicenvironments. environments. At the accumulations that form out of large geothermal systems "bottom" of VMS-producing systems are subvolcanic intrusions, which which produce produce focused focusedheat heatsources sourcesthat that "bottom" provide the energy for circulating hydrothermal fluids and leaching reactions, reactions, and and may may add addbase-metals base-metals into the geothermal system. Above Above the the subvolcanic subvolcanicintrusions, intrusions, the strata have undergone extensive highmetal depletion, alkali alkalimodification, modification,and andextensive extensive temperature alteration, which can include metal silicification. silicification. Over the last twenty years, a VMS ore ore deposit deposit model has been developed from the work of many researchers. Important features of the model Important features of the model include include the the following: following: • • • • • • deposits are associated VMS deposits associated with with submarine submarine volcanic volcanic rocks, and typically are are spatially spatially associated associated with felsic volcanic rocks in greenstone terranes. with age age or or petrochemistry petrochemistrybut butdo dooccur occurin indistinct distinct VMS deposits occur with no obvious correlation with provinces, camps or clusters. Deposits often occur at one or more distinctive favorable stratigraphic stratigraphichorizons horizonswithin withinaacamp. camp. These Deposits represent quiescent horizons represent quiescentperiods periods in in volcanic volcanic activity, activity, tops tops of volcanic volcanic cycles, cycles, or a major volcanicor aa sediment interface. The Thedistal distal extent extentof of ore ore horizons horizons may be marked by a siliceous, ferruginous, or tuff. base metal bearing bearing sediment sediment and/or and/or felsic felsic tuff. Footwall subvolcanic subvolcanic intrusions intrusionsare are typically typically sill-like, sill-like, with with irregularly irregularly distributed distributed porphyritic and non-porphyritic phases. The intrusions may contain minor porphyry copper occurrences. The intrusions may contain minor Synvolcanic fault controls are recognized in some deposits, and may control the location of rhyolite some Synvolcanic controls deposits. Proximal facies volcanic rocks associated with with domes that occur in the footwall of some deposits. synvolcanic faults are generally generally the most favorable favorable horizons for ore deposition. alteration may include semi-conformable and cloud-like Footwall alteration include a pipe-like zone and/or andlor widespread semi-confmable alteration. alteration. The massive-sulfide nature of the ore ore typically makes them conductors conductors of electricity, and therefore can be detected by airborne and ground-based electro-magnetic (EM) surveys. airborne feature of the ore deposit deposits have been based on only one feature Many exploration programs for for VMS deposits specifically, the blind drilling of airborne EM EM conductors. conductors. Although this method of exploration model, specifically, has discovered many major major deposits deposits in the the past, strictly strictly EM-based VMS exploration has many shortcomings. The The drawbacks drawbacks include include any any or or all of the following: • • • • Poor geologic control of conductor location block Poor ranking of conductors in a survey block High cost of drilling tens tens to hundreds hundreds of barren or graphitic EM conductors prior to discovery 1% of drilled EM conductors conductors lead to mines) Poor rate of discovery discovery (less than 1% 47 �This study study formulates formulates a GIS-based GIs-based exploration exploration model model that that incorporates incorporates all all facets facets of of the the VMS VMS ore deposit deposit Thesemaps maps have have ultimately ultimately led led to to the generation of specific s p e c i f VMS icps model into a series of digital maps. These exploration targets in the study area of northern Minnesota. The km The study study integrates integrates a new 5880 5880 km geologic compilation map with with mapped mapped alteration alteration zones, zones, over over 5400 5400 ground ground and airbome airborne EM EM anomalies, anomalies, and approximately 21,000 copper and zinc assays. Intermediate Intermediate maps maps used to predict VMS potential include stratigraphy, stratigraphy, heat generation, generation, alteration, Cu-Zn Cu-Zn geochemistry, geochemistry, and and EM EM conductors. conductors. The methodology inFigure ~ i &1.1. e methodology used used to to create create the the 015-based GIs-basedmodel modelisispresented presentedin STEP STEP I1 Build luild Spatial Spatial Data Base Base STEP STEP22 Collect Collect spatial Spatial Data Data and and Input Input into into GIS CIS Extraction of 015 CIS Extraction of Features Relevant Relevant to the VMS Ore to the VMS Ore Deposit Deposit Model Model S S BEDROCK BEDROCK GEOLOGY GEOLOGY S MAPPED MAPPED ALTERATION ALTERATION ZONES ZONES -H———— ———--H— I. Extractqtz.cpidote 2. Extract chlorite 2.Extractchlonu t.1.Reclassii RecUmfy 2. 2 Extract . m - contacts 3. Qeale buffers 3.0cçEbufiei 3. 3 .CYeate O m e bbuffers ufim I.1.Classifymiervals aassi& intervals 2. Grid soil & lake aSs 2.Gndsoil&lateseds 3. Buffer rock & axe 3.Bufferrock&core Data Data Processing Processing Preliminary Preliminary Maps Maps of of Selected Selected GIS GIs Features Features S ROCK, CORE, SOIL SOIL AIRBORNE ROCK. CORE. AIRBORNEAND AND & & LAKE LAKE SEDIMENT SEDIMENT GROUND GROUND EM EM CONDUCTORS GEOCHEMISTRY CONDUCTORS tontacts ;contscts Volcanic-Sedin.t contacts Greanstone Subvókar.ac 5eq lntrusio,.s Qsarfl- Chlorite A -A- Cu Zn Cu Zn Cu C"aC"a m Zna Epidote I I. Extnct ground EM 1-ExtractcrowdEM 2. airborne EM 2. Classil5, Classify airbornem soils Soik Lake Lake Rocks Reeks Gromd t EM EM EM EM Assign Assign Weights Weights of ofEvidence Evidence hny,gIits(O.on . t.) F-wriflte(0m-lm) STEP STEP33 Integration Integration Modelling Modelling Map Map Overlay Overlay to Produce to Produce Interrnedtate Intermediate Factor Factor Maps Maps 1 STRATIGRAPHY STRATIGRAPHY HEAT HEAT Factor Factor Factor Factor 1 ALTERATION ALTERATION Factor Factor 1 GEOCHEMICAL GEOCHEMICAL Factor Factor 1 EM EM Factor Factor Overlay of Overlay of Intermediate Intermediate Factor Factor Maps Maps to to Produce Producethe the Final Predictive Final Predictive Map Map Figure Figure 1. 1. VMS NJFEWflAL MAP VMS NERAL MINERAL-POTENTIAL MAP L Flowchart Flowchart for for the the 3-step 3-step GIS GIs modelling modelling of the VMS potential of the Archean Vermilion District District and and its its western western extension, extension, northeastern, northeastern, Minnesota. Minnesota. The Thethree threesteps stepsin inthe thestudy study are: are: 1) 1)Detailed Detailedgeologic geologicmapping mappingand andcompilation compilationof ofall allavailable availablegeological, geological, geochemical, geochemical,and and geophysical geophysical data. data. Conversion Conversionof ofall alldata datainto intoprimary primaryGIS GIsdatasets; datasets; 2) 2) Extracting Extractingand and enhancing enhancing the the features featuresof of the the primary primary datasets datasets that are important important for predicting VMS and 3) 3) Integration Integration of of the preliminary prelimhry maps using GIS GIs modelling p~dicting VMS deposits; deposits;and techniques techniquesthat that predict predictmineral mineralpotential. potential. 48 �LOW-GRADE LOW-GRADEMETAMORPHISM METAMORPHISMAND AND HYDROTHERMAL HYDROTHERMAL ALTERATION ALTERATION OF OF THE THE UPPER KEWEENAWAN PORTAGE LAKE VOLCANICS, MICHIGAN UPPER KEWEENAWAN PORTAGE LAKE VOLCANICS, MICHIGAN PUSCHNER, PUSCHNER, Ulrich, Ulrich, SCHMIDT, SCHMIDT,Susanne SusanneTh., Th., Mineralogisch-Petrographisches Mineralogisch-PetrographischesInstitut, Institut, Base!, J.. Department Department of Geological Basel. Switzerland Switzerlandand and BORNHORST, BORNHORST. Theodore Theodore J., Engineering Sciences, University, ~ n ~ i n e e rand and in~ Sciences,Michigan Michigan Technological ~echnolo~ical university, Houghton, ~ o u ~ h t o Michigan n,-~ichi~an 49931 4993 1 The ThePortage PortageLake LakeVolcanics Volcanicsin inthe theKeweenaw KeweenawPeninsu!a Peninsula of Michigan Michigan consists consists of aa thick thick succession interfiow sedimentary sedimentary successionof of subaerial subaerialtho!eiitic tholeiiticbasalt basalt !ava lava flows with scattered interflow !ayers. layers. These Theserocks rockshave havebeen beensubjected subjectedtotolow-grade low-grademetamorphic metamorphicand andhydrothermal hydrothermal alteration. emplaced during during this alteration alteration alteration.The Theworld-class world-classnative nativecopper copperdeposits depositswere were emplaced event. In the Keweenaw Peninsula, and elsewhere, the Portage Lake Volcanics are not event. In the Keweenaw Peninsula, and elsewhere, the widely widely exposed exposed at at the the surface surface because because of overlying overlying unconsolidated glacial sediments. However, Peninsula during However, subsurface subsurfacedrill drill core core were were obtained obtained in the Keweenaw Peninsula during exploration explorationfor for native native copper. Much Muchdrill drillcore, core,some someover over 100 100years old, still exists in excellent and locations. locations. Mr. Gordon Peterson excellent condition condition and and with original dri!l drill core logs and of of Calumet Calumetowns ownsand and continues continuesto to maintain maintain much much of the the existing existing drill drill core core and and allowed allowed us us to to sample samnlethe the core core for for our our research. Stoiber Stoiberand and Davidson Davidson (1959a, (l959a. b), in part, part. used a drill core portage Lake Lake Volcanics. Volcanics. corecross-section cross-sectionto topostu!ate postulatemetamorphic metamorphic zonation zonation within within the the Portage Four Fourdrill drillcore corecross crosssections sectionsthrough through the thePortage PortageLake Lake Volcanic Volcanic have been studied studied using using aa combination combinationof of microscopic microscopicinvestigation, investigation, XRD XRD technique, technique, and electron electron microprobe analysis, by Stoiber Stoiber & &Davidson Davidson(1959a. (1959a,b). b). These analysis, including including the drill core section studied by four are located located between Ahmeek and Copper Harbor and four dril! drill core core cross cross sections sections are provide km provide coverage coveragethrough through 3000m 3000m of of the the Portage Portage Lake Volcanics over a distance of 60 k m along alongstrike. strike. Interlayered Interlayeredsediments sedimentsand andse!ected selectedlava lavaflows flows can can be be used used as as marker marker horizons horizonsto to corre!ate correlatethe the stratigraphic stratigraphic zones zones between the different individual drill holes. Initial distinguished within Initial results resultsdemonstrate demonstratethat that two two main main minera! mineral associations associations can be distinguished the the amygdaloidal amygdaloidal tops tops of of the the basaltic basaltic !ava lava flows of the upper part of the Portage Lake Vo!canics Volcanics (above (above the the Greenstone Greenstone flow) flow) in the Copper Harbor to Ahmeek sections: (1) a zeolite-dominated laumontite-chlorite-corrensite±wairakite±quartz± zeolite-dominated assemb!age assemblage with laumontite-chlorite-corrensite±wairakite±quart calcite, with chloriteiquartz* chlorite±quartz± calcite, and and (2) (2) aa pumpellyite-epidote-dominated assemblage with calcite±prehnite±laumontite. The zeolite-dominated assemblage is more frequent in the the calcite±prehnite±laumontit upper upper part part of of the the section, section,but butwairakite wairakiteoccurs occurs only below the Hancock Conglomerate. The pumpellyite-epidote-dominated assemblage of the the lower part of the The pumpellyite-epidote-dominated assemblage is characteristic of sequence. sequence. Zeolite Zeoliteminerals mineralsare arepresent present throughout throughout the whole vertical section. Along strike, strike, aa temperature temperature gradient gradient is is observed observed with lower temperature conditions in the Copper Copper Harbor Harbor section section and and higher higher temperature conditions in the Ahmeek section near Ahmeek Ahrneek to the southwest. southwest. This Thiscorrelates correlateswith with the the distribution of native copper deposits increasing in frequency and size from Copper Harbor Ahmeek. increasing in frequency and size from Copper Harbor towards towards Ahmeek. The The most most abundant abundantphyllosilicates phyllosilicates in in the the upper part of the Portage Lake Volcanics are chlinochlore units of of aa lava lava chlinochlore and corrensite. Chlinoc!ore Chlinoclore is present in all morphological units flow, within all flow, i.e. in in the the amygdaloidal amygdaloidal flow top and in the massive interior, and within stratigraphic as determined determined by by electron electron stratigraphichorizons. horizons. The Themean meanof ofthe theinterlayered interlayeredcations cationsas microprobe microprobe analysis analysisis is around around 0.1 0.1 indicating indicatingon!y only very low mixed-layered mixed-layered components. components. Corrensite amygdaloidal flow flowtops. tops. In the eastern eastern most most Copper Copper Corrensite is more abundant in the amygdaloidal 49 �Harbor Harbor section, section, smectites smectites (saponite, (saponite,montmorillonite) montmorillonite)as as well well as asparagonite paragoniteand and pyrophyllite pyrophyllite are are also also present present in in the the zeolite-dominated zeolite-dominatedassemblage. assemblage. The complexmetamorphic metamorphicand andhydrothermal hydrothermal The Portage Portage Lake Lake Volcanics Volcanicswas was subjected subjectedto to aacomplex alteration event (Stoiber & Davidson, 1959, Bornhorst, 1997). The initial 1997). The initialresults resultsof ofthis this alteration event (Stoiber & Davidson, 1959, continuing continuing research research suggest suggest that the abundant laumontite-chlorite assemblage in most of the the upper upper part part of of the the Portage Portage Lake Lake Volcanics Volcanicswas was likely likely the the result result of of regional regional zeolite zeolite fades pumpellyite-prehnite-epidoteassemblage assemblageis is faciesmetamorphism metamorphism during duringburial. burial. The Thepumpellyite-prehnite-epidote likely In the the Copper CopperHarbor Harbor to to Ahmeek Ahmeek section sectionthe the likely related to a hydrothermal hydrothermal phase. In hydrothermal hydrothermalevent eventoccurs occursonly onlylocally, locally,but but when when present present itit is ispervasive. pervasive. References References . Bornhorst, Bornhorst, T.J., T.J., 1997, 1997,Tectonic Tectoniccontext context of of native native copper copper deposits depositsof of the the North NorthAmerican American Midcontinent Midcontinent rift rift system: system: Geological Geological Society Society of America Special Paper, 312, 312, p. p. 127127136. 136. Stoiber, E.S., l959a, 1959a.Amygdule Amygdulemineral mineralzoning zoningin inthe thePortage PortageLake Lake Stoiber, R.E. R.E. and and Davidson, Davidson, E.S., Lava v. 54, 54. p.1250Lava Series, Series, Michigan Copper Copper district. district. Part I: Economic Geology, v. p.12501277. 1277. Stoiber, R.E. and and Davidson, Davidson, E.S., E.S., l959b, 1959b.Amygdule Amygdulemineral mineralzoning zoningin in the thePortage PortageLake Lake Stoiber, R.E. Lava Lava Series, Series, Michigan Michigan Copper Copper district. district.Part Part H: IT Economic Economic Geology, v. 54, 54, p. p. 144414441460. 1460. 50 �Lake Superior's Superior's lUngs: Fault Systems Lake Rings: Clues Clues to to the the origin origin of of Polygonal Polygonal Fault Systems Deborah J. Wattrus Deborah E. E. Rausch Rausch and and N/gel NigeI J. Wattrus Large Lakes Minnesota Large Lakes Observatory, Observatory, University University of ofMinnesota Duluth MN 55812 55812 Duluth MN email: drausch2(did.umn.edu Polygonal Fault Fault Systems Systems (PFS) (PFS) are are aa recently recently recoenized recognized class class of of soft-sediment soft-sedimentstructures. structures. They result They result Polygonal from volumetric contraction contraction triggered triggered by by s&eresis-during syneresis during early early compaction compactionof ofultra-fine ultra-finesediments. sediments. PFS PFS from-volumetric were first described in Paleoeene Paleogene clavstones claystones from from the the North North Sea Seabasin basin (Cartwrieht. (Cartwright, 1994). In 2D cross1994). . sections, PES appear appear be be small small extensional extensional faults and in in map map view, view, the the faults faults have have aa polygonal polygonal appearance. sections, PFS faults and appearance. Cartwright and and Lonergan Lonergan (1996) measured measured the extensional Camwight extensional strain and determined determined that apparent apparent extension extension on any any given given horizon horizon was was uniform uniform in in all all directions. directions. They this that that the the fault fault systems systems could could on They concluded concluded from from this not not have have aa tectonic tectonic origin origin and and that that the the faulting faulting was was related related to to volumetric volumetric contraction contraction of of sediments sedimentsduring during early burial burial and and compaction. compaction. From North Sea, Cartwright Cartwright and and Dewhurst Dewhurst early From 3D 3D seismic seismic data data collected collected in the North (1998) identified identified specific specific criteria criteria of of PFS. PFS. These in other other (1998) These criteria criteria are are used used to to delineate delineate systems systems in sedimentary basins. Based normal faults faults with with small small sedimentary basins. Based on on these these criteria, criteria, PFS PFS are are typically typically close close networks networks of of normal throws that that occur occur over over aa large large area area in in sedimentary sedimentary basins. basins. The bound and and are are throws The systems systems are are layer layer bound occasionally tiered. They Theyonly onlyoccur occurin in very very fine-grained fine-grained sediments sediments (<2 (<2 micron). There Thereare areno noknown known modern analogues. analogues. - < Doughnut-shaped rings rings and and subcircular depressions have been observed in in the the side-scan records collected Doughnut-shaped subcircular depressions side-scan records collected in Lake Lake Superior (Bergson (Bergson and Clay, 1973; et al., 1984; Flood and and Johnson, Johnson, 1984; in 1973; Johnson et 1984; Flood 1984; Flood, 1989; 1989; and Anderson, 1997). The The described describedfeatures features are are 100-300 100-300 meters in diameter. diameter, 10-3- meters wide and 5 meters deep. Side-scan Side-scandata datacollected collectedby by the the Large Large Lakes Lakes Observatory Observatory (LLO) confirms confirms the widespread development of of ring ring structures structures throughout throughout much much of of the the basin. basin. The rings rings commonly commonly occur occur wherever wherever finefinedevelopment grained glacio-lacustrine glacio-lacustrine sediments sediments are are found. found. High-resolution reflection data data collected collected with with grained High-resolution seismic reflection boomer A and d echosounder echosounder systems systems show show that that the the nngs ringsare areassociated associated with with deformation deformationthat thattypically typically disrupts the the thir thifl Holocene Ilolocene surface sediments sediments and and elacio-lacustrine glacio-lacustrinesediments sedimentsbelow. below. The echosounder dismots data clearly shows that the underlying underlying glacio-lacustrine glacio-lacusmne exhibits extensive fracturing and small-scale small-scale ring or or faulting. Some Someof of these these fractures fractures and and faults faults continue to the surface where they coincide with a ring partial ring structure. Many, Many,however, however,are aretruncated truncated at at the the Holocene Holocene boundary. Throws Throwson onthe thesmall small faults are often less than 50 cm and the apparent displacement on these faults is typically normal. Some Some echosounder records show evidence apparently genetically unique deformation deformation sequences sequences in the of the echosounder evidence of apparently fine-grained sediments. sediments. These poorly resolved resolved horizon horizon with with little little coherent coherent These sequences sequencesare are separated separated by a poorly internal reflections. reflections. The well developed Thesequences sequencesabove aboveand andbelow belowexhibit exhibitwell developedinternal internalreflections reflectionsthat thatdo do not appear appear to be concordant. concordant: Both Bothsystems systemsshow showwell-developed well-developedfracture fractureand andmicro-faulting micro-faultingpatterns. patterns. observation suggests that these systems in the subsurface, and are This observation systems are confined to distinct intervals in therefore genetically genetically unique. unique. . ~ ~ ~ ~~ - ~ ~ It is proposed that the rings in Lake Superior expressions of PFS developed in the near surface surface Superior are surface expressions clays. AAcomparison comparisonwith withthe thecriteria criteriaoutlined outlinedby byCartwright Camwightand andDewhurst Dewhurst(1998) (1998)show showthat thatthe thenearnearsurface requirements for PFS development. surface sediment sediment and data from Lake Lake Superior Superior meet nearly all of the requirements development. The seismic data collected thus far indicate indicate that there are basinwide layer bound normal fault systems systems with small throws. The TheLake LakeSuperior Superiorglacio-lacustrine glacio-lacustrinesediments sedimentsare arecharacterized characterizedby by fine finegrain grainsize, size, high water content, content, and and aa clay clay fraction fractioncomposed composedpredominantly predominantlyof ofillite. illite. 51 �The following following model is proposed proposed to explain explain the the origin origin of the the rings rings on the floor of Lake Superior. High High sedimentation sedimentation rates associated with the retreat of ice ice out of the basin produced produced deposits deposits with with high high water water Sometimeafter after deposition, deposition, syneresis syneresis in the sediments sediments contents, which behaved like a colloidal fluid. Sometime tothe thedevelopment developmentof of fractures fracturesand and faulting faultingin in the the initiated volumetric volumetric contraction. This Thisprocess pocess led ledto initiated clays. The Thegrowth-fault growth-faultappearance appearanceof ofsome someof ofthe the faults faultssuggests suggests that that deformation deformation and and faulting faulting was was clays. syndepositional syndepositional and continued for some time. Since Since many of of the fractures fractures and and faults faults truncate truncate against against the the Holocene the deposition of Holocene boundary, we believe that most of the volumetric contraction ceased before the the the Holocene Holocene sediment. sediment. Some Somefractures fractureswhich whichcontinue continueto tothe thesurface surface appear appear to to act act as asconduits conduitsfor forthe the expressed expressed water. This Thisisissupported supportedby bysubmersible submersibleobservations observations(Flood, (Flood, 1989) 1989) that that appear appear to to suggest suggestthat that water in water venting venting from from the the lake lake floor floorremoved removed or or prevented prevented the the deposition depositionof of the the fine fineHolocene Holocenematerial materialin the the vicinity vicinity of of the the ring. ring. REFERENCES REFERENCES Anderson, graphic study Anderson,K.A., K.A., 1997, 1997,A A seismic seismic strati stratigraphic study of of western western Lake Superior Superior(M.S. (M.S. thesis). thesis): University University of of Minnesota, 74p. 74p. and Clay, Clay. C.S., C.S., 1973, 1973. Possible Possiblesyneresis syneresisorigin originof ofvalleys valleyson on the thefloor of ofLake Superior: Berkson, J.M.. f.M., and Lake Superior: Nature, Nature, 245:89-91. 24589-91. Cartwright, from geopressured Cartwright,f.A.. J.A., 1994, 1994.Episodic Episodicbasin-wide basin-widefluid fluid expulsion expulsionÂ¥fro geopressuredshale shale sequence sequencein in the the North 2222:447-450. :44 7-450. NorthSea Seabasin: basin:Geology, Geology, J.A., and sediments: Cartwright,.J.A., Cartwright. and Dewhursi, Dewhurst, D.N., D.N., 1998. 1998, Layer-bound Layer-bound compactionfaults faults infine-grained infine-grained sediments: Geol. 7. Geol.Soc. Soc. Am. Am. Bull., Bull., 110:1242-125 110:1242-1257. Cart-wright, .14. and Lonergan. Lonergan, L., Cartwright,J.A. L., 1996, 1996,Volumetric Volumetriccontraction contractionduring duringthe thecompaction compactionofofmudroc/cy: mudrocks: AA mechanism cale polygonal polygonalfault mechanismfor the the development development of regional-s regional-scale fault systems: systems: Basin BasinResearch, Research,v.8, v.8, p 183-193. 183-193. Flood. topography in Lake Superior: Superior: .1 Flood, RD., R.D., 1989. I989,Submersible Submersiblestudies studiesof of current-modified current-modified bottom topography J. Great GreatLakes Lakes Res., Res., 15:3-14. 15:3-14. Flood. C., 1984, for bedforms and Flood. R. R. D. and and Johnson, Johnson,TT.C., 1984,Side-scan Side-scan targets targetsin in Lake Lake SuperiorSuperior- evidence evidencefor sediment sediment transport: transport:Sedimentology, Sedimentology,v.v.31, 31,p. p. 311-333. 311-333. Johnson, and currents andfish Johnson,T. T.C.. C.,Halfman, Hal/man,J.D., J.D., Busch, Busch,W.H., W.H., and Flood, Flood,R.D., R.D., 1984, 1984,Effects Effects of bottom currents and fish on on sedimentation sedimentationin inaadeep-water. deepwater,lacustrine lacustrineenvironment.' environment:Geol. Geol.Soc. Soc.Am. Am.BulL, Bull., 95:1425-1436, 95:1425-1436. Mollard, patterns ononthe J.D.,1983, 1983,The Theorigin originofofreticulate reticulateand andorbicular orbicularpatterns thefloor floorofofthe theLake LakeAgassiz Agassiz basin basin Mollard. ID., in IT. and in Teller, Teller, J.T. andClayton, Clayton,L., L., eds.. eds.. Glacial GlacialLake Lake Agassiz: GeoL Geol. Assoc. Can. Can. Sp Sp. Paper, Paper,p. p. 355355374. 3 74. 52 �GEOLOGY OF THE TILDEN MINE, MINE,MARQUETTE MARQUEflE IRON IRONRANGE, RANGE, MICHIGAN MICHIGAN Glenn W. Scott Scott and Helene Helene M. Lukey, Lukey, Mine Engineering Engineering Department, Department, Tilden Tilden Mining Mining Company, P.O. Box 2000, 2000, Ishpeming, MI 49849 The Tilden hon Formation, Tilden Mine Mine produces produces iron iron ore ore from from the Negaunee Iron Formation, Menominee Menominee Group, Group, Marquette Range Supergroup in the Upper Peninsula of Michigan. As the base of the iron peninsula iron Marquette ~ a n ~ e ~ u p e r ~ r o u p formation is not exposed exposed at at the the Tilden Tilden Mine, the stratigraphic thickness is unknown. The The thickness exceeds 1000 feet and is probably underlain by and is gradational into thickness exceeds 1000 feet and is probably underlain bfand is gradational into Mine. The undifferentiated elastics and iron formation of the Empire Mine. The iron iron ore ore deposit deposit at at the the Tilden Mine hut, more Mine has has been divided divided into into geologic geologic domains based in part on lithology but, importantly on on the the metallurgical metallurgical response response based based on on bench bench tests tests and and in in the the processing processingplant. plant. The interaction of sedimentation sedimentation and diagenesis within growth fault controlled basin, metamorphism and metamorphism and supergene supergeneoxidation oxidationhas has resulted resulted in in aa complex complexsuite suiteof of ore oretypes, types,each each with specific, problems. Production specific, if not entirely entirely objective, blending characteristics and problems. Production since 1974 million long long tons tons of ore at a 1974 has been 126 126 million long tons of pellets from 355 million 1999, the Asof ofJanuary January1,1,1999, the proven proven and and probable long long range (30 (30 year) stripping ratio of 0.94. As mine mine plan reserve reserve was 628 628 million long tons of ore containing 233 million long tons of pellets at a stripping ratio 0.77. at a stripping ratio of of 0.77. Chlorite Chlorite schist schist defines defines the footwall footwall of the Tilden ore body. The The Martite Martite and and main main pit pit Carbonate Carbonate domains domainsdomains domainslie lie stratigraphically stratigraphicallybelow below the the CDffl CDffl Footwall Footwall and and Main Main Pit Pit Hanging wall intrusives. This is metallurgically "good ore" and is typically characterized This is metallurgically "good ore" and is typically characterizedby by high weight recovery (35-45+%), (35-45+%), low phosphorous, phosphorous, low slime slime iron and good grinding grinding media. The West PitJCDffl Pit/CDin Hematite domain in the west pit is stratigraphically stratigraphically between the CDffl CDIII this is is metallurgically metallurgically 'poor' 'poor' In contrast contrast to the martite domain, this hanging wall and footwall. In ore with higher slime iron, phosphorous and poor grinding. Mineralogically Mineralogically and and texturally, martite domain domain in in being being dominantly dominantly platey hematite hematite with with thin thin this domain domain differs differs from from the the martite (mm scale) chert laminae. The equivalent to the (mrn The Magnetite Magnetite domain domain is is the the stratigraphically stratigraphically equivalent West PitJCDIII. Pit/CDffl. Mineralogically, Mineralogically,the the ore oreconsists consistsprimarily primarily of of magnetite-siderite-chert magnetite-siderite-chertwith with variable hematite and silicates. silicates. This Thismineralogy, mineralogy, along along with with the the thin laminations, laminations,appears appears to to indicate a restricted basin and a reduced environment of deposition. environment There are several igneous igneous horizons horizonsatatthe theTilden TildenMine. Mine.The Theterm term'intrusive' 'intrusive' or or 'dike' 'dike' is is used for diabasic to porphyritic porphyritic to aphanitic aphanitic bodies, which vary from (semi) conformable sill-like horizons, usually interpreted as interpreted as synsedimentary sills but may be flows, to obviously crosscutting bodies, interpreted as dikes that may may be be feeders. feeders. The sills and majority of the crosscutting andlor the dikes appear appear to be of early early Proterozoic Proterozoic age, related to the Clarksburg Clarksburg Volcanics and/or Hemlock and Emperor Emperor Volcanics, but there there are are several several dikes that are interpreted interpreted to be of Keweenawan based on Keweenawan age agebased on the the magnetic magnetic signature. signature. 53 �AN ESTIMATE OF RATE DYNAMIC AND STATIC STATICSTRENGTH STRENGTH OF AGGREGATES: AN SENITIVITY OF SENITIVITY OF GEOLOGIC GEOLOGIC MATERIALS MATERIALS STANLEY J. VITTON, Department of & &Environmental Environmental Engineering, Engineering,GHATUPARTHJ GHATUPARTHI STANLEY J. S UBHASH, Department Mechanics, Michigan Michigan SUBHASH, DepartmentofofMechanical MechanicalEngineering—Engineering Engineering-Engineering Mechanics, Technological University, Technological University,Floughton, Houghton, Michigan Michigan49931 4993 1 The Michigan Michigan Department Department of ofTransportation Transportation (MDOT) (MDOT)has has initiated initiatedresearch research into into the dynamic dynamic fracture fracture characteristics of cement concrete concrete(PCC) (PCC) pavements. pavements. The main focus of this characteristics of aggregates aggregates in Portland cement this aggregateportion portion in PCC. research is to study the fracture fractureand and strength strcngth characteristics of the coarse aggregate wide range Due to the wide range of of rock rock types types inin Michigan, Michigan,there thereisisaacorresponding correspondingrange range of ofcoarse coarse aggregate aggregate used PCC. PCC. In used In addition, addition,aasignificant significant amount amountofofblast blastfurnace furnaceslag slag isis produced produced and used used in PCC. To study unconfined uniaxial uniaxial compressive of study the static static and dynamic dynan~icstrength strength of of the the aggregates. aggregates, unconfined compressive strength of three limestones, limestones, four fourdolomites. dolomites,two twobasalts basaltsand andthree three blast blastfurnace furnaceslags slags were were conducted. conducted. InIn addition, conditions. The addition,tests tests were wcre also also conducted conducted under dry dry and and saturated saturated conditions. The studies studies revealed that that the the aggregates exhibited exhibited the lowest compressive strcngth, strength,followed followed by the limestones and dolomites. slag aggrcgates aggregates exhibited exhibited the the highest highestcompressive compressivestrength. strength. The The compressive compressive strcngth strength data dataof of The basalt aggregates rocks were were found found to to have haveaastrong strongcorrelation correlationtotothe thebulk bulkdensities. densities. Both the dry thy dry and saturated rocks and saturated aggregates revealed revealed aahigher higher compressive compressive strength strength under under dynamic dynamic loads compared compared to to in dynamic compressive strcngth strength over over its static strength is the static loads. The Thepercentage percentage increase increase in identified as which may identified as aa measure measure of the rate sensitivity sensitivityofofthese these aggregates, aggregates, which may have have considerable considerable geologic interpretation. relevance in applications applicationssuch such as rock blasting, underground construction, and geologic interpretation. The aggregate used used in in the thestudy studywere wereobtained obtainedfrom fromquarries quarriesininMichigan, Michigan, Ohio Ohio and and Ontario, Ontario, while while blast furnnace ftrnnace slags obtainedfrom from steel steelmills mills in in Sault Stc. Ste.Marie. Marie, Ontario Ontario and and Detroit. Detroit, the blast slam were obtained Michigan. The limestones investigated Michigan. Thethree threelimestones investigated were from the Fibron Limestone Limestone Formation of of middle middle Silurain City Limestone Silurain age age (Port Inland Inland Quarry), Quarry), the the Roger City Limestone Formation of of middle middle Devonian Devonian age age (Presque Isle Quarry), Quarry), and the the Bay Bay Port Port Limestone Limestone of of late Mississippian Mississippian age age (Bay (Bay County County ~ Road (Presquc Isle oad Commission Quarry). The Dolomite Formation of middle Thedolomites dolomiteswere wcre from from the the Engadine Dolomite middle Silurian Silurian (Cedarville Quarry), age (Cedarvillc Quarry), the the Raisin Raisin River River Dolomite DolomiteFormation Formationofoflate lateSilurian Silurianage age(France (FranceStone Stone Quarry). the Lucas Dolomite Dolomite Formation of Quan-y), of middle middleDevonian Devonian age age (Rockwood Quarry), Quarry), and a stratigraphically later dolomite from the Lucas Formation (Denniston Quany). stratigraphically later dolomite from the Lucas Formation (Denniston Quarry).The Theigneous igneous rocks rocks tested were were from from the Portage Lake Lake Volcanics Volcanics (Moyle (Moyle Quarry) Quarry)and and the the Paleoproterozic Paleoprotcrozic volcanics near near Bruce Mines, Ontario Ontario (Ontario (OntarioTraprock Traprock Quarry). Quarry). The aggregates weresubjected subjectedtotouniaxial uniaxial compressive compressiveloads loadsatattwo twodifferent different strain strain rates. rates. Low Low strain aggrcgates were strain rate (static) wcre were conducted conducted on on aa MTS servohydraulic and high strain rate experiments experiments (static) sc~ohydraulicmachine machine and strain rate rate experiments (dynamic) (dynamic) were conducted conducted on on aa modified modified split Hopkinson Hopkinson pressure pressure bar (MSHPB). (MSHPB). The The experiments Split bar (also çalsocalled calledaa Kolsky Kolsky bar, bar, Kolsky, 1949) Split Hopkinson Hopkinson pressure pressure bar 1949)technique technique has has been been widely used for for dynamic testing testing of of materials materialsatathigh highstrain strainrates ratesininthe therange rangeofof10'-104 IO'-l 4 s"' s' (Ravichandran used (Ravichandran and and Subhash, 1995). It consists consists of aa striker strikerbar, bar, an anincident incidcntbar barand andaatransmission transmission bar, bar, as as shown shown Subhash, 1995). schematically in Figure I.1. The schematically in Thespecimen specimen totobe becharacterized characterized isis placed placed between between the incident incident and and transmission bars. bars.The Thefree freeend endof of the theincident incident bar bar isis impacted impactedby by the the striker striker bar, bar, which which is launched transmission launched from aa gas gun, at aa predetermined predetermined velocity. velocity. The pulse in in the incidcnt incident The impact impact generates generates aa compression pulsc from specimen,subjecting subjectingitit to to the the required required compressive compressiveloading. loading.AApart part of of bar which travels towards the specimcn, and the the rest rest is is rcflected reflected back back into into the the incident bar as a this pulse is transmitted to the transmitter bar and tensile pulsc. pulse. Strain Strain gages gages are are mounted mountedat at the the center center of of each bar to measure the magnitude and tensile each bar measure the magnitude and duration of of these these stress stress pulses. pulses. Based Based on on one-dimensional one-dimensional calculations, calculations, it can can be be shown shown that that the the magnitude of the transmitted pulse gives a measure of the stress to which the specimen is subjected the transmitted pulsc gives a measure which the specimen is subjected and the the magnitude and magnitude of the reflected reflected wave gives aa measure measure of the the strain strain rate rate within withinthe thespecimen. specimen. 54 �integrating Integratingthe the strain strainrate ratewith withrespect respecttototime timeyields yieldsthe thestrain strainin in the the specimen. specimen.Thus, Thus. the thestress-strain stress-strain a material can be obtained at high strain rates. response response of a material can be obtained at high strain rates. A A minimum minimum often of tensamples sampleswere weretested testedfor foreach eachaggregate aggregatetype type in in each each condition, condition, e.g. e.g. thy, dry,saturated, saturated, static, and dynamic dynamic conditions. The Theresults results of of the the static static and and dynamic dynamic testing for thy dry conditions conditions are arc presented in Figure slags arc are identified identifiedas as sample sampletypes types I.1. 2, 2, and 3; the 2, where whcre the the blast blast fi1rnace furnace sla&s the Figure 2, and the basalt and 6; the dolomites are sample types 7 through 10; lirnestones are sample types 4, 5, limestones are sample types 4, 5. and the dolomites arc 10; and the basalt sample sample types are II11and and12. 12. In addition, the bulk density of the rock types are are also alsopresented. presented. ItIt can can be be seen seen from from this thisdata datathat thatthe theslags slagsexhibited exhibitedthe thelowest lowest strength strength followed followed by the the limestones limestones and and addition, itit can can be be sccn seen that that there there is is aa good dolomites dolomites with withthe thebasalts basaltshaving havingthe thehighest higheststrength. strength. In In addition, good of the the slag!,, slags, all of the correlation with with bulk bulk density. density. With the partial exception cxception of the materials materials tested tested showed an increase indicating that that these materials are increase in in dynamic dynamic strength strength over o v a the static strength, strength, indicating rate rate sensitive. sensitive. The blast blast furnace furnacc slag slag showed showed relatively relatively low low strength strength both both in in satic satictesting testingand anddynamic dynamictesting. testing. However, it can be strength and strength in the range of the be seen that that the the air air cooled cooled slag had both low strcngth the limestoncs dolomites. The liniestones and dolomites. The two two data data sets sets shown shown for for the the air air cooled cooled slag slag represent represent samples samples form form two different blocks of slag slag material material assumed assunicd to have been been cooled cooled at different rates depending on where it was deposited. The blast fijrnacc slag is formed from the gangue gangue and and secondary secondaryconstiuents constiuents whcre deposited The blast furnacc slag is formed from the in iron ore as and has has aa chemical chemicalcomposition compositionof ofCaO, CaO,Si02, Si02, A103 Al03 as well as coke residue and limestone and and MgO with trace amounts of of.sulphur and some some alkalis alkalis (Smolczyk, (Smolczyk,1980). 1980). The final structure of sulphur and the slag depends on the cooling conditions conditionsof ofthe themolten moltenslag. slag. When When itit is allowed to cool slowly it forms into into aadense densecrystalline crystallinestructure structurewith with properties properties similar similartotorock. rock. When the molten molten slag slag isis quenshed quickly with limited amonts of water, it traps steam in the mass and produces a porous, quenshed quickly with limited amonts of water, mass and produces a porous, glassy properties. However, slag is formed it also glassy material material with poor mechanical propcriies. Howcvcr, when the air cooled slag has different rates of cooling depending on its location when when deposited deposited for for cooling. cooling. The The molten moltenslag slag exposed to the then the slag at the the atmosphere atmosphere (top) will cool more more quickly then the bottom bottom of ofthe thedeposit. deposit. The appearance of pores. The statification statification when when cooling cooling isis cieariy clearly observed observed by a color straification and the appearance The test data data shows shows this this with with aa significant significant increase increase in strength strcngth with test samples from two different blocks from from the the same same deposit. deposit, i.e., i.e., top topand and bottom bottom locations. locations. In the case of the tested, itit can can be be sccn seen that that the the trap trap rock rock basalt basalt has aa higher strength then the the basalt.s basalts tested, Portage Portage Lake Lake volcanic volcanic basalt. basalt. in In a simplied simplied framework it is assumed that the cooling rate of the trap trap rock basalt was longer which were were formed formed formed formed near near or or at the longer than the Portage Portage Lake voleanics, volcanics, which the surface. surface. This is also also seen in in the the crystalline crystallinc structure structurc of the two basalts, with the trap trap rock rock basalt basalt having a more uniform crystallinc crystalline structure structure and and the the Portage Lake Lake volcanics volcanics a more inconsistent and random crystalline crystallinestructure. structure. The The sedimentary sedimentarylimestones limestones and and dolomites dolomitesalso alsoshow showrate ratesensitivity. sensitivity. It is interesting interesting to note in the the limestones that the the static strength strength for the three three limestones limestones tested tested are relatively relatively close, close, while the the limestones that dynamic dynamic strength strength are archigher. higher. In particular, the Silurian limestone (Port Inland) showed significantly significantly higher rate sensitivity than does the Devonian limestone (Preque Isle) or the Mississippian limestone sensitivity (Prcque Islc) limestone With the exception However, this same trend is not seen in the doloinites. trend seen in the dolomites. With cxception of the the (Bay County). County). Howcvcr, this Silurian dolomite (Cedarville) (Cedarvillc)there thereisis an an increase increase in in both Silurian dolomite both the static static and and dyanmic dyanmic strength strength with with geologic geologic age. age. Previous research has has shown shown that that for Previous research for aa brittle brittle material, material, the the damage damage evolution evolution under under applied applied compressive microeracking at defects such a pores, inclusions, second compressive loading is intimately related to microcracking second phase particles, twin grain boundary intersetcions and triple point grain boundary boundary junctions junctions (Lanford, (Lanford, 1977). However, strain rate the the rate rate sensitivity of a However, under dynamic loading conditions above a critical strain atiributted to to the the inertia-dominated inertia-dominateddynamic dynamiccrack crackgrowth growthfrom fromprc-existing pre-existingflaws. flaws. Thus, material is attributted 55 �response responseofofaadefect defecttotoloading loadingisisinertia inertiadependent, dependent,i.e. i.e,the thetime timeitittakes takestotoovercome overcomethe theinertia inertiaofofthe the strength. Therefore, microcrack, more defects are loaded in the material leading to higher microcrack, more defects are loaded in the material leading to higher strength. Therefore, ititisis speculated speculated that thatififdynamic dynamicloading loadingstresses stressesaalarger largerportion portionofofa amaterial's material'sdefects, defects,then thenititmay maybebe lithification may possible possiblethat thatgeologic geologicmechanism mechanismsuch suchasasformational formationalhistory, history,compaction compactionand and lithification maybebe better bcncrobserved observedunder underdynamic dynamicfracture fracturetesting. testing. References References Koisky, Kolsky,H., H.,1949, 1949,AnAnInvestigation Investigationofofthe theMechanical Mechanicalproperties propertiesof ofMaterials MaterialsatatVery VeryHigh HighRates Ratesofof Loading. Proc.R.R.Soc. Sue.London, London,B62, B62,pp. pp.676-700. 676-700. Loading,Pmc. Ravichandran. Ravichandran,G. G.and andSubhash, Subhash, G., G., 1995, 1995, A A Micromechanical MicromechanicalModel Modclfor forHigh HighStrain StrainRate RateBehavior Behavior 32 Ceramics,lifiernalional International.Journal JournalofofSolids Solidsand andSiructures. S/ruc/ure.~. 32[17/IS], [I 71181,pp. pp.2627-2646. 2627-2646. ofofCeramics, Smolczyk, 1950, Smolczyk,H.G.. H.G., 1980.Slag SlagStrcture Strctureand andIdentification IdentificationofofSlags, Slags,Inter. Inter.Congress Congressononthe theChcmistry Chemistryofof Cement, editions Septima, Cement,7"7*ed. ed.1980. 1980.Paris Parisa aeditions Septima,pp. pp.3-17. 3-17. High HighSpeed, Speed,44Channel Channel Digital DigitalOscilloscope Oscilloscope PR040) (NicoletPRO4O) (Nicolet I I Wheatstone Bridge Strain gage - 1 Striker StrikerBar Bar - specimen m m Incident IncidentBar Bar I Transmission TransmissionBar Bar Modifidied ModifidiedSplit SplitHopkinson HopkinsonPressure PressureBar Bdrwit witMomentum MomentumTrap. Trap. Figure Figure I 1 0 = 0 S d 22 Q F - 700 V 0 H 0 ii Ji 1' 3.5 ID — 600 m U. 5 C 0 3.0 Stat 500 C., E Dens 0 0 > 0 C 0 1 400 I I 300 I .4: 2.5C I 0 C, V C 1 I I; 200 2' VA = 0 C 2.0 00 0 0 1.5 1 2 3 4 5 6 7 6 9 10 11 Sample SampleType Type Figure Figure22 Dynamic Dry Conditions. Dynamicand and Static Static Strength Strength Tests Tests in Dry Conditions. 56 56 12 13 �8 Palmer Palmer Gneiss; Gneiss; aa large, large, low-grade low-grade shear shear zone zone ' 1 ~ e b s t e rC. C. , L., 'Cambray 'cambray W. W. F., F., 2Scott, '~cott,0., G.,'~ordstrom, 'Webster, 2Nordstrom, P. P. 'Widson, 'Wilson, E. 'Dept. Sciences, Lansing, MI 48824 ' ~ e ~Geological Geological t. Sciences, Michigan State State University, University, East Lansing, 22Cleveland Cliffs Mining Co, lshpeming, MI Cleveland Cliffs Ishpeming, MI The Marquette Marquette Trough Trough is is interpreted interpreted to to be be an an asymmetric asymmetricrift-related rift-related basin; basin, truncated truncatedstratigraphy stratigraphy southern margin and a rollover on the southern rollover structure structure to to the the north north provides provides evidence evidencefor for this this interpretation, Marking interpretation. Markingthe thesouthern southernboundary, boundary,between between the the Archean Archeangneiss gneiss and and the the Early Early Proterozoic Proterozoic metasedimentary metasedimentary rocks of the Marquette Marquette Range Supergroup, is the Palmer Gneiss (PG). Van Hiseand andBayley Bayley(1897) (1897)first firstinterpreted interpretedthe the PG PG as as aa comminuited, comminuited, sericitzed and VanRise partly silicified silicified phase of of the lower lower Precambrian Precambrian gneiss. In In1968, 1968,Gair Gairand andSimmons Simmonsconcurred concurred with theft their interpretation and offered offered three three possible modes of genesis: (1) alteration and shearing of Precambrian Precambrian rock during during faulting, faulting, (2) (2) migration migrationof of fluids the contact contactbetween betweenlower lowerand and fluids along along the middle Precambrian rocks, and (3) alteration of of aa regolith regolith during duringfolding. folding. Since the the time time of of mapping by Gair Gair and Simmons, Simmons, Tilden Tilden open pit mining mining operations have exposed the PG. Large Large shear bands, with approximately approximately2.5 2.5 meter meter spacing, spacing,indicate indicatethe the importance importanceof of ductile ductile zone (Figure (Figure 1). Foliation deformation during the formation formation of the PG in a low-grade shear zone Foliation measurements show measurements show aa steep steep NNE NNE dipping dippingS-foliation S-foliation being being cut cutby by aa shallow shallowNNE NNE shear shearband, band, indicating a reverse sense sense of shear (Figure 2). The Thefolds foldsmeasured measuredin in the the adjacent adjacent iron ironformation, formation, at the Tilden Tilden and and Empire Empire mines, mines, consistently consistentlyplunge plunge gently gently towards towardsthe the WNW, WNW,reflecting reflectingthat that they were formed zone. The formed under under the same strain strain conditions conditions as the shear zone. TheNNE-SSW NNE-SSW compressive compressive direction direction corresponds corresponds to previous studies in the area (Myers, 1984). Whole Whole rock rock chemical analysis chemical analysis of of the PG PG suggests suggeststhat that these these altered altered rocks rocks are are not not granitic granitic in in origin, origin, but but rather rather basaltic. The age. The The basaltic basalticnature nature indicates indicates that the PG is of Proterozoic age. The Penokeon Penokeon Orogeny, collision of an island island arc arc with with the the Lake Lake Superior Superior Craton, Craton,caused caused closure closure of of the the Marquette MarquetteTrough Trough that reactivated preexisting normal faults and resulted in a reverse dip dip slip. slip. Strain Strain was was concentrated concentrated in in aa portion portion of of aa metadiabase metadiabasesill sillthat that was was dragged dragged into into the the shear shearzone zone causing causing 57 �alterationand andshearing shearingofofthe therock rocktotoform formthe the PG. A 34% loss of volume is the calculated effect alteration PG. A 34% loss of volume is the calculated effect alterationfrom fromthe themetadiabase metadiabaseprotolith protolith to the PG (Gresens, 1967). ofofalteration to the PG (Gresens, 1967). References: References: Gair, J. E., and Simmons, G. C, 1968, Palmer Gneiss-an example of retrograde metamorphism Gair, J. E., and Simmons, G. C, 1968, Palmer Gneiss-an example of retrograde metamorphism alongan anunconformity: unconfonnity:U.S. U.S.Geological GeologicalSurvey Survey Professional Paper, 600-D, p. D186along Professional Paper, 600-D, p. D186D194. D194. Gresens, R L., 1967, Composition-volume relationships of metasomatism: Chemical Geology, v. Gresens, R. L., 1967, Composition-volume relationships of metasomatism: Chemical Geology, v. 47-65. 2,2,p.p.47-65. G., 1984, Structural analysis of foliated Proterozoic metadiabase dikes in the MarquetteMyers, Myers, G., 1984, Structural analysis of foliated Proterozoic metadiabase dikes in the MarquetteRepublicregion regionofofNorthern NorthernMichigan: Michigan:Master's Master's Thesis, Michigan State University. Republic Thesis, Michigan StateofUniversity. Van Hise, C. R . , and Bayley, W. S., 1895, The Marquette iron-bearing district Michigan: U.S. Van Hise, C. R., and Bayley, W. S., 1895, The Marquette iron-bearing district of Michigan: U.S. 28, p. 608. GeologicalSurvey SurveyMonograph Monograph28, Geological p. 608. East West 2.5 Fig.11Footwall FootwallShear ShearZone, Zone,Tilden TildenMine Mine Fig. N\ . -*- (Palmer Gneiss) (Palmer Onees} o • Pole to mean C'-foliation Pole to mean C-foliation Pole to mean S-foliation Pole to mean S-foliation Slip direction Mean S-C' intersection A Slip direction * Mean S-C. intersection S-C' intersections S-foliation plane S-foliation plane Top of shear zone S-C' intersections \ Top of shear zone C'-foliation plane C-foliation plane StereonetofofFootwall FootwallShear ShearZone, Zone,Tilden Tilden Fig.22Stereonet Fig. 58 58 �IS ISTHE THEJANICE JANICELAKE LAKEUNCONFORMITY UNCONFORMITYAALINK LINKBETWEEN BETWEENTHE THEHEARNE HEARNE LAKE VOLCANIC ARC? CRATON CRATONAND ANDTHE THELA LARONGE RONGE- LYNN LYNN LAKE VOLCANIC ARC? YEO. YEO,Gary GaryM., M.,Saskatchewan SaskatchewanEnergy Energyand andMines, Mines, 1914 1914Hamilton HamiltonSt., St.,Regina, Regina,SK, SK, - S4P 4V4, gary.yeo@sem.gov.sk.ca The TheWollaston WollastonDomain, Domain,aaNE-trending, NE-trending, 75km 75 kmwide widebelt beltofof2076-1860? 2076-1860?Ma Maparagneisses, paragneisses, overlying Archean orthogneisses along the eastern edge of the Heame Craton overlying Archean orthogneisses along the eastern edge of the Hearne Cratonin in Saskatchewan, Saskatchewan,isis separated from 1910-1875 Ma arc volcanics and sediments of the La Ronge Domain by (a) (a) separated from 1910-1875 Ma arc volcanics and sediments of the La Ronge Domain by Archean Archeanorthogneisses, orthogneisses,poorly poorlyknown known supracrustal supracrustalrocks, rocks, and and minor minor Proterozoic Proterozoic plutons plutons of of the the Peter PeterLake LakeDomain, Domain,(b) (b)1855 1855Ma Mamonzogranite monzograniteto toquartz quartzdiorite diorite of of the the Wathaman Wathaman Batholith, Batholith, and and (c) (c)the theRottenstone RottenstoneDomain, Domain,aamigmatitic migmatiticbelt beltof of sediments, sediments,similar similarto to those those of of the the La La Ronge Ronge Domain, Domain, intruded intrudedby by tonalite. tonalite. The Theboundary boundarybetween betweenWathaman Wathaman Batholith Batholith and and Wollaston and Lake Shear Zone. While this Parker Peter PeterLake Lakedomains domainsisisthe theNeedle NeedleFalls Falls-- Parker Lake Shear Zone. While thisisisaafundamental fundamental break breakbetween betweenancient ancientcrust crustto tothe thewest westand andjuvenile juvenile crust crustof of Trans-Hudson Trans-Hudson Orogen Orogen to to the the east, east, there thereisisno noevidence evidencethat thatititisisaacollisional collisionalsuture. suture. Within Within the theWollaston Wollastonsupracrustal supracrustalsuccession, succession,the the Janice Janice Lake Lake Conglomerate, Conglomerate, which can be traced discontinuously for 245 km along the eastern margin of the belt, be traced discontinuously for 245 km along the eastern margin of the belt, marks marks aa major major unconformity. The conglomerate is polymictic, with clasts derived from underlying unconformity. The conglomerate is polymictic, with clasts derived from underlying strata strata predominating. predominating.Locally, Locally,ititincludes includesfanglomerate. fanglomerate.Although Althoughthe theconglomerate conglomeratecannot cannotbe betraced traced westward, westward,younger youngercalc-silicate calc-silicatebearing bearingstrata, strata,locally locally resting restingon on various various older olderlithostratigraphic lithostratigraphic units, units,mark markthe thepresence presence of of aa profound profound unconformity. AAcorrelative correlativeunconformity unconformityhas hasbeen been recognized recognizedininthe theManitoba Manitobasegment segmentof of Wollaston Wollaston Domain, Domain, another another200 200 km km to to the the northeast. northeast. In In the the absence absence of of any any evidence evidence for for thermal thermal uplift (ie. igneous igneous activity), uplift and extension extensionin in Janice Janice Lake Lake time time must must be be due due to to mechanical mechanical flexure flexure of the crust. AAprobable probablecause causeisis Lynn Lake tectonic tectonicloading loadingof of the the eastern eastern margin margin of of the the Hearne H e m e craton craton as as the the La Ronge --Lynn Lake volcanic volcanic arc LaRonge Ronge-arcconverged convergedtowards towardsitit over overan an east-dipping east-dippingsubduction subduction zone. zone. Volcanism Volcanismininthe theLa Lynn LynnLake Lakearc arcbegan beganabout about1910 1910Ma Maand andreached reachedaa peak peak about about 1880 1880Ma. Ma. Following Followingarc-craton arc-craton collision collisionat at about about1860 1860Ma, Ma, the the sense senseof of subduction subductionwas was reversed reversed and the Wathaman Batholith was was emplaced emplaced at at about about 1855 1855 Ma. Geochemical Geochemicalevidence evidencesuggests suggests that the La Ronge segment of the thearc arcdeveloped developedin in aatransitional transitionalcrust crust setting, setting, close close to to or or partly partly over marginal continental crust. Tectonic Tectonic loading loadingof of the the outboard outboard margin margin of of the the Heame Heame Craton Craton would have resulted in an inboard flexural bulge, with consequent normal faulting and erosion over its crest and deposition inboard flexural bulge, with consequent normal faulting of coarse clastics on its flanks. As convergence proceeded, the bulge of coarse clastics on its flanks. As convergence proceeded, the bulgewould would have have migrated migrated cratonward cratonward and and died died out, out, creating creatingaa diachronous diachronous regional unconformity. Preservation Preservationof of coarse coarse clastics clasticswould would be be favoured favoured only only on the the trailing, eastern flank of the bulge, however. The The approaching approachingvolcanic volcanic arc arc would would have have become a bather barrierto tothe the open open ocean, ocean, and and accumulation accumulation of evaporites evaporitesand and carbonates carbonates in in the the resulting resulting restricted restricted basin would have been increasingly favoured. This by the the This hypothesis hypothesisalso also explains explains the the dramatic dramatic change change in sedimentation recorded by Wollaston of Wollastonparagneisses. paragneisses. Following Following2100 2100Ma Marifting, rifting, the the widespread widespread Lower Sequence of psammopelites, psammopelites,pelites, pelites, and and feldspathic feldspathic psammites accumulated accumulated on a passive margin. As As arcarcahead of of craton craton convergence convergenceproceeded, proceeded,coarse coarseclastics clastics were were shed shedoff off aa migrating migrating flexural flexural bulge bulge ahead the arc. Upper Sequence calc-silicate rich strata subsequently accumulated in the developing the arc. Upper Sequence calc-silicate subsequently in the developing foreland basin. basin. foreland 59 �- (a) Rift Rift Sequence (Courtenay (Courtenay - Cairns Cairns Lake LakeBelt): Belt):2100 2100 Ma Ma Rift Stretched continental crust sediments—_ * + t ÷ j* + %+ + L+ + ± Superior Cratori? - Hearne Craton ÷+ ÷ + %+ + A+ + + ±1 ++ ÷ 4• + Oceanic crust - ,.., La Range Volcanic Volcanic Arc Arc (1910- 1860 Ma) (b) Lower LowerSequence: Sequence:2100 2100- 1880 1880Ma I - (c) Upper Sequence: 1880 1880-1860 1860 Ma Ma Restricted basin Restricted basin Uplift and erosion (uncónforrMy) Uplift (uncbnformity) (d) Cord illeran Margin: 1860 - 1830 Ma Foreland sedimentation tharnan (1855 Ma) Reference Reference +'>, : <;s Yeo, ci. G. 1998. 1998 A systems tract approach oach to totthe stratigraphy of paragneisses isses in in the the southeastern southeastern Wollaston Domain: Wollaston Domain: Saskatchewan Saskatchewan Energy Energy and and Mines, Mines, Misc. Misc. Rep. Rep. 98-4, 98-4, p36-47. p36-47. 60 �LOW-RESOLUTION OFF EARLY PROTEROZOJC LOW-RESOLUTION SEQUENCE SEQUENCE STRATIGRAPHY STRATIGRAPHY O EARLY PROTEROZOIC PARAGNEISSES, PARAGNEISSES, WOLLASTON WOLLASTON DOMAIN, DOMAIN, SASKATCHEWAN SASKATCHEWAN YEO, Saskatchewan YEO, Gary Gary M., M., SaskatchewanEnergy Energy and andMines, Mines,1914 1914Hamilton HamiltonSt., St.,Regina, Regina,SIC, SK. S4P S4P4V4, 4V4, gary.yeo @sem.ov.sk.ca The Wollaston Wollaston Domain 75 km km wide wide belt belt of of of of early early Proterozoic Proterozoic paragneisses, paragneisses, The Domain is is aa NE-trending, NE-trending, 75 overlying Archean orthogneisses the eastern eastern edge edge of of the overlying Archean orthogneisses along along the the Hearne Hearne Craton; Craton; west west of of the the TransTransHudson Hudson Orogen. Orogen. Bimodal Bimodal volcanics volcanics near near the the base base of of the the paragneiss paragneiss succession succession give give aa U-Pb U-Pb zircon zircon age granitic plutons plutons range range from from 1.81 to 1.84 Ga. A age of of 2.08 2.08 Ga. Ga, while while minor minor gabbro gabbro and and granitic 1.81 to 1.84 Ga. A threethreeyear, year, 1:20,000 1:20,000 scale scale mapping mapping project project was was begun begun in in 1997 1997in in the the southern southern part part of of Wollaston Wollaston Domain Domain to to improve improve understanding understanding of of the the stratigraphy stratigraphy and and structure structure of of the the guesses, gnesses, which which underlie underlie the unconformity-type unconformity-type uranium the uranium deposits deposits of of Athabasca Athabasca Basin. Basin. A A basic basic strategy strategy of of the the project project is is to to map outward from Archean inliers so as to be able to work out the stratigraphic succession in map outward from Archean inliers so as to be able to work out the stratigraphic succession in spite of of complex complex deformation deformation and spite and high high metamorphic metamorphic grade. grade. Four episodes of of regional regional deformation deformation can can be be distinguished. distinguished. D Dll involved Four episodes involved isoclinal isoclinal folding, local local basement basement ovenhmsting, overthrusting, and and transposition transposition of of primary primary layering layering into into the the SSIl foliation, foliation. folding, D2 involved involved tight tight to D2 to isoclinal isoclinal folding folding about about NE-trending NE-trending axes, axes, superposition superposition of of aa strong strong 52 S2 foliation, and and local local thrust thrust faulting. faulting. D3 foliation, D3 involved involved local local open open folding folding about about SW SW to to NW N W axes, axes, and and local brittle-ductile fault fault movement. movement. D4 D4 involved involved minor minor sinistral sinistral offset offset on on aa series series of of N N to to NNW NNW local brittle-ductile trending brittle trending brittle faults. faults. Two lower granulite facies) (upper amphiboIite amphibolite - lower Two episodes episodes of of high high T T -- low low P (upper metamorphism are associated associated with Dl and Peak with Dl a n 1)2 d ~ deformation. def&mation. 2 Peak P-T PITconditions, conditions. reached reached during during metamorphisk are D2, 750°C and and 55 kbar. kbar. Along the eastern margin of the belt, however, metamorphic D2, were were about about 750° Alone the eastern margin of the belt. however, metamorphic grades are are relatively relatively low lower amphibolite facies). grades low (upper (upper greenschist greenschist - lower Two widespread Two widespread paragneiss paragneiss successions successions are are distinguished distinguished in in Wollaston Wollaston Domain: Domain: aa lower lower Pelite Pelite Unit, Unit, and and an overlying overlying Psammite Psammite Unit. Unit. These These can can be be subdivided subdivided into into at at least least eight eight subunits subunits in the the Foster Foster -- Daly lakes area. Two lakes area. Tworestricted restricted successions successions are are also also recognized: recognized: aa basal basal package package in (Courtenay Lake Lake and (Courtenay and Souter Souter Lake Lake Formations), Formations). preserved preserved locally locally along along the the eastern eastern margin margin of of the the domain, and the heterogenous Hidden Bay Assemblage of interlayered quartzites, pelites. domain. pelites, amphibolites. calc-silicates, amphibolites, calc-silicates, and and marbles, marbles, which which likely likely overlies overlies or or is is correlative correlative with with the the Psammite Unit Unit near near Wollaston Wollaston Lake. Lake. The demonstrated to to Psammite TheCourtenay Courtenay Lake Lake Formation Formation has has been been demonstrated be a rift succession, while a passive margin margin setting is inferred inferred for for the the overlying overlying Pelite and Attempts to to determine the depositional setting of individual lithologic units, Psanunite units. Attempts Psammite however, have been restricted to to the the low-grade low-grade rocks rocks along along the eastern margin of of the the domain. domain. Although sequence Although sequence stratigraphy stratigraphy has has become become aa standard standard technique technique for for analysis analysis of of platform platform and craton margin strata, there have been few attempts to apply it to metamorphosed and deformed rocks. rocks. As which deformed As demonstrated demonstrated here, here, itit has has great great potential potential in in interpreting interpreting strata strata in in which obliteration of primary primary features limits application of facies models or comparison with modern obliteration modem sedimentary environments. environments. Two major sequence boundaries subdivide the succession. succession. The The unconformity on Archean basement can be recognized recognized without without difficulty. difficulty. The Thebase baseof ofthe theJanice JaniceLake LakeConglomerate Conglomerate(Unit (Unit Wo) marks a second unconformity. unconformity. To To the the west, west, where where the the conglomerate conglomerate is is absent, this unconformity corresponds to the base base of of the the calcareous rocks rocks that that overlie overlie it. it. Within each unconformity sequence, the mapped lithofacies lithofacies units can be asigned to a succession of of systems systems tracts. 61 �Lower Sequence: Lower Sequence:Ferruginous Ferruginous(garnet), (garnet),anoxic anoxic(graphite), (graphite), muddy muddy sediments sediments of Unit Wna fine upward upward into aluminous (clay-rich) (clay-rich) muddy muddy sediments sediments of of Unit Unit Wnps. Wnps. Such retrogradation is consistent with transgressive systems is consistent with aa transgressive systems tract. tract. The quartzite quartzite beds in Unit Unit Wna may be stormstormgenerated flows. Less aluminous muddy sediments of Unit Wns are transitional through Unit Wnsp to Unit Wrn Wm sands. Such a progradational sequence is consistent with a highstand systems systems tract. Upper Sequence: Upper Sequence:Polymict Polymictconglomerate conglomerateof ofUnit UnitWo Womarks marksaaperiod periodof of uplift uplift and and succession. Although erosion. Most Most clasts clasts are are derived derived from the underlying sedimentary succession. restricted to the eastern margin of Wollaston Domain, Domain, the Janice Janice Lake Lake Conglomerate Conglomerate can can be be for at at least least 245 245 km. km. This unit is interpreted interpreted as as alluvial alluvial fan fan deposits. deposits. Nontraced along strike for tract. An marine conditions indicate that it is a lowstand systems tract. An upward upward decrease decrease in in clastic input is suggested between calc-silicate calc-silicate bearing bearing Unit Wmc Wrnc and and calc-silicate calc-silicate rich rich Unit Unit Wnc. Wnc. This suggests another transgressive transgressive systems tract. tract. The rocks may may be evaporitic The calc-silicate calc-silicate rich rich rocks deposits. LIthelpay LuhtMY C, 0 C C, C. C, Cl) IC, C. a Wnc lnterDretatipn Gale-silicate Gale-silicate rocks rocks Wrnc Wo Transgressive Transgressive Systems Tract Systems Tract Gale-silicate -bee ring Calc-silicate-bearing arkose 1 I - Gale-silicate-bearing Calc-silicate-bearing conglomerate and arkose Lowstand Lowstand Systems Tract Systems Tract C, 0 C C, Wnsp = C. C, Cl) Wns C, -I0 Wnps Wna I Arid zone zone alluvial fan deposits alluvial deposits Coastal sands sands Arkose and subarkose wbarkose Highstand Highstand Systems Tract Systems Tract Biotite psarn mite and psammopelite Transitionat Transitional shell shelf sands sands and muddy muddy sands and Biotite psammopeiite psammopelite and psammite psammite Shelf muddy sands Shell Sillimanite-cordieriteiimanite-cordieritebiotule psammopelite biotite ~ sammo~eiite and psammite Graphite-garnet-biotute psammopelite and psammopeiite and pelite pelite with quartzite interbeds shell muddy Outer shelf muddy sands sands and sandy muds Transgressive Transgressive Systems Tract Unconformity strain zone) (and high high strai Unconformity (and Basement 1 Carbonate-cemented Carbonate-cemented coastal sands coastal Uplift Uplift and and erosion erosion Unconformity Unconformity Wrn Sauna or restricted restricted basin basin Salina calcareous sediments calcareous sediments and and evaporites evaporites Anoxic shell shelf muds muds and and sands sands Uplift and erosion erosion Granitoid rocks rocks with with local local amphibolite, amphibolite, metased. metased. xenoliths, chsrnockie xenoliths, and charnockile References Delaney. G GD.. and Tisdale. Tisdale, D D., 1997, Geological investigations investigationsof ofthe theCounenay CourtenayLake. Lake. D Jankovic, Jankovic, Z., Z . MacNeil, MacNol. A.. A . McGowan, McGowan. J.. J .and 1997. Geological Cairnss Lake Fold Belt and the H Hill Embayment, Johnson Johnson River River Intier. Intier, Wollasion Wollaston Domain. Domain, northern northern Saskatchewan. Saskatchewan: C i l l Lake Embayment, Saskatchewan SaskatchewanEnergy Energy and and Mines, Mines. Misc. Misc Rep. Rep 97.4. 97-4,p90-1O1. p90-101 Tran, G.M., Geology of of the the Burtudge BurbidgeLake Lake'- northern Lake area, aica. eastern eastern Wollaston Domain Yeo, O M . . 1997. 1997. Geology northern Upper Upper Foster Lake Wollasion D omn Tran, H.T. H T and and Yeo, (NTS 74A-14): Saskatchewan Energy and Mines. Misc. Rep. 97-4. 97.4. p72-89. N T S 74A-14)- Saskatchewan Tran, H.T., Yco, G., Bradley. S. and Lowry, Jl*, 1998. Geology of the Daly-suttle-Middle Foster lakes asea, eastern wollaston S. and Lewry. J.F. 1998. o f the My-Sutile-Middle area. eastern Wollasion Tran. H.T.. Yeo. G.. and.12): -12)Saskatchewan Saskatchewan Energy E n e m and and Mines, Mmcs, Misc. Misc Rep. Rep 98-4. p36-47. Domajn (NTS Domain (NTS 74A-5, 74A-5, -I -Il.I, and p36.47. paragneisses iinn the the southeastern southeastern Wollaason Domain: Saskaichewal Saskatchewan G., 1998. A systems systems tract approach to to the stratigraphy of of para&sses Yeo, G.. 1998, A ~oliaston Domain: Energy and Mines, Misc, Rep. 98.4. Mines. Misc. 984. p36.47. p3647. . . 62 � https://digitalcollections.lakeheadu.ca/files/original/23d76f1727e6bc080350abdf1335866d.pdf 54d5b014057f0f987f6c366c6fc057ee PDF Text Text Trip 1: Early EarlyProterozoic Proterozoic Strata Strata of the Maquette Marquette Iron of Range, Michigan Ishpeming Trip 22:: Archean Arc1 Greenstone Belt and Ore, G01d Golf Mineralization H-<o<a 3: Tilden Trip 3: Tilden and and Empire Empire A tc.* ~ c . \6 6 A\~ Mines 0f the Marquette Mines of Marpette Iron Range, Michigan Trip 4: Paleozoic andIGlacial Glacial Geology from1 Au Train to to Grand Marais,3, Michigan I on ~Lake Institute on ake Superior Geology 45th 45th Annual Annual Meeting Meeting Ramada da IInn nn Rama Marquette,,Michigan Maquette ~ichigan May 4-8, 4-8, 1999 1999 Sponsored by Sponsored Northern Northern Michigan Michigan University and Michigan University and Michigan Technological University Proceedings Volume 45: Field Trip Guidebook Theodore J. Bornhorst, editor �45TH ANNUAL MEETING SUPERIOR GEOLOGY GEOLOGY INSTITUTE ON LAKE SUPERIOR Volume 45 consists of Part Part 1: 1:Program Programand and Abstracts Abstracts Part 2: 2: Field FieldTrip Trip Guidebook Guidebook Reference to material in this volume should follow the example below: Holm, D., 1999, 1999, Characterization and timing constraints of post-Penokean Ams, D. and Hoim, meso-scale structures in the Watersmeet and Republic gneiss domes of northern Michigan (abst.): (ahst.): Institute Institute on on Lake Lake Superior Superior Geology Proceedings, 45th Annual Meeting, Marquette, MI, v. 45, part 1, 1, p. 2. 2. Volume 45 Published and distributed by Institute on Lake Superior Geology Mark .G. Mark Jirsa, Jirsa, Secretary-Treasurer, Secretary-Treasurer,I.L.S I.L.S.G. Minneosta Geological Survey 2642 University Avenue St. Paul, MN USA 55114-1057 551 14-1057 (612)-627-4780 email: jjrsaOOl @tc.umn.edu email: jirsaOOl @tc.umn.edu ILSGwebsite websitehtto://www.eeo.mtu.edu/ereat http://www.geo.mtu.edu/great lakes/ilse/ lakes/ilsg/ ILSG ISSN 1042-9964 1042-9964 All volumes volumes are are available for photocopying All photocopying costs costs from Michigan Technological Technological University University Library Library Archives Archives �INSTITUTE ON LAKE SUPERIOR GEOLOGY PROCEEDINGS Volume 45 45 Part 2: Field Trip Guidebook �CONTENTS PROCEEDINGS VOLUME VOLUME 45 PROCEEDINGS PART 2: FIELD TRIP GUIDEBOOK GUIDEBOOK PART2: FIELDTRIP Editor: Theodore J. Bornhorst Field Trip 1 :: Early Proterozoic Strata Strata of the Marquette Marquette Iron Iron Range, Range, Michigan Leader: W.F. W.F. Cannon Early Proterozoic Strata Strata of the Marquette Iron Range by W.F. Cannon ........................ 22 Field Trip Trip 2: 2: Archean Archean Ishpeming Ishpeming Greenstone GreenstoneBelt Belt and and Gold Gold Mineralization, Mineralization,Michigan Michigan Leaders: T.J. Bornhorst, D.J. Duskin, R.C. Johnson, R.A. R.A. Mahin, Mahin, T.O. TO. Quigley, Quigley, and and G.W. G.W. Scott and An Introduction to the Archean Ishpeming Greenstone Belt, Michigan by R.C. Johnson and 12 T.J. Bornhorst .......................................................................................... 12 Introduction to the Ropes Gold Deposit in the Ishpeming Greenstone Belt, Michigan abstracted from R.A. Brozdowski by T.J. Bornhorst, S.E. Bair, and G. W. Scott ............ 29 29 Geologic Field Excursion to the Ishpeming Greenstone Belt by R.C. Johnson and T.J. T.J. 45 Bornhorst ............................................................................................... Bomhorst 45 Shear Zones and Gold Mineralization in the South Half of the Ishpeming Greenstone Belt, Michigan by D.J. Duskin and T.O. Quigley ................................................57 Examples of Gold Mineralization in the Northern Block of the Ishpeming Greenstone 97 Belt by T.O. Quigley and R.A. R.A. Mahin ............................................................... 97 of the the Marquette Marquette Iron Range, Michigan Field Trip 3: Tilden and Empire Empire Mines of Leaders: G.W. Scott, P.M. Nordstrom and H.M. Lukey Geologic setting of the Tilden and Empire Mines, Michigan by T.J. Bornhorst ............ 108 108 Geologic Field Trip to the Tilden Mine by G.W. Scott and H.M. Lukey .................... 114 114 Geologic Field Trip to the Empire Mine by P.M. Nordstrom ..................................129 129 Trip 4: Paleozoic and and Glacial Glacial Strata Strata from Au Au Train Train to Grand Grand Marais, Field Trip Marais, Michigan Michigan Leaders: R.S. Regis and J. Anderton Regis and J. Anderton Paleozoic and Glacial Geology from Au Train to Grand Marais, Michigan by R.S. Regis and J. J. Anderton ......................................................................136 136 �Field Trip Trip11 Early Early Proterozoic Proterozoic Strata Strataof of the the Marquette MarquetteIron IronRange, Range,Michigan Michigan Leader: Leader:W.F. W.F. Cannon Cannon 2 �FIELD TRIP #1 EARLY PROTEROZOIC PROTEROZOICSTRATA STRATA OF OF THE THEMARQUETTE MARQUETTEIRON IRONRANGE RANGE LEADER: W.F. GEOLOGICAL SURVEY W.F.CANNON, CANNON, U.S. GEOLOGICAL INTRODUCTION preserved in the Marquette Iron Range Early Proterozoic strata of the Marquette Range Supergroup preserved the south edge of the the Archean Superior craton. record the tectonic evolution of a continental margin along the That evolution spans an imprecisely known period culminating in suturing with volcanic arcs to the south at about 1855 Ma. Ma. Lower the Chocolay Chocolay Group Group(Stops (Stops1,2,3,5), 1,2,3, 5), are mostly a sequence of Lower parts parts of the section, the pure quartzite, dolomite, and shale believed to have formed in a stable cratonic setting. They They were were deposited deposited now most commonly a fault (Stop I). unconformably on Archean greenstone and granite, but the contact is now 1). A low angle unconformity separates the Chocolay Group from the overlying overlying Menominee Group (Stop 5). The Menominee Group consists of a basal quartzite-shale unit (Stop 5) overlain overlain by the principal principal ironproducing unit in the range, the Negaunee Iron-formation The Menominee Menominee Group Groupwas was Iron-formation (Stops 6, 7, 10). The deposited in actively subsiding grabens, probably on the rifting continental margin, although a foreland researchers in in the the region. region. Strata basin setting is possible and is favored by some current researchers Strata of of the the Baraga Baraga 9, 11) are widely accepted accepted as products products of sedimentation sedimentation in in aa foreland foreland basin basin during during 8,9, Group (Stops 4, 8, preserved in in northern northern Wisconsin. Wisconsin. The accretion of volcanic terranes now preserved The Baraga Baraga Group Group which lies lies with with aa prominent low angle unconformity unconformity on rocks of the the Menominee Menominee Group Group (Stop (Stop 4), 4), is is composed composed mostly mostly of a prominent turbidite sequence of graywacke and shale (stop 11), and conglomerate conglomerate (stop (stop 4), 4), lesser 1I), quartzite and lesser black black shale shale (stop 8), and minor volcanic rocks rocks (stop (stop 9). 9). The Marquette Iron Range Range is is aa complexly complexly folded folded and faulted, faulted, gently gently west-plunging west-plunging synclinorium synclinorium composed of strata of the Marquette Range Supergroup and intrusive intrusive diabase diabase sheets. sheets. Deformation Deformation occurred occurred during the Penokean orogeny at about 1850 Ma. The synclinorium is bounded on both the north and 1850 Ma, The synclinorium is bounded on both the north and south south by Archean volcanic and granitic rocks, which are most most commonly in in fault fault contact contact with with the the Early Early Proterozoic strata. The Theintense intense penetrative penetrative deformation deformation seen seen in in the the Early Early Proterozoic Proterozoic strata strata isis not not present present in in the Archean basement rocks, which were last penetratively deformed in in Lake Lake Archean Archean time. time. This has led to penetratively deformed the interpretation that the Archean rocks behaved as relatively relatively rigid, rigid, fault-bounded fault-bounded blocks blocks during the Penokean orogeny, and were structuraliy highly deformed deformed Early Early Proterozoic Proterozoic strata. By structurally decoupled from the highly By that interpretation, the Marquette synclinorium is a graben with with respect respect to to Archean Archean basement basement rocks. rocks. The synclinorium synclinorium of of Early Early Proterozoic Proterozoic rocks rocks lies lies within within that that graben. graben. localities that that have have been been visited visited by by literally literally The Marquette area contains many classic geologic localities thousands of geologists during the past century. The TheInstitute Institute on on Lake Lake Superior SuperiorGeology Geology has has visited visited the the area area during several past meetings, but the the Marquette Marquette Range stratigraphy has has not not been been examined examined since sincethe the Twenty-first Annual Meeting in 1975. 1975. This Thisguidebook guidebook isis based based very very largely largely on on the the guidebook guidebook prepared prepared for for that meeting and other guidebooks guidebooks (Cannon (Cannon and and others, others, 1975; 1975; Prinz, Prinz, and and others, others, 1975; 1975;Margeson, Margeson,1989; 1989; Morey, 1989). 1989). For Forthose thosewho whohave havetoured toured the the region region before, before, the the stops stops will will be be largely largely the the same same and and the the rocks rocks familiar; only the interpretations have changed. The stops are arranged, arranged, to to the the extent extent practical, practical, to to view view the the beginning at the base of the the section section and and progressing progressing up-section. up-section. To streamline logistics and stratigraphy beginning minimize driving distances a few stops are out of proper stratigraphic order. 3 �a n - 00 1 1 22 33 44 1 5 miles 5miles 4- C; �frt? + C- 7p2 5 �Stop 1. Enchantment EnchantmentLake LakeFormation Formationand andArchean Archeanvolcanic volcanic rocks. rocks. The Enchantment Lake Formation is is a discontinuous unit at the base of of the Marquette Marquette Range Range Supergroup. ItIt consists arkose, and and slate. slate. Most geologists who have studied it Supergroup. consists of of conglomerate, conglomerate, graywacke, arkose, believe that itit is of of glacial glacial origin, origin, although although that that interpretation interpretation remains remains in in dispute. dispute. It conformably underlies underlies hills immediately immediately south south of of stop stop 1, I, but the Mesnard Quartzite. The The transition transition to the Mesnard can be seen in hills will not be visited on this trip. hip. Immediately north of the Old State State Road are are several exposures of of basal basal conglomerate conglomerate of of the the fragments of of tonalitic tonalitic gneiss gneiss and and greenstone. greenstone. The matrix is Enchantment Lake Formation, containing fragments is rich rich in in chlorite. The The conglomerate conglomerate can can be be seen seen in in sharp sharp vertical vertical contact contact with slaty, highly sheared greenstone greenstone of the underlying Mona Schist (Archean). About About 100 100 feet feet north of the contact, the Mona becomes mostly massive massive greenstone. Both Both the the greenstone greenstoneand and conglomerate conglomerate are are strongly strongly foliated foliated near the contact and and steeply steeply plunging to vertical lineations lineations are are common on on foliation surfaces. surfaces. The contact of the Mona Schist and Enchantment Lake Lake Formation Formation has has been been interpreted interpreted as as aa fault. In In addition to the highly sheared rocks along along the contact, contact, the two units units are are stratigraphically stratigraphically "back "back to to back". back". Pillows in the Mona Schist Schist in in nearby nearby outcrops outcrops are are uniformly uniformly north-facing, north-facing, whereas whereas the the Enchantment Enchantment Lake Lake and overlying formations are south-facing. The present attitude of the rocks is probably a result of vertical The present attitude of rocks is probably a of vertical rise of aafault faultblock blockof ofMona MonaSchist Schistand and passive passive draping draping of of the the Enchantment Enchantment Lake Lake Formation Formation over over the block. block. Stop 2. Mesnard Quartzite Quartzite and and Jacohsvitle Jacobsville Sandstone. Sandstone. In this lakeshore outcrop the Mesnard Quartzite, the basal orthoquartzite unit of of the the Chocolay Chocolay Group, is overlain with with aa sharp sharp angular angular unconformity unconformity by by the the Middle Middle Proterozoic Proterozoic Jacobsville Jacobsville Sandstone. Sandstone. strikes eastward and and dips dips White-weathering vitreous Mesnard Quartzite in beds 4 to 88 inches thick strikes 80°S to vertically. vertically. Cross 80's to Cross beds beds and and ripple ripple marks indicate top direction to the south, in conformance with the position of the quartzite on the north limb limb of the Marquette synclinorium. Reddish Jacobsvillle Jacobsvillle Sandstone Sandstone dips 10-15° 10-15' eastward to southeastward and the contact with the Mesnard is visible at several places on on the the outcrop. Patches of Jacobsville are plastered against the north side of the quartzite outcrop, and on the south side outcrop, and on the south side the contact is steep. Evidently Evidently during during deposition deposition of of the the Jacobsville, Jacobsville, the Mesnard at this locality formed formed a low topographic ridge, much as it does now. Flat-lying Flat-lying Jacobsville Jacobsville is is on the flank of Mt. Mesnard, about about 3/4 miles west of here and 370 feet above current lake level, which indicates a pre-Jacobsville topographic Vs miles relief and ruggedness approximately the same same or or greater than that adjacent to the present present shore shore of of Lake Lake Superior. Superior. 3. Kona Kona Dolomite Dolomite Stop 3. on the the Mesnard Mesnard Quartzite. Quartzite. The formation is mostly dolomite, The Kona Dolomite lies conformably on fine- to to medium-grained medium-grained clastic clastic rocks. rocks. The outcrops on the north siliceous dolomite, and lesser amounts of fineside of the highway show exceptionally exceptionally well preserved sedimentary textures indicative of of aa shallow shallow water water to to intratidal environment, probably open tidal tidal flats at at this this stratigraphic stratigraphic interval. interval. Many beds are algal carbonates with large stromatolites stromatolites exceptionally well displayed and accentuated by differential differential weathering. weathering. visitors. Other Please help preserve these features for future visitors. Other well preserved features features include dessication and rip-up structures, ripple marks, and rip-up beds beds probably probably produced produced by tidal currents. 6 �4. Goodrich Quartzite Stop 4. This stop, for the sake of economizing economizing driving drivingtime, time, isisnot notininproper properstratigraphic stratigraphicorder. order. We We have have temporarily skipped over the Menominee Group and will examine the basal conglomerate of the Baraga Group. At At this this stop, stop,just just south southof ofthe the village village of of Palmer Palmer and and near near the the old old Isabella Isabella Mine, Mine, coarse coarse conglomeratic conglomeratic Goodrich Quartzite are exposed. exposed. The rocks forming the lower part of the Goodrich The Goodrich Goodrich Quartzite, Quartzite, the the basal basal unit unit of the Baraga Group, unconformably overlies the Negaunee Iron-formation. Here Here aa basal basal conglomerate containing clasts of iron-formation iron-formation as as much as 0.5 0.5 m m diameter can be seen. The angular angular clasts clasts have have features features seen. The indicative of brittle fracturing, implying that the Negaunee Negaunee was was well well lithified lithified prior prior to to the the time that that these clasts were eroded. Well Wellrounded rounded clasts clastsof ofvein vein quartz quartz are are also also common, common, most most likely likely derived derived from from aa locally locally exposed basement basement terrane. terrane. The lack of other basement rock types in clasts may mean that the surface of The lack of in clasts may mean that the surface of vein material material persisted persisted in coarse fragments. The exposed basement rocks was deeply weathered so that only vein The exposures seen here are only a few feet above the unconformity and and Negaunee Negaunee Iron-formation Iron-formation was mined from small pits immediately immediately south south of of the the outcrop. outcrop. Conglomerate Conglomerate units such as seen here are typically lenticular. They range up to a few hundred feet thick locally but pinch out along strike so that quartzite commonly lies on the Negaunee with with little or no conglomerate. The Thecoarse coarseconglomerate conglomeratelenses lenses may mayhave have been been alluvial alluvial fans fans along along the the scarps scarpsof offaults faultsthat that were active during deposition. The Thefeatures featuresseen seenhere here suggest suggestaa rather rather long long time time interval interval across across the the unconformity and renewed deposition during reactivation reactivation of basement faults. One One interpretation interpretation isisthat that the the Menominee Group was deposited during continental margin and that the Baraga Group, continental breakup on a rifting margin beginning with the Goodrich Quartzite, was deposited deposited in in aa foreland foreland basin basin at at the the outset outset of of collision collision with with volcanic terranes now now seen seen in in northern northern Wisconsin. Wisconsin. 5. Chocolay Group and lower Group (Ajibik Quartzite Quartzite and Siamo Slate) lower part part of of Menominee Group Stop 5. The series of roadcuts along U.S. Highway much of the the stratigraphy stratigraphy of the the lower part of Highway 41 exposes much the Marquette Range Supergroup. The Theeasternmost easternmost cut cut shows shows aa few few tens tens of of feet feet of of green green chloritic chloritic slate slate Formation. Here mapped as the upper part of the Enchantment Lake Formation. Here the the Enchantment Enchantment Lake Lake differs differs significantly from that seen at stop I1 in in being being much much thinner, probably not much thicker than the section section exposed here, and it lacks conglomerate beds. The The Enchantment Enchantment Lake Lake grades grades over over aa few few feet feet into into the the overlying vitreous quartzite of the Mesnard Mesnard Quartzite, essentially similar similar to to that that seen seen at at stop stop 2. 2. The The Ajibik Ajibik Quartzite is exposed in the next set of roadcuts to the west. Although Although similar similar in in appearance appearance to to the the Mesnard, Mesnard, the Ajibik in general is somewhat more feldspathic. The The geology geology near near here here was was mapped mapped in in detail detail by by Willard Willard Puffett Puffen to document the rdationship relationshipbetween between the the two two quartzite quartzite units. units. Puffett Puffett was was able able to to trace trace aa distinctive distinctive granule bed, about 10 Ajibik and show that the the Ajibik Ajibik cuts across the 10 inches inches thick near the base of the Ajibik Mesnard at a low angle, demonstrating the angular unconformable nature of the the contact. contact. The The thinness thinnessof ofthe the Chocolay Group here and the absence of the Kona Dolomite reflect reflect the westward westward beveling of the group by the Ajibik. Within Withinaamile milewest westof ofhere herethe theChocolay ChocolayGroup Group isis absent absent and and the the Ajibik Ajibik rests rests directly directly on on Archean rocks. The Ajibik grades upward into the Siamo Slate seen in in roadcuts roadcuts about about 1000 feet feet farther farther west west on the south side ofthe of the highway. highway. The The Siamo Siamo is is a lithologic lithologic mixture of slate, slate, arkosic arkosic quartzite, and graywacke, generally in beds from a few inches to a few feet thick. thick. The The rock rock here here has has aa prominent prominent cleavage cleavage that that dips dips southward more steeply than bedding, consistent with the location location on the north limb limb of of the the Marquette Marquette synclinorium. An An interesting interesting and and controversial controversial feature feature of of this outcrop outcrop is is lenses lenses of graywacke that lie lie within 7 �the cleavage and transect bedding. Various Variousauthors authors have have interpreted interpreted these these as as clastic clastic dikes dikes or or as as lithified lithified beds beds tectonically forced into into cleavage cleavage planes. planes. Stop 6. 6. Negaunee Iron-formation (carbonate fades) facies) This stop, near the old Athens Mine, shows carbonate carbonate facies facies iron-formation iron-formation typical typical of of much much of of the the lower lower part of the Negaunee Iron-formation. Iron-formation. The here consists consists mainly mainly of of alternating alternating laminae laminae of of The iron-formation iron-formation seen here sideritic carbonate and 5% minnesotaite, which imparts a and chert, chert, but but some some layers layers contain contain as as much much as as 115% greenish color. Some Somelayers layerscontain contain small smallamounts amounts of of magnetite, magnetite, but but in in general general the the rock rock isis only only weakly weakly foot thick, are are also also present present in in the exposure. exposure. magnetic. Thin Thin sills sillsof ofaltered altered metadiabase, metadiabase, less less than than aa foot This exposure provides striking striking examples of the effects of of surface surface weathering weathering and and oxidation oxidation and and replacement of sideritic iron-formation. iron-formation. The The weathering weathering has has taken place in in the relatively few decades decades since since the road cut (originally a railroad cut) was excavated. Oxidation Oxidation has has advanced advanced inward inward from from cracks cracks and and exposed surfaces, first converting converting layers layers rich rich in in carbonate carbonate into into yellow yellow brown brown to to reddish reddish brown brown goethitic goethiticiron iron laminae into iron iron oxide. oxide. These oxide, and eventually changing even chert laminae These surficial surficial effects effects are analogous analogous to to those that formed the "soft ores" of the region. The major ore bodies in the eastern end of the iron range, The major ore bodies in the eastern end of the iron range, including those in the immediate immediate vicinity vicinity of of this stop, stop, were replacement masses of of iron iron oxides oxides and and hydroxides formed by low low temperature alteration alteration of of carbonate carbonate and and silicate silicate iron-formation iron-formation accompanied accompanied by by dissolution and replacement replacement of of chert. chert. Stop 7. Negaunee Negaunee Iron-formation (hematite-jasper (hematite-jasper fades) facies) Classic exposures exposures of of the jaspillitic upper upper part part of of the the Negaunee Iron-formation Iron-formation are are seen seen on on Jasper Jasper Knob. Knob. These exposures just north north of of the the major major fold fold axis of the exposures are in in the central part of the Marquette synclinorium, just district. The Theknob knob itself itself isison on aa west-plunging west-plunging anticline anticline so so that the base of the Goodrich Goodricb Quartzite wraps around the west end of the knob and is is exposed in in some of the fenced mined areas areas near near the the base base of of the the west west slope. The alternating thin thin beds beds of of specular hematite and bright The iron-formation iron-formation here here characteristically characteristically has alternating jasper. Specularite red jasper. Speculariteplates plates are are strongly strongly oriented oriented parallel parallel to bedding in the hematitic layers. Folds having drag folds on their limbs and themselves being drag folds on the limbs of still still larger larger folds are strikingly shown shown in in many many exposures. exposures. Fold axes are horizontal horizontal to to gently gently plunging. plunging. Small breccia zones are common and appear to lie roughly along axial axial planes planes of of folds. folds. In these zones fractured jasper beds are surrounded by crystalline hematite. hematite, theories have have been been advanced advancedfor forthe theorigin originofofthejaspillite. oxidation of Two theories the jaspillite. One advocates oxidation siderite-chert iron-formation iron-formation during the post-Negaunee erosional interval and its recrystallization recrystallization to to Penokean deformation deformation and and metamorphism. metamorphism. This theory accounts for patches of specularite-jasper during Penokean specularite-jasper within carbonate facies iron-formation near contacts between the two rock types, types, as, as, for for example, in the Cliffs Shaft mine about '/2 ¼ mile mile to to the the west. west. The second theory advocates a primary origin for the hematite and jasper in in a more more oxidizing oxidizing depositional depositional environment than that responsible responsible for for carbonate carbonate iron-formation. The Theeven-bedded even-bedded and and thin-bedded thin-bedded nature nature of of the jaspillite on Jasper Knob is very similar to iron-formation. ItIt contrasts the bedding characteristics of carbonate iron-formation. contrasts sharply sharply with wavy bedding and oolitic structure common common in injaspillite jaspillite elsewhere elsewhere in the district. district. At At the the Kloman Kloman Mine, near Republic (stop 10), lo), a convincing case can be made made for aa primary primary depositional depositionalorigin originofjaspillite. ofjaspillite. Here, at Jasper Knob, an origin origin by oxidation of carbonate iron-formation iron-formation is more possible. 8 �Stop 8. Michigamme Michigamme Formation, Formation, lower lower slate slate member. member. black sulfidic slate well well represented by the The lower part of the Michigamme Formation consists of black outcrops along the south side of Highway 41. This Thisoutcrop outcropisisnear near the the axis axisof of the the Marquette Marquette synclinorium, synclinorium. Subtle beds show mostly low dips and gentle folds with nearly horizontal axes. A A steep steepaxial axial plane planecleavage cleavage is well developed. Very Very fine-grained fine-grained pyrite pyrite is is abundant in many beds. This This black black slate slate facies facies lies lies on on the the Goodrich Quartzite and is overlain by the Greenwood Iron-formation Iron-formation Member, a lean silicate-magnetite iron-formation. iron-formation. Stop 9. Michigamme Michigamme Formation, Formation, Clarksburg ClarksburgVolcanics Volcanics Member. Member. The Clarksburg Volcanics is is a local unit resting on the Greenwood Iron-formation Member. The volcanic rocks are restricted to the south limb of the Marquette synclinorium, where they are exposed along length of about about 15 miles miles and and range range up upto toabout about 1I mile mile thick. thick. The absence of volcanic rocks on the a strike length away, shows that that deposition deposition in in the the Marquette trough trough north limb of the synclinorium, only a few kilometers away, was very assymetrical at this time and and that that the the trough trough was was probably probablymuch muchdeeper deeperon onthe thesouth. south. Lenses of lean iron-formation horizons throughout throughout the the volcanic volcanic section section indicate that that iron-formation at numerous stratigraphic horizons volcanism was mostly or or entirely entirely submarine. submarine. The volcanics are almost exclusively mafic and pyroclastic. pyroclastic. This This outcrop outcrop along alongthe the north north side sideof ofthe the which layering is shown shown distinctly by by variations in abandoned railroad grade is representative of the unit in which highly vesicular mafic rock, rock, and and the the matrix is rich in fragment size, most fragments are angular pieces of highly carbonate minerals. Fragments of argillite and chert are also present. Rocks here are Fragments of argillite and Rocks here are metamorphosed metamorphosed to to garnet grade and original mafic minerals are converted to amphibole and biotite. biotite. In In spite spite of ofthe the recrystallization, original igneous textures are very well preserved, even at microscopic microscopic scales. Low Low outcrops south of the railroad grade show lean, impure iron-formation typical of lenses that that are widespread iron-formation typical in the the Clarksburg Volcanics. Volcanics. The rocks consist of of alternating alternating layers layers of of magnetic magnetic biotite biotite argillite argillite and and siliceous in beds with abundant abundant fine-grained fine-grained muscovite. muscovite. Stop 10. Negaunee Iron-formation at a t Kioman Kloman Mine. Mine. the Negaunee Negaunee Iron-formation. Iron-formation. Interbedded Exposures here are of the uppermost units of the Interbedded on on aa scale scale iron-formation, aa wavy-bedded, wavy-bedded, oolitic oolitic jaspillite jaspillite of one to three feet are two contrasting lithologic types of iron-formation, and even-bedded cherty grunerite-magnetite gmnerite-magnetite iron-formation. The The features features seen seen in in this this exposure exposure were were instrumental in Hal James' formulation formulation in in the 1950's 1950'sof ofhis his concepts concepts of of sedimentary sedimentary facies facies of ofiron-formation iron-formation mineral assemblages assemblages during during high high grade grade metamorphism. metamorphism. A as well as oxygen-buffering by contrasting mineral A critical feature feature very well shown shown here here isis aa correspondence correspondence between between bedding bedding characteristics characteristics and andoxidation oxidation state. Relatively Relatively reduced reduced rocks, rocks, now now grunerite-magnetite grunerite-magnetite assemblages, assemblages, are invariably invariably in thin, even beds, beds, probably reflecting deposition in in quiet, quiet, poorly poorly oxygenated oxygenatedwater. water. Interbeds ofjaspillite ofjaspillite invariably invariably show show irregular wavy wavy beds and oolites are common, common, especially especially in in thicker thickerjasper jasper beds. beds. These These units are are believed believed to to formed in agitated, more oxygenated oxygenated water. water. James have formed James used used this correspondence of sedimentary sedimentary features features and mineral assemblages to argue argue that the different different lithologies lithologies and and their oxidation oxidation states, states, although although metamorphosed to sillimanite grade grade here, here, still still reflect reflect differences differences in in primary primary depositional depositional settings. settings. 9 �Stop 11. Michigamme Michigamme Formation. the Michigamme Michigamme Formation Formation in in the the axis axis of the the This roadcut shows very highly deformed deformed schist of the northwest-plunging Republic trough. The The rock rock here here consists consists of of iron-rich iron-rich metasediments, now now consisting of of biotite-garnet-amphibole schist. The The light-colored light-colored bladed bladed amphibole amphibole is is commonly commonly mistaken for for sillimanite, sillimanite, but although this exposure is is within the sillimanite sillimanite metamorphic zone, detailed petrographic petrographic examination examination failed to find sillimanite in this outcrop. Thicker has failed Thicker beds, beds, up up to a few few inches, inches, of impure impure quartzite form form readily traceable marker beds and reveal very tight folds with amplitudes many times greater than beds and reveal very folds with amplitudes many times greater than wavelength, and highly attenuated limbs. Fold Fold axes, axes, even even in in adjacent adjacent folds, folds, diverge diverge by by as as much much as as 600. 60'. The The compression of of the the metasediments metasediments between between two two blocks blocks of of very tight folding is caused, in large part, by compression Archean gneissic rocks that flank flank the Republic Republic trough on on the northeast northeast and and southwest southwest sides. sides. References Cannon, W.F., Gair, J.E., Klasner, J.S., and Boyum, B.H., 1978, Marquette Marquette Iron Iron Range: Range: Twenty-First Twenty-First Annual Institute on Lake Superior Geology, Field Trip Guidebook, p. 125-173. 125-173. Prinz, W.C., Gair, J.E., and Cannon, W.F., 1975, 1975, Greenstone: Twenty-First Annual Institute Institute on on Lake Lake p.41-85. Superior Geology, Field Trip Guidebook, p. 41-85. and metal metal occurrences, occurrences, Michigan's Michigan's upper peninsula: Society of Margeson, G.B., 1989, Precambrian Precambrian geology and Economic Geologists Geologists Fall Field Conference, 154 154 p. Morey, G.B., 1989, 28"' International Geological 1989, Early Proterozoic rocks of the Great Lakes region: 28th 63 p. Congress Field Trip Guidebook T145, T145,63 10 �Field Field Trip Trip22 Archean ArcheanIshpeming IshpemingGreenstone GreenstoneBelt Beltand andGold Gold Mineralization, Mineralization,Michigan Michigan Leaders: Leaders:T.J. T.J.Bornhorst, Bornhorst,DJ. D.J.Duskin, Duskin,ftC. R.C.Johnson, Johnson,R.A. R.A. Mahin, Mahin,T.O. T.O.Quigley, Quigley,and andG.W. G.W.Scott Scott 11 �An Introduction Introduction to to the the Archean Archean Ishpeming Ishpeming Greenstone GreenstoneBelt, Belt, Michigan Michigan Rodney Johnson & & Associates, Associates, Inc., 1550 1550 Baldwin Baldwin Ave., Ave., Negaunee, Negaunee, MI MI Rodney C. Johnson, Rod Johnson 49866 Theodore of Geological Theodore J. J. Bornhorst, Bornhorst, Department Department of Geological Engineering Engineering and and Sciences, Sciences, Michigan Technological Technological University, University, Houghton, Houghton, MI MI 49931 49931 Introduction Introduction The Ishpeming belt (IGB) (1GB)covers coversan anarea area of of about about 300 300 km2 km2 and and is located Ishpeming greenstone greenstone belt located in Michigan's Upper Upper Peninsula. Peninsula. It is is on on the the southernmost southernmost edge of the Superior Superior province and is is part of the composed of of tholeiitic tholeiitic pillowed pillowed basalt basalt the Wawa Wawa subprovince subprovince (Figure 1). 1). The belt is composed flows and lesser amounts to felsic volcanic flows amounts of intermediate intermediate to volcanic rocks rocks and minor minor clastic clastic and and chemical sedimentary sedimentary rocks. rocks. The 1GB IGB is about 2,700 2,700 Ma Ma (Bornhorst (Bornhorst and and Johnson, Johnson, 1998; 1998; Hammond, 1978). 1978). The The1GB IGB is is north north of of the the Great Great Lakes tectonic zone, an Archean Archean continentcontinentcontinent In Michigan, the Archean rocks south Michigan, the Archean rocks south of the the Great Great Lake Lake continent suture suture (Sims, (Sims, 1991). In tectonic tectonic zone zone are composed composed of old old Archean Archean gneiss gneiss and and migmatite migmatite intruded intruded by younger younger Archean granites granites and and is is part part of of the the Minnesota Minnesota River River Valley Valley Subprovince Subprovince(Sims, (Sims,1996). 1996). Tholeiitic to caic-alkalic calc-alkalic volcanism volcanism of of the the 1GB IGB belt resulted resulted from from north-directed north-directed subduction subduction (Hoffman, 1989; Card, 1990) Archean time. Successive Archean Archean accretion resulted in 1990) during Archean southward younging younging of of greenstone greenstone belts belts in in the the Superior Superior province. province. The Great Lakes tectonic tectonic zone (GLTZ) km long longArchean Archean paleosuture paleosuture (Sims, (Sims, 1991), 1991), the the most most significant significant (GLTZ) is aa 1,000 1,000km Archean boundary within within the the U.S.A. U.S.A. Sims Archean tectonic tectonic boundary Sims (1991) (1991) has has studied studied aasignificant significant shear shear zone, just south south of of the the 1GB, IGB, that he has interpreted as the only exposed segment of of the the GLTZ GLTZ (Figure 1). The buried. A The Minnesota Minnesota segment segment of the GLTZ is buried. A result result of of detailed detailed and andregional regional field studies, studies, including including structural structural geology, geology, is is the the synthesis synthesis of of the the lithostratigraphy lithostratigraphyof ofthe the1GB IGB in close close proximity proximity to to the the Great Great Lakes Lakes tectonic tectonic zone. zone. Structure Structure stratigraphic correlation correlation depends depends on on good good understanding understanding of of deformation deformationhistory. history. Accurate stratigraphic studies have have shown shown that that the the rocks rocks of of the the 1GB IGB have been effected by six periods of of Recent studies deformation (Table (Table 1)(Johnson, l)(Johnson, 1993; 1993;Nachatilo Nachatilo and and Bauer, Bauer, 1993). 1993).D1 Dldeformation deformation Archean deformation produced axial-planar cleavage (Johnson produced recumbent folds with a flat-lying penetrative, nearly axial-planar and Bornhorst, 1991). 1991). D2 D; deformation deformation produced large-scale strike-slip strike-slip shear shear zones zones that that Dadeformation deformation produced produced upright uprightfolds folds define the the boundaries boundaries of of lithostratigraphic lithostratigraphic blocks. blocks. D3 define axial-planar cleavage cleavage that is is developed best best in in felsic felsic volcanic volcanic (Figure 2) with a near-vertical axial-planar D4 deformation is associated with the intrusion of late interflow sedimentary units. D4 and interfiow trondjhemite-granite suite suite plutons plutons that that reorient reorient earlier earlier fabrics. fabrics. D5 Dsdeformation deformation produced produceddipdiptrondjhemite-granite slip motion in shear shear zones. zones. D6 D6 deformation deformation produced produced crenulation cleavage, cleavage, local local kink kink bands, bands, Archean rocks rocks of of the the 1GB IGB were not significantly deformed TheArchean and strike-slip shear zones. The Orogeny (Nachatilo (Nachatilo and and Bauer, Bauer, 1993). 1993). during the Early Proterozoic Penokean Orogeny strata of the southwest southwest block (Figure (Figure 2) 2) form form aa steeply steeply west-plunging synformal synformal The strata This fold foldisisdefined defined by by the the orientation orientation of bedding. Fieldwork Fieldwork has has identified identified northnorthanticline. This 12 �' I '00 700 I I 700KM Explanation Explanation Phanerozoic Phanerozoic 111111 - Precambrian Precambrian 1----] Middle Proterozoic Proterozoic volcanic and sedimentary sedimentary rocks rocks Early Proterozoic Proterozoic sedimentary and volcanic rocks V//A - Grenville front boundary boundary fault fault Sedimentary rocks rocks -- Fault Margins of Great Lakes Lakes tectonic tectonic zone; zone; short dashed where inferred inferred or or covered covered greenstone-granite complexes Archean greenstone-granite complexes Archean gneiss gneiss and and local local granitoid granitoid rocks rocks Regional geologic setting of the Archean Ishpeming greenstone belt and Archean Figure Figure 1: Regional Great Lakes tectonic zone. 13 �____ 87°30' - 0 I T. 50 2 Mi. 0 1 2 3 K m . N T. 49 N ++++++ T. ++++++ +++++++ 48 N. 46°30 Explanation Explanation Mona Mona Formation Formation Upper tuff unit lapilli-tuff unit unit and and lapilli-tuff IN '1 Basalt flow unit unit Kitchi Formation Kitchl Formation Tuff unit and lahar I- — — —l lahar unit unit Basalt flow unit Proterozoic Proterozoic Sedimentary Sedimentary rocks rocks Archean Archean Intrusive Rocks intrusive Rocks Granodiorite k'."J Granodiorite Ii Tonalite Tonalite Deer Lake Lake Peridotite Peridotite P Sedimentary Rocks Rocks £S&S1 Timiskaming-type Timiskaming-type sedimentary sedimentary rocks Volcanic Rocks Rocks Lighthouse Lighthouse Point Point Basalt Basalt Basalt Basalt V//I Reany Lake pyroclastic Reany Lake pyroclasticunit unitand and Fire Center mine iron-formation iron-formation unit unit PL.JLJ Nash Creek glomerophyric glomerophyric basalt unit unit a I\ /1 0 I I a I Bedding Bedding Overturned Overturned bedding bedding Facing of pillows pillows 'C Plunging anticline anticline syncline 'C Plunging syncline Plunging overturned anticline anticline \ Shear zone & - 87 f 87+ A % Figure 2: Generalized Figure Generalized geologic geologic map map of o f the the Ishpeming Ishpeminggreenstone greenstone belt. belt. 14 �Table Table1.1.Summary Summaryofofdeformation deformationsequence sequenceand andvolcanic/magmatic volcanic/magmaticactivity activity and and constraining constraining age agedates datesfor forthe the Ishepming greenstone belt. belt. Event Event - Characteristic Characteristic Volcanic/Magmatic Activity Volcanic/Ma~matic Activity Age Dates Ace Dates Undeformed post-tectonic post-tectonic granite granite 2585 Ma Ma D6 Kink Kink bands bands with with north-easterly north-easterly striking kink kmk planes and steeply plunging hinges D5 Ds Dip slip motion along shear shear zones. D4 D4 Folding associated Folding associated with with intrusion of plutons. plutons. D3 Upright Upright folding. folding. Appinite suite D2 Strike-slip Strike-slip shearing. shearing. Trondjheniite-granite suite Trondjhemite-granite DI Recumbent Recumbent folding. folding. Tonalite suite Extrusion of mafic and felsic volcanic rocks. 15 2668.4+2.11-1.8 Ma 2668.4+2.1/-1.8 2705.8+/-1.6 Ma �northeast-lacing beds in tuffaceous tuffaceous units on the northern northern limb limb of of th'is this D3 D3fold. fold. Tuffs Tuffs of and northeast-facing the southwest block have a well-developed axial planar foliation foliation (S4. (53). Pillow basalts of of the the southeast block consistently consistently young to the north and and have have aa poorly poorly developed developed cleavage. cleavage. Interfiow sedimentary rocks rocks dip dip steeply steeply to to the the north. north. Tuffs that overlie the basalts basalts have two Interflow well-developed cleaviges cleavages that defini define a-moderately a moderately plunging plunging D3 D3synform. synform. The The strata of the well-developed sedimentary and felsic volcaniclastic volcaniclastic rocks define a north block have been re-folded, and sedimentary pattern (Johnson (Johnson and and Bornhorst, Bornhorst, 1991). 1991). The low density of broad Z-shaped outcrop pattern sedimentary rocks necessitates relying on pillows pillows in pillow pillow lava for defining younging directions (Figure 2). AAwell-developed well-developed foliation foliation in in amphibolite-facies amphibolite-faciesbasalt basalt was was subsequently refolded by D3 deformation. deformation. D3 folds are are upright, upright, open open to to tight tight and have D3 folds shallow to steeply plunging fold axes. axes. Stratified Rocks Stratified lithostratigraphic blocks that comprise Bornhorst (1988) subdivided the 1GB IGB into three lithostratigraphic comprise units bounded boundedby bydeformation deformationzones zones (Figure 3). 3). As internally consistent stratigraphic units As aa whole, the belt is bounded to the south by Early Proterozoic Proterozoic sedimentary rocks along the Carp River Falls shear zone, a structure that was active from the Archean Archean through the Proterozoic (Puffett, 1974). 1974). The Thesouthwest southwest block block isis bounded bounded to to the the west west by by Archean Archean granitoid rocks. This Thiscontact contact may may be be aafault fault since since the the boundary boundary is is not not transitional transitional lit-par-lit lit-par-lit as found near the intrusive greenstone-granitoid contact of of the the north north block block of of the greenstone greenstone-granitoid contact belt (Johnson and Bornhorst, Bomhorst, 1991). The The Carp Carp River River shear zone separates the southwest interpreted to be a dextral shear zone with uncertain block from the southeast southeast block. It is interpreted displacement. The stratigraphic displacement. The dextral dextral Dead Dead River River shear shear zone separates separates the two southern southern blocks from the north block. The north block is bounded bounded to the east, north north and west by Archean granitoid rocks that have intrusive relationships relationships with with the volcanic rocks rocks (Wilkins and Bornhorst, 1992; 1992; Johnson and Bomhorst, Bornhorst, 1991). 1991). Southwest Block The southwest block is composed of mafic flows and intermediate to to felsic volcanic and volcaniclastic rocks of the Kitchi Formation which are subdivided into vokaniclastic into three three informal informal units: units: basalt flow, tuff, and lahar units (Figure 44 and 5). The The basalt flow unit, at at the base of of the the Kitchi Formation is composed of 1,800 1,800 meters of dominantly pillowed basalt flows, flows, but but includes massive basalt flows as well. The The basalts basalts are are dominantly dominantly magnesian tholeiites tholeiites composed of hornblende-actinolite-albite-chlorite-clinozoisite hornblende-actinolite-albite-chlorite-clinozoisitewith minor amounts amounts of of sphene, sphene, magnetite, and (or) pyrite. The The base base of of the the basalt basalt flow flow unit unit is is in probable fault fault contact contact with with beds of of chert and and dacite dacite to to rhyolite rhyolite tuff tuff are are interlayered interlayered Archean granitoid rocks. Several thin beds basalt flow unit. Near near the middle of the hasalt Near the the top ton of of the the basalt flow flow unit is is aa 30-meter30-meterplagioclase phenocrysts phenocrysts up up to to 22 cm cm in in diameter. diameter. The The thick glomerophyric flow containing plagioclase uppermost basalt flows intercalate with intermediate volcanic rocks of the overlying tuff intermediate overlying unit. - - tuff unit. Formation is composed composed of of 5,000 meters meters of of tuff tuff with with lesser lesser amounts amounts ~Thei tuff e unit of the Kitchi Formation of interlayered lahar with the thicker beds of lahar separated separated into into the the lahar lahar unit. unit. The The tuff tuff unit unit ash tuffs, tuffs, dominantly dominantly calccalcand lithic lithic tuffs, with minor lapilli-ash and ash consists of crystal and alkalic andesite in composition. The Therocks rocks are are schist schist composed composed of of quartz-feldspar-sericitequartz-feldspar-sericitechlorite containing quartz and feldspar phenocrysts. phenocrysts. Pillowed Pillowed basalt basalt flows flows interfinger interfinger with with 16 �T 50 N T 49 N T 48 N 4630 R2SW R2JW 0 R26W 5 111111 R25W 10 KILOMETERS Explanation Explanation Middle Proterozoic Proterozoic (900-1,600 (900-1,600 Ma) Ma) Metavolcanic rocks Metavolcanic rocks LAttJ ,Jacobsville JacobsvilleSanstone Sanstone(Midcontinent (Midcontinentrift rift system) system) Early Proterozoic Proterozoic (1,600-2,500 (I ,600-2,500 Ma) Ma) I Sedimentav Sedimentary rocks 0 rocks of of Marquette MarquetteRange RangeSupergroup Supergroup I Archean -- - Contact Contact +f - Shear zone Approximate trace of of axial axial plane of plunging plunging anticline anticline +i f- Approximate trace of axial axial — plane planeof ofplunging plungingsyncline syncline I÷++4 Granitoid Granitoid rocks rocks Figure 3: Lithostratigraphic Figure Lithostratigraphicblocks blocks of the the Ishpeming Ishpeminggreenstone greenstone belt. belt. 17 �8r4 8fl0 AAA A Y 4t32'30" 66 A AA A Aa A A'St'/?s —4 A A A A A A A'S {/? z A A A A A 2- A A A A A% M&io - - r4 2' - A A 'a a FJA.Je5 A C - - eq -, -— aa , V 0lshpemiig e3cy 0 88 KILOMETERS KILOMETERS 0 I Explanation Explanation Early Proterozoic Proterozoic 0Undifferentiated Undifferentiated metasedimentary metasedimentary rocks rocks I LAAAAAJ Tuff unit Tuff unit ---- Basalt flow flow unit unit Basalt + App~ximate Approximate trace of axial plane of anticline anttcl~ne I Archean Archean - Contact Contact Undifferentiated Undifferentiatedrocks rocks —r Metagabbro Metagabbro 65 Strike and dip dip of of beds beds "- Shear zone Shear zone Deer Lake Peridotite Peridotite Kitchi Formation Formation Lahar unit unit #— Note: All rocks are metamorphosed metamorphosed Figure 4: 4: Generalized geologic map of southwest block block of the Ishpming n s t o n e belt. Mi. of the southwest lshpeming p greenstone 18 �METERS METE1 0 0 500 500] 1000 1000 Tuff unit Lahar unit unil unit cc: Lahar unit J Basalt flow unit Formation within the southwest block Figure Figure 5: Generalized stratigraphy of the Kitchi Formation of the Ishpeming greenstone greenstone belt. belt. 19 �the tuff unit near the top and base of the unit. The Thelahar lahar unit unit of of the the Kitchi Kitchi Formation Formation isis composed of of beds and and lenses lenses of of polymictic volcanic conglomerate containing clasts of thinly laminated, laminated, porphyritic, porphyritic, and and holocrystaline holocrystaline rocks that are calc-alkalic andesite to dacite dacite in in composition. The polymict volcanic volcanic conglomerates conglomerates are are more more common near the base of the Thepolymict tuff unit unit and and are are interpreted interpreted as as lahar lahar deposits deposits (Bornhorst (Bomhorst and andJohnson, Johnson,1993). 1993). Southeast Southeast Block Block The southeast southeast block is is composed of basalt flows and felsic volcanic and volcaniclastic rocks of the Mona Formation, which which is subdivided subdivided into informal units: basalt flow, lapiffi-tuff, and and upper tuff units (Figure 6). The Thebasalt basalt flow flow unit, unit, at at the the base of the Mona Formation, is a 5,500-meter-thick 5,500-meter-thick sequence of pillowed with minor massive basalt flows. The basalts are dominantly magnesian tholeiite basalt flows. Within the basalt flow unit are thin interfiow interflow chert-magnetite iron formations and schistose schistose quartz-sericite quartz-sericiterocks rocks (tuffs). (tuffs). The most notable occurrence of interflow sedimentary sedimentary rocks in the Mona Formation occurs north of the Pine Hill Hill Quarry Quarry (Figure (Figure 6). 6). At Atthis thislocation, location,chert-magnetite chert-magnetiteand andchert-pyrite chert-pyriteiron-formation iron-formationare are intercalated with graphite and quartz-sericite schist (tuffs) (tuffs) as as interflow interfiow units. units. Diamond quartz-sericite schist drilling drilling along along this interval interval has indicated that these units have a combined thickness of approximately approximately 10 10 meters. meters. The Thelapilli-tuff lapilli-tuffunit unitoverlies overliesthe thebasalt basalt flow flowunit unit (Figure (Figure7) 7)and andisis composed of calc-alkalic rhyolite lapilli in a quartz-sericite-chlorite matrix supporting quartz of calc-alkalic quartz-sericite-chlorite and, less commonly, feldspar phenocrysts. This unit is interpreted as subaqueous pyroclastic flow and fall fall deposits deposits (Bornhorst (Bornhorst and Johnson, 1993). 1993). The Thelapilli-tuff lapilli-tuff unit unit grades gradeslaterally laterally and vertically into the upper tuff unit. This quartz-feldspar-sericiteThisunit unitisiscomposed composedof of quartz-feldspar-sericitechlorite chlorite schist schist with with aa bulk bulk calc-alkalic calc-alkalic dacite dacite composition. composition. It is interpreted as interbedded tuffs tuffs and and volcanic volcanic derived derived graywacke (Bomhorst and Johnson, 1993). The The combined combined thickness thickness of of these these felsic felsicvolcanic volcanic and andvolcaniclastic volcaniclastic units units isis900 900meters. meters. North North Block Block The The north north block block isis composed composed of of the the 3,100-meter-thick 3,100-meter-thick Lighthouse Point Basalt (Figure 8). 8). The Lighthouse Point Basalt is dominantly pillowed tholeiitic-magnesian basalt flows with The Lighthouse Point Basalt is dominantly with minor minor massive massive flows. flows. AAglomerophyric glomerophyric(Nash (NashCreek) Creek)basalt basaltflow flowoccurs occursnear nearthe thebase base (Figure 9). 9). Within Withinthe theLighthouse LighthousePoint PointBasalt, Basalt,the theReany ReanyLake Lakepyroclastic pyroclastic unit unitisiscomposed composed (Figure of interbedded interbedded ash ash to to lapilli lapilli ash ash tuffs tuffs of of caic-alkalic calc-alkalic dacite composition. ItItisisininturn tumoverlain overlain of by thinly bedded chert-magnetite chert-magnetiteiron-formation iron-formation by the the Fire FireCenter Centermine mineiron-formation iron-formationunit unit—- aa thinly with with lesser lesseramounts amountsof ofassociated associatedchlorite-magnetite chlorite-magnetite schist schistand andchert-pyrite chert-pyrite iron-formation. iron-formation. Over-lying Over-lyingthe the iron-formation iron-formation isisintercalated intercalated chlorite-carbonate-feldspar chlorite-carbonate-feldspar and andquartz-chlorite quartz-chlorite schist schist (tuffs). (tuffs). Above Aboveand andbelow belowthe theiron-formation iron-formationare areisolated isolatedlenses lensesand andpods podsof ofgraphitegraphitepyrite pyrite schist. schist. This Thisinterval internalofofsedimentary sedimentaryrocks rocksisispoorly poorlyexposed, exposed,but buthas hasan anestimated estimated thickness thickness of of approximately approximately 100 100meters. meters. The TheReany ReanyLake Lakepyroclastic pyroclastic unit unitalso alsocrops cropsout outinin the the vicinity vicinity of of Hills Hills Lakes Lakes where where itit is is composed composed of lapilli and ash tuffs of calc-alkalic dacite to to rhyolite rhyolite composition. composition. The TheLighthouse LighthousePoint PointBasalt Basaltrepresents representsaasuccession successionofofsubaqueous subaqueous tholehite tholeliitebasalt basalt flows flowswith with intercalated intercalated distal distal to to nearly nearly proximal proximal felsic felsic volcanic volcanic rocks rocks (Johnson (Johnson and andBornhorst, Bornhorst,1991). 1991). 20 �3fl5 8740 a 4 ___ -A t ......................... A A A A A A £e/Lt, —— A A fr - & DeadRiver StorageBasin AA 8t25 87% 87'30 \D A A. -- - A A A Ah —---" A A A A A SMarquette -_ 4632 30 - - - -C4 • - - - - - - - "s/i CARP RIVER FALLS SHEAR ZONE 0Negaunee 10lshpeming 4630 10 KILOMETERS KILOMETERS j0 5 0 1 Explanation Explanation 0Undifferentiated Undifferentiated metasedimentary rnetasedimentary rocks rocks I I V//A Lapilli-tuff unit unit Basalt flow unit unit ARCH EAN ARCHEAN - -- L1 Undifferentiated rocks Undifferentiated rocks - Metagabbro Metagabbro Mona Formation Formation '4-+ unit LA.AI Upper tuff unit — Contact--Dashed where concealed concealed Shear zone Approximate trace of axial axial plane plane of of plunging syncline plunging syncline Note: All rocks rocks are metamorphosed metamorphosed Figure 6: Generalized geolgic map of off the Ishpeming greenstone greenstone belt. o f the southeast block o 21 �Upper upper tuff unit *tuff unit Lapilli-tuff Lapilli-tuff unit unit METERS METERS 0 Basalt Basalt * flow flow unit unit 500 1000 Figure 7: 7: Generalized Generalizedstratigraphy stratigraphy of of the the Mona Mona Formation Formation within within the the southeast southeast block block Figure the Ishpeming Ishpeming greenstone greenstone belt. belt. of the 22 �_____ ______ ______ 0 10 5 Explanation Explanation 0Sedimentav rocks of Middle and EaW Proterozoic ages ages Sedimentary rocks of Middle and Early I Fire Center center mine iron-formation and and Reany Lake pyroclastic pyroclastic units units I ARCHEAN ARCHEAN a to a a] Nash Nash Creek glomerophyric glotnerophyric basalt basalt unit unit where inferred - Fault—Dashed Fault-Dashed where inferred --- Reany Creek Reany Creek Formation Formation Granodiorite near Rocking Granodiorite Rocking Chair Lakes Lakes Gneissic granite Gneissic granite to to tonalite tonalite Strike and dip of overturned overturned bedding bedding %5 +f I.. Lighthouse Point Lighthouse Point Basalt Basalt Hills Lakes Hills Lakes pyroclastic pyrodastic unit unit Shear Shear zone zone — '-{T - Approximate Approximate trace of axial plane plane of plunging plunging anticline anticline Approximate Approximate trace trace of of axial axial plane planeof of plunging syncline plunging syncline Figure 8: Generalized geologic map map of of the north block of of the Ishpeming Ishpeming greenstone greenstone belt. belt. 23 �Local unconformil unconformity + Hills Lakes pyroclastic unit unit Main lithology of Lighthouse Point Basalt Fire Center mine iron-formation unit Reany Lake pyroclastic pyroclastic unit unit Nash Creek glomerphyric basalt unit Base unknown unknown Figure 9: Generalized stratigraphy of the Lighthouse Point Basalt within the north block of the Ishpeming greenstone belt. 24 �Stratigraphic Relationships Stratigraphic Relationships Among Among Lithostratigraphic Blocks Blocks Stratigraphic correlations made across major maior shear zones are tenuous. The folded rocks in in the the southwest block comprise a steep westward plunging synformal anticline, with tops to bedding on the north limb facing northward (Figure (Figure 2). 2). Pillows Pillows in in the the southeast southeast block block consistently consist&tly young to the north, thus strata strata of the southeast block could be be interpreted interpreted as as younger than and overlying strata of the southwest southwest block. block. Displacement Displacement along along the the Carp Carp River shear zone may have eliminated eliminated some of of the the stratigraphic stratigraphic section. section. However, itit could could also be argued that the stratigraphy of the southwest and southeast blocks may have been juxtaposed by thrusting along the Carp River shear zone making stratigraphic relationships between them indeterminate but evidence for thrusting is lacking. & The stratigraphic stratigraphic units of the north block and the southeast block face each other other across across the the Dead River Basin, Basin, but but are separated separated by bythe theDead DeadRiver Rivershear shearzone zone (Figure (Figure 2). 2). The strata strata of the southeast block consist of pillowed and massive tholeiitic basalt flows overlain calcoverlain by by calcalkalic tuffs and lapilli-tuff. lapilli-tuff. The alkalic The Lighthouse Lighthouse Point Basalt and the Mona Formation may represent the opposing limbs of a synform. This This is is much much more more speculative speculative than the stratigraphic correlation between the southeast southeast and and southwest southwest block. block. If each block is stratigraphically unique and represents distinct and separate separate strata, strata, the the total total stratigraphic thickness is 16,000 16,000 meters. If all three blocks are correlatable with one another, the cumulative cumulative thickness thichess isis 13,000 13.000meters. meters. Intrusive Intrusive Rocks Rocks have been been intmded intruded by by multiple multiple episodes episodes of of Archean Archean mafic and felsic The rocks of the 1GB IGB have magmas. The and peridotite peridotite sills. sills. The gabbros Theoldest oldest intrusive intmsive rocks rocks are synvolcanic gabbro and are chemically indistinguishable from the surrounding basalt flows and likely represent subvolcanic intmsions intrusions (Bornhorst (Bornhorst and and Johnson, Johnson, 1993). 1993). The Deer Lake Peridotite is intmded intruded subvolcanic between the basalt and tuff units of the Kitchi Formation. Formation. Contacts with surrounding volcanic rocks are conformable, conformable, but lack of quench textures (spinifex), pillow structures, structures, or or other features typical tpical of ultramafic flows lead to the interpretation of the peridotite body as as aa (Bornhorst and and Johnson, Johnson, 1993; 1993; Brozdowski, Brozdowski, 1990, 1990,Bornhorst Bomhorstet et al., al., 1986). 1986). The sill complex (Bornhorst peridotite is lherzolite lherzolite to harzburgite in composition (Rossell, 1983) 1983) and similar to ultramafic ultramafk flows and sills from other Archean greenstone belts. sills from other Archean greenstone belts. The oldest felsic intrusive intmsive bodies are foliated diorite to granite gneiss batholiths that intrude intmde the margins of the greenstone belt. These These intrusions intmsions post-date post-date mafic volcanism in the greenstone belt and were deformed along with volcanic rocks during the earliest stage of quartz-feldspar porphyry dikes cut deformation. Archean granodiorite to syenite stocks and quartz-feldspar greenstone belt and Dzthrough through D4 D4deformation deformation the greenstone and are are interpreted interpreted to tobe besynchronous synchronouswith withD2 (Wilkin and Bornhorst, Bornhorst, 1992). 1992). 25 �Discussion Structural Stmctural studies studies in the 1GB IGB have led to a more accurate picture of stratigraphic thickness within lithostratigraphic Iithostratigraphic blocks and and possible possible correlations correlations between between blocks. blocks. Geological studies prior to 1988 1988 (Bornhorst) (Bomhorst) did not recognize the stratigraphic significance of lithostratigraphic Iithostratigraphic blocks. The Therecognition recognition of of the the synformal synfonnal anticline anticline in in the the southwest southwest block allows the true thickness of the volcanic strata of the Kitchi Kitchi Formation to be estimated 1,500 meter thickness estimated by Morgan at 6,800 6,800 meters, a four fold increase increase over over the the ++1,500 and DeCristoforo (1980). polyphase deformation in the Lighthouse Point (1980). The recognition of polyphase Basalt of the north block lead to a thickness estimate of 3,000 meters, compared to an earlier estimate of 4,500 meters (Morgan (Morgan and DeCristoforo, DeCristoforo, 1980). 1980). The stratigraphy of the 1GB can be be compared compared to tothat thatof ofthe theAbitibi Abitibigreenstone greenstonebelt. belt. In IGB can particular, the volcanic stratigraphy can be compared to the Tisdale Group, which stratigraphy compared the Tisdale Group, which isis composed of komatiites and basaltic komatiites at the base overlain overlain by tholeiitic basalts, calc-alkalic volcaniclastic volcaniclastic rocks rocks (Pyke, (Pyke, 1982). The Tisdale subsequently overlain by calc-alkalic Tisdale Group Group has a total thickness of from 950 to 4,000 meters. The The 1GB IGB has a similar volcanic stratigraphy: tholeiitic basalt (the basalt basalt flow units units of of the the Kitchi Kitchi and and Mona Mona Formations, Formations, and the Lighthouse Point Basalt) at the base, succeeded by calc-alkalic volcaniclastic rocks rocks (the (the tuff unit of the Kitchi Gtchi and and the upper upper tuff tuff unit unit of the Mona Mona Formations, Formations, and and the the Reany Reany Lake Lake pyroclastic unit of the Lighthouse Point Point Basalt). Basalt). The stratigraphy is even more comparable ifif the Deer Lake Lake Peridotite Peridotite represents represents subvolcanic subvolcanic sills sills analogous analogous with with komatiitic komatiitic volcanism. volcanism. Thus, in terms of lithology lithology and and thickness thickness the the stratigraphy stratigraphy of of the the 1GB IGB is directly comparable comparable to the Abitibi greenstone belt. The The1GB IGB represents an Archean volcanic arc assemblage as characterized characterized by Thurston Thurston and and Chievers Chievers (1990). (1990). The environment of deposition is important in identifying identifying potential potential ore deposits deposits with'in within the JOB. Significant occurrences of volcanogenic massive sulfide deposits are not known in the IGB. the 1GB. Chalcopyrite and sphalerite associated with chert in the Reany Lake pyroclastic unit of IGB. Chalcopyrite the Lighthouse Point Basalt was interpreted by Johnson and Bomhorst Bornhorst (1991) as a massive sulfide deposit incorporated incorporated in the pyroclastic pyroclastic flow. flow. Typically, Typically, volcanogenic massive greenstone belts belts that that have have aa greater greater than than average average volume volume of of sulfides are hosted in greenstone volcanic and sedimentary sedimentary rocks rocks (Franklin, (Franklin, 1990). The intermediate volcanic, felsic volcanic The JOB IGB is favorable for VMS deposits because of its considerable considerable stratigraphic stratigraphic thickness. thickness. The The Ropes Ropes 18901s,and more recently from 1985 1985 to to 1989, 1989,is is gold mine, which was in production in the 1890's, 1990). controlled by a shear zone (Bornhorst (Bomhorst et al., 1986; 1986; Brozdowski, 1990). Card (1990) suggested that the Superior province grew by Archean accretion, accretion, and and that that of north directed directed subduction. subduction. A similar sequence of Archean volcanism was the result of of events events could explain the Archean geology of the IGB. 1GB. The mafic to felsic volcanism could have subduction. Extra-belt been the result of northward directed subduction. Extra-belt tonalite tonalite batholiths, batholiths, intra-belt intra-belt granodiorite stocks, and appinite stocks intrude intrude the the greenstone greenstone belt. belt. The transition transition from from subduction to collision is is preserved by the volcanic rocks (subduction (subduction related arc) arc) and and by by intrusive rocks as described by Wilkin and and Bornhorst (1992) (1992) (transition from from subduction subduction to to collision). 26 �1GBcan canbe beplaced placedininan ananalogous analogousmodem modemenvironment. environment. As As a The volcanic strata of the IGB directed subduction subduction thick thick accumulations accumulations of pillowed tholeiitic tholeiitic basalt basalt flows flows result of north directed (basalt flow units of the Kitchi and Mona Formations and the Lighthouse Point Basalt) were were Subsequently, dome dome complexes complexes developed developed on on the the mafic mafic erupted from shield volcanoes. Subsequently, shield volcanoes. Subaqueous pyroclastic and lahar flows (tuff and lahar units of the Kitchi Kitchi Formation, upper tuff and lapilli-tuff units of the Mona Mona Formation, Formation, and and pyroclastic pyroclastic units units of of the Lighthouse Point Basalt) erupted erupted from the dome complexes. The Therocks rocksofofthe the1GB IGB Continent-continent collision collision caused caused the the represent coalescing volcanoes in an arc complex. complex. Continent-continent cessation of volcanism volcanism and and complex complex post-deposition post-deposition deformation. deformation. Summary Summary The Tshpeming Ishpeming greenstone belt consists of subaqueous tholeiitic basalt to calc-alkalic calc-alkalic rhyolite, 10-15 !an in total thickness. thichess. U-Pb U-Pb zircon zircon data data establish establish the the age age of of volcanism volcanism at at 10-15 km 2705.8 +1—i .6 Ma Ma (Bornhorst Johnson, 1998). The 2705.8+/-1.6 (Bomhorst and Johnson, The volcanic rocks were enveloped by tonalite suite suite plutons just before before and and during the beginning of recumbent folding folding (Table (Table 1). 1). Strike-slip Stnke-slip shear shear zones zones and and the the deposition deposition of clastic sediments into pull-apart basins followed followed recumbent folding. The folded. A The belt belt rocks rocks were were then upright folded. A trondhjemite trondhjemite to granite suite suite intruded the belt and folded earlier fabrics. The timing of deformation is constrained with UThe timing of deformation is with Udata yielding yielding an an age age of of 2668.4+2.1/-1.8 2668.4+2.1/-1.8 Ma for an intrusion in this suite suite (Bornhorst (Bornhorst Pb zircon data and Johnson, 1998). 1998). Intrusions Intrusions of of hornblendite homblendite to to syenite syenite (appinite (appinite suite) suite) and and continued continued movement along shear zones represent represent the end of of deformation. deformation. An undeformed post-tectonic granite (Sims,1991). 1991). granite intrusion intrusion has has an an age age of of 2585 2585 (Sims, These events are readily interpreted in a modem modern plate plate tectonic tectonic context. context. North-directed arc. Subduction-related subduction produced a volcanic arc. Subduction-related tonalite intruded the arc just prior to collision of a small km long long small continent continent from from the south (southern complex) along the 1000 1000 kin Great Lakes tectonic zone (suture). Collision Collision occurred occurred over over an an extended period of time with multiple deformation and magmatic events. events. The The appinite appinite suite suite is characteristic of modem collision-related magmatism. magmatism. Continuing Continuingstudies studiesof ofthe the1GB IGB and and vicinity vicinity will will help help to to unravel unravel the tectonic evolution evolution of of the the southern southem edge edge of of the the Superior SuperiorProvince. Province. Acknowledgments Acknowledgments We thank thank Shannon Shannon E. E. Bair Bair for forcomputer computerdrafting drafting of of the thefigures. figures. We References References T.J., 1988, 1988,Geological Geological overview overview of of the Marquette greenstone greenstone belt, Michigan Michigan ThirtyThirtyBornhorst, T.J., annual meeting, meeting, Institute Institute on Lake L&e Superior Geology, Field Trip Guide Books, Books, part part 2, 2, p. p, fourth annual Al-A31. ALA3 1. 1998, Archean evolution of the southern southem edge of the Superior Superior Bornhorst, T.J., and Johnson, R.C., 1998, 30, p. AProvince in Michigan: Geological Society of America Abstracts with Programs, v. 30, 159. 159. and Johnson, R.C., 1993, 1993, Geology of volcanic rocks in the southern half of of the the Bornhorst, T.J. and U S . Geological Survey Bulletin 1904-P, 1904-P, l3p. 13p. Ishpeming greenstone belt, Michigan: U.S. 27 �Bornhorst, T.J., Shepeck, Shepeck, A.W., and Rossell, D.M., 1986, 1986, The Ropes gold mine, Marquette County, Michigan, U.S.A.-- an Archean hosted lode gold deposit: MacDonald, A.J. (ed), (ed.), Proceedings of Gold '86, an International Symposium on the Geology of Gold, Toronto, p.213-227. bend, Marquette greenstone Brozdowski, WA., R.A., 1990, 1990, Ropes pyritic gold deposit in a dilational bend, belt, Michigan: University of Western Ontario, London, Ph.D. dissertation, 592 p. Card, K.D., 1990, 1990, A review of the Superior Province of the Canadian Shield, a product of of Archean Archean accretion: Precambrian 56. accretion: PrecambrianResearch, Research,v.48, v.48,p.99-1 p.99-156. Card, K.D. and Ciesielski, A., A,, 1986, 1986, DNAG No.1. Subdivisions of the Superior Superior Province Province of of the the Canadian Shield. Shield. Geoscience Geoscience Canada, Canada, v.13, v.13, p.5-13. Franklin, J.M., 1990, massive sulphide sulphidedeposits, deposits,inin Ho, Ho, S.E., SE., Robert, 1990, Volcanic-associated massive Robert, F., F., and and Groves D.I. (eds.), Gold and and Base-Metal Base-Metal Mineralization in the Abitibi Ahitibi Subprovince, Subprovince, Canada, Canada, With Emphasis 1-242. Emphasis on on the the Quebec Quebec Segment, Segment,Short ShortCourse CourseNotes, Notes,p.21 p.211-242. Hammond, R.D., 1978, 1978, Geochronology and origin of Archean rocks in Marquette County, Upper Upper Michigan: University of Kansas, Lawrence, Kansas, M.S. thesis, 69 p. University Kansas, Lawrence, Kansas, M.S. thesis, 69 p. Hoffman, P.F., 1989, 1989, Precambrian Precambrian geology and tectonic tectonic history history of North America, America, in in Bally, Bally, A.W., A.W., and Palmer, A.R. (eds.), (eds.), The geology geology of North North America: America: an an overview: overview: Boulder, Boulder, Colorado, Colorado, Geological society society of America, The Geology Geology of North America, America, v.4, v.4, p.447-512. p.447-512. Johnson, R.C., 1993, structural, tectonic, tectonic, and and economic economic studies studies of the Archean Johnson, R.C., 1993, Stratigraphic, Stratigraphic, structural, the Archean Ispheming greenstone belt, Marquette County, Michigan: Michigan Technological University, University, Houghton, Ph.D. Dissertation, Dissertation, 119 119 p. Johnson, R.C. and Bornhorst, of the the Ishpeming Ishpeming Bomhorst, T.J., 1991, 1991, Archean geology of the northern block of Bulletin 1904-F, 20 p. greenstone belt, Marquette County, Michigan: U.S. Geological Survey Bulletin Morgan, J.P., J.P., and and DeCristoforo, DeCristoforo, D.T., 1980, 1980, Geological Geological evolution of the the Ishpeming Ishpeming Greenstone Greenstone Belt, Michigan, Michigan, U.S.A.: U.S.A.: Precambrian Research, Research, v.11, v.11, p.23-41. p.23-41. Nachatilo, Nachatilo, S.A., and Bauer, R.L., 1993, 1993, Structural Structural analyses analyses of Archean Archean rocks rocks in in the theNegaunee Negaunee area, Michigan ---- constraints constraints on Archean versus versus Proterozoic Proterozoic deformation: deformation: U.S. U.S. Geological Geological Survey Survey Bulletin Bulletin 1904-0. 1904-0. Puffett, W. p., U.S. P., 1974, 1974, Geology of the Negaunee Quadrangle, Marquette County, Michigan: U S. Geol. Survey Professional Paper 788, 53 p. Survey Professional Paper 788,53 Pyke, D.R., 1982, Timmins area, District of Cochrane: Ontario Geological Survey 1982, Geology of the Tirnrnins Report 219, 219, l4lp. 141p. Rossell, Rossell, D.M., 1983, 1983, Alteration of the Deer Lake peridotite in the vicinity vicinity of the the Ropes Ropes mine, mine, Marquette County, Michigan: Michigan Technological University, Houghton, Michigan, County, Michigan: Michigan Technological University, Houghton, Michigan, M.S. M.S. thesis, thesis, 83 83 p. p. Subprovince (Archean (Archean Gneiss Gneiss Terrane): Terrane): in in Archean Archean and and Sims, P.K., 1996, 1996, Minnesota River Valley Suhprovince Proterozoic Proterozoic Geology of the Lake Superior Superior Region, U.S.A., U.S.A., 1993: 1993: U.S. U.S. Geological GeologicalSurvey Survey Professional Professional Paper Paper 1556, 1556,p. p. 14-23. 14-23. Sims, P.K., 1991, Great Lakes Lakes tectonic tectonic zone zonein inMarquette Marquettearea, area,Michigan Michigan--implications - implications for for Archean tectonics in north-central United States: U.S. US. Geological Geological Survey Survey Bulletinl9O4-E, Bulletin1904-E, 77 p. p. Thurston, P.C., and Chievers, K.M., 1990, Secular variations in greenstone sequence development, P.C., Chievers, 1990, Secular variations greenstone sequence development, emphasizing emphasizing Superior Superior Province, Province, Canada: Canada: Precambrian Research, Research, v.46, v.46, p. 21-58. 21-58. Geology and and geochemistry geochemistry of of granitoid rocks in the T.3., 1992, Geology Wilkin, R.T., R.T., and Bornhorst, Bornhorst, 713., Archean Northern complex, complex, Michigan, Michigan, U.S.A.: U.S.A.: Can. Journal of Earth Science, Science, v. v. 29, 29, p. 167416741685. 1685. 28 �INTRODUCTION INTRODUCTION TO THE THE ROPES GOLD DEPOSIT DEPOSIT IN THE THE ISHPEMING GREENSTONE GREENSTONE BELT, MICHIGAN ABSTRACTED H PERMISSION FROM GEOLOGY OF ABSTRACTED WIT WITH OF THE ROPES ROPES GOLD GOLD DEPOSIT BY R.A. BROZDOWSKI BROZDOWSKI (1988) (1988) Theodore J. Bornhorst and Shannon E. Bair, Department of Geological Engineering Engineering and and Sciences, Michigan Technological University, Houghton, MI 49931; 4993 1; and Glenn Glenn Scott, Scott, Tilden Tilden Mining Company, company, Ishpeming, MI 49849 Introduction Introduction The southwest part of the greenstone belt (Figure 1) 1) consists of, of, from west west to to east: east: pillowed and massive basalt, hypabyssal gabbro, dacite tuff and subordinate tuff breccia, breccia, fine-grained peridotite peridotite of of the the Deer Deer Lake Lake Peridotite. Peridotite. Volcanic rocks strike serpentinitic, fine-grained northeast, with a major bend to the east northeast at the Bjork-Lundeen prospect, 1.5 1.5km km west west of the Ropes deposit. Rocks face consistently to the southeast, based on pillows and graded and graded bedding The Ropes deposit is southeast of a transition from dominantly basalt to the the northwest northwest to dominantly dacite tuft to the southeast (Figure (Figure 2). 2). Meter-thick banded iron formation formation and and several meter thick greywacke are within the dacite up section from the interlayered basalt basalt and dacite. Sericite Sericiteprogressively progressively overprints overprints and and finally finally totally pesudomorphs feldspar feldspar phenocrysts in dacite tuff as the Ropes gold deposit is approached from the west. The deposit is in quartz-sericite-chlorite quartz-sericite-chloriterock which is interpreted as quartz-sericite altered altered dacite dacite tuff. tuff. Major Rock Types within Major within the the Ropes Ropes Deposit The four major rock types in or immediately bounding the Ropes deposit are are (Figures (Figures quartz-sericite-chlorite rock which encloses ore, strikes 070' 070° 3 and 4): 1.) fine grained quartz-serieite-chlorite overall, but 080' 080° where where it hosts the the Ropes deposit, deposit, and and dips dips steeply steeply south south or vertical. This This rock type is bounded north and south by 2.) fine grained carbonate-quartz-chlorite rock which is massive to compositionally layered on a scale of several millimeters. It and the the quartz-sericite-chlorite rock are locally complexly interlayered, particularly in the west part quartz-sericite-chlorite of the deposit, and have have generally generally sharp sharp contacts. contacts. Carbonate-quartz-chlorite rock is flanked successively, on both the north and south sides by, 3.) fine grained, massive to moderately rock, and 4.) 4.) fine foliated, carbonate-talc rock, fine grained, grained, serpentinitic peridotite which commonly original olivine olivine and and lesser lesserpyroxene. pyroxene. This is variably has pesudomorph texture after original with carbonate-talc carbonate-talc rock. rock. Contacts between carbonatecarbonate-bearing near its contacts with quartz-chlorite rock and carbonate-talc rock and between the carbonate-talc rock and the serpentinitic peridotite are generally gradational over one to several meters. Quartz-Sericite-Chlorite Quartz-Sericite-ChloriteRock Rock The main host for gold is light green, massive to slightly foliated, fine grained, quartz-sericite-chlorite thick on the 800 quartz-sericite-chlorite rock. This This rock rock is is aa 080°-striking 080'-striking layer up to 40 m thick mine level, but narrows toward the surface and toward the east to less than 6 m thick. It is is level. 29 �0 km 2 EXPLANATION EXPLANATION PROTEROZOIC PROTEROZOIC 1 graywacke and slate 2 quartalte ARCH LAN 3 serpentinillc peridotila 4 volcanIc conglomerate, subordinate daclte tuti 5 banded Iron lorniatton 6 daclle tutt, lull breccta and ltows 7 tonallIe and granodlorlte a gabbro 9 basalt 10 gtornerophyrlc basalt Figure1:1:Geologic Geologicsetting setting theRopes Ropesmine mineininthe thesouthwest southwestblock blockofofthe theIshpeming Ishpeming Figure ofofthe greenstone greenstonebelt. belt. 30 , �EXPLANI4 TION EXPLANA JION 0 0 0 ii GRAYWACKE GRAYWACKE 10 QUARTZITE QUARTZITE w F- 0 a- SERPENTINITIC PERIDOTITE LJ SERPENTINI11C PERIDOTITE EIJ CARBONATE RICH, SERPENnNITIC CARBONATE RICH, SERPENTINITICPERIDOTITE PERIDOTITE 1 a - CARBONATE TALCROCK ROCK ED CARBONATE-TALC COMPOSITIONALLY LAYERED TO MASSIVE COMPOSKIONALLY LAYERED MASSIVE CARBONATE QUARTZ - CHLORITE CHLORITE ROCK ROCK CARBONATE- QUARTZ - - TUFF, TUFF TUFF BRECCIA, BRECCIA, AND FLOWS; FLOWS; DACITE TIJFF, (ALTERED TO QUARTZQUARTZ-SERICITESERICITE- CHLORITE CHLORITE ROCK IN INROPES ROPESDEPOSE!) DEPOSIn S " = - ED BANDED BANDEDQUARTZ QUARTZ - MAGNETrrE MAGNETITE IRON FORMATION FORMATION GRAYWACKE ANDSILTSTONE SILTSTONE ED GRAYWACKE AND E1J FINE GRAINED FINE GRAINEDGABBRO GABBRO 1' L S BASALT (INCLUDES (INCLUDES PILLOWED PILLOWED AND GLOMEROPHYRIC VARIETIES) VARIETIES) PROJECTION TO SURFACE OF OF OREBODIES OREBODIES UPDIP PROJECTION TO SURFACE A = main main ore ore zone zone northwest ore B = northwest ore zone zone Figure 2: Surface Surfacegeologic geologic map mapof o f the the Ropes Ropes mine m i n e and vicinity. 31 �1 300 level 1152 level 200 Ft CI I 61 M I N 200E 400E 6006 BODE I000E 12006 EXPLANATION EXPLANATION I i I I 2 I i 3 i a [5 a Quartz-Sericite SerpentiniticFeridotite Peridotite L41 Quartz-Seridte chlorite Rock -. Serpentinitic Chlorite Rock locally carbonate carbonate rich rich locally Carbonate-Talc Carbonate-Talc Rock Rock Carbonate-Quartz -Carbonate-Quadz Chlorite Rock Rock Chlorite I Orebody Orebody >2 2 , g /tonne ,tonne Au Au Figure 3: Geologic Geologicplan planmap mapof ofthe the300 300and and1152 1152levels levels of of the the Ropes Ropes mine. mine. Figure 32 �ROPES R O P E S MINE MINE 600 E E Cross-Section Cross-Section Looking LookingS80°W S80° i Elevation Elevation(ft) ( 1 Serpentinitic Peridotite Peridotite Serpentinitic locallycarbonate carbonaterich rich locally I a 121 Carbonate-Talc Carbonate-TalcRock Rock 1400 1401 I 1200 1201 Carbonate-Quaflz -Carbonate-Quartz Chlorite Rock Rock Chlorite 3 4 i Quartz-Sericite Quartz-Sericite ChloriteRock Rock Chlorite Orebody 151 Orebody gltonneAu Au >>22g/tonne 1000 1001 C 0mine minelevels levels 800 80C Scale 0 600 600 100 200ff 61 m 100 400 40C 200 ft 61 m 200 200 Sea SeaLevel Level -200 -200 Figure Figure4:4:Geologic Geologiccross crosssection sectionofofthe theRopes Ropesmine minelooking loohngS80°W S80° along alongthe the600E 600Esection. section, 33 �complexly interlayered with carbonate-quartz-chlorite rock, especially west of the Ropes main ore zone and in the mine. Randomly Randomlyoriented orientedto tomoderately moderately aligned, aligned, 11to to22 mm, mm, angular, rectangular mats of felted sericite up to 2 mm in length, comprised of individual sericite grains less than 10 10 microns in size, and minor quartz, are enveloped in this matrix and have an external habit identical to plagioclase In dacite dacite tuff tuff immediately immediately plagioclasephenocryst. phenocryst. In west of the deposit there are plagioclase phenocrysts. The Therectangular rectangular mats matsare areinterpreted interpreted as as pseudomorphs after the plagioclase phenocrysts. phenocrysts. The rock is layered locally with 5 mm thick, lens-like, sericite-rich sericite-rich and and alternating alternating more more chlorite-rich chlorite-rich laminae. laminae. Sericite quartz-sericite-chloriterock rock decreases decreases Sericite in in relict relict feldspar feldspar and and lithic lithic clasts clasts in in the the quartz-sericite-chlorite gradationally westward from the Ropes main ore zone over a distance of of several hundred meters into aa large large body of of dacite dacite tuff tuff that that has has moderately moderately aligned aligned twinned twinned plagioclase plagioclase phenocrysts phenocrysts with with only only very very minor minor sericite sericite internal to the phenocrysts, bipyramidal quartz, and volcanic volcanic rock rock fragments fragments in in an an aphanitic aphanitic quartz-sericite feldspar-chlorite matrix. The Thedacite dacite tuff outside of the Ropes deposit, proper, has local lapilli-sized fragments as well as tuff tuff outside of the Ropes breccia breccia layers. layers. Gold abundance abundance is generally greatest where very fine to aphanitic pyrite is 5 to 8% 8% of the the rock, rock, quartz quartz and and sericite sericite are are most most abundant, and chlorite is least abundant. However, However,this this generality generality does does not not apply apply to to the the chioritic, chloritic, pyritic, pyritic, eastern eastern most most rim rim of of the the Ropes Ropesmain mainore ore zone. zone. Anastomosing Anastomosingquartz quartzveinlets veinletsless lessthan than11mm mmthick thickmake makeup upseveral severalpercent percentof ofthe the rock. rock, Rock Rockoutside outsideofofthe theore orezones zonescontains containsless lessthan than2% 2%very very fine finepyrite. pyrite. The The overall overall 070° quartz-sericite-chlorite rock rock which which hosts hosts the theRopes Ropes 070' striking striking trend trend of of quartz-sericite-chlorite main main and and northwest northwest ore ore zones zones interfingers interfingers with with carbonate-quartz carbonate-quartz chlorite chloriterock rockwest westof ofthe the deposit, and does not continue west of the northwest ore zone (Figure 2). East of the deposit, deposit, and does not continue west of the northwest ore zone (Figure 2). East of the deposit, the the trend trend of of quartz-sericite-chlorite quartz-sericite-chlorite rock does not continue at surface beyond 300 m east of the the Ropes Ropes main main ore ore zone. zone. A A zone zone of of carbonate-talc carbonate-talc rock rock within within the the serpentinitic serpentinitic peridotite peridotite marks marks the the east-northeast east-northeastprojection projection of ofthe thetrend. trend. Carbonate-Quartz-Chlorite Rock Carbonate-Quartz-Chlorite Rock Carbonate-quartz-chlorite Carbonate-quartz-chloriterock rock envelopes envelopes the the quartz-sericite-chlorite quartz-sericite-chloriterock rockand andisis complexly complexly interlayered interlayered with with it, it, particularly particularly toward toward the the west west end end of of the the Ropes Ropesdeposit. aeposit. Carbonate-quartz-chlorite rock rock isis restricted restricted largely largely to to the the deposit deposit although although layers layersup uptoto22mm Carbonate-quartz-chlorite thick thick occur occur up up to to 400 400 m m west of the Ropes main ore zone. The The thickest thickest layers layersof of carbonatecarbonatetalc talc rock rock are are immediately immediately north north and and south south of of the the deposit; deposit; however, however, thin thin layers layersof oftalc-rich talc-rich rock rock are are at at numerous numerous localities localities atat the the contacts contacts of, of, and and also also within within serpentinitic serpentiniticperidotite peridotite along along the the northwest northwest contact contact of of the the peridotite peridotite up up to to 500 500 m m away away from from the the deposit. deposit. Carbonate-quartz-chlorite Carbonate-quartz-chlorite rock rock contains contains abundant abundant ferroan ferroan dolomite, dolomite, and andlesser lesserquartz quartz and and chlorite. chlorite. Minor Minorsericite sericiteisisininsome someparts partsand andvariable variable amounts amounts of of talc talc in inothers, others,but butthese these two quartz-sericite-chloriterock rock two minerals minerals are are generally generally mutually exclusive. Contacts Contactswith with quartz-sericite-chlorite are are sharp sharp in in aa direction direction across acrossthe the strike strikeof of the the rock rock type, type, but but are arelocally locallygradational gradationalalong along strike. strike. Barren Barrenmilky milkyquartz quartzveins veinsup uptoto10 10cm cmthick thickoccur occurlocally locallyand andare areconformable conformabletotothe the contact. contact. Minor Minorcarbonate carbonateminerals mineralsare arepresent presentininthe thequartzquartz-sericite-chlorite sericite-chloriterock rocknear nearits its contacts contacts with with the the carbonate-quartz-chlorite carbonate-quartz-chloriterock, rock, and and minor minor sericite sericiteisispresent presentininthe the carbonate-quartz-chlorite carbonate-quartz-chlorite rock rock near near its itscontacts contactswith with quartz-sericitequartz-sericite-chlorite chloriterock. rock. Carbonate-quartz-chlorite rock rock has has up up to to150 150ppb ppb Au, Au, and and quartz quartzrich richparts partsof ofthe thecarbonatecarbonateCarbonate-quartz-chlorite 34 �quartz-chlorite rock only very locally contain several percent pyrite and up to 11 ppm Au. Locally the carbonate-quartz-chlorite rock is brecciated, with angular fragments of carbonate carbonate veined carbonate-quartz-chlorite rock in a chlorite rich matrix particularly at the south south side side of of the deposit. Carbonate-quartz-chlorite rock is generally compositionally layered on a scale of several millimeters, but but locally locally is is massive. massive. Layering is defined by hy fine to medium grained white cxbonate carbonate lenses, chlorite foliation, foliation, and and quartz-chlorite quartz-chloriterich richlaminae. laminae. The rock is white increasingly talcose toward the contact with carbonate-talc rock. Coarse grained, barren, white dolomite veins up to 5 cm thick cut across all other features and are several percent of rock. Carbonatethe rock. Carbonate- quartz-chlorite quartz-chlorite rock rock is is not only within the deposit but also in a zone up to 60 m thick on its north side, where it contains thin layers of the other three main rock types. Carbonate-quartz-chlorite types. Carbonate-quartz-chlorite rock rock is is in local lenses along the northeast striking contact of and northwest northwest of of the the Ropes Ropesdeposit. deposit. The carbonatethe Deer Lake Peridotite, southwest and quartz-chlorite rock is interpreted as an alteration product of a serpentinitic peridotite protolith. Serpentinitic Peridotite and Carbonate-Talc Serpenrinitic Carbonate-Talc Rock Rock quartz-sericite-chlorite rock and carbonate-quartz-chlorite rocks are The interlayered quartz-sericite-ihlorite are bounded on north and south by elongate masses of serpentinitic peridotite. There are also several smaller serpentinitic peridotite bodies enclosed within the carbonate-quartz-chlorite of the Ropes Ropes deposit. deposit. The largest of these bodies is the north wall of rock on the north side of the Ropes deposit. The The rock rock is is dark dark gray gray to green, green, mostly fine grained, with major serpentine, subordinate talc and carbonate, carhonate, minor chlorite, and accessory chromite and magnetite. The The serpentine serpentine isis aa mixture mixture of of felted felted chrysotile and oriented plates of antigorite. Commonly it has a relict texture of 11 to 3 mm serpentine pseudomorphs after major olivine pyroxene. The and subordinate pyroxene. The pseudomorphs pseudomorphs are surrounded by rims of talc, carbonate and accessory accessory fine-grained fine-grained magnetite magnetiteand andchromite. chromite. Locally the the rock rock is is dark green, minerals and felted textured serpentine which which lacks lacks aa pseudomorphic pseudomorphic texture. texture. The serpentinitic peridotite is increasingly carbonate-rich toward its borders, although the relict texture is commonly preserved. Fibrous continuously preserved. Fibrous chrysotile chrysotile veinlets in the serpentinite are locally pseudomorphed by carbonate. Au abundance is typically less than 30 ppb, even in close proximity to the deposit. deposit. Massive to well foliated, gray to dark green, very fine grained carbonate-talc rock margins of of serpentinitic serpentinitic peridotite. peridotite. Chlorite, serpentine, magnetite and occurs around the margins pyrite are minor phases. phases. The carbonate-talc The carbonate-talc rock has gradational contacts over one to several meters with serpentinitic peridotite and with talc-rich parts of the carbonate-quartz-chlorite subequal. The carbonate rock. Talc Talc and and carbonate carbonate contents contents are generally generally subequal. carhonate mineral in the ferroan dolomite dolomite with with minor minormagnesite. magnesite. Au abundance is carbonate-talc rock is dominantly ferroan from less than 30 ppb to approximately 100 100 ppb locally in the carbonate-talc rock. The Gold Ore g/ton Au, including production and The Ropes deposit is 2.8 million tonnes with 3.24 glton reserves. There Ninety five percent percent of the ore all categories of reserves. There are are two two main types of ore: 1.) 1.) Ninety dispersed pyrite pyrite in in quartz-sericite-chlorite quartz-sericite-chloriterock. rock. This ore has several subtypes is gold with dispersed proportions of of quartz, quartz, sericite, sericite, chlorite, chlorite, and andpyrite. pyrite. 2.) Five percent of the ore is with varying proportions 35 �auriferous quartz veins with saccharoidal saccharoidal texture and tetrahedrite, pyrite, galena, galena, and and chalcopyrite. ItItisisgenerally generallyatator ornear nearthe thesouth southside sideof of the the deposit. deposit. Dispersed Pyrite in Quartz-Sericite-Chlorite Gold With Dispersed Quartz-Sericite-ChloriteRock Rock The Ropes main ore zone is steeply dipping, 335 m in maximum strike length, 12 12 m in average thickness, and 600 m in known down dip extent (Figures 4). It is within the quartz-sericite-chlorite rock, and gold is associated associated with dispersed pyrite with only minor quartz veins. veins. The Ropes main ore zone can be divided longitudinally into three ore subtypes section. Subtype (Figure 5). These These subtypes subtypes are, are, from west to east, in long section. Subtype 11 is is light light gray gray to to pale green, siliceous quartz-sericite rock, containing minor lenses of of light gray cherty quartz rock with pyrite, tetrahedrite, chalcopyrite and galena. galena. Pyrite Pyrite abundance abundance does does not not correlate correlate with gold grade. Subtype quartz-sericite-chlorite Subtype11has hasan aninterdigitating interdigitatingcontact contact with with barren barren quartz-sericite-chlorite and carbonate-quartz-chlorite rock to the west, and a gradational contact to the east with is 6.8 6.8 glton. g/ton. Subtype subtype 2. The average gold concentration is Subtype 2 is is light green green quartzquartzsericite-chlorite-pyrite rock with a positive correlation between fine-grained pyrite pyrite content content and Au abundance. Subtype 2 is the thickest part of the ore zone and, therefore, therefore, the most volumetrically important. ItIt has with subtype 3. The has aa relatively relatively sharp contact to the east with The average Au concentration ranges from 2.6 glton, g/ton, rarely up to 6.2 glton. g/ton. Subtype dark Subtype33isisdark green quartz-chlorite-pyrite consistently large fine-grained fine-grained pyrite pyrite content. ItIt has quartz-chlorite-pyrite rock with a consistently has barren quartz-sericite-chlorite rock rock to to the the east. east. The a sharp contact with barren The average average Au Au concentration is is 10.3 10.3g/ton. glton. Zones of greater than 4 g/ton glton Au abundance, defined by assay data, are are contained contained centrally within the Ropes main ore zone. The middle parts of these greater than The middle parts of these greater than 44 g/ton glton Au Au zones strike echelon fashion at approximately strike across the 080° 080' trend of the orebody in en echelon 055°, 055', whereas their extremities have a nearly 070° 070' strike, thus forming forming a low low angle angle sigmoidal sigmoidal pattern across the ore ore zone. zone. This This pattern pattern is is most most pronounced pronounced at at and and below below the the 1152 1152level level toward the east end of of the the Ropes Ropes main main ore ore zone zone (Figure (Figure 6). 6). Above the 650 level, ore is separated from carbonate-quartz-chlorite rock on on the the north north by slightly auriferous quartz-sericite-chlorite rock, but but below below the the 650 level level the east end of the ore is in contact with carbonate-quartz-chlorite carbonate-quartz-chlorite rock (Figure 3). 3). The north side side of of the the ore ore strikes 070° 070" and dips very steeply north whereas the contact between quartz-sericite-chlorite quartz-sericite-chlorite rock and the carbonate-quartz-chlorite rock strikes 080° 080Âwithin the deposit and dips dips steeply steeply south. Therefore, Therefore, atatdepth, depth, the the ore ore abuts abuts the the carbonate-quartz-chlorite carbonate-quartz-chlorite rock contact contact on on the the north, with the east limit of the Ropes main ore zone plunging to the west at at deeper deeper levels levels along the line defined by the intersection of the planes of the orebody and the the carbonatecarbonatequartz-chlorite rock contact on the north. Below Below the the 1284 1284 level, the 25 25 to 40 m m thick quartzquartzsericite-chlorite rock thins markedly to the east and interfingers along strike strike to to the the east east with with quartz-sericite-chlorite rock rock plunges plunges carbonate-quartz-chlorite rock. The The thickest thickest part part of of the the quartz-sericite-chlorite approximately 45° 45' east and is coincident with the bend in strike of the trend of quartz-sericite-chlorite rock from 080° 070° east of the main ore zone. 080' within the deposit back to 070' zone. There are several other gold bearing bodies bodies near the the main main ore ore zone zone (Figure (Figure 5). The The northwest ore zone is a small ore zone of 101,000 tons tons with with 4.6 4.6 glton g/ton Au, Au, 170 m m northwest northwest of of the Ropes main ore zone, and is dominantly ore subtype 2. 2. Auriferous, Auriferous, pyritic pyritic quartzquartz36 �_____ _____ long section section of of the the Ropes Ropes deposit in in the the Vertical long N80° plane, with gold gold ore ore subtypes: subtypes: N80°E 1 1 2 3 - QUARTZ SERICITE SERICITE ROCK, ROCK, QUARTZWITH WITH GRAY GRAY QUARTZ QUARTZ LENSES LENSES QUARTZ SERICITE -- CHLORITE CHLORITE ROCK, ROCK, QUARTZ -- SERICITE GOOD Au AU :: PYRITE CORRELATION CORRELATION - QUARTZ CHLORITE - PYRITE PYRITE ROCK ROCK QUARTZ -- CHLORITE FigureS: Figure 5 : Vertical Vertical long long section section of of the the Ropes Ropes mine mine in in the the N80°E N8O0E showing gold gold ore ore subtypes. subtypes. 37 37 �0 50 meters meters ROPESMINE MINE 1152 1152 LEVEL LEVELPLAN PLAN ROPES tonne Au Au ore orezones zones >> 44 gg I1tonne I 0quartz quartz-- sericite sericite-- chlorite chloriterock rock 1 Figure6:6:Geologic Geologicplan planmap mapofofthe the1152 1152level levelofofthe theRopes Ropesmine mineshowing showinghigh highgrade gradeore orezones. zones Figure 38 �Ropes Ropes Mine Mine Level 1620 Level Geologic Plan Geologic Plan IN ul m C L, + cu "1" 50 50 m EXPLANATION EXPLANATION serpentinitic peridotite a serpenthitic peridotite C >2g/tonneAu 0 > 2 g I tonne Au carbonate - talc rock C] >4g/tonneAu 0 > 4 g I tonne Au a carbonate - quartz - chlorite rock rock - A,, Au LIII quartz rock quartz -- sericite -- chlorite rock auriferous quartz vein (Lower Secondary Secondary Drift Drift Vein) Vein) LI microcrystalline a rnicrocrystallinecarbonate carbonate - quartz rock rock Figure 7: Geologic Geologic plan plan map map of of the the 1620 1620 level of the Ropes mine. mine 39 �south south of of the the central central part part of of the the main main ore ore zone. zone, Non-Aunferous Non-Aur$erous Veins Veins Milky white, white, vitreous, vitreous, medium to coarse-grained quartz veins commonly cut the east northeast foliation in the quartz-sericite-chlorite quartz-sericite-chlorite rock at a high angle. Some Somedip dipsteeply, steeply, whereas others are shallowly dipping veins offset in north side up stepwise fashion along foliation parallel structures. The The veins veins are are not not auriferous auriferous and are commonly up to 10cm 10 cm thick and several meters in length. They are approximately 1% of the quartz-sericite-chlorite quartz-sericite-chlorite rock. rock. Quartz Quartzininthe theveins veinshas hasfull fullextinction extinctionunder undercrossed crossedpolars, polars,simple simplegrain grainboundaries, boundaries, and locally locally has has minor minor pyrite. pyrite. Banded dolomite veins commonly strike 015' 015° and dip nearly vertical. vertical. They cut the auriferous quartz veins. Massive, planar sided, coarse crystalline white white dolomite veins up to 10 10 cm thick thick cut cut all all other other veins veins and andare areundeformed. undefonned. Structure Structure An 065° 065' striking striking near vertical foliation is defined by planar alignment of sericite and chlorite chlorite in in the the quartz-sericite-chlorite quartz-sericite-chlorite rock. This Thisfoliation foliationisisinterpreted interpreted as astectonic tectonic schistosity schistosity or or SS fabric fabricbecause because the the sericite sericite and and chlorite chloriteare aresecondary secondaryminerals mineralsthat thatcut cutand and locally locally displace displace relict relict primary primary magmatic features, such as domains of former feldspar phenocrysts, phenocrysts, and and lithic lithic fragments. fragments. Locally LocallyS-C S-Cfabric fabricisisdeveloped, developed, especially especiallyin incarbonatecarbonatetalc talc and and carbonate-quartz-chlorite carbonate-quartz-chlorite rocks. rocks. Contacts Contacts between between the the quartz-sericite-chlorite quartz-sericite-chlorite rock rock and and rocks rocks derived derived from fromaaperidotite peridotite protolith protolith defined defined planes planes of of weakness weakness that favored development of a shear zone. The The orientation orientation of of reactivated reactivated zones zones of of weakness such as contacts between rock types are not strictly strictly governed governed by by the the exact exact orientation orientation of of the the stress stress that that causes causesthe the movement. movement. Ore Ore Related Related Structure Structure The The Ropes Ropes main main ore orezone zone trends trends 080° 080' overall, overall, but but is is internally internally comprised comprised of of 055° 055' to to 065° 065O trending trending zones zones of of greater greater Au concentration, concentration, which host most of the gold. These Theseoccur occur in glton Au Au concentration concentrationwithin within ore ore in low low angle angle sigmoidal sigmoidalzones zones of of greater greater than than 44 g/ton characterized characterized by by dispersed dispersedpyrite pyrite or or in in auriferous auriferous quartz-tetrahedrite quartz-tetrahedrite oblique obliqueshear shearveins veinswith with similar similar geometry geometryand andorientation. orientation. Zones Zones with withgreater greaterthan than44g/ton @on gold goldconcentration1 concentration, defined defined by by assay assaydata, data,are aremore more centrally centrally within within the the Ropes Ropes main main ore zone (Figures 6 and 7). The Themiddle middle parts partsof ofthese thesehigher higher grade grade zones zones strike strikeen en echelon echelon across acrossthe the overall overall 080° 080" trend trend of of the the ore orebody bodyatatapproximately approximately 055" whereas their extremities extremities nearer nearer the contacts contacts with the the carbonate-quartz-chlorite carbonate-quartz-chlorite rock rock 0550 strike strike approximately approximately070°, 070°forming formingaalow low angle angle sigmoidal sigmoidal pattern pattern across acrossthe thezone, zone,which whichisis most most prominent prominent on onthe theeast eastend endof ofthe the1152 1152level level (Figure (Figure6). 6).This Thisgeometry geometryisisconsistent consistentwith with greater movement movementbeing beingaccommodated accommodated near nearthe themargin margin of ofmore moreductile ductiledeforming deforming greater carbonate-quartz-chlorite carbonate-quartz-chloriterock, rock,where wherethe theoriginally originally formed formed 055° 055' to to065° 065' schistosity schistositywas was flattened 080' plane plane of of shear, shear, represented represented by by the the contact contact between between contrasting contrastingrock rock flattened to tothe thelocal local080° types. types. 40 �sericite-chiorite sericite-chlorite rock rock was was encountered encountered in in deep deep drilling drilling at at the the Ropes Ropes deposit, deposit, in in aa zone zone separate separate from, from, and and deeper deeper to to the the east east of, of, the the Ropes Ropes main main ore ore zone. zone. Ore Ore Minerals Minerals and and Textures Textures and and Metal Metal Distribution Distribution Gold is with pyrite pyrite less less than than 100 100 microns microns in in diameter diameter dispersed dispersed throughout throughout the the mass mass of of the the quartz-sericite-chlorite quartz-sericite-chloriterock, rock, and and with with this this pyrite pyrite on on fractures, fractures, foliations, foliations,and and within within and and along the margins of millimeter-thick quartz veinlets. veinlets. Native Native gold, gold, of of variable variable fineness, fineness, isis 11 to to 10 10micron micron grains grains isis attached attached to to the the surface surface of of the the fine fine grained grained pyrite, pyrite, included included as asround round blebs hlebs within within fine fine grained grained pyrite, pyrite, on on fractures fractures within within fine fine grained grained pyrite, pyrite, and andatatgrain grain boundaries boundaries of of fine fine grained grained quartz quartz and and sericite. Minor Minorcoarse coarsegold goldisison onfractures fracturesin inquartz quartz veins. veins. Silver Silver isis in in electrum, electrum, native native silver silver isis with with fine fine pyrite, pyrite, argentiferous argentiferous tetrahedrite, tetrahedrite, argentiferous argentiferousgalena, galena,and andrare raredyscrasite. dyscrasite. Pyrite Argentiferous galena, galena, Pyrite is is 97% 97% of of the the metallic metallic minerals followed by 1% chalcopyrite. Argentiferous argentiferous tetrahedrite, tetrahedrite, sphalerite, millerite, bravoite, magnetite, and argentiferous tetrahedrite, tetrahedrite, sphalerite, millerite, bravoite, magnetite, and rutile rutile are are present present in in trace trace amounts. amounts. The The gold gold is is silver silver bearing and metallic silver is gold bearing. bearing, The The low low bulk bulk Au/Ag AulAg of of 0.65 0.65 for for the the Ropes Ropes gold gold deposit deposit differs differs from from ratios ratios reported reported for for Precambrian Precambrian gold-quartz gold-quartz veins veins and and lodes lodes (Boyle, (Boyle, 1979), 19791,which which range range from from1.37 1.37to to12.5 12.5and and average average about about 4.2. 4.2. Both BothAu Auand andAg Aghave havecontinuous continuouspositively positivelyskewed skewed log log normal normal distributions, distributions, each each with with aa single singlemaximum, maximum, interpreted interpretedasasindicating indicatingonly onlyone oneperiod periodof of mineralization. mineralization. Distribution. Distribution.Geometry Geometryand andAu Au Concentration ConcentrationofofAur?ferous Auriferous Quartz QuartzVeins Veins Auriferous Auriferous quartz quartz veins veins are are concentrated concentrated at the south side of the deposit. Individual Individual veins average 20 cm thick and extend 12 m horizontally and 15 m vertically, but collectively veins average 20 cm thick and extend 12 m horizontally and 15 m vertically, but collectively these these veins veins are aregrouped groupeden enechelon, echelon,and andlocally locally they they form formore oreshoots shoots88to to11 11mmthick, thick,with withthe the quartz quartz-sericite-chlorite rock. The Theveins veinsstrike strikeen en quartz veins veins separated separated by by auriferous auriferous foliated foliated quartz-sericite-chlorite echelon echelonatat055° 055' to to065° 065' across acrossthe the080° 080' trend trend of of the the quartz-sericite-chlorite quartz-sericite-chloriterock rocktype, type,dip dip vertically geometry. They vertically to to 85° 85' south, south, and and have a slight sigmoidal geometry. They occur occur progressively progressively further further east east in in successively successively deeper deeper mine mine levels. Eight Eightmajor majorshoots shootsmade madeup upof of these these veins veins were were mined mined in in the the 1800's 1800'sand and 1890's 1890's above above the 800 level. Broderick Broderick (1945) (1945)implied implied that that single single large largequartz quartz veins veinsranged ranged up up to to11 11m m thick, thick, 60 60m m in in horizontal horizontal extent, extent,and and76 76m m vertical vertical extent, extent,but but examination examinationof of the the old old mine mine workings workings concurrent concurrent with with development developmentof ofthe the modern modern Ropes Ropes mine, mine, and anddetailed detailedhistoric historic descriptions descriptions indicate indicate the the old old stopes stopeswere were ore oreshoots shoots made made up upof of aaseries seriesof ofsmaller smallerindividual individualquartz quartz veins veins and and intervening interveningauriferous auriferousfoliated foliated quartz-sericite-chlorite quartz-sericite-chlorite rock. rock. The Theveins veinsare aresaccharoidal saccharoidalwhite white to tolight lightgray grayquartz, quartz,with with pyrite pyrite and andminor minorargentiferous argentiferoustetrahedrite, tetrahedrite, galena, galena, and and chalcopyrite; chalcopyrite;and and rare rare free freegold, gold, molybdenite, molybdenite, dyscrasite, dyscrasite, tourmaline, tourmaline, and and native silver. Overall, Overall,metal metal concentrations concentrations in inthe the veins veins range rangefrom from20 20to to75 75g/ton gltonAu Au with with Au/Ag AulAgratio ratio from from 0.08 0.08 to to 1.76. 1.76. Thequartz quartzveins veinshave havegreater greaterconcentrations concentrationsof ofgold goldthan thanmost mostof ofthe thegold goldwith withdispersed dispersed The quartz-sericite-chloriterock, rock,but butthe thezones zonesof ofgreater greaterthan than44g/tonne gltonneAu Au pyriteininquartz-sericite-chlorite pyrite concentrationin inthe thequartz-sericite-chlorite quartz-sericite-chloriterock rock are are not not generally generally zoned zoned about about the theveins, veins, concentration rather,they they occur occurmore morecentrally centrallywithin within the the quartz-sericite-chlorite quartz-sericite-chloriterock rock north northof ofthe theveins. veins.In In rather, some instances, Au concentrations of greater than 4 glton in the gold with dispersed pyrite in some instances, Au concentrations of greater than 4 g/ton in the gold with dispersed pyrite in quartz-sericite- chlorite chloriterock rockextend extendalong alongthe the same same055° 055' to to 065° 065' trend trend eastward eastwardfrom fromthe the quartz-sericitewest end endof ofold oldstopes stopeswhich which mark markconcentrations concentrations of of quartz quartz veins, veins,particularly particularlyfor forold oldstopes stopes west 41 �westward plunging plunging auriferous auriferous quartz-tetrahedrite quartz-tetrahedrite veins Vertically dipping, steeply westward occur progressively further east in successively deeper levels of the mine. These banded, auriferous, tetrahedrite-bearing tetrahedrite-bearing quartz veins with aligned schistose inclusions of sericitequartz-chlorite wall rock are at or near the south side of the main main ore zone and have an orientation the same as the zones of schistosity parallel gold concentration with dispersed pyrite, which are more centrally within within the the trend trend of of quartz-sericite-chlorite quartz-sericite-chloriterock. rock. The veins are oriented nearly parallel to the schistosity, and therefore therefore nearly nearly perpendicular to the inferred maximum principal principal stress. They They contain schistose septa identical in composition to the quartz-sericite-chlorlte wall rock and have quartz with undulose undulose extinction and subgrain development textures indicating grain size reduction. reduction. These characteristics are compatible with an origin as oblique shear veins formed in the direction of of shear movement movement (Hodgson, 1989). Oblique shears Oblique shear shear veins veins are are inclined inclined to to shear shear zone zone margins margins in the same same way as as PP shears and show the same sense of en echelon stepping as the sense of of movement movement across the zone containing them (Hodgson, 1989). In In the case of the Ropes deposit, this indicates a dextral component in plan view. An An origin origin for for the the quartz-tetrahedrite quartz-tetrahedrite veins, veins, and and the the en en echelon echelon zones of Au concentration with dispersed pyrite, pyrite, as tension tension shears, extension extension veins, or Riedel shears in a sinistral shear zone is not indicated indicated because the the low low angle orientation of the tips of the veins with respect to the shear zone margin, margin, and the similar orientation of of the ends of the en echelon zones of Au concentration with dispersed dispersed pyrite, pyrite, is not not consistent with the sense of rotation expected in sinistral sinistral shear. shear. The sigmoidal zones of gold concentration with dispersed dispersed pyrite pyrite and the oblique shear veins have orientations nearly parallel parallel to the S fabric, fabric, consistent with with an origin as P shears. PP shears commonly form at approximately 10° 10' to the shear shear zone zone boundaries, boundaries, after after peak peak deformation, when resistance to shear is decreased decreased by by the the presence presence of of well well developed planar fabric (Tchalenko, (Tchalenko, 1970). 1970). Inferred Direction of Shear Movement direction in aa shear zone is commonly considered considered to to be he in in the the line line perpendicular to the intersection of of the shear zone margin with the plane plane of of the the oblique obliqueSS fabric, and the sense of movement is inferred from the the sense of of obliquity obliquity of of the the fabric fabric to the shear zone margin. In In aaductile ductile environment environmentthe the acute acute angle angle between between the the SS fabric fabric and and the the shear plane faces away from the direction of movement. The The sigmoidal sigmoidal zones zones of of greater greater gold gold concentration with dispersed pyrite follow an SS fabric, fabric, interpreted as as aa largely largely compressive, compressive, ductile feature. Contacts Contactswith with nonfoliated nonfoliated serpentinitic serpentinitic peridotite peridotite north north and and south south of of the the Ropes deposit are considered considered shear planes parallel to the C C direction. direction. The The orientation orientation of of the the SS fabric relative to the C direction indicates a dextral dextral component component of of movement movement when when viewed in the horizontal plane, with the south south side side of of the zone moving west southwest southwest and and the the north north side moving east northeast. (Figure 8). Offsets Offsets along along northeast northeast trending trending faults faults in in the the deposit, deposit, and the Z sense of warping of layering between between the the rock rock types types at at the the south south side of of the the deposit (Figure 7) 7) also support support a dextral component of movement. Localization of the Deposit Within Within a Dilational Jog quartz-sericite-chlorite rock strikes strikes 070° 070' overall overall but but 080° 080" where where itithosts hosts The trend of quartz-sericite-chlorite the main ore zone (Figure 8). This This080° 080Âpart part of of the the trend trend acted acted as as aa dilational dilational jog or or releasing releasing of movement movement in in plan view view (Figure (Figure 8). 8). bend during the indicated dextral component of Releasing bends are are commonly commonly sites sites for for ore ore deposits, deposits, even even if if the the main main fault fault trend trend isis barren barren 42 �0 1 km r N 2 3 1 1 Ropes deposit quartz vein prospect - 18 svucture anspg, s'rn obliqu, m, v.i,,., nd.s nht Loan 01 gvSr Au conc.,Wi,flon slTh .p.nnpjfl, Numbered quadrants mathed by dashed lines are ranges of possible orientations of horizontal components of maximum principal stresses which could account for dextral components of strain at: I. Bjork -- Lundeen prospect 1. Lundeen prospect 2. 2. I-I -18 18Structure Structure 3. Ropes Ropesdeposit deposit 4. range 4 Possible Poss~ole rangeof 01 horizontal horizontalcomponent component 01 maximum maxfmumprincipal pr,nc~paI stress which wnlcn could co~ld of stress s ~ m ~ l t a n e o usatisfy sat~sfv s l ~ requirements requmvnentsfor lor simultaneously sites 2, and sqles 1, 1.2. and33 above above Figure 8: 8: Structural Structuralmodel model for for the the Ropes Ropes mine, mine. 43 �(Sibson, 1987). Both Both the the sigmoidal sigmoidal zones zones of of Au Au concentration concentration with with dispersed dispersed pyrite, pyrite, and and the the auriferous quartz veins are interpreted as indicating local extension within aa dilational dilational jog. jog. There is a component of positive volume change in the shear zone, in an otherwise compressive tectonic regime. Increased Increased pore pore pressure pressure within within the the shear shear zone zone as as microfractures filled with fluid, combined with the extensional effect effect of of the dilational jog, jog, local extension. extension. Slip jogs involves extensional may have contributed to local Slip transfer in dilational jogs fracture openings localized in the jog, jog, or step over. Formation Formation of of dilational dilational jogs involves involves local loss of cohesion, or brittle failure, in an an otherwise ductile ductile environment. environment. The trend of quartz-sericite-chlorite rock which hosts hosts the Ropes main ore zone narrows to the east of the deposit, as well as toward toward surface up up dip from the deposit, where it 70° south dip at shallow shallow levels. levels. This bends from a near vertical dip at depth to a 70" This change change in in thickenss and attitude may have caused barrier barrier conditions conditions for ascending hydrothermal fluids, thickens addition to to the the dilational jog jog controlling resulting in another control on localization of ore in addition of the overall setting setting of of the the deposit. deposit. References References Anderson,E.M., Anderson, E.M., 1951, 1951,The The Dynamics Dynamics of of Faulting: Faulting: London, England. Oliver and Boyd, Boyd, 191 191 p. p, Geochemistry of Gold and Its Deposits: Deposits: Geological Geological Survey of Boyle, R.W., 1979, 1979, The Geochemistry Canada Bulletin 280, 584 p. 280,584 p. Broderick, T.M., 1945, 1945, Geology Geology of the Ropes Ropes Gold Mine, Mine, Marquette Marquette County, County, Michigan: Michigan: Economic Geology, Geology, v.40, v.40, p.115-128. p.115-128. of Economic Brozdowski, R.A., 1989, 1989, Geology of The Ropes Mine, Society of Economic Geologists Geologists Fall Fall Field Conference Guidebook, Guidebook, Precambrian Geology and and Metal Metal Occurrences, Occurrences, Michigan's Michigan's Upper Peninsula Field Conference, Conference, p. 38-75. Groshong, R.H., Jr., 1988, 1988, Low Temperature Temperature Deformation Mechanisms Mechanisms and and Their Their v. 100, p. 1329-1360. 1329-1360. Interpretation: Geological Society of America Bulletin, v.100, Hodgson, C.J., 1989, 1989, Patterns Patterns of of Mineralization, Chapter Chapter 3, 3, in: in: Mineralization and Shear Zones, Geological Association of Notes v.6, Montreal, Quebec, May 12-14, 12-14, p.51-88. Lisle, R.J., 1989, Dike Sets: Geol. 1989, Paleostress Analysis from Sheared Dike Geol. Soc. Soc. Am. Am. Bulletin, Bulletin, v. v. 101, p. 968-972. Sibson, R.H.,1987, Earthquake Rupturing as a Mineralizing Agent in Hydrothermal Systems: Systems: Geology, v.15, v. 15, p 701-704. 701-704. Tchalenko, J.S., 1970, 1970, Similarities Similarities Between Shear Shear Zones of Different Magnitudes: Magnitudes: 625-1640. Geological Society Society of of America America Bulletin, Bulletin,v.81, v.81,p.1 p.1625-1640. 44 �Geologic Field Field Excursion Excursion to to the the Ispheming Greenstone Greenstone Belt Rodney C. Johnson, Johnson, Rod Rod Johnson Johnson & & Associates, Associates, Inc., Inc., Negaunee, MI MI 49866 49866 Theodore J. Bornhorst, Department Department of Geological Geological Engineering Engineering and and Sciences, Sciences, Michigan Michigan Technological Technological University, University, Houghton, Houghton,MI MI49931 4993 1 Introduction Introduction This geologic field excursion to the Ishpeming Ishpeming greenstone belt consists consists of of 88 selected selected stops stops (Figure 1). These These stops stops are are designed designed to to cover cover the major varieties of volcanic lithologies found in the belt and several localities of structural structural significance. significance. Johnson and and Bornhorst Bomhorst (this volume) provide an overview of the greenstone belt and explanation of stratigraphic stratigraphic terminology used in this guide. AA detailed detailed topographic topographic map map is is provided provided to to identify identify the the location of the stops, however users should refer to larger scale maps in order to navigate stops, order to navigate roads to the stops. stops. Trip Stops Field Trip Stops Stop Stop A -- Lighthouse Point Purpose: View: View:1)1)amphibolite amphibolite schist schist of of Lighthouse Lighthouse Point Point Basalt, Basalt, 2) 2) Dead Dead River Shear Shear Zone, Zone, and 3) cross-cutting relationships of Archean and Proterozoic rocks (Figure 2). Note: You Guard to to visit visit this this locality. locality. No rock You must must obtain obtain permission permission from from the U.S. Coast Guard rock hammers or specimen collecting are allowed at this this stop. Park Park in in the the parking lot across across from from the Coast Coast Guard Guard office. office. At this stop excellent shoreline exposures on Lighthouse Point and along the adjacent beach provide a view of Archean and Proterozoic Proterozoic rocks rocks (Figure (Figure 3). 3). Gair Gair and and Thaden (1968) (1968) mapped these rocks in detail. detail. Part I. The Theoutcrop outcrop in in the the middle middle of of the the beach beach near the parking lot is composed of Archean thinly layered, amphibolite schist (basalt). The basalt here is part of the Lighthouse Point generally strike E-W E-W and and dip dip 70°N 70°N. The layers in Basalt. The The layers layers on on Lighthouse Lighthouse Point generally highly flattened flattened pillows. pillows. The flattening is interpreted to be a the basalt are interpreted as highly result of north verging recumbent folding (Johnson and Bomhorst, 1991). 1991). Part 11. II. The Theoutcrop outcropon on the the southwest southwest end end of of the beach is composed of chlorite-sericite schist, again part of the Lighthouse Point Basalt (see Johnson and Bornhorst, Bomhorst, this volume). The chlorite-sericite schist is interpreted to be deformed pillow basalt and are Kinematic indicators indicators (S-C fabric in particular) metamorphosed to greenschist facies. metamorphosed facies. Kinematic indicate a dominantly left-lateral shear sense, but on close inspection right-lateral and northnorthindicators are also present. present. This locality is interpreted to be the eastern extension of side-up indicators block of of the Ispheming greenstone the Dead River Shear Zone that separates the northern block belt from the southern 1). southern two two blocks blocks (Figure (Figure 1). 45 �____ T. 50 N T. 49 N T. 48 N. 46°30' R.28W. R.27W. R.26W. R.25W. Explanation Explanation Proterozoic Proterozoic Sedimentary rocks 0Sedimentary rocks I Mona Mona Formation Formation Upper tuff unit unit and lapilli-tuff lapilli-tuff unit unit Basalt Basalt flow unit unit Kitchi Formation Formation Kitchi Tuff unit and lahar unit Basalt unit Basalt flow unit l\ /1 I Archean Intrusive Intrusive Rocks Rocks k''-4 Granodiorite Granodiorite 177771 Tonalite Tonalite 1 Deer Lake Peridotite Deer Lake Peridotite Sedimentary Rocks Sedimentary Rocks K"Xl Timiskaming-type Timiskaming-typesedimentary sedimentaryrocks rocks Volcanic Rocks Rocks Lighthouse Lighthouse Point Point Basalt Basalt Basalt Basalt V/A Reany Reany Lake pyroclastic pyroclastic unit unit and and Fire Center mine mine iron-formation iron-formation unit unit HP_Li Nash NashCreek Creek glomerophyric glomerophyric basalt basalt unit unit 3 L\1 m Bedding Bedding 87 f Overturned bedding bedding 87+ Facing of pillows A Facing pillows Plunging anticline Plunging anticline Plunging syncline syncline 'C Plunging overturned anticline Plunging overturned Shear zone 1 @ Location Location of stops stops & - A % Figure 1: 1: Generalized geologic geologic map of the Ishpeming greenstone belt showing location location of stops. stops. 46 �R. 25W. T. 48 N. Figure 2: 2: Stop StopAA- Lighthouse Lighthouse Point Point Figure 47 �100 0 200m 0 50rn I Explanation Middle Middle Proterozoic Proterozoic /,%j Keweenawan Keweenawandiabase diabasedike dike Early Early Proterozoic Proterozoic 'dt*S Metadiabase Metadiabasedike dike Archean Archean I Intrusive IntrusiveRocks Rocks Rhyolite Rhyolite 0 I Lighthouse Lighthouse Point PointBasalt Basalt Basalt, Basalt,outcrops outcropsinindarker darkershade shade Figure Figure3: 3:Geologic Geologicmap mapof of Lighthouse LighthousePoint Point (modified (modifiedfrom fromGair, Gair,1959). 1959). 48 I �Within the basalt dominated outcrop there are lenticular pods of quartz-chert (hematite (hematite The interpretation of the the quartz-chert stained) and small texturally different different quartz quartz veins. veins. The with deposition deposition of of the the basalts. basalts. Small Small quartz quartz pods is not clear, they could be synchronous with veins are found in rocks throughout throughout the the Ishpeming Ishpeming greenstone greenstone belt. belt. Most of of these were were Archean deformation deformation and and metamorphism. metamorphism. However, at at least some some likely emplaced during Archean 1998). were emplaced during the Proterozoic (Bornhorst (Bomhorst and others, 1998). Part III. 111. The main outcrops of Lighthouse Point are northeast of the first two localities. localities. This trip will only only visit visit the the shorline shorline ouctrops ouctrops on on the the northeast end of of the the beach, beach, near near aa house. house. Here, Archean thinly layered, amphibolite schist (basalt) is cut by a tabular, porphyritic rhyolite dike trending roughly parallel to the layering. The layered basalt is part of the the Lighthouse Point Basalt and the rhyolite is interpreted as synchronous with the synorogenic late trondjhemite-granite suite plutons that reorient earlier fabrics. are The Archean rocks are cut by two N-S trending Proterozoic diabase dikes that are metamorphosed to greenschist facies as compared to the surrounding Archean rocks that are are metamorphosed to amphibolite facies. The The crest crest of the ridge (not visited today) is underlain by a relatively unmetamorphosed E-W E-W trending diabase dike dike of Keweenawan Keweenawan age. age. White Bear Drive Stop B -- White Drive Purpose: View Viewthe theupper uppertuff tuffunit unitof ofthe theMona Mona Formation Formation and andS3 S3crenulation cleavage cleavage (Figure (Figure 4). 4). The rocks at this stop are part part of of the the upper upper tuff tuff unit unit of of the the Mona MonaFormation. Formation. The rocks of the quartz-feldspar-sericite-chlorite schist. schist. Thin primary layering subupper tuff unit consist of quartz-feldspar-sericite-chlorite parallel to S S,foliation foliationcan canbe beobserved observed in in some some of the outcrop in this area. Based Based on on the the presence of sand-sized, sand-sized, lenticular lenticular volcanic volcanic fragments, and lack of sand-sized quartz quartz and and feldspar in an intact framework, most of these schist beds are interpreted as tuff. On the north side of the outcrop nearest White Bear Drive we can see the hinge of a fold. This allows us to see S3 crenulation crenulationcleavage. cleavage. On the limbs of westward plunging F3 fold. S and Bornhorst, this volume, andS3 S3are are parallel parallel and and indistinguishable (see Johnson and Bomhorst, the fold S, Table 1). 1). Stop C - Tower Purpose: View Viewthe thelapilli-tuff lapilli-tuff unit unitof ofthe theMona MonaFormation Formation and andS3 S3crenulation cleavage cleavage (Figure 5). 5). Note: Park Parkatatthe thecommunication communicationtower towerand andwalk walkabout about lOOm loom downhill in a southerly direction to aa series series of of outcrops outcrops alongside alongside the the hill. hill. The rocks at this stop are part of the lapilli-tuff lapilli-tuff unit unit of of the the Mona MonaFormation. Formation. The rock is composed of calc-alkalic rhyolite lapilli in a quartz-sericite-chlorite matrix with quartz and and 49 �R. 26W. T. 48 N. Figure 4: Stop Stop B-White B-White Bear Drive Drive R. 26W. T. 48 N. Figure Figure 5: 5: Stop Stop C-Tower C-Tower 50 �R. 26W. 1.48 N. Figure Figure 6: 6:Stop Stop D-Shunk D-Shunk Furniture Furniture R. 26W. T. 48 N. StopE-Tuff E-Tuff Unit Unitof ofthe theKitchi KitchiFormation Formation Figure7:7:Stop Figure 51 �less commonly, feldspar phenocrysts. Flattened Flattened apple apple green green lapilli lapilli (muscovite (muscoviteand and chlorite; chlorite; Puffet, 1974) are also common at this stop. This This unit unit isis interpreted interpreted as as subaqueous subaqueous pyroclastic flow and fall deposits. At this stop an earlier shallowly easterly dipping foliation (S1) (Si) is crenulated by a later E-W foliation and and elongated elongated in the (S3). The The lapilli lapilli are are flattened along the S1 S, foliation trending foliation (53). direction of the intersection of Si and S3. The Dead River Shear zone (D2, D4) is interpreted 83. The Dead River Shear zone (Dz,D4) to be located on the north side of the tower hill. Stop D - Shunk Furniture Purpose: View Viewpillowed pillowedbasalt basalt of ofthe theMona MonaFormation Formation (Figure (Figure 6) 6) Note: Park Parkalongside alongsideof ofthe theroad roadacross acrossfrom fromShunk ShunkFurniture, Furniture, but but be be very very careful careful of oftraffic. traffic. There are excellent views of relatively undeformed pillowed pillowed tholeiitic basalt in outcrops pillowed basalts are part part of of the the basalt basalt flow flow unit unit of of the the Mona Mona alongside of U.S. The pillowed 30 cm cm X X 1.5 1.5 m; m; Formation. The Thepillows pillows are are ellipsoidal ellipsoidal in in shape shape with with dimensions dimensions about about 30 E-W and and dips dips 85' 85° N. N. Cusps on pillows indicate attitude of bedding strikes approximately E-W that stratigraphic top is to the north. There There are are characteristic characteristic well-developed well-developed pillow pillow rinds, rinds, radial fractures, and interpillow void spaces filled with quartz and carbonate. carbonate. Overall, Overall, pillowed and massive tholeiitic basalt is the dominant lithology in the Ishpeming greenstone belt. Stop EE— Kitchi Formation Stop - Tuff Unit of the Kitchi Purpose: View View tuff tuff unit unit of of the the Kitchi Kitchi Formation Formation and and evidence evidence for for superposed superposed folding folding (Figure 7). 7). Kitchi At this stop we will see some of the lithologic variation within the tuff unit of of the the Kitchi folding. Cross-bedding, Formation and evidence for large scale folding. Cross-bedding, graded graded bedding, and and cut cut and and Kitchi Formation, Formation, but but in in this this area area clearly clearly indicate indicate tops tops to to fill structures are uncommon in the Kitchi the east. AA foliation foliation atatright right angles angles to to bedding bedding indicates indicates this this area area is is near aa vertically vertically plunging fold nose. Stop F - Teal Teal Lake Purpose: View View the theCarp CarpRiver River Falls Falls shear shear zone, zone, tuff tuff unit unit of of the Kitchi Kitchi Formation and and Ajibik Ajibik 8). Quartzite of the Menominee Group of the Marquette Range Supergroup (Figure (Figure 8). At this stop we will cross the Carp Carp River Falls shear zone from Lower Proterozoic metasedimentary rocks into Archean We will will begin the the traverse in in the the Archean metavolcanic metavolcanic rocks. rocks. We steeply dipping Ajibik Quartzite of the Lower Proterozoic Marquette Marquette relatively undeformed steeply TheCarp CarpRiver River Falls Falls shear shear zone zone is is aa high high strain strain zone zone indicated indicated by by Range Supergroup. The The foliation foliation in in the the schist schist is is near near vertical vertical and and has has aa quartz-sericite-feldspar-chlorite schist. The steep mineral lineation. As As the the shear shearzone zone isis traversed traversed there there is is aa noticeable noticeable increase increase in in the the 52 �R. 27W. T. 48 N. Figure 8: 8: Stop Stop F-Teal F-Teal Lake Lake R. 27W. T. 48 N. Figure Figure 9: 9: Stop Stop G-Alorig G-Along Deer Deer Lake Lake 53 �amount of chlorite in the schist. On Onthe thenorth north side side of of the the Carp Carp River River Falls Falls shear shear zone zone isis quartz-feldspar-sericite-chlorite schist schist of of the the tuff tuff unit unit of of the the Kitchi Kitchi Formation. Formation. Stop G --Along Along Deer Lake Lake Purpose: View View lahar laharunit unitof ofthe theKitchi Kitchi Formation Formation (Figure (Figure 9). 9). At this stop polymictic volcanic conglomerate is exposed in in a glacially glacially polished polished outcrop along the north side the road road towards towards Deer Deer Lake. Lake. Park alongside of the road. road. This This volcanic volcanic part of the the lahar lahar unit unit of of the the Kitchi KitchiFormation. Formation. On the polished polished surface, the conglomerate is part volcanic clasts vary from 25 cm X 35 cm in cross section, section, down down to to the the size of the matrix grains. Larger 1.2mminincross crosssection, section, are are present present in in the the nearby nearby lake lake Larger clasts, clasts, up up to to 60cm 60 cmXX1.2 shore outcrops. Clasts Clastsgreater greater than than 11cm cm make make up up about about 20 20 % % of of the the rock rock and and there there isis an an obvious stratification in clast clast size. Shape very angular to to rounded. rounded. The Shape of clasts varies from very The volcanic clasts are caic-alkalic calc-alkalic andesite andesite to dacite dacite in composition with with most most clasts clasts at at this this locality being near the andesite to dacite division division with with small small chemical chemical variation variation (Bomhorst (Bornhorst unpublished holocrystalline. The unpublished data). Most Most clasts clasts are are porphyritic and some are holocrystalline. The volcanic volcanic conglomerate is interpreted as a subaqueous lahar deposit. deposit. Stop H - Shoreline of Deer Lake Purpose: View ViewDeer DeerLake LakePeridotite Peridotite (Figure (Figure10). 10). road. Park Note: There There isis aa small small pullover pullover on on the east side of the road. Park and and walk walk along along the the road road to to the north where you can find a small small trail that leads through the woods to to the the tip tip of of the the point. point. Outcrops occur along the shoreline of this small point into into Deer Lake. At this this stop stop serpentized peridotite of the Deer Lake Peridotite is exposed along the shore shore of of Deer Deer Lake. Lake. Elongate pods of massive serpentized peridotite with closely spaced spaced fractures fractures are are surrounded surrounded by 2 to 5 cm wide ribbons of 11 to 22 mm subparallel veins of cross cross cross-fiber serpentine serpentine (Bornhorst fine(Bomhorst and others, 1986). 1986). The Thedark dark grey grey to to green green massive massive serpentized serpentized peridotite peridotite is is fineto medium-grained and lacks obvious foliation. lit In hand sample one can find well-developed well-developed 1 to to 55 mm mm pseudomorphs pseudomorphs after olivine and and pyroxene pyroxene outlined outlined by by thin thin selvages of white carbonate or talc. No No spinifex spinifex or or pyroxene "string beef' beef' textures textures indicative indicative of of komatiite komatiite flows have been found. The Thechemical chemical composition composition indicates indicates an an olivine olivine to to pyroxene ratio ratio from about 0.25 to 2 and most samples samples are are in the harzburgite field and and some some are are in in the the Iherzolite lherzolite field. The The Deer Deer Lake Lake Peridotite Peridotite is is interpreted interpreted as as a sill complex. This locality is only a few 100 Ropes gold gold mine. mine. The 100 meters from the Ropes The shear shear zone zone that that hosts hosts the Ropes gold mine projects immediately immediately north north of of this this small small point. point. The The serpentized serpentized peridotite has a strong, veinlets and and streaks streaks strong, contorted contorted foliation with magnetite as irregular irregular veinlets along the foliation foliation planes indicating the shear zone that hosts gold at at the Ropes ~ o ~mine mine e s continues into the Deer Lake Peridotite. Adjacent Adjacent to to the the gold gold ore ore body body the the serpentized serpentized peridotite is altered altered to a talc-carbonate rock (Bornhorst (Bornhorst and and others, this this volume). volume). 54 �R. 26W. T. 48. N. Figure 10: 10:Stop Stop HH- Shoreline Shoreline of of Deer Deer Lake Lake Figure R. 26W. T. 48 N. Figure 11: 11:Stop StopI-I-County CountyRoad Road510 510 Figure 55 �StopiStop I -County County Road Road 510 Purpose: View Purpose: View fire fire Center Center Mine Mine iron-formation iron-formation of the Lighthouse Point Basalt (Figure (Figure 11). 11). Note: Park Parkalongside alongsideof ofthe theroad, road, but but be be careful careful to to watch watch for for logging logging trucks trucks moving moving at at high high speeds. There are several outcrops outcrops on both sides of County Road 510 510 at at this stop. stop. The The particular particular outcrop of interest is near the top of the slope slope on the east side of the road, itit may may take take aa little little effort to find the iron-formation. iron-formation. At At this this stop stop the the Fire Fire Center Center Mine Mine iron-formation iron-formation unit unit of of the the Lighthouse Point Basalt is overlain and underlain by massive tholeiitic basalt of of the the Lighthouse Point Basalt. The Theiron-formation iron-formation isis approximately approximately 1.5 1.5 m m thick thick and and strikes strikes N30°W. N30¡WItIt isis composed composed of of fine, fine, black black chert chert that contains scattered magnetite grains and pyrite. To Tothe thewest west of of this thislocality, locality, the the iron-formation iron-formation is is layered layered with 1.5 1.5 cm cm thick chert chert and and magnetite bands. Despite Despitevariable variable magnetite magnetite content, content, this this iron-formation iron-formation produces produces aa strong strong magnetic signature signature and can be traced from north to south in this stratigraphic stratigraphic block block of of the the greenstone belt. This marker. The This iron-formation iron-formation is an excellent stratigraphic marker. The basalt is dark dark amphibolite green to black, fine-grained and relatively non-foliated and metamorphosed to amphibolite grade. grade. Acknowledgments We thank Shannon E. Bair for computer drafting of the figures. References Bomhorst T.J., T.J., Shepeck, Shepeck, A.W., A.W., and and Rossell, Rossell, D.M., D.M., 1986, 1986, The The Ropes Ropes gold gold mine, mine, Marquette Marquette County, Michigan, gold deposit: deposit: MacDonald, - an Archean hosted lode gold MacDonald, E.J. E.J. Michigan, U.S.A -(ed.), Proceedings of Gold '86, an International Symposium on the Geology of Gold, Toronto, p. 13-227. Toronto, p. 2213-227. Bornhorst, Bomhorst, T.J., Thorpe, R.I., R.I., and Johnson, R.C., 1998, 1998, Lead isotope study of veins in in the the Archean Ishpeming greenstone belt, Michigan: Econ. Geol., v. 93, pp. 102-107. 102-107. Gair, J.E. and Thaden, R.E., 1968, 1968, Geology of the Marquette and Sands Sands Quadrangles, Quadrangles, Marquette County, Michigan: U.S. Geol. Survey Professional Paper397, 77 p. Johnson, R.C. and Bornhorst, T.J., 1991, Archean Archean geology geology of of the the northern Johnson, R.C. Bomhorst, T.J., northern block block of the the Ishpeming belt, Marquette Ishpeming greenstone greenstone belt, Marquette County, County, Michigan: Michigan: U.S. U S . Geological Geological Survey Survey Bulletin 1904-F, 1904-F, 20 20 p. p. Puffett, W. P., 1974, 1974, Geology of the Negaunee Quadrangle, Marquette County, Michigan: U.S. 53 p. U S . Geol. Survey Survey Professional Professional Paper Paper 788, 788,53 p. 56 �Shear Shear Zones Zones and and Gold Gold Mineralization Mineralization in the the South South Half Half of of the the Ishpeming Ishpeming Greenstone GreenstoneBelt, Belt, Michigan Michigan by D.J. Duskin Duskin Consulting Consulting Geologist Geologist 210 21 0 Union Street Street Camden, SC 29020 29020 and and T.O. T.O. Quigley President Minerals Minerals Processing Processing Corporation Corporation 4547 4547 County County Road Road 601 60 1 Champion, MI 49814 49814 57 �Foreword Foreword Much of this field trip is devoted to gold occurrences. occunences. At Atmost moststops, stops,the theguidebook guidebookdescriptions descriptions will include the the results results of of sampling sampling and and assaying assayingfor forgold. gold. The term "grab sample" means a small reconnaissance-style sample deliberately chosen chosen by by an an exploration exploration geologist to have, in his experience, the maximum chance of yielding gold in the assay because of the presence of alteration, sulfides, sulfides, gossan, gossan, etc.. etc.. If our reported result contains contains gold, gold, this this doesn't doesn't necessarily mean mean that that someone someone else's else's subsequent sample the same value. value. This phase of sampling is deliberately subjective, and and gold gold distribution distribution is is far far will give the from uniform in Archean deposits, deposits, varying varying greatly greatlywithin withinsmall smalldomains, domains,even evenatatproducing producingmines. mines. More representative sampling will will be be discussed at some of the field trip stops as either "channel" or "chip" samples across an exposure. exposure. Channel front diamond blade rock saw cuts or hand-cut Channel samples, samples, taken either from by hammer and moil, are meant to be volumetrically equivalent to a ¼ '/2 split of BQ (1.5-inch (1 S-inch diameter) drill core. Chip Chipsamples samples are are more more representative representative of the exposure than a grab sample, but not as rigorously taken as a channel sample. sample. Unless otherwise specifically noted, all gold values reported herein are from samples samples crushed, ground, split and assayed at the former Callahan Mining Corporation assay lab in the Humboldt (Ropes) Gold Mill, now owned and operated by Minerals Processing Processing Corporation Corporation (MPC). (MPC). MPC's MPC's assays assays were were all all done by the fire fife assay-gravimetric finish finish method, method, using usingaa 1-assay I-assay ton ton (29.167 (29.167 gram) gram) subsplit. subsplit. MPC's MPC's lower detection limit by by this this technique technique isis ,003 .003 odton oz/ton (103 (103 ppb) ppb) Au. Au. The terms "no gold", gold, or or "below "below detection" refer to this limit. In Insome somecases caseswhere wherewe we felt feltititimportant importantto toknow knowthe thepresence presenceof ofeven evenlower lower Au values, separate subsplits were sent to Chemex Labs in Vancouver, B.C. for analysis by a lower detection limit technique, noted herein where so done. This This technique technique is is the the fire fire assay assay -- atomic absorption finish fmish process (FA-AA), (FA-AA), and has has aa lower lower detection detection limit limit of of 10 10ppb. ppb. Chemex Labs also performed whole rock rock major major oxide analyses. analyses. Results Results from samples samples taken at (Grunsky, 1983; Jensen, 1976) some of the field trip stops are shown on Jensen cation plots (Gnmsky, 1976) that have been routinely used for characterizing the lithochemisby lithochemistry of of the the rocks rocks from from this this region (Bomhorst (Bornhorst and Johnson, 1993). Figure lisa 1993). 1 is atemplate templateJensen Jensenplot. plot.The TheJensen Jensenplots plotswere weredone doneby byconsulting consulting geologist geologist R.A. Campbell, Burlington, Ontario, using "Newpet" "Newpet" software developed and distributed by the Earth Sciences Sciences Department of Memorial Memorial University Universityof ofNewfoundland Newfoundland.. Mr. Mr. Campbell's Campbell's assistance assistance is is gratefully acknowledged. acknowledged. 58 �Chemex Chemexalso alsodid didaa32-trace 32-traceelement elementanalysis analysisusing usingthe theX-ray X-rayfluorescence fluorescencetechnique techniquefor forall all samples samplessubmitted submittedfor forassay; assay;results resultsare arediscussed discussedwhere wheresignificant. significant. All thinsections sectionsexamined examinedwith withthe theassistance assistanceofofR.L. R.L. Allpetrographic petrographicdescriptions descriptionsare arefrom frompolished polishedthin Bamett BamettofofBameti BamettGeological GeologicalConsulting, Consulting,London, London,Ontario. Ontario. Mineral Mineralcompositions compositionswere weredetermined determined using JEOLModel Model733 733five fivespectrometer spectrometerscanning scanningelectron electronmicroprobe. microprobe.Mineral Mineral usingBaniett BamettGeological's Geological'sJEOL percentages percentagesare arevisual visualestimates; estimates;no no point point counts counts were done. Mr. Mr.Bamett's Barnett'svaluable valuablehelp helpisisalso alsogratefldly gratefully acknowledged. acknowledged. FeO* + 1102 Cation % Jensen (1976) PJ203 A1203 Mgo MgO Figure 1: 1 :Fields Plot Figure Fieldsfor forthe theJensen JensenCation Cation Plot 595 9 I �CONTOUR INTERVAL 20 FEET 0.. ) 20 Zr.C .5 r CQ 0 t%Z 0 0 �Stop SC-I: SG1: Variably basalt of ol'Mona Mona Formation Formation near near M Marquette Harbor Variably sheared sheared pillow basalt a q u e t t e Harbor (Location 2). (Location map, Figure Figure 2). Gair and (1968) and Thaden Thaden (1 968) last mapped this locality and assigned it to the lower member of the Mona Schist. Recently (1993)have have revised revised the the stratigraphic stratigraphic nomenclature nomenclature of of the the Recently Bornhorst Bomhorst and and Johnson Johnson (1993) lshpeming Ishpeming Greenstone Belt; this locality would now be assigned to the informal basalt flow of the Mona lowermost unit unit.. Pillowed Formation, its lowermost Pillowedmetabasalts metahasaltsof ofgreenschist greenschistfacies facies are are the the predominant Ethology lithology in this this unit. unit. The exposures here are best seen in a three-part sequence. After After parking in in the the abandoned abandoned railroad railroad yard, head east and take the trail along the south side of of Whetstone Whetstone Brook Brook to to the the lakeshore lakeshoreto tosee seeglacially glacially polished outcrops of pillow lavas. The Theoutcrops outcropsdisplay displaywell wellpreserved preservedglacial glacial grooves grooves and andstriae striaetrending trending 198°; 19S0; some of the grooves contain contain classic classic examples examples of of crescent-shaped crescent-shaped chaffer chatter marks pointing down-ice. down-ice. The two outcrops here show show a layer of massive massive non-pillowed non-pillowed basalt basalt conformably conformably sandwiched sandwiched between two pillowed pillowed layers. layers. The layering shown by by this "sandwich" "sandwich" trends trends approximately approximately 305'. 305°. The The pillows are pristine and and undeformed, undeformed, some some of of them them up up to to six six feet feet across. across. They display internal cooling cooling fractures, well preserved rinds, and cusps cusps consistently consistently showing showing tops to to the the north. north. The next point of interest is about about 100 feet feet south south on on the the rocky rockypoint pointof of the thelakeshore. lakeshore. Please he be careful while making your wav way southward over over this this slioperv slippery outcro~. outcrop. On On the the south south side side of of this this rocky rocky point point are pillowed basalt and thin discontinuous lenses of chert interbedded interbedded with the pillows, pillows, some someforming forming pillow selvages. Some brecciated red Someof ofthe thechert chertcontains contains patches patches of ofbrecciated red jasper; this this texture is is probably the the deformation. None of result of soft sediment deformation. of these cherts is magnetic. A A great great deal of of excess calcite is also present with the pillows. This This probably probably contributed contributed to to the the development development of of shearing, shearing, which which can can be be to be increasing rapidly rapidly in in intensity intensity southward southwardin inthis thisoutcrop. outcrop. We think that the calcite and clearly seen to the result result of of degassing degassing and and seawater seawaterprecipitation precipitationininand andnear nearaaflow flowtop. top. In our experience chert lenses are the such interpillow zones containing discontinuous thin cherts are very common. path from the beach beach westward westward back back to to the the railroad railroadyard yard the the progressive progressive deformation deformation of of Taking the path best seen. seen. Entering pillow basalt here is perhaps best Entering the yard, the outcrop exposure on the right (north) again marks the north edge of the developing shearing. Here Here the rock is moderately and variably sheared but the outlines of pillows can still be seen seen despite a penetrative penetrative foliation striking striking 290°, 290°85°N 85'N (photo, (photo,Figure Figure3). 3). The pillow rinds are pinkish weathering calcite, and the pillows here are much smaller than those earlier seen to the north, again a possible contributing contributing factor to the development development of of shearing shearing here. here. 61 �Figure northwest Figure3.3.Looking Looking northwestatatsheared shearedgreenstone greenstoneoutcrop outcropononeast eastside sideofofrail railyard, yard,Stop StopSG-1. SG-1. Hammer Hammerpoint pointtouching touchingcalcareous calcareousflattened flattenedpillow pillowrind. rind. Moving is is itsitswest Movingwest westacross acrossthe therailroad railroadyard, yard,the thethird thirdpart partofofthis thisstop stop westwall. wall.The The features here features here 50yards yardswest westofofhere hereand andabove aboveususininthe theU.S. U.S.Highway Highway4141road roadcut; cut; aremore morecontinuously continuouslyexposed exposedabout about50 are that vexydangerous dangeroushigh highspeed speedIraffic trafficlocation locationand andwe've we'vechosen chosentotoshow showthe thesame sameunits unitsinindetail detailatat thatisisa avery this instead. thisrail railyard yardexposure exposure instead. 62 �'V p/I/sawed 4.ilr 1 n.bbl caicgreo4Jt —-—---— ZontS ii 4n14 a 71 Careen /1/1 Th— r--sheand 2 i4/ pt/al- p//Iota c,_.rinds —'-c re/ic4.. p1/ala 0 20 —'- Peer 7R2-2 - -4-- /1/in Jo.sper £he-rt 71 P/I/a Ec/kt p//loots v Shea rtd X Acsay .capIe ® u/kale- rock mj#trtJ ,tiaiQa d1 te. e-'-- ,-._, _iCoeapsss and Tape. ao Figure Figure4. 4. Compass Compassand andtape tapemap mapofofwest westwall wall of of railroad railroad yard, Stop SG-l. SG-1 63 Ca inple -Thea £ —A... ....LL__ C -at ny �Figure 4 is a map of the rail rail yard yard exposure. exposure. From From north north to to south, south, the the first exposed exposed unit unit is is dark blackish green non-amygdaloidal non-aniygdaloidal pillowed pillowed basalt basalt with with calcareous calcareousepidote-rich epidote-richpillow pillowrinds. rinds. The pillows are quite large, some up to four feet across. Blobs Blobs of ofcalcite calcite and and calcite-cemented calcite-cemented breccia fill some some of the pillow interstices. Within Withinthis thisunit unitthere thereare areshallow shallownorth-dipping north-dipping rubbly rubbly zones zonesthat thatmay may represent represent irregular flow tops. Many Many of ofthe thepillow pillowcusps cuspsindicate indicatefacing facingto to the thenorth, north, but butthis thisisiseasily easilyarguable arguableatatthis this outcrop. The Thepillows pillows themselves themselvesare arenot not megascopically megascopically foliated. foliated. Moving southward, there is an 8-foot covered interval critical to the relationship between the pillowed basalt and the next unit. unit, In In the the Highway Highway 41 exposure exposure to the west, this zone is occupied by a sheared-off lens of gray chert and highly sheared rusty weathering chlorite chlorite rock rock with with abundant abundant oxidized oxidized pyrite along along shear shearplanes. planes. (grab (grab sample sample HIWAY-1, HIWAY-l, .013 fine cubic pyrite ,013 ozlton Au). The The remainder remainder of the unit, exposed past the covered interval here, is highly sheared and light gray-green, with with aa strong Si S foliation foliation striking 269", 269°, 8O0N, 80°N, defined by chloritic slip planes with vertically plunging lineations. Oblique Obliqueto toS1 Sl and dipping more shallowly to the north are discontinuous blobs of white calcite and feathery seams of light brow!] ankerite. ankerite. These appear to to be flattened and and sheared-out sheared-out pillow pillowrinds. rinds. Both textural features are cut brown S1,striking strikingapproximately approximately300' 300°and and dipping dipping 30'-4O0N, 30°-40°N, defined by by a tensional cleavage conjugate to Si, seamlets. There thin fibrous calcite searnlets. There are are also also wide wide spaced spaced flat-dipping flat-dipping relief relief fractures fractures with with fibrous fibrouscalcite. calcite. At first sight this this rock rock might mightbe betaken takenfor foraamafic mafictuff. tuft However, However, itit lacks definitive features of such a protolith, i.e., lapilli, crystals, lithic fragments or bedding. Instead, Instead,itit appears appearsto tohave have been been derived derived from shearing of massive rock, save for the relict pillow rinds, which here indicate that the pillows were quite small and probably more susceptible to shearing than the more massive large pillows to the north. Southward there is another another 10-foot covered covered interval. interval, in In the Highway 41 exposure to the west, a highly sheared rusty sericite-chlorite-ankerite sericite-chlorite-ankeritezone with traces of oxidized cubic cubic pyrite pyrite occupies occupies this. this. (grab sample HIWAY-2, .009 odton ozlton Au). Past Past this this covered covered interval, the exposure is of moderately sheared gray-green rock. rock. SSifoliation vertical; foliationisisapproximately approximately270°, 270° vertical;the theshallow shallowdipping dipping cleavage cleavage isis weakly developed. Blobs Blobsof ofchlorite chlorite in in the the plane plane of SSlshow showvertical verticalalignment, alignment,and and the the rock rock carries carries disseminated coarse cubic pyrite with vertically aligned aligned calcite calcite pressure pressure shadows; shadows; in in thin thin section section both both of of these exhibit porphyroblastic development, rather than a pre-existing textural fabric. This Thischlorite-spotted chlorite-spotted unit has a higher MgO content than the adjacent rocks, plotting in in the the komatiitic field field (see (seeJensen Jensencation cation plot, Figure Figure 5). 5). Further south within this sheared rock rock is a 1.5 -foot bed of non-magnetic jasper chert that almost 285°, dipping 73'N. 73°N. The surely surely defmes defines an an original originallayering layering(So). (So). It strikes 285', Thepatchy patchy bright bright red red jasper zones zones 64 �in this chert are brecciated in in places, as seen seen in in the the outcrop outcrop east east across across the the railroad railroad tracks. tracks. The chert is is not foliated, but carries a trace of fine cubic pyrite on late white quartz fractures. A A grab grab sample sample here here yielded yielded oziton gold gold;;FA-AA 0.003 0 3 odton FA-AA re-analysis re-analysis by by Chemex Chernex gave gave 51 5 1ppb ppb Au. Au. Good examples of relict pillows are a couple of feet north of the hanging wall of the chert. This sheared lithology southern contact contact is is hthoáogy grades grades southward southward into intonon-foliated non-foliatedpillowed pillowedbasalt. basalt. The basalt's southern This a thin discontinuous 0-4" 0-4" nonmagnetic non-magnetic chert-jasper breccia breccia that that gave gave no no gold gold in in aa grab grab sample. sample. This Beyond its its cherty cherty southern southern contact contact the the foliation-resistant pillow basalt unit is about 15 feet thick. Beyond remainder of the exposure consists consists of of highly highly sheared, sheared, light light gray-green gray-green rock with arguable arguable relict relict pillow pillow "vein" S i at 260°, 260°70°N. 70°NWithin this southernmost part of the exposure is a 2-foot calcareous "vein" textures, S1 oblique to S,, S, strike foliation. A grab sample strike 315°, 3 15O, 72°N. 72'N. ItItexhibits exhibitsslickensides slickensides and and drag of the S1 S, foliation. yielded no gold. In In thin thin section sectionthis this rock rock consists consists of of alternating alternating parallel bands of of fine fine grained grained calcite and and plagioclase aggregates in nearly equal amounts, both carrying interstitial high magnesian chlorite that constitutes about 10% 10% of the rock. The Thecalcite calcitebands bands contain contain lozenge-shaped lozenge-shaped ankeritic ankeritic domains. Accessory minerals include apatite, zinc-rich cbromite chromite and abundant chromian titanornagnetite titanomagnetite commonly displaying atoll textures, a feature of of ultramafic rocks. rocks. Other Other indications indications of of an an ultramafic ultramafic parentage parentage are are the the high magnesian chlorite and a trace element analysis that includes 296 ppm Ni, an unusually high result in comparison to the nickel content found in in the dozens dozens of of other other rocks rocks analyzed analyzed in in the the study studyfor forthis thisfield field trip. We altered ultramafic ultramafic dike. dike. Gair and Thaden (1968, Wethus thus consider consider this this "vein" to be be a hydrotherrnally hydrothemally altered pl. 1) showed southernmost end of of what what was was then then the the p1.1) showed an an outcrop outcrop of of "hornblende "hornblende lamprophyre" at the southernmost Highway 41 road cut. Their Their outcrop outcrop cannot cannot be be found found today, today, but hut this this dike dike may may represent represent its its eastward eastward extension. extension. Marked on Figure 4 are five samples taken for for whole rock major oxide oxide and and 32-element 32-element trace trace analysis. Two Two are are from from unfoliated unfoliated pillow pillow basalts, basalts, two are are from from the the sheared sheared lithologies, and the fifth is The whole whole rock rock results results are are shown shown on Figure 5. from the ultramafic dike. The Discussion In this exposure chemically very similar rock rock units unitsexhibit exhibit shearing shearingthat that isispreferentially preferentially developed in weaker units (zones of small pillows, lava interfiow interflow beds, more more magnesian magnesian layers). layers). to shearing. The Intervening more massive buttress units are resistant to The sense sense of of shearing shearingisis nearly nearly vertical. vertical 65 �FeO* + 1102 SAMPLES Cation % Cation % Jensen Jensen(1 (1 976) 976) S RRCVN • PR-i A V RR-2 RR-3 • RR-4 S BK V A1203 MgO Figure 5: Stop SG-1 Figure 5: Stop SG-1 66 �Gait outcrop on on their their map map of of the the Marquette Marquette quadrangle, quadrangle,but but didn't didn't Gair and Thaden (1968) noted this outcrop indicate shearing or or slaty lithology lithology here. here. A couple of of miles miles to to the the southwest southwest they did describe describe east-west shearing at the southern contact of the Mona greenstones and the overlying overlying Proterozoic Proterozoic Enchantment EnchantmentLake Lake conglomerate. Mapping Fails Mapping the the next next quadrangle quadrangle west, Puffett (1974, p.43) recognized the Carp River Falls shear zone near that same contact and correlated it with the shearing in the Marquette quadrangle quadrangle by Gair and Thaden. Both previously noted by Both authors authors pointed pointed out out that that the the sense sense of of shearing shearing isis strongly strongly vertical. Puffett and noted noted (p. (p. 45), 45), "The "The Puffett (1974) (1974) also also recognized recognized and named the Dead River shear zone and projection of the shear zone to the southeast southeast across the Marquette quadrangle quadrangle meets meets aa conspicuous conspicuous rereentrant in the shoreline of of Lake Lake Superior." Superior." Thus Thus neither neither of of these these two two major major later-recognized later-recognized block boundary shear zones is actually actually shown shown on onGair Gait and andThaden's Thaden's 1968 map. map. This This was was left left to to later later authors of maps at various smaller scales, who usually place the Dead River shear shear zone zone as as aa west-northwest west-northwest trend trend meeting the lakeshore at the approximate contact with the Mona formation basalt basalt flow unit of the southern unit of of the the northern northern block block of of the the Ishpeming lshpeming Greenstone Belt. This block and the Lighthouse Point basalt unit This the Dead Dead River River shear shear zone zoneabout about'/< ¾mile miledue duenorth northof ofthis thisfield fieldtrip tripstop. stop. (Figure 2) 2) The would place the The zone seen here is a probable parallel, sympathetic sympathetic structure. Discussing the structural framework of the Ishpeming Greenstone Belt, Bornhorst Bomhorst and and Johnson Johnson (1993, event (Dl) (Di) was was recumbent recumbent folding. folding. This was followed by (1993, p. 4) stated, stated, "The oldest recognized event axes (Dz). (D2). Shear zones are interpreted to to have formed during and upright folding about east-west oriented axes later than 1)2 andmay mayhave havebeen beenreactivated reactivatedduring duringthe theearly earlyProterozoic Proterozoic Penokean Penokean deformation." deformation." They D; and They "Rock units in all blocks are also cut by by minor shear zones. zones. These went on to point out, out, "Rock These high-strain high-strain zones zones are characterized by by rocks that that are more more intensely intensely foliated foliatedthan thanthe thesurrounding surroundingrocks." rocks." We agree with these interpretations and offer the exposure at this stop as a case in point. point. Whether Whether or or not not this this particular particular major or minor one has yet yet to to be be determined. To shear zone is a major To the the west west itit projects projects into into an an urbanized urbanized area area with no outcrop. outcrop. The sheared lithologies at this locality exhibit varying levels of propylitic alteration (pyrite, carbonate, chlorite); significant significant trace amounts of gold can can be found where pyrite andlor carbonate are are well developed. As Asaapractical practicalmatter, matter, further further exploration exploration for for gold gold in in this this shear shear zone zone isis ruled ruled out out by by its its location. 67 �56-2: Stop SG2: Dead with gold gold mineralization mineralization at at Marquette Marquette Mall (Location map, Dead River Shear Zone with Figure 2). Figure 2). After parking at the head of the ski trail in the southeast comer corner of the Marquette Mall parking lot, walk about 500 feet ESE up the trail to an outcrop knob knob of undeformed metabasalt. This This greenstone greenstone has unusually high high number number of of blocky blocky hematitic hematiticfractures. fractures. The northern limit limit of of this this outcrop outcrop is isaa cliff an unusually cliff face face that is controlled by by foliation foliation planes planes striking striking280° 280°,85% 85°S. This cliff cliff face face represents representsthe thesouthern southern margin margin of of was exposed exposed in man-made outcrops in the bank of the Holiday Inn parking lot until it was shearing that was recently covered covered by by landscaping. landscaping. R. Brozdowski (personal communication, 1988) reported anomalous recently gold values from Callahan's Callahan's sampling sampling of of that that sheared sheared rock. rock. Returning about 400 feet westward down the trail, a makeshift wooden bridge crosses crosses the stream leading to the motel's duck pond. Intermittently Intermittentlyexposed exposed along along the the stream stream are are small outcrops outcrops of sheared chiorite-sericite rock rock with with brown brown carbonate carbonate weathering weathering rinds rinds and and oblique oblique quartz quartz veinlets. veinlets. Some chlorite-sericite Some of of this this phenocrysts, indicating aa crystal crystal tuff tuff protolith. protolith. Of rock has small gray quartz phenocrysts, Of three threewidely widely spaced spacedgrab grab took here, here, one one gave gave ,023 .023 odton or/ton Au, the the other other two two were were below below MPC's MPC's detection limits. The samples we took The sample that yielded the gold value was taken aa couple of feet below the little wooden bridge. Mail parking lot and moving west along its south bank, bank, there is a 650 foot Back at the Marquette Mall rock. Bornhorst long exposure of highly sheared rock. Bomhorst and Johnson (1993, p. 9) 9) gave aa brief description of this "..... tuffaceous rock and a graphitic zone crop out." outcrop: ". rock is badly badly slumped northward towards the the parking parking lot throughout throughout This sheared phyllonitic rock much of its length. At At its its eastern easternend end itit is is more more hematitic hematitic than elsewhere, elsewhere, with reddish reddish foliation foliation planes planes in the the relatively relatively underformed underformed greenstone greenstone to to the the east. east. Throughout resembling the fractures in Throughout its its entirety entirety there on foliation planes, planes, with with much much of the rock exhibiting a is a strong presence of chlorite and sporadic sericite on color due due to to weathering weathering of of chlorite chloriteand andferroan ferroancarbonate. carbonate. Where not not slumped, slumped, foliation maroon-reddish color measurements measurements cluster cluster closely closely about about 280°, 280°vertical. vertical. probable tuff There are some zones of chlorite-poor rock with small quartz eyes indicating aa probable protolith, similar to that found in the stream to the east. Near Near the the eastern eastern end end of of the the outcrop outcropisisan an irregular irregular to 2 feet layer of nearly pure graphite. Our Our grab sample of the graphite gave foliation-parallel two inches to no Au. One Oneorortwo twofeet feetacross acrossfoliation foliationfrom fromthe thegraphite graphiteisisaathin thinsemi-parallel semi-parallel layer layer of of rusty rusty to to black gossan with selvages of massive and semi-massive semi-massive pyrite carbonate carbonate breccia. breccia. From From the the pyrite-gossan pyrite-gossanzone, zone, Gleason (1986) reported values values up up to to 22 gmltonne gmltonne (.06 (.06 ozlton) or/ton) Au. Au. Our Our grab grab sample sample of of the the gossan g o w n gave gave .037 gave .028 .028 odton or/ton Au. Au. A 0 3 7 or/ton odton Au, Au, while our sample of pyrite breccia selvages gave A polished polished thin thin section section sample of the pyrite breccia shows shows that there are are two generations generations of of pyrite pyrite in in aa quartz-calcite-chlorite quartz-calcite-chlorite 68 �gangue, with the calcite late late in in the the paragenetic paragenetic sequence. sequence. The first generation pyrite is large and often framboidal, corroded and and partially partially replaced replaced by by quartz quartz and and second secondgeneration generationpyrite pyrite in in smaller, smaller,euliedral euhedral crystals. Heavy as inclusions inclusions in the second generation pyrite and as tiny Heavy traces traces of of covellite covellite (CuS) (CuS) occur occur as gangue. Other free grains in the gangue. Other inclusions inclusionswithin within pyrite pyrite found found by by the the microprobe microprobe include include chalcopyrite and sphalerite as as lesser trace sulfides. Accessory Accessory minerals minerals found found in in this this breccia breccia are are monazite monazite and and ilmenite ilmenite rutile, a common hydrothermal alteration. alteration. No grains common result of hydrothermal partially altered to mtile, grains of of free gold were found in this thin section sample. Besides Besides the the highly anomalous gold in in the corresponding corresponding assay assay grab grab sample, sample, anomalous trace trace elements elementswere werepresent present(ppm): (ppm): Ag-3; Ag-3; As-252; As-252; Cu-1700; Cu-l700; Pb-68; Zn-124; Uthe following anomalous 10. 10. About 150 150 feet from the western end of this exposure is a 2-foot complexly brecciated chloritecalcite-quartz-graphite vein vein with with strong strong vertical verticalhematitic hematiticslickensides slickensidesand andminor minorcopper copperstain. stain. The slickensided hangingwall of of this this vein vein strikes, strikes, 290' 290° 75'N. 75°N. Our Our grab grab sample sample yielded .005 .005 ozlton odton Au, Au, as aswell well as 244 pprn ppm Cu and 100 ppm Zn. Study Study of ofaa corresponding corresponding polished thin section did not fmd any gold grains but confirmed the presence of of very fine grained grained chalcopyrite chalcopyrite and and covellite covellite in in irregular irregular trains trains following late fractures and rimming graphite. This This vein vein appears to be cut cut off off by by aa northerly northerly cross cross fault. fault. About 75 feet from the western end of the exposure is a thin discontinuous vein vein of similar composition; our grab sample sample yielded yielded no no Au. Au. About 100 100 yards southward and uphill uphill from the west end off the Mall exposure is a slumped-in and partially flooded small open open cut cut next next to to the the north-flowing creek. In to the the south south wall of of this old mine pit there is an exposed patch of of rusty rusty weathering, weathering, ankeritic phyllonite similar to the Mall exposure exposure lithology. lithology of the the ore ore mined. mined. This There are no dumps or other present evidence of This is is the the Eureka Eureka mine mine (Williams, (Williams, 1890). 1890). Gair and Thaden (1968, p. 69) provided the following: "The "Theprincipal principaloccurrence occurrenceof ofiron ironininthe the Marquette-Sands area was at the old Eureka mine ... ... Perhaps Perhaps as much as several hundred tons of earthy goethitic iron ore was mined from from aa shear shear zone zone about about 55 feet feet wide wide in in chloritic chloritic slaty slatygreenstone greenstone (Mona (Mona Schist). Secondary Secondaryquartz quartz and and iron-bearing iron-bearing carbonate carbonate were deposited along the shear zone in places, prior to formation of of the iron ore. The Theore oreresulted resultedfrom fromoxidation oxidationof ofcarbonate carbonateand andchlorite, chlorite,the theconcentration concentration of iron, iron, and and the the removal removal of of silica, silica, MgO, MgO, CaO CaOand and alumina alumina by by ground groundwater watercirculating circulatingalong alongthe theshear shear zone. Virtually Virtually all all mining mining was was done done prior to to 1880, 1880,and and whatever whatever ore ore was extracted was shipped shipped to to local smelters. There were several unsuccessful attempts to to renew renew mining mining in in the the 1880's and charcoal-burning smelters. three test holes were drilled at the site in the 1920's". 1920's". 69 �Discussion Discussion This (1968, P1, 1). Their Thislocality locality was was last last mapped by (3air Gair and Thaden (1968, PI. 1). Their map shows shows only only aa couple couple of members of of small small exposures exposureshere, here, marking marking the contact between their Lower and Lighthouse Point members of the the Mona MonaSchist. Schist.We Webelieve believethat thatthe theoutcrops outcropshere hererepresent representthe thesouthern southernmargin marginof ofthe theDead DeadRiver Rivershear shear zone. zone. The Thepresence presenceofofgold goldand andbase basemetals metalsininthe theMaIl Mallexposure exposureisisdirect directevidence evidence of of their their localization localization by by major major shear shearzones, zones, and and we we suspect suspect that some some of of the the ore ore produced produced by by the the Eureka Eureka mine mine may have have been, been, in in aftersuluides. sulfides. part,gossan gossanafter part, The The trend trend of of shearing shearingprojects projects from from here here westward westward along along the the valley of an E-W creek. creek. No No exposures exposures could could be be found found along along itit except except aa few few outcrops outcropsof of aanarrow narrowE-W E-W magnetic magnetic diabase diabase dike dikeon on its its south 4,200feet feetwest west of ofhere, here, probably probably emplaced emplaced atator orclose closeto tothe thesouthern southernmargin margin of of the theshear shearzone. zone. southside side4,200 Thatdiabase diabaseisisnot noton onany anypublished publishedmap. map. That StopSG—3: SG3: Stop Archean metavolcanics metavolcanics (Location map, Keweenawan diabase in hornfelsed, foliated Archean (Location map, Figure Figure2). 2). Loojeuvs &4.cr '4 C -c -a C U '4 '7 sphero,Sc 'a 0" S'a 6atc h. h b It pg S -c 1 -z Lj :W40/e t-k/Occay scnsp.'e SM 0 U. a m rr AFPCg m&>C o% . reE Figure6. 6. Sketch Sketch cross cross section of outcrop outcrop at at Stop StopSG-3, SG-3,looking lookingeast. east. Figure 70 �This outcrop was last mapped by by Gair Gair and and Thaden Thaden (1968, (1968,P1. PI. 1) 1) as as Late Late Precambrian Precambrian diabase diabase with with a small adjoining patch of site" belonging to their lower unit of the Lighthouse Point Member of "fel "felsite" Member of of the the Mona Schist. There There is no outcrop description in their text. This This one one is is part of of a series series of outcrops outcrops of of a long narrow diabase dike that Gair and and Thaden traced eastward eastward into into Lake Lake Superior Superior at atLighhouse Lighhouse Point. Point. Here it strikes 250° 250°, dip 8O0N. 80°N. ItIt appears not appears to to be be about about 50 50 feet feet thick, thick, although its southern contact is not exposed. The Thediabase diabaseisisnon-foliated non-foliatedand andisismagnetic magneticthroughout. throughout. Figure 6 isisaa sketch sketch cross cross section section of this exposure. At Atthe thesouth south end end the the diabase diabase is is medium medium jointing grained, has a felty texture and contains rare vugs lined with drusy calcite. calcite. It shows shows rectangular rectangularjointing that controls incipient spheroidal spheroidal weathering weathering at the erosion surface; surface; the joint planes planes are are hematitic hematitic and and calcareous. The The diabase diabase grain grain size size lessens lessens northward to a fme fine grained but still magnetic chilled margin at the host rock contact. This Thiscontact contactisisstrongly strongly foliated foliated parallel parallel to to the the dike dike margin margin for for aa few few inches. inches. Beyond that the immediate immediate host host rock rock is is aablack, black,non-magnetic, non-magnetic,aphanitic aphanitichomfels homfelswith with strong strong foliation foliation defined mostly by prominent pink K-spar-quartz laminae. This Thisrock rock carries carries aa trace trace of of disseminated disseminated extremely fine-grained pyrite, and some some discontinous discontinous foliation-parallel foliation-parallel lenses lenses of of coarsely coarselyciystallized crystallized quartz. greenish gray. gray. Outward to the north, the host rock becomes rusty weathering, quartz. It weathers light greenish "baked"-lookthg, all the while exhibiting strong foliation planes closely slaty and less "baked"-looking, closely clustered clustered around around 280°, 80%. 80°N. The source 280" source of of the rusty color appears to be numerous foliation partings of ferroan carbonate; no pyrite is visible on a fresh surface. An An irregular irregular 4-6 4-6 inch inch foliation-parallel foliation-parallel quartz-iron oxide oxide vein marks marks the transition northward to non-thennally non-thermally altered altered grainy-light grainy-light greenish-gray greenish-gray sheared sheared rock rock with with rusty rusty 275°, 85¡ 85°N continuing continuing to the northern limit of the exposure. exposure. foliation planes at 275¡ Four grab samples of the host rock are shown on the sketch sketch cross cross section. section. Two Two of of these these were were clearly below below MPC's MPC's detection limit for gold (samples DBS-2 DBS-2 and and 3). 3). The other two returned values at or near the detection limit and were re-analyzed by Chemex using FA-AA, returning weakly anomalous gold values: DBS-l, DBS-1,51 5 1ppb; ppb;HFEL, HFEL,33 33ppb. ppb.No Nounusual unusualtrace traceelement elementsignatures signatureswere werepresent presentininany anyof ofthese these samples. samples. Study of a polished thin section of a sample corresponding to assay sample HFEL HEEL confirmed that that the pink bands consist of quartz phenocrysts phenoctysts in a K-spar groundmass. The The darker-colored darker-colored domains domains of of this this phenociysts/ciystals of albite-oligoclase, rock have a porphyritic texture with scattered phenocrysts/crystals albite-oligoclase, quartz quartz (often (often in in with pyrite), pyrite), and and composite composite large large grains grains or or fragments fragments of of quartzquartz- plagioclase. plagioclase. The dark biminerallic grains with color appears to be mainly due to biotization biotization of of numerous numerous foliation-defining foliation-defining chlorite-quartz-plagioclase chlorite-quartz-plagioclase to ruhle. rutile. The laminae and to an overall content of about 5% opaques: pyrite and ilmenite altered partially to The nature of the coarse grained components suggests that that this this rock rock was was originally originallyaa crystal crystallithic lithictuff. tuff. The 71 �thermal metamorphic effects effects near the contact of the diabase diabase dike dike appear appear to be limited limited to to prograde prograde alteration of chlorite to biotite biotite and recrystallization recrystallization of quartz and feldspar. feldspar. Figure 7 is a Jensen cation plot of of the the whole rock analyses analyses of the four four samples samples taken at this stop. stop. Discussion Discussion The foliation in the weakly hornfelsed country rock here is at an oblique angle to the diabase diabase dike, can best best be be seen seen at at the the top top of of the the outcrop outcropknob). knob). Because of this, we (which can webelieve believethis thisfoliation foliationprepreexisted the emplacement of the dike. We We also also believe believe that much of the foliation foliation in the host rock here is the result of shearing, shearing, and that this outcrop lies near the middle of the Dead River River shear shear zone. The The southern southern boundary of this shear zone would lie lie near near the the Marquette Marquette Mall Mall outcrop outcropatatStop Stop SG-2. SG-2. The emplacement of this diabase diabase dike dike within such such aa large large zone zone of of weakness weaknesswould would not notbe beunexpected. unexpected. Gaff and and Thaden Thaden (1968), (1968), though noting shearing in in their their text text from from place place to to place in the Archean Gair Quadrangle, did did not not emphasize emphasize its its importance, importance,nor norillustrate illustrateititon ontheir theirmap. map. They They rocks of the Marquette Quadrangle, also found very few outcrops of their lower unit of the Lighthouse Point Member, most of which are are shown as "felsite". "felsite". About About 1200 1200feet feet NNW NNW of ofhere, here, theft their map map shows showsaa cluster cluster of of 44 small small outcrops outcrops on on aa low low bill, "felsite". The hill, marked as ""felsite". TheWestwood WestwoodMall Mall and and its its parking parking lots lots now cover cover them, but we suspect that have exhibited exhibited shearing shearingsimilar similartotohere. here. About 500 feet further north in in the the woods woods these outcrops would have and sewer line line were were being being constructed constructed in in the the fall fall of of 1998. The behind the Mall, a new east-west street and The construction equipment tore up the tops of several small subcrops of sheared rock whose composition chioritic to nearly pure pure massive massive sericite. ItIt was was not not possible possible to to obtain obtain dips, but the strike varies from highly chloritic of the shearing is east-west. east-west. These exposures lie he near the probable northern contact contact of of the the Dead Dead River River shear zone. Since Since it's it's unlikely unlikely that that they'll they'll be bestill still available availablein in the the spring spring of of 1999, 1999, we we offer offer Stop Stop SG-4 SG-4 as as an alternative. alternative. 72 �FeO* + 1102 SAMPLES • • Cation Cation % % Jensen (1976) Jensen (1 976) A DBS-1 DBS-2 DBS-3 v HEEL BK A1203 A1203 MgO Figure 7: Stop SG-3 Figure 7; Stop SG-3 73 �Stop Stop SG-4: SG4: margin of Dead River Shear Northern margin Shear Zone on Wright Wright Street Street (Location map, Figure Figure 2). 2). This is an optional optional stop stop that that is is included included here here for for those those interested interestedin in seeing seeing more moreof of the theDead DeadRiver River Zone in in the the Marquette Marquette area. area. This exposure be Shear Zone exposure is very close to a busy highway, and care should be exercised when when visiting visitingit. it. About About 65 65 feet feet of of sheared sheared greenstone greenstone is exposed here on the east side of Wright Street. The The lower lower level level of of the the exposure exposurewas wasrecently recently created created by by widening wideningthe the road, road, and andwas wasnot notavailable available to Gair and Thaden (1968), who showed showed the the upper upper part part of of the the exposure exposureas as aa lithology lithologylabeled labeled"interlayered "interlayered ) massive basalt(?)" on on their their map map(P1. (PI. I1). The degree of shearing is variable here, here, but but much much of of itit is is intense. intense. Foliation The Foliation becomes generally more strongly developed from north to south, and strikes E-W, E-W, nearly nearly vertical. vertical, hi In the the small small cut above above the forms similar to to the the sheared shearedpillows pillows at at Stop StopSG-1 SO-i are present. Figure lower ledge, curvilinear sheared forms Figure 88 isis a photograph of pencil cleavage cleavage developed developed further further south south as as shearing shearing intensifies. intensifies. There Thereisisweak weakcarbonate carbonate alteration in some of the most heavily sheared rock, but it is not pyritic; we didn't sample samplefor forgold goldhere. here. Figure 8. Figure 8. Looking Lookingdue dueeast eastatatpencil pencilcleavage cleavagedeveloped developedby by shearing shearingin in greenstone greenstonein in low low roadcut, east side side of of Wright Wright Street, Street, Stop StopSG-4. SG-4. 74 �The northern part of this outcrop consists of nearly chaotic chloritic foliation that appears to result from the contact with a shear-resistant dike of non-magnetic gabbro that is only weakly foliated. Only Onlyaa couple of feet of this gabbro gabbro are exposed exposed in the roadcut, but a larger, glacially smoothed outcrop outcrop of of itit lies lies above above and and aa few few yards yards east east of of here. here. ii D is c u S 5 Discusssion About 400 feet to the north, on the east side of of the road road is an an outcrop outcrop of of relatively undeformed metabasalt; glacially polished outcrops outcrops of similar rock are exposed exposed in the south side side of the gravel pit being development aa couple couple of of hundred hundred yards yards further furtherENE. ENE. These rocks appear appear to to be be converted to a housing development Point basalt basalt typical typical of of this this area area(Johnson (Johnsonand andBomhorst, Bornborst,1991). 1991). So we would would place place the the Lighthouse Point northern boundary of the Dead River Shear Zone, if not at this outcrop, then no more than a few hundred feet north of here. Figure Figure22shows showswhat what we we think think are are the the approximate approximatelimits limits of of the the shear shear zone, zone, which which corresponds roughly to the lower unit imit of the Lighthouse Point Point Member of of the the Mona Mona Schist of of Gair and corresponds Thaden (1968), (1968), in turn corresponding roughly to the "Eureka Group" of Williams Williams (1890), nomenclature nomenclature that was discarded by Van Hise and Bayley (1897). This Thiswould wouldmake makethe theDead DeadRiver RiverShear ShearZone Zone3,000 3,000 feet wide at this point. Puffett Puffett (1974, (1974, p. p. 44) 44) noted noted that that the the Dead Dead River River Shear Shear Zone four miles west- northwest of here is about 3,400 3,400 feet feet wide. wide. To better define define and and understand understand the the Dead Dead River River Shear ShearZone Zone in in this this important importanthinge hinge area areabetween between the northern and southern blocks of the Ishpeming Greenstone Belt, Belt, remapping re-mapping and and structural study study of of existing and newly newly created created outcrops outcrops are areneeded, needed,before beforeit's it's too too late. late. The ex-urban growth around Marquette, Marquette, although presently presently creating creating new outcrops, outcrops, will will eventually eventually result in a net loss loss of of exposures. exposures. 75 I I I �Figure 9. Location map for Stops SG-5,6 and? 76 �Stop SG-5: Stop SG5: Progressively shearedlaltered sheared/altered metavolcanics metavolcanics in Carp River Falls Shear Shear Zone. 9). (Location map, Figure Figure 9). Puffett Puffett (1974) (1974) last mapped this this locality locality as as "undifferentiated "undifferentiated greenstone" greenstone" within within the Carp River Falls shear zone. Dealing Dealing with with this this exposure exposurespecifically, specifically,he he included included aa photograph photograph of ofits itsmost mostsheared sheared part, and described it as follows (p. 43): "Sericite, and altered part. "Sericite,chlorite, chlorite,carbonate, carbonate,and andleucoxene leucoxeneare arethe the products. Weathered brown from from oxidized oxidized most conspicuous alteration products. Weathered surfaces are commonly stained brown minerals. Quartz-carbonate iron minerals. Quartz-carbonate veinlets fomi form an an anastomosing network in some rock. Copper Copper minerals minerals are present locally. Analytical Analytical data datasuggest suggestthat that the the altered altered rocks rocks are are enriched enriched in CaO CaO and Co2 Co2(Table 12)." value, 90 90 ppm. ppm. W.A. Puffett's analysis from here gave an anomalous copper value, W.A. Bodwell Bodwell (personal (personal communication, 1998) 1998) reports having noted trace chalcopyrite and copper staining here, hut but they are not not easily found. found. This exposure was a 1988 follows (Bornhorst (Bomhorst and 1988 I.L.S.G. field trip stop stop described, described, in part, as follows others, 1988): 1988): "The "The rocks rocksin in this this roadcut roadcut have have aa well developed close-spaced foliation (N75-88W, dip 7089°S) which produces produces a slate-like appearance. appearance. These 89OS) These rocks rocks are are within within the Archean Carp River Falls Shear Zone. There two lithologies lithologiesin in this outcrop: chlorite chloriteschist schistfrom fromaabasaltic basaltic parent, parent, and and quartz-sericite quartz-sericite Thereare are two schist from a rhyolitic parent." Bornhorst Bomhorst and others (1988) also pointed out that near the very eastern end of the outcrop are probable relict pillow rinds. As probable Asseen seenelsewhere elsewhereon onthis thisfield fieldtrip, trip,these these are are gradually gradually obliterated obliterated by by shearing as one moves westward westward along the outcrop. outcrop. The The chioritic chloritic sheared sheared metabasalt metabasalt gives way progressively to more and and more more rusty rusty weathering weathering carbonate-sericite carbonate-sericitealtered altered rock; rock; we we believe believe itit represents represents aa progressively progressive change from from propylitic propylitic alteration alterationto to weak weak potassic potassicalteration alterationin in aa rock rock of ofthe thesame sameparentage. parentage. progressive lighter colored colored carhonate-sericite carbonate-sericitealtered alteredrock rocknear nearthe thewestern westernend endthere thereisisaa 6-inch 6-inch pyritic In the lighter pyritic carbonate zone. Our Our grab grab sample sample of of this this gave .005 oz/ton odton Au (sample STOP 3). AA70-foot 70-footchip chip sample sample across across the entire entire sericite-carbonate sericite-carbonate altered altered zone here (not including including the pyritic pyritic grab grab sample) sample) gave gave undetectable AU Au (sample STOP-3A) STOP-3A)..An AnFA-AA FA-AAre-analysis re-analysisby by Chemex Chemex of of the the pyritic pyritic grab sample gave a value of 90 ppb Au, Au, confirming MPC's MPC's results. We Wealso alsotook tookwhole wholerock rocksamples samplesfrom fromthe thevery veryeastern eastern exposure in the pillowed pillowed greenstone greenstone(sample (sample 5-1), 5-l), one onefrom frommoderately moderatelysheared shearedrock rocknear nearthe the end of the exposure (5-I), and one at the western end of the outcrop in approximately the same spot sampled and middle (5-1), photographed by Puffett (our sample 5-3). Whole Whole rock analyses analyseswere were also also done done on on the the two samples samples taken for gold assay; sample STOP-3A is the themost mostrepresentative representativeof of the the light light colored colored sheared sheared rock at at the western end of the exposure, exposure, since since itit was aa composite composite chip chip sample sampleacross across 70 70 feet feetof ofit. it. Figure 10 10 is a 77 �Jensen plot showing the Note that that sample sample STOP STOP33 is is anomalously anomalously high high in in iron iron because becauseititwas was Jensen the results. results. ((Note chosen chosen for for best bestpyrite pyritecontent). content). + FeO* FeO* + 1102 Ti02 Cation% % Cation Jensen (1976) (1 976) Jensen SAMPLES SAMPLES • 5-1 5-1 • 5-2 5-2 5-3 5-3 "v STOP3 STOP 3 A A • STOP-3A STOP-3A PJ203 MgO Figure Figure 10: 10: Stop Stop SG-5 SG-5 78 �A polished thin section was made of a specimen corresponding corresponding to to pyntic pyritic grab grab sample sample STOP STOP 33 that yielded anomalous gold. Megascopically Megascopicallythis thisspecimen specimenisisaa chlorite-sericite chlorite-sericiteschist schist hosting hostingaa foliationfoliation¼-inch veinlet veinlet of of carbonate. carbonate. Under conformable %-inch Under the microscope microscope and electron microprobe the host rock is 1%tiny tiny a fine grained mixture of 50% quartz, 30% 30% chlorite chlorite and 20% 20% niuscovite/sericite, muscovite/sericite, containing containing about about 1% rutile grains. The grains of disseminated chalcopyrite, as well as trace amounts of tiny disseminated mtile The dolomite with high-manganese cores, with about carbonate veinlet is a mosaic of coarse grains of zoned dolomite 10% high-iron chlorite. chlorite. This 10% interstitial high-iron This vein vein assemblage assemblage hosts hosts schlieren texturally similar to the host rock except that these fragments have no quartz. quartz, consist primarily of zoned ankerite with dolomitic cores and contain about 25% opaques, mostly Mn and Fe oxides, but also abundant fine fine grained euhedral pyrite and subordinate chalcopyrite. One Oneof of the thechalcopyrite chalcopyritegrains grains was was found found to to have have inclusions inclusions of of nonnonand one 3-micron grain of of high-gold high-gold electrum electrum (photo, (photo, Figure 11). This argentiferous tetrahedrite and Thiswas was the the only gold grain found in in this this sample. sampie Arsenopyrite Arsenopyritewas wasnot notfound, found,but but trace traceelement elementanalysis analysis of of the the corresponding assay sample yielded 170 170 ppm As. 'I, 3e+ Figure 3, White Figure II. 11.Polaroid Polaroidphoto photoofofbackscattered backscatteredelectron electrondetector detectorimage, image, sample sample STOP 3. White vertical scale bar at lower left is 10 10 microns. Bright 3-micron grain of of high gold gold electrum electrum in in (teØ. Black chalcopyrite (cpy) that also contains larger inclusions of tetrahedrite (tet). Black areas areasare are gangue. gangue. 79 �Discussion Puffett (1974, (1974, P. p. 43-44) 43-44) noted noted copper copper minerals minerals at at more more than one locality in the Carp Carp River Falls shear zone, an encouraging base metal association from kom the gold exploration explorationpoint point of ofview, view,since sincegold gold mineral here. here. This occurs directly with a copper mineral This outcrop outcrop is is close close to to the the north notth margin margin of the shear shear zone. zone. margin appears to be marked by bluffs that are coneolled controlled by by E-W foliated greenstone, across across The south margin behind Ball Moving and Storage, makiig making the the zone zone about about 400 400 feet wide wide at at this this point point, mostly mostly the highway behind covered by U.S.Highway 441. 1. We Wewill will cross crossthe the highway highway here to to see see more more evidence evidence of of gold gold mineralization. mineralization. Stop SG-6: Stop SG& Gold-bearing gtacial erratic erratic (Bill Bodwell'sBoulder) Boulder).. Location Gold-bearing glacial (Bii Bodwell's Location map, map, Figure Figure 9. first noticed noticed and and sampled sampled by by Bill BillBodwell Bodwellininthe the1980's. 1980's. His This was fust His attempts attempts to find fmd a bedrock source in the immediate immediate vicinity were unsuccessfid. unsuccessful. However, However, judging judging by byits itslarge largesize sizeand andangularity, angularity,this this boulder hasn't traveled traveled very very far. far. The Theeastern easternend endof ofthe theboulder bouldershows showsthe thepreserved preservedremains remainsof ofan an original bedrock surface. snrface. This rectangular 5 by 10 I0 foot foot block block was was unearthed unearthed in in sandy sandy glacio-fluvial glacio-fluvial sediments sedimentsduring during site site clearing for the moving and storage facilities, and and has has lain lain in inits itspresent presentposition positionfor forabout about 30 30years. years. It up to to one one foot thick. thick. The consists of about about 50% 50% close-spaced, close-spaced, sub-parallel sub-parallelen en echelon white quartz veins up veins contain l0%-20% 10%-20%light lightbrown brownweathering weathehgferroan ferroancarbonate carbonateininblobs blobsand andseamlets, seamlets,lesser lessersmall small masses of dark green euhedral euhedral chlorite, chlorite, and rare rare small small blobs blobs of of very very late late stage stage black tourmaline. tourmaline. Interstitial to the quartz veins, veins, the the other 50% 50% of of the the boulder boulder is is made made up up of of highly highly foliated foliated "horses" "horses" of of chloritic greenstone, showing an impressive impressive internal internal continuity continuity of of foliation throughout carbonate-altered cblontic with oblique tensional development in a boulder. The vein g", consistent with the boulder. The style style of veining is "ladder veining", sheared sheared rock. rock. chalcopyrite, and abundant malachite The quartz veins contain contain patchy blobs of pyrite with lesser chalcopyrite, staining. Bill Bill Bodwell's Bodwell'sinitial initialgrab g a bsample sampleof ofthe thesulfide-bearing sulfide-bearingquartz quartz gave gave .267 ,267oz/ton odton Au; Au; two later later 10,000 ppm Cu, samples odton and .006 01/ton, odton, respectively. All All of of these these samples samples yielded ++lO,OOO samples yielded .036 oz/ton Ch showing that copper, although obviously mineralogically associated with gold here, does not necessarily have a direct assay correlation (Bodwell, 1988, written written communication). communication). Our grab sample sample of the quartz.006 odton oz/ton Au. Au. Our greenstone yielded 005 sulfide gave ,006 Our sample sample of of the the barren-looking host greenstone ,005o1/ton odton Au. Au. 80 �This This wide range of gold gold assay results demonstrates the "nuggety" distribution dihbution of gold gold within within this this boulder, boulder, aa phenomenon phenomenon that that requires requires special special assaying assaying techniques techniquesto toobtain obtainaarepresentative representativevalue. value. However, Carp However, the the main point of of this this field fieldtrip trip stop stopisisthat that this this boulder boulderlikely lkeiy caine cameeither eitherfrom from the the Carp River from a similar zone nearby. It's River Falls Falls shear zone or &om It'sinteresting interestingto to note notethat that glacial glacialstriae striae on on the the pillowed greenstone greenstone near near the USGS USGS benchmark benchmark at at the eastern eastern end end of of the theStop StopSG-5 SG-5outcrop outcrop across across the the pillowed highway highway are are nearly nearly east-west, east-west, paralleling the shear zone. The Thestrongest strongestinference inferencefrom h m the theevidence evidenceatat both both this this stop stopand and the the preceding preceding one one isis that that the the Carp CarpRiver River Falls Fallsshear shear zone zone isis in in itself itself aagold goldexploration exploration target. target. Stop StopSG-7: SG7: Basal Basal Proterozoic Proterozoic Enchantment EnchantmentLake Lakeconglomerate conglomeratein in contact contact with Carp Carp River River Falls Falls shear shear zone zone in in Archean Archean metabasalt. metabasalt. (Location (Locationmap, map,Figure Figure9). 9). This This spot spot has has been been aa classic classic field field trip trip stop stop for for seeing seeingthe the features features of of the the Mesnard Mesnard and and Ajibik Ajibk quartzites U.S 41 41roadcut. roadcut.But Butour ourinterest interestlies liesthither furtherdown-section down-sectionininthe theProterozoic, Proterozoic,atatits its quartzites in the US. contact contact with with the Archean. This Thislocality localitywas waslast lastmapped mappedby byPuffett hffett(1974), (1974),who whoshowed showedthe theArchean Archean rocks rockshere here as as Mona Mona Schist, Schist, now now included included in in the the informal informal lower lower basalt flow flow unit unit of of the Mona Mona Formation Formation of of Bornhorst Bomhorst and and Johnson Johnson (1993). (1993).Puffett's Puffett'smap map(P1. (PI.1)1)shows showsthe theCarp CarpRiver RiverFalls Fallsshear shearzone zoneasasaaline line passing passingabout ahoutaa half half mile mile north north of ofhere, here, but but he hestated statedin in his his text text (p.43), (p.431, "The "The zone zoneisis indicated indicatedon on the the map map as asaanarrow, narrow,well well defmed defmedstructure, structure,but but the the symbol symbolmarks marks only only the the north north limit limit of of aa broad broad zone zoneof of sheared sheared and andaltered alteredlower lowerPrecambrian Precambrianrocks." rocks." After M e rparking parking at atthe the intersection intersectionof of U.S. U.S. 41 4 1and andChippewa ChippewaDrive, Drive, walk walk about about100 100feet feetnorthwest northwest past pastthe thebillboard billboardto to aaseries seriesof oflow lowoutcrops outcropsininthe thewoods. woods.The Thefirst fistone oneencountered encounteredisisglacially glaciallypolished, polishe4 and andexposes exposesthe theCarp CarpRiver RiverFalls Fallsshear shearzone zoneinincontact contactwith withthe theoverlying overlyingbasal basalconglomerate conglomerateofofthe the Enchantment EnchantmentLake L&e Formation. Formation.Both Botheast eastand andwest westofofhere herethere thereare arenumerous numerousnearby nearbyexposures exposuresofofthe the shear shearzone zonelying lyingnorth north of of this this contact. contact. AAwidth widthofofabout about200 200feet feetofofthe theshear shearzone zoneisisexposed exposedininthe the immediate immediatevicinity. vicinity. Shearing Shearingintensity intensity isis variable; variable; relict relict pillow pillow textures texturesare are present present in in exposures exposuresabout about 150 150yards yardsto to the the east. east. Here Hereatatthis thisexposure, exposure,the theArchean Archeangreenstone greenstoneshows showsstrong strongfoliation foliationstriking striking280°, 280° 85° 85' S. S.AAparallel parallelfoliation foliationalso alsoaffects a f k t sthe thelowermost lowermostpart partofofthe theoverlying overlyingconglomerate. conglomerate. The Theconglomerate conglomerateisisabout about55feet feetthick, thickbut butisissomewhat somewhatpatchy patchyand anddiscontinuous, discontinuous,grading grading intermittently intermittentlyboth bothupwards upwardsand andlaterally laterallyinto intocoarse coarsegrained gainedquartzite quartzitecontaining containingonly onlyoccasional occasionalsmall small 81 �cobbles or pebbles. The Theentire entireunit, unit, including includingthe the quartzite, quartzite,isis20-50 20-50 feet feetthick, thick forming formingan an erosionally erosionally resistant E-W ledge. South South of of it, it, aa 100-foot-wide 100-foot-widelow lowswale swaleoccupies occupiesthe theinterval interval presumed presumed to to represent represent erosionally recessive sericite slate that constitutes the upper part of erosionally of the the Enchantment Enchantment Lake Formation Fornation in in this area (Puffett, 1974); 1974); the very uppermost uppermost part part of of this thislithology lithologycan canbe beseen seenbelow belowthe theMesnard Mesnardquartzite quartzite back at the the highway. highway. The basal conglomerate conglomerate can be traced traced westward westward from fromhere here at at the the initial initial outcrop outcrop for forabout about250 250feet. feet. layers of of itit consist consist of of +80% The coarsest layers +8O% white vein qquartz u a cobbles. ItIt bears bears aa striking striking physical resemblance to the low-pyrite varieties of the Witwatersrand resemblance Witwatersrand conglomerate conglomerate of of South South Africa, Afiica, and and occurs occurs in in a similar geologic setting. setting. We We took tookthree threegrab grabsamples samplesof of conglomerate ffom from these these outcrops. outcrops. Two of them retumed returned detectable gold: gold: ,003 .003 odton ozlton and and ,005 .005 odton. or/ton. These These two two were were re-analyzed by Chemex and returned retumed 65 ppb and 37 ppb Au, respectively. respectively, These Theseresults resultssuggest suggestaa problem problem with with nugget nugget effect effect at the low end of the detectability scale, which would not be surprising surprising in the case of placer gold. No sulfides sulfides are are megascopically visible in this rock, nor were any found found under under the the binocular binocular microscope microscope in in an an examination examination crushed rejects rejects from from our our grab grabsamples, samples. We of the cmshed Wehave havenot not done doneany any further further testing to ddetermine e t d e the internal domains hosting these low but anomalous gold values. intemal Discussion Diicussion Although it may tum turn out that the the traces traces of gold gold in this rock are caused solely by the foliation event it, aa paleoplacer paleoplacer origin originseems seemstotous ustotobe be much more more likely. likely. For economic concentrations of of that affects it, paleoplacer gold to be found found in in the the Enchantment Enchantment Lake Lake conglomerate, conglomerate, the first fust consideration consideration would be be deposits in the Archean tmane terrane were actually in existence prior prior to to the erosional events whether or not gold deposits deposition of of the the conglomerate. conglomerate. The in that led to the deposition Theonly onlyknown known significant significant gold mineralization in region occws occurs in veins at the Champion iron mine; the Proterozoic rocks in this region the Proterozoic Proterozoicisis otherwise otherwise notable for its paucity of gold showings. Bornhorst Bomhorst and and others others(1998) (1998) have have attempted attempted to constrain constrain the dates for some some of the mineralization in the Archean rocks by lead isotope studies, obtaining obtaining a model model age age 2,455 to to 2,510 2,s 10Ma Ma for forgalena galenathat thatoccurs occurswith with gold goldin inthe theRopes Ropesmine, mine, but but also alsomentioning mentioningstructural stn~ctural of 2,455 evidence suggesting that that the the Ropes Ropes gold gold was was deposited deposited at at about about 2,690 2,690 Ma. Ma. They also pointed out that ages are poorly constrained for the timing timing of the the rifting that that initiated initiated the earliest Marquette MarquetteRange RangeSupergroup Supergroup sedimentation that this conglomerate represents. Whatever the absolute age(s) of the thegold-bearing g o l d - b e ~veins gveinsin in the the Archean Archean rocks rocks of of the the Ishpeming Ishpeming belt, this this conglomerate conglomerateprobably probablypost-dates post-datesthem. them. Evidence for this is is the the high high preponderance preponderance greenstone belt, 82 �of vein quartz within it at this this locality, locality, the the presence presenceof of anomalous anomalous gold gold in in it, it, and its apparently unconformable relationship with the Carp River Falls shear zone, which was demonskated demonstrated to be goldpaleoplacer-typegold golddeposit depositwithin withinthis this mineralized at Stop SG-5 Whether or or not not a paleoplacer-type SO-S of of this field field hip. trip. Whether for exploration exploration is is another another matter. There Thereare arevery veryfew fewexposures exposuresofofthe theEnchaniment Enchantment unit is a prime target for Lake Formation, and especially few hin conglomeratic few of of this this tthin conglomeraticphase. phase. The many samples needed needed for for assaying, provenance provenancestudies studiesand andthe thelike likewould would nearly nearly all all have have to to be be obtained obtainedby byexpensive expensivesubsurface subsurface assaying, means means over over aa wide wide area. area. Stop Stop SC-8: SG8: Lithology-seleetive Lithology-selectiveductile ductile shearing s h e a ~ with g boudinage i in n the Carp Carp River River Falls F a h shear shear 12). zone. (Location (Locationmap, map,Figure Figure12). This This locality locality was last mapped mapped by Cannon and and Klasner Klasner (1977), (1977), shown shown as as part part of of their their Precambrian Precambrian mafic-uleamafic complex. complex. The Therevised revisedstratigraphy stratigraphyof ofthe theArchean Archeanrocks rocksof ofthe theIshpeming Ishpeminggreenstone greenstone W mafic-ultramafic belt (Bomhorst (Bomhorst and and Johnson, Johnson, 1993) 1993)assigns assignsitit to to the the informal informal basalt basalt flow flow unit unit of of the the Kitchi Kitchi Formation. Formation. Beginning about us 118 mile mile east east of of here, Cannon and Klasner mapped a large area area of of Precambrian W serpentinite, serpentinite, now considered considered part of the Deer Lake peridotite. About About1/4 I14 mile to the northeast are a number of outcrops outcrops of of aa very very fresh fiesh looking, looking, nearly nearly unfoliated biotite biotite tonalite tonalite intrusion inhusion that we informally refer to as the "Mockler tonalite", named named for for an an adjacent adjacent small gold gold prospect; Cannon Cannon and and Klasner Klasner mapped mapped that as Only a few hundred feet south of here Cannon and Klasner's outcrop area as Precambrian W felsic rocks. rocks. Only Klasner's map shows shows a fault fault contact contact between the Archean and the Proterozoic. Bornhorst Bomhorstand andJohnson Johnson(1993) (1993) included included this this in the Carp Carp River River falls falls shear shear zone. zone. This expowe was was not not available available to previous workers. ItItwas wascreated createdonly onlyvery very recently, recently, when when the This exposure Marquette Marquette County County Road RoadCommission Commissionstraightened straightenedthe the approach approachto tothe therailway railwayunderpass underpassimmediately immediately south of of us. The TheRoad RoadCommission Commissiondumped dumpedaavery very large large pile pile of of excess excess blasted blasted rock from fiom this new south roadcut roadcut in aa gravel pit that lies a couple of hundred hundredyards yardssoutheast. southeast. Figure 13 13 is a detailed detailed map of of this this exposure. exposure. ItItshows showsaawide widevariety varietyof ofrock rocktypes, types,which which are are affected affected by by an an east-west east-west zone zoneof ofductile ductileshearing shearingthat that varies varies from fiom weakly weakly to to intensely intenselydeveloped, developed, dependiig on the rock type. There Thereisisevidence evidencehere here for formultiple multiple deformation: deformation: aa nearly nearly horizontal horizontal 52 S2 depending crenulation crenulationcleavage cleavageisis superimposed superimposedon onaavertical verticalS1 S, cleavage, and and is is mimicked mimickedatatall allscales scalesby bynearnear- horizontal horizontal boudins boudinsof offoliation-resistant foliation-resistantveins veinsand androck rockunits units (see (seephoto, photo,Figure Figure15). 15). 83 �___ _____ Figure 12. Location map for Stops SG-8, 9 and 10. 32'30' -______ 16/8 I ii I ." •- -: r a H o ?& 0 "N 7r"X)r 0 002 —C S 0 1000 - 60 - —-—— 2000 0 — ,g) 45 3000 I -5 I— - 2/± - —. T. 48 N. T. 47 W 1 MILE 3 601)0 7000 FEET i KILOHErER CONTOUR INTERVAL 20 FEET 47'JO' GREENWOOD 1.2 ML I,rrrnlOB—GE0LOGICAL SUSVEV - WASHING TON, Oj._ISSS—NS SI 5,3565 84 46030) 87°45' �I Figure Figure 13. 13. Plan Planmap map of of Marquette Marquette County Highway 8. Higbway "CL" roadcut, Stop SGSO-S. AT noMttrEtY $$(AC merA&454Lr .2 tocM1, LI/E4%'Y rot,4r6) o — 4u•rt1 E . yo Sd' m'Ms.4Lr ed fri 6Th 54$4tr So uOiW3 p0SN}fRY — -. 'tI—.& /tt1 c. A cm.' "/ ftttt H ti7t,t $4F1c r.mb4/c ch(orb'è jAta.rtd ..,&caJt -J 4 0 iay .cwt#4e6D MErA34re4. r >- L o I piEr I 'U mer4 ,nE.'r5 x .45S0-q SancpI' wI'ok rock sQflpI& T 0 50 5' Pit 1-owib flttT$ &4At- wE'Y C.npast and flp'. JA /0/98 C) 85 �Figure IS. 50-8. Near Figure 15.Looking Lookingeastward eastwardat at roadcut roadcut exposure, Stop SG-8. Nearcenter centerof of photo photo is is mega-boudined blocky tonalite porphyry with with brecciated south south margin margin of of tonalite tonalite fragments, fragments, quartz, chlorite and tourmaline. tounnaline. Tonalite Tonalitemega-boudin mega-boudinisisenveloped envelopedin in highly highly sheared sheared basalt basalt that also hosts horizontally horizontally boudined boudined quartz-carbonate quartz-carbonatevein veinnear nearleft leftside sideof ofphoto. photo. Highly sheared talcose ultramafic ultramafic is is at far far right. right. This exposure exhibits lithologic and structural features too numerous to be described described in detail in will be be briefly briefly mentioned. mentioned. Near the south end, end, a 15-foot this paper; the highlights will %foot layer of virtually piliowed metabasalt and undeformed graywacke and minor pink chert lies between weakly foliated pillowed moderately sheared metabasalt. rnetabasalt The not clear. clear. The southern Thegraywacke grayacke has graded bedding, but tops are not contact of the sediments sediments with the basalt appears to be conformable, conformable, but the northern contact contact with the by pyritic pyritic slickensides. slickensides. Moving northward past a mbbly rubbly interval of moderately sheared basalt is marked by near-outcropping sheared sheared basalt, shearing becomes progressively more intense, and seems to be centered on a 4-foot none can be be found at the 4-foot zone of talcose. talcose,papery-thin papery-thin sheared sheared ultrarnafic ultramafic rock. Although none outcrop itself, aapiece pieceof ofthis thisrock rock found foundin in the the gravel gravel pit pit waste waste pile pile contained contained cross-fiber cross-fiber chrysotile. On On the Jensen Jensen cation cation plot plotof ofour ourwhole wholerock rockanalyses analysesfrom fromthis thisexposure, exposure,the thesample samplefrom from this talcose material material 86 �plots in the peridotitic komatiite field (see Figure 14). This This rock rock isis either either an apophysis apophysis of the Deer Lake peridotite, or a tectonically emplaced slice slice of it; it; it exhibits exhibits vertical vertical rodding rodding but but horizontal horizontal slickensides. slickensides. Because of the structural deformation here, it's problematic problematicwhether whether the the boudined bondined tonalite tonalite porphyry is a dike or a sill. This Thisisisaafme fmegrained grained dark dark pinkish pinkish gray gray rock with up to about 20% small phenoctysts, which which in in thin thin section sectionare areall allhighly highlysericitized sericitizedalbite-oligoclase. albite-oligoclasa The feldspar phenocrysts, The groundmass composition is: a few few percent opaques, opaques, (about (about ¾ % rutile rude pseudomorphing pseudomorphing ilmenite ilmenite and ¼ '/4 porphyroblastic porphyroblastic cubic pyrite); one third anhedral quartz; quartz; one one third third sericitized sericitized albitic albitic plagioclase; plagioclase;and andone onethird thirdinterstitial interstitial chlorite, sometimes as pressure shadows of pyrite, defining a clear thin section-scale foliation. This This isis an an albitic rock that yielded yielded 6.04% 6.04% Na20, Na20, the study. A the highest highest of any of the whole rock analyses done for this study. A K20 content of 1.39% K20 1.39% is is probably probably entirely entirely due due to to sericite; sericite; no no K-spar K-spar was was found. found. A zone of of interest interest in in the the sheared sheared metabasalt metabasaltand and marked marked on onthe themap mapisisaathin thininterval intervalrich richinin disseminated disseminated coarse coarse magnetite magnetite coexisting coexisting with randomly randomly oriented oriented coarse coarsecubic cubicpyrite pyrite that thatappears appearsto tohave have developed porhyroblastically porhyroblastically without pressure shadows in a low strain environment. Another Another isis aa few few feet feet of sheared sheared greenstone greenstone with minor minor iron iron carbonate, carbonate, aa trace trace of of fine finedisseminated disseminatedpyrite pyriteand andrare raremalachite malachite spotting. spotting. This exposure has been sampled for gold more than once by MPC; MPC; the locations locations of of the latest three grab samples are shown on the map. None None of of the the samples samples yielded yielded gold above above detection limits. Despite Despite the favorable-looking favorable-looking shearing, shearing,the the lack lack of of significant significantaccompanying accompanyingcarbonate carbonatealteration alterationmay maybe hean an important reason for the lack of of detectable detectablegold gold in in it, it, even even where where copper copperminerals mineralsare arepresent. present. Figure 14, 14, the Jensen Jensen plot plot for for the the whole whole rock rock samples samplestaken takenfrom fromthis thisexposure, exposure,illustrates illustratesthat thatthe the greenstone samples, samples,regardless regardless of of degree degreeof of shearing, shearing, plot plot closely closely together together in in the the high iron tholeiite tholeiite field. Our sample of the tonalite porphyry from here is shown repeated on the Jensen plot of Figure 16 for the purpose of comparison comparison with with two two other other felsic felsic intrusive intrusive bodies bodies in in this this vicinity, vicinity, the the Mockler Mockler tonalite tonalite(sample (sample "MOCK") mine (sample (sample"PG-1"). "PG-I"). (Note: "MOCK) and andthe the tonalite tonalite porphyry at the Peninsula Peninsula mine (Note: Accessory Accessory zircon zircon was noted in thin sections sections of all of of these these rocks). rocks). The shearing be traced westward only shearing seen here cannot be Iraced traced eastward, and can he only a few few hundred feet, where it is partially exposed in the old highway highway cut. Beyond Beyond that that point, point, scattered scatteredoutcrops outcrops in in the the hills to the west are of unfoliated metabasalt. The Theductile ductileshearing shearingseen seen at at this this stop stop might might represent represent only only a branching splay of the Carp Carp River River Falls Falls shear shear zone, whose whose main expression expression may may well well lie lie south southof of here here at at the Proterozoic Proterozoiccontact. contact. 87 �FeO* + 1102 SAMPLES Cation % Cation YO Jensen Jensen (1976) (1 976) • PC-B-i • PC-B-2 £ PC-8-3 V PC-UM • PC-PFWO AJ203 MgO Figure 114: 4: Stop Stop SG-8 SG-8 Fe0* FeO*++1102 m02 Cation Cation % % Jensen Jensen (1976) ( 1 976) . w SAMPLES • MOCK K C K • RC-PI-WO m K-RtNO £ PJ203 A203 PG-I MgO Figure 6 Figure116 88 �Stop SG9: SG.9: Peninsula Peninsula gold gold deposit (Location (Location map, map, Figure Figure 12). 12). In In contrast to the broad anastomosing anastomosing ductile shear shear zones seell seen so far on this field trip, the Peninsula gold deposit occurs in a discrete brittle-ductile structure. Also Alsoin in contrast contrast to to the the nearly nearly east east -west regional shear zones seen so so far, the Peninsula Peninsula structure structure strikes strikes north north -- northeast. northeast. This structure structure cuts a by Cannon Cannou variety of host hostrocks, rocks, regionally regionally mapped mapped as as Precambrian Precambrian W massive massive and layered felsic rocks by and Klasner (1977) and subsequently subsequently assigned to the informal basalt flow flow unit of the Kitchi Formation (Bornhorst 1993). At (Bomhorst and Jobnson( Johnson(l993). At aa scale scale useful usefbl for exploration, exploration, the the geology geology is is quite quite complex. complex. History Gold was discovered at the Peninsula in the 1880's, 1 8 8 0 ' but ~but ~ there there was was never never any any commercial commercial production. All Allinformation informationon onthis thisperiod periodconsists consistsof ofold oldnewspaper newspaperarticles articleswith withvery verylittle littleuseful useful technical data except the probable depths of the two old main shafts on the property, 69 feet for the North shaft and 80 feet for the South Shaft (Fountain, (Fountain, 1992). 1992). There Therewas wasno nomodem-day modem-dayactivity activityat at the the Peninsula Peninsula by Callahan Miing Mining Corp. in 1981. MPC until it was purchased by MPCpurchased purchasedthe thetracts tractsencompassing encompassingthe the Peninsula and the nearby Michigan gold mine mine from from Callahan Callaban in in 1995; 1995;included includedin inthis thisacquisition acquisitionwere wereall all reports on these properties. properties. The following of Callahan's Callahan's drill d d l core, core, exploration data data and internal rqorts following synopsis part of a of the work done by Callahan is based on an analysis and summary of this acquired data in p~ detailed report for MPC (Duskin, (Duskin, 1997). 1997). detailed mapping, rock sampling and and Between 1981 1981 and 1984 1984 Callahan conducted surface reconnaisssance recomaisssance mapping geophysics. An An early early phase, phase,called calledthe the "granitoid "granitoid program" program'' was wasfocused focusedon onintensive intensivechip chipand and channel channel sampling of the tonalite porphyry, thought to be a pluglike intrusion with potential for large tonnage, tonnage, open inmsion with open pittable gold mineralization. Disappointing pittahle Disappointingresults resultsled led to to aare-focus re-focus on on the the steeply steeplydipping dipping"siliceous "siliceous exhalite" originally tested by the oldtimers, termed tenned the "Target exbalite" "Target Zone" by Callahan. Callahan. The The subsequent subsequent testing No detailed mapping or trenching Irenching were were done. done. Three program consisted entirely of angle core drilling. No mee shallow test holes under the old workings workings in 1984 1984returned returned sub-ore sub-oregrade gradeassay assayresults resultsbut but encouraging encouraging Shaft, returned returned 15.6 slant feet feet grading grading .216 thicknesses. In In 1985 1985the the fourth fourth hole, under the old South Shak ,216 oz/ton odton Au. Intensive Intensiveclose close spaced spaced drilling drillingwas wasthen then carried carried out out through through 1987, 1987, resulting resulting in about 20,000 feet of core in a total of 37 37 holes. boles. During D U Mthe the ~ course courseof ofthis thisdrilling drillingaapresumed presumedblind bliid en enechelon echelon termed the the "Back Zone", was was discovered. stratabound zone zone of of mineralization, mineralization, tenned The purpose purpose of of the the drilling drilling was was to to quickly quickly prove prove up up aa ramp-minable ramp-minableshallow shallowtonnage tonnageof ofhigh higb grade grade ore to augment augment or eventually eventually replace the ore being fed to the mill by the Callahan's mining operation 89 �o -. + • 441+ .7, -c - +- '"ir' '.4 .+ _+4'4 ,.—r' + 4 ++ + - ++ 4 5 + + 4 *4, - - - : 4 + - +4 + : -J 4 + • + + 4 4 + '4 4 , • + 4 + 4 + + 4* 4 o 4 4 .:-:+ ';* 55*4., 44 4 4 + •4 4 'i, 4 v-I i' +',. ,.' -4, 4 + 4 '"Mbc+ - " : + + 4 4 + + + I + + 44 4* ,1 V t -4+- '1-c-ct 7-c'? ++44•V +44 V 4,1-c-c 4 '4 -, 'ç+-c," "v' 414+ +4 "4 v,'ç.,'ç'v,'çc,v,vvv. v.vV'+ 744 +VT+ vvvV'?41vc,# 4', vvvuv c,'?c, :++ + SVV"V+IZQPQV 43'? '+24 WV. v tIn''? Vt ct -. +_.: _..., c,t 'Vt WV' 4 ' :- I \ 441*4 V14V 4 +444+4 ,'1vvvu vvvVvVtvv'?cvVtIc, Vvtv+,+-vvvVvtv,-V 'VVYVU" + Ct.'. 4 • + +-'. 44 4+-.' \\ c,t —': :'47—WãWet ,c-c1 +444+' 4; .>VQ,V+ 4 -: 4 + ' -'v-ct'-c-c VVvr+v +WtVVv V VWWVV WVV'?" V vcVvvvv V-c VvvV W4,VVv" 4' + ;+, 4 '.,tc':::4.'c:: Qc,vvvvVv'?M'?%4V?(,+ '—_.,.i,,,, + + jI V" '+++4I* '4*'• 4 wv,,-+ 9FtV++ 14 V4+- ..''?.t,,' "-cv — 43 ,, -'4" 0 .c(V4,VVV vc'vo vwt V-c 4-c vW+4.4..,v++% •-cfl-c ' Vt''? vwv 554,4 4 -' C" 'C 44+4* +44 4. 4444 +4 ',1W'-4-4-4'-4 w +44444444 '''.,.'.' t+* ' * 4 + 4 ++ +'+ 4+4+4+ -i B Zârie 1'+ 7+44. J -.f+JTh 4, .r+ -r 4 4 4 444''' ++ 4-- 4 + + -u'1°'' 'inC cm:. (.,U,4w vc, 1 •-'..C•0 0 7449?V 4* *4 r ' U'OO 4i4444'4+5 44_•+ .' C>>t>000( wvvv ,'', • +4.4+4+ + + 4 54+•4.'+ 1+4 ,I+ 44+4+4+5 — +4' 4-474 + 4- 4+ + 4 f'- I:$4f] ., 4+44+4+44.4 t. mafic intrusive 200 feet Geology Peninsula Gold Project Minaals Processing Corporation 100 Trench Outcrop Fault Gold—bearing zone Laminated felsic tuf I Greenstone Metagabbro Magnetic Figure 17: Stop SG-9 0 r Tonalite porphyry �at the the Ropes Ropes deposit. deposit. There Therewas wasno noattempt attemptatatstepout stepoutdrilling, drilling,and andthe the deposit deposit remains remains open open at at depth depth and and at in both both directions directions along along strike. strike. Callahan Callahan eventually eventually calculated calculated a reserve of 51,700 5 1,700tons tons grading grading .141 ,141 01/ton odton in in in the the outcropping outcropping Target Target Zone Zone segment segment of of the the structure, structure, and and an an additional additional 41,200 41,200 tons tonsgrading grading.082 ,082 oz/ton asfar far as as obtaining obtaining aa mining mining permit in 1988 1988 from from the first fust odton in in the the Back Zone. Mining Miingplans planswent went as level level of of authority, authority, Marquette County. But Butthe thedeclining decliningfortunes fortunesof of the the Ropes Ropes mine mine and and ofofthe the corporationitself itself led ledtotoaasuspension suspensionofofactivity activityatatthe thePeninsula Peninsulafrom fromthen thenuntil untilthe thesale saletotoMPC. MPC. corporation Taking in the the early early Taking aa new new approach approach not influenced influencedby bythe thestratabound strataboundgold goldmodels modelsin in vogue vogue in 1980's, MPC MPCbegan began its its re-evaluation re-evaluation of of the the property property by rock sampling sampling and mapping mappingon onaagrid gridat atl"=lOO' l"=lOO' 1980's, in in 1996. 1996.This Thisled ledtotothe thediscovery d i s w v wof ofthe theBack BackZone Zoneininoutcrop outcrop(now (nowrenamed renamed the the "B "B Zone"), Zone"), and and the the recognition recognition that that itit is is simply simply aa fault fault offset offset of of the the Peninsula Peninsula structure. structure. Further Furtherwork workincluded includeddigging diggingand and channel "=20' mapping channel sampling sampling 11 11backhoe backhoe trenches, trenches, detailed detailed11"=20' mappingalong alongthe theentire entireexposed exposedstructure, strncture, petrographic petrographic and polished section studies studies of mineralized core intervals, complete re-logging of many of the thecore coreholes, holes,and andcorrelation correlationof ofthe therevised revisedcore corelog loggeology geologywith withthe thesurface surfacemapping. mapping. Figure Figure17 17isisaa geologic geologicmap mapcompiled compiledfrom fromthis thiswork. work. Lithology Lithology The The oldest oldest of of the the rocks rocks hosting hosting the the deposit deposit area area appear appear to to be be an an interlayered interlayered sequence sequence of of greenstones greenstones and a laminated felsic felsic tuft'fragmental tuWfiagmental unit. unit. The Thetuff'fragmental tuWEapentd isisnon-magnetic non-magnetic and and erosionally one very v q small outcrop outcrop just east of the North Shaft Shaft can be found found on on surface; snrface; it erosionallyrecessive. recessive. Only one was wasalso alsoencountered encounteredin in Trench Trench 77 northwest northwest of of there. there. InIndrill drillcore corethis thisgray grayrock rockisisextremely extremelywell wellbanded banded and andoften oftencarries canies coarsely coarselyfragmental kagmental heterolithic heterolithic layers. layers. ItItexhibits exhibitsgraded gradedbedding, bedding,but buttops topsand andbottoms bottoms in inthis thissequence sequencehave have not not yet yet been worked worked out. out. The Thegreenstones greenstonesare aretypical typicalmetabasalts metabasaltsand andoften oftenexhibit exhibit amygdules amygdulesin in drill drill core. core. The Themany manystructural structuraldislocations dislocationsininthe theimmediate immediatePeninsula Peninsulavicinity vicinitymake makeitit difficult difficultto todetermine determineaastrike striketrend aendfor forthese theseunits, nnits,but butaadistinctive distinctivebedded beddedchert cherthorizon horizonwithin withinthis this sequence sequencethat that outcrops outcropsabout about1000 1000feet feetnorthwest northwestgives givesan anattitude attitudeof of035°, 035",72°SE. 72"SE. The Thecoarse coarsegrained grainedmetagabbro metagabbrowest westofofthe theNorth NorthShaft Shaftappears appearstotobe beaasmall smallplug plugororsill sillintruding inin~diig the thegreenstone; greenstone;itit contains containslarge large angular angular fragments Eagments of of greenstone. greenstone.This Thismetagabbro metagabbroisisnon-magnetic non-magneticand and completely completelyamphibolitized, amphibolitizedwith withpseudomorphs pseudomorphsofofcalcic calcicamphibole amphiboleafter afterorthopyroxene orthopyroxenefaintly faintlypreserved. presewed. ItIt contains contains 2-5% 2-5Yo coarse coarse cubic cubic porphyroblastic porphyroblastic pyrite pyrite that that often often hosts microscopic inclusions inclusions of of 6 chalcopyrite; chalcopyite; tiny tinyfree Eeegrains grainsof ofchalcopyrite chalcopyriteare arealso also present present in in traces. The Therock rockcontains containsabout about5% 5% chlorite chloritereplacing replacing amphibole, amphibole, indicating indicatingretrograde retrogrademetamorphism. metamorphism. 91 �The porphyry. This The most most erosionally erosionally resistant resistant of of the the Peninsula Peninsula structure structure host rocks is the tonalite porphy~~. This rock rock has has been been affected affectedby by both both metamorphism metamorphismand andhydrothermal hydrothermalprocesses, processes,especially especiallywhere whereititlies liesclose close to the the auriferous auriferous structure, structure, where gold values ranging from fiom a few to a few hundred ppb can often be found to in init. it. In In mapping mapping near near this this structure, structure, itit is is often often difficult diff~cultto todistinguish distinguishaadivision divisionbetween between the the two two alteration alteration effects, effects, but but one one of of the the principal principal hallmarks hallmarks of of the thehydrothermal hydrothermalalteration alterationisis an an increase increasein in chlorite chloriteas as replacements, replacements, seamlets, s e d e t s ,enhanced enhanced foliation foliation partings p d g s and and slickensides. Other Other such such effects effects are are elevated elevated content content of of fme fine grained grained disseminated disseminatedpyrite, pyrite,carbonate carbonatereplacement replacementof ofthe thegroundmass, groundmass,and andwhite whitequartz quark veining. veining. Strong Strongalteration alterationseldom seldompersists persistsfor formore morethan thanaafew fewfeet f& from fromstructural structuralcontacts. contacts. The Theporphyritic porphyriticnature natureof ofthis thisrock rock isisoften oftentoo toosubtle subtletotoreadily readilydiscern discernininhand handspecimens, specimens,even even in in relatively relatively fresh fiesh samples. In Inthin thinsections sectionsof ofthese, these,the thephenocryst phenocrystcontent contentvaries variesfrom from5% 5%to to 30%, 30%, and and these these are are all all heavily sericitized, sericitized, corroded albite-oligoclase albite-oligoclase crystals. The The groundmass groundmasshas has about about 20% 20% anhedral anhedral quartz, quark, within within aa very very fme fme grained grained mixture mixtureof ofbiotite biotitealtering alteringto to chlorite, chlorite, chlorite, chlorite,albite, albite, muscovite/sericite, primarily of of rutile rutile and and pyrite. Very muscovitelsericite, and opaques consisting primarily Very rare rare tiny tiny grains grains of of probable thin probablemolybdenite molybde~tehave been found in a few hand specimens. In In one polished t hin section of altered altered tonalite, tonalite,microscopic microscopicinclusions inclusionsof ofmixed mixedchalcopyrite-pyrrhotite chalwpyrite-pymhotitewere werefound foundininpyrite. pyrite. Because Becauseof ofits itsrelative relativeerosional erosionalresistance, resistance,this thisunit unit has hasbeen been "overmapped" "overmapped"ininthe thepast, past,and andisis probably probablyalso alsosomewhat somewhatovermapped overmappedas asdepicted depictedon onFigure Figure 17. 17. InInseveral severaldrill drillholes holespassing passingunder under outcrops outcropsof oftonalite, tonalite, none none was was encountered encountered in the subsurface. subsnrface. In Insome someof ofthe thetrenches trenchesitit was was found foundthat that low by outcrops outcropsof oftonalite tonalitewere wereunderlain underlamby bysubcrop subaopof ofthe theolder oldergreenstone-tuff greenstone-tuff lowareas areassurrounded surroundedby sequence sequencethat thatthis thishypabyssal hypabyssalintrusive intrusiveinvades. invades. In In drill drillcore coreintercepts, intercepts, its itscontacts contactswith withthis thissequence sequence are areusually usuallybrecciated brecciated and/or and/or slickensided, slickensided, but butaafew fewof ofthem themshow showinjections injectionsof ofthe thetonalite tonaliteinto intothe the greenstone. greenstone. The Thetonalite tonaliteporphyry porphyryisisnot notaamassive massiveplug, plug,but butrather ratheraacluster cluster of of dikes/sills. diiedsills. The Theyoungest youngestrock rock unit unitisismapped mappedas as"magnetic "magneticmafic mafic intrusive", intrusive", but butititisisprobably probablyaametadiabase. metadiabase. This Thisblack blackrock rockisisvery veryweakly weaklyfoliated foliated(except (exceptimmediately immediatelyadjacent adjacenttotocontacts), contacts),exhibits exhibitsan an equigranular equigranular texture, texture,and andcontains containsmegascopic megascopicfresh fieshbiotite. biotite. ItItcarries carriesaafew fewpercent percentcoarse coarseeuhedral euhedralpyrite, pyrite,asaswell wellasas ubiquitousdisseminated disseminated magnetite, magnetite, sometimes sometimes up up to to 10%. 10Y0.InIndrill drillcore coreititcontains containstrains bainsand andscreens screensof of ubiquitous angular fragmentsofofthe thetonalite. tonalite. angularfragments 92 �Structure Mineralization Structure and and Mineralization The Peninsula Peninsula fault fault structure structure affects affects all of of the the rock rock types, types, inclusions inclusionsof ofwhich whichcan canbe be found foundwithin within it. We Werecognize recognize two two basic basic lithotypes within the structure: quartz-carbonate quartz-carbonate ("q-c"), ("q-c"), and chlorite("ccpy"). Angular carbonate-pyrite ("copy"). Angular breccias are common in the q-c lithotype. Host Hostrock rock fragments fragments are are less abundant, but sometimes ccpy lithotype, usually as boudins. The sometimes present present in the copy Theq-c q-clithology lithology predominates Target Zone; Zone; itit exhibits predominates where the fault fault structure structure intersects tonalite, primarily in the Target exhibitsmostly mostly brittle brittle texures and has abundant abundant albite as veinlets and breccia matrix. The Theprimary primary carbonate carbonate is is dolomite, dolomite, with lesser ankerite, rhodochrosite and and late late calcite. calcite. Silica content is highly variable. variable. The q-c usually contains chioritic slip planes planes,,and contains chloritic and sometimes sometimes lenses lenses or intetfingering interfmgering layers of copy. Megascopic Megascopic metallic minerals in the q-c are pyrite, very minor chalcopyrite, minerals chalcopyrite, and, and, rarely, visible visible gold. gold. There Thereisis aa strong strong conelation correlationof ofgold gold assays assays with with pyrite, but it is not 1:1. 1:1. In Inmicroscope microscopeexamination examinationof of high high gold gold -- high pyrite samples, gold gold is is found found as as tiny tiny inclusions in pyrite, either alone or with sphalerite. Other Otherinclusions inclusions in pyrite samples with visible gold, the pyrite are are argentiferous argentiferous galena, galena- pyrrhotite pyrrhotite and and chalcopyrite. chalcopyrite. In low pyrite samples gold appears appears to to be be later later than than pyrite, pyrite, occurring occurring as as free freegrains grains in in chlorite chloriteand and carbonate, carbonate,associated associatedwith with chalcopyrite. chalcopyrite. characteristically displays The dominant mineral in the copy ccpy lithology is chlorite, and this rock characteristically ductile textures. This Thislithology lithologyseems seemsto to dominate dominatethe the fault fault zone zone where where itit intersects intersects greenstone or and in in the the deeper deeper levels levels of of the the Target Target Zone Zone segment. segment. There is metadiabase, both in the B Zone segment and albite in the matrix of this rock, rock, but but virtually virtuallyno noquartz. quartz. In the B Zone, Zone, most most of which which involves involves faulting adjacent to the metadiabase, much of the chlorite can be be seen in thin thin section to to be be replacing replacing biotite. biotite. The The ccpy is generally much higher in pyrite than the q-c, and also contains copy contains several several percent percent magnetite in the B Zone segment, probably probably derived derived directly from the metadiabase. In Inpolished polished section sectionmagnetite magnetite and and pyrite pyrite exhibit coexisting mutual mutual textures. textures. Chalocopyrite in this thislithology. lithology. Gold, Chalocopyrite also occurs as aa minor sulfide in where found so far, occurs occurs as as inclusions in pyrite, and in quartz and and carbonate carbonate veinlets. that the the composition composition of of the the Peninsula Peninsula gold gold structure's structure's gangue is is very very host host The clear impression is that rock-dependent. ItIt appears appearsthat that aa great great deal deal of of host host rock has has been been assimilated assimilated into into the structure, with the the metatonalite metatonalite contributing contributingto tothe thequartz quartzin inthe theq-c q-clithology. lithology. Similarly, the the availability of free silica in the of ferromagnesians fenomagnesians in more mafic host rocks promoted the lack of free silica and the abundance of rocks were were intersected. intersected. The abundance of dolomite in the q-c and of development of chlorite where those rocks chlorite cepy indicates introduction of magnesium chlorite in the copy indicates a very strong introduction magnesium into into the the hydrothermal system system that that operated within the operated the Peninsula Peninsula fault fault structure. structure. 93 �Zone strikes 020' 020° and dips steeply SE. On The Target Zone Onthe thesouthwest southwestititisisoffset offsetby by aa later laterhigh high angle fault which also rotates it into Zone). into an an orientation orientation trending trending 050°, 050°still stilldipping dippingsteeply steeplySE SE(the (theBBZone). Vertical movement on on this this cross crossfault faulthas has exposed exposedthe thegold goldstructure structurehere hereatataadifferent differentdepth depthlevel, level, porphyry. Where involving mostly mostly the the magnetic magnetic metadiabase in its footwall instead of the tonalite porphyry. Where intersected of breccia. These intersected in drilling, late faults such as this one consist of narrow zones of Thesefaults faults form form an an interconnecting pattern. Their interconnecting Their traces traces in in some somecases cases yield yield anomalous anomalous gold gold in in grab grab sampling, sampling, but it's not not clear whether this represents a later later mineralizing mineralizing event or mineralization mineralization remobilized remobilized from from the the Peninsula Peninsula structure. structure. SG-1O: Michigan Michigan Gold Mine Mine (Location Stop SG10: (Location map, map, Figure Figure12). 12). MPC has done only a limited amount of work so far on this gold occurrence, MFC occurrence. ItItisisincluded included as as an an optional field trip stop stop if time permits. There Thereare arevery veryfew fewoutcrops outcropsin inthe theimmediate immediatemine mine vicinity; vicinity; most most of them are metabasalts metabasalts of the the Kitchi Kitchi Formation Formation informal informal basalt basalt flow flowunit, unit, cut cutby by tonalite tonaliteporphyry porphyrysimilar similar to that at the Peninsula mine. These Theserocks rocksare are cut cut by by at at least least five five parallel, east-west, nearly vertical quartz veins. Compared Comparedto toother othergold golddeposits depositsininthis thisarea, area, the theMichigan Michigan veins veins are are very very low low in in pyrite, and represent a low sulfide sulfide hydrothermal system. There Thereisis no no particular particular correlation correlation of of gold gold with with pyrite, and and this deposit represents a case of severe nugget nugget effect. effect. These These veins veins are are famous famous for for carrying carrying free free gold, which can only be be found today today through through very verydiligent diligenteffort efforton onthe theold oldmine minedump. dump. Examination of of the the dump rocks and three core holes drilled by Callahan in the the 1980's 1980's shows shows that the theveins veinsrepresent representbrittle brittle structures. Angular are open open space space filling filling textures. Black Blacktourmaline tourmalineisisaacommon common Angular breccia breccia is is common, common, as as are vein mineral here; other exotic exotic minerals minerals found found here include include coarse coarse molybdenite, molybdenite, powellite, powellite, scheelite, scheelite, chalcopyrite chalcopyrite and a bismuth telluride. This Thismineralogy mineralogy is is typical of of the the igneous igneous assemblage often found in the roots roots of of porphyry porphyry copper copper deposits. deposits. Recorded 19th 19&Century production from from here was 856 856 ounces ounces of gold. Most Most of ofthis this appears appears to have come come from from the the No. No. 66 Shaft Shafton onthe the "main" "main"vein, vein,which whichwas wasmined minedfrom fromthe the55-foot 55-footlevel levelupwards. upwards. the property property by by MPC, MPC, nothing According to records inherited with the nothing happened at at this this mine again until the 1930's, when aa financially financially struggling strugglingsmall local stock company company deepened deepened the the No. No. 66to to 250 250feet feetand and drifted laterally east and west along the the vein. vein. The blocked-out ore was never mined except for bulk sample sample tests. tests. It is not clear exactly exactlywhere where in in the the mine the bulk samples came from. from. The Thereason reasonthat thatthe the bulk tests were done done is that the company's company's underground underground grab grab and and channel channelsamples samplesall allshowed showedextreme extreme 94 �assay variability. variability. Two Two bulk tests, tests, first first of of aa 1-ton 1-ton sample sample and and then of a 10-ton 10-ton sample, were done in Floughton by by Michigan Michigan Tech., Tech., which which determined determined aa grade grade of of approximately approximately .3 odton oz/ton Au. Au. A Houghton A third third test test was was done in 1937 at a flotation flotation mill SI! on-site Tech.. This on-site under under the the supervision supervision of Michigan Tech.. This also indicated a oziton. At Atthis this point point the the stock stock company company tried tried to to pay pay its its debts debts and move to commercial mining grade of .3 odton. by floating a new stock issue in a difficult fmancial financial and regulatory environment. The The various variousrecords records and and correspondence in MPC's files files show showthat that this this company companywas was still still frying trying to get approval to market its stock the summer of of 1941. As began, the the War War Production Production Board Board prohibited all as late as the Assoon soonas asWorld WorldWar War11 TI began, gold nothing apparently apparently ever ever happened happenedat atthe the Michigan Michigan after afterthat thatexcept exceptthe thethree threecore core gold mining in the U.S.; nothing holes by Callahan. Callahan. holes drilled drilled by 95 �References References Cited Cited Bornhorst, Bornhorst, T.J., T.J., Thorpe, Thorpe, R.I. R.I. and and Johnson, Johnson,R.C., R.C., 1998, 1998,Lead LeadIsotope Isotopestudy studyof ofveins veinsin inthe theArchean Archean Ishpeming Ishpeming(}reenstone GreenstoneBelt, Belt,Michigan: Michigan:Economic EconomicGeology, Geology,vol.93, vol.93,p.102-107. p.102-107. Cannon,W.F., W.F., and and Klasner, Klasner, J.S., J.S., 1977, 1977,Bedrock BedrockGeologic Geologicmap mapof ofthe thesouthern southernpart part of ofthe theDiorite Diorite Cannon, and Champion 7 Vz-minute quadrangles, Marquette County, Michigan: U.S. Geological and Champion 7 %-minute quadrangles, Geological Survey Survey Miscellaneous MiscellaneousInvestigations InvestigationsSeries SeriesMap Map1-1058, 1-1058,scale scale1:24,000. 1:24,000. Duskin, D.J., D.J.,1997, 1997,Peninsula Peninsula gold gold prospect: prospect: private privatereport reportfor forMinerals MineralsProcessing Processing Corporation, Corporation, Duskin, 21p. 2lp. in the the Upper Upper Peninsula: Peninsula: Lake Lake Superior Superior Port Port Cities Cities Fountain, W., 1992, 1992,Michigan Michigan gold gold mining mining in Fountain, Press, Press, l68p. l68p. Gair, Gair, J.E., and and Thaden, Thaden, R.E., R.E., 1968, 1968,Geology Geology of of the the Marquette Marquetteand and Sands SandsQuadrangles, Quadrangles, Marquette Marquette County, Tip., scale County, Michigan: Michigan: U.S. US. Geological GeologicalSurvey SurveyProfessional Professionalpaper paper 397, 397,77p., scale1:24,000. 1:24,000. Gleason, Gleason, R.J., R.J., 1986, 1986,Extended Extended Phase Phase 11Exploration Explorationof of the the Dead Dead RiverRiver- Ishpeming Ishpeming Greenstone Greenstone Belt, CallahanMining Mining Corp. Corp. private privatereport, report, 57p. 57p. Belt,Marquette Marquette County, County, Michigan: Michigan:Callahan Grunsky, E.C., 1981, 1981, No.16No. 16- An algorithm algorithm for for the classification classification of subalkalic subalkalic volcanic rocks Grunsky, using 1using the the Jensen Jensen cation cationplot: plot: Ontario OntarioGeological GeologicalSurvey SurveyMiscellaneous MiscellaneousPaper Paper100, 100,p.6 p.6165. 65. subalkalicvolcanic volcanicrocks: rocks: Ontario Ontario 1976, A new cation plot for classifying Jensen, L.S., 1976, c1assi'ing subalkalic Division 66,22p. 22p. Division of of Mines Mines Misc. Misc.Publication Publication##66, Johnson, Johnson, R.C., R.C., and and Bornhorst, Bomhorst, T.J. 1991, 1991,Archean Archean geology geology of of the the northern northern block of of the Ishpeming Ishpeming greenstone belt, Marquette County, Michigan: U.S. Geological Survey Bulletin 1904-F, greenstone belt, Marquette County, Michigan: US. Geological Survey Bulletin 1904-F, 20p. 20p. Puffet, Puffet, W.P., W.P., 1974, 1974,Geology Geologyof ofthe theNegaunee NegauneeQuadrangle, Quadrangle,Marquette MarquetteCounty, County,Michigan: Michigan:U.S. U.S Geological Geological Survey Survey Professional Professional Paper Paper 788, 788,53p., 53p.,1:24,000. 1:24,000. Van Van Hise, Hise, C.R., C.R., and and Bayley, Bayley ,W.S., W.S.,1897, 1897,The TheMarquette Marquetteironiron-bearing bearingdistrict districtof ofMichigan: Michigan:U.S. U.S. Geological 6O8p. Geological Survey SurveyMonograph Monograph28, 28,608~. Williams, G.H., 1890, 1890, The greenstone greenstone schist areas of the Menominee and Marquette regions of Michigan: 62, 2'llp. Michigan: U.S. Geological Survey Bulletin 62,241~. 96 �Examples of of Gold Gold Mineralization in the N orthern Mineralization in Northern Block of of the the Isbpeming Ishpeming Greenstonc Greenstone Belt Thomas 0. 0. Quigley and A. Mnhin Mahin and Robert Robed A. Minerals Minerals Processing Corporation, Champion MI Corporation, Champion 97 �Minerals Processing Corporation's High High Point, Point., Silver Silver Creek West, and Gold Bluff Bluff gold properties properties the in in the the northern northern m arm of of the the Ishpeming Jshpeminggreenstone greenstonebelt. belt. The field trip stops are intended to to all lie in the illudrate Host lithologies, structure, illustrate the the various various styles styles of of gold gold mineralization mineralization found found in in this thispart part of of the the belt. belt. Host mineralization, and alteration discus^ and and compared. compared. controls of mineralizatioq alteration will be discussed High Point Gold Prospect The High Point prospect was discovered diswvered by by MPC MPC during duringreconnaissance rewnnaissanceexploration explorationactivities activities from Callahan Callahan Mining Corp's working out fiom Corp's Silver SilverCreek CreekWest Westoccurrence occurrencelocated located1500 1500ft. ft.totothe thesoutheast. southeast, High grade surface mineralization at High Point is hosted by aa prominent knob knob of of intrusive intrusive felsic felsic porphyry porphy~~ escarpment of the the Dead Dead River Shear Zone Zone (DRSZ). (DRSZ). Subsequent along the topographic escaynent Subsequent exploration activities at the prospect included detailed mapping and and sampling, sampling, geochemical surveys, trenching, and diamond drilling. Figures Figures 11and 22 show show the the geology geology of of the the High High Point Point prospect. prospect. Lithology Rock types rocks, types at the High Point Point prospect prospect consist wnsist of of felsic, felsic, porphyritic porphy5ticintrusive in-ive rocks, within within aa sheared sheared of matic mafic flows and tuffacwus tuffaceous rocks and minor interfiow sequence of interflow sediments sediments corresponding correspondingto to the the Lighthouse Point Basalt of Johnson and Bomhorst Bornhorst (1991). The TheHigh High Point Point intrusive intrusiveis is aa fme fme to to medium medium grained, massive to weakly foliated foliated feldspar feldspar porphyry porphyq stock stock consisting consisting of of small small phenocrysts phenoaysts of of albite albitein in aa matrix of quartz, albite, potassium feldspar and carbonate. In thin In addition additionto to the the main main stock, stock, numerous numerous thin stringers of pporphyry o ~ h y r yof similar composition composition are are present present in the surrounding mounding volcanics. These mafic volcanic volcanic rocks consist of fine h e grained, chlorite, chlorite, sericite, sericite, and and amphibole amphibole schists, schists, and and are are probably probably altered altered basalts and related kff"wus tuffaceous rocks that have been variably sheared and deformed. Structure types defonnation related All rock t ypes at the High Point prospect have been subjected to shearing and deformation related to to the DRSZ. Mafic units exhibit steeply dipping shear related foliations striking NW-SE, parallel to the Mafk units exhibit steeply dipping shem foliations the ancillary shearing oblique obliqueto tothe themain main DRSZ DRSZtrend. trend. This shearing is main DRSZ, as well as E -- W striking ancillaq~ through 33 (see Figure 21, 2), which expose the flanking volcanic d section well exposed in backhoe trenches 11 &rough o n to the High High Point Point iintrusive. and dikes exhibit exhibit adjacent to n h i v e . The The High High Point Point Intrusive I n h i v e body and related sills and intense brittle deformation deformation manifested manifested by irregular stockwork fracturing, quartz -- carbonate carbonate veining, veining, and and brecciation. Irregular Irregularcontacts contactsof ofthese these felsic felsicunits unitsobserved observedin indrill drillcore, core,as aswell weU as as aa lack lackof ofcontinuity continuity between pporphyry units have been o r p h y ~units ~ in outcrops, drill holes, and trenches, kenches, suggest that these felsic units extensively disrupted and possibly rotated as as large competent wmpetent blocks within this portion of the DRSZ. Alteration Alteration Ubiquitous Ubiquitous carbonate carbonate alteration alteration consisting consisling of of calcite calcite and dolomite-ankerite is present throughout the the High High Point Point P Prospect, pervasive replacement replacement of of rock rock matrix, and as sheared section at the r o s p q occurring as a pmasive discrete fillings in both mafic and felsic muscovite, as discrete veins and fracture fiacture mlings felsic units. Coarse, secondary muswvite, as well as black shiny carbon mineral are wmmonly commonly associated with with carbonate carbonate veining veining in in the the High High an unknown black Point area. Examples of the carbon mineral mineralcan be readily readily found in the talus slope below the the mineralized that mineral is present present throughout throughout the the ha! this mineral outcrops of High Point intrusive, intrusive, and drilling has revealed t 98 �________ VVVVVVV vVVVVvVVVVVVvVVvVVVvyvVVtVVtvvVVvVvvVVVVVVVVVflVVVVVVVVVVV V V V V 9 V V V V V 9 V V V V V V V V V V VV V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V VVVVVVVVVV VV VVVWVVVVVVVVVVVVVVVVVVVVVVVVVVVVWVVVVVVVVVVVVVVVVVVVVVVVVVVVV VVVVVVVVV VVVVVVVVVVVVVVVVVWWWVVGVVV v VVVVVVVVVVVVVV V VVVVVV MINERALS PROCESSINGv' VcVVVVVVVV VVVVVVVVVVVVVVV74VVVVVVVV 9VVVVVVVVVVVVVVVVVVVVVVVV 7VVVVVVVVVV VVVVV CORPORATION Geology of the High Point Prospect ala t'ia eei.Iila Pi 11aM Rnjn 4n %0 — .'J — dd tone ci shearing. Strike and dip of inflation. 0 ¼0 Stzika end dip *1 .i or joint. VWVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVWVVVVVVVVVVVVWVVVVWVVVVVVV VVVVI V cv V V V V V V V V V V V V V V V V V V V V 9 V V V V V V V V V V V V V V V V V WV V V V V V V V V V V V V V WV 49 V V V ' V V V flY V V V V V V V V V V V VV V V V V V V V V V V cv c V V V iii VVVVr '' ea',,T, V igure narge VVVVVVVVVVVVVVV EVVVV 4VVVV VVV V r' I VVVVVVVVVVVVVvVVVVVVTwwVVVVVVVVVV VVVVVVVVVVVVVVVVVV VVVVVVVVVV - Thvv44 cv?,y1j.t,t÷ V VVV - 'VWVWVV&V \r.VVV*1 \74 ttt 4 V + 4 4 + % i " 4 4 V + + 4 4 4, • c + +* Hiab Point Intrusive Porpbyiy, Slice to madimu pained, reddish brown porphyriuo rock p,mpo.ed of 1 to 5 = sibite pbgiooryat. in a matrix of elba.. K—feitpar. a + * t '+ . VVVV VVVVVVVVVVVV VVV VVWVVVVVVV VVVVVVVVVVVV VV VVVVVVVVVV VVVVVVVVVVVV3'VV VVVVVVVVVV VVVVVV 3VVVVw. 9VVVVVVVVV + + * -€ - VVVWiVtI4 VVVSq4/VV9tN44 IVVVV4'W4iVV VVVVVVVVVV 9 VVVVVrVnVVVVV VVVVVVVVVV + VVVVVVVVVVVV t 9 1 4\VVWVWVV9VVVVVVWVVVVVV tt"VJPV?VVVVVVVVVWVVVVVVVVVVV 'V9JryV VV"?.flVV9VVVI "VW VVV V V V V 9' Fgpa_r V quarts, otuacovite and carbonat.. Parnitvely franturod, end veined with quartz, ten-can dolornito. ankorita. end en action black carbon minaret. liii j4j4j }li.h Point Intrusive Bericit. end cerbonata alias-ed porph1ry, cut by bieguler etockwork Veina and mesas of gray quartz. iron carbonates, minor pyrita, minor guiana, bismuth tafluride., and gold. VVVV VVV VVWVVV VVr,4VV.W VVVVVVVVVV rVVVV)_J,tc%.VV 44 VVVVVVVV t 4vvvc-vint VVr%nV gVVVVVVVVV VVt VVVVVVVVV VVWVVVVVVVV VVVVVVVVVV * 'VVVVV VVVt VVVVVV' VVVVVVVVV VVVVVVVVVV VVVV$t.'V-M, VVVVV VVVVVVVVVV VVVVVVVVVV flVVVVVV-t -VtflVWttt$VVV V%VPVVVWVVV VVVVVVVVVV VVVVVSVq VVVVVVVVVV t'Q.b7J VØ V VVV V 499 Vt V VVvVV-yg.VVVlV LVuvv',aVareVvVVVVVVVVVV VVVVVVVVVV 4 tVtV4VV-, VVVVVV 44 V' t.%fl Vflfi-VV Fj.+*9 + VVVVVVW V V; t$VVVY,VVVVV4V . tV,r-VVVVVV%V V uVVVVVVVVVV9V VVVVWVVVVVVVVW VVVVVVtVVVVVVV tI4SVVVVVVVVVV.aL \ViVVVVVVVVI V'VVVVVVVV \VLVVVVVfl VVV'VW WVVVVVVVVV Id WVVVVVVVVV VVT/V V 4LVVV V V VV'V VVVVVV VVVVVVV Ii V V - - 4 9 ÷ 9+ 9 -r 9 VVVVV + + -4 - wVr' -. - t - 'LV'kVV VV 1 t - - + 9 4 4 V4kVJV 'tJLVvN V VVV 4 9 V / tV VVV/ V '%VLVVVV V VVVVVVVVVVVVVVVVVV T''VVVVVVVVV1 VVVVVVVV' VVVVVVVVVV N o 25 - rVV -, V VVVVVVVV V V V V V V 44 V V V VV VVVVVVV V V V V V V V V VVV 50 Scala In S'bet Basalt. Indesita EJ - Fine grelnad chlorite, earlclt carbonate ecbi.te. Massive to roustad end fractured with atockwork carbonate — carbon mineral yams and dissemlatatad pyrtte where .haared, Locaily containing gray quarts veining ecd gold inlnarellsauon, Sequence contains minor mntarfior shexty. pyritln sediments- I Figure Figure 1. 1. Geology of High High Point Point �0 0 '4 + 4 4 4 4 4 4-4-4 MPC Trench in ppb. 7 4+4 444 1 + + ÷ + -1- -4- ± +4- —E 'Intt. 4 4 4 ——3— 4. ± Gold Values (ppb) CORPORATION Oda\E.ctMc Psat Geel 0 25 0 IT I Scale in Feet 0 Channel Chip and Trench Samples Figure 2. Detailed Geology of High Point 4 44 tt* 4+4444 4 44444444 * 4 rTTI Channel sample with gold value Basalt, Andesite 4 4 +t.tt.t-,-ttttt4 444 4-4+4 4 4 T:] High Point Intrusive IThrfl vfl C 4+44 Trench 1 50 MINERALS PROCESSING __ �flanking volcanic section as well. Within Withinthe themineralized mineralizedportion portionof ofthe theHigh HighPoint Pointintnisive, intmsive,microprobe microprobe analysis has shown that zoned zoned dolomite-ankerite dolomite-ankeriteis isintimately intimatelyintergrown intergrownwith with barium bariumbearing bearingK-feldspar K-feldspar Tourmaline is also locally present in quartz veins, in the matrix matrix of brecciated porphyry, and muscovite. Tourmaline and intergrown intergrown with chlorite chlorite along along fracture ffacture surfaces surfacesin in sheared sheared volcanic volcanicrocks. rocks. Mineralization Mineralization gray Surface mineralization at High Point is confmed to the porphyry, and consists of irregular gray quartz veining in altered porphyry porphyry on on the the southeast southeast side sideof ofthe themain mainknob. knob. Here channel chip chip sampling sampling of of exposed exposed outcrops outcrops (Chip (Chip Samples Samples A, A, B, B, and and C) C) returned retumed significant significant gold gold values values including 28 ft. &. of of .09 .09 ozlton, .35 oz/ton odton including including 4 ft of 1.25 1.25 ozlton odton (Figure 2). The Themineralized mineralized quartz quartz from ffom these odton, and 14 14 ft. of .35 exposures gay, cut cut by by iron ironcarbonate carbonateveinlets, veinlets, and andcontains containssmall smallamounts amountsof ofpyrite, pyite, chalcopyrite, chalcopyrite, exposures is dark dark gray, galena, sphalerite, galenq sphalerite, and electrum electnun as as well well as as an an exotic exoticsuite suiteof of bismuth bismuth tellurides, tellurides,lead leadbismuth bismuth tellurides, tellurides, and an unknown lead-bismuth-silver sulfide. The Thegeometry geometryof ofthis thismineralized mineralizedzone zonehas has not not yet yet been been defined, and defmed, although although its its extent extent has has been been partially patially confined confmedby by trenching ~~enching andwide widespaced spaceddrilling. drilling. Silver Creek Creek West Fiver Silver Creek West is a gold occurrence variably altered and and foliated foliated mafic mafic rocks. rocks. In occmnce within variably In 1988 1988 million tons @O.O37optAu @0.037optAu for Silver I. 1 &ion Silver Creek Callahan Mining Corporation calculated a resource of 1.1 controls. Surface West but never exploited it because of unresolved concerns over gold continuity and coneols. Surface trenching exposes exposes gold mineralization eenching mineralidon in in ductile ductile sheared sheared and and carbonate-chlorite-sericite-pyrite carbonate-chlonte-sericite-pyritealtered altered basalt near the margin of a felsic porphyry. In Indrill drillhole hole intersections, intersection&mineralization mineralizationappears appears to to be stratabound seatabound within 170 170 foot foot section section of ofmoderately moderatelycarbonate carbonatealtered, altered,massive massivemedium mediumgrained grainedmafics. mafics, Mineralization in both trenches eenches and and drill drill core core will be be observed observed at at this this stop. stop.The Thegeneral general geology geologyof ofSilver Silver Creek West is 3. is shown shown in in Figure Figure3. Lithology The lithologies in the immediate vicinity of the Silver Creek Creek West West prospect prospect consist consist of of aa number number of of generally generally northwest striking, striking steeply dipping dipping units, largely largely of the Archean Lighthouse Lighthouse Point Basalt (Johnson and Bornhorst, largeexpanse expanseof offine finegrained, graind Bomhorst, 1991). 1991).To Tothe thenortheast northeastof ofthe theSCW SCWprospect prospectisisaalarge pillowed basalt. Nearer Nearerto tothe theprospect prospectoccurs occursaamedium mediumto tofine finegrained grainedequigranular equigranularchiorite-albitechlorite-albitecarbonate carbonate unit, variously interpreted interpreted as as an ophitic ophitic basalt, aa sheared sheared carbonated carbonatedbasalt, basalt, and and aa hypabyssal hypabysd mafic pyrrhotite and hosts gold mafic intrusive intrusive or sill. The Theunit unit isisgenerally generally massive, massive, contains contains significant significant pynhotite mineralization mineralization at depth in drill core. The Thenext nextunit unit to to the the southwest southwestis is aa narrow narrow band of of chlorite-albitechlorite-albitephyllite interpreted interpreted as as aa ductile ductile sheared shearedbasalt. basalt. This unit hosts hosts gold mineralization mineralization sericite-carbonatephyllite trenches. This exposed in surface trenches. Thisbasalt basalt is is bounded bounded to to the the southwest southwest along along the cliff cliff face face by aa rhyodacite rhyodacite porphyry which which consists consists of of quartz, q w feldspar, feldspar,and andchlorite chloritephenocrysts phenocrystsin in aa felsic felsicgroundmass. groundmass.Drill Bill intercepts show an additional sequence sequence of chiorite-albite-sericite chlonte-albite-sericite phyllite and quartz-feldspar quartz-feldspar intrusive intrusive to the southwest. In Inatatleast least one onehole holethe the latter latter felsic felsic is is in in fault fault contact contact with Proterozoic Proterozoic sediments sediments which have have been down-thrown dom-thrown to form form the the basin basin to to the the southwest. southwest. 101 �_ __ ___ ____ VVVVVVVVVVV VVVVVVVVVP VVVVVVVV VVVVVV VVVV MINERALS PROCESSING CORPORATION --vvwVvvVvVt VVVnVflVVV vVVVVVVVVVVV VVVVVVVVVVVVV. uqVVVVVVVVVVVVV VVVV VV VV Vt Vt Vt V VV 1 —— V V V V V V V Vt PVV VVV % V V V V VVV VPVVV VVVV Silver Creek West — a'—.- 1, 1VVVVVVVV VVVVVVVPV 'VVVVVVVV Dt V V V V V V V V V V V V V V V 'VVVVVVVVVVVVVV V V V V V V V V V V V V V V V V! V V V V V V V V V V V V V V rt' °o°bZj V V V V V VP VVV V VY V V Geology — ni-la VVVVVVVVVVVVVW%R%i - VVVVVVVVVVVVV0 p V V V V V V V V V,S V V V V V V VVVVVVVV•&. qa4VV LV V VV V Vt VS%E VVVVV VVVVVV9V '1 r VVVVV Me Reversely polarized 0 a N441 Sedimantan rock. S*cii4 Undifferentiated. Berega Group V÷A FeolnaiRocks I Mafic Volcanic Rook. lv • Hvnabvsai Vafic Intnstve Rocks • Enterwediate Vo4genãc ggçp SedimoctmHoon • — Fault ihowing downthron aide Outcrop Shearing Trench — 200 0 Figure 3. 3. Geology Geology of of Silver Silver Creek Creek West West Figure 200 Scale in feet 400 V Vi �Structure and mineralization mineralization At the surface, gold mineralization exposed exposed in trenches by Callahan occurs within a ductile ductile sheared mafic mafic volcanic volcanic near the margin of of aa felsic felsic porphyry. This Thisshearing shearingstrikes strikes300° 300' and andmineralization mineralization is is considered C-type replacement. In Incontrast contrastto tosurface surfaceexposures, exposures,mineralized mineralizedintervals intervalsin in drill drill core core are are in in an unfbliated, weakly altered altered medium medium grained grained mafic maficunit; unit; there thereisislittle littleresemblance resemblanceto to mineralization mineralizationin in unfoliated, weakly depth to to the the easthortheast east/northeast from from the sheared basalt. Gold-bearing intervals that in drill intercepts occur at depth the trenched intervals intervals do not project project upwards upwards along alongobvious obviousstructural structuralplanes planesto tothe thesurface surfaceworkings. workings. The exact controls to the mineralization at at depth depth have havenot not been beenproved. proved. Intersection lineations lineations of of plunge down to to the east and offer offer one possible explanation for joining joining surface various structural features plunge and drill-hole gold (Johnson, 1991). MF'C has hasshown shownthat thatanomalous anomalousmineralization mineralization fJohnson 19911Limited Limitedwork workby bvMPC . occurs in outcrop outcrop to to the the east eastof o fthe thehigh gradetrenches trenchesatatthe theup-dip up-dip(along (alongSSlfoliation) projection of of the the occurs high grade 1 foliation) projection mineralized drill core mineralized core intervals, intervals, suggesting suggesting that the trench and and drill drill core core intercepts interceptsare are not connected. connected. Alteration The mineralized zone The mineralized zone is is associated associated with with weakly weakly disseminated disseminatedcalcite+chlorite+sericite+pyrite calcite+chlorite+sericite+pyrite alteration. Locally, Locally,minor minorFe-carbonate Fe-carbonateisisobserved observedatatsurface surfaceand andin infelsic felsicunits units encountered encountered in in drilling. drilling. Additionally, Additionally, Callahan Callahan geologists geologists identified identified an an early early epidote-pyrrhotite epidote-pynhotite(spilite) (spilite)event eventin in the the basalts. basalts. Gold Gold Bluff BluffProspect Prospect Gold Bluff Bluff is one of Minerals Processing Corporation's Corporation's active gold properties properties (see (see Figure Figure4). 4). Gold Bluff trending, 1000 foot wide shear zone in mafic mafic volcanics. volcanics. A A 100 to to 200 foot Bluff is is located located within a 3000 300' trending, wide wide felsic felsic porphyry porphyry (the (the GB GBporphyry) porphyry) intrudes intrudesthe theshear shearzone. zone.Within Withinthe thebounding boundingshear shearoccur occureasteastwest and northeast striking mineralized subsidiary shear zones. zones. Outcrop Outcropsampling sampling routinely yields yields gold in the Oppm range. range. Trenching the lppm lppmto to1lOppm 1998 intersected intersected significant significant widths of disseminated Trenching and drilling drilling in 1998 low low grade gold gold mineralization mineralization the GB porphyry and higher grade gold in mafic host rock. Mineralization in both mafic felsic hosts hosts in in different different structural structural orientations orientations will will be be seen seen at at Gold Gold Bluff. Bluff. mafic and felsic Lithology Three lithologies are found at Gold Gold Bluff: Bluff:fine-grained fine-grained mafic mafic volcanic; volcanic; medium medium to tocoarse-grained coarse-grained mafic volcanic, referred to as aa gabbro; volcanic, referred gabbro; and andfelsic felsicporphyry. porphyry. The The mafic maficunits units are aremetamorphosed metamorphosed to to facies and both both can be massive massive to to strongly strongly foliated. foliated. They greenschist fades They belong belong to to Lighthouse Lighthouse Point Point Basalt Basalt as as described by by Johnson and Bonthorst (1991). The felsic unit described Johnson and Bornhorst (1991). unit isis aarhyodacite rhyodacite porphyry porphyry interpreted interpreted as as intrusive. Where unit that that divides the the two two mafic mafic units, units, itit is is dubbed dubbed the the GB intrusive. Where it appears appears to be aa continuous continuous unit throughout the theproperty. property. Contacts porphyry. Smaller Smallerfelsic felsic dikes dikes are are also observed in various orientations throughout Contacts strike northwest and have have steep dips. A Areversely reverselypolarized polarized dike dikeof ofKeeweenawan Keeweenawan age age is is interpreted interpreted from from data to lie just just to the north of the outcropping rocks. rocks. The magnetic data The only only other other unit unit of of significance significance in in the the area is the Reany Lake Pyroclastic (Johnson (Johnson and and Bomhorst, Bomhorst, 1991) which which occurs occursjust just off off the Gold Bluff property to the the north and and west west in in association association with with gabbro gabbro flows. flows. 103 �104 Bluff Gold of Geology — .tgnlflcnt I thoy Figure 4. soda- tat wits. — .siclt. flax. Isyaxat a .aanta acp.alUon. .od.duo to basaitia pobabi. of rook mle4 and The.. u4—eet.d rse,. patoad. o.tdf to Ito. Rnnko Vnlnin MaNe Locally 33 Sec 28 Sec — — foto gr4. inn to coax.. iocr.iln .44 sonata. &aiztt.). (t. t.rbcats S44h .lnd .04 1.t.d broesalad. fr.osr.d. locally qwtL and/ot Mltpc or pb.toetyM. otth pakUo to SqWjnmiIar bO4a th04o .04 — dfr. rflo]a to t.Itth rot to wtittt nedja to vest jeus. Rocks Intrusive Feisic palnb.d Vfl 0 4 Ir' 4 0 0 • 44 + 4 V H '-•t• 01' 4t ciw g- flçf -, V V '7 VVVVV V V S÷ '4 :÷. V '-——V c'flu V — :rn7t; a-+-.. -. ?bfl ?V fl) t;vytV - • - w 4 VV +, 4 VVV o' -t 4 4 4 47 4÷ — 4 fl3 V 01 •vv-r 4_, , .4_ .4, 4__4 t hJt& + • 0 1çJ •lC. t9-ç, VV +, 4 ¶VV1.V • V9VVfl' C VW C" 1'- - + VVVVV C) flt * 4 2 4 vVVW VVV9? q feet in 300 200 .—Iy 9U Scale 100 4Sa 0 100 —I 0*t4M. GEOLOGY Project Bluff Gold Corporation Processing Minerals —. S WE N �Structure The orientation orientation of of brittle brittle and and ductile ductile shear shear structures, structure&felsic felsicporphyries, porphyries, and and quartz quartzveins veinsisis with a system of of conjugate shears shears that that control control gold gold mineralization, mineralization. Bounding geometrically consistent with shear features, mostly penetrative foliation in mafic rocks rocks and along felsic contacts, contacts, strike strike 300' 300°. . Within this, subsidiary stnctures structures and a variety of of mineralization mineralization styles are are observed. observed. For For example, example, the the GB GB trends 3200 and appears porphyry m d s 320' appears to occupy an oblique shear shear position (probably R-type Reidel). Mineralization 300 °and and is C-type Mineralization in Trench 5 and DDH GB-3, and along felsic contacts, strikes 300' replacement veining. R'-type R'-typeshears shearsare arepresent presentas asnortheast northeaststriking strikingductile ductilemicro-shears micro-shears and and as as numerous numerous mineralized quartz veins. Numerous Numerousductile ductileshears shearsand and minor minor felsic felsic porphyries porphyries that that strike strike 080° 080' to 1000 100' or S-type S-type structures. structures. may be either either P or quartz-carbonate-tounnaline veins White quartz-carbonate-towmaline veins with with fibrous fibrousstructure structnreperpendicular peqendicular to to the the vein vein walls walls are are interpreted features. These intqreted as extensional fmtures. Thesetypically typically dip dip in in the the 30° 30' to to40° 40' range rangeand andindicate indicateaahigh highangle angle component veins. Gray Gray quartz quartz veins veins are are often often mineralized, component to the shearing event responsible for these veins. defonned, deformed, and andcross-cut cross-cutby bywhite whitequartz quartzveins veinsindicating indicatingmultiple multipleshearing shearingand andvein veindeposition depositionevents. events. Alteration Alteration A zoned carbonate alterationhalo halo occurs occursat atGold GoldBid. Bluff Distal Distalalteration alterationin in the the mafic ma& volcanics consists cousists of of disseminated disseminated calcite calciteand and weak weak Fe-carbonate, Fe-carbonate, (ankerite-dolomite). (ankerite-dolomite). As As the the GB GB porphyry porphyryisis approached, Fe-carbonate approached, Fe-carbonate increases increases and can can be pervasive to the point point of of flooding floodingand and the theassemblage assemblage sericite+white quartz+pyrit&tourmaline quartz+pyrite+tounnaline appears, sencitewhite appears, largely largely in in the the felsics. felsics. Lithologic Lithologic textures texturesare are generally generally intact. Within Withinthe theGB GBporphyry poxphyryaazone zoneof ofstrong saongsericite+pyrite+Fe-carbonate sericite+pyrite+Fecarbonate occurs occurswith with porphyritic porphyritic sometimes completely completely replaced by fine textures sometimes fine grained gained sericite. sericite. Mineralization Mineralization occurs in all three lithologies lithologies and consists consists of quartz+gold+pyrite+Ag&Bi-tellurides. qm?z+gold+pyrite+Ag&Bi-tellurides. mineralization (i.e., In the metabasalts C-type replacement mineralkalion (i.e., highly silicified and sulfidized sulfidized zones wnes with with is generally generally confined confined to thin thin ductile ductile shears shears at at or or narrow foliation-parallel foliation-parallel quartz quartz veins, lenses lenses and pods) is proximal proximal to to felsic felsic contacts. contacts.Similar, Similar, although althoughoften oftenhigher higher grade, grade, mineralization minerahation occurs occursat atthe thegabbro/felsic gabbro/felsic contact. Significant SignificantC-type C-typevein vein mineralization mineralizationnot associated associated with a felsic contact also occurs within sheared gabbro south of the GB porphyry (S.7'@O. 188opt optAu Auand and17'@0.077 l7'@0.077 opt sheaxed (S.T@O. 188 opt including including 7.5'@0.1480pt w/VG). wNG). 7.5'@0.l4Sopt In In the GB porphyry, mineralization mineraIizatiou is is found found in in quartz quartz vein vein stockwork stockwork and and tends tends to to be be associated associated with disseminated pyrite and strong alteration. Gold-bearing Gold-bearing veins veins are gray g a y colored compared compared to to barren veins, although although white whitequartz quartzveins veinsare arean animportant importantindicator. indicator. One ggray ay white quartz-carbonate-tourmaline veins, quartz vein 0.4 feet wide assays 20-50 opt Au and has yielded impressive impressive visible visible gold gold specimens. Recent Recent drilling in the GB porphyry by MF' MPC significant m mineralization (224'@0.Ol8opt including C intersected sigificant i n e r h t i o n (224'@0.018opt including 1l'@O,l78 and 3'@0.l22 3'@0.122 opt). opt). ll'@O.178 105 �References R.C., 1991. proposal: Stage Johnson, KC., 1991. Silver Creek West drilling proposal: Stage III. 111. Western Western Mining CorporationlCallahan CorporatiodCallahanMining Corporation Corporation unpublished unpublished report, report, February, F e b m ~1991. 1991. , R.C., and Bodorst, Bornhorst, T.J., Johnson, R.C., T.J., 1991, 1991, Archean geology geology of of the northern northern block of of the Ishpeming Greenstone belt, Marquette Marquette County, County, Michigan: Michigan:U.S. U.S.Geological GeologicalSurvey SurveyBulletin Bulletin1904-F, 1904-F,2Op. 20p. 106 �Field Field Trip Trip33 Tilden Marquette Iron Tilden and and Empire EmpireMines Mines of the Marquette Iron Range, Range,Michigan Michigan Leaders: Leaders:G.W. G.W.Scott, Scott, P.M. P.M. Nordstrom Nordstrom and andH.M. H.M. Lukey Lukey 107 �Setting of of the the Tilden Tilden and and Empire Empire Mines, Michigan Michigan Geologic Setting Theodore Theodore J. Bornhorst, Bornhorst, Department Department of of Geological Geological Engineering Engineering and and Sciences, Sciences, Michigan Michigan Technological Technological University, University, Houghton, Houghton, MI MI 49931 4993 1 Introduction Introduction The Tilden and Empire Mines are located in the Marquette Iron hon Range Range of Michigan's Michigan's Upper Upper Peninsula (Figure (Figure 1). 1). Mining Miningbegan beganatatthe theEmpire EmpireMine Minein in 1963 1963and and at at the the Tilden TildenMine Minein in 1974. This This is is the the first first Institute Institute of of Lake Lake Superior Superior Geology Geology field trip to either of these iron mines. The TheMarquette Marquette hon IronRange Rangehas hashad had aalong long and and varied varied mining history (Boyum, (Boyum, 1988). 1988). reserves are are sufficient sufficient to to sustain iron mining Iron mining began in the mid-1800's and current reserves until at at least least 2030. 2030. Stratigraphic Setting Stratigraphic Marquette Marquette Range Range Supergroup Supergroup The Tilden and Empire Mines produce iron ores from the Early Proterozoic rocks of the Marquette Range Supergroup (Figure 2). The The Marquette Range Supergroup consists of sedimentary sedimentary rocks with minor minor amounts amounts of cf volcanic volcanic rocks and is is subdivided subdivided into into three three groups: groups: to youngest. youngest. The Chocolay, Menominee and Baraga Groups, from oldest to The rocks rocks of of the the on older older Archean Archean basement. basement. Marquette Range Supergroup were unconformably deposited on are separated separated by by minor minor angular angular The three groups of the Marquette Range Supergroup are and informal informal units. units. The unconformities. The The groups groups are are subdivided subdivided into formal formations and The from the base upward, of conglomerate Chocolay Group Group consists, consists, from conglomerate (Enchantment (EnchantmentLake Lake Formation), quartzite quartzite (Mesnard (Mesnard Quartzite), Quartzite), dolomite dolomite (Kona (Kona Dolomite), Dolomite),and and slate slate (Wewe (Wewe Slate). Slate). The TheMenominee MenomineeGroup Groupunconformably unconformably overlies overlies the the Chocolay Chocolay Group Group and and consists, consists, from the base upwards, upwards, of local local conglomerate, conglomerate, quartzite quartzite (Ajibik (Ajibik Quartzite), Quartzite),slate slate (Siamo (Siamo Slate), and iron formation (Neguanee Iron IronFormation). Formation). The Baraga Group is is the the youngest youngest of of Menominee Group. Group. The the three groups and unconformably overlies the Menominee The Baraga Baraga Group Group includes a variety of stratigraphic units. In In the the Marquette Marquette area area itit consists, consists,from from the the base base upwards, of quartzite quartzite (Goodrich (Goodrich Quartzite), Quartzite), slate slate (Lower (Lower Slate Slate of the the Michigamme Formation), iron formation (Greenwood Iron hon Formation), Formation), Formation), mafic pyroclastics (Clarksburg (Clarksburg Volcanics), Volcanics), iron formation, formation, slate slate (Upper (Upper Slate Slate of the Michigamme Michigamme Formation), Formation),and and iron iron formation (Bijiki Iron Formation). hon Ironore orehas has been been mined mined from from iron formations within the Menominee and Baraga Groups. However, However, most most production production has has come come from from the the Negaunee Negaunee hon Formation Iron Formation of of the the Menominee Menominee Group, Group, including the Tilden and Empire Mines. Diabase Dikes Dikes and and Sills Sills stratified units of the Marquette The stratified Marquette Range Supergroup Supergroup are commonly commonly intruded intruded by Early Early the form form of of dikes dikes and and sills. sills. Early Proterozoic diabase (metamorphosed) in the Early Proterozoic Proterozoic diabase dikes and sills sills are particularly particularly common in vicinity of the Tilden and and Empire Empire Mines. Mines. Volcanics of of the the Baraga Baraga Group, Group, 10 to to 20 20 km km Diabase is also associated with the Clarksburg Volcanics 108 �% LAKE SUPERIOR srow or EXPLANATION EXPLANATION Paleozoic Paleozoic Undifferentiated Paleozoic Undifferentiated Paleozoic Rocks Rocks Keweenawan Keweenawan UndifferentiatedKeweenawan Keweenawan Rocks Rocks [----1 Undifferentiated Early Proterozoic Early_Proterozoic ji Undifferentiated UndifferentiitedEarly EarlyProterozoic ProterozoicRocks Rocks Archean Archean Undifferentiated Undifferentiated Archean Rocks Rocks I I Marquette iron Figure 1: Geologic setting of the Marquette Figure iron range. range 109 �Explanation Explanation Archean Proterozoic Proterozoic Middle Proterozoic Early Proterozoic Archean Metadiabase Undifferentiated Keweenawan sandstone ItLIMI Undifferentiated Keweenawan sandstone Metadlabase I and diabase diabase Marquette Range Supergroup Marquette Range Supergroup Baraga Group Group Baraga rc::.i Undifferentiated Undifferentiated Menominee Group Group Menornmee Negaunee Iron Negaunee Iron Formation Formation Siate I>'">I Siamo Slate Quartzite LL.Jj.JI Ajibik Quartzite Chocolay Group Group Chocolay Undifferentiated Undifferentiated 1++1 0 I !1 Generalized geology in vicinity Figure 2: Generalized vicinity of the Tilden Tilden and and Empire Empire Mines. Mines. 110 �west of the mines. Several Severalgeologists geologists have have suggested suggested that Early Proterozoic Proterozoic diabase diabase intrusions intrusions throughout the Marquette region are the same age as the Clarksburg Clarksburg Volcanics Volcanics of of younger than than the theNegaunee NegauneeIron hon Formation. Formation. However, some of the the Baraga Group and younger diabase sills at the Tilden and Empire Mines may have been lava flows (Scott (Scott and Lukey, Lukey, this volume). This Thisinterpretation suggestsmultiple ages of Early Proterozoic Proterozoic diabase diabase and interpretation suggests multiple ages of deposition of of iron iron formation. formation. The Hemlock Formation, mafic magmatism during the time of Formation, south of the Marquette area in the Iron hon Mountain area, consists of mafic volcanics that is (Morey, 1996). This supports correlative with the Menominee Group (Morey, supports the interpretation interpretation of mafic magmatism during the Menominee Group in the Marquette area. metamorphosed) UKC> dikes also {UUL LLLCLUWL~LLUXAI, uu cut LUL the rocks rocks diabas~(not Later Middle Proterozoic Proterozoic Keweenawan diabase and cut cut the the Negaunee Negaunee Iron hon Formation at the Tilden and of the Marquette Range Supergroup and Empire Empire Mines. Mines. Negaunee Fonnation Negaunee Iron Formation The Neguanee hon Iron Formation Formation consists consists of of aa variety of iron rich rocks in the Marquette Marquette area (Figure 2). The Themajor major types types of of iron iron formation formation are are carbonate iron-formation (iron carbonate carbonate and chert with minor magnetite), oxide iron formation (hematite or magnetite and chert), magnetite-banded iron formation (laminated magnetite and chert), hematite banded iron formation (laminated hematite and chert), silicate iron formation (iron silicate minerals formation minerals and types (Gair, (Gair, 1975). The TheNegaunee NegauneeIron hon Formation is chert) and combinations of these types described in detail by Scott and Lukey (this volume), Nordstrom (this volume), and Gair (1975). The Theorigin originof of the the iron iron minerals minerals in in the the Negaunee hon Iron Formation Formation isis aa complex complex processes. combination of primary sedimentary depositional, diagenetic, and metamorphic processes. The The maximum maximum thickness thickness of of the the Negaunee Negaunee Iron Formation is about about 1150 1150meters, meters, although although to assess assess (Gair, (Gair, 1975). 1975). The Negaunee hon folds and faults make thickness difficult to Iron Formation is thickest thickest in in vicinity vicinity of the Tilden and Empire Mines where is occurs occurs in in the the nose nose km across across (Figure (Figure2). 2). of a regional west plunging plunging syneline synclineabout about 66 km Age of the Marquette Range Supergroup Morey (1996) has provided a stratigraphic synthesis of the Marquette Range Supergroup. Supergroup. Supergroup is Although the age of the onset of sedimentation of the Marquette Range Supergroup 2,219+1-4Ma. Ma. The age uncertain, he has suggested that the Chocolay Group is younger than 2,219+/-4 age the Menominee Menominee Group Groupisis 1,910+/-10 1,910+/-10Ma. Ma. Deposition of the of volcanic rocks correlated with the Baraga Group Baraga Group ended endedby by1,852+1-6 1,852+/-6Ma. Ma. Depositional and and Tectonic Tectonic Setting Setting The Marquette Range Supergroup is interpreted to have been deposited in several stages (Morey, 1996; Schulz and and others, others, 1993). 1993). The Thesediments sedimentsof of the the Chocolay Chocolay Group Groupwere were deposited or intrarift intrarift stage stage (Figure (Figure3). 3). The sediments deposited during a late intracratonic or sediments of the Menominee Group were deposited on a passive continental margin in normal listric fault bounded rift basins (Cambray, 1991). The The Negaunee Negaunee hon Iron Formation Formation mined mined at at the the Tilden Tildenand and 111 �Late lntracratonic Intracratonic or Intrarift lntrarift Stage Stage Chocolay Group Archean Basement Basement Rift Stage Rift Stage Menominee Group Group Spreading Spreading Center Archean Basement Foredeep Foredeep Basin Basin Stage Stage Archean Basement Continent-arc Continent-arc Collision matic Oceanic Crust Figure Figure 3: Depositional Depositional and and tectonic tectonic setting of the Marquette Marquette Range Range Supergroup Supergroup (modified (modified from from Morey, Morey, 1996). 1996). 112 �Empire Mines was deposited deposited during this stage stage and some some faults faults at the mines may may be be related related to to this stage (Scott and Lukey, this volume). Elsewhere, Elsewhere, and and perhaps perhaps in in the Marquette Marquette area area too, too, tholeiitic basaltic basaltic magmatism also also accumulated in these rift basins. The Negaunee Negaunee Iron Iron Formation basin(s) are are located along the Great Great Lakes tectonic tectonic zone, zone, aa late late Formation depositional depositional basin(s) others, 1980). Passive margin deposition transitioned into Archean suture zone (Sims and others, deposition deposition of sediments sediments and and minor volcanic rocks in a foredeep foredeep basin stage stage that that formed formed as as aa result of Penokean collision collision of the Wisconsin Wisconsin magmatic arc with the edge edge of the the continent continent (Morey, 1996). The The continent-arc continent-arcsuture suture zone zone approximately approximately follows the MichiganWisconsin border. The TheBaraga BaragaGroup Groupwas was mostly mostly deposited deposited during during the foredeep foredeep stage stage as as turbdites. turbdites. The Marquette Range t h s t belt of the 1850 1850Ma Ma Penokean Penokean Range Supergroup Supergroup is is within the fold and thrust orogen (Sims, 1996). The The Tilden Tilden and and Empire Empire Mines are located on the south limb of the Marquette Synclinorium, an east-west trending, west plunging syncline that is about 6 km km across and 55 lun km long long parallel parallel to tothe thefold foldaxis. axis. The synclinorium, like likethe theNegaunee NegauneeIron hon Formation depositional basin(s), may have been controlled by the Archean Great Lakes tectonic zone. The Archean structures structures were subsequently folded during The iron iron formation formationand and Archean the Penokean Orogeny. The The Penokean Penokean orogenic orogenic event produced complex folds and faults and greenschist facies metamorphsim metamorphsim of the rocks at the Tilden and Empire Mines Acknowidegments Acknowldegments for comments comments on on aa version versionof ofthis thispaper. paper. Shannon Bair I thank Rod Johnson and Glenn Scott for and Chris Pascoe Pascoe helped helped with with drafting draftingof of the the figures. figures. References References Boyum, B.H., 1988, 1988, Marquette Marquette Mineral District of Michigan Mining History and Geology: Geology: Meeting, Institute on Lake Lake Superior Superior Geology Geology Proceedings, Proceedings, 34th 3 4 Annual Annual ~ Meeting, Marquette, Marquette, MI, MI, v. v. 34, part 2, p. B1-B33. Bl-B33. Cambray, F.W., Mancusco, J.J., J.J., and Slitor, W., 1991, 1991, Detachment faulting and the origin of asymmetric depositional depositional pattern of the Marquette trough (abstract): Institute on Lake Clalre, WI, v. 37, part 1, Superior Superior Geology Geology Proceedings, Proceedings, 337th7 Annual Annual ~ Meeting, Eau Claire, 1, p. 17-18. 17-18. Gair, J.E., J.E., 1975, 1975, Bedrock Bedrock geology and ore deposits of the Palmer Quadrangle, Quadrangle, Marquette Marquette County, Michigan: U.S. U S Geological Survey Professional Paper 769, l59p. 159p. Continental Margin Assemblage: Morey, G.B., 1996, Assemblage: 1996, Early Early Proterozoic Proterozoic Penokean Orogen - Continental U.S. U S . Geological Geological Survey Survey Professional Professional Paper Paper 1556, 1556,p. p. 30-44. 30-44. Sims, P.K., Card, K.D., Morey, G.B., and Peterman, Z.E., Z.E., 1980, The Great Lakes tectonic major crustal crustal structure structure in in central central North North America: Geological Society -A major Society of zone —A v.91, p.690-698. America Bulletin, v. 91, Pt. pt. 1, p. 690-698. Structure of the Continental Sims, P.K., 1996, Continental Margin: Sims, 1996,Early Early Proterozoic ProterozoicPenokean Penokean Orogen Orogen -- Structure U.S. p.44-51. U S . Geological Survey Professional Paper 1556, p. 44-51. 113 �GEOLOGIC GEOLOGIC FIELD TRIP TRIP TO THE THE TILDEN MINE Engineering Department, Tilden Mining Glenn W. Scott and Helene M. Lukey, Mine Engineering Mining Company, P.O. Box 2000, Ishpeniing, MI 49849 Ishpeming, Introduction Introduction This geologic primary structural and geologic field trip to the Tilden Mine is designed to show the primary stratigraphic features of the iron ore bodies and the implication implication of these geologic stratigraphic geologic features features in mine planning and ore control. The The interaction interaction of sedimentation sedimentation and and diagenesis diagenesis within within and supergene oxidation oxidation has has resulted resulted in in aa growth fault controlled basin, metamorphism and complex suite of ore types, each with specific, specific, if not entirely entirely objective, blending blending characteristics and problems. Although the mining operation tends toward characteristics Although the mining operation tends toward "brute "bruteforce", force", some of the blending concerns can be quite subtle and sensitive. sensitive. For the purpose of mine modeling, the deposit has been divided divided into into geologic geologic domains domains 3;Table Table1) 1)based based in in part part on on lithology lithology but, more importantly importantly on the (Figures 1,2 1 , 2and and3; bench tests tests and and in in the the processing processing plant plant (Table (Table 2). 2). The metallurgical response based on bench The primary p r i m w ore/waste orelwaste parameters are weight recovery (the percentage of each ton of ore that is turned into concentrate concentrate and pellets; i.e. the value of that ton of ore) and grade grade (does (does the the ore contain sufficient sufficient iron and silica silica grade). Other factors which can effect the plant operation and pellet quality are mineralogy (as related to total oxides and loss on ignition); trace chemical ignition); chemical composition (in particular phosphorous but also also Mn and and alkalis); alkalis); crude, magnetic and slime iron; and autogenous grinding feed. It should be noted that essentially all based on rather involved bench tests that may not directly these data are essentially directly reflect the plant response. response. Structural Geology Geology Ththis thisregion, region,the the Marquette Marquette Range Range Supergroup Supergroup is is in in aa narrow narrow east east west trending basin In basin known as the Marquette Synclinorium. This basin lies along the Great Lakes Tectonic This lies along originally defined by Sims and others (1980) as a Late Archean crustal scale Zone as originally suture with Superior Superior Province Province granite-greenstone granite-greenstone terrain to the north and older older Southern Southern Complex gneiss gneiss to the the south. south. The The northern northern margin displays a more or less complete complete stratigraphic stratigraphic succession from the base of the Chocolay Group through to the top of the Menominee Group. The Thesouthern southernmargin margin is is truncated; truncate& the the Negaunee Iron Formation Formation at Menominee group abuts directly against the high grade Archean Gneiss. Gneiss. the top of the Menominee This sharp sharp contact contact is is marked marked by outcrops outcrops of the Palmer Gneiss, Gneiss, described described as as a "comminuted, sericitised sericitised and partly silicified phase of the lower Precambrian gneiss" "comminuted, gneiss" use and (Van Hise andBayley, Bayley, 1897). 1897). since that time time has exposed a large face (Stop2) (Stop2) of the chioritic chloritic schists schists Mining activity since that form part of the Palmer Gneiss. ItIt displays displays mega-shear mega-shear bands, the spacing spacing between 114 �NORTH INTRUSIVE 4-- - 35 I.A±J -, — — -4' - HEI'l �F1 : H lN 1 rC it. 4. CDIII PIT MAGNETIU CD-Ill I 420 L?1AIN /3 1/ — a H 1420 / / H DOOIAIN vii Jat 2 7/ / (_ CROSS SECN' 'N iT I OOP$A1S1 V 2 zi IIAIH 'F 7- CN- N, L — 3 JF -' 'H' 1 (i11T"fH ,-_ -! - IZO F — — TILDEN MINE Figure 1-21.000 DATE: 1 3/11/99 " IDSnAINJ: v1 '' - .1 400 0 I = *ivwW C:: g - 000 I ) .( 1 — 20 - ' PIT F = 800 I 1000 C w OanoIa BoeIdorIts :: flT Me Tot �TILDEN HEMATITE NORTH-SOUTH CROSS-SECTION -13,100 -13,10( - TILDEN MAGNETITE MAGNETITE EAST-WEST CROSS-SECTION CROSS-SECTION -16,000 WES WE' — �________ _________ PIT PIT I LOOKING WEST WEST LOOKING NORTIOWEST DOMAIN (500) HANGINGWALL ZONE (470) L MAGP11TITI DOMAIN 1(420) x4 rooTwALi. ZONE Li ri (410) HEMATITh DOMAIN $NOWAU. CONTACT 20111 DOMAIN INTRUSOVE (200) (230)(240) I_____ L CLAY ZONE (350) HEMATITE - MARTIn DOMAIN (350) MAIN PIT CARIONATE IRON FOMIATIOII (340) FOOTWALL CLASTICS (480) FOOTWALI. IRON FOM4ATIOII DOMAIN — I (310) FOOTWAU. AND MANOINOWALI. (12Q(250) 200 400 600 800 L FIGURI 2 CDIII PIT PIT CDIII LT LnqKING NORTH NORTH LOOKING I -. - ORIGINAL GR0tMD r I CDIII HANGINGWALL AND FOOTWALL E-:J (230) (250) ,/ HEMATITE r' / (440) 1 INTRUSIVE (460) HAGNETITE (420) Figure 2 HANGING WALL ZONE (470) 0 200 I I 400 I 600 I 800 I FIGuRE 5 000 FT. 1000 F �North North S outh South coolly UT PIT .tt I FT NW UUVU 600 rT EAST PIT +1200 FT a DIAGRAMATIC STRATIGRAPHIC STRATIGRAPHIC SECTION SECTION OF OF TILDEN DIAGRAMATIC T I L D E N MINE MINE West Looking Lookin9 est - W Refer R e f e r to t o tcxbIe t ~ k d e11 Figure 33 �Mine. The Table 1: 1: Geologic domains domains and subdomains at the Tilden Mine. The domains domainsare arebased based on on lithology and metallurgical response. The numbers for each domain correspond to those lithology metallurgical The numbers for each domain correspond to those shown on Figures 1, 2, and 3 and are used as drill core codes and in modeling of the deposit 1,2, using MEDSYSTEM. MEDSYSTEM. NORTHWESTDOMAIN DOMAIN 500 NORTHWEST The Northwest Domain is stratigraphically above CD IIll/West I W e s t pit hanging wall metadiabase metadiabase (250) and below North Intrusive Intrusive (270); (270); this this domain domain includes includes numerous numerous dikes dikes and and one one mappable igneous igneous horizon, horizon, the the West Intrusive Intrusive (260). (260). 550 West Hematite 550 Hematitedomain, domain,restricted restrictedto tothe theFar Far West West Extension, Extension, is dominantly 40% and and variable variable but but elevated elevated hematite chert with mixed goethite, recoveries around 40% and metallurgical metallurgical phosphorous. Contact Contact with with 530 530 domain is defined by a thin intrusive and change. change. 530 (Hematite)-Goethite (Hematitel-Goethitedomain domainincludes includesflot flot"ore" "ore"(531) (531)and and WIF WIF (532). (532). 531 Plot Flot"ore" "ore"that thatisisdominantly dominantlygoethite-chert goethite-chert with with weight weight recovery recovery of of iron iron in in the the low 30 to -mid 40 40 %; %; variable, but generally generally high silica silica and and phosphorous. phosphorous. 532 WIF WIF"ore" "ore"isisoxidized oxidizedmartite/goethite; martitelgoethite;low low weight weight recovery, recovery,high high silica silica and and phosphorous; low heads indicate original iron formation may have been carbonate(?). heads formation have been carbonate(?). Sulfates Sulfates are are locally locally common common in in bench bench faces. faces. 520 Magnetite horizons. This Magnetitedomain domainisisdominantly dominantlymagnetite-carbonate magnetite-carbonate with silicate horizons. may be flot and/or andor mag ore in part depending on liberation. liberation. 510 Clastic horizon is at the contact with top of 5 10 Clastic horizon is at the contact with top of the the CDffl/West CDIIT/West pit pit hanging hanging wall (250) syncline.This This horizon horizon isis in in part part flot lot ore. in a local ('1) (?) syncline. ore. 400 CDIII-WEST CDIII-WESTPIT PITDOMAIN DOMAIN This domain is stratigraphically stratigraphically between CDIIJIWest C D W e s t pit hanging wall metadiabase metadiabase (250) (250) and and Thisdomain domainincludes includesnumerous numerous small small dikes dikes and and sills, sills, aa Keweenawan Keweenawan CDIII footwall (230). This dike and the West Pit Marker horizon (240). Footwallclastic clasticzone zonealong alongMain Mainpit pit footwall footwall (100). (100). Consists of dominant martite 480 Footwall clastics with coarse coarse quartzite/conglomerate quartzitelconglomerate and interbedded martite-hematite martite-hematitechert. chert. 470 Hanging Hangingwall wallzone zonealong alongbase baseofofCDIII/West CDIIIlWestpit pithanging hangingwall wallmetadiabase metadiabase(250). (250). WIF Defined as W F due to very fine grain size andlor andor oxidization. Dikedomain domainisisdefined definedasasaanortheast northeasttrending trendingzone zone of of chioritic chloritic dikes dikes and and 460 Dike associated oxidized associated oxidized and unoxidized iron formation. 450 South South Hematite Hematitedomain domainoccurs occursininthe thesouth southpart part of of CDffl CDIII and and the the West pit. It contains flot oreof of variable variablemetallurgy metallurgy and and WIF and is dominantly thin bedded, fine lot ore grained hematite-martite chert although some zones may be oxidized oxidized carbonate. carbonate. 451 Goethite Goethitezones zoneswithin withinhematite hematitedomain. domain.These Thesezones zonesare areassociated associatedwith with folding folding and faulting. A goethite zone along the footwall is present but not broken out as A goethite zone along the footwall as aa domain domain at at this this time. time. 452 Goethite Goethitezone zonealong alongCDIII CDIIIfootwall footwallsouth southof ofKeweenawan Keweenawan dike. This This zone zone typically typically has high slime slime Fe, Fe, may be oxidized oxidized Carbonate Carbonate (430) (430) domain domain along along intersection of dike and footwall. dike footwall. 118 �440 North Northhematite hematitedomain domainconsists consistsofoffine finegrained grainedoxidized oxidizedmartite-hematite martite-hematite chert with numerous dikes. The Theboundary boundary between between this this domain domain and the Magnetite Magnetite domain domain (420) trends northeast and dips steeply Plot ore in part. steeply south. Flot Carbonatedomain domainisiscarbonate carbonateflot flotore orewith withlow lowmagnetite magnetite content, high weight 430 Carbonate recovery and low concentrate grade. Fault Fault bounded bounded on on north and south south but apparently apparently gradational down dip to the west into magnetite domain (420). gradational (420). 420 Magnetite Magnetitedomain domainconsists consistsofofmagnetite-carbonate magnetite-carbonateand and magnetite magnetite silicate-chert silicate-chert with variable variable oxidation and grain size. size. This domain has relatively sharp sharp boundaries boundaries with with other domains. Domain Domaingenerally generallydefined defined by by magnetite magnetite content, content, not ore type, so so it contains potential flot contains flot ore. ore. 421 West Westpit pitmagnetite magnetitedomain domainisisananisolated(?) isolated(?)zone zoneof ofhigh highgrade grademagnetite magnetite in the west pit. It is defined defined by drilling drilling and and blast blast pattern pattern data. data. Footwallzone zoneisisdefined definedasasthe themagnetite-silicate magnetite-silicate horizon horizon at at the contact contact with the 410 Footwall CDIII Footwall metadiabase metadiabase (230). (230). It is typically waste or low grade grade due to low low magnetite content or poor liberation. content or poor liberation. MAIN PIT PIT DOMAIN DOMAIN 300 MAIN The Main Pit Pit Domain Domain contains contains iron iron formation formation units stratigraphically stratigraphically below the the CDffl CDIII footwall metadiabase (230) and/or the East pit hanging wall metadiabase (200). (200). This domain domain includes numerous includes numerous small small mafic mafic intrusives. intrusives. 370 Hanging Hangingwall wallcontact contactincludes includeszones zonesof oferratic erratic metallurgy metallurgy along along the base of the CDffl footwall CDIII footwall (230) (230) or or East East pit pit hangingwall hangingwall(200). (200). 360 Transition Transitionzone zonebetween betweenCDffl CDIIIfootwall footwall(230) (230)and and East East pit hanging wall (200). This zone consists consists of variably variably oxidized oxidized hematite hematite iron formation formation and mafic intrusives. intrusives. Restricted to north side side of East East pit. 350 Hematite-martite Hematite-martitedomain domainininEast Eastpit pitconsists consistsofofvarious varioustypes types of of martite martite chert. This domain includes includes horizons of magnetite-carbonate iron formation and thin dikes. Gradational transition over 20-50 feet to Carbonate iron formation (340). Gradational 340 Carbonate is is stratigraphically Carbonateiron ironformation formation stratigraphicallybelow belowthe thehematite-martite hematite-martitedomain domain It consists of martite-carbonate-chert with variable in East pit. (350) (350) East pit. consists martite-carbonate-chert variable magnetite/martitelFe silicate content. This magnetite1martiteFe This unit is defined by magnetic Fe, weight recovery and total oxides. ItIt has has lower lower weight recovery and higher concentrate grade than CDIII CDIII carbonates carbonates (430) (430) and and may may be be magnetite magnetite ore ore in in part. part. 330 Clay Clayzone zoneisisdefined definedasasthe thehorizons horizonsof of iron ironformation formationoutlined outlinedas as waste waste due due to to high high silica from montmorillonite (or other) interference. interference. This does not differentiate differentiate nonliberating hematite material. This zone may be stratigraphically stratigraphically controlled. controlled. Includes nonliberating Includes some some flot flot ore ore within within boundaries. boundaries. 320 East East pit pitclastics clasticsare aremixed mixedsiliceous siliceous clastics clastics and iron formation. This This unit unit includes includes oxide and carbonate horizons. ItIt may may be be the the stratigraphic stratigraphic equivalent equivalent of carbonate domain (340) down plunge to the west (7). (340) down plunge to the west (?). 321 Clay similar Clay zone zone in in clastics.is c1astics.i~ similarto to clay clay zone zone (330) (330) but in clastic clastic domain. domain. Footwalliron ironformation formationdomain domainconsists consistsofofvariably variablyoxidized oxidizedoxide oxide iron iron 310 Footwall by erratic metallurgy. formation and coarse clastics. This domain is typified by 311 31 1 Earthy Earthyfines finesare arehigh highgrade grade (>50 (>50 weight recovery recovev and >50 head Fe) oxidized zones zones are are controlled controlledby structures structures within within the the footwall footwall domain. domain. 119 �200 INTRUSIVE INTRUSIVEDOMAINS DOMAINS These domains are used for correlations of the iron formation domains, and structural trends. They appear to be conformable at the the scale scale of of the the ore orebody. body. These rocks are generally which vary vary from from diabasic diabasic to to porphyritic porphyritic interpreted as intrusives, they consist of mafic rocks which to aphanitic. All horizons appear to thin to the west and south. Contacts tend to be All horizons appear Contacts tend to be sheared sheared if and locally oxidized. Contact Contact metamorphism metamorphism of the iron formation is minimal and, if present, results in finer grained iron iron formation. Synclinal structures and intersections with Synclinal dikes have focused oxidation of the iron formation. formation. 270 North intrusive is a poorly defined North intrusive is a poorly definedhorizon horizonatatthe thetop topof of the the Northwest Northwest zone zone (500). (500). West intrusive 260 West intrusiveisisaapoorly poorlydefined definedbut but mappable mappable horizon within the Northwest zone zone (500). (500). CDIIIIWest pit 250 CDIII/West pithanging hangingwall wallisisaarelatively relativelyeasily easilymappable mappablehorizon horizon and and along along principle horizons for stratigraphic with the CDIII footwall footwall (230) (230) is one of the principle stratigraphic correlations between the CDffl correlations CDIII pit and the the Main Main pit. 240 West Westpit pitmarker markerisisa athin thinbut butcontinuous continuoushorizon horizon within within the the CDIII/West CDin/West pit stratigraphy (300). ItIt isis interpreted interpreted to to extend extend from from the the Foster Foster Lake Lake slot slot through through the the West West pit. 230 Top Topof ofthe theCDIII CDIIIfootwall footwalldefines definesthe thebase baseofofthe theCDIIIfWest CDIO/West pit pit domain domain (400) (400) while the base defines the top of the Main pit east domain (300). (300). 220 Chloritic Chloriticand anddiabase diabasedikes dikesand andthin thinsills sillsoccur occurininall alldomains. domains. This Thisunit unitincludes includes east-west trending 30+ foot thick Keweenawan Keweenawan dike in CDIII. 200 East Eastpit pithanging hangingwall wallisisseparated separatedfrom fromthe theCDffl CDIIIfootwall footwall (230) (230) by the Transition zone (360) iron formation. This Thisunit unit occurs occurs along along the the north north side side of of the the East East pit, pit, the the base base Main pit pit East East domain domain (300) (300) for for mining mining and and planning planning of this horizon marks the top of the Main purposes PIT FOOT WALL DOMAIN 100 MAIN MAIN PIT FOOTWALL 100 rocks which which are are separated separated from from the the iron iron This domain consists of Archean metamorphic rocks formation formation domains domains by an an east-west east-west trending, north-dipping north-dipping high angle angle fault. fault. 121 Chloritic Chloriticschist schistisisthe thedominant dominantfootwall footwallrock rocktype typeexposed exposed within within the the pit pit and and in in of the the CDIII CDffl footwall horizon horizon (230) within the drill holes. This rock may the extension of the fault zone. zone. 111 111 Granite Granitegneiss gneissoccurs occurssouth southofofthe thechloritic chloriticschist schist(121) (121)but but isisonly onlypoorly poorly exposed exposed in the pit. This Thisdomain domainhas hasnot not been been used used in in the the drill drill hole hole codes. codes. 999 OVERBURDEN OVERBURDENDOMAIN DOMAIN This domain domain consists consists of of Quaternary Quaternary overburden, overburden, rock rock fill fill and and broken broken bench bench material. material. 120 �Table Table 2: Glossary Glossary of terms terms and and abbreviations. abbreviations, Natural The amount of material recovered from the material fed Natural Weight Weight Recovery Recovery -- The fed into into the the concentrator In other words, words, it's the tons of concentrate concentrate made made (measured (measuredas as filter filter concentrator circuit. circuit. In cake) from tons of crude crude ore ore used (measured (measured by #3 belt scale). scale). Metallurgical Metallurgical Weight Weight Recovery Recovery (Met. (Met. Wt. Wt. Rec.) Rec.) - Calculated Calculated by comparing comparing the iron losses losses (as tailings) tailings) with the iron iron content content of the crude crude ore fed into into the concentrator concentrator circuit circuit (i.e., (i.e., the the head Fe). The formula formula used for for this this calculation calculation is called the iron balance balance formula, formula, or or sometimes sometimes called called the the concentration concentration formula. formula. (Head Fe - Tail Fe) Wt. Rec. = x 100 (Grade - Tail Fe) Grade Grade -- Also Also called the concentrate grade, is a chemical measurement (assay) of the total total iron oxide oxide of the concentrate. Iron oxide oxide is found in iron minerals such such as hematite hematite (Fe203), (Fe203), magnetite magnetite (Fe304), (Fe304),geothite geothite(Fe203 (Fez03** OH), OH), and iron iron carbonate carbonate(FeCO3). (FeC03). Concentrate Silica Grade Grade - The chemical Concentrate chemical measurement measurement (assay) (assay)of of the the % % SiO2 Si02in the the concentrate. concentrate. When When aalower lowerconcentrate concentratesilica silicagrade grade is is achieved, achieved, the losses losses in iron units (tailings) (tailings) increases. increases. content of the crude ore fed into the concentrator concentrator circuit. circuit, Grade -- The assayed iron content Head Grade concentrator's ability Iron Iron Recovery Recovery(Fe (FeRec.) Rec.) -- A calculation of the efficiency of the concentrator's ability to recover the iron available. This is calculated by comparing the Met. Wt. Rec., at some iron available. grade, with the head Fe of the crude crude ore. For example, (Met. Wt. Rec. x Grade % Fe) Grade % % = .................................. % Fe Rec, Rec. = (Head (Head % Fe) Percent Percent Magnetic Magnetic Iron Iron Recovered Recovered (% (% Mag. Fe Rec.) - The calculation calculation of the efficiency of recovering recovering the magnetic magnetic iron that was in the feed (crude ore). The Met. Wt. Rec., Rec., at at some some iron grade, is compared compared with the magnetic potential (i.e. (i.e. head) of the crude crude ore. For example, example, (Met. Wt. WI Rec. % Fe) Rec. xx Grade Grade % Rec = .................................. % Mag Fe Re (Head Mag % Fe) Tailings Tailings -- The The product lost in the process. Tailings always includes iron, because iron is always associated with many other minerals (silica, phosphate, carbonate, etc.). 121 �Flot (flotation) -- Flot ores are are the martite, martite, hematite, hematite, geothite geothite and and carbonates, carbonates that are are treated treated The final stage of the by selective chemical chemical processes to achieve Fe and silica grade. The magnetite process magnetite process is is flotation flotation to to achieve achieve target target silica silica grade. grade. to low low weight weight recovery recovery andlor and/or high WIF (waste iron formation) formation) -- hon Iron formation that due to silica cannot be treated in the plant to produce economic economic concentrate. concentrate.Rarely, Rarely, phosphorous phosphorous levels are too high to be treated. treated. The percent of the crude iron that that is concentrated concentrated in in the the Davis Davis Magnetic Magnetic Magnetic iron - The Tube Test (DMTT). (DMTT). %MagFe %MagFe == DMTT Wt. Rec. Rec. x DMTT Grade Grade However, in in the the Tilden Tilden ores ores an The assumption assumption is that all of this occurs as magnetite. However, appreciable and is is carried carried into the DMTT appreciable amount of hematite is up locked with the magnetite and Fe content by by 1-2% points points and concentrate. This tends to over estimate the magnetic Fe therefore therefore the over over estimate estimate weight weight recovery. Satmagan susceptibility and and is is the the Satmagan - The satmagan magnetic iron content is measured using susceptibility actual magnetite content content of of the the crude crude or or concentrate. concentrate. Domain - The The deposit deposit is divided into volumes of rock with similar similar metallurgical metallurgicalresponse. response. These are usually stratigraphic stratigraphichorizons horizons but may be fault fault bounded bounded or or nonconformable nonconfonnable alteration/oxidation alteratiodoxidation zones. The The domains domains are are the basis for for the economic economic and and planning planning models. models. 122 �CIstructures structuresisison onthe theorder orderofof2.5 2.5meters, meters,most mostcommonly commonlysuch suchstructures structuresare arehand hand Cl specimen specimentotomicroscopic microscopicininscale. scale.They Theyindicate indicatethe thepresence presenceofofa alarge, large,reverse reversedip dipslip, slip, ductile ductileshear shearzone zonewith withaaNNE NNEtotoSSW SSWtransport transportdirection. direction.The Thefact factthat thatthe theNegaunee Negaunee hon IronFormation Formationisisthrust thrustup upon ontotothe theolder olderArchean ArcheanGneiss Gneissindicates indicatesthat thatthere theremust musthave have been beenprior priorlocal localsubsidence subsidencehere heretotoform formthe thesedimentary sedimentarybasin, basin,aaclassic classicbasin basininversion inversion pattern pattern(Gillchrist (Gillchristand andothers, others,1987). 1987). Folds Foldsmeasured measuredwith withininthe theNegaunee NegauneeIron IronFormation Formationhave haveaxial axialsurfaces surfacesand andhinge hingelines lines that WNW,indicating indicatingaasimilar similarstrain strainenvironment environmenttotothat thatdetermined determined thattrend trendeast-west east-westtotoWNW, from the shear zone. The folding and shearing are thought to have occurred at from the shear zone. The folding and shearing are thought to have occurred atthe thesame same time. time. The Theiron ironformation formationisismostly mostlyaachemical chemicalor or biochemical biochemical deposit. deposit. The Theclastic clasticlenses lensesthat that in the south, thinning and occur occurininthe theformation formationare arethickest thickestand andmost mostnumerous numerous in the south, thinning and becoming becomingless lessfrequent frequentto tothe thenorth north (Breithart, (Breithart, 1983). 1983). The Thepattern patternsuggests suggestsan an asymmetric asymmetricrift riftbasin basinwith withaabreakaway breakawayfault faulton on the thesouthern southernmargin margin and andaarollover rolloveron onthe the north side. The principal source of clastic sediments would have been derived from the north side. The principal source of clastic sediments would have been derived from the southern southernmargin marginthat thatwas wasbeing being uplifted uplifted along along the the fault. fault. This Thisepisode episodeisisrelated relatedto tothe the development of a passive margin during the early Proterozoic. Subsequent compression development of a passive margin during the early Proterozoic. Subsequent compression during duringthe thePenokean PenokeanOrogeny Orogenyinverted invertedthe thebasin basinand andinitiated initiatedreverse reversefaulting faultingthat thatgave gave rise to the shear zone (Cambray, 1978). To the east, in the Harvey Quarry, a similar rise to the shear zone (Cambray, 1978). To the east, in the Harvey Quarry, a similar pattern patternof ofyounger youngerover overolder olderthrusting thrustingisisobserved observedsuggesting suggestingthat that this thisrelationship relationshipisis rift and later thrusting common commonalong alongthe theSynclinorium Synclinoriumand andconfirms confirmsthe theearly early rift and later thrustingpattern. pattern. Mining Mining Ore 50'x50'x45' Orereserve reserveand and long long range range mine mine planning is done in MEDSYSTEM using 5O'x50'x45' blocks based primarily on development drill holes which are on a nominal 300 blocks based primarily on development drill holes which are on a nominal 300foot foot spacing. spacing.Weekly Weeklyplans plansand anddaily dailyore orepredictions predictionsare arebased basedon onblast blastpattern patternsamples, samples, which which are areon on about about 90 90 foot foot centers. centers. Reconciliation Reconciliationof of ore, ore,waste wasteand and pellet pellet production production between betweenthe the'model" "model"and andactual actualproduction productionshows showsless lessthan than1% 1%difference. difference. The Theannual annualmining miningtask taskisisto tomove moveapproximately approximately38 38million million long longtons, tons, about about20 20million million long longtons tonsof ofore oreand and18 18million millionlong longtons tonsof ofstripping strippingwaste, waste,to to produce produce 7.8 7.8million millionlong long tons tonsof of flux fluxpellets pellets per per year. year. Production Productionsince since1974 1974has hasbeen been 126 126million million long longtons tonsof of pellets pellets from from355 355million million long long tons tons of of ore ore at at aa stripping stripping ratio of 0.94. As As of of January January1,1, 1999, 1999,the the proven proven and and probable probablelong longrange range (30 (30year) year) mine mine plan plan reserve reservewas was628 628million million long long tons tons of of ore ore containing containing233 233 million million long long tons tons of of pellets pellets at at aa stripping strippingratio ratio of of 0.77. 0.77. The P&H 2800 2800(38 (38 cubic cubic yard) yard) cable cable shovels, shovels, three three P&H P&H Theloading loadingfleet fleetconsists consistsof of two twoP&H 2100 2100 (l7cubic (17cubicyard) yard)cable cableshovels shovelsand andtwo two994 994(21 (21cubic cubicyard) yard)Caterpillar Caterpillarwheel wheel loaders. loaders. The The hauling haulingfleet fleetconsists consistsof of three three 793 793 (240 (240 ton) ton) Caterpillar Caterpillar trucks, trucks, seven seven 789 789 (190 (190 ton) ton) Caterpillar trucks and six Haulpack 630E (170 ton) trucks. Drilling is done with one trucks. Drilling is done with one Caterpillar trucks GD120 120drill. drill. P&H and and one oneGD P&H 123 �Field Trip Trip Stops Stops - . stops to be visited and their locations cannot he be determined determined until until iust just urior prior to The actual stoos the field trip. The generic stop descriptions are provided in this guide and the actual The generic stop descriptions are provided in this guide and the actual localities to be visited will be provided provided to participants on on the the day day of the the field field trip. trip. Stop 1 - Pit Service Building Before beginning the tour of the Tilden pit, the mine safety rules will be reviewed and and aa short video of the Tilden Tilden Mine Mine operation operation will be shown. shown. The The video, not not particularly particularly geologic, describes pelletizing processes that have describes the mining cycle, milling and pelletizing have aa major major impact on the ore quality determinations. determinations. Stop 2 -- Main Main Pit Footwall Shear Shear Zone Zone Chlorite schist defines the footwall of the Tilden ore body. body. This This chlorite chlorite schist schist is is described described in the literature literature (Van Hise and Bayley, 1897; 1897; Gair, 1975) 1975) as part of the Palmer Palmer Gneiss and marks the boundary between the Archean Compeau Creek gneiss gneiss of the the Southern complex and the early Proterozoic sediments. This This contact contact extends extends for for approximately seven miles east from the the Tilden Mine. Gair Gair and Simmons Simmons (1968) (1968) made madeaa complete complete study study of this rock and proposed three possible modes of origin: 1) 1)alteration alterationand and shearing shearing of Precambrian rock during during faulting; faulting; 2) migration of fluids fluids along along the contact contact between lower and middle Precambrian rocks; and 3) alteration of a regolith during lower Precambrian alteration regolith during folding. folding. At Atthe theTilden TildenMine, Mine,there thereappear appearto to be be two two varieties varieties of of gneiss, gneiss, aa southern southern sericitic phase related to a granitic protolith and a northern chloritic phase with a diabasic diabasic protolith. protolith. At this locality surface cut by the the more more locality one one can see see the dominant, dominant, steep steep north dipping dipping S surface gently north dipping Cl 4). The C/ surface (Figure 4). The intersection intersection between the S and Cl C/ plunges plunges WNW. The gently to the WNW. The movement movement plane plane on the shear shear zone is at right angles to this intersection and the intersection of this plane and the boundary of the shear zone (determined from drill records) gives the direction of movement (marked with a filled (determined filled triangle on Figure 5). The Thepattern patternof of the the SS surfaces surfaces and and shear shear bands bands give the sense sense of movement (Figure 4). Chemical Chemical analyses analyses indicate that the protolith for the shear zone was most likely to have been one of the diabase bodies found interlayered with the banded iron formation. formation. (-60') foliation foliation in the chlorite schist creates a bench and slope slope The steep steepnorth northdipping dipping(—60°) stability concern. This 45° final wall slope with 35 Thisisis controlled controlled by by triple triple benching to a 45" 35 foot catchers. Proper Properclean cleanup up along alongthe the toe toe line line is is critical critical to avoid "undercutting" the the foliation. foliation. 124 �West 2.5 Shear Zone (Palmer IPaImrGness) GneW Figure Figure 4: 4: Footwall Footwallshear shearzone, zone,Tilden TildenMine. Mine. Pole Poleto to mean mean C-foliation C'-foliation S * U Pole poleto tomean meanS-foliation ~-fo~atim Slip direction Mean ~ e a S-C S-C. n intersection intersection S-C intersections S-foliafion plane Top of shear zone ' .C-foliation C'-foliation plane p4ane "Nonnal intersection \ N o m d totoS-C' S-C/intersection Figure5: 5: Stereo Stereo projection projection of of shear shearzone zonestructures. structures, Figure 125 �Stop 3 -- Martite and Main Pit Pit Carbonate CarbonateDomains Domains The Martite and main pit Carbonate Carbonatedomains domains lie lie stratigraphically stratigraphically below below the the CDffl CDEI Footwall and Main Pit Hangingwall intrusives. This This is is metallurgically metallurgically "good "good ore" ore" and and isis typically characterized characterized by high weight recovery (35-45+%), (35-45÷%), low phosphorous, low slime slime iron and good grinding media. With Withcrude crudeFe Fe around around 35%, 35%, iron iron recovery recovev averages averages over over 70%. The The'best' 'best'ore oreisiscentered centeredon onthe theMain MainPit Pitanticline anticlineand andconsists consistsof of thick thick bedded bedded (2(24 cm) chert with variable variable martite martite and magnetite magnetite plus subordinate subordinatecarbonates carbonatesand and silicates. silicates. The boundary between the Martite and Carbonate domains is based on higher magnetite Carbonate domains higher magnetite content; content; the Carbonate Carbonate domain domain contains contains low calculated calculated total oxides oxides that that may may be be due due to to iron silicates. silicates. The Thevariation variationin inmagnetite magnetitecontent content may may reflect reflect either either the the environment environment of of deposition deposition or or diagenesis; diagenesis;itit is is unlikely unlikely to be due due to supergene supergene effects. This domain comprises reserve. The mine plan comprises about 56% of the ore reserve. The~mine plan attempts attempts to to control control the blend percentages to about 50% of this domain. This This has has been been only only partially partially successful due to pellet quality concerns when blending with other types of ore. The successful planned final final depth depth of of the the Main Main pit pit is is about about sea sea level level so so the the pit will will be be about about 1000 1000feet feet deeper. deeper. As the base of the iron formation is not exposed at the Tilden Mine, the stratigraphic stratigraphic thickness is unknown. The Thethickness thickness exceeds exceeds 1000 1000feet feet and and is probably underlain by and is gradational into undifferentiated clastics and iron formation formation of the Empire Empire Mine. "Intrusive" Igneous Horizons Stop 4 - Mafic "Intrusive" formation and the rapid Due to the lack of clear stratigraphic markers within the iron formation rapid facies changes in the iron formation, several igneous horizons are used for stratigraphic stratigraphic and structural correlation correlation(Figure (Figure 1). 1). The term 'intrusive' 'intrusive' or or'dike' 'dike'isisused usedfor forgreen greenand andgray-green gray-green mafic mafic igneous igneous rocks, which vary from (semi) (semi) conformable sill-like sill-like horizons to obviously crosscutting crosscutting bodies. bodies. Since the fabric varies from diabasic diabasic to porphyritic to aphanitic, the conformable conformable bodies bodies are usually interpreted as synsedimentary sills sills but but may maybe beflows. flows. The sills range in thickness thickness from a few tens of feet to at least 600 600 feet with the thicker horizons horizons thinning thinning to to the south. Dikes, typically 55 to to 20 20 feet feetthick. thick. The Dikes,obviously obviously crosscutting crosscutting bodies, are typically dikes may be feeders. feeders. The dikes and thinner parts of the sills are commonly altered altered to chlorite and carbonate retain primary primary igneous igneoustextures. textures. Contacts cabonate while the thicker parts retain Contacts with the iron formation are variably sheared and altered. the iron formation are variably sheared and altered. The principal principal sills sills are are the the Main pit pit hangingwall, hangingwall, CDffl CDEI footwall footwall and the CDffl CDEI hangingwall. hangingwall. The Thesills sillsand andmajority majorityof of the the dikes dikes appear appear to be of early Proterozoic Proterozoic age but, there are several dikes that are interpreted to be of Keweenawan age based on the and fracture fracture zones. zones. The igneous event magnetic signature. Dikes Dikes commonly fill faults and Clarksburg, that emplaced emplaced the sills sills and majority of the dikes may be related to the Clarksburg, Hemlock and Emperor Volcanics. 126 �Dilution from dikes, especially along shallowly dipping contacts, is the most common internal waste. waste. kcipient type of internal Incipientrecrystallization recrystallization due due to to contact contact metamorphism metamo~hismand hangingwall. quartz overgrowing magnetite results in high silica waste along the CDIII hangingwall, Stop 5 - West West Pit/CDIJI Pit/CDZZZ Hematite Hematite Domain The West Pit/CDffl PivCDIII Hematite Hematite domain domain in in the the west pit is is stratigraphically stratigraphicallybetween between the the CDffl hangingwall and footwall. This domain extends from CDffl along the north limb CDIII This domain extends CDIII pit anticline. In contrast to the martite domain, this is and over the crest of the Main pit metallurgically 'poor' ore orewith with higher higher slime slime iron, iron, phosphorous and poor grinding. Fe), due to the the high crude Fe Although weight recoveries are relatively good (about 36% Fe), below 60%. 60%. Mineralogically of 40%, the iron in this domain is below Mineralogically and texturally, this domain differs from the martite domain in being dominantly platey hematite with thin (mm scale) chert laminae. Presumably, the bedding thickness reflects the original sedimentary sedimentary features but it is unclear as to the nature of the oxidation event that produced the hematite hematite (related (related to to deposition, deposition,diagenesis diagenesisor or supergene supergenealteration). alteration). With a thickness thickness of about about 600 600 feet, this domain contains contains 27% of the ore reserve but, due due to blending concerns for phosphorous, slimes or grinding, it is often difficult to maintain phosphorous, maintain that percentage percentage in in the the blend blend going going into into the the processing processing plant. plant. Stop 66 -- Magnetite Domain The Magnetite PitJCDffl hematite hematite Magnetite domain domain is the stratigraphically equivalent of the West PivCDIII but is essentially essentially restricted to the CDffl CDIII syncline (basin?). Magnetite Magnetiteore orequality qualityisis determined (magnetic determined by the percentage of the crude iron that occurs as magnetic iron (mapetic concentrate determined by the Davis Magnetic Magnetic Tube TubeTest). Test). On average, the Magnetite domain contains 27% magnetic iron with 37% total total crude crude iron. iron. With about 90% magnetic magnetic iron recovery recovery in the plant, weight recoveries are 34-36% iron. Mineralogically, Mineralogically, the ore consists primarily of magnetite-siderite-chert with variable hematite and silicates. consists primarily silicates. This This mineralogy, along along with the thin laminations, laminations, appears to indicate a restricted basin and a reduced environment environment of deposition. deposition. respect to to the There is some scientific scientific controversy as to the role of diagenesis with respect hematite and and siderite. siderite. Within the Magnetite domain relationship between magnetite, hematite there is a faultldike-bounded faulvdike-bounded zone of low magnetite siderite ore; it is uncertain whether this is the protolith of the magnetite ore. The The southern southern extension of the CDffl CDIII magnetite into the hematite domain contains the highest highest grades grades (-30 (-30 magnetic iron). The The sharp sharp contact, not structural, structural, indicates indicates that this is some sort of redox front but it is unknown unknown whether it is is of of diagenetic diageneticor or supergene supergene origin. origin. Approximately reserves are are in in magnetite. magnetite. Development of the Approximately 23% of the Tilden ore reserves of pushbacks pushbacks along along the thewest westwall wallininthe theintrusive. intrusive. The The wall is reserve involves a series of planned planned to move move back about about 800 800 feet and the pit is to be he deepened about 600 feet. 127 �Acknowledgments Acknowledgments We thank Bill Cambray Cambray and Cheryl Webster for their contribution of the Structural Stmctural Geology section in the Introduction and structural structural discussion in Stop Stop 2. References References Cambray, F. W., 1978, 1978, Plate Tectonics as a model for the environment environment of deposition deposition and and deformation Northern Michigan: Michigan: deformation of the the early early Proterozoic Proterozoic (Precambrian X) of Northern Geological 7, p. 376. 376. Geological Society Society of America Abstracts with Programs, Programs, v.10, no. 7, Breithart, M. S., S., 1983, 1983,Significance Significanceof of the the distribution distribution of of clastic clastic lenses lenses within within the the Breithart, Negaunee Iron Formation Formation at the eastern end of the of the Palmer Basin, Marquette Marquette Synclinorium, Synclinorium, Northern Michigan: Masters Masters thesis, Michigan Michigan State State University. University. Gair, J.E., J.E., 1975, 1975,Bedrock Bedrock geology geology and ore deposits deposits of the Palmer Palmer Quadrangle, Quadrangle, Marquette Marquette County, Michigan: U.S. U S . Geological Geological Survey Survey Professional Professional Paper 769, lS9p. 159p. Gair, J. E. and and G. G. C. C. Simmons, Simmons, 1968, 1968,Palmer Palmer Gneiss Gneiss -- an an example of retrograde retrograde metamorphism metamorphism along along and and unconformity: U.S. US. Geological Geological Survey Survey Professional Professional Paper, Paper, 600-D, 600-D, p. D186D186- D194. D194. Gillchrist, Gillchrist, Ralph, Ralph, Mike Mike Coward Coward and and Jean-Louis Mugnier, Mugnier, 1987, 1987,Structural Structural inversion inversionand and its controls: controls: examples examples from from the the French French Alpine foreland foreland and and the the French French Alps: Alps: Geodinamica Acta (Paris) v.1, no. 1, p. p.5-34. 5-34. Sims, P.K., Card, K.D., Morey, G.B., and Peterman, Z.E., 1980, The Great Lakes tectonic zone A major crustal cmstal structure structure in central North America: Geological Society of zone -- A America bulletin, V. V.91, 91, Pt. pt. 1, 1, P.690-698. p. 690-698. Van Hise, C.R., and C.R., and Bayley, W.W., 1897, 1897, The Marquette iron-bearing district of Michigan: Michigan: U.S. U.S. Geological GeologicalSurvey Survey Monograph Monograph 28, 2 8 ,608p. 608~. 128 �Geologic Field Field Trip Trip to the Empire Empire Mine Mine , M. Nordstrom, Nordstrom, Empire Empire hon IronMining MiningPartnership, Partnership,P.O. P.O. Box Box 38, 38, Palmer, Palmer, MI MI 49871 49871 Paul M. Introduction Introduction Empire mine commenced operations in 1963; 1963; over the past thirty-five years the operation The Empire has produced nearly 200 200 million long tons of pelletized iron concentrates. The TheEmpire EmpireMine Mine represents repr&ents a partnership partnekhip of steel making m G n g and iron iion mining interests and is partially oowned h e d and operated by the Cleveland-Cliffs Iron hon Company. The Thereserve reserveis is about about 600 600 million million long long tons tons of magnetite magnetite ore. ore. The Thecurrent currentannual annualproduction productionisis8.4 8.4million million long longtons tonsof of pellets pelletsfrom from27.5 27.5 million long tons of ore. Han hon Han (1975) (1975)provides provides aa detailed description of the Negaunee Iron Formation at at the the Empire Empire Mine. Mine. Stratigraphic hon Formation Stratigraphicrelationships relationships within the Negaunee Iron Formation in in the the vicinity vicinity of of the the Empire Empire Mine Mine are: are: • Upper UpperUndifferentiated UndifferentiatedSeries Series(+ (+1500'): 1500'):intermixed intermixedsequence sequenceof ofcarbonate, carbonate,silicate, silicate, clastic clastic and "lean" "lean" carbonate carbonateassemblages. assemblages. • Clastic Clastic horizon (200-300'); (200-300'); graywacke graywackeand and feldspathic feldspathicquartzite quartzite interbedded interbedded with with chert, iron carbonate carbonate minerals minerals and and magnetite. magnetite. • Carbonate Carbonate horizon horizon (350-500'): (350-500'): alternating alternating bands bands of of magnetite-chert magnetite-chert and and chert, chert, with with siderite siderite or or ankerite ankerite distributed distributedthroughout. throughout. Local, Local, minor minor beds beds of of riebeckite riebeckiteand and aegerine-augite aegerine-augite are present contact. present near near the the upper upper contact. .. • Silicate Silicate horizon horizon (400-500'): alternating alternating laminae laminae of of magnetite, magnetite, carbonate, carbonate, greenish greenish microstilpnomelane), and chert. micro- to to sub-microscopic sub-microscopiciron iron silicate silicateminerals minerals(principally (principally stilpnomelane), and chert. - . • Lower h w e r Undifferentiated undkferentiated Series Series (700-1500'): (700-1500'): intermixed intermixid sequence sequence of carbonate, carbonate, silicate silicate minerals and and clastics clastics (clastics (clasticsincrease increasetoward toward base base of of interval). internal). The The basal basal contact contact of of the the Lower Lower Series Series is is transitional with the underlying Siamo Slate. The The Siamo Slate is represented in the mine area by quartz-arkose and graywacke. Siamo Slate is represented in the mine area graywacke. A A geologic geologicmap map and and section sectionof of the theEmpire EmpireMine Mine(Figure (Figure11and and 2) 2) illustrate illustratethe therelationships relationships between between the the various variouslithologic lithologicunits. units. hon Ironformation formationatatthe theEmpire EmpireMine Mineisistransected transectedby by aa number number of of near-vertical, near-vertical, chioritized chloritized metadiabase are metadiabase dikes. dikes.The Thedikes dikesoccupy occupyfaults, faults,believed believed to to be be of of early earlyProterozoic Proterozoicage, age,which which are probably related related to to basin basin development. development. The Thedikes dikeshave havehad hadaasignificant significantlocalized localizedimpact impact on on ore ore probably processes. processes. The Thefine finegrain grainsize sizeofofmagnetite magnetite(5 (5p.) p)in in iron iron formation formationadjacent adjacentto tothe thedikes dikes represents represents aa "selvage" "selvage" which which was was apparently apparently impervious impewious to subsequent subsequent processes that influenced influenced growth of of the the magnetite magnetite grains grains in in other portions of the iron formation. Effective Effectiveliberation liberationof of growth magnetite in in these these"fine-gralned" "fine-grained"rocks rockscan canrarely rarelybe beachieved. achieved. magnetite The iron iron content content of of the the iron iron formation formation at at the Empire Empire mine is fairly uniform, about 32-35%. The The The amount amount and and grain-size grain-size of of magnetite magnetite determines determines ore quality. Ore-forming Ore-formingprocesses processescontributed contributed and remobilized remobilizediron ironby bydiagenetic diageneticor orlow-grade low-grademetamorphic metamorphicreactions reactionsof of recrystallization recrystallizationand and and replacement. Oxidation Oxidationofofsiderite sideritehas hasresulted resultedininlarge-scale large-scalereplacement replacementby by magnetite, magnetite,but but replacement. 129 �N N N N P1010ED 04/05/99 Formation Iron Fine—Grained 2000 Horizon Carbonate UOji.OWtiOJ Iron UOJI Oxidized PdZlplXn Formation Horizon Ctastics Series Undifferentiated Upper Siamo 1 Figure Series Undifferentiated Lower Horizon Silicate \ Dike Diabase Keweenawan SiLt/Dike Mc-taciiobase - LEGEND GEOLOGIC —- '9tfV.K 's.-.) ' Eevation) Bench (+940 —22000 ---r- ç—1j . - \(iJ l — -. GEOLOGY MINE EMPIRE —18000 (7bt -- 20 t 'tion _J 'A' -- —14000 Resource IV CD —10000 Cu 0 0 0 Li 00 0 nJ o o o Li Li EIIJ ��of the the magnetite magnetite in in these these enriched enriched chert layers are also replaced by magnetite; the grain size of zones is coarser than magnetite in zones not effected by replacement. coarser magnetite zones effected replacement. the Empire Empire Mine. Mine. NonSecondary oxidation has a profound impact on ore quality at the Non- or or weakly weakly characteristic of of oxidized oxidized zones zones are are typically magnetic iron minerals (e.g., martite, hematite) characteristic rejected during primary magnetic separation. Incipient Incipient or or partial partial oxidation oxidation of of magnetite magnetitegrains grains then in flotation flotation stages. may cause them also to be rejected, if not in magnetic separation, then extent of of oxidation oxidation and and predict predict its its impact impact on on plant plant However, it is often difficult to recognize the extent recovery. The Theoxidation oxidationisisthought thoughtto to be be largely, largely,ifif not not entirely, entirely, the the result result of of supergene supergene processes. processes. Circulation Circulationof ofthe theoxidizing oxidizingfluids fluidswas wascontrolled controlled by by faults, faults,dike dikecontacts contactsand andclastic clastic beds and lenses. However, the boundaries of oxidized zones are very irregular and difficult to However, the boundaries of oxidized define. define. all Empire Empire iron iron ores, ores, including including the the Due to the very fine grain size of magnetite in virtually all carbonate ores, extremely carbonate extremely fine fine grinding grinding (93-95% (93-95% passing 500 500 mesh) is is required required to to achieve achieve sufficient sufficient liberation of magnetite for product specifications. Process Process control control is is dominated dominated by by the the specification for Si02 Si02 in the final product. product. Smaller Smalleraverage average grain grain size size of magnetite magnetite in in silicatesilicateliberation. In type ores means that they must be ground finer to achieve acceptable liberation. In general, general,the the becomes significantly significantly more finer grain size also implies increased work index so that grinding becomes costly. costly. Field Trip Trip Stops Stops be determined determined until untiljust just prior prior to the field The actual stops to be visited and their locations cannot be trip. The Thegeneric genericstop stopdescriptions descriptionsare areprovided provided in in this this guide guideand and the the actual actual localities localities to to be be visited will be provided to to participants participants on on the the day day of the the field field trip. trip. Stop Overlook Stop 11 -- Dispatch Overlook Mining development pit, with with significant contributions contributions from development at Empire has focused on the Main pit, satellite satellite pits to the north north (CD-I), (CD-I), east east (Section (Section 20) 20) and and west (Southwest (Southwest Extension), Extension),and and expanding expanding development development to the northwest (CD-V). The The bottom bottom bench bench of of the the Main Main pit pit is is approximately pit will will ultimately ultimately be be developed approximately 1,000 1,000 feet below the dispatch overlook, and the pit 750 feet below the present mining mining level. level. entirely in in the the Carbonate Carbonate and and Silicate Silicate The bottom bench of the Main pit is developed almost entirely horizons. horizons. The The iron iron formation formation in in this this area area strikes strikes about about N35°E N35"E and and dips dips 30-35° 30-35' to to the the northwest. northwest. Stop 2 -- First Class Carbonate Carbonate The Carbonate i) Carbonate unit is gray to dark gray, relatively coarse-grained (magnetite grain size: 20-30 p) and characterized characterized by "thick" (5-25 mm), indistinct magnetite-rich and chert-rich layers with subordinate The term term "carbonate" "carbonate" is is utilized utilized at at the the Empire Empire Mine Mine to to subordinate siderite and ankerite. The c 10) 10) with very good liberation characteristics (low concentrate of indicate soft ores (work index < Si02). These high-grade ores ores have havebeen beenthe the"life-blood" life-blood' of Si02). of Empire's Empire's production. production. 132 �Stop 33 -- First Class Silicate Class Silicate Silicate iron ironformation formationconsists consists thin laminae (2 mm) of magnetite (minor tominor very minor of of thin laminae (2 mm) of magnetite (minor to very amounts greenish or or brownish-gray brownish-gray mixtures mixtures of of amounts of silicate, carbonate, chert, and clastics) and greenish silicate minerals, siderite, magnetite, chert and clastics. The The silicate silicate minerals minerals are are dominantly dominantly stilpnomelane stilpnomelane and and minnesotaite. The TheSilicate Silicatehorizon horizoncommonly commonlyhas has aa greater greaterabundance abundanceof of carbonate p) is finer carbonate minerals minerals than the Carbonate Carbonate zone. The Thegrain grainsize sizeof of the the magnetite magnetite(5-20 (5-20 L') than in the Carbonate-type. The term "silicate" at the Empire Mine is applied to ores Carbonate-type. The term "silicate" at the Empire Mine is applied to oreswith withwork work index> 10and andliberation liberationcharacteristics characteristicsthat thatare areinferior inferiorto to carbonate carbonateores; ores; silicate silicate or or yield yield index >10 higher higher concentrate concentrate of of Si02. Si02. Stop4Stop 4 -CD-V CD-V Future development in two areas: the the "pillar" "pillar" between the development of the mine will be principally focused in Main Main pit and and the the Section Section 20 20 pit pit (west (west wall), wall), and and the the north north and and northwest northwest areas areas of of the the Main Main pit pit termed CD-V. CD-V. Development Developmentof ofthe thewest westwall wallincludes includesrelocation relocation of of the the existing existing rail rail lines linesaround around the the east side side of the Section Section 20 pit; the "pillar" area will then be excavated to expose ores in the wall and, and, more more importantly, importantly,below below the the bottom bottom of of the the present present pit. pit. The The CD-V CD-V pit pit is is being being developed developed in in the the Upper Upper Series Series horizon, horizon, characterized characterizedprincipally principallyby by carbonate-type ores and clastic bands and lenses. Much of the yellowish carbonate material carbonate-type Much of the yellowish carbonate material contains contains very very little little magnetite magnetite ("lean" ("lean" carbonate), carbonate),consisting consistingdominantly dominantlyof of siderite sideriteand andchert. chert. However, very very rich rich carbonate carbonate ores ores are are also also present present in in the the push-back of of the the Main Main pit pit wall. wall. However, Current Current development development is is just below below aa large large diabase diabase sill sill that that is is 200-400 200-400 feet feet thick. thick. Several Several of of these these large, generally generally conformable, conformable, tabular tabular diabase diabase bodies bodies intrude intrude the the Negaunee Negaunee Iron Iron Formation Formation in in this this large, area of the mine. Predictably, Predictably, "fine-grained" "fine-grainer effects effectsare are extensive extensiveadjacent adjacentto to these these large largeigneous igneous bodies. bodies. In In the the CD-V CD-V area areaof of the themine, mine, iron ironformation formationand and interlayered interlayeredsills sillshave have been been folded foldedinto intoseveral several smallsmall- to moderate-scale moderate-scale anticlinal anticlinal and synclinal features. Fold Fold axes axes plunge plunge 10-20° 10-20' toward toward the the northwest. northwest. Bounding BoundingCD-V CD-V to tothe thenorth northisisaalarge largearea areaunderlain underlain by by magnetite-bearing magnetite-bearingiron iron formation formationthat, that, because because of of its its extreme extremehardness hardness (work (workindex index18-20) 18-20)and and poor poor liberating liberatingqualities, qualities,isispresently presently regarded regarded as as aa resource, resource, not not aa reserve. reserve. The Thenature natureof ofthe themetallurgical metallurgicalproblems problemsin in processing processing these these ores ores isis under under investigation investigationby by Cleveland ClevelandCliffs' Cliffs'research researchgroup. group. Stop 5- CD-I CD-I CD-I has has been been an an important importantsource sourceof of very veryhigh high quality quality(high (high weight weight recovery, recovery,low lowconcentrate concentrate Si02)carbonate carbonatematerial materialfor forblending blendingwith with lower lowerquality qualityores ores from from other otherlocations locationsin in the themine. mine. Si02) Ironformation formationininCD-I CD-Istrikes strikesnorth-south north-southand anddips dipsabout about30° 30' to to the the west. west. Two TwoKeweenawan Keweenawan hon age (1.1 (1.1 Ga) Ga) diabase diabase dikes dikes cross the pit in a westerly to southwesterly direction. These two two dikes dikes age direction. These are are coarser-grained coarser-grainedand and fresher fresherin in appearance appearancethan than the the other, other, older older early early Proterozoic Proterozoic diabase diabase dikes. dikes. 133 �The current phase of pit development development in CD-I will be completed in 1999. High High quality qualitycarbonate carbonate but its potential potential development development is is hindered hindered by by the the large large ore extends to the west beneath the pit wall but amount of stripping required. About About 750 750feet feet of of overlying overlying waste waste rock rock would would have have to to be be mined mined in in order to access access the ore. ore. References References Han, Tsu-Ming, 1975, Petrology of of Iron-Formation hon-Formation at the Empire 1975, Lithology, Stratigraphy, and Petrology Deposits of of the the Palmer Palmer Quadrangle, Mine, in Gair, Jacob E., Bedrock Geology and Ore Deposits Marquette Marquette County, County, Michigan, Michigan, U.S.G.S. U.S.G.S. Prof. Prof. Paper Paper 769, 769, pp. pp. 76-106. 76-106. 134 �Field Trip Trip 4 and Glacial Geology Geology from from Au Au Train Train to Paleozoic and Grand Marais, Grand Marais, Michigan Leaders: and J. J. Anderton Anderton Leaders: R.S. Regis and �______ ______ ____ ______ _____0144 ____On ______________________________________ ___________ ____________________________ ________ Pt(ISIOCENE NOMENCLATURE SYSTEM ERA a S z STAGE SERIES RRCMI U w_ i QUATERNARY kRISTOCINR EM STRATIGRAPHIC IN MICHIGAN MICHIGAN STFUTIGRAPHIC SUCCESSION SUCCESSION IN PPAUOZOIC ~ Z C Mmmi U THkOl &E4T 8 m . •• — I________ I__________ ——— DNI 3 DNR CSD I OUTCROP NOMENClATURE SUSSURFAa NOMtNCLATURE IECWW SCPtI11CJTOfflThI*tItSOOflS IWF 0. L GROIOGIC ITIMnTRAIIGAAPHFCI TIME F &LF4UFU& ORUET 0IT FORMATION MEMBER - R T44 kpl, — •.s — -19- .4- - coaflflI INFOPJI&AI. TERMS •fl 9 942 —fl — —d — — I....a — 4.4 04.44 •— •Afl •SC4MAL l1LU SILATGEAWIIC poslifli 4- 09,4 —. I-—. — 4-1 1, 1=- — ii 0._a. Rh 4.. b.,,Cs0L4 04442* 021 442 Tt 1Ja' ...u_o ._0fl4 — .4 S. — Os.— -.4-S— — ____._....__O4 4 ca hød5.S.. 44-,— .1 4-2 *4 —_____ - w.—9 89- -, — '4— 02S4 4I Go 04 SR. _•4_ — 0— O'la EXPLANATION C- — — — -' I flICMdfl p—,.— %%,— 4- 0-4.4 — waaflø fl.,O —. —, — — — —. a — .- 4d 4—— —, — # . . . — —I 42 --- CHART CHART 11 1964 ¶964 2071 em �ILSG Field Field Tdp Trip ##4from 1999 ZLSG 4 -Paleozoic Paleozoic and andPleistocene Pleistocene Geology Geologyfrom Au Train Train to Grand Grand Marais, Michigan Road log log Overview Ovewiew - - Au Train Train channel Profile of Au Stop 11 stop -- Au Au Train Train Falls Falls Stop stop 22 - - Beach Beach ridges and and Stop stop 33 -- Au Train-Whitefish Train-Whitefish Channel Channel Grand Grand Island Overlook Overlook Stop stop 44 -- Stop stop 55 -- Stop 6 -- Stop 77 stop -- Break Stop 88 stop -- Stop 9 stop Stop 10 stop 10 Stop 11 stop 11 ---- Stop 12 stop 12 -- US4 US41I S. (view to east at 2.3-3 ml.) mi.) continue 16.2 ml to M94, F. then E. 16.2 mi 18.2 mi. ml. to Forest Forest Lk. Rd. Rd. Return to Forest Lk. Rd. N 7.4 mi. to M28 M28 F. 2.5 mi. to H58, then to M28 E. Washington St. St. (1.2 mi.), to Munising Munising Falls (0.55 (0.55 mi.) mi.) Munising Munising Falls Falls Return to H58 (0.55 mi.), E to Miners Castle Rd (3.7 mi.), then N to Miners Castle Castle (4.9 mi.) mi.) Miners Miners Castle Castle Return to H58, continue continue F E to to Bear Bear Trap Inn 9 (3.6 mi.), mi.), keep keep left left and and continue on H58 (end of pavement another 4.3 mi.) to Kingston Corner Comer (11.2 (1 I .2 mi. beyond pavement end) Kingston Kingston Outwash Outwash Plains Plains N on H58 to 2-track on left (7.1 mi.) Proceed on 2 track 0.1 mi. & park park mi. & Walk due W 400 yds (strenuous) (strenuous) Turtle Return to H58,2.1 H58, 2.1 mi. to dropoff Turtle Lake Kettle into channel, @ 6.7 mi. is edge of upper outwash terrace, @ @ 8.7 mi. turn left on Log Slide Slide Rd.. Continue Continue to Log Slide Slide (0.75 ml.) mi.) Picnic area area @ Grand Sable @ NW side Grand Sable Lk. Log Slide Slide Return to to H58. H58. FE on on H58 H58 to to parking lot for Sable Creek falls and Ghost forest (5.6 mi.). Ghost Ghost Forest Sable Sable Falls Falls To Grand Marais (1.7 (1.7 mi.) Grand Harbor Grand Marais Harbor Nipissing Bluff E-W M77 to M28 (22.6 E-W channels channels crossed crossed by by M77 M77 (22.6 mi.) To Munising (22.4 mi.) To Marquette (44 mi.) 137 �Detailed Detailed Road Road Log. Log. of Au Au Train Train Whitefish Channel Overview of From the WashingtonfBaraga Washingtonmaraga St. St. intersection in downtown Marquette travel S on baraga St St US41). Along (turns into US4l). Along U.S. U S . 41 41 at at about about 2-3 2-3 miles miles south south of Marquette, a view to the east reveals a profile of channel cut by proglacial meltwater that flowed eastward along the margin of the Marquette (Grand Marais) ice. ice. The the Onota The feature feature was referred to by Hughes (1971) as the outlier. Water Waterspilled spilledinto intothe theAu Au Train TrainWhitefish Whitefish channel channel southward southward to to (present) (present) Lake Lake Michigan after passing through this channel. Continue on U.S. 41 south 16.2 miles to M94, turn east. Continue 16.2 Travel to 29.9 29.9 mi. to to Chatham, Chatham, turn turn SS at at stopsign, stopsign, travel travel to 30.9 30.9 mi and and turn FE on on M94, M94, continue continue to 34.4 34.4 mi and and turn N N on on Forest Forest lake lake rd., travel 0.1 mi to dirt road on right, follow to to gated gated parking area. Walk Walkto to falls. falls. Au Train Train Formation Formationand andAu AuTrain-Whitefish Train-WhitefishChannel, Channel,Beach Beachridge ridgecomplex complex Return to Forest Forest lake lake Rd., Rd., turn N. Follow Follow Forest Forest Lake Rd. At 6.3 miles and all the way to M28, note the beach ridge complex. Turn TurnEE on on M28 M28 and and travel travel to to the Grand Island overlook parking lot on the N side side of M28. M28. Grand Island Island Overlook Overlook From the Grand Island overlook, return return to M-28 and and travel travel east east 2.5 2.5 miles milesto toMunising. Munising. Turn on Dogpatch restaurant restaurant is on Ge the right at H-58 after passing the Holiday gas station on the left (the Dogpatch the intersection...excellent vittles! In In aa rustic, Li'l Abner Abner setting). setting). Continueon on H-58 H-58 an an additional additional intersection ...excellent vittles! . Continue 0.55 miles to to Washington Washington St., St., turn turn left left and and proceed to Munising Munising Falls Falls visitors visitors center center parking lot lot (0.3 miles). miles). Munising Falls Return to H-58 from the parking lot and and proceed proceed north north 3.7 3.7miles milesto toMiners MinersCastle CastleRoad. Road. Turn left. Travel Travel 4.9 4.9 miles miles to to the the Miners Miners Castle Castle parking parking lot. Miners Castle turn east (left). (left). Travel 5.9 miles to the Bear Trap Inn, veer left to to Return to H-58 (4.9 miles) and turn stay on H-58. End End of of pavement pavement 4.4 4.4 miles miles from the Bear Trap Inn. Continue Continueto to Kingston Kingston Corner Comer junction (11.2 (1 1.2 miles). Kingston Plain Travel north on H-58 7.1 miles to the small 2-track road on the left. Proceed Proceed about about50 50yards yardson on the road and park. Walk Walk about about 55 minutes minutes due due west (this (this walk may be strenuous for some). We We will remain at this stop for about ½ hour to allow time to walk to the bottom of the kettle. stop ?h NOTE: At 2.1 miles miles drop-off drop-off onto ontosurface surfaceB B (Hughes). (Hughes). NOTE: At 6.7 miles miles steep steephill hill marking markingedge edge of of surface surface C. C. Turtle Lake Return to H-58 and continue north. north. Travel 8.75 miles to the the Log Log Slide SlideRoad. Road. Turn north. Travel Travel 0.75 miles to the the Log h g Slide Slide parking lot. 139 �Log Slide Slide Return to H-58. Proceed Proceed east. east. NOTE: At 3.7 miles, the Grand Grand Sable Sable dunes dunes are areon onthe theleft left(north). (north). At At the the intersection intersection of of Newburg and William Hill roads, turn north on William Hill road and travel 0.7 miles to the Sable Falls parking lot. Walk Walkto tothe theGhost Ghostforest forest then then to to Sable Sable Falls. Falls. Ghost Forest and and Sable Sable falls Return to parking parking lot lot and and travel travel to to Grand Grand Marais Marais (east). (east), harbor Grand Marais harbor Follow M77 to M28, M28, turn turn e, e, follow follow to to US41, US41, N N to to Marquette. Marquette. Meitwater Meltwater channels channels Observe the outwash channels along M-77 appearance M-77 as as we we proceed proceed south. south. The "terrace"-like appearance of the topography is due due to M77 M77 crossing crossing proglacial meltwater meltwater terraces at right angles. angles. Streamfiow was to the east. Note Streamflow Note the the gentle gentle southward southward slope slope of individual individual terraces, marked by drained, swampy swampy areas areas near near their their southern southernedge. edge. poorly drained, 98 miles to Marquette. Sleep! Sleep! Grand Marais to M-28 M-28 south south on on M-77 M-77 West on M-28 M-28 to to U.S. U.S. 41 41 North on U.S. 41 to to Marquette Marquette 140 �—— — o — .r HIAWATHA InKEe - sera I. HP a Cenle 33 . FOREST 1 C —;-__--_---- NATIONAL -p /t t jIIu —H- -, _\_ 1 / LftlIaReavat - _ — . S -H ogon '° I-vanM -' t9 Ø4jfrurcShk . . -- a I I C C •— Ct-p I ft jt" rLi /' (\ "]' ' U\ In SI q Visitot Information Center a.0suadUtFI 0eSS— Mlnerecautle Stop 5 ta ° :1* diiiL ayeH ml - 4:) - r..r° ra TSI — tC Pu,u4LS.u.Hur '55d F PISM urea. LAKESUPERIOR UP Huu.p.flntS M,nuIee Falls 6*8.0 W5.0. wtt MUNISING HRHO 32*3)4* A re,pr'..d rt°t:d unttt _ _____ 3 C — ' 1 t / ( . C '>' - S I I C a - IA -- SUPERIOR C C -. S . STATE '\' A.a.uPuH . - . .- . FOREST '. .u Stop 9 -- s__s I 23 --— . j. ___1iS I. ¼ H- - -- I C / SIeP.IIS o 0LE' Ø1 "C-\' Log SI' ;r . S'.. ,CCk . SENEY NATIONAL WILDUFE REFUIE H LAKE r-t. ( 4' Hurricane River Stop 8 ___________________________________ jSop7 / 4)-Stop6 '— 'a CO&1 IWulvurulle Beach _____ — -- - STATEFOREST uurwPa.Heef - A 3 Sane 3 _eSdue lAKE SUPERIOR EuThHuth — -C. .H 5-' top 11 nterStop 12 Grand Sable GRAND t3ais rfiIrnCTMUSeUnt _______ �and Pleistocene Geology Geologyfrom fromAu AuTrain Trainto toGrand Grand Marais, Marais, Michigan Michigan Paleozoic and (FIELD TRIP #4) (FIELD TRIP#4) by Robert S. Regis and John B. B. Anderton Anderton Northern Michigan University University The geological history of the Pictured Rocks National Lakeshore (PRNL) (PRNL) is limited limited to to intervals of geologic two intervals geologic time; time; the Pleistocene Pleistocene epoch epoch when glaciers reworked reworked and and mantled mantled the the underlying underlying bedrock bedrock with with aa nearly nearly continuous continuous veneer veneer of drift, drift, and the the late late Proterozoic, Proterozoic,Cambrian Cambrian and early Ordovician periods when sediments deposited in shallow seas became the the sandstones sandstones that form the "pictured rocks" section section of of the the lakeshore. lakeshore. Late the Central Upper Late ProterozoiclCambrian/Ordovician Proterozoic/Cambrian/OrdovicianHistory of the Peninsula Peninsula including including PRNL PRNL Hamblin (1958), who who performed performed a Most material for this section was compiled from Hamblin detailed study of the Cambrian rocks outcropping in the PRNL and elsewhere elsewhere in northern Michigan. Michigan. The The Paleozoic Paleozoicgeologic geologic history history of of the the PRNL PRNL is is best described described as as aa period period of of eroding eroding highlands highlands and sedimentary sedimentary deposition deposition into into transgressing transgressing empeiric empeiric seas. seas. The The Northern Northern Michigan Michigan Highlands Highlands and the the Wisconsin Wisconsin Arch Arch stood stood as as eroding erodingremnants remnants of of former former highlands highlandsand and mountains, trending east-west across northern Michigan and more north-south across Wisconsin, trending east-west across northern Michigan and more north-south across Wisconsin. respectively. Encroaching respectively. Encroachingshallow shallowseas seas(the (theSauk Sauk transgression) transgression) during during the late Proterozoic Proterozoic and and throughout the Cambrian were depositional journey's end for of eroded sediments. the journey's most the sediments. Bedrock Bedrock is is best best exposed exposed in in the the pictured pictured rock rock section section where where bluffs bluffs rise rise up up to to 200' 200' above above short Lake Superior and extend seventeen miles from Munising to to the the Beaver Beaver Basin. Basin. For aa short distance distance inland from from the the escarpment, escarpment, usually usually no more than several several hundred hundred yards, bedrock may may occasional outcroppings. outcroppings. Elsewhere Elsewherein in the the National National Lakeshore Lakeshore bedrock is is only only found found be seen in occasional in the vicinity of the the Grand Grand Sable Sable bank, where where it forms a low bluff around around the the north north and and east east side side of Au Sable Point, Point, and and in in the the gorge gorge at at Sable Sable Falls. On this field trip, we will observe the rock rock formations formations in in reverse reverse order. order. The first rocks we will see are the youngest youngest exposed in the the park (western (western end) and last rocks we see see are are the oldest (eastern end). The TheJacobsville Jacobsvillesandstone, sandstone,of of questionable questionablelate-Proterozoic late-Proterozoicto to lower lower and and age, is is the the oldest oldest formation formation in in the the lakeshore lakeshore (more (more than 520,000,000 520,000,000years years old). old). middle Cambrian age, It is a feldspar-rich, quartz quartz sandstone, sandstone, deep deep red in color color with white mottlings. Although Although the the Jacobsville has a thickness thickness of of 1100 1100feet, feet, only only the top several several feet rise above above lake level, level, and and only only Jacobsville Onthis thistrip, trip,we we will will see see the the Jacobsville Jacobsville at at Sable Sable Falls Falls (Stop (Stop 10). 10). in the eastern part of the park. On The early-to-middle early-to-middle Cambrian-age Cambrian-age Munising- formation underlies most of the the lakeshore, lakeshore, mostof and forms most of the significant bedrock outcroppings in in the the park. park. It lies unconformably on top of the Jacobsville Jacobsville sandstone. The TheMunising MunisingFormation Formation isis divided divided into into two two members, members, the the lower lower and 5) 5) members members (500 (500 -Chapel Rock Rock (Stops (Stops 10 10and and 5) 5) and and the the upper upper Miners Miners Castle Castle (Stops (Stops 44 and 520,000,000 520,000,000 years old). old). Except Except where where the friable Miners Castle member is being rapidly rapidly undermined by wave action it forms slopes rather than cliffs. undermined forms slopes rather cliffs. In the western half of the "pictured rocks" the late Cambrian Au Train formation formation cropscropsout only along the top edge of the cliff (480 The Au Au Train Train is is aa light light along edge (480 500,000,000 500,000,000 years old). The brown or white, hard, dolomitic dolomitic sandstone sandstone usually highly present, it forms forms a highly glauconitic. glauconitic. Where present, - 142 �cap cap rock rock on on the the weaker weaker Miners Miners Castle Castle member. We Wewill will observe observethe the"type "type locality" locality" of ofthe theAu Au Train Trainformation formationatatAu AuTrain TrainFalls Falls(stop (stop1)1)on onthis thisfield fieldtrip. trip. All All formations formationsdip diptoward towardthe thesouth southaadegree degreeor ortwo twoas as shown shownin in the thephoto photohere here(view (viewisis toward toward the the east). east). All Allthe theformations formationsalso alsoarise ariseeastward eastwardso sothat thatthe theAu Au Train Trainformation formationisismissing missing east of of Miners MinersCastle castlebut but east the the Jacobsville, Jacobsville,which whichisis below below lake lake level levelat atMiners Miners Castle, Castle,carries carriesabove abovelake lake level. East Eastof ofMiners Miners level. beach, it is well exposed beach, it is well exposed beneath beneath the the conglomerate conglomerateof of ChapelRock Rockand andin inthe the Chapel gorgeat atSable SableFalls. Falls. gorge Thereare aremany many There waterfalls along along the the waterfalls escarpment escarpmentformed formed mostly mostly by the the Au Au Train Trainformation, formation, by but all all are arefed fedby by small small but streamswith with small small streams watersheds and and are are only only watersheds imposingin in the the spring springand and imposing during wet wet periods. periods. during MunisingFalls Falls(stop (stop4) 4) isis Munising view View along along the thePictured PicturedRocks Rocksshowing showing the the Munisiug Munisine formation formation dipping dipping to to the the the most most unusual unusual and and the south south at at aalow lowangle. angle. interesting of the the falls. falls. ItIt isis interesting - of an an extremely extremelywell-developed well-developedcap-rock cap-rockwaterfall waterfall with withthe the Au Au train train Formation Formation forming forming aa shelf shelf that that overhangs overhangs the the Miners Miners Castle Castle member member by by 25 25 to to 30 30 feet feet permitting observers observers to walk behind behind the the torrent torrent which which falls falls from fromthe the center center of of the the overhang overhang as as aa concentrated concentrated stream stream and then then descends descends another another 30 30 to to 40 40 feet feet in in aa series series of cascades. AArecent recent collapse collapseof of aa section section of this formation formation onto onto the walkway prompted Park officials to close a part of the walkway. Some well developed the walkway officials of the walkway. Some potholes potholes may may be seen seen in the cataract section. Miners MinersFalls Falls has has developed developed where where the Miners Miners River River drops from the upper to the broad lower valley. The fall is vertical and about 30 feet high. drops from the upper to the broad lower valley. The fall is vertical and about 30 feet high. Chapel Chapel Falls Falls isis fed fed by by aa tributary tributary to to Chapel Chapel Lake Lake that courses down the valley slope where there is no cap rock. The fall is a long cascade, about The fall is a long cascade, about 90 90 feet feet high that broadens fan-like from ten feet at the top top to to 30 30 feet feet at at the the bottom. bottom. Bridalveil Bridalveil Falls Falls and Spray Spray Falls are formed where streams streams fall, fall, about 90 feet, directly into Lake Superior, adding greatly to the beauty of the Pictured Rocks. At about 90 feet, directly into Lake Superior, of At Sable Sable Falls, Falls, the the stream stream draining draining Grand Grand Sable Sable Lake Lake cut into the Chapel Rock and Jacobsville Jacobsville formations formations where where they they lie lie beneath beneath sand sand dunes. dunes. The Thedeeply deeplywooded woodedsetting settingisisexceptionally exceptionally beautiful beautiful and and is is but but aa short shortwalk walk from from the the lakeshore lakeshore and and the the parking lot. The The Pictured Pictured Rock Rock escarpment escarpmenthas has been been carved, carved, by frost action and wave erosion erosion of the the present present lake, lake, as as well well as as higher higher stages, stages,into into aa variety variety of shore shore cliff features features such as: stacks, stacks, caves, caves, sea arches, and promontories. These Thesefeatures features have have been named: named: Lovers LoversLeap, Leap,Rainbow RainbowCave, Cave, Grand Grand Portal, Portal, Miners Miners Castle, Castle, Chapel Chapel Rock, Rock, The The Battleships, Battleships, Flower Vase and Indian Drum Cave. Cave. Most Most of of the the features features can can be be reached reached by hiking but only Miners Castle is accessible accessible by automobile. The The best way to appreciate appreciate the cliffs cliffs is by boat tour. Increased Increased accessibility accessibilityby by land land will will not not diminish diminish the the advantage advantageof of viewing viewingthe thecliffs cliffsfrom fromthe thewater. water. 143 �Late Quaternary QuaternaryHistory Historyof of the the Central CentralUpper UpperPeninula Peninulaincluding includingPRNL PRNL The Great Lakes region experienced experienced repeated glaciations glaciations and concommittant concommittant inundation inundation The the Quaternary. Quaternary. For by proglacial lakes throughout the For the the purposes purposes of of this fieldtrip, the final major (i.e. 11,800 BP) BP) marks marks the the beginning beginning of of deglaciation of the area during post-Twocreekan times (i.e. is most most relevant. relevant. Between 11,800 the portion of Late Quaternary history that is 11,800and 11,500 11,500 the Superior covered the the Upper Peninsula (Farrand and Drexler 1985). Superior Ice Lobe nearly completely covered 1985). By about 11,500 11,500 BP, retreating retreating glacial ice, followed followed closely by the high stands stands of Glacial Lake Lake Futyma, 1981). 1981). The retreat of ice continued, Algonquin, had nearly left the region (Larsen, 1987; Futyrna, opening up large portions of the Lake Superior basin in what is large portions the Lake Superior is known as as Glacial Glacial Lake Lake Duluth Duluth (Phases A and B), allowing allowing discharge discharge from from Glacial Glacial Agassiz during during Post Algonquin Algonquin times times (ca. (ca. 10,700 BP); however, evidence of this period was largely obliterated by a final readvance of ice into the Lake Superior Superior basin. The Theglacial glacialreadvance readvance is is known known as as the the Marquette Marquette advance advance (Drexler (Drexler et al. 1983). 1983). Shoreline features created by these earlier lake stages in the Lake Superior Superior basin were Shoreline largely destroyed during during the Marquette Marquette Advance (Hughes, 1978; Drexler, 1981; 1981; Farrand and and Drexler, 1985). 1985). By Byabout about9800 9800BP, BP, the theice icehad had begun begun to to retreat retreat again, again, resulting in a stepwise stepwise drop in lake lake levels levels which initiated initiated the the Post Post Duluth Duluth Phase Phase lakes lakes (Farrand (Farrand and and Drexler Drexler 1985). 1985). After about 9,800 BP, Marquette Advance ice had retreated far enough to the north to 9,800 Marquette Advance ice north to allow allow lake lake water from the western part part of Superior Superior basin to drain eastward and southward, southward, eventually eventually However, catastrophic catastrophic discharge discharge creating the Main Minong Phase (Farrand and Dexler, 1985). However, from Lake Agassiz early in the Nipigon Phase is thought to have breached a till barrier at at the the east east end of the Lake Superior Superior basin, basin, causing causing aa decline decline in Lake Minong resulting in at least least five five or or six six Teller and and Thorleifson, Thorleifson, 1983). The The post-Minong levels levels in the Superior Superior basin (Farrand, 1960; Teller lowering was finally finally stopped stopped as resistant resistant bedrock at the Sault Sault outlet outlet was reached, reached, culminating culminatingat at the Houghton Houghton Low around around 8000 8000 BP, BP, the the lowest lowest lake lake phase in the Superior Superiorbasin. basin. Finally, Finally, as as the the ice left the south south shore shore of of the the basin, basin, Glacial Glacial Lake Lake Minong formed formed along the margins margins of the the retreating glacier reaching its maximum expression by about 9500 BP; however, shortly reaching expression 9500 shortly afterwards afterwards the retreating retreating ice ice front front in the eastern eastern Great Lakes revealed the isostatically depressed depressed known as as the the Houghton HoughtonLow. Low. During this North Bay outlet, resulting in falling lake levels known period, lake lake levels levels in the the Superior Superiorbasin dropped to their their all all time time lowest lowest elevation, elevation, which which may may their valley valley channels. channels. By about 8000 BP the bedrock have allowed many rivers to deeply incise their sill at the Sault Sault acted acted to to halt halt falling falling lake lake levels levels in in the the Lake Superior Superior basin; however, however, by then isostatic isostatic uplift had had acted acted to to raise raise the the elevation elevation of of the the North Bay outlet, outlet, resulting resulting in rising rising lake lake levels in the southern southern basins. Eventually, Eventually, the the rising rising lake lake levels levels reached the elevation elevation of the Sault Sault Outlet Outlet and and lake lake levels levels in the Superior Superior basin began to gain in elevation. During During this this period, period, known as as the the "Nipissing "Nipissing Great Lakes," water in the basins basins of Lake Superior Superior and Lakes Huron and Michigan joined together to create create one one huge lake, lake, which which was controlled controlled by the North Bay outlet. Notwithstanding, Notwithstanding, the the North North Bay Bay outlet outlet continued continued to to rise rise and and eventually eventually lower lower drainages drainages to to the the south became active active commencing commencing the three-outlet (Chicago, (Chicago, Port Huron, and North Bay) phase of Lake Nipissing (Nipissing (Nipissing I), I),which reached its maximum elevation between 4700 and 4000 BP BP (Hansel et al. 1985). 1985). During Duringthe the Nipissing Nipissing Maximum, Maximum, flooding flooding of many tributary river valleys embayments along the coastal areas of the Upper Peninsula (Anderton 1993a). created large embayments 1993a). The Nipissing II IIphase, marking a slight drop in levels, was attained about 4000 years BP as continued continued isostatic isostatic uplift uplift raised the North Bay outlet above the altitude of the two southern southern II phase, lake-levels began to outlets (Hansel et al. 1985, 1985, Larsen 1985b). Following Following the the Nipissing II w �fall as as downcutting downcuttingprogressively progressively lowered lowered the Port Huron outlet and water volumes fluctuated fluctuated in in the the basins. basins. AAshort shortpeak peakininlake-levels lake-levelsatatabout about3200 3200BP BPresulted resulted in in the the Algoma Algoma phase, phase, but but by by about 2200 2200 BP BP lake-levels lake-levels in in the the Lake Lake Michigan-Huron basin had fallen below the elevation elevation of of the the rebounding rebounding sill sill at at the theSault, Sault,separating separatingthe the Superior Superiorbasin basin from from the the lower lowerGreat Great Lakes Lakesand and initiating 1960). Modern Modem lake-levels lake-levels in in both the the Lake Superior Superior and Lake initiating the Sault Sault level (Farrand 1960). Michigan-Huron Michigan-Huronbasins basinswere werefinally finallyattained attainedby byabout about2000 2000years yearsago. ago. Larsen Larsen (1985), (1985), working working in the the southern southern portions of the Lake Michigan-Huron basins, has has also also proposed an an alternative alternativemodel model of of Holocene Holocene lake-level lake-level fluctuations, fluctuations, which suggests suggests aa complex record of climate-related changes that lasted between 200 and 300 years complex record of climate-related changes that lasted between 200 and 300 years each each and and fluctuated fluctuated with with an an amplitude amplitudeof of 11to to 22 meters above the apparent mean lake level. In In the the last last 7000 7000 years at least least eight eight episodes episodes of high water periods separated by intervening low water periods are are recognized recognized (Larsen (Larsen 1985). 1985). periods Stop 1. Au Au Train TrainFormation Formationat atAu Au Train TrainFalls Falls At At Au Au Train Train Falls, Falls, aa 30 30 meter meter thick thick section section of the 100 100 meter thick early Ordovician Ordovician Au Train Formation (aka Trempealeau Formation) is exposed. exposed. The The formation consists consists of 2 parts, the upper and lower members. At Atthis thisstop, stop,we we will will examine examine the the lower lower section section at the lower lower falls, falls, and walk to to an overview overview of of the the upper upper section, section, which is is exposed about 200 200 meters upstream, upstream, but is is relatively inaccessible. inaccessible. The TheAu Au Train TrainFormation Formation lies lies unconformably unconformably upon the Munising Formation, Formation, and and is is representative representative of of the the continued continued Sauk Sauk sea sea marine marine transgression transgression that that began began in in the the late late Proterozoic. The environment of The environment of depositionwas was aa shallow shallow shelf shelf deposition platform platform with with fluctuating fluctuatingsea sea levels. levels. Paleogeographically, Paleogeographically,the theregion region was was equatorial with with the the equator equatortrending trending equatorial nearly north-south north-south compared compared to to nearly modern modem orientation. orientation. Note the the step-like step-like character character of falls (also (also in in the the upper upperfalls) falls) the falls created by the alternating alternating layers layers of of dolomite, dolomite, shale, shale, and and sandstone sandstoneof of varying varyingdegrees degrees of of resistance. Near areaaseries seriesof ofthin thinbeds bedsof of glauconitic glauconiticsandstone sandstone Nearthe thebase baseofofthe thelower lowerfalls fallsare (hydrous (hydrous silicate silicate of iron iron and potassium) potassium) and shale. Glauconite Glauconiteisisdisseminated disseminatedas asflakes flakes throughout the sandstone. sandstone.Glauconite Glauconite comprises comprises as much as 35% of some beds in the lower lower Individual beds Individual beds vary vary section and imparts a speckled green to dark green color to the formation. section and imparts a speckled green to dark green color to the formation. from pure sandstone sandstone to pure dolomite (Hamblin, 1958). Dolomitic Dolomitic beds are are bluish gray in color. An intraformational conglomerate(?) with pebbles of sandstone and dolomitic intraformational conglomerate(?) dolomitic sandstone sandstone are are found in a few of the units near the base of of the section (see (see cross cross section sectionon onnext nextpage). page). At the base of the lower lower Au Au Train, Train, the the contact contact with the underlying Miners Castle member of the Munising Formation is easy to recognize because the friable sandstone is quite quite different in Munising appearance from the hard, resistant dolomitic sandstone sandstone of the Au Train. Fossils are rare in the formations. Following deposition of the Au Train Au Train but become abundant in later formations. Formation was was aa minor minor regression regression that that formed formed an an unconformity. unconformity. 145 �eq a. a Cl) I- a a Cl) �Footage Footage Section section - top Upper Train rep of of ~ m Au AU s~ r r i nr7.11. a n Sandy grayi.h..blu. to to dark 4-10 Inch*. inches [hick. thick, some M d y dolomite, dtilmnitq, grayi#h-blu. dark blue, h h à §b.ds bed# 4-10 miu interbedded buff fin., ttoo mediurn.grained 1 iwft finemedium-grained sandstone Â¥andçtounits. ualte, .everal ¥¥vr horizons contain contain abundant • '/. 120 120 Description Detcription of of lithology lithology ,1auconite and and a uthl'enic pyrite authigenic pyrite Iglauconit. Olagco.ddc Gl.ueonldc . Sand.tone bed6""thick, S u d i t o i r bed thick, buff buff to to light gray, medium- to flne-grain.d, fine-grxincd, well wellrounded rounded well sorted and well sorted Sand.tone bed, same above S a d i t o n e bad, #Â¥mas above 110 110 Sandstonebed, bed, mame same ,a.iabove Suidçton above Ito 110 Authigenic pyrite filling vu ge Pure dolomite I::.: • 90 90 - :1 Highly glauconitic Authigenic pyrite filling wg. 80 80 70 70 - Sandstoneb ibed thick,finefine-totomedium-(rained, medium-grained,buff bufftotolight light gny, gray, well rounded thick, rounded Skndatonà d 8"8" sad well a dw e l l sorted sorted _ Abundant tuthigenic pyrite -: • - 60 60 Highly glauconitic of Uppç Upper An Au Train Train FP41. 1Base !am ef dln interysi (Covered :overed interval - 50. 50 40 40 - betweenUpper Upperaud andLowr LowerAu AuTrain Train fills Falls between -2 and" dolomite thick,generally generally Sandy dolomit,to todolomitic dolomiticsanditone, sandstone,buff bufftotobluish-#ray, bluieh-gray,bçd beds2"-t" 2"-" thick, separated by thin thin !.me* lenses of of silt silt or c p r t t e d by or clay, clay, bedding bedding planes plane* are ere undulatory, undulatory, percent of sand sand b e ' à § r lthroughout throughout à the section, #action, glauconite gluuconit. extremely extremelyabundant abundant mmpeclaily vane, the especially near the base Glauconità Glauconit. and clay galls Crest of of 1.ow.r lower Au Train Train Falls Crçs Fall. 30 30 Olauconit. concentrated along several horizons, sandy units ezhitat email-scale croci bedding, thin shale lense. separate unite 20 20 10 10 00 - _ _ Clauconite is is extremely 10-30 percent percent of of many many extremely abundant abundant as iitt conslitutee Gl*ucÈnit coirtltu1.m between bçtwç10-30 unit. thin uaite Had, weather, dark brown, beds Iron. thick, several horizons contain pebbles of .and.tone and dolomitic .sndstone which ,.pparently constitute an intraformational conglomerate IBase of Lower Au Train F'41s Columnar Columnarsection sectionofofthe theAu AuTrain TrainFormation FormationatatAu AuTrain Trainfalls falls(1-tamblin, [Hamblin, 1958) 1958) 147 147 �Stop Stop 2. Au Au Train-Whitefish Train-WhitefishChannel, Channel, Barrier, Barrier,and andBeach BeachRidge RidgeComplex Complex Train Whitefish Whitefish Channel Channel Au Train The The Whitefish-Au Whitefish-Au Train Train Channel Channel isis aa former formerproglacial proglacial lake lake spillway spillwaythat that drained drainedacross across the the central central Upper Upper Peninsula. Peninsula. From FromLittle LittleBay BayDe DeNoc, Noc,north northto toAu Au Train TrainBay Bay on on Lake Lake Superior, Superior, kilometers the channel is over 60 kilometers (36 miles) in length and ranges between about 2.5 the channel is over 60 kilometers (36 miles) in length and ranges between about 2.5 kilometers (1.5 miles) to 6.7 6.7 kilometers kilometers (4 (4 miles) miles) in width. The TheWhitefish WhitefishRiver, River,aatributary tributaryto to Little LittleBay Bay De De Noc, Noc, drains drains the the southern southerntwo-thirds two-thirds of of the the Channel. Channel. The Thenorthern northernthird thirdisisdrained drainedby bythe theAu Au Train Train River, River, aa tributary tributary to to Lake Lake Superior. Superior. Most Mostof ofthe thechannel channelbottom bottomisisunderlain underlainby by thin thin deposits of of gravel gravel or or till till over over bedrock. bedrock. deposits The The lowlands lowlands of of the the Au Au Train-Whitefish Train-WhitefishChannel Channel were were first first noted notedby by Foster Fosterand andWhitney Whitney (1851), (1851), but itit was was Winchell Winchell (1871) (1871) who who interpreted interpreted the feature as a meltwater spillway. Russell Russell (1904), (1904), however, identified identified the the channel channel sides sides as as kame kame terraces terraces having having been been deposited deposited by by aa thin thin lobe of ice that had stretched stretched across the Upper Peninsula. The The channel channel has has since since been been studied studied by by a number (1929), Hough Hough (1958), (1958), Drexler Drexler et et al. al. (1983), (1983), Farrand Farrand and and number of workers workers including including Leverett Leverett (1929), Drexler (1985), (1985), and Hughes Hughes (1990). (1990). Incidently, Incidently,Leverett Leverett (1929) (1929) indicates indicates that during during the 1920s, 1920s, government government engineers engineers considered considered cutting cutting aa ship ship canal canal through through the the Au Au Train-Whitefish Train-WhitefishChannel Channel and connecting connecting Lake Lake Superior Superior to to Lake Lake Michigan, thus avoiding avoiding the Sault Sault Locks. According to Hughes (1990), (1990), the Au Train-Whitefish Channel reached its its full expression expression According during during the Marquette Marquette Advance Advance about 10,000 10,000 BP. At At this this time, time, lake lakelevels levels in in the the Lake Lake MichiganMichiganHuron basins as retreating retreating ice ice had revealed revealed the the basins were were extremely extremely low low (Chippewa-Stanley (Chippewa-Stanley Low) as isostatically isostatically depressed depressed North North Bay Bay outlet, outlet, allowing allowing water water to nearly drain from the southern southern basins. basins. Before, Before, during, during, and, and, for for an an unknown unknown period period after after the the Marquette Marquette Advance, Advance, proglacial proglacial lake lake water water drained across the central Upper Peninsula to the lower lakes, creating the Au Train-Whitefish central Upper Peninsula lakes, creating Train-Whitefish Channel. Hughes Hughes(1990) (1990)has hassuggested suggestedthe the Channel Channel extends extends under Little Bay De Noc and Green Bay to a large, Chippewa-level off the east east side side of of the the Door Door Peninsula. Peninsula. The Chippewa-level delta submerged off The Channel was abandoned abandoned at at around around 9700 9700 BP when retreating retreating ice allowed allowed lake water to reach the the Sault Outlet Outlet on the the east east end end of of the the Superior Superior basin. Barrier Au Train Barrier During Nipissing Nipissing times, times, lake lake water water flooded flooded some some 66 kilometers kilometers (3.6 miles) miles) into into the the northern end of the the Au Train-Whitefish Train-Whitefish Channel Channel creating creating Au Train Relict Relict Bay, which is is one one of of Nipissing II wave-cut bluffs the largest Nipissing embayments in the central Upper Upper Peninsula. Peninsula. Nipissing and cliffs, cut cut in bedrock bedrock and and unconsolidated unconsolidated sediments sediments at at 192 192meters, meters, rim much much of of the the east east and and west sides of the former embayment; however, in places along the southeast fringes, the shoreline sides the former embayment; southeast fringes, the shoreline is less developed, appearing by eolian eolian activity. activity. In fact, small relict appearing to have been modified by along the the southeast southeast side of Au foredunes may be present along Au Train Lake. Lake. A massive mid-bay barrier extends extends from from northwest northwest of of Paulson Paulson Lake, Lake, east east to to Joel Joel Creek, Creek, forming forming the the northern northern boundary boundary of of modern Au Train Lake. The Thebarrier, barrier, having having an an elevation elevation of 186 186 meters on its north side side and 189 189 its south south side, side, reaches reaches an an elevation elevation at at its its crest crest between 195 195 and and 192 192 meters meters and and aa meters on its maximum elevation dune remnant. remnant. In elevation of 201 meters on what appears to be a dune In places, places, the barrier severely eroded eroded by by the the Au Au Train Train River. River. has been severely The barrier barrier created created aa huge huge lagoon lagoon between the barrier proper and the southern southern rim of of the the The bay, now occupied occupiedby byAu Au Train TrainLake, Lake, Paulson Paulson Lake, Lake, and surrounding surrounding wetlands. Based on the depth of Au Train Train Lake Lake (USDA, (USDA, 1938), 1938),during during Nipissing II times, times, the lagoon would have have had had aa maximum maximum depth depth of of at at least least7.5 7.5 meters meters (25 (25feet), feet), an an average average depth depth of 33 to to 44 meters meters (9.8 (9.8 to to 13 13feet), feet), miles). Drainage may have exited from the and covered about 6 square kilometers (2 square miles). 148 148 �Au Train Lake $Iapnck Cr. AU TRAIN- CisySand Cliffs Baa n SPI LLWAY Cr. WhIt.fish P. SCALE SCALE kilomstsrs I a , j I 0 -I $ IQ Ovation In ff11 vs Little Bay Do Noc AuTrain-Wiifeflsh Spiliway AuTrain-Whitefish Spillway (Hughes, (Hughes, 1971) 1971) 149 �D Moraines and channels in the Marquette area and the Eben-Chatham channels (Hughes, 1971) �SUBMARINE VALLEY IN IN THE THE FLOOR FLOOR OF GREEN GREEN BAY BAY I 11 SCALE I. I. ItI• KIlO S•I SI I SSIk,Ic lIt,SI - S S4—• 151 �lagoon on its its northeast northeast corner comer near near the the eastern eastern end end of of the the Au Au Train Train Barrier Barrier or or at at some somemidmidposition breach in the barrier, where the Au Train River runs through today. today. Nipissing Nipissing II IIlevels levels may have modified the the northern northern edge edge of the the barrier, which is is at an an elevation elevation of of 186 186meters. meters. Au Train TrainBeach BeachRidge Ridge Complex Complex The The Au Train Train Beach Beach Ridge Ridge Complex Complex occupies occupies the the area area between between the the barrier barrier and and the the modern shoreline of Lake Superior. The complex is composed of some 50 separate recessional modem shoreline Superior. The complex is composed of some 50 separate recessional ridges which form a compact, arcuate shaped "corrugated plain" plain' as first first termed by Bergquist Bergquist (1936). The rIdges, which range in elevation between 186 and 184 meters, are currently being (1936). The ridges, which range in 186 184 eroded eroded by Lake Lake Superior, Superior, which has has created created an irregular irregular line of actively forming coastal coastal foredunes on the northern end of the complex. The The Au Au Train Train River has also also meandered through the ridges ridges resulting resulting in in erosion erosion and and truncation, truncation, suggesting suggesting that that the the river's river's present present course course isis relatively recent. The Theridges ridgesare areparallel parallel to toeach each other, other, and and there there is is no no evidence evidence of cross-cutting cross-cutting relationships, relationships, indicating indicatingfluctuating fluctuatinglake lake levels. levels. Grand Island/Powell Stop 3. Grand Island/PowellPoint Point Scenic Scenic Overlook moraine. The This stop stop is on the crest of a Marquette Advance moraine. The overlook overlook affords affords aa view view of Grand Island, Island, Grand island Island Harbor, Harbor, Murray Bay, and Sand Point. Sand Point, a large cuspate spit, is visible in the east channel of Grand Island Harbor. Harbor. The The spit, spit, which ranges in elevation elevation between 185 185 and 183 183 meters, appears appears to to have formed in post-Nipissing times, times, and and is is still still being being actively modified by wave-action wave-action from from Lake Lake Superior. Superior. It is composed of a number of lesser bathers that deve]opment of of the the spit. A sandy barriers that appear appear to to have have created smaller smaller lagoons during the development A massive Nipissing I, wave-cut bluff at 192 meters is present along the southeast side of the spit, 192 southeast side the spit, Nipissing I, which extend extend to the the northeast northeast from marking the start of the modem cliffs of the Pictured Rocks, which Sand Point. Point. On Grand Island, Island, Nipissing Nipissing I wave-cut bluffs developed in bedrock at 192 192 meters meters rim rim much of the southern portions of island. Wave-cut Wave-cut caves, caves, shoreline shorelinestacks stacks and and arches, arches, and pocket pocket shoreline along beaches are present along the Nipissing I shoreline along the west side of Murray Bay. WaveWavecut bluffs at 192 192 meters meters rim portions portions of the the Island's Island's western high ground, known locally as as the the Thumb. Thumb. AAwell-developed well-developedsystem systemof ofstream streamgullies gullieshas has dissected dissected the the eastern eastern hillsides hillsides of of the the main lobe of the island, as well as much of the hillsides on on the the Thumb. Thumb. These These narrow, narrow, deep deep incised valleys end abruptly at the Nipissing I shoreline, indicating that they formed in preabruptly shoreline, Nipissing, Houghton Low times Nipissing, times when stream stream base levels were extremely extremely low. Cuspate Cuspate barriers barriers grading grading from the Nipissing I bluffs to about 186 186 meters are are also also visible visible on Grand Island. The Cuspate Bather, Barrier,which which isis located located Themost most obvious obvious of of these is the Duck DuckLake Lake Cuspate on the east side of the main lobe of the island. ItIt isis composed composed of of a number number of large ridges ridges that trend east to west on the south side, and north to south on the east east side of of the the barrier. barrier. Duck Duck Lake, Lake, a small inland lake, lake, is is aa lagoon lagoon remnant remnant associated associated with the the barrier. The The Muskrat Muskrat Point Point Cuspate Cuspate Bather, Barrier, formed formedoff off the the southwestern southwestern end end of of the the Thumb, also contains a lagoon remnant, now a small swampy pond. Wave Waveaction actionfrom fromLake Lake Superior Superiorhas has eroded the southeastern southeastern tip of the the Duck Lake Cuspate Cuspate Barrier, the the southern southern limb limb of the the Muskrat Muskrat Point Point Cuspate Cuspate Barrier, Barrier, and and the the southeastern southeastern tip tip of of the the Williams WilliamsLanding Landing Spit, Spit, exposing exposing imbricated imbricated gravel gravel and and cobble cobble beds beds in in both cases. These Theseeroded erodedareas areasshow showup upas ashigh, high, sandy sandy bluffs bluffs along along the modern shoreline. The Grand Island Beach Ridge Complex, a complex of at least 25 beach ridges ranging ranging in tombolo between the Thumb Thumb and and the the main main part part elevation between 186 186 and 183.5 183.5 meters, meters, forms a tombolo of the island. 1i Inplaces placeson onthe thesouthern southernedge edgeof of the the complex, complex, beach beach ridges are submerged by 152 1 52 �Stops 3 & 4 Stop 5 153 �Lake Superior, Superior, while while the the northern northern edge edge is is currently currently being eroded eroded by the modern modem lake, lake, creating creating small coastal foredunes foredunes along along Trout Trout Bay. of Grand Island Island presents presents an an enigma. enigma. Some A steep cliff visible high on the west side of Some workers (Don and Eschman, 1971) have suggested it is a wave-cut bluff from Lake Algonquin. (Dorr and Eschman, 1971) have suggested it is a However, such such aa feature feature would would have have been been destroyed destroyed during during the the Marquette Marquette Advance. Advance. Alternatively, the escarpment may be a remnant of an ice marginal stream channel Alternatively, escarpment may stream channel created created during during the retreat of Marquette Marquette Advance Advance ice ice or or itit may may have been created created by headward headward erosion erosion and and retreat retreat of a caprock on the the highest highest portions portions of of Grand Grand Island. Island. Stop 4. Munising Munising Falls Falls At Munising falls, the Au Train Formation (see (see stop stop 11for for a more complete complete description) description) meters thick. Conformably forms a resistant resistant caprock caprock of of dolomitic dolomitic sandstone sandstoneabout about 55 meters Conformably below is the weaker Miners Castle member of the Munising Formation, which has has been been undercut undercut to to form form a steep high-walled valley. Munising MunisingCreek Creek plunges plunges 10 10meters meters over over the the falls falls in in its its short short journey to to Lake Superior. At plunge pools. pools. They were At the the base base of of the the falls falls are are several potholes and plunge abraded into the sandstone sandstone by by hard cutting cutting tools tools carried from from the overlying overlying caprock caprock and and from from glacial drift. Visitors Visitorsused usedtotowalk walkbehind behindthe thefalls, falls,underneath underneath the the overhanging overhanging caprock, caprock, but but aa recent collapse collapse of aa section section onto onto the the walkway walkway prompted prompted park officials officials to to close close that that part part from from the the general general public. public. The Munising Munising formation formation represents represents aa transgression transgression of of the the shallow shallow Sauk Sauk sea sea onto ontothe themidmidcontinent region in Cambrian Cambrian time. The Theupper upper Miners MinersCastle Castle Member Member isis aa light light gray gray to to white white colored, poorly poorly sorted, sorted, cross-bedded sandstone sandstone that comprises comprises the the upper upper 30 30 meters meters of of the the Munising Formation. In the lower part of the member, thin thin lenses lenses of of blue blue shale shale are are common, common, but but become scarce scarce in the the upper upper portions. portions. Quartz Quartz grains grains dominate dominate the the lithology lithology (>95%) (>95%) of of the the Miners Miners Castle member, member, with feldspar comprising most of the remainder. Most Most quartz quartz grains grains exhibit exhibit straight extinction and are likely of igneous origin. origin. ItIt is is not not uncommon to find find 1-3 1-3 cm cm thick thick beds beds of shale alternating with 10cm 10 cmthick thickbeds bedsof of coarse coarsesandstone sandstoneand and even even conglomerate! conglomerate! Especially Especially in the lower parts of the Miners Castle member. Higher Higher up in the member, member, fewer beds of shale and conglomerate conglomerate are are found, found, and and beds beds consist consist mostly mostly of well-sorted, well-sorted, cross-bedded sandstones sandstones with a greater degree of mineralogical and textural textural maturity. maturity. Some Some individual individual beds are are quartz quartz arenites, but in sum the formation formation is poorly sorted. Quartz Quartzisis also also the the dominant dominant cementing cementing agent, agent, but with only only slight slight secondary secondary enlargement of the detrital detrital grains, grains, it is porous porous and and friable. friable. Authigenic Authigenic pyrite pyrite is is common common enough enough in in some some of the middle middle beds that it is is megascopically megascopically discernable. Discussion Discussionof ofthe theMiners MinersCastle Castlemember member continues continuesat at the the next next stop. stop. Stop Stop 5. 5. Miners Miners Castle This stop stop examines examines the the type locality locality of the Miners Castle member of the Munising Munising formation (see description in stop 4). Sedimentary structures such as mud cracks (mostly Sedimentary structures as (mostly in the shale beds), ripple marks marks and cross cross bedding bedding (as seen at this stop) suggest an alternating alternating fluvialllacustrine environment of deposition. Significantly, fluvial/lacustrine Significantly,the ripple marks and cross bedding in the Miners Castle Castle member member indicate indicate aa transport transport direction direction from the east-northeast, east-northeast, as as opposed opposed to to the northwesterly transport direction direction measured in the older Chapel Rock member (see (see figure figure illustrating cross-bedding directions). The The abrupt abrupt change change in in transport transport directions directions in the two members provides provides further further evidence evidence of an unconformity that separates separates them. The change change in transport direction also also suggests suggests that that the the Wisconsin Arch and the the Northern Michigan Michigan highlands, highlands, 1544 15 �Miners Castle Castle Member Member of the Munising Formation Au Train Train Formation Formation dolomitic, yellow-brown, coarse-and coarse-and medium medium grained with much 148 'Sandstone, very very dolomitic, 148 ' -152' 152' Sandstone, very coarse and little fine, fine, scattered scattered very veryfine fineand andfine finequartz quartzpebbles. pebbles. Some irregularly bedded. bedded. Some large zones conglomeritic. Massive Massive bedded and irregularly sandstone sandstone pebbles. pebbles. Munising Formation, Miners Munising Formation. Miners Castle Castle Member Member 144' - 148' 148' Sandstone, 144' Sandstone, light light yellow gray, medium grained grained with much fine fine and and aa little little fine, fine, even bedded and thin-medium thin-medium beds. Another bed as above above 140' - 144' haccessible 122' - 140' Inaccessible Light yellow-gray sandstone, coarse and very coarse grained with a trace of 121'- 120' granules. Cross-bedded Cross-beddedin inmedium-thin medium-thin beds. beds. Light yellow-gray sandstone medium to fine grained with traces traces of granules. granules. 115'- 121' bedding. Poorly sorted with much fine Medium - thick bedded with cross bedding. material in lower foot foot of bed. 112' - 115' coarse to to medium medium grains. grains. Trace Light yellow gray sandstone, coarse Trace of granules. Massive, cross cross bedded. 111' -112' Light yellow gray sandstone, poorly sorted with sizes ranging from silt silt to to granules. Abundant Abundant limonite limonite staining. Appears Appears to to be be burrowed. Narrow Narrow ledge. ledge. 108' - 111' Light brown gray sandstone. Medium to coarse grained grained with much much fines. fines. Massive. Trace Traceof of granules. granules. Light yellow gray sandstone. Poorly Poorly sorted sorted with grains grains ranging from silt silt to to 106' - 108' granules. Abundant Abundantburrows burrowsbest bestseen seenfrom from overhangs. overhangs. 103' - 106' Light yellow brown sandstone. Medium Medium to to coarse coarse grained grained with with few few granules. granules. Massive bedded. Massive bedded. Same as above above except except more silty and abundant abundant burrows. 99' - 103' 95' - 99' gray sandstone. sandstone. Silty Light yellow gray to gray Silty medium to fine grained with thin beds of red, blue and green shale. Poorly Poorly sorted. sorted. 40' - 95' gray sandstone. sandstone. Poorly sorted to well well sorted beds. beds. The Light yellow gray The upper upper part, part, of medium to fine grained beginning at the top of Miners Castle pinnacle, consists of wed wee1sorted sortedbeds beds of of sandstone, sandstone,interbedded interbedded with with poorly poorly sorted sorted beds beds of of sandstone sandstone containing abundant silt and thin beds of silty shale, and conglomeritic beds with silty shale pebbles. The The upper upper 66 feet contains burrowed beds. At At 10-12 10-12feet feet below the top, are prominent poorly sorted, shaly, conglomeritic and cross are prominent conglomeritic and cross bedded bedded sandstones. sandstones. 20' - 40' Gray sandstone. Poorly Poorlysorted, sorted,medium medium grained grained with with abundant abundant thin thin blue blue shale shale partings. Some conglomerate. Cross bedded. Some conglomerate. Cross bedded. 10' - 20' Light yellow gray sandstone, coarse coarse grained, grained, well well sorted. sorted. Contains Contains clay pebbles. pebbles. Formation, Chapel Munising Formation, Chapel Rock Member Light yellow gray sandstone. Medium 0' 0' -- 10' 10' Medium to to fine fine grained grained with with aa trace trace of of coarse, coarse, wellwellsorted sandstaone. Cross Cross bedded. bedded. Lake Level 183 m (602') Level == 183 (602') 155 �+ Inland—0Prairie du du Chlen Chien inland PJI members arise to to the the east east so that that JacobsvllIe All members Jacobsville is above lake Is above lakelevel levelat atGrand GrandMarais Maraisand andthe theAu AuTrain Train fornialion formationIsismissing missingfrom fromThe the top top of ofcliffs, cliffs. — 7 a —— —— - - Unconformity - -I- —- —— Light brown, hard, dolomilic sandstone. Giauconillc. Au Train Formation Resistant Resistant A Upper 5-12 5-12 meters meters Upper Moderate Moderateto tovery vewdoiomitlc doiommcwttti with O L O gronJies ~ on0 pebDie5. qUarlz granuies and pebbles. Burrowed br the ELnowea "for tnebirdC Diros" i Soft riable ,,poorly pooW sorted sorted Soft, ffriable silty silly, shaM shov, sandstone, often often cross-bedded. cross-bedded, Light yellow-gray color Light yellow-gray color imparted importedby bylimonite. llmontte.(In (In varying varying amounts amounts Miners Castle Member C 0 About 3Dm to lake 0 U- 0) C Cl, C 0' A ——— _Uo9rmjy_ —— Pink. light Pink, Ughtbuff buffto to brown, brown, medium medium omoquommcsandstone. sandstone. grainedorthoquartzWc grained Thin blue shale shale beds beds 2-3 2-3cm Thin blue cmthick. thick. Mudcracks Mudcrocksininshale shalebeds. beds. ""bdsôh"é'(°h€ri Uórtz quartzite, and chert w/minor amounts of siate, BiF, basalt and granite) Chapel Rock Memberô a —————a—— c b a Jacobsville Jacobsville Formation Formation I - —a——— a I Nigular unconformity Nkosic to Arkosic quartz quartz sandstone. sandstone. Red Redto red-brown abundant mottles red-brownwith abundant mottles and where on0sireaks, s ~ ~ o < sespecially es~ecioirv . wnere permeable. Th cmessvaries vanesgrecztts greatly, Denneooie.Thickness from > 1m to 500 m, from>lmtoSOOm. �____________________________________________________________________________________ Ga rat Zircon looter, 5.çtton Dcecflptioaotltthotogy ntis 0 I DeIsmilic ttnd.tae. light brown, Very resistant 3ssso1 kU TRAIN formation tencanlormity • • ISO ''.•.' Top of MUNISINC t.rmatlea Miner's C..tj. nwmb.r teStIsa. .Mte, m.dlwn-grasn.4, friable, ma.!,. badtsg (I' tot thick), fonti vertical di/l. — Details of ..dimtntsry strvcttr.s ob•c,and hyw.ath.riag; ataytadte appnrt be cr*tt.b.dd*d 120 Sandstone wMt. mSwm.praintd. ..U coned, masal,. bedding (I' tel thick). email cavee and arch... friable N'&marc'aa tat;. tcrsdeaa along bedding pta. Smelt—scale crass bedding to,. 11.4 stains ales; certain hastens .... b6. i" bed. very resistant. contains abtaadaat n.atris Sandstone. pr.tn,iaaatly poorly sorted ttt. is gray. tniaor streak. .1 red, blee and grssn, 5... of Mint's Castle piradle 55 Coarse .and.sene conc.atrat.d along cantata bedding p1.... Abundant day pail.;. deposited parallel to the cro.* bedding cbann*I. and Ian... shine cflgtOrnhrstO iO Small ceagiarat. lens.. So. Coarse eand and conglomerate I..... S Slut, grita, red, and yells'. colorado. foflo.tn bedding ,a.'... S.ndst.. gray to grayteh'.bl.a., m.tum,grain.d, pearly .on.d amall_.caIa crosa bedding 4".,i" thick ..parat.d by thin blue hal. I.n.e. l14"-t" thick '4 3° (approalmaisly 2030 percent .hal*), torn, coarse and fin. coagloanarat. / 20 ''i L'&3c 10 , & ¶cat.r t*v.l of bk. r / y I, ( 1 pa.' of Itt.,'0 tail. m..r unonntor Top of Chapel Rock rna,.bor 5aad.tots, pink, coors.-grainsd, nil sorted, rounded, abundant clay pellet. Mud cracks 4"-S' In tonnetar dontloped in beds of bIn. shale ines than I' thick. Clay pellet. sandseont light brown to buff well carted large •cale cross bedding It thick j - Superior Columnar Columnarsection sectionof ofthe theMiners Miners Castle Castle member memberof ofthe theMunising MunisingFormation Formation at atMiners MinersCastle Castle(after (afterHamblin, Hamblin,1958) 1958) 157 157 �Paleogeography member of Paleogeographyduring duringdeposition depositionof of the the Miners Miners Castle member of the the Munising Munising Formation Formation(after (afterHamblin, Hamblin, 1958) 1958) £IPLSNAtION bSSJ V tsuSe ndI.* d— - — •_N_ •m Caib — — fl4 —*• lö? I— I*aj •f — t* un—*. _*._ cni .4 * t tS3a, La - is 0 iG it sai—. t— '0 •0S.iii Cross bedding bedding directions MinersCastle Castlemember member of the the directions in the Miners Munising (after Hamblin, Homblin, 1958) Munising FormatHon Formatiion (after 1958) 158 158 �which which provided providedthe thesource sourcefor forsediments sedimentsduring duringthe the Jacobsville Jacobsvilleand and Chapel Chapel Rock Rock deposition, deposition,had had eroded erodedenough enoughthat thatthe thetransgressing transgressingSauk SaukSea Seasubmerged submerged most most of of the the Upper Upper Peninsula peninsula and and Wisconsin. Wisconsin. Stop6. 6. Kingston Kingston Plain Plain Stop Stop Stop66 isison onthe theKingston KingstonPlain, Plain,an an extensive extensiveoutwash outwashplain plain that that isislocated locatedbetween betweenthe the towns towns of of Munising Munising and and Grand Grand Marais. Marais. The ThePlain Plainpresents presentsaaunique, unique,prairie-like prairie-likesetting setting compared compared to to much much of of the the more more heavily heavily forested forested areas of the Upper Peninsula. The The multitude multitude of of pine pine stumps stumps preserved presemed on on the the Kingston Kingston Plain Plain serve serveas as testimony testimony to to the the intensity intensity of of historic historic clear-cut clear-cut logging. logging. A (Leverett,1911, 1911, A number numberof of workers workershave havestudied studiedthe the geology geology of of the the Kingston Kingston Plain Plain (Leverett, 1929; 1929;Bergquist, Bergquist, 1936; 1936;Martin, Martin, 1957; 1957;Hughes, Hughes, 1968; 1968;Drexler, Drexler, 1981, 1981, Drexler Drexler et et al., al., 1981; 1981;Blewett Blewett and and Rieck, Rieck,1987; 1987;Blewett, Blewett,1994), 1994),which which has has resulted resulted in in aa wide wide range range of of theories theories concerning concerningits its formation formationhistory. history.Largely, Largely,the thecontroversy controversyhas hasfocused focusedon onthe theinterpretation interpretationof ofthe theMunising Munising Moraine, which the Kingston Plain is part of (Blewett, 1994). According to Hughes Moraine, which the Kingston Plain is part (Blewett, 1994). According to Hughes (1968 (1968and and unpublished unpublishedwork), work),and andlater laterby by Blewett Blewett (1994), (19941,whom whom has has done done the the most most current currentQuaternary Quaternary geology geology research research in in the the area, area,the the Plain Plain initially initially formed formed when when Marquette Marquette Advance Advance ice ice probably probably reached reached an an equilibrium equilibriumand andsouthward southwardflowing flowingmeltwater meltwater streams streamsbuilt built the the Upper Upper Kingston Kingston Outwash I). Ice Outwash Plain Plain surface surface at at about about 10,000 10,000 BP (Phase 0. Ice retreated retreated somewhat somewhat from this position, opening opening up drainage drainage to to the the east east between between the the ice ice front front and higher ground to the south, south, creating creating aa channel M-77. The channel at at 283 283 m m that that currently currently parallels state highway M-77. The upper upper portions portions of of the the Kingston Kingston Lake Lake Kettle KettleChain Chainformed formedatatthis thistime. time. Next Next (Phase (PhaseII), lI), the the ice ice margin margin retreated to a new position along along the Lower Kingston Kingston Plain, opening even lower drainage outlets from the west. Consequently, water flowing to the Plain, opening even lower west. Consequently, water east east between between the the ice ice front front and and the the now now abandoned abandoned Upper Upper Kingston ice-contact ice-contact slope, slope, formed formed aa broad, broad, kame kame terrace terraceatatabout about285 285m, m,which whichslopes slopestotothe theeast. east. Marginal Marginal retreat retreat of of the the ice ice from from highlands highlands south south of Grand Grand Marais Marais allowed allowed still still lower lower outlets outlets to to open open (Phase (PhaseIII), I Q ,which which resulted resulted in in the the incision incision of a channel channel at 250-253 m, into the the Lower Lower Kingston Plain Plain surface. surface. The TheHurricane Humcane River River currently currently follows this channel. Also Alsoatatthis this time, time, some some meitwater meltwaterspilled spilledsouthward southward from from the the Lower Lower Kingston Kingston ice-contact ice-contact slope slope south south of of Beaver Beaver Lake, Lake, forming forming the the Long Long Lake Channel. The Thelower lowerportions portionsof ofthe theKingston KingstonLake LakeKettle Kettle Chain, Chain, including includingKingston KingstonLake Lakebasin basinmust musthave haveformed formedduring duringthis thisphase. phase. IV,and and55 of of Blewett, 1994), 1994), the ice withdrew to a position position several several Eventually(Phase (PhaseIV, Eventually kilometers kilometers north north of of the the Lower Lower Kingston Kingston ice-contact ice-contact slope, slope, allowing allowing meltwater meltwater drainage drainage across across aa broad terrace heading at 230 m, known as the Beaver Basin surface. Further retreat of ice broad terrace heading at 230 m, known as the Beaver Further retreat of ice allowed allowed even even lower lower channels channelsat at 225 225 and and 215 215 m m to to be be incised incised into into the the Beaver Basin surface. surface. Finally, the glacier retreated to a position just south of Portal Point, but eventually Finally, glacier eventually further retreat retreat allowed allowedlake lake water water in in the the western western Superior Superiorbasin basin to to flow flow eastward, eastward, abandoning abandoning the the Au Au Train-Whitefish Channel, into the eastern basin, forming an early version of Lake Minong. Train-Whitefish Channel, into the eastern basin, forming an early of Lake Continual Continual retreat retreat allowed allowed Lake Lake Minong Minong to expand, expand, eventually eventually filling the entire Superior Superior basin. 159 �- :'-——".. T't"- -: J. i r- P B - Phdse I Surface —'P. s5QfcEIev.: go RE -. - / "<k, -\' ;. 41t' -<k - Y - I Stops Stops66&&77 160 160 *An �a' — 'I', GoS 90 930 Pr-e dank7 k" //ey 'P. Pre C/ezcjaf L'a//ey? Grand Sa6/ Lke 1' '9 The Kingston Outwash Plain and meltwater channels near Grand Marais, Michigan Ti, '7 C 4 Ml g5 "V.. �ice 5 cc' a'icsA .rr lace 0 a-s tn1 art ordj-/o,a4y Chaewst2ng O7 eat La r i' 0 ,C7Ma,/ack.. I A '4 - - C /aives eat teverd . .noph-ig cc' tciiorh .ntrt.ct - Pi-erent level lake So,ntyor ,,v—*_5- rect;oAt vieai ea..rt ,s,i-tA end 07C sc (a SorjCacej 'lope ecjt'ora - .c4,,tt.n fan .&9nt5' iwein) -Present lake 4/fC 0 -I- 2 -.1. -A. 162 - 3 -I-- �0' Mosaic Mosaic of of digital digital elevation elevationmodels models(DEM) (DEM)processed processedtotoshow showrelief relief(illumination (illuminationfrom fromSE) SE) �; .—.I 0 / I,I) Stop 7.7.Turtle Lake (actually an unnamed pond, soso called stop lbrtle Lake (actually an unnamed pond, calledononthis thistrip tripbecause becauseon on previous previous visits itit was observed observed that many many turtles turtles make make itit their their home) home) occupies occupies aa deep deep kettle kettle in in the the Kingston KingstonOutwash Outwash Plain. The kettle kettle is is the the deepest deepest (though (though not not the the largest) largest) in in aa south-southeast/north-northwest south-southeasthorth-northwesttrending trending chain of kettles that (presumably) were formed as ice stagnated in aa now-buried now-buried bedrock bedrock valley. valley. A from bedrock bedrock and a minimum minimum seismic study conducted by Regis in the 1980's revealed no reflection from sediment thickness of 60 meters. The kettle is several hundred meters in diameter diameter and and is is at at least least 40 40 meters deep. deep. Because Because the the kettle kettle chain chain extends extends across across (cuts) (cuts) each each of of the the outwash outwash and and drainage drainagesurfaces, surfaces, the stagnant Marais) ice was was building building stagnant ice must have been in place from &om the time the Marquette (Grand Marais) surface, and was buried by sediments sediments as the main ice ice mass mass retreated. retreated. the upper Kingston outwash surface, 00 m m hike hike through through the the woods, woods, and and ifif descending descending into into the the kettle, kettle, aa strenuous strenuous This stop requires a 2-3 2-300 return hike. 164 164 �Stop 8. Log Log Slide Slide From the to the the east. east. Two the Log Slide, the Grand Sable complex is visible to Two geomorphic geomorphic features, in this this view: view: The features, the "Banks" "Banks" and the "Dunes" are prominent in The Grand Sable Sable Banks consist of a steep, north-facing exposure of glaciofluvial sediments ranging in thickness consist of a steep, north-facing exposure of glaciofluvial sediments ranging in thickness from from 30 30 to to 100 100 meters, meters, rising rising directly directly above above the the shoreline shoreline of Lake Superior. Superior. To Tothe thesouth southaway awayfrom fromLake Lake Superior, Superior, the Banks form form aa high high terrace terrace surface, which is overlain by the Grand Sable Dunes, a dune dune field field "perched" "perched"up up to to 100 100meters meters above above the the lake lake and and covering covering about about 10 10square square kilometers. kilometers. The The deposit deposit that that underlies underlies the the dune duneplateau, plateau, exposed exposed in in the the Grand Grand Sable Sable Banks, Banks, has has been been variously variously interpreted interpreted as as aa crevasse crevasse filling filling or or kame kame terrace terrace that incorporated incorporated heterogeneous heterogeneous glaciofluvial glaciofluvialdebris debris transported transported by by rivers rivers flowing flowing west to east east along along an ice marginal channel channel (Drexler, (Drexler, 1981; 1981;John D. D. Hughes, Hughes, personal communication). Within Within the the central central portion portion of of the the Banks, Banks, stratigraphy stratigraphy of the the lower lower third third of of the the section section is obscured by colluvial and aeolian aeolian sand; sand; at at the the east and and west ends ends of the banks, the section is better exposed. Upper Upper portions portions of of the the banks banks are several thin thin gravel gravel beds. beds. thterbedded are dominated dominated by coarse, coarse, cross-bedded cross-bedded sand and include several Interbedded silt silt and and sand sand comprise comprise the the middle middle third of the deposit. The Thelower lower third third is is dominated dominated by by fine fine textured textured material, material, primarily primarily silt. silt. AAsurface surfacegravel gravel layer layer is is presently partially exposed in a "stripped "stripped plain" in the central plateau. This This layer layer is is often iron-stained and contributes contributesto toaalag lag that that periodically periodically armors armors the the bluff bluff crest crest as as the the slope slope retreats retreats southward southward (Farrell and Hughes, Hughes, 1985). 1985). The The Grand Grand Sable SableDunes Dunescontain contain evidence evidence of of buried buried soils, soils, which which record record episodes episodes of of geomorphic geomorphic change change consisting consistingof of soil soil profile profile development development followed followed by soil profile profile burial burial (Anderton (Anderton and Loope, Loope, 1995). 1995). In Inmost mostcases, cases,an an entire entire episode episode is is preserved presewed as a buried soil profile or or organic organic accumulation, accumulation,capped capped by by relatively relatively finer sediments, sediments, such as silt and very fine to fine sand, which is capped by medium to coarse eolian cross-bedded sand. The contact between the sand, which is capped cross-bedded sand. upper upper soil soil horizon, horizon, usually usually an an 0/A OIAhorizon, horizon,isisoften oftenvery verysharp, sharp,suggesting suggestingrelatively relativelyrapid rapidburial. burial. Such Such sedimentary sedimentaryand andpedological pedologicalevidence evidencestrongly stronglysuggests suggeststhat that periods periodsof ofstability stabilityare are represented represented by by forest forest vegetation vegetation colonizing colonizing landscape landscape surfaces surfaces within the Dunes, while while periods periods of of instability instabilityare are characterized characterizedby by the the burial burial of trees and associated soil profiles and organic organic accumulations. accumulations. A A soil soil catena, catena, consisting consistingof of aa sequence sequenceof of buried Spodosols, Spodosols, may be found found outcropping outcropping in in various various places places throughout throughoutthe the Dunes, Dunes, especially especially at at the Log Slide Slide locality. The Thecatena, catena,referred referredto to as as the the Sable Sable Creek Creek Soil Soil(Anderton (Anderton and and Loope, Loope, 1995), 1995),is is traceable traceable across across much of the the Bluff top top and and outcrops outcrops sporadically sporadicallywithin within the the Dunes Dunes reflecting reflecting the preservation presemation of a past soilscape soilscape which began forming forming immediately immediatelyafter after deglaciation deglaciation about about 10,000 10,000 years ago, but was isolated from further further surface surface weathering weatheringunder under eolian eolian sediments sediments beginning about 5000 5000 years ago. The Theburial burial event event seems seems to to have have been been initiated initiatedby by the the massive massive destabilization destabilization of the the Bank Front Front during during the the high lake levels of the Nipissing Great Lakes. high lake levels of the Nipissing Great Lakes. While While there there may may have have been been some some eolian eolian activity activity immediately following deglaciation, deglaciation, the evidence tends to support the hypotheses of Farrell and Hughes (1985) who suggest evidence tends to support the hypotheses of (1985) suggest that the Grand Grand Sable Sable Dunes Dunes are are no no older older than 5500 5500 BP. This Thisisisin in contrast contrast to to Marsh Marsh and and Marsh Marsh (1990) (1990) who who argue arguethat that the the Grand GrandSable SableDunes Dunes began began forming forming as as lake lake levels levels rose from from the pre-Nipissing pre-Nipissing lows lows sometime sometimearound around 9000 9000 BP. BP. Marsh Marshand andMarsh Marsh(1990) (1990)maintain maintain that that sand sand nourishment nourishment to to the the Dunes until around around 3500 3500 BP. BP. However, Dunes increased increased from from 9000 9000 BP BP to to 5500 5500 BP and remained high until However, the the presence presenceof of the theSable SableCreek CreekSoil, Soil,which which isis developed developed into into glaciofluvial glaciofluvialsediments, sediments,suggests suggests that that the the Bluff Bluff top topwas wasaastable, stable,forested forestedenvironment environment from from deglaciation deglaciationuntil about about 5000 5000BP BP when when eolian eoliansand sandbegan began to to cover coverthe the upper upper surface. surface. Rising Risingpost-Houghton post-Houghtonlevels levelsapparently apparentlyhad hadlittle little 165 165 �a. t 0 0 2 0) 0) 0 D C 0 0 KN 0' a. 166 �effect on the Banks until well into into the the mid-Holocene, mid-Holocene, suggesting that wave action did not begin to modify the bluff front until lake levels were relatively high during the Nipissing Great Lakes. Stop 9. Ghost Forest Subsequent, Subsequent, post-Nipissing post-Nipissing fluctuations fluctuations in lake level also also seem to have influenced influenced the the Bank front, causing changes in the sediment available for eolian transport on the upper surfaces causing upper surfaces of the Grand Sables, Sables, resulting in a discontinuous discontinuous occurrence occurrence of buried soils soils and organic organic layers layers of various ages within the Dunes. In In addition addition to to the the major major burial event event that covered the Sable Creek Soil at least as early early as around 4500 BP, radiocarbon dates from the Ghost Forest profile also more recent recent burial burial at at about about 100 100years yearsago. ago. Also a record a burial event at about 3500 BP and a more radiocarbon date from other localities localities in the Dunes suggest that eolian activity also took place at about 1500 1500BP, BP, and and at at about about 700 700years years ago. ago. While, these these data data may not be conclusive, an analysis of radiocarbon dates from the Grand Sable Dunes strengthens strengthens interpretations. Beginning Beginning with the investigations investigations of Bach (1978), (1978), researchers working in the Grand Sable Dunes have been collecting and radiocarbon dating Sable collecting the study study area. area. In general, the dates seem to preserved wood, bulk soil, and charcoal from the roughly delineate delineate major times of increased eolian activity, documenting burial events within the Grand Sable Sable Dunes, and likely likely recording fluctuations in the lake levels of the Lake Superior Superior basin. Based on the available available data, data, episodes of increased eolian activity occurred at about about 5300, 5300, 2100, 1500, 500, and 4600, and 3500 3500 RCYBP, RCYBP, and and may also also have occurred at about 2500, 2500,2100, 1500, 1000, 1000,500, and RCYBP. The 100 RCYBP. The first first three three episodes episodesseem seem to to coincide coincide well well with with rising rising Nipissing, Nipissing, Nipissing NipissingI,I, and Nipissing II II phases, which are recognized throughout the southern Great Lakes (Larsen, 1985). The other, The other,later laterepisodes episodesalso alsocompare compare favorably favorably to to previous research research which which indicates indicates fluctuating fluctuating lake levels levels in in the the Lake Lake Michigan basin during the last 3000 years (Larsen, 1985). 1985). Larsen (1985) (1985) has suggested suggested that Holocene lake level fluctuations are linked to climatic climatic changes changes that affected the the temperature temperature and moisture moisture patterns influencing the water balance of the lakes. lakes. Holocene climate of proxy proxy indicators suggest climate changes changes in the the Great Great Lakes, as revealed by a varity of alternating alternating episodes episodes of cool, cool, moist moist and and warm, dry conditions (Swain, 1978; 1978; Bernabo, Bemabo, 1981). 1981). Comparison of burial burial events events within within Grand Grand Sable Sable system system with lake level and climate climate data data from from the the Great In general, general, Great Lakes Lakes region for for the the last last 3000 3000 years indicate a general correlation of events. In of cool, wet climatic increased eolian activity in the dunes appears to correlate with periods of conditions, conditions, while while periods periods of of relative relative stability stability and soil development development are are associated associated with times times of of of shore-zone warm, dry climatic conditions. These These findings findings do not fit Larsen's (1985) model of changes during rising and falling lake levels. In the Larsen model, increased precipitation In the Larsen model, increased precipitation and and lowered temperature result in high lake levels that cause tributary stream aggradation, marsh establishment, and soil profile development in in eolian eolian sands. sands. Decreased precipitation and raised establishment, temperatures cause temperatures cause low lake levels that result in tributary stream erosion, marsh extinction, and soil profiles becoming buried by eolian sands. While While Larsen's model seems to work for most coastal areas, perched dunes, dunes, such as the Grand Sable Sable system, appear to be reacting differently to climatically-induced climatically-induced changes changes in in lake lake level. level. been a nearly nearly constant constant rain rain of of eolian eolian sediments sediments with with In general, there seems to have been relatively short-lived periods of soil development and forest colonization within the Dunes since about 3000 BP. The Thedistinct, distinct,buried buried soil soil profiles profiles of different ages within the Dunes argues for the Bank Bank front. front. Otherwise, the fact that only relatively high lake levels are influencing the Otherwise,ifif the the of stability, but rather a Bank front was continually being undercut, there would be no evidence of 157 �homogeneous deposit of eolian sands. Thus, Thus,the the evidence evidence of of geomorphic geomorphic changes changes from from the the Grand Sable Dunes seems shore of seems to to be be an an indicator indicator of lake lake level changes changes along along the the south southshore of Lake Lake Superior. Th In sum, the buried soil profiles reflect fluctuations fluctuations in the amount amount of sediment sediment available available Hughes, 1985). The for eolian transport within the Dunes (Farrell and Hughes, The availability availability of sediment sediment Bank face face (Marsh (Marshand andMarsh, Marsh,1987) 1987).. High depends on the condition of the Bank High lake levels result in increased wave action, which undercuts the bank toe slope, slope, causing mass mass wasting, wasting, destabilizing destabilizing for wind wind deflation. deflation, If enough sediment is available the Bank face and presenting a fresh surface for on the the high high terraces. terraces. Low lake for a long enough period of time the result is dune development on lake levels allow the Bank face to stabilize stabilize and and perhaps perhaps become become vegetated, vegetated, resulting resulting in in aa reduction reduction or or near elimination of available sediment. Thus, Thus, dune dune building building on the the high high terraces is retarded, retarded, - on - terraces allowing stabilization stabilization and surface surface weathering to begin, as well as forest vegetation vegetation to to colonize colonize the fresh landscape surfaces. Thus, Thus, soil soil profiles profiles formed formed in in the the Grand Grand Sable Sable Dunes during during periods periods basin. These of low lake levels in the Lake Superior basin. These soil soil profiles profiles and in many cases the forests themselves were, however, buried by eolian sediments, sediments, eliminating further soil development development and and forest growth, the result of high lake levels destabilizing the Bank front. result lake destabilizing Stop Stop 10. 10. Sable Falls At Sable Sable Falls, Falls, the the Jacobsville Jacobsville Sandstone Sandstone forms forms the the base of the the section section exposed exposed in in the the gorge. The The Jacobsville Jacobsvillesandstone sandstoneisisaa redbed redbed deposit deposit of of questionable questionable late Proterozoic-early Proterozoic-early Cambrian age named by Lane and Seaman (1907) after the Keweenaw Peninsula town of Jacobsville. There, There,the the"Portage "PortageSandstone" Sandstone"that thatwas was used used as as ornamental ornamental and and construction construction stone stone was quarried. ItIt isisexposed exposedall all along alongthe the southern southern shoreline shoreline of Lake Superior Superior from the Keweenaw Peninsula to Munising. However, However, from from Munising Munising to Beaver Bay, it is below lake level, spare for a few occurrences slightly above above water. water. At Sable Falls and south of Grand Marais, good exposures are found. Thickness Thicknessof of the the Jacobsville Jacobsville appears appears to be extremely extremely variable variable and largely controlled by the underlying Precambrian topography, upon upon which which the Jacobsville unconformably for the most part. For example, rests unconformably example, Hamblin (1958) (1958) cites an instance instance of aa 22 wells, one of which drilled loot of of Jacobsville Sandstone, and the other well drilled drilled through 11100' through only 46' 46 of and the wells are only of the the same same rock rock before before reaching Precambrian quartzite, and 10 miles apart (T47N, (T47N, R1E). RlE). The Jacobsville can be easily recognized recognized by by its its striking striking red redto to red-brown red-browncolor. color. Much of the Jacobsville is mottled with white reduction splotches and streaks. The The color color change change is is abrupt abrupt and distinct. Most of the mottles are related to variations in permeability of the beds. That is, Most of the mottles are permeability of the beds. is. where the beds have the greatest permeability, leaching and white coloration is the greatest. greatest. Most fine grained beds and shale lenses in the Jacobsville are are not not leached leached at at all. all. Leaching most joints, and and cross-bedding. cross-bedding. Spherical often ffollows o l l o k bedding planes, fractures such as joints, Spherical spots spotiofof leaching appear to be controlled controlled by a dark grain or pebble at occurs at the center of the spots. The grains are probably of some some lithology that have a composition sufficient to produce a reducing reducing environment. environment. Texturally, Texturally, the the Jacobsville Jacobsville Formation Formation ranges from conglomerate conglomerate to a fine grained grained siltstone. The by Hamblin Hamblin (1958). (1958). The Theformation formationhas has 44 different different facies, as described by conglomerate conglomerate facies facies occurs occurs mostly at the base of the formation, formation, especially especially where it lies unconformably unconformably on Precambrian rocks (Presque Isle near Marquette, for example), but may also be found found scattered scatteredthroughout throughoutthe theyounger younger beds. beds. Thickness of these beds can be as great as 5 meters. The Themost mostcommon commonof of the the44 facies facies(laterally (laterally and vertically) is a red to reddish brown, medium-grained, medium-grained, lenticular lenticular bedded sandstone. In In some some places, places, lenticular lenticular bedded sandstones sandstones are are 168 �over meters in thickness. Trough over 100 100 meters Troughcross cross bedding bedding and and ripple marks are are common in this this facies. facies. bedding in in the the formation formation accentuate accentuatethe thestructures. structures. The Selective leaching along cross bedding occurrence environment of occurrence of ripple marks, mud cracks cracks and clay pebbles clearly indicate a fluvial environment deposition. AAmassive deposition. massivefacies faciesof of the theJacobsville Jacobsville is is characterized characterized by persistent, thick beds of relatively structureless structureless sandstone, sandstone, many over 3 meters in thickness. At Victoria Falls, a measured measured section of 200 meters of Jacobsville Jacobsville consists consists almost almost entirely entirely of 3 meter thick beds of sandstone. sandstone. Some laterally persistent oscillation ripple marks are found in the beds and support support a lacustrine lacustrine environment of of deposition. deposition. Finally, Hamblin Hamblin recognized recognizedaared redsiltstone siltstonefacies. fades. Though not not common (except (except at Laughing Fish Point and Agate Falls), beds of siltstone siltstone and shale occur occur interfingered with other facies and is interpreted as a quiet-water lacustrine environment facies environment of deposition. deposition. Paleogeographically, the Jacobsville Sandstone represents a continental red bed sandstone sandstone from the the Northern NorthernMichigan Michiganhighlands. highlands. As shown in the deposited by streams flowing northward from paleocurrent (cross (cross bedding and ripple marks) map produced by Hamblin (1958) there is very little deviation in trend (of indicators) indicators) from the mean. An unconformity, in some some places a disconformity, but as on Grand Island, an angular unconformity, separates separates the Jacobsville Jacobsville from the overlying Chapel Rock Member of the Munising Formation. At At Grand Grand Island Island the the Jacobsville Jacobsville dips a few degrees northward, and the Chapel Rock member of the Munising Munising Formation dips a few degrees southward, but at this stop, stop, bedding appears appears to to be be parallel parallel or or nearly nearly parallel parallel at at the the contact. contact. The lower two to fifteen foot section of the Chapel Rock member is a basal conglomerate, fifteen foot conglomerate, formation 90% vein quartz, quartzite quartzite and chert with lesser amounts of slate, basalt, granite, iron formation and sandstone. The Theupper upper 49 49to to 60 60feet feet isis pink pink to to light bluff to brown, medium grained orthoquartzitic sandstone blue-clay and and silty siltyshale shalebeds. beds. The beds in the top 20 orthoquartzitic sandstone with several thin, blue-clay very dolomitic dolomitic and andfrequently frequently"burrowed." "burrowed." Limonite imparts a light or 40 feet are moderately or very yellow-gray color to the member member and occasionally occasionally red to some some thin beds. beds. Northern Michigan The (lower) Chapel Rock member represents a period when the Northern Highlands (to the south) were eroding and shedding their sediments into a shallow empeiric sea south) to the north (see page). The (see the figures on the next page). The Chapel Chapel Rock member is composed composed almost entirely of quartz (mostly of igneous origin; Hamblin, 1958) with minor amounts of feldspar. (mostly minor of tourmaline, and abundant zircon Heavy minerals in the member include well-rounded grains of (20-50% of the total heavies, both altered and unaltered), magnetite, hematite, and ilmenite. Silica is the predominant cement in most places and occurs as secondary overgrowths of varying degrees. Farther Farthereast, east,near nearTahquamenon TahquamenonFalls, Falls, porosity porosity in in the the Chapel Chapel Rock Rock member member isis reduced reduced to almost zero due to the silica cement. Texturally, Texturally, the Chapel Rock member is well-sorted, medium-grained sandstone. Most Most grains grainsare are 1/4 114to 1/2 112 mm diameter. Color Colorgradually graduallyranges ranges sedimentary from white to buff to red. Large Large scale scale trough cross bedding is the most abundant sedimentary The size size of these troughs range from I1 to to nearly nearly 200 200 structure in the Chapel Rock member. member. The meters in width (mean about about 10 10 meters). Megascopically, Megascopically,they they are are useful useful for for differentiating differentiatingthe the Miners Chapel Rock Member Member from the underlying Jacobsville Formation and the overlying Miners Castle member. At At Miners MinersCastle, Castle, they they are are abundant. abundant. They Theyrepresent represent deep deep erosional erosional channels channels that were cut into the older units. Near Near the the base base of the Chapel Rock member, pebbles become abundant in the cross-beds that fill the troughs. Hamblin cross-beds Hamblin(1958) (1958)suggests suggeststhat thatthey theyare areindicative indicative along aa cuspate cuspate shore. shore. Ripples marks are less abundant and of marine deposition in embayments along are both current-formed and oscillatory. Where Wherecurrent-formed, current-formed,they they indicate indicate aa mean mean transport transport direction to the northwest with little little standard standard deviation about that mean (see figure illustrating illustrating cross-bedding directions directions in the the U.P.). This Thisindicates indicatesaafairly fairlysteep steepgradient gradientwith with little little cross-bedding 169 �11 UPLI6*TION S V •t•*d O$Iø *1 — bs#' an a4 — biSS d.ttli. 4 A'SM I.l.. s4 I 474 t1 isa— ii—.— ii I——— iii ii. *i• tsis. 0 G••t_ — .9 0 •? to Cross bedding bedding directions directions in in the the Jacobsville Jacobsville Sandstone Sandstone (after (after Hamblin, Hamblin, 1958) 1958) Paleogeographyduring during deposition depositionof of the theJccobsville JacobsvilleSandstone Sandstone(after (afterHamblin, Hamblin, 1958) 19581 Paleogeography 170 170 �1 Cflt*N*?IO V — •Iadad — ..d at d.(i.IIa I tetIa .1 n. - g*.• •.n k..—.' - • ••—' I— 4, t%St fr.at If SpI —a. Tnqt.....q P•fIi t — — Mtt • ft !a.f •— If so I-. Is ma'.. Cross Crossbedding beddingdirections directionsininthe theChapel ChapelRock Rockmember member ofofthe theMunising MunisingFormation Formation(affer (afterRamblin, Hamblin,1958) 1958) Paleogeography Paleogeographyof ofNorthern NorthernMichigan Michiganduring during deposition deposition of the the Chapel ChapelRock Rockmember member 958) ofthe theMunising MunisingFormation Formation(after (afterHamblin, Hamblin, 1958) of 171 171 �meandering. meandering.InInthe theupper upperpart partofofthe theChapel ChapelRock Rockmember, member,mudcracks mudcracksare arefound foundininthe thefinefinetextured cm in in diameter diameterand and the theintervening intervening textured sandstone sandstoneand andthe theshale shalebeds. beds. They Theyare areabout about55cm cracks cracksare arefilled filled with with sand. sand. Paleogeographically Paleogeographicallyand andenvironmentally, environmentally,they theyindicate indicatedeposition depositionin in aa shallow shallowwater, water, subaerial subaerialenvironment environmentwith with the the sea sea regressing regressing northward. These Thesebeds bedsgrade grade directly into the Miners Castle Member and suggest a minor regression of the Chapel Rock directly into the Miners Castle Member and suggest a minor regression of the Chapel Rocksea. sea. Compact Compact till till isis found found overlying overlying the the bedrock in this valley. The Thetill tillisisreddish reddishin incolor, color,and and texturally ranges from clay to cobbles. It is likely Marquette age (about 10,000 YBP) and was texturally ranges from clay to cobbles. It is likely Marquette age (about 10,000 YBP) and was deposited deposited as as the the Munising Munisingmoraine moraineand and Kingston Kingston outwash outwash plain were being deposited (Phase I1of of thi previous previousdiagrams). diagrams). the Grand Marais Marais Harbor Harbor Stop 11. Grand Grand Grand Marais Marais Harbor Harborisisthe theonly only Harbor Harbor of of Refuge Refuge between between Marquette Marquetteand and Sault SaultSt. St. Marie. Native Americans. Americans. Later, Marie. Originally, Originally,the theprotected protectedbay bay was was aa summer summer camping place for Native French voyageurs used the bay as stopping place to either prepare for, or recuperate French voyageurs used the bay as stopping place to either prepare for, or recuperate from, from, the the dangerous dangerouspaddle paddle along alongthe thecliffs cliffsof of the the Pictured Pictured Rocks Rocksduring during their their travels travelsalong alongthe thesouth southshore shore of of Lake Lake Superior. Superior. The Thetown townitself itselfwas wasestablished establishedininthe the1860's 1860'sby by the the fishing fishing and and lumber lumber industries industriesas as aa boom boom town. town. However, However,by by1910, 1910,the thelumber lumberindustry industrydeclined declinedand and Grand Grand Marais Marais became became mainly mainly a commercial commercial fishing fishing port. Eventually, Eventually,this thisindustry i n d u s e also also declined declined and and the the community community became became all all but aa ghost ghost town. Today, Today,Grand Grand Marais Maraisis is seeing seeing aa renaissance renaissance as as aa summer and and winter winterrecreation recreation town. town. summer Recent, Recent, soon-to-be-published soon-to-be-publishedresearch research at at the the Drexler-Carter Drexler-Carter(D-C) (D-C)site site(Anderton, (Anderton,in in progress), fill section section exposed exposed along along the the front front of of the the eastern eastern portion portion of of the theGrand Grand progress), aa paleo-valley paleo-valleyfill Sable Banks, suggests that prior to the Nipissing transgression, the Grand Marais area may Sable Banks, suggests that prior to the Nipissing transgression, the Grand Marais area may have have consisted consisted of of extensive extensive lagoon. lagoon. Evidence Evidencefor forthe thelagoon lagoonconsists consistsof of extremely extremelythick. thick. Compressed Compressed peat peat deposits, deposits,dated datedatatbetween betweenapproximately approximately7000-6000 7000-6000BP, BP, which which have have been been found foundatatthe theD-C D-C site, site, on on the the floor floor of of Grand Grand Marais Marais Harbor Harbor by by divers, divers, and and during during drilling drilling of wells wells to to the the east east of of the the Harbor. Harbor. The Thepre-Nipissing pre-Nipissinglagoon lagoonwas waslikely likelyprotected protectedbehind behindananextremely extremelylarge largebay baybarrier, barrier, which which may have have extended extended from from as as far far west west as as the the ancestral ancestral Grand Sable Sable Banks Banks or or Au Au Sable Sable Point. Point. The The high high ground groundand andassociated associateddune duneforms, forms,which which begin begin at at Lonesome LonesomePoint Point and andextend extendto tothe the east for for several several kilometers, kilometers, may may be be remnants remnants of of this this older older barrier, bamer, which was was apparently apparently breached breached during rising Nipissing levels. Much Muchfurther furtherwork work isis needed needed to to verify verify these these interpretations. interpretations. The The Nipissing Nipissing transgression transgressioninundated inundated the the Grand Grand Marais Marais area, area, flooding flooding the the mouth mouth of of the the Sucker Sucker River River and and extending extending inland inland for for several several kilometers kilometers south south of of the the high high ground ground that that begins begins at at Lonesome Point. Steep, Nipissing-age, wave-cut bluffs at an elevation of about 195 m (640 ft) Lonesome Steep, Nipissing-age, wave-cut bluffs at an elevation of about I95 m (640 ft) rim the the southern southern edges edges of the town and harbor. Much Much of of the the town town appears appears to to have have been been built built on on aa Nipissing wave-cut wave-cut platform. platform. Nipissing Recently, Recently, Grand Grand Marais Marais Harbor Harbor has has been been the the focus focus of a great great deal of concern by local local residents West Bay. Bay. From residents in regard to the condition of West Fromthe thetime time of of the the earliest earliest written written records and and maps maps of of Grand Grand Marais, Marais,the the bay bay was was originally originally portrayed as relatively relatively deep water lagoon, protected protected behind behind aa low, low,sandy sandybaymouth baymouth barrier barrier that that extended extended from from the the west west side side of of the the current current town, town, east east to to Lonsome Lonsome Point. Point. AAsmall smallchannel, channel,which whichallowed allowedwater waterto to pass pass from from the the bay bay into into the to the harbor. harbor. This the waters waters of of lake lake Superior, Superior, provided provided the only navigable entrance to This natural natural channel, channel, adequate adequate for for several several centuries centuries for for Native Americans Americans and Voyageurs in canoes, was was prone to sand sand bar development, development, which which interfered interfered with commercial shipping shipping associated associated with with the the lumber 800ts. Consequently, various harbor lumber and and fishing fishing industries industries that that began began in in the the late late 11800's. protection protection structures, structures, including includingeventually eventually aa large large breakwater, were built beginning beginning as as early early as as 172 17 2 �L Stop 1 — jt/ioravel Pita - et; Grav4f+t I -. - 'a $k — 3__'_ RM868 - — r -. 3 87O •1 lIlT H V-- -. — Eastward-slopingmeliwater meltwaterdischarge dischargechannels channels southofofGrand Grand Marais Easiword-sloping south Morals 17 3 �1883 1883 to stop sand bars from from blocking blocking the the entrance entrance to the bay (USCOE, (USCOE, 1980). 1980). , Over Over the the years, years, the the harbor harbor protection protection structures structureshave have effectively effectively blocked blocked coastallycoastallytransported transported sand, sand, that that would would have have otherwise otherwisebeen been conveyed conveyed by by littoral littoral drift drift processes, processes,from from replenishing the eastern portion of the original bamer. Consequently,this this part part of of the the barrier barrier has has replenishing barrier. Consequently, been removed removed by by wave wave erosion, erosion, exposing exposing nearly nearly the the entire entire east east end end of the the bay bay to to Lake Lake Superior. Superior. On the west side side of of the the structures, structures,sand sand eroded erodedfrom from the the Grand Grand Sable Sable Banks Banks and and transported transportedto to the the east east has has been been deposited. deposited. Since Since 1961 1961the the beach beach in this area has been growing at a rate of 38,000 cubic m per year (USCOE, 1980). According (1990),the the beach beach has has widened widened by by 1980). Accordingto to Marsh Marsh (1990), 2500-3000 2500-3000 m, m, and, and,ifif left leftunchecked, unchecked,will willeventually eventuallywiden widen to to the thepoint pointwhere wherethe thebeach beach effectively cuts off wave-contact wave-contact with the Grand Sable Banks. ifIf this thishappens, happens,the thesand sandsupply supplyto to effectively the the Grand Grand Sable SableDunes, Dunes,which which isisultimately ultimately tied tied to to lake lakelevels levelsand andbluff bluff recession, recession,will willbe be artificially reduce reduce dune dune nourishment nourishment rates. In Ineffect, effect,the theDunes Dunesmay may cease cease to to exist. exist. artificially Stop stop 12 12 Stop Stop 12 12is is not not really really aa stop, stop, but but an an overview ovewiew of of channels channels as as we we leave leave Grand Grand Marais Marais southward southward on on M-77. M-77. Eastward Eastwarddischarging dischargingstreams streamscut cutchannels channelsatatsuccessively successivelylower lower elevations elevations as the the Marquette Marquette (Grand (Grand Marais) Marais) ice ice retreated retreated into the Lake Superior basin. At least least 88channels channels as basin. At are are recognized recognized from from 207 207 meters meters to 271 meters. Leverett Leverett(1929) (1929)and andothers othersmistakenly mistakenlyinterpreted interpreted these these surfaces surfaces as as Lake Lake Algonquin wave-cut platforms. Hughes Hughes (1968), (19681,through through use use of of aneroid aneroid barometers and and topographic topographicmaps, maps,determined determined they they slope slopeeastward eastward and and asserted assertedthat that they theyare are barometers glacial glacial spillways. spillways. Further, Further,Hughes Hughesrecognized recognizedthe thefact factthat, that, in in profile, profile, the the surfaces surfacesactually actually slope slope southward southward toward toward the the next next escarpment escarpment(characterized (characterized by by poorly poorly drained drained swampy swampyregions regions at at the the south theywere werewave-cut wave-cutterraces, terraces,the the slope slope in in profile profile would would be be in in the the south edge edge of of the the surfaces). surfaces). ifIfthey opposite direction. opposite direction. 174 �REFERENCES Bach, Bach, D.P. 1978. 1978.Plant Plant Communities, Communities,Habitats Habitatsand and Soil Soil Conditions Conditionsof of Grand Grand Sable SableDunes, Dunes, Pictured Rocks National Lakeshore, Michigan. Unpublished Masters Thesis, Michigan Pictured Rocks Lakeshore, Unpublished Master's Thesis, Michigan Technological Technological University, University, 180 180p. p. Bergquist, Bergquist, S.C. S.C. 1936. 1936.The The Pleistocene Pleistocenehistory history of of the the Tahquamenon Tahquamenon and and Manistique Manistique drainage drainage region of hblication 40, 40, of the the northern northern peninsula peninsula of of Michigan. Michigan. Michigan Michigan Geological Geological Survey Survey Publication Geological GeologicalSeries Series34, 34,part part11pp.7-148. pp.7-148. Blewett, Blewett, W.L. W.L. and and R.L. R.L. Rieck. Rieck. 1987. 1987. Reinterpretation Reinterpretationof aa portion of the the Munising Munising Moraine Moraine in in northern northern Michigan. Michigan.Geological GeologicalSociety Societyof ofAmerica AmericaBulletin Bulletin98:169-175 98: 169-175 Blewett, Blewett, W.L. 1994, 1994,Late Late Wisconsin Wisconsin History History of Pictured Rocks Rocks National Lakeshore Lakeshore and Vicinity. Vicinity. National Park Park Service Service PIRO P R O 94-01, 94-01,8 p. National 8 p. Carey, L. 1993. 1993. Interim Interimsoil soilsurvey surveyof ofPictured PicturedRocks RocksNational NationalLakeshore, Lakeshore,Alger AlgerCo., Co.,Michigan. Michigan. USDA, USDA, Soil Soil Conservation ConservationService, Service,Marquette, Marquette,MI. MI. Drexler, Drexler, C.W. C.W. 1981. 1981.Outlet Outletchannels channelsfor for the the post-Duluth post-Duluth lakes lakes of of the the Upper Upper Peninsula Peninsula of of Michigan. PhD PhD dissertation, dissertation, University University of of Michigan, Michigan, Ann Ann Arbor. Arbor. 295 295 p. p. Michigan. Ells, Ells, G.D., G.D., 1967, 1967,Correlation Correlationof of Cambro-Ordovician Cambro-Ordovician Rocks Rocks in in Michigan. Michigan. in in Correlation CorrelationProblems Problems of the Cambrian and Ordovician Outcrop Areas, Northern Peninsula of Michigan, Michigan Cambrian and Ordovician Outcrop Peninsula Basin Geological } pp. Geological Society SocietyAnnual Annual Field FieldExcursion, Excursion,M.E. M.E. Ogstrom Ogstromand andA.E. A.E.Slaughter, Slaughter,{eds. {eds.] pp. 42-57. Farrell, Farrell, J.P. and and J.D. J.D. Hughes Hughes 1985. 1985. Long Long Term Implications, Implications, From a Geomorphological Geomorphological Standpoint, Location at at Grand Grand Sable SableLake. Lake. Contract Report Standpoint, of Maintaining Maintaining H-58 in its Present Location PX60000-4-0839, PX60000-4-0839, submitted submittedto tothe theNational NationalPark ParkService, Service,78 78p.p. Hamblin, Hamblin, W.K., 1958, 1958,Cambrian Cambrian Sandstones Sandstonesof of Northern Michigan. Michigan Michigan Dept. of Natural Natural Resources Pub. 51, 51, 146 146p. p. Resources Larsen, Larsen, C.E. 1985. 1985.Lake Lake Level, Uplift, Uplift, and Outlet Incision, The Nipissing and Algoma Great Lakes. In Quaternary Calkin. Quaternary Evolution Evolution of of the the Great GreatLakes, Lakes, edited edited by by P.F. P.F. Karrow Karrow and and P.E. P.E. Calkin. Geological GeologicalAssociation Associationof of Canada CanadaSpecial SpecialPaper Paper30. 30. Leverett, U.S. Geological Survey Leverett, F. 1929. 1929.Moraines Moraines and and shorelines shorelines of the Lake Superior region. U.S. Prof. Paper Paper 154-A. 154-A.72 72 p. p. Levin, H.L., H.L., 1996, 1996, Earth Earth Through Through Time. Time. 5th 5~ ed., Saunders, Saunders, 568 568 p. Marsh, W.M. 1990. 1990.Nourishment of Perched Sand Sand Dunes and the Issue of Erosion Control in the Great Lakes. Environ. Environ.Geol.Water GeoLWaterSci. Sci.16(2): 16(2):155-164. 155-164. 175 �Marsh, W.M. and B.D. Marsh 1988. 1988. Wind Erosion and Sand Dune Formation Formation orj on High Lake Superior Bluffs. Geografiska GeografiskaAnnaler Annaler 69A 69A (3-4):379-391. (3-4):379-391. Schaetzl, R.J. and D.L. Mokma 1988. 1988. A Numerical Index of Podzol and Podzolic Soil Development. Development. Physical Physical Geography Geography 9(3):232-246. 9(3):232-246. USCOE, 1980, 1980, Final Detailed Detailed Project Project Report on Shore Shore Damage of Grand Grand Marais Harbor, Harbor, Section ifi, in, Detroit, Detroit, MI MI Detroit Detroit District. Michigan, Section 176 � Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology: Proceedings, 1999 Description An account of the resource Institute on Lake Superior Geology. Marquette, Michigan. May 4-8, 1999. Creator An entity primarily responsible for making the resource Institute on Lake Superior Geology Date A point or period of time associated with an event in the lifecycle of the resource 1999 Format The file format, physical medium, or dimensions of the resource PDF Language A language of the resource English https://digitalcollections.lakeheadu.ca/files/original/c95bde6817524fac0b7a49791755d404.pdf 8b20a8aef357cb8f905e7b7c0b74dba7 PDF Text Text 4 FORTY-SIXTH ANNUAL MEETING St ON LAKE SUPERIOR : SINSTITUTEGEOLOG S — Si h1*r Bay,Ontario — S May. 08-13,2000 44 '1 44 . 4. ••r :4 .tssi.:. S S S S PROCEEDINGS VOLUME 46 45 /4 4 /4 /4 4/4 PARfl: PROGRAMAND ABSTRACTS �46th I.L.S.G. rogram & Abstracts 2000 May 8-13, 8-13,2000 Hosted at at Lakehead University Universi by the Geology Department Thunder Bay, Ontario Canada �46thAnnual Annual Meeting Meeting 46th 1 II INSTITUTE ON LAKE SUPERIOR GEOLOGY U Volume 46 contains the following parts: parts: Part 1: 1: Program and Abstracts Part 2: Field Trip Guidebook — Geology of the Mesoproterozoic Sibley Group 1 -Geology 22 — PGE-Cu-Ni Mine - Geology Geology of the Lac Lac des des Ties Iles PGE-Cu-Ni Mine 33 — Geoarcheology and deglaciation history the Thunder Thunder Bay Bay area - Geoarcheology history of the 44 — - Geology Geology of the Paleoproterozoic Gunflint Formation 55 — - Glacial Glacial history and regional till sampling sampling in the Archean Shebandowan Shebandowan Greenstone Greenstone Belt 6— Finlayson Lake Greenstone Greenstone Geology of of the Archean Archean Steep Steep Rock Rock Lake Lake - Finlayson 6 -Geology Belt - Reference ce to the material material in in this this volume volume should should follow follow the the example examplebelow: below: A., 2000. Chemistry and mineralogy of Lower Chowlay Chocolay G roup rocks, Dickinson Dickinson Argast, A,, County, Michigan, Michigan, [abstract]: [abstract]: Institute Institute on on Lake Lake Superior SuperiorGeology GeologyProceedings, Proceedings,46th 46* Annual Meeting, Meeting, Thunder Thunder Bay, Bay, ON, ON, 2000; 2000; v.46, v.46, Part Part 1, 1, p. p. 1-2. 1-2. Volume 46 is published by the Institute on Lake Superior Geology and distributed by the Institute Institute Secretary-Treasurer. Secretary-Treasurer. Jirsa (term (term 1994-2000) 1994-2000) Mark Jirsa Minnesota Minnesota Geological Geological Survey Survey 2642 University Avenue St. Paul, MN USA 55114-1057 55 114-1057 (612)-627-4780 (6 12)-627-4780 email: jirsaOO iirsa00liätc.umn.edu 1(Sitc.umn.edu http://www.ilsgeology.org ILSG website: website: htto://www.ilsgeologv.org All volumes are are available available at at the the cost costofphotocopying ofphotocopyingfrom from Michigan Michigan Technical TechnicalUniversity UniversityArchives. TSSN 1042-9964 1042-9964 ISSN �INSTITUTE LAKE SUPERIOR SUPERIOR GEOLOGY GEOLOGY INSTITUTE ON ON LAKE CONTENTS Proceedings Proceedings Volume Volume 46 46 Part Part 1—Program I-Program and Abstracts Abstracts Editor: Stephen Editor: StephenA. A. Kissin Kissin Institutes on Institutes on Lake Lake Superior Superior Geology, Geology, 1955-2000 1955-2000 ..... . ...... iii ... By-Laws of the Institute Lake Superior Geology .... By-Laws of the Institute on on Lake Superior Geology . . . .v Constitution of Constitution of the the Institute Institute on on Lake Lake Superior Superior Geology Geology Goldich Goldich Medal Guidelines Guidelines Past Goldich Medallists Medallists ................... ..... . . . vivi ... ..... viii Vlll Goldich Medal Committee ............... ... viii ..... Vlll Citation Recipient....... Citation for 2000 2000 Goldieh Goldich Medal Recipient ...... ix . ................. . . . xi ... .............. . . . . . . xii ... xiii . . . . . . Xlll Student Paper Student Paper Awards Awards Committee Committee ..... ... . . . . . . Xxiii lll Student Travel Travel Awards Awards Student Travel Travel Award Application Application Form Form Student Paper Student Paper Awards Awards Board of Directors Directors ............. Xiv . . . . . . xiv Local Committees Committees ..... . . . . . . xiv ................................. Report of the the Chair Chair of of the the 45th 45th Annual Annual Institute Institute Meeting ...... Program ........................................ Banquet Speaker Banquetspeaker Abstracts Abstracts .... xv .......XV ...... xvi ...... Xix xix ... 1 . 1 �ON LAKE LAKE SUPERIOR SUPERIOR GEOLOGY, GEOLOGY, 1955-2000 1955-2000 INSTITUTES ON # DATE PLACE CHAIRS 1 1955 1 1955 Minneapolis, Minneapolis, Minnesota Minnesota Floughton, Michigan 1956 1956 Houghton, 3 1957 1957 East Lansing, Lansing, Michigan Michigan 44 1958 1958 Duluth, Duluth, Minnesota Minnesota 5 1959 1959 Minneapolis, Minneapolis, Minnesota Minnesota C.E. Dutton Dutton A.K. Sneigrove Snelgrove B.T. Sandefür Sandefiir R.W. Marsden Marsden G.M. Schwartz & C. Craddock Craddock (note (note Schwartz & 2 2 66 77 88 9 9 10 10 11 11 12 12 13 13 14 14 15 15 16 16 17 17 18 18 19 19 20 20 21 21 22 22 23 23 24 24 25 25 26 27 28 29 30 30 31 31 32 32 33 33 34 34 35 35 36 37 38 1960 1960 Madison, Madison, Wisconsin 1961 1961 1962 1962 1963 1963 1964 1964 1965 1965 1966 1966 1967 1967 1968 1968 1969 1969 1970 1970 1971 1971 1972 1972 1973 1973 1974 1974 1975 1975 1976 1976 1977 1977 1978 1978 1979 1979 1980 1980 1981 1981 1982 1982 1983 1983 1984 1984 1985 1985 1986 1986 1987 1988 1988 1989 1989 1990 1990 1991 1991 1992 1992 Port Arthur, Arthur, Ontario Ontario Houghton, Michigan Houghton, Michigan Duluth, Duluth, Minnesota Minnesota Ishpcming, Ishpeming, Michigan St. Paul, Minnesota Minnesota Sault Ste. Marie, Michigan Michigan East Lansing, Michigan Superior, Superior, Wisconsin Oshkosh, Wisconsin Thunder Bay, Ontario Ontario Duluth, Minnesota Houghton, Michigan Madison, Wisconsin Sault Ste. Marie, Ontario Marquette, Marquette, Michigan Michigan Minnesota St. Paul, Minnesota Thunder Thunder Bay, Ontario Ontario Wisconsin Milwaukee, Wisconsin Minnesota Duluth, Minnesota Eau Claire, Wisconsin East Lansing, Michigan International Falls, Minnesota Houghton, Michigan Wausau, Wisconsin Kenora, Ontario Ontario Wisconsin Rapids, Wisconsin Wisconsin Wawa, Ontario Ontario Marquette, Michigan Marquette, Duluth, Minnesota Minnesota Thunder Bay, Ontario Ontario Eau Claire, Claire, Wisconsin Wisconsin Hurley, Wisconsin I correction) correction) E.N. Cameron Cameron E.G. Pye Pye A.K. Snelgrove Snelgrove H. Lepp Lepp A.T. Broderick Broderick P.K. Sims & R.K. Hogberg R.W. White W.J. Hinze H i e A.B. Dickas A.B. G.L. LaBerge LaBerge M.W. Bartley & E. Mercy M.W. D.M. Davidson Davidson Kalliokoski J. Kalliokoski M.E. Ostrom Ostrom P.E. Giblin Giblin J.D. Hughes Hughes M. Walton Walton M.M. Kehienbeck Kehlenbeck U. Mursky G. D.M. Davidson Davidson P.E. Myers Myers W.C. Cambray W.C. D.L. Southwick Southwick D.L. Bomhorst T.J. Bornhorst LaBerge G.L. LaBerge Blackburn C.E. Blackburn J.K. Greenberg J.K. Greenberg Sage E.D. Frey & R.P. Sage J. S. Klasner J.C. Green Green M.M. Kehlenbeck Kehlenbeck P.E. Myers Myers A.B. Dickas Dickas �39 40 41 42 43 44 45 1993 1994 1995 1996 1997 1998 1999 46 2000 D.L. Southwick Southwick T.J. Borithorst Bornhorst M.C. Smyk L.CI.Woodruff Woodruff L.G. R.P. Sage, W. Meyer J.D. Miller, M.A. Jirsa T.J. Bornhorst, R.S. Regis S.A. Kissin, P. Fralick Eveleth, Minnesota Minnesota Houghton, Houghton, Michigan Michigan Marathon, Ontario Ontario Cable, Wisconsin Wisconsin Sudbury, Ontario Ontario Minneapolis, Minnesota Marquette, Marquette, Michigan Michigan Thunder Bay, Ontario Ontario 11 �CONSTITUTION CONSTITUTION LAKE SUPERIOR SUPERIOR GEOLOGY GEOLOGY OF THE INSTITUTE ON LAKE (Last amended by the the Board—May Board-May 8, 1997) 1997) Article II Article Name The name of the organization organization shall be the "Institute on Lake Superior Geology". Geology". Article II Article I1 Objectives Objectives The objectives objectivesof of this this organization organizationare: are: A. To Toprovide provideaameans meanswhereby whereby geologists geologists in in the the Great Great Lakes Lakes region region may exchange ideas and scientific scientific data. B. To Topromote promotebetter better understanding understanding of of the geology of the Lake Superior Superior region. region. C. To Toplan plan and and conduct conduct geological geological field field trips. Article III Article 111 Status && No part of of the income income of the organization organization shall shall insure insure to the benefit benefit of of any any member or individual. In the event of dissolution, the assets of the organization shall organization shall be distributed distributed to to _________(some free (some tax free organization). organization). (To avoid Federal and State income taxes, the organization should be not educational, but only "scientific" or "educational, but also also "non-profit") "non-profit") Anno. 290.01, 290.01, subd. subd. 44 Minn. Stat. Stat. Anna. Minn. Stat. Stat. Anno. 290.05(9) 290.05(9) 1954 1954 Internal Revenue Revenue Code Code s.501(c)(3) s.501(~)(3) Article IV Article Membership Membershir The membership of the organization shall consist of persons who have registered for an annual meeting within the past three three years, and those who indicate interest in being a member according to guidelines approved by the Board of Directors. Directors. Article V Article Meetings Meetings The organization shall meet once aa year. year. The The place and exact date of each meeting will will be be designated designated by by the the Board Board of of Directors. Directors. Article VI Directors Directors The Board of Directors shall consist of the Chair, Secretary-Treasurer, and the last three past Chairs; but if the board should at at any any time time consist consist of of ... I"I ll �fewer than five persons, by reason of unwillingness or inability of any of the above persons to serve as directors, the vacancies on the board may be be filled by the Chair so as to bring the membership of the board to five members. members. Article ArticleVII VII Officers Officers The The officers officers of of this this organization organization shall shall be be aa Chair Chair and and Secretary-Treasurer. Secretary-Treasurer. A. A. The TheChair Chairshall shallbe beelected electedeach eachyear year by by the the Board Board of of Directors, Directors, who shall give due consideration to the wishes of any group that shall give due consideration to the wishes of any group that may may be be promoting the next annual His/her term of office as Chair will promoting the meeting. Hisher term of office as Chair will terminate terminate at at the the close close of of the the annual annual meeting over over which which he/she helshe presides, or or when when his/her hisiher successor shall have been appointed. He/she Helshe will then then serve serve for for aa period period of of three three years years as as aa member member of of the the Board Boardof ofDirectors. Directors. B. The Secretary-Treasurer shall be elected at the annual B. The Secretary-Treasurer shall be elected at the annualmeeting. meeting. His/her Hisiher term term of of office office shall shall be four four years, or until his/her hisher successor successor shall shall have been appointed. have been appointed. Article ArticleVIII VIII Amendments Amendments This This constitution constitutionmay may be be amended amended by by aa majority majority vote vote (majority (majority of of those those voting) voting)of of the the membership membershipof ofthe theorganization. organization. iv �BY-LAWS OF THE INSTITUTE ON LAKE LAKE SUPERIOR GEOLOGY I. Duties Dutiesof of the the Officers Officers and Directors A. ItItshall shallbe bethe theduty dutyof ofthe the Annual Annual Chairman Chairman to: 1. Preside Preside at at the the annual annual meeting. meeting. 2. Appoint Appointall allcommittees committeesneeded needed for for the the organization organizationof of the the annual annual meeting. meeting. 3. Assume Assumecomplete completeresponsibility responsibility for forthe the organization organizationand and financing financing of of the the annual meeting over which he/she presides. helshe B. ItItshall shallbe bethe theduty dutyof ofthe theSecretary-Treasurer Secretary-Treasurerto: 1. Keep Keep accurate accurateattendance attendance records records of of all all annual annual meetings. meetings. 2. Keep Keepaccurate accuraterecords records of of all all meetings meetings of, and correspondence correspondence between, the Board of Directors. Directors. 3. Hold Holdall allfunds fundsthat that may may accrue accrue as as profits profits from from annual meetings meetings or field field organization and operation trips and to make these funds available available for the organization operation of future future meetings meetings as as required. required. C. ItItshall shallbe bethe theduty dutyof ofthe the Board Board of of Directors Directorsto to plan locations locations of of annual annual meetings meetings and to advise advise on the organization organization and financing financing of all meetings. II. Duties 11. Duties and and Expenses Expenses A. Regular $5.00or or less less on on an an annual annual basis basis shall shall be assessed assessed Regularmembership membershipdues duesof of $5.00 each each member member as as determined determined by by the the Board Boardof of Directors.. Directors.. B. Registration Registrationfees feesfor forthe the annual annual meetings meetings shall shall be determined by the Chair in Directors. The consultation with the Board of Directors. The registration registration fees fees can include include determined by expenses to cover operations outside of the annual meeting as determined the Board of Directors. ItItisisstrongly recommended that registration strongly recommended that registration fees fees be be kept at a minimum minimum to to encourage encourage attendance attendance of of students. students. III. Rules 111. Rules of of Order The rules contained contained in Robert's Rules of Order shall govern this organization organization in in all all cases cases to which which they they are are applicable. applicable. IV. Amendments Amendments by aa majority majorityvote vote (majority (majorityof ofthose thosevoting) voting)of of These by-laws may be amended by the membership membership of of the organization; organization; provided provided that such such modifications modifications shall shall not not conflict with the constitution as presently adopted or subsequently amended. constitution v �GOLDICH GOLDICH MEDAL MEDAL GUIDELINES GUIDELINES (Adopted (Adopted by the the Board Board of of Directors, 1981; 1981;amended amended 1999) 1999) Preamble Preamble The The Institute Institute on on Lake Lake Superior Superior Geology Geology was born in 1955, 1955, as documented documented by the fact fact that that the 27th annual meeting was held in 1981. The Institute's continuing objectives are to 27th annual The Institute's continuing objectives are deal with those those aspects aspects of of geology that are related geographically geographically to Lake Superior; Superior; to encourage encourage the discussion discussion of of subjects subjects and sponsoring sponsoring field trips that will bring bring together together geologists geologists from from academia, academia, government surveys, surveys, and industry; and to maintain an an informal informal but but highly highly effective effectivemode modeof ofoperation. operation. During During the the course course of of its its existence, existence, the the membership membershipof of the the Institute Institute (that (that is, is, those those geologists who indicate an interest in the objectives of the ILSG by attending) geologists indicate an interest objectives attending) has has become become aware aware of of the the fact fact that that certain certain of of their their colleagues colleagues have made particularly noteworthy noteworthy and and meritorious meritoriouscontributions contributionsto to the the understanding understanding of of Lake Lake Superior Superiorgeology geology and and mineral mineral deposits. deposits. The The first first award awardwas was made made by by ILSG ILSG to to Sam SamGoldich Goldich in in 1979 1979for for his his many many contributions contributionsto to the the geology geologyof of the the region regionextending extendingover over about about 50 50 years. years. Subsequent Subsequent medalists medalists and and this this year's year's recipient recipientare arelisted listedin inthe thetable tablebelow. below. Award Award Guidelines Guidelines 1) 1) The Themedal medalshall shallbe beawarded awardedannually annuallyby bythe theILSG ILSGBoard Boardof ofDirectors Directorsto toaageologist geologist whose whose name name is is associated associated with a substantial substantial interest in, and contribution contributionto, to, the the geology geology of the Lake Superior region. of the Lake Superior region. 2) The TheBoard Boardof ofDirectors Directorsshall shall appoint appoint the Goldich Medal Committee. The The initial initial appointment appointment will will be be of of three three members, members, one one to to serve serve for for three three years, one one for for two two years, years, and and one one for for one one year. year. The Themember memberwith withthe thebriefest briefestincumbency incumbencyshall shallbe be chair chair of of the the Nominating Committee. After the first year, the Board of Directors shall appoint Nominating Committee. After the first year, the Board of Directors shall appoint at at each each spring years. In histher third third year year this this spring meeting meeting one one new new member member who will serve for three years. In his/her member member shall shall be be the the chair. chair. The TheCommittee Committeemembership membershipshould shouldreflect reflect the the main main fields fieldsof of interest interestand andgeographic geographicdistribution distributionofofILSG ILSGmembership. membership. 3) 3) By Bythe theend endofofNovember, November,the theGoldich GoldichMedal Medal Committee Committeeshall shall make make its its recommendation recommendationto to the theChair Chairof ofthe the Board Board of of Directors, Directors, who who will will then then inform inform the the Board Board of the nominee. of the nominee. 4) 4) The TheBoard BoardofofDirectors Directorsnormally normallywill willaccept acceptthe thenominee nomineeof ofthe theCommittee, Committee,inform inform the the medallist, medallist,and andhave haveone onemedal medalengraved engravedappropriately appropriatelyfor for presentation presentation at at the the next next meeting meetingof ofthe theInstitute. Institute. vi �I sources, such 5) ItIt isisrecommended recommendedthat that the the Institute Institute set set aside aside annually from whatever sources. funds as will be required to support the continuing continuing costs of this award. award. Nominating Nominatine Procedures Procedures 1) The The deadline deadline for for nominations nominations is November 1. Nominations shall be taken at any time by the Goldich Medal Committee. Committee Committee members members may themselves themselves nominate nominate candidates; however, Board members may not solicit for or support individual candidates; individual nominees. 2) Nominations Nominationsmust mustbe bein inwriting writing and and supported supported by by appropriate appropriate documentation documentation such such as as letters of recommendation, lists of publications, curriculum vita's, and evidence of lists curriculum evidence contributions contributions to Lake Superior Superior geology and to the Institute. Institute. 3) Nominations Nominationsare arenot notrestricted restricted to to Institute Instituteattendees, attendees, but are open to anyone anyone who has worked on and contributed to the understanding of Lake Superior Superior geology. Selection Selection Guidelines Guidelines 1) Nominees Nominees are are to to be be evaluated evaluated on on the the basis of their contributions contributions to Lake Superior geology (sensu (sensu lato) lato) including: including: geology a) importance importanceof ofrelevant relevantpublications; publications; b) promotion promotionof ofdiscovery discoveryand andutilization utilizationof of natural natural resources; resources; c) contributions contributionsto tounderstanding understandingof of the the natural natural history history and and environment environmentof of the the region; region; d) generation generationof ofnew new ideas ideas and and concepts; and e) contributions contributionsto to the the training training and and education education of geoscientists geoscientists and the public. 2) Nominees Nomineesare areto tobe beevaluated evaluatedon ontheir their contributions contributionsto to the the Institute Institute as as demonstrated demonstrated by attendance at Institute meetings, presentation of talks and posters, and service attendance at Institute of talks service on on Institute Institute boards, committees, committees, and and field field trips. trips. 3) The Therelative relativeweights weightsgiven givento to each each of of the foregoing foregoing criteria must remain flexible and at the discretion discretion of of the the Committee Committee members. members. 4) There Thereare areseveral severalpoints pointsto tobe beconsidered consideredby by the the Goldich Goldich Medal Medal Committee: Committee: a) An attempt should be made to maintain a balance of medal recipients from An attempt should be made to maintain a balance of from each each of the the three three estates—industry, estates-industry, academia, and government. government. b) ItItmust mustbe benoted notedthat thatindustry industrygeoscientists geoscientistsare are at at aa disadvantage disadvantage in that much of of their work in not published. 5) Lake Canada. This LakeSuperior Superiorhas has two two sides, sides, one one the U.S., U.S., and the other Canada. This is is undoubtedly undoubtedly one of the Institute's great strengths and should be be nurtured nurtured by by equitable equitable recognition recognition of of excellence excellence in in both both countries. countries. vii �GOLDICH MEDALISTS MEDALISTS 1979 Samuel Samuel5. S.Goldich Goldich 1990 Kenneth C. Card Card KennethC. 1980 not not awarded awarded 1980 1991 William Hinze 1991 1981 1981 Carl E. Dutton, Jr. 1992 William William F. F. Cannon 1982 Ralph RalphW. W. Marsden Marsden 1993 Donald Donald W. W. Davis Davis 1983 Burton BurtonBoyum Boyum 1994 Cedric CedricIverson Iverson 1984 Richard RichardW. W. Ojakangas Ojakangas 1995 Gene GeneLaBerge LaBerge K. Sims Sims 1985 Paul Paul K. 1996 David David L. L. Southwick Southwick 1986 G.B. G.B.Morey Morey 1997 Ronald P. Sage Sage Ronald P. 1987 Henry HenryH. H. Halls Halls Petennan 1998 1998 Zell Peterman 1988 Walter WalterS. S.White White 1999 Tsu-Ming Tsu-Ming Han 1989 Kalliokoski 1989 Jorma Jorma}Calliokoski 2000 John 2000 John C C. Green Green GOLDICH MEDAL COMMITTEE COMMITTEE1999-2000 1999-2000 members (Committee (Committeemembership membershipextends extendsthrough throughthe the meeting meeting year year shown; shown; replacement replacement members that year. year. Note will be selected during the Board of Directors' annual meeting of that Note that that the the 1999 Board of Directors) Directors) term of each each committee committee member was extended 11 year by the 1999 John Klasner John Klasner(2000) (2000) Western Illinois Illinois University, Macomb, Illinois Illinois Mark Mark Smyk Smyk(2001) (2001) Ontario Ontario Geological Geological Survey, Survey, Thunder Thunder Bay Bay Johnson (2002) Rod Johnson (2002) Rod Johnson and Associates, Associates, Negaunee, Michigan viii viii �CITATION John C C.Green, Green, Medal Recipient 2000 Goldich Medal John C. C. Green, Green, Professor Professor Emeritus Emeritus of of the the Department Department of ofGeological GeologicalSciences, Sciences, University of Minnesota-Duluth, has had a long, distinguished, and on-going career has distinguished, and on-going careerin in the the Lake Superior Superior region. He received his B.A. B.A. Degree Degree in in Geology Geology from from Dartmouth DartmouthCollege Collegeinin 1953. 1953. He at the the University University of of Oslo Osloon onaaFulbright FulbrightFellowship. Fellowship. He then spent a year amid Norskies at Ph.D. in 1960, both both from from Harvard Harvard University. University. With received his M.A. in 1956 1956 and his Ph.D. With his his dissertation on Appalachian geology of an area in New Hampshire/Maine not quite dissertation on Appalachian geology of in New Hampshire/Maine not quite finished, New Englanders Englanders John and Jan moved to Duluth in 1958 1958 when John accepted the position at UMD. position UMD. Most Mostofofhis hissubsequent subsequentsummers summers were were spent spent mapping mapping in in northern northern He did the Archean portion of the Minnesota for the Minnesota Geological Survey. the the Minnesota Geological Survey. the Archean Gabbro 1966), the first first detailed map map Gabbro Lake 15' 15' Quadrangle Quadrangle(Green, (Green, Phinney, Phinney, and Weiblen, 1966), in the that was undertaken by the MGS the remapping remapping of the the Precambrian Precambrian that undertaken by MGS under under the the leadership of Paul Sims. Whereas Minnesota has been his primary working domain, Whereas Minnesota has been his primary working domain, he he has taught at the Wasatch-Uinta Wasatch-Uinta Geology Field Camp in Utah for six summers, has spent a year year as as aaNASA-NRC NASA-NRC Senior Senior Research Research Associate Associate at the the Johnson Johnson Space Space Center Center in in including study Houston, and has traveled widely — Japan, Iceland, and Namibia — to Iceland, and Namibia - to study and has traveled widely - including volcanic volcanic rocks. rocks. John has served as Head of UMD's Department Department of Geology, has served served on on dozens dozens of university committees, and has chaired several of them. them. He Hewas wason onthe theAll-University All-University Council has chaired chaired the the Board of of Directors Council on Environmental Environmental Quality for eight years. He He has of the Lake Superior Basin Studies Center. He has taught 20 different courses, including Superior Studies He has taught 20 different courses,including several that that deal with geology, one one of of his many He has several with environmental environmental geology, many interests. interests. He has supervised 13 supervised 13 M.S. theses theses and has served served on on 61 61 M.S. and and 55 Ph.D. examining examining committees. committees. He has has authored authored or or co-authored co-authored more more than than 100 100publications, publications, maps, maps, field field guides, guides, and and abstracts. abstracts. John's has been been the theNorth NorthShore ShoreVolcanic VolcanicGroup Group—John's main main scientific scientific interest interest has physical volcanology, stratigraphy, physical volcanology, structure, structure, petrology, petrology, and and chemistry chemistry—- and and the the in general. general. Indeed, the name of John C. Green is synonymous Midcontinent Rift System in with with the NSVG. NSVG. He is a most careflil geologist. Today, careful and observant field geologist. Today, at the the age age of of 68, 68, long long would have have hung hung up up their field boots after many geologists would boots and settled for writing the paper about about their their glory glory days, he is still out their leading field trips, umpteenth summary paper beating the bush and the bugs, and hunting for the outcrops that that will will bring bring him him a little beating John doesn't just look at the rocks! He is the rare "complete closer is the rare "complete closer to the the truth. truth. John doesn't just look at the rocks! naturalist", for he knows every species of flora and and fauna fauna as as well. well. And, And, he he isis very very active active in preservation preservation ooff nature for the enjoyment enjoyment of all. His foresight, initiative, and drive resulted in in the the preservation preservation of of Sugarloaf Sugarloaf Cove/Point CoveIPoint near Schroeder on the North Shore, Shore, with its unique geology, geology, flora, flora, and cultural cultural history, as aa protected protected Natural and and Scientific Scientific ix �Site. Site. He was was instrumental instrumental in the the founding founding and and continuance continuance of SICA, SICA, the Sugarloaf Sugarloaf Interpretative Center Association. Interpretative Center Association. One of his his more more recent recent accomplishments accomplishments is his book on on the the geology geology and and natural natural history of the state parks of the North Shore, entitled "Geology on Display". history state parks of the North Shore, entitled "Geology on Display". It was was published by the 1996. published the Minnesota MinnesotaDNR DNR in in 1996. John has been one one of of the the strongest strongest supporters supporters of the the Institute Institute on on Lake Lake Superior Superior Geology over the last several decades. decades. He presented his first talk there in 1959, 1959, and has added 28 abstracts abstracts since since then. then. It is because of the dedication and support of people people like like John, that the ILSG has remained such a viable and healthy organization for nearly half remained such a viable and healthy organization for nearly half aa century. century. geologist, the the laboratory John C. Green Green—- the the tireless tireless field field geologist, laboratory scientist, scientist, the John C. demanding teacher, teacher, the knowledgeable and and cooperative colleague, the educator of the cooperative colleague, the the most deserving deserving of the public, the complete complete naturalist, naturalist, and and the the advocate advocateof ofILSG ILSG—- is is most Goldich Medal! Medal! All of us us assembled assembled here, as well well as as wife wife Jan, Jan, daughters daughters Martha Martha and and Sarah, sons-in-law Joe and Richard, and five grandchildren, are proud of you! and five grandchildren, are proud of you! Congratulations, J.C.! Congratulations, J.C.! x �4 EISENBREY EISENBREYSTUDENT STUDENTTRAVEL TRAVELAWARDS AWARDS The The 1986 1986Board Board of ofDirectors Directorsestablished established the the ILSU ILSG Student Student Travel Travel Awards Awards to to support support student Institute. The student participation at the annual meeting meeting of the Institute. Thename name "Eisenbrey" "Eisenbrey" was was added Eisenbrey(1926-1985) (1926-1985)and andutilize utilize added to to the theaward awardinin1998 1998totohonor honorEdward EdwardH.H.Eisenbrey contributions contributions made made to to the the 1996 1996Institute Institutemeeting meetingininhis hisname. name. "Ned' "Ned"isiscredited creditedwith with discovery of significant massive sulfide sulfide deposits deposits in in Wisconsin, significant volcanogenic volcanogenic massive Wisconsin, but his his talents as scopewas was much muchbroader—he broader-he has been described as having unique talents as an an ore ore finder, finder, scope geologist, geologist, and and teacher. teacher. These Theseawards awardsare areintended intendedtotohelp helpdefray defraysome someof ofthe thedirect directtravel travel costs costs of of attending attending Institute Institute meetings, meetings, and include include a waiver waiver of of registration registration fees, fees, but but exclude exclude expenses expenses for for meals, meals, lodging, lodging, and and field field trip registration. registration. The Thenumber numberofofawards awards and and value value are aredetermined determined by by the theannual annualChair Chairininconsultation consultationwith withthe theSecretarySecretaryTreasurer. Treasurer.Recipients Recipientswill willbe beannounced announcedatatthe theannual annualbanquet. banquet. The The following followinggeneral general criteria criteria will will be be considered considered by the annual annual Chair, Chair, who who is is responsible responsible for forthe theselection: selection: 1) 1) The Theapplicants applicantsmust musthave haveactive activeresident resident(undergraduate (undergraduate or or graduate) graduate)student studentstatus statusatai the thetime timeof ofthe theannual annualmeeting meetingof ofthe theInstitute, Institute,certified certifiedby by the thedepartment departmenthead. head. who are the senior author author on on either either an oral or poster 2) Students who poster paper paper will be be given given 2) favored favoredconsideration. consideration. 3) 3) ItItisisdesirable desirablefor fortwo twoorormore morestudents studentsto tojointly jointly request requesttravel travel assistance. assistance. 4) 4) InIngeneral, general,priority prioritywill willbebegiven giventotothose thoseininthe theInstitute Instituteregion regionwho whoare arefarthest farthestaway away from from the the meeting meetinglocation. location. 5) Each Eachtravel travelaward awardrequest requestshall shallbe bemade madeininwriting writingtotothe theannual annualChair, Chair,arid andshould should explain need, student and author status, and other significant significant details. details. The The form form below is is optional. optional. Students Students can can use use the the form form below, below, or or consult consultthe the Institute's Institute's web webpage page(ilsgeology.org) (ilsgeology.org) to to apply. apply. Successful Successful applicants should contact the meeting chair to receive receive their their awards awards during during the the meeting. meeting. xi �- p INSTITUTE SUPERIOR GEOLOGY INSTITUTE ONLAKE ONLAKE SUPERIOR GEOLOGY flTenbrey Eisenbrey Student Student Travel Travel Award Award Application Application Student StudentName: Name: Date: Date: Address: email: _____________________ Department DepartmentHead-Typed Head-Typed Department DepartmentHead-Signature Head-Signature Educational Educational Status: Status:________________________ Are the senior seniorauthor authorofofanan oral or poster paper? NO_._ oral or poster paper? Y YES— E S NOAre you the Will Will any any other otherstudents studentsbe betraveling travelingwith withyou? you? Who? Who? ________________ Statement Statementof of need need (use (useadditional additionalpage pageififnecessaiy) necessary) to: Pleasereturn returnto: Please xl' xii �STUDENT PAPER AWARDS ., Each year, the Institute selects the best of the student presentations and honors presenters Funding for for the award registrations of the with aa monetary monetary award. award. Funding award is generated from registrations annual meeting. The TheStudent StudentPaper Paper Committee Committeeis is appointed appointed by by the the annual annual meeting meeting Chair Chair in such a manner as to represent a broad range of professional and geologic expertise. manner as to represent a broad range of professional and geologic expertise. Criteria for Criteria for best best student studentpaper—last paper-last modified by the Board Board in in 1997—follow: 1997-follow: 1) The contribution contribution must be demonstrably demonstrably the work of the student. student. 2) The student student must present the contribution contribution in-person. 3) The Student Student Paper Committee Committee shall shall decide decide how many awards to grant, and whether or not to give give separate separate awards awards for for poster poster vs. vs. oral oral presentations. presentations. 4) In cases of multiple senior author, author, or or multiple student student authors, the award will be made to the senior * the award will be shared equally by all authors authors of the contribution. contribution. 5) The The total amount of the awards is left to the discretion of the meeting Chair and typically is about Secretary-Treasurer, but typically about $300 $300 US. US. 6) The Secretary-Treasurer Secretary-Treasurer maintains, and will supply supply to the Committee, Committee, a form form for for the the numerical ranking of presentations. This This form form was was created created and modified modified by Student Student Paper Committees Committees over several several years in an effort to reduce the difficulties difficulties that may arise from selection background. The selection by raters of diverse background. The use of the form is not required, but is required, is left left to the the discretion discretion of the Committee. Committee. 7) The names of award recipients shall Chair's report report shall be included as part of the annual Chair's that appears appears in in the the next volume of the Institute. Institute. Student Student papers papers will will be be noted noted (*) (*)on onthe the Program. Program. STUDENTPAPER PAPER AWARDS AWARDS COMMITTEE COMMITTEE 2000 STUDENT Manfred M. Kehlenbeck, Chair Chair Manfred Lakehead Univers University, Thunder Bay, Ontario Terrence TerrenceS. J. Boerboom Boerboom Minnesota Geological Survey, St. Paul, Minnesota Graham C. Wilson son Turnstone urnstone Geological Geological Ltd., Campbellford, Campbellford, Ontario Ontario ... xiii Xlll �2000 BOARD 2000 BOARD OF DIRECTORS meeting year shown in parentheses) parentheses) (Board membership extends through the meeting Stephen Stephen A. Kissin, Chair Chair(2003) (2003) Lakehead University, Thunder Thunder Bay, Ontario Ontario Theodore J. Bornhorst Bornhorst(2002) (2002) Michigan Technological Technological University, University, Houghton, Houghton, Michigan Michigan James James D. D. Miller Miller (2001) (2001) Minnesota Geological Geological Survey, Survey, St. St. Paul, Paul, Minnesota Minnesota Ronald Ronald P. P.Sage Sage(2000) (2000) Ontario Geological Geological Survey Survey Mark Mark Jirsa Jirsa(2000) (2000) Institute Institute Secretary-Treasurer Secretary-Treasurer Minnesota Geological Survey, St. Paul, Minnesota LOCAL COMMITTEES LOCAL COMMITTEES 2000 GENERAL GENERAL CO-CHAIRS: CO-CHAIRS:Stephen StephenA.A.Kissin Kissinand andPhilip PhilipW. W. Fralick Fralick PROGRAM PROGRAMAND AND ABSTRACTS ABSTRACTS EDITORS: EDITORS:Stephen StephenA. A. Kissin Kissin FIELD FIELD TRIP TRIPCOORDINATOR COORDINATORAND AND GUIDEBOOK GUIDEBOOKEDITOR: EDITOR:Philip PhilipW. W. Fralick Fralick Assistance to Assistance to the thelocal localcommittee committeewas wasprovided by by the thefollowing individuals: individuals: Karen Karen Farther Farrier - Lakehead Lakehead University Manuscript preparation, accounting accounting and communications Sam Lakehead University Sam Spivak Spivak -- Lakehead Graphics Graphics Becky Rogala Lakehead University Rogala - Lakehead Manuscript preparation Manuscript preparation and and audio-visual audio-visual Mark Smyk Northern Development Developmentand arid Mines Mines Smyk -- Ministry of Northern Organization Organization xiv �SESSION CHAIRS (in order order of appearance): appearance): Wed. AM Manfred Kehlenbeck Lakehead Lakehead University University— - Thunder Thunder Bay Dick Ojakangas University University of of Minnesota Minnesota— -Duluth Duluth Wed. PM Bornhorst Ted Bornhorst Houghton Michigan Technological TechnologicalUniversity University— - Houghton Mike Mudrey Wisconsin Geological Wisconsin Geologicaland and Natural NaturalHistory HistorySurvey Survey— -Madison Madison Thurs. Thurs. AM AM Andrew Mitchell Andrew Mitchell Thunder Bay DST DST Consulting ConsultingEngineers Engineers— - Thunder Bay Tony Naldrett Naldrett Consultant — Toronto Consultant - Toronto Thurs. Thurs. PM PM Mark Mark Smyk Smyk Bay Ontario Ontario Geological GeologicalSurvey Survey— - Thunder Thunder Bay Jim Jim Miller Miller Minnesota Minnesota Geological GeologicalSurvey Survey— - Duluth BANQUET SPEAKER SPEAKER 2000 2000 Bruce Bruce Simonson Simonson Oberlin Oberlin College College "Depositional "Depositional settings settings and and Early Diagenesis Diagenesis of Large Precambrian Precambrian Iron Formations" Formations" xv �REPORT MEETING OF REPORTOF OF TilE THE45"' 4SthANNUAL ANNUAL MEETING OF THE THE INSTITUTE INSTITUTE ON LAKE SUPERIOR SUPERIOR GEOLOGY GEOLOGY Marquette, Marquette, Michigan Michigan REPORT ON MEETING OF THE INSTITUTE INSTITUTE REPORT ON THE THE 45TH 45TH ANNUAL MEETING SUPERIOR GEOLOGY, MARQUETTE, ON LAKE SUPERIOR MARQUETTE, MICHIGAN MICHIGAN The 45th Annual Annual Meeting Meeting of the Institute Institute on on Lake Lake Superior Superior Geology Geology was held in in Marquette, Marquette, Michigan May 4-8, 1999 at the Ramada Inn. The meeting was co-sponsored by Michigan May 4-8, 1999 at the Ramada Inn. was co-sponsored by Northern Northern Michigan University Michigan Technological Technological University. University. Robert S. Regis and Theodore J. University and Michigan Bornhorst co-chaired the annual meeting. meeting. The The meeting was well attended with 120 geoscientists registering from 10 provinces, and and Switzerland. Switzerland. The registering from 10 different states, states, three Canadian Canadian provinces, The meeting meeting consisted of two pre-technical session and two post-technical session field trips, two days of of consisted pre-technical session and two post-technical session field hips, two days technical sessions, sessions, and and an an evening evening banquet. banquet. The Proceedings Volume 45 was published 1- Program Program and and Abstracts Abstracts was was Proceedings Volume published in two two parts. parts. Part 1Theodore J. J. Bornhorst. Bornhorst. There edited by Robert S. Regis and Theodore There were were 33 published abstracts for the 22 oral Part 22 -- Field Trip Trip 22 oral and and 11 11poster posterpresentations presentations given given in in the the technical technical sessions. sessions. Part Guidebook, was Guidebook, was edited edited by by Theodore Theodore J. Bornhorst. There There were were 44 field field trips hips and and for for two two of ofthem them there were were separate separate papers papers with with supplementary supplementary information. information. The Theguidebook guidebookcontained contained several several colored maps and cross-sections. Field hips trips continue to be an important component component of of the the annual annual meeting meeting of ILSG ILSG evidenced evidenced by by 76% of attendees participating in 11 or more field hips. trips. The Theguidebook guidebook contains contains44field fieldtrips: hips: Field trip 11 was titled titled "Early "Early Proterozoic Proterozoic strata of the Marquette iron range, Michigan" and was lead W.F. Cannon. Cannon. This one-day one-day field field trip hip had had 30 30 participants participants who visited classic classic lead by by W.F. geologic localities in in the Marquette Marquette area. area. Field trip "ArcheanIshpeming Ishpeming greenstone greenstone belt belt and and gold goldmineralization, mineralization, Field trio 22 was wastitled titled"Archean Michigan" and and was was lead by T.O. Michigan" T.O. Quigley, Quigley, R.A. R.A. Mahin, Mahin, D.J. D.J. Duskin, Duskin, R.C. Johnson, Johnson, T.J. Bornhorst, and G.W. Scott. This two-day field trip had 28 participants and visited localities This two-day field hip had localities focused on lithology, New data focused lithology, structure, structure, and and gold goldmineralization. mineralization. New data on on several several gold gold prospects were presented. Field trip 3 was Field was titled titled "Tilden "Tilden and and Empire Empire Mines of the the Marquette Marquette iron iron range, range, Michgian" Michgian" and was lead by G.W. Scott, P.M. Nordstrom, Nordstrom, and H.M. H.M. Lukey. Lukey. This This one-day one-day trip trip was was very very well attended with 45 participants. We Wethank thank the the Tilden Tilden Mining Mining Company Company and and the the Empire Empire Partnership for for allowing allowingaccess accesstotothese thesetwo twoiron ironmines. mines. Participants of this Iron Mining Mining Partnership Participants of one-day field trip visited localities within the open-pit mines. This was the first time trip visited localities This was the first timeILSG ILSG has visited either of these two mines. mines. The The last last time time aa Michigan Michigan iron iron mine mine was visited by an ILSG field trip hip was was 27 27 years years ago ago in in 1972. 1972. Field Field trip 44 was wastitled titled"Paleozoic "Paleozoic and and glacial glacial strata shata from from Au Au Train Train totoGrand Grand Marais, Marais, field trip Michigan" and was lead by R.S. Michigan" and R.S. Regis Regis and and J.J. Anderton. Anderton. This one-day one-day field hip had 16 16 This is the first participants. This first time time ILSG ILSG has had had a field field trip hip in in the the Munising Munising area area and and participants were able to see participants were see and and learn learn about about the the geology geology of ofthe thePictured Pictured Rocks RocksNational National Lakeshore. xvi �by 99 out of of the the 120 participants participants at at the the annual annual meeting. meeting. Nelson Ham The banquet was attended by along the the southern edge of the of St. St. Norbert College gave an excellent talk on glacial processes along Laurentide ice sheet. A Ahighlight highlight of of the the banquet banquet was was the awarding of the 1999 1999 Goldich medal to Tsu-Ming Han for his contributions contributions to the North American iron ore industry. It is is interesting interesting to note that Tsu-Ming delivered a paper at the first ILSG annual meeting in in 1955 1955and and another another at at this this meeting some 45 years later. meeting some years later. The technical sessions sessions included two invited papers: "Cross "Cross Margin Transport Transport in in Lake Lake Superior" Superior" presented by by Sarah presented Sarah Green Green and and 'A "ANew NewApproach ApproachtotoHistorical HistoricalReconstruction: Reconstruction: Combining Combining Descriptive and Experimental Paleolimnology" presented presented by by Charlie Charlie Kerfoot. Kerfoot. We We are are grateful grateful to to associated with Cliffs Mining Services, the solicited presentations made by industry industry personnel associated Services, Company and and Empire Iron Iron Mining Mining Partnership. Partnership. Two students were selected Tilden Mining Company selected for Ulrich Puschner (Basel, Switzerland) was awarded awarded $150 for his outstanding presentations. presentations. Ulrich Puschner (Basel, Switzerland) was his paper on metamorphism of the the Portage Portage Lake Lake Volcanics Volcanics in in Michigan Michigan and and Joy Loughry poster paper metamorphism of (Univ. Minnesota-Moms) Minnesota-Morris) was was awarded awarded $150 $150 for for her oral presentation presentation on the Crowduck Crowduck Lake David Ams (Kent State Univ., Group, Kenora, Kenora, Ontario. Ontario. Three students, David Univ., Ohio), Ohio), Dyana Dyana Czeck Czeck (Univ. Minnesota), and Ulrich Puschner were awarded Eisenbrey Eisenbrey Student Travel Travel Awards Awards for a total of $800. All Allof ofthese thesestudents studentspresented presented papers papers at at the meeting. The Institute's Board of Directors met on May 6, 1999 1999 and the following is an overview overview of the meeting. meeting. 1. 1. Accepted the Report Report of of the the Chairs Chairs for for the the 44th 44th ILSG. ILSG. 2. 2. Accepted the 1998-99 1998-99 ILSG financial report. There was discussion on methods to encourage student attendance through monetary and other means. A lower cost for encourage students students to attend field field trips, just like like meeting registration, was acceptable. acceptable. 3. 3. Discussed guidelines guidelinesfor for the the Goldich Goldich Award. Award. Committee for one year. 4. 4. Maintained the composition composition of the Goldich Medal Committee 5. Discussed Discussed recasting of the Goldich Goldich Medal. 6. Discussed and approved 2000 meeting location in Thunder Bay with Kissin and Fralick 6. Fralick as co-chairs. co-chairs. 7. Discussed the possibility of a 2001 meeting in Sault 7. Sault Ste Marie, Michigan. Michigan. 8. 8. Discussed defraying defraying the meeting meeting expenses expenses for the secretary-treasurer. secretary-treasurer. The The Board Board (registration, field trips, and approved waiving registration for all meeting events (registration, banquet). 9. Approved a Corporate Resolution for the secretary-treasurer to conduct investments. 9. 10. Reaffirmed Reaffirmed Board commitment to the ILSG Newsletter Newsletter and Web Web page. page. The The Web Web page page Bomhorst. will be reorganized reorganized by Laurel Laurel Woodruff Woodruff and and Ted Ted Bornhorst. 11. Agreed that only one of the two co-chairs co-chairs would continue continue as as primary primary board boardmember. member. Ted Bornhorst will will be be the the representative representative from from the the 1999 1999meeting. meeting. We wish to thank all of the people the field trips and made people who lead and prepared materials for the presentations. While field trips continue continue as as the the traditional traditional strength strength of of ILSG, ILSG, the the presentations presentations provide a forum for discussion of Lake Superior geology. Of Of course, course, there there are are many many others others who who to the success of aa meeting we acknowledge for contributing to meeting including including the session session chairs, chairs, best best student paper paper award award committee, committee, ILSG ILSG Board Boardmembers, members,and andthe the staff staff of of the the Ramada Ramada Inn. Inn. We We volunteer help help with with registration. registration. Last we thank all of especially thank Suki Smaglik for all of her volunteer those who attended attended the the 45th 45th annual annual ILSG ILSG meeting, meeting, for for without without them there there would would not not be be an an JLSG. ILSG. Our "break pre-meetinginterest. interest. As a result "break even" even" budget budget model was based based on indicated indicated pre-meeting result of of xvii �than expected expected and and some some unexpected unexpected additional additional monies, monies, the meeting generated higher attendance attendance than generated cash flow flow of a few thousands of dollars dollars for for ILSG. !LSG. aa positive cash thousands of personally satisfied satisfied with the overall We are personally overall outcome outcome of of the the meeting meeting and and feel feel ititwas was aa success. success. waswell well worth worth the the effort. effort. We While itit is is aa lot lot of of work work to toput put on on the the ILSG ILSG annual annual meeting, meeting, itit was We encourage others to consider being chair or co-chairs of the ILSG annual meeting. encourage others to consider being chair or co-chairs of the ILSG Regis Ted Bornhorst Bornhorst and and Bob Bob Regis Co-Chairs of the 45th Co-Chairs 45th Annual ILSG xviii �CALENDAR PROGRAM CALENDAR OF OF EVENTS EVENTS AND PROGRAM MONDAY, 2000 MONDAY. MAY MAY 8. 8.2000 I I I 0800-1800 0800-1800 PRE-MEETING FIELD FIELDTRIPS TRIPS PRE-MEETING All field trips trips leave leave and and return returneach eachevening eveningto to Lakehead Lakehead University University Alifleld 1) 1) MESOPROTEROZOIC MEsoPRoTERozoIc SIBLEY SIBLEYGROUP GROUP(Day (DayOne) One) Leaders: Philip Philip Fralick, Fralick, Mark Smyk Smyk TUESDAY. 2000 TUESDAY. MAY MAY 9. 9.2000 I 0800-1800 0800-1800 PRE-MEETING PRE-MEETING FIELD FIELDTRIPS TRIPS 1) MESOPROTEROZOIC MESOPROTEROZOIC SIBLEY SIBLEYGROUP GROUP(Day (DayTwo) Two) Leaders: Leaders: Philip Philip Fralick, Fralick, Mark Mark Smyk Smyk 2) LAC LAC DES DES ILES ILES MINE MINE (I) (I) Leader: Moe Lavigne Moe Lavigne 3) 3) GEOARCHEOLOGY GEOARCHEOLOGY OF OF THE THUNDER THUNDER BAY AREA Leaders: Brian Phillips, Phillips, Scott Scott Hamilton, Hamilton, Joe Stewart, Stewart, Pat Julig, Julig, Bill Bill Ross Ross I 1700-2200 1700-2200 REGISTRATION AT LAKEHEAD UNIVERSITY POSTER POSTER SESSION/CASH SESSIONICASH BAR MIXER Agora/Faculty AgoraPaculty Lounge Lounge xix xix �WEDNESDAY. 2000 WEDNESDAY. MAY MAY 10. 10.2000 0800-1200 0800-1200 REOISTRATION/PRE-REGISTRATION REGISTRATION/PRE-REGISTRATIONPACKET PACKETPICK-UP PICK-UP I: GEOLOGICAL PROVINCE Session I: GEOLOGICAL OVERVIEW OF THE WESTERN SUPERIOR PROVINCE University Centre Theatre Theatre 0900-0915 0900-0915 WELCOME AND OPENING OPENING REMARKS REMARKS 0915-0945 091 5-0945 Kehlenbeck, Kehlenbeck, M.M. M.M. Rocks of ofthe theQuetico QueticoSubprovince Subprovinceand andAdjacent AdjacentTerranes Tenanes A Review of Structures in Rocks 0945-10155 0945-101 Davis, Davis, D.W. D.W. Western Superior Province: Geochronologic GeochronologicAspects Aspects 1015-1040 1015-1040 COFFEE COFFEE BREAK AND AND POSTER POSTER SESSION SESSION 1040-1110 1040-1 110 Tomlinson, Tomlinson, K. K. Western Superior Superior Province: Province: Volcanological Volcanological Aspects Aspects 1110-1140 1110-1140 Fralick, P.W. P.W. Fralick, Western Superior Superior Province: Province: Sedimentological Sedimentological Aspects Aspects 1140-1200 1140-1200 Czeck, D.M.* and Hudleston, P.J. P.J. A Sequence Transpression in the the Seine River River Conglomerates Conglomerates Sequence of Folding and Dextral Transpression 1200-13 1200-1310 10 LUNCH BREAK 1310-1330 1310-1330 R.R. Davis, D.W., Amelin, Y., V., Nowell, Nowell,G.M. G.M. and and Parrish, Parrish, R.R. Hf Isotope Study Study on on Zircons Zircons from from the the Western Western Superior SuperiorProvince Province 1330-1350 1330-1350 Pettigrew, N.T.*, Hattori, Hattori, K.H. and and Percival, Percival, J.A. Intrusion: a POE-bearing Samuels Lake Intrusion: PGE-bearing Quetico-type Quetico-type Intrusion Intrusion in in the the Central, Central, Western Quetico Subprovince, Subprovince, Northwestern Ontario 1350-1410 1350-1410 Wilson, A.C. Archean Diamond Exploration Targets in the Michipicoten Greenstone Greenstone Belt, Wawa, Ontario Ontario 1410-1430 1410-1430 R.P. and and Crabtree, Crabtree, D.C. Morris, T.F., Sage, R.P. Kimberlite, Base Metal, Gold and Carbonatite Exploration Targets, Derived from Overburden Heavy Mineral Data, Killala Lake Area, Northwestern Northwestern Ontario Ontario xx �1430-1450 Karkkäinen, N. and Bornhorst, Bornhorst, T.J. Karkkainen, Small Mafic Intrusions of the Lake Superior Region: Targets for Titanium Titanium Exploration 1450-15 10 COFFEE BREAK AND POSTER SESSION 1510-1530 Bajc,A.F. Bajc, A.F. Results of Regional Till Sampling Sampling in the Western Part of the Shebandowan Shebandowan Greenstone Belt, Northwestern Ontario Greenstone Northwestern Ontario 1530-1550 Cannon, W.F. and Cannon, and Woodruff, Woodruff, L.G. Some Some Factors That Control the Distribution of Mercury in the Near-surface Environment of Isle Royale National Park, Michigan-earth, Wind and Fire 1550-1610 Larson, P.C. Larson, P.C. and andMooers, Mooers, H.D. H.D. Commination Commination and Dilution of Glacial Indicators: Rate Estimates Estimates from from Field Field Data Data 1610-1630 Peterson, D.M. Peterson, D.M. Geometry Between Gold-rich Gold-rich Subsidiary Subsidiary Shear Shear Zones Zones and Major Geometry Between Major Structures: Structures: Implicationsfor Archean Lode-Gold Mineral Exploration Implicationsfor 1630 ILSG Business Business Meeting Meeting 1800-1930 MIXER (Residence Cafeteria) Cafeteria) MIXER Cash Bar 1930-2130 ANNUAL BANQUET BANQUET AND AWARDS PRESENTATION Residence Cafeteria Cafeteria • Announcement Announcementofof47th 4T11 Annual Meeting location location • Banquet BanquetSpeaker: Speaker: Dr. Dr. Bruce BruceSimonson, Simonson,Oberlin Oberlin College College "Depositional Settings Settings and Early Diagenesis Diagenesis of Large Precambrian Iron Formation" xxi �THURSDAY. MAY 11.2000 GEOLOGICAL OVERVIEW OF THE WESTERN SUPERIOR PROVINCE II; Session 1 1: University Centre Theatre Theatre 0900-0930 0900-0930 Lavigne, M.J. The Lac Mine, Northwestern Ontario Lac des des Ties lies Mine, Ontario 0930-1000 0930-1000 Fralick, Fralick, P.W. Western Western Superior Superior Province: Province: Proterozoic Proterozoic Sediments Sediments 1000-1030 1000-1030 Kissin, Kissin, S.A. S.A. Vein-type Deposits of the Thunder Bay Area 1030-1050 1030-1050 COFFEE BREAK AND POSTER POSTER SESSION SESSION 1050-1110 Pueschner, U.* and Schmidt, Pueschner, U.* and Schmidt,S.T. S.T. the Keweenawan Keweenawan Portage Portage Lake Lake Volcanics Volcanics on Low-grade Metamorphic Zonation of the the Keweenaw Keweenaw Peninsula, Peninsula, Michigan Michigan 1110-1130 Rampe, .LS.* Rampe, J.S.* and Holm, Holm, D.K. Modelling the Effect of Intermediate Intermediate Temperature Temperature (350°-500°C) (350'-500°C Mazatzal-Age Mazatzal-Age Thermal Overprinting Superior Region Thermal Overprinting on Argon Diffusion from the Southern Lake Superior 1130-1150 Hudak, G.J., Hudak, G.J., Morton, Morton, R.L. RL. and and Dahlberg, H. Preliminary Volcanology and PreliminaryVolcanology and Hydrothermal HydrothermalAlteration AlterationStudies Studiesat at the the Five Five Mile Lake, Eagles Nest, and Quartz Hill Hill Prospects: Prospects: Implications Implications for forVMS-Style VMS-StyleMineralization Mineralization Eagles in Northeastern Northeastern Minnesota Minnesota 1150-1300 LUNCH LUNCH 1300-1320 Green, Green, J.C. J.C. Mystery Faults Faults of the Cascade River, North Shore, or What Is this Granite Doing Doing Here? Here? 1320-1340 K.J. and Schulz, KJ. and Nicholson, Nicholson, S.W. Lithogeochemistry Lithogeochemistry and and Paleotectonic Paleotectonic Setting Setting of the Bend Massive Massive Sulfide Sulfide Deposit, Deposit, Northern Wisconsin 1340-1400 Van Schmus, Schmus, W.R., W.R, MacNeill, MacNeill, L.C., Holm, and HoIm, D.K., D.K., Boerboom, Boerboom, T.J. and xxii �Jirsa, M.A. Jirsa, M.A. The 1787-1772 1787-1772 Ma Ma East-Central East-Central Minnesota Minnesota Batholith: Batholith: Precursor Precursor to to Crustal Crustal Stabilization in the Lake Superior Region Superior Region 1400-1420 Schweitzer, D.J.*, Schnieder, D.A., and Schweitzer, D.A., Boerboom, Boerboom,T.J., T.J., Holm, Hoim, D.K. BK. and Van Schmus, Schmus,W.R. Assessing the Extent of Assessing the of Early Early Proterozoic Proterozoic Penokean Penokean Versus Versus -A770-1760 -1770-1760 Ma Ma Metamorphism Metamorphism in East-Central East-Central Minnesota Minnesota 1420-1440 HoIm, D.K.. D.K., Hodees. Hodges, K.V. K.V.and and Hurtado, Hurtado, J. Holm. Results of40Ar/39Ar Laser Microprobe Mapping of of~Muscovite from Precambrian ~ e s u l tof s40k~9i~& ~ iec r o ~ r o~~e bAge e Mapping u s c o v i t from e ~recambrian Bedrock of the Southern Lake Superior Region 1440-1500 Kissin, S.A., Okamoto, RG. Okamoto, M., Addison, W.D. W.D. and and Brumpton, Brumpton, R.G. A Possible Sudbury Ejecta Layer in the Gunflint Formation, Northwestern Ontario Sudbury Ontario 1500-1520 COFFEE BREAK AND POSTER POSTER SESSION SESSION 1520-1530 STUDENT AWARDS STUDENT AWARDS 1530-1550 North, J. North,J. Nature of Logan Nature and Distribution Distribution of Logan Diabase Diabase Sills and Gabbro Gabbro Channels in the the near Thunder Thunder Bay, Bay, Ontario: Ontario: Brief BriefComparison Comparisonto toNoril'sk Noril'sk Keweenawan Rift near 1550-1610 Jr. Medaris, L.G. Jr. The Barren Barron Saprolite: Confirmation Confinnation of of Mature Mature Chemical Chemical Weathering Weathering in the Source Source Paleoproterozoic Quartz Arenites in the Lake Superior Region for Paleoproterozoic 1610-1630 Argust,A. Argust, A. Chemistry Chemistry and Mineralogy of Lower Chocolay Group Rocks, Dickinson Dickinson County, County, Michigan Michigan 1630-1650 Simonson, B.M. Simonson, Conglomerates and Intraformational Intraformational Breccias The Distinction Between Flat Pebble Conglomerates Breccias in Precambrian Precambrian Iron Formations Iron Formations xxiii �POSTER PRESENTATIONS POSTER PRESENTATIONS B.R.* and Saini-Eidukat, Saini-Fidukat, B. Bandli, B.R* B. Nomarski DIC Microscopy Analysis of of Plagioclases Plagioclases from from the Duluth Microscopy and Structural State Analysis Complex, Minnesota .J andHavholm, H~VUUIII!, K.G. Beaster, K.F.*, Kohn, J.D.* J.D.t and K.G. Wind or Water? Water? Paleoenvironment of the Proterozoic oterozoic Hinckley Sandstone, Sandstone, Northeastern Minnesota Minnesota Harris, Harris, E.C.A.* E.C.A.* and Wirth, Wirth, K.R. K.R region, eastern eastern Mesabi Mesabi Range, Range, Minnesota Petrogenesis of granitic rocks of the Dunka River region, Hauck, S.A., S.A., Bite, A. and Severson, M. Hauck, and Severson, Copper Pyroxene Homfelsed Homfelsed Archean Archean Giants Giants Range RangeBatholith Batholith Footwall Footwall of of opper Mineralization of Pyroxene thee Keweenanwan South Kawishwi Intrusion, Duluth Complex, NE Minnesota -- Archean Archean orr Keweenawan Mineralization? Mineralization? Jerde, E.A. Jerde, E.A. Magmatic for Nathan's Series: An Initial of the Magmatic Origins Origins for Nathan's Layered Layered Series: Initial Reassessment Reassessment of the or Plutonic Materials Midcontinent Rift's First FirstMaj Major Materials Kean, Kean. W.F. and Luther, Luther. F. F. Additional Paleomagnetic PaleoniagneticStudies Studiesofof aa Proterozoic Proterozoic Diabase DiabaseDike, Dike, in in Pifher Pifher and Irwin Irwin Townships,,Lake Nipigon District, District, Ontario Wirth, K.R. Kennedy, B.C.*, Wirth, K.R and and Vervoort, Vewoort, J.D. Petrogenesis Petrogenesis of the Midcontinent Midcontinent Rift Granophyric Granophyric Complexes Complexes of Northern Minnesota Minnesota Luther, Luther, F.R. Low-grade Metamorphismofofan an Archean Archean Debris Debris Flow, Flow, Irwin, Irwin, Pither Low-grade Metamorphism Pifher and and Meader Meader Townships, Lake Nipigon Region, Ontario Jr. and Medaris, L.G. Jr. and Fournelle, Fournelle, J.H. Tourmaline-bearing QuartzVeins Veinsin in the the Baraboo Baraboo Quartzite: Quartzite: A A New New Occurrence Tourmaline-bearing Quartz Occurrence of the Alkali-deficient Tourmaline, Foitite Morris, R.P. and Crabtree, Morris, T.F., Sage, Sage, RP. Crabtree,D.C. D.C. Kimberlite, Base Metal, Gold and Kimberlite, and Carbonatite Carbonatite Exploration Exploration Targets, Targets, Derived Derived from from Heavy Heavy Mineral Data, Killala Killala Lake Lake Area, Northwestern Ontario Ontario Peterson, Peterson, D.M. Geologic and GIS-based Geologic GIS-based Lode-gold Lode-gold and Volcanogenic Volcanogenic Massive Sulfide Sulfide (VMS) (VMS) Mineral Mineral xxiv xxiv �Potential Maps Maps of aa Portion Potential Portion ofofthe theArchean ArcheanVermillion VermillionGreenstone GreenstoneBelt, Belt,Northeastern Northeastern Minnesota Minnesota E.H.*, Wirth, Wirth, K.R. and Phillips, EX.*, and Morey, Morey, G.B. G.B. Petrogenesis of the Enigmatic Aurora Sill, Mesabi Range, Minnesota Petrogenesis Enigmatic Minnesota Sage, Sage, R.P. The "Sandor" Diamond Diamond Occurrence, Occurrence, Michipicoten Michipicoten Greenstone Greenstone Belt, Belt, Wawa, Wawa, Ontario Ontario Stott, Stott, G. and and Berdusco, Berdusco, B. B. Precambrian Features under the James Bay and Hudson Bay Lowlands Vervoort, Vewoort, J.D., Wirth, Wirth, K.R. and andKennedy, Kennedy, B.C. Isotopic Constraints Constraints on on the Origin of Granophyre in the Midcontinent Isotopic Granophyre Complexes Complexes in Midcontinent Rift Ritt of nNortheastern or the astern Minnesota Wilson, Wilson, A. A. Archean Diamond Exploration Targets in the Michipicoten Greenstone Belt Wolbers, Wolbers, J.A.* and and Seifert, Seifert, K.E. K.E. Petrogenesis of the Silver Creek Midcontinent Rift Petrogenesis Creek Dike: a 1.1 Ga Intrusion in the Midcontinent L.G., Attig, Attig,J.W. J.W.and andCannon, Cannon,W.F. WI. Woodruff, L.G., Geochemical Impacts of an Undisturbed Mineral Deposit -- Results from the Bend Deposit, Chequamegon Chequamegon National Forest, Forest, Wisconsin xxv XXV �FRIDAY. 2000 FRIDAY. MAY 12. 12.2000 0800-1800 0800- 1800 POST-MEETING FIELD TRIPS TRIPS All field field trips leave and and return returneach eachevening eveningto to Lakehead Lakehead University, University,except exceptfor Field an overnight stay in Field Tr4p Trip 6, which will involve an in Atikokan on May 12 (at (at the the participant's expense). participant's expense). 4) PALEOPROTEROZOIC PALEOPROTEROZOIC GUNFLINT GUNFLINTFORMATION FORMATION Leaders: Pier Pufahi and Philip Fralick Pier Pufahl and Philip 5) 5) QUATERNARY QUATERNARYGEOLOGY, GEOLOGY,SHEBANDOWAN SHEBANDOWANBELT BELT Leader: Andy Andy Bajc Bajc - - 6) 6) STEEP STEEPROCK ROCK - FINLAYSON FINLAYSON - LUMBY LUMBY BELTS BELTS (Day (Day One) One) Leaders: Denver Stone, Kirsty Tomlinson, Ray Bernatchez Denver Stone, Kirsty Tomlinson, Ray Bematchez and Philip Philip Fralick Fralick 7) 7) LAC LAC DES DES ILES ILES MINE MINE(II) (11) Leader: Moe Lavigne Moe Lavigne SATURDAY. 2000 SATURDAY. MAY MAY 13. 13.2000 0800-1800 0800- 1800 FIELD FIELD TRJP TRIP66(Day (DayTwo) Two) xxvi xxvi �Chemistry Group Rocks, Chemistry and and Mineralogy Mineralogy of Lower Chocolay Group Dickinson County, Michigan Michigan ARGAST, Anne, Department of Geosciences, Geosciences, Indiana-Purdue Indiana-Purdue University University Fort Fort Wayne, Wayne, Fort Wayne, Wayne,IN IN46805-1499, 46805-1499,argast(%ipfw.edu argastipfw.edu Fort Rocks of the Chocolay Dickinson County, County, Michigan. Michigan. Most Most Chocolay Group rest on Archean basement in Dickinson likely accumulated accumulated in the interval 1.9-2.1 Ga. ago, the Group consists of the basal, partly 1.9-2.1 consists partly conglomeratic, Fern Creek Formation, overlain conglomeratic, overlain by Sturgeon River Quartzite Quartzite and and Randville Randville Dolomite. Chocolay Group likely represents deposition terrigenous and shallow deposition in temgenous shallowmarine marine environments with later environments later units of the Marquette Range Supergroup Supergroup (Menominee (Menominee and and Baraga Baraga Groups) transitioning upwards into deeper water, more active and isolated environments. environments. There are two alternate alternate views for the origin origin of the Fern Creek Creek Formation. Some Some interpret interpret the the Fern Creek (or its putative equivalents of Reany Creek and Enchantment Lake Formations putative equivalents Lake Formations farther north in Michigan) as examples examples of glaciogenic glaciogenic sediments, possibly synchronous synchronous with with Gowganda glaciation (e.g., Pettijohn, 1943; Gair, 1981; Young, 1983; Ojakangas, 1984). Gowganda glaciation Pettijohn, 1943; 1981; 1983; 1984).Others Others interpret the Fern Creek as originating, originating, for for example, in fluviatile, fluviatile, occasionally occasionally torrential torrential environments (e.g., environments (e.g., Bayley et al., 1966; 1966; Larue, 1981; 1981; Mattson Mattson and Cambray, Cambray, 1983). 1983). This This is is an an important determination, determination, affecting affecting aspects aspects of regional and interregional interregional correlation correlation on on aa significant scale (e.g. Ojakangas, 1988). 1988). This study focuses focuses on on the bulk and and mineral chemistries chemistries of the Fern Fern Creek Creek Formation Formation and and Sturgeon Sturgeon River Quartzite Quartzite exposed exposed along an an access access road road to aa hydroelectric hydroelectric dam dam located located about about33 km northeast of Loretto, Loretto, Michigan. The purpose is to further further evaluate evaluate the glaciogenic glaciogenic vs. vs. fluviatile origins fluviatile origins of the Fern Fern Creek Formation, Formation, and to characterize characterize the materials materials to to constrain constrain possible source source areas. Fern Creek rocks are dominated by quartz, sodic sodic plagioclase, K-feldspar, antiperthite, antiperthite, and and micas micas of di- and trioctahedral trioctahedral composition. Sturgeon River is predominantly quartz with minor illite. composition. Sturgeon River quartz with minor illite. Zircon, tourmaline, tourmaline, fluor-hydroxy fluor-hydroxy apatite, apatite, pyrite, rutile, mtile, fluorite, fluorite, monazite, huttonite huttonite and and aa fluorfluorREF mineral occur Zircon and tourmaline tourmaline dominate in the hydroxy REE occur in the Fern Creek Formation. Zircon Sturgeon Sturgeon River River Quartzite. Quartzite. The tourmaline compositions compositionsvary systematically systematically with with vertical vertical position position throughout throughout the the Fern Fern Creek/Sturgeon CreekBturgeon River River sequence. sequence. This This suggests suggests a gradual gradual denudation denudationof of the the source source area areaover over an an extended length of time. extended length Bulk chemistries chemistries for for the the Sturgeon SturgeonRiver River Quartzite Quartzite are are consistent consistent with with an an origin origin as as aa well well sorted sorted sediment in a nearshore marine environment. Fern Creek chemistries are typical of unsorted chemistries unsorted materials materials that have have not undergone undergone hydraulic hydraulic differentiation, differentiation,and and are are similar similarto to chemistries chemistriesfrom from the underlying Carney Lake Gneiss. underlying Carney Lake Gneiss. I �REE distributions distributions are highly highly variable in the Fern Creek Formation, ranging from nearly flat (normalized to PAAS) to extremely steep (e.g. chondrite normalized LdYb La/Yb of 101.5). (normalized 101.5). These These distributions result from enrichment in LREEand Tb-rich minerals. distributions Th-rich The bulk chemistry and presence of REE minerals are consistent with proximal derivation derivation from the underlying (pegmatite-rich) Carney Lake Gneiss. The trends in tourmaline composition composition suggest association association with a process capable of deep erosion over an extended period of time. The similarity similarity of the Fern Creek to stratigraphically equivalent rocks at some distance north suggests suggests a process of significant significant areal extent. Taken together, these results strongly support support the origin origin of the Fern Creek Formation as a proximal till derived from the underlying gneiss. References References iron-bearing Bayley, R.W, Dutton, C.E., and Lamey, C.A., 1966, Geology of the Menominee iron-bearing Counties, Wisconsin: district Dickinson County, Michigan and Florence and Marinette Counties, 96 pp. U.S. Geological Survey Professional Professional Paper Paper 513, 513,96 Gair, J.E., 1981, Lower Proterozoic Proterozoic glacial deposits deposits of Northern Michigan, in Hambrey, M.J., University and Harland, W.B., eds., Earth's pre-Pleistocene glacial record: Cambridge University 803-806. Press, p. 803-806. Lame, D.K., 1981, 1981, The The Chocolay Chocolay Group, Group, Lake Superior Superior region, U.S.A.: Sedimentologic Sedimentologic evidence for deposition deposition in basinal and platform settings settings on an Early Proterozoic Proterozoic craton: Geological v. 92, 92,p. p. 417-435. 417-435. Geological Society Society of of America America Bulletin, Bulletin, Part Part 1,1, v. F.W., 1983, The The Reany Reany Creek Creek Formation: Formation: aa mass-flow mass-flowdeposit deposit of of Mattson, S.R. and Cambray, F.W., possible post Menominee possible Menominee age age [abs.], [abs.], in in Proceedings, Proceedings,29th 29' Annual Institute Institute on Lake Lake Superior Superior Geology, p. 27. 27. Ojakangas, R.W., 1984, 1984, Basal Lower Proterozoic glaciogenic formations, formations, Marquette Marquette Range Annual Supergroup, Upper Peninsula, Michigan [abs.]: in Proceedings, 30th Institute Upper Peninsula, Michigan [abs.]: in Proceedings, 30' Institute on Lake Superior Superior Geology, Geology, Wausau, Wausau,WI, WI,p.p. 43. 43. Ojakangas, R.W., 1988, 1988, Glaciation: Glaciation: An uncommon "mega-event" "mega-event" as a key to intracontinental intracontinentaland and intercontinental intercontinental correlation correlation of Early Proterozoic basin fill, North American and Baltic Baltic New perspectives in basin analysis: Cratons, in Kleinspehn, K.L., and Paola, C., eds., New Springer-Verlag, p. 431-444. 43 1-444. Pettijohn, F.J., 1943, 1943, Basal Basal Huronian Huronian conglomerates conglomerates of Menominee and Calumet Calumet districts, districts, Michigan: Journal of. Geology, v. 51, p. 387-397. Michigan: Geology, 51, 387-397. Young, G.M., 1983, Tectono-sedimentary history of Early Proterozoic Proterozoic rocks of the northern Proterozoic geology of the Great Lakes Great Lakes region, in Medaris, L.G., Jr., ed., Early Proterozoic Lakes region: Geological Society of America Memoir 160, p.15-32. 2 �RESULTS OF REGIONAL TILL TILL SAMPLING IN THE WESTERN PART OF THE SHEBANDOWAN GREENSTONE BELT, NORTHWESTERN NORTHWESTERN ONTARIO ONTARIO THE GREENSTONE BELT, Bajc, A.F., Sedimentary Geoscience Section, Ontario Geological Geological Survey, Survey, Ministry Northern Development and Mines, 933 Ramsey Lake Lake Road, Road, Sudbury, ON of Northern P3E 6B5 6B5 A project of Quaternary Quaternary geological geological mapping mapping and regional till sampling sampling was was undertaken undertaken over the western part of of the Shebandowan Shebandowan greenstone greenstone belt during during the the 1999 1999field field season. season. Quaternary Quaternary mapping helped to characterize characterize the surficial surficial materials materials present within within the the study area and to reconstruct the ice flow history associated with with these these deposits. deposits. This This information information provided a framework framework by which which the regional materials materials compositional compositional datasets could be interpreted. interpreted. The Theregional regional sampling sampling program program provided provided information informationon on the background concentrations concentrations of various elements elements in in till as as well as as on on the the number number and and character character of gold grains grains and base metal indicators indicators in in till. The study area contains a relatively simple record of Quaternary events. events. Most Most deposits deposits were laid down during the final retreat of ice from the the area. Ice flow indicators Ice flow indicatorssuggest suggest aa consistent consistent pattern of flow flow towards towards the south south across across the eastern eastern half half of of the the study study area, area, becoming more westerly towards the west. Till cover is generally thin and discontinuous Till cover is generally thin and discontinuous over most parts of the study area. Thicker, Thicker, more more extensive extensive till till is is present present within within Hagey, Hagey, Conacher Conacher and Blackwell townships. Till Tillsamples samplescollected collected from from these these townships townships are are less less representative representative of local local geology geology than are are samples samples collected collected from from areas areas of of very very thin thin till. till. Geochemical Geochemical and mineralogical mineralogical interpretations interpretations should should consider consider these these local local variations variationsin in till character. character. The till sampling sampling program program has has clearly clearly identified identified aa number number of of precious precious and and base base metal metal exploration belt. Higher exploration targets within the western Shebandowan greenstone belt. Higher density density follow-up follow-up sampling sampling surveys surveys are are recommended recommended for for these these areas areas to to determine determinethe the the anomalies and more more precisely precisely define define potential potential source sourcerocks. rocks. The significance of the regional character character of this this survey survey will assist assist with with the the interpretation interpretationof of local, local, property-scale property-scale geochemical datasets by placing results results into into context context with with respect respect to to regional regional background. background. geochemical datasets The results of this survey survey clearly clearly support support the the use use of of till compositional compositional surveys surveysfor for mineral mineral exploration. exploration. The Theapproach approachfor forfollow-up follow-upsampling samplingprograms programs will will be be dictated dictated by by the the type type of mineralization mineralization sought sought and and the the local local conditions conditionswithin within aa given given property. property. 3 �NOMARSKIIDIC DICMICROSCOPY MICROSCOPY AND AND STRUCTURAL STRUCTURAL STATE ANALYSIS ANALYSIS OF NOMARSKI OF PLAGIOCLASES FROM THE PLAGIOCLASES THE DULUTH COMPLEX, MINNESOTA MINNESOTA BANDLI, Bryan R., Department of Geosciences, Geosciences, North Dakota Dakota State Statc University, University,Fargo, Fargo, ND 58105-5517 USA. (Bryan_Bandli@ndsu.nodak.edu). (Bryan.Bandli@ndsu.nodak.edu). SAINI-BIDUKAT, SAINI-EIDUKAT, Bemhardt, Department Bernhardt, Department of Geosciences, North Dakota State University, Fargo, ND 58105-5517 58 105-5517 USA. (sainieid@badlands.nodak.edu) (sainieid@badlands.nodak.edu) Nomarski Differential Differential Interference Interference Contrast Contrast (DIC) (DIC) and and structural structural state state XRD XRD analyses analyseswere were carried out on samples of Duluth Complex Anorthositic Series rocks. rocks. Nomarski DIC is a reflected light technique technique which uses aa double-crystal double-crystal prism prism to to split split plane-polarized plane-polarizedlight lightto to produce a dark-light shading that highlights microrelief. To use the method on mineral in acid to to produce produce microtopography. microtopography. Anderson Anderson specimens, polished sections are etched in (1983), Pearce and Kolisnik (1990), (1990), and Pearce Pearce (1994) (1994) discuss the the importance importance of of interference interference imaging in showing that high-resolution high-resolution mineral mineral zonation patterns, patterns, that that are are correlatable correlatable with with varying An content, record complicated complicated dissolution dissolution and reaction reaction events events that that occurred occurredduring during crystal growth. growth. Polished block and thin sections sections were prepared from three three samples samples of of the the anorthositic anorthositic series taken near its contact with the Duluth Layered Series, Series, along along Skyline Skyline Parkway Parkway in in Duluth above Oneota Cemetary (Stop 1-5 of Miller, 1995). Sample 1 is an inclusion containing more than 95% plagioclase; plagioclase; sample sample 22 is the the host host gabbroic gabbroic anorthosite, anorthosite, and and sample sample three is from the outcrop used for zircon zircon age age dating dating (Paces (Paces and and Miller, Miller, 1993). 1993).Polished Polishedthin thin sections were prepared on orthogonal orthogonal axes axes to expose expose a variety variety of of crystallographic crystallographic orientations, etched in 48-50% HBF4 HBF4 for approximately seven minutes (adapted from the method of Anderson, 1983), 1983), and then imaged imaged using aa Nomarski Nomarski mirror mirror cube cube and and prism prism slider slider on an Olympus Olyrnpus BX-60 petrographic petrographic microscope. Although some some oscillatory oscillatory zoning was was observed, observed, the the most most common common zoning zoning observed observed was was patchy. This type of zoning zoning could be considered considered a hiatus hiatus type type of of discontinuity discontinuityas as defined definedby by Castro and de la Rosa (1994). Irregular Irregular zoning zoning of of plagioclase plagioclase is is common common in in the the Anorthositic Anorthositic Weiblen, 1990) 1990)and and could could indicate indicate significant significant disruptions disruptions of of the the Series (Miller and Weiblen, crystallizing surface surface due to dissolution dissolution and and reaction events events as as the the crystal crystal mush mush moved moved from from deep to shallow chambers. chambers. Plagioclase Plagioclase structural structural analyses analyses were were carried carried out out on on the the three three samples samples plus plus one one additional additional plagioclase separate separate from from the the Duluth Duluth Anorthositic AnorthositicSeries Series (Sample (SampleD-12) D-12) and and one onefrom from the the plagioclase Gabbro Lake area area (sample (sample BL-27). BL-27). Samples Samples were were crushed crushed and and plagioclase plagioclasewas was hand-picked, hand-picked, powdered, and diffraction patterns collected on a Philips Automatic Vertical Diffractometer Characterization Laboratory at the Materials Characterization Laboratory at NDSU. Scans Scans were were made made from from 6.0 6.0 to to 65 65 degrees 20 using using a step step of of 0.02° 0.02' 28, 20, Cu Ka Karadiation radiation (X= (\= 1.54184 1.54184 angstroms), angstroms), a diffracted diffracted 4 �beam beam monochrometer, monochrometer, and and aa sealed sealed proportional proportional counter. counter. Patterns were were matched matched to to aa plagioclase in the PDF reference file and refined using Garvey's Gamey's (1986) (1986) implementation implementation of the program of Appleman and Evans (1973). Refined A131 and yvalues yvalues were used to determine (1983). Despite determinethe the structural structuralstate state based based on on figures figures from from Kroll Kroll(1983). Despite the the potential potential difficulties difficultieswith with refining refining zoned zoned minerals, minerals, the the results results (with (with the the exception exception of of sample sample 3-99, 3-99, for for which are in in "low" "low' (lowwhich An An content content is yet to be measured) indicate that all the samples are (lowtemperature, temperature,or orordered) ordered)structural structuralstate state(Table (Table1). 1). samnie 1-99 90.180 (-i-I- An 65* 0.084) 2-99 90.247 (+1- 0.204) 62* 3-99 90.299 BL-27 89.843 D-12 90.213 (+1- 0.130) (+1- 0.190) (÷1- 0.218) n.m. 55 63 Table Refinedgamma gamma and values plagioclase separates of Duluth Complex Anorthositic Series rocks. Table 1. Refined and AnAn values for for plagioclase separates of Duluth Complex Anorthositic Series rocks. *from *from Miller Miller (1995); (1995);n.m. n.m. not not measured. measured. Errors Errors reported reported as two standard standard deviations, deviations, because no internal standard wasused. used. standardwas References Referencescited: cited: Anderson, Anderson, A.T., A.T., Jr., Jr., 1983, 1983,Oscillatory Oscillatoryzoning zoning of of plagoclase: plagoclase: Nomarski Nomarski interference interferencecontrast contrast microscopy microscopy of of etched etched polished polished sections: sections: American AmericanMineralogist, Mineralogist,v. v. 68, 68, p. p. 125-129. 125-129. Appleman, Appleman,D.E. D.E. and andEvans, Evans, H.T., H.T., Jr., Jr., 1973, 1973,Job Job 9214: 9214: indexing indexingand and least-squares least-squaresrefinement refinement of 67 pp. 20,67 pp. (NTIS (NTIS of powder powder diffraction diffraction data. data. U.S. U.S. Geological GeologicalSurvey, Survey, Computer Computer Contrib. Contrib. 20, Doc. Doc. PB2-16188) PB2-16188) Castro, A,,and and de dela laRosa, Rosa,J., J., 1994, 1994,Nomarksi Nomarksi study study of of zoned zoned plagioclases plagioclasesfrom from granitoids granitoidsof of Castro,A., the the Seville SevilleRange Range batholith, batholith, SW SW Spain. Spain. Petrogenetic Petrogenetic implications: European European Journal Journalof of Mineralogy, Mineralogy,v.v.6,6,p.p.647-656. 647-656. Garvey, Garvey,R.G., R.G., 1986, 1986,LSUCRIPC LSUCRIPCLeast Leastsquares squaresunit unitcell cellrefmement refinementwith with indexing indexingon onthe the personal personalcomputer. computer.Powder PowderDiffraction, Diffraction,v. v.I1no. no.1., I.,p.p.114. 114. Kroll, Kroll,H., H.,1983, 1983,Lattice Latticeparameters parameters and anddeterminative determinativemethods methods for for plagioclase plagioclaseand andternary ternary feldspars, ed., Reviews Reviews in in Mineralogy, Mineralogy,v.v.2, 2 ed., feldspars, in in Ribbe, P.H., P.H., ed., 2, 2nd ed., p. p. 101-119. 101-119. Miller,J.D., J.D., Jr., Jr.,1995, Duluth Complex Complex at at Duluth, Duluth, in in Miller, Miller, J.D. J.D. Jr., Jr., ed., ed., Field FieldTrip Trip Miller, 1995, The Duluth Guidebook Guidebookfor forthe theGeology Geologyand andOre OreDeposits Depositsof of the theMidcontinent MidcontinentRift Riftin in the the Lake Lake Superior SuperiorRegion. Region.Minnesota MinnesotaGeological GeologicalSurvey SurveyGuidebook GuidebookSeries, Series,no. no. 20, 20,p.p.123-169. 123-169. Miller,J.D., J.D., Jr., Jr., and andWeiblen, Weiblen, P.W., P.W., 1990, 1990,Anorthositic Anorthositic rocks rocks of of the the Duluth DuluthComplex: Complex: Miller, Examples Examplesof of rocks rocks formed formed from from plagioclase plagioclase crystal mush: Journal Journalof of Petrology, Petrology,v.v.31, 31,p.p. 295-339. 295-339. Paces,J.B., J.B., and andMiller, Miller,J.D., J.D., Jr., Jr., 1993, 1993,Precise PreciseU-Pb U-Pb ages agesof of Duluth DuluthComplex Complexand andrelated related Paces, maficintrusions, intrusions,northeastern northeasternMinnesota: Minnesota:Geochronological Geochronologicalinsights insightsto tophysical, physical, mafic petrogenetic,paleomagnetic paleomagneticand andtectono-magmatic tectono-magmaticprocesses processesassociated associatedwith withthe the1.1 1.1Ga Ga petrogenetic, Midcontinent Midcontinentrift rift system: system: Journal JournalofofGeophysical GeophysicalResearch, Research,v.v. 98, 98,p. p. 13,997-14,013. 13,997-14,013. Pearce, Pearce,T.H., T.H., 1994, 1994,Recent Recentwork workon onoscillatory oscillatoryzoning zoningin in plagioclase, plagioclase,in inParson, Parson,I., I.,eds., eds., Feldsparsand andTheft TheirReactions: Reactions:Dordrecht, Dordrecht,Kluwer, Kluwer,p.p.3313-349. Feldspars 13-349. Pearce,T.H., T.H., and andKolisnilc, Kolisnik, A.M., A.M., 1990, 1990,Observations Observationsof of plagioclase plagioclasezoning zoningusing using Pearce, interference EarthScience ScienceReviews, Reviews,v.v.29, 29,p.p.9-26. 9-26. interferenceimaging: imaging:Earth Grierguided guidedus usinincarrying carryingout outthe theplagioclase plagioclasestructural structuralanalyses. analyses.BSE BSE Acknowledgements:Dean DeanGrier Acknowledgements: gratefully NDEPSCoR EPSCoRthrough throughNSF NSFgrant grant#OSR-9452892. #OSR-9452892. gratefullyacknowledges acknowledgesfinancial financialsupport supportfrom fromND 5 �Wind or Water? Paleoenvironment Paleoenvironmentof the Proterozoic Proterozoic Hinckley Sandstone, Sandstone, Northeastern Minnesota Northeastern Beaster, K.F., Geology K.F., Kohn, Kohn,J.D., J.D.,and andHavholm, flavholm,K.G., K.G.,havholke@uwec.edu, havhoIkwuwcc.edu, Geology Department, University of Claire, Eau Claire, WI 54702-4004 o f Wisconsin-Eau Wisconsin-Eau Claire, The Proterozoic Hinckley Sandstone is a mineralogically mature, fine-grained quartz (Tryhorn & arenite representing late stage Keweenawan rift fill sedimentation. Previous work (Tryhom Ojakangas, 1972) interpreted the Hinckley Sandstone as a shallow-water lacustrine deposit. 1972) shallow-water Recent recognition of adhesion structures, formed by sand blown onto a damp surface, suggest be partially partially eolian eolian (wind-deposited). (wind-deposited). The purpose of this project was to that this formation may he reevaluate the environment(s) of deposition environmentfs) of deposition of of the the Hinckley Hinckley Sandstone. Sandstone. The study area is located in Pine County, MN, near the town town of Sandstone, where five sections along the Kettle River were measured and described in detail. Four major facies were identified: planar-bedded sandstone (PS), cross-stratified sandstone (CSS), trough crossstratified sandstone sandstone (TCS), and pebbly trough cross-stratified sandstone (PTS). The planarbedded facies comprises a complexly interbedded sequence of crinkly and mottled strata, preserved ripple forms, and packages of thin (2-6 mm.), planar, inversely graded laminae. Crinkly and mottled strata are interpreted as adhesion structures (Hunter, 1973, 1980). 1980). Ripples have low ripple indices (<10) ( 4 0 ) and steep, normally graded foresets, indicating a subaqueous origin (Hunter, 1977). Planar (Hunter, Planar laminae laminae are are identified identified as as climbing wind-ripple laminations (Hunter, 1977). Therefore, the planar-bedded facies represents an environment where where conditions conditions alternated from wet to damp to to dry. Fine nature of Fine grain grain size, large set size, and the compound nature large sets suggest a large-scale eolian dune dune origin origin for for the the cross-stratified cross-stratifiedsandstone sandstonefacies. facies. In addition, wind-ripple and adhesion lamination were identified in tangential dune apron toe-sets. Trough cross-strata cross-strata and pebbly trough cross-strata facies contain predominantly medium-grained sandstone; pebbles cm.are arecommon commonininthe thepebbly pebbly trough trough cross-stratified cross-stratified facies. facies. pebblesup uptoto44cm. Adhesion and wind-ripple laminae are absent in these relatively small (up to 2 m. wide and 20 cm. thick), simple trough cross-strata. cross-strata. Coarse Coarse grain grain size and nature of strata suggest a subaqueous subaqueous origin origin for for these these facies. facies. Exposures of the Hinckley Sandstone are incomplete and widely separated, limiting detailed correlation of sections. The m. for The thickest thickest exposure exposure measures measures 30 m., whereas a thickness of 150 m. this formation is reported from a well in the the area area (Morey, (Morey, 1977). 1977). However, using a regional regional structural dip of 50 (Tryhorn and Ojakangas, Ojakangas, 1972) the the measured measured sections sections stack stack to to give a 5' or less, (Tryhom sediment sequence sequence that alternates alternates from depositon in subaqueous to subaerial and back to subaqueous conditions several times. Small Small (mm.(mm.- to cm-scale) cm.-scale)extensional extensional faults faultsand andcm.cm.-to to dm.-scale fluid-escape structures are associated associated with with the the subaqueous subaqueousto tosubaerial subaerialtransition. transition. This vertical sequence sequence suggests suggests a rift setting setting with a shifting mosaic of eolian and fluvial, and possibly lacustrine, environments. lacustrmne, environments. Reference Reference cited: cited: by climbing climbing adhesion adhesionripples. ripples. Journal of Sedimentary Sedimentary Hunter, R.E., 1973, Pseudo-crosslamination formed by Petrology, v. v.43, p. 1125-1127. 43, p. 1125-1 127. R.E., 1977, Basic types types of of stratification stratificationininsmall smalleolian eoliandunes. dunes.Sedimentology, Sedimentology,v.v.24, p.361-387. Hunter, R.E., 24, p. 361-387. 6 �- I I I - Hunter, — an Hunter,R.E., R.E., 1980, 1980,Quasi-planar Quasi-planaradhesion adhesionstratification stratification an eolian eolian structure structure formed in wet sand. Journal Journal of of Sedimentary SedimentaryPetrology, Petrology, v. v. 50, 50, p. p. 263-266. 263-266. Morey, Morey, G.B., G.B., 1977, 1977,Revised Revised Keweenawan Keweenawan subsurface subsurface stratigraphy, southeastern Minnesota. Minnesota Minnesota Geological 2lpp. Geological Survey SurveyReport Report of of Investigations Investigations 16, 16,27pp. Tryhorn, Tryhorn, A.D. A.D. and and Ojakangas, Ojakangas, R.W., R.W., 1972, 1972,Sedimentation Sedimentation and petrology of the Upper Precambrian Hinckley Hinckley Sandstone Sandstoneof of east-central east-central Minnesota. In, In,P.K. P.K. Sims Simsand andG.B. G.B. Morey, Morey,eds., eds., Geology Geologyof of Minnesota: Minnesota: AACentennial CentennialVolume, Volume,Minneapolis, Minneapolis,Minnesota MinnesotaGeological GeologicalSurvey, Survey,p. p. 431-435. 431-435. 7 �Some factors factors that control the distribution of mercury in the near-surface environment National Park, Park, Michigan-earth, Michigan—earth, wind, and fire environment of Isle Royale National W.F. W.F. Cannon, Cannon,U.S. U.S. Geological GeologicalSurvey, Survey,Reston, Reston,VA VA L.G. U.S.Geological GeologicalSurvey, Survey,St. St.Paul, Paul,MN MN L.G. Woodruff, Woodruff,U.S. The The concentration concentrationof of mercury mercury in in soils soils on on Isle Isle Royale Royale is is highly variable variable in in response response to to many understood. Our Our studIes studies during during the many factors factors whose affects and interplay are poorly understood. past them. Interest past three three years years have helped to clarify some of them. Interest in in mercury mercury distribution distribution on on the island was heightened in the mid 1990's when potentially dangerous mercury levels the island was heightened in the 1990's when potentially dangerous mercury levels were were discovered discovered in in game game fish fish from from six six of of the island's island'sinland inlandlakes. lakes.Because Becausethese thesesix six lakes were clustered in the north central part of the island, whereas lakes on other parts lakes were clustered in of the whereas lakes on other parts of of the the island island showed showed lower lower mercury mercury levels levels in in game game fish, fish, we initially initially suspected suspected that that previously previously unrecognized unrecognized bedrock bedrock mercury concentrations concentrationswithin within the the watersheds watersheds could could be be aa cause. cause. Small Smallnative nativecopper copperdeposits depositswith withsignificant significantmercury mercury content content are are distributed distributed throughout throughout the the island. island. To Totest testthis thishypothesis hypothesiswe we sampled sampledsoils soilsaround around the the Minong Minong Copper 19&century century mining mining operations, operations, and and within within the the watershed watershed CopperMine, Mine,the thelargest largestofofthe the19th of of Sargent SargentLake, Lake, one one of of the the mercury mercury impacted impacted lakes. lakes. Results Results presented presented last last year year (Cannon (Cannon and and Woodruff, Woodruff, 1999) 1999)showed showed that that aa bedrock bedrock source source for for mercury mercury to to the the impacted impactedlakes lakesisis unlikely. The TheMinong Minongcopper copperdeposit depositproduces produces aa very very pronounced pronounced copper copper anomaly anomaly in in soils, Lake watershed. watershed. We soils, but a comparable comparable anomaly anomaly is absent within the Sargent Lake We did did find, find, however, however, that that a-horizon a-horizon soils soilsin in the the Sargent Sargent Lake Lake watershed watershed contain contain very very high high mercury mercury concentrations. concentrations. Because Because aa bedrock bedrock source source is is unlikely unlikely we we concluded concludedthat that mercury mercury concentrations concentrationswere were largely largelyfrom fromairborne airbornedeposition. deposition. Additional Additional sampling samplingand and analyses analyses have have been been conducted conducted in in the the past past year year to to address address the the question ~uestionof of why whv mercury mercurv concentrations concentrationsare are so so variable variable in in soils soils ifif the the source sourceof of mercury mercurv is atmospheric atmospheric pollution. pollution. There There isis aa very very high high correlation correlationbetween between organic organiccarbon carbonin in soils soils and total mercury content (R=0.9). Thus, it appears that factors that control carbon and total mercury content Thus, it appears that factors that control carbon sequestration sequestrationin in soils soils also also control mercury sequestration. One One factor factorthat that could could influence influence both bothisis the the history history of of forest forest fires. fires. During Duringaafire firemuch muchof ofthe thecarbon carbonon onthe theforest forestfloor floorand and in in surface surface soil soil layers layers may may be be burned, burned, thus thus liberating liberating adsorbed adsorbed mercury mercury as as aa gas gas or or fine fine particles that would be transported out of the area. After a fire, the gradual accumulation particles that would be transported After a fire, the gradual accumulation of of organic organic carbon carbon and and mercury mercury can can require require decades decades or or longer longer to to reestablish reestablish pre-fire pre-fire concentrations. A large portion of Isle Royale burned in 1936, including part concentrations. A large portion of Isle Royale burned in 1936, including partof ofour ourstudy study area, burned in in historic historic times. times. Soils area, whereas the remainder of our study area has not burned Soils within within the the 1936 1936burn burn area area invariably invariably have low carbon and mercury content. All Allsamples samples are are below the mean concentration concentration for the island. Some Somesoils soilsoutside outside of of the the bum burn have have comparably comparably low low values values but most are significantly higher. Further Further sampling samplingis is being being conducted fire to to conducted to to seek seek additional additionalconfirmation confirmationof of the the importance importance of of this this 64-year-old 64-year-old fire current currentmercury mercurydistribution. distribution. is Finally, Finally,the thedistribution distributionof ofmercury mercuryin ingeological geologicalmaterials materialsas asaa possible possiblefactor factorinin controlling controllinemercury mercurv accumulation accumulation in in the aquatic aauatic food web needs to be addressed. High Hi& mercury mercuryconcentrations concentrationsin ingame gamefish fishare arethe theend endresult result of ofsuccessive successiveaccumulations accumulations through various steps in the food web. Mercury concentrations inngame gamefish fishtissue tissuemay maybe be - - .. 8 �as lake water. water. Furthermore, as much as one one million times higher than in the ambient lake Furthermore, mercury mercury enters enters the food food web web as as an an organic organic compound, compound, mostly mostly dimethyl dimethyl mercury, mercury, rather rather than the more common inorganic forms. As a result, may researchers discount the than As a result, may researchers discount the concentrations concentrationsof of mercury mercury in in the the terrestrial terrestrial environment environment as as aa factor factor controlling controllingmercury mercuryin in fish. Rather, the efficiency of methylation processes within a lake and its watershed and fish. Rather. the efficiency of methvlation orocesses within a lake and its watershed and complexities complexitiesin in the the food food web web are are commonly commonly cited cited as as causes causes for for widely widely variable variablemercury mercury content Ourresults resultsnow now show showthat that there there are aremuch much higher higher content of of fish fish between nearby lakes. Our mercury mercury concentrations concentrationsin in soils soilsin in the the watersheds watershedsof of Sargent SargentLake Lake (high (highmercury mercury in in fish) fish) than than in in soils soils around around nearby Lake Richie (low mercury in fish). Although Althoughthese theseresults results address address only only two two lakes, lakes, we we suggest suggestthat that itit would would be be premature prematureto to discount discountvariations variationsin in mercury mercury in in watershed watershed soils soilsas as aa factor factorin in determining determiningthe the intensity intensityof of mercury mercury accumulation accumulationin in related related aquatic aquaticecosystems. ecosystems. . < 4. SARGENT LAKE4J MINONG MINE .:•—. LAKE RICHIE 6 6 0 l2MiIes East half of soil thickness) thickness)in inaaof Isle Isle Royale. Royale. Relative Relative mass mass of of mercury mercury (concentration (concentrationxx soil horizon shown fire. Native Native shown by black squares. White White area areawas was burned burned in in 1936 1936forest forest fire. copper copper mines mines and and prospects prospects shown shown by pick and and hammer hammer symbol. symbol. '. a 7 3 Reference Reference Cannon, W.F., W.F., and and Woodruff, Woodruff, L.G., 1999, 1999,Mercury Mercury distribution distribution in in bedrock, bedrock, native native copper copper Cannon, ore and soils—Isle soils-Isle Royale National Park, Michigan (abs): Institute on Lake Superior Superior Geology Proceedings, 45, p.9-10. p.9-10. Geology Proceedings, v. v. 45, 9 �A SEQUENCE OF FOLDING AND DEXTRAL TRANSPRESSION IN THE SEINE RIVER CONGLOMERATES. Peter J., J., Department Department of of Geology Geology and and Geophysics, Geophysics, University University of of CZECK, Dyanna M. and HUDLESTON, Peter Minnesota, 310 3 10 Pillsbury Drive SE, Minneapolis MN, 55455, 55455, czecO019@tc.umn.edu czec0019@tc.umn.edu GEOLOGICAL INTRODUCTION The Superior Province is subprovinces that that are are is comprised of of approximately approximately east east -- west trending subprovinces defined by lithological contrasts, metamorphic grade, and structural boundaries (Card and Ciesielski, 1986). A A popular popular tectonic tectonic model for for this this province province is is the the successive successive accretion accretion of of island island arcs arcs and and their their associated sediments sediments(Floffluan, (Hoffman, 1989). 1989). metavolcanic Wabigoon Wabigoon and the This study focuses on the boundary between the metavolcanic metasedimentary Quetico subprovinces near Mine Centre, Ontario. The Quetico contains mostly amphibolite facies metasediments, and the Wabigoon contains greenschist facies metavolcanics. Distinctive rocks found along this boundary are the Seine River Metasedimentary Metasedimentary Group including the conglomerates. From River conglomerates polymictic Seine River conglomerates. From stratigraphic evidence, the Seine River deposition of of the the volcanic volcanic sequences sequences(Poulsen (Poulsenetetal., at., 1980). 1980). Preliminary formed late following the deposition attempts at absolute absolute dating dating have have confirmed confirmed this this (Davis (Daviset et al., al., 1989). 1989). Previous structural work along this boundary described a dominant subvertical foliation (Poulsen, 1986; Tabor Tabor and and Hudleston, Hudleston, 1991). 1991). These features are interpreted interpreted to to transecting large folds (Poulsen, have formed during dextral transpression. STRUCTURAL INTRODUCTION The term "transpression" was first introduced by Harland (1971) to describe motion that is obliquely convergent, or motion motion partitioned partitioned into into both both convergent convergentand andstrike-slip. strike-slip. In In Harland's original obliquely convergent, described the development development of of folds folds and and strike-slip strike-slip structures structuresdue duetotooblique obliqueconvergence. convergence. A A paper, he described Marchini (1984). This specialized case of transpression was introduced by Sanderson and Marchini This idealized idealized definition of transpression has been used to describe homogeneous deformation consisting of orthogonal simple shear and pure shear components with constant volume and confined deformation deformation boundaries, possibly corresponding to the strain in deep ductile shear zones that accommodate oblique convergence. Quetico boundary seems to be related to this more specific The dominant foliation along the Wabigoon - Quetico type of transpression transpression based based on on the flattening flattening fabrics fabrics and asymmetric features indicating simple simple shear shear in in subhorizontal plane. the subhorizontal One of the questions addressed by this study is the relationship between the folds and the dominant foliation. Clearly, Clearly,itithas hasbeen beendemonstrated demonstratedthat that folds foldscan canform formin intranspression transpression Flarland Harland transpression fabric, or are they an (1971). Are Are the the folds folds at at this this boundary boundary consistent consistent with the rest of the transpression earlier structural feature? FOLIATION/ BEDDING INTERSECTIONS The Seine Group has well defined bedding indicated by fine grained layers interbedded with the recognized pebble conglomerates. conglomerates. In In this this analysis, analysis, the the major major fold fold in in the Seine River conglomerates was recognized on the basis of of structural structural facing facing between between bedding bedding and and the the dominant dominant foliation foliation(Shackleton's (Shackleton'srule rule—- see Borradaile (1976)). This Thisfold foldisisconsistent consistent with with the the observed observed 'way "wayup" up" indicators indicators in in the the conglomerates conglomerates based on graded bedding. bedding. 10 �If this fold were to have formed as a result of the transpression, transpression, the foliation! foliation1 bedding throughout the the field field area. area. In fact, the plunge of the intersection varies intersections should be consistent throughout throughout the field area. The The intersection intersection angle angle is is generally small because both foliation and bedding throughout are steep; steep; however, however, locally the the angle angle is is quite quite large large nearing nearing 75" 75°atatthe thehinge. hinge. This suggests a more history, and that the folding occurred prior to the major transpressive event. complex history, TECTONIC IMPLICATIONS As described elsewhere in the Superior province, the sequence of initial folding followed by homogeneous transpression (and possibly later stage amplification of the strike- slip slip shear shear component component reasonable for for the the WabigoonWabigoon-Quetico Queticoboundary. boundary. It remains an along discrete shear zones) seems reasonable interesting question whether the folds and the homogeneous transpression were two entirely abrupt, separate events or rather an evolution of structural styles during different stages of the same same oblique oblique collisional event. The fact that the initial folding folding is seen in the Seine Group has important implications for for the the relative timing between this change in in structural structural style style and and Seine Seinedeposition. deposition. Some researchers have pull-apart basins formed as suggested that the Seine Group could have been syntectonically deposited in pull-apart as (Poulsen, 1986). The a result of wrenching (Poulsen, The early folding folding of the Seine seems to indicate that it was deposited earlier in the deformation sequence, prior prior to to at at least least some some of ofthe thefolding. folding. Additionally, the overall geometry of the interconnected faults faults seems seems to indicate that during dextral wrenching, the the Seine Seine Group's "basin" "basin" would would have have been been in in aa compressive compressive rather rather than than an an extensional extensional regime regime required required of ofaapullpullbasin. This apart basin. Thissuggests suggests that that aa different different sedimentation/ sedimentation,tectonic model for the Seine Group is needed. J., 1976. the Palaeozoic Palaeozoic rocks rocks of of the the Malaguide Malaguide Complex Complex Borradaile, G. J., 1976. "Structural "Structural facing" (Shackleton's rule) and the nederlandse akademie akademie van van wetenschappen wetenschappen 79B, 798, near Velez Rubis, SE Spain. Proceedings, Koninklijke nederlandse 330330336. 336. A., 1986. the Superior Province Province of of the the Canadian Canadian Shield. Shield. Geoscience Geoscience Card, K. D., Ciesielski, A,, 1986. DNAG Subdivisions of the Canada 13, 5-13. 13,5-13. Davis, D. W., Poulsen, Poulsen, K. H., Kamo, Kamo, S. S. L., L., 1989. 1989.New New insights insightsinto into Archean Archean crustal crustaldevelopment developmentfrom from geochronology 97, 379-398. geochronology in the Rainy Lake area, area. Superior Superior Province, Canada. Journal of Geology 97,379-398. 8., 1971. 27-42. Harland, W. B., 1971.Tectonic Tectonictranspression transpressionin in Caledonian Caledonian Spitsbergen. Spitsbergen. Geological Magazine 108, 108,27-42. Hoffman, P. F., 1989. America. In: In: Bally, Bally, A. A. W., W., Palmer, A. A. R R. ( Hoflinan, 1989. Precambrian geology geology and tectonic history of North America. Eds.), The geology geology of of North America; an overview. The geology of North America A, pp. 447-512. Poulsen, K. Fl., example of of an an Archean Archean Subprovince Subprovinceboundary boundary in inNorthwestern Northwestern H., 1986. 1986. Rainy Lake Lake Wrench Zone: An example Ontario. (Eds.), Tectonic evolution of greenstone greenstone belts belts Technical Report Ontario. In: de Wit, M. J., Ashwal, L. D. (Eds.), 86-10, pp. 177-179. 177-179. Poulsen, K. H., Borradaile, Archean succession succession at at Ramy Rainy Lake, Lake, Borradaile, G. J., Kehlenbeck, M. M., 1980. An inverted Archean Ontario. Canadian Journal of Earth Sciences 17, 17,1358-1369. 1358-1369. D., 1984. Transpression. Transpression. Journal Journal of of Structural Structural Geology Geology 6,449-458. 6,449-458. Sanderson, D. J., Marchini, W. R. D., Tabor, J. R., Hudleston, P. J., 1991. Deformation at an Archean subprovince boundary, northern Minnesota. 1991. Tabor, Hudleston, Canadian Journal of E Earth Sciences 28,292-307. arth 28,292-307. 11 �A A Hf Hf isotopic isotopic study study on zircons from from the the western western Superior Superiorprovince province D.W. D.W.Davis, Davis,Y. Y.Amelin, Amelin,Earth EarthScience ScienceDept., Dept.,Royal RoyalOntario OntarioMuseum, Museum,100 100 Queen's Queen's Park, dond(drom.on.ca, G.M. G.M. Nowell Nowelland andR.R. R.R. Parrish Parrish Park. Toronto, Toronto. ON, ON. M5S 2C6 dond@,rom.on.ca. NERC NERC Isotope Isotope Geoscience GeoscienceLaboratory, Laboratory,Kingsley Kingsley Dunham Dunham Centre, Centre, Keyworth, Keyworth, Nottingham, Nottingham, NG12 NG12 5GG 5GGUK UK U-Pb U-Pb dating datingof of zircon zirconhas hasprovided provided some someof of the the most most important importantconstraints constraintson on geological geological interpretations interpretationsininthe theSuperior Superiorprovince. province.Zircon Zirconalso alsocontains containsHf, Hf,whose whose176Hf "'Hf isotope isotopecan can be beused used to toinfer inferolder oldercrustal crustal provenance provenance of of the magma from which it crystallized. crystallized. Early Hf studies by Corth and Stott (1993, 1996) in the Uchi subprovince show a distribution studies by Corfu and Stott (1993, 1996) in the distribution of EHf EHf values valuesthat thatrange rangebetween betweendepleted depletedmantle mantleand and aa3.0 3.0Ga Gacrustal crustalgrowth growth curve, curve, reflecting reflecting contamination contaminationfrom fromthe theNorth NorthCaribou Caribouterrane terraneand andsupporting supportingthe themodel modelof ofan anAndean-type Andean-type tectonic tectonicsetting. setting.These Thesestudies studiesused usedthermal thermalionization ionizationmass massspectrometry, spectrometry,which whichisis laborious laboriousand andrequires requireslarge largequantities quantitiesof ofzircon. zircon.The Therecent recentdevelopment developmentof ofmulti-collector multi-collector inductively inductivelycoupled coupledmass massspectrometry spectrometrynow nowpermits permitsprecise precisemeasurement measurementof ofHf Hfisotopic isotopic analyses analyseson onsingle singlezircon zircongrains grainsas assmall smallas as22micrograms microgramswith withanalysis analysistimes timeson onthe the order order of 10 minutes per sample. of 10 minutes per sample. Using Usingthe thePlasmaS4 Plasma54instrument instrumentatatthe the NERC NERC lab, lab,Hf Hf isotopes isotopeswere weremeasured measured on on dated datedzircons zirconsfrom from62 62separate separaterock rockunits unitsin inthe thewestern westernSuperior Superiorregion regionplus plus25 25detrital detrital zircon zircongrains grainsfrom fromthe theca. ca.2700 2700Ga GaQuetico Queticometasediments metasedimentsand andpre-2800 pre-2800Ma Maquartz quartz arenites. arenites.The Themass massspectrometer spectrometeranalyses analyseswere weredone doneover overaafour fourday dayperiod. period.This Thisexpands expands the available Hf database in the transect area by a factor of four. the available Hf database in the transect area by a factor of four. Most Mostdata datafrom fromthe theSW SWWabigoon Wabigoonsubprovince subprovincecluster clustertightly tightlyaround aroundaadepleted depleted mantle value of +6S. +6.5. This confirms that assemblages south of the Wiimipeg mantleEHf Enfvalue Winnipeg River River subprovince subprovincewere weremagmatically magmaticallyemplaced emplacedininan anoceanic oceanicenvironment. environment.Analyzed Analyzedzircons zircons from fromthe theSturgeon-Savant Sturgeon-Savantarea areashow showaatrend trendof ofincreasing increasingcontamination contaminationby byolder oldercrust crust with younger age (decreasing Eflf down to +2). with younger age (decreasing Ein-down to +2). Younger Youngerplutons plutonsand andvolcanics volcanicsfrom fromthe thecentral centralWabigoon Wabigoonsubprovince subprovince(2693-2750 (2693-2750 Ma) mostly scatter around values of +4 ± 1, showing the possibility of slight Ma) mostly scatter around values of +4 1, showing the possibility of slight contamination contaminationby byolder oldercrust. crust.Zircons Zirconsfrom fromthe theolder older3000 3000Ma Maold oldrocks rocksininthe theLumby Lumby Lake Lakebelt beltcluster clustertightly tightlyaround aroundaadepleted depletedmantle mantlevalue value(at (at3.0 3.0Ga) Ga)ofof+5.5. +5.5.Zircons Zirconsfrom from ca. ca.2.9 2.9Ga Gaplume-derived plume-derivedrocks rocksgive giveslightly slightlymore moreenriched enrichedvalues valuesdown downtoto+4. +4.The Theresults results from fromLumby LumbyLake Lakeoverlap overlapwith withthose thosefrom fromsimilar similaraged agedrocks rocksininthe theNorth NorthCaribou CaribouLake Lake area of the Sachigo subprovince, supporting the suggestion that the Lumby Lake and area of the Sachigo subprovince, supporting the suggestion that the Lumby Lake andNorth North Caribou Caribouterranes terranesare arecorrelative. correlative. The TheHf Hfdata datafield fieldfrom fromthe theWabigoon Wabigoonand andSachigo Sachigosubprovinces subprovincesoverlaps overlapsclosely closely with withthat thatofofCorfu Corfuand andStott Stott(1996) (1996)from fromthe theUchi Uchisubprovince. subprovince.InIncontrast contrastzircons zirconsfrom fromthe the Winnipeg WinnipegRiver Riversubprovince subprovinceare arenoticably noticablymore moreenriched, enriched,with withEHf EHfvalues valuesof ofabout about00toto -2 over an age range of 2721 Ma to 3225 Ma. An EHf value of—2 value found on 3.2 Ga Ga -2 over an age range of 2721 Ma to 3225 Ma. An Eufvalue of -2 value found on3.2 zircons zirconsfrom fromthe theCedar CedarLake Lakegneiss gneissimplies impliesaaca. ca.3.5 3.5Ga Gacrustal crustalprotolith, protolith,ininagreement agreement with withresults resultsofofNd Ndwhole wholerock rockisotopes isotopes(Henry (Henryetetal. al.1997). 1997). AAsingle singleanalysis analysisfrom fromthe thewidespread widespread2722 2722Ma Mavolcanics volcanicsininthe theShebandowan Shebandowanbelt belt shows showsaadepleted depletedsignature, signature,asasdoes doesaa2726 2726Ma Mavolcanic volcanicfrom fromthe theMelchett MelchettLake Lakebelt, belt,ininthe the middle middleofofthe theEnglish EnglishRiver Riversubprovince. subprovince.However, However,2723 2723Ma Mavolcanic volcaniczircons zirconsfrom fromthe the Separation SeparationLake Lakegreenstone greenstonebelt, belt,which whichmay mayunderlie underlieEnglish EnglishRiver Rivermetasediments, metasediments,show showaa * 12 �more enriched enriched value value of of -2, -2, suggesting suggesting that that itit is is contaminated contaminatedby by much much older olderWinnipeg Winnipeg River-type River-type crust. crust. Results Results on on detrital detrital zircons zirconsfrom from quartz quartz arenites arenites in in the the North North Caribou Caribouterrane terranegenerally generally overlap An analysis analysis from from 3250 3250 Ma Ma old old zircon zirconin inthe the overlap the the data data field field from from rocks rocks in in this this area. area. An Jutten Jutten quartz quartz arenite arenitein in the the northern northern Wabigoon Wabigoon subprovince subprovinceshows shows still still older oldercontamination contamination (E1 ( h fof of 0), 0), suggesting suggestingthat that 3.0 3.0 to to 3.3 3.3 Ga Ga detrital detrital zircons zircons in this unit were derived derived from from the adjacent adjacent Winnipeg Winnipeg River River subprovince. subprovince.Ca. Ca. 3.2 3.2 Ga Ga zircons zircons from from the the Quetico Quetico metasediments metasediments also show enriched values down to —3, while the youngest zircons at ea. 2.7 also show enriched values down to -3, while the youngest zircons at ca. 2.7 Ga show variable variableenrichments enrichmentsfrom from+5 +5 to to +0, +0, again againsuggesting suggestingaa source sourcepartially partiallyin inthe theWinnipeg Winnipeg River subprovince. River subprovince. The values at at ca. ca. 2.7 2.7 Ga Ga and and 3.0 Ga provides The general general coherence coherenceof ofthe thehighest highestEHf Enfvalues reasonable reasonable constraints constraintson onthe theevolution evolutionof ofthe thedepleted depletedmantle mantlereservoir reservoirduring duringthe thelate late Archean. These results agree with the linear growth trend defined by other Hf Archean. These results agree with the linear growth trend defined other Hf measurements measurements from from younger younger rocks rocks and and from from early early Archean Archean rocks rocks in in Greenland Greenland (Vervoort (Vervoort and Blichert-Toft 1999). They give evidence for a major depletion of the mantle or and Blichert-Toft 1999). They give evidence for a major depletion of the mantle atator before 4.0Ga. Ga.IfIfcontinental continentalcrust crustwas wasthe thecomplimentary complimentaryenriched enrichedreservoir, reservoir,this thisimplies implies before 4.0 that that such suchcrust crustwas wasof ofan anextent extentcomparable comparableto to the thepresent present during duringthe theearly earlyArchean. Archean. Otherwise, Otherwise, the the enriched enriched reservoir reservoir may may have have been been oceanic oceanic crust crust that that was was subducted subductedand and stored Archean. stored in in the the deep deep mantle mantle until until the the end end of of the the Archean. Continuing Hf isotopic studies on precisely Continuing Hf isotopic studies on precisely dated dated zircons zirconspromise promiseto to be beaapowerifil powerful tool tool for forresolving resolvinglocal localand andglobal globalproblems problemsininEarth Earthhistory. history. REFERENCES REFERENCES Corfu, Corfu, F. F. and and Stott, Stott,G. G. 1996. 1996. Hf Hf isotopic isotopic composition composition and and age age constraints on the evolution evolution of of the the Archean Archean central central Uchi subprovince, subprovince, Ontario, Canada. Precamb. Res. 78: 535363. 63. Corfli, Corfu,F. F. and and Stott, Stott,G. G.1993. 1993.Age Ageand andpetrogenesis petrogenesisof oftwo twolate lateArchean Archeanmagmatic magmaticsuites, suites, northwestern northwesternSuperior Superiorprovince, province,Canada: Canada:Zircon ZirconU-Pb U-Pb and andLu-Hf Lu-Hf isotopic isotopicrelations. relations. Jour. Petrol. 34: 8 17-838. Jour. Petrol. 34: 817-838. Henry, Henry,P., P., Stevensen, Stevensen,R., R., Larbi, Larbi,Y., Y.,and andGariepy, Gariepy,C. C.1997. 1997.Use Useof ofNd Ndisotopes isotopestotoquantify quantify 700 in the 700 Ma Maof ofcrustal crustalgrowth growthfrom from3.4 3.4to to2.7 2.7Ga Gain thewestern westernSuperior Superiorprovince province (Ontario, (Ontario,Canada). Canada).Western WesternSuperior SuperiorTransect TransectWorkshop WorkshopReport Report#63: #63:27-36. 27-36. Vervoort, Vervoort,J.D. J.D. and andBlichert-Toft, Blichert-Toft,J.J.1999. 1999.Evolution Evolutionof ofthe thedepleted depletedmantle: mantle:Hf Hfisotope isotope evidence from juvenile rocks through time. Geochim. Cosmochim. Acta evidence from juvenile rocks through time. Geochim. Cosmochim. Acta63: 63:533533556. 556. 13 �MYSTERY FAULTS FAULTSOF OFTHE THE CASCADE RIVER, RIVER, NORTH NORTH SHORE, MYSTERY SHORE, OR, OR,WHAT WHATIS IS THIS THIS GRANITE DOING HERE? HERE? GREEN, GREEN,John JohnC., C., Department DepartmentofofGeological GeologicalSciences, Sciences, University University of Minnesota Duluth, Duluth, MN 55812, jgreen@d.umn.edu jpreen@d.umn.edu Duluth, MN 55812, The The Cascade Cascade River in Cook County, MN, exposes exposes a homoclinal section section about about 13 13km km long longacross across normal-polarity rocks rocks of of the the North North Shore Volcanic Volcanic Group. Group. With normal-polarity With an an average dip of about 12 degrees, this this amounts amounts to a thickness of about 2.4 km. In degrees, Inits itslowest lowest mile mile and and aa major major tributary, tributary, it cuts across several faults faults that that involve the the great, great, cuesta-forming cuesta-forming Terrace Terrace Point Point basalt flow and the thick, underlying Good Good Harbor Harbor Bay Bay sandstone. sandstone. red sandstone and the At the top of of the the cascades, cascades, the conformable contact between the big red also visible visiblefour four miles miles to to the the east at the big Good overlying flood basalt flow, also GoodHarbor HarborBay Bayhighway highway fault. As the the river river starts starts cut, is dropped down to to the the north northaafew fewfeet feetalong alongaa high-angle, hii-angle, east-west fault. cascades,the the top top of of the sandstone can be seen under the to enter enter the the gorge and plunge down the cascades, 100m m into intothe thegorge, gorge,the thebasalt basaltisis interrupted interruptedby byaaremarkable remarkablebreccia brecciaof ofpink, pink, basalt. But Butless less than than 100 apparent fault contact medium-grained granite that occurs in apparent contact with the the volcanic volcanic rocks. rocks. Locally, Locally, fragments of of basalt basalt and and perhaps perhaps sandstone are caught up up in the breccia, but but most of it is granite. On uncontaminated granite. Onthe thedownstream downstreamside, side,abundant abundantjoints joints in in the the adjacent adjacent basalt imply aa upstream, but but itit isis covered. covered. On steep fault contact dipping upstream, On the the upstream upstream side, side, the bounding bounding fault apparently dips much apparently much more more gently gently downstream, downstream,implying implying aa wedge of of granite granite emplaced emplaced mechanically mechanically flows. A fault contact a kilometer or or so to to the northeast between the sandstone into the lava lava. flows. sandstone and aa rhyolite may be related to this event. event. There are two conceivable sources for this granite, granite, which which must must be be far far from from its original original site siteof of There crystallization. related to to the roof zone zone of crystallization. If itit is Keweenawan, and related of the the Duluth Duluth Complex Complexthat thatisis Eagle Mountain, Mountain, ititwould would have havemoved movedupward upward at at least least2.4 2.4km. km. Otherwise exposed up-dip (inland) at Eagle it could be part of the the Archean Archean basement, basement, in in which case it would have come up much farther, perhaps 10 km. ItItisismedium-grained, medium-grained,hypidiomorphic hypidiomorphicwith with minor minor micrographic micrographic patches patches and and its its ferromagnesian silicate is is Fe-rich biotite, which is rare in Keweenawan granites. Petrographically Petrographically itit rocks of of is not a good match for for either either the the Eagle Eagle Mountain Mountain granophyre granophyre or or typical typical Archean granitic rocks the region. Its from, but but not not how it Itsabundant abundantzircons zircons should should solve solve the problem of where it came from, got to to its its present present position. position. x---- ----- Geologic sketch map from from Green, Green, 1996, Geolom Diswlav: Geology Geology Geology on Display: Minnesota's North and Scenery of Minnesota's Shore State Parks, MN Dept. of of Nat. Nat. Resources, Resources. St. St. Paul Paul . .. . ............... ............... .... - Fault 880 Sandstone I 14 �PETROGENESIS OF RiVER REGION, PETROGENESIS OF GRANITIC GRANITIC ROCKS ROCKS OF OF THE THE DUNKA RIVER REGION, EASTERN EASTERNMESAI3I MESABI RANGE, MINNESOTA MINNESOTA HARRIS, Emily HARRIS, Emily C. C. A., A,, Macalester Macalester College, College, St. St. Paul, Paul, MN 55105, 55105, eharris@macalester.edu; eharris@macalester.edu; Wirth, Wirth, Karl Karl R., R., Geology Geology Department, Department, Macalester Macalester College, College, St. St. Paul, Paul, MN MN 55105, 55105, wirth@macalester.edu throughout the the Midcontinent Midcontinent Rift (MCR) Middle Proterozoic felsic igneous rocks are exposed throughout and the eastern Mesabi Range of northern northern Minnesota. Minnesota. Many of these are considered to be Keweenawan (1,100 Ma) in age. However, However,recent recent studies studies suggest suggest that some some early Middle Proterozoic felsic magmatism may have have occurred occuned in the region region as as well well (Phillips (Phillipset. et.al., al.,this thisvolume). volume). To better understand understand the history of felsic magmatism in northern Minnesota, samples from the Dunka River Region, which is situated along the base of the Duluth Complex, have been collected and studied. Felsic Felsicrocks rocksthat thatintrude intrudegabbros gabbrosof of the the Dunka Dunka River River Region Region have have coarse coarse graphic graphic textures, textures, chemically distinct complexes in the Midcontinent Midcontinent Rift. and are chemically distinct from granophyre complexes Granitic rocks were were collected collected from from along along the the eastern eastern Mesabi Mesabi Range Range between between Babbitt Babbitt and and Aurora, Minnesota. Granite with coarse graphic texture was collected from stockpiles within the Granite with coarse graphic within the Biwabik Iron Iron Formation Fonnation and gabbro mining operations operations along the contact between metamorphosed Biwabik gabbro withingabbro gabbroand andiron ironformation formation(Strandlie, (Strandlie,pers. pers. of the MCR. These Thesegranites granitesoccur occuras assmall smalldikes dikeswithin conimunication). They communication). Theyare arecommonly commonly pegmatitie, pegmatitic, and are composed of potassium feldspar or sodic %), and biotite biotite (5-10 (5-10 %). %). Feldspars Feldspars are variably altered altered to plagioclase (40-50 %), quartz (35-45 %), clay minerals and biotite is altered to chlorite. These Thesegraphic graphic granites granites are are mineralogically mineralogically and and texturally comparable comparable to some some felsic felsic granophyres of the Midcontinent Rift (Kennedy (Kennedy et al., this vol.). Other vol.). Othergranitic graniticrocks rocks were were collected collected from ____________________________________ IS 15 nearby outcrops of the eastern eastern margin of of the the Archean Aurora Sill Archean Giants Giants Range Range batholith batholith for for comparison. comparison. MCi felt The Archean granites are finefine- to to mediummediumgnnophyres 10 s 10 grained, and consist consist of of plagioclase plagioclase (20-40%), (20-40%), amphibole amphibole (<15%), (<15%), quartz quartz (10-40%), (10-40%), potassium potassium S. C feldspar feldspar (5-20%), (5-20%), biotite biotite (5-10%), (5-lo%),and andFe-Ti Fe-Tioxox- % ides ides (<5%), (<5%), and and commonly commonlyexhibit exhibitstrained strainedquartz quartz C+ Giants Range bathofith and weakly foliated textures. These These granites granites are are 9a MCRmalIc texturally texturally and and compositionally compositionallysimilar similarto to grangrangnnophyrs gra=ophy= ites of the western Giants Range batholith (e.g., Boerboom and Zartman, 1993). 1993). 60 70 80 o 40~ " " " " 50 SO" " " " " (50 ' 70 80 The graphic graphic granite granite dikes dikes from from the the Dunka Dunka SiO.* drt. %). o) St02(wt. . characterized by a greater Pit are characterized greater silica silica content Figure Figure 1. Plot of Na20+K20 Na 20+1(20 (total (totalalkalis) versus (Si02=70.8-78.7 wt. (Si02=70.8-78.7 wt. %) %) than than isis observed observed in in MCR MCR SiO2 comparing Dunka granites (open Dunks graphic granites (open gi02 wt. %; Kennedy et al., this this ggranophyres r a n o ~ h ~ r(66-76 es circles) with rocks from the Giants Range Giants Ranee vol.) and and the Aurora Aurora Sill Sfflsyenites syenites (54.6-60.7 (54.6-60.7 wt.%; W.%; batholith (Boerboom and Zartman,1993), Zartman,l993), the the Potassium,sodium, sodium,and and Phillips et al., this vol.). Potassium, Aurora Silt Sill (Phillips et at., al., this vol.), and granogranocalcium concentrations of the granite phyre complexes of the MCR (Kennedy granite dikes dikes (Kennedy et et al., al., (K20=2.98-5.16 wt. wt.%; %; Na2O=3.20-6.48 Na20=3.20-6.48wt, wt, %; %; this vol.). this vol.). (1(20=2.98-5.16 * 1 - - - I - - ~~~ ~ 15 �CaO=O.28-1 .09wt. wt.%) %)are aresimilar similar to to those those of of Ca0=0.28-1.09 the MCR granophyres and the Aurora Sill (Figgranophyres ure 1). 1). However, However,most mostof ofthe thegraphic graphic granites granites have high high A1203 Al203 concentrations concentrations and are are have peraluminous, similar to those of the Aurora Sill; MCR granophyres are peraluminous to metaluminous. The metaluminous. Thegraphic graphicgranites granites also have Ti02than than rocks rocks of of the the MCR MCR and and the the AuAuless Ti02 ron Sill. The graphic granites are compositionS i l l . The graphic granites are compositionrora ally similar to those of of the the Giants Giants Range Range batholith, although although they have slightly slightly higher higher 59 Si02 concentrations Si02 valSi02 concentrations (Giants (Giants Range Si02 ppm) ues %). ues range from 52.9-73.6 wt. %). 2. Plot PlotofofNb Nbversus versusY Y comparing comparing Dunka On On trace element element diagrams diagrams (e.g. ZrITiO2 Zr/Ti02 Figure 2. circles) with other graphic granites (open circles) other felsic felsic Nb/Y) the graphic graphic granites granites are are distinct distinct from from vs. Nb/Y) Minnesota; Figure 1 for data rocks of northern Minnesota; see 1 Aurora Sill syenites syenites (Phillips (Phillips et al., al., this this vol.), references. and references. but plot with the Duluth Complex Complex granophyres granophyres ((Kennedy ~ e k e et d al., ~ this vol.). However, However, whereas whereas major element concentrations of Dunka graphic granites are similar similar to those of felsic rocks of the MCR granophyre granophyre complexes, complexes, their trace trace element element compositions are more similar to those of mafic rocks of the MCR granophyres. A plot of Nb vs. Y compositions similar mafic rocks of the MCR granophyres. of (Figure 2) reveals the depletion in graphic granites of both Nb and Y with respect to MCR granogranoY/Nb and and ThfNb ThINb ratios ratios of of the the graphic graphic phyres and Aurora Sill Sill syenites. syenites. Furthermore, Furthermore, the the greater greater Y/Nb batholith and and suggest suggest that that these these rocks rocks contain contain granites are most similar similar to those of the Giants Range batholith a more significant crustal crustal component component than do the MCR granophyres and Aurora syenites. similar in mineral and major element element composicomposiGraphic granites of the Dunka River Area are similar lions to felsic granophyres associated with with the the Midcontinent MidcontinentRift. Rift. However, the low trace element tions concentrations of the concentrations the Dunka Dunka graphic graphic granites granites are are more more similar similar to the the mafic mafic rocks rocks of of the the granophyre granophyre complexes. Therefore, Therefore,we weconclude concludethat thatthe thegraphic graphicgranites granites of of the the Dunka Dunka Region Regionare areunrelated unrelatedto to Midcontinent Rift Rift and and to to the theAurora Aurora Sill. Sill. Alternatively, they the other felsic granophyres of the Midcontinent sirnimight be partial melts of country rock along the base of the Duluth Complex. The The chemical chemicalsimilarity between the Dunka graphic granites and the Giants Range Range batholith batholith granite granite suggests suggests that have produced produced the themelts meltsfor forthe thegraphic graphicgranite. granite. Alternapartial melting of Archean granites could have tively, the graphic graphic granites might be the melting products of nearby argillite, argillaceous siltstone, (e.g., Morey, Morey, 1972). 1972). and fme-grained fine-grained feldspathic feldspathic graywacke graywackeof of the the Virginia Virginia Formation Formation (e.g., j References Zartman, Robert E., E., 1993. Geology, geochemistry, and and geochronology geochronology of the central Boerboom, Terrence J. and Zartman, Giants Range batholith, batholith, northeastern Minnesota. Minnesota. Canadian Canadian Journal Journal Earth Earth Science, Science,v. 30, 30, p. p. 2510-2522. 2510-2522. Kennedy, Bryan Bryan C. wirth, Wirth,K., K., and andVervoort, Vervoort,J,J,this thisvolume. volume.Petrogenesis Petrogenesisof ofthe theMidcontinent MidcontinentRift Riftgranophyric granopbyriccomcomplexes of northern Minnesota. P.K. and and Morey, Morey, G.B., GB., Morey, GB., G.B., 1972. 1972.Mesabi MesabiRange. Range.A A Centennial CentennialVolume, Volume, Minnesota Geological Survey, Sims, P.K. editors, Minnesota Minnesota Geological Geological Survey, Survey,p. 204-217. 204-217. Phillips, Phillips, Erin Erin H. and and Wirth, Wirth, K., K., this this volume. volume. Petrogenesis Petrogenesisof of the the Enigmatic EnigmaticAurora AuroraSill, Sill,Mesabi MesabiRange, Range,Minnesota. Minnesota. 16 �Pyroxene Homfelsed Copper Mineralization in the Pyroxene Hornfelsed Archean Giants RangeBatholith Range Batholith Footwall Keweenawan Footwall of the Keweenawan Intrusion, Duluth South Kawishiwi NE Minnesota - Archean or Keweenawan Mineralization? South Intrusion, Dnlnth Complex, Complex, NE - Steven Miller Trunk Trunk Steven A. A. Hauck, Hauck, Natural Resources Resources Research Research Institute, Institute, University University of of Minnesota, Duluth, Duluth, 5013 Miller Andy Wallbridge Mining Company Fielding Road, Hwy., Duluth, MN, 5581 1, Bite, Wallbridge Mining Company Ltd., 129 129Fieldiing Road, Lively, Ontario, 55811, 5013 P3Y 1L7, 1L7, and Mark Severson, Natural Resources Research Institute, University of Minnesota, Duluth, Duluth, 5013 55811. Hwy., Duluth, MN, 1. Miller Trunk Hwy., MN, 5581 Copper sulfide mineralization mineralization in Ga)Giants GiantsRange RangeBatholith Batholith(GRB) (GRE)footwall footwallofthe of theSouth SouthKawishiwi Kawishiwi Copper sulfide in the the Archean Archean (—2.7 (-2.7 Ga) intrusion the Keweenawan intrusion (SKI) (SKI) of ofthe Keweenawan (1.1 Ga) Ga) Duluth Duluth Complex Complex (DC) (DC) of of northeastern northeastern Minnesota Minnesotawas was first first discovered discoveredby by exploration companies 960s and 1970s, exploration companiesdrilling drillingin in the the I1960s 1970s, and and was was recently reported reported in in outcrop outcropin in the the Spruce SpruceRoad Roadarea area E.. H.. H.. Dahlberg, 1998). The firstdescribed (pers. corn., E.. The Cu mineralization was first described by by Bonnichsen Bonnichsen et et al. al. (1980) and and further further described Severson (1994). According occurs as disseminatedsulfides disseminated sulfides described by Severson Accordingto to Severson Severson (1994), the copper mineralization occurs within the uppermost 330 ft. of the ORB, GRB, although in some some drill drill holes holesthe thefirst first 100 100ft. ft.may maybe bebarren. barren. Chalcopyrite Chalcopyrite is the dominant sulfide. However, pynhotite is However, in in the the overlying overlying SKI rocks pyrrhotite is the dominant dominant sulfide. Also, Also, at at or or just just above above the basal contact contact of of the SKI SKI and within the upper 100 100 ft. of the GRB, semi-massive to massive sulfides sulfides can occur, occur, e.g., drill holes BL-95-1, BL-95- I, K-1. K-I. These Thesesulfides sulfidesare aregenerally generallypyrrhotite-rich, pyrrhotite-rich, and andmay may have have Ni>Cu N i X u values values exceeding exceeding1% 1%Ni Ni and/or contain sulfides GRE, that these andlor Cu. Cu. Severson Severson(1994) (1994)suggests, suggests,based basedon onthe thelinearity linearityof ofthe the drill drill holes holes that contain sulfides in the GRB, drill northeast-trending belts drill holes define define two northeast-trending belts or or faults. Also Alsocoincident coincidentwith withthese theselinear linearzones zonesare areelevated elevatedplatinumplatinummineralizationin inthe theoverlying overlyingSKI. SKI. Bonnichsen Bonnichsen et et al. al. (p. (p. 563,1980) 563,1980) state state these Cugroup element (PGE) and Cr mineralization enriched enriched zones zones in the the GRE ORB "probably "probably resulted resulted because because fractional fractional crystallization crystallizationcaused caused Cu Cu enrichment enrichmentin in the the melts melts as as they trickled downward through a decreasing decreasing thermal thermal gradient beneath the complex". AAsimilar similarmodel modelfor forCu-PGE Cu-PGE mineralization has been proposed for the footwall mineralization in the Sudbury Complex Complex (Naldrett, (Naldrett, et et al., al., 1999) mineralization In 1997, Nickel,began beganaa new new exploration explorationprogram programon ontheir theirSpruce SpruceRoad Road INCO's U.S. U.S. subsidiary, subsidiary,American AmericanCopper Copper&&Nickel, 1997, INCO's Cu-Ni deposit east of Ely, MN. Based copper intercepts Based on on their previous drilling, drilling, INCO identified fifteen s 1% 1% copper interceptsin in the footwall, footwall, which suggested suggested footwall footwall copper copper exploration potential. Recently Recentlyat at Spruce SpruceRoad, Road, several several 1960-era 1960-eradrill drill holes holes geophysical surveys. surveys. Two drill holes (13648,34889) (13648,34889) were probed using a 3D Borehole Borehole Pulse were found to be open for geophysical geophysical survey, was identified identified at at 375131 375m (1,230 (1,230 ft.;Ravenhurst, ft.;Ravenhurst, 1997). EM geophysical survey, and in hole 13648, 13648, a footwall conductor was In 1999, injoint drilled DDH WM-lto WM- ito 1,754 ft. ft. At 183 1999, Wallbridge Mining Mining Company, in joint venture venture with INCO, drilledDDH 183 ft. and 330 footwall copper mineralization was intersected, ft. beneath the basal contact contact of the Duluth Complex Complex (919 ft.), footwall intersected,i.e., i.e., 79.5 79.5 ft. of 1.91% Cu and and 10.5 10.5 ft. ft. of of 0.55% 0.55% Cu. Cu. Nickel, PGEs, and Au ft. of 0.17% Cu, which includes includes 1.1 1.1 ft. 1.91% Cu Au are are minor minor constituents. constituents. The footwall to the DC DC is is generally generally composed composed of of felsic, felsic,intermediate, intermediate, mafic, hybrid hybrid rocks, and and scattered scattered homfelsed homfelsed inclusions of which which may maybe berelated relatedtotothe theGRB. GEE. In the Spruce Road area, Green (1970) inclusions (Severson, (Severson, 1994), 1994), some or all of of granitic rocks, rocks,i.e., i.e.,porphyritic porphyriticandnon-porphyritic. andnon-porphyritic. Both types types are are composed composed ofhomblende-biotite ofhornblende-biotite reports two types ofgranitic adamellite, monzonite, gabbroic adamellite, monzonite,granodiorite, granodiorite,and andseveral severalmafic maficrocks. rocks. All Allrock rocktypes typesare arecut cut by a variety of dioritic and gabbroic dikes, which may have been metamorphosed by the SKI. In In the the vicinity vicinity of of the the Spruce Spruce Road Cu-Ni Cu-Ni deposit, deposit, the the GRB ORB metamorphosed (Green, (Green, 1970). 1970). The has been contact metamorphosed The GRE ORB rocks rocks intersected intersected in WM-linclude WM-Iinclude several several of of these these rock rock types, e.g., porphyritic porphyritic quartz quartz monzonite, monzonite, quartz quartz monzonite, monzonite, granodiorite, granodiorite, monzonite, monzonite, possible Archean Archean Knife Knife Lake Lake Group homfels inclusions, norites, which which are are less lesscertain certainin inorigin. origin. All of these rock types inclusions, as well as melanorites and norites, have undergone pyroxene homfels hornfels (opx+cpx+amphibole) (opx+cpx+amphibole)contact contactmetamorphism. metamorphism. Of primary interest, however, is mineralization within itself. mineralization within the the footwall footwallmelanorites/norites, melanorites/norites, and and the the footwall footwallGRB GRB itself. The melanorites with the the granitoid granitoid rocks, rocks, and and include include rounded rounded fragments fragments of of melanorites and norites have sharp intrusive contacts with them. Petrographically, the melanorites are composed of opx and cpx (30-60%), amphibole (—1-80%), zoned and Petrographically, the melanorites are composed of opx and cpx (30-60%), amphibole ( -1-SO%), uuzonedplagioclase (10-60%),biotite biotite(4-8%), minorquartz(1-3%), oxides(<1-2%), (4-2%), unzoned plagioclase (10-60%), minor quartz (1-3%), apatite(<!%), apatite (<1%), muscovite muscovite (4-3%), (<1-3%), oxides (<1-3%). Opx early. Amphibole and sulfides (4-3%). Opx and and cpx cpx are are subhedral subhedral to euhedral and crystallized early. Amphibole generally rims the pyroxenes, Anhedralto to subhedral, subhedral,zoned zoned pyroxenes, and in some some instances instances corrodes corrodes the pyroxenes producing anhedral crystals. Anhedral 17 �and unzoned plagioclase may show show undulose extinction, extinction, occur occur as equant grains, and are often altered altered to clay clay minerals. Anhedral to subhedral flakes of biotite follow the crystallization of plagioclase and can be porphyroblastic. suhhedral flakes porphyroblastic. Quartz Quartzisis the last silicate to crystallize. Apatite occurs as euhedral rods, both very early and late in the sequence. last silicate to crystallize. Apatite occurs as euhedral rods, both very early and late in the sequence. Oxides (magnetite> ilmenite) and and sulfides sulfides are are last last to crystallize. The The oxides oxides and and sulfides sulfides are are often associated with alteration (magnetite >ilmenite) of the pyroxenes pyroxenes to to biotite, biotite, muscovite, muscovite,talc talc(?), (?),and andchlorite chlorite(?) (?)along alongfractures fracturesand andalong alongthe therims rimsininthe theopx. opx. The sulfides, in order of appearance, appearance, are are millerite, millerite, sphalerite, sphalerite,and andhomite bomiteandlor and/orchalcopyrite. chalcopyrite. The The sulfides sulfides also also occur occur as as interstitial to net-textured net-textured grains that surround surround opx. opx. The melanorites also contain net-textured magnetite. The A similar sulfide and silicate mineralogy occurs in the footwall footwall host host rocks. rocks. These These rocks rocks can can have have feldspar feldspar phenocrysts phenocrysts with a plagioclase plagioclase matrix matrix and minor minor quartz quartz ((5%), amphibole, opx, biotite, oxides (magnetite, ilmenite), ~ 5 % )amphibole, , opx, biotite, oxides (magnetite, ilmenite), and andsulfides sulfides (chalcopyrite, hornite, bomite, millerite, pentlandite, sphalerite). The blebs and anddiscontinuous discontinuous veins, veins, generally generally The sulfides occur as blebs associated with biotite, plagioclase, opx, ±amphibole ±amphibole, ±quartz ±quartz, and oxides. The The plagioclase plagioclase in in these these zones zones is equant and equigranular, and exhibits sutured contacts with the larger plagioclase. The copper mineralization in the granitoid and noritic rocks have Cu:Ni ratios of 8-32:l, 8-32:1, compared to the 3.3:1 3.3: 1 in the overlying DC. The Pd:Pt ratio is these rocks is 1-15:1, compared to the 3:1 in the overlying DC. Data from The Pd:Pt ratio is these rocks is 1-15:1, compared 3: 1 Data fromSeverson Severson (1994) on other footwall intercepts give Cu:Ni and Pd:Pt ratios of 2-9:1 and 2-7:1, respectively, indicating footwall intercepts give Cu:Ni and Pd:Pt of 2-9:1 and 2-7:1, respectively, indicating that thatCu Cuisis enriched, and Ni and PGEs are depleted in WM-1 when compared to other footwall samples. Comparison Comparisonof ofCu:Ni Cu:Niinin all GRB samples indicates a tendency for Cu enrichment and Ni depletion with depth beneath the the basal contact. contact. geology. Possible The origin of this this mineralization mineralization opens opens new and and exciting exciting aspects in Duluth Complex geology. Possible emplacement scenarios scenarios include: include: I) A relict of Archean emplacement. The 1) Themodel model isis supported supported by both sharp contacts contacts and net-textured sulfides sulfides and oxides in the mafic rocks rocks of the footwall. Duluth Complex Complex sulfide. sulfide. The observed depletion of nickel and enrichment enrichment 2) Keweenawan fractionation of Duluth depth below below the the footwall-Duluth footwall-Duluth Complex Complex contact, contact, and andthe theassociation associationof ofnet-textured net-texturedand anddiscontinuous discontinuous of copper with depth biotite-muscovite-talc(?)-chlorite(?) alteration support this possibility. vein sulfide with biotite-muscovite-talc(?)-chlorite(?) feeder-dike' totothe 3) Proximity to a "feeder-dike" theDuluth DuluthComplex. Comvlex.This Thiscase caseisissupported supportedby bythe thepresence presence of ofmagmaticmagmatichigh-grade sulfide intersections intersections not not unlike unlike those those from from Voisey's Voisey's Bay and Sudbury. These looking high-grade Theseintersections intersections are are associated with mafic assemblages not normally part of granitoid intrusions. Regardless ofpreferred genetic model, developments indicate significant new exploration exploration potential Regardless ofpreferredgenetic model, these recent developments potential for the Duluth Complex. Y., 1980, Geologic setting, mineralogy, mineralogy, and geochemistry of magmatic Bonnichsen, B., Fukui, L. M., and Chang, L. L. Y., magmatic D., ed., ed., Proceedings Proceedings ofthe of the fifth fifth quadrennial quadrennial IAGOD IAOOD symposium, symposium, sulfide deposits, Duluth Complex, U.S.A, in Ridge, J. D., Alta, Utah, 1978: Stuttgart, Schweizerbart'sche Schweizerbart'sche Verlagsbuchhandlung, Verlagsbuchhandlung, v. v. 1, p. p. 545-565. Snowbird, Aka, 3. C., C., 1970, Lower Precambrian rocks of the Gabbro Green, J. Gabhro Lake Quadrangle, Quadrangle, northeastern northeastern Minnesota: Minnesota: Minnesota Minnesota Geological Survey, Special Special Publication Series, Series, SP-13, SP-13, 96 96 p. Naldrett, A. J., Asif, M., Schandl, E., Searcy, T., Morrison, G G.. G., Binney, Binney, W. P., P., and and Moore, Moore, C., C., 1999, Platinum-group Platinum-group to the the origin origin of of different different ore ore zones zones and and to to the the exploration exploration for for elements in the Sudbury ores: Significance with respect to orebodies: Economic Economic Geology, Geology, v.v.94, footwall orehodies: 94, no.2, no. 2, p. p. 185-210. 185-210. Ravenhurst, B., 1997, 1997, Geophysical Geophysical survey survey report report for for American American Copper Copper & & Nickel Nickel Inc., Inc., Spruce SpruceRoad Road project: project: Division Division of Lands and Minerals, Minnesota Minnesota Department Department of of Natural Resources, Resources, Open-File Report, 26 26 p. p. of the South South Kawishiwi Kawishiwi Intrusion, Intrusion,Duluth DuluthComplex, Complex,northeastern northeastern Minnesota: Minnesota: Severson, M. J., 1994, Igneous stratigraphy ofthe 210 Duluth, Natural Resources Research Research Institute, Institute, Univ. Minnesota, Minnesota, Duluth, Duluth, Technical Technical Report NRRI/TR-93/34, NRRIITR-93/34,2 10p. p. 18 �An Archean age argument is supported by by::I)1)sharp sharpintrusive intrusivecontacts contactswith with xenoliths xenolithsof ofhost host rock rock in in the the melanorites; melanorites; and 2) magmatic-looking net-textured sulfides and oxides in the melanorites. A Keweenawan argument magmatic-looking sulfides the A Keweenawan argument includes includesthe the previous examples and is also supported by: 1) depletion ofNi and enrichment of copper with depth beneath 1) ofNi and enrichment of copper with depth beneath the the basal basal previous examples and is contact of the DC (as might be expected of a downward differentiating differentiating sulfide sulfidemelt meltthrough through fractures fractures and and faults); faults); 2) 2) association of net-textured and discontinuous vein sulfides with biotite-muscovite-talc (?)-chlorite (?) alteration; and (?) net-textured discontinuous sulfides 3) possible emplacement Keweenawan"feeder "feeder dike" dike" to the DC. Evidence Evidencewould wouldinclude includethe thepresence presenceof ofmagmatic magmatic emplacementof of aa Keweenawan pyrrhotite-pentlandite-chalcopyrite mineralization not unlike that of the Sudbury and Voisey's Bay, as well as the pyrrhotite-pentlandite-chalcopyritemineralization not unlike that of the Sudbury and Voisey's Bay, as well association of mafic assemblages not normally part of the granitoid intrusions. In In any any event, event, the the presence presenceof of Cu-rich Cu-rich well below the basal contact of the SKI (300-425 ft.) ft.) is interesting and suggests a new exploration mineralization well potential for potential for the the DC. DC. Table I. WM-l 1. Footwall Footwall Copper Copper Intersections in WM-1 Interval Ni Interval Feet Cu Cu Ni Co Ag Au Pt Pd ppm ppm ppb ppb ppm ppm (ft.) (ft.) (ft.) (ft4 P P ~P P ~P P ~P P ~P P ~P P ~ppb P P ~ 1102.0-1 107.3 15 2.7 54 152 1102.0-1107.3 5.3 5.3 4872 237 15 2.7 54 59 152 1251.6-1252.7 1.1 19100 592 23 5.7 154 1251.6-1252.7 1.1 19100 592 23 5.7 99 69 69 154 1251.6-1262.1 1251.6-1262.1 10.5 10.5 5496 5496 1249.0-1328.2 1249.0-1328.2 79.2 79.2 1655 1655 421 421 144 144 19 22 22 12 12 2.7 2.7 0.9 0.9 43 43 21 21 23 23 99 69 37 �Results of ^ArP9Ar 40ArP9Ar laser laser microprobe microprobe grain-scale grain-scale age mapping Results of mapping of of muscovite muscovite from from Precambrian bedrock Precambrian bedrock of the southern Lake Superior Superior region HOLM, Daniel Daniel K., K., Department of Geology, HOLM, Department of Geology, Kent State University, University, Kent. Kent, OH OH 44242; 44242; V., and HURTADO, Jose, both at Department of Earth, Atmospheric, and HODGES, Kip V., Cambridge, MA, 01239 Planetary Sciences, Massachusetts Institute of Technology, Cambridge, 01239 step-heating mineral mineral ages collected collected in the 4OArP9Ar step-heating the The results results of of over over 100 100conventional conventional40Ar/39Ar 1990's 1990's from the southern Lake Superior region have greatly increased our understanding of the thermal and deformational deformational history of the crust crust following following the the Paleoproterozoic PaleoproterozoicPenokean Penokean Step-heating experiments, experiments,however, however,may may not not effectively effectively reveal reveal the internal orogeny. Step-heating distribution of radiogenic radiogenic argon within crystals, distribution crystals, but instead can mask mask important importantdetails details of of the the GCA). samples thermal history (Hodges (Hodges et al., 1994, 1994, Geology; Geology; Hodges Hodges and and Bowring, Bowring, 1995, 1995,GCA). step-heating experiments experiments on four We compare results of of previous previous conventional conventional 40Ar/39Ar 40ArP9Ar step-heating muscovite grains grains with with new new results results from from in in situ spot analyses analyses using using the Ar-ion Ar-ion laser laser muscovite microprobe (see table below; age in Ma, uncertainties at two sigma). Samnle s ?A!u2k 97CM7 97CM7 97CM13 97CM 13 96DR8 96DR8 97DR7 97DR7 Location LQ&Qll Republic, MI Dunbar, inb bar, WI Little Falls, WI Neillsville, WI Step-heating ae stemheathe .@g 1716±5 171 6 s plateau olateau 1366±6 iear-plateau near-plateau 1366&6 plateau 11614±5 6 1 4 s plateau 1518 total gas Age gradient 1707-1660 1373-1327 1373-1327 1652-1563 1652-1563 1923-1414 1923-1414 The form 97CM7suggests suggestsincorporation incorporationof ofexcess excess40Ar ^Ar form of of the the age agegradient gradient of of sample sample97CM7 along the grain boundaries. boundaries. Core Core spot spot ages ages of of 1660-1680 1660-1680 Ma Ma are are consistent consistent with a nearby nearby biotite plateau spectrum age (Schneider (Schneider et et al., al., 1996, CJES). CJES). Laser Laser microprobe microprobe results results from the other three samples samples reveal varying degrees degrees of age gradients gradients (see (see Figure Figure 11for forexamples). examples). Sample 96DR8, which yielded an excellent plateau age of 1614±5 1 6 1 4 s Ma, revealed a >90 m.y. my. age gradient. gradient. The The weighted weighted mean mean of the the 10 10 spot spot fusion fusion ages ages is is close close to to the the plateau plateau age. age. Step-heating therefore appears to have homogenized age gradients present in in this this sample. sample. 97CM13 gives gives aa near plateau plateau age of 1366±6 The spectrum of sample 97CM13 1 3 6 6 s Ma, but shows age increasing from from 1334 Ma Ma to to 1383 Ma. Ma. In increments generally increasing I n situ spot spot laser analyses analyses allow us to verify the existence of an age gradient (from at least 1373±12 1373k12 Ma to to 1327±9 1327BMa) Ma) with with significant clarity. Sample Sample97DR7 97DR7yielded yielded aa saddle-shaped saddle-shapedage age spectra spectra with with aa minimum minimum age age increment of 1484±2 Ma (Fig. I). increment of 1484zk2 Ma (Fig. 1). Ar-ion Ar-ion laser microprobe data reveals a spectacular 500 500 m.y. age gradient from core (oldest) to to rim. rim. The my. The youngest youngest ages ages preserved (down to 1414±21 1414±2 Ma) are are obtained from what what is probably a real grain boundary. A difficult problem in in interpreting interpreting 40ArP9Ar 4OArP9Ar age gradients is whether they formed formed during during slow cooling or resulted resulted from from superimposed, superimposed, episodic episodic reheating. reheating. If the entire core to to rim rim south transect, transect, Fig. Fig. 1) 1)reflects reflectssimple simpleslow slow gradient of sample 97DR7 (along the center to south then the implied implied cooling cooling rate rate is is about about 0.3 0.3 K1m.y. K/m.y. This cooling with volume diffusion, then This rate rate is is remarkably similar to the integrated cooling rate estimated for the Proterozoic terrain of the southwest United United States (Hodges et al., southwest a]., 1994). 1994). However, However, consideration consideration of the overall overall geologic context within which these dated samples exist suggests suggests aa more more complex complex episodic episodic thermal history. Case studies studies (i.e., (i.e., Hames Hames and and Cheney, 1996, 1996, Los) Bos) suggest that the form of history. Case grain-scale 'chrontour' 'chrontour' maps maps should should help help to distinguish between between simple and more detailed grain-scale complex thermal histories. histories. Whatever Whatever their their mechanism mechanism of of formation, formation, the the documentation documentation of large and quantifiable quantifiable age gradients gradients in this this study study indicates indicates promise promise for forobtaining obtaining more more detailed information about the Proterozoic thermal evolution of the the southern southern Lake Lake Superior Superior region. 20 �97DR7 Muscovite Muscovite 10000 C-) C, 0 '5 1550 C 0) 1500 1450 0 20 60 40 80 100 % Ar cumulative O h ^Ar cumulativereleased released , - , r , I . I 1 . , , , s 10000 10000 9GDRBMuscovite Muscovite 96DR8 100 100 10 10 I - , I I , , , . 1 , , , , I I . , , 8 , , , I I 96-DR-8 96-DR-8 Muscovite Muscovite - 1650 1 6~0 1600 1600 , I * s - I1, = 1614±5 Ma 1550 t91615Ma 1500 I 0 20 40 60 80 lao % cumulative released released % 30Ar ^Ar cumulative Figure 1. Figure 1.Spot Spotfusion, fusion,single singlecrystal crystalage agedata data(this (thisstudy) study)and andcorresponding corresponding conventional conventional incremental-release data (after Romano, Kent State M.S. thesis, incremental-release data Romano, thesis, 1999) 1999) for fortwo twomuscovite muscovite 97DR7 is coarse samples samples from from the southern southern Lake Lake Superior Superior region. region. 97DR7 coarse muscovite muscovite from from a muscovite gneiss of probable Archean age collected at Sylvan Lake west of Neillsville, muscovite gneiss of probable Archean collected Sylvan Lake west of Neillsville, Wisconsin. 96DR8 Wisconsin. 96DR8 isis muscovite muscovite from from aa late late pegmatite pegmatite dike dike collected collected at at Little Little Falls Falls south of Eau Claire, Wisconsin. Wisconsin. Grain images are video stills of individual (001) cleavage fragments Grain images are video stills of individual Ar-ion laser fusion micron-diameter (outlined), and the dots indicate the position of ca. 100 (outlined), and position 100 fusion pits, each labeled with its measured age (age uncertainties are variable but generally less age (age uncertainties are variable but generally lessthan than Unlabeled pits pits in in 96DR8 too little gas 10 Ma at two two sigma). sigma). Unlabeled 96DR8 produced produced too gas for for effective effective measurement. 21 �PRELIMINARY HYDROTHERMAL ALTERATION PRELIMINARY VOLCANOLOGY VOLCANOLOGY AND HYDROTHERMAL ALTERATION STUDIES STUDIESAT AT THE THE AND QUARTZ QUARTZHILL HILL PROSPECTS: PROSPECTS: FIVE MILE LAKE, EAGLES NEST, AND IMPLICATIONS IMPLICATIONS FOR FOR VMS-STYLE VMS-STYLE MINERALIZATION MINERALIZATION IN NORTHEASTERN MINNESOTA GEORGE .1. HUDAK GEORGE J. HUDAK Department 54901 Departmentof of Geology, University iversityof of Wisconsin Wisconsin Oshkosh, Oshkosh, Oshlcosh, Oshkosh, WI WI 54901 RONALD L. MORTON MORTON Geology Duluth, MA' MN 55812 55812 Geology Department, Department, University Universityof of Minnesota Minnesota—Duluth, -Duluth, Duluth, HENK DAHLBERG Minnesota Department MN Department of of Natural Natural Resources, Resources, Minerals MineralsDivision, Division, Hibbing, Hibbing, MN The Five The Five Mile Mile Lake, Lake, Eagles Eagles Nest, Nest, and and Quartz Quartz Hill Hill prospects prospects occur occur ininArchean Archean metavolcanic metavolcanic and and metasedimentary rocks rocks the the western western half half of the Vermilion District District of northeastern northeastern Minnesota. Minnesota. The metasedimentary TheFive FiveMile MileLake Lake prospects occur within the lower member of the Ely Greenstone Sequence, whereas the Quartz and the Eagles Nest prospects Hill prospect Detailed diamond relogging, Hill prospect occurs within the the Bass Bass Lake LakeSequence Sequence (Peterson, (Peterson, 1999). 1999). Detailed diamond drill core core relogging, petrography, and lithogeochemistry lithogeochemistry have have been been recently recently completed completed at at these these prospects prospects (Hudak (Hudak and and Morton, Morton, 1999) to: petrography, a) better understand the geology (in particular, particular, the physical physical volcanology) volcanology) associated associated with with these these prospects; prospects; b) b) identify identi& and evaluate the metamorphosed hydrothermal alteration mineral assemblages that occur at each of of these these prospects; prospects; and c) evaluate the potential for volcanogenic massive sulfide (VMS) and/or gold mineralization in in these these areas. areas. During our investigation, investigation, a total of eight eight diamond diamond drill drill holes holes from from the the three threeprospects prospects listed listedabove abovewere were and sampled sampled for petrographic petrographic and and lithogeochemical lithogeochemicalanalysis. analysis. Four diamond drill holes from the Five Mile relogged and Lake prospect prospect (SXL-1, (SXL-l, SXL-2, SXL-4), two two diamond diamond drill drill holes holes from from the the Eagle's Eagle'sNest Nestprospect prospect(EN-4 (EN4 SXL-2, SXL-3, SXL-3, and and SXL-4), EN-7), and two diamond diamond drill holes from the Quartz Hill prospect (QH-84-2 and QH-85-4) were relogged with and EN-7), an emphasis emphasis on identi'ing identifyinglithology, lithology,volcanic volcanicfeatures featuresand andtextures, textures,metamorphosed metamorphosedhydrothermal hydrothermal alteration alteration mineral assemblages, assemblages, and syn-volcanic thin sections (33 from Five syn-volcanic and post-volcanic post-volcanic fault zones. Sixty-four thin Five Mile Mile Lake, 18 from Eagles Nest, Nest, and 13 Lake, 13 from from Quartz Quartz Hill) Hill) were were evaluated evaluated for lithology, lithology, alteration alteration types, primary primary and textures, mineral modal analysis, and fragment secondary textures, fragment modal analysis. In In addition, addition, thirty-eight thirty-eight lithogeochernical lithogeochemical (both major and trace element) were were also also performed performed (22 (22 from from Five Five Mile Mile Lake, Lake, 10 from Eagles Nest, and 6 analyses (both from Quartz Hill). Hill). andesite/basalt lava flows and scoriaThe Five Mile Lake region is composed of massive to amygdaloidal andesitehasalt rich volcaniclastic deposits, locally quartz-phyric dacitic to rhyolitic lavas and flow breccias, and and volcaniclastic volcaniclastic and and chemical sedimentary sedimentary rocks. rocks. Quartz-feldspar Quartz-feldspar porphyry porphyry and and diahase diabase dikes are also present throughout chemical throughout the the volcanic volcanic and sedimentary sequence. textures indicate indicate that that these these rocks rocks likely sequence. Volcanic Volcanic textures likely were formed within a deep deep water water Hydrothermal alteration alteration minerals minerals include include epidote, epidote, iron-rich chlorite, magnesium-rich (>500meters) environment. Hydrothermal magnesium-rich Stringer and and semi-massive sulfide mineralization mineralization are are associated associated with with two chlorite, and quartz quartz (silicification). (silicification). Stringer semi-massive sulfide chlorite, assemblage, and and b) an epidote - iron distinct alteration assemblages: a) an early early iron-rich iron-rich chlorite chlorite - sericite sericite - quartz assemblage, iron carbonate assemblage assemblage associated associatedwith with late late sulfide sulfide remobilization. remobilization.The TheFive FiveMile Mile Lake Lake prospect prospect appears appears to to be be a carbonate "Noranda-type" VMS system based on on the criteria discussed by Morton and Franklin (1987). (1987). Two diamond drill holes in the Two diamond drill holes investigated investigated in the Eagles Eagles Nest Nest region region comprise comprise massive massive to toamygdaloidal amygdaloidal andesite/basalt lava flows and associated flow andesiteibasalt flow breccias/hyaloclastite, breccias/hyaloclastite, mafic mafic to to intermediate intermediatevolcaniclastic volcaniclasticsediments, sediments, Algoma-type mixed mixed facies facies (sulfide, (sulfide, oxide, oxide, silicate) silicate) iron iron formations Algoma-type formations and associated associated chert horizons, horizons, interbedded interhedded semi-massive to massive sulfide deposits, deposits, and rhyodacite rhyodacite to rhyolite tuffs and/or siltstone and debris flow deposits, semi-massive tuffaceous volcaniclastic porphyry intrusions intrusions and and diabase diabase dikes dikes are also volcaniclastic sediments. sediments. Quartz Quartz feldspar feldspar porphyry also present. present. textures suggest that the rocks in the area were also formed in a deep water Volcanic textures water (>500 (>500 meters) meters) environment. environment. A A massive sulfide is overlain overlain by by approximately approximately 2 meters (6 feet) of sulfide four meter thick intersection of pyrite-rich massive approximately 22 22 meters meters (73 (73 feet) feet) of of mixed mixedfacies faciesAlgoma-type Algoma-typeiron iron (py)-chert exhalite, which is overlain in turn by approximately formation. The The host host and and footwall footwall rocks rocks to to the sulfide sulfide mineralization mineralization comprise sericite-, iron-rich chlorite-, kaolinitecarbonate- (dolomite (dolomite andlor and/or ankerite) ankerite) and and locally chlorite-alteredfelsic felsic ash ash tuffs tuffs or tuffaceous carbonatelocally magnesium-rich magnesium-rich chlorite-altered tuffaceous volcaniclastic sediments. sediments. Based Based on on the volcanology alteration present, present, the Eagles Nest prospect volcaniclastic volcanology and hydrothermal hydrothermal alteration prospect represent aa "Noranda-type" also appears to represent "Noranda-type" VMS VMS system. system. - 22 - �The two diamond diamond drill holes investigated investigated at the Quartz Quartz Hill Hill prospect prospect comprise comprise amygdaloidal amygdaloidal to massive massive basalt/andesitelava lava flows, flows, mixed mixed volcaniclastic and chemical (including chert, chert, silicate-fades basaltlandesite volcaniclastic and chemical sediments sediments (including silicate-facies iron iron formation, oxide-facies iron formation), and felsic tuffs and/or andlor tuffaceous volcaniclastic sediments. sediments. Massive Massiveto tosemisemimassive sulfide sulfide mineralization mineralization occurs occurs between between 89 89 and and 150 feet feet in exploration diamond drill drill hole QH-84-2, exploration diamond QH-84-2, and is hosted within felsic felsic ash tuff or tuffaceous tuffaceous sedimentary sedimentary rocks. rocks. The relative abundance of felsic tuffs and The and tuffaceous tuffaceous material, coupled with hydrothermal alteration assemblages comprising andalusite, chloritoid, and hon-rich iron-rich chlorite, chlorite, suggests that that the the Quartz Hill mineralization suggests mineralization may represent represent either either a) aa "Mattabi-type" "Mattabi-type" VMS VMS system system (Morton (Morton and and Franklin, 1987); or or b) an ancient shallow subaqueous or subaerial high-sulfidation high-sulfidation system (White and Hedenquist, Franklin, Mattabi-type VMS VMS systems systems are characteristic 1990, Sillitoe et al., al., 1996). 1996). Mattabi-type characteristic of shallow shallow water water 1990, 1995; 1995; Sillitoe, Sillitoe, 1995; Sillitoe meters water water depth) depth) hydrothermal hydrothermal systems, systems, reflect reflect extreme extreme acid acid leaching leaching of the rocks by ((<500 6 0 0 meters by the the hydrothermal hydrothermal fluids, and and may he be analogous to "high fluids, "high sulfidation" sulfidation" epithermal epithermal systems within shallow water environments. environments. Such Such hydrothermal systems may produce gold-rich massive sulfide deposits (Hannington et et al., 1997). 1997). The results of this study indicate that the Vermilion region of northeastern Minnesota contains at least least two examples (Five (Five Mile Lake and Eagles examples Eagles Nest) Nest) of ofdeep deepwater, water,flow-dominated flow-dominated VMS VMSsystems systems(Noranda-type). (Noranda-type). The The Quartz Hill prospect may represent either a volcaniclastic dominated, shallow water "Mattabi-type" VMS system, or Continued re-evaluation re-evaluation of of a shallow subaqueous subaqueous or possibly possibly subaerial subaerial high high sulfidation-type sulfidation-type epithermal epithermal system. system. Continued mapping, diamond these these prospects prospects via detailed detailed field mapping, diamond drill core core relogging, petrographic analysis, analysis, and lithogeochemical studieswill willbe be performed performedduring duringthe the next next year year to further evaluate lithogeochemical studies evaluate and better better understand understand these potentially ore-forming hydrothermal systems. References References Hannington, M., Poulson, K. H., Thompson, J., & Sillitoe, R. H., (1997): Volcanogenic gold gold and and epithermal-style epithermal-style environment. In In Barrie, Barrie, C. C. T., T.,and Hannington, M. D. (Eds.), (Eds.), Volcanic mineralization in the VMS environment. Associated Massive Sulfide Deposits: Processes and Examples in Modern Modem and and Ancient Settings: Settings: GACGACMDD-SED Short Course Volume, 183-214. 183-214. J., & Morton, R. L., (1999): Bedrock and glacial drift Hudak, G. J., drift mapping for for VMS VMS and and lode lode gold gold alteration alteration in in the the Vermilion — Big Fork Greenstone Belt: Part A: Discussion of Vermilion -Big of lithology, lithology, alteration, alteration, and and geochemistry geochemistry atatthe the Five Mile Lake, Eagles Nest, Nest, and Quartz Hill Hill Prospects. Prospects. Minnesota Department of Natural Resources Project 326 Open File Report, 136 136 pages. L., & & Franklin, Franklin, J. J. M., M., (1987): (1987): Two-fold Two-fold classification of Archean volcanic-associated massive sulfide Morton, R. L., Geol., 82, 1057-1063. 1057-1063. deposits. Econ. Geol., D. M., (1999): Economic geology and lode gold and volcanogenic massive sulfide Peterson, D. sulfide prospects prospects of ofthe the western Vermilion District, St. Louis and Lake Counties, northeastern Minnesota. Minnesota Minnesota Exploration Exploration Conference 1999, Lode Gold and Massive Sulfide Sulfide Prospects Prospects in in the the Archean Archean Western Western Vermilion Vermilion District District of of Northeastern Minnesota, Abstracts Abstracts Volume, Volume, 3-5. 3-5. Fl., (1995): (1995): The influence of magmatic-hydrothermal models on exploration strategies Sillitoe, R. H., strategies for volcanoSillitoe, Deposits, Mineralogical In Thompson, Thompson, 3. J. F. H., H., (Ed.), Magmas, Mamas. Fluids, Fluids. and Ore Deoosits, Mineralogical Association Association plutonic arcs. In of Canada Short Course Volume 23,511-525. Sillitoe, R. H., H., Hannington, Hannington, M. M.D., & Thompson, Thompson, J.J. F. F. H., H.,(1996): (1996): High High sulfidation deposits in the volcanogenic D., & massive sulfide environment. Econ. 204-2 12. Econ.GeoL, Geol.,91, 91,204-212. White, N. C., &Hedenquist, 3. W., (1990): Epithermal environments and styles of mineralization: variations and J. W., and their causes, and guidelines for exploration. Jour. 445-474. Jour.Geochein. Geochem.Res., Res., 36, 36,445-474. N. C., & Hedenquist, J. 3. W., W., (1995): Epithermal gold deposits: deposits: styles, characteristics, and Exploration. SEG White, N. Newsletter, 23,9-13. Newsletter, 23, 9-13. 23 �MAGMATIC ORIGINS ORIGINS FOR FOR NATHAN'S NATHAN'S LAYERED SERIES: AN INITIAL REASSESSMENT REASSESSMENT OF OF MAGMATIC THE MIDCONTINENT RIFT'S FIRST FIRST MAJOR PLUTONIC MATERIALS JERDE, Eric JERDE, Eric A. (e.jerde@morehead-st.edu), (e.jerde@morehead-st.edu),Dept. Dept. of ofPhysical Physical Sciences, Sciences, Morehead MoreheadState State University, Morehead, KY 40351 Morehead, 4035 1 In recent years, great greatstrides strideshave havebeen beenmade madein indeciphering decipheringdetails detailsof ofthe the evolution of of the the Midcontinent MidcontinentRift Rift (MCR.),and andthe theemplacement emplacementof ofthe theDuluth DuluthComplex Complexintrusions intrusionsand andassociated associatedlavas. lavas. The The vast vast majority majority of of System (MCR), Ripley, 1996), material produced by the MCR was between 1099 1099and 1095 1095Ma Ma (Miller (Miller and Smyk, 1995; 1995; Miller and Ripley, an indication of how vigorous the system was. The Theearliest earliestmaterials, materials,however, however, were wereproduced producedininexcess excessofof88 years previously, previously, and and because because of their relative rarity are incompletely understood. understood. million years One of the most interesting period of rifting is is known known informally informallyas asNathan's Nathan's Layered Layered interesting intrusions of this early period Series after after the the work of Nathan (1969). HIs still stands as the most intensive study of the rocks, comprising His still stands as the most intensive study of the rocks,comprisingfield field descriptions, petrographic petrographic analysis, and some microprobe and optical determinations of m mineral compositions. No ineral compositions. No work has has been been performed performedon on any anyrocks rocksof ofthis thisseries. series. A U-Pb zircon date of 1106.9  ± .6 .6 Ma has been geochemical work obtained for one (Paces and Miller, 1993), confirming its antiquity in in the the MCR MCR. Nathan one of of the the units units of this series series (Paces Nathan (1969) identified 27 separate the letter designations A-Z and AA in their inferred separate units, which are given the the discordancies discordancies observed. observed. Nathan's chronological order based on the Nathan'sown owninterpretation, interpretation,plus plus those thoseof ofsubsequent subsequent observers, gradational variations, whBe while others others may be sparse dikes or observers,offers that many of the separate separate units may be gradational "sport" varieties, Nathan (1969). varieties, to to use use the theterminology terminology of ofNathan (1969). In terms of major magma bodies, bodies, the the series seriescomprises comprisesfour fourprincipal principalunits: units:A-B A-B(troctolites (troctolitestotogabbronorites), gabbronorites),G0 (oxide-rich olivinegabbro), gabbro),M M (gabbronorite), (gabbronorite), and andP-Q P-Q((troctolites troctolites to to gabbros). gabbros), These (oxide-rich olivine These units units are are present present in in amounts amounts great enough great enough to to permit permit their their being assigned as the principal events in the evolution of the series. Typical Typicalmodal modal abundances and m mineral compositionsfor forthese thesegroups groupsare aregiven givenininTable Table1.1. One One of of the the more more interesting interestingaspects aspects of of ineral compositions the presence of large amounts of of Fe-Ti oxides in many many of of the the units. units. There Thereare areseveral severalunits units that that this series is the occasionally have have in in excess excess of of 30% 30% Fe-Ti Fe-Ti oxides. oxides. Among Among the the major major units, only only Unit 0Gisisnotable notablefor foraahigh high abundance of oxide phases, in some locations exceeding 60%. 60%. This abundance Thisoccurs occursin in aa fairly fairly primitive magma (forming abundant olivine of of Fo.io.50 Fo,0 and andlittle littlequartz quartzororgranophyre), granophyre),resembling resemblingthe theother othermajor majormagmas magmasin inother otheraspects. aspects. 1. TyDical Tvoical Mineral Abundances Abundancesfor for NLS NLS and and Model ModelResults. Results. Table 1. Model g ! Q l & g Q e & & g fl IEli Q! AizQn&Qa Eiz Li! u A-B P-Q 42 42 53 53 Fo AAn6.e lien,; FRY? 58 58 33 An,4 Fo Feat 3 Mg9 Mg*4,9 -—- ~ % 4 3 M 59 --- 62 An555 32 Mg71 M&wl 13 _ 77 24--- Mg53 Mg An55.43 G 22 3 5 8 Fo 11 —- -_ 6kb 6kb 47 io, 10% An Afl63 .. — 46 Fo62 FO(Q 22 6kb 6kb 49 49 14 14 Fo F06o 4 An62 &2 51 6 5 1kb i 14 Mg3 !izAnQ -15% 6kb 2 Q Qi l An Fo 48 6 An54 Fo45 55 Mg, M ~È-~ 33 33 Mg --- M~ Mg5g 343 --2816 Mg.57 Petrographic data data from Nathan (1969). Petrographic (1969). Modelling of magma crystallization was was made made to to explore possible possible scenarios scenarios for for the the unusual unusual abundance abundance of of oxide oxide in the the material material comprising comprisingUnit UnitG. 0. This Thismodelling modelling was was done donewith with the the MAGFOX MAGFOX program program(J. (1.Longhi, Longhi, pers. pa. minerals in comm.). AAtypical typicalcomposition compositionof ofaa Logan Logansill sill (Table (Table 2) 2) was was used used as as aa parent parentsince sinceitit represents representsthe theonly onlycommon common composition in the region that is even remotely remotely coeval coeval(the (theLogan Loganssills havebeen beendated datedatat1108 1108-1./.2 composition that is ills have ; by Davis and and Sutcliffe, 1985). 1985). constraints are poor Nathan's series, series,some someinteresting interesting Although the constraints poor since since no liquid compositions are known for Nathan's features features of of the the modelling are present. Using Usingthe theLogan Logancomposition compositionofofTable Table2,2,materials materialsresembling resemblingthose thoseof ofunits units P-Q, can to 6 kb. In A-B, can be generated generated through through crystallization crystallization at depths depthscorresponding corresponding to Inthis, this, A-B A-B and and P-Q A-B, M, and P-Q, P-Q 24 �I I I   I M after crystallizatiow These shown in can can be be formed fanned after after 10-15% 10-15% crystallization, crystallization, and and M after —25% -25% crystallization Theseresults results are are shown in Table Table I1 adjacent to the the petrographic petrographic observations of Nathan Nathan (1969). (1969). In adjacent to observations of Inthis thismodel, model,the theoxide oxidephases phasesappear appear after after28% 28% crystallization, are moderately moderately abundant abundantthereafter, thereafter,consistent consistentwith withthe thefe few percentofofoxides oxidespresent presentininP-Q P-Q.ItIt crystallization, and and are w percent is interesting interesting to to note existed at at 66 kb, kb, then then the the P-Q P-Q magma must mustbe beolder olderthan thanM, M, in in that ififonly onlyone onemagma magma chamber chamber existed is note that contradiction to the or that that contradiction to the original original mapping. mapping. IfIfNathan Nathan(1969) (1969)isiscorrect, correct,ititimplies impliesthat thatmultiple multiplechambers chambers existed, existed, or recharge was was taking taking place, place, both both of of course course plausible. plausible. At recharge At 66kb, kb,oxides oxidesdo donot notform formininabundances abundancesakin akintotothose thoseseen seen in in >50% crystallization, that point, the 0 until until >SO% crystallization,and and at at that point, the the liquid liquid isis far far more moreevolved evolved than than could could have have produced produced the Unit G responsible for for the the smaller intrusive dikes to other other minerals other minerals seen. seen. Such Such liquids may be be responsible smaller intrusive dikes and and sills sills assigned assigned to other units units enlarge the stability field for and modelling at I kb showed oxide phases, by Nathan. However, lower pressures by However, lower pressures enlarge stability field for oxide and modelling at 1 suggested by by the model 0 are ilmenite appearing appearing after afterless less than than 20% 20% crystallization. crystallization. Materials Materials akin akin to those of Unit G are suggested ilmenite crystallization, Thus 0 may evolution in shallower magma chamber. after after approximately 40% crystallization Thus Unit G may reflect evolution in aa shallower chamber. The tremendous variation and the the gradational gradational nature nature of the rocks is The tremendous variation in in cumulate cumulate textures on on aa small small scale, and rocks is of (and presage) the the later later Anorthositic Anorthositic Series Duluth Complex reminiscent reminiscent of (and in in this this case case may presage) Series rocks rocks of the the Dulutb Complex suggested suggested to to have formed formed as asinjections injectionsof of plagioclase plagioclasecrystal crystalmushes mushes(Miller (Millerand andWeiblen, Weiblen,1990). 1990). Thus, Thus, the the rocks rocksof of Nathan's Nathan's have Layered Series representmaterial material (of (of Logan Logan composition?) in the Layered Series may represent composition?) that that was was emplaced emplaced at at several several levels levels in the crust crust in in the initial phases of rifting. Evolution in these disparate chambers would lead to some magmas having unusual the initial phases of rifting. Evolution in these disparate chambers would lead to some magmas having unusual amounts amount's of nxidm ..--- of -.oxides. -.----. It composition similar It isis interesting interestingto to note note that that the the models models produced produced a liquid composition similar to to the the Grand Grand Portage Portage dikes (composition given Green al., 1987) after after35-40% 3540% crystallization 0. (composition given by G reen etetal., crystallization at at 1kb, lkb, roughly roughly equivalent equivalent to that of Unit G. Perhaps theGrand Grand Portage PortageDikes Dikes are arethe thehypabyssal hypabyssalsibling siblingofofthe theplutonic plutonicUnit Unit G 0 seen further inland. Perhaps the seen farther inland. may - F Table 2. Paren t ('am nnsitinn IIwit in M odelling and one result I wt nerrenti MnO MaO Na,O !c22 un!! 1Q2 1122 A1223 1Q fzQs Logan Login 49.0 49.0 3.40 3.40 13.1 13.1 15.6 15.6 0.22 0.22 5.60 7.45 2.52 2.52 1.16 1.16 0.38 40% 53.9 2.49 13.3 15.1 0.25 2.11 7.07 2.96 1.82 0.62 Grand 52.5 Grand 52.5 Portage Portage 2.48 13.4 13.4 13.3 '13.3 0.19 0.19 3.68 3.68 6.72 6.72 3.17 1.79 1.79 0.48 0.48 @1kb Logan composition from from Geul Logan composition Ged (1970). (1970). Grand of 17 Grand Portage Portage isis an average of 17dikes dikes (Green (Greenet al., 1987). 1987). Citedi References Cited: ItH.,1985, U-Pb Davis, D.W. and Sutclift'e, Sutcliffe, RH.,1985, U-Pb ages ages from the Nipigon Plate and and northern northern Lake Lake Superior: Superior: Geological Geological Society of Davis, AmericaBulletin, 96,1572-1579. 1572-1579. America Bulletin, v. 96, of Mines, Mines, Geological Geological of Devon Location: Ontario Geul, J.J.C., J.J.C., 1970, 1970, Geology of Devonand and Pardee Panfee Townships and Stuart Location: Ontario Department of Geul, Report 52p. Rewrt 87, 87.520. RE., Pesonen, and Wilband, Green,k., Bomhocst, Ti., T.J.,Chandler, Chandler, V.W., Mudrey, Mudrey, MJ., Mya-s, P.E., PisaKtt.LJ., Wilband,J .T., 1987, 1987, Mi., Jr., Myers, Li,, aud iC., Bomhorst IT., Region: Evidence Evidencefor forevolution evolutionof of the the Middle Middle Proterozoic Proterozoic Midcontinent Mideontinent Rift Rift in Keweenawan Keweenawan Dykes of the Lake Superior Region: in 289-302. North America, in Halls, North Halls, H.C. KC.and and Fahrig, Fahrig,W.F., W.F., eds., cds., Mafic MaficDyke DykeSwarms: Swarms:Geol. Geol.Soc. Soc. Canada Camda Spec. Spec. Paper Paper 34, 34,289-302. examples of Wieblen, P P.W., Anorthositic rocks of the D Duluth Miller, J.D., Jr. and Wieblen, .W.,1990, Anorthositic ulufliComplex: Complex: examples of rocks rocks formed formed from &omplagioclase plagioclase Miller, 3D., Jr. Petrology, v. v. 31.295-339. 31,295-339. crystal crvstalmush: mush: Journal of Prtrolotv. Trip Guidebook for in Field Tnp Miller, M.,,1995, Gabbroic intrusions M ~ I IJ.D., ~D. JJrJr. and Smyk, St& M 1995,Gabbroic mmisioiis of the the international mieniatid boundary area, in forthe die region: Minnesota Geoloacal Geological Survev Swvey Guidebook geology deposits of of the Midcontinent emloev and ore dcoosits Mdcooement Rift Rift in m the Lake Lake Superior Suoenorrefflon, ~Series e r iNo. e ~20,171-181. 20, ~ o171-181 . USA, In 1996, Layered intrusions Miller, J.D., J.D., Jr. and bley, intmions in in the Duluth Duluth Complex, Complex,Minnesota, Minnesota, USA, in Cawthorn, Cawthoin, KG., ed., Miller, Ripley,EM., EM., 19%. ItO., ed., Layered Intrusions: Amsterdam. Amsterdam, Elsevier, Uvered Intrusions: Elsevier, 257-301. 257-301. Cook County, The geology geology of a portion portion of the the Duluth Duluth Complex, Cook County,Minnesota: Minnesota: Ph.D. Ph.D. dissertation, dissertation.University University ~ a t h a H.D., n . - ~1969, . ~ . , The Nathan, Minneapolis, l9Sp. of Minnesota, Minnesota. Minueawlls. 1980. related mafic mafic intrusions, northeastern J,D., U-Pb ages Paces,J.B. and ~Miller, i e rJ.D.: , Jr., 1993, 1993"Precise Precise U-Pb ages of ofDuluth Dulufli Complex and related intrusions, northeastern Paces, Minnesota: Minnesota:Geochronological Geochronologicalinsights insightstotophysical, physical,petrogenetic, petrogenetic,paleomagnetic paleomagncticand andtectono-magmatic tectono-magmaticprocesses processes associated associated Midcontinent Rift System: of Geophysical GeophysicalResearch, Research,v.v,98,l3,997-14,013. withthe with the 1.1 1.1Ga MidcontinentRift System:Journal of 98,13,997-14,013. . I . - 25 - �INTRUSIONS OF OF THE THE LAKE LAKE SUPERIOR SUPERIOR REGION: REGION: TARGETS TARGETS FOR FOR SMALL MAFIC INTRUSIONS TITANIUM EXPLORATION KARKKAINEN, Nub, Geological -E N, Niilo, Geological Survey Survey of Finland, 02150 Espoo, Espoo, Finland; Finland; BORNHORST, BORNHORST, Theodore, J., Department of Geological Engineering and Sciences, Michigan Technological University, University, Houghton, oughto on, MI MI 49931 4993 1 - magmatic ilmenite deposits are often in or closely associated with Globally, high-grade magmati anorthosite complexes. Large layered mafic intrusions host Ti-rich magnetite in Fe-Ti oxide anorthosite rocks. There There are are many mafic intrusions intrusions throughout throughout the Lake Superior Superior region that range in in age age from Archean to Proterozoic. However, most of these intrusions are not associated with However, many are are relatively relatively small. small. Despite these characteristics, small mafic anorthosite complexes and many amounts of Ti, based intrusions of the Lake Superior Superior region have the potential to host significant amounts on analogy with a recently documented documented occurrence occurrence in in Finland. Finland. The Finnish Koivusaarenneva Koivusaarenneva gabbro is a small mafic intrusion, not associated with an anorthosite complex, that contains inferred resources of 44 Mt with 15 % ilmenite and 5% V-rich magnetite 50m (an (an additional additional 44 44 Mt Mt are arepossible possible to to aa depth depth of of magnetite (0.7 (0.7 wt. wt. % % V) V) to aa depth depth of of 1150m 300m). The Thefeasibility feasibilityof of mining mining this this ore ore body body is is currently currently being investigated by a Finnish mining mining company. company. Koivusaarenneva gabbro is an elongated sill-like intrusion, 0.5 to 1 km thick and 3 The Koivusaarenneva km long long parallel to strike. strike. The Theintrusion intrusionproduces produces aa 44 mGal mGal gravity gravity anomaly anomaly and a clear clear NE-SWNE-S oriented aeromagnetic anomaly anomaly in the low-intensity low-intensity background. The intrusion was emplaced at 1881 Ma Ma into tonalitic bedrock and is part of a gabbro province province interpreted to to have formed in tensional zones zones in in the the vicinity vicinity of of or or marginal to to convergent convergent plate boundaries. boundaries. The The Koivusaarenneva gabbro is part of a 12 km long chain of intrusions that were emplaced along along a Koivusaarenneva 12 krn NE-SW oriented oriented fault fault zone. The The intrusion intrusion is is mostly composed composed of of modally modally layered layered mediummediumgrained and metamorphosed metamorphosed gabbro and gabbronorite. The magmatic layers have been tilted to a subvertical orientation and faults faults segment segment the intrusion. The intrusion is divided into into three three zones zones on the basis of geochemistry geochemistry and petrography. Fe-Ti oxide-rich layers are common in the lower middle zones. ratioisisconstant constantfor forall allrock rocktypes types in in the the lower zone at and middle zones. The TheTi02/Fe2O3TOT Ti02/FezOsToT ratio about 0.2. The Themiddle middlezone zonecontains containsthe the 22 km km long long ilmenite ilmenite ore body that grades between 8 and an ratioofofthe themiddle middlezone zoneisisbetween between0.25 0.25and and0.50. 0.50. The The upper upper 48% ilmenite. ilmenite. The TheTiO2/Fe2O3TOT Ti02/Fe203ToT ratio zone consists consists of P-rich gabbro and leucogabbro. gabbro The genetic genetic model model of the intrusion intrusion and ore body begins with primary tholeiitic tholeiitic mafic mafic magma emplaced emplaced in a deep deep magma chamber at or near the base of the crust. In the deep chamber, crystal-liquid crystal-liquid fractionation, fractionation,under under conditions conditionsof ofvery verylow lowf02, fm, generated Ti-enriched parent magma averaging about 3.3% TiOz. Ti02. This parental magma was was successively successively injected injected into into each each of of the three zones zones of the the intrusion. intrusion. Variation Variation within the lower lower and upper zones is consistent consistent with with closed-system closed-system crystal-liquid crystal-liquid fractionation fractionation of two separate separate batches of magma magma from the deep deep chamber. chamber. The middle zone contains the voluminous voluminous ilmenite ore. The overall composition of nonnon non-mineralized gabbro mineralized gabbro from the middle zone is compositionally compositionally similar to non-mineralized gabbro 26 �from the lower zone, however, the weighted average composition composition of the middle zone is Fe than than the the weighted weighted average averagecomposition compositionof ofthe thelower lowerzone. zone. The considerably higher in Ti and Fe volume of magma magma in the the middle middle zone zone is is far far too small small to yield the the large large mass mass of of ilmenite ilmeniteunless unless the parent magma had extreme extreme Ti and Fe. Instead, the extreme excess excess of Ti and Fe Fe in in the the middle middle zone can be explained explained by a parent magma with elevated, but rather normal Ti and Fe Fe (like (like the the lower and upper zones) that flowed through the partly crystallized magma chamber. After an zones) crystallized chamber. After an oxide-rich immiscible immiscible liquid was was extracted from the magma this this dense dense material material sank sank to to the the floor floor of the shallow magma chamber to form a Fe-Ti oxide rich matrix between a silicate framework. shallow magma chamber form oxide silicate framework. The Ti and Fe depleted depleted magma magma was then replaced by a new batch of parent magma magma and and so so on. on. . The middle middle zone zone represents represents a channel channel for for magma flow with multiple multiple episodes episodes required required for for accumulation accumulation of the large mass mass of of oxides. A similar similar magma magma flow flow through through model model has has been been proposed to generate generate gabbro-hosted gabbro-hosted Ni-POE Ni-PGE sulfide sulfide mineralization mineralization at Norilsk, Norilsk, Russia Russia (Naldrett (Naldrettet et al., 1996) 1996) and Voisey's Bay, Canada Canada (Evan-Lamswood (Evan-Lamswood et al., 1998). 1998). an For Ti-deposits in small mafic intrusions, an open magma flow through system system is is an essential essential genetic genetic component component to produce produce economic economic size size deposits. Continuity Continuity of of gabbro gabbro intrusions intrusions as a chain or along a fault zone is a favorable geologic geologic setting. setting. An An overall overall elevated elevated Ti Ti content content of of non-mineralized non-mineralized gabbros gabbros is is another another favorable favorable characteristic characteristic indicating elevated elevated Ti in in the the parent parent magma. Small mafic intrusions of the Lake Superior region can be tested for potential to Small mafic intrusions of the Lake Superior can tested for potential to host host Ti Ti resources using an an exploration exploration model based on the Finnish Koivusaarenneva Koivusaarenneva gabbro gabbro intrusion. intrusion. 27 �ADDITIONAL PALEOMAGNETIC STUDIES OF OF A A PROTEROZOIC PROTEROZOIC DIABASE DIABASE DIKE, PIFTIER AND ANDIRWIN IRWINTOWNSHIPS, TOWNSHIPS,LAKE LAKE NIPIGON NIPIGON DISTRICT, DISTRICT, ONTARIO IN PIFHER Kean, W. F., Department Department of of Geosciences, Gwsciences, University University of of Wisconsin-Milwaukee, Wisconsin-Milwaukee, Milwaukee, Milwaukee, WI WI 53201, and Luther, F., Geology Geology Department, UW UW Whitewater, Whitewater, Whitewater, Whitewater, WI WI 53190 53190 An approximately 50 m wide porphyritic diabase dike outcrops outcrops in and Pither in western westernIrwin Irwinand Pier Townships in the Nipigon district and is locally known as greenspar. The dike has a characteristic Townships the Niigon district and is locally known as greenspar. characteristic outcrop as a result of saussaritized greenish mottled appearance in outcrop saussaritizedplagioclase glomerophenocrysts in in aadiabase diabasegroundmass groundmass(Luther, (Luther, 1997). 1997). The dike strikes due north and dips dips vertically, and is is found foundas assegments, segments,off-set off-setby byeast-west east-westfaulting faulting(Mackasey, (Mackasey,1975). 1975). There There are are no radiometric this rock or radiometric dates dates on this or associated associated rocks, rocks,although, although,itit isiscut cutby by aalarge largemiddle middle or or late late Proterozoic sill Proterozoic sill in Irwin Township (Mackasey, 1975). 1975). We previously reported reported palco magnetic studies on on four sites f from (Thomas and paleomagnetic rom this area (Thomas others 1998). Five additional sites were collected in the summer of 1999, primarily others 1998). Five additional sites were collected in the summer of 1999, primarily from from locations locations Pither Township. The The nine nine sites sites span span approximately approximately27km. 27km.of ofthe thedike. dike. Samples Samples from each in Piflier location were subjected suljected to (A.F.) and thermal demagnetization demagnetization. The to both both alternating alternating field (A.F.) The samples samples demagnetization temperatures show one one primary magnetic direction which is removed at demagnetization temperaturesof of 5100 570 C. A.F. A.F. demagnetization demagnetizationof ofsamples samplesup up to to60 60mT, mT,also alsogives givesonly onlyone onedirection directionof ofmagnetization.. magnetization the magnetometer the samples are quite viscous and require a long wait time in the However, the between measurements. All All but two two locations locationsshow shownormal normal polarity polarity with with northwesterly northwesterly declinations (200°-300°) and inclinations of 40° to 80°. One site in southern declinations (200"-300") and inclinations 40" to 80'. One site in southernPifher PifterTownship Township appears 100°, and appears to to be be reversally reversallymagnetized magnetizedwith withdeclinations declinationsof of90° 90"totolOOo, andinclinations inclinationsof of -75°. -75'. The The southern southern most location near Beardmore produces producesinconsistent inconsistentresults. results. Overall, the themagnetic magnetic directions directions are are consistent with Keweenawan paleomagnetic paleomagnetic directions directions in the Lake Superior SuperiorRegion. Region. References References Luther, F., 1997, Pither. 1997,The The Petrology Petrology of of Greenspar: Greenspar: A A Proterozoic Proterozoic Porphyritic Diabase Dike; Pifher and Irwin Townships, Lake Nipigon Nipigon District, District, Ontario., Ontario., [abstract]: [abstracti: Institute on Lake Superior Geology Proceedings, Proceedings,v.v.43, Part l,p. 43, 1,p. Macjçasey, W.O., 1975, Gwlogy Geology of of Dorothea, Dorothea, Sandra, and Irwin Townships, District of Thunder Mackasey, W.O., Bay; Ontario division of Mines, rpt rpt 122 2294, 83 p. 122 with map 2294,83 Kean, W., W.,and and Luther, Luther, F., F., 1998, 1998,Paleomagnetic Paleomagnetic Studies Studiesof ofaaProterozoic ProterozoicPorphyritic Porphyritic Thomas, C., Kean, Diabase Dike, Pifher Nipigon District, District, Ontario.,[abstract]: Ontario.,[abstractj: Institute P i i and Irwin Townships, Lake Nipigon Institute on on Lake Superior Part 1 l,p. ,p. 119. 119. Superior Geology Proceedings,v.44, Part 28 �PETROGENESIS PETROGENESIS OF OF THE THE MIDCONTINENT MIDCONTINENTRIFT RIFTGRANOPHYRIC GRANOPHYRICCOMPLEXES COMPLEXESOF OF NORTHERN NORTHERNMINNESOTA MINNESOTA ---- - - Kennedy, & Wirth, Kennedy,Bryan Bryan C., C., (bkennedy@macalester.edu) (bkennedy@macalester.edu) & Wirth,Karl Karl R., R., Geology Department, Macalester College, St. Jeff D., St. Paul, MN (wirth@macalester.edu); Vervoort, Vervoort,Jeff D., Department Department of Geosciences, University (vervoort@geo.Arizona.edu) University of of Arizona, Arizona, Tucson, Tucson, AZ AZ (vervoort@geo.Arizona.edu) The The hypabyssal hypabyssal granitic granitic rocks rocks of northeastern Minnesota, Minnesota, commonly commonly referred referred to to as as granophyres, granophyres, are are an an important important component component of of the the Midcontinent Midcontinent Rift Rift (MCR) (MCR)of of northeastern northeastern Minnesota. Minnesota. InInthis thisstudy study we we examine examine eight eight intrusive intrusive complexes complexes associated associated with the the rift, rift, including including detailed detailedtransects transects across across the the Misquah Misquah Hills Hills and and Greenwood Greenwood Lake Lake granophyres. Evidence Evidence from petrographic, major and trace element element data data suggest suggest a combination of of magma magma mixing, chemical differentiation, and partial melting melting of of MCR MCR and and older granophyrecomplexes. complexes. older crust crust to to form form the thegranophyre The in 30to to150 150km km in The granophyre granophyre complexes complexes are generally concordant intrusions intrusions that thatrange rangefrom from 30 surface area and are up up to to 1000 meters metersthick. thick. The The complexes basaldiorite, diorite, monzodiorite, monzodiorite, complexes typically consist of basal and quartz monzodiorite monzodiorite and and upper uppergranite graniteand andgranodiorite. granodiorite. The The contact contact zones zones between the mafic and felsic rocks are are compositionally compositionally and and structurally structurally heterogeneous; heterogeneous; evidence evidence of ofmutual mutual intrusive intrusive relationships relationships between between the mafic mafic and and felsic felsic rocks can often be found within the the same same outcrop. outcrop. Porphyritic Porphyriticrhyolire rhyolite overlies granitic of the the complexes complexes (e.g., (e.g., Misquah Misquah Hills, Hills,Greenwood Greenwood Lake, Lake, Eagle EagleMountain, Mountain, granitic rocks rocks of several of and Lima Lima Mountain). Mountain). Thus Thusfar, far,only onlyone oneof ofthe thecomplexes complexeshas has been been dated dated (Swamper (SwamperLake=Greenwood Lake=Greenwood Lake granophyre, granophyre, 1107 1107 Ma Ma ±Â1.1, reversed magnetic polarity; Davis and Green, 1.1, reversed magnetic polarity; Davis Green, 1997); 1997);the the other othercomcomplexes are undated but are generally considered considered to to have have normal normal magnetic magnetic polarity, polarity,with withthe the exception exception of of the the Misquah MisquahHills Hillsbody bodywhich whichisispossibly possibly reversed. reversed. The monzodiorite, monzodiorite, monzodiorite, and diorite) diorite) are The mafic maficrocks rocks of the complexes complexes (e.g., quartz monzodiorite, are distindistinguished by by abundant (15-35 vol.%) significant (1 (1-3 vol.%) apatite. apatite. The -3 vol.%) Thefelsic felsic vol.%) interstitial pyroxene and significant rocks sodic plagioclase plagioclase (30-40 (30-40 vol.%), vol.%), orthoclase orthoclase (25-35 vol.%), and quartz (20-30 (20-30 vol.%). rocks are composed of sodic vol.%). Plagioclase Plagioclase commonly exhibits exhibits normal normal and andoscillatory oscillatorycompositional compositionalzoning; zoning;many manyplagioclase plagioclasegrains grainsare are mantled mantled by by thin thinrims rimsof ofsodic sodicplagioclase. plagioclase. Anhedral Anhedral pyroxene pyroxene and andsmall smallFe-Ti Fe-Ti oxides oxides make make up up less less than than 10% 10% of the rock. All All of of the the rocks rocks in in the the complexes complexes are characterized characterized by granophyric intergrowths of alkalifeldspar feldspar and and quartz. quartz. Intergrowths Intergrowths are are interinter_________________________________________________ 10.0 stitial or more commonly .. stitial between between plagioclase plagioclase or more commonly radiate radiate from from euhedral euhedral plagioclase phenocrysts. phenocrysts. -quartz monzodiorite, monzodiortie, and diorite n monzodiorite, diorite • 8 0 lntergrown quartz ranges from coarse and Intergrown quartz ranges from coarse and mimioo -granophyric -granophyricgranite graniteand and granodiorite granodiorite 7.0 crographic crographiccuneiforms cuneiformsto to fine fine and andradially radiallygragranophyric $ 6.0 6.0 o O nophyricshapes. shapes. The granophyre complexes Thegranophyre complexesdisplay displayaa wide wide 5.0 5Q5.0 0 S silica ino 0 o b $ ' % As %). As inSiO2 (47-76 wt range of range of Si02 (47-76 wt %). cc$°°o 4Z 4.0 4.0 creases, TiOz, MnO, MgO, CaO, and creases, 1107, Fe203, CaO, and 33.0 0 2 0 increases. There P20sdecrease, whereas K P205 K20 There 20 2.0 • m • hare notable compositional "gaps" in lincompositional "gaps" in the lin• .^ ~ ~ 10 l . •~ % ear trends on Harker variation variation diagrams diagrams be0.0 tween the the mafic mafic and and felsic felsic portions portions of each each of of 65,0 55.0 60.0 55.0 60.0 65.0 70.0 75.0 50.0 the complexes (Figure 1). Although rocks in (Figure 1). Although rocks S (w - .i n- 7 (wt,%) .. r... % .,l Si02 the lower complexes have have relalower portions of the complexes relaFig. II - K20 K 2 0(wt. (wt.%) %) versus versusSi02 (wt.%) %)diagramfor diagramforall allsamples Si02 (wt. samples tively low Si02 (47-62 wt. /o) most are tively Si02 (47-62 wt.%) are silica silica illustrating the compositional compositional from the granophyre ganophyre complexes, complexes, illustrating over-saturated over-saturated and all but two two samples samples congap between the rocks of ofthe the intrusions. intrusions. Note the mafic and felsic felsicrocks Note tam normative quartz. The granitic rocks tain The granitic rocks in trend of the major element element variation variation with increasing the linear trend the the upper portions of the complexes are SiO the upper portions of the complexes are SiO z2SiOz.Full Fulldata dataset setin inKennedy Kennedy (2000) (2000) Si02. 1 - ~ Za?aj o & +',PQJ .-• •• ~ - - \ . . . 29 �rich (66-76 wt.%), and and are are slightly to metaluminous metaluminous to slightly 2.0 2.0 1.8 0 1,8 peraluminous. peraluminous. The The mafic mafic and and felsic felsic 1.6 0 - rocks plot along linear — linear trends trends on on most most —— F 1.4 incompatible incompatible trace trace element elementdiagrams diagrams .975 0 0 (e.g., U, U, Nb, Ce, (e.g., Ce, and and Zr Zr vs. vs. SiO SiO2); 2); Q 1.2 0 LZ .950 compositional compositional"gaps" "gaps" between between the the mamaI.0 • -quartz -quartz monzodiorite, monzodiorite,monzomonzofic evident in fie and and felsic felsic rocks rocks are also evident in 0.8 diorite, and diorite these these plots. plots. The Themafic maficand andfelsic felsicrocks rocks o granophyric granite and 0.6 0-6 from from all of the granophyre granophyre complexes complexes granodiorite exhibit -mixing 0.4 trend; exhibit negative negative Nb-anomalies Nb-anomalies on on 0.4 mixing trend; values values at at tick tick ,— marks indicate the fraction of multi-element variation variation diagrams. diagrams. o,7 marks indicate the fraction of 0.2 felsic component component The Th/Nb and The small small variation variation in in the the Th/Nb and ______________ 0.0 M 50.0 £00.0 150.0 200.0 250.0 M 50.0 100.0 150.0 200.0 250.0 Ce/Nb ratios between the the mafic mafic and Ce/Nb felsic rocks suggests that the source of S iO 2/C aO felsic rocks that the source SiOzICaO the hyperbolic Fig. Fig. 22 - KzO/Fe203* KzO/FeaO3* versus versus SiO2/CaO SiOdCaO diagram showing the hyperbolic the the Nb Nb depletion depletionisis common common for both mixing trend for all samples from the granophyre concordant mixing trend for all from the granophyre complexes, concordant the mafic and felsic rocks. the mafic and felsic rocks. withhmixing mixing model. The Themixing mixingmodel model isis based based on on two two end endmember member Although , ~ ~ the the~major ~ ~ ~with majorh element parental compositions (triangles), Sonju Lake (M) and parental compositions (triangles), the intrusion (M) and an an variation variation might mightbe beexplained explainedby bycryscrysaverage of the three most felsic granitic samples (F). (F). The dashed line isis The dashed line average three most tal tal fractionation fractionationofofaatypical typicalMCR MCRmamathe the NSVG the mixing model with the NSVG olivine olivinetholeiite tholeiiteasasthe themafic maficcompowmpofic fie component, component, itit seems seems unlikely that nent, nent,which which isis not not concordant concordantwith with the the data. data. fractionation fractionationofofmafic maficmelts meltscould couldacaccount 10.15% of the intrusive rocks countfor for the theformation formationof of this this large large volume of granitic rock (approximately 10-15% of SiO 27(e.g., (e.g., of the the MCR MCR exposed exposed in northeast northeast Minnesota). Minnesota). Furthermore, Furthermore,on onplots plotsof ofmajor major elements elements versus versus SiO K20; change during during thetrend trendof ofthe thedata datashould shouldchange changein inslope slope as as the the fractionating fractionatingassemblages assemblages change K20;Figure Figure 1) 1) the crystallization. of SiO SiO 22 concentrations concentrations which crystallization. Instead, Instead, the thedata dataplot plotalong alonglinear lineartrends trends over over aa wide range of may may indicate mechanical mixing. mixing. On Ondiagrams diagrams of of major element ratios, the themafic maficand andfelsic felsic rocks rocks plot along vs.SiOz/CaO, SiO2/CaO,Figure Figure2). 2). These These trends trends can be modeled modeled by along hyperbolic hyperbolic trends trends (e.g., (e.g., K2O/Fe2O3* K2O/FezOj* vs. mixing of a mafic mafic (Miller (Miller &Chandler'scomposite & Chandler's composite Sonju Sonju Lake intrusion intrusion parental parental magma (1997)) (1997)) and and felsic felsic (average (average of the the most most evolved evolved samples samples from the granophyre complexes) end-members. IfIf aa more more primitive primitive olivine olivinetholeiite tholeiitefrom fromthe theNSVG NSVGisisused usedas asthe themafic mafic end end member, member, the the resulting resulting model does not fit fit the the data data for for the the granophyre granophyre complexes. complexes. Samples Samplesthat thatdo donot notplot plotalong alongthis thismixing mixingline linelikely likely evolved evolved from mixing products SiO2/CaO z/CaOand andK1O/Fe2O3. K20/Fe203. productsby by fractional fractionalcrystallization, crystallization,which whichwould wouldincrease increaseSiO An A n origin origin for for the the granophyre granophyre complexes complexes by mixing is consistent with the field relationships relationships and and linear trends trends on on major major element elementdiagrams. diagrams. However, However,although although aa combination combinationof ofmagma magma mixing mixing and and fracfractionation tionationmay may explain explainthe theorigin originof ofthe thegranophyre granophyrecomplexes, complexes,itit does does not not explain explainthe thesource sourceof of the thelarge large volume of felsic end-member necessary for mixing. Nd isotope data from the granophyre complexes volume of Nd isotope data from the granophyre complexessugsuggest that that the thefelsic felsic component component may may be be the the result result of of fractionation fractionation or or melting melting of of contemporaneous crustal materials (Vervoort, (Vewoort, this volume). Further Further examination examinationisisneeded neededto todetermine determinean anultimate ultimatesource sourcefor for the the felsic material material and and it's it's relationship relationshiptotothe thegenesis genesisof ofthe thegranophyres. granophyres. * ,-- - References References Davis, Green, J. C. 1997. North American 1997. Geochronology Geochronology of the North American Midcontinent MidwntinentRift Riftininwestern westernLake LakeSu Su Davis, D. W. & Green, perior perior and andimplications implicationsfor forits itsgeodynamic geodynamicevolution, evolution,Can. Can.J.J. Earth EarthSci. Sci.34: 34:476-488 476-488 Kennedy, Kennedy, B.C. 2000. 2000. Petrogenesis Petrogenesisofofmidcontinent midcontinentrift riftgranophyres: granophyres:evidence evidencefrom fromgeochemistry geochemistryof ofKeweenawan Keweenawan granitic northern Minnesota. MacalesterCollege, College, graniticigneous igneous complexes complexes of northern Minnesota. Senior SeniorHonors HonorsThesis, Thesis,Macalester Saint SaintPaul, Paul,MN. MN. Miller, J.D. & Chandler, V.W. Miller,J.D. &Chandler, V.W. 1997. 1997.Geology, Geology,petrology, petrology,and andtectonic tectonicsignificance significanceofofthe theBeaver BeaverBay BayComplex, Complex, northeastern, 3 12. northeastern,Minnesota. Minnesota.Geological GeologicalSociety SocietyofofAmerica AmericaSpecial SpecialPaper Paper312. 30 �A A POSSIBLE POSSIBLESUDBURY SUDBURY EJECTA EJECTA LAYER LAYER IN IN THE THEGUNFLINT GUNFLINT FORMATION, FORMATION,NORTHWESTERN NORTHWESTERNONTARIO ONTARIO Stephen Thunder Bay, Ontario, Stephen A. A. Kissin, Kissin,Department Departmentof ofGeology, Geology, Lakehead University, Thunder Ontario, Makoto International University, Makoto Okamoto, Okamoto,Faculty Facultyof ofEconomical Economical Sciences, Sciences, Kyushu International University, Kitakyushu, Kitakyushu, Japan, Japan,William William 0. D.Addison, Addison,Thunder ThunderBay, Bay,Ontario Ontarioand andGreg GregR. R.Brumpton, Brumpton, Thunder ThunderBay, Bay, Ontario Ontario Previous Previous studies studiesrevealed revealed that that the the Gunflint Gunflint lapilli lapilli tuff tuffunit, unit, located located in in the the Kamistiquia KamistiquiaGorge Gorgenear near Thunder Bay, Ontario, is unlikely to represent an ejecta layer from the Sudbury impact event. Thunder Bay, Ontario, is unlikely to represent an ejecta layer from the Sudbury impact event. Euhedral unit have 878±2Ma (Fralick tuffunit haveyielded yieldedaadate dateof of11878i2Ma (Fralicket et al., al., Euhedralzircons zirconsextracted extractedfrom fromthe thelapilli lapillituff 1998) 1998) presumed presumed to to be be the the age age of of sedimentation sedimentation based based on on aa volcanic volcanic source source for for the the zircons, zircons,too tooold old to 4Ma (Krogh to be be associated associatedwith withthe theSudbury Sudburyevent eventatat1836±1 1836il4Ma (Kroghet et al., a1.J1984) 984) However, However,aadistinct distinctash ash layer higher in section has been layer approximately approximately lOOm loom stratigraphically higher been recently recently discovered discovered and and investigated. investigated. A A grey greyash ashlayer layer11.5 11.5cm cm thick thick isis exposed exposed in in two two locations locationson on the southern southernside side of of the the Kaministiquia Kaministiquia Gorge. Gorge. The Thelayer layerisiscomprised comprisedmostly mostlyofoffriable fiableillite illiteand andmontmorillonite, montmorillonite, but but separation separation of of particulate mudball lapilli, lapilli, crystal crystal fragments fragmentsand and particulatefractions fractionsrevealed revealedthe thepresence presenceof of lithic lithic fragments, fragments, mudball green spherules spherules with a glassy appearance. The The latter latter were were found to be glauconite, glauconite, which also has has been described The crystal crystalfragments fragments described in in the the Gunflint Gunflint lapilli tuff unit (Hassler & Simonson, 1989). The are are mostly mostly pure pure potassium potassium feldspar, feldspar, with with minor minor quartz, quartz, calcite calcite and and various various other other minerals. minerals. The proximal The glauconite glauconitepellets pelletsand andpotassium potassiumfeldspar feldsparclasts clasts have have been been interpreted interpreted as as indicative indicativeof of aaproximal submarine submarine volcanic vent (Hassler & Simonson, 1989). In Inparticular, particular, the the sodium-free sodium-free potassium potassium feldspar feldspar clasts have have been bee; interpreted interpreted as as the theproduct productofofmetasomatic ietasomatic replacement replacement of i f igneous igneous plagioclase. Also present are quench-textured basaltic clasts and occasional devitrified vesicular plagioclase. Also present are quench-textured basaltic clasts and occasional devitrified vesicular glass glass fragments. fragments. However, However,large, large,unstrained unstrainedquartz quartzclasts clastsand and fresh fresh plagioclase plagioclase clasts clasts suggest suggest that aa somewhat somewhat more more felsic felsicvolcanic volcanic source sourcehas has also alsocontributed contributedto to the the particulate particulateassemblage. assemblage. Analyses enclosing shales Analysesof of samples samplestaken taken at at various variousintervals intervals in in the the layer, layer, as as well as as of the enclosing shales at one of of the locations, locations,revealed revealed the the presence presenceof of anomalous anomalousJr Ir (1.9 ppb) ppb) as well as other other PGEs PGEs and and Au. Au. On On the Ni are in the the ash ash layer layer as as compared compared the other other hand, hand, the the chalcophiles chalcophilesCo, Co,Cr Crand andNi are significantly significantly depleted depletedin to which have to the enclosing enclosing shales. AAstrong strongenrichment enrichmentin in Zr Zr indicates indicates the presence of zircons, which have opportunitiesfor for dating dating of of the the material been been separated separated and and should should again again provide provide opportunities material as well well investigation for shock features. features. Shock Shock features features in in the the particulate particulate fraction have not so far far been been identified, and the depletion depletion of of chalcophiles chalcophiles is contrary contrary to expectations expectations in an an ejecta ejecta layer layer(e.g. (e.g. Hildebrand, 1993). However, However, Simonson Simonson et et al. al. (1998) (1998) noted noted that that the the Acraman Acraman impact impact layer layer in in Australia Australia lacks lacks this this enrichment. enrichment.This Thislayer layeras aswell well as as aa layer layer seen in the Hammersley Basin can be explained explained as as the result of submarine impact. As As the the Sudbury Sudbury Structure Structureis is believed to have formed formed in in aa marine marine shelf shelf environment, environment, the the possibility possibilityof of an an impact impact origin origin for for the the Gunflint Gunflintash ash layer layer remains remains open. open. To To date, date, however, however, the evidence evidence is in favor favor of of aa volcanic volcanic source source for for the the ash ashlayer, layer,but but geochronology geochronology will will be be needed neededin inorder orderto to exclude excludethe the possible possible impact impact origin originof of the the layer. layer. 31 �References References Kissin, S.A. Fralick, P.W., Kissin, S.A. and Davis, D.W. (1998) (1998) The age and provenance of the Gunflint Gunflint lapilli lapilli Inst. tuff. Program and Abstracts, 44th Lake Superior Geol, Minneapolis, 66-67. Program and Abstracts, 441hInst. Minneapolis, 66-67. vocaniclastic strata in two Hassler, S.W. and Simonson, B.M. (1989) Deposition and alteration of vocaniclastic Proterozoic iron-formations in Canada. Can. J. Earth Sci. 26, 1574-1585. 1574-1585. lare, early Proterozoic Cretaceous/Tertiary boundary boundary impact impact (or (or the the dinosaurs dinosaurs didn't didn't have a Hildebrand, A.R. (1993) The CretaceousITertiary Roy. Astronom. Astronom.SOC. Soc.Can. Can.g, $, 77-118. chance). J.J. Roy. 77-1 18. Krogh, T.E., T.E., Davis, D.W. and Corfu, F. (1984) Precise U-pb U-pb zircon and baddeleyite ages for the Sudbury area, -446 in The Deposits ofthe of the Sudbury Sudbury Structure, Sudbury area, p.431 p. 43 1-446 The Geology Geology and Ore Deposits Structure,edited editedby by E.G. E.G. Pye, A.J. Naldrett and P.E. Giblin, Ont. Geol. Surv. Spec. Vol. 1, 6O3p. and P.E. Giblin, Ont. Geol. Surv. Spec. Vol. 1 , 6 0 3 ~ . Reeves, S. S. and Hassler, S.W. (1998) anomalybut but Simonson, B.M., Davies, Davies, D., Wallace, Wallace,M., Reeves, Simonson, B.M., Hassler, S.W. (1998) Iridium Iridium anomaly nor shocked quartz quartz from fromLate LateArchean Archeanmicrokrystic microkrysticlayer: layer:Oceanic Oceanicimpact impact ejecta? Geology , ejecta? Geology 2 , 1 19595198. 198. 32 �COMMINUTION COMMINUTION AND AND DILUTION DILUTION OF GLACIAL GLACIAL INDICATORS: INDICATORS: RATE ESTIMATES ESTIMATES FROM FROM FIELD FIELD DATA DATA LARSON, LARSON, Phillip Phillip C., C., and and MOOERS, MOOERS, Howard D., Department D e p m e n t of Geological Sciences, University of Minnesota,Duluth, Duluth,MN MN55812, 55812,plarson2@d.umn.edu plarson2@d.umn.edu Minnesota, Introduction: Introduction: Tracing Tracing glacial glacial indicator indicator trains has long long been been recognized recognized as as being being fundamentally fundamentally a problem problem of of interpreting of an an indicator indicator with with increasing increasingtransport transportdistance distancefrom fromits itssource, source. The interpreting the diminishing diminishing concentrations concentrations of dispersal dispersal pattern of of an an indicator indicator is is aa function function of of the the processes processes of dilution dilution and comminution acting on that indicator indicator during during transport. transport, Numerous Numerousstudies studieshave haveestablished establishedempirically empiricallythat thatdispersal dispersalpatterns patternstypically typicallytake a ethe theform formofofan an exponential exponential decay curve. However, However,while while an anindicator indicator ideally ideally comprises comprises all debris debris with with aa particular particular lithologic lithologic or or geochemical geochemical signature signature derived from a unique source, source, in practice most indicator studies examine only a portion portion of of the the size sizefractions fractionsof ofaatill. till. This This approach approach precludes precludes precise precise quantification quantification of the the systematics systematics of of indicator indicator transport, transport, ininparticular particular quantification quantification of dilution dilution and and comminution comminution rates. This Tbis is due due to to aanumber numberofoffactors; factors;previous previousindicator indicatortracing tracing studies studies have have often often been been limited limited to to examination examination of of only only aa portion of the size size fractions containing containiig an indicator, and in in many many cases cases recognition recognition of of aa unique unique indicator indicator derived derived from from aa specific specific bedrock bedrock source source has has proven proven difficult. dificult. The Thelack lack of due to to dilution (largely a function of specific specific data data relating relating the the relative relative changes changes in in indicator indicator concentrations concentrations due dilution (largely function of interaction interaction with the bed) bed) and and comminution comminution (largely (largely a function function of the the material material properties properties of of the tbe indicator) indicator) have have precluded precludedsatisfactory satisfactoryquantitative quantitativemodeling modelingofofglacial glacialdebris debristransport. transport. The The present present study study attempts attempts to address address these these questions questions by examining examining dispersal dispersal of clasts derived from from aa quartz-epidote District, Minnesota. Minnesota. Bedrock qum-epidote altered basalt unit in the Vermilion District, Bedrock within the study area consists consists of of aa metavolcanic-dominated E-W. Glacial Glacial drift in the area area isis thin thin (0-10 (0-10 m), m), and and metavolcanic-dominated greenstone belt belt striking striking roughly roughly E-W. records advance moving moving from from the the W NNE. records aa single single advance advance of the Rainy Rainy Lobe, with the last glacial advance E . Samples Samples were were collected from basal basal tills, tills, and screened to separate separate ibree three adjacent adjacent size size fractions fractions(1-2, (1-2,2-4, 2-4, and and 4-16 4-16 mm). mm). The collected from screened to The quartz-epidote altered basalt basalt indicator indicator is is visually quartz-epidote altered visually distinct distinct and was was easily easily separated separated from from each eachsize sizefraction; fraction; concentrations represent mass proportions, proportions. A A transport transport distance distance for each each sample sample was was calculated calculated by measuring measuring the the distance distance from from each eachsample samplelocation locationalong alongthe theflow flowpath pathto tothe thesource sourcearea. area. Dispersal: Dispersal: The The concentration concentration of an an indicator indicator in in aa particular particular size size fraction fraction of of till till is is aa function function of of the the change change in in concentration due due to to dilution, dilution, and and the the change change inin concentration concentration due due to to comminution. comminution. The The relationship relationship concentration between dispersal, dispersal, dilution, dilution, and and comminution comminution can can be be between expressed expressedas: as: CD1SJ = (I) Grs~ = CDm CDTLCCOM ' CCOM (1) C Dispersal Dispersal curves cuwes for for the the indicator indicator inineach eachparticle particle 0 size size fraction fraction in in the the study study area areawere weregenerated generatedby byplotting plotting C indicator indicator concentration concentrationagainst againsttransport transportdistance distance(T, (T,inin1cm) !an) 5 of the largest (Fig, 1). I). Concentrations Concentrations of largest clasts clasts (4-16 (4-16 mm) mm) C (Fig. 0 decreased with transport transport distance distance at the the fastest fastest rate, rate, while while 0 decreased medium-sized (2-4 (2-4 mm) mm) clast clast concentrations concentrations decreased decreased at aa medium-sized rate. Dispersal Dispersal of of the the smallest smallest clasts clasts (1-2 (1-2 mm) mm) has has aa lower rate. curious km curious form; form;concentrations concentrationsincrease increaseduring duringthe thefirst fust—5 -5 km transport, then then decrease decrease with with increasing increasing transport transport of transport, 0 4 12 distance. distance. 3 Dilution: Dilution Dilution reduces reduces indicator indicator concentration concentration by by Tmnsporl Distance Distance(km) (km) Dilution: eroding erodiig and incorporating incorporating material from from the the bed bed downflow downflow Figure I . Dispersal Dispersal Curves Cuwes Figure 1. from the mdicator indicator source. source, AAdilution dilutioncurve curveshould should ideally ideally QE doler Allered Basalts Quarlz-Epidote Altered Basalts take a e the form fonn of of aapower power function, function, with with the the exact exact slope slope of of the dilution dilution curve curve at any any given given point point in in space space and and time time aa the function function of of distance distance from from the the source source and and the instantaneous instantaneous rate of erosion and entrainment. These 'hese are aredependent dependent on on a number and the the basal basal thermal thermal regime. regime. Deviations number of of variables, variables, including including bed bed roughness, roughness, bed composition, composition, and Deviations of of aa Trans - 33 �dilution curve from from a power function of represent deviations deviations from from uniform uniform conditions. conditions. However, over relatively short distances on a bed of uniform composition it is short distances on a bed of uniform composition it is , 0.04 reasonabletoto assume assume aa uniform rate of erosion reasonable uniform rate erosion and and S incorporation. incorporation. An estimate of the in the An estimate of the dilution dilution rate rate in 0.03 Vermilion District was obtained by calculating calculating the the total total =05 S proportion of indicator Ct indicator material in the the 1-16 1-16mm nun fraction fraction 5 (Fig. 1). of the total sample pig. I). Regression Regression analysis analysisof of these these E0,02 a> C, S C data results in the following equation equation for dilution: 0 C) CD = O.O348e)2T (2) S S too Clearly, this dilution rate represents non-ideal 2 conditions. This Thisisisdominantly dominantlyan anartifact artifactof of not not Transport Distance Qan) TrawxtDi&nce (km) the—I mm and and +I6 +16mm including the -I nun mm size size fractions in the Figure 2. Dilution Curve Figure 2.Dilution Curve calculations, calculations, however however itit represents represents aa reasonable reasonable Quartz-Epidote Altered Altered Basalts Quartz-Epidote Basalts estimate. estimate. Comminution: Comminution:Comminution Comminutionalters altersthe theconcentration concentrationofofan anindicator indicatorwithin wifimaaparticular particularsize sizefraction hction as asparticles particlesare are than broken down by by clast-on-clast clast-on-clast and andclast-on-bed clast-on-bed interactions. interactions. Larger clasts tend to comminute comminute more more rapidly rapidly than smaller clasts. Larger Largerclasts clastsare arenot notlost lost to to the the system system as as they break down, but are transferred to smaller smaller size size classes. classes. The concentration of an indicator in a particular size fraction hction at a given transport distance is thus a function of of the the of larger larger clasts, clasts, and the rate at rate at which particles are being Kmg added to the size size class class by comminution comminution of at which which particles are leaving the size particles size class class by by comminution comminution to $asmaller smaller clasts. clam. + Comminution rates for indicator Comminution rates indicator clasts in the the OM' I j4no • 1-zm + by rearranging Vermilion Dislxict District were were calculated calculated by Vermilion rearranging • 2-4mm 2-4equation (I) CCOM equation (1)and andsolving solvingforfor CcoM at at each sample sample m' • 4-18mm 4.16location using the 3). CCoM the model modelCDIL CDL (Fig. 3). CCOM for the largest particles showed poor poor correlation correlation with with transport transport F EM,4 e + distance; this is is probably probably aa result result of of significant significantvariation variation 0 S in indicator boulder concentrations in indicator concentrations along along individual individual flow lines resulting from variable rates of addition addition by boulder comminution to this size size class. class. Medium-sized particles showed increase in in CCOM CcoMwith particles showedaarelatively relativelysharp sharp increase om transport distance, reflecting the higher rate transport rate of Transport TtansprlDistance D i m(km) (h) comminution of the comminution the coarsest coarsest particles. particles. The smallest smallest Figure 3. Comminution ComminutionCurves Cu~es particles showed a lower rate of of increase increase reflecting reflecting the Quartz-Epidote Altered Basalts Quartz-Epidote Altered Basalts lower rate at which coarser comminuted to coarser particles are comminuted this size size class. class. Conclusions: Conclusions: Results of indicator indicator tracing in in the the Vermilion Vermilion District District illustrate illustrate the importance importance of segregating sepgating the effects of dilution and comminution comminution to modeling the concentration of an indicator at a particular transport distance. Dilution affects affects the concentration Dilution concentration of each each particle particle size s i z equally, equally, however however comminution comminution rates rates strongly strongly and and differentially influence influence indicator indicator concentrations concentrationsin in differing d i f f e ~size g sizeclasses. classes. Since clast comminution is controlled controlled by by grain size and and material material properties, the the concentration concentration of an indicator indicator at aa particular particular transport transport distance distance can can be be seen seento to be dominantly dominantly aa function function of of the the initial initialparticle particle size sizedistribution distribution and and the the material material properties properties of of the the indicator. indicator. basalts. However, This study focused on dispersal of quartz-epidote qnartz-epidote aaltered l t e d hasalts. However, several severalother otherdispersal dispersaltrains trains were recognized in the the Vermilion Vermilion District Dislxict during during the course course of of the thestudy, study,including includingankerite-sericite ankerite-sericite lode lode goldgoldassociated alteration, and iron formation, associated formation, each each displaying displaying aa unique unique dispersal dispersalpattern. pattern. Given an identical identical glacial glacial of the the differing material material properties properties of of the the indicator. These environment, these differences must be a reflection of Thesedata data underscore that may accompany application of of a general underscore the potential potential problems problems that accompany application general regional regional dispersal dispersal model model to to interpreting particular localized interpreting localized glacial glacial indicator indicator trains. 6 4 0 . 12 10 + + 0A4 + !03it !! Em' :: 2 34 4 6 $ 10 12 �1 Low-grade Low-grademetamorphism metamorphismof of an anArchean Archeandebris debrisflow, flow, Irwin, Irwin, Pifher, Pifher, and and Meader Meader Townships, Lake LakeNipigon NipigonRegion, Region,Ontario Ontario 1 I I Frank FrankR. R.Luther, Luther,Geography Geographyand andGeology GeologyDepartment, Department, UW-Whitewater, hitewater, Whitewater, Whitewater, WI 53190 lutherf@mail.uww.edu lutherf@mail.uww.edu This hisextensive extensivebody bodyof of volcanic volcanic breccia brecciaisislocated locatedeast eastof of Lake LakeNipigon Nipigoninin northern northernIrwin, Irwin, southeastern southeasternMeader, Meader, and and aa large largepart partof of southern southernand andeastern eastern Pifher PiiherTownships. Townships. ItItisis10-25 10-25km kmeast eastof ofLake LakeNipigon Nipigonand and17-28 17-28km kmnorthnorthnortheast northeastofofBeardmore. Beardmore. Mapping Mappingby byMackasey Mackasey(1975), (19751,Kresz Kreszand andZayachivsky Zayachivsky (1989), (1989),and andas as aapart partof of this thisstudy studyshows showsthis thisrock rockto tobe beenclosed enclosedininaathick thick sequence sequenceof of typical typicalgreenstone greenstoneassemblage assemblagerocks. rocks. Metamorphism, Metamorphism,probably probably caused causedby by large large granitic granitic intrusives intrusivesin in Pifher Piiher Twp, increases thenortheast. northeast. increases totothe The Thebreccia brecciawas was thoroughly thoroughly described described and andan anorigin originas asaasubaqueous subaqueous debris debrisflow flowwas wasdocumented documentedby byLuther Luther(1998). (1998). In Inbrief, brief,the the breccia brecciaconsists consistsof of rounded roundedrock rock fragments fragments ranging rangingfrom from <1 e l mm mmto toover over11mmininaavery veryfine-grained finegrained matrix. matrix. Larger Larger fragments fragments tend tend to to be besimilar similar to tothe thematrix matrixinincomposition compositionwhile while smaller smaller fragments fragmentsvary vary in incomposition compositionfrom from that that of of the thematrix matrixto tomafic maficto toultramafic ultramafic to tocarbonate-rich. carbonate-rich. The The matrix matrixof of the thebreccia brecciawas wascomposed composedof of quartz quartzgrains, grains, euhedral euhedral and and broken brokencrystals crystals of of plagioclase, plagioclase, aa glass glassshard/pelitic shardlpelitic fraction fraction (now (now replaced below), and replaced by by the the greenschist greenschist metamorphic assemblage described below), and pyroxene pyroxene and/or andlor hornblende hornblende (now (now chlorite) chlorite) up up to to 0.5 0.5 mm. mm. Representative Representativewhole whole rock rock analyses analyses of of the the portion portionof of the thebreccia brecciaaffected affectedby bylow-grade low-grademetamorphism metamorphism are are presented presented in in Table Table 1; 1;these these analyses analyses show show the the average average (igneous (igneous rock rock equivalent) equivalent) composition compositionof of the thebreccia brecciato tobe bedacitic. dacitic. A A metamorphic metamorphic assemblage assemblage of of Mg-Fe Mg-Fechlorite, chlorite, high-Al high-A1to to high-Fe high-Feepidote, epidote, aluminous aluminous actinolite, actinolite, phengitic-paragonitic phengitic-paragoniticsericite, albite, albite, high-Fe high-Fe biotite, biotite, and and minor minor KK feldspar feldspar over-prints over-printsthe the primary primarytexture textureand andreplaces replacesthe the glass/pelitic glasslpelitic fraction; fraction; EMPA EMPAanalyses analysesof of the themore morevoluminous voluminousmetamorphic-origin metamorphic-originminerals mineralsare are given givenininTable Table2. 2. Most Most of of the the metamorphic metamorphic minerals, minerals, as as isisto to be beexpected expectedinin greenschist greenschist facies faciesrocks, rocks, show showminor minorvariation variationininindividual individualminoral mineralcompositions; compositions; two two minerals, minerals,plagioclase plagioclaseand andepidote, epidote, show showmajor majorcompositional compositionalvariation. variation. Plagioclase varieties: — an5 an5and and— an40. a m . The Thelarge largeplagioclase plagioclase Plagioclaseisispresent presentinintwo two varieties: crystals an40and and are are partially partially sausuritized; saussuritized; these these grains grains crystalshave haveaacomposition compositionofof— anw are are interpreted interpretedas as relict relictigneous igneousplagioclase. plagioclase. The The small small matrix matrix grains grains of of plagioplagioclase an5;these thesegrains grains are are interpreted interpreted as metamorphic metamorphic plagioclase plagioclase generatgeneratclase are are —ans; ed of the the breccia brecciamatrix. matrix. The The variation variation in inepidote epidotecomposition composition ed by by recrystallization recrystallizationof probably probably represents represents crystallization crystallization of of epidote epidote over over aa range rangeof of temperatures; temperatures;high-Al high-A1 epidote epidote usually usually crystallizes crystallizes at at aahigher highertemperature temperaturethan thanthe thehigh-Fe high-Feepidote. epidote. - - - 35 - �In Insummary: summary: aa classic classic greenschist greenschistfacies faciesmineralogy mineralogyof of albite, albite,epidote, epidote, muscovite, muscovite, biotite, biotite, actinolite, actinolite, and and chlorite chlorite overprints overprints the the primary primarymineralogy mineralogyof of the the debris debris flow flow which whichincluded includedfinely-crystalline finely-crystallinequartz, quartz,glass glassand andaltered alteredglass glass(pelite), (pelite), and anddetrital detrital quartz, quartz, plagioclase, plagioclase, and and mafic mafic minerals minerals grains. grains. Table Table1.1. Whole Whole rock rock analyses analyses of of the the samples samples of of the thevolcanic volcanicbreccia brecciawhich whichwere were affected by low-grade metamorphism. Analyses by XRAL. affected by low-grade metamorphism. Analyses by XRAL. S102 AL203 CAO NA2O K20 K20 FE203 T102 P205 P205 LOl S102 AL203 CAO MOO MGO NA20 FE203 FEO FEO MNO MNO TI02 LO1 SUM SUM 0.58 0.15 3.20100.3 3.40 1.72 6.25 4.30 0.08 16.0 3.84 4.00 LN97-3-1A 61.0 61.0 16.0 3.84 4.00 3.40 1.72 6.25 4.30 0.08 0.58 0.15 3.20100.3 LN97-3-1A LN97-6-1B LN97-6-19 63.2 63.2 16.5 16.5 6.03 99.8 6.03 2.15 2.15 3.53 3.53 1.49 1.49 5.30 5.30 3.10 3.10 0.07 0.07 0.51 0.51 0.11 0.11 0.85 0.85 99.8 Table Table2.2. Chemical Chemical analyses analyses of major major greenschist-facies greenschist-facies minerals minerals in in the the volcanic volcanic breccia. breccia. Analyses Analysesby byelectron electronmicroprobe microprobeat atUW-Madison. UW-Madison. hi-Al epidote biotite chlorite 37.9 35.2 25.3 0.3 1.7 0.1 23.9 23.9 28.0 16.8 20.9 10.6 10.6 10.9 10.9 5.6 20.2 23.2 MnO MnO 0.2 0.2 0.2 0.2 0.2 0.3 MgO MgO 13.4 13.4 10.3 16.3 CaO CaO 11.3 11.3 actinohte actinolite b-Al lo-AIeDidote eoidote 5102 Si02 55.5 55.5 37.0 37.0 1102 Ti02 0.0 0.0 ----- A1203 A1203 5.1 5.1 FeO FeO Na20 Na20 K20 K20 TOTAL TOTAL 1.6 1.6 0.1 23.1 23.I 24.3 ----- 0.2 0.2 = 97.9 97.9 96.2 95.1 95.1 0.1 0.1 0.4 9.9 0.1 94.4 86.7 REFERENCES REFERENCES Kresz, D.U. and Zayachivsky, B., 1989, Precambrian geology, Barbara, Meader and Rfher Pifher Townships; 2536-2537,91 p. Townships; Ontario Ontario Geological Geological Survey, Survey, rpt 270 with maps 2536-2537,91 Mackasey, Mackasey, W.O., 1975, 1975, Geology of Dorothea, Sandra, and Irwin Townships, District of Thunder 2294,83 p. Thunder Bay; Bay; Ontario Ontario Division Division of Mines, rpt 122 with map 2294,83 1998,An An Archean Archean subaqueous subaqueous heterolithic heterolithic debris debris flow, flow, Irwin, Irwin, Pifher, and Meader _______ ,1998, Townships, 44thAnnual Annual Institute Instituteon on Townships,Lake Lake Nipigon Nipigon Region, Region, Ontario Ontario(extended (extendedabstract): abstract):44th Lake 44. Lake Superior Superior Geology Geology meeting, meeting, Minneapolis, Minneapolis,Minnesota, Minnesota,v. v. 44. 36 �ThE THEBARRON BARRONSAPROLITE: SAPROLITE:CONFIRMATION CONFIRMATIONOF OF MATURE MATURE CHEMICAL WEATHERING IN IN THE THE SOURCE SOURCE FOR FOR PALEOPROTEROZOIC PALEOPROTEROZOIC QUARTZ QUARTZ AREMTES ARENITESIN INTHE THELAKE LAKE SUPERIOR REGION SUPERIOR REGION MEDAMS, MEDARIS,L.G., L.G., Jr., Jr.,Dept. Dept.ofofGeology Geology&&Geophysics, Geophysics,Univ. Univ. of ofWisconsin-Madison, Wisconsin-Madison, 1215 W. W. Dayton Dayton Street, Street,Madison, Madison,WI, WI,53706, 53706,medaris@geology.wisc.edu medarisgeology.wisc.edu It It has has long long been been inferred inferred that that the the post-1760 post-1760 Ma Ma supermature supennature Baraboo, Bamboo, Sioux, Sioux,Barron Barren and and related related quartz arenites were derived from sources that experienced mature chemical weathering, due quartz arenites were derived from sources that experienced mature chemical weathering, dueto to the in the the near near absence absence of of feldspar feldsparin in the the arenites arenites and and predominance predominance of of kaolinite kaolinite and and pyrophyllite pyrophyllite in the -- - argillaceous 1983; Ojakangas Weber, al., 1986). Paleosols argillaceous strata strata (Don, (~ott,1983; Ojakangas and ~ e b e r1984; , Southwick et al., Paleosols have been recognized recognized beneath beneath the the Baraboo, Baraboo, Sioux, Sioux, and and Barron Barren quartzites quartzites (Medaris (Medaris et et al., al., 1997; 1997; Southwick of aa continuing continuing investigation investigationto to Southwick and and Mosler, 1984), 1984), and and the the present present report report is is part of determine determine the chemical chemical maturity maturity of such such paleosols paleosols and to evaluate evaluate the climatic climatic conditions conditions associated associated with genesis genesis and and deposition deposition of the supermature supennature sedimentary sedimentary rocks. rd- ,xL ------rA-A large large outcrop outcrop of of red saprolite saprolite occurs occurs beneath beneath the A Barton Ban-on Quartzite Quartzite on on the the south southbank bank of of Weirgor Weirgor Creek, Creek, —lOOm east of Reichel Road, in Rusk County (NWI/4, -100m east of Reichel Road, in Rusk County (NW114, R7W). The closest basement Sec 9, 9, T36N, R7W). basement exposure exposure SWll4,Sec SWI/4, is milesouth, south,where wheremetatonalite metatonalite crops out on is —J/2 -% mile on both both sides of Reichel Road. Although Althoughthe the saprolite saprolite and and metametamile apart, tonalite tonaliteoutcrops outcropsare areV2 %mile apart, the the close close similarity similarityin in their textures textures (Fig. 1) 1) makes the metatonalite metatonalite a reasonable reasonable protolith orotolith candidate. The Themetatonalite metatonalitehas has inequigranular ineauimular - texture, texture, in in which which large large quartz quartz grains grains are are surrounded surrounded by aa medium- to fine-grained fine-grained granoblastic granoblastic assemblage assemblage of quartz, Figure 1. Polished Barron polishedslabs slabsof ofBarron plagioclase, hornblende, homblende, epidote, and opaques. opaques. The texture of metatonalite metatonalite protolith protolith and and saprolite; saprolite; metatonalite has has been preserved preserved in in saprolite, saprolite, in which which large large light light grains grainsare are quartz. quartz. the metatonalite relict quartz grains are surrounded by a matrix of finer-grained quartz grains are surrounded finer-grained Table 1. 1. Chemical Chemical Analyses Analyses quartz, quartz, kaolinite-rich kaolinite-rich domains domains (after (after feldspar), feldspar), hematite-rich hematite-rich domains domains % (2) (3) (2) (3) (1) 70.25 67.85 -10 Si02 (after (after hornblende), hornblende), traces traces of of sericite sericiteand tiny, euhedral euhedral crystals of Sioz ~i02 67." 0.24 70.25 0.29 -10 0 crandallite-florencite(aluminophosphate (aluminophosphateminerals). minerals). A1203 14.00 16.15 o crandallite-florencite AIQs 7.05 +9 5.63 Fe203 The The most striking strikingfeature featureof of the the saprolite saprolite chemical chemical composition composition $;FMnO 0.00 0.07 -100 0.09 -97 Mgo 2.41 is the virtual absence absence of of MnO, MnO, MgO, MgO, CaO CaOand andNa2O Na20 and the low MOO 2.41 0.09 -97 concentration concentration of K20 K20 (Table 1). 1). The Thechemical chemicalindex indexof ofalteration alteration o3 -100 -lE 0:58 57 1.17 (CIA) for the saprolite is 95.7, illustrating the high degree x2o (CIA) for the saprolite is 95.7, illustrating the degree of of KSJ 1.17 0.58 -57 ',05 m5 maturity maturity attained attained by by chemical chemical weathering weathering of of the saprolite saprolite [CIA [CIA == I'% LO1 0.70 5.53 100.06 99,91 100*A1203/(AI2O3+CaO+Na2O+K2O) on a molar basis]. The isocon Sum lOO*Al203/(Al203+CaO+Na~0+K20) on a molar basis]. The isocon sum 99.91 1m.w method (Grant, 1986) 1986) has been used to evaluate evaluate the magnitude magnitude of ppm ..à 14 8 -52 8 -52 Rb 14 chemical changes associated with weathering, taking metatonalite chemical changes associated mitatonalite as Sr 321 90 -76 Sr protolith for for the saprolite. saprolite. When Whenoxides oxidesand andelements elementsin insaprolite saprolite Ba 251 31 -89 Ba 91 00 80 91 Zr 80 and and protolith protolith are areplotted plottedagainst againsteach eachother other(Fig. (Fig.2), 2),A1203, A1203,TiO2 Ti02 and and Barron metatonaliie metatonalite protolith (1) Barren pmtoli Zr lie on a straight straight line, line, an an isocon, isocon, along along which the relative concenBarron sapmliie saprolite (2) Barren trations of trations of the defining defining constituents constituents remained remained constant constant dunng during % change relative (3) %change relativeto to isocon isocon r ,. 37 : ;5 7: ;3 4: i:: �weathering. The Therelatively relativelysmall smalldeviation deviationof ofFe203 Fe203 40 40 / from the isocon may be he due to an unrepresentative unrepresentative concentration of Fe203 in the the assumed assumed protolith, rather o 5*SiO: concentration Fez03 in < / O.5'5i02 30 than than its mobility. The The isocon slope of 1.15 1.15 <D 30 + .corresponds corresponds to a 13% 13% mass mass decrease decrease during during 0. weathering, weathering, when MnO, MnO, MgO, MgO, CaO CaO and and Na20 Na20were were 20 2o effectively effectively completely completely removed, removed, and and K20, K20, P205, P20s, Rb, Rh, Sr Sr g and Ba were substantially substantially removed from from metatonalite metatonalite 5 m (Table 1). 1). 0 . m (Table 10 -Fe2%'".2oow2o5 •OPSr The Baraboo Barahoo Quartzite Quartzite is is also also underlain underlain by by aa 7 •SOl'Rb saprolite saprolite on Baxter Baxter Hollow Hollow granite granite (Medaris (Medaris et et al., al., e° • ,'lo*T102 I!•• 00 1997), and application application of the the isocon isocon technique technique to to the the 1997), 00 10 20 30 40 10 20 30 40 Baraboo saprolite yields Barahoo saprolite yields an an isocon isocondefined definedby byAl203 A1203 Barron protolith protolith and Ti02 Ti02with a slope of 1.14, 1.14,corresponding corresponding to to aa 12% 12% diagram for Barron Figure 2. Isocon mass decrease, similar similar to to that that for for the the Barron Barron saprolite. saprolite. protolith saprohte; oxides in wt%, protolith and saprolite; oxides m Chemical maturity matunty of of Baraboo Barahoo saprolite saprolite is comparable comparable to that elements elements in ppm. of Barron saprolite, as as indicated indicated by the "" levels of depletion in in MnO, MnO, MgO, M ~ O CaO, :C~O, 6 60 60 - A % Na20, Sr Sr and and Ba Ba (Fig. (Fig. 3). Depletion Depletion of Si02 Si02 ".. % 40 both saprolites, is --'10% -10% in both saprolites,and and—'30% -30% Fe203 Fe203 § 40 : A A - 20 20 Baraboo. Following was removed at Barahoo. Following 9 weathering of the the Baraboo Barahoo saprolite, saprolite, K20 £2 2-rn 00 Rb were reintroduced by hydrothermal -20 and Rh hydrothermal -20 fluids, which were channeled channeled along along the the §' sub-Baraboo unconforrnity unconformity during suh-Barahoo 60 metamorphism. metamorphism. -80 The Barron and other other contemporaneous contemporaneous b00 . " m " m . n saprolites exhibit a high degree of &  ¡ & m0 Q0 ' &a ' 0S , Q Q Q Q ° mineralogical mineralogical and and chemical chemical maturity, maturity, x g ~ us s a ~ + similar Figure 3. Comparison of similar to that found found in in present-day o f chemical changes changes in in Bàrron Barren and Baraboo saprolites; s~prolites;elements and oxides arranged arranged weathering zones zones in in humid, humid, warm warm climates, climates, in order order of decreasing ionic ionic radius. radius. where intense chemical chemical leaching leaching is is characteristic, characteristic, in demonstrating the existence of a tropical environment in the Lake Superior region at thereby demonstrating -4750-1650 Ma. -1750-1650 Ma. Such Suchaaclimate, climate,combined combinedwith withaastable stabletectonic tectonicsetting, setting,was wasessential essentialfor for supermature features of the Baraboo, Barron and Sioux quartzites. generating the supennature Acknowledgments Acknowledmzents I'm indebted to Mike Mudrey, who drew my attention to the Barron saprolite saprolite and provided a locality map, and to Jim Grant, who enlightened me regarding the isocon method. References Dott, Don, R.H. Jr. (1983) (1983) Geol. Soc. Soc. Amer. Memoir 160, 160, 129-141; 129-141; Grant, J.A. (1986) (1986) Econ. Econ. Geol., Geol., v. 81, 1976-1982; 1976-1982; Medaris, L.G. Jr. eta!. et al. (1997) (1997)43rd 43rd Inst. Inst. Lake Lake Superior Superior Geol., Geol., 39-40; 39-40; Ojakangas, R.W. and Weber, R.W. (1984) Minn. Geol. Sun'., Surv., Rept. Rept. Inv. Inv. 32, 32, 1-15; 1-15;Southwick, Southwick, Mossler, J.H. D.L. et al. (1986) Geol. Soc. Amer. Bull., v. 97, 1432-1441; 1432-1441; Southwick, D.L. and Mossier, (1984) Minn. Geol. Sun'., Surv.,Rept. Rept.Inv. Inv. 32, 32, 17-44. 17-44. - - / - - 7 - - I • • . . . . . 0 - 38 - �TOURMALINE-BEARING VEiNS IN THE BARABOO QUARTZITE: A NEW TOURMALINE-BEARING QUARTZ VEINS OCCURRENCE OCCURRENCE OF THE ALKALI-DEFICIENT TOURMALINE, TOURMALINE, FOITITE FOITITE MEDA1US, L.G., Jr.and FOURNELLE, J.H., Department of Geology & Geophysics, Geophysics, MEDARIS, L.G., Jr.and FOURNELLE, J.H., Department University University of Wisconsin-Madison, 1215 1215 W. Dayton Street, Madison, WI, 53706; 53706; medarisgeology.wisc.edu; johnfgeology.wisc.edu medaris@geology.wisc.edu; johnf@geology.wisc.edu Tourmaline Tourmaline has long long been known to occur in quartz veins in the Baraboo Baraboo Quartzite, Quartzite, but its its chemical composition and geological significance have have never neverbeen been assessed. assessed. The present investigation was undertaken to characterize this tourmaline, as part of an ongoing project to establish the mineralogical and chemical composition of Proterozoic rocks in the Baraboo Bamboo Range. Three samples of tourmaline-bearing tourmaline-bearing quartz quartz veins from low in the Bamboo Baraboo stratigraphic stratigraphic section section on the south limb limb of the Baraboo Baraboo syncline syncline were were selected for investigation: investigation: sample I) 1)aa folded, folded, 1.5 1.5 cm thick quartz quartz vein in in the the regolith regolith of of the the Baraboo Baraboo paleosol from drill core 613 in Baxter Hollow; sample 2) a 3 cm thick quartz vein in quartzite in the SE ¼ 'A Sec. 29, Baxter Hollow; and sample 3) 3) a 3 mm nun thick, tourmaline-rich tourmaline-rich quartz vein in quartzite and metapelite in the NW ¼ 'A Sec. 35, Pine Hollow. All All three samples samples are mineralogically mineralogically simple, consisting of quartz, quartz, specular specular hematite hematite and and tourmaline. tourmaline. Tourmaline Tourmaline occurs in clusters clusters of small, randomly Figure 1. Photomicrograph tourmaline, Photomicrograp~of oftourmaline, oriented oriented pleochroic pleochroic crystals crystals (Fig. (Fig. 1), l), in in which which sample 3, plane polarized light = pale o= 0 pale to medium medium grayish grayish blue blue and and EE == colorless. colorless. Tourmaline analyses Tourmaline analyses were obtained obtained by wavelength Table Electron microprobe Table 1. Electron microprobe analyses analyses 5X50 instrument, quartz veins dispersive tournialine from quartz of tourmaline veins in in dispersive spectrometry on a Cameca SX50 using instrument, using Baraboo Quartzite Barabw Quartzite a 15 kV accelerating voltage, a 10 nA beam current, a beam 15 accelerating voltage, a 10 nA beam current, a beam 2 3 sample 1 2 3 sample p.m.the the(bpZ pZ reduction diameter of 1 pm, reductionsoftware, software, and a wt% combination of natural and synthetic combination synthetic minerals minerals as as standards. standards. 35.14 Si02 35.14 35.23 35.18 35.18 tourmalme by electron Analysis of tourmaline Analysis electron microprobe microprobe presents presents aa 34.86 2O3 35.05 A ~ O S 34.00 34.00 35.05 34.86 14.59 challenge because 13.98 FeO 13.98 12.35 12.35 14.59 to measure measure Li Li and and H H directly, directly, FeO challenge because of of the inability inability to 1.11 0.00 MgO 0.49 0.49 1.11 0.00 MgO the difficulty in determining B and 0 0 precisely, and the Na20 0.62 0.55 0.47 uncertainty in valency of transition elements. Nevertheless, Nevertheless, 8 Sum 84.23 84.29 85.09 results are achieved by normalizing acceptable acceptable results achieved normalizing cations cations to to 24.5 cations p.fu. cationsp.~u. 5.96 6.00 5.96 Si 6.00 5.96 5.96' Si oxygens HzO- and and B203-free B203-free basis and assuming assumingaa oxygens on an H206.85 6.98 6.96 Al 6.96 6.85 6.98 A1 B atoms atoms and and 331 0 atoms stoichiometric formula containing 33 B 10 1.75 2.07 Fe 2.00 2.07 Fe 2.00 1.75 ofF). (in the absence of F). Tourmaline Tourmalinewas wasanalyzed analyzed for for Si, Si, Ti, Al, Mg 0.12 0.28 0.00 Fe, Mn, Mg, Ca, Na, K, F, and Cl, but only Si, Al, Fe, Mg, and Na 0.20 0118 ohs N~ 0.20 0.18 0.15 occupancy are Fe-rich, Fe-rich, site occupancy TheBamboo Baraboo samples samplesare Na were detected (Table 1). The 5.96 6.00 5.96 T(6) 6.00 5.96 5.96 T(6) the Y-site is effectively effectively filled filled with Fe and Al A1 (means (means and 6.00 &00 6.00 Z(6) z(6) 6.00 6.00 6.00 standard deviations are 2.96±0.05, 2.96±0.08 and 2.98±0.06 2.96+0.05,2.96+0.08 2.98L0.06 3.01 3.03 y(3, 2.97 for the three samples), samples), indicating indicating the absence of Li, and the o.is 0120 0.18 0.18 0.15 X(O-1) 0.20 x(0-1) X-site is largely vacant, containing containing subordinate subordinate amounts of Na. ,. 1 r° 39 �Thus, Thus, the the composition composition of of Baraboo Barabootourmaline tourmalineisis close close to type foitite and lies in a field near the foitite corner type foitite and lies in comer of a plane defmed definedby by the the tourmaline tourmaline end-members, end-members, , foitite foitite [O(Fe2+2A~)Aksi60~~(Bo3)3(oH)3(oH)], foitite[D(Fe22Al)Al6SioOia(BO3)3(OH)3(OH)J, schorl 8(B03)3(OH)3(OH)J, schorl[Na(Fe2s)AloSisOj paCFe2+~)A&O~s@O~)3(0I^OH)], and and their their Mg Mg counterparts counterparts(Fig. (Fig. 2). 2). >c All three Baraboo Baraboosamples samplesare arealkali-deficient, alkali-deficient,with with All three A cores 0.5<X0c0.9, 0.5<X60.9, although althoughthey theyeach eachoccupy occupyaadifferent different field field in Fig. 2. The TheFe Fe##ininsample sample33isis99-100 99-100 and and in in sample is 90-95, 90-95, but but sample sample22isismore moremagnesian, magnesian, sample 22 is with the Fe# of rims being 70-85 and of cores, the Fe# of rims being 70-85 and of cores, 35-55. 35-55. The The effective effectiveoperation operationof of the thecoupled coupledexchange, exchange, 90 100 60 70 60 50 40 30 X(D) Fe# Y(A1) ==X(Na) X(Na) + + Y(Fe+Mg), Y(Fe+Mg), is is illustrated illustrated by by X(D) ++Y(Al) F& 4 schorl schorl the linear linear covariance covariance of of Al A1 and and Fe+Mg Fe+Mg (Fig. (Fig. 3). 3). Figure 2. Fipe 2-Compositional variation of Most analyses extend from the theoretical composition of analyses extend from theoretical composition Baraboo tourmaline Bamboo tourmalineininterms termsof ofX-site X-site foitite (6 Al, foitite (7 Al, Al, 22 Fe+Mg) Fe+Mg) toward toward that that for forschorl schorl(6 Al, 33 vacancy, 0, and Fe#, I 0O*Fe/(Fe+Mg) vacancy, 0, and Fe#, 100*Fe/(Fe+Mg) Fe+Mg), although although the the cores cores of of tourmaline tourmaline in in sample sample33 are are 3.C more more aluminous, aluminous,extending extendingtoward toward88Al Aland and11Fe+Mg, Fe+Mg, * type fofte \.... 0 sanplel which, which, in in the the absence ofF of Fand andLi, Li, suggests suggeststhe the presence presence of of A sample2 \.. 4 . an [D(Fe2Al2)Al6Si6O13(BO3)3(OH)3O]. anoxy-foitite oxy-foititecomponent component [D(Fe2+A12)A16S&O18(B03)3(OH)30]. • b. 2.5 25 Foitite Foititeisis aa relatively relativelyuncommon uncommonvariety varietyof oftourmaline tourmaline %Nq : t that has been described from late-stage hydrothermal veins that has been described from late-stage hydrothennal veins ? ?jk associated associatedwith with granitic granitic pegmatites pegmatites and and from from metasomatized metasomatized If wall wall rocks rocks in in areas areas of of Pb-Zn mineralization. Such Suchfoitite foitite "- 2.0 2o : differs differs from from that that at at Baraboo Barabooin incontaining containingappreciable appreciable quantities quantities of of Ti, Ti, Mn, Mn,Ca, Ca, Li Li and and F, F, depending dependingon onlocality, locality, A and andin in being beingassociated associatedwith withaavariety varietyof ofsilicate silicateminerals, minerals, 1.5 6.0 6.5 7.0 7.5 6o 65 7o 75 includingalbite, albite,adularia, adularia,micas micasand andzeolites, zeolites,among amongothers. others. including Al, A aptu am In In contrast, contrast,the theBaraboo Baraboofoitite foititeisischemically chemicallysimple, simple, 3.0 30 1Vs8mp183 containing containingonly onlySi, Si,Al, Al,Fe, Fe,Mg Mgand andNa Na(+ (+B+O+H), B+O+H),and andisis •\\ accompanied accompanied solely solely by quartz quartz and hematite. The Thechemical chemical . \ 2.5 25 constitution constitutionand andalkali alkalideficiency deficiencyof ofBaraboo Barabootourmaline tourmalineare are •.•.c,,v "\..v %.. V V, consistent consistentwith with its its environment environmentof of formation formationin in hydrohydroÈ. a t 2.0 thermal thermal veins veinswithin withinBaraboo Baraboometasedimentary metasedimentaryrocks. rocks. a, & 20 + During post-Penokean folding and low-grade metaDuring post-Penokean folding and low-grade metae U. LL. 3%. morphism, morphism, hydrothermal hydrothermalfluids fluids migrated migrated through through the the 15 -\\ 1.5 v chemically chemicallymature mature Baraboo Baraboo sequence, sequence, which which consists consists almost almost +... V\ B entirely entirelyof of5i02, SiOa,Al203 A1203and andFe203, Fe203, where where they theyscavenged scavengedBB % .1 1.0 10 and and the the few fewother otherelements elementsnecessary necessaryto toprecipitate precipitatequartz, quartz, 60 65 7o 75 8o 6.0 6.5 7.0 8.0 7.5 Al, apfu am hematite, hematite, and and alkali-deficient alkali-deficient tourmaline. Thus, Thus,the the Figure 3. Covariance Covarianceof ofFe+Mg Fe+Mg Figure3. chemical chemicaland and mineralogical mineralogicalcompositions compositionsof ofhydrothermal hydrothermal (Y-and and Z-sites) Z-sites) (Y-site) and and Al A1(Y('V-site) quartz quartzveins veins in in the the Bamboo Baraboo range range are are inherited inherited in in part part from from in Bamboo tourmaline; scale in in Baraboo tourmaline; scale in Proterozoic Proterozoicweathering weatheringand andsedimentation sedimentationprocesses. processes. . & I . , . '%. . t\;\\ \\ .  % ¥. ! atomsper performula formulaunit unit atoms 40 �Metal, Gold Gold and and Carbonatite Carbonatite Exploration Kimberlite, Base Metal, Exploration Targets, Derived From Overburden Overburden Heavy Mineral Data, Killala Lake Area, Northwestern Ontario. MORRIS, MORRIS, T.F., SAGE, SAGE, R.P., Ontario Ontario Geological Geological Survey, 933 933 Ramsey Ramsey Lake Lake Road, Road, Sudbury, ON. P3E tom.morris(ändm.gov.on.ca; CRABTREE, D.C., D.C., Ontario Geoscience Sudbury, ON. P3E 6B5, tom.morrisfaindm.gov.on.ca; P3E 6B5. Laboratories, 933 Ramsey Lake Road, Sudbury, Sudbury, ON., P3E ABSTRACT Since Since 1993, 1993, the Ontario Ontario Geological Geological Survey Survey (OGS) (OGS) has has conducted conducted aa series series of of Quaternary Quaternary mapping mapping and overburden overburden sampling sampling programs programs to assess assess the diamond potential of the the Kapuskasing Kapuskasing Structural Structural A suite suiteof of heavy heavy Zone (KSZ). Kimberlite Kimberlite is is recognized as as the primary host host rock for for diamond. diamond. A minerals (Cr-pyrope minerals (Cr-pyrope garnet, garnet, chromite, cbromite, Mg-rich ilmenite, ilmenite, Cr-diopside Cr-diopside and and forsteritic forsteriticolivine) olivine)are are commonly KIMs). commonlv associated associated with kimberlite kimberlite and and are are referred referred to to as as kimberlite kimberlite indicator indicator minerals minerals(KIMs). The typeiand types and distribution distribution of of KIMs KIMs recovered recovered from from these these programs programs clearly clearly demonstrate demonstratethat that the the KSZ has excellent potential for hosting diamond-bearing kimberlite. kimberlite. To To date, date, 22 occurrences occurrences of of diamond-bearing rock and diamond-bearing and kimberlite kimberlite have have been been discovered discovered in in the the Wawa Wawa area area (KWG (KWG Resources Resources 1nc.ISpider Resources Resources Inc., Inc., Press Press Release, Release, 20/08/97; 20108197; Canabrava Canabrava Diamond Corporation, Corporation, Press Press Inc./Spider Band-Ore Resources Release, 09/12/97; 09/12/97; Band-Ore Resources Ltd., Press Press Release Release 10/02/00; 10102100;Pele Pele Mountain Resources Resources Release, Inc., Press Release, Release, 10/02/00). 10/02/00). Following the success success of the KSZ Quaternary Quaternary mapping mapping and overburden overburden sampling sampling programs, programs, plans to assess similar structures in other parts of Ontario were made. One such assess similar structures in other parts of Ontario were made. One sucharea, area, the the TransTransfrom Michigan Michigan north-northeast north-northeast into intoOntario, Ontario,west west of of Superior Tectonic Zone (TSTZ), extends from Marathon. Marathon. Kimberlites Kiberlites straddle straddlethe the structure structurewithin within Michigan Michigan and and ultramafic ultramafic lamprophyres, lamprophyres, which may belong "clan", are recognized in association with the TSTZ in belong to the kimberlite "clan", Ontario. Ontario. The The TSTZ TSTZ represents represents aa fault fault system system that that has has accommodated accommodated emplacement emplacementof of upper upper mantle derived derived material. material. To To evaluate evaluate the the kimberlite kimberlitepotential potential of of the the TSTZ TSTZ in in Ontario, Ontario,an an area area between the towns of Marathon and Terrace Terrace Bay was chosen for Quatemary Quaternary mapping mapping and and stream stream sediment and till sampling. Collected overburden samples were processed to determine KIM sampling. Collected overburden samples determine KIM signatures. signatures. In addition addition to KIMs, KIMs, overburden overburden samples samples were were processed for for metamorphic metamorphic or or magmatic magmatic gold grains and carbonatite indicator massive sulphide indicator minerals (MMSIM ), ), gold indicator minerals. minerals. The MMSIMs MMSIMs are are associated associated with with 33 main types types of of base base metal deposits: deposits: 1) 1)volcanosedimentary volcanosedimentary massive sulphides sulphides in in high grade grade regional regional metamorphic metamorphic terrain; terrain; 2) skarn skarn and and greisen greisen deposits; deposits; and and 3) magmatic Ni-Cu sulphides. The composition of gahnite, a rare and important MMSIM magmatic sulphides. The composition of gahnite, a rare and important MMSIM associated with metamorphosed associated metamorphosedvolcanosedimentary volcanosedimentarymassive massive sulphide sulphide deposits, deposits, can can also also be used used to indicate indicate the presence presence of of rare-element rare-element pegmatite. pegmatite. The The presence presence and and distribution distribution of of MMSIMs MMSIMs and gold grains grains recovered recovered from from overburden overburden sampling samplingprograms programs completed completed over over the the KSZ KSZ defined defined several several important important base base metal, metal, rare-element rare-element pegmatite pegmatite and and gold gold exploration exploration targets. targets. This presentation presentation provides provides data data and preliminary preliminary interpretations interpretations on the types types and and distribution distribution of kimberlite kimberliteheavy heavy mineral mineral indicators indicators(KIMs), (KIMs), metamorphic metamorphicor or magmatic magmatic massive massive sulphide sulphide indicator minerals (MMSIMs), gold grains grains and carbonatite carbonatite indicator indicator minerals minerals from from modern modem Registered Trademark, Trademark, Overburden Overburden Drilling Drilling Management Management Drilling Drilling Limited Limited 41 �alluvium and till samples collected in the Killala Lake area, northwestern Ontario. This data can be used to focus exploration efforts for kimberlite (diamonds), base metal, gold and carbonatite (rare earth )) deposits. A total of 230 modern alluvium and 82 till samples were collected from across the study area. modern alluvium and till samples, 8 bedrock samples were collected In addition to the modem and processed to characterize their heavy mineral composition. Orientation surveys surveys were were completed over the Prairie Lake Lake Carbonatite, Carbonatite,Major MajorGeneral GeneralResources ResourcesLtd.'s Ltd's Spider Spider Lake Lake base base metal deposit and Cameco Gold Inc.'s Inc.'s Empress EmpressHill Hill gold gold deposit. deposit. These Thesesurveys surveysdefined defined overburden heavy mineral signatures signatures associated with each deposit type. Bedrock and overburden overburden data obtained from the orientation surveys surveys were used to help interpret heavy mineral signatures signatures derived from the regional overburden sampling program. From the overburden samples, samples, a number of kimberlite indicator minerals, base metal indicator minerals, carbonatite iiidicator minerals, carbonatite indicator minerals and a few gold grains were recovered. This data will be released in number 6013, 6013,April April2000. an Ontario Geological Survey Open File Report, number 2000. 42 �Nature and distribution distribution of ofLogan Logandiabase diabase sills sills and and gabbro gabbro channels channelsin inthe the Keweenawan Rift near Thunder Thunder Bay, Bay,Ontario: Ontario: Brief Brief comparison comparison to to Noril'sk Noril'sk - Jon NorthAtlantic AtlanticNickel NickelCorp. Corp. Jon North North- North Flat-lying Flat-lyingLogan Logandiabase diabasesills, sills, vertical verticaldiabase diabasedikes, dikes, and andlayered layeredgabbro gabbrocomplexes complexes intrude intrudelower lowerProterozoic Proterozoicand andArchean Archean rocks rocksin inthe thenorthwest northwestcorner cornerof ofthe theKeweenawan Keweenawan Rift. These 1108 Ma plutonic and hypabyssal rocks are broadly contemporaneous Rift. These 1108 Ma plutonic and hypabyssal rocks are broadly contemporaneouswith with thick thick sequences sequences of of flood flood basalt basaltof of the the North NorthShore ShoreVolcanic VolcanicGroup Groupand andOsler OsierGroup Groupinin northern northernLake LakeSuperior Superiorand andthey they ostensibly ostensibly represent representthe themagma magmapathways pathwaysfor forthe theflood flood basalts. basalts. South Southof of Thunder ThunderBay, Bay,aarugged, rugged,upland uplandarea areaof ofdiabase-capped diabase-cappedcuestas cuestas occupies occupies aa 70 70 km km by by30 30km kmnortheast-trending northeast-trendingtopographic topographicfeature feature--the the"Logan "LoganBasin" Basin"-between betweenThunder ThunderBay Bayand andthe theMinnesota Minnesotaborder. border. The The Keweenawan Keweenawan diabase diabase caps caps of of the above valleys valleys and and canyons canyons thecuestas cuestas(i.e. (i.e. the theLogan LoganSills) Sills) commonly commonly rise rise 15Cm 150 mabove underlain underlainby by deeply deeply eroded erodedflat-lying flat-lying sedimentary sedimentaryrocks rocksof of the the lower lowerProterozoic ProterozoicRove Rove Formation Formation of of the the Animikie Animikie Group. Group. Northwest Northwestof ofthe theLogan LoganBasin BasinArchean Archeangranitoid granitoidrocks rocks of of the the southern southernSuperior SuperiorProvince Provinceform form low low rolling rolling hills. Southeast Southeastofofthe theLogan LoganBasin Basin the thetopography topographyisisaarugged ruggedupland uplandarea areaof ofnortheast-trending, northeast-trending,linear linearridges ridgesformed formedby by vertical verticaldiabase diabaseand andgabbro gabbrodikes dikesintruded intrudeddiscordantly discordantlythrough throughthe theflat-lying flat-lyingstratigraphy stratigraphy of ofthe the Animikie AnimikieGroup. Group. Geological Geologicalmaps mapsofofthe thearea area(e.g. (e.g.Smith Smithand andSutcliffe Sutcliffe1987;1989, 1987;1989, Pye and Fenwick 1963) tentatively illustrate NE-trending faults on the NW and Pye and Fenwick 1963) tentatively illustrate NE-trending faults on the NW andSE SEsides sides of the Logan Basin. of the Logan Basin. The I), high TheLogan LoganBasin Basinhas hasrelatively relativelyhigh hightotal totalmagnetic magneticintensity intensity(TM (TMI), highmagnetic magneticrelief, relief, and andlong longwavelength wavelengthmagnetic magneticanomalies anomaliescaused causedby bythe theflat-lying, flat-lying,magnetic magneticdiabasediabasecapped cappedcuestas. cuestas. North Northofofthe theLogan LoganBasin Basinaadomain domainofoflow lowTMI TMIand andlow lowrelief reliefcoincides coincides with Archeancrust crustand andsoutheast southeastof ofthe theLogan LoganBasin Basinhigh highTMI, TMI,high highrelief, relief,and andhigh high with Archean frequency magnetic anomalies coincide with the area of northeast-trending diabase frequency magnetic anomalies coincide with the area of northeast-trending diabase dikes. dikes.Northeast Northeastand andeast-trending east-trendingfaults faultscan canbe beinterpreted interpretedas asthe theboundaries boundariesofofthe the Logan LoganBasin. Basin. The Thejuxtaposition juxtapositionofofthe thedomain domainofofsills sillsofofthe theLogan LoganBasin Basinagainst againstthe thedomain domainofofdikes dikestoto the thesoutheast southeastcan canbe bereconciled reconciledininan anhypothetical hypotheticalcross crosssection sectionififthe theLogan LoganBasin Basinisis interpreted interpretedas asaagraben grabenwhere wherethe thetransition transitionfrom fromvertical verticaldikes dikesto tohorizontal horizontalsills sillsisis preserved preservedat atdepth. depth.The Thenortheast-trending northeast-trendingdikes dikestotothe thesoutheast southeastare arevertical verticalconduits conduits to tothe theLogan Logansills sillsunroofed unroofedby byerosion erosionininaahorst-like horst-likefeature. feature. In In1995 1995aalocal localprospector prospectordrilled drilledaa499 499mmvertical verticalhole holeinto intothe theLogan LoganBasin BasinininBlake Blake Township Townshipand anddiscovered discoveredan anunusual, unusual,131 131m-thick m-thicksulphide-bearing sulphide-bearinggabbro gabbrobelow belowthe the Logan Sills. Lighifoot and Lavigne (1995) compared the geology and texture of the Logan Sills. Lightfoot and Lavigne (1995) compared the geology and texture of the gabbro gabbroto tothe thefertile fertileNi-Cu-PGE Ni-Cu-PGEbearing bearingsub-horizontal sub-horizontalgabbro gabbrochannels channelsbelow belowthe theflood flood basalt basaltsequence sequenceatatNoril'sk, Noril'sk, Siberia, Siberia,the theworld's world'slargest largestnickel nickeldeposit. deposit. North NorthAtlantic AtlanticNickel Nickelsubsequently subsequentlydetermined determinedthat thatthe thegabbro gabbrowas wasaaflat-lying, flat-lying, northeast-trending, northeast-trending,taxitic-textured taxitic-texturedintrusion intrusionup upto to130 130mmthick thickwhich whichpinches pinchesdown downtotoaa normal-looking normal-lookingLogan Logandiabase diabasesill sillless lessthan than20 20mmthick thickwithin within300 300mmofofthe thecentre centreofofthe the intrusion. intrusion.An Aninterpretation interpretationisisthat thatthe thetransition transitionfrom fromvertical verticaldikes dikestotohorizontal horizontalsills sillsinin the theLogan LoganBasin Basinisisaccompanied accompaniedby byhorizontal horizontalmagma magmachannelling. channelling.This Thisdiscovery discovery underscores underscoresthe thepotential potentialfor forNoril'sk-type Noril'sk-typeNi-Cu-PGE Ni-Cu-PGEconcentrations concentrationsininthe thebasal basalpart partofof the Rift theflood floodbasalt basaltsequence sequenceofofthe theKeweenawan Keweenawan anidea ideathat thathas hasbeen beendeveloped developed Rift—-an and andadvanced advancedby byseveral severalworkers workersininthe thepast pastdecade. decade. 43 �References Lightfoot, P.C., and Lavigne, M.J., M.J., 1995. Nickel, Nickel, copper, and platinum group element mineralization in Keweenawan intrusive rocks: new targets in the Keweenawan mineralization Keweenawan intrusive Keweenawan of the Thunder Bay Bay region, northwestern Ontario. Ontario Ontario Geological Geological Survey Survey Open Open File File Report Report 5928, 32 32 p. p. Fenwick, K.G., KG., 1963. Pye, E.G., and Fenwick, 1963. Atikokan-Lakehead Atikokan-Lakehead Sheet, Map Map 2065, Ontario Department of Mines, Geological Compilation Series, scale 1:253,440. Smith, A.R., and Sutcliffe, R.H., Smith. A.R., R.H., 1987. Keweenawan Keweenawanintrusive intrusive rocks rocks of the Thunder Bay Bav area, in in Summary of Fieldwork, 1987. Ontario OntarioGeological Geological Survey Survey Miscellaneous Miscellaneous Paper paper 137, p. 248-255. A.R., and Sutcliffe, R.H., R.H., 1989. Precambrian Smith, A.R., Precambrian geology of Keweenawan Keweenawan intrusive intrusive River area; area; Ontario rocks in the Crystal Lake-Pigeon River Ontario Geological Survey, Map P. 3139, Geological Series - Preliminary Preliminary Map, Map, scale scale 1:50,000. 1:50,000. Geology 1987. 44 �GEOMETRY BETWEEN GEOMETRY BETWEEN GOLD-RICH GOLD-RICHSUBSIDARY SUBSIDARY SHEAR SHEAR ZONES ZONES AND MAJOR MAJOR STRUCTURES: STRUCTURES: IMPLICATIONS IMPLICATIONS FOR FOR ARCHEAN ARCHEAN LODE-GOLD LODE-GOLD MINERAL MINERAL EXPLORATION EXPLORATION DEAN M. PETERSON Department of Geology, University Minnesota -- Duluth University of Minnesota Detailed GIS-based spatial spatial analysis analysis of of the three Detailed GIs-based three largest largest Archean Archean lode-gold lode-gold mining mining camps camps in in Canada Canada has led led to to the thedevelopment developmentof ofsome someseemingly seeminglyfundamental fundamentalgeometric geometric relationships relationships between major structures, subsidiary subsidiary structures, structures, and and gold gold mineralization. mineralization. The lode-gold between lode-gold mining camps included in the Lake, and Hemlo Hemlo the analysis analysis are arethe theTimmins, Timmins,Kirkland KirklandLake Lake—- Larder Lake, districts, which together have produced approximately 115 million ounces of gold. Additional analysis of the Noranda, Cadillac, and Malarctic analysis Malarctic gold camps camps of northwestern northwestern Quebec Quebec corroborate corroborate the original original findings, and add strength to the implications implications of the model. The relationships relationships link the location and and size size of gold location gold mines mines with with the the sense sense of ofshear, shear,stress stressregime regime(compression (compression verses verses extension verses parallel), and and distance to to the the major major structures structures in in each each camp. camp. Additional analysis doduments the the relationship relationship between between the the location location and size of gold gold mines mines to to the the orientation orientation and and distance distance to subsidiary subsidiary shear zones in the the camps. camps. These relationships allow the the exploration exploration geologist to quickly develop specific lode-gold lode-gold structural structural targets targets based based on on the the simple simple analysis anaiysis of of geologic maps. geologic maps. For this study, study, the major major shear shear zones zones were were divided divided into into compressional, compressional, extensional, and shear parallel segments based on regional maps and and literature literature review review (see (see inset map of parallel segments based regional geological geological maps Figure 1). GIS GIs analysis analysis has has shown shown that that essentially essentially all all of of the the gold gold in in the the camps camps has has been been mined mined a). In within 6 kilometers of major major shear shear zones zones (Table (Table 1la). In addition, addition, greater greaterthan than 98% 98%of of the the gold gold in in the three camps has been mined from zones of regional compression (Table (Table lb). ib). Together, these facts lead to the conclusion conclusion that lode-gold lode-gold mineral exploration programs should be targeted upon areas areas within 6 kilometers kilometers of compressional compressional zones zones of of major major shear shearzones. zones. Inset H .4P N\ 1St Order Shear Shear Zone Zone. 1st Order '\\ yP&IeI 212T 21.27' - \' \ 163.670 163.67' Figure 11 Figure , 59.700 \_ * — -- \ Order Shear Compression Compression Zone 1st Order Zone 7 Regional shear shear zone zone classification classification(inset (insetmap) map)and androles rulesfor forclassifying classiing the geometry of Regional geometry of associated with with major major structures. Angles subsidiary shear zones spatially associated Angles are are measured measured along along arcs from points, in the same direction as the major structures ares Son) inferred intersection points, structures sense of shear. shear. 45 �Proximity (A) (A) and and structural structural (B) (B) relationships Table I1 Proximity relationships between between lode-gold lode-gold mines mines and and major major structures structuresfor for Timunins, Kirkland Lake-Larder Lake, and Hemlo gold the T iins,K i a n d Lake-Larder gold camps, camps, Ontario. Ontario. - A) Proximity Proximity Analysis A) Analysis Distance to Major Structure <3 < 3Kilometers Kilometers 33—6 - 6 Kilometers Kilometers >6 > 6Kilometers Kilometers B) Structural Structural Analysis Analvsis Mined Gold (million Oz) (million Oz) Structural Structural Setting Selling Mined Mined Gold Gold Gold (million Oz) (million Oz) Gold Gold 83.108 83.108 32.100 32.100 0.235 0.235 72.0% 72.0% 27.8% 27.8% 0.2% Compressional Extensional Extensional Parallel 113.539 113.539 1.877 1.877 0.027 98.35% 1.63% 1.63% 0.02% 0.02% ((%) %) Table Table 22 Proximity Proximity (A) and structural structural (B) relationships relationships between lode-gold mines and subsidiary subsidiary structures for the T Timmins, structures i i n s , Kirkland LakeLarder Lake, Lake, and Hemlo Hemlo gold gold camps camps A) Proximity Analysis A) Proximity Analvsis Distance to Subsidiary Shear <200 < 200meters meters 200—250 - 250 meters meters 200 Mined Gold 1%) ('%) (millionOz) Oz) (million Gold 108.417 108.417 5.869 5.869 0.5 10 0.510 1.157 93.91% 93.91% 5.08% 5.08% 0.44% 0.44% 1.00% 250—300 250 - 300 meters > 300 meters Bl Structural Analvsis (250 m Buffer1 Subsidiary Shear Mined Gold t%> (%) Gold Angle ~ n & (million Oz) (million Oz) 106.252 106.252 160° - 180 180° 160¡ 140°- 160° 120°- 140° 1.398 100° - 120° 80° - 100° 0.000 0.400 0.000 60° - 80° 400 - 60° 20° - 40° 0° - 20° 0.006 0.860 4.170 1.200 92.97% 92.97% 1.22% 0.00% 0.35% 0.00% 1.05% 0.01% 0.75% 3.65% (%) 1%) - For this study, the relationship between the major and has been defined and subsidiary subsidiary structures structures has defined by by measurements along an arc in the same same direction as the the dominant dominant sense sense of shear shear of the the major major structure (Figure (Figure 1). 1). Each structure Each subsidiary subsidiary shear zone segment has has an an associated angle to the major segment associated angle major structure stored in its associated GIS database. structure stored its associated GIs database. The location of gold mines and their production records in each camp were spatially records spatially linked linked with shear zone. zone. The the nearest subsidiary subsidiary shear The outcome outcome spatial analysis analysis isis given 2. of this this spatial given in in Table Table 2. Approximately 99% of the gold in the camps has Approximately been mined been mined within within 250 meters meters of of aa subsidiary subsidiary In addition, (Table 2a). 2a). addition, subsidiary subsidiary shear zone (Table high-angle (160°-180°, Figure 1) 1) structures structures in the (1600-180°Figure compressional zones of of the major compressional zones major structures structures account for ofall all gold gold mined mined in the camps -93% of for —93% (Table 2b). Table 33 display the Area calculations in Table usefulness of this analysis usefulness analysis for gold gold exploration. exploration. Programs that that would would be targeted upon the highhighPrograms subsidiary in in regional regional compresangle (160°-l80°) (160"-1800) subsidiary sional zones would would reduce reduce the the total total area to be sional zones be Similar targeting criteria in explored by 99%. explored by 99%. lode-gold exploration programs would greatly lode-gold exploration programs greatly lower lower cost, cost, reduce reduce time, time, and and would would quickly quickly vector the program vector program into areas areas of of high high lode-gold lode-gold mineral potential. - Table 33 Tabulation Table Tabulation of of the the total total GIS GIS map map area, area,areas areaswithin withm 250 250meters metersof ofhigh-angle high-anglesubsidiary subsidiaryshears, shears, and explorationwould wouldbe be targeted targetedonly onlyinin and percentage percentage of of the the map map area area reduced reduced ifif exploration the most favorable favorable areas areas of of the the three three mining mining -camps. . ~Mining Camp Camp Hemlo Hemlo Timmins Timmins Kirkland GIS Map 2 Area ( ~ m ) Area (Km2) 2280.00 4106.00 2414.00 ~~ 5 Buffer Around Hi-angle0 Percentage Reduced from ~ Area (Km2) of2SOm Total Map Area Area (> 1607 Subsidiary Shears Shears Total Map 160°) Subsidiary " 5.55 5.55 19.04 19.04 24.94 24.94 46 99.70% 99.70% 99.42% 99.42% 98.70% 98.70% �GEOLOGIC GEOLOGICAND AND GIS-BASED GIs-BASED LODE-GOLD LODE-GOLD & & VOLCANOGENIC VOLCANOGENICMASSIVE MASSIVE SULFIDE SULFIDE(VMS) (VMS) MINERAL MINERAL POTENTIAL POTENTIALMAPS MAPS OF OFA A PORTION PORTION OF OFTHE THEARCHEAN ARCHEAN VERMILION VERMILIONGREENSTONE GREENSTONEBELT, BELT, NORTHEASTERN NORTHEASTERNMINNESOTA MINNESOTA DEAN M. PETERSON Department Departmentof ofGeology, Geology,University Universityof of Minnesota Minnesota -- Duluth Duluth Lode-gold Lode-gold and VMS VMS mineral mineral potential potential maps maps of of aaportion portionofofthe theArchean ArcheanVermilion Vermiliongreenstone greenstone belt belt in in northeastern northeastern Minnesota Minnesota have have been been developed developed from from the the digital digitalincorporation incorporationof ofthe thelodelodegold and VMS ore deposit models into spatial analysis of a new 1:100,000 1:100,000 scale geologic map of the the study study area. area. The TheGIS-based GIs-basedmineral mineralpotential potentialmodels modelsincorporated incorporatedfuzzy-logic fuzzy-logic techniques techniques of of map map combination combination using using variations variations of of knowledge-driven knowledge-driven (ore deposit deposit model) model) and and data-driven data-driven (analog (analog mining camp analysis) techniques. Inference Inferencenets nets depicting depictingthe the mineral mineral potential potential models models are presented in Figure 1 (lode-gold model) and Figure 2 (VMS model). are presented in Figure 1 (lode-gold model) and Figure 2 (VMS model). .a STEP I Weld Spatial STEP 2 Dab Ptocasmg a Collect Spatial Date and Input into GIS GEOLOGIC GEOLOGIC STRUCTURES STOUCTURES Extraction of GIS Features Relevant to the VMS Ore 2. Cetass burg — I. Recteifr —— ladoeda I.tOtder Prolimhiaiy Maptof Selected GIS Peanuts PoId 3rd Ott Aalt Shass l.ExwtRoekTypa 2. Crate baea I. Babies mapped eats 2. Qeeiy Database 3. C* 3. Ct.att Baits —I S Aakarile SandIa SadISt —I— ASisli' QFP Pynte Afida — Sa GOLD GOLD ANOMALIES ANOMALIES COMPETENCY COMPETENCY CONThASTS CONTRASTS LthSse I. Emct C-to 2. Reclassify Types LCeateBaff froesG ____________________ Tails —— tabs. .a S0 ALTERATION ALTERATION ASSOCIATED ASSOCIATED HOST ROCKS HOSTROCKS it Soils !ssa)cael s_ — Wth_ of Evidence Fany tut(O.lI Map Overlay %Overlay STEP STEP33 to Peedsice = * Intenuedisee Factor Maps STRUCTURAL STRUCTURAL F- Integration i i HOSTROCK HOSTROCK ALTERATION ALTERATION Factor F- Factor 1 GEOChEMICAL GEOCHEWCAL Factor Facto' i ROCK RHEOLOGY ROCKRHEOLOGY Factor Factor Modelling UwkOsss Overlay of Intermediate Factor Maps to Pyodece the Final Predictive Map 04 Map Weightings Map Woifligs - LODE MAY LODEGOLD GOLDMINERAL MINERALPOTENTIAL POTBtTTUU. MAP Figure Figure1.1. Inference Inference net for for the the three-step three-step GIS GIs model model of ofthe thelode-gold lode-gold mineral mineral potential potential of of the theArchean Archean Vermilion Minnesota. The Vermilion greenstone belt, Northeastern, Minnesota. The three three steps stepsare: are:1)1)Detailed Detailedgeologic geologic mapping Conversion of all all data datainto intoprimary primary mapping and compilation compilation of of all all available available geologic geologic data. data. Conversion GIS datasets;2) 2) Extracting Extractingand and enhancing enhancingthe the features featuresof ofthe theprimary primarydatasets datasetsthat thatare areimportant important GIs datasets; for predicting deposits; and and 3) Integration predicting lode-gold lode-gold deposits; Integration of the the preliminary preliminary maps maps using using GIS GIS modeling modeling techniques techniques that that predict predict mineral mineralpotential. potential. 47 �Build Build Collect Collect Spatial Spatial Inpul Data and Input Data Spatial Spatial into into GIS GIs Database Extraction Extractionof of GIS GIS Features Features Relevant Relevan to to ±e theVMS VMS Ore Ore Deposit Deposit Model Model . . sTEP 1 • S MAPPED BEDROCK 13EDROCK GEOLOGY GEOLOUY ALTERATION Al.Tf-RATION ZONES ZONES ROCK, ROCK.CORE, CORE.SOIL SOIL REGIONAL REGIONAL & LAKE MAGNETICS A LAKk SEDIMENT SEDIMENT MAGNI-IlCS GEOCHEMISTRY GEOCHEMISTRY . AIRBORNE & GROUND EM GROUNDFM CONDUCTORS CONDUCTORS ————t——t——±—H— , II . ~Reclaatij e~la~~ify 2 Extract 2. EM contacts con3. Create Cteate buffers buflrs I. I. ~Classi . ~ExtractAitTypes x t r a c t A l ~1~classifyinterval~ ~ ~ intervals 2. Create buffers 2. Grid soil & lakes 3. Buffer rock Ctassifr 3. Classify rock & & core core II. .CompileTF C . & T FMae M ~ ~1.IExtractEM .E~~~~~~EM 2. Reduce pole Claaai' Reduce to topole 2. Classify 3. Calculate Calculate 2VD 2VD 4. Extract Polygona ExtractPolygons STEP 22 STEP Data Data Processing roccssiog Preliminary Preliminary Maps Maps of of Selected GIs GIS Selected Features Features contacts contacts 1 Quattz- contacts contacts Epedote Volcanic-Sedsment contacts Greenstone Subvolcanic Sequences Inmiatotia Msnerattzed Intxut,ves * -Na20 ChIotite Pyrite Cu Zn Soila Cu Zn Lake Cu Zn Rocks Core 2"VD Ground Aithonse Mag EM EM Assign Weights Weights Assign Evidence of Evidence Fair weights to - I) Fuzzyweighs(0-1) Map Overlay MapOverlay STEP 3 STEP Integration itegration Modelling toddling to Produce Produce to Intermediate Intermediate Factor Maps 1 Overlay of Intermediate Intermediate Factor Maps to Produce Produce the Final Predictive Predictive Map 1 1 STRATIGRAPHY STRATIGRAPHY HEAT HEAT Factor Factor Factor Factor 1 ALTERATION ALTERATION Factor Factor rILiTt 1 GEOCHEMICAL GEOCHEMICAL Factor Factor VVMS M S MINERAL POTENTIAL MAP 1 MAGNETIC Factor Factor  EM Factor 0.20 VMSTARGETSI TARGETS !____rIVMS GIS modeling of the VMS potential of the mineral mineral potential of of Figure 22 Inference network for the 3-step GIs the Archean Vermilion greenstone belt. belt, Northeastern, Northeastern, Minnesota. Minnesota. The The three three steps steps in in the thestudy study are: 1) 1) Detailed Detailed geologic geologic mapping mapping and compilation compilation of of all all available available geological, geological,geochemical, geochemical,and and geophysical data. data. Conversion Conversion of of all all data data into into primary GIS GIs datasets; datasets; 2) Extracting and enhancing the features of the primary VMS deposits; and 3) the primary datasets that are important important for predicting predicting VMS Integration of of the preliminary Integration preliminary maps maps using GIS GIs modeling modeling techniques techniques that predict predict mineral mineral potential. Unlike similar greenstone belts of the Canadian Shield in Canada, the Vermilion greenstone belt Minnesota remains remainsunder-explored. under-explored. One One of the main reasons reasons that the area has of northeastern northeastern Minnesota seen little mineral exploration was was due due to to the the discovery of of the the great great iron iron deposits in in the the 1870's. Iron companies acquired great tracks of land and essentially prevented precious and base land and essentially prevented precious and base metal metal exploration for over a century. Recent Recentmapping mappinghas hasshown shownthe thepresence presenceof ofvery verylarge-scale large-scaleVMS VMS alteration systems (> 19 19km km strike strikelength lengthofofsemi-conformable semi-conformable quartz-epidote quartz-epidote alteration) alteration) and and gold-mineralized shear systems analogous to Archean gold camps camps in Canada. The mineral potential potential maps maps integrate integrateimportant important geological, geological, geochemical, geochemical, and and geophysical geophysical data data mapped and compiled by the author with ore deposit models. The Theore oredeposit depositmodels modelshave havebeen been largely developed developed from from the the study study of mines within the Canadian Shield and western Austrailia. largely Areas on the the maps maps with with high high modeled modeled VMS VMS and and lode-gold lode-gold mineral mineral potential potential should shouldbe beexamined examined in detail by or in by exploration exploration geologists geologists because because they they possess possess geologic geologic features features indicative indicative of ofmaj major districts. mining districts. 48 �Samuels Lake Intrusion: Intrusion: A PGE-bearing Quetico-type Intrusion Intrusion in the Central, Western Western Quetico Quetico Central, Subprovince, Northwestern Ontario Ontario •I 2 Keiko H. ~ Hattori Neil T. Pettigrew pettigrewl,, Kbiko a t t o r iand and ' John John A. Percival perciva12 I I. Ont Canada, 1. Dept Dept.of ofEarth EarthSciences, Sciences, University University of Ottawa, Ont. Canada,KIN KIN 6N5 6N5 Survey of of Canada, Canada, Ont. Canada, Canada, KIA 0E8 2. Geological Survey OE8 An array of mafic-ultramafic mafic-ultramafic bodies known as the Quetico Quetico intrusions intrusions occur occur along alongthe the northern boundary of the Quetico Quetico subprovince subprovince adjacent adjacent to the Wabigoon Wabigoon belt (Fig. (Fig. 1). 1). This This study study identified two previously has identified previously unrecognised unrecognised Quetico-type Quetico-type intrusions, intrusions, the Samuels SamuelsLake Lake and and Redhorse Lake intrusions, intrusions, in the central central western Quetico Quetico subprovince subprovince (Fig. (Fig. 1). 1). The The Redhorse Redhorse the westernmost westernmost identified identified example example of of this this type type of of Lake intrusion near the US-Canada border is the intrusion. intrusion. The Quetico Quetico intrusions intrusions characteristically characteristically contain contain hornblendite, hornblendite, clinopyroxenite, clinopyroxenite, minor minor appinite and are accompanied appinite accompanied by Cu-Ni-PGE mineralization. mineralization. Larger intrusions intrusionsalso also contain contain wherlite, gabbro, diorite diorite and syenite. syenite. They are distinguished distinguished from from the mafic-ultramafic mafic-ultramafic intrusions intrusions in the Wabigoon Wabigoon and Wawa Wawa greenstone greenstone belts, such such as as the Bad Vermilion Vermilion Lake, Lake, Grassy Grassy Portage, Portage, and Shebandowan Shebandowan complexes, complexes, by their their lack lack of of associated associated komatiities komatiities and and anorthosites. anorthosites. The 1999 1999 field field season season consisted consisted of reconnaissance reconnaissance work on on the Quetico Quetico intrusions intrusionsalong along the subprovince subprovince boundary boundary and alkaline alkaline intrusions intrusions surrounding surrounding the Samuels SamuelsLake Lake intrusion. intrusion. Grid Grid mapping and sampling were conducted on the Samuels Lake intrusion which was chosen for sampling conducted Samuels Lake intrusion which was chosen for detailed work due to recent recent exploration exploration activity activity and a lack of previous previous work. The Samuels Lake intrusion has an NE-SW elliptical form, displaying Samuels Lake intrusion an elliptical displaying weak weak concentric concentric zoning with a wehrlite wehrlite core core partially surrounded surrounded by hornblendite hornblendite and and completely completely surrounded surrounded by by marginal clinopyroxenite. clinopyroxenite. These These units units are are cut cut by small small dikes dikes and and plugs plugs of of diorite. diorite. The The wehrlite wehrlite and clinopyroxenite clinopyroxenite phases show show aa gradational gradational change change in modal modal abundance abundance of of constituent constituent and clinopyroxene clinopyroxene (Mg# (Mg# =0.83) =0.83) with with minerals. The wehrlite is composed primarily of olivine and minor phlogopite. Sulphides % Sulphides(pyrrhotite, (pyrrhotite,chalcopyrite chalcopyriteand and pentlandite) pentlandite) constitute constitute up up to to 30 30 vol vol % with olivine olivineand andclinopyroxne clinopyroxne suggest suggest the the separation separation of of . The intergrowth textures of sulphides with inuniscible (<0.06 immisciblesulphide sulphide melt melt from fromthe the parental parental silicate silicate magma m a m a .. Low Ni f< 0.06 wt%) wt%)in in olivine suggests suggests sulphide sulphide saturation saturation prior prior or or during during olivine olivine crystallization. crystallization. The The clinopyroxenite clinopyroxenite contains contains clinopyroxene (Mg#= 0.84) variable amounts of olivine (Fo76) and minor phlogopite with clinopyroxene (Mg#= variable amounts of olivine (Fojyj) and minor phlogopite with trace trace sulphides. sulphides. Homblendite Hornblendite isis composed composedof of euhedral euhedralpargasite pargasite to to magnesio-hornblende, magnesio-hornblende,phlogopitic phlogopitic biotite, and plagioclase plagioclase with with trace trace titanite, titanite, pyrrhotite, pyrrhotite, chalcopyrite, chalcopyrite, and and ilmenite. ilmenite. Diorite Dioriteconsists consists of plagioclase, magnesio-hornblende, biotite, quartz, and microcline with trace pyrrhotite, magnesia-hornblende, biotite, quartz, and microcline with trace pyrrhotite, chalcopyrite, ilmenite, magnetite, ilmenite, magnetite, titanite, titanite, and and zircon. zircon. .. intrusion has undergone The Samuels Samuels Lake ~akeintrusion undergone high temperature, deuteric deuteric processes processes and and lowlowtemperature temnerature retrograde retrograde alteration. alteration. The The deuteric deuteric phase chase resulted resulted in in the the formation formation of of hornblende hornblende after clinopyroxene, clinopyroxene, and andthe theformation formationof oflow-Ti low-Timagnetite. magnetite.ItIt may may have have been been accompanied accompanied by by serpentinization of olivine. temperature alteration Low temperature alteration produced tremolite-actinolite tremolite-actinolite alteration of homblende, hornblende, chlorite after hornblende homblende and biotite, and carbonate alteration. Alteration style and biotite, and carbonate alteration. Alteration style and and intensity intensity vary vary between between phases phases and and appear appear to to partially partially control control Cu-PGE Cu-PGE mineralization. mineralization.In In the the clinopyroxenite clinopyroxenite - . - 49 �phase, Cu-POE Cu-PGE mineralization mineralization occurs occurs in areas areas of intense intense alteration alteration of the border border phase phase with with associated associated chalcopyrite-rich chalcopyrite-rich blebs, inter-fingered with actinolite actinolite and calcite. In wehrlite, wehrlite, high high levels of POE mineralization occur in chalcopyrite-rich zones instead of pentlandite-pyrrhotite levels PGE mineralization occur chalcopyrite-rich pentlandite-pyrrhotite rich zones. Small veinlets in wehrlite wehrlite may contain significant significant amounts amounts of of apatite apatite and and Small chalcopyrite chalcopyrite veinlets with elevated POE contents. POE mineralization in the wehrlite phase is associated with the elevated PGE contents. PGE mineralization associated with the immiscible immiscible sulphide sulphide liquid, liquid, indicating indicating a magmatic origin. In the clinopyroxenite clinopyroxenite border border phase, phase, POE mineralization PGE mineralization is is associated associated with apatite, carbonate carbonate and Cu enrichment enrichment suggesting suggestingthat that volatile-rich PGEs. volatile-rich fluid, fluid, most most likely likely deuteric deuteric in origin, origin, were responsible responsible for for concentration concentration of of PGEs. The The Quetico-type Quetico-type intrusions intrusions may not be restricted restricted to the Quetico Quetico belt. The The east-west east-west trending array array of of intrusions intrusions may be extended extended eastward into the Wabigoon Wabigoon subprovince subprovince to to include include Lac Des Milles Lac, Demar Lake, Lake, Taman Taman Lake, Legris Legris Lake, Lac Des Iles, Iles, and and Tip Tip Oabbro Gabbro mafic-ultramafic 1). These These intrusions intrusions have similar similar lithological lithological assemblages assemblages to to the the mafic-ultramafic intrusions intrusions (Fig. (Fig. 1). Quetico Quetico intrusions. intrusions. The The mineralization mineralization observed in Samuels Samuels Lake and other other Quetico Quetico intrusions intrusions are also also similar similar to to that that in in the the clinopyroxenite clinopyroxenitephase phaseof ofthe theRoby Robyzone zoneininthe the2692 2692+41-2 +4/-2 Ma Ma Lac Lac Des Ties mafic-ultramafic complex. If this correlation is supported by geochronology lies mafic-ultramafic geochronology in in progress, progress, it will establish 2692 Ma. establish that that the the Quetico Quetico and and Wabigoon Wabigoon subprovinces subprovinces were proximal proximal by 2692 Figure Figure1.1.Simplified Simplifiedgeological geologicalmap mapof ofWestern WesternWabigoon, Wabigoon, Quetico, Quetico, and Wawa subprovinces. Solid, finely dashed line outlines the distribution of similar subprovinces. Solid, finely dashed line outlines distribution similar mafic-ultramafic mafic-ultramafic intrusions intrusions across across the the Quetico-Wabigoon Quetico-Wabigoonsubprovince subprovince boundary. 50 �PETROGENESIS PETROGENESISOF OFTHE THEENIGMATIC ENIGMATICAURORA AURORASILL, SILL, MESABI MESABI RANGE, RANGE, MINNESOTA MINNESOTA PHILLIPS, PHILLIPS,Erin ErinH. H.(ephillips@macalestetedu); (ephillips@macalester.edu);Wirth, Wirth,Karl KarlR., R.,Geology GeologyDepartment, Department, Macalester Macalester St. Paul, Paul, MN MN 55105 and Morey, G.B., Minnesota Geological 55105 (wirth@macalesteiiedu); (wirth@macalester.edu); and College, St. 2642 University Ave., Ave., St. St. Paul, Paul, MN MN 55114 Survey, 2642 syenitic Aurora Sill intrudes the Biwabii Biwabik Iron-formation Iron-formation of ofthe the Mesabi Mesabi Range Range in in northern northern MmMinThe syenitic nesota. Since the work of White (1954), (1954), the sill sill has has commonly commonly been assigned assigned aa Mesoproterozoic Mesoproterozoic (Keweenawan) because of its resemblance to "red rocks" associated associated with the Midcontinent Midcontinent Rift in the (Keweenawan) age because Duluth region. The Theage ageof ofthe thesill sillisisimportant important to to understanding understanding the the timing timing of of ore ore formation formation in in the Mesabi Mesabi Range. In In1999, 1999,Morey Moreyproposed proposedaaconceptual conceptualmodel model in in which which the the high-grade high-grade ores ores formed formed in in aa regional regional ground-water Strandlie (1999) argued that ground-water system system during during Paleoproterozoic Paleoproterozoic time. Subsequently, Subsequently,Graber Graber and Strandlie nearby appears nearby high high grade grade ores ores must must have have formed formed in Mesoproterozoic Mesoproterozoic or or later later time, because the the Aurora Sill appears to have controlled controlledore-forming ore-forming processes processes in nearby mines. New New data datafrom from this this study study imply imply that that the the Aurora Aurora mineralogically and Sill is quite different, both mineralogically and geochemically, geochemically,from from known known felsic felsic rocks rocks in in the Midcontinent Midcontinent Rift. Thus, Thus,the the age ageof of the the sill sillremains remains uncertain uncertain and the possibility exists that it was intruded soon after deposition of the Biwabik deposition Biwabik Iron-formation Iron-formation in in the the Paleoproterozoic Paleoproterozoic(ca (ca 1878 1878Ma; Ma; Fralick, Fralick,1998). 1998). The Aurora The Aurora Sill Sill extends extends approximately approximately 5.6 5.6 km km along along the the Mesabi Mesabi Range Rangenorth northof ofthe thetown townof ofAurora and ranges in thickness thickness from 6 to 37 m m (White, (White, 1954). The sill was recently exposed exposed during during mining operaoperations in the NW 1/4, NE 1/4, T58N, R15W. RISW. Due to the inaccessibility inaccessibility of the exposures 114,NE 114, sec 3, T58N, ofthe exposuresalong along vertical vertical walls in the mine, samples were collected collected from from large largeblocks blocksalong alongaabench benchimmediately immediatelyabove abovethe thesill. sill. In In all, ten samples samples were studied from this locality along with seven samples from core (16.4-38.1 m in depth) studied locality in see 2, T58N, R15W. R15W. Fine-grained, porphyritic porphyritic syenite syenite from from the the margin margin of of 114, NE 1/4, 114, sec drilled in the NW 1/4, the sill exhibits exhibits trachytic trachytic texture; texture; mediummedium- to to coarse-grained coarse-grained syenite syenite in in the the interior interior of of the sill sill lacks lacks aligned aligned microcrysts (0.2-1.0 (0.2-1.0 mm) mm) plagioclase. Sodic Sodicplagioclase plagioclase(albite) (albite)occurs occurs as as phenoerysts phenocrysts (3.0-7.0 (3.0-7.0 mm) and microciysts Maiic minerals minerals in in finefineand is extensively altered. Mafic ' ' ' ' # ' ' ' ' # ' ' ' ' ' ' ' ' ' I15s grained samples include grained include small small (0.1-0.3 (0.1-0.3 mm) mm) and and euhedral aegirine; euhedral aegirine;fibrous fibrousamphibole amphiboleand andchlorite chloriteare are common in the coarse-grained samples. Other Otherminminerals oxides, Fe sulfide, sulfide, and zirerals are are K-feldspar, K-feldspar, Fe-Ti oxides, 3 ,oI0 con. con. The presence presence of two two feldspars feldspars indicates indicates $ 1 w subsolvus crystallization at at high highPH2@ H2Q 0 Coarse-grained Coarse-grained samples samplescontain contain50 50 modal modal per- S Mideontinent Rift sodic plagioclase plagioclase and modal percent cent sodic and 5-20 5-20 modal percent K-feldK-feld1 Granophyre Complexes Complexes 4: 5 Graaopbyre spar and are classified as syenite syenite and alkali-feldspar alkali-feldspar a? Lester Lester River Sill Sill these rocks insyenite. CIPW CIPW norm norm compositions compositions of ofthese clude 68-82 percent alkali alkali feldspar feldspar (Or (Or ++ Ab), 5-10 Endion Sill sill o 4o , , , , , , , , , , I , , , , percent plagioclase (An), and as much as 5 percent 50 60 70 SO 60 70 80 nepheline; they are are classified classified as as nepheline-bearing nepheline-bearing Si02 %) Sio, (wt. %) alkali syenite or or nepheline-bearing nepheline-bearing syenite. syenite. FineFinegrained samples from along the margin ofthe sill congrained from margin ofthe con- Figure Figure1. 1 Na20+K20 NaO+K20(total (totalalkalis) alkalis) versus Si02 SiO comparwmpartain 74-84 percent normative alkali feldspar (Or + + ing Aurora Sill rocks rocks with with those ofthe of the Endion Endion Sill, Lester Lester (<4%) normative quartz quartz and River Sill (Green, 1972; JJerde, Ab) with small amounts (<40h)n0rmative 1991), Midwntinent Midcontinent Rift d e , 1991), are classified classified as alkali alkali feldspar feldspar syenite. granophyre complexes complexes (Kennedy (Kennedy et al., this volume), volume), and granophyre Samples from from the Aurora Sill exhibit granophyric dikes of the Dunka River region (Harris exhibit little little granophyric (Harris and and percent; Wirth, this this volume). volume). Symbols: inverted geochemical inverted triangles triangles == drill geochemicalvariation variation(SiO2 (SO2= = 54.6-60.7 wt. percent; (open = = fine-grained; Y= = 9-19 Mg# mine (open fine-grained;half-filled half-filled== core; diamonds diamonds == mine 9-19 ppm) suggesting suggesting that that the core; Mg# == 0.40-0.75; Y filled = coarse-grained). = warse-grained). medium-grained; filled medium-grained; intrusion has undergone relatively little intrusion little internal frac- 1.0 6 , 51 �tionation. Al 203 con159 so tionation.All Allofofthe thesamples samplesexhibit exhibithigh highA120i concentrations centrations and andare areperalurninous peraluminous to tometaluminous. metaluminous. There There appears appears to tobe beno nocorrelation correlationbetween betweenstrucstructural tural position position in in the the sill silland andcomposition. com~osition.Marked Marked 00 0 differences differencesexist existininthe theconcentrations concentrationsofofsome someeleelements drillcore coreand and I (e.g., K20 K 2 0and and Si02) SO2) between between drill ments(e.g., mine minesamples samples.Such Suchdifferences differencescould couldreflect reflectdifferdiffer- S-S ent entdegrees degreesof ofalteration alterationororfractionation fractionationalong alongthe the & I length the sill. lengthof ofthe sill.Alternatively, Alternatively,the thedifferences differencesmight might reflect reflectthe thepresence presence of oftwo twoseparate separateintrusions, intrusions, but but this thisisisconsidered consideredunlikely unlikelybecause becausethese thesesample sampletolocalities are well within the 5.6 km expanse of the sill calities are well within the 5.6 km expanse ofthe sill o 0 as 10 15 asdefined definedby by'White White(1954). 0954). "(}"') yhmm) The Aurora Sill is distinct from the felsic com~ h e ~ & o sillis r a distinct from the felsic comFigure2.2.Nb Nbversus v a ~Yplot Yplot s comparing comparingAurora AuroraSill Sillsamples samples plexes plexesofofthe theMidcontinent MidcontinentRift Riftininseveral severalrespects. respects. Figure withanalyses analysesofofMidcontinent MidcontinentRift Riftgranophyre granophyrecomplexes complexes The andquartz with Thesill silllacks lacksthe thegranophyric granophec textures texturesand (Kennedyetetal.5 flmvolume). volume).Symbols SymbolsasasininFigure Figure1.1. al., this that thatare an.typical typicalofoffelsic f&iC rocks r o c kofofthe theMidcontinent Midcontinat (Kennedy $ I - I I Rift. Rift.The Thesill sillisisweakly weaklysaturated saturatedtotoundersaturated undersaturated ininsilica silicaand andcontrasts contrastssharply sharplywith withthe thesilica-saturated silica-saturatedrift-related rift-relatedgranophyres. granophyres. Furthermore Furthermorethe theAurora Aurora Sill Sillisischaracterized characterizedby byhigh hightotal totalalkalis alkalis(Figure (Figure1),I),Nb/Y NbN(Figure (Figure2), 2).and andCe/Zr CeIZrcompared comparedwith withfelsic felsicrocks rocks of ofthe theMidcontinent MidcontinentRift. Rift. The Thesill sillalso alsolacks lacksthe thenegative negativeNb Nbanomalies anomaliesthat that are arecharacteristic characteristicof ofthe the MidcontinentRift Riftgranophyres. granophyres.This Thisproperty propertysuggests suggeststhat thatthe thesill sillwas wasderived derivedfrom fromaamantle mantlemelt meltthat thatdid did Midcontinent Nband andRb, Rb,low-Y low-Ycomposition compositionof ofthe thesill sillisis notinteract interactsignificantly significantly with withcrustal crustalmaterials. materials. The Thehigh highNb not theAnimikie Animildeforelandbasin. foreland basin. typicalofofwithin plategranites m i t e s(Pearce, (Pearce,1984), 1984).consistent consistentwith withemplacement emplacementininthe typical within plate Nd Ndisotope isotopeand andU-Pb U-Pbzircon zirconstudies studiesare areininprogress progressand should provide additional constraints on the petrogenesis genesisand andage ageofofthe theAurora AuroraSill Sill(Phillips, (Phillips,ininprep). prep References ReferencesCited Cited D.W.Davis, Davis,1998, 1998,The TheAge Ageand andProvenance Provenanceofofthe theGunflint Gunflimt LapilliTuff. Tuff.Institute Instituteonon Fralick.P.W., P.W.,S.A. SA.Kissin, Kissin,and and11W. Fralick, Lapilli Lake LakeSuperior SuperiorGeology, Geology,Proceedings Proceedingsand andAbstracts, Abstracts,p.p.66-67. 66-67. Strandlie,1999, 1999,Where Whereace arethe theMetamorphosed MetamorphosedNatural NaturalOrebodies Orebodiesofofthe theMesabi Mesabi Graber,Ronald RonaldG. G.and andAlan AlanI.J.Strandlie, Graber, Range? ononLake 17-19. Range?Institute Institute LakeSuperior SuperiorGeology, Geology,Proceedings Proceedingsand andAbstracts, Abstracts,p.p.17-19. Green,J.C., J.C.,1972, 1972,North NorthShore ShoreVolcanic VolcanicGroup: Group:ininAACentennial CentennialVolume, Volume,Minnesota MinnesotaGeological GeologicalSurvey, Survey,Sims, Sims,P.K. P.K. Green, and andG.B. G.B.Morey, Morey,editors, editors,p.p.294-332. 294-332. andK.R. K.R.Wirth, Wirth,this thisvolume, volume,Petrogeneis Petrogeneisofofgranitic graniticrocks rocksofofthe theDunka DunkaRiver Riverregion, region,eastern easternMesabi Mesabi Harris,E.E.and Harris, Range, Range,Minnesota. Minnesota. Jerde,Eric EricA., A,,1991, 1991,Geochemistry Geochemistry andPetrology PetrologyofofHyuabyssal RocksAssociated Associatedwith withthe theMidcontinent MidcontinentRift, Rift,NorthNorthJerde, and 1-lypabyssal Rocks easternMinnesota. Minnesota.Unpublished Unpublished Ph.D.Thesis, Thesis, UniversityofofCalifornia, California,Los LosAngelas. Augelas. eastern Ph.D. University W i d ,and andJ.J.Vervoort, Vervoort,this thisvolume, volume,Petrogenesis PetrogenesisofofMidcontinent MidcontinentRift RiftGranophyres: Granophyres:EviEviKennedy,Bryan BryanC., C.,K.K.Wirth, Kennedy, dence dencefrom fromgeochemistry geochemistryofofKeweenawan Keweenawangranitic graniticigneous igneouscomplexes complexesofofnorthern northernMinnesota. Minnesota. — Product Morey,G.E., G.B.,1999, 1999.High-Grade High-GradeIron IronOre OreDeposits Depositsofofthe theMesabi MesabiRange, Range,Minnesota MinnesotaProductofofaaContinental-Scale Continental-Scale Morey, Proterozoic 133-142. ProterozoicGround-Water Ground-WaterSystem. System.Economic EconomicGeology, Geology,VV94, 94,p.p.133-142. Pearce,l.A., J.A.,NEW. N.B.W. Harris, Ymdle, 1984, Trace element discrimination diagrams tectonic intetprePearce, Harris, and8ndA.G. AG. Tindle, 1984, Trace element discrimination diagrams forfor thethe tectonic interpretation rocks. Journal of Petrology, V 25, p. 956-983. tationofofgranitic granitic rocks. Journal of Petroloev. V 25. D. 956-983. . , Phillips, Aurora Phillips,Erin ErinH., Hi,ininprep, prep,Petrogenesis Petrogenesisofofthe theEnigmatic Enigmatic AuroraSill, Sill,Mesabi MesabiRange, Range,Minnesota, Minnesota,Unpublished Unpublishedhonors honors thesis, MN. thesis,Macalester MacalesterCollege, College,St.St.Paul, Paul.MN. White, Geological White,David DavidA., A.,1954, 1954,The Stratigraphy Stratigraphyand andStructure Structureofofthe theMesabi MesahiRange, Range,Minnesota. Minnesota.Minnesota Minnesota Geological Survey University ofofMinnesota SurveyBulletin Bulletin38. 38.Minneapolis: Minneapolis:The The University MinnesotaPress, Press.p.n.63-66. 63-66. -. . 52 �I I I Low-grade metamorphic o the Keweenawan Portage Low-grade metamorphic zonation zonation of Lake Lake Volcanics Volcanics on on the the Keweenaw Keweenaw Peninsula, Peninsula, Michigan Michigan U. U. PUESCHNER* PUESCHNER* and and S. S. Th. Th. SCHMIDT SCHMIDT Mineralogisch-Petrographisches Mineralogisch-PetrographischesInstitut Institut Universitaet Universitaet Basel/Switzerland BaseWSwitzerland *UPUESCITNER@l1njbch *U.PUESCHNER@unibas.ch The The metamorphic-hydrothermal metamorphic-hydrothermal alteration and mineral assemblages assemblages of the the Portage Portage Lake Lake Volcanics Volcanics (PLV) (PLV) on onthe theKeweenaw KeweenawPeninsula Peninsulain in Michigan Michigan have been been studied studiedover overthe thelast last - have ten decades due to the occurrence of native copper deposits. Jolly & Smith (1972) ten decades due to the occurrence of native copper deposits. Jolly & Smith (1972) illustrated illustrated for for the the first firsttime timethe theexistence existenceofofa asystematic systematiczeolite-prehnite-pumpellyite zeolite-prehnite-pumpellyite sequence in the PLY. In their Eagle Harbor drill core profile sequence the PLV. In their Eagle Harbor drill core profilethey theyidentified identifiedaazonation zonation from from zeolite zeolite facies facies in in the the upper upper part part of of the thesection sectiontotoaaprehnite-pumpellyite prehnite-pumpellyite facies facies in in stratigraphically lower parts. In this study, three diamond-drill core profiles (Fig. A) A) stratigraphically lower parts. In study, three diamond-drill through through the the PLY PLV were were re-examined re-examined using using aa combination combination of of different different techniques techniques such such as as electron electronmicroprobe microprobeanalysis analysisand and special specialX-ray X-ray powder powder diffraction diffraction techniques. techniques. The Theprofile profile include the Ahmeek, the Copper Harbor, and the Eagle Harbor profile and penetrate penetrate 5000 5000 include the Ahmeek, the Copper Harbor, and the Eagle Harbor m m of of vertical vertical section section in in the the PLY, PLV, and and are are spaced spaced over over aa distance distance of of 50 50kin. km. The Theprofiles profiles were were correlated correlatedusing usingcharacteristic characteristicinterfiow interflowsediments sedimentsand and selected selectedlava lavaflows flowsas asmarker marker horizons. horizons. Macroscopic of 614 samples indicate at least three Macroscopic and microscopic microscopic examination examination of samples indicate three and and possibly possibly up upto tofive fivedifferent differentstages stagesofofalteration alterationbased based on onthe thevarious variousamygdule amygduleinfilling infilling sequences sequences that that can canbe bedifferentiated differentiatedduring duringthe thedeposition depositionand andsubsequent subsequenttilting tiltinghistory history of of the the PLY. PLV. AA vertical vertical as as well well as aslateral lateral zonation zonation is observed observed in the PLY PLV on on the the Keweenaw Keweenaw Peninsula. Peninsula. From From the the approximately approximately southwest southwest to to the the northeast, northeast, three three different different low-grade metamorphic zones can be observed. In the vicinity of the former low-grade metamorphic zones can be observed. In vicinity the former copper copper mining mining region region and andadjacent adjacenttotothe theKeweenaw KeweenawFault, Fault,aapumpellyite-prehnite pumpellyite-prehnitefacies, facies,also also with abundant epidote is observed. Further to the north and stratigraphically below the with abundant epidote is observed. Further to the north and stratigraphically below the Greenstone Greenstone Flow Flow aapumpellyite-prehnite pumpellyite-prehnite facies facies isis present. present. The The flows flows above above the the Greenstone to the the northeast northeast toward toward the the Keweenaw Keweenaw Tip Tip show show aa zeolite zeolite Greenstone Flow and and thrther further to facies facieswith with laumontite, laumontite,phyllosilicates phyllosilicates(mainly (mainlyclinochlore) clinochlore)and andcalcite. calcite. I I I I I I The The alteration alterationof of the the PLY PLV starts startswith withthe thefilling fillingof ofsmall small(<3 (<3 mm) mm) vesicles vesicles and and the the lining lining of 1). During During deposition deposition and and of larger largervesicles vesicleswith withphyllosilicates phyllosilicates(alteration (alterationstage stage 1). subsequent subsequent burial burial metamorphism metamorphism of the the sequence sequence (alteration (alteration stage 2), 2), Section Section CC and andDD (Fig.: A ++ B) B) experienced experienced conditions conditions of ofthe thepumpellyite-prelmite pumpellyite-prehnitefacies facies(assemblage (assemblage (Fig.: A pumpellyite-prehnite-epidote-albite) which the sequence. sequence. The The pumpellyite-prehnite-epidote-albite) which is the highest observed in the beginning beginning thrusting-event thrusting-event of of the the PLY PLV combined combined with with the theformation formationof ofthe theKeweenaw Keweenaw Fault Fault tilts tilts the thePLV. PLV. AAnew newalteration alterationstage stage33with withaafluid fluidprobably probablyoriginating originatingfrom fromthe the Lake LakeSuperior Superiorsyncline synclinenow nowcrosscuts crosscutsand andoverprints overprintsthe the earlier earlierburial burialmetamorphic metamorphicstage stage 53 �2, especially especially in in the the Section SectionAAand andB.B.Afterwards Afterwardsa anew newNINW-SSE NNW-SSE directed directed fracture fracture system was set up on the Keweenaw Peninsula. Along these fracture zones a hyper salinar ore containing containing fluid deposited deposited in open open pore pore space space quartz, quartz, native native copper copper and and calcite calcite conditions similar to the the temperature temperature conditions conditions of of alteration alteration (alteration stage 4) under conditions stage 3. The low-grade history of of the the Keweenaw Peninsula ends ends with with a low-grade metamorphic metamorphic history Keweenaw Peninsula meteoric, cold and low salinity fluid that precipitates the final calcite and late zeolites in veins veins and and still still open open amygdules amygdules(alteration (alterationstage stage5). 5). Differentiation of of Jolly, W. T. a. a. S., S., Raymond Raymond E., E., 1972, 1972, Degradation Degradation and Metamorphic Metamorphic Differentiation the Keweenawan Journal of the Keweenawan Tholeiitie Tholeiitie Lavas Lavas of of Northern Northern Michigan, Michigan, U.S.A.: U.S.A.: Journal Petrology, v. 13, 13, p. p. 273-309. 273-309. Paces, 1988, Magmatic Magmatic processes, processes, evolution and and mantle mantle source characteristics characteristics Paces, contributing to the petrogenesis contributing to petrogenesis of the the Midcontinent Midcontinent Rift basalts: basalts: Portage Portage Lake Lake Volcanics, Keweenaw Peninsula, Michigan [Ph.D. thesis]: Michigan Michigan Technological Technological University, 413p. 4l3p. 54 �A: SW NE [m] -o Top of Portage Lake Volcanics -1000 Hancock Congi. Calumet & Hecla Congl. - 2000 3000 Bohemia Congi. - 4000 Keweenaw Fault - 5000 0 n n. Portage Portage Lake Lake Volcanics Volcanics b q 1- 1 I—-i [ 1 1 1 D57 1 I D57I bigger single flows in in the the PLV PLV Interflow sediments used as marker horizons Interflow horizons other interfiow interflow sediments sediments with with in inthe the PLV PLV sampled drill cores Figure: A: :Back ~ ~ a rotated c k stratigraphic cross-section cross-sectionof of the the Portage PortageLake LakeVolcanics Volcanics (PLV) (PLV) along along strike strike from from Houghton in the southwest to the Keweenaw Point in the the northeast. With With the the sampled sampled drill cores Houghton appropriate marker and appropriate marker horizons. horizons. (Modified (Modifiedfrom from Paces Paces1988) 1988) Simplified sketch of cross-section B: Simplified cross-sectionfor easy easy reference. reference. 55 55 �Modeling Modeling the effect of intermediate intermediate temperature temperature(350°-500°C) (350'-500°C Mazatzal-age Mazatzal-age thermal thermal overprinting on on argon argon diffusion in hornblende from the southern Lake Superior Superior region. region. RAMPE, RAMPE, J.S., and and HOLM, HOLM, D.K., D.K., Dept. Dept. of of Geology, Geology, Kent Kent State State University, University,Kent, Kent,OH OH44242 44242 Mica Ar/Ar ArIAr ages from the southern Lake Superior region leave little question that that much much of of the the Penokean Penokean orogenic belt was thermally and deformationalty deformationally overprinted by Mazatzal (ca. (ca. 1650 1650 Ma) orogenic activity to the south (Holm et al., 1998; 1998; Romano et al., 2000; Loofboro Loofboro and and HoIm, Holm, 1998). Complete Complete resetting resetting of of mica mica argon argon systematics systematics from from bedrock bedrock samples samples beneath beneath 1750-1630 1750-1630 Ma deformed heating to temperatures deformed quartzites quartzites suggests suggests widespread widespread heating temperatures above 350°C 350° during during Mazatzal-related metamorphism. Hornblende Ar/Ar ages ages from within the Mazatzal-related metamorphism. Hornblende ArIAr the deformed deformed region region (Romano, 2000) are predominantly (Romano, 1999; Mancuso, Mancuso, 2000) predominantly younger younger than the the Penokean Penokean Orogeny, Orogeny, scattering from 1800-1638 1800-1638 Ma (Fig. (Fig. 1). 1). Three of of the the hornblende hornblende separates separates analyzed analyzed were were completely completely reset, probably by localized localized hydrothermal fluid-related activity associated with the the Mazatzal postRomano et et al. al. (2000) (2000) have have postulated postulated that the the scatter scatter of of the the other other postMazatzal Orogeny. Orogeny. Romano Penokean Penokean hornblende hornblende ages ages (over (over aa 70 70 m.y. m y . interval) interval) reflect reflect variable variable retention retention of of radiogenic radiogenic argon argon associated associated with episodic episodic loss loss after after initial initial closure closure during during Penokean Penokean time. time. In In order order to to test test this this hypothesis, hypothesis, we we have have modeled modeled various various thermal thermal histories histories by by imposing imposing peak peak temperature temperature excursions excursions from over orogenic orogenic timescales timescales (20-80 (20-80 my.). m.y.). The from 350°-600°C 350'-600° over The effectiveness effectiveness of diffusion diffusion in in resetting ArIAr ages is primarily a function of the the peak peak temperature temperature and its its duration; duration; the the model model resetting Ar/Ar results results are are fairly insensitive insensitive to choice of initial and final conditions (Lootboro (Loofboro and and Holm, Holm, 1998). 1998). The suggests that the distribution of hornblende Ar/Ar ArIAr ages ages (Fig. (Fig. The results of modeling modeling (Fig. (Fig. 2) suggests 1) 1) is consistent with the the partial partial resetting resetting of of argon argon systematics systematics at at ca. ca. 1650 1650Ma Maby bythermal thermalpulses pulses reaching temperatures between between 350' 350° and and 460°C 460°C. A recent petrologic investigation of reaching maximum temperatures the the deformed deformed Baraboo Baraboo quartzite quartzite indicates indicates metamorphic metamorphic temperatures temperatures of of 320°-390°C 320'-39O0C (Medaris (Medarisetet al., 1998). 1998). We Wedo donot notconsider considerititimprobable, improbable, therefore, therefore, that that bedrock bedrock regions regions underlying underlying the the deformed deformed quartzites quartzites reached reached even higher higher temperatures temperatures (between (between 400° 400' and and 460°C) 460°Cduring during Mazatzal-age Mazatzal-age overprinting. overprinting. The Themodeling modelingsuggests suggeststhat thataacombination combinationofofelevated elevatedtemperatures temperatures (350°-460°C (350'-460° with with gradients gradients in inTmax) Tmax)and and localized localized fluid-activity fluid-activity associated associated with withMazatzal Mazatzal deformation deformation provide provide the simplist simplist explanation explanation for preservation preservation of of the the scattered scatteredPenokean, Penokean, intermediate, intermediate, and and Mazatzal Mazatzal hornblende hornblende age agedata data(Fig. (Fig.1). 1). We We present present aa conceptual conceptual model model for for Mazatzal Mazatzal orogenic orogenic overprinting overprinting of of the thesouthern southernLake Lake Superior Superior region (Fig. 3). The Thegeometry geometrywe wedepict depictfor forcollision collisionof ofthe theMazatzal Mazatzalprovince province isisinin concert concert with with that that recently recently proposed proposed for for Mazatzal Mazatzal collision collision in in the thesouthwest southwestU.S.A. U.S.A. (Selverstone (Selverstone et al., 1999) 1999)and and accounts accounts for for southward southward vergence vergence of of folds folds in in the the Baraboo Baraboo quartzite quartzite (Dalziel (Dalziel and and et al., Dott, 1970). 1970). Our Ourresults resultscorroborate corroboratethe thehypothesis hypothesisproposed proposed nearly nearly 20 20years years ago agoby by Dott Dott(1983) (1983) Dott, that that the the severity severity of of quartzite quartzite deformation deformation in in Wisconsin Wisconsin was was related related to tocollision collisionfrom fromthe thesouth southatat —1630 -1630 Ma. Ma. References References Dalziel, Wisc.Geol. Geol.and andNat. Nat.Hist. Hist.Surv. Surv.Inf. Inf.Circ. Circ.14, 14,164 164p.p. Dalziel, I.W.D., I.W.D., and and Dott, Dott, R.H. R.H.,Jr., Jr.,1970, 1970,Wisc. Dott, Dott, R.H., R.H., Jr., Jr., 1983, 1983,Geo. Geo.Soc. Soc.Am. Am. Mem. Mem. 160, 160,129-141. 129-141. Holm, D.K., D.K., Schneider, Schneider,D., D., and and Coath, Coath,C.D., C.D., 1998, 1998,Geology Geology 26, 26,907-910. 907-910. Hoim, Loofboro, 1-82. Loofboro, J.J. and andHolm, Holm,D.K., D.K.,1998, 1998,Inst. Inst.Lake LakeSuper. Super.Geo. Geo.44, 44,881-82. Mancuso, C., 2000, 2000, Kent Kent State StateUniversity University MS MS thesis, thesis,97 97p. p. Mancuso, C., Medaris, L.G., L.G., Jr., Jr., Brown, Brown, PB., P.B.,and andBunge, Bunge,R.J., R.J.,1998, 1998,Inst. Inst.Lake LakeSuper. Super.Geo. Geo.44, 44,89-90. 89-90. Medaris, Romano, Romano, D., D., 1999, 1999,Kent KentState StateUniversity University MS MSthesis, thesis,92 92p.p. K.A., 2000, 2000, Precambrian Precambrian Geology Geology(in (inpress). press). Romano, D., D., HoIm, Holm, D.K., D.K., and and Foland, Poland, K.A., Romano, Selverstone, J., J., Pun, Pun, A., A,, and and Condie, Condie, K.C., K.C., 1999, 1999, Geol. Geol. Soc. Soc. Am. Bull. 111,590-606. I l l , 590-606. Selverstone, 56 �Partial resetting Partial resetting associated associated with with Mazatzal Mazatzal deformation deformation A> i 0'/ dBiW3-0 a Y 7 -'9Q Inu Li Penokean metamorphism /\ Penokean metamorphism (not reset) reset) (not \ c' 500 500 6- complete resetting Locally complete resetting associated associated with with Mazatzal Mazatzal deformation deformation 'S \ Age of Age of metamorphism metamorphism \ \ 2 \ \ a E C) H 4 /1 v 100• 100 25 25 -- 'L \ 300 0 and deformation of quartzites Toward localized localized younger — — D Toward resetting associated associated resetting with with 1100 1100Ma Ma rifting rifting 9 I , 1850 1850 I I P P 1550 1650 1650 1550 Age Age (Ma) (Ma) south of of the the Mazatzal-age Figure 1. Summary Summary of from bedrock bedrock south Figure 1. of hornblende hornblendeAr/Ar ArIAr dates dates from Mazatzal-agethermal thermal front front Romano et al. (after Flomano, Romano, 1999, 1999, and and Mancuso, Mancuso,2000). 2000).Temperature-time Temperature-timepath pathmodified modifiedafter after Romano eta\. (after (2000). Note gap gap in spread of ofhornblende hornblendedates datesbetween between 1733 1733 and and 1646 1646 Ma. (2000). Note in spread Ma. 1850 1750 1750 0 Hornblende — .5 (C S 1750 20 Ma excursion intervals C) 0) S (0 0 PC 0) 1650 80 Ma excursion intervals C S S 5'—— mica ages 2 PC C. C. araooo — < 1550 metamorphism metamorphism I I I I I I I I I I 600 400 500 200 300 200 300 400 500 600 Maximum thermal excursion excursion Maximum temperature reached during thermal (°C) PC) Figure 2. Results Resultsof of modeling modelingaa Mazatzal-age Mazatzal-age (1650-1600 (1650-1600 Ma) thermal overprinting on minerals which the Penokean orogeny orogeny (1850-1830 (1850-1830Ma). Ma). Dashed Dashed curves show effect on initially cooled shortly after the apparent age data for for 20 m.y. m.y. temperature temperature excursions excursionsat atvarious variouspeak peaktemperatures. temperatures. Solid lines for 80 m.y. m.y. excursions. Squares Squaresrepresent represent hornblende hornblende age age data data as in figure 1. Stippled Stippledregion regiondepicts depictsthe thetemperature temperature range necessary to thermally reset reset hornblende hornblende to the degree degree indicated indicated by the the hornblende hornblendeage age data. data. N undeformed Barron undeformed Barren quartzite \ quartzite deformed quartzites sS 1650-1630 0 Ma Ma Mazatzal Orogeny Orogeny C Figure 3. Conceptual Conceptualmodel model(no (no scale) scale) for the Mazatzal orogeny and andits its effects effectsin inthe the southern southernLake Lake Superior region. Modified Modifiedafter after Romano Romano et al., 2000. Collision Collisioncauses causescrustal-scale crustal-scaledeformation, deformation, south-verging of the the Penokean orogenic orogenic belt. belt. CM is continental south-vergingfolds, and heating of much of continental margin, MT is Marshfield Marshfield terrane, MP is Mazatzal Mazatzai province, NFZ is Niagara Niagara fault zone, WMT WMT is is Wisconsin Wisconsin magmatic terrane. magmatic 57 �THE 'SANDOR' 'SANDOR' DIAMOND DIAMOND OCCURRENCE, OCCURRENCE, MICHIPICOTEN MICHIPICOTEN GREENSTONE GREENSTONEBELT, BELT, WAWA, WAWA, preliminary study) ONTARIO study) ONTARIO ( a preliminary SAGE, R.P., R.P., Ontario Geological Survey, Survey, 933 933 Ramsey Lake Road, Sudbury, Sudbury, Ontario, P3E 6B5 (705-670SAGE, 5949; ron.sage@ndm.gov.on.ca) ron.sagendm.gov.on.ca) The "Sandor" diamond occurrence is is the first discovery of a bedrock diamond source in the Michipicoten region since since veteran prospector "Mickey" "Mickey" Clement reported in 1993 1993 the presence of alluvial diamond diamond in the the Michipicoten River to the Ontario Geological Survey. River to the Ontario Geological Survey. A xenolith-bearing lamprophyre dike Surmacz and and Geologist Geologist dike was observed by veteran prospector Sandor Sumiacz Marcefle Marcelle Hauseux along highway 17 17 in Lalibert Township north of Wawa, Ontario on June 22, 1993 1993 (S. ( S. Surmacz, Prospector, personal communication, 1997). area 1997). Exploratory work was being conducted in the area under an Ontario Prospector Assistance Program (OPAP) grant. Diamond indicators were not found but a sample for geochemical analysis returned results interpreted as unfavorable for the presence of diamond diamond (S. Prospector, personal personal communication, communication,1997). 1997). Surmacz, Prospector, Surmacz resampled the the Wawa Wawa dike dike in in 1995. The The sample was was sent to the Saskatchewan Surmacz and and 1-lauseux Hauseux resampled Research Council's diamond recovery laboratory and 6 gem diamonds recovered (press release, KWG Resources Inc., June 6, 1996; 1996; S. S. Surmacz, Sunnacz, Prospector, personal communication, 1997). 1997). The ground was claimed from the Algoma Central Railroad and optioned to Spider Resources Inc. (S. Surmacz, Prospector, 1997). personal communication, 1997). The occurrence represents a unique discovery  42 Ma, L. Heaman, discovery of diamond in Archean (2703 ± University of Alberta, unpublished data) rocks of non-kimberlite composition. The dikes are commonly data) non-kiiberlite of characterized by rounded mafic xenoliths up to 11 metre in size set in a fme fine grained groundmass. The The xenoliths are commonly widely spaced spaced throughout the lamprophyre dike but hut in some dikes the xenoliths may be densely packed. The The xenoliths weather in in positive relief and commonly are enveloped in a dark biotite-rich reaction rim up up to to several several centimeters centimeters in in width. The dikes have been subjected to regional metamorphism and tectonism and actinolite-talc-carbonateassemblage. Primary and the xenoliths altered altered to an actinolite-talc-carbonate mineralogy and texture are are not preserved in in the matrix or xenoliths. Geochemically the dikes dikes approximate aa basalt or or mildly alkalic basalt magma and the xenoliths pyroxenite. These diamoniferous dikes dikes are are concentrated concentrated in in the northwest-central region of the Michipicoten Greenstone Belt largely restricted to to aa synformal anticline anticline centered in Lalibert Township. The dikes are concentrated near and east of of the Dickenson Dickenson Lake Lake syenite syenite stock, but a genetic relationship between the stock and dikes dikes remains to be firmly firmly established. established. The The dikes are are both conformable and crosscutting to the deformed supracrustal supracmstal stratigraphy. The The lamprophyre lamprophyre dikes are are up to 5 m in width and cannot be traced any great great distance in the bush % of of the bush covered terrain. Diamond Diamond is is distributed erratically in the dikes and only 10 10 % dikes have yielded diamond. diamond. The dikes likely source source for for dikes do do not contain contain mineral mineral compositions compositions characteristic of kimberlite and are not a likely the kimberlite kiiberlite indicator indicator minerals found found in in the region. Geophysical Geophysical methods have proven ineffective in in locating the dikes. The presence of actinolite and anomlous Cr and Ni contents in the soils are the most effective prospecting techniques. 58 �Lithogeochemistry Lithogeochemistry and and Paleotectonic Paleotectonic Setting Setting of the Bend Northern Wisconsin Massive Sulfide Deposit, Northern Klaus J. Schulz Nicholson Schulz and Suzanne W. Nicholsov U.S. Geological Geological Survey Survey 954 National National Center Center Reston, VA 20192 20192 The Bend massive sulfide sulfide deposit deposit is is located located in in Taylor Taylor County, County, Wisconsin, Wisconsin, approximately approximately miles north-northwest north-northwest of Medford, Medford, in in the the Chequamegon ChequamegonNational NationalForest. Forest. Exploration 19 miles conducted between 1985 1985and and 1994 1994by by the the former former Jump Jump River River Venture, Venture, and and more more recently recently by by Sharpe Sharpe Energy Energy and and Resources, Resources, identified identified 3.9 3.9 million million short short tons tons of of ore ore grading grading 1.87% 1.87% copper, copper, 0.09 0.09 opt gold, and 0.39 opt silver (DeMatties and Rowell, RowelI, 1996). We conducted a geochemical study conducted of the volcanic section section hosting hosting the the massive massive sulfide sulfidedeposit deposit using using 90 90 samples samplescollected collected from from drill drill core. The (e.g., REE, REE, Zr, Hf, Ta, Ti) to Thestudy studyemphasized emphasizedinterpretation interpretationof ofstable stabletrace traceelements elements(e.g., help determine determine primary primary magmatic magmaticaffinities affinitiesand and the the tectonic tectonic setting settingof of the the volcanic volcanic section section hosting the Bend deposit. deposit. massive sulfide The Bend massive sulfide deposit deposit occurs occurs in a felsic felsic volcanic-dominated volcanic-dominatedsequence, sequence,located located along the southern flank of a mafic volcanic succession (DeMatties, 1994). The felsic volcanic (TDeMatties. 19941. alone sequence is up to 2000 feet thick, steeply dipping, south facing, andlow&greenschist and lower greenschist facies. facies. The stratigraphic footwall consists of bedded felsic tuffs tiffs (sericite stratieraohic (sericite schist) schist) with with interbedded interbedded quartztiffs (quartz-sericite crystal tuffs (quartz-sericiteschist) schist)overlain overlain(stratigraphic (stratigraphichangingwall) hangingwall)by mafic rnafic to felsic feisic fine tuffs tiffs and sediments. feldspar-phyric lava flows, felsic volcaniclastic layers, and associated fine sediments. Alteration, consisting sericitization, and and sulfidation, sulfidation, is is most most intense intense near near the the Alteration, consisting of of silicification, silicification,sericitization, massive sulfide, particularly in the footwall sericite and quartz-sericite schist. Mafic Mafic to to intermediate dikes intermediate dikes and and sills sills (?) (?) are are present present locally. locally. The Bend volcanic sequence volcanic sequence ranges ranges from from basalt basalt through through andesite andesite to to dacite-rhyodacite dacite-rhyodacite with dacite-rhyodacite most abundant. Both Boththe the mafic mafic and and felsic felsic rocks rocks are are calc-alkaline calc-alkaline in FeOt and and high high La/Yb LdYb (>15) (>IS) and and Th/Yb Th/Yb (>0.65) ratios (i.e., Barrett and character with decreasing decreasingFeOt MacLean, 1999). 1999). Mafic Maficrocks, rocks,mostly mostlyandesite andesitewith with minor minor basalt, basalt, are are characterized characterized by enriched enriched 6), relatively flat heavy-REE, prominent depletion of Ta, Zr, Hf, and Ti, ([La/Yb]n 6), relatively flat prominent depletion light-REE light-REE ([LaIYb]n and enriched Th on a primitive primitive mantle mantle normalized normalizedplot plot(Fig. (Fig.la). la). Relatively unaltered Relatively hangingwall felsic hangingwall felsic volcanic volcanic rocks rocks partly partly overlap overlapcompositionally compositionallywith with the the most most evolved evolved andesites andesites 3-4 vs -2-3 --2-3 for the andesites) light-REE(i.e., (i.e.,[La/Sm}n [LaISmln— 3-4 andesites) but have have slightly slightlygreater greaterenrichment enrichmentininlight-REE and no to small negative negative Eu anomalies. On Onaaprimitive primitive mantle mantle normalized normalized plot the the hangingwall patterns similar Nb,Ta, and Ti, felsic rocks show patterns similar to the the andesites andesiteswith with prominent prominent depletions depletions in Nb, lesser lesser depletion depletion of Zr-Hf, and enriched Th (Fig. la). la). The Thehangingwall hangingwalldacite-rhyodacite dacite-rhyodacitewas was plagioclase+pyroxene+Fe-Ti oxide oxide from from the the andesite. andesite. probably derived derived through throughfractionation fractionationof of plagioclase+pyroxene+Fe-Ti Footwall sericite sericite and and quartz-sericite quartz-sericite schist schist show show compositional compositional effects effects of of alteration alteration with with more variable Si02,higher higher K20/K20+Na20, K20/K20+Na20,and andhigher higherSScontent contentthan thanmost mosthangingwall hangingwallfelsic felsic variable Si02, rocks; footwall footwallsamples samplesalso alsoshow showincreasingly increasinglypositive positive Eu Eu anomalies anomalieswith with increasing increasingSScontent. content. felsic rocks rocks mostly mostly have have depleted depleted heavy-PEE heavy-REEand and generally generallylower lowertrace traceelement element The footwall felsic abundances than hangingwall felsic rocks (Fig. ib). lb). Although Althoughthe theheavy-REE heavy-REEdepletion depletionin in the the footwall samples samples could could be be aa result result of of alteration, alteration,some some samples samples with with depleted depleted heavy-PEE heavy-REEshow showno no compositional evidence of significant alteration (e.g., no high S content and/or high K20/K20+ compositional evidence of significant alteration (e.g., no high S content andlor high K20/K20+ Na2Oratio). ratio). Also, studies Na20 studies of of PEE REE mobility mobility associated associated with alteration below massive sulfide deposits have identified deposits identified mobility mobility of of lightlight- and and middle-PEE middle-REE but but general general stability stability of the heavy-PEE heavy-REE (Barrett and MacLean, 1999). (Barren 1999). Thus, Thus,the thecompositional compositionaldifference difference between footwall and hangingwall felsic hangingwall felsic rocks rocks may may reflect reflect primary primary differences. differences. ~ - -~ - - 59 �''JIlT Roc4P.imwve RoWPrirnitiveMantle Mantle 1000 1000 RocWPrlmiffve Mantle Rock/Primit!ve Mantle 1000 1000 b. 100 100 100 Hangingwall Felsic Rocks Hangingwall Felsic Rocks 10 10 10 1 1 Footwall Felsic Rocks .1 .1 I .1 II,, ThNbTsLaCeNdZr HfSmTiGdTbDy Y HoErTmYbLu Figure I.1. Primitive Primitivemantle mantlenormalized normalized(Kerrich (Kemchand andWyman, Wyman,1997) 1997)trace traceelement element plots plots for for Bend a) a) basalt-andesite basalt-andesite and hangingwall b) hangingwall hangingwalland and footwall footwallfelsic felsicrocks. rocks. hangingwall felsic felsic rocks rocks and and b) The Bend volcanic rocks plot as as a typical orogenic caic-alkaline calc-alkaline basalt-andesite-daciterhyodacite suite suite on on various various compositional compositionaldiscrimination discriminationdiagrams diagrams(e.g., (ex., Th-Hf-Ta; Th-Th-Ta; Th-Tb-Ta: . - . Th-FIf-Ta; Th/Yb-TaIYb). the T l k - T ~ I Y ~Further, Further, ). theenriched enrichedlight-REE light-REEand andhigh highTh T bof o fthe theBend Bend suite suiteare arecharacteristic characteristic Acontinental continental setting setting of orogenic sequences in continental settings settings (Barrett (Barren; and MacLean, 1999). 1999). A volcanic rocks rocks in in the the Bend Bend sequence. sequence. In is also supported by the abundance of felsic calc-alkaline volcanic addition, the volcanic rocks hosting the Bend massive sulfide deposit are similar compositionally to those in the Monico Monico area area in in north-central Wisconsin Wisconsin that that host the Pelican deposit (Sims (Sims and and have others, 1989). Neodymium Neodymium isotope isotope data data for for felsic felsic volcanic volcanic rocks in the Monico area have contribution from from an an enriched enriched crustal crustal source. source. The negative epsilon Nd indicating a contribution lithogeochemistry suggests that the Bend deposit deposit and probably other massive sulfide deposits associated with felsic centers in northern Wisconsin (e.g., (e.g., Flambeau, Pelican, Crandon) evolved in an extensional continental setting, possibly a continental back-arc back-arc basin in a southwarddirected subduction system. References cited: lithogeochemistry, and and hydrothermal hydrothennal Barrett, T.J. and MacLean, W.H., 1999, 1999, Volcanic sequences, lithogeochemishy, alteration in some bimodal volcanic-associated massive sulfide sulfide systems: systems: in in Bame, Bathe, C.T. C.T. and alteration Hannington, M.D., editors, Hannington, editors, Volcanic-Associated Volcanic-AssociatedMassive Massive Sulfide SulfideDeposits: Proceeses Proceeses and and Examples in Modem v.8, 8, p. p. 101-131. 101-131. Examples Modem and and Ancient Ancient Settings: Settings: Reviews in Economic Geology, v. DeMatties, T.A., 1994, 1994,Early Early Proterozoic Proterozoicvolcanogenic volcanogenicmassive massive sulfide sulfide deposits depositsin in Wisconsin: Wisconsin:an an overview: Economic Economic Geology, Geology, v. 89, 89, p. 1122-1151. 1122-1151. DeMatties, T.A. and Rowell, W.F., Wi., 1996, 1996,The TheBend Benddeposit: deposit:an an Early Early Proterozoic Proterozoic copper-gold VMS deposit: in LaBerge, LaBerge, G.L., G.L., editor, editor,Volcanogenic Volcanogenicmassive massive sulfide sulfidedeposits depositsof of northern northern Wisconsin: Wisconsin; aa commemorative volume: commemorative volume: Institute Institute on onLake LakeSuporior SuperiorGeology Geology Proceedings, Proceedings,v. v. 42, 42, Part Part 2, 2, p. p. 143143159. 159. Kenich, R. Kemch, R. and andWyman, Wyman,D.A., D.A., 1997, 1997,Review Reviewof of developments developmentsin in trace-element trace-element fmgerprinting fingerprinting of geodynamic geodynamic settings settingsand and their theirimplications implicationsfor formineral mineralresources: resources: Australian AustralianJournal Journalof of Earth Earth 465-487. Sciences, v. 44, p. 465-487. inthe theevolution evolutionof ofthe thePenokean Penokeanorgogen: orgogen:isotopic isotopic Schuiz, K.J. and and Ayuso, Ayuso, R. R. A., A,,1998, 1998,Crustal Crustalrecycling recyclingin Schulz, evidence evidence for for Archean Archean contributions contributionsto tocrustal crustalgrowth growthin inthe the Pembine-Wausau Pembine-Wausau terrane, terrane,northern northern Wisconsin Wisconsin [abstracti: [abstract]: Institute Instituteon onLake LakeSuperior SuperiorGeology GeologyProceedings, Proceedings,44th 44&Annual Annual Meeting, Meeting, Minneapolis, MN, 1998; v. 44, p. p. 113-114. 113-114. v.44, Sims, P.K., Schmus, W.R., Schulz, Schulz, K.J., and and Peterman, Z.E., 1989, 1989,Tecton-stratigraphic Tecton-stratigraphicevolution evolution of P.K., Van Schmus, the Early Proterozoic ProterozoicWisconsin Wisconsin magmatic magmatic terranes terranes of of the the Penokean Penokean Orogen: Orogen: Canadian Canadian Journal Journal of of Earth Sciences, Sciences, v. 26, 26, p. 2145-2158. 2145-2158. 60 �Assessing the extent of Early Proterozoic Mametamorphism metamorphism Proterozoic Penokean Penokeanversus versus—1770-1760 -1770-1760Ma in east-central east-central Minnesota Schweitzer, Kent, OH; OH; Schneider, Schneider, D.A., Schweitzer, D.J., DJ., Dept. Dept. of of Geology, Geology, Kent State University, Kent, DA., Dept. of Earth and Environmental Sciences, Lehigh Lehigh University, University,Bethlehem, Bethlehem,PA, PA;Boerboom, Boerboom, TJ., T.J., Minnesota Geological Survey, Survey, 2642 2642University UniversityAvenue, Avenue,St. St.Paul, Paul,MN, MN;Holm, iloIm,D.K., D.K.,Dept Dept of Geology, Kent State University, Kent, Kent, OH; OH; and and Van Van Schmus, W.R., W.R., Dept. Dept. of of Geology, Geology, University of Kansas, Lawrence, KS. Combined field, field, geophysical, geophysical, and and drill drill core core data from the internal zone Combined zone of of the the Penokean Penokean orogen in Minnesota Minnesota have have greatly greatly increased increased our our knowledge knowledge of of this this orogen's orogen's architecture, architecture,allowing allowing new correlations with the Wisconsin Wisconsin magmatic magmaticterranes terranesto to the the east east (Jirsa (Jirsa et et a]., al., 1995). Lowcorrelations with Lowgrade, greenschist structural panels panels have been identified as greenschist metamorphic, metamorphic, structural as arcuate, arcuate, imbricate imbricate thrust sheets in the poorly exposed, southeast portion of of the zone (Jirsa and Chandler, 1997). To To the west and and north, north, these these structural structural panels panels have have been been intruded intruded by bv abundant abundant post-tectonic vost-tectonic granitoids granitoids that make up aa physically physically continuous continuous mass of bedrock bedrock over over 7000 7000 km2 k d (Figure (Figure 1). 1). Holm et al. al. (1998) (1998) have have documented documentedwidespread widespread post-Penokean post-Penokean (1770-1760 (1770-1760 Ma) Ma) amphibolite amphibolite metamorphism in in the the plutonic plutonic region. region. Our goal is facies metamorphism Ourgoal is to to determine determine the the extent extent of of Penokean Penokean and/or —1760 Mametamorphism metamorphismwithin withinthe the lower-grade lower-grade terranes terranes to the southeast. andlor -1760 Ma We have have dated dated five five mineral mineral separates separates from from recently recently obtained obtained drill core housed housed at at the the Minnesota Geological Geological Survey. Survey. Sample Minnesota Sample localities localities and and associated associated dates dates are are plotted plotted on on figure figure11 (Southwick, 1994; Holm Holm and Lux, together with results from prior thermochronologic thennochronologic studies (Southwick, HoIm et et al., a!., 1998). 1998). Biotite from a biotite-hornblende 1996; Holm biotite.-hornblende granodiorite (EC-8) intruded by the post-tectonic Foley batholith batholith yielded yielded aa plateau plateau age age of 1846±6 post-tectonic Foley 1846k6 Ma Ma (four (four age age increments increments 75% of of the gas released). released). In constituting over 75% In spite spite of of its its proximity proximity to to the the Foley Foley intrusion, intrusion, this this unaffected by by it thermally. thermally. Biotite rock was apparently unaffected Biotite from from aa biotite biotite schist schist(EC-22) (EC-22)sampled sampled east of the Foley batholith yielded a plateau age of 1863±7 1863k7 Ma (eight age increments constituting over 72% of the gas released). Biotite Biotitefrom from aa tonalitic tonalitic orthogneiss orthogneiss (EC-27) yielded a plateau age increments, 81% 81% of of total total gas gas released). released). A 1820± Ma (seven increments, A hornblende hornblende separate separate from the of 1820±7 same drill core sample of orthogneiss yielded a plateau age of 1831±6 1831k6 Ma (seven (seven increments; from a meta-quartzdiorite (EC-l6) yielded 64% of total gas gas released). released). Lastly, hornblende hornblende from meta-quartzdiorite (EC-16) yielded aa 81% of of total total gas gas released). Only plateau hornblende date of 1838±5 1838k5 Ma (seven increments, 81% Only one one other Penokean Penokean Ar/Ar ArIAr age age on hornblende hornblende has been reported from east-central Minnesota Minnesota (1854±5 (1854s Ma plateau plateau date date on on the thePhilbrook Philbrook pluton; pluton; Southwick, Southwick, 1994). 1994). Five previously previously published published hornblende Ar/Ar ArIAr dates dates from from the theplutonic plutonicterrane terreneall allyielded yieldedpost-Penokean post-Penokeandates datesofof—1760 -1760 Ma Ma (Holm et al., 1998). 1998). This is the ArIAr cooling cooling ages ages in in east-central east-central the first first study study totodocument document biotite biotite Penokean Penokean Ar/Ar Minnesota. These Thesedata datafirmly firmlyestablish establishaaPenokean Penokean age age of ofmetamorphism metamorphism for for the the lower lower grade grade structural panels panels described described by by Jirsa and Chandler Chandler (1997). (1997). While While preliminary, preliminary, these these data data also also indicate thermal overprinting overprinting atat either either ca. ca. 1760 Ma Ma (as documented indicate no significant significant thermal documented to the northwest in the plutonic terrane) or at ca. 1630 1630 Ma (as (as long documented documented in adjacent adjacent parts of the the orogen in Wisconsin). ItItappears appearsthe the1770-1760 1770-1760Ma Ma barrovian barrovian amphibolite amphibolite metamorphism metamorphism in in the the internal zone of east-central east-central Minnesota may be restricted restricted to to the the deepest deepest exposed exposed region region which which also contains contains abundant abundant post-tectonic post-tectonic plutons. plutons. If so, so, this this would would be be consistent consistent with with the the interpretation of Holm et al. (1998) (1998) that the deepest deepest exposed region was probably the thickest thickest portion portion of the the Penokean Penokean orogenic orogenic belt belt which which became became unstable unstable and and rapidly rapidly collapsed collapsed (i.e., (i.e., unroofed) at —1760 Ma. The temporal and spatial association of post-tectonic plutons, barrovian -1760 Ma. —1760 Mametamorphism, metamorphism,and andmidcrustal midcrustal unroofing unroofing suggest suggest aa genetic link between -1760 Ma between thermal thermal softening of the overthickened overthickened crust and post-orogenic collapse in the hinterland of the Penokean Penokean orogen. 61 �QiDKeweenawan Supergroup Paleoproterozoic C:) LIWe Falls Fonnation Mixed supraaustai rot and shearS granitic rocks. Mets.plutomc rocks may be Archean an part 0 HilIman tonalite (may be Archean in part) c1 Archean/Proterozoic? Undiffetentiated — Minnesota. Filled Figure 1. 1.Generalized Generalized bedrock bedrockgeology geology map map of of east-central Minnesota. Filled circles circles represent localities Lux, 1996; HoIm etal., (after HoIm Holm and Lux. 1996;Holm etal.. 1998) 1998)which reoresent localities of of prior crior results results (after yielded ca. 1760-1750 Ma dates onmica on mica and hornblende. Also plotted the 1854 1854Ma M a Ar/Ar Arlk clotted is is the vikded 1760-1756% plateau age on hornblende from the Penokean Philbrook Philbrook pluton pluton (southwick, (Southwick, 1 1994). Open 994). Open circles from study. LF circles represent localities localities and results f rom this study. LF is is Little Little Falls, SC SC is St S tCloud. Cloud . ~ ~ ~ - ~ ~ - ~ ~~~~ References References HoIm, D.K.,and D.K.,and Lux, Lux, D.R., DR., 19%, 1996,Core Corecomplex complexmodel model proposed for gneiss dome development Holm, Paleoproterozoic Penokean Penokean omgen, orogen, Minnesota: Minnesota Geology, 24,343-346. 24,343-346. during collapse of the Paleopmteromic D.K., Darrah, K.S., K.S., and Lux, Lux, D.R., DR., 1998, -1760 Ma Holm, D.K., 1998,Evidence for widespread -1760 Ma metamorphism and 870-1820 Ma) morphism and rapid rapid crustal crustal stabilization stabilizationof ofthe theEarly EarlyProterozoic Proterozoic(1 (1870-1820 Ma) Penokean Penokean Orogen, Minnesota: Minnesota American 60-81. 298,60-81. Omgen, AmericanJournal Journalof ofScience, Science,298, M. and Jirsa, M., andChandler, Chandler,V., V,1997, 1997,Scientific Scientifictest testdrilling drillingand andmapping mappingin ineast-central east-centralMinnesota, Minnesota, 1994-1995: Sununary Summaryof oflithologic lithologicresults: results:M~M. Mlnn.Geol. Geol.Smv. Suit Info. Circular 105 pp. 1994-1995: Circular42, 42,105 .,Chandler, Chandler, V, V, and andBoerboom, Boerboom,11, T., 1995, 1995,Extension Extension of of the the Wisconsin Wisconsin magmatic terranes Jim, M Jirsa, lvi, into the Minnesota Minnesota segment of the Penokean omgen: orogen: Institute Institute on on Lake Lake Superior Superior Geology, 4 1st Annual Meeting, 41st Meeting, Marathon, ON, 42.25-26. 42,25-26. D.L., 1994, geochronologic studies studies of of Precambrian Precambrianterranes terranes in inMinnesota: Minnesott Southwick, D.L., 1994,Assorted geochronologic A potpourri of timely information: information: in in Short Short Contributions Contributions to the the Geology Geology of of Minnesota, Minnesota, 1994, 1994, Investigations 43, 1-19. edited by D.L. Southwick, Minnesota Geological Survey Report of Investigations 43,l-19. 62 �THE THE DISTINCTION DISTINCTION BETWEEN BETWEEN FLAT FLAT PEBBLE PEBBLECONGLOMERATES CONGLOMERATES AND AND INTRAFORMATIONAL INTRAFORMATIONALBRECCIAS BRECCIASIN INPRECAMBRIAN PRECAMBRIANIRON IRONFORMATIONS FORMATIONS Bruce BruceM. M.Simonson Simonson Geology GeologyDepartment Department Oberlin OberlinCollege College Oberlin, USA Oberlin,01144074-1044 OH 44074-1044USA Iron Ironformation, formation,aa distinctive distinctivetype type of of rock rock restricted restricted largely to the Precambrian, Precambrian, consists consists mainly of oi silica (in the form of ehert) and such iron-rich minerals as hematite, magnetite, siderite, andlor silica (in the form of chert) and such iron-rich minerals as hematite, magnetite, siderite, and/or greenalite. greenalite. Lamination Laminationand andthin thinbedding beddingare arecommonplace commonplacein inthese theserocks rocksas asevidenced evidencedby by the the near-universal use of the term BIF (for "Banded Iron Formation"), but a substantial minority near-universal use of the term BIF (for "Banded substantial minority of of iron iron formations formations(particularly (particularlythose thosein inNorth North America) America)are are actually actually GIF GIF(for (for "Granular "GranularIron Iron Formation"). Formation"). Such Suchunits unitshave havecoarsely coarselyclastic clastictextures textures(analogous (analogousto tothose thoseof ofcarbonate carbonate grainstones and oolites) and display structures such as cross-bedding rather than grainstones and oolites) and display structures such as cross-beddmg rather thanbanding. banding. CentimeterCentimeter-to todecimeter-scale decimeter-scalelayers layersrich richin insiliceous, siliceous,flat, flat,pebble-size pebble-size fragments fragmentsare areminor minor constituents of both BIF and GIF. The fragments are clearly intraformational in origin constituents of both BIF and GIF. The fragments are clearly intraformational in originand andare are frequently bothsettings, settings, frequentlycited citedas asevidence evidenceof ofdeposition depositionin inshallow, shallow,high-energy high-energyenvironments environmentsininboth but but they they have have very very different differentorigins. origins.The Thefragments fragmentsin in GIF GIFoccur occurin in flat flatpebble pebbleconglomerates conglomerates where where they they generally generally have havewell-rounded well-roundedends, ends, form formaa grain-supported grain-supported framework, framework,and andoriginated originated via via erosion erosionand and reworking reworkingatatthe the sediment-water sediment-waterinterface. interface.In In contrast, contrast,the the fragments fragmentsin inBIF BIF occur in breccias where they are generally much more angular, isolated in a matrix of less occur in breccias where they are generally much more angular, isolated in a matrix of less siliceous siliceousmaterial, material,and andoriginated originatedafter afterdeposition depositionand andatatleast leastshallow shallowburial burialvia viapartial partial liquefaction. liquefaction.The Thedominance dominanceof ofeherty chertyfragments fragmentsin inboth bothbreccias brecciasand andconglomerates conglomeratesindicates indicates that thansilica-poor silica-poorones onesin inboth bothBIF BIFand and that silica-rich silica-richlayers layerswere weresignificantly significantlymore morecohesive cohesivethan GIF. GIF. Many Manv of of the the flat flat pebbles nebbles in in GIF GIF are are identical identical to to nodular nodular masses masses known known as aschert chertpods in -podsin underlying sediments, suggesting they are exhumed nodules. Petrographic evidence such as underlying sediments, suggesting they are exhumed Petrographic evidence such as large differences in minus-cement porosity inside large differences vorositv vs. outside the pods and dramatic thinning- of . individual individuallayers layersas as they they exit exitpods podsindicates indicatesthe the pods podsformed formedvia via early earlysilica silicacementation. cementation.This This widespread widespread early early cementation cementationisis probably probably due dueto to high high silica silicaconcentrations concentrationsin inPrecambrian Precambrian seawater seawater which, which, unlike unlike today, today, caused caused silica silicato to difftise diffuse into into (rather (rather than than out out of) of) sediment sedimentpore pore waters (Siever, 1992, Geochimica et Cosmochiiica Acta 56,3265-3272). waters (Siever, 1992, Geochimica et Cosmochimica Acta 56, 3265-3272). . 63 �PRECAMBRIAN FEATURES UNDER THE THE JAMES JAMES BAY BAY AND HUDSON BAY BAY LOWLANDS LOWLANDS PRECAMBRIAN Geological Survey, Ontario Geological Lake Rd, Rd, Sudbury, Sudbury, ONP3E ON F3E 6B5 Greg Stott, Brian Survey, 933 Ramsey Lake Brian Berdusco, Ontario stottg(ägov.on.ca ;brian.berdusco)ndm.gov.on.ca stottgf%~ov.on.ca brian.berdusco@ndm.eov.on.ca Large cover of of both both the James Bay and and Hudson Large parts parts of the the Superior Superior Province Province underlie underlie the Phanerozoic Phanerozoic cover James Bay Hudson Bay lowlands. This, This, plus plus the the poor poorbedrock bedrockexposure exposureininthe theeastern easternand and northernmost northernmostexposed exposed Sachigo Sachigo Subprovince, Subprovince,has has permitted only limited with permitted limitedsuccess success to date date in correlating correlating the regional geology of of the the Uchi-Sachigo Uchi-Sachigo superterrane superterrane with parts project is planned number of geological the Superior Superior Province Province in in northwestern northwestern Quebec. Quebec. AA project planned to examine examine aa number parts of the problems in addition addition totoregional regionalcorrelations correlations by byreprocessing reprocessing the the ODMJGSC ODMIGSC aeromagnetic aeromagnetic data the 1960s 1960s problems data of of the covering the region mainly north of of Latitude Latitude 510. 51'. This will willbe be done done in in co-operation co-operation with with Regional Regional Geophysics Geophysics staff of under the the lowland cover of the the GSC. GSC. This This poster poster isis intended intended to to outline outlinesome some of of the the Precambrian Precambrian features under cover that might might be he enhanced when the reprocessing is completed. Features and Interpretive Interpretiveissues: issues: the substrate substrateof of the the northern northern Hudson Hudson Bay 1) The complex, broad high magnetic magnetic susceptibility susceptibility that that characterizes characterizes the I) [HBL] isissimilar Lowlands [HBL] similartotothat thatofofthe thePikwitonei Pikwitoneigranulite granulitedomain domainand and adjacent adjacent domains domains in Manitoba. We suspect that that this this region region of old broad range range of of Archean Archean ages ages including includingpre-3.8 pre-3.8 Cia Ga crust crust (cf. (cf. suspect old crust, crust, containing containingaa broad B6hm Geology, JanIOO), JanIOO), continues and underlies underlies the early early BOhm et et al., Geology, continuesinto intothe the northernmost northernmostpart partof of Ontario Ontario and Inlier. Proterozoic Sutton Inlier. Orogen [THO] [THO] are in parts parts of of Manitoba and the the HBL. Re-interpretation 2) Elements of the Trans-Hudson Orogen are exposed exposed in Re-interpretation of the aeromagnetic aeromagnetic patterns patterns that that characterize characterize this this region region suggests suggests several points to to consider: consider: 3) 3) River Belt BeltofofManitoba Manitobadoes doesnot notappear appeartotocontinue continueeastwards eastwards across across the northern northern a) The Proterozoic Fox River HBL but border and and probably probably links links to the hut continues continues north north to to Hudson Hudson Bay Bay near near the the Manitoba-Ontario Manitoba-Ontario border the Internides of the main THO linear gravity gravity anomaly just just offshore, north north of Ontario. Strata seem to reappear reappear as of Strata from from the Trans-Hudson Orogen Orogen offshore offshore in Hudson Bay seem asfolded folded units units north of b) h) the Sutton Inlier the Inlier and anddiscontinuously discontinuouslyextend extend southwards southwards towards the inlier. inlier.These These features features need need further further discrimination to to establish establish their full extent. discrimination full extent. as aa single, single, continuous continuous package packageof of rock rock as as currently c) The Sutton Sutton Inlier Inlier has has for formany manyyears years been been treated treated as c) portrayed on on geological maps maps (compiled (compiled from from the the original original work by H. Bostock GSC Paper Paper 70-42). But a portrayed Bostock -- GSC closer examination of patterns in in its its vicinity vicinity suggests the Sutton Inlier Inlier is of the the aeromagnetic patterns suggests the is not not aa single single large large domain but aa set set of ofsouthward-convex-shaped southward-convex-shaped inliers resemble "klippe-like" domain inliers strung strung along along aa line. They resemble features comprising shallow shallow water platform sedimentary depositsincluding including iron iron formation, and features comprising sedimentary deposits and gabbroic gahhroic sills that southwardsonto ontothe theArchean Archeancraton cratonfrom from the the main main THO belt that might might have have been been transported transported southwards belt offshore. Smaller of the currently defined aeromagneticallyapparent apparent to the north and east east of defined Smaller "klippe" "klippe"are arealso alsoaeromagnetically Sutton Inlier. The the Nastapoka NastapokaFormation Formationon onthe the eastern easternshore shoreofof Hudson HudsonBay Bay (cf. (cf. Sutton The strata strata resemble resemble the Chandler Res.1989) 1989)and andpart partofofthe thePovungnituk PovungnitukGroup Group of of Cape Cape Smith Smith Belt. Chandler and and Parrish, Prec. Res. Beneath the James JamesBay BayLowlands Lowlands [JBL], [JBLJ,there thereisis aa strong strong likelihood likelihood that that the Beneath the the English English River River and and Quetico Quetico metasedimentarysuhprovinces subprovincesconverge converge underthetheJames JamesBay BayLowland Lowlandcover cover and and that that the metasedimentary under the Wabigoon Wabigoon Subprovince pinches pinches out out eastwards. eastwards.Discrimination Discriminationof of this this has hasimportant importanttectonic tectonic implications implications for the original Subprovince original that the converging nature of these these subprovinces. subprovinces. It is compelling compelling to to suggest suggest that converging English English River River and and Quetico Quetico nature resemble telescoped telescoped inter-arc inter-arc basins basins that that wrap wrap around the the east eastend endof of aa Wabigoon Wabigoon microcontinent microcontinent and and continue continue 'basins in Quebec. The merged merged English English R R - Quetico R. and and Opinaca Opinaca R. R. "basins" Quebec. The Quetico isis on-line with with the the Nemiscau Nemiscau R. on-line conceptually conceptually analogous analogous to to the the modern modem basins basins of of the the Solomon Solomon and and Coral Coral seas seas respectively north north and and south south of of east east Papua-New Papua-New Guinea. Guinea. The Sachigo and Uchi Uchi subprovinces to merge merge under under the the eastern easternHBL HBL and and JBL JBL suhprovinces of northwestern Ontario appear appear to and continue as aa single single domain domain and and emerging emerging as asthe theMinto Minto domain domain of northern Quebec. continue eastwards eastwards as Quehec. the eastern easternHBL, HBL,flanked flankedto tothe thewest westby byaafault faultsimilar similar in orientation A major major indenter-like indenter-likefeature feature occurs occurs under the to the the Miniss Miniss Fault. Fault. This Thisfeature, feature, like likeother other"indenters" "indenters"ininnorthwestern northwestern Ontario Ontario (e.g., (e.g., at Savant Savant Lake, Lake, Shebandowan and Manitouwadge), Manitouwadge), closes closesnorthwards northwardsand andborders bordersaa"bow-tie" "bow-tie" aermagnetic aermagneticpattern patternon on its its north Shehandowan and flank, which flank, whichappears appears to to reflect reflectshortening shorteningand andexhumation exhumationofofdeeper deepercrust. crust. - 4) 4) 5) 5) 64 �THE THE 1787-1772 1787-1772MA MAEAST-CENTRAL EAST-CENTRAL MINNESOTA MINNESOTA BATHOLITH: BATHOLITH: PRECURSOR TO TOCRUSTAL CRUSTALSTABILIZATION STABILIZATION IN IN THE THEL.L.SUPERIOR SUPERIOR PRECURSOR REGION REGION - - VAN both bothatatDept. Dept.of ofGeology, Geology,Univ. Univ.ofof VAN SCHMUS, SCHMUS, W.R., W.R.. and andMacNEILL, MacNEILL,L.L.C.,C., ofofGeology, Kansas,Lawrence, Lawrence,KS, KS,55045 55045(rvschmus@ukans.edu); (rvschmus@ukans.edu);HOLM, HOLM.D.K., D.K.,Dept. Dept. Geology,Kent Kent Kansas, both at Minnesota BOERBOOM, JIRSA. M.A, M.A. both at Minnesota State Univ., State Univ.,Kent, Kent,OH; OH; BOERBOOM,T.J. T.J.and and JIRSA, Geological MN GeologicalSurvey, Survey,2642 2642University UniversityAvenue, Avenue,St. St.Paul, Paul,MN The Thecause causeofof—1760 -1760 Ma magmatism in the the southern southernL. L.Superior Superiorregion regionhas hasbeen beenconsidered considered considerable amagmatic enigmatic because of its apparently abrupt nature and the enigmatic because of its apparently abrupt nature and the considerable amagmatichiatus hiatus(—60 (-60 m.y.) m.y.) that that separates separates itit from from Penokean Penokean orogenesis. orogenesis. The Theage age of of post-Penokean post-Penokeanmagmatism magmatisminin Wisconsin, Wisconsin,commonly commonlyreferred referredtotoasas"1760 "1760Ma", Ma",was wasobtained obtainedby by dating datingmg-sized mg-sizedfractions fractionsfrom from here new U-Pb data rhyolites and and epizonal epizonal granites granites (Van (Van Schmus, Schmus, 1980). 1980). We Wereport report here new U-Pb datafrom fromthe the rhyolites recently recentlycharacterized characterizedmidcrustal midcrustalplutonic plutonicterrane terraneinineast-central east-centralMinnesota. Minnesota. This Thisportion portionofofthe the exposed exposed Penokean Penokean orogen orogencontains containsnumerous numerouspost-tectonic post-tectonicplutons plutonsof ofvarying varyingcomposition compositionand and Based on field, drill-core, geometry(Jirsa (Jirsaetetal., al.,1995; 1995;Boerboom Boerboometetal., al.,1995; 1995;1998). 1998). Based on field, drill-core,petrologic, petrologic, geometry and andgeophysical geophysicalstudies, studies,about about20 20separate separateintrusive intrusivebodies bodieshave havebeen been identified identifiedand andcategorized categorized granites; into 1)granodioritic granodioritic to todioritic dioriticrocks; rocks;2)2)gray gray granites; 3) 3)red redgranite granite into four four separate separate groups: groups: 1) quartzofeldspathic dikes. plutons, plutons,dikes dikesand andsills; sills;and and4)4)late laternafic maficplugs plugsand anddiabasic diabasictoto quartzofeldspathic dikes. 0.34 Richmond Granite Intercepts at 4 grains 'N f?2- 0±0&1772.3±1.OMaI MSWD= 1.6; P = a16J 0.32 1 grain 1720 0.30 0.28 600 TV" .1 grains 0.26 0.24 3.5 3.7 3.9 4.3 4.1 201 207 4.5 4.7 4.9 p y 235 U(J Pb!235 Fiyure 1.1. Plot Plotof ofzircon zircondata datafrom fromthe theyounger youngerand andolder olderends endsof ofthe theage agespectrum spectrumfor forthe theEastEastFigure Central CentralMinnesota Minnesotabatholith. batholith.Data Datafor forother otherunits unitsfall fallininbetween betweenthese thesetwo twopopulations. populations. 65 �Our new results are mostly U-Pb isotope dilution analyses from single zircons zircons weighing weighing 11 to to 5 micrograms, with with several several hand-picked, hand-picked,air-abraded air-abradedgrains grainsselected selectedfor foreach eachrock rockunit. unit. For the most part the analyses are not influenced by by xenocrystic cores, cores, although in a few cases they may be present. All units of the post-tectonic suite, the "East-Central All units of the post-tectonic suite, the "East-Central Minnesota Minnesota batholith" (ECMB), (ECMB), yield ages within the range 1772±1 Ma (Richmond Granite) to 1787±3 1772i1 to 1787i3 Ma (Warman (Wannan Granite). The relative relative ages within this range correlate with relative relative ages based based on on field fieldrelationships relationships known). Thus, (where known). Thus, all all units units of of the the ECMB ECMB (St. Cloud Cloud Red Granite, Granite, Reformatory Reformatory Granodiorite; Granodiorite; Rockville Granite, Freedham Tonalite, Anne Lake Lake Granite, Granite, plus plus others) others) were emplaced within 15 15 m.y. are determining determining ages from from surrounding surrounding basement rocks, using using m.y. In Inaddition additionto to the the ECMB, ECMB, we we are outcrop and and drill core samples; preliminary results resultsyield yield an an age age of of 1800±8 Ma for a foliated samples; preliminary 1800± Ma foliated tonaiite tonalite within the Hillman migmatitecomplex. We are are also also redetermining redetermining U-Pb U-Pb ages ages of of several several units of the "1760 Ma" suite in Wisconsin using using single-crystal single-crystal analyses, analyses, but but at at this this time our data definitive. are not definitive. Our results suggest that the "1760 Ma" magmatic event is only only the the last last stage stageof ofaa—25 -25 m.y. m.y. episode of post-tectonic magmatism. These post-tectonic magmatism. These data data significantly significantlyreduce reduce the the time-lag time-lagbetween between the the end of Penokean orogenesis and the onset of post-tectonic plutonism, bringing it into into line line with with thermal and physical physical models which which predict predict characteristic characteristic time-lags time-lags of of only only 20-40 m.y. m.y. (England thermal Thompson, 1984; and Thompson, 1984;Turner Turneret et aL, al., 1992). 1992). Intrusion of the Intrusion of the ECMB ECMB was was associated associated with withwidespread widespread midcrustal midcrustal amphibolite amphibolite and directly preceeded preceeded rapid rapid exhumation exhumation (i.e., (i.e., collapse?) collapse?) and and crustal crustal stabilization metamorphism and In some (Holm al., 1998). 1998). In some regions regions of incipient incipient collapse collapse (i.e., (i.e., India-Asia India-Asia today), today), the the (Holm et al., overthickened crust crust contains contains aa partially partially molten molten layer layer ponded ponded in in its mid-section (Nelson et al., overthickened al., 1996). manner we suggest the overthickened overthickened Penokean crust was was invaded invaded by by 1996). In analagous manner midcrustal melts melts which which probably probably affected affected its its rheological rheologicalbehavior. behavior. Vast magma midcrustal magma intrusion intrusion may may dramatically decreased crustal viscosity and and promoted promoted collapse (as recently proposed for have dramatically the Canadian Cordillera; Cordillera; Vanderhaeghe Vanderhaeghe et et aL, al., 1999). 1999). References Boerboom, T., T., Jirsa, Jirsa, M., M., and Holm, Hoim, D.K., D.K., 1998, Early Early Proterozoic Proterozoic Intrusive Intrusive Rocks of EastBoerboom, Superior Gwl. Geol. Fieldtrip Fieldtrip Guidebook, Central Minnesota: Institute on L. Superior Guidebook, v. 44, 44, p. 3-30. 3-30. Boerboom, T.J., T.J., Setterholm, D.R., D.R., and Chandler, V.W., V.W., 1995, Bedrock geologic map, plate 2 G.N., Project Project Manager, Manager,Geologic Geologicatlas atlasofofSteams SteamsCounty, County,Minnesota: Minnesota: Minnesota of Meyer, G.N., SurveyCounty CountyAtlas AtlasSeries, Series,C-10, C-b, Part A, Geological Survey scales 1:200,000 and l:lOO,000.England, P.C., and and Thompson, A.B., 1984, scales 1:200,000 and l:lOO,OOO.England, P.C., Thompson, A.B., 1984, PressurePressuretemperature-time paths paths of of regional regional metamorphism metamorphism I.I. Heat Heat transfer transfer during during the evolution of temperature-time regions of thickened continental crust: Journal of Petrology, v. 25, p. 894-928. 894-928. Holm, Darrah, K., K., and and Lux, Lux, D.R., D.R., 1998, 1998,Evidence Evidence for forwidespread widespread—1760 -1760 Ma Ma Holm, D.K., D.K., Darrah, metamorphism and and rapid crustal stabilization of the Early metamorphism Early Proterozoic Proterozoic (1870-1820 (1870-1820 Ma) Ma) Penokean orogen, Minnesota: American American Journal Journal of Science, Science, v. 298, p. 60-81. 60-81. Jirsa, M.A., 1995, Bedrock geologic map of easteastM.A., Chandler, V., V., Cleland, J., J., and Meints, J., J., 1995, central Minnesota: Minnesota Minnesota Geological Geological Survey Survey Open-File Open-File Report Report 95-1. Nelson, K.D., K.D., and 27 others, 1996, Nelson, 1996, Partially molten middle crust beneath beneath southern southern Tibet: Tibet: Synthesis of project INDEPTH results: Science, v. 274, p. 1684-1688. 1684-1688. and Foden, Foden, J., J., 1992, 1992,Some Some geodynamic geodynamic and and compositional compositional Turner, Sandiford, M., Turner, S., S., Sandiford, M., and constraints on "postorogenic" magmatism: 1-934. constraints magmatism:Geology, Gwlogy,v.v.20, 20,p.p.93 931-934. Vanderhaeghe, O., 0., Teyssier, Vanderhaeghe, Teyssier, C., and and Wysoczanski, Wysoczanski, R., 1999, 1999, Structural Structural and and geochronologic geochronologic constraints on the role of partial melting during during the formation formation of the the Shuswap Shuswapmetamorphic metamorphic Thor-Odin dome, British Columbia: Columbia: Can. J. Earth Sci., core complex at the latitude of the Thor-Odin 17-943. v. 36, 36, p. p. 9917-943. W.R., 1980, Van Schmus, W.R., 1980, Chronology of igneous rocks associated with the Penokean orogeny orogeny in Wisconsin: Geol. Geol. Soc. Soc.America AmericaSpec. Spec. Paper Paper 182, 182,p. p. 159-168. 159-168. 66 �ISOTOPIC CONSTRAINTS CONSTRAINTS ON THE ORIGIN ORIGIN OF GRANOPHYRE GRANOPHYRE COMPLEXES COMPLEXES IN IN THE THE MIDCONTINENT RIFT OF NORTHEASTERN MINNESOTA. MIDCONTINENT OF NORTHEASTERN MINNESOTA. Vervoort, Jeff D., Department Department of of Geosciences, Geosciences, University of of Arizona, Tucson, Tucson, AZ AZ Vewoort, JeffD., (ve~oor@,geo.arizona.edu); Wirth,K.R., K.R.,(wirth@macalester.edu) (wirth@macalester.edu) & & Kennedy, Kennedy,B.C., B.C., (vervoort(geo.arizona.edu); Wirth, (bkennedy@macalester.edu). Macalester College, St. Paul, MN (bkennedy@macalester.edu). The magmatism of the Midcontinent Midcontinent Rift (MCR) (MCR) is dominantly dominantly mafic in composition compositionin in both both the volcanic In some someportions portionsof of the the MCR, MCR, volcanic sequences sequences as well as the underlying intrusive rocks. In however, the magmatism magmatism is strongly strongly bimodal with significant significant volumes of rhyolite rhyolite and and granite granite in in [1]. This This is is particularly true true for for the northeast northeast limb of the North association with basalt and gabbro [I]. km thick thick volcanic volcanic stratigraphy stratigraphy Shore Volcanic Group Group (NSVG) (NSVG) where where approximately approximately 25% 25% of of its its 6.4 6.4 km other evolved evolved lavas lavas 121. [2]. Evolved intrusive rocks rocks are also also prominent prominent in in is composed of rhyolite and other the northwest flank of the MCR and occur in compositionally stratified complexes. Although these stratified complexes. Although northwest flank are dominantly granitic (granite-granodiorite) (granite-granodiorite) in composition, composition, intermediate intermediate rocks rocks (monzodiorite(monzodioritein the the lower lower portions portions of ofthese thesecomplexes. complexes. Collectively these quartz monzodiorite) predominate in granophyres because of their their characteristic characteristic gmophy& granophyrictextures. textures. The field relahave been termed graniphyres tions, petrography, and geochemistry of these complexes are described by Kennedy petrography, and geochemistry of these complexes are described by Kennedyet et al. al.[3]. [3]. similarities to to the the overlying overlying volcanic volcanic rocks. rocks. Both The granophyres granophyres have striking geochemical similarities 8-67% SiO2 S i O (Fig. 1). 1). have a strong bimodal character illustrated by the compositional gap at 558-67% The chemistry of the rhyolites and the granites are similar in many many respects (e.g., (e.g., major and trace elements, REE patterns) which begs the interpretation elements, interpretation that the granophyric granophyric complexes complexes represent represent the magma chambers chambers that supplied supplied the large rhyolitic rhyolitic eruptions. eruptions. The NSVG lavas are isotopihe NSVG i&topi7 (Fig. 2). 2). Most cally heterogeneous (Fig. Granophyre ~ r a u ~ p ~Complexes Complexes qm values close close basalts e.,, values basaltshave haveinitial initial£Nd • Greenwood 6 1 &wood Lake Lake [4, 5]. 51. to zero or slightly negative negative [4, A A Misquali MiquahHills Hilh The rhyolites, <)Eagle The rhyolites, in in contrast, contrast, typically typically 5 @ EagleMountain Mountain • Pine 0 PineMountain Mountain eNd have strongly strongly negative negativeinitial initial8Nd 4 o Finland values (down values (down to -15) - 15)indicating indicatingthey they were formed formed in in large large part by the the par- 1(20 4 t 3 a NSVG Rhyolites melting and tial melting and assimilation assimilation of of older (most Archean) crustal crustalrocks. rocks. (most likely likely Archean) C> 2 In contrast contrast to to the the NSVG lavas, lavas, the granophyres are more isotopi1I NSVG transitional baaalia b:alts cally homogenous both individually cally homogenous individually 0 and and collectively collectively (Fig. 2). With one one 60 65 75 80 exception hiexception all all granophyres granophyres have have iniSi02 similar to most values 0 tiale., 0 to to -5, -5, similar tial aNd values Figure 1. Si02 S i Ovs. vs. 1(20 K,0 plot illustrating illustrating the linear array basalts in the MCR [7]. [7]. The The isotoisotoand bimodal distribution of of and bimodal distribution of the the granophyre granophyre pic homogeneity of individual gracompositions. Also shown are data for NSVG tholeiitic Also shown are data for tholeiitic nophyres is demonstrated by the and and transitional basalts [5] rhyolites [6]. Few samples Few samples transitional [5] and compositions of two granophyre (4 of 5 are from the Finland granophyre) plot in the SiO2 from Finland granophyre) plot in the Si02 complexes (Misquah complexes (Misquah Hills and and gap (58% to 67%). (58% to 67%). Greenwood Lake) Greenwood Lake) located located in in central central - n .I - 455055 67 �Cook County. The The Greenwood Greenwood Lake Lake granophyre granophyre ranges from monzodiorite to granite ((Si02 S i O 52% to 72%) but has initialENd values of-0.7 of -0.7to to -1.9 -1.9 (Fig. (Fig. initial 2), an extremely extremely small small range considerconsidering analytical analytical reproducibility reproducibility (±0.3 (±0.to to 0.4&Nd G units). The Greenwood Lake ENd 0.4 NSVG Rhplites granophyre was emplaced emplaced during during the the early stage stage of the MCR MCR (1107 (1 107Ma; Ma; [8]). [8]). A Misquah Hilb granophyre The Misquah Hills granophyre has not Misquah granophyre been dated but is likely of similar age because it intrudes because intrudes a sequence sequence of older, 45 50 55 60 65 70 75 reversed polarity lavas. These new isotopic data, in in conconFigure 2. 2. Si02 Figure S i O vs. vs. initial initial EENd of the granophyre junction with major and trace-element chemistry and previously published Nd complexes. Also Also shown shownare are data data for for NSVG NSVG tholeiitic tholeiitic results from 6 other granophyres [6], basalts [5] and rhyolites [6]. Note the Nd isotopic basalts and rhyolites [6]. Note the isotopic [6], homogeneity of the Misquah Hills and Greenwood provide important constraints on the origin of the granophyre Lake granophyres granophyres over over a wide wide range range of of Si02. SiO. granophyre complexes. complexes. First, the granophyres graniphges are isotopically i~ot6~icall~ distinct from the rhyolites. Although Althoughtwo twoof ofthe theCook Cook County County rhyolites rhyolites(Kimball (KimballCreek, Creek,Red Red Rock) Rock) about -5, this is more negative (Eagle have initial G~ values than all but one of the granophyres ENd genetically related to Q = = -7.7). -7.7). This indicates that the granophyres are probably not genetically Mtn., initialENd Mm., initial the rhyolite flows and do not represent the the magma magma chambers chambersfrom fromwhich whichthe therhyolites rhyoliteswere werefed. fed. It is important to note, however, that the granophyres most thoroughly studied thus far (Misquah likely older older(-1 (l 107 Hills, Greenwood Lake) are likely 107Ma). Ma). ItItisisnot notuntil untilthe themain mainstage stageof of the the rift rift (<1100 ( 4 100 &Nd values were erupted erupted in in the the NSVG. NSVG. Ma, [9]) [9]) that thatthe thelarge largerhyolites rhyoliteswith withthe thehighly highlynegative negative Secondly, homogeneity. This Secondly, the two granophyres granophyres examined thus far show remarkable isotopic homogeneity. This implies that these complexes have been derived from a single parental parental magma magma or or from from magmas magmas that are isotopically similar. In In the the case case of of the the granophyre granophyre complexes, complexes, the high silica portions could not have been derived to a significant significant extent from melting or assimilation of older crust. Rather, Rather,ititisis complexes were were produced more probable that the limited isotopic variations within the granophyre complexes minor assimilation. assimilation. It remains by fractionation and mixing processes with only minor remains possible possible that that the the granitic rocks have been derived by partial melting and cannibalization cannibalization of crystallized crystallized MRS magmas lower in the crust, but this seems unlikely considering the refractory nature of such rocks and the availability of less refractory older crust present in the lower crust, especially during early stages stages of of rifling. rifting. sd - sd References: References: [I] [1] J.C.Green, 1982, Geol. Soc. Soc. Am. Am. Mem., Mem., 156,47. 156, 47. [2] J.C.Green and T.J.Fitz, J. Volcan. Geotherm. Geothenn. Res., 54, 177 177 (1993). [3] [3] B.C.Kennedy, B.C.Kennedy, K.R.Wirth, K.R.Wirth, and and J.D.Vervoort, J.D.Vervoort, this this vol[4]J.C.Brannon, J.C.Brannon, Ph.D. Ph.D. thesis, thesis, St. St. Louis Univ. (1984). [5] L.Dosso, L.Dosso, Ph.D. thesis, Univ. ume. [4] (1984). [5] (1984). [6] [6]J.D.Vervoort J.D.Vervoortand andJ.C.Green, J.C.Green, Can. Can. J. Earth Earth Sci.,34, Sci.,34,521 [7]S.W.Nicholson, S.W.Nicholson, Minn. (1984). 521 (1997). [7] K.J.Schulz, and and J.C.Green, J.C.Green,Can. Can.J.J.Earth EarthSci., Sci.,34,504 34, 504(1997). (1997). [8] D.W.Davis and S.B.Shirey, K.J.Schulz, J.C.Green, Can. J. Earth Sci., Sci., 34,476 [9] J.D.Miller J.D.Millerand andJ.D.Vervoort, J.D.Vervoort,ILSG ILSG abstr, abstr, (1996). (1996). J.C.Green, 34, 476 (1997). [9] 68 �ARCHEAN DIAMOND EXPLORATION EXPLORATION TARGETS IN IN THE THE MICHIPICOTEN MICHIPICOTEN GREENSTONE GREENSTONE BELT, BELT, WAWA, ONTARIO WILSON, A. A. C., C., Ministry Ministryof ofNorthern NorthernDevelopment Development and and Mines, Mines, Ontario Ontario Government Government Complex, Complex, Highway 101 101 East, East. WILSON, Porcupine, Ontario, P0N 0. Bag IHO (705-235-1614; (705-235-1614;ann.wilsonfiindm.gov.on.ca ann.wilson(ndm.gov.on.ca Bag 3060, 3060, South South Porcupine, PON 1HO P. 0. Traditionally, diamond exploration Traditionally, exploration in Ontario Ontario has focussed focussed on following following KIM dispersion dispersion trails trails and and on on examining geophysical targets believed to be related related to to kiiberlite kimberlite pipes. pipes. However, the 1993 announcement 1993 announcement of the discovery of three three alluvial alluvial diamonds diamonds recovered recovered from the the Michipicoten Michipicoten River River area area south south of of Wawa Wawa has has led led to tothe the search for much different diamond diamond sources. sources. Surmacz and the frst first discovery discovery of of Archean-aged Archean-agedbedrock bedrock occurrence occurrence of of In 1995, 1995, S. Sunnacz and M. Hauseux made the diamonds in Ontario kilometers north of the town of Wawa. Following diamonds Ontario in Lalibert Lalibert Township, Township, approximately approximately 20 kilometers Following this this discovered nine the same geographic geographic region region in announcement, Spider announcement, Spider Resources Resources discovered nine more diamondiferous dikes in the 1997. Early Earlythis this year, year, Ban-Ore Resources Ltd. announced the discovery of at least one more diamondiferous diamondiferous dike dike Township. To date, commercial stones, 42 macrodiamonds (the GQ Occurrence), macrodiamonds and in Musquash Musquash Township. date, 55 commercial and 338 338 Occurrence), this this time time in microdiamonds have microdiamonds have been recovered recovered from from bedrock bedrock in in an an area area approximately approximately30 30 square square kilometers in size. size. Presently, bedrock in in the the area. The Presently, there there appear appear to to be two two types types of Archean diamond-bearing bedrock The "Sandor"Sandoractinolite+talc+carbonatexenoliths xenolithsranging ranging type" dikes dikes are are mafic mafic in in composition compositionand and contain contain up up to to 35% 35% spectacular spectacular actinolite+talc+carbonate up to 11 metre in size. The fine-grained matrix is foliated. These These cocks rocks have been classified as lamprophyres. These These dikes are detailed P. Sage Sage of of the the Ontario Ontario Geological GeologicalSurvey Surveyin in Sudbury. Sudbury. detailed in in another another featured featured poster poster by by R. R. P. The second type of dike, the GQ-type, is more strongly strongly foliated than the first frst type and and very very closely closely resembles the intennediate intermediate to to felsic felsic tuffs tuffs that host them. The Thematrix matrixisisfute-grained, fme-grained,actinolite-rich actinolite-richand andup up to to 60% 60% subangular, supracrustal xenoliths have been observed in the the dikes. dikes. These rocks have yet to be classified. These Both maximum length length is is unknown. unknown. The 10 m wide) and their maximum Both types typesof ofdikes dikesare aretypically typicallynarrow narrow(0.1 (0.1— 10 dikes trend both northeast northeast and and northwest northwest and, and, as as yet, seem seem to to display display no no readily readily recognizable recognizablegeophysical geophysicalsignature. signature. Both types types of of dikes dikes are are metamorphosed metamorphosedand and are are presumed presumed to to be be slightly slightlyolder older than than the the supracrustal supracrustalrocks rocksthat thatthey they diopsides or KIM signature signatureassociated associatedwith with these these occurrences occurrences is unusual in that there are no chrome diopsides intrude. The TheKIM chrome chrome pyrope gamets garnets related related to the dikes. Chromite Chromiteand andilmenite ilmeniteare are the the only only two two indicator indicator minerals minerals presently presently thought to be associated associated with with these these occurrences. occurrences. These dikes are concentrated concentrated in the central part of the Michipicoten Michipicoten greenstone greenstone belt and and are are related related to to aa synfonnal anticliiie centred on Leclaire Township. These dikes appear to be related to a nappe structure lying synformal anticline centred These dikes appear to be related to a nappe structure lying on on the the northwest shoulder of the Kapuskasing Structural Zone. northwest shoulder of the Kapuskasing Structural Zone. The recent discovery discovery of Archean-aged diamondiferous diamondiferousdikes dikes within the Kapuskasing Kapuskasing Structure Structurein in Ontario Ontario has significantly significantlyinfluenced influenced diamond diamond exploration explorationin in the the province. province. - 69 �DIAMONDS AND AND DIAMOND DIAMOND EXPLORATION EXPLORATION IN IN NORTHERN NORTHERN ONTARIO ONTARIO WITH WITH A FOCUS ON THE MICHIPICOTEN MICHIPICOTENGREENSTONE GREENSTONE BELT, BELT, WAWA, WAWA, ONTARIO WILSON, C., Ministiy Ministryof ofNorthern NorthernDevelopment Development and and Mines, Mines, Ontario Ontario Government Complex, Highway 101 101 East, East. WILSON, A. A. C., P. 0. 0.Bag Bag3060, 3060,South SouthPorcupine, Porcupine,Ontario, Ontario,PON PON1HO 1HO(705-235-1614; (705-235-1614;ann.wilson(1iindm.gov.on.ca) ann.wilson@,ndm.eov.on.ca) Since 1899, it has been postulated that the source of the alluvial diamonds found south of Lake Superior originated east or west of James Bay. Blue Blue (1899) (1899) made this suggestion based on the fact that many of the gravels and boulders found in the glacial drift of Wisconsin, Michigan and Ohio, for example, are identical in composition and structure to the materials north of the Great Lakes. The first in hedrock, bedrock, in in Ontario, Ontario,was wasin in 1914. 1914. Microscopic diamonds were f i r t reported occurrence of diamond in reported by the Ontario Department of Mines in a sample collected from the claims of D. O'Connor in inReaume Reaume Township, located approximately 32 kilometers north of Porcupine. Exploration for diamonds in Ontario began in earnest in about 1960 1960 when the presence of of diamond indicator minerals (KIMs) (KIM5) was was discovered discovered by by Selco SelcoExploration Explorationininthe theMoose MooseRiver Riverdrainage drainagebasin. basin. The first confirmed kimberlite kimherlite was discovered in Hogg Township by Aquitaine Company of Canada iii in 1974. 1974. The James Bay Lowlands have been a diamond exploration hotspot since the late 1980's 1980's and andhas hascontinued continued up to to the the present. present. Exploration has involved companies such as KWG Resources, Resources, to be a favourite exploration target up Spider Resources, Ashton Mining of Canada, Continental Precious Metals, Garde Mining Exploration, B. B. P. Ltd. Exploration kimberlite clusters, the Resources Canada Ltd. and Monopos Ltd. Exploration has led to the discovery of several kiiherlite most notable being the cluster situated in the the Attawapiskat Attawapiskat River Riverarea. area. During the winter winter of 1999-2000, Monopros Monopros Ltd. has been in the process of extracting a 10,000 10,000 tonne bulk hulk sample from the Victor kimberlite kimherlite pipe. Traditionally, diamond exploration has focussed on following KIM dispersion trails and on examining examining related to kimberlite kimberlite pipes. pipes. However, the 1993 announcement announcement of the discovery of geophysical targets believed to be related three alluvial diamonds recovered from the Michipicoten River area south of Wawa has led to the search for for much much different diamond sources. In 1995 Surmacz and M. Hauseux made the first discovery of Archean-aged bedrock occurrence of of 1995 S. Surmacz diamonds in Ontario in Lalibert Township, approximately approximately 20 20 kilometers kilometersnorth northof ofthe thetown townof ofWawa. Wawa. Following this announcement, Spider Resources discovered nine more diamondiferous dikes in the same geographic geographic region in in 1997. Early Early this this year, year, Ban-Ore Ban-OreResources ResourcesLtd. Ltd.announced announcedthe the discovery discovery of of at at least leastone onemore more diamondiferous diamondiferous dike dike Township. To (the GQ Occurrence), this time in Musquash Township. To date, 5 commercial commercial stones, 42 macrodiamonds and 338 from bedrock hedrock in an area area approximately 30 square kilometers in in size. microdiamonds have been recovered from Archean diamond-bearing diamond-bearingbedrock bedrockin inthe thearea. area. The "SandorPresently, there appear to be two types of Archean actinolite+talc+carhonate xenoliths ranging ranging type" dikes are are mafic in in composition composition and and contain contain up to 35% 35% spectacular actinolite+talc+carbonate fme-grained matrix matrix is foliated. foliated. These rocks have been classified as lamprophyres. lamprophyres. up to 11 metre in size. The fine-grained The second type of dike, the GQ-type, is more strongly foliated than the first type and very closely tuffs that that host host them. them. The matrix is fine-grained, actinolite-rich actinolite-rich and up up to 60% resembles the intermediate to felsic tuffs been observed observed in in the the dikes. dikes. These rocks have yet to be classified. classified. subangular, supracrustal xenoliths have been their maximum maximum length length is is unknown. unknown. The 10 m wide) and their The Both types types of ofdikes dikesare aretypically typicallynarrow narrow(0.1 (0.1—- 10 dikes trend both northeast and northwest and, as yet, seem to display no readily recognizable geophysical signature. Both types of dikes are metamorphosed and are presumed to be he slightly older than the supracrustal rocks rocks that they they intrude. The TheKIM KIMsignature signatureassociated associated with with these these occurrences occurrences is unusual in that there are no chrome diopsides or chrome pyrope garnets related to the dikes. Chromite presently ilmenite are the only two indicator minerals presently Chromite and and ilmenite thought to be associated with these these occurrences. occurrences. The recent discovery of of Archean-aged Archean-aged diamondiferous dikes within the Kapuskasing Structure Structure in in Ontario Ontario will significantly influence influence the way that future future diamond exploration is conducted in the province. 19-124. in Report of the Ontario Ontario Bureau Bureau of of Mines Mines 1900, 1900,p.1 p.119-124 Blue, A. 1899. 1899. Are Arethere there diamonds diamondsin inOntario?; Ontario?; in Gibson, T. W. 1914. 1914. Statistical StatisticalReview Review of of the the Mineral Mineral Industry Industry of Ontario for 1913; 1913; in Ontario Bureau of Mines Annual Report 1913, v. v.23, p.47. 23, pt. pi. 1, 1, p. 47. 70 �PETROGENESIS OF THE SILVER SILVER CREEK DIKE: A 1.1 1.1 Ga INTRUSION IN THE MIDCONTINENT MIDCONTINENT RIFT RIFT SEIFERT, Karl Karl E., E., Department Departmentof ofGeological Geological& &Atmospheric Atmospheric WOLBERS, Jennifer A., and SEIFERT, State University, Ames, Iowa 50011 50011 Sciences, Iowa State The Lake Superior at Silver The Silver Creek dike is a mafic intrusion exposed along Lake Silver Creek cliff north of Duluth, Minnesota. Silver Creek cliff is comprised of several lava flows of the North Shore Silver Creek cliff is comprised Volcanic Group, which the the dike dike intrudes, intrudes,and andaathin thinoverlying overlyingsill. sill. Previous Previous misinterpretations misinterpretations of of the result result of of an an unseen unseen upper upper contact, a contact that is Silver Cliff as a single sill were the is now clearly exposed as the result of of aa tunneling tunneling project project in in the the early early 1990s. 1990s. Samples from field work and drill cores taken during the tunneling project have been analyzed for both maior major and trace trace elements. elements. Major Maior element data from both XRF analysis analvsis and microprobe work indicate that that the Silver Creek dike is is aa diabase consisting predominantly of of labradorite, augite, olivine, and Fe-Ti oxides. The The rare rare earth earth element (REE) pattern of the dike's upper chill zone mimics that that of the the overlying flows flows suggesting suggestingcontam&tion contamination inthe theupper upperpart part of of be aa primitive primitive olivine olivine tholeiite the dike. REE REEpatterns patternsofofthe thelower lowerchill chillzone, zone, however, however, show show itit to to be dataindicates indicatesthat that the theSilver SilverCreek Creekdike dikeformed formed from from with no significant contamination. Isotope Isotopedata a relatively primitive and uncontaminated 0. uncontaminated mantle magma with eNd values near near 0. in 71 �Geochemical Impacts of An An Undisturbed Undisturbed Mineral MineralDeposit Deposit— - Results From the Bend Deposit, Chequamegon National Forest, Wisconsin Laurel G. Woodruff, Woodruff, USGS, USGS,St. St.Paul, Paul,MN MN55112 55112(woodruff@usgs.gov), (woodruffusgs.gov), John John W. W. Attig, Attig, Wisconsin Geological and Natural History Survey, Madison, WI 53705, 53705, and William F. F. Cannon, USGS, Reston, VA 22092 The Bend copper-gold massive sulfide deposit in the Medford District District of of the Chequamegon National Forest, Taylor County, Wisconsin, is hosted by Early Proterozoic volcanic rocks. Pre-Pleistocene Pre-Pleistocene saprolite and 100-120 100-120 feet of Quatemary Quaternary materials materials cap cap the mineralized horizon. Ore Oreconsists consists of of massive massive pyrite with varying amounts amounts of bornite, arsenopyrite, chalcocite, and rare gold-silver chalcopyrite, tetrahedrite-tenantite, bornite, tellurides. The ore zone at Bend contains significant concentrations of arsenic, significant concentrations arsenic, bismuth, bismuth, mercury, tellurium, thallium, and antimony antimony in addition to the principal ore metals, metals, copper copper and gold. Because Because these these elements elements are atypical of surrounding bedrock, the Bend deposit was selected selected to examine examine natural elemental dispersion from a buried massive massive sulfide sulfide in in aa glaciated region. The The influence influence of the Bend mineralization on its environment is characterized using a geochemical characterized geochemical database database that that includes: includes: 1) bedrock, sulfide bedrock, massive sulfide provided by Sharpe Energy and Resources and saprolite obtained from diamond drill core provided and the Jump River Joint Venture; 2) regional samples samples of Quaternary Quaternary materials materials from from Taylor County (Attig, subsurface Quaternary Quaternary material and weathered weathered (Attig, 1993); 1993); and 3) subsurface bedrock from the area of the Bend deposit obtained by rotasonic drilling. The USGS drilled five rotasonic rotasonic drill holes through the Quaternary deposits deposits along along aa 0.5 0.5 mile the strike strike of of the the Bend Bendmineralized mineralizedzone. zone. Nearly mile linear transect that crossed the Nearlv continuous core was recovered from every borehole, including up to several feet of weathered weathered bedrock. The core reveals a complex Quatemary Quaternary section section that that consists consistsof of two two till sheets, several intervals of gray, clay-rich lake sediments, and several several areas areas of of outwash. The older till is gray, carbonate-rich carbonate-rich till of the Marathon Marathon Formation Formation and and the the younger is reddish brown till of the Copper Falls Falls Formation. Formation. The Marathon Marathon Formation Formation both underlies and overlies lake sediments and and outwash. outwash. Bedrock Bedrock in the area area of the Bend deposit is directly overlain by the lowermost outwash outwash unit, whereas the Marathon Marathon Formation Formation is draped over metagabbro metagabbro that forms a bedrock high to the south south of of the the deposit. Detailed Detailed geochemical geochemical analyses analyses of the different units show that in addition to obvious color and grain size distinctions, higher average Ca content also distinguishes the Marathon Marathon Formation Formation from from the the Copper Copper Falls Falls Formation. Formation. As much as 20 meters of saprolite saprolite is preserved over massive sulfide sulfide at the Bend Bend deposit. deposit. Saprolite is leached of most metals that are concentrated in unweathered massive sulfide. Cu, Zn, Au, As, Se, and Te concentrations all decline sharply across the transition from massive sulfide sulfide to saprolite, saprolite, whereas whereas Ba, Mn, Ti, Cr, Ni, and V increase increase markedly. Anomalous Anomalous concentrations concentrations of those elements elements within the massive sulfide sulfide are are mostly mostlv lacking in overlying glacial deposits suggesting that the saprolite saprolite cap prevented physical rock by by glacial glacial movement. movement. However, in the glacial deposits directly dispersal of sulfide rock relatively high high values valuesof ofHg Hgand andxAu arefound. found. This over the Bend mineralization, relatively u are of these these elements elements from frommassive massivesulfide. sulfide. In the downsuggests hydromorphic dispersion of 72 �ice direction from the Bend deposit, gold concentrations are found increasingly increasingly higher in the Quaternary section (Figure 1). The The overall pattern for Hg is less distinct, although Hg values found in both Marathon Formation and Copper Falls Formation tills from rotasonic core are markedly higher than Hg values measured in regional subsurface subsurface samples from the same formations. same formations. This study shows that although regional background geochemical values in glacial glacial deposits can be influenced by anomalous concentrations of elements in bedrock, the nature of both the sources and the sinks for various elements controls the dispersion dispersion subcrop of a massive patterns. ItIt also alsoshows showsthat that understanding understanding the character character of the subcrop sulfide and the intervening surficial stratigraphy is critical in geochemical exploration exploration in In the the case caseof of the the Bend Bend deposit, deposit, metal-depleted saprolite caps the deposit this terrane. In and has protected massive sulfide from glacial dispersion, so sulfide-bearing boulders boulders and traditional metal (such as Cu and Zn) anomalies have not developed even in till near the bedrock surface. However, However, more more subtle subtle dispersion dispersion patterns of other elements elements may indicate the presence of sulfide mineralization. sulfide mineralization. North Ta-4 Ta-4 Ta-3 1350 South South Ta-5 I—...—- Ta-2 1330 Ta-i rr L.r CF 1310 . — •• 1290 — —— C I 'S — A I 0 'U — - C) C — Ow . 1270 ow a) w 1250 1230 •-- -. ailay S.----- ls. t Bedrock surface - OH. . , - —___ - -S d deposit 1210 Figure 1: Cross-section of Bend Bend deposit. deposit. CF = Cross-section of of the the surficial surficial section in the area of = Copper = outwash, Falls Formation, MA = = Marathon Marathon Formation, ow = outwash, ls Is == lake lake sediments, sediments, Ta drill hole. hole. Striped designates each rotosonic drill Striped areas areas show locations locations of samples with greater than .01 .O1 ppm Au. Vertical Verticaland andhorizontal horizontaldimensions dimensionsare are not not to to scale. scale. Reference: Attig, Attig,J.1.W., W., 1993, 1993, Pleistocene Pleistocene geology of Taylor County, Wisconsin: Wisconsin Wisconsin Geological Geological 27 p. and Natural History History Survey Survey Information InformationCircular Circular65, 65,27 p. 73 � https://digitalcollections.lakeheadu.ca/files/original/632fe39ce4815072b1d96ab2bd49fbae.pdf 99b9247da40b34d2229b67c16bf62343 PDF Text Text FORTY-SIXTH ANNUAL MEETING INSTITUTE ON LAKE SUPERIOR .GEOLOGY. thunder Bay, Ontario May 08-13; 2000 * 4 1 PROCEEDINGS VOLUME 46 4 PART 2: FIELD TRIP GUIDE BOOKS S �46th I.L.S.G. 46th I.L.S.G. ield Guide May 8-13, 2000 �46thANNUAL ANNUAL MEETING MEETING INSTITUTE INSTITUTE ON LAKE SUPERIOR GEOLOGY GEOLOGY Volume 46 consists of Part Part 1:1:Program Programand and Abstracts Abstracts Part Part 2: 2: Field FieldTrip Trip Guidebook Guidebook References References to to material material in in this this volume volume should shouldfollow follow the the example examplebelow: below: Ams, Holm, D., 2000, Characterization Characterization and timing constraints of Arns, D. and Hoim, post-Penokean meso-scale structures structures in the Watersmeet and Republic gneiss Institute on Lake Superior gneiss domes domes of northern Michigan (abst.): Institute Superior, Geology 46"' Annual Annual Meeting, Meeting, Thunder Bay, Ontario, Ontario, GeologyProceedings, Proceedings,46th v.46, p.2. part 1, 1, p. 2. v. 46, part Volume Volume 46 46 Published Published and and distributed distributedby by Institute Institute on on Lake Lake Superior Superior Geology Geology Mark Jirsa, Jirsa, Secretary-Treasurer, Secretary-Treasurer, I.L.S.G. I.L.S.G. Minnesota Minnesota Geological Survey Survey 2642 University Avenue Avenue St.Paul,MN USA 55114-1057 551 14-1057 St. Paul, MN USA (612) 627-4780 627-4780 (612) email: jirsa00ltc.unm.edu email:jirsaOOl0,tc.urnn.edu ILSG ://www.Qeo.mtu.edulgreat lakes/ilsg/ ILSG website: website:httn httw://www.geo.mtu.edu/great lakedilsd ISBN ISBN 1042-9964 1042-9964 All volumes are available availablefor photocopying photocopying costs costs from Michigan Michigan Technological TechnologicalUniversity UniversityLibrary Library Archives �INSTITUTE INSTITUTE ON LAKE SUPERIOR GEOLOGY PROCEEDINGS PROCEEDINGS Volume 46 Field Trip Guidebook Compiled by: Philip Fralick �TABLE OF TABLE OF CONTENTS CONTENTS The trip description coded to to aid aid The first first page page of of each each field field trip description is is colour colour coded finding it finding it in in the the book. book. Field Trip Field Trip 1) 1) Mesoproterozoic Sibley Sibley Group Group (yellow) Mesoproterozoic (yellow) Field Trip Field Trip 2) 2) Lac des des Iles Iles Mine Mine (purple) (purple) Lac Field Trip Field Trip 3) 3) Geoarcheology of of the the Thunder Thunder Bay Bay Area Area (red) (red) Geoarcheology Field Trip Field Trip 4) 4) Paleoproterozoic Gunflint Gunflint Formation Formation (blue) (blue) Paleoproterozoic Field Trip Field Trip 5) 5) Quaternary Geology Shebandowan Belt Belt (orange) (orange) Quaternary Geology Shebandowan Field Trip Field Trip 6) 6) Steep Rock-Lumby Belts Belts (green) (green) Steep Rock-Lumby �GEOLOGY OF THE MESOPROTEROZOIC SIBLEY SIBLEY GROUP GROUP Philip Fralick, Lakehead University Mark Smyk, Ontario Geological Survey This of a This trip trip will will examine examine the depositional depositional architecture architec playa-alluvial playa-alZuvialsuccession successiondeveloped developed in in an an intracratonic intra basin. The pristine sequence records the effects basin. The pristine sequence records the effectsof ofboth both climate climateand andtectonics tectonicson onsediment sediment character. character. �Geology and and Stratigraphy Stratigraphy of the Mesoproterozoic Sibley Group Group by Philip Mark Smyk2 and Mariah Mariah Mailman' Philip Fralick1, ~ralick',Mark smyk2 and ail man' 1 a Amethystine stromatolite, Rossport Rossport Formation, Formation, Dorion, Dorion, Ontario Ontario Amethystine stromatolite, 1 'Department of '~e~artrnent of Geology, Geology, Lakehead LakeheadUniversity, University,Thunder ThunderBay Bay ON ON P7B P7B 5E1 5E1 2Ontwio '0ntario Geological Survey, Survey, Resident Resident Geologists Geologist'sProgram, Program,Ministry Ministryof ofNorthern NorthernDevelopment Development and 435 S. and Mines, Mines, Suite SuiteB002, B002,435 S. James James St., St., Thunder Thunder Bay ON ON P7E P7E 657 6S7 �FOREWORD FORE WORD This field of the This field guide guide is is intended intended to to provide provide aacomprehensive comprehensive overview overview of the Sibley Sibley Group Group Trip stops have been sedimentary rocks, their stratigraphy and associated mineral deposits. sedimentary rocks, deposits. Trip selected on on the basis of representivity many other key and noteworthy sites selected representivity and accessibility; accessibility; many cannot be easily accessed but are described in the text. This guide follows in the footsteps of a number of previous field trips that have focused on the Proterozoic geology geology of the Thunder Bay area. Field stop descriptions have incorporated Proterozoic incorporated and built upon and Kustra Kustra et et al. (1972), Kustra built upon those found in guide guide books written written by Franklin Franklin and Kustra (1972), aL (1982) and Kissin (1990), as as well as other, unpublished unpublished reports. (1977), Franklin ci et al. contact the staff of the Please contact the Resident Resident Geologist's Geologist's Office, Thunder Bay South District, (807) 475-1331, 475-133 1, for current information regarding regarding permitted permitted access access to private and staked properties. A CKNO WLEDGEMENTS ACKNOWLEDGEMENTS The authors would like like to thank Scott for for assistance in the the scanning of The authors would thank J. J. Scott assistance in scanning 1I digitization digitization of K. Farrier Farrier and and S. Warren photographs and maps. S. Spivak drafted some of the the diagrams. diagrams. K. Warren photographs the Ruby Lake quarry was permitted permitted by Ruby Lake provided word processing support. Access to the Marble Ltd. Ltd. Discussions with J. Franklin, Marble Discussions with Franklin, H. Poulsen, Poulsen, other other geologists geologists and prospectors prospectors are appreciated. appreciated. �INTRODUC HON INTRODUCTION Sedimentaryrocks rocksof of the the Sibley discontinuouslyoutcrop outcropon on the the north north shore shore of Lake Sedimentary Sibley Group Group discontinuously Lake Superior and around Lake Nipigon (Figure 1). The flat-lying to gently dipping, elastic-carbonate Superior and around Lake Nipigon (Figure 1). The flat-lying to gently dipping, clastic-carbonate succession oval area disconfonnably overlying succession occupies occupies aa broad broad oval disconformably to unconformably unconformably overlying Mesoproterozoic, Paleoproterozoic and Neoarchean rocks. Mesoproterozoic, Paleoproterozoic and Neoarchean rocks. Sibley Group rocks are located located in in the the Southern Southern Province Province of of northwestern northwestern Ontario. Ontario. The first first documentation of these red beds was by Logan (1863). Hunt (1873) divided Logan's Upper documentation of these red beds was (1863). Hunt divided Logan's "Upper Copper-Bearing Series" into the Animikie and Keweenaw groups; the Sibley was assigned to the latter. descriptions of of the the Sibley Group were were provided provided by by Bell (1870), Parks (1901) and latter. Other descriptions Wilson (1910) who compared the Sibley Sibley (then called "Nipigon "Nipigon Series") Series") to to similar similarKeweenawan Keweenawan and Paleozoic rocks rocks elsewhere. elsewhere. Despite similarities to these younger rocks, the Sibley Sibley Group Group rocks were different enough to preclude assigning them a definite Precambrian age. Subsequently, they were were given given the the name name "Sibley by Tanton Subsequently, they "Sibley Group" Group" by Tanton (1931) (1931) after after the the Sibley Sibley Peninsula on on which which they they are are well-exposed. well-exposed. The Peninsula The Sibley Sibley Group Group rocks were studied studied by Hawley Hawley (1930) and Moorhouse (1960), but they were not mapped comprehensively comprehensively until Coates Coates (1972). (1972). Mcllwaine (1971a,b; (1971a,b; 1975) 1975) also also studied studied the the Sibley Group Mcllwaine Group rocks. Franklin Franklin (1970) pursued pursued a doctoral thesis thesis regarding regarding the the metallogeny of Proterozoic rocks in the doctoral metallogeny of Proterozoic rocks the Thunder Thunder Bay Bay District, District, including the Sibley Group. Thereafter, he described the stratigraphy and depositional setting of the Sibley Group (Franklin et al. 1980). 1980). The Thetectonic tectonic setting, setting, source source areas, areas, and and environments environments of deposition Keweenawan strata strata in in the the Lake Lake Superior region were were published published by by Morey Morey and deposition of Keweenawan Superior region and A doctoral thesis by Cheadle (1985) was executed on the "lower Ojakangas (1982). the "lower Ojakangas (1982). A doctoral thesis by Cheadle (1985) was executed Keweenawan" Sibley Sibley Group. Group. In all of of these these studies studies the Sibley Sibley Group Group rocks are presented as Rift. ift. This Thisnotion notionwas was refuted refuted by by Fralick Fralick and and Kissin Kissin being genetically related to the Keweenawan R (1995). (1995). Outcrop exposure and drill core lithologies provide evidence that the Sibley Sibley Group Group sedimentary sedimentary rocks are continuous, near-horizontal strata there were deposited in an an extensive, extensive, low-lying low-lying area. area. For this reason, the structure structure in in which which the the sediments sedimentsaccumulated accumulated will will be be called called the the Sibley SibleyBasin. Basin. The Sibley Basin is elongate elongate in a northwesterly northwesterly direction, extending extending from from the north north shore shore of of Lake Lake It Superior in the south, along the western mar margin of Lake Nipigon, to Armstrong in the north. in Nipigon, It discontinuouslywithin withinaa15 15000 000km km area area (Franklin et eta?. crops out discontinuously al. 1980). 1980). The Theexposed exposed basin basin is now 175 175 km wide and and 400 400 km km long. long. 5 The fluvial fluvial and and lacustrine lacustrine strata strata of of the the Sibley SibleyGroup Groupare aredivided dividedinto intothree threesubhorizontal subhorizontal formations: 1) the the lowermost is the Pass Lake Formation; 2) the Rossport Formation overlies it, formations: followed by 3) 3)the the Kama KamaHill Hill Formation. Formation. The Pass Lake Formation varies from 0 to 50 50 m in in thickness thickness and and consists consists of of basal basal conglomerate conglomerate It was and beds of quartz arenite. arenite. It was deposited deposited in in aashallow shallowlacustrine lacustrine and upward-thinning upward-thinning beds of quartz 135 m thick Rossport Formation is composed The 135 thick Rossport composed of a a!. 1980). environment (Franklin et a!. 1980). lower, arenaceous, red red dolomite unit, a central and stromatolitic unit, and an lower, arenaceous, dolomite unit, central chert-carbonate chert-carbonate and stromatolitic unit, upper, argillaceous, argillaceous, red red dolomite dolomite unit. unit. These These beds were were laid down in aa shallow, shallow, saline saline basin basin of of Fifty metres of purple shale constitute the Kama Hill fluctuating size (Franklin et a!. fluctuating a!. 1980). constitute Kama Hill 1980). Fifty metres �Formation. Formation. Desiccation Desiccationcracks, cracks,evaporite evaporiteclasts, clasts,and and mud mud chip chip conglomerates conglomerates in this purple purple shale were formed on mudflats that were subaerially exposed and dried out periodically (ibid). formed on mudflats that were subaerially exposed dried periodically (ibid). The The lack lack of of fossils fossils in in the the Precambrian Precambrian presents problems in age determination and in interpreting that are are present present in in the Sibley Group rocks the environment environment of of deposition. deposition. The stromatolites stromatolites that (Hoffman (Hoffinan 1969) 1969) can can occur occur in in both marine and fresh water, but the red beds are characteristic of a continental setting (Morey (Morey and Ojakangas Ojakangas 1982). 1982). The thick dolomite and gypsum gypsum sequences sequences formed formed during during intermittent, intermittent, arid arid periods. 33 Ma, to 1537 Historically, has been been postulated postulated that that the the Sibley SibleyBasin Basin(1339 (1339+1+/- 33 1537 Ma; Franklin Historically, itit has 1978a; Davis and Sutcliffe 1985, respectively) developed because of crustal attenuation Sutcliffe 1985, attenuation caused caused described itit as a failed by the Keweenawan Rift System. System. Franklin Franklin et et aL al. (1980) described Keweenawan Midcontinent Rift arm, radial radial to aa paloplume palasoplumeininrelation relationthe theKeweenawan Keweenawan Rift. Rift. Morey Morey and Ojakangas (1982) described a sedimentary-tectonic framework for the Sibley Basin that is is related related temporally temporally and and sedimentary-tectonic framework for the Basin that spatially Rift; the Sibley subsidence of of the Sibley Basin developed developed as renewed renewed subsidence spatially to the Keweenawan Keweenawan Rift; Conversely, Cheadle (1986) described Animikie Animikie Fault Block Block Basin Basin (2200-2000 (2200-2000 Ma). Ma). Conversely, Cheadle (1986) described the tectonic framework of the Sibley Basin as broad, subsiding sag where significant fault control is lacking at the Sibley Group red beds are representative of the the margins. margins. It was thought that the Sibley earliest evidence Event. However, evidence of the Keweenawan Rift Event. However, this this interpretation interpretation is problematic. The oldest igneous activity that that isis related relatedtoto rifting riflingisisdated datedatat-1l 109 igneous activity 109 Ma Ma (Davis (Davis and and Sutcliffe Sutcliffe 1985) aa minimum minimum of of -230 230 My Myafter afterthe theSibley SibleyGroup Groupwas was deposited. deposited. Conceptually, Conceptually, aa thermal thermal dome precedes rifting, rifling, and erosion will will occur. occur. It is reasonable to presume that volcanism can be be that volcanism can is preceded by sedimentation on this this time time scale, scale,but butthe thepalasodrainage palodrainage system sedimentation on system will will develop develop away away from the dome. This palocurrent work This conflicts conflicts with the palasocurrent workdone doneby by Cheadle Cheadle (1986) (1986) in which the palodrainage palasodrainagesystem systemflowed flowedtoward towardthe thethermal thermal welt. welt. It is unreasonable to hypothesize that the pre-rifi pre-rift basin was was overlying overlying the the thermal thermal welt. welt. Thus, the presumed presumed relationship relationship of the Sibley Sibley Basin with the Keweenawan Rift must must be be reappraised. reappraised. The two events are not related directly Keweenawan Rift the Sibley Sibley Basin Basin may may be be genetically linked to the Kissin 1995). 1995). Alternatively, Alternatively, the genetically linked (Fralick and Kissin heating event represented by the igneous complex formed at Redstone Point, Lake Nipigon heating represented igneous complex formed Redstone Point, (1537 Ma; Davis SibleyGroup Group Davis and and Sutcliffe Sutcliffe 1984) 1984)as as these these igneous igneous rocks rocks lay lay directly directly under under the the Sibley Basin (Fralick (Fralick and Kissin 1995). 1995). In Inthis thisscenario, scenario,mantle mantle upwelling upwelling would cause partial melting of the lower crust, forming the Redstone Point Point felsic felsic magmas. magmas. The higher heat heat flow flow would would cause cause thermal doming the dome. dome. Upon relaxation of the crust an infracratonic infracratonic doming with erosive stripping of the This basin basin, basin, the Sibley Sibley Basin, Basin, would would be be created. created. This basin would would be be saucer-shaped saucer-shaped without without pronounced pronounced boundary boundary faults. faults. This Thiscorresponds correspondsto to the theactual actual basin basin geometry. geometry. REGIONAL REGIONAL GEOLOGY GEOLOGY The Sibley Group rocks rest unconformably on the Paleoproterozoic Animikie Animikie Group Group (Rove (Rove and and Gunflint Formations) and Ojakangas 1982). Formations) and Neoarchean granitic and volcanic rocks (Morey and The Sibley Formation, a series of trough crossSibley Group Group possibly correlates correlates with the Puckwunge Formation, crossstratified, medium-grained, lensoid sandstones occurring in northeastern Minnesota (Mattis medium-grained, lensoid sandstones occurring northeastern Minnesota (Mattis overlies Paleoproterozoic Rove Formation Formation shales shales and 1972). This formation formation unconformably unwnformably overlies Paleoproterozoic Rove 1972). �represents braided fluvial fluvial channel channel deposition, deposition, exhibiting exhibiting strong, strong, represents South South Saskatchewan Saskatchewan River-type, River-type, braided unimodal flow to the southeast. unimodal flow to the southeast. The and sandstone unit, the the Pass The Sibley Sibley Group Group consists consists of of aa lower lower basal basal conglomerate conglomerate and sandstone unit, Pass Lake Lake Formation, and an Formation, aa middle middle unit unit of of red red and and buff buff dolomitic dolomitic mudstone, mudstone, the the Rossport Rossport Formation, Formation, and an upper unit, unit, the to siltstone to upper the Kama Kama Hill Hill Formation, Formation, consisting consisting of of purple purple shale shale and and buff-coloured buff-coloured siltstone fine has subdivided the Sibley to the fine sandstone. sandstone. Cheadle Cheadle (1986) (1986) has subdivided the Sibley Group Group according according to the different different sedimentary facies. From sedimentary facies. Frombottom bottom to to top: top: Pass Lake Lake Formation Formation Pass Loon Loon Member Member (basal (basal conglomerate conglomerate facies) facies) Fork Bay Member (plane-bedded Fork Bay Member (plane-bedded facies facies and and cross-bedded cross-beddedfacies) facies) Rossport Formation Rossport Formation Channel facies and Channel Island Island Member Member (cyclic (cyclic facies and mudstone mudstone facies) facies) Middlebrun Bay Bay Member Middlebnm Member (stromatolite (stromatolite facies) facies) Fire Hill Hill Member Fire Member (chalcedonic (chalcedonic mudstone mudstone facies) facies) Kama is constituted wholly by by aa laminated shale facies facies and and thus, Kama Hill Hill Formation Formation is constituted wholly laminated shale thus, is is not not subdivided. subdivided. PASS FORMATION PASS LAKE LAKE FORMA TION The Loon Lake Member Member is is best best exposed exposed at at Pass Pass Lake Lake in in aa cliff, The Loon Lake cliff, consisting consisting of of the the Pass PassLake Lake At the Formation, which which runs runs parallel parallel to to the the Canadian Canadian National National Railway. Railway. At the north north end end of of the the Formation, outcrop, the the Paleoproterozoic Rove Formation Formation is is visible visible where where the the lowermost lowermost beds beds of of the the Pass outcrop, Paleoproterowic Rove Pass Lake Formation disconformably overlie overlie it. it. The The Pass Pass Lake Lake Formation Formation ranges ranges in in thickness thickness from from Lake Formation disconformably The Loon facies, consists consists of of thin, 18 thin, lensoid lensoid 18 to to 50 50 m. m. The Loon Member, Member, the the basal basal conglomerate conglomerate facies, conglomeratic sediment. sediment. ItItisisthickest near the the basin basin margin margin and and it it thins thins down to conglomeratic thickest(-'-15 (-15 m) near to about about Animikie taconite, taconite, jasper, jasper, and and shale one metre in the the basin basincenter center(Cheadle (Cheadle 1986). 1986). Animiie shale clasts clasts one metre in predominate in in the over Animikie rocks whereas predominate the conglomerate conglomerate deposited deposited over Animikie rocks whereas vein vein quartz quartz vein, vein, granite, and metavolcanic rock clasts dominate where an Archean basement is present clasts dominate an Archean is present (Cheadle (Cheadle and are are < 5 30 30 cm cm in in diameter diameter (Morey (Morey and and Ojakangas Ojakangas 1982) 1982) and are in in matrixmatrix1986). The The clasts clasts are 1986). The polymictic polymictic matrix matrix isis brick-red brick-red (Franklin (Franklinetet aal. and it support (Cheadle 1986). 1986). The t. 1980) 1980) and it is is composed of of medium medium to to coarse coarse grains grains of of quartz, quartz, hematite, hematite, and and mudstone mudstone(Cheadle (Cheadle1986). 1986). The The composed conglomerate has has calcareous calcareous cement. cement. The with deeper and conglomerate The lenses lenses have have scour scour and and fill fill structures structures with deeper and They are narrower channels channels cut proximal narrower proximal to the the source. source. They are cross-bedded cross-bedded with with rare rare dolomite dolomite mudstone layers. layers. At with the the lower lower half half of of the the bed bed in in clast clast mudstone At Silver Silver Islet, Islet, the the lensoids lensoids are are graded graded with support that changes into matrix support in the upper half (Cheadle 1986). support that changes into matrix support in the upper half (Cheadle 1986). The Fork Fork Bay Bay Member Member is is dominated dominated by by medium-grained medium-grained sandstones sandstones which which are are subdivided subdivided into into aa The lower plane-bedded plane-bedded facies facies and an an upper cross-bedded facies. ItItoverlies overliesthe theLoon LoonMember. Member. Plane Bedded Facies: 20 Plane Bedded Fades: 20 to to 80 80m mofofplaneplane-totocross-bedded cross-bedded quartz quartz arenite arenite overlie overlie the the basal basal conglomerate (Cheadle 1986). This buff-coloured sandstone consists of well-sorted, rounded to This buff-coloured well-sorted, �well-rounded, coarse-grained, mature subarkose to sublitharenite sublitharenite well-rounded, finefine- to to coarse-grained, mature quartz quartz arenite arenite to to subarkose and Ojakangas Ojakangas 1982). Minor quartzfragments fragmentsare arepresent. present. As (Morey and Mmor chert and polycrystalline quartz layers that that separate the 5 cm well, there are thinly interbedded dolomitic mudstone and siltstone layers to 11 m thick thick sandstone sandstone beds. The Theupper upper 55to to 10 10cm cmof ofsome somesandstone sandstonebeds beds contain contain mud mud chips. chips. The plane-bedded facies hosts an array of sedimentary structures plane-bedded facies hosts an array of sedimentary structures including including horizontal laminations and and parting laminations parting lineations, lineations, isolated isolated planar planar cross-beds, cross-beds, straightstraight- to to sinuous-crested sinuous-crested oscillation ripples, rare current ripple laminations, rare sand volcanoes and dewatering tubes, and and oscillation volcanoes and dewatering tubes, desiccation desiccation cracks cracks and halite halite clasts clasts in thinly interbedded mudstone (Cheadle 1986). Cross Bedded Fades: Facies: The Thecross-bedded cross-bedded facies facies is is 55 to to 10 10m m thick, thick, with with mediummedium- to large-scale, large-scale, cross-bedded arenite intercalated beds at cross-bedded intercalated with with 1 m thick thick planar, planar, cross-stratified cross-stratified sandstone sandstone beds Rossport, Red Rock, Channel Island, contains scoured scoured bases and Rossport, Island, and Pass Lake. Lake. This facies contains The ripple-marked ripple-marked and and mud-cracked mud-crackedquartz quartz arenite arenite beds beds have have pebbly (Cheadle 1986). 1986). The pebbly infill (Cheadle horizontal to to wavy wavy stratification stratificationwith withminor minortrough troughcross-beds. cross-beds.The Thepalaeocurrents palocurrents of the crosshorizontal beds at Pass Lake and the Enterprise Mine are scattered, with predominantly south-southeast and east-southeast directions, east-southeast directions, respectively (Morey and Ojakangas 1982). aL (1980) logged the 50 m m cliff cliff exposure exposure at at Pass PassLake. Lake. Twenty metres of of buff arenite Franklin et al. rest on the basal lensoid conglomerate. This This buff buff arenite arenite thins upward from 1 m thick beds at the base to to 1 to 22 cm The sequence alternates alternates upwards upwards from from dolomite dolomite to cm thick thick beds beds at at the the top. top. The base arenite. They recorded 1 m of red dolomitic arenite to 13 m of buff arenite; 1 m of thinly bedded They recorded 1 m of red buff arenite; 1 m of thinly bedded m of of buff buff arenite; arenite; 1 m m of ofred reddolomitic dolomiticarenite; arenite;and and 77m mof of red dolomite and dolomitic arenite; 10 m thickly bedded buff arenite. arenite. AAsharp sharpincrease increase in inthe thedolomite dolomite content, content, aa pigmentation pigmentation change, and a gradual decrease in clastic content mark the upper boundary of the Pass Pass Lake Lake Formation Formation et aL al. 1980). 1980). (Franklin (Franklin et At Redstone Point, along the At the Lake Lake Nipigon Nipigon shoreline, shoreline, the Pass Pass Lake Lake Formation Formation overlies an anorogenic, felsic intrusive-extrusive granitoid suite (Fralick and Kissin 1996, unpublished data) dated at 1537 One metre below the dated 1537 Ma Ma (Davis (Davis and and Sutcliffe Sutcliffe 1985). 1985). One the volcanic-sedimentary volcanic-sedimentary contact, the the igneous rocks rocks are weathered weathered and and the the feldspar feldspar exhibits exhibits aa chalky, chalky, cream cream to to salmon contact, colouration. There There is aa 30 colouration. 30cm cmthick thickvolcanic volcanic breccia breccia with with caliche caliche overlying overlying the the weathered weathered rocks. The igneous rocks. ThePass PassLake LakeFormation Formation isis lacking lacking the the basal basal conglomerate conglomerate at Redstone Redstone Point. Point. Instead, there are fine-grained, red sublithareniite to subarkosic sandstones with massive or Instead, fine-grained, sublitharenite to subarkosic sandstones with massive or parallel-laminated beds. beds. Trough These strata contain very angular, Trough cross-stratification cross-stratification is rare. These volcanic clasts clasts with with oxidized oxidizedrims rimsand andweathered weathered granular to pebble to small cobble-sized, felsic volcanic pebbles. feldspars. There Thereare arealso alsoless lessabundant abundant clasts clasts of of hematitic hematitic mudstone, chert, and caliche pebbles. The basal pebble sandstone sandstone is poorly sorted with subrounded to rounded sand and small angular grains. The hascarbonate carbonatecement cement with with clay filling between the grains. The beds range from from 11 to pebbles. ItIt has 80 cm thick, with a mean thickness of 15 cm. They They are are laterally laterally continuous continuous over several metres with rare, 1 m wide channels. Tightly Tightlypacked packed quartz quartzarenite arenite overlies the basal pebbly sandstone. contains minor minor clay clay in the matrix, The quartz arenite is well-sorted, well-sorted, subangular to subrounded, contains and has quartz cement. The Thearenites arenitesare are folded folded slightly slightly and and plunge at <15° <15' to the east east (Fralick (Fralick and Kissin 1996, 1996, unpublished data). �ROSSPORT FORMA FORMATION ROSSPORT TION The The Rossport Rossport Formation Formation consists consists of of red, red, fine-grained, fine-grained, arenaceous arenaceous and and clayey clayey dolomite. dolomite. It It lies lies disconformably on the the Pass Pass Lake Lake arenite arenite (Franklin (Franklin et et at. al. 1980) disconformably on 1980) and and it it has has aamaximum maximum vertical vertical outcrop of 134 m at marker bed bed (Morey and outcrop of 134 m at its its type type locality locality where where it it is is lacking lacking the the stromatolite stromatolite marker (Morey and Ojakangas 1982). North Ojakangas 1982). North of of Nipigon Nipigon and and north north of of Dorion Dorion there there have have been been 300 300 m m and and 180 180 m m of of the et aL al. (1980) (1980) logged logged and and subdivided subdivided the the the Rossport Rossport Formation Formation drilled, drilled, respectively. respectively. Franklin Franklin et Rossport Formation, Formation, starting starting with with aa lower, lower, argillaceous argillaceousred reddolomite dolomiteunit. unit. They recorded 11 m m of Rossport red through the the sequence, there is red conglomerate conglomerate at at the the base. base. Upwards Upwards through sequence, there is 96 96 m mof ofwell-bedded, well-bedded, brick and arenaceous dolomite intercalated intercalatedwith withbuff buff to to white white arenite, arenite, 11 to to 22 m m of brick red red dolomite dolomite and arenaceous dolomite of black chert, gray chert, and buff calcite that alternate from 0.5 to I cm thick laminae with a lowblack chert, gray chert, and buff calcite that alternate from 0.5 to 1 cm thick laminae with a lowamplitude is topped amplitude crinkle crinkle structure. structure. The The sequence sequence is topped with with 37 37 m m of ofmassive massive red reddolomite dolomite and and arenaceous and irregular arenaceous dolomite dolomite that that fractures fracturesconchoidally. conchoidally. Spherical Spherical and irregular reduction reduction spots spots are are noted throughout the the upper upper and and lower lower units. units. The noted throughout The top top of of the the Rossport Rossport Formation Formation is is marked marked by by aa change as it gradually to to zero. zero. The change in in the the dolomite dolomite content content as it decreases decreases gradually TheRossport Rossport Formation Formation rests rests unconformably on Archean Archean rocks rocks at at Kama Kama Bay. Bay. At unconformably on At this this location, location, as as well well as as at atPass PassLake, Lake, Red Red Rock, and Moseau Mountain, the Rossport's basal, basal, red red conglomerate conglomerateis is not not present. present. Cheadle has divided Cheadle (1986) (1986) has divided the the Rossport Rossport Formation Formation into into four four facies facies from from base base to to top: top: Channel Island Member (cyclic and mudstone facies) Channel Island Member (cyclic and mudstone facies) Middlebrun Middlebrun Bay Bay Member Member (stromatolitefacies) facies) (stromatolite Fire Fire Hill Hill Member Member (chalcedonic (chalcedonic mudstone mudstone facies) facies) Cyclic of the Member isis 77 to It has has Cyclic Facies: Fades: The The cyclic cyclic facies facies of the Channel Channel Island Island Member to 30 30 m m thick. thick. It rhythmic layering of mudstone and dolomite couplets or mudstone and sandstone sandstone couplets. The The couplets couplets range range from from 5 5 to to 160 160 cm cm in in thickness. thickness. The Thesandstone sandstoneand anddolomite dolomiteare areend endmembers members that that have throughout the the sequence. sequence. The have various various degrees degrees of of mud mud interlayering interlaye~g throughout The dolomitic dolomitic mudstone mudstone is is red, massive, friable, and tabular. ItIt isis common common to to both both cycles. cycles. The Themudstone mudstoneoften oftencontains containssilt silt or or sand sand and and rounded, rounded, reworked reworked pebble-sized pebble-sized carbonate carbonate clasts clasts (Cheadle (Cheadle 1986). 1986). Red Red Rossport Rossport Formation Formationmudstones mudstones with with diagenetic diagenetic evaporite evaporite minerals minerals formed by by precipitation precipitation in in aa clastic-dominated, clastic-dominated, sabkha sabkha setting setting Mudstone The Mudstone Facies: Fades: The mudstone mudstone facies facies has has �red red and and buff, buff, dolomicritic dolomicritic mudstone mudstone and and claystone claystone that that are are dominantly dominantly massive massive and and friable, friable, but but have local desiccation cracks and adhesion ripples, and small, scattered gypsum and anhydrite have local desiccation cracks and adhesion ripples, and small, scattered gypsum and anhydrite nodules. One to nodules. One to two two metre metre thick thickfinefine-totomedium-grained, medium-grained, poorly poorly sorted, sorted, subarkose subarkose and and sublitharenite with 10% matrix are sporadically introduced into the dolomicritic mudstone. matrix are sporadically introduced into the dolomicritic mudstone. sublitharenite with These subarkose and sublitharenite beds have powering-down powering-down sequences: scoured scoured bases, bases, planeplanebedded laminations, ripple cross-laminations, and horizontal, suspended load laminations bedded laminations, ripple cross-laminations, and horizontal, suspended load laminations (Cheadle 1986). (Cheadle 1986). Stromatolite Facies: Fades: The Stromatolite Thestromatolite stromatolitefacies, facies,or or the the Middlebrun Middlebrun Bay Member, which is located in the middle of the Rossport Formation, is 80 to 140 cm thick. The Thegray graychert-carbonate, chert-carbonate,cryptcryptalgal laminate is laterally laterally extensive, extensive, but but commonly commonly brecciated. brecciated. The base of this this marker marker bed bed has has alternating layers of light gray dolomite, dark gray chert, and crinkly laminae. Moving upwards, Moving upwards, there is a thin thin chert-carbonate chert-carbonate breccia, a layer layer of of laterally laterally linked linked and and domical domical single single parabolic - parabolic stromatolites, and a dark gray chert band (Cheadle 1986). stromatolites, gray chert band (~headle1986). I 1 I I I I Plan Plan view view section of stromatolitic marble garganicus), (Conophyton garganicus), Wolatko Lake Mudsione Facies:. Facies:. The upper Rossport Rossport Formation is is composed composed of of the the chalcedonic chalcedonic Chalcedonic Mudstone mudstone facies. At the base, there are local lenticular, intraformational, paraconglomerate mudstone facies. the base, there are local lenticular, intraformational, paraconglomerate wedges that that contain contain subangluar subangluar mudstone mudstoneclasts clastsup uptoto22 m m in in diameter. diameter. Reduction Reduction spheres spheres are wedges comnon. Red of common. Redand andbuff, buff,dolomicritic dolomicriticmudstone mudstone and and claystone claystone are the dominant constituents constituents of this sequence. They lack the coarse silt and sand that is present in the Channel island mudstone They lack the coarse silt and sand that is present in the Channel Island mudstone and claystone. Dolomite Dolomitepartially partially replaces replaces 22 cm cm diameter diameter chalcedony chalcedony nodules that are are found found in in weathering horizons. resistant weathering horizons. I �KAJVL4 HILLFORMATION FORMATION KAMA HILL The The Kama Kama Hill Hill Formation Formation consists consists of of aa single single (laminated (laminated shale) shale) facies, facies, and and thus thus is is not not subdivided subdivided into constituent members. The original thickness of the Kama Hill Formation is unknown into constituent members. The original thickness of the Kama Hill Formation is unknown as as the the top top is is truncated truncated by by the the Logan Logan diabase diabase sills. sills. The The Kama Kama Hill Hill laminated laminated shale shale has has aa minimum minimum thickness of 46 46 m m (exposed at Kama Kama Hill), Hill), and and this this varies, varies, depending dependingon onthe the location. location. At thickness of (exposed at At Red Red Rock, Island, and and Stewart StewartLake Lakeititisis 20 20 m m thick, thick, and and at at Albert Albert Lake Lake itit is is 45 Rock, Channel Channel Island, 45 m m thick thick (Franklin et a!. (Franklin et al. 1980). 1980). The Thehorizontally horizontallylaminated, laminated, buff buff arkosic arkosicsiltstone siltstoneto to fine finesandstone sandstoneare are exposed in aa cliff exposed in cliff face face along along the the shore shore of of Kama Kama Bay, Bay, east east of of Nipigon Nipigon (Franklin (Franklin et et aL al. 1980; 1980; Cheadle 1986). The base base is istransitional transitional over over 11 to to 22 mmand andcontains containsvery verylittle littlecarbonate. carbonate. 1986). The Reduction spots are common, as are mud cracks and ripple marks on bedding surfaces Reduction spots are common, as are mud cracks and ripple marks on bedding surfaces(Franklin (Franklin et et al. al. 1980). 1980). Breccia Brecciadikes dikesare arepresent presentand andmud-chip mud-chipconglomerates conglomeratesare are associated associated with with the the siltsiltdominated layers (Morey and Ojakangas 1982; Cheadle 1986). The siltstone layers contain dominated layers (Morey and Ojakangas 1982; Cheadle 1986). The siltstone layers containthe the following ripple cross-laminations, low amplitude ripple following sedimentary sedimentary structures: structures: erosional erosional bases, bases, ripple cross-laminations, low amplitude ripple marks, normal grading, marks, centimetre centimetre to to decimetre decimetre fining-upward fining-upward sequences, sequences, normal grading, and and suspensate suspensate caps caps (Cheadle This formation formation locally locally contains contains (Cheadle 1986) 1986) evaporite evaporite casts casts and andraindrop raindropimpressions. impressions. This carbonate algal stromatolites stromatolites at at the the base base of of the carbonate algal the facies facies in in the the north north half half the the basin basin (Morey (Morey and and Ojakangas Ojakangas 1982; 1982; Cheadle Cheadle 1986). 1986). Enigmatic mudcracks(?) in distal bar Formation bar sequences sequences of of the Kama Hill Formation �.1 I I I I LEGEND L EGEND Conglomerate Basal Conglomerate Dolomite lolomite Sandstone Sandstone Gypsum Siltstone __jshale p Mud Chip Conglomerate Mud 1 Stromatolites , Diabase Magnetic Granite Granite Magnetic Gneiss �DEPOSITIONAL SYSTEM DEPOSITIONAL SYSTEM The Sibley were laid The Sibley Group Group sediments sediments were laid down down in in aa broadly broadly subsiding, subsiding, ovoid ovoid basin basin during during the the Mesoproterozoic. There is no evidence of high relief along any exposed segments of Mesoproterozoic. There is no evidence of high relief along any exposed segments of the the basin basin margin. At the margin. At the time time of of deposition, deposition, the the Sibley Sibley Basin Basin was was situated situated very very close close to to the theequator equator (unpublished paleomagnetic paleomagnetic results, results, Dr Dr G. G. Borradaile) Borradaile) and and was was subjected subjected to to an an arid arid to (unpublished to semiarid semiarid climatic regime. These These factors factors combined combined to to dictate dictate the the types climatic regime. types of of depositional depositional systems systems which which developed the basin's developed during during the basin's active activelifetime. lifetime. The large-scale The large-scale depositional depositional patterns patterns are are quite quite clear clear and and will will be be discussed discussed in in the the following followingpages. pages. However, superimposed on this are complex, and sometimes conflicting, data provided smallHowever, superimposed on this are complex, and sometimes conflicting, data provided by by smallscale structures. This complexity scale structures. This complexity is is aa result result of of the the excellent excellent preservation preservation on on minute minute structures structures in in both outcrop outcrop and and core. the field trip we we will will investigate possible solutions solutions to to some some of of the the both core. During During the field trip investigate possible conflicting data, data, and and hopefully conflicting hopefully gain gain further further insight insight into into how how aaMesoproterozoic, Mesoproterozoic, low-relief, low-relief, interior drainage interior drainage depositional depositional system system operated. operated. Pass Lake Formation Disagreement exists exists as as to to the the depositional depositional setting setting of of the the Pass Pass Lake Formation Disagreement Pass Lake Lake Formation. Franklin et al. Formation. Franklin al. (1980) (1980) believed believed that that the the sandstones sandstones and and conglomerates conglomerates were were deposited deposited in a shallow shallow lacustrine lacustrine environment, environment, whereas Cheadle (1986) put forward an alluvial fan-fluvial fan-fluvial model. This controversy model. This controversy may may be be due due to to different differentdepositional depositional processes processes operating operating in in different different places. The The sequences of powering-down beds,such such as as at at Pass places. sequences of powering-down beds, Pass Lake, Lake, appear appear to to represent represent lacustrine sheet sheet sandstones sandstones laid laid down down during during the the waning waningphases phases of of storm storm events events in in aa shallow lacustrine shallow lacustrine environment. environment. The The lensoid, planar and lacustrine lensoid, large-scale large-scale planar and trough trough cross-stratified cross-stratified sandstones, sandstones, present on present on Channel Channel Island Island and and neighbouring neighbouring islands, islands, probably probably represent represent transverse transverse bar bar sand sand flats flats in aa braided they may may be be aeolian. in braided fluvial fluvial system. system. However, However, they aeolian. The The rapid rapid lateral lateral variations variations in in thickness of of the the unit, thickness unit, not not related related to to position position relative relative to to the the basin basin margin, margin, reflect reflect infilling infilling of of basement depressions depressionsrather rather than than clastic clastic wedges wedges advancing advancing from from sediment sediment entry points. points. A A picture picture arises than sands sands evening evening the the topography topography arises of of the the Pass Pass Lake Lake Formation Formation representing representing first first gravels gravels than by by infilling infilling basement basement valleys valleys through through braided braided stream stream transport transport to to a a shallow shallow lacustrine lacustrine environment. environment. In with In the the upper upper portion portion of of the the Pass Pass Lake Lake Formation, Formation, the the sheet sheet sandstones sandstones become become interbedded interbedded with red, red, silty silty shales. shales. This This represents represents aa more moreoffshore offshoredepositional depositional setting setting where where muds muds deposited deposited between storm events events are are not not eroded by the the next as is between storm eroded by next storm, storm, as is the the case case in in the the nearshore nearshore multimultistorey sandstones. The upward decrease in the sand-to-mud ratio indicates deepening sand-to-mud deepening water water and and culminates in the siltculminates silt- and and mud-dominated mud-dominated Rossport Rossport Formation. Formation. The the Pass The contact contact between between the Pass Lake Lake Formation Formation and and the the Rossport Rossport Formation is is gradational, with the the name Formation gradational, with name change change occurring occurring where where the the mudstone mudstone begins begins to to dominate. Upward Upward through through the the sequence, indicationsofof dessication dessicationbegin begin to to appear dominate. sequence, indications appear (eg. (e.g. mudcracks and and early, authigenic gypsum growth). at this point, growth). Commonly at point, interlayering, interlayering, with with mudcracks millimetre- to to centimetre-scale dolomite laminae, laminae, begins begins to to occur. In drill millimetrecentimetre-scale dolomite occur. In drill cores cores from from areas areas closer to to the of dolomite closer the basin basin center, center, an aninterlaminated, interlaminated, decametres decametres thick, thick, succession succession of dolomite and and gypsum occurs gypsum occurs at at this this stratigraphic stratigraphic position. position. The The dessicated dessicated mudstones mudstones probably probably represent represent periodically exposed exposed mudflats mudflats surrounding surrounding the the lake. lake. This This necessitates necessitates aa regressive regressive phase phase in in the the periodically lower Rossport Rossport Formation, Formation, for for which there is friable siltstone lower which there is some some evidence evidence (e.g. (e.g. brick-red, brick-red, friable siltstone at at Rossport Formation �The dolomite-mudstone horizon). horizon). The dolomite-mudstone interlayered interlayered sequence represents represents aa shallow shallow offshore offshore environment. Muds Muds were were washed washed in during rainy periods and dolomite environment. dolomite precipitated during dry periods as the saline lake shrank. The deeper offshore only received clastic elastic input during the latter stages off this depositional episode. It was probably dominated by chemical depositional was probably dominated chemical precipitation precipitation of dolomite during the wetter periods when the lake salinity decreased, and by rainout of of gypsum gypsum crystals on the bottom during dry intervals with increasing salinity. Alternatively, both may have precipitated during dry intervals with hiatuses representing wet periods. The control in this model is the MgICa Ca2 inhibits dolomite precipitation and a period of removal MgKa ratio. The presence of ca2+ of Ca2 ca2+via viagypsum gypsumprecipitation precipitationisisneeded needed before before dolomite dolomite will will begin begin to precipitate. precipitate. Thus, first gypsum then dolomite would precipitate during the dry intervals in this scenario. gypsum then dolomite would precipitate during the scenario. Oxygen Oxygen and and carbon stable isotope analysis is needed to choose between the two alternatives. Sandstone beds beds appear interbedded with the upper dolomite-mudstone sequence or overlie it. Sandstone interbedded with dolomite-mudstone sequence These are interbedded with red mudstone and the unit is laterally extensive is laterally extensive throughout throughout the the basin. basin. Closer to to the southern Closer southern edge edge of of the thebasin, basin,repetitive repetitivesandstone sandstone assemblages assemblages occur occur (metre(metre- to to decametre-scale thicknesses) with intervening intervening decametre-scale, decametre-scale, dolomite-mudstone dolomite-mudstoneassemblages. assemblages. Here, trough trough cross-stratification and channelling channellingare are much much more more evident evident in the Here, cross-stratification and the sandstones. sandstones. in this unit show higher variability than in the Pass Lake Formation. They appear Paleocurrents in to have a southern southern source as opposed to the northern source for the Pass Lake. Lake. The The sandstones sandstones appear to represent renewed renewed tectonism tectonism and basin tilting, resulting resulting in sediment influx from the south, with sand dunes migrating through scour scour channels channels during duringhigh-discharge high-dischargeevents. events.Whether Whether this was was subaerial subaerial or or shallow shallow subaqueous subaqueous deposition deposition is difficult difficult to determine determine as as strandline strandline position is position is obscure. obscure. Metres to decametres of mud-chip mud-chip conglomerates conglomerates overly the sandstones, sandstones, either interbedded interbedded with Mudstone the upper sandstones sandstones or interbedded interbedded with with siltstones siltstones overlying overlying the upper upper sandstones. sandstones. Mudstone fragments are up to metres in diameter, although this is rare, and have variable compositions. metres in diameter, although is rare, and have variable compositions. This unit appears appears to thicken thicken towards towards the the basin basin center centerand and may mayrepresent representupraising upraisingand andrecycling recycling (cannibalization) of basin margin lithofacies; perhaps a further stage of the north-down (cannibalization) of basin margin lithofacies; perhaps the north-down basin basin tilting, represented by the sandstone sandstone wedges, upraising upraising the southern southern basin margin. The mudblock conglomerate is succeeded in some areas by red mudstones mudstones with mudcracks mudcracks and abundant gypsum veins veins and nodules nodules representing representing clastic clastic sabkhas. sabkhas. In In other otherareas, areas, aabrecciated brecciated and and stromatolitic stromatolitic carbonate occurs at this this horizon. horizon. This Thisrepresents representssubaqueous, subaqueous,shallow shallowlacustrine lacustrine deposition, with teepee teepee structures reflecting reflecting periodic periodic drying. drying.These These units units are are overlain overlain by by the deposition, with Kama Hill Formation. Formation. The Kama Hill Formation consists of red to Kama Hill Formation: to purple purplemudstones mudstones with with Formation: sandstones. The ripple-laminated, coarse-grained siltstones to fine-grained interlayered, coarse-grained siltstones fine-grained sandstones. The interlayered, ripple-laminated, abundance of rippled units increases upward until they are replaced by sandstones, which abundance of rippled increases upward are replaced by sandstones, which continue the coarsening- and thickening-upward trend. The sandstones sandstones and and all all stratigraphically stratigraphically higher units are only present in core. The Kama Hill mudstones represent subaqueous in core. The Kama Hill mudstones represent subaqueousdeposition deposition in a non-saline lake. The coarseningcoarsening- and and thickening-upward sequence sequence reflects reflects delta delta progradation progradation and eventual establishmentof of aa fluvial delta delta top environment, forming the the base base of the next environment, forming next and eventual establishment formation. formation. �This is This is a a new new unit unit which which has has not not been been formally formally named. named. It It is is present present only in drill core from under Nipigon Bay. At some localities, the coarsening-upward only in drill core from under Nipigon Bay. At some localities, the coarsening-upward delta delta Nipigon Bay Bay Formation: Formation: Nipigon sequence of planar cross-stratified, sequence of the the Kama Kama Hill HillFormation Formationisisoverlain overlainby bylarge—scale, large-scale, planar cross-stratified, mediummediumgrained sandstones of the Nipigon Bay Formation. These reach approximately 500 m m in grained sandstones of the Nipigon Bay Formation. These reach approximately 500 in thickness. Mudstone Mudstone and and siltstone siltstone interlayers interlayersare are almost almost totally totally absent absent with with the thickness. the entire entire unit unit consisting of of multi-storey multi-storey sandstones. sandstones. Colour Colour varies varies from red to consisting from red to white, white, with with the the white white sandstones sandstones containing gypsum gypsum grains grains and containing and aa gypsum gypsum cement. cement. Research Research continues continues on on this this formation, formation,with with the the present data data indicating indicating an an aeolian aeolian origin origin with with possibly possibly some some braided braided fluvial fluvial reworking. reworking. present At other other locations, locations, aa different different assemblage the one one described described above the Kama At assemblage separates separates the above from from the Kama Hill Hill Formation. This assemblage overlies the coarsening-upward sequence of the delta and is Formation. This assemblage overlies the coarsening-upward sequence of the delta and is composed of of finingcomposed fining- and and thinning-upward, thinning-upward, multi-storey multi-storey sandstones, sandstones, metres metres to to decametres decametres thick, thick, separated by by friable red mudstones In some some places, places, minor minorcarbonate carbonate separated friable red mudstones and and fissile fissile green green mudstones. mudstones. In layers are layers are associated associated with with the the green green mudstones. mudstones. The The sandstones sandstones are are fluvial fluvial channels; channels; the the intervening mudstones mudstonesrepresent representaa rare intervening rare example example of of a apreserved preservedPrecambrian Precambrianfloodplain floodplain succession with with sun-baked, poorly developed succession sun-baked, poorly developed soils, soils, friable friable units, units, and and small, small, open-water open-water ponds. ponds. The possibly possibly aeolian The aeolian sandstones sandstones lie lie sharply sharply on on top top of of this this unit. unit. �FIELD TRIPROAD TRIP ROAD LOG Stop Number Locality Road Lop Loe (kin) (km) DAY DAY ONE STOPS 1-1 1-1 Remote Drill Core Storage Facility (Generalized (GeneralizedStratigraphy) Stratigraphy) 1-2 1-2 Thunder North (Downtown) Thunder Bay Bay North (Downtown) (Architectural Stone) (Architectural Stone) Junction JunctionHighways Highways 11-17 11-17 and and 587 587 0.0 km (Reset) (Reset) 1-3 Pass Pass Lake Lake (Railway) (Railway) (Animikie/Sibley disconformity; disconformity; Pass PassLake Lake Fm. Fm. Type Type section) section) 5.9 to 6.1 km 1-4 Pass Pass Lake Lake (Highway) (Highway) (Lacustrine (LacustrineSheet Sandstones, Sandstones, Rossport Rossport Fm.) 8.6 km 1-5 Pass Pass Lake Lake (Highway) (Highway) (Lacustrine (LacustrineChannel ChannelSandstones, Sandstones,Rossport Rossport Fm.) Fm.) 9.9 km Pass Pass Lake Lake Crossroad Crossroad 10.3 km Pass Pass Lake Lake (Highway) (Highway) (Subaerial (SubaerialSiltstone-Caliche, Siltstone-Caliche,Rossport RossportFm.) Fm.) 10.7 km Junction JunctionHighways Highways11-17 1 1 - 17and and587 587 0.0 0.0 km km (Reset) (Reset) Turn-off Turn-off(Pipeline (Pipelineroad) road) to to Enterprise Enterprise Pb-Zn Pb-Zn Mine Mine 14.5 14.5 km km 1-6 1-7 �DAY TWO TWO STOPS STOPS DAY Stop Number Stop Number Locality Road Lou Road Loe (kin) (km) Junction of Junction of Highways Highways 11 11 and and 17, 17, east of Nipigon east of Nipigon 0.0 km km (Reset) 0.0 (Reset) 2-1 Gurney (Highway) Gurney (Highway) (Archean-Sibley (Archean-Sibley Unconformity, Unconformity, Debris Debris Flows, Flows, Pass Pass Lake Lake Fm.) Fm.) 39.9 km km (east of 39.9 (east of junction) junction) 2-2 Kama Kama Hill Hill (Highway) (Highway) junction) junction) (Interbedded (Interbedded Dolostone-Siltstone, Dolostone-Siltstone, Rossport Fm.) Fm.) 21.4 km (east of (east of 21.4 km 2-3 Ruby Ruby Lake Lake Marble Marble Quarry Quarry (Turn-off from Highway Highway 17; 17; (Turn-off from 4.1 4.1km km into into site) site) 6.8 km (east of (east of 6.8 km junction) junction) 2-4 2-5 2-6 Junction Junction of of Highways Highways 11 11and and 17, 17, east of Nipigon east of Nipigon 0.0 km (Reset) (Reset) Junction Junction of of Highways Highways 11-17 11-17and and 585 585 4.0 km Turn-off Turn-off to to Moseau Moseau Mountain Mountain 5.0 5.0 km km 17.5 17.5-- 18.5 18.5 km Moseau Moseau Mountain Mountain (Pass Lake Fm. & Evaporite Breccias, Rossport Fm) (Pass Lake Fm.& Evaporite Rossport Fm) Junction Junction of of Highways Highways11-17 11-17and and 628 628 0.0 km km (Reset) (Reset) Pull-off Pull-off parking parking lot; lot; start start of of hiking hiking trail trail 6.3 6.3 km km Lloyds Lloyd's Lookout Lookout q'Shales &Rippled RippledSiltstones, Siltstones, Kama Kama Hill Hill Fm.) Fm) (Shales & (walk (walk trail trail 0.8 0.8 km) km) Junction Junction of of Highways Highways 11-17 11-17 and and 628 628 0.0 km (Reset) (Reset) Big Big Squaw Squaw Creek Creek Road Road Cut Cut (Hornfelsed Rossport Fm.) Fm.) (Hornfelsed 10.2 10.2 km km �(JAY (((UNDER I Rossport Fm 0 0: 12 LAKE Map 2232 NIPIGON-SCHREIBER SHEET, MNDM Miles I P (CO (V 2-5 Lloyd's — (IV4 /.k fACQ,' r Thunder Bay, Ontario a a C I Ic-i -t 1. CD -a. ij 0 CD c-) C/D 0 0 0 0 C/D I-I I ILSG 2000 Sibley Group Field Trip �FIELD FIELD TRIP TRIP STOP STOP DESCRIPTIONS DESCRIPTIONS STOP Y REMOTE GE SITE, SITE, CONMEE STOP 1-1: 1-1: THUNDER THUNDERBA BAY REMOTE DRILL DRILL CORE CORESTORA STORAGE CONMEE TOWNSHIP TO WNSHZP This This stop stop presents presents an an opportunity opportunity to to view view stratigraphic stratigraphic sections sections of of the the Sibley Sibley Group Group in in diamond diamond drill drill core core donated donated to, to, or or collected collected by, by, the the MINDM from private MNDM from private sector sector mineral mineral exploration exploration projects. projects. Core Core from from three three drill drill holes holes has has been been selected selected from from different of different parts parts of the the Sibley Sibley depositional basin to provide depositional basin to provide geographically and stratigraphically geographically and stratigraphically distinct of the distinct sections sections of the constituent constituent formations formations and and their their characteristic characteristic lithologies. lithologies. The The latest latest drilling drilling was was completed completed by by Falconbridge Falconbridge Limited Limited and and joint-venture joint-venture partners, partners,Canmine Canmine Resources Corporation and and Red Red Engine Engine Resources, Resources, in in 1997 1997 from from the the ice ice of of Nipigon Nipigon Bay Bay as as part part Resources Corporation of an exploration program designed to evaluate the area's nickel potential (Assessment files, of an exploration program designed to evaluate the area's nickel potential (Assessment files, Thunder Thunder Bay Bay South South District, District, Thunder Thunder Bay). Bay). Falconbridge Falconbridgebecame becameinterested interestedin inthe thearea areainin1991 1991 and started a compilation and regional reconnaissance program which delineated a 23 km k m long long and started a compilation and regional reconnaissance program which delineated a 23 magnetic anomaly adjacent adjacent to to the the North Airborne and and ground ground magnetic anomaly North Shore Shore fault fault (see (see map map below). below). Airbome geophysical geophysical surveys, surveys, geological geological mapping, mapping, soil soil sampling sampling ensued, ensued, culminating culminatingin in the thedeep deepdrilling drilling program program (ibid). (ibid). No Nosignificant significantnickel nickelnor norcopper coppervalues values were were returned returned from fromthe thedrilling drillingand andthe the Exploration Licence of of Occupation on Lake was allowed allowed to to lapse lapse in Exploration Licence Occupation on Lake Superior Superior was in 1999. 1999. Despite Despite the the disappointing results, the the drilling disappointing results, drilling provided provided aa plethora plethora of of new new stratigraphic stratigraphic information information on on the the Sibley Sibley Group Group in an an area area where where no no drilling drillinghad had previously previously taken taken place. place. Most Most remarkable remarkable of of these these results were the unexpectedly thick sections (up to 850 m) that dwarfed previously measured results were the unexpectedly thick sections (up to 850 m) that dwarfed previously measured sections total thickness thickness on on the the mainland, mainland, as well well as as the the presence presence of of upper upper units units of of sections and and estimated estimated total the the Sibley Sibley Group Group which which had had not notpreviously previously been been recognized recognized (i.e. (i.e. proposed proposed Nipigon Nipigon Bay Bay Formation). Formation). Drill Drill Hole Hole CYP 96-01 CYP 96-01 NB NB 97-02 97-02 NB NB 97-03 97-03 NB NB 97-04 97-04 NB NB 97-05 97-05 (m) Overburden (m) Overburden Sibley Group (m) Sibley Group (m) 2.0 2.0 22.7 22.7 30.0 28.6 28.6 73.2 73.2 62.56 62.56 452.5 452.5 75.0* 75.0* 858.2 858.2 635.1* 635.1* Total Length (m) Total Length (m) 464.5 464.5 677.8 677.8 105.0 105.0 1015.6 1015.6 744.2 744.2 Table showing thicknesses of Sibley Sibley Group Group intersected intersected by Falconbridge Falconbridge Limited's Limited'sdrilling. drilling. (*hole did not reach reach Archean Archean basement) basement) �Vert Island Black Bay Peninsula Osler Osier Group Group FZ1 0 5 km LI Loqari Logandiabase diabase / North Shore / North ShoreFault Fault 1 Archean Archean I 'High" 1 Magnetic 'High" r] Sibley Group Group D -Sibley I/ Map of of Nip Nipigon drilling Map igon Bay, Bay, showing showing location location of Falconbridge Falconbridge Limited's drilling The sedimentary rocks unconformably overlieaa red red granite which has been sedimentary rocks unconformably overlie granite which been subjected subjected to to Mesoproterozoic, equatorial equatorial weathering. weathering. Mesoproterozoic, Two drill holes, completed Noranda Inc., Inc., will also be examined at this stop. completed by Noranda stop. These holes were drilled drilled north of of Dorion Dorion on on aa north-south north-south line, line, with with one one lying lying approximately approximately 15 15 km km closer closer to to Thus, the three holes examined will show basin margin basin center center than than the the other. other. Thus, three holes examined will show basin margin the basin (Falconbridge), (Falconbridge), medial medial (Noranda) (Noranda) and and basin basin center center (Noranda) (Noranda) lithofacies lithofacies assemblages. assemblages. The basin-center drill hole bottoms out in diabase which is overlain by millimetre- to centimetrescale layers layers of dolomite and gypsum. This This assemblage assemblage is is approximately approximately 40 m thick and contains pink to to red, red, clay-rich clay-rich interlayers interlayers near near its its top. top. This is overlain overlain by by aashale-siltstone shale-siltstone sequence sequence containing containing abundant abundant layers layers of of intraformational intrafonnationalconglomerate conglomerate(mud-chip (mud-chipconglomerates). conglomerates). �The The medial medial drill drill hole holeoverlies overliesa agneissic gneissicbasement. basement. An An upward-fining, upward-fining, conglomerate conglomerate to to sandstone sandstone to to siltstone, siltstone, trend trend is is well well developed developed in in the the Pass Pass Lake LakeFormation Formation present present in inthis thishole. hole. Approximately 50 m m of and red Approximately 50 of inter-laminated inter-laminated dolomite dolomite and red mudstone mudstone overlie overlie the the Pass Pass Lake Lake siltstone containing layers of mud-block succeeded by a Formation. in is turn, This, Formation. This, in turn, is succeeded by a siltstone containing layers of mud-block conglomerate. conglomerate. AAbrecciated brecciatedand andstromatolitic, stromatolitic,white whitecarbonate carbonateunit unitoverlies overliesthe theclastic clasticrocks rocksand and this this is is succeeded succeededupwards upwards by by siltstones siltstonesof of the the Kama Kama Hill Hill Formation. Formation. The drill hole hole is is similar The basin-marginal basin-marginal drill similar to to the the medial medial hole hole except: except: 1) 1) the the basal basal Pass Pass Lake Lake is is almost entirely missing; 2) the upper carbonate unit is not present; 3) it contains approximately almost entirely missing; 2) the upper carbonate unit is not present; 3) it contains approximately 550m Sibley strata strata not not previously previously discovered discovered elsewhere elsewhere in in the thebasin. basin. The The upper upper 550m of of overlying overlying Sibley strata consist of a coarsening-upward, deltaic sequence at the top of the Kama Hill Formation. strata consist of a coarsening-upward, deltaic sequence at the top of the Kama Hill Formation. This This is is overlain overlain by by aawell-developed well-developedfluvial fluvialsystem systemwith withmulti-storey, multi-storey, powering-down, powering-down, channel channel sandstone sandstone assemblages assemblagesimbedded imbedded with with floodplain floodplain fines. fines. Overlying Overlyingthis thisisisaasandstone sandstonesuccession succession hundreds dry aeolian aeoliansystem. system. hundreds of of metres metres thick thick which which was was probably probably deposited deposited in in aa dry We We will will spend spend the the morning morning in in the the core coreyard yardexamining examining the the various variouslithofacies lithofacieswhich whichare arepresent present in in each each of of these these lithofacies lithofacies assemblages. assemblages. For Foraa more more detailed detailed description descriptionof of the the sequence sequence see see the the section section on on "Depositional "Depositional Environment". Environment". STOP(S) STOP(.) 1-2A,B,C: 1-2 A,B.C: THUNDER THUNDERBAYNORTH(DOWNTOWN) BAY NORTH (DOWNTOWN) This This series series of of stops stops is is designed designed to to show showexamples examples of of rough rough and and cut cutdimension dimensionstone stonethat thatwas was quarried quarried from three different different sites sites around around the turn turn of of the the century. century. Sibley Sibley Group Group stone stone quarries quarries are are listed listed below below (summarized (summarized from from Hinz Hinz et et al. al.1994): 1994): Quarry Rock Type Cooke Point George George Point Point La Grange Island Nipigon River Quarry Ouarrv Island Island Ruby ~ u bLake ~ yake Simpson SimpsonIsland Island Vert Vert Island Island Wolf Wolf River River Marble Gray Gray sandstone sandstone Red sandstone Variegated marble White White sandstone sandstone Variegated Variegated marble marble Buff Buff sandstone sandstone Red Red sandstone sandstone Buff Buff sandstone sandstone . - Years of Operation 1931-1 940(?) Late 800s(?) Late 118001s(?) 1882-1 1882-1883(?) 883(?) 1883-1910(?) 1883-l910(?) Late 800's(?) Late11800's(?) 1998-present 1998-present 1904-1912 1904-1912 1881-1912 1881-1912 1913-15; 1913-15;1921-31 1921-31 �These three three architectural stops along along one one of Thunder These architectural stops Thunder Bay's major arteries provide an opportunity see aa Bay's major arteries provide an opportunity toto see variety of of Sibley Sibley Group Group stones stonesand and note note the the effects effects of of variety The architectural of exposure. almost a century of exposure. The are taken website descriptions are taken from from aa walking walking tour tour website developed and and maintained by the developed maintained by the Local Local Architecural Architecural Conservation Advisory Committee conservation Advisory Committee (L.A.C.A.C.) (L.A.C.A.C.) (http://www.city.thunder-bay.on.caltbwalkingtourf). (http://www.city .thunder-bay.on.ca/tbwalkingtour/). STOP 1-2A: 432 432 RED RED RIVER RIVER ROAD retaining wall wall is constructed with distinctive, brickThis retaining red Vert red Vert Island Island sandstone sandstone (Nipigon (Nipigon Bay Bay Formation?). Formation?). was aa popular stone used in the construction of many many This was grand homes and other buildings in Fort William grand homes and other buildings in Fort William (now (now Thunder Bay South). PORT ARTHUR STOP 1-2B: PORT ARTHUR INSTITUTE (401 INSTITUTE (401 Red Red River Road) COLLEGIATE COLLEGIATE This building, building, constructed constructedin in 1909 1909of ofSimpson SimpsonIsland Island buff buff sandstone (Nipigon Bay Formation?), is an example of the Queen Anne Anne style that that was was in in common common use use from from the the 1880s l880s I to the 191 Os.wWhen the school school board board made made the initial to 1910s. h e n the initial planning for the building, it was decided that it should be "erected for posterity, and and not not be be of the 'shack' order", resulted in the shack' order", so so they they chose chose the the stately stately Queen Queen Anne style. style. Alterations in 1925 resulted addition of four more classrooms, and more renovations to the north and south in 1953 south 1953 and 1962 1962 created other other rooms. rooms. These These alterations alterationsused usedstone stoneinin an an attempt attempt to to blend created blend into the original original building, but a gynmasium gymnasium planned in 1964 1964 and completed in 1974, 1974, provoked controversy as its design was incompatible incompatible with the rest of the school. school. ' �Port Arthur after its Port Arthur Collegiate _. ..sgiate Institute, Institute, shortly rtly after its construction co ..-.. ~ c t i o n STOP J-2C: South) STOP 1-2C:TRINITY TRINITYUNITED UNITEDCHURCH CHURCH(30 (30Algoma Algoma Street Street South) This building, building, constructed constructed in in 1906, was formerly known as as the This 1906, was formerly known the Trinity Trinity Methodist Methodist Church, Church, and and became the Trinity United Church after the United Church of Canada was formed became the Trinity United Church after the United Church of Canada was formed in in 1925. 1925. Constructed of of rough rough cut, Constructed cut, Simpson Simpson Island Island buff buff sandstone sandstone (Nipigon (Nipigon Bay Bay Formation?), Formation?), this this structure is is an an example 890s to to the the that was was popular popular from from the the 11890s structure example of of the the Late Late Gothic Gothic Revival Revival style style that 1 940s.The Theunusual unusualtower tower features featuresvery very narrow narrow windows, windows, four four buttresses, buttresses, each each capped capped with with aa 1940s. pyramid shaped shaped finial, finial, and and an an extremely extremely sharp sharp hexagonal hexagonalspire. spire.The The rest rest of of the the building also pyramid building also features very very steeply pitched roofs, features steeply pitched roofs, and and arched arched windows windows in in the the Gothic Gothic style. style. �TrinityUnited UnitedChurch Church Trinity STOPI-2D: I-2D:MASONIC MASONICHALL HALL(262-270 (262-270Red RedRiver RiverRoad) Road) STOP Builtinin1910 1910and andalso alsoknown knownas asShuniah ShuniahLodge, Lodge,this thisstone, stone,brick brickand andconcrete concretebuilding buildingreplaced replaced Built the 1909.The Thefirst firstfloor floorisismade madeof ofcut cutNipigon Nipigon theold oldMasonic Masonictemple templethat thatwas wasdestroyed destroyedby byfire fireinin1909. River Rivermarble marble(Rossport (RossportFormation) Formation)and andthe theentrance entrancefeatures featurescarved carvedmarble marblepilasters pilastersand and decorative decorativepanels. panels.Originally Originallythere therewas wasaadome domeon onthe theroof roofover overthe theentrance, entrance,which whichhas hassince since been been removed. removed.The Thecentral centralportion portionof ofthe thebuilding buildinghas hasaaMansard Mansardroof roofof ofFrench Frenchdesign. design.The The building's building'swindows windowsare aredecorated decoratedwith withalternating alternatinground roundand andtriangular triangularpediments pediments above above them. them. Conmercial Commercialspace spaceoccupies occupiesthe theground groundfloor, floor,while whilethe thelodge lodgeisislocated locatedabove. above. �STOP STOP 1-3: 1-3:PASS PASS LAKE LAKE (RAILROAD (RAILROADCUT) CUT) j ' Ill 4 I A A cliff cliff on on the the far far side side of ofthe therailroad railroad tracks tracks contains the type type section of the contains the section of the Pass Pass Lake Lake A Formation. disconformity Formation. A low-angle low-angle disconformity separates the the basal separates basal conglomerates conglomerates of of the the Pass Pass lake Formation from the underlying, —1.9 Ga lake Formation from the underlvin~. -, -1.9 Ga Rove Formation shales (lower photo). The Rove Formation shales (lower r>hoto\. The maroon maroon and and green green shales shales exhibit exhibit extensive extensive weathering. evidence of Mesoproterozoic weathering. evidence of Mesooroterozoic The The basal basal conglomerate conglomerate thins thins and and thickens thickens laterally, laterally, pinching pinching down down to to pebbly pebbly sandstone sandstone in in places. places. Clasts Clastsare are generally generally surrounded surrounded and and by dominated local Gunflint dominated by local Gunflint formation formation lithologies. lithologies. The The matrix matrix is is poorly poorly sorted. sorted. The The conglomerates are overlain by a conglomerates are overlain by a thinningthinningupward of sandstone (ohoto at at uoward sequence seauence of sandstone beds beds (photo laterally left). left). Individual ~ndividualbeds beds are are reasonably reasonablylaterally continuous though sometimes sometimes lens lensout. out. They continuous thoueh They are by upper are dominated dominated bv uover flow flow regime regime parallel oarallel lamination with occasional lamination with occasional ripples ripples and and smallsmallscale scale dunes dunes on on their their tops. tops. . - ~ - A - ~ - of this this depositional environment of depositional environment Both alluvial fansequence is continuous. atuuence is continuous. Both alluvial fanThe braided fluvial and and shallow shallow lacustrine lacustrine (Cheadle (Cheadle 1986; 1986; Franklin Franklin 1980; 1980; respectively) respectively) While examining examining the the lithofacies lithofacies in in the the field depositional environments ments have have been been proposed. proposed. While field we we depositional em will discuss the merits incuts of of each each interpretation. interpretation. between the the Disconformity between Disconformity conglomerate basal basal conglomerate of of the the Pass Pass Lake Lake Formation Formation (cgl) (cgl) Rove and weathered and weathered Formation shales at Pass Pass Formation shales (reg) (reg) at Lake. A Lake. A concretion concretion (con) (con) is is shown at lower right. right. �STOP 1-4: (HIGHWA!) 1-4: PASS LAKE (HIGHWAY) This This is aa roadside roadside exposure exposure of of the the transition transition zone zone between between the Pass Pass Lake Lake and and Rossport Rossport Formations. Laterally Laterally continuous continuoussandstone sandstone beds interlayer in a red silty mudstone assemblage. The sandstones are often massive, though though one bed at this this location location contains contains low angle, angle. wispy, wisvv. trough trough cross-stratification. cross-stratification. Here again, two two possibilities possibilities exist exist for for the theenvironment environment of of deposition. deposition. The sandstones could represent crevasse fines during during flood-stage flood-stage failure failure crevasse splay sand sheets spread out on the floodplain fines of fluvial levee systems. Alternatively, Alternatively, the the sand sand sheets sheets may may represent represent shallow lacustrine storm deposits caused by rainfall event-driven fluvial sand delivery delivery to to a lacustrine system and storm event-driven fluvial surge ebb plus river river mouth mouth density density current current transport of sand offshore into a fair fair weather weather siltsiltdominated area. The merits of both interpretations will be discussed in the field. The merits of both interpretations will be discussed in I �STOP 1-5: WA 17 STOP 1-5:PASS PASSLAKE LAKE(HIGH (HIGHWAT) This This stop stop contains contains aa variety variety of of lithofacies. lithofacies. Large Largesandstone sandstonelenses lensescap capthe the outcrop, outcrop, overlying overlying siltstones, siltstones, a fairly continuous, continuous, non-silty, non-silty, purple purple shale shale and and aa massive, massive, lensoid lensoid dolomite. dolomite. tQ — .1 .: C :- :-A' - r1.a ff !$ ... . S . — ,. . . ',r - -. .1 .. ., Layer geometry geometry and and contact contact relations relations in in this this exposure exposure are are perplexing perplexingat at first first glance. glance. Take Layer Take some some time to to trace time trace out out the the geometries geometries of of the the various various lithologies. lithologies. Hopefully Hopefully we we will will be be able able to to answer answer the the following following questions questionswhile whileat at this thisstop: stop: Is the dolomite dolomite really lensoid? Is the lensoid? Is this this caused by structural disturbance? disturbance? How does does the the dolomite How dolomite correlate across across the road? Are any intraformational intraformational conglomerates present? What is their geometry? geometry? How could have this small small sequence sequence of various lithologies have formed? • STOP 1-6: WA 17 STOP 1-6:PASS PASSLAKE LAKE(HIGH (HIGHWAY) This is is aa small, small, crumbly crumbly outcrop outcrop of of red, red, friable friable silty-mudstone. silty-mudstone. Although Although it it is is not not impressive, impressive, its its This brick-red colour and friable nature are important. important. Commonly, outcrops such as this this one are brick-red interpreted as highly highly sunbaked that is is probably probably what what this this outcrop outcrop represents. represents. interpreted sunbaked soils soils / caliche, and that �1-7: ENTERPRISE STOP 1-7: ENTERPRISE MINE Peter MeKellar discovered aa lead-copper-mineralized veinatatthis thislocation locationinin 1865 1865 that that was was Peter McKellar discovered lead-copper-mineralized vein subsequently developed as as the the Enterprise Enterprise Mine Mine (Tanton (Tanton 1931). 1931). Between 1870 and 1876, subsequently developed 1876, two two shafts were sunk. No. 1 shaft reached a depth of 180 feet (55m); levels were run easterly 76 feet No. 1 shaft reached a depth of 180 feet and westerly westerly 66 feet feet at at aa depth depth of of 60 60 feet. feet. A A stope stope was made in the west drift and a winze was sunk in the east drift about 50 feet from the the shaft. shaft. A crosscut was driven from the the bottom bottom of the the shaft 115feet 5feetsouth. south. Shaft No. 2 was sunk 60 feet at a point approximately 300 feet west of No. 11 of shaft (ibid). The total total recorded productionfrom fromthe themine minewas wasaa single single shipment shipmentofof 167 167 tons tons made made to to The recorded production Swansea, Wales, in in 1875 (ibid) (ibid) that that averaged averaged37.4% 37.4%Pb. Pb. Although no no other metal metal credits credits are Swansea, Wales, listed, appreciable copper, copper, silver silver and and gold gold were were reported reportedin in some somesamples. samples. Analyses conducted by Dr. E.J. Chapman Chapman in in 1874 1874include include one of a large, large, average average sample sample of the vein near surface surface that that returned 41.84% Pb, 5.40% Cu, 3.2 ounce Ag per ton and 0.33 0.33 ounce Au per ton (Tanton 1931). Hawley (1930) reported Chapman's Chapman's estimate estimate of of part part of of the ore body near surface as 47.5% Pb, Hawley Pb, 10.0% Cu, 17 17 dwt. 12 12 grammes grarnrnes (sic) Au and 2 ounces ounces 22 dwt. Ag per ton. ton. Mining operations ceased ceased in in 1876; fire destroyed destroyed the the mine mine buildings buildings aa few few years years later (Tanton (Tanton Mining 1931). New owners dewatered and sampled the workings in 1884 but no ore was produced. 1931). New owners dewatered workings in but produced. 927(ibid). Dewatering, examination and subsequent suspension took place between between 1926 1926and and11927(ibid). The mine site was initially rehabilitated in 1926 and 1927 and is now monitored and maintained by the Canadian CanadianNational ~ a t i o n Railway. al~ailwa~. have described describedthe thelocal localgeology. geology. Ore-bearing Ore-bearingveins veinsoccupy occupyaa Hawley (1930) and Tanton (1931) have breccia breccia zone in in dolomitic dolomitic siltstones siltstones of the the Rossport Rossport Formation Formation that extends extends down into into the the has basement. This Archean granitic basement. This breccia breccia zone strikes 065° 065' and dips 70° 70' SE to vertically. ItIt has been traced traced over over 60 m and has an average width width of 1.2 m. Fracture-controlled offshoots and and been Fracture-controlled offshoots branch veinlets may occur in a metre-wide envelope envelope or or "shatter "shatter zone" zone" around aroundthe themain mainvein. vein. The veins tend to pinch-and-swell; pinch-and-swell; mineralized zones over 4 m wide have been noted (ibid). The veins consist consist of the the ore oreminerals minerals galena, galena, low-Fe low-Fe sphalerite sphalerite pyrite pyrite and chalcopyrite chalcopyrite in a Malachite has gangue assemblage of of quartz / amethystine gangue assemblage amethystine quartz, quartz, calcite calcite and and pink pink barite. barite. Malachite precipitated on chalcopyrite-rich, chalcopyrite-rich, weathered dump material. First synopsized by Hawley Hawley (1930) (1930) and and further further described described by Tanton Tanton (1931), (1931), "lead-zinc-barite "lead-zinc-barite termed, were were reported reportedas asearly earlyas as1866. 1866. Of Of the the approximately approximately veins" (LZBV), as they are locally termed, only the Dorion, Ogema and and Enterprise Enterprisedeposits depositswere weremined minedtotoany anydegree. degree. As 30 deposits, only Dorion, Ogema noted by Franklin Franklin and and Mitchell Mitchell (1977), (1977), these these small small deposits deposits comprise comprise a distinct distinct metallogenetic metallogenetic and is is spatially spatially confined confined to to the the group that has a consistent, simple vein mineralogy and structure, and basin. Although Sibley Group depositional basin. Although temporally temporally similar, similar, they are are clearly clearly distinguishable distinguishable from silver-bearing veins that occur near Thunder Animikie Group Group from silver-bearing veins Thunder Bay in in Paleoproterozoic Paleoproterozoic Animikie sedimentary rocks and Logan diabase. �Franklin Franklinand andMitchell Mitchell(1977) (1977)subdivided subdividedthese theseLZBV LZBVdeposits depositsinto intothree threeclasses: classes: (1) those that have the Sibley Group as at least one wall rock (1) those that have the Sibley Group as at least one wall rock(e.g. (e.g.Dorion DorionMine); Mine); (2) those that are entirely enclosed within Sibley Group rocks (e.g. Enterprise (2) those that are entirely enclosed within Sibley Group rocks (e.g. EnterpriseMine); Mine);and and lie within the Archean basement but near the unconformity with (3) those (3) those lie within the Archean basement but near the unconformity withbasal basal sandstones sandstonesof ofthe theSibley SibleyGroup Group(e.g. (e.g.Ogema OgemaMine). Mine). Wall Wall rock rockalteration, alteration,typically typicallybest best manifested manifested in in dolomitic dolomiticrocks, rocks, isistan tanto togrey greyinincolour colourand andisis characterized characterized by by enrichment enrichmentin in 5i02 Si02and andCaO, CaO,slight slightenrichment enrichmentin inFeO FeOand andMnO MnOand anddepletion depletion of ofMgO, MgO,CO2 CO2and andA1203. A1203.(ibid). (ibid). This This indicates indicates silicification, silicification, the the conversion conversion /I replacement replacement of of dolomite dolomite by by calcite, calcite, removal removal of of feldspar feldspar and and mica mica and and pyritization. pyritization. Metal Metalenrichment enrichmentis is noted noted in in altered alteredwall wallrock. rock. Archean K-Arages, ages,indicating indicatingthat thatvein-forming vein-formingfluids fluidswere were Archean wall wall rocks rockshave havereturned returnedArchean ArcheanK-Ar Sulphur-isotopic data not hot enough to re-equilibrate the argon in the analysed mica. not hot enough to re-equilibrate the argon in the analysed mica. Sulphur-isotopic dataindicate indicate equilibrium equilibrium between between galena galena and and sphalerite sphalerite yielding yielding aa depositional depositional range range of of35°-135°C, 35O-l35OC, and and disequilibrium disequilibriumbetween between sulphide-sulphate sulphide-sulphate pairs pairs (ibid). (ibid). Haynes Haynes(1988) (1988)noted notedthat thatfluid fluidinclusions inclusions in wt.%NaCI NaCl±+ in quartz quartzand andsphalerite sphaleriteindicate indicatedeposition depositionfrom fromNa-Ca Na-Cachloride chloridebrines brines(26-33 (26-33wt.% CaCl2 equiv. equiv. salinity) salinity)atattemperatures temperaturesofof90°-200°C. 9O0-20O0C. He He also alsosuggested suggestedthat thattwo twoisotopically isotopically CaCl2 distinct CaCl2 >> >> Theinclusion inclusionfluids fluidscontain containNaC1 NaCl>>CaCl2 distinctbrines brines were were involved involvedin invein veinformation. formation.The brines and Mississippi Valley-type present-day basinal to KC1, compositionally compositionally similar similar to present-day basinal brines and Mississippi Valley-type KCl, mineralizing mineralizingsolutions. solutions. High ratios 30.7) from from vein vein galena galena (Franklin (Franklin and and Mitchell Mitchell 1977) 1977)likely likelyreflect reflect 2 0 6 ~ b / 2 wratios ~ b ((s 30.7) High206Pb/2°4Pb radiogenic lead associated with the radioactive, Archean, Bowker granite. As noted radiogenic lead associated with the radioactive, Archean, Bowker granite. As notedby byFranklin Franklin (l978b), (1978b),the theEnterprise Enterprise Mine Mine has hasthe thedistinction distinction of ofbeing being the theonly onlyLZBV LZBVdeposit deposittotocontain contain appreciable U)portions portionsof ofthe theEnterprise Enterpriseveins veins appreciable amounts amounts of of uranium. uranium. U-enriched U-enriched(up (upto to540 540ppm ppmU) (Ruzicka (Ruzicka1976) 1976)are areapparently apparentlyspatially spatially associated associated with with galena galena and and likely likely reflect reflectproximity proximityto toaa uraniferous, uraniferous, basement basement source source rock. rock. Lead Leadisotopes isotopesare arehighly highlyanomalous, anomalous,yielding yieldingaa secondary secondary isochron isochron that that likely likely indicates indicatesaamixed mixedArchean-Paleoproterozoic Archean-Paleoproterozoic lead lead source source (Franklin (Franklin and and Mitchell Mitchell 1977). 1977). These These anomalous anomalous and and very very radiogenic radiogenic leads leads are are more more uranogenic uranogenic and and less less thorogenic thorogenic than than leads leads found found in insilver-bearing silver-bearing veins veins near near Thunder Thunder Bay Bay (Franklin (Franklinet et al. al. 1986) 1986)to to which they are temporally and tectonically similar. The LZBV mineralization is most which they are temporally and tectonically similar. The LZBV mineralization is mostlikely likely related 110 Ma). related to to dewatering dewateringof of the the Sibley Sibleybasin basin during during Keweenawan Keweenawanrifting rifting(ca.1090-l (ca.1090-1110 Ma). Franklin Franklin and and Mitchell Mitchell (1977) (1977) suggested suggested that that the the vein vein metals metals were were either either leached leached from from basement basement rocks rocks anchor andlor the breakdown breakdown of of Sibley Sibleysandstone sandstonematrix. matrix. Metals Metals and and sulphate sulphatemoved moved through through permeable permeable Pass Pass Lake Lake Formation Formation sandstone sandstoneas as chloride chloride complexes complexes (Haynes (Haynes 1988) 1988)in in aa very very saline saline brine, brine, and and precipitated precipitated in in veins veins at at the thesandstone sandstone pinch-out, pinch-out, at at or or near near the the Archean Archean unconformity. unconformity. Isotopically Isotopically light light sulphur, sulphur,perhaps perhapsof oforganic organicderivation, derivation,may mayhave havebeen beenlocked lockedinina aH2S-bearing His-bearing gas gas trap trap at the the pinch-out, pinch-out, and and caused caused the the metals metals to to precipitate. precipitate. �- - -s-- -:-ccc: H . . .. - .1.1.II.__•t ,'.' • - • - - • • . - . - . . , 1' .;•.'•• - - —- — — - KAMA HILL-FM-L-.. — I— H S accumulates ROSSPORT FM. pinch-out trap • ' • PASSLAKE FM : , f :.:c:::::::::I.:4;. Tt0Itso •,e.:,.f_.e_•._ . -,.•s• 4—BASEMENT '_.,—,-,—;.-; — 4-t4t'+ + + . + + - • + t + + + • • _ __ — — — Schematic formation of LZBVdeposits from Franklin Franklin and and Mitchell Mitchell (1977); fl977); scale is Schematic model model of of the formation ofLZBV depositsfrom is variable. variable. Amethyst The The largest largest deposits depositsof of amethyst, amethyst,Ontario's Ontario's provincial provincial mineral mineral emblem, emblem, occur occur in in the the Thunder Thunder Bay Bay area. area. Nearly Nearlyall allof ofthe themost mostextensive extensiveand andrichest richestdeposits depositsoccur occur in in aa 10 10km-long, km-long, northeastnortheasttrending trending belt belt near near the the margin margin of of the the Sibley SibleyGroup Group outcrop outcrop where where itit is is underlain underlain by by Hilma Hilma Lake Lake granite granite (Kissin (Kissin 1990; 1990; Garland 1994; 1994; Vos 1976). 1976). Other Otheroccurrences, occurrences,mainly mainly hosted hosted in in granite, granite, are also also spatially spatially related related to to Sibley Sibley outcrop. outcrop. These amethystine quartz, quartz, with minor calcite, calcite, barite and sulphide sulphide These deposits depositsconsist consist of of quartz quartz ++ amethystine minerals minerals (e.g. (e.g. pyrite, pyrite, chalcopyrite, chalcopyrite, bomite). They Theytypically typicallyoccupy occupyfaults faultsand and related related fracture fracture zones zones where where brecciation brecciation and open-space-filling open-space-filling are common (ibid). The The similarities similarities in in geologic geologic setting setting between between amethyst amethyst and and LZBV LZBV led led Kissin Kissin (1990) (1990) to to suggest suggest that that these these two two types types of of vein vein deposits in McArthur McArthur and deposits are are cogenetic. cogenetic. Fluid Fluidinclusion inclusionstudies studiesby by McArthur McArthur (1988), (1988), reported reported in Kissin Kissin (1988), (1988), indicate indicate that that most most amethyst amethyst formed formed near near the the surface, surface, at at temperatures temperatures ranging ranging from from 60' to 90' C. Kissin (1990) suggested that they may be low-temperature derivatives of LZBV, 60" to 90" C. Kissin (1990) suggested that they may be low-temperature derivatives of LZBV, poorer in in sulphides sulphidesand and richer richer in in quartz. quartz. The The presence presence of of aa uraniferous uraniferous basement granite granite (Franklin 1978) was probably crucial in the formation of local amethyst deposits (Kissin 1990), (Franklin 1978) was probably crucial in the formation of local amethyst deposits (Kissin1990), because radioactivity radioactivity is is essential essential in in causing causingelemental elemental substitution substitutionand and defects defectsin in the the quartz quartz crystal crystal that that impart impart the the purple purple colour colour (Garland (Garland1994 1994and and references referencestherein). therein). In In the vicinity vicinity of of the the Enterprise Enterprise Mine, Mine, amethyst amethyst veins veins and and breccia breccia zones zones occur occur in in basement basement granitoids granitoids and and in in overlying overlyingclastic clastic and and stromatolitic stromatoliticSibley Sibley Group Group rocks rocks at at or or near near the the unconformity. paper) basemetal-mineralized metal-mineralizedstromatolites stromatolites(see (seefrontispiece, this this paper) unconformity. Amethyst Amethyst± base the Ontario Gem Welch's Farm occurrence occurrence (C. Anderson, Anderson, pers. comm., 1999), 1999), at occur at the Welch's Company Company Amethyst Amethyst Mine Mine (Garland (Garland 1994) 1994)and in other other places places east east of of Ancliff Ancliff station station (Tanton (Tanton + �1931; J. Scott, Scott, pers. pers. comm., comm., 1999; Mcllwaine 1971 a,b). They They typically typically occur occur on on top top of of 1931; J. 1999; Mcllwaine 1971a,b). prominent granite granite knobs knobs which which may mayreflect reflectpaleotopographic paleotopographic"highs". 'highs. prominent The nearby nearby rockcut rockcut produced produced by the railway railway exposes exposes the the basal basal contact contact of the Sibley The by the of the Sibley Group Group with with underlying Archean Archean granitoids. A regolith regolith makes makes identification identification of of the the contact contact difficult difficult in places. underlvine eranitoids. A in vlaces. Unconformity betweenArchean Archeangranite granite (Gr) (Gr) and and trough Unconformity between trough crossbedded, crossbedded, Pass Pass Lake Lake Formation Formation quartz arenite (s.s.), Enterprise Mine quartz arenite ( i s . ) , Enterprise Mine Medium-grained,trough trough cross-stratified cross-stratifiedsandstones sandstoneslie lie directly directly on on the Medium-grained, the regolith. regolith. The The lensoid, lensoid, cross-cutting bed bed geometry the sandstone discernable. Flow cross-cutting geometry of of the sandstone units units is is easily easily discernable. Flow was was to to the the southeast southeast (Cheadlel986). Again the the exact these units units were were laid laid down (Cheadlel986). Again exact depositional depositional environment environment these down in in is is difficult to to ascertain. difficult ascertain. Without Without fossil fossil evidence evidence to to rely relyon onnear-shore, near-shore,trough troughcross-stratified, cross-stratified, sandstone sandstone sequences sequencesare aredifficult difficultto to differentiate differentiate from from South South Saskatchewan-type, Saskatchewan-type, braided braided fluvial, fluvial, channel channel sandstones. sandstones. Overall Overall unit unit geometry geometry can can give give some some clues, clues, as as can can variability variability of of paleocurrent paleocurrent directions and directions and associated associated minor minor lithofacies, lithofacies, but, but, thick, thick, laterally laterally persistent, persistent, monolithic monolithic successions successions assigning a depositional in serious problems of trough cross-stratified sandstone pose of trough cross-stratified sandstone pose serious problems in assigning a depositional environment. Are Are there there any any clues which would would help help in environment. clues in in this this assemblage assemblage which in deciding deciding between between the the two aforementioned two aforementioned environments? environments? �STOP GH WA 17 STOP2-1: 2-1:GURNEY GURNEY(HI (HIGHWAY) The basal unconformity between the Sibley Group and underlying granitoids is exposed at at this this location. A channel is eroded into basement, and containing matrix-supported visible, eroded into basement, and containing matrix-supported location. A channel is visible, conglomerates overlain by by cross-stratified sandstones. This This sequence sequence differs differs from the conglomerates overlain cross-stratified sandstones. the other other coarse-grained, basal sediments we have examined, as itit represents episodes of subaerial debris represents episodes of subaerial debris flow flow activity activity interspersed intersversed with with normal normal flash flashflood flood runoff. runoff. Unconformity between betweenweatheredArchean weatheredArchean granite (Gr) flows (Gr) and and Pass Pass Lake Lake Formation Formationdebris debrisjlows and Highway 17 17 at at Gurney Gurney and sandstones, sandstones,Highway Archean granitic granitic rocks are altered altered at the unconformity and likely represent a pre-Sibley weathered regolith. This weathered weathered regolith. This weathered paleosol, noted noted by Gill Gill (1926) (1926) and and Moorhouse Moorhouse(1960), (1960),was was locally described by Scott (1987). Friable, locally described Friable,blotchy, blotchy,red redand andgreen greengranite granitehosts hostsquartz-carbonate quartz-carbonate veins between exfoliation exfoliation blocks. Feldspars Feldsnarshave havebeen beenhematitized hematitizedand/or and/ordestroyed; destroyed:. ferromagnesian ferromagnesian minerals minerals have have been been chloritized chlohtized(ibid). (ibid). Limited sampling sampling of of drill drill core core from from the the Black Sturgeon Lake area area cited by Scott (1987) (1987) suggests suggests that this alteration may typically involve marked increases H20 and decreases in NazO, Na20, CaO and perhaps KiO. K20. increases in Fe203, MgO, H2O Paleomagnetic data suggest suggest that this weathering was equatorial (G. Borradaille, unpublished data, 1999). 1999). data, . As (1987) number of As noted noted by by Franklin Franklin(1 (1 978b), Scott (1 987) and Tanton (1948), (1 948), a number of uranium uranium occurrences occurrences are rocks within within the Sibley Sibley are associated associated with with altered altered Archean Archean granitoids granitoids and and overlying overlying sedimentary sedimentary rocks basin, basin, prompting prompting comparisons comparisonswith with the the Athabasca Athabasca basin basin in in Saskatchewan. Saskatchewan. Favourable Favourablelocal local parameters parameters for for supergene supergeneuranium uranium deposits depositsinclude: include: (i) (i) uranium-enriched uranium-enriched basement rocks rocks (quartz monzonites, (ii) onlaps onlapsof ofbasal basal Sibley Sibleysandstones sandstoneson onArchean Archeanpaleotopographic paleotopographic monzonites, pegmatites); pegmatites); (ii) "highs'; "highs";and and(iii) (iii)Keweenawan(?) Keweenawan(?)faults faultsthat thatextend extendto tothe thebasement basement(ibid). (ibid). �STOP 2-2: HILL STOP 2-2:KAIMA KAMA HILL Diabase-capped Kama Hill Hill provides Diabase-capped Kama provides an an excellent excellent roadside roadside exposure exposure of of the theRossport RossportFormation. Formation. The sequence is dominated by interlaminated dolostone and red shale with The sequence is dominated by interlaminated dolostone and red shale with layer layer thicknesses thicknesses ranging from from millimeters millimeters to to ten ten centimeters. There is is some ranging centimeters. There some cyclicity cyclicity in in layer layer thickness thickness variation variation up through the sequence, but it is not a strong trend. Some carbonate layers up through the sequence, but it is not a strong trend. Some carbonate layers contain contain coarsecoarse- to to medium-grained sand sand grains. grains. Dessication medium-grained Dessication cracks cracks are are present, present, but but not not common. common.Synsedimentary Synsedientary deformation manifests manifests itself itself as deformation as small small to to large large slump slump folds folds and and brecciation brecciation of of units unitsin inplaces. places.The The sequence is is intruded sequence intruded by by diabase diabase dykes dykes which which bake bake immediately immediately adjacent adjacent sediments. sediments. Some Some evidence has been been put put forward evidence has forward that that the the diabase diabase intruded intruded watery watery sediment, sediment, but but we we have have not not seen seen clear indications of of this. this. Structural clear indications Structural controls controls on on the the emplacement emplacement of of local local sills sills were were suggested suggested by by Antonellini and and Cambray Cambray (1992). (1992). AAchert-carbonate Antonellini chert-carbonate unit unit (Middlebrun (Middlebmn Bay Bay Member; Member; Cheadle Cheadle 1986) locally hosts hosts stromatolites 1986) locally stromatolites and and hydrocarbons. hydrocarbons. The Kama below the the upper The Kama Hill Hill sequence, sequence, exposed exposed below upper diabase diabase sill, sill, was was laid laid down down in in the theshoreshoreproximal region of a playa lake. Pieces accessible in a pile of excavated talus display proximal region of a playa lake. Pieces accessible in a pile of excavated talus display ripple ripple rainy periods, periods, water marks, mudcracks influx into into the the marks, mudcracks and and rare rare raindrop raindrop impressions. impressions. During During rainy water influx lake lake brought brought and and deposited deposited hematite-rich hematite-rich clays. clays. In In dry dry periods, periods, evaporation evaporation from from the the internal internal drainage system system resulted resulted in in lake contraction, hypersalinity hypersalinity and and the the precipitation drainage lake contraction, precipitation of of dolomite. dolomite. This This requires aa high high MgICa Mg/Ca ratio ratio in in the the lake lake water water and and may may have have been been the the result result of of the the precipitation precipitation of of requires gypsum in in the the central this is is unlikely are expected in the the gypsum central lake. lake. However, However, this unlikely as as higher higher salinities salinities are expected in shallower, marginal marginal areas areas then then the shallower, the lake lake center, center, and and thus, thus, gypsum gypsum precipitation precipitation should shouldbe be initiated initiated in triplets. The The lack in the the shallows, shallows, producing producing shale-gypsum-dolomite shale-gypsum-dolomite triplets. lack of of these these means means that that gypsum gypsum precipitation was was not not necessary necessary to to increase increase the the MgKa Mg/Ca ratio. ratio. The The ambient ambient ratio ratio in in the the lake lake water water precipitation itself must have been high enough for the precipitation of dolomite. itself must have been high enough for the precipitation of dolomite. Interlaminated red formed Interlaminated red shales shales and and dolostone, dolostone, formed by climatic cycling cycling in in aa semi-arid, semi-arid, playa playa lake by climatic lake environment, Rossport Rossport Formation, Formation, Kama environment, Kama Hill. Hill. I Reduction spheres in Reduction suheres in hematitic hematitic siltstone, siltstone. Kama Kama Hill, with black, carbonaceous, locally "",z,.-"*;.," """*""" radioactive centers. �STOP STOP2-3: 2-3:RUBYLAKEMARBLELTD. RUBY LAKE MARBLE LTD.QUARRY QUARRY The The Ruby Ruby Lake Lake Marble Marble Ltd. quany quarry isis owned owned and and operated operated by by Don DonMacAlpine MacAlpine and Gerald Gerald Landry. Landry. Variegated, Variegated, multimulticoloured, coloured, banded banded marble marble is is quarried quarried for for landscaping landscaping stone. stone. Approximately Approximately 175 175 tonnes tonnes of of marble marble were were quarried quarried and and shipped shipped in in 1998 1998(D. (D. MacAlpine, MacAlpine, personal personal communication,1999). 1999). The dimensions dimensions communication, of of the the largest, largest, transported transported block block was was 1.8 1.8 by by 0.50 0.50mm(= ( 1.8 1.8 tonnes) tonnes) (ibid). (ibid). by 0.75 0.75 by Approximately 398 tonnes Approximately 398 tonnes of of marble marble were were quarried quarried and and shipped shipped in in 1999 1999(D. (D. MacAlpine, MacAlpine, pers. pers. comm., comm., 2000). 2000). Additional Additional information information is provided orovided on on their their World World Wide Wide Website Website(http://www.rubylakemarble.com). (http://www.mbylakemarble.com). . This This marble marble consists consistsofofcontact contactmetamorphosed, metamorphosed,Mesoproterozoic, Mesoproterozoic, Rossport Rossport Formation Formation contact sedimentary rocks in the (Sibley (Sibley Group) Group) dolostone dolostone and and other, other, calcareous calcareous sedimentary rocks in metamorphic metamorphic aureole aureole of of Keweenawan Keweenawan diabase diabase sills. sills. It has previously been termed termed Nipigon Nipigon River from 1883 1883 to to ca. ca. 1910 at at a site on the River marble and was quarried quarried from the eastern eastern side side of of the the cxl. 1994). A Nipigon River, approximately km west west of the Ruby approximately 66 km Ruby Lake Lake quarry quarry (Hinz (Hinz et et al. similar, similar, hornfelsed homfelsed unit is is described described in more detail detail at Stop Stop 2-6. all Calcite, and opaque minerals were were noted noted in in thin section by Hinz Him et et al. Calcite, dolomite, epidote epidote and opaque minerals (1994) from the Nipigon Nipigon River River quarry. quarry. The The following following chemical analyses analyses for for Nipigon Nipigon River River (1994) - chemical marble also provided (wt.%)were werialso provided (ibid): (ibid): marble (wt.%) Sample Sample S102 SiO; 89MCK 35.21 35.21 89MCK 1 MgO MgO Ti02 A1203 A1203 Ti02 0.20 0.20 8.20 8.20 Fe203 Fe203 FeO FeO MnO MnO 2.04 2.04 0.05 0.05 23.56 23.56 CaO 26.74 Na20 0.00 1(20 2.38 P205 1.53 1.53 0.10 2.04 0.00 0.03 27.63 35.32 0.00 0.41 0.07 0.09 -09 -09 89MCK 29.19 5.20 -10 Shallow Shallow exploration explorationtrenches trenches on on the side side of the road leading to the top of Ruby Mountain (i.e. (i.e. top of marble. Fine-grained, of the the upper upper sill) sill)have have exposed exposed copper-mineralized copper-mineralized marble. Fine-grained, disseminated disseminated blebs blebs of of native native copper copper (0.1 (0.1 to to 1.0 1.0 mm mm occur occur along along calcite-coated, hairline fractures parallel to fractures are are most easily planes. Mineralized Mineralized fractures easily recognized recognized where where secondary secondary bedding planes. just formed. The top of the the adjacent adjacent (middle?) sill is exposed just (supergene) malachite has formed. Polygonal cooling coolingjoints joints (seen (seen in in plan plan view) view) and and the chilled, upper upper farther along along the the road. road. Polygonal are evident. evident. margin are Similar, calcareous units units have have been been noted noted near near a diabase sill contact at Similar, copper-mineralized, calcareous all Hughes Point, 2.5 km krn to the south-southwest south-southwest by Schnieders et al. (1996). At At this this location, location, 2 to 55 cm cm wide wide calcite calciteveins veinscontain containdisseminated disseminated covellite covellite (after (after chalcocite?) chalcocite?) and and malachite. malachite. �The Theorientation orientationof ofthe theveins veinsare areroughly roughlyparallel parallelto tojoints joints developed developedin inthe theadjacent adjacentsill. sill.Grab Grab samples have returned up to 3.065% Cu, nil Au and nil Ag (ibid). samples have returned up to 3.065% Cu, nil Au and nil Ag (ibid). Franklin Franklin (1970) (1970)described describedcopper-mineralized copper-mineralized stromatolitic stromatolitic units units near nearDisraeli DisraeliLake. Lake. AA variety varietyof ofcopper copperminerals, minerals,including includingdigenite, digenite,cuprite, cuprite,covellite, covellite,chalcopyrite, chalcopyrite,native nativecopper copper and and malachite, malachite, were were identified identified as as open-space open-space fillings fillings in in the the vuggy vuggy host hostrock. rock. Franklin Franklin (1970) (1970)suggested suggestedthat thatthe thecopper copperwas wasintroduced introducedepigenetically epigenetically from fromaagabbro gabbrototoperidotite peridotite The close spatial association plug plug that thatintruded intrudedthese theseSibley SibleyGroup Grouprocks. rocks. The close spatial associationofofcoppercoppermineralized mineralizedrocks rockswith withdiabase diabasesill sillcontacts contactsininthe thevicinity vicinityof ofRuby RubyLake Lakesupports supportsthis thistheory. theory. The Themarble marble isisstromatolitic, stromatolitic, though though this this is is difficult difficult to to ascertain ascertain in in most most places places due due to tothe the alteration. The stromatolites are best seen on the tops of beds where they protrude, causing alteration. The stromatolites are best seen on the tops of beds where they protrude, causing the thecontact contactto tobecome becomewavy. wavy. Hints Hintsof ofthe thepresence presenceofofstromatotactis stromatotactisare arevisible, visible,giving givingthe the impression impression that that aa large largeamount amount ofofthe thehorizontal horizontallayering layeringisisstromatolitic stromatoliticS-mat. S-mat. This This sequence sequence was was deposited deposited in inaastrandline strandlineproximal proximalposition positioninin the the playa, playa, as asdenoted denotedby bythe the horizon at other locations. This infers that lake size presence presenceof ofteepee teepeestructures structuresininthis this horizon at other locations. This infers that lake sizehad had stabilized stabilizedduring during this thisinterval, interval,eliminating eliminatingthe theseasonal, seasonal,large-scale large-scalefluctuations fluctuationsin inshoreline shoreline positioning. positioning. r . - - - - .— t - - — 4T : TI - - - -- Lz.L .. . Parallel-laminated ininpossible Parallel-laminatedcarbonate carbonatewith withvariable variableamounts amountsofofhematitic hematiticclays ...~, 'a possibleS-mat stromatolite, Ruby Lake Quarry �U MOUNTAIN MOUNTAIN STOP 2-4: 2-4: MOSEA MOSEAU The Moseau stop consists consists of of a number Moseau Mountain Mountain stop number of of small small outcrops outcrops extending extending from from the the granitic basement basement to to the middle granitic middle portion portion of ofthe theRossport RossportFormation. Formation. Cross-stratified sandstones of the Pass Lake multiLake Formation Formation overlie basement. The lower lower sandstones sandstones are multistorey, but as we proceed upwards mudstone mudstoneinterbeds interbedsbegin begin to to appear. appear. Above Above this thiszone zonethe the sandstones are replaced by siltstones, continuing the general upward-fining trend. trend. Higher Higher in the siltstone mudblock conglomerates conglomeratesbegin begintoto appear. appear. These, These, in in turn, turn, are are the siltstone succession succession mudblock upwardly transitional into evaporite block conglomerates, with gypsum and dolomite clasts. and dolomite clasts. Diabase intrusions intrusions occur occur above above this this zone. zone. The lower portion of of the sequence is similar to the one observed The lower portion sequence is similar to observed at Pass Pass Lake, Lake, where where a and conglomerates to to the the siltstones thinning and fining trend exists from the basal sandstones and of the basal Rossport Formation. This again represents expansion expansion of a playa playa lake lake system. system. The The intraformational conglomerates which which overly overly this this sequence may may have have been produced by two intrafonnational mechanisms. Dissolution Dissolution of of an evaporitic layer causing collapse of the overlying mechanisms. overlying units, units, or or tectonic raising of the basin margin margin resulting resulting in inthe theerosion erosionand and resedimentation resedimentation in in aa more more basin-center environment of the upper marginal sediments. The latter explanation explanation is preferred as this unit is regional in extent, occurring where it is unlikely that underlying evaporites ever existed. existed. STOP 2-5: S LOOKOUT 2-5: LLOYD LLOYD'S LOOKOUT This stop, located on the Nipigon-Red Rock hiking trail, features exposures of Kama Hill Formation shales and siltstones siltstones underneath a Logan diabase sill which caps the prominent mesa and provides the vantage point over Nipigon Nipigon Bay Bay known known as as Lloyd's Lloyd'sLookout. Lookout. From one can can view view the themouth mouthof ofthe theNipigon NipigonRiver Riverand andNipigon NipigonBay. Bay. The this vantage point, one locations of the quarries 1-2 are shown in the photograph photograph below. quarries mentioned in STOP 1-2 Viewfrom fromLloyd's Lloyd's Lookout, Lookout, looking looking east east over Nipigon Bay View I �This This is is one oneof ofthe thefew fewaccessible accessibleexposures exposuresof ofthe theKama KamaHill HillFormation. Formation.This ThisFormation Formationisis homogeneous, consisting of brick-red mudstones with interlayered, current-rippled, homogeneous, consisting of brick-red mudstones with interlayered, current-rippled,coarsecoarsegrained grained siltstones siltstones and and very veryfine-grained fine-grained sandstones. sandstones. The The rippled rippled layers layers are are usually usually centimeters centimetersto to aa few fewdecimeters decimetersthick. thick.The Themudstones mudstonesare arecommonly commonlymudcracked. mudcracked. Cheadle believed that that this this unit unit was was deposited deposited as as subaerial subaerialmudflats, mudflats, representing representing the the Cheadle (1986) (1986) believed final final stage stageof of infilling infilling of of the the lake lake basin. basin. However, However, new new drill drill core core data dataindicate indicate that that the the Kama Kama Hill Hill Formation Formation is is overlain overlain by by aaprograding, prograding,deltaic deltaicsequence. sequence. This Thisinfers infersa asubaqueous subaqueous position position for for the the Kama KamaHill HillFormation Formationwith withthe thesediments sedimentsrepresenting representingprodelta prodeltato todistal distalbar bar sequences. In core, core, these these are are overlain overlain by by distributary distributary mouth mouth bar bar and andfluvial fluvialchannel channel sequences. In sandstones. The mudcracks are problematic. They may be syneresis cracks, sandstones. The mudcracks are problematic. They may be syneresis cracks, formed formed subaqueously subaqueously through through differential differential swelling swelling of of clays, clays, or or they they may mayrepresent representlarge-scale, large-scale,water water level level fluctuations. fluctuations.Neither Neither of ofthese theseexplanations explanationsappears appearsadequate. adequate. The The view view over over Nipigon Nipigon Bay Bay isistowards towardswhere wherethe thedrillcore drillcoreexamined examinedthe theprevious previousday daywas was drilled. 20 m m of ofwater, water, drilled. During During Kama Kama Hill Hill time time we we would would be be standing standing in in approximately approximately 20 looking looking towards towards aa horizon horizon where where aa delta delta system system would would be be located, located, building building towards towards us. us. STOP STOP 2-6: 2-6:BIG BIGSQUAW SQUAWCREEK CREEKROAD ROAD CUT CUT This This stop stop presents presents the the opportunity opportunity to to view view stoped stoped blocks blocks of of Rossport Rossport Formation Formation dolostone dolostone in in aa diabase diabase sill, sill, as as well well as asthe theattendant attendantmetamorphic metamorphic and andmetasomatic metasomaticeffects. effects. Pale Pale green green to to buff, buff, thinly thinly bedded bedded dolostones dolostonesare are preserved preserved as as rafted rafted xenoliths xenolithsup up to to several severalmetres metres thick. thick. Rafied Formation in Rafted xenoliths of hornfelsed Rossport Formation in Logan diabase diabase sill, sill, Highway Highway 11-17 11-1 7 at at Big Squaw Creek Creek I I I I �Thin section Thin section petrography petrography reveals aa recrystallized, recrystallized, granoblastic granoblastic texture texture predominated predominated by epidote. Subhedral to euhedral, epidote porphyroblasts are typically 0.05 to 0.10mm size, mm in size, Subhedral porphyroblasts are typically 0.05 to 0.10 but they may reach 0.25 mm, in a fine-grained matrix of vermicularly intergrown quartz and feldspar. Large Large (<0.75 (<0.75 cm), cm), optically optically continuous, continuous, decussate, decussate, acicular biotite porphyroblasts Radiating, sheaf-like sheaf-like aggregates aggregates have have been are also overgrown are overgrown by by epidote. epidote. Radiating, been tentatively tentatively Minor sericite sericite and and calcite were were also noted. Epidote persists into the identified as chlorite. chlorite. Minor margin of of the the diabase. diabase. Joints fine-grained, chilled margin Joints and and fractures fractures surfaces may be coated coated with Large, consp conspicuous, lime-green clots of prehnite occur in epidote-calcite veins muscovite. Large, within the diabase. Little work has been done done on on contact contact metamorphic metamorphic effects in vicinity of the diabase diabase sills. sills. (1986) noted noted that that such metamorphism occurs in in calcareous calcareous rocks rocks over over a zone Sutcliffe (1986) Sutcliffe metamorphism occurs zone several metres metres thick, thick, where where bleaching bleaching is is its its most obvious manifestation. manifestation. Fine-grained, Fine-grained, calcmineral-bearing assemblages assemblages identified identified by by Sutcliffe Sutcliffe (1986) (1986) include include calcite, plus (in silicate mineral-bearing (in order of increasing metamorphic grade): talc, tremolite, serpentine (after forsterite) and increasing metamorphic grade): tremolite, serpentine (after forsterite) and Calculations by by Sutcliffe Sutcliffe (1986) (1986) suggest suggest that that the diabase sills were emplaced at diopside. diopside. Calculations shallow crustal crustal levels, levels, near near the the Archean-Proterozoic Archean-Proterozoicunconformity, unconformity,atataalithostatic lithostaticload loadof of shallow <0.4 adjacent 88OSC,while adjacent 0 . 4 kbars. The Thesills sillscrystallized crystallizedover over aa temperature temperature range of 1100° 1100" to 880°C, wall rocks may have experienced 665°C (ibid). experienced temperatures temperatures ranging ranging from 540° 540' to 665'C hornfels at Big Squaw Creek may may reflect simple The mineral assemblage in the calc-silicate hornfels significant recrystallization of sedimentary units without of the impure, impure, calcareous sedimentary units without significant compositional or compositional or volumetric volumetric changes changes (ci'. (cf. Hall and and MacKevitt MacKevitt 1962). 1962). �REFERENCES REFERENCES Antonellini, Antonellini, M.A. M.A. and and Cambray, Cambray, F.W. F.W. 1992. 1992. Relations Relations between between sill sill intrusions intrusions and and beddingbeddingparallel extensional shear zones in the Mid-continent rift system of the Lake parallel extensional shear zones in the Mid-continent rift system of the LakeSuperior Superior region; region; Tectonophysics, Tectonophysics,v.212, v.212,p.33 p.331-349. 1-349. Bell, Bell, R. R. 1870. 1870.Report Report on on the the geology geology of of the the northeast northeast side side of of Lake Lake Superior Superiorand and the theNipigon Nipigon district; district;Geological GeologicalSurvey Surveyof ofCanada, Canada,Report Reportfor for1866-1869, 1866-1869,p.3 p.313-364. 13-364. Cheadle, Cheadle, B.A. B.A. 1981. 1981. The The stratigraphy stratigraphy and and depositional depositional history history of of the theRossport RossportFormation Formation Stratotype, Channel Island, H.B.Sc. Stratotype, Channel Island, Thunder Thunder Bay Bay District, District, Ontario; Ontario; unpublished unpublished H.B.Sc. thesis, thesis, Lakehead LakeheadUniversity, University,Thunder ThunderBay, Bay,Ontario, Ontario,46p. 46p. . 1985. 1985. Stratigraphy Stratigraphy and and sedimentation sedimentation of of the the middle middleProterozoic ProterozoicSibley SibleyGroup; Group; unpublished Ph.D. thesis, University of Western Ontario, London, Ontario unpublished Ph.D. thesis, University of Western Ontario, London, Ontario .1986. sedimentation in in the the lower 1986. Alluvial-playa Alluvial-playa sedimentation lower Keweenawan Keweenawan Sibley Sibley Group, Group, Thunder Thunder Bay Bay District, District, Ontario; Ontario; Canadian Canadian Journal Journal of of Earth Earth Sciences, Sciences, v.23, v.23, p.527-542. p.527-542. Coates, Coates, M.E. M.E. 1972. 1972. Geology Geology of of the the Black Black Sturgeon Sturgeon area, area, District District of of Thunder Thunder Bay; Bay; Ontario Ontario Department 4lp. Department of of Mines Mines and and Northern Northern Affairs; Affairs; Geological Geological Report Report 98, 98,41p. Davis, Davis, D.W. D.W. and and Sutcliffe, Sutcliffe, R.H. R.H. 1985. 1985.U-Pb U-Pb ages ages from from the the Nipigon NipigonPlate Plateand andnorthern northernLake Lake Superior; Geological Society of America Bulletin, v.96, p.1572-1579. Superior; Geological Society of America Bulletin, v.96, p.1572-1579. Fralick, Fralick, P.W. P.W. and and Kissin, Kissin, S.A. S.A. 1995. 1995.Mid-Proterozoic Mid-Proterozoic basin basin development development in in central centralNorth North America: inProceedings, Proceedings, America: Implications Implicationsof of Sibley SibleyGroup Groupvolcanism volcanismand andsedimentation; sedimentation;in 1995 Program, Project Project 336, 1995 International International Geological Geological Correlation Correlation Program, 336, Petrology Petrology and and metallogeny RiftSystem, System,p.51 p.51metallogeny of of volcanic volcanic and and intrusive intrusive rocks rocks of ofthe theMidcontinent MidcontinentRift 52. 52. Franklin, J.M. 1970. of the Franklin, J.M. 1970. Metallogeny Metallogeny of the Proterozoic Proterozoic rocks rocks of of the the Thunder Thunder Bay Bay District, District, Ontario; Ontario;unpublished unpublished Ph.D. Ph.D. thesis, thesis,The TheUniversity Universityof ofWestern WesternOntario, Ontario,London, London,3O4p. 304p. 978a.The The Sibley Sibley Group, Group, Ontario; Ontario; iin Rubidium-strontium isochron .1 1978a. n Rubidium-strontium isochronage age studies, studies, Report Report 2, 2, Geological Geological Survey Survey of of Canada, Canada, Paper Paper 77-14, p.31-34. 978b.Uranium Uraniummineralization mineralizationinin the the Nipigon Nipigon area, area, Thunder . 11978b. Thunder Bay Bay District, District, Ontario; in Part A, A, Geological Survey of of Canada, Paper 78-lA, n Current Current Research, Research, Part Geological Survey Canada, Paper 78-lA, Ontario; i p.275-282. p.275-282. Franklin, J.M. and Franklin, J.M. and Kustra, Kustra, C.R. C.R. 1972. 1972. The The Proterozoic Proterozoic rocks rocks of of the the Lake Lake Superior Superior area, area, northwestern northwestern Ontario; Ontario; in in XXIV International International Geological Conference, Montreal, Montreal, Field Field Excursion Excursion C34 C34 Guidebook, Guidebook,p.20-46. p.20-46. �Franklin, Franklin, J.M. J.M.and andMitchell, Mitchell,R.H. R.H.1977. 1977.Lead-zinc-barite Lead-zinc-barite veins veins of ofthe theDorion Dorionarea, area,Thunder Thunder Bay District, Ontario; Canadian Journal of Earth Sciences, v.14, p.1963-1979. Bay District, Ontario; Canadian Journal of Earth Sciences, v.14, p.1963-1979. Franklin, Franklin, J.M., J.M., Kissin, Kissin, S.A., S.A., Smyk, Smyk,M.C. M.C. and andScott, Scott,S.D. S.D.1986. 1986.Silver Silverdeposits depositsassociated associated with withthe theProterozoic Proterozoicrocks rocks of of the the Thunder Thunder Bay Bay District, District, Ontario; Ontario;Canadian Canadian Journal Journalof of Earth EarthSciences, Sciences,v.23, v.23,p.1576-1591. p.1576-1591. Franklin, Franklin, J.M., J.M., Mcllwaine, Mcllwaine, W.H., W.H., Poulsen, KR. K.H.and andWanless, Wanless,R.K. R.K. 1980. 1980.Stratigraphy Stratigraphyand and depositional setting of the Sibley Group, Thunder Bay District, Ontario, Canada; depositional setting of the Sibley Group, Thunder Bay District, Ontario, Canada; Canadian CanadianJournal Journalof ofEarth EarthSciences, Sciences,v.17, v. 17,p.633-651. p.633-65 1. Mcllwaine, W.H., W.H., Shegelski, Shegelski, R.J., Mitchell, Mitchell, R.H. and and Platt, Platt, R.G. R.G. (1982) (1982) Franklin, J.M., J.M., Mcllwaine, Franklin, Proterozoic geology of the northern Lake Superior area; Field Trip Guidebook, Proterozoic geology of the northern Lake Superior Field Trip Guidebook, GACGACMAC MAC Annual AnnualMeeting, Meeting,Winnipeg, Winnipeg, '7lp. 71p. Garland, Garland, M.I. M.I. 1994. 1994. Amethyst Amethyst in in the the Thunder Thunder Bay Bayarea; area;Ontario OntarioGeological GeologicalSurvey, Survey,Open Open File FileReport Report5891, 5891,l9'7p. 197p. Gill, Gill, J.E. J.E. 1926. 1926.Gunflint Gunflintiron-bearing iron-bearing formation, formation, Ontario; Ontario; Geological Geological Survey Survey of of Canada, Canada, Summary Report, 27C, p.28c-88c. Summary Report, 27C, p.28~-88c. Hall, Hall, W.E. W.E. and and MacKevitt, MacKevitt, E.M. E.M. 1962. 1962.Geology Geology and and ore ore deposits deposits of of the the Darwin Darwin Quadrangle, Quadrangle, Inyo Inyo County, County, California; California; United United States StatesGeological Geological Survey, Survey,Professional Professional Paper, Paper, no.368. no.368. Hawley, Hawley, J.E. J.E. 1930. 1930.Lead Lead and and zinc zinc deposits, deposits, Dorion Dorion and and McTavish McTavish townships, townships, Thunder Thunder Bay Bay District; District;Ontario OntarioDepartment Departmentof of Mines, Mines, Annual Annual Report, Report, v.38, v.38, pt.VI, pt.VI, p.59-85. p.59-85. Haynes, Haynes, F.M. P.M. 1988. 1988. Fluid-inclusion Fluid-inclusion evidence evidence of basinal basinal brines brines in inArchean Archeanbasement, basement, Thunder Thunder Bay Bay Pb-Zn-Ba Pb-Zn-Ba district, district, Ontario, Ontario, Canada; Canada; Canadian Canadian Journal Journal of Earth Earth Sciences, v.25, v.25, p.1184-1894. p. 1184-1894. Sciences, in Hinz, Him, P., P., Landry, Landry, R.M. R.M. and and Gerow, Gerow, M.C. M.C. 1994. 1994.Dimension Dimension stone stone occurrences occurrencesand and deposits deposits in northwestern OpenFile FileReport Report5890, 5890,191p. l9lp. northwestern Ontario; Ontario Geological Survey, Open Hofmann, Hofmann, H.J. 1969. 1969. Stromatolites Stromatolites from the the Proterozoic Proterozoic Animike Animike and and Sibley Sibley Groups, Groups, Ontario; 33p. Ontario; Geological Geological Survey Survey of of Canada, Canada, Paper Paper 68-69, 68-69,33p. Hunt, Hunt, T.S. T.S. 1873. 1873. The The geonostical geonostical history history of the the metals; metals; Transactions Transactions of the the American American Institute Institute of Mining Mining and Engineering, Engineering, v.1, p.331-345. Kissin, Kissin, S.A. S.A. 1990. 1990.Granitoid-related Granitoid-related mineral mineral deposits deposits of the the western western Lake Lake Superior Superiorregion; region; 36th Annual Institute on Lake Superior Geology, 36th Annual Geology, Field Trip Guidebook, Guidebook, v.36, v.36, pt.2, pt.2, p.52-66. �Kustra, Kustra, C.R., C.R., Mcllwaine, McIlwaine,W.H., W.H., Fenwick, Fenwick,K.G. K.G. and andScott, Scott,J.F. J.F. 1977. 1977.Proterozoic Proterozoicrocks rocksof ofthe the Thunder Bay area, northwestern Ontario; Field Trip Guidebook, 23 Annual Institute Thunder Bay area, northwestern Ontario; Field Trip Guidebook, 23rdAnnual Institute on on Lake LakeSuperior SuperiorGeology GeologyMeeting, Meeting,Thunder ThunderBay, Bay,4'7p. 47p. Logan, Logan, W. W. 1863. 1863.Geology Geologyof ofCanada, Canada,Geological GeologicalSurvey Surveyof ofCanada, Canada,Report Reportof ofProgress Progressfrom from Commencement to 1863. Commencement to 1863. Mailman, Mailman, M. M. 1999. 1999.Cyclic Cyclicdeposits depositsofofthe theRossport RossportFormation, Formation,Mesoproterozoic MesoproterozoicSibley Sibley Group, Group, northwestern northwestern Ontario; Ontario; unpublished unpublished H.B.Sc. H.B.Sc. thesis, thesis,Lakehead LakeheadUniversity, University, Thunder ThunderBay, Bay,Ontario, Ontario,87p. 87p. Mattis, in Geology Geology of of Mattis, A.F. A.F. 1972. 1972. Puckwunge Puckwunge Formation Formation of of northeastern northeastern Minnesota; Minnesota; in Minnesota: Acentennial centennialvolume; volume;pAcl2-4l5. p.412-4 15. Minnesota:A MeArthur, McArthur, J. J. 1988. 1988. Fluid Fluid inclusion inclusion and and stable stable isotopic isotopic studies studies on on amethyst, amethyst, Thunder Thunder Bay Bay Amethyst Mine, Thunder Bay District, Ontario; unpublished B.Sc. thesis, Amethyst Mine, Thunder Bay District, Ontario; unpublished BSc. thesis,Lakehead Lakehead University, University,Thunder ThunderBay, Bay,Ontario, Ontario,lO8p. 108p. McArthur, McArthur, J. J. and and Kissin, Kissin,S.A. S.A.1988. 1988.Stable Stableisotope, isotope,fluid fluidinclusion inclusionand andmineralogical mineralogicalstudies studies relating relating to to the the genesis genesisof ofamethyst, amethyst,Thunder ThunderBay BayAmethyst AmethystMine, Mine,Ontario, Ontario,Canada; Canada;inin Geological Geological Society Society of of America, America, Annual Annual Meeting, Meeting, Denver, Denver, 1988, 1988,Abstracts Abstracts with with Programs, Programs,v.20, v.20,p.A40. p.A40. Mcllwaine, Mcllwaine, W.H. W.H. 1971a. 1971a. McTavish McTavish Township Township (East part of of north north half), half), District District of of Thunder Thunder Bay; Ontario Department of Mines and Northern Affairs, Preliminary Map 1, Bay; Ontario Department of Mines and Northern Affairs, Preliminary MapP.72 P.721, scale 1:15 840. 840. scale 1:15 (West part part of . 1971b. 197Ib. McTavish McTavish Township Township (West of north north halt), half), District District of ofThunder ThunderBay; Bay; Ontario Ontario Department Department of of Mines Mines and and Northern Northern Affairs, Affairs, Preliminary Preliminary Map Map P.720, P.720, scale scale 1:15 1:15 840. 840. . 1975. 1975. McTavish McTavish Township Township (south (south half), half), District District of of Thunder Thunder Bay; Bay; Ontario Ontario Department Department of of Mines Mines and and Northern Northern Affairs, Affairs, Preliminary Preliminary Map Map P.990, P.990, scale scale 1:15 1:I5 840. 840. Moorhouse, W.W. 1960. The Gunflint Moorhouse, W.W. Gunflint iron range in the the vicinity vicinity of ofPort PortArthur; Arthur;Ontario Ontario Department Department of of Mines, Mines, Annual AnnualReport, Report,v.69, v.69, pt.7, pt.7, p.1-40. p.1-40. Morey, Morey, GB. G.B. and andOjakangas, Ojakangas, R.W. R.W.1982. 1982.Keweenawan Keweenawan sedimentary sedimentary rocks rocks of ofeastern eastern Minnesota and northwestern northwestern Wisconsin; Wisconsin; in Tectonic Tectonic history of of the the Lake LakeSuperior Superior basin, Geological Geological Society Society of of America, America, Memoir Memoir 156, 156, p.135-146. p.135-146. Parks, W.A. 1901. 1901. Report of 1901 1901 on on the the geology geology and and natural natural resources resourcesof ofthe thesoutheastern southeastern Nipigon district; Geological Survey of Canada, Report. Nipigon district; Geological Survey of Canada, Report. Ruzicka, V. 1976. in Report 1976. Assessment Assessment of some some Canadian Canadian uranium uranium occurrences; occurrences; in Report of Ruzicka, V. Activities, Part A, Geological Survey of Canada, Canada, Paper Paper 76-IA, 76-lA, p.341-342, p.341-342. �Scott, J.F. J.F. 1987. Uranium occurrences of the Thunder Bay-Nipigon-Marathon area; Ontario Ontario Bay-Nipigon-Marathon area; l58p. Geological Survey, Open File Report 5634, 158p. Sutcliffe, R.H. R.H. 1986. The The petrology, mineral chemistry chemistry and and tectonics tectonics of Proterozoic Sutcliffe, petrology, mineral Proterozoic riftrelated igneous rocks at Lake Nipigon, Ontario; unpublished Ph.D. related igneous rocks Lake Nipigon, Ontario; unpublished Ph.D. thesis, thesis, The The University of Western Ontario, London, 325p. Tanton, T.L. T.L. 1931. Fort William and Port Arthur, and Thunder Cape map-areas, Thunder Tanton, map-areas, Thunder 222p. Bay District, Ontario; Geological Survey of Canada, Memoir 167, 167,222~. Radioactive nodules nodules in in sediments sediments of of the Sibley series, Nipigon, Ontario; 1948. Radioactive . 1948. Ontario; III, section IV, p.69-75. Transactions of the Royal Society of Canada, v.XLII, series 111, Wilson, A.W.G. 1910. 1910. Geology of the Nipigon Basin, Ontario; Canada Department of Mines, Geological Survey Branch, Memoir Memoir No. No. 1, 152p. l52p. 1 �GEOLOGY GEOLOGY OF OF THE THE LAC LAC DES DES ILES ILES PGE-Cu-Ni PGE-Cu-Ni MINE MINE Moe Lavigne, Lavigne, Matawin Matawin Mineral Mineral Exploration, Exploration, and and Moe Staff, Lac Lac des des lies lies Mines Mines Ltd. Ltd. Staff, North American American Palladium Palladium Limited's pit Lac Iles North Limited's open open pit Lac des des lies Mine Archean Lac Lac des des lies Iles mafic-ultramafic mafic-ultrama/Ic Mine is is hosted hosted by by the the Archean intrusive complex. This trip on PGE intrusive complex. This trip willfocus will focus on PGE mineralization mineralization as structure and and alteration. as it it relates relates to to intrusive intrusive lithology, lithology, structure alteration. �PaHadurn Geology Geology of the the Lac Lac des lies lles Palladium Deposit Regional Regional Geology Geology The age rocks rocks (2.6 (2.6 ttoo 3.0 The Lac Lac des des lies lles area is underlain by the Archean age 3.0 billion billionyears) years) of ofthe theCanadian CanadianShield.These Shield.These rocks are overlain and and intruded intruded by by Proterozoic Proterozoic age agerocks rocks(0.9 (0.9t o to 2.2 2.2billion billionyears) years)that thatformed formedduring duringrifting riftingof of the mafictto or have the continent, continent,centered centeredon onLake LakeSuperior.Archean Superior.Archean age. age. mafic o ultramafic intrusions that host, host. or have the potenpoten- tial are dispersed dispersedthroughout throughoutNorthwestern Northwestern 0ntario.These Ontario.These intrusions are are concentratconcentrattial to t ohost hostPGE PGE mineralization, mineralization,are ed malor faults.The 20km km south south of Lac Lac ed within within granite-greenstone granite-greenstone terrain, terrain, and and along major faults.The Quetico Quetico Fault, Fault, which passes passes 20 des lles, lies, is is coincident coincident with with a 180km southwesterly trend trend of PGE bearing intrusi0ns.A intrusions.A particular particular high high concentraconcentral80km southwesterly PGE bearing tion circular feature feature with with a 30 km tion of ofintrusions intrusionsoccur occurininthe theLac Lacdes des lies llesarea, area,aa number of intrusions define a circular diameter.These major crustal sutures are are important important as that can can only only originate originate from diameter.These major as they allow allow PGE PGE rich magma magma that the the earths earths mantle, mantle, ttoo move move upwards upwards into into the the crust. crust Proterozoic mineralization isis not not as as common common in northwestern northwestern Ontario. Ontario.However, However,potential potentialPGE PGE hosting hosting Proterozoicage age PGE PGE mineralization Proterozoic mafic tto voluminous.Gabbro Gabbrocovers coversor or intrudes intrudes most most of the Proterozoic age mafic o ultramafic intrusions are much more voluminous. Lake Superior Superior area, area, no no less lessthan than 20,000 20,000km2.A km2.A recent 8 g/t lies at Wolf Wolf g/t Pd+Pt Pd+Pt discovery discovery made made east of Lac Lac des lles Mountain in a Proterozoic ultramafic intrusion intrusion is testament testament tto o its its potential. potential. Proterozoic age age ultramafic The known knownmafic mafic to t oultramafic ultramaficintrusions intrusionsininthe theLac Lacdes deslIes llesarea area consists consists of of the the following; following; . • Lac des lles lies Complex Complex (—30km2) (-30km2) (Company ownership; ownership;100%) 100%) • Tib TibLake LakeIntrusion Intrusion(—25km2) (-25km2) (Company (Company ownership; ownership;65%) 65%) ..,~ ~, ~:~ ,. • Legris 2km2) (Company ownership; LegrisLake LakeIntrusion Intrusion(—.1 (-1 2km2) ownership; 0%) 0%) • Demars DemarsLake LakeIntrusion Intrusion(-.(-1I km2) km2)(Company (Companyownership; ownership;100%) 100%) . . • Taman Taman Lake Lake Intrusion Intrusion (CI (C Ikm2 km2 (Company (Company ownership; ownership;0%) 0%) DogRiver RiverIntrusions Intrusions(<I (< lkm2) km2) • Dog (Company (Company ownership; ownership;0%) • Buck BuckLake LakeIntrusion Intrusion(—Skm2) (-5km2) (Company (Company ownership; ownership;90%) 90%) • Wakinoo WakinooLake LakeIntrusion Intrusion(--II (-1 kni2) lkm2)(Company (Companyownership; ownership;100%) 100%) w) These intrusion intrusion are are similar similar in in that that they theyare arelate latetectonic tectonicand andintrude intrudetonalite tonalitegneiss, gneiss,have have tholeHtic tholeiitic affinity and contain rangingfrom from ultramafic ultramafic peridotitic contain phases phases ranging peridotiticand andpyroxenitic pyroxeniticcummulates cummulatestot omagnesian magnesiangabbro-norite gabbro-noriteand and iron-rich Taman and Dog Dog River intrusions. is found found in in all all except except the theTaman intrusions. iron-rich gabbro. gabbro. PGE PGE mineralization mineralization is des lles lies Complex consist of three The Lac des three distinct distinct lithological lithologicalpackages; packages; tto o the north, north, ultramafic ultramafic intrusions intrusionscencentered gabbroic intrusions intrusions immediately south of the lake and a horntered on on Lac Lac des des lies, lles, complex gabbroic lake (Mine Complex), Complex),and blende gabbro southwest southwest of of Camp Camp Lake Lake (Camp (Camp Lake Lake lntrusion).These three threesegments segments are are partially partiallyseparated separated by by tonalite of which consist of several septums.The ultramafic ultramafic intrusions intrusions consist consist of of two twocoalescing coalescingcentres, centres, each each of tonalite septums.The It conconintrusive phases.The northern ultramafic centre is nearly circular in plan with a diameter of nearly 4 km. It phases.The northern sist of interlayered interlayered dunite dunite and and wehrlite, wehrlite,olivine-clinopyroxenite olivine-clinopyroxeniteand andclinopyroxenite. clinopyroxenite.and and websterite websteritewhich whichdefine define aa �Palladium concentric concentric inward inward dipping dipping pattern.These pattern.These lithologies lithologies occur occurininseveral several cycles cycles which which probably probably represents represents fractional fractional crystallization of pulses.ItItalso alsorepresents representsrepeated repeatedopportunities opportunities tto concentrate PGE's. This crystallization of separate separate magma magma pulses. o concentrate PGE's. This part of sulphide rich rich horizons, occurrences (>1 part of the thecomplex complexdoes doeshave haveseveral several PGE's PGE's bearing bearing sulphide horizons, and and 55 PGE PGE occurrences (>I g/t g/t Pd+Pt) were discovered 999.The elliptical elliptical southern southern ultramafic wehrlite core Pd+Pt) were discovered in in I1999.The ultramafic centre centre consist consist of of aamassive massive wehrlite coresursur- rounded websterite.These two gabbronorite wedge on the the east side of of the the rounded by by websterite.These two centres centres are are partially partiallyseparated separated by by aa gabbronorite wedge on east side complex..This area PGE occurrences, occurrences,S 5 discovered discovered in in 1999. 1999. complex..This area contains contains 77 PGE The had been been considered considered tto a relatively homogenous hornblende hornblende be a relatively homogenous The Camp Camp Lake Lake Intrusion, Intrusion, south south of of Camp Camp Lake, Lake, had o be gabbro and barren barren of of PGE mineralization. Historically, Historically, itit had had been been subjected subjected tto minimal exploration. exploration. Stripping gabbro and PGE mineralization. o minimal Stripping along its northern northernmargin margininin1999 1999revealed revealedunexpected unexpectedcomplexity complexityand andsampling samplinggenerated generatedlow lowgrade gradePGE PGEassays. assays. along its The mineralcompositionally, texturally texturallyand and structurally structurallycomplex, complex,contains contains abundant abundant PGE PGE mineralThe Mine Mine Complex, Complex, which which isis compositionally, ization. Its Its composition composition ranges rangesfrom from anorthosite anorthosite to ization. t o clinopyroxenite, clinopyroxenite,gabbronorite gabbronoriteto t omelanonorite melanonoriteand andincludes includes magnetite rich rich gabbro.Textures gabbro.Textures include include equigranular equigranularfine fine to to coarse coarse grain, grain, porphyrytic, porphyrytic, pegmatitic. pegmatitic, varitextured varitextured magnetite units three textural units and and heterolithic-gabbro-breccia.These heterolithic-gabbro-breccia.Theselast last three texturaltypes typescoincide coincidewith withallallmineralized mineralizedzones. zones. Fragments in in these these breccias, breccias,which whichvary varyfrom fromaafew few centimeters centimeters tto Fragments o tens tens of of meters metersininsize, size, are are representative representative of of most most of of the the lithologies lithologiesininthe thearea.The area.Thebreccias breccias were were created created as as the result result of of an an energetic energetic influx influxof of aa volatile volatile charged, PGEbearing bearing magma, magma,which whichassimilated assimilatedofofaalarge largevolume volumeof of rock rock and disrupted Iayering.The charged. PGE and disrupted layering.The PGE PGE bearing bearing magma, typically aa melanogabbro, melanogabbro,isisthe the matrix matrix to magma, typically t o the thebreccia brecciaand andcontains containshigh highgrade gradePGE PGEvalues.The values.The accompanyaccompany- ing volatiles, volatiles, migrating migrating through through the altered the the chamber, chamber, produced varitextured varitexturedand andpegmatitic pegmatiticgabbro, gabbro,altered thesilicates, silicates, mobilized produced sulphide poor poor mineralization.The mineralization.The varitextured varitexturedgabbro gabbrothat thatdefines definesthe theoval ovalshape shape mobilized metals and produced of the to the the Roby Roby Zone.The Zone.The occurrence occurrenceof ofthese these key key rock rock types types and and mineralization mineralization the Mine Mine Complex, Complex, is host to throughout the it in throughout the Mine MineComplex Complexisisevidence evidence that thatthe themineralizing mineralizingprocess process affected it in its its entirety. entirety. POE Mineralization of PGE Mineralization of the the Mine Mine Complex Complex Zones of PGE PGE mineralization with delineated resources in the Mine Complex Complex consists consists of the Roby Roby Zone,Twilight Zone, Moore Zone. from 0.7 0.7 e/t g/t to Zone. Baker Zone and Moore Zone. Other Othermineralized mineralizedareas, areas, with values values from to 23 23 g/t g/t Pd Pd have have had limited amounts of surface sampling and anddiamond diamond drilling drilling and and require require follow-up. follow-up. Roby Zone Roby Zone The Roby Zone is is a bulk bulk mineable deposit with aa minimum minimum strike strike length length of of 850 850 meters, meters, width width achieving achieving 365 meters and aa depth depth of 500 meters.The zone and tto and potential exist for zone remains remains open at depth, and o the southeast and for new mineralization mineralization ttoo the the west. west. Ore (7.6% North Roby The Roby Zone contains three three distinct distinctore oretypes; types; Shear Shear Ore (7.6% of volume), North Roby Ore Ore(5.3% (5.3%of ofvolume) volume) and Breccia 1% % of volume). volume). Breccia Ore Ore(87. (87.1 Shear Ore Ore Shear Ore is is confined confinedtto portion of a IS Shear Ore o aa portion 15 to t o25 25meter meterthick thicklayer layerof ofpyroxenite pyroxenitethat thatisisthe thehanging hanging wall in the central portion Only aa 400 400 metre metre portion portion of portionof ofthe theRoby RobyZone, Zone,inincontact contactwith withaabarren barrengabbro gabbrohangingwall. hangingwall. Only of the the �Palladium mineralized, the the strike pyroxenite pyroxenite is is mineralized, strike extensions extensionsare are barren.The barren.Thepyroxenite pyroxeniteisisplanar, planar,trends trendsatat3410, 3 ~ ,is sis subvertical subvertical to t o aa depth depth of of 250 250metres metresand and shallows shallows to to the theeast eastat at greater greaterdepth. depth Higher grades(6 (6tto g/t Pd) are achieved achievedin inthose thoseportions portions o off the Higher PGE PGE grades o 12 12 g/t Pd) are the pyroxenite pyroxenite that that are arealtered alteredto t oaaschisschistose assemblage of actinolite-talc-chlorite (Shear Ore).The sulphide content of the pyroxenite, including the conconlose assemblage of actinolite-talc-chlorite (Shear Ore).The sulphide content of the pyroxenite, including the tained shears, shears,isis 1% 1%t oto3% 3%with with local localpatches patchesofofnet nettexture texture with with up tenor is tained up to to10% 10% sulphide. sulphide. The The PGE PGE tenor is not notproproportional to the sulphide content.The dominant sulphides are pyrrhotite, chalcopyrite and pentlandite. PGM are portional t o the sulphide content.The dominant sulphides are pyrrhotite, chalcopyrite and pentlandite. PGM are vysotskite (Pd,Ni)S Sb2As2 , kotulskite sperrylite PtAs2 ,and aa vysoiskite (Pd.Ni)S , isomerticite isomerticitePd PdIII ISb2As2 kotulskitePd(Te,Bi), Pd(Te.Bi). sperrylite PtAs2 , Merenskyite Merenskyite PdTe2 PdTe2 .and Pd arsenide,either either Stillwaterice Stillwaterite or segment of of the the pyroxenite pyroxenite is is juxtaposed juxtaposed Pd arsenide, o r Palladoarsenide. Palladoarsenide. The The higher-grade higher-grade segment with the higher grades found in the adjacent breccia ore.The PGE tenor of the pyroxenite declines with the higher grades found in the adjacent breccia ore.The PGE tenor of the pyroxenite declines dramatically dramatically ttoo the the south southwhere wherethe thedegree degreeof oftalc talcalteration alterationisisdiminished. diminished.ItI talso alsodeclines declines in in the theadjacent adjacent breccia breccia ore.The ore.The pyroxenite and and iits contained shears shears pinch pinch out out tto to pyroxenite t s contained o the the northwest.The northwest.TheShear Shear Ore Orehas has been been drilled drilledsystematically systematically rn a depth of 500 mecers.The pyroxenite separates the two other major ore types. a depth of 500 meters.The pyroxenice separates the two other major ore types. . . . North Roby Ore Ore N o r t h Roby North ofofthe Roby Ore is a zone whose gradual boundaries are It North thepyroxenite, pyroxenite,the theNorth North Roby Ore is tabular a tabular zone whose gradual boundaries aredefined definedby byassay. assay. It is 20 20 to to 40 The zone zone strikes strikes aatt 20' 20° with with its is 40 meters meters thick, thick, and and 200 200 meters meters long. long.The its footwall footwalldipping dippingto t othe theeast eastat at45° 45wall steeper. steeper.ItIt isishosted hostedby byaawide wide variety variety of of lithologies.At lithologies.At surface, it is is dominated dominated by by surface, it tto o 60° 60Âand and the the hanging hanging wall coarse grain Ieucogabbro containing irregular irregular masses massesof ofvari-textured van-textured gabbro gabbro and and medium medium tto gabbrocoarse grain leucogabbro containing o coarse coarse grain grain gabbronorite.The proportion with depth.The content of of this this norite.The proportionof ofnoritic noriticrock, rock,and andheterolithic heterolithicbreccia brecciaincreases increases with depth.The suiphide sulphide content mineralization ranges from trace amounts to 4%, and typically is less than 0.25%.The sulphide poor mineralization mineralization ranges from trace amounts t o 4%. and typically is less than 0.25%.The sulphide poor mineralization cannot visually visually be be distinguished distinguished from from the the barren cannot barren wallrock.The wallrock.The dominant dominantsulphides sulphides are are pyrrhotite, pyrrhotite.pentlandite. pentlandite,chalchalcopyrite and distribution isis erratic 1616 g/teft Pd) and weakens copyrite and pyrite.The Pd) and weakensininaaNE N Edirection. direction.Systematic Systematic pyrite.The PGE PGE distribution erratic(I( Itot o drilling of a maximum maximum depth depth of of 250 drilling of this this zone zone has has only only attained attained a 250 metres. metres. Breccia Ore Breccia O re Southwest of the Southwest of the Shear Shear Ore, Ore, Breccia Breccia Ore Ore isis contained containedwithin withinaamineralized mineralizedcomplex complexthat thatmeasures measures550 550by by350 350 meters.This complex complex contains contains intrusive intrusive lithologies lithologies that that range range in in composition composition from from clinopyroxenite clinopyroxenite tto meters.This o anorthosite anorthosite van-textured and tto o norite.Textures norite.Texturesinclude includeequigranular equigranular fine fine to t ocoarse coarsegrain, grain, porphyrytic, porphyrytic.pegmatitic. pegmatitic.vari-textured and heterolithheterolithic-gabbroic-breccia. Mineralization is concentrated in these last three textural types, but it is also commonly ic-gabbroic-breccia. Mineralization is concentrated in these last three textural types, but it is also commonlyfound found in medium grain grain norite norite and coarse grain grain gabbro-norite.All gabbro-norite.All of in clinopyroxenite, clinopyroxenite.melanogabbro, melanogabbro, medium and medium medium and and coarse ofthese these rocks types rocks types occur occur as as discontinuous, discontinuous, irregularly irregularlyshaped shaped pods pods with with rnaxiryium maximum dimensions dimensions of of 60 60 meters metersthat, that,when when viewed are in viewed on on aa map, map, are in fact fact aa larger largerscale scale of ofbreccia. breccia. The boundary tto the central portion of the sub-vertical contact with with the the The east east boundary o the central portion of the theBreccia Breccia Ore Ore is is defined defined by by the sub-vertical contact pyroxenite.The southeast extension of of the pyroxenite.The southeast extension the Breccia Breccia Ore Ore remains remains open.The open.The southern southern and and western western boundaries boundaries of of the Breccia Ore are all defined by a gradual decline in PGE content.As defined by the 0.7 g/t Pd assay cutoff, the the Breccia Ore are all defined by a gradual decline in PGE content.As defined by the 0.7 g/t Pd assay cutoff, the western and the the southern southern boundary boundary dips dips steeply steeplyt to the north. north. Due Due tto nature is sub-vertical sub-vertical and o the o the the gradual gradual nature western boundary boundary is distribution, these assaycutoff.The cutoffThe greatest potential of the PGE PGE distribution, these boundaries boundaries are subject to t o changes changes in assay potential for forexpanexpansion of of resources assuming similar similar continuity continuity of the extent extent of sion resources at at the the Roby Roby Zone, Zone.assuming of grade, grade, is is deepening deepening the of the the Breccia Breccia Ore. Ore. �Palladium The The most mostdefinitive definitive lithological lithologicalcontrol controlon onPGE PGEgrade gradewithin withinthe theRoby RobyZone Zoneisisthe thepresence presenceoro rabsence absenceof ofhetheterolithic-gabbro-breccia erolithic-gabbro-breccia and andaccompanying accompanying varitextured varitexturedgabbro.The gabbro.Thebreccia brecciaisisalways always ore, ore, and and other otherlithologies lithologies are are only only ore oregrade gradewhen when in inproximity proximitytot obreccia. breccia.However, However,mineralized mineralized breccia breccia on o n the the margin margin of of the theRoby RobyZone Zone can contain less than 0.7 g/t Pd.Also, with the exception of post mineralization dykes. every lithology can either can contain less than 0.7 g/t Pd.Also. with the exception of post mineralization dykes, every lithology can either be orwaste. waste.I-Ieterolithic-gabbro-breccia Heterolithic-gabbro-brecciaisiscomposed composedof offragments fragmentsaafew fewcentimetres centimetrestotoseveral severalmetres metresinin be ore oreor size area, size that that are are representative representative of ofmost mostof o fthe thelithologies lithologiesininthe the area.as aswell wellas asaafew fewexotic exoticclast.This clast.Thisbreccia brecciahas hasaa wide spectrum of characteristics. It can consist of sub-angular fragments crowded together with very little matrix wide spectrum of characteristics. It can consist of sub-angular fragments crowded together with very little matrix and to aa breccia of 10% sub-rounded fragments fragments (digested (digestedxenoliths) xenoliths) and and 90% 90%matrix. matrix. A A trantranbreccia consisting consisting of 10% sub-rounded and can can grade grade to sition matrix rich breccia is is observed in in the southern Breccia Ore. Melanogabbro matrix rich breccia observed the southern Breccia Ore. Melanogabbroisisthe the sition from frommatrix matrixpoor poortoto matrix Observed gabbroic gabbroic composition composition for for the matrixand andthe theintrusive intrusivemagma magma creating creating the the breccia. breccia. Observed the matrix matrixin inmany many localocations selectively assayed, tions isisinterpreted interpretedas asaacomposition compositionchange changedue dueto t odigestion digestionofofless lessmafic maficxenolithsWhere xenoliths.Where selectively assayed, the gltPd, Pd,and andthe thefragments fragments0.08 0.08 g/t g/t Pd.This Pd.This explains explains the the observation observation that that matrix matrixpoor poor the matrix matrixwill willcontain contain8 8g/t breccia is usually low grade. Pegmatitic gabbro occurs on the rims of the fragments, as irregular patches and breccia is usually low grade. Pegmatitic gabbro occurs on the rims of the fragments,as irregular patches andveinveinlets lets throughout. throughout. Breccia is transitional transitional into Breccia is into varitextured varitextured gabbro gabbroand and lacks lacks distinct distinct xenoliths.Varitextured xenoliths.Varitextured gabbroic gabbroic rock rock range range in in composition from leucocratic to melanocratic, and some varieties are fine grain with medium grain patches and composition from leucocratic t o melanocratic, and some varieties are fine grain with medium grain patches and veinlets, grain with with pegmatitic veinlets, medium medium grain grain with withcoarse coarseand andpegmatitic pegmatiticpatches patchesand and veinlets, veinlets. and and coarse coarse grain pegmatitic patches patches and Pegmatitic rock rock is occursas asconvex convexmultiple multiplelaylayand veinlets. veinlets. Pegmatitic is more more abundant abundant in in the the central central Breccia Breccia Ore. Ore.Here Hereititoccurs ers, and lenses lensesup upto to several severalmetres metresin inwidth width but but not not laterally ers, dykes dykes and laterally continuous.The continuous.The common common pairing pairing of of the the brecbreccia pegmatitic gabbro gabbro nnot cia and and varitextured varitextured gabbro, gabbro, and and its its associated associated pegmatitic o t only only key key to t o interpreting interpretingthe theprocess processthat that created the the ore, ore. but butalso alsoserves serves as as an important importantassociation association that that guides guides exploration. exploration. The 5%chalcopyrite. chalcopyrite.pentlandite. pentlandite,pyrrhotite pyrrhotiteand andpyrite. pyrite.ItItoccurs occursas asfine fine grain grain The Central CentralBreccia BrecciaOre O r econtain containup uptoto5% disseminations throughout. and as coarse blebs interstitial to pegmatitic clinopyroxene and plagioclase.The fine disseminations throughout, and as coarse blebs interstitial t o pegmatitic clinopyroxene and plagioclase.The fine sulphides are contained contained as asstreaks streakswithin within the the clinopyroxene clinopyroxene pseudomorph pseudomorph that that are are now now composed composed of of actinolite, actinolite, sulphides are chlorite chlorite and and talc.This talc.This late late introduction introductionof ofsulphide sulphideisis synchronous synchronous with with the the alteration alteration of ofpyroxene pyroxene to t oactinolite. actinolite. Twilight TwilightZone Zone The Twilight Zone TheTwilight Zone is is aa new new zone zone in inthe theprocess processof ofbeing beingdelineated delineated by by diamond diamond drilling.This drilling.This zone zone isisseparated separated from the Roby Zone on its western boundary by a 50 to 70 metre thick unit of massive homogenous from the Roby Zone on its western boundary by a 50 t o 70 metre thick unit of massive homogenousbarren barrengabgabbro from the broand and separated separated from the southeastern southeastern extension extension of of the theRoby RobyZone Zoneby byaa50 50metre metrethick thickbarren barrendyke.Twentydyke.Twentytwo t w odiamond diamonddrill drillholes holestotodate datehave havedelineated delineatedaazone zone 100 100 metre metrewide. wide, 200 200 metre metredeep deep and and 200 200 metre metrelong.The long.The zone remains open to the east, and south.This area has the same style of lithological complexity as the Roby zone remains open t o the east,and south.This area has the same style of lithological complexity as the Roby Zone, pegZone, but but isis dominated dominatedby byaaless less heterolithic heterolithicgabbronorite-breccia, gabbronorite-breccia,and and does does not notcontain containvaritextured varitexturedand andpegmatitic matitic gabbro.The gabbro.The most mostsignificant significant mineralization mineralization discovered discovered in in this this area area to t odate datewas was intersected intersectedby byholes holes99-040 99-040 and 99-I 71 This mineralization, containing 0.25% to 0.5 % pyrrhotite and chalcopyrite. assayed 2.38 g/t Pd over and 99-1 71 .This mineralization,containing 0.25% to 0.5 % pyrrhotite and chalcopyrite,assayed 2.38 g/t Pd over I I II 6 metres. metres. �North American America:Palladium Palladium Ltd Baker Z Zone Baker one The Baker surface is one one kilometer kilometereast eastofofthe theRoby RobyZone. Zone.Detailed Detailed surfacesampling samplinghas has defined defined aa mineralized mineralized The Baker Zone Zone is zone that that is wide and traced continuously continuously for for 250 meters. This defined by by is 100 100 meters meters wide and traced 250 meters. This northeast-striking northeast-strikingzone zone is is defined zone assaysgreater greaterthan than0.68 0.68g/t g/t Pd Pdand andgenerally generallycontains containsless lessthan than 11% sulphides.The Themineralized mineralizedrock rockconsists consistsof of assays % sulphides. gabbro, varitexcured varitextured gabbro. and heterolithic-gabbro-breccia.To heterolithic-gabbro-breccia.To date, date, the the highest highest grades are associated gabbro, gabbro, pyroxenite, and with aa pyrrhotite-chalcopyrite samplesover overaa 115 meter length with pyrrhotite-chalcopyriterich richarea areaat ataabreccia/gabbro breccWgabbro contact. contact. Channel samples 5 meter assayed4.69 4.69g/t g/tPd, Pd,0.27 0.27g/t g/tAu. 0.48g/t g/tPC. Pt.0.43% 0.43%Cu Cuand and0.39% 0.39% Ni.A3500 3500metres metres(20 (20holes) holes)diamond diamonddrilling drilling Au, 0.48 Ni.A assayed program conducted conducted during during the the winter winter of program of 1998-99 1998-99 tested tested the the zone zone over over aa 250 250 metre metre strike strikelength, length, tto o aa maximum maximum 200 metres, metres, at 50 50 metre metrespacing. spacing. depth of of 200 Moore Moore Zone Zone area 600 600 metres metres south south of of the the Roby Zone poorlyexposed exposedmineralized mineralized area area in in aa swampy area The Moore Moore Zone Zoneisis aapoorly Mineralization has hasbeen beenuncovered uncoveredin inan anarea area 100 100by by200 200meters metersand andappears appearstotohave havea aNW NW strike strike boundary. Mineralization basedon on the the alignment alignmentof of aa sulphide sulphiderich rich zone. zone. Where Where best exposed the the mineralization consist of of heterolithicheterolithicbased best exposed mineralization consist with less less than than 1% 1%disseminated disseminatedpyrrhotite pyrrhotiteand andchalcopyrite.An chalcopyrite.Aninterval intervalofofnorice norite within within the the gabbro-norite-breccia with breccia contains contains 22 to to 5% 5% pyrrhotite pyrrhotite and interval generated generatedfrom from diamond diamond drilling drilling is is and chalcopyrite.The chalcopyrite.The best best assay assay interval 24.3 meters meters grading I .39 g/t g/t Pd+PC+Au, Pd±Pt+Au, 0.29 0.291% Ni, and and 0 0.3 I 2%Cu. Cu.On Onthe the basis basisof of 17 diamond diamond drill drill holes, 24.3 grading 1.39 1% Ni, .3 12% holes, itit is is interpreted that interpreted that the theMoore MooreZone Zoneisisrod rodshaped, shaped,and and has has maximum maximum dimensions of 22m x 45m x 90m 90m deep. deep. �LH r Palladium Pafladium Ltd Northi American Geology of of the the Lake Lake Superior Superior Region Region , _ r -' • BuKkflr pN a 'i . Tc' ,\&wt3 çfra q Wi 80 km radius "( tii2f' t 'fl (Ii. 1 . . . . . . . . . . . METAL> FOR CLEAN AIR PAGE 7 . . ... . . . . .. .. . . . . .. .. . .. : �.. .,.. . . .. . . .. . . . . . C) cf 3 F EL SO D n (D -J .. Ct- North American . . PalladiumLt / .... Geology and mining claims of the Lac des lies area . .. ... . . . . . PAGE:a . . . . . .. . . . . . . . , ... . . . . .. ... . . . . .... . . . . . . . . . . . E T A L S F O R CLEAN A m . .. ... . ,,. . �. .. .. . . . North American Ltd. North ~ m e r i c Pallad:ium aPalladium ~ Ltd., Geology of the Lac des lies complex ...... ... . .. . �. . . Geology of the mine complex . . . ...... . North American Palladium Ltd. 1 �r a N Geology of the Roby Zone ......... 9 .. .. . it r Th / North American Palladium Ltd. �P Norite; equigranular Gabbro-norite; equigranular Meianogabbro-norite (10-35% plag) Ilelerolithic gabbro-norite breccia Gabbro; varitexiured Gabbro;equigranular Incipient heterolithic gabbro breccia Heterolithic qabbro breccia Leucogabbro/East Gabbro Anorthosite (<10% matics) Leucogabbro; porphyrltic Leucogabbro; varitextured Mine Sequence Felsic Dyke Folsic Plutonic Rocks U 025 U 04 Pyroxen lie Heterolithic melanogabbro breccia Melanogabbro Breccia Dykes •I Matic dykes (minor, narrow) lJ LI n 9 EEl lCd •J_ Archean LI Diabase Diabase Breccia Prot€iozolc 0 meters 12.5 Detailed geology, South Roby Zone 25 tit E 2 01 a In -J > if 7 North American Palladium Ltd. �I Surface South Roby RobyZone Zone (overlay (overlay on on "Detailed "Detailed geology, South Roby Zone") SurfacePd Pdassay, assay, South geology, South Roby Zone") Pd Grade 5 >5.0 g/tonne 5 2.50 to 5.0 g/tonne 5 0.70 to 2.50 g/tonne 5 0.35 to 0.70 g/tonne S <0.35 g/tonne z 0-5 > 0 12.5 meters w S mi U 0 00 25 SOUTH ROBY PIT (0 -5 n CM C, 31750 N 1) B �a F as -Q > 7 2 -J c3j fl 3 1 Roby Pit Level Plan Grade Model 0 North American Palladium Ltd 1' . . . PAGE'1.4 . . . .. . . . . . . ' . . . . .... ... . . . . .., .. + O K CLEAN A I R . .METALS . .... .. ..... . . . . .... . . . ., �-r Oeet burden ODD-i collar with driltitole taco downhote 0 25 SC. metera 15 . . . . . '---ft Oetirie 01 Proposed Pit Oetbne ot Current Pit and SurDace Pd Grade - 035 Do 0.70 gltortne Pd Grade - 0.70 to 2.50 g'tonne Pd Grade - 2.50 to 5.0 ylronne Pd Grade >5.0 gilcoine ' LEGEND ' e 0 0 . 20Dm 0 2 z 0 1 . . . .. . . . . 30Dm 400,0 A Roby Zone Grade Model Cross Section 509 N > IT North AmericanPallad:i.umLtd. F n (D 3 -I . . . . . . . . .. . . . .. . . . . �GEOARCHAEOLOGY AND DEGLACIATION GEOARCHAEOLOGY HISTORY OF AREA HISTORY OF THE THE THUNDER THUNDER BAY BAY AREA Brian Phillips, Lakehead University Scott Hamilton, Hamilton, Lakehead Lakehead University Joe Stewart, Lakehead University University Joe Stewart, Lakehead Pat Julig, Larentian University Pat Julig, Larentian University Bill Ross, Ross, Ontario Ontario Ministry Ministry of of Citizenship, Citizenship, Culture and Culture and Recreation Recreation Soon after after deglaciation, deglaciation, native nativepeoples peoples established established habitation habitation Soon examine how how the the local local geomorphology geomorphology will examine sites. This trip will these sites sites and and the the nature nature of of controlled the location of these human activities. activities. �Forty-Sixth Annual Meeting Forty-Sixth Meeting Institute on Lake Superior Geology Thunder Bay, Ontario, May 8-13th, 2000 PRE-MEETING FIELD TRIP 3 PRE-MEETING - Geoarchaeology of the Thunder Bay area - Field Guide Dr. Brian Phillips, Department Department of Geography, Geography, Lakehead Lakehead University. with Dr. Joe Joe Stewart, Stewart, Department of Anthropology, Anthropology, Lakehead University. Hamilton, DeDartment Department of Anthro~oloav, Anthropology, Lakehead Dr. Scott Hamilton. Lakehead University Universitv Dr. Pat Julig, Julig, ~ Department of Anthropology, Laurentian University e ~ a r t m e n t~ n t h r o ~ o lLurentian o~~, Mr. Bill Ross, Regional Archaeologist, Ministry Ministry of Culture Culture and and Recreation. Recreation. - Objectives: Objectives: field trip tripwill willfocus focuson onthe thedeglaciation deglaciationand and lake lake level level history history of of Thunder The field Bay and the area immediately to the west of of the thecity city -the the Kaministiquia Kaministiquia river river valley, Kakabeka Falls Falls and and the Marks Moraine. evidence of of Paleoindian Kakabeka Moraine. In particular, particular, evidence presence associated with the abandoned shorelines, river mouths and deltas associated with shorelines, river deltas of the 9.5 ka ka B.P.) will be examined examined (Figure (Figure 1). 1). Lake Minong stage of Lake Superior Superior (circa (circa 9.5 The trip trip will will start startby byaa visit visit to to the the former former mouth mouth of of the the Current Current River River at at the the north north end of the city, where the Simmonds and McDaid McDaid sites, both surface sites, are located. Then Then the trip will head head west, west, up valley, to the more famous Cummins site, a site,part partofof which which is is preserved preserved within aa fenced area. area. The third area to be be stratified site, the western extent of Lake visited will be the Rosslyn delta, on the Kam river, visited Rosslyn delta, the Kam river, the western extent of there the the trip trip will will travel travel west west to toKakabeka, Kakabeka,where whereearlier earlierLake Lake Beaver Beaver Minong. From there Bay features will be examined. Finally, the trip will mount the Marks moraine, features examined. Finally, trip will mount the Marks moraine, that will place the days observations in context with pre pre and post providing a view that Marquette ice marginal events and Paleoindian Paleoindian presence. - Introduction strongthough though not notexclusive exclusiverelationship relationshipofof Paleoindian Paleoindian habitation habitation There is aa strong in the Thunder Bay area areawith with the the shoreline shoreline features features of of the the post-glacial post-glacial lakes of of the Superior basin, particularly those those of of Lake Minong, established about 9.5 ka ka B.P. B.P. This is perhaps illusory in that they they occupied occupied aa number number of of habitats, habitats, some some far far from from aa lakeshore, but but in geomorphological mapping of of these these shorelines, it is these sites lakeshore, which have been most commonly found and reported. have been most commonly since early early excavations excavations by by MacNeish MacNeish (1952) (1952) by by both Evidence accumulated since amateur and professional archaeologists (Fox 1975, 1980: amateur archaeologists (Fox 1980; Dawson 1983; Julig, Julig, PIano Indians migrated north eastwards from from the the area area of of Dakota Dakota 1984) suggest that Piano cul-de-sac formed formed between between Lake Lake Agassiz, Agassiz, which which covered and Minnesota into the cul-de-sac of Manitoba and parts of Northwestern Ontario at at its maximum Northwestern Ontario maximum extent, extent, the most of lakes of of the Superior basin and the retreating margin of of the Laurentide ice sheet. These people, people, derived derived from from plains plains cultures cultures characterised characterisedby byaa fluted fluted point point lithic These �Sturgeon Pt u4ittgleton I _ p. uneII 1: Keeter T hn,e,r 250,000 tIIand .- B atne .t--- Li CT 1.tAdcKeIIAr Greenetone Pt T ARTHUR r 165 are Pt - S ç1\, - �technology, probablyfollowed followedgame gameinto intothe the area area as as hunters, technology, probably hunters, but but in in staying, staying, to form and in their diversified diversified in in their economic economic activities activities and their tool-kit tool-kit design, design, to form a a recognisable "Interlakes Composite" Composite"(Ross, (Ross,1994) 1994)characterised characterisedby byaa parallel recognisable "Interlakes flaked making particular particular use use of of the flaked point point lithic lithic technology, technology, making the red red (jasper) (jasper) taconite taconite material material found in the siliceous Gunflint Gunflint formation that outcrops in the Thunder Bay Bay area. area. Deglaclation Deglaclatlon HIstory History The broad details of of the the deglaciation of the the Superior Superior basin basin are are shown shown in deglaciation of Figure 2. As ice central of the the Rainy Rainy River River and and Superior Superior lobes lobes withdrew withdrew from from central Figure 2. ice of Minnesota, series of of recessional moraines were were left left in in place. recessional moraines place. The The Vermilion Vermilion Minnesota, aa series moraine, trending northwest northwest to southeast, southeast, across northeastern northeastern Minnesota, Minnesota, was was followed on the Canadian Canadian side of the border, and and the followed by by the Steep Steep Rock Rock moraine on Brule moraine (Figure (Figure 3). 3). Guided Guided by by these these ice Brule moraine ice marginal marginal positions, positions, Lake Lake Agassiz Agassiz found found an an early early eastern eastern outlet outlet through through the Arrow/Whitefish ArrowiWhitefish lakes corridor (circa (circa 11 11 ka and, shortly after, after,through throughthe theShebandowan Shebandowan lake corridor, corridor, ultimately ultimately using ka B.P.) B.P.) and, the Lake Lake Nipigon Nipigon spillways spillways around around 10.4 10.4 ka ka B.P. B.P. (Figure (Figure 2a) to enter enter Early Early Lake Lake Minong Minong (Teller, 1985) which occupied occupied the Superior Superior basin. basin. There is no to believe did not There is no reason reason to believe that that the the warming warming climate climate did not permit permit environmental conditions conditions to to become become suitable suitable for for the the immigration immigration of of animal animal herds and prehistoric peoples into the Thunder Bay region at this time. Only tenuous prehistoric peoples into the Thunder Bay region at time. evidence of such early occupation has been reported (Phillips, 1993: Ross, occupation reported (Phillips, 1993: Ross, 1994), 1994), evidence though corridor has has confirmed confirmed aa number of sites though recent recent work in in the Arrow/Whitefish ArrowIWhitefish corridor sites that most likely represent earlier Paleoindian presence. (McLeod and Phillips, represent an earlier Phillips, 1999). pers. communication, 1999). A major event in the the deglaciation deglaciation history of ofthe the Lake Lake Superior Superior basin basin was the the Marquette readvance readvance (9.9 (9.9ka ka B.P.), B.P.),during duringwhich whichthe the basin basin and and its margins were briefly reoccupied by ice (Figure 2b). Recessional of the Brule Recessional moraines northeast of moraine formed in in their their place. This event moraine were destroyed destroyed and and more more recent features formed of archaeological implications, implications, since since any also also has has considerable considerable archaeological any evidence evidence of occupation prior to to the the Marquette Marquettereadvance readvanceisislikely likelytoto have have been been buried buried or or occupation prior destroyed destroyed completely completely by by the the new new ice ice cover. cover. Hudson Bay ice pushed southwestward southwestward into into the the Lake LakeSuperior Superior basin, basin, to to halt on its southern shore (Drexler, (Drexler, Farrand Farrand and and Hughes, Hughes, 1983). 1983). Only in in aa small small portion of Whitefish Bay near near Sault Sault Ste. Ste. Marie, corner of of the basin, did Whitefish Bay Marie, in the southeast corner did Early an open 1985). The Early Lake Lake Minong Minong remain remain an open lake lake (Farrand (Farrand and andDrexler, Drexler, 1985). Marquette lobe pushed westward against against the the steep steep Minnesota shore shore and, and, where flowed northwest up up the the Kaministiquia Kaministiquia valley valley to the Marks moraine less obstructed, flowed moraine (Figure (Figure 3). This Thisprominent prominentridge, ridge,rising risingto to over over 470 470 m m (1550') (1550') in in places, places, curves curves from Lappe through and around around the the northwest of Kakabeka towards the the through Mokomon Mokomon and northwest of Kakabeka towards Pigeon River (Zoltai, l965b; Burwasser, 1977, 1980; Burwasser and and Pigeon (Zoltai, 1963, 1963, l965a, 1965a, 1965b; Ferguson, 1980). Contemporaneously, to the the west west of of the the Lake Lake Superior Superior basin, the Patricia ice lobe pushed district and and halted to to deposit the pushed towards the Rainy Rainy River district Dog Dog Lake moraine. This This runs runs northwest northwest from Lappe and holds up Hazlewood, One of Lappe, Lappe, where where the two moraines meet, a Hawkeye and and Dog Doglakes. lakes. East of Island, Hawkeye line of glacial debris known as the Mackenzie interlobate moraine can be known Mackenzie be traced traced �Figure Figure 22 The TheGlacial GlacialPeriod Periodand andEarly EarlyPeoples Peoples d) km Figure 1: The retreat of the glaciers in the Superior region nbetween between10,400 10,400and and9,500 9,500year yearago. ago.(from (from Phillips, Phillips, 1993, p. 95) �Figure Figure33 - -.... o0 • to . .-..- . . .. . .. . .. . . . .-... .... ....- . -. . . . - ... . . ... 4 0ma.. mile 40 I area showing moraines moraines and Mapof ofthe theQuetico—Nipigon Quetico-Nipigon area and direction direction of of ice ice movement movement FIG.5.5. Map Fm. duringvarious variousphases. phases. during - from from ZoItal, Zoltai, 11965a) 965a) �through Pearl and and on to through Pearl to the the Black Black Bay Baypeninsula. peninsula. A glacial glacial lake, lake, Lake Kaministiquia, was formed formed between between the the two two ice margins 1985) and, and, at Kaministiquia, was margins (Teller, (Teller, 1985) Lappe, off the the Lappe, a huge sand sand and gravel gravel delta delta was was built built by bysediment sediment pouring pouring off interlobate of debate, however, and and there there interlobate moraine. moraine. This Thisscenario scenarioisisstill stillthe the subject subject of is some the Marks and Dog may be older some field field evidence evidence that that the Marks and Dog Lake Lake moraines moraines may older features that that were were simply reoccupied reoccupied by ice ice ofofthe theMarquette Marquetteadvance advance(Jul19, (Julig, Lake McAndrews and Mahaney, Mahaney, 1990; 1990; Tickle, Tickle, 1996; 1996;Noble, Noble,pers.comm.1999). pers.comm.1999). Lake Agassiz, deprived advance of of the the ice, ice, expanded expanded in area area deprived of its its eastern outlets by the advance and depth (the Emerson phase), and flooded catastrophically through depth Emerson phase), and flooded catastrophically through the the Clearwater Saskatchewan and the Clearwater and and Athabasca Athabasca valleys valleys in in Saskatchewan and Alberta into the Mackenzie Mackenzieriver river and and the the Arctic Arctic ocean ocean (Smith (Smithand andFisher, Fisher,1993). 1993). The of Lake Lake The Marquette readvance obliterated evidence of the earlier phase phase of Superior's shoreline history in most of the basin. Shoreline sites that prehistoric basin. Shoreline sites that prehistoric most people people might might have have occupied in in the area of Thunder Bay before 9.9 ka B.P. B.P. may lie lie buried beneath Marquette deposits. In the four hundred years between 9.9 and 9.5 buried beneath Marquette deposits. the four ka B.P., about which which much more is known, B.P., aa period about known, ice withdrew from from the Lake Lake Superior basin (Figure 2c/2d). Then, again, Paleoindian people followed game up 2cl2d). Then, again, Paleoindian up Superior basin the Interlakes this time time to to settle, settle, in in part at Interlakes corridor and into the Thunder Bay region, this least, on the shores shores of of Lake Beaver Bay (Stuart, 1993) and Lake Minong Minong (Phillips, (Phillips, 1988). 1988). Shoreline Shoreline History History At At the peak peak of of Wisconsinan Wisconsinan glaciation, glaciation, the the northeastern northeastern margin of the the Lake Lake Superior was depressed by the the weight to a ice (isostatic (isostatic depression) depression) to Superior basin was depressed by weight of ice greater degree than the less heavily ice-loaded southwestern side. As a result, than the less heavily ice-loaded southwestern side. result, "rebound" (isostatic (isostatic recovery) since that time has been greater on the north north shore shore of the Lake Lake Superior Superior basin basin than than on on the the south south shore. shore. Lake shorelines which had had been originally originally horizontal horizontal became became progressively progressively tilted tilted along along an an approximate approximate northeast such that that aa shoreline shoreline of of the the same same chronological age increases in northeast axis, axis, such altitude northeast along along the western western shore shore of of Lake Lake Superior. Superior. AA altitude from from southwest to northeast theoretical archaeological site at Grand Marais, Marais, Minnesota, found at 184 184 metres, metres, just theoretical site just above abovethe the present present storm beach, will be of similar age to another theoretical site in contour (1 (117 in Terrace Terrace Bay, Bay, Ontario, Ontario, at the 300 metre contour 17 metres above the the present present lake), lake), with with both both sites sites lying lying on on the the same tilted tilted shoreline shoreline (Figure (Figure 4). As As the the Marquette Marquette ice ice lobe lobe wasted wasted back, the eastern and western sides sides of of the the basin (Figure 2c). 2c). On basin were exposed as separate entities (Figure On the eastern eastern side, a fairly fairly stable Mary's River outlet at Sault stable Lake Lake Minong, controlled by the height of the St. Mary's Ste. Marie, extended northwards up the Ontario shore and westwards along the the Michigan (Farrand and and Drexler, Drexler, 1985). 1985). Michiganshore shore as as they were exposed by by melting melting ice ice (Farrand In succession of of ever ever larger but In the enclosed western end of the basin, a distinct succession progressively lower level progressively lower level lakes lakes formed formed (Lakes (Lakes Duluth, Duluth, Highbridge, Highbridge, Moquah, Moquah, Washburn, Manitou, and and Beaver Beaver Bay), each extending Washburn, Manitou, Bay), each extending further further northeast northeast up the the Minnesota Minnesota - Ontario Ontario shore shore and east east along along the the Wisconsin Wisconsin shore shore (Farrand (Farrand and and Drexler, of these these water bodies bodies were formed Drexler, 1985). Along Along the the land-based land-based margins margins of various features, such as bluffs and beaches, by which the shorelines can various coastal features, can be ice vacated vacated the basin, basin, the the eastern eastern and and western western lakes lakes were were be traced tracedtoday. today. As ice �Figure Figure44 Shoreline ShorelineDiagram Diagramfor forthe thePigeon PigeonRiverlThunder RiverKhunderBay BayArea Area 1400 a; S I H (fromFarrand, Farrand,1960) 1860) (from FIgure Figure 55 ofpotential Figure pre-Nipissing transgression Figure10: 10:The Thepre-Nipissing transgressionand andthe theresulting resulting loss loss ofpotential archaeological archaeologicalsites sitesalong alongthe theshores shoresof ofLake Lake Superior Superior,(from (fromPhillips, Phillips, 1993, 1993, p. P. 100) 100) �united and the single shoreline of Lake Minong was formed around around the the basin basin about about (Figure2d). 2d). For the the next next 1,500 1,500years, years,water-levels water-levelsininthe the Lake Lake Superior Superior 9.5 ka B.P. (Figure basin declined as the St. St. Mary's Mary's sill sill was was eroded eroded down down to to bedrock. bedrock. A staircase of of Post-Minongshorelines shorelineswere wereformed, formed,the thelast lastand and lowest lowest ofof which which was was Lake Post-Minong Lake Houghton, about 8.0 ka B.P. Houghton, about 8.0 ka B.P. By 8.0 8.0 ka B.P., due to isostatic uplift, the the rising levels of of Lake Huron Huron had had By B.P., due isostatic uplift, flooded into the St. St. Mary's Mary's River River and and reversed reversed the the flow flow (Larsen, (Larsen, 1987). 1987). This backThis backflooding led Lake Superior Superior basin basin and and culminated culminated flooding led to slowly rising water-levels in the Lake in the Nipissing lake stage around 5,000 years years B.P.. B.P.. Known as the pre-Nipissing pre-Nipissing transgression, this rise in in water-level water-level was was imposed imposedupon uponaastill-tilting still-tiltingbasin. basin. On On the the north shore, shore, east east of of Dorion, Dorion,the therate rateofofisostatic isostaticuplift upliftremained remained more more rapid rapid than than the waters of of the the pre-Nipissing pre-Nipissing period, period, with with the the result result being being that that the the shoreline rising waters marking the Nipissing lies at at a lower all older older marking Nipissing maximum maximum level level lies lower altitude altitude than than all shorelines, including that of of Lake Lake Houghton. Houghton. On the south shore the pre-Nipissing pre-Nipissing transgression was more rapid than isostatic recovery, and wave action "inherited" rapid isostatic "inherited" the features of older older shorelines, shorelines, modifying modifying and and destroying destroying portions portions of them, them, the features of including pre-existing pre-existing shoreline shoreline archaeological archaeological sites sites(Phillips, (Phillips,1977). 1977). This This causes causes a chronological discontinuity which which is at its its greatest greatest near Duluth Duluth and and decreases decreases towards Dorion where the Houghton shoreline shoreline appears appears above above the the present waterlevel (Figure (Figure 5). 5). Thunder Bay, Bay, evidence evidence of of Paleoindian would normaliy Paleoindian activities activities would normally have have In Thunder been traced as aa continuum continuum from the high high Minong Minong shoreline shoreline to to the the Houghton Houghton (which lies just (which just below below present present water-level), water-level), but but the the pre-Nipissing pre-Nipissing transgression transgression reoccupied the the lower lower and and later later part part of of that that record record and and cut cut a prominent reoccupied prominent bluff on on which the General St. Joseph's Joseph's Hospital, Hospital, and Lakehead General Hospital, the Court House, St. Lakehead University are are built. built. The Nipissing shoreline can be traced traced up up the valley valley towards University Thus, the the later later part part of of the Mapleward road and across across towards towards Mount Mount McKay. McKay. Thus, record, the the important important transition transition into into the the Archaic Archaic period, period, and and aa good Paleoindian record, portion of the early Archaic is is missing missing in in Thunder Thunder Bay. Bay. portion Local HIstory History an interpretation of of the the detailed detailed history of of the the withdrawal withdrawal Figure 6 represents an of Superior (Marquette) ice from from the Thunder Thunder Bay area, area, based on currently known information, though though subject to As ice wasted information, to revision. revision. As wasted back back from from the Marks Marks Moraine, shorelines shorelineson onthe thesouth southside sideofof the the moraine moraine show show that that aa body body of of water water Moraine, between the the moraine moraineand andthe theice icefront front(Figure (Figure6a). 6a).This This proglacial proglacial lake lake collected between and there there is is tenuous tenuous evidence evidence has been named Lake Cedar Creek (Jahnke, 1993), and that with high high level level lakes Lake Duluth Duluth that it connected connected with lakes to the the south, south, perhaps perhaps of Lake Kaministiquia embayment embayment aa small equivalence. As ice equivalence. As ice withdrew withdrew from from the the Kaministiquia small The higher higher levels of of lake readvance to the Intola Moraine occurred (Figure (Figure 6b). The been traced traced into into this this moraine moraine (Stuart, (Stuart,1993), 1993),and andititisis likely likely that that Beaver Bay have been Paleoindian time, probably using the Marks Paleoindian peoples peoples entered entered the area at about this time, Marks causeway. As As ice ice withdrew withdrew further further and and water water level level declined declined to to the moraine as a causeway. of lake Beaver Bay (Figure 6c), the possibility of Paleoindian lower lower levels levels of lake Beaver Bay (Figure 6c), the possibility Paleoindian in the the occupation increases, increases,and and by by the the time Lake occupation Lake Minong Minong was was established established in Kaministiquia embayment, embayment,there thereisisplenty plentyofof evidence evidenceto to prove prove their their presence presence Kaministiquia �Figure Figure 66 An history in in the Bay area. area. An interpretation Interpretation of of post-Marquette post-Marquette history the Thunder Thunder Bay (a) Marquette toe and Proglaciat Lakes area 9,800 years 8.P (b) Intola Moraine Moraine and Upper Upper Lake Beaver Bey Bay circa circa ( bl irrtola 9,700 years years B,P. 9.700 B.P. Hudson Bay t.obe - KaWA7 M s—ar S.—, * — ;8.s'.a.yr — Uporlor Lobe'. 1= ,1-..t22i (c) Lower Cc) LowerLake LakeBeaver BeaverBay Baycirca circa9.600 9.600years yearsB.P. 89. Cd) Lake Minong, 9,500 years B.P. (from Phillips, Hill, Fralick Fralick and Ross, Ross, 1994) 1994) I �(Figure 6d). 6d). The withdrew from The shoreline shoreline diagram diagram (Figure (Figure 4) shows that only as ice withdrew sequence of post-glacial the Thunder Bay region did the sequence post-glacial takes lakes extend into the area. Beaver Bay shorelines shorelines can can be be found found at at Kakabeka, Kakabeka,but butonly onty the the lower tower levels levels extend eastwards eastwards through through the the city towards the Mackenzie Mackenzie River. River. A few notable sites associated with the Minong shorelineare arethe the Simmonds Simmonds A few notable sites associated with the Minona shoreline sites at at the the mouth mouth of of the the Current Current ~ River, Catherine and and McDaid sites i v e r the ,the Biloski, Biloski, Catherine Cummins sites sites along along the north side side of of the Kam embayment, embayment, and and the the collection of on the the Rosslyn Rosslyndelta deltaat atthe themouth mouthof ofthe the Kaminsitiquia Kaminsitiquia River, River,though though here here the the sites on association is less simple and deserved more analysis (Hamilton, 1996). Some association analysis (Hamilton, Paleoindian sites unrelated to to shorelines shorelines occur occur at at High High Falls Falls on on the the Pigeon river, 1982), at at Harstone Harstone Hill, Hill, near near the the junction junction of of the Whitefish on Dog Lake (McLeod, (McLeod, 1982), rivers and and near near Kakabeka Falls where where the the Kaministiquia Kaministiquia river river and Kaministiquia Kaministiquia rivers Kakabeka Falls might have been most logically logically crossed. crossed. The Crane site near Kakabeka, found The Crane found in a garden, provides providesaa rich richcache cacheofofbeautifully beautifullycrafted crafted10 10cm cmbiiaces bifacesin in aa vegetable garden, location apparently unrelated unrelated to to any any topographic topographic feature, feature, though though on on the the surface of of Beaver Bay Bay terraces. one of the higher Beaver Because of their antiquity and the acidic Because of their acidic nature nature of of the the Boreat Boreal forest forest soils, soils, (Figure 7), 7), though though itit is is very very only the lithic materials remain to be found at these sites (Figure likely that many other natural materials would have been been used used by these peoples. Field Excursion Stops - Stop 11 - Hillcrest Stop Hillcrest Park Park Lookout. from the the University will will be down Oliver Road and, and, just just below betow the The route from bluff, left left on on High High Street Streetand and up upthe thehill hill for for aa brief visit to Hillcrest Nipissing bluff, Hillcrest Park Park from where where the the city city can can be seen and Lake from seen in in context context with with local local topography topography and Superior. Superior. - Boulevard Park, Park, the the Bluffs Bluffs and and CentennIal Centennial Park Park Rd. Rd. Stop 2 and 3 - Boulevard will head northeast on on High High St., St., over overthe theSt.Joseph/Hillcrest St.Joseph/Hillcrest island island The route will lefton on Balsam BalsamSt, St,right righton onHudson HudsonAve Aveand and onto onto Arundel Arundel St. from from of Minong times, left which a left turn before Black which Black Bay bridge will lead lead to to the the Bluffs Bluffs Scenic Scenic Lookout. Lookout. Walk down to Simmonds site. Then board to cross Black Black Bay Bay bridge bridge and and turn turn left left on on Walk Centennial Park roadway. Centennial Park Rd Rd to visit the McDaid McDaid site, adjacent adjacent to the roadway. Simmonds (DcJh-4) and arid McDaid McDaid (DcJh-16) (DcJh-16) Sites Sites The Simmonds present Current Current river riverruns runsin in aa channel channel incised incised into into a gently lakeward The present formationthat thatfronts frontsaa bounding bounding rock rock wall, wall, a structural dipping shelf shelf of of Gunflint Gunflint formation structural feature, which now forms the 'bluffs' In the Minong feature, which 'bluffs' scenic scenic lookout lookout (Figure (Figure 8). 8). In Minong river carried carried much muchwater waterand andsediment sedimentfrom frominland inlandproglacial proglaciallake lake flows flows period, the river of coastal coastal history, a series and here evidence evidence supports supports a major 'bar building' building' phase phase of spits or or bars being being formed formed on on both both sides sides of of the the river riveras as water water level of river mouth spits generally declined. generally The highest lay against the bounding at The highest Minong Minong shoreline shoreline lay against the bounding rock rock wall at approximately the the 252 252 m m (827') (827') level level but but between between 240 240 and and 236 236 m (787-774') (787-774') a approximately of sand sand bars bars formed formedparallel parallel to to the the wall wall on on the the west west bank bank of of the the river river and and a series of matching series seriesofofbars barson on the the east east side side curve curve sharply sharply into into the the then then river mouth matching �Figure Figure 7 7 Paleolndlan bifaces from the Paleoindian blfaces the Thunder Thunder Bay Bay region. region. THE LAKEHEAD LAKEHEAOCOMPLEX COMPLEX —REFINED -REFINED BTFACES BIFACES — - DaJn—7 DfJg—1 Quetico Park Oliver O l t v e rLake Lake DeJj-6 DcJi—1 4% I'" OdJf—I tT;.rJTH:::i;:::tc:: OdJn— 7 DdJn—7 1995) (from Ross, 1995) �Figure 88 MOUTh Figure 9: The The changing changing geography geography and and present present characteristics characteristics of Figure of the and sites. (fivm Phillips,1988, 1988, p. p. 135) ~Simmonds i k n d and s ~McDaid c ~ asite; i d (from~hillips, LATER 4O RE LINE A. UPPER MINONG 250fl!m F .SLAOOO\ R.POSTMINONG SERIES OF MAJOR •ARS + FLAT SWALE • EASTERN 'US FORMEDPOST MINONG 217215n, 'F ALLUVIUM .&rMARSH C —Cr— CREST Pr CUFF u,4. DIRECTION OF SLOPE MODIFIED �from aa source source on on the thesame same rock rockwall wall east east of of the the point point where where itit is It from is cut cut by by the the river. river. It appears that the river entered Lake Minong along the rock wall, forming offshore appears that the river entered Lake Minong along the rock wall, forming offshore and beach and beach bars bars which which extend extend south south eastwards eastwards across across the the shallow shallow McVickers McVickers embayment to the southwest. Simultaneously, longshore transport embayment to the southwest. Simultaneously, longshore transport from from the the east east built bars partially across the river mouth at times, only to be later truncated by built bars partially across the river mouth at times, only to be later truncated by fluvial fluvial action. action. The Simmonds site site on on the the west west side, side,occurring occurringon on the the parallel parallel bars bars at at about about The Simmonds 236 m is matched in elevation and position position by by the the McDaid site on on the 236 m (774') (774') is matched in elevation and McDaid site the eastern curving bars bars (Figure (Figure 9). 9). Neither Neither site site is is aa long long term term habitation habitation site site but but show show eastern curving evidence of activity typical typical of of aa river river mouth mouth camping evidence of activity camping and and fishing fishingsite. site. Interestingly, Interestingly, the to have to the the the major major bar bar building building episode episode appears appears to have been been just just subsequent subsequent to occupation of these these two two sites, sites,two twolarge largecurving curvingbars bars being being formed formed at at 231 231 and and 227 227 occupation of m (758-745') flat topped one largely m (758-745') on on the the east east bank, bank, the the latter latter flat topped one largely aa subaqueous subaqueous feature that that was was probably probably contemporaneous contemporaneous with with the the supra-aqueous supra-aqueous ridge ridge form form of of feature the first. On the west bank a very long bar, now unfortunately truncated at its river the first. On the west bank a very long bar, now unfortunately truncated at its river mouth In south west, west, in in places places broadening broadening to over 100 (328') in end, runs mouth end, runs south to over 100 m m (328') in width. width. In almost text-like manner, the mean grain size and sorting characteristics along its almost text-like manner, the mean grain size and sorting characteristics along its length out from from the length confirm confirm that that itit prograded prograded out the mouth mouth across across the the McVickers McVickers embayment, probably mostly in subaqueous form. Some evidence of Paleoindian embayment, probably mostly in subaqueous form. Some evidence of Paleoindian activity has has been been found found on on the the crests crestsofofthese thesenewer newerbars barsbut butno no sites sites equivalent equivalent to to activity those those named. named. - Stop 4 Stop 4 - Mapleward Mapieward Road, Road, the the Cummlns Cummins Site. Site. The route to Balsam The route will will retrace retrace to Balsam and and join join the the Expressway Expressway heading heading southwest. Just Just beyond beyond the the John John St. St. traffic lights the the route southwest. traffic lights route crosses crosses the the Mcintyre Mclntyre River. On On the the north north side side the the low bluff will will be River. low Minong Minong bluff be seen, seen, with with aa housing housing development on on top. top. The The Biloski Biloski site siteoccupies occupiesthis thissurface surfaceand andaa small small sand sand bar bar at at development the bluff bluff foot, foot, where where a a small small embayment embayment and the and secondary secondary river river channel channel once once existed. existed. As the the Expressway Expressway curves curves to to the the south, south,the thearea areaon on the the right right holds holds a As a lengthy lengthy baymouth bar on the end of which lies the Catherine site, unseen from the baymouth of the road. On the summit summit of of Rabbit RabbitMMt (275m, 902'), 902'), which which rises t. (275m, rises behind behind the the embayment, embayment, lies lies the the the Irene site, a lookout lookout site. site. The route route turns turnswest weston on Oliver Oliver Road Road and and within within 22 km km rises The rises up up the the distinct distinct Minong bluff. A left turn on Mapleward Road sees a gentle descent to the Minong bluff. left turn on Mapleward Road sees a gentle descent to the Minong Minong shore again, and and the the Cummins Cummins site. site. Cummins Site 1) The Curnmins Site(DCJi(DCJi-l) By far far the Paleoindiansite site isis that that of of the By the most most significant significant Paleoindian the Cummins Cummins site site (Figure 10). Reported the site site has has been been excavated excavated and and Reported by by a local local collector collector in in 1962, 1962, the considered a has been the focus of thorough thorough examination. examination. Initially Initially considered a typical typical surface surface site, Julig Julig et et al. al. (1986) (1986) concluded concluded that that itit is is a a rare deeply stratified site, stratified Paleoindian Paleoindiansite, site, under use over over a under continued continued use a long long period periodof ofchanging changingenvironmental environmentalconditions. conditions. Though the the basic basic tool tool kit Though kit is is that that of of Plains PlainsPiano Pianoculture culture(Dawson, (Dawson, 1983; 1983; Julig, Julig, 1984), the the wide wide range range of of artefacts artefacts imply imply a a diversification diversification into into woodworking, woodworking, fishing fishing 1984), and beaver trapping in in addition addition to to the and beaver trapping the regular regular hunting hunting of of caribou caribou and and perhaps perhaps bison (McAndrews, (McAndrews, 1982). 1982). The The Cummins Cummins Site Site was was a major regional tool tool making making �Figure 9 tHE MCDAID SITE — geamorphologicl basis and archaeological test and transect locations PT — Pathway V metres • — Shovel • - Excavation in path U — Feature 'Ill' — Culling leil I sites — ((napping 'lotion - Spot height (in metre,) S Beach ridge axis —— — — Swale axis 236.6 236 6? baseline conlrol pabst — Elevation 0236 .7 transect A a I (from Stewart, Ross and Phillips, 1984) �and the the presence of of exotic exotic lithic components components implies implies aa broad geographical centre and interaction interaction with other groups in in the region region (Julig, 1984). 1984). While the the archaeological archaeological value value of of the Cummins Site was being While being revealed, revealed, broader questions questions remained remainedconcerning concerningthe thechoice choiceofofthis this and and other other sites sites for broader habitation, habitation, especially especially in in relation relationto to lake lake margin margin history. history. The Cummins CumminsSite Site occurs occurs across across the the surface surface of of several several large large sand sand and The which trend trend northeastwards from from the the then then Neebing river mouth across gravel bars which southerly gentle gentledip dip slope slopeof oflocal localGunflint Gunflintshales, shales,overlain overlain by by aa thin thin water water a broad southerly washed silty these shales shales occurs occurs jasper jasper taconite, taconite, the the major major source source of of tool till. In these silty till. making material. Figure I10 of the the site and and the the material. Figure 0 shows shows the the fenced fenced area of morphological details (Julig et al., 1990). Figure I 11 1990). Figure I shows the paleogeographic reconstructionofof events events believed believed to to have have formed formed the the area area (Phillips, (Phillips, 1982). reconstruction 1962). Longshore transport from from the the Neebing Neebing rivermouth rivermouth built built aa series series of of progressive bars bars Longshore across the shallow shallow water water rock rock shelf, shelf, recurving recurving into into aa minor across the minor river river valley valley which which sheltered embayrnent embaymentto tothe thewest westofofa arock rockisland. island. AA further further bar bar was was built built formed a sheltered front of of existing existing ones eventually crossed along the front crossed the the embayment embayment in in tombolo-Jike tombolo-tike form to to enclose enclose a small lagoon, the Cummins Pond, after after which which lower lower and and later later form Cummins Pond, the established established plan plan shape. shape. A pollen shoreline features mimicked the pollen core taken in the pond suggests etal., al., 1986). 1986). pond suggests this enclosure enclosure took place place before before 8.1 8.1 ka ka B.P. B.P. (Julig et of the the bars bars is not compatible with with a declining The plan shape and structure of water level indeed the the accumulation and building building up up of of these water level margin, margin, indeed accumulation and these large large features shelf slope slope is unlikely unlikely without without transgressive transgressivewave waveaction. action. Even features on a gentle shelf so, such action across across aa gentle gentle shore shoreslope slopewould would not notordinarily ordinarilybuild build up up a large so, large supra-aqueous bar without some initial encouragement to accumulate sediments supra-aqueous bar without some initial encouragement to accumulate sediments sheet. The along a line line rather rather than disperse them in a sheet. The key key to the existence of these bedrock control. control. While Julig (1984) determined bars in this location is underlying bedrock and ground ground radar studies that that variation variation in sediment through resistivity resistivity and sediment character character irregularities could could be be traced, traced, aa more of the and bedrock bedrock irregularities more simple reconstruction reconstruction of possible. By surveying surveying down downthe theexposed exposed bedrock bedrock dip dip slope bedrock profile is also possible. of the the site, site, the the trend trend (see inset, Figure showed that that beneath beneath the site must north of Figure111) I ) showed must typical of of many that that occur occur in in the the present present topography topography of of these these lie a marked marked rock step, typical flat-lying shales, and often sharpened by wave action. Accumulation of of sediments took place place firstly firstly against against this this rock rock step step and and subsequently subsequently over over the the top top of of it. ItIt is took possible that some till till remained remained in the the angle angle of ofthe thestep. step. To To the the west west of of Mapleward possible Road an an exposure exposure of of coarse coarse sand sand and and gravel gravel with with angular angular shale inclusions at the rear of the bar bar in contact contact with with bedrock bedrock suggests suggestsoverwash overwashof ofthe thetype typethat thatwould would be expected in this scenario (Figure 12). Once the linear feature was established, scenario (Figure 12). Once the established, longshore sediment supply would extend its length and add add to to the the lakeward lakeward face. face. overwash and and building building in elevation is characteristic characteristicof of wave wave action action either either However, overwash storm event eventor or aa transgressive transgressivelake lakemargin. margin. Again, Again,aa 'bar 'bar building' building' in an extreme storm evident at at Cummins. Cummins. Julig Julig (1984) (1984) found found confirmatory evidence of of period seems seems evident confirmatory evidence of these these bars bars during during the the process process of of their their accumulation, accumulation, aa buried buried layer of habitation of water worn being associated associated with with aa period of water worn taconite taconite artefacts artefacts being of overwash overwash and and construction. construction. While occupation of of the site when a lakeshore location offered advantages While of longshore of longshore access, access, lagoon fishing and perhaps perhaps water water transport, transport, a cremation cremation �(1982) Phillips by mapping morphological on based (1990), Mahaney and McAndrews, Julig, scions. sod aandarsi of Figure Onurio Superior. Lake nartbwzern size, UChSeCIOiS Cummins 5. 1don sad Quarry nct'ottons edge 3-Bog acovah1s Minwtg Z.Lower section DI & excavations Main I. pit Growl % cliff — Govt Ontero by purchased site of gortion Fenced ponns undisturbed Sot',. ridges beach Mong tower and middle at grawl-remoct) s.c destnjction site totol to severe of Areas ,-. — ridge beach Minong middle of limits Aggrazimate —— viSit7. the in elevatton Spot pit Gravel xe' zidg bcb and r.n*4 'ccc terrace wove or Bluff —.-, ridge beach Removed ridge Beach — Ouzailo, Supeñor, Lake oorthwzern she, Cummins 4. Fsguzt 500 9 NCThCS PMPOOSIGC1 OF TWP eat TIC$OCR OF CITY 10 Figure �Figure 11 Figure 11 + FLAT SWAtS '1 ALUWIUM MARSH CREST CUFF S ...d. DIRECTI ON OF 5LOFE MOCIFIED C. MAJOI •AR FORMATION FIIOFILE ACROSS CUMMINS Sits EAST OF MAPLEWARD ROSS ri,cit 0. POST MINONsMIN0R EARS FORMED MIMICKING PLANE SHAPE OF MAIN EAR 0—n.. Nh Figure 8: p. Fisure 8:The Thechanging changinggeography geographyand andpresent presentcharacteristics characteristicsofofthe theCummins Cummimsite. site. (from (from Phillips, PhiZlips, 1988, p. 133) 133) �Figure 12 12 Schematic representation representationofofthe the cliff cliff face at the the Cummins site. Schematic site W Wind wi nd Deflation -flation Lag Lag Fbviai Deposits 0—I 00 - Rich Berm Berm Gravel —Rich Magnetite — Rich Magnetita -Rich Foreshore Beach Foreshwe 8each Sands Sands .. . Foreshore Beach cc! Talus Slope Sands Backbeach Backbeach I—.zkl Bioturbated Sands Sands Massive, Heavily Massive, Bioturbated Dune Dune Sands Sands + +.. • 4+ + Bedrock Bedrvk Sedimentology Sediment010 y A wellwell-developed, exposed at the Cummins site. site. Figure30 30shows shows a eveloped, beach deposit is exposed schematicrepresentation representationofofthe thesandy sandy dii cliffface face where where the the strandline strandlinedeposits deposits are are visible. schematic cross-stratified sands sands of of the the foreshore foreshore dominate dominatethe the exposure. exposure. individual Low-angle, planar cross-stratified Individual Iarninae dipping 3' 3° to 12° 12' lakeward lakeward (original swash-backwash surfaces) are arranged atranged into into laminae dipping one another anotheratatvery verylow low angles. angles. The packets which erosively truncate one Theplanar planarcross-stratified moss-stratified both laterally laterally and and vertically vertically into massive sands through a bioturbated sands are transitional both bioturbated zone. the zone. thebioturbated bioturbatedarea area represents represents a sparsely sparsely vegetated vegetatedbackshore backshore environment environment while while massive sands the massive sands were were deposited depositedas asaeoliari aeolian dunes. foreshore sands sands are areerosively erosivelytruncated truncatedininthe theeastem easternportion portionofofthe the dii cliff by by The foreshore magnetite-richsands. sands. Internal internalstructures structuresindicate indicatethat thatthey theyare arealso alsoforeshore foreshoredeposits. deposits. The The magnetite-rich magnetite-richforeshore foreshorelaminae laminaewere wereformed formedduring duringregression regressionwhen whenstorm stormwave wave activiv activity magnetite-rich During the the lower portion of the beach, erosively erosivelytruncating truncating the older older deposits. During reworked the intervening periods periods of fair events sand was removed storm events removed and stored in offshore bars. In inte~ening weather, sand sand moved fmm fromthe theoffshore offshorebars barsback backonto ontothe thebeach beachand andwas waswinnowing winnowing by by weather, waves, producing small waves, produangthe themagnetite-rich magnetite-richlag lag deposits depositsfilling fillingscour scour truncations. truncations. The beach assemblage is is overlain overlainby by erosively erosively based, trough cross stratified stratified sands. sands. Major rainfall rainfallevents events caused after subaereal subaereal exposure exposure of of the area. fomed after area. Major These were formed to flow flow off the adjacent rock knoll knot!dissecting dissectingand and reworking reworkingthe theupper upper layer of beach streams to beach deposits. deposits. 3 Phillips, Fralick Fralick and and Ross, Ross, 1987. Phillips, C-12, From INQUA Field Guide GI 2, Eds. Geddes, Geddes, Kristjansson Kristjansson and and Teller. Teller. Eds. �burial site on on the the rock rock island (Dawson, (Dawson, 1983) was was dated to to 8,480 8,480 + 390 years BP. both just just below below the the aeolian aeolian sands sands that that characteristically characteristically Julig (1984) found artefacts both modified the topography of the the shoreline shoreline features features once once the offshore offshore shelf shelf was modified the topography of exposed by by lower exposed lower lake lake levels levels (circa (circa 8.0 8.0 ka ka BP) BP) and and in in peat peat that that accumulated accumulated in in Cummins Pond, Indications are are that that long longafter afterthe the lake lake ceased ceased to to lap the beach Cummins Pond. Indications beach face, the site remained remained used, used, at at least least until until 7.5 7.5 ka ka BP BP (Julig (Julig et et al., 1986). 1986). - Stop 5 Stop 5 (several) (several) - The The Rosslyn Rosslyn Delta Delta will retrace to to Oliver Oliver Road Roadand and continue continue west, west, turning turning south at the The route will towards Twin Twin City City Crossroads. Crossroads. Immediately Immediatelysouth south of of the the driving driving range, Golf Course towards the road road descends descends aa gentle gentle bluff bluff around around the the 268m 268m (880') (880') contour contour that that is is aa Beaver Beaver the Bay shoreline, shoreline, and and then, then, beyond beyond two two turns turns in in the the road, the descent down the the Bay road, the descent down shoreline arrives flat surface of the offshore shelf and offshore shelf Minong shoreline arrives on the extensive, extensive, flat surface of the Kaministiquia Kaministiquia River River where where it entered Lake Minong. This surface, 222m delta of the (729') at the mainly of sandy with boggy sandy silts, silts, with boggy areas (729') the crossroad, crossroad, is composed composed mainly the crossroads crossroads the the route route will will turn turn west west along underlain by more clayey silts. From the Hwy 11-17, Minong bluff, forming an abrupt rise at 11-17, where, within 2 kms, is again the Minong Hwy at the Weighscales. Weighscales. Turning Turning left, left,once once on on the the terrace, terrace, the the road leads south to several several fields in which the Dairy fields which lithic lithic material material has has been been found. found. These These sites, sites, the Dairy Farm, Farm, Breukelman Evergreen and and Halow A, A, B B and andC, C,will willbe beviewed viewed (Figure (Figure 15) 15) and and then of the Kaministiquia River River towards towards Stanley. Stanley. En the route will follow the north side of route the upper and lower Drezecky sites sites will will be seen (Figure (Figure 17), 17), as as well well as the Pawlick site site which whichcan canbe be seen seen on on the the south south side side of of the river. From Stanley Stanley the the river. From Pawlick Stanley delta and return return to Hwy 11-17. 11-17. route will climb the terraces of the Stanley The following extract extractofof text textand and figures figuresderives derivesfrom fromaa paper paper by by Scoff The following Scott Hamilton, that that has has been been submitted submitted for forpublication publication review review to to the theCanadian Canadian Journal Journal Archaeology. Entitled Entitled"Archaeological "ArchaeologicalPredictive PredictiveModelling Modellingininthe theBoreal BorealForest: Forest of Archaeology. No Easy the paper paper addresses addresses the the difficult difficult problem problem of of developing No Easy Answers", Answers", the developing archaeological predictive models to aid in Forest Class Class environmental environmental assessment. assessment archaeological Among the many many challenges challenges is is the issue of development sensitive Among development of temporally temporally sensitive palaeo-environmental reconstructions, and also models of land use that are are palaeo-environmental reconstructions, and also models of land use that relevant to the political economy and social organization of the ancient peoples relevant political economy and social organization of the ancient peoples under consideration. consideration. "The apparent apparent correlation correlation between between relict relict shorelines shorelines and Piano archaeological archaeological well known in the Thunder Bay area where landscape evolution has been sites is well been of some somestudy study(Figure (Figure 13). 13).While While many many Piano Piano shoreline shoreline archaeological archaeological the subject of documented,the thebest best known known ones ones are are lithic quarry and workshop sites sites are documented, such as the and McDaid Sites (Julig 1984, the Cummins, Cummins, Biloski, Simmonds, Simmonds, and 1984, 1994; 1994; MacNeish 1952; 1952; Hinchelwood Hinchelwood 1990; 1990; Phillips 1988, These large large sites sites are MacNeish 1988, 1993). 1993). These well removed shorelines, but but are are associated associated with with the the late late Pleistocene removed from modern shorelines, shorelines of Glacial Lake Minong Minong and and Gunflint Gunflint Formation Formation bedrock exposures that are suitable suitable for lithic lithic tool tool production production (Figure (Figure 13) 13) (see (see Julig, Julig, McAndrews McAndrews and and Mahaney 1990; Phillips Phillips 1988, 1988, 1993). 1993). However, However, these these special-purpose special-purpose sites sites do do not not characterize the full Paleo-Indian settlement pattern. Rather, the dense recoveries Paleo-Indian Rather, the dense recoveries �from, and ready visibility visibility of, of, these these sites has resulted in their over-representation over-representation in in the published literature. Archaeological Archaeological reconnaissance reconnaissance upon the published archaeological archaeological literature. agricultural fields fields within within the Kaministiquia River River delta delta has has revealed revealed a number number of PIano or or probable probable Plano Piano sites sites in a wide of landscape landscape contexts contexts (Hamilton Plano wide range range of 1996) (Figure (Figure 14). 14). Many Manyofofthese thesesites sitesare arefound foundatatorornear nearLake LakeMinong Minong shoreline shoreline elevations (i.e. 750 750 feet feet or or 228.6 A.S.L). When elevations (i.e. 228.6 metres metres A.S.L.). When placed in aa hypothetical hypothetical Minong- environmental Lake Minong environmentalcontext, they are found associated with: flowing into into sheltered sheltered coves coves of Lake Minong (Figure 15: 1) springs flowing 15: Breukelman Breukelman Evergreen); Evergreen); 2) on points of land that protruded protruded out out into Lake Minong to the the northeast northeast and and southeast of the Breukelman Evergreen site cluster (Figure 15); Breukelman Evergreen site cluster (Figure 15); well-drained sandy knolls surrounded surrounded by by poorly drained 3) upon low, low, well-drained sandy knolls poorly drained 3) upon floodplain/deltaic sediments floodplainldeltaic sediments (Figure (Figure 15: 15: Halow Halow C); 4) along deltaic deltaic backwater backwater channels (Figure 16: DbJi-8, DbJi-7, DcJi-28, DcJi32); 32); or 5) on sandy storm storm beaches beaches developed upon relict deltas (Figure (Figure 16: 16: DcJi-30, DcJi-30, DcJi-31). DcJi-31). Other sites were found upon: upon: 6) raised raised Pleistocene Pleistocene terraces terraces overlooking overlooking the the present present Kaministiquia Kaministiquia River River channel (Figure channel (Figure 17: 17: Pawlick); Pawlick); 7) on high high valley valley rims rims that offer panoramic views (Figure 17: Drezecky E); the Lake Minong shores shores (Figure (Figure 16: 16: Halow Halow A A 8) along draws leading down to the and B); and B); well removed from from late Pleistocene shorelines (Figure 9) along upland upland streams well (Figure 17: DcJj-12, DcJj-13); and 10) upon well-drained upland knolls knolls on what likely were formerly discontinuous permafrost uplands permafrost uplands (Figure (Figure 18: 18: Breukelman BreukelmanField: Field:lithic lithicscatters scatters AA to to E). E). Probable Plano Piano lithic lithic scatters scatters have have also also been been found found upon isolated bedrock Probable controlled knolls knolls offering offering panoramic panoramicviews viewsofof floodplains floodpiainsadjacent adjacenttoto the the Lake controlled Thisrange range of of landscape landscape characteristics Minong shoreline (Figure 19: 19: BeIluz Belluz Farm). This is certainly not exhaustive, but is sufficient to demonstrate demonstrate the the diverse diverse microhabitats microhabitats frequented by frequented by Piano Piano people. people. Lakehead Complex sites in terms of of a very These sites are consistent with other Lakehead preference for Gunflint Gunflint Formation Formation lithic lithic materials materials (Hamilton 1996). However, strong preference virtually all all of of these are well well removed are virtually these sites sites are removed from from bedrock bedrock exposures, exposures, are �Figure Figure 13 13 Late Rolocene watere Cuminins Site Bioski Site Brohn. Site .r Julig, MscAndr..,s nit Maimmy (l900tfl) Figure Figure66The The Correlation Correlationbetween betweenLakehead Lakfcu-u Complex bxinplexsites, sites, Minong Minong beaches beachesand andGunflint GunflintFormation Formationoutcrops outcrops(Hamilton (Hamilton1996:34) 1996:34) �Figure 14 Figure 14 Some Archaeological Sites associated with the Kaministiquia River Delta • Modern Hydrological System U Lake Minong water levels * Lakehead Complex Site SModern Community ..-M fl f Lake Superior distribution of confirmed confirmed or Figure 7 The distribution or probable probable Lakehead LakeheadComplex Complex sites sites on on the the Kaministiquia River (Hamilton1996: 1996:36) 36) Kaministiquia Riverdelta delta(Hamilton �Figure Figure 15 15 Figure or probable welldrained drained sediment sediment near near Figure 88 Confirmed or probable Piano Piano sites on well (750feet feetASL) ASL)(Hamilton (Hamilton 1996:59) 1996:59) LakeMinong Minongshorelines shorelines(750 Lake �1996:65). (Hamilton channels backwater deltaic along and beaches, storm Minong on Sites Piano probable and Confirmed 9 Figure 16 Figure �1). 1996:5 (Hamilton terraces river Holocene early and Pleistocene late along sites Piano possible and Confirmed 10 Figure 17 Figure �Figure 18 18 3ntour Interval = 10 metres AN 1 Figure PIano sites located located upon upon well-drained well-drained knolls knolls in in upland upland Figure11 11 Possible Piano regions (Hamilton1996:47). 199647). regions well well removed removedfrom fromLake LakeMinong Minong(Hamilton �Figure Figure 19 19 Figure Figure12 12Probable ProbablePiano Pianosites siteson on aa bedrock bedrock controlled controlledknoll knoll overlooking overlookingPleistocene Pleistocenefloodplain floodplainzone zone(Hamilton (Hamilton1996:57). 1996:57). �significantly smaller smaller than than the the quarry-workshop quarry-workshop sites sites (such (such as as the the Cummins Site), When compared compared to to the the quarry/workshop quarry/workshop sites, sites, these these and yield many fewer artifacts. When small sites also yield a much higher relative frequency of tools, preforms tools, preforms and small sites also a much higher relative frequency to debitage debitage (Hamilton (Hamilton 1996). This indicates that the small utilized flakes compared to sites represent represent encampments, encampments,hunting hunting stands stands and and food procurement procurement sites, rather than lithic extraction extraction and reduction stations. Such Such observations than lithic reduction stations. observations are hardly surprising, but they do serve as cautionary tales regarding surprising, but they do serve as cautionary tales regarding the dangers dangers of of predicting site distribution on the basis of our current incomplete heritage heritage inventory. inventory. predicting in that that they theydemonstrate demonstratethat thatmodelling modelling ancient ancient These examples are also important in human behaviour requires ongoing ongoing refinement, an understanding of human human human behaviour requires refinement, an understanding of forager behaviour, and a well-developed sense sense of of the the nature nature and and structure structure of of the forager behaviour, ancient landscape landscape and and its its microhabitats." microhabitats." As one leaves delta (Minong, (Minong, 9.5 9.5 ka ka B.P.) B.P.) and and moves As leaves the Rosslyn Rosslyn delta moves up the the present Kaministiquia valley, valley,one one is is in in effect effect travelling travelling back back in time. time. The Stanley delta was built into Lake Beaver Bay (9.7 ka ka B.P.), B.P.), the the first first Superior Superior lake to occupy occupy the area recently recently vacated vacated by Marquette Marquette ice ice retreating retreating from from the the Marks Marks moraine. moraine. Though perhaps less less conspicuous, conspicuous,Paleoindian Paleoindiansites sitesofofthis this period period are are also also Though perhaps present. present. The larger step back in paleogeography paleogeography is is to consider consider the area that lies lies to to the the west of the maximum of the Marks maximum position position of Marks moraine, moraine, an area area which which remained remained west of unglaciated by the Marquette readvance and and which which was was freed freed from from ice icearound around 11.0 unglaciated B.P. Figure Figure 20 20 shows shows the the projected projected maximum maximum position position of ofthe the Marquette Marquettelobe lobe and and ka B.P. the large area covered of aa pre-Marquette pre-Marquette lake lake (Early (Early Lake Lake Minong) , covered by the waters of by the the 1400' contour. contour. On Onthe thebasis basis of of some some long known sites, now approximated by both in the interior Phillips and and Hill (1995) both interior and and on on the the Minnesota Minnesota northshore, northshore, Phillips proposed that a Paleoindian Paleoindian routeway routeway along the Minnesota Minnesota shore turned inland of Judge Judge C. C. Magney MagneyState StatePark, Park,towards towardsNorth Northand andSouth South Fowl Fowl lakes lakes and and just east of in the the Whitefish number of more more recently recently discovered discovered sites in Whitefish Lake region. region. A number Whitefish -- Arrow Arrow Lake Lake corridor corridor supports supports the the contention contention that that Paleoindian peoples peoples Whitefish the Marquette advance, and were present in the area perhaps before and during the Marquette advance, that corridor continued continued to to be used into periods of of Lake that the corridor into the the post-Marquette post-Marquette periods Though conjectural, conjectural, there there is is the possibility that after Beaver Bay and Lake Minong. Though the retreat of of Marquette Marquetteice ice had hadbegun, begun,the the Marks Marks moraine moraine itself itself provided provided aa high ground from the the Whitefish Whitefish Lake Lake area area along along the north of the ground routeway routeway from north side side of Kaministiquia valley of Thunder Thunder Bay. Bay. Kaministiquia valley and andinto intothe the area areato to the the north northand andeast eastof - Stop 6 - Kakabeka Falls Just west west of of the the junction junction of of Hwy Hwy 11-17 and the Stanley turn off, off, the the huge Just structure of the Stanley delta is seen (Figure 21). This delta was formed as the structure of the Stanley delta seen (Figure 21). This delta was formed as Kaministiquia River Beaver Bay. Bay. A A well welt formed formed bluff representing a River entered Lake Beaver lower Beaver Bay phase of 260m (853'), runs along the north north side side of of the main main lower Beaver Bay phase of 260m (853'), runs along the highway, and the surface below below itit (250-2401% (250-240m;820-787') 820-787')is is heavily heavily exploited exploited by the sand and gravel industry. The present Kaministiquia river river has has incised incised deeply into into the delta, creating a terraced valley side. �Figure 2200 Figure -- 1 Kaminisliqili R. Valley 2 Whilefish Lake 4 N & S Fowl Lake 5 Pigeon R.Valley 6 Silver Mountain km SW99 (Alter Phillips and Hill Hamilton 1996) (After Phillips and Hill 1995:22-24, 1995:22-24, HuDUton 1996 Figure 13 Siteson onthe theUplands Uplands west westof ofThunder Thunder Bay Bay Figure 13 Some Piano Archaeological Sites �-4 ( Shorelines - Terraces - -1 r (from Grootenboer, 1972) — -C - Lower Beaver -4 cirumlinoid -r Upp, N N Beaver 88y N ( Figure 21 �Kakabeka Kakabeka village village is is built built on on the the floor (277m; (277m; 909') 909') of of an an old old distributary distributaryof of the the Kaministiquia Kaministiquia river which cut through a higher terrace still (Figure 21). Remnants of this at around around 300m 3Com(984') (984')overlook overlookthe the village. village. They They represent represent the this terrace surface at highest Lake and highest level level of of the Stanley delta, as asitit formed into Upper ~ a k eBeaver ~ e a v e Bay, r and on on the of the the Kaministiquia KaministiquiaRiver, River, the Crane Crane site was found. the one one on on the the west west side side of Kakabeka Falls is aa major major scenic scenic attraction. attraction. ItIt is is formed formed as as aa result of of a very was just just this sort of resistant chert bed that caps the the underlying, underlying, softer softer shales. shales. It was rock material material that was desired desired for tool making. Today, a fairly spectacular gorge lies lies below the falls (Figure 22). There has been some debate concerning the age of the below the falls (Figure gorge, since unless inherited and exhumed from from a previous stage of of river river incision to prior to there is only post to the the Marquette Marquette readvance, readvance, there post Beaver Beaver Bay Bay time time for it to develop its current grandeur. grandeur. An An interesting interestingabandoned abandonedfalls falls and and plunge plunge pool on the west side of the gorge lies lies aa short short walk walk from from the the Park Park Information Information Centre, and represents 'horseshoe' fall that would have been more spectacular than the represents a wide 'horseshoe' present present falls. falls. - Stop 7 (several) (several) - The The Marks Marks Moraine. Moraine. The route will turn on Hwy 590 just just west of of Kakabeka, Kakabeka, run run across across the the High Beaver Bay terrace terrace remnant remnant (look (lookfor forthe thecemetery cemeteryon onthe the right) right) and and due due west west until turning Moraine. As Asthe thedirt dirtroad road rises rises up up the the outer outer face face of of turning north north towards the Marks Moraine. the Marks (1400') and 442m Marks moraine it crosses crosses two distinct distinct benches at 42Gm 426m (1400') (1450'). These shorelines shorelines of of pro-glacial pro-glacial Lake Lake Cedar Cedar Creek Creek have been traced all (1450'). These along of the theMarks Marksmoraine moraine (Jahnke, (Jahnke, 1993) 1993) and and represent represent along the outer outer slope slope of stillstands in a lake earlier than and isolated of the the Superior Superior basin. isolated from the lakes of The Marks Marks Moraine. Moraine. Stopping on the 46Cm (1510'), (1510'),the the view view south south over over the Stopping on the crest crest around around 460m Whitefish valley and the the rugged rugged borderland borderland country country of of the mesa-like NorWesters Norwesters is spectacular. It is not hard hard to to envisage envisage ice ice pushing pushing its its way way up to the Marks Marks moraine, moraine, nor is it hard to imagine pro-glacial Lake Cedar Creek occupying the narrow strip hard imagine pro-glacial Lake Cedar between between the the moraine moraine and and the the retreating retreatingice icemargin. margin. The moraine is not a simple structure. structure. Although Although characterised characterised by by aa till which contains diagnostic pieces of Sibley red red sedimentary sedimentary rocks, along much its length the Marks joins and Marks moraine moraine joins and smothers smothers isolated isolated rock rock outcrops outcrops which which probably probably greatly top of of the the moraine greatly influenced influencedthe the extent to which the ice pushed inland. The top kettles. Spillway remarkable flat flat in many many places places and and is pockmarked pockmarked with with kettles. is remarkable Lake at one channels channels cross cross the the feature feature in in places suggesting suggesting that at one point point Lake Kaministiquia to the north was a few metres higher than Lake Cedar Creek to the south. The moraine moraine appears appears to to have have briefly briefly existed existedas as aa narrow south. The narrow ridge between between these two water bodies, and a possible post-Marquette routeway for animals routeway for animals and water Paleo-lndian Arrow/Whitefish region region and and Paleo-Indian people, from the unglaciated unglaciated (Marquette) ArrowIWhitefish Interlakes corridor. corridor. Interlakes The Conmee Pit and and spiliway. spillway. Conmee Pit Continuing north across across the the moraine and then turning east brings Continuing north moraine and brings the the excursion to to a distinct south across across the moraine excursion distinct channel cutting from from north to south surface. Here Conmee Township has has several several gravel gravel pits. from the huge surface. Here Conmee Township pits. Apart Apart from huge boulder beds that that suggest suggest high high discharges discharges down down this this spillway spillway at at times, times, the the pit boulder beds �Figure 22 (from Grootenboer, 1972) �reveals another another key key fact fact in in understanding local deglaciation. deglaciation. Figure Figure 23 23 shows shows a understanding local rare occasion occasion when when the the western western pit pit face rare face was cleanly cleanly exposed. exposed. Tickle Tickle (1996) (1996) interpreted the the sequence sequence as as consisting consisting of of two two tills separated interpreted separated by by fluvioglacial fluvioglacial sediments. However, However,while while one one till till was was of of diagnostic diagnostic Sibley Sibley origin, origin, the the underlying sediments. lobes. one was of a northern provenance typical of the Patricia or Rainy River lobes. Elsewhere, at several several places places along along the the Marks Marks moraine moraine aa thin thin Sibley till Elsewhere, at till is p'astered over over fluvioglacial fluvioglacial sediments. sediments.The The observation observationsuggests suggeststhat thatthe thebulk bulk of of plastered the Marks moraine may may in in fact fact be be pre-Marquette pro-Marquettein inorigin, origin,with with thin thin Marquette Marquette ice the moraine. moraine.This Thishas hasbeen beenthe the growing growing opinion opinion of another local field field reoccupying the Figure 3, 1999) and and looking the worker (T. (T. Noble, Noble, pers.communication, pers.communication, 1999) looking at Figure worker 3, the possibility of the Marks moraine moraine being being the the result result of of pushing pushing the the eastward eastward part part of of the the possibility former Brule moraine till contorted till former moraine is not not unreasonable. unreasonable. Several exposures exposures of contorted structures also support this idea. idea. Moraine scenic Marks Moraine scenic lookout. lookout. outofofthe thepit pitand andagain againatatthe thecrossroad crossroadbrings bringsone oneto toaa point point in in Turning left out the road from from which which a view east over the Kaministiquia Kaministiquia valley, valley, Thunder Thunder Bay city the distant distant Sleeping Sleeping Giant Giant is is obtained. obtained. The The stop stop is useful only to to impress impress and the of things. It is easy to imagine ice viewers with scale of ice grinding grinding past past Mt. Mt. McKay McKay and and flowing up to imagine imagine that that aa slight slight tilt tilt of of the the present lake flowing up the valley, just as it is easy to would bring the margin of the lake lake right right up up into into the the Kaministiquia Kaministiquia and and Whitefish Whitefish valleys. valleys. 1I I I ii I to the the crossroad, crossroad, the the route routewill will turn turn left leftdown down the the face face of of the Marks Marks Returning to road is is straight, straight,save savefor foraajog jogaround aroundaakettle kettlehole, hole, and, and, lower lower down, down, moraine. The road fields and along the stream stream beds. beds. Turning Turning right rightat at Hwy Hwy 11 11-17 rock is exposed in the fields -17 back to to Kakabeka Kakabeka Falls. Falls. From Fromhere herethe theroute routewill will take take Oliver Oliver brings the excursion back Road back back to to the the University. University.The The road road climbs climbs out out of of Kakabeka over the the Upper Kakabeka over Beaver Bay terrace remnant and then runs due east to Lakehead University. University. One landform of of note note on on the theway wayisis aa crag crag and and tail tail feature, feature, showing the flow up the the Kaministiquia Kaministiquiavalley. valley.The The'Old 'Old Barn' Barn' restaurant restaurantlies lieson on the thetail tail and and on on of ice up farmhouse, open open for for bed bed and and breakfast breakfast (another (another time, time, eh!). ehl). Just east the crag is the farmhouse, the north north of of Oliver Oliver Road Roadisisthe theMurillo Murillodrumlin drumlinfield, field, aa series series of of long of Murillo but to the narrow forms parallel to the the strike strike of ofthe theGunflint Gunflintshales, shales,suggesting suggestingaagood good deal deal of bedrock control. bedrock The The excursion excursion is over. over. We We hope that that participants participants obtained obtained an informed informed glimpse of of the the deglaciation deglaciation history history and and Paleoindian presence in in the area, and glimpse Paleoindian presence and enjoyed enjoyed doing doing so. so. I �Figure Figure23 23 . , Figure Figure1.1 1.1 The TheStudy StudySite Site--Conmee Cn-mee Township Townshipgravel gravelpit pi SOUTH Note: Note: Vertical Vertical sections sectionstaken takenatatnumbered numberedlocations. locations. (from Tickle, 1996) �References and and Bibliography Bibliography References Arthurs, D. 1979. 1979. An Archaic Archaicsite siteon onthe thewestern westernLake LakeSuperior Superiorshore. shore.Report Reporton onfile filewith withthe the Planning and Research Historic Planning Research Branch, Ontario Ministry of of Culture Culture and and Histonc Branch, Ontario Communication, Toronto, Toronto, 35 pp. pp. S. 1985 1985 Deglaciationchronology chronology and and revegetation revegetation in in northwestern northwestern Ontario. Bjorck, S. Ontario. Canadian Canadian Journal of of Earth EarthScience Science 22,850-871. 22, 850-871. G.J. 1977 1977 Quaternary Geology of the City of Thunder Bay Ministry of of Bay and and Vicinity. Ministry Burwasser, G.J. Natural Resources, Resources, Ontario Ontario Geological Survey Report 164. Geological Survey Report OR GR164. Burwasser, G.J. G.J. 1980 1980 Quaternary Quaternarygeology geology of of the the Onion Lake of Burwasser, Lake and Sunshine Sunshine area, area, District of Thunder Bay. Ministry of of Natural Resources, Ontario Ontario Geological Geological Survey Survey Report Report Bay. Ministry Natural Resources, MP94. MP94, Burwasser, G.J. G.J.and andFerguson, Ferguson,A. A.1980 1980Quaternary Quaternarygeology geologyof ofthe theOnion Onion Lake Lakeand andSunshine Sunshine Burwasser, area, District of of Thunder Bay. Ministry of of Natural area, District Bay. Ministry Natural Resources, Resources, Ontario Geological Geological MapP.2203, P.2203, Geological GeologicalSeries, Series, 11:50,000. :50,000. Survey Preliminary Map Clayton, L. L. 1983 1983 Chronology Chronologyof ofLake LakeAgassiz AgassizDrainage Drainage Lake Superior: jTeller, J.T., J.T., and and totoLake Superior: in.Teller, Clayton, Lee, Agassiz, Geological Geological Association of Canada Lee, eds., eds., Glacial Glacial Lake Agassiz, Canada Special Special 26, 291-307. Paper 26,291 -307. Clayton, L L. 1984 Pleistocene Pleistocenegeology geologyof of the theSuperior Superiorregion, region, Wisconsin, Wisconsin, Information InformationCircular No. 46, Wisconsin Geological Geological and Natural History HistorySurvey, Survey, Madison, Madison, Wisconsin. Wisconsin. N O. 46, Clayton, 5. 1982 Clayton, L. and Moran, Moran, S. 1982 Chronology Chronology of of Late LateWisconsin Wisconsin glaciation glaciation in middle North North America, Quaternary Science Science Reviews, vol. vol. 1, pp. pp. 55-82. 55-82. Dawson, K.C.A. K.C.A. 1983a Prehistory of Northern Bay Historical Historical Museum Museum Dawson, Northern Ontario. Ontario. Thunder Bay Society. Society. ite at Thunder Bay, Bay, Dawson, K.C.A. K.C.A. 1983b Cummins Site: A Late Palaeo-lndian (Piano) Site Dawson, Archaeology 39,3-31. 39, 3-31. Ontario. Ontario Archaeology Drexler, C.W., C.W., Farrand, Farrand,W.R. WA. and and Hughes, J.D. .J.D. 1983 1983 Correlation of Glacial Lakes in the Drexler, Correlation of Glacial Lakes the with Eastward EastwardDischarge DischargeEvents Eventsfrom fromLake LakeAgassiz AgassizjnTeller, j. Teller,J.T., J.T.,and and Superior Basin with Agassiz, Geological Geological Association Association of Canada Clayton, Lee, eds., Glacial Glacial Lake Agassiz, Canada Special Special 26, 309-329, Paper 26,309-329. Dudzik, M.J. M.J. 1993 1993 The State Archaeology Archaeology The Paieoindian Paleoindian tradition in northwestern northwestern Wisconsin, Wisconsin, State Regional Program, Siren, Siren, Wisconsin. Regional Former shorelines shorelines in in western western and and northern northern Lake Farrand, W.R. 1960 Farrand, W.R. 1960 Former Lake Superior Superior Basin, Basin, unpublished Ph.D. Michigan, Ann Ann Arbor Arbor Ph.D. Thesis, University University of Michigan, unpublished �Farrand, WA. and and Holocene Holocene history history of of the Lake Farrand, W.R. and Drexier, Drexler, C.W. C.W. 1985 Late Wisconsin Wisconsin and Lake jjj. Karrow,P.F. P.F.and andCalkin, Calkin,P.E. P.E. eds., eds., Quaternary Quaternary Evolution Evolution of the Superior basin, hKarrow, the Great Lakes, Lakes, pp. pp.17-32, 30. 17-32, Geological Geological Association Association of Canada, Special Paper 30. Fox, jj Nunn, Fox, WA. W.A. 1975 1975The ThePalaeo-lndian Palaeo-Indian Lakehead Lakehead Complex, Complex, h Nunn, P., P., &,Canadian ed.,Canadian Archaeological Association, Collected Papers, Historic Sites Branch, Ontario Ministry Ministry of Natural Resources, Archaeological Research 6, 29-53. 29-53. Natural Resources, Research Report Report 6, Fox, W.T. 1980 The Research Report Report 13, The Lakehead Lakehead Complex: New Insight Archaeological Archaeological Research Historical Planning and and Research Research Branch, Branch, Ontario OntarioMinistry MinistryofofCitizenship Citizenship and and Culture. Culture, 127-151. Toronto, 127-151. Grootenboer, Grootenboer, J. 1972 1972 Former Former Shorelines Shorelines in in the theKaministikwia Kaministikwia Plain Rain and the Geomorphology Geomorphology Area. H.B.A. H.B.A. Dissertation, Department Department of of Geography, of the the Kakabeka Kakabeka Falls-Stanley Area. Lakehead Ontario. Lakehead University, University, Thunder Bay, Bay, Ontario. Hamilton, J.S. J.S. 1996 Pleistocene Archaeological Sites Sites upon upon Hamilton, Pleistocene Landscape Landscape Features and PIano Piano Archaeological the Delta, Thunder Thunder Bay District, District,Lakehead Lakehead University University Monographs Monographs the Kaministiqula Karninistiquia River Delta, in Archaeology, Archaeology, 1. 1. Hinchelwood, the Brohm Hinchelwood, A. 1990 1990 1987 1987Observations Observationsatatthe BrohmSite Site (DdJe-1), (DdJe-I), Sibley Provincial Park, Conservation Archaeology North North Central Central Region Region Report 27, Ontario Ministry of Conservation Archaeology Report 27, Ontario Ministry Citizenship Citizenshipand Culture, Heritage Heritage Branch, Thunder Bay, Ontario. Huber, An overview of the 1992 An overview of the vegetational vegetational history history of the the Arrowhead Arrowhead region, region, Huber, J.K. 1992 northeastern Minnesota in Field trip trip guidebook guidebook for for the the glacial geology of of the in Field glacial geology Laurentian St. louis louis and andlake lakeCounties, Counties, Minnesota, Minnesota Laurentian divide area. St. Minnesota Geological Geological Survey, Field 55-64. Field Trip Guidebook 18, pp. 55-64. Huber, J.K. .1K. and Hill, Hill, C.L. CL. 1987 1987 A A pollen pollen sequence sequenceassociated associated with with Paleoindian Paleoindian presence in in northeastern 4, p. p. 89-91. 89-91. northeasternMinnesota, Minnesota,Current Current Research Research in in the Pleistocene, Pleistocene, vol. vol. 4, Jahnke, evidence of an on the flanks Jahnke, R. R. 1993. 1993. Initial evidence an early Post-Marquette Post-Marquette pro-glacial pro-glacial lake on flanks of or the Moraine, Thunder Thunder Bay, Bay, Ontario. Ontario. HBSc. the Marks Marks Moraine, HBSc. dissertation, Dept. of Geography, Geography, Lakehead Lakehead University, University, Thunder Bay, Ontario. Johnson, L.L. L.L. and and M. Stright Johnson, Stright (eds.) (eds.) 1991 1991 Paleoshorelinesand and prehistory: prehistory: an an investigation investigationof of method, CRCPress, Press, Boca BocaRaton, Raton,Florida. Florida. method, CRC Julig, P. 1984 Cummins Cummins Paleo-lndian Paleo-Indian Site and its its Paleoenvironment, Paleoenvironment, Thunder Thunder Bay, Bay, Canada. Canada. fi. Gramly,P.M., R.M.,ed., ed.,New New Experiments Experimentsupon upon the the Record of Eastern hGramly, Eastern Palaeo-lndian Palaeo-Indian 12, 192-209. 192-209. Culture, Archaeology of Eastern Eastern North North America 12, W.C. 1986 Geoarchaeological Geoarchaeological Investigations Investigations at at Julig, P.J., McAndrews, McAndrews, J.H., and Mahaney, Mahaney, W.C. the Cummins CumminsPaleoindian Paleoindian Site, Thunder Bay, Bay, Ontario Ontario Ia Paleoenvironments: Paleoenvironments: Site, Thunder Geosciences, Current Research in in the the Pleistocene Geosciences, Pleistocene 3, 79-80. 79-80. �1J.H.and andMahaney, Mahaney,W.C. W.C. 1990Geoarchaeology Geoarchaeologyofofthe the Cummins Cumminssite site Julig, PU., P.J., McAndrews, J.H. 1990 on the beach of ofproglacial proglacialLake LakeMinong, Minong,Lake LakeSuperior Superiorbasin, basin,Canada, Canada. NP. Lasca injN.P. Donahue, eds., Archaeological Geology of North pp. 21-50. and J. J. Donahue, Archaeological Geology NorthAmerica, America, pp. 21-50. Society of America, Centennial Centennial Special Special Volume Volume 4, 4, Ch.2. Ch.2. Geological Society Julig, P.J. 1994 1994 The occupations in in the the Julia, The Cunimins Cummins Site Site Complex Complex and and Paleoindian Paleoindian occupations Northwestern Lake Lake Superior Superior Region, Ontario Ontario 2, Ontario Northwestern Ontario Archaeological Archaeological Reports 2, Foundation, Toronto. Toronto. Heritage Foundation, Landmesser, C.W., C.W.,T.C. T.C.Johnson, Johnson, R.J. RU. Wold Wold 1982 Seismic reflection study of of recessional recessional Landmesser, moraines beneath beneath Lake Superior moraines Superior and their relationship relationship to to regional regional deglaciation, deglaciation, vol. 17, p. 173-1 173-190. 90. Quaternary Research, vol. G.E. 1987. 1987. Geological GeologicalHistory Historyofof Glacial GlacialLake LakeAlgonquin Algonquin and and the the Upper Great Lakes. Larsen, C.E. U.S. Geological Geological Survey Survey Bulletin Bulletin 1801, 1801, 36pp. 36pp. J.D. and and H.C. HG. Hobbs Hobbs 1992 1992 Glacial geologyofofthe the Laurentian Laurentian divide divide area, area,St. St. Louis Louisand and Glacialgeology Lehr, J.D. Counties, Minnesota, Minnesota Geological GeologicalSurvey Survey Field Field Trip Trip Guidebook Guidebook 18, 18, Lake Counties, prepared for the the 39th 39th Midwest prepared Midwest Friends Friends of the Pleistocene PleistoceneField FieldTrip. Trip. Leverett, F. 1929 of the the Lake Superior Superior basin, U.S.G.S. Professional Professional 1929 Moraines Moraines and shorelines of Leverett, 154-A. Paper 154-A. Leverett, F. and F.W. Sardeson Sardeson 1932 Quaternary geology geology of of Minnesota and and parts parts of of adjacent Leverett, states United United States States Geological GeologicalSurvey Survey Professional ProfessionalPaper Paper161, 161, U.S. U.S. Government Government states Printing Office, Washington Printing WashingtonD.C. D.C. T.R. Giles Giles 1993 1993 Recognizing Recognizing Late Late Pleistocene paleoshoreline Levson, V.M. D.E. Kerr, and T.R. levels from levels from geomorphic geomorphic and and stratigraphic stratigraphic records records of glaciofluvial glaciofluvial delta, delta, Current Current the Pleistocene, Pleistocene, vol. 10, p. 82-84 Research in the MacNeish,R.S. R.S. 1952 1952 AA Possible Early Site in the Thunder MacNeish, Thunder bay bayDistrict, District, Ontario, Ontario, National National of Canada CanadaBulletin Bulletin 120, 120, 23-47. 23-47. Museum of Matsch,C.L. C.L.and andA.F. A.F.Schneider Schneider 1986 1986 Stratigraphy and and correlation correlation of of the the glacial deposits deposits of Matsch, lobe complex in Minnesota and northwest Wisconsin in Sibrava, Sibrava, B., B., D.Q. the glacial lobe Quaternary glaciations glaciations in in the Northern the Northern Bowen, and Richmond (eds.). Bowen, and G.M. G.M. Richmond (eds.), Quaternary p. 59-64. 59-64. Hemisphere: Quaternary Science Reviews, vol. 5, p. McAndrews,J.H. JR. 1982 Holocene environment of of afossil afossil bison bison from from Kenora, Kenora, Ontario, Ontario, Ontario Ontario McAndrews, 41-51. Archaeology, 37, 41-51. McLeod, M.P. M.P. 1982 A Re-Evaluation of of the the Palaeo-Indian Palaeo-lndian Perception Perceptionof of the the Boreal Forest. McLeod, Forest. ArchaeologicalQuarterly, Quarterly,6,4, 6,4, 107-1 107-116. 16. Manitoba Archaeological Mulholland,S. S.and andE. E.Dahl DaN 1988 1988 Two projectile points points associated associatedwith with aa Two late late Paleoindian Paleoindian projectile Mulholland, GlacialLake LakeDuluth Duluthbeach, beach,Carton CarltonCounty, County,Minnesota, Minnesota,The The Minnesota Minnesota Archaeologist, Archaeologist, Glacial vol. 47, 47, no. 2, p. 41-50. 41-50. vol. �M. and andJulig, Julig, P. P. 1989. 1989. The The Identification Identification of Protein from a Newman, M. ProteinResidues Residuesin inLithic Liihic Artifacts Artiactsfrom a Stratified Boreal Forest Forest Site. Site. Canadian CanadianJournal JournalofofArchaeology Archaeology 13, 119-132. 119-132. Nordeng, S.C., S.C., A.P. A.P. Ruotsala, Ruotsala,S.A. S.A.Nordeng Nordeng1987 1987Buried Buriedbog bogdates datesthe the time time of of the Nordeng, establishment of postglacial post-glacial Lakes Lakes Houghton Houghton and Nipissing in inthe the western western arm armof of the the Section Geological Society of America Lake Superior basin, North-central Section America Abstracts Abstracts with Program,vol. vol. 19, 19, no. no.4, 4, p. 237. Phillips, B.A. BA. M. Science 195, 11-16. 11-16. Coastal Histories, Science M.1977 1977Shoreline ShorelineInheritance Inheritancein in Coastal Phillips, B.A.M. B.A.M. 1982 Morphological Mapping and and Palaeogeographic Reconstruction of of Phillips, Morphological Mapping Palaeogeographic Reconstruction Former Shorelines Shorelines between between Current Current River Riverand and Rosslyn, Rosslyn, Thunder Thunder Bay, Former Bay, Ontario, Ontario, Including Cummins CumminsSite SiteDcJi-1. DcJi-1. Report Report on file with Historical Historical Planning and Research Research of Citizenship Citizenship and Culture, Culture, Toronto, Toronto, Ontario. Branch, Ontario Ministry of Ontario. Phillips, B.A.M. Phillips, B.A.M. 1988 1988 Palaeogeographic Palaeogeographic reconstruction of shoreline shoreline archaeological archaeological sites sites around Thunder Bay, Ontario. Geoarchaeology: An International Journal, 3, 2, 127138. Phillips, 3A.M. 1993 A Time-Space Model for the Distribution of Shoreline Archaeological the Lake Superior Superior Basin. Basin, Geoarchaeology: An International Journal, 8, 8, 2, 2, 87Sites in the InternationalJournal, 107. 107. Phillips, B.A.M. 3A.M. and C. 1994 History and Glacial and and Shoreline and 1994 The The Geology, Geology, Glacial Shoreline History Phillips, and Hill, Hill, C. Archaeological Potential Potential of of the Lake Superior: Superior: a Archaeological the Minnesota Minnesota North North Shore Shore of Lake background paper paper for for Geomorphologial and Archaeological ArchaeologicalStudies Studies of of Individual background Gwmorphologial and Individual State Parks Report for for Minnesota State Parks on the North North Shore. Shore. Report Minnesota Department Department of Natural Natural and Recreation, Recreation, St. St. Paul, MN. MN. Resources, Division Division of Parks and SAM. and P.W. 1994. 1994. A Transgressive Event on Lake Minong, Northshore Northshore Phillips, B.A.M. andFraiick, Fralick, P.W. of Lake Superior Superior -- Possible Possible Evidence Evidence of of Lake Agassiz Inflow,circa circa9.5 9.5 KA KA BP. BP. Agassiz Inflow, of Journalof ofEarth EarthScience, Science,v.v.31, pp. 1638-1 1638-1641. Canadian Journal 31, pp. 641. Phillips, B.A.M., B.A.M., Hill, Hill, C.L., CL., Fralick, P.W. and and W.A. WA. Ross Shorelines and Ross 1994 1994 Post-Glacial Post-Glacial Shorelines Phillips, Fralick, P.W. alongthe the Northwestern Northwestern Shore Shoreof of Lake LakeSuperior, Superior,Guidebook Guidebookfor for Paleoindian Migration along Fieldtrip E, E, 13th Biennial Meeting Meeting of AMQUA, AMQUA, Minneapolis, Mn. . : 8A.M. and Hill, C.L. CL. 1995 The Phillips, B.A.M. Thedeglaciation deglaciation history history and and geomorphological character a partof ofthe the region region of ofthe the 'Interlakes 'InterlakesComposite' Composite' -some - somenew newevidence. evidence. Symposium Symposium of apart Paleoindian Occupations Occupations of of on 'Archaeology, geomorphology and Paleoenvironmet: Paleoindian Great Lakes, Lakes, 60th 60th Annual of the the Western Western Great Annual Meeting Meeting of the Sociaety Sociaety for for American American Archaeology, Minneapolis, Archaeology, Minneapolis,MN. MN. May May 3-7. W. 1980 AAPalaeo-lndian Oliver Lake Lake Area. Area. Wanikan, 80, 33. Ross, W. Palaeo-Indian Refined Refined Biface Biiace from the Oliver Wanikan, 80, Society. Thunder Bay Bay Chapter, Ontario Archaeological Society. �Ross, Composite:aa re-definition re-definitionofof the the initial settlement of of the Ross, W. W. 1995 The Interlakes Interlakes Composite: initial settlement Agassiz-Minong 76, 3-4. 3-4. Agassiz-Minong Peninsula. Wisconsin Wisconsin Archaeologist, 76, Sharp, Sharp, R.R R.P. 1953 1953 Shorelines Shorelines of the the glacial glacial Great Great Lakes Lakes in in Cook Cook County, County, Minnesota: Minnesota: American Science,251, 251,109-139. 109-139. AmericanJournal JournalofofScience, Smith, TO. 1993 Lake Agassiz: The northwestern outlet and Smith, D.G. D.G. and Fisher, Fisher, T.G. 1993 Glacial Glacial Lake Agassiz: The northwestern outlet and paleoflood, 9-12. 21,9-12. paleoflood, Geology 21, Stewart, Ross, W.A. W.A.and and B.A.M. SAM. Phillips Stewart, J.D., Ross, Phillips1984 1984Investigations Investigationsatatthe theMcoaid McDaidSite Site(DcJh(DcJh16), 16), Thunder Thunder Bay, Bay, Report Reportto to the theOntario OntarioHeritage HeritageFoundation. Foundation. Stewart, A.S. Kissin 1989 1989 Two Isolated Isolated Biface Biace Finds Finds from the the Stewart, JO., J.D., Ross, Ross, W.A. W.A. Faykes, Faykes, A and AS. . Thunder Thunder Bay Bay Area, Area, Wanikan, Wanikan, 89, 89, 3, 3, 4-6, 4-6, Thunder Ontario Thunder Bay Bay Chapter, Chapter, Ontario Archaeological ArchaeologicalSociety. Society. Stuart, 1993 Paleogeographical Paleogeographical reconstruction reconstruction of of Lake LakeBeaver BeaverBay Bayraised raisedshorelines shorelines Stuart, A.J, A.J. 1993 with correlation correlation to possible possible Paleoindian Paleoindian settlement, settlement. Thunder Bay Bay region, region, Ontario. Ontario. HBSc. HBSc. dissertation, dissertation,Dept. Dept.of of Geography, Geography,Lakehead LakeheadUniversity, University,Thunder ThunderBay, Bay,Ontario. Ontario. Taylor, on the the north north Superior shore, American Geologist, vol. Taylor, F.B. 1895 The Nipissing beach on 15, 304-314. 15, p. 304-314. Teller, Agassiz and its fts Influence Influence on on the the Great Great Lakes. Lakes. fi Karrow, Teller, J.T. J.T. 1985 1985Glacial Glacial Lake Agassiz Karrow, P.F., P.F., and Calkin, Calkin, P.E. P.E. eds., eds., Quatemary Quaternary Evolution Evolution of the Great Great Lakes, Lakes, Geological Geological Association Associationof of Canada CanadaSpecial Special Paper Paper 30, 30, 1-16. 1-16. Teller, Thorleifson, L.H. 1983. The - LakeSuperior Superiorconnection. connection. In Teller, J.T. and Thofieifson, The Lake Lake Agassiz -Lake In J.T. Teller and LakeAgassiz, Agassiz,pp. pp.261-290, 261-290, Geological Geological Association Association and L. Clayton, eds., Glacial Glacial Lake of 26. of Canada, Canada, Special Special Paper Paper26. Tickle, Tickle, R. R. 1996. 1996. Depositional Depositional Systems Systems developed developed during during Deglaciation:Evidence D8glaciation:Evidence from from aa portion of the Mats ON., NBA. Dept. of MarksMoraine, Moraine, Conmee Conmee Township, Township, ON., HBA. dissertation, dissertation, Dept. Geography, Geography, Lakehead LakeheadUniversity, University, Thunder Thunder Bay, Bay, Ontario. Ontario. Van Rise, CR. and C.K. Leith 1911 of the the Lake Lake Superior Superior Region, Region, United States States 1911 The Geology of Hise, C.R. Geological Survey, Washington. Washington. Geological 1901, Glacial Glacial Lakes America Bulletin, Bulletin, vol. vol. Winchell, N.H. 1901, Lakes of of Minnesota, Geological Society of America 12, 12, p. p. 108-128. 108-128. Wright, H.E., F-I.E.,1972a 1972aPhysiography PhysiographyofofMinnesota, Minnesota,ininSims, Sims,P.K. P.K.and andG.B. GB.Morey, Morey,eds. eds.Geology Geology of Minnesota: aCentennial volume, 561- 578. Minnesota: acentennial volume, Minnesota MinnesotaGeological Geological Survey, p. 561- Wright, history of of Minnesota, in Sims, Sims, P.K. P.K. and and G.B. GB. Morey, Wright, H.E. H.E. 1972b Quaternary Quaternary history Minnesota, in Morey, eds. eds. Geology of Minnesota: Centennial volume, Minnesota Survey, p. 515515Minnesota: aacentennial Minnesota Geological Survey, 547. 547. Wright, HE. HE. Tunnel hydrology of of the Superior Tunnelvalleys, valleys, glacial glacial surges and subglacial subglacial hydrology Superior lobe, lobe, Minnesota, 136, p. 251-276. 251-276. Minnesota, Geological Geological Society Society of America Memoir 136, �Wright, J.V. 1963 Survey along alongthe the North North Shore Shore of of Lake Wright, J.V. 1963 An An Archaeological Archaeological Survey Lake Superior. Superior. Anthropology Papers, of Canada3, Canada 3,Department Departmentof of Northern Northern Affairs Affairs Anthropology Papers, National National Museum of pp. and National National Resources, Ottawa, 99 pp. Wright, HE., Cushing 1973 Moines Lobe, Geological H.E., C.L. C.L. Matsch, Matsch, and and E.J. Gushing 1973 Superior Superior and and Des DesMoines Geological Society 136, p. 153-1 153-185 Society of America Memoir 136, 85 Wright, Wright, HE. H.E. WA. W.A.Watts Watts1968 1968Glacial Glacialand andvegetational vegetational history of northeastern Minnesota, Minnesota Minnesota Geological Society Survey Survey Special Special Publication Publication11. 11. - Zoltai, S.C. 1963. Glacial features of the Canadian Lakehead, Canadian Geographer Geographer 7, 7, 101 101ead, Canadian 115 Zoltai, S.C. 1 965a. Glacial features of the Quetico-Nipigon area, area, Ontario. Ontario. Canadian CanadianJournal Journal of of Earth Science Science 2, 247-269. 2,247-269. - Zoitai, Thunder Bay Surficial Geology, Geology, 1:506,880, 1:506,880, Map S265, Ontario Zoltai, S.C. S.C. 1965b. 1965b. Thunder Bay - Surticial Ontario Department of Lands and Forests. ! �TIlE PALEOPROTEROZOIC GEOLOGY OF THE GUNFLINT FORMATION Peir Pufahi, Pufahl, University University of British Columbia Fralick, Lakehead Philip Fralick, Lakehead University John Scott, John Scott, Ministry Ministry of of Northern Northern Development Development & Mines Formation records records a chemically dominated shelf shelf The Gunflint Formation silica- and iron-rich iron-rich precipitates precipitates predominate. predominate. sequence in which silicathe role role that both physical physical and and chemical chemical This frzp trip will examine the This processes played in invarious variousshelf shelf environments. environments. �FIELDTRIP 4 DEPOSITIONAL ENVIRONMENTS OF THE PALEOPROTEROZOIC GUNFLINT FORMATION Pier Pufahl and Philip Fralick INTRODUCTION INTRODUCTION Paleoproterozoic many other other important important Paleoproterozoiciron ironformations formationshost host most mostof of the the world's iron ore and many Consequentlythey theyhave havebeen beenthe the focus focus of of hundreds hundreds of of mineral deposits deposits (Simonson, (Simonson, 1985). 1985). Consequently mineralogical, geochemical investigations. However, in in spite spiteof ofthis immense mineralogical, geochemical and sedimentological investigations. this immense amount of data no consensus has ever ever been been reached reachedas asto totheir theirorigins. origins. Some Some of of this this controversy controversy stems accumulated stems from a poor understanding of the sedimentological environments in which they accumulated (Simonson, 1985). They were deposited during a unique interval of earth history when grossly They were deposited during a unique interval different prevailed (Cloud, 1973), and therefore therefore lack lack precise precise different atmospheric atmosphericand and hydrologic hydrologic conditions conditions prevailed modem analogues (Simonson, 1985; Fralick and Barrett, Barrett, 1995). Modem subaqueous hot-spring Modem subaqueous hot-spring activity has provided sediments, but but the provided clues to understanding understanding the precipitation precipitation of iron-rich sediments, mineralogy and deposits is not not comparable comparable to tothe thelarge largePaleoproterozoic Paleoproterozoic iron iron mineralogy and areal areal extent extent of modem deposits formations deposited during during the the Earths Barren, 1995). 1995). Detailed formations deposited Earth's early early history history (Fralick (Fralick and and Barrett, petrographic to interpreting the the paragenesis paragenesis of of petrographic investigations investigations of iron formations formations have contributed to syndepositional, diagenetic and metamorphic mineral mineral assemblages, assemblages, but but they they have done little to provide detailed, reliable information information regarding regarding iron iron formation formationdepositional depositionalprocesses. processes. Minerals Minerals and and provide detailed, textures are are commonly commonly altered altered to to secondary secondary assemblages, and when primary minerals minerals are are present present theoretical has been been limited in in its its ability to explain explain their genesis genesis (Fralick (Fralick and and theoretical chemical chemical modelling modelling has Barrett, Barren, 1995). 1995). Paleoenvironmental Paleoenvironmentalinferences inferencesfrom fromgrain-size grain-sizedistributions distributionsin in iron iron formations formationsare are also plagued with with uncertainty uncertainty because because they they are are commonly commonly indurated, indurated, diagenetically &genetically altered, altered, and and frequently fine-grained fine-grained (Simonson, (Simonson, 1985). 1985). Fortunately, these problems can be overcome by combining a regional basin analysis analysis of clastic clastic and and volcanic volcanicrocks rocks surrounding surroundingiron iron formations formations with detailed detailed examination examination of vertical and lateral facies trends within chemical sedimentary successions (Fralick and Barrett,1995). 1995).Associated Associated lateral faciestrends withinchemical sedimentary successions(Fralick and Barren, sedimentary settings. Sedimentary structures sedimentary and volcanic rocks provide data on depositional settings. structures and lithic processes, and lithic associations associationswithin within iron iron formations formationsgive give information on physical sedimentary processes, regional basin analysis analysis provides a framework framework within which to to view view iron ironformation formationgenesis. genesis. Combining these techniques techniques makes makes paleogeographic paleogeographic reconstruction reconstruction and depositional depositional modelling modelling Barrett, 1995). 1995). This is the same same approach approach used by sedimentologists sedimentologists and possible (Fralick and Barrett, stratigraphers to reconstruct clastic depositional systems. stratigraphers reconstruct clastic depositional systems. Location Location The United States, The main main portion portionofthe of theGunflint Gunflintiron ironrange rangeextends extends170 170km km northeast northeast from from the United States, at Gunflint Lake, to Loon Lake, Ontario (Figure 1). In Minnesota the Gunflint forms a narrow belt Gunflint 1). In narrow belt �Figure 1. Distribution Figure Distributionof ofPaleoproterozoic Paleoproterozoic iron iron formations in the Lake Superior region. Ontario f GUNFLINT — —— — -S. — — 1 4+( 41-4 i2H-' •4.* 4+4+444443+ +4. A . —. MESABI Cf 'of MARQUETTE t,++ •+ 4j 4+4+ 4 + 4 : 444 +44 CUYUNA F rocks younger than early Proterozoic Proterozoic early _____ Penokean plutonics L and volcanics early Proterozoic early ____ rocks, BIF in black Archean greenstone -::.:;:; Archean granite terrane .: +'4" + ... +44 . . . 444+ •831 •44 GOGEBIC .... .... . ..... . __ A . 44+ 44 4.44 4+ L5r '; I, Ii II.— / i/pt/li/I p/il, '1/ LEGEND — I, Archean Archean gneiss gneiss terrane terrane I / t t - . - Minnesota I- ON R1VE - Wisconsin. C. L100km I . �20 the southwest southwest by bythe theDuluth DuluthComplex. Complex. Erosional remnants 98 km 20 km long which is truncated to the northeast Ontario, suggest suggest apre-erosional apre-erosional continuation continuation of the the chemical chemical northeast of of Thunder Thunder Bay at Schreiber, Ontario, sedimentary rocks to the east. sedimentary rocks to the east. Previous Previous Work Work The Gunflint was discovered discovered in in 1850. 1850. The Gunflint iron formation was The earliest earliest recorded recorded geological investigation of the Gunflint by E. D. Ingall in 1887 (Ingall, 1888), who briefly briefly Gunflint was conducted by described the iron-bearing and Whitefish Whitefish Lake. Lake. Other early accounts iron-bearing strata near Silver Mountain and accounts were made by general by Smith Smith (1905) (1905) and Silver Silver (1906). (1906). Van VanHise Hiseand and Leith Leith in in 1911 1911presented presented aa general overview of the iron bearing rocks rocks in in the the Thunder ThunderBay Baydistrict. district. In 1924 J. E. Gill was was the the first to to describe its stratigraphy stratigraphynortheast northeastof ofSilver SilverMountain. Mountain. In 1927 describe the Gunflint in detail, and in 1926, its Gill published the chemical chemical sediments. sediments. T. L. Tanton described the published again, this this time on the origin of the iron prospects at at Mink Mink Mountain Mountain in in 1923, 1923, and in 1931 1931 gave an overview of the general geology in the vicinity vicinity of of Thunder Thunder Bay. Bay. In 1956 formation was divided by Goodwin Goodwin (1956) (1956) into six major sedimentary sedimentary facies 1956the iron formation facies representing Conglomerate Member, Member, 2) Lower Lower representingtwo twodepositional depositionalcycles cyclesand and four four members, members, a 1) 1)Basal Conglomerate Gunflint Member, 3) Upper Gunflint Member and 4) Upper Limestone Member; which are largely Gunflint Member, 3) Gunflint Member correlative with Broderick's (1920) four-fold division of of the Gunflint Gunflint iron iron range range in northeastern Minnesota and Wolff Wolffss (1917) subdivision of the the Biwabik Biwabii iron iron range range in in east-central east-central Minnesota Minnesota (Figure 2). He He also alsodrew drewattention attentionto to evidence evidence of widespread volcanism associated with deposition of the chemical sediments and suggested the iron iron and and silica. silica. In 1960 Goodwin of the chemical sediments and suggested a volcanic source for the 1960Goodwin and Moorehouse published the first detailed geological maps of the region (scale 1:31 1:31 680) 680) and and reported on the economic potential, mineralogy and chemistry of the iron bearing strata, and reported the economic potential, mineralogy and chemistry of the bearing modified modified Gunflint Gunflintstratigraphy stratigraphyinto intoaaLower Lowerand and Upper Upper Gunflint Gunflint depositional depositionalcycle cycle composed composedof of 10 10 members; a Basal Lower Taconite, Taconite, Local Local Lava Lava Basal Conglomerate, Conglomerate, Lower Algal Chert, Lower Shale, Lower Flows, Upper Algal Chert, Chert, Upper Jasper, Upper Shale, Shale, Upper Upper Taconite and an an Upper Limestone Limestone member. Fralick and Barrett Barrett (1995) (1995) have have recently recently redefined redefined Gunflint Gunflint stratigraphy stratigraphy as as simply simply member. consisting oncolitric cherts, cherts, and and cherty cherty consisting of of 22 members; members; aa lower lower member member comprised comprised of of basal algal and oncolitric They grainstones, and an upper member similar in stratigraphy to the lower member. grainstones, upper member similar in stratigraphy to the lower member. They have interpreted interpreted this this succession successionas as representing representing aa wave- and and tide-dominated inner shelf shelf sequence. Floran Lougheed (1983) (1983) and and Simonson Simonson Papike (1975), (1975), Randazzo Floran and and Papike Randazzo and Markun (1980), Lougheed (1987) mineral assemblages comprising comprising the the ~Gunflint (1987) have have given givendetailed detkled petrographic ietro&aphicdescriptions descriptionsof ofmineral unflint iron formation. The iron formation. TheGunflint Gunflintvolcanics volcanics(Goodwin, (Goodwin,1956,1960) 1956,1960)have havebeen been petrographically petrographically described described by Hassler and Simonson by Kissin and REE geochemistry geochemistry has been determined by Simonson (1989). Their TheirREE Fralick exhalative origin Fralick (1994). (1994). Carbon Carbonand andsulfur sulfurisotopic isotopicstudies studieshave have indicated a hydrothermal exhalative geochemical source for for the the iron iron (Cameron, (Cameron, 1983; 1983; Carrigan Camgan and as the source and Cameron, Cameron, 1991). 1991). Other geochemical investigations investigations(Kronberg (Kronbergand and Fralick, Fralick,1992) 1992)have have attributed attributedthe the alteration alteration of the Archean basement to the penetration penetration of Gunflint-derived fluids diagenetic or low-grade metamorphic metamorphic conditions. conditions. of Gunflint-derived fluids under under diagenetic Detailed accounts accounts of of The overlying Rove Formation Formation was was described described by Morey Morey (1967, (1967, 1969). 1969). Detailed microfossils preserved and Barghoorn (1 (1954), Barghoorn microfossils preservedwithin withinGunflint Gunflintchert chertare arerecorded recordedby Tyler andBarghoom 954), Barghoom and Tyler (1965a,b), Awramik (1976). (1976). (1965a,b), Edhom Edhom(1973) (1973) and and Awramik �S. I .... Basal con glomeraté Iron Formation Gunflint . . Formatin Pokagama Quartzite Biwabik Iron Formation 0 a = Dunham Formation Glen Township Formation lJttie Falls Formation Trout Lake Formation Mahnomen Formation Trommaid Formation Formation Rabbit Lake Virginia Formation Rove Morey, 1978 White, 1954 Goodwin, 1958 Cuna range Mesath range Gunflirite Northwest segment S a a Sunday Quartzite Bad River Dolomite Palms Formation Formation Iron- Ironwood Formation Tyler Aldrich, 1929 Blair / Ajibik Quartzite Siamo Slate Negaunee Iron Formation Goodrich Quartzite Michigamme Formation Cannon, 1986 Marquette range Archean rocks, undivided Bad River Dolomite Palms Formation Creek Formation Emperor Volcanic Compl Formation copp NFormation N Michigamrne GogeLNcrage For mation Reany Creek Enchantment Lake Formation Mesnard Quartzite Kona Dolomite Wewe Slate Quartz ite Ajibik Siamo Slate Formation Iron- Negaunee Goodrich Quartz ite Thaden, 1968 Marquette range Saunders Hemlock Formation Amasa and Fence River Formations Michigamme Slate Green stone Badwater Cannon, 1986 A7uPhft - Southeast segment Hemlock Formation Amasa and Fence River Formations Mithigamme Slate Dunn Creek Slate Badwater Greenstone Riverton Iron Formation Fortune Lakes Slate Stambaugh Formation Hiawatha Graywacke Cannon, 1986 ta Figure 2. Proposed correlations ofAnimikie Group sediments in the Lake Superior region. Fern Creek Formation Qua rtzite Sturgeon Randville Dolomite Fetch Formation Formation Iron- Vulcan Michigamme Slate Badwater Greenstone ratttn 2 j a a 0 0 C E a 0 0. = I! a d. 0 a = I 0 0, = 0 a = 2 2' 0 a = �Figure 3. Gunflint sediments sediments outcropping outcropping at at the spiiway Photographs of of Gunflint spillway section section near Figure 3. Photographs Kakabeka Falls. A) Trough Kakabeka Troughcross-stratified cross-stratified"lapihi" 'Tap(Â¥//i"tuff'(reworke volcaniclastic tuff (reworked rip-ups). rip-ups). ofseqAence. B) Large Largealgal algalbioherm biohermdraped drapedby by shales shales at at the the base of sequence. I A C! __ - -— , -: r' a -a / - - / - ;1 '. , - 2 - - I — _----1-_ - ft /; - I .-;C f �Several isotopic investigations investigations have have constrained constrainedthe the depositional depositionalage ageof of the the Gunflint Several isotopic Formation between between 1.6 1.6 and and 2.0 2.0Ga. Ga. Hurley Hurley et et al. at (1962) minimum age age of of deposition depositionof of (1962) obtained aa minimum about K-Ar methods, methods, to to which which they theyapplied appliedan anempirical empiricalcorrection correction factor, factor, about 1.6 1.6 Ga Ga by both Rb-Sr and K-Ar which allowed age of ofapproximately approximately 1.9 1.9Ga Ga(Stille and Clauer, Clauer, 1986). 1986). Faure and which allowed them to propose an age (Stille and approximately 1.6 1.6 Ga. Ga. More Kovach (1969) also obtained a Rb-Sr isochron age of approximately More recently, recently, Stille Stille Clauer (1986) obtained about 2.08 2.08 Ga Ga for forvolcanic volcanicslates slatesinterbedded interbedded and Clauer obtained a Sm-Nd isochron age of about with iron-bearing iron-bearing strata strata of the Guntlint Gunflint Formation. The absolute age age of of the the Gunflint Formation Formationwas wasdetermined determinedby byFralick Fralicketetal. a. (1998) by conducting U-Pb, single zircon geochronology on a sample population from reworked tuffs conducting U-Pb, single zircon geochronology on a population from reworked tuffs outcropping near Kakabeka Kakabeka Falls Falls (Figure (Figure3), 3),aaeuhedral euhedralzircon zircongave gaveaaconcordant concordantage ageof of1878 1878zE2 ± outcropping (Figure 4) which Fralick et al. at (1998) (Figure (1998)argue argueis is the the depositional depositional age of this unit. I KB-TUFF Brown euhedral zircons .34 .338 .336 .334 .332 5.2 5.25 5.3 5.35 5.4 5.45 fromthe theupper upperGunflint GunflintFormation Formation (Fralick, (Fralick,etetal, at 1998) 1998) Figure 4. Concordia Concordiadiagram diagramfor zirconsfrom Animikie Basin Paleoproterozoic sedimentary Animikie Basin Basin form a southwardsouthwardThe Paleoproterozoic sedimentaryrocks rocks deposited deposited in in the Animikie thickening wedge province, which truncated in in eastthickening wedge covering coveringthe the southern southernmargin marginof of the the Superior Superiorprovince, which is truncated central Minnesota and northern Wisconsin by the 'Penokean" magmatic terranes". Sedimentation Minnesota by the "Penokean" terranes". Sedimentation began approximately prior to to or during during approximately 2.1 2.1 Ga Ga ago ago and and ceased ceased roughly 1.85 1.85 Ga ago (Morey, 1983), prior the Penokean orogeny of Goldich et al. at (1961). The nature of the sediment varies considerably, The nature of the sediment varies considerably, (1961). ranging the thick thick successions successions of of ranging from from volcanic volcanic and and clastic to the chemical precipitates which form the iron formation. The Thetermination terminationof ofthe thePenokean Penokeanorogeny orogeny marked marked the onset onset of an intrusive igneous phase plutons into into the theAnimikie Animikiesediments sediments emplaced subduction subductionrelated related tonalitic tonalitic and granitic plutons phase which which emplaced related volcanics volcanics of the the Wisconsin Wisconsin magmatic magmatic terranes. terranes. The the basin basin was was The present form of the and the arc related �achieved around1.1 1.1 Ga Gaago achieved around ago (Silver (Silver and Green, 1972; 1972; Hanson, Hanson, 1975;Wanless Wanless and and Lovebridge, Lovebridge, 1978) 1978) System (King (King and Zietz, 1971) when a north-northwest trending trending branch of the Midcontinental Rift System Zietz, 1971) separated the Animikie Animikie sediments southeastern segment (Morey, 1983). separated sediments into a northwestern and southeastern 1983). of the Animikie Animikie Group unconformably unconformably overlays overlays the the Superior The northwestern segment of Province Malinomen Formation), Province and and consists consistsof a basal sandstone-siltstone (Pokegama (Pokegama Quartzite, Mahnomen Formation), (Gunflint, Biwabik, Biwabii, Trommald iron formations), and a thick, upper, formation (Gunflint, iron formation upper, shale-siltstone shale-siltstone sequence (Rove, Virginia and Rabbit Rabbit Lake Lake Formations). Formations). In east-central Minnesota the the Thomson Formation Rove, Virginia Virginia and and Rabbit Rabbit Lake Lake Formations, Formations, has has also also Formation which has similar similar lithology to the Rove, considered correlative and Ojakangas, Ojakangas, 1970). However, been considered correlativeto the Animikie Group (Morey and However, recent recent structural interpretations have complicated complicated the the correlation interpretations have correlation of these units units (Ojakangas, (Ojakangas, 1995); 1995); northward Penokean Orogeny Orogenyhas hasjuxtaposed units of ofdiffering differingEarly EarlyProterowic Proterozoic juxtaposedunits northward thrusting thrustingduring during the Penokean ages in east-central Minnesota (Southwick et al., 1991; Hemming, Hemming, Minnesota at, 1988; Southwick and Morey, 1991; 1994). Similar within the the southeastern southeastern segment segment in in Similarstructural structuralcomplexities complexities also hinder correlation within northern Wisconsin 1991 ; Gregg 1993). northern Wisconsin and and the the Upper Upper Peninsula Peninsula of Michigan (Klasner et al., at, 1991; 1993). The were deformed deformed during during the the Penokean Penokean orogeny, orogeny, The sedimentary sedimentary rocks rocks of the Animikie Basin were and Archean Archean rocks rocks along along the the forming a zone zone of of deformed deformed and and metamorphosed metamorphosed Paleoproterozoic and southern margin of the Superior Province Province (Sims, (Sims. 1991). It is thought to to have have occurred occurred between between 1.9 1.9 southern margin and Bickford, Bickford, 1985). In the Great Lakes region it and 1.76 Ga (Van Schmus, 1976; Van Schmus and forms the Late Archean crust crust of the the Superior Superior Province Province and and the the forms the transition transition between between the Late Archean Paleoproterozoic Paleoproterowic continental crust of the Mazatzal Mazatzal and Yavapai Yavapai provinces of the southwestern southwestern Deformation along United United States States(Sims (Simsand and Petennan, Peterman,1986; 1986;Hoffman, Hofflnan, 1989; 1989;Barovich Barovich et al., at, 1989). Deformation this thrusting and development of basement basement gneiss gneissdomes domes(Southwick (Southwicket etal., this tectonic tectonic front front involved involved thrusting at, Minnesota, northern northern Wisconsin Wisconsin and and the the 1988). Rocks of the orogeny are exposed in east-central Minnesota, 1988). Upper Peninsula Peninsulaof of Michigan. Michigan. The orogeny Animikie strata strata and and a southern orogeny consists consists of a northern northern zone of deformed deformed Animikie southern assemblage plutonic rocks rocks of of the the Wisconsin Wisconsin assemblageof of arc-related arc-related volcanics volcanics and synorogenic, calc-alkaline plutonic magmatic terranes, separated by by the Niagra Fault Zone Zone in in northern northern Wisconsin Wisconsin and the the Upper Upper Peninsula 1989) and by the Malmo Discontinuity in east-central Minnesota 1989) and by the Malmo Discontinuity in east-central Minnesota Peninsulaof of Michigan Michigan(Sims (Sims et al., at, (Southwick and and Morey, Morey, 1991). 1991). (Southwick The earliest sedimentological for the the Animikie Animikie Basin Basin relied relied on on the the work work of of James James sedimentological models for (1954) successions evolved from from aa "miogeosynclinal" "miogeosynclinal" sequence sequence (1954) who concluded concluded that iron formation successions to a "eugeosynclinal" during Paleoproterowic Paleoproterozoictime. time. James' James original ideas were then "eugeosynclinal" assemblage during discarded in the mid sediments in in aa rift-like rift-like basin basin discarded mid 1970's 1970's in in favour favour of of deposition deposition of Animikie Animikie sediments (Cambray, 1978). 1978). In Inthis thismodel modelrifling riftingwas wasfollowed followedby by the theultimate ultimateclosure closureof of the the basin basin with with the the and Penokean orogeny. orogeny. In the early early 1980's 1980's Young Young (1983), (1983), by analogy analogy with other Phanerozoic and Proterozoic tectonic and sedimentary history history of ofProterozoic Proterozoicdepositional depositionalbasins, basins, interpreted interpretedthe the tectonic Proterozoic rocks in the northern Great Lakes region in terms of an aulacogen model. model. Unfortunately, Unfortunately, there was no general consensus on the driving force which which caused caused the the basin basin to to form and the characteristic characteristic sediments analogous to rift/aulacogen depositional systems are absent. In In the mid 1980's 1980's continued sediments analogousto rifVaulacogen depositional systems research lead to to significant significant advancements advancements in understanding understanding the tectonics associated with basin development. These Thesestrides strideslead leadHoffman Hofflnan(1987) (1987)to tosuggest suggestdeposition depositionof ofchemical-sediments chemical-sediments �Icr\, member 2 member member!1 member member 4 member 3 member member S member 5 , I. p..) : I 20m C ra'< I I C) t ININ coarsening upwards sequence transgression basal transgression middle transgression upper transgression Stratigraphk section from holes Figure 5. Stratigraphic sectionthrough throughthe theGunflint GunflintFormation Formationbased basedon on drill drill core corefrom Figure is aafew few kilometres kilometres north north of the the US US border. barrfcr.A A detailed detailed discussion discusshn of the lithofacies is - - L (sen/o/In the tort 'ft Up vorlion graillsizg changes in water 4 tR1 In4n-nt clianees trends.,, L [_Z L L �occurred crustal loading occurredin inaamigrating migratingperipheral peripheralforeland forelandbasin basinwhich whichformed formedin in response response to crustal loading during during the thePenokean Penokeanorogeny. orogeny.Southwick Southwicketetatal.(1988) (1988)and andSouthwick Southwickand andMorey Morey(1991) (1991)then thenreinterpreted reinterpreted the at, 1960) 1960)in in Minnesota. Like LikeHoffman, Hoffinan,(1987) (1987) thesegment segmentof ofthe thePenokean Penokean orogeny orogeny (Goldich, et al., they peripheral foreland theyalso alsoconcluded concludedthat thatthe theAnimikie AnimikieGroup Groupaccumulated accumulatedin inaaperipheral forelandbasin basinand andthat thatiron iron formation formationdeposition depositionoccurred occurredin inthree threedistinct distincttectonic tectonic settings settings as as the orogeny evolved. evolved. Recently, Recently, Pufahl (1995, 1996) 1995) have Pufahl(1995, 1996)and andHemming Hemming(1994, (1994,1995) have demonstrated demonstrated that this tectonic tectonic model model is is no no longer 1995) contends longervalid. valid. Hemming Hemming(1994, (1994,1995) contendsthat thatnedodimium nedodimiumand and lead lead isotopic isotopic evidence evidence for for the provenance provenanceof ofthe theAnimikie AnimikieGroup Groupis isconsistent consistentwith with the the tectonic tectonic evolution evolution of of the Animikie Animikie Basin Basin beginning telescoped back arc basin basin that that beginningas asaapassive passivemargin marginwithin withinaaback backarc arc basin, basin, and and ending ending as a telescoped closed closedas asaa result resultof ofaa change change in in relative relative plate convergence convergence direction. Nevertheless, Nevertheless,in in recent recentyears years the theforeland forelandmodel modelhas hasreached reachedwidespread widespreadacceptance acceptancebased basedon on high high resolution resolution aeromagnetic data, aeromagneticdata, exploratory exploratory drilling drillingthrough through structurally structurally complex complex regions, and sedimentological sedientological investigations investigations of clastic (Hoffman, 1987; 1987;Southwick Southwicketal. etat,,1988; 1988; Southwick clasticunits unitsabove aboveand and below below iron-bearing strata (Hoffman, Southwick and andMorey, Morey,1991; 1991;Ojakangas, Ojakangas,1995). 1995). GUNFLINT GUNFLINTIRON IRONFORMATION FORMATION PALEOGEOGRAPHIC PALEOGEOGRAPHICRECONSTRUCTION RECONSTRUCTION Interpretations Interpretations presented presented here here are are based based on on the the study study of of core core drilled drilled through through the the Gunflint Guntlint Formation. Formation. The The Formation Formation has has been been divided divided into into five five members members based based on on distinct distinct lithofacies lithofacies associations associationsand andcontrolled controlledby bytransgressive-regressive transgressive-regressivecycles cycles(Figure (Figure5). 5). The The strand strand proximal proximal succession fine-grained, chemical chemical sediments sediments successionpresent presentin inNorthwestern NorthwesternOntario Ontarioofflaps offlapsto to the south as fine-grained, of 6 and and 7). 7). This of the the slope slopeare are approached approachedin in the the Cuyuna Cuyuna Iron Iron Range Range (Figures (Figures 6 Thischapter chapterwill will give give detailed detailed interpretations interpretations of the the depositional depositional environment environment of each each of of these these members; members; but but first first an an overview overviewof ofnearshore nearshoresedimentation sedimentationisispresented. presented. Lateral Lateral and and vertical vertical facies facies changes changes within within the the Gunflint Gunflint Formation Formation indicate indicate deposition deposition occurred on a shallow, non-barred, microtidal, storm-enhanced shelf with little influx. occurred on a shallow, non-barred, microtidal, storm-enhanced shelf with little clastic clasticinflux. Bedforms Bedformsand andinternal internalsedimentary sedimentarystructures structurescomposing composingGunflint Guntlintstrata strataare are directly directly comparable comparable to to those found in modern nearshore-to-shallow marine clastic and carbonate depositional those found in modern nearshore-to-shallow marine clastic and carbonate depositionalsystems. systems. These These facies facies trends trendsrecord recordenergy energytransformations transformationsfrom fromdeep deep water, water, offshore offshorewaves waves to to nearshore, nearshore, shoaling conditions. Minor shoaling waves waves under under fairweather fairweather and storm conditions. Minor mafic mafic flows (Goodwin, 1960) 1960) and extensive extensivetuffaceous tuffaceoushorizons horizons(Hassler (Hassler and and Simonson, Simonson,1989) 1989)within within the the Gunflint Gunflint indicate indicate that that the the region region was was volcanically volcanicallyactive activeat at the the time time of of basin basin formation formationand and subsidence subsidence (Fralick (Fralick and Barrett, Barren, 1995). 1995). The Themain mainmorphological morphologicalelements, elements,bedforms bedformsand andinternal internal sedimentary sedimentary structures structures along along aa typical typical beach-shoreface-offshore beach-shoreface-offshore profile profile are are shown shown in in Figure Figure 8. 8. The Thefollowing followingsummary summary of of these these features and Clifton Clifton et. et. al. at (1971). features is is taken taken from from Walker and Plint (1992), Einsele (1992) and (1971). The foreshore, also portion above above the the low low tide tide line line alsosynonymous synonymouswith with beach, beach, consists consistsof of the the portion Theforeshore, In the the foreshore foreshore zone zone the the most most and and is is dominated dominatedby by the theswash swashand andbackwash backwash of of breaking breakingwaves. waves. In high characteristic dipping seaward. seaward. On characteristicfeature featureis is parallel parallel to to low-angle low-angle cross-bedding cross-bedding dipping On non-barred non-barred high inner (upper foreshore), planar zone and energy an energyshorelines, shorelines,two twoplanar planarlaminated laminatedzones zonesoccur, occur,an an inner (upper foreshore), planar zone and an outer (lower foreshore) planar zone within the zone of breaking waves. waves. In between these outer (lower foreshore) planar zone within these two two zones zones depositional features. the the bedding beddingsurface surfacebecomes becomesrough roughdue dueto to dotted dotted and andirregular irregularerosional erosionaland and depositional features. �Figure 6. Figure 6. Location of driliholes depicted on Figure 7. �G-1 MGS-8 Mahnomen Fm (Cs) 18290 Rabbit Lake Fm (Vs) SW Is. • -.4 MGS-7 MGS-2 20m Figure 7. 30f TB0642 4 Gp,GIICS TB0218 Rove Fm (Vs) NE slope. The green units are fine-grained volcaniclastics. Other units are described in the text. sections are from logged driliholes (for locations see Figure 6). Sc and Si arc fine-graincd chemical sediments of the Lithofacies distribution across the Gunflint, Mesabi and Cuyuna shelf to slope depositional system. Stratigraphic Kakabeka Conglomerate (Cp) GF-3 ftp Pokegama Quartzite (Cs) MGS-5 Virginia Fm (Vs) �Morphological elements, and internal Figure & elements, bedforms bedforms and internal sedimentary sedimentary structures structures along along aa Figure 8. Morphological typical beach-shoreface-offs bore profile. @pica1beach-shoreface-offshoreprofile. - ZONE ZONE OF OF SHOALING SHOALING WAVES SURF ZONE fairweather wave base !Sn'9 P2 stormwave wave storm basebase S SHOREFACE SHOREFACE SHELF INNER SHELF (subtidal) (subtidal) Beciforms Bedforms Grain Grain size size - parallel - laminated laminated mud facies fades silt and and clay clay —I I lunate lunate outer assymetric HCS HCS assYmetric dune planar dune planar fades facies facies rippled facies facies fades facies facies fine sand sand - silt silt - - fine fine medium medium sand sand FORE FORE BACK BACK SHORE SHORE SHORE SHORE (supratidal) (intertidal) (supratidal) (intertidal) inner inner rough rough facies facies inner inner planar planar fades facies coarse coarse fine sand sand - fine mediurr fine fine mediur sand gravel sand gravel - - �In tidally tidally dominated dominated systems systems the the foreshore foreshore and and upper upper shoreface shoreface is is comprised comprised of of flaser, wavy, and In lenticular lenticular bedded bedded sands sands and and muds muds typical typical of of sedimentation sedimentation on inter- and subtidal, sand-mud subtidal, mixed sand-mud flats. The shorefacelies liesbelow belowlow lowtide tidelevel leveland andisischaracterized characterized by by day-to-day day-to-day sand transport transport Theshoreface above fairweather fairweather wave wave base. base. The above Theshoreface shoreface normally normally has has a concave-upward profile, which is is in in equilibrium with with the the waves waves that that shape On the the lower at depths equilibrium shape it. it. On lower shoreface, shoreface, at depths near near or or below below produced by storm fairweather wave base (10 (10 to 20 m), are long crested symmetrical symmetrical wave ripples, produced storm waves. Landward, Landward, on onthe themiddle middleshoreface, shoreface,these these inactive inactive ripples ripples pass into into active active ripples ripples which which become increasingly increasingly asymmetric, trough cross become asymmetric, irregular, and short-crested. Larger, trough cross stratified, stratified, lunate lunate zone. All dunes are frequently observed on the upper shoreface in the breaker zone. All these these bedforms bedforms are are associated with small-scale or larger-scale cross bedding which is is predominantly oriented oriented toward toward continuous agitation, the deposition of mud the land. Because Becausethe thesediments sedimentsof of this this zone zone are are under continuous base and has a lower istypically typically below below fairweather wave base is prevented. The The mud-dominated mud-dominated shelf is depositional slope foreshore and shoreface. Muds typicallyparallel parallel laminated laminated and and highly highly thanthe foreshore Muds are typically depositional slope than bioturbated bioturbated in in modem modem shallow-marine shallow-marinedepositional depositionalsystems. systems. In storm In storm enhanced enhanced shallow shallow marine marine non-barred, non-barred, environments environments the the shoreface shoreface processes processes described thus described thus far, far, as as well well as as processes processes in in deeper deeper regions regions of of the the inner inner and and outer outer shelves shelvesare are strongly strongly affected and and modified modified by by storm storm events events (Davis, (Davis, 1992; 1992; Einsele, Einsele, 1992). 1992). Storms affected Storms can can deposit deposit sandy sandy material in in the material the supra supra tidal tidal zone, zone, as aswell well as asgenerate generatespecial specialnon-channelized non-channelized flow flow conditions conditions in in deeper water. water. These deeper Thesehigh highenergy energyevents eventsfrequently frequently produce produce characteristic characteristic sheet-like sheet-like sand sand and and mud mud beds, called tempestites, of considerable lateral extent (Johnson and Baldwin, Baldwin, 1986; Morton, 1988; Nummedal, 1991; Einsele, 1992). Storm-generated bed forms show a the beach beach Nummedal, 1991; E i e l e , 1992). Storm-generated bed forms show adistinct distincttrend trend from fromthe into deeper water. The upper shoreface (surf zone) displays dunes oriented parallel to the shoreline. into deeper water. The upper shoreface (surf zone) displays dunes oriented parallel to the shoreline. When flow becomes channelized longshore erosively scour scour fairweather fairweather sediments, sediments, longshore and rip currents erosively depositing mediumand coarse-grained, trough and planar, cross stratified sands. The middle depositing medium- and coarse-grained, trough and planar, cross stratified sands. The middle shoreface is dominated dominated by both bedfonns with horizontal or shoreface both flat and swaley bedforms with nearly horizontal or low-angle low-angle swaley swaley cross cross stratification. stratification. Because Becauseof ofthe thecontinual continual agitation agitation of of sediments sediments through through wave wave action action on on the the upper middle shoreface record of of heavy heavy storms storms is isusually usuallydestroyed. destroyed. On the lower lower shoreface shoreface upper and and middle shorefacethe the record On the (5 to 20 20 m m water water depth), depth), the ebb currents currents reaches reaches about about the the same same magnitude magnitude as (5 to the stress stress of of storm storm surge surge ebb as the oppositely oppositely directed the directed storm-wave storm-waveinduced induced oscillatory oscillatory flow flow (Einsele, ( E i e l e , 1992). 1992). Thus, Thus,for foraashort shorttime, time, both currents both currents may stir stir up up sand sand and and mud, mud, which which are are redeposited redeposited as as parallel parallel and and hummocky hummocky crosscrossstratified fine-grained,graded gradedbeds bedsatat the the same same location location or or nearby nearby (Einsele, (Einsele, 1992; 1992; Dott Doll and stratified fine-grained, and Bourgeois, 1982). On inhigh-energy high-energy Bourgeois, 1982). Onthe theinner innershelf shelfand andparts partsof ofthe theouter outershelf, shelf,particularly particularly in environments, environments, storm-induced combined flow may still still generate HCS or thin, graded, graded, sandy sandy beds beds resulting from If the the suspension suspension originates originates from from storms storms and and resulting from laterally laterally flowing flowing suspension suspension currents. currents. If the sandy the sandy layers layers alternate alternate with with shelf shelfmuds, muds, the thestorm storminduced inducedbeds bedsrepresent represent distal distaltempestites tempestites (Aigner, Einsele, the current current component component of of the the (Aigner, 1985; 1985; E i e l e , 1992). 1992). At At greater greater water water depths depths on on the the shelf, shelf, the combined storm beds combined storm flow flow becomes becomes dominant, dominant,leading leadingto to current currentrippled, rippled, offshore, offshore, fine fine sand sand and and silt silt beds and graded graded rhythmites rhythmites (Aigner, and (Aigner, 1985; 1985;Einsele, Eisele, 1992). 1992). Since Sinceonly onlythe thelargest largeststorms stormscan canaffect affectthe the mud deposition. quiescence and mud deposition. storm events events are are followed followed by long long periods of quiescence sea floor in this zone, storm Paleogeographic interpretation interpretation Paleogeographic The following following discussion discussion has divided into into three three sections. sections. Each The has been been divided Eachdeals deals with with interpreting interpreting the vertical and lateral facies trends comprising the basal transgressive, middle regressive and upper upper the vertical and lateral facies trends comprising the basal transgressive, middle regressive and transgressive depositional cycles of the Gunflint Formation in detail. transgressive depositional cycles of the Gunflit Formation in detail. �Kakaheka Conglomerate Conglomerate ,tidal roB Figure 9. Figure 9. Paleogeographic reconstruction reconstruction of of the thedepositional depositional system system which which produced produced the Gunflint Formation. Formation. AAdetailed attributes of this this system system isispresented presented detaileddiscussion discussionof of the the attributes Gunflint -in &the iB &L text. - ?UOPtE �Basal transgression Basal transgression extends from from the the base base of ofthe theAnimikie Animikie Group Group to to the the This 20 m thick, upward fining cycle extends middle of base of the cycle a middle of member member 2. 2. The base cycle is is characterized characterized by the Kakabeka Conglomerate, a backshore, pebbly pebbly beach beach deposit deposit preserved preserved as as aa transgressive pebble lag lag (Figure This backshore, transgressive pebble (Figure 9). 9). This intraformational conglomerate conglomeratewas was formed formedby bythe the localized localizedweathering weatheringof ofthe thepeneplain. peneplain. Then, intraformational tirough subsequent intraclasts were reworked reworked (swash/backwash) these these intraclasts through subsequentstorm-generated storm-generatedwave waveaction action(swashlbackwash) crudely stratified, tidal, beach beach deposits. deposits. Ojakangas into crudely stratified, matrix supported, supported, pebbly, supra tidal, Ojakangas (1983) (1983) has similar interpretation conglomeratic unit forming the the base base of of the the Pokegama Pokegama quartzite quartzite made a similar interpretation of the conglomeratic Mesabi iron range in the Mesabi range in east-central Minnesota. Minnesota. As the basal transgression progressed these coarse beachface beachface sediments sediments were were on-lapped on-lapped by by the fineparallel laminated laminated grainstones grainstones(GP (GP facies) facies) of of member member 1. The the fine- and and medium-grained, medium-grained, parallel 1. The hematite-enriched transition from member I may indicate a change in the redox state of of the hematite-enriched transition from member 1 may indicate a change in the redox state the depositional system level or, or, more more simply, simply,ititmay mayrepresent representoxidation oxidationof of depositional system accompanying accompanying a rise in sea level iron-bearing sediments by late phase oxygen-rich meteoric waters percolating along this lithologic sediments meteoric waters percolating along this lithologic boundary. Deposition boundary. Deposition of of this this thin, thin, parallel parallel bedded bedded grainstone unit marks the onset of intertidal intertidal sedimentation on the the foreshore foreshore (Figure (Figure 9). 9). Member Member l's m) and and apparent apparent lack lack of of sedimentation 1'srelative relative thinness thinness (5 m) desiccation features features suggests suggests that that the the intertidal zone was was not well developed Animikie desiccation intertidal zone developed on the Animikie (tidal range range <2 <2 m) m) (Reineck (Reineck and and shoreline, a situation typical of modem, microtidal environments (tidal Singh, may have have developed developed LLHstromatolitic stromatoliticbioherms bioherms(Ad (Ad facies) facies)at at the base of member 1 may Singh, 1975). 1975). LLH modem, colunmar in much the same way modem, columnarstromatolites stromatolitesand and crypt crypt algal algal knolls knolls form form in the low energy Here columnar intertidal flats flats of intertidal of Shark Shark Bay, Bay, Australia Australia (Hoffman, (Hoffinan, 1976; 1976; Schreiber, Schreiber, 1986). 1986). Here columnar stromatolites develop in the lower intertidal zone in response response to to strong strong wave wave action. action. An oncolitic stromatolites oncolitic mats marks the the transition transition into into the the subtidal subtidal (Figure (Figure horizon (Ao facies) facies) above these intertidal, algal mats Ginsburg (1960) (1960) compared compared modem modern 9) (1 (Logan al.,1964; 1964; Ginsburg, Ginsburg, 1960, 1960, Sellwood, Sellwood, 1986). 1986). Ginsburg ogan et aT, oncolites from Devonian, Permian and Precambrian Precambrian examples examples and oncolites from the the Florida Florida and and Bahama Bahama Banks to Devonian, found they formed I1 to or near near the the subtidal subtidaltransition. transition. Oncolites to 88 feet below mean water level at or from the Paris Paris Basin, Basin, aa non-barred, non-barred, shallow shallow epeiric epeiric sea sea covering covering much much of of northwestern northwestern Europe Europe during the Middle Jurassic, were also determined to characterize this transition (Seliwood, Middle Jurassic, were also determined (Sellwood, 1986). 1986). The overlying flaser and wavy bedded grainstones(Gf, (Gf, Gw Gw facies) of bedded chert-carbonate chert-carbonate grainstones member 2 represents represents subtidal deposition on a shallow, shallow, storm stormenhanced, enhanced,non-barred non-barred shoreline shoreline (Figure (Figure member subtidal deposition 9). 9). Similar Similar Middle Middle and and Upper Upper Cambrian, Cambrian, cyclic, cyclic, peritidal, peritidal, carbonate, carbonate, tempestites tempestites compose compose the the Elbrook Virginia Appalachians Appalachians (Demicco, (Demicco, 1985, 1985, Koerschner Koerschner Elbrook and Conococheague ConococheagueFormations Formations in the Virginia This 1.6 and Read, 1989). krn thick thick carbonate carbonate sequence sequence formed formed on on the the Cambro-Ordivician Cambro-Ordivician 1989). This 1.6 km continental shelf; aggraded, rimmed, mature, passive passive continental continental margin margin (Koerschner (Koerschner and and Read, Read, continental shelf; an aggraded, 1989). Cycles thickness, possess possess aa sharp sharp basal basal transgressive transgressive 1989). Cyclestypically typically range range between between 11 and and 7 7m m in in thickness, surface, are overlain overlain by aa subtidal subtidal unit unit passing passing into into aa tidal tidal flat flat cap, and record regional changes changes in time (Koerschner and and Read, Read, 1989). 1989). Three types of cycles are sea level over very short intervals of time recognized, thin-based cycles. The been recognized, thin-basedcycles, cycles,thrombolitic-based thrombolitic-basedcycles, cycles,and and thick thick based based cycles. The latter latter has has been divided into three grainstone-based and and digitate-algal-bioherm-based,grainstone-based three subcycles, subcycles, those which are digitate-algal-bioherm-based, Ribbon-carbonate-basedcycles cyclesmost most closely closely resemble resemble member member 33 and ribbon-carbonate-based. and ribbon-carbonate-based. Ribbon-carbonate-based therefore will will be be the the only only cycle cycle fully fully reviewed reviewedin inthe thefollowing followingdiscussion. discussion. For Foraafull fulltreatment treatmentof of these cyclic, peritidal carbonates carbonates the reader is referred to Koerschner and Read (1989). �Ribbon-carbonate-based cycles are composed Ribbon-carbonate-based cycles composed of 11 to to 55 m m thick thick fining fining and andcoarsening coarsening upwards upwards sequences sequencesof of flaser flaserbedded beddedcoarse-grained coarse-grainedcarbonate carbonate grainstones/packstones grainstonedpackstones which are are comprised of wavy and lenticular bedded finer-grained pelletal carbonates and rare lime comprised wavy and lenticular bedded finer-grained pelletal carbonates rare lime conglomerates dolomitic mudstones. The conglomeratesintimately intimately interbedded interbedded with with parallel laminated dolomitic The term term comes comes from the field appearance ofthe alternating thin-beds (ribbons) of differentially weathered limestone from the field appearanceofthe alternatingthin-beds (ribbons) of differentially weathered limestone and are and dolostone dolostone (Demicco, (Demicco, 1983). 1983). The Thecoarse-grained coarse-grainedribbon ribboncarbonates carbonatesand and pellet pellet packstones packstones are most similar to the flaser bedded, chert-carbonate grainstones (Of facies) of member 3 in the similar to the flaser bedded, chert-carbonate grainstones (Gf facies) of member 3 Gunflint Gunflintchemical chemicalsedimentary sedimentarysuccession. succession.Coarse-grained Coarse-grainedribbon ribboncarbonates carbonatesare areunlayered unlayered or or have have low-angle cross bedding with weakly graded foreset laminae. Pellet packstones have horizontal low-angle cross laminae. Pellet packstones horizontal laminations, ripple cross-lamination. cross-lamination. Mud-poor pelletlaminations,climbing climbingwave-ripple wave-ripple lamination and wave ripple rich ribbon ribbon carbonates carbonates and and mud-and-pellet mud-and-pellet rich ribbon ribbon sequences sequences most most closely closely resemble resemble wavy wavy bedded, bedded, chert-carbonate chert-carbonategrainstones grainstones(Gw (Gwfacies). facies). Mud-poor, Mud-poor, pellet-rich pellet-rich ribbon ribbon carbonates carbonatesare are planar laminated; pellet laminated;mud-and-pellet-rich mud-and-pellet-richribbon ribbonsequences sequencesare arecomprised comprisedofwavy of wavyand andlenticular lenticular bedded pellet layers interbedded interbedded with with form-discordant form-discordant wave ripples, ripples, starved ripples and and parallel parallel laminated laminated dolomite mudstones mudstones(Koerschner (Koerschnerand and Read, Read, 1989). 1989). dolomite Demicco Demicco (1983) (1983) and Koerschner Koerschner and Read (1989) (1989) interpret these ribbon sequences as recording storm-influenced subtidal settings floored floored by byinterlayered interlayered recordingshallowing shallowingby by aggradation aggradationfrom from storm-influenced Coarsening-upward lime muds muds and and sands, sands, to to subtidal subtidal sand sand flats flats on on aa mature, mature, rimmed rimmed shelf shelf. Coarsening-upward sequences sequencesrecord record low lowenergy, energy, subtidal subtidalconditions, conditions,followed followedby upward-shallowing upward-shallowinginto into high high energy energy shallow zone). Koerschner contend these these shallowwater water settings settings (ie. surf zone and breaker zone). Koerschner and Read (1989) contend coarse-grained coarse-grained ribbon ribbon carbonates carbonatesformed formed on on sand sand flats, flats, where coarse layers were reworked during storms and stormitidalenergy energy waned. waned. The rare lime lime and less less common common fine fine layers layerssettled settledout outasasstorm/tidal conglomerate conglomerate interbeds interbeds were deposited deposited during storms storms which swept semi-lithified, peloidal sands from transgressive from shallow shallowsubtidal subtidaland and intertidal intertidal settings. settings. Fining Finingupwards upwards ribbon ribbon sequences sequences record transgressive high-energy conditions that gave way way to low energy energy conditions conditionsas asthe theplatform platform shallowed. shallowed. Finegrained response grained peletal, peletal, ribbon ribbon carbonates carbonatesformed formed on lower energy, subtidal, mixed sand flats in response to wave wave action action and and grade gradelandward landward into into mixed mixed sand-mud sand-mud flats flats (Koerschner (Koerschner and Read, 1989). 1989). Fairweather with deposition of of thick thick storm layers of Fairweather reworking of wave rippled beds alternated with pelletal pelletal sediment sedimentwith withamalgamated amalgamatedhurnmocky hummocky cross cross stratification, stratification, and wave and current ripples (Koerschner (Koerschnerand and Read, Read, 1989). 1989).Similar Similarclastic, clastic,shallow, shallow,tidal tidal marine marine sequences sequences occur occur in the modem North Sea (Demicco, 1983). North Sea (Demicco, 1983). Unlike wavy Unlike the the Cambrian Cambrian ribbon ribbon carbonates carbonatesof the Virginia Appalachians these flaser and wavy bedded bedded chemical chemicalsediments sedimentsdid didnot notform formby by aggradation aggradationof of upward upward shoaling shoaling grainstone grainstonesequences. sequences. Instead, vertical changes reflect a rate of relative sea level level rise which which changes in iron formation facies reflect consistently consistently outpaced outpaced that that of of sedimentation. sedimentation. Middle regression regression Middle This extends to This minor, minor, 20 20 m m thick, regressive regressive phase begins in the middle of member 2 and extends 3. It records the gradual onset of shallowing from the middle shoreface to the the top top of of member member 3. the lower lower foreshore. foreshore. The Thesubtidal subtidalmixed mixedgrainstone-chemical grainstone-chemicalmud mud flats flats of member 2 coarsen upwards to packages) of flaser bedded chert-carbonate grainstones. grainstones. These to 33 m thick packages) These coarse coarse packages packages formed formed during duringregressive regressiveofflap offlapof oflower-middle lower-middleshoreface shorefacefacies faciesand, and,like likethe theflaser flaser bedded bedded package packageat atthe the base base of of this this member, member, represent represent dune dunedevelopment development in in the the surf surf zone zone of of the upper shoreface. shoreface. �As level continued continued to these flaser flaser and and wavy wavy bedded bedded chert-carbonate chert-carbonate grainstones As sea sea level to fall fall these grainstoneswere were transgressed by the coarser, higher energy, stromatolitic and oncolitic, chert-carbonate, transgressed by the coarser, higher energy, stromatolitic and oncolitic, chert-carbonate,grainstones grainstones of member member 3. 3. These Thesestorm stormenhanced, enhanced,crypt cryptalgal, algal, lower lower foreshore foreshoredeposits depositsmark mark the the regressive regressivepeak peak of of this middle cycle. Like the Gf facies chert-carbonate grainstones of member 2, those of this middle cycle. Like the Gf facies chert-carbonate grainstones of member 2, those comprising comprising member 3 3 also member also record record lunate lunate dune dunedevelopment developmenton onaasubtidal/lower subtidal/lowerintertidal intertidalchert-carbonate chert-carbonate grainstone flat. grainstone flat. The 6 6 cm cm thick, thick, Gi Gi facies grainstone bed bed at at the the base base of of member member 33 in in O'Connor O'Connor township township is is facies grainstone The interpreted as being a proximal tempestite deposited in this shallow, high energy environment. This interpreted as being a proximal tempestite deposited in this shallow, high energy environment. This coarse, storm storm generated lag is coarse, generated lag is thin, thin, possesses possesses aa base base which which appears appears to to erosively erosively scour scour the the flaser flaser bedded chert-carbonate 2, contains contains abundant abundant magnetitemagnetite- and and hematite-rich hematite-rich bedded chert-carbonate grainstones grainstones of of member member 2, rip ups ups and and pinches pinches laterally laterally towards rip towards Blake Blake township; township;all all are are characteristics characteristicsof of nearshore nearshoresheet sheetstorm storm deposits (Einsele, deposits (Einsele, 1992). 1992). With Withaareturn returnto tofairweather fairweatherconditions conditionsthis this storm stormbed bed provided provided an an ideal ideal substrate for stromatolites to colonize. ItIt was not easily easily reworked reworked by by normal normal shelf shelf currents currents substrate for stromatolites to colonize. was firm firm and and not (Fralick, 1988). was however however not not (Fralick, 1988). Development Developmentof of crypt crypt algal algal knolls knolls on on this this intraformational intraformational lag lag was widespread as they they colonized colonized only only the the most most stable stableregions regions sheltered shelteredfrom from fairweather fairweatherreworking. reworking. widespread as The thin, thin, discontinuous, discontinuous, stromatolitic which developed developed above above this this basal, basal, crypt crypt The stromatoliticand and oncolitic oncolitichorizons horizons which algal layer layer suggests suggests subtidal, on aa substrate substrate that that underwent underwent periodic, periodic, algal subtidal, stromatolitic stromatolitic growth growth occurred occurred on fairweather reworking. reworking. The LLH and and SS SS fairweather The development development of of these these poorly poorly formed, formed, discontinuous discontinuous LLH stromatolitic on shallow, shallow, high high energy, energy, subtidal subtidal sand sand sheets sheets stromatolitic bioherms biohermsis is typical typical of of crypt crypt algal algal growth growth on of the the upper upper shoreface (Pratt et et a al., of shoreface and and lower lower foreshore foreshore (Pratt !., 1992). 1992). Upper transgression transgression possibly The upper transgressive member 3, through through member member 5, The upper transgressivecycle cycleextends extendsfrom fromthe thetop top of of member 5, possibly to the top of the Rove Formation; Formation; a vertical distance of almost 700 m. m. Its onset is rapid and Its onset is rapid and records records the those of of the the lower lower shoreface. shoreface. The the progressive progressive onlap onlap of of offshore offshore facies facies over over those The thin thin package package of of hummocky cross stratified chertand hematite-rich grainstones (GHCS facies) at the base of of hummocky cross stratified chert- and hematite-rich grainstones (GHCS facies) at the base member 4 in O'Connor township marks this transition. Modern hummocky-cross-stratified sands member 4 in O'Connor township marks this transition. Modem hummocky-cross-stratified sands occur below wave base base on of storm dominated, shallow shallow marine marine occur below fairweather fairweather wave on the the lower lower shoreface shoreface of storm dominated, depositional systems transition to the the inner inner shelf shelf (Hamblin (Hamblin and and Walker, Walker, 1979; Dott and depositional systemsat ator or near near the the transition 1979; Dott and Binsele, 1992; Davis, Davis, 1992). The Bourgeois, 1982; 1982; Aigner, Aigner, 1985; 1985; Walker and Plint, 1992; Emsele, The parallel parallel bedded, hematite-rich, (Gp facies grainstone) to to which which this this basal basal bedded, hematite-rich,slaty slatyand and cherty chertyiron iron formation formationbeds beds (Gp facies grainstone) hummocky-cross-stratified unit of storm storm deposition deposition near near this this change change hummocky-cross-stratified unit grades grades are are also also characteristic characteristic of They are (Figure 9). 9). They are identical identical to to modern, modem, proximal proximal offshore, offshore, storm-sand storm-sand layers layers (proximal (proximal tempestites) and their their more distal equivalents, rhythmites (distal (distal tempestites) tempestites) (Reinick (Reinick and and tempestites) equivalents, graded rhythmites Singh, Aigner, 1985). these storm 1985). Like Likethe theparallel parallel bedded bedded chemical chemical sediments sediments these storm beds beds also also Singh, 1972; 1972; Aigner, posses a sharp instances may may grade grade upward upward into into plane planelaminated laminatedmud mud layers. layers. In In posses sharp base, and in some instances modern tempestites laminated rhythmites, rhythmites, in modem tempestitesthe the sand sandlayers layersare are either either evenly evenly laminated laminated or or are laminated in which which the lower lower laminae grade upwards upwards into into thinner thinner and and finer-grained finer-grained the laminae are thicker thicker and coarser-grained coarser-grainedand grade laminae (Reinick (Reinick and and Singh, 1972). marine, storm storm laminae 1972). They They are are the the result result of of deposition deposition from shallow marine, generated turbidity currents Finsele, 1992; generated currents (Reinick (Reinick and Singh, 1972; 1972; Walker, 1984; Einsele, 1992; Davis, 1992). 1992). The fine- and medium-grained chert- and hematite-rich hematite-rich grainstone grainstone beds beds comprising comprising this this member member represent episodic development of storm-generated, chemical sedimentary beds. Thicker, episodic development Thicker,graded, graded, coarser beds developed during the most severe storms. The hematite-rich slaty The hematite-rich slaty iron iron formation formation packages between fairweather sedimentation sedimentation on lower packages between storm-grainstones storm-grainstonesrepresents representsthe the return return to fairweather on the lower �Figure precipitation of iron-rich iron-rich sediments on ontheAnimikie theAnimikie shelf. shelf Figure10. 10. Depositional modelfor modelfor the theprecipitation The model is based on data from the Gunflint and Gogebic data collected collectedfrom Gogebic iron ranges. Samples from these areas were Samples from were examined examined in thin section section under under a petrographic petroeraphic - microscope and using aa scanning scanningelectron electronmicroscope microscope with an an energy dispersive dispersive systent rock analysis was also performed performed using an system. Whole Whole rock an inductively inductively coupled plasma. plasma. During Duringthe thePaleoproterozoic Paleoproterozoicthe theworld world ocean ocean contained much less oxygen than enabledFeu FC and thanatatpresent present(Cloud, (Cloud,1973). 1973). This enabled and MC Mnutotostay stayininsolution. solution. Oxygen present in appreciable quantities in the Oxygen would only be present the shallow, near shore environment environmentwhere wherereasonable reasonablelevels levelsof of light light penetration penetrationtotothe thebottom bottomwould would allow allow the cyanobacteria colonies in this area to thrive, and thus release oxygen (1). During the cyanobacteriacolonies in this area to and oxygen (1). During storm pressure system system over overthe the area area storm events events the the combined combined effects of an extreme low pressure and forcing caused the buildup of water in in aa strand-proximalposition. strand-proximal position. Upon and wave waveforcing Upon waning this water would be returned returned to the mid and and outer outer shelfvia shelf via waning of of the the storm storm event event this aa storm storm surge surgeebb ebbflow. flow. The Thedeliverance deliveranceof ofmore morehighly highlyoxygenated oxygenated waters waters to to the the deeper deeper shelf would would cause the the rapid rapid precipitation of iron iron oxide oxide (probably (probably as as iron iron hydroxyoxide) (2). At high high Fe/Mg FdWg ratios ratios itit isis likely likely that that precipitation precipitation of iron iron hydroxyoxide)(2). compounds during this rapid precipitation phase. phase. As oxygen and/or compoundswould would dominate dominate during this rapidprecipitation and/or iron iron is/are Mare depleted depleted precipitation precipitation rate slows slows and silica silica gel, gel, which which isis slowly slowly but but constantly from seawater, seawater, increases increases in in importance importance as as aa constituent constituent of of constantlyprecipitating from the phase due the layer. layer. Finally, Finally,with withdepletion depletionof of iron iron in in the the aqueous aqueousphase due to to precipitation, and a resu king decrease in Fe/Mg ratio, manganese anda resultingdecrease in the the FdWgratio, manganesebegins beginsto to effectively effectivelyscavenge oxygen forming MnO-rich Mn 0-rich tops some layers. layers. If another another storm occurs before oxygenforming tops to some storm event occurs the the Fe/Mg F e w ratio ratioof of seawater seawater has hasdecreased decreased to to the thepoint point manganese manganeseprecipitation precipitation becomes sign jficant the the upper upperportion portion of of the thelayer layer will willnot notform. form. Smaller Smaller storms storms are are becomes significant able precesses to operate in the mid-shelfarea, mid-shelf area, whereas ableto to cause causethe theabove aboveprecesses whereasonly only large large storms shelf Thus, storms effect effect the outer shelf. Thus,manganese manganeseconcentrations concentrationsare are much much higher higherin in rocksformed environment A) sequencefound rocksformed in inthe theouter outershelf-slope shelf-slope environment. A) Typical Typicalsequence found in in the mid-shelf (1cm) cm) chemical chemical sediment sediment laminae laminae directly directly overlay overlay storm mid-shelf where where thick thick (1 generated from the outer mid-shelf. mid-shelf C) B)Typical Typical laminae from C)Thin, Thin, generated granule granule lenses. lenses. B) complete laminae deposited in the outer helf complete laminae deposited helf. �- 1. Fairweather FairweatherPeriod Period - Stratified Stratified Water Water Body Body 02 02 020202 F' 02 — PC' — Mn PC' PC' PC' PC' Mn" -I- FC' 2. Storm - Destratitied Water Body 4 ÷+ PC' Fe' Fe" cmC cm 20 C.mm - mm -5 Ca fl • 'e3'•4 + 5102 Massive Fe304 Granular -o 1e304 2MnO Fe304 + Si02 Massive Fe304 0 Trough Trough Cross-Stratified Fine to Medium Grained Sand Grained - Ripple Laminated d and Massive Very FineS Fine Grained Sand to Coarse - Grained Grained Silt Silt Coarse - - Thinly to Thickly Laminated Mud �shoreface. storm layers lay directly shoreface. In In Blake Blake township township these chemical sedimentary storm directly on the the grainstones grainstonesof ofmember member2. 2. The The pulse pulse of of volcanism volcanism preserved preserved in in the the middle middle of member member 4 as a 6m thick volcaniclastic volcaniclastic horizon deposition. Its (Vsfacies) facies) occurred occurred swiftly, swiftly, outpacing iron formation deposition. Its sharp sharp bottom bottom contact contact horizon(Vs indicates indicatesrapid rapid influx influxof of volcaniclastic volcaniclasticmaterial material into into the depositional depositional system. Its Its gradational gradational upper contact waned gradually gradually over overtime. tuff contactsuggests suggestsdelivery delivery of of volcaniclastics volcaniclastics to the basin waned time. The The lapilli lapillituff (Vt reworked and deposited deposited (Vtfacies) facies)which whichcharacterizes characterizesthe thebase baseof ofmember member 55 is is interpreted interpreted as being reworked by by storms stormsfrom from more more proximal, proximal, nearshore nearshore settings. settings. The Themassive, massive,mediummedium-to tocoarse-grained coarse-grainedchertchertcarbonate grainstone bed (Gi facies) characterizing its base is similar to the (Gi facies) characterizing stromatolitic,storm storm carbonate grainstone stromatolitic, generated itslower lowerbounding boundingsurface surfaceisiserosive erosiveinto intothe thetop topof of member member44 and and itit also also generatedlag lagin inmember member3;3;its pinches laterally towards Blake Township. beds within the Gunflint pinches laterally towards Blake Township. Similar Similar volcaniclastic volcaniclastic beds Gunflint Formation Formation have have also alsobeen been interpreted interpreted by by Hassler Hassler and and Simonson Simonson(1989) (1989) as being being reworked reworked and and redeposited by turbidity currents. redeposited by turbidity currents. The The coarsening coarsening upwards, upwards, massive massive and andcross-stratified, cross-stratified, chert-carbonate, chert-carbonate, grainstone grainstone successions onlapof ofinner innershelf shelffacies facies over those of successions(Gc (Gcfacies) facies)of ofmember member55mark markthe theprogressive progressiveonlap the GHCS thelower lowershoreface shorefaceparallel parallel laminated laminated and and hummocky cross-stratified grainstones (Gp and GHCS facies) facies)(Figure (Figure9). 9).Similar Similarsuccessions successionsdeposited depositedinincoastal coastaland andopen openmarine marineenvironments environmentscomprise comprise the Sussex and and Shannon Shannon sandstones sandstones of the Powder Powder River River Basin Basin in in northern northern the Upper Upper Cretaceous, Cretaceous, Sussex Wyoming. Wyoming.InInWyoming Wyomingcycles cyclescoarsen coarsenfrom fromsilty siltyoffshore offshoremudstones, mudstones,through throughthin thin bedded, bedded, stormstormand and tide-generated tide-generated sandstones sandstones to to cross cross stratified stratified sandstones sandstones with with abundant abundant clay clay rip-ups. rip-ups. The The depositional depositionalmodel modelproposed proposedfor forthe theShannon Shannonand andSussex Sussexsandstones sandstonesconsists consistsof ofmigrating, migrating,offshore offshore bar Hobson barcomplexes complexesthat thathave havetheir theirsurfaces surfacescovered coveredby bysand sandwaves waves(Berg, (Berg,1975; 1975;Brenner, Brenner,1978; 1978;Hobson etetal., al.,1982; 1982;Davis, Davis, 1992). 1992).With Withsome somemodification modificationthis thismodel modelmay may also also be be applied applied to to the the coarsercoarsergrained, grained,shoaling shoalingupwards upwardssequences sequencescomprising comprisingmember member55 of of the the Gunflint GunflintFormation. Formation. Like Like bar bardevelopment development in inclastic clasticsystems systemsgenesis genesisof ofchemical chemicalsedimentary, sedimentary, offshore offshorebar bar complexes 2)the the complexesdepends dependson, on,1)1)the theavailability availabilityof ofcoarse coarsesediment sedimentto to offshore offshore environments environments and and 2) presence presenceof ofcurrents currentscapable capableof ofshaping shapingthis thissand sandinto intowell well defined, defined, migrating, migrating, sand sandbodies. bodies. In In elastic, clastic,shallow shallowmarine marinedepositional depositionalsystems systemscoarse coarsedetritus detritusisissupplied suppliedtotooffshore offshoreareas areasby bystorm storm Tillmanand andMartinson, Martinson,1984) 1984)and andtide tide(Spearing, (Spearing,1976; 1976;Brenner, Brenner,1978; 1978; (Berg,1975; 1975;Walker, Walker,1984; 1984;Tillman (Berg, Seeling, 1978) generated currents. However, unlike their clastic counterparts, bar genesis Seeling, 1978) generated currents. However, unlike their clastic counterparts, genesis in in chemical chemicalsedimentary sedimentarysystems systemsrelies relieson onoffshore offshoreprecipitation precipitation of of iron iron oxides oxides and and silica silica gel gel from from upwelled 1993),and andsubsequent subsequent upwelledenriched enrichedbottom bottomwaters waters(Borchert, (Borchert,1950; 1950;Drever, Drever,1974; 1974;Morris, Moms,1993), current currentreworking reworkingof ofthe thesubstrate substrateinto intorip-up rip-upgrains grains(Simonson (Simonsonand andGoode, Goode,1989) 1989)(Figure (Figure10). 10). Consequently this process does not depend on systems tracts capable of delivering coarse detritus Consequently this process does not depend on systems tracts capable of delivering coarse detritus to development in in a chemical sedimentary, tothe theshelf shelf.Bar Bar development a chemical sedimentary,shallow-marine shallow-marinedepositional depositionalsystem systemisis therefore governed by the position of areas on the shelf with high, primary chertlcarbonate therefore governed by the position of areas on the shelf with high, primary chert/carbonategrain grain production production and and the the presence presence of ofcurrents currentsable abletotorework reworkthese thesechemical chemical precipitates precipitates into into bar bar complexes. The relative stratigraphic position of member 5, over facies characteristic of the lower complexes. The relative stratigraphic position of member 5, over facies characteristic of the lower shoreface graingenesis genesis were were below below shoreface(Gp (Gpand andGHCS GHCSfacies), facies),suggests suggeststhat thatregions regionsofofprimary primarygrain fairweather wave base and that storm-generated, oscillatory flow was responsible for reworking fairweather wave base and that storm-generated,oscillatory flow was responsible for reworkingthe the substrate substrateinto intorip-up rip-upgrains. grains. Finer-grained, Finer-grained,shoaling shoalingupwards, upwards,chemical chemicalsedimentary sedimentarysuccessions successionsfrom fromthe theGogebic GogebicIron Iron Range in Wisconsin (Figure 11) (Pufahl, 1994) are identical to the coarsening upwards successions Range in Wisconsin (Figure 11) (Pufahl, 1994) are identical to the coarsening upwards successions �comprising comprising the Sussex Sussex and Shannon Shannon sandstones in Wyoming. They They are regarded regarded as as being being the distal equivalents equivalentsto to the the coarse coarse offshore offshore bars bars of of the the Gunflint Gunflint(Pufahl, (Pufahl, 1994). 1994). Cycles in Wisconsin Wisconsin coarsen from parallel laminated, magnetite-rich slaty iron formation in much much the the same same way way the the Cretaceous Cretaceous upwards shoaling shoaling cycles coarsen from shelf mudstones. 4 Sm.g 0*4. Dunes 400, 'S 4% 44. ) O4 Figure 11. Figure 11. Development Development oof f coarsening- and thickeningcoarseningthickening- upwards sequences in sequences in Wisconsin Wisconsinwere were related related to progradation of lunate dune offshoreprogradation dominated sand sand sheets. sheets. In a broad sense these offshore In offshore regions of primary grain genesis are similar to the "subtidal carbonate factories" factories" responsible responsible for for grain grain production production on on modem modem and some carbonate some ancient ancient carbonate carbonate platforms (James and Kendall, 1992). The upwards cycles cycles The coarse coarse nature of the Gunflint shoaling upwards suggests bar bar development developmentoccurred occurredatator or near near aa "grainstone "grainstone in Blake and O'Coimor O'Connor townships suggests factory. As with shoreline shoreline retreat retreat factory". Asthe thetransgression transgressionprogressed progressedthese thesefactories factoriesmigrated migrated in step with resulting in the deposition complexes further ftrther from regions of primary deposition of slightly finer-grained bar complexes production. The absence of of aa mud mud grain production. Theexistence existenceof of these these offshore offshore factories factories would account for the absence dominated offshore, offshore, which is so clastic shallow-marine shallow-marine depositional systems dominated so characteristic characteristic of of clastic (Walker and Plint, 1992). 1992). Alternatively, these coarsening upwards, upwards, GGc fades grainstone c facies grainstone successions successions may represent paracycle development (Vail et al., aL, 1977) in regions regions of high high primary primary grain grain genesis genesis during during relative relative sea level rise. Paracycles form in response to a punctuated rise in overall sea level. They record a Paracycles form in response sea level. period of rapid, relative sea level rise rise and still stand, followed by another another relative relative rise rise with with no no period followed by �significant that accumulates accumulates is is coarsening coarsening upwards, upwards, significantfall fall intervening intervening (Vail (Vail et aL, al.,1977). 1977). The sequence that suggestive of mediumsuggestive of continued continued shoaliñg shoaling as sediments sediments accumulate accumulate to sea level. In In this this scenario scenario the the medium-- as grained, chert-carbonate grainstonesatat the the base base of coarsening upward cycles would represent chert-carbonate grainstones grainstone deposition deposition during during rapid rapid sea level level rise as regions of primary primary grain grain genesis genesis migrated migrated shoreward. The coarse-grained tops of cycles would represent accumulation of grainstones shoreward. The coarse-grained tops of cycles would represent accumulation of grainstonesnear near to to base level where breaking waves were able to winnow the substrate during an interval of relative calm. At Atthe thepresent presenttime timedata dataisisinsufficient insufficient to to choose choose whether whether an autocyclic autocyclic (depositional) or allocyclic allocyclic (sea (sea level) level) mechanism mechanism governed governed cycle cycle genesis. - The transitional facies into the the The Gi Gi facies facies chert-carbonate chert-carbonategrainstone erainstonebeds which characterize transitional overlying Rove in Blake township are interpreted as representing deposition from suspension clouds overlying Rove in Blake township are interpreted representing suspension generated generated by either either storms storms or or tectonism tectonism during during basin subsidence. subsidence. OVERVIEW OVERVIEW OF OF THE THEANIMIKIE ANIMIKIEBASIN BASIN The iron-bearing iron-bearing rocks in Ontario and Minnesota Minnesota form form aachemical chemicalsedimentary sedimentary wedge wedge which which fines fines and and thickens thickens from from coarse, coarse, wave wave reworked, reworked, nearshore deposits of the Gunflint Formation to offshore, offshore, parallel-laminated, chemical mudstones of the Trommald iron formation in east-central Minnesota. to those those observed observed in in Minnesota. Lateral Lateral and and vertical vertical facies facies transitions transitions are directly comparable to modem They record modem shallow shallow marine marine clastic clastic and and carbonate carbonate depositional depositional systems. systems. They record energy energy transformations nearshore, shoaling shoaling waves waves under under fairweather fairweather and transformationsfrom fromdeep deepwater, water, offshore offshorewaves waves to nearshore, storm storm conditions. conditions.Paleocurrent Paleocurrentdata datasuggests suggestsdeposition depositionof of chemical chemical sedimentary sedimentaryrocks rocks occurred occurred on on a southward southward facing facing paleoslope. paleoslope. Nearshore rocks consist Nearshore chemical chemical sedimentary sedimentary rocks consist of of successions successions of ofstrand-proximal strand-proximal stromatolites, flaser and wavy and parallel parallel bedded stromatolites, flaser wavy bedded bedded chert-carbonate chert-carbonate grainstones, grainstones, and bedded and hummocky non-barred, microtidal, microtidal, hummocky cross cross stratified stratified hematite-rich cherty grainstones deposited on aa non-barred, storm-enhanced storm-enhancedshoreline shorelinewith with little little clastic clastic influx. Deposition Depositionon on the the inner shelf was dominated by accumulation accumulationof of coarse-grained, coarse-grained,shoaling shoalingupwards, upwards, chert-carbonate chert-carbonategrainstone grainstone successions, successions,possibly possibly associated with offshore bar development. development. Their Their genesis is believed to have been the result of a unique physiological conditions Animikie unique interplay interplaybetween betweenthe the chemical chemicaland and physiological conditionswhich which prevailed prevailed in in the Animikie Basin Basin during during iron iron formation formation deposition. deposition. They Theyformed formedin in proximal proximal offshore offshore areas areas where iron oxide and precipitating, and precipitates into and silica silicagel gel were actively precipitating, and where currents could rework chemical precipitates into rip-up grains grains and redistribute redistribute them. The Theshoaling shoalingupwards upwards successions successions replace replace a mud dominated offshore, offshore, which which is is so so characteristic characteristicof of modern modem shallow-marine shallow-marine depositional depositional systems. systems. Distally, finer-grained and less lesswell welldeveloped. developed. They They Distally, coarsening coarsening upwards upwards sequences sequencesbecome become finer-grained coarsen medium-grained, massive massive and and coarsen from from parallel parallel laminated, laminated, magnetite-rich chemical slates to medium-grained, cross offshore crossstratified, stratified,chert-carbonate chert-carbonategrainstones, grainstones,and andare areinterpreted interpretedas as recording recording progradation of offshore bar complexes of high high primary primary complexeson on the the middle middle shelf shelf below below fairweather fairweather wave base, away from areas of grain paracycledevelopment developmentduring during an an overall overall rise rise grain genesis. genesis. These Thesesuccessions successionsmay mayalso alsorepresent representparacycle in in relative relative sea sealevel. level. Monotonously parallel and slatescharacterize characterizedeposition deposition Monotonouslyinterbedded interbeddedparallel and wavy wavy bedded chemical slates of of iron iron formation formation on on the the outer outer shelf shelfand and slope-break. slope-break. The Thepresence presence of ofrare, rare, massive massive and and cross cross �stratified, magnetite-rich slaty slaty laminations laminations indicates indicates that that stratified, chert-carbonate chert-carbonate grainstones grainstones and graded, magnetite-rich storm-generated storm-generatedturbidity turbiditycurrents currentsoriginating originatingon onthe theinner innershelf, shelf,where where grain grain production productiontook tookplace, place, were for moving moving sediment sediment onto ontothe thedistal distalshelf. shelf The were an an important important mechanism for The existence existenceof of iron iron formation 1963) in in the the Cuyuna Cuyunanorth north range suggests that mass mass flow flow processes processes formation breccia breccia (Schmidt, (Schmidt, 1963) generated the distal distal shelf shelf were important generatedby by large-scale large-scalerotational rotationalslumping slumpingand and sliding sliding on the important in moving sediment sediment from from the the slope slope to to the the rise. rise. Sedimentological Sedimentologicaland andstratigraphic stratigraphicdata datado do not not provide provide definitive definitiveanswers answers to to the the question question of of plate plate tectonic tectonic setting settingof of the the Animikie Animikie Basin. However, However,paleogeographic paleogeographicreconstructions reconstructionsbased based on vertical vertical and and lateral lateral facies facies transitions transitions present within the Gunflint, Gunflint, Biwabik, and Trommald iron formations, and clastic units above and below strata indicate indicate that that it is is doubtful doubtful formations, and below iron-bearing iron-bearing strata deposition migrating peripheral depositionof ofthe theAnimikie AnimikieGroup Groupoccurred occurredwithin withinaamigrating peripheral foreland foreland basin basin (Figure (Figure 12). 12). There There is is an an apparent apparentlack lackof ofcoarse coarseturbiditic, turbiditic, deltaic, deltaic, and and fluvial fluvial sediments, adjacent to the fold thrust thrustbelt, belt, and and paleocurrent paleocurrentdata data from from clastic clastic and chemical sedimentary rocks are are opposite opposite to what should formation was shouldbe be observed observed if if deposition deposition took took place in a foreland basin. Also, Also, if if iron iron formation was to have accumulated accumulated in aa foreland foreland setting setting it it should should be be restricted restricted to the distal distal shelf shelf as as weathering weathering of the peripheral peripheralbulge bulgewould wouldshed shedclastics clasticsinto intonearshore nearshoreregions regionseffectively effectivelypreventing preventingdeposition depositionof ofiron iron formation. formation. Facies Faciestransitions transitionswithin withiniron-bearing iron-bearingmembers membersindicate indicatethe the contrary, contrary, a complete complete shelf shelf sequence, sequence, not not just just the the distal distalportion portionis ispresent. present. Although Animikie Basin Basin in Although data datais is insufficient insufficient to to reconstruct reconstruct the the tectonic tectonic evolution evolution of the Animikie detail, detail,some somegeneral generalinferences inferencesregarding regarding the the depositional depositionalhistory history of of the the Paleoproterozoic Paleoproterowic strata stratain in Ontario Ontarioand and Minnesota Minnesotacan canbe be made. made. Sedimentation Sedimentationwithin withinthe thebasin basinbegan beganwith withan anoverall overallmarine marine transgression the transgressiononto ontothe thesouthern southernmargin marginof ofthe the peneplained peneplainedSuperior Superior province province and deposition deposition of the Pokegama Mahnomen Formation. Formation. The PokegamaQuartzite Quartzite and and Mahnomen ThePokegama PokegamaQuartzite Quartziteaccumulated accumulated in in nearshore, tidally tidally influenced influencedsettings settingswhile whilethe theMahnomen Mahnomen Formation Formation was deposited deposited simultaneously in more distal shelf regions by pelagic settling and waning turbidity currents. This clastic deposition deposition distal shelf regions by pelagic settling turbidity This phase of clastic was probably associated with the early history of basin development. Through time, the continuing was probably associated with the Through continuing erosion erosionof of the the source sourceterrain terrainresulted resultedin in aa reduction reduction of of clastic clastic supply supply to to the the basin, basin, to to aa point pointwhere where iron iron formation formation began to accumulate accumulate on the the shelf, shelf, away away from from the the land-derived land-derived clastics clastics still still accumulating iron formation formation was accumulatingin inthe thenearshore. nearshore.Once Oncethe thehighland highlandwas wasreduced reducedto to near near base base level level iron then then able ableto to accumulate accumulatein inthe thenearshore, nearshore, and anddirectly directlyonlap onlapthe the Superior Superiorprovince. province. Deposition Deposition of of iron ironformation formation occurred occurred during during an anoverall overalltransgressive transgressive marine marine event event punctuated punctuatedby by two twominor minorregressions. regressions.Lateral Lateralfacies faciestransitions transitionswithin withiniron-bearing iron-bearingunits unitsrecord recordaa complete reworked nearshore nearshore deposits deposits completeshelf shelfsequence sequencewhich which fines finesand and thickens thickens from from coarse, coarse, wave wave reworked to the distal distal shelf. shelf. The to parallel-laminated parallel-laminated chemical chemical mudstones mudstones of of the The existence existence of of minor minor mafic mafic flows flows and three volcaniclastic volcaniclastic horizons horizons in in the the and tuffaceous tuffaceous horizons horizons in in the the Gunflint, Gunflint, and andthe thepresence presence of of three Ounflint Gunflintand andMesabi Mesabiiron ironranges rangesindicate indicatethat that the the region region was was volcanically active active during during subsidence subsidence in in this this phase phaseof of basin basin development. development.The Thethickest thickestvolcaniclastic volcaniclastichorizon horizonforms formsthe thebase baseof ofthe theRove, Rove, Virginia, Virginia, and and Rabbit Rabbit Lake Lake Formations, Formations, and and marks marks the the transition transition from from chemical chemical to to clastic clastic sedimentation. sedimentation.This Thisvolcanic volcanicepisode episodestopped stoppediron ironformation formationgenesis genesisby bychoking chokingthe thedepositional depositional system with volcaniclastics. As relative sea level continued to rise deposition within system with volcaniclastics. As relative sea level continued to rise deposition within the the Animikie Animikie Basin Basin was wasdominated dominatedby byaccumulation accumulationof of clastics clasticsderived derived from from the the north north by by southward southward flowing flowing turbidity turbiditycurrents. currents. �Figure asaamigrating migratingperipheral peripheralforeland basin. Arguments Figure 12. 12. The TheAnimikie Animikie Basin Basin modelled modelled as foreland basin. against this tectonic interpretation are presented in the text. SOUTH SOUTH NORTH - - FOLD THRUST BELT FOLD - AND AND - THRUST BELT I FORELAND FORELAND BASIN I migrating to north PERIPHERAL PERIPHERAL BULGE • •iiui sediment sediment eroded eroded from from fold fold -- and and thrust thrust belt belt mmmm — ---- Â¥I - fluvio deltaic flu I sedts .N sediment eroded eroded from sediment from peripheral bulge N SUPERIOR PRO VINCE nearshore clastics ff011 formation and coarse turbidites 0 20km 10 I I I �FIELD TRIP FIELD T R I P STOPS STOPS ITINERARY ITINERARY Stop 11 Stop Stop 22 Stop Stop 33 Stop Stop 44 Stop Stop S5 Stop Stop Stop 66 Stop 77 Stop Stop 88 stop Stop 99 Stop Stop 10 Stop 10 Stop Stop11 11 Stop 12 Stop 12 V V V V V V V V V V V V V V V V V V V V VV V V V V V Basal stromatolites; Nolalu Nolalu turnoff turnoff Basal unconformity, unconformity, stromatolites; Basal unconformity, unconformity, stromatolites; stromatolites; Kaministikwia Kaministikwia Riven Riverr Basal TufTs,volcaniclastic volcaniclastic sediments; sediments; Kakabeka Kakabeka Falls Tuffs, Falls Ribbon chert-carbonate; chert-carbonate; Kakabeka Kakabeka Falls Falls Ribbon Basal Basal pebbly pebbly sandstone; sandstone; Oliver Oliver Road Road Stromatolites, beachrock; beachrock; Rupert Rupert Street Stmmatolites, Collapse breccia; breccia; base basc of High Street Park Collapse of High Street Park Grainstones, stromatolites; Current River Slumped ribbon ribbon chert-carbonate; chert-carbonate; road road to to Pass Pass Lake Lake Slumped Rippled Rippled siltstones; siltstones; Transcanada Transcanada Highway Highway Grainstones with volcanic volcanic sherds; sherds; Terry Teriy Fox Fox Lookout Lookout Grainstones with Grainstones; Transcanada Transcanada Highway Highway near near Current Cunent River Grainstones; River V V V V pa Thunder V o 00 V Bay 000 0 hwy. 1h17 ÷ g —- Lake Superior LEGEND ARCHEAN PROTER OZOIC PROTEROZOIC km km V V 111 0 0 V V V 'I ++ V V V 5 5 Rove Rove Formation Formation Eorniation Gunflint Formation 10 0021 Gunflint Figure 13. Geology Geology of the Thunder ThunderBay Bay area. area Figure 13. Rocks ~ ~ +1 ~ Granitic a ? i Rocks + i c I ~olcanics IV V VJ Volcanics �STOP STOP 11 Proceed west weston onHighway Highway 11/17 11/17(the (the Transcanada) Transcanada) (Figure 13)past Kakabeka Falls and over the Kaministikwia Kaministikwia River. A A few few hundred metres metres past this bridge there is a turn off to Nolalu. Take outcrop Takeitit and and proceed proceed approximately approximately100 100metres, the main outcrop is is on on the the right. right. Chemical Archean granitoids granitoids at Chemical sediments sedimentsof of the Gunflint Gunflint Formation rest directly on Archean this location. This thin conglomeratic conglomeratic Thishighlights highlightsthe the sporadic sporadicdevelopment development ooff the the thin veneer comprising the Kakabeka Conglomerate Conglomerate in inthis thisarea. area. Stromatolites, Stromatolites, including including veneer comprising some these basal basal cherty cherty carbonates. carbonates. The some large forms, are developed in these The stromatolitic stromatoliti; ribbon carbonates carbonates are are abruptly abruptly overlain overlain by aa grainstone grainstone succession. succession. STOP STOP22 Proceed Proceed back to to the the Transcanada Transcanada Highway turn right and cross the Kaministiquia Bridge. Turn park to to end end of Turnleft lefton onroad road immediately immediately after after bridge bridge and drive through park road. road .AApath pathleads leadstotothe thefirst firstset setofofrapids rapidsabove abovethe the bridge. bridge. The The rapids rapids are are formed by Archean granitoids. The The slow-water slow-waterarea area to to the the south southisis underlain Gunflint Formation. Formation. River Riverlevel level will will govern govern what can can be observed observed underlain by the Gunflint at at this this stop. stop. The Kakabeka Kakabeka Conglomerate Conglomerate is patchily developed overlying the Archean Archean at at this stop. stop. Silicified Silicifiedstromatolites stromatolitesare aredeveloped developedon on the the Conglomerate Conglomerateor or directly directly on on the granitic granitic basement. This Thisisis the the location location from from which samples collected in cyanobacteria described in in the theliterature. literature. The in the 1950's 1950's yielded the first Gunflint cyanobacteriadescribed samples samples were were from from the the silicified silicified stromatolites stromatolites and were described by Tyler and and Barghoom (1954). (1954). Barghoorn STOP STOP33 Proceed back to the Transcanada Highway Highway and andturn turn left. left. Turn Turn right right into into the the main main park entrance entrance and and park in in the parking lot. Walk Walkto tothe theobservation observationplatform. platform. The The river river gorge gorge is is composed composed of of aa sequence sequence of of volcaniclastic volcaniclastic shales shales (less (less resistant, darker darker units) units) and and tuffs tuffs (more (moreresistant, resistant, lighter lighter coloured coloured units). units). This sequence sequence represents the ma] or volcaniclastic horizon present in the upper Gunflint Formation represents the major volcaniclastic horizon present the upper Formation and and is is traceable traceable to to the thesouth souththrough throughthe theMesabi MesabiRange. Range. Basalts outcropping outcropping approximately probably correlatable this unit. approximately30 30kilometres kilometresto tothe the southwest southwestare are probably correlatablewith withthisunit. Unpublished Unpublishedgeochemistry geochemistry(major, (major,trace, trace, REE REEanalysis) analysis)performed performedon onsamples samplesof of the the basalt basalt indicate indicate aa tholeiitic tholeiiticcomposition composition with a deep deep mantle source source (plume). STOP STOP44 Walk Walk back back to to the the southeast southeastside sideof ofthe the parking parking lot. lot. An forms this this cut. cut. Folds An outcrop outcrop of of ribbon ribbon chert-carbonate (ankerite) forms Folds and faults faults are present deformation. Commonly, present and and probably probably represent soft sediment deformation. Commonly, past studies studies of of the the Gunflint GunflintFormation Formationhave havedescribed describedthis thistype typeof ofsediment sedimentas asforming formingin inaadeep, deep, quiet quiet water water environment environment and and this this may may be becorrect. correct. However, similar carbonate carbonate sequences intertidal or or lagoonal lagoonal subtidal subtidal sequences are often often attributed attributed to deposition deposition in intertidal environments. environments.The Theevidence evidencesupporting supportingaadeep deepor or aa shallow shallow setting setting will be discussed at at this thisstop. stop. �a tS-s -:. - 4 ., - . -A ..r!'efl j _ Figure wave rip-up rip-up Figure 14. 14. Stop Ston6. 6.A) A)Intraformational Intraformationalbeachrock beachrockconglomerate conglomerateformed by by storm wave of partially partially cemented cementedsurficial surficial layers layers of of medium-grained medium-grainedsand sandin in the the upper upper beach beach of environment B) stromatolites with withmedium-grained medium-grainedan and B)Plan-view Plan-viewof of cabbage-sized cabbage-sizedstromatolites environment. granule sand sand layers layers banked bankedup upagainst againstthem. thea granule 1 �STOP STOP 55 Turn right onto the Transcanada and turn left onto Oliver Road. Turn Turn left on Turn Transcanada and Road and and right right on on Dawson DawsonRoad. Road. Drive 7.4 km km to to aa small small dirt dirt pull pull off off Mapleward Road on the left and park. Walk down the path for a few minutes until you see a small, Walk down rounded outcrop. belonging to to the the Kakabeka Kakabeka Conglomerate. Conglomerate. It This is an outcrop of pebbly sandstone belonging overlies Archean granitic rocks and represents gris (angular, granitic debris), probably probably @tic debris), overlies Archean granitic rocks and represents surrounding basement. weathered from surroundiig STOP 66 STOP Turn left left on Dawson Turn right right on the Turn Dawson Road Road and and proceed proceed to to Thunder Thunder Bay. Bay. Turn onthe Oliver Road. Road. Continue down Oliver Road Turn right right at at Hill Hill Transcanada and left on Oliver the comer corner with with Queen Queen Street. Street. This Street and stop at the This is private property so stay on the outcrop. outcrop. the base base of of the the This is a no hammer stop. The The middle middle stromatolitic stromatolitic unit forming the the Gunflint Gunflint Formation Formationoutcrops outcropsatatthis thislocation. location. The carbonate upper member of the stromatolites are excellently preserved and are visible visible in in both both cross-section cross-section and and plan plan stromatolites forms Carbonate sands sands are are banked up against the cabbage-sized forms view (Figure 14). Carbonate and eventually eventually overwhelm overwhelmthem. them. The stromatolites established themselves themselveson on aa and stromatolites established silicified horizon horizon which at other other locations locationsshows showsevidence evidenceof ofsubaerial subaerialexuosure. exposure.This This silicified provided the necessary hard substrate substrate to to allow allowthe thestromatolites stromatolitesto todevelop. develop. The unit providedthe maj or stromatolitic horizon is overlain by beachrock which has been disrupted by a major stromatolites have been been ripped ripped up up along along with with storm event. Pieces Pieces of of the underlying stromatolites overlying partially cemented carbonate sands. The beachrock formed in inthe theupper upper the overlying beach environment beach environment due to salt salt spray spray from from breaking breaking waves waves depositing depositing meniscus meniscus cements at grain grain contacts contacts of the sand. This partially cemented cements This partially cemented sand sand layer layer on the upper disrupted into slabs when when a large storm caused waves waves to to break in this tbis beach was dismpted environment. environment. STOP STOP 77 Proceed back back to to Oliver Oliver Road. Road. Turn right. Turn the Turn left left on Crown Street (at the junction with near where where it bends bends right right and and &s turns into into Dixon Dixon junction with John John Street) Street)and and drive drive to ;ear Street. Drive one block northwest on a small lane to the base of the High Street Park Drive one block northwest to the base of the High Street Park hill. outcrop forming forming the base of the hill bill is composed composed of carbonate carbonate breccia breccia with with some The outcrop chert clasts. Wavy laminated dripstone forms discontinuous layers blanketing some Wavy laminated dripstone discontinuous of the the roof roof of a of the breccia piles. This Thisisisaacollapse collapse breccia breccia formed by failure of cavern in the ankerite. A number of failures occurred as denoted by the the ankerite. A number of failures occurred as denoted by cavern which were were deposited depositedon ondebris debrispiles pileson onthe thecavern cavernfloor floor interlaminated dripstones, which by roof leakage. This Thisunit unit forms forms the the upper layer of the Gunflint Formation's lower correlatable with with STOP STOP 6. 6. Maximum Maximum member and is often overlain by stromatolites comelatable regression of the Animikie sea occurs at this internal interval leading to to subaerial subaerial exposure exposure Gunflint sediments sediments in the the Thunder ThunderBay Bayregion. region.Evidence Evidenceof ofsubaerial subaerialexposure exposurehas has of Gunflint not been obserned observed in drill core examined from south of the U.S. border. �A) Sills jfied stromatolitic stromatolitic reef reef overlain ales Figure 8.8. A) Siliszjied overlain by by black blackvolcaniclastic volcanicIasticsit shales Figure I15. S . Stop Sto~ (background). plane of ankeritic grainstones. grainstones. -' B) -' Hummocky Hummoc'- bedding ' ?ddingplane of ankeritic �STOP 8 Drive turn left. left. Proceed to its end and andturn turnleft leftonto onto Cumberland. Cumberland. Drive to Oliver Road and turn Continue River. Turn Continue driving driving on on Cumberland Cumberland to to Current Current Ever. Turnleft left onto onto Arundel Arundel Street Streetand and proceed to the cement bridge across the Current Rver. River. Immediately Immediately before crossing crossing the onto Lyon Blvd. Pull to the right into one of the the parking parking m the bridge bridge turn turnleft onto Pull off offto as. areas. Walk to the river then then down down river river to to the the end end of of the the outcrop. outcrop. You are standing on the Paleoproterowic Paleoproterozoic sea sea floor. floor. The The uneven topography of the ankerite ankerite grainstones grainstones reflects reflects the presence of dunes and hummocks produced by the storm which moved moved thi the carbonate sands over over the the bottom 15). carbonate sands bottok (Figure (~igure-15). storm events events which Silicified dot this this irregular irregularseascape. seascape. This forms forms the the lower lower Silicified colonies of stromatolites dot unit of of a tripartite assemblage. Large Large silicified silicifiedstromatolites stmmatolites occur occur on the other side of directly overlying overlying these is aavolcaniclastic volcaniclastic silty silty shale. shale. A description description of the the bridge and directly of this this sequence sequence and and its itsinferred inferreddepositional depositional environment environment follows (taken from Fralick, 1989). Fralick, 1989). Major Major Subdivisions Subdivisions Coalesced domal stromatolites form the microbial bioherms bioherms present present in the outcrops outcrops on on Current underlain by by parallel parallel CurrentRiver. River. They Theyrepresent representaaportion portionof ofthe the upper upper stromatolitic stromatoliticzone and are underlain laminated complicated laminatedand and hummocky hummocky cross-stratified cross-stratified carbonate carbonate grainstone which has undergone a complicated history and dolomitizition-ankeritizition. dolomitization-ankeritization. The bioherms history of of silicification, silicification, neomorphic neomorphic spar growth and are are draped draped by by graded, graded, centimetre-scale centime&-scale beds beds of of dark grey siltstone siltstone and shale shale (Figure (Figure 16). 16). Carbonate Carbonate Grainstone Grainstone The grainstone grainstone is divisible divisible into into three three types: types: 1)1)hummocky hummocky cross-stratified; cross-stratifie4 2) parallel parallel laminated; 3) massive, massive, highly highly silicified. silicified.Flummocky Hummockycross-stratification cross-stratificationgenerally generally has has wavewavelaminated; and and 3) lengths, lengths,between between0.5 0.5 and and 11m. ItItcommonly commonlycontains containsoval oval and and rectangular rectangular rip-up rip-up clasts clasts which, which, when when abundant, abundant,fine fineupwards upwards in in individual individualbeds. beds. Often Oftensilicified silicifiedrip-ups rip-upsor or mixtures mixtures of of silicified silicified and and nonsilicified silicified rip-ups rip-ups are arepresent present in in non-silicified non-silicified beds. beds. Parallel they are less less abundant. The The Parallellaminated laminated grainstone grainstone also also contains rip-up clasts, but they appearance of these units is nodules. The is complicated complicated by the presence of silicified nodnles. Thegrainstone grainstone develops bend around around silicified silicified areas. areas. The compaction compaction developsaa wavy wavy lamination laminationwhere where the the parallel laminae bend deformed but can be deformed laminae laminae are similar similar in inappearance appearance to tohummocky hummocky cross-silicification, cross-silicification, but be distinguished distinguishedby by the thepresence presence of of low-angle low-angleerosive erosive truncations truncationsin in the latter. Silicification is very verycommon. common. One bed bed near near the the base base of of Silicificationof of layers layers and patches within layers is the silicification, culminating, culminating, at at its its top top in the measured measured section section exhibits exhibitsaa marked upward increase in silicification, aa layer layer of of massive massive chert. chert. Another Anotherhighly highlysilicified silicifiedzone zonedirectly directlyunderlies underlies the the stromatolite stromatolitebearing bearing horizon. horizon. Silicified Silicifiedlaminae laminaepresent presentin inother other beds beds have have sharp, sharp, flat top surfaces and sharp irregular bottom bottomsurfaces. surfaces.Water Waterescape escapestructures, structures,common commonthroughout thoughout the the hummocky hummocky cross-stratified cross-stratifiedunits, units, sometimes brittlelydeform deformthese thesesilicifled silicifiedlayers, layers, forming forming cross-cutting cross-cutting carbonate carbonate dikes dikes which rotate sometimesbrittlely plug-like, plug-like,angular angularchert chertblocks blocksto toone oneside. side. �Figure 16. of1 the stromatolitic sequence on Current Current Figure 16. Stratigraphic Stratigraphic sections sections at at varying vafying scales scales 0 the stromatolztzc sequence on River. LEGEND L EGEND I: m Limestone I 90 - Dolomite Dolomite ____ wavy Lamination Hum mocky Cross Cross -Hummocky Stratification Stratification El V / 1 I F / F F / I F F / F so - II, __________ Section A Intrabasinal lntrabasinal Rip—up Clasts Rip-up Clasts Escape Fluid Escape Fluid Structures structures 0 ARCHEAN BASEMENT SECTION B SECTION SECTION A SECTION m I F F Pyrite Lenses % Lenses ___ Pyrite .tr I / / / I / 1' / / 1 I I 1 / / II / I / / I F Hemispherical Hemispherical ___ .Stromatdi+es Stromatolites :° I I 60 - Chert Light/Dark Chert LightIDark e0 I / Parallel to rj Parallel Wavy Lamination Co I F = Interbedded flTInterbedded Siltstone and Siltstone and Shale Shale t1 ''I'll - — — — -JROVE FORMATION -- m m 12 — I--- . -- - - -- - - -I - 7' i/i .9 — 6 17 / ., '.-1 //I ,—. / •6 3 .3 0 0 A PAP SpA P A p PAPA PA P A P A ASASASSA AS P �Microbial Bioherms Bioherm The base of of the thebioherm biohem bearing bearing layers layers isiscomposed composed of ofparallel parallellaminated laminated carbonate carbonate containing isolated isolated centimetre centimetre to todecimetre-scale decimetre-scale (maximum (maximum grainstone, wavy silicified in patches, containiig 20cm 20 cmhigh high xx 50 50 cm cm wide) wide) domal domal stromatolites. stromatolites. They They represent represent the the SH-C/LLH-C SH-CLLH-C type of Logan et aL mounds. Some 01. (1964). Grainstone Grainstonelaminae laminae are are banked banked up against the sides of the microbial mounds. Some mounds are buried by the grainstone while others, after being partially covered, re-established by the grainstone while others, after being p d a l l y covered, re-established themselves. stromatolites begin themselves. The Theamount amountof ofgrainstone grainstonedecreases decreasesupward upward and and the cabbage sized stromatolites laterally merging. This trend resulted in the development of up to 1 m high, sometimes overlapping, This trend resulted in the development of up to 1mhigh, sometimes overlapping, microbial mounds scattered oover bottom. In v e ~the bottom. In areas areas where where early stromatolites stromatolites flourished raised platforms were built (up to 0.5 m above the surrounding bottom) on which large, coalescing. coalescing, domal domal platforms were built (up mounds grew, grew, giving giving aa total total relief reliefabove abovethe the adjacent adjacentarea area of of well well over over1I m. m. All stromatolites stromatolites have internal structures poorly preserved. presewed. have been extensively silicified silicified and internal structuresare are poorly Lamination in the strornatolites, when visible, is caused by the alternation of light and dark stromatolites, caused by the alternation of light and dark chert chert bands. bands. Pyrite layers and irregular blobs are present in the mounds, in places forming up to 20% of the rock. A A stratiform, stratifom,discontinuous discontinuouspyrite pyritelayer layeralso also drapes drapes some some mounds; the larger ones. Lenses Lenses in this unit attain attain thicknesses thicknesses up to to 10 I0 cm. Siliciclastics Siliciclastic, microbial biohems. bioherms. Centimetre-scale beds Siliciclastic, fine f i e grained Centimetre-scale graded graded beds -grained rocks drape the microbial comprising this comprising this unit unit are are internally internally composed composed of of alternating altemating matrix-rich and and matrix-poor, millimetremillimetrescale laminae. The angular qtmtz quartz Thematrix-rich matrix-richlayers layerscontain containfine fine grained, grained, sand-sized sand-sized clasts clasts of very angular and orthoclase containing orthoclase floating floating in aa matrix m&ix composed composed of 02very very fine finegrained graineddark darkmaterial mat-erial-cont&ng disseminated pyrite and 1I to 2% non-carbonate carbon. Matrix-poor layers are composed 2% non-carbonate Matrix-poor layers are composedof of very very (maximum size size 0.1 0.1 mm). mm). The The angular quartz, orthoclase, biotite and muscovite in clast support (maximum thinly bedded rocks are overlain by an indistinctly laminated unit which fills in hollows between thinly bedded rocks are overlain by an indistinctly laminated unit which fills in hollows between the the stromatolite stromatolite mounds. mounds. It is aapoorly poorly sorted, sorted,matrix matrix supported supported silty silty shale shale with bedding bedding parallel muscovite flakes muscovite flakes up up to to 55 mm in length. Shallow, Shallow,saucer saucershaped shapedscours scoursoccur occur in in its its upper upper portion. portion. The scours carbonate grainstone laminae thick. The scours are overlain overlain by carbonate laminae up to 1 cm thick. The grainstone grainstone unit unit is is 10 10 cm in thickness and is interbedded with and overlain by well laminated silty shale similar to the silty similar to thickness and is interbedded lower lower unit. unit. DEPOSITIONAL ENVIRONMENT DEPOSITIONAL ENVIRONMENT AND HISTORY HISTORY Sta2e A -- Storm Dominated Sheq Stage Shelf The presence presence of hummocky hummocky cross-stratified cross-stratifiedcarbonate carbonate grainstone grainstone with with the the absence absence of of wavewaveripple weather wave-base wave-base ripple reworked reworkedtops topsindicates indicatesthat thatthe thesand sandblankets blankets were were deposited depositedbetween between fair fair weather and stom storm wave-base in arelatively a relatively silicicIastic-free siliciclastic-free environment environment (Figwe (Figure 17). The The carbonate, carbonate,or or its its precursor was likely precipitated, eroded and abraded in shallow water areas and transported into deeper water during storm events. Silicified Silicifiedrip-up rip-up clasts clasts and and crusts cmstsdenote denote that that silicification silicificationwas was syndepositional (as degree of silicification increasing towards the syndepositional (asproposed proposed by by Simonson, Simonson,1986); 1986);the the degree bed indicates indicates that that seawater seawater(sensu (sensulato) lato)was wasprobably probablythe thesilicieing siliciing agent. top of the bed �/ - ...: . , .. D. sediments present at Stop 8. Figure Figure17. 17. Developmentalphases DeveIopmentaIphasesofofthe theenvironment environmentrepresented represented by by sedimentspresent atS-. Each text. Each stage stage is is discussed discussed in in the the text. I �- Stare B - Stabilization Siii4fication of Stabilization and Siliczfication Stase of the Substrate If seawater seawater was the the agent agent causing causing alteration alteration of of the the carbonates carbonatesthen then silicification silicification should should increase with length of time the unit was positioned close to the water-sediment interface. increase with length of time the unit was positioned close to the water-sediment interface. This This scenario implies implies that that the the stromatolites formed by by alteration of scenario the massive massive chert chert underlying underlying the stromatolites formed alteration of carbonates during an interval of lowered deposition rate. This is best explained by increasing the carbonates during an interval of lowered deposition rate. This is best explained by increasing the water depth depth so so that that the the bottom bottom sinks sinks below below storm storm wave-base wave-base and and slower slower deposition deposition rates rates ensue. ensue. - Stage C Stase C - Stromatolite Stromatolite Development Development Stromatolites grew on the silicified, silicified, stabilized stabilized substrate, substrate, forming forming inverted-bowl shaped mounds on the Some mounds the bottom. bottom. The lack of of wave wave agitation agitation and and firm firm substrate substrate proved proved ideal. ideal. Some stromatolites were occasionally buried by grainstone laminae brought in by seaward flowing bottom stromatoliteswere occasionally buried grainstone laminae seaward bottom currents generated during storm events. storm Stage D D --Large Large Microbial Bioherm Stase Bwherm Growth Growth Individual stromatolite Individual stromatolitecolonies coloniescoalesced coalesced upwards, upwards, forming forminglarge large mound mound shaped shapedstructures structures irregular bottom bottom topography. topography. Little and creating an irregular Little sediment sediment accumulated accumulated between between the stromatolite stromatolite mounds mounds as as storm storm events events became became less less capable capableof of supplying supplyingdetritus detritusto to the the area. area. - State E - Volcaniclastic VolcaniclasticBurial Growth of of the bioherms was abruptly terminatedby bythe the deposition depositionofof aa series Growth bioherms was abruptly terminated series of volcaniclastic sediment blankets. The Thedraping drapingof of these these blankets blankets over the mounds rather than their than sedimentation from bottom ponding in intermound intermound areas indicates indicates rain out from above rather than currents. currents. feldspars) and and sorting sorting (large The composition (fresh detrital biotite), angularity (subhedral feldspars) well formed crystals floating in a dark fine grained groundmass) all point to little or no reworking floating a dark fine grained of pyroclastic pyroclastic material. asrain rainout outfrom fromash ash landing landingon on material. The Thelayers layersmay mayhave havebeen beendeposited depositeddirectly directlyas the water's surface or as storm deposited layers. The latter is preferred, as the microlaminations can surface or as storm The latter is microlaminations can then be explained explained by fluctuations fluctuationsin in the the energy energy level. level. The Thepresence presenceof of significant significantquantities quantitiesof ofnonnoncarbonate carbon in the groundmass of these rapidly deposited units units may may reflect reflect the the settling settling of of the the volcaniclastics a composition similar siinilarto vegetable soup. soup. The occasional occasional volcaniclasticsthrough throughaa sea sea with acomposition to microbial vegetable large storm event eroded saucer shaped scours into the the fine fine grained grained material material and andtransported transported minor minor amounts of carbonate amounts carbonate in in from from the the shore-proximal shore-proximal zone. zone. Lateral Implications As water depth depth increased, increased, the bottom changed changed from from a wave dominated dominated condition condition to to aa zone zone sedimentation and of slow sedimentation and thus pervasive silicification. silicification.This Thisled ledto to stabilization stabilizationof of the the sediment sedimentand and stromatolites began to develop. With time (increasing water depth) large domal structures grew. With time (increasing water depth) domal structures stromatolites stromatolitic zone of the Gunflint Formation by This succession succession was was brought brought to an end in the upper stromatolitic pyroclastic eruptions eruptionsand and volcaniclastic volcaniclasticdeposition depositiondrastically drastically altering altering bottom bottom conditions. conditions. STOP 99 STOP Turn left on Lyon Blvd. Blvd. Then Arundel. Turn nodder Ave Then right on Anmdel. Turn left on Hodder Ave and and drive drive Transcanada. Turn to the Transcanada. Turnright righton onthe theTranscanada Transcanadaand anddrive driveto to the the Pass Pass Lake Laketurnoff. turnoff. Turn right here and proceed till you encounter encounter a large outcrop of contorted contorted ribbon ribbon chert-carbonate on chert-carbonate on your your left. left. �Figure 18. ribbon chedearbonate chert-carbonate layers. B) SStop Chert-carbonate silt to1 ~11. 1. Chert-carbonate A)-.Ston 9. Disrupted ribbon Figure 18. A) to pebbly pebbly grainstone grainstone with rip-ups of volcaniclastic sediment and fingernail-sized volcaniclastic sediment and fingernail-sized, shards. shards. 1 ____ - :' F&: . 1; _ �a- Jrrce —— _____ w L a: r Ia r TTTT 'a — , -a / 't* ;c •- 5'4 '1r * J * a a : s A .1-1 — at' Efl C_ - \ ' - — — * — a- a- I a -, Figure 19. A) internal laminations medium-grained sandstone stormlayer. layer. The Figure A)Close-up Close-upof internal laminations in a medium-grainedsandstonestorm one third third of of the thelayer layercontains containsupperflow upperfiowregime regimeparallel parallellaminations. laminations. The bottom one two thirds thirds Ic hummockycross-stratified cross-stratjfiedwith withlow lowangle angletruncation truncationsurfaces surfaces upper two is hummocky B) Outcrops Outcropsat at Stop Ston 12. 12. clearly visible. B) �Laterally chert and and ankerite ankerite layers layers occur occur in Laterally continuous continuous interlaminated interlaminated chert in this this outcrop outcrop (Figure 1 8A). These layers are intensely folded and disrupted in places. The outcrop outcrop (Figure 18A). These layers are intensely folded and disrupted in places. The provides an excellent opportunity to discuss if the deformation is synsedimentary, provides an excellent opportunity to discuss if the deformation is synsedimentary, why the the ribbon ribbon chert-carbonates preferentially exhibit exhibit this this type type of of deformation deformation and and why chert-carbonates preferentially what type type of depositional environment what of depositional environment is is represented represented by by these these sediments. sediments. STOP 10 STOP 10 Proceed back back to Bay. Drive Proceed to the the Transcanada Transcanada and and turn turn left, left, towards towards Thunder Thunder Bay. Drive approximately 1.8km road on on the the right. right. Turn approximately 1.8km until until you you reach reach the the small small side side road Turn right right onto onto the the dirt dirt road road and and park. park. Volcaniclastic, Volcaniclastic, very very fine-grained fine-grained sandstones, sandstones, siltstones siltstones and and mudstones mudstones occur occur at at this this location. These location. Theseare areprobably probably correlatable the gorge gorge at correlatablewith with the the interval interval examined examined in the Kakabeka Kakabeka Falls. Falls. The Thevery veryfine-grained fine-grainedsandstones sandstonesand and coarse-grained coarse-grained siltstones siltstonesare are ripple ripple laminated laminated with with well well preserved preserved internal internal structure structure (see (see polished polished hand hand samples). samples). Again possible depositional Again the the possible depositionalprocesses processes which which may may have have operated operated to to form form this this unit unit will be discussed in the field. will be discussed in the field. STOP STOP11 11 Turn right onto onto the the Transcanada Transcanada and and proceed proceed towards towards Thunder Thunder Bay. Bay. Turn Turn right Turn right right at at the the access road to the Terry Fox lookout. Park in the parking lot and walk to the large access road to the Terry Fox lookout. Park in the parking lot and walk to the large outcrop outcrop on on the the highway; highway; watch watch out outfor forcars. cars. A the Gunflint Gunflint Formation Formation at at this this location. location. The A large large diabase diabase sill sill overlies overlies the The Gunflint Gunflint grainstones present here contain large rip-up fragments and both small grainstones present here contain large rip-up fragments and both small and and large large sherds of of volcanogenic volcanogenic origin origin(Figure (Figure 18B). l8B). Volcaniclastic present near near Volcaniclastic shales shales are are present sherds the top of the sedimentary this has has been been described described by by some some workers workers as as the top of the sedimentary sequence sequence and and this However, interbedding interbedding of of volcaniclastic the the Gunflint-Rove Gunflint-Rove contact. contact. However, volcaniclastic shales shales and and chemical sediment the upper upper portions portions of of the the Gunflint Gunflint Formation, Formation, and chemical sediment is is common common in in the and if more sediment occurs occurs at at aa higher higher level level this this cannot cannot be be defined defined as as the the if more chemical chemical sediment contact. contact. STOP STOP 12 12 Turn Turn right right on on the the Transcanada Transcanada and and drive drive across across the the bridge bridge over over the the Current CurrentRiver. River. Proceed up hill on the other Proceed up the the hill on the other side. side. A Arock rock cut cutoccurs occursat at the the top top of of the the hill hill and and there there is a dirt dirt pull-in. pull-in. Park is a Park here. here. This grainstone those examined on the the Current Current River This grainstone sequence sequence is is similar similar to to those examined on River (Figure (Figure 19). There may be metre-scale thinning-upwards cycles present here 19). There may be metre-scale thinning-upwards cycles present here hut but they they are are indistinct. indistinct.Thin, Thin, discontinuous discontinuouschert chertlayers layersoccur occurat at the the tops tops of of these these possible possible cycles. cycles. In places this this chert very carbon-rich, carbon-rich, possibly possibly denoting denoting aa slowing slowing of of the the deposition deposition In places chert is is very rate allowing accumulation of organic carbon. This outcrop provides an opportunity rate allowing accumulation of organic carbon. This outcrop provides an opportunity to discuss to discuss some some of of the the possible possible process process which which could could drive drive deposition deposition of of this this type type of of assymetric cycle (Figure 20). assymetric cycle (Figure 20). Turn right Turn left left on on Turn right on on the the Transcanada Transcanada and and proceed proceed to to Oliver Oliver Road. Road. Turn Oliver Road Road and and turn turn right right at at the the road road leading leading to to the the main main entrance entrance of of Lakehead Lakehead Oliver University. University. �Figure modelfor for nonbarred nonbarred portions portions of Figure 20. 20. Generalized Generalized depositional depositional model of the the strand-proximal strand-proximal Gunflint shelf during deposition of upper Gunflint chemical sediments. Gunjlint shelf during deposition of upper Gunflint chemical sediments. Karstified Area Karstified Stromatolites,ij Stromatolites //'.i:;.: I— attcc - .. - .. . C- —r--rr. —. 'C. / 7 .•. Grainstone Reworked ( Reworked Chemical Chemical Sediment)) Sediment C L. Fine — Grained Fine - Grained Chemical Chemical Sediment Sediment �.+. REFERENCES REFERENCES Ainger, T., 1985, Hi., Neugebauer, in Friedman, Neugebauer,AND AND Seilacher, Ainger, 1985, Storni Storm Depositions! Depositional Systems Systems in Friedman, H.J., A., cds.. eds., Lecture Lecture Notes Notes Earth Earth Sciences, Sciences, 3: SpringerSeilacher, A., Verlag, Berlin, 174 p. Verltg. Awrnwik, S.M., S.M., 1976, Gunflint Gunflint stromatolites: stromatolites: microsfossils microsfossilsdistribution distribution in inrelation relationto tostromatolite stromatolitemorphology, morphology,ininWalter, ed., Siromatolites: Stromatolites: Awrawik, Walter, M.R., MR., ed., Elsevier, 11-320. p.331 1-320. E w e r , Amsterdam. Amsterdam,p. 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E .Tectonostrati-hicevolution , Tectonostratigraphic evolutionofthe terranes of the v. 26, 26, p. p. 2145-2158. 2145-2158. tcrranes of the Penokean Penokean orogen: Canadian Canadian Journal Journal of ofEarth EarthSciences, Sciences, v. Lake iron iron bearing bearing district, district,ininAnnual AnnualReport: Report:Ontario OntarioBureau Bureau of ofMines, Mines,V. v. 14, 14, p. p. 254-260. Smith, W.N., W.N., 1905. 1905, Loon Loon Lake AND McSwiggen, P.,P., 1988, Geological map (scale 1:250 000) the Penokean Southwick, DL., D.L.,Morey, Morey,GB., G.B., AND McSwiggen, 1988, Geological map (scale 1250 000)ofof the PenokcanOrogen, Orogen,central centraland andeast-central east-central Minnesota, and accompanying text: Minnesota Geological Geological Survey, Survey. Report Report of of Investigations Investigations R1-37. RI-37. - Southwick, DL., Tectonic Imbrication and Forcdeep the ANDMorey, Morey,GB., G.B.,1991, 1991, Tectonic Imbrication and ForedeepDevelopment Developmentillin thePenokean PenokeanOrogen, Orogen,East-Central East-CentralMinnesota Minnesota- An An Southwick, D.L.,AND Interpretation Based Based on onRegional RegionalGeophysics Geophysicsand andthe theResults Results Test-Drilling:United UnitedStates StatetGeological GeologicalSurvey SurveyBulletin Bulletin 1904-C, 1904-C, 17 17P ofof Test-Drilling: p. Southwick, DL, Ontario: Canadian Canadian Southwick, D L AND , ANDDay, Day,W.C., W.C.,1983, 1983,Geology Geologyand andpetrology petrologyof ofProterozoic Proterozoicmafic maficdikes, dikes,noi'th-central "01th-central Minnesota Minnesota and and western western Ontario: Journal of of Earth v.20, 20, p. p. 622-638. 622-638. Earth Sciences, Sciences, v. Structures and and Stratification Stratification Spearing, D.R., 1976, 1976, Shallow Shallowmarine marinesands, sands, in in Depositional EtepositionalEnvironments Environmentsas asInterpreted Interpretedfrom from Primary Primary Sedimentary Sedimentary Structures Sequences: SEPMShort Short Course2, Course 2,p.p.103-132. 03-132. Sequences: SEPM Stille. P., N., 1986, provenanceof of the theargillites argillites of of the the Gunflint Ounflint Formation AND Clauer, N., 1986, Sm-Nd isochron-age isochron-age and provenance Formationin inOntario, Ontario,Canada: Canada: Geochemica Geochemica Stille, P., AND et Cosmochemica Aeta, v. v.50, 50,p. p. 1141.1146. 1141-1 146. Cosmochemica Acta, DAy., 1986, Deep Seas, Edition: Blackwell. Reading,HG., H.G.,ed., cd..Sedimentary SedimentaryEnvironments Environmentsand andFacies, Facies, Second Second Edition: Blackwelt, Oxford. Oxford, Stow, D.A.V., 1986, DeepClastic Clastic Seas,ininReading, p. 399-444. 399-444. Tanton, Bay District, District, Ontario: Arthu,gad andThunder ThunderCap Cap Map Map Areas, Areas, Thunder Thunder Bay Ontario".Geological Geological Survey Survey of ofCanada Canada Tanton, T.L., T.L., 1931. 1931,Fort Fort William Williamand andPort PortArtIrn, Memoir 167. Memoir 167. Tanton, T.L., T.L., 1923, C. inSummary Summary Report: Report: Geological Geological Survey Survey of of Canada, Canada, Part C, Tanton, 1923, Iron Iron formation formationat at Gravel Gravel Lake, Lake, Thunder Thunder Bay Bay District, District,Ontario, Ontario,in p. 1-75. 1-75. TilIman. R.W., complex, Salt Salt Creek Creekanticline anticline area, area,Powder PowderRiver Riverbasin, basin,Wyoming, Wyoming, AND Martinsen, Martinsen, ItS., R.S..1984, 1984,The TheShannon Shannonshelf-ridge shelf-ridgesandstone sandstone complex. 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W.R., 1976, North America: America: Philosophical of the the Royal Royal 1976, Early Early and and middle middle Proterozoic Proterozoic history historyof ofthe theGreat Great Lakes Lakes area, area, North Philosophical Transactions Transactions of Society of London LondonSeries Series A, A, v. v. 280, 280, P. p. 605-628. 605-628. Van Schmus, Schmus,W.R., W.R.,~AND Biclsford. ME., Proterozoicchronology chronologyand andevolutionofthe evolution of theMidcontinentregion.North Midcontinentregion,North America, America,in inKroner, Kroner,A., A,,ed., cd., ~~B i c K f oM.E., r d , 1981,Proterozoic Precambrian plate tectonics. tectonics, Developments Developments in in Precambrian Precambriangeology: geology:Elsevier, Elsevier,New New York, York, p. 261-296. Precambrian plate Van Hise, CR., 52. 641 AND Leith, Leith, (1K., C.K.,1911, 1911,The Thegeology geologyofofthe theLake LakeSuperior Superiorregion: region: United UnitedStates States Geological Survey Monograph 52, 641 p. p. C.R., AND Walker, RU., AND Punt. Plint, AG.. A.G.,1992, 1992,Wave Waveand andStorm-Dominated Storm-DominatedShallowMarine Shallow MarineSystems, Systems,ininWalker, Walker,RU., R.G.,ANDJame5, AND James,NP., N.P.,eds., eds.,Facies FaciesModels Models Walker, R.G.,AND Response to SeaLevel Sea LevelChange: Change:Geological GeologicalAssociation Associationof of Canada, Canada,Stittsville, Stittsvilte,p. p.219-238. Response to 219-238. �Walker, R.G., 1984, 1984,Shelf Shelfand andShallow ShallowMarine MarineSands Sandsin inWalker, Walker,kG., R.G.,ed., ed.,Facies FaciesModels ModelsSecond Second Edition: Edition:Geological GeologicalAssociation AssociationofofCanada, Canada, Stittsville, p.141-!70. Stiltsville. p.141-170. AND Lovebridge, Wanless, R.K. R.K. AND Lovebridge, W.D., W.D., 1978, 1978,Rubidium-strontium Rubidium-strontiumisotopic isotopicage agestudies, studies,Report2, Report 2,(Canadian (CanadianShield): Shield):Geological GeologicalSurvey SurveyofofCanada Canada Paper 77-14, 77-14, 70 70 p. p. Young, early Proterozoic the nonhem northern Great Great ILakes region,in inMcdans, Medaris,LL.G., ed., Early .oung, G.M., G M ,1983, 1983,Tectono-sedinientaiy Ternno-sedimeniar) history histor> of ofearly Proicrowic rocks of ofthe akcs region. G , cd , Early Proterozoic ProteroxoicGeology Gcolog) of ofthe thc Great Great Lakes Lakes Region: Region Geological Geological Society Socicty of of America America Memoir Memoir160, 160,p.p 15-32. 15-32 �GLACIAL GLACIAL HISTORY AND REGIONAL TILL SAMPLING SAMPLING IN THE THE ARCHEAN SHEBANDO WANGREENSTONE GREENSTONE BE BELT SHEBANDOWAN I Andy Bajc, Ontario Geological Survey , Shebandowan Greenstone Belt, Belt, host host to to aa variety variety of of The Shebandowan precious and andbase basemetal metal deposits, deposits, is is aa bedrock-dominated bedrock-dominated terrain, terrain, typically typically having having thin thin till cover. Till Till sampling sampling techniques and resultant grain and resultant gold grain and other other heayy heavy mineral results will be examined in an exploration context context. �INTRODUCTION INTRODUCTION The Shebandowan Shebandowan greenstone greenstone belt, which is located located immediately north and west of the city of Thunder exploration and mining spanning well over 100 Thunder Bay, has a rich history of mineral exploration 100 years. The belt is host to a variety of precious and base metal mineral deposit types. Past producers The mineral deposit types. include the Inco Inco Limited Shebandowan Shebandowan Ni-Cu-PGE mine, the the North Coldstream Coldstream Cu-Au-Ag Cu-Au-Ag mine mine include and the Ardeen Au mine which which was was northern northernOntario's Ontario's first first gold gold producer. producer. Exploration activity activity within response to the depressed prices of precious precious and within the belt has has currently currently subsided, subsided, largely largely in response base metals and the inability ofjunior mining companies to raise hinds for exploration. metals of junior mining companies to raise funds for exploration. In response to this this recent decline decline in mining mining and mineral mineral exploration exploration activity activity within the belt, the Ontario and drift drift geochemistry geochemistry Ontario Geological Geological Survey Survey initiated a 2 year program of Quaternary geology and to further evaluate potential of of the the Shebandowan belt. belt. This was accomplished evaluate the mineral resource potential by detailed (1:50 (1 :50 000 000 scale) scale) Quaternary geology mapping and regional humus and c-horizon till sampling. sampling. Quaternary Quaternarymapping mapping was was undertaken undertaken to to document document the the distribution distribution and character of the various Quaternary deposits within the study area and to reconstruct various Quaternary deposits within the study area and to reconstruct the the ice ice flow flow history history associated associated with with these these deposits. deposits. This Thisinformation informationisis essential essentialfor forthe thesuccessful successfulimplementation implementationof ofmineral mineral exploration drift prospecting prospectingtechniques techniquesand and surficial surficialgeochemistry. geochemistry. explorationprograms programsutilizing utilizing drift During During the the 1998 1998field field season, season, Quaternary Quaternary mapping mapping and and regional regional humus humus and and c-horizon c-horizon till till sampling sampling were conducted conducted over the eastern half of the belt within within the area area covered by the Sunshine Sunshine (52A/l2) (52N12)and andKakabeka KakabekaFalls Falls (52A/5) (52N5) 1:50 1:50000 000scale scaleNational NationalTopographic Topographic System System (NTS) map sheets sheets (Figure (Figure 1). 1). The Theresults resultsof ofthis thiswork workare aresummarized summarizedin in Ontario OntarioGeological Geological Survey Survey Open File Report (Bajc 1999a). 1999a). Mapping Mappingand andsampling samplingwas wasextended extendedwestward westward into into the the Shebandowan Shebandowan Report 5993 5993 (Bajc (52B/9) (52B19) and and Burchell Lake (52B/10) (52B110) 1:50000 1:50 000scale scaleNTS NTS map map areas areas during during the 1999 1999 field season. An OGS OGS preliminary preliminary map map containing containing information information on on the the regional regional distribution distribution of of gold gold grains grains in in till till within was released during the fall of 1999 1999 (Bajc 1999b). The The complete complete mineralogical mineralogical and within this this area area was geochemical geochemicaldatasets datasetsfor for the the western western half half of of the the belt belt were were released released in in the the spring spring of of 2000 2000 (Bajc (Bajc 2000). 2000). BEDROCK BEDROCK GEOLOGY GEOLOGY A A comprehensive comprehensivesummary summaryof of the the bedrock bedrock geology geology of of the the study study area area is is contained contained in papers by Sutcliffe Sutcliffe (1991), (1991), Williams Williams (1991) (1991) and Williams Williams aetall al.(1991) (1991)as aswell wellas asininnumerous numerousOntario Ontario Geological Geological Survey Survey maps maps and reports. The Thestudy studyarea areacontains contains33 distinct distinct bedrock domains domains (Figure (Figure 2). 2). The Thecentrally-located centrally-locatedWawa WawaSubprovince, Subprovince,within withinwhich whichthe the Shebandowan Shebandowan greenstone greenstone belt occurs, occurs, is is fault-bounded fault-boundedto to the the north north by by metasedimentary metasedimentaryand and felsic felsic intrusive intrusive rocks rocks of of the the Quetico Quetico Subprovince Subprovinceand and unconformably unconformably overlain overlain to to the the south southby by Paleoproterozoic Paleoproterozoicmetasedimentary metasedimentaryrocks rocks of Animikie Group Group(Gunflint (Gunflintand and Rove Roveformations). formations). of the the Animikie Metasedimentary Metasedimentaryrocks rocks of of the the Quetico QueticoSubprovince Subprovinceconsist consistprimarily primarily of of turbiditic turbiditic wackes, wackes, arkoses migmatites. PostQuartzarenites arenites and and their their associated associated paragneisses varaeneisses and miematites. Post- to to arkoses and and quartz - syndepositional syn&msitiorkl felsic felsicplutons plutonsconsisting consistingof offeldspar-megacrystic feldspar-megacrysticgranite, granite, granodiorite granodiorite to to tonalite tonalite and and monzonite monzonite comprise comprise aa notable notable proportion proportion of this domain. Narrow, Narrow, elongate elongatelenses lensesof of tholeiitic, tholeiitic, �PROTEROZOIC Jacobsville Gp.- sandstone, shale, wnglomerate [_\j Keweenawan mafic intrusive rocks Sibley Gp.-conglomerate,sandstone,shale I Animikie Gp.-wacke, shale, iron formation, limestone, minor volcanics Oskr Gp.- basalt, wn@omerate, a h s e a ARCHEAN Fekic lntmsbe ro& 0 Queliw Subpvinm mtasedimnta~yrocks UI tig goon Subpvince supracrustal rocks Wawa Subprovince supramstat rocks [S3 1998 Survey Area 19gg suwey Area Figure 1. Figure 1.Location Locationmap mapofofstudy studyarea areaincluding includingregional regionalbedrock bedrockgeology. geology. mafic metavolcanic the Quetico metavolcanic rocks and associated gabbroic rocks occur locally within the Subprovince (Brown 1995, 1995, Osmani Osmani 1997). 1997). Metasedimentary rocks of the Gunflint Gunflint and Rove formations (Animikie Group) were deposited Metasedimentary Minnesota, Wisconsin Wisconsin and and Michigan Michigan(Sutcliffe (Suteliffe 1991). 1991). The within a large basin that extends into Minnesota, Gunflint Formation unconformably on the Archean Formation contains a basal unit of conglomerates conglomerates that rest unconfommbly jasper, basement and are overlain by interbedded argillites, argillite tuffs, cherts, algal cherts, jasper, carbonates, hematite, hematite, magnetite magnetitetaconite taconiteand andsilicate silicate(aconite. taconite. The carbonates, ferruginous carbonates, consists of black, locally pyritic shales and grades upward upward into into overlying Rove Formation consists interbedded black shale and arkosic greywackes. greywackes. Mesoproterozoic "Logan" diabase sills intrude the the Mesoproterowic diabase sills intrude resistant cap rocks of large mesas in the southern part of the map Rove Formation Formation and form the resistant area. Silver Silvermineralization mineralizationoccurs occurslocally locally along along normal normal faults faults that intersect the contact between Rove Formation shales and Logan diabase diabase sills. sills. Notable silver silver deposits deposits occur in the southeastern corner of the Kakabeka Falls NTS map area, in the vicinity of Badger and Beaver mountains mountains comer Mountain group group of of deposits) deposits) and and Silver SilverMountain. Mountain. Accessory minerals minerals in in these these (Creswel or Rabbit Mountain calcite; quartz; quartz; barite; barite; and and fluorite fluorite deposits include: acanthite; pyrite; sphalerite; marcasite; galena; calcite; (Franklin eta/. 1986). et at 1986). �Figure 2. Bedrock geology of the study area, Archean metasedimentary rocks (Wawa Subprovince) Aichean mafic intruswe rocks Archean felsic4ntennediate intrusive rocks Archean meta volcanic rocks * CmsweuRabbit Mtn. Ag deposits Snodgrass Lake Au deposit Shebandowan Ni-Cu-POE mine North Coidsiream Cu-Au mine Animikie Op. Metasedimentaiy rocks Archean metasedirnentary rocks (Quetici, Subprovince) Ardeen Au mine Keweenawan malic intrusive rocks �can be be subdivided into into 2 contrasting Supracrustal rocks of the Shebandowan greenstone belt can packages of metavolcanic metasedimentary rocks: 1) metavolcanic and metasedimentary 1) an older suite of mafic to felsic metavolcanic metavolcanic rocks rocks belonging to the Greenwater Greenwater assemblage; and 2) a younger suite of metasedimentary to the the Shebandowan Shebandowan assemblage. assemblage. The metasedimentary and metavolcanic rocks belonging to Greenwater assemblage consists primarily of tholeiitic basalts with intercalated intermediate to Greenwater assemblage felsic and Stott 1998). Mineralization felsic metavolcanic metavolcanic rocks and minor komatiites (Corfu and Mineralization at the Shebandowan, North Coldstream and Ardeen mines are all contained within Greenwater assemblage Ni-Cu-Pt-Co mineralization mineralizationatatthe theShebandowan Shebandowanmine mineisishosted hosted by by aa gabbrogabbroassemblage rocks. Ni-Cu-N-Co peridotite sill. Cu-Au-Ag mineralization at the North Coldstream mine is hosted by a highly peridotite Cu-Au-Ag mineralization at the North Coldstream mine is hosted silicified silicified gabbro. Gold Goldmineralization mineralizationatatthe the Ardeen Ardeen mine mine occurs occurs in in composite composite quartz veins developed along the Pete and Ardeen fault systems. Advanced exploration developed Pele exploration in the form of a decline ramp ramp and and bulk sample sample was was undertaken undertaken at the Snodgrass Snodgrass Lake Au prospect in central Moss Township mid-i 980s. Gold Township during the mid-1980s. Gold mineralization mineralizationoccurs occurs in in sheared sheared felsic felsic metavoicanic metavolcanic rocks and in sheared and fractured diorite, gabbro and and quartz-feldspar quartz-feldsparporphyries. porphyries. The Haines gabbroanothosite within the the belt. belt. The intrusion, anothosite complex complex represents one of the larger mafic intrusions within which occurrences. which is is crudely layered, is host to a number of Ni-Cu-PGE occurrences. The overlies (i.e., (i.e., fault contact) and The younger, Shebandowan Shebandowan assemblage assemblage both unconformably overlies intrudes intrudes the Greenwater Greenwater assemblage. ItIt consists consists of of alkalic alkalic metavolcanic metavolcanic and intrusive rocks (tuff, breccia, breccia, syenite, syenite, lamprophyre, lamprophyre, quartz quartz and and feldspar feldspar porphyry, granodiorite and diorite) as well as metasedimentary metasedimentaryrocks rocks (arkoses, (arkoses, wackes, conglomerates conglomeratesand oxide-facies oxide-facies iron iron formation) formation) at 1998). This assemblage is interpreted as representing an intracratonic (Schneiders ci' This assemblage is interpreted as representing an intracratonicbasin basin (Schneiders et al. assemblage assemblagesimilar similarto to the the Timiskaming Timiskamingassemblage assemblageof of the the Abitibi Abitibi subprovince. subprovince. Most Most gold gold occurences occurencesin in the the eastern eastern portion portion of of the the Shebandowan Shebandowangreenstone greenstone belt display display a close close spatial association with rocks of the Shebandowan assemblage. The style of gold mineralization is spatial association The style of not unlike and Kirkland Lake. Lake. Gold i m m i nand i I&kland unlike that which occurs in such such prolific mining camps as ~Tinimins mineralization mineralizationisiscommonly commonly associated associatedwith with both both large large scale scale and and subsidiary subsidiarystructures structures and structurally-controlled en echelon echelon quartz veins, structurally-controlledintrusions. intrusions. Mineralization Mineralizationoccurs occursin in sheeted sheetedand and en stockwork stockworkzones zones and and vein vein breccias breccias as as well well as as in in stocks, stocks,dikes dikes and and sills sillsof of felsic felsicto to mafic mafic composition. composition. Gold Goldisisalso alsocommonly commonlyassociated associatedwith: with: sulphide-rich, sulphide-rich,quartz-veined quartz-veined replacement replacement zones zonesin in oxide oxidefacies faciesiron iron formation; formation;silica-flooded, silica-flooded,hydrothermally-altered hydrothermally-altered zones; and and ironcarbonate with shear shear and and permeable permeablezones. zones. Associated carbonate and and pyrite-rich zones commonly associated with alteration alteration and and accessory accessory minerals minerals thclude: include: iron iron carbonate; carbonate; silica; silica; potassic potassic feldspar; feldspar; sericite; sericite; arsenopyrite; tourmaline; tourmaline; and and green green mica (Schneiders (Schneiders et aL al. 1998). 1998). hematite; chlorite; chlorite; calcite; calcite; pyrite; pyrite; arsenopyrite; hematite; Notable Notable exploration explorationprograms programs within within the the current currentstudy study area area are are highlighted highlighted in the last 55 annual annual volumes volumes of of the Resident Resident Geologist's Geologist's Report Report of of Activities. Activities. as GLACIAL GLACIALGEOLOGY GEOLOGY The The study study area area isis characterized characterizedby by several several contrasting contrasting physiographic physiographic regions, each defined by a unique unique suite suite of of overburden overburden conditions. Most Mostof ofthe theBurchell BurchellLake Lakeand and Shebandowan Shebandowan NTS map areas areas as as well well as as parts parts of of the the Sunshine Sunshineand and Kakabeka Kakabeka Falls Falls NTS NTS map map areas areas are are characterized characterized as as bedrock-dominated drift occur occur in in morainic morainic belts, bedrock-dominated terrains. Local Localaccumulations accumulationsof of thick thick drift glaciofluvial glaciofluvial complexes complexesand and narrow, narrow, structurally-controlled structurally-controlleddepressions depressions within within these these uplands. �Thicker within g. Thicker deposits deposits of of till and/or andlor glaciofluvial glaciofluvial and glaciolacustrine sediments occur within#? isolated isolatedbasins basins(Figure (Figure3). 3). AAnorthern northernlowland, lowland,here herereferred referredtotoasasthe the"Kaministikwia "Kaministikwiabasin", basin", occupies Kaministikwia rivers. AA occupies the the valleys valleys of of the the Oskondaga, Oskondaga, Matawin, Shebandowan and Kaministikwia southern In the the northern northern southern basin is centred centred on the valleys of the Whitefish and Kaministikwia rivers. In basin, basin, thick thick deposits deposits(several (several tens tens of of metres) of of glaciofluvial glaciofluvial and glaciolacustrine glaciolacustrine sediments sediments drape a rugged surface rolling landscape landscapewith with sporadic sporadicbedrock bedrock rugged Archean ~ r c h e bedrock bedrock k surface producing producing aa gently rolling outcrops. The low relief observed within the southern basin is a reflection of not only outcrops. The low relief observed within the southern basin is a reflection of not onlythe theflat-lying flat-lying Proterozoic glaciolacustrine deposits that ~roterizoicsediments sedimentsthat that underlie underlie the the region, but the extremely extremely thick glaciolacustrine have have infilled infilled deep deep valleys valleys along the Whitefish Whitefish and Kaministikwia Kaministikwia rivers. Overburden Overburdenthicknesses thicknesses along along these these valleys valleysexceed exceed 60 60m m in in places. places. The The erosional erosionaland and depositional depositionalfeatures features preserved within the study area record a complex history history of of ice ice flow flow events events associated associated with the Wisconsinan glaciation ( T i m 44).1 The Theentire entirestudy study -daciation (Figure area area was was initially initially affected affected by southward flowing "northern" "northern" ice. Striae striaeassociated associatedwith with this thisevent event generally 220° Az. In In the the southern southernhalf half of of the the Sunshine Sunshinemap map area, area, there there is is generally range between 190° 190Âand 220' widespread evidence for an older ice flow event towards 160° Az. The age and significance of this widespread evidence for an older ice flow event 160" significance event event is is not not known. known. Pauses Pauses in in the the general general retreat retreat of of the the "northern" "northern" ice ice lobe lobe resulted resulted in in the the formation formationof oflarge, large, which traverses arcuate, arcuate, recessional recessional moraines moraines in northwestern Ontario. The TheBrule Brule Creek CreekMoraine, Moraine, Aldina A l d i and Marks Marks townships townships in the west-central portion of the Kakabeka Falls NTS map area, represents represents one one of these stilistand stillstand positions (Figure 4). This Thismoraine morainehas hasbeen beentraced tracedwestward westward into the Shebandowan by aa discontinuous discontinuous belt belt of Shebandowan and and Burchell Lake map areas where it is defined by morainic associated ice-contact ice-contact stratified stratifieddeposits. deposits. West of the stud study morainic ridges composed of till and associated with the Eagle Finlayson Moraine (Zoltai area, the Brule Creek Moraine has been correlated area, the Brule Creek Moraine has been correlated with the Eagle Finlayson Moraine (Zoltai 1965). 1965). A A large large ice-contact delta delta built off of this moraine along the south shore of Burchell Lake is graded to to an an upper upper level level of of glacial Lake Agassiz. The Theupper uppertopset topsetsurface surfaceof of the the delta deltaoccurs occursat at an an elevation as1 (1500-1550 feet aasl). s h To To the east of Marks Township, the Brule elevation of 457 to 472 m asl Creek Creek Moraine Moraine has has been been overridden overridden and obscured obscured by by aayounger younger glacial glacial advance advance (Figure (Figure 4). Isolated deposits of northern provenance and Isolated buried deposits denosits of of ice-contact ice-contact stratified stratified glaciofluvial elaciofluvial dewsits possibly associated and associated with with the the Brule Creek Moraine occur in south-central Conmee A d west-central Oliver Olivertownships. townships. - A second major moraine of northern affinity (Dog Lake Lake Moraine) Moraine) is is situated situated in in the the northeastern northeastern corner 4). Unlike comer of of the Sunshine Sunshine map map area area in the vicinity of Dog Lake (Figure 4). Unlike the Brule Creek Moraine, this moraine was constructed following a minor readvance of the "northern" ice lobe. Moraine, this moraine was following Glaciolacustrine readvance. In In the the vicinity vicinity Glaciolacustrine sediments sediments underlying till north of the moraine support aa readvance. of Dog Lake, the moraine is fronted by a lobate, flat-topped, flat-topped, glaciofluvial glaciofluvial plain plain believed believed to to be be deltaic deltaic in origin. The Theelevation elevationof ofthis thisplain plain(460 (460m masl) as]) defines definesthe the level level of of Glacial GlacialLake Lake Kaministikwia, a large large glacial lake which occupied the Kaministikwia basin. Distinctive Distinctivered red clays, clays, associated with this to, but not above, this this glacial lake can be found throughout the basin up to, elevation. �IA r— I' - - r- — C— Figure 3. Digital elevation model of the Shebandowan greenstone belt and surrounding region. ,1 c— r-— C —- — - — -- r - �f(i ij Figure4.4. Ice Ice flow flowpatterns, patterns, major major moraines moraines and and surface surface tills tills of of the the study study area. area. Onlapping Onlappingpattern pattern of ofarrows arrowsindicates indicates age age relationships relationships Figure of ice flow events. Light grey zone indicates area where older, northern till exposed on surface. Dark grey zone indicates of ice flow events. Light grey zone indicates area where older, northern till exposed on surface. Dark grey zone indicates area affected by Marquette readvance (southern area by Superior lobe; northern area by northern lobe). area affected by Marquette readvance (southern area by Superior lobe; northern area by northern lobe). �Kaministikwia was was topographically-supported topographically-supportedon on its its southern southernflanks flanksby by an anupland upland Glacial Lake Kaministikwia in the northwestern corner of the Kakabeka Falls NTS area area (Figure (Figure 3) 3) as as well as as by glacial glacial ice ice advancing out of the Superior basin and whose terminal position is defined by the Marks moraine TheMarks Marksand andDog DogLake Lake moraines moraines are are correlative correlative and and merge merge east east of of the the study study area area (Figure 4). The Striaeassociated associatedwith withthe theadvance advanceof of the the Superior Superiorlobe lobe along the McKenzie interlobate Moraine. Striae range from from 3400 340ÂAz in the northeast to 240 240° Az in the southwest attesting to the lobate nature of the Thisadvance advanceisiscorrelated correlatedwith withthe theMarquette Marquettestadial, stadial,aalate-glacial late-glacial surge surgeevent event that that Superior lobe. This resulted in much of the Superior basin being reoccupied by glacial ice about 9.9 ka With the the ka BP. BP. With corner of the Kakabeka Falls area, the Superior lobe advanced into a exception of the northeast comer glacial lake, referred to as "Glacial Lake OtConnortt Numerous sections sectionsalong along the the O'Connor" (Zoltai (Zoltai 1963). Numerous Whitefish River expose thick sequences of glaciolacustrine rhythmites beneath a fine-textured Superior lobe till. Shorelines Shorelinesof ofthis thisglacial glaciallake lakehave havenot notbeen beenidentified. identified. Superior (Patrician) provenance has been observed throughout the the entire study area. area. On Till of northern (Patrician) On less than than aa few metres metres Archean terrane, beyond the limits of the Superior lobe, the till is generally less thick and has a silty sand texture with low to to moderate moderate stone-content. stone-content. Till cover is significantly more extensive thicknesses of 5 extensive over over the east-central part of the Shebandowan Shebandowan NTS map area where thicknesses to 10 m m are are not not uncommon. uncommon. Unoxidized till generally has an olive-grey olive-grey to to buff-grey buff-greycolour. colour. The contains a high proportion of clasts clasts that are locally-derived. Faceted Faceted and andstriated striatedclasts clasts till often contains within the till indicate deposition by ice flowing towards towards the the south-southwest. south-southwest. Many exposures of this northern till contain remnant clasts Sibley Group metasediments. These Thesedistinctive, distinctive,reddishreddishclasts of Sibley coloured siltstones occur most commonly east of Thunder Bay on the Sibley Peninsula and on the Peninsula and to to aa Nipigon basin. basin. Sibley Sibley Group Group clasts clasts in in northern northern lesser extent, northeast of the study area in the Lake Nipigon till are likely derived from the Nipigon basin, 70 km to the northeast. The Thenon-magnetic non-magneticheavy heavy mineral concentrates concentrates of northern tills are rich in clinopyroxenes derived from Nipigon diabase within this same same area. area. Northern till is buried beneath variable thicknesses of glaciolacustrine deposits deposits and Superior Superior lobe till within the Whitefish-Kaministikwia Whitefish-Kaministikwia lowland. lowland. InInthis thisarea, area,the thetill tillisissignificantly significantlyfinerfinertextured owing owing its fineness fineness to the underlying shales shales and argillites argillites of the Rove and Gunflint formations. formations. Stony Stonytill, till,of ofpresumed presumednorthern northernprovenance, provenance,exposed exposedatatwater waterlevel levelalong alongWhitewood Whitewood Creek in east-central O'Connor Township, contains abundant striated and faceted boulders boulders indicating ice ice flow flow toward toward the the southwest. southwest. Superior Lobe till is easily recognized recognized by its fine-texture fine-texture and and high high proportion proportion of of Animikie Animikie Group Group metasedimentary clasts in the pebble fraction. Even Evenin inareas areasunderlain underlainby by Archean Archean terrane, terrane, as occur in the northeast corner comer of the Kakabeka Falls area, Superior lobe till contains only a small proportion of the underlying rock rock types types in in the the pebble pebble fraction. fraction. This has important implications for programs of mineral exploration utilizing till as a sample medium. medium. The The colour of the unweathered till ranges from dark grey, to brown to reddish-brown. Sections Sectionsof oftill, till,containing containinglayers layerswith with all all 33 reflect the the melting melting out out of of colours have been observed in Oliver and Gillies townships and probably reflect debris bands derived from different bedrock sources. Superior Superiorlobe lobetill till also alsovaries varies markedly markedly in in texture. In InOliver OliverTownship, Township,near nearthe thecommunity communityof ofMurillo, Murillo,Superior Superiorlobe lobetill tillisissilty siltyand andcharged charged with shales shales of the Gunflint Formation. The The till till in in this this region region is is fluted fluted and has been observed to reach thicknesses thicknesses of 7 to 8 m. Similarly, Similarly,to to the the south south in in Scoble Scoble and Gillies Gillies townships, Superior �Lobe till is silty and charged with clasts of Rove Formation shale and Logan diabase. diabase. Along Along the the Whitefish contains less less than than 5% 5% clasts. clasts. This Whitefish River valley, the till is silty to clayey and contains This is is primarily primarily due due to to the the thick thick deposits depositsof ofglaciolacustrine glaciolacustrinesilts siltsand andclays claysthat that were overridden overridden and and incorporated incorporated into the basal debris layers of the advancing Superior lobe. into the basal debris layers of the advancing Superior lobe. OVERVIEW OVERVIEWOF OFFIELDTRIP FIELDTRIPAGENDA AGENDA The landforms, landforms, deposits depositsand and features featureswe we will will be be visiting visitingtoday today will will provide provide an an overview overviewof of the the The important belt. An important elements elements of of the Quaternary Quaternary geology of the Shebandowan greenstone belt. An attempt attempt will willbe be made madeto toillustrate illustratehow howthe thevarious variousQuaternary Quaternarydeposits depositscan canbe beutilized utilizedfor forprograms programs of of mineral mineral exploration. Figure Figure55 shows showsthe the locations locations of of the the fieldtrip fieldtrip stops. The Thefieldtrip fieldtripwill willbegin begin in in the anderosional erosional thecentral centralpart part of ofthe thebelt beltwhere wherewe wewill willhave havean anopportunity opportunitytotoview viewdeposits depositsand features features associated associated with the advance and retreat of northern ice. Some Sometime timewill willbe be spent spent demonstrating demonstratingsampling samplingtechniques, techniques,discussing discussingthe thepros prosand andcons consof ofvarious varioussample samplemedia, media,and and finally, reviewing strategies for property-scale surficial geochemical surveys. We will proceed finally, reviewing strategies for property-scale We will proceed easterly easterlyinto intothe the centre centreof ofthe thenorthern, northern,Kaministikwia Kaministikwiabasin basinwhere wherewe wewill willhave havean anopportunity opportunitytoto view deposits of glacial Lake Kaministikwia and discuss the glacial lake history of this view deposits of glacial Lake Kaministikwia and discuss the glacial lake history of thisand andother other proglacial proglacial water water bodies bodies within within the the region. region. From Fromhere, here,we wewill willproceed proceedsouth southand andwest westwhere wherewe we will have an opportunity to view deposits and landforms associated with the advancing Superior will have an opportunity to view deposits and landforms associated with the advancing Superior lobe. lobe. Stops Stopswill willinclude: include:1)1)aasection sectioncut cutinto intoice-marginal, ice-marginal,subaquatic subaquaticdebris debrisflows flowsdeposited depositednear near the the outer outer limit limitof of the the Superior Superiorlobe; lobe; 2) 2) aa Superior Superiorlobe lobe ice-contact ice-contact delta delta which which has has been overridden overridden by by the the advancing advancingSuperior Superiorlobe; lobe;3) 3) an anexposure exposureof ofSuperior Superiorlobe lobetill tilloverlying overlyingnorthern northerntill; till;4) 4)aa panoramic panoramicview viewof ofthe thelowland lowlandinto intowhich whichthe theSuperior Superiorlobe lobeadvanced; advanced;and and5)5 )aastop stoptotoview viewthe the morphology morphologyof of the the Marks Marks Moraine. Moraine. At Atseveral severalofofthese thesestops, stops,we wewill willhave havean anopportunity opportunityto to discuss discussthe thesuitability suitabilityof ofSuperior Superiordrift driftas asaasample samplemedium mediumfor forprograms programsofofmineral mineralexploration. exploration. The The trip trip will will conclude concludeby by travelling travellingsouthward southwardinto intothe theWhitefish WhitefishRiver Riverlowland lowlandwhere wherewe wewill willlook look at ataa couple coupleof ofexposures exposuresthat thatillustrate illustratethe thedrastic drasticfacies facieschanges changesthat thatoccur occurininboth boththe theSuperior Superiorand and northern northernlobe lobetills tillsas asyou youproceed proceedinto intothe thelowland. lowland.The Thesignificance significanceofofglaciolacustrine glaciolacustrinedeposits deposits exposed exposedatateach eachof ofthe thesites siteswill willbe bediscussed discussedasaswell. well. The proceed westward The bus bus will will depart depart the the Lakehead Lakehead University Universitycampus campus and andproceed westwardalong alongHighway Highway 102 102 towards Kaministikwia where it will meet Highway 11/17 at Sistonens Corners. Turn right onto towards Kaministikwia where it will meet Corners. Turn right onto Highway Highway 11/17 11/17and andproceed proceed west west 16.0 16.0k,ns. kms. Turn Turnleft left on on aasmall small sideroad sideroadsituated situatedapproximately approximately 150 m before before (east (eastof) of)aa sign sign on on the the north northside side of of the the highway highway marking marking "Bylund "BylundPit Pit Road". Road". 150to to 200 200 m Follow Follow the the main mainroute route on on this this sideroad sideroadfor approximately approximately 300 to 400 m. Park Park vehicle vehicle at aa large large area areaofofstripped strippedoutcrop. outcrop.This Thisisisthe theBylundAu Bylund Auoccurrence. occurrence. �Figure 5. Location map of fieldtrip stops. S- C S r r— C - S——-- 5-- - - �Stop 1: Bylund Trenches 1: Property-Scale Sampling Programs: Programs: Bylund Regional and property-scale property-scale surficial sampling sampling programs programs have been been widely used in in the the mineral mineral Unfortunately, aa significant significant proportion of of the surveys exploration sector for many years. Unfortunately, surveys Clearly, many many of of the failures failures undertaken within glaciated terrain have met with mixed results. Clearly, some effort been put forth forth in in understanding understanding the the nature nature of of the the could have been avoided had some development of of sampling sampling strategies strategiesand and Quaternary deposits present within a given area prior to the development At this thisstop, stop,discussion discussionwill will focus focuson on the the various various sampling sampling implementation of sampling programs. At media that are available to the explorationist explorationist for for aa surficial surficial geochemical geochemical perspective, perspective, how how they they are are collected and at what density and finally, the analytical techniques employed for their analysis. Trenches exposed adjacent to the Bylund Au occurrence occurrence will be used to demonstrate demonstrate the the various various media and sampling techniques. The Bylund Au property is located at the west end of the Dawson Road Lots and is is jointly operated by Freewest Resources Resources & & Co. Co. Inc. Inc. and and Greater Greater Lenora LenoraResources ResourcesCorp. Corp. Mineralization is contained within a 125 125 m wide, east-trending east-trending zone zone of of carbonate-altered, carbonate-altered, mafic mafic to to ultramafic ultramafic metavolcanic rocks transected by a stockwork stockwork of northeast-trending northeast-trending quartz-carbonate quartz-carbonate veins veins (Schneiders et al. 1998). 1998). Finely Finelydisseminated disseminatedarsenopyrite arsenopyriteis is commonly commonly associated associated with with Theauriferous auriferousdeformation deformationzone zoneoccurs occursatator ornear near the theunconformable unconformablecontact contact mineralization. The between Greenwater assemblage metavolcanics metavolcanics to the south south and and Shebandowan Shebandowanassemblage assemblage metasedimentary rocks to Channel sampling sampling over over the the main main Bylund occurrence occurrence has to the north. north. Channel 14 m. Outcrops Outcropsexposed exposedin intrenches trenchessituated situatedboth both east eastand and returned values of up to 4.1 g/t Au over 14 showing have have returned returned values values typically typically in in the the range range of of 11 to to 22 g/t g/t Au Au over over several several west of the main showing metres. Notable Notableintersections intersectionsinclude includeaa 0.54 0.54m m section, section,just west west of of the the main main showing, showing, assaying assaying 123 123 g/t Au (C. Bishop, Freewest Resources, personal communication, communication, 1998). 1998). The trenches west of the main Bylund occurrence occurrence were opened to investigate investigate Au and As anomalies in soil. They They expose expose aa variety of material types. Two Two facies facies of till have been recognized; buff-grey, stone-poor, silty sand till till of of semi-local semi-local composition composition and and aa stony, stony, silty siltysand sandtill tillof of a light, buff-grey, local composition. The Thelower, lower,local localtill till contains containsabundant abundantclasts clastsof of mineralized, mineralized, iron-carbonate iron-carbonate altered sericite schist. ItIt isis restricted restricted to to the the up-ice up-ice side side of of aa prominent prominent bedrock bedrock ridge ridge where erosive erosive forces beneath the advancing ice were greatest. Compressive Compressive subglacial subglacial forces forces resulted in the the till till section. This lower, local till being sheared up into the upper parts of the This till till contains contains anomalous concentrations concentrations of gold gold grains grains and appears appears to reflect reflect local local mineralization mineralization along along aa major major break. The Thebuff-grey buff-grey till till reflects reflects semi-local semi-local bedrock bedrock sources sources and and was was probably derived from the melting out of debris bands in the basal portions of the glacier. Red Red glaciolacustrine glaciolacustrine clays clays overlie overlie the till in low-lying ground to the north. Drift Drift thickness thickness diminishes diminishes to to less less than than aa metre metre at at the the crest crest of the hill consists of deeply deeply weathered weathered till. till. Soil samples collected for mineral exploration exploration bill and consists from this area returned anomalous Au and As values. The The bedrock bedrock surface surface drops drops off off abruptly abruptly to the south and is overlain by deposits of different character. Stratified Stratified deposits depositsare are intimately intimately associated associated with the till and were probably deposited in a leeside cavity beneath beneath the the glacier. glacier. These deposits deposits have been modified by glaciolacustrine processes. A A thin thin layer layer of of red clay overlies overlies the till package gravel. This and is capped by nearshore deposits of sand and gravel. This example example clearly clearly demonstrates the variabilty that that can can occur occurover oversuch suchaashort shortdistance. distance. Clearly, Clearly, an understanding of complexity and variabilty the origin of samples samples collected is required required for for the the successful successful interpretation interpretation of of geochemical geochemicaldatasets. datasets. �Sample Media Four distinct media types are commonly sampled for programs of mineral exploration utilizing These include: include: 1) 1) vegetation; 3) b-horizon b-horizon soil; and 4) surficial geochemistry. These vegetation; 2) 4) c-horizon c-horizon humus; 3) 2) humus; till. till. Vegetation Vegetation consists of the leaves, needles, twigs, twigs, cones cones or or bark bark of ofplants. plants. The premise behind behind sampling plant tissue for mineral exploration exploration is that the root systems of the plants will absorb metals contained in groundwater, overburden and bedrock at a given sample site. The The geochemical geochemical response of the plant tissue will therefore provide information on the chemistry of the the substrate on which the plant is growing. Vegetation Vegetationsampling samplingisisbest best utilized utilized in interrains terrainswhere where overburden overburdencover cover is thin (less than 3-4 m) and locally locally derived derived and and where where root root systems systems are are able able to to penetrate penetrate fractures fractures and joints Inareas areasof of thick, thick, exotic exoticoverburden, overburden,vegetation vegetation sampling samplingmay may fail fail to to evaluate evaluate joints in bedrock. In local mineral potential. For Foraaregional regionalor orproperty-scale property-scale sampling sampling program, program, itit is is important important to to collect collect samples of the same organ from plants plants of of the the same same species, species, size size and and outer outer appearance. appearance. Humus is a black to brownish-black, organic-rich material derived from the decomposition of forest lifter. litter. It consists of unidentifiable remains of leaves, roots, needles, bark, etc. that that have have been been Humus sampling sampling broken down into a fine-grained, amorphous material by by organisms and and bacteria. bacteria. Humus is, in many respects, similar to that of vegetation sampling. Being Being aa first first derivative derivative of of vegetation, vegetation, geochemical results geochemical The same same basic basic rules apply to humus resultsshould shouldmimick mimick those of vegetation. The sampling as well. Drastic changes changes in vegetation type could different well. Drastic could produce humus with different Bothhumus humusand andvegetation vegetationare aresusceptible susceptibleto tocontamination contaminationfrom fromanthropogenic anthropogenic compositions. Both 11117 between between the the Shebandowan Shebandowan Nisources of metal. Humus Humussamples samplescollected collectedalong alongHighway Highway 11/17 Cu-PGE mine and the hamlet of Sunshine are strongly contaminated. Hauling Hauling of of Ni-Cu concentrate concentrate in aa corridor corridor aa few few hundered hundered metres metres on on either eitherside sideof of the the highway highway within within along the highway resulted in values are enriched in humus. The Thec-horizon c-horizontill tilldata datawithin withinthis thiscorridor corridorhighlights highlights which metal values the true geochemical geochemical signature signature for for this this area. area. B-horizon soil soil is usually usually the the dark, dark, orange-brown orange-brown material material encountered encounteredat at aa depth depth of of 5-10 5-10 cm cmin in immediately below the humus layer. Often Oftenthere therewill willbe be aathin thinhorizon horizon of ofgreyishgreyishthe soil profile immediately Thislayer layer(Ae) (Ae)has has been been coloured mineral matter (Ae horizon) between the humus and B-horizon. This leached of most metals by the downward percolation of groundwaters rich in humic and fulvic downward percolation of groundwaters rich in humic and fulvicacids acids derived from the decomposition of the overlying organic organic debris. debris. The B-horizon is the the zone of of accumulation accumulation of of clays clays and and iron iron and and manganese manganese oxides oxidesderived derivedfrom fromthe theweathering weatheringof ofunstable unstable mineral grains. They Theyare are excellent excellentscavengers scavengers of metals. B-horizon B-horizonsoils soilscan can be be developed developed on on parent materials of local to non-local composition. For For the successful successful implementation of b-horizon soil sampling surveys, surveys, it is critical that the the parent parent materials materials be be composed composed of of till, till, that that they they have have characteristics and that they be of a local provenance. provenance. ItIt is similar grain size characteristics is also also important important that the till cover not exceed 2 to 3 m in thickness. C-horizon till is the relatively unaltered sediment that underlies the B-horizon. In In well-drained well-drained upland environments, oxidation can extend down several meters meters into into the theprofile. profile. Metals contained within sulphide sulphide minerals in this zone zone of of weathering weathering are generally liberated by oxidation and �hydromorphically dispersed downwards The upper part of the c-horizon will often often be downwards in in the the profile. profile. The enriched in metals compared surveys are compared to to the the b-horizon. b-horizon. C-horizon till sampling surveys are best best undertaken undertaken in areas where till thicknesses are less than The till till will will often often exhibit exhibit aa local local than aa few metres. metres. The signature under these In areas areas where where till cover is thicker, more more expensive expensive sampling sampling these conditions. In methods, such as reverse circulation drilling, are required to obtain till samples from the bedrockloverburdeninterface. interface. bedrock/overburden Analytical Methods explorationutilizing utilizing drift drift prospecting prospecting Analytical methods employed for programs of mineral exploration techniques and surficial geochemistry geochemistry vary depending upon the commodity commodity sought sought and and the sample sample media collected. collected. For the Shebandowan Shebandowan regional sampling sampling survey, 2 samples samples of of screened screened c-horizon c-horizon mm) were collected for analysis. A A 10 10kg kg sample sample was was collected collected for for heavy mineral till (-5 mm) concentration and subsequent subsequent gold grain, kimberlite indicator indicator mineral mineral (KIM) (KIM) and and metamorphic/magmatic massive sulphide indicator mineral Heavy mineral (MMSIM) determinations. determinations. Heavy minerals were separated from the -2 mm fraction of these these samples samples by wet gravity gravity tabling, producing producing a "table concentrate", then further concentrated to >3.2 B3.2 S.G. using density-dependent density-dependent settling settling in in methylene iodide. Magnetic Magneticminerals minerals were were removed removed from from the the methylene iodide heavy mineral concentrate (HMC) using an automagnet. automagnet. At the tabling stage, a preliminary count of gold grains, including size size and shape shape determinations determinations Thegold goldgrain graincontent contentof ofthe thetable tableconcentrates concentrateswas was further further refined refined using using aa was undertaken. The secondary panning samples containing greater panning procedure procedure to to obtain obtain more more exact exact grain grain counts. counts. Only samples 10 gold grains at the tabling stage were panned. The Thesize sizeand and shape shape of of the the panned panned gold gold grains grains than 10 table concentrate. concentrate. were recorded before they were returned to the table A second 2 to 3 kg sample of screened screened c-horizon till was was also also collected collected for for -63 -63 micron micron fraction fraction geochemical determinations. A A 30 30gg split split of of the the -63 -63 micron micron (silt (silt and and clay) clay) size size fraction fraction of this till (1NAA). The neutron activation activation matrix was sent for instrumental neutron activation analysis (INAA). dataset provides excellent information information on on the the Au, As, As, Sb Sb and and REE REE content of the samples. A second 20 to 25 g split was sent for Pt-Pd-Au determinations by by Pb Pb fire fire assay1ICP-MS assay/ICP-MS finish. finish. A third split was sent for geochemical geochemical determinations determinations by ICP-OES and ICP-MS following following aa standard standard aqua regia digestion. These Thesemethods methods produce produce high high quality quality data data on on the the base metal content of the samples. Similar Similaranalytical analyticalmethods methodsshould shouldbe be employed employedfor for vegetation, vegetation, humus humus and and b-horizon b-horizon sampling sampling programs programs as as well. well. For Forhumus, humus,the the—177 -1 77 im pmfraction fractionisisanalyzed analyzedrather ratherthan thanthe the—63 -63 .tm pm fraction. Humus Humussamples samples are are briquetted briquetted prior to neutron activation analysis. Vegetation Vegetation samples are either macerated and briquetted or ashed prior to analysis. analysis. Sampling Strategies The density at which samples samples are collected collected is dependent dependent upon the the style style of of mineralization mineralization sought. sought. Dispersal trains originating from lode gold type mineralization are significantly smaller than those originating from mineralization are significantly smaller than those originating from volcanogenic massive sulphide-type mineralization. mineralization. The The surface surface area area of mineralized rock available for erosion is probably the the most most important factor that that effects effects the the scale scale of of the dispersal train produced. The Theorientation orientationof ofthe themineralized mineralizedstructure structureor or horizon horizon with with respect respect to to �regional ice ice flow flow is is parallel parallel to to regional ice flow also effects the geometry of the the dispersal trains. trains. IfIf regional the strike of mineralization, the resulting dispersal train will be long, narrow and difficult to identify. IfIf regional regional ice ice flow flowisis perpendicular perpendicular to to the the strike strikeof of mineralization, mineralization, then then the the resulting resulting Theposition positionof ofmineralization mineralizationwith withrespect respectto toregional regionalice iceflow flowisisalso also dispersal train will be wider. The important. For For example, example, ifif the the mineralized mineralized horizon is shear-hosted and situated within a deep ravine perpendicular to ice flow, the advancing glacier may not not actively actively erode erode the the mineralized mineralized rock rock and therefore not produce a dispersal plume from mineralization. Dispersal trains originating from lode gold deposits of northern Ontario are typically less than than 200 to 300 m in length. Sampling Samplingprograms programs aimed aimed at at locating locating these types of deposits should Asample samplespacing spacingof of200 200m mand and therefore be designed to be able to identify a train of this length. A 100 m, staggered on 100 100 m lines, is recommended to recognize a potential target of this preferable 100 Onceaadispersal dispersaltrain trainisisidentified, identified,the thedensity densityatatwhich whichsamples samplesare arecollected collectedshould shouldbe be type. Once increased to to 25 25 m or 50 m spacing to more clearly define the source of the Dispersal increased the gold gold anomaly. anomaly. Dispersal arising from base metal targets can reach several kilometres in length. AAsampling samplingprogram program trains arising aimed at identifying this type of dispersal train could involve collecting samples initially at 500 m spacing then increasing the density to to 100 m in anomalous areas to to further refine refine exploration exploration targets. targets. Bylund Case Case Study High density, property-scale property-scale soil soil sampling sampling was was undertaken undertaken over over and and adjacent adjacentto to the theBylund Bylund Au Au occurrence at the west end of Dawson Road Lots. Fifty-three Fifty-threeb-horizon b-horizon soil soilsamples sampleswere were collected collected at 100 100 m centres centres on 6 grid lines spaced 100 100 m apart. The Theproperty propertyisischaracterized characterizedas asaa bedrockbedrockaccumulations of drift drift (till and stratified deposits) deposits) in low-lying areas. dominated upland with local accumulations Most soils soils are developed in till, however, a small small proportion have stratified sands sands and pebbly sands as their parent material. These These samples sampleswere were included included in in the geochemical dataset since most of them were derived from the washing and sorting of of the the underlying underlyingtill. till. Soil samples were not collected from a poorly drained drained wetland wetland that that occurs occurs along along the the west-central west-centraledge edgeof ofthe the property. property. The sample grid is oriented at 150° 150' Az and extends extends approximately 500 m south of the mineralized zone into unaltered Keewatin mafic metavolcanics. The Thesilt siltand andclay clayfraction fractionof ofthe thesoil soilsamples samples were extracted extracted and and geochemically geochemically analyzed analyzed by instrumental instrumental neutron neutron activation activation analysis analysisand and ICPICPfollowing a standard aqua OES following aqua regia digestion. digestion. Contoured plots of Au and As content derived by neutron activation analysis are presented in Figures 6. Other Otherelements elementsdid did not not show show aa close close spatial spatial association with mineralization. The The patterns depicted by Au and As clearly delineate the position of mineralization with little or no evidence for down-ice down-icedispersal dispersalbeyond beyond aa few few hundred metres. Similar Similarscales scalesof of glacial glacial dispersal dispersal have been previously documented at the Matachewan Consolidated Mine in northeastern Ontario (Bajc 1997) as well as at numerous other gold properties within the province. Gold Gold values values quickly quickly 100 to 200 metres down-ice (south-southwest) decline to below detection limit (<2 ppb) within 100 from the Bylund occurrence. Arsenic Arsenic values values display display aa slightly slightly more more diffuse diffuse pattern. L1W west of of the the documented documented Au Au W and L2W, immediately west Elevated Au and As values along L1 zones suggests a possible possible westerly westerly extension extension of ofmineralization. mineralization. A sample on L1 L1W that returned W that returned aa �1280 ppb and an As value of of 88.4 88.4 ppm was was collected collected from from the the fringes fringesof of aa poorly poorly gold value of 1280 This anomaly anomaly may have been drained wetland and contained significant amounts of organic matter. matter. This A produced by hydromorphic dispersion from from mineralized mineralized till till and and rock rock on on the the upland upland to to the the north. north. A anomalous Au and As response response was was obtained, obtained, however, however, from from an an upland upland soil soil 100 100metres metresto to similar anomalous the east. These These22anomalies anomaliesmay may suggest suggest aa second second auriferous auriferous structure structure is is present to the south; south; this warrants further investigation. A A strong strong Au Au and and As As anomaly anomaly along along the the central portions of lines lines 2E and 3E suggest suggestthe the potential potential for for another, another, as as yet yet undiscovered, undiscovered, zone zone of of mineralization. mineralization. recommendations can be drawn drawn from from the the Bylund Bylund soil soil sampling sampling Several conclusions and recommendations orientation survey. orientation survey. 1. Soil geochemistry is a viable exploration tool for shear-hosted 1. shear-hosted gold gold in in the the eastern eastern portion of the Shebandowan Shebandowan greenstone belt. The Thetechnique techniqueisisbest best suited suitedto to upland upland properties with thin till cover and abundant abundant outcrop. outcrop. 2. AAsample samplespacing spacingofof100 100mmstaggered staggeredon onadjacent adjacentlines linesappears appearsto to be be sufficient sufficient for for the the delineation delineation of potential potential mineralized mineralized zones. zones. 3. Gold and arsenic appear to be the elements elements best suited suited for for target delineation delineation at at the the Bylund occurrence. Neutron fraction (-63pm) (-63tm) of Neutron activation activation analysis of the silt and clay fraction of soil provides excellent contrast between background and anomalies. anomalies. 4. 4. There is very little little evidence evidence in in support support of of down-ice down-ice dispersal dispersal of of gold gold greater greater than than 100 100to to 200 m from mineralization. Ultra-high Ultra-highdensity densitysampling sampling(i.e. (i.e.20 20to to 25 25 m m spacing) spacing)would would be required to more more clearly clearly define define glacial glacial dispersal dispersal patterns. patterns. Turn vehicles vehicles around around and proceed proceed back out to the highway. highway. Turn Turn right, right, drive 3.1 km km east along Highway 11/17 and stop at aa rockcut rockcut along alongthe the sides sidesof of the the highway. highway. �A *C) • <2 • <2 Auu mineralization mineralization 5' •<2 • <2 Au (ppb) • <2 • <2 .3 • <2 .3 100 m .3 • • <2 .3 13 • <2 B .1.8 / .2.9 100 m •2O .19 4.6 S /.12 Figure Figure6. 6.Geochemical Geochemicalresponse responseofofb-horizon b-horizonsoils soilsover overand andadjacent adjacentto to the the Bylund Au contoured Au data: B: contoured occurrence, Dawson Road Lots. A: occurrence, Lots. A: data: B: contoured As As data. data. �Stop 2: Crossing Crossing Striation Striation Site Site At this stop, we will have an opportunity to look at an outcrop outcrop of of Shebandowan Shebandowanassemblage assemblage The striation striation metasediments that are well striated and flanked by red, glaciolacustrine clay clay and and till. till. The displays a consistent consistent pattern pattern of of flow flow towards towards the the south south record for the western half of the belt displays There is, is, however, however, evidence evidence for an older becoming progressively more westerly westerly towards towards the the west. west. There ice flow event towards the the southeast in the the southwestern comer corner of of the the Sunshine Sunshine map map area. area. The significance of this event is There are are several several examples examples of of southeasterly-oriented southeasterly-oriented (160° (160' is not not known. known. There Az) striae striae crossed by south-southwesterly south-southwesterly (200-210° (200-2 10' Az) striae striae on on the the outcrop outcrop surface surface (Figure (Figure 7). 7). This older older event event is is In most cases, the older striae are preserved on faceted, protected protected surfaces. surfaces. This krn since it has been observed observed at many sites sites spanning spanning aa 30 30 km recognized as regionally significant since The till till overlying overlying the bedrock surface is distance and is not apparently related to topography. The deposited by ice flowing flowing towards towards the the south-southwest south-southwest (latest (latest ice ice fow fow direction). direction). believed to be deposited on the the southeast southeast end end of of the the roadcut roadcut have have striated striated upper upper surfaces surfaces Faceted boulders within the till on indicating flow towards the south-southwest. This Thisinformation informationisis critical critical for for the the successful successful followfollowanomalies obtained obtained from from this this area. area. up of till anomalies Figure Figure7. 7. Crossing Crossingglacial glacialstriae striaewere werefrequently frequentlyencountered encounteredin in the the Dawson Dawson Road Lots area. In In this example, example, an an older older set set of of striae striaeoriented oriented at at 160° 160' Az Az are are crossed crossed by by aa main main set set oriented oriented at 210° Az. 210' Az. Proceed 11/17 to to Sunshine Sunshine Cross Cross Rd. Rd. Turn left left (north) onto onto sideroad sideroad and Proceed 9.8 9.8km krn east on Highway 11/17 stop by the railway railway tracks. tracks. �Stop 3: Contamination and and Implications for Mineral Exploration 3: Humus Contamination At this stop, we will have an opportunity to discuss humus sampling and potential problems that may arise with contamination. This This site site was chosen for the stop because it is located adjacent to to aa railway siding that was used by 1NCO INCO for for loading loading Ni-Cu Ni-Cu concentrate from the Shebandowan mine onto railway cars. During Duringthe the early early days days of of production production at at the the Shebandowan Shebandowanmine mine site, site, Ni-Cu Ni-Cu concentrate was hauled by dump truck from the mill to the railroad siding at Sunshine, a distance of Thedispersal dispersalof offine fineparticulate particulate suiphides sulphidesby by wind wind from from these these trucks trucks approximately 50 km. The resulted in a strong Ni-Cu geochemical anomaly in humus following the Highway 1117 corridor Highway 111/17 corridor and extending several hundred metres in from the highway. The The Ni-Cu Ni-Cu geochemical geochemical datasets datasetsfor for Sunshine and Kakabeka Kakabeka Falls Falls map areas areas clearly illustrate illustrate this contamination (Figure 8). The The the Sunshine geochemical datasets c-horizon till are are drastically drastically different different from from those those portrayed portrayed by by the the humus humus datasets for c-horizon and reflect the primary geochemical geochemical signature signature associated associated with with local local variations variationsin in bedrock bedrock source source and 7). 7). It would appear appear that there is no signal of contamination within c-horizon rocks (Figures (Figures 66 and rocks till. till. All humus sampling programs should consider the potential for problems associated with contamination. Contamination Contaminationcould couldarise arisefrom fromaavariety varietyof ofsources sourcesincluding including road road dust dust from from local local logging operations, wind-blown tailings dust and airborne fallout from smelters. In In the the Sudbury Sudbury area, there is a strong geochemical signature in humus associated with the smelting operations. The The element associations associations closley mimmick those of the ore mined in Sudbury. Fine Finecolloidal colloidalsulphide sulphide element particles particles are dispersed by wind and quickly oxidize in the surficial environment where they are available for adsorption by humic and falvic fulvic acids acids present present within withinthe thehumus. humus. Should a potential source of contamination be identified within a given area, it is recommended that the explorationist avoid sampling humus and that c-horizon till be considered considered as the preferred sampling medium for surficial geochemical geochemical surveys. surveys. Proceed back back out out to to Highway 11/17. Turn Turnleft left (east) (east)and anddrive drive 3.0 3.0km krn to the intersection intersection of Proceed Highways 11/17 and and 102 102 (Sistonens (Sistonens Corners). Corners).Turn Turnleft leftonto ontoHighway Highway102 102and andproceed proceed7.0 7.0kms krns before turning left into a sand sand and and gravel gravel pit pit owned owned and andoperated operatedby byTowland-Hewitson Towland-Hewitson Construction Ltd. Construction Ltd. �Figure 8. Comparison of humus and c-horizon till geochemical datasets for nickel, Note the strong anthropogenic signature in humus along the Highway 11/17 corridor. This anomaly is attributed to the haulage of Ni-Cu concentrate from the Shebandowan mine to a railway siding near the village of Sunshine. �Stop 4: Glacial Lakes of the Shebandowan Shebandowan Area Portions of of the the Shebandowan Shebandowan study study area were inundated by glacial lakes. These Theseinclude: include:1) 1) Portions glacial Lake Agassiz Agassiz in in the the extreme extreme western western part part of of the the study study area; area; 2) glacial glacial Lake Kaministikwia Kaministikwia glacial and high high level level Superior Superiorbasin basin lakes lakes within the Sunshine Sunshine map area; area; and 3) glacial lakes O'Connor and within Thedeposits depositsof ofthese theseglacial glaciallakes lakes are are considered considered aa Kakabeka Falls map area (Figure 9). The within the Kakabeka hinderance to to programs programs of ofmineral mineralexploration explorationutilizing utilizingsurficial surficialgeochemistry geochemistryininthat thatthey theyconceal conceal hinderance and prevent access to deposits of till till and the the underlying underlying bedrock. bedrock. The composition of glaciolacustrine deposits deposits does does not not closely closely reflect reflect the the composition compositionof ofthe theunderlying underlyingbedrock bedrockand and glaciolacustrine should should therefore not be sampled for mineral exploration purposes. Expensive Expensiveoverburden overburdendrilling drilling techniques must must be be utilized utilized to to sample sample the the glacial glacial tills tills that that underlie underlie these these deposits. deposits. techniques Glacial Glacial Lake Lake Agassiz Agassiz was was the the largest largestof ofseveral severalwater waterbodies bodieswhich whichbordered borderedthe thesouthern southern margin margin of of the the Laurentide Laurentide Ice Sheet Sheet during Late Wisconsinan glaciation. ItItinundated inundatedan anarea areaof of approximately11million million square squarekilometres, kilometres,although althoughatatno noparticular particulartime timedid didititoccupy occupythis thisentire entire approximately area. The Thelake lakeformed formedapproximately approximately12.0 12.0ka kaBP BPwhen whenice iceof ofthe theDes DesMoines MoinesLobe Loberetreated retreatednorth north area. of the the continental continental drainage drainage divide divide in in eastern eastern North Dakota Dakota and northwestern northwestern Minnesota ponding of water water within within the the Red Red River River valley. valley. Progressive Progressiveexpansion expansionofofthe thelake lakeinto intoManitoba, Manitoba,Ontario Ontarioand and parts of of Saskatchewan Saskatchewan occurred occurred with with further further retreat of the ice margin. Glacial GlacialLake LakeAgassiz Agassiz parts extended into into northwestern northwestern Ontario, Ontario, fronting fronting the the retreating retreating ice margin in low-lying areas. Deposits Deposits extended of glacial glacial Lake Lake Agassiz Agassiz occur occur in in the the southwestern southwesterncorner cornerof ofthe theBurchell BurchellLake Lakearea areaalong alongthe the of Obadinaw Obadinaw River River valley. The Thedeposits depositsare aremainly mainlycoarse-textured coarse-texturedwithin withinthis thisregion, region,however, however, varved clays clays have been reported from the banks of Tilly Creek. An Anice-contact ice-contactdelta deltabuilt built off off of ofthe the Brule Creek Creek Moraine Moraine south southof ofBurchell BurchellLake Lakegrades gradesto to an anupper upper level levelof ofLake LakeAgassiz Agassiz(Herman (Herman Brule level) at at 457 457 to 472 m m asl. asl. All Allareas areassouth southofofthis thismoraine moraineand andbelow belowthis thiselevation elevationwould wouldhave have level) been been inundated inundatedby by Lake LakeAgassiz. Agassiz. Glacial Glacial Lake Lake Kaministikwia Kaministikwiaoccupied occupiedthe thevalleys valleysof ofthe theOskondaga, Oskondaga,Matawin, Matawin,Shebandowan Shebandowan and Kaministikwia Kaministikwiarivers riverswithin within the the Sunshine SunshineNTS NTS map maparea. area. The Thelake lakewas wasbordered borderedon onits its and northeastern northeasternand andsoutheastern southeasternflanks flanksby byglacial glacialice iceof ofthe thenorthern northernand andSuperior Superiorlobes, lobes,respectively. respectively. High High ground ground bordered bordered the the lake lake to to the the north, north, west west and and southwest. southwest. The TheDog DogLake Lakeand andMarks Marks moraines morainesmark mark the thepositions positionsofofthe thenorthern northernand andSuperior Superiorlobes, lobes,respectively respectivelywhile whileLake Lake Kaministikwia Anice-contact ice-contactdelta deltaconstructed constructedoff offofofthe theDog DogLake LakeMoraine Morainejust just Kaministikwiawas was in in existence. existence. An south of Dog Lake and whose upper surface occurs at 460 m as! defines the glacial Lake south of Dog Lake and whose upper surface occurs at 460 m as1 defines the glacial Lake Kaministikwia Kaministikwiawater water plane. plane. The Thelake lakeformed formedwithin withinthe thestudy studyarea areaapproximately approximately10.0 10.0ka kaBP BPwhen when the northern and Superior lobes readvanced into the study area during a late-glacial surge event the northern and Superior lobes readvanced into the study area during a late-glacial surge event (Marquette (Marquetteadvance). advance). The Theextent extenttotowhich whichnorthern northernand andSuperior Superiorice iceretreated retreatedprior priortotothis thisevent eventisis not not known. known. Meltwaters Meltwatersinitially initiallydrained drainedfreely freelysouthward southwardoff offofofthe theretreating retreatingnorthern northernice icemargin margin depositing depositingthick thicksequences sequencesof ofglaciofluvial glaciofluvialsands sandsand andgravels gravelswithin withinthe thevalleys valleysofofthe the Kaministikwia Kaministikwiaand and Oskondaga Oskondagarivers. rivers. Progressive Progressiveincision incisionofofthe thefluvial fluvialdeposits depositsoccurred occurredasasbase base levels levelsdropped dropped within within the the lower lowerreaches reaches of of the the valleys. valleys. Thick Thicksections sectionsofofglaciofluvial glaciofluvialsand sandand and gravel gravelare areclearly clearly visible visible from from Highway Highway 102 102within within the the Kaministikwia Kaministikwia river valley. These Thesedeposits deposits are are terraced terraced and and draped draped by by distinctive, distinctive,red red clays clays associated associated with with glacial glacial Lake Kaministikwia. The The red red clays clayswhich whichcan canreach reachthicknesses thicknessesof ofseveral severaltens tensofofmetres, metres,are aretypically typicallyfaintly faintlylaminated laminatednear near the the base base and and become become progressively progressively more more massive massive upwards. upwards. The Thered redpigmentation pigmentationofofthe theclay clayisis �showing the locations of the major moraines and glacial lake basins. Figure 9. Digital elevation model of the Thunder Bay area �derived from Sibley Group metasedimentary source source rocks rocks eroded eroded by bythe the Superior SuperiorLobe. Lobe. Glacial Lake Kaministikwia probably probably lasted no no more more than than aa few few hundred hundred years. years. The lake overflowed westward into the Glacial Lake Agassiz basin via a narrow lowland lowland through through Shebandowan ShebandowanLake. Lake. Red clays within the eastern Lake Agassiz basin attest to this influx influx of of sediment sediment from from Lake Lake GlacialLake LakeAgassiz Agassizmust musthave havefallen fallenfrom fromaa Herman Herman to to aa lower lower level level prior prior to to the the Kaministikwia. Glacial development Kaministikwia. development of Lake Kaministikwia. occupied the the valleys valleys of the Whitefish Whitefish and Kaministikwia Kaministikwia rivers within Glacial Lake 0'Connor O'Connor occupied the Kakabeka Falls map area. The Thelake lakefronted frontedthe the advancing advancingSuperior Superiorlobe lobe during during the the Marquette Marquette the Whitefish Lake Lake basin. basin. Shorelines advance and probably drained westward through the Shorelines associated associated with this glacial lake have not been identified. Fine Finetextured textured glaciolacustrine glaciolacustrinedeposits depositshave have 0'Connor occur both however been observed beneath till up to 350 350 m asl. Deposits Deposits of of glacial glacial Lake O'Connor above and below till of the Superior lobe. This Thissequence sequenceisistypical typical of of an an ice ice advance-retreat advance-retreat cycle cycle within a glacial lake basin. Thick Thicksequences sequencesof ofsilty siltyto to clayey clayey grey grey rhythmites, in places exceeding 30 m in thickness, occur beneath fine-textured Superior lobe till within the Whitefish River lowland. Excellent Excellentexposures exposuresof ofthese thesedeposits depositsoccur occurin inlarge largesections sectionsalong along the the Whitefish Whitefish River River valley and its tributaries. Grey, Grey, rhythmically rhythmically laminated laminated clays clays associated with Lake O'Connor overlie Superior lobe till. These Thesedeposits depositshave havebeen been observed observedto to exceed exceed several several metres metres in in thickness. thickness. As the Superior lobe continued to withdraw from the Whitefish-Kaministikwia lowland, low-level outlets to the Superior Superior basin were uncovered resulting in the termination of glacial Lake Lake O'Connor. O'Connor. Superior basin waters inundated low-lying areas up to This represents represents the the to approximately 260 260 m m asl. asl. This approximate level of glacial Lake Minong, the highest water water body body to to occupy occupy the the entire entire perimeter perimeterof of the Lake Superior basin. Very Verylittle littleisisknown knownabout aboutthe thelate-glacial late-glacialhistory history of of glacial glacial lakes lakes within within Clearly,additional additional studies studiesare are required. this southern basin. Clearly, At this fieldtrip stop, we will look at a section of glaciolacustrine deposits associated with glacial Lake Kaministikwia. These Thesered, red, clay clay rich rich deposits depositsare are unusual unusual in in that that they were deposited in the ice ice margin. margin. The approximately 100 m of water less than a few kilometres from the The clays clays are rhythmically rhythmically laminated laminated at at their their base, where where they they overlie overlie glaciofluvial glaciofluvial deposits depositsof of sand sandand and gravel, gravel, and quickly become massive upwards. There Thereisis very very little little indication indication of of aa proximal proximal ice ice margin. margin. That is, there are very few dropstones and few few indications indications of of seasonal seasonal variability variability in in sediment sediment load. load. Proceed Proceed back back to to Highway 11/17 (Sistonens Corners) and turn left (south). Travel Travel 5.2 5.2km south along along Highway 11/17 and turn turn left on Teitto Road. The The next stop consists of aa road cut on the southeast ofthis this intersection. intersection. southeast corner cornerof Subaquatic Flow Till and Stop 5: Subaquatic and Glaciolacustrine Glaciolacustrine Sediment Sediment of the Superior Superior Lobe Lobe lobe. This At this site, we begin to see the first direct evidence of the advancing Superior lobe. This 22 m m well exposure is located less than a few kilometres from the Marks Moraine and consists of well overlying massive massive dark dark grey grey silty siltytill. till. These sediments were laminated, gritty, pebbly silt and clay overlying likely deposited at the bottom of of Lake Lake Kaministikwia Kaministikwia as as bottom-hugging bottom-hugging density density currents currents and and as as ice-rafted debris (rainout) in water depths of about 50 m. Red Red Sibley SibleyGroup Group siltstones siltstonesand and dark dark grey shales derived from the Gunflint Formation Formation are are clearly clearly recognizable recognizable in in the pebble pebble fraction fraction and and provide direct evidence for a Superior lobe provenance. Red, Red, massive massive clays clays associated associated with with glacial glacial �Lake Kaministikwja Kaministikwia occur less less than a few few kilometres kilometres to Clearly, the to the the north north of of this this site. site. Clearly, the composition of the Superior Superior lobe lobe flow flow tills tills and and glaciolacustrine glaciolacustrine deposits do not reflect the composition deposits do not reflect the This material material should should not not be be sampled sampled for for mineral mineral composition of the underlying Archean bedrock. bedrock. This exploration purposes. purposes. exploration Turn vehicles around and and turn turnleft left onto onto Highway Highway 11/17. 11/17. Proceed south 5.3 5.3 km krn to Proceed south to Briggs Briggs Drive Drive and and turn right (east) (east) into into Conmee Township sand and gravel pit. Park vehicle at gate i f locked. sand and gravel pit. Park vehicle at gate j/locked. OverriddenSuperior SuperiorLobe LobeDelta Delta and and Conmee Conmee Base Stop 6: Overridden Base Metal Metal Float Float exposed sequence sequence At this stop, we will be looking at aa section of of Superior Superior lobe lobe drift. drift. The exposed consists of approximately 6 to 7 m of well sorted, steeply-dipping sand and gravel of Superior consists Clastsin in the the lower lower provenance overlain by 2 to 3 m of silty Superior lobe till till (Figure 10). Clasts glaciofluvial unit consist primarily of Proterozoic Proterozoic metasedimentary metasedimentaryrocks. rocks. Numerous cobbles and Numerous cobbles and Gunflint Formation Formation and and Sibley Sibley Group Group are are recognizable. Foreset beds dip boulders belonging to the Gunflint recognizable. Foreset beds dip The delta delta was steeply to the north and are likely of deltaic origin. origin. The was probably probably built built proglacially proglacially into into an early phase of glacial Lake Kaministikwia. The feature therefore represents a location at which Lake Kaministikwia. The feature advancing Superior Superior Lobe Lobe stalled stalled prior prior to to reaching reaching its its maximum maximumposition positionatatthe theMarks MarksMoraine. Moraine. the advancing The delta is actually located within only 3 km krn of the Superior Superior lobe limit limit and and occurs at an elevation was of about 375 m asl, asi, 85 m below below the the maximum maximum elevation elevation of of Lake LakeKaministikwia. Kaministikwia. The delta was overridden by the Superior lobe resulting in the truncation of the foreset unit and removal of the overridden Superior resulting in the truncation of the foreset unit and removal of the Severalmetres metresof ofsilty, silty,Superior SuperiorLobe Lobe subglacial subglacialtill till was was deposited deposited on on top top of of the the sands sands topset beds. Several and gravels. gravels. of large large angular angular to rounded boulders boulders on on the the pit pit floor. floor. significanceis the occurrence occurrence of Of particular significance from the lower lower glaciofluvial glaciofluvial unit unit and, and, in in some some cases, do not appear to The boulders were extracted from have been transported very far. Some Someof of the the larger larger boulders boulders measure measure several several metres across and still still display striated surfaces. The The boulders boulders are are derived derived from from both both Archean Archean and and Proterozoic Proterozoic source source rocks. Several Severalboulders bouldersof ofsuiphidized sulphidizediron ironformation formationand andmassive massivepyrite pyriteof ofArchean Archeanage agewere were discovered in the boulder piles. One Oneof ofthe theboulders bouldersmeasured measuredover over11m min indiameter diameterand and consisted consisted of massive pyrite and magnetite with 10 to 15% sphalerite disseminated in pyrite-rich sections. 10 15% sphalerite disseminated in pyrite-rich sections. throughout the Sphalerite was also concentrated concentrated along along fractures fractures and and adjacent adjacent to to quartz quartz veinlets veinlets throughout the rock. rock. 18 pprn ppm Cu, Cu, 19 ppm Two samples samples from from the pyrite-rich pyrite-rich zones zones returned returned values values of: of: 1) 1)5.13% 5.13% Zn, Zn, 18 19 pprn 16 ppm Cu, 20 ppm Pb, 245 ppb Au and Pb, 260 ppb Au and 0.5 pprn ppm Ag; and 2) 2)2.85% 2.85% Zn, Zn, 16 pprn Cu, 20 pprn Pb, 245 ppb Au and 0.5 0.5 magnetite-rich zone returned values of 850 ppm Zn, 25 ppm Cu, ppm Ag. A sample from the pprn A sample from magnetite-rich zone returned values of 850 pprn Zn, 25 pprn Cu, 55 ppm <0.2 pprn ppm Ag. Ag. A second second sulphidized sulphidized iron iron formation formation boulder boulder measuring measuring pprn Pb, 25 ppb Au and <0.2 pyrite, returned of pyrite, returned values approximately 0.5 m in diameter and consisting almost exclusively of values of of 140 140 ppb Au Au and ppm Zn, 8 pprn ppm Cu, 11 ppm Pb, 710 ppb 1 1 pprn and <0.2 <0.2 ppm pprn Ag. Ag. pprn There are two possible source source areas areas for for the the boulders. boulders. Superior Superior lobe lobe striae striae in in the the immediate immediate eroded and transported vicinity of the pit are oriented at 320 to 330' 330° Az. IfIf the the boulders boulderswere were eroded and transported by by from they could have been Superior ice, then there is a 5 km window towards the southeast from which which they could have been beyond the the 55 krn km limit. limit. Alternatively, derived. Proterozoic derived. Proterozoic metasedimentary metasedimentary rocks rocks outcrop outcrop beyond Alternatively, the the �Figure 10. Figure 10. Steeply-dipping Steeply-dippingforeset foresetbeds beds of of aa delta delta constructed constructed along the margin of the advancing Township). Silty Superior lobe (Conmee Township). Silty Superior Superior lobe lobe till caps the sequence. boulders could have initially been eroded by northern ice from a source source to the north-northeast north-northeast of of the the pit then remobilized by the Superior lobe. remobilized Superior Exploration work during 900s along the lower reaches of Brule Creek, 4 to 5 km during the early 11900 Sulphur Company and General General north-northeast of the gravel pit, by B.L. Morrison, the Davis Sulphur Chemical Company resulted in the discovery discovery of "seven "seven lenticular masses of brecciated, brecciated, banded iron iron formation, in which pyrite has replaced replaced aa considerable considerable part part of of the the rock" rock" (Carter (Carter 1990b). 1990b). The largest of these masses has a maximum width of 23 m and is 244 m long. Other Other discoveries discoveriesinclude includeaa 21 21 magnetite and and pyrrhotite pyrrhotite and andaa99m mwide wideby by 15 15m mlong longzone zoneof of m wide body of pyrite containing magnetite magnetite-pyrite-jasper ironstone. ItIt is is possible possible that that the the boulders boulders found found within the gravel pit were derived from this area and that sphalerite was was not not recognized recognized in in the the rock. rock. It is is not yet clear whether sulphides indicate proximity to to a VMS style zone zone of of mineralization. mineralization. Further the sulphides Further work is is required required to assess the mineral potential of this area. assess south on onHighway Highway111/I 0.5km. k,n. Turn Turn left leftatatHolland HollandRoad Roadand andproceed proceedeastward eastwardfor for Continue south 1 4 77for for 0.5 roadcut at 2.6 km to a roadcut at the the top top of of aa steep steep decline. decline. �7: Superior Northern Till Stop 7: SuperiorLobe Lobe Till Overlying Northern The purpose of this stop is to compare and contrast contrast tills tills of Superior and and northern northern provenance provenance programs. Exposed with respect to drift prospecting programs. Exposed in in the the road cut cut on on the the south south side side of of the the road road is is approximately fairly high approximately 2 to 3 m of brown, blocky, silty Superior lobe till containing aa fairly from the the Gunflint Gunflint Formation, Formation, 2.5 2.5 km km to to the the southeast. southeast. Despite concentration of platy clasts derived from Despite its apparent subglacial origin, this this till till contains few clasts clasts of of Archean Archean supracrustal supracrustal or or intrusive intrusive composition. An olive grey, silty sand till of northern provenance is exposed in a few An olive grey, silty sand of northern provenance few places beneath the the Superior Superior lobe lobetill. till. Most of the clasts in this this lower till till are are of of an an Archean Archean directly beneath affinity. A quartz-feldspar porphyry is exposed in outcrop further down the slope and probably A quartz-feldspar porphyry is exposed in outcrop underlies the northern till in this exposure. exposure. lobe till till as as aa sample medium medium for for mineral mineral Under discussion is the suitability of Superior lobe exploration programs over Archean terrain terrain in in the the eastern eastern portion portion of of the the belt. belt. Compositionally, Compositionally, lobe till till overlying overlying Archean Archean bedrock bedrockdoes doesnot notreflect reflectlocal localbedrock bedrocksources. sources. This is likely Superior lobe attributed to the fact that the Superior lobe was was surging into into aa glacial glacial lake, lake, overriding overriding fine-grained fine-grained glaciolacustrine deposits and shales. The fine-grained substrates and proglacial lakes faciltated The fine-grained substrates proglacial lakes faciltated the process. Subglacial erosion close to the the ice ice margin margin was wasreduced reduceddue dueto toaafloating floatingice iceshelf shelf surging process. and reduced subglacial pressures caused by buoyancy. subglacial pressures caused by buoyancy. The background concentrations of gold grains grains and and elements elements in in Superior Superior lobe lobe till till are are different from those of northern tills. It is therefore advisable to treat datasets significantly different containing information from both tills as separate populations populations for anomaly anomaly recognition. recognition. Even more more importantly, it is recommended that only the the lower lower northern northern till till be be sampled sampled in in Archean Archean areas areas affected by the the Superior lobe. In In areas areas of of thick thick overburden, overburden, it may be necessary to employ expensive drilling techniques to sample the the lower till. till. Alternatively, Alternatively, itit may may be be possible possible to access access by backhoe backhoe trenching trenching off offof ofoutcrops outcropsininthe thearea. area. To To the the south south of of the the Whitefish Whitefish River River the lower till by lowland, Superior lobe till till is exposed exposed on on the the flanks flanksof ofthe theLogan Logandiabase diabasesills. sills. The The till till in in this this area area appears to reflect local variations in bedrock geology geology and and may maybe be suitable suitablefor for exploration exploration programs programs for such commodities commodities as as silver silver or or the platinum group group elements. elements. the vehicles vehiclesaround around andproceed and proceed back to to Highway Highway 11/17. 11/17. Drive Drive straight through Turn the through Highway down Holland Holland Road Roadfor for 3.2 Ion. kin. Stop at top of hill adjacent to aa microwave 11/17 down microwave tower on on the side of of the the road. road. Proceed pit adjacent adjacent to the south side Proceed to to the the north north edge edge of the sand and gravel pit microwave tower. microwave tower. 8: Scenic Brule Creek Moraine Stop 8: Scenic Lookout and Extension of the Brule this site provides provides the the fieldtrip fieldtrip participant participant an an excellent excellent view view of of the the lowland lowland The view afforded at this advanced during during the the Marquette MarquetteAdvance. Advance. Thunder Bay is located into which the Superior Lobe advanced km to to the the east east and and the the Logan Logandiabase diabasesills sillstotothe thesouth southare areapproximately approximately12 12km kmaway. away. Our over 30 km point is is situated situated approximately approximately 120 120mm above the Whitefish Whitefish River River lowland lowland to tothe the south southand and vantage point the east. east. The 230 m above Lake Superior to the The upland upland on which we stand stand clearly clearly affected the Superior Lobe causing it to halt less than 3 km to the northeast of this advancing Superior this location. location. �-s- —— — -- - - - — - _ - - ____________ -- - _ — — _ - --- - ---j -- -c 1--: -t.... -: _ r— -- - C *S- - — Cr----: I Ic- - ___ - 1- - ____ T_i_± - —- - C --S-- 2__ _ - Figure 11. Figure 11.Oblique Obliqueaerial aerialview viewofofthe theWhitefish-Kaministikwia Whitefish-Kaministikwialowland lowlandinto intowhich whichthe theSuperior Superior the Marquette Marquette stadial. stadial. Mesas of Proterozoic diabase are visible lobe advanced during the in the distance. distance. Exposed in the sand and gravel pit before us is a sequence of glaciofluvial sand and gravel of northern provenance which is in turn overlain by a unit of glaciolacustrine sand and finally by m of silty silty Superior Superior Lobe Lobetill. till. Paleocurrents in the lower glaciofluvial unit suggest approximately 3 m flow towards the the south-southwest. south-southwest. This is consistent with with aa northward northward retreating retreatingice icemargin. margin. The The glaciolacustrine sands glaciolacustrine sands are likely associated associated with with an an early early phase phase of of glacial glacial Lake Lake Kaministikwia. Kaministikwia. A number of buried sand and gravel deposits of northern provenance occur in southern and and adjoining adjoining Oliver Oliver Township. Township. It is possible that these deposits southeastern Conmee Township and Brule Creek Moraine, a significant ice-marginal feature represent the eastern extension of the Bmle the retreating northern northern ice ice lobe. lobe. The eastern extension of the Brule Creek Moraine associated with the beyond the Marks Moraine was obscured by Moraine obscured the the advancing advancing Superior Superiorlobe. lobe. The next next stop stop is is optional optional depending depending upon upon the the condition conditionof ofthe theroad road intothe intothe site. site. Continue Continue west kmdown downHolland HollandRoad RoadtotoSovereign SovereignRoad. Road Turn Turn right right and and drive drive north north 800 800 m. m. Turn another 1. 1.77 km logging road road and follow follow ititfor for approximately approximately600 600m. m. Turn right right at at vv in in road road and andfollow follow road road left on logging (-1 kin,). km). Park Parkvehicle vehicleatatcrest crestofofhilL hill. to the north until you reach reach aa clearcut clearcut('-4 �9: Marks Moraine Stop 9: This fieldtrip stop is located on on the the crest crestof ofthe theMarks MarksMoraine, Moraine,aaterminal terminalmoraine moraineassociated assoiated 9.9 to 10.0 10.0ka ka BP, BP, with the advancing Superior lobe. The The moraine moraine was was constructed constructed approximately approximately 9.9 during the Marquette Stadial. ItIt has has been been traced traced across across the study study area area from the southwestern southwestern corner comer of Marks Township to the southeastern comer corner of of Ware Ware Township. Township. Mapping is required toward Mapping is required toward the the its position. position. Where it is best developed, the moraine consists consists of of multiple multiple till till southwest to determine its ridges within a zone 11 to 2 km wide (Figure 12). It is composed primarily of fine-textured till and 12). It is composed primarily of fine-textured till and relatively low low profile. profile. In other areas, the moraine consists of a stacked therefore displays a relatively sequence of flow tills tills and glaciolacustrine deposits deposits with with no no apparent apparent geomorphic geomorphic expression. expression. The moraine here is assymetrical in cross-section, displaying a steeper slope on the ice-contact side. side. Fine-textured glaciolacustrine sediments sediments interbedded interbeddedwith with Superior Superior southeastern, ice-contact tills occur bevone beyong themoraine morainetotothe thenorthwest. northwest. To To the the southwest southwest are areaa series seriesof of lobe flow tills . the that follow follow the the ice ice margin margin and and terminate terminate in in the the Kaministikwia Kaministikwia basin basin to to northeast-trending channels that the northeast. Paleocurrents flow towards towards the the Paleocurrentsof of cross-bedded cross-bedded sands within these channels indicate flow Kaministikwia basin. Kaministikwia basin. - Drive back out to the intersection of of Sovereign SovereignRoad Roadand andHolland HollandRoad Roadand andproceed proceedsouth south on on Sovereign Road for for 4.9 km km till tillyou youreach reach Highway Highway 590. 590. You have have now now descended descended into into the the Whitefish-Kaministikwialowland, lowland,aavertical verticaldrop dropof ofover over100 100in. m. Sovereign SovereignRoad Roadchanges changes into into Whitefish-Kaministikwia Highway 595 595 beyond beyond Highway Highway590. 590. Continue south on Highway 595 595for for an an additional 4.9 Highway 4.9 km. km. Smith Road Road and and drive drivefor for 3.0 km km till tillyou you reach reach a bridge crossing Turn left on Smith crossing Whitewood Whitewood Creek. Creek. Park the far side Park the vehicle vehicle on the right on on the far side of of the bridge. bridge. Stop 10: 10: Whitewood Creek Section At this stop, we will be looking at a section along the banks of Whitewood Creek that exposes 2 glaciolacustrinedeposits. deposits. The section has has not not been been studied studied tills with intervening and superimposed glaciolacustrine in detail. however, demonstrate demonstrate the the drastic drastic changes changes that that occur occur in in till till character character as as one one detail. It does, however, Archean upland upland to tothe thenorth northinto intothe theWhitefish-Kaministikwia Whitefish-Kaministikwialowland. lowland. At At this this proceeds off of the Archean stop, we will concentrate on on the the lower lower till till unit. unit. The steepness of the section makes makes itit difficult difficult to to units. These units will be viewed at the next fieldstop. view the upper stratigraphic units. At river level is exposed a dark grey, sandy silty silty subglacial subglacial till till presumed presumed to to be be of of northern northern its subglacial subglacial origin. origin. A affinity. The The till till contains contains numerous numerous faceted and striated clasts supporting its stone line or boulder pavement is clearly visible visible aa few metres metres above above water water level level when when the the section section is is towards the the well exposed. Striations Striationson on the the upper upper surfaces surfaces of the boulders indicate ice flow towards southwest. Most Shales, wackes, wackes, ironstones ironstones Mostof of the the clasts clasts are are derived from the Gunflint Formation. Shales, jaspers are commonly seen in the the river river bed. bed. Only and oolitic jaspers Only aa small small proportion proportion of of the clasts clasts are are derived from Archean terrain terrain to to the the north. north. Oxidation of sulphide minerals contained contained within within Gunflint Formation shales has caused a rusty appearance appearance to to the the till till up up to to the the high high water water line line in in the the section. The that which which is is observed observed over over Thecharacter character of of this this lower lower northern till is quite different from that to the the north. north. The owing its its fineness fineness to to the the texture texture Archean terrain to The till is significantly finer textured, owing displays aa much much darker darker colour. colour. It is similar of the underlying source rocks and displays similar in that it appears appears to �:Jw •w&w • fl LjflMj f'? 2 Figure 12. Figure 12. Aerial Aerialphoto photoof ofthe thearea areasurrounding surrounding Stop Stop 9. 9. The Themorainie morainicridges ridgesform form part part of of the the Marks Moraine. Moraine. The Theesker eskerridge ridgewas wasdeposited depositedby bysouthward southwardflowing flowingnorthern northernice. ice. Superior lobe till flanks flanks the eastern eastern margin of of the the esker. esker. �reflect local bedrock sources sources and and therefore is is aa suitable suitable sampling samplingmedium medium for for programs programs of of mineral mineral exploration exploration within within this thisarea. area. The The lower, lower, northern northern till till is is overlain overlainby by aa sequence sequence of of silty silty glaciolacustrine glaciolacustrinesediments sedimentswhich which were were likely deposited within glacial Lake O'Connor. These sediments have likely deposited within glacial Lake O'Connor. These sediments have not not been been looked lookedat at in in any any detail. There Thereare arenumerous numeroussections sectionsalong alongthe thevalley valleyof ofthe theWhitefish WhitefishRiver Riverand andtributaries tributariesthat that expose Lake O'Connor sediments. These sections require detailed examination to expose Lake O'Connor sediments. These sections require detailed examination to assist assist with with unravelling unravelling the the history history of of this this glacial glacial lake. lake. Higher toclayey clayey Higher up up in in the the section, section, immediately immediatelyabove abovethese these stratified stratifieddeposits depositsisisaamassive massivesilty siltyto diamicton pebbles. This diamicton with with aa few few grits grits and and pebbles. Thisunit unitisisinterpreted interpretedto tobe beaatill tillderived derivedfrom fromthe the Superior Superior lobe. lobe. The Thetill till owes owesits itsfine fine texture texture to to the the incorporation incorporation of of large large volumes volumes of of glaciolacustrine deposits into the base of the advancing Superior lobe. This till barely glaciolacustrine deposits into the base of the advancing Superior lobe. This till barelyresembles resembles other other examples examples of of Superior Superior lobe lobe till which we we have have seen seen earlier earlier in this fieldtrip. fieldtrip. The Thepebbles pebbleswithin within this till are easily recognized as derived from the Animikie and Sibley Group metasediments to the this till are easily recognized as derived from the Animikie and Sibley Group metasediments to the northeast and east. This till does not carry a signature of local geology because the ice that northeast and east. This till does not cany a signature of local geology because the ice that deposited deposited it it did did not not have have access access to to the the bedrock bedrock surface. surface. The The upper, upper, Superior Superior lobe lobe till till is is overlain overlainby by aa second secondsequence sequenceof ofglaciolacustrine glaciolacustrinedeposits. deposits. These These sediments sedimentswere were probably probably also also deposited deposited within within glacial glacial Lake O'Coimor O'Connor as as the the Superior Superiorlobe lobe ice ice margin margin retreated retreated eastward eastwardout out of ofthe thelowland. lowland. Turn vehicles around around and and go go back back to to Highway Highway595. 595 Turn for 3.3 km. Turn Turn the the vehicles Turnleft left and and drive drive south southfor 3.3 km. Turn left on Blaikie Rd and Bluikie Rd. anddrive drive east eastfor for 1.8 1.8kin. km.The Theexposure exposurewe wewill will be belooking lookingat atisiscut cutinto intothe the River valley valley and and is is located located on on the southwest southwest side side of the the road. road walls of the the Whitefish Whitefish River Stop Stop 11: 11: Whitefish WhitefishRiver RiverSection. Section. At At this this stop, stop, we we will will have have an an opportunity opportunityto to look look at, at, in in detail, detail, aa section section through through glacial glacial lake lake O'Connor anupper upper sequence sequenceof of glaciolacustrine glaciolacustrineand and fluvial fluvial O'Connor sediments, sediments,Superior SuperiorLobe Lobetill till and and an deposits. deposits. The Thelower lower33toto44mmconsists consistsofofdark darkgrey, grey,well welllaminated laminatedsilt siltand andclay claylikely likelydeposited deposited with are the the Rove Rove and and with glacial glacial Lake Lake O'Connor. O'Connor. The Theclays claysare areclark dark grey grey because because their their source source rocks rocks are Gunflint Gunflint Formations. Formations. Glacial GlacialLake LakeO'Connor O'Connorformed formedproglacially proglaciallywhen when the the Superior Superiorlobe lobe advanced advanced into intothe the Whitefish WhitefishRiver Rivervalley valleyproducing producingaaclosed closedbasin basinsupported supportedon onits itsnorthern northernand andsouthern southern flanks flanksby by high high ground. ground. The Thelake lakemust musthave havedrained drainedwestward westwardout outthrough throughthe theWhitefish WhitefishLake Lakebasin. basin. Eventually Eventually the the advancing advancingSuperior Superiorlobe lobeoverrode overrodethese thesedeposits depositsproducing producingaafine-textured, fine-textured, clayey silt till with few pebbles. This till is very similar to that exposed clayey silt till with few pebbles. This till is very similar to that exposedatatnear nearthe thetop topof ofthe the section at Stop 10. The till is massive, dense, compact and was probably deposited subglacially section at Stop 10. The till is massive, dense, compact and was probably deposited subglacially under of under active active ice. ice. The Thefineness finenessof ofthe theunderlying underlying deposits deposits would would have have prevented prevented the the infiltration infiltration of subglacial meltwater into the substrate. This would help to promote the development of a thin layer subglacial meltwater into the substrate. This would help to promote the development of a thin layer of of water water at at the the glacier glacier sole sole which which would would have have facilitated facilitated the the rapid rapid advance advance of of ice ice into into the the basin. basin. As As mentioned earlier, the base of the advancing Superior lobe was isolated from the bedrock surface by mentioned earlier, the base of the advancing Superior lobe was isolated from the bedrock surfaceby �several tens tens of of metres metres of of intervening intervening sediment. sediment. As this ice not several As aa result, result, the the till till deposited deposited by by this ice does does not display a signature of local bedrock within the Whitefish River valley. display a signature of local bedrock within the Whitefish River valley. Superior Superior lobe lobe till till is is abruptly abruptlyoverlain overlainby by approximately approximately22m mof ofdark darkgrey greyclay clayand andsilty siltyclay. clay. This sediment was deposited proglacially beyond the retreating margin of the Superior lobe. The This sediment was deposited proglacially beyond the retreating margin of the Superior lobe. The dark grey grey colour colour of of this this sediment sediment suggests deposition within dark suggests deposition within aa closed closed basin basin (i.e. (i.e. aa basin basin not not linked linked with Lake with Lake Superior). Superior). This overlain by by approximately approximately 11 m m of of pebble pebble gravel. gravel. The This unit unit is is abruptly abruptly overlain The gravel gravel is is wellwellmoderately sorted and probably of fluvial character. A low-water phase is implied by this unit. unit. moderately sorted and probably of fluvial character. A low-water phase is implied by this Perhaps the the eastward eastward retreat retreat of of Superior Perhaps Superior ice ice beyon beyon d d Stanley Stanley Hill Hill in in southwestern southwestern Paipoonge Paipoonge Township resulted resulted in in the the drop drop of of glacial glacial Lake Lake O'Connor O'Connor to to aa lower Township lower level. level. This Thiswould wouldhave have resulted resulted in the influx of coarse sediments from uplands east of the site (Little Pig and Badger Mountains) in the influx of coarse sediments from uplands east of the site (Little Pig and Badger Mountains) resulting resulting in in the the superposition superposition of of fluvial fluvial sands sands and and gravels gravelsover over clay. clay. The fluvial fluvial unit unit is abruptly overlain overlain by reddish-coloured glaciolacustrine The is abruptly by reddish-coloured glaciolacustrine deposits deposits of of silt, silt, very very fine minor clay. clay. The fine sand sand and and minor Theupper upper surface surfaceof of this thisglaciolacustrine glaciolacustrineplain plain occurs occurs at at approximately approximately 275 275 m m asl, as], well well above above the the highest highest elevations elevations reported reported for for Superior Superiorbasin basin lakes lakesin in the theThunder ThunderBay Bay area. area. These Thesedeposits depositswere werelikely likelylaid laiddown downwithin withinaa confined confined basin basin with with aa configuration configurationsimilar similarto to that of of glacial glacial Lake Lake O'Connor. O'Connor. The of the the sediment sediment that Thesignificance significanceof of the the colour colour and and textural textural change change of is is unclear. unclear. Clearly, Clearly,additional additionalwork work is isrequired requiredto to obtain obtainaa better better understanding understanding of of the the glaciolacustrine history glaciolacustrine history of of this this region. region. �REFERENCES REFERENCES Baje, Bajc, A.F. A.F. 1997. 1997. A Aregional regionalevaluation evaluationof ofgold goldpotential potentialalong alongthe the western western extension extensionof of the the Larder Larder Lake-Cadillac Break, Matachewan area: results of regional till sampling; Ontario Lake-Cadillac Break, Matachewan area: results of regional till sampling; OntarioGeological Geological Survey, Open Open File File Report Report 5957,50 5957, Sop. Survey, p. Bajc, A.F. 1999a. 1999a.Results Resultsof ofregional regionalhumus humusand andtill tillsampling samplingin inthe theeastern easternpart part of of the the Bajc, A.F. Shebandowan greenstone greenstone belt, belt, northwestern northwestern Ontario; Ontario; Ontario Shebandowan Ontario Geological Geological Survey, Survey, Open Open File File Report W2p. Report5993, 5993,92p. Baje, Ak'. 1999b. Bajc, A.F. 1999b. Gold Goldgrains grainsin intill, till,western western Shebandowan Shebandowan greenstone greenstone belt, belt, northwestern northwestern Ontario; Ontario; Ontario Geological Geological Survey, Survey, Preliminary Preliminary Map Map 341 3417, 000. Ontario 7, scale scale 1:100 1:100 000. Bajc, A.F. 2000. Bajc, A.F. 2000. Results Resultsof ofregional regionaltill tillsampling samplingin inthe thewestern westernpart part of ofthe the Shebandowan Shebandowan greenstone northwestern Ontario; Ontario; Ontario greenstone belt, belt, northwestern OntarioGeological GeologicalSurvey, Survey, Open Open File File Report Report 6012, 6012, 82p. 82p. Brown, Brown, G.H. G.H. 1995. 1995. Precambrian Precambriangeology, geology, Oliver Oliverand and Ware Ware townships; townships;Ontario Ontario Geological GeologicalSurvey, Survey, Report 294, 'tSp. Report 294,48p. Carter, Carter, M.W. M.W. 1990b. 1990b. Geology Geologyof ofForbes Forbesand and Conmee Conmeetownships; townships;Ontario OntarioGeological Geological Survey, Survey, Open Open File Report 5726, 158p. File Report 5726, 188p. Corfu, F. and and Stott, Stott, G.M. G.M. 1998. 1998.Shebandowan Shebandowangreenstone greenstonebelt, belt,western westernSuperior SuperiorProvince: Province: U-Pb U-Pb Corfu, F. ages, tectonic implications, and correlations; Geological Society of America, ages, tectonic implications, and correlations; Geological Society of America, v.110, v.110, no.11, no. 11, p. 1467-1484. p.1467-1484. Franiclin,J.M., J.M.,Kissin, Kissin,S.A., S.A.,Smyk, Smyk,M.C. M.C.and andScott, Scott,S.D. S.D. 1986. 1986. Silver Silver deposits deposits associated associated with with Franklin, Proterozoic rocks rocks of of the the Thunder Thunder Bay Bay District, District, Ontario; Ontario; Canadian Proterozoic Canadian Journal Journal of of Earth Earth Sciences, v.23, No. No.10, 10, p.1576-1591. p. 1576-1591. Osmani, LA. l.A. 1997. Osmani, 1997. Geology Geologyand andmineral mineral potential: potential:Greenwater GreenwaterLake Lake area, area, West-Central West-Central Shebandowan Greenstone Greenstone Belt; Belt; Ontario Ontario Geological Geological Survey, Survey, Report Report 296, 296, 135p. l35p. Shebandowan Schneiders, B.R., Scott, Scott, J.F. J.F. and and Smyk, M.C. 1998. Schneiders, B.R., Smyk, M.C. 1998. Report Report of ofActivities, Activities,1997, 1997,Resident ResidentGeologist Geologist Program, Thunder Thunder Bay Bay South South Regional Regional Resident Resident Geologist's Geologist's Report: Program, Report: Thunder Thunder Bay Bay South South District; Ontario District; Ontario Geological Geological Survey, Survey, Open Open File File Report Report 5971, 5971, 56p. 56p. Sutcliffe, R.H. 1991. Sutcliffe, R.H. 1991. Proterozoic Proterozoicgeology geologyof ofthe the Lake Lake Superior Superiorarea; area; in in Geology Geology of of Ontario, Ontario, Ontario Ontario Geological Geological Survey, Survey, Special Special Volume Volume 4, Part Part 1, 1, p.627-660. Williams, H.R. H.R. 1991. Quetico in Geology Geology of of Ontario, Ontario, Ontario Ontario Geological Geological Survey, Survey, QueticoSubprovince; Subprovince;in Special Special Volume Volume 4, Part Part 1, 1, p.383-404. p.383-404. �Williams, T.L. and and Sage, Sage, R.P. R.P. 1991. Wawa Subprovihce; Williams, H.R., H.R., Stott, Stott, G.M., Heather, K.B., K.B., Muir, T.L. in in Geology Geology of of Ontario, Ontario, Ontario OntarioGeological Geological Survey, Survey, Special Special Volume Volume 4, Part Part 1, 1, p.485-542. Zoltai, Zoltai, S.C. S.C. 1963. 1963.Glacial Glacialfeatures featuresof ofthe theCanadian CanadianLakehead Lakeheadarea; area; Canadian Canadian Geographer, Geographer, v.7, p.101-115. p.101-115. Zoltai, Zoltai, S.C. S.C. 1965. 1965.Glacial Glacial features featuresof of the the Quetico-Nipigon Quetico-Nipigonarea, area, Ontario; Ontario; Canadian Canadian Journal Journal of of Earth Earth Sciences,v.2, v.2, p.247-269. p.247-269. Sciences, �GEOLOGY OF STEEP GEOLOGY OF THE THE ARCHEAN ARCHEAN STEEP ROCK LAKE FINLAYSON LAKE LAKE ROCK LAKE - FINLAYSON GREENSTONE BELT GREENSTONE BELT Denver Denver Stone, Stone, Ontario Ontario Geological Geological Survey Survey Kirsty Tomlinson, Survey of of Canada Canada Kirsty Tomlinson, Geological Geological Survey Ray Ray Bernatchez, Bernatchez, Atikokan Atikokan Resources Resources Inc. Inc. Philip Fralick, Fralick, Lakehead Philip Lakehead University University 3.0 Ga Ga assemblage qfIc tofelsic fr/sic This 3.0 assemblageconsists consistsof ofu/tram ultramaflc metavolcanic rocks and metavolcanic and chemical-clastic chemical-clasticsedimentary sedimentaryrocks. rocks. This trip will will examine examine the the structural, structural, petrographic, petrographic, sedimentaological sedimentaologicaland andeconomic economicaspects aspectsof ofthese theserocks. rocks. �FIELDTRIP 2B FIELDTRIP 2B MESOARCHEAN GEOLOGY GEOLOGY OF OF THE THE STEEP STEEP ROCK ROCK MESOARCHEAN LUMI3Y LAKE LAKE TERRAINS TERRAINS LUMBY INTRODUCTION INTRODUCTION The various Mesoarchean Mesoarchean sequences sequencesin in the Canadian Canadian Shield are remarkably similar, being composed metabasalt-quarta areniate-iron areniate-iron formation formation association association or a 2) 2) metavolcanicmetavolcanic:1) a metabasalt-quartz composedof of either either :1) shallow water carbonate association (Thurston and Chivers, 1990; Erikisson et al, 1994). Worldwide shallow water carbonate association (Thurstonand Chivers, 1990;Erikisson Worldwide examples of these associations associations typically overlie overlie granitoids granitoids or volcanic volcanic successions successions intruded by granitoids and include theoleiitic basalt and komatiite, quartz arenite, iron formation, pelite and/or include theoleiitic quartz andlor stromatolitic carbonate. carbonate.Conglomeratic Conglomeraticintervals intervalstypically typicallyare arepresent presentatat the the base base of stromatolitic of these these successions. Included in this association successions. association are the Manjeri Formation at the base of the Ngezi Group in Zimbabwe, the Paddy Market in the the Pilbara Zimbabwe, Paddy Market Supersequence Supersequence in Pilbara Block, Block, and and numerous numerous local local occurrences in in Superior Province Province (Erikisson (Erikissonetet al, al, 1994; 1994; Erikisson Erikisson et et al, al, 1996). Of the various occurrences various examples examplesof of this this association associationthe the carbonate carbonatedominated dominatedSteep SteepRock Rock assemblage assemblagehas has received receivedthe the most most detailed analysis. Thurston and Chivers (1990) and Erikisson et al (1994, (1994, 1996) 1996)concluded concluded that that associations associations of of this this type type are areformed formedas asaa stable stableplatform platform undergoes undergoesrifting. rifting. Data amassed in recent years on these Mesoarchean sequences Data amassed in recent years on these Mesoarchean sequenceshas, has, in in some some cases, cases, led led to to an an elaboration of of preexisting preexisting tectonic tectonic models, models, while while in in other other cases cases has has caused new models to be elaboration be proposed. This new data on the Steep Rock-Lumby Lake Terrains is briefly reviewed at the end of proposed. This new data on the Steep Rock-Lumby Lake Terrains this article, with the the new article, with with the aim aim of of familiarizing familiarizing the field field trip participants participants with new ideas on Mesoarchean tectonics being developed in this area. First we will stan with an introduction to and Mesoarchean tectonics developed First we will start with an description of the the belt. belt. General Geology General Geology The Steep Steep RockRock- Lumby Lumby Lake within the the District District of of Rainy Rainy River, River, extending extending Lake Terrain Terrain is located within Atikokan to the northeast, northeast, and approximately 200km Thunder Bay, Ontario 200km to the west of Thunder from Atikokan and lying approximately TheSteep SteepRock RockGreenstone GreenstoneBelt Beltlies liesimmediately immediatelyto tothe thenorth north of of Atikokan, near the (Figure 1). The (Figure 2). 2). It was first described by Smyth (1891) as a Quetico-Wabigoon subprovince boundary (Figure vertical succession ferruginous strata strata and and volcanics. volcanics. The The belt vertical succession of conglomerate, conglomerate, limestone, limestone, ferruginous belt unconformably overlies the 3003-+5Ma (Davis and Jackson, 1988) tonalitic Mannion Marmion batholith (Wilks, 1986; Stone et al., 1992). 1992). The The greenstone greenstone Belt comprises comprises the following following five Formations defined (1959),Wilks Wilksand andNisbet Nisbet(1986) (1986)(Figure (Figure3) 3)and andStone Stoneetetal. al.(1992). (1992). defined by Jolliffe Jolliffe (1959), 1. quartzo-feldspathic conglomerates 1. Basal quartzo-feldspathic conglomeratesand andsandstones sandstonesof ofthe theWagita WagitaFormation Formation(up (upto to 150 150 m thick) thick) representing representing alluvial alluvial fan fan deposits deposits on on fault fault scarps scarps (Wilks, 1986); 2. Stromatolitic limestones of the Mosher Carbonate Formation(up (uptoto 500 2. Stromatolitic limestones of the Mosher Carbonate Formation 500 m m thick) thick) representing deposition deposition in in aa shallow shallowsea; sea; 3. Iron formation formation and carbonate-Mn-Fe-rich carbonate-Mn-Fe-rich mudstones mudstones of the the Jolliffe Ore Ore zone zone Formation Formation (up to 400 m thick) representing representing deposition deposition in a subsiding subsiding basin (Wilks, 1986); 1986); 4. an unusual high-Mg, predominately pyroclastic komatiitic unit named the Dismal Ashrock 4. predominately pyroclastic komatiitic Formation (50-400 m thick) representing Formation representing shallow water volcanism; and �n -.4 I n t —. —. Location (if nit itihohan /Steep Ruck area in relation to the belt structure of Superior Province. �p.. * N e Geology of the Steep Rock (a and south-east of A), Finlayson Lake (B) and Lumby Lake (C,S, E) areas. t :!lj Figure 2. �5. . aa sequence the Witch Witch Bay Bay Formation Formation (ca. (ea. 55 sequence of mafic and minor felsic metavolcanics called the km km thick) thick) representing representingsubaqueous subaqueousvolcanism. volcanism. T 200 nil] 200 m .1. B. B. Metosedirnents Metosediments WITCH WITCH BAY BAY FORMATION FORMATION (( rnetavolcanics metavolconics , metasedirnents) metosediments I Possible Possible Tectonic break Tectonic break Lovas LOWS ? DISMAL DISMAL ASHROCK ASHROCK JOLLIFFE JOLLIFFE ORE ORE ZONE ZONE Pyrite Pyrite lenses lenses Buckshot Buckshot ore ore Mammil lose Mommillose St ro m a to ites Stromatolites ./ / / Fault MOSHER CARBONATE 1 WAGrTA FORMATION i clostics I unconformity MARMtON COMPLEX MOSHER MOSHER CARBONATE CARBONATE WAGITA WAGITA FORMATION FORMATION clastics ( ctostics) I MARMION MARMION COMPLEX COMPLEX Giant Columnar Columnar Stromalolites Oncolites Strotiform Stromatolites Pseudomorphs otter Gypsum (Atikokania) Branching & Hemispherical Stromotolites Xenoliths Xenoliths Eorly Early Mofic Mafic Dykes Dykes Figure Rock area. area. The sedimenta,y units Figure3.3. A) A) Geology Geology of the Steep Rock Thewhite white area area south south of of the the sedimentary unitsisis mostly mafic volcanics. B) Stratigraphy of the Steep Rock succession. (Mod j/ied mostly B) Stratigraphy of the Steep Rock (Modified afterWllks Wilksand andNisbet, Nisbet,1986).. 1986).. after The The age ageof ofthe theSteep SteepRock Rock Group Group isis fairly fairly well well constrained. constrained. Six Sixdetrital detritalzircons zirconsfrom fromthe the basal basal sandstones sandstonesare areca. ca. 2999 2999 Ma Ma (reported (reported by by Fralick Fralick and and King, King, 1996 1996 as as D. D. Davis, Davis, personal personal communication). The Dismal Ashrock Formation contains zircons which are 2997 Ma, and these communication). The Dismal Ashrock Formation contains zircons which are 2997 Ma, and these may Davis, personal personal communication, communication,1998). 1998). Sphenes Sphenes in in the the Marmion Mannion (D. Davis, may also alsototobe bedetrital detrital(D. batholith batholithhave havebeen beendated datedatatca. ca.2950 2950Ma, Ma,possibly possiblyreflecting reflectingthe theage ageof of hydrothermal hydrothermalactivity activity that that generated JolliffeOre Orezone zoneFormation Formation(D. generatedthe theJolliffe (D. Davis, Davis,persona personacommunication, communication, 1998). 1998). The TheFinlayson FinlaysonLake LakeGreenstone GreenstoneBelt Beltajoins ajoinsthe theSteep SteepRock RockBelt Belttotothe thenorthwest northwestand and extends extendstotothe thenortheast. northeast.ItItisisapparently apparentlyan anextension extensionof ofthe theSteep SteepRock Rock sequence, sequence, but with with the the sedimentary sedimentaryassemblage assemblageon ontop topofofthe thevolcanic volcanicpile pilerather ratherthan thanbelow belowit, it,as asisisthe the case case with with Steep Steep Rock. Rock. The thanthe theSteep SteepRock Rockgreenstone greenstonebelt belt and andthe therocks rocks are are atat The belt belt isismore moredeformed deformedthan greenschist greenschistto toamphibolite amphibolitefacies. facies.The Thebelt beltisisthought thoughttotooverlie overliethe theMarmion Marmionbatholith batholith(Stone (Stone et., et., 1992) 1992)but butthe thecontact contacthas hasbeen beenintruded intrudedby bysynsyn-to to post-volcanic post-volcanic tonalite tonaliteand and granodiorite granodiorite(Stone (Stoneand and Kamineni,1989). 1989).The Thebelt beltisisdominated dominatedbybysheared, sheared,pillowed pillowedand andmassive massivemafic maficlava lavaflows flowsand and Kamineni, banded bandedamphibolites. amphibolites.Thin Thinunits unitsofoffelsic felsictuff tuffand andmetasedimentary metasedimentarvrocks rocksare are intercalated intercalatedwith with the the mafic maficvolcanics volcanicsapproximately approximatelyhalfway half waythrough throughand andatatthe thetop topofofthe thesequence sequence(Fralick (Fralickand andKing, King, 1996). Six detrital zircons from the upper sedimentary sequence give an age range of 3002 to 2997 1996). Six detrital zircons from the upper sedimentary sequence give an age range of 3002 to 2997 �Ma Ma (Fralick (Fralick and and King, King, 1996) 1996)and and aa rhyolite rhyolite near near the the top top of of the the volcanic volcanic sequence sequencehas has been beendated datedat at 2932± 2 Ma (Davis, 1993). 2932  2 Ma (Davis, 1993). The The Luinby LumbyLake LakeGreenstone GreenstoneBelt Beltextends extendseastwards eastwardsfrom fromthe thenorth northend endof of the the Finlayson Finlayson belt (Jackson. (Jackson, l985a, belt 1985a,b). b). The Thesouthern southernmargin margin of ofthe the belt belt is is in in faulted faulted contact contact with with the the Marmion Marmion batholith (Sage (Sage et et al., al., 1974). pillow batholith 1974). The Thebelt beltisisdominated dominated by by mafic mafic volcanic volcanic rocks, rocks, particularly particularly pillow lavas, but but also of komatiite komatiite flows flows and and pyroclastics, pyroclastics, felsic felsic volcanic volcanic units units and and lavas, also contains contains sequences sequences of chemical the chemical and and clastic clastic sediments. sediments. The Thenorthern northernhalf half of of the the belt belt generally generally youngs youngs to to the the south south and and the southern the belt belt dipping dipping steeply. steeply. Jackson a,b) suggested southernhalf half to to the the north, north, with with both both halves halves of of the Jackson(1985 (1985a,b) suggested that represented aa syncline along the the sedimentary sedimentary units units at the centre centre of of the the that the the belt belt represented syncline with with the the fold fold axis axis along at the belt. This model model appears to be belt. This appears to be oversimplified oversimplified as as there there are are reversals reversals in in younging younging direction direction and and because the the stratigraphy stratigraphy in in the the two two halves halves of of the the belt belt is is rather rather different. different. We because We suggest suggest that that these these opposing tectonic boundary simple synclinal synclinal axis, opposing sequences sequencesmay may be be separated separatedby by aa tectonic boundary rather rather than than aa simple axis, but but do not not discount do discount that that the the two two halves halves of of the the belt belt are are related. related. A felsic of the the southern southern half half of of the the belt belt contains contains 2999 2999 Ma Ma tuff unit unit low low in in the the stratigraphy stratigraphy of A felsic tuff zircons and and aa felsic felsic tuff tuff slightly slightly higher higher in the stratigraphy stratigraphy contains range 2999 zircons in the contains zircons zircons in in the the range 2999 to to 2903 2903 Ma (Davis (Davis and and Jackson, Jackson, 1988). 1988). However the 2999 2999 Ma Ma component component in in each eachof ofthese these felsic felsic units units is is Ma However the thought to be be detrital detrital (D. (D. Davis, 1998). This higher felsic felsic unit unit is thought to Davis, personal personal communication, communication, 1998). This higher is also also along 1 50-m-thickunit unit(Jackson, (Jackson, 1985a) 1 985a)ofofsandstones, sandstones,carbonates carbonatesand and chert chert along strike strike from from aa ca. ca. 150-m-thick thought to thought to have have formed formed in in aa shallow shallowwater water environment environment (lower (lower sedimentary sedimentary sequence; sequence; Fralick Fralick and and King, 1996). 1996). A thick thick felsic the belt belt contains contains zircons zircons that that give give aa maximum maximum felsic tuff unit in the northern northern half of the U-Pb age age of of 2973 2973 ma ma (D. (D. Davis, An upper upper sedimentary sedimentary sequence sequence U-Pb Davis, personal communication, communication, 1997). 1997). An described by Fralick and King (1996) m of chert-rich chert-rich iron formation formation and elastic described (1 996) comprises comprises 500-700 500-700 m clastic sedimentary rocks rocks that that occur The upper part of sedimentary occur along along the the centre centre of of the the greenstone greenstone belt. belt. The upper part of the the upper upper sedimentary sequence sedimentary sequence comprises comprises elastic clastic turbidites turbidites with conglomeratic conglomeratic intervals intervals (Fralick (Fralick and King, 1996). 1996). The mafic rocks of of the belt are The mafic and and ultramafic ultramafic volcanic volcanic rocks the Lumby Lurnby Lake Lake greenstone greenstone belt are at at amphibolite the northen northen margin margin of of the the greenstone greenstone amphibolitegrade grade close close to to the the Norway Lake Lake pluton and along along the belt, but but otherwise otherwise they they are are generally generally at at greenschist greenschist facies. facies. They belt, They are, are, in places, places, well well exposed exposed and, and, although original stratigraphic stratigraphic and textural features features are frequently frequently very well preserved, preserved, although often often sheared, sheared, original and textural particularly in particularly in the the northern northern half half of of the the belt. belt. Steep Rock Rock Group Group Steep Smyth (1891) was the the first first to to describe describe the the Steep Steep Rock Rock succession succession which which he he interpreted interpreted as as aa Smyth (1891) was monoclinal assemblage monoclinal assemblageof ofnine nineformations formationsthat that unconformably unconformablyoverlie overlieaa granitic graniticbasement basementcomplex. complex. The unconformable basal (1912), though he reduced reduced Smyth's Smyth's nine 912), though Lawson (1 The unconformable basal contact contactwas was confirmed confirmedby Lawson formations formationsto to four four by eliminating eliminatingthe the dubious dubious upper upper units. units. Moore Moore (1939), (1939), Jolliffe Jolliffe (1966), (1966), Shklanka Shklanka (1972) and Wilks Wilks and Nisbet (1988) (1972) andNisbet (1988)all all redefined redefined the constituents constituents of of the Steep Steep Rock Rock Group, Group, with with the the presently accepted presently accepted definition definitionbeing being the thelatter. latter. Intrusive Igneous Atikokan area Intrusive IgneousRocks. Rocks.The The Atikokan areastraddles straddlesan anelongate elongatebelt beltof ofmetasediments metasedimentsto to the the (Quetico Subprovince) south (Quetico Subprovince)and andmixed mixedmetavolcanic, metavolcanic,gneissic, gneissic,and andfelsic felsicplutonic plutonicrocks rocksto tothe the north north tonalite and and "old" 'old" tonalite (Wabigoon Subprovince). PrePre- to to synvolcanic synvolcanic intrusions intrusions include mafic tonalite tonalite �(which predate the the Marmion Marmion batholith batholith and and Lefteye Lefteye stock stock as as well wellas astonalite tonalite (which predate the Steep Steep Rock Rock Group) Group) in in the and Syn-to postvolcanic postvolcanic intrusions and mafic mafic tonalite tonalite gneisses gneisses of of the the Dashwa Dashwa gneiss gneiss complex. complex. Syn-to intrusions encompass a broad range of felsic to mafic magmas. "Young" tonalite, which postdates postdates some some encompass a broad range of felsic to mafic magmas. "Young" tonalite, which supracrustal the Nevison, Nevison, Righteye, Righteye, Wasp, Wasp, Bow, Bow,and andHardtack Hardtackintrusions. intrusions. The TheLittle Little supracrustal rocks, rocks, occupies occupies the Eye stock is composed of homblendite and diorite. Granodiorite and granite occur in the Diversion, Eye stock is composed of hornblendite and diorite. Granodiorite and granite occur in the Diversion, Margaret, Margaret, Bewag, Bewag, Eye-Dashwa, Eye-Dashwa, and and White White Otter Otterfelsic felsic intrusive intrusive centres. centres. 'Old" tonalite is so-named because it is the dominant " O l d tonalite is so-named because it is the dominant lithology lithology of of the the basement basement complex complex upon which rocks of the Steep Rock Group were deposited. Along northeastern margins upon which rocks ofthe Steep Rock Group were deposited. Along northeastern marginsof of the the Steep Steep Rock of several metres to to metasandstone metasandstone and and Rock belt, belt, the the "old" "old" tonalite tonalite is is gradational gradational over over distances distances of several metres meaconglomerate at the base of the Steep Rock Group. Here the plutonic basement is tan-brown in meaconglomerate at the base of the Steep Rock Group. Here the plutonic basement is tan-brown in colour, shear zones zones and and abundant abundant metagabbro metagabbro dykes. dykes. colour, schistose, schistose, highly highly fractured, fractured, and and transected transected by by shear The dykes are are fewer fewer and and the the "old" in the The metagabbro metagabbro dykes " o l d tonalite tonalite is is less less intensely intensely foliated foliated in the central central Marmion this tonalite, tonalite, collected collected near the Ontario Ontario Hydro Marmionbatholith batholith(Marmion (MarmionLake Lakearea). area).A Asample sampleof ofthis generating station, has has aa U/Pb generating station, Ma (Don (Don Davis, Davis, Royal Royal Ontario Ontario Museum, Museum, U/Pb titanite titanite age age of of 2953 2953 Ma pers.comm., 1987). Granodiorite of the Margaret and Diversion stocks intrudes the northern northern and and pers.comm., 1987). Granodiorite of the Margaret and Diversion stocks intrudes the southern southern margins margins of of the the Marmion Marmion batholith. batholith. "Old" "Old" tonalite tonalite is is typically typically mediummedium- to to coarse-grained, coarse-grained, foliated, foliated, and and inequigranular inequigranular to to porhpyritic. quartz (25%), (25%), and andbiotite biotite(20%). (20%). Plagioclase Plagioclase and porhpyritic. It is composed composed of plagioclase (40%), quartz biotite are extensively altered to epidote, sericite, and chlorite in shear zones and biotite are extensively altered to epidote, sericite, and chlorite in shear zones and adjacent adjacent to to the the Steep Steep Rock Rock belt. belt. Sedimentary SedimentaryRocks Rocks The Rock Group Group and and consists consists of of pebble pebble and and cobble cobble The Wagita Wagita Formation Formation forms forms the the base base of of the the Steep Steep Rock conglomerates interbedded with with poorly poorly sorted sorted sandstones sandstones conglomerateswith withangular angularto to subrounded subrounded igneous igneous clasts clasts interbedded and fine-grained chioritic in thickness thickness from from 0 0 to to 150 chloritic layers. The clastic clastic sequence varies in 150 m and and is is overlain overlain by by carbonates. carbonates. The by dark dark kerogen The Mosher Mosher Carbonates Carbonates are are well well banded banded with with the the layering layering accentuated accentuated by kerogen streaks silica-rich ridges ridges at at some some locations. locations. The The laminated laminated carbonates carbonates streaks or millimetre-scale elevated silica-rich are are commonly commonly brecciated brecciated where where they they are are spatially spatially associated associated with with fault fault zones zones and and mafic mafic dykes. dykes. Calcite, and brecciated brecciated units, units, although although the the ankerite and dolomite dolomite are are present in both the laminated and Calcite, ankerite breccias Stromatolites are breccias often often have have an anadditional, additional,late-stage late-stagedolomitic dolomiticpore-filling pore-filling cement. cement. Stromatolites common forms dominating dominating in in the the lower lower portions portions and common throughout the carbonate carbonate unit, with smaller forms larger mounds, 33 m m in in diameter, diameter, becoming becoming abundant abundant near the upper contact (Wilks and Nesbit, 1988). very irregular irregular upper upper contact contact 1988).The Theentire entirecarbonate carbonateassemble assembleisisup upto to 500 500 m m in thickness with a very which which may may be be karstified. karstified. A rock" overlies the A 100 100to to 300 300 m m thick thick succession successionof of manganiferous manganiferous strata strata termed termed "paint "paint rock" overlies the Mosher Jolliffe Ore Zone, Zone, consists Mosher Carbonates. Carbonates. This Thissuccession, succession,which whichforms formsthe thelower lower unit of the Jolliffe consists of of an an of of an earthy, earthy, poorly poorly consolidated consolidated mixture mixture of of goethite, goethite, hematite, hematite, chert, chert, kaolinite, kaolinite, illite, illite,calcite, calcite, gibbsite gibbsite and and pyrolusite pyrolusite (Huston (Huston 1956). 1956). The Thepaint paintrock rockisisoverlain overlainby by the the upper upper unit unit of of the the Jolliffe Jolliffe Ore Ore Zone, Zone, the the Goethite GoethiteMember, Member, which, like like the paint rock, is composed of of goethite goethiteand and hematite, hematite, but versus 5%, 5%, respectively). respectively). The Goethite Goethite Member, Member, which which but has has aa lower lower manganese manganesecontent content(3.8% (3.8%versus is is 50 50 to to 100 100m m thick, thick,occurs occursboth bothas asbrecciated brecciatedmasses masses and and alternating alternating layers layers of goethite goethite and and chert chert (McIntosh mafic to ultramafic pyroclastics and minor (Mclntosh 1972). 1972).The The Goethite Goethite Member is is overlain by mafic thin thin flows flows of of the the Dismal Dismal Ashrock. Ashrock. �and Nisbet Nisbet (1 (1988) suggest the the depositional depositional environment environment of ofthe small Wilks and 988) suggest the basal basal clastics elasticsis is a small alluvial fan fan with alluvial with aa fluvial fluvial system system that that has has incised incised and and backfilled backfilled channels channels into into the the underlaying underlaying Marmion Complex. Complex. As Marmion As clastic clastic deposition deposition in in the the area area ended, ended, carbonate carbonate began to accumulate in a shallow marine setting. setting. The depth is indicated indicated by byprobable probablegypsumpseudomorphs, gypsum pseudomorphs,possible possible shallow The shallow shallow depth desiccated ripples, ripples, and and polygonal polygonal cracks cracks on on large (Wilks and and Nisbet, desiccated large stromatolites stromatolites (Wilks Nisbet, 1988). 1988). Stromatolite sabkha to Stromatolite morphology may reflect adaptations to various water depths ranging from sabkha subtidal (Wilks (Wilks and and Nisbet Nisbet 1988). 1988). The of the the carbonate carbonate assemblage assemblage suggests suggests an an subtidal The karstified karstified surface surface of interval of subaerial exposure that caused dissolution of the upper units. It is also possible that acidic interval subaerial exposure dissolution upper units. It is also possible that acidic pile and and dissolved dissolved the the carbonates. carbonates. In solutions moved downward through the sediment pile In either either case case the manganiferous manganiferous paint represent residual residual deposits deposits left leftbehind behindafter after carbonate carbonate dissolution. dissolution. paint rock may represent published concerning concerning the the origins origins of ofthe theoverlying overlyingJolliffe JolliffeOre OreZone. Zone. The The alternating alternating layers layers Little is published unit is a classic classic iron iron formation; formation; modified modified through through dissolution dissolution or or as as of chert and Goethe suggest this unit paleosol. aa paleosol. Volcanic Rocks Formation has has been been Volcanic Rocks The Thestratigraphy stratigraphyand andpetrography petrography of of the the Dismal Dismal Ashrock Ashrock Formation Komatiiticlapilli lapillituff tuff makes makesup up over over 80% 80% of of the the described by by Schaefer described Schaefer and Morton Morton (1991). (1991). Komatiitic formation, the remainder comprising comprising lenses lenses of komatiitic komatiitic volcanic volcanic breccia, breccia,komatiitic komatiiticvolcaniclastic volcaniclastic formation, sandstone and siltstone, and mafic to to ultrarnafic ultramafic lava lava flows. flows. The rocks are classified classified as as komatiites komatiites as defined defined by by Arndt Arndt and and Nisbet Nisbet (1 (1982) although spinifex spinifex texture texture is is rare. rare. Wilks (1986) recognized recognized as 982) although possible pseudomorphs after spinifex layering layering' in in one one lava lava flow, flow, and and Schaefer Schaefer and Morton Morton (1991) (1991) possible described lapilli fragments. The nvroclastic pyroclastic rocks of of the the Dismal DismalAshrock Ashrock described micro-spinifex micro-sninifex texture in lanilli Formation thought to have formed by explosive explosive volcanism volcanism caused causedby byultramafic ultramaficmagma magmaentering entering Formationare are thought water-saturated sediments Formations (Schaefer water-saturated sedimentsof of the the Mosher Mosher Carbonate Carbonateand and Jolliffe Ore Zone Formations (Schaeferand Morton, 1991). 1991). The Thetop topof ofthe the Dismal Dismal Ashrock Ashrock Formation Formation is in most places tectonised or invaded original stratigraphic stratigraphic relation-ship relation-ship occurs between between the the Dismal Dismal Ashrock Ashrock Formation-and Formation-and by sills but an original al., 1992; Tomlinson Tomlinson ct ct al., al., 1996% 1996a; TomlinTomlinthe Witch Bay Formation near Strawhat Strawhat Lake (Stone et al., son, 1996). 1996). The Witch Witch Bay Formation dominates Rock greenstone greenstone belt belt and and is is aa sequence sequence of of The dominates the Steep Rock mainly mafic mafic lavas lavas comprising comprising up up to to 80% pillow pillow lavas. lavas. Flyaloclastite and massive massive lava lava flows flows also also mainly Hyaloclastite and beds of of felsic felsic pyroclastic pyroclastic volcanics. volcanics. The occur along with rare beds The volcanics volcanics have have in places been highly suggested that the original thickness thickness was was likely likely to to approach approach 55km kmbased based folded. Stone et al. (1992) suggested of pillow pillow lavas lavas at at the the south south end end of of Steep SteepRock RockLake. Lake. The Witch Bay on a homoclinal sequence of Formation is is overlain by by intermediate to felsic felsic volcanics volcanics whose whose age age and and contact contact relationship relationship with with Formation intermediate to the underlying predominantly mafic mafic sequence sequence is unknown. - A. Finlayson and and Lumby Lake Greenstone Belts Finlayson Early reconnaissance surveys surveys were were conducted conducted in in the the Lumby Lumby Lake Lake area area by by the the Geological Geological Early Canada in the late andthe thearea areareceived receivedconsiderable considerableattention attentionfrom fromprospectors prospectors Survey of Canada late 18001s, 1800's, and 890ts.L. L.F.F.Kindle Kindletraversed traversedthe thearea areain in 1937 1937for forthe theG.S.C. G.S.C.and andthe therocks rocksof ofthe thebelt belt were were in the 11890's. by Jackson (1985). mapped for the Ontario Ontario Geological survey by Woolverton (1960) and again by Volcanic Rocks Volcanic mafic lava lavaflows flowspredominating. predominating. These These Volcanicrocks rocks dominate dominate the belts, with mafic commonly have massive lower portions and pillowed upper portions and are rarely capped by commonly have massive portions and pillowed upper portions and are rarely capped by brecciated flow flow tops. tops. Individual flows (ca. (ca. 44 m ni thick) by thin thin units thick) are are often often separated separated by units of of iron iron brecciated Individual flows �formation and and iron iron rich rich material material is is also also common commonbetween betweenpillows. pillows. Variolitic Variolitic lavas lavas are are found found in in the the formation upper portion of the stratigraphy. upper portion of the stratigraphy. Two komatiite komatiite horizons horizons occur occur in in the the northern northern half half of ofthe thebelt. belt. The The main main komatiitic komatiitic sequence sequence Two is 120 m thick and includes numerous komatiite flows, pillowed and hyaloclastite portions of flows, flows, is 120 m thick and includes numerous komatiite flows, pillowed and hyaloclastiteportions of variolitic intervals, subordinate mafic flows and inter-flow sediment comprising chert and iron variolitic intervals, subordinate mafic flows and inter-flow sediment comprising chert and iron formation (Tomlinson et al., 196b). Komatiite flows are 4 to 13 m thick with cumulate bases and formation (Tomlinson et aL, 196b). Komatiite flows are 4 to 13 m thick with cumulate bases and spinifex-textured tops. tops. This can be be traced traced for for ca. ca. 13 13 krn km along along spinifex-textured This predominantly predominantly ultramafic ultramafic sequence sequence can strike and and occurs occurs on pluton but but itit thins thins to to the This main main strike on either either side side of of the the Van Van Nostrand Nostrand pluton the east. east. This komatiite overlain by by further further mafic maficflows flowsand andiron ironformation. formation. An An upper upperkomatiitic komatiitic unit unit komatiite sequence sequence is is overlain is present east east of of the the Van Van Nostrand Nostrandpluton, pluton, near near the the top top of of the the stratigraphy. stratigraphy. This upper komatiitic komatiitic is present This upper unit varies varies along along strike. 20 m m thick thick bedded beddedpyroclastic pyroclasticunit unit(similar (similar unit strike. To Tothe the east east itit comprises comprises aa Ca. ca. 20 to the the Dismal Dismal Ashrock Ashrock Formation) Formation) interbedded interbedded with with thin thincarbonates carbonatesand andiron ironformation. formation. West West of ofthis this to is ca.20 20m mthick thickcoarse-grained, coarse-grained,unsorted unsortedand andmassive massiveultramafic ultramaficpyroclastic-flow pyroclastic-flowunit. unit. Both Both units units is aa ca. overlie a prominent prominent ca. l0-m-thick, chert-rich chert-rich iron iron formation formation and and are are themselves themselves overlain overlain by by further further overlie a ca. 10-m-thick, amygdaloidal amygdaloidal mafic mafic lavas. lavas. The main sequence in the the southern southern half half of of the the belt belt isis less lesswell wellknown. known. Fine-grained Fine-grained pillow pillow The main sequence in lavas are common but the exposure is not good enough to establish flow thickness or lavas are common but the exposure is not good enough to establish flow thickness or flow flow stratigraphy. phenocrysts up upto to22cm cmacross. across. Poorly Poorly exposed exposed stratigraphy. Some Somelava lava units units contain contain plagioclase plagioclase phenocrysts komatiite flows unknown extent near the the top top of ofthe thestratigraphy stratigraphy(which (whichwere werefirst firstrecorded recorded komatiite flows of of unknown extent occur occur near by Jackson, I 985a). Thin felsic tuff units (10-20 m) occur intermittently in the southern stratigraphic by Jackson, 1985a). Thin felsic tuff units (10-20 occur intermittently in the southern stratigraphic sequence. sequence. Sedimentary Rocks Sedimentary RocksAAthin thinsuccession successionofoffelsic felsicmetavolcanic metavolcanicunits units and and matasediments matasediments occurs occurs approximately halfway halfway through through the the volcanic volcanic sequence sequenceand and aa thicker thicker assemblage assemblage of of elastics clastics and and approximately chemical sediments caps the In the the Finlayson Finlayson Lake Lake area area the the lower chemical sediments caps the assemblage. assemblage. In lower clastic clastic wedge wedge thickens and and coarsens coarsens to to the the southwest, and is is gradational in this this direction to aa felsic thickens southwest, and gradational in direction to felsic volcanic volcanic assemblage (Figure (Figure 4). Beds vary in thickness assemblage 4). Beds vary from from centimetre-to centimetre-to metre-scale metre-scale in thickness averaging averaging approximately 70 Coarse-grainedsand sand with with felsic felsic volcanic-clast pebble bands bands approximately 70 centimetres. centimetres. Coarse-grained volcanic-clast pebble dominates, except except in in volcanic volcanic proximal proximal areas areas where units gain gain in in importance. importance. dominates, where the the conglomeratic conglomeratic units Clastic mud drapes drapes and and mafic centimetre-scaleinterbeds. interbeds. The The sandstones sandstones are are Clastic mud mafic tuffs tuffs rarely rarely form form centimetre-scale massive and and nongraded. nongraded. Bed the metasedimentary massive Bed thicknesses thicknesses decrease decrease to to the the northeast northeast where where the metasedimentary succession thin and sporatically present. succession is thin �Little Falls! Little Falls 1Finlayson Finlayson Lumby Lumby i V A . . "V V v v Oo:,.. . V " "V V v v V oO...... A . VV . VV V V v v v v v + ++ + +±± + 500 m V V V V V V V V V v V v VV v v V v ; .. v v V V V V vV vV v V V V v Scale V V V V V V V vV V v ±±+ V vV vV ±± ± ± ++± + + + V V IV V V ~ v v v v l'V " V " V v Vv Vv v v I V vV I , 61 Iv V V v Vv V v v V V V V v v .....:::...: .... ...-.... V V V v V V v V V V V V v v v vv v v. v v vV vV "V v V " V v V v V v V vV "V "V vV v V vV v V v V v V v V v vv v v v v v v v vV vV vV vV vV vV vV vV vV vV v v V v K\"+";;v:v V v v V v v v v + + v + + + + + v v v v v v + + + + + + + V V V + + + + + + + v+ v + + + + + Figure Figure 4. 4. V ~ I" vV vV vV vV vV vV vV vV vV vV vV vV vV vV vV v ~ v V v V v V v V v V v V v V v V v V v V vV vV vV vV vV v ~ lv vv vv vV vV vV vV vV vV vV vV vV vV vV vV vV v l v v v v v v v v v v v v v v v v I lYlafic Volcanic Volcanic Mafic Rocks Rocks AA Felsic Volcanic Faisic Volcanic Rocks Rocks 1+ +1 Intrusive lntruslveRocks Rocks IV V '' v IV ±1 4 km Iron Iron Formation Formation Carbonate 11111111111 Carbonate Lo0oal Conglomerate Conglomerate Pf1 Sandstone/Siltstone Sandstonepiltstone A J Agglomerate Agnlomerate .. l;".;,vvy .:,.v V + + - " - V V V v A V V v + V " " v + v+ + Reconstruction of Finlayson andLumby -.sect;anal Reconstruction of the the units unitsin in the Finlayson andLumbyLakes Lakesarea areain incross trim-sectional view. view. The The stratigraphy stratigraphyisIs discussed discussedin in the thetext. text. These sediments sediments represent represent aa clastic clastic debris apron formed adjacent to, and deriving sediment These sediment Sedimenttransport transport was was via via subaqueous subaqueous sediment-water sediment-water dispersions dispersions from, a felsic volcanic centre. Sediment with to keep keep material material in in suspension. suspension. This with dispersive dispersivepressure pressure probably acting acting as an important force to This requires a high slope, slope, possibly higher higher than 10 in the the area areathe theflows flowswere weregenerated. generated. Subarial Subarial requires 10 degrees, in mafic mafic volcanism volcanism was was also also active activeduring during this this depositional depositional interval. interval. The Lake area area is is also also laterally laterally transitional transitional metasedimentary belt belt present present in the Lumby Lake The lower lower metasedimentary to felsic centimetres, and felsic volcanics. volcanics. Sandstones Sandstonespresent presenthere hereare are more more thinly bedded, averaging 15 centimetres, do mafic ash interbeds. They nongraded varieties exist. do not contain containmafic ashinterbeds. Theyare aremassive, massive,and and both both graded graded and andnongradedvarieties exist. The elastic with cherts cherts and and carbonates. carbonates. The cherts are laminated laminated on on the the clastic sequence is intercalated with centimetrerecrystallized with with fractures, fractures, which may centimetre-and and subcentimeter-scale subcentimeter-scaleand and the the carbonates carbonates are recrystallized represent represent original original bedding, bedding, spaced spaced on on the the decimeter-scale. decimeter-scale. This provides very little information information on ondepositional depositionalenvironment. environment. The sandstones sandstones This sequence sequence provides suggest imply aa water water suggest deposition depositionby by grain grainflows flows and and high-density turbidity turbidity currents, currents, but this does not imply depth. Personal Personalexperience experienceindicates indicatesthat thatArchean Archean carbonates carbonates associated with cherts usually form in shallow shallow water water environments, environments, though though this this remains remains speculative. speculative. The capping cappingsedimentary sedimentarysequence sequencepresent present at Finlayson The Finlayson Lake conformably conformably overlies overlies pillowed and massive mafic volcanic pile in the volcanic rocks in the the southern and central area and an agglomeratic pile north. Basal by centimetre centimetre thick thick DE DE Basalcherts chertsand andoxide oxidefacies faciesiron iron formation formation are are succeeded upwards by turbidites. turbidites. These gradually gradually thicken and coarsen coarsen up sequence sequence to decimeter decimeter thick, graded graded beds beds Above this this zone zone internal, internal, scallop-shaped scours, composed of medium-grained sand. Above scours, filled filled with coarse-grained sand, appear in the beds. These Theseare areoverlain overlainby by pebble pebble and and cobble cobble conglomerates conglomerates with sandstone sandstone interbeds interbeds and lenses lenses (Figure (Figure 5). 5). The coarsening coarsening upward upward trend trend in in this this zone is interrupted grainsize to medium-grained medium-grained turbidites, turbidites, which again gradationally gradationally interruptedby by abrupt abruptdecreases decreasesin grainsize coarsen coarsen upwards upwards to to conglomerates. conglomerates. + + v �m. F-6 200 S. s. graphite graphite sulfides sulfides C. C. ch e ii chert 9. 9. 150 F-5 1I-I- +H Diabase Diabase 1 Conglomerate Conglomerate Sandstone with with lil Sandstone L°o°o°S! I.E I.F. Iron IronFormation Formation ________ [i.cJ j1 - ________ L: fl—El :1 L _______ H 1 bc?ed ________ 100 Al I scours scours Sandstone Sandstone ( turbidites ) (turbidites) Siltstone Siltstone Slate SlateCouplets Couplets Slate Slate MaficVolcanic Volcanic Mafic Rocks Rocks Agglomerate Agglomerate F-4 F-2 F-i c.Is. ___ oJ c..,g.js. 0 FigureS. sections through Figure 5. Stratigraphic Stratigraphicsections throughthe theuppersedimentary uppersedimentaryunit unitononFinlayson FinlaysonLake. Lake.Section Section F-i logged other F-1was was loggednear nearthe thenorth-east north-eastend endofofthe thelake. lake.The The othersections sectionsare arespaced spaced along approximately iO10 km alonga aline linerunning running approximately krntotothe thesouth-west south-westand andending endingwith withF-6 F-6inin the lake. system braidedfault systemlongitudinally longitudinallycutting cuttingthe thebelt belt thesouthern southernportion portionofthe of the lake.AAbraidedfault inintwo twonear nearitsitscenter centerdownsteps downstepsthe thestratigraphy stratigraphyininaanorth-east north-eastdirection. direction. 1 I < 1 I �3,000 Ma 2,925 Ma ?7 A v V V v Vv v V v " V V .—C?\ 1 Little LittleFalls FallsI1Finla Finlayson yson V I? I? — —— r fle?.o.o.c?1o.? OOoooo C v ?1 ? I V v V v V v Lumby Lumby vV / Iron Iron Formation Formation IJJJfflJJCarbonate Carbonate J'I Conglomerate Conglomerate Sandstone Slate % H I Agglomerate Siltstone I/Slate Agglomerate ..... Sandstone ____ ____ Siltstone IAAI Felsic F e M cVolcanic VolcanicRocks Rocks Mafic VolcanicRocks Rocks l"I Mafic Volcanic Figure for the Figure6.6. Sequential Sequential environmental environmental reconstruction reconstruction of of the the depositional depositional environments environments for the lower sedimentary units of the the Finlayson Finlayson and lower sedimentary sedimentary units units(A) (A) and and upper uppersedimentary units (B, C) of and Lumby Lumby Lake greenstone greenstone belts. Processes Processes are arediscussed discussedin in the the text. text. Il �This After cessation cessation of of mafic This succession succession represents represents shoreline shoreline progradation progradation (Figure (Figure 6). 6). After mafic volcanism a starved succession of chemical sediments was deposited. The first clastics delivered volcanism a starved succession of chemical sediments was deposited. The first clastics delivered to silts transported transported by by low-density low-density turbidity turbidity flows. flows. This This probably probably represents represents to the the area area were were clays clays and and silts prodelta debris, though sediment transport via storm currents is possible. Shoreline progradation prodelta debris, though sediment transport via storm currents is possible. Shoreline progradation caused caused the the coarsening coarseningupwards upwardssuccession successionto to form, form, culminating culminatingin in deposition depositionin in depths depthseffected effectedby by wave induced scour. Delta top sands and gravels cap the sediment pile and show the effects lobe wave induced scour. Delta top sands and gravels cap the sediment pile and show the effects of of lobe outbuilding outbuildingand andabandonment, abandonment,with withsubsequent subsequentsubsidence subsidenceinduced induced flooding floodingand and aa return return to to deeper deeper water water conditions. conditions. This Thisisisfollowed followedby byaaminor minorcoarsening coarseningupward upward sequence sequenceformed formedby by another anotherstage stage of is capped capped by by aa final final flooding flooding event. event. of delta delta lobe lobe outbuilding outbuildingin in the the area area and and the the entire entire sequence sequence is The The sedimentary sedimentary sequence sequence capping capping the the assemblage assemblage of of mafic mafic volcanic volcanic rocks rocks in in the the Lumby Lumby Lake thick sequences sequences ofboth ofboth clastic and chemical chemical units units (Figures (Figures 5). 5). WayLake greenstone greenstonebelt belt consists consistsof of thick clastic and up up directions directionsare are ambiguous ambiguous though though itit is is more more likely likely that that the the iron iron formation formation and and chert chert assemblage assemblage overlies overlies the the elastic clastic assemblage. assemblage. AAthin thinzone zoneof ofchert chertand andiron ironformation formationseparates separates the the volcanic volcanic sequence sequence from from the the clastic clastic sediments. sediments. The Theclastics clasticsconsist consistof of decimeter-scale decimeter-scaleturbidites turbiditeswith with metremetrescale the map map area. area. Structural scale conglomeratic conglomeratic interbeds interbeds in in the the western western portion portion of of the Structural complexities complexitiesand and scarcity if aa coarsening coarsening upwards upwards trend trend exists. exists. In scarcity of of outcrops outcrops prohibited prohibited ascertaining ascertaining if In the the west west the the elastic clasticsuccession successionis isoverlain overlainby by thinly thinly laminated laminatedcarbonates carbonates with with submillimeter-scale submillimeter-scale clay clay drapes drapes commonly commonly developed developed between between laminations. A Avery very chert-rich chert-rich iron formation formation assemblage assemblage overlies the carbonates in the west, lies on clastics in the central area and directly overlies the carbonates in the west, lies on clastics in the central area and directly overlies the the volcanic volcanic succession successionin inthe theeastern easternoutcrop outcroparea area(Figure (Figure4). 4). Scarcity of outcrop Scarcity of outcrop in in this this belt belt inhibits inhibits aa detailed detailed reconstruction reconstruction of of depositional depositional environments. environments.After Aftercessation cessationof ofvolcanic volcanicactivity activityaaclastic clasticsediment sedimentstarved starved interval interval was was followed followed by by deposition depositionvia via high high and and low low density density turbidity turbidity currents. currents. Conglomeratic Conglomeraticunits unitsprobably probably represent represent the the most most strand-proximal strand-proximaldepositional depositional events. events. This Thisisisfollowed followedby bythe thesequence sequencebecoming becomingrapidly rapidly sediment sedimentstarved starvedwith withcarbonates carbonatesdeposited deposited in in the the shallower shallower areas, areas, in in the the west west where where the the strandstrandproximal and cherty cherty iron iron formation formation in in deeper deeper areas. areas. With proximal conglomerates conglomerates are are located, located, and With increasing increasing water migrated westward westward until until iron iron formation formation diachronouscontact contactwith with the the iron iron formation formation migrated water depth depththe the diachronous was was being being deposited depositedthroughout throughoutthe theentire entirearea. area. Structure Structure The from mainly mainly The east-trending east-trending Quetico-Wabigoon Quetico-Wabigoonboundary boundary separates separatesturbidites turbiditesto the south from volcanic rocks to the north and is a first order lithostrnctural feature in the area. Regional volcanic rocks to the north and is a first order lithostructural feature in the area. Regional juxtaposition subduction-related accretion accretion of ofthe juxtapositionof ofthe thecontrasting contrastinglithologies lithologieshas has been been attributed attributed to subduction-related the Quetico sedimentary prism against the Wabigoon volcanic arc about 2695 Quetico sedimentary prism against the Wabigoon volcanic arc about 2695 Ma Ma ago. ago. Quetico Quetico metasediments metasedientshave have pervasive pervasive easterly easterly structural structural trends trends and and isoclinal, isoclinal, mainly mainly easteast- facing facing folds folds possibly possibly generated generated by by strong strongdextral dextraltranspression transpression after after accretion. accretion. In contrast, contrast, Wabigoon WabigoonSubprovince Subprovinceshows showsmegascopic megascopicoval ovalstructures structuresdefined definedby by concentrically concentricallyzoned zoned structural structural trends trends in in gneisses gneissesand and supracrustal supracrustal rocks rocks that that envelope envelope felsic felsic plutons. plutons. Metavolcanic belts are are situated situatedbetween between some someoval oval structures. structures. This Thisstructural structuralpattern pattern may may have have originated originated due to rise of granites overlying granitesand and gneisses gneissesin incentral centralparts parts of of oval oval structures structures attendant attendant with slumping of dense overlying supracrustal supracrustalrocks rocksinto intomarginal marginaltroughs troughs(metavolcanic (metavolcanicbelts). belts). The Theaxes axesof ofsix sixmajor majorfolds foldsextend extendalong alongthe the central central Finlayson Finlayson belt belt whereas whereas the the Steep SteepRock Rock belt belt shows showsfewer fewerfolds foldsand andaa thick thick east-younging east-youngingsequence sequenceof of pillow pillow lavas lavas that that may may be be aa deformed deformed lava lavacone. cone. �A complex system system of of oblique-slip and and normal normal faults faults and and metagabbro metagabbro dykes dykes transect transect the the A Marmion batholith. batholith. The Samuel fault fault appears appears to to have have The dip-slip, dip-slip, probably probably reverse reverse displacement displacement Samuel uplifted a segment segment of the batholith batholith into into the the central central Steep SteepRock Rockbelt. belt. Displacements Displacements of of reverse reverse sense sense uplifted are also likely to have occurred on the Atikokan fault, which is conformable conformable with the the upper upper ashrock ashrock transcurrent fault fault in in the the vicinity vicinity of of contact in the Steep Rock Mine area. Clear Clear evidence evidence of a major transcurrent the Quetico-Wabigoon Quetico-Wabigoon boundary has not been found. Metamorphism Metamorphism Supracrustal belts of Metavolcanic belts belts have Supracrustal belts of the the Atikokan Atikokan area areaare aremetamorphically metamorphically zoned. zoned. Metavolcanic greenschist assemblages (chlorite-epidote-plagioclase-calcite+actinolite) and and greenschist assemblages at their cores (chlorite-epidote-plagioclase-calcit&actinolite) amphibolite assemblages assemblages at the margins margins (hornblende-plagioclase-biotite-ilmenite-sphene). (hornblende-plagioclase-biotite-ilmenite-sphene). amphibolite Metamorphic grade grade does does not not change but increases Metamorphic change abruptly abruptly at at the the Quetico-Wabigoon Quetico-Wabigoon boundary boundary but increases garnet, and and staurolite staurolite isograds. isograds. Pressuresteadily southward as defined by biotite, Ca-amphibole, garnet, Presswetemperature conditions conditions in in the the garnet garnet zone zone of of Quetico Quetico metasediments metasediments fall fall in in the the upper upper greenschist greenschist temperature facies facies (P-230-430 (P230-43O MPa, MPa,T-419-577T). T4l9-577°C). Except for greenschist greenschist assemblages assemblages in the Marmion Marmion batholithadjacent batholith adjacent to to the the Steep Steep Rock Rock belt, belt, Except tonalites and gneisses show amphibolite conditions (P=560 (P=560 MPa, MPa, T=7OO0C). T=700°C). tondites The high metamorphic grade of of gneisses gneisses within within oval oval structures structures is is attributed attributedto totheir theirhaving having risen from deeper crustal levels, as well as proximity to felsic plutons. Thermal anomalies risen from deeper cmstal levels, as well as proximity to felsic plutons. Thermal associated with oval structures associated structures and and plutons, plutons, superimposed superimposed on regional burial metamorphism, metamorphism, may may be responsible responsible for the metamorphic zonation of these supracrustal belts. metamorphic zonation supracrustal Tectonic Environment Environment Tectonic Nisbet (1988) were were the the first first to to attempt attempt to to define definethe the tectonic tectonic environment environment that that Wilks and Nisbet Mesoarchean Mesoarchean strata strata in the Atikokan Atikokan area area formed in. They believed that a rift, formed by extension extension of the Marmion Marmion block, is the most most probable probable tectonic setting for the the Steep Steep Rock Rock Group. Group. This explains explains the outcrop outcrop patterns patternsof of the the Steep SteepRock Rock and and nearby nearby Lumby Lumby Lake Lake successions successionsas as having having formed formed on on two two adjacent tilted tilted blocks. blocks. Alternatively, may have have taken taken place place on on aa stable stable divergent divergent adjacent Alternatively, sedimentation sedimentation may continental margin with all or only the lower lower part of of the the Wagita Wagita Formation Formation representing synrift symift deposits and the remainder of the succession up to and including the Joliffe Ore Zone reflecting deposition on a post-fift post-rift stable shelf. shelf. If the overlying Witch Bay Bay volcanics volcanics are areautochthonous, autochthonous,they they deposition record renewed extension. extension. A study by Thurston and Chivers (1990) put the Steep Rock and Lumby Lake Belts into a A * group they they termed sequences. These These consist, consist,from frombase basetoto top, top, of of quartz group termed platform platform sequences. quartz arenite arenite ± carbonate, oxide facies iron formation and komatiitic to tholeiitic volcanic rocks. They strornatolitic strornatoliticcarbonate, oxide facies formation komatiitic to tholeiitic volcanic rocks. They compare these with plateform sequences at modern passive margins. compare sequences at modem at the the base base of of the the Steep Rock Fralick and King (1995) correlate the sedimentary sequence at Group with sediments at the top of the Finlayson-Lumby Lake Lakevolcanic volcanicsuccession. succession. This implies chronologicallyseparated separatedby byaasedimentation sedimentationevent. event. They They also also two periods of mafic volcanism chronologically record subsidence of of an an oceanic oceanic plateau plateauand andthe thelong longperiod period stress that the sedimentary sequences record plume driven volcanism is consistent consistent with with coupling coupling of ofthe thearsenosphere arsenosphereand andlithosphere. lithosphere. If If they they of plume are correct this would plate tectonics during the the early are would negate negate the the operation operation of of modern-style modem-style plate tectonics during Mesoarchean. Mesoarchean. �Hollings et a1 a! (1998) agree with Fralick and and King's King's (1995) stratigraphic interpretation but advance a more elaborate model model to to explain explainthe thegeochemistry geochemistryof ofvolcanic volcanicand andintrusive intrusiveunits. units. They They which is mainly mainly represented represented by by the the believe that an arc arc developed, developed, due to subducting lithosphere, which deformed and eroded remnants of the intrusive tonalites. tonalites. Failed subduction of an ocean plateauridge complex al, 1998). complex juxtaposed the the arc arc intrusives intrusives with the metavolcanics (Hollings et a], 1998). Kusky and Hudleston (1999) agree with with Hollings Hollings et et d al (1998) (1998)that thatthe thetonalites tonalitesrepresent representan an sedimentary succession formed formed aa shallow shallow water water carbonate carbonate eroded arc remnant, but suggest that the sedimenw platform during arc rifling and the Dismal Ashrock is an allochthonous melange overridden by rifting Dismal allochthonous ovemdden by structurally structurally emplaced emplaced arc arc volcanics volcanics of of the the Witch Witch Bay Bay Formation. Formation. Based on extensive geochemical analysis of volcanic units Tomlinson et al(1999) al (1999) propose that the unfractionated high-Mg basalts basalts represent represent high unfractionated basaltic basaltic komatiites and spinifex-textured spinifex-textured high-Mg degree partial melts of the upper mantle. These are geochemically similar to lavas erupted forming empted forming degree These the modem Ontong-Java Ontong-Java oceanic plateau. Enriched Enriched basaltic basaltic komatiites represent a deep mantle plume source that has not been mixed with depleted Volcanism in in the the Steep Rock, has not been mixed with depleted mantle. mantle. Volcanism Rock, Finlayson and Lumby Lake belts most likely occurred in a continental setting and was probably proceeded by rifling rifting of of the continental continental crust. The Thestratigraphy stratigraphyof of the greenstone greenstone belts and evidence of magmatism may 70 Ma) Ma) indicates indicates that more than one pulse of plume magmatism of protracted protracted volcanism volcanism (ca. (ca. 70 have been involved (Tomlinson et a!, 1999). have been involved (Tomlinson et al, 1999). Obviously opinions tectonic setting the the succession succession developed developed in. The fieldtrip Obviously opinionsdiffer differas as to the tectonic will provide a form where these ideas can be discussed on the outcrop. provide a form where these �REFERENCES: REFERENCES: - Antevs, E.,l951, E.,l951, Glacial Glacialclays claysininSteep SteepRock RockLake, Lake, Ontario, Ontario,Canada; Canada; Geological Society Sooiety of America Bulletin, Bulletin,v. v.62, 62,nono.ID, 10,p. p. 1223-1262 1223-1262 Brain, Canadian Mining JJournal, v. 61, 61, p. p. 573.574. 573-574. 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Biotite-gmct gemhernomew in biotite; biotite; American American Mineralogist, v. Minedogist, v. 70, 70, p. p. 272-278. 272-278. Irvine, TN., of rocks; Journal of of Eanh Earth guidetotothe thechemical chemicalclassification clw~ificalio of ~the the common common volcanic vd-ic r o c k , Canadian Canadian Journal T.N.,and andBaragar, Baragar,W.R.A.. W.RA., 1971. 1971,AAguide Sciences, v. v. 8, p. 523-548. Scienoes, 523-548. Jackaon,M.C., MC., I1985% 985&, Geology of western ppart. Districtsof ofKenom Kenoraand andRainy RainyRiver River. Ontario Ontario Gedogioal Geological Survey, Survey, Open Open of the the Lumby Lake Area, h a , western a t Diwicts Jackson, File Report 5514. 5514, 178 178 pp. Jackson. M.C., MC.. 1985b., and Ralny Rainy River; River; Ontario Ontario Geological Geological Survey, Survey, Open Open Kenom and Jackson. 1985b.. Geology Geology of ofthe theLumby LumbyLake LakeArea, h a ,eastern eastern part, p a districts of Icenora File Report 5535. 5535, 142 142 pp. Jackson, M.C., MC., 1985, Districts of Kenom Kenora and and Rainy Rainy Rivcr: River: Ontario Ontario Geological Geological Survey. Survey. Open Open Jacbon, 1985, Geology Geology of of the the Lumby Lumby Lake Lakearea, area, western wesern ppart, a t , Districts File Report File Report 5534. 5534. Jolliffe, A.W., Rock Lake. Lake. Economic Geology Geology 50 50(4), Jolliff', A.W., 1955, 1955,Geology Geology and and iron imn ores ores of of Steep Steep Rock (a), 373-398. 373-398. Jolliffe, A,W,, Rock Lake, Lake, Ontario: Ontario: in in The The relatiomhip relationship of of rnineralimion mineralization to ofthe the Steep Sleep Rock Rock Group, Group, Steep Steep Rock lo Precambrian Prccmb"an Jolliffe, A.W., 1966, 1966,Stratigraphy Stratigraphy of stratigraphy in certain m mining ofOntario and Quebec, PrecambrianSymposium, Symposium,edited editedby byA.M. A.M.Goodwin, Goodwin,Geological Geological Association Association smtigraphy i n i n gareas m m of Ontarioand Qmebeo, Precambrian Paper No. No. 3, 3, p. p. 75-98. 75-98. of Canada, Special Paper Jolliffe, A.,W., oresof ofSteep Steep Rock Rock Lake: Lake: Economic Economic Geology, G d o g y , v.50, v. 50,no.4, "0.4. p.373-398. p.373-398. Jolliffe, A..W., 1955, 1955,Geology Geologyand andiron imnores c h e a n granitic updm K m i n m i , D.C.. D.C., Thivierge. Thivierge, R.H., RH.. and and Stone, Stone, D., Kaminent. D., 1988, Development Developmentof of aa camlwtic cataclasticfault faultmne zoneininan ankArchean graniticplulm pluton of of the the SSuperior structural, geochemical geochemical and andgeophysical geophysicalcharacte"stics: characteristics:AAmerican JournalofofScience, Science, v.v.288, 288, p.p.458.494. 458494. Province: smclural, mdJournal Lakespluton, ploton, Atikokan, Alikokan,northwestern norlhwestern Ontario, Canada; Canada; Kmineni, D.C., 1982,AAstudy sNdyofofrock mckalteration alterationininthe theEye-Dashwa Eye-Dmhwa Lakes Kansineni, D.C.. and and Dugal, Dugal, J.J.B., JIB., 1982, ('heroical Geology, v. 36, 36, p. p. 35-57. 35-57. Chemical Geology, v. Kmineni, D.C., and andStone, Stone, D.,1983, D.,1983, The The ages ages of fmcturos in tlse the Eye-Dmhwa o Mineralogy Kamineni, D.C., of fractures Eye-DashwaLakes Lakespluton. pluton.Atikokan, Atikokan, Canada; Canada;Contribuliom Contributionstto Mineralogy and Petrology, Petrology, v. v. 83, p. 237-246. and D., 1983, granite kKamineni, i n m i , D., 1983, Sulphur-isotope Sulphur-isolop~geochemistry gco~hemistryof fracture-filling xiacNre-filling gypsum mpaum in in an an-'Archean --A~chean granitenear near Atikokan, Atikokan,Ontario. Ontario.Canada: Canada: Chemical Geology Geology v.39.p.263-272. Chemical v.39.p.263-272. Kimberley, M.M.,and andSorbara, Sorbma,J.P., J.P..1976, Post-Archean weathering of pmceedings of of the the 1976 1976Geotraverse Geomve-=Kimberley, MM., 1976, Post-Arehean of Steep Steep Rock Rock Gmup GroupIron Iron Fornation: Formation: proceedings Conference, University University of Conference, of Toronto, Tomnto, Toronto, Toronto, Ontario, Ontario, p. p. 128-136. 128-136. �Lawson, AC., Lake, Ontario Canada, Canada, Memoir28, Memoir 28, P. P 7-15. A.C., Geological GeologicalSunYey SunYey011912, of 1912,The TheGeology Geologyof of Steeprock SteepmckLake, 7-15 Legget, MW., 1953, Ontario, Canada: Canada: Economic Economic Geology, Geology, v.v. ha% R.F., R.F., and and Bartley, B d e y , M.W., 1953,An An engineering engineering study study of of glacial glacialdeposits dewsits at at Steep Steep Rock Rock Lake, Lake, Ontario, 13-540. 48, 48. p. p. 5513-540. Miller, v. I2,p. 12,p. 304-317. 304-317 Miller, Wa., W.G.,1903, 1903.Iron lmnranges rangesofofnorthern northernOntario; Ontario;Ontario OntarioBureau Bureauof ofMines, Mines, v. Moore, Mcare, ES., E X , 1939, 1939,Geology Geologyand and ore oredeposits depositsof ofthe theAtikokan Atikokan area: area: Ontario Ontario Department Department of Mines, Mines, v. 48, 48, Pt. pt. 2, 2, P.p. 1-34 1-34(published (pblished 1940). 1940). Accompanied 48a, scale scale 1I inch inch to to II mile. Accompanied by by Map Map 48% mile. Moore, ES., Canada, Transaction Section 4,3rd 4,3rd series, v.32, E X ,1938, 1938,The TheSteeprock SteepmckSeries: Series: Royal Royal Society Society of ofcanada, series, v. 32,p. p. 11-23. 11-23, Morris, M.V., 1986, Geochemistry and tectonic seuing setting of the Steep Rock Rock greenstone greenstone belt, belt,Atikokan, Atikokan,Canada; Canada;BSc. B.Sc.thesis, thesis,Carleton CarletonUniversity, University, 1986, Geochemistry Ottawa, Oltawa,Ontario, Ontario,162p. l62p. Roberts, T,L. and Bartley, replacement in in deep deep seated seated iron iron ore deposits Lake Superior Superior region; Economic hnomic B d q . M.V, M.V, 1943, 1943. Hydrothermal Hydmthennal replacement dewsits of the Lake Geology, v' 38. Geology, v 38. p. p. 1-24. 1-24, their geology, geology, ppetrography, Schaefer, 5.J., S.J.. Morton, Morton, P., P., 1991, 1991. Two Two komatiitic komatiiticpyroclastic pymclastic units, units, Superior Superior Province, Fmvince, northwestern northwestern Ontario: their e m p p h y , and correlation. Canadian CanadianJournal Journal of of Earth EatthScience Science 28, 1455-1470. 1455-1470. Schaefer, Schaefer, S.J., 1989, 1989, AAcomparison comparisonofoftwo twoultramafic ultramaticpyroclastic pyroclasticrock rock units, units, northwestern northwestem Ontario: Ontario: Institute Institute of of Lake Lake Superior SuperiorGeology Geology Proceedings, v. 35, p. 77. Proceedings, Shklanka, Shklanka, R., 1972, 1972, Geology Geology of of the the Steep SteepRock RockLake Lakearea, area,District DistrictofofRainy RainyRiver; River;Ontario OntarioDepartment Department of ofMines Minesand andNorthern NorthernAffairs, Affiirs, Geological Report93, Report 93, I l4p. Smith, Sheet. Geological Survey of Canada, sscale Smith, W.H.C., W.H.C., and Mclnnes, W., 1897, Seine River Sheet. Suwey ofcanada, a l e I1:253 12.53440. 440, of America Bulletin, v. v.4, Smith, 4, p. p. 333-348. 333-348 Smith, W.H.C., 1893, 1893, The Archean Archean rocks rocks west of of Lake Superior; Geological Geologicd Society Society ofAmerica Smith, Smith, F.G., 1942, 1942, Notes Notes on on the the iron iron ores ores of of Steeprock Steepmck Lake, Lake, Ontario; Ontario; University Universityof of Toronto Toronto Studies, SNdies, Geological Geologid Series, Series, No. 47, Contributions Contributions to 1-75. to Canadian CanadianMineralogy, Mineralogy,p.p.771-75, Sniyth, Lake, Ontario; Ontario;American AmericanJournal JournalofofScience, Science,v.v.42, 42, p.p.317-331. 3 17-331 Smyth, H.L., H.L., 1891, 1891, Structural Structural geology of Steep Rock Lake, Stone, Stone, D., Kamineni, Kamineni, D.C., D.C., Brown, Brow, A., A., and and Everitt, Eveflit R.A., R.A., 1989, 1989. A A comparison comparisonof of fracture h t u r e styles stylesin in two two granitic graniticbodies bodiesof ofthe theSuperior Superiorprovince; province; Canadian Journal of Earth Sciences. Sciences. v. 26, p. 387-403, 387403. Canadian Stone, 1982, Fractures and fracmre fracture in in fillings fillings of of the the Eye-Dashwa Fye-Dashwa Lakes Lakes pluton, pluton, Atikokan, Atikokan, Ontario; Ontario; Canadian Canadian Journal Journal of of Stone, U.. D., and and Kamineni, Kamineni,D.C., D.C.,1982, Earth Sciences, Sciences, v 19, 19, p. 789-803. Stone, the Atikokan Canada, Map 1666A, Stone, D., Kamineni, Kamineni, D.C., D.C., 1989, 1989,Geology Geologyof ofthe Atikokan area, area, Ontario Ontario Geological Geological Survey Suwey of ofCanada, 1666A, scale 1:50,000. 1:50,000, Precambrian geology geology of of the Atikokan m area, Ontario. Geological Survey of Stone, Stone. D., D., Karnineni, Kamineni. D.C., D.C., Jackson, Jackson,M.C., M.C., 1992, 1992, Precambrian a, northwestern northwestern Ontario. Bulletin 405, 106 Canada, Bulletin I06 pp. 35, p.p. 131-141, 1941,Origin Tanton,T.L., 1941, Origin to the hematite deposits at Steep Rock Rock Lake, Lake, Ontario; Ontario;Royal RoyalSociety Societyof ofCan* Canada,Transactions, Transactions,v.v.35, 131-141. Tanton,T.L., Lake, Ontario; Ontario; Royal Royal Society Society of of Canada, Canada,Transactions, Transactions,Section Section4,4,v.v.20, Tanmn,T.L., 1926, 1926, Recognition Recognition of the Coutchiching near Steep Rock Lake, 20, p. 39-49. 3949. Tanton,T.L., of Canada, Canada,Transactions, Transactions,v.v.40. p. 103-112 103-112. Steep Rock Lake; Royal Society of 40. p. Tant0n.T.L.. 1946, 1946. The iron iron ore at Steep Tanton,T.L., 1927, 1927, Mineral Mineral deposits deposits of of Steep Steep Rock Rock Lake Lake map-area, maparea, Ontario; Ontario; Geological Survey Swwey of of Canada, Canada, Summary Summay Report, 1925, 1925,Pt. pt. C, C, p. p. Tanton,T.L., IC-lIC, IC-IIC. Secular variation variation in greenstone Canada. P.C., Chivers, Chivers, K.M., K.M., 1990, 1990, Secular Thurston, P.C., greenstone sequence development development emphasizing Superior Province. Province. Canada. Research 46,2148. 46,21-58. Precambrian Research and Chivers, Chivers, K.M., K.M., 1990, 1990,Secular Secular variation variation inin greenstone greenstone sequence sequence development development emphasizing cmphmizing Superior Province, Canada: Canadc Thurston, P.C., and Research:v.v.46, p.21 Precambrian Research: 46, p. 21 -58. -58. 'Tomlinson, K.Y., Thurston, P.C., P.C., Hughes, Hughes, D.J., D.J., Keays, R.R., 1996%The central Wabigoon Wabigom region: pemgenesis of mafic-ultramafic matic-ultram&k rocks Tomlinson, KY.. Thurston, R.R., l996a, region: petrogenesis rocks in in the Steep Rock, Lumby Lake and O h n g a Lake greenstone In: Harrap, Harap, R.M., RM., Obonga greenstone belts belts (continental (continental rifIing rifting and drifIing drifting in in the Archean). Archean). In: Helmstaedc e p m #53. Helmstaedt, H. Fds.), (Eds.), Western Superior Transect Transect Second SecondAnnual AnnualWorkshop Workshop(Oci (Oct.20-2l,l995), 20-21,1995),Lithoprobe LithoprobeRReport #53.Lithoprobe Lithoprobe Secretariat, University of British Secretariat British Columbia, pp. 65-73. �Tonilinson, KY., 1996,The belts, northem northern Ontario, Ontario, Canada: Canada: unpublished Tom1ins.m. K.Y., 1996,megeochemistry geochemisryand andtectonic tectonicsetting sewingofofearly earlyPrecambrian Precambriangreenstone greenstone belts, unpubliihd PhD thesis. University University of PhD thesis, of Portsmouth, P o ~ m o u t hUK, U& , pp.287. pp.287. Tomlinson, K.Y., KY., Hughes, I) the Hughes, Di., D.J.,Thurston, 'Ibumton,P.C., P.C., 1996b, 1996b,Metavoleanic Metavolcanic rocks rocks of of the the central c e n d Wabigoon Wabigmn Subprovince: Subprovince: 1) the Lurnby Lumby Lake Lake Tomlinson, greenstonebelt. belt.1": In: Summary Summas'yofofFicidwork other Activities, 1996. Geological Survey pp. 60-63. 60'63. greenstone Fieldwork and and otherAdvities. 1996. Ontario OntarioGeological SurveyMiscellaneous MiscellaneomPaper Paper 166. 166, pp. tJgIow, vicinity of Rook No. No. 8, 12th IInternational I, 12th n t m d o n a l Geological GeologicalCongress, Congress,issued iswed Uglow, W.L., W.L.. 1913, 1913, Geology Geology of of the the vioinity ofSteep Steep Rock Rock Lake: Lake: in in Guide Guide Rook 8, Pt. pt. I, by the Canada, p. by the Geological Geological Survey S w e yof ofCanada p. 46.53. 46-53. Walcott, Ontario. CanadCanada;Geological GeologicalSurvey Surveyof ofCCanada. Memoir 28, 28, p. p. 16onfossils fos?.ils from tiomlimestone limestoneof o fSteeprock Steeprock Series, Series, Ontario, d a . Memoir 16W d w C CD,, C.D.,1912, 1912,Notes Noteson 23. 23. Wilks, ME., NorthwesternOntario: Ontario:aaMajor Major Archean ArcheanUmwfonnity Unconformity an an Archean Archean Swomatolites, Stromatolites, TheGeology Geologyof ofthe theSteep Steep Rock Rock Group Group Norrhwestem Wilks, M.E..1936, 1986,The MSc thesis. Saskatchewan, MSG thesis. University University Saskatchewan.Saskatoon, Saskmon,Saskatchewan, Saskatchewan, 206 206 pp. pp. Wilks, unconformity and Wilb. ME,, M.E.,and andNisbet, N i s ME.G., E.G.,1988, 1988,Stratigraphy Stratigraphyofofthe theSteep SteepRock Rock Group, Group, northwestern northwestern Ontario: Ontario: aa major major Archean Amhean unwnfomity and Archean Amhem stromatolites: smmatolites: Canadian Canadian iournal l o ~ aofoflEarth EaOhSciences. Sciences,v.v.25, 25,P. p. 370-391. 370-391. Wilka, ME., major Archean Wilks, M.E., 1986, 1986,The Thegeology geologyofofthe theSteep SteepRock RockGroup, &up, northwestern mxthwestem Ontario: Ontario: a a major Archem unconformity unwnfomity and and Archean Archean stromatolites; smmmliies; M.Sc. thesis, MSe. thesis,University UnivmityofofSaskatchewan, Saskatchewan,Saskatoon, Stskmon,Saskatchewan, Saskatchewan,206 206P. p. Wilks, ME,, ofofthe Wilb, M.E.,Nisbct, Nisbet,E.G., E.G.,198$, 1988,Stratigraphy Shati-hy theSteep SteepRock RockGroup, Group,northwestern norrhwestmn Ontario Ontario major major Archean Archean unconformity unwnfomity and and Archean Archean stromatolites. 370-391, smmatoliies. Canadian Canadian iournal JournalofofEarth EarthScience:25, Scimce:25,370-391. Wright, CM,, the Steeprock Queen'sUniversity, University,Kingston, Kingston, Ontario. Ontario. zonesininthe thehangingwall hangingwallofofthe Steeprock Ore %Zone; Zone; M.Sc. M.SG thesis, thesis, Queeds Wright, C.M.,1959. 1959,Pyrite Pyritezones I I �Day 1 Field Field Trip Tnp Stops Stops Field stops in the Lumby Lake Lake greenstone greenstone belt. From Thunder Bay travel NW to to Upsala Upsala on onTrans-Canada Trans-Canada Highway Highway 17. 17.Approximately Approximately 15-16 km further west of Upsala at the Graham road, turn left (west) onto the Graham logging logging road, gravel road to Sapawe. This road initially follows the Firesteel Sapawe. follows Firesteel River (Provincial Series Pakashkan Lake sheet). After approx. approx. 24-25 24-25 km you come to aa junction junction map 52G15E, 52G/SE, Pakashkan where the main gravel Sapawe-Upsala road turns south (left), continue ahead (right gravel Sapawe-Upsala south (right hand hand fork) and after km you you come come to to aabridge bridgecrossing crossingBrush Brush Creek Creek (Provincial (Provincial Series Series map map after 22 km 520/SW, Gulliver River sheet). You are at the eastern extent of the greenstone belt and 52G/SW, Gulliver eastern within the northern half of the stratigraphy (Figure 1). 1). This generally east-west bush road runs across most of the length of the Lumby Lumby Lake Lake greenstone greenstone belt (at a low low angle angle to to the the strike) before turning turning south near Lumby Lake and joining the gravel north-south Premiere south joining Lake road (near km 33), main Sapawe-Upsala Sapawe-Upsala road (between km 33), which which later later rejoins rejoins the main 12 and 13). 13). Stop 643340E 5435920N: 5435920N:BIF, BIF,ultramafic ultramafic fragmentals, fragmentals, mafic Stov 1: 1: UTM UTM zone zone 15, 15,643340E maficlavas. lavas. reachthe thefirst firststop stop(Figure (Figure 1). 1).ItIt is is on the From Brush Creek Creek continue continue another another —2km -2km totoreach left side of the road next next to to aa clearing clearing for for parking, parking, and opposite opposite aa road road heading heading off off on on the the right hand side. The area area was was burnt burnt over over in in 1995 1995but but new new trees have been planted and are are growing Om thick growing fast. fast. A A large largeoutcrop outcropofofbanded-iron banded-ironformation formationisisclose closetotothe theroad road(—4 (-10m unit) and then further from the road (south) is an ultramafic fragmental sequence then further from the road (south) is an ultramafic fragmental sequence(—20m (-20m thick). South of this are are mafic mafic amygdaloidal amygdaloidal flows, followed by mafic pillow lavas. The ultramafic fragmental ultramafic fragmental sequence sequence is unusual unusual and and ofofdebatable debatable origin. origin. It comprises comprises predominantly ultramalic ultramafic lithic, lithic, lapilli-sized lapilli-sized fragments fragments within an an ultramafic ultramafic matrix matrix (bulk (bulk composition 12 to 21 wt. % MgO, 38-45 wt. % Si02). a ranges from ungraded to graded 12 21 wt. % MgO, % Si02). It ranges from ungraded to graded and would be breccia. In places it shows be classified classified un-genetically un-genetically as aa volcaniclastic volcaniclastic breccia. shows evidence and quenched but was was itit generated evidence of hyaloclatite hyaloclatite textures textures and quenched fragments fragments but generated by by predominantly pyroclastic or autoclastic processes and has it been resedimented? predominantly pyroclastic or autoclastic processes and has it been resedimented? in high-field strength strength Geochemically these are Al-depleted komatiites komatiites which are enriched in elements and light rare earth earth elements elements (REE) (WE) but depleted depleted in in heavy heavy REE. See See Tomlinson Tomliison (l999a) for et al. (1999a) foraadetailed detaileddescription descriptionof of the the chemistry chemistry and and petrogenesis. petrogenesis. Stop 2: UTM 637750E 5437300N: 5437300N: komatiite komatiite flows, interflow interfiow BIF BIF and and chefl. Stov UTM zone zone 15, 15,637750E chert. From stop I continue a further —6 km west along the road to a small turn off on the right. stop 1 continue a further -6 km west Follow Follow this thistrail trailfor for—200 -200 m then then turn turn left leftonto ontoanother anotherfrail trailand andafter after—300 -300 m you you come cometo to the outcrop or a large, flat, flat, glacially glacially polished polished outcrop. outcrop. ItIt may may be be possible possible to drive drive right right to outcrop the trails may be too too muddy. muddy. At this this stop stop (Stop (Stop 2, Figure Figure 1) 1) there there are are several several large large �mapof ofthe theLumby LumbyLake Lake greenstone greenstone belt belt showing showing trip trip localities Figure 1: GeoLogical Geological map Jackson, 1985 (geochronology from Davis and Jackson, 1985 and and Tomlinson Tomlmson et et a!., al., 1999b). 1999b). -t Norway Lake Pluton Ma 3014 Ma A Red Paint /3014 -t , /, Irene-Eltruit me-Eltruit Lakes ikes Complex Complex Lake -s - — .1 Gargoyle Lake 4' Norway A Lake V 'Bar 1 2 Divers ion W 1 T 1 -- L - 2997 r'---a 0,-1c,11', Jefferson Lake !U..::A - Stnç * Sapawe4 Post-tectonic granite Post-tectonic granite Pre-tectonic tonalite-gabbro intrusive complex Pre-tectonic granodioritegranodioritetonalite gneiss I . ) - r. Marmion Batholith 3003. Ma Sedimentary edimentary rocks - Brush Brush Creek I Lake -1 .4 - <2963 ~ 2 9 6 3Ma 6 kilometers h/f. ' Sapawç * Komatiitic ir Komatiiticrocks rocks Intermediate felsic ..a. ~termediateto felsic A pillow ~nging Pillow p younging volcanic rocks direction direction * volcanicrocks rocks * U-Pb U-Pbage age Mafic volcanic --f-i— 4 4- 1 Faults Faults Fold axis: Fold . axis: . syncline anticline, anticline, Q Trip @ Triplocality locality Roads Roads ala �komatiite outcrops that have been stripped by a prospector (Ray Bernatchez) and hence the rocks are well exposed with beautifully preserved textures and field relationships visible. The first large outcrop contains komatiite flows, some are thin (<lm thick) with thin pillowed flow tops (sequence youngs to the SW\ Spinifex texture is rare on this outcrop. Spherules are common as are ancient cracks that facilitated devitrification. Walk through the bush towards the NW and several outcrops occur along the way, the first on a small scarp shows wonderful spinifex texture. Several small outcrops show banded iron formation (BIF) and evidence of komatiite lava flowing into water-laden sediment. Peperite textures, evidence of autobrecciation, ultramafic pillows etc. are all beautifully preserved. Continue to the NW and a large outcrop of spinifex textured komatiite occurs. Flows are separated by thin units of cherty BIF. A 9m thick flow contains randomly orientated pyroxene crystals (altered to actinolite) which in the top 3m of the flow are spinifex textured with branching acicular blades. Generalized log of these outcrops shown in Figure 2 (from Tomlinson et al., 1996 and 1999b). Compositionally many of these flows are basaltic komatiites (18-13 wt. % MgO) and spinifex textured high-Mg basalts (9-12 wt. % MgO). Chemically they differ from the ultramafic fragmental units and are Al-undepleted with REE profiles that are generally unfractionated (Tomlinson et al., 1999a). Stons 3 and 4: UTM zone 15, 0637345E 5437130N, and further along the road: Mafic lava flows. Return to the east-west road that runs across the greenstone belt, reset the odometer and continue driving west. Many outcrops along the road between the komatiite locality and the culverts at Pinecone and Cryderman Lakes (2 large lakes either side of the road after a -7.5 km stretch) show well preserved mafic lava flows. This lithology (having undergone various states of strain) makes up the majority of the greenstone belt stratigraphy. We will stop at a couple of localities before Pinecone and Crydennan Lakes. In the first (just -300 m from where we rejoined the road, Stop 3, Figure I), flow top breccia is overlain by a massive flow base, which is in turn overlain by pillowed lava, which is capped by thin hyaloclastite. The complete flow is 3.8 m thick with 80 cm of pillowed flow. Further along the road several outcrops show undefonned pillows where younging direction is easy identified (any of these outcrops are suitable for stop 4). Pillows are small with convex upward tops and bases that project downwards into the interstices between the underlying pillows. 4 S t o 5: ~ UTM zone 15,633266E 5435600N: view. Continue along the "main" road until reaching the culverts at Pinecone and Cryderman Lakes (Stop 5, Figure 1, -7-7.5 km from Stop 3), reset the odometer again. These lakes occur at the central east-west axis of the belt within sedimentary units that separate the northern and southern halves of the belt. Unfortunately the sedimentary units are poorly exposed but the view is nice and so is the water if its warm enough for swimming! �Figure 2. Lower komatiite unit, northern assemblage, Lumby Lake greenstone belt (after Tomlinson et al., 1999a). Metres SW Pyroxene cumulate Pillowed and spherultitc flows Komatiite flow Ultramafic hyaloclastite Komatiite flow Ultramafic hyatoclastite Interbedded komatiite flows, iron formation and chert Komatiite flow Komatiite flow Gabbro with increasing proportion of tremolite needles towards the top �Stoo 6 (if time allows): UTM zone 15,628130E S433850N: mafic lavas. Continue following the road through the belt. The road heads predominantly west and is now cutting the southern stratigraphy at a low angle to the strike. Unlike the northern half of the belt (which youngs southwards), the southern half youngs predominantly to the north. Mafic pillow lavas again make up the majority of the stratigraphy and an outcrop 6 km further on shows good younging to the north (Stop 6, Figure 1). - Stoo 7: UTM zone 15,624000E 54323SON: felsic volcanics a t S margin of the belt. Continue on this "main" road for a further -6 lan as the road eventually turns south into the felsic volcanic sequence at the base of the greenstone belt. Close to Lumby Lake itself a large property has been cleared by Ray Bernatchez and the felsic sequence is well exposed (Stop 7, Figure 1). Felsic volcanic rocks make up a relatively thick sequence here. They have been dated at 3 localities along the southern margin of the belt (including this site) and range from 3001 to 2997 Ma which is within error of the age of the Marmion tonalite. The felsic volcanics are quartz porphyritic and range from fragmental units to massive rhyolite flows. Continue south and this road rejoins the main gravel logging road down to Sapawe (Provincial Series map 52B/NW, Marmion Lake sheet). A few km south of Sapawe the road rejoins Trans-Canada Highway 11. To the right is Atikokan and to the left is Thunder Bay. References Davis, D.W. and Jackson, M.C., 1988. Geochronology of the Lumby Lake greenstone belt: a 3 Ga complex within the Wabigoon Subprovince, northwest Ontario. Geol. Soc. Am. Bull., 100, 818-824. Tomlinson, K.Y., Hughes, D.J., Thurston, P.C. and Hall, R.P. 1999a. Plume magmatism and crustal growth at 2.9 to 3.0 Ga in the Steep Rock and Lumby Lake area, western Superior Province. Lithos, 4611, 103-136. Tornlinson, K.Y., Davis, D.W., Thurston, P.C., Hughes, D.J. and Sasseville, C. 1999b. Geochemistry, Nd isotopes and geochronology from the Central Wabigoon Subprovince and North Caribou Terrane: regional correlations leading towards a Mesoarchean reconstruction; in Harrap, R.M. and Helmstaedt, H. (eds.). Western Superior Transect 1999 Annual Meeting, Lithoprobe Report #70. Lithoprobe Secretariat, University of British Columbia, p 136-146. Tomlinson, K.Y., Hughes, D.J. and Thurston, P.C. 1996. Metavolcanic rocks of the central Wabigoon subprovince: 1) the Lumby Lake greenstone belt. Summary of Fieldwork and Other Activities, 1996. Ontario Geological Survey Misc. Paper 166: 6063. �Day 2. Atikokan area Day 2 features several unique geologic features spanning 300 My of Archean crustal evolution in the southern Wabigoon and adjacent Quetico Subprovinces. Stops include the early Mannion tonalite batholith, a rare Archean unconformity, the Steep Rock Group (a platform sequence) and contiguous metavolcanic and metasedimentary rocks (Figure 1). An attempt is made to examine the structural relation between these rocks. Most stops are in vicinity of the abandoned Steep Rock Iron Mine from which approximately 100 Mt of ore were removed from 1944 to 1979. Starting from the junction of Highways 622 and 11B in Atikokan, proceed west on Highway 11B turning north on Mercury or O'Brien Streets. Continue past the Atikokan airport in the direction of the abandoned Steep Rock Iron Mine. After passing under the Canadian National Railway overpass, follow the paved perimeter road along the west side of the mine area. The perimeter road curves around the north end of the mine area and crosses back and forth beneath a power line. Stop where the road crosses the power line for the second time approximately 1 1 km from the start and walk west to low, rounded outcrops interspersed with grassy areas. The Marmion batholith -The Marmion batholith is a large oval tonalitic intrusion extending at least 50 km east of Atikokan. Dated at 3003+5 Ma (Davis and Jackson 1988) it is among oldest felsic intrusions in the western Superior Province and together with the pre 2900 Ma Dashwa Gneiss Complex and Finlayson and Lumby greenstone belts (Davis and Jackson 1988) is part of the enigmatic older volcanoplutonic terrane in the central Wabigoon Subprovince. The Marmion batholithformed a mature erosional surface on which the Steep Rock Group was unconformably deposited at an unknown but probably early stage in tectonic evolution of the area. Although rare in central parts of the Marmion batholith, metagabbro dikes are voluminous near the Steep Rock belt; some dikes seem topredate whereas others postdate the Steep Rock Group. Although the tectonic environment at the time of Steep Rock Group deposition is speculative, the dikes imply extensional conditions, possibly in a rifted continental margin setting (Wilks and Nisbet 1988). Felsic and mafic varieties of tonalite are mapped in the Marmion batholith near the Steep Rock belt. These are typically medium-grained, weaklyfoliated and inequigranular to porphyritic rocks composed of essentially plagioclase, quartz and biotite and up to 20% hornblende in the mafic variety. The Marmion batholith is cut by ductile-brittlefaults and primary minerals are extensively converted to sericite, carbonate, chlorite and actinolite within a few kilometers of the Steep Rock belt. Stop 6-1 The Marmion batholith adjacent to the Steep Rockgreenstone belt Exposures in the power line right-of-way are somewhat poor due to an intense brown weathering rind but consist of tonalite interspersed with about 30% metagabbro dikes. Dikes are irregular in shape and are mainly north-northeast trending. All rocks are well foliated to friable and intensely altered to greenschist minerals. A few hundred meters south of this locality the Marrnion tonalite is overlain by the Steep Rock Group. �Figure 6-1 : Geologic map of the Atikokan area shown field-trip stops 6-8 and 6-9. See Figure 63 for other stops. �From the power line, follow the perimeter road south for a distance of about 1 km where the pavement ends at an abandoned railway crossing. Park in the grassy area near the end of the paved perimeter road and walk west along the tom-up railway roadbed. Unconformity at the base of the Steep Rock Group - Greenstone belts are interspersed with voluminousfelsic plutonic rocks throughout the Superior Province and in the majority ofplaces the boundaries between the two are marked by sharp contacts where plutonic rocks intrude and contact metamorphose greenstone belts. Rarely is the base of an Archean greenstone belt preserved although in recent years, an increasing number of localities have been identified where the bases of greenstone successions lie unconformably on felsic plutonic rocks. Most arc: found in remote northwestern parts of the Superior Province. The Steep Rock Lake locality represents an extremely rare, well-preserved and accessible example of an Archean unconformity and is featured in several stops on this trip. Stop 6-2 Unconformity at the base of the Steep Rock Group Outcrops at this locality illustrate the gradational nature of the unconformity at the base of the Steep Rock Group. After walking about 200 m along the tom-up railway line past outcrops of mainly tonalite, you will see a large vertical rock-cut on the east side. Foliated, relatively fine-grained dark metagabbro makes up most of the vertical rock-cut and, at the north end of the vertical rock-cut, is transitional to friable, buff, gritty metasandstone (Wagita Formation). The metasandstone is poorly bedded, but locally shows clasts, and is well foliated, possibly indicating enhanced shearing at the unconformity. The metasandstone can be traced north from the large vertical rock-cut through a series of small exposures to a rounded outcrop of metaconglomerate that lies opposite piles of mine-waste placed on the railway line. Boulders of the conglomerate are mainly tonalite and a dark rock that is probably metagabbro. Beyond the metaconglomerate, rock-cuts show carbonate breccia, tonalite and metagabbro. Outcrops at the obstructed tunnel entrance are metamorphosed pelite and sandstone. The section along the railway line passes back and forth through the unconformity at the base of the Steep Rock Group. Rarely is the unconformity sharply defined as the transition from tonalite to sandstone takes place gradually over distances of a few meters. Return to the vehicle and drive .9 km south along the gravel perimeter road past remains of the head frame of the Hogarth shaft and a branch road (east) to the former concentrator plant area. Turn right and proceed .2 krn down the pit access ramp. Turn right at the first side-road and proceed .15 km and park at the outside of a sharp bend overlooking the pit. Walk down the gravel slope following an erosional channel past several rusty, jagged outcrops of tonalite and gabbro. Follow a flagged trail from the base of the gravel slope about 100 m westerly through a wooded area and emerge onto a north-facing slope interspersed with outcrops, boulders and �Mafic Metavolcanii rocks Dismal Ashroch Jolliffe Ore Zone Mosher Carbonate Wagita Formation (elastics) Stromatolites Unconforrnity Marrnion Batholith Early Mafic Dikes Figure 6-2: Schematic diagram (after Wilks and Nisbet 1988) showing the Steep Rock Group and contiguous supracrustal rocks of the Steep Rock belt. See legend of Figure 6-1.The upper ashrock contact is locally faulted and may represent a depositional time-gap. �small trees. The Steep Rock Group -The Steep Rock Group was extensively studied over the past century (see reviews in Stone et at. 1992) and although early workers offered diverse structural and stratigraphic interpretations, later studies showed the Group to be a homoclinal succession of 4 formations totalling about 1000 m thickness. These include,from the base up, the Wagita Formation (conglomerate, sandstone), Mosher Carbonate Formation (limestone, dolostone), Jolliffe Ore Zone Formation (goethitic ironformation) and Dismal Ashrock Formation (ultramafic pyroclastic rocks) as shown in Figures 6-2 and 6-3. The lower boundary of the Steep Rock Group is defined by an unconformity at the base of the Wagitaformation. Although sheared locally (e.g. Stops 6-2, 6-3) other exposures of the unconformity (e.g. Stop 6-5) show little deformation and preclude tectonicjuxtaposition of the Steep Rock Group onto the Marmion batholith. The upper contact of the Dismal Ashrock Formation is highly deformed in the Steep Rock Mine area and is widely cited as the upper boundary of the Steep Rock Group (e.g. Jolliffe 1955). The Steep Rock Group is cited as a prime example of a platform sequence (Thurston and Chivers 1990); platform sequences typically comprise a succession of quartz arenite and conglomerate, stromatolitic carbonates, iron formation and komatiitic rocks unconformably overlyingfelsic plutonic rocks. Most are concentrated in the Sachigo Subprovince and arepre 2.8 Ga in age having preceded the mafic, mafic tofelsic and "Timiskaming-type" volcanic sequences that make up many greenstone belts. Geologic evidence suggests platform sequences developed in shallow water in a tectonically stable environment, involving initial erosion of widespread older sial followed by rift-related volcanism (Thurston and Chivers 1990). Stop 6-3 Marmion tonalite; Wagita Formation; Mosher Carbonate Formation; stromatolites The outcrop at the end of the trail is made up of buff, gntty, poorly bedded Wagita Formation and altered friable tonalite that gives way westerly over a distance of about 10 m to grey, bedded Mosher Carbonate Formation. Note that the contact between clastic metasedimentary rocks and Mosher Carbonate Formation is sharp; rocks are strongly foliated possibly due to shearing at this locality. About 10 m north at the low end of the outcrop, spherical and pseudocolumnar stromatolites, typically .1 m in diameter are exposed in bedded Mosher Carbonate Formation. A west-sloping face of the outcrop provides a lateral section showing the concentric domical shape of stromatolites. Apexes of the stromatolite domes indicate a westerly younging direction. Continue down the slope to a level area and turn west toward the edge of the Hogarth open pit. Observe large domical stromatolites exposed on the west-facing wall of the upper bench. Rather than returning to the vehicles by the way in which you came, proceed northerly from the level area along a steep access ramp to the perimeter road. Walk along the perimeter road to the vehicles and drive .1 km up the hill and park in a level place at the observation area. Walk to the west side of the observation area. Stop 6-4 Scenic lookout and overview of the Steep Rock Group The partly flooded Hogarth pit can be seen to the north and the Roberts pit to the extreme �Figure 6-3: Detailed geologic map of the Steep Rock Mine area showing l o c a t i o n s o f field-trip stops 6-1 to 6-7. Dlsmel Ashrook Formatton Pyrite Zones JoIHffe Ore Zone Formati Moaher Carbonate Format! Wagita Formation ~onai~te - ./--ROÈ -Railway, ..... - ' abandoned railws Water lavç*l1 9 8 8 ) Geological contact / Fault ^%/Aautt + zone synctirr outcrop �south. Looking west, the west arm of Steep Rock Lake can be seen beyond a concrete retaining dam. This stop provides an overview of the west-facing Steep Rock Group and adjacent, eastfacing mafic metavolcanic sequences in the Mine area (Figure 6-3). The scenic lookout is situated on Marmion tonalite at the base of the Steep Rock Group. The Wagita Formation is not well exposed and can be either absent or else thin at this locality. In either case, the base of the Steep Rock Group extends along the foot of the slope immediately in front of you. Grey outcrops underlying the tree-covered hill 200 to 300 m west, and the rusty jagged bluff to the northwest are Mosher Carbonate Formation. The Jolliffe Ore Zone Formation is almost completely flooded; a few reddish outcrops at the northwest end of the Hogarth pit may be remnants of the ore-zone. Dark grey-greenish outcrops on the west side of the pit near the centre of the mine area are Dismal Ashrock Formation. The sides of large heaps of waste rock on the west side of the mine area also appear to be mainly ashrock. The upper bench road that runs nearly fill1 length of the west side of the mine area approximately marks the contact between Dismal Ashrock Formation and adjacent mafic metavolcanic rocks. East-facing pillow lavas and metagabbro dikes underlie most of the tree-covered ridge beyond the upper bench road. The failed slope at the northwest end of the Hogarth pit is in metagabbro and tonalite. Proceed back down the side-road, turn right and continue 1 km down the main access ramp. Park at the junction with the first bench road to the left and walk about 300 m south along the bench road. Stop 6-5: Unconformity; Wagita Formation; Mosher Carbonate Formation The large bluff east of the bench road is composed of tonalite cut by thick, inclined metagabbro dikes. Near the top and at the south end of a small talus slope about 300 m from the parking area, steep-westerly dipping beds of rusty Mosher Carbonate Formation lie in unconformable contact with tonalite along the face of the bluff. Normally grey crystalline tonalite becomes buff, friable and altered possibly reflecting development of a regolith at the unconformity. Mafic minerals and plagioclase are altered to chlorite, sericite and carbonate however, the rock retains a relict igneous texture shown by unaltered quartz grains dispersed throughout the medium. The unconformity is marked by a transition over a distance of 1 to 2 m from altered tonalite through gritty sandstone (Wagita Formation) to bedded Mosher Carbonate Formation. Quartz content increases up-section in the sandstone; quartz grains are nresent in the lower carbonate beds. The unconformitv is relatively unfaulted at this locality. Proceed back up the main ace of Dismal Ashrock in the west wall of the Hogarth pit Stop 6-6:Dismal Ashrock Formation mine to exposures �The soft, dull, dark green rock is representative of Dismal Ashrock Formation. The ashrock is composed of dark, poorly sorted, subrounded lapilli and is essentially a lapilli tuff that rarely shows stratification. Some lapilli are zoned possibly due to accretionary growth under subaerial conditions. Although Jolliffe (1955) noted thin lava flows in this unit, the ashrock is dominantly pyroclastic and in this regard is rare among Archean komatiites. Continue north on the upper bench road along the west side of the mine area. Turn right and proceed .3 km down a pit-access ramp turning right at an ensuing junction and continue .2 km down toward the flooded Hogarth pit. Park by two power poles mounted in cement-filled tires. Walk north along the access road curving up-hill to the west. From the end of the road follow a flagged trail north over heaps of mine waste to the edge of the pit. Supracrustal assemblages and the structure of greenstone belts - Although originally interpreted as homoclinal successions, geochronology has shown in recent years that many greenstone belts are composites of supracrustal assemblages differing in age by up to 300 Ma (e.g. Corfu and Andrews 1986; Ayres and Corfu 1991). Although assemblages are major structural components of greenstone belts, mapping of assemblages has proven difficult because many contain similar types of volcanic and sedimentary rocks and all have been affected by late metamorphism and deformation. Boundaries between assemblages tend to be poorly exposed and can be either depositional time gaps orfaults with large possible displacements. In certain well-studied areas such as the Favourable Lake greenstone belt, assemblage boundaries appear to be mainly thrust faults (Ayres and Corfu 1991). At the present locality, the boundary between the Dismal Ashrock Formation and contiguous mafic metavolcanic rocks could possibly be an assemblage boundary. Favourable evidence includes the major lithologic and structural transition from the west-facing Steep Rock Group to a thick sequence of east-facing mafic lava flows and gabbro. The boundary isfaulted in the mine area (Figure 6-3; Stop 6-7),which implies that the mafic sequence could have been tectonically juxtaposed with the Steep Rock Group. Since neither the Steep Rock Group or metavolcanic rocks of the Steep Rock belt are dated, the depositional time gap at their mutual contact is unknown. Stop 6-7: Deformation at the contact between the Dismal Ashrock Formation and mafic metavolcanic rocks The pale green to white outcrops west of the upper bench road are pillowed mafic flows. East of the flooded Hogarth pit are bluffs of rusty Mosher Carbonate Formation. Approximately where the trail ends, the Steep Rock Group curves abruptly west and is cut off by a large metagabbro dike and the Bartley fault at the northwest end of the Hogarth pit. Metagabbro and tonalite make up most of the large bluff beyond the northwest end of the pit; a large rock slide in this area forced the mine to close prematurely in 1979. Provided that the slope of waste rock to the north is not flooded, carefully move down the �slope. Observe the pale-green to white friable rock that shows good pencil structure in the bench wall to your left. The pale green rock is probably a deformed mafic metavolcanic flow. Moving farther down the slope you pass beside soft, crumbled and crenulated, variably rusty to dark green rock of uncertain type. Upon reaching the lower bench level, go west along the bench to apoint where it is overrun with talus. The darkgreen deformed rocks in the pit wall behind the bench are Dismal Ashrock Formation and metagabbro. A rusty zone, which is possibly a pyrite lense, can be seen high on the pit wall. Several subvertical, north-trending mesoscopic faults cut up through the ashrock and are eroded back into the pit wall. The area illustrates severe deformation at the upper ashrock contact in the mine area, Mineral lineations plunge down-dip westerly implying a dominant component of dip-slip faulting. Proceed back up the ramps to the upper bench road and go south on to the paved perimeter road in the direction of Atikokan. Turn right on the road leading to the seaplane base on Steep Rock Lake and continue up a steep hill and around several curves for .4 km. Park in a turn-out on the left side of the road and walk east about 50 m through small trees to low outcrops in an abandoned gravel pit. Stop 6-8 (optional) Pillowed mafic metavolcanic flows This outcrop lies within the Canadian Charelston Gravel Pit that was strip-mined for ironore gravel in 1960 to 1965. Pillowed mafic metavolcanic flows that underlie the western half of the Steep ~ o c belt k are represented in this outcrop. Pillows typically have a pale-green, homogeneous interior and very-fine-grained, dark-green rims. Several varieties of pillows ranging from large "mattress" to small spherical types occur together. The younging direction derived from pillow shapes in this outcrop and elsewhere in the western Steep Rock belt is east. A massive flow occurs at the extreme west end of the outcrop. The rock is calc-alkaline basalt. Proceed back to Atikokan and south to the junction of Highway 11B and Highway 11. Turn west on Highway 11 and continue for 1.2 kilometers turning north on a forest access road just before Kemuel Lake. Drive about 200 m into a clear-cut and reforested area, park at the first turn-out and walk to a low outcrop on the east side of the road. Stop 6-9 (optional) Quetico metasedimentary rocks This outcrop shows several types of bedding and sedimentary features. Thick beds (up to 1. m) of fairly homogeneous metawacke occur at the north side of the outcrop. Bed thickness is generally .05 to .2 m; many beds are graded from a coarse sandy base to a grey, laminated, silty top. Truncated beds and crescentic scour channels filled with coarse material are present. Clasts (up to .2 m) of fine-grained, white chert or possibly �felsic volcanic material fill some scoured channels. Grainsize gradation and scour channels indicate that the local younging direction is south. The outcrop is cut by quartz veins, fractures and small-displacement faults. The mafic mineral is mainly biotite. Metamorphic grade increases south with the appearance of amphibole and garnet about 1. krn distant. End of Day 2. Return to Atikokan or Thunder Bay. �References Ayres, L.D. and Corfu, F. 1991. Stacking of disparate volcanic and sedimentary units by thrusting in the Archean Favourable Lake greenstone belt, central Canada; Precambrian Research, v. 50, p. 221-238. Corfu, F. and Andrews, A.J. 1987. Geochronological constraints on the timing of magmatism, deformation, and gold mineralization in the Red Lake greenstone belt, northwestern Ontario; Canadian Journal of Earth Sciences, v. 24, p. 1302-1320. Davis, D.W. and Jackson, M.C. 1988. Geochronology of the Lumby Lake greenstone belt: a 3 Ga complex within the Wabigoon Subprovince, northwest Ontario; Geological Society of America, Bulletin, v. 100, p. 818-824. Jolliffe, A.W. 1955. Geology and iron ores of Steep Rock Lake; Economic Geology, v. 50, p.373-398. Stone, D., Kamineni, D.C. and Jackson, M.C. 1992. Precambrian geology of the Atikokan area, northwestern Ontario; Geological Survey of Canada, Bulletin 405, 106p. Thurston, P.C. and Chivers, K.M. 1990. Secular variation in greenstone sequence development emphasizing Superior Province, Canada; Precambriari Research, v. 46, p. 21-58. Wilks, M.E. and Nisbet, E.G. 1988. Stratigraphy of the Steep Rock Group, northwestern Ontario: a major Archean unconformity and Archean stromatolites; Canadian Journal of Earth Sciences, v. 25, p. 370-391. � Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology: Proceedings, 2000 Description An account of the resource Institute on Lake Superior Geology. Thunder Bay, Ontario. May 8-13, 2000. Creator An entity primarily responsible for making the resource Institute on Lake Superior Geology Date A point or period of time associated with an event in the lifecycle of the resource 2000 Format The file format, physical medium, or dimensions of the resource PDF Language A language of the resource English https://digitalcollections.lakeheadu.ca/files/original/6069db93fb592ecdd4a76f58a9bae2c9.pdf 47805a815831581565df2436ab456bb3 PDF Text Text 47th Annual Meeting Institute on Lake Superior Geology Proceedings Volume 47 Part 1—Program and Abstracts Madison, Wisconsin • May 9—12, 2001 �INSTITUTE ON LAKE SUPERIOR GEOLOGY U 47th Annual Meeting May 9-12, 2001 Madison, Wisconsin Hosted by: University of Wisconsin-Extension Wisconsin Geological and Natural History Survey University of Wisconsin-Madison Department of Geology and Geophysics Proceedings Volume 47 Part 1 - Program and Abstracts �47th Annual Meeting Institute on Lake Superior Geology Volume 47 contains the following parts: Part 1: Program and Abstracts Part 2: Field Trip Guidebook 1- Sedimentologic, Tectonic and Metamorphic History of the Baraboo Interval: New Evidence from Investigations in the Baraboo Range, Wisconsin 2 - Geology, Ore Deposits, and Cultural History of the Upper Mississippi Valley Zinc-Lead District 3 - Economic Geology of the Baraboo and Waterloo Quartzites of Southern Wisconsin Reference to the material in this volume should follow the example below: Medaris, L.G., Jr., 2001, Precambrian geology of S. Wisconsin: A panorama from the Baraboo Range, [abstract]: Institute on Lake Superior Geology Proceedings, 47th Annual Meeting, Madison, WI, 2001, v. 47, Part 1, p. 51. Volume 47 is published by the Institute on Lake Superior Geology and distributed by the Institute Secretary-Treasurer: Mark Jirsa Minnesota Geological Survey 2642 University Avenue St. Paul, MN USA 55114-1057 (612) 627-4780 email: jirsa001tc.umn.edu ILSG webstite http://www.ilsgeology.org/ ISSN 1042-9964 Cover Illustration: Van Hise Rock, Abelmans Gorge, SW1/4, sec. 28, T21N, R5E, Sauk County, Wisconsin from Salisbury, R.D., and W. W Atwood, 1900, The Geography of the Region About Devil's Lake and the Dalles of the Wisconsin: Wisconsin Geological and Natural History Survey Bulletin V. Plate IX. Charles R. Van Hise and Charles K. Leith used this outcrop as a laboratory to demonstrate the fundamental geometric relationship between slaty cleavage and bedding at an outcrop-scale for inferring larger-scale structures. See Field Trip Stop 4, this meeting, Part 2, p. 17. �CONTENTS Proceedings Volume 47 Part 1—Program and Abstracts Editor: Michael G. Mudrey, Jr. iv Institutes on Lake Superior Geology, 1955-2001 Constitution of the Institute on Lake Superior Geology v By-Laws of the Institute on Lake Superior Geology vi An Obituary for Samuel S. Goldich by Bruce R. Doe vii Goldich Medal Guidelines xiv Goldich Medalists XV Goldich Medal Committee xvi Citation for 2001 Goldich Medal Recipient by Gene L. Laberge xvii Eisenbrey Student Travel Awards xviii Student Travel Award Application Form xviii Student Paper Awards xix Student Paper Awards Committee xix xx Membership Criteria Board of Directors Xxi Local Committee XXi Session Chairs XXU Banquet Speaker Xxii Report of the Chair of the 46th Annual Institute Meeting xxiii Program XXiV Xxiv Xxiv Wednesday May 9 Thursday Morning May 10 Thursday Afternoon and Evening May 11 xxv xxvi xxvii Friday Morning May 11 Friday Afternoon May 11 Saturday May 12 XXVfl xxviii List of Poster Presentations Abstracts 1 111 �INSTITUTES ON LAKE SUPERIOR GEOLOGY, 1955-2001 # 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 DATE PLACE 1955 Minneapolis, Minnesota 1956 Houghton, Michigan 1957 East Lansing, Michigan 1958 Duluth, Minnesota 1959 Minneapolis, Minnesota 1960 Madison, Wisconsin 1961 Port Arthur, Ontario 1962 Houghton, Michigan 1963 Duluth, Minnesota 1964 Ishpeming, Michigan 1965 St. Paul, Minnesota 1966 Sault Ste. Marie, Michigan 1967 East Lansing, Michigan 1968 Superior, Wisconsin 1969 Oshkosh, Wisconsin 1970 Thunder Bay, Ontario 1971 Duluth, Minnesota 1972 Houghton, Michigan 1973 Madison, Wisconsin 1974 Sault Ste. Marie, Ontario 1975 Marquette, Michigan 1976 St. Paul, Minnesota 1977 Thunder Bay, Ontario 1978 Milwaukee, Wisconsin 1979 Duluth, Minnesota 1980 Eau Claire, Wisconsin 1981 East Lansing, Michigan 1982 International Falls, Minnesota 1983 Houghton, Michigan 1984 Wausau, Wisconsin 1985 Kenora, Ontario 1986 Wisconsin Rapids, Wisconsin 1987 Wawa, Ontario 1988 Marquette, Michigan 1989 Duluth, Minnesota 1990 Thunder Bay, Ontario 1991 Eau Claire, Wisconsin 1992 Hurley, Wisconsin 1993 Eveleth, Minnesota 1994 Houghton, Michigan 1995 Marathon, Ontario 1996 Cable, Wisconsin 1997 Sudbury, Ontario 1998 Minneapolis, Minnesota 1999 Marquette, Michigan 2000 2001 CHAIRS C.E. Dutton A.K. Sneigrove B.T. Sandefur R.W. Marsden G.M. Schwartz & C. Craddock E.N. Cameron E.G. Pye A.K. Sneigrove H. Lepp A.T. Broderick P.K. Sims & R.K. Hogberg R.W. White W.J. Hinze A.B. Dickas G.L. LaBerge M.W. Bartley & E. Mercy D.M. Davidson J. Kalliokoski M.E. Ostrom P.E. Giblin J.D. Hughes M. Walton M.M. Kehienbeck G. Mursky D.M. Davidson P.E. Myers W.C. Cambray D.L. Southwick T.J. Bornhorst G.L. LaBerge C.E. Blackburn J.K. Greenberg E.D. Frey & R.P. Sage J. S. Kiasner J.C. Green M.M. Kehienbeck P.E. Myers A.B. Dickas D.L. Southwick T.J. Bornhorst M.C. Smyk L.G. Woodruff R.P. Sage, W. Meyer J.D. Miller, M.A. ursa T.J. Bomhorst, R.S. Regis S.A. Kissin, P. Fralick M.G. Mudrey, Jr., B.A. Brown Thunder Bay, Ontario Madison, Wisconsin iv �__________ CONSTITUTION OF THE INSTITUTE ON LAKE SUPERIOR GEOLOGY (Last amended by the Board—May 8, 1997) Article I Name The name of the organization shall be the "Institute on Lake Superior Geology'. Article II Objectives The objectives of this organization are: A. To provide a means whereby geologists in the Great Lakes region may exchange ideas and scientific data. B. To promote better understanding of the geology of the Lake Superior region. C. To plan and conduct geological field trips. Article III Status No part of the income of the organization shall insure to the benefit of any member or individual. In the event of dissolution, the assets of the organization shall be distributed (some tax free organization). to (To avoid Federal and State income taxes, the organization should be not only scientific" or "educational, but also "non-profit") Minn. Stat. Anno. 290.01, subd. 4 Minn. Stat. Anno. 290.05(9) 1954 Internal Revenue Code s.501(c)(3) Article IV Membership The membership of the organization shall consist of persons who have registered for an annual meeting within the past three years, and those who indicate interest in being a member according to guidelines approved by the Board of Directors. Article V Meetings The organization shall meet once a year. The place and exact date of each meeting will be designated by the Board of Directors. Article VI Directors The Board of Directors shall consist of the Chair, Secretary-Treasurer, and the last three past Chairs; but if the board should at any time consist of fewer than five persons, by reason of unwillingness or inability of any of the above persons to serve as directors, the vacancies on the board may be filled by the Chair so as to bring the membership of the board to five members. Article VII Officers The officers of this organization shall be a Chair and Secretary-Treasurer. A. The Chair shall be elected each year by the Board of Directors, who shall give due consideration to the wishes of any group that may be promoting the next annual meeting. His/her term of office as Chair will terminate at the close of the annual meeting over which he/she presides, or when his/her successor shall have been appointed. He/she will then serve for a period of three years as a member of the Board of Directors. B. The Secretary-Treasurer shall be elected at the annual meeting. His/her term of office shall be four years, or until his/her successor shall have been appointed. Article VIII Amendments This constitution may be amended by a majority vote (majority of those voting) of the membership of the organization. V �BY-LAWS OF THE INSTITUTE ON LAKE SUPERIOR GEOLOGY I. Duties of the Officers and Directors A. It shall be the duty of the Annual Chairman to: 1. Preside at the annual meeting. 2. Appoint all committees needed for the organization of the annual meeting. 3. Assume complete responsibility for the organization and financing of the annual meeting over which he/she presides. B. It shall be the duty of the Secretary-Treasurer to: 1. Keep accurate attendance records of all annual meetings. 2. Keep accurate records of all meetings of, and correspondence between, the Board of Directors. 3. Hold all funds that may accrue as profits from annual meetings or field trips and to make these funds available for the organization and operation of future meetings as required. C. It shall be the duty of the Board of Directors to plan locations of annual meetings and to advise on the organization and financing of all meetings. II. Duties and Extenses A. Regular membership dues of $5.00 or less on an annual basis shall be assessed each member as determined by the Board of Directors.. B. Registration fees for the annual meetings shall be determined by the Chair in consultation with the Board of Directors. The registration fees can include expenses to cover operations outside of the annual meeting as determined by the Board of Directors. It is strongly recommended that registration fees be kept at a minimum to encourage attendance of students. III. Rules of Order The rules contained in Robert's Rules of Order shall govern this organization in all cases to which they are applicable. IV. Amendments These by-laws may be amended by a majority vote (majority of those voting) of the membership of the organization; provided that such modifications shall not conflict with the constitution as presently adopted or subsequently amended. vi �AN OBITUARY FOR SAMUEL S. GOLDICH By Bruce R. Doe Goldschmidt Medal winner in 1983, Samuel Stephen Goldich died 20 December 2000 at his apartment in Applewood, Colorado (a suburb of system in granites. In brief, the theory says that as the Denver), less than a month before his 92nd birthday. Sam, as he was widely known, received early fame with his 1938 paper in the Journal of Geology on rock weathering based on his Ph.D. thesis, an amazing paper that continued to receive citations into the 1990s, more than 50 years later. In short, he deter- mined that the resistance of igneous minerals to weathering was the inverse of the Bowen Reaction Series, that is minerals crystallized at lower temperatures were more resistant to weathering than those crystallized at high temperatures (and pressures). In other words the last minerals to crystallize (e.g., the most resistant was quartz followed by orthoclase, etc.) from a melt were the most resistant to weathering, a sequence that became known as the Goldich Stability Series (for a short discussion of this on the web see the 1996 web site of Pamela Gore at http://www.dc.peachnet.edu/_pgore/ge0logY/geo 10 1/weáther.htm. A few years later in 1941, a second, two-part widely utilized paper was published by pressure on minerals and rocks is released through uplift and erosion, they expand and make lead that is not in the structure accessible to removal by crustal fluids. No biography of Goldich would be complete without mention of Sam's interest in the 3 ,500-myr.-old rocks of the Minnesota River Valley Sandell and Goldich on the trace-element concentrations in igneous rocks, also in the Journal of Geology. This pioneering paper introduced the dithizone colorimetric analytical technique for trace-element determination to earth science and resulted in some of the most precise trace-element data in extent at the time and for decades thereafter. Sam got into an argument with the late Paul Gast which originally appeared to be 3,800 myr. old in his collaborations with Ed Catanzaro and later Tom Stern (see Goldich, Hedge and Stern, 1970). This discovery over which radiometric dating system on biotite led to the search of other areas in the U.S. for would be more susceptible to alteration by weathering -- the K-Ar system or the Rb-Sr system -- and a bet ensued. Paul thought that K-Ar would be more affected by weathering which would open the structure of biotite and let the argon escape which was not bound in the structure. Sam, however, thought that Rb-Sr would be more affected because the Sr that did perancient rocks with the discovery by others of these sorts of rocks in Michigan and Wyoming. He ended his research career with a very important paper on the air abrasion method ofpreparing zircons for U-Th-Pb dating (Goldich and Fischer, 1986) that has become not fit in the structure would be subject to ion exchange. Thus a paper resulted (Goldich and Gast 1966). Incidentally, Goldich won the bet. A very important paper with Mudrey (first presented in abstract form at the Geological Society of America Interior Distinguished Service Award in 1965, and he much used for Pb isotope tracer work as well as zircon dating. Sam received the Department of was a founder of the Institute of Lake superior Geology and received its first Goldich Award in 1980. S.S. Goldich received an AB from the University meeting as Goldich and Mudrey, 1969, with the paper appearing in 1972) using dilatancy to help explain the ofMinnesotain 1929 andeventuallyaPh.D. in 1936. In between, he earned an M.A. from Syracuse University in 1930 (and was to receive their Alexander Winchell Award in Geology in 1977), spent two years as an assistant in geology at the then Missouri discordance of the U-Th-Pb ages in zircons never received the acclaim it deserved because of its original publication in a Russian book, but Doe was later to make use of it in explain the U-Th-Pb whole-rock vii �School of Mines, 1930-1932 (now, University of Missouri at Rolla) where his association with Garrett Muilenburg resulted in his first paper (Muilenberg and Goldich, 1933), and was a fellow at Washington University in St. Louis where he published a paper with Carl Tolman (Tolman and Goldich, 1935). While a graduate student at the University of Minnesota, he was a chemist in the famous Rock Analysis Laboratory. In the period 1936- 1941, Sam rose from instructor to Associate Professor at the Texas Agricultural and Mechanical College which resulted in a number of papers on Texas geology and developed his research interest in iron ore in a paper with Barnes, Goldich, and Romberg (1949) that resulted in the milestone papers with Henry Lepp (Lepp and Goldich, 1959, 1964). He served World War II in the U.S. Geological Survey in exploration and study of laterite and bauxite in a number of unusual locations that eventually resulted in a series of papers during a second tour with the U.S. Geological Survey in 1947 and 1948. There were papers with Hendricks and Nelson in 1946 on a portable differential thermal analysis unit for bauxite exploration, with Bergquist on aluminous latentic soil ofthe Dominican Republic in 1947, and with Bridge in 1948 on the bauxite of Babelthuap Island in the Palau Group of islands. time and became the founding Branch Chief of the famous Branch of Isotope Geology in 1960, now defunct. Initially, the Branch was located at the old National Bureau of Standards site on Connecticut Avenue and Van Ness in Washington. D.C. (Now the site of the University of the District of Columbia). This location also allowed Goldich to make a close association with Bill Shields of the National Bureau of Standards (NBS, now called the National Institute of Standards and Technology) who was involved in using mass spectrometry for redeterniining atomic weights. This association resulted in much upgrading of the instrumentation in the Branch plus the building of new equipment known as Shields Mass Spectrometers and led the late Paul Gast to say that Shields was the most valuable employee in the Branch of Isotope Geology and the U.S. Geological Survey didn't even have to pay him. The core of this Branch came from the old Nucleonics Group that had been headed by Frank Senftle. In addition to Senftle, there were Irving Friedman, Henry Faul, Lorin Stieff, Thomas Stern, and Meyer Rubin, among others. Stieff shortly left to pursue his interest in world peace. Faul and Goldich had a falling out and Faul eventually left for the University of Pennsylvania. Quickly added were Ron Kistler (who was soon to return to the U.S. Geological Survey in Menlo Park, Sam rejoined the University of Minnesota in California), Carl Hedge, Edward Catanzaro, and 1948 and became Professor and Director of the Rock Analysis Laboratory the following year, a position he became a professor at the University of Alberta, Bruce Doe. Catanzaro soon left to join Shields at the NBS. The NBS site had to be abandoned because of their move to Gaithersburg and the Assistant Chief Geology for the Central Region came up with a small building (Bldg. 21) at the former WW II munitions plant in Lakewood, Colorado, known as the Denver Federal Center. John Rosholt was already there, and Irving Friedman in 1963 was the first to move. John Stacey and Mitsunobu Tatsumoto were soon added Harry Gehman who went to work in the oil industry, and Bruce Doe moved there in 1963 followed by and Richard B. Taylor who became Chief of the Branch of Central Mineral Resources in the U.S. Geological Survey. Master's candidates included most of the others in 1964, including Robert Zartman and John Obradovich. During a period of years after this move, Senftle and Stern stayed in Washington, D.C., and joined other groups. Meyer Rubin was to remain there in the Branch with his carbon- 14 operation. A popular way for the Branch of Isotope Geology to acquire researchers was from other branches. was to hold until his departure in 1959 (and was to receive the Minnesota Outstanding Achievement Award in 1985). Notable among his Ph.D. graduate students were Ralph Erickson who was the founding Branch Chief of the U.S. Geological Survey's Geochemical Exploration Branch, Ronald Burwash who Gary Ernst, currently a Professor at Stanford Univer- sity, and Zell Peterman a former Chief of the late Branch of Isotope Geology, U.S. Geological Survey, and a pioneer in strontium isotope geochemistry. During this period and in collaboration with Alfred Zell Peterman, for example, came from a branch Nier of the University of Minnesota, Sam organized called Geochemical Census (and, for example, later in Menlo Park, Marvin Lanphere from the Branch of a potassium-argon facilty in the Department of Alaskan Geology, and Brent Dalrymple from the Geology with Halfdon Baadsgaard (who also later became a professor at the University of Alberta). A Branch of Theoretical Geophysics) [History of the Branch of Isotope Geology after Sam ceased being Branch Chief in 1964 is not covered here.] . number of important papers resulted from this collaboration. Upon leaving the University of Minnesota, Doe, a Post-doctoral Fellow at the Geophysical Laboratory, was hired under an agreement that the Goldich joined the U.S. Geological Survey a third viii �U.S. Geological Survey would acquire a 12-inch Shields solid-source mass spectrometer. After that, however, most equipment was obtained as a result of cooperation with other organizations. Stieff, for example, arranged an investigation of uranium series disequilibrium in soils that freed up money for a second 12-inch Shields mass spectrometer and the stayed on for a fifth year to get a bachelors degree in geological engineering but decided to go on and get a master's degree somewhere else as well. I went in to Sam to tell him this and I was going to ask him not to prejudice others that I would ask to write letters of reference for me. I only got out that I was thinking of going somewhere for a master degree when he interrupted that he thought that was a great idea and he could get me a good deal at Missouri School of Mines. He added that I would never regret it. So I decided to leave it at that, and he was right, I never did regret it. Later when I mentioned I wanted to go on for a Ph.D., he suggested I apply to Caltech and that he would have a place for me in the isotope lab when I flunked out. Well, I did apply and was accepted, but, fortunately, never flunked out. Once I told Dick Taylor about my confusion concerning the unexpected result over the master's degree proposal, and he replied, that Sam was hard on me because he thought I had potential. I had noticed that with certain students of little accomplishment he would talk about building of a clean laboratory for isotopic investigations in Denver. A program with Saudi Arabia freed up money for a 6-inch Shields solid-source mass spectrometer. Money was obtained from the Japan-U.S. Scientific Cooperation Program for an argon mass-spectrometer (for an entertaining account of the argon mass spectrometer, you are referred to Glynu's book "The Road to Jaramillo"). After his tour as Chief of the Branch of Isotope Geology, Goldich was to leave the U.S. Geological Survey again and from 1964-1965 joined Pennsylvania State University as Professor of Geology and Geochemistry and Director of the Mineral Constitution Laboratories, for which he was to hire his former associate Oliver Ingamells. The State University of New York began upgrading their faculty and Sam moved to the State University of New York at Stony Brook as Professor of Geology from 1965 to 1968 at which he oversaw the building of another isotope fishing, hunting, movies, and the like, but that he eventually even wrote papers with some of the people he was hardest on. However, he left an ill will with many which probably accounts for this remarkable scientist not winning more honors than he did. But there were a lot of us that learned to overcome Sam's laboratory and hired Gil Hanson who became a professor there. Restless, he moved to Northern outbursts and to regard him as a friend and wonderful scientist. Illinois University in 1968 as Professor of Geology until his retirement in 1977 as emeritus and where he organized yet a fourth isotope geology laboratory. He was to move to Denver in his retirement and became emeritus at the Colorado School of Mines. Sam was a fellow of the American Geophysical Union, Mineralogical Society of America, and Geological Society of America. Selected Bibliography of Samuel Stephen Goldich Bridge, Josiah, Goldich, Samuel S., Preliminary report on the bauxite deposits of Babeithuap island, Palau group, p. 46, 1948. Cameron, R.L., Goldich, S.S., Hoffman, J.H., Radioactivity age ofrocks from the Windmill islands, Budd coast, Antarctica, Stockholm Contributions in Geology, 6, p. 1-6, 1960. No discussion of Sam would be complete with- out some mention of his famous personality. Although Sam could be very generous, he was prone to giving unsolicited good advice or opinions. This Goldich, Samuel S., A study of rock-weathering, 97 p., 1936. Thesis Doctoral from University of Minnesota, Minneapolis, Minneapolis, MN, United States advice or opinions was often given in a tone that the recipient would take as criticism or, even, condemnation. Perhaps all those who were close to Sam, and even many more distant, experienced this at some time or other. He was prone to allergies which did not improve his disposition. I recall once in a class when he repeatedly asked some question in an increasingly agitated and loud voice punctuated by his blowing his Goldich, Samuel S., Authigenic feldspar in sandstones of southeastern Minnesota, Journal of Sedi- nose as one student after another he called upon Goldich, Samuel S., A study in rock weathering, couldn't answer it. He finally said with a cute smile, "This class sure is stupid when I don't feel good." I Journal of Geology, 46(1), p. 17-58, 1938. recall thinking he didn't like me when I was an Goldich, Samuel Stephen, Bergquist, Harlan Richard, undergraduate at the University of Minnesota. I had Aluminous latentic soil of the Sierra de Bahoruco mentary Petrology, 4 (2), p. 89-95, 1934. ix �area, Dominican Republic, West Indies, U. S. Geological Survey Bulletin, B 0953-C, p. 53-84, 1947. Goldich, Samuel S., Lidiak, Edward G., Hedge, Carl Goldich, Samuel Stephen, Origin and development of aluminous latente and bauxite, Geological Society of America Bulletin, 59 (12, Part 2), p. 1326, 1948. area, Journal of Geophysical Research, 71(22), p. E., Walthall, Frank G., Geochronology of the midcontinent region, United States; [Part] 2, Northern 5389-5408, 1966. Goldich, Samuel S., Muehlberger, William R., Goldich, Samuel Stephen, Bergquist, Harlan Richard, Lidiak, Edward G., Hedge, Carl E., Geochronology of the midcontinent region, United States; [Part] 1, Scope, methods, and principles, Journal ofGeophysical Research, 71(22), p. 5375-5388, 1966. Aluminous lateritic soil of the Republic of Haiti, West Indies, U. S. Geological Survey Bulletin, B 0954-C, p. 63-111, 1948. Goldich, Samuel Stephen, Oslund, Eileen H., Composition of Westerly granite G- 1 and Centerville diabase W-l, Geological Society of America Bulletin, 67(6), p. 811-815, 1956. Goldich, S.S., Geochronology in the Lake Superior region, Inst. Lake Superior Geology, 13th Ann., East Lansing, Mich., 1967, p. 13, 1967. Goldich, S.S., Ingamells, CO., Suhr, N. H., Anderson, D. H., Analyses of silicate rock and mineral standards, Canadian Journal of Earth Sciences, 4(5), p. 747-755, 1967. Goldich, Samuel Stephen, Nier, Alfred Otto C., Problems of the division of Precambrian time, Institute on Lake Superior geology, April 21-22, 1958., p. 11, 1958. Goldich, S.S., Mudrey, M.G., Jr., Dilatancy model Goldich, Samuel Stephen, Nier, Alfred Otto C., for discordant U-Pb zircon ages, Abstracts with Baadsgaard, Halfdon, Three-fold division ofPrecam- Programs - Geological Society of America, Part 7, p. 80, 1969. brian in the Lake Superior region, Transactions American Geophysical Union, 39 (3), p. 516, 1958. Goldich, S.S., Nier, A.O., Washburn, A.L., A (super Goldich, S.S., Hanson, G.N., Geology of the Saganaga-Northern Light Lakes area, Minne- 40) /K (super 40) age of gneiss from McMurdo sota-Ontario, Inst. Lake Superior Geology, 15th Sound, Antarctica, Transactions - American Geophysical Union, 39 (5), p. 956-958, 1958. Ann., 1969, Tech. Sess. Abs., p. 16, 1969. Goldich, S.S., Geochronology in the Lake Superior region, Geochronology of Precambrian stratified rocks--Internat. Conf., Edmonton, Alberta, 1967, Papers, Canadian Journal of Earth Sciences, 5 (3, Part 2), p. 7 15-724, 1968. Goldich, Samuel S., Nier, Alfred 0., Baadsgaard, Halfdon, Hoffman, John H., Krueger, Harold W., The Precambrian geology and geochronology of Minnesota, Bulletin - Minnesota Geological Survey, 193 p., 1961. Goldich, S.S., Geochronology of the Minnesota-Ontario border region, Summary of fieldwork, 1969, Information Circular - Minnesota Geological Survey, 7, p. 30, 1969. Goldich, S.S., Hedge, C.E., Dating of the Precambrian of the Minnesota River valley, Minnesota, Journal of Geophysical Research, 67 (9), p. 3561-3562, 1962. Goldich, S.S., Ages of rocks assigned to the Penokean orogeny in Minnesota, Summary of fieldwork, 1969, Information Circular - Minnesota Geological Survey, 7, p. 31, 1969. Goldich, S.S., Hedge, C.E., Investigations in Rb-Sr dating, Journal of Geophysical Research, 67 (4), p. 1638, 1962. Goldich, S.S., Age of the Precambrian rocks of southwestern Minnesota, Summary of fieldwork, Goldich, S.S., Ingamells, C.O., Comparative determi- nations of potassium and rubidium, Transactions American Geophysical Union, 44(1), p. 109, 1963. 1969, Information Circular - Minnesota Geological Survey, 7, p. 31, 1969. Goldich, S.S., Gast, P. W., Effects of weathering on the Rb-Sr and K-Ar ages of biotite from the Morton Goldich, S.S., Hanson, G.N., Hailford, C.R., Mudrey, Gneiss, Minnesota, Earth and Planetary Science M.G., Jr., Re-interpretation of the structure of the Letters, 1(6), p. 372-375, 1966. Saganaga-Northern Light Lakes area, x �Minnesota-Ontario, Special Paper Society of America, p. 114, 1969. - Goldich, Samuel S., Ages of Precambrian Banded Geological Iron-Formations, Precambrian iron-formations of the world, Economic Geology and the Bulletin of the the Morton and Montevideo gneisses and related Society of Economic 1126-1134, 1973. rocks, southwestern Minnesota: Geological Society of Amenca Bulletin, 81, p. 367 1-3696, 1970. Goldich, S.S., Hedge, C.E., 3 ,800-Myr granitic gneiss Goldich, S.S., Hedge, C.E., and Stern, T.W., Age of Geologists, 68 (7), p. in south-western Minnesota, Nature (London), 252. (5483), p. 467-468, 1974. Goldich, Samuel S., Turek, Andrew, Hanson, Gilbert N., Peterman, Zell E., Correlation of early Precam- brian basins of the Canadian shield, Geological Goldich, S.S., Hedge, C.E., Interpretation ages in Association of Canada-Mineralogical Association of Canada, Joint Annual Meetings, Abstracts of Papers, p. 27, 1970. Minnesota — Reply: Nature, 257 (5528), p. 722-722, 1975. Goldich, S.S., Bodkin, J.L., Fluorine in Cenozoic Goldich, S.S., Lunar and terrestrial ilmenite basalt, Science, 171 (3977), p. 1245-1246, 1971. volcanic rocks of Ross Island and vicinity, Antarctica; a progress report, Bulletin - Dry Valley Drilling Project (DVDP) (6), p. 6-7, 1975. Goldich, S.S., Geochronology in Minnesota, Geology of Minnesota; A Centennial Volume, p. 27-37, 1972. Goldich, S.S., Doe, Bruce R., Delevaux, M.H., Possible further evidence for 3.8 b.y.-old rocks in the Minnesota River valley of southwestern Minnesota, Goldich, S.S., The Penokean orogeny [abstr.], Proceedings and Abstracts - Institute on Lake Superior Geology, Annual Meeting, 1972. Open-File Report - U. S. Geological Survey, OF 75-0065, p. Il, 1975. Goldich, S.S., Treves, S.B., Suhr, N.H., et al., Geochemistry of Cenozoic volcanic-rocks of Ross-Island and vicinity, Antarctica: Journal of Geology, 83 (4), p. 415-435, 1975 Goldich, S.S., Ages of Precambrian iron-formations, Precambrian iron-formation symposium, Abstracts and Field Guides, p. [19], 1972. Goldich, Samuel S., Geochronology and geochemistry, Field Trip Guide Book for Precambrian Migmatitic Terrane of the Minnesota River Valley, Guidebook Series - Minnesota Geological Survey, 5, p. 17-41, 1972. Goldich, S.S., Precision and accuracy in silicate analysis, National Bureau of Standards Special Publication (422), p. 79-89, 1976. Goldich, S.S., Problems in dating old Precambrian rocks, Program with Abstracts - Geological Associa- Goldich, Samuel S., Fallacious isochrons and wrong numbers, North-Central Section, 6th Annual Meeting, tion of Canada; Mineralogical Association of Canada; Canadian Geophysical Union, Joint Annual Meeting, 1, p. 71, 1976. Meeting: Geological Association of Canada, 29th annual meeting; Mineralogical Association of Canada, 21st annual meeting, Edmonton, Alberta, Canada, May 19-21, 1976. Abstracts with Programs - Geological Society of America, 4 (5), p. 322, 1972. Goldich, S.S., Mudrey, M.G.,Jr., Model' rasshireniya dlya ob"yasneniya nesoglasnykh urano-svintsovykh vozrastov v tsirkonakh; Dilatancy model for explain- Goldich,. S.S., Peterman, Z. E., Geology and geochemistry of the Rainy Lake area, Gorton, M. P. (editor), Archean geochemistry, Precambrian Research, 11(3-4), p. 307-327, 1980. Meeting: Archean geochemistry field conference, Ontario and Minnesota, Canada, Aug. 2-17, 1978. ing discordant uranium-lead zircon ages, Ocherki sovremennoy geokhimii i analiticheskoy khimii, p. 415-418, 1972. Goldich, S.S., Hanson, G.N., Hallford, C.R., and Mudrey, M.G., Jr., Early Precambrian rocks in the Saganaga Lake-Northern Light Lake area, Minnesota-Ontario. Part II. Petrogenesis, Doe. B.R. (edi- Goldich, S.S., Wooden, J.L., Geochemistry of the tor) and Smith, D.K. (editor), Studies in Mineralogy Archean rocks in the Morton and Granite Falls areas, and Precambrian Geology, Memoir - Geological southwestern Minnesota, Gorton, M. P. (editor), Archean geochemistry, Precambrian Research, 11 Society of America, l3S,p. 151-178, 1972. (3-4), p. 267-296, 1980. Meeting: Archean geochemxi �istry field conference, Ontario and Minnesota, brian iron formations, Economic Geology and the Bulletin of the Society of Economic Geologists, 59 Canada, Aug. 2-17, 1978. (6), p. 1025-1060, 1964. Goldich, S.S., Wooden, J.L., Origin of the Morton Gneiss, southwestern Minnesota; Part 3, Geochronol- Lepp, Henry, Goldich, Samuel S., Kistler, Ronald W., A Grenville cross section from Port Cartier to Mount Reed, Quebec, Canada, American Journal of ogy, More, G. B. (editor), Hanson, Gilbert N. (editor), Selected studies of Archean gneisses and lower Proterozoic rocks, southern Canadian Shield, Special Paper - Geological Society of America (182), p. 77-94, 1980. ISBN: 0-8137-2182-2. Science, 261 (8), p. 693-7 12, 1963. Lepp, Henry, Goldich, Samuel S., Origin of Precambrian iron formations, Kvenvolden, Keith A. (editor), Goldich, S.S., Wooden, J.L., Ankenbauer, G.A., Jr., Geochemistry and the origin of life, Benchmark Levy, T.M., Suda, R.U., Origin of the Morton Papers in Geology, 14, p. 195-209, 1974. (reprint) Gneiss, southwestern Minnesota; Part 1, Lithology, More, G. B. (editor), Hanson, Gilbert N. (editor), Ludwig, K.R., Zartman, R.E., Goldich, S.S., Gentry, Robert V., Lead retention in zircons; discussion and reply, Science, 223 (4638), p. 835, 1984. Selected studies of Archean gneisses and lower Proterozoic rocks, southern Canadian Shield, Special Paper - Geological Society of America (182), p. 45-56, 1980. ISBN: 0-8137-2182-2. Peterman, Z.E., Goldich, S.S., Hedge, CE., and Yardley, D.H., Geochronology of the Rainy Lake Goldich, S.S., Hedge, C.E., Stern, T.W., Wooden, Region, Minnesota-Ontario, Doe, B.R. (editor) and J.L., Bodkin, J.B., North, R.M., Archean rocks of the Granite Falls area, southwestern Minnesota, More, G. Smith, D.K. (editor), Studies in Mineralogy and Precambrian Geology, Memoir - Geological Society of America, l35,p. 193-215, 1972. B. (editor), Hanson, Gilbert N. (editor), Selected studies of Archean gneisses and lower Proterozoic rocks, southern Canadian Shield, Special Paper - Sandell, Ernest Birger, Goldich, Samuel S., The rarer Geological Society of America (182), p. 19-43, 1980. metallic constituents of some American igneous ISBN: 0-8137-2182-2. rocks, American Mineralogist, 24(12, Part 2), p. 12, 1939. Goldich, Samuel S., Determination of ferrous iron in silicate rocks, Chemical Geology, 42 (1-4), p. Sandell, Ernest Birger, Goldich, Samuel Stephen, The rarer metallic constituents of some American igneous 343-347, 1984. rocks; Part 1, Journal of Geology, 51. (Part 1), p. 99-115, (Part 2), 167-189, 1943. Goldich, S.S., and Fisher, L.B., Air-Abrasion experiments in U-Pb dating of zircon: Chemical Geology, v. 58: (3), p. 195-215, 1986 Shields, W.R., Garner, E.L., Hedge, C.E., Goldich, S.S., Survey of Rb (super 85)/Rb (super 87) ratios in minerals, Journal of Geophysical Research, 68 (8), p. Hanson, G.N., Goldich, S.S., Early Precambrian Rocks in the Saganaga Lake-Northern Light Lake 233 1-2334, 1963. Area, Minnesota-Ontario; Part II, Petrogenesis, Doe. B.R. (editor) and Smith, D.K. (editor), Studies in Mineralogy and Precambrian Geology, Memoir Geological Society of America, 135, p. 179-192, 1972. Shields, W.R., Goldich, S.S., Garner, E.L., Murphy, T. J, Natural variations in the abundance ratio and the atomic weight of copper, Journal of Geophysical Research, 70 (2), p. 479-49 1, 1965. Jenks, William F., Goldich, S.S., Ignimbrites in Stern, T.W., Goldich, S.S., Newell, M.F., Effects of southern Peru, Geological Society of America Bulletin, 65 (12, Part 2), p. 1271, 1954. weathering on the U-Pb ages of zircon from the Morton Gneiss, Minnesota, Earth and Planetary Science Letters, 1(6), p. 369-371, 1966. Lepp, Henry, Goldich, Samuel Stephen, The chemistry and origin of iron formations, Economic Geology and the Bulletin of the Society of Economic Geologists, 54 (7), p. 1348-1349, 1959. Stuckless, John S., Weiblen, Paul W., Goldich, Samuel S., A petrogenetic model for the alkalic rocks from the Ross Island area, Antarctica, Dry Valley Drilling Project (DVDP) Seminar-i, Bulletin - Dry Valley Drilling Project (DVDP), 4, p. 52-53, 1974. Lepp, Henry, Goldich, Samuel S., Origin of Precamxii �Stuckless, J. S., Miesch, A.T., Goldich, S.S., Weiblen, P. W., A Q-mode factor for the petrogen- esis of the volcanic rocks from Ross Island and vicinity, Antarctica, McGinnis, Lyle D. (editor), Dry Valley Drilling Project, Antarctic Research Series, 33, p. 257-280, 1981. ISBN: 0-87590-177-8. Wooden. J.L., Goldich, S.S., Suhr, N.H., Origin of the Morton Gneiss, southwestern Minnesota; Part 2, Geochemistry, More, G. B. (editor), Hanson, Gilbert N. (editor), Selected studies of Archean gneisses and lower Proterozoic rocks, southern Canadian Shield, Special Paper - Geological Society of America (182), p. 57-75, 1980. ISBN: 0-8137-2182-2. Yardley, D.H., Goldich, S.S., Preliminary review of Precambrian shield rocks for potential waste repository, YIOWIISUB-436712, p. (unpaginated), 1975. xlii �G0LDICH MEDAL GUIDELINES (Adopted by the Board of Directors, 1981; amended 1999) Preamble The Institute on Lake Superior Geology was born in 1955, as documented by the fact that the 27th annual meeting was held in 1981. The Institute's continuing objectives are to deal with those aspects of geology that are related geographically to Lake Superior; to encourage the discussion of subjects and sponsoring field trips that will bring together geologists from academia, government surveys, and industry; and to maintain an informal but highly effective mode of operation. During the course of its existence, the membership of the Institute (that is, those geologists who indicate an interest in the objectives of the ILSG by attending) has become aware of the fact that certain of their colleagues have made particularly noteworthy and meritorious contributions to the understanding of Lake Superior geology and mineral deposits. The first award was made by ILSG to Sam Goldich in 1979 for his many contributions to the geology of the region extending over about 50 years. Subsequent medalists and this year's recipient are listed in the table below. Award Guidelines 1) The medal shall be awarded annually by the ILSG Board of Directors to a geologist whose name is associated with a substantial interest in, and contribution to, the geology of the Lake Superior region. 2) The Board of Directors shall appoint the Goldich Medal Committee. The initial appointment will be of three members, one to serve for three years, one for two years, and one for one year. The member with the briefest incumbency shall be chair of the Nominating Committee. After the first year, the Board of Directors shall appoint at each spring meeting one new member who will serve for three years. In his/her third year this member shall be the chair. The Committee membership should reflect the main fields of interest and geographic distribution of ILSG membership. The out-going, senior member of the Board of Directors shall act as liaison between the Board and the Committee for a period of one year. 3) By the end of November, the Goldich Medal Committee shall make its recommendation to the Chair of the Board of Directors, who will then inform the Board of the nominee. 4) The Board of Directors normally will accept the nominee of the Committee, inform the medalist, and have one medal engraved appropriately for presentation at the next meeting of the Institute. 5) It is recommended that the Institute set aside annually from whatever sources, such funds as will be required to support the continuing costs of this award. Nominating Procedures 1) The deadline for nominations is November 1. Nominations shall be taken at any time by the Goldich Medal Committee. Committee members may themselves nominate candidates; however, Board members may not solicit for or support individual nominees. 2) Nominations must be in writing and supported by appropriate documentation such as letters of recommendation, lists of publications, curriculum vita's, and evidence of contributions to Lake Superior geology and to the Institute. 3) Nominations are not restricted to Institute attendees, but are open to anyone who has worked on and contributed to the understanding of Lake Superior geology. xiv �Selection Guidelines 1) Nominees are to be evaluated on the basis of their contributions to Lake Superior geology (sensu lato) including: a) importance of relevant publications; b) promotion of discovery and utilization of natural resources; c) contributions to understanding of the natural history and environment of the region; d) generation of new ideas and concepts; and e) contributions to the training and education of geoscientists and the public. 2) Nominees are to be evaluated on their contributions to the Institute as demonstrated by attendance at Institute meetings, presentation of talks and posters, and service on Institute boards, committees, and field trips. 3) The relative weights given to each of the foregoing criteria must remain flexible and at the discretion of the Committee members. 4) There are several points to be considered by the Goldich Medal Committee: a) An attempt should be made to maintain a balance of medal recipients from each of the three estates—industry, academia, and government. b) It must be noted that industry geoscientists are at a disadvantage in that much of their work in not published. 5) Lake Superior has two sides, one the U.S., and the other Canada. This is undoubtedly one of the Institute's great strengths and should be nurtured by equitable recognition of excellence in both countries. xv �GOLDICH MEDALISTS 1979 Samuel S. Goldich 1990 Kenneth C. Card 1980 not awarded 1991 1981 Carl E. Dutton, Jr. William Hinze 1992 William F. Cannon 1982 Ralph W. Marsden 1993 Donald W. Davis 1983 Burton Boyum 1994 Cedric Iverson 1984 Richard W. Ojakangas 1995 Gene LaBerge 1985 Paul K. Sims 1996 David L. Southwick 1986 G.B.Morey 1997 RonaldP.Sage 1987 Henry H. Halls 1998 Zell Peterman 1988 Walter S. White 1999 Tsu-Ming Han 1989 Jorma Kalliokoski 2000 John C. Green 2001 John S. Kiasner GOLDICH MEDAL COMMITTEE Mark Smyk (2001) Ontario Geological Survey, Thunder Bay Rod Johnson (2002) Rod Johnson and Associates, Negaunee, Michigan Frank Luther (2003) University of Wisconsin, Whitewater James D. Miller, Jr., as out-going senior member of Institute Board of Directors, is liaison between Goldich Medal Committee and the Board through the 2001 meeting. xvi �CITATION FOR JOHN S. KLASNER 2001 G0LDICH AWARD RECIPIENT It is my distinct honor and privilege to present this citation for John Klasner, the 22 recipient of the Goldich Award. John was born and raised in the Upper Peninsula. He received his Bachelor's degree in 1957 from Michigan State. His first professional job was with [NCO in the bush of northern Ontario in the summer of 1957. From 1958 to 1962, John worked for Geophysical Services International reducing seismic data for petroleum exploration in New Mexico, Texas, Wyoming, and then overseas in Libya and Muscat. He returned to Michigan State in 1962 and earned a Master's degree in geophysics in 1964. His thesis, mapping a bedrock valley by gravity methods was sponsored by the Groundwater Branch of the U.S.G.S. For the next five years, John worked for the Standard Oil Company of California as an exploration geophysicist stationed mainly in Anchorage, Alaska. He worked in the Cook Inlet area, and while in Anchorage, he met and married his wife, Gretchen, who also happens to be from the U.P. of Michigan. He worked in California from 1967 to 1969, and was then transferred back to Alaska in 1969 to work on the North Slope oil project. In the fall of 1969, he left Standard Oil to work on a Ph.D. at Michigan Tech., under Jo Kalliokoski. His dissertation was on the structure and metamorphism in the western Marquette range. I believe that John's thesis was the first study to show that the Early Proteozoic rocks are detached from the Archean in the area, suggesting large-scale horizontal tectonics. Upon completion of his Ph.D. in 1972, John joined the faculty at Western Illinois University, where he taught for 27 years. He resumed his contact with the U.S.G.S. spending summers mapping in northern Michigan. John as applied his knowledge of geophysic and structural geology to solving problems in the Precambrian of a number of areas including he Marquette range, the Gogebic range, the Feich trough, several areas in Wisconsin and in the Trans-Hudson Orogeny. He has done a broader range of structural studies in the Lake Superior region than anyone I know. He has a long list of publications (49) resulting from his work, in addition to the teaching and administrative load at an undergraduate university. He received the highest awards offered by Western Illinois University for his teaching and research. An indication of the esteem with which he was held at Western is shown by his being named Director of their Honors Program from 1994-1998. He introduced many students to the mysteries of he Precambrian by leading a field trip to the area every year. Especially important, I think, has been his role of introducing undergraduate students to professional activities by supervising fourteen Senior theses and six Honors theses during his years of teaching at Western. I have had the privilege of working directly with John since the mid-i 980s, doing field work in Wisconsin and northern Michigan. During this time John has been the mentor for a number of NAGGED/USES Summer trainees and volunteers. I have found John to be an exceptionally dedicated teacher of young geologists, as well as being a very competent geologist himself, a very good woodsman, and a pleasant fellow to work with. Therefore, it is with great pleasure that I present the 2001 recipient of the Goldich Award for "Outstanding Contributions To The Geology Of The Lake Superior Region", John S. Klasner. Gene L. LaBerge xvii �_______________________________ _____________ ___________ EISENBREY STUDENT TRAVEL AWARDS The 1986 Board of Directors established the ILSG Student Travel Awards to support student participation at the annual meeting of the Institute. The name "Eisenbrey" was added to the award in 1998 to honor Edward H. Eisenbrey (1926-1985) and utilize substantial contributions made to the 1996 Institute meeting in his name. 'Ned" Eisenbrey is credited with discovery of significant volcanogenic massive sulfide deposits in Wisconsin, but his scope as much broader—he has been described as having unique talents as an ore finder, geologist, and teacher. These awards are intended to help defray some of the direct travel costs of attending Institute meetings, and include a waiver of registration fees, but exclude expenses for meals, lodging, and field trip registration. The number of awards and value are determined by the annual Chair in consultation with the Secretary-Treasurer. Recipients will be announced at the annual banquet. The following general criteria will be considered by the annual Chair, who is responsible for the selection: 1) The applicants must have active resident (undergraduate or graduate) student status at the time of the annual meeting of the Institute, certified by the department head. 2) Students who are the senior author on either an oral or poster paper will be given favored consideration. 3) It is desirable for two or more students to jointly request travel assistance. 4) In general, priority will be given to those in the Institute region who are farthest away from the meeting location. 5) Each travel award request shall be made in writing to the annual Chair, and should explain need, student and author status, and other significant details. The form below is optional. Successful applicants will receive their awards during the meeting. enbrey INSTITUTE ONLAIE SUPERIOR GEOLOGY Student Travel Award Application Student Name: Date. Address: DpartmentHead-Tjped EducationalStatns:__________________ Department Head-Si gnature Are you the senior author of an oral or poster paper? YES_ NO___ Will any other students be traveling with you? Who? Statement of need (use additi onal page Efnecessary) Please xviii rdurn to: �STUDENT PAPER AWARDS Each year, the Institute selects the best of the student presentations and honors presenters with a monetary award. Funding for the award is generated from registrations of the annual meeting. The Student Paper Committee is appointed by the annual meeting Chair in such a manner as to represent a broad range of professional and geologic expertise. Criteria for best student paper—last modified by the Board in 1997—follow: 1) The contribution must be demonstrably the work of the student. 2) The student must present the contribution in-person. 3) The Student Paper Committee shall decide how many awards to grant, and whether or not to give separate awards for poster vs. oral presentations. 4) In cases of multiple student authors, the award will be made to the senior author, or the award will be shared equally by all authors of the contribution. 5) The total amount of the awards is left to the discretion of the meeting Chair and Secretary-Treasurer, but typically is in the amount of about $300 US. 6) The Secretary-Treasurer maintains, and will supply to the Committee, a form for the numerical ranking of presentations. This form was created and modified by Student Paper Committees over several years in an effort to reduce the difficulties that may arise from selection by raters of diverse background. The use of the form is not required, but is left to the discretion of the Committee. 7) The names of award recipients shall be included as part of the annual Chair's report that appears in the next volume of the Institute. Student papers will be noted on the Program. STUDENT PAPER AWARDS COMMITTEE Dave Meineke - Committee Chair Menden Engineering LLC Anne Argast Indiana University - Purdue University Fort Wayne Thomas J. Evans Wisconsin Geological and Natural History Survey Steve Kircher Nicolet Minerals xix �MEMBERSHIP CRITERIA FOR THE INSTITUTE ON LAKE SUPERIOR GEOLOGY Approved May 8, 1997 A. Membership in the Institute on Lake Superior Geology requires either participation in Institute activities, or an indication on a regular basis of interest in the Institute. Those individuals registering for an annual meeting will remain as members for 4 years unless: 1) they indicate no further interest in the Institute by responding negatively to the statement on meeting circulars "Remove my name from the mailing list"; or 2) two successive mailings in different years are returned by the postal service as address unknown. B. Those individuals who have not registered for an annual meeting in the past 4 years must indicate an interest in the Institute by postal, electronic , or verbal correspondence with the Secretary-Treasurer at least once every two years. Such individuals will be removed from the membership if they indicate no further interest in the Institute or two successive mailing in different years are returned by the postal service as address unknown. C. The Secretary-Treasurer will maintain a list of current members. The list will include the date of the beginning of continuous membership, dates of returned mail, dates of last contact (expression of interest), and the date membership expires, barring a change of status initiated by the member. Those individuals who have become members of ILSG by Section B will have an expiration date listed at 2 years from the upcoming meeting. For example, a member who expresses interest in September of 1997 (the next annual meeting is May, 1998) will have an expiration date of May, 2000, unless the member contacts the Secretary-Treasurer or attends an annual meeting. D. "Member for Life" status is granted to individuals who have been (nearly) continuous participants of the ILSG meetings for 15 years, Goldich Medal recipients, or those who have served as meeting chairs. This status will be further maintained unless the individuals indicate no further interest in the Institute, or 4 mailings in different years are returned by the postal service as address unknown, or they are deceased. E. All members will be mailed the First Circular for the Annual Meeting and the ILSG Newsletter. The Chair of the annual meeting may opt to send the first circular to additional individuals. All returned mail should be reported to the Secretary-Treasurer. F. The Secretary-Treasurer can designate any individual who is on the ILSG membership list (mailing list) as of January 1, 1997 as a member for life based on participation in ILSG activities. G: Members are strongly encourage to send address corrections to the Secretary-Treasurer to avoid unintentional lapse of membership. xx �2001 BOARD OF DIRECTORS (Board membership through the close of the meeting year shown) Michael G. Mudrey, Jr., General Chairman (2004) Bruce A. Brown, Co-chair Wisconsin Geological and Natural History Survey Stephen A. Kissin (2003) Lakehead University Theodore J.Bornhorst (2002) Michigan Technological Univensty James D. Miller, Jr. (2001) Goldich Liaison Minnesota Geological Survey Mark A. Jirsa - Executive Secretary (2002) Minnesota Geological Survey 2001 LOCAL PLANNING COMMITTEE Michael G. Mudrey, Jr. - Co-chair Bruce A. Brown - Co-chair Robert H. Dott, Jr. - Program Co-chair L. Gordon Medaris, Jr. - Program Co-chair Kathleen M. Zwettler - Meeting Coordinator Assistance to the local committee was provided by the following individuals from the Wisconsin Geological and Natural History Survey: James M. Robertson Director and State Geologist - Wisconsin Geological and Natural History Survey Virginia Trapino Office Support Mindy James Publication Preparation Susan Hunt Graphic Arts Michael L. Czechanski Program and Technical Assistance xxi �2001 SEssioN CHAIRS (In order of appearance) James M. Robertson - Geologic Overview of Southern Wisconsin Wisconsin Geological and Natural History Survey D.L. Daniels - Geophysical Overview and Earliest Archean Evolution U.S. Geological Survey William F. Cannon - Geology and Hydrogeology ofArsenic in Domestic and Public Water Supplies U.S. Geological Survey Michael D. Lemcke - Geology and Hydrogeology of Arsenic in Domestic and Public Water Supplies Wisconsin Department of Natural Resources Suzanne W. Nicholson - General Geology U.S. Geological Survey Dean Rossell - Developments in Understanding Keweenawan Geology Kennecott Exploration Company D.K. HoIm - Thermo-Tectonic History of 1800 to 1200 Ma post-Penokean to Pre-Keweenawan Rocks Bowling Green State University D.A. Schneider - Thermo- Tectonic History of 1800 to 1200 Ma post-Penokean to Pre-Keweenawan Rocks Syracuse University Eric Jerde - Developments in Understanding Archean Geology and Ore Depo.its Morehead State University 2001 BANQUET SPEAKER Thomas C. Hunt University of Wisconsin - Platteville A Practical Exercise in Metallic Mine Reclamation - Ladysmith, Wisconsin xxii �REPORT ON THE 46TH ANNUAL MEETING OF THE INSTITUTE ON LAKE SUPERIOR GEOLOGY Thunder Bay, Ontario The 46tl Annual Meeting ofthe Institute on Lake Superior Geology was held in Thunder Bay, Ontario, May 8-13, 2000, at Lakehead University. The meeting was sponsored by the Department of Geology, Lakehead University, with the assistance of the Ministry of Northern Development and Mines, Thunder Bay office. The meeting was co-chaired by Stephen A. Kissin and Philip W. Fralick. The meeting was attended by approximately 200 geoscientists from the United States and Canada. The meeting consisted of two days of technical sessions, with welcoming reception and annual banquet, and after six field trips before and after the technical sessions. The Proceedings Volume 46 was published in two parts. Part 1 - Program and Abstracts was edited by Stephen A. Kissin. There were 41 published abstracts for the 23 oral and 18 poster presentations given in the technical sessions. Part 2 - Field Trips was edited by Philip W. Fralick. There were six field trips, one before and after the technical sessions; all were described by the respective leaders in Part 2 of the Proceedings. The field trips were as follows: Mesoproterozoic Sibley Group was lead by Philip Fralick and Mark Smyk. This was a two day trip. 1) Lac des Iles Mine was lead by Moe Lavigne. The trip was run before and after the technical sessions. 2) Geoarcheology of the Thunder Bay Area was lead by Brian Phillips, Scott Hamilton, Joe Stewart, Pat 3) Julig, and Bill Ross. Paleoproterozoic Gunflint Formation was lead by Peir Pufahl and Philip Fralick. 4) Quaternaiy Geology, Shebandowan Belt was lead by Andy Bajc. 5) Steeprock-Finlayson-Lumby Belts, a two day trip was lead by Denver Stone, Kirsty Tomlinson, Ray 6) Bernatchez and Philip Fralick. The annual banquet was held in the Residence Dining Room at Lakehead University. The dinner speaker was Bruce Simonson of Oberlin College whose talk "Depositional Settings and Early Diagenesis of Large Precambrian Iron-Formations" was enthusiastically received. The 2000 Goldich Medal was awarded to John C. Green for his contributions to the geology of the North Shore Volcanic Group. The technical sessions included seven invited papers on the Canadian side of Lake Superior: M.M. Kehlenbeck "A Review of Structures in Rocks of Quetico Subprovence and Adjacent Terranes", D.W. Davis "Western Superior Province: Geochronologic Aspects", K. Tomlinson "Western Superior Provence: Sedimentological Aspects", M.J. Lavigne "The Lac des Ties Mine, Northwestern Ontario", P.W. Fralick "Western Superior Province: Proterozoic Sediments", and S.A. Kissin "Vein-type Deposits of the Thunder Bay Area". Student awards were given to Neil Pettigrew (University of Ottawa) for his oral presentation on the Samuels Lake intrusion, Ontario and to K.F. Beaster and J.D. Kohn (University of Wisconsin-Eau Claire) for their poster presentation on the Hinckley Sandstone of Minnesota. The Eisenbrey Student Travel Award was distributed among eight student presenters of oral and poster papers. The Board of Directors of the Institute met for a brief business meeting on May 10. Michael Mudrey of the Wisconsin Geological and Natural History Survey agreed to host the 2001 meeting at Madison, Wisconsin. The term of the Secretary-Treasurer, Mark Jirsa, expired at this meeting. He was nominated for an additional term as Secretary-Treasurer. There being no further nominations, he was acclaimed for an additional term. We wish to thank all those who contributed to the success of the meeting. The field trip leaders who contributed to the guidebook in Part 2 and devoted time and energy to field trips merit our special thanks. The session chairs are thanked for their services as well as the awards committee. We are especially grateful to Mark Smyk and staff of the Ministry of Northern Development and Mines, Sam Spivak, Karen Farrier, Becky Rogola and France Lagroix of the Department of Geology for invaluable assistance. Finally we would like to thank all participants whose attendance made the meeting a financial success. Stephen A. Kissin and Philip W. Fralick Co-Chairs of the 46th Annual ILSG xxiii �CALENDAR OF EVENTS AND PROGRAM WEDNESDAY MAY 9 8:00 am. FIELD TRIP 1: Sedimenlology, Tectonic and Metamorphic History of the Baraboo Interval: New Evidence from Investigations in the Baraboo Range, Wisconsin L.Gordon Medans, Jr. and Robert. H. Dott, Jr. University of Wisconsin - Madison Return of Trip 1 6:00 p.m. 5:00 p.m. - 8:00 p.m. Registration 7:00 p.m. - 10:00 p.m. Welcoming Reception, Cash bar and Poster Setup THURSDAY MAY 10 7:00 a.m. - 9:00 a.m. REGISTRATION 8:10 a.m. INTRODUCTORYREMARKS M.G. Mudrey, Jr. Chairman Wisconsin Geological and Natural History Survey SESSION I: GEOLOGIC OVERVIEW OF SOUTHERN WISCONSIN Session Chair: J.M. Robertson, Director and State Geologist, Wisconsin Geological and Natural History Survey 8:20 a.m. MICKELSON, D.M. and Clayton, L., (University of Wisconsin - Madison; Wisconsin Geological & Natural History Survey) Recent Advances in Understanding the Glacial Record of Wisconsin 9:05 a.m. BYERS, C.E. (University of Wisconsin - Madison) Overview of Paleozoic Geology in Southern Wisconsin 9:50 a.m. MEDARIS, L.G., Jr. (University of Wisconsin - Madison) Precambrian Geology of S. Wisconsin: A Panorama from the Baraboo Range 10:35 a.m. COFFEEBREAK AND POSTER SESSION SESSION II: GEOPHYSICAL OVERVIEW AND EARLIEST ARCHEAN EVOLUTION Session Chair: D. Daniels, US. Geological Survey 10:55 a.m. CHANDLER, Val W. and MUDREY, M.G., JR. Overview of Aeromagnetic Mapping: Minnesota (Chandler) and Wisconsin (Mudrey) 11:20 a.m. CANNON, W.F., Daniels, David L., Snyder, Stephen L., and Nicholson, Suzanne W. A Preliminary Interpretation of New Aeromagnetic and Gravity Data in Wisconsin 11:40 a.m. VALLEY, J.W., Peck, W.H., King, E.M, Graham, C.M., and Wilde, S.A. The Cool Early Earth: Oxygen isotope Evidence for Continental Crust and Oceans on Earth at 4.4 Ga NOON LUNCH BREAK ILSG BOARD MEETING (by invitation) POSTER SESSION xxiv �SESSION III: GEOLOGY AND HYDROGEOLOGY OF ARSENIC IN DOMESTIC AND PUBLIC WATER SUPPLIES Session Chairs: W.F. Cannon (U.S. Geological Survey) M.D. Lemcke (Wisconsin Department of Natural Resources) 1:30 p.m. NORDSTROM, D. Kirk (US. Geological Survey) Overview of Arsenic Occurrences and Processes in Controlling Mobility in Groundwater 2:30 p.m. MUDREY, M.G., Jr., Brown, B.A., Freiberg P.G., and Simo, J.A. Mississippi Valley-Type Mineralization in the Fox River Valley, Eastern Wisconsin 2:45 p.m. GOTKOWITZ, M.B., Schreiber, M.E., and Simo, J.A. Contrasts in the Geologic and Hydrochemical Occurrences of Arsenic contamination of Groundwater in Eastern Wisconsin 3:05 p.m. KANIVETSKY, Roman ilydrogeochemical Modeling of Arsenic in Minnesota Ground Water 3:25 p.m. KOLKER, Allan, Cannon, W.F., Haack, S.K., Westjohn, D.B. and Woodruff, L.G. Hydrogeologic Setting of Elevated Arsenic in Southeastern Michigan 3:45 p.m. COFFEE BREAK AND POSTER SESSION SESSION IV: GENERAL GEOLOGY Session Chair: Suzanne W. Nicholson, US. Geological Survey 4:05 p.m. WOODRUFF, L.G., Attig, J.W., and Cannon, W.F. Geochemistry of Quaternary Deposits in North-central Wisconsin: Geochemical Exploration and Provenance Analysis 4:25 p.m. FAUBLE, Philip, and Lien, Jennifer Some Observations from the Williams Quarry Exposure: Evidence of Debris Flow Deposits in the Parfeys Glen Formation? 4:45 p.m. BOERBOOM, Terrence J. and Jirsa, Mark A. Stratigraphy of the Paleoproterozoic Denham Formation Basalt, Arkose, and Dolomite 6:00 p.m. a Continental Margin Assemblage of ICE BREAKER - MIXER Cash Bar 7:00 p.m. • • • • ANNUAL BANQUET AND AWARD PRESENTATION Announcement of 48th Annual Meeting Location Memorial on Samuel Stephen Goldich 1909 - 2000 2001 Goldich Award Presentation to John Kiasner Banquet Speaker: Thomas Hunt, University of Wisconsin Platteville A Practical Exercise in Metallic Mine Reclamation - Ladysmith, Wisconsin Participants who are not registered for the banquet are welcome to join for the speaker xxv �FRIDAY MAY 11 8:10 a.m. INTRODUCTORYREMARKS M.G. Mudrey, Jr. Chairman Wisconsin Geological and Natural History Survey V: DEVELOPMENTS IN UNDERSTANDING KEWEENA WAN GEOLOGY Session Chair: Dean Rossell, Ken necott Exploration Company SESSION 8:20 a.m. MILLER, James D., Jr. The Duluth Complex: What it Is, What it Ain't, and What We Still Don't Know 8:50 a.m. GREEN, J.C., Davis, D.W., and Schmitz, M.D. Three New Zircon Dates for the Midcontinent Rift, North Shore, Minnesota: More Data, More Questions 9:10 a.m. ROGALA, B., and Fralick, P.W. A Metamorphosed Evaponte Sequence from the Sibley Basin SESSIoN VI: THERMO-TECTONIC HISTORY OF 1800 TO 1200 MA POST-PENOKEAN TO PRE-KEWEENA WAN ROCKS IN THE MIDWEST Session Chairs: D.K. Holm, Kent State University D.A. Schneider, Syracuse University 9:30 a.m. HOLM, D.K., Van Schmus, W.R. and MacNeill, L.C. Age of the Humboldt granite, northern Michigan: Implications for the origin of the Republic metamorphic node 9:50 a.m. VAN SCHMUS, W.R., MacNeill, L.C., Hoim, D.K., and Boerboom, T.J. New U-Pb Ages from Minnesota, Michigan, and Wisconsin: Implications for Late Paleoproterozoic Crustal Stabilization 10:10 a.m. COFFEEBREAK AND POSTER SESSION 10:30 a.m. SCHWEITZER, D., Hoim, D., Van Schmus, W.R. and Boerboom, T. Results of Igneous Thermometry and Barometry on the East-central Minnesota Batholith: Evidence for Post-emplacement Exhumation and Cooling 10:50 a.m. NAYMARK, Alissa, Singer, Brad, and Medaris, L.G., Jr., Recognition of Post-1630 Ma Fluiddriven Metamorphism in Baraboo Interval Quartzites by Means of Laser Probe 40Ar/39Ar Geochronology 11:10 am. DAVIS, Peter B., Williams, Michael L., Bownng, Samuel A. and Ramezani, Jahan Middle Proterozoic Tectonic History of the Central Tusas Mountains, Northern New Mexico, and Comparison with the Baraboo Interval, Southern Lake Superior Region 11:30 a.m. WILLIAMS, M.L., Jercinovic, M.J., and Karlstrom, K.E. Proterozoic Tectonic History of Southwestern North America: Insight from Microprobe Monazite Geochronology 11:50 a.m. HOLM, D., Jercinovic, M.M., and Williams, M. Initial Results of In Situ electron Microprobe (EMP) Age Dating of Monazite from the Southern Lake Superior Region: Confirmation of Widespread Geon 17 Metamorphism xxvi �_____ 12:10 p.m. SCHNEIDER, D.A., Hoim, D.K., and Hamilton, M.A. Directing Timing Constraints of Paleoproterozoic Metamorphism, Southern Lake Superior Region: Results from Shrimp U-Pb Dating of Metamorphic Monazites 12:30 LUNcH BREAK POSTERS removed after Lunch SESSION VII: DEVELOPMENTS IN UNDERSTANDING ARCHEAN GEOLOGYAND ORE DEPOSITS Session Chair: 2:00 p.m. Eric Jerde, Morehead State University HUDAK, George J., Peterson, Dean M., and Morton, Ronald L. New Volume Calculations for the Pyroclastic Eruptions Associated with the Sturgeon Lake Caldera Complex, Northwestern Ontario: Implications for the Scale of Archean Volcanic Processes 2:20 p.m. PETERSON, D.M., Gallup, C., Jirsa, M.A. and Davis. D.W. Correlation of Archean Assemblages Across the U.S.-Canadian Border: Phase I Geochronology 2:40 p.m. JIRSA, Mark A. and Chandler, Val W. Geophysical Answers to Geologic Queries in the Superior Province of Northern Minnesota C., Mason, John K., Schnieders, Bernie R., and Stott, Greg M. A Synopsis of Archean and Proterozoic Platinum Group Element Mineralization in the Thunder Bay District, Ontario 3:00 p.m. SMYK, Mark 3:20 p.m. COFFEEBREAK SESSION VIII: GEOLOGIC SETTINGS OF WEEKEND FIELD TRIPS 3:40 p.m. MUDREY, M.G., Jr., Hunt, T.C., and Czechanski, M.L. Overview of Field Trip 2: 4:00 p.m. Upper Mississippi Valley Zinc-Lead District BROWN, B.A., Luther, F.R., Courter, S.M., Schmitt, J.W., and Lien, 3. Field Trip 3: Economic Geology of the Baraboo and Waterloo Quartzites 5:00 p.m. SINGER, Brad (Department of Geology and Geophysics) Tour of Weeks Hall, University of Wisconsin (Transportation provided) SATURDAY 8:00 a.m. and Weeks End Refreshment Seminar MAY 12 FIELD TRIP 2: Upper Mississippi Valley Zinc-Lead District M.G. Mudrey, Jr. and Thomas C. Hunt Wisconsin Geological and Natural History Survey and University of Wisconsin - Platteville 8:00 a.m. 6:00 p.m. TRIP 3: Industrial Mineral and Aggregate Resources of the Baraboo Interval Quartzites Brown, B.A., Luther, F.R., Courter, S.M., Schmitt, J.W., and Lien, 3. Wisconsin Geological and Natural History Survey, University of Wisconsin - Whitewater, Mathy Construction, Kraemer Company FIELD Return of Field Trips xxvii �POSTER PRESENTATIONS BESKAR, Shawn The Blake Gabbro: A taxitic-tectured gabbro sill south of Thunder Bay, Ontario BIHARI, D.B. and Kissin, S.A. Alteration and Pge-au Mineralization in the North Roby Zone, Lac Des lies Mine, Northwestern Ontario BOERBOOM, Terrence J. Redefined Volcanic and Sedimentary Stratigraphy of the Northern St. Croix Horst in Pine County, Minnesota, and the Application of Arcview to Geologic Mapping BROWN, B.A. and Czechanski, M.L. GIS Applications for Resource Inventory and Land-use Planning in Wisconsin BUCHHOLZ, Thomas W., Faister, Alexander U., and Simmons, Wm. B. Recent Developments in the Mineralogy of the Nine Mile Piuton, Wausau Complex CANNON, W.F. and Woodruff, L.G. Regional Arsenic Anomalies Shown by NURE Stream Sediment and Hydrogeochemical Data in Northern Wisconsin and Michigan CHANDLER, Val W. and Morey, G.B. Paleomagnetic Study of Paleoproterozoic Rocks in the Animikie Group, Northern Minnesota DAHL, D. Structure, Stratigraphy and Punctuated Evolution of Minnesota's Mineral Exploration Archives DANIELS, David L., Nicholson, Suzanne W., Cannon, William F., and Kucks, Robert P. New Aeromagnetic Map of Wisconsin Examined by Regional Context Jerde, Eric A., SAL VATO, DanielJ., Thole, Jeff and Wirth, Karl R. The Early Gabbroic Series of the Midcontinent Rift System: Continued Assessment of Magmatic Origins JOHNSON, Dave Distribution of Arsenic in Wisconsin Groundwater JOHNSON, J.R. and Kissin, S.A. Fluid Inclusion Evidence for a Role for Hydrothermal Activity in the Roby Zone, Lac Des lies Mine, Northwestern Ontario KELLY, Colleen, and Kean, William F. Rock Magnetic Studies of Phyllitic Zones from the Baraboo Syncline, Wisconsin KNOBELOCH, Lynda, Warzecha, Charles. and Nelson, Shelli Health Surveillance in a Community Affected by Arsenic-Contaminated Water LARSON, Phillip C. Potential for Copper Mineralization in the Animikie Group, Minnesota LIVELY, Richard and Morey, G.B. Contributions to the Cultural Geography of the West Mesabi Range, Northern Minnesota xxviii �MILLER, J.D., Jr., Wahi, T.E., Green, J.C.,Chandler, V.W., Severson, M.A., and Peterson, D.E. Digital Geologic Map of Northeastern Minnesota and Associated Databases in GEMS - a Modified Arcview Format MUDREY, M.G., Jr., and Brown, B.A. Structure of the Buried Precambrian Basement in Southwest Wisconsin and Its Influence on Regional Paleozoic Geology and Zinc-Lead Mineralization MUDREY, M.G., Jr., Brown, B.A. and Daniels, Daniels L. Preliminary Analysis of Aeromagnetic Data in Southern Wisconsin: The Role of Precambrian Basement in Paleozoic Evolution NEMITZ, Michael B. and Larson, Phillip C. Mineralogical Variations in Iron-formation in the Thermal Metamorphic Aureole of a Diabase Dike NEWKIRK, Trent T., Hudak, George J., and Hauck, Steven A. Preliminary Lava Flow Morphology Studies at the Five Mile Lake Vms Prospect, Archean Vermilion District, Ne Minnesota: Implications for Volcanic Processes, Volcanic Paleoenvironments, and VMS Exploration NICHOLSON, S.,W., Boerboom, T., Cannon, W.F., Wirth, K., and Isachsen, C.E. A New Look at the 1.1. Ga Chengwatana Volcanics in the St. Croix Horst, Minnesota and Wisconsin ODETTE, Jason D., Hudak, George J., Suszek, Thomas, and Hauck, Steven A. Preliminary Evaluation of Hydrothermal Alteration Mineral Assemblages and Their Relationship to VMS-style Mineralization in the Five Mile Lake Area of the Archean Vermilion Greenstone Belt, Northeastern Minnesota PETERSON, Dean M., Gallup, Christina, Jirsa, Mark A. and Davis. Donald W. Correlation of Archean Assemblages Across the U.S.-Canadian Border: Phase I Geochronology PEYCHAL, C., Kean, W.F., and Schaper, D. Magnetic Survey Near Waterloo Wisconsin PHILLIPS, Erin H., Wirth, Karl R., Veroort, J.D. Gehrels, G.E. Nd and U-Pb Isotope Studies of the Syenitic Aurora Sill, Mesabi Range, Minnesota REID, Daniel D. Freeze/Thaw Testing of Carbonate Aggregate Sources in Wisconsin - a Status Report SANDLAND, Travis 0., Wirth, Karl R., Vervoort, Jeff D., Gehrels, George E., Kennedy, Bryan C. and Harpp, Karen S. Roles of Fractional Crystallization and Assimilation in the Production of Midcontinent Rift Granophyres SMYK, Mark C., Stewart, Jennifer and O'Brien, Mark S. Platinum Group Element Exploration in Northwestern Ontario SNYDER, Stephen L., Ervin, C.Patrick, Geister, Daniel W., and Daniels, David L. A New Gravity Map of Wisconsin SOOFI, M.A. and King, S.D. Post-rift Evolution of the Midcontinent Rift System: Some Numerical Experiments Weissbach, Annette E., HEINEN Elizabeth M., and Lauridsen, Keld B. A Study of Well Construction for Arsenic Contamination in Northeast Wisconsin xxix �Industry and Informational Displays CRONK, William J. Layne Northwest, W229 N5005 DuPlainville Rd, Pewaukee, WI 53072. Phone (262)-246-4646 KIRCHER, Steve Crandon Mine Development, Nicolet Minerals, 7 N. Brown St, 3rd Floor, Rhinelander, WI 54501. Phone (715)365-1450 (Rhinelander office), (715)478-1516 (Crandon office) STEWART, Jennifer Ontario Geological Survey Resident Geologist Program, Northwestern Ontario District, Suite B002, 475 South James Street, Thunder Bay ON, P7E 6E3. Phone (807) 475-1108 SUNDEEN, S. Paul Michigan Department of Environmental Quality, Geological Survey Division, 735 E. Hazel Street, P.O. Box 30256, Lansing, MI 48909. Phone(517) 334-6959. xxx �The Blake Gabbro: A taxitic-textured gabbro sill south of Thunder Bay, Ontario. Shawn Beskar - University of St. Thomas South of Thunder Bay, Ontario, plutonic and hypabyssal rocks associated with the Keweenawan Rift (1109 Ma to 1086 Ma) intrude sedimentary rocks of the Lower Proterozoic (1.9 Ga) Animikie Group. Prior to the discovery of the Blake Gabbro, the igneous terrane south of Thunder Bay was thought to have been comprised of five distinct intrusions (Lightfoot and Lavigne, 1995): (1) Logan Sills; (2) Arrow River Dikes; (3) Pigeon River Dikes; (4) Crystal Lake Gabbro; (5) Pine River - Mount Molly Intrusion. Discovered in 1995, the Blake Gabbro is situated south of Thunder Bay within Blake Township, some 60 km north of the Duluth Complex. The region is characterized by northeast trending diabase-capped ridges and deeply eroded valleys. Positioned beneath a sequence of flat lying Logan Sills, the Blake Gabbro intrudes the argillites of the Rove Formation (Animikie Group). Since 1995, diamond drill cores that intersect the Blake Gabbro have been recovered and logged. From these cores it has been determined that the Blake Gabbro is a northeast trending, sub-horizontal sill of limited plan width but unknown strike length. The maximum thickness intersected by holes drilled thus far is 131 m. The sill thins to less than 20 mat its margins, some 300 m from the center of the body. Samples of the Blake Gabbro have been taken from the diamond drill cores for petrologic study. Initial studies indicate that the Blake Gabbro is a taxitic-textured sulphide-bearing sill. Plagioclase and pyroxene are present in roughly equal quantities. Elongate, cumulus plagioclase grains of variable size are enclosed by optically continuous intercumulus pyroxene. Initial analyses of plagioclase yield compositions ranging from An72 to An83. Minor amounts of olivine and biotite are present. Sulphide minerals consist of pyrrhotite and chalcopyrite. Preliminary whole-rock geochemical data obtained through XRF spectroscopy is presented in Table 1. The significance of the Blake Gabbro is realized upon comparison with the igneous terrane of Noril'sk, Siberia. The region south of Thunder Bay is thought to be equivalent in many respects to the geologic setting of the Noril'sk region and as such, may host large magmatic sulphide deposits (Lightfoot and Lavigne, 1995). The Keweenawan Osler Group Volcanic rocks are similar in composition to the Nadezhdinsky Formation lavas at Noril'sk. Both exhibit large degrees of crustal contamination (as evidenced by their high silica content and LaJSm ratio) and are depleted in nickel and copper. At Noril'sk, chonoliths (subvolcanic tube-like magma channels) containing mineralized picrites and gabbros served as feeders to the overlying sequence of flood basalt. It is thought that the Blake Gabbro may represent such a conduit. Chalcophile elements (nickel, copper and platinum group metals) missing from the associated Osler Group Volcanic sequence may reside within the Blake Gabbro, although the preliminary geochemical data presented seems to suggest otherwise. REFRENCES Lightfoot, P.C. and Lavigne, Jr., M.J. 1995. Nickel, copper, and platinum group element mineralization in Keweenawan intrusive rocks: new targets in the Keweenawan of the Thunder Bay region, northwestern Ontario: Ontario Geological Survey, Open File Report 5928, 32p. 1 �Table 1. Preliminary whole-rock geochemistry Major element data presented as weight per cent Trace element data presented as parts per million BP98.1-1 BP99.1-2 BP99.2-1 BP99.3-2 BP99.3-3 Si02 Ti02 51.42 49.29 48.54 52.00 51.18 3.38 1.43 1.27 2.95 3.48 A1203 13.84 18.52 15.18 13.65 13.97 Fe203 MnO MgO CaO Na20 1(20 P205 17.09 10.91 11.14 16.15 16.54 0.17 0.14 0.14 0.15 0.16 4.44 6.72 10.16 4.62 4.92 6.76 10.85 9.11 6.68 6.99 2.97 2.81 1.58 2.67 2.70 1.35 0.66 0.61 1.38 1.33 0.59 0.25 0.18 0.44 0.48 Total 102.01 101.58 97.91 100.69 101.75 Sc 23.5 25.6 25.2 25.8 26 V 337.5 224 200.9 343.9 396.8 Cr 51.8 157.1 104.8 63.1 48.9 Co 46.1 46.7 58 47.5 45.9 Ni 67 121.1 207 79.2 74.5 Zn Ga Rb Sr 162.5 82.9 90.6 172.9 144.9 25.6 20.9 19.5 23.9 24.1 54.9 23.2 29.3 61.3 60.8 425 306.6 348.8 447.4 501.8 Y 42.4 23.5 20.4 36.2 36 Zr 277.8 101.7 87.3 243.6 233.5 Nb 30.8 9.7 9.5 27.4 28.9 Ba 337.7 167.4 189.3 390.9 442.8 La Ce Pb 32.4 6.6 4.2 32.3 24.8 79.5 26.6 25.6 77.3 68.7 7.5 3.9 3 10.1 6.7 2 �ALTERATION AND PGE-AU MINERALIZATION IN THE NORTH ROBY ZONE, LAC DES ILES MINE, NORTHWESTERN ONTARIO BIHARI, D.B. and KISS1N, S.A., Department of Geology, Lakehead University, Thunder Bay, ON, P7B 5E 1, stephen.kissin(Z)lakeheadu.ca The Lac des Ties Complex appears as a linear zone of mafic plutons that trend east to northeast and extends from Lake Nipigon to Atikokan in northwestern Ontario (Sutcliffe, 1986). The complex is situated in Archean granitoids that consist of gneissic tonalites, medium-grained hornblende diorites and quartz diorites. The Lac des lies Complex occurs in a circular outcrop fashion that is approximately 30 km in diameter and is the largest of a series of mafic to ultramafic intrusions (Sutcliffe, 1986). The Roby Zone was the initial site of mining at the Lac des Iles Mine. The North Roby Zone is its narrow northward extension. The North Roby Zone contains a narrow strip (<50 m) of anomalously high PGE and Au values associated with sparse sulfides called "noseeum ore". Five stripped outcrops approximately 50 x 10 m were studied in the North Roby Zone. Alteration of primary pyroxene to talc and pink coloration of recessively weathered plagioclase is strongly suggestive of hydrothermal alteration. The five stripped outcrops reveal a northeasterly striking, steeply dipping sequence of leucogabbro, varitextured gabbro, pyroxenite and east gabbro. A total of 32 hand samples were collected and studied in thin section. From these 21 were selected for whole-rock and trace element analysis in order to compare chemistry of altered and unaltered samples. Hydrothermal alteration appears to have affected the primary ortho- and clinopyroxenes of the host rocks, progressively converting them to talc. Other petrographic indications are obscure, as regional metamorphism has overprinted the Lac des Iles Complex and its mineralized rocks. The grade of metamorphism is the albite-epidote subfacies of greenschist facies as evidence by incipient breakdown of plagioclase to sericite and clinozoisite, chioritization of pyroxenes and formation of tremolite-actinolite, as well as minor metamorphic albite. Chlorite coronas surround mafic minerals, and develops decussate assemblages of chlorite and tremolite-actinolite. Minor penetrative deformation is evident in undulatory extinction in plagioclase and development of weak schistosity. The development chlorite coronas and general overprinting ofmafic minerals by chlorite and sericitization ofplagioclase are significant in distinguishing hydrothermal alteration from subsequent regional metamorphism. Analysis did not generally reveal striking compositional variations in the host rocks; however, more detailed analysis did show chemical effects of alteration. Chondrite-normalized REE plots revealed that all REEs were depleted relative to unaltered rocks; however, in most altered leucogabbro and varitextured gabbro, minor to insignificant depletion of Eu relative to other REEs produced an apparent positive Eu anomaly in the plots. Other chemical changes, because of their subtle expression and obscurity owing the problem of closure, were examined by use of Pearce Element Ratios. Most notable was Na-depletion due to alteration, which can be distinguished from igneous fractionation effects in plagioclase. Sutcliffe, R.H., 1989. Magma Mixing in Late Archean Tonalitic and Mafic Rocks of the Lac des lies Area, Western Superior Province. Precambrian Research, vol. 44, pp.81-101. 3 �________________ REDEFINED VOLCANIC AND SEDIMENTARY STRATIGRAPHY OF THE NORTHERN ST. CROIX HORST IN PINE COUNTY, MINNESOTA, AND THE APPLICATION OF ARC VIEW TO GEOLOGIC MAPPING BOERBOOM, Terrence J. (Minnesota Geological Survey, boerb001@unm.edu) Pine County, Minnesota, located on the northwestern margin of the St. Croix horst (Fig. 1), contains bedrock that ranges from Archean to Pale ozoic in age; however, most of the county is underlain by rocks of the Midcontinent rift (Fig. 2). The southeastern half of the county is underlain by mafic volcanic rocks ofthe St. Croix horst, late rift-filling sedimentary rocks (Hinckley Sandstone and Fond du Lac Formation) underlie the central part of the county, and the far northwestern corner is made up of Archean and Paleoproterozoic rocks (see Boerboom, this volume). This county was recently remapped by the Minnesota Geological Survey (MGS)', but due to generally poor outcrop, the mapping relied heavily on geophysical and drill hole data. ArcView GIS software proved useful in manipulating and integrating these data, particularly the 4000 water wells contained in the MGS County Well Index database. This presentation is intended to demonstrate the usefulness of Arc View software in map construction, and also to show the results of our mapping efforts in T Pine County. Other Arc files to be demonstrated /1 — Pine include maps of depth to bedrock, bedrock topography, and surficial geology. Craddock (1972) provides a review of the history I — St. — -) Croix horst of investigations in Pine County and environs, but some of the notable early accounts of the bedrock geology in Pine County are by Upham (1888) and Hall (1901); Grout (1910) described the occurrence of copper mineralization in Pine County. The most recent published geologic map to cover Pine County is 1:250,000scale(Moreyandothers, 1981). Ourlatest mapping coincides with other mapping projects in — N / Sedimentary rocks Intrusive rocks Volcanic rocks \ I Figure 1. Location of Pine County relative to the St. Croix horst and the Mid-continent rift system. adjacent Wisconsin (Wirth and others, 1998, Cannon and others, 2001; Nicholson and others, 2001). Geologists on both sides of the border benefited greatly from the recent acquisition of high-resolution aeromagnetic data in Wisconsin by the U.S.G.S., as summarized by Cannon and others (2001). The St. Croix horst, part of the Mesoproterozoic Midcontinent rift system, is comprised of subaerial mafic volcanic rocks that have traditionally been lumped together as the Chengwatana volcanic group. Based on our work and that of the U.S.G.S., the term "Chengwatana" is now restricted only to those volcanic rocks that lie between the Douglas and Pine Faults (Fig. 2). The demarcation of Keweenawan sedimentary vs. volcanic rocks (i.e. the Douglas Fault) is welldefined on the basis of water well information. The Pine Fault (Fig. 2) lies inboard of and parallel to the Douglas Fault, and as summarized by Cannon and others (2001), may have been a bounding fault that controlled the distribution of graben-fill volcanic rocks. Hall (1901) first applied the term "Chengwatana Series" to a series of steeply dipping basalt flows and interfiow conglomerates exposed along the Snake River near Pine City, Minnesota. According to Hall: "Thefloodofl898, which tore away the dam at the foot of Cross Lake and poured down the [Snake] river a vast volume of water, cleaned out in an admirable manner for examination the channel of the stream for several miles." In this stretch of outcrop, Hall recognized 65 steeply dipping lava flows and five interfiow conglomerates. Our remapping of this now less wellexposed sequence identified 37 basalt flows that range from 10 to 300 feet thick, and six interfiow conglomerates that range from 10 to 100 feet thick, although a more careful examination might reveal more flows. The interfiow conglomerates contain abundant round boulders of Keweenawan-type granophyric granite and porphyritic basalt from an unknown source. This sequence, the type locality for the Chengwatana volcanic group, dips about 65 degrees east, and starts within a few hundred feet of the Douglas Fault. Other outcrops of the redefined Chengwatana group that are adjacent to the Douglas Fault dip 40 to 70 degrees east, and those near the Pine Fault dip 10 degrees west. This change in dip is consistent with aeromagnetic patterns that indicate the presence of a southward-plunging syncline that merges 4 �into an unnamed fault (Fig. 2). Hall (190 l)recognized this syncline on the basis of the change in dip direction. Sedimentary rocks similar to the Fond du Lac Formation are present on top of the horst at the south edge of the county. The Minong volcanics (approximately 1094 Ma; Nicholson and others, 2001), part of the northeast-plunging Ashland Syncline, lie to the east of the Chengwatana group, and are interpreted to be younger. The Minong volcanics are distinguished from an unnamed central pile of volcanic rocks to the north by divergent linear aeromagnetic patterns. Aeromagnetic lineaments imply that the rocks in this central panel are folded into a doubly-plunging anticline (Fig. 2). Several aeromagnetically-inferred, reverse-polarized diabase dikes cut all three of the volcanic packages in Pine County. The Hinckley fault (Fig. 2) is proposed as a splay from the Douglas Fault that has displaced the contact between the Hinckley Sandstone and Fond du Lac Formation slightly upward, based on outcrop and geophysical data. North of this fault, the Hinckley Sandstone is typical cliff-forming, uniform and fine-grained quartz arenite, whereas south of the fault the sandstone forms subdued outcrops, is slightly more feldspathic, and contains scattered cobbles of quartzite and minor agate. Locally, tributary streams have cut through Sandstone similar to Fond du Lac Formation Figure 2. Simplified geologic map of Pine County, Minnesota. this feldspathic sandstone and exposed conglomeratic sandstone, typified by trough cross-beds with trough bases lined by small basalt cobbles, which is assigned to the underlying Fond du Lac Formation. Although the major distribution of rock types has not changed significantly as a result of this mapping effort, we have been able to refine the volcanic stratigraphy of the St. Croix horst. Pronounced linear trends on aeromagnetic maps, essentially parallel to the strike of bedding in volcanic rocks, outline the different volcanic basins, and the measured orientations of volcanic flows in outcrops match those aeromagnetic trends. 'This work was done as part of the Pine County Geologic Atlas (Minnesota Geological Survey, County Atlas Series, in prep.), which includes data base, bedrock geology, surficial geology, Quaternary stratigraphy, depth to bedrock and bedrock topography, and mineral resource plates. References: Cannon, W.F., Daniels, D.L., Nicholson, S.W., Phillips, J., Woodruff, L.G., Chandler, V.W., Morey, G.B., Boerboom, T.J., Wirth, K., and Mudrey, M.G., Jr., 2001, New map reveals origin and geology of North American Mid-continent rift: Eos, v. 82, no. 8, p. 97. Craddock, C., 1972, Keweenawan geology of east-central and southeastern Minnesota, in Sims, P.K., and Morey, G.B., eds., Geology of Minnesota—A centennial volume: Minnesota Geological Survey, p. 4 16-424. Grout, F.F., 1910, Keweenawan copper deposits: Economic Geology, v. 5, p. 471-476. Hall, C.W., 1901, Keweenawan area of eastern Minnesota: Bulletin of the Geological Society ofAmerica, v. 12, p. 312342. Morey, G.B., Olson, B.M., and Southwick, D.L., 1981, East-central Minnesota, bedrock geology: Minnesota Geological Survey, scale 1:250,000. S.W., Boerboom, T.J., Cannon, W.F., and Wirth, K., 2001, Reinterpretation of the Chengwatana volcanics in Nicholson, the St. Croix horst, Minnesota and Wisconsin: Geological Society ofAmerica, North-Central Section Abstracts, 35" Annual Meeting. Upham, W., 1888, The geology of Pine County, in Winchell, N.H., and Upham, W., eds., The geology of Minnesota: Minnesota Geological Survey Final Report 1, v. 2, p. 629-645. Wirth, K., Cordua, W.S., Kean, W.F., Middleton, M., and Naiman, Z.J., 1998, Field guide to the geology of the southeastern portion of the Midcontinent rift system, eastern Minnesota and western Wisconsin: Institute on Lake Superior Geology 44" Annual Meeting, Minneapolis, Minn., Proceedings, v. 44, Pt. 2, Field trip guidebook, P. 33-75. 5 �STRATIGRAPHY OF THE PALEOPROTEROZOIC DENHAM FORMATION-A CONTINENTAL MARGIN ASSEMBLAGE OF BASALT, ARKOSE, AND DOLOMITE BOERBOOM, Terrence J., and JIRSA, Mark A. (Minnesota Geological Survey, boerb001@umn.edu andjirsa001@umn.edu) The Paleoproterozoic Denham Formation, as originally defined, consists of metamorphosed quartz-rich sedimentary rocks, dolomite, and mafic volcanic rocks (Morey, 1978). The type locality of the Denham Formation, in northwestern Pine County, Minnesota (see figure 2 in Boerboom, this volume), consists of a series of outcrops in and near an abandoned glacial outwash channel. The Denham Formation forms a pronounced linear, positive aeromagnetic anomaly that can be traced from the exposures for 40 miles to the west, to Mille Lacs Lake (Boerboom and others, 1999). This anomaly follows the northern margin of the Archean McGrath Gneiss (2550±14 Ma, Van Schmus and others, this volume). The anomaly is produced by scattered chert-magnetite clasts within fragmental volcanic rocks. The Denham area was mapped as part of the Pine County Geologic Atlas (see Boerboom, this volume). In addition to the outcrops, 15 exploratory drill cores and cuttings holes were utilized in the map interpretation, as was information from water wells contained in the Minnesota Geological Survey County Well Index. The results will be published on the forthcoming 1:100,000 scale geologic map of Pine County, which will include a 1:12,000 scale inset map of the type locality. The rocks of the Denham Formation have undergone regional, amphibolite-grade metamorphism and at least two periods of deformation. The mafic volcanic rocks are amphibolitic, but contain well-preserved primary features. The granular sedimentary rocks are recrystallized, but retain much of their primary grain size, shape, and composition. In contrast, layers interpreted as pelitic sedimentary rocks are recrystallized to garnet-staurolite-sericite schist. Dolomite is completely recrystallized to marble, with local relict bedding features. The first of two deformation events was synchronous with metamorphism to the garnet zone of the amphibolite facies (Holm, 1986). It produced Si foliation that typically is parallel to bedding, and a locally strong, shallowly plunging, stretching lineation. The second deformation folded Si and bedding along steeply dipping axes, and was concurrent with or followed by peak metamorphism that produced staurolite. In the Denham valley, the stratigraphic sequence dips variably to the north, having local F2 folds with overturned limbs. North of the valley, bedding and SI in graywacke are nearly horizontal, and are deformed into open F2 folds with local crenulation features. North of these graywacke outcrops, the bedding dips to the south, defining a broad, regional-scale, F2 syncline. Despite deformation and metamorphism, the stratigraphy of the Denham Formation forms a coherent package that is shown schematically on Figures 1 and 2. In this discussion, metamorphic rock names, and the prefix "meta" are omitted for clarity. The base of the Denham consists of interbedded siltstone and cross-stratified pebble conglomerate. This is overlain by coarse-grained and locally conglomeratic arkose that apparently pinches out laterally. The arkose is interbedded 3-10 Graphitic argillite with amygdaloidal basalt flows that grade stratigraphically upward (northward) from Dolomite >500 massive, to pillowed, to fragmental. The volcanic rocks are thickest at the eastern limit of outcrop, where at least four flows of nearly 1000 feet total thickness were c Fragmental mafic ol anic rocks 700 recognized and thin westward to two flows of 300 feet total thickness This distribution implies that the eastern exposures are nearest to the vent, which may lie beneath the DOIOiIC ad arkose Fond du Lac Formation (Fig 2) The overlying arkosic and pelitic strata apparently Shale 350 pinch out to the east where the volcanic package thickens, and are not present in drill holes to the north and east of the Denham valley. The northern-most outcrops in the Pillowed basalt valley consist of very pure dolomite, now marble, having ptygmatically folded and 3001000 strongly lineated quartz veins. Drill cores show that the dolomite is at least 500 feet Dolomiticarkose 200 thick, and is overlain by graywacke that is exposed discontinuously to the north for some distance. The contact between dolomite and overlying graywacke is marked by Siltstone 1100 a thin layer of graphitic argillite. Field and petrographic observations imply that clastic detritus in the Denham Formation was derived in large part from a weathering residuum on the subjacent McGth McGrath Gneiss. Near the contact with the Denham Formation, the McGrath grades Gneiss : abruptly from granite gneiss containing quartz, orthoclase, plagioclase, and biotite; to Figure 1. Stratigraphic column of strongly foliated, quartz- and sericite-rich schist that contains orthoclase, but no Denham Formation; thicknesses plagioclase. The arkosic parts of the Denham Formation similarly lack plagioclase, in feet. Ok 6 �Fraacvolc Dolomitic mble —----- — I r' -— t. t_ L - - Lva_3. Explanation Metagraywactse (Palnoproterozotc) Strike and dip of inclined bedding showing younging dtrection Direction and plunge of lineations inctnding Outcrop — elongate metamohic mineeds, fold axes. on McOuath Ofleiss Strike and dip of Fl cleavage Map urea — Fond On Lac Formation _ Conlacl between basalt flows and elongate pillows -r-10 Strike and dip of inclined bedding, youngiug direclion unknown —r_ Strike and dip of overturned bedding. in this example beds top to northeast but dip southwest Geologic contact Figure 2. Simplified geologic map of the Denham Formation and adjacent McGrath Gneiss. and are composed of quartz and orthoclase grains, together with scattered clasts of granitic gneiss. Studies of saprolite developed beneath Cretaceous sedimentary rocks on Precambrian crystalline rocks in southwestern Minnesota may provide an analog (Setterholm and others, 1989). These studies demonstrate that plagioclase is one of the first minerals to alter to kaolinitic clay during the weathering process, and that orthoclase and quartz are the most resistant to weathering. The basal Cretaceous strata locally consists of reworked saprolite, including beds of cross-stratified sandstone and nearly pure kaolinitic shale, Exposures of basal Cretaceous sedimentary rocks locally contain detrital orthoclase and quartz derived by slight reworking of grus-textured, weathered granite. We infer that the same process occurred in the Paleoproterozoic by erosion and reworking of weathered McGrath Gneiss into beds of arkose and kaolinitic shale. These were subsequently metamorphosed to produce recrystallized arkose and staurolite-garnet- sericite schist. Weathering of orthoclase may have liberated potassium for the inferred conversion of kaolinite to sericite during metamorphism. The Denham Formation is interpreted to represent a rift-margin assemblage deposited during the Paleoproterozoic, genetically similar to, and perhaps temporally equivalent with, the Chocolay Group in Michigan. In this setting, the McGrath Gneiss was part of the continental margin that was weathered and eroded to provide detritus to an evolving rift basin undergoing active, shallow water volcanism. Interbedded arkose and dolomite higher in the stratigraphic section represent foundering of the shelf and deepening water, possibly by subsidence of localized grabens. The lack of arkose in the upper, dolomite-dominated part of the sequence indicates that deposition of coarse detritus was restricted to the shallow, nearshore environment adjacent to the McGrath. The sedimentological gradation of dolomite to graywacke stratigraphically upward indicates further deepening water and associated turbidite deposition. The deformation of the Denham Formation is inferred to be the product of basin closure during the Penokean orogeny. References: Boerboom, T.J., Severson, M.J., and Southwick, DL., 1999, Bedrock geology of the Mule Lacs 30 x 60-minute quadrangle, eastcentral Minnesota: Minnesota Geological Survey Miscellaneous Map Series M-l00, scale 1:100,000. Holm, D.K., 1986, A structural investigation and tectonic interpretation of the Penokean Orogeny: east-central Minnesota: Unpubi. M.S. thesis, University of Minnesota, Duluth, 114 p. Morey, GB., 1978, Lower and Middle Precambrian stratigraphic nomenclature for east-central Minnesota: Minnesota Geological Survey Report of Investigations 21,52 p. Setterhoim, DR., Morey, GB., Boerboom, T.J., and Lamons, R.C, 1989, Minnesota kaolin clay deposits—A subsurface study in selected areas of southwestern and east-central Minnesota: Minnesota Geological Survey Information Circular 27, 99 p. 7 �GIS Applications for Resource Inventory and Land-use Planning in Wisconsin B. A. Brown and M. L. Czechanski Wisconsin Geological and Natural History Survey Madison, Wi Wisconsin has recently enacted comprehensive "smart growth"land-use planning legislation that specifically requires counties and local units of government to consider metallic and nonmetallic mineral resources as they develop and adopt a comprehensive plan by 2010. Wisconsin's nonmetallic mine reclamation rules take effect in 2001. These rules contain provisions to protect undeveloped aggregate deposits from zoning changes, and designate end uses for reclaimed sites, both of which link reclamation into the planning process. Implementation of mandatory reclamation and planning for future supplies both require an inventory of active production sites. In addition, planning requires analysis of geologic data to identify location, extent, and quality of undeveloped resources. Wisconsin Geological and Natural History Survey (WGNHS) is working in cooperation with the U.S. Geological Survey and several state agencies to compile a spatial database of active operations and historic mineral production sites. This database will ultimately link existing state and federal databases containing a variety of information on location, lithology, formation, engineering testing, permit status etc., through a common identification number. WGNIHS is also working with county and local governments to inventory active sites and to assemble and assess the quality of geologic data available for comprehensive planning. Computerized geographic information system (GIS) technology provides powerful new tools for inventory and analysis of mineral resource information. Digital coverages showing the locations of mines, pits, and quarries, and spatial databases documenting the character and extent of deposits can now be easily incorporated with bedrock and surficial geology, soils maps, water resource maps and an ever increasing variety of other environmental, cultural, and political coverages to feed directly into the land-use planning process. We will present an interactive demonstration of statewide and county geologic and mineral resource coverages recently produced by WGNHS and discuss some of the land-use planning methodologies and applications currently under development. 8 �Field Trip 3:Economic Geology of the Baraboo and Waterloo Quartzites Bruce A. Brown (1), Frank R. Luther (2), Susan M. Courter (3), James W. Schmitt (4), and Jennifer Lien (5) Field trip 3 will examine the aggregate and industrial mineral resources of the Proterozoic Baraboo and Waterloo Quartzites of southern Wisconsin. In the Waterloo area twenty miles east of Madison, we will visit a large quarry that produces construction aggregate, breakwater stone, and railroad ballast. We will travel from waterloo to the Baraboo area, where we will visit five operations that produce a variety of aggregate products. The Proterozoic quartzites of southern Wisconsin have long been recognized for their unique hardness and durability, refractory properties, and resistence to weathering. Quarries have operated in both areas for more than a century, producing a variety of industrial mineral products and aggregates. Today the major uses for this hard and durable rock are railroad ballast, riprap and breakwater stone, and crushed stone base material. A variety of specialty aggregates ranging from seal coat chips to leachate collection and filter bed material are produced as well. Stop 1 will be the Michels Materials Waterloo quarry. This operation was opened in 1988 as a source of large stones with high resistance to freeze-thaw loss, for use in constructing breakwaters and erosion control structures on the great lakes. Bedding in the Waterloo Quartzite is up to 2 meters thick and joints are widely spaced, allowing blocks up to 10 tons or larger to be quarried. Crushed material was first produced only as a means of disposing of undersize waste rock. Michels continues to produce breakwater stone that exceeds all Corps of Engineers specifications, but much of the current output is crushed for ballast and construction aggregate. We will not have time to visit the historical quarries located to the south of the Michels Quarry which were operated in the early 1900s for refractory blocks, but the geology of the Michels quarry and the old quarry area is described in previous guidebooks by Luther(1992, 1997). From Stop 1 we will travel northwest across the glaciated landscape of Dane and Colombia counties to the Baraboo Range. This drive will provide a look at a classic drumlin landscape. Stop 2 will be the Williams quarry, operated by the Kraemer Company. This quarry is located on the north limb of the Baraboo syncline and utilizes the Parfreys Glen Formation, a time-transgressive, near-shore deposit that accumulated around the Baraboo bluffs during Cambro-Ordovician submergence. This quarry produces aggregate that is essentially a quartzite gravel. As the quarry goes deeper into the hillside, more solid quartzite is being quarried. The sedimentary structures in the coarse basal conglomerates and overlying sandstones are spectacular. Stop 3 will briefly examine the 1760 Ma. Rhyolite that underlies the quartzite, 9 — �exposed in a road cut on STH 33. Stop 4 will be Milestone Materials Jesse Pit a combination gravel pit/quartzite quarry on top of the south range. Lunch will be in Devils Lake State Park, near the site of a former refractory and abrasive quarry. After lunch we will visit Milestone's Fox Ridge pit and asphalt plant where a variety of quartzite aggregate products are produced, sold, and made into asphalt paving materials. We will next visit the Martin-Marietta Rock Springs Quarry, a historic operation now a major producer of ballast. We will finish at the Kraemer Co, LaRue Quarry on the south range near the site of the historic Sauk and Illinois iron mines. LaRue Quarry contains many examples of sedimentary and tectonic structures as well as examples of quartzite weathering. The trip will return to the Sheraton by 6:00 PM. Luther, F.R., (1992 ), The Waterloo Quartzite at the old Portland Quarry: in The 56th Annual TriState Geology Field conference Guidebook to the Geological setting of whitewater, Wisconsin and surrounding Area, Jack Travis, ed. P51-61. Luther, F.R. (1997), The Precambrian Waterloo Quartzite, Dodge and Jefferson Counties, Wisconsin—Petrology, Structure, and Industrial Use: in Mudrey, M.G., Jr., ed. Guide to Field Trips in Wisconsin and Adjacent areas of Minnesota., 31st Meeting Northcentral Section, Geol. Soc. Am., Madison, WI, p.31-35. 1) Wisconsin Geological Survey, Madison, WI 2 )UW-Whitewater, Whitewater, Wi 3) Michels Materials, Inc., Brownsville, WI 4) D.L.Gasser Construction, Baraboo, WI 5) The Kraemer Company, Plain, WI 10 �RECENT DEVELOPMENTS IN THE MINERALOGY OF THE NINE MILE PLUTON, WAUSAU COMPLEX BUCHHOLZ, Thomas W., 1140 12th Street North, Wisconsin Rapids, Wisconsin 54494; FALSTER, Alexander. U., and SIMMONS, Wm. B., Department of Geology and Geophysics, University of New Orleans, New Orleans, Louisiana 70148 The mid-Proterozoic Wausau Complex is composed of four intrusive centers; from north to south the Stettin, Wausau, Rib Mountain and Nine Mile plutons (Meyers eta!, 1984). The Stettrn intrusion is the oldest and most ailcalic, and the three other plutons are progressively younger and more siicic. The youngest and most silicic intrusion, the Nine Mile Pluton, is an epizonal anorogenic and heterogeneous granitic intrusion, with locally abundant pegmatites, aplites and miarolitic zones. Miarolitic cavities in the pegmatites as well as miaroles within some phases of the granite attest to shallow levels of emplacement of the pluton. At the surface the granite is altered to a friable disaggregated material called "grus" that is extensively quarried for use as road gravel. The Nine Mile pluton's pegmatites and aplites contain a wealth of mineral species (Faister, 1981, 1987, Hanson et a!., 1998. Buchholz eta!., 1999, 2000), which show heterogeneous distribution for many species across the pluton. Titanium oxide species, such as anatase, brookite, and rutile are abundant in pegmatites of the northern part of the pluton and are far less conspicuous in the central and southern portions. Anatase is by far the most abundant polymorph in the Nine Mile pluton. Niobium and tantalum mineralization is sparse in the northern part of the pluton but becomes more abundant in the central and southern parts. As in most other anorogenic pegmatites, Nb> Ta in Nb-Ta oxides such as ferrocolumbite, uranopyrochlore, liandratite/petschekite, and ilmenorutile. However, in some small-scale, restricted environments late-stage Ta-enrichment is manifest as tapiolite, manganotantalite, strueverite and microlite. In Ta-rich areas there is also a dramatic increase in the abundance of fluorite (Buchholz et al., 1999, 2000). In these areas fluorite becomes a significant mineral phase in some of the pegmatites and even in the adjacent granite. Beryllium mineralization is dominated by phenakite and bertrandite. Rare beiyl, bavenite, and euclase tend to be more common in the northern segment of the pluton. Lithium mineralization is absent but elevated contents of Li are found in micas (lithian biotite and zinnwaldite) in the central and northern parts of the pluton, typically associated with more fractionated mineral species of the Nb-Ta oxides. LREE-mineralizalion in the Wausau complex is dominated by phosphates of the monazite group, rhabdophane, and by carbonates of the bastnaesite group. HREE-minerals are essentially restricted to xenotime-group minerals. Both LREE and HREE minerals are found throughout the pluton. Unlike the REEminerals in the South Platte district in Colorado (Simmons Ct a!., 1987), REE-minerals in the Nine Mile pluton occur as small crystals and grains throughout the pegmatites (Hanson et a!., 1998), whereas in the South Platte pegmatites, they form large masses in the core margin and in the replacement units. Zirconium mineralization is restricted to zircon, which occurs as an accessory mineral throughout the complex, but more HI-enriched examples are restricted to the high-F environments. Tin is exceedingly rare but it has been found as cassiterite in the central and western portions of the pluton. Some Nb-Ta-oxide minerals from this area contain elevated Sn-content, as well. Manganese mineralization tends to increase from north to south throughout the pluton. The mineralogy of the Nine Mile pluton exhibits some unusual geochemical trends which are not commonly seen in other anorogenic intrusive systems (which are typical NYF-type environments, i.e. Nb, Y, and F-enriched): The strong Ta-, HI- and minor Li-, and Sn-enrichment, if only in localized environments, are far more characteristic of LCT-type pegmatites (Li, Cs, and Ta-enriched). REFERENCES: Buchholz, T. W., Falster, A. U. & Simmons, Wm. B. 1999. Ta, Nb, U, Y, and REF Minerals of the Koss Quariy, Marathon County, Wisconsin: The 26th Rochester Mineralogical Symposium, Abstracts of Contributed Papers. p.6. Buchholz, T. W., Falster, A. U. & Simmons, Wm. B. 2000. Additional Mineralogy of the Koss Quarry, Miarathon County, Wisconsin: The 27" itochester Mineralogical Symposium, Abstracts of Contributed Papers. P.S. 11 �Falster, A. U. 1981. Minerals of the Wausau Pluton: The Mineralogical Record, 12, P. 93-97. Faister, A. U. 1987. Minerals of the Pegmatitic Bodies in the Wausau Pluton, Marathon Co., Wisconsin: Rocks and Minerals, 62, p. 188-193. Faister, A. U., Simmons, Wm. B., Webber, K. L., & Buchholz, T.W. (in press). Peginatites and Pegmatite Minerals of the Wausau Complex, Marathon C., Wisconsin: Special volume published by the Societa Italiana di Scienze Naturali Hanson, S.L., Faister, A.U., Simmons, W.B., Webber, K.L., Buchholz, T. 1998. Rare-Earth-Element (REE) Mineralization of Pegmatites in the Wausau Complex, Marathon County, Wisconsin: The 25th Rochester Mineralogical Symposium, Abstracts of Contributed Papers. p. 12. Myers, P.E., Sood, M.H., Berlin, L.A. & Faister, A.U. 1984. The Wausau Syenite Complex, Central Wisconsin: Thirtieth Annual Inst. On Lake Superior Geology, Field Trip Guidebook 3. Simmons, W. B., M. T. Lee, and R. H. Brewster 1987. Geochemistry and evolution of the South Platte granitepegmatite system, Jefferson Co., Colorado. Geochimica et Cosmochimica Ada, 51, 455-471. 12 �CAMBRO-ORDOVICIAN STRATIGRAPHY OF SOUTHERN WISCONSIN: SEQUENCE STRATIGRAPHY RULES Byers, C.W., Dept. of Geology and Geophysics, University of Wisconsin, Madison, WI 53706 cwbyers(i).geology.wisc .edu During the past decade, the series of sandstone and carbonate formations that range from Late Cambrian through Late Ordovician have been reinterpreted n terms of the tenets of sequence stratigraphy. Older interpretations relied heavily on the facies concept; while some facies changes are still accepted, others have been superceded by the recognition of subtle unconformities, both at formation contacts and within formations. The new approach breaks the stratigraphic column into numerous unconformity-bounded units, indicating many short-term sea level fluctuations. These cycles are shorter by more than an order of magnitude than the major cratonic sequences originally defined by Sloss. Rarely do they local sequences show the full range of features expected in a complete cycle: subaerial weathering, lowstand sediments, marine transgressive surface, zone of maximum flooding, offlapping deposits. More typically, the sequences are asymmetric and truncated, with their thicknesses dominated by only one phase of the transgressive-regressive cycle. For example, the Cambrian Jordan Sandstone consists of two shaling-upward marine packages (offlaps) separated by a surface of transgression; lowstand and transgressive deposits are lacking. In contrast, the Ordovician St. Peter Sandstone is composed mostly of eolian lowstand and transgressive marine strata, with a thin cap of Glenwood Shale representing the zone of maximum flooding. Offlapping strata are thin or absent. Because southern Wisconsin lies on the flank of a cratonic dome, minor unconformities might be expected to grade into comformable sections downdip into the surrounding basins but reappear on other cratonic highs, if the cycles are eustatic in origin. 13 �A preliminary interpretation of new aeromagnetic and gravity data in Wisconsin W. F. Cannon, David L. Daniels, Stephen L. Snyder, Suzanne W. Nicholson, USGS, Reston, VA This geologic sketch map showing Precambrian basement terranes of Wisconsin is an early interpretation of newly acquired and compiled gravity and aeromagnetic data. Geophysical data are supplemented by bedrock mapping in the north and by limited drill hole information and erosional windows through Paleozoic cover in the south. The map presents a new picture of parts of the Precambrian basement in the southern and western parts of the state where it is largely concealed by a thin cover of Paleozoic strata. The map allows inferences on the mineral resources of shallowly buried basement rocks. NOKEAN FOLD AND THu rfI'?* • Most of the basement of Wisconsin is composed of rocks formed or modified during the Penokean orogeny, roughly 1850 m.y. ago. Isotopic evidence indicates that Penokean crust extends throughout the southern part of the state where younger granite, rhyolite, 14 �and quartzite lie unconformably on it. All were folded and metamorphosed in the foreland of the Mazatzal orogen sometime after 1760 Ma. Archean crust can be confidently traced as far south as the Trempealeau fault, but there is no geologic or isotopic evidence for it south of that structure. The southern Penokean Province appears to be entirely juvenile crust formed during the Penokean orogeny and composes the continental basement for the slightly younger Mazatzal orogeny. Large anorogenic granite plutons were intruded at about 1450 Ma and mafic plutons of unknown age also are widespread. Finally, dikes of diabase, probably related to the Midcontinent rift, cut all other Precambrian units. Diabase dikes Midcontinent Rift- sandstone in flanking basins. I. I Midcontinent Rift- basalt flows and conglomerate in central horst (1100 Ma). Anorogenic granite plutons and related rhyolite. Roughly 1 450 m.y. old. Mafic plutons of unknown age. Identified by circular to ovoid corresponding magnetic and gravity anomalies. Quartzite, lesser argillite and schist, minor iron-formation. Unconformable on 1 760 Ma rhyolite and granite. Strongly folded and variably metamorphosed. Granite plutons (1 760 Ma). Post -orogenic plutons with respect to Penokean orogeny. Rhyolite and epizonal granite (1760 Ma). Contains undifferentiated areas of younger quartzite. Strongly folded in Mazatzal orogeny. ROCKS OF PENOKEAN OROGEN Fold and thrust belt- Early Proterozoic metasedimentary and metavolcanic rocks and Archean basement gneisses. 1' . VV v I V V• Y Pembine-Wausau terrane- Early Proterozoic metavolcanic rocks, syntectonic granite. Archean basement lacking or discontinuous. Marshfield terrane- Archean gneiss and infolded Early Proterozoic metavolcanic and granitic rocks. Mostly granitic gneisses based on low gravity values. Ia—I V V V V V K V V *VXXXX V V V 1.' Marshfield terrane- Archean gneiss and infolded Early Proterozoic metavolcanic and granitic rocks. Mostly mafic gneiss based on high gravity values. Southern Penokean terrane- poorly known unit with high gravity and magnetic anomalies. Probably mostly mafic metavolcanic rocks. Contains undifferentiated areas of quartzite and 1 760 Ma rhyolite and granite. Southern Penokean terrane- poorly known unit with low gravity and magnetic anomalies. Probably mostly felsic rocks. Contains undifferentiated areas of quartzite and 1 760 Ma rhyolite and granite. Northern limit of Paleozoic strata. 15 �REGIONAL ARSENIC ANOMALIES SHOWN BY NURE STREAM SEDIMENT AND HYDROGEOCHEMICAL DATA IN NORTHERN WISCONSIN AND MICHIGAN W. F. Cannon, USGS, Reston, VA L. G. Woodruff, USGS, Mounds View, MN A regional arsenic anomaly in northeastern Wisconsin and the upper peninsula of Michigan is identified in the NURE (National Uranium Resource Evaluation) surveys of stream sediments and ground water. The anomalous region is about 250 miles long in north-south direction and as much as 75 miles wide. Examination of the anomaly with regard to bedrock and glacial geologic features suggests that it is a composite anomaly caused by two different bedrock sources of arsenic and variations in glacial dispersal of arsenic-rich bedrock. The two sources differ in their expression. One, the Michigamme anomaly is expressed mostly in stream sediments and to a lesser degree in well water. The other, the Fox River Valley anomaly is expressed strongly in well water, but has almost no stream sediment signature. Figure 1. Composite arsenic anomaly map of northern Wisconsin and Michigan. Base is map of glacial lobes. The shaded semi-transparent surface shows a combined anomaly from both NURE stream sediment and well water data. The surface shows the more anomalous of the two data sets relative to the regional mean values of 2 ppm As for stream sediments and 0.65 ppb As for well water. Only areas with arsenic above regional mean values are shown in the 3-D surface. The surface is defined by about 3200 stream sediment analyses and 3500 well water analyses. Black unit is arsenic-bearing Michigamme Formation and heavy line is the outcrop trace of arsenic-bearing Ordovician sandstone. 16 �Figure 2. A. The Fox River Valley anomaly shown by well water. Anomaly lies mostly west of arsenic-rich sandstone. B. The Michigamme anomaly shown by stream sediments. Anomaly location is controlled by location of Michigamme Formation and glacial features. Base map as in Figure 1. Arrows show direction of ice movement. The Fox River Valley Anomaly The Fox River Valley arsenic anomaly is best shown by NURE well water data and is only weakly expressed in stream sediment data (see Figure 2A). Arsenic values range up to a maximum of 60 ppb in well water. Interestingly, a great majority of the wells that show high arsenic in the NURE data lie west of the outcrop trace of the gently eastdipping arsenic-bearing sandstone and also west of the area where more recent data has identified an arsenic problem in wells. Bedrock in the western area of the anomaly is mostly Cambrian sandstone and Precambrian crystalline rocks, mostly granite. No arsenic source is known in these rock units. The anomaly is mostly within the area once occupied by the Green Bay glacial lobe and lies in a down-ice direction from the Ordovician sandstone. Glacial transport of arsenic-rich bedrock into the anomalous area appears to be a significant factor, suggesting that the immediate source of arsenic in well water west of the outcrop of the arsenic-rich sandstone is the unconsolidated glacial deposits. The Michigamme Anomaly The Michigamme anomaly is most strongly expressed in stream sediment date, but also occurs in well water data (Figure 2). It is geographically restricted by a combination of bedrock and glacial geology. The northern extent of the anomaly in stream sediments coincides very closely with the northern extent of the outcrop belt of black slate within the Precambrian Michigamme Formation. The eastern extent of the anomaly in northern Michigan is defined by the western margin of the Green Bay glacial lobe, which did not cross arsenic-rich bedrock. To the south, there appears to be glacial dispersal of arsenicrich bedrock in both the Green Bay lobe and Langlade sublobe in northern Wisconsin where high arsenic values in stream sediments and well water extend more than 50 km south of the outcrop belt of the Michigamme black slates. 17 �PALEOMAGNETIC STUDY OF PALEOPROTEROZOIC ROCKS IN THE ANIMIKIE GROUP, NORTHERN MINNESOTA CHANDLER, Va! W. and MOREY, G.B. (Minnesota Geological Survey, chand004@umii.edu and moreyOol @urnn.edu) A pilot study was conducted to investigate paleomagnetism in the Pokegama Quartzite and Biwabik Iron Formation of the Paleoproterozoic Animikie Group. The quartzite was sampled at six sites along the northern margin of the central and western parts of the Mesabi Iron Range, with three to six oriented samples collected per site. The iron-formation was sampled at four sites at a small outlier located to the north of the range near Pike Mountain; one to five oriented samples were collected per site. All samples are fresh and unoxidized. Core and cube specimens were cut from the field samples, and selected specimens were subjected to stepwise alternating-field and thermal demagnetization. Observed directions of magnetization have considerable scatter, and some specimens, especially those of magnetite-rich ironformation, were highly unstable during stepwise demagnetization. Nonetheless, several samples of each unit yielded stable, well-clustered magnetizations which appear to be associated with the early history of the rocks. With regard to thermal demagnetization, high blocking temperatures (greater than 600°C) are consistent with hematite as the primary carrier of stable magnetizations in the iron-formation. Hematite is an original or very early diagenetic phase in the iron-formation, whereas magnetite is a diagenetic phase that may have formed much later in the paragenetic scheme. Unstable or nulled magnetizations above thermal levels of 550°C in Pokegama specimens imply that magnetic minerals other than hematite may be present. Correcting for a bedding dip average of 100 to the south, the stable magnetizations of the Biwabik and Pokegama formations are directed at declinations/inclinations averaging 242°/65° and 289°/85°, respectively. The Pokegama (alpha=13.3°) and Biwabik (alpha=21.9°) directions cannot be discriminated from each other, or from a direction derived previously for the stratigraphically equivalent Gunflint Iron Formation in Canada (Symons, 1966) within 95 percent confidence. Our Animikie directions also overlap with those of a secondary imprint recognized to the north in the pre-Animikie Kenora-Kabetogama dikes by Halls (1986), who attributed it to a regional episode of hydrous alteration that tended to be more pronounced southwards, towards the Animikie basin. A working model that is consistent with present observations proposes that the diagenesis and subsequent magnetization of the Animikie rocks were accompanied by a regional groundwater flow system that may have been concentrated near the base of the Animikie sequence. The results of this study indicate that further paleomagnetic work on the Animikie Group rocks will be valuable, although the selection of sites that will produce useful results may pose some problems. Remaining tasks include determining if the Pokegama and Biwabik directions are truly indiscriminate from each other as well as from other Paleoproterozoic directions reported in the area. Ultimately, tightly constrained paleopole(s) can be combined with high-resolution radiometric dating to significantly improve the Paleoproterozoic apparent polar wander path for North America. References Cited: Halls, H. C., 1986, Paleomagnetism, structure, and longitudinal correlation of middle Precambrian dykes from northwestern Ontario and Minnesota: Canadian Journal of Earth Sciences, v. 23, p. 142-157. Symons, D. T. A., 1966, A paleomagnetic study of the Gunflint, Mesabi, and Cuyuna Iron Ranges in the Lake Superior Region: Economic Geology, v.61, p. 1336-1361. 18 �AN OVERVIEW OF AEROMGANETIC MAPPING IN MINNESOTA CHANDLER, Val W., Minnesota Geological Survey,2642 University Avenue, St. Paul, MN 55114, chand004@tc.umn.edu AN OVERVIEW OF AEROMAGNETIC MAPPING IN WISCONSIN MUDREY, M.G. Jr., Wisconsin Geological and Natural History Survey, 3817 Mineral Point Rd., Madison, WI 53705, mgmudreyfacstaff.wisc.edu MINNESOTA Magnetic methods have long been used by geologists in Minnesota to help investigate poorly exposed Precambrian bedrock. In fact, the Cuyuna iron range, which was discovered in 1904 by dip needle, was the first mining district in the United States that was discovered wholly by a geophysical method. The first large-scale magnetic project in Minnesota occurred after WWII, when George M. Schwartz, then-director of the Minnesota Geological Survey (MGS), made a cooperative arrangement with the U.S. Geological Survey (USGS) for surveying in Minnesota using the newly developed aeromagnetic method. Although the priority of this early work was locating new iron ore resources, the usefulness of the new method in mapping Precambrian geology was quickly realized, and by 1950 over 40,000 square miles of northern Minnesota had been covered by this method. During the 1960s while P.K. Sims was director of the MGS, the USGS aeromagnetic coverage over the entire state was completed, and an integrated program of geologic mapping using aeromagnetic and gravity data began. These efforts culminated in 1970 with the publication of a state geologic map, the first since 1932. By the mid-1970s, the potential of the USGS aeromagnetic data had been realized, and newer, higher resolution data were needed. Through the efforts of Matt Walton, thendirector of the MGS, and Robert Hansen, then-executive director of the Legislative Commission on Minnesota Resources (LCMR), a new state-wide program of high-resolution aeromagnetic surveying began in 1979. Funding came primarily from the LCMR, with additional contributions of data from the USGS, the U.S. Steel Corporation and the Geological Survey of Canada. The data obtained earlier was flown at one mile line spacing with 1000 feet terrain clearance, and much of the new flying was conducted at ¼ mile spacing with 500 feet terrain clearance. In addition, the new data were digital and could be readily subjected to a variety of computer-based processing and enhancement schemes to assist in geologic interpretation and mapping. State-wide coverage was completed in 1991, and the new aeromagnetic data, used in conjunction with an improved gravity database, have dramatically improved Precambrian geologic mapping in Minnesota. Virtually all Precambrian bedrock in the state has been re-mapped at a scale of 1:1,000,000 or larger. The new geologic maps, as well as the geophysical data used to help make them, will be useful to a variety of scientific and economic investigations for many years to come. WISCONSIN T.C. Chamberlin's geologic staff began mapping in northern Wisconsin in 1 870s as a continuation of mapping in southern Wisconsin. It was recognized that conventional geological techniques did not provide sufficient information in the glacially covered, 19 �geologically complex areas of the Gogebic Range. C.E. Wright of the Chamberlin's Wisconsin Geological Survey sent a sketch to instrument maker Gurley in Troy, New York, of a "dipping needle" based on a design he had seen from Sweden. The determination of the inclination and declination of the magnetic field had been well established by R.D. Irving and C.R. Van Hise of the U.S. Geological Survey by the late nineteenth century. Beginning in 1913, W.O. Hotchkiss and colleagues of the Wisconsin Geological and Natural History Survey (WGNHS) mapped large areas of northern Wisconsin by conventional and magnetic methods to assess mineral value for taxation. In 1935, C.K. Leith, R.J. Lund and A. Leith of the U.S. Geological Survey were able to produce a reasonable regional geologic map of the Lake Superior Precambrian that was based on conventional geology, mineral mapping, and extensive magnetic surveys. Using fluxgate magnetometers developed during World War II, G.P. Wollard and his students undertook regional magnetic surveying in Wisconsin in the early 1 960s. Of note is the regional aeromagnetic map of Wisconsin in 1964 by R.W. Patenaude and colleagues, who mapped Wisconsin on a 1 0-km line spacing at 1 000-m elevation. Prior to 1972, more detailed surveys were limited to small areas in support of geologic programs in southwestern and central Wisconsin. In 1972, the WGNHS received a small grant from industry to initiate surveys in central Wisconsin. With this seed and grants from Upper Great Lakes Regional Commission, the WGNHS and University of WisconsinOshkosh professor John Karl conducted a survey of a large area in northern Wisconsin. That survey was completed in 1977 with a "fill in the holes" grant from U.S. Geological Survey. These and subsequent public surveys were flown on lines spaced at 805 m, and oriented in a north-south direction. The altitude was draped to topography at 305 m above ground level. Interest in massive sulfide exploration in the 1 980s resulted in many private surveys, some of which were released to the public through the WGNHS. More recently (1997-1999) the U.S. Geological Survey completed surveying the remaining land areas of Wisconsin. The release of these data on CD-ROM is stimulating a reevaluation of the regional geologic fabric of the upper Midwest. Remaining activities include an adjustment of the surveys to a common base and preliminary analysis of the newly acquired data by W.F. Cannon and others. The statewide coverage, used in conjunction with an improved gravity database, will dramatically improve subsurface geologic mapping. The new geologic maps as well as the geophysical data used to help make them will be useful to a variety of scientific and economic investigations for many years to come. 20 �STRUCTURE, STRATIGRAPHY AND PUNCTUATED EVOLUTION OF MINNESOTA'S MINERAL EXPLORATION ARCHIVES David Dahi, Minnesota Department of Natural Resources, Hibbing, Mn 55746 A project to catalog the content of Minnesota DNR's mineral exploration archives for remote digital access has provided a substantial "opportunity" to better understand the structure and relationships among some 15,000 unpublished mineral exploration documents, and the 100+ exploration programs they came from. Together, the documents provide a mosaic of evolving mineral exploration methods and exploration models, and reflect changing exploration focus over time. In the past, archive users have often had a sense of déjà vu when researching Minnesota's archives. Indeed, during the process of cataloguing the content, some 25% of the archive documents were found to be duplicates of existing information. In some cases, more than a dozen copies of a document existed in the files. The lineage of that duplication, and the reasonable geographic motif that led to that duplication offer several lessons about the geographic nature of mineral exploration programs and storage of mineral exploration data for future geologic research. Within government activities, remote access to the mineral exploration archive provides a needed input for more responsive local and regional planning, and offers a historical background for mineral resource management decisions. In conjunction with online comparison to other data sets such as DOQ's, DRG's, DEM's, Public Land Survey, aeromagnetic data, published geologic maps, land use, landsat, soils and other baseline information, the exploration data archives can now be used to more quickly glean unique insights about previous exploration efforts and to develop insights for new geologic and geophysical exploration and research. 21 �NEW AEROMAGNETIC MAP OF WISCONSIN EXAMINED IN A REGIONAL CONTEXT DANIELS, David L., daveusgs.gov, NICHOLSON, Suzanne W., swnichusgs.gov, CANNON, William F., wcannonusgs.gov, U.S. Geological Survey, MS 954 National Center, Reston, VA 20192, and KUCKS, Robert P., rkucksusgs.gov, U.S. Geological Survey, MS964 DFC Box 25046, Denver, CO 80225 The new aeromagnetic map of Wisconsin portrayed in color at a scale of 1:500,000, is the result of digitally blending grids of 22 surveys flown between 1948 and 1999. The composite grid features 1) a resolution of 250m, and 2) a common elevation; all surveys were either flown at, or digitally continued to, an elevation of 1000 ft (305m) above mean terrain, prior to assembling into a state grid. The U.S. Geological Survey acquired all recent data in the state (1988 to 1999) amounting to about 77,000 line-miles. The digital flightline data for three of these surveys have recently been released on CD-ROMs. These data were added to earlier USGS surveys and 4 surveys acquired by Wisconsin Geological and Natural History Survey. Flight lines are V2-mile apart or less for 95% of the state, giving the aeromagnetic map nearly uniform specifications, and making the map an excellent tool for USGS mineral resource investigations. The Wisconsin grid has also been digitally blended with data from surrounding areas (Chandler, 1991; Hildenbrand and Kucks 1984, 1991) to form a preliminary regional aeromagnetic map of the North-Central US that reflects the structure of Precambrian basement rocks (see gray-scale index map). The regional map will be shown in color at a scale of 1:2,000,000. The aeromagnetic data of the region include some surveys of widely spaced flight lines (3 to 6 miles), particularly in northwestern and central Illinois, Lake Michigan, and the lower-peninsula of Michigan. These are areas where higher resolution data would be helpful to better define basement geology. Aeromagnetic features observed within Wisconsin can be traced into surrounding states. These features include: 1) high-amplitude linear anomalies that record the upturned edges of Keweenawan basaltic lava flows of the Midcontinent Rift System, and the smooth magnetic field of the flanking sedimentary basins; 2) abundant, narrow, linear magnetic anomalies probably generated by diabase dikes show a variety of trends across the region. (These anomalies are prominent only in areas of high-resolution surveys); 3) a strong ENE directed aeromagnetic fabric in areas of exposed Precambrian rocks in northern Wisconsin, Minnesota, Michigan's northern Peninsula, and Canada that records highly-deformed basement rocks, and 4) an aeromagnetic fabric with no preferred trend and abundant circular to arcuate anomalies, that characterizes much of the area to the south. This undirected fabric may reflect large areas of anorogenic igneous rocks, although part of the undirected fabric could also be due to greater depth to basement and lower survey resolutions. In SE Wisconsin a belt of high overall magnetic intensity lies within this area of undirected fabric, and is characterized by a series of high-amplitude, oval to circular anomalies. The belt continues SW through Illinois and Iowa into Missouri and eastward across Lake Michigan (A-A' on figure). The circular to oval anomalies suggest plutonic complexes in the basement. References Chandler, V. W., 1991, Shaded-relief aeromagnetic anomaly maps of Minnesota: Minnesota Geological Survey, one sheet, Scale 1:1,000,000 Hildenbrand, T.G., and Kucks, R.P., 1984, Residual total intensity magnetic map of Ohio: U.S. Geological Survey Geophysical Investigations Map GP-096 1, 1 sheet, scale 1:500,000. Hildenbrand, T.G., and Kucks, R.P., 1991, Total intensity magnetic anomaly map of Missouri: U.S. Geological Survey Open-File Report 9 1-0573, 1 sheet, scale 1:500,000. 22 �Aeromagnetic Anomaly Map of Wisconsin and North-Central US 0 0 _950 9O0 100 0 100 kilometres NAD27/LCC9O 23 200 -85° �MIDDLE PROTEROZOIC TECTONIIC HISTORY OF THE CENTRAL TUSAS MOUNTAINS, NORTHERN NEW MEXICO, AND COMPARISON WiTH THE BARABOO INTERVAL, SOUTHERN LAKE SUPERIOR REGION DAVIS, Peter B 1, WILLIAMS, Michael L. 1, BOWRING, Samuel A 2, and RAMEZANI, Jahan 2. (1) Department of Geosciences, Univ of Massachusetts, Amherst, MA 01003, davis@geo.umass.edu, (2) Earth, Atmospheric & Planetary Sciences, Massachusetts Institute of Technology The similarity between mid-Proterozoic quartzite units of the Baraboo interval, and the Ortega quartzite of northern New Mexico, has been recognized for decades. (Dott 1993) These similarities are based on sedimentary characteristics and the general timing of deposition, deformation and metamorphism of these supermature quartzite units. Plate tectonic reconstruction models also show a trend in the bedrock's mantle separation age extending from southwest to northeast that conned these units (Hoffman 1988)(Figure#1). However, drawing a meaningful correlation between them across a 1000-mile separation is extremely difficult because of the sparse nature of intervening exposure, and the uncertainty in the absolute timing of events. Recent work in the Tusas Mountains of northern New Mexico has further constrained the tectonic history that affected the Ortega quartzite among other units, and therefore might shed light on this tentative correlation. Recent field mapping, geochronology, and petrologic analysis in the Tusas Mountains has focused on a northwest-southeast striking enigmatic discontinuity that is suspected to be a late mesoproterozoic ductile fault. This fault juxtaposes higher-grade complexly deformed supracrustal immature to mature metasediments (which includes the Ortega Quartzite) and felsic metavolcarncs found to the south against lower grade immature metasediments and mafic to felsic metavolcanics and intrusives found to the north (Figure#2). Ductile deformational features present in these rocks can be grouped into three generations. A strong bedding parallel Si foliation is ubiquitous, however there are few Fl folds. D2 features include reclined tight to isoclinal folds with a strong SW dipping S2 foliation, and SW plunging stretching lineations (L2), sub-parallel to fold axes. Kinematic indicators suggest transport on this lineation was to the northeast. These structures are more pronounced south of the discontinuity. D3 produced east-west trending open folds, and either a newer crenulation clevage, or reactivated S2. In some localities D3 has reoriented F2 folds into F3 folds. Metamorphic conditions can also be grouped into three generations. Ml conditions reached lower greenschist facies. M2 reached greenschist facies conditions. M3 is discontinuous across the NW-SE discontinuity from upper greenschist to the north, to sub-amphibolite to the south (550°C -4.5kb). Timing of all three of these deformational events were traditionally correlated with the 1.67-1.65Ga. Mazatzal orogeny. New geochronologic data of 1.67-1.69 Ga. for the wealdy deformed Tusas granite suggests that the strong fabric in the Moppin group host rock was produced before 1.69Ga., possibly during the Yavapai Orogeny. A refined age for the Tres Piedras Granite, and a syn-Di dike, is now 1.67-1.69 Ga. is interpreted to suggest that the first two fabrics in the granite, directly correlative to regional fabrics, are indeed the result of 1.68-1.65Ga. Mazatzal tectonism. These data also further constrain the age of deposition of the Hondo group, which includes the 24 �______ Ortega Quartzite, to older than 1.69 -Ga., but younger than the underlying 1.70-1.7lGa. Burned Mountain rhyolite. D3, which increases in strength from north to south across the Tusas Mountains, was recently constrained in the southern Tusas Mountains to 1.45-1.4OGa. (Bishop et al 1996). Island-Arc Related Mafic to Felsic Volcanic sediments and Intrusives Moppin Group > 1.775 The earliest tectonic event represents an Tusas Granite early phase of thrusting with syn-orogenic - 1.69 Ga plutonism, and is primarly preserved to the SuDracrustal northern portion of the Tusas Mountains. The Sediments Tres second event, thought to be the result of additional and Volcanics thrusting with folding, is preserved throughout the Granite —1.68 Ga Tusas Mountain range. The third event significantly overprinted much of the range both Vadito Group tectonically and thermally. Its effects are thought to -1 .7OGa. be the result of the burial of the range to the midcrust leaving the region to eventually cool isobarically, erasing much of the evidence for earlier events. The effects of this overprint however 5 Miles strong in the southern half of the tusas, diminish at the discontinuity, and are poorly preserved to the Figure#2 - Tectonic Map of north. the Tusas Mountains, NM This refined model for the central Tusas Mountains of northern New Mexico provides a better framework with which to compare the Baraboo Interval in future regional models. Both regions contain a generalized supracrustal sequence of rhyolite followed by laminated to massive supermature quartzites. Constraint on the timing of deposition of both quartzite units is between approximately 1.71 — 1.65 Ga. Deformation is constrained between deposition and respective thermal event around 1.45 Ga. (Medaris 1996). Some have suggested that folding of the Baraboo Interval occured around approximately 1.63 Ga., during which whole-rock Rb-Sr systems were reset over the region. The 1.45 Ga. thermal and tectonic event that affected both the southern Lake Superior Region and the Tusas Mountains is thought to be related to the anorogenic magmatic event (Anderson / 1992). Proterozoic rocks of northern New Mexico and the Baraboo Interval of the southern Lake Superior region are windows into important tectonic processes. These widely separated windows reveal different structural levels and foreland proximities along a general orogenic trend across the Laurentian margin as it grew through mid-Proterozoic time. Hoffman, Paul F. United plates of America, the birth of a craton; early Proterozoic assembly and growth of Laurentia, Annual Review of Earth and Planetary Sciences. 16; P 543-603. 1988. Bishop, Jennifer L. Williams, Michael L. Lanzirotti Antonio. A doubly-looping P-T-t-D history for Proterozoic rocks of northern New Mexico and implications for the tectonothermal behavior of the mid-crust. Abstracts with Programs, Geological Society of America. 28; 7, P 495. 1996. Medaris, Gordon L. The Baraboo Quartzite, Wisconsin; Proterozoic deposition and deformation in the Lake Superior region. Abstracts with Programs, Geological Society of America. 28; 7, P376. 1996. Dott, Robert H. The Proterozoic red quartzite enigma in the north-central United States; resolved by plate collision?. In: Early Proterozoic geology of the Great Lakes region. Memoir Geological Society of America. 160; Pages 129-141. 1983. 25 �SOME OBSERVATIONS FROM THE WILLIAMS QUARRY EXPOSURE: EVIDENCE OF DEBRIS FLOW DEPOSITS IN THE PARFEYS GLEN FORMATION? PHILIP FAUBLE, Wisconsin Department of Natural Resources, P.O. Box 7921, Madison, WI 53707-7921; faub1p@dnr.ta.wi.us JENNIFER LIEN, Kraemer Company, P.O. Box 235, Plain, WI 53577; jen1@mhtc.net A quarry located on the north side of the North Range of the Baraboo Syncline, originally developed to mine sand and gravel from a quartzite conglomerate of the Parfeys Glen Formation, has recently expanded into insitu Baraboo Quartzite. This provides a unique opportunity to observe the Precambrian/Cambrian unconformity and its relationship to the overlying conglomeratic deposits. The undifferentiated quartzite conglomerate and conglomeratic sandstones, found in a broad band on both sides of the North and South Ranges of the Baraboo Hills, were first described in detail by Dalziel and Doll (1970). They inferred that these deposits were the result of wave action eroding and transporting quartzite from the ancestral Baraboo Hills during the transgression of the late Cambrian seas. These deposits were later named the Parfeys Glen Formation by Clayton and Attig (1990), who split the formation into three componants: a talus conglomerate directly adjacent to the quartzite, a conglomeratic sandstone and a sandstone unit that may or may not contain quartzite pebbles. Although the causes of the lithologic differences between individual units within the Parfreys Glen Formation were unclear, they considered it likely that the talus conglomerate was considerably older than the adjacent sandstone. At present, the quarry exposure is approximately 15 meters high, 200 meters long, north facing, and generally oriented east to west, parallel to the axis of the North Range. There are three distinct lithologies exposed along the quarry wall consisting of the following, from the base of the exposure to the top: in-situ quartzite, a very coarse quartzite conglomerate, and a finer-grained conglomeritic sandstone that gradually transitions to a quartz sandstone. In-situ Quartzite: An irregular, rounded outcrop of Precambrian Baraboo Formation quartzite is exposed at the base of the quarry wall. The quartzite is slightly overturned and dips steeply to the north. The main quarry face roughly parallels the strike of the quartzite beds. The largest exposure of quartzite is located just west of the center of the quarry wall and seems to represent a subdued topographic high prior to deposition of the upper units. The most striking feature of the in-situ quartzite is the presence of a smooth, rounded, gently undulating erosional surface along the unconformity between the quartzite and the overlying conglomerate. This smooth surface is present on both vertical and near horizontal exposures. While smooth, this surface is not varnished and does not possess striations, gouges or percussion marks. There are no obvious potholes developed in this surface, but shallow features that resemble small chutes are common. Quartzite Conglomerate: Overlying the smooth erosional unconformity developed on the quartzite is a quartzite conglomerate of varying thickness. This deposit is thinnest on the top of the quartzite knob, thickest within two low areas that flank the western and eastern sides of the knob, and gradually thins to the east. An exposure along the western wall of the quarry indicates that the coarse conglomerate gently dips northward. The conglomerate is composed of large rounded boulders of quartzite suspended in an unsorted, massive matrix of sand, rounded pebbles and cobbles. The conglomerate is poorly cemented and seems to lack any materials smaller than fme sand. All lithic fragments larger than sand are composed of quartzite or quartzite breccia likely derived from the nearby outcrops of Baraboo quartzite. Depending on quarry development, exposed quartzite boulders can range in size from 1 to over 3 meters in diameter along their longest visible axis. The largest boulders appear to be concentrated along the quartzite unconformity and the upper margins of the deposit. As first noted by Dalziel and Dott (1970), boulders larger than 2 meters in diameter tend to be angular or posses smooth rounded features on only one or two faces. Boulders smaller than about 2 meters are almost always completely rounded. There does not appear to be any clear fabric or preferred orientation to the larger clasts within the deposit. Con glomeratic Sandstone: Above and to the north of the quartzite conglomerate is a conglomeratic sandstone that thickens dramatically from less than 3 meters directly above the quartzite knob to over 10 meters at the northern edge of the quarry. It is composed of horizontally bedded strata consisting of alternating layers of sand and pebble conglomerate. This deposit is likely marine in origin, with abundant glauconite, sometimes occurring in thin, discrete beds, and abundant Scolithus burrows in the sand layers. The sandstone is visibly finer grained and much better cemented than the quartzite conglomerate directly below it. In contrast to the large boulders found in the conglomerate, an informal, random sampling of 50 of 26 �the largest pebbles visible in the sandstone exposed along the eastern wall of the quarry indicated a long axis diameter no greater than 20 cm. On the eastern wall of the quarry, the horizontally bedded conglomeratic sandstone can be seen to neatly onlap the north-dipping conglomerate. Above the conglomerate/sandstone unconformity, sandstone drapes the larger boulders projecting above the surface of the conglomerate deposit. Along the southern wall of the quarry, the conglomeratic sandstone extends from above the quartzite conglomerate to within about 3 meters of the top of the exposure where it abruptly transitions into a well cemented quartz aremte. This transition does not appear to be an unconformable surface, but likely reflects the drowning of the pebble source as the transgressing Cambrian seas covered the crest of the North Range. Conclusions: The exposure at the Williams Quarry clearly preserves and records three distinct events that occurred on the North Range of the ancestral Baraboo Hills sometime from the late Precambrian to the late Cambrian. First, topographically prominent exposures of the Baraboo quartzite were eroded into a series of smoothly rounded features. Second, conditions changed and a deposit of unsorted quartzite debris (conglomerate) covered the erosional surface, filling in swales and blanketing hillslopes. Lastly, a marine deposit of sand and locally derived pebbles covered the conglomerate. The exact processes and conditions that produced the smooth weathering/erosional surface on the insitu quartzite are unknown. However, the nature of the unsorted quartzite conglomerate indicates that it is likely the result of subaerial mass movements of material eroded from the crest of the North Range. We suggest that these movements took place in the form of large debris flows that generally followed drainages developed in the ancestral Baraboo Hills. Similar coarse conglomeritic deposits adjacent to Precambrian erosional surfaces have been noted in other regions. The lowest facies of the Upper Cambrian Lamotte Sandstone in southeast Missouri contain locally-derived rhyolite boulder conglomerates that have been interpreted as small alluvial fans formed, in part, by debris flows adjacent to the incised Precambrian bedrock of the ancestral St. Francois Mountains (Houseknecht, 1978). The wide grain size distribution of the conglomerate, ranging from fine sands to boulders several meters in diameter, the larger clasts suspended within in an unsorted mass, and the lack of clear grading or internal structures, closely matches the description of debris flow deposits in Coussot (1996). Debris flow deposits result from the rapid transport and mass emplacement of a highly viscous slurry of water and debris. Evidence that the quartzite debris within the conglomerate was transported and not merely weathered in place can be seen at the unconformable boundary between the in-situ quartzite and the conglomerate, just east of the bedrock knob. The in-situ quartzite at this location contains an extensive quartzite breccia, very similar to the Rock Springs (Abelman's Gorge) breccia (Dalziel and Dott, 1970). Directly above the smooth, truncated surface of the eroded breccia lie large boulders and cobbles that contain no evidence of breccia. Conversely, large grains of breccia-bearing material can be seen in the conglomerate mass above other areas that lack insitu breccia. Many other questions concerning this conglomerate deposit remain unanswered. The source area and the processes that produced the rounded boulders, cobbles and sand that make up the conglomerate remain enigmatic. Thick deposits of Paleozoic and glacial sediments obscure the most likely quartzite source areas near the crest of the North Range, south of the quarry. If the top of the conglomeratic sandstone represents the drowning of the uppermost quartzite outcrop, then the source area for the conglomerate may be no more than a few meters higher in elevation than the top of the quartzite knob exposed in the quarry wall. This doesn't preclude the formation of a debris flow because, once a flow is initiated, it may continue to move over slopes as low as a few degrees. Debris flows are also very poor rounding agents, so it is likely that the quartzite clasts in the deposit were already rounded smooth prior to transport. Features visible in a few of the conglomerate boulders suggest that at least some of the rounding occurred in situ, possibly in response to intense chemical weathering. REFERENCES CITED Clayton, L. and Attig, J., 1990, Geology of Sauk County, Wisconsin, Wisconsin Geological and Natural History Survey Information Circular No. 67, 68p. Coussot, P. and Meunier, M., 1996, Recognition, classification and mechanical description of debris flows, Earth Science Reviews, 40, 209-2TJp. Dalziel, I.W. and Dott, R.H., 1970, Geology of the Baraboo District, Wisconsin, Wisconsin Geological and Natural History Survey Information Circular No. 14, l64p. Houseknecht, D. and Etheridge, F., 1978, Depositional history of the Lamotte Sandstone of southeast Missouri, Journal of Sedimentary Petrology, v.48, 5'75-586p. 27 �Contrasts in the Geologic and Hydrochemical Occurrences of Arsenic Contamination of Groundwater in Eastern Wisconsin Gotkowitz, M.B., Wisconsin Geological and Natural History Survey, 3817 Mineral Point Road, Madison, WI 53705 mbgotkow(facstaffvisc .edu Schreiber, M.E., Department of Geological Sciences, Virginia Tech, Derring Hall, Blacksburg, VA 24061 mschreib@vt.edu Simo, J.A., Department of Geology and Geophysics, University of Wisconsin-Madison, 1215 W. Dayton St, Madison, WI 53706 simo@geology.wisc.edu Clusters of arsenic-impacted wells are found in two areas of eastern Wisconsin: the Fox River valley (east-central Wisconsin) and the Lake Geneva (southeastern Wisconsin) areas. In the Fox River valley area, arsenic concentrations up to 12,000 ppb have been measured in groundwater from a CambrianOrdovician sandstone aquifer. An arsenic-rich, sulfide-bearing, secondary cement horizon (SCH) is commonly present at the top of the Ordovician St. Peter Formation. Within the SCH, arsenic occurs in pyrite and marcasite and in iron oxyhydroxides, but not as a separate arsenopyrite phase. Whole rock concentrations of arsenic within the SCH range from about 15 to 675 ppm. Core samples show that arsenic-bearing minerals are also present in the St. Peter below the SCH. Several pieces of evidence support the hypothesis that oxidation of sulfides is the cause of high (>100 ppb) concentrations of arsenic in well water, including 1) the presence of arsenic-bearing sulfides in the aquifer, 2) water-chemistry data that show a positive correlation between arsenic, iron, and sulfate and negative correlation between arsenic and pH; and 3) nearly identical sulfur isotopic signatures in pyrite and dissolved sulfate. There is a strong correlation between high arsenic concentrations and the occurrence of intersecting elevations of the SCH and water levels within wells. This relationship provides the basis for our conclusion that atmospheric oxygen, introduced to the SCH through well boreholes, provides an oxidant to the system. However, the cause of more commonly encountered, moderate (less than 100 ppb) arsenic concentrations found in wells in the Fox River valley is not well understood. The variability of the thickness of the SCH and the associated mass of arsenic within the sulfides, as well as the local availability of an oxidant to fuel the oxidation of the SCH, may contribute to the spatial variability in arsenic concentrations in well water. The available water quality data are not sufficient to determine if other geochemical mechanisms, such as desorption or reductive dissolution of arsenic-bearing iron oxyhydroxides, control the moderate arsenic concentrations measured in well water. Our preliminary work in the Lake Geneva area indicates that geologic and hydrogeologic conditions leading to a cluster of arsenic-impacted wells in this part of the state are not similar to those in the Fox River valley. Arsenic has been detected in wells open to the Quaternary deposits, Silurian dolomite, and the Cambrian-Ordovician sandstone. Arsenic concentrations in rock samples from these aquifers range from 1.4 to 18 ppm; aqueous concentrations in well water range up to 80 ppb. Water chemistry in arsenic-contaminated wells is not consistent with sulfide oxidation, and sulfide mineralization has not been observed in rock samples collected from the area. These results indicate that other geochemical mechanisms of arsenic release, such as the reduction of arsenic-bearing iron-oxides, may also affect Wisconsin groundwater supplies. 28 �THREE NEW ZIRCON DATES FOR THE MIDCONTINENT RIFT, NORTH SHORE, MINNESOTA: MORE DATA, MORE QUESTIONS GREEN,J. C., Geological Sciences, U. of MN Duluth, Duluth, MN 55812; DAVIS, D. W., Royal Ontario Museum, 100 Queen's Park, Toronto, ON M5S 2C6; and SCHMITZ, M. D., Earth and Planetary Sciences, MIT, 77 Mass. Ave., Cambridge, MA 02139 U/Pb zircon dates have been obtained for three significant localities on the North Shore, with some intriguing results. Skeletal zircons from the residual monzodiorite at the top of the Duluth Complex Layered Series in Duluth give an age of 1098.5 +7-1.3 Ma (DWD). This agrees with the 1099.3 +1- 0.3 Ma age determined by Paces and Miller (1993) from a segregation within the upper part of the Layered Series. The skeletal crystal habit assures that the new date represents the magma crystallization age, not inherited crystals. A rhyolite flow exposed below the quarry in the southeast flank of Canton Peak near Tofte gives an age of 1092.6 +7- 2.0 Ma (DWD). This is the youngest date yet found on the North Shore. This local volcanic sequence is apparently isolated by faults from the rest of the North Shore Volcanic Group, including its uppermost sequence, the Schroeder-Lutsen basalts (SLB), and it is clearly younger than the 1096.6 +7- 1.9 Ma Palisade thyolite (Davis and Green, 1997) which unconformably underlies the SLB. Since it has been suested (Miller et al., 1995) that the SLB is an outlier or fringe of the Portage Lake Volcanics (dated at 1096-1094 Ma, Davis and Paces, 1990), this new date implies either that the SLB is actually considerably younger that the PLy, or that the Canton Quarry sequence is an isolated remnant of heretofore unrecognized late volcanic activity on the Minnesota flank of the MRS. Furthermore, the dated quarry sequence is intruded by the anorthosite-xenolith-bearing Carlton Peak diabase, making that still younger and not coeval with dated units of the Beaver Bay Complex (—1096 Ma, Paces and Miller, 1993). Finally, a granite xenolith in the Terrace Point basalt flow southwest of Grand Marais (Green, 2000) is dated at 1096.7 +7- 1.4 Ma (MDS). Since this medium-grained, upper mesozone-looking biotite granite must have crystallized slowly at many kilometers' depth before being broken off and carried to the surface in the basalt magma, this implies that the Terrace Point flow (basal flow of the SLB) is significantly younger than the granite. A coarse-grained segregation vein within the Schroeder basalts was collected and processed, but no zircons or baddelyite were found, leaving the SLB still undated. Davis, D.W. and Green,J.C., 1997, Geochronology of the North American Midcontinent rift in western Lake Superior and implications for its geodynamic evolution: Can. Jour. Earth Sci., v. 34, p. 476-488 Davis, D.W. and Paces,J.B., 1990, Time resolution of geologic events on the Keweenaw Peninsula and implications for development of the Midcontinent Rift system: Earth and Planet. Sci. Lett., v. 97, p. 54-64 Green, J .C., 2000, Mystery faults of the Cascade River, North Shore, or, What is this granite doing here? (abs.): Proceedings Vol. 46, Part 1: Abstracts and Programs, Forty-Sixth Ann. Institute on Lake Superior Geology, Thunder Bay, ON, p. 14 Miller,J.D.,Jr., Chandler, V.W., Green,J.C. and Witthun, K., 1995, The Finland Tectonomagmatic Discontinuity - a growth fault marking the western margin of the Portage Lake volcanic basin of the Midcontinent Rift System: Basement Tectonics 10, R.W. Ojakangas, A.B. Dickas, andJ.C. Green, Eds, Kiuwer Acad. Publ, p. 35-40 Paces,J.B. and Miller,J.D.,Jr., 1993, Precise U-Pb ages of Duluth Complex and related mafic intrusions, northeastern Minnesota: Geochnonological insights to physical, petrogenetic, paleomagnetic, and tectonomagmatic processes associated with the 1.1 Ga Midcontinent Rift System: Jour. Geophys. Res., v. 98, No. B8, p. 13,997-14,013. 29 �INITIAL RESULTS OF IN SITU ELECTRON MICROPROBE (EMP) AGE DATING OF MONAZITE FROM THE SOUTHERN LAKE SUPERIOR REGION: CONFIRMATION OF WIDESPREAD GEON 17 METAMORPHISM IIOLM, D., dhohnkentedu, Dept of Geology, Kent State University, 44242; JERCINOVIC, MJ., and WILLIAMS, M., both at Dept of Geosciences, University of Massachusetts, Amherst MA 01002. Introduction. Like most orogens the pattern and degree of Proterozoic metamorphism preserved across the 1870-1830 Ma Penokean orogemc belt is highly variable. Widespread 1630 Ma metamorphism of much of the Penokean arc terrane in Wisconsin is well documented (see Table 4 of Peterman et aL, 1985, CMP; Romano Ct al., 2000, PreC. Res.) and intrusion of the 1470 Ma Wolf River batholith certainly produced thermal overprinting in central and southern Wisconsin (Hohn et aL, 1998, GSAA; Naymark et al., 2001, GSAA). Until recently, intermediate pressure and higher temperature metamorphism both north and west of the arc terrane (northernmost Wisconsin, northern Michigan and east-central Minnesota) has been attributed to the geon 18 Penokean orogeny (see overview by Geiger and Guidotti, 1989, Geoscience Wise.). Three separate studies published in the late 1990's however (Schneider Ct al., 19%, CJES; Marshak et al., 1997, Geology; Hohu et al, 1998, AJS) proposed the presence of a widespread geon 17 amphibolite facies overprinting metamorphism. In order to better establish the timing of intermediate grade metamorphism in the southern Lake Superior region we have started a microprobe monazite geochronology study of selected metamorphic samples from east-central Minnesota and northern Wisconsin. EMP dating of monazite is currently emerging as a rapid and accurate means of geochronology, and is most easily applied to the Precambrian where the Pb concentration often allows greater analytical precision (Williams et al., 1999, Geology). Results from East-Central Minnesota. We dated monazite from a staurolite-garnet schist outcrop (sample MN-29) of the metamorphosed Little Falls Formation collected on the Mississippi River (Blanchard Dam). Thermobarometric, thermochronologic, and textural analysis of this and other metamorphosed samples led Hoim et al. (1998) to suggest two episodes of aniphibolite facies metamorphism (a geon 18 Penokean Ml and a younger geon 17 M2 associated with emplacement of abundant post-tectonic plutons). Eight EMP spot analyses on three separate monazite grains yielded U-Th-P ages ranging from 1755 Ma down to 1667 Ma (mean age of 1719±21 Ma). The oldest ages are concordant with abundant Ar/Ar mica and hornblende ages obtained throughout the region (Holm and Lux, 19%, Geology; Holin et al., 1998). Results from northern Wisconsin. We have obtained preliminaiy data from two drill core samples of garnet-biotite-sillimanite schist located near Park Falls, Wisconsin. These samples occur within the recently identified fault-bounded, sillimanite-bearing panel just north of the Niagara Fault zone (Park Falls subterrane of Cannon et al., 1998, ILSG). Sample PF-2-311 yielded a mean age of 1710 ± 43 Ma from 3 spots analyzed on 3 separate monazite grains. Sample BL-2-252 (located nearby the first sample) revealed a distinctly chemically zoned grain which produced distinct core and rim ages of 1805 Ma and 1695 Ma, respectively. Finally, we sampled the well known kyanite bearing outcrop located near the town of Powell. This sample is located within the higher pressure fault-bounded panel identified as the Powell subterrane (Cannon et at., 1998). Ten EMP spot analyses on three separate grains yielded U-Th-Pb ages ranging from 1780 Ma to 1747 Ma(meanageofl765±7Ma). Implications and Conclusions. These data, although preliminary, appear to support other independent evidence for a widespread, post-Penokean, geon 17 intermediate grade metamorphic event in the southern Lake Superior region. In Minnesota, geon 17 metamorphism appears concentrated within the internal zone of the orogen where abundant post-tectonic pistons exist To the north in the medial zone earlier geon 18 metamorphic ages are preserved in garnet grade rocks (Schneider et aL, 2001, ILSG). In northern Wisconsin the age of metamorphism in stmctural panels preserving contrasting metamorphic conditions (sillimarnte versus kyanite) appears to be dominantly geon 17 although a hint of earlier Penokean metamorphism is apparently preserved (see also Schneider et at., 2001, for Penokean metamorphic ages preserved north of the Niagara Fault zone in northern Michigan). These metamorphic age constraints suggest that the fault-bounded subterranes may have been juxtaposed after the Penokean orogeny during a period of widespread metamorphism followed by rapid unrooflng (as suggested by geon 17 cooling ages). We conclude that microprobe monazite geochronology holds promise for unraveling the timing of multiple metamorphic and tectonic events in the Lake Superior region as it has for other portions of the southern maigin of Laurentia (Williams et at, 2001, ILSG). 30 �AGE OF THE HUMBOLDT GRANITE, NORThERN MICHIGAN: IMPLICATIONS FOR THE ORIGIN OF THE REPUBLIC METAMORPHIC NODE. HOLM, D., dholni(ã)Jentedu, Dept of Geology, Kent State University, 44242; VAN SCLIMUS, W.R.. and MacNEILL, LC., both at Dept of Geology, Univ. of Kansas, Lawrence, KS, 55045. The alkali-feldspar granite near Humboldt lies on the northeastern edge of a large negative gravity anomaly that is roughly coincident with the Republic metamorphic node. Hornblende, muscovite, and biotite Ar/Ar ages obtained from country rock across the node are 1720-1670 Ma (Schneider et al., 1996). A whole-rock Rb/Sr minimum age of 1733±25 Ma on the granite was reported by Schulz et a!. (1988). The concordance of the whole-rock age with the thermochronologic data led Schneider Ct a!. (1996) to infer that post-tectonic plutonism caused the metamorphic nodal pattern. In order to test this hypothesis we obtained both U-Pb and Ar/Ar mineral age data on the Humboldt granite. U-Pb zircon results. Six single-grain analyses of somewhat tuibid grains separated from a sample of the Humboldt granite (AGR-1, collected by Holin) yielded a linear array on a conventional U-Pb concordia diagram. All six analyses yield an upper intercept age of 1806±21 Ma, although one analysis is clearly off the line. Elimination of this analysis yields an age of 1805 ±7 Ma with a lower intercept of 181 ±64 Ma. One analysis is nearly concordant and has a 201Pb/206Pb age of 1802±6 Ma. Thus it is clear that the ciystallization age is significantly older than the Ar retention age or the Rb/Sr age, which is a common situation in the Lake Superior region. Ar/Ar mica results. We dated three separate mica fractions obtained from a coarse phase of the granite. In order to test for possible intraciystalline age gradients we furnace step-heated the rim and core portion of a coarse muscovite grain. Both analyses yielded essentially concordant plateau dates of 1712±6 Ma (core) and 1703±6 Ma (rim). Step-heating of biotite did not produce a reliable plateau age. Incremental ages increased monotonically with the three highest temperature fractions giving ages between 1670 and 1700 Ma (amounting to 45% of the gas released). These Ar/Ar results are comparable to mica Ar/Ar ages from older bedrock of the Republic region and are concordant within error with the whole-rock Rb/Sr age of the Humboldt granite. The crystallization age of the granite indicates that it could not have provided the heat source needed to form the Republic metamorphic node at ca. 1720 Ma. We cannot rule out the possibility of a younger geon 17 intrusive body existing in the Republic subsurface, although thus far geon 17 plutons have been documented predominantly south of the Niagara fault zone (the sole exception being the 1781 Ma Parlc Falls granite, Van Schmus et a!., 2001). A combination of anomalously high basement heat production rates (Attob, 2000) together with basement remobilization (i.e., Marshak et al., 1997) might better explain geon 17 low P/high T metamorphism in the Republic region of northern Michigan. Attob, K, 2000, Contrasting Metamorphic Record of Heat Production Anomalies in the Penokean Orogen of Northern Michigan: Journal of Geology, v. 108, p. 353-36 1. Marshak, S., Tinkham, D., Allcmim, F., Bruekner, H., and Bornhorst, T., 1997, Dome-and-keel provinces formed during Paleoproterozoic orogenic collapse — core complexes, diapirs, or neither?: Examples from the Quadrilatero Ferrifero and the Penokean orogen: Geology, v. 25, p. 415-418. Schneider, D., Holin, D., and Lux, D., 1996, On the origin of Early Proterozoic gneiss domes and metamorphic nodes, northern Michigan: Canadian Journal of Earth Sciences, v. 33, p. 1053-1063. Schulz, K.J., Sims, P.K., and Peterman, Z.E., 1988, A post-tectonic rare-metal-rich granite in the Southern 34th Ann. Inst. on L. Superior Geology, Marquette, Michigan, Complex, Upper Peninsula, Michigan: p. 95-96. Van Schmus, W.R, MacNeill, L.C., Holm, D.K, and Boerboom, T.J., 2001, New U-Pb ages from Minnesota, Michigan, and Wisconsin: Implications for Late Paleoproterozoic Crustal Stabilization: 47th Ann. Inst. on L. Superior Geology, Madison, Wisconsin, May (this volume). 31 I �206PbI 0.20 0.22 0.24 0.26 0.28 0.30 0.32 0.34 3.0 3.4 3.8 207Pb/235U 4.2 4.6 5.0 5.4 �NEW VOLUME CALCULATIONS FOR THE PYROCLASTIC ERUPTIONS ASSOCIATED WITH THE STURGEON LAKE CALDERA COMPLEX, NORTHWESTERN ONTARIO: IMPLICATIONS FOR THE SCALE OF ARCHEAN VOLCANIC PROCESSES GEORGE J. HUDAK Department of Geology, University of Wisconsin Oshkosh, Oshkosh, WI 54901 DEAN M. PETERSON and RONALD L. MORTON Department of Geology, University of Minnesota — Duluth, Duluth, MN 55812 The Archean Sturgeon Lake Caldera Complex (SLCC) of northwestern Ontario comprises a north-facing, homoclinal, bimodal sequence of caldera-associated, greenschist facies metamorphosed, volcanic, intrusive and sedimentary strata with a composite thickness of nearly 4500 meters and a strike length of at least 25 km (Morton et al., 1991). At least three volcanic-associated massive sulfide (VMS) orebodies were formed as sub-seafloor replacement deposits within quartz-phyric, pumice-rich vitric tuff within the caldera complex (F-Group, Mattabi, and Sturgeon Lake); the other three orebodies in the region (Sub-Creek Zone, Creek Zone, and Lyon Lake) were probably formed from structural deformation of the Sturgeon Lake Mine (Hudak, 1996). Hudak et al. (2000) have divided the caldera complex into three stratigraphic sequences. The Pre-Caldera Sequence (PCS) is composed of a 200-2100 meter thick succession comprising subaerial basalt lavas, scoria-rich volcaniclastic rocks, and very minor rhyolite lavas that formed prior to the development of the caldera complex. The Early Caldera Sequence (ECS) comprises a 650-1300 meter thick succession of subaerial rhyolite ash tuff formed immediately prior to the formation of the caldera edifice, and subaqueous coarse heterolithic breccias, syneruptive aphyric and quartz-phyric pumice-rich vitric tuffs, and andesitic to rhyolitic lava flows formed prior to, or simultaneously with, the Mattabi VMS orebody. The Late Caldera Sequence (LCS) is composed of a complex, 500-1500 meter thick succession of subaqueous quartz- and quartz-plagioclase-phyric vitric tuffs, andesitic to dacitic lava flows, lava domes, and cryptodomes, resedimented syn-eruptive ash-rich mudstones, ash-rich sandstones, and lithic lapillistones, as well as post-eruptive tuffaceous mudstones, tuffaceous sandstones, tuffaceous breccias, and Algoma-type iron formations. Hudak (1996) and Morton et a!. (1998) have shown that the stratigraphic relationships indicate that the SLCC developed in a manner similar to the well-known caldera cycle of Smith and Bailey (1968). The presence of exceptionally well-preserved, often delicate primary lithological textures within the volcaniclastic and volcanic rocks in the south Sturgeon Lake region has allowed individual rock units and their facies equivalent deposits to be distinguished and correlated. These features indicate that only minor amounts of structural deformation, and probably only minor amounts of diagenetic compaction, have occurred within the intracaldera volcaniclastic rocks that are present. This, along with the steeply dipping nature of the strata in the region, allows us to make very accurate areal calculations of the volcaniclastic strata by means of geographic information system (GIS) analysis. Although arguably speculative, volumes of individual pyroclastic eruptions have been calculated using the average thicknesses of each of the major pyrociastic units within the caldera, the strike lengths of the pyroclastic units, and the assumption that these units were deposited within a circular caldera. The results of our eruption volume calculations are contained in Table 1. These calculations indicate that, although Archean in age, the SLCC eruptions were similar in scale to eruptions associated with Cenozoic arc-associated caldera systems. Thus, it appears that not only was the caldera cycle (Smith and Bailey, 1968) established by Late Archean time, but that pyroclastic eruptions occurring in Archean arc-associated caldera complexes were similar in scale to pyroclastic eruptions occurring in more recent arc-associated caldera systems. Although more detailed studies on the evolutionary processes and eruptive volumes need to be completed at other well-preserved Archean caldera complexes, these results may provide us with clues to Late Archean tectonic and petrological processes. 33 �Table 1. Volume Estimates of Sturgeon Lake Caldera Complex Eruptions Eruption Caldera Sequence - Estimated Jackpot Lake High Level Lake Bell River Mattabi Lower "L" Middle "L" Upper "L" Eruption Pre-Caldera Early Caldera Early Caldera Early Caldera Late Caldera Late Caldera Late Caldera VoIu(J 8.8 16.6 1.9 28.7 3.0 6.9 3.5 Table 2. Volume Estimates of Historic Caldera Eruptions Estimated Volume (km3) Source Pinatubo (1991) Krakatau(1883) Santorini (3.6 ka) Tambora (1815) Vandever Mtn Tuff(T-J) Taupo (1.8 Ka) Kuwai (—1450) 4-5 10 25-30 25 13-26 30-35 32-39 Lipman, 2000 Lipman, 2000 Lipman, 2000; Cas and Wright, 1987 Cas and Wright, 1987 Kokelaar and Busby, 1992 Lipman, 2000: Cas and Wright, 1987 Monzier et al., 1994 References Cas, R. A., and Wright, J. V., 1987. Volcanic Succession Modern and Ancient: Allen and Unwin, London, 528 pp. Hudak, G. J., 1996. The physical volcanology and hydrothermal alteration associated with late caldera volcanic and volcaniclastic rocks and volcanogenic massive sulfide deposits in the Sturgeon Lake region of northwestern Ontario: unpublished Ph. D. dissertation, University of Minnesota, Minneapolis, MN, 463 pages. Hudak, G. J., Morton, R. L., Peterson, D.M., and Franklin, J. M., 2000. The Sturgeon Lake Caldera Complex, northwestern Ontario: volcanological evolution of an Archean shallow water VHMS belt: Volcanic Environments and Massive Sulfide Deposits, CODES Special Publication 3, p. 8991. Kokelaar, P., and Busby, C., 1992. Subaqueous explosive eruptions and welding of pyroclastic deposits: Science, v. 257, p. 196-201. Lipman, P. W., 2000. Calderas, in Sigurdsson, H., 2000, Encyclopedia of Volcanoes: Academic Press, San Diego, CA, p. 643-662. Monzier, M., Robin, C., and Eissen, J.-P., 1994. Kuwae (—1425 A. D.): the forgotten caldera: Journal of Volcanology and Geothermal Research, v. 59, p. 207-218. Morton, R. L., Walker, J. S., Hudak, G. J., and Franklin, J. M., 1991. The early development of an Archean submarine caldera complex with emphasis on the Mattabi ash-flow tuff and its relationship to the Mattabi massive sulfide deposit: Economic Geology, v. 86, p. 1002-1011. Morton, R. L., Hudak, G. J., Walker, J. S., Jongewaard, P. K., and Murphy, C. M., 1998. The stratigraphy and physical volcanology of the Archean south Sturgeon Lake Caldera Complex, northwestern Ontario: Geological Association of Canada / Mineralogical Association of Canada Annual Meeting Abstract Volume 23, p. A-128. Smith, R. L., and Bailey, R. A., 1968. Resurgent Cauldrons, in Coats, R. R., Hay, R. L., and Anderson, C. A. (eds), Studies in Volcanology (Howell Williams Volume), Geological Society of America Memoir ll6,p. 153-210. 34 - �A PRACTICAL EXERCISE IN METALLIC MINE RECLAMATION LADYSMITH, WISCONSIN T.C. HUNT, Director of the Reclamation Program, School of Agriculture, University of Wisconsin - Platteville, Platteville, WI 53818 hi November 1968, Great Lakes Exploration, a subsidiary of Kennecott, intersected copper mineralization along the Flambeau River south of Ladysmith, Rusk County, Wisconsin. This discovery and the rising environmental consciousness in Wisconsin lead to nearly 25 years of legislation, engineering evaluation, and environmental regulation. The Flambeau Mine was officially opened on July 31, 1993 and by the time the mine closed in 1997, 1.9 million short tons of ore averaging 8.9 percent copper and 0.10 ounces per ton of gold were produced. This mine was unique in that all of the ore was shipped directly to smelters for metal recovery; there was no beneficiation on site. Reclamation of the property was an important part of the entire mine design. Kennecott Minerals' Flambeau Mine reclaimed their open pit copper mine under the jurisdiction of Wisconsin's environmentally sensitive metallic mining laws; viewed by many as the most strict in the nation. It was alleged that Wisconsin's modem mining laws were prohibitive, but the Flambeau Mine provides a case study demonstrating viable mining and reclamation within the constraints of rigorously protective regulatory requirements can happen. The mine site is adjacent to the Flambeau River, one of Wisconsin's premier whitewater canoeing area, and surface water, groundwater, and mine water were important parts of the mining plan. Reclamation was initiated prior to the completion of mining. Flambeau backfilled the open pit mine by layering mine waste and carbonate rock to control acidity, and reclaimed the surface using state-of-the-science ecological restoration methods. The company brought the surface of the former pit to its approximate original contour, rebuilt pre-existing intermittent stream channels, restored native plant communities, and developed a series of biofilters and wetlands to enhance runoff quality. A trail system opens the site to the public for recreational pursuits such as hiking, cross-country skiing, and bird watching. Bald Eagles and black bears are frequent visitors to the site. Visual inspection and statistical analysis of surface water samples present evidence that the site is stabilizing and that the hydrologic system is functioning as designed. Monitoring continues on the site, and the vegetative sampling results indicate the reclaimed mine site is on the desired trajectory for plant community development, diversity, cover, plant frequency, and productivity. This experience of a well designed and implemented mine plan, in conjunction with a well designed and implemented reclamation plan, illustrates that mining and the environment can be compatible. 35 �THE EARLY GABBROIC SERIES OF THE MIDCONTINENT RIFT SYSTEM: CONTINUED ASSESSMENT OF MAGMATIC ORIGINS JERDE, Eric A. (e.jerde@moreheadst.edu) and SAL VATO, Daniel J. (student), Department of Physical Sciences, Morehead State University, Morehead, KY 40351; THOLE, Jeff and WTRTH, Karl R., Geology Department, Macalester College, St. Paul, Minnesota, 55105 The Early Gabbroic Series of the Midcontinent Rift System (MCR), informally known as Nathan's Layered Series (after Nathan, 1969), is comprised of numerous tabular and sheetlike intrusions just south of the Gunflint Trail in the vicinity of Poplar Lake in extreme northeastern Minnesota. A U-Pb zircon date of 1106.9 ± 0.6 Ma has been obtained for one of the units in this series (Paces and Miller, 1993), making it among the oldest materials associated with the MCR. Nathan (1969) identified 27 separate units, given the letter designations A-Z and AA in their inferred chronological order. The chronological sequence was based entirely on cross-cutting relations and discordances observed. Nathan's own interpretation, plus those of subsequent observers, offers that many of the separate units may be gradational variations, while others may be unique dikes or "sport" varieties (to use the terminology of Nathan (1969). In terms of major magma bodies, the series comprises four principal units: A-B (troctolites to gabbronorites), F-G (oxiderich olivine gabbro), M (gabbronorite), and P-Q (troctolites to gabbros). These intrusions can fairly be considered to be the principal events in the evolution of the Early Gabbroic Series. A key aspect of this series of rocks, and one that led to the initial interest by H.D. Nathan in the I 960s, is that several units contain large amounts of Fe-Ti oxides. In places oxides comprise in excess of 30% of the rock (Nathan, 1969). However, among the major units listed above, only unit F-G is notable for high abundances of oxide, exceeding 60% at some locations. Until now, no chemical analyses have been available for rocks of this series. Initial modeling of magma crystallization (Jerde, 2000) indicated that the principal units of the early Gabbroic Series may be the result of polybaric fractionation. In such a scenario, units A-B, and P-Q formed through crystallization at 6kb, with M being more fractionated. Unit F-G could have formed through extensive crystallization at I kb. Oxide phases are favored by fractionation at lower pressures, resulting in the higher amounts seen in unit F-G. Results of initial chemical analyses of bulk rocks (Table I) show that both high-Al, low-Ti magmas and low-Al, high-Ti magmas are present. This is an interesting result since it has long been assumed that the earliest magmatic products of the MCR were all low-Al and high-Ti. The oxide-rich nature of the Early Gabbroic Series was consistent with this assumption. From this data, it is evident that unit F is significantly different from other units. Unit F may not be associated with unit G at all, and chemically resembles unit A. This is in marked contrast to the interpretation of Nathan (1969). The mineral data for some of the units shows variations consistent with multiple magma compositions, particularly among the pyroxenes. Among pyroxenes, Unit C appears to resemble unit P-Q, which was originally thought to be much younger. The variations in pyroxene may be additional evidence for polybaric fractionation since pressure differences plays a pivotal role in the appearance of pyroxene during crystallization. Most plagioclase is of an intermediate An content, showing very little variation among the units. The latest field efforts have focused on individual units, simply to be systematic. These probably represent individual intrusions, and once each one is examined, they can hopefully be placed into a broader context. The first unit examined was P-Q. This unit is present in a single band extending from Poplar Lake for approximately 15 kilometers to the west. It is interesting to note that Unit P is repeated, appearing twice in the magma stack (Fig. 1). In both instances, unit J is present stratigraphically above unit P. Unit J has a very characteristic appearance, with large ophitic pyroxenes (up to 3 cm) and layers of coarse grained material rich in Fe-Ti oxides. In most instances, when the top of unit P is approached, material very much like unit J appears, suggesting a gradational contact. A definitive contact between P and J was not observed at any location. Chemical and mineral data, along with the completely gradational contact between P (a gabbro) and Q (a troctolite), indicate that these are probably a single intrusion that has undergone continuous fractionation. It is possible that J is simply a late stage fractionate, formed after P. The repetition of unit P may reflect a second pulse of magma, or perhaps faulting (a thrust?) There is some indication that the two layers of unit P may have different compositions (compare sample JO-I 4 with the other unit P compositions in Table 1). One sample does not a difference make, however, and a definitive answer awaits further analysis, which will take place in the next few weeks. Recent studies of anorthosite-bearing intrusions such as the Kiglapait, Laramie, and Nain intrusions, has indicated that anorthosites can be produced through fractionation of a high-Al, high-Fe basaltic magma at pressures of 9-14 kb (Scoates and Lindsley, 2000). Such high-Al, high-Fe melts are consistent with low degrees of melting of an Fe-rich, enriched mantle source. Fractionation of these melts at higher pressures (-.14 kb) produces strong Fe enrichment and oxide phases appear early in the crystal assemblage. In such a scenario, the fractionation produces a ferrodiorite which is in equilibrium with anorthosite of an intermediate (An) composition. Enrichment of Si during fractionation is only possible at low pressures due to a thermal divide between cpx and opx at pressures in excess of 5 kb (Scoates and Lindsley, 2000). If such a magma were parental to the Early Gabbroic Series, it suggests that some of the Early Gabbroic Series units may be siblings to the liquids that formed the anorthositic series further to the south. Additional crystallization modeling is definitely needed to explore this new potential source for rocks of the early stages of the Midcontinent Rift development. 36 �! Table 1. Preliminary whole-rock compositions of major units from the Early Gabbroic Series Sample Unit J9-38 J9-35 J9-32 J9-31 J9-2 J9-46 J9-24 J9-23 J9-22 J9-29 A A B B C F G G G SiO2 TiO2 48.26 0.25 49.47 0.25 52.63 52.66 50.02 51.53 45.65 0.55 0.75 0.50 47.31 4.79 3.05 M M M 47.06 51.48 48.09 48.29 47.03 P 48.31 io-io P P jo-il J0-14 J9-26 J0-4 J0-13 J9-48 J9-41 J9-40 J9-37 J9-27 J9-17 1.12 3.73 20.01 17.93 13.84 16.36 16.23 15.08 15.74 12.98 15.14 49.20 49.30 0.41 1.83 1.80 1.72 22.98 18.93 22.28 17.52 15.56 15.40 14.53 p 4743 2.15 15.81 P 49.63 51.70 1.15 J Q Q Q Y 1.61 1.30 2.64 48.17 49.03 0.93 0.17 0.12 16.14 14.68 18.48 19.92 52.76 1.75 18.81 1.28 1.32 1.54 1.11 1.31 1.26 1.99 1.99 2.19 0.92 1.38 1.26 1.62 1.50 1.45 1.50 1.67 1.30 1.21 1.72 1.48 1.70 !P MiiQ M CaO 8.50 8.80 10.24 7.42 8.72 8.38 13.27 13.26 14.60 6.14 9.20 8.41 10.79 9.98 9.68 9.97 11.14 8.63 8.03 11.46 9.85 11.30 0.12 0.14 0.23 0.16 0.15 0.16 0.18 9.40 10.70 8.03 8.99 8.24 4.73 0.21 5.41 0.20 0.08 0.13 0.09 0.15 0.16 0.16 0.17 0.16 0.15 0.17 0.16 0.14 0.10 4.88 8.75 1.76 7.43 1.76 10.25 7.23 7.15 7.84 6.46 7.25 7.64 8.16 7.12 4.38 8.92 9.05 8.38 11.08 11.68 12.05 9.60 9.39 7.36 9.58 9.74 10.15 7.71 12.81 12.09 12.60 11.80 12.94 12.79 7.12 7.66 3.07 2.94 2.75 2.13 2.65 2.46 2.34 2.87 2.68 3.13 3.80 2.85 3.78 2.63 2.40 2.44 2.32 2.45 2.32 2.58 3.14 3.38 2.92 0.25 0.23 0.22 0.42 0.25 0.33 0.61 0.81 0.93 0.98 0.29 0.48 0.29 0.23 0.37 0.21 0.29 0.19 0.47 0.35 0.35 2.15 !Qs 0.04 0.03 0.02 0.13 0.03 0.10 0.57 0.31 0.76 0.07 0.03 0.11 0.07 0.02 0.13 0.06 0.03 0.02 0.11 0.07 0.06 0.22 99.33 99.39 100.51 100.83 100.37 100.64 98.96 99.14 99.32 99.44 99.40 99.26 98.47 100.07 99.90 100.22 99.39 99.72 100.32 99.00 99.11 99.19 61.8 62.6 62.1 63.0 61.8 60.7 35.9 39.0 34.4 31.0 55.9 24.7 59.9 53.2 53.7 55.3 47.7 56.9 59.9 52.8 53.2 37.8 Analyses performed by XRF in the Department of Geology, Macalester College, St. Paul. Fig. 1. North-South cross section of the Early Gabbroic Series. Note the P-J-Q-P-J sequence in the center of the figure. (from Nathan, 1969). 0 I,,,, References Cited: Jerde, E.A., 2000, Magmatic origins for Nathan's Layered Series: An initial reassessment of the Midcontinent Rift's first major plutonic materials: Institute on Lake Superior Geology Proceedings, 46th Annual Meeting, Thunder Bay, ON, May, v.46, part 1, p. 24-25. Nathan, H.D., 1969, The geology of a portion of the Duluth Complex, Cook County, Minnesota: Ph.D. dissertation, University of Minnesota, Minneapolis, i98p. Paces, J.B. and Miller, J.D., Jr., 1993, Precise U-Pb ages of Duluth Complex and related mafic intrusions, northeastern Minnesota: Geochronological insights to physical, petrogenetic, paleomagnetic and tectono-magmatic processes associated with the 1.1 Ga Midcontinent Rift System: Journal of Geophysical Research, v.9 8, 13,997-14013. Scoates, J.S. and Lindsley, D.H., 2000, New insights from experiments on the origin of anorthosite: EOS Transactions American Geophysical Union, v.81, no.48, Fall meeting Supplement. 37 �GEOPHYSICAL ANSWERS TO GEOLOGIC QUERIES IN THE SUPERIOR PROVINCE OF NORTHERN MINNESOTA JIRSA, Mark A., and CHANDLER, Va! W. (Minnesota Geological Survey, jirsa001@umn.edu, and chand004@umn.edu) Geophysical investigations play a vital role in geologic mapping in Minnesota. As an example, we present here some highlights from mapping the Archean Superior Province in parts of Minnesota where the bedrock is covered by glacial sediment as thick as one hundred meters. Mapping of the well-exposed Archean rocks, restricted largely to the Vermilion district of northeastern Minnesota (Fig. 1), was completed in the 1960s and 1970s. Although significant, this outcrop mapping suffered from two shortcomings: 1) the area constitutes less than 5 percent of the state's bedrock crust; and 2) the Vermilion district may not be representative of the remainder of the state's Archean terranes that comprise two-thirds of Minnesota's first crystalline bedrock beneath glacial materials. These shortcomings were recognized in the late-1970s by Dr. Matt Walton, then-director of the Minnesota Geological Survey (MGS). He lobbied the State Legislature and received funding (from 1979 to 1991) to collect high-resolution, state-wide, aeromagnetic data. Additional aeromagnetic data were contributed by the U.S. Geological Survey and U.S. Steel Corporation. This was coupled with efforts to map areas having poorly exposed bedrock, acquire gravity and corresponding rock properties data, and drill test-borings in locations determined by the new geophysical data and geologic models. After two decades, the results include aeromagnetic coverage of the entire state, a grid containing more than 56,000 gravity stations (averaging 1 station per 2 square kilometers, state-wide), 2,500 determinations of density and magnetic susceptibility from nearly every major rock type, and drill cores representing many of Minnesota's Archean rocks. Armed with these data, the MGS has produced variably detailed geologic maps, and in the process raised important questions about the nature of the Archean crust. Some of those questions are described below. International VERMILION DISTRICT Falls 9o COOK- ,-' >GRB NIA HORN LIJthb2 SIDE LAKE // Archean rocks Mafic to felsic stocks Granitoid batholiths Schist of sedimentary protolith Gneiss and schist I L_ 0 (JJ Layered mafic complexes 6Omi 0 I I 80km MAP AREA Metavolcanic and metasedimentary rocks Figure 1. Simplified geologic map of the Superior Province in Minnesota showing the location of various map areas discussed in the text (LLSD—Leech Lake structural discontinuity, GRB—Giants Range batholith). 38 �Early thrust nappes Mapping in the Cook-Side Lake area (Fig. 1) was directed toward understanding the geological relationships between well exposed strata of the Vermilion district on the east, and the less well exposed rocks of the Side Lake area on the west. The volcanic sequences in each district are lithologically similar, and form broad anticlines that face stratigraphically toward one another. The intervening area, covering some 600 square kilometers, is marked by scattered exposures composed of turbiditic graywacke and slate of the Lake Vermilion Formation. These strata are complexly deformed, and their interpretation required detailed structural and sedimentological analysis. The model that evolved requires that much of the Lake Vermilion Formation has been exhumed to a level that exposes the bottom of a very large thrust nappe. Geophysical data are consistent with this interpretation, as they show graywacke as a relatively thin, geophysically translucent package overlying a "floor" of denser and variably magnetic volcanic strata. Leech Lake structural discontinuity The broadly folded volcanic and turbiditic strata of the Lake Vermilion Formation are part of what is known as the Soudan belt. The Soudan belt is structurally contrasted with the adjacent Newton belt to the north; the latter forms a series of fault-bounded, mostly north-facing, homoclinal panels. The Newton belt differs further in containing komatiitic flows and abundant mafic to ultramafic sills that are not present in rocks of the Soudan belt. In the well-exposed Vermilion district, the belts are dissected by late faults to such an extent that the relationships between the belts, and even the exact position of the boundary, are unclear. Extending the belts westward away from the zone of coinciding faults using a combination of geophysical, outcrop, and drilling data identified a major state-wide discontinuity between the two contrasting structural panels. Although its origin is not well understood, the informally named Leech Lake structural discontinuity (LLSD on Fig. 1) is considered a significant Archean crustal suture. Mud Lake syncline Because Archean bedrock in the area known as the Virginia horn (Fig. 1) is mantled by magnetic iron-formation of Paleoproterozoic age, recent mapping in the area relied almost exclusively on gravity modeling. The Archean bedrock is marked by a large fold, thought by earlier geologists to be an anticline, having graywacke in the core and volcanic strata on the limbs. The graywacke is complexly folded, and detailed structural analyses of cleavage/bedding relationships produced a contrasting model of an early-formed, west-facing, upright syncline. The model was tested by a series of gravity profiles that outlined a broad synform that is cored by relatively low-density graywacke, and flanked by inwardfacing limbs of dense basaltic crust. Gravity modeling indicates that graywacke along the syncline axis is I to 2 kilometers thick and underlain by dense basaltic rocks. Geophysical boundary at 5 kilometers Constrained by physical property data, gravity and magnetic model studies of Superior Province rocks in Minnesota indicate that density and magnetization contrasts associated with the Archean greenstone-granite belts do not appear to extend to depths much greater than about 5 kilometers. This depth is surprisingly shallow, given the width and near-vertical geometry of most of these belts. The belts could be cut off by low-angle thrusting, which would be consistent with the convergent-margin, accretionary models advanced by the Canadian Lithoprobe project. On the other hand, the limited depth extent of the greenstone-granite belts could represent enhanced assimilation and mixing of magmatic and supracrustal components at depth. Support for the latter model exists along the southern boundary of the Giants Range batholith with adjacent supracrustal sequences that are complexly invaded and locally migmatized by granodiorite. In either interpretation (low-angle thrust or assimilation at depth), gravity and magnetic model studies indicate the observed anomaly signatures associated with Superior Province rocks in Minnesota are largely the result of sources within the uppermost crust, and that relatively little anomaly signature is apparently produced by middle or lower crustal rocks. In summary, aeromagnetic data, supplemented by gravity and rock property data, have had tremendous impact on mapping Superior Province rocks in Minnesota. These geophysical data provide an essential framework and continuity that is not typically available from existing drill holes and outcrops. During the last 20 years, much of the Archean bedrock outside of the Vermilion district has been remapped at scales of 1:1,000,000 or greater. This ongoing mapping provides a valuable base for launching mineral exploration programs, and has revolutionized our understanding of early crustal evolution in Minnesota. 39 �DISTRIBUTION OF ARSENIC IN WISCONSIN GROUNDWATER DAVE JOITThSON, Drinking Water & Groundwater Bureau, Wisconsin Department of Natural Resources, PU Box 7921, Madison, WI 53707-7921, johnsdmmail0.dnr.state.wi.us Iii the past ten years, with the recognition of elevated arsenic is drinking water supplies, the Wisconsin Department of Natural Resources has analyzed over 15,000 individual samples statewide. Ten percent of the statewide samples exceed the 10 ppb proposed Arsenic standard by the US Environmental Protection Agency; in some large areas over 25 percent of the samples exceed this level. The objectives of the sampling program were to (1) identify the distribution of elevated arsenic in water supplies, (2) identify concentration ranges and causes for the elevated arsenic, and (3) determine construction guidelines to minimize the problem. The arsenic appears in many settings, the most common presently recognized being a northeast trending belt in east-central Wisconsin, west of the Fox River. The highest concentration in Wisconsin, 15,000 ppb, occurs in this belt. Numerous studies (Burkel, 1993; Pelczar, 1996; Simo and others, 1996,1997; and Gotkowitz, 2001) have identified a sulfide-rich horizon recognized near the top of the St. Peter sandstone. This appears to be the principal source for As in this area. However, there are some elevated levels west of the western-most subcrop extent of the sandstone unit. Other areas of elevated As have been identified. These include relatively shallow domestic water wells in Silurian dolomite in southeast Wisconsin and in association with the Upper Mississippi Valley Zinc-Lead District in southwestern Wisconsin. Elevated concentrations are also noted in isolated areas in central and northeastern Wisconsin associated with unique Precambrian units. Township-based surveys of private wells in east central Wisconsin are continuing and may permit greater delineation of the As problem and possible solutions. Burkel, R.S., 1993, Arsenic as a Naturally Elevated Parameter in Water Wells in Winnebago and Outagamie Counties, Wisconsin: unpub. MS thesis, University of Wisconsin -Green Bay, 111 p. Pelczar, J.S., 1996, Groundwater Chemistry of Wells Existing Natural Arsenic Contamination in East-central Wisconsin: unpub. MS thesis, University of Wisconsin - Green Bay, 206 p. Simo, J.A., Freiberg, P.G., and Freiberg, KS., 1996, Geologic Constraints on Arsenic in Groundwater with Applications to Groundwater Modeling: Wisconsin Water Resources Center GRR 96-01, 57 p. Simo, J.A., Freiberg, P.G., and Schreiber, M.E., 1997, Stratigraphic and Geochemical Controls on the Mobilization and Transport of Naturally Occurring Arsenic in Groundwater: Implications for Water Supply Protection in Northeastern Wisconsin: Wisconsin Water Resources Center GRR 97-05, 56 p. Gotkowitz, M.B., Scbrieber, M.E., and Simo, J.A., 2001, Contrasts in the geologic and hydrochemical Occurrences of Arsenic contamination of Groundwater in Eastern Wisconsin: thstitute on Lake Superior Geology, this meeting. 40 �FLUID INCLUSION EVIDENCE FOR A ROLE FOR HYDROTHERMAL ACTIVITY IN THE ROBY ZONE, LAC DES ILES MINE, NORTHWESTERN ONTARIO JOHNSON, J.R. and KISSIN, S.A., Department of Geology, Lakehead University, Thunder Bay, ON, P7B 5E 1, stephen.kissin@lakeheadu.ca The Roby Zone of the Lac des lies Pd-Pt-Ni-Cu-Au mine, located 90 km north-northwest of Thunder Bay, Ontario, was the initial locus of mining activity in the deposit. A striking aspect of the deposit in the early stages of mining was the coincidence of the ore zone with an envelope of taicose hydrothermal alteration (Michaud, 1998). Based on evidence of hydrothermal activity in off-set and deep, copper-rich deposits at Sudbury (Molnar et al., 1997) and observations of felsic veins and pods in the Roby Zone, an investigation of fluid inclusions was undertaken. From a suite of samples collected during pre- and early-mining stages at the Roby Zone by Michaud (1998), hand specimen were selected. The hand samples used in the study were selected based on the presence and abundance of transparent minerals. From three hand samples (Lac-3 18, Lac-304 and Lac-46) taken from the heterolithic gabbro located in the middle of the Roby Zone, a number of doubly polished sections 100 tim-thick were prepared. Sample Lac-3 18 is composed of pegmatitic gabbro that contain minor veins of plagioclase and epidote. Sample Lac-304 consists of a quartz vein that is emplaced within a medium-grained gabbro. The last sample used, Lac-46, is a mediumgrained gabbro crosscut by a felsic vein network. Four distinct types of fluid inclusions were observed occurring in plagioclase and quartz: Type I - Single-phase liquid inclusions. These were extremely numerous in quartz but less common in plagioclase. Type II- Two-phase inclusions (liquid + vapour). The most numerous type of inclusion that was found in plagioclase. The majority had a vapour content of 10-20% by volume. Type III - Polyphase inclusions (liquid + vapour + one or more solid phases). These were fairly abundant but of a small size, occurring only only in plagioclase. Numerous inclusions contained a single crystal, but only a few were found to contain more than one solid. Type IV - CO2-rich polyphase inclusions. These were found only in plagioclase, containg two solids, two liquids and vapour. The fluid inclusion studies revealed the following results: Type II and Type III inclusions - All but one inclusion homogenized to vapour in the range of 540.1-352.4°C, but the majority were in the range 460-420°C. Eutectic temperatures generally clustered about -52°C or -49.8°C, the eutectics 41 �for NaC1-CaC12-H20 and CaCl2-H20, respectively. Some lower eutectic temperatures observed may be the result of the small size of the inclusions and difficulty in detecting the first melting. Some eutectic temperatures were near -21.2°C, the NaCl-H20 eutectic temperature. These may be attributed to lower concentrations of CaC12, such that its effects on melting behaviour were difficult to observe. Most inclusions in this group had final melting temperature of about -20°C, indicating moderate to high salinity. Type IV inclusion - Only one inclusion containing liquid CO2 and two daughter crystals was observed. The eutectic temperature was -54.2°C with a final melting temperature of -15.5°C, consistent with data from Type II and II inclusions. Liquid CO2 homogenized to vapour at 26.8°C, and the remaining vapour homogenized to liquid at 160.8°C. However, the daughter crystals melted at 230°C and >412°C, the decrepetation temperature. Type I - Liquid only inclusions exhibited eutectic temperatures of--34°C and final melting temperatures of-15°C. These inclusions, although saline, were clearly formed at low temperature. The study has shown that high-temperature saline fluids are intimately associated with the ore zone of the Roby Zone. It is suggested that the present distribution of the ore is the result of hydrothermal activity associated with these fluids, possible of late magmatic origin. Similarity to fluids active in the Sudbury Igneous Complex is striking. Michaud, M.J., 1998. The Geology, Petrology, Geochemistry and Platinum-Group ElementGold-Copper-Nickel Ore Assemblage of the Roby Zone, Lac des Iles Mafic-Ultramafic Comples, Northwestern Ontario. M.Sc. thesis, Lakehead University, Thunder Bay, Ontario. Molnar, F., Watkinson, D.H., Jones, P.C. and Gatter, I., 1997. Fluid inclusion Evidence for Hydrothermal Enrichment of Magmatic Ore at the Contact Zone of the Ni-Cu-PlatinumGroup Element 4b Deposit, Lindsley Mine, Sudbury, Canada. Economic Geology, vol. 92, pp. 674-685. 42 �HYDROGEOCHEMICAL MODELING OF ARSENIC IN MINNESOTA GROUND WAT1R KANTVTSKY, Roman, Minnesota Geological Survey, 2642 University Avenue, St. Paul, MN 55114, kaniv001@umn.edu Concentrations of dissolved arsenic in Minnesota ground water commonly exceed 3pJL (micrograms per liter) and are as much as 157 piL. The highest regional concentration of arsenic in ground water (0.06—91 WL, mean 6 iJL) is documented in the western part of the state where the aquifers are buried bodies of sand, gravel, and silty sand that form discontinuous lenses beneath lake deposits and within till. Because of this, the hydrogeochemical modeling was performed for ground water systems in this part of the state. A surface complexation two-layer sorption model was used to assess the mechanisms controlling arsenic distribution in the Quaternary buried artesian aquifer and Cretaceous aquifer systems. The release of sorbed arsenic from iron hydroxides into ground water was estimated by this model. The test data consisted of concentrations of total arsenic and iron in solution; this was derived from the arsenic and iron concentrations in the geological materials. The result of this model is the distribution of arsenic—either adsorbed to the surface of iron hydroxides, or into the aqueous phase. These results suggest that mobility of arsenic is promoted by the onset of suboxic conditions in aquifer systems where iron hydroxides have sorbed arsenic. The reduction of ferric oxides and hydroxides, together with the reduction of As(V) to the less-strongly adsorbed and more mobile As(III), can release adsorbed arsenic into ground water. This reduction may be driven by microbial degradation of natural organic matter in aquifer systems. The source of the arsenic in the Quaternary buried artesian system is presumed to be iron oxides and iron hydroxides that have sorbed arsenic, and form amorphous coatings on mineral grains within the silty-clayey till. Despite the fact that the arsenic concentrations in the ground water and the geological materials used for the modeling were far apart, the modeling illustrates that the process is probably generic and not limited by geography. Based on the sorption model and the classification of aqueous environments, the working model that explains the mech anisms controlling distribution of arsenic in ground water can be applied to five hydrogeochemical systems in Minnesota: the Quatemary buried artesian aquifer system, the Quaternary water-table aquifer system, the Cretaceous aquifer system, the Paleozoic- Mesoproterozoic artesian basin aquifer system, and the Precambrian crystalline rock aquifer system. The thick, silty-clayey, glacial and lacustrine sediments that cover the aquifers of the Quatemary buried artesian aquifer system in west-central Minnesota serve as a geochemical transition zone associated with the transformation from oxic to suboxic conditions. In areas where the tifi is more sandy, suboxic conditions may not exist. Variability in arsenic concentrations in water of the Quaternary buried artesian aquifer system can thus be explained by changes in geochemical conditions, the variability of the arsenic content in the sediments, or by variability in Waters within other distribution of chemical reductants (e.g. buried organic matter). hydrogeochemical systems typically have low concentrations of arsenic, possibly because they are associated with oxic conditions. High arsenic concentrations associated with sulfide mineralization or iron-formation may be present locally within any system, but are not a regional feature of any of these hydrogeochemical systems. Reference: Kanivetsky, R., 2000, Arsenic in Minnesota ground water: hydrogeochemical modeling of the Quaternary buried artesian aquifer and Cretaceous aquifer systems: Minnesota Geological Survey Report of Investigation 55, 23 p. 43 �ROCK MAGNETIC STUDIES OF PHYLLITIC ZONES FROM THE BARABOO SYNCLINE, WISCONSIN. KELLY*, COLLEEN, AND KEAN, WILLIAM F., Department of Geosciences, University of Wisconsin —Milwaukee, P.O. Box 413, Milwaukee, WI 53201 wkean@uwm.edu. (* student) Preliminary paleomagnetic results based on limited sampling from a variety of rock types, suggested that the magnetism of the metamorphic rocks associated with the Baraboo Syncline was pre-folding (Kean and Mercer,1986). The detailed work by Mercer (1984) based on data from two quartzite sites on the southern limb and one site on the northern limb shows the magnetism is carried by hematite, and is prefolding. The unfolded magnetic direction is Dec=201°, Inc.66.4°, Alpha 95=10.7°, which gives a paleopole at 257°E, 4.67°N. This is consistent with 1.75Ga. paleopole positions for North American. However, the sites with phyllites were notably inconclusive (Mercer, 1984). Possible causes are; incomplete structural information, synfolding magnetization that was not recorded by the quartzite, or anisotropy of magnetization caused during the folding. Nonetheless magnetic results from these strata could provide important clues about the actual age of folding of the syncline. The phyllites result from metamorphism of clay rich layers within the original sandstone sequence, which could also contain hematite, magnetite or maghemite. It is likely that the magnetic mineralogy, and/or the magnetic direction in these layers will change during metamorphism and folding, and therefore provide information on the deformation history. A suite of oriented hand samples, collected from phyllitic zones on both sides of the Baraboo Syncline, were subjected to a variety of magnetic measurements to determine the magnetic mineralogy and remanent directions. The measurements included stepwise alternating field (AF) and thermal demagnetization to either 1 OOmT or 750° C respectively, magnetic susceptibility, saturation isothermal remanent magnetization (SIRM), and hysteresis properties. A.F. demagnetization was mostly ineffective. However, the thermal demagnetization results show that the majority of samples have a single magnetic direction that reduced to zero intensity between 650-750° C. The SIRM plots never reach saturation by 300mT, and the coercivity of remanence (Hcr) values derived from the hysteresis ioops are in the range of 140-180 mT. These results are indicative of hematite with an effective grain size of 15-20 tm, that probably developed during metamorphism. However, additional studies are required to exclude the possibility that the differences in the magnetic directions for these sites are not due to local structural conditions, or anisotropy of remanent magnetization. References: Mercer, D.,1984, Paleomagnetism of the Baraboo Quartzite, Unpublished MS thesis, UWMilwaukee, 294 pp. Kean, W.F. and Mercer, D., 1986, Preliminary Paleomagnetic Study of the Baraboo Quartzite, Wisconsin, Geoscience Wisconsin, Vol. 10, p 46-53. 44 �HEALTH SURVEILLANCE IN A COMMUNITY AFFECTED BY ARSENICCONTAMINATED WATER Lynda Knobeloch and Charles Warzecha, Wisconsin Department of Health and Family Services Shelli Nelson, University of Wisconsin-Madison, Environmental Toxicology Center Many private drinking water wells in Wisconsin's Fox River Valley contain naturally occurring arsenic at levels of health concern (Riewe et al., 2000). In 1993, the Department of Health and Family Services surveyed water use and health status among families living in the affected region. Data from this initiative indicated that residents whose daily arsenic intakes exceeded 50 were approximately three times more likely to report skin cancer than residents with lower arsenic intakes (Haupert et al, 1996). The Department is currently re-evaluating arsenic exposure and health status in this region. Three factors make this re-evaluation timely. In January, the U.S. EPA lowered the safe drinking water standard for arsenic from 50 ug/L to 10 ugIL, increasing the number of wells that will be deemed "unsafe." Since the previous study, the number of families that use private wells in this region has increased dramatically. In addition, repeated sampling of several wells in this region seems to indicate that arsenic concentrations are increasing over time, possibly due to regional drawdown of the aquifer. The current study covers a much broader geographic region than the 1993 study. Each township within Outagamie and Winnebago Counties has been encouraged to participate. To date, nearly 2000 families have submitted well water samples for arsenic analysis and completed a 4-page health questionnaire. This larger study population, combined with a more extensive list of health outcomes, is expected to provide us with a better understanding of arsenic exposure and its impact on the health of families that consume water from private wells in these counties. References: Riewe T, Weissbach A, Heinen L, and R Stoll, 2000. Naturally occurring arsenic in well water in Wisconsin. Water Well J; (June 2000): 24-31. Haupert TA, Wiersma JH, and JM Goldring, 1996. Health effects of ingesting arseniccontaminated groundwater. Wisc Med J; 95(2):100-104. 45 �HYDROGEOLOGIC SETTING OF ELEVATED ARSENIC IN SOUTHEASTERN MICHIGAN KOLKER, Allan, and CANNON, W. F., U.S. Geological Survey, Reston, VA, 20192, HAACK, S. K., and WESTJOHN, D. B., U.S. Geological Survey, Lansing, Ml, 48911, and WOODRUFF, L. G., U.S. Geological Survey, Saint Paul, MN 55112. High levels of arsenic, up to nearly 40 times the EPA standard of 10 tg/L, are present in southeastern Michigan, primarily in private supply wells. To investigate the problem, the USGS, in collaboration with the University of Michigan, sampled more than 100 wells, including public, private and monitoring wells, and examined aquifer materials in the region for possible sources of arsenic. The study area includes nearly all of Genesee, Huron, Lapeer, Livingston, Oakland, Sanilac, Shiawassee, Tuscola, and Washtenaw counties, which have a combined population of more than 2 million. Of the wells sampled by the USGS, more than 50% exceed the current EPA drinking water standard [1,2]. Most of the affected wells are completed in the Marshall Sandstone (Mississippian), the principal bedrock aquifer in the region, but in some cases, water in overlying glacial aquifers, or in the Saginaw (Pennsylvanian) aquifer is also affected. Problem wells are concentrated in the eastern and southeastern parts of the Marshall Sandstone subcrop belt, where Marshall Sandstone is in direct hydrologic contact with permeable Pleistocene glacial deposits [3]. To investigate the possible relation between the composition of aquifer materials and the arsenic content of well water, test wells were drilled by the USGS in Huron County (H-i 5D) and Lapeer County (LP-1), in areas known to contain wells with high arsenic contents. Both of the USGS wells show a gradual decrease in total arsenic of well water with depth, but redox conditions (Eh = 29 to 68 mV; [1]) and the fraction of As III (88 — 100% of total arsenic), measured for H-15D, are essentially uniform with depth (Figure). There is no distinct correlation between arsenic contents of aquifer materials, sampled in 4.5 ft. (1.37 m) intervals (0 to —300 ppm), and waters, sampled largely in 50-ft. (15.7 m) intervals (Figure). Likewise, there is no correlation between As concentration and total Fe or SO4 contents of waters in the region, even for samples having the highest (>50 p.g/L) As contents. These findings suggest that in-situ pyrite oxidation in the Marshall aquifer is very limited. In the Marshall Sandstone and in portions of the overlying Michigan Formation, pyrite is locally present as a cement whose texture indicates growth that has displaced framework sand grains. This pyrite locally contains highly arsenic-enriched domains (up to 8.5 wt. %) occurring as overgrowths on pyrite framboids having much lower arsenic contents, that are in turn incorporated into pyrite having low arsenic contents [4,5]. Well cuttings containing arsenic-rich pyrite have arsenic contents as high as 1020 ppm. Investigation of till samples containing iron oxy-hydroxides, probably derived from weathering of arsenian pyrite, shows arsenic contents up to about 0.7 weight percent. This indicates that a portion of the arsenic is retained during the weathering/oxidation process, or that arsenic is concentrated on oxide surfaces by adsorption of aqueous arsenic. Because there are multiple sources of arsenic in glacial and bedrock aquifers that are in hydrologic continuity, no single process may explain the overall distribution of arsenic in southeastern Michigan wells. Reduction of iron-oxyhydroxides in glacial aquifers by exposure to suboxic ground water, is one possible source. In-situ pyrite oxidation, while limited, may locally be enhanced by high concentrations of bicarbonate [6]. Because of the extreme arsenic contents of some Marshall Sandstone pyrite, small amounts of in-situ oxidation could result in contamination of bedrock aquifers. Likewise, we cannot rule out sporadic paleo-oxidation of pyrite in the bedrock aquifer, resulting from lower water table levels that existed following the last glaciation. All of these processes have likely contributed to the widespread but sporadic nature of arsenic enrichment in southeastern Michigan ground water. 46 �Compilation of analytical results for arsenic in USGS test cores H-15D (A), and LP-1 (C) and waters in corresponding packed intervals (B and D). Data represent bedrock intervals only, with the exception of glacial drift recovered from LP-i. Brackets on water data indicate size of packed intervals represented. Results for arsenic speciation in H-i 5D waters are from M.-J. E a C 0 10 Kim [21. 20 30 Sandstone! 40 Siltstone 60 70 Mudstone 80 90 100 Shale 110 120 As (ppb) As (ppm) Carbonate References and selected geochemical characteristics of [1] Haack, S. K., and Trecanni, S. L., 2000, Arsenic concentration U.S. Geological Survey Water Resources in southeastern Michigan: ground water and aquifer materials Investigations Report 00-4171, 38 p. groundwater of southeast Michigan: Ph.D. dissertation, [2] Kim., M.-J., 1999, Arsenic dissolution and speciation in University of Michigan, Ann Arbor, Ml, 201 p. framework of the Michigan Basin Regional Aquifer [3] Westjohn, 0. B., and Weaver, T. L., 1998, HydrogeologiC Professional Paper 1418, 47 p. System: U.S. Geological Survey pyrite in the Mississippian 0. B., Woodruff, L. G., 1998, Arsenic-rich [4] Kolker, Allan, Cannon, W. F., Westjohn, Michigan ground water: Geological Society anomalous arsenic in southeastern Marshall Sandstone: Source of of America, Abstracts with Programs, v. 30, no. 7, p. A-59. D. B., Haack, S. K., and Kim, M.-J., 1999, Arsenic in [5] Kolker, Allan, Cannon, W. F., Woodruff, L. G., Westjohn, EOS, American Geophysical Union southeastern Michigan ground water: Results of USGS test drilling: S147. Transactions, v.80, no.17, p. S146ions and arsenic dissolution by ground water: [6] Kim, M.-J., Nriagu, Jerome, and Haack, S. K., 2000, Carbonate 34, p. 3094-3100. Environmental Science and Technology, v. 47 �POTENTIAL FOR COPPER MINERALIZATION IN THE ANIMIKIE GROUP, MINNESOTA LARSON, Phillip C., Department of Geological Sciences, University of Minnesota, Duluth, MN 55812, plarson2@d.umn.edu Numerous lines of evidence have recently been identified suggesting the potential occurrence of Keewenawanstyle native copper or White Pine-style sediment-hosted copper mineralization in Animikie Group sediments in northeastern Minnesota. Native copper has been reported in quartz veins in the Wanless Mine near Buhi (Gruner, 1946). Copperstained rocks have been observed in the Butler Taconite mine (D. Ridgeway, pers. comm.). Hydrothermally altered diabase dikes in the National Steel Pellet Company (NPSC) Mine have elevated copper values (up to 950 ppm) associated with a chlorite-quartz alteration assemblage (Larson et a!., 1999). In addition, diagenetic pyrite from the NSPC mine frequently displays a secondary coating of chalcopyrite on bedding and fracture surfaces. Finds of float native copper in glacial drift have occasionally been reported in the Nashwauk-Keewatin area of the Mesabi Range (L. Mattson, pers. comm.). It is improbable that this float copper is derived from the Lake Superior basin, as surficial drift on the Mesabi has either a northwest or northeast provenance; this strongly suggests a local provenance. These occurrences suggest the northern flank of the Animikie Basin has hosted a regional-scale mineralizing event by an oxidized cupriferous fluid. The timing of copper mineralization is constrained by cross-cutting relationships. A series of diabase dikes intrude iron formation in the NSPC mine area; these dikes are tentatively dated at —1 100 Ma. Overprinting of alteration on well-developed cooling structures and post-emplacement shears in the dikes suggests that the copper mineralizing event significantly post-dated dike emplacement — similar to native copper mineralization on the Keewenaw peninsula. In contrast, in the South Stevenson Mine a diabase dike intruded the natural iron-ore body, indicating dike emplacement post-dates natural iron-ore formation. These relationships demonstrate that the natural iron-ore forming hydrothermal event is distinct from, and preceded the copper mineralizing event. Morey (1999) has presented a model for formation of natural iron ore deposits on the Mesabi by alteration of Biwabik iron-formation by a hydrothermal fluid generated deep in the Animikie basin. A topographically-driven regional hydraulic flow system was induced by recharge in a highland area bounding the southern end of the basin. Fluids evolved deep in the basin were driven through relatively permeable strata at the base of the Animikie Group (Pokegama Quartzite). These fluids leaked upward along fractures through the overlying iron-formation toward the margins of the basin. Concentration of iron oxides in natural iron-ore deposits is chiefly a function of silica removal from unaltered iron-formation. Evidence suggests that this process extended down into the underlying Pokegama Quartzite as well. Holway (1956) reported that at the Auburn Mine, quartzite underlying the natural iron-ore was composed of friable sand from which the silica cement had been leached. In most exposures, the Pokegama Quartzite is a well-cemented low permeability rock. However, local desilicification associated with natural iron-ore formation has enhanced permeability along numerous fault- and fracture-hosted fluid pathways. During formation of the Midcontinent Rift System, rift-filling volcanics, intrusives, and sediments truncated the southern margin of the Animikie basin. The base of the Animikie Group is thus presumably in contact with volcanic rocks in the deeper portions of the rift — the likely source of oxidized cupriferous brines associated with Keewenawan native copper mineralization. These same fluids may have been driven from the western flank of the rift into the basal portion of the Animikie Group (Fig. 1). Fluid flow would have been focused through the basal clastic units, and in particular along the preexisting desilicified faults and fractures. Overlying iron-formation and shale units would have served as effective aquitards. Two scenarios for concentration of significant copper mineralization by cupriferous brines discharging along the northern margin of the basin present themselves (Fig. 2). High-porosity, high-permeability desilicified clastic sediments within the Pokegama Quartzite may be the host for native copper mineralization similar to that observed in brecciated flow-tops and conglomerate beds on the Keewenaw Peninsula. Interaction of the brines with the iron-sulfide-rich base of the Virginia Formation may have produced stratiform copper sulfide mineralization similar to that observed at the White Pine and Presque Isle deposits. 48 �Midconfinent Rift Mesabi Range Animikie Basin NW OronfoIBaieId Group Sec mentsJSE Figure 1. Schematic cross-section of post-Midcontinent Rift flow system through the Animikie Basin. k\ \ White Pine-Style Copper Sulfide \Tirginia Formation Figure 2. Schematic cross-section of copper mineralization, showing relationship between faults/fractures, desilicification, and Animikie Group sediments. References Gruner, J.W., 1946, Mineralogy and Geology of the Mesabi Range: Office of the Commissioner of the Iron Range Resources and Rehabilitation, St. Paul, 127 p. Hoiway, W., 1956, Auburn Mine, in Schwartz, G.M., ed., Geol. Soc. America Annual Meeting Field Trip 1 Guidebook, p.160-167. Larson, P. C., Hanttula, J. E. and Price, J. 5., 1999, Proterozoic mafic dikes, western Mesabi Range, Minnesota. GSA Abstracts with Programs, Vol. 31, No. 7: 106. Morey, G. B., 1999, High-grade iron ore deposits of the Mesabi Range, Minnesota — product of a continental-scale Proterozoic ground-water flow system. Economic Geology 94: 133-142. 49 �CONTRIBUTIONS TO THE CULTURAL GEOGRAPHY OF THE WEST MESABI RANGE, NORTHERN MINNESOTA LIVELY, Richard and MOREY, G. B. (Minnesota Geological Survey, lively@umn.edu and morey001@umn.edu) The Mesabi Iron Range in northern Minnesota has produced more than 4 billion tons of iron ore and taconite from more than 500 mines since ore was first shipped from that area in 1892. Mining, confined to an east-northeast trending strip of land some 100 miles long and 4 miles wide, has greatly modified both the original landscapes and associated cultural features. In this discussion, we assess some of those changes utilizing modern data and topographic maps prepared during the summer of 1899 and 1900 by E.C. Bebb and his assistants D.L. Fairchild and Louis B. Weed, all topographers with the U.S. Geological Survey. The original data were compiled at a scale of 1:16,000 with a contour interval of 20 feet, and ultimately were published at a scale of 1:50,000 in 1903 (Leith, 1903). The original field maps were obtained from the Cuyler Adams estate in the early 1 980s and were subsequently digitized by Emily Bauer of the Minnesota Geological Survey to produce a pre-mining topographic rendition of the western half of the Mesabi range. The digitized maps were prepared as part of a larger study of the hydrologic character of the western Mesabi range sponsored by the Legislative Commission on Minnesota Resources (LCMR). We compare here the 1899-1900 data with topographic data obtained in 1999 by the Minnesota Department of Natural Resources as part of the Mesabi Elevation Project (DNR, 1999). As one would expect, open-pit mining has considerably changed the topographic expression along the range, mainly by increasing the relative abundance and depth of topographically low areas and redistributing topographically high areas. In general, the overall spread in elevation has increased by over 100 feet due to excavation, while over the same period the maximum elevations on the range have not changed substantially. When comparing data from the 1999 high-resolution digital surface with data from the 1899-1900 survey in undisturbed areas, the resulting correlations are virtually identical. This implies a high degree of accuracy among the early topographers mapping the Mesabi range. In 1899-1900, the western half of the range contained 11 mines and 17 mine dumps totaling about 0.7 square miles, or about 0.3 percent of the 230 square miles originally mapped. In 1986, mining activity accounted for about 140 square miles of pits and dumps, or about 60 percent of the study area. Mining also significantly affected drainage patterns along the range. In 1899-1900, mapped streams totaled over 400 miles in length, whereas only about 220 miles could be found compiled from topographic maps between 1970 and 1990, a reduction of about 45 percent. Much of that loss involved the removal of headwaters for many southward flowing streams along the south side of the Laurentian drainage divide. The transportation features mapped on the range in 1899-1900 were primarily limited to about 170 miles of roads or trails and about 125 miles of railroad track. Most of these extended along the strike of the range, with relatively little development crossing the range. By 1990, railroad mileage had doubled to about 250 miles and road length had increased to over 1000 miles. In addition to the increased transportation network, mining created and destroyed many towns as it progressed. For example, the entire town of Hibbing was moved several miles after a rich deposit was discovered under its streets. The Leith (1903) report also included a geologic map that was designed to aid exploration by delineating the boundaries of the Biwabik Iron Formation. That map, compiled during the spring of 1900 and the summers of 1900 and 1901, relied heavily on the support and geologic information, especially test pit and drill hole data, provided by J.U. Sebenius of the Lake Superior Consolidated Iron Mines (now owned by the U.S. Steel Corporation). Although the geologic data base was limited to relatively few, sparsely distributed data points, the resulting geologic map read very much as it does today, even with more complete data now available. The only significant difference involves a better understanding of faults and related structures which became apparent as extensive test drilling and mining progressed. References Cited: Leith, C.K., 1903, The Mesabi iron-bearing district of Minnesota: United States Geological Survey Monographs, v. 43, 316 p., 1 plate. Minnesota Department of Natural Resources, 1999, Mesabi Elevation Project: St. Paul, Minnesota. 50 �PRECAMBRIAN GEOLOGY OF S. WISCONSIN: A PANORAMA FROM THE BARABOO RANGE MEDARIS, L.G., Jr., Dept. of Geology & Geophysics, Univ. of Wisconsin-Madison, 1215 W. Dayton Street, Madison, WI, 53706, medarisgeology.wisc.edu Red, supermature quartzites, most notably the Baraboo, Barron, and Sioux Quartzites, have long been recognized as a distinctive Proterbzoic feature in the southern Lake Superior region, signifying depositionon a stable craton under conditions of intense chemical weathering in the presence of significant free atmospheric oxygen (Dalziel & Dott, 1970; Ojakangas & Weber, 1984; Southwick et al., 1986). The quartzites were inferred to be post-1750 Ma in age, to have been folded and metamorphosed at —4630 Ma, and locally intruded by granitic rocks at —1450 Ma. The term, Baraboo interval, was introduced by Dott (1983) for this succession of sedimentation, deformation, metamorphism, and intrusion in the time span of 1750 to 1450 Ma. In the last six years investigators at Wisconsin and Dan Holm at Ball State and coworkers have taken a renewed interest in the Baraboo interval, and their results substantiate the original concept and framework of the Baraboo interval and provide a more detailed understanding of the disparate geological processes that shaped the southern Lake Superior region in mid-Proterozoic time. Igneous Basement of the Baraboo Range The Baraboo Quartzite is underlain by diorite, granite, and rhyolitic lavas and pyroclastic rocks (Dalziel and Doff, 1970). U-Pb zircon ages of the Baxter Hollow granite and rhyolite are indistinguishable, and taken together, yield an age of 1749±12 Ma (Van Wyck, 1995). Chronologically and petrologically, the Baraboo basement is correlative with the subalkalic granite and rhyolite suite of the Fox River Valley (Smith, 1978; Anderson et a!., 1980). Igneous textures are well preserved in all units of the Baraboo basement, although igneous minerals are partly to completely replaced by a variety of greenschist facies minerals. Typically, biotite is replaced by chlorite, plagioclase is transformed to albite and fine-grained epidote (saussurite), and intermediate alkali feldspar is recrystallized to a fine-grained mixture of near end-member microcline and albite. Hornblende in diorite is partly replaced by chlorite and intergrown actinolite and cummingtonite. Paleosols beneath the Quartzites Mature paleosols have been recognized beneath the Sioux, Baraboo, and Barron quartzites (Southwick & Mosler, 1984; Medaris et al., 1997; Medaris, 2000). Among these paleosols, the Barron represents the best standard for comparison, because it has not been affected by later hydrothermal alteration, as have the other two. The Barron paleosol is a saprolite, derived from Penokean metatonalite and consisting of quartz, kaolinite, hematite, traces of sericite, and crandallite-florencite. During weathering of the metatonalite protolith, A12O3, TiO2 and Zr remained immobile, Na20, CaO, MgO, and MnO were effectively removed, Ba, Sr, K2O, and Rb were substantially reduced, and SiO2 decreased by 10%. The extreme chemical maturity of the Barron saprolite is reflected in a high value, 95.7, for its Chemical Index of Alteration. The Baraboo Quartzite is underlain by saprolite, which varies in thickness from 30 to 50 feet and was derived from granite and rhyolite. Chemical changes in the Baraboo saprolite relative to protolith are similar to those in the Barron, except for subsequent hydrothermal addition of K20 and Rb. Common features of the Barron, Baraboo, and Sioux paleosols are the absence of feldspar and a high degree of mineralogical and chemical maturity, similar to present-day weathering profiles in warm, humid climates, where intense chemical leaching is characteristic. Such a climate and a stable tectonic setting were essential for generating the supermature features of the Baraboo interval quartzites. Depositional Age and Composition of Baraboo Interval Sedimentary Rocks A post-Pen okean depositional age for the Baraboo interval quartzites was long recognized, and recent analyses of detrital zircon grains in the Baraboo, Barron, Flambeau, McCaslin, and Sioux quartzites have yielded numerous U-Pb ages from 1782 to 1712 Ma (Van Wyck, 1995; Doff et a!., 1997; Holm Ct al, 1998), demonstrating that the quartzites were deposited no earlier than —1710 Ma. The chemical maturity of the Baraboo 51 �interval sediments was previously inferred from the near absence of detrital feldspar, the abundance of pyrophyllite or kaolinite, and the predominance of zircon, magnetite, rutile, and apatite among heavy minerals. Recent analyses of Barron, Baraboo, and Sioux siltstone, pelite, and their metamorphosed equivalents, yield compositions consisting essentially of Si02, Al203, Fe203, hO2, and H20, with a Chemical Index of Alteration ranging from 96.8 to 98.6. The extreme chemical maturity of the Baraboo interval sediments reflects a source region that experienced extensive chemical leaching and produced detritus consisting largely of quartz, kaolinite, and hematite. 1630 Ma Folding and Metamorphism There was widespread Rb-Sr isotopic resetting at 1635±33 Ma in Fox River Valley granite and rhyolite, Baxter Hollow granite, and Baraboo rhyolite (Dott & Dalziel, 1972; Van Schmus et al., 1975; X = 1.42*1011 yr'), and it was suggested that such isotopic resetting was coincident with folding of the quartzites, which might represent an eastern expression of the Mazatzal deformation in the southwestern U.S. (Dott, 1983; Van Schmus et al., 1993). The extent of foreland deformation is marked by a 1630 Ma tectonic and thermal front in northern Wisconsin, which was located on the basis of 40ArP9Ar cooling ages of mica and hornblende in basement rocks and the distribution of folding in quartzites (Holm et al., 1998; Romano et al., 2000). The Barron Quartzite is unfolded and unmetamorphosed, consisting of quartz, kaolinite, and hematite. In contrast, the Baraboo Quartzite and underlying basement have been folded and recrystallized under low grade conditions (Medaris et al., 1998). The coexistence of quartz and pyrophyllite in the metasedimentary rocks requires a temperature between 285°C and 360°C, at 1 kbar and unit activity of H20. Post-i 630 Ma HydrothermalActivitv Muscovite grains from sub-Baraboo metasaprolite and from muscovite-pyrophyllite-diaspore veins near the base of the Baraboo quartzite yield discordant 39Ar release spectra, with well-defined plateaux ages at 1456±11 and 1467±11 Ma, respectively (Naymark et al., 2001a). An apparent whole-rock Rb-Sr isochron age for saprolite and pedogene is 1336±75 Ma. These data provide the first substantial evidence for a Wolf River-age imprint on the Baraboo Range, due to the effects of hydrothermal fluids that probably were driven along the sub-Baraboo nonconformity by heat from regionally extensive Wolf River magmatism. Muscovite grains from two samples of Sioux pipestone, which contain the assemblage muscovitepyrophyllite-diaspore, also yield discordant 39Ar release spectra, but with significantly younger plateaux ages of 1370±10 and 1268±11 Ma (Naymark et al., 2001b). The geological significance of such ages is unclear at present, and the —100 m.y. difference in ages of the two samples, which occur at the same stratigraphic level, deserves further investigation. Nevertheless, these recent 40ArP9Ar results reveal the existence of important, and possibly widespread, post-1630 Ma hydrothermal activity in the southern Lake Superior region. References Anderson, J.L. et al. (1980) Contrib. Mineral. Petrol., v. 74, 3 11-328; Dalziel I.W.D. & Dott R.H., Jr. (1970) Wis. Geol. Nat. History Survey, Inf. Circ. 14, 164 pp; Dott, RH. Jr. (1983) Geol. Soc. Amer. Memoir 160, 129-141; Dott, R.H., Jr. & Daiziel, I.W.D. (1972) Jour. Geol., v. 80, 552-568; Dott, R.H., Jr. et al. (1997) Geol Soc. Amer. Abstr. with Progr., v. 29, No. 4, 13; HoIm, D. et at. (1998) Geology, v. 26, 907-910; Medaris, L.G. Jr. et al. (1997) 43rd Inst. Lake Superior Geol., 39-40; Medaris, L.G., Jr. et al. (1998) 44th Inst. Lake Superior Geol., 89-90; Medaris, L.G., Jr. (2000) 46th Inst. Lake Superior Geol., 37-38; Naymark, A. et at. (2001a) Geol. Soc. Amer. Abstr. with Progr., v. 33, No. 4, in press; Naymark. A. et al. (2001b) 47th Inst. Lake Superior Geol., in press; Ojakangas, R.W. & Weber, R.W. (1984) Minn. Geol. Surv., Rept. mv. 32, 1-15; Romano, D. et at. (2000) Precambr. Res., v. 104, 25-46; Smith, E.I. (1978) Geol. Soc. Amer. Bull., v. 89, 875-890; Southwick, D.L. et at. (1986) Geol. Soc. Amer. Bull., v.97, 1432-1441; Southwick, D.L. & Mossler,J.H. (1984) Minn. Geol. Surv., Rept. mv. 32, 17-44; Van Schmus, W.R. et at. (1975) Geol. Soc. Amer. Bull., v. 86, 1255-1265; Van Schmus, W.R. et al. (1993) Precambrian: Conterminous U. S., Geol. North America, v. C-2, 270-28 1, Geol. Soc. Amer.; Van Wyck, N. (1995) Ph.D. thesis, Univ. Wis.-Madison, 280 pp. 52 �RECENT ADVANCES IN UNDERSTANDING THE GLACIAL RECORD OF WISCONSIN MICKELSON, D.M., Department of Geology and Geophysics, University of Wisconsin, Madison, WI 53706. Micke1son(dgeology.wiscedu and CLAYTON, LEE, Wisconsin Geological and Natural History Survey, 3817 Mineral Point Rd., Madison, WI 53706. Several glacier advances reached Wisconsin, but the record of early ice advances is sparse, probably because of their age (some >800 ka) and the intense erosion that took place during the subsequent glaciations (Alden, 1918; Bleuer, 1970; Baker, etal., 1983; Miller, 2000). Thin, discontinuous till occurs on the uplands in west central and central Wisconsin and south of the late Wisconsin deposits in southern Wisconsin. Most have been classified into the lithostratigraphic system adopted by the Wisconsin Geological and Natural History Survey, as have younger 1987; Clayton, 1991). More detailed mapping of 1984; Attig, glacial deposits (Mickelson, Quaternary deposits at 1:100,00 scale has been published for about 18 counties, and several more are in preparation. South and west of late Wisconsin and older glacial deposits lies the Dnftless Area. Recognized as driftless since before 1850 because of its higher relief and lack of erratics, the Driftless Area has Paleozoic bedrock close to the surface with only a cover of bess less than lOm thick. The landscape has been produced by fluvial erosion since at least sometime in the Tertiary. Although Black (1960) suggested that it was glaciated, there is no evidence that it was (Mickelson etal., 1982). In all likelihood, the Driftiess Area remained unglaciated because it had to cross the deep, east-west Lake Superior basin. Much ice was diverted into the Green Bay Lobe and into the Superior Lobe, which extended westward into Minnesota. The Driftless Are a is surrounded by glacial deposits of different ages, but was never surrounded by ice as portrayed by early workers and many textbooks. Late Wisconsin ice advanced into the northeast Wisconsin by about 24,000 radiocarbon years ago based on model results and radiocarbon dates in Illinois. There is a relative lack of radiocarbon dates from this time period in Wisconsin, probably because permafrost was thick and tundra vegetation covered the landscape. Also, ice was particularly erosive behind the ice margin compared to further south in Illinois. Although there is no radiocarbon record of the advance, model results indicate that the Lake Michigan Lobe advanced more quickly than lobes that had to traverse the deep, east-west oriented Lake Superior basin (Cutler, j., 2000a) because extensive calving slowed their advance. Ice of the Green Bay, Langlade, Chippewa, and Superior Lobes clearly advanced onto permafrost. This argument is based field observation of ice wedge casts (Clayton, etal., 1997), the lack of buried wood, (Attig, etal., 1989) evidence of tundra vegetation (Maher and Mickelson, 1996), and model results (Cutler, etal., 2000b). Landforms near the maximum ice extent appear to reflect subglacial conditions at the time they were deposited. Relief within end moraines increases from about 10 m in southern Wisconsin to more that 60 m in northern Wisconsin, reflecting the wider, longer lasting frozen-bed zone near the margin. Drumlins are extensive, probably because slow melt out of subglacial permafrost caused inhomogenities in the bed and differential streamlining. Tunnel channels left by large, probably catastrophic, flows of water from beneath the ice sheet are common along the outer margins in press). Tunnel channels and drumlins appear to be absent farther south in (Clayton,., 1999; Cutler, Illinois where buried wood and model results indicate there were warmer temperatures during the advance to the maximum ice advance position. The length of time that ice remained at or near the maximum position in the Green Bay lobe is debated. Maher and Mickelson (1996) have argued that ice remained near its maximum extent until about 15, 000 years ago and that subsquent deglaciation was rapid based on radiocarbon dates from Devils Lake and northeastern Wisconsin. Colgan submitted) argue that deglaciation was slower based on cosmogenic dates and time needed for (1999; Colgan glacial landform development. References Cited Attig, J.W., Clayton, Lee, and Mickelson, D.M., (Eds.), 1988, Pleistocene stratigraphic units of Wisconsin 198487: Wisconsin Geological and Natural History Survey, Information Circular 62, 61 pp. Attig, J.W., Mickelson, D.M., and Clayton, Lee, 1989, Late Wisconsin landform distribution and glacier-bed conditions in Wisconsin, Sedimentary Geology, v. 62, p. 399-405. Baker, R.W., Biehl, J.F., Simpson, T.W., Zelazny, L.W., and Beske-Deihl, S., 1983, Pre-Wisconsinan glacial 53 �stratigraphy, chronology, and paleomagnetics of west-central Wisconsin: Geological Society of America Bulletin, v. 94, p. 1442-1449. Clayton, Lee, Attig, J.W., and Mickelson, D.M., and Johnson, M.D., 1991, Glaciation of Wisconsin: Wisconsin Geological and Natural History Survey, Educational Series 36, 4 p. Clayton, Lee, Attig, J.W., Mickelson, D.M., 1997, Conditions around the margin of the Green Bay lobe during the height of the Wisconsin glaciation: j: Mudrey, M.G., Jr., Guide to Field Trips in Wisconsin and adjacent areas of Minnesota, 31st Annual meeting of the North-central section, Geological Society of America: Wisconsin Geological and Natural History Survey, p. 23-30. Clayton, L, Attig, J.W., and Mickelson, D.M., 1999, Tunnel channels formed in Wisconsin during the last glaciation: In Mickelson, D.M. and Attig, J.A., (Eds.), Glacial Processes Past and Present: Geological Society of America Special Paper 337, p. 69-82. Colgan, P.M. and Mickelson, D.M., 1997, Genesis of streamlined landforms and flow history of the Green Bay lobe, Wisconsin, USA: Sedimentary Geology, v. 111, p. 7-25. Colgan, P.M., 1999, Reconstruction of the Green Bay Lobe, Wisconsin, United States, from 26,000 to 13,000 radiocarbon years B.P. :In Mickelson, D.M. and Attig, J.A., (Eds.), Glacial Processes Past and Present: Geological Society of America Special Paper 337, p. 137-150. Colgan, P.M., Bierman, P.R., Mickelson, and Caffee, Marc, submitted, Variation in glacial erosion near the southern margin of the Laurentide Ice Sheet, south central Wisconsin: implications for cosmogenic dating of glacial terrains: Geological Society of America Bulletin, Cutler, P.M., Colgan, P.M., Mickelson, D.M., and MacAyeal, 2000a, Influence of the Great Lakes on the advance of the southern Laurentide Ice Sheet at the last glacial maximum: Geological Society of America, 2000 Abstracts with Programs, v. 32, no. 7, p. A-330 Cutler, P.M., MacAyeal, D.R., Mickelson. D.M., Parizek, B.R., and Colgan, P.M., 2000b, A numerical investigation of ice-lobe-permafrost interaction around the southern Laurentide Ice Sheet: Journal of Glaciology, v. 46, no. 153, p.311-325. Cutler, P. M. Clayton, Lee, Mickelson, D.M., Colgan, P.M., and Attig, J.W., in press, Tunnel Channels and Associated Fan Deposits in Wisconsin, U.S.A.: Insights into the Plumbing of the Southern Laurentide Ice Sheet: Quaternary International. Maher, L.J. Jr., and Mickelson, D.M., 1996, Palynological and radiocarbon evidence for deglaciation events in the Green Bay lobe, Wisconsin: Quaternary Research, v. 46, p. 251-259. Mickelson, D.M., 1997, Wisconsin's glacial landscapes: In Ostergren, R.C. and Vale, T.R., Wisconsin Land and Life: Madison, University of Wisconsin Press, p. 35-48. Mickelson, D.M., Clayton, Lee, Baker, R.W., Mode, W.N., and Schneider, A.F., 1984, Pleistocene stratigraphic units of Wisconsin: Wisconsin Geological and Natural History Survey, Miscellaneous Paper, 84-1, 199 pp. Miller, J.W., 2000, Glacial stratigraphy and chronology of central southern Wisconsin, west of the Rock River, Wisconsin: Madison, Wisconsin, M.S. Thesis, University of Wisconsin, 147 pp. Mickelson, D.M.,Knox, J.C. and Clayton, Lee, 1982, Glaciation of the Driftless Area: An evaluation of the evidence, In: Quaternary history of the Driftless Area , Knox, J.C., Clayton, Lee, and Mickelson, D.M. (Eds.), Wisconsin Geological and Natural History Survey, Field Trip Guidebook 5, p. 155-169. 54 �THE DULUTH COMPLEX: WHAT IT IS, WHAT IT AIN'T, AND WHAT WE STILL DON'T KNOW p Miller, James D., Jr. (Minnesota Geological Survey, do Natural Resources Research Institute, 5103 Miller Trunk Highway, Duluth, MN 55811 mille066@tc.umn.edu) Since the time of the first Minnesota state geological survey over 100 years ago (1.), several generations of geologists have worked to unravel the mysteries of the igneous rocks of northeastern Minnesota: the aptly named Duluth Complex. Each new level of understanding was brought about by new data or concepts about geological processes. With early survey studies recognizing the general distribution of igneous rock types, Grout's (2-5) work in the Duluth area established many of the broader geologic relationships of the Complex and developed fundamental concepts about how layered mafic intrusions form, many of which are still held today. However, one of his principle ideas, that the Duluth Complex is a large singular lopolithic intrusion (2), was proven to be an oversimplification by a flurry of geologic mapping conducted throughout the complex in the 1950s to 1970s (6-13). These studies were spurred by efforts to establish a geological framework within which to understand the Cu-Ni sulfide deposits first discovered in the late-1940s. Despite the fact that these deposits have yet to prove of economic importance, this intense period of geologic mapping served to formalize the general intrusive stratigraphy of the Duluth Complex and showed it to be a multiply intruded igneous system. In the early-70s, acceptance of the plate tectonic theory and recognition that the Duluth Complex was part of an intracontinental rift system created a new paradigm within which to evaluate the magmatic and tectonic history of the Duluth Complex (14). The current generation of Duluth Complex studies have focused on five major objectives: 1) 2) 3) 4) 5) to interpret the geologic picture of the vast, poorly exposed central part of the Duluth Complex using high-resolution aeromagnetic data acquired in the early-1980s (15, 16); to unravel the intrusive history of the complex with high resolution U-Pb dating of its gabbroic, anorthositic and felsic rocks (17, 18); to delineate the internal igneous stratigraphy of the various layered intrusions of the Duluth Complex with core logging, detailed mapping, and petrologic studies (19-22); to map the intrusive components of the hypabyssal Beaver Bay Complex and distinguish these from intrusions of the deeper Duluth Complex (23); and to evaluate the potential for economic base- and precious-metal deposits in areas of known mineralization and in other unexplored areas of the Duluth Complex (25). This presentation will highlight some of the new ideas that have emerged from this generation of studies and that are currently being summarized in a new 1:200,000-scale digital geologic map of northeastern Minnesota (see Miller and others, this volume) and in a companion report to be published in summer, 2001. It will also attempt to clarify some misconceptions about the Duluth Complex and point out where more study is needed. Keweenawan intrusive igneous rocks compose more than 60 percent of the bedrock geology of northeastern Minnesota, but only about half of them constitute the Duluth Complex. The Duluth Complex refers to those intrusions that were emplaced into the base of the comagmatic volcanic edifice of the North Shore Volcanic Group. Intrusions emplaced higher within the volcanic pile are not considered part of the Duluth Complex, but rather belong to the Beaver Bay Complex or, where isolated, are identified as individual subvolcanic bodies. Other than scattered, isolated masses of strongly hornfelsed mafic volcanic rock, the Duluth Complex is virtually a continuous mass of intrusive igneous rock. Within that mass, four general rock series are distinguished on the basis of age, dominant lithology, internal structure, and structural position within the complex. Each rock series was multiply emplaced and, where possible, individual intrusion names are assigned. 55 �Early Gabbroic Series—layered sequences of dominantly gabbroic cumulates occurring along the northeastern contact of the Duluth Complex. With their reversed magnetic polarity and 1108 Ma U-Pb zircon age (17), these rocks were evidently emplaced during the early magmatic stage of rifting. Two intrusive units are currently recognized: Nathan's Layered Series and the Cucumber Lake gabbro, though the latter has not been mapped in detail (10). Felsic Series—massive granophyric granite with lesser intermediate rocks occur in a semicontinuous string of elliptical bodies along the eastern and central roof zone of the complex. Ongoing age dating and isotopic studies of these various bodies by Vervoort (26, 18) suggest that most were emplaced during early magmatic activity between 1109 to 1102 Ma and came from Paleoproterozoic to Mesoproterozoic crustal sources. The position of major mafic and anorthositic intrusions beneath these granophyre bodies suggests that the felsic rocks acted as density barriers to mafic magmas, thus causing their plutonic emplacement. Anorthositic Series—a structurally complex suite of foliated, but rarely layered plagioclase-rich gabbroic cumulates that evidently formed by multiple emplacement of plagioclase crystal mushes (27). The erratic internal structure of these rocks typically precludes distinguishing individual intrusive bodies. Although commonly intruded by and included in layered series rocks, U-Pb dating indicates comparable ages of 1099 Ma for both rock series (17). Layered Series—previously referred to as the troctolitic series (9-12, 14), this suite is composed of troctolitic to ferrogabbroic cumulates that constitute at least 11 major mafic layered intrusions. These intrusions display a range of internal differentiation from poorly differentiated troctolitic bodies, such as the Partridge River and South Kawishiwi intrusions, to the well-differentiated Duluth Layered Series and Greenwood Lake intrusion (21). Interpretations of geophysical data over the unexposed central and southern parts of the complex (15, 16; Miller and others, this volume) have led to the recognition of several previously unidentified layered intrusions: the Boulder Lake, Western Margin, Greenwood Lake, and Osier Lake intrusions. Aeromagnetic data also imply that emplacement of the thick sheet-like intrusions of the layered series occurred by sequential overplating of previous intrusions beginning in the northwestern part of the complex and progressing southeastward. Although much has been learned about this enormous and complex igneous system in the past 20 years, much more remains to be done before a complete picture of the Duluth Complex is developed. Vast areas of the Duluth Complex and associated subvolcanic intrusions are unmapped in detail (especially in the BWCA). Age dating studies have only begun to unravel the emplacement history of Duluth Complex intrusions and their possible relationship to higher subvolcanic intrusions and volcanic rocks. Determining the igneous stratigraphy of the newly recognized, but poorly exposed layered intrusions (and their potential for stratiform POE deposits) will require systematic drilling and geochemical studies. These and other challenges await the next generation of geologists who dare to tackle the mysteries of the Duluth Complex. References: 1) Winchell, 1899, MGS Final Rpt IV; 2) Grout, 1918a, Am J Sci 46, p.516; 3) Grout, l9l8b, J Geol 26, p.626; 4) Grout, 1918c, J Geol 26, p.481; 5) Grout, 9l8d; J Geol 26, p.439; 6) Grout, Sharp, & Schwartz, 1959, MGS Bull 39; 7) Taylor, 1964, MGS Bull 44; 8) Green, Phinney & Weiblen, 1966, MGS Misc Map M-2; 9) Phinney, 1972, Geol of Mimi: Cent Vol, p.335; 10) Phinney, 1972, Geol of Minn: Cent Vol, p.346; II) Davidson, 1972, Geol of Minn: Cent Vol, p.354; 12) Bonnichsen, 1972, Geol of Minn: Cent Vol, p.361; 13) Green, 1982, MGS Geologic Map of Minn—Two Harbors sheet; 14) Weiblen & Morey, 1980, Am J Sci 280-A, p. 88; 15) Chandler, 1990, Econ Geol 85, p.816; 16) Miller & Chandler, 1999, MGS Misc Map M-l0l; 17) Paces & Miller, 1993, J Geophys Res 98, p.13997; 18) Vervoort & others, this volume; 19) Severson & Hauck, 1990, NRRIJGMIN-TR-89-1l; 3) Severson, 1994, NRRI/TR-93/34; 21) Miller & Ripley, 1996, Layered intrusions, Elsevier, p.257; 22) Severson & Miller, 1999, MGS Misc Map M-91; 23) Miller & Chandler, 1997, GSA Spec Paper 312, p.73; 24) Hauck et a!., 1997, GSA Spec Paper 312, p.137; 25) Miller, 1999, MGS Inf Circ 44; ) Vervoort & Green, 1997, Can J Earth Sci 34, p.521; 27) Miller & Weiblen, 1990, J Pet 31, p.295. 56 �DIGITAL GEOLOGIC MAP OF NORTHEASTERN MINNESOTA AND ASSOCIATED DATABASES IN GeMS—A MODIFIED ARCVIEW FORMAT Miller, J.D., Jr.', WahI, T.E.', Green, J.C.2, Chandler, V.W.', Severson, MA.3, and Peterson, D.E.23 1) Minnesota Geological Survey, 2642 University Ave., St. Paul, MN 55114; 2) Department of Geological Sciences, University of Minnesota—Duluth, Duluth, MN 55812; 3) Natural Resources Research Institute, 5013 Miller Trunk Highway, Duluth, MN 55811 The Minnesota Geological Survey (MGS), in collaboration with the Natural Resources Research Institute and the Department of Geological Sciences at the University of Minnesota—Duluth, is currently compiling geologic, structural, drill hole, geophysical and geochemical data from northeastern Minnesota into an ArcViewbased GIS called GeMS (Geologic Mapping System). The main focus of this project, which is being funded by the Minnesota State Legislature through the Minerals Coordinating Committee, is to develop a new 1:200,000-scale geologic map of the Duluth Complex and related Keweenawan igneous rocks. The map will be available in summer, 2001 either as a 1:200,000-scale paper map, as a downloadable image from the MGS website, or as part of a CD-ROM that will also include all related data compiled for the study in an ArcView format. A companion report addressing the geology and mineral potential of the Duluth Complex will also be published in summer, 2001. This presentation will give an overview of the basic components of GeMS and will describe the types and attributes of database themes included in the compilation. GeMS was initially conceived to be a user-friendly interface to the UNIX workstation-based Arclnfo software for the purpose of digitally storing, retrieving and imaging geologic, geophysical and geochemical data (WahI and others, 1995, 1997). GeMS was recently converted to ArcView 3.2 for this and other MGS mapping projects because of ArcView's common usage as GIS software on the PC platform, its expanded data management capabilities, and its more flexible and easy-to-use graphical user interface. ArcView manages geographical data as point, line and polygon themes and links them to attribute tables containing related information. ArcView is particularly well-suited to making geologic maps because of its ability to sort data and interpretive themes by various attributes, to graphically portray the sorted data in a variety of ways, and to accommodate various types of base images (DRGS, geophysical images, orthophotos, etc.). The types of data and information themes that are part of the current version of GeMS are listed in Table 1. Table 1. Data and interpretive themes included in GeMS Data type Theme type Attributes outcrop polygon field station ID, geologist, date visited, data source, observational detail, exposure, outcrop types, # of photos taken, # of samples taken, rock type, field description, map unit sample point field station ID, geologist, date sampled, data source, form of sample, rock type, map unit, field description, # of thin sections, petrographic description, related data available (probe, whole rock, isotope, assay, geochron, rock properties) structure point field station ID, geologist, date measured, data source, structure type, attitude, confidence level, # of averaged measurements, display scale drill hole point drill hole ID, date drilled, logged by, date logged, lessee, elevation, azimuth, plunge, depth, depth to bedrock, first bedrock, last rock type, present core location, core diameter map lines line geologist, date, basis 1, basis 2, line type (e.g., fault, contact, dike, fold axis, etc.) map unit polygon geologist, date, basis I, basis 2, map label, unit description miscellaneous varied gossan zones, test pit locations, etc. 57 �The advantages of moving from the old cartographic methods of making geologic maps to a GIS-based approach are numerous: GIS maps are easily updated as new data become available; several types of geologic data can be viewed at the same time; and different bases can be used. In addition to these advantages common to all GIS map systems, GeMS incorporates some other unique and useful features. These include source identification data (who, when and where). This is especially important for the compilation of the 1:200,000-scale geologic map of northeastern Minnesota, which compiles data and interpretations from various sources. Another important feature is the identification of the basis for geologic interpretations (map lines and units). For example, possible options for the basis for a fault interpretation are: topographic expression, inference from aeromagnetic data, geologic unit offset, air photo lineament, or local observation in outcrop. The database for northeastern Minnesota available on CD-ROM will not be a complete compilation. The amount of detailed outcrop data alone that could potentially be compiled totals more than 100,000 polygons. This project is concentrating on data from areas that are open to mineral exploration. The completeness of the database at present is shown in Table 2. In addition to continuing to fill out this database, some other elements that will be added to GeMS include 1) compiling all geochemical data and linking that database to outcrop and drill hole samples, 2) allowing for drill core logs to be added to the database, and 3) exporting portions of the system to handheld devices to allow direct data capture in the field. As new mapping is conducted and new data are acquired, updated versions of the regional geologic map and its associated database will be published—a task made easier having moved our map-making into the digital age. Table 2. Current Status of Data Compilation for the Northeastern Minnesota Geologic Map Area Data Type Outcrop Areas where compilation nearly complete Areas yet to be compiled —23,000 Duluth area, southern and central Duluth complex, Allen and Babbitt quadrangles, Beaver Bay Complex' Northern Duluth Complex (map areas of Phinney, Davidson, Foose, and Nathan), North Shore (Green) Units compiled Total units to be compiled to date >100,000 Samples2 —5000 —3600 Duluth area, central Duluth Complex, Allen quadrangle, Beaver Bay Complex Samples of Green and misc. uncatalogued samples and thin sections stored at the MGS Structure —8000 >5000 All published geologic maps Unpublished mapping by Phinney and Green —600 All drill hole data are compiled Drill Holes —600 ')only about 30% of outcrop polygons in the Beaver Bay Complex have their attributes compiled, the unattributed polygons thus mark only outcrop location. 2) includes well-located samples for which geochemical, petrographic, or rock property data exist and/or for which a preserved sample or thin section exists. References: 1995 ESRI International User Conference Proceedings, http://www.esri.comllibrary/userconf/proc95/to200/p I 67.html Wahl, T.E., Miller, J.D., Jr., Bauer, E.J., 1995, Bedrock geologic mapping using Arclnfo: Wahi, T.E., Miller, J.D., Jr., Jirsa, M.A., Boerboom, T.J., Chandler, V.W., Runkel, A.C., Dahl, D., Severson, M.J., 1997, Geologic mapping System (GeMS): a digital approach to bedrock geologic mapping: Institute on Lake Superior Geology, Proceedings v. 43, part 1, p. 59-60. 58 �STRUCTURE OF THE BURIED PRECAMBRIAN BASEMENT IN SOUTHWEST WISCONSIN AND ITS INFLUENCE ON REGIONAL PALEOZOIC GEOLOGY AND ZINC-LEAD MINERALIZATION MUDREY, M.G. Jr., Wisconsin Geological and Natural History Survey, 3817 Mineral Point Rd., Madison, WI 53705, mgmudreyfacstaff.wisc.edu BROWN, B.A., Wisconsin Geological and Natural History Survey, 3817 Mineral Point Rd., Madison, WI 53705, babrown1@facstaff.wisc.edu Our recent geologic mapping in southwestern Wisconsin has focused on pre-Sinnipee Group formations in the area north of the historic upper Mississippi Valley Zinc-Lead Mining District and south of the Wisconsin River. The regional stratigraphy can be clearly defined from outcrop exposures and mineral exploration boreholes in the Tunnel City Group, Trempealeau Group, Prairie du Chien Formation, and Ancell Group. The relatively easy recognition of the various units, in some cases to within 3 meters of a member or formation boundary, permits recognition of repeated sections and offset. This and previous mapping permits delineation of regionally significant anticlines, synclines, and faults, some which of have throws of more than 30 meters. Our analysis of recent aeromagnetic data (Bracken and Nicholson, 2000) leads to a better understanding of folds and faults recognized in outcrop and permits extrapolation of some of the faults to the east at least 60 km. The large, broad folds mapped by previous workers were based on detailed examination of mineral exploration boreholes and outcrops. Many of the structures correlate with basement linear features defined from the aeromagnetic data. Some of the folds, such as the Allamakee Anticline, may be related to readjustments along Keweenawan and older structural features (such as the Belle Plaine Fault and equivalent structures). Other east-west features, notably the Meekers Grove Anticline, are coincident with aeromagnetic linears at depth. These linear features are probably faults or faults with coincident dikes and probably define the north tectonic edge of the Illinois Basin. Regional map analysis suggests a steepening of dip of the Paleozoic rock units southward into the Illinois Basin along these regional deep basement trends, from a regional dip of less than 10 feet per mile to over 20 feet per mile. The large, quiet magnetic areas in western Grant County and else where in southern Wisconsin may reflect Wolf River-age plutons within the basement because the signatures are circular and apparently undeformed. Wolf River-aged material has been drilled along the state line between Wisconsin and Illinois (Coates, and others, 1983). Some of the linear anomalies defined from aeromagnetic data are coincident with positive linear gravity anomalies (Geister and Ervin, this meeting) and are probably wide, mafic dikes. The location of the plutons and faults probably influenced sedimentation patterns during the early Paleozoic by localizing the large reef/carbonate bank deposits of Middle Ordovician (Ludvigson and others, 1983). It is commonly thought that deep brines from the Illinois Basin gave rise to the hydrothermal solutions responsible for mineralization (Bethke, 1986). Localization of the 59 �zinc-lead mineralization along small faults and folds in mine workings is well documented from detailed mine mapping; however, this mineralization does not appear to be related to the deeper, more extensive basement structures. Heyl and others (1970) concluded that the broad-scale folds and faults were responsible for the tectonic framework, but the structures controlling ore deposition were developed by solution and slumping during the mineralization stages. We propose that the broader tectonic elements at the periphery of the Illinois Basin controlled ascent of the hydrothermal brines and therefore distribution of regional mineralization. The rapid cooling of those ascending brines along fractures resulted in the mineral concentrations. In this model it is the details of depth of burial of individual geologic units, their uplift and cooling history, and timing of transit of the hydrothermal brines, that are important in mineralization rather than host-rock lithology and local structure. Bethke, C.M., 1986, Hydrologic constraints on the genesis of the Upper Mississippi Valley mineral district from Illinois Basin brines: Economic Geology, v. 81, p. 233-249. Bracken, R.E., and Nicholson, S.W., 2000, Aeromagnetic Surveying in Wisconsn 1998-99: Digital Data Files: U.S. Geological Survey Open-File Report 99-527. Coates, M.S., Haimson, B.C., Hinze, W.J., and Van-Schmus, W.R., 1983, Introduction to the Illinois Deep Hole Project: Journal of Geophysical Research. B, v 88, no. 9, p. 7267-7285. Heyl, A.V., Broughton, W.A., and West, W.S., 1970 (1st edition), Geology of the Upper Mississippi Valley Base-Metal District, Wisconsin Geological and Natural History Survey [3Td Information Circular 16, 45 p. (some sections revised by M.G. Mudrey, Jr. in 1978 edition]). Ludvigson, G.A., Bunker, B.J., Witzke, B.J., and Garvin, P.L, 1983, A burial diagenetic model for the emplacement of zinc-lead sulfide ores in the Upper Mississippi Valley, USA: in Kisvarsanyi, G., Grant, S.K., Pratt, W.P., and Koenig, J.W., eds. International conference on mississippi valley type lead-zinc deposits (proceedings volume): University of Missouri Rolla, Rolla, MO, p. 497-5 15. 60 �___ PRELIMINARY ANALYSIS OF AEROMAGNETIC DATA IN SOUTHERN WISCONSIN: THE ROLE OF PRECA1'LBRIAN BASEMENT IN PALEOZOIC EVOLUTION MUDREY, M.G. Jr., Wisconsin Geological and Natural History Survey, 3817 Mineral Point Rd., Madison, WI 53705, mgmudreyfacstaff.wisc.edu BROWN, B.A., Wisconsin Geological and Natural History Survey, 3817 Mineral Point Rd., Madison, WI 53705, babrown1@facstaff.wisc.edu DANIELS, David L., U.S. Geological Survey, MS954 National Center, Reston, VA 20192, dave@usgs.gov The new aeromagnetic map of Wisconsin was the result of digitally blending the data from 22 surveys flown between 1948 and 1999. The most recent survey (74,000 line-kilometers), acquired by the U.S. Geological Survey, covers much of southern Wisconsin. The flight line data for four surveys acquired by the U.S.Geological Survey during the past 12 years have been released on CD-ROMs. These data were added to earlier U.S.Geological Survey surveys and 4 surveys acquired by Wisconsin Geological and Natural History Survey. Flight lines are 800m apart or less for 95% of the state, giving the aeromagnetic map nearly uniform specifications. All surveys were either flown at or continued to an elevation of 305m above terrain prior to assembling into a state grid. The data interval of the grid is 250 m. In this presentation, we show a preliminary analysis of the aeromagnetic data south of 4350, for which there is little Precambrian information either from outcrop or cuttings from deep boreholes. The resulting aeromagnetic map of southern Wisconsin illustrates the structure of the Precambrian rock underlying Paleozoic and Pleistocene cover. The pattern generally represents a complex Precambrian terrane that may include middle Proterozoic granite-green stone terrace containing small, circular anomalies related to the Wolf River batholith. The dominant Precambrian bedrock in eastern Wisconsin is quartzite of the Baraboo type. This unit is generally magnetically transparent; as a result, the magnetic signature originates from rock of the basement to the quartzite. In places, notably Fond du Lac County, folding within some units in the quartzite sequence is evident and suggests interbedded slate and iron formation. Other basement features include well defined faults in the Precambrian, some of which are clearly of Paleozoic age. In eastern Wisconsin the Waukesha and related faults define a basement terrace boundary, and in southern Wisconsin the Beloit and other faults define the northern edge of the Illinois Basin. The lack of relationship between the Upper Mississippi Valley Zinc-Lead District mineralization and Precambrian basement suggests a minor role for pre-Paleozoic elements during localization of the zinc-lead mineralization. 61 �MISSISSIPPI VALLEY-TYPE MINERALIZATION IN THE FOX RIVER VALLEY, EASTERN WISCONSIN (Modified from 42nd ILSG - Cable, Wisconsin, 1996) MUDREY, M.G. Jr., Wisconsin Geological and Natural History Survey, 3817 Mineral Point Rd., Madison, WI 53705, mgmudrey@facstaff.wisc.edu BROWN, B.A., Wisconsin Geological and Natural History Survey, 3817 Mineral Point Rd., Madison, WI 53705, babrownl@facstaff.wisc.edu FREIBERG, P.G., Department of Geology and Geophysics 1215 W. Dayton St., Madison, WI 53706-1692, SIMO, J.A., Department of Geology and Geophysics 1215 W. Dayton St., Madison, WI 53706-1692, simo@geology.wisc.edu Regional NURE (National Uranium Resource Evaluation Program) geochemical data suggest that anomalous concentrations of arsenic and other mineral exploration path-finder elements are present in the area southwest of Green Bay, Wisconsin where the Sinnipee, Ancell and Prairie du Chien Groups are the uppermost bedrock units. In addition, fluorine levels in groundwater have been known to be high in the Fox River valley between Green Bay and Appleton, where fluorite and other Mississippi Valley-type minerals are reported to be present in well cutting from the Sinnipee Group. Areas of significantly elevated values occupy northwestern Outagamie County and adjacent areas. A clearly defined nickel province that spatially corresponds to the arsenic province suggests that a polymetallic (As, Co, Mo, Ni, Th, V) hydrogeochemical province exists in eastern Wisconsin and may relate to documented faults (Mudrey and Bradbury, 1992). The geology differs from the better documentd five-element (Ni-Co-As-Ag-Bi) veins (Kissin, 1993) by being carbonate hosted rather than shale or volcanic hosted, but are similar in essential mineralogy and elements. Economic concentrations of Mississippi Valley-type mineralization have not been found in Wisconsin outside of Grant, Iowa, and Lafayette Counties, but geologic logs of 16 mineral exploration holes and more than 600 water wells in eastern Wisconsin contain reports of minor mineralization. In addition, more than 100 occurrences of sulfide minerals have been reported from outcrops and quarries throughout southern and eastern Wisconsin (Brown and Maass, 1992). A fairly continuous horizon of sulfide mineralization has been observed in quarries and drill core from Kenosha to Green Bay. Mineralization within this horizon consists of sulfide-cemented sandstone and sulfide infills of vugs and molds of fossil. The principal concentration of mineralization has been observed at or near the top of the St. Peter sandstone, but scattered mineralization is known throughout the Paleozoic section in this region. References: Brown, B.A. and Maass, R.S.. 1992, A reconnaissance survey of wells in eastern Wisconsin for indications of Mississippi Valley type mineralization: Wisconsin Geological and Natural History Survey Open-file Report WOFR 1992-3, 31 p. 62 �Kissin, S.A., 1993, Five-element Ni-Co-As-Ag-Bi) Veins: in P.A. Sheahan and M.E. Cheny, Ore Deposit Models, Volume II, Geoscience Canada Reprint Series,V. 6, p. 87-98. Mudrey, M.G., Jr., and Bradbury, K.R., 1992, Evaluation of NURE hydrogeochemical data for use in Wisconsin groundwater studies: Wisconsin Geological and Natural Histoly Survey Open-file Report WOFR 93-2, 61 p., 1 computer diskette. Mudrey, M.G., Jr., Brown, B.A., Freibeg, P.O., and Simo, J.A, 19%, Mississippi ValleyType Mineralization in the Fox River Valley, Eastern Wisconsin (abs.): Institute on Lake Superior Geology Proceedings, 42nd Aimual Meeting, Cable, WI, 1996, v. 42, part 1, p. 38-39 63 �OVERVIEW OF FIELD TRIP 2: UPPER MISSISSIPPI VALLEY - ZINC-LEAD DISTRICT, WISCONSIN MUDREY, M.G., Jr., Wisconsin Geological and Natural History Survey, 3817 Mineral Point Road, Madison WI 535705, mgmudreyfacstaff.wisc.edu HUNT, T.C., Director of the Reclamation Program, School of Agriculture, University of Wisconsin - Platteville, Platteville, WI 53818, huntt@am.uwplatt.edu CZECHANSKI, M.L., Wisconsin Geological and Natural History Survey, 3817 Mineral Point Road, Madison WI 535705, mlczecha@facstaff.wisc.edu First recovery of galena in the Driftless Region of Wisconsin, Illinois, Iowa and Minnesota was by native Americans for ornamentation about 1000 C.E. By 1690 Europeans recognized the lead deposits and began mining in what is now known as the Upper Mississippi Valley Zinc-Lead District. In the early 19th century, this was the premier lead mining district in North America. Numerous immigrant groups and developers were attracted to the area and the resulting population in flux played a major role in the eventual formation of the states of Illinois, Iowa, and Wisconsin. Initial mineral development (late 1 8111 and early 1 9t11 centuries) consisted of collecting surface occunences of galena and digging down until the excavations became unstable (badger holes). Deeper mining techniques were initiated, but were hampered by inadequate dewatering techniques below the water table. By 1850 lead production had peaked. Metallurgical developments and techniques to dewater mines led to significant zinc production (zinc ore became abundant with depth), initially from smithsonite (dry bone) and since 1900 from spha!erite. Peak production years were 1917 and 1952. The last mine, Eagle-Picher's Shullsburg Mine, ceased production in 1978. Since 1900, the U.S. Bureau of Mines, the U.S. Geological Survey, and the Wisconsin Geological and Natural History Survey have collected large amounts of information from the area including, detailed mine maps, drillhole locations, assay data, and geologic logs. This information is summarized in detailed geologic maps. Al Hey! and others (1959) summarized information up to 1950s in U.S. Geo!ogica! Survey Professiona! Paper 309. Hey! and others (1970) prepared a shorter summary of Professional Paper 309 for the Geological Society of America Meeting in Milwaukee in 1970. In addition, a large number of detailed geologic maps were prepared for southwestern Wisconsin and adjoining Illinois and Iowa. This area is the type-locality of the Upper Mississippi Valley Zinc-Lead mineralization. Warm (135 to 180 °C) mineral-bearing saline bnnes from the south (Illinois Basin) and southwest (Iowa Basin) are responsible for the mineralization. The bulk of the mineralization occuned in Middle Ordovician strata during the PermianlPennsylvania. Dominant mineralization occurred in the Decorah shaly dolomite, with significant quantities of mineralization in the over- and under-lying dolomite strata. Deeper mineralization has not been significantly tested. Local ore controls include minor folds and faults. Mineralization occurs as replacement and breccia filling in vertical fractures and crevices (gash veins), 64 �dipping fracture planes (pitches) and horizontal bedding planes (flats). Gash veins not uncommonly occur directly over the pitch and flat structures. There is a general vertical ore zonation with lead in the vertical veins and zinc concentrated in pitches and flats. Replacement and solution breccia are common, leading to some bonanza-type mineralization. Wall rock alteration is minimal, suggesting no widespread thermal events, but rather joint and fracture localization. The field trip will examine the Potosi Hill Ordovician geologic exposure, where the entire section from the Ancell Group through the Sinnipee Group is exposed; the Platteville Mining Museum and Bevan Mine which illustrates the regional geology and historical mineral development techniques; Pendarvis State Historical Site which captures the 1 830s spirit of mining;; and modem reclamation at the Shullsburg site, location of the last producing zinc mine in Wisconsin (1978). Heyl, A.V., Agnew, A.F., Lyons, E.J., and Behre, C.H., Jr., 1959, The Geology of the Upper Mississippi Valley Zinc-Lead District: U.S. Geological Survey Professional Paper 309, 310 p. Heyl, A.V., Broughton, W.A., and West, W.S., 1970 (1st edition), Geology of the Upper Mississippi Valley Base-Metal District, Wisconsin Geological and Natural History Survey Circular 16, 45 p. (some sections revised by M.G. Mudrey, Jr. in 1978 (3"' edition)) 65 �RECOGNITION OF POST-1630 MA FLUID-DRIVEN METAMORPHISM IN BARABOO INTERVAL QUARTZITES BY MEANS OF LASER PROBE 40Ar/39Ar GEOCHRONOLOGY NAYMARK, ALISSA, SINGER, BRAD and MEDARIS, L.G., Jr., Deptartment of Geology and Geophysics, Univ. of Wisconsin-Madison, 53706, anaymark@students.wisc.edu, bsinger@geology.wisc.edu, medaris @geology.wisc.edu. The southern Lake Superior region experienced many transformations during Proterozoic time Quartzites of the Baraboo interval, most notably the Baraboo, Barron, and Sioux, are known to have been deposited between —1710 Ma and 1630 Ma on 1750 Ma and older igneous and metamorphic basement. Quartzites south of Hoim et al.'s (1998) tectonic front were folded at —1630 Ma and, presumably, subjected to low-grade metamorphism at the same time. The present investigation, using CO2 laser probe 40Ar/39Ar incremental-heating methods, was undertaken to evaluate the timing and extent of low-grade metamorphism in Baraboo interval sedimentary rocks. Surprisingly, little evidence for 1630 Ma metamorphism was found in the analyzed samples; instead, a strong signature of post-1630 Ma hydrothermal activity was discovered in the Baraboo and Sioux quartzites. The Baraboo Ranke: Age spectra for three muscovite samples are discordant in the low temperature gas steps, but gave similar plateau ages. The initial 30% of gas released typically gave apparent ages beginning at —900 Ma in the vein material, and —1200 Ma in the samples from Baxter Hollow, rising to concordant plateau ages for the last 70% of the gas released. Muscovite from hydrothermal muscovite-pyrophyllite-diaspore veins in the base of the Baraboo Quartzite yielded a plateau age of 1467±10 Ma (±2) [Figure 1A]. Samples of muscovite from metasaprolite at Baxter Hollow yielded plateau ages of 1456± 11 and 1461± 12 Ma [Figure 1B]. These data provide the first evidence that hydrothermal metamorphism coeval with the Wolf River Batholith affected rocks in the Baraboo Range. We propose that this hydrothermal activity was due to large-scale movement of fluids through the crust, driven by heat provided by Wolf River granitic magmatism. The Denzer diorite is a member of the —1750 Ma igneous suite underlying the Baraboo Quartzite. It preserves an igneous texture, but was weakly recrystallized, presumably at 1630 Ma, with plagioclase partly replaced by albite and epidote, biotite altered to chlorite, and hornblende altered to actinolite, cumrningtonite, and chlorite. Biotite from the Denzer diorite yielded a plateau age of 1746±12 Ma [Figure 1D] that may reflect time since crystallization. In contrast, two samples of hornblende with intergrowths of actinolite, cummingtonite, and chlorite, yielded ages of 1596±16 and 1427± 15 Ma, which represent partial and complete Ar resetting of what may have been a 1630 Ma metamorphic assemblage. Sioux Ouartzite: Numerous samples of fine-grained metasedimentary rocks (pipestone) were collected from Pipestone National Monument, SE Minnesota. They were analyzed optically and via electron microprobe, X-ray diffraction, and X-ray fluorescence to determine their mineralogical and chemical compositions. Most samples contain the assemblage: muscovite-pyrophyllite-diaspore, similar to pipestone and hydrothermal veins at Baraboo. Owing to the fine grain size (—20 pm), less than 0.0 1mg whole rock samples were prepared from the SiOux pipestone based on X-ray diffraction patterns indicating that muscovite was the only potassium bearing phase present. The samples with K20 greater than 4.0 wt. % and more than 35% modal muscovite were chosen for the whole rock 40Ar/39Ar experiments. The plateau ages are slightly discordant at low and high temperature. However, 80% of the gas gave plateau ages of 1370±10 and 1268±10 Ma [Figure 1C], suggesting that post-1460 Ma hydrothermal activity affected the Sioux Quartzite. The geological significance of these ages from Sioux pipestone remains unclear, and the 100 million year difference in age between two samples from the same stratigraphic level is problematic. A possible explanation is differential loss of argon via diffusion from multiple small domains within the submicroscopic mica. The Barron Quartzite: The Barron Quartzite, which is located north of Holm et al.'s (1998) inferred tectonic front, was unaffected by 1630 Ma folding and metamorphism, and consists 66 �. predominately of quartz, kaolinite, and hematite. Although muscovite is rare in the Barron sedimentary rocks, t was found immediately beitw the Barron Quartzite in a vein, which cuts metatonalite basement. Both the vein and metatonalite are now saprolites, having been weathered in Proterozoic time, and the muscovite is partly replaced by kaolinite. The muscovite yields a plateau age of 1808±14 Ma, demonstrating that the Barron Quartzite and underlying basement were not affected by 1630 Ma folding and metamorphism, or by 1460 Ma hydrothermal activity. Conclusions: "°Ar/39Ar analyses of Baraboo and Sioux samples that were affected by Kmetasomatism, including Baraboo metasaprolite and hydrothermal veins and Sioux pipestone, revealed post-1630 Ma hydrothermal activity in the southern Lake Superior region. Hydrothermal fluids attending Wolf River magmatism exploited the nonconformity separating quartzites from underlying plutonic and metamorphic rocks. The muscovite 40Ar/39Ar plateau ages most likely record low temperature crystallization (—300CC) assemblages in metasaprolite, hydrothermal veins, and pipestone. Further investigation will be necessary to delineate the scale of this potentially regional fluid flow in the crust. Reference: HoIm, D., D. Schneider, and C. D. Coath. (1998) Age and deformation of Early Proterozoic quartzites in the southern Lake Superior region: Implications for extent of foreland deformation during final assembly of Laurentia. Geology, v. 26, p. 907-9 10. A 2000- B I I I I 2000 OOBOWIa muscovite vein 1467.14 ± 10.55 Ma . 96BHIA Baxter Hollow muscovite 1461.23 ± 11.79 Ma and 1455.93± 11.45 Ma • 1600 : 1600: 4 120O- 1200 , 8O0 800- 400- 400- I I I 20 40 60 C I 80 , 0 iC 0 I 0 uuu. D I OOPNMOI muscovite whole rock 1370.42± 10.30 Ma 20 40 I 60 I 80 10 0 • 1600- 1600- ,1200- OODDOI Denzer Diorite biotite 1742.02±11.91 Ma 1200 I: OOPNMO3 muscovite whole rock 1268.10± 10.63 Ma 800- 400 0 I I I I 20 40 60 80 100 0 20 40 60 80 100 Cumulative 39Ar released % Cumulative 39Ar released % Figure 1: Age spectra discussed in text. Weighted mean plateau ages are reported with 2 errors. 67 �MINERALOGICAL VARIATIONS IN IRON-FORMATION IN THE THERMAL METAMORPHIC AUREOLE OF A DIABASE DIKE NEMITZ, Michael B., and LARSON, Phillip C., Department of Geological Sciences, University of Minnesota, Duluth, MN 55812 The Biwabik Iron-formation in the National Steel Pellet Company Mine near Keewatin, Minnesota is cut by a series of diabase dikes. It has previously been empirically observed that iron-formation adjacent to these dikes is characterized by enhanced magnetite weight recovery, increased silica liberation indices, and increased oxidation reflected as elevated Fe3!Fe2 ratios. Granular cherty iron-formation samples were collected in two transects perpendicular to the contact of a 5-rn thick diabase dike. The transects extended 25-rn along the strike of the ironWhole-rock geochemical analyses, x-ray diffractometry, and reflected- and formation. transmitted-light polarizing microscopy were used to assess the geochemical, mineralogical, and textural variations in iron-formation adjacent to the dikes. Microscopy indicates an increase in the total amount of primary, euhedral magnetite, hematite, and total Fe-oxide proximal to the dike. The total Fe-oxide content appears to increase in an exponential fashion. This trend is reflected in the whole-rock Fe content. Magnetite occurs in all samples as primary euhedral crystals. Hematite occurs in a number of forms: euhedral to subhedral pseudomorphs after magnetite (martite), as inclusions in the lattice of euhedral magnetite, or as fine grained aggregates. Previous to iron-formation oxidation, a hydrothermal alteration event was focused along the dike margins. Alteration leached Mg, Ca, and Na from both diabase and iron-formation. This event is reflected in the iron-formation whole-rock Mg content, which show that Mg has essentially been removed in the 5-m zone proximal to the contact. Carbonate, and to a lesser extent Fe-silicate minerals have essentially been removed in the 10-rn zone proximal to the dike contact. Conversely, void space is most abundant in this same zone. This suggests void space has been created at the expense of carbonate and silicate phases. The increase in void space also correlates with the increase in hematite proximal to the This suggests that the oxidation of Fe-oxides to hematite may be related to increased porosity and permeability of iron-formation due to void space. Increased porosity allowed dike. oxidizing meteoric waters to circulate deeper along the dike margins. This study demonstrates that thermal metamorphism of iron-formation by diabase dikes has caused an increase in Fe-oxide content, both magnetite and hematite, thus resulting in increased magnetite weight recovery. However, the increase in void space due to alteration has allowed increased circulation of meteoric waters, resulting in oxidation of iron-formation and elevated hematite content relative to magnetite. 68 �PRELIMINARY LAVA FLOW MORPHOLOGY STUDIES AT THE FIVE MILE LAKE VMS PROSPECT, ARCHEAN VERMILION DISTRICT, NE MINNESOTA: IMPLICATIONS FOR VOLCANIC PROCESSES, VOLCANIC PALEOENVIRONMENTS, AND VMS EXPLORATION TRENT T. NEWKIRK, GEORGE J. HUDAK Department of Geology, University of Wisconsin Oshkosh, Oshkosh, WI 54901 STEVEN A. HAUCK Natural Resources Research Institute, University of Minnesota-Duluth, Duluth, MN 55811 As part of a two-year grant from the Minerals Coordinating Committee (MCC, State of Minnesota), we have undertaken a field-based, detailed investigation of the pillow lava morphology at the Five Mile Lake volcanic-associated massive sulfide (VMS) prospect, which is located approximately 15 miles southwest of Ely, Minnesota. This prospect is situated within a greenschist facies metamorphosed assemblage of Archean subaqueous, primarily mafic metavolcanic and metasedimentary rocks. These rocks lie within the Lower Member of the Late Archean Ely Greenstone (Peterson and ursa, 1999). Historically, several mineral exploration programs have been conducted in the Lower Member of the Ely Greenstone to evaluate its potential for VMS-style base metal mineralization. One of the most significant exploration programs to date was completed in 1995, when Teck Exploration Ltd. intersected stringer Zn-Cu mineralization in several diamond drill holes at the Five Mile Lake prospect. Cas (1992) and Gibson et al. (1999) and have emphasized the importance to mineral exploration programs of determining the volcanic environments associated with VMS mineralization. Gibson et al. (1999) have noted that mapping the orientation of synvolcanic dikes and sills is perhaps one of the most effective means to identify synvolcanic fault zones in lava flow dominated volcanic settings. This mapping is important because these structures not only dictate the location of volcanic vent sites, but also commonly control the locations of hydrothermal discharge sites responsible for making VMS deposits. Unfortunately, the generally small size of synvolcanic dikes and sills relative to their volcanic products commonly makes them difficult to recognize, especially in ancient volcanic sequences that are often plagued by a general lack of outcrop. Thus, detailed facies mapping in volcanic sequences is also an essential part of determining vent proximal volcanic environments. Studies of modern subaqueous mafic lava flows indicate that flow morphology has a direct relationship to effusion rate, cooling rate, and the slope upon which the lavas are erupted (Kennish and Lutz, 1998). In areas with relative fast effusion rates, sheet flows commonly occur in vent proximal environments, and grade laterally into pillow lavas farther from the volcanic vent. Slow effusion rates favor the immediate development of pillow lavas. It is interesting to note that several studies (Ballard and Moore, 1977; Ballard et al., 1981; Hekininan, 1984; Hekinian et al., 1989) have found that the glassy margins on vent proximal sheet flows tend to be thicker than the glassy margins on more distal, pillow lavas formed from the same eruption. This occurs because pillow lavas generally contain more crystals or crystal nuclei that inhibit the formation of their glassy outer margins (Kennish and Lutz, 1998). Therefore, the thickness of the glassy margins on these lava flows is also a general indicator of proximity to the volcanic vent. We have undertaken detailed mapping (1:120 scale to 1:5000 scale) of the physical characteristics of the extremely well-preserved, relatively undefonned pillow lavas at the Five Mile Lake prospect. We have measured various features of these pillows, including pillow shape, pillow horizontal dimensions, pillow vertical dimensions, pillow vesicularity, pillow selvedge characteristics, and the thicknesses of chlorite-rich, formerly glassy pillow margins. The relatively high vesicularity of these 69 �pillows (commonly between 10% and 15%), as well as the local presence of multiple pillow selvedges suggests that these flows were formed in a relatively shallow (<1 km) subaqueous environment. Our detailed mapping has also allowed us to identify several shallow mafic dikes that can be seen undergoing a vertical, then lateral transformation into pillow lavas. These regions represent the volcanic vent sites from which these pillow lavas issued. In several vent-proximal locations, chert-rich exhalite horizons (up to 1 meter thick) containing traces to several percent pyrite, sphalerite, and chalcopyrite are also present. Thus, our results support the relationships between proximal volcanic environments and mineralization in lava flow dominated sequences indicated by Gibson et al. (1999). Of particular interest are the results of our measurements of the thickness of the formerly glassy margins surrounding the pillows at the prospect. We have found that the formerly glassy pillow margins are clearly thicker (>4.5 cm thick) at these vent proximal locations than they are at locations that appear to be more distal to volcanic vents (where they are generally 2cm or less in thickness). Our results suggest that in pillow-dominated sequences, the thickness of the glassy margin surrounding pillow margins may be an accurate indicator of proximity to volcanic vents. This has significant implications for mineral exploration, as this measurement can be easily and quickly completed during field mapping, and may provide an effective means to identify regions within monotonous pillow sequences that are more likely to contain VMS mineralization. References Ballard, R. D., and Moore, J. G., 1977. Photographic Atlas of the Mid-Atlantic Ridge Rift Valley: Springer-Verlag, Berlin, 114 p. Ballard, R. D., Francheteau, J., Juteau, T., Rangin, C., and Normark, W.,1981. East Pacific Rise at 21°N: the volcanic, tectonic and hydrothermal processes of the central axis: Earth and Planetary Science Letters, v. 55, p. 1-10. Cas, R. A. F., 1992. Submarine volcanism: eruptions styles, products, and relevance to understanding the host rock successions to volcanic-hosted massive sulphide deposits: Economic Geology, v. 87., p. 5 11-541. Gibson, H. L., Morton, R. L., and Hudak, G. J., 1999. Submarine volcanic processes, deposits, and environments favorable for the location of volcanic-associated massive sulfide deposits: Reviews in Economic Geology, v. 8, p. 13-5 1. Hekinian, R., 1984. Undersea Volcanoes: Scientific American, v. 251, p. 46-55. Hekinian, R., Thompson, G., and Bideau, D., 1989. Axial and off-axial heterogeneity of basaltic rocks from the East Pacific Rise at 12°35'N — 12°5 1 'N and 1 1°26'N — I l°30'N: Journal of Geophysical Research, v. 94, p. 17437-17463. Kennish, M. J., and Lutz, R. A., 1998. Morphology and distribution of lava flows on mid-ocean ridges: a review: Earth Science Reviews, v. 43, p. 63-90. Peterson, D. M., and Jirsa, M. A., 1999. Bedrock Geological Map and Mineral Exploration Data, Western Vermilion District, St. Louis and Lake Counties, Northeastern Minnesota. 70 �A NEW LOOK AT THE 1.1 GA CHENGWATANA VOLCANICS IN THE ST. CROIX HORST, MINNESOTA AND WISCONSIN Nicholson, S.W., U.S.Geological Survey, MS 954, Reston, VA 20192; Boerboom, T., Minnesota Geological Survey, St. Paul, MN 55114; Cannon, W.F., U.S.Geological Survey, MS 954, Reston, VA 20192; Wirth, K., Macalester College, St. Paul, MN 55105; and Isachsen, C.E., University of Arizona, Tucson, AZ 85721 The 1.1 Ga Midcontinent rift system (MRS) hosts several classes of hydrothermal and magmatic mineral deposits (Nicholson et al, 1992). Recent speculation about the presence of additional magmatic mineral deposits (e.g., "Voisey's Bay"-type Ni-Cu: Schulz et al., 1998) has focused attention on the chemical compositions of rift-related basalts and the identification of central volcanic complexes as exploration tools. The St. Croix horst contains the most southerly exposure of volcanic and sedimentary rocks related to the MRS, but until now, sufficient chemical, age, and geophysical data have not been available to characterize adequately the nature of the volcanic rocks (Nicholson et al., 1997) and their relationship to the regional chemical stratigraphy established for the MRS. North of the St. Croix horst rift-related rocks are exposed around the margins of Lake Superior and detailed stratigraphic sections are well established. Outcrops within the St. Croix horst are sparse: the volcanic rocks are nearly all subaerial basalt flows and were previously assigned to the Chengwatana Volcanics. Recently Cannon et al. (2001) used new detailed aeromagnetic imaging and age determinations to subdivide the former Chengwatana Volcanics into three units, the Minong Volcanics, the Clam Falls Volcanics, and a newly redefined Chengwatana Volcanics (Fig. 1). More than 200 chemical analyses are now available for volcanic rocks in the St. Croix horst. When compared to the regional chemical stratigraphy recognized previously around western Lake Superior (Nicholson et al., 1997), the three new volcanic units appear to be related as follows. The youngest unit, the Minong Volcanics, is most similar to the Portage Lake Volcanics (1096-1094 Ma). Both are dominated by low- TiO2 (less than about 2.0 wt. % Ti02 ) basalts with low abundances of incompatible trace elements. Like the Portage Lake Volcanics, the Minong Volcanics contain few rhyolites, but a rhyolite flow near the base of the section has been dated at about 1094 Ma (Lake Nelson rhyolite; Zartman et al., 1997), an age comparable to the upper Portage Lake Volcanics. Basalts with depleted compositions similar to N-MORB occur as flows near the top of the Minong Volcanics and as dikes in the upper Portage Lake Volcanics. The Clam Falls Volcanics unconformably underlie the Minong Volcanics. It is also dominantly lowTi02 basalts, but high-Ti02 basalts (more than about 2.5 wt % Ti02; increased abundances of incompatible trace elements) are more common in this unit than in the overlying unit. The high- TiO2 basalts are similar in composition to high- Ti02 basalts in the Portage Lake Volcanics and the underlying upper Kallander Creek Volcanics (dated at 1098 Ma) to the northeast. Although chemically similar, the metamorphic grade of the Clam Falls Volcanics is considerably higher than the Minong, Portage Lake, or Kallander Creek Volcanics, suggesting that this unit represents exhumation of a more deeply buried portion of the rift (Wirth et al., 1997). A rhyolite flow at the base of the Clam Falls Volcanics yields ages between 1100 and 1102 Ma (unpub. data: K.R. Wirth). The oldest unit, the newly redefined Chengwatana Volcanics now confined to the volcanic rocks between the Pine and Douglas faults, is dominated by high- Ti02 basalts with accompanying intermediate and felsic volcanic rocks. This association of high-hO2 basalts with intermediate and felsic volcanic rocks has been postulated elsewhere in the rift to be related to central volcanic complexes, sites of prolonged shallow magma chamber development and accompanying volcanism. The Amnicon gabbroic and granophyric intrusive complex southeast of Duluth cuts the base of the redefined Chengwatana Volcanics and most likely represents the magma chamber for a central volcanic complex. The extensive granophyre in the Amnicon pluton is identical chemically to an overlying rhyolite flow. This rhyolite flow has a preliminary date of about 1106 Ma (unpub. data: C.E. Isachsen). Southwest of the Amnicon pluton the Chengwatana Volcanics include more low- hO2 basalts as the influence of the localized Amnicon magmatic system diminishes. In conclusion, the high- Ti02 basalts and related intermediate and felsic rocks of the redefined Chengwatana Volcanics in the St. Croix horst were probably erupted from localized magmatic sources from about 1107 to about 1102 Ma. This was followed by outpourings of voluminous low-Ti02 flood basalts characteristic of the main stage of nfting of the MRS after about 1102 Ma, now represented in the St. Croix 71 �horst by the Clam Falls and Minong Volcanics. Central volcanic complexes in the St. Croix horst, such as the Amnicon complex, may be potential exploration targets for Cu-Ni sulfide mineralization, if further study can show the availability of a source of sufficient sulfur to produce segregation of Cu-Ni+PGE metals. References: Cannon, W.F., Daniels, D.L., Nicholson, S.W., Phillips, J., Woodruff, L.G., Chandler, Va!, Morey, G.B., Wirth, K.R., and Mudrey, MG., Jr., 2001, New map reveals origin and geology of North America Midcontinent rift: EOS, v. 82, no.8, pp. 97-101. Davis, D.W., and Green, J.C., 1997, Geochronology of the North American Midcontment rift in western Lake Superior and implications for its geodynamic evolution: Canadian Journal of Earth Sciences, v. 34, pp. 476-488. Davis, D.W., and Paces, J.B., 1990, Time resolution of geologic events on the Keweenawan Peninsula and implications for development of the Midcontinent rift system: Earth and Planetary Science Letters, v. 97, pp. 54-64. Nicholson, S.W., Cannon, W.F., and Schulz, K.J., 1992, Metallogeny of the Midcontinent rift system of North America: Precambrian Research, v., 58, pp. 355-386. Nicholson, SW., Shirey, S.B., Schulz, K.J., and Green, J.C., 1997, Rift-wide correlation of 1.1 Ga Midcontinent rift system basalts: implications for multiple mantle sources during rift development: Canadian Journal of Earth Sciences, v. 34, pp. 504-520. Schulz, K.J., Cannon, W.F., Nicholson, S.W., and Woodruff, L.G., 1998, Is there a "Voisey's Bay" —type Ni-Cu sulfide deposit in the Midcontinent rift system in the Lake Superior region?: Mining Engineering, v. 50, pp. 57-62. Wirth, K.R., Vervoort, J.D., and Naiman, Z. J., 1997, The Chengwatana Volcanics, Wisconsin and Minnesota: petrogenesis of the southernmost volcanic rocks exposed in the Midcontinent rift: Canadian Journal of Earth Sciences, v. 34, pp. 536-548. Zartman, R.E., Nicholson, SW., Cannon, W.F., and Morey, G.B., 1997, U-Th-Pb zircon ages of some Keweenawan Supergroup rocks from the south shore of Lake Superior: Canadian Journal of Earth Sciences, v. 34, pp. 549-561. Upper Michigan and North-Central Wisconsin Northwest Wisconsin Eastern Minnesota Fig.!: Stratigraphic column for Keweenawan volcanic rocks on the south shore of western Lake Superior. The U-Pb age dates are from the following sources: a, Davis and Green, 1997; b, Zartman et a!., 1977; c, Davis and Paces, 1990; d, unpublished data, K.R. Wirth; e, unpublished data, C.E. Isachsen 72 �OVERVIEW OF ARSENIC OCCURRENCES AND PROCESSES CONTROLLING ARSENIC MOBILITY IN GROUND WATER D. Kirk Nordstrom U.S. Geological Survey Boulder, CO Introduction Arsenic concentrations in ground waters can range from less than a few jtg/L to tens or even hundreds of mg/L in locally contaminated environments. Both anthropogenic and natural sources for arsenic in ground waters occur in many locations world-wide. Natural sources are causing or have caused poisoning of populations in India, Bangladesh, Chile, Argentina, Mexico, Taiwan, Mongolia, Japan, and China. Mining activities are responsible for arsenic poisoning in Thailand. Arsenic mass poisoning in Bangladesh is the largest known, affecting nearly 30 million people. The processes that enrich arsenic in minerals and in ground waters are complex but apprehensible. Sources The geochemical cycle of arsenic from magmatic-hydrothermal processes through weathering, sedimentation, and diagenesis transforms the element in a number of ways that produces an array of present-day natural sources. Probably the single most abundant mineral source of arsenic is arsenian pyrite. Pyrite is ubiquitous in the earth's crust, occurring in sedimentary, metamorphic, and igneous rocks. Arsenic has a strong affinity for the sulfur site in pyrite, substituting up to about 10 wt. % regardless of whether the origin is sedimentary or hydrothermal (Kolkar, Nordstrom, and Goldhaber, 2001). Arsenopyrite contains higher concentrations of arsenic (39-53%) but it is a much rarer mineral. Arsenopyrite and arsenian pyrite are commonly found in association with gold mineralization so that gold mining frequently releases arsenic to the environment. Other arsenic-rich minerals include orpiment, realgar, and enargite. Weathering of these minerals in oxidizing environments solubilizes arsenic as As(III) and ultimately as As(V). Arsenate, or As(V), has a strong adsorption affinity for hydrated iron oxides (Pierce and Moore, 1982) and in oxidized sediments iron oxides can be a source of soluble arsenic if they undergo reductive dissolution during early diagenesis. Geothermal springs are commonly enriched in arsenic, containing 0.1-5 mgIL dissolved arsenic (as both As(llI) and As(V)). Geothermal power plants often have to deal with proper disposal of arsenic-enriched waste waters. Anthropogenic sources of arsenic are numerous. The primary source of industrial and commercial arsenic was arsenic trioxide that was produced as a by-product of metal mining and processing, primarily from copper smelting. More than 300,000 tons of flue dust containing an average of 6.5% arsenic were piled at Anaconda, Montana before removal and disposal. Stockpiles of arsenic trioxide still exist without proper containment and are releasing soluble arsenic to ground waters. Several arsenic insecticides (copper, lead, calcium, magnesium, zinc, and sodium arsenites and arsenates), herbicides (sodium arsenite and methanearsonate, disodium methanearsonate, and cacodylic acid), dessicants (arsenic acid), wood preservatives, animal feed additives, drugs, chemical weapons, and alloys were produced for many years. Roxarsone, an organic arsenical, is still widely used today to 73 �clean parasites out from the stomachs of pigs and poultry (Garbarino et al., 2001). Transformations and Processes Arsenic in surface and ground waters occur dominantly as either arsenite, As(III), or arsenate, As(V). Reduction of arsenic occurs with the possible formation of several methylated species, the most prevalent being monomethly- and dimethylarsenic acid. Several microorganisms including species of fungi, algae, and bacteria catalyze the reduction of arsenic. Methylated arsenic is volatile and is released to the atmosphere in open systems. Oxidation of arsenic is also catalyzed by microbes and it has been demonstrated that soluble As(III) and arsenic sulfide minerals such as arsenopyrite and orpiment can be catalytically oxidized to soluble As(V). More than 25 species of arsenic-oxidizing bacteria have been identified and many more are believed to exist. In geothermal waters, the dominant form of dissolved arsenic is As(ffl) at the source of the discharge but this can be oxidized rapidly to As(V) by microbes that survive at temperatures of 50-95°C. Little is known about the breakdown of feed additives such as roxarsone. Although there has been research on the transformations of arsenic within humans and some other organisms, the pathways are very complicated and much remains to be learned. Water quality environments that encourage solubilization and mobility of arsenic are high pH and oxic conditions, anoxic or moderately reducing conditions with no sulfate reduction, anoxic with strongly reducing conditions with little to no sulfate present, or strongly acidic oxidizing conditions (below the normal solubility of hydrated iron oxides). Environments that encourage low arsenic mobility are moderately acidic to neutral and oxidizing conditions, or organic rich sulfate-reducing environments. References Garbarino, J.R., Rutherford, D.W., and Wershaw, R.L. (2001) Degradation of roxarsone in poultry litter. USGS Workshop on Arsenic in the Environment, Feb. 21-22, 2001, website: wwwbrr.cr.usgs.gov/Arsenic Kolkar, A., Nordstrom, D.K., and Goidhaber, Mi. (2001) Occurrence and micro-distribution of arsenic in pyrite. USGS Workshop on Arsenic in the Environment, Feb. 21-22, 2001, website: wwwbrr.cr.usgs.gov/Arsenic Pierce, M.L. and Moore, C.B. (1982) Adsorption of arsenite and arsenate on amorphous iron hydroxide. Water Res. 16, 1247-1253. 74 �PRELIMINARY EVALUATION OF HYDROTHERMAL ALTERATION MINERAL ASSEMBLAGES AND THEIR RELATIONSHIP TO VMS-STYLE MINERALIZATION IN THE FIVE MILE LAKE AREA OF THE ARCHEAN VERMILION GREENSTONE BELT, NORTHEASTERN MINNESOTA JASON D. ODETTE, GEORGE J. HUDAK, THOMAS SUSZEK Department of Geology, University of Wisconsin Oshkosh, Oshkosh, WI 54901 STEVEN A. HAUCK Natural Resources Research Institute, University of Minnesota Duluth, Duluth, MN 55811 The Minerals Coordinating Committee (MCC, State of Minnesota) recently awarded a two year grant to geologists from the Natural Resources Research Institute (NRRI), University of Minnesota — Duluth (UMD), and the University of Wisconsin Oshkosh to further characterize the geology, volcanology and metamorphosed hydrothermal alteration mineral assemblages associated with several volcanic-associated massive sulfide (VMS) prospects in the Vermilion District of northeastern Minnesota. This investigation includes detailed outcrop mapping, diamond drill core relogging, petrography, lithogeochemistry, and geophysical rock property evaluations. The Five Mile Lake prospect occurs approximately 15 miles southwest of Ely, Minnesota, and is situated within greenschist facies metamorphosed, primarily mafic Archean metavolcanic and metasedimentary rocks within the Lower Member of the Ely Greenstone Sequence of the Vermilion District. In 1994, Teck Exploration Ltd. intersected volcanic-associated massive sulfide (VMS) — style stringer Zn-Cu mineralization in three of four diamond drill holes completed at this prospect. Subsequent studies by the Minnesota Department of Natural Resources (Hudak and Morton, 1999) have suggested that mineralization at Five Mile Lake may be representative of that associated with "Noranda-type" (Morton and Franklin, 1987) Archean VMS We are currently completing a detailed investigation of the mineralogical, chemical, and spatial characteristics of the metamorphosed synvolcanic hydrothermal alteration that occurs at the Five Mile Lake VMS prospect. Our two-month long field program was completed during the summer, 2000, and consisted of two investigative phases. The first phase comprised GPS-assisted geological mapping of the entire prospect at 1:5000 scale, with more detailed mapping of surface-mineralized zones at 1:120 scale. During our mapping, special attention was paid to alteration mineral assemblages present, and their apparent relationships to locally extremely well-preserved volcanic and volcaniclastic textures. Hand samples were collected from each outcrop. When appropriate, a portable, hand-held diamond drill was used to collect samples of adjacent alteration mineral assemblages. During the second phase of our investigation, Teck Exploration Ltd.'s four diamond drill holes (SXL-1, SXL-2, SXL-3 and SXL-4) were relogged, with particular emphasis being paid to the alteration mineral assemblages and textures present. Laboratory investigations are currently being conducted. All outcrop and drill core samples have been slabbed and investigated using a binocular microscope for alteration mineral assemblages, alteration textures, and alteration paragenesis. Prepartion of one hundred ninety-seven thin sections is currently being completed, and petrographic analyses on the available thin sections are being performed. Fiftythree samples have been analyzed for major and trace elements by ALS Chemex Labs, Inc. (Sparks, NV). At the present time, we have concentrated our alteration studies on the mafic pillow lava flows and the diabase dikes that occur at the prospect. Based on our preliminary field and laboratory investigations, all pillow lavas at the prospect have undergone varying degrees of hydrothermal alteration. This is consistent with the findings of Peterson (personal communication, 2001), who indicates that the prospect resides within a semiconformable alteration zone comprising at least 3 0km2 of rocks. Three distinct hydrothermal alteration mineral assemblages occur within the pillow lavas at the prospect. The 75 �least altered assemblage (LA) comprises pillow lavas containing a mineral assemblage composed of albite + epidote + chlorite in proportions which are consistent with greenschist facies metamorphism of an original basalt or basaltic andesite lava composition. A quartz + albite + epidote ± actinolite assemblage (QAE) occurs within pillow cores, whereas a chlorite + actinolite ± epidote assemblage (CA) occurs in pillow selvedges and within the matrix to pillow breccia and pillow hyaloclastite. Locally, CA assemblage filled amygdules occur within the QAE assemblage pillow cores, and these rocks seem to have a close spatial relationship to thin (<Im thick) mineralized exhalite horizons at the prospect. Textural relationships indicate that the paragenesis of these alteration mineral assemblages is early QAE followed by later CA. Mass balance analysis of least altered pillow lavas, QAE assemblage, and CA assemblage rocks have been performed using constant Al203 and best fit (based on Al203, Ti02, Zr, Nb) isocons (Grant, 1986). Relative to least altered rocks, QAE assemblage rocks illustrate gains in Si02 and Na20 and decreases in CaO, Fe203, FeO, MgO, MnO, K20, H20, Cu and Zn. These trends may be indicative of regional silicification and spilitization from rapidly heated, downwelling, silica-saturated hydrothermal fluid. Relative to least altered rocks, CA assemblage pillows are enriched in MgO, Fe203, FeO, Zn and H20 and are depleted in Si02, K20, and CaO. These results may represent an alteration assemblage formed from the mixing of cooler, downwelling Mg-rich seawater and hotter, upwelling Fe- and Zn-rich evolved hydrothermal solutions within permeable pillow selvedges and hyaloclastite. Locally, diabase dikes are altered to epidosite. The epidosite zones comprise rounded to oval, 0.1-1.5 meter diameter patches composed of pale green epidote (locally up to 40%) + quartz ± actinolite. Preliminary x-ray diffraction analyses indicate that the major epidote mineral in these epidosite zones is clinozoisite. Least altered diabase is composed of a mineral assemblage containing quartz-clinochloreferroactinolite and albite with only minor (<5%) epidote and clinozoisite being present. Mass balance analysis of the least altered diabase and the epidosite alteration patches have also been performed using constant A1203 and best fit isocons. These analyses indicate that the epidosite zones are enriched in CaO, and simultaneously depleted in Na20, K20, MnO, Fe203, FeO, 5, Zn, and Cu relative to the least altered diabase. We initially interpret these epidosite patches as areas of locally high water:rock ratio alteration within lower semi-conformable alteration zones associated with high temperature base metal leaching. At the present time we are in the process of further characterizing the alteration mineral assemblages by means of our laboratory studies. Petrographic and x-ray analyses to further characterize the mineralogy and paragenetic sequences of the alteration mineral assemblages is just beginning, and further geochemical classification of the alteration mineral assemblages will be performed. It is believed that in the long term, further characterization of the alteration mineral assemblages at the Five Mile Lake VMS prospect will provide exploration companies with additional data that is needed to conduct efficient and effective mineral exploration programs for VMS deposits in the Vermilion District. References Grant, J. A., 1986. The isocon diagram — a simple solution to Gresen's equation for metasomatic alteration: Economic Geology, v. 81, p. 1976-1982. Hudak, G. J., and Morton, R. L., 1999. Bedrock and Glacial Drift Mapping for VMS and Lode Gold Alteration in the Vermilion — Big Fork Greenstone Belt, Part A, Discussion of Lithology, Alteration, and Geochemistry at the Five Mile Lake, Eagles Nest, and Quartz Hill Prospects: Minnesota Department of Natural Resources Project 326, 136 pages. Morton, R. L., and Franklin, J. M., 1987. Two-fold classification of Archean volcanic-associated massive sulfide deposits: Economic Geology, v. 82, p. 1057-1063. 76 �CORRELATION OF ARCHEAN ASSEMBLAGES ACROSS THE U.S.-CANADIAN BORDER: PHASE I GEOCHRONOLOGY PETERSON, Dean M. (Natural Resources Research Institute, Duluth, MN, dpetersl@nrri.umn.edu); GALLUP, Christina (University of Minnesota-Duluth, cgallupd.umn.edu); uRSA, Mark A. (Minnesota GeologicalSurvey,jirsa OOl@umn.edu); and DAVIS, Donald W. (Royal Ontario Museum, Toronto, ON, dond@rom.on.ca) Past attempts at temporal correlation of Archean stratigraphic assemblages between rocks of the geochronologically well constrained Shebandowan district of Ontario, and the Vermilion district of northeastern Minnesota (Figure 1), have invariably suffered because: I) at some scale, one greenstone belt looks pretty much like another; 2) rocks of both districts are dissected by faults having poorly known displacements; and 3) little geochronologic data exists for the Minnesota rocks. Nevertheless, detailed analyses reveal that there are significant stratigraphic and lithologic relationships in each assemblage that can be compared and contrasted. Recent U-Pb geochronology is beginning to shed light on similarities and differences between assemb1aes in the two areas. Here we report the results of two sets of U-Pb dates on zircons from the Vermilion district. - Locatkrn of dated samples A 2683±1.4 Ma Porphyry B 2722±0.9 Ma Rhyohte - '11'" S ,',S - ''/, .'.''.." '.Northern Light Gneiss Saganaga piuton" ,t,A,,, 'l Western SupedoProvtce Vermilion " wgoo, District A R B j IIIIlIU4Soudari 'Btgfork ' Shebandowan 's.Stratigraphic faong p'Lake \,.... ,E!y',' , ;)" 'SSS, ,-,5555 s'Gmnitnd truss S.ipeno r Twnisiiameg lype conglomeratic sequences - ''" 'S" Wawa Subprovlnce Vermilion Ouetico Subprovlnce Wabigoon Subprovince Graywactra Metasvcanic rocks J1111 Graywacke -KomatutcTholeidic NEWTON BELT Shebandowan GREETCHELL Cak-akaiic/thoIeitic/komatiitic Migmatite. schist. and j granite TIf1 Graywacke SOUDAN BELT SAGANAGONS ASSEMBLAGE LI Cac-aalcflhoeiiiic Figure 1. Preliminary correlation of Late Archean stratigraphic assemblages and belts through the Vermilion district, showing the location of dated samples. Inset shows the map location within the western Superior Province. Rocks of the Vermilion district are subdivided on the basis of stratigraphic and structural contrast into two distinct domains, known as the Soudan and Newton belts (Figure 1). The Soudan belt contains large, broad folds involving caic-alkalic and tholeiitic volcanic strata overlain by, and locally interdigitates with, turbiditic rocks. In contrast, the Newton belt consists of elongate, northeast trending, and mostly northward-younging volcanic and volcaniclastic sequences. Volcanic rocks of the Newton belt differ from those of the Soudan in containing locally abundant komatiitic flows and peridotitic sills (eg. the Newton Lake Formation and Deer Lake sequence). The belts are fault-bounded, and the relationship between stratigraphic units within each belt is largely conformable, though faults obscure contacts locally. In its eastern extension, the Soudan belt is continuous with the Saganagons assemblage that terminates against the Saganaga pluton and Northern Light Gneiss (Figure 1). The Newton belt extends discontinuously eastward into the Shebandowan district, and broadens to form the approximately 2720 Ma-old Greenwater and Burchell assemblages2. The Greenwater assemblage is similar to the Newton Lake Formation in younging predominantly to the north, and in containing komatiitic and tholeiitic flows, mafic and ultramafic sills, and local intermediate to felsic flows and pyroclastic rocks. The Burchell is lithologically similar and temporally identical, but youngs to the south. Intrusions in both districts vary from felsic porphyries demonstrably related to volcanism, to large plutons emplaced posttectonically. Both districts contain unconformable, Timiskaming-type sequences composed of caic-alkalic volcanic rocks, conglomerates, and finer grained sedimentary rocks. These Timiskaming-type rocks include the roughly 2690 Ma-old Shebandowan assemblage2 and some lithologic components of the Knife Lake Group in Minnesota5. Periods of generally N-S-directed compression resulted in three major deformation events that are recognized in rocks of both districts. The earliest, D1, produced broad, locally recumbent folds within the Soudan belt and major fault zones throughout the region. The affect of D1 on rocks of the Newton belt and much of the Greenwater assemblage appears to have been thrust imbrication of large crustal blocks, resulting in mainly northward stratigraphic facing. Field relationships indicate that uplift, faulting, and the deposition of Timiskaming-type sequences in local fault-bounded basins occurred late in D1 deformation. The 77 �second deformation event, D2, produced synchronous regional metamorphism, foliation development, and structures having largely dextral asymmetry. D2 has been constrained in the Vermilion district to the time period 2674 Ma to 2685 Ma', and between about 2680 and 2685 Ma in the Shebandowan2. The abundant NE- and NW-trending faults that dissect the stratigraphic assemblages are assigned to D3. The two samples from the Vermilion district were selected mainly because they had the potential to produce zircons, and their ages would constrain the timing of volcanism and D2 deformation. Zircons were separated and isotopic compositions were measured at the Royal Ontario Museum in Toronto, using methods developed by Krogh4. The samples include: A) Newton belt a weakly deformed, irregularly shaped quartz-feldspar porphyry (QFP) body intruded discordantly along a basalt - iron-formation contact within the Pac Man Pond gold prospect. Field relationships indicate that the QFP was emplaced late in D2, because it (and similar intrusions) cuts D2 shear zones but is itself weakly deformed. B) Soudan belt — quartz-phyric rhyolite lava flow in the Fivemile Lake VMS prospect. Zircons from the Newton belt sample were very light brown, small (—lig after abrasion), subhedral, prismatic, and typically cracked. The zircons from the Soudan belt sample were colorless to light brown or pink, larger (—2-3 jig after abrasion), euhedral to subhedral, prismatic, and except for rare grains having a clear core, lacked observable internal cracks. After abrasion, three zircons from each sample were chosen for dating: the results are shown in Figure 2, plotted as 2 sigma error ellipses relative to concordia. Regression and age calculation follow the method of Davis (1982). Only one zircon without cracks could be found in the Newton belt sample. This produced the datum that is closest to concordia (Figure 2). It defines a line with the other data, giving an upper concordia intercept age of 2681 +1-4 Ma. All data have 207Pb/206Pb ages that are indistinguishable within error. Under the assumption that Pb loss was recent, the average 207Pb/206Pb age of 2683.0 +1- 1.4 Ma gives a more precise value for the age of intrusion. The age of this late-syn D2 porphyry constrains the maximum age of D2 in the Vermilion district, and is similar in age to parts of the Giants Range batholith, which forms the southern margin of the Vermilion greenstone belt. Data from the Fivemile Lake rhyolite zircons plot very near concordia and have indistinguishable 207Pb/206Pb ages that average to 2722.6 ± 0.9 Ma. This is the first age ever reported for the Ely Greenstone, and is similar in age to rhyolites in the Greenwater assemblage in Shebandowan district. .528 .526 D .524 .522 .520 .518 13.4 13.5 13.6 11.6 13.7 12 12.4 12.8 207Pb/235LJ 207Pb/235u Figure 2. U-Pb isotopic compositions for A) Porphyry in the Newton Belt, and B) Rhyolite in the Soudan Belt. 7 The ca 2720 Ma volcanic rocks in the Vermilion and Shebandowan districts, as well the Manitouwadge area6' farther to the east, together with the bracketed ages for D2 deformation that are nearly identical in all three districts, implies that this terrane defines a major orogen extending more than 600 kilometers. The individual belts have been strongly attenuated by deformation and pluton emplacement, but further U-Pb dates will continue to explore the Vermilion district in this broader orogenic context. Boerboom, T.J., and Zartman, R.E., 1993, Geology, geochemistry, and geochronology of the central Giants Range batholith, northeastern Minnesota: Can. J. Earth Sci. 30:25 10-2522. 2Corfu, F., and Stott, G.M., 1998, Shebandowan greenstone belt, western Superior Province: U-Pb ages, tectonic implications, and correlations: GSA Bulletin 110:1467-1484. 3Davis, D.W., 1982. Optimum linear regression and error estimation applied to U-Pb data. Can. J. Earth Sci. 19: 2141-2149. 4Krogh, T.E., 1982, Improved accuracy of U-Pb ages by the creation of more concordant systems using an air abrasion technique. Geoch. et Cosmochim. Acta, 46: 637-649. 5iirsa, M.A., 2000, The Midway sequence: a Timiskaming-type pull-apart basin deposit in the western Wawa subprovince, Minnesota: Can J. Earth Sci., 37: 1-15. 6Zaleski, E., van Breemen, 0., and Peterson, V.L., 1999, Geological evolution of the Manitouwadge greenstone belt and WawaQuetico subprovince boundary, Superior Province, Ontario, constrained by U-Pb zircon dates of supracrustal and plutonic rocks: Can. J. Earth Sci., 36: 945-966. 7Davis, D.W., Schandi, E.S., and Wasteneys, H.A. 1994. U-Pb dating of minerals in alteration halos of Superior Province massive sulfide deposits: syngenesis vs. metamorphism. Contrib. Mineral. Petrol. 115: 427-437. 78 �MAGNETIC SURVEY NEAR WATERLOO WISCONSIN PEYCHALI*, C., KEAN', W. F., and SCHAPER2*, D., Department of Geosciences, University of Wisconsin -Milwaukee'4 Department of Geological Engineering, University of Wisconsin-Madison2. wkean@uwm.edu. (* student) The Waterloo Wisconsin area continues to provide geologic puzzles because of its diverse bedrock. The Waterloo Quartzite is considered by most as cotemporaneous with the Baraboo Quartzite. A mafic dike is known from two quarries about I mile apart along a north-south strike, and a course feldspar rich pegmatite appears within 200 meters north of the quartzite quarry in which the mafic dike is found ( Luther, 1977). The age relationship of these three rock types is still uncertain. Although, the two igneous rock types intrude the quartzite, and both are dated in the range of 1.45 -1.5 Ga.(Van Schmus Ct al. 1978, Brown, 1986), which is not necessarily the primary age. Paleomagnetic studies on the mafic dike provide a well defmed magnetic direction of Dec.=299°, Inc.-43°, N=26, Alpha 95= 13.7°, giving a paleopole at 323° E, 2° S which is interpreted as a 1 .7Ga. pole position ( Schaper and Kean, 1999). The object of this study is to better define the magnetic basement in this region in an attempt to clarify the relationship of these known rocks. A proton precession magnetometer survey covered most of the roads in the vicinity of the old Portland Quarry and the Michels Materials Quarry. A first survey covered the region at 0.1 mile spacing. Selected locations were resurveyed at 50 meter spacing , and also with a continuously reading rubidium vapor magnetometer. Finally one road section was covered at 2 meter spacing. Well construction reports, and the aeromagnetic maps of the region were examined for additional insight into the bedrock. The aeromagnetic maps show a general decreasing trend from west to east, with a 200 nT. anomaly northeast of the Michels Materials quarry, where well construction reports indicate granite. The ground magnetic survey shows a 1000 nT. negative anomaly along Highway 19 that appears to be in line with the known outcrops of the mafic dike. The negative value is consistent with the negative remanent magnetism of the mafic dike. Similar anomalies are found on the south-north section of Hubbleton Rd. which passes in front of the active quarry. We interpret these as the extension of the dike to the north east of its known outcrop. References: Brown, B.A., 1986, The Baraboo Interval in Wisconsin, Geoscience Wisconsin, vol.10, p.1-15. Luther, F. R., 1997, the Precambrian Waterloo Quartzite, Dodge and Jefferson Counties, WisconsinPetrology, structure and Industrial Use. In "Guide to Field Trips in Wisconsin and Adjacent Areas of Minnesota". M.G. Mudrey Jr. Field Trip Coordinator, Wisconsin Geological Survey, 114 pp. Schaper, D., and Kean, W., 1999, Multiple Magnetic Directions in a Proterozoic Dike near Waterloo, Wisconsin. Fall Meeting AGU, abstracts. Van Schmus, W.R., 1978, Geochronology of the Southern Wisconsin Rhyolites and Granites, Geoscience Wisconsin, vol.2, p.1 9-24. 79 �Nd and U-Pb Isotope Studies of the Syenitic Aurora Sill, Mesabi Range, Minnesota Phillips, Erin H., Macalester College, St. Paul, MN 55105; Wirth, Karl R., Geology Department, Macalester College, St. Paul, MN 55105, wirth@macalester.edu; Vervoort, J.D., and Gehrels, G.E., Dept. of Geosciences, University of Arizona, Tucson, Arizona 85721 The Aurora sill is a concordant tabular intrusion approximately 5.6 km long and 6 to 37 meters thick that occurs in the Mesabi Range north of the town of Aurora, Minnesota. The age of the sill is unknown, but it has commonly been assigned a Mesoproterozoic (Keweenawan) age because it intrudes the 1.88 Ga Biwabik Iron-Formation and also because of its resemblance to granitic complexes, or "granophyres", of the Midcontinent Rift (White, 1954). The geochemistry and petrography of the Aurora sill, however, are quite different from the Midcontinent Rift (MCR) granophyres (Phillips et al., 2000). The isotopic data presened ho indicate that the Aurora sill has a Keweenawan age and represents a unique occurrence of this rock type in this portion of the MCR. Mineralogically, the Aurora sill is composed of albite, potassium-feldspar, aegerine, fibrous amphibole, chlorite, Fe-Ti oxides, Fe sulfide, and zircon and is classified as a syenite (< 5% normative quartz or nepheline) based on both modal and normative compositions. The presence of two feldspars indicates subsolvus crystallization and high P1120. Albite is the predominant mineral in the sill and is aligned in fine-grained samples to produce a trachytic texture. Samples from the Aurora sill exhibit limited geochemical variation (Si02=54.6-60.7 wt. percent; Mg#=0.4-0.75; Y=9-19 ppm) suggesting that the intrusion has undergone relatively minor internal fractionation. A large sample from the Aurora sill yielded only a few small (60-80 jim) anhedral zircons. No crystal faces were present on any of the zircon grains but it is unknown whether this is due to fracturing during processing or resorption of the grains. Four U-Pb zircon analyses (each consisting of 3 small zircons) yield a highly discordant regression with an upper intercept of 2970 ± 257 (2; Figure 1). It is improbable that this date represents the age of the sill because it intrudes the Biwabik Iron-Formation that was deposited about 1.88 Ga(Fralick et al., 1998). Our interpretation is that the zircons grew in the Mesoproterozoic with significant inheritance from Archean basement rocks in the region. The lower intercept of 1190 ± 466 Ma indicates either zircon growth or a Pb loss event in the Mesoproterozoic that may have coincided with intrusion of the sill. Nd isotopes provide further evidence of a Keweenawan age for the Aurora sill. A slightly negative epsilon Nd value of -1.4 results when an age of 1,100 Ma is assumed for the Aurora sill (Figure 2). If older ages are used, they result in unrealistically high ENd values (ENd = +9 @ 1,800 Ma; ENd +23 @ 2,900 Ma). The Nd isotopic results corroborate a Keweenawan age (1 .1 Ga) for the sill and are consistent with the lower U-Pb zircon intercept. If the Aurora sill is Keweenawan in age, the sill would represent a relatively rare type of alkaline 0.6 magmatism in this part of the MCR. The sill is distinct from the granophyric complexes of the northwestern limb of the MCR in several respects. The granophyric 0.5 textures and modal quartz that are typical of felsic rocks of the MCR are not present in the Aurora sill. The sill is saturated to weakly undersaturated in silica and 0.4 contrasts sharply with the silica saturated granophyres of the MCR. Furthermore the Aurora sill is characterized by high total alkalis, Nb/Y, and Ce/Zr compared with felsic rocks of the MCR. The Aurora 0.3 sill also lacks the negative Nb anomalies that are 0.2 characteristic of the MCR granophyres. Although there are no known Keweenawan syenites in the vicinity of the Aurora sill, there are several alkaline intrusions in 01 the northern portion of the MCR. The Coldwell Complex and Killala Lake Complex both contain 7Pb* 1235U syenites that have been dated at 1108 ± I Ma, near the Figure 1 80 �beginning of continental rifting (Heaman and Machado, 1992). Preliminary examination of the geochemistry of these syenites displays similarities to the Aurora sill. For example, they exhibit high Nb/Y ratios and follow similar trends on many geochemical plots (e.g., Na20+K20 versus Si02) and have broadly similar ENd values to the early E Nd (T) gabbro phases from the Coldwell complex (Heaman and Machado, 1992). Unlike the Coldwell and Killala Lake complexes, the Aurora sill is not known to be associated with carbonitites. The Nd isotopic and trace-element signatures of the Aurora sill suggest that it was derived from a mantle melt that did not interact with older, evolved crustal materials. However, this Age (Ga) Figure 2 . interpretation is not fully consistent with the U-Pb zircon data which clearly indicate inheritance of older zircons. It is difficult to imagine a process whereby zircons are inherited from the extant Archean crust without that crust contributing a highly unradiogenic Nd isotopic signature (negative ENd values) to the sill. This could occur if the assimilant was of low concentration and/or near-chondritic Sm/Nd ratios, but neither is a common characteristic of zircon bearing lithologies. One possibility is that the zircons in the Aurora sill have been derived from small degrees of assimilation of Biwabik Iron-Formation or related Animikie sediments. These sediments have variable but generally less negative E values in the Mesoproterozoic (Hemming et al., 1995) and also contain Archean detrital zircons. The age of the Aurora sill is important to understanding the timing of ore formation in the Mesabi Range. In 1999, Morey proposed a conceptual model in which the high-grade ores formed in a regional ground-water system during Paleoproterozoic time (1.6-2.5 Ga). Subsequently, Graber and Strandlie (1999) argued that nearby high grade ores must have formed in Mesoproterozoic or later time, because the Aurora sill appears to have controlled ore-forming processes in nearby mines. Since a Keweenawan Aurora sill could not have had an effect on ore-formation in the Paleoproterozoic, it is not possible at this time to confirm the model described by Morey (1999). References Cited Fralick, P.W., S.A. Kissin, and D.W. Davis, 1998, The Age and Provenance of the Gunflint Lapilli Tuff. Institute on Lake Superior Geology, Proceedings and Abstracts, p. 66-67. Graber, Ronald G. and Alan J. Strandlie, 1999, Where are the Metamorphosed Natural Orebodies of the Mesabi Range? Institute on Lake Superior Geology, Proceedings and Abstracts, p. 17-19. Heaman, L.M. and Machado, N., 1992, Timing and origin of the Midcontinent rift alkaline magmatism, North America: Evidence from the CoIdwell complex. Contributions to Mineralogy and Petrology, V 110, p. 289-303. Hemming, S.R., S.M. McLennan, and G.N. Hanson, 1995, Geochemical and Nd/Pb isotopic evidence for the provenance of the Early Proterozoic Virginia Formation, Minnesota. Implications for the tectonic setting of the Animikie basin, Journal of Geology, V 103, p. 147-168. Morey, G.B., 1999, High-Grade Iron Ore Deposits of the Mesabi Range, Minnesota Product of a Continental-Scale Proterozoic Ground-Water System. Economic Geology, V 94, p. 133-142. Phillips, E.H., 2000, Petrogenesis of the Enigmatic Aurora Sill, Mesabi Range, Minnesota, Unpublished honors thesis, Macalester College, St. Paul, MN. Phillips, E.H., K.R. Wirth, and G.B. Morey, 2000, Petrogenesis of the Enigmatic Aurora Sill, Mesabi Range, Minnesota. Institute on Lake Superior Geology, Proceedings and Abstracts, p. 51-52. White, David A., 1954, The Stratigraphy and Structure of the Mesabi Range, Minnesota. Minnesota Geological Survey Bulletin 38. Minneapolis: The University of Minnesota Press, p. 63-66. 81 �Freeze/Thaw Testing of Carbonate Aggregate Sources in Wisconsin — A Status Report Daniel D. Reid, Wisconsin Department of Transportation, 3502 Kinsman BlvcL, Madison, Wisconsin 53704-2507 Delamination and deterioration of exposed pavement aggregates has been a common occurrence on highways in southern and northeastern Wisconsin, and has lead to highway maintenance problems in some areas. The principal cause of these problems is crushed stone aggregate produced from Sinnipee Group (Galena, Decorah and Platteville Formations) rock. Prior to 1999, the Wisconsin Department of Transportation (WisDOT) excluded entire formations and members of the Sinnipee Group as a way of mitigating pavement problems. A study of carbonate aggregate resources in Wisconsin, conducted jointly by WisDOT and the Wisconsin Geologic and Natural History Survey (WGNHS), provided a comprehensive analysis of the stratigraphy and geologic properties of Sinnipee Group aggregate resources. This study identified significant regional variability in Sinnipee Group rock, and concluded that the laboratory freeze/thaw test was the most effective method of identifying problem aggregate sources. Based on this data, WisDOT concluded that specifications excluding certain formations and members were not appropriate for uniform application across the entire state. As a result of the WisDOT/WGNHS carbonate aggregate study, WisDOT developed and implemented specifications for freeze/thaw testing of carbonate aggregate sources used in pavements and bridge decks in October 1999. These specifications mandated freeze/thaw testing in counties where Sinnipee Group rock outcrops, and set the threshold for loss at 18% by weight. Included in the specifications was a clause that allows WisDOT to waive freeze/thaw testing for existing aggregate sources determined to be in the Silurian System or Prairie du Chien Group of carbonate rocks. WisDOT has now completed over 230 independent freeze/thaw tests on carbonate aggregate source material throughout Wisconsin. To date, the results indicate that freeze/thaw testing is performing it's intended function. A decrease in pavement problems has been reported and WisDOT now has an effective method of controlling aggregates that produce excessive delamination. As an added benefit, aggregate producers are now permitted to use aggregate sources that were excluded prior to establishment of the freeze/thaw testing specification, so long as material from these sources tests under the 18% threshold. References: - AASHTO, 1996, Standard Specification for Soundness of Aggregates by Freezing and Thawing, American Association of State Highway and Transportation Officials (AASHTO) Designation T 103-91. Brown, Bruce A., 1999, Aggregate Resources of the Sinnipee Group in Eastern and Southern Wisconsin, Wisconsin Geologic and Natural History Survey Open-File Report 1999-07. Ostrom, M.E., 1967, Paleozoic Stratigraphic Nomenclature for Wisconsin, Wisconsin Geological and Natural History Survey Information Circular No. 8. 82 �___ A Metamorphosed Evaporite Sequence from the Sibley Basin Rogala, B. and Fralick, P.W., Department of Geology, Lakehead University, Thunder Bay, Ontario The Sibley Group sediments were deposited in a subsiding infracratonic basin (Fralick and Kissin, 1995), between 1339 ± 33 Ma (Franklin et aL, 1980) and 1537 +10-2 (Davis and Sutcliffe, 1984). The Group is divided into three main Formations: Pass Lake, Rossport, and Kama Hill, representing deposition in a braided fluvial-mudflatplaya environment (Cheadle, 1986). This study concentrates on a section through a lateral correlative of the cyclic facies contained in the Rossport Formation present in the Noranda drill core NI-92-5. The location of this drill hole is north f iier cored sections and outcrops of the cyclic facies and represents a more basin center environment. The cyclic facies to the south consists of alternating layers of dolomite and red shale with individual layers, in the approximately 40 m thick assemblage, varying from mm- to dm- In drill hole NI-92-5 the layering is at a similar scale, but is composed of alternations between what was first thought to be dolomite-rich and gypsum-rich scale. intervals, reflecting wet and dry seasonality in the central playa environment. SEM and XRD analysis indicate that the sequence was metamorphosed. The progression of metamorphic facies, from lower to higher T towards the diabase sill, is represented by the following reactions: 3CaMg(C03)2 + 4SiO2 +1H20 —' lMg3Si4Oio(OH)2 + 3CaCO3 + 3CO2 and 5Mg3Si4O10(OH)2 + 6CaCO3 + 45i02 —. 3Ca2Mg5Si8O22(OH)2 + 6C02 + 2H20 or 2Mg3Si4Oio(OH)2 +3CaCO3 - lCa2Mg5Si8O22(OH)2+ lCaMg(CO3)2 + 1CO2 +1H20 or 5CaMg(C03)2 + 8SiO2 + 1H2O — lCa2Mg5Si8O22(OH)2 + 3CaCO3 + 7CO2 (Winkler, 1974) Near the diabase sill pargasite, a hornblende with the composition (Na,K)01Ca2Mg4A13Si6O22(OH), is the dominant mineral. Clinochlore, a chlorite mineral with a composition of Mg5Al5Si3Oio(OH), is found throughout the metamorphic series. Both of these minerals are common in magnesian carbonate metamorphism (Pattison and Tracy, 1991). Primary layering has been masked in places by metamorphism. However, S.E.M. analysis clearly shows the mineralogical layering, reflecting variations in primary geochemical constituents between individual laminae. ICP-AES analysis also highlights the varying compositions of the sequence. CafMg ratios indicate the precipitation of gypsum in many layers. This is supported by the observation of gypsum using S.E.M.. The pargasite tends to exist in layers with differing K and Na proportions. The K and Na ions may reflect the incorporation of KCI and NaC1. Both of the minerals are found in S.E.M. sections taken near the middle of the sequence. 83 �Clastic input is variable between individual layers, but shows an increase at the top of the section. This increase is related to the appearance of sand sheets, which typically mark the end of the cyclic facies in the Sibley Group sediments. Clastic material becomes slightly enriched in elements denoting a mafic rock source up-section. The source material is distinctly alkalic. Alkalic material may be associated with plume activity, which would fit the infracratonic theory for the formation of the Sibley Basin. However, suitably alkalic rocks have not been found in the area. References Cheadle, B.A. 1986. Alluvial-playa sedimentation in the lower Keweenawan Sibley Group, Thunder Bay District, Ontario. Canadian Journal of Earth Sciences, 23, 527-542. Davis, D.W. and Sutcliffe, R.H. 1984. U-Pb ages from the Nipigon Plate and Northern Lake Superior. Geological Society of America Bulletin, 96, 1572-1579. Fralick, P. and Kissin, S. 1995. Mesoproterozoic basin development in central North America: implications of Sibley Group volcanism and sedimentation at Redstone Point. in: Petrology and metallogeny if volcanic and intrusive rocks of the midcontinent rift system, Proceedings of the International Geological Correlation Program, Project 336. Franklin, J.M., Mcllwaine, W.H., Poulsen, K.H. and Wanless, R.K. 1980. Stratigraphy and depositional setting of the Sibley Group, Thunder Bay District, Ontario, Canada. Canadian Journal of Earth Sciences, 17, 633-651. Pattison, D.R.M. and Tracy, R.J. 1991. Phase equilibria and thermobarometry of calcareous, ultramafic, and mafic rocks, and iron formations. In: Kerrick, D.M. (ed.), Contact Metamorphism. Reviews in Mineralogy, 26, Mineralogical Society of America. Winkler, H.G.F. 1974. Petrogenesis of Metamorphic Rocks. Springer-Verlag: New York Inc., 320 p. 84 �Roles of Fractional Crystallization and Assimilation in the Production of Midcontinent Rift Granophyres. Sandland, Travis 0., (tsandland@Macalester.edu) & Wirth, Karl R., Geology Department, Macalester College, St. Paul, MN, 55105; Vervoort, Jeff D. & Gehrels, George E., Department of Geosciences, University of Arizona, Tucson, AZ, 85721; Kennedy, Bryan C., Geology Department, Macalester College, St. Paul, MN, 55105; Harpp, Karen S., Department of Geology, Colgate University, Hamilton, N 13346 The granitic complexes of the Midcontinent Rift (MCR), commonly termed granophyres, comprise a significant portion of the Duluth Complex in northern Minnesota. They range in size from 30 to 150 km2 in surface area and are ito 2 km in thickness. They consist of basal diorite and monzodiorite and progress upward to quartz monzodiorite, granodiorite, and granite. This study focuses on the petrogenesis of four of these complexes: the Greenwood Lake, Misquah Hills, Eagle Mountain, and Pine Mountain granophyres. Miller and Vervoort (1996) identified two magmatic stages in this part of the MCR. An "early stage" from 1108-1105 Ma, and a "main stage" from 1100-1094 Ma. The four granophyre complexes addressed in this study have U-Pb zircon ages consonant with this chronology. The older granophyres include the Misquah Hills and Greenwood Lake complexes with ages of 1106±6 Ma and 1106±3 Ma (±2 sigma), respectively, and were emplaced during the early stage of the rift. The Eagle Mountain and Pine Mountain granophyres have ages of 1098±4 and 1095±4 Ma, respectively, and were emplaced during the main magmatic stage. The granophyre complexes vary widely in composition (47-76 wt. % Si02) and plot as linear Figure 1 trends on many Harker variation diagrams. The two granophyre groups (early and main stage) are indistinguishable on such plots and appear to have ci very similar major element chemistry. Incompatible trace elements (including REEs) are enriched in both groups (Fig. 1) although the early stage granophyres generally have higher concentrations than the late stage granophyres. Incompatible trace element ratios (e.g., LaISm, Gd! Yb) are similar for both groups. The early stage granophyres have initial epsilon Nd values between 20 0 and -2. In contrast, the main stage granophyres are isotopically enriched, with initial epsilon Nd Ce values between -3 and -8. The isotopic and incompatible element data suggest both fractional crystallization (FC) and La Pr Tb Ho Tm Lu Nd Sm Gd Dy Er Yb Eu assimilation fractional crystallization (AFC) processes were involved in the evolution of the granophyres. The large negative epsilon Nd values associated with the main stage granophyres indicate AFC processes and suggest contamination by an isotopically enriched source, possibly felsic Archean crustal rocks. This trend can be seen on a graph of epsilon Nd vs. La/Yb (Fig. 2). The main stage granophyres also 85 �have low Nb/Y ratios, a signature of contamination by crustal materials, whereas the Nb/Y ratios of the early stage granophyres are positively correlated with La/Sm (Fig. 3). If AFC processes were active during the formation of the early stage granophyres, the assimilant was likely limited to juvenile mafic rocks with little or no isotopic enrichment. Alternatively, the enrichment of incompatible elements in the early stage granophyres could be the result of fractional crystallization. Figure 2 Figure 3 20 2 FC AFC 10 - Early Stage. Main Stages 0 I -8 -7 I I I -6 -5 -4 I -3 -2 0 I -l La/Sm Epsilon Nd These data support the model of rift evolution as presented by Vervoort and Green (1997). The early stage granophyres were formed either by fractional crystallization, or by AFC processes with contamination from a high Sm/Nd mafic crust. During this stage of rift evolution, the middle to upper crust was probably relatively cold, so assimilation likely occurred in the lower crust due to local heating as a result of plume initiation. The main stage granophyres are isotopically contaminated and suggest AFC processes at work. During this time period, renewed rifling resulted in higher ambient temperatures in the crust, and allowed melting of older, low Sm/Nd, negative epsilon Nd sources, possibly at middle to upper crustal levels. References: Miller, J.D., Vervoort, J.D., 1996, The latent magmatic stage of the Midcontinent rift: a period of magmatic underplating and melting of the lower crust. in 42" Annual Meeting of the Institute on Lake Superior Geology, Cable, Wis., May 1996, Proceedings volume 42, pp 33-35. Vervoort, J.D., and Green, J.C., 1997, Origin of evolved magmas in the Midcontinent rift system, northeast Minnesota: Nd-isotope evidence for melting of Archean crust: Canadian Journal of Earth Sciences, v. 34. p. 52 1-535. 86 �DIRECT TIMING CONSTRAINTS ON PALEOPROTEROZOIC METAMORPHISM, SOUTHERN LAKE SuPERIOR REGION: RESULTS FROM SHRIMP U-PB DATING OF METAMORPHIC MONAZITES Schneider, D.A., Syracuse University, Syracuse, NY, 13244; Hoim, D.K., Kent State University, Kent,OH, 44242; Hamilton, M.A., Geological Survey of Canada, Ottawa, ONT, K1A 0E8 The age of the Penokean Orogeny has been firmly established for decades by U-Pb pluton age data. The timing of post-orogenic cooling and lower temperature overprinting has only been established more recently by thermochronologic investigations. Little information exists however on the timing of the initial higher-grade metamorphism which affected the southern Lake Superior region during the Paleoproterozoic. Such information represents a critical missing link in our understanding of the tectonothermal evolution of the crust during and after Penokean orogenesis. Monazite, a REEphosphate accessory mineral, grows as a common metamorphic phase under amphibolite and higher-grade conditions. Monazite U-Pb ages typically yield reliable estimates for the timing of peak metamorphism, although post-metamorphic deformation and fluid flux can cause lower-temperature monazite dissolution and reprecipitation. We utilized the SHRIMP at the GSC in Ottawa to obtain geochronologic information on distinct mineral domains (e.g., core vs. rim) which may have resulted from particular tectonothermal events. Through BSE imaging prior to analyses, we found rim textures spatially distinct from interior replacement textures. Age data are indistinguishable from each domain and replacement probably resulted from the same thermal event as the rim neogrowth. Age data summarized below include only concordant or near concordant analyses and are reported as weighted average 207Pb/206Pb ages. In this pilot study we separated monazite from an amphibolite grade Paleoproterozoic metasedimentary unit in east-central Minnesota (Kettle River locality) and from an Archean gneiss unit within the Peavy district, northern Michigan (Foster City locality). From Kettle River, a sample of garnet schist, which attained peak metamorphic conditions of 470-520°C at 5-6 kbar, yielded distinct populations of core and rim U-Pb ages from 5 monazite grains. Eight analyses of core domains yielded a U-Pb monazite crystallization age of 1834.0 ± 6.1 Ma, reflecting primary metamorphic growth. A population of monazite rimlreplacement spot analyses, characterized by a higher Th content, yielded an age of 1792.9 ± 4.3 Ma. This latter age is similar to the --1799 Ma U-Pb crystallization age of the deformed Hillman tonalite (Van Schmus et aL, 2000, ILSG and this volume). From Foster city (>500°C and -4 kbar), an Archean muscovitebiotite quartzofeldspathic gneiss yielded a monazite core age of 1834.4 ± 6.0 Ma from 10 spots on 11 grains. Rimlreplacement domain analyses from 9 spots yielded a younger age of 1809.8 ± 6.3 Ma, consistent with high-temperature cooling in the region (1800 and 1785 Ma Ar-Ar hornblende ages; Mancuso et al., ILSG, 1997). 87 �These metamorphic U-Pb ages are the first reliable metamorphic dates from this Paleoproterozoic orogen and indicate a widespread thermal pulse at 1835 Ma in response to accretion-induced crustal thickening. Albeit preliminary, the age of metamorphism is remarkably consistent across the orogen. High temperature Ar-Ar cooling ages on hornblende (—500°C) from north of the Watersmeet dome (—1822 Ma; Schneider et a!., 1996, CJES) and biotite Ar-Ar dates from low-grade structural panels from east-central Minnesota (1840-1830 Ma; Schweitzer et al., 2000, ILSG) are compatible with an —1835 Ma orogenwide metamorphic episode. Further accessory mineral geochronology holds promise for better elucidating the timing of syn-orogenic metamorphism, as well as overprinting post-orogenic events throughout the southern Lake Superior region. Foster City Archean mu-bi quartzofeldspathic schist 0.35 -a a. (p 0 0.33 1: 0.31 0.29 0.27 4.2 4.4 0.33 0.31 0.29 0.27 4.2 4.4 4.6 88 4.8 5.0 5.2 5.4 5.6 �RESULTS OF IGNEOUS THERMOMETRY AND BAROMETRY ON THE EAST-CENTRAL MINNESOTA BATIIOL1TH: EVIDENCE FOR POST-EMPLACEMENT EXHUMATION AND COOLING SCHWEiTZER, D., and HOLM, D., both at Dept of Geology, Kent State University, Kent; OH, 44242; VAN SCHMUS, W.R., Dept. of Geology, Univ. of Kansas, Lawrence, KS, 55045; BOERBOOM, T., Minnesota Geological Survey, 2642 University Avenue, St Paul, MN The internal zone of the Penokean orogen in east-central Minnesota was invaded by abundant plutonism associated with emplacement of the East-Central Minnesota Batholith (ECMB) from 1787 to 1772 Ma (Van Schmus et al., 2000). Previous application of the Aluminum-in-Hornblende (AH) igneous barometer on phases of the ECMB indicate paleodepth estimates that increase from -13 km in the north (Ka-F, K-4; Fig 1) to —18 km in the southeast near St Cloud (SC, Fig 1). HoIm aed others (1998) inferred the depth differences to reflect intrusion of different magma bodies into a rapidly unroofing terrane. This interpretation predicts that progressively younger intrusions would have been emplaced into progressively shallower crustal levels. At the time, their hypothesis was limited by the lack of any precise age data on the intrusions analyzed. The new ECMB U-Pb age data provides the framework to directly test this hypothesis of syn-emplacement exhumation. We selected six new samples for further All barometric work (starred localities, Fig 1). Anderson and Smith (1995) outlined the important role that temperature plays in the Al concentration in homblende rims and proposed a temperature correcting calibration for the application of the All barometer. For this reason, the Hornblende-Plagioclase (HB-PL) thermometer (Blundy and Holland, 1992) was applied to these samples in order to derive a crystallization temperature estimate for each sample. The Schmidt (1992) pressure calibration is presented in order to compare the effects of the temperature correction. The electron microprobe at the University of Maine, Orono was used to obtain compositional data used for both the All barometer and HB-PL thermometer. The results of these analyses are summarized in the following table. Table 1: Summary of igneous thermometry and barnmetry results Schmidt, 1992 A and S, 1995 An Content B and H, 1990 kbar kbar Temperature (°C) (%) Sample ID Rock Unit EC-31 EC-15 DS-99-17 EC-1 EC-35 RFG-7 Granodiorite Mafic Plug Anne Lake Granite Watab Quartz Dionte Granodiorite Reformatory Granodiorite 789 ± 38 49.99 59.35 31.29 40.01 43.77 51.31 884±38 745 ± 38 6% *38 746±38 783 ± 38 9.1 ± 0.6 13.4 ± 0.6 6.0 ± 0.6 5.2 ± 0.6 8.5 ± 0.6 6.4 ± 0.6 <6.5 ± 0.6 <6.2 ± 0.6 4.9 ± 0.6 5.0 ± 0.6 <7.2 ± 0.6 4.4 ± 0.6 Assuming an average overburden density of 2.7 g/cc, the pressures recorded by EC-31, EC-15, and EC-35 correspond to maximum emplacement depths of 23-27km. These appear to be anomalously high when compared to other rocks in the area and probably reflect artificial Al enrichment of hornblende crystal rims due to high An content. Samples RFG-7 (17 lan), EC-1(18 km), and DS-99-17(18 km) when corrected for temperature effects appear to preserve viable estimates since they are consistent with prior barometric results. In the north, samples Ka-F and K-4 (Fig. 1) both preserve emplacement depths of 13 km but their UPb ages are 15m.y. apart. To the south, all samples near St Cloud record —17-18 km depths regardless of age. This implies that very little uplift occurred during emplacement of the ECMB. The bulk of post-tectonic exhumation appears to have occurred after the emplacement of the ECMB. Preserved pressure differences probably reflect deeper emplacement depths (from N to 5) across the batholith. Post-emplacement exhumation is consistent with cooling ages being 10-20 m.y. younger than ciystallization ages from the batholith (Holm et al, 1998; Hohn and Lux, 19%). The ECMB was likely completely exhumed prior to deposition of the Early Proterozoic (1750-1650 Ma) red quartzites. 89 �MSD: \ ,.- H It Is MN-29 :-::•::-: 18±2km I— A l P16 (MSD) ,c—I___ -. ', / -, — _s. \ I_,>. — — -%.— •5_ , / 5—— '11787* ' I<23*2km s. ,,-I, ' 5. Is ; '___-\ I_/'-'/ — I S —' — I -. Is * I <24t2kI — c-i J , — ;, _, __'_l 10 km F/ /' , _s— -, I, / — .,- —. — I / \ 'I'—'' ' — —' - Figure 1: Simplified map of the internal zone of the Pekonean orogen. Localities marked with filled circles represent samples analyzed by Hotm et at (1998). Starred localities denote samples analyzed in this study. MSD = Malmo Structural Discontinuity; SC = St. Cloud; LF = Little Falls. U-Pb ages after Van Schmus et at (2000). References Anderson, J.L, and Smith, D.R, 1 995,The effects of temperature and oxygen fugacity on the Al-in -hornblende barometer: American Mineralogist, v.80, p.549-559. Blundy, J.D., and Holland,TJ.B, 1990, Calcic amphibole equilibria and a new amphibole-plagiodase geothermometer: Contributions to Mineralogy and Petrology, v.104, p.208-224. HoIm, D.K.and Lux, D.R, 1996, Core complex model proposed for gneiss dome development during collapse of the Paleoproterozoic Penokean orogen, MN: Geology, v.24, p.343-346. Holm, D.K., Darrah, KS., and Lux, D.R., 1998, Evidence for widespread —.1760 Ma metamorphism and rapid stabilization of the Early-Proterozoic (1870-1820 Ma) Penokean orogen, MN: Am. J. of Science, v.298, p.60-81. Schmidt, M.W., 1992, Amphibole compoisition in tonalite as a function of pressure: an experimental calibration of the Al-in-hornblende barometer: Contributions to Mineralogy and Petrology, v.1 10, p. 304-310. Van Schmus,W.R., MacNeill, LC., Holm, D.K, Boerboom,TJ., and isa, M.A., 2000,The 1787-1772 Ma east-central Minnesota batholith: Precursor to crustal stabilization in the Lake Superior region: ILSG proceedings, 46th annual meeting, Marquette, Ml, v.46, p.65-66. 90 �A SYNOPSIS OF ARCHEAN AND PROTEROZOIC PLATINUM GROUP ELEMENT MINERALIZATION IN THE THUNDER BAY DISTRICT, ONTARIO SMYK, Mark C., MASON, John K. and SCHNIEDERS, Bernie R., Ontario Geological Survey, Ministry of Northern Development and Mines, Suite B002, 435 James St. South, Thunder Bay, ON P7E 6S7, and STOTT, Greg M. Ontario Geological Survey, Ministry of Northern Development and Mines, Willet Green Miller Centre, 933 Ramsey Lake Road, Sudbury, ON P3E 6B5 Platinum group element (PGE) occurrences and deposils are in both Neoarchean and Mesoproterozoic mafic to ultramafic intrusive rocks in the Thunder Bay District. Recent, dramatic increases in platinum group metal prices have prompted renewed interest in PGE exploration, resulting in the discovery of hitherto unknown hosts and styles of mineralization. Archean There are four broad, temporally diverse settings for Neoarchean PGE mineralization: (1) Pre-tectonic, mafic to ultramafic, subvolcanic(?) intrusions intimately associated with greenstone belts of various ages in the Wawa and Wabigoon subprovinces; (e.g. Haines Gabbro (Shebandowan belt / Wawa; 2722 Ma); Core Zone gabbro (Obonga Lake belt / Wabigoon; 2733 Ma); (2) Post-tectonic, mafic to ultramafic intrusions (Ca. 2692 Ma), related to late plutonism in the Wabigoon Subprovince, hosted by gneissic tonalite-granodiorite (e.g. Lac des Iles and Tib Lake complexes; Buck Lake, Legris Lake, etc.); (3) Syn- to post-tectonic, mafic to ultramafic intrusions (a.k.a. "Quetico-type"; Ca. 2680 to 2688 Ma) hosted by Quetico subprovince metasedimentary rocks (e.g. Samuels Lake, Kawene, Nym Lake, Chief Peter Lake, North Elbow Lake, etc.); and (4) Mafic intrusive rocks occurring within syn- to post-tectonic, diorite-monzodiorite-monzonite suites with sanukitoid affinity (ca. 2680 to 2685 Ma), within the Wabigoon, Quetico and Wawa subprovinces (cf Stern eta!. 1989) (e.g. Roaring River Complex; Entwine Lake). Disseminated to locally net-textured chalcopyrite, Fe-sulphides, pentlandite and magnetite typically characterize PGE-mineralized zones, which are commonly associated with intrusive contacts, polyphase intrusive breccias, as well as sheared and hydrothermally altered zones. Proterozoic Mesoproterozoic intrusive rocks associated with the Midcontinent Rift locally range in age from Ca. 1108 Ma (e.g. reversely polarized Coldwell alkaline complex; Logan diabase) to ages younger than the magnetic polarity reversal that occurred between 1105 and 1102 Ma (Davis and Green 1997). A tabulated synopsis is provided below: 91 �Intrusion I Lithology Mineralization Style Local Examples (Associated PGE-Miizeralized Areas) Disseminated sulphides and native metals in lherzolite, dunite, peridotite,_etc. Disseminated and blebby sulphides in medium- to coarsegrained, varied-textured (a.k.a._'taxitic')_gabbro Disseminated sulphides in Crspinel-bearing cumulate layers Disseminated, intergranular sulphides, fracture fillings Disseminated sulphides in medium- to coarse-grained Leckie Lake (Wolf Mountain); Hele Township; Eva-Kitto townships Normal Magnetic Polarity <(1005-1102 Ma): U Layered(?) picritic ultramafic intrusions Layered gabbro-anorthositic intrusions: Crystal Lake Gabbro Pine Point - Mount Mollie Gabbro Pigeon River and Arrow River diabase dykes Crystal Lake Gabbro (Great Lakes Nickel deposit) (Cr-spinel-bearing, anorthositic gabbro above Cu-Ni deposit) (Mount Mollie; Pine River) (Wallenius; Naomi Island; Jarvis Point) gabbro_and_diabase Reversed Magnetic Polarity (ca. 1108 Ma): Logan diabase sills, cone sheets Tholeiitic to alkaline complexes: CoIdwell alkaline complex Killala Lake alkaline complex I Sparse, disseminated sulphides Disseminated sulphides in medium- to coarse-grained, van-textured (a.k.a. taxitic) gabbro Disseminated sulphides in massive Fe-Ti-oxide cumulate layers (Numerous) Coldwell Two Duck Lake gabbro (Marathon deposit); Geordie Lake gabbro Eastern Border gabbro (Skipper Lake zone) Killala Lake Border gabbro (Sandspit Killala) (Unknown) The development of tectono-magmatic models for these various suites of intrusions is the focus of ongoing research as part of the Ontario Geological Survey's Operation Treasure Hunt. These new data will provide new insights into late Archean subprovince accretion in the Superior Province, as well as the development of the Midcontinent Rift. They will help to elucidate possible links between the age, geochemistry and setting of these intrusive rocks and PGE mineralization processes in order to generate new exploration targets. References Davis, D.W. and Green, J.C. 1997. Geochronology of the North American Midcontinent rift in western Lake Superior and implications for its geodynamic evolution; Canadian Journal of Earth Sciences, V.34, p.476-488. Stern, R.A., Hanson, G.N. and Shirey, S.B. 1989. Petrogenesis of mantle-derived, LILE-enriched Archean monzodiorites and trachyandesites (sanukitoids) in southwestern Superior Province; Canadian Journal of Earth Sciences, v.26, p.1688-1712. 92 �PLATINUM GROUP ELEMENT EXPLORATION IN NORTHWESTERN ONTARIO SMYK, Mark C., S11IWART, Jennifer and O'BRIEN, Mark S. Ontario Geological Survey, Ministry of Northern Development and Mines, Suite B002, 435 James St. South, Thunder Bay, ONP7EÔS7 During the past two years there has been a marked increase in exploration for the platinum-group elements (PGE) In iiorthwestern Ontario. This interest has been driven by a dramatic rise in the prices of platinum and palladwm. Renewed PGE exploration in northwestern Ontario has also been significantly influenced by the sticcess of North American Palladium Ltd.'s Lac des Ties Mine which produced 95 116 ounces of -palladium at àcash cost of US$142 per ounce in 2000. Recent exploration efforts at Lac des lies have increased 'the measured and indicated resource to 145 600 000 tonnes at an average grade of 1.57 glt Pd, 0.17 g/t Pt, 0.12 g/t Au, 0.06% Cu and 0.05 % Ni. There were over 100 PGE exploration programs in northwestern Ontario in 2000, accounting for more than half of all mineral exploration in the region. These exploration efforts have focused on both Archean and Proterozoic mafic to ultramafic intrusions, leading to the discovery of many occurrences in areas andlor intrusions not previously known to host PGE mineralization. PGE occur in two main geological settings: (1) In Archean pre- to post-tectonic, mafic to ultramafic intrusions. Mineralization may be associated with: • Disseminated "sparse" sulphides in gabbro and ultramafic rocks (e.g. Lac des Ties complex and satellite intrusive complexes), and localized by: • Zones of hydrothermal alteration • Heterolithic, igneous breccia zones Contact zones and tectonic structures • • (Semi-) massive, sheared, copper-nickel sulphide deposits (e.g. Shebandowan and Thierry mines) in ultramafic rocks • Sheared ultramafic rocks with copper-nickel sulphides intruding banded iron formation (e.g. Trout Bay) • Chromitite within layered ultramafic complexes (e.g. Big Trout Lake, Chrome Lake?) (2) In Mesoproterozoic, Midcontinent Rift-related (ca. 1108-1102 Ma), mafic to ultramafic intrusions near Lake Superior and Lake Nipigon. Mineralization may be associated with: • Disseminated suiphides in coarse-grained to pegmatitic, van-textured ("taxitic") gabbro (e.g. Great Lakes Nickel; Marathon deposits) • Massive oxide (± sulphide) units in layered gabbro (e.g. Coidwell Eastern Gabbro (Ti-Fe-oxides); Great Lakes Nickel (Cr-spinel) • Disseminated sulphide ± native metals in olivine-rich, layered ultramafic rocks (e.g. Wolf Mountain) 93 �A NEW GRAVITY MAP OF WISCONSIN SNYDER, Stephen L., U.S. Geological Survey, MS 954 National Center, Reston, VA 20192, ssnyder@usgs.gov; ERVIN, C. Patrick, Dept. of Geology and Environmental Geosciences, Northern Illinois University, DeKalb, IL 60115, pervin@niu.edu; GEISTER, Daniel W., Dept. of Geology and Environmental Geosciences, Northern Illinois University, DeKalb, IL 60115, z011561@students.niu.edu and DANIELS, David L., U.S. Geological Survey, dave(usgs.gov. A new Bouguer anomaly gravity map of Wisconsin has been created from more than 37,000 gravity measurements collected between 1948 and 2000. North of latitude 44° N., more than 28,000 stations were compiled by C.P. Ervin and M.E. Thompson. These data include stations from the Wisconsin Geological and Natural History Survey (WGNHS), the National Geophysical Data Center (NGDC), the Defense Mapping Agency (DMA), the U.S. Geological Survey (USGS), and Northern Illinois University. An approximate station interval of 1 mile (1.6 km) was established where possible, but limited access in some areas necessitated a greater spacing. Prior to 1999, the data south of latitude 44° N. totaled approximately 3800 stations from NGDC and from G. Randy Keller (University of Texas-El Paso). The current USGS effort, begun in 1999, is directed at upgrading the gravity coverage of the state south of latitude 44° N. with a station density comparable to that north of 44° N. To date we have added measurements at more than 6800 stations to achieve a nominal spacing of one to two miles. These new data replace the older data from the NGDC, which had a nominal station spacing ranging from 3 to 10 miles. Nearly all of the data are tied to the Wisconsin First-order Gravity Base Station Network, which is in turn tied to the International Gravity Standardization Network-1971 (IGSN-71). The 37,000 measurements were then gridded at an interval of 500 m. The map will be presented at a scale of 1:500,000. The rationale for upgrading the gravity coverage has been to provide higher resolution data to assist in the interpretation of the basement geology of southern Wisconsin, much of which is hidden by glacial and Paleozoic cover. The gravity data, along with aeromagnetic data, give clues to the structural evolution of the Precambrian crust, making the map an excellent tool for USGS mineral resource studies. Some notable features shown on this map include 1) the Midcontinent gravity high, which has one of the steepest gravity gradients in the conterminous US, 2) the flanking gravity lows corresponding to Keweenawan sedimentary basins, 3) gravity highs over inferred mafic plutons in southeastern Wisconsin, 4) a low centered on the Precambrian Wolf River batholith, but much larger in area, 5) gravity lows associated with granitic intrusives just south of latitude 46° N., and 6) several northwest trending linear features of unknown origin. The accompanying figure shows a shaded relief image of the gravity map. 94 �92° -89 -91 880 -87° .5 .15 H 21 .— .55 — 27 .— 30 .— 33 — 35— 35 I 42 * 45 — -45 * 51 54 * 50 53 -55 72 -51 500 milhigals 50 50 0 klometres NA027/ LGC-90 95 �POST-RIFT EVOLUTION OF THE MIDCONTINENT RIFT SYSTEM: SOME NUMERICAL EXPERIMENTS Soofi, M. A., and King, S. D., Department of Earth and Atmospheric Sciences, Purdue University, West Lafayette, Indiana 47907 The Midcontinent Rift system (MCR) is a major geological and geophysical feature of North America. This 1.1 b.y. old feature (Nicholson and Shirey, 1990; Klewin and Shirey, 1992) is believe to have evolved first through the tensile forces of a rift origin and then through the compressive forces from the intraplate collision between North America and Grenville Tectonic Zone (GTZ) (Van Schmus and Hinze, 1985). The support for interaction between the MCR and GTZ come from the geophysical studies which reveal thrust faults and folds along the length of the MCR (e.g., Zhu and Brown, 1986; Chandler et al., 1989; Mariano and Hinze, 1994; Allen et al., 1997). Also, the time of thrusting along the MCR is constrain to be 1060 Ma (Bornhorst et al., 1988; Cannon et al., 1993) which is comparable to 1100-1060 Ma (Easton, 1992) as the duration of collision in Grenville Tectonic Zone. A quantitative study of the late-stage deformation of the MCR under the Grenville tectonism has been performed by Soofi and King (1999). They showed that the distance between the MCR and Grenville Tectonic Zone is appropriate for the two geologic provinces to interact. We expand on their findings and investigate the contribution of various factors involved in the collision between North America and the Grenville terranes, including: the size of colliding terranes of Grenville Tectonic Zone; the location of collision along the eastern and southern boundaries of North America; and the angle of convergence between North America and the Grenville terranes. We use a 2D, viscous, finite element model that treats lithosphere as a thin sheet with stresses averaged over its thickness (for computational method see Houseman and England (1986); for program location see the web site http : / /www.earth.monash.edu.au/Research/Basil). The model boundaries coincide with pre-Grenville collision boundary of North America. In the model, the MCR is considered as a low-strength block. We consider the present shape of the MCR as the shape at the time of collision with the Grenville terranes. The models are run to represent 21 Ma of convergence at the rate of 50 mm/yr. To constrain the model results we use geophysically observed uplifts along the reverse faults of the MCR (e.g., Zhu and Brown, 1986; Mariano and Hinze, 1994; Allen et al., 1997). These uplifts are compared with crustal thickening along the MCR in the model. Based on our results we conclude that the 3 to 8 km variation in uplift along the thrust faults of the western arm of the MCR is the consequence of size of the colliding terranes and location of the collision. The model results also suggest that deformation along the eastern arm of the MCR was comparable to that along the western arm. The reason we do not observe such deformation in geophysical studies (e.g., Zhu and Brown, 1986) is, perhaps, due to later surficial and/or tectonic processes. We also conclude that collision with Crenville Tectonic Zone was active along both the eastern and southern boundaries of North America. This resulted in the non-linear shape of the western arm of the MCR and may also played a role in the formation of the Belle Plaine fault. We do not observe any significant difference in the model results for different angles of convergence between North America and Grenville terranes. Consideration of other constraints in addition to the uplift along the MCR may help to determine the dominant angle of convergence between North America and Grenville Tectonic Zone. 96 �REFERENCES CITED Allen,D.J., Hinze, W.J., Dickas, A.B., and Mudrey, M.G.Jr., 1997, tnterMedgeophysical modeling of the North American midcontineñt rift system: New thterpretations for western Lake Superior, northwestern Wisconsin, and eastern Minnesota: Geological Society of America Special Paper 312, p. 47-71. Bornhorst, T J, Paces, J B, Grant, N K, Obradich D, and Thiber, NkK 1988, Ae f native copper mineralization, Ke-weenaw Peninula, Michigan: conmi (eiogy, v 83, p. 41,19-625 Cannon, W.F., Petermn, Z.E., and Sims, P.K., 193, Crustal- sca1ethrtisting and origin of the Montreal river monocline - A 35 km thick cross-section of the midcontinent rift in northern Michigan and Wisconsin: Tectonics, v. 12, p. 728-744. Chandler, V.W., McSwiggen, P.L., Morey, G.B., Hinze, W.J,, and Anderson, R.R., 1989, Interpretation of seismic reflection, gravity, and magnetic data across middle Proterozoic mid-continent rift system, northwestern Wisconsin, eastern Minnesota, and central Iowa: American Association of Petroleum Geologists Bulletin, v. 73, p. 261-275. Easton, R.M., 1992, The Grenville Province and the Proterozoic history of central and southern Ontario, in Thurston, P.C. Williams, H.R., Sutcliffe, R.H., and Stott, G.M., eds., Geology of Ontario, Ontario Geological Survey, Special volume 4, Part 2, p. 715-904. Houseman, G., and England, P., 1986, Finite strain calculations of continental deformation 1. Method and General results for Convergent zone: Journal of Geophysical Research, v. 91, p. 3651-3663. Klewin, K.W., and Shirey, S.B., 1992, The igneous petrology and magmatic evolution of the midcontinent rift system: Tectonophysics, v. 213, p. 33-40. Mariano, J., and Hinze, W.J., 1994, Structural interpretation of the Midcontinent Rift in eastern Lake Superior from seismic reflection and potential-field studies: Canadian Journal of Earth Sciences, v. 31, p. 619-628. Nicholson, S.W., and Shirey, S.B., 1990, Midcontinent rift volcanism in the Lake Supe- rior region: Sr, Nd, and Pb isotopic evidence for a mantle plume origin: Journal of Geophysical Research, v. 95, p. 10851-10868. Soofi, M.A., and King, S.D., 1999. A modified beam analysis effect of lateral forces on lithospheric flexure and its implication for post-rift evolution of the Midcontinent Rift System, Tectonophysics, v. 306, p. 149-162. Van Schmus, W.R., and Hinze, W.J., 1985, The midcontinent rift system: Annual Reviews of Earth and Planetary Science, v. 13, p. 345-383. Zhu, T., and Brown, L.D., 1986, Consortium for continental reflection profiling Michigan surveys: Reprocessing and results: Journal of Geophysical Research, v. 91, p. 11477-11495. 97 �THE COOL EARLY EARTH: OXYGEN ISOTOPE EVIDENCE FOR CONTINENTAL CRUST AND OCEANS ON EARTH AT 4.4 Ga VALLEY, JW*, PECK, WH, KING, EM, Dept. of Geology + Geophysics, Univ. of Wisconsin-Madison; GRAHAM, CM, Dept. of Geology + Geophysics, Edinburgh Univ., Scotland; and WILDE, SA, School of Applied Geology, Curtin Univ., Bentley, Western Australia, * valley@geology.wisc.edu Zircons preserve the best record of U-Pb crystallization age and oxygen isotope ratios of igneous rocks. The I8O of non-metamict zircon is unaffected even by hydrothermal alteration and high-grade metamorphism. Ion microprobe analysis of detrital zircons from the —3 Ga Jack Hills metaconglomerate (Narryer Gneiss Terrane, Yilgarn Craton, Western Australia) yield UPb ages from 3.1 to 4.4 Ga (Fig. 1, SHRIMP II, ref 1) and I8O from 5 to 8 %c (Cameca 4f, ref 2). The 18O of these zircons averages 6.3, and is 1 per mil higher than that in equilibrium with the mantle and that of normal Archean granitic zircons (Fig. 2; 5.3±0.3 %o, 5.5±0.4 %, respectively; ref 3). The distribution of mantle-like vs. mildly elevated 18O values for magmas is constant from 2.7 to 4.4 Ga, and on 4 continents (Fig. 2). The age of 4.404 ±0.008 Ga from one 200 im zircon is >99% concordant and represents the oldest recognized terrestrial material. This crystal is zoned in ö'80 (5.0±0.7% vs. 7.4±0.7%o) and REEs (La=0.3 to 13.6 ppm), and contains inclusions of Si02. REE patterns are HREE enriched with positive Ce and negative Eu anomalies; calculated melts are LREE enriched. Taken together, these results suggest crystallization from a quartz-saturated granitic magma and thus the existence of continental crust, possibly in a setting like Iceland. The high 6180 portion of the crystal would be in equilibrium with a magma at 6'80(WR)= 8.5-9.5%. There is no known mantle reservoir with such high values. '8O(WR) values above 8.5 are typical of "S-type" granites that have melted or assimilated material that was altered by low temperature interaction with water at the surface of the Earth (i.e., weathering, diagenesis, low T hydrothermal alteration). Thus the high 18O value of the 4.4 Ga zircon suggests that surface temperatures were cool enough for liquid water suggesting that the early steam-rich atmosphere condensed to form oceans at that time. The evidence for liquid water and oceans at 4.4 Ga suggests a Cool Early Earth. This contrasts with the Hot Early Earth and global magma oceans envisioned at 4.5-4.3 Ga based on: an impact origin of the Moon (4.45-4.50 Ga), core formation, higher Hadean radioactive heat production, and intense early meteorite bombardment (4). The surface of the Earth cools quickly to form a crust by radiation, but a magma ocean caused by these processes would persist beneath the initially thin crust for up to 400 m.y. (5) and might erupt as massive flood basalts in response to major meteorite impacts, boiling surface waters. The thermal contrasts presented by these lines of evidence are minimized if the Moon and core formed earlier (-.4.5 Ga), if the Moon formed by a process not involving a Mars-size impactor, or if the early meteorite bombardment was less intense or irregular in timing. It is possible that periods of Cool vs. Hot Early Earth alternated, with 98 �boiling of early oceans after major impact events followed by periods of cooler surface conditions. If life evolved in these seas, multiple extinctions before 3.9 Ga are suggested. 1.20 Detrital Zircon from Jack Hills Metaconglomerate W74 1.10 1 .00 0.90 0.80 206Pb 238 U 0.70 0.60 0.50 0.40 0.30 0.20 0.10. 207 Pb/ 235U -I- 7 A + 6 dD A AAA £ V A 5.3±0.3%c A V A a mantle zircon A A £ V A 0 V A Jack Hills Australia V Barberton + Superior Province 4, Scotland ® Manitouwadge Miscellaneous 2 2600 , 2800 3000 3200 3400 3600 3800 4000 4200 4400 U-Pb Zircon age (Ma) (1) SA Wilde, JW Valley, WFI Peck and CM Graham (2001) Evidence from Detrital Zircons for the Existence of Continental Crust and Oceans on the Earth 4.4 Gyr Ago. Nature. 409: 175-178. (2) WH Peck, JW Valley, SA Wilde, and CM Graham (2001) Oxygen Isotope Ratios and Rare Earth elements in 3.3 to 4.4 Ga zircons: Ion Microprobe Evidence for Early Archean high ö'80 Continental Crust. Geochim Cosmochim Acta, in press (3) WH Peck, EM King, JW Valley (2000) An Oxygen Isotope Perspective on Precambrian Crustal Growth and Maturation. Geology 28: 363-366; EM King, JW Valley, DW Davis, GR Edwards (1998) Oxygen isotope ratios of Archean plutonic zircons from granite-greenstone belts of the Superior Province. Precam, Res 92: 365-387. (4) HN Pollack (1997) Thermal Characteristics of the Archaean, in: de Wit and Ashwal (eds), Greenstone Belts, 223-232 (5) Y Abe (1993) Physical State of the Very Early Earth. Lithos 30:223-235 99 �NW U.PB AGES FROM MINNESOTA, MICHIGAN, AND WISCONSIN: IMPLICATIONS FOR LATE PALEOPROTEROZOIC CRUSTAL STABILIZATION. Van Schmus, W.R. (rvschmus@ku.edu) and MacNeil!, L.C., both at Dept. of Geology, Univ. of Kansas, Lawrence, KS 66045, HoIm, D.K., Dept. of Geology, Kent State University, Kent, OH 44242; and Boerboom, T. J., Minnesota Geological Survey, 2642 University Avenue, St. Paul, MN 55114. As part of a comprehensive study of the East-Central Minnesota Batholith (ECMB: Boerboom and HoIm, 2000; Van Schmus et al., 2000), we have also determined the U-Pb ages of several rock units spatially and (or) temporally related to it. The samples fall into several categories (Table 1): (a) the McGrath Gneiss to the NE of the ECMB in Minnesota; (b) Penokean basement east of the ECMB in Minnesota; (c) post-Penokean, preECMB rocks in Michigan and Minnesota, and (d) units coeval with or slightly younger than the ECMB in Minnesota and Wisconsin. Table 1. New U-Pb Results from Minnesota, Michigan, and Wisconsin. Sample Description MNOO-07 McGrath Gneiss, 2 mi. west of McGrath, MN 2550 ± MNOO-0 1 Bradbury Creek granodiorite, 4 mi. south of Onamia, MN Tonalitic gn., Hillman migmatite?, 3 mi. south of Onamia, MN 1877±15 Ma MNOO-02 MN99-09 MNOO-03 MNOO-04 AGR-1 Late tonalite in Hiliman migmatite, 3 mi. SE of Lastrup, MN Late tonalite in Hilhnan migmatite, 9 mi. west of Onamia, MN Late tonalite in Hilhnan migmatite, 13 mi. west of Onamia, MN Humboldt granite, Humboldt, MI (Hoim et aL, 2001) Multiple East Central Minnesota Batholith Van Schmus et al. (2000); Boerboom and HoIm (2000) Includes EC-2, -4, -5, -15, and -25 of Jima and Chandler (1997) 1787 to 1772 Ma PF-99 VS73-37 Two-mica granite, Park Falls, Wisconsin Radisson Granite, Radisson, Wisconsin 1781 ± 14 Ma 1776 ± 08 Ma V573-08 VS77-251 VS79-85 Amberg granite, 1 mi. north of Amberg, WI Lugerville granite, 1.5 mi. SW Lugerville, WI Lugerville granite, Rock Carry Rapids, 1 mi. E Lugerville, WI 1754±llMa VS74-13 VS73-02 Observatory Hill rhyolite, 6 mi. south of Montello, WI Montello granite, Montello, WI Age 14 Ma 1853 ± 10 Ma 1798 ± 03 Ma (composite of 3 samples) 1805 ± 07 Ma all 1749 ± 04 Ma (pooled; all near) concordia) 1759 ± 02 Ma 1746 ± 03 Ma The oldest sample is from the McGrath Gneiss and confirms the late Archean age of that unit. Two samples yielded Penokean ages. The Bradbury Creek granodiorite was dated by Goldich and Fischer (1986) at 1869 ± 5 Ma, and our date of 1877 ± 15 Ma is fully consistent with this. A tonalitic gneiss (MNOO-02) about one mile to the north-northwest and mapped as Hillman migmatite yields a slightly younger, but statistically indistinct, age of 1853 ± 10 Ma. We are investigating the possibility that the Bradbury Creek unit is more extensive than originally defmed from aeromagnetic data. In any case, normal Penokean ages occur in eastern Minnesota, and Penokean basement forms the host crust for the ECMB. Several samples yielded U-Pb ages close to 1800 Ma. The main units in Minnesota are late, undeformed (or less deformed) tonalite phases within the Hillman migmatite complex. Samples from three separate localities yield a mean age of 1798 ± 3 Ma for the tonalite. For one of these samples (MNOO-03), the tonalite dated intrudes deformed tonalitic gneiss similar to that of MNOO-02. At this time it appears that the protolith for the Hillman migmatite may be Penokean, but that it was extensively injected by younger tonalite about 1800 Ma. 100 �Further detailed sampling and geochronology will need to be done to test this option. In Upper Michigan an alkali-feldspar granite near Humboldt yielded an upper intercept age of 1805 ± 7 Ma (HoIm et al., 2001). These rocks define a distinct post-Penokean, pre-ECMB phase of magmatism in or near the southern part of the Superior Province. As reported previously (Van Schmus et al., 2000), the East-Central Minnesota Batholith was emplaced within a relatively short span of time 1787 to 1772 Ma. We analysed zircons from several drill core samples to the east (ursa and Chandler, 1997), and the results (Table 1: EC-2, EC-4, EC-5, EC-1 5, EC-25) extend the known area of the ECMB. In order to determine whether units of the so-called "1760 Ma" suite of rocks in Wisconsin are coeval with the ECMB, we reanalysed several samples from the senior author's collection, plus one new sample (PF-99) using single-grain analyses and more precise techniques. The westernmost samples (PF-99 and VS7337) yield ages indistinguishable from those of the ECMB, suggesting that magmatism of that episode extended into northwestern Wisconsin. In contrast, samples of the "1760 Ma suite" farther east (Amberg and Lugerville granite) or farther south (Montello granite, Observatory Hill rhyolite) are distinctly younger, with ages of 1746 to 1759 Ma. These ages are similar to Ar-Ar ages reported for the ECMB and indicate that the last major magmatic pulse occurred about 1750 Ma. Our new data now suggest three main post-Penokean pulses of magmatism about 25 m.y. apart in the southern Lake Superior region: ca. 1800, 1775, and 1750 Ma. The origin of the thermal energy in each case is presently unknown. These ages fall within the time span of magmatism in the Yavapai province-northern Central Plains orogen (inner accretionary belt; Van Schmus et al., 1993) and could correlate with pulses of northward directed subduction associated with southward growth of the continent. Interestingly, the two older and deeper-seated igneous bodies currently reside in Paleoproterozoic crust unaffected by younger Mazatzal deformation and reheating (Holm Ct al., 1998). It appears that localization of the earlier pulses in the west (Minnesota) and north (northwesternmost Wisconsin) may have contributed to the overall greater exhumation of these areas (compared to most of Wisconsin) and that this in turn may have dramatically strengthened the crust in those regions. We note that crustal remelting and thinning after orogenesis can both contribute to an overall stronger continental lithosphere. One fmal note: reference to a "1760 Ma" suite in Wisconsin should probably now be discontinued, since those rocks formed in two pulses at about 1775 and 1750 Ma. Boerboom, TJ., and Holm, D.K., 2000, Paleoproterozoic intrusive igneous rocks of southeastern Stearns County, central Minnesota. Minn. Geol. Survey, Rept. mv. 56, 36p + 1 map. Goldich, S.S., and Fischer, L.B., 1986, Air-abrasion experiments in U-Pb dating of zircon. Chemical Geology, v. 58, p. 195-215. Holm, D., Schneider, D., and Coath, C., 1998, Age and deformation of Early Proterozoic quartzites in the southern Lake Superior region: Implications for extent of foreland deformation during final assembly of Laurentia: Geology, v. 26, p. 907-910. Holm, D., Van Schmus, R., Boerboom, T., and Jirsa, M., 1999, Role of post-Penokean granite genesis in crustal stabilization in the Lake Superior region, north-central United States. Geol. Soc. America Abstracts with Programs, v. 31, no. 7, p. A-259. Hoim, D.K., Van Schmus, W.R., and MacNeill, L.C., 2001, Age of the Humboldt granite, northern Michigan: Implications for the origin of the Republic metamorphic node. 47th Ann. Inst. on L. Superior Geology, Madison, Wisconsin, May (this volume). Jirsa, M.A., and Chandler, V.W., 1997, Scientific test drilling and mapping in east-central Minnesota, 19941995: summary of lithologic results. Minn. Geol. Survey, Inf. Circular 42, lO5p. Schweitzer, D.J., Schneider, D.A., Boerboom, T.J., HoIm, D.K., and Van Schmus, W.R., 2000, Assessing the extent of Early Proterozoic Penokean versus 1770-1760 Ma metamorphism in east-central Minnesota. Abstracts, 46th Ann. Inst. on L. Superior Geology, Lakehead University, Thunder Bay, Ontario, May Van Schmus, W.R., Bickford, M.E., and Condie, K.C., 1993, Early Proterozoic crustal evolution, in Reed, J.C., Jr., Bickford, M.E., Houston, R.S., Link, P.K., Rankin, D.W., Sims, P.K., and Van Schmus, W.R., editors, Precambrian: Conterminous U.S. Geological Society of America, The Geology of North America, v. C-2, p.270-281. Van Schmus, W.R., MacNeill, L. C., Holm, D.K., Boerboom, T.J. and Jima, M.A., 2000, The 1787-1772 Eastcentral Minnesota batholith: precursor to crustal stabilization in the L. Superior region. Abstracts, 46th Ann. Inst. on L. Superior Geology, Lakehead University, Thunder Bay, Ontario, May 101 �A STUDY OF WELL CONSTRUCTION FOR ARSENIC CONTAMINATION IN NORTHEAST WISCONSIN ANNETTE E. WEISSBACH, Wisconsin Department of Natural Resources, DNRNortheast Region, Waste Management, Remediation, and Redevelopment, 1125 N Military Aye, P0 Box 10448, Green Bay WI 54307-0448 ELIZABETH M. HEINEN, Wisconsin Department of Natural Resources, DNRManitowoc Field Station, 2220 E CTH V, Mishicot WI 54228, and KELD B. LAURIDSEN, Wisconsin Department of Natural Resources, DNR-Northeast Region, Waste Management, Remediation, and Redevelopment, 1125 N Military Aye, P0 Box 10448, Green Bay WI 54307-0448 Arsenic has been detected in approximately one third of the private drinking water wells in the Fox River valley of Northeast Wisconsin. Concentrations detected are some of the highest found naturally occurring in the world. Research has indicated that presently 3.5% of the wells in Outagamie and Winnebago counties exceed the current drinking water standard of 50 ppb, whereas close to a quarter of the wells may exceed the proposed standard of 10 ppb. Department of Natural Resources study results indicate the geochemical phenomenon causing the elevated levels of arsenic in groundwater of this region is associated with oxidation of a sulfide-mineralized zone located at the top of the deep sandstone aquifer system. A regional decline in water levels may have exposed this sulfide rich zone to oxidation from air within the open boreholes of water wells extending through this zone. This oxidation process can initiate a chemical reaction similar to acid mine drainage. Recommendations have been developed for constructing wells within a delineated advisory area. This guidance recommends constructing wells with well casing pipe to extend through the sulfide rich zone. This study compared arsenic concentrations of wells constructed according to the guidance, with wells constructed to traditional construction standards. Additionally, this study examined data to determine if it was better to replace a contaminated well with a new one, or to reconstruct the existing well with a liner. The results of this study indicate that the guidance gives adequate protection for wells constructed in the arsenic advisory area and that liners are successful at reducing arsenic concentrations, although not as successful eliminating arsenic contamination. 102 �holes from which the samples were collected, and so may be from near the base of the Quaternary section. Low ppm Th I wt.% K ratios, which is the case for most of these high K tills, in general are indicators of potassic alteration. Because there is no apparent process that would create this type of alteration pattern following till deposition, it is proposed that tills with high K contents may have a local bedrock source area characterized by potassic alteration. This alteration may indicate pre-Marathon Foration paleoweathering of bedrock or may indicate local or regional hydrothermal alterat4on. Because potassic alteration can indicate precious metal and sulfide mineralization, high-K tills in the lower part of the Marathon Formation, characterized by some high Zn values, may aid in mineral exploration. Provenance Discrimination Samples of Quatemary deposits collected from closely spaced rotasonic boreholes in the area of the Bend massive sulfide deposit in Taylor County proved to have unique geochemical characteristics that could be xpiained by differences in source areas and sedimentological processes (Woodruff and others, 2000). The classification of till samples in the field was successfully duplicated by discriminate analysis of the geochemical data set using the statistical software package 5+. Based on geochemistry and grain size, samples from the Bend area were statistically grouped into either Copper and a Lake Falls or Marathon Formations, easily distinguishing a carbonate provenance Superior provenance. The same type of discriminate analysis was run on the regional data set using identical element and grain size input. When compared to field classifications, results gave an error of 17% (correctly predicting 38 of 46 samples) for a carbonate provenance and an error of 5% (correctly predicting 51 of 54 samples) for a Lake Superior provenance. However, the utility of this statistical approach for the regional Quaternary data set is questionable. Geochemical fingerprints of correlated Mg Cu and Ti for and Ca values for samples from the Marathon Formation and correlated samples from the Copper Falls Formation identified in the Bend section are less distinct compared to the in the regional data set. Several factors inherent to the regional data, as These Bend area data, may complicate correlations and discrimination of provenance. factors include differences in the method of sample collection (rotasonic core vs. auger glaciofluivial or sampling), which could influence sample identification (e.g., till vs. glaciogenic debris samples), and the existence of geochemical outliers, such as the high larger geographic area K tills, in the regional data set that may reflect sampling from a with many diverse bedrock sources of material. References Wisconsin Attig, J. W., 1993, Pleistocene geology of Taylor County, Wisconsin: Geological and Natural History Survey Bulletin 90, 35 p. Woodruff, L.G., Attig, J.W., and Cannon, W.F., 2000, Geochemical impacts of an undisturbed mineral deposit — results from the Bend deposit, Wisconsin [abs]: Institute on Lake Superior Geology Proceedings, v. 46, p. 72-73. 105 � https://digitalcollections.lakeheadu.ca/files/original/0274839d2a0a8f7d05473aabb3bc01b8.pdf c44648cb3e8f8088276e886956c5658f PDF Text Text ���CONTENTS CONTENTS Proceedings Volume 47 Part 22 -— Field Trips Interval: New TripI: Tripi: Sedimentologic, Tectonic and Metamorphic Metamorphic History of the Baraboo Interval: 11 Evidence from Investigations in the Baraboo Range,Wisconsin Investigations Locality 1: Baxter Hollow 10 10 Locality 2: Hydrothennal Veins, Hwy 12 14 14 Hydrothermal 15 Locality 3: Quartzite and Metapelite, Hwy 12 15 Locality 4: Abelman's Gorge 17 17 Leaders: L. Gordon Medaris, University of Wisconsin -- Madison Madison Robert H. Dott, Jf., Madison Jr., University of Wisconsin -- Madison Trip Trip 2: Geology, Ore Deposits, and Cultural History ofthe of the Upper Mississippi Valley Zinc-Lead District Stop 1: Platteville Mining Museum and Rollo Jamison Museum Stop 2: Potosi Hill - Ordovician Sinnipee Group Stop 3: New Diggings Lead Digs Stop 4: Shullsburg Mine Site - Metallic Mine Reclamation Reclamation Stop 5: Pendarvis State Historical Site (Mineral Point) Historical 23 30 30 36 36 39 Leaders: M.G. M.G. Mudrey, Mudrey, Jf., Jr., Wisconsin Geological and Natural History Survey Thomas C. Hunt, University of Wisconsin - Platteville of the Baraboo and Waterloo Quartzites Trip Trip 3: Economic Geology Geology of Quartzites of Southern Southern Wisconsin Stop Stop 1: Michels Michels Materials Waterloo Quarry Stop 2: The Kraemer Co. Williams Quarry Stop Stop 3: 1,760 1,760 MaRhyolite Ma Rhyolite Stop Stop 4: Milestone Materials Jesse Pit and Quarry Stop Stop 5: Milestone Materials Fox Ridge Asphalt Plant and Sales Yard 6: Martin Marietta Aggregates Aggregates Rock Springs Quarry Stop 6: Stop 7: Kraemer Company LaRue Quarry Leaders: Leaders: Brown, Wisconsin Geological and Natural History HistOlY Survey Bruce A. Brown, Survey University of Wisconsin - Whitewater Frank R. Luther, University Whitewater Susan M. M. Courter, Michels Materials Susan Materials James W. Schmitt, D.L. Gasser Construction Construction Company Jennifer Lien, The Kraemer Company 111 43 46 47 49 50 50 51 52 �This page intentionally left blank �Field Trip 11 Sedimentologic, Tectonic and Metamorphic History of the Baraboo Interval: Interval: New Evidence from Investigations in the Baraboo Range, Range, Wisconsin Wisconsin by H. Dott, Dort, Jr. Ir. L. Gordon Medaris, Ir. Jr. and Robert H. Department of Geology and Geophysics Geophysics of Wisconsin Wisconsin -- Madison Madison University University of Devil's Lake State Park View to the west along the East Bluff, Devil's State Park (foreground), Devil's Lake (background) Baraboo Quartzite (foreground), (background) �This page intentionally left blank �FOREWORD Baraboo Range began began in 1852 and and culminated in in 1970 with with Geological investigations of the Baraboo Dott. publication of the monograph, "Geology of the Baraboo District, Wisconsin", by Dalziel and Dott. Despite its publication 30 years ago, this monograph (with accompanying accompanying detailed geological map) Despite remains today the most comprehensive Baraboo Range. comprehensive treatise on the Baraboo In 1996 a paleosol at the base of the Baraboo Baraboo Quartzite Quartzite was discovered in a drill core taken in in Baxter Hollow (Medaris et al., 1996). Recognition of ofthe paleosol, which was developed at the expense the developed expense of granite underlying the quartzite, resolved any remaining question about the relative ages of the Baraboo Quartzite and Baxter Hollow Granite. This discovery subsequently inspired aa new look at the Baraboo techniques that that were unavailable unavailable 30 years ago, and led led to Baraboo Range, using a variety of analytical techniques depositional age of the quartzite, paleoclimatic environment, elaboration of a number of topics, including depositional data substantiate substantiate the sedimentary geochemistry, and conditions and age of metamorphism. The new data Dott and provide additional insight into into the processes conclusions originally drawn by Dalziel and Don discovery is is responsible for producing the Baraboo Range. An especially exciting and unexpected unexpected new discovery the occurrence of 1,460 Ma hydrothermal activity along the base base of the Baraboo Quartzite, presumably driven by heat from Wolf River-type magmatism. Proterozoic evolution of of the Baraboo This field guide provides an updated overview overview of the Proterozoic and describes several key outcrops, from which Range, based on the results of our recent investigations, investigations, many of the new data were obtained. INTRODUCTION Red, supermature quartzites, most notably the the Baraboo, Barron, and Sioux Quartzites, Quartzites, have long long been recognized as distinctive and important Precambrian Precambrian features in the southern Lake Superior Superior region. region. The physical and chemical characteristics of the quartzites signify deposition in a stable cratonic setting under conditions of intense chemical weathering in the presence of significant free oxygen in the post-1,750 1997; FloIm Holm et a!., al., atmosphere. These quartzites are now known to be post-I, 750 Ma in age (Dott et al., 1997; ~ 1,630 Ma (Romano eta!., et al., 2000), and locally intruded 1998), to have been folded folded and metamorphosed at 1,630 I,450 Ma (Dott and Dalziel, 1972). by granitic rocks at ~—l,450 1972). The term, Baraboo interval, was introduced by Dott DoU (1983) for this distinctive sequence of sedimentation, deformation, and metamorphism in the time span of 1,450 to 1,750 Ma. Our recent investigations not only validate the concept ofthe of the Baraboo interval, but also provide greater insight into the disparate geological events that shaped shaped the southern Lake Superior region in mid-Proterozoic time. REGIONAL SETTING Baraboo interval sedimentary rocks are widely distributed in the southern Lake Superior region metamorphic rocks. The (Fig. 1), where they lie nonconformably on 1,750 Ma and older igneous and metamorphic the general southerly direction paleocurrents, the increase in sediment thickness from north to south. south, direction of paleocurrents, suggest that chemical maturity of the sedimentary rocks, and the occurrence of mature paleosols suggest deposition occurred occurred on a stable, passive continental margin on the southern edge of a Proto-North American craton (Dott, (Dort, 1983). All of the quartzites in Figure 1, 1, except for the Waterloo, have yielded detrital zircon grains with V-Pb ages in the range, 1,712 to 1,778 et aL, 1997; Holm et al., U-Pb 1,778 Ma (Dott (Dort eta!., 1998; Van Wyck, 1995), and these data, combined with the inferred time offolding of folding and deformation at —1,630 Ma, established from 4°ArP9Ar 40Ar/39Ar cooling ages of basement amphibole and mica (Romano ~1,630 (Romano et al., (Dort and Dalziel, Dalziel, 1972; Van Schmus et al., 1975), 2000) and Rb-Sr resetting of granite and rhyolite (Dott eta!., 3 3 �constrain deposition to between between 1,710 and 1,630 Ma. Subsequently, constrain deposition of of the the Baraboo Baraboo interval interval sediments sediments to 1,710 and 1,630 Ma. Subsequently, the easternmost quartzites at McCaslin and Waterloo were intruded by granitic rocks of Wolf Wolf River River age age the easternmost quartzites at McCaslin and Waterloo were intruded by granitic rocks of Dalziel, 1972; 1972; Van Wyck, Wyck, 1995). 1995). (Dott and Daiziel, 200 km NO (, ~-S~D~--~ MN Barron (300 m) • • 1630Ma McCaslin \ A ,. Flambeau ­ ~..t f~-Water - - ­ '~ to ­ B"abOO(~ml ~I ~_MI NE , )~ IA I IN Figure Figure 1. 1. Distribution Distribution of of Baraboo Baraboo and and correlative correlative quartzites quartzites in in the the southern southern Lake Lake Superior Superior region, with meters, and region, with average average paleocurrent paleocurrent directions, directions, thicknesses thicknesses in in meters, and paleosol paleosol localities localities quartzites have (*). Heavy (*). Heavyline lineisisthe theinferred inferred1,630 1,630Ma Matectonic tectonic front, front, south south of of which which quartzites have been folded (Hoim been (Holm et al., aI., 1998) 1998) IGNEOUS IGNEOUS BASEMENT BASEMENT OF OF THE THEBARABOO BARABOO RANGE RANGE The Baraboo Baraboo Quartzite Quartzite is diorite near the town town of of Denzer, Denzer, by The is underlain underlain by by diorite near the by granite granite in in Baxter Baxter both limbs of the syncline in its Hollow, and and by rhyolitic lavas and pyroclastic rocks beneath both limbs of the syncline in its eastern eastern part part Hollow, granite and rhyolite are (Daiziel and Dott, 1970). U-Pb zircon ages of the Baxter Hollow (Dalziel and Dott, 1970). U-Pb zircon ages of the Baxter Hollow granite and rhyolite are indistinguishable, and indistinguishable, and taken taken together, together, yield yield an an age age of of1,749±12 1,749±12 Ma Ma (Van (Van Wyck, Wyck, 1995). 1995). Chronologically Chronologically subalkalic granite and rhyolite rhyolite suite suite of of and petrologically, petrologically, the the subalkalic granite and is correlative correlative with with the and the Baraboo Baraboo basement basement is Fox River River Valley Valley igneous igneous suite suite contains the Fox Fox River River Valley Valley (Smith, (Smith, 1978; Anderson etet al.. a!.. 1980). 1980). The The Fox the 1978; Anderson contains analyses demonstrate that both metaluminous and peraluminous types, and recent chemical both metaluminous and peraluminous types, and recent chemical analyses demonstrate that Denzer Denzer diorite is peraluminous. peraluminous. and and rhvolite rhyolite is is both both peraluminous peraluminous and and diorite is is metaluminous, metaluminous, Baxter Baxter Hollow Hollow granite granite is the Baxter Hollow metaluminous (Fig. (Fig. 2A). 2A). Among Ma silicic silicic lithologies. lithologies. the Baxter Hollow granite granite is is the the metaluminous Amongall allthe the1,750 1,750 Ma Fe-number and least differentiated differentiated in as measured by fe-number in terms terms of of major major elements, elements, as measured by and Ca-number Ca-number (Fig. (fig. 2B). 2B). least of Figure 2. containing 625-700 The Denzer diorite is metaluminous and plots outside the scales The Denzer diorite is metaluminous and plots outside the scales of Figure 2. containing 625-700 ppm ppm Sr. Sr, 0.62-0.68. of 0.70-0.82, Fe-number of 0.46-0.48. and Ca-number of an A1203/(K20+Na20+CaO) AbO)(K20+Na20+CaO) value of 0.70-0.82, fe-number of0.46-0.-l8. and Ca-number of 0.62-0.68. 44 �• 1000 1000 G Baxter Hollow granite rhyolite booBara R granite Hollow Baxter G R Baraboo rhyolite GG C C G cC G G G E P P 100 100 R P 0. 0 .3 R E P R en P C/) R R R R R P granites peraluminous ~~ P P '--- peraluminous granites A M1 MJi'l.MM;P ~ ~ metaluminous granites A graniteS metaluminous PP I"" I"" I"" j_.IPH 1.0 1.1 10 0.9 1.0 I 1.2 I"" 1.3 I",.;"" 10 0.9 1.1 1.2 1.3 1.0 1.0 * + 0.9 + R + C G ~ ".).. R 0 ~~, G a) 0.7 ~...,,~. <$)~ G G a) 0.6 0.61 G 0 E rhyolite boo Bara R granite Ho/low Baxter G B G Baxter Hollow granite B R Baraboo rhyolite t! , o ! , I, ,! I I !! I!!!, I , ! ! ! I, ! I I I CaO/(CaO+Na20) molecular 0.3 0.2 0.1 0.5 0.1 0.2 03 molecular CaO/(CaO+Na20) ! !! !,!! 0.4 ('3 0 " G Li:::J u E G G ~ 07 ~ 0 G ~ G 0+ U- "'' 'oj>. G 0.8 0.8 R 0 ~ * ~ ++ R +,. 0OJ u. R + ++ * + OJ u. metaluminous granites R 6" Cl :2: + 0 granifes meta/uminous J. / + / 0.4 0.5 0 impublished) Medaris, 1980; al., et Anderson 1978; Smith, from (data suite Valley River Fox the of granites (F) peraluminous and (M) metaluminous with (R) rhyolite Baraboo and (G) granite Hollow Baxter of comparison Chemical 2. Figure Figure 2. Chemical comparison of Baxter Hollow granite (G) and Baraboo rhyolite (R) with metaluminous (M) and peraluminous (P) granites of the Fox River Valley suite (data from Smith, 1978; Anderson et aI., 1980; Medaris, unpublished) quartz. q, saussurite; and albite mostly plagioclase, p, hornblende; h, biotite; after chlorite c, biotite; b, feldspar; alkali a, Abbreviations: polarizers). crossed matrix, aphanitic m microphenocrists (plagioclase rhyolite Baraboo and light), polarized plane (micrographic, granite Hollow Baxter light), polarized plane granular, (subhedral diorite Denzer in textures typical of Photomicrographs 3. Figure Figure 3. Photomicrographs of typical textures in Denzer diorite (subhedral granular, plane polarized light), Baxter Hollow granite (micrographic, plane polarized light), and Baraboo rhyolite (plagioclase microphenocrysts in aphanitic matrix, crossed polarizers). Abbreviations: a, alkali feldspar; b, biotite; c, chlorite after biotite; h, hornblende; p, plagioclase, mostly albite and saussurite; q, quartz. 5). (Fig. cummingtonite and actinolite intergrown and chlorite by replaced partly is hornblende diorite. Denzer In 4). (Fig. albite and microcline end-member near of mixture fine-grained extremely an into exsolves feldspar alkali intermediate and (saussurite), epidote fine-grained and albite to transformed is plagioclase chlorite, by replaced is biotite Typically, minerals. facies greenschist of variety a by replaced completely to partly being minerals igneous with extensive, is lization recrystal- although 3), (Fig. basement Baraboo the of units all in preserved well are textures Igneous Igneous textures are well preserved in all units of the Baraboo basement (Fig. 3), although recrystal­ lization is extensive, with igneous minerals being partly to completely replaced by a variety of greenschist facies minerals. Typically, biotite is replaced by chlorite, plagioclase is transformed to albite and fine-grained epidote (saussurite), and intermediate alkali feldspar exsolves into an extremely fine-grained mixture of near end-member microcline and albite (Fig. 4). In Denzer diorite, hornblende is partly replaced by chlorite and intergrown actinolite and cummingtonite (Fig. 5). 5 5 �Figure image, showing Figure 4. 4. Back-scattered electron image, partial of relict igneous alkali feldspar, partial replacement of af (Or 54-70) 54-70) by by K-feldspar, K-feldspar, k (Or 98.5) and albite, af(Or ab p, epidote epidote and and albite albite after plagioclase; ab (Ab (Ab 96.4). 96.4). p, q, q, quartz. of amphibole in Denzer Denzer Figure 5. 5. Ca Ka X-ray map of diorite. Relict igneous hornblende, hb, partly partly chI, and an intergrowth of replaced by chlorite, chl. actinolite, act, and cummingtonite, cum. cum. BARABOO QUARTZITE QUARTZITE BARABOO The 1,500 m thick and is overlain conformably by by the black black Seeley Seeley Slate Slate The Baraboo Quartzite is 1,500 (100 (100 m), m), Freedom Freedom Dolomite Dolomite with with iron formation formation (300 m), Dake Quartzite (65 m), and Rowley Creek Slate The Baraboo Baraboo Quartzite Quartzite consists consists of of 85-90% 85-90% of of quartz quartz arenite and subordinate subordinate quartz wacke, wacke, Slate (45m). (45m). The which are characterized by prominent cross bedding bedding (Fig. (Fig. 6) 6) and and ripple ripple marks, marks, 5-10% 5-10% of ofconglomerate, conglomerate, and Although all of of these rock types have experienced low and 5-10% 5-10% of of siltstone siltstone (Fig. (Fig. 7) 7) and and pelite pelite (Fig. (Fig. 8). 8). Although grade 9) and grade metamorphism, metamorphism, the the original original clastic elastic textures are remarkably well-preserved in quartzite (Fig. 9) Figure 6. 6. Typical Typical quartzite quartzite with with prominent prominent Figure cross bedding. metasiltstone with with refracted refracted Figure 7. Typical metasiltstone Coin isis 2.5 2.5 cm in in cleavage, Field cleavage, Field Locality Locality 2. 2. Coin diameter. 6 6 �fL. Figure 9. Photomicrograph of typical quartzite with relict clastic texture (partly crossed polarizers). polarizers). crossed (partly texture elastic relict with quartzite typical of Photomicrograph 9. Figure __j cleavage. crenulation and folds chevron with Larue, near Metapelite 8. Figure Figure 8. Metapelite near Larue, with chevron folds and crenulation cleavage. polarizers). crossed (partly texture elastic relict with metasiltstone of Photomicrograph 10. Figure Figure 10. Photomicrograph of metasiltstone with relict clastic texture (partly crossed polarizers). intense experienced that region source a implies sediments interval Baraboo the of maturity chemical ages average to compared and shale average to normalized extreme The 11). (Fig. different of shale are compositions rock the when apparent readily (Al:O±K2O+Na2O+CaO) is maturity chemical of degree remarkable The Al203/ 100*molar = CIA radius. ionic decreasing 98.6. to 96.8 from ranging high, exceptionally of order in arranged Elements ages. the over shale average to compared and 1985) McLeiman, and (Taylor is Alteration of Index Chemical The MnO. and MgO, CaO, Na20, K20, of concentrations shale average to normalized rocks, metasedimentary interval Baraboo fine-grained of Compositions 11. Figure low extremely with H20, and Ti02, Fe203, A1203, Si Al Fe Mg Ca Na K Si02, of entirely almost consist chemically 0.001 quartzites Barron and Sioux, Baraboo, the Cenozoic & from metapelite and Metasiltstone minerals. Mesozoic heavy among apatite and rutile, magnetite, 0.01 Paleozoic zircon, of predominance the and pyrophyllite, x of abundance the feldspar, detrital of absence Proterozoic near the from inferred been long has Quartzite x 0.1 Archean Baraboo the of maturity chemical The -9-Sioux metasiltstone. and quartzite in domains interstitial in and 8), (Fig. cleavage crenulation Barron of development by accompanied is it where Baraboo layers, metapelite in only occurred has lization recrystal- Complete 10). (Fig. metasiltstone metasiltstone (Fig. 10). Complete recrystal­ 10 F r - - - - - - - - - - - - - - - - - - _ lization has occurred only in metapelite layers, Baraboo -.­ CIA 50.4 - 65.4 where it is accompanied by development of Barron crenulation cleavage (Fig. 8), and in interstitial .. < SIOUX domains in quartzite and metasiltstone. ".ii x -; The chemical maturity of the Baraboo ~ Archean f 0< Quartzite has long been inferred from the near ~ 0.1f~ ProterozOic absence of detrital feldspar, the abundance of x ii PaleOZOIC pyrophyllite, and the predominance of zircon, E CIA 96.8 - 98.6 magnetite, rutile, and apatite among heavy "' MeSOZOIc & CenozoIc minerals. Metasiltstone and metapelite from the Baraboo, Sioux, and Barron quartzites 0.001 I~-----'---~--~-----'---~--'---' K Na Ca Mg Fe AI Si chemically consist almost entirely of SiOl, Ah03, Fe 203, Ti0 2, and H 20, with extremely low Figure I 1. Compositions of fine-grained Baraboo interval metasedimentary rocks. normalized to average shale concentrations of KlO, Na 20, CaO, MgO, and (Taylor and McLennan, 1985) and compared to average MnO. The Chemical Index of Alteration is shale over the ages. Elements arranged in order of exceptionally high, ranging from 96.8 to 98.6. decreasing ionic radius. CIA = 100*molar AbO/ The remarkable degree of chemical maturity is (AbOJ+K20+N a20+CaO) readily apparent when the rock compositions are normalized to average shale and compared to average shale of different ages (Fig. 11). The extreme chemical maturity of the Baraboo interval sediments implies a source region that experienced intense 10 ~ 1i~ ; ~ : ~I ~ , .. ~ . ' 7 7 �chemical leaching leaching and and produced produced detritus detritus consisting consisting largely largely of of quartz, quartz, kaolinite, kaolinite, and and hematite. hematite. chemical The persistent question about the about the grains of of the Baraboo Quartzite and relative ages of I Individual Individual grains of detrital detrital zircon zircon 0.34 ft from basal Baraboo Quartzite 0.34 from basal Baraboo Quartzite Baxter Hollow granite was was resolved resolved with with the the 1800 Ma UW Radio genic Isotope Lab ! UW Radiogenic Isotope Lab 1800 Ma discovery of the sub-Baraboo paleosol in drill of paleosol in drill Analyst: Analyst: Ron Ron Schott Schott 0.30 0.30 core from Baxter Hollow (Medaris (Medaris et et al., aI., 1996) 1996) 1600 and subsequent recognition of 1600 Ma _ _ 1761 (3) I of saprolite saprolite in in b ,i712(4) •1866 (3) outcrop in Baxter Hollow granite granite and and in in 0.26 1715(12) 6 rhyolite beneath the east end 1400 MaZ °.'222 1691(2) -1866 (3) end of of the syncline syncline 1400 Ma - 1691 (2) al., 1997). (Medaris et aI., 1997). Further Further confirmation confirmation i< •17793 . 0 0.22 -1779 (3) post-1,750 Ma depositional age of of the post-l,750 of the 0- ° CD o quartzite was provided by U-Pb analyses of N M. detrital zircon from near the base of of the the 0.18 1000 Ma quartzite, using the single-grain evaporation quartzite, 1740 (6) -1740 (6) technique. Among Among the the seven seven grains grains analyzed, analyzed, _~I --'---'----'I_ _ J 0.14 0.14 six are slightly discordant and and one one is is more more so, so, 5.0 4.0 3.0 2.0 1.0 1.0 2.0 3.0 4.0 5.0 age of with one onegrain grainyielding yieldinga 207Pb/206Pb a 207Pbpo6Pb age 2O7pb* 207 * / 235 pb U 1,691 1,866 Ma, and the other other six six ranging rangingfrom from 1,691 12. V-Pb U-Pb concordia concordia diagram diagram for Figure 12. Figure for detrital detrital zircon zireon to 1,779 Ma (Fig. (Fig. 12; Dott et al., 1997). Although a1., 1997). Although grains from the Baraboo Quartzite. 207Pb/206Pb ages are grains from the Baraboo Quartzite. 207Pbfo6 Pb ages are of 1,691 1,691 and 1,715 1,715 Ma are the two grains with ages of given for for individual individualgrains; grains;2cr 2 standard given standard deviations deviations are are shown in parentheses. highly radiogenic and may not be reliable, the the in parentheses. shown other grains are "well behaved" and indicate indicate that + hematite deposition may ~ 1,710 Ma. may have have begun begunas aslate lateasas—4,710 qtz :!: hematite diorite, qtz diorite, rutile rutile silt stone granite svanbergite siltstone 5vanbergite granite METAMORPHISM & petite pelite albite & rhyolite & & rhyolite albite r ' I ~ ,I / - ­ 01.1- ;;~; ~i~) "" II 1 chlorite chlorite pale osol pri actinolite paleosol Although the structure of of the Baraboo Range actinolite cummingtonite has been been well well studied, studied, little attention has has been cummingtonite has been mc epidote epidote devoted to metamorphism, other than identifying identifying titanite titanite pyrophyllite in metapelite. ItItisis now (kaolinite, now known known that that all all (kaolinite, retrograde) lithologic units in the Baraboo Baraboo Range Range have have been been retrograde) recrystallized to varying hydrothermal varying degrees degrees by by low-grade low-grade veins metamorphism (Medaris et at., 1998). Because of veins metamorphism (Medaris et aI., 1998). Because Si02 the extreme chemical maturity maturity of ofmany many rock rock types, types, the critical mineral assemblages can be adequately represented in in the the system, system, K20-Al203-Si02K 20-Ab03-SiOz- H20 H20 (KASH) (Fig. 13). Baraboo quartzite. metasiltstone, (KASH) (Fig. 13). Baraboo quartzite. metasiltstone, and metapelite metapelite contain contain quartz and pyrophyllite plus plus and svanbergite (a hematite, rutile, and svanbergite accessory hematite, (a strontian strontian aluminophosphate-sulfate diagenetic diagenetic mineral); mineral); of quartz, quartz, muscovite, muscovite, hematite, hematite, metapaleosol consists of usp and rutile; rutile; hydrothermal hydrothermal veins veins near of the the and near the the base base of A1203 K20 quartzite contain pyrophyllite, muscovite, and Figure compositions and diaspore; and the metaigneous basement is Figure 13. 13. Rock Rock compositions and mineral mineral assemblages assemblages in in the Baraboo Range, projected into the system, the Baraboo Range, projected into the system, KASH. KASH. characterized by quartz, microcline, microcIine, and and albite albite Abbreviations: dsp, diaspore; mc, microcline; ms, mus+/- muscovite, hematite, chlorite, epidote, epidote, titanite, titanite, Abbreviations: dsp, diaspore; me, microeline: ms, mus­ covite; qtz, covite; qtz, quartz: quartz: pri. prl. pyrophyllite. pyrophylJite. actinolite, and cummingtonite, depending depending on on 8 8 ,I �2. and I Localities Field on sections following the in further discussed are results new significant These magmatism. River Wolf from pulse thermal a by driven fluids hydrothermal of activity the to related presumably found, was Ma —1,460 at overprint strong a metamorphism, Ma 1,630 for evidence obtaining than rather However, 2001). eta!., (Naymark Ma 1,630 of age metamorphic a confirm to effort an in taken under- recently was Range Baraboo the in muscovite (1975). and hornblende of investigation 40Ar/39Ar A eta!. Schnius Van and (1972) Dalziel and Dott north. the to ages older and Penokean from south the in of data from recalculated intercept and Age ages post-Penokean separates front tectonic the 2000); Valley. River Fox the and Baraboo from rhyolite and granite for isochron Rb-Sr 15. Figure a!., et Romano 1998; al., et (Holm rocks basement in biotite and hornblende of analyses 40Ar/39Ar 6 S I Ri.r by established was 1) Fig. (see Wisconsin central 40 30 20 10 0 0.6 and northern in deformation foreland of extent 0.70254 the marking front tectonic a of position The 8.1 = MSWD 0.8 time. this at occurred also Range Baraboo the of metamorphism and folding that implication 1.0 5 the with 1993), Schmusetal., Van 1983; (Dott, U.S. southwest the in orogeny Mazatzal 1.2 w Ma -—1,650 the with associated deformation foreland to related metamorphism low-grade 1.4 regional by caused was resetting Rb-Sr granite Valley River Fox V such that suggested was it Subsequently, ). yr rhyohte Valley River Fox = Ma(X 33 ± 1,635 of age isochron BaxterHollowgranite 1.42*10.11 1.6 rhyolite Baraboo A Rb-Sr whole-rock apparent an yielded Valley River Fox the and Baraboo from rocks igneous that and Ma —l,650 at region Superior Lake southern the in systems Rb-Sr of resetting widespread was there that recognized Schmus Van 1975 in However, Range. Baraboo the from available were ages mineral metamorphic meaningful no recently, Until metamorphism. Baraboo of conditions the precisely more establish to effort an in underway are studies inclusion Fluid respectively. C, 335 1. = a(H20) for calculated and 290 and 50°C, 3 and 275 to lowered be would KASH, system, the in reactions Stable 14. Figure limits temperature these example for 0.9 0C T activity, H20 reduced At 345°C. and 305°C 450 400 350 300 250 to further temperature the restrict between 0 Quartzite Baraboo the of base the at veins hydrothermal in diaspore and pyrophyllite of coexistence The kbar. 1 at 60°C 3 and 285°C 2 between to metamorphism of temperature the constrains kyanite) or (andalusite phase 4 aluminosilicate an or kaolinite of absence the in pyrophyllite and quartz of association stable The 14. Figure in shown topology 6 the in resulting 1989), al., et (Brown base data thermodynamic GeoCaic the of means by activity H20 unit at calculated been have 8 KASH, system, the in equilibria Phase product. retrograde a is it that indicate relations textural occurs, kaolinite Where composition. bulk specific specific bulk composition. Where kaolinite occurs, textural relations indicate that it is a retrograde product. Phase equilibria in the system, KASH, 8I I \ , , have been calculated at unit H 20 activity by means of the GeoCalc thermodynamic data base (Brown et al., 1989), resulting in the 6 topology shown in Figure 14. The stable association of quartz and pyrophyllite in the .... (IJ absence of kaolinite or an aluminosilicate ~ 4 phase (andalusite or kyanite) constrains the 0: temperature of metamorphism to between 2 285°C and 360°C at 1 kbar. The coexistence of pyrophyllite and diaspore in hydrothermal veins at the base of the Baraboo Quartzite oI V /1 <1 /' I I I I restrict the temperature further to between 250 300 350 400 450 305°C and 345°C. At reduced H 20 activity, T,oC 0.9 for example, these temperature limits Figure 14. Stable reactions in the system, KASH, would be lowered to 275 and 350°C, and 290 and calculated for a(HzO) = I. 335°C, respectively. Fluid inclusion studies are underway in an effort to establish more precisely the conditions of Baraboo metamorphism. Until recently, no meaningful metamorphic mineral ages were available from the Baraboo Range. However, in 1975 Van Schmus recognized that there was widespread resetting of Rb-Sr systems in the southern Lake Superior region at ~ 1,650 Ma and that igneous rocks from Baraboo and the Fox River Valley yielded an apparent whole-rock Rb-Sr I l. Baraboo rhyolite 1.6 Y Baxter Hollow granite isochron age of 1,635 ± 33 Ma (A = 1.42*10. 11 n Fox River Valley rhyolite yr· l ). Subsequently, it was suggested that such 'V Fox River Valley granite 1.4 Rb-Sr resetting was caused by regional <0 low-grade metamorphism related to foreland ~ (f) 1.2 deformation associated with the ~ 1,650 Ma !'--­ Mazatzal orogeny in the southwest U.S. (Dott, <Xl (jj 1.0 1983; Van Schmus et aL, 1993), with the implication that folding and metamorphism of 0.8 the Baraboo Range also occurred at this time. MSWD = 8.1 The position of a tectonic front marking the 0.6 I I I I I I I I extent of foreland deformation in northern and o 10 20 30 40 central Wisconsin (see Fig. 1) was established by Rrfl7j sr8 6 40 ArP9 Ar analyses of hornblende and biotite in basement rocks (Holm et al., 1998; Romano et aL, Figure 15. Rb-Sr isochron for granite and 2000); the tectonic front separates post-Penokean ages rhyolite from Baraboo and the Fox River Valley. in the south from Penokean and older ages to the north. Age and intercept recalculated from data of A 40ArP9 Ar investigation of hornblende and Dott and Dalziel (1972) and Van Schmus et al. muscovite in the Baraboo Range was recently under(1975). taken in an effort to confirm a metamorphic age of 1,630 Ma (Naymark et al., 2001). However, rather than obtaining evidence for 1,630 Ma metamorphism, a strong overprint at -1,460 Ma was found, presumably related to the activity of hydrothermal fluids driven by a thermal pulse from Wolf River magmatism. These significant new results are discussed further in the following sections on Field Localities 1 and 2. i . . . I — 9 9 ! I �DESCRIPTIONS OF FIELD TRIP LOCALITIES LOCALITIES Figure 16. Figure 16. Perspective Perspective sketch sketch map map of of the the Baraboo Baraboo Range, Range, showthg showing Field Field Trip Trip Localities Localities 1-4, 1-4, of quartzite in in the Baraboo syncline syncline (light stipple), extent extent of of glacial glacial drift drift (parallel (parallel distribution of lines), and and important from map map by by L.J. LJ. Maher). Maher). lines), important geographic geographic features features (modified (modified from Locality 1: Locality 1: Baxter Baxter Hollow Hollow 1 (SW¼, Sec 33, 33, T1IN, TI iN, R6E; (SW i4, Sec R6E; Figure Figure 17) 17) Relation Baxter Hollow Hollow granite granite and and Baraboo Relation between between Baxter Baraboo Quartzite; sub-Baraboo paleosol Quartzite: sub-Baraboo paleosol Note that the are on Note that the outcrops outcrops in in Baxter Baxter Hollow Holloware 0!l private private property, and and pennission permission is is required required for for access. access. property, Although the contact contact between Although the between Baxter Baxter Hollow Hollow granite and Baraboo Quartzite is not exposed, granite and Baraboo Quartzite is not exposed, the the closest outcrops of the two rock types being 20 closest outcrops of the two rock types being 20 feet feet apart, (1970) concluded and Doff Dott (1970) concluded that that quartzite quartzite apart, Daiziel Dalziel and is nonconformable on on granite, granite, because is nonconformable because of of the the absence absence of granitic granitic dikes dikes in quartzite, quartzite of in quartzite, quartzite xenoliths xenoliths in in granite, and contact metamorphic effects in quartzite. granite, and contact metamorphic effects in quartzite. The uppermost uppermost outcrops outcrops of of granite granite in The in Baxter Baxter Hollow Hollow are commonly commonly sheared, sheared, most most likely are likely due due to to concenconcen­ tration along the tration of of deformation deformation along the quartzite-granite quartzite-granite Figure 17. Locality 1, 1, Baxter Baxter Hollow. Hollow. Symbols: Symbols: PEg, PEg, contact by differential slip between two competent contact by differential slip between two competent Baxter Hollow Hollow granite; granite P€b, Quartzite Baxter PEb, Baraboo Quartzite; lithologic units units during during folding. folding. lithologic €, Upper Cambrian quartzite conglomerate and E, and conglomeratic sandstone. This and other locality locality maps from from Dalziel and Dott, 1970. maps 1970. 10 10 �i; I :1 W rrIiiI I . ' c: 11: 18. Composite figure figure of Baraboo Baraboo paleosol paleosol in Drill Drill Core 613. Left: boxed drill core with recovered Figure 18. quartzite. regolith, and saprolite; saprolite; center: enlargements of selected selected intervals; right: photomicrographs of quartzite (note (note Si0 Si022 overgrowths on on detrital detrital quartz quartz grains). grains). regolith (pedogene). and saprolite. quartzite saprolite. 11 11 �The lower lower 100-200 The 100-200 meters meters of of quartzite quartzite exposed exposed uphill uphill to to the the north north are are characterized characterized by by red, red, Pebbles average 30 sand-sized strata strata with with pebble mm in in sand-sized pebble beds beds and and lenses. lenses. Metapelite Metapelite layers layers are are rare. rare. Pebbles average 30 mm formation, milky vein quartz pebbles diameter, but range up to 68 mm near the base. Throughout the diameter, but range up to 68 mm near the base. Throughout the formation, milky vein quartz pebbles The latter probably predominate, predominate, but but red red cherty cherty granules granules and and pebbles pebbles form form aa persistent persistent lesser lesser component. component. The latter probably Stratification here here consists include include clasts clasts of of devitrified devitrified groundmass groundmass from from the the 1,750 1,750 Ma Ma rhyolites. rhyolites. Stratification consists mostly mostly 50 cm thick. Several these of long, low-angle oflong, low-angle cross cross bedding bedding in in both both tabular tabular and and wedge wedge sets sets up up to to 50 cm thick. Several sets sets of ofthese overturning beneath beneath their their upper upper truncation truncation surfaces. surfaces. A cross laminae show syndepositional overturning A sudden sudden and disturbed pore fluid and grain increase of shear velocity scoured grains from the bed increase of shear velocity scoured grains from the bed and disturbed pore fluid and grain packing, packing, causing incipient of the the cross cross bed bed set set deformed deformed as as ififentirely entirely causing incipient liquefaction liquefaction during during which which the the upper upper part part of characteristic of fluvial and liquid. Such Such distorted distorted cross liquid. cross bedding bedding in in low-angle low-angle sets sets is is most most characteristic of fluvial and tidal tidal sands, sands, which experience large fluctuations of current velocity. which experience large fluctuations of current velocity. holes drilled drilled in The quartzite-granite quartzite-granite contact The contact was was penetrated penetrated by by eight eight holes in 1959 1959 by by the the U.S. U.S. Army Army the contact was recovered in only one drill core, this this Corps of Engineers, and although material from the contact was recovered in only one drill core, single recovery was fortuitous, because it demonstrates the the existence existence of of aa paleosol paleosol beneath beneath the the quartzite quartzite pebbly quartzite is separated from underlying (Medaris et et al., al., 1997). 18) overlying overlying pebbly quartzite is separated from underlying (Medaris 1997). In In drill drill core core 613 613 (Fig. (Fig. 18) pedogenic zone, zone, consisting consisting of of fine-grained foot-thick, reddish-purple pedogenic granitic saprolite saprolite by byaa2Y2 2\12 foot-thick, fine-grained hematite, hematite, largely Granitic texture is preserved in saprolite, but biotite quartz, and muscovite (± (± kaolinite). kaolinite). Granitic texture is preserved in saprolite, but biotite is is largely replaced by by muscovite. muscovite. The replaced by hematite, and and feldspar feldspar isis completely completely replaced The first first feldspar feldspar to to appear appear in in of the saprolite drill core is feet below below quartzite. quartzite. Note that is —30 ~30 feet that the the uppermost uppermost part part ofthe saprolite (2" (2" thick) thick) has has aa not in to that pronounced pronounced planar planar fabric fabric (Fig. (Fig. 18), 18), which which is is similar similar in in style, style, if if not in scale, scale, to that in in the the sheared sheared granite granite outcrops. change in oxides, has been calculated by Chemical weathering weathering in as % Chemical in the the paleosol, paleosol, expressed expressed as % change in oxides, has been calculated by unweathered granites from outcrops —30 and 40 feet comparing paleosol paleosol samples comparing samples to to the the average average of of two two unweathered granites from outcrops ~30 and 40 feet (Fig. to be be immobile during weathering below the contact, contact,and andnormalizing normalizingtotoA1203, Al z0 3, which which is is assumed assumed to immobile during weathering (Fig. in K. effective removal of Mg, Ca, and Na, and enrichment 19). 19). The The most most notable notable chemical chemical changes changes are effective removal of Mg, Ca, and Na, and enrichment in K. o ,.-----~-o-r~ 0 o 1 ,--------,---___, 0.1 o ~ .::."....;~. ,.~..... :. " :.......'::..1­ 0 a ~ I s s ss ~ 10 {g :.~ 10 10 ••••••••••••••••••••• •••• ••••••• J••••• GG .. ..... 100 ~--'-'---'--'-~~...L.......l.-o-~ 20 -20 -40 -60 -60 -40 -20 00 20 01 — ~ ..~ s 10 :~ L ..; 100 -100 -100 100 0 0 100 300 200 200 300 % change Fe203 % change Fe203 L . 50 25 0 100-100......--e~-'-'-'-'~~~.L........J -50 -25 -75 -100 -75 -50 -25 MgO 0 25 50 % change Lot % change MgO 0.1 ,------,-~-___, 0 0 0 III ~ 0.1 01 01 b 10 I r i G % % change change S102 5i02 01 ~ 0 0 I peciogenic pedogenic zone z one, 0 0 0 i,: ~(·········t·:~ i·····:::,;::··l···:~·····r·1· 1 I 0) -c 0 1 0 .- s saprO1!t 10 10 is athered ,·················,1; ·········,··;;=:,:;;;;····;1'· ·················'···1·;·'· I_t_II• GIG 100 100 -100 -100 -80 -80 -60 -60 ' -40 -40 -20 -20 % change % change CaO CaO 00 1 0 -60 ' -1000-60 -100 -80 -60 -40 -40 -20 -20 % change Na20 % change Na20 ' 0 0 100-80 -60 -40 -20 0 ' 20 . -80 -60 -40 -20 0 K20 20 % change % change K20 19. Depth Depth variation variation in in % % change change of of selected selected oxides Figure Figure 19. oxides in in the the Baraboo Baraboo paleosol. paleosol. 0, O. pedigene; pedigene; S, S, saprolite; saprolite; G, G, granite. granite. 12 12 40 60 40 60 �___________________________________________ occurred, but that K K was We believe that all feldspar was altered during weathering weathering and that K leaching occurred, reintroduced during later metasomatism 80 " - - - - - - - - - - - - - - - - - - - - - - - , 80 associated with fluid flow along the sub­ subsaprolite comparison comparison 60 Baraboo nonconformity. Note that detrital E 0 ]i feldspar is rare in quartzite, and interlayered, interlayered, 0 ec. 40 0. fine-grained metasedimentary metasedimentary rocks are are 20 .S for impoverished in K, Na, and Ca. A model for ~ \ ,c .... t.:: / .s0 01 Baraboo saprolite, prior to K metasomatism, metasomatism, ~ is is provided by the Barron saprolite, saprolite, which is ~ -20 a) ~ unmetamorphosed and located located north of the the • asa) -40 0) g> inferred 1,630 Ma tectonic front (Fig. 1; 1; 0 ~ -60 -c Medaris, 2000). A comparison comparison of the two ':!< o -80 saprolites (Fig. 20) reveals a close similarity similarity in most elements, except for the enrichment of -100 ! ¥ ! '.~! ! -100 p AI Si Rb Al Rb KK Ba Sr Na Ca Ca Mg Mn Fe Fe Ti K and Rb in Baraboo saprolite. The chemical features of the Baraboo and Barron paleosols Figure Figure 20. Chemical Chemical comparison of are similar to those of modern-day, modem-day, mature weathering weathering Baraboo and Barron saprolites. Baraboo profiles developed in warm, humid climates. profiles - (,l C.) 1 I 1 An apparent whole-rock Rb-Sr isochron age of the paleosol is 1336 ± ± 75 Ma (Fig. 21), and 39 Ar release spectrum, with a well-defined plateau plateau at at metasaprolite yields a discordant 39Ar muscovite from metasaprolite evidence for 1,456 ± 11 (Fig. Naymark et aI., These data provide the substantial evidence a 1,456 11 Ma 22; al., 2001). These data provide first substantial ± Wolf River imprint on the Baraboo Range, due most likely to the the effects of hydrothermal hydrothermal fluids that were channeled along the sub-Baraboo nonconformity and driven by a thermal pulse generated by regionally sub-Baraboo nonconformity extensive magmatism of Wolf River age. 0.85 j ~ 0.80 (jj r-:- co (jj 075 0.75 z 7 I a0 I I 1 I I 1500 ro 1500 1)3 ~ 1250 1250 OJ 'E ~ « 0. c. 0. c. I I I I I 55 I ! L1J~ 1456 t 11 ± Ma (2o) Ma (20") 1000 1000 750 muscovite, metasapralite, metasaprolite, Baxter Hollow muscovite. Hallow 500 500 250 250 o 0 I 6 ThnJI IF=!' C) Cl «0 MSWD = 19.4 194 2 4 2 3 3 4 TI- - - - - - - - - - - - - - - - - - - - , 1750 1750 UW Radiogenic Radio genic Isotope Lab Lab Analyst: Ron Schott — 0.70817 0.70817 ~ 0.70 0.70 \ 2000 I Baraboo paleosol paleosol R regolith (pedogene) (pedogene R S s saprolite saprolite UW Rare Gas Geochronology UWRare Geochronoiogy Lab Lab Analyst: A!,ssa Alissa Naymark Naymark 4naIySf: +-I---~--~---~-------< 40 60 80 20 40 60 80 100 o 0 7 Cumulative 39Ar Released (%) 87; Sr 86 Rb87! Sr86 Rb Figure 21. Rb-Sr whole-rock isochron for the paleoso!. metamorphosed Baraboo paleosol. 39Ar Figure 22. 22. 39 Ar release spectrum for muscovite from the the metamorphosed Baraboo saprolite. 13 13 �Locality 2: 2: Hydrothermal Hydrothermal Veins. Locality Veins, Hwy Hwy 12 12 (SW1/4, NE1/4, Sec Sec 34, 34, TIIN, Ti iN, R6E; (SWl/4, NEl/4, R6E; Figure Figure 23) 23) Diaspore.-muscovite-pyrophyllite veins in in Baraboo Baraboo Diaspore-muscovite-pyrophyllite veins Quartzite, near the base of the section Quartzite, near the base ofthe section Figure 24. 24. Diaspore-muscovite-pyrOPhyllite Figure Diaspore-muscovite-pyrophyllite veins veins in quartzite. in quartzite. Figure 23. Locality 2, 2, hydrothermal hydrothennal vems veins in in quartzite. Symbols as in in Fig. Fig. 17. 17. of Baraboo Quartzite, which contains a A few few meters meters east east of dark red A of Highway Highway 12 12 is is aa dark red outcrop outcrop of Baraboo Quartzite, which contains a consisting of of diaspore, diaspore, muscovite, muscovite, and network ofthin, of thin, white network white hydrothermal hydrothermal veins veins (Fig. (Fig. 24), 24), consisting and pyrophyllite. pyrophyllite. occurring in the center and intergrown muscovite The veins veins are are commonly zoned, with The with diaspore occurring in the center and intergrown muscovite and and pyrophyllite along the margins (Fig. 25). Kaolinite pyrophyllite along the margins (Fig. 25). Kaolinite of pyrophyllite. occurs as a retrograde replacement of pyrophyllite. Locally, fine-grained fine-grained sedimentary Locally, sedimentary rocks, rocks, originally quartz-bearing, near the base of the originally quartz-bearing, near the base of the quartzite have quartzite have been been pervasively pervasively replaced replaced by by diaspore, diaspore, muscovite, pyrophyllite, pyrophyllite, and muscovite, and hematite, hematite, resulting resulting in in aa soft, purple-red stone that was quarried by Native soft, purple-red stone that was quarried by Native Americans for for the the production production of of pipes. pipes. This Americans This mineral mineral assemblage is the same assemblage is the same as as that that in in the the classic classic pipestone (catlinite) (catlinite) from pipestone from the the Sioux Sioux Quartzite Quartzite (Medaris et al., 1999). The formation (Medaris et a!., 1999). The formation of of diaspore diaspore at at 300-350°C in in the the Si0 Si02-rich 300-350°C environment of of the the 2-rich environment Baraboo and and Sioux Baraboo Sioux quartzites quartzites is is surprising, surprising, and and requires the influence of a fluid phase with an requires the influence of a fluid phase with an 0.01. extremely extremely low low activity activity of of Si02, Si0 2, on on the the order order of of 0.0 I. 14 14 05mm Figure 25. Photomicrograph of hydrothermal vein in Figure 25. Photomicrograph ofhydrothennal vein in quartzite (plane (plane polarized polarized light). quartzite light). Abbreviations: Abbreviations: dsp, dsp. diaspore; ms, muscovite: prl, pyrophyllite. diaspore; ms. muscovite: prl. pyrophyllite. �Muscovite in a sample discordant 39Ar 39Ar release release spectrum with with a Locality 2 yields a discordant sample of vein from Locality " --------------------, 2000 plateau age of 1,467 ± 11 Ma, which is within ± 11 error of the 1,456 Ma plateau age for muscovite muscovite 1750 1750 from metasaprolite. Thus, the age of fluid 1500 1500 ..... ro activity responsible for K-metasomatism in the 1250 Baraboo 1,460 Ma. J1250j ~ -1,460 Baraboo Range is well established at ~ 1467 ±± 11 Ma Ma (2o) (20-) 1467 Although migration was C woo 1000 Although large-scale fluid migration !!! probably probably driven by heat from Wolf River fr 750 muscovite, ms-pu-dsp ms-prl-dsp vein, vein, Hwy Hwy 12 12 muscovite, c( magmatism, magmatism, the source and composition of such 500 500 fluid fluid remains remains unknown unknown and deserves UW Rare Gas Gas Geocivoao!ogy GeochronologyLab Lab 250 250 UWRare investigation. Analyst: Alissa Nayma,* Naymarl< AaaIyst I f 11 AhSSa Figure 26. Ar release spectrum for muscovite from ~ 26. 39 Ar hydrothermal vein in quartzite, Highway 12. o 0 I o 0 I 20 20 40 40 60 60 80 80 100 100 CumulatIve 39Ar Released (%) (%) Cumulative Ar Released Locality 3: Quartzite Quartzite and and Metapelite, Metapelite, Hwy 12 12 (NWI/4, IS, Ti TlIN, Figure 27) 27) (NW 1/4, Sec 15, iN, R6E; Figure Sedimentary and metamorphic metamorphic features of Baraboo Baraboo Sedimentwy quartzite and metapelite metapelite "~:Ji£b Qal 27. Locality 3. 3, Baraboo quartzite and meta­ metaFigure 27. Symbols: Qal. Qal, Quaternary alluvium; other pelite. Symbols: as in in Fig. Fig. 17. 17. symbols as outcrop at Locality Locality 3, illustrating illustrating aa Figure 28. Quartzite outcrop cross sets reactivation surface (left arrow) and contorted cross (right arrow). DO NOT HAMMER HAMMER THIS OUTCROP! OUTCROP! The upper stratigraphic stratigraphic portion portion of the Baraboo quartzite differs in several ways ways from the lower part part at at The Baxter Hollow. Pebbly layers are rare, metapelite layers are Baxter are more more common common (the thickest known zone zone isis exposed here), here), and the style style of stratification is different. Cross bedding exposed bedding is higher angle and occurs occurs in in sets sets mostly 10 10 to 20 em cm thick. thick. Master Master bedding surfaces are commonly defined by by thin metapelite. metapelite. Individual mostly are slightly slightly concave upward; sets of cross laminae are mostly tabular, but some cross laminae are some troughtrough­ shaped sets occur in this roadcut. Excellent asymmetric. sinous-crested sinous-crested ripple ripple marks marks visible visible on one of the exposure indicate the dominant dominant south-flowing surface in the middle ofthe south-flowing paleocurrent direction typical typical the entire foonation. formation. for the On the south-facing south-facing cliff, several reactivation surfaces and some some contorted cross cross sets sets are are exposed exposed in cross section section (Fig. 28). Reactivation surfaces are convex-up in convex-up truncations truncations of of cross bed sets. They They spasmodic activation of of dune forms that produce cross bedding, and are most most characteristic characteristic of indicate spasmodic 15 15 �tidal systems. The dominant migrate to to form form cross cross laminations, laminations, then tidal systems. The dominant tidal tidal flow flow activates activates dunes, dunes, which which migrate then the subordinate flow partially erodes the dunes to form the convex-up surface. When the tide the subordinate flow partially erodes the dunes to form the convex-up surface. When the tide turns turns again, again, the dominant of cross cross laminae, which bury the reactivation laminae, which bury the reactivation the dominant flow flow reactivates reactivates the the dunes dunes to to form form aa new new set set of surface. In this cliff, the the dominant dominant currenf current'ss bed bed shear occasionally disturbed disturbed the the grain grain packing packing and and caused deformation of cross laminae as described for Locality 1. Reactivation surfaces suggest that the the caused deformation of cross laminae as described for Locality I. Reactivation surfaces suggest that origin, indicating a gradual northward of the Baraboo Quartzite is of of shallow marine origin, indicating a gradual northward upper half of transgression during formations are are all all considered considered to to be transgression during its its deposition; deposition; the the overlying overlying Precambrian Precambrian formations be normal normal marine. manne. of the A the north end of of the the roadcut roadcut and and at at the the top A prominent prominent metapelite metapelite layer layer is is exposed exposed at at the north end top of the hill, hill, metapelite (Fig. where one one can can see see excellent exposures of and crenulation crenulation cleavage cleavage in in metapelite (Fig. 29). of chevron folds folds and 29). where Red quartzite quartzite boudins are common Red common in in metapelite, metapelite, as as are are folded folded white white quartz quartz veins, veins, which which appear appear to to inferred to to have have originally originally been emanate from from the the quartzite quartzite boudins. boudins. The metapelite is inferred been aa kaolinite-rich kaolinite-rich in metapelite, which shale, based on its bulk chemical composition. composition. The which is is shale, The present present mineral mineral assemblage assemblage in metapelite, with subordinate typical for metapelite metapelite throughout throughout the the Baraboo Baraboo Range, Range, consists consists mostly mostly of ofpyrophyilite pyrophyllite with subordinate small grains grains of of rutile, and minor tablets of of black black hematite, hematite, small rutile, and minor amounts of recrystallized quartz, thin tablets retrograde kaolinite (Fig. 30). Trace of retrograde kaolinite (Fig. 30). Trace amounts amounts of Ce-bearing svanbergite, SrAl3(P04)(S04)(OH)6, Ce-bearing svanbergite, SrAb(P0 4 )(S04)(OH)6, 10 to 20 20 !-lm tm in 10 to in diameter, diameter, are are scattered scattered through through the the metapelite (Medaris and Fournelle, 1998). The metapelite (Medaris and Fournelle, 1998). The occurrence of this this diagenetic diagenetic aluminophosphatealuminophosphate­ occurrence of sulfate mineral is significant because itit bears bears on on sulfate mineral is significant because the phosphorus flux in the oceans. the phosphorus flux in the oceans. Figure 30. Back-scattered Back-scattered electron electron image image of of Figure 30. metapelite. Locality 3. Abbreviations: h, hematite; metapelite, Locality 3. Abbreviations: h, hematite; k, k, kaolinite; kaolinite; p, p, pyrophyllite; pyrophyllite; q, q, quartz. quartz. Figure 29. Chevron folds folds and and crenulation crenulation cleavage cleavage in metapelite, Locality 3. The scale The scale coin, coin, enhanced enhanced by a black circle, is is 2.5 cm em in in diameter. diameter. DO NOT HAMMER THIS THIS OUTCROP! OUTCROP! 16 16 �Gorge Ableman's 4: Locality Locality 4: Ableman's Gorge dunes eolian Cambrian Upper Rock; Hise Van zone; breccia quartzite quartzite; in layers metasiltstone and ripples Quartzite; Baraboo and conglomerate Cambrian Upper between unconformity Angular 31) Figure R5E; T12N, 28/29, Sec (SW1/4, (SW1I4, Sec 28/29, T12N, R5E; Figure 31) Angular unconformity between Upper Cambrian conglomerate and Baraboo Quartzite; ripples and metasiltstone layers in quartzite; quartzite breccia zone; Van Hise Rock; Upper Cambrian eolian dunes 17. Fig. in as symbols other Group; City Tunnel Cambrian Upper €tc, Sandstone; Galesville Cambrian Upper Eg Symbols: dunes. eolian Cambrian and unconformity angular 4D, Rock; Hise Van 4C, breccia; quartzite 4B, Quartzite; Baraboo and conglomerate Cambrian Upper between unconformity angular 4A: Gorge. Ableman's 4, Locality 31. Figure Figure 31. Locality 4, Ableman's Gorge. 4A: angular unconformity between Upper Cambrian conglomerate and Baraboo Quartzite; 4B, quartzite breccia; 4C, Van Hise Rock; 4D, angular unconformity and Cambrian eolian dunes. Symbols: €g Upper Cambrian Galesville Sandstone; Etc, Upper Cambrian Tunnel City Group; other symbols as in Fig. 17. N 8r,,",,- Zon. ­ 1100 1000 .11m:['p'f~~ ~11.EJ51in ",?,.iH I~~~900 D B C 1 ? ? A t 'kh~s;;~.. 5'/0 iOcx ~:;;:;r~I']1'l' ~'fq 'Opo Feel o Feel (1970). Dott and Daiziel from Modified gorge. the of ends both at cliffs quartzite buried against butting strata Cambrian of symmetiy the Note D. - A letters by indicated Localities Field with west), (lookmg Gorge Ableman's through section Cross 32. Figure Figure 32. Cross section through Ableman's Gorge (looking west), with Field Localities indicated by letters A-D. Note the symmetry of Cambrian strata butting against buried quartzite cliffs at both ends of the gorge. Modified from Dalziel and Dott (1970). features guartzite unconformity; angular Precambrian - Cambrian Upper 4A: 4A: Upper Cambrian - Precambrian angular unconformity; quartzite features Baraboo. the of part upper marine, the in be must exposures These waves. by formation indicate which symmetric, and straight relatively are these of crests the 3, Locality at ripples the Unlike bed. overlying the of bottom the on ripples the of casts are see we What board. wash great-grandmother's resembling face marked ripple spectacular a is quarry the of end south The help). (binoculars boulders quartzite coarse with conglomerate Cambrian Upper brown-weathering and quartzite red between unconformity angular the see can one cliff, the of top the At face. quarry old the in visible layers metasiltstone thin and sets bed cross with quartzite vertical exposes 32) (Fig. Gorge Ableman's of side west the on quarry abandoned The The abandoned quarry on the west side of Ableman's Gorge (Fig. 32) exposes vertical quartzite with cross bed sets and thin metasiltstone layers visible in the old quarry face. At the top of the cliff, one can see the angular unconformity between red quartzite and brown-weathering Upper Cambrian conglomerate with coarse quartzite boulders (binoculars help). The south end of the quarry is a spectacular ripple marked face resembling great-grandmother's wash board. What we see are casts of the ripples on the bOllom of the overlying bed. Unlike the ripples at Locality 3, the crests of these are relatively straight and symmetric, which indicate formation by waves. These exposures must be in the marine, upper part of the Baraboo. 17 17 �4B:Qjiartzite 4B: Quartzite Breccia Breccia Zone Exposed on on the the west west wall wall of the gorge Exposed of the gorge is is aa quartzite breccia zone, consisting of angular red quartzite breccia zone, consisting blocks cemented cemented by a stockwork of of white quartz blocks generally massive, veins (Fig. (Fig. 33). 33). The vein quartz is generally massive, veins of which but locally euhedral quartz crystals, some crystals, some of which (a member member of are coated by dickite (a of the the kaolinite kaolinite are The mineral group), group), occur occur in mineral in vugs vugs (Fig. (Fig. 34). 34). The growth euhedral quartz crystals commonly show growth euhedral indicates that zoning, and and preliminary investigation indicates that zoning, isotopes quartz is is zoned zoned with with respect to oxygen isotopes as as quartz 18 from 9.33 to 18.95 %o ranging well, with 18O from 9.33 to 18.95 %0 well, with 8 0 in one one crystal crystal (Fig. (Fig. 35). 35). S. (VSMOW) in (VSMOW) S. W. W. Bailey Bailey for fluid reported a temperature of of 105-107°C 105-1 07°C for fluid reported inclusions in in the the quartz, quartz, which which is inclusions is consistent consistent with with the the (cited in in Daiziel Dalziel and and Doft, Dott, 1970). 1970). presence of dickite (cited In some some places places in in the the breccia breccia zone, zone, the the In slightly separated quartzite fragments fragments appear quartzite appear to to be be slightly separated the impression pieces of a jigsaw jigsaw puzzle, giving the impression that that fragmented by some the quartzite might have been fragmented by the some Perhaps brecciation type of explosive activity. Perhaps brecciation was was type (sub critical caused by passage of a low-temperature caused by passage of a low-temperature (sub critical fluid to to the the vapor vapor point) hydrothermal hydrothermal fluid point) fluid from from the the fluid shallower levels field as it migrated from deeper to shallower levels field Figure 33. Quartzite breccia, Martm-Marietta Quarry, north limb limb of of the the along the the quartzite strata in the north along Figure 33. Quartzite breccia, Martin-Marietta Quarry, along strike and 1 km east of Field Locality 4B. hydrothermal activity activity is syncline. The The age age of such hydrothermal along strike and ­ I km east of Field Locality 4B. is syncline. Precambrian, because unknown, other other than than being being Precambrian, unknown, because Cambrian conglomerate. breccia boulders and cobbles of quartzite occur in in the the overlying overlying Upper Upper Cambrian conglomerate. boulders and cobbles of quartzite breccia occur 1 mm 1 mm UW Stable Isotope Lab Analyst: Mike Spicuzza UW Stable Isotope Lab Analyst Mike Spicu2Za Sample Sample 98BB20 986820 breccia, Figure 34. 34. Polished Polished slab Figure slab of of quartzite quartzite breccia, crystals. showing aa vug vug lined lined by by euhedral euhedral quartz showing quartz crystals. VSMOW) in a zoned Figure 35. Variation of ö'°O 180 (%o, of8 (%0, VSMOW) in a zoned Figure 35. Variation breccia. euhedral quartz crystal from the quartzite euhedral quartz crystal from the quartzite breccia. 18 18 �here. quartzite the in seen be also can cleavage widely-spaced a and bedding Cross idea. better a have not do We surface. unusual this produced probably time Pleistocene during silt and sand by blasting wind that suggested Twenhofel W.H. 1930s, the In polish. exceptional an has quartzite of outcrop an highway, the of west just and north meters 100 32). (Fig. gorge the of end south the at spring artesian the behind seen is as quartzite the in joints and bedding along weathering by produced fissures down filtered that sand Cambrian as it interpret We saw? just we that unconformity the below feet 200 least at here doing this is What quartzite. vertical of outcrop an within quartzite of fragments angular enclosing sandstone quartz Cambrian-type tan-colored, has exposure subtle a zone, breccia the of north meters 100 About alteration. hydrothermal of episodes unrelated and different two by affected was Range Baraboo the whether or 2), and 1 (Localities quartzite the of base the modified that fluids River-age Wolf the of variants shallower and cooler are brecciation with associated fluids thermal hydro- the whether seen be to remains It Range. Baraboo the in feature widespread a thus is breccia quartzite of development The syncline. the throughout scattered are vertical, and east-west oriented also features, breccia incipient with veins quartz thin and kilometers, of-20 distance a over syncline Baraboo the of limb north the along occur breccia quartzite of Outcrops dip. vertical and strike east-west an with layers, quartzite adjoining the to parallel oriented and thick meters -400 is zone breccia The The breccia zone is ~ 100 meters thick and oriented parallel to the adjoining quartzite layers, with an east-west strike and vertical dip. Outcrops of quartzite breccia occur along the north limb of the Baraboo syncline over a distance of ~20 kilometers, and thin quartz veins with incipient breccia features, also oriented east-west and vertical, are scattered throughout the syncline. The development of quartzite breccia is thus a widespread feature in the Baraboo Range. It remains to be seen whether the hydro­ thermal fluids associated with brecciation are cooler and shallower variants of the Wolf River-age fluids that modified the base of the quartzite (Localities 1 and 2), or whether the Baraboo Range was affected by two different and unrelated episodes of hydrothermal alteration. About 100 meters north of the breccia zone, a subtle exposure has tan-colored, Cambrian-type quartz sandstone enclosing angular fragments of quartzite within an outcrop of vertical quartzite. What is this doing here at least 200 feet below the unconformity that we just saw? We interpret it as Cambrian sand that filtered down fissures produced by weathering along bedding and joints in the quartzite as is seen behind the artesian spring at the south end of the gorge (Fig. 32). 100 meters north and just west of the highway, an outcrop of quartzite has an exceptional polish. In the 1930s, W.H. Twenhofel suggested that wind blasting by sand and silt during Pleistocene time probably produced this unusual surface. We do not have a better idea. Cross bedding and a widely-spaced cleavage can also be seen in the quartzite here. Rock Hise Van 4C: 4C: Van Hise Rock wall). washboard ripple-cast the did (as south the to be also must up' 'way the that shows geometry its for this, with consistent is bedding cross truncated The Gorge. Ableman's of south the to axis its with syncline a be must structure that that infer then We fold. large a surface axial the parallels roughly cleavage the that assume we part, a is Rock Hise Van which of structure larger the see not can we Although beds. quartzite two these of each in visible is bedding cross addition, In side. either on quartzite pink coarser the into (flattens) refracts cleavage this that note degrees; 20 about northward dipping cleavage slaty shows it Today Quartzite. Baraboo the of part upper the mudstone silty a as deposited was rock the of center the in band dark vertical The within Landmark. Historical National a declared was Rock the 1999, May, In annually. visit professionals and students of hundreds century; a for trips field for stop essential an this made has Rock Hise Van of clarity and accessibility The 'way-up.' or direction facing structural determining for marks ripple and bedding cross folds, drag cleavage, of use the perfected team Their region. Superior Lake the of geology Precambrian the study to side. either on quartzite cross-bedded by bounded layer central Leith later and Hise Van by headed Madison slaty the with east, the from viewed Rock Hise Van 36. Figure at office district a 1882 in established had Survey Geological U.S. the mining, iron of commencement the by Stimulated country. vegetated in clear not typically are which structures, larger-scale inferring for clue scale outcrop- an as bedding and cleavage slaty between relationship geometric fundamental the demonstrate to laboratory a as this used Leith, K. Charles protégé, his and Hise Van R. Charles sign. historic 1999 a and plaque metal 1923 a by described as geologists, for interest historic special of is 36) (Fig. highway the of side east the on Rock Hise Van Van Hise Rock on the east side of the highway (Fig. 36) is of special historic interest for geologists, as described by a 1923 metal plaque and a 1999 historic sign. Charles R. Van Hise and his protege, Charles K. Leith, used this as a laboratory to demonstrate the fundamental geometric relationship between slaty cleavage and bedding as an outcrop­ scale clue for inferring larger-scale structures, which are typically not clear in vegetated country. Stimulated by the commencement of iron mining, the U.S. Geological Survey had established in 1882 a district office at Figure 36. Van Hise Rock viewed from the east, with the slaty Madison headed by Van Hise and later Leith central layer bounded by cross-bedded quartzite on either side. to study the Precambrian geology of the Lake Superior region. Their team perfected the use of cleavage, drag folds, cross bedding and ripple marks for determining structural facing direction or 'way-up.' The accessibility and clarity of Van Hise Rock has made this an essential stop for field trips for a century; hundreds of students and professionals visit annually. In May, 1999, the Rock was declared a National Historical Landmark. The vertical dark band in the center of the rock was deposited as a silty mudstone within the upper part of the Baraboo Quartzite. Today it shows slaty cleavage dipping northward about 20 degrees; note that this cleavage refracts (flattens) into the coarser pink quartzite on either side. In addition, cross bedding is visible in each of these two quartzite beds. Although we can not see the larger structure of which Van Hise Rock is a part, we assume that the cleavage roughly parallels the axial surface a large fold. We then infer that that structure must be a syncline with its axis to the south of Ableman's Gorge. The truncated cross bedding is consistent with this, for its geometry shows that the 'way up' must also be to the south (as did the ripple-cast washboard wall). 19 19 �and Eolian 4D: Basal Basal Cambrian Cambrian Unconformity and 4D: Eolian Dunes Dunes basal Cambrian unconformity again to the northeast (Fig. From Van Van Hise Hise Rock, From Rock, we we can can see see the the basal Cambrian unconformity again to the northeast (Fig. Cambrian conglomerates. high on on the the wall wall at the top 32, right right end). end). The flat-lying strata high 32, at the top of of the the Gorge Gorge are are Cambrian conglomerates. against buried cliffs of sandstones underlie As shown shown in in Figure 32, Cambrian sandstones As underlie these these and and terminate terminate against buried cliffs of eolian dunes blown up these sandstones quartzite. Large, Large, sweeping cross beds in these quartzite. sandstones were were formed formed within within eolian dunes blown up fell from the cliffs to be angular blocks blocks of of quartzite quartzite occasionally against those those cliffs. cliffs. Rare, Rare, scattered angular against occasionally fell from the cliffs to be Late Cambrian sea is sand. The The arrival the encroaching buried, but but not abraded, abraded, by by the the eolian sand. buried, arrival here here of ofthe encroaching Late Cambrian sea is quartzite conglomerate, which is underlain by a recorded by by aa sharp boundary with overlying rounded recorded rounded quartzite conglomerate, which isparallel underlain by a vertical called Skolithos (closely spaced, straight, sandstone layer with marine worm burrows called sandstone Skolithos (closely spaced, straight, parallel vertical 20 mm from 33 -- 20 tubes about 1 mm in diameter and from mm long). long). pocket sample of most of the important features of the Ableman's Gorge provides a geologic vest of the Ableman's Gorge provides a geologic vest pocket sample of most of the important within afeatures mountain deposited, then then folded folded and Baraboo Hills. Baraboo Quartzite was deposited, and metamorphosed metamorphosed within a mountain Ma ago), an elliptical ring of quartzite hills eroded. In range, which was then eroded. In Late Late Cambrian Cambrian time time (500 (500 Ma ago), an elliptical ring of quartzite hills range, encroached, those hills were converted deposits. Then, began to be buried by wind deposits. began Then, as as the the Cambrian Cambrian sea sea encroached, those hills were converted tropics at about 15 degrees south latitude, so islands. North North America America lay to an an atoll-like atoll-like ring ring of of islands. lay in in the the tropics at about 15 degrees south latitude, so to cliffs and tore away quartzite huge waves, waves, which the sea passing tropical storms generated huge which broke broke upon upon the sea cliffs and tore away quartzite and occasionally swept rounded boulders blocks. Repetition of this scenario rounded boulders up up to to 1.5 1.5 m m in in diameter, diameter, and occasionally swept blocks. the islands became buried by sediment, for half haifaa kilometer some of them them offshore for kilometer or or so. so. Gradually Gradually the islands becameisland burieddisappeared by sediment, some by the clearly revealed than here. The Abieman testimony of which is nowhere more clearly revealed than here. The Ableman island disappeared by the buried until the end of the Ordovician highest islands end of Cambrian time, but the highest islands were were not not finally finally buried until the end ofthe Ordovician end Period (ca 440 Ma). Acknowledgments departmental Weare are indebted indebted to to many of our departmental We many ofour encouragement, and and colleagues for for their interest, encouragement, colleagues John Fournelle technical expertise. These include technical expertise. These include John Fournelle (rare gas (electron microprobe,1, Brad Singer Singer (rare (electron microprobe), Brad gas isotopes), isotopes), John Valley (stable Clark Johnson (radiogenic isotopes), Clark Johnson isotopes), John Valley (stable inclusions), who isotopes), and and Phil Phil Brown Brown ('fluid (fluid inelusiam), who isotopes), made possible possible provided analytical analytical facilities facilities that that made provided Baraboo data. data. Several acquisition ofmany of man of the new ofthe new Baraboo Several acquisition analyses that that are are students and technicians pe/formed performed analyses students Alissa Nayniark, reported here, reported here, including ineluding Robb Robb Bunge, Bunge, Alissa Naymark. Brian Hess Hess prepared prepared Ron Schott, Schott, and and Mike Mike Spicuzza. Spicuzza. Brian Ron thin sections, sections, and and Mary various types of high-quality thin ofhigh-quality Mmy composite diagram of the Diman created created the Diman the marvelous marvelous composite diagram ofthe Baraboo paleosol. Geological and and Natural Natural From the From the Wisconsin Wisconsin Geological provided access to the History Survey. Histor.v Survey. Bruce Bruce Brown Brown provided access to the examination which Baxter Hollow Hollow drill drill core. core, the Baxter the examination of ofwhich take aa new look at at the to take prompted us to new look the Baraboo Baraboo Range, Range, the Barron and Mike Mudrey informed us of paleosol, and Mike Mudrey informed us of the Barron paleosol. unmnodJIedBaraboo Baraboo which serves serves as as aa model for unmodified which model for paleosol. 20 20 Cleopatra's Needle, overlooking Devil's Lake Cleopatra's Needle, overlooking Devil's Lake �REFERENCES Anderson, J.L., Cullers, RL. Anorogenic metaluniinous metaluminous and peraluminous peraluminous granite R.L. and Van Schmus. Sciunus. W.R W.R. (1980) Anorogenic plutonism Wisconsin, USA: Contrib. Mineral. PetroL, 311- 328. plutomsm in the Mid-Proterozoic of Wisconsin, Petrol., v. 74, p. 311PTA-system: a Geo-Calc Geo-Calc software software package package for the Brown, T.H., Berman, RG. and Perkins, E.H. (1989) PTA-system: Mineral., v. 74, 74, p. p. 485-487. 485-487. calculation and display of activity-temperature-pressure phase diagrams: Amer. Mineral., Jr. (1970) Geology of the Baraboo Wisconsin: Wisconsin Geol. GeoL Nat. Dalziel, I.W.D. and Dott, RH. Jr. Baraboo district, Wisconsin: History Survey ml. Inf. Circ. 14. 164 pp. 164 pp. in the the north-central north-central United States: States: Resolved Resolved by by plate plate Dott, RH., R.H., Jr. (1983) The Proterozoic red quartzite enigma in collision?: GeoL p. 129-141. Geol. Soc. Amer. Memoir 160. p. 129-141. Dott, R.H., Jr. and Dalziel, I.W.D. (1972) Age and correlation of of the Precambrian Baraboo Baraboo Quartzite of Wisconsin: 552-568. Jour. Geol., v. 80, p. p. Dott, R.H., Jr., Medaris, L.G., Jr. and Schott. RC. (1997) Doll, Schott, R.C. (1997) Post-1760-Ma deposition of the Baraboo Quartzite: Confinnation Soc. Amer. Abstracts with Programs, Programs. v. Confirmation from detrital zircon ages and new field evidence: Geol. Soc. 29, No.4, No. 4, p.13. p.13. of Early Proterozoic Proterozoic quartzites in in the Holm, Hoim. D.K., Schneider, C.D. (1998) Age and deformation of Schneider, D. and Coath, C.D assembly of southern Lake Superior region: Implications Implications for extent of foreland defonnation deformation during final assembly Laurentia: Geology, V. p. 907-910. v. 26, p. 907-9 10. Dott, R.H.. Jr.. J.H.. Johnson, C.M., Schott, R.C. R.e. and and Baumgartner, L.P. (1996) Age Age Medaris, L.G., Jr., Dolt, Jr., Fournelle, J.H.. CM., Schott, Inst. Lake Superior Geol., Abstracts with and geological significance Superior Geol., significance of the Baraboo Quartzite Quartzite: 42nd 1nst. Programs, v. 42, p. p. 31-32. Dott, R.H.. RH.. Jr. Jr. and McSweeney, K. (1997) (1997) The sub-Baraboo paleosol, Medaris, L.G., Jr., Baumgartner, Bau.mgartner, L.P.. Dolt weathering and metasomatism: metasomatism: 43rd 43rd Inst. Lake Lake Superior Superior Wisconsin: Geochemical Geochemical evidence for Proterozoic weathering 43. p. 3 9-40. p. 39-4U. Geol., Abstracts with Programs, v. 43, R.J. (1998) Post-1.76 Ga low-grade metamorphism of Medaris, L.G., Jr., Brown, Brown. P.B. and Bunge. Rl of the the Baraboo Baraboo v. 44, p.89-90. p.89-90. Quartzite: 44th Inst. Lake Superior Geol.. Abstracts with with Programs, v. Medaris, L.G., Jr., and Foumelk Svanbergite in the Baraboo Baraboo Quartzite: Quartzite: Significance Significance for diagenetic Fournelle. lH J.H. (1998) Svanbergite Geol., Abstracts with processes and phosphorous flux in Precambrian oceans: 44th Inst. Lake Superior GeoL Programs, V. v. 44, p.91-92. p.91-92. Medaris, L.G., Jr., Fournelle, 1.H., Broihan, J.H. (1999) Chemical and mineralogical J.H., Boszhardt, R.F. and Broihan. comparison of Baraboo, Barron. and Sioux argillite. metapelite and pipestone: 45th Inst. Lake Superior Geol., Abstracts with Programs. Programs, v. 45. p.35-36. p.35-36. Medaris, L.G., L.G.. lr. Jr. (2000) The Barron saprolite Confirmation of mature chemical weathering in the the source for Mcdaris, saprolite: Confirmation Paleoproterozoic Paleoproterozoic quartz arenites aremtes in the Lake Superior Superior region: 46th Inst. Lake Superior Geol., Abstracts with Programs, v. 46, p. p. 37-38. Naymark, A., Singer, B.S. and Medaris, L. G.. Jr I) Recognition of Wolf River-age metamorphism in the L.G.. Jr. (200 (2001) Baraboo Ar thermochronology: thermochronology: Geol. Soc. Amer. Abstracts with Programs, v. V. 33, Baraboo Range by means of 4°ArP~ 40Ax/39Ar no. 4, in press. Romano, D., Holm, D.K. and Foland. K.A. (2UOO) (2000) Determining the extent and nature of Mazatzal-related overprinting of the Penokean orogenic belt in the southern Lake Superior region, north-central USA: Res.. v. 104, p. Precambrian Res., p. 25-46 Smith, E.I. El. (1978) Precambrian rhyolites and granites in south-central Wisconsin: Field relations and Smith, 875-890. geochemistry: GeoL Geol. Soc. Amer. Bull Bull.... v 89. p. 875-890. SR. and McLennan, Crust: Its Composition and Evolution: Blackwell, BlackwelL Taylor. S.R McLeiman. S.M. SM. (1985) (1985) The ContlI1ental Continental Crust: Oxford, 312 pp. pp. Van Schmus, W.R., Thurman. E.M. and Peterman. Z.E. ZE (1975) Geology and Rb-Sr chronology chronology of Middle Wisconsin: Geol. Soc. Amer. Bull., v. 86. Precambrian rocks in eastern and central Wisconsin BulL, V. 86, p. p. 1255-1265. K.C. (1993) Early Proterozoic Jr. Proterozoic crustal evolution, in Reed, J.C. J.e. Jr Van Schmus, W.R., Bickford. M.E. and Condie. K.c. U.S.. Geology of North America, America. V. v. C-2, GeoL Geol. Soc. Amer., Boulder, et al., eds., Precambrian: Conterminous US. Boulder, p. 270-281. 21 �This page intentionally left blank �Field Trip 2 Geology, Ore Deposits, and Cultural History of the Upper Mississippi Valley Zinc-Lead District M.G. Mudrey, Jr. Wisconsin Geological and Natural History Survey 3817 Mineral Point Road Madison, Wisconsin 53705-5100 T.C. Hunt University of Wisconsin–Platteville Platteville, Wisconsin 53818 Headframe of a southwestern Wisconsin zinc-lead mine circa 1930 (photograph courtesy of Platteville Mining Museum). �This page intentionally left blank �BACKGROUND The Upper Mississippi Valley Zinc-Lead District of southwestern Wisconsin was one of the oldest continuously producing mining districts in the United States. The largest and most productive parts of the district extended across five Wisconsin counties and into small areas in Illinois and Iowa (fig. 1). Over 1.2 million tons of zinc and nearly 100,000 tons of lead were recovered from the Wisconsin part of the Upper Mississippi Valley District from 1910 to 1974. Heyl and others (1959) suggested that an additional 250,000 tons of zinc and 350,000 tons of lead were produced in the Wisconsin part of the district from 1800 to 1910. This field trip provides a geologic, mineral deposit, and cultural overview of the area. Numerous detailed geologic reports about the area have been prepared; the most significant by Heyl and others (1959) documents ____ R5W. 4 5, 8Z0p8 5 3 4 (lW. 2 ShE. 3 2 N. 5.3* - %J 98 AL CR A N FORD E H 111151(1150 So..0411. , 04 WyLodfl CL 20. Elk.d.,° :: a F/\ PlO1( PlO / 20. Pb ,-51k90R00 'I 90 DE L A W 0th_I:. 08 -: J P1.5*—rn. J3UQU( \ sal 0 U B UO 80* Yb..t a S —- S. RSW '4.zo MADISON — I M(H..b smo LA F A 3 4 1 Y 0AV A S / 084101118(0 — E TT E I N A 4W O \J0 •s I I Pb O L.m (0.o V*-. PSZ.o13 c1( I oa.,...jus. ! CX,I2 SM_S ON 08_i —— b.l W110 P111th do. 5u a—. L F'-—— 94 1*, 0 Ce,18 Mi Pt1 -+44 ,1L58 FL A M A K 97 ° / E Pd - ESS 6 RAE INDEX TO MINES AND DIGGINGS 25 . M0.,.IC,.d01.I08 2 LO&1(jIfld000000 3. Cop,., 0111 ,.io. 4 R1500IO0 00.4 O!W1080 5. 3.01410118011*44401(1 7 24 N 6. AlI leAd Clj1(11884101111t 51(101 C 0100W Il1llI• 9 W800$AORId(OIOOdd00lII*l ID 11111. IIIC5*ff11 0* dIgolliR II W00dlO.4I..d10040 8. f( 12. 0It11Cnl*0p IS. D.195W1111110141 IS. 5101. 811ff SWOOP - IS $o!dgqOfl q00.0, IN 05.100111. 4IWIP II 0(1.10 dIRgIIOP IS St*d(01(&IOSddMO. IS. 1010 C SSZAOP 20. M4*0..A9W14( 2!. *40041.444111(1 22. ML C41011 4089040 250808* 0 EXPLANATION 081.180. .1.5840.4110.110. dSIII.404 e1 — Ml.,, 01 5.1858010 1R Geltilt 0.00,89, and P1110.4111. MIt. II 0410 II• P00.1, dli CA.,, 410115 — 41.0800 Pb.Z0,Co.d 16.4. 8AO0 (005011.01 II Figure 1. Map of the main part of the Upper Mississippi Valley district and mineral deposits in outlying parts of the district (from Heyl and others, 1959). 25 �hundreds of mineral properties in Illinois, Iowa, and Wisconsin. A good summary of the geology and economic geology of the district is given by Heyl and others (1970). In addition, more than 1,000 square miles have been mapped and published on standard 1:24000, 7.5-minute topographic maps. The zinc and lead mines of southwestern Wisconsin are part of the earliest producing zinc-lead mining districts in the United States—the Upper Mississippi Valley District. There was a small amount of production in Minnesota. Production prior to 1800 was very small. Use of galena and mining were reported by explorers Jean Nicolet in 1634 and by Nicolas Perrot in 1692. In 1788 Julien Dubuque obtained permission from the Sauk and Fox to work lead mines in the area.. Dubuque mined, with these Native Americans as his labor force; on the west bank of the Mississippi River in the vicinity of what is now Dubuque, Iowa. This was the principal mining center in the region from 1788 until Dubuque’s death in 1810. Early white miners worked the properties during the summer, returning south in the winter. The arrival of permanent settlers in 1825, the Winnebago Peace Treaty, and “lead rush of 1827” established the Upper Mississippi District as a major mineral producer and resulted in the states of Illinois, Iowa, and Wisconsin. The Mexican War of 1847, the California Gold Rush of 1849, and the cholera epidemic of 1854 brought the district into decline, but by 1859 and the Civil War, production of lead increased, and eventually zinc production began. The District remained in production until 1978 with the close of the Eagle–Picher mine in Shullsburg. Around 1906, systematic mine mapping was begun by students and faculty at the Wisconsin School of Mines, now University of Wisconsin–Platteville. In 1946 systematic geologic mapping was initiated by the U.S. Geological Survey. This mapping program was built on the foundation of mine maps and mineral reserve studies initiated during World War II. Thecooperation of the former U.S. Bureau of Mines, U.S. Geological Survey, University of Wisconsin–Platteville, and the Wisconsin Geological and Natural History Survey led to the development of the Wisconsin Mineral Development Atlas (Heyl and Broughton, 1980). In a series of atlas plates at 1:2400-scale, mine workings, drillhole location and number, and selected surface features (roads, lead digs) are shown. The details of the 30,000 drillholes are kept in approximately two dozen large binders covering Grant, Iowa, and Lafayette Counties. Supplemental data include Green County. From this information Broughton (1991) estimated 12.5 million tons of ore averaging 4.94 percent zinc and 0.47 percent lead remain in place. There are large areas for which there is no modern (post-1900) mineral exploration, and little exploration data below the base of the Sinnipee Group. The field trip begins in Madison, Wisconsin, proceeds to Platteville (Stop 1) for an overview of the district at the Bevan Mine and Rollo Jamison Museum. The Bevan Mine is an 1845 lead mine that produced more than two million pounds of lead ore in one year. The underground workings include two dioramas, one of an 1840 crevice lead mine, and the other a 1935 zinc mine. The Mining Museum includes numerous dioramas about the regional geology and mineral deposits and how miners in the 19th century went about their business. The geologic stop will be west of Dickeyville on Potosi Hill (Stop 2). This road cut contains St. Peter sandstone (Ancell Group) at the base to Galena Dolomite (Sinnipee Group) at the top. Two stops will be made to address mine reclamation: west of New Diggings (Stop 3), where one of the few remaining area of lead digs is preserved (natural reclamation from 1840), and Shullsburg at the former Eagle–Picher mine (Stop 4). The trip ends with a tour of Pendarvis State Historical Site (Stop 5), which recreates the cultural–historical setting of the 1840s mine district. 26 �Geology and Ore Deposits The zinc-lead deposits of southwestern Wisconsin are a classic example of the stratabound Mississippi Valley-type zinc–lead deposits. Although deposits are known to occur in most of the lower Paleozoic dolomite and sandstone units of the district (fig. 2), all important ore bodies are in the Middle Ordovician Sinnipee Group (Platteville, Decorah, and Galena Formations). The most common type of deposits (fig. 3) are (1) those associated with vertical or steeply inclined joints or fractures, called gash-vein or crevice deposits (dominantly galena with gangue); j E ZINC ' z I/ i/z / / 2 A/,/ / I a % k Average Maquoketa shale 200 - Dolomite, buff, cherty; argillaceous near base 110 108—240 // Dolomite, yellowish-buff, thin-bedded, shaly 40 Dolomite, yellowish-buff, thick-bedded;.Receptacuhtes in middle 80 / - ,, 7 /7 1" Dolomite, drab to buff; cherty; Receptac-uhies near base Platteville formation Limestone and dolomite, brown and grayish; green, sandy shale and phosphatic nodules at base St. Peter sandstone Sandstone. quartz, coarse, rounded - 105 35—40 . DISCONFORMITY -DISCONFORMITY— -r-±7 225 - Dolomite, limestone, and shale, green and brown; phosphatic nodules and bentonite near base Decorah formation —— 90 // "/ / 1 0 Dolomite, buff, cherty; Pentamerizs at top, Shale, blue, dolomitic; phosphatic depauperate fauna at base Galena dolomite > — thickn:ss, Description 40+ — — ----- -- Prairie du Chien group (undifferentiated) Trempealeau formation 280- Dolomite, light-buff, cherty; sandy near base and in upper part; shaly in upper part .: - 0— 320 240 .____________ — Sandstone, siltstone, and dolomite 120-150 Franconia sandstone Sandstone and siltstone, glauconitic 110—140 Dresbach sandstone Sandstone •I- -/- - a z a, 0. 0. Eau Claire sandstone 60-- 140 Siltstone and sandstone Mount Simon :.' . -- - -. -: sandstone ---:-:-:•:•:-: Sandstone 700— 1050 440— 780 Figure 2. Simplified stratigraphic section showing relative quantitative stratigarphic distribution of lead and zinc in the Upper Mississippi Valley district (from Heyl and others, 1978). 27 �(2) those associated with inclined and horizontal fractures, called pitch-and-flat deposits (sphalerite with minor galena and gangue); (3) those consisting of fine galena and sphalerite scattered through the country rock, called disseminated deposits; and (4) those consisting of angular breccia or country rock fragments cemented with ore and associated minerals, called breccia deposits. The most abundant gangue minerals with the ore minerals sphalerite and galena are calcite, pyrite, marcasite, barite, and rarely, chalcopyrite. The ore deposits are contained within weak structures produced by gentle folds in the dolomite strata. The ore deposits show vertical and regional zoning. Copper, barium, nickel, and arsenic are abundant in the east-central part of the district. Vertically, lead is greater in the higher part of a mineralized area; zinc, iron sulfides, nickel, and secondary dolomite are more abundant in the deeper deposits, generally conforming to structural control. Details of the district and individual properties are given in Heyl and others (1959). The paragenetic sequence generally involves early deposition of quartz, dolomite, pyrite, marcasite, with several generations of sphalerite and galena. Mineralization of cobaltite, chalcopyrite, chalcocite, millerite, and enargite are reported. Late gangue minerals include most of the calcite. It should be emphasized that the paragenetic sequence is generalized. Deposition Figure 3. Diagrammatic plans and sections illustrating typical patterns of gash-vein lead deposits and underlying pitch-and-flat deposits of the arcuate and linear types and their stratigraphic position to one another (from Heyl and others, 1978). 28 �throughout the general sulfide period took place under conditions of rhythmic oscillations in composition and temperature; such oscillations formed the hundreds of minute color bands characteristic of all the main minerals. McLimans and Barnes (1975) argued that most, if not all, of the distinct color-banding in sphalerite from mine to mine and area to area are coeval. McLimans and Barnes (1975) also argued for warmer temperatures (up to 220°C) for the deposits; most other workers suggest 150°C is a maximum. The high temperature seems to be supported by Rowan and Goldhaber (1996) and Zimmerman (1986), but their interpretations differ. Rowan and Goldhaber argued for a relatively thin Paleozoic cover (less than 3,000 ft) and higher heat flow; Zimmerman argued for normal geothermal gradients, but deeper burial (up to 9,000 ft). Outside of ore deposits, regional maximum temperatures were quite low (50 to 90°C, Blabaum, 1995). I interpret these data to suggest that the region was not deeply buried, but that high temperature mineral solutions were restricted to faults and fracture channels, and hence ore deposits were hotter than country rock. Extensive lead isotope data clearly identify disconformable lead that varies regionally; lead isotope ratios are lowest to the west and south. It is generally thought that mineralization of the district occurred during the Permian–Pennsylvanian by long-distance transport of metalbearing brine from the adjacent Illinois basin. FIELD TRIP The field trip (fig. 4) will visit the Platteville Mining Museum and Bevan Mine (Stop 1), examine Ordovician geologic exposures at Potosi Hill (Stop 2), an historic unreclaimed area of 1830 lead digs near New Diggings (Stop 3), modern metallic mine reclamation at the former Shullsburg zinc–lead mine/mill site (Stop 4), and the Pendarvis State Historical Site (Stop 5). 1 Platteville Museum 2 Potosi Hill 3 New Diggings 4 Shullsburg Reclamation 5 Pendarvis Miles Figure 4. Map showing field trip stops. 29 �Stop 1: Platteville Mining Museum and Rollo Jamison Museum Location: SW¼NE¼ sec. 15, T3N, R1W, Grant County, Wisconsin (Platteville 7.5-minute topographic quadrangle, 1952). Authors: Modified from <http://platteville.wi.us/visitors/mining.html>. Description: The Platteville Mining Museum traces the development of lead and zinc mining in the Upper Mississippi Valley through models, dioramas, artifacts, and photographs. The surface part of the museum includes dioramas of mining and a mosaic of maps showing at 1:24,000scale the geology and structure of the district. The underground part of the Museum, the Bevan Lead Mine, is an 1845 lead mine which produced over two millions pounds of lead ore in one year and is accessed via a walk down decline. The diorama in the mine shows how mining was accomplished in the 1840s (crevice deposits of lead), and in the 1930s (pneumatic jackleg drill and mine carts). Ceiling bolting is extensive in the underground mine; this was a demonstration area in the the past on how to install mine bolts. A small, reconstructed headframe and hoist and a track with a 1931 locomotive and ore cars complement the underground mine. For more information contact: City of Platteville Museum Department, 405 East Main Street, P.O. Box 780, Platteville, Wisconsin 53818; telephone (608) 348-3301. Stop 2: Potosi Hill—Ordovician Sinnipee Group Location: Roadcut at east side of U.S. Highway 61 in the SW¼NW¼ sec. 7, T2N, R2W, Grant County (Potosi 7.5-minute topographic quadrangle, 1972; fig. 5). Author: M.G. Mudrey, Jr. (modified from Ostrom, 1987). Description: This is an excellent and easily accessible exposure of the upper part of the Ancell Group (St. Peter and Glenwood Formations), and the Sinnipee Group (Platteville, Decorah, and Galena Formations). The exposure consists of a lengthy roadcut on the north side of U.S. Highway 61 (Whitlow and West, 1966). At least two east–west faults are recognized in the outcrop, the most significant (about 10 ft of throw) is in the valley between the upper and lower exposures. On the southeast wall of the quarry at the north end of the roadcut can be seen an example of the pitch and flat structure that hosts the zinc–lead mineralization in the district. The Sinnipee Group is the principal host of zinc and lead mineralization in the Upper Mississippi Valley Base-Metal District. This locality has been well studied over the past 50 years. The lithologic description given is that of Agnew (1956) with modifications by Ostrom (1978, 1987), and Mudrey (field reviews from 1976 to 2001). Agnew’s descriptions are the most comprehensive; however, slumping and outcrop deterioration and road construction have changed the ease with which individual parts of the exposure may be examined. The description given here is a composite of Agnew, Ostrom, and Mudrey. Figure 6 shows the geologic section exposed at the Potosi Hill roadcut. The base of the section consists of Ordovician sandstone of the St. Peter Formation, Ancell Group. The Tonti Member is the thickest of the three members in the formation, and consists dominantly of friable fine- to coarse-grained sandstone. Thickness can vary considerably, from absent to over 100 m thick. This variation is attributed to deposition on an erosional surface; deep channels are recognized elsewhere, particularly in La Crosse County. The basal unit of the St. Peter is the clay-rich Readstown Member, which may be a reworked residual regolith. The upper unit of the St. Peter seen in this exposure is a bluish-gray silt to shale, which can be locally absent in the region. Bioturbation in this unit is common. The St. Peter appears to be a near-shore deposit (Dott and others, 1986). In the area of the type locality, St. Paul, Minnesota, it appears to be entirely ma30 �rine; in the Madison and southern Wisconsin area, it is entirely nonmarine (Winfree and Dott, 1983). Locally the St. Peter is well cemented, ranging from silica to hematite. Habermann (1978) interpreted most of the cementation as the result of duricrust development during the Ordovician; however, in places, notably in area of known zinc-lead sulfide mineralization, some of the hematite cementation appears to be the result of weathering of minor sulfide ore bodies. The overlying Ordovician Sinnipee Group consists of a basal Figure 5. Topographic map showing location of field trip stop 2. Platteville Formation of several dolomitic limestone to dolomite members; the Decorah Formation, a shaly dolomite; and the uppermost Galena Formation, a vuggy weathering cherty dolomite. The lowermost member of the Platteville Formation, the Pecatonica Member, was formerly quarried in the district for building stone. The Quimbeys Mill Member, the uppermost member of the Platteville, is a sublithographic dolomite. This less than 1-m thick bed is very distinctive and is used through the district as a marker to the base of the mining horizon (termed glass rock because of the conchoidal fracture). The overlying Decorah Formation has been defined in many ways over the years, but is generally mapped as the shaly, dolomite part of the Sinnipee Group. Members in the Decorah are easily recognized. The basal Spechts Ferry Member is overlain by the Guttenberg (pronounced GUT-ten-berg), which contains brown petroliferous shale partings. This unit recognized in the mining district as “oil rock” and is a source bed for petroleum in Iowa and Michigan, where it is deeply buried. The Ion Member overlies the Guttenberg and consists of a blue unit and a gray unit, both very shaly dolomite with minimal structural strength. Most of the zinc–lead mineralization in the area is hosted by the Ion. Thickly bedded Galena Dolomite overlies the Decorah Formation. In the absence of fossils, field mapping has relied on the abundance of chert to divide the unit into a lower cherty unit and an upper relatively non-cherty unit that is less dolomitized. Weathering of the unit results in a mosaic outcrop pattern (locally termed honey-comb), where the less dolomitic parts of the rock are dissolved in preference to the harder, more indurated well-dolomitzed parts. Receptaculites is abundant in the upper, non-cherty part of the section. 31 �120GALENA FORMATION 110- Potosi Hill Section SE ¼NW ¼ sec, 7, T2N, R2W Grant County, Wisconsin 100OECORAH FORMATION Ion Member 90- 80Guttenberg Member 70 60- 50McGregor Member 40- 30- 20Pecotonice Member 10- 0- Figure 6. Potosi Hill section; explanation on following pages. (Modified from Ostrom, 1987.) 32 �Thickness (ft) Unit Member thickness (ft) Sinnipee Group Galena Dolomite Formation Prosser Member (upper cherty) Dolomite, olive drab to light brown, thick- to thin-bedded; medium to coarse grained, vuggy, abundant white chert; Receptaculties near top 4.0 44+ (lower Receptacultities Zone) Lower Receptaculities Zone; dolomite, olive drab to light brown, thickbedded, medium-to-coarse-grained, abundant chert, abundant Receptaculities 16.0 (lower cherty) Lower Cherty Zone; dolomite, olive drab to light brown, thick- to mediumbedded, medium to coarse grained, bands of chert nodules 14.5 (buff) Lower Buff Zone; dolomite, light brown, slight green mottling; thickbedded 9.5 Decorah Formation Ion Dolomite Member (Gray Unit) Dolomite, olive to gray, medium- to thick-bedded, vuggy, green shale partings throughout, sparry calcite present 13.5 (Blue Unit) Blue unit; dolomite, purplish gray, medium grained, slightly fossiliferous. Green shale present as partings, and as 0.5-ft bed, 0.8 ft below the top of the Interval, calcite present 5.1 Shale, green dolomitic shale in middle of interval 0.9 33 19.5 �Guttenberg Limestone Member (Oil Rock) Limestone, pinkish to purplish brown, fine grained to sublithographic, fossiliferous, upper 1 ft fine- to medium-grained, red-brown shale present as parting, calcite and limonite and iron sulfide present in small amounts 4.6 Shale metabentonite, brownish orange, crumbly, sticky when wet 0.1 Limestone, purplish brown, sublithographic, thin wavy bedding, fossiliferous, brown carbonaceous shale present as thin beds and partings, calcite and limonite present. Thin metabentonite bed at base 9.6 Limestone, brown gray, fine grained, thick-bedded 1.0 15.3 Spechts Ferry Shale Member (Clay Bed) Shale, orange gray, calcareous, and limestone, tan gray, fine grained, limestone 0.4 to 0.7 ft from base of unit 0.8 Limestone, gray, fine-grained, thin bedded 0.6 Shale, gray, green, brown, fissile, some beds fossiliferous, limestone present as thin lenses near middle of the interval 3.2 Limestone, tan with iron oxide mottling, fine grained, thin bedded 0.8 Shale, gray-green-brown. Fissle with thin lenses of gray fine-grained limestone 1.7 Limestone, dark to light gray, thin-bedded fossiliferous 0.7 Shale, brown-green-orange-gray, brown carbonaceous shale parting at top 0.5 Limestone, purplish brown, fine grained, thin-bedded, very fossiliferous, fucoids at base 0.5 Metabentonite, orange, sticky when wet, with brown shale partings 0.2 8.8 Platteville Formation Quimbys Mill Member (Glass Rock) Limestone, dark purplish gray, sublithographic, thick-bedded, conchoidal fracture, irregular upper surface, shale at base 0.8 0.8 14.5 29.5 MacGregor Member (Magnolia) Limestone, light grayish tan, fine grained, dense, partings of yellowish platy shale, very fossiliferous, thin 2 in. to 10 in. beds, upper 5 ft generally thicker bedded 34 �Mifflin Sub-member Limestone, light grayish brown, fine-grained, dense, medium-to thickbedded, discontinuous partings of bluish-green shale, fossiliferous 15.0 Pecatonica Member (Quarry Beds) Dolomite, brownish gray, fine to medium crystalline, sugary texture, thinto medium-bedded, even-bedded, beds 0.1 to 18 in. thick. Weathered surface shows distinct but discontinuous thinner beds 3.0 Dolomite, bluish, medium-grained, granular, sugary textured, argillite 1.0 Dolomite, brownish gray, fine to medium grained, crystalline, sugary texture, thin- to medium-bedded, even-bedded, beds about 1in. thick 8.0 Dolomite, bluish, medium grained, granular, sugary textured, argillite 1.0 Dolomite, brownish gray, fine to medium grained, crystalline, sugary texture, medium-bedded, even-bedded, beds thicker than 1 ft 6.0 Dolomite, brownish gray, fine to medium grained crystalline, sugary texture, medium-bedded, even-bedded, beds about 1 ft thick 4.6 23.6 Ancell Group Glenwood Formation Hennepin Member (Shale) Very silty, sandy dolomite, yellowish brown, abundant phosphatic pellets up to 2 mm in diameter, scattered round medium quartz sand grains, poorly sorted, iron-oxide cemented 0.5 0.5 1.5 1.5 Harmony Hill Member Silty shale, brown and bluish green grading downward to bluish green with some redish brown, little rounded medium grained quartz sand, abundant pale green clay in matrix, reworked/bioturbated texture St. Peter Formation Tonti Member (Sandrock) Sandstone, light yellowish gray, very fine to medium grained, Some light brown stains cross-bedded Base of exposure in drainage ditch 35 �Stop 3: New Diggings Lead Digs Location: Intersection of County Highways J and W, NW¼NW¼SW¼ sec. 22, T1N, R1E, Lafayette County, Wisconsin (New Diggings, 7.5- minute quadrangle, 1952). Author: M.G. Mudrey, Jr. (2001). Description: The area south of the intersection was extensively mined for residual lead in the early part of the nineteenth century. Miners would dig down over surface occurrences of galena, collecting residual galena in the soil. At some depth, depending principally on the competency of the soil profile, the dig was abandoned, and another started adjacent to the first dig. Once the area had been mined, miners would move on to another area. There was no attempt at active reclamation or deeper mining at that time. The area north of the intersection was also heavily mined; however, leveling of the surface in the 1950 for agriculture effectively removed the topographic evidence of mining. The high alkalinity of the soil derived from the Sinnipee Group dolomite neutralizes any acid mine drainage from oxidation of sulfide minerals and allows native plant communities to rapidly recover. Stop 4: Shullsburg Mine Site—Metallic Mine Reclamation Location: East side of Lafayette County O, 2 miles south of Shullsburg, sec. 22, T1N, R1E, Lafayette County (Shullsburg 7.5-minute topographic quadrangle, 1972; fig. 7). Authors: T.C. Hunt (University of Wisconsin–Platteville) and M.G. Mudrey, Jr., 2001. Figure 7. Topographic map showing location of field trip stop 4. 36 Description: The Shullsburg/Blackstone mining unit, known as the Calumet and Hecla Mine, was discovered by Calumet and Hecla Consolidated Copper Company about 1947 in a systematic exploration drilling program. In 1949 a 360-ft shaft was sunk on the property. Typically in the Upper Mississippi Valley Base Metal District, small ore bodies were identified from surface drilling exploration, and cross-cuts and drifts were driven from existing areas of mining to the newly discovered ore. Mining was by room and pillar methods. Within the mine, many of the drifts converged to make this complex one of the largest producers in the district. Galena and sphalerite were the principal ores mined. The most abundant minerals associated with the ore minerals were calcite, pyrite, marcasite, barite, and more rarely, chalcopyrite. Some individual intersections of ore over a 10-ft vertical interval assayed at more than 14 percent zinc. The ore was processed by �an on-site 1,000 ton per day flotation mill. The mill feed ranged between 4 to 6 percent zinc (Heyl and others, 1959; Heyl and others, 1970). The ore was hosted in the Decorah Formation, about 280 ft below the pre-mine groundwater surface. Groundwater pumping to dewater the mine ranged from 4 to 17 million gallons per day, and the cone of depression extended over 12 square miles (Evans and Cieslik, 1985). Eagle Picher Company (EP) acquired the properties in 1954. EP operated this site continuously from 1954 until 1979. The zinc and lead ore body was accessed via a decline that was built to replace the shaft that had succumbed to fire. EP extracted about 1,000 tons per day using a modified room and pillar mining method and processed about 1,500 tons of ore per day in the flotation mill on-site. Ore was also received from the nearby Bear Hole mine (Reinke, 1977). In the 1970s Wisconsin changed its mining laws to require a reclamation plan and financial bonding in conjunction with a mining permit. On April 18, 1978, Eagle-Picher received a permit from the Wisconsin Department of Natural Resources to mine zinc and lead at the site. The permit to mine was secured with only an approved reclamation plan; no bond was required because the operation was permitted as a nonconforming project site (Wisconsin Department of Natural Resources, April 18, 1978). The permitted mining site covered 72 acres at the Shullsburg mine and mill and an additional acre at the Blackstone pump site. In 1981, Inspiration Development Company gained ownership of four nonconforming mining units in southwestern Wisconsin that were under permit with the Wisconsin Department of Natural Resources; the Shullsburg site was among them. Most of these sites had been developed decades earlier, but were not permitted until mid to late 1970s. Only the Bear Hole (a nearby ore deposit) and Shullsburg units produced ore after they were permitted. The mining units in southwestern Wisconsin were shut down permanently during the period of 1978 and 1979. IDC never produced ore after its purchase, but IDC assumed responsibility for the reclamation of the site which is currently in different phases of reclamation. The primary environmental concerns were groundwater and surface water pollution, dusting from waste piles, stockpiles, and roads, aesthetics, and safety concerns. Following the closure of the Shullsburg Mine water quality in some nearby private watersupply wells deteriorated. Affected wells were located within the cone of depression created by pumping to keep the underground mine dewatered (fig. 8). Following mine closure, groundwater from these wells showed increased levels of sulfate, iron, calcium, magnesium, and total dissolved solids. The mechanism of contamination was postulated to be the following sequence: (1) oxidation of sulfide minerals, (2) formation of soluble sulfate mineral phases, (3) breakdown of carbonate host rock by acid produced during sulfide oxidation, and (4) dissolution of soluble materials by groundwater with rock strata that was previously dewatered during active mining (Evans and others, 1983; Evans and Cieslik, 1985). The impacted wells were reconstructed or abandoned and new wells constructed into the underlying sandstone aquifer. Numerous relic mine waste piles exist nearby this site including flotation tailings, jig tailings, waste-rock piles, and junk piles. Waste materials were trucked off-site as merchantable by-product for purposes of construction or agriculture. The decline portal has been backfilled, graded, top dressed, and stabilized with vegetation. Surface drainage has been restored so that runoff water is discharged from the sites without significant erosion. Observable subsidence or caving has not occurred at this site. This site predates topsoil salvage requirements. Generally, no topsoil remained for redistribution during reclamation activities, but where it was available in the form of dikes and berms, it was used to topdress the site. IDC has routinely used cow manure as a substitute for topsoil to 37 �Figure 8. Areal extent of cone of depression developed around the Bear Hole and Shullsburg Mines, Lafayette County, Wisconsin (from Evans and others, 1983). \- 38 �effect the establishment of vegetation. Most of the site is reasonably well stabilized with vegetation. Introduced pasture grasses and legumes were approved by the Wisconsin Department of Natural Resources for the last phase of final reclamation at this site. During the first phase of final revegetation in the mid-1980s, native trees, grasses, and shrubs were installed (Hunt, 1989). The area is not sited in wetland habitat. Proximity to a perennial stream may have impacted the riparian area, but baseline data are scarce. Presently, the impacts appear minimal. The existence of on-site settling ponds have created wetland habitat, albeit small and of marginal value. The designated post-mining land use for the nonconforming Shullsburg mining unit is a designated wildlife area, which is compatible with the adjacent land-use pattern. The current management level of this site is low, but there are plans to conduct a prescribed fire to help revitalize the native vegetation. The topography of this site is modified by waste piles, steepsided settling ponds, and relic mine openings and artifacts. The existing vegetation is a mixture of introduced agronomic pasture grasses and native vegetation. The existing oak groves present on these sites represent pre-settlement vegetation. Stop 5: Pendarvis State Historical Site (Mineral Point) Location: NE¼SE¼ sec. 31, T4N., R3E., Iowa County, Wisconsin (Mineral Point 7.5-minute topographic quadrangle, 1980). Author: Modified from <http://www.shsw.wisc.edu/sites/pend/>. Description: The museum consists of a series of stone buildings salvaged in the 1930s by Robert Neal and Edgar Hellum, who formed a partnership to acquire, restore and rebuild the few remaining cottages built by Cornish miners in the early 19th century. Their venture, called Pendarvis after an estate in Cornwall, preserves the regional cultural legacy of the early history of mining in southwest Wisconsin. Early miners extended their search for lead from northwestern Illinois into Wisconsin around 1820. Much of this early mining was seasonal, with the miners returning south to Illinois in the winter. With the arrival of Cornish miners and their families in the late 1830s, small villages were developed in proximity to the lead mines. In the hill south of Pendaris is the Merry Christmas Mine. Originally lead was mined by recovering residual galena from the soil (the hill side is extensively pockmarked with lead digs). Once the residual lead was recovered, the early miners sunk shallow shafts (above the water table) to recover galena from bedrock. For more information: Pendarvis State Historical Site, 114 Shake Rag Street, Mineral Point, Wisconsin 53565; telephone (608) 987-2122. REFERENCES Agnew, A.F., 1956, in Agnew, A.F. and Sloan, R.E., The Ordovician Rocks of Southwestern Wisconsin and Northeastern Iowa: Second Day, October 29, 1956: in G.M. Schwartz, ed., Guidebook for Field Trips, Minneapolis Meeting, 1956, Geological Society of America, p. 86. Blabaum, J.M., 1995, Origin and maturation of the organic matter in the Middle Ordovician Guttenberg Member of the Decorah Formation of southwestern Wisconsin: Geoscience Wisconsin, v. 15, p. 71–76. 39 �Broughton, W.A., 1991, Zinc and lead reserves of southwest Wisconsin: The undrilled lead digs of southwest Wisconsin: Wisconsin Geological and Natural History Survey Open-File Report 1991-5, 11 p. Dott, R.H., Jr., Byers, C.W., Fielder, G.W., Stenzel, S.R., Winfree, K.E., 1986, Aeolian to marine transition in Cambro-Ordovician cratonic sheet sandstones of the northern Mississippi Valley, USA: Sedimentology, v. 33, no. 3, p. 345–367. Evans, T. J., and Cieslik, M.J., 1985, Impact of groundwater from closure of an underground zinc–lead mine in southwest Wisconsin: Wisconsin Geological and Natural History Survey Miscellaneous Paper 81-01, 16 p. Evans, T.J., Cieslik, M.J., and Hennings, R.G., 1983, Investigation of the effects of recent mine closings on ground-water quality and quantity in the Shullsburg area: Wisconsin Geological and Natural History Survey Open-File Report 1983-1, 36 p. Habermann, G.M., 1978, Mineralogic and textural variations of the duricrust in southwestern Wisconsin: Ph.D. thesis, University of Wisconsin–Madison, 153 p. Heyl, A.V., Jr., Agnew, A.F., Lyons, E.J., Behre, C.H., Jr., and Flint, A.E., 1959, The Geology of the Upper Mississippi Valley Zinc-Lead District: U.S. Geological Survey Professional Paper 309, 310 p. Heyl, A.V. (and Broughton, W.A.), 1980, A very brief history of the Wisconsin mineral development atlas; general information and procedures concerning zinc-lead atlas: Wisconsin Geological and Natural History Survey Open-File Report 1980-4, 6 p. Heyl, A.V., Broughton, W.A., and West, W.S., 1970 (revised 1978), Geology of the Upper Mississippi Valley Base-Metal District: Wisconsin Geological and Natural History Survey Information Circular 16, 45 p. Hunt, T.C. 1989. Mined land reclamation in Wisconsin since 1973: Ph.D. Thesis. University of Wisconsin–Madison. 194 p. McLimans, R.K. and Barnes, H.L., 1975, Sphalerite stratigraphy in the upper Mississippi Valley Pb-Zn deposits: Economic Geology, v. 70, no. 7, p. 1324–1325. Ostrom, M.E., 1978, Potosi Hill Exposure, in Geology of Wisconsin: Outcrop descriptions: Wisconsin Geological and Natural History Survey, Gr-7/2N2W, 4 p. Ostrom, M.E., 1987, Middle Ordovician rocks at Potosi Hill, Wisconsin: in Geological Society of America Centennial Field Guide–North-Central Section, 1987, p. 201–204. Reinke, G. H., December 2, 1977, Eagle-Picher Industries, Shullsburg Mine and Mill Unit Environmental Impact Assessment Worksheet, Bureau of Solid Waste Management: Wisconsin Department of Natural Resources. 40 �Rowan, E.L. and Goldhaber, M.B., 1996, Fluid inclusions and biomarkers in the Upper Mississippi Valley zinc-lead district; implications for the fluid-flow and thermal history of the Illinois: U.S. Geological Survey Bulletin B 2094-F, 34 p. Whitlow, J.W., and West, W.S., 1966, Geology of the Potosi Quadrangle, Grant County Wisconsin, and Dubuque Country, Iowa: U.S. Geological Survey Bulletin 1123-I, p. 533–571. Winfree, Keith, and Dott, R.H., Jr., Progress on the St. Peter Sandstone of the Upper Midwest, in M.G. Mudrey, Jr., Field Trip chairman, Sedimentology of Ordovician Carbonates and Sandstones in Southwestern Wisconsin: Field Trip Guide Book, 17th Annual Meeting, North-Central Section, Geological Society of America, 1983, p. 4–13. Wisconsin Department of Natural Resources. April 18, 1978, Order Number EX-78-32B. Eagle Picher Industries, Shullsburg Mine and Mill Unit Permit to Mine: Wisconsin Department of Natural Resources, 3 p. Zimmerman, R.A., 1986, Fission-track dating of samples of the Illinois drill-hole core: U.S. Geological Survey Bulletin 1622-J, p. 99–108. 41 �This page intentionally left blank �Field Trip 3 Economic Geology of the Baraboo and Waterloo Quartzites of Southern Wisconsin Bruce A. Brown Wisconsin Geological and Natural History Survey 3817 Mineral Point Road Madison, Wisconsin 53705-5100 Frank R. Luther Department of Geology University of Wisconsin–Whitewater Whitewater, Wisconsin 53190 James W. Schmitt D.L. Gasser Construction Box 441 Baraboo, Wisconsin 53913 Susan M. Courter Michels Materials Box 128 Brownsville, Wisconsin 53006 Jennifer Lien The Kraemer Company Box 235 Plain, Wisconsin 53577 Stockpiles of Baraboo quartzite aggregate, LaRue Quarry. �This page intentionally left blank �INTRODUCTION The Proterozoic quartzites of the Baraboo Range and the Waterloo area of south-central Wisconsin have been quarried for a variety of industrial mineral products since the late nineteenth century. The extreme hardness and refractory properties of the quartzite were recognized early, and both areas were important producers of refractory blocks, mill liners, grinding pebbles, and several types of abrasives. Grinding pebbles were used in place of steel balls in mills that ground clay and feldspar for the ceramic industry, talc and other materials for the cosmetic industry, and in grinding some types of metallic ores. The most commonly used pebbles were flint beach pebbles, of 3 to 4 inch size, hand picked from beaches in northern Europe. Wisconsin State Geologist William O. Hotchkiss was instrumental in promoting Wisconsin quartzite for this use when European sources were cut off during World War I. The early operations used pebbles from gravels derived from the Paleozoic conglomerates formed adjacent to the quartzite, such as we will see at Stop 2. Later, grinding balls were manufactured by crushing quartzite, sorting for size and shape, and tumbling to achieve rounding. The last grinding ball producer, the Baraboo Quartzite Co., ceased operations in the 1980s. Today the primary uses for southern Wisconsin quartzite are crushed stone aggregates, riprap, and breakwater stone. For many years, the hard, abrasive nature of the quartzites, which average 98 percent silica, made crushed quartzite aggregates prohibitively expensive due to excessive wear on crushing and screening equipment. Crusher jaws quickly wore out, and steel wire screens commonly had to be replaced daily to maintain tight gradation specifications. Modern alloys, combined with the use of plastic and rubber-faced screens, have increased efficiency and reduced the cost of producing railroad ballast and a variety of quartzite aggregates. This guide is intended to provide the locations of stops and a brief outline of the geology and industrial process and products at each site that we intend to visit. Additional handouts relating to products, testing, specifications, and history will be provided on the day of the trip. We have not provided lengthy detailed descriptions of each stop because the geologic features we will see will likely change with the next round of production blasting. Wherever possible, we have tried to cite appropriate published field guides that explain the geology of nearby or relevant exposures regularly accessible to the public. We will visit six active quarrying operations on Trip 3. Company personnel will be available to describe the geology, mining, and processing methods, and the products made at each site. Because these are active operations, we will need to sign releases and MSHA Part 46 site-specific hazard training forms. Please remember to observe all safety rules and use common sense at all times. Especially stay away from highwalls and unstable slopes. GEOLOGY OF THE QUARTZITES Because the primary focus of this trip is on economic geology, only a brief discussion of the geologic history of the Baraboo Interval rocks of southern Wisconsin is included. The reader is referred to general discussions of tectonic setting (Greenberg and Brown, 1984) and previous guides to the Waterloo area (Luther, 1992, 1997), and the Baraboo area, (Dalziel and Dott, 1970; Malone and others, 1997; Medaris and Dott, 2001). The Baraboo and Waterloo quartzites belong to a group of sedimentary rocks that include the McCaslin, Rib Mountain, Necedah, Hamilton Mound, Flambeau, Barron quartzites of Wisconsin and probably the Sioux Quartzite of Minnesota that were deposited on continental crust formed during the 1850 Ma Penokean Orogeny (Greenberg and Brown, 1984). For many years the exact age of the quartzites and the timing of deformation and metamorphism were hotly debated. Recent work summarized by Medaris and Dott (2001) suggested that the Baraboo quartzite was deposited on a basement of 1,760 Ma. Granite and rhyolite, and that most of the 45 �metamorphism and alteration at Baraboo resulted from hydrothermal activity accompanying emplacement of the Wolf River Batholith at around 1,460 Ma. The Waterloo quartzite is intruded by pegmatite of Wolf river age, suggesting that recrystallization and metamorphism may also be related to this regional thermal event. Both the Baraboo and Waterloo quartzites have been deformed. The Baraboo Range is an isolated doubly plunging syncline, slightly overturned to the south. The Waterloo outcrop area is in the nose of a broad eastward-plunging synclinal structure. Several possible models for deposition and subsequent deformation of the quartzites have been proposed (Greenberg and Brown, 1984), but the timing of deformation is still debated, possibly related to a regional 1,630 Ma thermal event, but certainly after about 1,720 Ma and prior to the 1,460 hydrothermal event. Quartzite, presumably equivalent to the Baraboo and Waterloo quartzites, is the most common basement lithology found in deep wells throughout southeastern Wisconsin. The stratigraphic sequence at Baraboo consists of 1,500 m of red to purple quartzite, overlain by 100 m of Seeley slate, 300 m of Freedom Formation (dolomite and carbonate iron formation), 65 m of pebbly Dake quartzite and 45m of Rowley Creek slate (Dalziel and Dott, 1970). The quartzite is well exposed, but the overlying formations, except for the Dake, are known from drill core. All of the operations that we will visit are located in or adjacent to the lower quartzite sequence. The upper formations are rarely exposed, being covered by Paleozoic sandstone and Quarternary glacial deposits in the interior of the syncline. At Stop 5 we will be near the site of two of the three historic iron mines that, before closing in the early 1920s, briefly mined the iron-rich carbonates of the Freedom Formation. The stratigraphic sequence at Waterloo is poorly known because of very limited exposure. Most of the known outcrop is in the area of the Michels Quarry, and consists of gray pebbly quartzite interbedded with argillite beds that have been metamorphosed to andalusite schist (Luther, 1992, 1997). Outlying exposures that show less evidence of metamorphism tend to have the more typical reddish-purple color and hydrothermal quartz veins and breccias seen in other Baraboo Interval quartzites. Stop 1: Michels Materials Waterloo Quarry Location: NE¼ sec. 33 and NW¼ sec. 34, T 9N, R13E, 1.5 miles east of Portland, Wisconsin, on Highway 19 (Waterloo 7.5-minute topographic quadrangle, 1976; fig. 1). Leaders: Frank Luther, Sue Courter, and Bruce A. Brown. Description: The Michels Waterloo Quarry was opened in 1988 by the Edward E. Gillen Co., a Milwaukee marine contractor involved in harbor and breakwater construction on the Great Lakes. Exploratory core drilling indicated massive bedding up to 2 to 3 m thick that would allow for quarrying of individual blocks up to 20 tons in size. The rock was tested and proved to exceed all Corps of Engineers specifications for breakwater stone, particularly resistant to freezethaw. The quarry site is also conveniently located for transport of blocks to Milwaukee by rail or truck. The Gillen Co. soon began to accumulate a large pile of waste blocks too small for breakwater stone and brought in crushing equipment to convert this material into railroad ballast and other aggregate. A rail loading facility was built 1 mile south of the quarry and ballast is moved to the site by truck. The rock is gray to reddish gray in color and consists of thoroughly recrystallized sandsized quartz grains, with interlayered pebbly beds containing pebbles of quartz, chert, jasper, and iron formation up to 1.5 cm. The massive beds of quartzite are separated by 10 to 20 cm argillite beds that are metamorphosed to andalusite schist. The thick east-dipping beds facilitate quarrying of large blocks, but make maintaining a flat quarry floor difficult. Glacial till overburden thickens to the east, and contains abundant quartzite boulders. Paleozoic sandstone conglomerate similar to the Parfreys Glen Formation of the Baraboo area locally occurs above the quartzite. 46 �- _S - 55 ± -_S— — _ 44' S -. -S — __-44-44.'44— 44—7895 .- - -44- -44-44- z 7 Figure 1. Topographic map showing location of Stop 1. We will examine the quarry geology, mining and crushing methods, and products. From Waterloo we will proceed northwestward across the glaciated terrane of eastern Dane County to Stop 2, located at the east end of the Baraboo Range. On the way we will have the opportunity to see some classic examples of drumlins and other glacial landforms. Stop 2: The Kraemer Co. Williams Quarry Location: SE¼, NE¼ sec. 19, T 12 N, R 8,E, Columbia County, south side of Highway 33, about 3 miles west of I-90-94 (Pine Island 7.5-minute topographic quadrangle, 1975; fig. 2). Leaders: Jennifer Lien, Phil Fauble, and Bruce A. Brown. Description: The Williams Quarry and another operation 0.5 mile to the west were opened in the Parfreys Glen Formation (Clayton and Attig, 1990), a time-transgressive, proximal conglomeratic facies formed by wave and storm action around the Baraboo Range as it was slowly buried by advancing seas during Cambrian and Ordovician time. The face at Williams has now advanced far enough into the hillside to expose three distinct lithologic units. At the base of the highwall, steeply dipping quartzite of the Baraboo north range is exposed. The quartzite is cut by numerous white quartz veins, and is locally brecciated and recemented by white hydrothermal quartz. Vugs and cavities in the breccia zone are filled with white clay and lined with quartz crystals up to 20 cm in length. Overlying the quartzite is a poorly sorted deposit consisting of clasts ranging from sand sized up to rocks 2 to 3 m in diameter (fig. 3). Fauble and Lien (2001) suggested that this unit may have originated as a debris flow because of the lack of sorting and evidence of reworking by wave action. Overlying and lapping onto the debris flow are beds of pebble conglomerate and sandstone typical of the Parfreys Glen Formation. These beds contain sedimentary structures and trace fossils typical of near-shore marine deposits. Abundant glauconite suggests equivalence to 47 �Figure 2. Topographic map showing location of Stop 2. Figure 3. South face at Williams Quarry showing Parfreys Glen conglomerate over coarse debris flow deposit overlying quartzite with white veins and clay pockets. the Tunnel City Group away from the Baraboo Range. The Williams Quarry produces base course and a small amount of riprap. Crushing and screening are done intermittently as needed, using portable equipment, typical of smaller quarry operations throughout rural Wisconsin. 48 �Stop 3: 1,760 Ma Rhyolite Location: SW¼ NE¼ sec. 33, T12N, R7E, south side of Highway 33, 0.3 mile east of the Lower Narrows of the Baraboo River (Lewiston 7.5-minute topographic quadrangle, 1975; fig. 4). Leader: Bruce A. Brown. Description: We will stop at this old road cut to examine the rhyolite that underlies the Baraboo quartzite. This rock is typical of the 1,760 Ma metarhyolites exposed along the northern and southern edges of the Baraboo syncline and to the northeast in the Fox Valley (Smith, 1978). The rock is a dark reddish brown color, and contains quartz and feldspar phenocrysts in a fine-grained matrix. A tuffaceous texture is visible in thin section and hand specimen. Flattened shards and pumice fragments can commonly be seen on weathered outcrop surfaces. The rhyolites have never been commercially quarried in the Baraboo area, but were extensively quarried for paving blocks in the Fox Valley at Berlin and Utley (Buckley, 1898). As you return to the van, notice the shoulder stone along Highway 33, and the aggregate used in the asphalt pavement. The shoulder stone was likely from the Williams Quarry. The asphalt contains a high percentage of quartzite aggregate. Wisconsin Department of Transportation has been slow to embrace quartzite as an aggregate for asphalt mixes because the dense quartzite does not absorb asphalt readily. This pavement seems to be performing well with no sign of deterioration after several years of use. Quartzite has performed well as a concrete aggregate in Wisconsin, outlasting the cement and fine aggregate matrix in many older pavements and sidewalks. In the city of Madison, examples of concrete with quartzite aggregate poured in the 1920s are still in use. We will drive through the Lower Narrows, where the Baraboo River cuts through the North Range on our way to the next stop. The quarry at the southwest end of the narrows was formerly operated by the Baraboo Quartzite Co., a producer of mill linings and grinding balls for many years. This was the last active producer of grinding media in the district, closing in the early 1980s. 15 Figure 4. Topographic map showing location of Stop 3. 49 �Figure 5. Topographic map showing location of Stop 4. Stop 4: Milestone Materials Jesse Pit and Quarry Location: NW¼SW¼ sec. 15, T11N, R7E, 2 miles east of Highway 113 on south side of Tower Road, Sauk County (Baraboo 7.5-minute topographic quadrangle, 1994; fig. 5). Leader: Jim Schmitt. Description: The Jesse Pit began as a sand and gravel operation along the terminal glacial moraine. As the pit was deepened, quartzite was exposed and quarrying began (fig. 6). We are now on the south limb of the Baraboo syncline, and the exposed quartzite dips to the north at a low angle. The dip surface is a bedding plane, and in many areas excellent examples of ripple marks can be seen. The Jesse Pit is an example of a growing number of sand and gravel operations that have encountered bedrock and begun to produce crushed stone as well. Gravel is in demand for concrete aggregate, but many gravel deposits lack sufficient coarse crushing material needed to achieve the percentage of fractured surfaces required to meet modern asphalt mix design standards. Operations such as Jesse Pit have the advantage of being able to furnish concrete and asphalt aggregate plus a variety of other products ranging from sand to landscape boulders. We will examine the quartzite exposures and look at some of the glacial material as well as discuss some of the reclamation activities currently in progress at this site. Stop 5: Milestone Materials Fox Ridge Asphalt Plant and Sales Yard Location: SE¼ sec. 22, T12N, R6E, 2 miles north of Baraboo on Fox Hill Road. Leader: Jim Schmitt. Description: At this stop we will examine some of the 27 products produced at or marketed from the Fox Ridge Pit. The list includes seven quartzite products ranging from base course to washed sand and seal coat chips. The quartzite is hauled in to this site from quarries such as Jesse and Rock Springs. We will discuss the uses of quartzite for asphalt aggregate as well as a variety of construction uses. 50 �Figure 6. View of Jesse Pit looking north. Working face in quartzite in foreground; sand and gravel face behind. Slope in background near tree line is reclaimed pit area ready to be seeded. Stop 6: Martin Marietta Aggregates Rock Springs Quarry Location: SW¼ sec. 28, T11N, R5E, 0.5 miles north of Rock Springs (Rock Springs 7.5-minute topographic quadrangle, 1975; fig. 7). Leader: Bruce A. Brown (Joe Michels). Description: This large quarry was opened in 1958 by the C&NW Railroad and operated exclusively as a source of railroad track ballast for many years. At the time it was opened, the large-scale production of quartzite ballast was not considered cost effective because of the hard abrasive nature of the rock and the resulting equipment maintenance costs. The C&NW ultimately proved that the superior performance of quartzite under heavy, high-speed train traffic was worth the cost in the long run. The angular quartzite interlocked to form a stable, welldrained track base, and did not break down under heavy traffic, as did limestone. Today the quarry is operated by Martin Marietta and it remains one of the largest ballast producers in Wisconsin. We are directly across the Upper Narrows of the Baraboo River from Van Hise rock, and several historic quarries. The old quarries are now part of a Wisconsin Department of Natural Resources Natural Area; originally they were operated for refractory and grinding media, with some early limited production of crushed stone. The bedding in this area is nearly vertical as at Williams Quarry (Stop 2). Hydrothermal breccias cemented with white quartz are also common in this area. The geology of the Narrows area is described in detail by Medaris and Dott (2001) as Locality 4 of Field Trip 1 for this meeting. We will tour the quarry and examine the large modern crushing and screening plant. As we drove up the hill to the quarry office, we passed a large pile of fines created by years of ballast production. This material at one time was eroding and washing into the Baraboo River 51 �below. It has since been stabilized and vegetation is beginning to take hold. Does anyone have any good ideas for marketing this stuff? Stop 7: Kraemer Company LaRue Quarry Location: NW¼ sec. 22, T11N, R5E (Rock Springs 7.5-minute topographic quadrangle, 1975; fig. 8). Leaders: Jennifer Lien and Bruce A. Brown. Description: The LaRue Quarry is a historic operation Figure 7. Topographic map showing location of Stop 6. in the Baraboo region. At one time this quarry produced ballast and was connected to the C&NW at North Freedom by the track now used by the Midcontinent Railway Museum. We may be in time to see the steam train pull into the quarry on one of its runs. Foundations of the old permanent crushing and screening plant and a few ruined buildings remain from the earlier operation. As we drive into LaRue Quarry, the leaders will point out the sites of two historic iron mines also served by this branch line from 1900 to around 1922. The quarry is now operated on an as-needed basis with portable crushing and screening equipment. The principal product is construction aggregate, primarily base course. Rue Figure 8. Topographic map showing location of Stop 7. 52 �Figure 9. East face of LaRue Quarry, showing unconformity of Paleozoic sandstones overlying Baraboo quartzite. LaRue is on the south range and, as at Jesse Pit, beds dip at a low angle to the north. Thin argillite beds are present and cross-bedding can be seen on joint surfaces. Ripple marks are common on bedding surfaces. LaRue provides some excellent views of the unconformity between the quartzite and the onlapping Cambrian-Ordovician sandstones (fig. 9). Alteration along joint surfaces has produced a weathering pattern similar to spheroidal weathering in granite. Rounded boulders can be found in which the interior is fresh purple quartzite surrounded by a rind of bleached sandstone resembling the overlying sediments. REFERENCES Buckley, E.R., 1898, On the Building and Ornamental Stones of Wisconsin: Wisconsin Geological and Natural History Survey Bulletin 4, 544 p. Clayton, L., and Attig, J.W., 1990, Geology of Sauk County, Wisconsin: Wisconsin Geological and Natural History Survey Information Circular 67, 68 p. Dalziel, I.W.D., and Dott, R.H., Jr., 1970, Geology of the Baraboo District, Wisconsin: Wisconsin Geological and Natural History Survey Information Circular 14, 164 p. Fauble, Philip, and Lien, Jennifer, 2001, Some Observations from the Williams Quarry Exposure: Evidence of Debris Flow Deposits in the Parfreys Glen Formation: Abstracts, 47th Annual Institute on Lake Superior Geology, Madison, Wisconsin, p. 26–27. 53 �Greenberg, J.K., and Brown, B.A., 1984, Cratonic sedimentation during the Proterozoic: An anorogenic connection in Wisconsin and the upper Midwest: Journal of Geology, v. 92, p. 159–171. Luther, F., 1992, The Waterloo Quartzite at the old Portland Quarry, in Travis, J., ed., the 56th Annual Tri-State Geology Field Conference, Whitewater, Wisconsin, p. 51–61. Luther, F., 1997, The Precambrian Waterloo Quartzite, Dodge and Jefferson Counties, Wisconsin—Petrology, Structure, and Industrial Use: Field Trip 5, in Guide to Field trips in Wisconsin and Adjacent Areas of Minnesota: Wisconsin Geological and Natural History Survey, Madison, Wisconsin, p. 31–35. Malone, D.H., Van Wyck, N., and Nelson, R., 1997, Field guide for field trip leaders to the Baraboo district, Wisconsin: Field Trip 3 in Guide to Field Trips in Wisconsin and Adjacent Areas of Minnesota: Wisconsin Geological and Natural History Survey, Madison, Wisconsin, p.13–22. Medaris, L.G. Jr., and Dott, R.H., Jr., 2001, Field Trip 1:Sedimentologic, Tectonic, and Metamorphic History of the Baraboo Interval: New Evidence from investigations in the Baraboo range, Wisconsin: 47th Annual Institute on Lake Superior Geology, Madison, Wisconsin, p. 1–21 (this volume). Smith, E.I., 1978, Introduction to Precambrian rocks of south-central Wisconsin: Geoscience Wisconsin, v. 2, p. 1–15. 54 � Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology: Proceedings, 2001 Description An account of the resource Institute on Lake Superior Geology. Madison, Wisconsin. May 9-12, 2001. Creator An entity primarily responsible for making the resource Institute on Lake Superior Geology Date A point or period of time associated with an event in the lifecycle of the resource 2001 Format The file format, physical medium, or dimensions of the resource PDF Language A language of the resource English https://digitalcollections.lakeheadu.ca/files/original/2456791f0692913e884f16bfdc1fc9dd.pdf b199c02ed01d1fc8083e26b36a355f3f PDF Text Text INSTITUTE ON LAKE SUPERIOR GEOLOGY 48TH ANNUAL MEETING PROCEEDINGS VOLUME 48 PART 1 –PROGRAM AND ABSTRACTS Kenora, Ontario – May 12-16, 2002 �INSTITUTE ON LAKE SUPERIOR GEOLOGY 48TH ANNUAL MEETING PROCEEDINGS VOLUME 48 PART 1 –PROGRAM AND ABSTRACTS �CONTENTS Proceedings Volume 48 Part 1—Program and Abstracts Editors: K. O’Flaherty, C. Storey Institutes on Lake Superior Geology, 1955-2002.............................................................. iii Constitution of the Institute on Lake Superior Geology......................................................v By-Laws of the Institute on Lake Superior Geology........................................................ vii Membership Criteria for the Institute on Lake Superior Geology................................... viii Goldich Medal Guidelines ................................................................................................. ix Past Goldich Medalists ...................................................................................................... xi Goldich Medal Committee................................................................................................. xi Citation for 2002 Goldich Medal Recipient ..................................................................... xii Eisenbrey Student Travel Awards .....................................................................................xv Student Travel Award Application Form ........................................................................ xvi Student Paper Awards..................................................................................................... xvii Student Paper Awards Committee .................................................................................. xvii Board of Directors ......................................................................................................... xviii Local Committees .......................................................................................................... xviii Session Chairs.................................................................................................................. xix Banquet Speaker ................................................................................................................xx Report of the Chair of the 47th Annual Institute Meeting.................................................xx Program............................................................................................................................xxv Abstracts ..............................................................................................................................1 Cover Photo: Regina Mine, 1918. The Regina Mine produced intermittently from 1895 to 1943, yielding over 8,000 ounces gold and 1,460 ounces silver. The Regina was the first mine in North America to use cyanide for gold recovery. ii �INSTITUTES ON LAKE SUPERIOR GEOLOGY, 1955-2002 # 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 DATE 1955 1956 1957 1958 1959 1960 1961 1962 1963 1964 1965 1966 1967 1968 1969 1970 1971 1972 1973 1974 1975 1976 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 PLACE Minneapolis, Minnesota Houghton, Michigan East Lansing, Michigan Duluth, Minnesota Minneapolis, Minnesota Madison, Wisconsin Port Arthur, Ontario Houghton, Michigan Duluth, Minnesota Ishpeming, Michigan St. Paul, Minnesota Sault Ste. Marie, Michigan East Lansing, Michigan Superior, Wisconsin Oshkosh, Wisconsin Thunder Bay, Ontario Duluth, Minnesota Houghton, Michigan Madison, Wisconsin Sault Ste. Marie, Ontario Marquette, Michigan St. Paul, Minnesota Thunder Bay, Ontario Milwaukee, Wisconsin Duluth, Minnesota Eau Claire, Wisconsin East Lansing, Michigan International Falls, Minnesota Houghton, Michigan Wausau, Wisconsin Kenora, Ontario Wisconsin Rapids, Wisconsin Wawa, Ontario Marquette, Michigan Duluth, Minnesota Thunder Bay, Ontario Eau Claire, Wisconsin Hurley, Wisconsin Eveleth, Minnesota Houghton, Michigan Marathon, Ontario Cable, Wisconsin CHAIRS C.E. Dutton A.K. Snelgrove B.T. Sandefur R.W. Marsden G.M. Schwartz & C. Craddock E.N. Cameron E.G. Pye A.K. Snelgrove H. Lepp A.T. Broderick P.K. Sims & R.K. Hogberg R.W. White W.J. Hinze A.B. Dickas G.L. LaBerge M.W. Bartley & E. Mercy D.M. Davidson J. Kalliokoski M.E. Ostrom P.E. Giblin J.D. Hughes M. Walton M.M. Kehlenbeck G. Mursky D.M. Davidson P.E. Myers W.C. Cambray D.L. Southwick T.J. Bornhorst G.L. LaBerge C.E. Blackburn J.K. Greenberg E.D. Frey & R.P. Sage J. S. Klasner J.C. Green M.M. Kehlenbeck P.E. Myers A.B. Dickas D.L. Southwick T.J. Bornhorst M.C. Smyk L.G. Woodruff iii �43 44 45 46 47 48 1997 1998 1999 2000 2001 2002 Sudbury, Ontario Minneapolis, Minnesota Marquette, Michigan Thunder Bay, Ontario Madison, Wisconsin Kenora, Ontario R.P. Sage, W. Meyer J.D. Miller, M.A. Jirsa T.J. Bornhorst, R.S. Regis S.A. Kissin, P. Fralick M.G. Mudrey, Jr., B.A. Brown P. Hinz, R.C. Beard iv �CONSTITUTION OF THE INSTITUTE ON LAKE SUPERIOR GEOLOGY (Last amended by the Board—May 8, 1997) Article I Name The name of the organization shall be the "Institute on Lake Superior Geology". Article II Objectives The objectives of this organization are: A. To provide a means whereby geologists in the Great Lakes region may exchange ideas and scientific data. B. To promote better understanding of the geology of the Lake Superior region. C. To plan and conduct geological field trips. Article III Status No part of the income of the organization shall insure to the benefit of any member or individual. In the event of dissolution, the assets of the organization shall be distributed to _________ (some tax free organization). (To avoid Federal and State income taxes, the organization should be not only "scientific" or "educational, but also "non-profit") Minn. Stat. Anno. 290.01, subd. 4 Minn. Stat. Anno. 290.05(9) 1954 Internal Revenue Code s.501(c)(3) Article IV Membership The membership of the organization shall consist of persons who have registered for an annual meeting within the past three years, and those who indicate interest in being a member according to guidelines approved by the Board of Directors. Article V Meetings The organization shall meet once a year. The place and exact date of each meeting will be designated by the Board of Directors. Article VI Directors The Board of Directors shall consist of the Chair, Secretary-Treasurer, and the last three past Chairs; but if the board should at any time consist of fewer than five persons, by reason of unwillingness or inability of any of the above persons to serve as directors, the vacancies on the board may be filled by the Chair so as to bring the membership of the board to five members. Article VII Officers v �The officers of this organization shall be a Chair and Secretary-Treasurer. A. The Chair shall be elected each year by the Board of Directors, who shall give due consideration to the wishes of any group that may be promoting the next annual meeting. His/her term of office as Chair will terminate at the close of the annual meeting over which he/she presides, or when his/her successor shall have been appointed. He/she will then serve for a period of three years as a member of the Board of Directors. B. The Secretary-Treasurer shall be elected at the annual meeting. His/her term of office shall be four years, or until his/her successor shall have been appointed. Article VIII Amendments This constitution may be amended by a majority vote (majority of those voting) of the membership of the organization. vi �BY-LAWS OF THE INSTITUTE ON LAKE SUPERIOR GEOLOGY I. Duties of the Officers and Directors A. It shall be the duty of the Annual Chairman to: 1. Preside at the annual meeting. 2. Appoint all committees needed for the organization of the annual meeting. 3. Assume complete responsibility for the organization and financing of the annual meeting over which he/she presides. B. It shall be the duty of the Secretary-Treasurer to: 1. Keep accurate attendance records of all annual meetings. 2. Keep accurate records of all meetings of, and correspondence between, the Board of Directors. 3. Hold all funds that may accrue as profits from annual meetings or field trips and to make these funds available for the organization and operation of future meetings as required. C. It shall be the duty of the Board of Directors to plan locations of annual meetings and to advise on the organization and financing of all meetings. II. Duties and Expenses A. Regular membership dues of $5.00 or less on an annual basis shall be assessed each member as determined by the Board of Directors.. B. Registration fees for the annual meetings shall be determined by the Chair in consultation with the Board of Directors. The registration fees can include expenses to cover operations outside of the annual meeting as determined by the Board of Directors. It is strongly recommended that registration fees be kept at a minimum to encourage attendance of students. III. Rules of Order The rules contained in Robert's Rules of Order shall govern this organization in all cases to which they are applicable. IV. Amendments These by-laws may be amended by a majority vote (majority of those voting) of the membership of the organization; provided that such modifications shall not conflict with the constitution as presently adopted or subsequently amended. vii �MEMBERSHIP CRITERIA FOR THE INSTITUTE ON LAKE SUPERIOR GEOLOGY Approved May 8, 1997 A. Membership in the Institute on Lake Superior Geology requires either participation in Institute activities, or an indication on a regular basis of interest in the Institute. Those individuals registering for an annual meeting will remain as members for 4 years unless: 1) they indicate no further interest in the Institute by responding negatively to the statement on meeting circulars "Remove my name from the mailing list"; or 2) two successive mailings in different years are returned by the postal service as address unknown. B. Those individuals who have not registered for an annual meeting in the past 4 years must indicate an interest in the Institute by postal, electronic , or verbal correspondence with the Secretary-Treasurer at least once every two years. Such individuals will be removed from the membership if they indicate no further interest in the Institute or two successive mailing in different years are returned by the postal service as address unknown. C. The Secretary-Treasurer will maintain a list of current members. The list will include the date of the beginning of continuous membership, dates of returned mail, dates of last contact (expression of interest), and the date membership expires, barring a change of status initiated by the member. Those individuals who have become members of ILSG by Section B will have an expiration date listed at 2 years from the upcoming meeting. For example, a member who expresses interest in September of 1997 (the next annual meeting is May, 1998) will have an expiration date of May, 2000, unless the member contacts the Secretary-Treasurer or attends an annual meeting. D. "Member for Life" status is granted to individuals who have been (nearly) continuous participants of the ILSG meetings for 15 years, Goldich Medal recipients, or those who have served as meeting chairs. This status will be further maintained unless the individuals indicate no further interest in the Institute, or 4 mailings in different years are returned by the postal service as address unknown, or they are deceased. E. All members will be mailed the First Circular for the Annual Meeting and the ILSG Newsletter. The Chair of the annual meeting may opt to send the first circular to additional individuals. All returned mail should be reported to the Secretary-Treasurer. F. The Secretary-Treasurer can designate any individual who is on the ILSG membership list (mailing list) as of January 1, 1997 as a member for life based on participation in ILSG activities. G. Members are strongly encourage to send address corrections to the Secretary-Treasurer to avoid unintentional lapse of membership. viii �GOLDICH MEDAL GUIDELINES (Adopted by the Board of Directors, 1981; amended 1999) Preamble The Institute on Lake Superior Geology was born in 1955, as documented by the fact that the 27th annual meeting was held in 1981. The Institute's continuing objectives are to deal with those aspects of geology that are related geographically to Lake Superior; to encourage the discussion of subjects and sponsoring field trips that will bring together geologists from academia, government surveys, and industry; and to maintain an informal but highly effective mode of operation. During the course of its existence, the membership of the Institute (that is, those geologists who indicate an interest in the objectives of the ILSG by attending) has become aware of the fact that certain of their colleagues have made particularly noteworthy and meritorious contributions to the understanding of Lake Superior geology and mineral deposits. The first award was made by ILSG to Sam Goldich in 1979 for his many contributions to the geology of the region extending over about 50 years. Subsequent medalists and this year's recipient are listed in the table below. Award Guidelines 1) The medal shall be awarded annually by the ILSG Board of Directors to a geologist whose name is associated with a substantial interest in, and contribution to, the geology of the Lake Superior region. 2) The Board of Directors shall appoint the Goldich Medal Committee. The initial appointment will be of three members, one to serve for three years, one for two years, and one for one year. The member with the briefest incumbency shall be chair of the Nominating Committee. After the first year, the Board of Directors shall appoint at each spring meeting one new member who will serve for three years. In his/her third year this member shall be the chair. The Committee membership should reflect the main fields of interest and geographic distribution of ILSG membership. The out-going, senior member of the Board of Directors shall act as liaison between the Board and the Committee for a period of one year. 3) By the end of November, the Goldich Medal Committee shall make its recommendation to the Chair of the Board of Directors, who will then inform the Board of the nominee. 4) The Board of Directors normally will accept the nominee of the Committee, inform the medalist, and have one medal engraved appropriately for presentation at the next meeting of the Institute. 5) It is recommended that the Institute set aside annually from whatever sources, such funds as will be required to support the continuing costs of this award. ix �Nominating Procedures 1) The deadline for nominations is November 1. Nominations shall be taken at any time by the Goldich Medal Committee. Committee members may themselves nominate candidates; however, Board members may not solicit for or support individual nominees. 2) Nominations must be in writing and supported by appropriate documentation such as letters of recommendation, lists of publications, curriculum vita's, and evidence of contributions to Lake Superior geology and to the Institute. 3) Nominations are not restricted to Institute attendees, but are open to anyone who has worked on and contributed to the understanding of Lake Superior geology. Selection Guidelines 1) Nominees are to be evaluated on the basis of their contributions to Lake Superior geology (sensu lato) including: a) importance of relevant publications; b) promotion of discovery and utilization of natural resources; c) contributions to understanding of the natural history and environment of the region; d) generation of new ideas and concepts; and e) contributions to the training and education of geoscientists and the public. 2) Nominees are to be evaluated on their contributions to the Institute as demonstrated by attendance at Institute meetings, presentation of talks and posters, and service on Institute boards, committees, and field trips. 3) The relative weights given to each of the foregoing criteria must remain flexible and at the discretion of the Committee members. 4) There are several points to be considered by the Goldich Medal Committee: a) An attempt should be made to maintain a balance of medal recipients from each of the three estates—industry, academia, and government. b) It must be noted that industry geoscientists are at a disadvantage in that much of their work in not published. 5) Lake Superior has two sides, one the U.S., and the other Canada. This is undoubtedly one of the Institute's great strengths and should be nurtured by equitable recognition of excellence in both countries. x �PAST GOLDICH MEDALISTS 1979 Samuel S. Goldich 1991 William Hinze 1980 not awarded 1992 William F. Cannon 1981 Carl E. Dutton, Jr. 1993 Donald W. Davis 1982 Ralph W. Marsden 1994 Cedric Iverson 1983 Burton Boyum 1995 Gene LaBerge 1984 Richard W. Ojakangas 1996 David L. Southwick 1985 Paul K. Sims 1997 Ronald P. Sage 1986 G.B. Morey 1998 Zell Peterman 1987 Henry H. Halls 1999 Tsu-Ming Han 1988 Walter S. White 2000 John C. Green 1989 Jorma Kalliokoski 2001 John S. Klasner 1990 Kenneth C. Card 2002 Ernest K. Lehmann GOLDICH MEDAL COMMITTEE Ron Sage (2002) Ontario Geological Survey, Thunder Bay Rod Johnson (2002) Rod Johnson and Associates, Negaunee, Michigan Frank Luther (2003) University of Wisconsin, Whitewater James D. Miller, Jr., as out-going senior member of Institute Board of Directors, is liaison between Goldich Medal Committee and the Board through 2001 meeting. xi �Citation Ernest K. Lehmann 2002 Goldich Medal Recipient Ladies and Gentlemen: Thank you for being here tonight for this presentation of the 2002 Sam Goldich Medal Award. I don’t know if any of you remember the late Ben Dickerson. He was one of the premier exploration geologists of his time and wrote a column in Skillings Magazine. The column was called “ News and Rumor” from the Bush”. It described current exploration activity particularly in the U.S. during those golden years of the 70’s and 80’s. At that time the exploration community was quite small and Ben seemed to know everyone. His column was cryptic in that he would commonly refer to people in coded names. For example there was the “Great Buawa” and the “Shallow”. Ernie was referred to as the “Bearded Sphinx of the North”. I’m not sure where that came from, but know the “Sphinx” was “good copy” because he was so active in minerals exploration particularly in the Great Lakes region in those days as he certainly is today. I had the good fortune and privilege of working with Ernie for nearly two decades. Over the years, his list of accomplishments in the field of economic geology and mineral exploration are many. It would take hours to go through them all. So I will focus on those related to the Lake Superior area. First some background. Ernie was born in 1929 in Heidelberg, Germany. He was educated in the public schools of New Rochelle, N.Y., attended Williams College, in Williamstown, Massachusetts, where he graduated cum laude in geology in 1951. He attended graduate school in geology at Brown University, Providence, Rhode Island, and completed the Owners and Presidents Management Program of the Harvard Business School in 1984. He is married to Sally Willius Lehmann (his better half) and resides in Minneapolis, Minnesota. Prior to founding Ernest K. Lehmann & Associates, Inc. in 1967, Ernie was an independent consultant and partner in a Minneapolis based geological consulting firm from 1958 to 1967. In 1950, he began his career by working first as a miner and then as geologist for the Signal Mining Company at Bannack, Montana. From 1951 to 1958, Ernie worked for Kennecott Copper Corporation and its exploration arm, Bear Creek Mining Company. Ernie served on active duty as a Terrain Intelligence Analyst in the U.S. Army Corps of Engineers from 1953 to 1955 and was awarded the Commendation Ribbon for his service. As many of you know, Ernie is the founder and CEO of North Central Mineral Ventures Inc. and of Ernest K. Lehmann & Associates, Inc. Under his leadership, the firms have engaged in the planning, management and execution of mineral exploration programs, mineral deposit development, mine appraisal and mineral economic studies particularly in the Great Lakes region. These activities have spanned most important hard mineral commodities including ferrous, non-ferrous, precious and strategic metals, industrial minerals and fertilizer raw materials. In the course of their activities, he and his firms have been active in staffing and managing exploration and mine development projects and acquiring private and public mineral xii �lands in both the U.S. and abroad. The latter activities have included claim staking, mineral leasing of private, state and federal lands, and the creation and management of mineral joint ventures. In addition to extensive work in the US and Canada, he has conducted and managed exploration and consulted on mine development and evaluation Central and South America, Africa and Europe. In the course of his activities, Ernie has specialized in exploration management and in mineral deposit appraisal evaluation. He has appeared extensively as an expert witness on mineral property appraisal and taxation and on mining claim related issues. He has served on advisory committees to the Office of Technology Assessment of the U.S. Congress on strategic and critical minerals and to the state of Minnesota on direct reduction of iron ores. On behalf of the American Institute of Professional Geologists, he has testified before Congress on strategic minerals issues and on the 1872 mining law. He has also testified before Congress on issues related to the Federal Land Management Policy Act. Before state legislative committees, he has testified on mineral property appraisal, mineral taxation, mineral leasing and mine permitting. In 1985, he became president of the American Institute of Professional Geologists, also serving the Institute in various other capacities on a state and national level. In 1987, he was awarded the Ben H. Parker Medal, the Institute’s highest award for service to the geological profession, and has been awarded Honorary Membership in the Institute in 1997. He currently serves as Chairman of the AIPG Foundation and is a member of numerous other technical and professional bodies including the Society of Economic Geologists, The Mining and Metallurgical Society of America, the Society of Mining Engineers, the Society for Geology Applied to Mineral Deposits, and the Northwest Mining Association. He is a registered geologist in Minnesota, California, Georgia, Delaware and Alaska and is accredited by the European Federation of Geologists. He is currently a director and president of the Minnesota Exploration Association and serves on various committees in Minnesota in this capacity, including the Blue Ribbon Committee on Minnesota Minerals, the advisory board to the Natural Resources Research Institute and the State Mapping Advisory Committee. Ernie has also been actively involved in mining related environmental issues again particularly in the Great Lakes area. He was instrumental in initiating and executing the Minnesota Mining Permit Simulation Project, a joint state agency, environmental community and industry effort to examine the mine permitting process and problems for non-ferrous metal mining. He helped organize a workshop on financial assurance in the mining industry in cooperation with the Minnesota DNR, MPCA, and the Audubon Society. Ernie and the firm were consultants to the state of Maine charged with developing metal mining regulations, spanning activities from exploration through operation and closure. The firms were also involved in mine permitting activities in Wisconsin. To this I would like to mention some of the major projects Ernie has initiated in the Great Lakes area. These include: 1) Exploration programs for Bear Creek in 1952, 1956 through 1957 for copper and copper nickel in the Duluth and Mellon Gabbro Complexes and Nonesuch Shale. xiii �2) Copper and copper-nickel exploration projects carried out for Cerro Corp. from 1967-1969 in Wisconsin, Minnesota, Michigan and Ontario. He was one of the first explorationists to recognize the Wisconsin greenstone belts as an important VMS target . 3) VMS exploration joint ventures in Wisconsin that spanned from 1975 through 1993. Such notables as Getty, Chevron, Denison Mines and Asarco were involved with these projects. Under the supervision of the late Ned Eisenbrey, these programs resulted in three discoveries. Of these one, the Bend copper-gold deposit, is and remains potentially economic. The huge amount of geologic and geophysical data generated from these exploration projects also resulted in establishing a more detailed “geologic framework” that was previously lacking for the Wisconsin greenstone belts. This work culminated in several papers published in “Economic Geology” in 1990 and 1994. 4) At the same time Ernie was managing a major VMS program in Archean of northern Minnesota from 1979 through 1985 for Getty and Billiton. 5) From 1980 to 1982, Ernie was one of the first to seriously begin exploring the interior of the Duluth Complex for copper-nickel and PGMs through a joint venture with Billiton. His exploration efforts continue today with a focus on the Birch Lake PGM project that involves Impala. I believe Ernie’s past and present exploration and political efforts in Minnesota today has resulted in the current brisk activity within the Duluth Complex and transformed the state to an attractive part of the world to conduct environmentally responsible mining projects. Finally, with deep gratitude and respect, I would like to present this year’s Sam Goldich medal recipient, my mentor and friend, Ernest K. Lehmann. Theodore A. DeMatties xiv �EISENBREY STUDENT TRAVEL AWARDS The 1986 Board of Directors established the ILSG Student Travel Awards to support student participation at the annual meeting of the Institute. The name "Eisenbrey" was added to the award in 1998 to honor Edward H. Eisenbrey (1926-1985) and utilize substantial contributions made to the 1996 Institute meeting in his name. "Ned" Eisenbrey is credited with discovery of significant volcanogenic massive sulfide deposits in Wisconsin, but his scope was much broader—he has been described as having unique talents as an ore finder, geologist, and teacher. These awards are intended to help defray some of the direct travel costs of attending Institute meetings, and include a waiver of registration fees, but exclude expenses for meals, lodging, and field trip registration. The number of awards and value are determined by the annual Chair in consultation with the Secretary-Treasurer. Recipients will be announced at the annual banquet. The following general criteria will be considered by the annual Chair, who is responsible for the selection: 1) The applicants must have active resident (undergraduate or graduate) student status at the time of the annual meeting of the Institute, certified by the department head. 2) Students who are the senior author on either an oral or poster paper will be given favored consideration. 3) It is desirable for two or more students to jointly request travel assistance. 4) In general, priority will be given to those in the Institute region who are farthest away from the meeting location. 5) Each travel award request shall be made in writing to the annual Chair, and should explain need, student and author status, and other significant details. The form below is optional. Successful applicants will receive their awards during the meeting. xv �INSTITUTE ON LAKE SUPERIOR GEOLOGY Eisenbrey Student Travel Award Application Student Name: Date: Address: email: Department Head-Typed Department Head-Signature Educational Status: Are you the senior author of an oral or poster paper? YES Will any other students be traveling with you? NO Who? Statement of need (use additional page if necessary) Please return to: xvi �STUDENT PAPER AWARDS Each year, the Institute selects the best of the student presentations and honors presenters with a monetary award. Funding for the award is generated from registrations of the annual meeting. The Student Paper Committee is appointed by the annual meeting Chair in such a manner as to represent a broad range of professional and geologic expertise. Criteria for best student paper— last modified by the Board in 1997—follow: 1) The contribution must be demonstrably the work of the student. 2) The student must present the contribution in-person. 3) The Student Paper Committee shall decide how many awards to grant, and whether or not to give separate awards for poster vs. oral presentations. 4) In cases of multiple student authors, the award will be made to the senior author, or the award will be shared equally by all authors of the contribution. 5) The total amount of the awards is left to the discretion of the meeting Chair and SecretaryTreasurer, but typically is in the amount of about $300 US. 6) The Secretary-Treasurer maintains, and will supply to the Committee, a form for the numerical ranking of presentations. This form was created and modified by Student Paper Committees over several years in an effort to reduce the difficulties that may arise from selection by raters of diverse background. The use of the form is not required, but is left to the discretion of the Committee. 7) The names of award recipients shall be included as part of the annual Chair's report that appears in the next volume of the Institute. Student papers will be noted on the Program. STUDENT PAPER AWARDS COMMITTEE Peter Hollings Lakehead University, Thunder Bay Neil Pettigrew Avalon Ventures Ltd., Thunder Bay Connie Dickens U.S. Geological Survey, Reston, Virginia xvii �BOARD OF DIRECTORS Mike Mudrey, Chair Wisconsin Geological and National History Survey, Madison, Wisconsin 2000-2003 Steve Kissin, Department of Geology, Lakehead University, Thunder Bay, Ontario 1999-2002 Ted Bornhorst, Department of Geological Engineering and Sciences, Michigan Tech UniversityHoughton, Michigan 1998-2001 Jim Miller, Minnesota Geological Survey, St. Paul, Minnesota Secretary-Treasurer Mark A. Jirsa, Minnesota Geological Survey St. Paul, Minnesota LOCAL COMMITTEE Peter Hinz, Ontario Geological Survey, Resident Geologist Program, Kenora, ON Richard C. Beard, Northwest Mineral Development Services, Kenora, ON Charles E. Blackburn, Blackburn Geological Services, Kenora, ON Christine Blackburn, Blackburn Geological Services, Kenora, ON Kevin O’Flaherty, Kenora, ON Carmen Storey, Ontario Geological Survey, Resident Geologist Program, Red Lake, ON Kathleen McGowan-Hinz, Kenora, ON SESSION CHAIRS xviii �Alasdair Mowat Emerald Fields Resource Corp., Kenora Philip Fralick Lakehead University, Thunder Bay Mark Smyk Ontario Geological Survey, Thunder Bay 2002 BANQUET SPEAKER L. Harvey Thorleifson Geological Survey of Canada Ottawa, Ontario The Search for Diamonds in Canada xix �REPORT OF THE CHAIR OF THE 47TH ANNUAL MEETING Michael G. Mudrey, Jr. Chair ILSG 2001 The 47th Annual Institute on Lake Superior Geology was jointly hosted by the University of Wisconsin-Extension Geological and Natural History Survey and the University of WisconsinMadison, Department of Geology and Geophysics , in Madison, Wisconsin on May 9-12, 2001. Principal local committee members were M.G. Mudrey, Jr . and B.A. Brown, co-chairs, Robert H. Dott, Jr, and L.Gordon Medaris, Jr., Program-cochairs, and Kathleen M. Zwettler, Meeting Coordinator. Other principal individuals are listed in the Proceedings Volume. Attendance at ILSG 2001 A total of 172 professionals and student professionals attended the meeting, 82 of whom preregistered by the April 2, 2001 deadline. A total of 26 students were registered, ten of whom requested and received travel assistance and had registration fees for the meeting waived. Eisenbrey Student Travel Awards 2001 Ten students requested and received travel assistance form the Eisenbrey Student Travel Award Fund established to support student participation at the Annual Institute. Details, including criteria and application forms are available at the Eisenbrey website. Justin Johnson- Lakehead University-Thunder Bay, Ontario - Fluid Inclusion Evidence for a Role for Hydrothermal Activity in the Roby Zone, Lac Des Iles Mine, Northwestern Ontario; Becky RogalaLakehead University-Thunder Bay, Ontario - A Metamorphosed Evaporite Sequence from the Sibley Basin ; Dan BihariLakehead University-Thunder Bay, Ontario - Alteration and Pge-au Mineralization in the North Roby Zone, Lac Des Iles Mine, Northwestern Ontario; Phillip LarsonUniversity of Minnesota-Duluth - Potential for Copper Mineralization in the Animikie Group, Minnesota; Michael NemitzUniversity of Minnesota-Duluth - Mineralogical Variations in Iron-formation in the Thermal Metamorphic Aureole of a Diabase Dike; Lisa LarsonUniversity of Minnesota-Duluth - ; Muhammad Asif Soofi- Purdue University, West Lafayette, Indiana - Post-rift Evolution of the Midcontinent Rift System: Some Numerical Experiments; Jason OdetteUniversity of Wisconsin-Oskosh - Preliminary Evaluation of Hydrothermal Alteration Mineral Assemblages and Their Relationship to VMS-style Mineralization in the Five Mile Lake Area of the Archean Vermilion Greenstone Belt, Northeastern Minnesota ; xx �Trent Newkirk- Dan Schweitzer- University of Wisconsin-Oskosh - Preliminary Lava Flow Morphology Studies at the Five Mile Lake Vms Prospect, Archean Vermilion District, Ne Minnesota: Implications for Volcanic Processes, Volcanic Paleoenvironments, and VMS Exploration ; and Kent State University-Kent Ohio - Results of Igneous Thermometry and Barometry on the East-central Minnesota Batholith: Evidence for Post-emplacement Exhumation and Cooling . Meeting Summary The 47th Annual Institute on Lake Superior Geology Annual Meeting was held at the Sheraton Madison Hotel, the same location as the 1973 meeting. The 2001 meeting focused on hydrogeologic aspects of arsenic in groundwater, completion of acquisition of detailed regional geophysical data; the evolution of the post-Penokean--pre-Keweenawan crystal terrane of North American in 36 poster displays and 32 oral presentations . The two days of technical sessions were preceded by Field Trip 1 - Sedimentologic, Tectonic and Metamorphic History of the Baraboo Interval led by L. Gordon Medaris, Jr. , and Robert H. Dott, Jr. (University of Wisconsin-Madison) , and followed by Field Trip 2 - Upper Mississippi Valley Zinc-Lead District led by M.G. Mudrey, Jr. (Wisconsin Geological and Natural History Survey) and Thomas C. Hunt (University of Wisconsin-Platteville) and Field Trip 3- Industrial Mineral and Aggregate Resources of the Baraboo Interval Quartzites lead by Bruce A. Brown (Wisconsin Geological and Natural History Survey), Frank R. Luther (University of Wisconsin Whitewater), James W. Schmitt (D.L. Gasser Construction), Susan M. Courter (Michels Materials) and Jennifer Lien (The Kraemer Company) The meeting began with regional geologic summaries of the Pleistocene of Southern Wisconsin by D.M. Mickelson (University of Wisconsin-Madison) and Lee Clayton (Wisconsin Geological and Natural History Survey), Sequence Stratigraphic Analysis of the Paleozoic by Charles E. Byers (University of Wisconsin-Madison), and Stratigraphic Metamorphic Analysis of the Baraboo Supracrustal Rocks of the Midcontinent by L. Gordon Medaris, Jr. , (University of Wisconsin-Madison). A series of invited presentations completed the morning of the first technical sessions three presentations on Aeromagnetic Investigations in the Midwest. An invited presentation on an Overview of Arsenic Occurrences and Processes Controlling Arsenic Moblity in Ground Water by D. Kirk Nordstrom (U.S. Geological Survey) kicked off a special session of four papers on the Hydrogeologic Setting of Elevated Arsenic and Heavy Metals in Public and Private Water Supplies. General geologic topics completed the first day. Three papers on the Midcontinent Rift started Friday morning, followed an invited symposium of eight papers on the Thermo-Thermo-Tectonic History of 1800 to 1200 Ma post-Penokean to pre-Keweenawan. The remaining significant presentations dealt with Archean and platinumgroup element geology. The technical program was completed by 3:30 p.m. Proceeding including Pat 1 (Programs and Abstracts) and Part 2 (Field Trip Guidebook) are available from the Institute . xxi �Best Student Paper Awards for 2001 were presented to Alissa Naymark-University of Wisconsin-Madison (oral) Cash award plus chrome-plated drill bit donated by Layne Northwest Travis Sandland-Macalester College (poster) and Erin H. Phillips-Macalester College (poster) Jason D. Odette-University of Wisconsin-Oshkosh (poster) Trent T. Newkirk-University of Wisconsin-Oshkosh (poster) After the break, Mike Mudrey and Bruce Brown regaled the remaining participants with previews of their field trips (Field Trip 3 was eventful in that a flat tire delayed the trip ( How many ILSGers does it take to change a tire? Three! Frank Luther, Mark Jirsa and Bob Reszka), and Brad Singer and John Valley lead the group on an impressive tour of the isotope laboratories in the Department of Geology and Geophysics. The entire group of ILSG participants remaining in Madison joined the Department of Geology and Geophysics Geology Club for the annual Departmental Picnic courtesy of contributions from Steve Kircher, Crandon Mine Development, Nicolet Minerals, and William J. Cronk, Layne Northwest, directly to the Institute. Annual Banquet and Goldich Award At the Annual Banquet Bruce R. Doe prepared, and Michael G. Mudrey, Jr. delivered a biographic history of Samuel S. Goldich who passed away on 20 December 2000 in Applewood, Colorado. After the introduction of 6 of the surviving Goldich recipients, an upbeat Gene LaBerge presented John S. Klasner with the Goldich Medal for 2001 for his contributions to the Institute and Lake Superior Geology. Tom Hunt illustrated the successful reclamation of the Ladysmith Mine of Kennecott Mining for the after dinner address, and Peter Hinz of the Ontario Geological Survey invited participants to the 48 th Annual Meeting in Kenora . Best Student Paper Awards for 2001 Alissa Naymark-University of Wisconsin-Madison (oral) Cash award plus chrome-plated drill bit donated by Layne Northwest Travis Sandland-Macalester College (poster) Erin H. Phillips-Macalester College (poster) Jason D. Odette-University of Wisconsin-Oshkosh (poster) Trent T. Newkirk-University of Wisconsin-Oshkosh (poster) Details of presentations are found in the Proceedings and Abstracts for the 47 th Annual Meeting . After the break, Mike Mudrey and Bruce Brown regaled the remaining participants with previews of their field trips (Field Trip 3 was eventful in that a flat tire delayed the trip ( How many ILSGers does it take to change a tire? Three! Frank Luther, Mark Jirsa and Bob Reszka), and Brad Singer and John Valley lead the group on an impressive tour of the isotope laboratories in the Department of Geology and Geophysics. The entire group of ILSG participants remaining in Madison joined the Department of Geology and Geophysics Geology Club for the annual Departmental Picnic courtesy of contributions from Steve Kircher, Crandon Mine Development, Nicolet Minerals, and William J. Cronk, Layne Northwest, directly to the Institute. xxii �Proceedings including Part 1 (Programs and Abstracts) and Part 2 (Field Trip Guidebook) are available from the Institute . Institute on Lake Superior Geology c/o Mark Jirsa, Executive Secretary 2642 University Avenue St. Paul MN 55114-1057 Phone: (612)-627-4539 Fax: 612-627-4778 e-mail: jirsa001@maroon.tc.umn.edu xxiii �48th ANNUAL MEETING INSTITUTE ON LAKE SUPERIOR GEOLOGY Program xxiv �Program of Events Sunday, May 12 06:00 - 18:00 Field Trip 1: Tanco Rare-Element Pegmatite, Southeastern Manitoba Monday, May 13 08:00 - 18:00 Field Trip 2: Quaternary Geology of Southeastern Manitoba 08:00 - 16:00 Field Trip 3: Structure and Sedimentology of the Seine Conglomerate, Mine Centre Area, Ontario 17:00 – 20:00 Registration – Best Western Lakeside Inn, Kenora, Ontario 19:00 – 21:00 Ice-Breaker Social and cash bar in Best Western Lakeside Inn (Authors at poster from 19:30 to 21:00) xxv �TUESDAY MAY 14 8:00 a.m. - 9:00 a.m. REGISTRATION 9:00 a.m. INTRODUCTORY REMARKS P. Hinz Co-Chairman, Ontario Geological Survey, Kenora, ON TECHNICAL SESSION I: Session Chair: Mark Smyk, District Geologist, Ontario Geological Survey, Thunder Bay, ON 9:15 a.m. K. O’Flaherty, (Consulting Geologist, Kenora, ON) A Brief History of Mining in Northwestern Ontario 9:40 a.m. M. Sanborn-Barrie*, T. Skulski, N. Rayner, (Geological Survey of Canada, Continental Geoscience Division, Ottawa, ON) and J.R. Parker (Ontario Geological Survey, Precambrian Section, Sudbury, ON) 300 my evolution of the Red Lake greenstone belt, western Superior Province, Ontario: A synthesis of current constraints on volcanism, sedimentation, deformation, metamorphism and gold mineralization 10:05 a.m. COFFEE BREAK AND POSTER SESSION 10:30 a.m. B. Nitescu*, A. R. Cruden, A. R. Bailey, (University of Toronto, Toronto, ON) Crustal Models of the Western Superior Province from gravity and Lithoprobe seismic data. 10:55 a.m. P. Fralick* and King, (Department of Geology, Lakehead University, Thunder Bay ON) Mesoarchean evolution of Western Superior Province: evidence from metasedimentary sequences near Atikokan 11:20 a.m. M.A. Jirsa, (Secretary-Treasurer, ILSG, St. Paul, MN) A Report on the Secretary-Treasurer's Position LUNCH BREAK ILSG BOARD MEETING (by invitation) POSTERS xxvi �TECHNICAL SESSION II: Session Chair: Alasdair Mowat, Emerald Fields Resource Corp., Kenora, ON 1:30 p.m. D. Peterson, (Natural Resources Research Institute, University of Minnesota-Duluth, Duluth, MN) Cu-Ni-PGE Mineralization in the South Kawishiwi intrusion; North Western Minnesota. Variation due to magmatic processes. 1:55 p.m. B. Rogala*, P.W. Fralick, and G. Borradaile, (Department of Geology, Lakehead University, Thunder Bay, Ontario) New Information from the Sibley Group 2:20 p.m. M.A. Jirsa, (Minnesota Geological Survey, St. Paul, MN) Archean and Paleoproterozoic mafic intrusions in Minnesota 2:45 p.m. COFFEE BREAK AND POSTER SESSION 3:15 p.m. C. Sturm, (Geology Department, Oberlin College, Oberlin, OH) Petrographic study of the Otter Tail Pluton, Superior Province, Northwestern Ontario 3:40 p.m. G. Ferguson* and A. Woodbury, (Department of Civil Engineering, University of Manitoba, Winnipeg, MB) Ground water and heat flow in an interlobate moraine in southwestern Manitoba. 6:00 p.m. SOCIAL - Cash Bar 7:00 p.m. Annual Banquet and Award Presentation • Announcement of 49th Annual Meeting Location • 2002 Goldich Award Presentation to Mr. Ernie Lehmann, Citation by Ted Dematties • Banquet Speaker: Dr. L. Harvey Thorleifson, Geological Survey of Canada The Search for Diamonds in Canada xxvii �WEDNESDAY MAY 15 8:00 a.m. - 9:00 a.m. REGISTRATION 9:00 a.m. INTRODUCTORY REMARKS R.C. Beard Co-Chairman, Northwest Mineral Development Services, Kenora, ON TECHNICAL SESSION III: Session Chair: Philip Fralick, Lakehead University, Thunder Bay, ON 9:15 a.m. H.H. Woodard, (Department of Geology, Beloit College, Beloit, WI) Internal structures within crustal structural slabs, Quetico-Wawa subprovince junction, Quetico Provincial Park, Ontario 9:40 a.m. R. Bernatchez, (Atikokan Resources Ltd., Atikokan, ON) Mesoarchean Base Metal Systems in the Atikokan Area, NW Ontario, Canada 10:05 a.m. COFFEEBREAK AND POSTER SESSION 10:30 a.m. L.G. Medaris*, B.S. Singer, P.E. Brown, B.R. Jiccha, M.E. Smith, (University of Minnesota-Duluth, Duluth, MN) Wolf River age brecciation in the Baraboo quartzite, Wisconsin: implications for Proterozoic tectonics in the Lake Superior Region. 10:55 a.m. D.L. Southwick, (Minnesota Geological Survey Ret’d, Pacitas, NM) Inferences from the Hattemberger deep drill hole, Carlton County, Minnesota, pertinent to regional stratigraphy and mineral potential of the western segment of the Penokean Orogen 11:20 a.m. J.C. Pedersen* and D. Bubar, (Avalon Ventures Ltd., Toronto, ON) Mineralogy and zonation of the Big Whopper Pegmatite, Separation Rapids, Kenora area, Ontario 11:45 a.m. M.A. Jirsa (Secretary-Treasurer, ILSG, St. Paul, MN) Discussion on the Secretary-Treasurer’s Position 12:00 Presentation of Student Paper Awards LUNCH BREAK 3:00 p.m. FIELD TRIP 6: Departs for Red Lake. xxviii �Thursday, May 16 08:00 - 18:00 Field Trip 4: Industrial Minerals and Paleozoic Geology of Southeastern Manitoba 08:00 - 16:00 Field Trip 5: Separation Rapids Rare-Element Pegmatite Field, Ontario 08:00 - 18:00 Field Trip 6: Geology of the Red Lake Camp, Ontario xxix �POSTER PRESENTATIONS Boerboom, T. (Minnesota Geological Survey, St. Paul, MN) The influence of Archean crustal structures on the margin of the Penokean orogen near Pope County, west central Minnesota. Buchholz, T.W. (Wisconsin Rapids, WI); Falster, A.U. and Simmons, Wm. B. (University of New Orleans, New Orleans, LO). Heterogeneity in the nine mile pluton, wausau complex, in terms of high field strength element (hfse) and rare earth element (ree) bearing minerals Cannon, W.F., (USGS, Reston, VA), Laberge, G.L. (Oshkosh, WI), Klasner, J. S., (Macomb, IL), Dicken, C., (USGS, Reston, VA) Geology of the western Gogebic Iron Range, northwest Wisconsin—a record of sedimentation and deformation across the northern margin of the Penokean orogen Fein, E., (Oberlin College, Oberlin, OH) Anisotropy of magnetic susceptibility in the Otter Tail Pluton, Northwestern Ontario Hubin, J. and Cordua, Wm. (University of Wisconsin, River Falls, WI) SEM imaging of fossil nannobacteria from the supergene zone, Flambeau copper mine, Ladysmith Wisconsin. Johnson, J.R., Kissin, S.A. and Hollings, P. (Lakehead University, Thunder Bay, ON) VHMS mineralization and alteration within the Lumby Lake Greenstone Belt, Northwestern Ontario Kelso, I.S. and Kissin, S.A. (Lakehead University, Thunder Bay, ON) Geology and fluid inclusion studies of the Thunder Bay Agate Mine, Northwestern Ontario Miller, J.D. Jr.1, Green2, J.C., Severson3, M.J., Chandler1, V.W., Peterson3, D.M., Hauck3, S.A., and Wahl1 T.E, (1Minnesota Geological Survey, St. Paul, MN, 2University of Minnesota–Duluth, Duluth, MN, 3Natural Resources Research Institute, Duluth, MN) New geologic map and report of the Duluth Complex and related rocks, Northeastern Minnesota. R.L. Patelke, (Natural Resources Research Institute, University of Minnesota-Duluth, Duluth, MN) Digital Drill Logs for the Duluth Complex - Lithology and Assays Peterson, D.M. (Natural Resources Research Institute, University of Minnesota-Duluth, Duluth, MN) xxx �3-Dimensional View Through a Mineralized System: the South Kawishiwi Intrusion, Duluth Complex Severson, M.J. (Natural Resources Research Institute, University of Minnesota-Duluth, Duluth, MN) The Mine Permitting Process in Minnesota - Who, What, Where, and When. Sikkila, K. (Wisconsin Department of Transportation, Superior, WI) Description of a pegmatite occurrence on the eastern margin of the Mellen Granite, State Highway 13, Ashland County, Wisconsin Woodruff, L.G. (U.S. Geological Survey, Mounds View, MN), W. F. Cannon, and C. Dicken, (U.S. Geological Survey, Reston, VA) Impact of fire on the forest floor and mineral soils, Snowbank Lake, Minnesota COMPANY POSTERS Avalon Ventures Ltd., 111 Richmond Street West, Suite 1116 Toronto, ON M5H 2G4 Emerald Fields Resource Corp., 1546 Pine Portage Road Kenora, ON P9N 2K2 Goldcorp Inc., Balmertown, ON P0V 1C0 Nuinsco Resources Ltd., 940 The East Mall, Suite 110 Toronto, ON M9B 6J7 xxxi �48th ANNUAL MEETING INSTITUTE ON LAKE SUPERIOR GEOLOGY Abstracts ��Mesoarchean Base Metal Systems in the Atikokan Area, NW Ontario, Canada Bernatchez, R. (Atikokan Resources Ltd., Atikokan, ON) A high grade silver-lead-zinc showing was discovered in the Lumby Lake Metavolcanic Belt in 1995 between Lumby and Herontrack Lake 100 meters north of the creek. Three grab samples taken from this showing returned an impressive 109, 189 and 416 oz Ag/ton with other samples assaying up to 25% lead and 8% zinc. Since 1995, Atikokan Resources has carried out an extensive exploration program consisting of detailed geological mapping, a 2292 sample soil survey, a 150 whole rock geochemical survey, ground geophysical magnetic, electromagnetic and IP surveys ( dipole-dipole and Gradient array) and seven diamond drill holes over a grided area 6.6 km E-W x 2 km N-S from the west end of Lumby Lake to Hutt Lake. All but one diamond drill hole was drilled on the main high grade silver discovery area located between Lumby and Herontrack Lake. This work has been successful in defining a world class size VMS system. This VMS system is located near the base of the southern limb of the Lumby Lake Mesoarchean Syncline. The detailed geological mapping has identified at least three major eruptive cycles of predominantly intermediate to felsic pyroclastic and tuffaceous rocks. Each eruptive cycle has been terminated with the deposition of exhalative chert and sulphides. These three horizons have been identified as the Lumby-Spoon Lake (southern), Delos Lake and Pond Lake (northern) Horizon. Most of the mechanical stripping has been focused on the Lumby-Spoon Lake Horizon. The rocks along the top portion of the Delos and Pond Lake Horizon are poorly exposed. Limited portions of these two horizons were exposed by mechanical stripping near the west end of each lake. The main high grade silver-lead-zinc discovery is located at the top of the Lumby-Spoon Lake Horizon. This horizon has now been traced from outcrop exposure and drilling for a distance of about 3 kilometers. This east-west striking horizon consists of four or more repeated sub-cycles of quartz-phyric rich tuff, lapilli tuffs (coarse and fine) and chert mineralized with sulphides. The mineralization is contained within the chert, tuff, lapilli tuff in the form of dissemination, stringers and semi-massive sulphide beds. The sulphide mineralization consists of sphalerite, galena, native silver, acanthite, pyrite, chalcopyrite and pyrrhotite. Each felsic cycle vary in thickness from a few meters up to 20-25 meters. The upper cherty portion of each cycle contains is usually mineralized with sphalerite, galena and acanthite and/or native silver. The tuff lapilli tuff section generally contains stringer mineralization of chalcopyrite, pyrite with minor sphalerite, galena acanthite and pyrrhotite. The main high-grade silver showing within the Lumby-Spoon Lake has been drill-tested with six diamond drill holes along an east-west strike length of 450 meters and down to a depth of about 400 meters. Mineralization has been intersected in drill holes over core length from 13 metres and up to 82 metres. Native silver was observed in all six holes drilled on the Lumby-Spoon Lake Horizon. The zone is still open at depth and on strike east and west. In total, the four or five sub-cycles forms a mineralized horizon measuring over 200 meters wide in the main showing area. 1 �The geology, mineralogy and alteration of the mapped area have shown strong similarity with other VMS deposits found in the Archean, Properozoic, and Mesozoic Eras. The drilling and litho geochemical rock sampling have shown the classic footwall chloritic and/or talc alteration, sericitic hanging wall alteration, zinc-lead-silver capping and footwall stringer mineralization. The litho geochemical rock sampling has shown areas of sodium depletion and potassium enrichment within the felsic rocks and magnesium enrichment within the footwall. This alteration appears to be parallel to sub-parallel to the stratigraphy. The felsic volcaniclastic rocks appear to have been derived from a highly fractionated sub-volcanic magma and have been classified as F2 and F3a type rhyolites. A new exploration program of detailed geological mapping, litho geochemical sampling and ground geophysical magnetic and electromagnetic surveys carried out in the Richardson Lake area, 12 km east and on strike with the main high grade silver-lead-zinc showing, has identified similar thick felsic bi-modal volcaniclastic rock sequence. It appears to be the easterly extension of the island arc at Lumby Lake. Several new base metal occurrences have been discovered and are contained within similar quartz- spheric felsic volcaniclastic rocks similar to those found in the Lumby Lake felsic rocks. K. Tomlinson of the GSC (2000) has identified island arc volcanism in both the southern and northern limb and at the base of the Lumby Lake Meta-volcanic Belt. The work carried out to date by ARI has shown that at least two island arc systems may exist within the lower portion of the southern limb of the Lumby Lake Meta-volcanic Belt. It is however, conceivable, that more that two island arc systems exist in this part of the belt. Recent work by ARI in the upper portion of the belt, north of the synclinal fold axis in the northern limb, has identified a different style of mineralization contained within sulphide facies IF. It is unknown at this time if thses two styles of mineralization are related to the same volcanic event of island arc building. It is possible that the Lumby Lake Meta-volcanic Belt may contain world class size VMS deposits yet undiscovered. The results obtained to date by Atikokan Resources Inc. shows that potential. ARI continues to explore and find previously identified and unidentified VMS base metal style mineralization in the Lumby and Finlayson Lake Meta-volcanic Belts. 2 �The influence of Archean crustal structures on the margin of the Penokean Orogen near Pope County, West-central Minnesota Boerboom, T. J., (Minnesota Geological Survey, St. Paul, MN (boerb001@umn.edu)) The western margin of the Paleoproterozoic Penokean Orogen in Minnesota is marked by a series of northwest-trending, sawtooth-like fault offsets, or tear faults, that correspond in space but not in orientation to major northeast-oriented block-bounding shear zones within the adjacent Archean Minnesota River Valley subprovince (Figs. 1A and 1B). The origin of these tear faults, whether reactivated Archean structures or structures formed strictly during the Penokean orogenic cycle is not clear, but they may be the result of scissors faulting focused on preexisting weaknesses in the crust during pre-orogenic extension. Pope County was mapped as part of the Minnesota Geological Survey County Atlas geologic mapping program, supported by the Minnesota Department of Natural Resources, Division of Waters. The bedrock there is deeply buried beneath glacial drift, and there are only six drill cores available for the 720 square mile (1900 square kilometer) county. Considering this, it is obvious that any geological interpretation of the bedrock geology is more accurately described as an interpretation of geophysical characteristics; nevertheless, several prominent structural features can be recognized from the geophysical data. The Minnesota River Valley subprovince has been subdivided into four blocks⎯the Benson, Montevideo, Morton, and Jeffers blocks, from north to south (Fig. 1B). Each of these crustal blocks is separated by straight and narrow east-northeast-trending geophysical lineaments that have been shown to be north-dipping shear zones (Southwick and Chandler, 1996; Southwick, 2002). In Minnesota, the western margin of the Paleoproterozoic Penokean Orogen changes from a northeast orientation that is roughly parallel to the Wawa-Minnesota River Valley subprovince boundary, to a north-south orientation that is perpendicular to the regional fabric of the adjacent Minnesota River Valley subprovince (Figs. 1A and 1B). The major sawtooth-notched offsets, or tear faults, are most prominent in the north-south stretch, where they coincide spatially with the major block-bounding faults in the adjacent Minnesota River Valley subprovince. The most prominent of these tear faults, which crosses southern Pope County, separates the Archean Montevideo block from Paleoproterozoic rocks on the east, but to the west is contained wholly within the Archean Benson block (Fig. 1B). The eastern extent of this fault is occupied by a continuous string of late- to post-tectonic, mafic to felsic intrusions (Fig. 1B) that have been verified by drilling to consist of intrusive rocks similar to those exposed in outcrops to the northeast, near St. Cloud. The concentration of these late, ovoid intrusions along the fault plane implies that the fault extends to deep crustal depths. The tear faults may have formed strictly during Paleoproterozoic time, or they may be reactivated Archean structures that were oriented parallel to the direction of maximum extension and shortening of the Penokean deformational belt. In either case, the coincidence of the orogen-bounding tear faults with major northeast-trending shear zones in the adjacent Minnesota River Valley subprovince 3 �implies that preexisting structures in the Archean crust provided zones of weakness which ultimately controlled the location of the tear faults. Boerboom, T.J., in prep., Bedrock geology of Pope County, Minnesota, plate 2 of Harris, K.L., project manager, Geologic atlas of Pope County, Minnesota: Minnesota Geological Survey County Atlas C-15, Part A, scale 1:200,000. Southwick, D.L., 2002, Geologic map of pre-Cretaceous bedrock in southwest Minnesota: Minnesota Geological Survey Miscellaneous Map M-121, scale 1:250,000. Southwick, D.L., and Chandler, V.W., 1996, Block and shear-zone architecture of the Minnesota River Valley subprovinces: Implications for late Archean accretionary tectonics: Canadian Journal of Earth Sciences, v. 33, no. 6, p. 831–847. 4 �Heterogeneity in the Nine Mile Pluton, Wausau Complex, in terms of high field strength element (hfse) and rare earth element (ree) bearing minerals. Buchholz, T.W., (1140 12th Street North, Wisconsin Rapids, Wisconsin 54494); Falster, A. U., and Simmons, Wm. B., (Department of Geology and Geophysics, University of New Orleans, New Orleans, Louisiana 70148). The Nine Mile pluton is the youngest and most silica-rich of the four intrusive centers of the Wausau complex which is exposed in Marathon County, Wisconsin. The complex is approximately 1.5 Ga in age and locally abounds in pegmatitic veins, aplites, and miarolitic granite. Mineralization in these environments is commonly varied and complex, though in many cases restricted to small localized environments. Notable variation exists in minerals of HFSE (high-field strength elements such as Nb, Ta, Zr, Th, etc) and to a lesser extent in REE (rare earth elements). The type and size of these mineral-rich bodies varies throughout the pluton. In the northern portion of the pluton, large and well-defined pegmatitic bodies with sizeable miarolitic cavities containing a wealth of accessory phases are abundant. The size of pegmatitic veins and the size of miarolitic cavities are smaller in the central and southern parts of the pluton. Small miarolitic cavities in dikes and in granite typically host the accessory minerals in the northern area of the pluton. Significant variation also exists with respect to Nb, Ta, Ti and REE+Y mineralization. In the northern part of the pluton, titanium oxides such as anatase, brookite, and rutile are widespread but they are rare in the central and southern portions. Columbite-tantalite group minerals are most abundant in the central part of the pluton, but less abundant in the south and rare in the northern part. Conversely, Nb-dominant euxenite-group minerals which are rare to absent in the central portion of the pluton are more abundant in the southern and western parts. REE+Y mineralization appears to be more evenly distributed but may be slightly more abundant in the central part. Zircons are widespread in small amounts throughout the pluton and show Hf-enrichment, particularly in close proximity with fluorite. This enrichment occurs mainly in the central portion of the pluton, although isolated instances have been noted in the south and west. Cassiterite is exceedingly rare but has been noted in the central, western, and northern parts of the pluton. Fluorite is most abundant in the central portions of the pluton where it is commonly intimately associated with the Nb-Ta-oxide minerals microlite, manganotantalite and tapiolite. The link of high fluorine activity and more highly evolved geochemical fractionation of Nb-Ta oxides is obvious. Such highly fractionated oxides generally do not occur in NYF-type (NYF = Nb-Y-F enriched pegmatites typical of anorogenic origin) pegmatitic environments. The localized niches of high F activity are where these uncommon HFSE and REE phases occur. In all cases of exotic mineralogy, the central portion of the Nine Mile pluton differs from the rest of the pluton; it is also only here that several high-Sc phases have been found. These observations may indicate that the central portion of the Nine Mile pluton may actually be a fifth intrusive center. Buchholz, T. W., Falster, A. U. & Simmons, Wm. B. 1999. Ta, Nb, U, Y, and REE Minerals of the Koss Quarry, Marathon County, Wisconsin: The 26th Rochester Mineralogical Symposium, Abstracts of Contributed Papers. p. 6. 5 �Buchholz, T. W., Falster, A. U. & Simmons, Wm. B. 2000. Additional Mineralogy of the Koss Quarry, Marathon County, Wisconsin: The 27th Rochester Mineralogical Symposium, Abstracts of Contributed Papers. p. 5. Buchholz, T. W., Falster, A. U. & Simmons, Wm. B. 2001. Minerals of the Ladick East Quarry, Marathon County, WI: The 28th Rochester Mineralogical Symposium, Abstracts of Contributed Papers. p. 7. Falster, A. U., Simmons, Wm. B., Webber, K. L., & Buchholz, T.W. 2000. Pegmatites and Pegmatite Minerals of the Wausau Complex, Marathon C., Wisconsin: Memorie della Societa Naturali e del Museo di Storia Naturale di Milano, V. 30, p. 13-28. Hanson, S.L., Falster, A.U., Simmons, W.B., Webber, K.L., Buchholz, T. 1998. Rare-EarthElement (REE) Mineralization of Pegmatites in the Wausau Complex, Marathon County, Wisconsin: The 25th Rochester Mineralogical Symposium, Abstracts of Contributed Papers. p. 12. Keppler, Hans. 1993. Influence of fluorine on the enrichment of high field strength trace elements in granitic rocks: Contributions to Mineralogy and Petrology, V. 114, p. 479-488. Myers, P.E., Sood, M.H., Berlin, L.A. & Falster, A.U. 1984. The Wausau Syenite Complex, Central Wisconsin: Thirtieth Annual Institute On Lake Superior Geology, Field Trip Guidebook 3. 6 �Geology of the western Gogebic Iron Range, northwest Wisconsin—a record of sedimentation and deformation across the northern margin of the Penokean orogen Cannon, W. F., (USGS, Reston, VA), Laberge, G. L. (Oshkosh, WI), Klasner, J. S., (Macomb, IL), Dicken, C., (USGS, Reston, VA) Between 1991 and 1994 we examined most bedrock exposures of the iron-bearing strata and adjacent units of the western Gogebic Iron Range. We also compiled previously mapped features from both published and unpublished sources, including very detailed but unpublished mapping performed by iron mining companies in the 1950’s. The Gogebic Iron Range is a steeply north-dipping monocline of Early Proterozoic strata. The monocline formed at about 1.1 Ga (Cannon and others, 1993) by crustal-scale thrusting on faults such as the Atkins Lake-Marenisco fault that tilted gently south-dipping rocks formed during the Penokean orogenic cycle at approximately 1.9-1.8 Ga. The map pattern therefore is a cross section of the Penokean features that passes from a weakly deformed depositional and deformational foreland in the east to parts of the Penokean orogen toward the west that were more tectonically active both during deposition and deformation. Early Proterozoic strata lie unconformably on Late Archean granitic and metavolcanic rocks. The Bad River Dolomite, the oldest of the Early Proterozoic strata, is thin and discontinuous in the east but forms a continuous unit several hundred meters thick in the Grandview area in the west. Eastward from the Mt. Whittlesey area the basal unit of the Palms Formation commonly is a chert breccia composed of a residuum of chert fragments formed by dissolution of the cherty dolomite of the Bad River. Elsewhere, basal argillite beds of the Palms lie directly on the Bad River or Archean basement. The Palms Formation maintains uniform character and thickness 7 �across the mapped area. It consists of thin-bedded argillite and argillaceous quartzite in the lower part and thick-bedded quartzite in the upper part. The Ironwood Iron-formation lies conformably on the Palms. Near Upson and eastward the Ironwood displays a five-fold internal stratigraphy (Huber, 1959) of wavy-bedded jaspillitic iron-formation interlayered with even-bedded carbonate and silicate iron-formation. This stratigraphy cannot be traced far west from Upson. Jaspillitic and hematitic units are rare from Mt. Whittlesey to the west where the iron-formation has more reduced mineral assemblages and scarcer cherty units. The westernmost exposures in the Grandview area are banded gruneritemagnetite iron-formation, which becomes progressively leaner and more argillaceous to the west. The iron-formation in the Grandview area is roughly 2,000 feet thick as opposed to 5001000 feet farther east. In the westernmost part of the Grandview area drill holes show that the iron-formation becomes interbedded with black shale. The Tyler Formation overlies the Ironwood, probably with low angle unconformity, and is predominantly a thick sequence of graded-bedded turbidites. Most Early Proterozoic rocks were variably metamorphosed at about 1.1 Ga in the broad contact aureole of the Mellen intrusive complex. The Early Proterozoic stratigraphic section records deposition in a relatively shallow stable basin through the time of deposition of the Palms Formation. Beginning with deposition of the Ironwood, there are indications of tectonic instability and variations of depositional setting across the area. In the east iron-formation appears to have accumulated under tectonically quiescent conditions allowing widespread deposition of similar lithologies and thicknesses. To the west, deposition appears to have been in progressively deeper water and to have had a greater input of fine-grained clastic material. Also, in the Grandview area, sills of diabase as much as 1,000 feet thick occur in the iron-formation and enigmatic beds of breccia, possibly of volcanic derivation, suggest a proximity to tectonically and volcanically active parts of the Penokean orogen. The situation may be similar to the eastern end of the Gogebic Range where sills and volcanic rocks interfinger with the Ironwood. Finally, deposition of the turbidites of the Tyler Formation marks the transgression of a deep-water foreland basin in advance of accreting volcanic arcs of the Wisconsin magmatic terranes to the south. Penokean structures are much more intensely developed in the west than the east. Near Upson, structures are limited to rare outcrop-scale folds in the iron-formation and to moderately to weakly developed cleavage in more argillaceous units of the Tyler Formation. From near Mt. Whittlesey to the Mineral Lake area folding and faulting are much more intense. Folds with wavelengths of hundreds of meters are discernable in the iron-formation. Bedding plane faults and other originally low angle faults are also common including a basal decollement that separates the Early Proterozoic strata from Archean basement. Folds appear to have been mostly assymetrical to recumbent and north-verging, but Middle Proterozoic tilting has rotated these so that they are now mostly overturned structures. Likewise, original low angle thrusts are now tilted toward the north and have the appearance of down-to-the-north normal faults. Cannon, W.F., Peterman, Z.E., and Sims. P.K., 1993, Crustal-scale thrusting and origin of the Montreal River monocline—a 35-km-thick cross section of the Midcontinent rift in northern Michigan and Wisconsin: Tectonics, v.12, p. 728-744. Huber, N.K., 1959, Some aspects of the origin of the Ironwood Iron-formation of Michigan and Wisconsin: Econ. Geology, v. 54, p. 82-118. 8 �Anisotropy of magnetic susceptibility in the Ottertail pluton, Northern Ontario. Fein, E. M., Sturm, C. L., and Czeck, D. M., (Department of Geology, Oberlin College, Oberlin OH, 44074, Elizabeth.Fein@oberlin.edu.) INTRODUCTION In this project, we studied the structural and magnetic fabrics of the Ottertail Pluton, a small, granitic body within the Superior Province of Northern Ontario. The Ottertail, part of the Algoman suite of quartz monzonites and granodiorites, intruded surrounding rocks along the Wabigoon- Quetico subprovince boundary around 2.7 Ga (Davis et. al, 1989). From field observation, the emplacement of the Ottertail Pluton has been considered to be postdeformational due to the lack of obvious deformation fabrics evident in other types of rocks at the subprovince boundary (e. g. Davis et. al, 1989; Poulsen, 2000) . This interpretation has been used to constrain the timing of final deformation along the Wabigoon- Quetico boundary associated with microplate collision. PROBLEM On a much smaller scale, within deformed conglomerates, such as the nearby Seine River conglomerates, rigid clasts often display evidence of much less deformation than do more yielding clasts. Could a similar situation have occurred on a much larger scale in the case of the Ottertail Pluton? The relatively rigid granite body could appear to be relatively undeformed in the field, even if the timing of its emplacement was pre- or syn- tectonic. This situation, where a pluton appears undeformed despite being present during deformation, has been shown to occur in some modern tectonic regimes (e. g. Paterson & Tobisch, 1988). A more detailed study of plutonic fabrics and relationships is needed to conclusively determine whether a pluton is truly “post deformational.” METHOD In August of last year, we sampled an E-W (and a shorter N-S) transect across the Ottertail Pluton traveling along Highway 11, west of Mine Centre, Ontario. Thirty-one sites were sampled resulting in a total of 218 oriented cores. The anisotropy of magnetic susceptibility (AMS) of these granitic cores was measured to learn about the history of pluton emplacement. The measurements were conducted at the Institute of Rock Magnetism at the University of Minnesota in Minneapolis in January of this year. In general, the samples have a significant AMS signal contributed by multi-domain magnetite. The AMS data has been complied to describe the attitude of the variations in magnetic fabric within the Ottertail Pluton. To determine the relative timing of pluton emplacement with respect to the deformation, we need to show whether the magnetic fabric is concordant with preserved, regional patterns of strain. The AMS study will be combined with our field observations and a detailed petrographic study. 9 �RESULTS Regional deformation fabrics have already been well documented in the literature (e. g. Poulsen, 1986; Poulsen, 2000; Czeck, 2001). Preliminary results of our study suggest that the magnetic fabrics preserved in the Ottertail are related to the pattern of regional strain fabrics in the surrounding rocks. Further work will be conducted to analyze the magnetic fabrics to determine if, indeed, the Ottertail Pluton is pre- or syn- tectonic. If it can be shown to be pre- or syntectonic, this will have major implications for the relative timing of larger, regional, tectonic events. In particular, if the Ottertail is representative of other Algoman plutons and the deformation is truly regional, it may not be possible to constrain the end of regional deformation using the date of the intrusion of Algoman granites. Czeck, D. M., 2001. Strain analysis, rheological constraints, and tectonic model for an Archean polymictic conglomerate: Superior province, Ontario, Canada. Unpublished Ph. D. Thesis, University of Minnesota, 245 p. Davis, D. W., Poulsen, K. H., Kamo, S. L., 1989. New insights into Archean crustal development from geochronology in the Rainy Lake area, Superior Province, Canada. Journal of Geology 97, 379-398. Paterson, S. R., Tobisch, O. T., 1988. Using pluton ages to date regional deformations: problems with commonly used criteria. Geology 16, 1108-1111. Poulsen, K. H., 1986. Rainy Lake Wrench Zone: An example of an Archean Subprovince boundary in Northwestern Ontario. In: de Wit, M. J., Ashwal, L. D. (Eds.), Tectonic evolution of greenstone belts Technical Report 86-10, pp. 177-179. Poulsen, K. H., 2000. Archean metallogeny of the Mine Centre - Fort Frances area. Ontario Geological Survey Report 266, 121. 10 �Groundwater and Heat Flow in an Interlobate Moraine in Southeastern Manitoba. Ferguson, G. A. G. and Woodbury, A. D., (Department of Civil Engineering, University of Manitoba) Significant deviations from conductive heat flow occur in the Sandilands area of southeastern Manitoba as a result of groundwater flow. The main geological feature of this area is an interlobate moraine that coincides with the subcrop belt of the Ordovician Winnipeg Formation, which is composed primarily of sandstone and shale. The sandstone of the Winnipeg Formation acts as a regional aquifer and regional hydraulic head distribution and hydrogeochemistry suggest that the Sandilands area is an important source of recharge to this aquifer. The sandstone aquifer of the Winnipeg Formation is hydraulically separated from overlying hydrostratigraphic units west of the subcrop belt by a layer of low permeability shale that is thought to be continuous [Betcher, Third Canadian Hydrogeological Conference (1986)]. Overlying the Winnipeg Formation is the Ordovician Red River Formation, consisting of dolomitic limestone in the study area. The Red River Formation contains an aquifer in its upper extent, and previous studies have suggested that the Sandilands is an important recharge area for this aquifer as well. However, Sandilands recharge is likely less important as this carbonate aquifer is also recharged through the tills between the moraine and the Red River [Render, Cdn. Geotech. Journal, v.7, 243-274 (1970)]. Till is also present at the base of the moraine and acts as a semi-confining layer. Deviations from conductive heat flow provide a means of determining the direction and magnitude of groundwater flow, and are an efficient method of investigating the interaction between the groundwater in the surficial glaciofluvial sediments and the underlying bedrock aquifers. The geothermal gradient in the Superior Province of southeastern Manitoba is approximately 0.01 OC/m, which corresponds to a heat flow of approximately 40 w/m2. Temperature profiles through the moraine in the Sandilands area show geothermal gradient as low as 0.006 OC/m below the depth of penetration for seasonal variations. One-dimensional analytical models indicate that downward flow of groundwater must be occurring at a rate of approximately 10-8 m/s to produce the observed curvature seen in the temperature profiles. Analysis of temperature profiles in areas adjacent to the moraine has also aided in interpreting the hydrostratigraphy of the area. Temperature profiles through the upper shale unit of the Winnipeg Formation are linear, indicating that this layer is does act as an effective confining layer for the underlying sandstone aquifer. Discharge areas and areas of predominantly horizontal flow are present in the area surrounding the moraine, suggesting that not all of the water infiltrating into the moraine reaches the underlying bedrock aquifers. 11 �Mesoarchean Evolution of Western Superior Province: Evidence from Metasedimentary Sequences near Atikokan P. Fralick and D. King, (Department of Geology, Lakehead University, Thunder Bay, Ontario, P7B 5E1. (pfralick@mercury.lakeheadu.ca)) Paleogeographic reconstructions based on data supplied by metasedimentary sequences provide a basis for understanding plate movements which create sedimentary basins. This combined with geochronology allows sequential developmental stages of a craton to be deciphered. Thus, by interpreting environment of deposition from outcrop data, inferring provenance from composition of the metasedimentary units, and determining the age of detrital zircons and zircons in interstratified or intrusive igneous rocks insight will be gained on the plate movements which caused a basin to subside. The above was applied to a Mesoarchean sequence of metasedimentary and metavolcanic rocks to evaluate the degree basin subsidence could be explained by lateral plate motions similar to those of today. The succession is on the southern margin of Wabigoon Subprovince in the Atikokan area. It consists of, along an eighty kilometre belt from southwest to northeast,: 1) the Steep Rock Group, 2) the Little Falls assemblage, 3) the Finlayson Lake greenstone belt, and 4) the Lumby Lake greenstone belt. The oldest rocks exposed in the sequence are pre 3014 Ma tholeiitic basalts (Figure 1), the dominant rock type in the entire assemblage. The lowermost sedimentary units at Little Falls and Finlayson consist of felsic agglomerates and tuffs laterally transitional into resedimented sandstones and conglomerates. Transport was via high density mass flows, though some beds are possibly traction current deposits. Mafic ash layers are present throughout. Age determinations of the resedimented, pyroclastic zircon population gave 2996.9+-0.8 Ma, almost identical to the compositionally similar (Stone et al, 1992) Marmion Batholith which intrudes the area. Felsic volcanism also occurs in the Lumby Lake belt at this time. Plume derived (Hollings et al, 1999; Hollings and Wyman, 1999; Tomlinson et al, 1999) mafic and ultramafic volcanism continued in these belts until some time after 2828 Ma (age determination of Tomlinson et al, 2001) when eruptions ceased and a coarsening upward sedimentary sequence of iron formation, DE turbidites, classic turbidites, wave reworked coarse sandstones, and conglomerates was deposited at Finlayson The geochemistry and detrital zircon population (2997 to 3002 Ma) of the Finlayson sandstones is similar to Wagita Formation sandstones, a fluvial unit deposited in incised paleovalleys at the base of the Steep Rock Group (Wilks and Nisbet, 1988). Eroded basalt and tonalite detritus moved through Steep Rock paleochannels building out the shoreline in the Finlayson area. The sequence at Lumby consists of classic turbidites interbedded with lenses of mass flow conglomerates near the top, overlain by carbonate and finally iron formation. Although sedimentation was probably sinchronous in the Lumby area, geochemistry indicates komatiites were important source material here. Clastic sedimentation in the entire area was ended by transgression causing backfilling of Wagita fluvial channels followed by rapid drowning of the entire source area. Stromatolitic carbonates were deposited in the shallow ocean, but as subsidence continued the carbonate was replaced by iron formation. At 2780 Ma (Tomlinson et al, in press) the next volcanic cycle began with eruption of the komatiitic Ashrock Formation. 12 �The mafic and ultramafic volcanic units which dominate the sequence have been ascribed to plume generated melts erupting in an ocean plateau environment (Tomlinson et al, 1999; Hollings and Wyman, 1999; Hollings et al, 1999); the presence of the tonalites being ascribed to slab melting in a proximal subduction zone (Hollings et al, 1999). Though this model is elegant the new age determinations render it unworkable. Generation of felsic melts by plume induced heating at the base of the overthickened basaltic plateau is compatible with tonalite geochemistry (Smithies, 2000). The sedimentary sequence is consistent with an ocean plateau environment, but definitely not representative of a continental rift or cover sequence as previously suggested (Thurston and Chivers, 1990). Partial melts generated in the lower basaltic crust formed magma chambers 2 to 4 km below surface. This magma fed felsic eruptions which, in turn, supplied most of the material for clastic layers lower in the volcanic pile. Eruption of the enclosing basalts was probably sporadic, as the area was effected by pulses of plume activity (Tomlinson et al, 1999). These events lasted for over 185 Ma, into the Neoarchean, until finally terminated by a hiatus in volcanic activity. With the end of plume driven crustal heating, thermal decay induced subsidence lead to subareal to shallow water clastic sedimentation and shallow to deep water chemical sedimentation. The existence of an ocean plateau for over 185 Ma, and possibly over 235 Ma, calls into question the operation of plate tectonics in the Mesoarchean, and especially fast spreading, multiple ridge-subduction zone configurations proposed for this time period. If the plateau was undergoing lateral plate motion subduction zones must have been well spaced and movement slow to allow for its activity over such a long time period. This is difficult to reconcile with a hotter earth requiring greater heat dissipation capacity than today. The other alternative: conventional plate tectonics did not operate in the Mesoarchean, but rather heat loss was achieved through vigorous plume activity, possibly similar to the early history of Mars, needs to be explored further. Hollings, P. and Wyman, D., 1999. Lithos, 46, 189-213. Hollings, P., Wyman, D. and Kerrich, R., 1999. Lithos, 46, 137-161. Smithies, R.H., 2000. Earth and Planetary Science Letters, 182, 115-125. Stone, D., Kamineni, D.C. and Jackson, M.C., 1992. Geological Survey of Canada, Bulletin 405. Thurston, P.C. and Chivers, K.M., 1990. Precambrian Research, 46, 21-58. Tomlinson, K., Davis, D.W., Hughes, D.J. and Thurston, P.C., 1998. Lithoprobe Report 65, 35-47. Tomlinson, K., Hughes, D.J., Thurston, P.C. and Hall, R.P., 1999. Lithos, 46, 103-136. Tomlinson, K., Davis, D.W., Stone, D. and Hart, T., 2001. Lithoprobe Report, Western Superior. Tomlinson, K., Davis, D.W., Stone, D. and Hart, T., in press, Precambrian Geology. Wilks, M.E. and Nisbet, E.G., 1988. Canadian Journal of Earth Sciences, 25, 370-391. 13 �EEl Felsic Volcanic Rocks LUMBY SOUTH Mafic Volcanic ______________ 2997 Ages in Million years 2828 WALLACE I AVP Grarntic Rocks —e .— —C U— —U U— FINLAYSON LITTLE PAT T c Fig. 1. Stratigraphic sections of Mesoarchean rocks in the Atikokan area, and Wallace Lake in the Uchi Belt for comparison. Geochronology mainly from Tomlinson et al and Stone et al. 14 �SEM Imaging of fossil nannobacteria from the supergene zone, Flambeau Copper Mine, Ladysmith, Wisconsin Hubin, J. and Cordua, Wm., (Department of Planet and Earth Science, Univeristy of Wisconsin-River Falls, 410 South Third Street, River Falls, WI 54022 william.s.cordua@uwrf.edu) The discovery of nannobacteria fossils in Eocene supergene zones in Chilean copper deposits (Sillitoe et. al. 1996) suggested that similar, but older, nannobacteria may be found in the Cambrian to Precambrian supergene zone of the Flambeau Copper Mine in Rusk County, Wisconsin. A scanning electron microscope study of samples from the Flambeau Mine focused on the interface between pyrite and replacing chalcocite. Images revealed several sites where spherical clusters 0.1 to 0.3 microns in diameter are present (fig. 1). Through visual comparison with existing studies and elimination of other possible explanations, these clusters are interpreted as being nannobacteria. The age of supergene mineralization at the Flambeau Mine predates the 525 million year old Mt. Simons sandstone (May and Dinkowitz, 1996), and may pre-date the deposition of the Barron quartzite (1750 - 1650 m.y.) (Medaris, 2000). If these are fossil nannobacteria rather than recent contaminants, it suggests that the involvement of nannobacteria in the supergene enrichment processes spans a considerable proportion of geologic time. Acknowledgements: We are indebted to Jeff Thole and his assistants for their help in using the SEM facilities at Macalester College for this study. REFERENCES: May, E.R., and Dinkowitz, S., 1996. An overview of the Flambeau supergene enriched massive sulphide deposit: geology and mineralogy, Rusk County, Wisconsin, in LaBerge, G., edited, Volcanic massive sulphide deposits of northern Wisconsin: A commemorative volume: Insitute on Lake Superior Geology Proceedings, 42nd Annual Meeting, Cable, WI, v.42, part 2, p. 67-93 Medaris, G. 2000, The Barron saprolite: confirmation of mature chemical weathering in the source for Paleoproterozoic quartz arenites in the Lake Superior region [abstract] Institute on Lake Superior Geology Proceedings, 46th Annual Meeting. Thunder Bay, ON, 2000, Part 1, p.37-38. Sillitoe, R.H., Folk, R.L. and Saric, N. 1996, Bacteria as mediators of copper sulphide enrichment during weathering, Science, vol. 272, p. 1153-1155. 15 �Figure 1. Suspected nannobacteria (arrows) at pyrite-chalcocite interfaces, Flambeau Mine, Ladysmith, Rusk County, Wisconsin. 20,000x. White bar = 5 microns. 16 �Archean and paleoproterozoic mafic intrusions in Minnesota Jirsa, M. A., (Minnesota Geological Survey, St. Paul, MN (jirsa001@umn.edu)) Exploration for platinum group elements (PGEs) in Minnesota has been restricted largely to mafic intrusions of the Mesoproterozoic Duluth Complex for obvious reasons—the complex contains known PGE occurrences associated with previously outlined coppernickel deposits. A new study by the Minnesota Geological Survey and the Natural Resources Research Institute is looking beyond the Duluth Complex to describe and analyze the varied mafic, ultramafic, and alkalic intrusions in the northern, western, and east-central parts of the state. Although geochronologic data are sparse, field and geophysical evidence implies that most of these intrusions are Archean and Paleoproterozoic in age. Figure 1 shows some of the major mafic intrusions and intrusive complexes for which some “ground truth” (drill core or outcrop) exists. They have been identified by surface and geophysical mapping and drilling; however, few have been thoroughly described and analyzed, and almost none have been analyzed for PGE content. Some of the intrusions are lithologically similar to the so-called “Quetico” and “Atikokan” intrusions that are the targets of exploration in Canada. The geologic settings of significant PGE deposits worldwide include mafic to ultramafic plutons associated with volcanic arc terranes in linear orogenic belts (Ural-type), differentiated sill complexes, ophiolitic complexes, syn- to late-orogenic layered complexes (Bushveld and Stillwater), and alkalic to subalkalic composite plutons. This study is designed to characterize the various types of mafic intrusions in Minnesota and perhaps broaden the exploration horizon to similarly diverse terranes throughout the state. At present, an inventory containing the pertinent facts from more than 150 individual intrusions and thousands of diabasic dikes has been compiled and sampling work has begun. As an initial classification, the known intrusions are grouped below first according to the age of host rocks into which they were emplaced, and secondly on the basis of perceived temporal, mineralogic, and geometric attributes. ARCHEAN HOST ROCKS Layered mafic-ultramafic complexes—Pre-tectonic with respect to the main metamorphic and foliation-forming deformation event. The rocks are variably metamorphosed and have tholeiitic to komatiitic compositions. The presence of layering and cumulate textures indicates differentiation. Rock types include peridotite, pyroxenite, gabbro, anorthositic gabbro, and diorite. Examples: Deer Lake Complex and Newton Lake Formation. Subvolcanic mafic sills—Pre-tectonic, variably metamorphosed, presumably hypabyssal intrusions associated with greenstone sequences. They typically are tholeiitic in composition. Rock types include gabbro, pyroxenite, and diorite. Example: Thistledew Lake sequence. Amphibolitic sills and dikes—Typically schistose, metamorphosed, and folded, narrow sills emplaced into schists and granitic intrusions of the Quetico subprovince. Example: Ash River amphibolite. 17 �Grygla p/Won ( Ash River amphibo/ite N - I Dever P'U(UT' *,'/ N.anLn. Thistledow Lake, Duluth Complex and related rocks sills 1• r, p EXPLANATION anomaly / Aeromagnetic interred lobe diaba&c dike iiiiuuiij,ipuui or containing, matic phases '\\\- &Y Drill hole Figure 1. Generalized map of Minnesota showing the distribution of mafic intrusions and drill holes that intersect mafic intrusive rocks. Many of the drill holes encountered intrusions too small to be depicted. Intrusions discussed in the text are labeled. 18 �Lamprophyric, pyroxenitic, peridotitic, and hornblendic intrusions—Syn- to post-tectonic dikes and small, irregular bodies. They typically are coarse-grained, oxide-rich, and strongly magnetic. Example: Dead River pluton. Sanukitoid composite stocks—Discrete, post-tectonic intrusions of the dioritemonzodiorite-monzonite-syenite suite. They typically are quartz-poor to quartzabsent. Most contain intrusive phases and enclaves of amphibole-pyroxene-micabearing diorite, locally of lamprophyric affinity. Example: Gheen pluton. Tonalite-diorite intrusions—Discrete, probably post-tectonic intrusions that contain magmatic phases varying from tonalite to diorite and gabbro. Example: Grygla pluton and associated intrusions. PALEOPROTEROZOIC AND ARCHEAN HOST ROCKS Gabbroic intrusions—Discrete, typically semicircular intrusions containing gabbro, troctolite, anorthositic gabbro, and pyroxenite. Typically show as magnetic and gravity highs. Example: Lake Washington intrusion. Pyroxenite and peridotite plugs and stocks—Small, circular to irregularly shaped bodies having strong magnetic signatures. Commonly occur along linear geophysical discontinuities. Examples include many of the small intrusions in central Minnesota shown on Figure 1 by drill hole locations. Age is uncertain, but one is known to intrude ~1780 Ma granite. Diabasic to gabbroic dikes—Northwest-trending dikes are part of the Kenora– Kabetogama swarm of Paleoproterozoic age, other dike orientations are both Paleoproterozoic and Mesoproterozoic in age. Outcrop mapping indicates dike widths between 30 and 100 meters. Larger dikes commonly are composite and have central zones composed of granodiorite and leucogabbro. This classification will undoubtedly be modified with the addition of new petrographic and geochemical data. This study is funded by the Minnesota Legislature on recommendation of the Minnesota Minerals Coordinating Committee. 19 �VHMS mineralization and alteration within the Lumby Lake Greenstone Belt, Northwestern Ontario Johnson, J.R., Kissin, S.A. and Hollings, P., (Department of Geology, Lakehead University, Thunder Bay, ON, P7B 5E1, jrjohnsonca@yahoo.ca) The Lumby Lake greenstone belt, located approximately 40 km northeast of Atikokan, Ontario or 165 km northwest of Thunder Bay, Ontario, has been a focus of exploration activity for the past one hundred years. The belt is located within the Wabigoon Subprovince of the Superior Province. The Wabigoon Subprovince is interpreted as a collage of 3 – 2.7 Ga plutonic and volcanic terranes (Blackburn et al, 1991), where as Tomlinson et al. (1999) have shown that the Lumby Lake belt ranges in age from 28983001 Ma. Mapping done by Jackson (1985a, b) showed the belt to be dominated by mafic metavolcanic units that occur as massive to pillowed flows. Thin felsic volcanics and clastic sedimentary units were found throughout the belt along with less common ultramafic flows, located only towards the top of the stratigraphic sequence. The Marmion Lake Batholith intrudes the southern portion of the belt. In the past the belt has been discounted as a possible base metal camp due to the relative absence of felsic material. Hollings and Wyman (1999) documented that the mafic volcanic rocks of the belt are similar to those of modern ocean plateau tholeiites (associated with plume related volcanism), consistent with the presence of komatiites. These authors invoked a complex model of plume arc interaction to account for the presence of arc related felsic volcanics within the plateau related mafic pile. Hollings and Wyman (1999) reported the presence of two distinct types of felsic volcanic rock, and comparable plutonic phases, within the Lumby Lake belt. One suite characterized by fractionated heavy rare earth elements (HREE) are comparable to the nonprospective F1 of Lesher et al (1996), the second suite with unfractionated HREE are similar to the variably prospective F2 of Lesher. The F2 felsic horizons are found within the Lumby Lake-Spoon Lake area and further to the south within the Marmion Lake Batholith. Tomlinson et al. (1999) found that the felsic volcanics of belt, in general, give the oldest ages, ~3 Ga. Additional exploration, by Atikokan Resources Inc. (ARI) in the last decade, and mapping done, in conjunction with ARI, in the last year has identified the presence of additional extensive felsic flows, tuffs and breccias, predominately in the southern portion of the belt. Areas of base metal mineralization, such as the stratabound pyrite, sphalerite, galena, chalcopyrite, native silver and silver sulphides found in the Lumby Lake-Spoon Lake area (0.2 to 9.2% Zn, #0.25% Cu, 1.0% Zn and 37 – 60 g Ag/t) and the pyrite, sphalerite, galena, pyrrhotite and chalcopyrite located near Richardson Lake, have been revealed. It should be noted that there is a scarcity of volcanic hosted massive sulphide deposits of 3 Ga or older 20 �Examination of the area near Richardson Lake indicates that silicification and chloritization of the rocks is common with sericite alteration less prominent. This alteration is consistent with that found in the footwalls of volcanogenic massive sulphide deposits (Franklin, 1993). Additional mapping and geochemical work, involving the classification of newly located felsic horizons and the relative enrichment and depletion of elements, will lead to further defining these zones of alteration and enable the location of additional base metal targets. In addition geochemical analyses will be utilized to further define the tectonic setting of the Lumby Lake belt. Blackburn, C. E., Johns, G. W., Ayer, J. A., Davis, D. W., 1991, Wabigoon Subprovince, Geology of Ontario. Ontario Geological Survey, p. 303-382. Davis, D. W., and Jackson, M. C., 1988, Geochronology of the Lumby Lake greenstone belt: A 3 Ga complex within the Wabigoon Suprovince northwestern Ontario: Geological Society of America Bulletin, v. 100, p. 818-824. Franklin, J. M., 1993, Volcanic-associated massive sulphide deposits, in Kirkham, R. C., Sinclair, W. D., Thorpe, R. I. And Duke, J. M., eds, Mineral Deposit Modeling: Geological Association of Canada, Special Paper 40, p. 315-334. Hollings, P. and Wyman, D., 1999, Trace element and Sm-Nd systematics of volcanic and intrusive rocks from the 3 Ga Lumby Lake Greenstone belt, Superior Province: evidence for Archean plume-arc interaction. Lithos, v. 46, p. 189-213. Jackson, M. C., 1985a, Geology of the Lumby Lake area, eastern part, districts of Kenora and Rainy River: Ontario Geological Survey Open-File Report 5535, 122 p. Jackson, M. C., 1985b, Geology of the Lumby Lake area, western part, districts of Kenora and Rainy River: Ontario Geological Survey Open-File Report 5534, 151 p. Lesher, C. M., Goodwin, A. M., Campbell, I. H. and Gorton, M. P., 1986, Trace-element geochemistry of ore associated and barren, felsic metavolcanic rocks in the Superior Province, Canada; Canadian Journal of Earth Sciences, v. 23, p. 222-237. Tomlinson, K. Y., Davis, D. W., Thurston, P. C., Hughes, D. J., and Sassevile, C., 1999, Geochemistry, Nd isotopes and geochronology from the Central Wabigoon Subprovince and North Caribou Terrane: regional correlations leading towards a Mesoarchean reconstruction. Lithoprobe Report #70, p. 136-152. 21 �Geology and fluid inclusion studies of the Thunder Bay Agate Mine, Northwestern Ontario Kelso, I.S. and Kissin, S.A., (Department of Geology, Lakehead University, Thunder Bay, ON, P7B 5E1 stephen.kissin@lakeheadu.ca.) The Thunder Bay Agate deposit, located northeast of Thunder Bay at the junction of Highway 11/17 and Highway 527, was discovered in 1989 and developed into the presently operating Thunder Bay Agate Mine in 1996. The deposit is hosted by carbonate units of the Gunflint Formation and is 50m NE of the contact with olivine diabase of a Logan sill. The deposit has a surface exposure of approximately 160 x 200m. Throughout the area of the deposit, intermittent open spaces within the host-rock are filled by agate. Where the agate-filled spaces are larger than ~50cm high and 1m wide, there is evidence of collapse brecciation of the host-rock. The largest agate-filled space observable is 2m wide and 0.8m high. Thunder Bay agate consists of three mineral phases: chalcedony, quartz, and graphitic carbon. The chalcedony can be subdivided into three varieties: dark amorphous silica (opal-A?), light amorphous silica (opal-CT?), and translucent chalcedony. A consistent sequence of mineralization is observed throughout the deposit. From nucleation against the host-rock, the sequence is rhythmic bands of dark amorphous silica, light amorphous silica, translucent chalcedony, and quartz-encased, carbon-filled vugs. As mineralization radiates from several points of nucleation, interference patterns generate complex, sinuous structures in the sequence. Although graphitic carbon is not always present, where it does occur it is never observed to be in direct contact with the host-rock or chalcedony. Quartz is often the final stage of mineralization and is rarely observed to be in direct contact with the host-rock. The rhythmic bands of each stage vary from <1mm to >1cm in width. Three types of fluid inclusions, according to the terminology of Roeder (1984), were found: Type I: Single-phase inclusions containing liquid or vapour only. A number of Type I inclusions in quartz were examined; however, no change on freezing was observed. Type II: Two-phase liquid + vapour inclusions. These are the most common type of inclusion in quartz. Eutectic, final melting and homogenization temperatures were obtained. Type III: Three-phase liquid + vapour + solid(s) inclusion. Not observed in this study. Type IV: CO2-rich inclusions. These homogenized at <31°C and decrepitated when heated above homogenization. Type II Two-phase inclusions (liquid and vapour) are the most numerous type of primary inclusion within the quartz. These inclusions range in size from 4 to 50µm along their greater axis and have a vapour content of 2% to 7% by volume. All Type II inclusions in the quartz homogenized to liquid in the range of 70 - 190°C with the majority homogenizing within the range of 100-130°C. The eutectic and final melt temperatures of the inclusions clustered around two ranges. Inclusions with eutectic temperatures of 22 �approximately -52°C exhibited final melt temperatures of -34 to -27°C; inclusions with eutectic temperatures of approximately -25°C exhibited final melt temperatures of -7 to 2°C. The higher temperature cluster corresponds to the NaCl-H2O eutectic temperature of -22°C. The lower temperature cluster of inclusions likely contain minor amounts of CaCl in solution with NaCl. The chalcedony is almost exclusively devoid of inclusions; however, one cluster of about 10 Type IV inclusions containing liquid CO2 were observed in translucent chalcedony. In these inclusions the frozen CO2 displays visible melting at -45°C and homogenized at 30.2°C. Fluid inclusion studies indicate the quartz was emplaced in a low-temperature, lowpressure hydrothermal environment. The timing of deposition is delimited between the age of the Gunflint Formation (1878 ma; Fralick et al., in press) and the Logan sills (1109 ma, Davis and Sutcliffe, 1985) which have been seen to contain xenoliths of typical agate (J. Scott, pers. com., 2001). The deposition of agate appears to be related pervasive, early silicification of the Gunflint and other Lake Superior-type iron formations, as summarized by Simonson (1987). He proposed that silicification most likely could be attributed to siliceous thermal springs located nearer the axis of the depositional basin. Compaction could expel the siliceous solutions to the margins of the basin. Rimstidt and Barnes (1980) showed that silica precipitation in response to declining is most effective in a decrease from 300 to 100°C, the approximate mean homogenization temperature observed in quartz-hosted inclusions. Silica solubility also decreases sharply with a drop in pH. Organic matter producing CO2 at the site of agate deposition would both cause silica precipitation and dissolution of carbonate host rocks through lowering of pH. Evidence of such CO2 production is seen in the presence of Type IV CO2-rich inclusions. Organic matter also is believed to provide a favourable nucleation site for silica (Knoll, 1985). Thus, the occurrence of the agate deposit in carbon-rich carbonate rocks can be explained. Fralick, P., Davis, D.W., and Kissin, S.A. in press. The Age of the Gunflint Formation, Ontario, Canada: Single zircon U-Pb age determinations from reworked volcanic ash. Canadian Journal of Earth Science. Davis, D.W., and Sutcliffe, R.H. 1985. U-Pb ages from the Nipigon Plate and northern Lake Superior. Geological Society of America Bulletin 96: 1572-1579. Knoll, A.H. 1985. Exceptional preservation of photosynthetic organisms in silicified carbonates and silicified peats. Philosophical Transactions of the Royal Society of London B311: 111-122. Rimstidt, J.D., and Barnes, H.L. 1980. The kinetics of silica-water reactions. Geochimica of Cosmochimica Acta 44: 1683-1699. Roedder, E. 1984. Fluid Inclusions. Mineralogical Society of America, Reviews in Mineralogy 12. Simonson, B.M. 1987. Early silica cementation and subsequent diagenesis in arenites from four Early Proterozoic iron formations of North America. Journal of Sedimentary Petrology 57: 499-511. 23 �WOLF RIVER-AGE BRECCIATION IN THE BARABOO QUARTZITE, WISCONSIN: IMPLICATIONS FOR PROTEROZOIC TECTONICS IN THE LAKE SUPERIOR REGION Medaris, L.G., Jr., Singer, B.S., Brown, P.E., Jicha, B.R., and Smith, M.E., (Dept. of Geology and Geophysics, Univ. of Wisconsin-Madison, 1215 W. Dayton St., Madison, WI, 53706; medaris@geology.wisc.edu) Breccia zones in Baraboo Quartzite, consisting of angular red quartzite fragments cemented by a stockwork of white quartz veins (Fig. 1), are distributed sporadically throughout much of the Baraboo Range. Larger breccia zones tend to be parallel with associated bedding, although thin quartz veins with incipient breccia features are commonly crosscutting. The most prominent breccia zone, which is well exposed in Ableman's Gorge, is ~100 meters thick and can be traced for ~20 kilometers along the north limb of the Baraboo syncline. Figure 1. Quartzite breccia, Martin-Marietta Quarry, north limb, Baraboo syncline Figure 2. Back-scattered electron image of muscovite (light gray), kaolinite (dark gray), and quartz (medium gray) in breccia cement The breccia cement is composed predominantly of massive vein quartz, which is occasionally accompanied by small amounts of specular hematite. Locally, euhedral quartz crystals occur in vugs, some of which are partly to completely filled by kaolinite. Muscovite is rare in the breccia cement, but muscovite was recently discovered by Phil Fauble in an outcrop of breccia in the bluffs east of Devil's Lake on the south limb of the syncline. At this locality, prominent flakes of muscovite appear to be in textural equilibrium with kaolinite and quartz (Fig. 2). 24 �The stable coexistence of quartz, kaolinite, and muscovite is limited to temperatures below ~300oC by the reaction, kaolinite + quartz = pyrophyllite + H2O (Fig. 3). Quartz in the muscovite bearing sample contains abundant H2O-rich fluid inclusions, which yield homogenization temperatures from 160 to 186oC and freezing point depressions from 1.2 to -6oC, corresponding to maximum salt contents of ~6.8 wt%. The fluid inclusion isochores, combined with phase equilibrium constraints, indicate that brecciation occurred in the range, 200 to 280oC and 500 to 2000 bars (depths of 2 to 8 kms). Such a temperature range is lower than that for the predominant quartz + pyrophyllite assemblage in the Baraboo Quartzite, which is stable between about 300 and 375oC at 2000 bars. Step-heating, using a defocused CO2 laser, of a single muscovite grain (ca. 0.01 mg) from the breccia cement yielded a discordant 40Ar/39Ar age spectrum with a well-defined plateau at 1,459  3 Ma (Fig. 4), representing ~75% of the gas released during the heating experiment. This result is concordant with 40Ar/39Ar plateau ages of 1,456  11 and 1,467  11 Ma for muscovite from metasaprolite and muscovite-pyrophyllite-diaspore hydrothermal veins in the Baraboo Quartzite (Naymark et al., 2001). Results of this investigation provide additional evidence for post-1,630 Ma hydrothermal activity in the Baraboo Range, which was first recognized by Naymark et al. (2001) and attributed to the influence of 1,465 Ma Wolf River magmatism (Medaris, 2001). We suggest that emplacement of the 1.5-1.4 Ga transcontinental belt of A-type granites, of which the Wolf River batholith is part, provided the heat flux necessary for promoting regionally extensive fluid flow and accompanying hydrothermal alteration along permeable channels. It now appears that such hydrothermal activity extended far beyond the Baraboo Range, including the Sioux Quartzite in Minnesota (Naymark, et al., 2001) and the Athabasca Basin in northern Canada (Kotzer, et al., 1992). Further research may reveal that much of the Paleo- and Mesoproterozoic crust of the Lake Superior region was affected by areally extensive, but stratigraphically restricted, hydro- thermal alteration at 1.5-1.4 Ga. REFERENCES Kotzer, T.G., et al. (1992) Can. Jour. Earth Sci., v. 29, p. 1474-1491; Medaris, L.G., Jr. (2001) 47th Inst. Lake Superior Geol., p. 51-52; Naymark, A., et al. (2001) 47th Inst. Lake Superior Geol., p. 66-67. 25 �New geologic map and report of the Duluth Complex and related rocks, Northeastern Minnesota. Miller, J. D. Jr.1, Green2, J. C., Severson3, M. J., Chandler1, V. W., Peterson3, D. M., Hauck3, S. A., and Wahl1 T. E, (1Minnesota Geological Survey, 2642 University Avenue, St. Paul, MN 55114, 2Department of Geological Sciences, University of Minnesota–Duluth, Duluth, MN 55812, 3Natural Resources Research Institute, 5013 Miller Trunk Hwy., Duluth, MN 55811) The Minnesota Geological Survey (MGS), in collaboration with the Natural Resources Research Institute, recently published a 1:200,000-scale map of the Duluth Complex and related rocks of northeastern Minnesota (Miller and others, 2001). A companion report (Miller and others, 2002) describes the geology and mineral potential of the map area; a CD-ROM included in the report contains the digital map and related field data. This ambitious project was funded by a grant from the Minnesota State Legislature on the recommendation of the Minerals Coordinating Committee. The two products constitute the largest and most complete summary of the Duluth Complex ever produced and, undoubtedly, will serve as a benchmark for future exploration and research. Several developments in the last 20 years led to the regional compilation. Gravity and high-resolution aeromagnetic data were used in conjunction with test drilling to map the vast, poorly exposed central part of the Duluth Complex. Detailed mapping in the better exposed part of the complex, as well as core logging along the its base, led to a greater understanding of the petrologic, structural, and stratigraphic relationships in the intrusive sequence. High-resolution radiometric dating aided the unraveling of the intrusive history of the complex, and advances in Geographic Information Systems (GIS) greatly enhanced our ability to produce complicated geologic maps. Finally, and perhaps most importantly, the sharp upturn in PGE exploration in the Duluth Complex created an urgent need for an improved geologic framework. The new geologic map (Miller and others, 2001) has two major components: a 1:200,000-scale geologic map that delineates over 270 rock units, and a more generalized 1:500,000-scale geologic map that classifies the rock units into 72 time–stratigraphic entities. Additional information on the two sheets include (1) a map that shows the relative density of outcrops and drill-out locations; (2) a 1:500,000 gray-scale image of the first vertical derivative of the aeromagnetic data; (3) an index to 64 published and unpublished mapping projects used in the present compilation; (4) a correlation diagram for the 72 generalized time–stratigraphic units; (5) brief descriptions of rock units; and (6) a list of references. Because of the importance of gravity and magnetic data to this compilation, a related map that presents the second vertical derivative of gravity data (color) superimposed on reduced-to-pole, first vertical derivative of magnetic data (black and white) is available at a scale of 1:200,000 (Chandler, 2001). The maps can be purchased from the Minnesota Geological Survey in paper form; they are also available 26 �as PDF files from the Minnesota Geological Survey web site (http://www.geo.umn.edu/mgs). The new 1:200,000-scale geologic map is a greatly improved representation of the Duluth Complex and related rocks; for example, the southern and northwestern parts of the complex are depicted as a composite of several intrusions consisting of troctolitic and gabbroic cumulates. Much of the PGE exploration has focused along the base of the complex in two cumulate bodies, the South Kawishiwi and Partridge River intrusions, but several similar bodies defined by the new map may become exploration targets. The map refines our understanding of the geology of the upper part of the complex, which consists of a partially disrupted cap of anorthositic rocks and several felsic intrusions. Improvements have also been made in mapping the Beaver Bay Complex and other hypabyssal intrusions in the volcanic roof of the complex. One of the bodies, the Sonju Lake intrusion, is being explored for PGE potential, and the new map identifies other mafic cumulate and gabbroic bodies in the roof zone that may be worthy exploration targets. Finally, the North Shore Volcanic Group, which forms the roof (and locally the footwall) of the complex, has for the first time has been subdivided into informal formational entities on a geologic map. The companion report (Miller and others, 2002) provides a general description of the Duluth Complex and related rocks as portrayed on the map and assesses the potential for nonferrous mineral deposits. The following is a summary of each of the 8 chapters and appendix: 1, Formal definition of nomenclature and stratigraphic and rock-type classifications used to describe the Mesoproterozoic (Keweenawan) rocks of northeastern Minnesota; 2, History of geologic mapping and mineral exploration of the complex; 3, Geophysical attributes of the complex and related rocks; discussion of how data aid interpretation of the buried bedrock geology and deeper geologic structure of northeastern Minnesota; 4, Geology and structure of the Archean and Paleoproterozoic rocks that form the footwall of the complex; 5, Volcanic stratigraphy and structure of the comagmatic North Shore Volcanic Group, into which the Duluth Complex and related intrusions were emplaced; 6, Geologic, structural, and stratigraphic relationships of various intrusions of the complex; 7, Geology of the Beaver Bay Complex and related hypabyssal intrusions; 8, Types of mineralization that are known to occur or may occur in the complex and related Keweenawan rocks; identification of new exploration target areas; and Appendix, Description of digital image and data layers included in the CDROM (back pocket) and an explanation of how to access the GIS layers within the ArcView program. The CD-ROM accompanying the report contains ArcView -based digital compilations of the geologic map and related field data, as well as base maps and geophysical images. Attribute tables identify the source and character of data that accompany the geology layers (map units, geologic faults, and contacts) and the related field data layers (outcrops, structure measurements, drill holes, and field samples). An MGS-created 27 �extension, GeMS (Geologic Mapping System) allows the abbreviated attribute data to be decoded. Compilation of all pertinent field data was not possible under the time constraints of the two-year project. Priority was given to data from explorable areas of the Keweenawan in northeastern Minnesota. Updated versions of this digital compilation will be produced as warranted by new mapping and additional archiving of older field data. Chandler, V.W., 2001, Superimposed magnetic on gravity anomaly map of the central Duluth Complex and western part of the Beaver Bay Complex, Lake and St. Louis Counties, Minnesota: Minnesota Geological Survey Miscellaneous Map Series M-120, scale 1:200,000. Miller, J.D., Jr., Green J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.M., and Wahl, T.E., 2002, Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geological Survey Report of Investigations 58, 207 p. + compact disc in back pocket. Available by the end of March 2002. Miller, J.D., Jr., Green J.C., Severson, M.J., Chandler, V.W., and Peterson, D.M., 2001, Geologic map of the Duluth Complex and related rocks, northeastern Minnesota: Minnesota Geological Survey Miscellaneous Map Series M-119, 2 sheets, scales 1:500,000 and 1:200,000. 28 �Crustal models of the Western Superior Province from gravity and Lithoprobe seismic data Nitescu, B., Cruden, A.R., Bailey, R.C., (Department of Geology, University of Toronto, Toronto, ON M5S 3B1, bnitescu@geology.utoronto.ca) The western part of the Superior Province (WS) is characterised by a regional pattern of linear, EW-trending, fault-bounded belts containing distinctive rock types, structures, ages, metamorphic conditions, and geophysical attributes. Detailed geological, geochemical and geochronological studies in the past two decades have led to the formulation of the “accretion hypothesis” in which the WS is interpreted to have assembled progressively from north to south by collisions with various arc, oceanic and continental fragments. To test this hypothesis at depth a Lithoprobe transect was established in the WS, and acquisition of seismic refraction and reflection data, as well as detailed gravity observations were made along several profiles crossing the WS subprovinces. The refraction/wide-angle reflection data indicate a crustal thickness of 4043 km and relatively high average crustal velocities of 6.6-6.7 km/s (White et al. 1997). A high velocity basal crustal layer (7.5-7.7 km/s) was imaged west of Lake Nipigon in the southern WS beneath the Wawa, Quetico and central Wabigoon subprovinces (White et al. 2001). In general, on the seismic reflection sections the lower crust is homogeneously reflective, the middle crust is heterogeneous and strongly reflective, and the upper crust is heterogeneous and often weakly reflective. The Moho is well imaged and shows a flat or slightly dipping geometry below the Berens River Subprovince, a crustal root below the Uchi Subprovince and an upwarp beneath the metasedimentary English River Subprovince (ERS) and its along strike continuation in the Wabigoon indentor. The Bouguer gravity data in the WS show variations between -10 and -90 mgal, gravity highs occurring over the metavolcanic belts, and gravity lows in most of the areas underlain by felsic intrusions. An exception is represented by the metaplutonic Berens River Subprovince, which is characterised by a regional gravity high. The most outstanding gravity feature observed in the WS is a prominent Bouguer gravity high (>20 mgal) coincident with the length of the metasedimentary ERS. The lack of detailed correspondence between this anomaly and the surface distribution of the metasediments suggests a deep linear source (Nitescu and Cruden 2001). A significant positive anomaly also occurs in the Winnipeg River Subprovince east of Kenora, and broadly parallels the surface distribution of the Lount Lake batholith. It is not clear whether this anomaly is related to the ERS gravity anomaly, or some other deep-crustal feature. The aim of the present study is to investigate the crustal structure of the WS based on gravity models constrained by seismic information and to infer the processes responsible for its present-day configuration. We present results obtained from a gravity inversion algorithm (Pilkington and Crossley 1986) employed for the determination of the Moho topography in the WS, and 2.5-D gravity forward models of the WS crust obtained along N-S seismic lines 1 and 2b. The inversion model of the Moho topography reveals the existence of EW-trending crustal roots below the Uchi and Wabigoon subprovinces (Moho depths of 43-44 km) separated by a parallel rise of the Moho to depths of 37-39 km below the ERS. In the 29 �region underlain by the high velocity basal slab the Moho deepens from 41 km below central Wabigoon Subprovince to 47 km below the northern Quetico Subprovince. These results generally agree with the Moho geometry observed along the seismic profiles, although in detail there are differences. For instance, the N-S extent of the Moho upwarp below the ERS is overestimated in the inversion model, which clearly demonstrates that the ERS gravity anomaly cannot be accounted for only by the Moho rise as observed on the reflection sections. The 2.5D gravity forward model along reflection line 2b extends from the Winnipeg River Subprovince to the Berens Subprovince along the Red Lake road. The model indicates that north of the ERS the observed Bouguer gravity field combines the effects of the Moho topography and upper crustal structures (greenstone belts and various intrusive bodies), with no major density variations occurring at the mid- and lower crustal levels in this segment of the crust. Due to poor reflectivity below the ERS, the seismic data does not constrain well the location at depth of the ERS anomaly source, which may correspond to a transparent region observed between 2-4 s TWT, above a zone of weak mid-crustal reflectivity. The lower density of the rocks exposed at surface in the region of the Winnipeg River Subprovince anomaly indicates that its source is also buried. This anomaly could be caused either by a dense body, which may be related to the source of the ERS anomaly, or by uplifted mid-crustal rocks as suggested by the seismic fabric at the southern end of line 2b. In the latter case the modelling results show that the uplifted Winnipeg River Subprovince mid-crustal rocks should extend to the south below the northern Wabigoon Subprovince. The 2.5D forward gravity model of the WS crust along reflection corridor 1 extends for 500 km from the northern Wawa Subprovince to the Sachigo Subprovince. In the model, Moho topography and the interface relief between major crustal layers (as revealed by the refraction data) cause long wavelength gravity anomalies. Short wavelength anomalies are generated by upper crustal metavolcanics and granitoids. The high-density (3.1-3.2 g/cc), high-velocity basal slab imaged in the southern half of the corridor is not reflected in the long wavelength gravity field as a maximum, since the effect of the positive density contrast created in the lower crust is cancelled by the combined effects of the southward deepening of the Moho and the downward depression of the intra-crustal interfaces above the slab. The present study shows that the gravity field in the WS can be successfully modelled in accord with seismic reflection/refraction data by upper crustal density variations and undulations of the Moho and the interfaces between major crustal layers. The structural grain of the subprovincial configuration is not paralleled by density variations in the middle and lower crust, but is reflected in the Moho undulations. The cause of the crustal roots beneath the granite-greenstone subprovinces is not clear. They may be related to greenstone belt loading, but this depends on whether the lower crust was strong enough after the formation of the greenstone belts to prevent the relaxation of the initial Moho depressions through flow. The Moho upwarp below the ERS may be preserved from a ca. 2710-Ma-old extensional episode in the WS which led to the uplift of the Winnipeg River Subprovince and the formation of sedimentary basins on its flanks (Nitescu and Cruden 30 �2001). The deepening trend of the Moho towards the Quetico and Wawa subprovinces is probably related to the load imposed at the lower crustal level by the observed highvelocity/high density basal slab. This slab was previously interpreted as a remnant of a subducted Archean oceanic plate (White et al. 2001), but it could alternatively represent a Proterozoic igneous underplating episode related to Mid-continental rift magmatism. It is likely that the crustal structure of the WS contains not only elements preserved from Kenoran tectonism, but also younger post-orogenic effects. Therefore caution should be used when interpreting the features revealed by the seismic data solely in terms of processes responsible for the accretionary stage of the Superior Province. Nitescu, B., and Cruden, A.R. 2001. Gravity models of the English River Subprovince: implications for its deep structure and tectonic origin. In 2001 Western Superior Transect Seventh Annual Workshop, University of British Columbia (in press). Pilkington, M., and Crossley, D.J. 1986. Determination of crustal interface topography from potential fields. Geophysics, 51, p.1277-1284. White, D., Asudeh, I., Kay, I., Forsyth, D., and Roberts, B. 1997. Preliminary results of the 1996 Lithoprobe Western Superior seismic refraction survey. In 1997 Western Superior Transect Third Annual Workshop, University of British Columbia, Lithoprobe Report #63, p.106. White, D., Musacchio, G., Helmstaedt, H.H., Harrap, R., Thomson, C., Sol, S. 2001. Remnants of Archean subduction in the Western Superior Province: Results from combined Lithoprobe deep seismic studies. In 2001 Western Superior Transect Seventh Annual Workshop, University of British Columbia (in press). 31 �Digital Drill Logs for the Duluth Complex - Lithology and Assays Patelke, R. L., (Natural Resources Research Institute, University of Minnesota Duluth, 5013 Miller Trunk Highway, Duluth, MN 55811-1442) Since 1988 the Natural Resources Research Institute (NRRI) staff has logged 950 Duluth Complex drill holes of the about 1,500 holes with core available (of the approximately 2,200 holes recorded as penetrating the Complex). This project involves transcribing all of the NRRI logs, plus our interpretation of company logs for drill holes with no core remaining, into a consistent digital format. This downhole “from-to” interval format includes logged rock type, map unit (reconciled from cross-sections and geologic maps where available), and the interval distance up from footwall. This is combined with other data tables, being consolidated from older reporting and projects, which will include: all drill hole specific information (location, company, year drilled, etc.); down hole survey data; all original assays (usually copper-nickel +/sulfur); secondary assays on selected intervals; all available PGE plus gold assays; and eventually all whole rock geochemistry and microprobe data. The format will allow this data to be sorted and filtered geographically, by deposit, by company, by depth or distance from footwall, or other criteria. This data format is intended primarily for use in 3-D mining software and is available on CD-ROM as Excel spreadsheet files, ASCII text files, and as a Gemcom for Windows (Microsoft Access) database. NRRI staff will also format this database to the users needs if practical. The database will also be posted on the NRRI Economic Geology Group website at: http://www.nrri.umn.edu/egg/. The ultimate purpose of this work is to improve the ability to do statistical and spatial analysis of Duluth Complex geology by rapidly combining lithological and assay data for both scientific study and deposit evaluation. This analysis breaks down into two broad categories. Simple 2-D comparisons, such as: comparisons between intrusions or deposit areas; rock type versus grade; ratio of a particular rock type which is mineralized versus part not mineralized; assay grade of a particular unit; or grade as a function of distance from footwall (see Table 1 for examples from Babbitt and Serpentine deposits). More complex 3-D analysis can also be done by using this data to create lithological and grade surfaces, solids, or block models, then intersecting these models; compositing grade by rock type, map unit, level, or distance from a particular point or surface; assessing linear and 3-D variography; contouring by level or distance from a point; and other manipulations of the raw data. Most mining software is very flexible, and the overall data layout will allow NRRI or others to add distance or point data (distance from collar, such as a thin section location) or from-to interval data (such as alteration, grain size, or other assayed elements) as separate tables that can then be compared to the existing data set. The utility in this is the time savings in answering the numerous small questions that arise as well as easing modeling efforts on the Duluth Complex Cu-Ni sulfide and Fe-Ti oxide deposits 32 �Table 1: Consolidated Rock type versus Copper, Nickel, Sulfur, and Copper:Nickel ratio, for Babbit and Serpentine Deposits, St Louis County Minnesota. Surface drill holes only. “Consolidated Rocktypes” are condensed from over 1,000 individual rock types over about 25,000 lithological intervals. “Percent Mineralized” assumes that company sampled all mineralized zones. This is essentially true at Babbitt-Serpentine, but not always true for other deposit areas. Grades are only for the mineralized portions, no unsampled footage (zero value?) was included in the average. "Consolidated Rocktype" Overburden (glacial drift) OUI (Oxide Ultramafic Intrusions) Anorthositic rocks Augite Troctolites Contaminated Rocks Total footage of rock type Percent of rock type in deposit Total mineralized footage of rock type Total unmineralized footage of rock type Percent mineralized of rock type Average copper grade of rock type Average nickel grade Average sulfur grade Average Cu:Ni of rock type of rock type ratio 10,122 2,824 1.8 0.5 447 2,377 0 16 0.31 0.10 1.92 3.29 63,510 63,210 30,792 11.3 11.2 5.5 18,151 28,443 18,248 45,359 34,767 12,544 29 45 59 0.31 0.42 0.44 0.09 0.10 0.11 0.66 1.14 2.25 3.32 4.02 4.06 Gabbroic Rocks Mixed Duluth Complex (not logged) Pegmatitic Rocks Ultramafic Rocks Troctolitic Rocks Dikes, Basaltic Veins, Granitic Massive Sulfides Semi-Massive Sulfides Virginia Formation 3,022 166,344 0.5 29.5 1,348 70,695 1,674 95,649 45 42 0.33 0.39 0.08 0.10 0.85 1.03 4.32 3.74 710 24,473 143,059 698 2,428 415 343 24,760 0.1 4.3 25.4 0.1 0.4 0.1 0.1 4.4 139 7,732 63,929 181 870 413 341 6,279 571 16,741 79,130 517 1,558 2 2 18,481 20 32 45 26 36 100 99 25 0.36 0.36 0.38 0.08 0.29 2.02 1.45 0.38 0.10 0.11 0.10 0.02 0.08 0.57 0.43 0.10 0.71 0.74 0.90 0.35 1.04 12.68 9.69 2.43 3.53 3.17 3.62 2.58 3.51 3.86 4.17 3.04 Bedded Pyrrhotite Unit Virginia Sill MG Sill Sill in Biwabik Iron Formation Biwabik Iron Formation Pokegama Quartzite Giants Range Granites Basalts Massive Chlorite Faults Massive Graphites Hybrids (Hornblendite, etc.) Orthopyroxenites Massive Oxides (not in OIUs) Serpentinites deposit total: deposit averages: 3,834 1,686 9,537 284 6,976 20 420 2,951 12 630 10 505 111 49 76 563,811 0.7 0.3 1.7 0.1 1.2 0.0 0.1 0.5 0.0 0.1 0.0 0.1 0.0 0.0 0.0 100 2,928 311 1,129 4 507 906 1,375 8,408 281 6,469 20 301 2,404 4 443 5 412 20 23 57 330,500 76 18 12 1 7 0 28 19 67 30 60 18 82 55 25 40 0.13 0.09 0.10 0.03 0.11 0 0.14 0.20 0.19 0.36 0.41 0.31 0.44 0.52 0.47 0.04 0.05 0.03 0.01 0.03 0 0.05 0.08 0.06 0.10 0.08 0.12 0.14 0.13 0.11 4.25 0.35 0.49 0.14 0.44 0 0.29 0.85 0.34 1.15 1.31 1.67 2.29 0.56 1.62 2.50 1.58 2.45 2.67 3.05 0 2.34 2.65 2.75 3.66 4.58 2.86 3.79 4.40 4.72 0.39 0.10 1.29 3.66 24.40 34,742 25,445 60,187 5.20 34,314 25,873 60,187 37.10 34,532 25,655 60,187 47.00 34,414 25,773 60,187 119 547 8 187 6 93 91 27 19 223,192 maximums: number of assayed intervals: number of unassayed intervals: total number of intervals: 33 �Mineralogy and Zonation of the Big Whopper Pegmatite, Separation Rapids area, Kenora, Ontario Pedersen, J. C. P.Geo., (Consulting Geologist, Avalon Ventures Ltd.) and Bubar, D. S. (President and CEO, Avalon Ventures Ltd.) The Big Whopper pegmatite is the largest in a field of rare metal pegmatites occurring in the Separation Lake metavolcanic belt, situated approximately 60 km north of Kenora, Ontario. This greenstone belt occurs at the boundary of the English River and Winnipeg River sub-provinces of the Superior structural province. It is interpreted to be the eastern continuation of the Bird River Greenstone belt in Manitoba, which hosts the Cat Lake rare metal pegmatite field and the famous Tanco pegmatite. The Big Whopper pegmatite is 100% owned by Avalon Ventures Ltd. Exploration work to date has defined indicated and inferred resources totalling 11.6 million tonnes grading 1.34% Li2O, 0.30% Rb2O and 0.007% Ta2O5. The deposit is now at the feasibility study stage as a potential producer of lithium-rich feldspars, tantalum and mica. The Big Whopper is a vertically-oriented pegmatite dyke hosted by deformed metavolcanic rocks of lower to middle amphibolite facies metamorphism (amphibolite). A north-directed compressional tectonic event produced flattening and a strong vertically oriented regional schistosity striking WNW at the Big Whopper. This schistosity is folded about a sub-vertical axis. The Big Whopper itself exhibits high-strain features and a tight S-fold geometry, with the thickened central portion of the pegmatite coinciding with the hinge zone, and attenuated limbs extending to the east and west. The pegmatite has been traced over a strikelength of 1.5 km and ranges from 10 to 80 m in width. The dominant economic minerals in the deposit are petalite and columbite-tantalite, the principal ore mineral of tantalum. Petalite is a rare lithium aluminosilicate mineral used in certain specialty glass and ceramics applications, such as thermal shock resistant cookware. Tantalum finds its major use (approximately 60%) in the manufacture of miniature tantalum capacitors, which are currently in high demand for use in popular electronics products such as computers and cell phones. The deposit also contains significant quantities of rubidium potash (Rb-K) feldspar and albite, (both used in the glass and ceramic industries); lepidolite, an important source of rubidium in the chemicals industry, as well as the tin mineral cassiterite. Other potentially valuable industrial minerals include muscovite mica (some with elevated lithium) which may comprise up to 15% of the ore, spodumene averaging 3-5% occurring as SQUI (spodumene-quartz intergrowth) replacement rims on petalite, cassiterite, pale pink spessartine garnet and high-purity quartz. Minor accessory minerals include apatite, zircon, gahnite, monazite, xenotime, rare sulphides, sulphosalts, and thorite. Mineralogical zoning observed in the Big Whopper is characteristic of highly evolved rare metal pegmatites, with well-developed wall zones and internal intermediate zones classified according to their dominant constituent minerals (Figure 1). The mineralogical zones of the Big Whopper identified to date are Wall Zone (predominantly albitite), Megacrystic Feldspar and Quartz-Mica marginal Zones and Petalite (intermediate) Zone. The Petalite Zone is the largest zone defined to date, comprising approximately 80% of the volume of the Big Whopper. Its essential mineralogy consists of petalite, Rb-K-feldspar, albite, quartz and mica. Sub-zones are defined based on subtle textural and mineralogical variations such as the presence of lepidolite mica. 34 �Lepidolite-rich zones tend to occur on the periphery of the deposit and are typically enriched in tantalum. THE BIG W-IOPPER PEGMATITE Detailed Geology lro 700W N I 600W 300W ( EngIMi River .— IiaseIine—* PETALITE ZONE Ato D: A - petalte (white) k-feldspar quartz a lb te B - petalte (pink) k-feldspar albite I C- petalite (blue-grey and pink) albite mica 0 - peta lite lepido lite a lb te (ta ntalum-rich zone >0.01 % Ta 0) I:.: I interdigitated PSLL ZONE (albitite) and FE LDSFR, QUARtZ-MICA ZON ES 100 amphibolite host rocks metre stripped outcrop Fie I: BiWhcwer Geology 35 / �Cu-Ni-PGE Mineralization in the South Kawishiwi Intrusion, Northeastern Minnesota Variation due to Magmatic Processes Peterson, D. M., (Economic Geology Group, Natural Resources Research Institute, University of Minnesota, Duluth email: dpeters1@nrri.umn.edu) INTRODUCTION The Mesoproterozoic South Kawishiwi intrusion (SKI), which is exposed in a 32 x 8-kilometer arcuate band along the northwestern margin of the Duluth Complex, is composed dominantly of troctolitic cumulates. Footwall rocks to the intrusion include, from north to south, the Paleoproterozoic Virginia and Biwabik Iron Formations, and the Late Archean Giants Range batholith. Regional crosscutting features and remnant pillars and xenoliths indicate that the SKI intruded along the boundary between Mesoproterozoic volcanic and Anorthositic Series rocks and the older footwall strata. The SKI abuts the troctolitic Partridge River intrusion on the southwest, is inferred to be semi-conformable to the later Bald Eagle intrusion to the east, and abuts and cuts older Anorthositic Series rocks to the northeast. The basal mineralization within the SKI occurs dominantly in heterogeneous zones of troctolitic, gabbroic, noritic, and ultramafic rocks. This heterogeneity is a function of many factors, including footwall assimilation, dehydration and volatile fluxing from the footwall strata into the overlying magmas, chilling, and repeated magma injection. Miller and Severson (2002) subdivide the intrusion into five major map units from the base upwards: 1) a heterogeneous basal contact zone of sulfide-bearing troctolitic, gabbroic, and noritic rocks; 2) ophitic augite-troctolite; 3) poikilitic leucotroctolite; 4) ophitic troctolite; and 5) homogeneous troctolite. Severson (1994) and Zanko et al. (1994) have further subdivided the marginal zone of the intrusion into 17 different units, with sulfide mineralization dominantly confined to four units: the Basal Augite-Norite (BAN), Basal Heterogeneous (BH), Updip Wedge (UW), and Ultramafic 3 (U3) units. Minor mineralization higher up in the igneous stratigraphy of the intrusion occurs locally in the Ultramafic 1 (U1) and Ultramafic 2 (U2) units. Recent research has identified two dominant styles of mineralization (“Open” and “Confined”) within the intrusion that have distinctive differences in their igneous stratigraphy, metal contents, timing, and mode of origin. Simplified regional geologic and mineralization style maps for the SKI are presented in Figure 1. 36 �Figure 1. Simplified regional geologic (left) and basal mineralization style (right) maps of the South Kawishiwi intrusion. Disseminated basal mineralization within the SKI has traditionally been divided into five distinct deposits: 1) Serpentine, 2) Dunka Pit, 3) Birch Lake, 4) Maturi, and 5) Spruce Road. The disseminated sulfides occur as interstitial grains that average about 1-5% (visual estimation) and range from trace amounts to 10%, with local zones of massive sulfide at the basal contact. Major sulfides are pyrrhotite, chalcopyrite, cubanite, and pentlandite. Pyrrhotite is generally the dominant sulfide, especially closer to the basal contact. Recent work by Wallbridge Mining Company and the author have defined a potential additional deposit area east of the Maturi deposit, informally named the Maturi Extension deposit. Although the style of mineralization in all of the deposits is dominated by disseminated Cu-Ni sulfides, differences occur between the deposits in igneous stratigraphy, Cu-Ni and PGE grade, mineralization thickness, and contained tonnes. Regional analysis of the drill hole assay data for all of the deposits of the South Kawishiwi intrusion has led to the identification of two main styles of mineralization associated with the base of the intrusion (Fig.1). These mineralization types include: 1. “Open” – vertically extensive (> 450 meters) mineralization with low - moderate Cu-Ni grade and low Au+PGE grades. Cu-Ni grades typically increase towards the basal contact although the mineralized zones are typically erratic in their spatial extent and grade and commonly interfinger in a random pattern with zones that are barren of sulfides. Restricted zones of massive sulfide occur locally at the basal contact. This erratic pattern of mineralization, in part, mirrors the lithologic heterogeneity of the basal units. Examples of this open style include the Spruce Road, Serpentine, and Dunka Pit deposits. 2. “Confined” – vertically restricted (< 150 meters) mineralization with moderate - high Cu-Ni grades and moderate to very high (locally) Au+PGE grades. Cu-Ni grades typically are the highest near the top of the mineralized zone (units U3 and BH) and gradually decrease with 37 �depth toward the basal contact, and no zones of massive sulfide at the basal contact have been identified. For example, the upper portion of the mineralized zone within the Maturi deposit consistently exhibits copper values in excess of 1.0% that decrease to ~0.25% at the basal contact. Examples of the confined style include the Maturi, Maturi Extension, and the Birch Lake deposits. The differences between the styles of mineralization reflect the different histories for the rocks hosting the mineralization. It is hypothesized that the early Open-style of mineralization reflects repeated small injections of sulfur-saturated, highly contaminated magma. In contrast, the Confined-style of mineralization reflects much larger batches of more primitive magma that may have incorporated much of its sulfur by assimilation of Open-style mineralized rocks (or remnant magma). Miller, J.D., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.M., and Wahl, T.E., 2002, Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geologic Survey, Report of Investigations 58, 200 p. Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., and Peterson, D.M., 2001, Geologic map of the Duluth Complex and related rocks, northeastern Minnesota: Minnesota Geological Survey Miscellaneous Map Series M-119, nominal scale 1:200,000. Peterson, D.M., 2002, Copper-Nickel grade maps for the Spruce Road deposit, South Kawishiwi Intrusion, Duluth Complex: Natural Resources Research Institute, University of Minnesota, Duluth, Report of Investigations, NRRI/RI-2002/03, 99 p. Severson, M.J., 1994, Igneous stratigraphy of the South Kawishiwi intrusion, Duluth Complex, northeastern Minnesota: Natural Resources Research Institute, University of Minnesota, Duluth, Technical Report NRRI/TR 93/34, 210 p. (with plates) Zanko, L.M., Severson, M.J., and Ripley, E.M., 1994, Geology and mineralization of the Serpentine copper-nickel deposit, Duluth Complex, Minnesota. Natural Resources Research Institute, University of Minnesota, Duluth, Technical Report, NRRI/GMIN-TR-93-52, 90p. 38 �3-Dimensional View Through a Mineralized System: the South Kawishiwi Intrusion, Duluth Complex Peterson D. M., (Economic Geology Group, Natural Resources Research Institute, University of Minnesota, Duluth email: dpeters1@nrri.umn.edu) A 35x8x2 km three-dimensional AutoCAD™ model of the basal contact surface of the South Kawishiwi intrusion (SKI) has recently been completed. The model is based on the location of ~ 800 drill hole piercing points into footwall strata and geophysical data (Figure 1). Integration of regional geological, geophysical, and geochemical features with the 3-D model has led to new ideas on possible feeder channels for magmas of the northern SKI. The interpreted master magmatic feeder channel of the northern SKI is fed from the central Mid Continent Rift through the Bald Eagle Intrusion gravity high, into a dike-like body of troctolitic rocks (herein termed the BEI Trough) cutting older Anorthositic Series rocks (Fig. 1). The 3-D model has been instrumental in the development of new ideas on the location and origin of the Cu-Ni-PGE mineralization within the SKI. Integration of the magmatic feeder channel concept with lineament analysis and recent PGE assays has led to the development of conceptual models for the formation of the Spruce Road (Peterson, 2002) and Maturi (Peterson, 2001) deposit areas. Early, contaminated magmas that formed the Spruce Road deposit were deflected to the north by a pillar of older Anorthositic Series rocks (Fig. 1) that is located at depth within the northern portion of the SKI (Fig. 2). Moreover, PGE-enriched Cu-Ni mineralization of the Maturi Extension deposit is located beneath the pillar, and a conceptual model for the formation of this deposit area is given in Figure 3. Figure 1. Regional shaded relief 3-D model of the SKI 39 �. Figure 2. Conceptual magma flow model for the Spruce Road area. Figure 5. Conceptual model for the formation of the Cu-Ni deposits of the northern SKI. Sheet-like mineralization of Maturi located to the left of the diagram, and the PGE-enriched Maturi Extension located under the Anorthositic Series block. Peterson, D.M., 2001, Development of a conceptual model of Cu-Ni-PGE mineralization in a portion of the South Kawishiwi Intrusion, Duluth Complex, Minnesota: Laurentian University – Society of Economic Geologists, Second Annual PGE Workshop, Sudbury, Ontario. Peterson, D.M., 2002, Copper-Nickel grade maps for the Spruce Road deposit, South Kawishiwi Intrusion, Duluth Complex: Natural Resources Research Institute, University of Minnesota, Duluth, Report of Investigations, NRRI/RI-2002/03, 99 p. 40 �New Information from the Sibley Group Rogala, B., Fralick, P.W., and Borradaile, G. (Department of Geology, Lakehead University, Thunder Bay, Ontario) The Sibley Group sediments were deposited in a subsiding intracratonic basin (Fralick and Kissin, 1995) between 1537 +10-2 Ma (Davis and Sutcliffe, 1984) and 1339 ± 33 Ma (Franklin et al., 1980). It is presently thought that these sediments were deposited closer to 1537 Ma. The Group was previously divided into three main Formations: Pass Lake, Rossport, and Kama Hill. The Pass Lake Formation consists of the conglomeratic Fork Bay Member and the sheeted sandstones of the Loon Lake Member, representing a braided fluvial environment (Cheadle, 1986). The Rossport Formation is separated into the Channel Island, Middlebrun Bay, and Fire Hill Members. The Channel Island Member is a cyclic dolomite-shale unit interpreted to be playa lake sediments (Cheadle, 1986). The Middlebrun Bay Member is a stromatolitic unit, considered a marker bed for the Sibley Group. It represents a period of relatively stable water and salinity levels. The Fire Hill Member consists of mudcracked red silt with mudchip conglomerates and sand sheet incursions. It signifies a time of tectonic tilting of the basin. The Kama Hill Formation is not subdivided, and is composed of purple shales and siltstones interpreted as mud flat deposits (Cheadle, 1986). The Nipigon Bay Formation is a new addition to the Sibley Group, overlying the Kama Hill Formation. The exposure of this unit is quite limited. It has only been found to crop out on some of the islands in Nipigon Bay, in particular Quarry Island and Vert Island. The Nipigon Bay Formation has also been found in several holes drilled by Falconbridge Limited in 1997. These holes intersected approximately 500 m of well-sorted medium sandstone. The colour of the sandstone can be buff, red, orange, or purple. The sandstone has cross-stratification, which varies from low- to high-angle. The thickness of cross-stratification sets is also variable, ranging from 20 cm to 300 cm, with an average thickness of 100 cm. Figure 1 SiO2 content varies through the Sibley stratigraphy. The upper 500 m is the Nipigon Bay Formation. Si O 2 C o n t e n t 50. 00 55. 00 60. 00 65. 00 70. 00 75. 00 80. 00 85. 00 90. 00 95. 00 1 00. 00 0 1 00 200 300 400 500 600 700 800 900 1 000 Extensive geochemistry was conducted on the sandstones throughout the Sibley stratigraphy, from the Pass Lake Formation to the Nipigon Bay Formation. The geochemistry shows an 41 �interesting series of element peaks within the Pass Lake Formation, indicating sharp, contrasting influxes of source material. However, source-rock modeling has not proven conclusive at this point. The geochemistry indicates an abrupt change at the base of the Nipigon Bay Formation. This Formation is much less variable and shows a different source-rock character than that of the Pass Lake Formation. This is portrayed in Figure 1, using SiO2 content, although the same pattern is repeated in all of the elements that were used for this analysis. The Channel Island Member of the Rossport Formation has also produced some interesting information. Paleomagnetic analysis was conducted on a 50 cm section of core from Noranda’s NI-92-7 drill hole. The inclinations were calculated and graphed to show the variations. The result was the chart in Figure 2 (left), which defines a curve that is believed to represent secular variation. This work will be followed up with isotope geochemistry and scanning electron microscopy for further interpretations. Figure 2 Paleomagnetic inclinations measured at 1 cm intervals along a 50 cm length of core. Inclination (degrees) -50 -40 -30 -20 -10 0 10 20 30 40 0 5 10 15 20 25 30 35 40 45 References Cheadle, B.A. 1986. Alluvial-playa sedimentation in the lower Keweenawan Sibley Group, Thunder Bay District, Ontario. Canadian Journal of Earth Sciences, 23, 527-542. Davis, D.W. and Sutcliffe, R.H. 1984. U-Pb ages from the Nipigon Plate and Northern Lake Superior. Geological Society of America Bulletin, 96, 1572-1579. Fralick, P. and Kissin, S. 1995. Mesoproterozoic basin development in central North America: implications of Sibley Group volcanism and sedimentation at Redstone Point. in: Petrology and metallogeny if volcanic and intrusive rocks of the mid-continent rift system, Proceedings of the International Geological Correlation Program, Project 336. Franklin, J.M., McIlwaine, W.H., Poulsen, K.H. and Wanless, R.K. 1980. Stratigraphy and depositional setting of the Sibley Group, Thunder Bay District, Ontario, Canada. Canadian Journal of Earth Sciences, 17, 633651. 42 �300 my evolution of the Red Lake greenstone belt, western Superior Province, Ontario: A synthesis of current constraints on volcanism, sedimentation, deformation, metamorphism and gold mineralization. Sanborn-Barrie, M.1, Skulski, T.1, Rayner, N.1 and Parker, J.R.2 (1Geological Survey of Canada, Continental Geoscience Division, 601 Booth Street, Ottawa, ON, K1A 0E8; 2Ontario Geological Survey, Willet Green Miller Centre, Sudbury, ON, P3E 6B5) msanborn@NRCan.gc.ca The Red Lake gold camp preserves an extensive record of magmatic and sedimentary activity from 3.0-2.7 Ga, and evidence of multiple episodes of deformation, hydrothermal alteration and metamorphism. The oldest volcanic rocks, which host its major lode gold deposits, are komatiite and tholeiitic basalt of the Balmer assemblage which includes minor 2.99-2.96 Ga intermediate to felsic volcanics and chemical metasedimentary rocks. Some of these basaltic rocks have trace element compositions (Th/Nb) reflecting local interaction with a differentiated crust, however, continental basement has not yet been observed in the belt. A thick sequence of 2.94-2.91 Ga intermediate to felsic calc-alkaline flows and pyroclastic rocks, interbedded with basalt, komatiite and stromatolitic carbonate comprise the overlying Ball assemblage. Ball volcanic rocks show trace element variations (e.g. Th/Nb) in mafic to felsic volcanic rocks that are also consistent with contamination by differentiated crust. Relatively juvenile εNd values of +0.8 to +1 in these felsic rocks suggest that the contaminant was relatively young (~3 Ga) crust. Ball ultramafic rocks have depleted εNd values of +2.2 and may reflect intra-arc extension and upwelling of depleted mantle-derived magma. Quartz-rich fuchsite-bearing clastic rocks of the c. 2.9 Ga Slate Bay assemblage have detrital zircon profiles (GSC SHRIMP, n=140 grains; and Corfu et al., 1998) that indicate derivation from predominantly Ball (with lesser Balmer) sources. This clastic sequence, which extends the length of the belt, overlaps the Ball/Balmer contact, however, north of Red Lake (McInnes Lake), Ball-age volcanic rocks and iron formation are in depositional contact with the Balmer assemblage (Corfu et al. 1998). Ball volcanism was followed by the deposition on Balmer substrate of the 2.894 Ga Bruce Channel assemblage comprising calc-alkaline intermediate pyroclastic rocks overlain by an upward-fining sequence of clastic sediments and chert-magnetite iron formation. Felsic tuff has depleted εNd values of +2 (Henry et al. 1999), reflecting a change between 2.99 and 2.89 Ga toward isotopically depleted magmatism that may reflect crustal growth at a juvenile continental margin (cf. Henry et al. 2000). The overlying Trout Bay assemblage comprises a lower sequence of basalt, 2.85 Ga intermediate pyroclastic and epiclastic rocks, iron formation and an upper sequence of pillowed tholeiitic basalt. The upper basalt sequence has depleted, MORB-like trace element profiles reflected in positive εNd values of +2, similar to model Archean depleted mantle. The Trout Bay and Ball assemblages face toward each other across an interpreted tectonic contact that may reflect telescoping across the continental-oceanic interface. Following a hiatus of some 100 my, the onset of extensive calc-alkaline volcanism is recorded by the Confederation assemblage with c. 2.75-2.74 Ga shallow marine to subaerial calc-alkaline mafic to felsic volcanic rocks of the McNeely sequence, overlain and interstratified by c. 2.74 Ga submarine, mixed calc-alkaline and tholeiitic basalts and FIII-type rhyolite of the Heyson assemblage. McNeely volcanic rocks have an εNd value of +1 to +0.8, whereas Heyson volcanic rocks have depleted εNd values of +2.9. These geochemical and isotopic data are consistent with establishment of a Neoarchean shallow marine arc (McNeely) on the existing Mesoarchean 43 �continental margin sequence, with local intra-arc extension and eruption of high temperature F3 rhyolite and submarine tholeiitic basalt (Heyson). A depositional relationship between the Confederation and Mesoarchean assemblages is supported by a c. 2.75 Ga felsic dyke that cuts the Balmer assemblage, and by Mesoarchean inheritance in c. 2.74 Ga volcanic rocks (Corfu et al., 1998). An angular unconformity is indicated by opposing facing of Balmer and Confederation strata in the Madsen area, consistent with overturning of the Balmer (D0) prior to Confederation volcanism. Local occurrences of coarse clastic rocks, such as ironstone-derived conglomerate on Wolfe Bay, may represent basal conglomerate marking this unconformity. Typically, however, the interface between Neoarchean and Mesoarchean volcanic rocks is covered by a c. 2.735 Ga clastic sequence, the Huston assemblage. Detrital zircon populations from conglomerate samples of the Huston assemblage from the Balmertown and Madsen areas show single, prominent (50-60 grains), c. 2.74 Ga detrital age peaks that indicate derivation from the Confederation assemblage. Undated conglomeratic rocks that underlie the youngest volcanic assemblage in the belt (c. 2.733 Ga Graves) are correlated with the Huston assemblage, as are argillaceous to turbiditic rocks that conformably overlie c. 2.744 Ga McNeely volcanic rocks in central Red Lake. Shallow marine to subaerial, 2.73 Ga calc-alkaline, intermediate to felsic volcanic rocks and synvolcanic plutons of the Graves assemblage reflect Andean-style arc magmatism that culminated in the c. 2.72-2.70 Ga Kenoran orogeny. The youngest supracrustal rock in the belt is a pebble conglomerate, once thought to correlate with the “Austin tuff” ore horizon. This garnetiferous metasedimentary rock yielded a diverse detrital zircon population profile (n=56 grains), with ages that correspond to the Balmer, Ball, Trout Bay and Confederation assemblages, and younger. The maximum depositional age of this sample is 2700 ±6 Ma, the upper intercept of a 5-spot regression for the youngest detrital zircon analysed. This young supracrustal sequence provides a new maximum age for penetrative deformation and amphibolite-facies metamorphism in the Red Lake area. Its relationship to gold mineralization is no longer certain. The Red Lake greenstone belt displays evidence of several episodes of deformation, interpreted to be closely linked with extensive hydrothermal activity and gold mineralization. Nonpenetrative deformation (D0) appears to have involved overturning of the c. 2.99 Ga Balmer assemblage, possibly related to recumbent folding, prior to Neoarchean volcanism. The main stages of penetrative deformation were imposed after ca. 2.74 Ga volcanism. These resulted in northerly trending, south-plunging F1 folds and associated S1/L1 fabrics, superimposed by eastto northeast-trending D2 structures (F2/S2/L2) in western and central Red Lake, and by southeasttrending folds and fabrics (the “mine trend”) in eastern Red Lake. D1 may be bracketed between 2.744 Ga, the age of Confederation volcanic rocks in central Red Lake that contain F1 structures, and c. 2.733 Ga, the age of the Graves volcanic sequence that does not appear to have been affected by D1. It seems probable that deposition of the Huston assemblage took place synchronous with, and as a response to deformation (D1) and that D1 is linked to a change in plate dynamics that took place between Confederation and Graves volcanism. An important constraint on the timing of D2 is provided by the relationship of regionally extensive D2 fabrics to the 2718 ±1 Ma Dome Stock. Supracrustal rocks adjacent to the stock, and occurring as xenoliths within the stock, contain a penetrative S2 fabric. The stock itself contains a weak throughgoing NE-striking foliation coplanar to S2 observed elsewhere. These fabric relationships suggest that the main cleavage-forming stage of D2 predated intrusion of the Dome stock at 2718 Ma, but that shortening was sustained beyond its emplacement. Penetrative strain 44 �appears to have outlasted c. 2700 ±6 Ma, the maximum age of young unconformably overlying conglomeratic rocks that display a penetrative tectonic foliation coplanar to D2 fabrics throughout central Red Lake. Metamorphic mineral assemblages within this sample and surrounding rocks include staurolite-cordierite-garnet-bioite from pelitic compositions and orthoamphibole-garnet from mafic compositions, indicating that amphibolite facies metamorphism also outlasted ca. 2700 ±6 Ma. We interpret tectonometamorphism of the Red Lake belt between c. 2.718 and 2.7 Ga to be the result of orogenic activity related to collision between the c. 3 Ga North Caribou terrane and the c. 3.6-3.0 Ga Winnipeg River terrane, to the north and south of the Red Lake belt, respectively. If the dated “Austin tuff” sample represents an intact relict of the gold ore horizon mined from the Madson area, these new data would suggest gold mineralization of young, unconformably overlying clastic rocks after ca. 2.7 Ga. However, this is in contrast to the timing of gold mineralization in the Balmertown area, which has been shown to predate 2714 ±4 Ma, the age of a post-ore porphyritic dyke from the New Red Lake (formerly A.W. White) mine (Corfu and Andrews, 1987). Corfu, F. and Andrews, A.J., 1987. Geochronological constraints on the timing of magmatism, deformation and gold mineralization in the Red Lake greenstone belt, northwestern Ontario. Can. J. Earth Sci. 24, 13021320. Corfu, F., Davis, D. W., Stone, D. and Moore, M., 1998. Chronostratigraphic constraints on the genesis of Archean greenstone belts, northwestern Superior Province, Ontario, Canada. Precam. Res. 92, 277-295. Henry, P., Stevenson, R.K., Larbi, Y. and Gariepy, C., 2000. Nd isotopic evidence for Early to Late Archean (3.4-2.7 Ga) crustal growth in the western Superior Province (Ontario, Canada). Tectonophysics, v.322, p.135-151. 45 �The Mine Permitting Process in Minnesota - Who, What, Where, and When. Severson, M. J., (Natural Resources Research Institute, University of Minnesota Duluth, 5013 Miller Trunk Highway, Duluth, MN 55811-1442 e-mail: mseverso@nrri.umn.edu) Interest in the Cu-Ni-PGE deposits of the Duluth Complex has been recently renewed due to advances in hyrometallurgical processes and increased PGE metal prices. Economic evaluations are currently being conducted for several of the deposits, and in some cases, initial steps have also been taken in order to obtain the various permits to mine. In Minnesota, the permitting process for a non-ferrous, metallic mine is complicated; involves dealing with numerous different federal, state, and local agencies; and as yet, has not been carried through to completion for establishment of either an open pit or underground mine. This project outlines each of the necessary permitting steps that are needed to develop a metallic mine, and provides time lines, or length of the various determination processes, for each type of permit that would be required. Also included are lists of agencies, contact persons, phone numbers, and e-mail addresses. All of the data generated in this project will be used to produce a report, and supplemental pamphlet, that fully describes what each permit entails. At the forefront of the mine permitting process in Minnesota is an environmental review process consisting of preparation of an Environmental Impact Statement (EIS), and because an EIS is mandatory, it must be preceded by preparation of a scoping Environmental Assessment Worksheet (EAW). Many of the permits that are mining-related require this process to be completed (Table 1). The scoping EAW is designed to identify potentially significant issues (including possible environmental, sociological, economic, and health risk impacts) that will be associated with a proposed mine and will need further study in the EIS. Thus, the EAW is a “blueprint” for the EIS because it sets limits on what will be discussed further and at what level of detail. It is a standardized six-page questionnaire (31 questions), accompanied by numerous supplemental pages to fully answer the questions, and “typically” takes 90-120 days to complete. The EIS, which typically takes 280 days to complete (legally mandated time), is a thorough study of the issues defined in the EAW and provides information regarding environmental impacts and how they can be avoided/minimized. It considers and sets forth a series of “reasonable” alternatives (including the “no-build” alternative), possible permit conditions, and possible mitigation measures. The EIS does not approve or disapprove of a project; rather, it provides information and alternatives. A determination on the adequacy of the information provided in the EIS must be made before it can be used in determining whether to grant or deny any mine-related permit applications. The Minnesota Department of Natural Resources (DNR) is the Responsible Government Unit (RGU) for both the scoping EAW and EIS. The EAW/EIS process should begin as soon as possible because the entire process could take around 1.5 years before a final adequacy decision can be made. Furthermore, no decisions on most mine-related permits can be made until after the EIS process has been completed (actually 25-90 days after the adequacy decision). It is extremely important to make early contact with all of the agencies that are responsible for granting permits for several reasons that include: 1. to insure that all issues relating to the various permits are included for study in the EIS; 2. to begin preparation on some of the permits so they can be issued as quickly as possible after the EIS is complete (some permits can take up to 1.5 years (or more) before decisions to grant or deny can be made); 3. to avoid duplication of efforts 46 �for the various permits and establish cooperation between the agencies; and 4. to get the public informed and involved to hopefully minimize later delays in the permitting process. Table 1: List of potential permits required to establish a non-ferrous, metallic mine in Minnesota. Permit Agency Approximate Application determination time* Permit to Mine** DNR Minerals 7-8 months WCA-Wetlands Replacement Plan** DNR Minerals 4-6 months Water Appropriation Permit** DNR Waters < 2 months Public Waters Work Permit** DNR Waters < 2 months Dam Safety Permit** DNR Waters < 2 months Part 70 - Air Quality Permit** MPCA 1.5 years NPDES/SDS Stormwater Permits (two - for Construction and Industrial Activity) NPDES/SDS Wastewater Permits** ( two - for industrial process wastewater and sewage) Hazardous Waste Permit** MPCA < 2 months? MPCA 1.5 years MPCA 1-3 years Section 404 Permit (discharges to wetlands)** ACOE Section 10 Permit ? (Affects to “navigable” waters of the US) Section 401 Certification (wetland certification needed before 404 permit can be issued) Water Treatment Plant Permits ACOE 120 days (unless Fed. EIS required?) ? MPCA 60-120 days ? MPCA ? Local Permits - zoning, construction, bonding, etc varies “short” period * = Minimum time to complete (assuming optimal conditions) after all data that are required to be submitted for the permit are complete ** = Permit that by itself may require a mandatory EAW and/or EIS. Abbreviations: DNR = Department of Natural Resources; MPCA = Minnesota Pollution Control Agency; ACOE = Army Corps of Engineers; NPDES = National Pollutant Discharge Elimination System; SDS = State Disposal System; WCA = Wetland Conservation Act. 47 �Description of a pegmatite occurrence on the eastern margin of the Mellen Granite, State Highway 13, Ashland County, Wisconsin. Sikkila, K., (Wisconsin Department of Transportation, District 8, Superior, Wisconsin) A highway construction project supervised by the Wisconsin Department of Transportation, District 8 office took place during the 2001 field season on State Highway 13 immediately north of the City of Mellen in central Ashland County. Construction activity began at the intersection with State Highway 169, extending northward for 3480 meters. The project consisted of a realignment within the existing right-of-way and necessitated the blasting, excavation and removal of approximately 33000 cubic meters of rock. Bedrock geology in the project area consists of intrusive rocks of the Lower Keweenawan (approximately 1.1 Ga) Mellen Complex. The highway right-of-way is positioned over the contact between two discrete rock bodies within the Complex: the gabbroic rocks of the Potato River Intrusion (to the east); and the younger Mellen Granite (to the west). A previously existing roadcut along the right-of-way indicated the presence of an intrusive breccia at this intrusive contact. The primary lithology of the breccia matrix in this previously existing roadcut appears to be granodiorite, with xenoliths of gabbro, lesser amounts of mafic volcanic rocks, and minor amounts of metasediments of probable Lower Proterozoic age. New exposures resulting from construction activity indicate an additional complexity to this brecciated contact margin, culminating in the presence of a small pegmatite body approximately 520 meters to the north of the original intrusive breccia exposure. The petrography of the pegmatite itself appears to consist primarily of coarse-grained potassium feldspar (perthite), quartz and biotite, with accessory epidote, apatite and other currently unidentified minerals. An irregular central “core” contains abundant quantities of myrmekite, various unidentified phosphates(?), and an unidentified pale lilac-colored mineral. Analyses will be performed on these minerals in the near future. The intrusive breccia in the areas proximal to the pegmatite displays moderate to strong potassium metasomatism, with plagioclase altered to potassium feldspar/perthite/myrmekite and the generation of epidote as a residual by-product, the alteration of mafic minerals in gabbroic xenoliths to biotite and/or phlogopite and hornblende, etc. Anastomosing swarms of small irregular felsite (aplite?) dikes also occur in this zone. Evidence for later-stage cataclasis and a retrograde chlorite event is represented by occurrences of chloritized mafic fault gouge and the presence of books of chlorite replacing biotite phenocrysts in areas proximal to these gouge zones. Irregular localized bodies of intrusive rocks of intermediate composition (syenite, monzonite, monzonite porphyry, monzogabbro, quartz diorite, etc.) also occur along the perimeter of this breccia/pegmatite zone. Widely disseminated sulfides (pyrite/chalcopyrite) are also noted. 48 �Inferences from the Hattenberger deep drill hole, Carlton County, Minnesota, pertinent to regional stratigraphy and mineral potential of the western segment of the Penokean Orogen Southwick, D.L., (Minnesota Geological Survey (retired); davidsouthwick@earthlink.net) The Hattenberger core hole, drilled vertically to a depth of 7,440 feet (2,268,3 meters) near Kettle River in Carlton County, Minnesota, penetrates a sequence of metamorphosed and deformed sedimentary and volcanic strata of Paleoproterozoic age within the fold and thrust belt of the Penokean orogen. Continuous core recovered from the depth interval 803 to 7,440 feet (244.8 to 2,268.3 meters) provides a rare depth-dimensional view of the lithostratigraphy and structures in rocks that are not well exposed at the surface. The cored interval below depth 4,780 feet (1,457.3 meters) consists predominantly of quartz-rich mica schist, calcareous mica schist, and dolomitic marble that are interstratified on coarse to fine scales. Relatively minor amounts of mafic volcanic rock (both flows and fragmental types) and recrystallized cherty iron-formation are interstratified with the dominant quartzose, micaceous, and calcareous metasedimentary rocks in the upper part of this lowermost major division of the core. In the upper major division of the core, above depth 4,780 feet, the predominant rock types are (1) dark-colored, thinly bedded graphitic mica schist and phyllite; (2) non-magnetic, graphitic, silicate-carbonate-sulfide iron-formation, parts of which are characterized by prominent porphyroblasts of Fe-rich garnet and amphibole; and (3) an assortment of mafic volcanic rock types that in general form thin stratigraphic units interbedded with the metasedimentary components. Sills of metadiabase chemically identical to the volcanic rocks occur throughout the whole core, in both the lower sequence of quartzose and calcareous schists and the upper sequence of graphitic and iron-rich rocks. Altogether, the sills and allied metavolcanic rocks amount to about half of the total footage drilled, with the greater part being sills. The lithologic and structural attributes of the lower sequence of rocks in the Hattenberger core match well with those of the Denham Formation at its type locality (Morey, 1978; Boerboom and Jirsa, 2001). Likewise, the attributes of the upper sequence match well with those of the Glen Township Formation, the type section for which is contained in several cores drilled about 23 miles (37 kilometers) along regional strike to the west of the Hattenberger locality (Morey, 1978). The assumed Denham–Glen Township contact in the Hattenberger core is occupied by a thick sill of metadiabase, the margins of which are strongly sheared. This leaves the nature of the formational contact open to question. It could be a conformable contact invaded by the sill, with the shearing at sill margins due to mechanical contrasts during complex, multi-stage regional deformation, or it could be a tectonic contact invaded by the sill that was later reactivated. 49 �Highly deformed, variably metamorphosed stratotectonic complexes of calcareous metasedimentary rocks, carbonate-silicate-sulfide iron-formation, and mafic metavolcanic rocks are the regional hosts for economic mineral deposits in several important mining camps, including the Homestake district in South Dakota (Caddey and others, 1991) and the Broken Hill and Mount Isa districts in Australia (Ashley and others, 1998; Painter and others, 1999). The mechanical contrasts among the various rock types in these complexes, together with the geochemical reactivity of the calcareous and iron-rich strata, provide fertile ground for mineral deposition from spatially associated hydrothermal systems. Clearly, the geological attributes revealed in the Hattenberger core are not of themselves meaningful indicators of mineral-deposit probability. However, those attributes taken in conjunction with mineralogical evidence of hydrothermal activity in the Cuyuna mining district (McSwiggen and others, 1995; Melcher and others, 1996) and the presence of barren massive sulfide deposits in the Glen Township area (Han, 1968) suggest that broad areas of east-central Minnesota are intrinsically prospective for a variety of ore-deposit types. Ashley, P.M., Lottermoser, B.G., and Westaway, J.M., 1998, Iron-formations and epigenetic ironstones in the Palaeoproterozoic Willyama Supergroup, Olary domain, South Australia: Mineralogy and Petrology, v. 64, p. 187-218. Boerboom, T.J., and Jirsa, M.A., 2001, Stratigraphy of the Paleoproterozoic Denham Formation—a continental margin assemblage of basalt, arkose, and dolomite [abs.]: Institute on Lake Superior Geology, 47th Annual Meeting, Proceedings, v. 47, Program and Abstracts, pt. 1, p. 6-7. Caddey, S.W., Bachman, R.L., Campbell, T.J., Reid, R.R., and Otto, R.P., 1991, The Homestake gold mine, an Early Proterozoic iron-formation-hosted gold deposit, Lawrence County, South Dakota: U.S. Geological Survey Bulletin 1857-J, p. J1-J67. Han, T.M., 1968, Ore mineral relations in the Cuyuna sulfide deposit, Minnesota: Mineralium Deposita, v. 3, no. 2, p. 109-134. McSwiggen, P.L., Morey, G.B., and Cleland, J.M., 1995, Iron-formation protolith and genesis, Cuyuna range, Minnesota: Minnesota Geological Survey Report of Investigations 45, 54 p. Melcher, F., Morey, G.B., McSwiggen, P.L., Cleland, J.M., and Brink, S.E., 1996, Hydrothermal systems in manganese-rich iron-formation of the Cuyuna North range, Minnesota: Geochemical and mineralogical study of the Gloria drill core: Minnesota Geological Survey Report of Investigations 46, 59 p. Morey, G.B., 1978, Lower and Middle Precambrian stratigraphic nomenclature for east-central Minnesota: Minnesota Geological Survey Report of Investigations 21, 52 p. Painter, M.G.M., Golding, S.D., Hannan, K.W., and Neudert, M.K., 1999, Sedimentologic, petrographic, and sulfur isotope constraints on fine-grained pyrite formation at Mount Isa Mine and environs, northwest Queensland, Australia: Economic Geology, v. 94, no. 6, p. 883-912. 50 �Petrographic Study of the Ottertail Pluton, Superior Province, Northwestern Ontario Sturm, C. L., Czeck, D. M and Fein, E., (Oberlin College Geology Department, Oberlin Ohio 44074. Claire.sturm@oberlin.edu) INTRODUCTION This study concentrates on the Ottertail Pluton at the Wabigoon-Quetico subprovince boundary in the western Superior Province near Mine Centre, Ontario. The Ottertail is one of the Algoman plutons dated at 2686 Ma (Davis et. al, 1989), which have been used to constrain the termination of deformation at the subprovince boundary (e. g. Davis et. al, 1989; Poulsen, 2000). Our goal is to study the mineralogy and microstructures of the pluton in thin section. At the macroscopic level the pluton seems largely undeformed because of the general lack of macroscopic deformation fabrics. This has led many researchers to interpret the Ottertail Pluton to be posttectonic. However, it has been shown that determining the relationship between pluton emplacement and deformation requires more detailed analysis (Paterson and Tobisch 1988). We are using petrographic analysis combined with a magnetic fabric study to better determine the relationship between the pluton emplacement and deformation at the Wabigoon-Quetico boundary. ANALYSIS Using optical petrography and a vibrating sample magnetometer (VSM), we determined the mineralogy and magnetic mineralogy of the pluton. In general, the pluton ranges from quartz monzonite to granite to granodiorite. Mineralogy consists of: quartz, plagioclase, microcline, microcline perthite, hornblende, myrmekite, and some zircon. There are also minor amounts of sericite, clinozoisite, magnetite (and other opaque minerals) in some portions of the pluton. The grain sizes vary. Many quartz crystals have undulose extinction and some have subgrains, which suggest slight deformation by dislocation processes. Minor metamorphism is indicated by sericite in plagioclase crystals, and microcline perthite. At the macroscopic level, the pluton seems largely undeformed, but our study of anisotropy of magnetic susceptibility (AMS) and microstructures show that there is some evidence for deformation. Davis, D. W., Poulsen, K. H., Kamo, S. L., 1989. New insights into Archean crustal development from geochronology in the Rainy Lake area, Superior Province, Canada. Journal of Geology 97, 379-398. Paterson, S. R., Tobisch, O. T., 1988. Using pluton ages to date regional deformations: problems with commonly used criteria. Geology 16, 1108-1111. Poulsen, K. H., 2000. Archean metallogeny of the Mine Centre - Fort Frances area. Ontario Geological Survey Report 266, 121. 51 �Internal structures within crustal structural slabs, Quetico-Wawa subprovince junction, Quetico Provincial Park, Ontario Woodard, H.H. (Department of Geology, Beloit College, Beloit, Wisconsin 53511) The rocks of the Wawa subprovince, adjacent to the Quetico-Wawa subprovince junction, in the region between Agnes Lake and McKenzie Lake in Quetico Provincial Park, Ontario can be grouped into two discrete structural slabs. Detailed mapping of the ductile borders of these slabs indicates that the slabs are approximately 2 km thick and probably form the limbs of major recumbent folds. The base of each slab is defined by a ductile structural discontinuity, possibly representing the lower limb of a nappe or thrust surface. The northeastern McKenzie Lake slab rests upon the underlying Agnes Lake slab. The rocks which make up these structural slabs are amphibolite-grade, migmatitic, mica-quartz-feldspar-garnet schists, amphibolite, and tonalitegranodiorite. All of these rocks contain major deformational structures which were formed before, during and after the emplacement of the enclosing crustal slabs. The earliest recognizable structures are found only in the tonalite-granodiorites. These structures are layers of centimeter-scale thickness, and most are only recognized on outcrop surfaces where the layers are accentuated by superficial weathering. These layers are probably caused by centimeter-scale shearing within the tonalites. Both field measurements and aerial photo interpretation demonstrate that these centermeter-scale layers occur in “structural packages” which often have thicknesses on a scale of hundreds of meters. Hundreds of strike and dip measurements made on these layers, in conjunction with interpretation of large-scale, detailed, aerial photographs, demonstrate that the layers are deformed into a complex series of folds with typical wave lengths ranging from one to two kilometers. Many of the folds appear overturned, but some are upright, and most plunge northeastward, with occasional reversal of plunge. Interlayered and infolded with the tonalites are schists and amphibolites, and primary structures in these meta-sedimentary and metavolcanic rocks sometimes allow determination of stratigraphic top. All of the internal layers and folds are cut and smeared by the last recognizable ductile deformation along the subprovince junction. The internally folded rocks are later broken into individual kilometer-scale structural blocks which are bounded by steeply dipping faults. These faults are rarely exposed in outcrop, but some cut and offset the Quetico-Wawa subprovince ductile junction. Thus, a brittle phase of deformation followed the last ductile deformation at the subprovince junction and the emplacement of the crustal slabs. These faults, which cut the internal folds of the McKenzie Lake crustal slab, appear unrelated to the Burntside Lake brittle fault zone. This fault has been traced in outcrop about 100 km northeastward from its type locality on Burntside Lake, Minnesota and is mapped as terminating against the Quetico-Wawa junction in the vicinity of McKenzie Lake. The internal faults typically lack the intense centimeter-scale brecciation and the intense hydrothermal oxidation so characteristic of the Burntside Lake fault zone. Further, the strike orientation of seventy-five mapped internal faults appear geometrically related to the ductile folding of the Quetico-Wawa junction (and crustal slab development) rather than to the northeast-striking Burntside Lake fault. 52 �Thus, the internal structures within the McKenzie Lake crustal slab initially appear to record intense centimeter-scale shearing within all the rocks. This shearing is followed by ductile folding and complete recrystallization and then by recumbent overfolding of the sheared and folded sequence to produce the crustal slab. Following slab emplacement, the rocks are broken into kilometer-scale fault blocks, the strikes of which suggest a relationship to a later stage of compression across the subprovince junction. The latest ductile deformation along the QueticoWawa subprovince junction cuts the crustal slabs, and all their rocks and internal structures. The Agnes Lake structural slab, which underlies the McKenzie Lake slab, contains a similar suite of rocks and has similar internal structures to the above-described McKenzie Lake slab. However, within the Agnes Lake slab the internal structures are dominated by a series of northeast-striking, gently northwest-dipping, ductile shear zones chiefly at the contacts with tonalite and schistose rock units. These ductile shears are themselves recumbently folded and overturned toward the southeast. Although fieldwork continues on the Agnes Lake slab, the current interpretation of these internal ductile shears is that they represent folded sections of an earlier Quetico-Wawa subprovince junction, which have been sheared and folded into the Agnes Lake slab. 53 �Impact of fire on the forest floor and mineral soils, Snowbank Lake, Minnesota Woodruff, L.G., (U.S. Geological Survey, Mounds View, MN, 55112 (woodruff@usgs.gov)), Cannon, W.F., and Dicken, C., (U.S. Geological Survey, Reston, VA 20192) We are investigating the geochemical effects of fire on the forest floor and mineral soils in an area of the Superior National Forest near Snowbank Lake that was burned in a fuel reduction prescribed fire by the USDA Forest Service on October 11, 2000. The forest around Snowbank Lake, in northern Minnesota, is a mixed stand of older-aged balsam fir, spruce, aspen, red pine and jack pine. Many trees, particularly mature balsam and spruce, were blown down by a severe storm in July 1999. The scheduled Snowbank Lake fire gave us the opportunity to establish 10 study sites in the proposed burn area. At each site we described and collected soils prior to the fire, immediately after the fire, and seven months after the fire to quantify physical and chemical changes in the forest floor as a result of the burning. Initial sampling was in July 2000. At each site, forest floor material (living moss, forest litter, and/or humic layers) and mineral soil horizons (A-, E-, B- and C-horizons as available) were collected for analysis. The thickness of each layer was measured, and representative samples were collected for density calculations. The fire burned much of the fine fuel at the surface and affected 8 of the 10 sample sites. On October 12, prior to any post-fire rainfall, we resampled the sites. Burn severity (a qualitative assessment of the heat pulse directed towards the ground during a fire) was estimated and where possible samples comparable to pre-burn samples were collected. The sites were resampled seven months later in May 2001, following a winter of moderate snow cover. The fuel load at the surface and the moisture content of the forest surface controlled the major impact of the fire on the forest floor. Fire severity was high (100% of organic forest floor material consumed, mineral soil exposed) at 4 sites, moderate at 1 site (some forest floor material burned, mineral soil not exposed), and light at 3 sites (surface material charred with minimal loss at the forest floor). Two sites were untouched by fire. One of the elements of interest in this study was mercury. Gaseous elemental mercury in the atmosphere is transferred into a forest environment by wet and dry deposition onto the forest floor, creating a repository of mercury that is bound to organic carbon compounds. Recent studies show that smoke plumes from forest fires carry substantial mercury, but the origin of that mercury was in question (Friedli et al., 2001). Our preburn sampling shows that much of the mercury at the forest surface is bound to organic material on the forest floor and in organic mineral soils. Severe fire can vaporize much of this mercury as the organic compounds are combusted. Based on pre-burn analyses and measured thickness and density of organic material and assuming that all mercury in the organic layer is emitted during high severity burns when the forest floor is burned down to the mineral soil, average mercury emissions from the Snowbank Lake burn were about 2 mg/m2 of burned surface. This estimate only takes into account mercury bound to organic material on the forest floor and does not include burned foliage or woody fuels. Mercury bound to organic material in A-horizon mineral soils was not lost, even at sites of high burn severity, indicating that the fire’s thermal pulse did not heat soils above the 357oC vaporization temperature of mercury. Because the footprint of an historic fire was apparent in our study of soil geochemistry in Isle Royale National Park (Woodruff and Cannon, 2001) we anticipated that exposure of mineral 54 �soils by fire would lead to loss of both mercury and carbon over time. Loss of organic matter should be accelerated by solar heating of previously shaded soils and by lower input rates of organic matter because of loss of forest canopy. Interestingly, analyses of A-horizon soils collected at Snowbank Lake in May 2001 show no carbon loss and marked increases in mercury rather than decreases (Figure 1A). The reasons for this increase are unknown, although we speculate that it may be the result of leaching of mercury from the overlying ash layer or possibly from enhanced sorption of mercury by exposed organic material in mineral soils. The fact that carbon does not increase concomitant with mercury suggests that physical incorporation of ash and adsorbed mercury is not the cause of higher mercury concentrations. Lead, which much like mercury has an atmospheric source and is enriched in the forest floor and organic mineral soils, is enriched in the uppermost mineral soil samples collected in October after the fire compared to preburn values (Figure 1B), but comparable soil samples collected the following May show a slight decrease in lead, although most values are still higher than July 2000 levels. The increase of lead in soils immediately following the fire may be the result of the concentration of residual lead remaining after the burning of forest vegetation. Lead sorbed onto organic material may not volatilize in the fire, but could collect on surface soils. Additional sampling is planned for May 2002 to monitor the evolving geochemistry of the forest surface. This research shows that there are both immediate and long-term impact from fire, including loss of volatile elements and profound element movement at the forest surface. Figure 1. Mercury and lead versus carbon in A-horizon soil samples from the area of the Snowbank Lake prescribed fire. Friedli, H., L. Radke and J. Lu, 2001, Mercury in smoke from biomass fires: Geophysical Research Letters, v. 28, p. 3223-3226. Woodruff, L.G. and Cannon, W.F., 2001, The effect of fire on mercury and carbon in forest soils: results from northern Michigan and Minnesota: Geological Society of America Abstracts with Programs, v.33, p. A186. 55 � https://digitalcollections.lakeheadu.ca/files/original/9e1527ceb6b24c1bb2230ab8c4120149.pdf 125fa414cc8aa8747725d63da098aa94 PDF Text Text INSTITUTE ON LAKE SUPERIOR GEOLOGY 48th Annual Meeting Proceedings Volume 48 Part 2 - Field Trip Guidebook Kenora, Ontario – May 12-16, 2002 �INSTITUTE ON LAKE SUPERIOR GEOLOGY 48th Annual Meeting May 12-16, 2002 Kenora, Ontario Hosted by: Peter Hinz and Richard C. Beard Co-Chairs Sponsored by the Ontario Geological Survey Proceedings Volume 48 Part 2 - Field Trip Guidebook (Compiled by Blackburn Geological Services) �48th Annual Meeting Institute on Lake Superior Geology Volume 48 contains the following parts: Part 1: Program and Abstracts Part 2: Field Trip Guidebook 1 - Tanco Rare-Element Pegmatite, Southeastern Manitoba 2 - Quaternary Geology of Southeastern Manitoba 3 - Structure and Sedimentology of the Seine Conglomerate, Mine Centre Area, Ontario 4 - Industrial Minerals and Paleozoic Geology of Southeastern Manitoba 5 - Separation Rapids Rare-Element Pegmatite Field, Ontario 6 - Geology of the Red Lake Camp, Ontario Reference to the material in this volume should follow the example below: Lichtblau, A. and Storey, C.C. 2002. Geology of the Red Lake Camp, Ontario: Institute on Lake Superior Geology Proceedings, 48th Annual Meeting, Kenora, Ontario, 2002, v. 48, Part 2, p. 121-138. Volume 48 is published by the Institute on Lake Superior Geology and distributed by the Institute Secretary-Treasurer: Mark Jirsa Minnesota Geological Survey 2642 University Avenue St. Paul, MN USA 55114-1057 (612) 627-4780 email: jirsa001@tc.umn.edu ILSG webstite http://www.ilsgeology.org/ ISSN 1042-9964 Cover Illustration: Geologists examining a dump at the Gold Hill mine, Kirkup Township, 13 km southeast of Kenora, in 1914. Between 1886 and 1893 this mine produced 1090 ounces of gold from 220 tons milled. Four shallow shafts were sunk to a combined depth of 258 feet: the headframe for one of them is seen in the picture. The winning of gold from narrow discontinuous quartz veins was typical of the numerous small-scale mines of the Kenora goldfields in the latter part of the 19th and the early 20th century. ii �CONTENTS Proceedings Volume 48 Part 2 – Field Trips Trip1: The Tanco Rare-Element Pegmatite, Southeastern Manitoba ....................................1 Leaders: Staff, Tantalum Mining Corporation of Canada, Ltd. Trip 2: Quaternary Geology of Southeastern Manitoba..........................................................23 Stop 1: Striated outcrop, West Hawk Lake ................................................................26 Stop 2: West Hawk Lake, Till Section .......................................................................27 Stop 3: West Hawk Lake, Meteorite Impact Structure ..............................................28 Stop 4: Sapping Channels (Upper Cambell Beach) ...................................................30 Stop 5: Upper Campbell Beach of Lake Agassiz .......................................................31 Stop 6: Interglacial Site at Grunthal...........................................................................32 Leaders: E. Nielsen, Manitoba Geological Survey G. Matile, Manitoba Geological Survey Trip 3: Structure and Sedimentology of the Seine Conglomerate, Mine Centre Area, Ontario ........................................................................................................................37 Stop 1: Basal facies, low deformation, Shoal Lake road............................................60 Stop 2: 2D view, sandy lenses, dextral shear, Forest Tour road ................................61 Stop 3: 3D view, moderate deformation, Horsecollar Junction, Hwy 11 ..................61 Stop 4: Ultra deformed conglomerate, Hwy 11 .........................................................61 Stop 5: Small fold, Seine River bridge, Hwy 11 ........................................................62 Leaders: D. Czeck, Oberlin College P. Fralick, Lakehead University Trip 4: Industrial Minerals and Paleozoic Geology of Southeastern Manitoba ....................69 Stop 1: Sungro horticultural shagnum peat bog and plant .........................................90 Stop 2: Cold Spring Granite dimension stone quarry and plant.................................90 Stop 3: Gillis Tyndall Stone quarry and plant............................................................91 Leaders: J. Bamburak, Manitoba Geological Survey R. Bezys, Manitoba Geological Survey iii �Trip 5: Separation Rapids Rare-Element Pegmatite Field, Ontario ......................................95 Stop 1: Big Mack pegmatite.....................................................................................107 Stop 2: Separation Rapids pluton .............................................................................109 Stop 3: Big Whopper pegmatite ...............................................................................110 Stop 4: Marko's pegmatite........................................................................................115 Stop 5: James' pegmatite ..........................................................................................116 Leaders: C. Blackburn, Blackburn Geological Services D. Bubar, C. Pedersen, K. Rees, Avalon Ventures Ltd. C. Galeschuk, Tantalum Mining Corporation of Canada, Ltd. A. Mowatt, Emerald Fields Resource Corp. T. Pryslak, A..P. Pryslak Geological Services Trip 6: Geology of the Red Lake Camp, Ontario ...................................................................121 Stop 1: Meso-neoarchean contact, Woodland Cemetery road .................................130 Stop 2: Calcite carbonatized pillowed flows, Sandy Bay road ................................130 Stop 3: Cofederation/Balmer assemblages contact, Suffel Lake road .....................130 Stop 4: Madsen deposit, power line outcrops ..........................................................131 Stop 5: Buffalo deposit.............................................................................................133 Stop 6: Howey mine .................................................................................................134 Stop 7: Howey Bay-Flat Lake deformation zone.....................................................134 Stop 8: Redcon carbonate zone, Nungesser road .....................................................135 Leaders: A. Lichtblau, Ontario Geological Survey C. Storey, Ontario Geological Survey Staff, Goldcorp Inc. - Red Lake Mine Staff, Placer Dome North America - Campbell Mine iv �Field Trip 1 The Tanco Rare-Element Pegmatite, Southeastern Manitoba Peter Vanstone Chief Geologist Steven Young Mill Superintendent Roland Simard Mine Superintendent Carey Galeschuk Project Geologist Alistair Gibb Chemical Plant Superintendent Tantalum Mining Corporation of Canada Limited P.O. Box 2000 Lac du Bonnet, Manitoba R0E 1A0 "Giraffe" underground at the Tanco mine. �INTRODUCTION Pegmatites throughout the world range in age from late Archean (2,500-2,800 million years) to Miocene (5-23 million years) (Cerny 1989a). Within Canada, there are noticeable concentrations of rare-element pegmatites associated with the following orogenic events: 1. the Kenoran Orogeny (2,750-2,550 million years) in the Archean Superior Province; 2. the Hudsonian Orogeny (1,800 – 1,600 million years) in the Churchill Province; and, 3. the Grenville Orogeny (1,200 – 900 million years) in the Grenville Province. The pegmatite being commercially exploited by Tantalum Mining Corporation of Canada Limited (Tanco) is an example of an extremely fractionated, pollucite-bearing pegmatite which was emplaced during the Kenoran Orogeny. The Tanco pegmatite is located at Bernic Lake in the Canadian Shield of southeastern Manitoba, approximately 180 kilometres by paved and all-weather gravel road northeast of Winnipeg (Figure 1). The nearest communities, Lac du Bonnet and Pinawa, are located approximately 60 kilometres and 75 kilometres, respectively, from the minesite. Tanco has been a significant producer of tantalum concentrates, ceramic-grade spodumene concentrates, pollucite and other materials since the late 1960’s. More recently cesium N (icY ii Winnipeg IdIlUVI du Bois Lac PiIqIM if I Seauj ic o 20 ——— 10 Figure 1. Location of Tanco 2 20 30 40 40 30 50 �chemicals have been produced at the minesite. During this time, the pegmatite has been the subject of numerous studies because of its very limited low temperature alteration, lack of postemplacement, structural deformation and its absence of weathering effects. The geographic location of the pegmatite and the willingness of Tanco to allow access to its extensive diamond drill core library, as well as, the underground workings have also been contributing factors. For more information on the Tanco pegmatite and pegmatites in general, the reader is referred to the Canadian Mineralogist issues by Berry (1972), Cerny (1982), Martin and Cerny (1992), and Anderson et al (1998). Also, Brown and Ewing (1986) edited an issue of the American Mineralogist focused on pegmatites and granitic rocks, and Moller et al (1989) edited the proceedings of a workshop on the lanthanides, tantalum and niobium. Brisbin (1986) discusses pegmatite intrusion mechanisms and Ercit (1986) discusses tantalum mineralogy. An overview of the Tanco pegmatite and the mining/milling operations was published by Crouse, et al (1979) and Thomas (1984) completed a fluid inclusion study of the Tanco pegmatite. Tanco is 100% owned by Cabot Corporation of Boston, Massachusetts and is operated by the Cabot Specialty Fluids division headquartered in Houston, Texas. TANCO HISTORY In 1928, Jack Nutt Mines staked and explored the Bernic Lake area pegmatites for tin. During the following two years, shaft sinking began and a small tin concentrator was established on what is now the Tanco minesite. Feed for this mill came from the nearby exposed pegmatites. At this same time, a four hole drill program was underway to explore the pegmatites at depth. It was during this program that the Tanco pegmatite was intersected in Hole #3. Work on the property continued through 1930, but poor economic conditions forced the company to abandon the property. The claims subsequently reverted back to the Crown. In 1955, Montgary Petroleum Corporation Limited acquired the property and completed an extensive surface drill program. Over the next couple of years, a power transmission line from Pointe du Bois and a mine access road were constructed. Also, the sinking of a threecompartment shaft began and some surface facilities were built. In 1957, American Metals Company, Limited optioned the property from Montgary and completed a surface drill program. It was during this work that the internal zonation of the pegmatite was recognized and documented (Hutchinson 1959), and pollucite was identified. During 1959 and 1960, Chemalloy Minerals Limited (formerly Montgary) completed both surface and underground drill programs, and extracted small quantities of pollucite and quartz. In 1961, the mine was placed on care and maintenance and then allowed to flood in 1962. In 1966, Chemalloy started to evaluate the tantalum potential of the pegmatite. Extensive diamond drilling was carried out from surface and underground over the subsequent three years. The initial result of this activity was the formation in 1967, of Tantalum Mining Corporation of Canada Limited (Tanco), a joint venture between Chemalloy and Northern Goldfield Limited. In 1969, construction of a 500 ton per day tantalum gravity concentrator was completed and Tanco began commercial production. Production of ceramic grade spodumene concentrates began on a pilot scale in 1984, and went commercial in 1986 when the new spodumene concentrator was completed. Although the lithium potential of the pegmatite had been investigated over the years for the production of 3 �ceramic grade spodumene concentrates and lithium carbonate, none of these investigations proceeded beyond the feasibility stage. The Tanco joint venture remained in place until 1993 when Cabot Corporation acquired 100% of the operation. Up to 1993, different companies, in addition to the original two companies, were involved in the joint venture. These include: Manitoba Development Corporation (1972-1993), Kawecki Berylco Industries/Cabot (1974-1993) and Hudson Bay Mining and Smelting Co. Ltd. (1978-1993). In 1996, Cabot formed the Cabot Specialty Fluids division and started construction of the cesium brine plant. In 2001, the plant was expanded to allow for the manufacture of conventional cesium chemicals. RARE-ELEMENT PEGMATITE FORMATION Rare-element pegmatites, like the Tanco pegmatite, occur in synclinoria of metavolcanicmetasedimentary sequences that separate granitoid batholiths from gneissic tonalities. The pegmatites evolve from late orogenic, peraluminous (A/[C+N+K]>1), S-type granites. The resultant pegmatite fields are situated on the lower portions of relatively steep geothermal gradients (±40°-50°C/km.). The complex type pegmatites are commonly emplaced in low pressure/high temperature facies (upper greenschist to lower amphibolite) metamorphic terrains with emplacement generally at a depth of four to six kilometers (Cerny 1989a). The parental, fertile granite is late- to post-tectonic and post-dates the peak of regional metamorphism. These granites are leucocratic and although commonly equigranular, may be porphyritic. At depth, they are biotite bearing and grade upward or laterally into a two-mica granite or a muscovite granite which is capped by a coarse grained to pegmatitic, megacrystic Kfeldspar, graphic leucogranite (Cerny and Meintzer 1988). This pegmatitic granite stage is an integral step in the formation of rare-element pegmatites. The pegmatitic melt forms within the parental, fertile granite through the process of magmatic differentiation. This melt collects in the upper portion of the fertile granite pluton, with the volatiles and other liquidus-depressing constituents such as H2O, F, Li, B and P increasing outward from the parental granite. These constituents, plus even small amounts of exsolved supercritical fluid, reduce the viscosity of the melt. The lower the liquidus temperature and the less viscous the melt, the more mobile the melt becomes and the further out it will migrate. Melt migration occurs when there is sufficient internal pressure and the magma reservoir is tapped by a tectonic disturbance of the outermost solidified shell of the pegmatitic granite. Pegmatite groups form a regional zonation around the parental granite with the complexity of the pegmatites increasing away from the parental granite (Cerny 1991b) (Figure 2). Rare-Element Pegmatite Classification Pegmatites can be divided into two types, the lithium-cesium-tantalum (LCT) type and the niobium-yttrium-fluorine (NYF) type. Over the years, the LCT type pegmatites have been well studied because of their economic significance. These pegmatites have been an economic source of lithium, tantalum, tin, cesium and rubidium minerals with mica, quartz and feldspathic sand by-products. The LCT type of pegmatites can be subdivided into classes based on mineralogy/chemistry and complexity (Table 1). 4 �I I I I / / / / / / / / / 1 / / ' Li,Be,Ta,Sn (Rb,Cs) I \ . Figure 2. Schematic section of a zoned fertile granite-pegmatite system. 1. fertile granite; 2. pegmatitic granite; 3. barren to beryl bearing pegmatites; 4. beryl-type, columbite- to phosphate-bearing pegmatites; 5. complex spodumene (or petalite) bearing pegmatites with Sn, Ta, ±Cs; 6. faults. (modified from Cerny 1989b) LCT type pegmatites have a number of chemical characteristics that distinguish them from the NYF type pegmatites. Some of these features include the following: • the tantalum content exceeds the niobium content; • the tin content can equal tantalum content; • they contain low levels of the light and heavy rare earths; • the pegmatites are enriched in boron and alkali elements; and, • they have low levels of uranium and thorium. Mineralogical characteristics can also be used to distinguish between LCT type and NYF type pegmatites. Some of the characteristics of the LCT type pegmatites include: • a general absence of fluorite (fluorine is tied up in minerals such as topaz, lepidolite, amblygonite); • a common occurrence of lithium and phosphate minerals; • the presence of tourmaline; 5 �• • simpler oxides of tantalum and niobium ±tin with essentially no rare earth element (REE) content; and, beryl can be present. The Tanco pegmatite is a good example of the complex type-petalite subtype of LCT pegmatite and is probably the most studied pegmatite of its type. Pegmatite Types Subtypes Characteristics Examples Beryl (i) (ii) beryl-columbite beryl-columbite-phosphate • relatively simple Greer Lake, MB PEG Group, NWT Complex (iii) spodumene petalite complex internal zonation lithium rich diverse mineralogy primary crystallization is greater than secondary replacement bodies Hugo, SD; Harding, NM (iv) • • • • Complex (v) amblygonite Lepidolite AlbiteSpodumene Albite • high fluorine activity Tanco, MB; Bikita, ZM, Peerless, SD Brown Derby #1, CO Kings Mountain, NC • least common • generally small Hengshan Field, P.R.C. Table 1. Classification of LCT type pegmatites. (modified from Cerny 1991a) GEOLOGIC SETTING The Bernic Lake pegmatite group, of which the Tanco pegmatite is a member, is one of a number of such groups comprising the Winnipeg River Pegmatite Field located in the Archean Bird River Greenstone Belt of the western Superior Province in the Canadian Shield (Figure 3). The Bird River Greenstone Belt is bounded on the north by the English River Subprovince, a belt of highly metamorphosed metasediments and metavolcanics rocks, and mafic to felsic batholiths and plutons (Beakhouse 1991a). To the south, this belt is bounded by the pluton-dominant Winnipeg River Subprovince (Beakhouse 1991b). The Bird River Greenstone Belt is comprised of six formations of the Rice Lake Group. In general terms, the belt consists of mafic to felsic metavolcanic and derived metasedimentary rocks all of which have been intruded by synvolcanic to late tectonic mafic to felsic intrusives. Of these formations, the Eaglenest Formation is the oldest and the Booster Lake Formation is the youngest (Trueman 1980). The six formations are listed below in chronological order. 1) Booster Lake Formation: metapelite and metagreywacke - unconformity 2) Flanders Lake Formation: lithic meta-arenites and metaconglomerate - unconformity 3) Bernic Lake Formation: felsic to mafic metavolcanic and metasedimentary units, felsic and mafic intrusive with porphyry units 6 �- unconformity4) Peterson Creek Formation: metarhyolites and clastic components 5) Lamprey Falls Formation: metabasalts, Bird River Sill, metagabbro 6) Eaglenest Lake Formation: metamorphosed volcanic wacke IVIdflROUd / / / JIILdFIU Sachigo Subprovince Bird River greenstclne belt Berens River SubDrovince --- 250 kflometres _________ Superior Province Southern Province and Nipigon Plate Phanerozoic basin sequence .___— Subprovince boundary Figure 3. Regional geological setting of the Tanco pegmatite (modified from Beakhouse 1991b). 7 �Trueman (1980) defined four major structural events in the area. The first two events were episodes of east-west folding, the second event being associated with the emplacement of large regional batholiths. These events led to associated prograde, low pressure/high temperature metamorphism (Abukuma type) in the Bernic Lake Formation. The third event was major eastwest faulting, with associated retrograde metamorphism. The last major event was a second faulting episode that propagated a series of northwest trending faults. With this event there was localized retrograde metamorphism. Overall the metamorphic grade throughout the Bernic Lake Formation is upper greenschist to lower amphibolite facies (Cerny, et al 1981). Numerous synvolcanic intrusive units intrude the Bird River Greenstone Belt. They range in size from a kilometre up to about 25 kilometres and generally display an elongated east-west shape. The compositions of these intrusives vary from body to body and even within an individual intrusive, and include gabbro, anorthositic gabbro, diorite, quartz monzonite, granodiorite and granite. Quartz and quartz-feldspar porphyries are also found throughout the belt. The Bernic Lake Formation, which is in fault contact with the Booster Lake Formation to the north and the Peterson Creek Formation to the south, consists of a complex array of layers of metamorphosed basalt, andesite, dacite, rhyolite, iron formation, conglomerates, volcanic wackes and sandstone. Lateral continuity is not common among most of the rock types and is only persistent in the mafic to intermediate metavolcanics and, in part, the iron formations. Other volcanic units in the area seem to be composed of flows of limited lateral extent. Syn- to post-tectonic emplacement of granite and pegmatitic granite stocks throughout the Bird River Greenstone Belt provided the source for a number of rare-element pegmatite groups found within the belt. Of all the pegmatites identified in the area, the largest and most economically significant ones occur within the Bernic Lake pegmatite group situated within the Bernic Lake Formation. The major pegmatites of this group include the Tanco, Dibs, Buck, Coe, and Pegli. All of these pegmatites have an east-west elongation, are horizontal to sub-horizontal in orientation and are hosted by either mafic intrusives or associated mafic metavolcanic units. TANCO PEGMATITE GEOLOGY The Tanco pegmatite, situated at the western end of Bernic Lake, is an extremely fractionated, rare-metal, complex type-petalite subtype, LCT pegmatite and is hosted by a late-stage, subvolcanic, metagrabbro (Tanco amphibolite) intrusive. The age of the Tanco pegmatite is 2,650 – 2,550 million years (Cerny 1989a). The pegmatite is completely blind or buried, and only sub-crops in a limited area in the bottom of Bernic Lake. Based on hundreds of diamond drill hole intercepts, the pegmatite has a maximum length of approximately 1,990 metres and a maximum width of 1,060 metres (Figure 4), and is up to 100 metres thick. The total tonnage of the pegmatite has been calculated to be approximately 25 million tonnes (Stilling, 1998). Emplacement is hypothesized to be within the pressure shadow of an easterly trending, dual plunging, anticlinal axis (Cerny, et al 1981). More recent data has given rise to the possibility that dilation may have been aided by a number of pre-existing faults that could have freed up the overlying block of metagabbro, thus allowing the intruding pegmatitic fluid to “hydraulically” lift the overlying host rock. Initial intrusion of this fluid appears to be into a sub-horizontal joint 8 �set that is ubiquitous throughout the Bernic Lake area. The bi-lobate shape of the pegmatite may also be influenced by a possible sinistral offset of the host anticline. OS 1.0 0.6 Tanco Mineral OS 1.0 NI kIIon,41n LI I cd7a-'----T—--—7 I - C' -' 7)-i 0 LI '—-' L7 f Lake Figure 4. Plan view of the Tanco Pegmatite Pegmatite Zonation Internally, the pegmatite is composed of nine discrete mineralogical zones with the different ores of economic interest – those of tantalum, spodumene, cesium and rubidium – each essentially occurring in different zones. Characteristic textures and mineralogical assemblages distinguish each zone. The pegmatite is the host to approximately 100 different minerals (Cerny, et al 1998). A coloured longitudinal section of the Tanco pegmatite is appended and a brief description of each of the internal zones is given below. A more complete description of the pegmatite zonation and mineralogy can be found in Cerny, et al (1998). Of the nine zones comprising the pegmatite, only the Border and Wall Zones occur as concentric shells enveloping the entire pegmatite. When combined, however, the Lower and Upper Intermediate Zones also form a concentric shell within the pegmatite. The Central Intermediate and Lepidolite Zones have both been subjected to late stage micaceous alteration. Unlike the stereotypical zoned pegmatite, the location of the Quartz Zone or core is in the upper portion of the dike for most of the mine and in its more traditional, central location only in the western portion. Border Zone (Zone 10) This is a very thin zone (a few to 30 centimetres) that envelops the pegmatite and is composed of fine-grained, saccharoidal assemblage of albite and quartz with lesser to rare tourmaline, apatite, biotite, beryl and triphylite. In places it may have a layered appearance. 9 �Wall Zone (Zone 20) The Wall Zone consists of very coarse grained, brick-red perthite, quartz, fine to coarse-grained tourmaline, albite, brown to greenish muscovite books and accessory white beryl. In general, the hanging-wall Wall Zone is thinner and coarser grained than the footwall Wall Zone. Increased albite (cleavelandite) content and bands of “footwall” aplite characterize this latter Wall Zone. The tin content of the zone generally exceeds the tantalum content. Aplitic Albite Zone (Zone 30) This zone is one of the main tantalum ore zones and is most prominent in the eastern portion of the pegmatite. The dominant mineral is a pale blue to white, fine-grained, saccharoidal albite. Quartz is a common constituent with subordinate to rare minerals including muscovite, Taoxides (predominantly wodginite), beryl and apatite. Texturally, undulating layers of the saccharoidal albite distinguish the zone. Where the Aplitic Albite Zone is in contact with the Quartz Zone, the contact is generally characterized by increased tantalum along the contact and within the albite in very close proximity to the contact (“nugget effect”). In some places, white beryl crystals that have grown off the albite into the quartz mark this contact. Lower Intermediate Zone (Zone 40) The locally known Mixed Zone is characterized by both its diversity of minerals and grain size. The mineral assemblage that distinguishes this zone is comprised of coarser grained microclineperthite and SQUI pseudomorphs (Spodumene-QUartz Intergrowth after primary petalite) in a finer grained matrix of albite, quartz and micas. Assemblages consisting of quartz pods containing amblygonite and/or spodumene, and radial rims of cleavelandite and lithianmuscovite around feldspar rich assemblages are less common. Common subordinate to rare minerals include lithian-muscovite, lithiophilite, lepidolite, petalite and Ta-oxides. The generally gradational contacts with the tantalum and spodumene zones allow for the selective mining of this zone for both tantalum and spodumene, but only under strict grade control. Upper Intermediate Zone (Zone 50) The Spodumene Zone as it is referred to, has evolved from the Lower Intermediate Zone and is comprised of very coarse-grained microcline-perthite and SQUI with lesser spodumene blades, quartz and amblygonite. Subordinate to rare mineralogy consists of pollucite, lithiophilite, albite, lithian-muscovite, petalite, eucryptite and Ta-oxides (predominantly tantalite). The SQUI, which is an oriented intergrowth of cogenetic spodumene and quartz, has resulted from the isochemical breakdown of the primary petalite. This process occurs under decreasing pressure conditions during the cooling of the intrusion (London (1986). This zone displays the largest crystals in the pegmatite with the petalite pseudomorphs (SQUI) attaining lengths up to approximately seven metres and the microcline-perthite reaching up to ten metres in length. Central Intermediate Zone (Zone 60): The MQM (Muscovite and Quartz after Microcline) Zone is another of the main tantalum ore zones. The zone is comprised of microcline-perthite, quartz, albite and muscovite with 10 �subordinate to rare beryl, Ta-oxides (predominantly wodginite), spodumene, sulphides, and apatite. The minerals are medium to coarse grained. Quartz Zone (Zone 70) This is a massive, monomineralic zone with accessory spodumene (SQUI) and amblygonite. When in contact with the tantalum zones, the contact is commonly characterized by increased tantalum concentration (“nugget effect”) Pollucite Zone (Zone 80) The Pollucite Zone is a sub-zone of the Upper Intermediate Zone with a gradational contact occurring between the two, and consists of a monomineralic core of pollucite enveloped by an assemblage of SQUI, microcline-perthite, amblygonite, petalite and interstitial pollucite. Polygonal fracturing with lithian-muscovite and/or quartz filling is not uncommon within the core. Accessory minerals include apatite and albite. This is the zone from which the cesium ore is mined. Lepidolite Zone (Zone 90) This zone forms two flat lying, east-west elongated sheets within, (at least in part) the Central Intermediate Zone and in contact with the Upper Intermediate Zone. The mineral assemblage consists of fine-grained, purple lithian-muscovite and lepidolite with lesser microcline-perthite, and subordinate to accessory albite, quartz, beryl and Ta-oxides (predominantly microlite). The zone is mined for tantalum and has been mined, on a limited scale, for rubidium. MINING The heart of the Tanco pegmatite is situated some 60 metres (~200 feet) below Bernic Lake, and is accessible from surface either via a shaft or via a 400 metre (~1,300 foot), 20 percent decline. Mining is carried out using the “room and pillar” method. The mine’s shallow depth contributes to lower inherent ground stresses and generally stable ground conditions. After weighing these factors, and considering the diverse mineralogy encountered at Tanco, it was decided that the Room and Pillar mining method would provide the optimum approach for economic extraction at Tanco. The first pillar design saw pillars 16 metres square (50 ft. x 50 ft.), with mining rooms also at 16 metres wide. As mining progressed over the years, ongoing rock mechanics studies showed that the rooms could be increased to 22 metres or 72 feet, without excessively loading the pillars. Pillar reduction has now been done successfully throughout the mine and continues as an integral part of the mining plan. Two-boom hydraulic jumbos perform all drilling for drifts, slashes, benches and arches. During the initial top slice development, the roof is carefully arched, utilizing smooth blasting techniques. The roof arches allow residual ground stresses to be redirected to the post pillars. Ground stress in the Tanco mine is considered low, relative to other hard rock mines, and as such, rock bolting is rarely required. At Tanco, the roof of mature mine workings may often average 20 metres (~65 feet) above the working levels below, and in places, may reach 30 metres (~95 feet). These high backs are 11 �carefully monitored throughout mining operations, utilizing custom designed aerial lift devices (referred to as Giraffes). Where suited, mining is carried out, utilizing a single boom Simba longhole drill. In particular, the longhole method has been the primary approach to pillar reduction. The broken ore is transported utilizing 5 yd3, 6 yd3 and 7 yd3, load-haul-dumps (LHD’s) – mobile, front-end loader units - and a 20 ton truck to various ore-passes, which are located throughout the mine. The ore is broken on grizzlies (metal grates at the top of the ore pass), utilizing either mobile or stationary hydraulic rock breakers. The ore is then passed to an underlying tramming level where it is transported to the shaft by a train of 4 ton, Granby style, side dump ore cars, and hoisted to surface coarse ore bins via 4 ton Kimberly style skips. Tantalum and spodumene ores are stored in one of two loading pockets and skipped on a daily basis up the two-compartment shaft, into dedicated surface coarse ore bins. The mine however, must produce and provide three distinct ores to the mill. To overcome the limitation of the system, one loading pocket and associated coarse ore bin is emptied weekly and an appropriate tonnage of pollucite ore is batched through. Mine ventilation air is downcast from surface through one of two vent raises, one being, in part, the Jack Nutt shaft from 1929/30 and the other, a 1.8-m (6 foot) diameter bore-hole raise. The exhaust mine air up-casts through the access decline. Total fresh air volume exceeds 5300 m3 per minute (190,000 ft3 per minute) and is appropriate for the operation of Tanco’s fleet of diesel mining equipment. A fleet of personnel carriers and service trucks supports mining operations. Tanco maintains all of its mine equipment at its own on-site facilities. MINERAL PROCESSING Due to land constraints, the concentrator is constructed on a peninsula formed by two bays on Bernic Lake. The building is multi-floored, with equipment on a total of six levels. The major items of concentration equipment are on two levels, with feed preparation equipment, filters and driers, on the upper levels, with pumps on the lower levels. The first stage of processing, common to all four mineral products, is crushing, where the coarse ore from underground (-300 mm in size) is broken down to –12 mm. in size. The tantalum, spodumene and pollucite ores are crushed into separate fine-ore, storage bins. The new dry grinding plant supplies ground pollucite for the cesium formate plant. Different processes concentrate each ore. Tantalum is processed by gravity concentration, a process that makes use of the fact that tantalum minerals are much heavier than the waste minerals. Spodumene, on the other hand, is primarily processed by flotation, which makes use of the different physical and chemical characteristics of the surfaces of the various minerals. Pollucite is ground and then subjected to acid leaching and other chemical processing to produce cesium chemicals. Tantalum Processing (Figure 5.) There are three main elements in the gravity concentration of Tanco’s minerals: liberation of the values from the gangue or waste rock; feed preparation of the ground product into different size fractions; and concentration of the different fractions. At Tanco, the plant is split effectively into four fractions – grinding/spiral circuit, coarse sand circuit, fine sand circuit and slime circuit. 12 �Fine ore is first ground to pass 2 mm. The –2 mm. product passes to the spirals, which recover the coarse, free, tantalum minerals, which may otherwise have been ground too fine for effective recovery. The spiral tailing is sized at 0.30 mm. by a Linatex hydrosizer with the underflow recirculating to the main grinding mill. Figure 5. Tanco’s tantalum gravity separation flowsheet. Effective feed preparation is essential for satisfactory separation on shaking tables, and this is carried out with cyclones, followed by Bartles-Stokes hydrosizers. The hydrosizers contain four spigots and an overflow. The spigot products, or sand fractions, are distributed to further banks of spirals. These spirals each produce a low-grade concentrate, a recirculated middling, and a 13 �tailings product. Falcon concentrators scavenge the fine sand tailings products. This centrifugal separator is one of the newest concentration devices, confirming Tanco’s commitment to “leading edge technology” in the pursuit of performance. Rougher concentrates from all sections are collected in a storage tank from which the cleaner section is fed at constant flowrate and density. Classification in cyclones and a hydrosizer sizes feed to four cleaner tables, which produce a fine, 35% Ta2O5 concentrate, a recirculated middling, and a tailing. Overflows from the various cylcones along with the Stokes hydrosizer overflow constitute the feed to the ultrafines circuit. These are thickened in another bank of cyclones and treated on a Mozley MultiGravity Separator (MGS). The MGS produces a rougher concentrate, upgraded on Bartles CrossBelts. Overall recovery of tantalum ranges from 69-72%. During the summer months accumulated tailings can be processed along with the ore; the same flowsheet being used. Recovery from the tailing portion of the feed is of the order of 30%, upgrading the feed from 0.05% to 30% Ta2O5. The specifications of a typical tantalum concentrate produced at Tanco is given in Table 2. Element Typical Concentrate (wt. %) Ta2O5 35% - 38% SnO2 14% - 18% Nb2O5 5% - 8% TiO2 2% - 4% Table 2. Typical tantalum concentrate Tantalum Markets Tanco’s tantalum concentrates are shipped to the Cabot Performance Materials facility in Boyertown, Pennsylvania for conversion to the metal or tantalum compounds. The major uses for tantalum are in the electronics industry and for cutting tools. High quality capacitors are the major single use for tantalum. Europe is the major consumer of tantalum carbide used in production of hardmetal alloys for cutting tools. Other tantalum alloys are important constituents of aero engines, and for acid resistant pipes and tanks used in the chemical industry. One minor but important use of tantalum is in the medical industry, for “spare-part” surgery – tantalum “pins” are used for such areas as hip-joint replacements, as it is the only metal that is not rejected by body fluids. Spodumene Processing (Figure 6) After crushing to –12 mm., the heavy medium Triflo circuit rejects the feldspar from the –12 mm. +0.5 mm. range. Ferrosilicon and magnetite as a 70:30 mixture are used with a feed density of 2.74 kg/l. and effective density of separation of 2.65 kg/l. The –0.5 mm. fraction continues to the grinding circuit. The sink product and the –0.5 mm. fraction are ground in closed circuit with a 2 mm. primary screen and a Linatex hydrosizer with an approximate cut point of 150 micron. Rougher and cleaner spirals recover coarse free tantalum within the grinding circuit. A 5 foot, low intensity drum magnet removes ground steel produced during the grinding process. The grinding circuit product is scavenged for tantalum by two Falcon concentrators. Tantalum from the Falcon concentrators is upgraded on a Double Deck Holman Table with the tailings and middlings returning to the grinding circuit. Coarse tantalum from the cleaner spiral is also 14 �upgraded on a single Holman Table. The tantalum recovered from the spodumene circuit is a valuable by-product. Figure 6. Schematic flowsheet of the spodumene circuit Prior to the amblygonite flotation stage, in order to control phosphate levels, the pulp is deslimed by single stage cycloning. Due to the nugget-like appearance of the amblygonite, close control of this flotation stage must be maintained. Starvation quantities of collector are used based on 15 �feed tonnage and previous tails assays. Starch is used as a depressant for spodumene at pH 9.2. A spodumene-phosphate by-product called Montebrasite is produced to meet market requirements. This concentrate is subjected to wet high intensity magnetic separation to remove weakly magnetic iron materials. This concentrate is pumped to a belt filter and propane fired rotary drier. The dried concentrate goes to a storage bin prior to bagging or bulk shipping to meet the customers’ requirements. Mica is then removed with a single flotation stage. This step assists in the removal of K2O from the final concentrate product. The mica flotation tailings are two stage cycloned to remove starch. Two conditioning stages for automatic pH control and collector addition are carried out prior to rougher flotation. The rougher concentrate goes on to two or three stages of cleaning to The Wet High Intensity Magnetic Separator (WHIMS) non-magnetic fraction is thickened by two stages of cycloning and stored in a holding tank prior to tonnage controlled feeding to the belt filter and propane fired rotary drier. Final product handling is carried out utilizing air slides and dense phase pneumatic pumping to storage bins. The final product can be shipped to the customer in 25 kg, 1,000-kg bags, or bulk, via road or rail. The concentrate is sold to markets worldwide. Water used within the circuits is either fresh (from Bernic Lake) or re-cycled pond overflow depending on the section of the plant. Spodumene Product Specifications Clients can accept different levels of impurities, depending on their specific use of the material. Specifications of Tanco’s 7.25%, -200 Mesh, 6.8%, and Spodulite grade concentrates are shown in Table 3. Spodumene Markets Customers specify tight impurity levels for the use of spodumene concentrates in the glass and ceramics industries, and the process for the production of these concentrates is based on removal of contaminant minerals. Spodumene can be used either as a feedstock for the production of lithium carbonate and metal, or directly, in its mineral form, in the glass and ceramics industries. Since the development of the “salars” in the USA and Chile, most lithium carbonate is recovered from these sources, and little spodumene is now used for chemical production. Lithia is a very powerful flux, especially when used in conjunction with potash and soda feldspars. In ceramics, lithium lowers thermal expansion and decreases the firing temperature. Glasses containing lithia are much more fluid in the molten state than those containing proportionate amounts of sodium or potassium. Lower viscosity and faster melting can be utilized to improve glass quality in terms of fewer defects such an unmelted or partially melted raw material grains, and more rapid removal of small bubbles. Lower viscosity can permit the glassmaker to run a forming machinery at a higher rate, or create more elaborate products such as some perfume bottles. In frits and glazes, lithia is used to reduce the viscosity and thereby increase the fluidity of the coatings. This reduces maturing times and lowers firing temperatures. Small amounts of lithia also increase gloss. 16 �Element/Sizing 7.25% Grade -200 Mesh 6.8% Mesh Spodulite Li2O 7.25 ± -0.1% 7.10% ± -0.2% 6.80% min. 5.00% min. 0.15% max. 0.08% max. 0.10% max. 0.06% ± -0.01% Fe2O3 Na2O 0.35% max. 0.30% max. 0.45% max. 0.75% max. K2O 0.30% max. 0.60% max. 0.40% max. 0.75% max. P2O5 0.27% max. 0.40% 0.27% max. 0.20% max. MnO2 0.04% max. 0.06% max. 0.04% max. 0.05% max. Al2O3 24.0% min. 25.0% min. 23.0% min. 20.0% typ. Tyler 20 Mesh 0.0% max. Tyler 28 Mesh Trace max. Tyler 48 Mesh 1.0% max. Tyler 200 Mesh 50.0% min. 10.0% max. 55.0% min. 80.0% typ. Table 3. Tanco’s spodumene products specifications. CESIUM FORMATE Cesium formate is a clear, water soluble fluid with a specific gravity of 2.3 g/cm3 (i.e. it is two and one third times heavier than water) and a viscosity similar to water. It is used in the oil drilling industry as a drilling fluid, where the properties of low viscosity, high specific gravity and complete solution confer significant benefits over traditional mud (bentonite) based drilling fluids in deep wells greater than 4,575 metres (15,000 ft.). Use of cesium formate eliminates formation damage, particularly skin formation while drilling through the reservoir (oil bearing) rock. This results in improved hydrocarbon flows to the well giving better daily production from the well, in addition to enhanced recoveries from the reservoir in the long term - that is more hydrocarbons may be extracted from the well before well stimulation techniques become necessary. From an occupational health and safety perspective, there are considerable benefits to the use of cesium formate. The pH is between 10 - 11, and skin contact, although undesirable, has no immediate consequences. Low mammalian toxicity is an added benefit. The low environmental toxicity of cesium formate makes it the fluid of choice in areas where environmental sensitivities are particularly acute. Cesium Formate Plant The cesium formate pilot plant was designed, built and commissioned in 1996/97 in response to a potential market for formate brines. The focus of plant production was aimed at the oil and gas industries’ demand for a high-density, solids free drilling fluids. The plant was designed to 17 �readily incorporate process changes and modifications enabling it to produce a wide variety of cesium-based products, thus allowing Tanco and Cabot to rapidly respond to these future markets. The original plant was designed to produce 500 barrels/month of 2.3 g/cm3 specific gravity cesium formate. In 1999, expansion of the plant allowed for the production of 700 bbl/month. In 2001, the plant underwent a further expansion in order to accommodate the manufacturing of conventional cesium chemicals. Since Bernic Lake is a headwater lake and therefore very susceptible to environmental damage, the plant design minimizes environmental impacts on the surrounding area. All areas of the plant are contained to capture any spilled material, and wastes are stored in a lined disposal cell, which eliminates the discharges to the lake. Cesium Formate Manufacture Pollucite ore is mined from the Tanco mine along with the spodumene and tantalum ores. The mine contains approximately 75% of the worlds proven reserves of pollucite. The ore is crushed to –12 mm., and then dry ground in a ball mill to a powder form. Utilizing a series of acid/base reactions, the cesium is extracted from the pollucite ore and converted to a high-density, cesium formate solution. The final product is shipped by container to Aberdeen, Scotland and Bergen, Norway for use by the drilling industry in the North Sea, and to Houston, Texas for use in the Gulf of Mexico. Markets Cesium chemicals are currently used primarily in catalyst and chemical synthesis applications. While current worldwide demand for fine cesium chemicals is approximately 700,000 pounds a year, it is expected that new applications in the oil, gas and chemical industries for these products will increase in demand by more than ten-fold. TANCO UNDERGROUND TOUR STOPS Due to the constant changes underground as a result of on-going mining activities, the stops for the tour will not be determined until closer to the date of the tour. The tour participants will receive a tour stop handout upon arrival at the minesite. 18 �REFERENCES Anderson, A.J., Groat, L.A. and Simmons, W.B.Jr. (eds.) (1998): Granitic Pegmatites: The Cerny – Foord Volume. The Canadian Mineralogist, vol. 36, pt. 2. Beakhouse, G.P. (1991a): Winnipeg River Subprovince, in Thurston, P.C., Williams, H.R., Sutcliff, R.H. and Stott, G.M., (eds.), Geology of Ontario: Ministry of Northern Development and Mines, Special Volume 4, Part 1, pp.239-278. Beakhouse, G.P. (1991b): Winnipeg River Subprovince, in Thurston, P.C., Williams, H.R., Sutcliff, R.H. and Stott, G.M., (eds.), Geology of Ontario: Ministry of Northern Development and Mines, Special Volume 4, Part 1, pp.279-302. Berry, L.G. (ed.) (1972): The Tanco Pegmatite at Bernic Lake, Manitoba. The Canadian Mineralogist, vol. 11, pt. 3. Brisbin, W.C. (1986): Mechanics of pegmatite intrusion. The American Mineralogist, vol. 71. N°s. 3 and 4, pp. 644-651. Brown, G.E., Jr., and Ewing, R.C. (eds.) (1986): R. H. Jahns Memorial Issue: The mineralogy, petrology, and geochemistry of granitic pegmatites and related granitic rocks. The American Mineralogist, vol. 71, N°.’s 3 and 4. Cerny, P. (ed.) (1982): Granitic Pegmatites in Science and Industry. Mineralogical Association of Canada Short Course Handbook 8. Cerny, P. (1989a): Characteristics of pegmatite deposits of tantalum, in Moller, P., Cerny, P. and Saupe, F., (eds.), Lanthanides, Tantalum and Niobium: Society for Geology Applied to Mineral Deposits, Special Publication 7, Springer-Verlag, pp.195-239. Cerny, P. (1989b): Exploration strategy and methods for pegmatite deposits of tantalum, in Moller, P., Cerny, P. and Saupe, F., (eds.), Lanthanides, Tantalum and Niobium: Society for Geology Applied to Mineral Deposits, Special Publication 7, Springer-Verlag, pp.274-302. Cerny, P. (1991a): Rare-element Granitic Pegmatites. Part II: Regional to Global Environments and Petrogenesis. Geoscience Canada, vol. 18, pp. 49-67. Cerny, P. (1991b): Rare-element Granitic Pegmatites. Part 1: Anatomy and Internal Evolution of Pegmatite Deposits. Geoscience Canada, vol. 18, pp.68-81. Cerny, P., Ercit, T.S. and Vanstone, P.J. (1998): Mineralogy and Petrology of the Tanco Rareelement Pegmatite Deposit, Southeastern Manitoba. International Mineralogical Association, Field Trip Guidebook B6, 17th General Meeting, Toronto, Ontario, Canada. Cerny, P. and Meintzer, R.E. (1988): Fertile granites in the Archean and Proterozoic fields of rare-element pegmatites: crustal environment, geochemistry, and petrogenetic relationships, in Taylor, R.P. and Strong, D.F. (eds.), Recent advances in the geology of granite related mineral deposits: Canadian Institute of Mining and Metallurgy, Special Volume 39, pp. 170-207. Cerny, P., Trueman, D.L., Ziehlke, D.V., Goad, B.E., and Paul, B.J. (1981): The Cat LakeWinnipeg River and Wekusko Lake Pegmatite Fields, Manitoba. Manitoba Department of Energy and Mines, Mineral Resources Division, Economic Geology Report ER80-1. Crouse, R.A., Cerny, P., Trueman, D.L. and Burt, R.O. (1979): The Tanco Pegmatite, Southeastern Manitoba. The Canadian Mining and Metallurgy Bulletin, Feb. 1979, pp.142-151. 19 �Ercit, T.S. (1986): The simpsonite paragenesis: the crystal chemistry and geochemistry of extreme Ta fractionation. Ph.D. thesis, University of Manitoba, Winnipeg, Manitoba, Canada. Hutchinson, R.W. (1959): Geology of the Montgary Pegmatite. Economic Geology, vol. 54, pp. 1525-1542.. London, David (1984): Experimental phase equilibria in the system LiAlSiO4-SiO2-H2O: a petrogenetic grid for lithium rich pegmatites. American Mineralogist, vol. 69, pp. 995-1004. Martin, R.F. and Cerny, P. (eds.) (1992): Granitic Pegmatites. The Canadian Mineralogist, vol. 30, part 3. Moller, P., Cerny, P. and Saupe, F. (eds.) (1989): Lanthanides, Tantalum and Niobium. Society for Geology Applied to Mineral Deposits, Special Publication N°. 7, Springer-Verlag, New York, NY. Stilling, A. (1998): Bulk composition of the Tanco Pegmatite at Bernic Lake, Manitoba, Canada. M.Sc. thesis, University of Manitoba, Winnipeg, Manitoba, Canada. Thomas, A.V. (1984): A petrological and fluid inclusion study of the Tanco pegmatite, S.E. Manitoba. M.Sc. thesis, University of Toronto, Toronto, Ontario, Canada. Trueman, D.L. (1980): Stratigraphy, structure, and metamorphic petrology of the Archean greenstone belt at Bird River, Manitoba. Ph.D. thesis, University of Manitoba, Winnipeg, Manitoba, Canada. 20 �21 �22 �Field Trip 2 Quaternary Geology of Southeastern Manitoba Erik Nielsen and Gaywood Matile Quaternary Geologists Manitoba Geological Survey Industry, Trade and Mines 360-1395 Ellice Avenue Winnipeg, Manitoba R3G 3P2 Canada Extensive wetlands that started to form in response to mid-Holocene climate change, are a common feature of the southeastern Manitoba landscape. The photo was taken approximately 25 km east of Sandilands. �INTRODUCTION The climatic, geomorphic and ecological changes that have occurred in northwestern Ontario, southeastern Manitoba and Canada in general, over the last 100 000 years have been nothing short of spectacular. The Sangamonian Interglacial, which was not unlike the present interglacial, lasted from approximately 125 000 to 75 000 years ago, and ended with the advance of the Laurentide Ice Sheet in what was the greatest ecological catastrophe to befall Canada in recent geological time. The Laurentide Ice Sheet flowed southward out of Quebec and Nunavut and covered most of Canada and the northern parts of the United States as far south as New York City and De Moines, Iowa. The southern ice margin fluctuated periodically throughout the Wisconsinan, but northwestern Ontario and southeastern Manitoba and most of the rest of Canada were locked in the icy grip of the continental ice sheet until almost 10 000 years ago. For an estimated 65 000 years, northwestern Ontario and southeastern Manitoba lay devoid of trees, grasses and all living things, under a one kilometre thick ice mass! Ameliorating climate in the late Pleistocene saw the rapid northward and northeastward retreat of the ice margin and the establishment of glacial Lake Agassiz between the retreating ice margin and the high ground to the south, east and west. Lake Agassiz, at times over 200 m deep persisted for about 4 000 years from approximately 11 700 to 7 700 years BP and occupied all of Manitoba below the Cretaceous Escarpment, as well as much of northwestern Ontario. The flat fertile plains of the Red River valley and parts of northwestern Ontario, such as the Fort Francis area, resulted from the deposition of thick deposits of deepwater glaciolacustrine sediments. The numerous beach deposits in northwestern Ontario, southeastern Manitoba and elsewhere, record successive lake levels. Water levels recorded by the beaches relate to differential isostatic rebound and stepwise drainage of Lake Agassiz into the Gulf of Mexico, the Great Lakes and the Arctic Ocean before the lake finally drained into Hudson Bay. POSTGLACIAL VEGETATION AND CLIMATE Little is know of the postglacial vegetation of southeastern Manitoba despite the extensive and detailed work in the region by the Geological Survey of Canada and the Manitoba Geological Survey over the last fifteen years. Information on the postglacial climate and vegetational history of the region is inferred form a single pollen diagram from Hayes Lake near Kenora (McAndrews, 1982). The vegetational history of Hayes Lake suggests that the area was invaded by spruce forest immediately upon deglaciation and regression of Lake Agassiz. The early spruce forest changed to pine, birch, poplar and alder forest after 10 000 years BP. Based on the available data from Hayes Lake, open, mixed woodland existed in the northwestern Ontario and southeastern Manitoba during the early to mid-Holocene. Spruce and fir increased at the expense of alder after about 3 600 years BP, in response to a cooling climate. The present vegetation has remained relatively stable for the past 3 600 years. QUATERNARY GEOLOGY OF SOUTHEASTERN MANITOBA Twelve thousand years ago all of Manitoba, except possibly isolated areas above the Manitoba Escarpment, was completely covered by glacial ice, which at it's maximum extended as far south 24 �as Des Moine, Iowa. Rapid glacial retreat, caused by the rapid amelioration of climate, was enhanced by a proglacial lake environment, which promoted accelerated ice beak-up by means of iceberg calving along the glacier margin. By ten thousand years ago the ice margin was at the south end of Lake Winnipeg, and southeastern Manitoba was ice-free. As ice retreated into southeastern Manitoba it divided into two glacial lobes, the Rainy Lobe which advanced from the northeast and the Red River Lobe which advanced from the northwest. Sediments deposited by the Rainy Lobe typically have a sand-rich matrix and Precambrian-rich clast lithology, whereas sediments carried by the Red River Lobe are typically silt-rich and predominantly Paleozoic carbonate clasts, reflecting the lithologies of the bedrock that the glacier was advancing over. The interlobate position between these two ice-lobes is defined by large, sorted sand deposits in the south, the Sandilands Moraine, and sand and gravel deposits further north (Figure 1). Retreat was rapid, commonly followed by minor glacial readvances that eroded or destroyed previously deposited recessional moraines. Figure 1. Field trip stops plotted on a Digital Elevation Model of a portion of southeastern Manitoba Glaciation in Manitoba blocks the natural northward drainage and consequently a proglacial lake, glacial Lake Agassiz, formed as the ice front retreated north of the continental divide in South Dakota. Lake Agassiz existed for about four thousand years, from about 11 700 years before present (BP) until about 7 700 years BP when it finally drained into Hudson Bay (Thorleifson, 1996). Paleostrandlines and associated radiocarbon dates from southeastern 25 �Manitoba document much of Lake Agassiz history (Figure 1). The initial phase of Lake Agassiz, the Lockhart Phase, during which time the lake drained southward into the Gulf of Mexico, and encompasses the highest levels of the lake lasted until about 11 000 years BP. The Lockhart Phase in southeastern Manitoba is represented by numerous, but poorly defined strandlines along the higher parts of Sandilands Moraine and by most of the clay deposited in the Red River valley to the west. The Lockhart Phase was followed by the Moorhead Phase which ended about 9 900 years BP. The Moorhead Phase is characterized by relatively low water levels, due to glacial retreat in the Lake Nipigon area of northwestern Ontario, which allowed drainage through lower outlets to Lake Superior and the Atlantic Ocean. Several, well-developed beaches and wave-cut escarpments and at least one in-filled abandoned river channel represent the Moorhead Phase north and west of the Sandilands Moraine. The following Emerson Phase, spanned the interval from about 9 900 to 9 300 years BP. A glacial readvance in northern Ontario blocked the eastern outlets. This caused the level of Lake Agassiz to rise to approximately the level it was at the end of the Lockhart Phase. The elevation difference was the result of about 1 000 years of isostatic rebound. Lake Agassiz again drained south into the Gulf of Mexico via the Mississippi River. Four prominent lake levels formed during the Emerson Phase, the Norcross, Tintah, Upper Campbell and the Lower Campbell. The Upper and Lower Campbell levels are the bestdeveloped strandlines in Lake Agassiz and can be traced in this region around the Sandilands Moraine and eastward almost to the Ontario border. A great deal of erosion occurred along the Sandilands Moraine at this time as a result of the prevailing winds coming from the northwest, across the open expanse of the lake. The final phase of Lake Agassiz, the Morris Phase, is represented by a series of regularly spaced, moderately well developed strandlines (Figure 1). The final drainage of the lake, occurred as successively lower eastern outlets opened, first draining through Lake Superior and then through more northerly outlets to the north Atlantic, until the final drainage into Hudson Bay about 7 700 years BP. During the rise and fall of Lake Agassiz water levels, the Sandiland Moraine, a large generally unconfined sand aquifer, rapidly became saturated and de-watered. The rapid de-watering caused the formation of sapping channels throughout the moraine, one of which is truncated by the Upper Campbell, and is therefore clearly related to the final drainage of Lake Agassiz from the area. FIELD TRIP STOPS STOP 1 - STRIATED OUTCROP, WEST HAWK LAKE Northwestern Ontario and the adjacent parts of southeastern Manitoba have been glaciated numerous times throughout the Pleistocene. Each successive glaciation in large part removes the evidence of previous glaciation. Previously deposited sediments are stripped away and bedrock outcrops are molded and striated such that they record only the most recent events. Consequently, the terrestrial glacial record is largely incomplete. The outcrop of pillow basalt at this stop was striated and polished by the last ice flow to affect this area (Figure 2). The striations, orientated towards 230º, are common throughout the area and 26 �record glacier movement out of Hudson Bay towards the southwest. Striations are typically fine scratches on the gentle stoss sides of this outcrop. The plucking and steep sides at the down glacier side of the outcrop, indicates that the ice flow was towards the southwest and not the northeast. Minor variations in striation direction across the outcrop is due to topographic deflection at the glacier sole and is not related to different glacial events. In addition to striations, numerous p-forms, areas that have been eroded by subglacial meltwater under hydraulic head imposed by the glacier, are found on the outcrop. Figure 2. Striated outcrop at West Hawk Lake showing ice flow towards 230º. Striations are best preserved under till or glaciolacustrine sediments. Once glacial polish and striations have been exposed to weathering they don’t usually last very long. STOP 2 – WEST HAWK LAKE, TILL SECTION Till is the material that is directly deposited by the action of glacier ice although there may be considerable influence of subglacial meltwater. It consists of a wide variety of grain sizes from clay to boulders and may be considered as being generally unsorted (Figure 3). Till is generally derived primarily by the comminution of the immediately underlying bedrock with only very small components originating from various up glacial sources. This is the case with the till in the West Hawk Lake area, which is sandy in texture and was derived primarily by the comminution of the underlying volcanic rocks. A small proportion of the till was derived from granitic rocks that outcrop approximately 5 km to the northeast. The till is generally not calcareous, but in places Paleozoic carbonate erratics derived from the Hudson Bay Lowland, 650 kilometres to the 27 �northeast, can be found testifying to long distance glacial transport. Carbonate erratics are however rare, both because of the long glacial transport and the low survival rate of these soft lithologies, but also because of dissolution by near surface weathering in the time since the area was deglaciated. Carbonate erratics, which may be found in the scree were probably ice rafted and subsequently deposited in the glaciolacustrine sediments, which are common in the area. Carbonate erratics are probably not derived from the till at this site. Figure 3. Section at West Hawk Lake exposing sandy till of northeast provenance. STOP 3 - WEST HAWK LAKE, METEORITE IMPACT STRUCTURE West Hawk Lake is almost 4 km wide and nearly circular in shape. It was drilled in the 1960s by the Dominion Observatory and found to be approximately 100 m deep and contain approximately 100 m of sediment overlying the Precambrian basement. The circular shape and great depth of the lake, as well as the presence of shock-metamorphosed quartz and other features indicates the lake was formed by a meteor impact. The oldest sediments in the crater are of Cretaceous age indicating the impact occurred prior to that time, possibly in the Paleozoic. Because the lake is so deep compared to its diameter, it has been believed for many years that sediment deposited in the lake would be protected from erosion during glaciation. Glacier ice moving over the lake would shear over the top of the lake and not penetrate to the bottom. Previously deposited sediment would therefore be unaffected by glaciation. In addition, the great depth and the fact that there are no major rivers draining the lake means that sediments entering the lake would not be eroded or flushed through the lake. Consequently, the possibility exists that a complete Holocene, glacial and pre-glacial record spanning perhaps millions of years 28 �might be preserved in the sediment in-fill at the bottom of the lake. Jim Teller from the Department of Geology, University of Manitoba, has undertaken coring of the upper 15 m of the sediment in-fill and is planning to core the remaining 75-85 m to elucidate the glacial history, the history of glacial Lake Agassiz and climate variability of the mid continent over possibly the last million years or more. En route to Stop 4 we drive west on the Trans Canada Highway. Approximately 12 km west of Falcon Lake we leave the area affected by glaciation from Hudson Bay and northwestern Ontario. The ice flow direction changes to southeasterly (145º) and the associated till becomes fine textured and highly calcareous, having been derived by the comminution of Paleozoic carbonate bedrock in the Manitoba Interlake, northwest of Winnipeg. We will have an opportunity to observe diamicton, similar to this till, in the Grunthal pit at Stop 6. Point of Interest In some sections of the low boreal forest, the construction of the Trans-Canada Highway had a considerable effect on drainage and the height of the water table. Where the Trans-Canada Highway becomes divided, just west of Falcon Lake, a small stand of eastern white cedars (Thuja occidentalis) in the median illustrates the impact of hydrological change on the local vegetation. The growth of these trees began to be affected following highway construction in 1981. Although most of these trees had been growing since the early 1800s, the elevated water table caused their ringwidth and wood density to decline by 50 percent within two to three years. While some cedars were able to survive under the raised water table for several years, the last tree had succumbed to flooding by 1993. Although most are still standing upright, these trees have been dead for 10 to 20 years. Figure 4. Composite tree-ring density curves for eastern white cedar (Thuja occidentalis) from Falcon Lake and East Braintree. 29 �In other areas of southeastern Manitoba, which marks the western limit of cedars, trees growing around undisturbed wetlands can live up to 350 years and possibly longer as is the case around East Braintree. These long-lived cedar trees can potentially provide records of changes in environmental conditions since the mid-17th century and may greatly improve our understanding of the natural variability of climate, forest fire frequency and insect infestations in this region (Figure 4). STOP 4 - SAPPING CHANNELS (UPPER CAMPBELL BEACH) The Upper Campbell beach is Lake Agassiz's most prominent strandline and defines the Sandilands Moraine as an island about 9 500 years BP (Figure 1). This phase of Lake Agassiz relates to the final drainage of the lake as progressively lower eastern outlet were opened by glacial retreat until the lake finally completely drained into Hudson Bay. The lunch spot is located on the back or landward side of the Upper Campbell beach, on the shore of a small lake. This small lake is located at the down slope end of what is believed to be a sapping channel. The head of this channel is 5 km to the southeast (Figure 5). This site clearly indicates that the sapping channel is truncated by the Upper Campbell beach, making it older than the beach. k Peat r%%Upper Cajnpbell WavecuffScarp 'I C SaLing els '1 ciofluvial Norcross Sand . Navecut $carp 5km Figure 5. Surficial geological map draped on a Digital Elevation Model of a portion of southeastern Manitoba. Stop 4 is located at the lower end of the sapping channels. The sapping channels are typically infilled with peat. Conical depressions are found above and below the Norcross wavecut scarp. Sapping channels have only been recognized in the Sandilands area, and are only found above the Upper Campbell beach, in areas where silty sands and fine sands are the predominant 30 �sediments. During times of high lake levels in Lake Agassiz, prior to the formation of the Upper Campbell beach, a high water table would have existed in the Sandilands. With the drop in lake level to the Upper Campbell beach, the water table would have dropped accordingly. This reequilibration of the water table would have taken place rapidly, perhaps in a decade or less, resulting in high gradients in the local hydrogeologic system. These high gradients made it possible to mobilize silt and transport it from the hydrogeologic system. The surficial expressions of this transport are the sapping channels, created where groundwater discharge occurred. Numerous conical depressions occur in the Sandilands that are also likely the result of groundwater movement and are believed to be contemporaneous with the sapping channels. These conical depressions occur almost exclusively in silty sands, above and below the Norcross escarpment, 5 km to the southeast. Although the term piping is commonly used to refer to flushing of sediment from beneath a dam in civil engineering terms, piping can occur in natural settings when there is upward movement of groundwater under high gradients in silts and silty sands (Higgins, 1982). The high gradients enable silt to be removed from the sediment matrix and depressions were formed as the remaining sediment collapses under the influence of gravity. Following a drop in the level of Lake Agassiz, groundwater flowing from higher elevations to lower elevations may have become semi-confined as overlying sediments became finer. The resulting upward gradient combined with presence of overlying silty sands suggests that piping is a viable mechanism for forming these conical depressions. STOP 5 - UPPER CAMPBELL BEACH OF LAKE AGASSIZ During the Emerson phase of Lake Agassiz the Sandilands Moraine was subjected to a tremendous amount of shoreline erosion. The prevailing winds were from the northwest and Sandilands was an island in the southeast part of the Lake Agassiz basin. As a consequence, the Sandilands Moraine was subjected to waves with a fetch in access of 300 km. Evidence of this erosion are two 20 m high wave-cut scarps, the Upper Campbell and Norcross scarps, on the northwest flank of the moraine. Large well-developed spits are situated on both ends of these scarps. The largest of these spits is found south of the lower of the two scarps, the Upper Campbell scarp. Figure 6. Ground penetrating radar profile across the Upper Campbell spit. This west to east profile is 150 metres wide with 30 metre thick foreset beds and several metre thick topsets. There is fine-grained glaciolacustrine sediment at the base of the foresets. 31 �This stop is located on the crest of the southern spit. The spit is approximately 15 k long, 8 k wide and 30 m thick and composed predominantly of sand with minor amounts of gravel. Ground penetrating radar surveys carried out by the Geological Survey of Canada and the Manitoba Geological Survey show the structure of the spit to be large foreset beds which prograde southward in the core and westward on the west flank (Figure 6). Topset beds are several metres thick. STOP 6 - INTERGLACIAL SITE AT GRUNTHAL Although the exposure at the Grunthal pit is poor, it is interesting because musk ox (Ovibos moschatus), extinct bison, (possibly Bison antiquus) and wooly mammoth (Mammuthus primigenius) (Figure 7) bones have been dredged from below the water table. Wood and a variety of organic-rich, fine-textured silt and silty-clay sediments have also been recovered during gravel extraction, but the stratigraphy of the site is speculative. Figure 7. Lower M1 molar of Mammuthus primigenius (V2554) from the Grunthal pit. (A) Lateral aspect. (B) Occlusal aspect. The sediment above the water table and the sand and gravel extracted by the dredge is interpreted to be late Wisconsinan, ice-proximal, glaciofluvial sediment deposited 11-12 000 years ago, when the last glacier ice to affect the area was waning. These sand and gravel deposits are in part capped by diamicton that may have been deposited as debris flows in a proximal glaciofluvial environment. 32 �Little is known about the sediments underlying the late Pleistocene sediments below the water table. The various bones, wood and the organic-rich sediments dredged from the bottom of the pit suggest the underlying deposits are of mid-Wisconsinan interstadial or possibly Sangamonian Interglacial age. Radiocarbon dating of a wood sample gave a finite age of 44 020 ± 1 030 years BP (GX-27643) suggesting an interstadial age, although dates in this range are close to the limit of the radiocarbon technique and must be accepted with some trepidation. The presence of bones of Mammuthus primigenius and Bison antiquus strongly suggest an interglacial age. In addition, a lophar index of 9 (number of lophs per 100 mm of mesiodistal crown length) of an M1 mammoth molar from the deposit (Graham Young and Ed Dobrzanski, per com 2002) is similar to an M1 or M2 molar of Sangamonian age from Bird, Manitoba, (Nielsen et al. 1988) further suggesting an interglacial age for the deposit. Two samples of organic-rich mud, dredged from below the water table, were submitted to Paleotec Services in Ottawa for macrofossil analysis. The plant macrofossil evidence suggests a forested environment dominated by spruce trees and the presence of sedges, buckbean and mosses further indicates the area was poorly drained. The presence of bark beetles (Scolytidae) agrees with the plant fossil evidence of a forested environment. The insect fossils, specifically rove beetles are in agreement with the plant macrofossil data suggesting a stream or slowly moving water in a poorly drained area possibly a pond or wet depression. The absence of aquatic submergent plants and other typical aquatic faunal elements suggest the pond was temporary rather than permanent. The water-worn bones in association with finer textured organic-rich silt and silty clay suggest the deposit may be in part a point bar. Alternatively, the bones became abraded when they were incorporated into the overlying glaciofluvial sediments. Interpretation of the floral and faunal macrofossil assemblage indicates the climate at the time of deposition was probably warm. This conclusion is based on the abundant macrofossil remains of spruce along with bark beetles suggesting the climate was at least warm enough for the growth of boreal forest. However, the absence of fossil evidence of other boreal taxa, specifically pine and deciduous trees such as birches and alders is puzzling. The forest, being composed of only spruce, resembles the boreal forests in northern regions. This, along with the presence of fossil rove beetles (Eucnecosum) which have distributions restricted to northern boreal, arctic, or alpine areas suggests the climate may have been cooler than today. The macrofossil evidence and the presence of M. primigenius, Bison antiquus and Ovibos moschatus at the site are taken to indicate a northern or boreal steppe or steppe-tundra environment. All the taxa from the site except the Pleistocene megafauna can be found living in southern Manitoba today, with the rove beetle being at its southern limit. It is therefore concluded from the macrofossil evidence that the deposit at Grunthal is probably of interglacial age, and it is tentatively assigned to the Sangamonian. Point of interest If time permits a small detour will be made to the Dawson Trail, which was the first ‘road’ linking Fort Garry to eastern Canada. 33 �Simon James Dawson, an engineer and land surveyor, was given the task in 1857 of surveying the country between Lake Superior and the Red River valley in Manitoba. Dawson subsequently proposed a route from Port Arthur’s Landing, which later became Port Arthur and then Thunder Bay, that would use waterways and roads, to prepare the way for the railroad and thereby forestall northward expansion by aggressive American interests. Figure 8. Map showing the Dawson Trail between Port Arthur’s Landing (Thunder Bay) and Fort Garry (Winnipeg). Construction of the Dawson Trail in Manitoba was started in 1868 under the direction of John Allan Snow, as a make-work project after several years of repeated crop failure in the Red River valley, but was then hastened because of potential trouble with the métis. The 1200 man army of Colonel Garnet Joseph Wolseley, which was sent west from Upper Canada in 1870 to quell the métis uprising led by Louis Riel, was in part employed to help finish the construction of the Dawson Trail (Figure 8). The army worked on the road to the point where it was passable and arrived in the Red River settlement in August of 1870. Interestingly the army traveled from Fort Francis to Fort Garry via Lake of the Woods and the Winnipeg River and did not use the Dawson Trail from the Northwest Angle, and across Sandilands. The trip from Port Arthur’s Landing to 34 �Fort Garry lasted approximately one month and was made by approximately 1600 travelers in 1873. With the completion of the Canadian pacific Railroad in 1885, the Dawson Trail was quickly forgotten after having being used for only a few years and never really being finished as Dawson originally envisioned it. However, much of the trail is still in use either as bush roads or snowmobile trails. Parts of the Trans-Canada Highway between Richer and Winnipeg also follow the original road. The road is especially well preserved in sections of Sandilands where in some boggy areas the original corduroy can still be found. ACKNOWLEDGEMENTS We would like to thank Graham Young and Ed Dobrzanski of the Manitoba Museum of Man and Nature for their analysis of the vertebrate bones from the Grunthal pit and their help in making the Sangamonian age assignment of the deposit. We would also like to thank David Riddle of Manitoba Historic Resources Branch for supplying the Dawson Trail map. Grant Ferguson of the Department of Engineering, University of Manitoba wrote the very eloquent description of the formation of the sapping channels for us. REFERENCES Higgins, Charles G. 1982. Piping and sapping: development of landforms by groundwater outflow. pp. 18-59. In: Groundwater as a Geomorphic Agent. R.G. LaFleur (ed.) 1982. Allen and Unwin, Inc. London, U.K. Kerr, D.G.G. 1975. Historical Atlas of Canada. Third revised edition. Thomas Nelson & Sons (Canada) Ltd. McAndrews, J.H. 1982. Holocene environment of a fossil bison from Kenora, Ontario. Ontario Archaeology, vol. 37, pp.41-51. Nielsen E., Churcher, C.S., and Lammers, G.E. 1988. A wolly mammoth (Proboscidea, Mammuthus primigenius) molar from the Hudson Bay Lowland of Manitoba. Canadian Journal of Earth Sciences, vol. 25, pp. 933-938. Thorleifson H. 1996. Review of Lake Agassiz History. In, Teller, J.T., Thorleifson, L.H., Matile, G. and Brisbin, W.C. eds. Sedimentology, geomorphology and history of the central Lake Agassiz basin. Geological Association of Canada Field Trip B2, 101p. 35 �36 �Field Trip 3 Structure and Sedimentology of the Seine Conglomerate, Mine Centre Area, Ontario Dyanna Czeck Department of Geology Oberlin College 52 W. Lorain Street Oberlin, OH 44074 Philip Fralick Department of Geology Lakehead University Thunder Bay, ON P7B 5E1 Moderately deformed Seine conglomerate containing metavolcanic and granitoid clasts, with clast tiling due to dextral deformation. Hwy 11, 1 km east of Horsecollar Junction. �FOREWORD This trip will examine sites related to the development and deformational history of a synorogenic sedimentary unit, the Seine Conglomerate. The unit extends across the Canada/ United States border in the Rainy Lake region, an area that has sparked interest and controversy for American and Canadian geologists for over a century. The Seine is of significant interest because it preserves important structural and sedimentological clues that may lead us to a better understanding of the tectonic history of the Archean Wabigoon-Quetico subprovincial boundary. REGIONAL SETTING Introduction and Tectonic Setting The central portion of the Superior province is characterized by alternating subprovinces of metavolcanic-plutonic and metasedimentary natures (Fig. 1). Figure 1. The Superior Province. From Card and Ciesielski (1986). One popular tectonic interpretation for the central portion of the Superior Province is of repeated island arc, microcontinent collisions. The collisions are evidenced by rocks that can be interpreted as arc sequence subprovinces (metavolcanics) and their corresponding accretionary prism subprovinces (metasediments) (Langford and Morin, 1976; Hoffman, 1989; Percival and 38 �39 Figure 2. Simplified Geologic Map of Mine Centre area showing the extent of the Seine. Geology compiled from Wood, 1980a & b, Stone, 1998a & b, and Czeck, 2001. �Williams, 1989; Card, 1990; Hoffman, 1990). In general, the ages in the greenstone belts are similar along strike, but differ systematically across strike (Hoffman, 1989). This is consistent with the island-arc accretion model. A history of southward accretion has been proposed to explain the juxtaposition of Superior Province terranes (Langford and Morin, 1976; Percival and Williams, 1989; Card, 1990). The Seine Conglomerate, located in the Rainy Lake region of the western Superior Province, was deposited along the boundary between the Wabigoon metavolcanic/plutonic subprovince and the Quetico metasedimentary subprovince (Fig. 2). The structural observations along the Wabigoon–Quetico boundary are consistent with an oblique island-arc microplate collision circa 2.7 Ga. In this part of the Superior Province, it seems likely that first the Quetico acted as a subduction prism during accretion of the Wawa to Wabigoon. Then, it was effectively shifted from the subduction prism setting to a back-arc setting, as subduction shifted (Percival and Williams, 1989). In the Rainy Lake region, a series of lithostratigraphic terranes were assembled together along structurally controlled, stratigraphically discordant boundaries during the collisions. The boundary between the Wabigoon and Quetico Subprovinces in this region is divided into three primary blocks by dextral wrench faults (Poulsen, 1986). Each of the small terranes and subterranes may have undergone a somewhat unique history of formation and deformation. The Quetico Fault forms the northern boundary, separating the granite-greenstone terrain of Wabigoon Subprovince from the Coutchiching Group argillites and the Seine Conglomerate. The Seine River-Rainy Lake Fault forms the southern boundary of these sedimentary sequences with the Quetico turbiditic metasediments to the south. The wedge-shaped area lying between the two major fault systems is itself dissected by splays off the major east-west faults, which isolate the lithic units, destroying stratigraphic integrity. This problem has resulted in historical speculation on the lateral equivalency of the Seine and Coutchiching sediments (Merritt, 1934) and the Coutchiching and Quetico (Lawson, 1913). Geochronology Davis et al (1989) used U-Pb, single zircon geochronology to bracket the ages of the Coutchiching and Seine between 2704+-3 to 2692+-2 and 2696+5-3 to 2686+2-1 respectively. Even though an overlap in age existed, Davis et al (1989) believed that structural considerations indicated that the Coutchiching is slightly older than the Seine. Further detrital zircon geochronology of the Seine was conducted by Davis and reported in Fralick and Davis (1999). Of the two samples analysed one was from the sandstone dominated lithofacies (collected near the Seine River bridge) and the other was from the conglomerate dominated lithofacies (collected near Horsecollar Junction on Highway 17). The detrital zircons in both samples give very consistent ages with the sandstone lithofacies clustering at 2693+-1 (Fig. 3) and the conglomerate lithofacies clustering at 2692+-1 (Fig. 4). These detrital zircon ages are similar to the Bear Pass pluton, a granitic mass which outcrops near the Seine 40 �Conglomerate (Fralick and Davis, 1999). Metamorphic titanite from the pluton gave an age of 2684+-5. Figure 3. Isochron showing U-Pb detrital zircon ages from the upper Seine Group (supplied by D. Davis). Figure 4. Isochron showing U-Pb detrital zircon ages from the lower Seine Group (supplied by D. Davis). Detrital zircons from the nearby Quetico and Coutchiching metasediments have an age distribution that is very different than that from the Seine Conglomerate. Their youngest ages are 2699+-1 for the Quetico and 2704+-3 for the Coutchiching with the population in both rock units extending back past 3000Ma. This is in sharp contrast to the Seine zircons which indicate 41 �dominance of a single source. This source may have evolved slightly through time as the age for the upper sandstone is slightly older than the lower conglomerate, though within error. This inverse age stratigraphy may reflect erosive unroofing of slightly older segments of the source igneous body. In any case, the Seine must be 2692 Ma, or younger, and its detritus was derived from a different source than the Quetico and Coutchiching (Fralick and Davis, 1999). The Bear Pass pluton is a good candidate for the source of the sediment except its zircons have lower Th/U ratios than the Seine. The Seine’s Th/U ratios of 0.74 to 1.07 are more typical of alkaline igneous rocks (Fralick and Davis, 1999). Alkaline igneous rocks 2692 Ma in age are present to the east of the area in the Shebandowan region. Metamorphic titanite in the Rice Bay Dome has given an age of 2693-+3 Ma , coeval with or predating the Seine (Davis et al,1989). The Bear Pass intrusion also predates the Seine and is late-tectonic, probably emplaced into the Rice Bay Dome after the Coutchiching turbidites had been overturned (Fralick and Davis, 1999). Thus, the Seine was deposited after an early period of folding and metamorphism. What can we learn from the Seine? The Rainy Lake region has been metamorphosed, generally to greenschist grade, and deformed in response to the Archean microplate collisions. Most of the preserved deformation is of a ductile nature, and thus occurred at significant depth. The Seine itself is metamorphosed and significantly deformed. From the geochronology and sedimentological evidence (to be described below), we know that the Seine was deposited in a dynamic convergent plate setting. From the significant flattening fabrics and structural evidence, we know that the Seine was subsequently buried and deformed at mid-crustal levels. Therefore, the Seine preserves a record of a conglomerate’s journey through the crust within a dynamic convergent zone. We can hope to interpret from the Seine a relatively late stage record of the microplate collision history at the Wabigoon- Quetico tectonic boundary through a history of syn-deformational deposition, burial, and deformation. Through analysis of the sedimentology, early structures, and late structures, we can hope to interpret various portions of the conglomerate’s path through the crust. SEDIMENTOLOGY OF THE SEINE The Seine Conglomerate was interpreted to have been deposited in a fluvial system by Wood (1980). This system undergoes a gradual transition from conglomerate dominated near its base to sandstone dominated near its top. Channels in the lower portion of the section are Scott type (Miall, 1978), with gravel and cobbles forming both longitudinal bars and interbar channels. Through cross-stratified, medium-grained sandstones, representing chute channels, commonly form small lenses in these sequences. The coarse-grained lithofacies association is transitional upwards into successions which contain thicker layers of trough cross-stratified sandstone. This represents a transition from gravelly main channels to channels dominated by the migration of sand dunes. With further fining upwards, the Formation becomes sandstone dominated. The sandstones are organized into stacked, trough cross-stratified lenses, with rarer large-scale, planar cross-stratified layers and conglomeratic horizons. This reflects development of a South Saskatchewan type braided river (Miall, 1978) with dune migration prevalent in the channels and only minor development of sandy transverse and gravelly longitudinal bars. To further clarify the 42 �relationship between the lithofacies present in the rocks and the environments in which the sediment was deposited, the remainder of this section will discuss the types of sediment deposited in the differing sub-environments of gravelly to sandy braided streams. Braided rivers are multichannel streams with large width to depth ratios that commonly develop in high slope areas, such as alluvial fans and proglacial outwash plains. This type of channel pattern is generally caused by a combination of factors which include: large diurnal and seasonal fluctuations in discharge; high slope, or a rapid increase in slope; high discharge velocities; the dominance of bedload sediment (sand and gravel) in transport; and, meagre vegetation on the floodplain. These factors result in the stream being easily able to erode its banks, spread out laterally and choke its channel with coarse sediment building midchannel islands. The channel morphology of braided rivers is characterised by a series of channels and bars which are occupied at various levels of discharge (Williams and Rust, 1969). During most of the year, with normal to low discharge, one, two or more channels will snake through the assemblage of bars. However, during peak discharge all the previously dry bars and minor channels will be overtopped and the river will develop only one large channel. Flood events, such as this, are the intervals when the coarsest sediment, generally composing the bars, but also flooring the channels in high energy systems, will move. Hammer and Smith (1983) found that bedload sediment transport increases at an exponential rate with river discharge. As the majority of coarse sediments are transported in bedload during high discharge events, with reduction in discharge the ability of the river to continue transporting this material down gradient is also reduced (Burton, 1989). At this point the largest material in transport stops moving. This produces a low velocity shadow downstream from the sedimented material where more detritus accumulates. This process leads to the formation of gravel bars, termed longitudinal bars, within the river channel. If the channel is in a relatively high slope area, the sand and finer material in transport will only be deposited if it is trapped or carried into the pores between the gravel and sedimented as matrix. Here, the main channel is pebble, cobble or boulder dominated with this detritus arranged into a stacked sequence of commonly irregular lenses. These lenses represent areas of scour and deposition on the bottom of a larger channel. Some gravel lenses may be formed by either migration of coarse-grained dunes or lateral fill into scour pits producing cross-stratification. In less energetic systems trough cross-stratified sand lenses will interbed with the gravel. This represents sand in bedload tractive transport as dunes being deposited and not re-eroded. Further reduction in energy levels of the system will eventually cause the main channel deposits to become a series of stacked, trough cross-stratified sandstone lenses. In braided streams carrying gravel, longitudinal bars will develop, splitting the flow at low stage (Figs. 5 & 6). These bars are lozenge-shaped mounds of pebbles, cobbles or boulders in clast support with a coarse-grained matrix. They are usually internally massive and nongraded, though occasionally parallel lamination is present. While the bars are submerged during maximum discharge events they are subjected to extremely turbulent flow, during which time they are modified by both erosion and deposition. Such processes result in the head of the bar being continually reworked producing a well sorted and coarser grained deposit (Burton, 1989). A significant decrease in velocity over the length of the bar results in a corresponding decrease in the size of material being deposited. 43 �Figure 5. Photograph of the North Saskatchewan River in Alberta. The main channel (A) is cutting around a longitudinal bar which grades from a coarse head (B) to a finer pebble tail with a veneer of darker coloured sand (C). The bar tail in the foreground also has darker sandy areas mantling it (C) and is cut by chute channels (D); one of which is building a chute delta (E). If main channel erosion was not occurring at F this would be the site of a bar edge sand wedge supplied by the nonconfined overbar flow. During waning flow, the bar tail will be shielded from the main current in the river as the bar top, in the center of the bar, begins to become emergent. This may cause a thin sand sheet, or patches of sand in lower areas, to be deposited over the bar tail. As the flow stage continues to drop small channels will sporadically develop cutting across the upper surface of the bar, from a main channel upstream to another main channel downstream. These are chute channels which commonly fill with trough cross-stratified sand produced by dunes migrating down the channels. They form sand lenses in the gravelly longitudinal bar. Where chute channels rejoin the main channel sediment may accumulate as a large avalanche face building out into the deeper main channel (chute delta). This produces a large-scale, planar cross-stratified sand or gravel deposit banked up against the side of the longitudinal bar. Similar, but laterally extensive, deposits can also be formed by unconfined sheet flow over the only slightly submerged surface of the bar. Again, where this flow enters the deeper main channel a large-scale, planar cross-stratified wedge of sediment will be banked up against the bar. This is called a bar edge sand wedge (Figs. 5 & 6). All of the above may be present in sand dominated braided systems, but the longitudinal bars will be subordinate and may be absent. The main bar forms in these systems are transverse bars. These are large sand waves, features similar to continuous crested, long wavelength dunes. 44 �Figure 6. Block diagram of a coarse-grained braided stream schematically showing: coarse bar head (A); finer bar trail with thin sand patches which accumulate in areas shielded from the current during waning flow (B); chute channel with dunes (C); chute delta (D); bar edge sand wedge (E); and main channel with dunes (F). Gravelly braided stream deposits similar to these dominate the lower Seine Conglomerate. Figure 7. Block diagram of a sand dominated braided stream schematically showing: the main channel with dune migration producing trough cross-stratification (A); transverse bar migration and stacking producing sandflats which are internally planar cross-stratified (B); minor (to no) development of gravelly longitudinal bars (C). Sandy braided stream deposits, and especially the main channel facies, dominate the upper Seine Conglomerate. Where they are abundant they will pile up next to one another clogging the channel with sand and producing large areas of sand flats (Smith, 1970; Miall, 1985) (Fig. 7). Internally, they are composed of large-scale (commonly>1m thick sets), planar cross-stratification. Transverse bars are quite laterally continuous and are interbedded with other large-scale, planar cross-stratified sand units, in sand flats (downstream accretion macroforms of Miall, 1988), or successions of smaller lenses of trough cross-stratified sand, representing dune migration on the channel floor 45 �(Fig. 7). The transverse bars are mostly active during higher discharge. During low discharge, their tops may become emergent and eroded, and dunes mantle their surface. The next flood event will often result in the bar building in a somewhat different direction. This will result in an apparent change in the angle of the cross-stratification (a reactivation surface). This sloping surface may also show evidence of erosion and contain small lenses of trough crossstratification. This completes the general overview of sediment deposits associated with channel sequences in braided streams. Floodplain lithofacies are usually minor to nonexistent in these successions as the fine grained deposits have little preservation potential. Braided streams commonly avulse, changing the position of their channel and combing the floodplain, eroding the fine-grained deposits and forming stacked channel sequences. The Seine Conglomerate contains lithofacies corresponding to all of the channel subenvironments discussed above. Conglomerate dominated longitudinal bar-channel sequences are the most common in outcrop. However, sandier channel sequences are not rare and gain importance higher in the Formation. STRUCTURAL GEOLOGY Deformed primary fabrics Bedding is often difficult to discern in the Seine. Where it can be identified, it is displayed by variations in grain sizes, often indicated by fine-grained layers interbedded with pebble conglomerates. In some cases, cross bedding can be seen within larger sandy layers. In general, bedding strikes approximately east – west, and is subvertical (Fig. 8). This orientation is similar to bedding attitudes measured throughout much of the Superior Province (Poulsen, 1986; Hudleston et al., 1988; Bauer and Bidwell, 1990; Tabor and Hudleston, 1991; Bauer et al., 1992; Jirsa et al., 1992; Bauer and Hudleston, 1995; Hudleston and Bauer, 1995). Bedding is generally subvertical and subparallel regardless of lithology. However, this fact does not necessarily indicate deformation of a continuous stratigraphic sequence. Based on opposing stratigraphic facing in adjacent rocks, an unconformity between the volcanic units and the base of the Seine conglomerate can be identified (Stop 1) (Lawson, 1913; Poulsen et al., 1980; Poulsen, 2000). There are some exceptions to the general EW, vertical bedding orientation. In the area along Shoal Lake (Stop 1), the bedding strikes more NE/ SW with a shallower (~65°) southeasterly dip. The orientations of the bedding within the Shoal Lake area and adjacent rocks to the north and south combine to create a large, gentle S structure, with shallower dips on the middle section of the S. Several folds on the scale of hundreds of meters have been identified within the Seine metasediments based on stratigraphic facing (scour beds and cross-beds) and lithologic similarity (Hsu, 1971; Wood et al., 1980a; Wood et al., 1980b; Poulsen, 2000). These folds have vertical limbs and are typically upright and isoclinal. The trends of the hinges are roughly parallel to the foliation (EW). The plunges of the hinges are unknown, and cannot be constructed due to the subparallelism of the limbs. In a few locations (see Stop 5), one can see small scale folds that may display horizontal bedding at the hinges. 46 �Figure 8. Equal area stereonets showing structural fabric data near Mine Centre, Ontario. A) Poles to bedding. 46 measurements. B) Poles to foliation. 142 measurements. C) Mineral lineations. 123 measurements. D) Intersections between foliation (cleavage) and bedding. 44 measurements. From Czeck (2001). Ductile deformation Ductile deformation (as evidenced by rocks with pronounced foliations and mineral lineations) is pervasive throughout the entire Wabigoon–Quetico boundary zone. The overall dominance of the foliation over the lineation creates an S-L type fabric. However, strain is also localized into an anastomosing network of more discrete shear zones, including two main zones of localized shear and displacement. These are the Seine River - Rainy Lake Fault and the Quetico Fault, which diverge to the west and merge to the east (Fig. 2). An anastomosing pattern of smaller shear zones links the major shear zones shown in anastomosing, gentle S-like shapes (Fig. 9). The locations of these smaller shear zones have been determined by linear features observed on electromagnetic anomaly maps or are identified as the discordant boundaries of apparently independent lithostratigraphic terranes (Poulsen, 1986; Poulsen, 2000). Direct observation of these smaller shear zones is difficult because they are typically under water or buried beneath recent sedimentary deposits and not exposed, presumably because they are highly erodable. The presence of discrete shear zones implies some strain partitioning between shortening and wrench components of ductile deformation along the Wabigoon–Quetico boundary. It is probable that the discrete shear zones are zones with relatively high wrench influence. Conversely, it is probable that the wide zones of deformation between the shear zones have undergone deformation with a stronger shortening influence. In some instances, small (on the order of a few meters), secondary shear zones may be seen. 47 �Figure 9. Schematic diagram illustrating structural features of Rainy Lake Wrench Zone. Short solid arrows identify downward facing units. From Poulsen (1986). Foliation is moderately to well developed in all rocks of the region, with the exception of some late stage plutons. It is especially well developed along the major shear zones and the Seine River – Rainy Lake and Quetico Faults. The foliations are largely subvertical and at a low angle both to bedding (except in the hinge of folds) and to the subprovince boundary (Fig. 8). An exception to the subvertical foliation is in the area along Shoal Lake (Stop 1). Here, like the bedding, the foliation strikes more NE/ SW with a shallower dip (~65°). Like the bedding, the combined orientations of the foliations within this Shoal Lake area and adjacent rocks to the north and south create a large, gentle S structure, with shallower dips on the middle section of the S. Unlike the bedding, the cleavage is not folded by the major upright folds. It is, however, affected locally by crenulations. In general, there is no consistently oriented intersection lineation between bedding and foliation throughout the Seine. Instead, this lineation defines a great circle corresponding roughly to the planes of foliation and bedding. This is to be expected in the situation in which bedding and foliation are subparallel because slight variations in orientation of the two planar features will have a significant effect on the orientation of the intersection lineation. Typically, chlorite or amphibole forms a mineral lineation, which varies in intensity from weak to strong depending on location. The lineation plunge is highly variable across the Wabigoon– Quetico boundary without any clear systematic change from east to west or from north to south, although there are local domains of similar lineation plunge (Czeck, 2001). The highest concentrations of lineation orientations plunge steeply to the east, and their mean orientation is 66°/076. However, there are also significant numbers of westward and shallowly plunging lineations. The range of lineation orientations is great enough that the “average lineation” may have little geologic meaning. 48 �The lineation referred to thus far is the mineral lineation, a lineation due to the preferred alignment of mineral grains or clusters of grains. This lineation is present in most rock types along the Wabigoon–Quetico boundary. Within the Seine conglomerate, there are, in fact, two distinct linear elements that can be measured independently: the mineral lineation and that defined by the long axes of the conglomerate clasts. Both can be considered penetrative features of the rock fabric. The long axes of clasts within the conglomerate are generally coincident with the mineral lineation as would be expected if the conglomerate clasts were an accurate recorder of strain and the mineral lineation reflects the stretching direction Relatively late-stage sinistral and dextral crenulations locally affect the cleavage. These crenulations are fairly small (usually cm scale). They are most abundant in the most highly sheared rocks. This correlation and the relative timing of the crenulations makes it seem likely that they formed during the latest stage of a continuing saga of transpression. Features of deformed conglomerates The Seine Group provides an excellent opportunity to observe the effects of competency contrasts on deformation. These natural competency contrasts allow us to obtain structural information that is unavailable in more homogeneous rocks, making the conglomerates are excellent tools for structural analysis and tectonic interpretations. Within the conglomerates, asymmetric shear sense indicators are prevalent. In general, these are either in the form of asymmetric pressure shadows at the ends of clasts, wrapped foliation indicating rotation of the most rigid clasts, and clast tiling. All of these shear-sense indicators are most evident on the subhorizontal plane, regardless of lineation orientation. They indicate dextral sense of shear. In general, the conglomerate clasts have been strongly flattened, although the degree of flattening is strongly dependent on lithology. The intensity of flattening strain varies greatly through the field area. We will be viewing several degrees of deformation on the various stops of the fieldtrip. TECTONIC STORY OF THE SEINE METASEDIMENTARY SEQUENCE AND SURROUNDING ROCKS Plate Collisions and Sediment Deposition Comparing the zircon populations of sedimentary units in the Rainy River area with other rock groups in the region generates some interesting trends. The zircon populations of turbidites and conglomerates on the northern margin of Wabigoon Subprovince ( Savant Group and Ament Bay Formation) exhibit similarities with the Coutchiching metasediments (Davis, 1997). The conglomeratic units near the northern margin of Wabigoon Subprovince represent braided fluvial systems (Turner and Walker, 1973; Devaney, 1999). The sediments they were transporting were deposited in the same time bracket (Davis et al, 1988) as some of the sedimentary units near Rainy Lake. If the sedimentary units in the Rainy Lake area represent detritus shed off of upraised blocks during collision-orogeny, as appears to be the case (Davis et al, 1989), 49 �Figure 10. Interpretive sketches showing subduction and plate convergence along the southern margin of Wabigoon Subprovince. The accretionary prism built at 2700 Ma of trench turbidites is represented by the Quetico. Northward subduction in the Schreiber and Shebandowan areas ceased at approximately 2720 Ma, but restarted in the Shebandowan area at 2692 as immanent collision ceased subduction in the Quetico trench. After collision of the Wawa-Abitibi terrain with the Quetico-Wabigoon assemblage, orogenic uplift affected the area. During this interval small basins opened on both sides of the suture zone and accumulated coarse fluvial deposits including the Seine Conglomerate. understanding the sequencing of sedimentary pulses is key to deciphering the tectonic forces which formed the Seine Basin and uplifted its source area. To understand this sequencing, it is necessary to outline the tectonic history of the region, and the sedimentary response to tectonism, from 2720 Ma to 2685 Ma. Sedimentary sequences deposited between 2720 and 2685 Ma in, and adjacent to, Wabigoon Subprovince record the final phases of subduction and collision of this area with landmasses to the north and south. Examined basin fill sequences are divisible into three depositional systems tracts. Sediments in the Beardmore-Geraldton area record progradation of braided streams and fan/braid delta complexes from a volcanically active area to the north (Devaney and Fralick, 1985; Devaney, 1987; Devaney and Williams, 1989). The outbuilding sequence fed detritus to a poorly structured, turbidite ramp/fan assemblage in the forearc basin (Barrett and Fralick, 1989), from which it was rerouted into the Quetico trench, via multiple channel systems (Fralick et al., 1992) (Fig. 10). Sediment geochemistry confirms that the calc-alkaline volcanic rocks present in the Onaman-Tashota area, to the immediate north of the forearc basin, were the source of the sediment (Fralick and Kronberg, 1997). Lower Zr and Y values in sandstones from this area compared to analyses of rocks from the western trench (data from Sawyer, 1986) indicate less involvement of older felsic crust as a sediment source. Zircon geochronology (data from D. Davis) demonstrates that the northern Quetico trench received sediment between approximately 50 �2705-2699 Ma, whereas the forearc basic continued to accumulate sediment until at least 2696 Ma. No zircons older that 2828 were found in these sequences. In contrast the zircon population of samples from the western Quetico (Davis et al., 1990) (Fig. 10) contains both 2900 and 3000 Ma zircons, indicating the erosion of older tonalites in this area. A sedimentary assemblage present to the east of Terrace Bay, in Wawa Subprovince, contains a poorly structured turbidite succession which correlates as the distal equivalent of the Quetico trench deposits (Purdon, 1995) (Fig. 10). The geochemistry of these rocks is very similar to the trench and forearc assemblages to their north, with lower Zr and Y than the trench sandstones to the west. Their geochemistry does not match local sources in the Hemlo and Winston Lake areas. Detrital zircon geochronology matches the Quetico trench sediments to the north, with the exceptions that the main zircon population, which probably reflects age of deposition, is 3 Ma younger, and the sandstones contain a 2900 Ma population. Similar turbidites are present in two other areas of the northern Wawa Subprovince; Shebandowan and Manitouwadge. At the former they are younger than 2700 Ma (F. Corfu, pers. comm.), and at the latter they are younger than 2692 Ma (E. Zaleski, pers. comm.). In the Shebandowan belt, the turbidites are succeeded by a 2692 Ma (Corfu and Stott, 1986), high-Na volcanic assemblage interlayered with near-shore, moderate-to high-slope marine deposits. These are in turn succeeded by <2686 Ma (Corfu and Stott, 1998) braided stream conglomerates eroding crystalline basement. This depositional system tract records 2709-2693 calc-alkaline arc volcanism on the southern margin of Wabigoon Subprovince and its erosion and transport to the Beardmore-Geraldton forearc basin and Quetico trench at approximately 2700 Ma. A trench-full state developed at approximately 2696 Ma and the sediment apron expanded to the south covering tholeiitic basalts and a starved clastic-chemical sequence in the area east of Terrace Bay. Similar sequences in Shebandowan and Manitouwadge probably represent overflow in these areas as well. Diachronous overflow younging to the east suggests oblique closure of the arcs comprising northern Wawa Subprovince with the Quetico trench. Deep-marine sediments and volcanic assemblages in the Shebandowan area were upraised at 2692 Ma to surface levels, while deepmarine sedimentation continued in the Manitouwadge area, further indicating west-side-first scissor closure. The second tract encompasses the English River Subprovince and Warclub Group, on the northern margin of Wabigoon Subprovince. The depositional systems which formed these two units were very similar. They are both primarily composed of unstructured medial to distal turbidite assemblages. The Warclub Group thickens and coarsens upward from a 10m thick basal zone composed of a starved slate-chert assemblage that overlies mafic volcanics. Near its top, minor interbedding with ashes of the Berry River Volcanics occurs. It is sharply overlain by the volcanic unit, which is mostly composed of grainflows of felsic volcanic detritus. The Warclub Group is laterally continuous to the east of Dryden, but does not lithically correlate with sediments in the Minnitaki Lake area. Zircon geochronological patterns (Davis, 1995) for the Warclub Group and English River sediments exhibit a variety of ages. This is in contrast to patterns for sediments on the southern margin of Wabigoon Subprovince, which show a clustering of young ages, and indicates that syndepositional volcanism was not an important sediment source. The youngest detrital zircon age determination for the Warclub Group is 2716 Ma (Davis, 1995), which is in agreement with its stratigraphic position below the 2712 Ma Berry 51 �River Volcanics (Davis, 1995). Volcanics interbedded with the Warclub Group near Vermilion Bay give an age of 2716 Ma (Davis, 1995). The youngest detrital zircon present in the English River assemblage is 2705 Ma (Davis, 1995). Turbidites of the English River Subprovince and the Warclub Group accumulated in a deep water setting; in the case of the latter, accumulation occurred directly on a mafic assemblage. The turbidites fed from the erosion of local rocks which were upraised, probably tectonically. Deposition of the Warclub Group ceased at 2712 Ma when a felsic volcanic episode effected its basin. English River sediments continued to accumulate until at least 2705 Ma (Davis, 1995). The Warclub Group was most likely deposited in a remnant ocean basin between the Wabigoon and Winnipeg River landmasses. The English River sediments may represent either a remnant ocean basin or a classical trench. The third depositional systems tract includes the Abram, Minnitaki, Savant, Sturgeon and Conglomerate Lake Groups, and possibly the Crowduck and White Partridge Bay Groups. These sedimentary sequences represent high-slope, fan delta deposits fed into an east-west linear trough which developed south of the north margin of Wabigoon Subprovince. Basal units are dominated by erosion products from the immediately adjacent, underlying lithologies. There is a rapid upward increase in the amount of felsic volcanic detritus, with some sequences almost entirely composed of this material. Reworked sedimentary clasts, representing all fan delta lithofacies, are important constituents of some sequences. Ages of youngest zircons are variable, ranging from post 2707 Ma (Stott and Davis, 1999) for Conglomerate Lake (probably depositional age) to 2699 Ma for Crowduck Lake (D. Davis, pers. comm.). Zircon populations are varied, indicating erosion of older units rather than penecontemporaneous volcanism. The basin system represents proximal foreland basin deposits which were overridden by thrust sheets. Multiple periods of thrusting are indicated by cannibalism, and variation in youngest zircon ages. The Conglomerate Lake assemblage was deposited between 2703 and 2709 Ma; the basal Savant Group at 2704 Ma (Davis, 1995). These ages are similar to ages for cessation of sedimentation in the English River assemblage, and indicate foreland thrusting may be linked to closure of the English River oceanic system. The three depositional systems tracts are interrelated due to the controlling tectonic processes. Closure of the Warclub remnant ocean initiated development of a foreland thrust belt on the northern margin of Wabigoon Subprovince. During the same period, extensive calc-alkaline volcanism commenced on the southern margin of the subprovince, with commencement of north directed subduction, and fed a systems tract which delivered sediment to two other subprovinces. Probably oblique closure of Wawa arc systems terminated the southern depositional system at 2692 Ma in the west, and resulted in upraising of deep-water environments to shallow depths. By 2686 Ma oceanic deposits, which had formed only 14Ma previously, were being eroded by streams draining the Wawa-Quetico-Wabigoon collision zone. The depositional environment of the Coutchiching turbidites is consistent with a source-distal, off fan or braid delta setting. Its sedimentology is also consistent with a distal Quetico ramp setting but its position on the northern, source-proximal basin margin makes this scenario unlikely. The Coutchiching most likely was deposited as a submarine apron to the south of fandeltas fed by thrust-faulting on the northern margin of the Wabigoon subprovince. The age 52 �distribution of zircons from the Coutchiching Group (Davis et al., 1989) is similar to that of coarse-grained metasedimentary sequences present on the northern boundary of the Wabigoon subprovince, all of which have youngest detrital zircons of 2704 Ma, a few m.y. earlier than the Quetico (Davis 1995, Davis 1990). None of these metasediments show the concentration of ages less than 2710 Ma that is characteristic of the Quetico metasediments, suggesting that they are slightly earlier. The Seine Conglomerate represents the youngest pulse of sedimentation in the evolving collision zone between the Wawa-Abitibi oceanic volcanics, the Quetico trench sediments and the Wabigoon craton. The west side first, scissor closure of the Wawa-Abitibi terrain with the Wabigoon resulted in compression and metamorphism in the Rainy Lake area at 2692 Ma while the Shebandowan area 200 km to the east was just starting to be uplifted and the Manitouwadge area 500km to the east still had an active trench system and subducting ocean floor. Scissor closures such as this denote oblique collision or collision of a promontory. In either circumstance, transpression, or partitioning of oblique deformation into boundary-parallel (wrench) and boundary-perpendicular components (folding or thrust faulting) is likely to result (e. g. Harland, 1971). As blocks slide past one another in wrench settings, strike-slip basins can form as dilation zones open at fault curves, terminations with stepovers, or extensional duplexes. Small rifts can also open due to lateral terrain escape from the compression zone. There are several late-stage conglomerates similar to the Seine in the Superior Province, many bearing a striking resemblance to one another. They are known as Timiskaming type conglomerates (e. g. Pettijohn, 1943). Based on sedimentological evidence, including the large clast size, relative rarity of cross-bedding in quartzites, and the predominance of relatively immature graywackes interbedded with the conglomerates, Timiskaming type conglomerates have long been recognized as forming in a dynamic, tectonic environment (Pettijohn, 1943). Specifically, researchers have concluded that the conglomerates may have formed synkinematically, possibly in wrench related basins (Poulsen, 1986; Poulsen, 2000). This conclusion is based on the fact that, like the Seine Group conglomerates, other Timiskaming type conglomerates are often found along major wrench zones within the Superior Province. Detailed provenance studies of Timiskaming-type conglomerates have supported the wrench basin interpretation, in that source areas have been located both north and south of the deposition area (Legault and Hattori, 1994). Thus, due to the scissor-closure interpreted from the Seine clasts’ provenance and the association of the Seine and other Timiskaming type conglomerates with major wrench zones, it may be appropriate to compare the early structures and stratigraphy in the Seine group to those of more modern strike-slip basins. The aerially limited basin into which the Seine was deposited most likely opened due to some wrench-related process, while the general compression in the area upthrust a possibly yoked source terrain. Plate collisions and Deformation In general, the rocks at this boundary show a history of upper crustal stacking and wrenching followed by ductile transpression. The stacking is evidenced by significant amounts of upright bedding and early folds. As deformation continued and rock units became buried, ductile deformation became dominant. This part of the deformation sequence created the dominant S-L 53 �fabrics and was responsible for most of the strain in the rocks. Continued ductile transpression resulted in late-stage crenulations and kinks in the foliation fabric. There is some evidence for minor, brittle structures that formed during the waning moments of deformation. Along the Wabigoon- Quetico boundary, there are two general deformational phases of collision evidenced by the structural field observations. Note that the distinction of two deformational phases is NOT meant to imply that there were two stages of collision, but only that the structural style evolved during the collisional history. The first phase can be interpreted to have included shortening or stacking of strata, frequently coinciding with boundary-parallel motion, in the upper crust. After the first phase and corresponding crustal thickening, the second phase of deformation that created the dominant structural fabric is interpreted to have involved deeper, ductile transpression. The structures preserved in the Seine and surrounding rocks are a result of both their upper-crustal and deeper level deformation. Structural Evidence of the Tectonic Nature of the Seine Basin The sedimentological evidence supports the interpretation that the Seine Group was deposited in a dynamic, tectonic environment. The structural evidence supports this conclusion as well. The Seine is the latest of supracrustal rocks to have been deposited at the Wabigoon–Quetico boundary (Poulsen et al., 1980; Davis et al., 1989; Fralick and Davis, 1999). Despite not containing as many folds as its neighboring rock units, the Seine has also undergone deformation that caused the bedding to become vertical. Even though both the Seine and its neighboring rock units are vertically oriented, the reversal in stratigraphic facing between the base of the Seine and the directly adjacent volcanic units (Lawson, 1913; Poulsen et al., 1980; Poulsen, 2000) implies that the earlier strata were tilted, at least in part, prior to Seine group deposition. Therefore, it can be interpreted that the early stacking deformation began before and endured during deposition of the Seine. Thus the Seine group was deposited in a dynamic, tectonic environment, most likely in a basin formed through strike-slip faulting processes that would be expected at an obliquely convergent margin. The specific type of strike-slip basin (pull-apart, duplex related, fault splay, …) in which the Seine was deposited remains unclear. However, the geometries of the structures within the Seine Group may provide clues as to the nature of this basin. The dominant vertical nature of the bedding implies that the bedding was probably tilted during the first, upper-crustal stage of deformation by some means (folding, faulting, or both). The few relict folds within the Seine suggest that at least part of the basin was undergoing shortening during basin evolution. This observation leads to the conclusion that the basin was most likely not a simple extensional pullapart basin (Poulsen, 1986), but rather some other type of wrench-related basin that contained significant areas undergoing shortening during its formation. If we assume that shear zones form in relatively weaker zones of rocks, the present–day location and orientations of the bounding ductile shear zones may be indicative of the earlier brittle faults. If we follow this logic, the gentle S-shapes of the bounding faults suggest a restraining bend rather than a releasing bend in a dextral strike-slip regime (Fig. 11). This type of restraining bend would likely be associated with thrusting, overturned folding, and localized areas of subsidence and sediment accumulation (Christie-Blick and Biddle, 1985). This scenario seems 54 �Figure 11. A) Bends in dextral strike-slip fault resulting in either a restraining bend and corresponding thrust faults, overturned folds, and sediment accumulation or a releasing bend and the corresponding normal faulting and pull apart basin. Based on Christie-Blick and Biddle, 1985. B) Possible basin that may have formed if Quetico Fault to the north had relatively more displacement than the Seine River - Rainy Lake Fault to the south. likely for the Seine basin. Alternatively, one could imagine that both the southern Seine RiverRainy Lake Fault and the northern Quetico Fault were active master-faults and the secondary features (now seen as secondary shear zones) were insignificant in the early history (Fig. 11b). In this scenario, one might expect a basin to form at the fault intersection if the displacement on the Quetico Fault was significantly more than the displacement along the Seine River-Rainy Lake Fault. Alternatively, it is possible, given the scarcity of folds within the Seine, that the Seine could have been deposited in a pull-apart basin; however, this would require that the sense of motion along the faults was sinistral rather than dextral during this early history. There is no evidence for such a change in sense of fault motion, but a switch in motion sense would be possible given that such evidence would likely have been later obliterated by the dominant ductile fabrics. A change in fault sense would require a change in plate motion or the geometry of the boundary. Given the above three scenarios, it may not be possible to prove any one of them, but it seems that the first scenario, that of a basin forming at a restraining bend, is most likely. The original nappe-like nature of the folds and the likelihood of thrusting (see below) support this scenario. Evidence of Upper Crustal (Brittle) Deformation: Upright bedding as a result of “Stacking” The major piece of evidence for the first phase of deformation is the ubiquitous steep bedding. One might consider two structural end-members, folding and faulting, that could cause the subvertical tilting of strata. After folding, one would expect to see repetition of stratigraphy and opposing stratigraphic facing directions in adjacent strata. Even in areas with upright, isoclinal folds, one might also expect to see some areas with horizontal bedding corresponding to the fold hinges. After faulting, one would also expect to see repetition of stratigraphy, but not necessarily reversals in stratigraphic facing directions. Within the Seine and surrounding regions, it seems likely that the tilting of bedding was achieved, to some degree, through both faulting and folding, a combination referred to here as “stacking” (Czeck, 2001). 55 �There is significant evidence for folding throughout the Superior Province in general and the Wabigoon–Quetico boundary in particular. Evidence for folding includes visible hinges of some folds and opposing directions of stratigraphic facing (Hooper and Ojakangas, 1971; Bauer, 1985; Poulsen, 1986; Hudleston et al., 1988; Bauer and Bidwell, 1990; Tabor and Hudleston, 1991; Bauer et al., 1992; Jirsa et al., 1992; Bauer and Hudleston, 1995; Hudleston and Bauer, 1995). Several large, upright and isoclinal folds have been identified within the Seine metasediments based on stratigraphic facing (scour beds and cross-beds) and lithologic similarity (Hsu, 1971; Wood et al., 1980a; Wood et al., 1980b; Poulsen, 2000). The present–day upright orientation of folds is consistent with either originally nappe-like or more upright folds. In either case, the second phase of deformation, bulk ductile transpression, would have the effect of rotating the fold limbs from steep to subvertical orientations. However, the combination of several observations including the present-day juxtaposition of adjacent right-way-up and overturned folds (Poulsen et al., 1980; Borradaile, 1982; Poulsen, 2000), suggest that the original orientations of many of the folds were nappe-like. There is less direct evidence for thrusting, due to a general lack of sedimentary markers that would enable one to recognize duplicated sequences of rock. Instead, the most convincing pieces of evidence for the existence of faults are observations that suggest that all of the vertical bedding could not be a consequence of folding alone. First, within the Seine metasedimentary sequence, folds are much less evident than those reported in other areas, but the dominance of vertical bedding is still the rule. Second, unlike in rocks faulted by brittle mechanisms, folded rocks often leave a record of strain. It has been shown in some areas within the Superior Province that after “unstraining” the folded component of the preserved strain, the strata are still dipping vertically (Schultz-Ela, 1988). Thus, folding alone is insufficient to account for the vertical bedding. The lack of evidence for faults may indicate that stacking occurred entirely by folding. However, it is also true that if thrust faults were originally present, one would not expect to see evidence of them, other than tilted bedding, after the major fabric-forming deformation (phase 2) took place. Thus, it may be impossible to conclusively determine to what degree the initial stacking event consisted of folds or thrusts. If modern arcs are a suitable analogy to describe the tectonics of the Wabigoon–Quetico boundary, we may consider that while both folding and faulting play a role, faulting appears to be the dominant process at modern arcs (Karig et al., 1979; Karig et al., 1980). Thus, in all likelihood, both thrusts and folds were operational during the stacking phase of deformation at the Wabigoon–Quetico boundary, and it is possible that faulting, although not directly evident, was the dominant process. The relative contributions of faulting and folding to the overall structure may not be equivalent across granite-greenstone terranes, nor even across different portions of the same boundary, such as the Wabigoon–Quetico subprovince boundary. Lack of key exposures and a detailed stratigraphic framework, allied with obfuscation caused by overprinting fabrics from the second phase of deformation, may make it difficult to ascertain the relative contributions of folding and faulting. The term “transpression” was first introduced by Harland (1971) to describe obliquely convergent motion between two crustal blocks, or motion partitioned into convergent and strikeslip components. Harland, in his original analogue experiments, demonstrated this type of partitioning by generating folds within a deforming medium bounded by two rigid obliquely 56 �convergent plates. In an analogous region of deforming rocks, oblique plate collision may partition motion into strike-slip zones parallel to the boundary and folds and thrusts whose strikes rotate during deformation. The probable contemporaneous history of convergence and strike-slip motions along the Wabigoon–Quetico boundary, as manifested by both faulting/folding (stacking) and wrenching, suggests that Harland-type transpression may have been occurring during the stacking phase of deformation. At least this is likely during the stacking phase that occurred contemporaneously with Seine Group deposition. It should be noted that the “stacking” in the Seine may or may not be contemporaneous with any of the “stacking” record within adjacent lithologic domains, even within the Quetico and Seine River - Rainy Lake fault-bounded wedge. In fact, the opposite facing directions found between the Seine and a directly adjacent volcanic unit within the wedge reinforces the idea that at least some of the stacking in adjacent units occurred prior to Seine deposition. Deeper Ductile deformation: Homogeneous Transpression with Variable Extrusion The second phase of deformation formed the dominant structural fabric in the Seine. It formed the penetrative foliation and lineation fabrics, as well as most of the recorded strain. Figure 12. Idealized transpression model according to Sanderson and Marchini (1984) In general, the foliation is subvertical, and the individual clasts within the Seine conglomerates display a flattening fabric with a subvertical plane of flattening. On subhorizontal planes, the clasts commonly form dextral shear sense indicators, regardless of lineation orientation. Most of these fabric features along the boundary are consistent with a type of ductile transpression first described by Sanderson and Marchini (1984). Sanderson and Marchini (1984) provided a mathematical description of a specialized case of transpression: homogeneous deformation consisting of orthogonal simple shear and pure shear components (Fig. 12). In addition to these two components, their model involves constant volume and confines deformation to a vertically bounded zone. Such an idealized scenario is, perhaps, most likely to correspond to strain in deep, vertical ductile shear zones during oblique convergence. The structural fabrics for this model were predicted by Fossen and Tikoff (1993) and are summarized in Fig. 13. The consistencies of fabrics observed along the Wabigoon–Quetico boundary with the homogeneous transpression model suggest that at least the Wabigoon–Quetico boundary as a whole has undergone quasi homogeneous transpression. 57 �Fig. 13. Generalized transpression. Strain (simple shear and pure shear components) and fabrics (foliation, lineation, conglomerate clast asymmetry) based on Sanderson and Marchini (1984) and Fossen and Tikoff (1993). The front and back sides of the boxes are parallel to the deformation zone boundaries. The mineral fabrics are shown in the general case with fabric oblique to the deformation zone boundaries. As strain accumulates, the foliation becomes progressively closer to subparallel with the deformation zone boundaries. Ellipses (some with “tails”) represent schematic clast traces on each plane. Based on Czeck (2001). Significantly, the mineral lineations along the Wabigoon–Quetico boundary are neither vertical nor horizontal, as predicted by the Sanderson and Marchini (1984) transpression model; they plunge between 0-90° in both east and west directions (Fig. 8). The most likely way to create variable obliquely plunging lineations of this type is through a combination of Sanderson and Marchini style transpression with nonvertical extrusion (Fig. 14) (Czeck, 2001). In the original Fig. 14. Schematic view of strain and deformation fabrics for monoclinic transpression with nonvertical extrusion. Light colored ellipses represent schematic clast traces on each plane. (a) Simple view of transpression with nonvertical extrusion. (b) Schematic view of transpression with overall bulk vertical extrusion and localized zones of nonvertical extrusion. Dark colored ellipses represent schematic "hard" zones that influence local extrusion directions. The relative location of Fig. 8a is indicated. Based on Czeck (2001). 58 �Sanderson and Marchini style transpression and subsequent models (Sanderson and Marchini, 1984; Fossen and Tikoff, 1993; Robin and Cruden, 1994; Dutton, 1997; Jones and Holdsworth, 1998; Lin et al., 1998), extrusion of material was assumed to be vertically upwards. This assumption is logical because, in general, one would expect the direction towards the earth’s surface to provide the least resistance for material movement. However, rocks at depth may have other boundary conditions that cause local pressure gradients to deviate from this first-order assumption. For many reasons, such as the anastomosing of shear zones and influences of large lithologically diverse bodies, the local pressure gradients at depth may cause rocks to extrude in nonvertical directions. Given this model, the large range of lineation orientations in the Seine is not surprising considering the lithologically diverse nature of the subprovince boundary and the intricately anastomosing shear zones. As noted by some authors (e. g. Bauer et al., 1992), there may be a final stage of deformation involving amplification of strike-slip motion along wrench zones. This late stage of deformation has often been described as brittle in nature, and thus represents rock exhumation. The presence of brittle faulting of dextral strike-slip motion without contemporaneous thrusting (Kennedy, 1984), may suggest that the latest stage of deformation was almost entirely strike-slip. However, some late-stage brittle faults associated with N-S shortening have been observed (Tabor and Hudleston, 1991). There may also have been an amplification of shortening between the major strike-slip faults, thus creating a more discretely partitioned deformation toward the end of the tectonic history (Tabor and Hudleston, 1991). Evolution of tectonic styles The stacking and the homogeneous transpression most likely developed as an evolution of structural styles during different stages of the same oblique collisional event. The presently exposed rocks undoubtedly underwent a voyage through different zones in the crust as evidenced by their deposition at the surface and ductile deformation at some depth. This gradual change in position within the crust is probably responsible for the observed evolution in deformation styles. It is important to note that the two phases of deformation described here are similar to the commonly discussed “D1” and “D2” described throughout the Superior Province. While specific structural details are different, the general trend of folds overprinted by intense ductile flattening fabrics is common (Poulsen, 1986; Hudleston et al., 1988; Bauer and Bidwell, 1990; Tabor and Hudleston, 1991; Bauer et al., 1992; Jirsa et al., 1992; Bauer and Hudleston, 1995; Hudleston and Bauer, 1995). Locally, rocks at all the Superior Province microplate boundaries may have undergone the same general story of tectonic stacking and wrenching in the upper crust followed by crustal thickening and ductile transpressive flattening as the rocks were deformed at deeper levels. While this general story may be similar, it is important to consider that the regional or even local correlation of these “D1” and “D2” events is probably inappropriate. While the Seine metasedimentary group was forming in a wrench-related basin and surrounding rocks were being stacked, other rocks- even those located along the same microplate boundary or even within the same fault-bounded wedge- may have already reached a deeper level of the crust and began a more ductile, homogeneous phase of transpression. Thus, it is dangerous to correlate exact styles of deformation (e.g. folds) in the Seine with those in either of the adjacent Wabigoon or Quetico subprovinces. 59 �FIELD TRIP STOPS (Stops are located by UTM co-ordinates based on NAD 83, UTM Zone 15) Drive to outcrops located on the west side of Shoal Lake Road, south of Mine Centre. (0526600E 5394850N) STOP 1 - BASAL FACIES OF THE SEINE CONGLOMERATE, EXAMPLE OF LOW DEFORMATION At this location, we are near the basal contact of the Seine. Based on opposing stratigraphic facing in adjacent rocks, an unconformity between the volcanic units and the base of the Seine conglomerate can be identified (Lawson, 1913; Poulsen et al., 1980; Poulsen, 2000). The clasts within the Seine include more tonalite than is typical further up in the section. A possible saprolite may be observed between the volcanics and the Seine (C. Hemstad, pers. comm.). Strain at this location is unusually low. Foliation and bedding attitudes are atypical, subparallel with a more NE/SW strike and shallower (~65°) southeasterly dip than is typical for the rest of the Seine’s area. In addition, the clasts appear to be much more angular and irregularly shaped than the rest of the Seine. This is a highly unusual outcrop of Seine that poses several interesting questions that are open for discussion. We pose some here with the hope of encouraging some reflection and discussion at the outcrop. The orientations of the bedding and foliation within the Shoal Lake area and adjacent rocks to the north and south combine to create a large, gentle S structure, with shallower dips here on the middle section of the S. There are several different ways to interpret this sigmoidal map pattern. The various interpretations have bearing on the timing of the geometrical arrangement of structures with respect to the phases of deformation, and therefore will also influence our interpretations of the nature of the Seine basin. Five interpretations are suggested here: 1) The variation in orientation of foliation around the Shoal Lake area may be due to a late stage, gentle folding event. This is suggested by the similar changes in strike and dip of both foliation and bedding. The fold axis of this structure is approximately 55°/077. 2) As suggested by Poulsen (2000), the faults and small-scale shear zones could be equivalent to those created in fault models of strike-slip systems such as those of Tchalenko (1970), Lowell (1972), and Wilcox et al. (1973). If this were the case, since these models involve Mohr-Coulomb failure, the orientations of the faults would have been determined relatively early in the deformation history, during Harland-type transpression, when the rock would have behaved in Mohr-Coulomb fashion. In this scenario, the Rainy Lake – Seine River fault was the “master fault,” and the Quetico and smaller shear zones formed later as second order conjugate shears. Since there is significant, deeper level, ductile deformation recorded along these same faults, they must have been reactivated during ductile deformation because they were zones of weakness. 3) Alternatively, the orientation of the faults may have been determined during the later ductile transpressive stage of deformation. In this case, the orientation of the major faults and the minor 60 �shear zones would constitute a mega-scale S-C feature creating the sigmoidal pattern seen on the map (Fig. 9). 4) It is also possible that the largest faults were formed by brittle processes, buried and then reactivated as ductile shear zones, with the smaller shear zones being formed later purely by ductile means. 5) Yet another interpretation is possible. It could be that the sigmoidal shape of the foliation is related to lithological contrasts, with foliation wrapping around more rigid bodies. In such a scenario, the rocks to the east of the Bad Vermilion intrusive complex (the large plutonic unit in the west-central portion of Fig. 2) were caught in a large strain shadow region (Borradaile and Dehls, 1993). This hypothesis is consistent with the lower strain found in this area. The apparent “wrapping” of the fabrics around this intrusion seems analogous to the “wrapping” of foliations and lineations around relatively rigid conglomerate clasts at a smaller scale. Not surprisingly, there exists a similar asymmetry in shape between the rigid conglomerate clasts and large rigid units such as the Bad Vermilion intrusive complex. In addition, the shapes of the clasts here are much more angular than the rest of the Seine clasts. If one were to consider studies that model nonspherical clasts in deformation, we would expect more deformed outcrops of Seine to have barrel or bone shaped deformed clasts (Treagus et. al, 1996; Treagus and Lan, 2000). As we will see at the next outcrops, this is not the case. Why? Drive East along Hwy 11, right onto Forest Tour Road. (0536650E 5398850N) STOP 2 - TWO-DIMENSIONAL HORIZONTAL VIEW OF SEINE CONGLOMERATE, SANDY LENS AND DEXTRAL SHEAR INDICATORS This outcrop allows us to see a large 2D view of the Seine. The deformation here is typical of much of the field area, the granitoid clasts being fairly undeformed while the volcanic clasts reflect significant flattening. Minor amounts of quartzite, BIFs, and other lithologic clast types are also observable. Dextral shear sense indicators are prominent including asymmetric pressure shadows and clast tilings. Many sandy beds and channels, often with graded bedding, can be located within this outcrop. Is it possible to determine stratigraphic facing here? Drive East along Hwy 11 to Horsecollar Junction. (0542150E 5398500N) STOP 3 - THREE-DIMENSIONAL VIEW OF MODERATELY DEFORMED SEINE CONGLOMERATE This outcrop allows us to see a 3D view of the Seine. The deformation here, as at the last stop, is typical of much of the field area with average strain and dominant flattening fabrics. Here we can observe the subparallel, undulatory nature of bedding and foliation. The lineation plunge here is relatively steep (~78°E). The dextral asymmetric indicators are most prominent on the subhorizontal plane. 61 �Drive East along Hwy 11. (0559500E 5398900N) STOP 4 - ULTRA DEFORMED SEINE CONGLOMERATE WITH ALTERATION The conglomerates here are extremely deformed. Both volcanic and plutonic clast types are extremely flattened. Many clasts are flattened beyond recognition, giving the rock a striped appearance. Is it possible to estimate strain in a rock that is this deformed? The lineation plunge here is typical of the field area (~44°E). While this lineation is “average” stretching lineation for the Seine, it is in no ways representative of the typical deformation due to the wide variance in lineation orientation. The lineation orientation does not vary systematically across the field area and is not directly related to the amount of strain (Czeck, 2001). Again, the dextral asymmetric indicators are most prominent on the subhorizontal plane. This is the case throughout the field area, regardless of lineation orientation, a result that one would not expect in either an ideal strike-slip shear zone or homogeneous transpression. The combination of subhorizontal asymmetric shear indicators and variable lineation orientations makes a deformation model of quasi homogeneous transpression with a variable extrusion direction most likely (Czeck, 2001). The extensive carbonate alteration in this rock suggests that fluid flow was an important factor during deformation. This implies the possibility of volume loss in these ultra deformed rocks. There was probably a symbiotic relationship between fluid localization and enhanced deformation. Drive back West along Hwy 11, stop just east of Seine River bridge. (0551850E 5398700N) STOP 5 - SMALL FOLD IN SEINE At this stop, we can observe the sandier facies more typical of the upper part of the Seine sequence. Cross beds and localized deposits of gravel can be observed. The cross beds allow us to recognize stratigraphic facing. The cross beds are deformed, and a good example of a meter-scale fold evident. This fold is asymmetric and upright with its axis oriented 6°/S83W. This subhorizontal fold axis orientation is similar to fold axes orientations described in the Quetico Subprovince and contrasts with the subvertical fold axes described in the Wabigoon subprovince (Borradaile, 1982). Localized prolate strain, atypical of the field area, is evident in the fold hinge. Down the road, just east of this outcrop, another small fold can be observed in the sandy facies. Does this fold have the same subhorizontal fold axis? 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Ontario Geological Survey Preliminary Map P. 2201, scale 1:15840. 67 �68 �Field Trip 4 Industrial Minerals and Paleozoic Geology of Southeastern Manitoba James D. Bamburak and Ruth K. Bezys Manitoba Geological Survey 360-1395 Ellice Avenue Winnipeg, Manitoba R3G 3P2 Interior of the Manitoba Legislative Building, Winnipeg, built in the early 1900s. Decorative dolomitic limestone, the world famous "tapestry stone" with its unique mottled appearance, is still being quarried from the Selkirk Member, Red River Formation in the Garson area. �FOREWORD Currently, the Province of Manitoba has 18 industrial mineral processing plants and quarries, excluding the production of aggregate (Fig. 1 and Table 1). The 2001 estimated value of industrial mineral production in the province is $78.1 million, including aggregate production that forms slightly less than half of the total. The $78.1 million represents 7% of the province’s mineral production. Fourteen of Manitoba’s industrial mineral processing plants and quarries are situated in southeastern portion of the province. Figure 1. Industrial mineral producing plants and quarries in southern Manitoba Industrial minerals produced in southeastern Manitoba include lithium, cesium, tantalum, sand, aggregate, dimension stone and peat. Peat is considered as a quarry mineral under the Manitoba Mines and Minerals Act. Lithium, cesium, and tantalum are produced for export from the Tanco mine in the Bernic Lake pegmatite (Field Trip No. 1, this volume). Sand and aggregate are quarried for use in local construction. Dimension stone and peat are quarried for local consumption and export. Two companies, Sun Gro Horticulture Canada Ltd., and Premier Horticulture Ltd. extract horticultural quality sphagnum peat. The locations of the peat operations are shown on Figure 1 and listed in Table 1. Five companies, Cold Spring Granite (Canada) Ltd., Gillis Quarries Limited, Carrieres Polycor Inc., Manex Granit Ltd. and Canital Granite Ltd. quarry dimension 70 �Table 1 INDUSTRIAL MINERAL PROCESSING PLANTS AND QUARRIES 2002* No. 1 2 Location Elma (P&Q) Whitemouth (Q) Company Sun Gro Horticulture Canada Ltd. Carrieres Polycor Inc. 3 Bernic Lake (P&Q) Tantalum Mining Corporation of Canada Ltd. 4 Winnipeg (P) Westroc Inc. 5 6 Brandon Winnipeg (P) Nexen Inc. Canital Granite Ltd. 7 Moss Spur (Q) 8 Lac du Bonnet (P&Q) dimension stone 9 Garson (P&Q) 10 Medika (Q) Sun Gro Horticulture Canada Ltd. Cold Spring Granite (Canada) Ltd. Gillis Quarries Limited Manex Granit Ltd. 11 Meditation Lake (Q) Manex Granit Ltd. 12 PR 307&309 (Q) Manex Granit Ltd. 13 14 15 16 17 18 Julius North (P&Q) Caribou Cluster (Q) Giroux (P&Q) Harcus (Q) Faulkner (P&Q) Flin Flon (P) Sun Gro Horticulture Canada Ltd. Premier Horticulture Ltd. PremierHorticulture Ltd. Westroc Inc. Graymont Western Canada Ltd. Hudson Bay Mining and Smelting Co., Limited Product Peat moss Granite, dimension stone Tantalum oxide, spodumene and amblygonite concentrates, cesium and rubidium ores and cesium formate Gypsum wallboard Sodium Chlorate Granite, dimension stone Peat moss Granite, Tyndall stone Granite, dimension stone Granite, dimension stone Granite, dimension stone Peat moss Peat moss Peat moss Gypsum Lime, limestone Sulphur (P) Plant (Q) Quarry * excludes aggregate producers stone. Gillis quarries a dolomitic limestone, the renowned "Tyndall Stone", and the other companies quarry granite. Locations of the stone quarries are shown on Figure 2. INTRODUCTION This one day field trip is designed to visit three industrial mineral sites, including one Lower Paleozoic site. The sites that will be visited include: • the sphagnum peat bog harvesting operation and plant of SunGro Horticulture Canada Ltd. near Elma (Stop 1); 71 �• the granite dimension stone quarry and plant of Cold Spring Granite (Canada) Ltd. near Lac du Bonnet (Stop 2); and, • the Ordovician Tyndall Stone quarry and plant of Gillis Quarries Limited at Garson (Stop 3). Figure 2. Granitic dimension stone quarries in southeastern Manitoba. Sphagnum Peat Canada holds more than a third of the world’s peat resources with 1 223 834 km2 of peatlands, or 12 percent of the total land mass (Tarnocai et al., 1995). Approximately 40% of Manitoba's surface is covered with peat deposits, many of which are inaccessible and/or of uneconomic thickness and quality to be harvested (Dixon and Stewart, 1988). Nevertheless, Manitoba holds vast reserves of peat suitable for horticultural or energy peat production. Two companies, Sun Gro Horticulture Canada Inc. (Stop 1) and Premier Horticulture Inc. harvest 6 bogs in southeastern Manitoba (Fig. 1) for horticultural quality peat. An aerial view of a typical peat extraction area is shown in Figure 3. Peat companies hold almost 4000 hectares of leases in good standing, and 5500 hectares of pending leases in the Interlake and southeastern Manitoba. Peat production in Manitoba 72 �in 2001 was estimated to be worth over $25 million, and this figure is sure to increase as new companies bring their bogs into production. Figure 3. Caribou cluster peat extraction area of Premier Horticulture Inc. Horticultural quality sphagnum is the preserved, but undecomposed, remains of sphagnum moss plants. Sphagnum deposits accumulate in areas of poor drainage where the rate of atmospheric precipitation exceeds the rate of evapotranspiration, i.e. the low boreal forest climatic zone. The accumulation of sphagnum occurs within an acidic, nutrient poor environment, above the level of the local water table. The characteristics of sphagnum that allow it to survive in this environment are the same characteristics that make it valuable to the horticultural industry as a growing medium and soil conditioner (Schmidtke and Bamburak, 1996). These characteristics are: 1. the ability to absorb approximately 20 times its weight in water; 2. a high capacity for cation exchange; 3. a fibrous structure that introduces volume and pore space to a soil mix; 4. compressibility; and, 5. the ability to resume its precompression volume after compression is released. Sphagnum moss is composed largely of rigid walled hyaline cells. The function of these cells is to absorb and hold water. Since the sphagnum must get all its nutrients from the 73 �nutrient poor atmospheric precipitation, it is able to absorb approximately 20 times its weight in water and has a high cation exchange capacity. These properties make it a valuable growing medium in places like Texas where the native soil is fine and does not retain moisture. The hyaline cells are compressible and will resume their shape even after being compressed to a 10:1 ratio. The sphagnum moss can be compressed into bales for efficient transport. These properties are retained after the plant has died and even when it is slightly humified. Since the sphagnum accumulation takes place in an acidic, oxygen poor environment, it is possible for several metres of relatively undecomposed sphagnum to develop. The high quality bogs in southeastern Manitoba accumulated sphagnum to a maximum depth of approximately 2.5 m over a period of 4000 years. A bog must be prepared before peat extraction occurs. First the trees and roots are removed, ditches are dug, and the bog is drained. It may take two years to prepare a bog for harvesting. Sun dries the surface of the bog, which is then raked using a harrower (or cultivator) to loosen the surface peat. This loose, dry sphagnum peat is lifted from the surface with vacuum harvesters (Fig. 4). The harvesters empty the peat into stock piles, or winrows. The stock piled peat is either moved into the plant for processing or is stored in plastic "silage" tubes for future processing when unfavourable weather prohibits harvesting. (Peat can't be extracted if the weather is too wet or too dry, because quarrying equipment can't operate on a bog that has been saturated by rain or melt water and sparks from equipment can ignite peat dust in hot, dry weather). Figure 4. Conga line of peat vacuum harvesters on Elma bog of Sun Gro Horticulture Canada Inc. Once in the plant, the peat is dried (if necessary). Popped perlite is added as a volumizer. The peat is treated with surfactants, which increase absorptive capacity, and fertilizers. 74 �The type and quantity of the chemicals added to the peat are dependent on the intended end use. Specialized mixes are available for several applications, i.e. soil mix for violet plants. The peat is baled, shrink-wrapped on pallets and stored in the warehouse before being loaded onto semi trailers. Some of the peat is sold in local markets, but most goes to nurseries and greenhouses in the southern United States. The peat companies take advantage of backhaul rates by shipping the peat south in trucks that bring produce to Manitoba from the southern United States. Ninety percent of the peat from Sun Gro's Manitoba quarries is exported to Texas. If the Sun Gro operations in Alberta or New Brunswick are unable to produce because of bad weather or labour problems, Manitoba peat is diverted to markets west or east of Texas to cover the shortfall. Both Sun Gro Horticulture and Premier Horticulture operate harvesting operations and plants across Canada (Schmidktke and Bamburak, 1996). Granitic Dimension Stone Four companies quarry Precambrian granitic dimension stone at 5 sites in southeastern Manitoba (Fig. 2). They include: Cold Spring Granite (Canada) Ltd., Carrieres Polycor Inc., Manex Granit Ltd. and Canital Granite Ltd. At all sites, the granitic outcrops possess unique physical features that permit the quarrying of large blocks of stone Schmidtke (1993). They include: 1. widely spaced, preferably orthogonal fractures that will allow removal of blocks with a minimum trimmed size of 2.0 m by 1.25 m by 1.25 m; 2. widely spaced, or preferably the absence of, veining; 3. homogeneous, attractive and fashionable colours and textures; 4. the absence of minerals that pluck when polished, or oxidize and cause unsightly rust spots when exposed to the elements; 5. road access; 6. proximity to transportation routes, finishing facilities and markets; and, 7. acceptable strength values that meet ASTM standards (as per Annual Book of ASTM Standards). Two of the granitic dimension stone cutting and polishing plants are located in southeastern Manitoba (Fig. 1 and Table 1). One plant (Stop 2) is adjacent to the quarry near Lac du Bonnet (Cold Spring Granite), and the other plant is situated in Transcona, on the east side of Winnipeg (Canital Granite). Tyndall Stone 75 �At Garson, Tyndall Stone is quarried by Gillis Quarrries Ltd. (Fig. 1 and Table 1), east of the plant. This famous dimension stone, sometimes called “tapestry stone”, occurs in the lower half of the 43 m thick Selkirk Member of the Ordovician Red River Formation. Its unique appearance comes from a matrix of light-coloured limestone, with mottled areas of dark dolomitic limestone distributed uniformly throughout. The horizontal beds are approximately 60 to 100 cm thick, beneath 1 to 6 m of overburden consisting of soil, lake clays and stony glacial till. The upper four beds in the quarry (total thickness 2 to 4 m) have either a buff and golden-brown buff matrix. The fifth layer down has a matrix, which is transitional into the underlying beds that have a steel-gray matrix (Wilkins, 1986; Coniglio, 1999). After the overburden is stripped, the stone is extracted using two 2.5 m diameter circular saws, drilling and wedging. The 6 to 8 tonne stone blocks are taken into the plant where they are cut and finished using diamond tipped saws or are ground, sheared or split into a variety of products. Finishes available are rubbed (machine smoothed), bushhammered, pointed face, rough cut, sandblasted, split and rustic. Stone is usually marketed as dimension stone, which is cut and shaped to specification or as random ashlar, which are pre-cut into standard or random shapes to be set into mortar (Wilkins, 1986; Coniglio, 1999). 76 �REGIONAL GEOLOGICAL SETTING The Province of Manitoba is completely underlain by 3.0 to 1.7 billion year old Precambrian rock, which is overlain in the southwest and northeast by younger (less than 570 million years old) Phanerozoic sedimentary rocks (Fig. 5). Pleistocene and Holocene deposits, younger than 2 million years in age cover most of these earlier rocks. Industrial minerals in Manitoba range, in age and form, from Precambrian dimension stone to Holocene peat deposits. Figure 5. Principal geological domains of Manitoba Within southeastern Manitoba, the Precambrian surface is exposed in the east half of the area and is known as the Precambrian Shield. The surface dips to the west (Fig. 6), where it is covered by an increasing thickness of Lower Paleozoic strata which also dip gently to the southwest at approximately 2.8 m per km (Fig. 7). The eastern edge of the Paleozoic outcrop belt represents the Manitoba Lowland or First Prairie Level, which is 77 �bounded on the east by the Precambrian Shield, and on the west by the Manitoba Escarpment. Figure 6. Precambrian structure contour map. The Manitoba Escarpment forms the eastern edge of the Second Prairie Level, which is underlain by Cretaceous strata dipping gently to the southwest at 1.5 to 1.9 m per km. The actual Escarpment is composed of soft, easily eroded sands and shales in the lower part of the Cretaceous, underlying a resistant shale cap (Odanah Member of the Pierre Shale). In southeastern Manitoba, industrial minerals are mined from Precambrian, Lower Paleozoic and Holocene localities. Granite quarries (Fig. 2) are situated within late tectonic granitic batholiths and plutons on the western edge of the Archean Superior Province. The Ordovician Tyndall Stone quarry (Gillis Quarry, Fig. 1) occurs within the eastern edge of the Paleozoic outcrop belt. The harvested spahagnum peat bogs (Fig.1) 78 �are located in areas of thick glacio-lacustrine sediments that overlie Archean terrane or Ordovician strata. Figure 7. Structure cross section, southern Manitoba. Precambrian Basement The Precambrian of southeastern Manitoba is comprised of younger Archean granitic batholiths or plutons that have intruded the older greenstone, sedimentary and gneissic formations of the Wabigoon, Winnipeg River, Bird River and English River domains of the Superior Province (Fig. 5). The structure contours on the buried Precambrian surface are shown in Figure 6. All granitic dimension stone production to date has been derived from the Winnipeg River and Bird River domains. The Medika and Betula Lake plutons are situated within the Winnipeg River Domain. The Lac du Bonnet Batholith (LDBB), located to the northwest, lies within the Bird River Domain. For the purpose of the visit to Cold Spring’s Lac du Bonnet Quarry (Stop 1), the following will focus on the latter. The LDBB (outlined on Fig. 2) is the youngest intrusion in the Winnipeg River area (Tammemagi et al., 1980). The batholith is a predominantly pink granite that extends over approximately 1000 km2 from Pointe du Bois southwestward beneath Paleozoic cover. The largest exposures of the batholith occur east of the town of Lac du Bonnet, but isolated outcrops are found as far west as the farmlands directly north of Beausejour. Many of the outcrops of Lac du Bonnet granite have widely spaced fractures, which makes them potential sources of dimension stone (Schmidtke, 1993). 79 �The geology of the Lac du Bonnet Batholith (LDBB) has been the subject of intensive study by Atomic Energy of Canada Ltd. (AECL) as a research site for geological, geotechnical and hydrogeological studies to determine the potential for storing nuclear waste at depth in granite. The regional geology has been described, in detail, by Tammemagi et al. (1980) and McCrank (1985). Lower Paleozoic Stratigraphy The Lower Paleozoic beds (Table. 2) exposed in southeastern Manitoba form part of the Manitoba outcrop belt that is located on the northeastern edge of the Western CanadaSedimentary Basin (WCSB). The WCSB is a composite feature which includes both the Elk Point Basin (Fig. 8), centered in south-central Saskatchewan (which controlled Devonian deposition), and the Williston Basin, centered in northwestern North Dakota (which controlled the depositional patterns throughout the remainder of postCambrian time). Since the Manitoba outcrop belt appears to be situated on the northeastern edge of the sedimentary basin, and roughly parallels the regional structure contours, one might surmise that the strata comprising the outcrop belt would be relatively uniform in lithology. The outcrop would also represent marginal shelf-type deposits relative to the thicker, more basinal sedimentary sequence found to the southwest in the subsurface. However, this is not the case for most Paleozoic formation in southwest Manitoba. The outcrop belts, particularly the Ordovician and Devonian, show marked changes in both thickness and lithology, indicating a complex and varied tectonic and depositional framework (Bezys and McCabe, 1996). The outcrop succession is not marginal to the depositional basin, but rather exposes a series of dip sections of the basin, which show the maximum possible isopach and lithofacies variation. As well, the directions of the dip sections are opposite: basinal Ordovician outcrops occur at the southern end of the outcrop belt, whereas basinal Devonian outcrops occur at the northern (or northwestern) end of the outcrop belt. Regional strike of the Paleozoic strata is approximately north-south, and regional dip increases gradually and uniformly from about 2.6 m/km in the eastern part of southeastern Manitoba to 4.2 m/km in the western part. Despite the regional structural dip to the southwest, isopachs of the Winnipeg and Red River formations all trend eastwest and thicken to the south at up to 0.3 m/km (Fig. 9B and 10B, respectively). This indicates a major change in tectonic framework subsequent to early Paleozoic time, as mentioned previously. The present north-south structural trend probably developed during late Paleozoic to early Mesozoic, due to uplift with associated erosion and eventual exposure of Precambrian bedrock in southeastern Manitoba. A detailed discussion outlining: the evolution of the complex pattern of structural trends in the Lower Paleozoic; a regional tectonic control for apparent anomalies in facies trends; and other related structural and stratigraphic anomalies are described in Bezys and McCabe (1996). 80 �Table 2. Geological formations in Manitoba. 81 �Figure 8. Major structural features of the Elk Point and Williston basins. Ordovician Winnipeg Formation The Winnipeg Formation, a quartzose sandstone interbedded by green, waxy shale with sand and silt interbeds, is exposed in outcrop east at the northeast end of the southeastern Manitoba area, on Black Island and near Seymourville. Structure contours and isopachs 82 �Figure 9A. Winnipeg Formation structure contour map. 83 �Figure 9B. Winnipeg Formation isopach map. 84 �Figure 10A. Red River Formation structure contour map. 85 �I * I •1 Red River Formation IsopachMap - Sm —— Churchill Superior :---- BoundaryZone Outcrop belt suborop I LTt1,1OJW zk Figure 10B. Red River Formation isopach map. 86 I N _______ kilonwtres �for the Winnipeg Formation are shown in Figures 9A and B, respectively. The formation was described, in detail, by McCabe (1978). Red River Formation The Red River Formation consists of two principal subunits, the lower Red River and upper Red River strata (Table 2). In the vicinity of the south basin of Lake Winnipeg, the lower Red River (Baillie, 1952) can be subdivided into three mappable members: a lower Dog Head Member, a medial Cat Head Member, and an upper Selkirk Member (=Tyndall Stone). Lower Red River strata consist of light grey to yellowish- and brownish-buff, prominently mottled, fossiliferous, commonly cherty, dolomitic limestones. The upper Red River strata consist of dolomite and argillaceous cherty dolomite, designated as the Fort Garry Member. A thin, high calcium limestone bed occurs locally at the top of the Fort Garry Member. Stucture contours and isopachs for the Red River Formation are shown in Figures 10A and B, respectively. At Garson, Gillis Quarries Limited (Stop 3) is actively quarrying the Red River Formation (Selkirk Member) for its dimension stone (Tyndall Stone). Stony Mountain Formation The Stony Mountain Formation is subdivided into three members, in ascending order: the Gunn, Penitentiary, and Gunton (Table 2). The Williams Member was once included within the Stony Mountain Formation; however, standardized correlations established for the new Atlas of the Western Canada Sedimentary Basin (Norford et al., 1994) have placed the Williams into the overlying Stonewall Formation (Bezys and McCabe, 1996). The Gunn Member consists of greyish-red to purplish-grey, fossiliferous, calcareous shale with interbeds of relatively clean, fossiliferous limestone. The Penitentiary Member consists of yellowish- to reddish-grey, fossiliferous, argillaceous dolomite. These two members together comprise the lower Stony Mountain. The upper Stony Mountain (Gunton Member) consists of a buff, finely crystalline, sparsely fossiliferous, nodular-bedded dolomite that is relatively uniform in thickness and lithology. All three members are exposed in the Mariash Quarry and abandoned City of Winnipeg quarries at Stony Mountain, an erosional outlier of the Stony Mountain Formation, located 7 km southwest of the Town of Stonewall, near the western edge of the southeastern Manitoba area. The Gunton Member acts as a cap rock for a shallowly buried, east-facing, north-trending escarpment (Gunton Escarpment), 4 km east of the Town of Stonewall. The Gunton Member is extensively used for crushed stone, extracted from quarries in the Stony Mountain and Stonewall areas. The stone has also been used to construct buildings in Winnipeg, Stony Mountain (including the Federal Penitentiary) and in Stonewall. 87 �Ordovician/Silurian Stonewall Formation The Williams Member, is the basal unit of the Stonewall Formation (Bezys and McCabe, 1996). It represents the oldest of a series of so-called “para-time-stratigraphic” markers; thin sandy and/or argillaceous beds that can be traced for many hundreds of kilometres throughout most of the Williston Basin (Table 2, Fig. 8). These marker beds provide the primary means for stratigraphic subdivison of Upper Ordovician and Silurian strata and probably represent deposits related to brief periods of shoaling or even slight uplift and erosion (i.e. non-sequences) (Porter and Fuller, 1959). The Williams Member consists of buff to grey to red, sublithographic dolomudstone. The lower Stonewall beds, above the Williams Member, consist of pale yellowish-grey to yellowish-brown, faintly mottled, medium- to thin-bedded, finely crystalline dolomite with sparse, poorly preserved fossils. A sandy argillaceous marker bed, the “t-marker” or “t-zone”, separates the lower Stonewall Formation from the upper Stonewall Formation. The upper Stonewall Formation consists of light brown to grey, laminated to thin-bedded, sparsely fossiliferous microcrystalline dolomite, which is capped by a grey to buff dolomudstone marker bed, the Upper Stonewall Marker. According to Bezys and McCabe (1996), the t-marker within the upper part of the Stonewall Formation, also marks the position of the Ordovician-Silurian boundary in the Williston Basin. This was confirmed in biostratigraphic studies, based upon outcrop and subsurface investigations. The formation was previously quarried in the Town of Stonewall for lime and aggregate production. Silurian Interlake Group The Interlake Group, consisting of the Fisher Branch, Moose Lake, Atikameg, East Arm and Cedar Lake formations (in ascending stratigraphic sequence), is exposed near the western margin of southeastern Manitoba area. In the subsurface, the group consists of yellow-orange to grey, fossiliferous, oolitic, stromatolitic dolomite, interrupted by sandy, argillaceous marker beds. Jurassic and Cretaceous South of the City of Winnipeg, within the Manitoba Lowland, a large area of Jurassic sediment infills a major pre-Mesozoic channel in the Paleozoic erosion surface. Also, many small Cretaceous outliers have been noted in karst features penetrated by water wells (Bannatyne, 1988). Recent Six peat quarries (Fig.1) are located in areas of thick glacio-lacustrine sediments that overlie Archean terrane or Ordovician strata. Over the past 4000 years, sphagnum plants have contributed organic matter that has accumulated to a maximum depth of approximately 2.5 m. 88 �Near Elma, Sun Gro Horticulture Ltd. (Stop 1) harvests sphagnum peat from spring to fall from a drained bog. Sphagnum peat is also produced on a seasonal basis at 5 other sites, on a seasonal basis, by Sun Gro Horticulture Canada Inc. and Premier Horticulture Inc. 89 �FIELD TRIP STOPS Leave Kenora, on Trans Canada Hwy, travel west for 110 km to Prawda, at junction of Trans Canada Hwy and PR 506. Continue on Trans Canada Hwy (west) to Hwy 11 (11 km), turn right (north) to Sun Gro Horticulture entrance road (12 km), turn left (west), park in visitor’s parking area. STOP 1 – SUNGRO HORTICULTURE SPHAGNUM PEAT BOG AND PLANT The Elma bog (Fig. 1 and Table 1), quarried by Sun Gro Horticulture Canada Ltd., is approximately 3000 acres (12.14 km2) in area and has been in production since 1969. The on site plant was completed in 1972. Peat is quarried using the vacuum harvesting method described above. The bales are loaded at the plant into semitrailers and shipped to the southern United States. A small percentage of the peat is sold for local consumption at retail stores in Manitoba. Reserve estimates have not been published by the company. Leave Sun Gro parking area, travel east on exit road back to Hwy 11, turn left (north) for 50 km to Cold Spring Granite entrance road, turn left (west), park in visitor’s parking area. STOP 2 – COLD SPRING GRANITE DIMENSION STONE QUARRY AND PLANT The quarry and plant of Cold Spring Granite (Canada) Ltd. (Fig. 2) are located in the south central area of the LDBB on a 1220 m long 6 to 8 m high ridge approximately 10 km south of the Town of Lac du Bonnet (Fig. 2). The quarry is accessed via Highway 11. The Precambrian monadnock (Bezys et al., 2001) was first quarried from 1933 to 1949 by a local, Ivor Peterson, for tombstones. An American company, Cold Spring Granite Ltd., reopened the quarry in 1959 and has produced stone from the ridge to the present time. The product is a fine grained pale rose granite sold under a variety of trade names including Lac du Bonnet, Canyon Rose, Colonial Rose and Canadian Mist. Even grained rock is used for monuments and building stone, textured or variegated rock is used for tiles and countertops. The plant at Lac du Bonnet is equipped to make grave markers, countertops, paving and landscaping material and structural panels. Blocks are shipped to the Minnesota plant for finishing into headstones, mausoleums, monuments, columbariums, structural panels, tiles, custom design industrial work, paving and landscaping material. Fine grained, even textured Lac du Bonnet granite is prized for grave markers, monuments and headstones, because sandblasted letters and designs stand out well. It is also a preferred rock for precision industrial applications because it takes a very tight smooth polish, i.e. precision milling surfaces. Rock is sold locally from the Lac du Bonnet quarry, and internationally through Cold Spring Granite's office in Cold Spring, Minnesota. 90 �Figure 11. Granite dimension stone blocks being removed from Cold Spring Quarry in 1982. Prior to 1987, rock was removed from the quarry with wire saws and moved into the plant using hoists (Fig. 11). Since 1987, blocks have been removed by drilling and blasting. Sections of the outcrop are drilled off with portable, track-mounted, hydraulic drills. The drilled sections are then separated from the outcrop by blasting. The blast must move the section of rock without shattering it or inducing microfractures. The sections of rock are then drilled and wedged into smaller blocks that are moved to the plant with a 988 Cat loader. The plant has a 10 wire slab saw, a Salvatore 16 head polishing machine, a JB 110 granite milling machine, a 24" diamond saw and 6', 2', and 1' hydraulic splitters. Blocks are cut into slabs with the wire saw. The slabs are cut and polished with the diamond saw and the polishing machine for use as structural stone. Polished slabs are also manufactured into paving stone and grave markers with the hydraulic splitters and into countertops and furniture with the milling machine. Raw blocks are shipped to plants in Montreal for manufacture into granite tile and to the plant in Cold Spring, Minnesota to be processed for all other applications. Leave Cold Spring parking area, travel east on exit road back to Hwy 11, turn right (south) to PTH 44, turn right (west) to Garson (total 55 km) arrive Gillis Quarries, park in visitor’s parking area. STOP 3 – GILLIS TYNDALL STONE QUARRY AND PLANT Gillis Quarries Limited quarries an 8 m thick section of pale yellowish brown, dolomite mottled, burrowed, fossiliferous micrite of the Selkirk Member of the Ordovician Red River Formation at Garson (Fig. 1 and Table 1). In the quarry, the well known “Tyndall Stone”, is extracted using two 2.5 m diameter tungsten carbide-toothed circular saws, 91 �(Fig. 12), followed by wedging of the blocks along the bedding planes. The stone is finished in the plant, as described earlier. Three colours of stone are produced from various parts of the quarry – buff (a light creamy beige with pastel brown mottles) and golden buff (possibly due to ground water) from the upper beds. And, gray (a pale bluish grey with gray-brown mottles) from the lower beds (Wilkins, 1986; Coniglio, 1999). Figure 12. Carbide toothed circular saw in Gillis Tyndall Stone Quarry. The quarry is noted for its well-preserved fauna of large cephalopods, gastropods, corals, stromatoporoids, bryozoans, crinoids, trilobites, brachiopods, bivalves and calcareous algae, etc. A waste pile is available to hunt for fossils. Gillis Quarries Limited has been a family-owned business since 1915 when August Gillis and his son, Charles, acquired a quarry property in Garson. The stone was finished in Winnipeg, until 1968 when the Garson plant was built. Gillis Quarries has owned all the quarry property in Garson since 1973. The company estimated that based on the 1986 production rate, it had at least 100 to 125 years of stone in reserve (Garson and District History Book Committee, 1990). References: Corehole M-3-69 (internal government core logs) (Garson Quarry, 15-3-136EPM). Go east on Hwy 44, 7 km to Hwy 12, turn right (south) and continue 38 km to the Trans Canada Hwy. Turn left (east) and continue to Kenora (175 km). 92 �REFERENCES Baillie, A. D. 1952. Ordovician geology of Lake Winnipeg and adjacent areas, Manitoba. Manitoba Mines Branch Publication 51-6, 64 p. Bannatyne, B.B., 1980. Sphagnum bogs in southern Manitoba and their identification by remote sensing; Manitoba Energy and Mines, Economic Geology Report ER79-7, 103p. Bannatyne, B.B., 1988. Dolomite resources of southern Manitoba; Manitoba Energy and Mines, Economic Geology Report ER85-1, 4 maps, 39p. Betcher, R. N., McCabe, H. R., and Render, F. W. 1993. The Fort Garry aquifer in Manitoba. Manitoba Energy and Mines, Geological Report GR93-1, 15 p. Bezys, R.K. and McCabe, H.R. 1996. Lower to Middle Paleozoic stratigraphy of southwestern Manitoba – Field Trip Guidebook B4: Geological Association of Canada/Mineralogical Association of Canada Annual Meeting, Winnipeg, Manitoba, May 27-29, 1996. Bezys, R.K., Matile, G.L.D. and Keller, G.R. 2001. Investigation of Precambrian monadnocks (NTS 62I/1 and 62/8); in Report of Activities 2001, Manitoba Industry, Trade and Mines, Manitoba Geological Survey, p. 133-137. Coniglio, M. 1999. Manitoba’s Tyndall Stone; in Wat on Earth; University of Waterloo, Department of Earth Sciences, Spring 1999, pp. 15-18. Dixon R.J. and Stewart, J., 1988. Peatland inventory of Manitoba: III- Interlake region using LANDSAT thematic mapper; Manitoba Department of Mines and Natural Resources, Surveys and Mapping Branch, 21p. Garson and District History Book Committee 1990. Garson, then and now 1890-1990; Derksen Printers Ltd., pp. 14-26. McCabe, H.R., 1978. Reservoir potential of the Deadwood and Winnipeg Formations, southwestern Manitoba, Manitoba Energy and Mines, Geological Paper GP 78-3, 54p. McCrank, G.F.D., 1985. A geological survey of the Lac du Bonnet Batholith, Manitoba; Atomic Energy of Canada Limited, Report AECL-7816, 63p. Norford, B.S., Haidl, F.M., Bezys, R.K., Cecile, M.P., McCabe, H.R., and Paterson, D.F. 1994. Middle Ordovician to Lower Devonian strata of the Western Canada Sedimentary Basin, in Geological Atlas of the Western Canada Sedimentary Basin, G.D. Mossop and I. Shetson (compilers), Calgary, Canadian Society of Petroleum Geologists and Alberta Research Council, p. 109-127. Porter, J.W. and Fuller, J.G.C.M. 1959. Lower Paleozoic rocks of the northern Williston Basin and adjacent areas. American Association of Petroleum Geologists Bulletin, Vol. 43, No. 1, pp. 124-189. Schmidtke, B.E., 1993. Granitic dimension stone potential of southeast Manitoba; Manitoba Energy and Mines Economic Geology Report ER93-1, 52p. 93 �Schmidtke, B.E. and Bamburak, J.D., 1996. Industrial minerals of southeast Manitoba – Field Trip Guidebook B7, Geological Association of Canada/Mineralogical Association of Canada Annual Meeting, Winnipeg, Manitoba, May 27-29. 1996. Tammemagi, H.Y., Kerford, P.S., Requeima, J. and Temple, C.A., 1980. A geological reconnaissance of the Lac du Bonnet Batholith; Atomic Energy of Canada Limited, Report 6439, 68p. Tarnocai, C., Kettles, I.M., Ballard, M., 1995. Peatlands of Canada; Geological Survey of Canada, Open File 3152. 1:6 000 000 map with marginal notes. Wilkins, C., 1986. Manitoba’s magnificent limestone graces Canada’s finest buildings; Canadian Geographic, v. 106, no. 1, pp. 28-37. 94 �Field Trip 5 Separation Rapids Rare-Element Pegmatite Field, Ontario Charles Blackburn Blackburn Geological Services Site 130, Comp. 21 Kenora, Ontario P9N 3W Don Bubar President and CEO Avalon Ventures Ltd. 1116-1111 Richmond Street West Toronto, Ontario M5H 2G4 Carey Galeschuck Project Geologist Tantalum Mining Corporation of Canada Limited Box 2000 Lac du Bonnet, Manitoba R0E 1A0 Alasdair Mowatt President Emerald Fields Resources Corporation 1546 Pine Portage Road Kenora, Ontario P9N 2K2 Chris Pederson Consulting Geologist Karen Rees General Manager Avalon Ventures Ltd. 777 Red River Road Thunder Bay, Ontario P7B 1J9 Tony Pryslak A.P. Pryslak Geological Services 15 Hunterspoint Rd. Winnipeg, Manitoba R3R 3B6 Aerial view of the Big Whopper pegmatite, from the southeast �FOREWORD Separation Lake and surrounding area has in recent years emerged as, if not the most, certainly among the most, important host to rare-element pegmatites in Ontario. The Separation Rapids pegmatite field (Figure 1), located where the English River forest access road crosses the English River near Separation Rapids, was first discovered in the 1993 field season by Fred Breaks of the Ontario Geological Survey (OGS). However, the presence of beryl-bearing pegmatites had been known long before roads were pushed into this region, since at least the 1930s, when a Geological Survey of Canada field crew working its way along the lakes and waterways of the English River noted beryl "in a large pegmatite dyke cutting volcanics on the east shore of English River 2 miles northwest of Separation rapids" (Stockwell 1932). Access in those days was difficult, and so possibilities of exploitation lay dormant for 50 years. Separation Lake and the surrounding area was included in a 37 000 km² reconnaissance survey of the present English River and Winnipeg River subprovinces in the 1970s (Breaks and Bond 1993), and although other previously known rare-metal pegmatites were examined in some detail, little attention was paid to the pegmatites at Separation Lake. Then, in the 1980s, Carmen Storey of the OGS, as part of a broad evaluation of the industrial mineral potential of a large part of northwest Ontario, sampled what was possibly the same dike as Stockwell had examined and noted accessory red garnet and apatite. He also found lithium and berylium assay values in other pegmatites recently exposed along the newly opened right-of-way for the English River road (Storey 1990). It was not until late in the field season of 1993, when Fred Breaks, following a long summer of investigation of the Raleigh lake pegmatites, and taking the opportunity to visit the OGS field camp of Charlie Blackburn at Separation Rapids, knowing of the beryl mineralization and suspecting that the pegmatites there might show some further characteristics of the prized rare-element group, that the real potential began to emerge (Breaks 1993). Blackburn was completing the second year of a broad geological survey of the never-before-mapped Separation Lake greenstone belt. He and Jeff Young (Blackburn and Young 1993) had become involved in the possibilities of base metal potential in the belt, at that time being explored by Champion Bear Resources Ltd., and had not realised that other more exciting and exotic metals lay beneath their feet. Tony Pryslak, working for Champion Bear, had also encountered beryl-bearing pegmatites in their base metal exploration program. A half-day of fieldwork convinced Breaks that he was on to something, and in the next few days he laid the foundation for what was to eventually develop over the next 5 field seasons into the discovery of the Big Whopper and Big Mac pegmatites and numerous other bodies. Breaks (1993) was quick to realise that the Separation Rapids pluton, exposed on islands and along the shoreline of the English River, bears striking similarity in size, constituent granitic units and mineral content to the peraluminous Greer Lake pluton of the Winnipeg River Pegmatite District in Manitoba, the location of the Tanco Mine. He related the pegmatites, that had up to that time only been discovered on the 96 �97 Figure 1. General geology and distribution of rare-element groups in the Bird River-Separation Lake metasedimentary-metavolcanic belt, southeast Manitoba and northwest Ontario. Orthopyroxene-in isograd outlines the Umfreville-Conifer lakes granulite zone. Figure taken from Figure 1 in Breaks and Tindle (2002). �east side of the river, to the pluton, and called the complete package the Separation Rapids Pegmatite Field. In the following field season Breaks began what was to become a detailed investigation of the area around Separation Rapids, in partnership with colleagues Andy Tindle (Breaks and Tindle 1994) and Yuanming Pan (Breaks and Pan 1995). Thanks to painstaking work, discovery of the Big Whopper pegmatite was made by Breaks and Tindle in 1996 on the west side of the English River (Breaks and Tindle 1996, 1997). Following announcement in 1996 of the discovery, the Big Whopper was staked by local prospectors Bob Fairservice and Jim Willis. Further expansion of the Separation Rapids pegmatite field was made to the west. Discovery of the Big Mack was made in 1998 by two other local prospectors, Al Mowatt and Phil Thorgrimson. Meanwhile, Tantalum Mining Corporation of Canada Ltd. (Tanco) geologists Carey Galeschuk and Peter Vanstone were further exploring the numerous pegmatites on ground earlier investigated by Champion Bear to the east of the river. Tanco continues to explore under a joint venture agreement with Gossan Resources Ltd. Other companies that became major stakeholders included Avalon Ventures Ltd. (Big Whopper), Emerald Fields Resources Corp. (Big Mack) and Champion Bear Resources Ltd. (Marko's Pegmatite). Most recently Tony Pryslak and Seymour Sears, working for Champion Bear Resources, enabled further expansion of the field to a minimum 6.5 km strike length, with their discovery of a number of rare metal pegmatites (e.g. the Glitter Zone) further to the west. INTRODUCTION Breaks and Tindle (1997) have pointed out that: "Rare-metal class pegmatites of the complex-type and petalite-subtype represent the most desirable target for tantalum, cesium, rubidium and ceramic quality petalite in Archean terrain settings......Current economic interest is focussed upon the petalite potential.......The widest part of the Big Whopper Pegmatite averages 37% petalite over 60 m which is comparable to the world's premiere petalite deposit at the Bikita Pegmatite of southern Zimbabwe. The tantalum potential is also considerd significant as wodginite, the chief ore mineral for Ta at the Tanco Mine of southeastern Manitoba, is not only widespread in the Separation Lake area, but also exhibits compositional variation unlike any other pegmatite group on a global scale. Cesium.....also has high exploration potential as pollucite, the only ore mineral for the metal, has ...been verified in the area." So-called "fertile" granite/pegmatite systems are typically peraluminous and of an S-type heritage (Breaks and Tindle 1997). The 4 km² Separation Rapids pluton represents a classic example of a fertile granite. It has generated a rare-element pegmatite field with a minimum presently known east-west dimension of 12 km that is 3 km at its widest. The pluton compares in size and constituent granitic units with the Greer Lake pluton (Cerny et al 1981) 55 km to the northwest in the Winnipeg River Pegmatite District of southeast Manitoba (Figure 1). Similarities include presence of cordierite, beryl, cassiterite and the common presence of primary layering between pegmatitic leucogranite, sodic aplite, 98 �99 Figure 2. Distribution of rare-element pegmatite mineralization in the Separation Rapids pegmatite group, and location of field trip stops 1 through 5. Modified from Figure 3 in Breaks and Tindle (2002). �potassic pegmatite and coarse grained granite. Beryl has been found at numerous places that constitute a zone in the southern portion of the pluton, either as a primary phase, or secondary with garnet, muscovite and biotite after cordierite. The pegmatites have recently (Breaks and Tindle 2002) been grouped into an eastern and a western subgroup, based on their position relative to the Separation Rapids pluton. However, both subgroups exhibit a beryl zone and a complex, petalite-bearing zone. The Big Whopper, Big Mack and Glitter Zone pegmatites are located within the petalite subzone in the western subgroup, and the Marko's and James' pegmatites are in the complementary subzone in the eastern subgroup. Audrey's pegmatite lies within the beryl zone, just outside the petalite zone. REGIONAL GEOLOGICAL SETTING The Separation Rapids pegmatite field is set in the heart of the Separation Lake greenstone belt (Figures 1 and 2). Metavolcanic and subordinate metasedimentary rocks occur discontinuously along the English River-Winnipeg River subprovincial boundary from the Ontario-Manitoba border in the west to western Lac Seul in the east, a distance of about 100 km. They represent the eastern extension of the Bird River greenstone belt in Manitoba (Cerny et al. 1981) (Figure 1). The Separation Lake greenstone belt (Figure 1 and 2; Blackburn and Young 2000; Blackburn et al 1994a,b) is the largest segment, extending from the east shore of Umfreville Lake to Helder Lake, a distance of 45 km, and with a maximum width of 5 km. It consists predominantly of a lower sequence of mafic metavolcanic rocks, with intercalated magnetite-bearing iron formations, a single discontinuous clastic metasedimentary unit, and overlying subordinate felsic metavolcanic rocks. Gabbro sills intrude the mafic metavolcanic sequence. A thin unit of polymictic conglomerate and sandstone lies along the northern margin of the belt. Along the length of the belt the volcanosedimentary assemblage faces predominantly to the north. However, the lower sequence is folded about the westerly plunging Separation Narrows anticline, while in the west folding has been about the easterly plunging Paterson Lake antiform. Metamorphic grade is amphibolite throughout the belt. Breaks and Tindle (2002) suggest on the basis of geochronology done in the Bird River portion in Manitoba that the belt has an age range of >2844 Ma to 2740 Ma, while the Separation Rapids pluton has been dated at 2646 +/- 2 Ma (Larbi et al 1999). The English River Subprovince, extending north from the Separation Lake belt to the Uchi Subprovince, is comprised of metasedimentary migmatites (50%), and felsic to intermediate plutonic rocks comprised of a tonalitic suite in the west and a granodiorite to granite suite in the east (Breaks 1991). Metamorphic grade varies from amphibolite to granulite, and has affected all rocks except 100 �those of a peraluminous suite (Breaks 1991). The Winnipeg River Subprovince, south of the Separation Lake belt, is comprised of felsic to intermediate plutonic rocks ascribed to two suites, an early tonalitic suite in the north and a later granitic suite in the south by Beakhouse (1991). Rocks of the tonalitic suite in the subprovince are metamorphosed to medium to high grade, while granitic suite rocks were either synchronous with or postdated regional metamorphism (Beakhouse 1991). GEOLOGY OF THE PEGMATITE FIELD The Separation Rapids pluton and the Separation Rapids rare-element pegmatite field lie entirely within the greenstone belt (Figure 2). There appears to be little direct relationship between the pegmatite field and the stratigraphy of the greenstone belt. However, deformation events could have provided convenient structural traps into which the pluton and the pegmatites were emplaced. A major folding event, represented by folding about the Separation Narrows anticline, preceded emplacement of the cross cutting Separation narrows pluton, dated at 2646 +/- 2 Ma, as noted above. De la Fuente (1998), in a study done for Tanco, interpreted three deformation phases, such that D1 and D2 predated emplacement of the Separation Narrows pluton. He interprets the pluton to therefore be parent to pegmatites that are only weakly deformed, such as the Marko's pegmatite. In his interpretation, the Separation Narrows pluton cannot be parental to the Big Whopper and Big Mack pegmatites, both of which are complexly folded. He suggests that the source of the Separation Narrows pluton may be at depth either within the greenstone belt or "within the mainly undeformed Winnipeg River subprovincelate granites outcropping to the south" (de la Fuente 1998). His analysis that the Treelined Lake granite was involved in D2 and D3 deformation is consistent with this granite being a possible source of the Big Whopper, Big Mack, James and other complexly deformed pegmatites. These structurally based interpretations of relative timing of emplacement of various pegmatites and granitic bodies differ from the interpretation of Fred Breaks and colleagues (eg. Breaks and Pan 1995; Tindle and Breaks 2000: Breaks and Tindle 2002), made on mineralogical and geochemical arguments, that there is a consistent evolutionary trend from the Treelined Lake granite complex through the Separation Narrows pluton, to the various pegmatite groups. Detailed discussion of all other aspects of the Precambrian geology of the Separation Lake area, such as lithology, stratigraphy, metamorphism and mineral deposits, other than those associated with the rare metal pegmatites, has been made by Blackburn and Young (2000). Breaks and Tindle (2002) have recently presented a detailed account of the Separation Narrows pegmatite field, from which much of the rest of this section is paraphrased or quoted in parentheses. 101 �Treelined Lake Granite Complex The Treelined Lake granite complex is a "peraluminous granite mass situated in the adjacent English River subprovince. This granite mass is an irregular-shaped, 3 to 23 by 63 kilometer mass situated mostly in the core of the Umfreville-Conifer lakes granulite centre." (Figure 1). There is an abrupt regional metamorphic discontinuity at the boundary between the English River subprovince and the Separation Lake greenstone belt, jumping from upper amphibolite in the greenstones to granulite in the migmatized clastic metasedimentary rocks to the north. Although not a field trip stop in the present guide, the boundary is described in detail as Stop 1-6 in the Western Superior Province Fieldtrip Guidebook for Precambrian '95 (Beakhouse et al 1995). Rocks of the Treelined Lake granite complex characteristically contain the metamorphic minerals garnet, orthopyroxene, cordierite, while the "southwestern apophysis consists mainly of garnetbiotite and muscovite-biotite granite with local, in situ pegmatite zones that contain rareelement-enriched mineralogy." Breaks and Tindle (2002) discuss such a pegmatite that occurs very close to the boundary adjacent to the Umfreville Road that contains "tourmaline, topaz, cassiterite, gahnite, fluorapatite,....microlite, manganocolumbite and manganotantalite." Separation Rapids Pluton The Separation Rapids pluton (Figures 2) has a "core of coarse-grained, potassiumfeldspar-porphyritic, garnet-biotite-muscovute granite that is enveloped by a larger area composed of various pegmatitic granite units." Variable textural and mineralogical features include: wide range in grain size, from aplite to potassic pegmatite and pegmatitic leucogranite (with potassic megacrysts up to one meter in diameter); graphic, plumose, radial, and unidirectional solidification textures; layering among units; a peraluminous mineralogy of cordierite, primary muscovite, biotite, garnet and schorl tourmaline; and metasomatic rare-element-rich biotite and muscovite along contacts with mafic metavolcanic host-rocks and enclaves. "Rare-element minerals.....are largely confined to...the southeastern part of the pluton. These comprise occurrences of green and white beryl, columbite-tantalite group minerals and cassiterite in potassic pegmatite, various sodium-rare-element-enriched pods and layers (albitite, albite trondhjemite and muscovite-quartz-cleavelandite pods) and more rarely in fine-grained leucogranite." Rocks of the pluton are conveniently exposed on the shore-lines and islands of the English River. Eastern Subgroup Pegmatites The eastern subgroup of pegmatites (Figure 2) covers a 7.5 km² area to the east of the English River. It "comprises a narrow, 0.5 by 5 kilometre, central axis of 11 complex type, petalite subtype pegmatites that is almost completely enveloped by zones of beryltype pegmatites." "The beryl zone contains dikes of pegmatitic leucogranite, potassic pegmatite and minor sodic aplite.....Green and white beryl....is the most widespread rareelement mineral. Cassiterite, ferrocolumbite and gahnite represent widespread accessory 102 �minerals. (In the beryl zone) Wodginite.....has only been documented in Audrey's pegmatite." Topaz has been found in four dikes in the beryl zone. The petalite zone pegmatites characteristically, in addition to petalite, contain a diverse population of oxide minerals, in particular wodginite. The 8 by 130 metre Marko's pegmatite in particular contains a diversity of wodginite species (viz. titanowodginite, ferrowodginite, ferrotitanowodginite, and tungsten-rich wodginite). Marko's pegmatite is notable also for its zonation, containing four primary zones and two replacement units. Features of the primary zones are dominated by megacrysts of petalite and of potassium feldspar. In the replacement zones: spodumene-quartz intergrowths (the SQUI so common at Tanco) occur within petalite megacrysts; lepidolite replaces muscovite; and oxide minerals are especially conspicuous. Southwestern Subgroup Pegmatites The southwestern pegmatite subgroup of pegmatites (Figure 2) occupies a 0.3 to 0.8 by 6.5 kilometre area to the west of the English River. It is divisible into two zones, a beryl type and a petalite subtype zone. The beryl zone contains numerous small and large pegmatites with major mineralogy similar to those of the eastern subgroup. The petalite zone contains nine relatively larger, deformed pegmatite lenses, the largest of which in surface outcrop are the 56 by 650 metre Big Whopper and the 30 by 100 metre Big Mack. "The initial resource estimate of Avalon Ventures Limited....revealed the Big Whopper pegmatite to contain 7.1 million tonnes with an average of 1.285% Li2O, 0.346%Rb2O and 0.007% Ta2O5 over a strike-length of 600 metres and to a depth of 200 metres." At the Big Mack, "a preliminary estimate of 300 000 tonnes averaging 30.5% petalite to a 65 metre depth has been indicated by the initial diamond drilling program." "Petalite content of the core of the Big Whopper and Big Mack pegmatites ranges from 30 - 60%.....Petalite is of optimum quality for use as a direct feedstock in the lithium glass and ceramic industry." Extremely low iron contents, a deleterious metal in the glass-making industry, are indicated in samples taken by Breaks and Tindle that analyzed at <5 to 123 ppm Fe. "Furthermore, Li2O contents (4.6 - 4.7 %) are somewhat higher than the 4.3% average of six Li2O analyses compiled from the literature." "Other phases in the petalite zone pegmatites include garnet...., cordierite, lepidolite, cookeite, spodumene, eucryptite, bikitaite, holmquistite, topaz and chrysoberyl. Oxide phases include cassiterite, gahnite, ferrowadginite, ferrotapiolite, ferrocolumbite, manganocolumbite, ferrotantalite, struverite, and yttro- and yttrian pyrochlore (Tindle and Breaks 2000). Potassium and sodium feldspar minerals, of high purity, represent potential valuable byproducts of the exploitation of petalite pegmatites in the area. Potassium feldspar from the southwestern subgroup also reveals a significant variation in Rb content.....(Those) from the Big Whopper pegmatite indicate a rubidium content mainly in the 1.5 to 2.0 wt. % range with a maximum value of 3.0 wt %." 103 �FRACTIONATION TRENDS IN RARE ELEMENT PEGMATITES Oxide "minerals of the columbite-tantalite group are the most common Nb-Ta species in rare element pegmatites" (Tindle and Breaks 2000). Following Cerny and Ercit (1985), Tindle and Breaks (2000) have used the columbite-tantalite quadrilateral (Fig. 3) to Figure 3. Columbite-tantalite quadrilateral. Vectors describe variation trends in beryltype and complex-type pegmatites. From Fig. 9, Tindle and Breaks (2000). analyse fractionation trends in both beryl-type and complex-type pegmatites of the Separation Narrows rare element pegmatite field. Fractionation trend may be strongly influenced by the activity of fluorine, as indicated in the evolution paths shown in the quadrilateral (Fig. 3). Changes in bulk chemistry, increase in temperature and decrease in pressure result in the formation of petalite, lepidolite and amblygonite subtype pegmatites, all noted for their high fluorine activity. Tindle and Breaks (2000) have subdivided the Separation Rapids pegmatites into Fesuites and Mn-suites on the basis of columbite-tantalite (oxide mineral) compositions. Figures 4, 5,and 6 demonstrate the use of this classification for the four pegmatites to be visited on the field trip. Data for those of the southwest sub-group complex-type pegmatites are shown on Fig. 4, and for the eastern sub-group complex-type pegmatites on Figs. 5 and 6. Data for a number of other pegmatites are also included on the diagrams, taken from Tindle and Breaks (2000). For clarity, envelopes have been added around the data for the Big Mack, Big Whopper and lepidolite unit of the Big Whopper (Fig. 4), Marko's pegmatite petalite core and outer layer (Fig. 5) and James' pegmatite (Fig. 6). 104 �Figure 4. Columbite-tantalite quadrilateral: major SW subgroup petalite pegmatites. Modified from Fig. 13a, Tindle and Breaks (2000). In Fig. 4, the Big Mack pegmatite data clearly fall in the Fe-suite, as do most of the pegmatites of the southwest sub-group. However, data for the Big Whopper pegmatite are spread over a large area of the quadrilateral, while data for the lepidolite unit show extreme fractionation along the high Mn side of the quadrilateral. It is suggested that the apparent randomness of Big Whopper data reflect the complex folding this pegmatite has undergone. The Marko's pegmatite (Fig. 5) is the only Mn-suite petalite-subtype in the eastern subgroup. According to Tindle and Breaks (2000), the fractionation trend is from the outer, earlier crystallizing, layered pegmatite-aplite unit toward the late crystallizing petalite rich core. In Fig. 6, the James' pegmatite data fall in the Fe-suite. The differentiation trend is subvertical in the diagram, indicating fluorine-poor conditions. However, the pegmatite is more evolved than the primitive dikes #9 and #10 (same diagram) and equivalent to approximately 50% of the samples from the Big Whopper (Fig. 4). 105 �Figure 5. Columbite-tantalite quadrilateral: eastern subgroup, manganese suite petalite pegmatite. Modified from Fig. 12b, Tindle and Breaks (2000). Figure 6. Columbite-tantalite quadrilateral: eastern subgroup Fe suite petalite pegmatites. Modified from Fig. 12a, Tindle and Breaks (2000). 106 �THE FIELD TRIP The trip commences in Kenora. Proceed north on Highway 659 to the turn-off on to English River Road, just south of Redditt. Take the English River Road to the turn-off on to the Sand Lake Road, about 7 km south of the Separation Narrows bridge. Proceed along the Sand Lake Road to the Emerald Fields Resources access road, to the Big Mack pegmatite (Fig. 2). STOP 1 - BIG MACK PEGMATITE The Big Mack pegmatite (Fig. 7) is complexly folded and compressed into a 35 m x 100 m lens with several prominent apophyses tapering to the south, southeast and west. These apophyses consist of non-petalite bearing sodic aplites, blocky potassic pegmatite and holmquistite bearing granitic units. Several units will be observed within the Big Mack: A) Wall zone of medium blocky quartz+plagioclase+muscovite+garnet+biotite. Cordierite is common in this unit and in the apohyses. It is generally altered to garnet+mica+holmquistite rich simplectites that give the unit and several metres of the interior petalite zone a spotted appearance. B) Petalite rich, medium to coarse blocky phase of quartz+plagioclase+Kspar+muscovite+petalite. The petalite varies up to 60% in this unit and is identified by light brown weathering. C) Chrysoberyl bearing petalite pegmatite. This unit is restricted to a lens at the south end of the trench. It is grey due to the dominance of biotite over muscovite. Petalite content is generally lower than in unit B (15-20%), and it is generally finer grained but still blocky in nature and contains sporadic lime green 1-15 m chrysoberyl crystals. D) Primary aplite layered with petalite pegmatite. Folds are defined by this unit. E) Replacement albitic unit seen as white weathering layers and pods of muscovite+garnet+albite+quartz+K-spar. This unit is best observed enveloping the mafic metavolcanic screens. F) Post deformation, fracture controlled to vein like features include bikitaite, holmquistite and eucryptite. The eucryptite can be recognized by its grey, recessive weathering, capped by a lacy network of quartz veining. Massive to pillowed mafic flows can be observed on the hill north of the parking area. 107 �108 Figure 7. The Big Mack pegmatite, showing location of units discussed in the text. �Return to the Sand Lake Road, and then to the Separation Narrows bridge. Board boats and travel down stream, passing through Separation Rapids, to Heart Island on the Separation River (Fig. 2). STOP 2 - SEPARATION RAPIDS PLUTON The Separation Rapids Pluton is interpreted by Tanco geologists to be a flat lying, sheetlike, layered, very fractionated pegmatitic granite. The interpretation of the pluton being a sheet rather than a stock-like body is based on an aeromagnetic survey flown for Tanco (Assessment Files, Ministry of Northern Development and Mines, Kenora) that shows a magnetic pattern of similar continuity, amplitude and intensity as in the surrounding volcanic rocks extending beneath the Separation Rapids Pluton. Figure 8. Location of Heart Island in the Separation Rapids pluton. Heart Island (Fig. 8) displays classic pegmatitic granite features such as “bird’s-foot mica”, megacrystic potash feldspars, pegmatitic vugs, and aplite banding. If time permits, Red Handed Island, the larger island to the west of Heart island, will be visited. This island features a flat lying lepidolite-bearing pegmatitic granite that postdates D1 and D2 deformation phases. It is affected by D3 phase open folds with E-W 109 �axes gently plunging to the west. It may represent an external facies of the Separation Rapids pluton. Proceed by boat from Heart Island to Avalon Ventures Ltd.'s boat landing on the west shore of the river. A short walk inland leads to the Big Whopper pegmatite (Fig. 2). STOP 3 - BIG WHOPPER PEGMATITE Introduction Following staking of ground over the Big Whopper by local prospectors Bob Fairservice and James Willis in 1996, the 560 acre property (since expanded to 4480 acres) was optioned to Avalon Ventures Ltd. which has earned a 100% interest, subject to a 2% NSR royalty interest retained by the vendors. Figure 9. Big Whopper pegmatite. Areas 1 and 2 of the field trip stop are indicated. In 1997 and 1998, Avalon Ventures Ltd. began exploring the property by conducting linecutting, ground magnetic, geological and geochemical surveys, overburden stripping, trenching, mineralogical studies and diamond drilling totalling 8,751 metres in 57 holes. This work delineated the Big Whopper (Fig. 9) over a strike length of 1.5 km with widths 110 �ranging from 10 m to 80 m and to a vertical depth of 250 metres where it remains open. Total indicated and inferred petalite resources are estimated at 11.6 million tonnes grading 1.34% Li2O, 0.30% Rb2O and 0.007% Ta2O5. In 1999, a pre-feasibility study was completed on the deposit by Micon International Ltd. which concluded that development of the deposit as a producer of petalite plus feldspars, mica and tantalum, was economically viable and recommended proceeding with a bulk sampling program and full feasibility study. This work has not yet begun. In 2001, in response to higher tantalum prices, Avalon conducted a program of 1,401 metres of diamond drilling in 12 holes, channel sampling, mineralogical and metallurgical studies to better define tantalum distribution within the Big Whopper pegmatite system. The dominant economic minerals in the deposit are petalite and columbite-tantalite, the ore mineral of tantalum. The deposit also contains significant quantities of rubidiumpotash feldspar, albite, lepidolite, and cassiterite. At the Big Whopper, which is hosted entirely within amphibolites, a north-directed compressional tectonic event produced flattening and a strong vertically oriented regional schistosity striking west-northwest. This schistosity is folded about a sub-vertical axis, with minor folds commonly observed both in the pegmatite and the amphibolite host rocks. The Big Whopper itself exhibits tight s-fold geometry, with the thickened central portion of the pegmatite coinciding with the hinge zone, and attenuated limbs extending to the east and west. Fold axes exhibit vertical to sub-vertical orientations. Parallel mineral lineations indicate a strong vertical stretching component, with aspect ratios in the order of 5:1 to 10:1. Small pegmatites flanking the Big Whopper commonly exhibit boudinage structures indicative of a high-strain environment. Mineralogy and Zonation Mineralogical zoning observed in the Big Whopper is characteristic of highly evolved rare metal pegmatites, with well-developed wall zones and internal intermediate zones classified according to their dominant constituent minerals (Figure X). Metallogenic zoning is closely related to mineralogic zoning. The predominant mineralogical zones of the Big Whopper are as follows: 1. Wall Zone (predominantly albitite) 2. Megacrystic Feldspar and Quartz-Mica marginal Zones 3. Petalite (intermediate) Zone 1. The Wall Zone is a narrow 1 to 10 metre wide endocontact zone of albitite consisting essentially of saccharoidal to aplitic albite with accessory K-feldspar, muscovite, quartz, and Mn-rich garnet (spessartine). Proximal dykes and stringers exhibit the same mineralogy as the Wall Zone albitite. Cassiterite and tantalum oxides are commonly associated with the albitite, along with rare gahnite (a zinc spinel). Albitite characteristic of the Wall Zone commonly occurs in intimate association with the Megacrystic Feldspar Zone and these zones together with the Quartz Mica Zone, likely reflect early, more primary phases of pegmatite development. 111 �2. The Megacrystic Feldspar Zone is confined to the eastern and western internal margins of the Big Whopper pegmatite and resembles pegmatitic granite. It consists predominantly of orange-pink to grey-white, coarse to megacrystic K-feldspar and coarse silvery mica aggregates in an albitic matrix. Highly elevated whole rock rubidium values averaging in excess of 0.3% Rb2O are attributed to Rb-K-feldspar as an essential mineral in the pegmatite. The Quartz Mica Zone occurs in intimate association with the Megacrystic Feldspar Zone, and is generally enveloped by or interdigitated with it. It contains 50% quartz, 30% muscovite, and 20% K-feldspar. Elevated whole rock lithium values in this zone (up to 0.5% Li2O) are attributed to the presence of micas of the lithian muscovite to lepidolite series. 3. The Petalite Zone is an intermediate zone of the Big Whopper pegmatite and is the largest defined to date, comprising approximately 80% of its volume. The essential mineralogy consists of petalite, Rb-K-feldspar, albite, quartz and mica. A crude but distinct petalite zoning can be identified within the Big Whopper pegmatite as tightly folded layers with progressive fractionation increasing eastward. Ribbon-like, white petalite displaying schlieren-like habit (Type A) grades to coarse pink and pink-white petalite (Type B), and to blue-grey to pink-grey petalite (Type C). A fourth sub-zone (Type D) is recognized based on the presence of the purple mica lepidolite and occurs peripheral to Types A-C, mainly to the south and east. This zone is enriched in tantalum, typically assaying greater than 0.01% Ta2O5. Type C is a very fine-grained, equivalent of Types B and A, which is very highly foliated, mica-rich unit and commonly occurs interlayered as coarse-grained bands and lenses within Types B and A. Petalite in this unit tends to be partially altered to spodumene-quartz intergrowth (“SQUI”) exhibiting a net texture. Mineralization The main economic minerals of the Big Whopper deposit are petalite, Rb-K-feldspar, albite, lepidolite and columbite-tantalite. The Big Whopper pegmatite system is characterized by unusually high purity end-member compositions of the constituent minerals, a feature reflecting the highly evolved chemistry of the system. Petalite (LiAlSi4O10) is almost stoichiometrically pure, averaging close to the theoretical maximum lithium content of 4.8% Li2O, with only traces of soda, potash, and iron. It averages about 25% of the ore, and varies from white to pale pink in colour. Albite averages 11% soda, 0.10% potash, 0.35% lime, 0.01% iron oxide (Fe2O3). It makes up 40% of the ore, and on average ranges from white to bluish-white in colour. Rb-K-feldspar constitutes 10 to 15% of the ore. Although known in other pegmatites where it generally exhibits perthitic intergrowth, the Big Whopper variety carries only 0.3-0.4% Na2O along with 2.8% Rb2O, 15-16% K2O and. It is generally grey-white in colour and typically occurs as large megacrysts in the petalite ore. Lepidolite (K Rb(Li,Al)2-3(AlSi3O10)(O,OH,F)2) is a distinct purple-coloured mica that occurs in marginal zones of the Big Whopper and in separate flanking dykes. The mineral is an ore of rubidium, containing up to 4% Rb2O, and can comprise up to 15% of the ore. 112 �Columbite-tantalite (Mn,Fe)(Nb,Ta)O6 Manganocolumbite and manganotantalite predominate with rare microlite and ferrocolumbite, all occurring as fine-grained dark brown opaques. Tantalum is well distributed through the deposit but is typically enriched to levels exceeding 0.01% Ta2O5 in marginal lepidolite-rich petalite zones and albitite dykes. Other potentially valuable industrial minerals include muscovite mica (some with elevated lithia) which may comprise up to 15% of the ore, spodumene averaging 3-5% occurring as SQUI replacement rims on petalite, cassiterite, pale pink spessartine garnet and high-purity quartz. Minor accessory minerals include apatite, zircon, gahnite, monazite, xenotime, rare sulphides, sulphosalts, and thorite. Field Stop Descriptions Area #1: Big Whopper Petalite Deposit Main Mass The large stripped exposure of the Big Whopper main mass reveals all of its major mineralogical zones and sub-zones. The Wall Zone and feldspathic zones are exposed on the northwest side of the outcrop. The coarse white and pink petalite zone (Type B) is well exposed in the trench in the central part of the outcrop. It is flanked by Type A and C petalite mineralization with Type D (lepidolite rich) occurring on the south side and in a separate exposure to the east. The surface trench along the top of the Whopper averages 1.58% Li2O, 0.33% Rb2O and 0.007% Ta2O5 across 58.90 metres. Amphibolite screens occur within and at the margins of the Big Whopper. These screens are commonly disjointed with mullioned terminations, but are continuous to depth. Narrow 2 to 5 cm wide albitic haloes characteristically rim most of these screens, and show remarkably little variation in width regardless of the size of the amphibolite screen. These rims are interpreted as reaction fronts or depletion haloes, in which are concentrated lithophile elements (specifically Li, Rb, and Cs) forming mica-rich (glimmerite) selvedges to the amphibolite screens. The white albitic rims are depleted in these elements, but commonly exhibit elevated tantalum values of up to 0.049% Ta2O5. Area #2: Lepidolite Dike Lepidolite-rich petalite pegmatite dikes occur flanking the Main Mass of the Big Whopper pegmatite system mainly to the east. These dikes tend to be enriched in tantalum relative to the Main Mass of the Big Whopper reflecting the greater abundance of more high-evolved tantalum minerals such as microlite and wodginite in the Lepidolite Dyke. Channel sampling of this outcrop has produced an average of 0.023% Ta2O5 over 3.15 metres, with individual assays up to 0.031% Ta2O5. Dark blue fluor-apatite is a common accessory mineral Return to Avalon's landing on the Winnipeg River, and by boat to the Separation Narrows bridge. Proceed north on the English River Road about 3 km to the Umfreville 113 �114 Figure 10. Marko's pegmatite, showing location of zones discussed in the text. �Road turn off. A skidder trail leads from the Umfreville Road to Marko's pegmatite (Fig. 2). STOP 4 - MARKO'S PEGMATITE Marko's pegmatite (Fig. 10) extends along strike for a distance of 190 m in an east-west direction. It has a shallow dip to the south and is discordant to the near vertical dip of the iron formation that is its host rock. The pegmatite has a maximum thickness of 15 m and a maximum down dip extension of 30 m. Drill intersections indicate in cross section that Marko's pegmatite occupies a tension fracture that extends from the gabbro/iron formation contact at surface and progresses south across the iron formation at a relatively shallow angle but near normal to the primary layering in the iron formation. In longitudinal section Marko's pegmatite plunges 5-10º to the west. A second pegmatite, the North Marko's pegmatite, lies along the gabbro/mafic metavolcanic contact at surface, 20 m to the north of marko's pegmatite. Diamond drilling shows that the pegmatite extends to a depth of 40 m as a single, near vertically dipping sheet and then splits into north and south dipping sections. The south limb increases in size and degree of differentiation in the down dip and easterly directions. The North Marko's pegmatite is essentially barren at surface, but drill results show that mineralogy and geochemistry changes abruptly with depth and association with either a roll or flattening of the dike. This is likely due to the entrapment of fluorine rich fluids along these structural features. Both pegmatites are pristine and undeformed by the folding events that affected the Big Mack and Big Whopper pegmatites. Marko's pegmatite is exposed as two lenses at surface (Fig. 10). This is due to a sigmoidal roll in the moderate dip of the intrusion to the south. The internal zones can be readily traced and differentiation trends established with confidence along the surface exposures and in drill core. Zones of Marko's pegmatite to be examined are as follows: A) Wall zone comprised of quartz+albite+muscovite+beryl. Accessory minerals include black oxides (cassiterite, wodginite), green tourmaline and apatite. B) Petalite rich core zone, containing up to 95% petalite. The amount of petalite is locally variable, as seen in the westernmost part of the outcrop which consists essentially of coarse blocks of K-spar up to 2 m across, with interstitial petalite. This represents the crest of the pegmatite. C) Layered pegmatite-aplite. Quartz+garnet+biotite aplite is interlayered with a coarser muscovite+beryl granite. Fine grained black oxide minerals are common in the albite rich unit. A 1.6 m channel sample assayed 0.165% Ta2O5 and 0.10% Sn. D) Grey, fine grained granite withj minor muscovite and fine grained black oxides. 115 �E) Muscovite replacement unit of K-spar. F) Albitization of the petalite core. North Marko's pegmatite will also be examined. It has a simple assemblage at surface of quartz+K-spar+albite+mica with minor bands of aplite. Return to the Umfreville Road/English River Road intersection. Proceed north on the English River Road to a 75 m trail on the left leading to James' pegmatite (Fig. 2). STOP 5 - JAMES' PEGMATITE This highly fractionated pegmatite (Fig. 11), intruded into mafic metavolcanics, is strongly deformed and folded. De la Fuente (1998) has described it as an example of pre D2 deformation phase pegmatite. The Treelined Lake Granite, which shows the same deformation, is suggested (de la Fuente 1998) to be source granite for pre D2 pegmatites and pegmatitic granites Figure 11. Location of James' pegmatite In addition to quartz and feldspars, minerals present include green beryl, lithiophilite, petalite, spodumene blades, abundant black tourmaline, curviplanar mica (“ballpeen 116 �mica”), which is assumed to be lithium rich, and possibly zinnwaldite. Ferrowodginite has been identified in this pegmatite (Fred Breaks, personal communication). This zoned pegmatite is approximately 30 metres by 2 metres. The joint venture of Tanco and Gossan Resources Ltd. drilled the dike in 1996. The best Ta205 grades obtained were 0.035% in drill core and 0.026% at surface. The pegmatite dips to the southwest at about 60° and appears to widen at depth. End of field trip stops. 117 �REFERENCES Beakhouse, G.P. 1991. Winnipeg River Subprovince; in Geology of Ontario, Ontario Geological Survey, Special Volume 4, Part 1, p.279-301. Beakhouse, G.P., Blackburn, C.E., Breaks, F.W., Ayer, J., Stone, D. and Stott, G.M. 1995. Western Superior Province Fieldtrip Guidebook, Precambrian '95; Ontario Geological Survey, Open File Report 5924, XXp. Blackburn, C.E. and Young, J.B. 1993. Geology of the Separation Lake greenstone belt; in Summary of Field Work and Other Activities 1993, Ontario Geological Survey, Miscellaneous Paper 162, p.68-73. Blackburn, C.E. and Young, J.B. 2000. Precambrian geology of the Separation Lake area, northwestern Ontario; Ontario Geological Survey, Open File Report 6001, 94 p. Blackburn, C.E., Young, J.B., Searcy, T.O. and Donohue, K. 1994a. Precambrian geology of the Separation Lake greenstone belt, west part; Ontario Geological Survey, Open File Map 241, scale 1:20 000. Blackburn, C.E., Young, J.B., Searcy, T.O. and Donohue, K. 1994b. Precambrian geology of the Separation Lake greenstone belt, east part; Ontario Geological Survey, Open File Map 242, scale 1:20 000. Breaks, F.W. 1991. English River Subprovince; in Geology of Ontario, Ontario Geological Survey, Special Volume 4, Part 1, p.239-277. Breaks, F.W. 1993. Granite-related mineralization in northwestern Ontario: I. Raleigh Lake and Separation Rapids (English River) rare-element pegmatite fields; in Summary of Field Work and Other Activities 1993, Ontario Geological Survey, Miscellaneous Paper 162, p.104-110. Breaks, F.W. and Bond, W.D. 1993. The English River Subprovince – an Archean gneiss belt: geology, geochemistry and associated mineralization; Ontario Geological Survey, Open File Report 5846, v. 1 and 2, 884 p. Breaks, F.W. and Pan, Y. 1995. Granite-related mineralization in northwestern Ontario: III. Relationship of granulite metamorphism to rare-element mineralization in the Separation Lake area of the English River Subprovince in Summary of Field Work and Other Activities 1995, Ontario Geological Survey, Miscellaneous Paper 164, p. 79-81. Breaks, F.W. and Tindle, A.G. 1994. Granite-related mineralization in northwestern Ontario: II. Detailed examination of the Separation Narrows (English River) rare-element group in Summary of Field Work and Other 118 �Activities 1994, Ontario Geological Survey, Miscellaneous Paper 163, p. 109-112. Breaks, F.W, and Tindle, A.G. 1996. New discovery of rare-element pegmatite mineralization, Separation Lake area, northwestern Ontario; Ontario Geological Survey, Open File Report 5946, 9p. Breaks, F.W. and Tindle, A.G. 1997. Rare-metal exploration potential of the Separation Lake area: an emerging target for Bikita-type mineralization in the Superior Province of NW Ontario; Ontario Geological Survey, Open File Report 5966, 27p. Breaks, F.W. and Tindle, A.G. 2002. Rare-metal mineralization of the Separation Lake area, northwest Ontario: characteristics of a new discovery of complex-type, petalitesubtype, Li-Rb-Cs-Ta pegmatite in Industrial Minerals in Canada, CIM Special Volume 53, p. 159-178. Cerny, P. and Ercit, T.S. 1985. Some recent advances in the mineralogy and geochemistry of Nb and Ta in rare-element granitic pegmatites; Bulletin Mineralogie, v. 108, p. 499-532. Cerny, P., Trueman, D.L., Ziehlke, D.V., Goad, B.E. and Paul, B.J. 1981. The Cat Lake-Winnipeg River and the Wekusko Lake pegmatite fields, Manitoba; Manitoba Department of Energy and Mines, Economic Geology Report ER80-1, 216p. de la Fuente, F. 1998. Structural analysis of the Tanco's Separation Lake property, western Ontario, Canada; report for Tantalum Mining Corporation of Canada Limited, 29 p. Larbi, Y., Stevenson, R., Breaks, F.W., Machado, N. and Gariepy, C. 1999. Age and isotopic composition of Late Archean leucogranites: implications for continental collision in the western Superior Province; Canadian Journal of earth Sciences, Vol. 36, p. 495-510. Stockwell, C.H. 1932. Beryllium deposits; p.126 in Geology and mineral deposits of a part of southeastern Manitoba, by J.F. Wright; Geological Survey of Canada. Memoir 169, 150p. Storey, C.C. 1990. An evaluation of the industrial mineral potential of parts of the districts of Kenora and Rainy River; Ontario Geological Survey, Open File Report 5718, 259p. Tindle, A.G. and Breaks, F.W. 2000. Tantalum mineralogy of rare-element granitic pegmatites from the Separation Lake area, northwestern Ontario; Ontario Geological Survey, Open File Report 6022, 387p. 119 �120 �Field Trip 6 Geology of the Red Lake Camp, Ontario Andreas Lichtblau and Carmen Storey Ontario Geological Survey Ministry of Northern Development and Mines 227 Howey Street, Box 324 Red Lake, Ontario P0V 2M0 Example of late gold vein stockwork forming part of the High Grade Zone, 32 Level, Red Lake Mine. Estimated contained gold in sample: 298 ounces (at 7,284 ounces gold per ton). Photo courtesy of Goldcorp Inc. �REGIONAL GEOLOGY The Red Lake District (Fig. 1) is underlain by Archean rocks of the Superior Province of the Canadian Shield. Rocks of four subprovinces are found in the Red Lake District: 1. Uchi Subprovince rocks in the Red Lake District comprise the Red Lake and BirchConfederation Lake greenstone belts in which the bulk of exploration and mining activity has taken place. The supracrustal rocks of the Red Lake greenstone belt can be subdivided into several assemblages with ages ranging from ca. 2990 Ma to ca. 2700 Ma (Table 1). Major granitoid intrusions show a range from ca. 2734 Ma to 2699 Ma (Table 2). 2. English River Subprovince rocks, south of the Uchi Subprovince, are predominantly metasedimentary and host minor intrusive rocks similar to those in the Quetico Subprovince. 3. To the north, the Berens River Subprovince formed the core of a microcontinent. This area is underlain by ca. 2750-2690 Ma felsic plutonic rocks interpreted as a magmatic arc formed at an Andean-style margin that culminated in the Kenoran orogeny. These plutonic rocks intruded an older subtratum (North Caribou terrane) on which Mesoarchean volcanic rocks of the Red Lake belt are also interpreted to have formed. 4. The Sachigo Subprovince comprises crustal blocks ranging from Paleoarchean (>3.4 Ga) to Neoarchean (ca. 2.7 Ga) in age. Figure 1. Western Uchi Subprovince (modified from Percival et al. 2000) 122 �123 Figure 2. General geology of the Red Lake greenstone belt (after Parker 2000). Tour stops as indicated. �GEOLOGY OF THE RED LAKE BELT (adapted from Sanborn-Barrie et al. 2001) The Red Lake greenstone belt (Fig. 2) is dominated by the (ca. 2990 Ma) maficultramafic Balmer assemblage, an oceanic plain sequence; minor calc-alkalic volcanic rocks of arc-like affinity terminate the assemblage. The majority of lode gold deposits in the camp are hosted by the basal mafic-ultramafic sequence. A later diverse lithologic association, the Ball assemblage, appears to represent a shallow marine, volcanic edifice built upon the Balmer substrate. Table 1. Summary of supracrustal lithologies and radiometric ages in the Red Lake greenstone belt (modified from Parker 2000; with new ages from Sanborn-Barrie et al. 2001 and Skulski et al. 2001; final error estimates are not cited for the new unpublished ages of T. Skulski). Supracrustal assemblage English River ? U-Pb Age (Ma) <2700±6 Rock types and descriptions References Polymictic pebble conglomerate. Thought to correlate with the Austin tuff, host to the Madsen gold deposit. Strongly calc-akaline rocks. Andesitic to dacitic pyroclastic rocks SanbornBarrie et al. 2001 Corfu and Andrews 1987 ConfederationGraves (north Red Lake) 2733±1.5 Huston (Cemetery) ConfederationHeyson (southeast Red Lake) ≤2743 Well-bedded argillite and turbiditic wacke; polymictic conglomerate. Basal sequence is commonly tholeiitic to calcalkaline with lobe-hyaloclastite rhyolite flows; intermediate pyroclastic rocks; basalt; and feldspar-phyric andesite. Calc-alkaline rocks are more abundant at higher stratigraphic levels. Skulski et al. 2001 Corfu and Wallace 1986 ConfederationMcNeely (central and SE Red Lake) Trout Bay 2742; 2748+10/-5 Dominated by calc-alkaline, intermediate lapilli-tuff breccia and lapilli tuff SanbornBarrie et al. 2001 2853 Lower tholeiitic basalt sequence with associated gabbroic rocks overlain by finegrained clastic metasedimentary rocks (wacke, argillite) interlayered with subordinate intermediate pyroclastic rocks and chertmagnetite iron formation. Overlain by tholeiitic, pillowed basalts. Strongly calc-alkaline intermediate pyroclastic rocks overlain by pebble conglomerate, thinly bedded wacke and capped by chert-magnetite iron formation Interlayered, feldspathic wacke, lithic wacke and argillite; lenses of pebble and cobble conglomerates and quartz-rich pebble conglomerate and quartz arenite. Typically calc-alkaline intermediate pyroclastic rocks and rhyolite flows; komatiitic to tholeiitic SanbornBarrie et al. 2001 2748+10/-5 to 2739±3 Bruce Channel 2894±1.5; 2894±2 Slate Bay ≤2916 Ball 2940±2; 2925±3 124 Corfu and Wallace 1986; Corfu and Andrews 1987 Corfu et al. 1998 Corfu and Wallace 1986 �Balmer 2992+20/-9; 2989±3; 2964+5/-1 basalt; overlain by chert-magnetite iron formation and dolomitic marble which contains stromatolites. Tholeiitic basalt, basaltic komatiite and komatiite interlayered with subordinate chertmagnetite iron formation; minor clastic metasedimentary rocks; minor intermediate to felsic pyroclastic rocks; and rhyolite. Corfu and Andrews 1987 Table 2. Summary of lithologies and radiometric ages for major granitoid intrusions in the Red Lake greenstone belt (modified from Parker 2000; new ages cited in Sanborn-Barrie et al. 2001 and elsewhere do not have final error estimates assigned, as this U-Pb data is not yet published). Granitoid intrusion Cat Island pluton U-Pb Age (Ma) 2699 Rock types and descriptions References Potassium feldspar granodiorite SanbornBarrie et al. 2001 Noble 1989 Walsh Lake pluton 2699 Killala-Baird batholith 2704±1.5 Hammel Lake batholith Dome stock 2717±2 2718±1 McKenzie stock 2720±2 Red Crest stock 2729±1.5 Potassium feldspar- and quartz-phyric monzogranite; xenolith-rich, diorite or granodiorite; possible oxidized phase at Ranger Lake with broad magnetic anomaly Potassium feldspar- and quartz-phyric monzogranite; xenolith-rich, diorite or granodiorite, diorite or granodiorite; oxidized, magnetite-bearing marginal phase. Potassium feldspar and quartz porphyritic monzogranite; associated anorthositic intrusion. Granodiorite and augite porphyritic diorite/gabbro. Augite porphyritic diorite-gabbro; some ultramafic rocks; granodiorite Augite porphyritic diorite-gabbro Little Vermilion batholith Douglas Lake pluton 2731±3 Hornblende tonalite-granodiorite 2734±2 Biotite tonalite Corfu and Andrews 1987 McMaster 1987 Corfu and Andrews 1987 Corfu and Andrews 1987 Corfu and Andrews 1987 Corfu and Andrews 1987 Corfu and Stone 1998 Widespread ca. 2894 Ma calc-alkaline volcanism is represented in Red Lake by the Bruce Channel assemblage. Overlying this is the ca. 2850 Ma Trout Bay assemblage which includes substantial basaltic and gabbroic rocks in western Red Lake which are prospective for PGE mineralization, and which includes minor intermediate pyroclastic rocks throughout central Red Lake. The Trout Bay assemblage may correlate with Woman assemblage rocks of the Confederation Lake belt. A regional angular unconformity is interpreted to separate the Mesoarchean assemblages from the Neoarchean Confederation assemblages. Volcanogenic massive sulphide mineralization is associated with the younger sequence. A significant number of felsic 125 �units are classed as FII and FIII type rhyolites, considered highly prospective for large (Kidd Creek/Noranda type) massive sulphide deposits (Parker 1999). A newly recognized component of the Neoarchean supracrustal package is the Huston sedimentary assemblage that includes polymictic cobble- to pebble-conglomerate and argillite; clasts include jasperoidal chert iron formation, massive sulfide pebbles, and mafic flow (?) rocks, as well as well-bedded, graded turbiditic wacke and argillite. The U-Pb age of detrital zircons give single age peaks of 2743 and 2746 Ma at the cemetery and Madsen sites respectively (Skulski et al. 2001), indicating erosion of pre-existing Confederation age rocks, and deposition after ca. 2743 Ma. Recent age dating (Skulski et al. 2001) has also yielded multiple ages of detrital zircons from a fragmental unit thought to correlate with the Austin "tuff" ore zone at the former Madsen mine. Most of the Meso- and Neoarchean assemblages exposed in Red Lake are represented in this unit. Maximum age of deposition is consequently ≤2700±6 Ma. DEFORMATION (adapted from Sanborn-Barrie et al. 2001) The Red Lake greenstone belt has undergone at least three phases of deformation: 1) D0, a non-penetrative, early (pre-2748 Ma) event involving overturning of the Balmer assemblage; 2) D1, (bracketed between 2733-2742 Ma) resulted in a north trending foliation that is axial planar to F1 folds and involved east-west shortening; and 3) D2, (ca. 2720-2700 Ma) resulted in a dominantly east- to northeast-striking foliation that refolds F1 folds. A local 'deflection' of S2 around the McKenzie stock created an east-southeast striking corridor of heterogenous strain forming the "Mine Trend", from Cochenour through the Balmertown area, hosting the major gold deposits of the camp. HYDROTHERMAL ALTERATION (adapted from Parker 2000) The Red Lake greenstone belt has been affected by a large-scale (10's of kilometres) hydrothermal alteration system, resulting in approximately contemporaneous a) strong to intense, distal calcite carbonatization that affects rocks of all ages; and b) less extensive (kilometres), proximal, strong to intense ferroan-dolomite and potassic alteration, found in almost all areas hosting gold mineralization. Carbonate alteration affects both the Dome (2718±1 Ma) and McKenzie (2720±2 Ma) stocks and is overprinted by calcsilicate, skarn-like alteration formed during the intrusion of the Killala–Baird batholith (2704±1.5 Ma) and the Walsh Lake pluton (2699 Ma). The significant carbonate alteration event is therefore bracketed between 2718 and 2704 Ma, during D2. The main macroscopic features of carbonate alteration are pervasive replacement of rock matrix, open-space filling/replacement of primary porosity (vesicles, pillow selvages, hyaloclastite matrix), filling of extension veins with massive, colloform, crustiform and cockade breccia textures, networks of variably oriented veins and "jigsaw puzzle" breccia veins. 126 �Multiple stages of carbonate alteration and veining have been recognized, indicating continuous carbonatization during D2 deformation. Potassic metasomatism takes the form of sericite/muscovite alteration in greenschistfacies rocks; in ferroan-dolomite altered ultramafic rocks fuchsite occurs instead of sericite. Potassic alteration in amphibolite-facies mafic and ultramafic rocks takes the form of pervasive biotite ± muscovite. Centimetre- to metre-wide, strong to intense, biotite ± calcite ± ferroan-dolomite ± disseminated pyrite alteration halos often enclose ferroan-dolomite veins in amphibolite-facies mafic rocks. Biotite altered zones in amphibolite-facies rocks are characterized by a diverse assemblage of aluminosilicate minerals such as andalusite, staurolite and cordierite, with garnet, chloritoid, cummingtonite and anthophylite. Barren, pervasive silicification within proximal alteration zones may be due to release and remobilization of silica during periods of pervasive carbonatization. The majority of gold deposits in the Red Lake belt are quartz and arsenopyrite rich selective replacement zones of colloform-crustiform ferroan-dolomite veins and breccia. GEOLOGY OF THE CAMPBELL-RED LAKE GOLD DEPOSIT (adapted from Dubé et al. 2002) Gold has been continuously produced from the Campbell-Red Lake (formerly known as the Campbell-Dickenson) deposit since 1948: current production levels and reserves are given in Table 3. Historical production figures for the Red Lake greenstone belt are shown in Table 4. Table 3. Current gold production and reserves, Red Lake greenstone belt Mine Production to end of 2000 Production in 2001 Tonnage Grade Tonnage @ Grade @ Total Commodity Reserves at end of 2001 Total Commodity Tonnage Grade Goldcorp Inc. Red Lake Mine 74 148 tons @ 1.57 ounces per ton 85 115 ounces Au 246 618 tons @ 2.26 opt Au (223 728 tonnes @77.50 g/t) 503 385 ounces Au 3 208 000 tons (2 910 000 tonnes) (1) 1.34 opt Au (46.04 g/t ) Placer Dome (CLA) Ltd. Campbell Mine 473 000 tonnes @ 15.7 g/t Au 229 408 ounces Au 438 000 tonnes @13.3 g/t 178 139 ounces Au 1 941 000 tonnes (2 139 600 tons) (2) 16.7 g/t Au (482 800 tons @ 0.388 opt Au) (1) News release, Goldcorp Inc. February 7, 2002 (2) News release, Placer Dome (CLA) Ltd. February 14, 2002 127 (0.487 opt Au) �Table 4. Historical gold production, Red Lake greenstone belt GOLD PRODUCTION IN THE RED LAKE GREENSTONE BELT to December 31, 2001 MINE YEARS PRODUCTION OF CAMPBELL RED LAKE GOLDCORP (DICKENSON) MADSEN COCHENOUR-WILLANS MCKENZIE RED LAKE HOWEY HASAGA STARRATT OLSEN H.G. YOUNG MCMARMAC GOLD EAGLE RED LAKE GOLD SHORE BUFFALO ABINO LAKE ROWAN RED SUMMIT MOUNT JAMIE 1949 - PRESENT(1) 1948 - PRESENT(1,2) 1938 - 1976, 1997(4) - 1999 1939 - 1971 1935 - 1966 1930 - 1941, 1957(7) 1938 - 1952 1948 - 1956 1960 - 1963 1940 - 1948 1937 - 1941 1936 - 1938 1981 - 1982 1985 - 1986 1986 - 1988 1935 - 1936 1976 TOTAL ORE MILLED (SHORT TONS) GOLD PRODUCED TROY OUNCES OUNCES PER TON 17,979,851 8,619,008 8,678,143 2,311,165 2,353,833 4,630,779 1,515,282 907,813 288,179 152,978 180,095 86,333 31,986 2,733 13,023 591 552 10,335,248 3,736,704 2,452,388 1,244,279 651,156 421,592 218,213 163,990 55,244 45,246 40,204 21,100 1,656 1,397 1,298 277 265 0.575 0.434(3) 0.283(5) 0.538(6) 0.277 0.091(8) 0.144 0.181 0.192 0.296 0.223 0.244 0.052 0.511 0.100 0.469 0.480 47,752,344 19,390,257 0.406 NOTES: (1) Includes final production figures for 2001. (2) For 1997, 1998 and 1999 no production due to strike by unionised employees. (3) From 1970, includes production from Robin Red Lake. (4) Includes clean up ore and materials from the mine site. (5) Historic grade, actual grade for 1999 was 0.14 ounce per ton gold. (6) Includes production from Annco and Wilmar properties. (7) Continuous production 1930 to 1941; includes 268 ounces recovered from clean up in 1957. (8) The ore mined at Howey, before sorting totalled 5,158,376 tons. The average production from run-of-mine ore was therefore 0.0817 ounce per ton gold. Alteration facies in the High Grade Zone at Goldcorp Inc.'s Red Lake Mine have been described by Dubé et al. 2002: 1. an outer, metre-wide, garnet-chlorite-magnetite alteration with chlorite-amphibole-andalusite and locally associated centimetre- to metre-wide 'bleached zone' containing andalusitemuscovite-quartz-ilmenite ...; 2. a proximal, centimetre- to metre-wide, massive to laminated, reddish-brown, biotite-carbonate alteration with disseminated pyrite (3-5%) and carbonate veinlets in well foliated basalt; and 3. a gold-rich, strongly foliated, silicified zone with abundant fine-grained arsenopyrite, sericite, and rutile, and lesser amounts of pyrite, pyrrhotite, magnetite, and stibnite (≤15%). This third alteration facies is adjacent to the silicified auriferous carbonate veins and replaces the biotite-carbonate-rich alteration. 128 �The chronology of gold-rich replacement textures suggests a syn-D2 mineralizing event, dominated by silicification of carbonate veins, contemporaneous with boudinage of the veins. The silicified carbonate veins are hosted mainly by basalt; areas of high-grade gold mineralization are controlled by F2 fold hinges deforming the basalt-ultramafic contact. Multiple periods of silicification and gold deposition overprint and replace the carbonatization in these lower pressure hinge zones. The extremely high grade ore (>2.0 oz/t Au) currently mined at Goldcorp Inc.'s Red Lake Mine, is possibly due to a combination of factors, including the presence of a lowpermeability ultramafic cap, allowing the build-up of very high fluid pressure in the footwall basalt; the high iron content of the tholeiitic basalt, creating a chemical, as well as structural, trap for the auriferous fluids; multiple D2 strain events; repeated episodes of gold deposition and remobilization into a low pressure F2 fold hinge hosting the High Grade Zone. SUMMARY OF STOPS, SURFACE FIELD TRIP, RED LAKE BELT The first outcrops after the underground tours will traverse both the proximal-distal alteration facies and the Neo–Mesoarchean boundary. ¾ The tour then continues to the south-central portion of the Red Lake greenstone belt, within the metamorphic aureole of the Killala-Baird batholith. ¾ Outcrops at the interface of Meso- and Neoarchean assemblages expose rock units similar to those mined at the past producing Madsen mine, a high temperature, disseminated, stratabound gold deposit, quite dissimilar to deposits in the 'Mine Trend'; ¾ this is followed by a visit to the Dome stock, a mineralized granodiorite intruded into the volcanics in the central portion of the belt; ¾ followed by stops in the town of Red Lake to view the site of the Howey mine, and to examine intensely deformed rocks within the purported Howey Bay – Flat Lake deformation zone; ¾ the last stops will be within strongly altered and veined Balmer rocks in the northeast portion of the belt. 129 �STOP 1 - MESO-NEOARCHEAN CONTACT Woodland Cemetery Road and Hwy. 125 (Fig. 2) These outcrops show altered relatively low-strain pillowed basaltic komatiite flows of the Balmer Assemblage unconformably overlain by polymictic conglomerate of the Huston assemblage. The exposures are in the transition from calcite carbonatization (distal alteration) to ferroan-dolomite (proximal) alteration. The pillowed and minor massive flows show extensive iron carbonate alteration as well as iron carbonate and quartz veins. Fuchsite is present in the central part of the outcrops on the west side of the highway (cemetery side). The Campbell Mine is approximately 1.5 km to the north While the mafic flows have not been directly dated at this locality, they are typically variolitic, and show a geochemical similarity with known Balmer age rocks elsewhere; the massive and pillowed flows here can be traced to Balmertown, where an intercalated rhyolite at the Campbell mine was dated at 2989 ± 3 (Corfu and Andrews 1987). Variolitic flows occur in the northern part of the outcrops on the east side of the highway. However, they are unconformably overlain by Huston polymictic conglomerate further south along the outcrop. The conglomerate contains a large proportion of rounded cherty, jasperoidal and pyritic fragments. It represents an apron of Confederation assemblage (McNeely age-2743 Ma) detritus deposited at the break in the paleoslope between the Confederation volcanic centre and its Balmer age substrate. STOP 2 - CALCITE CARBONATIZED PILLOWED FLOWS: DISTAL CARBONATE ALTERATION FACIES Outcrops on west side of Hwy. 125 and Sandy Bay Road (Fig. 2) Slightly deformed pillows of the Balmer assemblage show pervasive calcite carbonatization, calcite veins and pods. Amygdules are also filled (replaced?) with calcite. Jig-saw puzzle breccias (created by fluid overpressure at depth) are cemented by calcite. This represents the distal, outer halo of carbonate alteration. STOP 3 - CONTACT BETWEEN CONFEDERATION AND BALMER ASSEMBLAGES Suffel Lake Road and Hwy. 618 (Figs. 2 and 3) Exposures on the south side of the highway are part of the lowermost units of the Neoarchean Confederation assemblage. The outcrops here are amphibolite-facies 130 �tholeiitic, quartz-feldspar-porphyritic lapilli-crystal tuff, with thin, dark grey, collapsed pumice fragments; occasional lapilli sized lithic clasts are also observed. Strike of the rocks is generally northeast, facing and dipping steeply southeast. A sample from this unit, 800 m northeast of the intersection, gave an age of 2744 ± 1 Ma (Corfu and Andrews 1987). The north side of the road exposes highly altered tholeiitic, mafic volcaniclastic rocks of the Balmer assemblage. Abundant garnet and biotite rims clasts; minor andalusite is present. This outcrop, barren at this locality, forms part of the Austin "tuff" ore zone, described further below. STOP 4 - MADSEN DEPOSIT, POWER LINE OUTCROPS (Figs. 2 and 3) Time limitations of the tour do not permit a complete visit of the Madsen deposit; a brief description of the deposit follows: Geology of the Madsen Deposit (adapted from Dubé et al. 2000) Madsen is a stratabound, replacement-style, disseminated gold deposit, exhibiting two alteration facies, the mineralogy of which is now represented by two amphibolite-facies zones: 1) a pervasive aluminous, metre- to tens-of-metres-wide, low-strain, outer zone, containing andalusite-garnet-biotite-staurolite-amphibole; metre-wide stockwork amphibole veins and veinlets alternate with the pervasive alteration. Timing of this alteration is pre- to syn-D1, but its relationship to gold mineralization is not yet known; indeed, it could be classified as the amphibolite-facies equivalent of volcanogenic massive sulfide (VMS) type alteration, related to a Confederation age syn-volcanic hydrothermal alteration system; 2) an inner zone comprising a banded-laminated texture, characterized by bands of actinolite-hornblende-microcline-calcite-tourmaline, alternating with biotiterich bands. The amphibole is commonly randomly oriented. Diopside locally forms disseminated crystals up to 7-8 cm long, or veinlets. Ore zones occur within the inner alteration zone, and comprise finely layered, sulfide-rich lenses up to a few metres wide. Sulfides (8-10%) comprise pyrrhotite, pyrite and/or arsenopyrite with trace chalcopyrite, and are found as disseminations or veinlets parallel to lamination/foliation. Gold occurs in the native state as inclusions in silicate minerals and locally as coatings on sulfide minerals. Highest grade is found in areas of most intense alteration, represented by quartz-biotite-muscovite-microcline assemblage in mm-cm bands or layers. Crenulation of alteration bands, sulfides and calcite veinlets by S2 as well as the large-scale deformation and folding of Austin ore lenses by F2 folds are consistent with pre- to early D2 timing of gold mineralization. A minimum age on the deposit is 2699 ± 4 Ma (Corfu and Andrews 1987), the age of a cross-cutting post-ore granodiorite dyke. 131 �1/ I 400m / // J // //// d//// oO// / Figure 3. Geology of the Madsen mine area (modified from Dubé et al. 2000) 132 �Proximal alteration and style of mineralization may indicate the Madsen deposit to be related to higher temperature (400º-600ºC) gold deposits and gold-skarn deposits hosted by mafic volcanics (Parker 2000). South Austin Zone – Powerline Section The base of this series of poorly exposed outcrops is a well banded/layered example of Austin "tuff", from which the bulk of the 2.5 million ounces gold of the Madsen deposit were mined between 1938-1976 (Table 4). At this locality the Austin is a strongly altered (biotite, amphibole, garnet) mafic volcaniclastic/epiclastic rock, with wacke and conglomerate clasts, occupying the position of the unconformity between Balmer and Confederation assemblages. Further up the hill the Confederation age quartz-feldspar porphyritic lapilli tuff unit from Stop 1 forms the structural and stratigraphic hangingwall of the deposit and marks the beginning of Confederation time. Overlying this unit is an altered (biotite, garnet) polymictic conglomerate outcrop, part of the Huston assemblage, that yielded a single peak in detrital U-Pb zircon ages of ≤ 2746 Ma (Sanborn et al. 2001a). At the top of the hill, feldspar phyric tuff of the Confederation assemblage is exposed. STOP 5 - BUFFALO DEPOSIT - DOME STOCK MINERALIZATION (Fig. 2) The approximately 7 km diameter hornblende-biotite granodiorite stock (Table 2) has been dated at 2718 ± 1 Ma (Corfu and Andrews 1987) and is interpreted to have been emplaced during D2 (Sanborn-Barrie et al. 2001). The stock is variably iron-carbonate, sericite, and chlorite altered and deformed. Exposures to be visited (Figure 4) are at its southern contact; here it intrudes, and contains xenoliths of, foliated Balmer assemblage mafic volcanic rocks (Figure 4). The stock hosts several gold occurrences and two past-producing mines: the Red Lake Gold Shore produced 21,100 ounces gold, and the Buffalo Mine produced 1656 ounces gold. The Buffalo prospect was discovered in 1925 and explored several times since then. Note the adit reopened by Claude Resources Ltd. in October 1998 to further explore the Buffalo deposit. Gold is hosted within two sets of quartz-tourmaline-pyrite-calcite veins in conjugate orientation (centimetre-wide NE veins: 239°/73° N, and decimetre-wide NW veins: 119°/76° S; Pettigrew 1999). Their orientation may be as a result of the intersection of two previously interpreted (Durocher and Hugon 1983) deformation zones (St. Paul BayMartin Bay and Flat Lake-Howey Bay Deformation Zones). The dominant vein set strikes NW; primary quartz vein fill was replaced by tourmaline, concomitant with bleached pink metasomatic halos developing around tourmaline-rich portions of the veins. Gold is concentrated in the calcite-albite-sulfide halos, in particular at its outer fringe, where chalcopyrite and tellurides were deposited. A second stage of gold 133 �mineralization is associated with Bi-tellurides in fractures and cavity fillings in quartz and late fracture-filling pyrite, hosted within the qtz-tourmaline-pyrite-calcite veins. mafic volcanics granodiorite '---1 vein / shear Figure 4. Detailed geology of south side of Buffalo Pit (from Lavigne et al. 1986) STOP 6 - HOWEY MINE (Fenced in pit - drive by: Fig. 2) On the north side of Hammell Road a cement foundation marks the site the former Howey mine. Behind the fenced off area is the site of the crown pillar mined out in the final stages of the mine. The Howey Mine was the first producer (1930-1941) in the Red Lake camp and remains the lowest grade profitable gold mine in Canadian mining history (final average grade 0.08 opt Au, having produced 422 000 ounces gold). The Howey (and adjacent Hasaga) ore bodies occur in a boudinaged, variably sericitized and silicified quartz-feldspar porphyry dyke trending approximately 065°/80°S. Centimetre-wide, auriferous quartz veinlets trend 080°, making an angle of 15° with the contacts of the dyke and dip 80°S. Gold-bearing quartz veinlets formed as the last of three episodes of quartz veining. Gold is associated with pyrite-sphalerite-galena-tourmaline ± tellurides. Small flat outcrops between the highway and the fence are highly deformed intermediate rocks of the Howey Mine hanging wall. This site lies within the northeast trending Howey Bay – Flat Lake deformation zone and comprises Confederation age rocks. STOP 7 - HOWEY BAY-FLAT LAKE DEFORMATION ZONE (Fig. 2) The Howey Bay – Flat Lake deformation zone was defined by Durocher and Hugon (1983), and was interpreted to be part of a belt-wide system of transcurrent shear zones hosting most of the major gold deposits. Recent detailed work has led to a reevaluation of this concept (Sanborn-Barrie et al. 2000). 134 �This stop is approximately 750 m southwest of the Howey mine. Intense deformation at this stop has destroyed most primary textures that might be used to identify the rocks. The dominant rock type is mylonitized intermediate tuff. Pink felsic dikes that cut the intermediate rock are also mylonitized. Iron-carbonate veins are boudinaged and transposed into the shear direction. The far western extremity of the outcrops exposes deformed quartz-feldspar dyke (similar in appearance to the Dome stock) containing mafic xenoliths and quartz-tourmaline veinlets. STOP 8 - REDCON CARBONATE ZONE: Proximal ferroan-carbonate alteration; carbonate veining West and east sides of Nungessor Road (Fig. 2) This area is approximately 4 km north of the Campbell–Red Lake deposit, still within the proximal, ferroan-carbonate alteration facies. The outcrops are weakly foliated (145°), dominantly massive to pillowed Balmer assemblage basalts, occurring within the amphibolite-facies metamorphic aureole of the Walsh Lake pluton. The stripped area on the east side of the Nungesser road was mapped in detail (Figure 5) by Redcon Gold Mines in 1981 (assessment files) and now forms part of Goldcorp Inc.'s holdings. Here, a 1-2 m wide carbonate vein is exposed near its southeastern termination. The vein can be traced in outcrop and drilling for approximately 750 m to the west-northwest and will be seen at the next stop on the west side of the road. Gold occurs in north-northwest trending, irregular, centimetre-thick quartz-actinolite stringers within the carbonate vein. After an initial, pervasive biotite alteration event, cross-cutting relationships suggest the following sequence of formation (from Lavigne et al. 1986): 1. amphibole-quartz-calcite cross-fractures 2. quartz-calcite veins 3. ferroan-dolomite veins 4. mafic dyke 5. auriferous quartz veins Silicification evident in the pillowed flow on the northern half of the outcrop is barren and apparently not related to the gold-rich silicification event, rather, it may be due to local silica dumping following pervasive carbonate metasomatism. A "black line" fault occurs in the northern wall rocks of the main carbonate vein. A mafic (or lamprophyre) dyke (unit 4, above) cuts the vein, but is itself cut by late quartzactinolite-gold stringers. 135 �TYPE A VEINS TYPE C VEINS I \\\\\\\ss\\1 hb+q+pI+blo 1i2:J Pillow basalt Type A veins re aorom,e (type ci veine EI Matic dike (D) II Quartz ±Au (tvoe E) veina Fault Prominent Joints Figure 5. Detailed geology of the Redcon prospect (modified from Lavigne et al. 1986) The western outcrops are approximately 300 m west-northwest of the previous exposures. Things to note on the western series of outcrops: • differing colours of cross-cutting carbonate veins • colloform/crustiform textures in carbonate veins • andalusite-garnet-biotite alteration of pillows cut by calc-silicate veins (diopside ± calcite, quartz, tourmaline; retrograding to epidote, tremolite, actinolite/hornblende, magnetite) • calc-silicate veins cross-cut by carbonate veins • folding of carbonate veins by the "Mine trend" S2 136 �REFERENCES AND BIBLIOGRAPHY OF RECENT RESEARCH Chi, G., Dubé, B. and Williamson, K. 2002. Preliminary fluid-inclusion microthermometry study of fluid evolution and temperature-pressure conditions in the Goldcorp High-Grade zone, Red Lake mine, Ontario, in Current Research 2002C27, geological Survey of Canada, 14p. Dubé, B., Balmer, W., Sanborn-Barrie, M., Skulski, T. and Parker, J. 2000. A preliminary report on amphibolite-facies, disseminated-replacement-style mineralization at the Madsen gold mine, Red Lake, Ontario; in Current Research 2000-C17, Geological Survey of Canada, 12p. Dubé, B., Williamson, K., and Malo, M. 2001. Preliminary Report on the Geology and Controlling Parameters of the Goldcorp Inc. High Grade Zone, Red Lake Mine, Ontario; Geological Survey of Canada, Current Research 2001-C18, 13 p. Dubé, B., Williamson, K., and Malo, M. 2002. Geology of the Goldcorp Inc. High Grade zone, Red Lake mine, Ontario: an update, in Current Research 2002-C26, Geological Survey of Canada, 15p. Durocher, M.E. and Hugon, H., 1983. Structural geology and hydrothermal alteration in the Flat Lake-Howey Bay deformation zone, Red Lake area, in Summary of Field Work, 1983, Ontario Geological Survey, Miscellaneous Paper 116, p. 216 to p. 219. Gulson, B.L., Mizon, K.J. and Atkinson, B.T. 1993. Source and timing of gold and other mineralization in the Red Lake area, northwestern Ontario, based on lead-isotope investigations, Canadian Journal of Earth Science v. 30, pp. 2366-2379. Horwood, H.C., 1940. Geology and mineral deposits of the Red Lake area, in Fortyninth Annual Report of the Ontario Dept. of Mines, vol. XLIX, Pt. II, 231p. Lavigne Jr., M.J., Hugon, H., Andrews, A.J. and Durocher, M.E. 1986. Gold deposits of the Red Lake District, Relationships of gold mineralization to regional deformation and alteration in the Red Lake greenstone belt, Ontario, in Gold '86, Excursion Guidebook, ed. Pirie, J. and Downes, M.J., p.167 to p.211. Parker, J.R. 1999. Exploration potential for volcanogenic massive sulphide (VMS) mineralization in the Red Lake greenstone belt; in Summary of Field Work and Other Activities 1999, Ontario Geological Survey, Open File Report 6000, p.19-1 to 22-26. Parker, J.R. 2000. Gold mineralization and wall rock alteration in the Red Lake greenstone belt: a regional perspective; in Summary of Field Work and Other Activities 2000, Ontario Geological Survey, Open File Report 6032, p.22-1 to 22-27. 137 �Parker, J.R. 2001. Intermediate to Felsic Plutons in the Red Lake Greenstone Belt: Relationship to Deformation and Gold Mineralization; in Summary of Field Work and Other Activities 2001, Ontario Geological Survey, Open File Report 6070, p. 191 to 19-10. Penczak, R.S., and Mason, R. 1999. Characteristics and origin of Archean premetamorphic hydrothermal alteration at the Campbell Gold Mine, Northwestern Ontario, Canada, Economic Geology, v. 94. pp. 507-528. Penczak, R.S., and Mason, R. 1997. Metamorphosed Archean epithermal Au-As-Sb-Zn(Hg) vein mineralization at the Campbell Mine, Northwestern Ontario, Economic Geology, v.92, pp 696-719. Percival, J.A., Bailes, A.H., Corkery, M.T., Dubé, B., Harris, J.R., McNicoll, V., Panagapko, D., Parker, J.R., Rogers, N., Sanborn-Barrie, M., Skulski, T., Stone, D., Stott, G.M., Thurston, P.C., Tomlinson, K.Y., Whalen, J.B., and Young, M.D. 2000. An integrated view of Western Superior crustal evolution: highlights of 2000 NATMAP studies, in Summary of Field Work and Other Activities 2000, Ontario Geological Survey, Open File Report 6032, p.13-1 to p.13-17. Pettigrew, N., 1999. Structural and alteration history of the Buffalo Gold Deposit, Red Lake, Ontario; B.Sc. Thesis, University of New Brunswick, 154p. Pirie, J. and Downes, M.J., eds., 1986. Gold '86 Excursion Guidebook. Sanborn-Barrie, M., Skulski, T., and Parker, J. 2001. Three hundred million years of tectonic history recorded by the Red Lake greenstone belt, Ontario, in Current Research 2001-C19, Geological Survey of Canada, 32p. Sanborn-Barrie, M., Skulski, T., Parker, J. and Dubé, B., 2000. Integrated regional analysis of the Red Lake greenstone belt and its mineral deposits, western Superior Province, Ontario, in Current Research 2000-C18, Geological Survey of Canada, 16p. Skulski, T., Sanborn-Barrie, M. and Sanborn, N., 2001. New U-Pb geochronology in the Red Lake greenstone belt, Western Superior NATMAP, unpublished poster. Stone, D. and Hallé, J. 2000. Geology of the Blackbear, Yelling and Stull Lake areas, Northern Superior Province, Ontario, in Summary of Field Work and Other Activities 2000, Ontario Geological Survey, Open File Report 6032, p. 15-1 to 15-9. 138 � Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology: Proceedings, 2002 Description An account of the resource Institute on Lake Superior Geology. Kenora, Ontario. May 12-16, 2002. Creator An entity primarily responsible for making the resource Institute on Lake Superior Geology Date A point or period of time associated with an event in the lifecycle of the resource 2002 Format The file format, physical medium, or dimensions of the resource PDF Language A language of the resource English https://digitalcollections.lakeheadu.ca/files/original/144f6c8afe6c22b14b0587341cb54d7e.pdf c67d0082182c4da907410c829b7469e8 PDF Text Text 49TH ANNUAL INSTITUTE ON LAKE SUPERIOR GEOLOGY IRON MOUNTAIN, MICHIGAN MAY 7 — 11, 2003 Proceedings 49- a- Volume - ------PART -PROGRAMS PROGRAMS AND ABSTRACTS ABSTRACTS PART 11— AND �INSTITUTE INSTITUTE ON ON LAKE LAKESUPERIOR SUPERIOR GEOLOGY GEOLOGY 49TH ANNUAL MEETING MAY 7-11, 2003 IRON MOUNTAIN, MICHIGAN HOSTED HOSTEDBY: BY: LAUREL G. LAUREL G.WOODRUFF WOODRUFFAND AND WILLIAM WILLIAMF. F.CANNON CANNON Co-Chairs Co-Chairs U.S.GEOLOGICAL GEOLOGICAL SURVEY U.S. SURVEY With With assistance assistance from from Michigan MichiganTechnological TechnologicalUniversity University and and John JohnGartner, Gartner,Coleman ColemanEngineering EngineeringCompany Company Volume Volume 49 49 Part Proceedings and Abstracts Abstracts Part 11 — - Proceedings Compiled Compiledand and edited edited by byLaurel LaurelWoodruff, Woodruff,U.S. U.S. Geological GeologicalSurvey Surveyand and Theodore Theodore Bornhorst, Bornhorst, Michigan Michigan Technological University University Cover CoverPhoto: Photo:Berkshire BerkshireShaft, Shaft,Menominee MenomineeRange, Range,Michigan. Michigan.Photo Photofrom fromthe theMichigan Michigan Technological University Mining Engineering Department Collection. Technological University Mining Engineering Department Collection. �49TH 4gTHINSTITUTE INSTITUTE ON LAKE LAKE SUPERIOR SUPERIOR GEOLOGY GEOLOGY VOLUME VOLUME 49 49CONSISTS CONSISTSOF: OF: PART PART1: 1:PROGRAM PROGRAMAND ANDABSTRACTS ABSTRACTS PART2: 2: FIELD FIELDTRIP TRIPGUIDEBOOK GUIDEBOOK PART OVERVIEW: OVERVIEW: PALEOZOIC PALEOZOICSTRATIGRAPHY STRATIGRAPHYAND ANDTECTONICS TECTONICSALONG ALONG THE THE NIAGRA NIAGRA SUTURE SUTURE ZONE, MICHIGAN MICHIGANAND AND WISCONSIN WISCONSIN TRIP MAGMATIC 1:PEMBINE-WAUSAU PEMBINE-WAUSAU MAGMATICTERRANE TERRANE TRIP 1: TRIP 2: MENOMINEE MENOMINEEIRON IRONDISTRICT DISTRICT TRIP 2: TRIP 3:STRATRIGRAPHY STRATRIGRAPHYAND ANDSTRUCTURE STRUCTUREOF OFTHE THEIRON IRONRIVER RIVER—TRIP3: CRYSTAL CRYSTALFALLS FALLSBASIN BASIN TRIP - THE THE REPUBLIC REPUBLICMINE MINE TRIP4: 4: LIFE LIFECYCLE CYCLEOF OFAN ANIRON IRONDEPOST DEPORT— FROM FROM ORE ORE GENESIS GENESIS TO TO MINE MINERESTORATION RESTORATION Reference to to material in Part 1 1 should should follow the t h e example below: Rogala, Rogala, B., B., Fralick, Fralick,P., P., and and Borradaile, Borradaile,G., G., 2003, 2003, AA magnetostratigraphic magnetostratigraphicand andsecular secularvariation vari$ion 4gth study study of of the the Sibley Sibley Group Group [abstract]; [abstract];Institute Instituteon onLake LakeSuperior SuperiorGeology GeologyProceedings, Proceedings, 49 Annual Ml, v. v.49, Annual Meeting, Meeting, Iron Iron Mountain, MI, 49, part part 1, 1,p.65-66. p. 65-66. and distributed by Publishedby bythe the 49th 4gth Institute Instituteon on Lake Lake Superior Geology Geology a n d distributed bythe the Published ILSG Secretary-Treasurer: ILSG MarkJirsa Jirsa(through (through2003) 2003) Mark Minnesota Minnesota Geological GeologicalSurvey Survey 2642 University UniversityAvenue Avenue 2642 Paul, MN MN55114-1 55114-1057 St. Paul, 057 In In 2004 2004 contact: contact: USA USA JirsaOOl @tc.urnn.edu @tc.umn.edu JirsaOOl Peter Peter Hollings Hollings Lakehead Lakehead University University Department of of Geology Geology Department Thunder Thunder Bay, Bay, ON ON P7B P7B5E1 5E1 CANADA CANADA peter.hollinas@lakeheadu.ca Deter.hollinps@lakeheadu.ca ILSG website: http://www.ilscieolopy.orp htt~://www.ilsaeoloav.org ILSG ISSN 1042-9964 �CONTENTS CONTENTS PROCEEDINGS PROCEEDINGS VOLUME VOLUME 49 49 PART PART1—PROGRAM 1-PROGRAM AND AND ABSTRACTS ABSTRACTS Institutes Instituteson on Lake LakeSuperior SuperiorGeology, Geology,1955-2003 1955-2003............................................................ iviv Constitution Constitutionof of the the Institute Instituteon onLake LakeSuperior Superior Geology Geology ................................................... vivi vii By-Laws of of the the Institute Instituteon onLake LakeSuperior Superior Geology Geology ....................................................... vii By-Laws ... MembershipCriteria Criteria ...................................................................................................... Membership viH VIII GoldichMedal MedalGuidelines Guidelines............................................................................................... Goldich ix ix Goldich GoldichMedal MedalCommittee Committee ............................................................................................... xx Past Goldich GoldichMedallists Medallists ..................................................................................................xi Past xi Citation Citation for for 2003 2003Goldich GoldichMedal MedalRecipient Recipient..................................................................... xD xii Eisenbrey EisenbreyStudent StudentTravel TravelAwards Awards................................................................................. xiv xiv xiiv Student Travel Travel Award Award Application ApplicationForm Form ....................................................................... xHv Student Student Student Paper PaperAwards Awards................................................................................................... xv xv Student Paper Paper Awards Awards Committee Committee ................................................................................ xv xv Student SessionChairs Chairs .............................................................................................................. Session xv xv Board of of Directors Directors ........................................................................................................ Board xvi xvi Local Committees Committees......................................................................................................... Local xvi xvi BanquetSpeaker Speaker.......................................................................................................... Banquet xvi xvi xvii Report of the the Chair Chair of of the the 48th 48thAnnual AnnualMeeting Meeting............................................................ xvii Report Program ....................................................................................................................... Program )O(i List of of Contributors Contributors ....................................................................................................... List X)(iI xxii Abstracts ................................................................................................................... Abstracts iii III xxi ... )O(VItI xxvm �INSTITUTES LAKE SUPERIOR INSTITUTESON LAKE SUPERIOR GEOLOGY GEOLOGY # YEAR YEAR CHAIRS CHAIRS PLACE PLACE 1 1955 Minneapolis, Minneapolis,Minnesota Minnesota C.E. Dutton C.E. Dufton 2 1956 Houghton, Houghton,Michigan Michigan A.K. A.K. Snelgrove Snelgrove 3 1957 East EastLansing, Lansing,Michigan Michigan B.T. B.T. Sandefur Sandefur 4 1958 Duluth, Duluth,Minnesota Minnesota R.W. Marsden R.W.Marsden 5 1959 Minneapolis, Minneapolis,Minnesota Minnesota G.M. G.M. Schwartz Schwartz && C. C. Craddock Craddock 6 1960 Madison, Madison,Wisconsin Wisconsin Eli. E.N.Cameron Cameron 7 1961 Port Port Arthur, Arthur, Ontario Ontario E.G. E.G. Pye Pye 8 1962 Houghton, Houghton,Michigan Michigan A.K. A.K. Snelgrove Snelgrove 9 1963 Duluth, Duluth, Minnesota Minnesota H. H.Lepp Lepp 10 1964 lshpeming, Ishpeming,Michigan Michigan A.T. A.T. Broderick Broderick 11 1965 St. St. Paul, Paul,Minnesota Minnesota P.K. P.K. Sims Sims && R.K. R.K. Hogberg Hogberg 12 1966 Sault SaultSte. Ste.Marie, Marie,Michigan Michigan R.W. R.W. White White 13 1967 East EastLansing, Lansing,Michigan Michigan W.J. W.J.Hinze Hinze 14 1968 Superior, Superior,Wisconsin Wisconsin A.B. A.B. Dickas Dickas 15 1969 Oshkosh, Oshkosh,Wisconsin Wisconsin G.L. G.L. LaBerge LaBerge 16 1970 Thunder ThunderBay, Bay,Ontario Ontario M.W. E.Mercy Mercy M.W. Bartley& Bartley &E. 17 1971 Duluth, Duluth,Minnesota Minnesota D.M. D.M. Davidson Davidson 18 1972 Houghton, Houghton,Michigan Michigan J. J. Kalliokoski Kalliokoski 19 1973 Madison, Madison,Wisconsin Wisconsin M.E. M.E. Ostrom Ostrom 20 1974 Sault SaultSte. Ste.Marie, Marie,Ontario Ontario P.E. P.E.Giblin Giblin 21 1975 Marquette, Marquette,Michigan Michigan J.D. J.D. Hughes Hughes 22 1976 St. St.Paul, Paul,Minnesota Minnesota M. M.Walton Walton 23 1977 Thunder ThunderBay, Bay,Ontario Ontario M.M. M.M.Kehlenbeck Kehlenbeck 24 1978 Milwaukee, Milwaukee,Wisconsin Wisconsin G. G.Mursky Mursky 25 1979 Duluth, Duluth,Minnesota Minnesota D.M. D.M.Davidson Davidson 26 1980 Eau EauClaire, Claire,Wisconsin Wisconsin P.E. P.E.Myers Myers 27 1981 East EastLansing, Lansing,Michigan Michigan W.C. W.C.Cambray Cambray 28 1982 International InternationalFalls, Falls,Minnesota Minnesota D.L. D.L.Southwick Southwick 29 1983 Houghton, Houghton,Michigan Michigan T.J. T.J.Bornhorst Bornhorst iv �30 1984 Wausau, Wisconsin Wisconsin G.L. LaBerge G.L. LaBerge 31 1 985 Kenora, Ontario Ontario C.E. C.E. Blackburn Blackburn 32 1986 Wisconsin Rapids, Wisconsin Rapids, Wisconsin Wisconsin J.K. Greenberg Greenberg 33 1987 Wawa, Wawa,Ontario Ontario 34 1988 Marquette, Michigan Michigan J. S. Klasner Klasner 35 1989 Duluth, Minnesota Minnesota J.C. J.C. Green Green 36 1990 Thunder Bay, Bay, Ontario Ontario M.M. Kehlenbeck M.M. 37 1991 Eau Claire, Wisconsin P.E. Myers Myers 38 1992 Wisconsin Hurley, Wisconsin A.B. A.B. Dickas Dickas 39 1993 Eveleth, Minnesota Minnesota D.L. Southwick 40 1994 Houghton, Michigan Michigan T.J. Bornhorst Bornhorst 41 1995 Marathon, Ontario Ontario M.C. Smyk M.C. Smyk 42 1996 Cable, Wisconsin Wisconsin L.G. Woodruff 43 1997 Sudbury, Ontario Ontario Meyer R.P. Sage & W. Meyer 44 1998 Minneapolis, Minnesota Minnesota J.D. Miller & M.A. J.D. M.A. Jirsa Jirsa 45 1999 Marquette, Michigan Michigan Regis T.J. Bornhorst Bornhorst & R.S. Regis 46 2000 Thunder Bay, Ontario Ontario S.A. Kissin & P. Fralick S.A. Fralick 47 2001 Madison, Wisconsin Wisconsin M.G. Mudrey, Jr. & & B.A. B.A. Brown Brown 48 2002 Kenora, Ontario Ontario P. Hinz Beard Hinz & R.C. Beard 49 2003 Iron Mountain, Michigan Michigan L.G. L.G. Woodruff & W.F. Cannon Cannon E.D. Frey & R.P. Sage Sage E.D. V �__________(some CONSTITUTION OF THE INSTITUTE ON LAKE SUPERIOR GEOLOGY (Lastamended amendedby bythe theBoard—May Board-May 8,8,1997) (Last 1997) ArticleII Article Name The The name nameof of the the organization organizationshall shall be bethe the "Institute "Instituteon onLake Lake SuperiorGeology". Geology". Superior ArticleIII1 Article Obiectives Obiectives Theobjectives objectivesof of this thisorganization organizationare: are: The A. A. To Toprovide provideaameans meanswhereby wherebygeologists geologistsininthe theGreat GreatLakes Lakesregion regionmay may exchangeideas ideasand andscientific scientificdata. data. exchange B. B. To Topromote promotebetter betterunderstanding understandingofofthe thegeology geologyofofthe theLake LakeSuperior Superiorregion. region. C. field C. To Toplan planand andconduct conductgeological geological fieldtrips. trips. . Status No No part part of of the the income income of of the the organization organizationshall shallinsure insureto to the the benefit benefitof ofany any member memberor or individual. individual.In Inthe the event eventof of dissolution, dissolution,the theassets assetsof ofthe theorganization organization (sometax tax free freeorganization). organization). shall shallbe bedistributed distributedto to Article IllIll Article a (To (To avoid avoid Federal Federal and and State State income incometaxes, taxes, the the organization organizationshould shouldbe benot notonly only "scientific" "scientific"or or "educational, "educational, but but also also "non-profit") "non-profit") ArticleIV IV Article Article VV Article ArticleVI Vl Article Article VII Vll Article ArticleVIII Vlll Article Minn. Minn. Stat. Stat. Anno. Anno. 290.01, 290.01, subd. subd.44 Minn. Minn. Stat. Stat. Anno. Anno. 290.05(9) 290.05(9) 1954 1954 Internal InternalRevenue RevenueCode Codes.501 s.501(c)(3) (c)(3) Membership Membership The The membership membership of of the the organization organizationshall shall consist consist of of persons persons who who have have registered registeredfor for an an annual annualmeeting meetingwithin withinthe thepast pastthree threeyears, years, and andthose thosewho who indicate a member according to guidelines approved by the indicate interest interestin in being being a member according to guidelines approved by the Board Board of of Directors. Directors. Meetinas Meetings The The organization organization shall meet once a year. The Theplace placeand and exact exact date date of of each each meeting meeting will will be be designated designated by by the the Board Boardof of Directors. Directors. Directors Directors The The Board Boardof of Directors Directorsshall shall consist consist of of the the Chair, Chair, Secretary-Treasurer, Secretary-Treasurer,and andthe the last last three three past past Chairs; Chairs; but if the the board board should should at at any any time time consist consist of of fewer fewer than than five five persons, persons, by by reason reasonof of unwillingness unwillingnessor or inability inabilityof of any any of of the the above above persons persons to to serve serve as as directors, directors, the the vacancies vacancieson on the the board boardmay may be be filled filled by by the the Chair Chairso so as as to members hi^ of of the the board boardto to five five members. members. to bring brina the the membership 0fficeA Officers The officers officers of this organization organization shall shall be be aa Chair Chair and and Secretary-Treasurer. Secretary-Treasurer. A. The TheChair Chairshall shallbe beelected electedeach each year year by by the the Board Board of of Directors, Directors, who who shall shall give give due consideration consideration to the wishes of any group group that may may be be promoting promoting the the next next annual meeting. His/her Hislherterm term of of office officeas asChair Chairwill will terminate terminate at at the the close close of of the the annual annual meeting meeting over which which he/she helshe presides, presides, or when his/her hislher successor successor shall shall have been appointed. He/she Helshe will will then serve serve for aa period period of three years as a member member of the Board Board of Directors. Directors. B. The meeting. His/her TheSecretary-Treasurer Secretary-Treasurershall be be elected at the annual meeting. Hislher term of office shall be four years, or until his/her successor shall have term of office shall be four years, or until hislher successor shall havebeen been appointed. appointed. ~mendments Amendments This This constitution constitution may may be be amended amended by by aa majority majorityvote vote (majority (majorityof of those thosevoting) voting) of of the the membership membership of of the the organization. organization. vi �BY-LAWS OF BY-LAWS OFTHE THE INSTITUTE INSTITUTE ON LAKE LAKE SUPERIOR SUPERIOR GEOLOGY GEOLOGY I. Duties of the Officers and Directors I. Directors A. It shall be the duty of the Annual Annual Chairman Chairman to: to: I1.. Preside Presideat at the the annual annualmeeting. meeting. 2. Appoint Appointall allcommittees committeesneeded neededfor for the theorganization organizationof of the theannual annualmeeting. meeting. 3. Assume Assume complete completeresponsibility responsibility for for the the organization organization and and financing financing of of the the meeting over which annual meeting which he/she hershe presides. presides. B. It shall be the duty of the Secretary-Treasurer B. Secretary-Treasurer to: 1. Keep I. Keepaccurate accurateattendance attendance records recordsof of all all annual annual meetings. meetings. 2. Keep Keepaccurate accurate records recordsof of all all meetings meetings of, of, and and correspondence correspondence between, the Board of Directors. Directors. 3. Hold all funds Hold all funds that that may may accrue accrue as as profits profits from from annual annual meetings meetings or field trips for the organization and operation of of and to make these funds available for future future meetings meetings as as required. required. of the the Board of Directors to plan locations of annual C. It shall be the duty of C. organization and meetings and to advise on the organization and financing financing of of all all meetings. meetings. II. Duties 11. Dutiesand and Expenses Expenses $5.00 or or less less on on an an annual basis shall be A. Regular Regularmembership membershipdues dues of of $5.00 assessed each member as determined by the Board of Directors.. B. Registration Registrationfees fees for for the the annual annual meetings meetings shall shall be be determined determined by by the Chair in the Board Board of of Directors. Directors. The consultation with the The registration registration fees fees can can include include expenses to cover operations outside of the annual meeting as determined determined by recommended that registration the Board Board of Directors. Directors. It is strongly recommended registration fees be be attendance of students. kept at a minimum to encourage attendance Ill. Rules Ill. Rules of of Order Order The rules contained in Robert's Rules of Order shall govern this organization organization in in all cases cases to which which they they are are applicable. applicable. IV. IV. Amendments Amendments These by-laws by-laws may be amended by a majority majority vote (majority (majority of those those voting) voting) of of the the membership of the organization; provided that such modifications modifications shall shall not conflict conflict with the constitution constitution as presently presently adopted adopted or subsequently subsequently amended. amended. - Last 996 May, 11996 Last Amended Amended— May, vii vii �MEMBERSHIP MEMBERSHIPCRITERIA CRITERIA FOR FOR THE THE INSTITUTE ON LAKE LAKE SUPERIOR INSTITUTE SUPERIOR GEOLOGY GEOLOGY Approved May 8, 1997 8,1997 A. Membership Membershipin inthe theInstitute lnstituteon onLake LakeSuperior Superior Geology Geology requires requires either either participation participationin in Institute activities, activities, or an indication on aa regular basis of of interest interest in in the the lnstitute, Institute. Those Those individuals registering for an annual annual meeting meeting will remain remain as members members for for 44 years years unless: unless: 1) they indicate no further interest interest in in the Institute Institute by by responding respondingnegatively negatively to to the the statement on meeting meeting circulars circulars "Remove "Remove my my name name from from the the mailing mailinglist"; list";or or 2) 2)two two different years are successive mailings in different are returned returned by by the the postal postal service service as as address address unknown. unknown. B. Those individuals individuals who have have not not registered registered for an an annual annual meeting meeting in in the thepast past44years years must indicate indicate an interest interest in the Institute by postal, electronic, or verbal correspondence lnstitute by or verbal correspondence with the Secretary-Treasurer Secretary-Treasurer at least once every two two years. years. Such Suchindividuals individualswill will be be removed from the membership membership if they indicate indicate no no further further interest interestin in the the Institute lnstituteor ortwo two different years are successive mailing in different are returned returned by by the postal postal service service as as address address unknown, unknown. C. The The Secretary-Treasurer Secretary-Treasurer will maintain a list of current members. The The list listwill will include include the date of the beginning beginning of continuous continuous membership, membership, dates of returned returned mail, dates dates of last contact (expression of interest), and the date membership expires, barring a change of status initiated by by the the member. member. Those Those individuals individuals who have have become become members membersof of ILSG ILSG by by Section B will will have have an an expiration expiration date date listed listed at at 22 years years from from the the upcoming upcoming meeting. For For example, a member who expresses interest interest in in September of 1997 1997 (the (the next nextannual annual 2000, unless meeting is May, 1998) will have an expiration date of May, 2000, unless the the member member contacts contacts the Secretary-Treasurer Secretary-Treasureror or attends attends an an annual annual meeting. meeting. 0. "Member D. "Memberfor forLife" Life"status statusisisgranted grantedto to individuals individualswho who have have been been (nearly) (nearly) continuous participants of the ILSG meetings meetings for 15 years, Goldich Medal recipients, or those who have sewed served as as meeting meeting chairs. chairs. This maintained unless the This status will be further maintained individuals indicate no further further interest in the the lnstitute, Institute, or 4 mailings in different years are returned by the postal service as address unknown, or they are are deceased. deceased. E. All All members memberswill will be be mailed mailedthe the First FirstCircular Circularfor for the the Annual Annual Meeting Meetingand andthe theILSO ILSG Newsletter. The Chair of the annual meeting may opt to to send the the first first circular to Newsletter. additional individuals. All All returned returnedmail mail should shouldbe be reported reported to to the the Secretary-Treasurer. Secretary-Treasurer. F. The TheSecretary-Treasurer Secretary-Treasurercan candesignate designateany any individual individualwho who is is on on the the ILSG ILSGmembership membership list (mailing list) as of January 1, 1997 as a member for life based on participation participation in in ILSG ILSG activities. activities. corrections to the SecretaryG. Members Membersare are strongly strongly encouraged encouraged to send address corrections Secretay Treasurer to avoid unintentional unintentional lapse lapse of membership. membership. viii viii �GOLDICH GOLDICHMEDAL MEDALGUIDELINES GUIDELINES (Adopted by the the Board Board of Directors, (Adopted Directors, 1981; 1981; amended amended1999) 1999) Preamble Preamble documented by the fact that the 27th The Institute lnstitute on Lake Lake Superior Superior Geology Geology was born in 1955, as documented meeting was held in 1981. The annual meetino TheInstitute's Institute'scontinuing continuina objectives obiectivesare are to to deal dealwith with those those aspects of geiogy geology that are related geographically to to Lake Lake .&p&ior; Superior; to to encourage encourage the the discussion discussion of of ispects subjects and geologists from and sponsoring sponsoring field trips that will bring bring together geologists from academia, academia, government government surveys, and industry; maintain an informal industty; and to maintain informal but highly effective mode of operation. the course of its existence, the membership membership of the Institute During the lnstitute (that (that is, those geologists who indicate an interest in the the objectives of the ILSG ILSO by attending) attending) has has become aware of the fact fact that certain of their colleagues meritorious contributions to the colleagues have have made made particularly parlicularly noteworthy and meritorious understanding of Lake understanding Lake Superior Superior geology geology and and mineral mineraldeposits. deposits. The first award award was made made by by ILSO ILSG to Sam Goldich Goldich in in 1979 1979 for his his many many contributions contributions to to the the geology of the region region extending over about 50 years. Subsequent Subsequentmedallists medallistsand andthis thisyear's year'srecipient recipientare are table below. listed in the table below. Award Guidelines 1) The Themedal medalshall shallbe beawarded awardedannually annuallyby bythe the ILSG ILSGBoard Boardof of Directors Directorsto toaageologist geologistwhose whosename name is associated with a substantial interest in, and contribution contribution to, to, the the geology geologyof of the the Lake LakeSuperior Superior region. region. 2) The TheBoard Boardof of Directors Directorsshall shall appoint appoint the the Goldich Goldich Medal Medal Committee. The Theinitial initialappointment appointmentwill will of three three members, one one to to serve serve for for three three years, years, one onefor fortwo twoyears, years,and andone onefor forone oneyear. year. The The be of member with the the briefest incumbency incumbency shall shall be be chair chair of ofthe the Nominating Nominating Committee. Committee. After the first the Board of of Directors shall appoint at each spring meeting meeting one new member year, the member who will serve for three years. In the chair. chair. The Committee In his/her hisher third year this member shall be the Committee membership the main main fields fields of of interest interest and and geographic geographic distribution distributionof of ILSG ILSG membership. membership. The outshould reflect the going, senior senior member of the Board of Directors going, Directors shall act as liaison liaison between between the the Board Board and and the Committee for aa period Committee period of of one one year. year. 3) By Bythe the end end of of November, November, the Goldich Goldich Medal Medal Committee Committee shall make make its its recommendation recommendation to the of the Board of of Directors, who will then inform the Board of the nominee. Chair of 4) The The Board Boardof of Directors Directors normally normallywill will accept the nominee nominee of the Committee, Committee, inform inform the medallist, and have one medal engraved appropriately appropriately for presentation medal engraved presentation at the next next meeting meetingof of the the Institute. lnstitute. 5) recommended that the Institute lnstitute set set aside aside annually annually from from whatever whatever sources, sources, such such funds funds as as will will 5) It is recommended support the continuing continuing costs be required to supporl costs of of this this award. award. Nominating Procedures Nominatina Procedures shall take 1) The The deadline deadline for nominations is November 1. The The Goldich Goldich Medal Medal Committee shail at any anytime. nominations at time. Committee Committeemembers membersmay maythemselves themselves nominate nominate candidates; however, for or supporl support individual Board members may not solicit for individual nominees. nominees. 2) Nominations Nominationsmust mustbe bein inwriting writingand andsupported supportedby by appropriate appropriatedocumentation documentationsuch such as as letters letters of recommendation, publications, curriculum curriculum vita's, and recommendation. lists of ~ublications. and evidence evidenceof of contributions contributionsto to Lake Lake Superior geologiand geology and to the Institute. superior lnstitute. 3) Nominations Nominationsare are not not restricted restricted to Institute lnstitute attendees, but but are are open open to anyone who has worked on contributed to to the the understanding of Lake Superior geology. and contributed ix �Selection Guidelines Guidelines Selection 1) Nominees Nomineesare aretotobe beevaluated evaluatedon onthe thebasis basisofoftheir theircontributions contributionsto toLake LakeSuperior Superiorgeology geology 1) (sensulato) lato)including: including: (sensu a) importance importanceofofrelevant relevantpublications; publications; a) b)promotion promotionofofdiscovery discoveryand andutilization utilizationofofnatural naturalresources; resources; b) c) c) contributions contributionstotounderstanding understandingofofthe thenatural naturalhistory historyand andenvironment environmentof ofthe theregion; region; d') generation generat'onofofnew newideas ideasand andconcepts; concepts;and and d) e) and e) contributions contributionstotothe thetraining tra~ning andeducation educationof ofgeoscientists geoscientistsand andthe thepublic. public, 2) 2) Nominees Nomineesare aretotobe beevaluated evaluatedon ontheir theircontributions contributionsto tothe theInstitute lnstituteas asdemonstrated demonstratedby by attendance attendanceat at Institute lnstitutemeetings, meetings,presentation presentationof oftalks talksand andposters, posters,and andservice sewiceon onInstitute lnstituteboards, boards, committees, committees,and andfield fieldtrips. trips. 3) 3) The Therelative relativeweights weightsgiven giventotoeach eachofofthe theforegoing foregoingcriteria criteriamust mustremain remainflexible flexibleand andatatthe the discretionof of the the Committee Committeemembers. members. discretion 4) There Thereare areseveral severalpoints pointstotobe beconsidered consideredby bythe theGoldich GoldichMedal MedalCommittee: Committee: 4) a) a)' An Anattempt attemptshould shouldbe bemade madetotomaintain maintainaabalance balanceofofmedal medalrecipients recipientsfrom fromeach eachofofthe the three threeestates—industry, eslales-inoustry, academia, academia,and andgovernment. government. b) ItIt must mustbe benoted notedthat that industry industrygeoscientists geoscientislsare are at ataadisadvantage disadvantageininthat thatmuch muchofoftheir the.r b) work work ininnot notpublished. published. 5) 5) Lake LakeSuperior Superiorhas hastwo twosides, sides,one onethe the U.S., U.S.,and and the the other other Canada. Canada. This Thisisisundoubtedly undoubtedlyone oneof of the the Institute's Institute'sgreat greatstrengths strengthsand andshould shouldbe benurtured nurturedby byequitable equitablerecognition recognitionofofexcellence excellenceininboth both countries. countries. GOLDICH GOLDICH MEDAL MEDALCOMMITTEE COMMITTEE Serving Sewing through through the the meeting meeting year year shown shown in inparentheses parentheses Frank Frank Luther Luther(2003) (2003) University University of Wisconsin, Whitewater Ron Sage Sage(2004) (2004) Ron Ontario Ontario Geological Geological Survey Survey (retired) (retired) David DavidMeineke Meineke(2005) (2005) Meriden Meriden Engineering, Engineering, Hibbing, Hibbing, Minnesota Minnesota Steve Kissin,as asout-going out-goingsenior seniormember memberof ofInstitute lnstituteBoard Boardof ofDirectors, Directors,isisliaison liaison Steve Kissin, between Goldich Medal Committee and the Board through the 2004 meeting between meeting x �2003 2003GOLDICH GOLDICHMEDAL MEDALRECIPIENT RECIPIENT Klaus Klaus J. Schulz Schuiz U.S. Geological Geological Survey Survey Reston, Virginia Reston, Virginia GOLDICH GOLDICHMEDALISTS MEDALISTS 1979 1979 Samuel SamuelS. S. Goldich Goldich 1991 William WilliamHinze Hinze 1991 1980 1980 not notawarded awarded 1992 William WilliamF.F.Cannon Cannon 1981 1981 Carl Carl E. E. Dutton, Dutton, Jr. Jr. 1993 Donald DonaldW. W. Davis Davis 1982 1982 Ralph RalphW. W.Marsden Marsden 1994 Cedric CedricIverson Iverson 1983 I983 Burton BurtonBoyum Boyurn 1995 Gene GeneLaBerge LaBerge 1984 1984 Richard RichardW. W.Ojakangas Ojakangas 1996 I996 David DavidL.L.Southwick Southwick 1985 1985 Paul PaulK. K.Sims Sims 1997 1997 Ronald RonaldP. P.Sage Sage 1986 1986 G.B. G.B.Morey Morey 1998 Zell ZellPeterman Peterman 1998 1987 I987 Henry HenryH. H.HaIls Halls 1999 1999 Tsu-Ming Tsu-MingHan Han 1988 1988 Walter WalterS. S.White White 2000 2000 John JohnC. C.Green Green 1989 1989 Jorma JormaKalliokoski Kalliokoski 2001 2001 John John S. S. Klasner Klasner 1990 1990 Kenneth KennethC. C.Card Card 2002 2002 Ernest Ernest K. K. Lehmann Lehmann xi �CITATION KlausJ. J. Schulz Schulz Klaus 2003 Goldich Goldich Medal Medal Recipient Recipient 2003 Klaus Klaus Schulz Schulz has has had had aa long long and and productive productivecareer career spanning spanning more morethan than30 30years years as asaa geologist geologistininthe theLake LakeSuperior Superiorregion. region.He Hewas wasintroduced introducedto tothe thegeology geologythrough throughhis his educationin inthe thearea, area,he hecompleted completedgraduate graduatestudies studiesininthe theregion, region,performed performedseveral several education summers summers of of field field work work for for mining miningcompanies companies in in aa number number of different different areas, areas, and and has has conducted conductedextensive extensiveresearch researchas as aa scientist scientistwith withthe theU.S. US. Geological GeologicalSurvey. Survey.This This extensive extensiveand anddiverse diverse experience experiencehas hasmade madehim himaa real realauthority authorityon onthe thegeology geologyof ofthe the Lake Superior Superiorregion. region. Lake Klaus Klaus received receivedhis his B.S. B.S. degree degree in in geology geology from from the the University Universityof of WisconsinWisconsinOshkosh Oshkoshinin1971. 1971.He Hecompleted completedhis hisMasters Mastersdegree degreeatatthe theUniversity UniversityofofMinnesotaMinnesotaDuluth Duluthin in 1974, 1974,with with aa thesis thesis project project in in the the Vermilion Vermilion district districtof of northern northernMinnesota. Minnesota.He He received receivedhis his Ph.D. Ph.D. from from the the University Universityof of Minnesota Minnesotain in1977 1977with withaadissertation dissertationon onthe the petrology petrology of volcanic rocks rocks in the Vermilion district. Klaus spent the next two years as a National NationalResearch Research Council Council Research ResearchAssociate Associate with with NASA NASA at at the the Johnson JohnsonSpace SpaceCenter Center in in Houston, Houston, where he he studied Archean basaltic and ultramafic magma types as analogs analogs of early early planetary planetary crust. In In 1982, 1982, after three years as a faculty member member at Washington Washington University University in in St. St. Louis, Louis, Klaus Klaus resigned resigned his his teaching teachingposition positionand andjoined joinedthe theU.S. U.S.Geological Geological Survey Survey in in Reston, Reston, VA, fulfilling a long-standing long-standing dream dream of his. his. During During the next next twenty twenty years with with the the USGS USGSKlaus Klaus was was aa research research scientist scientist and and administrator administrator with with a strong strong interest interestin in the the geology geologyof of the theLake LakeSuperior Superiorregion. region. The The traits traits that that have have made made Klaus Klaus a success success were evident early early in his his career. In In his his undergraduate undergraduate days days at Oshkosh, Oshkosh, Klaus Klaus distinguished distinguished himself as as an an avid avid reader reader of the the geological geological literature. literature. As a junior in 1970, 1970, he wrote an outstanding outstanding research research paper paper discussing greenstone belts and modern modern island island arcs. He He discussing the similarities similarities between Archean greenstone worked worked several summers doing fieldwork for Bear Creek Mining Mining Company Company in in central central Wisconsin Wisconsin and and northern northern Michigan, Michigan, and for U.S. Steel Steel Corp. Corp. in in the the Vermilion Vermilion district district of of northern northern Minnesota. Minnesota. This combination of field work and and a thorough thorough knowledge knowledge of of the the literature literature has has continued continued to be a hallmark hallmark of his his professional professional career, career, and and has has led led to to aa number number of of significant significant contributions contributions to the geology geology of of the the Lake LakeSuperior Superior region. region. In William Spence In the summer of 1971, Klaus and William Spence discovered discovered the Lake Lake Ellen Ellen kimberlite working as exploration geologists in the kimberlite near Crystal Falls, Michigan, while working area. area. Klaus Klaus was very much involved involved in the recognition recognition of the rock rock as as aa kimberlite. kimberlite. This This the first first kimberlite kimberlitediscovered discovered in in the the Lake LakeSuperior Superiorregion. region. was the xii xv �His Masters thesis involved considerable mapping in the Ely greenstone greenstonebelt belt in in Minnesota, and and geochemical geochemical studies studies for his his Ph.D. Ph.D. dissertation dissertationshowed showedthat thatthe theNewton Newton Lake to komatiites. komatfltes. This was the first Lake Formation Formation was a high-magnesium high-magnesium basalt, similar to documented occurrence rocks in in the the Lake Superior region. documented occurrence of komatHtic komatiitic rocks 1980's, his field mapping and and geochemistry geochemistry of of rocks in the Pembine Pembine In the early 1980's, area of the Wisconsin Wisconsin magmatic magmatic terranes demonstrated demonstrated the presence presence of ophiolitic ophiolitic rocks. rocks. the Lake Superior region, Again, this was the first documented ophiolite in the region, and and showed that the Wisconsin magmatic magmatic terranes were, at least in part, an oceanic oceanic island island arc. arc. His His subsequent model model for the evolution of the Marquette Range Range Supergoup Supergoup on on the the continental continental margin during familiarity with the rocks during the Penokean Penokean orogeny is an extension of his familiarity rocks in in the region of the the geologic literature region combined combined with his encyclopedic knowledge of literature on the evolution of continental continental margins. margins. the GLIMPCE program, program, which which ultimately provided provided Klaus also contributed to the significant insight insight into the structure and origin of the Mid-continent Mid-continent rift, rift, and and into into its its magmatic origin origin and metallogeny. metallogeny. He has authored and co-authored more than than 120 publications, maps, maps, abstracts abstracts and field guides, including 1992, including field guides for the 1984, 1984, -1 992, and and 2003 2003 Institute Institutemeetings. meetings. Klaus' contributions have provided aa better better understanding understanding of the Archean, the the the Middle Proterozoic, and and the the Phanerozoic history of the Lake Early Proterozoic, the Lake Superior region. And he continues to be an active contributor on a global global stage, stage, taking takingthe the knowledge knowledge and experience experience that he has gained in the Lake Lake Superior region region and and applying applying itit to international international projects. projects. Therefore, itit is Schulzas as is my my distinct pleasure pleasure and honor honor to present present Klaus KlausJuergen JuergenSchulz the 2003 recipient recipient of the Goldich Medal Medal "For Outstanding Outstanding Contributions ContributionsTo To The The Lake Lake Superior Superior Region". Region". Submitted by Submitted by Gene Gene L. L. LaBerge LaBerge xui xiii �__________________________________________ __________________________ EISENBREY STUDENT TRAVEL AWARDS The The 1986 1986Board BoardofofDirectors Directorsestablished establishedthe theILSO ILSGStudent StudentTravel TravelAwards Awardstotosupport supportstudent student participation of the the Institute. Institute. The participation at the annual meeting of The name name Eisenbrey" "Eisenbrey"was was added added to to the the awardin in1998 1998totohonor honorEdward EdwardH.H.Eisenbrey award Eisenbrey(1(1926-1985) 926-1 985) and and utilize utilize substantial substantialcontributions contributions made madeto to the the 1996 1996Institute Institutemeeting meetingin in his his name. name. "Ned" "Ned"Eisenbrey Eisenbreyisiscredited creditedwith withdiscovery discoveryofof significant massive sulfide sulfide deposits deposits in in Wisconsin, significant volcanogenic volcanogenic massive Wisconsin, but his his scope scope was was much much broader—he broader-he has has been been described described as as having having unique unique talents talents as as an anore orefinder, finder,geologist, geologist,and and These awards are intended help defray some of the direct travel costs ofof teacher. These awards intended to help defray some of the direct travel costs teacher. attending Institute meetings, and include a waiver of registration fees, but exclude expenses attending Institute meetings, and include a waiver of registration fees, but exclude expenses for meals, meals, lodging, lodging, and andfield fieldtrip tripregistration. registration. The The annual annual Chair Chair ininconsultation consultation with with the the for Recipients Secretary-Treasurer determinesthe the number number of of awards be Secretary-Treasurer determines awards and andvalue. value. Recipients will be announcedat at the the annual annualbanquet. banquet. announced The The annual annual Chair, Chair, who who isisresponsible responsible for for the theselection, selection, will will consider consider the thefollowing followinggeneral general criteria: criteria: 1) 1) The Theapplicants applicantsmust musthave haveactive activeresident resident(undergraduate (undergraduateor or graduate) graduate)student studentstatus statusat at the the time time of of the the annual annual meeting meeting of of the the Institute, Institute, certified certified by by the department departmenthead. head. 2) 2) Students Studentswho whoare arethe thesenior seniorauthor authoron oneither eitheran anoral oralor orposter posterpaper paperwill willbe begiven givenfavored favored consideration. consideration. 3) ItIt is is desirable desirablefor for two two or or more morestudents studentsto to jointly jointly request requesttravel travelassistance. assistance. 3) 4) 4) InIngeneral, general,priority prioritywill willbe begiven giventotothose thoseininthe theInstitute Instituteregion regionwho whoare arefarthest farthestaway awayfrom from the the meeting meetinglocation. location. 5) 5) Each Eachtravel travelaward awardrequest requestshall shallbe bemade madeininwriting writingto to the the annual annualChair, Chair,and andshould shouldexplain explain need, need, student and author status, and other significant details. The Theform formbelow belowisisoptional. optional. Successful Successful applicants applicants will will receive receive their their awards awards during during the the meeting. meeting. - NSTITUTE ONLAKUPERIORGEOLOGY ONLAKESUPERIORGEOLOGY INSTITUTE EisenbreyStudent Student Travel Travel Award Application Application Date: Date: StudentName: Name: student Address: Address: email: Department Head-Typed Department DepartmentHead-Signature Head-Signature Educationalstatus: Status: Educational Are you author of anoforal poster paper? paper? YESNO-NO_ Are youthe thesenior senior author anororal or poster YES_ who? Who? Willany anyother other students studentsbe betraveling travelingwith withyou? you? Will statement Statementofofneed need(use (useadditional additionalpage pageififnecessary) necessary) Please Pleasereturn returnto: to: xiv �STUDENTPAPER PAPERAWARDS AWARDS STUDENT Each year, year, the the Institute Instituteselects selects the the best best of of the the student student presentations presentations and and honors honors Each presenterswith with aa monetary monetary award. Funding Fundingfor for the theaward awardisis generated generated from from registrations registrations presenters of the theannual annualmeeting. meeting.The TheStudent StudentPaper PaperCommittee Committeeisisappointed appointedby bythe theannual annualmeeting meeting of Chairin insuch suchaamanner manneras asto to represent representaabroad broadrange rangeof of professional professionaland andgeologic geologic Chair expertise. Criteria Criteriafor forbest beststudent studentpaper—last paper-last modified modifiedby bythe theBoard Boardinin2001—follow; 2001-follow: expertise. 1) The Thecontribution contributionmust mustbe bedemonstrably demonstrablythe thework workofofthe thestudent. student. 1) 2) 2) The Thestudent studentmust mustpresent presentthe thecontribution contributionin-person. in-person. 3) The TheStudent StudentPaper PaperCommittee Committeeshall shalldecide decidehow howmany many awards awards to to grant, grant, and and whether whether or or 3) not to to give give separate separate awards awards for for poster postervs. vs.oral oralpresentations. presentations. not 4) 4) InIncases casesofofmultiple multiplestudent studentauthors, authors,the theaward awardwill willbe bemade made to to the the senior senior author, author, or or the award awardwill willbe beshared sharedequally equallyby byall allauthors authorsof ofthe thecontribution. contribution. the 5) The Thetotal totalamount amountofofthe theawards awardsisisleft leftto tothe thediscretion discretionof of the themeeting meetingChair Chairand and 5) Secretary-Treasurer, but typically is in the amount of about $500 US (increase approved Secretary-Treasurer, but typically is in the amount of about $500 US (increase approved by Board, Board,10/01). 10101). by 6) 6 )The TheSecretary-Treasurer Secretary-Treasurermaintains, maintains,and andwill willsupply supplyto to the the Committee, Committee, aa form form for for the the numerical numerical ranking ranking of presentations. This Thisform form was was created createdand and modified modifiedby by Student Student Paper Paper Committees Committees over over several several years years in in an an effort to to reduce reduce the the difficulties difficultiesthat that may mayarise arise from from selection selectionby by raters raters of diverse background. The Theuse useof of the theform formisisnot notrequired, required,but butisis left to to the the discretion discretionof of the the Committee. Committee. left 7) The Thenames namesofofaward awardrecipients recipientsshall shallbe beincluded includedas as part partof of the the annual annualChair's Chair'sreport report 7) that that appears appearsininthe thenext nextvolume volumeof ofthe theInstitute. Institute. Student Student papers papers will will be be noted noted on on the the Program. Program. 2003STUDENT STUDENTPAPER PAPERAWARDS AWARDSCOMMITTEE COMMITTEE 2003 Theodore - Michigan TheodoreBornhorst Bornhorst - MichiganTechnological TechnologicalUniversity, University,Houghton, Houghton, Ml MI --- Chair Chair Kevin KevinSikkila Sikkila—-Wisconsin Wisconsin Department Department of of Transportation, Transportation,Superior, Superior,WI Wl Anne Purdue University University Fort Fort Wayne, Wayne, Fort Wayne, IN AnneArgast Argast—-Indiana IndianaUniversity University — - Purdue IN Tim St. Norbert TimFlood Flood— -St. Norbert College, College, De De Pere, Pere,WI Wl 2003 2003SESSION SESSIONCHAIRS CHAIRS Peter Geological Survey, Kenora, PeterHinz Hinz— - Ontario Geological Kenora,ON ON Eric Jerde Morehead State University, Morehead, Eric Jerde - Morehead State University, Morehead, KY KY James Miller Miller -- Minnesota Minnesota Geological GeologicalSurvey, Survey, Duluth, Duluth, MN MN James Mike MikeMudrey, Mudrey,Jr. Jr.— - Wisconsin Geological Geological and and Natural Natural History History Survey, Survey, Madison, Madison,WI Wl xv �2003BOARD BOARDOF OFDIRECTORS DIRECTORS 2003 Board Boardappointment appointmentcontinues continuesthrough throughthe theclose closeofofthe themeeting meetingyear yearshown shownininparentheses, parentheses,ororuntil untilaa successor successorisisselected selected Laurel Laurel Woodruff Woodruff Co-Chair Co-Chair2003 2003meeting meeting(2006) (2006) U.S. U.S. Geological GeologicalSurvey, Survey,St. St.Paul, Paul,MN MN Peter Hinz Hinz(2005) (2005) Peter Ontario OntarioGeological GeologicalSurvey, Survey,Kenora, Kenora,ON ON Jr. (2004) (2004) Michael C. G. Mudrey, Mudrey,Jr. Michael Wisconsin WisconsinGeological Geologicaland andNatural NaturalHistory HistorySurvey, Survey,Madison, Madison,WI Wl Stephen A. Kissin Kissin(2003) (2003) Stephen A. Lakehead LakeheadUniversity, University,Thunder ThunderBay, Bay,ON ON Hollings-Secretary-Treasurer(2006) (2006) Peter Hollings-Secretary-Treasurer Peter Lakehead LakeheadUniversity, University,Thunder ThunderBay, Bay,ON ON Mark A. A. Jirsa-Secretary-Treasurer-"emeritus" Mark Jirsa-Secretary-Treasurer-"emeritus" (in transition) transition) Minnesota MinnesotaGeological GeologicalSurvey, Survey,St. St.Paul, Paul,MN MN 2003LOCAL LOCALCOMMITTEES COMMITTEES 2003 General General Co-Chairs Co-Chairs Laurel LaurelC. G. Woodruff Woodruff—-U.S. U.S.Geological GeologicalSurvey, Survey,St. St. Paul, Paul, MN MN WilliamF.F.Cannon Cannon—-U.S. U.S. Geological GeologicalSurvey, Survey,Reston, Reston,VA VA William Program Program and Abstracts Abstracts Editors Editors Laurel U.S. LaurelG. G. Woodruff Woodruff-- -US.Geological GeologicalSurvey, Survey,St. St.Paul, Paul,MN MN Theodore J. Bornhorst — Michigan Technological University, Houghton,MI MI Theodore J. Bornhorst - Michigan Technological University,Houghton, Field Field Trip Trip Guidebook Guidebook Editor Editor WilliamF.F.Cannon Cannon— U.S. U.S.Geological GeologicalSurvey, Survey,Reston, Reston,VA VA William - Acting Acting Local LocalCommittee, Committee, Iron IronMountain Mountain John Coleman Engineering, Engineering, Iron IronMountain, Mountain, Ml MI JohnGartner Gartner—- Coleman Connie ConnieDicken Dicken— - U.S. U.S. Geological GeologicalSurvey, Survey, Reston, Reston, VA VA Sally - Oshkosh, WI Wl SallyLaBerge LaBerge— 2003 BANQUET SPEAKER Susan Susan Martin Martin Department Department of of Social Social Sciences Sciences Michigan Michigan Technological Technological University University Houghton, Houghton, Michigan Michigan The indigenous indigenous people of the Lake Superior Superior Basin: Basin: Understanding Understandingthe the links links among among environment, geology and religious belief belief xvi xvi �48TH Report Reportof of the theChair Chair of of the the 48'" Annual Annual Meeting Meeting 2002 Peter Hinz, Hinz, Co-Chair Co-ChairILSG ILSG2002 Peter The 48thAnnual AnnualInstitute Instituteon onLake LakeSuperior SuperiorGeology Geologywas was hosted hostedby by the the Ontario Ontario Geological Geological The Survey 2001. Principal Surveyon on May May9-12, 9-12,2001. Principallocal localcommittee committeemembers memberswere werePeter PeterHinz Hinzand and Richard C. C. Beard, Beard, co-chairs, co-chairs,Carmen CarmenC. C.Storey, Storey, and andKevin KevinO'Flaherty O'FlahertyProgram Programco-chairs, co-chairs, Richard CharlesE. E. Blackburn, Blackburn,Field FieldTrip TripCo-ordinator, Co-ordinator,M. M.Kathleen KathleenMcGowan-Hinz, McGowan-Hinz,Treasurer, Treasurer, Charles and Christine ChristineC. C. Blackburn, Blackburn,Secretary. Secretary.Other Other principal principalindividuals individuals are are listed listed in in the the and Proceedings ProceedingsVolume. Volume. 2001 AttendanceatatILSG ILSG2001 Attendance A A total total of of 97 97 professionals professionalsand andstudent student professionals professionalsattended attendedthe the meeting, meeting, 39 39 of of whom whom pre-registered pre-registeredby by the the April April 2, 2, 2001 2001 deadline. deadline. A A total total of of 88 students studentswere were registered, registered, 77of of whom whom requested requestedand andreceived receivedtravel travelassistance. assistance. Eisenbrey Student StudentTravel TravelAwards Awards2001 2001 Eisenbrey Seven Seven students students requested requested and and received received travel assistance assistance from the Eisenbrey Eisenbrey Student Student Travel Travel Award Award Fund Fundestablished establishedto to support support student student participation participationat at the the Annual Annual Institute. Institute. Details, website. Details, including includingcriteria criteria and andapplication applicationforms, forms, are areavailable availableat atthe theILSG ILSGwebsite. BogdanNitescu Nitescu Bogdan Claire Sturm Sturm Claire ElizabethFein Fein Elizabeth Justin Johnson Johnson Justin Becky Rogala Rogala Becky WilliamJahn Jahn William DanielaVallini Vallini Daniela University of Toronto, Toronto, Toronto, Toronto, ON ON University Oberlin OberlinCollege, College, Oberlin, Oberlin,OH OH Oberlin Oberlin College, College, Oberlin, Oberlin,OH OH Lakehead University, University, Thunder Thunder Bay, Bay, ON ON Lakehead Lakehead Lakehead University, University, Thunder Thunder Bay, Bay, ON ON University University of Minnesota Minnesota-- Duluth, Duluth, Duluth, Duluth, MN MN University University of Western Western Australia, Nedlands, Nedlands, WA WA MeetingSummary Summary Meetin The 48 Annual AnnualInstitute Instituteon onLake LakeSuperior SuperiorGeology GeologyAnnual AnnualMeeting Meetingwas washeld heldat atthe theBest Best Western Western Lakeside Lakeside Inn Inn and and Convention Convention Centre, Centre, the the same same location locationas as the the 1985 1985meeting. meeting. The The one-and-a-half one-and-a-halfdays days of of technical technicalsessions sessionswere werepreceded precededby: by:Field FieldTrip Trip11— - Tanco Rare-Element Rare-Element Pegmatite, Peamatite. Southeastern Southeastern Manitoba Manitoba led led by bv staff of the the Tantalum Tantalum Mining Mining Corporation by Quaternary Geology corporation of of Canada ~ a n a dLtd.; a~ t d .followed followed ; byField FieldTrip Trip22—-Quaternary Geologyofof Southeastern Southeastern Manitoba Manitoba led ledby by E. E. Nielsen Nielsenand and Gaywood GaywoodMatile Matile(Manitoba (ManitobaGeological Geological Survey); Survey); and and Field Field Trip 33- Structure structure and and Sedimentology Sedimentologyof the the Seine Seine Conglomerate, Conglomerate, Mine Mine Centre Centre Area, Ontario Ontario lead lead by by Dyanna Dyanna Czeck (Department (Department of Geology, Geology, Oberlin Oberlin College) College) and Philip Fralick (Department (Department of Geology, Geology, Lakehead Lakehead University) University) Due Due to the small number number of talks talks submitted, submitted, the the Technical Technical Session Session Chairs Chairs were were unable unableto to group group talks into into session session themes. The The meeting meeting began began with with an an anecdotal anecdotal history historyof of mining mininginin northwestern northwestern Ontario Ontario presented presented by by Kevin Kevin O'Flaherty, followed followed by by regional regional scale scale talks talks on on the Western Superior Province. The ~uperior~rovince. Theremainder remainderof of the the technical technicalsessions sessionsincluded includedaa broad broad range range of talks talks focusing focusing on on ground ground water, water, petrography, petrography, sedimentology, sedimentology, mineralogy mineralogyand and structural structural topics. The The final final session session ended ended at noon, noon, allowing allowing for for an an early early departure departureof of Field Field Trip 66 to Red Red Lake. Lake. Post Post meeting meeting trips trips included: included: Field Field Trip Trip 4— 4 - Industrial Industrial Minerals and and Paleozoic Separation Rapids Paleozoic Geology Geology of of Southeastern SoutheasternManitoba; Manitoba;Field FieldTrip Trip55— -Separation RapidsRareRareElement Pegmatite Geology of of the the Red Lake Camp. All Pegmatite Field, Ontario; and Field Field Trip 6— 6 - Geology field field trips ran ran smoothly smoothly considering considering the the frigid frigid conditions conditions of of early early May May in in northwestern northwestern Ontario. ILSG Secretary -Treasurer, Mark Jirsa was the lone participant Ontario. 1LSG Secretary -Treasurer, Mark Jirsa was the lone participantof of Field FieldTrip Trip66 successful in obtaining samples from Goldcorp's Red Lake Mine in Red Lake. He successful in obtaining samples from Goldcorp's Red Lake Mine in Red Lake. Hewas was able to do this by cunningly cunningly embedding embedding the samples samples in in the the back back of of his his neck. neck. Upon Upon returning to Kenora the samples were proudly displayed in a baggy kindly supplied supplied by by the staff of Red Red Lake's Lake's Margaret MargaretCochenour Cochenour Memorial MemorialHospital Hospitalemergency emergencyroom. room. - - - ~ xvii �Annual Banquet and Goldich Goldich Award At the Annual Annual Banquet Banquet Ted Ted DeMatties DeMatties presented presented the the citation citation for Ernest Ernest K. K. Lehmann, Lehmann, recipient recipient of the the Goldich Goldich Medal Medal for 2002 2002 for for his his contributions contributionsto to the the Institute Instituteand andLake Lake Superior Superior Geology. Geology. L. L. Harvey Harvey Thorliefson, Thorliefson, Geological GeologicalSurvey Survey of Canada, Canada, provided provided aa scintillating scintillating discussion discussion on The Search Search for Diamonds Diamondsin in Canada Canada for the the after after dinner dinner address. Laurel Laurel Woodruff Woodruff and and Bill Bill Cannon Cannon of of the the U.S. U.S. Geological GeologicalSurvey Survey invited invited 49th Annual in participants participants to the the 49Ih~ n n u aMeeting Meeting l in Iron Iron Mountain, Mountain, Michigan. Michigan. 2002 Best Student Paper Awards 1) Becky BeckyRogala Rogala--Lakehead LakeheadUniversity,Thunder University,ThunderBay, Bay, Ontario Ontario ($400, ($400, oral oral presentation) presentation) New in formation from from the Sibley New information Sibley Group Group OberlinCollege, College, Oberlin, Oberlin, Ohio Ohio ($50, ($50, poster; poster; Co-authors Co-authorsC.L. C.L. Sturm Sturm 2) Elizabeth ElizabethFein Fein--Oberlin Anisotropy of magnetic magnetic susceptibility susceptibility in in the the Ottertail Ottertailpiuton, and D.M. Czeck) Anisotropy pluton, Northern NorthernOntario Ontario 3) Claire ClaireSturm Sturm--Oberlin OberlinCollege, College,Ohio Ohio($50, ($50, oral; oral; Co-authors Co-authors D.M. D.M. Czeck Czeck and and E. E. Fein) Fein) Petro graphicstudy studyof of the the Offertail Ottertailpluton, pluton, Superior Superior Province, Province, Northwestern Northwestern Ontario Petrographic 2002 Eisenbrey Student Student Travel Awards Awards University of of Toronto, Toronto, Toronto, Toronto, ON ON ($250) ($250) 1) Bogdan BogdanNitescu Nitescu-- University 2) Claire ClaireSturm Sturm--Oberlin Oberlin College, College, Oberlin, Oberlin, Ohio Ohio ($200) 3) Elizabeth ElizabethFein Fein-- Oberlin OberlinCollege, College,Oberlin, Oberlin, Ohio Ohio($200) ($200) 4) Justin JustinJohnson Johnson--Lakehead LakeheadUniversity, University,Thunder Thunder Bay, Bay, ON ON($150) ($150) LakeheadUniversity, University,Thunder Thunder Bay, Bay, ON ON ($150) ($150) 5) Becky BeckyRogala Rogala--Lakehead 6) William WilliamJahn Jahn--University Universityof of Minnesota, Minnesota, Duluth, Duluth,MN MN($150) ($150) 7) Daniela DanielaVallini Vallini -- University University of Western Western Australia, Nedlands, WA ($400) 2002 Goldich Goldich Medal Recipient 2002 Recipient Lehmann Ernest K. Lehmann MTII MTU Archives Donation Donation A check for $100 Technological University Archives, as required $100 was sent to Michigan Technological required by Board agreement ($1 per participant per meeting), for maintenance of ILSG proceedings proceedings archives. Proceedings including Part 1 (Programs and Abstracts) Abstracts) and Part 2 (Field Trip Guidebook) Guidebook) are available from the Institute: Institute: Institute on Lake Superior Geology do c/oMark MarkJirsa, Jirsa, Secretary Secretary -- Treasurer Treasurer Minnesota Geological Survey Survey 2642 University 2642 University Avenue St. Paul MN 55114-1057 551 14-1057 Phone: 612.627.4539 612.627.4539 Fax: Fax: 612.627.4778 612.627.4778 jirsaool @tc.umn.edu e-mail: jirsa001 @tc.umn.edu xvhi xviii �4&h1 ANNUAL INSTITUTE LAKE SUPERIOR GEOLOGY BOARD 4dhANNUAL INSTITUTE ON LAKE BOARD OF DIRECTOR'S DIRECTOR'S MEETING MEETING Board of Directors Directors Peter Hinz (2002 (2002General GeneralChair) Chair) Michael Mudrey Mudrey (2001 (2001 Co-chair) Co-chair) Steve Kissin Kissin (2000 (2000 Co-chair) Co-chair) Laurel Woodruff: Proxy Proxy for Ted Ted Bornhorst Bornhorst (1999 (1999 Co-chair Co-chair and and liaison liaison with Goldich Goldich committee) committee) Mark Jirsa (Institute (InstituteSecretary-Treasurer) Secretary-Treasurer) Guests Guests Phil Fralick Fralick (2000 (2000 Co-chair) Co-chair) Carmen Storey storey (2003 Program program Chair) Kevin O'Flaherty (2003 (2003 Program Program Chair) Chair) Bill Cannon Cannon (proposed (proposed 2003 2003 Co-chairs) Co-chairs) Rod Johnson (Goldich Rod Johnson (Goldich Committee) Committee) Frank Luther Luther (Goldich (Goldich Committee) Committee) based on the secretaries' notes and recollection; recollection; any omissions or The following is based misstatements unintentionaL Motions by the the Board Boardof of Directors Directors are generally generally misstatements are unintentional. Motions by paraphrased—"approved" paraphrasedÑ'approve or or "accepted" "accepted implying implyingthat that aa motion motionwas was made, made, seconded, and passed unanimously. The and passed The expression expression "generally "generally agreed" carries carries less formality, be pursued. pursued. Some formality, but but indicates indicates a directive that will be Some issues issues that that were were resolved resolved after after the Board Boardmeeting, but but during during the conference conference are are included includedhere here for for closure. closure. MINUTES MINUTES 1. Accepted Acceptedreport reportof of the the Chairs Chairs for for the the 47th 47th ILSG, ILSG, Madison, Madison, Wisconsin; Wisconsin; as as printed printed in in the the Proceeding Proceeding Volume (Mudrey), and minutes of last Board meeting, meeting. May 10, 2001 (Jirsa) (Jirsa) 2. 2, Received, Received,discussed, discussed, and andaccepted accepted2001-2002 2001-2002 ILSG ILSG Financial FinancialSummary Summary (Jirsa). (Jirsa). (4gth 3. Discussed location—Iron Mountain, Discussedand andapproved approved2003 2003 ( 4 9 annual) meeting location-Iron Mountain, Michigan, and tentative co-chairs Laurel Woodruff and Bill Cannon, Cannon, USGS. As currently currently envisioned, envisioned, Ted Bornhorst Bornhorst will handle handle logistics logistics of field trips. 4. Approved ApprovedPeter PeterHinz Hinzas as on-going on-goingILSG ILSG Board Boardmember. member. 5. Discussed Discussedreplacing replacingRod RodJohnson Johnson as as the the "member "member from from industry" industry" on on Goldich Goldich Committee Committee (end (end of term term 2002) 2002) with several several candidates candidates including including Dave Dave Meineke Meineke of of Meriden Meriden Engineering, Hibbing, Minnesota. Dave later accepted the position and was welcomed, and Rod was thanked for for his service to to the the Institute, during during the the annual annual banquet. banquet. Dave's Dave's term will end after Goldich Goldich selection for the the meeting meeting of of 2005. 6. Discussed Discussedreplacement replacementof of Mark MarkJirsa Jirsa as as ILSG ILSGSecretary-Treasurer Secretary-Treasurer(end (endof of 4-year 4-year term term 2002). A new member to the Institute, Peter Hoflings, Lakehead University in Thunder Institute, Peter Hollings, Lakehead University in Thunder Bay, was installed installed as "Secretary-Treasurer in-training," pending pending a vote vote by by the the general general membership membership (as required required in in By-Laws). By-Laws). Because Because of his his newness newness to the the Institute, Institute,the the board board generally agreed that Peter Peter would serve serve 2 years of the the 4-year 4-year term term concurrently concurrentlywith with Mark Mark in a period of transition. At the end end of the the 22 years years (following (following the the 2004 2004 meeting), meeting),the the finances and records finances records of the institute, institute, and and responsibilities responsibilities of the the position positionwould would fall fall to to Peter. Peter. This was presented to to the membership after after the the Board meeting, and and was was generally accepted. accepted. 7. Other Otherbusiness: business: a) Discussed Discussed the offer by by Mike Mike Mudrey Mudrey to take take over over as as ILSG ILSG webmaster—It webmaster-lt was was generally agreed that Mike Mike could could do that, assuming assuming Ted Ted was was busy busy with with other other obligations and probably probably would not mind the relief. relief. Subsequent Subsequent discussions discussionsindicate indicate xix �that Ted Ted would like like to to continue continue in in this this endeavor, endeavor, and and has has already already paid paid in in advance advance for for 5 years of web service service to to continue. continue. ItIt remains remains in in Ted's Ted's hands. hands. b) Discussed efforts by Graham Wilson to list ILSG publications as part of his —Steve Kissin MINLIB project and website (www.turnstone.ca) (www.turnstone.ca)ÑStev Kissin volunteered volunteered to contact Graham and see if there is anything that the ILSG can and should do to assist, assist. c) Discussed Discussed the prospect prospect of extending extending aa "free "free ride" ride" to to annual annualGoldich GoldichMedal Medal recipients. It was generally generally agreed agreed that registration registration costs costs should should be be paid paid by by the the annual meeting annual meeting committee, and and that that lodging, lodging, meals, meals, and and travel travel costs costscould couldbe bepaid, paid, at the discretion discretion of the the annual annual meeting meeting chairs. chairs. d) Discussed Discussed the the ILSG ILSG Newsletter—Peter NewsletterÑPete Hinz Hinz has has offered offered to to write write itit beginning beginningin in that transition. The topic of of 2004 or so. He can coordinate with Ted Bornhorst about that whether the Newsletter Newsletter should should remain remain paper, or be be changed changedto to aa wholly wholly electronic electronic format was discussed discussed and and tabled. tabled. Most Most seemed seemed to to think think we we should shouldeventually eventuallyswitch switch with email notification. This This raised a further further to a web-based newsletter, perhaps with issue issue that members must be encouraged to notify the secretary-treasurer of changes in email email address address or or other other status. status. sampling—An issue was raised that that at at least one group of regular e) Questionable sampling-An meeting meeting participants participantshas has a tradition tradition of using using guidebooks guidebooksto to locate locateplaces placesfor formassive massive sampling programs. In this one case, samples are sold to Wards or other rock rock and mineral mineral specimen specimen dealers. The The problems problems are are 1) 1) some some of of the the localities localitiesdiscussed discussedinin guidebooks guidebooks are on private land (and therefore trespassing is likely), and 2) taking large amounts of sample from some localities limits the use of these sites sites to future generations. It was generally agreed that ILSG would print in their guidebooks a Policy Statement that warns of this "questionable "questionable sampling sampling practice." practice." Mark Mark Jirsa Jirsa will will create such language create language for inclusion inclusion in in future future guidebooks. guidebooks. abstracts—Peter f) Discussed Discussed digital submission of a b s t r a c t e e t e r Hinz warns from experience that for preparers, particularly this practice can easily turn into a nightmare for particularly if the submitters don't follow (or the host organization doesn't specify) rigid rigid guidelines guidelines for submission formats. This includes both text and illustration formats. Adjournment submission formats. both text and illustration formats. Adjournment Respectfully Respectfully submitted on January 27, 2003 to Peter Peter Hinz, Hinz, Chair Chair of of the the 48th 48thannual annual meeting, for incorporation into the Report of the Chair to appear in Proceedings Proceedings Volume 49. Mark Jirsa, Secretary-Treasurer, Institute on Lake Superior Geology xx �PROGRAM xxi �49th The following companies made made generous generous contributions contributions to to the the 49"' Annual Annual Meeting. Meeting. We thank them and and John John Gartner Gartner of the the Local Local Committee Committee for their commitment to the the Institute Institute on on Lake Lake Superior Superior Geology. Geology. For For almost almost 50 years this this organization organization has has thrived thrived through through the the sustained sustained interests interests of of individuals, and government agencies in the individuals, corporations, corporations, universities, universities, and government agencies the international geologic community. community.This Thisdedication dedicationtoto an an exchange of international geologic exchange of scientific ideas and aa passion passion for for field field trips trips (even (even in in driving driving rain rain or or snow) snow) has enabled enabled the ILSG ILSG to to fulfill fulfill one one of of its its primary primary objectives: objectives: to to promote promote better better understanding understandingof of the thegeology geology in in the the Lake Lake Superior Superiorregion. region. Kleiman Well Drilling, Kleiman Pump Pump &&Well Drilling,Inc. Inc. P.O. 704 P.O. Box 704 Iron Mountain, Michigan 49801-0704 49801-0704 Prime Prime Meridian Meridian Resources ResourcesLtd. Ltd. N7478 N7478 Niagara Niagara Lane Lane Lac, WI Wl 54935 54935 Fond du Lac, Coleman Coleman Engineering Engineering Company Company Circle Drive Drive 635 Circle Iron Mountain, MI Ml 49801 49801 xxii �WEDNESDAY MAY 7,2003 WEDNESDAY 7, 2003 8:00 TRIP 1: 1: WISCONSIN MAGMATIC TERRANE 8:00a.m. a.m.FIELD FIELD TRIP WISCONSIN MAGMATIC TERRANE(#1 (#1ININGUIDEBOOK) GUIDEBOOK) Klaus Klaus Schulz, Schulz, U.S. U.S. Geological GeologicalSurvey Survey Gene Oshkosh,emeritus emeritus GeneLaBerge, LaBerge,University UniversityofofWisconsin Wisconsin—- Oshkosh, - OF FIELD TRIP TRIP 2: 2:THE THEREPUBLIC REPUBLICMINE MINE— LIFE LIFECYCLE CYCLE OFAN ANIRON IRONORE ORE 8:00 a.m. a.m. FIELD 8:00 DEPOSIT (#4 IN INGUIDEBOOK) GUIDEBOOK) DEPOSIT FROM FROMGENESIS GENESIS TO RECLAMATION RECLAMATION(#4 William WilliamCannon, Cannon,U.S. U.S.Geological GeologicalSurvey Survey John John Meler, Meier,Cleveland ClevelandCliffs Cliffs Iron Iron Company Company 6:00 Return of 6:00 p.m. Return of Trips Trips11 and and22 4:00 p.m. p.m. - 8:00 Registration 4:00 8:00 p.m. p.m. Registration 7:00 p.m. p.m. -- 9:00 and Poster Poster Setup Setup 7:00 9:00 p.m. p.m. Ice Breaker Social and THURSDAYMAY 8,2003 THURSDAY 8, 2003 - 8:00 8:00 a.m. a.m.- 9:00 9:00a.m. a.m.REGISTRATION REGISTRATION Note: Note: Technical TechnicalSessions Sessionsare arein inWhite White spruce, Spruce,Pine PineMountain MountainResort Resort 4• *: Denotes Denotes Student Student Presentation Presentation 8:15 a.m. a.m.INTRODUCTORY INTRODUCTORYREMARKS REMARKS 8:15 Laurel G. G. Woodruff and and William WilliamF. F. Cannon, Cannon,Co-Chairs Co-Chairs TECHNICAL TECHNICALSESSION SESSIONII Session Session Chair: Chair: Jim Jim Miller, Miller, Minnesota Minnesota Geological Geological Survey, Survey,Duluth, Duluth,MN MN - 8:30 - Menominee Range 8:30 a.m. a.m. Harold HaroldBernhardt Bernhardt Menominee RangeHistorical HistoricalFoundation FoundationMuseum Museum A A brief brief history historyof of iron ironmining mining on on the theUpper UpperPeninsula's Peninsula'sMenominee MenomineeIron IronRange Range 9:00 9:00 a.m. Cannon, W.F., W.F., LaBerge, G.L. G.L. and Klasner, Klasner, J.S. J.S. Niagara Niagara suture suture zone, northern Michigan Michigan and and Wisconsin—tectonics Wisconsin-tectonics in in the the 1.85 1.85 Ma Ma arc-continent arc-continent collisional collisional boundary boundary K. 9:30 a.m. Schulz, K. A A Paleoproterozoic Paleoproterozoic suprasubduction suprasubductionzone ophiolite-island ophiolite-island arc complex complex in northeastern northeasternWisconsin Wisconsin 10:00 10:OO a.m. a.m.COFFEE COFFEEBREAK BREAKAND ANDPOSTER POSTER SESSION SESSION 10:40 a.m. am. Schneider, D.A., HoIm, D.K., D.K., O'Boyle, O'Boyle, C. C., Hamilton, M. and Jercinovic, Jercinovic, M. D.A., Holm, , Hamilton, M. Paleoproterozoic development of a gneiss dome corridor in the southern Lake Paleoproterozoic development gneiss dome corridor in the southern Lake Superior Superior region, region,USA USA 11:00 a.m. Holm, HoIm, D.K., D.K., Van Van Schmus, Schmus, W.R., W.R., MacNeill, MacNeill, L.C., L.C., Boerboom, Boerboom, T.J., 11:OO a.m. T.J., Schweitzer, D. and Schneider, D.A. Schweitzer, D. and Schneider, D.A. Late Paleoproterozoic Paleoproterozoic (1900-1600 (1900-1600 Ma) Ma) tectonic tectonic history history of of the the northern northernmidmidcontinent, continent, U.S.A.: Implications Implicationsfor for crustal crustalstabilization stabilization 11:20a.m. 11:20 a.m. Medaris, L.G., L.G., Jr. and and Dofl, Dott,R.H., R.H., Jr. Jr. The sedimentology, metamorphism, The Sioux Sioux Quartzite Quartzite revisited: sedimentology, metamorphism,geochemistry geochemistryand and the the origin origin of of pipestone pipestone 11:40p.m. 11:40 p.m. Smyk, Smyk, M.C. M.C. The planned activities activities and objectives objectives The Lake Lake Nipigon Nipigon Geoscience Geoscience Initiative Initiative— - planned xxiii �12:00 Meeting (by invitation) 12:OO p.m. p.m. Lunch LunchBreak— Break -Poster PosterSession Sessionand andILSG ILSGBoard Board Meeting (by invitation) TECHNICAL TECHNICALSESSION SESSIONIIII Session SessionChair: Chair:Mike MikeMudrey, Mudrey,Jr., Jr.,Wisconsin WisconsinGeological GeologicalSurvey, Survey,Madison, Madison,WI Wl 1:30 1:30 p.m.4 p.m. +Heggie, Heggie,C. G. and and Hollings, Hollings,P. P. Geochemistry Geochemistry.and and mineralization mineralizationof of the the Seagull Seagull Intrusion, Intrusion, Northern Northern Ontario Ontario +:. 1:50 Johnson, J.R., 1:50 p.m. p.m. + Johnson, J.R., Hollings, Hollings,P. P.and andKissin, Kissin,S.A. S.A. Mineralization Mineralizationof of the theNorton NortonLake LakeCu-Ni-POE Cu-Ni-PGEdeposit deposit 2:10 2 3 0 p.m. p.m. Miller, Miller,J.J.D., D., Jr. Jr. Petrology Petrologyand and POE PGE potential potential of of the theGreenwood GreenwoodLake Lake Intrusion, Intrusion,central centralDuluth Duluth Complex, Complex,Lake LakeCounty, County,Minnesota Minnesota 2:30 2:30 p.m. p.m. + + Joslin, Joslin,G.D., G.D., Miller, Miller,J.D., J.D., Jr. Jr. and andRowell, Rowell,W.F. W.F. Stratiform StratiformPd-Pt-Au Pd-Pt-Aumineralization mineralizationininthe theSonju SonjuLake LakeIntrusion, Intrusion,Lake LakeCounty, County, Minnesota Minnesota 2:50 2:50 p.m. p.m. + + Marma, Marma, J., Brown, Brown,P. P.and andHauch, Hauch,S. S. Magmatic Magmaticand and hydrothermal hydrothermalPOE PGE mineralization mineralizationof of the the Birch Birch Lake LakeCu-Ni-POE Cu-Ni-PGE Deposit Depositin in the theSouth South Kawishiwi, Kawishiwi,Duluth Duluth Complex, Complex, northeast northeast Minnesota Minnesota 3:10 3:10p.m. p.m.COFFEE COFFEEBREAK BREAKAND ANDPOSTER POSTERSESSION SESSION 3:30 3:30 p.m. p.m. Waggoner, Waggoner,1. T. AAhydrothermal hydrothermalcomponent componentofofIron IronFormations Formations—A -A Marquette Marquette Range Rangeperspective perspective 3:50 3:50 p.m. p.m. Tsu-Ming Tsu-MingHan Han Mode of occurrence Mode of occurrence of of trona trona and and thermonatrite thermonatriteand andtheir theirpossible possibleorigin originininthe the Negaunee Iron-Formation of the Marquette Range, Lake Superior District, Negaunee Iron-Formation of the Marquette Range, Lake Superior District,USA USA 4:10 4:10 p.m. p.m. Blaske, Blaske,A.R. A.R. Geology of Valley type mineralization mineralization at Bellevue, Geology of the the MississippiMississippi-Valley Bellevue, Michigan Michigan 4:20 4:20 p.m. p.m. +ç Larson, Larson,P. P. Mean Mean transport transport length length in in tills tills of of the the southern southernportion portion of of the theLaurentide Laurentideice icesheet: sheet: implications implications for for drift drift exploration explorationin in the the Lake LakeSuperior Superiorregion region .:. 4:50 4:50 p.m. p.m. + Marlow, Marlow, L., L., Mooers, Mooers,H. H. and andLarson, Larson,P. P. Glacial Glacial Lakes Aitkin and Upham: Upham: their their origin origin and and environmental environmentalhistory history 5:10 530 p.m. p.m. Trow, Trow,J. J. Five wan copper copper sulfides sulfides in in Ontario and Five gold gold possibilities in some Keweena Keweenawan Michigan Michigan xxiv �- - ICEBREAKER BREAKER— MIXER MIXER— CASH BAR CASH BAR 6:00 p.m. p.m. ICE 6:00 BANQUET 7:00 p.m. p.m. ANNUAL ANNUAL BANQUETAND ANDAWARD AWARDPRESENTATION PRESENTATION 7:00 Announcement of of 50th 50"'Annual Annual Meeting Meeting Location Location • Announcement Presentationto Klaus Klaus Schulz Schulz 2003 Goldich Award Presentation • Banquet Address Address • 2003 Banquet Dr. Technological University Dr. Susan Martin, Michigan Technological University The indigenous indigenous people people of of the the Lake Lake Superior Superior Basin: Understanding links among geology Understanding the links among environment, geology and and religious religious belief belief . Ing participants Meeting participantswho who are arenot notregistered registeredfor forthe thebanquet banquetare arewelcome welcometo tothe thebanquet banquetaddress address Meet FRIDAY 9, 2003 FRIDAY MAY 9,2003 TECHNICAL TECHNICAL SESSION SESSION Ill Ill Session Chair: Chair: Eric Jerde, Jerde, Morehead State State University, University,Morehead, Morehead, Kentucky Kentucky 8:20 8:20 a.m. a.m. INTRODUCTORY INTRODUCTORYREMARKS REMARKS Laurel 0. G.Woodruff Woodruffand andWilliam WilliamF. F.Cannon, Cannon,Co-chairs Co-chairs 8:30 a.m. a.m. Hollings, Hollings, P., P., Fralick, Fralick, P. P. and and Kissin, Kissin, S. S. Geochemista'y andgeodynamic geodynamic implications implications of of the the 1537 1537 Ma Redstone Redstone Point Geochemistry and Point anorogenic anorogenic granite, granite, Ontario, Ontario, Canada Canada 8:50 a.m. a.m. 8:50 Buttram, R.M. P.M. and Bjornerud, Bjornerud, M. M. Textural constraints on holith Textural constraints on the the origin of rapakivi rapakivi textures textures in in the theWolf WolfRiver RiverBat Batholith 930 a.m. 9:10 am. -:Sequence .Sandin, NA. N.A. and and Bornhorst, Bornhorst, T.J. T.J. Marquette County, Sequence of Precambrian Precambrian mafic dikes in Marquelte County, Michigan, with +: - emphasis emphasis on on the the Sugarloaf Sugarloaf Mountain Mountain and and Republic Republic areas areas 9:30 a.m. a.m. Jerde, Jerde, E.A. E.A. Gabbro/granophyre relations relations of of the the Crocodile CrocodileLake Lake Intrusion: Intrusion: a possible vent for the Hovland Hovland Lavas? Lavas? 9:50 a.m. < + Vislova, T. T. Evaluation of initial magma compositions for the Bald Eagle Intrusion Intrusion and associated rocks associated rocks 10:10 COFFEEBREAK BREAKAND AND POSTER POSTER SESSION SESSION 10:lOa.m. a.m.COFFEE 10:30a.m..:• Charkoudian, K., 10:30 a.m. +: Charkoudian, K., Tikoff, B. 6. and and Bauer, Bauer, R. R. Stike -slipseparation separation of of the the Burntside Burntside trondhjemite trondhjemite and and the Wakemup Stike-slip WakemupBay Bay tonatlite, tonatlite, Northern Northern Minnesota Minnesota -:. 10:50 a.m. a.m. + Garbowicz, Garbowicz, A. A. and Bjornerud, Bjornerud,M. M. Paleostress eastern part of the Paleostress inferences from slip vectors in the eastern the Wisconsin Wisconsin segment of the the Midcontinent rift rift 11:10 am.. :+e Potter, E.G. and Mitchell, Ri-I. 1 1 :10 a.m. E.G. and R.H. The rare and exotic mineralogy mineralogy of of the Western Subcomplex of the Deadhorse Deadhorse Creek Diatreme, Northwestern Northwestern Ontario Ontario xxv �11:30 a.m. a.m. Brown, Brown, B.A., Jr., Czechanski, M.L., 11:30 B.A., Mudrey, M.G., M.G., Jr., M.L., Reid, Reid, D.D. D.D. and and Hunt, Hunt, T.C. T.C. Highway construction, construction, mine reclamation, and land-use planning challenges in the the historic historic Upper Upper Mississippi Mississippi Valley Valley lead-zinc lead-zinc district district of of southwest southwestWisconsin Wisconsin 11:50a.m. 1 1:50 a.m. Wattrus, N. N. High-resolution High-resolution multibeam bathymetry in Lake Superior 12:10 p.m. p.m. LUNCH LUNCHBREAK— BREAK -POSTERS POSTERSREMOVED REMOVED AFTER LUNCH LUNCH TECHNICAL TECHNICAL SESSION SESSIONIV Chair: Peter Session Chair: Peter Hinz, Hinz,Ontario OntarioGeologicai GeologicalSurvey, Survey,Kenora, Kenora,ON ON 1:40 a.m. a.m. 1:40 Vatlini, D.A., N.J., Rasmussen, Rasmussen, B., B., Fletcher, I. + Vallini, D.A., McNaughton, N.J., I. and Griffin, B.J. B.J. xenotime U-Pb geochronology geochronology to to unravel unravel the the history history of of Proterozoic Using xenotime sedimentary basins: aa study study in in Western WesternAustralia Australia and and the the Lake Lake Superior Superior region 2:00 p.m. p.m. Kissin, Kissin, S.A., S.A., Vallini, Vallini, D.A., Addison, W.D. W.D. and and Brumpton, Brumpton, G.R. 2:00 D.A., Addison, G.R. zircon ages from the flint and and Rove Rove Formations, Formations, northwestern northwestern Ontario New zircon the Gun Gunflint Ontario 2:20 p.m. p.m. +Richardson, • Richardson, A., 2:20 A., Fralick, P. and Hollings, P. Sibley zircon and whole rock geochemical Sibley Basin sediment provenance using zircon methods: Possible Possiblesource source areas areas of of the the Pass Pass Lake Lake Formation Formation + 2:40 p.m. p.m. + Rogala, Rogala, B., B., Fralick, Fralick, P. P. and and Borradaile, G. 2:40 magnetostratigraphic and secular variation study of the A magnetostratigraphic the Sibley Sibley Group Group 3:00 p.m. COFFEE COFFEE BREAK BREAK p.m. Argast, Argast, A. 3:20 p.m. What does sediment chemistry tell us about rocks like those those from the the Fern Fern Creek Formation? 3:40 p.m. p.m. Bartnik, Bartnik, P. J. and 3:40 and Evans, B. W. Geology and hydro geology in in the the Kingsford, Michigan Geology hydrogeology Michigan area area 4:00 p.m. p.m. Presentation Presentation of Student Student Paper Paper Awards Ted Bornhorst, Michigan Technological University: University: Student Student Paper Paper Committee Committee SATURDAYMAY MAY10,2003 10, 2003 SATURDAY (#2 IN 8:00 a.m. a.m.FIELD FIELDTRIP TRIP 3: 3: MENOMINEE MENOMINEE IRON IRON RANGE (#2 IN GUIDEBOOK) Gene Oshkosh, emeritus Gene LaBerge, LaBerge, University University of of Wisconsin Wisconsin— - Oshkosh, John Kiasner, Klasner, University University of Western Western Illinois, Illinois, emeritus emeritus William Cannon, William Cannon, U.S. US. Geological GeologicalSurvey Survey 6:00 p.m. p.m. Return Return of Trip Trip 3 6:00 SUNDAYMAY MAY11, 2003 SUNDAY 11,2003 8:00 a.m. FIELD TRIP4:4:Iron Iron River River —Crystal Crystal Falls Falls Iron Iron District 8:00 FIELD TRIP District(#3 (#3ININGUIDEBOOK) GUIDEBOOK) LaBerge, University Oshkosh, emeritus emeritus Gene LaBerge, University of Wisconsin Wisconsin— - Oshkosh, John Klasner, Klasner, University of Western Western llllnois, Illinois, emeritus emeritus William Cannon, William Cannon, U.S. U.S. Geological Geological Survey Survey 6:00 p.m. p.m. Return Return of Trip Trip 4 6:00 EM �POSTER PRESENTATIONS PRESENTATIONS Brown, B.A., Brown, B.A., Czechanski, Czechanski, M.L., M.L., Mudrey, Mudrey,M.G., M.G., Jr. Jr. and and Reid, Reid,D.D. D.D. Wisconsin Wisconsin mineral resource resource GIS GIs and and related related digital digital map map and anddatabase databaseproducts products— -a progress report Boerboom, T. Boerboom, Bedrock geologic maps of Keweenawan Bedrock geologic Keweenawan volcanic volcanic and intrusive intrusive rocks rocks in in the the Lake wood, French French River, River, and and Knife Knife River 7.5' quadrangles, North Lakewood, 7.5'quadrangles, North Shore Shore of of Lake Lake Superior, Superior, Minnesota Minnesota Easton, P.M. Easton, P.M. Geology and mineral potential of Proterozoic intrusions in the Geology Proterozoic mafic intrusions the northern northern Grenville Province of Ontario Grenville Ontario Hart, T.R. Hart, rocks of of the Lake Nipigon and Crystal Keweena wan mafic and ultramafic intrusive rocks Keweenawan and . northwestern Ontario Lake areas, northwestern Ontario S.A., Oreskovich, Oreskovich, J.A. and Hauch, S.A., and Severson, Severson, M.J. M.J. Geology, drill holes, mineral leases, and geophysics in the andgeophysics the Duluth Duluth and and Beaver Beaver Bay Bay Compexes, northeastern Minnesota: Integration Integration of of various various GIS G I sdatabases databasesto totell tellaa story of the history of past and current current Cu-Ni-PGE Cu-Ni-PGE mineral mineral exploration exploration • Heiling, 4Heiling,C.D. C.D. Oaf vertLake Lake Volcanic VolcanicComplex, Complex,St. St Louis Louis County, Minnesota Peperites of the Gafvert •4. Hooker, Hocker,S.M., S.M., Hudak, Hudak, G.J., G.J., Odette, Odette,J.D. J.D. and andNewkirk, Newkirk,T.T. T.T. Chemistry of alteration mineral phases phases at at the Five Five Mile Mile Lake Lake volcanic-hosted prospect, NE Minnesota massive sulfide prospect, Minnesota Jirsa, M. .+:.Keatts, Keatts, M.J., Jirsa, M. and and Hoim, Holm,D. D. 40ArPArsingle-grain single-grainanalyses analyses of of Precambrian Precambrian mafic intrusions intrusions in Results of 4oArf'Ar in northern northern north-central Minnesota and north-central Minnesota • McKenzie, Â¥: McKenzie, M.A., M.A., Hoim, D.K., D.A. and and Jercinovic, Jercinovic, M. Holm, D.K., Schneider, D.A. I . of EMP monazite monazite geochronology geochronology in in E-C E-C Minnesota: Minnesota: Evidence Results of Evidence for for largelargescale geon 17 post-tectonic plutonism 17 metamorphism associated with post-tectonic •:-Metsaranta, Metsaranta,R., R.,Fralich, Fralich, P. P.and and Hollings, Hollings, P. P. investigation of Mesoarchean volcanic and and rnetasedimentary metasedimentary A geochemical investigation Mesoarchean meta metavolcanic Uchigreenstone greenstone belt belt rocks from the the Birch Birch — - Uchi . :+ Nicholson, and Cannon, Nicholson,S. S. W. W. and Cannon, W.F. W.F. and structure structure of Keweenawan Keweena wan rocks rocks of of the the St. St. Croix horst, northwestern northwestern Stratigraphy and Wisconsin Wisconsin W.,Parker, Parker,J.R., JR.,Straub, Straub, K.J. K.J. and and Tomllnson, Y. Stott, G.M., G.M., Davis, 13. D. W., Tomlinson, K. K.Y. goon Subprovince, Archean tectonostratigraphic tectonostratigraphicassemblages assemblages of of eastern eastern Waba Wabagoon Subprovince, northwestern north western Ontario Ontario xxvii �ABSTRACTS Xxviii �WHAT DOES SEDIMENT SEDIMENT CHEMISTRY TELL US ABOUT ROCKS FORMATION? LIKE THOSE FROM THE FERN CREEK FORMATION? Argast, Anne, Department of Geosciences, Geosciences, Indiana Indiana University Purdue Purdue University University Fort Wayne, Fort Wayne, IN 46805-1499, 46805-1499, Argast@ipfw.edu Argast@ipfw.edu Bulk chemical analyses are accepted and powerful tools for the the study of of metamorphic and igneous rocks. Bulk chemical chemical analyses analyses are are less accepted accepted and and less less widely widely used used for for the the study studyof of sedimentary rocks. This is at least partly the result of a widely-held view that chemical data result a widely-held view that chemical dataare are unreliable unreliable indicators indicators of sedimentary sedimentary events events due due to to the the post-burial post-burial diagenetic diageneticalteration alterationof of the the sediment. sediment. In recent years, this view has been reinforced reinforced with with studies studies indicating indicatingthe thepotential potential for for extreme diagenetic diagenetic alteration alteration of sediments, sediments, with with km-scale km-scale transport transportproposed proposedin insome somesystems systems (e.g., Wintsch and and Kvale, Kvale, 1994). 1994). Potassium is often singled-out as an especially mobile component in diagenetic systems. For example, Awwiller (1993), working in the Gulf Coast Tertiary, postulates an increase from 2.0 to 3.8 wt. percent K20 in mudrocks, due significantly to to the the transport transport of of K K as aspart part of of 1UJ0 pore volumes of fluid passing through the system in the depth range from 1500 to 4000 m below the surface. surface. Donnelly, 1987) maintains maintains KzO K20 is a generally conservative An alternate view (Argast and Donnelly, that observed variations in KzO K20 content preserve element in diagenetic settings, and that preserve compositional characteristics present at and before accumulation. Depending on your point-ofdiagenetically and metamorphically metamorphicaily altered view, the chemistry of diagenetically altered sedimentary sedimentaryrocks rocks may may (or (or not) provide provide useful information information about about provenance, provenance, weathering and other qualities of the may not) sedimentary system. system. Unconsolidated Unconsolidated sediments, sediments, delivered as turbiditic pulses of siliciclastic siliciclastic debris debris eroded eroded from from the the Himalaya Mountains, accumulated on the Bengal Pan Fan (DSDP 218) and now at subbottom subbottom depths from 12 12 to 729 m, produce chemical trends very similar to those previously noted in lithified lithified sedimentary rocks. rocks. The The similarity similarity in in chemical chemical trends trends across this broad (and metamorphosed) sedimentary range of conditions and environments suggests sedimentary chemical trends arise from fundamental conditions imposed upon the system before burial, and are are not necessarily necessarily the the result of extensive extensive diagenetic alteration alteration at depth. depth. Proterozoic, Lower Chocolay Group) is well exposed along the The Fern Creek Formation (Early Proterozoic, Sturgeon River near the dam northeast of Loretto, Michigan. These rocks have been interpreted interpreted as glaciogenic in origin, and the diamictites they contain used as evidence for glacially-derived dropstone units. Others have interpreted the Fern Creek Formation as nonglaciogenic nonglaciogenic in in origin origin lagoonal or or with the sediments accumulated in fluviatile environments grading upward into into lagoonal estuarine environments. Field and textural qualities (to be discussed discussed as as part part of of aa post-meeting post-meeting hon Range) fieldtrip in the Menominee Iron Range) support support aa glaciogenic glaciogenicorigin. origin. 1 �Bulk rock chemistry chemistry also also supports supports aa glaciogenic origin (Argast, (Argast, 2002) . The The chemical chemical data, data, including sediments from including the the absence absenceof of aacorrelation correlationbetween between K20 K2Oand andA1203, A1203show sediments from the Fern Creek Formation Formation were were deposited deposited without without extensive extensivesorting sorting or or demixing demixing of of hydraulically hydraulically coarsercoarserNa20/K20 and atomic + K)/Al K)/A1 atomic and (2Ca (2Ca ++ Na + and finer-grained fractions. Other data, including the NazO/K^O ratio ratio suggest suggest sediment sediment accumulated accumulated with with abundant abundant original original feldspar. feldspar. The The chemical chemical index index of of alteration (CIA) Gowganda diamictite diamictite (CIA) ranges ranges from from 50 50 to 61, 61, similar similar to the average average CIA of 57 in Gowganda matrices. The accessory accessory suite suite is complex complex and includes includes poorly rounded zircons. zircons. These These attributes attributes are consistent consistent with an an origin origin as as aa glacial glacial till. till. . and/or thorium were identified identified in the Several minerals enriched in rare earth elements (REE) andlor Fern Creek Creek Formation. Formation.These Theseinclude includemonazite, monazite,huttonite huttonite(monoclinic (monoclinicThS1O4) ThSi04) and a fluorfluorhydroxy-REE mineral. Th concentrations concentrations as high as 53 53 ppm were noted in one one bulk bulk analysis. analysis. Efforts to obtain obtain aa chemical chemical date date on these minerals minerals have so so far far been unsuccessful. unsuccessful. The Camey Carney Lake Lake Gneiss Gneiss is is aa chemically-compatible chemically-compatiblepossible possible source source for for the the Fern Fern Creek Creek Formation. Formation. REFERENCES REFERENCES Argast, A., A,, 2002, 2002, The The lower lower Proterozoic Proterozoic Fern Creek Creek Formation, Formation,northern northern Michigan: Michigan:mineral mineral and and J. Earth Earth Sci., Sci., v. v. 39, 39,p. p. 481481bulk geochemical geochemical evidence evidence for for its glaciogenic glaciogenic origin: origin: Can. Can. J. 492. 492. Argast, S. and Donnelly, Donnelly, T.W., T.W., 1987, 1987, The chemical chemical discrimination discrimination of of clastic clastic sedimentary sedimentary components: components: J. J. Sed. Sed. Pet., Pet., v. v. 57, 57, p. p. 813-823. 813-823. Awwiller, D.N., 1993, 1993, Illite/smectite Illitelsmectite formation formation and and potassium potassium mass mass transfer transfer during duringburial burial diagenesis of mudrocks: a study from the Texas Gulf Coast Paleocene-Eocene: Sed, diagenesis mudrocks: study Texas Coast Paleocene-Eocene:J.J. Sed. 501-512. Pet., v. 63, 63, p. 501-512. of Wintsch, R. P. and Kvale, C. M., 1994, 1994, Differential mobility of elements in burial diagenesis of siliciclastic J. Sed. Sed. Res., Res., v. v. 64A, 64A, p. p. 349-361. 349-361. siliciclasticrocks: rocks: 3. 2 �GEOLOGY AND HYDROGEOLOGY IN THE KINGSFORD, MICHIGAN MICHIGAN AREA BARTNTK, PATRICK PATRICK J., J., pbartnik0arcadis-us.com, pbartnik@)arcadis-us.com, ARCADIS ARCADIS G&M, (I&M, Inc., BARTNIK, Inc., Kingsford, Kingsford, Michigan, 49802; and bevans@arcadis-us.com, ARCADIS EVANS, BRUCE W., bevans@arcadis-u.s.com, ARCADIS G&M, G&M, Inc., Inc., Milwaukee, Milwaukee, Wisconsin, 53202 Wisconsin, 53202 Investigations have been undertaken in a portion of the City Investigations City of Kingsford, Kingsford, Michigan Michiganand and Michigan (the Breitung Township, Michigan (the study study area) area) to determine determine the the geologic geologic and and hydrogeologic of glacial sediments and and bedrock. bedrock. The hydroeeologic characteristics of The study study area area is is located located Dickinson in ~ i c k i n s oCounty County i in the south-central south-central Upper Upper Peninsula Peninsula of of Michigan. Michigan. 1EPA)t ffpT I 7 .' '. I . . / IUNGSEOkD .. ,*-. 4 C&y md L v". ] """ l—% Cit;ofKingsford 1. f.I if, P'1 \ — — Study Area * ty 1ownship Kinysford City Supply Well The ARCADIS investigations were largely completed between April 1997 1997 and January January 2001, but are continuing. During During the the investigations, investigations, over 300 300 soil borings were were completed, along with with 47 47 test test pits pits and and 9.5 9.5 miles miles of of geophysical geophysical study. study. The topography is by three three distinct distinct landform terraces (Upper, Lower, and Riverside), which characterized by range in elevation from approximately 1,120 feet above mean sea level (ft msl) to 1,045ftftmsl. msl. The Upper Terrace contains several isolated glacial kettles. approximately 1,045 elevation of of the the Menominee Menominee River River is is approximately approximately 1,037 1,037 ftft msl. msl. The The geology geology is The elevation 3 �comprised of unconsolidated glaciofluvial and glaciolacustrine deposits of clay, silt, sand, that exhibit complex horizontal horizontal and and vertical vertical spatial spatialvariability. variability. These and gravel that Michigamme Slate and the Lower sediments overlie the Middle Precambrian Michigarnme Formation. Depth to groundwater in the Precambrian metavolcanic Quinnesec Formation. unconsolidated deposits ranges from about 10 feet below land surface (bis) near the to more more than than 50 feet feet bls bis in in the theUpper UpperTerrace. Terrace. Groundwater flow Menominee River to follows irregular pathways toward the Menominee River, but generally flows from northeast to southwest. Vertical Vertical hydraulic hydraulic gradients range from +0.863 ft/ft in upland —0.012ft/ft ftlftnear nearthe theMenominee MenomineeRiver. River. Hydraulic Hydraulic conductivities conductivities range from iO3 areas to -0.012 10'~ per second (cdsec) (cm/see) to to 10" 10' cm/sec material to to 1.03 xx 1i03 centimeters per c d s e c for coarser-grain material 0'~ i05 0 "cm/sec c d s e c for for the the very very fine-grain sand and sandy silt. The Thebedrock bedrockisis ccm/sec d s e c to 3.94 x 1 considered impermeable. impermeable. Groundwater generally considered Groundwater flow velocities range from approximately 3 ft/day to 280 ft/day in coarser-grain units, units, and and from from approximately approximately0.1 0.1 fine-grain sand and sandy silt. ft/day to 3 ft/day in the very fine-grain To aid in the understanding of the complex complex geology within the study study area, area, threethreetopographic surface, dimentional modeling of the geology was undertaken using the topographic surface, and glacial glacial sediments. sediments. Thirteen geologic units identified from the bedrock surface, and borehole data were categorized in to 3 units, based on permeability and anticipated anticipated effects on groundwater flow. flow. The modeling and visualization of the geology were The Geostatistical Software Library (GSUB), (GSLIB), developed at Stanford completed using a Geostatistical FORTRAN programs, and Environmental Visualization System University, FORTRAN System (EVS) Development Corporation. Corporation. software developed by the C Tech Development 4 �GEOLOGY GEOLOGY OF OF THE THE MISSISSIPPI-VALLEY MISSISSIPPI-VALLEYTYPE TYPE MINERALIZATION MINERALIZATIONAT MICHIGAN BELLEVUE, MICHIGAN BLASKE, Allan R., Blaske Geoscience, BLASKE, Allan R., Blaske Geoscience, 8313 8313 Hartel, Hartel, Grand Grand Ledge, Ledge, Ml MI48837 48837 The Bayport Bayport Limestone is exposed exposed in in quarrying quarrying operations operations at at Bellevue, Bellevue, in in southwestern southwestern Eaton Baton County, Michigan. Mining has active around around Bellevue County, Michigan. Mining has been been active Bellevue since since the mid-1800's. Approximately 25 25 feet of the Bayport is exposed in the quarrying Approximately quarrying operations, and consists of a buff colored thin-bedded limestone. gray to buff limestone. The Bayport Bayport limestone limestone is is late Mississippian in age, age, and comprises the upper portions of the The Mississippian in the Grand Rapids Group. Grand Group. It is underlain underlain by the the Michigan Michigan Formation, Formation, also of of the the Grand Grand Rapids Rapids below the Grand Group. The The early earlyMississippian Mississippian Marshall Marshall Sandstone and Coldwater Shale lie below Rapids Group. The TheBayport Bayportisisoverlain overlainby bythe theearly earlyPennsylvanian PennsylvanianSaginaw SaginawFormation Formation(within (within the Michigan Basin), but covered only by glacial sand and gravel at the quarry site. gravel at the quarry site. Mineralogy of the deposit is simple, Mineralogy of simple, consisting consisting predominantly of pyrite, marcasite, and calcite. Pyrite is most commonly found as encrustations of cubic crystals, formed directly on limestone. limestone. Mamasite Marcasite is generally generally lighter in color than the pyrite, and often in iridescent, iridescent, platy crystal groups. Marcasite is by by far the dominant iron sulfide. sulfide. Two observed. Early Marcasite is dominant iron Two generations generations of calcite are observed. Early calcite is found as calcite as small small crystals crystals lining lining cavities cavities as as drusy drusycoatings. coatings. The second generation of calcite is found in large, large, euhedral euhedral crystals and cleavable masses. Trace Traceamounts amountsof ofsphalerite, sphalerite, barite, and fluorite are present. Fluorite Fluorite was was the the earliest earliest to to form, form, as as small small brown brown crystals crystals directly directly on the Pyrite was was formed on the limestone. limestone. Pyrite formed in association association with the the early early calcite. calcite. Marcasite and and sphalerite pyrite. Second sphalerite are later than the early calcite and pyrite. Second generation generationcalcite calcitebegan began slightly slightlyafter after the marcasite. Tiny Tiny crystals crystals of of marcasite marcasite can also also be be found found on on the the large large calcite, calcite, indicating indicating that that formation of of marcasite continued to to the end of formation marcasite continued of mineralization. mineralization. Barite Barite appears appears later than the sulfides, but before the end of the calcite formation. sulfides, calcite formation. Mineralizationisis present present predominantly predominantlyinin brecciated brecciated zones zones and and vein structures Mineralization structures within within the the The most Bayport Limestone. Limestone. The most common common type type of breccia breccia consists consists of of small, small, angular angular clasts clasts surrounded by by open-space open-spacefilling fillingofofsulfides sulfidesand andcalcite. calcite. A second type of breccia consists consists of of surrounded type of larger, rounded rounded clasts, clasts, with with the the interstitial larger, interstitial spaces spaces filled filled with aa muddy muddy limestone limestone and and pyrite. pyrite. Orientation and and size size of of the mineralized mineralized zones zones within within the the limestone limestone is is not not known, due to lack of Orientation within the the quarry quarry and and insufficient insufficienthistorical historicalmapping. mapping. Fine-grained iron sulfide is also exposure within observed as replacement structures, along apparent solution fronts within the massive massive limestone. limestone. The geochemistry of the the sulfides indicates indicates the the simplicity simplicity of of the the mineralization. mineralization. 36-element ICP geochemistry of analysis of of pyrite pyrite and and marcasite marcasite separates, separates, as as well well as a composite breccia sample, indicate very analysis concentrations of of trace trace elements. elements. Copper, lead and low concentrations and zinc zinc are are found found at at concentrations concentrations of of less less than 60 ppm; ppm; nickel nickel is is less less than than 30 30 ppm; ppm; and and cadmium cadmium and and cobalt cobalt less lessthan than 55ppm. ppm. Barium is generally less less than than 20 20 ppm. ppm. Manganese is high in the also low, generally the breccia breccia (385 (385 ppm), ppm), and and lower lower in the sulfide separates (64 to 109 ppm), while chromium is high in the sulfides (150 ppm) and low 5 �ppm). Arsenic in the breccia (31 ppm). Arsenic isis present present in in the the breccia breccia (7 (7 ppm), ppm), but but low low in in the the iron iron sulfides sulfides at at less than 5 ppm. ppm. Sulfur isotopic were analyzed analyzed on on separated samples of Sulfur isotopic compositions compositions were of pyrite, pyrite, marcasite, marcasite, and and sphalerite. Sulfur (6S) ofofthe Sulfur isotopic compositions (S^S) thesulfide sulfidephases phasesfrom fromthe theBayport BayportLimestone Limestone are for These are 14.5°/ 14S0Ioofor forthe the pyrite, pyrite, 12.8°/ 12.8°/o forthe themarcasite, marcasite, and and 19.8°/ 19.8'loofor forthe thesphalerite. sphalerite. These compositionsindicate indicatethat that the the mineralizing mineralizingfluids fluids were were basinal basinal brines brines from from within compositions within the the Unpublished data obtained from the surrounding Mississippian and Pennsylvanian Pennsylvanian formations. formations. Unpublished USGS (Westjohn, I). B., pets. pers. comm.) comm.) as part part of the the RASA RASA program program indicate indicate aa large large range range of of (Westjohn, D. 34 . 8SSSininsamples samplescollected collectedfrom fromthe theunderlying underlyingMarshal! MarshallSandstone Sandstone and and Michigan Michigan Formation, Formation, as well as the overlying Saginaw Formation Formationand andthe theJurassic Jurassic Red Red Beds. Beds. Pore well overlying Saginaw Pore water, water, whole-rock, whole-rock, sulfide, and sulfate sulfur isotope isotope compositions compositions for the the underlying underlying formations formations exhibit exhibitaverage average o/ 20°/c,, while the average 634S withinthe the overlying overlying formations formations is near 17 S^S within 17 'loo. S^S near 20°/oo Temperature Temperature of the mineralization has been determined using fluid inclusions in calcite (Panter, (Panter, K. 5., S., 2001). 2001). Calcite afforded afforded the only mineral phase with with inclusions inclusions for formicrothermometric microthermometric study. Temperatures Temperaturesof of homogenization homogenization indicate a bimodal distribution, with aa low low temperature temperature mean of approximately 5g°C 58°C, and and a high high temperature mean mean of of approximately 138OC. 138°C. The The mean mean temperature of all inclusions These temperatures are similar to those temperature of inclusions analyzed analyzed was was 107°C. 107OC. These temperatures are those compositions in authigenic minerals the Mississippian observed observed using using isotopic isotopic compositions authigenic minerals in Mississippian and and Pennsylvanian sandstones (Westjohn, (Westjohn, D. B., 1994). 1994). Fluid salinities salinities based based on on freezing freezing point point Pennsylvanian sandstones equivalent weight percent depression range from 2.6 to 9.5 equivalent percent NaCI. NaCl. The quarries quarries at at Bellevue Bellevue are are located located within within55 to to 66 miles miles to the north The north and and west west of of the the known known northwest end of the northwest the Albion-Scipio Albion-Scipio Oil Oil Field Field Trend. Trend. This oil oil field field (dolomitized (dolomitized fracture fracture and and solution cavities) structure is located within the Trenton-Black River (Middle (Middle Ordovician) Ordovician)rocks, rocks, 4,000 feet feet deeper deeper than than the the Bayport Bayport Limestone. Limestone. The structure structure is related related to to faulting faulting within within some 4,000 basement rocks. rocks. Evidence the basement Evidence of of the the structure, structure, however, however, is is present present in in the the lower lowerMississippian Mississippian Sunbury Shale Shale Formation, Formation, (approximately (approximately 3,000 3,000 feet feet higher higher than than the Middle Ordovician rocks), the Coldwater the Sunbury), and the the Coldwater Shale Shale (overlying (overlying the Sunbury), and the Marshall Marshall Sandstone Sandstone (overlying (overlying the If movements associated with this are evident Coidwater). Coldwater). If movements associated with this structure structure are evident in in the theformations formations immediately below the Bayport Limestone, it seems likely that the Bayport Bayport Formation Formation would would also also be affected by faulting associated with with the structure. Faulting Faulting associated associatedwith with the theTrend Trendisislikely likely responsible for small in the Bayport, allowing allowing for for brecciation, brecciation, subsequent subsequent fluid fluid responsible for small structures structures in the Bayport, precipitation of the mineralization. migration, and precipitation mineralization. REFERENCES REFERENCES Panter, K. 5., S., 2001. 2001. A Preliminary Preliminary Microtermometric Study of Fluid Inclusions in Calcite Calcite from data, Bowling Bowling Green Green State University, OH Bellevue, Michigan, unpublished data, of Westjohn, D. B., 1994, Michigan Basin RASA Solid-Phase Investigation, in Geohydogeology of Carboniferous Aquifers of the Michigan Basin, Great Lakes Section-SEPM, 1994 Fall Carboniferous Aquifers of the Michigan Section-SEPM, 1994 Fall Field Conference, September 23-24, 1994, 1994, Lansing, MI 6 �GEOLOGIC MAPS MAPS OF OFKEWEENAWAN KEWEENAWAN VOLCANIC AND INTRUSIVE ROCKS BEDROCK GEOLOGIC IN THE LAKEWOOD, FRENCH FRENCH RIVER, RIVER, AND KNIFE RIVER 7.5' QUADRANGLES, NORTH SHORE OF OF LAKE LAKE SUPERIOR, SUPERIOR,MINNESOTA MINNESOTA f., Minnesota Geological Survey, St. Paul, Paul, MN, MN, boerbOOI@umn.edu boerb001@umn.edu BOERBOOM, Terrence J., Geological Survey, Survey, with with partial partial funding funding by by the U.S. The Minnesota Minnesota Geological U.S. Geological Geological Survey Survey STATEMAP STATEMAP geologic mapping program, has recently published detailed bedrock geologic geologic maps maps of of three threequadrangles quadrangles located along the North North Shore of Lake Superior northeast of Duluth, Minnesota Minnesota (Fig. 1; located 1; Boerboom and others, 2002a, b). Field Field mapping mapping was was completed completedatat aa scale of 1:12,000, others, 1:12,000, and compiled compiled at a scale scale of of has shown that some some flow flow sequences sequences can can he be traced traced inland inland as as far as 10 1:24,000. This mapping 10 to 12 12 1:24,000. mapping has shown that of individual individual flows flows within within the thelarger larger flow flow units. units. Several kilometers, and has identified hundreds of Several mafic to felsic, subcordant to discordant discordant sills sills and and intrusions intrusions have have also also been been mapped. mapped. Prior to this mapping, bedrock geologic geologic maps maps for for this this area were at aa scale mapping, the only only published published bedrock scale of of (for example example Miller Miller and others, 2001). 2001), and other work was concentrated concentrated along the shoreline 1:200,000 (for shoreline of Lake Superior. Superior. Brannon Brannon(1984) (1984)sampled sampled160 160successive successivevolcanic volcanic flows, flows, starting startingabove abovethe theLester LesterRiver River geochemical study. study. Green sill and ending in Two Harbors, as part of an exhaustive geochemical Green and and others others (1977) (1977) included this this area area as part of aa more included more broad broad coastal coastal zone zone management management study. study. Schwartz and Sandberg Sandberg published a paper on the diabase sills near Duluth that included some of the (1940) published the sills sills mapped mapped during during this study. Sandberg Sandberg(1938) (1938) mapped mapped the the stratigraphy stratigraphy of the flows exposed exposed at the the shoreline shorelinefrom from Duluth Duluth to to Two Harbors, identifying some 180 180 lava lava flows. flows. Although all of these these studies studies made made some someincursions incursions inland from the shore, none of them provided systematic mapping away from the shoreline shoreline proper. Bedrock exposure in the map area varies greatly, from nearly continuous outcrop along the shoreline shoreline and many of the short short streams streams along along the the slope slope into into Lake Lake Superior, Superior, to to variably variablyabundant abundant outcrop outcropin in the the hills inland from the lakeshore. Throughout Throughout the the map map area, area, there there are are many many closely closelyspaced spacedstreams streams that that bedrock perpendicular perpendicular to to the the strike strike of of the the volcanic volcanic stratigraphy. stratigraphy. Hence, have eroded into the bedrock Hence, many of the individual flows could could be be traced traced for for a great individual flows great distance distance along along strike strike by tying tying them them together together from from one one streamcut to the next, in combination combination with with the the shoreline outcrops. outcrops. In In contrast, contrast,the the more more resistant resistantintrusive intrusive typically exposed exposedon onthe thetops topsand andslopes slopesofofhigh highhills. hills. The northeast northeast part part of of the map area is rocks are typically aeromagnetic poorly exposed and thus much of the bedrock geology in that area is constrained constrainedlargely largelyby by aeromagnetic data. data. Green (2002) has proposed a subdivision of the North Shore Volcanic Group into a series series of informal informal sequences and formations formations that that are are separated by major lithological breaks or by sequences and separated by lithological and and geochemical geochemical breaks by Within the the area of the maps shown here, these include the Larsmont basalts, Sucker River intrusions. Within intrusions. Lakewood lavas, lavas, and and the the Lakeside Lakeside lavas lavas (Fig. (Fig. 2). 2). The detailed detailed bedrock geologic geologic maps maps shown basalts, the Lakewood here subdivide these informal informal formations formationsinto into multiple multiple layers layers comprised comprisedof of lava lava flows of here subdivide these of similar similar documented. composition and texture in which multiple flow contacts have been documented. REFERENCES Boerboom, T.J., T.J., Green, J.C., J.C., and Jirsa, Jirsa, M.A., M.A., 2002a, 2002a, Bedrock Bedrock geology geology of of the French French River and Lakewood quadrangles, St. Louis County, Minnesota: Minnesota Geological Survey quadrangles, St. Louis County, Minnesota: Minnesota Geological Survey Miscellaneous Miscellaneous Map M128, scale 1:24,000. 1:24,000. Bedrock geology geology of of the Knife River quadrangle, St. Louis and Lake Counties, -2002b, 2002b, Bedrock Counties, Minnesota: Survey Miscellaneous Miscellaneous Map Map M-129, scale 1:24,000. Minnesota Geological Survey Brannon, J.C. J.C. 1984, Geochemistry Geochemistry of of successive successive lava lava flows flows of of the Keweenawan North Shore Volcanic Group: St. dissertation,312 312p. p. Group: St. Louis, Louis, Washington Washington University, University, Ph.D. dissertation, 7 �Green, Green, J.C., J.C., 2002, 2002,Volcanic Volcanicand andsedimentary sedimentary rocks rocksofofthe theKeweenawan KeweenawanSupergroup Supergroupininnortheastern northeastern Minnesota, Minnesota,Chapter Chapter55of of Miller, Miller,J.D., J.D., Jr., Jr., Green, Green,J.C., J.C.,Severson, Severson,M.J., M.J., Chandler, Chandler,V.W., V.W.,Hauck, Hauck,S.A., S.A., Peterson, T.E.,Geology Geologyand and mineral mineralpotential potentialof ofthe theDuluth DuluthComplex Complexand andrelated related Peterson,D.M., D.M., and andWahl, Wahl,T.E., rocks rocksof of northeastern northeasternMinnesota: Minnesota: Minnesota MinnesotaGeological GeologicalSurvey Survey Report Report of of Investigations Investigations 58, 58, p. p. 9494105. 105. Green, Green, J.C., J.C., Jirsa, Jirsa, MA., M.A.,and andMoss, Moss,C.M., C.M., 1977, 1977,Environmental Environmental geology of the North North Shore Shore of of Lake Lake Superior:Minnesota MinnesotaGeological GeologicalSurvey, Survey,99 99p.p. Superior: Miller, J.C.,Severson, Severson,M.J., M.J., Chandler, Chandler,V.W., V.W., and andPeterson, Peterson,D.M., D.M., 2001, 2001,Geologic Geologicmap map Miller, J.D., J.D., Jr., Jr.,Green, Green,J.C., of of the the Duluth Duluth Complex Complex and and related related rocks, northeastern Minnesota: Minnesota Minnesota Geological Geological Survey Survey MiscellaneousMap MapM-119, M-119,scale scale1:200,000. 1:200,000. Miscellaneous Sandberg, Sandberg, A.E., 1938, 1938, Section Section across across Keweenawan Keweenawan lava lava flows flowsatatDuluth, Duluth,Minnesota: Minnesota: Geological Geological Society of of America America Bulletin, Bulletin,v. v. 49, 49, p. p. 795-830. 795-830. Society Schwartz, Schwartz, G.M., G.M., and and Sandberg, Sandberg, A.E., A.E., 1940, 1940, Rock series series in in diabase diabase sills sillsatatDuluth, Duluth,Minnesota: Minnesota: Geological Society of America Bulletin, v.51, v. 51,p. p. 1135-1172. 1135-1172. Geological AmericaBulletin, 92' 47 47' map showing showing location location of of Figure 1.1. Index map Figure mapped quadrangles. quadrangles. Work Work is is currently currently in in progress on the the Two Two Harbors Harbors and and Castle Castle progress Dangerquadrangles. quadrangles. Danger Index map map showing showing the the Figure 2.2. Index relative positions positions of the the informal informal volcanic units of Green (2002). volcanic Green (2002). Intrusive rocks Volcanic rocks Boundary of informal volcanic units units volcanic 88 �MINERAL RESOURCE WISCONSIN MINERAL RESOURCEGIS CISAND AND RELATED RELATED DIGITAL DIGITALMAP MAP AND AND DATABASE PRODUCTS -A PROGRESS REPORT REPORT B:A.1,CZECHANSKI, CZECHANSKI,M.L.', Mi.', MUDREY, BROWN, B.A.', MUDREY,M.G., M.G., Jr.', ~ r . and ' ,andREID, REID,Daniel DanielD.2 D.' (1) (1) Wisconsin Geological and Natural History Survey, Univ. of Wisconsin-Extension, 3817 Survey, Wisconsin-Extension, 3817Mineral Mineral Point Road, Madison, Madison, WI 53705, 53705, babrownl@facstaff.wisc.edu, babrownl @facstaff.wisc.edu, (2) (2) Wisconsin Dept of Transportation, 3502 Transportation, 3502 Kinsman Blvd, Madison, Madison, WI WI 53704-2507 53704-2507 information on 1,302 A new Mines, Pits and Quarries (MPQ) database containing information throughout Wisconsin Wisconsin has has been been completed by the Wisconsin significant nonmetallic mining sites throughout Geological and Natural History Survey (WGNHS) in cooperation cooperation with the the U.S U.S Geological Geological Survey (USGS). Locations were digitized from county-based digital orthophotography orthophotographywherever wherever (MASIMIIILS available and by site site visits. visits. Data Datatables tableswere werelinked linkedtotoexisting existingUSGS USGSdatabases databases (MAS/MILS and MRDS) and to Wisconsin Department of of Transportation Transportation (WDOT) (WDOT) aggregate test data; this linkage of all previous digital and analog analog databases is the first updated updated inventory inventory since since1980. 1980. Future versions will be augmented with current site information, collected under the nonmetallic nonmetallic reclamation program of Wisconsin Department of Natural Natural Resources, and additional historic sets such as the Road Material Survey WGNHSIWDOT. Survey sites sites of of the the WGNHSIWDOT. orthophotography, soil, Georeferenced maps layered with digital geology, topography, orthophotography, soil, and so forth provide a valuable land-use planning resource. Concern Concernfor forsafety safetyand andconstruction construction reconstruction of U.S. Highway 151 through the historic Upper Mississippi problems in the reconstmction District, southwest Wisconsin, Wisconsin, made possible the scanning and Valley Base-Metal Mining District, georeferencihg of the the Wisconsin Wisconsin Mineral MineralDevelopment DevelopmentAtlas. Atlas. The Mineral Development Atlas georeferencing of is a detailed set of of 1,450 section-scale maps (1 (I inch equal inch equal 200 200 feet) feet) of of mine mine workings, workings, drill-hole drill-hole dating from 1900 until until mining ceased ceased in 1979. These maps were location and ancillary data dating maintained by the WONTIS and the the University University of Wisconsin-Platteville Wisconsin-Platteville and WGNHS and and were were scanned scannedby by the the WDOT. WDOT. All Wisconsin water well construction reports for 1936-1988 are now available available on CDRom. They provide an extensive data set for geologic mapping as well as environmental environmental and and water resource analysis. New WGNHS map products are being produced in digital digital form and a variety of analog maps including the 1:24,000 1:24,000 USGS geologic quadrangle maps of the lead-zinc lead-zinc district are being converted to digital as resources allow. digital as resources allow. This presentation will provide an interactive demonstration demonstration of these these data data sets sets and and GIS GIs layers, a review of available map data such as regional geophysics, and an update on the status of geologic geologic mapping mapping at at the the WGNHS. WGNHS. 9 �HIGHWAY CONSTRUCTION, CONSTRUCTION,MINE MINERECLAMATION, RECLAMATION,AND AND LAND-USE LAND-USEPLANNING PLANNING CHALLENGES CHALLENGES IN THE THE HISTORIC HISTORICUPPER UPPER MISSISSIPPI MISSISSIPPI VALLEY VALLEY LEAD-ZINC LEAD-ZINC DISTRICT OF OF SOUTHWEST SOUTHWEST WISCONSIN BROWN, B.A.1, M.L.', REID, B.A.', MUDREY, M.G., Jr.1, ~ r . ' CZECHANSKI, , M.L.', REID, Daniel Daniel D.2, D.', and HUNT, T.C.3, Wisconsin Geological Geological and T.c.~,(1) Wisconsin and Natural History Survey, Univ. of Wisconsin-Extension, Wisconsin-Extension, babrown1@facstaff.wisc.edu, 3817 Mineral Point Road, Madison, WI 53705, babrownl@facstaff.wisc.edu, mgmudrey@wisc.edu, (2) Wisconsin Dept. of of Transportation, 3502 Kinsman Blvd, Madison, WI mgmudrey@wisc.edu, 53704-2507, (3) Reclamation Program, Univ. of Wisconsin-Platteville, 712 Pioneer Tower, Platteville, Platteville, WI WI 53818 53818 The Upper Mississippi Valley Valley Lead-Zinc District of Wisconsin, Illinois, Iowa, and Minnesota produced nearly 10 10 million tons of lead-zinc ore from the 1820s 1820s until the last last mine mine closed closed in 1978. The district will probably never be mined again, but but problems problems related to mineralization and past mining activity pose significant problems for highway construction and post-mining land use. Specific hazards and engineering problems problems include include (1) Highly altered and unstable rock and construction, (2) leachate from shallow abandoned mine workings encountered during highway construction, roaster-pile waste and (3) locally degraded degraded groundwater groundwaterfrom fromlead-zinc lead-zincsulfide sulfidemines. mines. As rural residential development increases, increases, the abandoned workings, particularly poorly sealed sealed shafts, can be a hazard. Most low-sulfide low-sulfide waste rock has been recycled as aggregate, and carbonate-rich tailings overgrown with vegetation vegetation make it difficult to find any surface evidence of small, small, older mine sites sites that may cause cause problems. problems. High sulfate in groundwater samples was noted in 1978 following closure 1978 following closure and and flooding flooding in an an area where large mines had operated for more than 50 years and a drawdown cone had bad developed developed over a 20-square mile area. A well-replacement program near Shullsburg Shullsburg restored potable water supplies. Onsite reclamation consisted consisted of establishing establishing vegetation vegetation on the tailings tailings and and crushing crushing the the coarse waste rock for aggregate. Leachate from zinc roaster waste piles produced over 100 100 years Mineral Point. The roaster piles were resulted in highly acidic and metal-rich surface water near Mineral successfully reclaimed by surface grading and contouring contouring along along with neutralization and fertilization to allow vegetation to establish. This was accomplished accomplished at aa fraction fraction of of the the cost cost of of removal of the roaster waste waste piles. piles. Previously undiscovered sulfide sulfide mineralization mineralization and and associated associated rock rock alteration alterationexposed exposedduring during construction along U.S. Highway 151 near Mineral Mineral Point Point resulted in the unanticipated highway construction unanticipated structures. The need to identify identify areas areas of need for engineering redesign of major roadcuts and structures. potentially unstable slopes led to scanning of the Wisconsin Mineral Development Development Atlas, Atlas, which which has proven to be invaluable in identifying areas of mineralization, alteration, alteration, and and abandoned abandoned workings in the path of construction. These detailed maps (1 inch to 200 200 feet) feet) of of mine mine workings workings and exploration drillhole locations are now being used by county and regional planners planners and and zoning authorities to identify and incorporate potential mining related hazards hazards into into land-use land-use planning. planning. 10 �TEXTURAL CONSTRAINTS TEXTURES IN IN THE THE WOLF WOLF RIVER TEXTURAL CONSTRAINTSON ON THE THE ORIGIN ORIGIN OF RAPAKIVI TEXTURES RIVER BATHOLITH BATHOLITH R. Michele Buttram Buttrarn and and Marcia Marcia Bjornerud Bjomemd Geology Department, Department, Lawrence University, Appleton, WI 54912 The Wolf River Batholith of north-central Wisconsin, a 1.47 Ga composite anorogenic pluton, includes some of the world's finest examples of 'rapakivi' granite, granite,in in which which large large plagioclase. Although potassium feldspar crystals are mantled by plagioclase. Although rapakivi rapakivi granites graniteshave have origin of of this this distinctive distinctive texture, texture, both both in the been described for more than a century, the origin elsewhere, remains remains controversial. controversial. Some workers argue that Wolf River complex and elsewhere, point under equilibrium rapakivi mantles are coronae formed at a peritectic or eutectic point crystallization conditions. Others Others maintain that rapakivi rapaldvi textures record disequilibrium associated with magma mixing mixing and and or or sudden sudden changes changes in in pressure. pressure. While most previous investigations investigations have focused on the chemistry of rapakivi granites, this study study examined examinedthe thephysical physicalcharacter characterofofthe theWolf WolfRiver Riverrocks rocks—- specifically, specifically, the size, shape, orientation and distribution rapakivi-type feldspar crystals. Among distribution of the rapahvi-type Amongthe the most striking characteristics of these rocks is the large size of the feldspars (up to 7 cm in length). Statistical Statisticalanalyses analyses show show that there is no significant significant difference in size or aspect with and and without without the therapakivi rapakivimantle. mantle. However, the K-feldspar ratio between crystals with tend to be be rounder rounder (less euhedral) euhedral) than than non-rapakivi non-rapakivi cores of the rapakivi-type crystals tend grains, suggesting prior to to the growth of the suggesting that they experienced significant resorption prior plagioclase mantle. A Aweak weak grain grain shape shape fabric fabric and and random random juxtaposition of rapakivi rapaldvi and non-rapakivi grains must also be explained by any viable model for the origin of the grains also explained texture. Our Our data data appear appearto to be be most most consistent consistent with the magma mixing model, which is compatible with earlier geochemical studies of the Wolf River complex. compatible complex. 11 �Niagara suture suture zone, zone, northern northernMichigan Michiganand andWisconsin—tectonics Wisconsin-tectonics in Ma arc-continent arc-continent collisional boundary the 1.85 Ma W.F. G.L. LaBerge, W.F. Cannon, Cannon,(U.S. (U.S.Geological GeologicalSurvey, Survey,Reston, Reston,VA VA20192, 20192, wcannon@usgs.Rov) wcannon(Susss.sov) G.L. LaBerge, (University of of Wisconsin-Oshkosh Wisconsin- Oshkosh(retired) (retired)and andU.S. U.S.Geological GeologicalSurvey), Survey),John JohnS.S.Klasner Kiasner (Western Illinois University (retired) (retired)and and U.S. U.S.Geological Geological Survey) Survey) The Niagara suture suture zone, zone, as as used used here, here, is is aa belt belt varying varying in in width width from fromabout about66 km krnto to40 40km kmlying lying north of the Niagara fault. It separates the accreted accreted volcanic volcanic arcs arcs of the the Wisconsin Wisconsinmagmatic magmatic terranes (WMT) on the south from the autocthonous and parautochtonous continental margin metavolcanic rocks rocks of of rocks on the north. ItIt consists consists of of Paleoproterozoic Paleoproterozoic metasedimentary and metavolcanic upon which they they were the epicratonic Marquette Range Supergroup Supergroup and Archean basement rocks upon deposited. The TheArchean Archean rocks rocks constitiute constitiute the the southern southern margin of the Superior craton, which was rifted and eventually separated during extensional eventually separated during extensional phases of the Penokean orogenic orogenic cycle, and then thrust northward northward during Penokean convergence. The suture zone is marked by very high strain and widespread multiple steeply to vertically plunging folds. folds. The suture zone is one of whose hierarchy hierarchy of of component component parts parts is is shown numerous subdivisions of the Penokean orogen whose below. Michigamme subterrane subterrane Michigamme Foreland fold Foreland fold and thrust "Niagara \ Niagarasuture suture zone Park Falls panel /, I Watersmeet Watersmeet panel Beechwood Beechwood panel Iron River River panel Menominee panel cianel /1 north north PENOKEAN OROGEN\ — - Niagara Niagara fault --PENOKEAN OROGEN --ioihh — — \\ south\ Pembine-Wausau (northern ~ e m b i n e - ~ a u s terrane aterrane u (northernpart part of ofWMT) WMT) - --- Mars hfleld terrane (southern (southern part Marshfield part of ofWMT) WMT) The The map pattern shown here was derived from published detailed maps in the east east (Bayley and and others, 1966; Dutton, 1971; others, 1961) others. 1971: James and others 1968; and James and others. 1961) and from our recent work in the west, where outcrops are scarce but access to recent exploration recentwork exploration drill electromagnetic data have aided in information as well as proprietary detailed aeromagnetic and electromametic clarifying the geologic>el~tionships geologic relationships (Cannon clarifying (Cannon and and others, others,1998). 1998). Each of the five fault panels of the Niagara suture suture zone has a unique unique set set of characteristics. characteristics. Watersmeet panel- Paleoproterozoic Paleoproterozoic strata are mostly pelitic schists schists and and gneisses gneisses containing containing ferruginous near the the base. base. They ferruginous strata and locally dolomite near They were were deposited deposited on on aa basement basement of Archean gneiss. gneiss. Both basement and cover were deformed deformed into into gneiss gneiss domes. domes. High-pressure metamorphism metamorphism produced kyanite-bearing assemblages. assemblages. Park Falls panel- Generally similar to Watersmeet panel except that metamorphism metamorphismwas lower lower pressure and sillimanite-bearing sillimanite-bearing assemblages assemblagesare arepredominant. predominant. Beechwood panel- Consists of Paleoproterozoic Paleoproterozoic graywacke graywacke and shale and mafic volcanic rocks rocks in in equal parts. parts. Archean basement is not exposed. exposed. Folds are roughly equal are ENE-trending and have subhorizontal axes. Rocks are are in greenschist greenschistfacies. facies. hon Iron River River panel- Rocks are the Paint River Group, including the Badwater Greenstone. Archean basement is not exposed. Strata are multiply folded creating a complex complex fold fold interference interference map map pattern. Most Most fold fold plunge steeply. Metamorphosed Metamorphosedto to greenschist greenschist or or sub-greenschist sub-greenschist facies. 12 �_____________________________ 89' 90' 88' t 4, '4— 46'- 3i •"'$ — ., S, 1' nt titer-i' V '7 A•S 50 0 I I I 7 AL-) L nt nit n-i' rlLp,1reflorniflee A 7 7 ,a 7 50 L'/AtvAkva 7 AS 7 A•3 7 , CL, L LVA tY' tvA tvA C. 7 7 L KM - 7,,. 7 7 89' 88' Maoshowing showingthe thefive fivestructural structuralpanels panels(Park (ParkFalls, Falls.Watersmeet, Watersmeet, Beechwood. IronRiver, River. Map Beech wood, Iron and that the Niagara andMenominee) ~enominee) thatconstitute constitutethe ~ i a ~ asuture suture r a zone. zone.Faults Faultsthat thatbound boundthe thepanels panelsare are Flambeau FlambeauFlowage Flowagefault fault(FFF), (FFF),Powell Powellfault fault(PF), (PF),Elmwood Elmwoodfault fault(EF), (EF),Paint PaintRiver Riverfault faull(PRF), (PRF), Badwater Badwaterfault fault(SF), (BF),North NorthRange Rangefault fault(NRF), (NRF),and andSouth SouthRange Rangefault fault(SRF). (SRF). 9'l' Menomineejanelpanel-Rocks Rocksare areentirely entirelyof ofPaleoproterozoic Paleoproterozoic age. age.No NoArchean Archeanbasement basementisisexposed. exposed. Menominee Strain Strainwas wasextreme. extreme. Commonly Commonlyallallstructural structuralelements, elements,including includingfold foldaxes, axes,are aresubvertical. subvertical. Metamorphism Metamorphismisislower lowertotoupper uppergreenschist greenschistfacies faciesand andlargely largelypost-tectonic. post-tectonic. These Thesepanels, panels,and andthe theNiagara Niagarasuture suturezone zone that that they they constitute, constitute,differ differfrom fromthe theMichigamme Michigamme subterrane subterraneto tothe thenorth. north.There Therewas waslittle littlepenetrative penetrativedeformation deformationofofArchean Archeanbasement basementisisininthe the Michigamme Michigamme subterrane. subterrane. Paleoproterozoic Paleoproterozoicstrata stratawere weremoderately moderately to toweakly weakly deformed. deformed. Folds, Folds, for postforthe themost mostpart, part,are aresimple simpleand andgently gentlyplunging. plunging.Metamorphic Metamorphicgrade gradeisisvariable variableand andmostly mostlyposttectonic. Thus, the Niagara suture zone documents a range of tectonic styles unique to the very tectonic. Thus, the Niagara suture zone documents a range of tectonic styles unique to the very high strains strainsin inaabelt beltno nomore morethan than aa few few tens tens of of kilometers kilometerswide, wide, along alongwhich which differential differential high movement craton margin margin on on the the north north was was movement between between the the accreting accreting arcs arcs on on the the south south and and the the craton concentrated. concentrated. References References Bayley, Bayley,R.W., R.W.,Dutton, Dutton,CE., C.E.,and andLamey, Lamey,C.A., C.A.,1966, 1966,Geology Geologyofofthe theMenominee Menomineeiron-bearing iron-bearingdistrict, district, Dickinson DickinsonCounty, County,Michigan Michiganand and florence Florenceand andMarinette MarinetteCounty, County,Wisconsin: Wisconsin:U.S. U.S.Geological GeologicalSurvey Survey Professional 96p. ProfessionalPaper Paper513, 513.96~. Cannon, Cannon,WE., W.F.,LaBerge, LaBerge,G.L. G.L.Kiasner, Klasner,J.S., J.S.,and andSchulz, Schulz,K.J., K.J.,1998, 1998,Reinterpretation Reinterpretationof of the the Penokean Penokean 44th continental part of ofnorthern northern Wisconsin Wisconsin and Michigan Michigan (abs.): (abs.):Proceedings Proceedingsof of 44' Annual AnnualInstitute Institute continentalmargin margin in in part on onLake LakeSuperior SuperiorGeology, Geology,v.v.44, 44,p.p.52-53. 52-53. Dutton. Dutton,CE., C.E.,1971, 1971,Geology Geologyofofthe theFlorence Florencearea, area,Wisconsin Wisconsinand andMichigan: Michigan:U.S. US. Geological GeologicalSurvey Survey Professional 54 p.p. ProfessionalPaper Paper633, 633,54 James, James,H.L., H.L.,Clark, Clark,L.D., L.D.,Lamey. Lamey,C.A., C.A.,and andPettijohn, Pettijohn,EU., F.J.,1961, 1961,Geology GeologyofofCentral CentralDickinson DickinsonCounty, County, Michigan: Michigan:U.S. US. geological geological Survey Survey Professional ProfessionalPaper Paper 310, 310,176 176p.p. James, James, H.L., H.L., Dutton, Dutton, CE., C.E.,Pettijohn, Pettijohn,F.J., F.J., and and Weir, Weir,K.L., K.L.,1968, 1968,Geology Geologyand andore oredeposits depositsof ofthe theIron IronRiver River US. Geological Crystal Falls Falls district, district,Iron Iron County, County, Michigan: US. GeologicalSurvey SurveyProfessional Professional Paper Paper 570, 570,134 134p. p. -- Crystal 13 �Strike-slip separation Strike-slip separation of the Burntside Burntside trondhjemite and and the the Wakemup Wakemup Bay Bay tonalite, Northern Minnesota Karoun Charkoudian, Basil Tikoff Department of Geology and Geophysics, Wisconsin. Madison WI, WI, 53706 53706 Geophysics, University of Wisconsin, Robert Bauer Department of Geological GeologicalSciences, Sciences,University University of of Missouri, Missouri,Columbia, Columbia,MO, MO, 65211 65211 INTRODUCTION The INTRODUCTION TheVermilion Vermilionfault faultisisaalocal localtectonic tectonicboundary boundary in in the the southern southern Canadian Canadian Shield juxtaposing the Quetico subprovince (granites and schists) with the Wawa greenstones. Quetico subprovince greenstones. Burntside trondhjeniite The Bumtside trondhjemite and the Wakemup Bay tonalite are small, elliptical, Archean granites granites separated by 35 k km of right right lateral offset offset on on the the Vermilion Vermilionfault faultin innorthern northernMinnesota. Minnesota. The m of Vermilion fault is interpreted interpreted as initially initially active as a normal fault, juxtaposing the shallow shallow Wawa the north (figure 1, greenstone to the south with the deeper granites and migmatized schists to the 1, reactivated as a strike-slip trondhjemite from stage stage 1). 1). It was later reactivated strike-slip fault, separating the Burntside trondhjemite from the Wakemun Wakemup Bay tonalite (figure 1, stage 2). Although the Vermilion fault is the regional Although the Vermilion fault is the regional 1. . Bav, . boundary between between the the Quetico Quetico and Wawa Wawa subprovinces, the Haley fault lies to the south south of of the the Vermilion fault and contains contains Quetico schists schists that belong- [ Stage N s t a x II Queticp gnttanaschsts on the north side side of of the Vermilion vermilion fault fault (figure 1) 1). The purpose of this study is to compare pluton setting fabrics, fabrics composition, of emplacement setting, composition, and shape of the two plutons to to determine determine if they constitute constitute a piercing point on the Vermilion fault. In In addition, addition, we have 20km determined din on on the the Vermilion Vermilion fault, fault. constrained constrained ..---~ the dip I emplacement history of the Wakemup tonalite. tonalite, and the emplacement the determined determined a potential potential cause cause for the the isolated isolated fault fault block block Queticp Sge 2 granites and schists that now contains contains the the Wakemup WakemupBay Bay pluton pluton (figure (figure11, 44 Bumtslde stage 2). stage Wakenup 110 The Burntside Bumtside trondhjeimte trondhjemite is is aa small small lenticular lenticular s pluton that intruded intruded the schist schist that lies lies to to the the north north of of the the ShaawaLake Bumtside continuation of the Vermilion Burntside Lake fault, a continuation fault at its eastern end. The TheWakemup Wakemup Bay Bay pluton pluton is is aa biotite-bearing tonalite that intruded biotite-bearing intruded the schist schist that lies lies Figure Figure 11 just to to the the north north of of the the Haley Haley fault. fault. - - -çffr ~ ~ tr ~ n'" The Anisotropy of Magnetic Susceptibility (AMS) is a rapid, nonAMS ANALYSIS ANALYSIS used in in granitic granitic studies studies to to obtain obtain magnetic magnetic fabrics. fabrics. Principle destructive technique, commonly used Principle magneticfoliation foliationisisdefined definedasas k-k1 kmx-k,,,, AMS ellipsoid axes are defined as km>kht>k,nin. The magnetic thethe is defined defined as as the the orientation orientation of of km,. plane, and the magnetic lineation is Wakemup Bay Bay tonalite tonalite (500-8500pSI) (500-8500pSI) The bulk susceptibility varies widely in both the Wakemup Burntside trondhjemite (SOO-3500pSI). This range range of of susceptibility susceptibility is attributed to the and the Bumtside (500-3500pSI). This content throughout throughout these these bodies. bodies. The large variation in magnetite content The AMS foliations foliations parallel parallel the the in both both plutons. plutons. Lineations measured field foliation in Lineations in in the the Wakemup Wakemup Bay tonalite tonalite dip dip shallowly shallowly to the E and W, and lineations lineations in the Burntside Bumtside trondhjemite trondhjemite dip dip shallowly shallowlyto to the the ENE ENEand andWSW. WSW. consistently parallel Magnetic lineations consistently parallel the long long axis axis of of the the plutons. plutons. The Bumtside and Wakemup plutons were selected selected for a gravity gravity study study GRAVITY STUDY lithology (biotite (biotite schist) with with a significant and because they both contain aa single surrounding lithology gicc). In consistent density contrast (Adensity (Adensity == -0.08 -0.08 to -0.1 glcc). In addition, addition, the gravity gravity data allows allows us to model the dip of the Vermilion Vermilion fault. fault. 14 �Lacoste and Romberg Romberg gravity gravity meter meter model modelGGwas wasused usedfor forboth bothareas. areas. After A Lacoste Burntside pluton and corrections, a forward model approach was used to interpret the depth of the Bumtside Vermilion fault geometry using WinGLink, WinGLink, a geophysical geophysical interpretation interpretation software software program. program. The Burntside pluton pluton is a thick body body between between 2-3 2-3 km km in inthickness. thickness. The Vermilion fault is a steeplyBumtside dipping to to vertically vertically oriented orientedfeature. feature. Using a gravimetric gravimetric three-dimensional thee-dimensional iterative north dipping technique on on the the Wakemup Wakemup Bay Bay pluton pluton resulted resulted in in aa good goodfirst-order first-orderpicture pictureof ofthe thepluton. pluton. Most technique thin, less less than than 0.5km 0.5km thick. thick. There are two root zones of up to 4 km krn depth, of the pluton is very thin, both of which lie on the southern portion of the Wakemup pluton, furthest away away from from the the Vermilion fault. fault. INTERPRETATION We interpret the Burntside Bumtside and Wakemup plutons as as part part of of the the same same granitic complex complex prior prior to to strike-slip strike-slip faulting faulting on on the theVermilion Vermilionfault. fault. These igneous bodies are granitic composition and both both have undergone solid-state deformation. deformation. The The plutons have similar similar in composition settings. The structural settings. The Wakemup Wakemup Bay pluton intrudes an F3 fold hinge and the Burntside pluton has refolded F2 folds at its southern end. Given Given the the separation, separation, the the folding folding episodes episodes may may or or may may not correlate correlate exactly. exactly. The gravity inversion and AMS study on the Wakemup pluton provide constraints constraints on on pluton emplacement. The The pluton pluton has has an an average average thickness thickness of of 0.5 km. Because the pluton reflects the true thickness thickness of the pluton. pluton. The TheAMS AMS contains a roof of wallrock, this estimate reflects F3fold. fold. foliation and lineation parallel the the fold fold limbs limbs and and fold fold hinge, hinge, respectively, respectively, of of aa km-scale km-scale F3 Therefore we interpret the Wakemup F3 fold fold hinge. hinge. Wakemup as syntectonically intruding intruding an an F3 We use a forward gravity model to estimate the dip on the Vermilion fault, which dips 70° N N and and vertical. vertical. This interpretation requires that the section of the Vermilion fault between 70' Bumtside pluton was not active as a south-side down normal fault. south of the Burntside We propose the following tectonic tectonic model model (figure (figure 1). The Burntside Bumtside pluton and the Bay pluton pluton were were initially initially part part of of the the same same granitic granitic complex. complex. The Wakemup Bay The Vermilion Vermilion fault fault was was 1, stage 1), l), which which juxtaposed the the aznphibolite amphibolite facies facies Quetico Quetico initiated as a normal fault (figure 1, sub-province with with the the greenschist greenschist facies facies Wawa Wawa belt. belt. The The Wakemup Wakemup tonalite, tonalite, with with aa thick thick root root on on promontory in the the fault system. The its south side, acted as a promontory The Vermilion Vermilion fault fault was was then then reactivated as a strike-slip fault (figure 1, stage 2), cutting through the thinnest (NW) section of the Wakemup Bay pluton. This This created created the the fault-bounded fault-bounded block block that that contains containsthe theWakemup WakemupBay Bay pluton. Therefore, Therefore, itit isis evident evident that that the the pluton pluton shape shape has played a crucial role in controlling Vermilion fault orientation, both for the early normal faulting and later strike-slip faulting. REFERENCES REFERENCES Bauer, R.L., 1985. 1985, Norwegian Bay Quadrangle, St. Louis County, Minnesota. Minnesota MinnesotaGeological Geological Survey, Miscellaneous Map series. 1:24,000. Survey, Miscellaneous series, Map Map M-59, M-59, 1:24,000. Bauer, R.L., 1986, 1986, Multiple folding and pluton emplacement in Archean migmatites of the southern Vermilion granitic complex, complex,northeastern northeasternMinnesota. Minnesota.Can. Can.1. J.Earth EarthSci., Sci.,v.v.23, 23, p. 1753-1764. 1753-1764. R.L., and Bidwell, Bidwell, M.E., ME., 1990, Bauer, R.L., 1990,Contrasts Contrasts in the response to dextral transpression across the Queticoboundary in innortheastern northeasternMinnesota. Minnesota. Can. J. Earth Sci., v. v.27, Wawa subprovince boundary 27, p. p. 1521-1535. 1521-1535. Sims, MG., 1972, Sims, P.K., and Mudrey, M.G., 1972,Burntside Burntsidegranite granitegneiss, gneiss,Vermilion Vermiliondistrict, district,ininSims, Sims,P.K., P.K.,etetal., al.,eds., eds., Geology of Minnesota: A Centennial Centennial Volume: Volume: St. Paul, Minnesota Minnesota Geological Survey, p. 98-101. Vigneresse, emplacement by regional deformation: Tectonophyiscs, Tectonophyiscs, v. 249, p. 1995, Control of granite emplacement Vigneresse, J.L., 1995, 17 3-186. 173-186. 15 �MINERAL POTENTIAL POTENTIAL OF PROTEROZOIC PROTEROZOIC MAFIC GEOLOGY AND MINERAL MAFICINTRUSIONS INTRUSIONS IN THE THE NORTHERN PROVINCE OF ONTARIO NORTHERN GRENVILLE GRENVILLE PROVINCE ONTARIO R.M. EASTON, EASTON, Ontario OntarioGeological GeologicalSurvey, Survey,933 933 Ramsey Ramsey Lake Lake Road, Road, Sudbury, Sudbury,Ontario OntarioP3E P3E6B5, 6B5, mjke.easton@ndm.gov.on.ca imke.easton@ndm.eov.on.ca Since 1998, 1998, mafic intrusions near the Grenville Front in Ontario have been prime exploration targets targets for for Cu-Ni-PGE Cu-Ni-PGE mineralization. mineralization. To To assist assist in this this effort, effort, the the Ontario Ontario Geological Geological has conducted detailed mapping in in high high potential areas of of the Grenville Province between between Survey has and 2002. This poster summarizes 1999 and summarizes the results results of these these mapping mapping efforts. efforts. East Bull Lake East Lake intrusive intrusivesuite, suite,including includingthe theRiver RiverValley Valleyintrusion: intrusion:Country Countryrocks rocksto to East Bull Lake intrusive intrusive (EBU) (EBU)suite suiterocks rocks in in the thearea area are areinferred inferredto to be be mainly mainlyArchean Archean in in age, age, and and are grouped into 4 gneiss associations. Metamorphic grade is upper amphibolite associations. Metamorphic amphibolite facies; facies; country country rocks to the mafic mafic intrusions intrusions are are commonly commonlymigmatitic. migmatitic. The Paleoproterozoic Paleoproterozoic EBLI intrusions emplaced EBU suite consists of several mafic layered intrusions emplaced between 2490 and kin, roughly and 2468 2468 Ma Ma (James (Jameset et al. al. 2002) 2002)that that occur occurover overaadistance distanceofof—250 -250 km, centered on the present site site of Sudbury. Sudbury. The largest of these bodies in the Grenville Grenville is the River Valley intrusion, intrusion, which underlies underlies roughly roughly 100 100kin2 km2 of Dana and Crerar Crerar townships. townships. Previous Previousmaps maps correlated correlated mafic rocks west of Crerar Township with the River Valley intrusion. intrusion. This This study indicates that at least 3 separate separate intrusions are present, each emplaced emplaced into into different different country country rocks, and with different different stratigraphy stratigraphyand and mineral mineral potential. potential. EBU EBU suite suiterock rock types types range range in composition composition from anorthosite anorthosite to melanorite, troctolite and rarely peridotite; leucogabbronorite leucogabbronorite and gabbronorite dominante. The crystallization crystallization order order of primocryst phases is most commonly plagioclase (An80.62), olivine (Fo7659), orthopyroxene primocryst phases is most commonly plagioclase (An80.62), olivine (Fo76-59), orthopyroxene (En7558), titanomagnetite,and and clinopyroxene. clinopyroxene. In In Dana Dana Township, Township, the (En75.58),titanomagnetite, the River River Valley Valley intrusion intrusion locally exhibits primary mineralogy and well preserved igneous textures. textures. Phase Phase layering varies from cm- to m-scale, which is discernable discemable in outcrop, outcrop, and dm or or larger, larger, which which is is identified identified by by detailed mapping. Isomodal layering is most common; mineral and size graded graded layers are less common. Cryptic layering is well documented for the River Valley intrusion. Pearce-element Pearce-element chondrite-normalized REE diagrams illustrate illustrate that each ratio and chondrite-normalized each body formed formed from from one one or or more more cogenetic magmas (James et al. 2002). A high-Al, low-Ti tholeiite tholeiite composition composition can can explain explain the the dominant leucocratic rock compositions in in the the EBU EBU suite suite (James (James et et al. al. 2002). 2002). EBLI suite, contact-type Cu-Pd-Pt mineralization (1 Within the EBU (1 to 10 10g/t g/t Pd+Pt+Au) Pd+Pt+Au) occurs in the matrix of an inclusion and/or fragment-bearing gabbronorite to leucogabbronorite andlor fragment-bearing gabbronorite to leucogabbronoriteat at the base or side of the intrusions where the primary igneous contact contact is preserved. preserved. A A second, second, similar, zone of mineralization may occur 100-200 100-200 m above above the contact. contact. Examples Examples occur occur throughout the EBLI EBU suite, suite, however, the most consistent grades grades have been reported from the River Valley intrusion in Dana Township. Chalcopyrite and lesser lesser pyrrhotite pyrrhotite form form 1-3 1-3 % % sulfide, sulfide, either finely disseminated or as local cm-sized patches. PGE mineralization mineralization is is commonly commonly associated with suiphide. sulphide. Study of the East Bull Lake intrusion indicates indicates that that mineralization mineralization originates from the intrusion and subsequent subsequent dynamic dynamic mixing of S-saturated, S-saturated, inclusion-bearing, inclusion-bearing, second-stage second-stage (PGE enriched, i.e. 20-100 ppb PGE) magmas magmas that that entered enteredthe the magma magma chamber chamber carrying liquid sulfide droplets (James et al. 2002). Reef-style mineralization mineralization has has yet yet to to be be documented within the EBLI suite. suite. 16 �GeologicaJ history history between Sudbury Geological Sudburyand andRiver RiverValley: Valley:Archean Archeanrocks rocksininthis thisarea arearecord record aa sequence of events similar to that observed observed in the Levack Gneiss complex complex and and high-grade high-grade portions of the Quetico subprovince, subprovince, but unlike the Pontiac subprovince. The following geological history is inferred. After deposition of greywackes south of the Temagami Temagami greenstone greenstone belt, invasion by tonalitic to granodioritic plutons, probably accompanied accompanied by burial, burial, formed formed the the migmatitic migmatitic gneisses gneisses now represented represented by the Pardo Pardo and Red Cedar Cedar Lake Lake gneiss gneiss associations, associations,likely likely between 2685 and 2675 2675 Ma. This This was was followed followed by by a second second period period of of tonalitic tonaliticto togranodioritic granodioritic magmatism, deformation deformation and metamorphism metamorphism at at mid-crustal mid-crustal levels levels between between 2670 2670and and2660 2660Ma. Ma. products of of this this latter activity. activity. Subsequent felsic The Crerar gneiss association represents the products magmatism at roughly 2640 Ma was accompanied accompanied by emplacement emplacement of pegmatite pegmatiteveins. veins. A logical extension of this work is to interpret the gneiss associations as a southwarddeepening deepening section of the crust. As interpreted, interpreted, Archean metawackes metawackes exposed exposed immediately immediatelynorth north of the Grenville Front Front represent high-levels high-levels of of the the crust. crust. The The Pardo Pardo gneiss, gneiss, immediately immediately south of of the Grenville represents the middle part of aa 10-15 Grenville Front, represents 10-15 km km thick thick upper upper crustal crustallayer layerdominated dominated by supracrustal and intrusive rocks. The Red Cedar Cedar Lake gneiss and the the Street Street gneiss gneissassociation association represent the basal portion of this upper crustal layer, with with the the former former derived derived from from a sequence and the latter metasedimentary rock sequence latter from a greenstone greenstone sequence. sequence.Intrusive Intrusiverocks rocks of of the the Crerar gneiss association are part of aa 10-15 km thick thick middle middle crustal crustallayer. layer.This Thiscrustal crustalsection sectionisis 10-15 km roughly equivalent to that observed across the Wawa gneiss gneiss domain. domain. Emplacement Emplacementof ofEBLI EBLIsuite suite bodies occurs at several levels within this this crustal crustal section. section. Flett Township Townshipmafie maficIntrusions: Intrusions:Evidence Evidencefor foraamafic maficand andA-type A-typegranite granite magmatic magmatic province in the northern Grenville Province was discovered while examining mafic intrusions near Temagami that that occur in in Tomiko Tomiko domain, near near its contact with with the Grenville Front tectonic zone. rocks consist consist of of gneissic gneissic granite, granite, with with minor minor mafic mafic and quarztose gneiss and Proterozoic country rocks The Fall Fall Lake Lake intrusion intrusion consists consists of of little little metamorphosed metamorphosed gabbro and metaconglomerate. The Fanny Lake Lake intrusion intrusion consists consists of of olivinite olivinite and and troctolite. troctolite. Igneous texture is leucotroctolite. The Fanny well preserved, but metamorphic coronas occur occur around around primary olivine olivine and and clinopyroxene. clinopyroxene. indicates that that both both bodies bodies are are slightly slightly alkalic, alkalic, compositionally compositionally similar to the Geochemistry indicates diabase dike dike swarm swarm dated dated at at 1238  ± 44 Ma, Ma, and and have have affinities affinities to to within-plate within-plate basalts. Sudbury diabase intrusion yielded pristine baddeleyite, with with 3 concordant or just just slightly The Fall Lake intrusion slightly discordant 1235±22Ma. Ma.The TheFanny Fanny Lake Lake sample discordant grains grains giving givingan an average average207Pb/206Pb 2 0 7 ~ b / 2 0age 6age ~ bofof1235 yielded baddeleyite, baddeleyite, with with some some grains grains having thin thin zircon zircon overgrowths, consistent with the presence of corona textures in the body. Two concordant grains without overgrowths overgrowths gave an 1238  ± 2 Ma. Ma. average average 2207Pb/2°6Pb 0 7 ~ b / 2 fage fage i ~ of bof1238 Both intrusions are spatially associated with the A-type Mulock granite, granite, dated previously at 12444L3 Ma. Intrusions Intrusions of of similar similar age include include the the Sudbury dike dike swarm, Mercer anorthosite, 1244+4/.3Ma. the West West Bay Bay and and Powassan Powassan granitoid granitoid plutons. plutons. The The new new age age data data provides provides further evidence and the of aa bimodal bimodal magmatic province active from 1270-1235 for the presence of 1270-1235 Ma in the Laurentian margin of the Grenville Province. Province. The The tectonic setting is interpreted as an extensional extensional rift that of a continental continental arc active on on the southern margin of North America formed inboard of America between 1450-1300 Ma. Ma. This This setting setting resembles resembles that that of of the the Cenozoic Cenozoic Columbia River Basalt Group. Group. R.S., Easton, Easton, R.M., R.M., Peck, Peck. D.C. D.C. and Hrominchuk, Hrominchuk, J.L. J.L. 2002. 2002. The East Bull Lake intrusive James, R.S., intrusive suite: suite: remnants of a —2.48Ga Galarge largeigneous igneousand andmetallogenic metallogenicprovince provinceininthe theSudbury Sudburyarea areaof of the the Canadian Canadian Shield; Economic -2.48 v.97, p.1577-1606. Geology, v.97, 17 �PAIEOSmES5 INFERENCES FROM FAULT PALEOSTRESS INFERENCES FROM FAULTSLIP SLIPVECTORS VECTORSIN INTHE THEEASTERN EASTERNPART PARTOF OFTHE THE WIscoNSIN SEGMENT MIDCONTINENT Rwr WISCONSIN SEGMENTOF OF THE MIDCONTINENT Amy Garbowicz, Marcia Marcia Bjomerud, Bjomerud, 54912 Geology Department, Lawrence Lawrence University, Appleton, WI 54912 Building accurate models models for for both Building accurate both the opening opening and closing of of the the Midcontinent Midcontinent Rift Rift requires an understanding of the the evolution of regional stresses over over time. This study requires an understanding of evolution of regional stresses study paleostress indicators indicators in in the the portion portion of of the Rift exposed near focused on slickenfibers as paleostress northeasternmost Wisconsin. Wisconsin. The the southern shore of Lake Superior in northeastemmost The orientations orientations of slickenfibers were were used used to determine slickenfibers determine slip vectors vectors on outcrop-scale outcrop-scale faults faults within within riftriftrelated igneous and sedimentary rocks. Rocks sampled span the entire range of related and sedimentary rocks. Rocks sampled span the range the the Keweenawan Supergroup, from the Tyler Formation to the Freda Sandstone, with most of the sampling in the Porcupine Volcanics, the Kallander Creek Volcanics, and the Mellen Gabbro. for their their age, age, Gabbro. . The mineral composition of the slickenfibers was used as a proxy for based on the based the known known regional regional sequence sequence of of secondary secondary mineralization mineralization within within the the Rfit. Rfit. Chlorite and epidote slickenfibers slickenfibers were grouped together and considered considered older older since since these these were among were among the first first minerals minerals precipitated precipitated by by hydrothermal hydrothermal fluids following following the the main main of calcite and zeolite were considered to be magmatic interval. interval. Slickenfibers of be younger. younger. Some individual faults were observed to have multiple generations of slickenfibers slickenfibers with reactivation or or continuous continuous slip over aa protracted protracted different compositions, indicating either reactivation Data from the period of time. period time. Data the field field were were analyzed analyzed using using Fault Fault Kinematics Kinematics (by (by R. R. Allmendinger, Cornell Unviersity), a program that calculates best-fit paleostress tensors Allmendinger, Cornell Unviersity), a program that from fault slip slip information. information. The calculated tensors all indicate indicate normal normal stress stress regimes regimes (maximum principal stress subvertical), even even for the latest (maximum principal latest generations generations of of slickenfibers. slickenfibers. This contrasts with the the results This contrasts with results of studies studies on on the the Keweenaw Keweenaw Peninsula, Peninsula, which which have have documented two distinct stress documented two stress regimes. regimes. There, There, early early normal normal faulting faulting gives way to to far-field stresses associated with the Grenville reverse faulting, possibly as a response to far-field Orogeny. Orogeny. The The absence absence of of reverse-slip reverse-slip vectors vectors in in the the northeastern northeastern Wisconsin Wisconsin segment segment of of the Midcontinent Rift may reflect the misorientation of this part of of the the rift rift with withrespect respectto to those far-field stresses. stresses. . 18 �Mode of Occurrence their Possible Possible Origin in in the the Negaunee Occurrence of Trona and Thermonatrite and their Iron-Formation of the Marquette Range, Range, Lake Superior District, USA Tsu-Ming Han (Retired) (Retired) Research Research Laboratory, Cleveland-Cliffs Cleveland-CliffsInc. Inc. A white colored substance Negaunee Ironsubstance is often seen on the surface of the silicate-bearing Negaunee Formation of low metamorphic grade on the Marquette Range, Michigan. This substance is mostly of a mixture containing hydrous hydrous sodium sodium carbonates carbonates(trona (tronaand andthermonatrite) thermonatrite) It occurs as thin coatings coatings coatings along bedding (Figure 1-A), and in fractures cutting across the bedding; as coatings and colloform colloform clusters on bedding planes (Figure 1-B); and as contour patterns distributed between the fractures fractures of bedding surfaces. Furthermore, nearly pure trona was developed quickly as as dendrites dendrites and minute minute dots dots on on the the cut cut surfaces surfaces of of some somehand hand specimens specimensin instorage storage(Figure (FigureC). C). To To the writer's writer's knowledge, knowledge, these these minerals minerals have have not not been been previously previously reported reportedfrom from Precambrian Precambrian BIF BIF of of the the equivalent equivalentmetamorphic metamorphicgrade gradein in other otherdistricts. districts. 4' 4fr.! Figures Mode of of occurrence occurrence of of trona trona and and thermonatrite. thermonatrite. Figures1 1—- Mode A-As white B-As colloform bedding plane. plane A—As whitecoatings coatings along along bedding. bedding. B—As colloform clusters clusters on a bedding C- As specimenin in storage. storage. C— Asdendrites dendriteson onthe the cut cut surface surface of a hand specimen The iron-formation is composed of magnetite, siderite, siderite, ankerite, ankerite, and stilpnomelane.. stilpnomelane.. The Minnesotaite more than Na in in these these minerals minerals Minnesotaite is also locally present in noticeable quantities. quantities. K is more as is the case in nearly all of the As aa as the Precambrian iron-formations iron-formations of of low low metamorphic metamorphic grade. grade. As 19 �_____________________ general general rule, rule, stilpnomelane stilpnomelanecontains containsmore moreKK and and Na Na than than the theminnesotaite. minnesotaite. However, However, the the K20:Na20 ratio these minerals and K20:Na20 ratioinin these minerals andin..the in theiron ironformation& formationsmay may vary vary substantially.. substantially. . Based Basedon onthe theresults resultsfrom fromthe thehighly highlypurified purifiedwater waterleaching leachingtests testson onmore morethan thantwenty twentydifferent different samples, practicallyinsoluble insoluble samples, the theNa Nain inthe theiron-formation iron-formationisiswater-soluble water-solublewhereas whereasthe theKKisispractically (Figures 2A and and B). B).The TheXRD XRDand and analytical analyticaldata datashow showaagood goodcorrelation correlationbetween between the theamount amount (Figures2A of Na20, K20 and the the amounts amounts of ofNa20, K 2 0and andA1203 A1203(Figures (Figures3A 3Ato to C). C). of stilpnomelane stilpnornelaneand 1L40 0.35 0.30 0.25 0.20 0.15 0.34 p.06 0.04 0.30 y25.66x+2.1167 = 0.7495 25, 0.9 20. 15 0.7 ar. I In .BTst':T 5 0 0 0.05 0 0.10 0.15 0.1 0.2 0.3 20 0.4 0.5 0.6 0.7 0.8 0.20 - Figure Figure 22-AAand andBB Solubility Solubilityof ofK20 K 2 0and and Na20 Na20 in the the silicate-bearing silicate-bearingiron-formation iron-formationwith with high high in and low K20:Na20 K20:Na20ratios. ratios. and - 0.9 Figure33- A A to to CC Relationship Relationshipofofstilpnomelane stilpnomelane Figure to K20, K20,Na20 Na20and andA1203. A1203. to 5 . 5 0. 0. c/ 00 % #4203 Y m A1203 0.5 0.5 1.0 1.0 1.5 1.5 2.0 2.0 2.5 2.5 It It may be logically concluded concluded that most of the sodium was was derived derived from from the stilpnomelane, stilpnomelane, which which was leached leached out by meteoric water. The mixture mixture of of the the hydrous sodium sodium carbonates carbonates was then developed through evaporation under the atmospheric conditions. developed through evaporation under the atmospheric conditions. 20 I1 3.0 3.0 �and Ultramafic Ultrainafic Intrusive Intrusive Rocks of the Lake Nipigon and Keweenawan Mafic and Ontario Crystal Lake areas, northwestern Ontario 933Ramsey RamseyLake LakeRoad, Road, Sudbury, Sudbury,Ontario Ontario P3E R.,Ontario OntarioGeological GeologicalSurvey, Survey,933 Hart,Thomas ThomasR., Hart, 6B5; tom.hart@ndm.gov.on.ca tom.hart@ndm.gov.on.ca 6B5; The Keweenawan diabase sills in the the Lake Lake Nipigon and and Crystal Crystal Lake Lake areas, areas, northwest of Lake Lake Superior, Superior, consist of two two distinct distinct geochemical geochemical and and geographical geographicalgroups groups with each area also hosting a number of unique intrusions intrusions that suggest suggest different different tectonic tectonic processes. Mapping by Smith and Sutcliffe (1987) in in the the Crystal Crystal Lake Lake area area identified identified a processes. series of 6 Logan diabase sills >5 m thick that gently dip dip to to the the southwest, and intrude into the early Proterozoic Rove Formation. Formation. Northeast Northeast trending dykes of of the the Pigeon Pigeon River swarm range from olivine to quartz diabase in composition, and include dykes that crosscut the Logan sills and dykes that appear to merge with the sills. crosscut sills. The The layered layered gabbro gabbro — anorthosite anorthosite -- troctolite Crystal Lake Gabbro crosscuts and contains inclusions of Pigeon of the Logan sills can can be be subdivided into into a low Ti02 Ti02 -Th/Yb - ZrN Zr/Y River dykes. Samples of group and group (OGS (OGS2002). 2002). The high Ti02 group group and aa high high Ti02 Ti02-ThJYb -Th/Yb - ZrIY group The high Ti02 group waswas asbeing beingquartz quartz normative, comparable theLogan type sills Logan by Sutcliffe identified as normative, andand comparable to the to type by sills Sutcliffe (1991). Samples identified as Pigeon River dykes exhibit high degree of variability (1991). Samples identified as Pigeon River dykes exhibit a highadegree of variability suggestingthat thatthey they represent at least unrelated intrusions. Oneofsubset suggesting represent at least three three unrelated intrusions. One subset the of the Pigeon TiOz group of Logan sills, and and another another Pigeon River River dykes dykes is is comparable comparable to the the low low TiO2 subset is comparable to the Crystal Lake Lake Gabbro. Gabbro. Most Most of of aa third third subset subset of of dyke dykesamples samples are located along Highway 61 61 close close to to the the Pigeon Pigeon River, River, and and contain contain lower lower trace element than the the other other intrusions intrusions in the area. Gabbro samples from the abundances and ratios than Crystal Lake Gabbro intrusion display some overlap with the low Ti02 hO2 group groupof of Logan Logan Zr/Y, Th/Yb, and Th/Ta ThITa ratios. ratios. The Logan sills but also includes samples with higher ZrIY, diabase diabase sills sills are confined to the area to the south of Thunder Bay, with the Nipigon diabase diabase sills sills located locatedto to the the north. north. intrusive event in the the Lake Nipigon Nipigon area is probably The initial Keweenawan intrusive flat lying lying to to shallowly shallowly dipping dipping peridotites peridotites located located in the represented by the relatively flat Disraeli, Leckie Seagull -- Fox Leckie— - Seagull Fox Mountain, Mountain, Hele, and Kitto areas that form intrusions a Disraeli, few kilometres in diameter. The peridotites are composed of orthocumulate orthocumulate to mesocumulate mesocumulate textured wehrlite to lherzolite, containing containing 11 to 2% reddish brown mica mica and and commonly a discontinuous olivine gabbro border phase (e.g. Sutcliffe 1987; 1987; Hart et al. 2002). The Disraeli, Seagull and Hele Hele peridotites are characterized characterized by higher MgO and Zr/Y ThiTa ratios than the Nipigon diabase series of of ZrIY and Th/Yb values but lower Th/Ta diabase sills. sills. A series stratigraphically below the Nipigon sills, as exposed 0.5 to 3.0 3.0 m thick sills are located stratigraphically exposed along Highway 17 at Kama Kama Hill. Hill. These These sills have MgO, Th/Yb ThIYb and Th/Ta Itt/Ta values values intermediate intermediate between the peridotites and Nipigon sills, and subdivided subdivided into into higher higher Th/Ta Th/Ta and lower ThITa may be be possible possible with with additional sampling. These sills have Th/Ta subgroups may ThITa and LdYb La/Yb ratios comparable to the high TiO2 Th/Ta Ti02 group of Logan sills, but generally generally have lower trace element abundances. The The Kitto peridotite also has Th/Yb, Th/Ta and and Zr/Y ratios that overlap with these sills rather than the other peridotites. The olivine ZrIY these peridotites. The olivine tholelite m thick, thick, and are chilled against the peridotite peridotite tholeiite Nipigon diabase sills are up to 200 200 in work indicates indicatesthat that some some sills sills were were formed formed by bymultiple multiple pulses pulses of of intrusions. Previous work Hart et et al. al. 2002), but the chemistry of the sills over the magma (e.g. Sutcliffe, 1987; Hart 21 �entire Lake Nipigon area area displays displays little little variation. Geochemical Geochemical differences differencesbetween between the the peridotites and Nipigon diabase sills the variations observed in the sills are comparable to the Osler Ti02 values Osier Group volcanic rocks (e.g. Sutcliffe, Sutcliffe, 1991). The Nipigon sills have Ti02 values comparable but higher ThTa ThiTa and lower LaNb La/Yb Ti02 group of Logan sills but comparable to the low Ti02 ratios. The differences differences in the the geochemistry geochemistry of the diabase diabase sills sills between between the the Lake LakeNipigon Nipigon and Crystal Lake areas is similar to the differences observed observed in in the the volcanic volcanic rocks of of a number of flood basalt provinces (e.g., Mantovani et al. 1985). The regional extent of the geochemical groups within the Keweenawan Keweenawan intrusions intrusionsisisnot notknown. known. An initial examination Complex (Ripley examination of troctolites troctolites from from the Babbit deposit of the Duluth Complex (Ripley et al. ThITa, ThlYb, ThIYb, and ZrN Zr/Y ratios comparable comparable to to the Nipigon peridotites 1999) indicates Th/Ta, peridotites rather than the intrusions intrusions of the the Crystal Crystal Lake area. area. References Hart, T.R., terMeer, M., and Jolette, C. C. 2002. 2002. Precambrian Precambrian geology of Kitto, Eva, Summers, Summers, Dorothea Dorothea and andSandra Sandra Townships, 206 Townships, Beardmore Beardmore area, area, northwestern northwesternOntario; Ontario; Ontario Ontario Geological Geological Survey, Survey, Open OpenFile FileReport Report6095, 6095,206 p. P. L.S., de Sousa, M.A., MA., Civetta, Innocenti, F., 1985. Mantovani, M.S.M., Marques, L.S., Civetta, L., L.,Atalla, L., and Innocenti, 1985.Trace Trace element and strontium isotopic constraints constraints on the origin and evolution evolution of of Parana Paranacontinental continental flood flood basalts basalts of of Santa Santa strontium Catarina State (southern Brazil); Journal of of Petrology. Petrology, v.v.26, 26, p. 187-209. 187-209. Ontario Geological Survey. Survey, 2002 2002. Proterozoic Volcanic volcanic and Intrusive Data associated associated wilh with Oniano fntrusive Whole Rock Geochemical Daia the Keweenawan Kewcenawan Midcontinent Midconunent Rift, Rift.Lake Lake Superior Suoenor Area, Area. Ontario; Onlano. Ontario Onlano Geological Geological SurveyMiscellaneous Miscellaneous . Survey Release—Data 114. Release-Data 114. Ripley, EM., Sm-Nd, and Pb isotonic isotopic consl~aints constraints on mantle Rinlev. Larnbert. D.D., D.D.. and and Frick, Frick. L.R., L.R.. 1999. 1999. Re-Os, Re-0s. Sm-Nd. mantle and crustal ..r.., , E.M.. - ,Lambert, contributions to magmatic contributions magmatic sulfide sulfide mineralization in the Duluth Complex; complex; Geochimica Geochimica et et Cosmochimica Cosmochimica Acta, Acta, v. 62, p.3349-3365. p.3349-3365. v. Smith, AR. and 7987. andSutcliffe, Sutcliffe.RH. R.H. 1987.lCeweenawan Keweenawanintrusive intrusive rocks of the Thunder TTiunder Bay area; in Summary Summary of Field Smith. A.R. Work and Other 137, p. Other Activities, Activities. Ontario Ontano Geological Geological Survey Miscellaneous Paper paper-137. p.248-255. 248-255. Sutcliffe, LakeNipigon, Nioigon. Canada;Contributions Conmbuiions Sutclifie. R.H., R.H.. 1987. 1987.Petrology Pctrolow Middle Proterozoic Proterozoic diabase diabase and and picrites niintes from from Lake -.ofof Middle . - Canada; to Mineralogy p.201-211. Mineralogy and Petrology, v.96, p. 201-211. Sutcliffe, R.H., RH., 1991. 1991.Proterozoic Proterozoicgeology geologyof of the theLake Lake Superior Superior area; area; in Geology of Ontario, Ontario, Ontario Ontario Geological Geological Survey Special Volume Volume4, 4,Part Part!, 1,p.627-658. p. 627-658. 22 �DRILL HOLES, HOLES, MINERAL MJNERALLEASES, LEASES,AND ANDGEOPHYSICS GEOPHYSICSIN GEOLOGY, DRILL NTHE DULUTH AND BEAVER BAY COMPLEXES, NORTHEASTERN NORTHEASTERN MINNESOTA: MINNESOTA: INTEGRATION INTEGRATION OF GIS DATABASES DATABASES TO TELL A STORY OF THE HISTORY OF PAST AND VARIOUS GIs CURRENT CU-NT-PGE MINERAL EXPLORATION CURRENT CU-NI-PGE MINERAL EXPLORATION Steven Hauck, Julie Julie A. A. Oreskovich, Oreskovich, and and Mark J. J. Severson, Severson,Economic EconomicGeology GeologyGroup, Group, Steven A. Hauck, Natural Resources Resources Research Research Institute Institute (NRRI), (NRRI), University University of Minnesota, Minnesota, Duluth, Duluth, 5013 5013 shauck@nni.umn.edu Miller Trunk Highway, Duluth, MN 55811-1442, shauck@nrri.umn.edu Mineral exploration in the Duluth Complex began in 1948 on Spruce Road when two prospectors found sulfide mineralization. mineralization. Subsequent Subsequentcore coredrilling, drilling,geological geologicalmapping, mapping,and and airborne airborneand and ground geophysics by more than 28 exploration companies (including (including the NRRI, MGS MGS Dept. of Natural Resources, Minnesota Geological Survey, Minnesota Survey, and the DNR - Dept. Resources, Division Division of Lands Lands and Minerals), over the next 52 copper-nickeltplatinum-groupelement element 52 years years led led to to the the discovery discovery of copper-nickel±platinum-group mineralization along the basal contact of of the the Duluth Duluth Complex Complex (Fig. 1). Ten (PGE) mineralization Ten Cu-Ni-PGE Cu-Ni-PGE orPGE-Cu-Ni years. Over or PGE-Cu-Ni deposits depositswere were defined defined by by drilling drilling during these years. Over 2,142 2,142 drill drill holes holes have have with 1,666 of of these holes being drilled been drilled into the Duluth and Beaver Bay complexes with the basal basal contact. Over along the Over 954,000 954,000 ft. ft. of of drill drill core from the the basal basal contact contact has has been relogged by NRRI, and their results are discussed in many publications. publications. Geophysical Geophysical exploration exploration began began as as early as 1956, by Bear Bear Creek Creek Mining Mining Company, and and continues continues today. today. The State of Minnesota State Minnesota (MGS), with funding from the Legislative Commission on Minnesota Minnesota Resources, Resources, flew flew high high this area area as as well well as asthe therest restof ofthe thestate. state. The MGS has also resolution aeromagnetics over this collected and produced a gravity map covering both both complexes. complexes. Peak Peak exploration exploration(1966-1978) (1966-1978) began with the the leasing leasing of of State State of of Minnesota Minnesotamineral mineralrights rightsinin 1966 1966(Fig. (Fig. 1). 1). Exploration and began resource calculations) continued through development (drilling, bulk sampling, shaft sinking, resource In 1998, 1998, State State and and Federal Federal mineral mineral leasing and exploration exploration drilling drilling began to to increase increase with with 1978. In rise in price price of PGEs; PGEs; 2) possible use of new new hydrometallurgical hydrometallurgical techniques to more the: 1) rise recover copper copper and nickel; nickel; and and 3) 3) introduction introduction of of new new PGE PGE exploration exploration models efficiently recover (sulfide saturation; Miller et al., 2002) for intrusions in in the Beaver Bay Complex, i.e., at Sonju and the the Duluth Duluth Complex, i.e., Greenwood Lake Lake and and Layered Layered Series Series at at Duluth). Duluth). The Lake, and The maps maps poster illustrate the relationship between geology, geophysics, geophysics, drilling, drilling, and and mineral in this poster were produced produced in ArcView (GIs). (GIS). The The maps were were also also compiled compiled by by using using leasing and were the DNR's online information from: 1) the online (minarchive.dnr.state.mn.us) (minarchive.dnr.state.rnn.us)attributeattribute- and and GIS-based GIs-based database of non-ferrous minerals' information information and and State State mineral mineral rights rights holdings; holdings; 2) 2)U.S. U.S.Forest Forest leases, permits, permits, and applications database; database; and and 3) 3) NRRI NRRI in-house GIS data on the history Service leases, of Cu-Ni-PGE Cu-Ni-PGE mineralization. mineralization. Using the resulting GIs GIS database, the spatial relationships in the of in drilling, leasing, leasing, etc. with with time and place place were were then then combined combined with geological changes in information from Miller et al. (2002) to better understand the past and present exploration exploration areas areas to assist in in defining new new areas in in which to to explore explore for for non-ferrous non-ferrous minerals. and to References References Miller, J. D., Jr., Green, J.C., Severson, Severson, M.J., Chandler, V.W., V.W., Hauck, Hauck, S.A., S.A., Peterson, Peterson, D.M., D.M., and and Wahl, T.E., T.E., 2002, Geology and mineral potential of of the Duluth Complex and related rocks rocks of of 207 p. northeastern Minnesota: Minnesota Geological Survey Report of Investigations 58, 58,207 23 �s-ale so Deposit/Area as as as as as as N1i •ChiIL,LSSdJOeSideLHGLOi •1u0b14/HGs SPRUCE RD INCO & t '0 0 0 CU 0 as as t as 50 as as !essk;L Ij_ Rod & Mstsse-i R Spiace - Ieoo MATURI CU as Leek - Spease Rood —1— —f-—I— Il—eorCoeekHII BABBITT (M in no ma x) o(E9e! III AMAX (Mesabo) USS DUNKA ROAD I oeeoo (NorthMel) PolyNeti I I -— Ceeeo WYMAN CREEK USSIITa .1.1 11111 Ii: DUNKA PIT ; Begs yi,—os—foeesotios dells og 11 ErIch WETLEGS <ii AMAX * Duval Duval — fl"II WATER HEN(OUI) • Bear B e a rCreek Creek i_- - SOFILSONCRECK WESTERN MARGIN II It • : Exxon Exxon heIst Dodge! I I I • t/Des o,-e/Aoeecoe Seed • Other O t h e r Companies Companies I —— — LOCCOOS BIRCH LAKE 11 II hee,icoo She d/N(COR LONCNOSE (Out) -ii INCO•. SCATTERED Reosceek.. BeoCeeek••I OR I - • Neeost RECONNAISSANCE Ducol DRILLING SCATTERED DRILLING I USS toXXp • •Ul — / Si * so OS 0 5 Beg, Cooed o sO Si so I o ! '0 so C - Stereos S Ctsfffft ! GUNFLINT TRAIL = CONCENTRATED DRILLING St SI-il 0 320koJg-WhItesIde JJjOekJoyfl as 0 fl I-os--s-so Si LEGEND LEGEND • INCO INCO uss uss 0 0 Si 0 0 '0 0 0 0 0 0' 0 Ot 5 as 2 Figure 1. History Historyof of Cu-Ni-PGE Cu-Ni-PGEexploration explorationin in the the Duluth DuluthComplex Complex (after (afterMiller Milleret etal., al., 2002). 2002). as 0 - as Si �Geochemistry and Mineralization of the Seagull Intrusion, Northern Ontario Heggie, G., P., P., (Depanment of Geology, Lakehead Heggie, G.,and andHollings, Hollings, (Department of Geology, LakeheadUniversity, University,955 955Oliver Oliver Road, Thunder Bay, On, P7B 5E1, gheggie@mail.lakeheadu.ca) gheggie@mail.lakeheadu.ca) Platinum Group Group Elements Elements (POE-Platinum, (PGE-Platinum, Palladium, Osmium Osmium and and Iridium) Indium) have have seen seen substantial increases in demand over the last 30 years, as industrial and and commercial commercial users users have increased their consumption. consumption. Canadian Canadian production of these metals has until until recently recently been limited to by-products from from nickel Sudbury). Opening Opening of of the the copper mines (e.g., Sudbury). lies mine in Ontario Ontario demonstrated demonstrated the the Lac des fles economic PGE deposits potential for economic deposits in in Canada. Further work on deposit deposit and and exploration models is essential essential to to identifying identifying new targets and prospective prospective host rocks. rocks. 4 Lake N(pfgono 1'> •z SeaguU I Quetico Subprovrnce I Lake Superhr • • bIeyGro The Seagull Lake intrusion is found found within the n's Osler Grp Nipigon Embayment, approximately 70km 1). north east east of Thunder ThunderBay, Bay,Ontario Ontario(Fig. (Fig.1). Relative age dating places the age of the intrusion to be younger Seagull Lake intrusion younger than than Figure 1. Map Map showing location of Seagull Intrusion Intrusion Figure the Sibley Group sedimentary sedimentary sequence, sequence, (1339±33 (1339±3Ma) (Franklin et et al., al., 1980), 1980), and regional regionalgeology. geology. as part of the intrusion has been seen to cross cut Sibley Sibley stratigraphy. stratigraphy.A A chilled chilledmargin margin has has been been Intrusion and and the the younger younger Nipigon Nipigon Sills defining defining an upper age of of observed between the Seagull Intrusion approximately 1.1 Ga (Davis (Davis and and Sutcliffe, Sutcliffe,1985). 1985).This This falls falls within within the the time time of of mid-continental mid-continental region. Volcanic Volcanic activity activity was was responsible responsible for for production production of of thick thick rifting in the Lake Superior region. al., 1989) beneath beneath and around the shores of Lake Superior and the basaltic sequences (Cannon et al., mafic to ultramafic complexes complexes (e.g., Duluth complex). emplacement of numerous mafic C The Seagull Intrusion Intrusion is is currently currently under under exploration exploration by by East East West West Resource Resource Corporation, It is a ultramafic intrusion intrusion consisting consisting of of cumulate cumulate olivine, olivine, and oxide minerals with pyroxene layered ultramafic oikocrysts and interstitial feldspar. feldspar. Lithological Lithological phases phases include include dunites, lherzolites, olivine gabbronorites, gabbros, and and pryoxenites. pryoxenites. A A distinctive distinctive olivine gabbronorite is found within the but this exhibits chilled margins and is thought to post date the formation of the rest of intrusion but intrusion. the intrusion. 25 �Ni (ppm) Interval (m) Cu (ppm) 1160 269 4.0 375.0 WMOO-01 1413 501 408.0 4.5 1565 12.0 779 572.0 987 6.0 1180 546.0 WM98-02 1647 112 379.0 8.0 WM98-05 1841 1843 569.0 6.0 1455 1220 6.0 579.0 Figure2.2.Table Tableofofmetal metalcontents contentsfrom fromassay assay(Caven, (Caven,It,R.,2000) 2000) Figure DDH Depth (m) Pt (ppb) 307 336 363 535 336 693 458 Pd (ppb) 383 418 438 566 393 847 537 Mineralization Mineralizationoccurs occursininthe theform formofofPGE PGEminerals mineralsassociated associatedwith withdisseminated disseminatedFeNi FeNisulfides sulfides (pentlandite).Pentlandite Pentlanditeisisfound foundininhigher higherabundances abundancesatatdiscrete discreteintervals intervalsthroughout throughoutthe the (pentlandite). intrusion,with witha ageneral generalincrease increasetowards towardsthe thebase baseofofthe theintrusion. intrusion. intrusion, Work Workisiscurrently currentlybeing beingundertaken undertakentotounderstand understandthe thestratigraphy stratigraphyofofthe theintrusion, intrusion,the thenature natureofof the theplatinum platinumgroup groupmineralization, mineralization,and andformational formationalcontrols controlsononthe themineralized mineralizedzones, zones,which whichare are present presentininthe theintrusion intrusionininorder ordertotoaid aidininthe thedevelopment developmentand andrefinement refinementofofexploration exploration techniques,and anddeposit depositmodels. models. techniques, Cannon, J.C., Cannon,W.F., W.F.,Green, Green,A.G., A.G.,Hutchinson, Hutchinson,D.R., D.R.,Lee, Lee,M., M..Milkereit, Milkereit,B., B.,Behrendt, Behrendt,J.C., J.C.,Halls, Halls,H.C., H.C.,Green, Green, J.C., Dickas, Dickas,A.B., A.B.,Morey, Morey,GB., G.B.,Sutcliffe, Sutcliffe,R., R.,and andSpencer, Spencer,C., C.,1989, 1989,The TheNorth NorthAmerican AmericanMidcontinent Midcontinentrift rift beneath beneathLake LakeSuperior Superiorfrom fromGLIMPCE GLIMPCESeismic SeismicReflection Reflectionprofiling. profiling.Tectonics, Tectonics,v.v.8,8,p.p.305-332. 305-332. Caven, R.J., R.J., 2000, 2000,Progress ProgressReport Reporton onthe theWolf WolfMountain Mountainand andDisraeli DisraeliProperties Propertiesfor forEast EastWest WestResource Resource Caven, Corporation, Ltd. Corporation,Canadian CanadianGolden GoldenDragon DragonResources ResourcesLtd. Ltd.and andAvalon AvalonVentures VenturesLtd. Davis, Davis, D.W., D.W., and and Sutcliffe, Sutcliffe, R.H., R.H., 1985, 1985, U-Pb U-Pb ages agesfrom fromthe theNipigon Nipigonplate plateand andNorthern NorthernLake LakeSuperior. Superior. GeologicalSociety SocietyofofAmerican AmericanBulletin, Bulletin,v.96, v.96,p.p.1572-1579. 1572-1579. Geological Franklin, Franklin,J.M, J.M.McJlwaine, Mcllwaine,WIT., W.H., Poulsen, Poulsen,K.H., K.H., and and Wanless, Wanless,R.K., R.K., 1980, 1980,Stratigraphy Stratigraphyand anddepositional depositionalsetting settingofof the theSibley SibleyGroup, Group,Thunder ThunderBay Baydistrict, district,Ontario, Ontario,Canada. Canada.Canadian CanadianJournal JournalofofEarth Earth Sciences, Sciences,v.17, v.17,p.p. 633-65 1. 633-651. 26 �PEPERITES OF PEPERITES OF THE THE GAFVERT GAFVERTLAKE LAKE VOLCANIC VOLCANIC COMPLEX, COMPLEX, ST. LOUIS MINNESOTA COUNTY, MINNESOTA Heiling, Carrie Came D., Department Geological Sciences, Sciences, University of Minnesota Minnesota Duluth, Duluth, Department of Geological 1114 Kirby Drive, Drive, Duluth, Duluth, MN, 55812; cheiling@d.umn.edu cheiling@d.umn.edu The Gafvert Lake area, located within the Upper Ely member of the Ely Greenstone Greenstone of forms part of of aa the Vermilion District in Northeastern Minnesota (Figure 1) (Card, 1990), forms Morton (personal (personal communication) communication)has has large, Archean, felsic volcanic complex. Morton late stage caldera interpreted the complex to be a composite volcano that underwent late collapse. This Thisstudy studyhas has focused focused on on aa two two square square mile area in the central part of the complex. Here Herethe the complex, complex,from from the the oldest oldest to to the the youngest rocks, is composed of a) coarse, heterolithic breccias (interpreted (interpreted to represent meso-and mega breccias) breccias) (Morton, (Morton, personal communication), b) more than 3000 feet of massive to bedded pumice-rich communication), and beds beds of of chert chert and and massive to lapilli tuff, c) dacitic lavas and domes, and d) lenses and pyrite (Figure 2). The The breccias breccias and and lapilli lapilli tuffs tuffs have have been been intruded intruded by by aa semi-massive pynte swarm of feldspar porphyry dacite dikes that represent feeders to the domes domes and/or andlor flows. flows. Peperites are rocks formed by the in situ disintegration of magma intruding and mixing unconsolidated sediment sediment or or ash ash (Skilling (Skillingetetal., al., 2002). 2002). At Gafvert Lake the with wet unconsolidated dikes intruded and peperites formed near the top of the complex where dacite porphyry dikes mixed with wet, unconsolidated pumice-rich pumice-rich lapilli lapilli tuff. tuff. This mixing led to quenching quenching of the the dacitic magma magma and disruption disruption and and vesiculation vesiculation of the lapilli and fragmentation of tuffs. The Thepeperites peperites occur occur within within 100 100 feet feet of dike contacts though they form much more extensive areas where where several several dikes dikes occur occur close close together. together. Angular and finger-like blocks of dike material occur within the peperite, peperite, locally locally these these are are connected connectedto toaanearby nearbydike. dike. Macrotextures in outcrop and microtextures in thin section helped identify and classify classify the following fragment types and internal structures within the peperites: a) blocky juvenile juvenile fragments fragments with chilled chilled rims rims and and occasional occasional jig-saw fit fit textures, textures, b) b) platy platyto to ragged ragged juvenile juvenile fragments fragments with curviplanar curviplanar surfaces surfaces and and broken gas gas bubbles, bubbles, c) c) ameboid ameboidto to juvenile fragments, fragments, d) abundant pumice which exhibits variable vesicularity. vesicularity. globular juvenile Most of this pumice is juvenile to the lapilli lapilli tuffs tuffs but a small small percentage percentage contains contains feldspar feldspar to those those found found in in the the dikes possibly indicating local, rapid vesiculation crystals identical to of dike material. Close Close to to dike dike margins margins feldspar feldspar crystals are broken and internally with fractures fracturesfilled filledby bylapilli lapillituff. tuft Pumice, fractured with Pumice, close close to to dike dike contacts, may be broken or disaggregated into into several several small small jigsaw-fit jigsaw-fitpieces. pieces. Locally the ash matrix to radiating away from from dike margins. the lapilli tuffs is amygdaloidal with amygdules radiating References References K.D., 1990, A A review review of of the the Superior Superior Province Province of of the the Canadian Canadian Shield, a product of Card, K.D., Archean accretion: Precambrian Precambrian Research, Research,v.v. 48, 48,pp. pp. 99-156. 99-156. Ely, Minnesota, 2003, Mapquest, www.mapquest.com. Mapquest, www.mapquest.com. Morton, R.L., communication, University of Minnesota-Duluth. R.L.,2003, personal communication, White, J., J., McPhie, McPhie, J., J., 2002, 2002, Peperite: Peperite: a review of magma-sediment mingling, Journal Skilling, I., White, of Volcanology Volcanology and Geothermal GeothermalResearch, Research,xc v. 114, 114,pp pp 1-17. 1-17. 27 �___ \ 01 crane Lake Agnes Lake St. Lóujc Buyck--. 0% - !L h__c1tQ/iF 6urrit'i'1 : - For. . it Trout Lake I RObIF1SOr'' Forestcenter0 lear island st,L120K -—22 .••' -- ,Babbitt — — ;Emlrarrass Erit :-°_V S Virgirii&.J_c.Auroja 11001 9 1 L4SAbCIIA — . r Murphy Cit HoYt Lakes IToirñi . .0Yihyte Fifllaç_fl — (Mapqucst, 2003). Location of Gafvert Lake complex 1: Location Figure Figure 1: complex (Mapquest, 2003). Exp I a n at io fl Explanation - -? ' Tuft II - • Peperite samples samples Fault Zone '" ''Contacts Contacts / \" /\/ Railroad ' R a i l r o a dgrade grade Mud Road 4, "Mud Creek Road / -Â¥ 7 ) - p '*__ -ç. \ .'f -,Qg __DE" I 'S-- ,' , "- " N UI Tuff Qac / /A/ . Diab " Zrtc c'_ Bx N ,. - - - -. I Mbas -- Metabasalt Metabasalt Q Qfp-QtzfeldPorphyry - Qtzfeld Porphyry Cht - Black Chert Cht Diab -- Diabase Diabase Dikes Dikes Dior -- Diorite Dior Diorite Dac -- Dacite Dac Dacite Dikes Dikes luff Tuft Tuff - Lapilli Lapilli Tuff Bx -- Breccia Breccia Bx -- Siltstone &&Iron IronFm Fm 81st SIst - - I 01p , Sisi It METERS Wa to O 0 of Gafvert Lake Lake volcanic complex. Figure 2: Generalized map of a portion of 28 28 �CHEMISTRY CHEMISTRY OF OF ALTERATION ALTERATION MINERAL MINERAL PHASES PHASES AT THE FIVE MILE MILE LAKE LAKE VOLCANIC-HOSTED MASSIVE SULFIDE PROSPECT, NE MINNESOTA VOLCANIC-HOSTED MINNESOTA Hocker, J., Odette, Odette, J.J. D., D.,and andNewkirk, Newkirk, T. T.T., T.,Department DepartmentofofGeology, Geology, Hocker, S. M., Hudak, Hudak, G. J., University of Wisconsin Blvd., Oshkosh, WI 54901, hudak@uwosh.edu Wisconsin Oshkosh, Oshkosh, 800 800 Algoma Blvd., Alteration mineral assemblage assemblage mapping mapping at at the Five Mile Lake Alteration mineral Lake Prospect Prospect in in the the Vermilion Vermilion District of northeastern Minnesota has identified two distinct types of alteration alteration zones zones within within 2.7 2.7 billion year-old billion year-old volcanic volcanic and and volcaniclastic volcaniclastic rocks rocks associated associated with withvolcanic-hosted volcanic-hosted massive massive sulfide (VHMS) (VHMS) mineralization mineralization (Hudak (Hudak etet al., al., in press; al., 2001a, 2001a, 2001b; 2001b; Peterson, Peterson, press; Odette et al., alterationzones zones are are composed composed of of various 2001). 2001). Regional Regional semi-conformable semi-conformable alteration various proportions of ± amphibole ± chlorite + epidote quartz epidote 5 amphibole 5 chlorite ± quartz + i plagioclase plagioclase feldspar. feldspar. These regional, semiconformable alteration alteration zones zones are locally cross-cut by several relatively narrow, northeastsemiconformable northeasttrending disconformable alteration alteration zones composed composed of fine-grained fine-grained chlorite and/or sericite that are closely associated with synvolcanic synvolcanic fault fault zones. zones. Electron microprobe microprobe analyses analyses of of the various Electron various alteration alteration mineral mineral phases phases (epidote (epidote group group minerals, chlorite, chlorite, amphibole, amphibole, white white mica, mica, and and feldspar) feldspar) have have been been conducted conducted in an effort minerals, effort to to associated with the better understand hydrothermal processes processes associated the development development of the the semiconformableand and disconformable disconformablealteration alterationzones zonesatat the the Five Five Mile Lake prospect. semiconformable prospect. These These that: a) epidote group minerals range in composition from zoisite/clinozoisite zoisite/clinozoisite to analyses indicate that: pistacite; pistacite; b) chlorite chlorite is dominantly dominantly ripidolite; ripidolite; c) amphibole amphibole is is primarily primarily actinolite actinolite and and ferroferroactinolite, with magnesio-homblende and ferro-hornblende ferro-hornblendealso alsopresent; present;d)d) sericite sericite is fineactinolite, with magnesia-hornblende and finegrained muscovite; and e) e) plagioclase plagioclase feldspar feldsparisisdominantly dominantlyalbite. albite. Alteration mineral chemistry chemistry at at the the Five Mile Lake Alteration mineral Lake Prospect Prospect is remarkably remarkably similar similar to to that that from the the Noranda Noranda VHMS VHMS mining mining camp camp of of Canada, Canada, as as well well as as other other VHMS VHMS mining mining camps around the world. world. This alteration mineral chemistry chemistry suggests the presence of of aa complex, complex,long-lived long-lived hydrothermal system that that evolved (hundreds of of meters) hydrothermal system evolved from seafloor-proximal seafloor-proximal (hundreds meters) to to deeper deeper subseafloor environments (-1-3 (—4-3kilometers) kilometers)asasthe the volcanic volcanic rocks rocks were buried subseafloor environments buried by by apparently apparently mafic to intermediate volcanism and associated sedimentation. This rapid, dominantly effusive mafic suggests that in addition to the Five Mile Mile Lake Lake Prospect, Prospect, the the uppermost uppermost several several hundred hundred meters meters of the Lower Member of the Ely Greenstone also has excellent exploration potential for VHMS mineral deposits. deposits. References References Galley, A,, A., Bailes, A., Hannington, M., Hollc, G., Katsube, 1, Paquette, S., Bailes, A., Hannington, M., Holk, G., Katsube, J., Paquette, F., F., Paradis, Paradis, S., Galley, Santaguida, F., Database for Santaguida, F., and Taylor, Taylor, B., 2002, Database for CAMIRO CAMIRO Project Project 94E07: 94E07: Interrelationships between subvolcanic subvolcanic intrusions, intrusions, large-scale large-scale alteration alteration zones, zones, and VMS Interrelationships between VMS of Canada Open File Report 4431 (CD-ROM). deposits: Geological Survey of Hudak, G. J., Heine, J., J., Newkirk, Newkirk, T., T., Odette, Odette, J., J., and Hauck, S., S., in press. press. Comparative Comparativegeology, geology, stratigraphy, and lithogeochemistry lithogeochemistryof of the the Five Five Mile Mile Lake, Lake, Quartz Hill, and Skeleton stratigraphy, and Skeleton Lake Lake Vermilion District, District,NE NE Minnesota: Minnesota: A report to the Minerals Coordinating VMS occurrences, Vermilion Coordinating Committee, DNR Minerals Division, State State of Minnesota. 29 �Kranidiotis, Kranidiotis,P. P.and andMacLean, MacLean,W. W.H., H.,1987, 1987,Systematics Systematicsofofchlorite chloritealteration alterationatatthe thePhelps PhelpsDodge Dodge Massive MassiveSulfide SulfideDeposit, Deposit,Matagami, Matagami,Quebec: Quebec:Economic EconomicGeology, Geology,v.v.82, 82,p.p.1898-1911. 1898-1911. Odette, Odette, J. D., D., Hudak, Hudak, G. G. J., J., Suszek, Suszek, T., T.,and andHauck, Hauck, S. S.A., A,,2001a, 2001a,Preliminary Preliminary evaluation evaluation of hydrothermal alteration alteration mineral mineral assemblages assemblages and and their their relationship relationship to to VMS-style VMS-style hydrothermal mineralization mineralization in the Five Mile Mile Lake Lake area areaof ofthe theArchean ArcheanVermilion VermilionGreenstone GreenstoneBelt, Belt, NE NE 47th Minnesota: Institute Institute on on Lake Lake Superior SuperiorGeology, Geology, 47thAnnual Annual Meeting, Meeting, Proceedings ProceedingsVolume Volume Minnesota: 47, Part Part 1-Program 1-Programand and Abstracts, Abstracts,p. p. 75-76. 75-76. 47, Odette, Odette, J. D., D., Hudak, Hudak, G. G. J., J., Suszek, Suszek,T., T.,and andHauck, Hauck,S.S.A., A.,2001b, 2001b,Preliminary Preliminaryevaluation evaluation of of hydrothermal alteration alteration mineral mineral assemblages assemblages and and their their relationship relationship to to VMS-style VMS-style hydrothermal Archean Vennilion Vermilion Greenstone Greenstone Belt, Belt, NE NE mineralization in the Five Mile Lake Lake area area of of the the Archean mineralization Minnesota: Minnesota: Geological Society Society of of America America Abstracts Abstracts and and Programs Programs Volume Volume 33, 33, No. No. 6, 6,p. p. AA420. 420. Peterson, Peterson, D. M., M., 2001, 2001, Development Development of of Archean Archean lode-gold lode-gold and and massive massive sulfide sulfidedeposit deposit exploration exploration models using geographic geographic information information system system applications: applications: targeting targeting mineral mineral exploration exploration in northeastern northeastern Minnesota from analysis analysis of of analog analogCanadian Canadianmining miningcamps: camps: unpublished unpublished Ph. Ph. D. D. dissertation, dissertation,University Universityof of Minnesota, Minnesota,Duluth, Duluth, Minnesota, Minnesota,503 503p.p. 18-cm 16 14 H —I P 1111111 0.0 O4'.B 0.2 0.8 0.0 PSEUOOThUNPNGOT CQRVNOOflILIT! -J "ffiff. 12 10 — C— •FM.CnICaC!*I AU. o'A•*TPff C ow—5—o'.w TALC .C1tGTh 00 \ TOk..S. 6- 'RI 10 20 0.D 30 1.0 tM. — 3.0 2.0 4.0 5.0 6.0 A WTIG ON EPIDOTE NA C0RUIUM 1.0 0.9 0,8 0.7 0.6 ft. :? 0.5 0.4 0.3 0.2 0.1 ThCCLASE 0.0 K&A 6.0 analyses for for epidote-group epidote-group minerals minerals (A), (A),chlorites chlorites Figure 1.1. Summary of electron microprobe analyses Figure (B), amphiboles (C), and white white micas (D) from the Five Mile Mile Lake Lake Prospect Prospect and andselected selectedVHMS VHMS mines. Compositional fields for Noranda minerals determined from Galley et al. (2002). (2002). 30 �GEOCHEMISTRYAND AND GEODYNAMIC GEODYNAMICIMPLICATIONS IMPLICATIONS THE 1537 1537 MA GEOCHEMISTRY OFOFTHE MA POINT ANOROGENIC REDSTONE POINT ANOROGENICGRANITE, GRANITE,ONTARIO, ONTARIO,CANADA CANADA Hollings, Fralick, P.P.and andKissin, Kissin,S.S.(Department (DepartmentofofGeology, Geology,Lakehead LakeheadUniversity, University, 955 955 Hollings, P., P., Fralick, Rd., Thunder Thunder Bay, Bay, Ontario, P7B 5E1, 5E1, Canada; Canada; Peter.HollinRs@lakeheadu.ca) Oliver Rd., Peter.Hollings@lakeheadu.ca) The The Redstone Redstone Point granite granite is is a Mesoproterozoic felsic igneous Mesoproterozoic felsic igneous complex (1537+101-2 Ma; Davis Davis and complex (1537+10/-2 Ma; Sutcliffe, 1984) located in the the northern northern portion of the Sibley Sibley Basin on the west shore of Lake 1). It It is is shore Lake Nipigon Nipigon (Fig. (Fig. 1). unconformably overlain overlain by by arenites of unconfonnably the Pass Lake Sibley Lake Formation Formation of of the the Sibley Group. These sediments are in turn Group. These sediments are turn intruded and overlain overlain by by an an extensively extensively developed sills developed Sequence sequence of diabase diabase sills related to an early related to early stage stage of of the the MidMidevent. The Continent Rifting event. The entire entire sequence has been gently folded into aa shallowly, easterly plunging plunging succession succession of open synclines and anticlines, open synclines and anticlines, with with Figure 1. Map Map showing Figure showing the the location location of the the Redstone Redstone Point Point dips not usually usually exceeding exceeding 15°. 15". Outcrop Outcrop granite in relation relation to Proterozoic granite Proterozoic anorogenic anorogenic granite granite complexes of ofNorth North America. America. Modified after after Anderson complexes Anderson (1983) (1983) density density of of igneous igneous units units is very very good good along the shoreline of Lake Nipigon, in contrast contrast to to sedimentary sedimentary sequences, sequences,which which only only provide provide small, scattered scattered outcrops. outcrops. The igneous rocks of Redstone Point have been briefly described by Davis and and Sutcliffe Sutcliffe (1985), (1985), wherein they emphasised that the rocks are wherein they emphasised that are anorogenic anorogenic granites granites gradational gradational to rhyolites rhyolites and and fragmental rhyolites and and dacites. In fact, fragmental rhyolites fact, presently presently accessible accessible outcrop outcrop indicates indicates that that extrusive extrusive members dominate the the magmatic magmatic rocks rocks of of the area. Porphyritic texture with members dominate with volcanic volcanic features features including vesicles, flow flow structures, agglomeratic units, units, rubbly rubbly flow flow tops including vesicles, structures, agglomeratic tops and and segregation segregation cylinders differentiate extrusive extrusiverocks rocks from from more more limited cylinders differentiate limited exposures exposures of of uniformly uniformly textured textured intrusive rocks. As As contacts between between units units are are generally unexposed unexposed and the base base of of the section is nowhere exposed, thicknesses thicknesses of of units units and and of the nowhere exposed, the entire entire succession succession are areunknown; unknown;however, however, continuous outcrop outcrop in in cliff-forming cliff-formingunits unitsindicates indicatesthat that aa minimum of lOOm continuous minimum thickness thickness of 100m of volcanic rock is present in in the the area. area. The igneous rocks are distinctively brick red, red, suggesting suggesting the the dominace dominace of of ferric ferric iron iron in the The igneous rocks distinctively brick the various mineral hosts but especially in trace amounts in feldspars. feldspars. The The intrusive intrusivemember memberdisplays displays equigranular phaneritictexture texturewith withmost mostmineral mineralgrains grains11 to to 5 mm in diameter. equigranular phaneritic diameter. The volcanic rocks are true porphyries with phaneritic phenocrysts of alkali feldspar, quartz and hornblende hornblende in an aphanitic matrix of of the the same minerals. minerals. Quartz Quartz phenocrysts phenocrystsare areeuhedral euhedraland and 11 to to 33 mm in aphanitic matrix diameter associated with alkali feldspar phenocrysts occasionally exhibiting exhibiting synneusis synneusis twinning twinning of microcline. Homblende and magnetite are less as well as as albite-pericline albite-pencline twins indicative indicative of microcline. Hornblende less 31 �_________________________________________________ abundant and finer grained than in the intrusive rocks. Near flow tops the porphyries grade into textured aphanitic uniformly textured aphanitic rhyolites with sparse phenocrysts. The samples samples from the the Redstone Redstone Point Point intrusive intrusive complex complex are all all characterised characterised by high high Si02 SiO, contents (73-83 (73-83 wt%) wt%) and and elevated elevatedK,0 1(20and andNN;0 contents q 0 abundances abundances (2-7 (2-7 wt% wt% and and 0.2-3.5 0.2-3.5 wt% wt% respectively). respectively). They They are are typically typically LREE relatively LREE enriched enriched with with relatively unfractionated HREE HREE (La/Sm, (La/Smn = = unfractionated 2.8-5.1; Gd/Yb,, GdIYb == 1.1-1.6; 1.1-1.6; Fig. 2) characterised by elevated elevated and are characterised and Nb Nb contents. contents. Samples Samples Zr, Y and from the Redstone Redstone Point Point igneous igneous 02.13 complex fulfil fulfil the detailed complex detailed trace trace 02-H element criteria of of Whelan element criteria Whelan et al. al. y (1987) (1987) for anorogenic anorogenic granites. granites. o . I 1 . . . . Rb Tli Rb Ra BaTh Pr S SrNd U HISmEu, fl GdThOy Er YbLu U NbLa Nb La Cc Cc Fr t hid Zr Hf Sin Eu ll Gd 7% D! Tile Y ifc Er Yb Lu Al A1 U V V Sc SP Figure Representative primitive mantle normalised diagram diagram for Figure 2. Representative for samples from from the the Redstone RedstonePoint Pointigneous igneous complex Similarities Similarities between between Proterozoic Proterozoic basin sequences basin sequences (e.g., (e.g., Athabaska, Athabaska, Thelon, Hornby Bay and Sibley basin fill sequences) imply that basin genesis and developmental developmental controls were similar. The setting, setting, architecture, depositional systems and deformational deforrnational histories of all four basins strongly infer that they are intracratonic, forming as a result of heating cratonic lithosphere. The heating event is represented represented in in northern northern Canada by by numerous 1790 to 1730 Ma anorogenic, syenogranite batholiths and and comagmatic ash-flow tuffs occurring syenogranite batholiths occurring west of of Hudson Hudson Bay. In the western Great Lakes region a heating event produced the 1537 Ma Redstone western Great the 1537 Ma Redstone Point Point assemblage and and other 1500 records a assemblage 1500 Ma Ma anorogenic anorogenic batholiths. The southern southern mid-continent mid-continent records lithospheric heating event with anorogenic granite production production from from approximately approximately 1480 1480to to 1320 1320 Ma (Fig. 1). 1). These These events events outline outline aa progressive progressive southward southward displacement displacement of lithospheric heating from a maximum age of approximately 1750 ma in northern Canada to aa minimum minimum age age of of 1310 1310 Ma in the As heat transfer Ma the southwestern southwestern United United States. States. As transfer from from the the asthenosphere asthenosphere is the the only only heating, drift of North America over hotter than mechanism for producing extensive lithospheric heating, regional ages of of heating, drift rates of approximately 1.1 average asthenosphere is implied. Using regional 1.1 cm/year are necessary, and agree in magnitude with present to 1.4 cmlyear present rates. rates. REFERENCES Anderson, J., J., 1983. Proterozoic anorogenic anorogenic granite granite plutonism plutonism of of North North America. In: Medaris et Anderson, al., (Eds), Proterozoic Proterozoic geology. Geological Society Society of America America Memoir Memoir 161, 161,133-154. 133-154. Davis, D., D., and Davis, and Sutcliffe, Sutcliffe, R., R., 1985. 1985. U-Pb ages ages from from the the Nipigon Nipigon Plate Plate and and Northern Northern Lake Lake Superior. Bulletin, 96, Superior. Geological Geological Society of America Bulletin, 96, 1572-1579. 1572-1579. Whelan, J., Currie, Currie, K., K., and andChappeli, Chappell, B., B.,1987. 1987.A-type A-typegranites: granites:geochemical geochemicalcharacteristics, characteristics, discrimination Contributions to 407-419. discrimination and petrogenesis. Contributions to Mineralogy Mineralogy and and Petrology, Petrology, 95, 95,407-419. 32 �PALEOPROTEROZOIC (1900-1600 Ma) TECTONIC HISTORY OF LATE PALEOPROTEROZOIC OF THE THE NORTHERN NORTHERN FOR CRUSTAL STABILIZATION STABIUZATION MID-CONTINENT, U.S.A: IMPLICATIONS FOR HOLM, D.K., Dept. of Geology, Geology, Kent Kent State StateUniversity, University, Kent, Kent, OH OH44242; 44242;VAN VANSCHMIUS, SCHMUS, MacNEILL, L.C., Dept. of Geology, University of of Kansas, Lawrence, KS 66045; W.R., and MacNELL, Minnesota Geological BOERBOOM, T.J., Minnesota Geological Survey, 2642 University Avenue, St. St. Paul, Paul, MN MN SCHWEiTZER, D., Dept. of Geology, 55114; SCHWEITZER, Geology, Kent State State University, University, Kent, OH OH 44242; 44242; SCHNEIDER, D.A., Dept. of Geological Geological Sciences, University, Athens, SCHNEJDER, Sciences, Ohio University, Athens, OH OH 45701 45701 We propose that the the late late Paleoproterozoic Paleoproterozoic igneous and and deformational deformational history history preserved preserved in in the the southern Lake Lake Superior region is the southern Superior region the result resultofofnorthwest-directed northwest-directed convergence convergence during during and and New U-Pb zircon ages indicate indicate that that late to post-Penokean following geon 18 Penokean accretion. New magmatism began ca. 1800 1800 Ma and and generally generally migrated migrated southeastward southeastward across across the thenewly newlyaccreted accreted terrane. Magmatic Magmaticpulses pulses atatca. ca.1800, 1800,1775, 1775,and and1750 1750Ma Mamay maycorrelate correlatewith withnorthwest-directed northwest-directed growth of of the North North American mid-continent. We suggest subduction associated with southward growth suggest that geon 17 Yavapai-age slab rollback caused continental arc magmatism to step step southeastward southeastward between 1800 and 1750 Ma (Fig. (Fig. 1A). As As the the slab slab steepened, steepened, the reduced compressional compressional stresses thermal input allowed for collapse of the overthickened and increased thermal overthickened portions of of the thePenokean Penokean In northern collapse involved involved the the formation formation of of gneiss crust. cmst. In northern Wisconsin, Wisconsin, collapse gneiss domes domes and and their their exhumation panels brought brought up up from depth via exhumation within discrete fault-bounded panels via tectonic tectonic extrusion extmsion (Schneider (Schneider et al., al., ILSG, ILSG, 2003). 2003). Collapse of the thePenokean Penokeanorogen orogen—- and possibly temporary temporary cessation of crustal stabilization and deposition of Baraboo Interval cessation of slab slabsubduction subduction—- resulted in cmstal quartzites between between 1750 and 1650 in aa long-lived quartzites 1650 Ma. Ma. However, However, in long-lived orogen orogen model, model,renewed renewed tectonism to the the south south resulted resulted in in the the eventual accretionofof aa Mazatzal Mazatzal arc arc (Fig. tectonism to eventual accretion (Fig. IB) 1B) with with widespread deformation deformationand andmild mild reheating reheatingof of Penokean Penokean crust crust to to the north. The age of of this this widespread Ar/Ar step-heating studies on basement rock deformation is inferred from conventional ArIAr rock beneath beneath undeformed Baraboo BarabooInterval Intervalquartzites. quartzites. The 1900 to 1600 deformed and undeformed 1600 Ma tectonic history of the north-central United States, States, not not surprisingly, records the the southward growth and the north-central United surprisingly, records and tectonic tectonic development of the southern Laurentian margin. New and published 40Arf39Ar mineral agesdelineate delineatethe thenorthern northern and and western western extent of geon New 4 0 ~ r / 3 9mineral ~r ages geon crustal deformation. deformation. Interestingly, 16 crustal Interestingly, only lower-grade crust intruded by the the shallower-level shallower-level ca. 1750 Ma Ma plutons (and associated associated rhyolites) rhyolites) were weredeformed deformedsignificantly significantlyduring duringgeon geon 16. 16. Deeper level collapsed crust and crust pervasively invaded by the older magmatic pulses are level collapsed cmst and cmst pervasively invaded by magmatic pulses arelargely largely unaffected by Mazatzal deformation deformation and and reheating. reheating. We suggest suggest that post-orogenic post-orogenic intrusions intrusions and and crustal thinning was an important step in strengthening strengthening and stabilizing the the crust crust in inthe thesouthern southern Lake Superior Superior region. region. O'Boyle, Hamilton, and Jercinovic, 2003, 2003, Paleoproterozoic Paleoproterozoic development of of a Schneider, Holm, O'Boyle, gneiss dome corridor corridor in in the the southern southern Lake LakeSuperior Superiorregion, region,USA: USA: Institute on Lake Superior Geology Abstracts (this (this volume). volume). 33 �avapai subduction (ca. 1750 Ma) w -southward propagation of magmatism from 1775 Ma ECMB to 1750 Ma granites and rhyolites in Wl Mazatzal orogeny (1650-1630 Ma) deformation of during ost-Penokean" quartzites 8azatzal accretion " age Figure 11:: A) A) Subduction Subduction rollback model model proposed to explain magmatic magmatic age progression the Penokean convergence, preprogressionacross across the Penokean orogen, orogen, ca.Yavapai convergence, accretion. Mazatzal ECMB East-centralMinnesota Minnesota batholith. batholith. B) Mazatzal ECMB ==East-central B) Mazatzal Mazatzal accretion model (modified aL,2000) to to explain explain deformation deformation accretion model (modifiedafter after Romano Romano et al.,2000) of Baraboo Interval Interval quartzites quartzites and and southward southward growth growth of of Laurentia. Laurentia. 34 �GABBRO/GRANOPHYRE OF THE THE CROCODILE CROCODILELAKE LAKEINTRUSION: INTRUSION: A GABBROIGRANOPHYRERELATIONS OF POSSIBLE VENT FOR THE HOVLAND LAVAS? (e.jerde@morehead-st.edu), Department of Physical Sciences, Morehead State JERDE, Eric A. (e.jerde@morehead-st.edu). State University, Morehead, KY 40351 40351 One of the notable notable characteristics characteristics of the Midcontinent Midcontinent Rift Rift is the presence of large large amounts amounts of of felsic felsicmaterial. material. Indeed, the nature and origins of this abundant abundant silicious silicious material material has been the source source of numerous numerous studies studies(e.g., (e.g., Nelson, 1991; Green and and Fitz, Fitz, 1993; 1993; Vervoort Vervoort and andGreen, Green, 1997; 1997;Kennedy Kennedyeteta!., 2000;Sandland Sandlandetetal., al.,2001). 2001). To the Nelson. al., 2000, Series is a pronounced ridge south of the Early Gabbro Series ridge composed composed of of this this felsic felsic material, material, properly properlytermed termedaa Series layers are granophyre. The Early Gabbro Series are inc!ined inclined to the south, south, thus are below the granophyre granophyre stratigraphically. This This felsic felsic rock rock was was noted and and described described by Nathan (1969) (1969) as a very late-stage material, and has presumed to to have have formed formed significantly significantlyafter afterthe the gabbros gabbrosin inthe theregion. region. However, several severa! generally been presumed observations !ater than than the the granophyres. granophyres. These observationsindicate indicate that the gabbro was emplaced later These include include gradational gradational contacts, contacts, with some chilling of the gabbro. Another described as Another observation observation is is the the abundance abundance of material described as "intermediate "intermediate rock" by various investigators aL, 1959). This This materia! material is always found between the investigators in the past (e.g., Grout et al., gabbro gabbro and granophyre, granophyre,and is is presumably presumably the the result of of assimilation assimilationof of granophyre granophyreby by an an intruding, intruding,hot hotgabbro. gabbro. Investigations Investigations into into possible origins of the the granophyres granophyres(Sandland (Sandlandet et al., al., 2001; 2001; Karl Karl Wirth, Wirth,pers. pers.comm.) comm.) included included radiometric radiometric age age determinations, determinations, and and revealed revealed that that the the granophyres granophyresadjacent adjacentto to the theEarly EarlyCiabbro Gabbro Series Seriesare, are, like the gabbros, Ga),and andessentially essentiallycontemporaneous. contemporaneous. Because gabbros, among among the earliest earliest rocks of the the rift rift (—1107 (-1 107 Ga), Because silicious silicious materia! material generally is a late-stage product of magma evolution, evolution, the surprising antiquity of the the granophyres granophyres adds to questions thheir origin. questionssurrounding their The early age for the grar granophyres however, suggest suggest an an origin origin fifor the layered layered nature nature of of the the Early Early Gabbro Gabbro 3r the 1013hyres does, however, Series stratigraphically. Senes located below them stri :ti;graphically. Due granophyric material would have created created aa Due to to their their low low density, density, the granophyric . that .. . reiaraea . ~ ~ > - > . . . - -. ~ ~.~material . . coming ~~. up "from below. barrier retarded the buoyant. gamroic gabbroic up rrom below. These harrier mat me rise of buuyani These rising rising !iquids liquids would would have have beeni forced to spread !aterally, resulting in the the ;aP apparent layering that isis obserl observed, andproviding providing aa cap, cap, blocking blocking any any laterally, resultingin bee] red, and parent layeringthat further ler rise of gabbroic gabbroic material. material. furti east of of the the layered layered Earl: EarlyY (Gabbro Series of of Nathan Nathan (19 (1969) is another another occurrence occurrence of Immediately to the east 69) is Jabbro Series ~ . ~ ~ > & - L . c. . . , (i.e., . —1107 < granophyre, also determined Ga;Karl Kar!Wirth, Wirth,pers. pers. comm.). comm.). This 01 an early early origin origin n.e., -1 I U I Ga; This granopnyre, aeiermmea to be ne among among those mose of termed the the Crocodile Crocodile Lake Lake Intrusion by by Miller Miller et et al. al. (2001). (2001), and and the rocks are interpreted to be rock group has been termed geophysica! evidence, evidence, and and aa few few sample sample examinations examinations(Babcock, (Babcock, 1959). 1959). Work gabbroic based on geophysical Work done done between between 1913 edges of of this this intrusion, intrusion, and and indicates indicates that that they they are are basalt basalt lavas lavas and gabbroic 1913 and 1948 1948 included the very edges intrusions, along with "red "red rock" (Grout et al., 1959) 1959) that is now known to refer to granophyre. (1969), there is a body of gabbroic material stratigraphically stratigraphica!ly below below the Like the series mapped by Nathan (1969). granophyre. granophyre. During the past year, a reconnaissance reconnaissance was was made made into into the the Crocodile Crocodile Lake Lake Intrusion Intrusion to to examine examine some of the rock rock re!ations (Fig. 1). Traverses by forest forest blowdown, blowdown,but butthe theoutcrops outcropsare arenumerous. numerous. Several relations Traverses were greatly hampered by gabbro units are present, as well we!l as as aa band band of of "intermediate "intermediate material" materiaP' at at the the very very top top of of the the gabbro, below the granophyre. Within Withinthe thegranophyre granophyreitself, itself, several several bodies bodies of of gabbro gabbrowere were found found to to have have actually actuallyintruded intrudedthe the granophyre. In Inthe thecoarse-grained coarse-grainedinteriors interiors of of these these bodies, bodies,the thegabbro gabbroisisindistinguishable indistinguishablefrom fromthe thegabbros gabbros (i.e., below the granophyre granophyre stratigraphically). stratigraphically). observed further further north (i.e., south of of the the granophyre granophyre are are prominent prominent knobs knobs and and ridges ridges that that are are composed composed of basalt. These Immediately to the south These part of the Hovland Lavas, which are are reversely reversely polarized, polarized, and were extruded extruded during the earliest earliest are mapped as pan discontinuous bodies (and other stringers stringers and local dikes) period of the the rifting. rifting. It is perhaps possible that the discontinuous dikes) within within feeder conduits for the eruptive eruptive basalts basa!ts immediately immediately to to the the south south (shown (shown schematically schematically the granophyre represent the feeder are basaltic basaltic stringers stringers and and small small dikes. dikes. Surrounding in Fig. 2). In In several several other other places within the granophyre, there are Surrounding !arger gabbroic bodies bodiesare areobvious obviousreaction reactionzones zoneswhere wheregranophyre granophyrehas hasbeen beenassimilated assimilatedinto intothe thegabbro. gabbro. In In the larger to the the south, south,numerous numerous inclusions inclusions are arepresent present that that are are pinkish pinkish in in color, color. along with with one of the flows immediately to felsic stringers stringers and irregular masses of felsic felsic material. to assess assess the the relation relation between between the the gabbros gabbros within within the the granophyre granophyre and the lavas to the Further work is planned to south. IfIf this thisisisindeed indeed aa feeder feeder system, system, itit might provide provide insight insight into into the the mechanism mechanism of magma magma emplacement emplacement and and the the eventual "breakthrough" "breakthrough" to the surface, during during the the onset onset of of rifling. rifting. h he earl^ ~ .~~~ . ~ . ~ ~ ~ . ~ . ~. ~~~ ~ ~ ~~ ~~ . . ~ .~ ~ ~ . 35 P-. �ReferencesCited: Cited: References (1959)MS. M.S.thesis, thesis.University UniversityofofWisconsin, Wisconsin.Madison, Madison,4747p.p. Babcock,R.C., R.C..Jr. Jr.(1959) Babcock, Geothermal Research, Journal of Volcanological and Green. J.C. and Fitz. T.J., 1993. Journal of Volcanological and Geothermal Research,54, 54,177-196. 177.196. Green, J.C. and Fitz, T.J., 1993, Bulletin 39, 163p. G.M. 1959 Minnesota Geologica Survey Grout, F.F., Sharp, R.P., and Schwm, G.M. 1959 Minnesota Geologica Survey Bulletin 39, 163p. Grout, F.F., Sharp, R.P., and Schwartz, Union Jerde,BA. E.A.and andKennedy, Kennedy.B.C., B.C.,2000, 2000.American AmericanGeophysical Geophysical Union2000 2000Fall FallMeeting, Meeting,San SanFrancisco. Francisco. Jerde, and Wirth, KR. 2001, ILSG 47, 36-37. Jerde, E.A., Salvato. D.J. Thole. J., and Wirth, K.R. 2001. ILSG 47.36-37. Jerde, BA., Salvato, D.J, Thole, S., 29-30. Kennedy,B.C., B.C.,Wirth, Wirth,K.R., K.R.,and andVervoort, Vervoort,J.D., J.D.,2000, 2000,ILSG ILSG46, 46.29-30. Kennedy, M.J., Chandler, V.W., and Miller,J.D., J.D..Jr., Jr.,Green, Green,J.C., J.C.,Severson, Severson, M.J., Chandler, V.W., andPeterson, Peterson,D.M., D.M.,2001, 2001,Minnesota MinnesotaGeological Geological Miller, M119. SurveyMiscellaneous MiscellaneousMap MapSeries Series M-119. Survey 198p. University Nathan.HI)., H.D..1969, 1969. Ph.D.dissertation, dissertation. UniversityofofMinnesota, Minnesota.Minneapolis, Minneapolis, 198p. Nathan, Ph.D. University of Minnesota. Duluth. Nelson. N. 1991. M S . Thesis. University of Minnesota. Duluth. Nelson, N. 1991, MS. Thesis, K.S. 2001, LSG 47, 85-86. Gehrels, G.E., Kennedy, B.C., Sandland,TO., T.O.,Wirth, Wirth,KR., K.R.,Vervoort, Vervoort,J.D., J.D., Gehiels, G.E., Kennedy, B.C.,and andHarpp, H q p , K.S. 2001, ILSG 47,8546. Sandland, Vervoort,3D. J.D.and andGreen, Green.J.C., J.C.,1997, 1997.Canadian CanadianJournal JournalofofEarth EarthSciences, Sciences,34,521-535. 34.521-535. Vervoort, Lake, showing location of Fig.i.1. Reconnaissancegeologic geologicmap mapofofthe theregion regionjust justsouth southofofCrocodile Crocodile Lake, showing location of Fig. Reconnaissance Lake Intrusion gabbros sabhrobodies bodiesininthe thegranophyre granophyre thatforms formsthe thecap capabove abovethe theCrocodile Crocodile Lake Intrusion gabbros gabbro that andintermediate intermediaterocks. rocks. and '4 possible feeder for the Hoviand Lavas. Schematic N-Scross crosssection sectionofofFig. Fig.1 1showing showingthe the possible feeder for the Hovland Lavas. Fig.2.2.Schematic Fig. N-S 36 �East West Resource ResourceCorporation Corporation(EWR) (EWR) has has undertaken undertakenaa detailed detailedexploration explorationprogram program in the vicinity of Norton Lake, including an extensive drilling program. Detailed Detailed examination examination of of being paid paid to to the the mineralized mineralized 'main' zone drill core has been undertaken with special attention being zone to to determine the exact nature of the mineralization. Preliminary Preliminaryresults results indicate indicate the the deposit denosit consists consists of massive pyrrhotite with pentlandite, magnetite, chalcopyrite and pyrite. pyrite. The The platinum group element's element's (PGE's) (PGE's)are arefound foundforming formingdiscrete discreteplatinum platinumgroup groupminerals mineralsand andare arealso alsobelieved believedto to form a solid solution with the sulphides. sulphides. Results ~ e s u i tshow sshowthat that in inaddition additionto toprimary primarymineralization mineralization a secondary, hydrothermal, enrichment enrichment of PGE's PGE's has has taken takenplace. place. Mineral Mineral Pyrrhotite Pyrrhotite Pentlandite Pentlandite Pyrite Chalcopyrite Chalcopyrite Manganoan fllmenite Illmenite 1 Magnetite Michenerite Hessite Hessite Formula Fe,.$ Fei.S (Fe,Ni)9S8 (Fe,NihSs FeS2 CuFeS2 CuFeSi (Fe,Mn)Ti03 (Fe,Mn)Ti03 Fe304 1 Fe304 1 PdBiTe Ag2Te 1 Ag2Te Elements Minor Elements Ni Co Co Co Co Ni Notes Notes Main mineral mineral Secondary Secondary Trace Trace, also also veins More common common than magnetite, magnetite, easily mistaken for magnetite in polished section section 1 Sb, Pt Table 1: Table 1:Summary Summaryof of the mineralogy mineralogy of Norton Lake Lake deposit. deposit. Corfu F. and Stott G.M. 1996. 1996. Hf isotopic composition and age constraints constraints on on the evolution evolution of of the Archean Central Uchi Subprovince, Ontario, Canada. Precambrian Research, v. 78, p 53-63 East West Resources Resources Corporation, Corporation, 2001. 2001. Annual Annual Report. Report. PGM Ventures, Ventures, 2003. 2003. www.pgm-ventures.com Stott 0. G.M. M.and andCorfu Corfu F. F. 1991, 1991,Uchi Uchi Subprovince, Subprovince, in Geology of Ontario, Ontario, Ontario Geological Survey Survey Special Special Volume Volume 4, 4, Part Part1.1. 38 �STRATIFORM STRATIFORM Pd-Pt-Au Pd-Pt-AUMINERALIZATION MINERALIZATION IN IN THE THE SONJU SONJU LAKE LAKE INTRUSION, INTRUSION, LAKE COUNTY, MINNESOTA JOSLIN, 1114 Kirby JOSLIN,Gregory GregoryD. D.Department Departmentof ofGeological Geological Sciences, Sciences, University of Minnesota-Duluth, 11 14 Kirby Drive, MILLER, James D., Jr., Drive, Duluth, Duluth, MN MN 55812, 55812,email: email:ioslOOl3@d.umn.edu; josl0013@d.urnn.edu;MILLER, Jr., Minnesota Minnesota do NRRI, 5013 Miller Trunk TrunkHwy, Hwy,Duluth, Duluth,MN MN 5581 55811; and ROWELL, ROWELL, Geological Survey, c/o 1; and 6th William, St., Minneapolis, MN 55402. 55402. William, F., Franconia Minerals MineralsCorp., Corp.,12 12S. S. 6 St., The Sonju Lake intrusion intrusion (SLI) is a 1200 1200 m thick, closed-system, well-differentiated, tholeiitic, layered intrusion located within the Mesoproterozoic Midcontinent Rift-related Beaver Bay Complex of northeastern Minnesota (Miller and and Chandler, Chandler, 1997). 1997). In the late 1990's, 1990's. outcrop outcrop sampling by Miller (1999) indicated indicated the presence of meter-scale stratiform Pd-Pt-Au mineralized POE reef) within the oxide gabbro interval (or PGE gabbro unit of the SLI, located about 2/3 of the way up intrusion. In from the basal contact of the intrusion. In June of 2002 Franconia Minerals Corp. conducted exploratory drilling exploratory drilling through through the the Pd-Pt-Au Pd-Pt-An enriched enriched zone. zone. In hand sample, the mineralized interval appears as a homogeneous oxide gabbro, with no enrichment. However, geochemically the location of the visible indication of precious metals enrichment. mineralization is distinct. Three Three drill drill cores, cores, spanning spanning aa strike strike length of approximately 800 m, define define and are are correlated correlated on the the basis basis of of aa distinctive distinctive Cu-Au Cu-Au break datum (Fig. 1). 1). With the exception exception of localized Pt enrichment enrichment associated with an interval interval enriched in olivine olivine about about 110 110m m below the Cu-Au horizon, all Pd-Pt-Au enrichment occurs over an interval of 0 to 90 m below the defined datum (Fig. 2). In In general general precious precious metals metals peaks are stratigraphically stratigraphically offset from one another, progressing upward upward in in the the succession successionPd->Pt->Au. Pd-*Pt-)Au. Maximum Maximumgrades gradesin in 0.3m 0.3m long long core core samples are 410 ppb Pd, 275 ppb Pt, and 1080 ppb ppb Au. Au. Above the Cu-Au break, all precious metals are very strongly depleted. Strong Strong correlation correlation between Fe. Fe, Al A1 and modal olivine with precious metals peaks indicates indicates a possible possible connection connection between subtle subtle modal layering layering of of plagioclase, plagioclase, oxide, oxide, and and olivine olivine with withmineralization. mineralization. The oxide oxide gabbro-hosted POE PGE reef in the Sonju Sonju Lake intrusion intrusion shows shows marked similarities, similarities, with some differences, Skaergaard intrusion intrusion of of East East differences, to stratiform stratiform POE PGE mineralization in the Skaergaard Greenland (Andersen et al., 1998), 1998). the Rincon del Tigre Complex of Bolivia (Prendergast, (Prendergast, 2000), 2000). intrusions throughout throughoutthe the world. world. Whole rock and many other tholeiitic mafic layered intrusions geochemistry, clinopyroxene clinopyroxene and olivine compositions, compositions, and petrographic data are are consistent consistent with with an orthomagmatic origin for the mineralization related to the fractional fractional segregation segregation of of sulfide sulfide magma. The melt from silicate magma. The homogeneity homogeneity of the host rock, the thickness of the mineralized interval, and the offset of metal concentrations concentrations imply that sulfide sulfide saturation saturation was was passively passively thggered by fractional crystallization of the Sonju magma. Mungall (2002) recently triggered Mungall(2002) recently argued argued that that stratigraphic POE reefs can be satisfactorily stratigraphic offsets of Pd, Pt, Au and Cu peaks common to many PGE satisfactorily model of of sulfide liquation liquation and and settling. settling. The model shows explained by a kinetic model shows that that the the degree degree will be be controlled controlled by by kinetic kinetic factors, factors, such such as as the the diffusivity diffusivity of of of offset and metal enrichment will of sulfide supersaturation, sulfide sulfide droplet droplet size, and its its settling chalcophile elements, the degree of result in in variability of of the apparent apparent silicate/sulfide silicate/sulfide melt melt ratio ratio (R (R factor). factor). The velocity, which result The correlation correlation of multiple multiple peaks peaks of POE PGE with with subtle, subtle, broad broad modal modal variations variations may may be be related related to to repeated convective overturn caused by the crystallization of of magnetite magnetite in in an environment environment of of sulfide over-saturation, over-saturation, as suggested by Prendergast (2000) (2000) to explain a similar similar correlation correlation in in the the Rincon del Tigre Complex. Some Some evidence evidence of late-stage sulfide dissolution and remobilization remobilization exists, but it appears to have little to no effect effect upon upon the the distribution distribution of of precious precious metals. metals. 39 �SLO2-3 SLO2-2 5L02-1 mno,3 ibo,e Co'Aob,,ak rlTrm,T +70.0 — +70.0 +80.0 — +600 —+000 4105 — +40.0 +300 — +35.0 +10.0 — +20.0 .300,0 +30,0 — lddk!4- Fig. Fig.1:1:Correlation Correlationofofdrill drill cores 1, 5L02-2, coresSLO2SL02-1, SL02-2,and and SLO2-3 SL02-3showing showingdistinctive distinctive 00 1=11— .10.0 — ''0.0 -10.0 — '20.0 — 30,0 WWIII— — .40.3 -40.0 Cu-Au Cu-Aubreak. break.The TheCu-Au Cu-Au break breakisisused usedtotoprovide provideaa datum datumtotowhich whichall all stratigraphic stratigraphicplots plotsare are correlated, correlated,and andposition positioninin stratigraphy stratigraphyisismeasured measuredasas meters metersabove aboveororbelow belowCu-Au Cu-Au break. break. '50.0 -10,0 — '00.0 11214- — '70.0 -700 — '00+ 11414- Will- — '90,0 ._,I000 —-lion -100.0 112111— -110.0 —'120.0 -l 00.0 — -000,0 [It'i—' Cu'Au break 8 • Au(ppb) Cu ppm) mete's above Cu-Au break 5L02-3 SLO2-2 SL02-1 — +700 — +700 — +700 t00.0 — +600 — -+600 +50,0 —*500 —+50,0 +400 — 5400 — +400 300 — +30.0 — +30.0 200 — +20.0 — 420.0 — +100 — +10.0 +10.0 — -10.0 — -100 — -20,0 — - 00i 20,0 — -500 — -30,0 — 420 — '400 — .000 — -50.0 '600 — -00.0 -700 — -700 — .70.2 000 — .800 — -00.0 — 900 — -00.0 — -00.0 1000 __..,1000 —100.0 7100 —110.0 —-110.0 —-1200 —010.0 —'120.0 300 — '132.5 — -130.0 —00 -400 888 Fig. Fig.2: 2:Correlation Correlationof of Pd Pd and Pt in drill holes SLO2and Pt in drill holes SL021,1,SLO2-2, SL02-2,and andSLO2-3. SL02-3. Notice Noticemultiplicity multiplicityof of spikes spikesand andoffset offsetbetween between Pt Ptand andPd Pdpeaks. peaks. I 2 § § U Pd)ppb) W Po(ppb) References: References: Andersen, Rasmussen, Andersen. J.J. C. C. 0., O., Rasmussen,H., H.,Nielsen, Nielsen,T. T. F. F.D., D., Ronsbo, Ronsbo,J.J. G., G., 1998, 1998,The The Triple TripleGroup Groupand andthe the Platinova PlatinovaGold Goldand andPalladium PalladiumReefs Reefsininthe theSkaergaard SkaergaardIntrusion: Intrusion:Stratigraphic Stratigraphicand andPetrographic Petrographic Relations. 488-509. Relations.Economic EconomicGeology. Geology.Vol. Vol.93, 93,pp. pp.488-509. Miller, Miller,J.J. D. D. Jr., Jr.,1999, 1999,Geochemical GeochemicalEvaluation Evaluationof ofPlatinum PlatinumGroup GroupElement Element(PGE) (PGE)Mineralization Mineralizationininthe the Sonju 44,31 3 1p. p. SonjuLake Lake Intrusion, Intrusion, Finland, Finland,Minnesota: Minnesota: Minnesota Minnesota Geological Geological Surv. SUN.Information InformationCircular Circular44, of the Beaver Miller, J. D., Jr., and Chandler, V. W., 1997, Geology, petrology, and tectonic significance Miller, J. D.. Jr., and Chandler, V. W.. 1997, Geology, petrology. and tectonic significance of the Beaver Bay Bay Complex, Complex,northeastern northeasternMinnesota, Minnesota,inin Ojakangas, Ojakangas,R. R.W., W., Dickas, Dickas,A. A.B., B., Green, Green,J.J.C., C.,eds., eds.,Middle Middle Proterozoic Proterozoicto toCambrian CambrianRifting, Rifting, Central CentralNorth North America: America:Geological GeologicalSociety Societyof of America AmericaSpecial SpecialPaper Paper 312, p. 73-96. 312, p.73-96. Mungall, E.,2002, 2002,Kinetic KineticControls Controlson onthe thePartitioning Partitioningof ofTrace TraceElements ElementsBetween BetweenSilicate Silicateand andSulfide Sulfide Mungall,J.J. E.. Liquids. Liquids. Journal Journal of of Petrology. Petrology. Vol.43, Vol. 43, pp.749-768 pp. 749-768 Prendergast, Tire Complex, D., 2000,Layering 2000,Layering and Precious Metals Mineralization in the Rincon del Tigre Complex, Prendergast, M. M. D.. Eastern Bolivia. Economic Geology. Vol. 95, pp. 113-130. Eastern Bolivia. Economic Geology. Vol. 95, pp. 113-130. 40 �RESULTS OF SINGLE-GRAINANALYSES ANALYSESOF OFPRECAMBRIAN PRECAMBRIAN MAFIC MAFIC RESULTS OF40Ar/39Ar '")~r/^'>~r SINGLE-GRAIN INTRUSIONS IN IN NORTHERN NORTHERN ANI) AND EAST-CENTRAL EAST-CENTRALMINNESOTA MINNESOTA INTRUSIONS KEA'ITS, M.J., Kent, OH OH44242; 44242;JIRSA, uRSA, M., KEATTS, MJ., Dept. of Geology, Kent State University, Kent, St. Paul, MN 551 14-1057; Minnesota Geological Geological Survey, Survey, 2642 University University Avenue West, St. Minnesota 55114-1057; HOLM, HOLM, D., Dept. Dept. of of Geology, Geology, Kent Kent State StateUniversity, University, Kent, Kent,OH OH44242 44242 Age information from mafic mafic intrusive intrusive suites suites is critical Age information from critical for for proper proper interpretation interpretation of the the geologic history and for mineral deposit models in the Lake Superior region. region. As As part of an effort to evaluate evaluate PGE potential potential in mafic mafic intrusions intrusions in Minnesota, Minnesota, several several plutons plutons have been dated dated using the CO2 Ar/Ar incremental heating technique at the University of of Wisconsin-Madison Wisconsin-Madison COi laser ArIAr Rare Rare Gas Gas Geochronology Geochronology Laboratory. Laboratory. For late-stage late-stage shallow shallow plutons plutons containing containing primary primary magmatic homblende, Ar/Ar ArIAr mineral mineral ages are likely likely to to closely closely approximate approximate the crystallization crystallization magmatic hornblende, age. In regions regions with with aamore moreprotracted protractedthermal thermalhistory history(i.e., (i.e.,low-grade low-grademetamorphism, metamorphism, slowslowcooling, etc.), the Ar/Ar intrusions from ArIAr data provide minimum ages for the mafic plutons. Mafic intmsions Minnesota selected for for this study study represent aa broad range range of of geologic geologic settings, settings, including including1) 1)small small mafic supracrustal and and intrusive intrusive rocks rocks within mafic plutons plutons emplaced emplaced into Paleoproterozoic Paleoproterozoic supracrustal within the Penokean orogen (samples 264, R17); and 2) varied, varied, primarily latelate- to post-tectonic post-tectonic intrusions intrusions in in supracrustal rocks of the Archean Archean Wabigoon Wabigoon (samples Al, Al, B21, UBD) and Wawa (samples K15, LP, ANA) subprovinces of Superior Province. We We report report here here the initial results from LP, subprovinces of Superior Province. from eight eight separate intrusions (Fig. 1). separate intrusions (Pig. 1). A homblende East-central Minnesota. (R17) from East-central Minnesota. A hornblende grain grain (R17) from aa sample sampleofofmedium-grained medium-grained hornblendite from the Tibbett's Brook intrusion cutting the homblendite the East-central East-central Minnesota Minnesota batholith batholith in in Morrison Co. yields yields aa plateau date of 0.006 Ga from contiguous increments from 44 contiguous increments Momson Co. plateau date of 1.770 1.770 ±Â 0.006 constituting 74% of of the gas released. constituting 74% released. AAbiotite biotitegrain grain(264) (264)from fromaasample sampleofofcoarse-grained coarse-grained biotitic olivine gabbronorite gabbronorite cutting the Little Falls Formation in Morrison Momson Co. yields yields aa plateau plateau constituting 68% of the gas released. date of 1.791 ± Â0.008 0.008 Ga from 5 contiguous increments constituting Wawa Subprovince. A hornblende grain (ANA) from a sample sample of of prismatic prismatic homblende hornblendediorite diorite collected near Red Lake in Beltrami Co. yields a near-plateau date of 2.587 ±0.012 a . 0 1 2 Ga Ga in in 55 nonnoncontiguous incrementsconstituting constituting50% 50%ofofthe thegas. gas. A A biotite biotite grain grain (K15) (K15) from from a sample contiguous increments sample of biotite granodiorite porphyry collected in Norman Co. yields aa plateau date date of 2.639 2.639±Â 0.007 Ga 0.007 Ga from 6 contiguous increments constituting 79% 79% of of the gas released. A biotite grain (LP) from a sample of porphyritic syenite collected at the the Wawa-Quetico Wawa-Quetico subprovince subprovinceboundary boundary in in St. St.Louis Louis Co., 0.006 Ga from from 77contiguous contiguous Co., in the the Linden Linden Pluton, Pluton, yields yields aa plateau plateau date date of 2.666 2.666 ±Â 0.006 increments constituting constituting 88% of the gas released. grain (£321 (B21) from the Oaks intrusion leucodiorite sampled Wabigoon Subprovince. A hornblende grain near Fault in in Roseau 1 ±Â 0.008 0.008 Ga from 88 near the Vermilion Vermilion Fault Roseau Co. yields yields aa plateau plateau date date of of 2.67 2.671 contiguous increments increments constituting constituting75% 75%ofofthe thetotal totalgas gasreleased. released. A A hornblende homblende grain grain (Al) (Al) from the Black River gabbro, collected in Roseau Co., yields a plateau date of 2.685 ± 0.011 Ga from from 2.685  0.011 11 contiguous increments increments constituting constituting90% 90%of of the the total total gas gas released. released. A hornblende hornblende (UBD) (UBD) from a sample collected in Koochiching Co. north of the sample of homblende-biotite hornblende-biotite gabbro collected the Rainy Rainy LakeLakeSeine River Fault Fault yields yields aa plateau plateau date date of of 2.695 Seine River 2.695 ±  0.007 0.007 Ga from 66 contiguous contiguous increments increments constituting 49% of the gas released. released. 41 �_____________ __________________________ ___ The mineral age data data from mafic mafic plutons plutons from from the the Wabigoon Wabigoon subprovince subprovince are are synchronous synchronous Mafic plutons plutons from from the the with the last deformation event (D2) dated in the range 2.685-2.674 Ga. Ga. Mafic Wawa subprovince give an 80 m.y. Interestingly, the Linden m.y. age age range range from from 2.58 2.58 to to 2.66 2.66 Ga. Ga. Interestingly, Linden Pluton gives a biotite Pluton biotite date date concordant concordant (within (within error) with the youngest youngest mafic pluton from the Wabigoon subprovince. subprovince.The Theyounger youngerspread spreadofof ages ages from from the Wawa Wabigoon Wawa are are consistent consistent with with southward growth of the Superior Province Province during the the latest latest Archean. Archean. Mafic plugs plugs evident evident from aeromagnetic maps in in east-central Minnesota are comagmatic with the circa 1.775 aeromagnetic maps east-central Minnesota comagmatic with 1.775 Ga Ga EastEastcentral Minnesota batholith. batholith. Further constraining the temporal framework of mafic mafic intrusions intrusions may contribute to mineral deposit models models for for PGE may contribute to mineral deposit PGE in these these intrusions intrusions and and their their analogs analogs in in adjacent states states and and provinces. provinces. Fig.1 °Ar/"Ar age Fig.l "ArPAr age spectra: spectra: t, = plateau age, age, çt, == total total gas gas age. age. 2.0 2.0 2.0 R17 I 264 1.5 o t_.._1 :. e 1.8 1.0 0.5 3.0 Btite MSWD Biotite MSWD2.57 2.57 Amphibole MSWD 1.78 1.770±0.006 t, = 1.770  0.006 Ga Ga t9 = 1.653 1.653  ± 0.005 0.005 Ga p Ga [.1 t==1.791 t, 1.791±0.008 t 0.008 Ga Ga 1.782  ± 0.007 Ga t,t9 == 1.782 1.5 3° ANA .. o H- — 2 25 Amphibole MSWD MSWD 0.30 = 2.671 ± 0.008 Ga t9 = 2.705 ± 0.013 Ga Amphibole MSWD Amphibole MSWD 0.71 0.71 = 2.587 ± 0.012 Ga = 2.550 ± 0.009 Ga 2.0 3.0 2.0 3.0 K15 — t: 2.5 TTL..c.H 2.5 . 2 Amphibole Amphibole MSWD MSWD1.81 1.El Biotite MSWD MSWD 2.48 2.46 t, = 2.639 ±Â 0.007 Ga Ga 2.685 ± t, = 2.685 Â0.011 0.011 Ga Ga = 2.640 ± 0.007 Ga t = 2.700 ± 0.010 Ga 2.0 — LP % Ir11 — m —LI 2.5 2.5 2 Amphibole MSWD MSWD 1.03 1.03 ± 0.007 Ga t, = 2.695  = 2.730 ± 0.006 Ga Biotite MSWD 1.11 = 2.666 ± 0.006 Ga = 2.657 ± 0.006 2.0 - 0 0 2.0 100 0 $00 0 —. . 100 $ 20 20 30 30 40 50 SO 60 60 700 7 80 Cumulative "Ar "Ar released (%) 90 SO 42 10 10 20 20 30 40 S O 50 60 60 7 700 80 "Ar released Cumulative "Ar released (%) 90 90 300 100 �Ontario New zircon ages from from the the Gunflint and Rove Formations, Formations, northwestern northwestern Ontario Kissin, Kissin, S.A., Department Departmentof of Geology, Geology,Lakehead Lakehead University, University,Thunder ThunderBay, Bay,ON, ON,P7B P7B5E1 5E1Canada, Canada, stephen.kissin@lakeheadu.ca stephen.kissin@lakeheadu.ca ;;Vallini, Vallini, D.A., D.A., University University of Western Australia, Australia, 35 35 Stirling Stirling Hwy, Crawley, 6009, W.A., iWO, Canada; W.A., Australia; Australia; Addison, Addison, W.D., RR 2, Kakabeka Kakabeka Falls, Falls, ON, ON, POT 1W0, Canada; Brumpton, Brumpton, G.R.., 211 21 1 Henry Henry St, St, Thunder Thunder Bay, ON, P7E 4Y7, Canada. Canada. Previous work based on U-Pb U-Pb geochronology geochronology from presumed volcanogenic volcanogenic zircons zircons obtained obtained from a tuff layer at the lower/upper ± 2Ma, lowerlupper Gunflint Formation boundary yielded an age of 1878 1878  2Ma, believed to approximate the age of deposition of of the the unit unit (Fralick et et al., al., 2002). This This age age corresponds closely with the age of the correlative Hemlock Formation of Michigan (1874 ±  corresponds closely Schneider et al., 9Ma; Schneider a]., 2002). 2002). 9Ma; We report here preliminary age determinations determinations based on SHRIMP SHRIMP analyses of zircons extracted from three volcanic ash layers; one lying in the Gunflint Gunflint Formation, Formation, and and two within the overlying Rove Formation. The The Gunflint-Rove Gunflint-Rovecontact contact is an an important important reference reference point. Pufahl and Fralick (2000) placed it at the top of a sequence sequence of chert-carbonate chert-carbonate grainstones grainstones which which is overlain by carbonaceous shales of of the the Rove Rove Formation. Formation. The Gunflint Gunflint exposure exposure outcrops at Little Falls, on lOm (topographically) (topographically) below the -10m Little on the the south southside sideof of the theKakabeka KakabekaFalls FallsGorge, Gorge,— Gunflint lapilli tuff dated by Fralick et al. (2002). A Rove volcanic ash ash exposure exposure at at Oliver Oliver Creek Creek is estimated (stratigraphically)above abovethe theGunflint-Rove Gunflint-Rove contact. contact. Zircons were also estimated to be —70m -70m (stratigraphically) extracted from an ash layer within Falconbridge Pine River (PR98-1) (PR98-1) drilicore drillcore (688.24m (688.24m down down contact. hole), located -Am -4m above the Rove-Gunflint contact. The Oliver a mean 207Pb/206Pb age of of 1821 1821 ± ""~b/^Pbage  16 single The OliverCreek Creekzircons zirconsrecorded recorded a mean 16 Ma while a single age of 1840Ma 1840Ma was obtained from the drillcore PR98-1 sample. sample. The The errors errors cited cited are are at at the the one one standard deviation (10) ( l a ) and and 95% 95% confidence confidence level level and and the the analyses analysesare are less lessthan than5% 5%discordant. discordant. There recordedfrom from each each locality locality which are assumed to There are are also also two two younger younger ages ages of of —1786Ma -1786Ma recorded outliers. be outliers. The Little Falls zircons, zircons, which are are somewhat somewhat rounded rounded and and fractured, fractured,yielded yielded various various than 2000Ma. 2000Ma. Most Most of of the the ages ages are more more than 10% 10% discordant, discordant,and and these these samples samples ages, all older than may have suffered lead loss. As As well, well, there there are are some some indications indicationsof of admixture admixtureof of shalely shalelymaterial material in the ash layer at this locality. Older Older zircons zircons from from the the lapilli lapilli tuff tuff layer layer at at the the lower/upper lowerlupper Gunflint Gunflint contact (Fralick (Fralick et al., a]., 2002) 2002) were also found found to be admixed admixedwith with Paleoproterozoic Paleoproterozoic zircons. zircons. Using the stratigraphic column of of Pufahl and Fralick (2000), we estimate that the drillhole (PR9S-i) Gunflint lapilli (PR98-1)samples samplesare—i are -1 lOm 10m above the Gunflint lapilli tuff tuff layer layer containing containingthe the zircons zircons dated dated by Fralick et al. (2002), (2002),while while we we estimate estimatethe theOliver OliverCreek Creeksamples samplestotobebe—iSOm -150m above this same layer. The ages reported here indicate that a slow sedimentation sedimentationrate ratemust must have havebeen been required in order to account for the age difference difference between between the the lapilli lapilli tuff tuff of Pufahl and Fralick Fralick and the two sets of Rove dates reported here. here. This This slow Rove Rove sedimentation sedimentation rate is comparable that reported in banded iron formations formations of the early early Proterozoic Proterozoic Campbell CampbellGroup, Group,Griqualand, Griqualand, West Sequence, Sequence, South Africa (Barton et al., 1994). 1994). 43 �Oliver Creek reasonable agreement The Oliver Creek ages ages reported here are are in reasonable agreement with the the 1833 1833±Â 6Ma age reported by Schneider Schneider et a!. al. (2002) for the Tobin Lake Pluton, which is undeforined undeformed by Penokean deformation and intrudes presumed Hemlock Volcanic equivalents. However, However,the the with the the Penokean Penokean zircons from the Rove Formation suggest that volcanic activity associated with continued for for at at least least 40 40 m.y. m.y. Further Orogeny continued Furtherstudies studiesare are underway underway to clarify some of the questions raised by our results. results. Barton, E.S., Altermann, Smith, C.B. 1994. U-Pb Altermann, W., William, 1.5., I.S., amd Smith, U-Pb zircon zirconage agefor foraatuff tuff in in the Campbell Group, Griqualand West Sequence, South Africa: Implications for Early Proterozoic Proterozoic rock rock accumulation accumulation rates. rates. Geology Geology 22: 22:343-346. 343-346. Fralick, P., Davis, D.W., and Kissin, S.A. 2002. 2002. The Ontario, The age age of the Gunflint Formation, Ontario, reworked volcanic ash. Canadian Canada: single zircon U-Pb age determinations from reworked Journal of Earth Earth Science Science39: 39:1089-1091. 1089-1091. Pufahl, P. and Fralick, P. 2000. Fieldtrip 4: Depositional environments environments of the Paleoproterozoic Paleoproterozoic Gunflint Formation. Proceedings of the Institute Institute on on Lake Lake Superior SuperiorGeology, Geology, 46, 46, pt.2. D.A., Bickford, Bickford, M.E., M.E., Cannon, Cannon, W.F., W.F.,Schulz, Schulz,K.J., K.J.,and andHamilton, Hamilton,M.A. M.A.2002. 2002. Age of of Schneider, D.A., formations, Marquette volcanic rocks and syndepositional iron formations, Marquette Range Range Supergroup: Supergroup: implications Paleoproterozoic iron formations implications for for the tectonic tectonic setting setting of Paleoproterozoic formations of the Lake Lake Superior Superior Region. Canadian Canadian Journal Journal of of Earth Science Science 39: 39:999-1012. 999-1012. 44 �OF THE THE SOUTHERN SOUTHERN PORTION OF O FTHE THELAURENTIDE LAURENTIDE ICE ICE MEAN TRANSPORT TRANSPORT LENGTH LENGTH IN TILLS OF SHEET: IMPLICATIONS FOR DRIFT EXPLORATION EXPLORATION IN THE THE LAKE LAKE SUPERIOR SUPERIOR REGION REGION LARSON, Phillip Phillip C., C., Department of Minnesota, MN 55812, 55812, LARSON, Department of Geological Geological Sciences, University University of Minnesota, Duluth, Duluth, MN plarson2@'d.umn.edu plarson2@d.umn.edu Introduction Bedrock - till, till, Bedrock in the the Lake is typically typically covered covered by by aamantle mantleofofglacigenic glacigenicsediments sediments— Lake Superior region is outwash, that presents significant challenge challenge to successful successful application application of of surficial surficial outwash, and lacustrine lacustrinesediments sediments—- that presents a significant geochemical techniques techniques widely widely used used to help targets. The glacial glacial environment environment is is very very complex, complex, geochemical help generate generate drilling drillingtargets. with sediments produced produced by by a range of processes. with sediments processes. Till represents the ideal sampling sampling media media in in these theseenvironments, environments, direction) is attached to since a vector (ice flow direction) to the the composition composition at any location location indicating indicatingthe thedirection directiontotothe thesource source of any defined anomaly. anomaly. However, recent work has led to recognition that both the the magnitude magnitude of of aa till till geochemical geochemical anomaly andthe the potential potential transport transportdistance distancetotoits itssource sourcemay mayhave haveaawide widerange rangeofofvalues. values. This is a reflection of anomaly and transport distance of till-forming material, material, and is related the mean transport related to the the fundamental fundamental sediment sediment transport transport process process responsible for forming forming the till. responsible till. Theory The concentration concentration of an indicator (a distinct distinct lithologic or geochemical component derived from a discreet in till till is the direct product of the physical processes processes of of glacial glacial erosion, erosion, transport, transport, and and deposition. Indicator source) in Indicator concentrationisis controlled controlledby by aa number number of of variables, including substrate substrate hardness hardness and and the the efficacy of the glacial concentration variables, including erosional regime. ice flow indicator mass regime. Under Under steady steady state state ice flow conditions conditions and and uniform uniform bed bed erosion erosion rates, rates, indicator mass concentration c in (1) down-ice down-ice of of an an indicator indicator source sourceof of finite finiteflow-line flow-linelength lengthLL(1) (1) is: is: concentration ci intill till atatany anytransport transportlength lengthTT(1) — L (1) where X X isis the the erosion erosion length length scale scale (I). (1).For Fortills tills down-ice down-ice of of the the indicator indicator source, under steady state state conditions, conditions, the decrease &/ST assumes assumes aa quasi-exponential quasi-exponential form. form. decrease in indicator indicator concentration concentration with with increasing increasing transport transport length length bc/8T Erosion length length scale, scale, \X,isisrelated relatedto to the the spatial spatial bed bed erosion erosion rate rate E E ((mL3) and the thickness of the debris layer in Erosion m r ) and (m-1.): transport md 1d (mV2): (2) increases, so so does the mean transport XXis is closely related to the mean transport distance distance of till-forming material; as X X increases, distance. distance. vs. Long-Distance Long-Distance Transport: Transport: Examples Short- vs. Tills in the Lake Superior Superior region can be broadly broadly grouped grouped into two two categories categories based based on onthe themean meantransport transport length of the till forming material. mean transport length Tills characterized characterized by short-distance short-distance mean length are commonly commonly composed composed of of coarse-grained coarse-grained material containing abundant abundant angular angular clasts, clasts, and and display rapid rapid decrease in indicator material containing indicator concentration concentration with with transport transport length. This This isis exemplified exemplified by by tills tills overlying overlying the the Vermilion Vermilion greenstone greenstone belt of of northern northern Minnesota, Minnesota, which which displays displays rapid decrease decrease in in concentration concentrationofofnumerous numerousindicator indicatorlithologies; lithologies; X for range from 2.Oi0m. m. A ?for . thisthis tilltill range from 1.01.0 to to 2.0-103 rapid dispersal train composed composedofof clasts clasts of of Nipigon Nipigon diahase diabase in in till east dispersal train east of of Lake Lake Nipigon. Nipigon, Ontario Ontario displays displays similar similar characteristics. Dispersal in indicator indicatorconcentration; concentration;calculated calculated?\for forthis this Dispersal isis characterized characterizedby by aa similar similar rapid rapid decrease decrease in till range 102 m over relatively relatively soft greenstone greenstone to to 2.4-lo4 2.4 i04 m m over over hard hard diabase. diabase. Both range from from5.5 5.5-10' Both tills tills are are interpreted interpreted to to have formed by by erosion of of hard hard bed bed by quarrying and abrasion, and englacial transport. transport. Tills characterized by long-distance Tills characterized by long-distance mean mean transport transport length are are commonly commonly composed composed ofoffine-grained fine-grained material with aa relatively low abundance of rounded material with relatively low abundance of rounded clasts, and display display little little apparent apparent decrease decrease in inindicator indicator with transport transport length. length. Cretaceous shale grains in Des Moines Lobe tills of the Minnesota concentration with Minnesota River valley show relatively relatively little little decrease decrease in in concentration concentration along along the the flow flow axis axis extending extending down down the the valley valley (Matsch, 1972); the calculated X Xfor for this this till till is is 5.0-lo5 5.01 5 m. m. Carbonate-bearing Carbonate-bearing tills overlying the Canadian Shield north of Lake Lake Superior Superior (Thorleifson and and Kristjansson, Kristjansson, 1993) show similar long-distance transport of Paleozoic Paleozoic carbonate carbonateand andProterozoic Proterozoic greywacke clastswith withlittle little decrease decrease in in concentration; concentration;the thecalculated calculated1.Xfor forthis thistill till is is o.oio5 6.0-10m. m. Both tills are are greywacke clasts have deposited deposited from deforming subglacial sediment layers (deformation (deformation tills). interpreted to have 45 �Discussion Discussion tills are are characterized characterizedby by1).that that are 10 to to 100 times The data indicate deformation tills times higher than those of of thin, thin, coarse-grained tills. Tills characterized by intermediate intermediate values of the erosion erosion length length scale scale X 1. have not as as yet yet been been recognized in the Lake Superior region, and perhaps perhaps do do not not exist. This Thisgap gapininrecognized recognized values values suggest suggest there there are are two main processes by which which bed bed material material is is eroded eroded and and entrained, entrained,transported, transported, and anddeposited deposited— - entrainment entrainment and and transport by a deforming subglacial subglacial layer, and erosion by quarrying and abrasion and transport transport as as an an englacial englacial debris debris load with deposition deposition by lodgement or meltout. Deformation tills form with with little little accompanying accompanying erosion of underlying underlying hard hard bedrock. bedrock. Their formation is is consistent with with redistribution of unconsolidated regolithoror sediment derived from consistent redistribution of unconsolidated regolith sediment derived from aa preglacial preglacial reservoir. reservoir. Consequently,till till composition composition reflects reflectsthat that of of distant distant (>100 Consequently, (>lo0 km) km)bedrock. bedrock. Tills characterized characterized by short short mean mean restricted erosion erosion and and entrainment and and transport of hard bedrock. transport length indicate spatially and temporally restricted Their formation is consistent consistent with erosion erosion by quarrying quarrying and abrasion abrasion of of hard hard bedrock bedrockwith withsubsequent subsequentextensive extensive textural Till composition km) bedrock. bedrock. textural modification during during transport. transport. Till composition closely reflects reflects that thatofofnearby nearby(—10 (-10 1cm) Consequently, these tills have enormous potential value as geochemical sampling sampling media. Recognition of the process process responsible responsible for till till formation formation in in aa given given area areaisiscritical criticalfor forsuccessful successfulapplication application of surficial geochemical and boulder tracing exploration techniques techniques in the Lake Lake Superior Superior region. region. Limited scope scope orientation surveys surveys aimed aimed at at characterizing characterizingthe theerosion erosionlength lengthscale scale 1?. provide provide a means orientation means of of quickly quickly assessing assessing the potential for successful application of drift exploration techniques on both both regional and property scales. 46 �Glacial Glacial Lakes Lakes Aitkin and Upham: their origin origin and and environmental environmental history history PhillipLarson Larson LisaMarlow, Marlow,Howard HowardMooers, Mooers,&&Phillip Lisa Department Departmentof ofGeological GeologicalSciences, Sciences,University Universityof of Minnesota, Minnesota, Duluth, Duluth, Minnesota Minnesota55812 55812 Glacial Minnesota bounded Glacial Lakes Lakes Aitkin Aitkin and and Upham Upham occupied a basin in north-central Minnesota bounded on on the the north northby by the the Giants GiantsRange Range and and to to the the east, east, south, south,and and west west by by hummocky hummocky moraines moraines of of the the Rainy Rainy lobe, lobe, Superior Superior lobe, lobe, and St. Louis sublobe. The The lakes lakescaine came into into existence existence with the the retreat retreat of of the the Rainy Rainy lobe lobefrom fromthe theSt. St.Croix Croix phase phase sometime sometimeafter after15,000 15,000yr yr BP BP (Clayton (Clayton and and Moran, Moran,1982; 1982; Mooers Mooersand andLehr, Lehr, 1997). 1997).The Thebasin basinwas waslater lateroveridden overiddenby by the the St. St. Louis Louis sublobe sublobe from from the the northwest. northwest.With Withthe thewastage wastageofofthe theice iceof ofthe theSt. St.Louis Louissublobe, sublobe,the the basin basin was was again againoccupied occupiedby by lakes; as Lake Lake AitkinAJpham I and the later phase as lakes; the the earlier earlierphase phase is is referred referred to as as Lake AitkinlUpham Aitkin/Upham II.Sediment Sedimentof ofthe theearly earlylake lakephase phaseisispreserved preserved at at aa few localities. localities. One Onesuch such AitkinAJphamII. locality locality preserves preserves aa sequence sequence that helps redefine the glacial glacial chronology. AAsedimentary sedimentarysequence sequence located locatedin in the thenortheast northeastcorner comerof of the the Upham Upham basin basin reveals reveals sub-aqueously sub-aqueously deposited depositedRainy RainyLobe Lobe outwash outwashbeneath beneathglaciotectonically glaciotectonically deformed deformedfine-grained fine-grained lake lake sediments sediments deposited depositedby by the theSt. St. Louis Louis sublobe. sublobe. This, This,along alongwith withother othergeomorphic geomorphicrelationships relationships(P.C. (P.C.Larson, Larson,unpublished unpublisheddata) data) indicates indicatesthat thatthe theRainy RainyLobe Lobe ice ice margin margin was was coincident coincidentwith with the the Giants Giants Range Range when when the theSt. St. Louis Louissublobe sublobeadvanced advancedacross acrossthe thelake lakebasin. basin. Using @EM) the the elevation elevationof of lake lakebasin basin was was adjusted adjustedfor forisostatic isostatic Using aa Digital DigitalElevation Elevation Model Model (DEM) rebound based on the highest lake level, then tilted incrementally through several stages rebound based on the highest lake level, then tilted incrementally through several stagesto toassess assess beaches, beaches, inlets, inlets, and and outlets outlets over time. AAseries seriesof ofsuccessively successivelylower lower outlets outletsdraining drainingto tothe theSt. St. Louis Louis River Riverserved servedas as outlets outlets for for Glacial Glacial Lakes Lakes Aitkin Aitkin and and Upham Upham (Hobbs, (Hobbs,1983; 1983;Farnum, Farnum,1964; 1964; Wright, Norwood through through the the Embarrass Embarrass Wright, 1972). 1972).Meltwater Meltwaterentered enteredthe thelakes lakesfrom fromGlacial GlacialLake LakeNorwood gap, gap,and and later laterfrom fromGlacial GlacialLake LakeKoochiching Koochichingalong alongthe thePrairie PrairieRiver. River. During During this this time time Aitkin Aitkin and and Upham Upham were were confluent, confluent,and and the the outlet outlet was was established establisheddown down the the modern modem St. St.Louis LouisRiver. River. The The lakes lakes were were separated separated by by aa sill sill ca ca 11,500-10,100 11,500-10,100yr yr BP, BP, after after inflow inflow from from Koochiching Koochichingwas was divertedto to Glacial GlacialLake LakeAgassiz. Agassiz. diverted Extensive Extensive dune dune fields fields formed following initiation initiation of drainage drainage of the lakes. Granulometry Granulometry indicates throughout the thebasin. basin. Maximum indicates a 4p 4(pgrain grain size size signature characterizes dunes throughout Maximum dune dune amplitude metersand anddune dunemorphologies morphologiesrecord recordprominent prominentnorthwesterly northwesterlywinds. winds. Dunes amplitude is is —3 -3 meters overly overly source source areas, areas, which include include an underfiow underflow fan fan deposited deposited by by the the Prairie Prairie River Riverinlet inletand andthe the western margin marginof of Lake Lake Upham. Upham. western A sediment sediment core core collected collected from Hay Lake (93°W, (93'W, 52°N), 52ON), located located within aa dunefield dunefieldat atthe theedge edge of Glacial Glacial Lake Lake Upham, records three prominent peaks in whole-core whole-core magnetic magnetic susceptibility susceptibility between No clastic clasticinput inputisisevident evidentafter after6,600 6,600yr yrB.P., B.P.,suggesting suggestingdune dune between 10,100 10,100and and 6,600 6,600 yr BP. No stability. The timing of dunes within the basin has important implications for other dunes throughout Minnesota. Eolian Eolianevents eventsrecorded recordedin in the the core coreare areinterpreted interpreted as as the the result resultof of lake lake drainage drainage and exposure exposure of abundant abundant source material material during during Late Late Glacial Glacial and and Early Early Holocene Holocene rather rather than than landscape landscape destabilization destabilization because of mid-Holocene aridity (Keen et al., 1990; 1990; Grigal Grigal et et al., al., 1976; 1976;Dean Dean et et al., 1996; 1996; Dean, 1997). 1997). Additionally, Additionally,this thissediment sedimentcore core places places aa minimum minimum age on the drainage of of Glacial Lakes Lakes Aitkin Aitkin and and Upham Upham II. II. Lake Upham must have drained drained after 47 �age on the drainage of Glacial Lakes Aitkin and Upham IiT. I. Lake Upham must have drained drained after 11,500 B.P. and Lake Aitkin may have persisted until ca. 7,000 10,100 yr B.P. 7,000 yr 11,500 yr BP and before 10,100 BR BP. Clayton, L. and Moran, S.R. 1982, 1982, Chronology Chronology of late Wisconsinan glaciation glaciation in in middle middle North North America. Quaternary Quaternary Science ScienceReviews Reviews11(1), (I),55-82. 55-82. Ahlbrandt, T.S., Dean, W.E., Ahlbrandt, T.S., Anderson, Anderson, R.Y., R.Y., Bradbury, Bradbury, J.P., 1996. 1996.Regional Regional aridity aridityin in North North America during the middle Holocene. Holocene. The The Holocene Holocene 6 (2), 145-155. 145-155. Dean, W.E., 1997. 1997. Rates, Rates, timing, timing, and and cyclity cyclity of Holocene eolian activity activity in in north-central United United States: 1-334. States: Evidence Evidence from from varved varvedlake lakesediments. sediments.Geology Geology25 25(4), (4),33 331-334. H.E., Jr. Jr. 1964. A Late-Wisconsin Late-Wisconsin buried soil near Farnham, R.S., McAndrews, J.H., and Wright, H.E., Minnesota, and its paleobotanical setting. 393-412. Aitkin, Minnesota, setting. American American Journal Journal of of Science Science262, 262,393-412. Drexier, 1985. 1985. Late-Wisconsinan and Holocene Holocene History History of of the the Lake Lake Superior Superior Farrand, W.R. & Drexler, In Karrow, Karrow,Quaternary Quaternary evolution evolution of of the the Great Great Lakes, Lakes, Geological Geological Association Association of of Canada Canada Basin. In Special Paper 30, 30, 17-32. 17-32. D.F., Severson, R.C., R.C., Golz, G.E., 1976. Evidence Evidence of of eolian eolian activity activity in north-central Grigal, D.F., Geological Society Society of America America Bulletin Bulletin 87, 87, 1251-1254. 1251-1254. Minnesota 8,000 to 5,000 yr. ago. Geological Aitkin, Upham, and early Lake Hobbs, H.C. 1982, 1982, Drainage relationships of Glacial Lakes Aitkin, Agassiz in northeastern Minnesota. In In Teller, Teller,J.T., J.T., and and Clayton, Clayton,Lee, Lee,eds., eds.,Glacial GlacialLake LakeAgassiz: Agassiz: Geological Association of Canada Special Paper 26, 245-259. Canada Special Paper 26,245-259. K.L., Shane, L.C.K, L.C.K, 1990. A continuous record record of of Holocene Holocene eolian eolian activity and Keen, K.L., Lake Ann, east-central east-central Minnesota. Minnesota. Geological vegetation change at Lake Geological Society Societyof of America America Bulletin Bulletin 102, 1646-1657. 102,1646-1657. Mooers, Mooers, H.D., H.D., Lehr, J.D., 1997. 1997. Terrestrial record of Laurentide ice sheet reorganization reorganization during Heinrich events. events. Geology Geology 25 25 (11), (ll), 987-990. 987-990. H.E. 1972, Quaternary Quaternary history historyof ofMinnesota. Minnesota In In Sims, P.K., and Morey, G.B., GB., eds., Wright, H.E. eds., Geology of of Minnesota: A 15-578. A Centennial Centennial Volume: Volume: Minnesota MinnesotaGeological GeologicalSurvey, Survey,5515-578. 48 �and Hydrothermal HydrothermalPGE PGEMineralization Mineralizationof ofthe theBirch BirchLake LakeCu-Ni-PGE Cu-Ni-PGEDeposit Deposit Magmatic and in the South northeast Minnesota South Kawishiwi Intrusion, Duluth Complex, northeast Minnesota John Marma, Mama, Phil Brown and Steve Steve Hauck* Hauck* University of of Wisconsin, Wisconsin, Madison, Madison, Wisconsin Wisconsin 53706, USA Department of Geology Geology and Geophysics, University *Natat Resources Research *Natural Resources ResearchInstitute, Institute,University Universityof ofMinnesota, Minnesota, Duluth, Duluth, Minnesota 55811, 55811, USA USA The The Birch Birch Lake Lake Cu-Ni-POE Cu-Ni-PGE Deposit is located 12 12 miles south south of Ely, MN in the the South South Kawishiwi Intrusion (SM) (SKI) of of the the Duluth Duluth Complex (DC). The TheSKI SKIisisone oneof of two twolayered layered mafic mafic intrusions intrusions along along the the basal contact contact of the the DC to host sub-economic Cu-Ni-POE Cu-Ni-PGE deposits. deposits. Mineralization is is dominantly dominantly hosted by the U3 layer, the lower-most of three ultramaficultramafictroctolite packages characterized characterized as a zone of alternating ultramafic (picrite-peridotite) (picrite-peridotite) and troctolite troctolite horizons horizons with with lenses lenses and and pods of oxide-bearing oxide-bearing (>5%) (>5%) ultramafic ultramafic and/or and/or massive massive oxide. oxide. The purpose of this study study was to locate, describe, describe, and characterize the textural relationships relationships among platinum group group minerals minerals (POM), (PGM), sulfides, sulfides, and silicate silicate phases to help help delineate delineate the the relative significance significanceof primary and remobilized platinum group group element element (POE) (PGE) concentrations. concentrations. Samples Samples from from 4 drill drill holes transecting the Birch Lake Deposit were obtained obtained from from the the (NRRI) located located in in Duluth, Duluth, MN. MN. EMPA and Natural Resource Research Institute (NRRI) and detailed detailed c15.tm in petrography were used to locate POE PGE bearing minerals, averaging <l5yan in diameter, diameter,and andto to geochemistry. Identifying POM textural relationships with characterize the host mineral geochemistry. Identifying the PGM other phases is critical to understanding the mechanism mechanism by by which which PGMs POMs were were deposited. deposited. Data from this study study will aid exploration exploration in locating other other deposits deposits and and guide guide metallurgists metallurgists in in improving recovery recovery techniques. techniques. POEs occur most often as various Pd minerals with associated PGEs associated Pt, Pt, Os, Os, Ir, Ir, Ru, Ru, Au, Au, Ag, Ag, Te, Te, Bi minerals categories of silicate-sulfide-PGM silicate-sulfide-PGMtextural textural minerals and and were were grouped grouped into into the following following 4 categories relations: PGMs that occur in "halos" "halos" residing most commonly commonly in anorthite-enriched anorthite-enriched zones zones in• in relations: 1) PGMs primary plagioclase around either interstitial sulfide sulfide (dominantly (dominantly chalcopyrite), chalcopyrite), interstitial interstitial sulfide sulfide and silicate silicate (dominantly (dominantly chalcopyrite, chalcopyrite, clinopyroxene, clinopyroxene, and hydrous silicates silicates (amphibole (amphibole and and biotite)), or silicate or hydrous hydrous silicate) silicate) (Figure (Figure 1). This This style style of silicate (dominantly clinopyroxene or POMs identified. 2) 2) Remobilized RemobilizedPOMs PGMsthat thatoccur occurin in mineralization hosted 58% of the total PGMs chlorite, serpentine, or secondary magnetite. magnetite. This chlorite, This style style of of mineralization mineralization hosted hosted 21% 21% of of the the total total PGMs identified. 3) 3)Random RandomPOMs PGMsthat that occur occur in in poikilitic poikilitic anorthite-rich anorthite-richplagioclase plagioclase(An (An75-An 75-An POEs sometimes residing in disseminated 95) and clinopyroxene (Wo 30-Wo 50) with PGEs chalccipyriteor orhydrous hydroussilicate silicate pockets, pockets,but butno no association association with with "halos". "halos". This style of chalcopyrite POMs identified. 4) 4)In In interstitial interstitialsulfides sulfidesor orsilicates silicatesthat that mineralization hosted 11% 11% of the total PGMs include (?) textures, or include hydrous silicates, silicates, chalcopyrite, chalcopyrite, clinopyroxene, clinopyroxene, sulfides sulfides with symplectite symplectite ('1) calcite. This style calcite. style of mineralization mineralization hosted 10% 10% of the total total POMs PGMs identified. identified. POM concentrations in The following following is a summarized summarized model for the formation of high PGM the Birch Lake deposit. The SKI begins as a magma body that is replenished The SKI begins as a magma that is replenished with with multiple multiple saturated. The injections of magma, which becomes sulfur saturated. The magma magma body body is is relatively relatively POE PGE poor, poor, the conduits with a PGE POE enriched due to partial loss of sulfides during emplacement leaving the segregation of the total sulfide. The The sulfides sulfides in in the the magma magma body body scavenge scavenge available availablePOEs PGEs and and grains. Primary crystallize as disseminated, interstitial sulfide grains. Primary hydrous hydrous phases phases form form at at this this time time 49 �from a fluorine-rich, deuteric fluid. fluid. A Cu-, PGE-rich, sulfide-poor fluid A Cl-, Cu-, fluid enters enters the the magma magma chamber at its base via the original magma conduit(s) and/or faults. faults. The The fluid fluidmigrates migrates along along grain boundaries, and interacts with the larger interstitial sulfides. A A dynamic dynamicenvironment environmentisis created created in which the fluid, fluid, containing containing aa significant significantconcentration concentration of of dissolved dissolvedmetals, metals,begins beginsto to consume and use sulfur from the larger grains to produce more more sulfides. At At the the same sametime, time, the the fluid is reacting with neighboring neighboring grains, grains, specifically specifically plagioclase, plagioclase, and and through through aa cation cation exchange exchange reaction, alters the plagioclase rims, enriching them in calcium. This This reaction reaction causes causesaa volume volume loss that is filled with precipitated (±chlorite) producing the disseminated precipitated sulfides and POMs PGMs (±chlorite "halos" Finally, another anotherfluid fluid migrated migrated through through "halos" around the larger interstitial grains (Figure 1). Finally, the intrusion that remobilized POMs on a small scale. intrusion remobilized PGMs on a small scale. Based on evidence evidence solely from the Birch Lake deposit, deposit, POM PGM mineralization mineralizationappears appears "compartmentalized". This concentrated or "compartmentalized. This could could be the result of two possible mechanisms: 1) Areas of high PGM POM concentrations are dependent on on their proximal proximal distance to "feeder" "feeder" zones (i.e. conduits or faults) where fluids can be introduced; introduced; and/or 2) High POM PGM concentrations concentrationsare are that localize fluid movement. due to structural controls within these heterogeneous rocks that of primary primary vs. vs. remobilized remobilized This study contributes to the current debate on the roles of (deuteric?) PGE PGE mineralization mineralization in in layered mafic mafic intrusions. intrusions. For the Birch fdeuteric?) Birch Lake Lake deposit, deposit, the the data data suggest both mechanisms played important roles in the origin of the ore minerals. Figure Figure 11 a.) Photomicrograph Photomicrograph in in plane-polarized plane-polarizedlight lightof ofthin thinsection sectionBL BL89-2 89-22516.4 2516.4—- locations locations A-L. chalcopyrite and and pyroxene pyroxene cross-cut cross-cut by by vertical verticalchlorite chlorite veins, veins, all all of of which which are Large interstitial chalcopyrite by a disseminated, dominantly dominantly chalcopyrite chalcopyrite halo halo that that is is in in An-enriched An-enriched plagioclase surrounded by rims. Notice Notice all all PGMs, PGMs, except except one, either occur in the halo; in chlorite veins; in interstitial biotite; or in clinopyroxene. The The altered altered plagioclase plagioclase and altered pyroxene on the left side of the occurrences -this —thisincludes includesareas areaswithin withinthe theoriginal originalhalo. halo. This image are devoid of any PGM occurrences fluid event that that removed PGMs and altered the minerals, which it suggests a second alteration fluid passed through. Dashed line represents the extent of halo and An-enrichment in the adjacent plagioclase grains. grains. White stars represent PGM POM occurrences. occurrences. Cpx=Clinopyroxene, plagioclase Opx=Orthopyroxene, Bi=Biotite, Bi=Biotite, Chl=Chlorite, Chl=Chlorite, Cpy=Chalcopyrite, Plag=Plagioclase Plag=Plagioclase 50 �EMP MONAZITE RESULTS OF EMP MONAZITEGEOCHRONOLOGY GEOCHRONOLOGYIN IN B-C E-C MINNESOTA: MINNESOTA: EVIDENCE EVIDENCE FOR LARGE-SCALE GEON 17 17 METAMORPHISM ASSOCIATED WITH POSTPLUTONISM TECTONIC PLUTOMSM MCKENZIE, M.A., and HOLM, D.K., both at Dept. of Geology, Kent State University, Kent, OH; SCHNEIDER, D.A., Dept. of Geological Sciences, Ohio University, Athens, OH; OH, SCHNEIDER, JERCII4OVIC, M., M., Dept. of Geosciences, Geosciences, University of Massachusetts, Massachusetts, Amherst, JERCINOVIC, Amherst, MA MA Determination of the Determination the timing timing and and extent extent of of poly-phase metamorphism metamorphism is is essential essential in in unraveling the the tectonic tectonic history history of of a region. region. The The pattern and degree of metamorphism preserved across the Penokean orogenic belt in the southern Lake Superior region is highly variable. dates fromeast-central east-centralMinnesota Minnesotaindicate indicatewidespread widespread cooling cooling at -1760 0~r/3Q dates ~ r from Abundant 440Ar/39Ar 1760 Ma shortly after the emplacement emplacementof ofthe theeast-central east-centralMinnesota Minnesotabatholith batholith(ECMIB) (ECMB)atat—1775 -1775 Ma Ma (Hoim (Holm et al., 1998; in review). Yet YetU-Pb U-PbSHRIIvIP SHRIMP monazite ages from three localities localities across the northern MI, region (e-c MN, northern MI,and andnorthern northernWI) WI)record recordonly onlyaauniform uniform—1830 -1830 Ma metamorphic metamorphic episode and episode and aa secondary secondaryyounger younger—1800 -1800 Ma thermal pulse linked linkedto toaarecently recentlyidentified identified—1800 -1800 Ma magmatic event event (Schneider et al., in in review). review). This study utilizes the total Pb electron (EMP) monazite age dating technique to better constrain the extent of thermal thermal microprobe (EMP) overprinting surrounding the batholith. overprinting surrounding batholith. in situ situ metamorphic metamorphic monazite monazite ages ages from from three three For this study we obtained obtained in Paleoproterozoic metasedimentary garnet-staurolite schist samples and one garnet-cordierite garnet-cordierite from the the plutonic plutonic zone of of east-central east-centralMinnesota Minnesota (Figure (Figure 1). 1). Schist gneiss sample from Schist sample sample AMpredominantly elongate monazite grains displaying a mottled chemical variation in 016 contains predominantly Th content. content. This ± 33 Ma from 79 79 spots spots on on seven seven Y and Th This sample sample yielded yielded aa mean age of 1746 1746  Maand and 1760 1760Ma. Ma. A third grains. Two Two age age domains domainswere were recognized recognizedatat—1738 -1738 Ma third less less prominent —1780Ma Maage agedomain domainwas wasobtained obtainedon onsome somehigh highYYregions. regions. Schist Schist sample -1780 sample MN-29 MN-29 contains contains sub-euhedral monazite monazite displaying prominent prominent regions regions of of high highTh Th content. content. This This sample sample yielded a 10 Ma from 92 spots 1764±Â 10 spots on five grains. A A single singleprominent prominent age agedomain domain was was mean age of 1764 recognized atat— 1772 1772 Ma. Schist Schistsample sampleP-16 P-16contains containsmonazite monazitewith with very very irregular irregular grain grain recognized numerous inclusions, inclusions, and variable Th Th content. content. This sample sample yielded yielded aa mean age age of boundaries, numerous grains. Two 1772 1772±Â 33 Ma from 92 spots on seven grains. Two age domains are identified: aaprominent prominent age age domain at —1770 Maand andaasmaller smaller population population age domain at —1800 Ma. Lastly, the Sartell -1770 Ma -1800 Ma. Sartell euhedral monazite gneiss, sample S-2, contains euhedral monazite grains displaying displaying distinctive distinctive core/rim corelrim textures. textures.  3 Ma from 102 spots on seven grains. Three Three age age This sample yielded a mean age of 1756 1756 ± domains are identified: two 1750 Ma and 1770 1770Ma Ma and and aa third third less less twoprominent prominentdomains domainsatat— 1750 Ma on high U cores. cores. prominent domain domain at at —1800 -1800 Ma Mathermal thermalimprint imprint associated associated with intrusion reveal aa profound profound —1770 -1770 Ma Our EMP results reveal of the 1775 Ma Ma ECMB. ECMB. The The considerable considerable distance distance of of some some of these these samples samplesfrom from the the western western edge of of the exposed exposed batholith (30-40 km) and the absence of Penokean metamorphic ages absence metamorphic ages suggests that that the the thermal thermal pulse pulse must have been been dramatic. dramatic. However, KHowever, the the garnet-schist garnet-schist sample sample KR (east of Mille Lacs) that records only geon 18 18 SHRIMP ages lies north of the the region of of thermal thermal of the the batholith. batholith. We influence of We note note that that sample sample K-R K-R is is located located just north north of of the the Malmo MaimoStructural Structural Discontinuity (MSD) (MSD) and and sample sample AM-016 is is located located south south of of it. it. Our Discontinuity Our data data reveal reveal that that the the MSD MSD juxtaposes rocks of different metamorphic metamorphic age (geon 18 18 metamorphism to the north from from geon - - 51 �ba' C) CD . '° <o ,__._ toaaa C') Epp CD .-,. 0 •L rQ5o o ECDCD -t r> to u — — " Un J•fl1 tr4 0 t nfl ni C)g <'o— rM -t. flH hU >E;•<'z g :h UH 17 metamorphism to the south). We propose, therefore, that the MSD is a geon 17 structure that exhumed the plutonic terrane of east-central Minnesota. West of Mille Lacs, a significant portion of the MSD juxtaposes post-Penokean plutons to the south against older metamorphic rocks to the north. This clearly supports our interpretation that this structure was active well after Penokean orogenesis. z __c_ C C S S a a I Frequncy P-16 Composite - AllSamples All Ages Histogram F) 01 52 CD 0 0, 'C to 0 I Figure 1: Histograms of BMP Th-U-total Pb in situ monazite spot ages. . ¶. a 0 ) .:po S. H 0 a Frequency 5 5 Frequency a —— I, •1 0 C 0, a C, 0 3 <'c C, .. C) MN-29 Conposte AM-016 Composite t o C,, pp —— Holm, D.K., Dan-ah, K., andLux, D., 1998, American Journal of Science, 298,60-81. Holm. D.K., Van Schmus, W.R., MacNeill, L., Boerboom, T., Schweitzer, D., and Schneider, D.A., in review, Geological Society of America Bulletin. Schneider, D.A., Holm, D.K., O'Boyle, C.,Hamilton, M., Jercinovic, M., in review, Geological Society of America Special Volume "Gneiss Domes and Orogeny." �THE SIOUX QUARTZITE REVISITED: SEDIMENTOLOGY, METAMORPHISM, GEOCHEMISTRY AND THE ORIGIN OF PIPESTONE GEOCHEMISTRY MEDARIS, L.G., L.G., Jr., Jr., and DOTT, R.H., Jr., Dept. of Geology & Geophysics, University University of Wisconsin-Madison, Madison, medaris@geology.wisc.edu; Wisconsin-Madison, Madison,WI, WI,53706; 53706; medarisgeology.wisc.edu;rdott@geology.wisc.edu rdottgeoIogy.wisc.edu Red, supermature supermature quartzites of the Baraboo Interval were deposited after 1.75 1.75 Ga on a stable craton craton in in the presence of free atmospheric oxygen under under conditions conditions of of intense intense chemical chemical weathering. weathering. Some atmospheric oxygen Some quartzites quartzites (Baraboo and and Flambeau) Flambeau) were were folded folded and and recrystallized recrystallizedatat 1.63 1.63 Ga Ga (HoIm ci aT, al., 1998), 1998), and and many many (Baraboo (Holm et quartzites were hydrothemally hydrothermallyaltered alteredatat1.46 1.46 Ga Ga (Medaris (Medaris etci al., 2002, quartzites were 2002, in in press), press), presumably presumably in in response promoted by by continental continental scale scale A-type A-type magmatism. magmatism. These discoveries response to brine migration migration promoted discoveries have prompted us to reevaluate the sedimentology, metamorphism, and geochemistry of the Sioux Quartzite. Sedimentolozy The Sedimentolo.eg TheSioux SiouxQuartzite, Quartzite, which which isisseveral several hundred hundred meters meters thick, thick, is is composed composed mostly mostly of quartz sandstone with interstratified ci al., 1986). 1986). Heterogeneous Heterogeneous interstratified lenses of red mudstone (Southwick et cobble conglomerate occurs at the base and finer pebbly layers are scattered throughout the lower half half or so, whereas mudstones Sedimentary structures in the sandstones mudstones occur chiefly chiefly within the upper upper half. half. Sedimentary sandstones include predominant - 15 15 cm in in thickness, thickness, rare rare predominant festoon-style, festoon-style, nested trough cross cross beds beds averaging averaging 10 10— zones sets, a few examples zones of planar-tabular planar-tabular sets, examples of herring herring bone bone cross cross bedding, bedding, and and both both asymmetric asymmetric and and symmetric ripple marks. The mudstones are mostly massive, massive, but but parallelparallel- laminated laminated and andripple-laminated ripple-laminated varieties are also also present. present. In In most most cases, cases, quartz quartz silt silt and and fine fine sand in a finer varieties are sand grains grains are disseminated disseminated in rare graded graded laminations laminations are arealso alsopresent. present. Some mudstones mudstones show polygonal polygonal 'mud' 'mud' cracks, matrix, but rare cracks, and and the overlying sandstones commonly contain intraclasts ripped up from such cracked beds Interpretations of the Sioux depositional environment include shallow marine and braided fluvial Interpretations (Doff, 1983; 1986). In In the the latter scenario, the cross bedded et a!., a!., 1986). bedded sandstones sandstones represent represent river river (Dott, 1983; Southwick Southwick ci' channel deposits, and the mudstones, channel deposits, mudstones, slack water deposits in in ponds ponds between between active active channels. channels. However, However, this interpretation is inconsistent with the the rarity of scoured channel bases bases and and tabular sets of planar cross inconsistent with laminations, which would would have have formed formed by by laterally laterally migrating migrating bars, bars, and and the existence of wave laminations, which wave ripples, ripples, which expected in in the the sands sands of of aa braid braid plain. plain. Ojakangas (1984) suggested which are not expected Ojakangas and Weber (1984) suggested that that the the one-third of the Sioux formation formation was was deposited deposited in a shoreline marine setting with tidal influences, upper one-third accounting for the polygonal desiccation desiccation cracks, cracks, and the herringbone herringbone cross bedding, wave ripples, polygonal and thickness and extent of certain certain mudstone mudstone layers layers (now (now pipestone). pipestone). Interpretation Interpretation of of the the Sioux Sioux as asaa fluvial-to-marine fluvial-to-marine transgressive succession successionwould wouldconform conformtoto the present transgressive present interpretation interpretation of of the thecorrelative correlativeBaraboo Baraboo Quartzite, which has wave ripples ripples and and reactivation reactivation surfaces surfaces in in its upper upper half half (Medaris (Medaris et et al., al., in press). Metamorphism Metamorvhism Mineral Mineralassemblages assemblages in in finefine5 grained grained Sioux sedimentary rocks can be 5 expressed in the expressed in the system, system, KASH, KASH, as as portrayed portrayed in in a:4 0:4 Figure 1, where rock compositions are projected compositions projected onto the anhydrous onto anhydrous plane, plane, K-Al-Si, K-Al-Si, and and two two 3 critical dehydration reactions reactions are plotted. Additional phases include abundant hematite and Additional 22 a Ti02 TiO; phase, phase, either either anatase anatase in in the theCottonwood Cottonwood 1989), or rutile in Basin (CB) Basin (CB) (Stelz, (Stelz, 1989), in the Pipestone Basin (PB). The stable of 11 Pipestone (PB). The stable existence existence of the CB kaolinite in the CB (Stelz, (Stelz, 1989) 1989) requires temperatures below -300°C -100°C, whereas temoeratures whereas pyronvro250 350 T, 300 phyllite in in the the PB PB is is stable stable above above-300°C -100°C. The T, 0C OC phyilite 250 300 53 �quartz-pyrophyllite assemblageininthe the PB PB (0, (0, Fig. quartz-pyrophyllite assemblage Fig. 1), I), ininwhich which vermicular vermicular kaolinite kaolinite has has been been replaced replacedby bypyrophyllite pyrophyllite(Fig. (Fig.2A), 2A), most likely likely represents represents higher highertemperature, temperature, largely largelyisochemical isochemical most recrystallization of ofaaquartz-kaolinite quartz-kaolinite protolith, protolith, like likethat thatininthe theCB CB recrystallization The occurrence occurrence of muscovite muscovite in both both basins basins isis (0, Fig. Fig. 1). 1). The (C>, attributed to to K-metasomatism K-metasomatism related related to to 1.46 1.46Ga Gahydrothermal hydrothermal attributed (+,Fig. Fig.1)1)isisaametasomatic metasomaticrock rock composed composed of of activity. Pipestone Pipestone(+, activity. pyrophyllite, muscovite, diaspore, diaspore, hematite, hematite, and and rutile, rutile, ininwhich which pyrophyllite, former quartz quartz grains grains have have been been completely completely replaced replaced by bydiaspore, diaspore, former Because the Sioux 2B). Because the Sioux pyrophyllite, and muscovite (Fig. pyrophyllite, muscovite (Fig. 2B). Quartzite Quartzite is is largely largely undeformed undeformed and and lies lies north north of of the the extrapolated extrapolated trend et al., al.,1998), 1998), trend of of the the 1.63 1.63 Ga GaMazatzal Mazatzal tectonic tectonic front front (HoIm (Holm et we we suggest suggest that that all all metamorphic metamorphic features features of of the theSioux SiouxQuartzite Quartzite are are due due to to 1.46 1.46Ga Gahydrothermal hydrothermal activity, activity, rather rather than than aaMazatzal Mazatzal event. event. Geochemistry Geochemistq Where Whereunmodified unmodifiedby byK-metasomatism, K-metasomatism, finefinegrained grained sedimentary sedimentary rocks rocks of of the theBaraboo BarabooInterval Intewalare areremarkably remarkably mature, mature, being being practically devoid devoid of of K, K, Na, Na, Ca, Ca, Mg, Mg, and and Mn Mn (Fig. (Fig. 3), values of 97 to 99. 31, and and having Critical Index of Alteration Alteration values 99.InIn such 3) and and such rocks rocks the the wide wide range range ininproportion proportion of of Si Si to to Al A1(Fig. (Fig. 3) of quartz quartz to to kaolinite kaolinite in inthe the I), reflects the original original proportion proportion of quartz to to kaolinite, kaolinite, or orpyrophyllite pyrophyllite(Fig. (Fig. 1), quartz protolith sediments. protolith sediments. has stabilized stabilized K-metasomatism has muscovite muscovite in both the CB and and PB, PB, but but the muscovite-bearing muscovite-bearing rocks in the the CB CB 0 150 2 record aa lower lower temperature temperature and and higher higher record C ratio ratio of of Si/Al SiIAl compared compared to to pipestone pipestone in in 1uJ (Fig. 1). 1). The Theclassic classicpipestone, pipestone, the PB (Fig. o in addition addition to to substantial substantial KK contents, contents, in contains contains lower lower Si and and higher higher Al A1than than pyrophyllite in associated associated quartz quartz ++pyrophyllite that in samples (Figs. (Figs. 1 & 3). Assuming Assuming Zr Zr to to samples be an an immobile immobile element, element, isocon isocon be calculations indicate indicate that that the the mean mean calculations -50 pipestone pipestone composition composition was produced by by 65% Si02, S O 2 ,45 45 to to 55% 55% removal of 20 to 65% -100 Al Si Mn Fe 11 K Na Ca Mg Ti02, Ti02,35 35 to to 65% 65% Fe203, Fe203, and addition of 15 to 45% 45% Al203 A1203and and—800% -800% K20, K20, 15 pyrophyllite samples. samples. compared to the average average compositions compositions of of the two Si-rich Si-rich and and two two Al-rich Al-rich quartz quartz ++pyrophyllite compared The composition of of one one pipestone pipestone sample sample(*, (*, Fig. Fig. 1) 1) requires requires removal removal of of 68% 68% Si02 Si02 and and The reconstructed reconstructed composition addition addition of of50% 50%A1203 A1203during during metasomatism. metasomatism. Further investigation investigation is underway to provide a more more detailed detailed evaluation evaluation of of brine brinecompositions compositions and and metasomatic processes involved in this important, regional scale, 1.46 Ga hydrothermal event. metasomatic 1.46 Ga hydrothemal event. References Geol. Amer. References Dott, Dott,RH. R.H.Jr.Jr.(1983) (1983) Geol.Soc. SOC. h e r .Memoir Memoir160, 160,129-141; 129-141;HoIm, Holm,U. D. etetatal.(1998) (1998)Geology, Geology,v.v.26, 26, 907-910; Medaris, L.G., Jr. et at (2002) 48th Inst. Lake Superior Geol., 24-25; Medaris. L.G., Jr. eta!. L.G., Jr. et 01.(in (inpress) press) 907-910; Medaris, L.G., Jr. et aL (2002) 48th Inst. Lake Superior Geol., Jour. Ojakangas, R.W. Geol. Jour. Geol.; Ojakangas, RW. & & Weber, Weber,R.W. R.W.(1984) (1984)Minn. Mi. Geol. Sun'., S w . ,Rept. Rept.mv. Inv.32, 32,1-15; 1-15;Soutkwick, Southwick,D.L. D.L. etet at Amer. al.(1986) (1986)Geol. Geol,Soc. SOC. Amer.Bull., Bull.,v.v.97, 97,1432-1441; 1432-1441;Stelz, Stelz,D.E. D.E.(1989) (1989)M.S. MS.Thesis, Thesis,Wichita WichitaState StateUniv., Univ.,140 I40pp. pp. 850 54 �_____________ A geochemical investigation investigation of of Mesoarchean Mesoarchean metavoIcanic metavolcanic and and metasedirnentary metasedimentary rocks from from the the Rirch-Uchi Birch-Uchi greenstone greenstone belt Metsaranta, R., Hollings, P.P(Department of of Geology, Lakehead Metsaranta, R.,Fralick, Fralick,P.P.and and Hollings, . (Department Geology, LakeheadUniversity, Universio,Thunder ThunderBay Bay ON CAN, CAN, P7B 5 SE)) EI) Most Mesoarchean greenstonebelts belts in in the Western Most Mesoarchean grtGlfibL"fifie Western Superior Province are comprised comprised primarily of komatiite-tholeiite komatiite-tholeiite sequences and associated associated sedimentary sedimentary rocks (Thurston (Thurston and Chivers 1990). 1990). These These—2.9-3.0 -2.9-3.0 Ga Ga assemblages assemblages have have been been interpreted interpretedto to represent represent plume generated volcanism in oceanic plateau settings (for example, Hollings et al. al. 1999, 1999, Tomlinson et aL1999). a1.1999). This study is aa preliminary preliminary investigation investigation of metavolcanic metavolcanic and metasedimentary strata strata from from the Mesoarchean metasedimentary Mesoarchean Balmer assemblage assemblage of the the Birch-Uchi Birch-Uchi Rogers et et al. greenstone belt. belt. Rogers al. (2000) (2000) have have suggested suggested that, that, given given their theirgeochemical geochemical affinities and Nd isotopic evidence for contamination by older crust, volcanic rocks of the may represent represent aa continental continentalarc arcsetting. setting. This implies that the Balmer Balmer assemblage may Assemblage may represent a distinct tectonic setting from those proposed Assemblage may represent a distinct tectonic setting from proposed for other other B Mesoarchean rocks in the the Superior Superior Province. F'rovince. Sediment geochemistry geochemistry and and depositional depositional environment studies studies along along ¶C — / :t ...,/ with igneous igneous geochemistry will will be ..-:?;_ applied to provide further further constraint constraint on on applied / -, - — the possible tectonic setting of these the possible tectonic setting these — rocks. rocks. — r* '— •M ç' The The Birch-Uchi Birch-Uchi greenstone greenstone belt is located in the central located in central portion portion of of the the W It is Uchi (Fig.1). is Uchi Subprovince Subprovince (Fig.!). comprised comprised of three three volcanic units termed termed the Balmer, Balmer, Nanow Narrow Lake Lake and and spanning Woman assemblages, Woman assemblages, spanning approximately approximately 250 Ma. Ma. The Balmer Balmer assemblage is the assemblage the oldest oldest of these these volcanic units and volcanic units and has has U-Pb U-Pb zircon zircon ages from felsic volcanic horizons that suggest an an age of ca. suggest ca. 2975-2989 2975-2989 Ma Ma (Rogers et al., 2000). The stratigraphy (Rogers et al., The stratigraphy of the the Balmer Balmer assemblage assemblage is is divided divided into four four suites: suites: aalower lowersedimentary sedimentary sequence, aa mafic sequence, mafic volcanic volcanic suite and and petrographically distinct distinct felsic two petrographically felsic volcanic suites suites (Rogers volcanic (Rogers et et a1. al. 2000). 2000). Samples collected for for this study Samples collected study are are located in the the southern southern portion portion of of the the located 1- Location Location and generalized generalized geology geology of of study study Figure 1area and Birch-Uchi Greenstone belt (modified from from assemblage in the Woman Woman Balmer assemblage Stott and Corfu stott Cofi 1991) 1991) River/Bear RiverBear Lake area. These These comprise comprise 16 samples from the lower sedimentary sequence and 34 samples of the mafic mafic volcanic volcanic suite (2000). suite of Rogers Rogersetetal. a1.(2000). , J: - - —— L 144)t,Iun, Lake 55 �Field observations suggest that the the Balmer Balmer assemblage assemblage sedimentary rocks are turbiditic. turbiditic. Sediment geochemistry geochemistrywill will be be applied to constrain the source rocks compositions for Sediment constrain the these sediments. As As no no contact contact with with underlying underlying older rocks has been identified this might provide valuable about the preexisting provide valuable information information about preexisting older older crust. crust. Alternatively, the the may be be derived derived from the Balmer assemblage volcanics volcanics and and this this could support a sediments may hypothesis that that the the Balmer assemblage represents aa continental continental arc arc setting with hypothesis assemblage represents with the the sediments deposited in a fore-arc trench. trench. Volcanic rock samples samples appear The first first is aa Volcanic rock appear to fall fall into intotwo twocompositional compositional trends. trends. The tholeiitic trend trend comprised comprised of of primarily primarily tholeiitic tholeiiticbasalts basaltsand andandesites. andesites. The second tholeiitic second is is aa calc-alkaline trend trend of of andesitic to rhyodacitic The geochemistry of these calc-alkaline rhyodacitic compostion. compostion. The geochemistry of samples will be be applied setting for for these rocks samples will applied to suggest suggest a possible possible tectonic tectonic setting rocks and and implications of this in relation to to other other Mesoarchean Mesoarchean terranes. terranes. 400 C = C C * 300 Rhyolite Tr ac 0.1 N C Rhyodacite/ a >. 200 rachyA Andesite 0.01 N • I SIIJNP1 AndeslQ asa ..I 100 AIk.Bas SubAlkaline Basa t .001 .01 0.1 1 10 0 200 100 300 Zr ZT Nb/Y N bN Lithology Figure 3- Lithology Discrimination diagram for Assemblage Balmer Assemblage volcanics. Circles volcanics. Circles are q w e s are Tholeiitic trend ssquares caic-alkaline trend. calc-alkaline Figure 3-A plot of V vs Zr showing compostional compostional groups groups in Balmer Assemblage Assemblage volcanics. volca~cs. Circles are Tholeiitic trend squares are calc-alkaline calc-alkaline trend. trend. References: References: Hollings, P., Wyman, Wyman, D. D. and and Kerrich, Kerrich, R. R. 1999. Komathte-basalt-rhyolite Hollings. P,, Komatiite-basalt-rhyolite associations northern Superior Province greenstone belts: belts: significance of plume-arc volcanic associations interaction in the the generation generationof ofthe theproto protocontinental continentalSuperior SuperiorProvince. Province.Lithos Lithos 46: 137-162. interaction in 137-162. Rogers, N., McNicoll, V., V., van van Stall, C.R., C.R., and and Todinson, Tomlinson, K.Y. K.Y.2000. 2000. Lithogeochemical Lithogeochemical in the the Uchi-Confederation lJchi-Confederation greenstone greenstone belt, northwestern O Ontario: implications for studies in n h o : implications for Archean Archean Tectonics. Geological 16: 1lip. lp. GeologicalSurvey Surveyof of Canada, Canada,Current Current Research 2000-C 2000-C16: G.M., and andCorfu, Corfu,F. F. 1991. 1991. Uchi Subprovince. Tn: of Ontario, Ontario, special Stott, G.M., In: Geology of volume 4, 4, part part 1. Ontario OntarioGeological GeologicalSurvey, Sumey,pp pp 145-238. 145-238. P.C. and and Chivers, Chivers,K.M. K.M. 1990. 1990. Secular Thurston, P.C. Secular variations in greenstone sequence development emphasizing Superior Superior Province, Province,Canada. Canada. Precambrian Research. 46: 21-58 emphasizing Tomlinson, K.Y., K.Y.,Hughes, Hughes,D.J., D.J., Thurston, Thurston,PP.C., andHall, Hall,R.P. R.P. 1999. 1999. Plume Todinson, C , and magmatism and and cmstal crustal growth at 2.9 2.9 to 3.0 Gain Ga in the Steeprock and Lumby Lake Lake area, area, Western Western Superior Province. Province. Lithos 46: Superior 46: 103-136. 103-136, 56 �PETROLOGYAN1I ANJl PGE POTENTIAL GREENWOODLAKE LA= INTRUSION, INTRUSION, PETROLOGY POTENTIAL OF THE THE GREENWOOD CENTRAL CENTFULDULUTH DULUTH COMPLEX, COMPLEX,LAKE LAKECOUNTY, COUNTY,MINNESOTA MINNFSOTA MILLER, MILLER,James, James,D., D., Jr., Jr., Minnesota MinnesotaGeological GeologicalSurvey, Survey,mille066@tc.umn.edu milleO66@tc.umn.edu This This report report summarizes summarizes the results results of of aa petrologic petrologic and and metallogenic metallogenic study study of of drill drill core coreand and outcrop outcrop samples samples that that profile profile the the Greenwood Greenwood Lake Lake intrusion intrusion (GLI) (GLI) of of the the central centralDuluth Duluth Complex Complex (Fig. 1). The Thelittle littlethat thatwas wasknown knownabout aboutthis thisvery very poorly poorly exposed exposedlayered layeredmafic maficintrusion intrusionprior priorto to (Fig. 1). this this study study came came from from interpretation interpretation of of its itsaeromagnetic aeromagneticsignature, signature,seven sevendrill drillcores, cores,and andsparse, sparse, localized localized outcrop. outcrop. The Thepurpose purposeofofthis thisstudy studywas wastotoestablish establishthe theigneous igneousstratigraphy stratigraphyofofthe theGLI GLI and and to to evaluate evaluate its itspotential potentialfor forPGE PGEreef reefmineralization. mineralization. The The GLI GLI isis an anapproximately approximately two two kilometer-thick, kilometer-thick, sheet-like sheet-like intrusion intrusion that that dips dips gently (approximately (approximately10°) 10")to to the east east and covers an area of about in length) about 300 square square kilometers. For For this this study, study, 19 19bedrock bedrock drill drill cores cores (20 (20 to to 80' 8O'in length) were were acquired acquired in inearly early2002 2002along alongthe thewest—northwest-trending west-northwest-trending Erie/LTV ErieLTV railroad railroad and andpowerline powerline west west of of Lake Lake County County Highway Highway 22(Fig. (Fig. 1). I). Samples from these these cores cores and andfrom fromintermittent intermittent outcrops outcrops along the eastern eastern extent extent of of the the railroad railroad grade grade were weresubjected subjectedtotopetrographic petrographicstudy studyinin transmitted transmitted and reflected light, light, microprobe microprobe analyses of olivine olivine and and pyroxene pyroxene composition, composition, and and whole whole rock rock analyses analyses of of their theirlithogeochemistry, lithogeochemistry, including including platinum, platinum, palladium, palladium, and and gold gold concentrations. concentrations. The The results results of of the the drilling drilling and and petrographic petrographic study study show show that that the the igneous igneousstratigraphy stratigraphy of of the the GLI GLI can can be be grossly grossly subdivided subdivided into into aa lower lower troctolitic troctolitic zone zone(GLtr, (GLtr,0-650 0-650meters), meters),composed composed mostly mostly of of leucotroctolitic leucotroctoliticcumulates, cumulates,aa medial medial gabbroic gabbroiczone zone(GLog, (GLog,650-1800 650-1800meters), meters),composed composed of (GLfg,1800-2130 1800-2130meters), meters), of olivine olivine oxide oxide gabbro gabbro cumulates, cumulates,and and an an upper upper ferrogabbroic ferrogabbroiczone zone(GLfg, composed largely of magnetite magnetite gabbro gabbro (Fig. (Fig. 2). 2). The troctolitic troctolitic zone zone contains contains abundant, abundant, large large anorthositic and oxide gabbroic gabbroic inclusions, inclusions, presumably derived derived from from anorthositic anorthositic series seriescountry country rock. Although Althoughthe theGLI GLIisisaawell-differentiated well-differentiated intrusion intmsion that that formed formedas asan anopen openmagma magma system, system, microprobe data show that cryptic layering trends (such as Fo in in olivine, olivine,Fig. Fig. 2) 2) are are inconsistent inconsistent with formation by in situ crystallization crystallization differentiation. differentiation. This This and and other other evidence evidence (such (such as abrupt abmpt changes in lithology, lithology, leucocratic leucocratic compositions of troctolitic troctolitic rocks, rocks, and and suspect suspectcumulus cumulustextures textures of troctolitic troctolitic rocks) suggest suggest that that the the differentiated differentiated character character of of the theGLI GLIwas wasprobably probablyinherited inherited from from aa deeper deeper crustal crustal magma chamber, chamber, which which was itself itself undergoing undergoingopen opensystem systemdifferentiation. differentiation. The chemostratigraphy chemostratigraphy of chalcophile chalcopbile elements elements through through the the GLI GLI are are difficult difficultto tointerpret interpretinin such such a complex open magma system, but suggest that some potential for PGE reef mineralization may occur in the lower part of the gabbroic zone (Fig. 2). Below Belowthis thislevel, level,recharging rechargingmagmas magmas appear to have been undersaturated in sulfide, and copper copper and and sulfur sulfurconcentrations concentrationshigher higherin inthe the pabbroic zone (above 800 meters) indicate An unexpected unexpected result of this gabbroic indicate intermittent intermittent saturation. saturation. An study was the discovery of aa large, large, sulfide-bearing sulfide-bearing oxide oxide gabbro gabbroinclusion inclusionwithin withinthe thetroctolitic troctolitic zone. Aeromagnetic Aeromagnetic data data suggest suggest that this inclusion is aa conformable conformable tabular mass with a strike length of about 8 kilometers. The magnetic data further suggest The magnetic further suggest that that similar similar rock rock types types form form part of the footwall to the GLI, GLI. The The possibility possibility of of sulfur sulfur contamination contamination in in the the contact contact aureole aureole around this inclusion and along the base of the intrusion intmsion warrants further exploration of these areas for contact-type contact-typeCu-Ni-PGE Cu-Ni-PGE sulfide sulfidemineralization. mineralization. ., Funding for this project was provided to the Minnesota Geological Geological Survey Survey by aa grant grant from from the Minnesota State Legislature Legislature on the recommendation of the Minerals Minerals Coordinating CoordinatingCommittee. Committee. 57 �Generalized Generalized geology geology of of the the Greenwood GreenwoodLake Lake intrusion intrusionand and the the central ceneal Duluth Duluth Complex. Small Small dots denote denote drill d i l l hole hole and and Complex. diamonds denote denote outcrop outcrop locations locations along along diamonds ErieLTV railroad railroad tracks. tracks. Long Long dashed dashed lines lines Erie/LTV denote denote faults. faults.Intrusive Intrusiveunits unitsare: are: Figure 1. 1. Figure GLtr—-GLI GLtr-GLl troctolitic troctoliticzone zone GIog-GLIgabbroic gabbroiczone zone GIog—GLI GLtg—GLI GLfpGLlferrogabbroie ferrogabbroiczone zone MW—Mt. MW-Mt. Weber Weber granophyre granophyre CLLS—Cloquet CLLS-Cloquet Lake Lakelayered layeredseries series BEI-Bald Eagle Eagle intrusion intrusion BEI—Bald SKI—South SKI-South Kawishiwi Kawishiwi intrusion intrusion PAl—Partridge PRI-Partridge River Riverintrusion intrusion WMI—Western WMI-Western Margin Margin intrusion intrusion sew Layered Series Ferrogabbroic Femgatbr& Gabbroic Gabbmic S T-IIrc L____J Trociolluc Fetsc Series ri Morihositic l___J Series Norih Shore a Group Virginia Forniston Biwabik ranFormation Giants Range I,-: Granite o0 10 10 20 20 Kilometers Kilometers Sample locationo I I Au concentrations of Fo Foin in olivine olivine and and of of Cu, Cu, Pt + Figure 2. 2. Chemostratigraphy Chemostratigraphy of + Pd, Pd, and and Au wncentrations through thmugh the the Figure locations of of drill drill core Lake intrusion. intmsion. Stratigraphic Stratigraphic locations core (boxes) (boxes) and and outcrop outcrop (diamonds) (diamonds) Greenwood Lake samples and general are shown shown in in the left columns. g e n e d lithostratigraphy lithostratigraphy are columns. Large Largeinclusions inclusionsofofanorthositic anorthositic (ox gb) gb) are denoted. Abrupt series rocks (AS) and oxide gabbro (ox Abmpt increases increasesin inCu/Pd CuPd(arrows) (mows)may maymark mark sulfide saturation events. The The zone found most favorable favorable to to host host PGE PGE reef reef mineralization mineralizationis is identified. identified. 58 �Stratigraphy and the St. St. Croix Croix horst, horst, northwestern northwestern Stratigraphy andstructure structureof ofKeweenawan Keweenawan rocks of the Wisconsin U.S.Geological Survey, Sun'ey, Reston, VA S.W. Nicholson, and W.F.Cannon, U.S.Geologica1 The St. St. Croix Croix horst horst is is the the partially inverted inverted central graben graben of the the Midcontinent Rift System System (MIRS)that thatextends extendssouthwestward southwestwardfrom from western westernLake Lake Superior. Superior. It It is is bounded by the (MRS) Douglas fault on the the northwest northwest and and the Atkins Lake fault on the the southeast. southeast. Both Both are arenow now reverse faults, but may have been graben-bounding normal faults during rifting and and northern limit of of the the horst horst is is White's White's Ridge, a subsurface basement high volcanism. The northern evident in both seismic seismic and and gravity gravity data, data, which did not subside subside substantially substantiallyduring duringrifting rifting and against which rift volcanic and sedimentary rocks pinch out or become much much thinner. thinner. Lake Superior from the St. Croix White's Ridge Ridge effectively effectively separates separates the MRS in western Lake horst and the volcanic, sedimentary, sedimentary, and structural history of the two rift segments segments differ differ in several aspects. High-resolution aeromagnetic, gravity, and seismic data permit the the tracing of flow sequences for long distances and to great depth. This geometry geometry combined combined with with the the chemistry of the volcanic rocks allows us us to to decipher decipher aa volcanic volcanic stratigraphy stratigraphy in in spite spite of of widespread cover by glacial deposits and Paleozoic sedimentary rocks (Cannon (Cannon et et al., al., 2001). interpretations (Chandler Our interpretation, aided by previous gravity and seismic interpretations (Chandleret et al., al., is that that the original structure of of the St. Croix horst was an asymmetric graben, or 1989), is possibly a half graben, like those of the Lake Superior portion of the MRS. MRS. The The Lake Lake Owen Owen fault was a major growth fault fault on the southeast southeast side side of the the graben graben and and the the volcanic volcanicfill fill fault, The Douglas fault on the thickened toward and terminated against the fault. the northwest northwest side side of of the horst is not clearly a growth feature and may be simply a thrust formed formed during during rift rift inversion. Thrust displacement on the Douglas Douglas fault fault must must be be 20 km or more because it juxtaposes of the base of a thick volcanic sequence sequence over the younger younger Bayfield Bayfield Group. Group. Cannon et al. (2001) (2001) and Nicholson et al. (2001) used chemical chemical and and aeromagnetic aeromagneticdata datato to define the Minong Volcanics, the underlying Clam Clam Falls Falls Volcanics, and the Chengwatana Volcanics as three formations making up the graben-filling graben-filling volcanic volcanic sequence. sequence.The Thethree three have similar chemistry, chemistry, but were defined defined by structure structure and and geochronology. geochronology.The Thethree-part three-part division no longer seems justified justified and the Chengwatana and Clam Falls Volcanics are Volcanics, as earlier earlier defined, were restricted restricted combined into a single unit. The Chengwatana Volcanics, to a fault-bounded belt belt between between the theDouglas Douglasand andPine Pinefaults faultsand andtheir theirstratigra'phic stratigrahic not known known directly. directly. We now believe, based on seismic relationships to other volcanics were not data, that the Pine fault does not extend into the the northern northern part of of the horst, where the previously defined Chengwatana and Clam Falls units units appear to be a continuous depositional sequence of compositionally indistinguishable flows that we propose be called entirely Chengwatana. basalts about Chengwatana. The Minong Volcanics, a sequence sequence of low-Ti02 low-TiOibasalts about 33km kmthick, thick, overlie the Chengwatana, Chengwatana, along an apparent low angle angle disconformity disconforrnitybased based on onaeromagnetic aeromagnetic form lines. These form lines also show a disconfonnity disconformity within the Minong volcanics on the of the the Ashland Ashland syncline. syncline. A A lower lower unit, unit, not not present present on the northwest northwest limb, is southeast limb of mostly high-Ti02 high-Ti02 basalt. basalt. Based Based on on the the presence presence of of abundant abundanthigh-TiO2 high-Ti02 basalts basaltsand andmore more magmatic center center was was active in this area sometime evolved rocks, we infer that a localized magmatic flow in in the the upper upper part of of this sequence. A second, but before 1095 1095 Ma, the age of a rhyolite flow may have have existed existed on on the western margin of the graben near apparently older, volcanic center may 59 �_____ the Amnicon Amnicon Complex Complex where the Chengwatana Volcanics are mostly high-Ti02 high-Ti02basalts, basalts, andesites and rhyolites. rhyolites. Clastic sedimentary sedimentary rocks of the Oronto Group overlie the volcanic rocks. Only the basal unit, the Copper Copper Harbor Conglomerate, Conglomerate, is preserved in most of the St. St. Croix horst where as as much as 2 km of sandstone and conglomerate lie along the axis of the Ashland syncline. The krn sandstone conglomerate Ashland syncline. Copper Harbor thins to only a few tens of meters toward the northern end of the horst in the same areas where the volcanic section also shows substantial thinning. Apparently the area now comprising comprising the northern part of the St. Croix horst did not subside nearly as deeply deeply as parts farther to the southwest. southwest. This relatively positive relief persisted throughout throughout volcanic volcanic activity and deposition deposition of the Copper Harbor Conglomerate. Conglomerate. The overlying overlying Nonesuch Shale Shale maintains a relatively relatively uniform thickness around the northern part of the Ashland syncline, syncline, suggesting suggesting that that the the topographic topographic high high was was buried buried by by that that time. time. 91'00' EXPLANATION EXPLANATION 4V00' Bayfield and BayfieldGroup Group and equivalent sandstones Freda Sandstone Nonesuch Shale copper Harbor Conglomerate Gabbro and granophyre Minong Volcanicslow-Ti basalts Minong volcanicshigh-Ti basalts Chengwatana volcanics Kallander Creek Kallander CreekVolcanics Volcanicl Siemens Creek Volcanics volcanics Siemens Archean and Paleoproterozoic Paleopmterozoic 46'OO,i__ 46'O 92'OO' 92W 0 rocks 91°00 90 30 KM WY., Daniels, Cannon, Cannon, W.F., Daniels, D.L., D.L., Nicholson, Nicholson, S.W., Phillips, J., Woodruff, Woodruff, L.G., Chandler, Chandler, V.W., V.W., Morey, Morey, G.B., G.B., Boerboom, T., Wirth, K.R., and Mudrey, M.G., MG., Jr., Boerboom. Jr., 2001, 2001,New New map mapreveals revealsorigin originand andgeology geologyof ofNorth North American v. 82. 82, no. no. 8, 8, pp. pp. 97-101 97-101 American Midcontinent Midcontinent rift: rift: EOS, EOS, v. V.W., McSwiggen, P.L., P.L., Morey, Morey, G.B., GB., Hinze, W.J., W.J., and Anderson, R.R., R.R., 1989, Interpretation of Chandler, V.W., seismic seismic reflection, gravity, and magnetic data across Middle Proterozoic Mid-continent Rift system, northwestern Wisconsin, eastern Minnesota, and central Iowa: American Association of Petroleum Petroleum Geologists Geologists v. 73, 73, p. p. 261-275. 261-275. Bulletin, v. SW., Boerboom, W.F., Wirth, K. and Isachsen, C.E., C.E., 2001, A new look at the the 1.1 Ga Ga Nicholson, S.W., Boerboom, T., Cannon, W.F., Chengwatana horst, Minnesota Minnesota and Wisconsin, Institute Institute on Lake Superior Superior Geology, Chengwatana Volcanics in the St. Croix horst, 1,p. 71-72. 71-72. v. 47, part 1, 60 �TheRare Rareand andExotic ExoticMineralogy Mineralogyof of the theWestern WesternSubcomplex Subcomplexof of the the Deadhorse DeadhorseCreek Creek The Diatreme,Northwestern NorthwesternOntario. Ontario. Diatreme, G. Potter Potterand andRoger RogerH. H. Mitchell Mitchell EricG. Eric egpotter@mail.lakeheadu.ca egpotter@mail.lakeheadu.ca Dept. Dept.of of Geology, Geology,Lakehead LakeheadUniversity, University,955 955Oliver OliverRoad, Road,Thunder ThunderBay, Bay,ON. ON.P7B P7B5E1 5El The Themain main mineralized mineralizedzone zoneof ofthe thewestern westernsubcomplex subcomplexof ofthe theDeadhorse DeadhorseCreek Creekdiatreme diatreme exhibits complex complexmineralization mineralizationinvolving: involving: first first and and second second order ordertransition transitionmetals metals exhibits (specifically PEE; Be; Th; and U. The (specifically Sc, Ti, V, Cr, Mn, Fe, Zr and Nb); REE; Themineralization mineralization is Nbis manifested manifested by by the the presence presence of of the the following followingminerals: minerals: thortveitiite, thortveitiite,Sc-V-aegirine, Sc-V-aegirine, NbV-rutile, V-mtile, V-crichtonite, V-crichtonite, Ba-Mn-hollandite, Ba-Mn-hollandite, zircon, zircon, monazite-Ce, monazite-Ce, xenotime-Y, xenotime-Y, uraninite, uraninite, thorite, thorogummite, thorogurnmite, barite, barite, barylite, barylite, tyuyamunite, tyuyamunite, phenacite, phenacite, pyrite, pyrite, hematite, hematite, thorite, magnetite magnetite and and several several as as of of yet yet unnamed unnamed mineral mineral species species (Platt and Mitchell, Mitchell, 1996; 1996;Smyk Smyk et al., 1993; this study). Of interest in this presentation are: Nb-V-rutile, crichtonite et al., 1993; this study). Of interest in this presentation are: Nb-V-mtile, crichtoniteand and Sc-V-aegirine. Sc-V-aegirine. The The Nb-V-rutile Nb-V-mtile isis enriched enriched in in Cr2O3, with concentrations concentrations reaching Cr203, with reaching 30 6.49 6.49 wt.%. wt.%. The enrichment enrichment of of - Cr2O3 and Nb205 is similar to that Cr203 and Nb2O5 similar to that of of rutile mtile reported reported in in alkaline alkalineigneous igneous :1 20 rocks, as illustrated in an atomic rocks, as illustrated in an atomic - percent plotofof~Cr percent plot + r 3 4++' Nb5 Nb5+++ Ta5 ~ a ^ 1991). vs. Ti4 (Haggerty, vs. ~ i ^ (Haggerty, 1991). z 10 + However, Nb205 contents contents are are However, the the Nb2O5 - unusually unusually high high compared comparedto toalkaline alkaline M ~ ÑT ~ ~ ~igneous & ~ rocks igneous rocks in in general, general, with with " 00 29.32 concentrations reaching 95 100 80 85 90 50 55 55 60 65 70 75 75 80 85 90 95 100 concentrations reaching 60 65 70 50 Ti4 (Atomic%) Ti4* ( A ~ O ~%)C wt.%. Such such Nb2O5 Nb205 contents contents&re re wt.%. I similar similar to those those reported in in ilmenorutile, ilmenorutile, whiph which is historically found in pegmatites. pegrnatites. Also Also unique to the rutile is the distinct the Deadhorse Deadhorse Creek: Creek futile distinct enrichment enrichment of V2O3 V203 (up to 10.52 10.52 wt.%) and and the the lack lack of of tantalum. tantalum....' wt.%) : The Sc-V-aegirines Sc-V-aegirines present present at at Deadhorse Deadhorse Creek Creek contain contain the the highest highest reported reported The concentrations of Sc2O3 and and V203 V2O3(16.46 (16.46and and11.99 11.99wt.%, wt.%,respectively). respectively). The only only other other have been occurrences of of VV- and andSc-enriched Sc-enriched aegirine aegirine have been reported reported from from alkaline alkaline occurrences metasomatites metasomatites associated with iron-ore deposits in Ukraine (Valter et al., 1994; 1994;Pavlishin Pavlishin et al., presence thortveitiite and al., 2000). 2000). OfOfnote noteisisthethe presenceofofboth both thortveitiite(Sc2Si2Oi) (Sc2Si2O7) andSc-enriched Sc-enriched aegirine within the main mineralized zone. zone. Although Although the the source source of the the Sc Scin inthe theaegirine aegirine remains conjectural,itit appears appears that that the the Sc, V and Na remains somewhat somewhat conjectural, Na was was scavenged scavengedfrom from alteration of the main mineralized mineralized zone zone by Fe-rich fluids. fluids. The V-rich V-rich crichtonites crichtonites are best termed termed vanadium-rich vanadium-rich analogues of crichtonite-(Sr) crichtonite-(Sr) and and senaite-(Pb). The in the the crichtonites crichtonites isis peculiar, peculiar, as as the the presence of of Nb205in The enrichment enrichment in Nb2O5 Nb has has been been aa distinguishing distinguishing feature feature of of the the mantle-derived mantle-derivedend end members memberslindsleyite-(Ba) lindsleyite-(Ba) 61 �the crichtonites plot ininthe Interestingly, the theupper-mantle upper-mantle and mathiasite-(K) mathiasite-(K) (LIMA). (LIMA). Interestingly, and LIMA quadrant of FeO + Fe203 + MgO vs. Ti02 (Haggerty, 1991), near + F e B 3 + MgO vs. Ti02 (Haggerty, 1991), near the the LIMA LIMA LIMA quadrant of compositions compositionsdue dueto tothe thereplacement replacementof of iron ironby by vanadium. vanadium. The mtiles and and The Nb-enriched Nb-enriched rutiles have believed to crichtonite are crichtonite are believed to have formed fonned relatively relatively early early in in aa 30 30 crichtrmite (s$ Crichtnnite(Sr) multistage-alteration Armalcolite Ooadrant • (?EE) multistage-alteration sequence sequence of of the the diatreme — Deadhorse Creek &n,irc am,(Pb) w). Deadhorse Creek diatreme by by O 25 reaction reaction of of stoichiometric stoichiometricrutile mtilewith with Lovamgitc (Ca) + hydrous hydrous alkaline alkaline solutions solutions ennched enriched 20 in Nb Nb and and V. V. These These hydrous hydrous alkaline alkaline solutions solutions likely likely also alsoaltered altered Armalcolite Ouandrant DHC Crichinnite 15 zircon zircon to to an an unnamed unnamed hydrated hydrated awhich is zirconosilicate, calcium zirconosilicate, which is — calcium LIMACrptonites— with the found found in in association association with the 10 72 64 68 56 60 52 52 56 60 64 68 crichtonite and rutile. crichtonite and mtile. Textural Textural and and TtO2çVt. /o) TiO, (Wt.%) suggest that compositional data compositional data suggest that subsequent alteration alteration formed fonned the the Sc-V-aegirines Sc-V-aegirines and and imparted imparted the the pervasive pervasive subsequent hematitization hematitization to to the the main main mineralized mineralizedzone. zone. I I I I I I n - ~ i ~ b Crichtonite ~Crichtooite ~ ~ i t i ~ Non-Kimberlitic - - - p • - — I I I I I ' References References of the upper Oxide Minerals: Minerals: mineralogy of upper mantle. mantle. In: In: Oxide Haggerty, S.E. S.E. (1991): (1991): Oxide mineralogy petrologic and and magnetic magnetic significance. significance. Reviews Reviews in in Mineralogy, Mineralogy, 25, 25,Mineral. Mineral. Soc. Soc. Amer., 335-416. Amer., 335-416. Platt, Platt, R.G. R.G. and and Mitchell, Mitchell, R.H. R.H. (1996): (1996): Transition Transition metal metal rutiles mtiles and andtitanates titanatesfrom fromthe the Miner. Miner. Deadhorse Creek Diatreme complex, northwestern Ontario, Canada. Deadhorse Creek Diatreme complex, northwestern Ontario, Canada. Mag., 403-413. 60,403-413. Mag., 60, Pavlishin, Baldan, F.G., F.G., Bugaenko, V.M., Voznyak, Voznyak, D.K., D.K., Galaburda, Pavlishin, V.1., V.I., Baklan, Bugaenko, V.M., Galaburda, Yu, A., A., G.O., Mel'nikov, Kulchic'ka, Dekhtulins'ky, E.S., Donskey, O.M., Krivdik, S.G., Krivdik, G.O., Mel'nikov, Donskey, V.S., Radzivill, A, Ya. Ya. And And Zimbal, Zimbal, S.M. S.M. (2000): (2000): Science-based Science-based perspectives perspectives of of improvement of mineral mineral resources resourcesor orrare raremetals metalsinin Ukraine. Ukraine. Mineral., Journal, improvement of 22, no.1, no.l,5-20. (in Russian) Russian) 22, 5-20. (in Smyk, M.C., Taylor, R.P., Jones, Smyk, M.C., Taylor, R.P., Jones, P.C. P C . and and Kingston, Kingston, D.M. D.M. (1993): (1993): Geology Geology and and geochemistry geochemistry of the West Dead Horse Horse Creek Creek rare-metal rare-metal occurrence, occurrence,northwestern northwestern Ontario. Explor. Mining. Geol., 2, no. 3, 245-25 1. Ontario. Explor. Mining. Geol., 2, no. 3,245-251. Valter, V.M., Sharkin, O.P. O.P. and Yakolev, Valter, A.A., Khomenko, V.M., Yakolev, V.M. V.M. (1994): (1994): AA vanadian vanadian aegirine in alkaline metasomatites from Zheltye Vody. Vody. Doklady Doklady Akademii Akademii Nauk Nauk Ukrainy, Ukrainy, No. 3, 3, 110-116. 110-116.(in (in Russian) Russian) 62 �Sibley Sibley Basin Basin sediment sediment provenance provenanceusing using zircon zircon and andwhole whole rock rockgeochemical geochemical methods: methods: Possible Possible source sourceareas areasof of the thePass PassLake LakeFormation Formation Richardson, Hollings, P.P. (Department Geology, A.,Fralick, Fralick,P.,P.,and and Hollings, (Departmentofof Geology,Lakehead LakeheadUniversity, University,955 955 Richardson,A., Oliver OliverRd., Rd., Thunder ThunderBay, Bay,Ontario, Ontario,P7B P7B5E1, 5E1,Canada; Canada,ajrichar@mail.lakeheadu.ca) airichar@mail.lakeheadu.ca) The TheSibley SibleyGroup Groupconsists consistsof of Proterozoic Proterozoicsediments sedimentsthat that outcrop outcropdiscontinuously discontinuouslyover overaa 15000 15000sq. sq. km km region region in in the the area area surrounding surrounding central central and and southern southern Lake Lake Nipigon. Nipigon. Its Its age age is is bracketed Redstone Point Point Complex Complex(1537 (1537 bracketedby by the theunderlying underlyingRedstone ++lo/-2 101-2 Ma; +I-33 33Ma Ma Ma;Davis Davisand andSutcliffe, Sutcliffe.1985) 1985)and anda a1339 1339+1Rb-Sr Rb-Srage ageon ondiagenetically diageneticallyaltered alteredSibley Sibleysediments sediments(Franklin, (Franklin, 1978). SibleyGroup Groupwas was divided divided into into three three formations: formations:the the 1978).The The Sibley Kama Kama Hill Hill Formation Formation(top), (top),the theRossport RossportFormation, Formation,and and the the Pass PassLake LakeFormation Formation(bottom), (bottom),by by Franklin, Franklin, et et al. al. (1980). (1980).The The Kama Kama Hill Hill Formation Formationconsists consistsof of aalaminated laminatedshale shalefacies, facies,the the Rossport Rossportof of mudstone mudstoneand and stromatolitic stromatoliticfacies, facies,and and the the Pass Pass Lake Lakeof of aaconglomeratic conglomeraticfacies faciesand andaaplane-bedded plane-beddedor orcrosscrossbedded bedded sandstone sandstonefacies facies(Cheadle, (Cheadle,1986). 1986).This Thisstudy study investigates investigatesthe thesources sourcesthat thatfed fedsediment sedimentto tothe thePass PassLake Lake Formation Formationin in the thesouthern southernportion portion of of the the basin. basin. 30km LEGEND Proterozoic 1097 Ma rn Li_J Oslar Group 1110 Ma DI. base :I. ,1339Ma Regional Regional granitic granitic sources sourcesmay may include: include: the the Mesoproterozoic Mesoproterozoic Redstone Redstone Point Point anorogenic anorogenic intrusion, intrusion, Sample Locations Neoarchean Neoarchean peraluminous peraluminous Quetico Queticogranites, granites, and and Granite •,, Redstone Point Point Granite McKenzie McKenzie granites. granites. Of Ofthese, these,the the Redstone Redstone Point Point isis more more Artisan highly evolved than the others and contains abundant highly evolved than the others and contains abundant • Regional Granites ~ ~ it^^~ l ~ ~ ~ l Granitic Rocks Metasedimentaty zircon zircon and and aa distinct distinctgeochemical geochemical signature signaturewith withvery very elevated PassLakeFm. elevated values values for forthe thehigh highfield field strength strengthelements elements Rocks (HFSE). (HFSE). Samples Sampleswere were collected collectedfrom fromsurface surfaceexposures exposuresatatseveral several Figure 1. 1. Regional Regional geologic geologic map map with with sample sample Figure locations (Fig. 1). Representative samples of the Pass Lake locations (Fig. 1). Representative samples of the Pass Lake locations. locations. Formation Formation of of the the Sibley SiblevGroup Grouowere were taken taken from from aa cliff cliff section section directly directlyacross acrossfrom fromPass PassLake Lakeon on Hwy. Hwy. 587. 587. Individual Individualbeds beds were were grouped grouped into into assemblages assemblages consisting of up to 16 16 beds. Bed Bedthickness thicknessbecame became finer finer and and thinner up section. AAtotal totalof of 26 grained sandstone. sandstone. Two 26hand hand samples sampleswere were obtained from the Pass Lake cliff and consisted of fine to medium grained Two additional Pass Lake Formation samples were obtained from road cuts cuts further furtherup-section up-section that that consisted consistedof of medium medium grained grained sandstone. sandstone. Additional granitic samples were were obtained from road cuts along Hwy 11/17 11/17 and each location. Samples and Hwy. 527 (Fig. 1). One One sample sample was was taken taken from fromeach Samples of of Redstone Point sandstones, and and granite granite samples samples were were previously obtained by by P. P. Fralick from from the the English English Bay Bay region region of of Lake Lake Nipigon Nipigon (Fig. (Fig. 1). 1). S Sibley Graup 1537 Ma Granite and Rhyolite 1900 Ma AnImikle Group • 1 I - Figure x-Ray Figure 2. 2. Backscatter X-Ray SEM-EDS SEM-EDSimage image of of aa zircon zircon AR-01. fromsample sampleAR-Ol. from ICP-AES Plasma -- Atomic Emission ICP-AES (Inductively Coupled Plasma Emission Spectroscopy) Spectroscopy) Samples were cut into approximately 4 x 3 x 0.5 cm sections and crushed to included to aa fine fine powder nowder of <30 microns. Chemical Chemicalpreparation preparation included - hydrofluoric hydrofluoric acid acid digestion digestion to to remove remove all all silica silica and and allow allow complete complete solution solutionof of samples. Prepared samples were analysed at the Lakehead University Prepared samples were analysed at the Instrument Laboratory. Laboratory. 63 �A SEM-EDS SEM-EDS (Scanning (Scanning Electron ElectronMicroscope Microscope- - Energy EnergyDispersive Dispersive X-Ray X-Ray Microanalysis) Microanalysis) Samples Samples were were ground ground to to 30 30micron micron thin thinsections sectionsand andcut cutinto into discs discs suitable suitable for for the the SEM SEM stage. stage. Before Beforeanalysis, analysis,samples samples were carbon coated to prevent charge build- up while being analysed. Samples analysed for with were analysed. carbon Samples coatedwere were to prevent analysed charge for50 build50seconds seconds up while withan being an accelerating voltage of 20 KeV, and a beam current 0.475 accelerating voltage of 20 KeV, and a beam current ofof0.475 t, ..'$?Â¥: . .:. pA using using aa JEOL JEOL 5900 5900 SEM SEMwith withaasystem systemresolution resolutionof of139 139 pA . - .. eV, at at Lakehead Lakehead University Instrument Instrument Laboratory. Laboratory. Images Images eV, . .A* .:.. . . were taken using a backscatter-electron detector. Zircons were were taken using a detector. Zircons were .. analysed Zr,Y, Y,Th, Th,U, U,and andHf. Hf. analysed for for five five elements: elements: Zr, The The use use of of zircons zirconsin insediment sedimentprovenance provenancestudies studieshas hasbeen been 10Y+TKU IO-Y+T~+U WIO Hrlo limited to to work work done done by by Owen Owen(1987) (1987)which whichinvolved involved limited employing employing hafnium hafnium content of of detrital detritalzircons zirconsin indetermining determining zirconsfrom from Figure Figure3.3. SEM-EDS analyses of zircons the upper Jackfork the source source of of the hupper JackforkSandstone Sandstoneand andthe theParkwood Parkwood Pass PassLake LakeFm Fm sandstones sandstones(points), (points)'Redstone Formation. He Hecame cameto tothe theconclusion conclusionthat thathafnium hafniumcontent content Formation. Point Pointsandstones sandstones(squares), (squares),Redstone RedstonePoint Point of of these these zircons zircons agreed agreed with with optical opticaland andcathodoluminescence cathodoluminescence (+),and andregional regionalArchean Archeanand and Granites(+), Granites modal modal analyses, analyses, and and is is aa viable viable method methodfor forprovenance provenance Neoarchean Neoarcheangranites granites(triangles), (triangles). determination. determination. This This study study is is the the first first to touse useSEM-EDS SEM-EDSmethods methodsasaswell wellas as analyses for Y, Th, and U. Fig. 3 shows zircon analysis results. The majority of zircons plot at Zr/Hf ratio analyses for Fig. 3 shows zircon analysis The majority of zircons plot at Zr/Hf ratio of of approximately Y, Th, Th,and and U, U, but but aa significant significantpopulation populationshow showaaY, Y,Th, Th,UU approximately 40 40 with with relatively relatively low low amounts amounts of of Y, enrichment trend. The Thegeochemical geochemical signature signature of zircons from from both sandstones sandstones and and granites granites show show similarity, similarity, and and indicates indicates local local sourcing sourcingof of sediment sediment • £ with Archeanand and with aa possible possible influence influence of of regional regionalArchean Proterozoic Proterozoic felsic felsic igneous igneous intrusives. intmsives. 2000 Whole Whole rock rock interpretation interpretation of ofICP-AES ICP-AES ppmZr/%T02 ppm Zr/%T102 geochemistry geochemistry (Fig. (Fig. 4) 4) trends trends agree agree with with SEM SEM 1000 loGo distribution within samples. Immobile elemental distribution within samples. Immobile elemental element element ratios ratios of of the the Pass Pass Lake Lake sandstones sandstonestend tendto to - A fall on on aa mixing trend between between enriched enrichedand and nonnonfall I 125 75100 25 50 enriched 0 25 wm 75 Nb/%Ti02 ' 150 enriched sources. This Thisstudy studyhighlights highlights the the possible possible usefulness of using SEM-EDS generated data usefulness of using SEM-EDS generated datain in concert concert with with more more traditional traditionalchemical chemicalanalyses analysesin in Figure Figure4. 4. Immobile Immobileelement elementplot plotofofICP-AES ICP-AESanalyses. analyses. provenance provenancestudies. studies, Pass PassLake Lakesandstones sandstones(points) (points)plot plotininsimilar similarfield fieldtoto sandstones derived from, and overlying Redstone Point sandstones derived from, and overlying RedstonePoint granite granite (squares). (squares). Redstone (+), and and other other Redstonepoint point granite granite (+), granites granites (triangles) (triangles)are arealso alsoshown. shown. . I I I , 1 . A I I I 1 References References Cheadle, Alluvial-playasedimentation sedimentationininthe thelower lowerKeweenawan KeweenawanSibley SibleyGroup, Group,Thunder ThunderBay BayDistrict, District, Cheadle, BA. B.A.(1986) (1986)Alluvial-playa Ontario; Journal of of Earth EarthSciences, Sciences,v.v.23, p.527-542. Ontario; Canadian Journal 23, p. 527-542. Davis, Davis, D., and Sutcliffe, Sutcliffe,R., (1985) (1985) U-Pb U-Pb ages ages from from the Nipigon Plate Plate and Northern Lake Lake Superior. Superior.Geological GeologicalSociety Society of of America 1572-1579. America Bulletin, Bulletin, 96, 96, 1572-1579. Franklin, in Rubidium-strontium Rubidium-strontiumisochron isochron age age studies, studies. Report Report2,2, geological geological Franklin, J.M., (1978) (1978)The The Sibley Sibley Group, Group, Ontario; Ontario; in Survey of Canada, Canada, Paper Paper77-14, 77-14,p.p. 31-34. 31-34. Survey Franklin, McIlwaine, W.H., W.H., Poulsen, Poulsen, K.H. K.H. and and Wanless, Wanless. R.K. R.K. (1980) (1980)Stratigraphy Stratigraphyand anddepositional depositionalsetting settingofofthe the Franklin, J.M., Mcllwaine, Sibley 17,p.633-651. p.633-651. Sihley Group, Group, Thunder Thunder bay District District Ontario, Ontario, Canada; Canada; Canadian Canadian Journal Journal of of Earth Earth Sciences, Sciences,v.v. 17, Owen, Sedimentary Petrology, Vol 57, No.5., NO.^., 1987., 1987..p.p.831-838. 831-838. Owen, M. (1987) (1987) Hafnium in Detrital Zircons: Journal of Sedimentary Vol57, 64 �Magnetostratigraphic and Group A Magnetostratigraphic and Secular SecularVariation Variation Study Study of the the Sibley Group Rogala, P. and Borradaile, G. (Department of Geology. Lakehead University, Thunder Rogala,B., B.,Fralick, Fralick, P. and Borradaile, G. (Department of Geology, Lakehead University, ThunderBay, Bay, Ontario, P711 SEI, brogala@lakeheadu.ca) brogala@lakeheadu.ca) P7B 5E1, The Sibley Sibley Group Group is is aa red red bed bed sequence sequence that that was was deposited deposited in in aasubsiding subsidingintracratonic intracratonic basin (Fralick and Kissin, 1995) 1995) overlying, in part, a 1537+10-2 1537+10-2 Ma (Davis (Davis and and Sutcliffe, Sutcliffe, 1984) 1984) anorogenic granite-rhyolite complex. The Group was previously divided into three main anorogenic granite-rhyolite complex. The Group previously divided three main Rossport, and and Kama Kama Hill. Hill. An unnamed Formations: Pass Lake, Rossport, unnamed Formation and the Nipigon Bay Formation have have recently recently been been added. added. The Formation The Pass Pass Lake Lake Formation Formation consists consists of of the the conglomeratic conglomeratic Loon Lake Lake Member of the Loon Member and the the sheet-like sheet-like sandstones sandstones of the Fork Fork Bay Bay Member, Member, representing representing aa braided fluvial environment environment (Cheadle, (Cheadle, 1986). 1986). The Rossport Formation Formation is separated separated into the the braided Channel Island, Island, Middlebrun MiddlebrunBay, Bay,and andFire FireHill HillMembers. Members. The Channel Member is is a Channel Channel Jsland Island Member cyclic dolomite-shale dolomite-shale unit unit interpreted interpretedtoto be be playa cyclic playa lake lake sediments sediments (Cheadle, (Cheadle, 1986). 1986). The Middlebrun Bay Bay Member, considered a marker marker bed bed for for the Sibley Group, is aa stromatolitic Middlebrun stromatolitic unit unit that represents a migrating strandline. The The Fire Fire Hill Hill Member Member consists consistsof of mudcracked mudcrackedred red silt siltwith with mudchip conglomerates and sand sand sheet sheet incursions. incursions. It signifies a time of of tectonic tectonic tilting tilting of the the basin. The Kama basin. Kama Hill Hill Formation Formation is not not subdivided, subdivided, and is is composed composed of of purple purple shales shales and and siltstones interpreted as mud flat flat deposits deposits (Cheadle, (Cheadle, 1986). 1986). The The unnamed Formation is divided divided two unnamed unnamed Members. Members. These into two These represent represent a deltaic deltaic and fluvial fluvial environment. environment. The The Nipigon Nipigon Bay Formation Formation consists of of cross-stratified sandstonebeds, beds, and and is is thought to denote Bay cross-stratified sandstone denote an an acolian aeolian environment. environment. Samples were were taken taken from Samples from the Pass Pass Lake, Lake, Rossport, Rossport, Kama Kama Hill, Hill, and and Nipigon Nipigon Bay Bay Formations for a paleomagnetic study. study. The unnamed Formation was not sampled due to the lack of exposure. of exposure. The ThePass PassLake, Lake,Kama Kama Hill, Hill, and and Nipigon Nipigon Bay Bay Formation Formation were were used to to conduct conduct a preliminary of the the Sibley Sibley Group. Group. The TheRossport RossportFormation Formation was was preliminary study of the magnetostratigraphy of sampled from unoriented drill core, core, thus could only be used to study study secular secular variation. variation. The paleopoles calculated from the Pass Lake, Lake, Kama Kama Hill, Hill, and andNipigon NipigonBay BayFormations Formations have been been plotted along an apparent apparent polar wander path (APWP) defined defined by Elston Elston et et at. al.(2002) (2002) (Figure 1). 1). The samples samples from from the Pass Pass Lake Lake Formation Formation have been divided into into sample sample groups groups corresponding to Quarry Island, Transitional to to the Rossport Formation, and and an an outcrop outcrop atat Pass Pass The paleopole of the with a diagenetic Lake. Lake. The paleopole of the Quarry Quarry Island Island Group Group corresponds corresponds with diagenetic event at at 1978), and and the approximately approximately 1339±33 1339±3 Ma Ma (Franklin, (Franklin, 1978), the latter latter two two groups groups have havepaleopoles paleopoles associated with an The Kama has an older associated with an early earlyKeweenawan Keweenawan overprint. overprint. The Kama Hill Formation Formation has older discordant paleopole and and a younger younger paleopole paleopole that that isis located located within within the 1500 Ma section of the discordant paleopole that the the Sibley Sibley Basin Basin formed formed prior prior to to this, as is apparent polar polar wander path. is apparent path. This This suggests suggests that supported by by the recent discovery of sedimentary xenoliths within the 1537 supported 1537 Ma Ma Redstone Redstone Point Point granite. that lie on the APWP granite. The The Nipigon Nipigon Bay Bay Formation Formation has paleopoles paleopoles that APWP near near 1400 1400 Ma Ma and and 1100 Ma. Ma. The The first first paleopole paleopole may may be primary or related to the diagenetic diagenetic event that affected the Pass Lake samples at 1339 1339 Ma. The Thelatter latter paleopole paleopole correlates correlateswith with the the Osler OsierVolcanics. Volcanics. Formation revealed a The paleomagnetic study on a 90 cm core section from the Rossport Formation When this this curve was compared secular variation curve. curve. When compared to typical typical secular secular variation variation curves curves (Butler, 1998; Tauxe, Tauxe, 1998), the the time-span time-spanfor for Sibley Sibleydeposition depositioncan canbe beestimated. estimated. The 90 cm section was estimated to to represent represent 2500 2500 to to 3000 3000 years. years. This can can be be extrapolated extrapolated to to estimate estimate that that the Rossport Formation could potentially potentially represent represent 75 75 000 000 years years of of deposition. deposition. 65 �210'S — 240'S Figure 4.12 Paths (APWP) is plotted Figure 4.12 AAwell-defined well-defined Proterozoic Proterozoic Apparent Apparent Polar Polar Wander Paths al., 2002). The components are ThePass PassLake LakePCA PCAcomponents are designate designatewith withQI, QI,T, T, (after Elston et at., and 0 0 to the Quany to indicate indicatethe Quarry Island, Island, Transitiona', Transitional, and and Outcrop Groups. The The Kama Kama Hill Hill KH and and the the Nipigon Nipigon Bay Bay Formation Formation isisNB. NB. The PCA, PCB, and Formation is designate KH PCC components are denoted respectively by A, B or C after the Formation short form. form. Note that NB-C is a reversed reversed pole pole on on the the back back side of of the the globe. globe. Elston Blston et al. al. (2002) has provided a lower (Sl) (Si) and provided and upper upper (S2) (S2) Sibley Sibley Group pole based on data from Robertson well as as aa pole pole for for the the Keweenawan Keweenawan Osier Osler Group Group (Kl) (Ki) and lower Powder Mill (1973), as well Volcanics (K2). (K2). 1998. Paleomagnetism: magneticdomains domains to to geological Butler, R.F. R.F. 1998. Paleornagnetism: magnetic geological terranes, terranes, Department Department of of Geosciences Geosciences University of Arizona, (originallypublished published by by Blackwell University Arizona, http://www.geo.arizona.edu/Paleomag/bookl http://www.eeo.arizona.edu/Paleomag/book/ (originally Blackwell Scientific Publications Publications in 1992) 1992) Cheadle, B.A. BA. 1986. Alluvial-playasedimentation sedimentationininthe thelower lowerKeweenawan KeweenawanSibley SibleyGroup, Group,Thunder ThunderBay Bay District, District, 1986.Alluvial-playa 527-542. CanadianJournal Journalof ofEarth EarthSciences, Sciences, 23, 23,527-542. Ontario. Canadian Davis, D.W. and Sutcliffe, Suteliffe,R.H. RH. 1984. 1984.U-Pb U-Phages agesfrom fromthe theNipigon NipigonPlate Plateand and Northern Northern Lake Superior. Geological Geological Society of of America America Bulletin, Bulletin, 96, 1572-1579. 1572.1579. D.P., Enldn, Enkin, R.J., R.J., Baker, Baker, J. J. and Kisilevsky, Kisilevsky, D.K. D.K. (2002). Elston, D.P., (2002). Tightening Tighteningthe theBelt: Belt:paleomagnetic-stratigraphic paleomagnetic-stratigraphic constraints on deposition, correlation, and and deformation of of the Middle Proterozoic (ca. 1.4 1.4 Ga) Ga) Belt-Purcell Belt-Purcell United States States and andCanada. Canada. Geological Society ofAmerica Bulletin, 114, Supergroup, United 114, 619-638. 619-638. basin development development in in central central North North America: of America: implications of Fralick, P. and and Kissin, Kissin, S. S. 1995. 1995. Mesoproterozoic basin Sibley Group volcanism volcanism and sedimentation at Redstone Point. In: Petrology and metallogeny metallogeny of volcanic volcanic and intrusive system, Proceedings Proceedings of of the International International Geological Geological and intrusive rocks rocks of of the themid-continent mid-continent rift system. Correlation Program, Project 336. 336. in: Wanless, Wanless, R.K. R.K. and andLoveridge, Loveridge, W.D., W.D., Rubidium-strontium Rubidium-strontium 1978. The Sibley Group, Group, Ontario, Ontario, in: Franklin, J.M. J.M. 1978. report 2. 2. Geological 3 1-34. isotopic age studies, report Geological Survey Survey of Canada Canada Paper Paper77-14, 77-14.31-34. Robertson, W.A. WA. (1973a). Robertson, (1973a). Pole position position from thermally thermally cleaned cleaned Sibley Group sediments sediments and its its relevance relevance to to Proterozoic magnetic magneticstratigraphy. stratigraphy. Canadian Canadian Journal Journal of Earth Eon/i Sciences, Sciences 10, 10, 180-193. 180-193. Tauxe, L. 1998. 1998.Paleomagnetic Paleomagneticprinciples principlesand andpractice, practice,Kiuwer KluwerAcademic Academic Publishers, Publishers, Netherlands, Netherlands,299 299p.p. 66 �Mafic Dikes in Marquette County, County, Michigan with with Sequence of Precambrian Precambrian Mafic emphasis on the Sugar/oaf Sugarloaf Mountain and and Republic Republic Areas N.A. Sandin and T.J. Bornhorst Engineering and Bomhorst (Department (Department of Geological and Mining Engineering and Sciences, Michigan Technological University, Sciences, University, Houghton, Houghton, MI 49931) 4993 1) Precambrian mafic dikes are very common throughout Marquette County, Michigan. Michigan. These dikes Ga). Past Past studies studies by Kantor dikes have have ages agesfrom fromArchean Archean(—2.7 (-2.7 Ga) to middle Proterozoic Proterozoic (—1.1 (-1.1 Ga). Kantor (1968), Gair (1969), Cannon (1974), and Baxter and Bomhorst Bornhorst (1988) have suggested up to six suggested different mafic dike events in Marquette County. These events were interpreted to consist of (from old to young): 1) Archean mafic dikes post-Archean post-Archean volcanism volcanism and before Archean granitoid intrusions intrusions which cut the Archean volcanic rocks; 2) Archean mafic dikes that cut Archean granitoid intrusions, but are subjected to Archean deformation; 3) Archean mafic dikes sedimentary rocks of the that cut Archean basement rocks, but do not cut Early Proterozoic sedimentary dikes that cut Marquette Range Marquette Range Supergroup; 4) Early Proterozoic mafic dikes Supergroup sedimentary rocks rocks prior to Penokean metamorphism and deformation; 5-6) Supergroup 5-6) N-S and E-W Keweenawan mafic dikes. This study has confirmed much of Baxter and and Bornhorst Bomhorst (1988), (1988), however, new data indicate significant significant modifications. modifications. This study focused on the Sugarloaf Sugarloaf Mountain area near Marquette, Ml MI because because of of the the excellent exposures on shore and adjacent to Lake Superior, and previous work by Kantor exposures adjacent previous work by Kantor (1968), who identified mafic dikes of multiple ages. In the Sugarloaf Mountain area over over 300 mafic dikes intruding Archean tonalitic basement were identified and mapped using using aa GPS GPS and pace pace method. method. Dikes receiver and the compass and Dikes identified identified as as critically critically important important to to understanding the sequence of events were sampled for microscopic and chemical chemical study. study. Bornhorst (1988) interpreted interpreted thin, thin, discontinuous, discontinuous, tabular tabular mafic mafic bodies bodies at Baxter and Bomhorst in the the Sugarloaf Sugarloaf Mt. Mt. area as as being being Archean Archeanpost-plutoniclpre-deformation post-plutonic/pre-defonnation Wetmore Landing in above). While While this this interpretation interpretation is is still still possible, possible, the the favored favored mafic dikes (number 22 above). interpretation here is that these mafic bodies are xenoliths that were deformed deformed during during the the Archean along with the host plutonic rocks. plutonic rocks. In Marquette Marquette County, Baxter and Bornhorst Bomhorst (1988) (1988) as well as as previous previous workers workersrecognized recognized the numerous mafic intrusives intrusives that cut Marquette Marquette Range Supergroup Supergroupsedimentary sedimentaryrocks rocksprior prior to to Penokean metamorphism and deformation. deformation. These These were were presumed presumed to to be be of of generally generallythe the same same that in the Sugarloaf Mt. area, three age separate mafic intrusive age. This study indicates that intrusive events events of this age are present. Based on cross-cutting cross-cutting relationships, the sequence sequenceconsists consistsof of diabase diabase dikes trending N20¡E N20°E, diabase diabase dikes dikes trending trending N60¡E N60°E, and anddiabase diabase dikes dikes trending trending east-west. east-west. In In discriminated from addition to cross-cutting relationships, these groups can be discriminated from each each other otherby by trace trace elements. elements. N20°E diabase diabasedikes dikesare arethe theoldest oldest of ofthe theEarly EarlyProterozoic Proterozoicdikes. dikes. In the Sugarloaf Mt. The N2093 area, these dikes N20°E and range in width from NO593 to N20% from one one to to 25 dikes vary in trend from from N05°E 25 feet. feet. Mafic dikes of this age are are the the most most common common of of the Early Early Proterozoic Proterozoic dikes dikes in the Sugarloaf Mountain area. These These dikes dikes exhibit exhibit aa varying varying texture texture from from porphyritic porphyritic to to phaneritie phaneritic from fromthe the dike interiors interiors to the margins. They consist of hornblende, pyroxenes, pyroxenes, chlorite, chlorite, plagioclase, plagioclase, epidote, and sericite. The The REE REE patterns patterns are are enriched enriched in in light-REE light-REEwith with aa moderate moderateslope. slope. Compared to the REE patterns of the sills sills from from the the Marquette Marquette Range Range Supergroup, Supergroup,the theN20°E N2093 concentration of of light-REE, light-REE, is is less less depleted depleted in in heavy-REE, heavy-REE, and has a series has a higher concentration shallower slope. Thus, Thus, our our initial initial interpretation interpretationis is that that these these dikes dikes are are not not related relatedto tothe thesills. sills. 67 �The TheN60°E N60"Emafic maficdikes dikesare areintermediate intermediateEarly Early Proterozoic Proterozoic age. age. They Theyvary varyinintrend trendfrom from N45°E and N45% to N60t N60"E andrange rangeininwidth widthfrom fromone onetoto6060feet. feet.These Theseare arethe theleast leastcommon commonof ofthe theEarly Early Proterozoic Proterozoicdikes dikesininthe theSugarloaf SugarloafMountain Mountainarea. area.They Theygenerally generallyhave havethinly thinlyfoliated foliatedmargins margins with with aa massive, massive,fine-grained fine-grainedinteriors. interiors. These Thesedikes dikeshave haveaaphaneritic phaneritic texture textureand and consist consist of of hornblende, hornblende,pyroxenes, pyroxenes,chlorite, chlorite,plagioclase, plagioclase,epidote, epidote,sericite, sericite,and andminor minoramounts amountsofofcarbonate. carbonate. These Thesedikes dikescross-cut cross-cutthe theN20°E N20TE diabase diabase dikes. REE REEpatterns patternsare areenriched enrichedininlight-REE light-REEand andhave have aasteep steepslope. slope.Compared Comparedtotothe theN20°E N20%series seriesand andthe thesills sillsof ofthe theMarquette MarquetteRange RangeSupergroup, Supergroup, the theN60°E N60"E dikes dikes are are more more enriched enriched in light-REE light-REE with a steeper slope. slope. The TheN60°E N60"Edikes dikes are are more more depleted depletedin inheavy-REE heavy-REEthan thanthe theN20°E N20"Eseries. series.Our Our initial initialinterpretation interpretationisisthat thatthese thesedikes dikesare areaa distinct distinctmagmatic magmaticevent eventwith withrespect respecttotothe theearlier earlierN20°E N20%dikes dikesand andthe themafic maficsills. sills. The Theeast-east east-eastdiabase diabasedikes dikesare arethe theyoungest youngest series seriesof of the the Early Early Proterozoic Proterozoicdikes. dikes. They Theyvary vary in inwidth widthfrom fromfive fivetoto75 75feet feetwide. wide.These Thesedikes dikesgenerally generallyhave have thinly thinlyfoliated foliatedmargins margins with with aa massive, massive,fine-grained fine-grainedinterior, interior,although althoughtwo two dikes dikeshad had porphyritic porphyritic interiors. interiors. They They have have aa phaneritic phaneritictexture textureand andconsist consistof of homblende, hornblende,pyroxenes, pyroxenes,chlorite, chlorite,plagioclase, plagioclase,epidote, epidote,and and sericite. sericite.These Thesedikes dikescross-cut cross-cutthe theN20°E N20"Ediabase diabasedikes dikesand andthe theN60°E N60"Ediabase diabasedikes. dikes. Compared Compared to tothe theREE REEpatterns patternsof ofthe thesills sillsfrom fromthe theMarquette MarquetteRange RangeSupergroup, Supergroup,the theeast-west east-west dikes dikeshave haveaa higher concentration of light-REE and a steeper slope. The east-west dikes are depleted in the higher concentration of light-REE and a steeper slope. The east-west dikes are depleted in the heavy-REE heavy-REEcompared comparedtotothe theN20°E N20%dikes. dikes. They They are are lower lowerin in light-REE light-REEand andhave haveaashallower shallower slope than the N60°E series. Our initial interpretation is that these dikes are a distinct slope than the N60% series. Our initial interpretation is that these dikes are a distinctmagmatic magmatic event eventfrom fromthe theearlier earlierdikes dikesand andthe themafic maficsills. sills. There Thereare arethree threedistinct distinctmafic maficdike dikeevents eventsin in the theSugarloaf SugarloafMt. Mt.Area. Area.We Wepropose proposethat thatthese these dikes dikesare are not not related related to tothe themafic maficsills sillsthat that cut cut the theMarquette MarquetteRange RangeSupergroup. Supergroup.IfIf true, true, then then there theremust must be be atatleast least4, 4,and andlikely likelymore, more, Early Early Proterozoic Proterozoicmafic mafic magmatic magmaticpulses pulses in in the the Marquettearea. area. Marquette Two Twogroups groupsof of unmetamorphosed unmetamorphoseddiabase diabasedikes dikeswere were identified identifiedin inthe theSugarloaf SugarloafMt. Mt.area, area, consistent consistent with with Baxter Baxter and and Bornhorst Bomhorst (1988). (1988). These Thesedikes dikesare areKeweenawan Keweenawanin inage ageand and consist consist of of north-southtrending trendingseries seriesand andan aneast-west east-westtrending trendingseries. series. Both Both dikes dikeshave haveaadiabasic diabasictexture texture aanorth-south andvary vary from from10 10toto75 75 feet feet wide. wide. and In In the the Republic Republicarea, area,Baxter Baxterand and Bornhorst Bornhorst (1988) (1988) suggested suggestedthat thatsome somemetamorphosed metamorphosed mafic maficdikes dikes with with distinct distinctplagioclase plagioclase phenocrysts phenocrysts are are older older than than the themetamorphosed metamorphosedProterozoic Proterozoic dikes dikesin in the the Sugarloaf Sugarloaf area. area.They They proposed proposed that that these these dikes dikes might might correlate correlatewith withthe theMatechewan Matechewan dike dikeswarm swarmnorth north of of Lake Lake Superior Superiorin in Canada. Canada. We We tested tested this this hypothesis hypothesisby bydoing doingchemical chemical analysis analysis of these these dikes. dikes. The The REE REE data data for for these dikes dikes are are similar similar to to Matechewan Matechewandikes dikesfrom from elsewhere elsewhereand and support supportthe the hypothesis hypothesisproposed proposed by by Baxter Baxter and and Bornhorst Bornhorst(1988). (1988). References References Baxter, Baxter,D.A. D.A. and and Bornhorst, Bornhorst,TI., T.J.,1988, 1988,Multiple MultipleDiscrete DiscreteMafic MaficIntrusions IntrusionsofofArchean ArcheantotoKeweenawan KeweenawanAge, Age, western 2 pp. western Upper Upper Peninsula, Peninsula,Michigan: Michigan:Institute Institute on Lake Lake Superior Superior Geology Geology Proceedings Proceedingsand andAbstracts, Abstracts,v. v. 34, 34.2 pp. Cannon, Cannon, W.F., W.F.,1975, 1975,Bedrock Bedrock Geological Geological Map Mapof of the the Republic Republic Quadrangle, Quadrangle,Marquette MarquetteCounty, County,MI: MI:U.S. U S .Geological Geological Survey, Survey, Miscellaneous MiscellaneousInvestigations InvestigationsSeries SeriesMap, Map,1-862. 1-862. Gair, MI: U.S. US. Gair, J.E. J.E. and and Thaden, Thaden, RE., R.E.,1968, 1968,Geology Geologyof ofthe the Marquette Marquetteand andSands SandsQuadrangles, Quadrangles,Marquette MarquetteCounty, County, MI: Geological 77 pp. Geological Survey SurveyProfessional ProfessionalPaper Paper397, 397,77 pp. Halls, Halls, ll.C. H.C.and andPhinney, Phinney,W.C., W.C.,2001, 2001,Petrogenesis Petrogenesisof ofthe theEarly EarlyProterozoic ProterozoicMatachewan MatachewanDyke DykeSwarm, Swarm,Canada, Canada,and and Implications 22 Implications for for Magma Magma Emplacement Emplacementand Subsequent Subsequent Deformation: Deformation: Canadian CanadianJournal Journal of of Earth Earth Sciences Sciences 38, 38,22 pp. PP. Kantor, Swarms in in the the Sugarloaf Sugarloaf Mountain Mountain Area, Area, Marquette Marquette County, County,MI: MI:M.S MS. Kantor, J.A., J.A., 1969, 1969, Assimilation Assimilation and Dike Swarms Thesis, Thesis, Michigan Michigan Technological Technological University, University, Houghton, Houghton, MI, MI, 83 83 pp. pp. 68 �PALEOPROTEROZOIC DOME CORRIDOR CORRIDOR IN THE THE PALEOPROTEROZOIC DEVELOPMENT OF A GNEISS DOME SOUTHERN SOUTHERN LAKE LAKE SUPERIOR SUPERIOR REGION, REGION, USA USA SCHNEIDER, SCHNEIDER, D.A., Dept. of Geological Geological Sciences, Sciences,Ohio Ohio University, University, Athens, Athens, OH OH 45701; 45701; HOLM, D.K., and O'BOYLE, C., C., Dept. Dept. of of Geology, Geology, Kent State State University, Kent, OH 44242; HAMILTON, HAMILTON, M., M., Continental Continental Geoscience Geoscience Division, Division, Geological Geological Survey Survey of of Canada, Canada, Ottawa, Ottawa,ON ON Canada; and JERCINOVIC, MA 01003 01003 JERCINOVIC, M., Dept. of Geosciences, Geosciences, U-Mass, Amherst, Arnherst, MA Paleo-reconstruction of the Penokean orogen at Ca. Ma reveals reveals the presence of of a Paleo-reconstruction ca. 1750-1700 Ma narrow corridor corridor of Archean cored Paleoproterozoic gneiss domes just north of and parallel Paleoproterozoic gneiss domes of and parallel to to the the main suture Wisconsin, and and northern northern Michigan. Michigan. Penokean suture zone zone in Jvlinnesota, Minnesota, Wisconsin, Penokean (ca. (ca. 1850 1850Ma) Ma) metasedimentary rocks infolded within the domes metasedimentary domes give give predominantly 1750-1700 1750-1700Ma Ma cooling cooling ages and are overlain depositionally depositionally by ca. 1700 1700 Ma Baraboo Baraboo interval interval quartzites. quartzites. We We conducted conducted SHRIMP and total-Pb EMP geochronometry to obtain metamorphic timing constraints on U-Pb SHRIMP distinct monazite monazite mineral domains domains from from amphibolite amphibolite grade grade rocks rocks sampled sampledacross acrossthe the entire entirelength length of the gneiss dome corridor. Based Based on on metamorphic metamorphicmonazite monazite crystallization crystallization ages, ages, midcrustal midcrustal amphibolite facies metamorphism (Ml) (Ml) peaked peaked around around 1830 1830Ma Ma and and was was concurrent concurrent with with late late reliably recorded recorded at at ca. ca. 1800 Ma Ma (M2) and and Penokean plutonism; subsequent thermal pulses are reliably again at ca. 1765 1765 Ma (M3), (M3), both also also coeval coeval with magmatic magmatic activity. activity. The youngest monazite monazite ages overlap overlap with abundant abundant Ar-Ar mineral mineral age age data, data, which which indicate indicate widespread cooling of of the gneiss dome comdor corridor immediately immediately following M3. M3. We We propose propose that that the the lime during structural modification of of the the tectonically tectonically buried buried gneiss domes formed at this time continental margin rocks. In In our ourconceptual conceptual model model (Fig. (Fig. 1), l), northward northward vertical vertical extrusion extrusion of of aa decoupled midcrustal block containing the gneiss dome corridor accommodated gravitational collapse of overthickened crust. Elevated Elevatedcountry countryrock rock temperatures temperaturesaccompanied accompaniedwith with profuse profuse melting (i.e., intrusion of the East-central Minnesota batholith) promoted doming of the lower density Archean basement into the more dense overlying Paleoproterozoic metasedimentary rocks, ultimately enabling its complete decoupling from the remaining lower crust. This Thisprocess, process, primarily driven by buoyancy forces, allows for the redistribution of crustal mass from thick to horizontal crustal crustal extension. extension. Tectonic extrusion and crustal thin regions without significant horizontal thinning at this stage may have been facilitated by a decrease in horizontal compressive compressive stresses acting on the region from the south (i.e., Yavapai slab rollback as proposed by Holm HoIm et al., ILSG, 2003). 2003). In Inour ourmodel model(Fig. (Fig.1), l),the thefaults faultsbounding boundingthe thegneiss gneissdome domecorridor comdorare areca. ca.1765 1765 structures, although some, like the Niagara Fault zone, are am reactivated Penokean structures. Ma structures, We note that in east-central Minnesota, a significant portion of the Malmo Structural Structural discontinuity juxtaposes post-Penokean plutons to the south south against older older metamorphic metamorphic rocks to the north (west of Mille Lacs). This This clearly clearly supports supports our our interpretation that this structure structure (and the Flambeau Flowage fault equivalent in northern Wisconsin) Wisconsin) was active active well after Penokean orogenesis. orogenesis. D.K., Van Schmus, W.R., W.R., MacNeill, L.C., Boerboom, Boerboom, T.J., Schweitzer, Holm, D.K., Schweitzer, D., and and Schneider, D.A., 2003, Late Paleoproterozoic (1900-1600 Ma) tectonic history of the northern U.S.A.: Implications for for crustal crustal stabilization: stabilization: Institute mid-continent, U.S.A.: Institute on Lake Superior Superior Geology abstracts abstracts (this (this volume). volume). 69 �Penokean Penokean orogen, orogen, Ml: MI: 1830 2830 Ma Ma to to M2: M2:1800 1800Ma Ma $S NN warm, warm, Penokean Penokean orogen, orogen, M3: M3: 1765 1765 Ma Ma N N SS GNEISS DOME DOMECORRIDOR COftRlDOR GNEISS ARCHEAN GRANITE-GREENSTONE V.1SONSIN MAGMA11C TERRANE Quvenile island alt) ARCHEAN GNEISS PALEOPROTEROZOC ROCKS (supracwstal} F ~ g ~1. 1. x eSchematic SchemaucN-S N-Scross-sections cross-secttonsat at1830-1800 1830-1800Ma Ma(A) (A)and and Figure 11765 765 Ma Ma (6) (B) dep~cling depicting the [he proposed proposed evolution evoluttonot of the thegneiss gnelssdome 6ome corridor corndor in In northern northern Wisconsin. Wtsconsin. Note Noterelative relativelocations locattons ol of gray gray circles clrcies that represent depth of crustal blocks. that represent depth of crustal blocks. 70 �A Paleoproterozoic suprasubduction suprasubduction zone arc complex zone ophiolite-island ophiolite-island arc in northeastern northeastern Wisconsin Wisconsin Schulz, Klaus Schulz, Klaus J., J., (U.S. (U.S.Geological GeologicalSurvey, Survey,Reston, Reston,VA VA 20192, 20192, kschulz@usgs.gov) kschulz@us~s.~ov) The Paleoproterozoic Paleoproterozoic volcanic volcanic and and associated associated intrusive intmsive rocks rocks exposed exposed in in northeastern northeastern Wisconsin Wisconsin are are the the easternmost easternmost exposures exposures of of the the Pembine-Wausau Pembine-Wausau terrane, terrane, the the northernmost northernmost of of the the two two Wisconsin Wisconsin magmatic magmatic terranes terranes that that were were accreated accreated to to the the southern southern margin of the Archean Atchean Superior SuperiorCraton during during the the Penokean Orogeny Orogeny (Sims (Sims and and others, others, 1989). The The rocks rocks of of the the Pembine-Wausau Pembine-Wausau terrane terrane are are separated separated from the epicratonic sedimentary rocks of the Marquette Range Supergroup to the north in Michigan by the Niagara fault fault zone. zone. The volcanic rocks of the the Pembine-Wausau terrane terrane exposed exposed northeastern northeastern Wisconsin, Wisconsin, formed at about 1,870 1,870Ma Ma and and are are cut cut by a variety variety of intrusive intrusive rocks ranging ranging from from synsynvolcanic gabbros, diorites, and tonalities to syn-and post-tectonic granitoids (i.e., Dunbar Dunbar Oneiss Gneiss and related rocks). The Thevolcanic volcanicrocks rocks are are divided divided into into four four fault-bounded fault-bounded units, units, McAllister, Beecher, and Pemene formations. These the Quinnesec, McAllister, Theseunits units are are interpreted interpreted to record the evolution evolution of of aa Paleoproterozoic Paleoproterozoic suprasubduction suprasubductionzone zoneophiolite-island ophiolite-islandarc arc complex, the Pembine Pembine ophiolite-arc ophiolite-arc complex. complex. of The Quinnesec Formation is the oldest volcanic unit and consists predominantly of lava flows pillowed basalt flows and massive diabase, but includes andesite and rhyolite lava and fragmental rocks locally. Several Several large large gabbro gabbro sills sills are are present, present, particularly particularly near the Niagara fault zone, some with peridotite and pyroxenite layers. In In addition, addition, aa large large serpentinized peridotite-gabbro pendotite-gabbro body that produces a large large positive positive magnetic magnetic anomaly anomaly is is exposed exposed south south of Timms Timms Lake Lake (Morgan (Morgan County County Park) Park) east east of of Pembine, Pembine,Wisconsin. Wisconsin. is locally Serpentinized peridotite is dominant in the western part of this body where it is cut by coarse-grained (1-5 cm) dikes of of pyroxenite. Layered Layered and and massive massive gabbro gabbro and and masses of strongly foliated-lineated gabbro are dominant in the eastern part of the body where they are cut by numerous mafic dikes with cliabasic microdioritic textures; some diabasic to microdioritic some of the dikes dikes appear appear to to be be sheeted. sheeted. The rocks of the Quinnesec Formation appear to record the birth and youth stages of a suprasubduction zone ophiolite (Shervais, 2001). 2001). Rocks Rocks formed formed during during the initial phase of ophiolite evolution evolution typically include include layered layered and and isotropic isotropic plutonic plutonic gabbros, gabbros,sheeted sheeted dikes, and a "lower" "lowe?' volcanic volcanic section consisting consisting of low-K tholeiitic basalt and basaltic andesite with MORB MORE and primitive arc tholeiite tholeiite affinities. Gabbros Gabbros formed formed during during this stage are often ductilely deformed deformed (foliated (foliated or or boudinaged) boudinaged) in in response response to to syn-magmatic syn-magmatic extension. Rocks Rocks formed formed during during the the second second or or youth stage of ophiolite formation include intrusive mafic-ultramafic sills and diabase dikes, and an "upper" volcanic unit unit characterized by basalt and andesite with with highly depleted incompatible trace element compositions (i.e., low-Ti basalt, high-Mg andesite and boninite) (Shervais, (shemais, 2001). basalts and gabbros are tholeiitic, with generally low Compositionally, the Quinnesec basalts Ti02 and other high field strength element abundances, Ti02 other abundances, and and flat to extremely extremely light REE REE 71 �some of of the the basalts, basalts, gabbros, and and depleted patterns (Sims and others, 1989). Tn In addition, some Ti02 and REE REF abundances, abundances, but relatively high Cr and Ni andesites have very low Ti02 contents. The Thetrace traceelement element characteristics characteristicsof of the the mafic mafic rocks rocks overlap overlap those those of of mid-ocean mid-ocean ridge basalts and primitive island-arc tholeiite suites whereas the andesites show with fore-arc-related fore-arc-relatedboninites. boninites. The presence in in the upper upper part part of of compositional affinities with the Quinnesec Quinnesec Formation Formation of mafic mafic rocks rocks derived derivedfrom from highly highlyrefractory refractorymantle mantle isis particularly diagnostic of a relationship to the early stages of of intraoceanic subduction and formation (Shervais, 2001). 2001). This fomation in a forearc setting (Shemais, This also implies that the Quinnesec Formation and associated rocks did not form in a back-arc basin near or on the margin of the Superior Johnson, 1997), Superior Craton, as has recently been proposed (Van Wyck and Johnson, 1997), but rather formed as an intraoceanic ophiolite-arc system above a southward dipping (in present coordinates) coordinates) subduction subduction zone. zone. The McAllister, Beecher and Pemene formations consist of volcanic and volcaniclastic calc-alkaline rocks ranging from andesite (McAllister) to rhyolite (Pemene), all with calc-alkaline compositions characteristic of of mature mature oceanic oceanic arcs, arcs. These volcanic rocks and associated intrusives, such as the Newingham Tonalite and Twelve Foot Falls Quartz Diorite, appear appear compatible with the third or maturity stage of suprasubduction ophiolite evolution compatible suprasubduction zone ophiolite evolution (Shervais, 2001). Characteristic (Shemais, Characteristic of this stage are intrusive rocks, such as hornblende diorite, quartz diorite, and tonalite, as well as volcanic rocks ranging from basalt to diorite, to calc-alkaline caic-alkaline compositions. compositions. Volcanism typically becomes rhyolite, all with transitional to in these sequences. sequences. In many cases, rocks of this stage have not more silicic with time in of the subjacent ophiolite, ophiolite, but rather have been attributed attributed to postbeen considered part of ophiolite arc volcanism (Shervais, ophiolite (Shervais, 2001). 2001). It appears appears likely that growth of the Pembine ophiolite-arc complex was terminated terminated by its its collision with and obduction onto the passive passive southern southern margin margin of the the Superior SuperiorCraton. Craton. Because subduction appears to be largely driven by slab pull, the southward southward subduction subduction of oceanic lithosphere lithosphere attached to the Superior Superior continental continental margin would would have have pulled pulled the the continental lithosphere lithosphere along with it as it descended descended into into the the subduction subduction zone zone below below the the ophiolite-arc system. With With detachment detachment of of the the subducting subducting oceanic oceanic lithosphere, lithosphere, the the buoyancy of the continental lithosphere lithosphere would have led to its rapid uplift along along with with the the leading edge of the the ophiolite-arc ophiolite-arc complex complex ((Shervais, 2001). This S h e ~ a i s2001). , This stage stage is is recorded recorded by by the deformation sequence and by the intrusion of the synSF- to post-tectonic post-tectonic deformation of the ophiolite-arc sequence units of the the Dunbar Dunbar dome. dome. Shervais, J.W., 2001, Birth, death, and resurrection: the life life cycle cycle of suprasubduction suprasubduction zone zone ophiolites: ophiolites: Geochemistry Geophysics Geosystems, Geosystems, v01.2, vol.2, Paper Paper number 2000GC0~080. 2000GC000080. On-line publication publication at at htto://g-cubed.org. hm:I/g-cubed.org. Sims, W.R., Schulz, Schulz,K.J., K.J., and and Petennan, Peterman, Z.E., Z.E., 1989, 1989, Tectono-stratigaphic Tectono-stratigraphic evolution evolution of of Sims, P.K., Van Schmus, W.R., Proterozoic Wisconsin magmatic terranes of the Penokean Orogen: Canadian Canadian Journal the Early F'roterozoic Journal of of Earth Earth Sciences, v. v. 26, 26,p.p.2145-2'58. 2145-2158. Van Wyck, N., N., and Johnson, C.M., C.M., 1997, Common lead, Sm-Nd, and U-Pb constraints constraints on on petrogenesis, petrogenesis, crustal architecture, and tectonic setting of the Penokean orogeny (Paleoproterozoic) cmstal (Paleoproterozoic)in in Wisconsin: Wisconsin: Geological Society of America Bulletin, Bulletin, v, v. 109, p, p. 799-808. 72 �- PLANNW PLANNED ACTIVITIES ACTIVITIES AND THE LAKE LAKB NIPIGON GEOSCIENCE INITIATIVE OBJECTIVES SMYK, Mark C., Ontario Geological Survey, Ministry SMYK, Ministry of Northern Development Development and Mines, Suite 8002, 435 James Suite B002,43S JamesSt. St.South, South,Thunder ThunderBay, Bay,ON ONP7E P7E6S7, 6S7,and andmembers membersof ofthe theScientific Scientificand and Implementation Committees. Committees, Lake Lake Nipigon NipigonGeoscience GeoscienceInitiative, Initiative,c/o do Ontario Prospectors Association, 1000 1000 Alloy Drive, Drive, Thunder Bay, ON P7B 6A5 The Lake Nipigon Geoscience Initiative Initiative (LNGI) (LNGI) was created in 2002 as a $7.0 M Cdn. Cdn. project projectaimed aimed at at attracting mineral investment to the area around Lake Nipigon. The O Ontario Prospectors Association's attracting n t ~ Prospectors o Association's (OPA) portion of the project is funded funded through through an agreement agreement with the Northern Ontario Onmio Heritage Heritage Fund. The partnering with the Ontario Geological Survey (OGS), the Ministry of Northern Development and OPA is partnering Canadian Mining Industry Research Organization (CAMRO), (CAMIRO), Lakehead University, Mines (MNDM), the Canadian partners and communities in the L&e Lake Nipigon area. It will focus focus on four key as well as with private sector pmners objectives: Lake Nipigon Nipigon region region through collection collection of high Maintain and then increase mineral investment in the Lake 1. 1. quality geological data and provision of interpretations that meet the needs and priorities priorities of of the the mineral mineral industry and that maintain maintain or attract attractmineral mineral investment investmentto to Ontario; Ontario; exploration discovery discovery rate by addressing "masking and deep search search challenges challenges 2. Increase the mineral exploration and skill gap" in the area; area; recently recognized 3. Respond to, and evaluate, new and exciting mineral deposit models recently 3. recognized for for nickelnickelcopper, palladium-platinum, and gold-copper mineralization in the region; 4. Reinforce and demonstrate an innovative economic development model based on local local community, community$ partnerships in geoscience that result in mineral resource economic industry, and government partnerships economic development in the local communities, communities, the the region, region, and and Ontario. Ontario. The LNGI predominantly of Emhayment, which consists predominantly LNGI is focused on the Nipigon Basin Basin I1 Embayment, Mesoproterozoic, Midcontinent Midcontinent Rift-related, RiR-related, ultramafic ultramaficto mafic mafic intrusions intrusions that that have have intruded intruded Mesoproterozoic Sibley Sibley Group sedimentary sedimentary rocks and Archean Archean basement basement rocks rocks of of the the Quetico Queticoand and Wabigoon Wabigmn subprovinces. snbprovinces. comprehensive geoscience geoscience database database that that will will assist assist in in mineral mineral exploration. exploration. The The project will develop a comprehensive consultations that helped LNGI evolved through a series of community and industry consultations helped define define the the project project parameters. AAthorough thorough compilation compilationof of previous previous exploration exploratiou and and geological data data provided provided aa baseline baseline for for the project and identify potential potential gaps in the the geoscience database. database. The The main main components componentsof of the the initiative initiative include: • • • • • • • • Precambrian Section, Detailed geological mapping, undertaken by Precambrian Section, OGS OGS Airborne Airborne magnetic survey survey Gravity Gravity survey survey Quaternary (surficial) (surticial) case studies, studies, undertaken undertaken by Sedimentary SedimentaryGeoscience GeoscienceSection, Section,OC)S OGS Geochronology Geochronology Physical property studies studies Geographic Information Systems (GIs) (GIS) compilation Complementary Complementary research at Lakehead University • Sibley Fralick) Sibley Group studies (P. Fmlick) intrusion studies (P. Hollings; G. Borradaile) • Nipigon mafic intmsion Bomadaile) • Sulphide mineralization mineralization studies studies(S. (S. Kissin) Kissin) . 73 �The TheOntario OntarioGeological GeologicalSurvey Surveywill willhelp helpacquire acquireand and publish publish the the results results of of the the geoscience geosciencestudies studies as as maps, maps, reports,and and digital digitaldata datasets. sets.The Theinformation informationwill willthen then be be available available over over the the Internet Internet through though the tbe MNDM's MiWM's reports, ERMES ERMESand andCLAIMap CLAIMapsystems. systems.This Thisvaluable valuableinformation informationwill be be used to to globally globally market market the tbe resource resoutce potentialand andinvestment investmentappeal appealofofthe theLake LakeNipigon Nipigonregion. region. potential 74 �TECTONOSTRATIGRAPHIC ASSEMBLAGES OF EASTERN ARCHEAN TECTONOSTRATIGRAPHIC WABIGOON SUBPROVINCE, NORTHWESTERN ONTARIO ONTARIO STOTT Greg STOTT GregM., M., Ontario OntarioGeological GeologicalSurvey, Survey,Sudbury, Sudbury, ON, ON, P3E P3E 6B5 6B5 (greg.stott@ndm.gov.on.ca), DAVIS, Don. W., Department of Geology, University of of (greg.stott@ndm.gov.on.ca), DAVIS, of ON, PARKER, PARKER, Jack Jack R., R., Ontario Ontario Geological Geological Survey, Survey, Sudbury, ON, Toronto, Toronto, ON, STRAUB, Kristan J., Laurentian University, University,Sudbury, Sudbury,ON ONand andTOMLINSON, TOMLINSON, Kirsty STRAUB, Knstan J., Y., Geological Survey of Canada, Y., Canada, Ottawa, Ottawa, ON ON The Archean Wabigoon Subprovince Subprovince is a complex of volcanic and sedimentary sedimentary of Mesoarchean to Neoarchean age. The supracrustal assemblages and granitoid suites of of this subprovince, which includes the Onaman-Tashota greenstone easternmost part of greenstone belt east of Lake Nipigon, preserves a history of over 250 million years of volcanism. This area has recently recently been been treated treated to to aa regional regional mapping, mapping, geochemical geochemical and and geochronological part of of the the Western Western Superior NATMAP project. A 1:250 000 compilation synthesis as part map (Stott et al. 2002) arising from this project illustrates the subdivision subdivision of of the the OnamanOnamanTashota (0-T) (O-T) greenstone belt belt into into tectonostratigraphic tectonostratigraphic assemblages assemblages (Figure (Figure 1), I), based on stratigraphic geochronological and geochemical similarities similarities and stratigraphic correlations, geochronological and contact contact component of of this this map, map, summarized summarized in in Figure Figure 2, 2, is is the the relationships. A more interpretive component delineation delineation of the assemblages in terms of the environment environment of crystallization of of volcanic volcanic and plutonic rocks and deposition of sedimentary sedimentary rocks. This is based on on lithologic lithologic and and geophysical characteristics, whole-rock geochemical classification, and where available, available, Nd isotopic isotopic signatures. signatures. Onaman-Tashota greenstone belt straddles the width of the eastern The Onaman-Tashota eastern Wabigoon Wabigoon Subprovince between the English River and Quetico metasedimentary subprovinces. Subprovince metasedimentary subprovinces. ItIt isis mainly composed composed of Neoarchean Neoarchean(dominantly (dominantly2.74 2.74— - 2.72 2.72 Ga) basaltic and dacitic dacitic flows, flows, volcanic rocks occur autobreccia and pyroclastic rocks. Mesoarchean (3.05 (3.05 —2.92 - 2.92 Ga) volcanic occur in in the northwest and along the western margin of the belt. Widespread Nd isotopic isotopic evidence evidence in the northern part of the Onaman-Tashota belt suggests that Neoarchean volcanism volcanism erupted thmugh half of the belt through Mesoarchean basement. Basement in the northern half contains an older component component than than that that south of of the Humboldt Humboldt Bay High High Strain Zone. The 2.74 Ga Willet assemblage tholeiitic basalts of ocean floor affinity dominate 2.74 dominate the the northern northern half of the 0-T O-T belt. belt. This This assemblage is flanked to the the north north and and south by calc-alkalic of continental continental margin arc affinity that border metasedimentary subprovinces assemblages of subprovinces composed of of flysch-like wacke wacke derived derived from from the the erosion erosion of of the the 0O-T belt and plutons plutons - T belt during orogenesis at at circa circa 2.7 2.7 Ga. Ga. Most Most sedimentary units units within within the the 0-T O-T belt form the supracrustal assemblages, assemblages, reflecting reflecting erosion of the underlying volcanic and youngest supracrustal and plutonic rocks towards the English English River River and Quetico basins basins to the north and south. Reference Reference Stott, G.M., Davis, D.W., D.W., Parker, Parker, J.R., J.R., Straub, Straub, K.J. ILI. and Tomlinson, K.Y. K.Y. 2002. Geology and Assemblages, eastern eastern Wabigoon Wabigoon Subprovince, Subprovince, Ontario; Ontario; Ontario Ontario Geological Geological Survey, Tectonostratigraphic Assemblages, Preliminary Map P.3449, scale 1:250 1:250 000. KY., Stott, Tomlinson, G.M.and and Davis, D.W. 2000. Nd isotopes in the eastern Wabigoon subprovince: Tomlinson, K.Y., Stott, G.M. for crustal crustalrecycling recyclingand andcorrelations correlationswith withthe thecentral centralWabigoon; Wabigoon;in inHarrap, Harrap, R.M. R.M. and and implications for I-Ielmstaedt,H.H. H.H.(eds.), (eds.),2000, 2000,Western WesternSuperior SuperiorTransect Transect Sixth Sixth Annual Annual Workshop, Workshop, Lithoprobe Report Report #77, #77, Helmstaedt, Lithoprobe Lithoprobe Secretariat, Secretariat, University University of British British Columbia, Columbia, p.119-126. p.119-126. 75 �__________ Figure1.1. Figure Tectonostratigraphic Tectonostratigaphic assemblagesof of the the assemblages Onaman-Tashota Onaman-Tashota greenstone greenstonebelt beltand and Proterozoic Proterozoicdiabase diabasedike dike swarms, swarms,Eastern Eastern Wabigoon WabigoonSubprovince. Subprovince. i o__o 1e o Kilameires Presreetlo = 0' Moos Is mlanreediala nwIarasw ass.eblagos jmeppsdi&, Is aMOOsodeaso asseaamagas Niri5000labauseilIl Ares.., Uas1jsdtstad plusast — CSaO,c rn,100asbnaSaW assamb)aga, C— m 1T!2J EnlolsIlar — Heml MalmUp-Veaus unsabdisidad Nipiger iKasreeasaasl slabs. didps (0100 Ma) and Marathon diahaaa dId. errors 12101 and 2120 Ma) *4,-p .__+ + '-•l- + -_. to. Is-a as eJ it) Wtle thdahth) Oreanar leelsilda) Zi' lhrsaloaandlh.bee Isslaidle) canasru East )thseasr Marshall Shear elena Albad-OlSdhll 4 Z222 ElethasSRlcMly (saJbaOnaas) fl staladnewas dials,, sib, salem )244E hI 247.1 Ma) arId kasadasi tIlt ftaashlisn Uscsrredlled dbMs so andatasal age. possibly ralal,dIo Biac0005Il,a 2170 Ma) UI Malaffoeer (2101 and 2121 Ma) dika swaass Figure2. 2. Tectonic Tectonic Figure affinities affinitiesassigned assignedtoto volcanicand and volcanic sedimentary sedimentary assemblages assemblagesand and plutonic plutonicsuites. suites. + + 1- +.+,+.+ 0Nakina 4 )a rss--'a------C-a't + -a-/ p Lake Nopigon1., Is°—.1 + •4 + + + + + + + + -p 4Th—---rI-n- — + + + + + + + + + + + + + -p 4 •-0- Proterozoic i continental plume related a'nyMyM Ocean floor Oceanic unsubdivided Archean 4 Orogenic plutons I } continental arc conbnental margin arc + 4.4—cc + ::-:;1 Orogenic sediments 1 Unknown tectonic affinity 20 0 Kilomalyas Meaoarcliean continental unsubdivided 76 �FIVE GOLD GOLD POSSIBILITIES SOME KEWEENAWi4N KEWEENAWAN COPPER SULFIDES FIVE POSSIBILITIES IN IN SOME SULFIDES IN IN ONTARIO AND AND MICHIGAN MICHIG?d Prow, Trow, Jim, Jim, Geological Geological Sciences, Sciences, Michigan Michigan State State University, University, emeritus, 540 Lake Avenue *2, HanCock, emeritus, 540 #2, HancoCk, Michigan 49930 49930 Most fire—assayed fire-assayed "invisible" "invisible"gold, gold, from from .12 -12to to 2.50 2.50 oz oz Au/st, Au/st, occurs in "blue "blue chalcocite" chalcocite" (with (with minor covellite) covellite) but not in black black chalcocite chalcocite (with (with no covellite) covellite) on on the the adit, adit, 1st, lst, 2nd, 2nd, and and 3rd levels Mamainse Point, Point, Ontario. Ontario. levels of of the the Coppercorp Coppercorp mine mine at at Mamainse Both Both occur occur with with specular specular hematite. hematite. Copper Copper mineral mineral zoning zoning exextending from from carbonates and oxides through through native native copper, copper, black chaldocite and specularite, chalcocite specularite, "blue "blue chalcocite" chalcocite" and specularite, specularite, to to bornite and chalcopyrite is related to to nearness nearness to the the Keweenaw Keweenaw and related faults faults apparently down which circulated oxidizing solutions The solutions during during an an upward-migrating upward-migrating hydrothermal hydrothermal episode. episode. The former faults electrical anomalies, whereas former faults display positive SP electrical anomalies, whereas nearly perpendicular perpendicular cross faults faults with commercial commercial ores display display negative SP SP anomalies anomalies of this convective convective hydrothermal hydrothermal cell cell (Trow). (Trow). Such progressive hydrothermal fluids Such progressive oxidation of hydrothermal fluids is is suggested suggested for for the Keweenawan of Michigan by the USGS1s USGS's Woodruff, Woodruff, Cannon, Cannon, and and Back. Back. Ontario, Trow For Ontario, Trow deduces thermochemical thermochemical calculations calculations with with standard standard free free energies energies and typical activities activities for for constituents constituents (except (except for for oxygen, oxygen, whose whose activities activities are are the the unknowns). unknowns). These are arrayed on on a logarithmic logarithmic scale scale which mimics the the observed observed copper copper mineral mineral zones, zones, and in that sequence sequence AAuS U S -first ~firstoxidized oxidized to to deposit deposit gold gold at at the the which chalcopyrite chalcopyrite first same oxygen activity at which first oxidized to to covellite covellite and and specularite. specularite. At the the present present it it is is uncertain uncertain if if the the "blue chalcocite" chalcocite" exsolved exsolved ivto into chalcocite chalcodite and covellite from from llblue at low low temperatures, of original covellite digenite at temperatures, or if most of original covellite was was replaced replaced by by late late chalcocite chalcocite at at roughly roughly 2,500 2,500 times times the the oxygen oxygen activity activity at at which which covellite covellite originally originally formed. formed. Essentials Essentials for for gold at Coppercorp include include 1) 1) Keweenawan Keweenawan permeable permeable basaltic vesicular beds and conglomerates, conglomerates, 2) 2) felsite felsite intrusives intrusives with with permeable permeable border border breccias as as conduits conduits for for rising rising hydrohydrothermal solutions, solutions, 3) 3) nearness to the the Keweenaw Keweenaw and and related related faults faults with with positive positive SP SP anomalies, anomalies, 4) 4) mineralized mineralized cross cross faults faults with with ores ores yielding negative negative SP anomalies, anomalies, and 5) 5) "blue ''blue chalcocite". chalcocite". In Michigan, Michigan, field examination examination of ore ore deposits deposits and and structures structures mapped by the the USGS USGS shows shows that that the the major major lodes lodes (Baltic, (Baltic, Ashbed, Ashbed, Isle Royale, Pewabic, Osceola, Calumet Isle Royale, Pewabic, Osceola, Calumet conglomerate, conglomerate, and and Kearsarge) Kearsarge) and the Cliff, Cliff, Central, Central, and and Delaware Delaware fissure fissure deposits deposits all all displayS display negative SP anomalies. anomalies. The Keweenaw, Keweenaw, Hancock, Hancock, Mayflower, Mayflower, and and negative SF Gratiot—Suffolk 5? anomalies, anomalies, approapproGratiot-Suffolk faults faults all display positive SP priate for downward oxidative oxidative contamination contamination of of rising rising hypogene hypogene (not supergene) supergene) mineralization. (not mineralization. the best best matches to Canadian Canadian gold gold in From southwest to northeast the Michigan Michigan , so so far examined, examined, occur 1) 1) from from Mass Mass City City to to the the Indiana Indiana mine adjacent intrusives and the Keweenaw fault fault in adjacent to felsite intrusives in Houghton and and Keweenaw Counties the Allouez Ontonagon County, 2) In Houghton Gap Gap fault fault between Copper City and New Allouez is is near the Copper , 77 �2 City felsite felsite and and the the Keweenaw Keweenaw fault. fault. According to to Bornhorst, Bornhorst, page 132, 132, Randy Weege of C & H H thought thought that that this this fault fault perhaps perhaps was was aa fluid fluid pathway pathway for for 60% 60% of of the the district's district's copper copper production. production. Further, Further, it it replicates replicates and and improves improves upon upon the the best best geophysical geophysical signature signature at Coppercorp, Coppercorp, the the persistent persistent SB SB zone, zone, with with flanking flanking negative SP anomalies negative anomalies in in the the midst midst of of which which is is aa positive positive SP SP anomaly. anomaly. In Michigan, the positive "core" "core" anomaly splinters splinters the northern northern end end of of the the negative negative anomalies anomalies in the westward of offf the vicinity of vicinity of Abmeek. Ahmeek. This This part of the the district district contains contains arsenic, arsenic, which accompanies accompanies gold in in many western western mining mining camps. camps. 3) 3 ) In In 1999 1999 million tonnes tonnes of chalcocite Maki and and Bornhorst Bornhorst reported reported on on the the 4½ 4% million chalcocite amygduloids of the in drilled amygduloids the Gratiot Gratiot deposit deposit in in Keweenaw Keweenaw County, County, This lode where these these beds beds are are intruded intruded by by dacite dacite (felsite). (felsite). This lode appears appears at at the the intersection intersection with with the the southward southward extension extension of of Trow's Trow's negative negative SF SP anomaly anomaly as as observed observed at at the the Central Central mine mine and and 2¼ 2% miles miles 4) In Keweenaw County, County, the the USGS's USGS's Hank Hank Cornwall Cornwall on on the SSE. SSE. 4) to the pages 166—167 166-167 describes describes minor minor traces traces of of gold gold with with mainly mainly chalco— chalcocite and specularite specularite and some some covellite covellite and and chalcopyrite chalcopyrite in in an an amygduloid near near the the top top of of the the Greenstone Greenstone flow. flow. This This is is not not near near the Keweenaw fault, N.4°E. vertical fault fault, but it is is cut by a N.4OE. fault with a negative SF SP anomaly, anomaly, which must be intersected at depth by a a N.4°E., SF anomaly, anomaly, where it N.40~., 35°—45°NW. 350-45oNW. fault fault with a positive SP it is is exposed exposed to to the the east east of of the the vertical vertical fault. fault. There exists exists aa possipossibility for for aa horizontal horizontal ore ore shoot shoot at at these these faults' faults' intersection. intersection. These geologic map These four four possibilities possibilities are plotted on the latest geologic of the the Keweenaw Keweenaw peninsula, Peninsula, by by Cannon Cannon and and Nicholson. Nicholson. Not yet yet reconnoitered reconnoitered possililities possililities may may occur occur to to the the northeast northeast of of these. these. Remember, Remember, from from 1849 1849 to 1961 1961 the the old timers all missed the the Carlin Carlin "invisible" gold. ttinvisible" gold. Nevada is is now now the the biggest gold producing producing state state because of the observations, observations, thinking, thinking, and and Perseverance perseverance of of the the USGS's USGSss Ralph Ralph Roberts Roberts and and Newmont's NewmontlsJohn John Livermore. Livermore. REFERENCES REFERENCES CITED Bornhorst, T. P. J., 3., 1997, Tectonic context of native copper Bornhorst, 1997, Tectonic copper deposits deposits Midcontinent Rift System, System, in Geological of the North American Midcontinent in Geological 127-136. Society of America Special Special Paper Paper 312, 312, p. p. 127—136. Cannon, Nicholson, S. W., 2001, Geologic map of Cannon, W. F. and Nicholson, 2001, Geologic of the the Michigan, USGS USGS Geological Keweenaw Peninsula and adjacent area, area, Michigan, Geological Investigations Investigations Series Series Map Map 1—2696. 1-2696. Cornwall, Differentiation in lavas of the Cornwall, H. R., 1951, 1951, Differentiation the Keweenawan Keweenawan series Michigan, series and the origin of the copper deposits of Michigan, Geological Society no.2, p. p. 159—201. 159-201. v. 62, 62, no.2, Geological Society of of America America Bull. Bull. v. Maki, 3. Gratiot chalcocite Maki, J. C., 1999, 1999, The Gratiot chalcocite deposit, deposit, Keweenaw Keweenaw Peninsula, Michigan, Michigan Technological University, M.S. Peninsula, Technological University, M.S. Thesis, 71 Thesis, 71 p. p. Trow, Trow, 3., J., 1992, 1992, Inductive Inductive electrostatic electrostakic gradiometry gradiometry (IESG) (IESG) deciphers deciphers Keweenawan copper plumbing system, system, Soc. SOC. Mining, Mining, Metall. and and Expl. Phoenix Phoenix Meeting, Meeting, Preprint Preprint 92—32, 92-32, 22 22 p. p. Woodruff, L. G., Cannon, Cannon, W. W. F., and Back, 3. M., M., 1994, Woodruff, Back, J. 1994, Chalcocite Chalcocite Portage Lake volcanics, volcanics, Keweenaw mineralization in the Portage Keweenaw Peninsula, Peninsula, Michigan, Michigan, 40th 40th Ann. Inst. Inst. on on Lake Lake Superior Superior Geology, Geology, Houghton, Houghton, Abstracts, p - 77—78. 77-70. Abstracts, p. 78 �Using xenotime U-Pb geochronology tto unravel the the history of Proterozoic U-Pb geochronology o unravel Proterozoic sedimentary sedimentary basins: a study study in inWestern Western Australia Australia and and the the Lake Superior Region N.J., Rasmussen, B., Fletcher, dvallini@aeol.uwa.edu.au, McNaughton, N.J., Fletcher,I., I., Griffin, Griffin, B.J., B.J., Vallini, D.A., D.A., dvallini@cieol.uwa.edu.au, University of Western Western Australia, 35 University 35 Stirling StirlingHwy, Hwy,Crawley, Crawley, 6009, Australia Diagenetic xenotime ('(P04) (YPOJ is is aa trace trace constituent constituent in in aa wide wide variety variety of of siliciclastic siliciclasticsedimentary sedimentaryrocks. rocks, It typically forms pyramidal crystals of only a few microns in size, rarely exceeding exceeding 10 10 pm, pm, growing on (isostructural] detrital zircons. A recent study by Vallini et al. (2002) [isostructural] detrital (2002) showed showed convincing convincingpetrographic petrographic and age relationships that demonstrate this U-bearing phosphate phosphate could could begin begin forming forming in sediments at or just below the sediment-water sediment-waterinterface, interface, shortly shortly after burial. burial. A few years years ago ago itit was was discovered discoveredthat that itit is possible 0 pm possible to date xenotime crystals 1210 pmininsize, size,using usingthe theSHRIMP SHRIMP (Sensitive (SensitiveHigh HighResolution ResolutionIon Ion Microprobe), providing a robust Microprobe), providing robust isotopic isotopic age age for its its formation, formation, hence an age for for early early diagenesis diagenesisand andaa close proxy for sediment deposition. Xenotime is especially useful in that it has has very high high U U contents contents and remains remains closed to to radiogenic radiogenic parent-daughter parent-daughter mobility, unlike most most other other dateable dateablediagenetic diagenetic mineral. Diagenetic Diageneticxenotime xenotimeU-Pb U-Pbgeochronology geochronology has has the the potential potential to unravel unravel the the chrono-stratigraphy chrono-stratigraphy of unfossiliferous unfossiliferous sedimentary sedimentary basins, especially especially those sequences sequences devoid devoid of of dateable dateableinterlayered interlayered volcanic rocks. basins where where aa lack of aareliable main application application is in Precambrian Precambrian basins reliabletemporal temporal volcanic rocks. Its main framework hinders evolution and framework hinders an an understanding understanding of basin basin evolution and maturation, maturation, tectonic tectonic affiliations, metallogeny metallogeny and and value value as as exploration explorationtargets. targets. Xenotime also forms during post-diagenetic fluid flow events, such as alteration, mineralisation mineralisation and and metamorphism, as well as being metamorphism, as being aa magmatic magmatic mineral mineral and and aadetrital detritalheavy heavymineral. mineral.The Theexceptional exceptional its excellent range excellent properties in situ range of of formation formation conditions conditions of xenotime, xenotime, coupled coupled with with its properties for in situ geochronology, provide many many new new opportunities opportunities in in establishing establishing the the timeframe timeframe of events geochronology, provide events in inmany many hitherto hitherto poorly poorly understood understoodsedimentary sedimentarybasins. basins. Unusually coarse (up to 200 Unusually coarse 200 microns) microns) and and abundant abundant diagenetic diagenetic xenotime xenotime crystals c!ystals in inthe themetametasandstones of the greenschist greenschist facies Mount Mount Barren Barren Group, Group, southwestern southwestern Australia, Australia, allow allowthe thedetailed detailed study of xenotime within a phosphatic phosphatic sandstone interval and and is study xenotime and its host rock. rock. Xenotime occurs within present in multiple different styles styles - as cement present multiple morphologically morphologically different cement overgrowths overgrowths on zircons, zircons, pyramidal pyramidal overgrowths on on zircons, zircons, cement (no zircon) in shale laminations, replacement of shale (?) (7) intraclasts intraclasts overgrowths and as fluid events events from from early diagenetic to low and as xenotime xenotime crystals crystals within within intraclasts. intraclasts, Multiple Multiple fluid diagenetic to low temperature/early temperaturelearly hydrothermal, hydrothermal, prior prior to metamorphism, metamorphism, were recorded recorded within within single singlexenotime xenotimecrystals. crystals. U/Pb geochronology, accompanied by Ob~eNati0nS observations of of petrographic relationships relationships between SHRIMP UlPb of xenotime and other diagenetic the various various generations generations of xenotime and between between xenotime xenotime and diagenetic minerals minerafs and pyrobitumen,allowed allowedfor for the the construction construction of of a temporal for the diagenetic pyrobitumen, temporal framework framework for diagenetic and and early early hydrothermal events that that occurred within these rocks; (1) ca 1700 1700 Ma: deposition deposition of of partly partlyre-worked re-worked phosphatic siliciclasticsediments sedimentson on the the seafloor seafloor was was followed followed by by in-situ of the in-situ phosphatisation phosphatisation of the phosphatic siliciclastic sediments formation (mean (mean age age of of 1697i 1697± 77 Ma), (2) With burial, sediments and an initial period of xenotime formation burial, an an early pore-filling pore-filling carbonate cement was introduced into parts of the interval, i n t e ~ a las , well well as as early early diagenetic diagenetic cuboid pyrite growth, (3) ca 1650 1650Ma: Ma: during during burial burial diageriesis, diagenesis, aa fluidfluid- movement movementevent eventcaused causedthe the dissolution of of primary pore pore space space and formation of of xenotime xenotime (mean age of 1646 Ma),with with partial dissolution 1646 ±t 88 Ma), accompanying phosphate remobilisation, (4) Oil migration event, (5) Several accompanying Several silica silica cement cementgenerations generations introduced around this time, (6) ca ca 1560 1560Ma: Ma: minor minor addition addition of of xenotime xenotime rims rimsto toexisting existingovergrowths, overgrowths, (7) ca ca 1480 1480 Ma: Ma: addition addition of of xenotime xenotime cement cement (no (no zircon) zircon) in in shale shale interlaminations, interlaminations, (8) (8) ca ca1200 1200Ma: Ma: peak peak of metamorphism. metamorphism. Wavelength Dispersive Spectrometer (WDS) microprobe analysis of each type type of of xenotime xenotime showed showed a gradual from LREE enrichment enrichment to to MREE MREE enrichment, enrichment,with withtime. time. Due Due to to this this radational gradual change change from gradational nature, discrete boundaries between generations, based based on on chemistry, chemistry, could could not not be established, established. This study of diagenetic to hydrothermal hydrothermal xenotime dramatically dramatically improved improved the the estimated estimatedage agerange rangeof of the Mount Mount Barren Group, which was was previously previously constrained constrained to 1200 1200 Ma Ma(peak (peakmetamorphism) metamorphism)and and 1790 Ma Ma (youngest (youngest detrital detrital zircon zircon population), population), and and discounted discounted some some previous previous tectonic tectonic models models concerning the the timing of collision between between major major cratons cratons within western Australia Australia and andthese these cratons cratons with East East Antarctica. Antarctica. Using the information gleaned gleaned from the study of xenotime in the Mount Mount Barren Barren Group, Group, aa similar similar study study basin in in the the Lake is currently underway on another Proterozoic sediment-dominated basin Lake Superior Superior Region Region containing the the Marquette Range Supergroup Supergroup and and its its equivalents, equivalents, the the North Range, Mille Marquetle Range Mille Lacs Lacs and and containing The early Proterozoic strata consist of of three unconformity-bounded unconformity-bounded lithostratigraphic Animikie Groups. The lithostratigraphic groups consisting of glaciogenics, quartzites, dolomite, iron iron formation, greywacke greywacke and andshale shaleand andminor minor intercalated volcanics. Sedimentation Sedimentation is is thought thought to to have begun Ma (correlation of Chocolay intercalated volcanics. begun —2240 -2240 Ma Chocolay 79 �Group with with Gowganda Gowganda Fm, Fm, upper upper Huronian Huronian Supergroup, Ontario) (Fairbain (Fairbain et al., 1969) 1969) and and ceased ceased by by Group —1850Ma Ma(coinciding (coincidingwith with orogen-normal orogen-normalarc arc collision collision along along the the Niagara Fault zone and the Malmo -1850 Malmo Discontinuity, during during the the Penoken Penoken Orogeny) Orogeny)(Sims (Simsetetal., al., 1993). 1993).Part Partof of the the study study is is to to determine Discontinuity, determine if xenotime-rich horizons, such such as as that in the Mount xenotime-rich horizons, Mount Barren Barren Group, can be located located in in this this stratigraphy stratigraphy and to to document document the thesedimentological, sedimentological,structural structuralor orstratigraphical stratigraphicalfeatures featuresthat thatthey theyhave haveinincommon. common. Ceflain rock units units from from the the different different sequences sequences over over the whole whole region region were were targeted targeted for for xenotime xenotime Certain analysis using proposed proposed sedimentological sedimentoiogical controls for xenotime formation that were determined from the Mount Mount Barren BarrenGroup Groupstudy. study. One sedimentary feature favourable to xenotime formation may be the presence of large quantities of sedimentary sedimentary apatite apatite within within the the host host rock rock or or adjoining adjoining rocks. rocks. A A field fieid sample sample of of low low greenschist greenschist facies facies phosphatic chert-conglomerate,atat the the base base of the Baraga phosphatic chert-conglomerate, Baraga Group, Group, from from aa documented documented phosphorite phosphorite locality locality in in the Dead Dead River River Basin, Basin, northern norlhern Michigan, contains large quantities of xenotime ranging from <30 <30 pm pm pitted pittedovergrowths overgrowthson on detrital detritalzircons, zircons,to to>100 >I00micron micronxenotime xenotimecements. cements. Other rock units units that that contain contain xenotime xenotime overgrowths overgrowths and cements of appreciable appreciable size and quantity, were; were; (i) (i) quartzite quartzite beds beds ininseveral several drillholes drillholesthrough through the the Mahnomen Mahnomen Formation, Formation, Mille Mille Laos Lacs Group, Group, Cuyuna Range, Range, contain up up to to 50 50 xenotime xenotime crystals crystals per per thin thin section, section, some someofofthese theseup uptoto—60 -60 pm pmin in size, (ii) (ii) aa sandstone sandstone bed bed within drillcore from the base of the Baraga Group in Dead River Basin- its largest was 60 o b s e ~ e dwas 60 pm pm(Di) (iii) aagrit-pebble grit-pebbleconglomerate conglomerate and and very very coarse-grained coarse-grained largest xenotime xenotime observed sandstone sandstone outcrop outcrop at Slate Slate River River Kill Hill locality, locality, Baraga Baraga Basin, Basin, which is is assumed assumed to to lie lie at at the the base base of of the the xenotime grains grains per per thin thin section which are pm in in size, and (iv) Baraga Group, averaged averaged —15 -15 xenotime are up up to to —60 -60 prn conglomerate at Big Eric's Crossing locality, Baraga Basin, contains the basal basal Baraga Baraga Group hematitic conglomerate up to 55 xenotime xenotime grains grains per per thin thin section, section, some some of of these these are are up uptoto—100 -100 pm pm in insize. size. Pokegema Pokegema Quartzite Quartzite samples, samples, West West Mesabi Mesabi Range, Range, showed showed minor <30 pm prn xenotime xenotime overgrowths overgrowths on on zircons. zircons. All of of the therock rocksamples samplesdescribed describedabove aboveare arevery verycoarse-grained coarse-grainedsandstone/conglomerate sandstone/conglomerate beds beds which are either which either located located near near aastratigraphic stratigraphic boundary boundary and/or andlor are are interbedded interbedded with with shale shalebeds. beds. Xenotime Xenotirne from these localities were analysed on the SHRIMP SHRIMP and and revealed revealed several several age age groups; groups; (i) (i) xenotime xenotime in in the the Mahnomen MahnomenFormation Formationdrillcore drillcorerevealed revealedages agesofof—1870 -1870 Ma Maand and—1770 -1770 Ma (1760-1790 (1760-1790 Ma), (U) One large xenotime overgrowth from the Dead drillcore,gave gavean anage ageofof—2600 -2600 (ii) One Dead River RiverBasin Basindtilfcore, Ma, (Di) xenotime contained contained within within the the Slate River Hill outcrop (iii) xenotime outcrop yielded yielded ages ages of of —2500 -2500 Ma, Ma, (iv) the samples from Ma. The The samples from Big Big Eric's Eric's Crossing Crossingcontained containedxenotime xenotimeshowing showingages agesofof—2550 -2550 Ma Ma and and —1750 -1750 Ma. Ouartzite in in the the West West Mesabi Range, Range, contained contained xenotime xenotime with with an an age age of of Pokegem Quartzite sample from the Pokegema -2300 -2300 Ma Maand and—1770 -1770 Ma. Ma. Xenotime yielding yielding ages ages of of ca 2500 Ma Xenotime Ma or or older older may may be befrom fromrecycled recycleddetrital detrital(magmatic) (magmatic) grains. grains. The younger Ma (1760-1790 (1760-1790 Ma), Ma), occurs occurs in in xenotime from widespread localities younger age age of of —1770 -1770 Ma localities across across the Lake Lake Superior Superior Region Region and may may reflect reflect an an epigenetic epigenetic thermal thermal event event across across the the region. region. The The age age Maof of anorogenic anorogenic magmatism, magmatism, pluton pluton emplacement emplacement and and appears to correlate correlate with an an episode episode at at —1760 -1760 Ma gneissic doming gneissic doming recorded recorded throughout throughout Wisconsin, Wisconsin, northern northern Michigan Michigan and and central centralMinnesota. Minnesota. ItIt partial melting of crustal rocks postdates the Penoken Orogeny and involved paflial rocks as as aa result result of of continentcontinentcontinent or continent-arc collision to the south south of the the region region (Sims, (Sims, 1996). 1996). This This event event is is approximately approximately coeval with the development of the Central Plains Plains Orogen (1800-1630 Ma) to the south and may be a consequence consequence of the the accretion accretion of this this terrane terrane to to the the North North American American continent continent(Sims, (Sims, 1996). 1996). This study highlights highlights the sensitivity of in-situ xenotime geochronology to identifying identifying cryptic fluid flow events within within basins. basins. This This study study will be be ongoing ongoing in in 2003-2004. 2003-2004. Fairbafrn H.W., Hurley, P.M., P.M., Card, K.D. K.D. and and Knight, C.J., C.J., 1969, Correlation and radiometric ages of Fairbaim H.W., Nipissing metasediments with Proterozoic Canadian Journal Journal of Nipissing Diabase Diabase and Huronion Huronlon metasedlments Proterozoic events in Ontario: Canadian Earth Sciences, v. 6, p. D. 489-497. Sims, P.K., Proterozoic Penokean eds., Archean and P.K., 1996, 1996,Early Ear1y'~roterozoic PenokeanOrogeny, Orogeny,ininSims SlmsP.K. P.K.and andCarter, Cafier, L.M.I-1., L.M.H., eds., and Late Survey Late Proterozoic Proterozoic Geology Geoloav of the Lake Lake Superior Suoerior Region, Realon. U.S.A.. 1993: 1993:U.S. US.Geological Geoloaical S u ~ eProfessional vProtessional -, of - . U.S.A., u Paper 1556, p.28-60. 1556, p. 28-60. Sims, al., 1993, Region and Trans-Hudson Trans-Hudson Orogen, in J.C., Jr., Jr., and Slms, P.K., P.K., et al., 1993, The Lake Superior Region in Reed, Reed, J.C., and others, eds., eds., Precambrian:Conterminous Society of of America. America, the the Precambrian:Conterminous U.S.: US.: Boulder, Colorado, Geological - Souetv Geology of ~ North v. C-2, p. o r l America, hAmerica, v. p. 11-120. 11-120. Vallini, B.,. Kranez. Krapez, B.,. Fletcher. Fletcher, l.R., Vallini.. D., D...Rasmussen, Rasmussen. 5.. 1.R.. and and McNaughton, McNauahton. N.J.. 2002, 2002.. Obtaining Obtainina diagenetic diaoenetlc , . B.. - . N.J., ages from metamorphosed sedimentary rocks: U-Pb of unusually metamorphosed sedimentary U - ~ dating dating b unusually coarse coarse xenotime xenotirne cement cement in In phosphatic v.30, phosphatic sandstone: Geology, v. 30,p. p.1083-1086. 1083-1086. ~ ~~ 80 . ~-~ ~ �EVALUATION OF INITIAL MAGMA COMPOSITIONS COMPOSITIONS FOR FOR THE THE BALD BALD EAGLE EAGLE INTRUSION INTRUSION AND ASSOCIATED ROCKS VISLOVA, Tatiana, Tatiana, Department Department of of Geology Geology and and Geophysics, Geophysics, University University of of Minnesota Minnesota VISLOVA, funnel-shaped concentrically-zoned concentrically-zoned Bald the Duluth Duluth The funnel-shaped BaldEagle Eagle Intrusion Intrusion in in the Complex is characterized characterized by very very restricted mineral compositions, and consists consists of of only only two units: units: ananolivine—plagioclase olivine-plagioclase cumulate cumulateand andananolivine—plagioclase-clinopyroxene olivine-plagioclase-clinopyroxene cumulate (Weiblen, 1980). In terms terms of of differentiated differentiated units units (Weiblen, 1965; Weiblen Weiblen and Morey, 1980). expected in the Bald to be expected in a typical typical layered layered intrusion, intrusion, the Bald Eagle Eagle Intrusion Intrusion appears appears to be petrologically petrologically incomplete. incomplete. This This has has raised raised the question question whether the four-phase (olivineplagioclase-clinopyroxene-oxide)cumulates, cumulates, assigned assignedto to the the Greenwood Lake Intrusion plagioclase-clinopyroxene-oxide) found to to the south of the Bald Eagle (Miller et al., 2002), and granophyre granophyre found Eagle Intrusion Intrusion are genetically related to the Bald Eagle Intrusion Intrusion (Weiblen (Weiblen and and Morey, Morey, 1980). 1980). New petrographic studies and and microprobe analyses (Vislova, (Vislova, 2003) 2003) make make it New petrographic studies microprobe analyses possible to evaluate for the Bald possible evaluate parent parent magma magma compositions compositions for Bald Eagle Eagle Intrusion, Intrusion, and and quantitatively assess possible petrogenetic relationships between the Bald Bald Eagle Eagle Intrusion Intrusion and spatially associated rocks. Computer programs programs (MELTS, Ghiorso and Sack, Sack, 1995; 1995; Ariskin et et al., al., 1993) were used to investigate and COMAGMAT, COMAGMAT, Ariskin investigate these questions. questions. A A primitive North North Shore Volcanic Group olivine tholeiite tholeiite (P-melt) (P-melt) was used as as an an initial initial composition (Miller magma composition (Miller and and Ripley, Ripley, 1996). 1996). Equilibrium crystallizationofof P-melt, P-melt, calculated calculated by by MELTS Equilibrium crystallization MELTS at at I1 atm total total pressure and oxygen quartz-fayalite-magnetite pressure oxygen fugacity fugacity near near ororbelow belowthethe quartz-fayalite-magnetite buffer, buffer, reproduces the crystallization order and mineral assemblages observed in the Bald Eagle Intrusion. The The calculated calculated composition compositionof ofthe thefirst firstclinopyroxene clinopyroxene(mg (mg— 81) 81) equals the one of the first plagioclase and olivine observed, however calculated compositions of olivine are are much much higher than those This could be ascribed higher those observed. observed. This ascribed to to the the dynamics dynamics of ofcrystal-melt crvstal-melt - than segregation in in a flowing segregation flowing magma system. Until the crystals crystals suspended suspended in magma magma grow grow large enough they might be carried away, erupted, and found as phenocrysts in lavas. lavas. At -—7 At 7 % melt melt remaining remaining MELTS MELTS reproduces reproduces the most most evolved evolved mineral mineral compositions in in the Bald Eagle This suggests that the Bald compositions Eagle Intrusion Intrusion (Fig. (Fig. 1). This suggests that Bald Eagle Eagle Intrusion might might be be a complete sequence with with a few Intrusion complete crystallization crystallization sequence few percent percent remaining remaining melt. the question melt. It leaves leaves unanswered unanswered the question of of the the origin origin of of four-phase four-phase cumulate cumulate and and granophyre. Modelingshows showsthat that aa more more evolved evolved high high Ti Ti and and high high Fe melt Modeling melt (D-melt) (D-melt) is required for crystallization of the evolved evolved units units in in the the Greenwood GreenwoodLake Lake Intrusion Intrusion (Fig. (Fig.1). 1). This melt of P-melt in an melt can can be be produced produced by by fractional fractional crystallization crystallization of an intermediate intermediate magma chamber at 2-3 kbar total total pressure. Equilibrium crystallization of of D-melt D-melt at 1 atm Equilibrium crystallization atm reproduces reproduces the the crystallization crystallization order, the the appearance of Fe-Ti Fe-Ti oxides, oxides, and and the the compositions most of order, appearance of compositions ofof most of the the units units associated with the Bald Eagle Intrusion (Fig. I). 1). However, However, the the most most evolved evolvedrocks rocks in in the the Greenwood LakeIntrusion Intrusion(ferrogabbro (ferrogabbrowith withFoFo<c 50) 50) and Greenwood Lake and granophyre granophyre were were not not reproduced crystallization. These units might require fractional reproduced by equilibrium equilibrium crystallization. require fractional crystallization or assimilation. assimilation. - 81 �. 65 c. C x 0 I- 6•, >1 . ,. I • 4: A -. 265 65 A£As Zig, A • 1 4 .f s0 E , 6060 A 5 A AA - A . 55 55 1 <>Bald EagleIntrusion Intrusion oBald Eagle tGreenwood AGreenwoodLake LakeIntrusion Intrusion +Calculated from P-melt P-melt • Calculated from sCalculated A Calculatedfrom from D-melt D-melt I 30 30 40 40 50 60 50 60 Mg/ (Mg+Fe) (Mg+Fe)in in olivine olivine Atom % % Mgf 70 70 60 80 MgI(Mg+Fe) variations variations in in coexisting olivine and Fig. 1. Mg/(Mg+Fe) and clinopyroxene. clinopyroxene. References: References: Ariskin, A.A., Frenkel, M.Y., MY., Barmina, Barmina,G. G. S., S., and and Nielsen, Nielsen,R. R. L., L.,1993, 1993,COMAGMAT; COMAGMAT;aa differentiation processes. processes, Computers & Geosciences, 19 FORTRAN program to model magma differentiation (8), p. 1155-1170. 1155-1170. Ghiorso, mass transfer transfer in magmatic processes; IV, A Ghiorso, M. S., and Sack, Sack, R.O., 1995, 1995, Chemical mass revised and internally consistent thermodynamic model for the interpolation interpolation and internally consistent and extrapolation extrapolation elevated temperatures and pressures, of liquid-solid equilibria in magmatic systems at elevated Contributions to Contributions to Mineralogy Mineralogvand and Petrology, Petrolopy, 119 119(2-3), (2-3),p. p. 197-212. 197-212. Hauck, S.A., Peterson, Peterson, D.M., D.M., and Miller, J.D., Jr., Green J.C., Severson, Severson, M.J., Chandler, V.W., Hauck, Wahl, T.E., 2001, Geology and mineral potential of the Duluth Complex Complex and related related rocks rocks of Geological Survey SurveyReport Report of of Investigations Investigations 58,207 58, 207pp. pp. + + northeastern Minnesota: Minnesota Geological disc in back pocket, 2002. compact compact pocket, 2002. Miller J.D., Jr. and and EM. E.M.Ripley, Ripley,1996, 1996,Layered Layeredintrusions intrusionsofofthe theDuluth DuluthComplex, Complex,Minnesota, Minnesota, USA. In: In: Cawthorn Cawthom R.G. (ed.) (ed.)Layered Lavered Intrusions, Intrusions, 531 pp. Vislova, T., 2003, Petrology of the Bald Eagle Intrusion and associated rocks rocks and its its relevance relevance to to crystallization in dynamic magma chambers in in the the Midcontinent Midcontinent Rift, Ph.D. Dissertation, crystallization Dissertation, University of Minnesota, Minnesota, 226 226 pp. pp. Weiblen, P.W. and Morey, Ci. B., 1980,AAsummary summaryof ofthe thestratigraphy, stratigraphy, petrology, petrology, and and structure structure G. B., 1980, of the Duluth Complex. In: In: Irving, Irving, A. J., and Dungan, M. A. A. (ed.), (ed.),1980, 1980,The TheJackson Jacksonvolume, volume, p. 88-133. 88-133. American Journal of Science, Science,Vol. Vol. 280-A, 280-A, Part Part 1,1,p. funnel-shaped, gabbro-troctolite gabbro-troctolite intrusion in the Weiblen, P.W., 1965, 1965, A funnel-shaped, the Duluth Duluth Complex, Complex, Lake Lake Ph.D. Dissertation, Dissertation, University County, Minnesota, Ph.D. University of Minnesota, 161 161pp. pp. 82 �A Hydrothermal Component A Component of Iron Formations-A Marquette Range Perspective T.D., 141 141 Chippewa, Chippewa, Negaunee, MI MI 49866 49866 Waggoner, T.D., The origin of of Lake Lake Superior Superior banded banded iron iron formations formations (BIF) (BIF) has has been been aa contentious contentious issue issue for at least aa century century and and aa half. half. Concepts of origin include include weathering, weathering, volcanic and organic activity activity whereby ions are carried Clear organic whereby ions carried in in and andprecipitated precipitated from from solution. solution. Clear definition of of the definition the source, source, mode mode of of transport transport orordepositional depositionalmechanisms mechanisms isis generally generally lacking. This Thispaper paperwill willaddress addressthe the strong strongevidence evidence for for aa hydrothermal hydrothermal source for "hard ores" found in the upper parts of ores" of the the Negaunee Negaunee Iron Formation (NIP) (NIP)and by extension a possible source BIP portion. The source for for the precursor hematite in the BIF TheRange Range was was formed formedin in aa tectonically active area believed to be an extensional rifting environment not unlike those active an extensional rifting environment unlike those found in Fe-Oxide (Cu, U, U,Au, REE) and some VIIMS VHMS deposits deposits The Marquette Range portion of the the Lake Lake Superior SuperiorIron Iron District District displays displays many many features features similar to other large Lake Superior BIFs found around the world and, thus, making it an excellent study subject subject for for the source excellent study source and and role role played played by by igneous igneous and andsedimentary sedimentary processes. The TheNegaunee Negauneeand andbasal basalGoodrich Goodrichunits unitsexhibit exhibitBlIP, BIF, soft soft supergene supergeneenriched enriched concentrationsand and "hard "hard ores" ores" as massive concentrations massive bodies, bodies, banded banded jaspilites jaspilites and and detrital detrital "Hard ores" conglomerates. "Hard conglomerates. ores" are generally generally dense silver gray to black black massive massive metallic metallic magnetite or schistose metallic hematite associated with jaspilite jaspilite and contain in excess of of of the the origin of the 60% iron. iron. Discussion Discussion of the "hard "hard Ores" Ores" on on the theMarquette Marquette Range Range has has metamorphism or revolved revolved around around supergene supergene enrichment enrichment prior to metamorphism or hydrothermal hydrothermal enrichment associated associated with the Penokian Orogeny. Orogeny. of the field field geology geology do donot not support support with with either eitherof ofthese thesepositions. positions. First, the Many features of cobble and pebbles of jasper hematite in the the basal basal Goodrich Goodrich conglomerate conglomerate show show random random orientation of of the 'schistose' orientation 'schistose' hematite hematite indicating indicating the the schistose schistose nature nature of of the the hematite hematite not a result of metamorphism. In existed prior to emplacement and not In addition addition many many of of the the with the "hard rocks associated associated with "hard ores" ores" exhibit exhibit hydrothermal hydrothermal minerals minerals including including sericite, sericite, tourmaline, chlorite, chloritoid, high aluminous silicates, garnet, hematite, magnetite and tourmaline. The lower and Menominee below the the NIP lower Proterozoic Proterozoic Chocolay Chocolay and Menominee sediments sediments below NIF exhibit exhibit Specular, multiple examples examples of of high-grade hematite that that can be interpreted multiple high-grade hematite interpreted as as vents. vents. Specular, microplaty and and bytroidal bytroidalhematite hematiteare arefairly fairlycommon commonininmany manyoutcrop outcropareas. areas. Some Some of of microplaty All the sites these have been described previously in literature while others have not. previously others have not. All sites were subject to exploration for iron ore during century and most exhibit exhibit were exploration for during the the late late 119th9 century shallow shafts. The The major major components components are are chert, chert, jasper and and vein quartz along along with coarse specular, microplaty microplaty and and bytrioidal bytrioidal hematite hematite contained contained in in breccia breccia zones zones that exhibit specular, exhibit episodic reworking. reworking. There episodic There are are alterations alterations to the host host rock rock as as some some occurrences occurrences exhibit exhibit chlorite, silica, silica, k-spar and aluminous alurninous silicates. silicates. A large area in sections sections 21, 22, and and 23, 23, 47-26 47-26 contain contain multiple multiple enriched enriched hematite hematite sites breccia zones adjacent to northwesterly trending faults. that form two northwest trending breccia In addition there is aa In addition to the the silica silica flooding, flooding, brecciation brecciation and and hematite hematite concentration concentration there significant area area of of andalusite cordierite and and chloritoid chloritoid adjacent adjacent to to the eastern significant andalusite cordierite eastern linear linear 83 �breccia zone in section 23. These section 23. Theseminerals minerals are are present present in aa much much broader lower regional chlorite zone of metamorphism metamorphism and most likely are a result of the hydrothermal event that impacted the three square square miles miles referenced referenced above. above. A conglomerate in Sec. 22 and 23, 47-26 has been previously described as "unusual" and 23,47-26 is sandwiched sandwiched between between lower Chocolay Chocolay argillite argillite units. units. Clasts causing dimpling in the underlying argillite were described as rafted elasts clasts from a glacial interlude. interlude. It is unlikely reef growth growth during during the the same period of of that a glacial event coincided with significant algal reef time. The "unusual" is extremely coarse, tightly tightly packed packed and and shows no time. "unusual" conglomerate conglomerate is extremely coarse, sedimentary features, sedimentary features. In addition significant rinds of k-spar have formed on the the granite granite gneiss cobbles. The The cobbles cobbles and and matrix matrix contain contain euhedral euhedral magnetite, martite and specular hematite suggesting suggesting this this area was tectonically active and may well have been been an an active active vent area over a period vent period starting starting at the the earliest earliest extensional extensional period period and continued continued to be active beyond resembles some some of of the breccias beyond the Ajibik Ajibik time. time. The conglomerate conglomerate resembles breccias at Olympic Dam and could well be a hydrothermal breccia. REE chondrite normalized analysis of of the hematite vents match quite closely closely with with both both the hematite, mametite magnetite and combination the hematite. combinationhematite/magnetite hematitelmasnetite "hard ores" found found throughout the the Range. Range. Recent hematite confirms throughout Recent work work on on the the Brockman Brockman microplaty microplaty hematite confirms a hydrothermal hydrothennal origin due to the recognition of surrounding alteration to the host. - - Initially the the vent vent areas areas were were studied studied in in relation relation to to the "hard "hard Ores" Ores" but the fact fact that that the the vents are all hematite suggests they could be the source for the precursor hematite seed hematite the source for the precursor hematite seed cores that Han Han identified identified in low low metamorphic metamorphic grade grade BIF BIF units. units. These seed cores cores have have been been identified identified in in the theNegaunee, Negaunee, Biwabik, Biwabik, Brockman, Brockman, Solcoman, Sokoman, Temiscamie Temiscamie and and Kuruman Iron hon Formations. Formations. that the extensional phase of rifting ceased ceased near near the the end of NIP time It is quite plausible that ofNIF and and aa reverse reverse compressional compressional event event started started causing causing faulting faulting that that produced producederosional erosional material for the basal Goodrich conglomerate. conglomerate. Reference: Reference: Origin of of Magnetite hon Formations Han, T.H., 1988, 1988, Origin Magnetite in Precambrian Precambrian Iron Formations of Low Low Han, T.H., Metamorphic Grade, Grade, Proceedings Proceedings of of the Seventh Metamorphic Seventh Quadrennial Quadrennial IAGOD Symposium, Symposium, p. 641-656. 641-656. 84 �nigh-Resolution High-ResolutionMultibeam Multibeam Bathymetry Bathymetry in in Lake Lake Superior. Superior. N. J. N. J. Wattrus Large Large Lakes Lakes Obsewatory, Obseruotory,University Universityof of Minnesota, Minnesota, Duluth, Duluth,MN MN 55812 55812 Like all large large lakes, lakes, the the composition composition and and shape shapeof ofthe thelake lakefloor floorof of Lake Lake Superior Superior reflects the processes that well as as in in the past. that shape shape its its formation formation today as well Maps of of the the lake floor floor made made with withtraditional traditional echosounders echosounders lack the resolution resolution to these processes preserved preserved in in permit scientists to read the subtle subtle "fingerprint' "fingerprint" of of these the lake-floor. advent of of modem, modem, high-resolution high-resolution multibeam multibeam sonar has has lake-floor. The advent revolutionized the the mapping mappingof of the the sea-floor. sea-floor. In a traditional echosounder, echosounder, the depth depth to to the thelake-floor lake-floor below the ship ship is measured measured by by timing timing how how long long It it takes for an an acoustic acousticping ping to to travel travel to to and back to the ship. the lake-floor and ship. the delay, the deeper The longer the deeper the the lake This type of surveying lake floor floor is. This surveying provides high-resolution bathymetric information along the trackline trackline followed by the the survey survey boat. is followed by boat. Nothing Nothing is about the known about the lake lake floor floor either either side. side. High-resolution multibeams use a fan of acoustic acoustic beams beams (over (over100) 100)to measure the shape of the measure the shape of the lake lake floor floor along a "swath'. "swath".By sailing a series of overlapping swaths, It it is possible to achieve complete complete coverage coverage of the lake lake floor at high resolution. resolution. Backscatter Backscatter information informationcollected collected with with the bathymetric data can can be used to to create psuedo-sidescan images of the create psuedo-sidescan images These can can be used to map lakefloor. These map spatial variations variations in the spatial the composition composition of the the lake lakefloor. floor. from the the catalog catalog of ofmultibeam multibeam surveys surveys conducted conducted by the Examples, drawn from Large Lakes LakesObservatory, Observatory,are arepresented. presented.These Theseillustrate ifiustrate the the potential potential of of this this Large formapping mapping the the subtle subtle signal processes technology for signal of past geologic processes superimposed on superimposed on the the lake lakefloor. floor. 85 � https://digitalcollections.lakeheadu.ca/files/original/8525569c75ccfe6981beac1369d7b7e6.pdf 8131941cb506976ea49cb08392f013c4 PDF Text Text INSTITUTE ON ON LAKE LAKE SUPERIOR SUPERIOR GEOLOGY GEOLOGY INSTITUTE 49t 4 9 'Annual ~ n n u aMeeting Meeting l Proceedings Volume 49 Part 2- Field Trip Guidebook QuinnesecMine, Mine.Menominee MenomineeIron IronDistrict District Quinnesec Wausecapyritic pyriticslate slate Wauseca A' Refoldedfold foldstyle styleof ofIron IronRiver-Crystal River-CrystalFalls Fallsallochthon allochthon Refolded Iron Mountain, Mountain, Michigan Michigan Iron May 7-11, 7- 11,2003 May 2003 �INSTITUTE ON LAKE SUPERIOR GEOLOGY 49TH ANNUAL MEETING MAY 7-12, 2003 IRON MOUNTAIN, MICHIGAN HOSTED BY: HOSTED Laurel Laurel G. Woodruff and William William F. F. Cannon Co-chairs Co-chairs U.S. Geological Geological Survey Survey With assistance from Michigan Technological University University and and John Gartner, John Gartner, Coleman Engineering Engineering Company Company Local committee committee representative representative Proceedings Proceedings Volume Volume 49 49 Part 2 2— Field Trip Trip Guidebook Guidebook - Field Compiled Compiled and and edited editedby byWilliam WilliamF. F.Cannon, Cannon,USGS USGS Illustrations prepared Dickenand andStacy StacySaari, Saari,USGS USGS Illustrations preparedby byConnie ConnieL.L.Dicken �CONTENTS Proceedings Volume Volume 49 49 Proceedings Part 2— 2 - Field Trips Part Trips Overview—Paleoproterozoic stratigraphy and Overview-Paleoproterozoic stratigraphy and tectonics tectonics along along 1 the the Niagara Niagara suture suture zone, Michigan Michigan and Wisconsin Wisconsin. ....... .1 Trip Trip 1. 1. Pembine-Wausau Pembine-Wausau magmatic magmatic terrane Trip Trip 2. Menominee Menominee Iron Iron District District. ................. . 3333 .......................... . 47 47 Trip Trip 3. Stratigraphy Stratigraphyand and structure structureof of the the Iron Iron RiverRiver644 Crystal Crystal Falls Falls basin basin. ................................ . 6 Trip Trip 4. Life Lifecycle cycleof ofan aniron irondeposit—the deposit-the Republic RepublicMine Mine 877 from from ore ore genesis genesis to mine mine restoration restoration ................ . 8 Cover illustrations: illustrations: Wauseca Wauseca Pyritic Pyritic Member Member of Dunn Dunn Creek Creek Slate. AAsulfide sulfidefacies faciesiron-formation iron-formationfrom fromthe the Iron Iron River-Crystal River-Crystal Falls district, Michigan. Original Original photograph photograph appears appears in USGS in USGS Professional ProfessionalPaper Paper 570. 570. Schematic illustration illustration of cross-folding cross-folding in in the the Iron Iron River-Crystal River-CrystalFalls Falls district district as as depicted depicted USGS Professional ProfessionalPaper Paper 570. 570. in USGS Quinessec Mine in the Menominee Iron District at at Quinnesec, Quinnesec, Michigan. Michigan. Photograph by Photograph by Elizabeth Heinen. Heinen. Elizabeth �PALEOPROTEROZOIC STRATIGRAPHY AND TECTONICS ALONG THE PALEOPROTEROZOIC STRATIGRAPHY NIAGARA NIAGARA SUTURE ZONE, MICHIGAN AND WISCONSIN G.L. LaBerge1, ~ a ~ e r g eW.F. ' , cannon2, J.S.Klasne,3, ~lasne?, W,Ojakangas4 0jakangas4 Cannon2, K.J. K.J. Schulz2, Schuli, J.S. R.R.W. U.S. University Wisconsin-Oshkosh(retired) (retired)and and U.S. U.S. Geological GeologicalSurvey, Survey,2* U.S. University of Wisconsin-Oshkosh Geological Survey, Geological Survey, Western Illinois University (retired) and U.S. U.S. Geological GeologicalSurvey, Survey, University (retired) 4 University of Minnesota Minnesota Duluth Duluth (retired) (retired) INTRODUCTION INTRODUCTION The Niagara Niagara suture zone formed during the Penokean Penokean orogeny at about 1875 1875 Ma by collision of island island arcs of the Wisconsin magmatic magmatic terranes with the southern margin of the Superior craton and its epicratonic cover of the Marquette Marquette Range Supergroup. The Niagara Niagara fault, fault, generally generally considered considered the the principal principalsuture suture Supergroup. boundary, has an arcuate trace across northern Wisconsin defined defined primarily from geophysical data, sparse sparse outcrops, outcrops, and and widely widely spaced spaced drill drill core core data data(fig. (fig.1). 1).In In geophysical more detail, the Niagara fault is but one of a family of subparallel anastamosing anastamosing faults that bound bound structural panels of Paleoproterozoic Paleoproterozoic rocks, which together comprise the Niagara suture zone (fig. 2). The rocks within these panels have a distinctive structural style marked marked by by tight tight folds folds with with widely widely varying varying but but commonly commonly steeply plunging axes. These probably formed by refolding of simpler, gently plunging folds, which are widely recorded recorded in in correlative correlative strata strata north north of of the the suture suture zone. A blanket of glacial material material over most of the region, the on-lap of Paleozoic thick cover cover of of Keweenawan Keweenawanrocks rocksof of Paleozoic rocks rocks in northern northern Michigan Michigan and and aa thick the Midcontinent rift system in northwestern Wisconsin and eastern Minnesota Minnesota have obscured the suture zone over most of its length in the southern Lake Superior region. region. The area area described described in in this this guide guide is is the the only only area areawhere where extensive exposures allow direct observations observations of stratigraphic stratigraphic and and structural structural features along features along the collision collision zone. The purpose of these field trips is to examine exposures along and on both sides of the Niagara suture zone. Stops were chosen to illustrate: 1) the Paleoproterozoic Paleoproterozoic sedimentary sedimentary sequence sequence developed developed on on the the continent continent margin; margin;2) 2) magmatic terranes; and 3) igneous rocks that constitute parts of the Wisconsin magmatic some of the structural collision of the island island structural features produced as a result of the collision arcs with the continent margin, and their influence influence on interpretations interpretationsof the stratigraphy. stratigraphy. 1 �92' 84' 88' EXPLANATION 46' Pajeozoic strata- limestone, dolomite, sandstone shale Grenville Province- Middle Proterozoic gneiss and Ed'dáiplutonic rocks fiIi.± Midcontinent Rjft- Middle Proterozoic flood basalt, rhyolite, sandstone, and gabbroic intrusive rocks Anorogenic pluton- Middle Proterozoic granite, " anorthosite Marquette Range Supergroup andAnimikie GroupEarly Proterozoic metasedimentary and meta vol conic rocks. Iron ranges in black Wisconsin Maginatic Terranes- Early Proterozoic metavolcanic, metasedimentary rocks, andplutons Huronian Supergroup- Early Prolerozoic sedimentary rocks, gabbroic intrusions Superior Province- Archean granilic and - ' metavolcanic rocks — — — Gravity gradient, southern edge ofArchean craton MAJOR JR ON RANGES A Gogebic B i'il'arquette ( I \• C Menominee 42' D Iron River Crystal Falls E Cuyuna F Mesabi 2 TGGUnfltit 92' Figure Figure1. 1.Generalized Generalizedgeologic geologicmap mapof ofthe theLake LakeSuperior Superiorregion regionshowing showingthe themajor majoriron iron ranges. ranges. The Thesouthern southernpart partof ofthe theupper upperpeninsula peninsulaofofMichigan Michiganand andadjacent adjacentparts partsofofnortheastern northeastern Wisconsin Wisconsincontain containexposures exposuresofofArchean Archeanrocks rockswith withoverlying overlyingPaleoproterozoic Paleoproterozoic metasedimentary metasedimentaryand and metavolcanic metavolcanicrocks rocksof of the theMarquette MarquetteRange RangeSupergroup, Supergroup,as aswell well as as aa wide wide variety varietyof of volcanic, volcanic, sedimentary, sedimentary,and andintrusive intrusiverocks rocksof ofthe theWisconsin Wisconsin magmatic magmaticterranes. terranes. The The modern moderngeologic geologicframework frameworkofofthe thearea areahas hasbeen beenestablished established largely 1940'sthrough through largely by by mapping mappingby by the the U.S. U.S. Geological GeologicalSurvey Surveyconducted conductedfrom fromthe the1940's the Jamesand andothers, others,1968; 1968;Sims, Sims, the1980's 1980's(Bayley, (Bayley,and andothers, others,1966; 1966;Dutton, Dutton,1971; 1971;James 1990; 1990; Sims Sims and and Schulz, Schulz, 1993). 1993). Beginning Beginningininthe thelate late1970's, 1970's,the theimportance importanceof ofthe the Niagara Niagarafault fault zone zone as as aa suture suturebetween betweenoceanic oceanicterranes terraneson onthe thesouth southand andaacontinental continental assemblage assemblageon onthe thenorth northwas wasdocumented documented(Cambray, (Cambray,1978; 1978;Greenberg Greenbergand andBrown, Brown, 1983; Larue and Sloss, 1980; Larue, 1983). This led to a period of study of the 1983; Larue and Sloss, 1980; Larue, 1983). This led to a period of study of thestructural structural geology geology of of the the region regionin inorder order to todecipher decipherthe thestructural structuralevents eventsconsequent consequentto tosuturing suturing (Larue (Larue and and Ueng, Ueng, 1985; 1985;Sedlock Sedlockand andLarue, Larue,1985; 1985;Ueng Uengand andLarue, Larue,1987). 1987).This This guidebook guidebook draws draws heavily heavily on on these these previous previousstudies, studies,supplemented supplementedin inplaces placesby byour our own own observations. observations. 2 �8900 8800 - 8800' - Diabasedike dike Diabase (Middle (MiddleProterozoic) Proterozoic) of the the Niagara of Niagara fault /\/ Faults set set /Av A/Faults Faults Cambrian Cambrian El Munising MunisingSandstone Sandstone Paleoproterozoic 46 00, 0° Metagabbro Metagabbro Paint Grroup, Paint River Grroup, iron-formation iron-formationin inblack black Baraga Group Group Menominee MenomineeGroup, iron-formation iron-formation in in black black Chocolay Chocolay Group Group Wisconsin MagmaticTerranes MagmaticTerranes Archea n Archean Granite Granite and and gneiss gneiss BF BLF CT FT NF NRF -4530 45¡30PRF PRF SRF 30 0 30 60 Badwater fault fault Badwator Bush Lake Lake fault fault Bush Calumet trough Calumet trough Felch Felchtrough trough Niagara Niagara fault fault North North Range Range fault fault Paint Paint River River fault fault South South Range Range fault fault 90 Kilometers Kilometers Figure 2. Geologic Geologic map map of the Niagara Niagara suture suture zone and surrounding surrounding terranes terranes showing the location of named named features referred to in the text. Figure �CONTINENT CONTINENT MARGIN MARGIN SEQUENCE SEQUENCE A thick succession of dominantly dominantly sedimentary rocks that were deposited deposited on Archean basement is widely distributed in northern Michigan, Wisconsin and Minnesota, and, to a lesser extent, in northern northern Ontario. The succession records a wide variety of geologic iron-formations in environments and includes the extensive and economically important iron-formations the Lake Superior Superior region. region. STRATIGRAPHY Archean Archean Carney Lake Gneiss: The Carnev Theonly onlyextensively extensivelyexposed exposedArchean Archean unit unit in in the the field field trip trip area areais is the Carney 2). The The Carney Carney Lake, Lake, like like other other Archean Archean rocks rocks farther farther Carney Lake Lake Gneiss Gneiss (fig. (fig. 2). north, is exposed in the core of a Penokean bounds the Menominee Penokean structural uplift. It bounds Menominee Range on the north north and and is is the the basement basement on on which which the the Paleoproterozoic Paleoproterozoiciron-bearing iron-bearing sequence sequence of the Menominee Menominee Range Range was deposited. The Carney Lake Lake Gneiss is seen at stops 2-2 and 2-3, where it is exposed beneath the unconformity unconformity at the base of the Paleoproterozoic Fern Creek Formation. According to Treves (1 (1966) 966) granitic gneiss forms forms about 85 85 percent percent of the the unit. unit. Inclusions Inclusionsof of amphibolite, amphibolite, biotite biotite schist schist and and some some quartzite quartzite constitute constitute about about 10 10 percent, the the remainder remainder being being granodiorite granodiorite and and syenite syenite dikes. AmphiboUte inclusions are are more abundant in the northern Amphibolite inclusions northern part part of the the complex, complex, whereas biotite biotite schist is is more more common common in in the southern part. part. According to Davis Davis and and others others (1960) (1960) the Carney Carney Lake Lake Gneiss Gneiss is is about 2,700 million million years old. Foliation Foliation and tabular inclusions inclusions define a complex internal internal folding pattern pattern in the gneiss, which is mostly mostly a result of Archean Archean deformation. deformation. Paleoproterozoic Paleoproterozoic The Paleoproterozoic continent continent margin margin sequence is is comprised comprised of sedimentary and volcanic rocks rocks at at least least several severalkilometers kilometersthick thickininthe theMenominee Menomineeand andIron IronRiver River--Crystal Crystal Falls districts and probably much thicker in much of the area. The stratigraphic relationships are shown in figure 3, which compares the stratigraphy in the field trip area relationships with that in other well-studied well-studied areas to the north north and east. Originally Originally referred referred to as "Huronian" by (1911), and andlater lateras as"Animikie "AnimikieSeries" Series"by byJames James by Van Hise Hise and and Leith Leith (1911), (1958), (1 958), the sequence was renamed renamed the Marquette Range Supergroup, comprised of the Chocolay, Menominee, Menominee, Baraga Baragaand and Paint PaintRiver RiverGroups, Groups,by byCannon Cannonand andGair Gair(1970). (1970). The stratigraphic stratigraphic succession succession of the the first three three groups groups is is well well established. established. However, However, the stratigraphic stratigraphic position position of the Paint Paint River River Group, Group, along along with the Badwater Badwater Greenstone, is is of four less certain. The Paint River Group was originally considered the youngest of stratigraphically stratigraphically superposed superposed groups groups by by James James (1958), (1958), but but more more recent recent interpretations interpretations generally emplaced over the Baraga Group. generally consider consider it to be be an allochthon allochthon structurally structurally emplaced The Paint Paint River Group Group may may be be a distal, deeper water equivalent of the Menominee Menominee and Baraga Groups, which has been thrust onto the continental margin during Penokean compressive compressive deformation. deformation. 4 �Areas north and east of Areas north Menominee MenornineeRange Range Iron River-Crystal River-CrystalFalls Falls Iron field trip trip NortheasternWisconsin Wisconsin Northeastern 1 , allochthon allochthon Amberg Granite I 1833 Ma Diabase and gabbro ] 11 1 Goodrich Quartzite Ne 0.1 unconformity unconfmitY Amasa AmasaIron Iron - formation 1- — Negaunee ic lrontormation5 aj -- unconformity — I Piblk AjibikQuartzite Quartzite - ' -I860 Ma —1860 ...Ma 1 Volcanic Volcanic and and mafic mafic intrusive intrusiverocks rocks —1870 -1870 Ma Ma Ophiolite Dunn Creek Slate I Feich FelchFormation Formation Badwater Greenstone uhconfomiity unconformity Kona KonaDolomite Dolomite Randviile Dolomite Dolomite Randville Mesnard Quartzite Sturgeon SturgeonQuartzite Quartzite Enchantment Lake Formation Fern Fern Creek Creek Formation Formation giaciogenic gladogenicsediments sediments unconformity -- unconformity Granite Graniteand and gnelss gneiss n I unconformity 0n 1 Qieciogenlc sediments Archean Archean 1 Granite Graniteand and gneiss gneiss . Riverton Iron-Formation Riverton Iron-Formation 1874 Ma ISiamo Siamo Slate Slate 1 21 2 Hiawatha Graywcke CD z . Vulcan Vulcan Iron-formation iron-formation Volcanics FortuneLake LakeSlate Slate Fortune I 0. I Stambaugh Stambauah Formation Formation Michigamme Michigamme Formation Formation (0 1835 Ma u Diabase and gabbro 2 Michigamme Formation 0 Spikehom Creek Granite I 'Tobin Lake Granite Diabase and gabbro 1752 Ma 1 detachment detachment surface surface Camey Lake Gneiss —2700 Ma Camey!-$;p I Metasedimentary ro Metasedimentaryrocks Note: The Thestratigraphic stratigraphicrevisions revisions shown shown in in Note: diagram diagram are are not not official official USGS USGS revisions revisions to to stratigraphic stratigraphic usage usageas as of of2003. 2003. Figure 3. Correlation Correlation chart chart of Paleoproterozoic Paleoproterozoic strata strata in in Menominee Menominee and and Iron IronRiverRiverFigure Crystal Crystal Falls Falls and and surrounding surrounding terranes. terranes. Includes Includeschanges changesto to previous previoususage usagenot notyet yet officially officiallyadopted adoptedby bythe the USGS. USGS. Chocolay Chocolay Group Group Fern Creek Creek Formation: Formation: Francis FrancisJ.J.Pettijohn Pettijohn(1943) (1943) described describedand andnamed namedthe the Fern Paleoproterozoic Fern Fern Creek Creek Formation, Formation,verified verified itit as as aa basal basal sedimentary sedimentaryunit unit resting resting Paleoproterozoic unconformablyupon upon Archean Archean granitic granitic basement basement of of the the Carney Carney Lake LakeGneiss, Gneiss,and and unconformably suggested aa glacial glacial origin. origin. Additional Additional descriptions descriptionswere were made made by by Trow Trow (1948), (1948), by by suggested Freedman and and others others (1961) (1961) and and by by Bayley Bayley and and others others (1966). (1966). Prior Prior to to Pettijohn's Pettijohn'swork, work, Freedman the relationships relationships of these these sedimentary sedimentary rocks rocks to the granitic rocks of the region region were the debated debated by by Bayley Bayley (1904), (1904), Lamey Lamey (1937), (1937), and and Dickey Dickey (1936). (1936). Two other other basal basal Paleoproterozoic Paleoproterozoicformations formationsin in the the region, region,the theEnchantment EnchantmentLake Lake Two Formation Formation and and the the Reany Reany Creek Creek Formation, Formation, are are present present in in the the Marquette MarquetteTrough, Trough,50-55 50-55 miles miles north north of the the Fern Fern Creek Creek exposures. exposures. ItIt has has been beensuggested suggestedby byseveral severalworkers workersthat that these rock rock units unitsare arealso alsoglaciogenic glaciogenicand andcorrelative correlativewith with the the Fern FernCreek CreekFormation Formation these (e.g., Gair and and Thaden, 1968; 1968; Puffett, Puffett, 1969; 1969; Gair, 1981; 1981; Ojakangas, 1984). The two exposures of the Fern Creek Formation seen on these the Fern Creek Formation seen on these field field trips trips (stops (stops2-2 2-2and and2-3) 2-3)are are of more morethan than local localsignificance. significance.Correlation Correlationwith with the the Gowganda GowgandaFormation Formationininthe the of Huronian Huronian Supergroup Supergroup ca ca 120 120 miles miles to to the the east east in in Ontario Ontario and and with with the the Snowy Snowy Pass Pass Supergroup 900 900 miles miles to the the WSW in in Wyoming Wyoming has has been been proposed proposed by by various various workers Supergroup (e.g., (e.g., Puffett, Puffett, 1969; 1969; Young, Young, 1970, 1970, 1973, 1973, 1983; 1983; Ojakangas, Ojakangas, 1984, 1984, 1985, 1985, 1988; 1988; Roscoe Roscoe and Card, Card, 1993). 1993). Young Young (1970) (1970) suggested suggestedthat that all allof of these theseunits, units,and andothers othersininQuebec Quebec and 5 �and the NW Territories, Territories, are are remnants remnants of of aa continental-scale continental-scaleglaciation. glaciation. Furthermore, Furthermore, correlation correlation with glaciogenic glaciogenic units units on the Fennoscandian Fennoscandian Shield in Finland Finland and adjacent Karelia, Russia, has also been proposed (Marmo and Ojakangas, 1984; Ojakangas, Ojakangas and others, 2001). 1985, 1988; Ojakangas and others, 1991; 1991; Ojakangas strengthened by the presence of It must be emphasized that the correlations are greatly strengthened similar stratigraphic stratigraphic sequences sequences in in all of the above-named above-named areas and regions. This is is as follows, moving stratigraphically stratigraphically upward: glaciogenic rocks, paleosols paleosols (or remnants thereof), orthoquartzites, orthoquartzites, and and carbonates carbonates (Ojakangas, (Ojakangas,1997; 1997; Ojakangas Ojakangas and and others, others, 2001). Mafic 2.15 Ga cut cut all all the the aforementioned aforementioned sedimentary - 2.1 5 Ga Mafic dike dike swarms swarms dated dated at at 2.2 2.2 — sequences. It is possible possible that the glaciogenic glaciogenic units units of North North America and and the Baltic Baltic region (Marmo and Ojakangas, 1984) were formed on a single supercontinent, Kenorland, at about 2,300 Ma Ma (Ojakangas, (Ojakangas, 1988). 1988). The breakup breakup of Kenorland Kenorland occurred occurred at 2.2 — 2.1Ga Gawith withthe theemplacement emplacementof of the the Nipissing Nipissing mafic mafic sills sills and and dike dike swarms of of the - 2.1 Canadian Canadian Shield and the Jatulian mafic rocks rocks of the Fennoscandian Fennoscandian Shield. The Fern Fern Creek Formation Formation is exposed in only a few small areas adjacent to the Archean Carney Lake Gneiss Gneiss (figs. 2, 4). These These exposures exposures likely likely represent represent erosional erosional remnants remnants of more more widespread widespread glaciogenic glaciogenic deposits depositspreserved preservedin intopographic topographic lows lowson onthe theArchean Archean bedrock bedrock surface. Post-glacial Post-glacial weathering, erosion, and sorting sorting by wind and water resulted stratigraphic sequence of glaciogenic deposits, paleosol resulted in the stratigraphic paleosol (sericitic (sericitic schist), and quartz sand (now the Sturgeon Sturgeon Quartzite) Quartzite) that will be seen at stop 2-2 (Fern Creek locality) Black Creek Creek (sec. (sec. locality) and and stop stop 2-3 2-3 (Sturgeon (SturgeonRiver Riverlocality). locality).Two Twoother otherlocalities localities—- Black 6, T. 39 N., R. 28 W.), and Pine Creek (sec. 32, T. 41 N., N., R. 29 W.) W.) (Freedman (Freedman and and others 1961) are relatively relatively inaccessible inaccessible and and not as well exposed. These others 1961) — - are These four four small small areas of exposure are located along a 17-mile portion of the northwest-trending exposure located 17-mile portion northwest-trendingcontact between the Carney Lake Gneiss and these basal Paleoproterozoic units (fig. 4). The two field stops are within 5 miles of each other. Sturcieon Quartzite: At thick, light light colored, colored, Sturaeon Quartzite: At most most places places in in the the Menominee Menominee district district aa thick, vitreous quartzite forms the basal basal Paleoproterozoic unit on the Carney Lake Gneiss (Bayley and others, 1966). 1966). The type exposures exposures of the Sturgeon Sturgeon Quartzite Quartzite are along the Sturgeon Sturgeon River in the Felch Felch district, about six mites miles north of the Menominee district (see fig 2). In In the Menominee Menominee district the quartzite quartzite forms a continuous continuous belt belt along along the southwest margin of the Carney Lake Gneiss. ItIt is well exposed at stops 2-2 and 2-3. The Sturgeon Sturgeon Quartzite Quartzite ranges ranges from 1,000 1,000 to 2,000 feet thick, thick, is is composed composed primarily primarily of white, gray, green or pink vitreous vitreous quartzite, quartzite, commonly showing ripple marks and crossbedding. It is composed composed almost entirely of quartz. quartz. Trow (1948) (1948) showed that the crossbedding bedding data suggest that the quartz quartz sand sand of the Sturgeon Sturgeon was derived derived from the northwest, and Pettijohn Pettijohn (1957) (1957) showed that many of the Paleoproterozoic quartzites of the Lake Superior Superior region region had had a source source area area to the west or northwest. The Sturgeon Sturgeon Quartzite is considered considered to have formed during a marine marine transgression transgression onto the Archean craton, possibly to the northwest. Bayley and others (1966) interpreted the quartzite to (1966) interpreted be conformable conformable and gradational gradational with the Fern Fern Creek Formation, and to be conformable conformable with the overlying overlying Randville Randville Dolomite. Dolomite. Randville Dolomite: The TheRandville RandvilleDolomite Dolomiteoverlies overliesthe the Sturgeon SturgeonQuartzite Quartzitein in the the Menominee and Felch districts. It takes its name from exposures near Randville, north of of southeast-trending belts belts in the Menominee Menominee Iron Mountain. The dolomite occurs in three southeast-trending 6 �district because because of repetition repetition by by faulting faulting (Bayley (Bayley and others, 1966). 1966). ItIt is estimated to be be thick. Classic Classic exposures exposures of deformed deformed stromatolitic dolomite dolomite are are present present about 2,000 feet thick. at stop stop 2-6. The Randville Randville is composed mainly of massive massive clastic dolomite with thick- and thinbedded sandy dolomite, dolomitic and quartzose slate, and pebbly dolomite 3-12 conglomerate (Bayley (Bayley and others, 1966). 1966). Domal stromatolites 1-3 1-3 inches high and 3-1 2 common and widespread in the formation. They form reefs reefs as inches in diameter are common much much as 50 feet thick and and are are of great great aerial aerial extent. extent. The The stromatolites stromatolites are usually in thinbedded sandy dolomite and conglomeratic dolomite of shallow water origin. The clastic dolomite consists mainly of rounded rounded carbonate fragments that range range in size from sand to cobbles. Internally the fragments consist of fine-grained fine-grained dolomite. dolomite. Bayley and others (1966) (1966) stated stated that the the presence presence of of stromatolites, stromatolites, oscillation oscillationripple ripplemarks, marks,mud mudcracks cracksand and (1981) clastic beds indicates deposition in very shallow water. Larue (1 981) reported that remnants of anhydrite and gypsum gypsum crystals, along with the bedding features, suggest deposition of the Randville Randville Dolomite in a paleo-sabkha environment. Thus, the Chocolay Group may record continuous deposition from a glacial environment (the Fern Creek Formation) Formation) to an an arid arid sub-tropical sub-tropical environment environment(the (theRandville RandvilleDolomite). Dolomite). Menominee Group Menominee Feich Felch Formation: The TheFelch FelchFormation Formationisisaasericitic sericiticslate slateand andquartzite quartzitethat thatoverlies overliesthe the Randville Randville Dolomite. Dolomite. ItIt consists consists of thin-bedded thin-beddedsericitic sericitic slate slateand andphyllite phylliteand andintercalated intercalated thin-bedded thin-bedded quartzite, with the quartzite quartzite layers layers more more prevalent prevalent near near the top of the (Bayley and and others, others, 1966). 1966). ItIt is is about about 100 100 feet feet thick thick in in the the Menominee Menominee district, district, formation (Bayley formation but thickens to about 500 feet in in the Feich Felch district district to the north. Bayley and others (1966) considered the Felch Formation Formation to be correlative with the Ajibik Quartzite and Siamo Slate of the Marquette district and the Palms Palms Formation Formation of the Gogebic district. They state that although the Felch Felch Formation Formation is structurally concordant on the Randville Dolomite, both local and regional relationships suggest that the Felch Formation is unconformable on the Randville Dolomite. However, the Felch Formation is conformable and gradational gradational with the overlying overlying Vulcan Vulcan Iron-formation. Iron-formation. Iron-formation: The Vulcan Iron-formation: The Vulcan Vulcan Iron-formation Iron-formationis is the the major major iron-bearing iron-bearing unit of the iron-formation is divided divided into four units, two composed mainly of of Menominee district. The iron-formation granular iron-formation and two composed of slate and slaty iron-formation. In iron-formation composed In succeeding order the units are the Traders Iron-bearing Iron-bearingMember, the Brier Slate, the Iron-bearing Member, and the Loretto Curry Iron-bearing Loretto Slate. The Traders and Curry Members layers of granular granular jasper alternating alternating with layers layers of magnetite magnetite and and hematite. hematite. The The contain layers Brier and Loretto Members are mainly laminated siliceous iron-rich slate, which locally Loretto Members contains laminae of detrital quartz, feldspar, micas, micas, zircon and tourmaline. According to (1958), 958), the iron-formation iron-formation is about 1,000 feet thick, of which about 730 feet is is Dutton (1 ferruginous 330 feet, Loretto LorettoSlate Slate == 400 400 feet) feet) and and 270 270 feet feet is is ferruginous slate slate (Brier (Brier Slate Slate == 330 iron-formation (Traders granular iron-formation (Traders = 100 100 feet, Curry Curry = = 170 170 feet). The The Vulcan is is seen seen at 2-4 and stops 2-4 and 2-5. 7 �Group Baraga Group In the area of the field trips, rocks rocks of the Baraga Baraga Group Group are are mostly mostly in in the the Menominee Menominee Range, where they consist of a variety of rock types generally combined combined into into the Michigamme Formation. The belts underlain by the Michigamme Michigamme Formation are very poorly exposed, exposed, which accounts, at least in part, for for the lack lack of of detailed detailed mapping mapping of of what what may well be otherwise discernible map units. According to Bayley and others (1966) (1966) the Michigamme Formation consists chiefly of slate, especially quartzose, micaceous, and Michigamme graphitic varieties, subgraywacke, subgraywacke, quartzite, quartzite, conglomerate, conglomerate, dolomite, dolomite, dolomitic dolomitic quartzite, quartzite, iron-formation.More More recent recent exploration exploration drilling drilling also has has identified identified units units of and some iron-formation. rocks. An unconformity unconformity between between the Michigamme Michigamme and and underlying underlying Vulcan mafic volcanic rocks. Iron-formation is indicated indicated by the presence of widespread basal conglomerate, Iron-formation clasts of iron-formation iron-formationand and other other Menominee Menominee and and Chocolay Chocolay Group Group containing clasts lithologies, and by regional truncation truncation of pre-Baraga pre-BaragaGroup Group units units beneath beneath the the basal basal lithologies, Michigamme units. The Michigamme Michigamme Formation Formation is is the youngest youngest Paleoproterozoic Paleoproterozoic unit Michigamme preserved preserved in in the the Menominee MenomineeRange. Range. The Michigamme Michigamme Formation Formation extends extends westward westward and is present widely, although poorly exposed, in fault panels panels lying lying between between the Wisconsin magmatic magmatic terranes terranes and and the the Iron Iron River-Crystal Falls allochthon. has provided providedaa detailed detaileddescription description of of these these River-Crystal allochthon. Dutton Dutton (1971) (1971) has lithologically as their equivalents in the Menominee rocks, which are equally as varied lithologically Range. Farther north, the Michigamme Michigamme Formation is structurally beneath the Iron Iron RiverCrystal Falls allochthon and is widely exposed north and east of the allochthon. This area also has has considerable lithologic diversity, but graded-bedded graded-bedded graywackes, graywackes, pelitic politic schist and slate, and and impure impure quartzite become become more more dominant dominant toward toward the north north and and interlayered interlayered mafic volcanic rocks rocks and iron-formation iron-formationbecome become volumetrically volumetrically minor. minor. Total thickness of the Michigamme Michigamme Formation Formation is is not not known, known, but but itit is is probably probably several several thousand feet or more. Dutton Dutton (1971) (1971) stated stated that that the the Michigamme Michigamme might might be be as as much muchas as 20,000 feet thick in in the Florence, Florence, Wisconsin Wisconsin area. area. Barovich Barovich and and others others (1989) (1989) used used Nd Nd isotope data to show that the Michigamme Michigamme Formation Formation in in the field trip area area was derived derived from a Paleoproterozoic Paleoproterozoic source. Sims and others (1993) (1993) suggest that the source was the Wisconsin Wisconsin magmatic magmatic terranes terranes to to the the south, south, with with deposition deposition in in aa foredeep foredeep environment environment during docking docking of the the Wisconsin Wisconsinmagmatic magmatic terranes terranes with with the the continent continent margin. margin. Paint River Group Group Rocks assigned to the Paint River Group consist of about 6,500 feet of sedimentary sedimentary strata, which overlie as much much as 15,000 volcanic rocks. rocks. They form the of 15,000 feet of volcanic They form the bedrock bedrock of the Iron Iron River-Crystal River-Crystal Falls Falls district district (James (James and and others, 1968). 1968). Five Five formations formations were were assigned assigned originally to the Paint Paint River River Group. Here, Here, we add add a sixth sixth formation, the the Badwater Greenstone, as discussed in more detail below. The stratigraphic position position of the Paint Paint River River Group Group has has been been problematical problematical for more more than than half half aa century. century. The The group group was interpreted (e.g. Leith Leith and and interpreted to be be part part of the the Michigamme MichigammeFormation Formation in in older older reports reports (e.g. others, 1935), 1935), but but was interpreted interpreted to to be be aa separate separate group group (younger (younger than than the the Michigamme) by James and others (1968) Michigamme) (1968) and Cannon and Gair (1970). (1970). However, as discussed discussed below, below, more recent recent studies studies have have suggested suggested that the the Paint Paint River River Group Group may may be a fault-repeated fault-repeated sequence sequence correlative correlative with the Baraga Baraga and/or and/or Menominee Menominee Groups Groups (cf. (cf. Sims Sims and 1993). Below, Below, we provide provide evidence evidence that the Paint Paint River Group Group is an and others, others, 1993). is an 8 �structurally emplaced emplaced over the Michigamme Michigamme Formation Formation and consists of deepallochthon structurally water distal distal equivalents equivalents of the Menominee Menominee and Baraga Baraga Groups. Groups. The 6,500 feet of Paint Paint River River Group Group sedimentary sedimentary strata strata have have some some special special Larue and and Sloss Sloss (1980) (1980) point point out that presence presence of turbidites indicates indicates characterisitics. Larue that they were deposited in a subsiding basin. Likewise, 986) noted Likewise, Cannon Cannon (1 (1986) noted "the group consists of a very unusual unusual sequence sequence of extremely extremely ferruginous ferruginous slate, greywacke, and carbonate carbonate iron-formation. iron-formation.The The abundance abundance of pyritic pyritic and and graphitic graphitic slate, slate, the absence absence of more more oxidized oxidized facies facies of iron-formation, iron-formation, and and the the drastic drastic lateral lateral facies facies changes changes of of some some units suggest that the group was deposited in deep, anaerobic water in a tectonically environment." unstable environment." Badwater Greenstone: The The Badwater Badwater Greenstone Greenstone is is aa thick sequence of massive massive and units (agglomerates) (agglomerates) along with minor pillowed basalt, with the pillows and fragmental units interbedded slate and iron-formation iron-formation(James (James and and others, 1968). 1968). ItIt is is seen seen at stop stop 3-7. 3-7. interbedded The Badwater Badwater is estimated estimated to be be 3,000 3,000 to 8,000 feet feet thick, but but may may be be up up to to 15,000 15,000 feet thick in the Iron relative age of the Badwater Iron River area (James and others, 1968). The The relative Greenstone Greenstone has has been been uncertain uncertain for many many years, but but most most recent recent interpretations interpretations(e.g., 1993) consider consider it to be be correlative correlative with the lithologically and chemically Sims and others, 1993) Hemlock Formation. Formation. As such, itit would be be equivalent to part of the Menominee similar Hemlock Group Group deposited deposited about 1875 1875 Ma. We here here suggest suggest that that the the Badwater Badwater be be placed placed within the Paint Paint River River Group in contrast contrast to its its previous previous assignment assignment to the Menominee Menominee or Baraga Baraga Groups Groups for reasons reasons discussed discussed more more fully fully below. Dunn Creek Slate: The TheDunn DunnCreek Creek Slate Slate is is composed composed of 400 400 to 1,500 1,500 feet of strata strata between the Badwater Badwater Greenstone and the Riverton Riverton Iron-formation Iron-formation(James (James and others, 1968). It is seen at stop stop 3-5. 3-5. The The Dunn Dunn Creek Creek is is aa lithologically lithologically varied varied unit unit comprised comprised of of aa sequence sequence of well-bedded well-bedded to laminated laminated argillite argillite and cherty argillite, argillite, with units units of somewhat coarser impure impure quartzite, and and thin cherty iron-formations. iron-formations. The term "slate" is a misnomer misnomer in that most most of the rock rock has, has, at at best, best, aa moderately moderately developed developed cleavage, cleavage, and true slates are rare. rare. The upper upper part part of the the formation formation is is aa distinctive distinctive highly highly graphitic graphitic argillite and argillite breccia unit, the the Wauseca Wauseca Pyritic Member (stop (stop 3-3), 3-3), which is present throughout the Iron Iron River-Crystal River-Crystal Falls Falls district district and and forms the footwall of the Iron-formation (James and others, 1968). The Wauseca is an example of Riverton Iron-formation sulfide facies iron-formation sulfide iron-formationas as defined defined by by James James (1954). (1954). ItIt contains contains from from 15 15 to to 25% 25% Fe Fe from 10 and from 10 to 30% 30% S. In In the the Iron Iron River River area area parts parts of the the Wauseca Wauseca Member Member contain contain carbon that has has not been been altered altered to graphite, graphite, suggesting suggesting that that the the rocks rocks have have undergone undergone little, if any metamorphism. metamorphism. Tyler and and others others (1957) (1957) showed that some of the carbonaceous carbonaceous material material in the Iron Iron River River area area is, in in fact, coal, coal, and and as such, this is is one one of the oldest known occurrences of coal in the world. The lower part of the Dunn Creek known occurrences coal in The part Dunn Slate Slate is is poorly poorly exposed, but but evidently evidently varies varies considerably considerablyin incharacter characterwithin withinthe thedistrict district (James Badwater (James and others, 1968). 1968). The basal basal contact contact of the the Dunn Dunn Creek Creek with the Badwater Greenstone Greenstone is poorly poorly known, but probably probably conformable. conformable. The great lateral lateral changes changes in thickness of the Dunn thickness Dunn Creek suggest suggest that that itit was was deposited deposited over aa surface surface with considerable relief and that the relief was buried buried before before the later later phases phases of deposition deposition so that thin units units of the Wauseca Wauseca Member Member are are continuous continuous over over the the entire entire district. The The Dunn Dunn Creek is conformable conformable with and and locally locally gradational with the basal basal units of the Riverton Riverton Iron-formation. Iron-formation.This contact contact is is seen seen at at stop stop 3-3. 3-3. The The Riverton Riverton is is also also seen seen at stops stops 3-3 3-3 and 3-4. 3-4. 9 �The 10 10to to 800-foot-thick 800-foot-thickRiverton Riverton Iron-formation Iron-formationis is conformable Riverton Iron-formation: The and gradational with the underlying Dunn Creek Slate, and is the main iron-bearing unit in the Iron River-Crystal Falls district. It is described in detail by James and others (1 968). Where it is is oxidized oxidized it consists consists primarily of thin-bedded chert and iron-rich iron-rich (1968). carbonate, layers of stilpnomelane and disseminated graphite. However, in most areas iron-formationis is altered, altered, with with the the iron iron oxidized oxidized to limonite limonite and and goethite, goethite, and and the the chert chert the iron-formation being variably leached (James and others, 1968). The best natural natural exposure of the unoxidized phase of the Riverton is on the apron of the dam on the Paint River in Crystal Falls, Michigan Michigan (stop (stop 3-6). 3-6). The Hiawatha Hiawatha Graywacke Graywacke ranges in thickness from 0 -- 500 feet Hiawatha Grawacke: Graywacke: The and disconformably overlies the Riverton Iron-formation. Iron-formation.The basal part of the formation formation breccia of chert fragments in is a breccia in a graywacke graywacke matrix, matrix, which is well exposed along the Paint River in Crystal Falls (James and others, 1968). The chert fragments are commonly commonly an inch inch or more more in in size, but but locally locally range range up up to as much much as 2 feet in in length. length. Above the basal basal breccia breccia unit, unit, most most of the the Hiawatha Hiawatha Graywacke Graywacke is is a dark gray gray massive massive medium grained greywacke in which clastic grains are readily visible. In the western part Iron River River area area the graywacke graywacke is is particularly particularlycoarse coarse and and contains contains aa large large amount amount of the Iron of clastic feldspar (James (James and and others, others, 1968). 1968). They They suggested suggested that that the the change change from from chemical ironchemical to clastic deposition deposition was the result result of structural structural disturbance disturbance that halted halted ironformation formation deposition. deposition. Stambaugh Formation: The Stambauoh The Stambaugh Stambaugh Formation Formation is is about 100 100 feet thick and is composed composed of a lower lower laminated laminated cherty unit overlain by massive massive chloritic slate and some graywacke (James and others, 1968). 1968). Much of the unit is moderately moderately to strongly magnetic, a feature that was very helpful in resolving the structure in the district. helpful Fortune Lakes Slate: The The Fortune Fortune Lakes Lakes Slate Slate is is the uppermost uppermost formation of the Paint River Group and the youngest Precambrian Precambrian unit in the district (James and others, 1968). 1968). It also underlies underlies the largest area and, because of poor exposure, is the least known known formation. ItIt is is at at least least 4,000 feet feet thick thick and and is is composed composed dominantly dominantly of of slates slates with with interbedded graywacke and minor iron-formation. Graded-bedded graywacke composes Graded-bedded about 25 percent of the formation. Graded Graded units units range range in in thickness from 1-30 1-30feet (James and others, 1968). 1968). Stratigraphic Synthesis Stratigraphic Synthesisof of the the Marquette MarquetteRange Range Supergroup Supergroup The foundation foundation for modern modern stratigraphic stratigraphic terminology terminology of of the the Paleoproterozoic Paleoproterozoicstrata strata of of the the southern southern Lake Lake Superior Superior region region was established established by James James (1958), (1958), who defined the fourfourfold group group designation designation still still in in use use and and introduced introduced the term term Animikie Animikie Series Series for the the stratigraphically superposed sequence of Chocolay, Menominee, Baraga, presumably stratigraphically and and Paint Paint River River Groups. Groups. The term term "Marquette "Marquette Range Range Supergroup" Supergroup" was introduced introduced by Cannon and Gair (1970) (1970) to to replace replace "Animikie "Animikie Series" Series"in in compliance compliancewith with the the North North American Stratigraphic Stratigraphic Code; an an assemblage assemblage of groups groups is is aa supergroup, supergroup, not not aa series, series, and the name name "Animikie" "Animikie" was already already aa well established established group group name namein inMinnesota Minnesotaand and Ontario. In more recent years, two principal principal changes have been proposed. First is the recognition that the Menominee Group contains thick volcanic formations that are temporal equivalents of the major major iron-formations. iron-formations.This This relationship relationship is is best best documented documented in the eastern Gogebic range where the Emperor Volcanic Complex (Trent, 1976) Emperor 1976) interfingers with the Ironwood interfingers Ironwood Iron-formation Iron-formation and is unequivocally unequivocally part of the Menominee 10 �(Klasner and and others, 1998). 1998). Likewise, Likewise, the Hemlock Hemlock Formation, Formation, aa thick thick Group succession (Klasner rocks long long considered considered aa part part of the the Baraga Baraga Group, Group, was was succession of volcanic rocks reassigned reassigned to the Menominee Menominee Group Group (Cannon, 1986). 1986). Although Although relationships relationships are not not as definitive as in the Gogebic definitive Gogebic Range Range because because of structural structural complications complications and and poor poor exposures, the correlation correlation is supported supported by interbedded interbedded iron-formations iron-formationswithin the Hemlock and an extensive iron-formation (AmasaIFence River) River) overlying the Hemlock, Hemlock, iron-formation (Amasa/Fence which are believed to be the westward continuation continuation of the Negaunee Negaunee Iron-formation Iron-formationfrom the Marquette Marquette Range. Also, the Amasa Formation Formation is is unconformably unconformably overlain overlain by by a conglomerate lithologically lithologically indistinguishable from basal basal conglomerate conglomerate of the Goodrich Goodrich Quartzite of the Marquette Marquette Range. Range. Taken Taken together, together, the the weight weight of of evidence evidence suggests suggests that that Emperor and Hemlock Hemlock volcanic rocks rocks were erupted erupted simultaneously simultaneously with ironboth the Emperor formation deposition and interfinger interfinger with the Ironwood Ironwood and Negaunee Negaunee Iron-formations. Iron-formations. EmperorIIronwood assemblage assemblage and and the Hemlock/Amasa Hemlock/Amasa assemblage assemblage are are Both the Emperor/Ironwood unconformably overlain by a conglomerate marking marking the base base of the Baraga Baraga Group Group and and unconformably thus thus seem seem properly properly assigned assigned to to the the Menominee Menominee Group. Group. second principal principal change change to to stratigraphic stratigraphiccorrelation correlation is is the the recognition recognition that that the the Paint Paint A second River Group is likely an allochthon allochthon structurally emplaced over the Baraga Group and is therefore not necessarily therefore necessarily a younger sequence. sequence. As we propose propose here, certain lithologic similarities between Paint River units and Menominee and Baraga Group units suggest similarities that the Paint Paint River River is equivalent to both both the Menominee Menominee and Baraga Baraga Groups. The five formations originally assigned formations assigned to the Paint Paint River River Group Group lie in their entirety within the Iron Iron River-Crystal River-Crystal Falls basin (referred (referred to as the Iron Iron River-Crystal River-Crystal Falls district by James and others, 1968). 1968). Over the years there have have been been differences differences in in opinion opinion concerning concerning the relative ages of the strata strata within the Iron Iron River-Crystal River-Crystal Falls Falls basin, the underlying underlying Badwater Greenstone, and nearby nearby Baraga Baraga and Menominee Group strata. Leith Leith and others (1935) (1935) interpreted interpreted the sedimentary sedimentary rocks rocks of the district to to be be roughly roughly equivalent equivalent to the Michigamme Michigamme Formation, Formation, but but they they were were uncertain uncertain about about the the stratigraphic stratigraphic position position of of what they called Pentoga belts belts of greenstone greenstone (Badwater (Badwater Greenstone Greenstone called the Paint Paint River River and and Pentoga of current usage). James James (1958), (1958), however, however, proposed strata of the Iron Rivercurrent usage). proposed that the the strata of the Iron RiverCrystal Falls district comprised the uppermost the Paint River Group of four Paint River Group Crystal uppermost stratigraphic groups that make stratigraphic make up up the the Marquette Marquette Range Range Supergroup. Supergroup. The The Badwater Badwater Greenstone Greenstone was considered to be part of the Baraga Baraga Group. Larue Larue and and Sloss Sloss (1980) (1980) discussed sedimentation of the Marquette Marquette Range Range Supergroup Supergroup and and accepted accepted James' James' interpretation, interpretation, whereas others questioned questioned the stratigraphic stratigraphic position position of the Paint Paint River River Group. For example, Cambray Cambray (1978) (1978) suggested suggested that the Paint Paint River River Group Group is is stratigraphically equivalent to the Menominee 1992), stratigraphically Menominee Group. More More recently, Sims (1990, (1990, 1992), and Sims and Schulz (1993) (1993) proposed proposed that the Paint Paint River Group Group is the stratigraphic equivalent of the Baraga Baraga Group Group and and they correlated correlated the Badwater Badwater Greenstone Greenstone with the Hemlock Hemlock Formation Formation in in the the Menominee MenomineeGroup. Group. is some some lithologic lithologic similarity between between units units of the Paint Paint River River As shown in figure 3, there is Group and other nearby nearby successions, successions, the most most obvious obvious being being a thick and and extensive extensive banded iron-formation iron-formation overlain disconformably disconformably by clastic rocks rocks including including ferruginous conglomerate. In In this this regard, regard, the the Riverton Riverton Iron-formation/Hiawatha Iron-formation1HiawathaGraywacke Graywacke VulcanlMichigamme succession succession of succession of the Paint Paint River River Group Group is is similar to the Vulcan/Michigamme the Menominee Menominee Range Range and and the the Negaunee/Goodrich NegauneelGoodrichsuccession successionof of the theMarquette MarquetteRange. Range. We suggest suggest that that the the Riverton/Hiawatha RivertonIHiawathadisconformity disconformity is is equivalent equivalent to to the the Menominee/Baraga formations MenomineelBaraga unconformity unconformity elsewhere elsewhere and that the Riverton Riverton and older formations of the Paint Paint River Group Group are are equivalent equivalent to Menominee Menominee Group Group units, whereas the Hiawatha and younger parts parts are equivalent to the Baraga Baraga Group. Group. Thus, the thick clastic, 11 �units of the upper upper part part of the the Paint Paint River River Group Group are are correlatives correlativesof rocks rocks in part turbiditic, units of similar depositional setting setting in the Baraga Baraga Group. The Badwater Badwater Greenstone Greenstone may may be be the equivalent of the Hemlock Hemlock Formation Formation in the Menominee Menominee Group. We here here suggest suggest Badwater Greenstone should be placed placed in the Paint River Group rather than the that the Badwater Structural evidence presented Menominee Group. Structural presented below indicates that the Badwater is sequence, as indicated indicated by the widespread occurrence of part of the allochthonous sequence, steeply plunging integral part part of the Paint Paint plunging folds characteristic characteristic of the allochthon, and is an integral being the the volcanic volcanic base base on on which which younger younger units unitswere were deposited depositedwith with River succession, succession, being is more more aptly included included with the Paint Paint River Group rather than apparent conformity. ItIt thus is separated and thus has unknown unknown in other groups from which it is structurally separated stratigraphic relations. stratigraphic relations. Structure Structure The area of the field trips spans the Paleoproterozoic Paleoproterozoic suture separating the Archean Superior craton and Paleoproterozoic Paleoproterozoic epicratonic rocks on the north from Paleoproterozoic Paleoproterozoic oceanic and island arc rocks of the Wisconsin magmatic magmatic terranes on the south. The most commonly commonly accepted model model for accretion is is northward northward emplacement emplacement south-directed subducting slab, eventually leading to the of volcanic arcs over a south-directed of Penokean culmination of Penokeandeformation deformation as as the the arcs arcs collided collided with the southern edge of the Superior Superior craton. craton. The Niagara fault zone, a zone of intense intense shearing as much much as several kilometers kilometers wide in some areas, is the feature which most completely separates volcanic rocks on the south from epicratonic sedimentary rocks on the north north and is generally considered to be the surface surface trace trace of the the suture. suture. However, However, itit is is but but one one of of aa series series of of anastamosing anastamosingfaults faults structural panels in which volcanic and sedimentary rocks are intermixed. that bound structural Niagara These panels compose a belt as much much as 25 km wide, which we refer to as the Niagara northward suture zone. Available structural data indicates that these panels were thrust northward during collision and are allochthonous with regard regard to Archean basement rocks and, in some cases with regard regard to lower parts parts of the sedimentary sedimentary succession. succession. The allochthons are characterized by a complex structural structural history in which refolding refolding of early folds has resulted resulted in diversely diversely and and commonly commonly steeply steeply plunging plunging folds. folds. Terminology Terminology Because Because various names have been applied over the years to structural elements composing what we now refer to as the Niagara Niagara suture zone, the following discussion (1966) correlates the terms used used here with previous terminology. Both Bayley and others (1 966) and Dutton Dutton (1971) (1971) defined defined the structural structural panels panels of the region region and and the map map geometry geometry has has remained remained essentially unchanged unchanged since their work. They, however, did not use the term "Niagara fault", although they did recognize and map the fault as a major boundary boundary between between the dominantly volcanic rocks to the south and dominantly sedimentary rocks to the north. They did name name other faults and and also also applied applied names names to various structural structural blocks. We have have retained retained their names names throughout throughout most most of this this report. report. With With recognition recognitionof of and Ueng Ueng (1985) (1985) introduced introducedthe the significance of the Niagara Niagara fault as aa suture, suture, Larue Larue and the fault as term "Florence-Niagara "Florence-Niagara terrane" to encompass encompass the eight structural structural panels panels defined defined by Bayley and others (1966) roughly ten-kilometer-wide zone of (1966) and Dutton (1971) (1971) as a roughly rocks exhibiting internal deformation unique suture exhibiting very intense internal unique to the tectonics of the suture zone. An additional terrane, the Crystal Falls terrane, was proposed by Ueng Ueng and Larue Larue 12 �(1987) (1 987) and considered to be part of the terrane-boundary tectonic assemblage because its complex, although although less less intense intense deformational deformational history. history. In In this report report we have have used used of its the term "Niagara terrane and "Niagara suture zone" to encompass encompass both both the Florence-Niagara Florence-Niagara terrane Crystal Falls Falls terrane. The suture suture zone is is bounded bounded on the south by the Niagara Niagara fault zone. The rocks rocks of the suture suture zone zone all all show show a complexity complexity and intensity of folding much much exhibited by correlative strata strata farther north. north. The northern northern boundary boundary of the greater than exhibited suture zone is the Badwater Badwater fault and and Paint Paint River River fault, both both probably probably originally thrusts, which transported transported Paleoproterozoic RiverPaleoproterozoic strata strata northward. northward. We use use the term term "Iron "Iron RiverFalls allochthon" allochthon" as as an an equivalent equivalent to to the the "Crystal "CrystalFalls Fallsterrane" terrane"of of Ueng Uengand and Crystal Falls Larue Larue (1987) (1987) and and "Iron "Iron River-Crystal River-CrystalFalls Fallsbasin" basin"of of many manyolder older reports. reports. Niagara Niagara Fault Fault Zone Zone The Niagara Niagara fault zone is is a zone of highly highly strained strained rock most most commonly commonly up to a few hundred hundred meters meters wide wide although although there there are arefew few places placeswhere where ititisisexposed exposedand andits itswidth width can be estimated. Its Its location location (see (see fig. 1) 1) is is relatively relatively well defined in the regions regions covered by this field guide, but lack of outcrops outcrops to the west make make its its location location somewhat problematic. In In those regions regions its its location location is is based based largely on interpretation of aeromagnetic maps maps on which the fault is is expressed expressed as a discontinuity of structural structural patterns. On geologic maps maps itit is is generally generally portrayed portrayed as a single line line separating mostly mostly metasedimentary metasedimentary rocks rocks on the north north from metavolcanic metavolcanic rocks rocks on the south, but but itit is is likely likely to have numerous numerous splays extending southward into the volcanic terranes, only some of which have have been been seen seen in in outcrop. outcrop. On the field trips we will examine two of the best-exposed best-exposed areas of the fault zone. Highly sheared rocks of the fault zone are are well exposed exposed in in Piers Piers Gorge Gorge (stop (stop 2-1). Although this locality locality is about a kilometer south of the mapped mapped fault, there is is little little doubt that shearing shearing seen here is a splay of the Niagara Niagara fault. Rocks Rocks within the fault zone are severely flattened and stretched stretched and and foliation foliation strikes strikes generally generally WNW and and dips dips steeply steeply south. High High strain has has resulted resulted in in rotation rotation of fold axes axes to parallelism parallelism with the direction of maximum maximum Larue, 1985). 1985).Based Basedon onstretch stretchlineations, lineations,which whichplunge plunge600 60' elongation (Sedlock (Sedlock and and Larue, SW, Sims Sims and and Schulz (1993) (1993) suggest suggest that tectonic tectonic transport transport was northeastward, northeastward, perpendicular to the trace of the fault, and and onto onto the exposed exposed part part of the continental continental margin margin to the north. north. They They infer inferthat that the thefault faultzone zonedips dipsabout about700 70' to to the the south. south. The The fault fault zone is also seen at stop 3-1 3-1 at the Pine Pine River dam. There, very highly strained rocks rocks of the Michigamme Michigamme Formation Formationare are well well exposed. exposed. Although the Niagara Niagara fault has has been been widely accepted as a paleosuture, gravity studies by Attoh and and Klasner Klasner (1989) (1989) suggest suggest that that the the Archean Archean cratonic cratonicrocks rockscontinue continuesouthward southward at depth to a steep gravity gradient that trends northeastward northeastward across north-central north-central Wisconsin (fig. Wisconsin (fig. 1). 1). By By that that interpretation interpretationthe the Niagara Niagara fault fault is is aa major major thrust, thrust, which which has has transported arc rocks transported rocks for aa least least aa few few tens tens of of kilometers kilometersnorthward northward over over the the southern southern edge of the Superior Superior craton. Based Based on on gravity gravity model model studies, studies, Klasner Klasner and Osterfeld Osterfeld (1984) suggested that magmatic (1984) had had previously suggested magmatic domes, such such as the Dunbar Dunbar Dome, Dome, are allochthonous, detached at depth and thrust northward northward in the hanging hanging wall of the Niagara Niagara fault, which flattens flattens at depth depth toward toward the south. 13 �Niagara Niagara suture suturezone zone Both Both the the Menominee Menominee and and Iron Iron River-Crystal River-Crystal Falls Falls districts districts lie lie near near the the southern southern edge edge of of the exposed continental margin of the Penokean orogen and both lie within the Niagara the exposed continental margin of the Penokean orogen and both lie within the Niagara suture suture zone zone as aswe we define defineitithere. here.The Thecontinental continentalrocks rocksnorth northof ofthe theNiagara Niagarafault faultare are highly highly faulted faulted and and divided divided into into aa series series of of structural structuralblocks. blocks. Sedlock Sedlock and and Larue Larue(1985) (1985) refer refer to to these these blocks blocksas as "fault "faultbounded boundedtectonostratigraphic tectonostratigraphicterranes", terranes", but butwe we prefer preferto to use use the the term term "structural "structuralpanels" panels" because becausethey they do do not not fit fit the the strict strictdefinition definitionof of aa tectonostratigraphic tectonostratigraphic unit, unit,which whichisisdefined definedin inthe the Glossary Glossaryof of Geology Geologyas asaa"mixture "mixtureof of rock faultrock stratigraphic stratigraphicunits unitsresulting resultingfrom from tectonic tectonic deformation." deformation."Rather Ratherthese theseare arefaultbounded boundedslices slices of of rocks rocksor orrock-stratigraphic rock-stratigraphicsequences sequencesthat thathave haveaaunique uniquestructural structural signature. signature. Although Althoughboth bothdistricts districtsare arepart partof ofthe theNiagara Niagarasuture suturezone, zone,they theyare aredistinctly distinctly different differentfrom fromeach eachother otherstructurally. structurally.Thus Thusthey theywill willbe bediscussed discussedindividually individuallybelow. below. Menominee Menominee District District The The Menominee Menomineedistrict districtlies lieswithin within the the Niagara Niagarasuture suture zone zone at at the the southern southern edge edge of of aa series series of of uplifted upliftedblocks blocksof of Archean Archeanbasement basement(figs. (figs.1,1,2). 2).Those Thoserocks rocksnorth northofofthe the Menominee Menomineerange rangecontain contain fault-bounded fault-boundedtroughs troughsof of tightly tightlyappressed appressedPaleoproterozoic Paleoproterozoic strata—the Felch and and Calumet Calumet troughs troughs (fig. 2)-2 ) - down-faulted down-faulted between between blocks blocks of of Archean Archean strata-the Felch rocks. rocks.Studies Studiesby byKlasner Klasnerand andothers others(1989) (1989)and andKlasner Klasnerand andSims Sims(1993) (1993) suggest suggestthat that these Archean blocks were uplifted and thrust northward along Bush Lake fault, a these Archean blocks were uplifted and thrust northward along Bush Lake fault, a master masterfault fault that, that, they theysuggest, suggest,carried carriednow noweroded erodedPaleoroterozoic Paleoroterozoicand andArchean Archean rocks rocks onto onto the the continental continentalhinterland hinterlandto tothe thenorth. north.AAseries seriesof of south-verging south-vergingthrusts thrustsoccurs occursinin the the Felch Felchand andCalumet Calumettrough troughareas. areas. These Thesemay mayhave havedeveloped developedas as back-thrusts back-thrustsduring during the thenorthward northwardthrusting thrustingevent, event,or orthey theymay mayhave havedeveloped developedlater laterduring duringthe theMazatzal Mazatzal orogeny orogeny (HoIm (Holm and and others, others, 1999; 1999;Romano Romanoand and others, others, 2000). 2000). The The general generalstructure structureof of the theMenominee Menomineeiron irondistrict district(fig. (fig.4) 4)isisaasouth-facing south-facinghomocline homocline of of Paleoproterozoic stratain which stratigraphic arecreated createdby Paleoproterozoicstrata inwhich stratigraphicrepetitions repetitionsare bythree three major major faults (Bayleyand andothers, others,1966). 1966).The Thefaults faultscut cutthe the faultsand andby byfolding foldinginternal internalto tofault faultslices slices(Bayley folds foldslongitudinally, longitudinally,approximately approximatelyalong alongthe thefold foldaxes, axes,repeating repeatingthe thePaleoproterozoic Paleoproterozoic sequence sequencethree three times timesand andforming formingthree three"ranges". "ranges".Farthest Farthestnorth, north,the theCarney CarneyLake Lake Gneiss forms the core of a broad anticlinal structure. The Paleoproterozoic Gneiss forms the core of a broad anticlinal structure. The Paleoproterozoicstrata stratalie lie unconformably unconformablyon on the the gneiss gneissand anddip dipsteeply steeplyto to the the south southor orare areoverturned overturned(as (asat atstop stop 2-3) 2-3)and anddip dipsteeply steeplynorth northand andface facesouth. south.Farther Farthersouth, south,the thePaleoproterozoic Paleoproterozoicstrata strataare are repeated repeatedtwice twice by bymajor majorfaults faultsto toform formthe thetwo tworanges rangesof ofthe thedistrict. district.These Thesefaults faultswere were named 4). namedthe theNorth NorthRange Rangefault faultand andSouth SouthRange Rangefault faultby byBayley Bayleyand andothers others(1966) (1966)(fig. (fig.4). The Thefaults faultshave havesteep steepdips dipsatatthe thepresent presentlevel levelofofexposure exposureand andconsistently consistentlyshow showsouthsouthside-up side-updisplacement. displacement.Most Mostrecent recentinterpretations interpretations(e.g., (e.g.,Sims Simsand andSchulz, Schulz,1993) 1993) consider them to have been thrust faults, which were steepened by continued consider them to have been thrust faults, which were steepened by continuedshortening shortening of of the thethrust thrustpanels. panels.The Therocks rocksininthe thehanging hangingwall wall(south (southside) side)ofofthese thesefaults faultshave haveno no indications indicationsof of Archean Archeanbasement basementrocks, rocks,inincontrast contrastto tothe thearea areaimmediately immediatelyto tothe thenorth north where wherethe theCarney CarneyLake LakeGneiss Gneissisisan anintegral integralpart partofofthe thestructure. structure.The Thenorth northrange rangeand and south southrange rangepanels panelsmay maybe beallochthons allochthonsdetached detachedfrom frombasement basementand andthrust thrustnorthward northward over overthe themore moreautochthonous autochthonoussequence sequenceofofthe thenorthern northernpart partofofthe thedistrict. district.The The Menominee range is bounded on the south by the Niagara fault (stop 2-1), along Menominee range is bounded on the south by the Niagara fault (stop 2-I), alongwhich which itit isisinincontact contactwith withvolcanic volcanicrocks rocksof ofthe theWisconsin Wisconsinmagmatic magmaticterranes. terranes. 14 �87 4630' •r •( •( •( • . . a , 4552'3O — I • • '2 —Ferr Cre,!Ic Fm Fern Cr,ck rm 88 730" 87 5700 2 I 90 55 2 4 87 4630 I 00 5 10 ip 8 6 I I 1'O 10 12 12 I 15 15 Miles Miles Kilometers Kilometers EXPLANATION EXPLANATION Cambrian Cambrian MunisingSandstone Sandstone Munising North NorthofofNiagara Niagarafault fault Paleoproterozoic Paleoproterozoic SouthofofNiagara Niagarafault fault South Paleoproterozoic Paleoproterozoic Metadiabase Metadiabase E Michigamme Formation Michigamme Formation -graywacke graywacke LI. Badwater Greenstone Badwater Greenstone Vulcan VulcanIron-formation Iron-formation L HoskinsLake LakeGranite Granite Hoskins MarinetteQuartz QuartzDiorite Diorite ........ Marinette Metagabbro Metagabbro QuinnesecFormation Formation Quinnesec Randville RandvilleDolomite Dolomite Sturgeon SturgeonQuartzite Quartzite Fern FernCreek CreekFormation Formation - — fault fault Archean Archean Granitic Graniticrocks rocksand andgneiss gneiss 0 L:.' Carney Lake Gneiss Carney Lake Gneiss Figure Figure4. 4. Geologic Geologicmap mapof ofpart partofofthe theMenominee MenomineeIron IronRange Rangeshowing showingthe thelocation locationofof field trip stops. Geology is from Bayley and others (1966). field trip stops. Geology is from Bayley and others (1966). Thesefaults faultscontinue continuetotothe thenorthwest northwestinto intothe theFlorence, Florence,Wisconsin Wisconsinarea areaand andalong alongthe the These southside sideof ofthe theIron IronRiver-Crystal River-CrystalFalls Fallsdistrict, district,where whereDutton Dutton(1971) (1971)mapped mappedfour fourfaultfaultsouth boundedstructural structural"blocks". "blocks".He Henamed namedthe theblocks, fromnorth northtotosouth, south,the BruleRiver blocks,from theBrule River bounded block,the theKeyes KeyesLake Lakeblock, block,the thePine PineRiver Riverblock, block,and andthe thePopple PoppleRiver Riverblock. block.The The block, 15 �Menominee Menominee Range Range and contains, in in its its western portions, portions, an an extensive extensive dolomite dolomite (Saunders Formation) Formation) generally generally accepted accepted as the equivalent equivalent of the the Randville Randville Dolomite Dolomite of (Saunders Menominee Range. Range. The The occurrence occurrence of of these these shelf-deposited shelf-depositedrocks rocksin inthe the the Menominee southernmost and and structurally structurally highest highest thrust thrust panel panelof of the the continental continentalforeland forelandindicates indicates southernmost the continental continental shelf shelf originally originally extended extended substantially substantiallysouth south of of the the present presentNiagara Niagara that the fault. fault. - Iron District Iron River - Crystal Falls District 88 1800" EXPLANATION EXPLANATION North North of o f Niagara Niagara fault Tobin TobinLake LakeGranite Granite Metagabbro Metagabbro - Paint Group - undivided Paint River River Group undivided Fortune FortuneLake LakeSlate Slate Riverton Riverton Iron-formation Iron-formation Dunn DunnCreek CreekSlate Slate 463'00" Badwater BadwaterGreenstone Greenstone a Michigamme MichigammeFormation, Formation, graywacke graywackeand andvolcanic volcanicrocks rocks Michigamme MichigammeFormation Formation --quartzite quartzite Michigamme MichigammeFormation Formation --graywacke graywacke Amasa Iron-formation Hemlock Volcanics undivided undivided HemlockVolcanics Randville Dolomite Randville Dolomite :k.j_ Saunders SaundersFormation Formation Dickinson DickinsonGroup Groupundivided undivided ,n South South of of Niagara Niagara fault 4552'30" ,-,!;..:+ Bush Lake Lake granite Bush granite Granite and tonalite Graniteand tonalite Quinnesec Formation Formation Quinnesec EI] Metasedimentary Metasedimentaryrocks rocks - — faults faults • field fieldtrip tripstops stops 88 1800" 22 I I 55 22 00 I I I I 00 I 44 66 I I I F 55 Miles Miles 88 F I I I 10 10 Kilometers Kilometers NF-Niagara fault NF-Niagarafault SRF- South South Range Range fault fault NRF-North Range fault NRF-North Range fault BF-Badwater fault BF-Badwaterfault PRF-Paint River PRF-Palnt River fault fault CS-Commonwealthsyncline svncline CS-Commonwealth MA-Mastodon MA-Mastodonanticline anticline TBS-Tim TBS-TimBowers Bowerssyncline syncline Figure 5. 5. Geologic Geologicmap mapof of the theFlorence Florencearea areaand andeastern easternpart partof ofthe theIron IronRiver-Crystal River-Crystal Figure (1971)and and Falls district district showing showing the the location locationof of field fieldtrip trip stops. stops. Geology Geologyisisfrom fromDutton Dutton(1971) Falls James James and and others others (1968). (1968). 16 �The Iron Iron River-Crystal River-Crystal Falls District (fig. 1, 2, 5) is a triangular-shaped basin of tightly tightly triangular-shaped basin folded strata of the Paint Paint River Group. Structural studies by James and others (1968) (1968) showed that sedimentary strata of the Iron Iron River-Crystal River-Crystal Falls basin are tightly and multiply folded. Dips of less than 60 degrees are rare. For the most part the trends of the parallel or sub-parallel sub-parallel to the principal principal axis of the triangular triangular district. fold axes are parallel Stratigraphically overturned beds are common. There are a dozen or more faults having displacements measured in thousands of feet. In In addition, James and others (1968, (1968, page 87) noted "It is the opinion of one of the authors (FJP) that the folds in in the Michigamme Michigamme Slate are unrelated to those in the Badwater Greenstone and Dunn Creek suggestingthat thatthat thatthe theMichigamme Michigammehas hasaa different different structural structuralsignature signature than than slate...", suggesting Francis J. Peftijohn Pettijohn (FJP) the Badwater Greenstone and overlying Dunn Dunn Creek Creek Slate. Slate. Francis suggested that aa north-trending north-trendingfault fault separates separates the the Michigamme Michigamme from from the the futher suggested Badwater and Paint River Group strata along the eastern edge of the district. More recent structural studies (Ueng and Larue, 1987) identified identified four phases of deformation, including two major folding events that resulted in the extremely complex fold interference pattern characteristic of the district. This refolded interference refolded fold geometry geometry is exceptionally well displayed displayed at stop stop 3-6. 3-6. exceptionally Uncertainty concerning the stratigraphic position of the Paint River Group and Badwater (1990) (1993) to Greenstone, as discussed above, led Sims (1 990) and Sims and Schulz (1 993) to propose that a thrust (detachment) fault (the Badwater thrust fault) lies at the base of the Badwater Greenstone Greenstone and that the Badwater Badwater and and overlying overlying Paint Paint River River Group Groupstrata strataare are allochthonous, allochthonous, tectonically emplaced above Baraga Group strata of the continental margin. Although the Badwater thrust fault was not directly observed, they based their interpretation on unpublished geochemical data by K. J. Schulz, and and the the presence of of interpretation "extensive thrust faulting" faulting" of continental 'extensive continental margin margin strata. strata. To test this this interpretation, interpretation, Klasner Klasner and and others (1999) (1999) compared compared the structural structural signature signature (primarily orientation of fold axes) in the Paint River Group strata and Badwater Greenstone with the structural signature in underlying Baraga Group strata. Based on measurement of 123 measurement 123 fold axes throughout the Iron Iron River-Crystal River-Crystal Falls basin, they found that most most fold fold axes axes plunge plunge steeply steeply (approximately (approximately 85 degrees) north (fig. 6A). There There are are at least least three other trends in in the plot plot of the the fold fold axes, indicating indicating that the rocks rocks in in the the Paint River Group are multiply deformed. In contrast, fold axes in the area north and east of the Iron Iron River-Crystal River-Crystal FaIls Falls basin plunge at low angles to the west-northwest or east-southeast east-southeast (see (see fig. 6B). 6B). These These measurements measurementswere were made made mainly mainly in in Michigamme Michigamme Formation strata of the Baraga Baraga Group. The striking striking difference in in structural structural history history of these two regions separates regions supports the proposal proposal of Sims Sims (1990) (1990) that aa thrust fault separates these two regions. regions. Thus, the Paint Paint River including the Badwater Greenstone, Greenstone, River Group, including appears appears to be allochthonous, allochthonous, having having been thrust onto the autochthonous autochthonous Baraga Baraga Group Group strata. strata. The metamorphic metamorphic grade grade of rocks rocks surrounding surrounding the the Paint Paint River River Group Group ranges ranges from from greenschistto greenschist toamphibolite amphibolite(James (Jamesand andothers, others,1968; 1968;Bayley Bayleyand andothers, others, 1966; 1966;Dutton, Dutton, 1971), but Paint River Group rocks of the Iron Iron River-Crystal Falls district reach only greenschist or lower metamorphic metamorphic grade. This feature, too, suggests suggests that that there there is is aa fault fault with considerable displacement separating the Paint River Group from surrounding considerable displacement Paint surrounding rocks. rocks. 17 �A 0 0 •0 0 0 4 123 fold axes 123 fold axes in in rocks rocks of of the theIron IronRiver-Crystal River-CrystalFalls Falls 0 allochthon allochthon 0 .00 .0 0 f 36 Michigamme 36 fold fold axes axes in Michigamme Formation in footwall of allochthon. allochthon.Black Blackdot dotisis beddingbeddingcleavage cleavage intersection intersection at at stop stop 3-8. 3-8. Figure Figure 6. Stereoplots Stereoplots (lower (lower hemisphere hemisphere equal equal area area projections) projections) showing showing the the orientation orientation of fold fold axes axes within within the the Iron IronRiver-Crystal River-CrystalFalls Falls allochthon allochthon(A) (A)and and in in the the Michigamme Michigamme Formation Formation north north and and east of the the allochthon allochthon (B). (B). north-northeasterly-orientedcross cross section sectionconstructed constructedfrom fromunderground undergroundmapping mappinginin A north-northeasterly-oriented mine mine workings workings (James (James and and others, others, 1968) 1968)in inthe the Iron IronRiver Riverarea areaprovides providesan anexample exampleof of the complexity complexity of folding folding typical typical of of this this district. district. The The structure structureseen seenhere heresuggests suggeststhat thatthe the initial folding folding in in the the Iron Iron River-Crystal River-CrystalFalls Fallsbasin basin may may have have been beenrecumbent recumbent (fig. (fig. 7A). 7A). initial synclinein in Hiawatha Hiawatha Graywacke Graywackeand and overlying overlying The cross cross section section shows shows aa recumbent recumbent syncline The Riverton Iron-formation. Iron-formation.In Inthe the third thirddimension dimensionthis this fold foldplunges plungessteeply steeplyinto intothe thesection section Riverton NW). ItItisisstrongly stronglyoverprinted overprintedby byother otherfolds foldsof of diverse diverseorientations. orientations.The Thesyncline syncline (to the the NW). (to closes toward the southwest and the overturned upper limb in which the stratigraphically closes toward the southwest and the overturned upper limb in which the stratigraphically older Riverton Riverton Iron-formation Iron-formationlies liesabove above Hiawatha HiawathaGraywacke Graywacke suggests suggestsaa northward northward structural structural vergence. vergence. The The orientation orientationof of fold foldaxes axesmeasured measuredin inoutcrops outcropsnear nearthe themine mineare are shown shown in in figures figures 7B 7B and and 7C. 7C. 18 �___ ____ ___ NE A 1500" 1500' 1000' 1000' 1000 1 EXPLANATION deposits 500 500' 500 Metodiabose dike HioessthssGoywoeke Riverton Iron-Formation Dunn Creek Slate Mined ore bodies LEVE SEA LEVEL LEVEL 200 0 200 400 600feet Figure Figure7. 7.AA-Geologic Geologiccross crosssection sectionthrough throughthe theBuck BuckMine MineininIron IronRiver, River,Michigan, Michigan,about about 12 12miles mileswest westof ofCrystal CrystalFalls. Falls.Cross Crosssection sectionisisininsec. sec.1,1,T.T.42 42N., N.,R.R.35 35W., W.,and andsec. sec.6,6, T. T.42 42N., N.,R.R.34 34W. W.Reproduced Reproducedfrom fromJames Jamesand andothers others(1968) (1968)totoshow showlarge largerefolded refolded recumbent recumbentsyncline, syncline,the thevery verytight tightfolding, folding,and andwide widediversity diversityofofaxial axialsurfaces surfacesofoffolds. folds. Lower Lowerhemisphere hemisphereequal equalarea areastereograms stereogramsofoffold foldaxes axesininoutcrops outcropsofofRiverton RivertonIronIronformation formation(B) (B)and andHiawatha HiawathaGraywacke Graywacke(C) (C)near nearthe themine. mine. 19 �History of Iron History Iron Mining Mining the Menominee district district in 1848 by by two two explorers, explorers, J.W. J.W. Foster Iron ore was discovered in the and S.W. Hill, Hill, according according to Winchell Winchell (1895). (1895). However, However, iron iron mining mining did did not not begin begin until until 1870, when when N.P. N.P. Saxton started digging pits and trenches on the site of of the Breene the first first ore being shipped in 1873 (Bayley and and others, others, 1966). All All the the major Mine, with the mines had been located located by 1878. 1878. Production Productioncontinued continued until until 1958, 1958, with with aa total total production production from the district of approximately 85,000,000 tons (Bayley and others, 1966). 1966). Seven mines produced produced nearly 77,000,000 tons of ore, with a majority of the production production from the district coming coming from three three mines, mines, the the Chapin Chapin (27,500,000 tons), the the Penn Penn (21,700,000 tons) and the Aragon (11,200,000 (11,200,000 tons) (Dutton, 1958). 1958). Production Production from the Chapin Mine ended in 1934 1934 with a major collapse of the workings. The subsidence from Chapin this collapse collapse formed the lake lake on the north north side of Iron Iron Mountain. Mountain. A causeway across the district was Highways U.S.-2 and U.S.-141. Ore from the district lake now carries the traffic on Highways hauled hauled by rail to Escanaba, Ml, MI, from where it was carried carried by boat to steel mills mills on the lower Great Lakes. All of the ore shipped shipped from the district was high-grade high-grade natural natural iron iron ore. Although the iron-formation iron-formation in the Menominee Menominee district was studied as a possible source of beneficiating beneficiating ore (a "taconite "taconite ore"), ore"), no no commercial commercial operation operation has has been been undertaken. undertaken. Harvey Mellen, a United States land surveyor, first discovered discovered iron iron ore ore in in the the Iron IronRiverRiverCrystal Falls district in 1851 commenced in 1881 1851 (James (James and others, 1968). Mining commenced I881 and the first ore ore was shipped shipped in in 1882. 1882. Except Except for for the the early early years years of of mining, mining, when when many manymines mines recovered by underground underground were operated as small open pits, nearly all of the ore was recovered mining mining (James (James and others, 1968). 1968). Mine exposures and maps of the numerous mines greatly helped helped resolve the complex structure of the district. A major major hazard hazard to mining mining was caused by the pyritic slate of the Wauseca Pyritic Member of the Dunn Creek Slate, to air in the the mines. During the the 92-year which would burn spontaneously when exposed to period 974, approximately approximately 205,000,000 205,000,000 tons tons of iron ore were shipped from period from 1882-1 1882-1974, the district. district. Although there there were were 121 121 mines mines in in the the district, district, aa majority majorityof of the the ore ore came came from from about a dozen mines. about a dozen mines. In the Florence district, iron iron ore ore was recovered recoveredfrom from six six relatively relativelysmall small mines minesbetween between 1980. Only about about 8,000,000 8,000,000 tons tons of of predominantly soft soft hematite and limonite 1880 and 1960. ore with a high phosphorous produced (Dutton, 1971). The Florence phosphorous content was produced Florence mine was the most productive productive mine in the district with 3,680,000 tons shipped between 1880 and 1931. Nearly Nearly all of the production production from the district district was from the Riverton Riverton IronIronformation (Dutton, formation (Dutton, 1971). 1971). PEMBINE-WAUSAU MAGMATIC PEMBINE-WAUSAU MAGMATIC TERRANE Introduction Introduction northeastern Wisconsin The volcanic and plutonic plutonic rocks rocks that are widely distributed in northeastern south of the Menominee Menominee and Iron Iron River-Crystal River-Crystal Falls iron-bearing iron-bearing districts in Michigan are the easternmost easternmost exposures of a major major east-trending east-trending belt belt of volcanic and plutonic plutonic 20 �Pembine-Wausau terrane, the northernmost of the two Wisconsin Wisconsin rocks known as the Pembine-Wausau magmatic terranes (Sims and others, 1989). 1989). The occurrences in Marinette and Florence represent the best-exposed best-exposed portion of this suite of rocks in the Lake Counties, Wisconsin represent Superior region region and include a dismembered dismembered ophiolite sequence (Schulz, 1987). The magmatic terranes represent represent complex magmatic magmatic arcs accreted accreted to to the Wisconsin magmatic southern margin Paleoproterozoic Penokean Penokean margin of the Archean Superior Craton during the Paleoproterozoic orogeny orogeny (Sims (Sims and and others, others, 1989). 1989). The Pembine-Wausau Pembine-Wausau terrane is is mainly composed composed of tholeiitic and calc-alkaline volcanic and others, others, 1989). A A more rocks that formed between 1860 and 1889 Ma (Sims and restricted calc-alkaline volcanic succession restricted succession was deposited deposited between between 1835 1835 and and 1845 1845Ma Ma on the older rocks rocks along along the southern margin margin of the terrane terrane in in Marathon Marathon County. Granitoid rocks constitute nearly half of the exposed rocks in the terrane and range in age from about 1870 1870 to 1760 1760 Ma. These intrusive rocks are mainly mainly granodiorite and tonalite but include gabbro, diorite and granite (Sims and others, 1993). 1993). An older suite of granitoids ranging ranging in age from about 1870 1870 to 1840 1840 Ma is broadly syn-orogenic whereas post-collsional alkali-feldspar younger post-colisional alkali-feldspar granite suites were emplaced at about 1835 Ma and 1760 1760 Ma. The magmatic magmatic rocks rocks of the Pembine-Wausau Pembine-Wausau terrane terrane are are separated separated from from epicratonic rocks of the Marquette Marquette Range Range Supergroup Supergroup to the north north by the Niagara Niagara fault zone (see Bayley and others, 1966, and Dutton, 1971) and from the Marshfield magmatic terrane to the south along the Eau Eau Plaine Plaine shear zone (Sims (Sims and and others, others, 1989). 1989). General Geology Geology Volcanic rocks are relatively well exposed in the the northern and eastern part of Marinette County and eastern Florence Florence County in northeastern Wisconsin, where they form an arcuate belt around the large Dunbar gneiss-granitoid dome (fig. 8) (Sims (Sims and Schulz the Quinnesec Formation by James 1993). The supracrustal rocks, formally named the (1958), (1958), consist of metamorphosed metamorphosed basalt, andesite, dacite, and rhyolite lava flows and volcaniclastic rocks, and locally, sedimentary rocks including greywacke, graphitic slates, and iron-formation. iron-formation.Pyritic to pyrrhotitic pyrrhotitic massive sulfide bodies are also present Cummings, 1982; 1983). Gabbro Gabbro sills sills are are common, common, locally (Hollister and Cummings, 1982; LaBerge, 1983). particularly in the northern part of the sequence (Bayley and others, 1966). Serpentinite bodies, bodies, commonly with some some associated associated gabbros, gabbros, are are also also present presentlocally. locally. Jenkins (1973) (1973) noted noted that at least four lithologically distinct volcanic units could be defined in northeastern northeastern Wisconsin with three of the units units sufficiently sufficiently different different from the the lithologies (Prinz,1958; 1958;Bayley Bayleyand andothers, others, lithologies of the the type type area area of of the the Quinnesec Quinnesec Formation Formation(Prinz, 1966) to warrant their their separate designation. designation. He proposed proposed the the informal names McAllister formation, Beecher formation, and Pemene Pemene formation, listed listed in the order of progressively progressively (1973) that this this also represents the order of more silicic units, and Jenkins (1 973) suggested that decreasing age. More recently, DePangher (1982) (1982) proposed that the Quinnesec the Quinnesec Group consisting consisting of of five five lithostratigraphic lithostratigraphic units Formation be designated the having formational status. For the purposes of this field guide, the informal nomenclature proposed by by Jenkins Jenkins (1973) (1973) will will be be used used for for the the volcanic volcanic rocks rocksin inthe the area. area. proposed 21 �88" 0845" 87" 5830" 87° 4815" 45" 4100"- 45° 4100" I 88" 0845" 22 I I 87° 4815" 87° 58' 30" 00 ' I ' 22 I 88 , I 00 55 66 44 ' 10 10 55 10 10 'I I I I 15 15 12Miles 12 Miles I Kilometers Kilometers EXPLANATION EXPLANATION Munising MunisingSandstone Sandstone(Cambrian) (Cambrian) Paleoproterozoic rocks rocks north northof of Niagara Niagara fault fault Paleoproterozoic Michigamme MichigammeFormation Formation -graywacke graywacke Vulcan VulcanIron-formation Iron-formation Randville RandvilleDolomite Dolomite Paleoproterozoicrocks rocksof ofWisconsin Wisconsin Magmatic MagmaticTerranes Paleoproterozoic Terranes Intrusive trusiverocks rocks Volcanicrocks rocks Volcanic . Spikehorn SpikehornCreek CreekGranite Granite formation' . . . Rhyolite Rhyoliteand and dacite dacite"Pemene "Pernene formation" Bush BushLake LakeGranite Granite Basaltic andesitic breccia "McAllister "McAllister formation" formation" Basaltic and andesitic r"J Hoskins HoskinsLake LakeGranite Granite , ,,=.*: Rhyolite Beecher formation Rhyolite,felsic felsictuff tuff,graywacke graywackenBeecher formation" TwelveFoot FootFalls FallsQuartz QuartzDiorite Diorite Twelve Marinette MarinetteQuartz QuartzDiorite Diorite Serpentiniteand andgabbro, gabbro, base base of of ophiolite ophiolitecomplex complex Serpentinite NewinghamTonalite NewinghamTonalite QuinnesecFormation Formation Quinnesec ' NewinghamTonalite, NewinghamTonalite, . , Athelstane AthelstaneQuartz QuartzMonzonite Monzonite megacrystic megacrysticfacies facies .. . DunbarGneiss Dunbar Gneiss Metagabbro Metagabbro Volcanic Volcanicand andgranitic graniticrocks rocksundivided undivided - — faults faults •0 field fieldtrip tripstops stops Figure 8. 8. Generalized Generalizedgeologic geologic map map of of part partof of northeastern northeasternWisconsin Wisconsinshowing showingthe the Figure location locationof fieldtrip stops. of field tripstops. The units unitsof of the theDunbar Dunbargneiss-granitoid gneiss-granitoiddome domeintrude intrudethe thevolcanic volcanicrocks rocksininnorthern northern The MarinetteCounty, County, and andthe the Athelstane AthelstaneQuartz QuartzMonzonite Monzoniteintrudes intrudesthem themininthe thesouth. south. Marinette 22 �Small Small intrusive intrusive bodies bodies ranging ranging from hornblendite hornblendite and gabbro to quartz quartz diorite, diorite, granite granite and and calc-alkaline calc-alkalinelamprophyre lamprophyre are are widespread, widespread, particularly particularly in in the southeastern southeastern parts parts of of the the exposed exposedvolcanic volcanic sequence. sequence. To To the the north north and and northeast, northeast, the the volcanic volcanic sequence sequenceis is truncated truncated by by the the Niagara Niagarafault fault (Bayley (Bayley and and others, others, 1966; 1966; Dutton, Dutton, 1971), 1971),which which marks marksaa major major discontinuity discontinuity in in the rocks rocks of the the region. region. North North of this this fault, fault, rocks rocksof of the the Michigamme Michigamme Formation Formation and other units units of the Marquette Marquette Range Range Supergroup Supergroup occur, occur, along Archeangneissic gneissicrocks. rocks. alongwith with basement basementuplifts upliftsof of Archean The The supracrustal supracrustalrocks rocks and and associated associatedsubvolcanic subvolcanic intrusives intrusives are are variably variably altered altered to to greenschist facies mineral assemblages throughout the eastern outcrop area but contain greenschist facies mineral assemblages throughout the eastern outcrop area but contain assemblages assemblages as as high high as as amphibolite amphibolitefacies facies adjacent adjacent to to the the Dunbar Dunbar dome dome and andfurther further to to the the west. west. The The rocks rocksare areregionally regionallyfolded foldedon onnorthwest-trending northwest-trendingaxes, axes,but butthey theycommonly commonly lack lack aa strong strongpenetrative penetrativecleavage cleavage in in the the east. east. As As aa result, result, primary primarytextures textures and and structures structuresare are generally generally well well preserved preservedin in the the eastern eastern outcrop outcrop area. area. Units Units generally generally face face outward Dunbardome dome and and Athelstane Athelstane intrusion. intrusion. outwardfrom from the the margins marginsof of the the Dunbar Contacts Contactsbetween between the the various various volcanic units units are not not exposed, but but are are interpreted interpreted to to be be high-angle high-anglefaults faults (Jenkins, (Jenkins, 1973; 1973; Sims Sims and and Schulz, Schulz, 1993). 1993). Because Because of of uncertainties uncertaintiesin in the the amount amount of of displacement displacement on on these these faults faults and and the the complexity complexity of of folding, folding, detailed detailed correlations correlationsbetween between units units have have not not been been possible. possible. Present Present geologic geologic data data support support the the interpretationof of the the Quinnesec Quinnesec Formation Formation(as (as used used by by Jenkins) Jenkins) as as the the oldest oldest volcanic volcanic interpretation unit. unit. The The relative relativeages agesof ofthe theother otherunits, units,however, however,remain remainuncertain. uncertain.The Theunits unitsmay mayhave have been beenstructurally structurallyemplaced emplaced and and may may not not be be in in their their original original stratigraphic stratigraphic position. position. The The regional McAllisterformation formation may may be be younger younger than than the the regionalstructure structuresuggests suggeststhat that the the McAllister Beecher Beecher formation formation but but older older than than the the Pemene Pemeneformation. formation. Further Furtherwork work isisrequired requiredto to resolve resolvethe the age ageand andstratigraphic stratigraphicrelationships relationshipsof ofthese theseunits. units. Until Until relatively relativelyrecently recently the age age of the the volcanic volcanic rocks rocks in in northeastern northeasternWisconsin Wisconsin was was aa point point of of controversy. controversy.Van VanHise Hiseand andBayley Bayley(1900) (1900)and andBayley Bayley(1904) (1904)originally originally Archeanbecause becauseof ofthe thestriking strikingsimilarity similarityofofthese these interpretedthe the"Quinnesec "Quinnesecschists" schists"as asArchean interpreted rocks rocksto to Archean Archean rocks rockselsewhere elsewhere in in the the Lake Lake Superior Superior region. region. Van Van Hise Hiseand and Leith Leith (1911) subsequentlyassigned assignedthe theQuinnesec Quinnesecto toaapost-Michigamme post-Michigamme(i.e. (i.e. (1911)subsequently Paleoproterozoic) Paleoproterozoic) age age on on the the basis basis of of the the interpretation interpretationof of Hotchkiss Hotchkissand and others others (1915) (1915) that the Michigamme Formation graded upward into volcanic rocks in Florence County, that the Michigamme Formation graded upward into volcanic rocks in Florence County, Wisconsin.Dutton Dutton(1971) (1971)later laterreinterpreted reinterpretedthe therelationship relationshipininthis thisarea areaand andplaced placedaa Wisconsin. faultbetween betweenthe the volcanic volcanic units unitsto to the the south south and andthe the Michigamme MichigammeFormation Formationto to the the fault Bayleyand andothers others(1966) (1966) and andDutton Dutton(1971), (1971), favored favoredan anArchean Archean age, age,although although north.Bayley north. acknowledgingthat thatdefinitive definitivefield fieldevidence evidencewas was lacking lackingto to establish establishthe the age ageof of the the acknowledging Quinnesec QuinnesecFormation. Formation. Banks Rebello(1969) (1969) reported reportedaa U-Pb U-Pbzircon zircon age age of of 1,866±39 1,866±3Ma Mafor for aa felsic felsic Banksand andRebello volcanic sample sample from from south south of of the the Dunbar Dunbardome. dome. This Thisage, age, which which isisnot notresolvable resolvablefrom from volcanic the theages agesofofthe therocks rocksofofthe theDunbar Dunbardome dome(Sims (Simsand andothers, others,1984), 1984),isisnow nowgenerally generally taken takenas asthat thatofofthe thevolcanic volcanicsequence sequenceininnortheastern northeasternWisconsin, Wisconsin,although althoughthis thislocality locality isisisolated 1,870±5Ma Mawas was isolatedfrom fromthe themain mainareas areasof of outcrop. outcrop. More Morerecently, recently,an anage ageof of 1,870±56 obtained for the basalts of the Quinnesec Formation by whole-rock Sm-Nd isotopic obtained for the basalts of the Quinnesec Formation by whole-rock Sm-Nd isotopic systematics(Beck (Beckand andMurthy, Murthy,1991). 1991).Thus, Thus,the theage ageof ofthe thevolcanic volcanicrocks rocksinin systematics Archean as as northeasternWisconsin Wisconsinisisnow nowestablished establishedas as Paleoproterozoic Paleoproterozoicand andnot notArchean northeastern once oncethought. thought.Their Theirage ageisisgenerally generallysimilar similarto tothat thatobtained obtainedfor forthe themassive massivesulfide sulfide depositsnear nearCrandon, Crandon,Monico Monicoand andLadysmith Ladysmithto tothe thewest west (Sims, (Sims,1976) 1976)and andto toages agesof of deposits 23 �magmatic terrane terrane (Sims and other volcanic and and plutonic plutonic rocks rocks of the the Pembine—Wausau Pembine-Wausau magmatic others, 1989). 1989). Volcanic Units Units The four volcanic units that comprise the supracrustal sequence in Marinette and Florence Counties, as well as some of the granitoid bodies that intrude them, are briefly (1989), described below. Further information can be found in Sims and others (1 989), Sims and (1993). 993). Although the rocks are mostly others (1992), (1992), and Sims and Schulz (1 metamorphosed fades, the metamorphosed at greenschist facies, the prefix prefix "meta" "meta"is is generally generally omitted omitted below belowfor for simplicity. simplicity. Quinnesec Formation: The Quinnesec Formation is the dominant volcanic unit exposed in northeastern Wisconsin. Its Its stratigraphic thickness is not known known because because of uncertainties in the degree of folding and faulting but is probably on the order of several thousand meters. The Quinnesec Formation consists predominantly of basalt lava flows fragmental andesite and diabase in the north, but includes includes pillowed pillowed and fragmental andesite in in the the south. south. The The basalt is commonly pillowed, with the pillows locally variolitic. The pillowed flows are in commonly pillowed, pillows locally The pillowed flows are in some areas overlain by thick (tens of meters) sections of pillow breccia and hyaloclastite breccia (N. V2, ½, sec. 11, T. 37 N., A. R. 21 E., E., Faithorn Faithorn 7.5 minute minute quadrangle). quadrangle). Andesite Andesite breccia increases increases in abundance abundance southward southward in in the formation, formation, and and is is generally generally plagioclase plagioclaseand and clinopyroxene-phyric clinopyroxene-phyric and amygdaloidal. amygdaloidal. Fresh, Fresh, glacially glacially polished polished outcrops outcrops around aroundthe the new location location of the Kremlin mine pit east of Pembine (S. V2, ½, sec. 26, T. 37 N., R. 21 E., Faithorn Faithorn 7.5 minute minute quadrangle) quadrangle) provide provide excellent excellent exposures exposures of of andesite andesitebreccia. breccia.Felsic Felsic tuff and breccia breccia are also present locally in the Quinnesec, particularly in the northern Menominee River. Felsic Felsic fragmental fragmental units unitsare are also also present presentin inthe the portion near the Menominee LaSalle LaSalle Falls Falls area along along the Pine Pine River River in in Florence Florence County County (Bayley (Bayley and and others, others, 1966; 1966; Dutton, Dutton, 1971). 1971). tholeiitic, with with generally generally low low Ti02 Ti02 and and other other Compositionally, the Quinnesec basalts are tholeiitic, high-field-strength element abundances, and flat to extremely light rare-earth element high-field-strength rare-earth (REE) depleted patterns (Sims and others, 1989). In addition, some of the basalts basalts and andesites have have very very low low Ti02 Ti02and and REE REEabundances, abundances, but butrelatively relativelyhigh highCr Crand andNi Ni the andesites contents. The trace element characteristics of the mafic volcanic rocks overlap those of mid-ocean ridge basalt mid-ocean basalt (MORB) (MORB) and and primitive primitive island-arc island-arc tholelite tholeiite suites suites whereas whereasthe the with boninites (fig. 9), although none of the andesites show compositional affinities with andesites are as high in MgO as true boninites. The felsic volcanic rocks have low potassium compositions with relatively low REE flEE abundances abundances and flat REE REE patterns. patterns. They are are similar in in composition composition to tholeiitic plagiogranite/rhyolite plagiogranitelrhyolite (Schulz, 1987; Sims and others, 1989). 1989). The compositional data suggest that the original basaltic magmas that gave rise incompatiblerise to the Quinnesec rocks rocks were derived from a variably incompatiblevalue element-depleted mantle source. This is supported by a large positive epsilon Nd value 1991), indicative indicative of of derivation derivation from from aa mantle mantlewith with of 4.2 for the basalts basalts (Beck (Beck and and Murthy, Murthy, 1991), long-term depletion in light rare-earth rare-earthelements. elements. 24 �2.0 ' I -L 1.6 1.2 - Beecher Beecher andesites andesites A A Phanerozoic Phanerozoic boninites 0.8 Pemene Pemene - rhyol ites 0.4 0.0 40 50 60 70 80 S102 (wt. %) 2.0 I I I a I I I I 1.6 0 1.2 Beecher andesites 0.8 Pemene rhyolites Phanerozoic boninites 0.4 .1 L 0.0 0 50 100 150 200 Zr (ppm) Figure 9.Plots Plotsof of Ti02 TiOaversus versusSi02 SiOg(upper) (upper)and andZr Zr (lower) (lower)for forvolcanic volcanicrocks rocks(triangles) (triangles) Figure9. andrelated relatedgabbros gabbrosand anddiabase diabase(circles) (circles)from fromthe theQuinnesec QuinnesecFormation, Formation,northeastern northeastern and Wisconsin.Note Notethat thatseveral severalofofthe thesamples samplesplot plotininthe thefield fieldofofPhanerozoic Phanerozoicboninites. boninites. Wisconsin. Fields Fieldsalso also are areshown shownfor for Beecher Beecherandesites andesitesand andPemene Pemenerhyolites. rhyolites. Sedimentaryrocks rocksappear appeartotobe berare rarewithin withinthe theQuinnesec QuinnesecFormation. Formation.Where Wherepresent, present, Sedimentary they theyconsist consistmostly mostlyofofchert, chert,graywacke, graywacke,slate slateand andiron-formation. iron-formation.Iron-formation Iron-formationoccurs occurs asthin thinunits unitsinterlayered interlayeredwith withvolcaniclastic volcaniclasticsedimentary sedimentaryrocks rocksand andtuffs, tuffs,and andconsists consistsofof as 25 �interlayered interlayered chert and siderite (Cummings, 1978). 1978). A small massive massive sulfide deposit, containing containing pyrrhotite pyrrhotiteand and chalcopyrite, chalcopyrite, in in a felsic tuff, and and in in aa fine-grained fine-grainedblack black slate slate that that occurs occurs between between the the felsic felsic tuff tuff and and aa mafic mafic unit unit is is exposed exposed at at Pine Pine Rapids Rapids (locally (locally known "LaSalle Falls") Falls") on on the the Pine Pine River River (LaBerge, (LaBerge,1983). 1983). known as "LaSalle In In addition to volcanic rocks, the Quinnesec Formation Formation also contains a number of gabbroic gabbroic and and ultramafic ultramafic rocks rocks (Sims (Sims and and Schulz, Schulz, 1993). 1993). Numerous Numerous large large gabbro gabbro bodies bodies are are present, present, particularly particularly near near the Niagara Niagara fault zone. These These bodies bodies are are more more or or less less conformable conformable with with the the basaltic basalticflows flows and andprobably probablyrepresent representsynvolcanic synvolcanicsills, sills,although although Bayley Bayley and and others others (1966) (1966) considered considered the gabbroic gabbroic sills to be be "post-Animikie" "post-Animikie"in in age. age. Gabbro Gabbro and and anorthositic anorthositic gabbro gabbro comprise comprise the the bulk bulk of the the sills. sills. Some Some sills sills also also contain contain peridotite peridotite(serpentinite) (serpentinite) and and pyroxenite pyroxenite layers layers and magnetite-rich magnetite-richgabbro. gabbro. Trace Trace element element and and isotopic isotopic data data for for the the gabbros gabbros show show they are are comagmatic comagmatic with with the the basalts basalts of of the the Quinnesec Quinnesec Formation Formation(Schulz, (Schulz, unpublished unpublisheddata). data). Several Several serpentinized serpentinized peridotite peridotite bodies bodiesof of varying varyingsizes sizesoccur occurwithin withinthe thevolcanic volcanic succession, succession, but but the the largest largest and and best-exposed best-exposed occurs occurs south south of Timms Timms Lake Lake (Morgan (Morgan County County Park) Park) east east of Pembine, Pembine, Wisconsin. Wisconsin. The The body body trends trends east east from from outcrops outcropsin in the the NE. sec.19, 19,T. T. 37 37 N., N., R. R. 21 21 E., for for a distance of about %, sec. about 4.5 km kmto to the theNorth NorthBranch Branch NE. 1/4, Pemebonwon %, sec. 22, T. 37 37 N., N., R. R. 21 21 E. E. The The body body produces produces aa large large Pemebonwon River River in in the NE. ¼, magnetic magnetic anomaly. Serpentinized peridotite is dominant in the western part body anomaly. Serpentinized peridotite is dominant in part of the the body where (1-5 cm) dikes of pyroxenite pyroxenite now now replaced replaced by by where itit is is locally locally cut by by coarse coarse grained grained (1-5 amphibole. amphibole. The textures, although The serpentinite serpentinite generally generally shows few primary primary textures, although primary primary compositional compositional layering layering is is suggested suggested locally locally by differential differential weathering of bands bands in in some some outcrops. outcrops. Veins Veins of of carbonate carbonate and and cross-fiber cross-fiber asbestos asbestos are are common. common. Layered Layered and and massive massive gabbro, gabbro, along along with local local masses masses of strongly strongly foliated-lineated foliated-heated gabbro, gabbro, are are dominant dominant in in the the eastern eastern part part of of the the body. body. The The gabbroic gabbroic rocks rocks are are cut cut by by numerous numerousmafic mafic dikes, some of which appear to be sheeted, with diabasic to microdioritic textures. The dikes, some of which appear to be sheeted, diabasic microdioritic The strongly strongly foliated-lineated foliated-lineatedgabbro gabbro masses masses appear appear as as screens screens between between the the dikes. dikes. The The foliation in in the the gabbro gabbro screens screens is is at at aa high high angle angle to to the the contacts contacts of of the the body body with with the the foliation Quinnesec Quinnesec basalts. basalts. The The strong strong deformational deformationalfabric fabric of of the the gabbro gabbro screens screens is is not not present present in in most most of of the the associated associated rocks rocks of of the the body, body, including including the the dikes. dikes. This This suggests suggeststhat that the the deformation deformation of of the the gabbro gabbro in in the the screens screens occurred occurred prior prior to emplacement emplacement of the dikes dikes and and to to the the regional regionaldeformation. deformation.The Thetrend trendof of the the mafic maficdikes dikesisisabout aboutparallel parallelto tothe thetrend trendof of the serpentinite-gabbro serpentinite-gabbrobody body(—E-W) (-E-W) and generally at a high high angle angle to to the the foliation foliation of of the the the gabbro gabbro screens. screens. The The dikes dikes do do not not appear appear to to extend extend into into the the surrounding surrounding pillow pillow basalts. basalts. These These features features suggest suggest that that the the serpentinite-gabbro serpentinite-gabbro body body may may be be fault-bounded fault-bounded and and tectonically emplacedwithin within the the Quinnesec Quinnesec Formation. Formation. The The compositions compositions of of the the gabbros gabbros tectonically emplaced and and diabasic diabasic dikes dikes within within this this body bodyrange rangefrom fromMORB MORBto todepleted depletedisland-arc island-arctholeiite tholeiiteand and high-magnesiumandesite andesite with with boninitic boninitic affinities; affinities; these these mafic mafic rocks rocks are are similar similar in in high-magnesium composition compositionto to the the basalts basaltsin inthe the Quinnesec QuinnesecFormation Formation(fig. (fig.9). 9). The The lithologies lithologiesand andtheir their arrangement arrangementin inthis this ultramafic-gabbroic ultramafic-gabbroicbody, body, along alongwith with those those of of the the Quinnesec QuinnesecFormation Formationgenerally, generally,are are similar similar to to those those that that characterize characterize Phanerozoic Phanerozoic ophiolite sequences sequences(fig. (fig. 10). 10).This This includes includes(from (frombottom bottomto to top): top): mafic-ultramafic mafic-ultramafic ophiolite plutonic plutonic rocks, rocks, aa dike dike(sheeted?) (sheeted?)complex, complex,extrusive extrusivepillowed pillowedand andmassive massivebasalt basaltlava lava flows, flows, and and overlying overlying arc-related arc-relatedvolcaniclastic volcaniclastic rocks. rocks. Tectonized Tectonized ultramafic ultramafic rocks rocks representing suboceanic suboceanic mantle mantle have have not not been been recognized. recognized. Also, based based on on the present present representing exposure,the the ultramafic-mafic ultramafic-maficcumulates cumulatesand anddike dikecomplex, complex,although although present, present,are aremuch much exposure, less less extensive extensive than than in in the ideal ideal ophiolite sequence sequence (fig. (fig. 10). 10). This This may may be be aa function function of 26 �limited limited exposures exposures in in the region region and/or and/or lack lack of of preservation preservationof of the the lower lowerpart partof of the the ophiolite ophiolite sequence sequenceduring during later later magmatic magmaticand andtectonic tectonicevents, events.Incomplete Incompletesequences sequences are are common commonin in ophiolites ophiolitesfound found ininmany manyorogenic orogenic belts belts(Moores, (Moores,2002). 2002).The Thelithologic lithologic association associationobserved observedin in the the Quinnesec Quinnesec and andthe the compositions compositionsof of rocks, rocks,ranging rangingfrom from MORB MORE3 to depleted depleted island-arc island-arc tholeiite tholeiite and and high-magnesium high-magnesiumandesite andesiteof of boninitic boniniticaffinity, affinity, and plagio-rhyolite, are similar to Cenozoic suprasubduction zone ophiolites like those and plagio-rhyolite, are similar to Cenozoic suprasubduction zone ophiolites like those of of the the Coast Coast Ranges Rangesin inCalifornia California(Shervais (Shervaisand andKimbrough, Kimbrough,1985; 1985;Shervais, Shervais,2001). 2001). J 1 1 , ^ , I, Shallow-water Shallow-water or terrestrial terrestrial sedimentary sedimentaryrocks rocks Unconformity Unconformity Pelagic sediments Pelagic sediments sediments or or abyssal abyssal deep-sea fan sediments or or volcanic volcanicarc arc deposits deposits Mafic pillow breccia, Mafic pillow pillowlava, lava, pillow breccia, and and massive massive flows Mafic Mafic sheeted sheeted dike dike complex complex \( 1/ Massive plagiogranite Massive gabbro, diorite, and plagiogranite Cumulate Cumulatesection: section: ultramafic-mafic ultramafic-maficcumulates cumulatesatatbase, base, more toward top. more felsic toward top. Commonly Commonlycyclic, cyclic,common common contorted contortedlayering layeringand and other other evidence evidence for deformation deformation Petrologic Moho Ultramafic peridotite with Ultramafic tectonite; peridotite with discontinuous discontinuous layers of of dunite dunite (D) and and concentrations concentrations of chromite (Cr) chromite(Cr) Figure 10. 10. Schematic Schematic cross-section cross-section of of aa complete completeophiolite ophiolite(after (afterMoores, Moores,2002). 2002). Figure The Quinnesec Quinnesec Formation Formationis is intruded intruded by by units units of of the the Dunbar Dunbar dome dome including includingthe the Marinette Marinette Quartz Quartz Diorite Diorite and and the the Spikehorn Spikehorn Creek Creek Granite, Granite, as as well well as as by by the the Newingham Newingham Tonalite Tonalite and and Twelve Foot Foot Falls Falls Quartz Quartz Diorite Diorite (Sims (Sims and and Schulz, Schulz, 1993) 1993) and and numerous numerous 27 �To the the southeast the the Quinnesec is in small lamprophyre dikes and plugs locally. To apparent fault contact with the Pemene formation (Sims and Schulz, 1993). McAllister formation: The The McAllister McAllisterformation formationappears appearsto to be be in in fault fault contact contactwith with the the the area (Jenkins, 1973). It occurs in a roughly east-west belt adjoining volcanic units in the between the Pemene formation to the north and the Beecher formation formation to the south (Sims and Schulz, 1993). It ranges in thickness from about 300 meters in the west to W and ~.70-80'W 3,000 meters in the east with units facing north and generally striking N.70-80° consists of of calc-alkaline caic-alkalirie basaltic to to andesitic andesitic dipping near vertical. The McAllister consists volcanic breccia breccia with aa lithic lithic tuff matrix, matrix, and and locally locally pillowed pillowedand and massive massivelavas. lavas. Fragments in the breccia breccia are distinctive distinctive in in that they they generally generally contain contain large largepyroxene pyroxene crystals crystals that are now replaced replaced by hornblende. Amygdules are also common in many gradation in fragment fragments. Vertically, the McAllister formation shows no consistent gradation size. However, laterally, fragment fragment size size increases from from west west to to east east (Jenkins, 1973). Near the Menominee suggesting the Menominee River, blocks more than 15 cm in diameter are common, suggesting source area for this dominantly fragmental fragmental unit may be located located to the east in Michigan. source formation: The Beecher formation: The Beecher Beecher formation formation extends extends in a north-facing, east to southeasttrending belt belt south of the McAllister formation and north north of the Athelstane Quartz Quartz and Schulz, Schulz, 1993). The The formation formation is at at least 3,000 3,000 meters thick thick with aa Monzonite (Sims and thicker (2,000 - 3,000 m) lower unit consists dominantly of of lower and upper unit. The thicker caic-alkaline flows and and lesser calc-alkaline plagioclaseplagioclase- and pyroxene-phyric andesite and dacite lava flows interbedded felsic volcaniclastic rocks. The thinner (up to 300 m) upper unit consists of interbedded rounded ash, crystal tuff, lapilli tuff, and coarser fragmental rocks, some with distinctive rounded pink to white felsite fragments. Some units show grading whereas others are unsorted. Black slates are also present present locally in the upper upper part part of the the formation. formation. The lower part of the Beecher Beecher formation, where intruded intruded by the Athelstane Quartz Monzonite, faces away from the intrusion intrusion and and has has aa well well developed developed foliation foliation and and steeply steeply plunging lineation. Dikes plunging Dikes of Athelstane Quartz Quartz Monzonite Monzonite extend extend only only aa short short distance distance formation. into the Beecher formation. Pemene formation: The The Pemene Pemeneformation formationisisinterpreted interpretedto to be bethe the youngest youngest volcanic volcanic unit in eastern Marinette County. It occupies a broad broad oval oval area area whose whose outline outline is is well well in the the northern part part of of the Miscauno Island 7.5 7.5 minute expressed in the local topography in 2,000 meters thick and consists predominantly predominantly of of quadrangle. The Pemene is at least 2,000 thick (45 300 m) calc-alkaline calc-alkaline rhyolite lava flows typically composed of a flow-top (45 -- 300 hyaloclastite microspheruliticcentral central core. core. hyaloclastite breccia, an underlying flow-banded flow-banded unit, and a microspherulitic The microspherulitic microspherulitic core constitutes the bulk of each flow and contains plagioclase plagioclase phenocrysts, commonly in glomeroporphyritic clusters, clusters, and and locally locally phenocrysts phenocrystsof of blue blue and lower contacts contacts quartz. The flow-banded unit, where present, has gradational upper and and laminar to highly-contorted highly-contorted banding. The The top top of each each flow flow has has aa layer layer of of hyaloclastite hyaloclastite breccia 3 to 75 m thick that probably formed by quench fragmentation as the hot rhyolite flows sec. 13, 13, flows came came in in contact contactwith with cold coldexternal externalwater. water.Exposures Exposuresininthe theNE NE1/4, 14, NW ¼, 14, sec. T. 36 N., R. 21 E., E., (Miscuano (Miscuano Island Island 7.5 minute quadrangle) provide an excellent cross sectional view of one of these rhyolite flows. Here, large open outcrops show a transition from massive massive microspherulitic microspheruliticrhyolite rhyoliteupward upward into into flow-banded flow-banded rhyolite rhyolitebreccia brecciawith with clasts to about 30 cm (a "crackle "crackle breccia"?). breccia"?). This in turn grades grades upward upward into a spectacular fine hyaloclastite hyaloclastite consisting of black rhyolite fragments, some with fine internal banding and white rinds rinds (figs. 11 11 and and 12). 12). Thin Thin sedimentary sedimentary units unitswith with graded graded 28 �bedding are present between some rhyolite flows. The general characteristics characteristics of the Pemene rhyolites is similar to those of other submarine rhyolite lava flow and dome complexes such that on the Island Island of Ponza, Italy (Scutter and others, 1998). Autobrecciated Autobrecciated and and hyaloclastite hyaloclastitecarapace carapace banded and Flow banded and microspherulitic interior Figure 11. Schematic cross-section of of the the upper part of of a subaqueous rhyolite lava flow in the Pemene Pemene formation. formation. Photograph of the rhyolite hyaloclastite in the upper carapace of a Figure 12. Photograph subaqueous Pemene subaqueous Pemene rhyolite rhyolite lava lava flow. 29 �The Pemene Pemene formation shows little evidence of a penetrative structural fabric. The flows show a southward dip in the north and dip nearly vertically in the south. Jenkins (1 (1973) 973) interpreted the structure of the Pemene as an east-trending, asymmetric, doubly interpreted of plunging plunging syncline. syncline. Major Intrusive Intrusive Rocks Rocks A variety of intrusive rocks are present within the supracrustal sequence in northeastern Wisconsin. They range tonalite to range from synvolcanic synvolcanic gabbro, diabase, diorite, and tonalite syntectonic intermediate to felsic granitoids (including those related related to the Dunbar dome), lamprophyric lamprophyric dikes and plugs, and post-tectonic granites and diabase dikes. Twelve Foot Falls Quartz Diorite: The Twelve Foot Falls Quartz Diorite comprises a some 20 20 km km by by 55 km km in size size to to the the west west of of Beecher, Wisconsin Wisconsin large "sill-like" intrusion some of gray, gray, generally medium to to coarse-grained coarse-grained (Sims and Schulz, 1993). It is composed of quartz quartz diorite diorite containing containing subhedral subhedral crystals crystals of of sodic sodic andesine, andesine,subhedral subhedralhornblende, hornblende, and anhedral bluish quartz. In composition it is similar to calc-alkaline calc-alkaline andesites in the McAllister and Beecher formations (Sims (Sims and others, others, 1992). 1992). Atheistane Atheistane Quartz Monzonite intrudes the Beecher Athelstane Quartz Monzonite: The Athelstane formation and extends for an unknown formation and extends for an unknown distance distance to to the the south southand andwest west(Sims (Simsand andSchulz, Schulz, 1993). It consists dominantly of of medium- to to coarse-grained quartz monzonite and locally contains contains numerous numerous metavolcanic metavolcanicinclusions. inclusions. The The Athelstane Athelstane Quartz QuartzMonzonite Monzoniteisisdated dated at 1,836±15 1,836±1Ma Ma (Banks (Banks and and Cain, 1969). 1969). The The Amberg Amberg Granite, Granite,which which intrudes intrudesthe the Athelstane Quartz Monzonite west and north of of Amberg (Sims and Schulz, 1993), 1993), is dated 1,756±1Ma Ma (Van (Van Schmus, Schmus, 1980). 1980). dated at 1,756±19 Dunbar Dunbar Dome Dome Only a brief summary of of the aeoloav geology of of the Dunbar dome is presented presented here for the Note. Onlv purposes of the present trips. More More comprehensive comprehensive accounts accounts are are available available in in aa guidebook auidebook by bv Sims Sims and and others others (1984), (19841, and and in in U.S. U.S. Geological Geoloaical Survey Survey Professional ProfessionalPaper Paper 1517 by Sims and and others others (1992). (1992). 1517 bv The Dunbar dome is one of several granitoid domes in northern Wisconsin that have have cores of gneiss, migmatite migmatite and granitoid rocks rocks and are mantled mantled by metavolcanic metavolcanic and Pembine-Wausau magmatic terrane (Sims and others, metasedimentary rocks in the Pembine-Wausau 1985). Where ages have been determined, both the core of the domes and mantle or cover rocks are of Paleoproterozoic age. The Dunbar dome is a complex antiformal structure consisting of a central core of Dunbar Gneiss, Marinette Quartz Diorite, and from the core composed of Hoskin Lake Granite, and three lateral protuberances (lobes) from the west, west, Spikehorn Creek Granite on on the the east, and Newingham Bush Lake Granite on the Tonalite on the south (Sims and others, 1992). structural evolution of 1992). The intrusive and structural the dome spanned the relatively short time of about 30 Ma, from syn-tectonic syn-tectonic events at about 1865 1865 Ma to post-tectonic post-tectonic at about about 1835 1835 Ma. Ma. Conclusions Conclusions The volcanic and associated intrusive rocks in northeastern Wisconsin south of the fault, the Pembine ophiolite, are are interpreted to to record the the evolution of of a Niagara fault, 30 �Paleoproterozoicsuprasubduction suprasubductionzone zone ophiolite-island ophiolite-islandarc arc sequence. sequence. Shervais Shervais(2001) (2001) Paleoproterozoic has has shown shown that suprasubduction suprasubductionzone zone ophiolites ophiolites tend tend to to display displayaa consistent consistentsequence sequence of events tectonic processes. events during their formation formation and evolution in response response to similar tectonic processes. This This sequence sequence includes includesthe the following following(after (afterShervais, Shervais,2001): 2001): (1) Birth, Birth, which which entails entails the the initiation initiationof ophiolite ophiolite formation formation during during extension extension above above aa (1) reconfigured intraoceanic subduction zone. Rocks formed newly forming or reconfigured during during the the initial initial phase phase of ophiolite ophiolite formation include include layered layered and isotropic isotropic plutonic gabbros, sheeted dikes, and a "lower" volcanic section consisting lowplutonic gabbros, sheeted dikes, and a "lower" volcanic section consistingof oflowK K tholeiitic tholeiitic basalt basalt and and basaltic basaltic andesite andesite with with MORB MORB and and primitive primitive arc arc tholeiite tholeiite affinities. Gabbros Gabbros formed formed during during this this stage stage are are often often ductilely ductilely deformed deformed (foliated (foliated affinities. or or boudinaged) boudinaged)in in response responseto to syn-magmatic syn-magmaticextension. extension. (2) (2) Youth, Youth, during duringwhich which continued continuedmelting meltingof of previously previouslydepleted depletedasthenospheric asthenospheric mantle mantle occurs occurs in in response response to to increased increased fluid flux from from the the subducting subducting slab. Rocks Rocks formed during during the the second second phase phase of ophiolite ophiolite formation formation include include intrusive intrusive maficmaficformed ultramafic ultramafic sills sills and and additional additional dikes, dikes, and andan an"upper" "upper"volcanic volcanicunit unitcharacterized characterizedby by basalt and andesite with highly depleted incompatible trace element basalt and andesite with highly depleted incompatible trace element compositions compositions (i.e., (i.e., low-Ti low-Tibasalt, basalt, high-Mg high-Mgandesite andesiteand andboninite). boninite). (3) Maturity, Maturity,during duringwhich which the the subduction subductionzone zone stabilizes stabilizes and and the the rate rate of of crustal crustal (3) spreading slows. slows. Rocks Rocks formed formed during during this this phase phase include include hornblende hornblende diorite, spreading quartz quartz diorite, diorite, tonalite, and and volcanic volcanic rocks rocks ranging ranging from from basalt basalt to to rhyolite, rhyolite, all all with with transitional transitional to tocalc-alkaline calc-alkalinecompositions. compositions.Volcanism Volcanismtypically typicallybecomes becomesmore more silicic silicic with time. In In many many cases, these rocks rocks have have not been been considered considered part part of the subjacent subjacent ophiolite, ophiolite, but but rather rather have have been been attributed attributed to post-ophiolite post-ophiolitearc volcanism. volcanism. (4) Death, Death,which which results resultsfrom from the the demise demiseof of active activespreading spreadingand andsubduction. subduction.In Inthe the (4) case case where where death death results results from from collision collision with with an an active activeocean oceanspreading spreadingcenter, center, dikes dikes and and lavas lavas with with oceanic oceanic basalt basalt compositions compositionsmay maycrosscut crosscutand andoverlie overliethe the older older ophiolite-arc ophiolite-arcsection. section. (5) Resurrection, Resurrection,which which accompanies accompaniesemplacement emplacementby by obduction obduction onto onto aa passive passive (5) continental margin marginor or accretionary accretionary uplift uplift with with renewed renewedsubduction. subduction. In In the the case case continental where where death death is is the the result result of of collision collision with with aa passive passive margin, margin, death death and and resurrection of of the the ophiolite ophiolite sequence sequencemay mayoccur occuressentially essentiallysimultaneously. simultaneously. resurrection The rocks rocks of of the the Quinnesec Quinnesec Formation Formationappear appear to to record record the the first first two two stages stagesof of The suprasubduction zone zone ophiolite evolution. The presence presence in the upper upper part of the suprasubduction Quinnesec Formation Formation of basalt basalt and and andesite andesite lavas lavas and dikes derived from highly highly Quinnesec refractory mantle mantle is is particularly particularly diagnostic diagnostic of aa relationship relationship to to the the early early stages stages of refractory intraoceanic forearcsetting setting(Shervais (Shervaisand andKimbrough, Kimbrough, intraoceanicsubduction subductionand and formation formationin inaaforearc 1985; Beccaluva Beccaluvaand and Serri, Serri, 1988). 1988).This This further furtherimplies impliesthat that the the Quinnesec QuinnesecFormation Formationand and 1985; associated rocks rocks did did not not form form in in aa back-arc back-arcbasin basin near near the the margin margin of of the the Superior Superior associated Craton, but but probably probablyformed formedas asan anintraoceanic intraoceanicophiolite-arc ophiolite-arcsystem systemabove aboveaasouthward southward Craton, dipping dipping(in (inpresent presentcoordinates) coordinates)subduction subductionzone. zone. McAllister, Beecher Beecherand and Pemene Pemene formations formationsand and The calc-alkaline calc-alkalinevolcanic volcanic rocks rocks of of the the McAllister, The associated associatedintrusives intrusivessuch such as as the the Newingham Newingham Tonalite Tonalite and and Twelve Twelve Foot Foot Falls Falls Quartz Quartz Diorite appear appear compatible compatiblewith with the the third thirdstage stage(maturity) (maturity)of ofsuprasubduction suprasubductionzone zone Diorite ophiolite evolution. evolution. Shervais Shervais (2001) (2001) notes notes that that for for aa suprasubduction suprasubduction zone zone ophiolite ophiolite to to ophiolite reach maturity maturity requires requires that that the the ocean ocean basin basin being being subducted subducted be be large large enough enough to to reach complete completethe the first first two two stages stageswithout without disappearing. disappearing.This This suggests suggests that that the the 31 �Paleoproterozoic significant in size (at Paleoproterozoic ocean basin that was subducted was probably significant least many many hundreds hundreds of kilometers). kilometers). It appears likely that growth of the Pembine Pembine ophiolite-arc system system was terminated terminated by by its its collision obduction onto onto the passive passive margin margin of the the Superior Superior craton. craton. Since Since collision with with and and obduction subduction subduction appears appears to to be be largely largely driven driven by by slab slab pull, pull, the the southward southward subduction subduction of of oceanic oceanic lithosphere lithosphere attached attached to to the the Superior Superior continental continental margin marginwould would have have pulled pulled the the continental lithosphere along with it as it descended into the subduction zone below continental lithosphere along with it as it descended into the subduction zone below the the ophiolite-arc ophiolite-arcsystem. system. With With detachment detachment of of the the subducting subductingoceanic oceanic lithosphere, lithosphere,the the buoyancy buoyancy of the the continental continental lithosphere lithosphere would have have led led to its its rapid rapid uplift uplift along along with with the the leading leading edge edge of of the the ophiolite-arc ophiolite-arc system system (Shervais, (Shervais, 2001). 2001). This This interpretation interpretationsuggests suggests that the the volcanic volcanic and and associated associated rocks rocks of of northeastern northeastern Wisconsin Wisconsinare are allochthonous, allochthonous,as as is is also also suggested suggested by by gravity gravity and and magnetic magnetic data data for for the the region region (Klasner (Klasner and and others, others, 1985; 1985; Attoh Attoh and and Klasner, Klasner, 1989). 1989). This This stage stage is is recorded recorded by by the the deformation deformationof of the the ophiolite-arc ophiolite-arc sequence Dunbardome. dome. ItItisis sequence and and by by the the intrusion intrusionof the the syn-tectonic syn-tectonic units units of of the the Dunbar possible possible that the shallow-water shallow-water sedimentary sedimentary rocks rocks (carbonates) (carbonates) along along the the west west margin margin of of the Dunbar Dunbar dome dome (Sims (Sims and and Schulz, Schulz, 1993) 1993) represent represent Chocolay Chocolay Group Group rocks rocks of of the the Marquette Marquette Range Range Supergroup Supergroup that that were were uplifted upliftedfrom from the the continental continentalmargin marginbasement basement below below during during formation formation of of the the Dunbar Dunbardome. dome. 32 �FIELD FIELDTRIP TRIP1I PEMBINE-WAUSAU PEMBINE-WAUSAU MAGMATIC MAGMATICTERRANE TERRANE Klaus Klaus J. J. Schulz, USGS, Reston, VA VA and and Gene Gene L. LaBerge, University University of of Wisconsin-Oshkosh Wisconsin-Oshkosh (retired), (retired), Oshkosh, WI Wl and andUSGS USGS Pillowed flows of high-Mg high-Mg andesite of the Quinnesec Quinnesec Formation, Formation, part part of the Pillowed Pembine Pembine ophiolite ophiolite complex, complex, Quiver Quiver Falls, Falls, Wisconsin Wisconsin �FIELD FIELDTRIP TRIP1I PEMBINE-WAUSAU MAGMATIC MAGMATIC TERRANE TERRANE PEMBINE-WAUSAU Klaus J. Schulz, Schulz, USGS, Reston, VA and Gene L. LaBerge, University University of WisconsinWisconsinOshkosh Oshkosh (retired), (retired),Oshkosh, Oshkosh,WI Wland andUSGS USGS The Paleoproterozoic Paleoproterozoicvolcanic volcanic and andassociated associatedintrusive intrusiverocks rocksexposed exposedininthe theeastern eastern The part of of Marinette MarinetteCounty County in in northeastern northeastern Wisconsin Wisconsin are are the the easternmost easternmost exposures exposures of of part the Pembine-Wausau Pembine-Wausauterrane, terrane, the northernmost northernmost of the two Wisconsin magmatic magmatic terranes the (Sims (Sims and others 1989). 1989). The volcanic rocks are composed composed of tholeiitic tholeiitic and calc-alkaline volcanic and and volcaniclastic volcaniclastic rocks rocks that formed at about 1870 1870 Ma Ma (Sims (Sims and and others, others, 1989). 1989). volcanic They They are are cut cut by by aa variety variety of intrusive intrusive rocks rocks ranging ranging from from syn-volcanic syn-volcanic gabbros, gabbros, diorites, diorites, and tonalities tonalities to to syn-and syn-andpost-tectonic post-tectonicgranitoids granitoids(i.e., (i.e., Dunbar DunbarGneiss Gneissand andrelated relatedrocks). rocks). and The The magmatic magmatic rocks rocks of of the the Pembine-Wausau Pembine-Wausauterrane terrane are are separated separated from from the the epicratonic epicratonic sedimentary sedimentary rocks rocks of of the the Marquette Marquette Range Range Supergroup Supergroup in in Michigan Michigan by by the Niagara Niagara fault fault zone. The The lithologic lithologic units units present present in in eastern eastern Marinette Marinette County County and and their chemistry chemistry zone. strongly strongly suggest suggest that that these these rocks rocks represent represent aa Paleoproterozoic Paleoproterozoicsuprasubduction suprasubduction zone zone ophiolite, the the Pembine Pembine ophiolite ophiolite (Schulz, 1987; 1987; Sims Sims and others, 1989). 1989). The ophiolite ophiolite, and and associated associated island-arc island-arcrocks rocks were accreted accreted to the southern southern margin margin of the the Archean Archean Superior Superior Craton Craton during during the the Penokean PenokeanOrogeny. Orogeny. On On this this field field trip trip we will examine examine the major major lithologies lithologies that comprise comprise the Pembine Pembine ophiolite. This includes examples of ultramafic rocks (serpentinite), layered ophiolite. This includes ultramafic rocks (serpentinite), layered and and massive massive gabbros cut cut by by mafic mafic dikes, dikes, pillow pillow basalts basalts and and andesites, andesites, and and several overlying calcgabbros alkaline alkaline arc-related arc-relatedvolcanic volcanic and andvolcaniclastic volcaniclasticrocks rocks(fig. (fig.1-1). 1-1). 34 �88 0845' 8T 5830" 88°08'45" 8758'30" 87 4815" 45 41 00 22 t I 0 0 ' 5 ' 2 I 0 0 55 6 6 4 'II 5 5 ' 8748'15" 88 , II 10 10 10 10 •II 15 15 12 Miles Miles 12 II Kilometers Kilometers EXPLANATION EXPLANATION Munising Sandstone Munising Sandstone(Cambrian) (Cambrian) Paleoproterozoic north of Paleoproterozoic rocks north o f Niagara fault Formation-graywacke Michigamme Formation graywacke Iron-formation Vulcan lron-formation Dolomite Randville Dolomite Paleoproterozoic rocks of Terranes Paleoproterozoic of Wisconsin Magmatic MagmaticTerranes Intrusive trusive rocks rocks Volcanic rocks . E Spikehorn Creek Spikehorn CreekGranite Granite and dacite dacite "Pemene formation" formation" . . ...., Rhyolite Rhyoliteand Granite Bush Lake Granite Basaltic and andandesitic andesitic breccia breccia"McAllister "McAllisterformation" formation Hoskuns Lake Lake Granite Granite Hoskins Rhyolute tuff graywacke Beecher formation" formation Rhyolite, felsuc felsic tuff,graywackel'Beecher Athelstane Athelstane Quartz Monzonite Monzonite Twelve Foot FallsQuartz Quartz Diorite Diorite Twelve Marinette Quartz Diorite Diorite Marinette Serpentinite of ophiolite ophiolite complex complex Serpentinite and and gabbro, base of NewinghamTonalite jJ NewinghamTonalite Quinnesec Formation Quinnesec Formation NewinghamTonalite, NewinghamTonalite, megacrystic facies megacrystic .. . Metagabbro Metagabbro Volcanic and rocks undivided undivided and granituc granitic rocks — faults - faults • field field trip trip stops stops Figure 1-1. Generalized geologic map of part of northeastern northeastern Wisconsin Wisconsin showing showing the the location of field trip stops stops 1-1 1-1 through through 11-8. See figure figure 3-2 3-2(p. (p. 67) 67) for for location locationof of stops stops1-9 1-9 location -8. See and 1-10. 1-10. 35 �____ _______ __ ____________ ____ _______ ° '6 / 1' + ET. -r1I k N 32 ' - ' aveH Pt -f •. - --- I && • ./ — c( - - . ./ ;. L 9Xt I - — - N ' . . . \ . / - — * . .. ) - • 2Y -- I . ( T. \ - — Approximatel 10 miles to Dunbar - 26O . . k. . ••• -' - n — . '- iaN . - . .— . - l . IZ I K" w 3 I) 5000 0 — ,_ — 0 10000 METERS I I 10000 20000 30000 — -= 400CR FEET Figure Figure1-2. 1-2.Part Partofofthe theEscanaba Escanaba1:100,000-scale 1:I 00,000-scaletopographic topographicmap mapshowing showingthe thelocation location of of field fieldtrip tripstops stops1-1 1-1through through1-7 1-7and andsupplemental supplementalstops stops1-8 1-8and and1-11. 1-11. Stop side U.S. 141, SW NE Stop1-1. 1-1.Spikehorn SpikehornCreek CreekGranite Granite(west (west side U.S.Highway Highway 141, SW¼,?h, NE1/4 '%sec. Isec. , 36, 36,T. T.38 38N., N.,R. R.20 20E.). E.). The Theoutcrop outcropon onthe thewest westside sideofofU.S. U.S.Highway Highway141 141(fig. (fig.1-2) 1-2)isisrepresentative representativeofofthe the Spikehorn SpikehornCreek CreekGranite, Granite,aagray graytotopinkish pinkishgray, gray,finefine-totomedium-grained medium-grainedmassive massive granite granitewith withscattered scatteredpotassium potassiumfeldspar feldspargrains grainsas asmuch muchas as22cm cminindiameter. diameter.The The granite graniteisiscomposed composedofofplagioclase plagioclase(sodic (sodicoligoclase) oligoclase)with withweak weaknormal normalzoning, zoning, 36 �microclinemicroperthite, microperthite,quartz, quartz,biotite, biotite,sphene, sphene,and andopaque opaqueoxides. oxides.Accessory Accessoryminerals minerals microcline include zircon, allanite, apatite (rare), and fluorite (rare). The granite has sharp intrusive include zircon, allanite, apatite (rare), and fluorite (rare). The granite has sharp intrusive contacts contactswith with the the Quinnesec Quinnesecvolcanic volcanicrocks rocksand andthe theMarinette MarinetteQuartz QuartzDiorite. Diorite. The The Spikehorn SpikehornCreek Creek Granite Granite isis aa post-tectonic post-tectonicdiapiric diapiric intrusion intrusion on on the the northeast northeast side side of of the the Dunbar Dunbar dome dome with with an an age age of of 1,835±6 1,835*6 Ma Ma (Sims (Sims and and others, others, 1992). 1992). ItIt is is similar similar in in composition LittleTobin Lake Lake Granite, Granite, which which intrudes intrudes the the Badwater Badwater composition and and age age to to the the LittleTobin Greenstone Greenstonenorth north of of the the Niagara NiagaraFault Faultzone zone (Schneider (Schneiderand and others, others, 2002; 2002; see see also also field field trip trip 3, 3, stop stop 3-9). 3-9). Both Both the the Spikehorn Spikehornand and Little LittleTobin Tobin Lake Lakegranites granites represent represent"stitching" "stitching" plutons, plutons, emplaced emplaced after after collision collision of of the the Pembine-Wausau Pembine-Wausaumagmatic magmaticterrane terranewith with the the passive passive margin margin of of the the Superior Superior Craton. Craton. As As such, such, the the age age of of these these granites granites provides providesaa minimum minimumage agefor for the thePenokean PenokeanOrogeny. Orogeny. Stop Stop 1-2. 1-2. Exposures Exposuresof of Serpentinite, Serpentinite,Gabbro. Gabbro,and and Mafic Mafic Dikes Dikes(i.e., 0.e.. Ophiolite) Ophiolite)East East of (NW1/4, %, NW ¼, sec. sec. 22, 22ÂT. T. 37 37N., N.?R. R. 21 21E.) Em) ofPembine Pembine(NW % Follow Follow the the red red flags flags and and trail trail north north to to outcrops outcrops of of serpentinite serpentinite on on the the south south side side of of the the North NorthBranch BranchPemebonwon PemebonwonRiver. River.The Theserpentinite serpentiniteininthe thesurrounding surroundingoutcrops outcropsshows shows variable variable features features including includinglayering layering(fig. (fig. 1-3), 1-3),brecciation, brecciation,and andcarbonate carbonateand andserpentine serpentine veining. veining. Chromite Chromiteisisclearly clearly evident evident in insome some samples. samples. Some Some serpentinite serpentinite is is highly highly magnetic magnetic whereas whereas other other samples samples are are not; not; this this may may reflect reflectoriginal original variations variations in in the the proportion proportionof of olivine olivineand andpyroxene pyroxenein in the the ultramafic ultramafic rocks. rocks.Locally, Locally,dikes dikes of of pyroxenite pyroxenite (now (now altered alteredto to talc-serpentine) talc-serpentine)have havebeen beenobserved observedcutting cuttingthe theserpentinite. serpentinite. Figure Figure1-3. 1-3.Photograph Photographof ofnearly nearlyvertical verticallayering layeringininultramafic ultramafic(serpentinite) (serpentinite)rocks rocksat at stop 1-2. stop1-2. 37 �As we walk south we will first first see further exposures of serpentinite followed after a few hundred hundred meters meters by by a series series of of outcrops outcrops with with variable variable proportions proportionsof of layered layeredgabbro, gabbro, foliated-lineated gabbro, massive diabase and quartz diabase (dikes?), all cut by foliated-lineated reddish-brown-weathering reddish-brown-weathering mafic dikes (figs. 1-4 1-4 and 1-5). The foliation in the foliated gabbro is variable in strike between N.15 W. to N.15 E. and dips steeply either E or W. The mafic dikes generally strike about E-W parallel parallel to the overall trend of the serpentinite-gabbro body and dip steeply. The dikes do not appear to extend outside the body into the surrounding pillow basalts. The serpentinite-gabbro body appears to be fault-bounded and and tectonically tectonically emplaced emplaced within within the the Quinnesec Quinnesec Formation. Formation. Figure Figure 1-4. Photograph Photograph of rusty weathering mafic dikes with gabbro screens at stop 1-2 1-2 (Note, dike dike margins margins are are highlighted). highlighted). The lithologies lithologies and their arrangement in this ultramafic-gabbroic ultramafic-gabbroic body along with those of the Quinnesec Quinnesec Formation Formation generally are similar to those that characterize recent ophiolite sequences (Moores, 2002). This includes (from bottom to top): mafic-ultramafic mafic-ultramafic plutonic rocks, rocks, aa dike dike (sheeted?) (sheeted?) complex, complex, extrusive extrusivepillowed pillowedand andmassive massivebasalt basaltlava lava flows, and overlying volcaniclastic sedimentary rocks. The compositions of the gabbros and mafic island-arc tholeiite and MORB to depleted island-arc mafic dikes within this body range from MORB high-magnesium high-magnesium andesite with boninitic affinities; the mafic rocks rocks are similar in composition to the gabbros and basalts basalts in the Quinnesec Formation Formation (Sims and others, 1989; Schulz, 1987 1987 and unpublished unpublished data). The The lithologic lithologic association association and and chemistry chemistry are are similar to recent suprasubduction zone ophiolites like those of the Coast Ranges in California California (Shervais and Kimbrough, 1985; Shervais, 2001). These data, along with the presence of overlying overlying calc-alkaline caic-alkaline island-arc island-arc volcanic and volcaniclastic rocks, suggest formation of the Quinnesec Quinnesec as as a suprasubduction suprasubduction zone ophiolite associated with forearc extension during the early stages of subduction subduction and island arc formation. 38 �Figure Figure 1-5. 1-5. Close-up Close-up photo photo of of rusty rusty weathering weathering mafic mafic dike dike cutting cuttingfoliated foliated gabbro gabbro at at stop stop 1-2. 1-2. Note Note cleavage cleavage development development along along the the upper upper margin margin of of dike dike (dike (dike contacts contactsare are hightlighted). hightlighted). Stopl-3. Stop1-3.Pillow Pillowbasalt basaltofofthe theQuinnesec QuinnesecFormation. Formation,Quiver QuiverFalls Fallson onthe theMenominee Menominee River sec. 24, T. 37 River(NE (NE1/4, %, SW SW 1/4, %, sec. 37 N., N., R. R.21 21E.) E.) - Basalt Basaltand andandesite andesitepillow pillow lavas lavasand and pillow pillow breccias brecciasare are common common in in the the Quinnesec Quinnesec Formation. three-dimensionalview viewofofpillows pillowscan canbe beseen seenininthis thisexposure exposurealong alongthe the Formation.AA three-dimensional banks banksof of the theMenominee MenomineeRiver Riverat at Quiver QuiverFalls Falls(fig.1-6). (fig.l-6). Here, Here,south-facing south-facingelongate elongate closely closely packed packedbasalt basaltpillows pillowscan can be be seen seen with with excellent excellent preservation preservation of of features features due due to to the thelow lowdegree degreeof ofdeformation. deformation.Locally, Locally,the thebasalt basalthere hereisisalso alsohighly highlyvariolitic variolitic(fig. (fig.1-7) 1-7) with with large large(1-2 (1-2cm) cm)round roundpinkish pinkishsiliceous-appearing siliceous-appearingvarioles. varioles. Chemically Chemicallythe the basalt basalt isis characterized (9.8 wt.%), wt.%), and andvery very low low Ti02 Ti02(0.35 (0.35 wt.%), wt.%), Zr Zr (35 (35 characterizedby byrelatively relativelyhigh highMgO MgO(9.8 ppm), ppm),and andREE REE(flat (flatpattern patternatat—6 -6 xx chondrites) chondrites) contents. contents. ItIt is is similar similar in in composition composition to to some someof of the themassive massivediabasic diabasicgabbro gabbroseen seenat atstop stop1-2. 1-2. 39 �Fig1-ire 1-6. Figure 1-6. Photograph Photograph of of pillow pillow basalt basalt in in the the Quinnesec Quinnesec Formation Formation at at Quiver Quiver Falls Falls on oi the the Menominee MenomineeRiver River(stop (stop1-3). 1-3). FigiJre 1-7. 1-7.Photograph Photographof of variolitic variolitic basalt basalt at at Quiver Quiver Falls Falls on on the the Menominee Menominee River River (stop (s1 Figure 1-3). 1-3)I. 40 �Stop 1-4. 1-4. Andesite Andesite Breccia Brecciaat atthe theNew NewKremlin KremlinMine MinePit Pit(5(S½, Vs,sec. sec.26, 26,T.T.37 37N., N.,R. R. Stop 21 21 E.) E.) The fresh fresh glacially glacially polished polished outcrops outcrops around around the the new new Kremlin Kremlin mine mine pit pit at at this this stop stop The provide excellent exposures of andesite breccia breccia in the upper upper part part of the Quinnesec Quinnesec provide Formation. The Kremlin Kremlin mine mine processes processes the Quinnesec rocks rocks for roofing roofing granules. granules. The The Formation. andesite breccia breccia consists consists of of angular angular to to sub-rounded sub-roundedvolcanic volcanic fragments fragments ranging rangingfrom from andesite about 55 cm cm to to at at least least 40 40 cm cm across across in in aa matrix matrixof of 0.5 0.5 to to 2.0 2.0 cm cmhyaloclastite hyaloclastitefragments. fragments. about The sub-rounded sub-roundedfragments fragments and and hyaloclastite hyaloclastitematrix matrix suggest suggest that that the the unit unit may mayrepresent represent The andlor subaqueous subaqueous debris debris flows. flows. Note Note the the black black pyroxene pyroxene phenocrysts phenocrysts in in pillow breccias brecciasand/or pillow some of of the the andesites andesites clasts. clasts. Locally Locallyto to the the west, west, similar similar rocks rocksare are interlayered interlayeredwith with some rhyolite rhyolite flows flows and and tuffs. tuffs. Stop1-5. 1-5.McAllister McAllister Formation Marek Road %,sec. sec.22, 22,T.T.36 36N., N.,R.R.21 21 Stop Formation onon Marek Road (NE(NE 1/4,%, NENE 1/4, E.) The McAllister McAllister formation formation consists consists of of caic-alkaline calc-alkaline basaltic basalticto to andesitic andesitic volcanic volcanic breccia breccia The with aa crystal crystal lithic lithic tuff tuff matrix, matrix, and and locally locally pillowed pillowedand and massive massive lavas. lavas. This outcrop outcrop is is with typical of of the the McAllister McAllister volcanic volcanic breccia. breccia. Fragments Fragments in in the the breccia brecciaare are distinctive distinctive typical because they they generally generally contain contain large large pyroxene pyroxene crystals crystals that that are are now now replaced replaced by by because hornblende. Amygdules Amygdules are are also also common common in in many many fragments. fragments. The The size size of of fragments fragments in in hornblende. the McAllister McAllister formation formation increases increases from from west west to to east. east. Near Near the the Menominee Menominee River, River, blocks blocks the over 15 15 cm cm in in diameter diameter are are common common suggesting suggestingthe the source source area areafor for this this dominantly dominantly over fragmental fragmental unit unit may may be be located locatedto to the the east east in in Michigan. Michigan. Stop onon Marek Road (NW 1/4,%,NW Stop1-6. 1-6.Beecher BeecherFormation Formation Marek Road (NW NW1/4, %, sec. sec. 26, 26,T. T. 36 36N., N.,R. R.21 21 E.) E.1 The Beecher Beecherformation formationconsists consistsof oftwo twounits: units:aathick thick(—2,000-3,000 (-2,000-3,000 m) m) lower lower unit unit The consisting consisting dominantly dominantly of of calc-alkaline calc-alkaline plagioclaseplagioclase- and and pyroxene-phyric pyroxene-phyric andesite andesite and and dacite lava lavaflows flows and andlesser lesserpyroclastic pyroclasticrocks, rocks,and andaathinner thinner(—300 (-300 m) upper upper unit, which which dacite consists consists of of interbedded interbedded felsic felsic ash, ash, tuff tuff and andcoarser coarser fragmental fragmental rocks. rocks.The Theexposures exposuresat at this lapillituff, tuff, and and this stop stop are are in in the the upper upper unit unit and and show show bedded bedded crystal crystal tuff tuff (fig. (fig. 1-8), 1-8),lapilli coarser coarser units, units, some some with with distinctive distinctive rounded rounded pink pink to to white white felsite felsite fragments fragments (fig. (fig. 1-9). 1-9). Some Some units units are are graded graded whereas whereas others others are are unsorted. unsorted. Graded Graded bedding bedding in in some some layers layers indicates tops tops to to the the northeast. northeast. Jenkins Jenkins(1973) (1973) suggested suggestedthat that at at least leastsome someof of the the rocks rocks indicates of of the the Beecher Beecherformation formationwere weredeposited depositedsubaerially. subaerially. 41 �4L - -. — —. - 4 —? S — -;.. —-— :;-— :-— :t_ —. ;:;b -- - ,- —- ,- -. __4 --- . __( . : Figure 1-8. 1-8. Photograph Photograph of bedded bedded crystal crystal tuff in in the the upper upper unit unit of of the the Beecher Beecherformation, formation, Figure stop stop1-6. 1-6. Figure 1-9. 1-9.Photograph Photographof of aa distinctive distinctive fragmental fragmental unit unitwith with rounded roundedpink pinkto to white white felsite felsite Figure 1-6. fragmentsininthe theupper upperpart partof ofthe theBeecher Beecherformation, formation,stop stop1-6. fragments 42 42 �Stop 1-8. 1-8. Pemene Pemene Formation Formationat at Pemene Pemene Falls Fallson onthe theMenominee MenomineeRiver River(NW (NW corner, corner, Stop Sec. sec. 23, T. 37 37 N., N., R. R. 25 25 W.) W.) Exposed along along the bank bank of the the Menominee Menominee River River at Pemene Pemene Falls Falls is is rhyolite rhyolite of the Exposed Pemene Formation. Formation. The The rocks rocks are are dark dark gray gray to to reddish reddish gray, gray, contain contain few few feldspar feldspar Pemene phenocrysts, and are generally microspherulitic. Phenocrysts, many of which are phenocrysts, and are generally microspherulitic. Phenocrysts, many glomeroporphyritic, consist consist mainly mainly of euhedral euhedral to subhedral subhedral albite; however, phenocrysts phenocrysts glomeroporphyritic, blue quartz quartz are are present present locally. locally. The The microspherules microspherules consist consist of radial radial intergrowths intergrowths of of blue quartz quartz and and albite. This phase phase of the the Pemene Pemene probably probably represents represents the devitrified devitrified interior interior portion portion of of aa rhyolite rhyoliteflow. flow. In In areas areas to to the the west, Pemene Pemene rhyolite rhyolite lava lava flows show internal internal gradations gradations from massive massive microspherultic rhyolite rhyolite at their their centers centers to flow-banded flow-banded rhyolite rhyolite and and finally autobreccia autobreccia microspherultic and hyaloclastite hyaloclastite carapaces. carapaces. This suggests suggests the Pemene Pemene rhyolite rhyolite flows were deposited deposited and subaqueously. Locally, Locally, felsic dikes dikes are found found cutting cutting the rhyolite. rhyolite. subaqueously. SupplementalStop Stop 1-8. 1-8. Dunbar Gneiss Dunbar, Wisconsin Supplemental Gneiss West and North of Dunbar, Wisconsin Stop 1-8a. 1-8a. Dunbar Gneiss on the West Side of of County CountyRoad Road U U Stop of the the Intersection Intersection of Y4, SW SW ¼, Y4, sec. sec.26, 26,T. T.37 37N., N.,R. R.18 18E.). E.). withU.S. U.S.Highway Highway8 8(SW (SW1/4, with The low low outcrops outcrops here here are are composed composedmainly mainly of of megacrystic megacrysticgranite granite gneiss gneiss that that contains contains The rafts of layered layered amphibolite amphibolite (fig. (fig. 1-10). 1-10). This This lithology lithology is is similar similar to to that that dated dated from from an an rafts outcrop to to the the north northwith with aa U-Pb U-Pbzircon zircon concordia concordia upper upper intercept intercept age age of of 1,862±5 1,862~5 Ma outcrop Ma N. (Sims and and others, others, 1992). 1992).Lineation Lineationin inthe theamphibolite amphiboliteplunges plungesgenerally generallyabout about20-25° 20-25' N. (Sims 85-90' E. E.Locally, Locally, the the amphibolite amphiboliteisisrefolded refoldedby byfolds foldshaving havingN. N.50° 50' W. W.steep steepaxial axial 85-90° N. 70° 70' W. foliation. foliation. The The granite granitegneiss gneiss surfaces. The The granite granite gneiss gneiss has pervasiveN. hasaa pervasive surfaces. composition, and is interpreted plutonic protolith protolith (Dunbar Gneiss) tonalitic in Gneiss) is is tonalitic in composition, and is interpreted as a plutonic (Dunbar (Sims (Sims and and others, others, 1992). 1992). Stop1-8b. 1-8b.Migmatitic MigmatiticDunbar DunbarGneiss Gneiss (Center sec.15, 15,T.T.3737N., N.,R.R.18 18E.). E.). Stop (Center sec. The exposures exposures on on the the east east side side of of the the road roadare are of of migmatitic migmatiticDunbar DunbarGneiss. Gneiss.The Thegneiss gneiss The here here consists consistsmainly mainly of of compositionally compositionally layered layered rocks, rocks, biotite biotite gneiss, gneiss, and and lesser lesser amphibolite, intruded intrudedby by megacrystic megacrystic biotite biotitegneiss, gneiss, granite granitepegmatite, pegmatite, and and aplite. aplite. All All amphibolite, N. 50-55° 50-55' W. at at 90°. 90'. The The foliation foliation is is defined defined by by rocks are are deformed deformed with with foliation foliation striking striking N. rocks biotiteand andhornblende hornblendealignment alignmentand andisisgenerally generallyparallel paralleltotocompositional compositionallayering. layering. biotite 43 �_ — ..— a.— * it,r * -- qr * - ..._ - ..- ' -. ,—. - •* - - I . -. — .4 T Figure 1-10. 1-10. Photograph Photograph of an an outcrop outcrop of of megacrystic megacrystic Dunbar Dunbar Gneiss Gneiss with with folded folded Figure amphibolite amphiboliterafts raftsat atstop stop1-8a. 1-8a. SupplementalStop Stop1-9. 1-9. Sulfide Sulfide deposit depositat at"LaSalle "LaSalleFalls" Falls"on onthe thePine PineRiver. River.(NW (NW Supplemental %, SE 1/4, SE%, ¼,sec. sec.30, 30,T.39 T.39N., N.,R.18 R.18E.E.See Seefigure figure3-1 3-1and and3-2 3-2for for location.) location.) The Pembine-Wausau Pembine-Wausauterrane terraneisishost hostto to aanumber numberof of volcanogenic volcanogenicmassive massivesulfide sulfide The deposits, deposits, two two of of which whichare areknown knownin innortheastern northeasternWisconsin Wisconsin(Cummings, (Cummings,1978; 1978;LaBerge, LaBerge, 1983). 1983). The The deposit deposit on on the the Pine PineRiver Riverisisthe theonly onlyknown knownnaturally naturallyexposed exposedmassive massive sulfide sulfidedeposit depositin inWisconsin. Wisconsin. LaSalle Falls) Falls) on on the the Pine Pine River River occurs occurs in in The deposit deposit at at Pine Pine Rapids Rapids(locally (locally known known as as LaSalle The 3-2). the Quinnesec Quinnesec Formation Formationabout about one one mile mile south south of of the the Niagara NiagaraFault Faultzone zone (fig. (fig. 3-2). the LaSalleFalls Fallsis is formed formed where where the the Pine Pine River River flows flows over over aa resistant resistant unit unit of of rhyolite rhyolite LaSalle breccia brecciaonto onto an an easily easily eroded eroded unit unit of of sulfide-bearing sulfide-bearing schist schist that that occurs occurs between betweenthe the rhyolite rhyolite and and aa unit unit of of mafic maficvolcanic volcanic rocks. rocks.Removal Removalof of the the easily easily eroded eroded schist schist has has formedaa narrow narrowgorge gorge on on the the Pine PineRiver Riverfor for several severalhundred hundredfeet feet below belowthe the falls. falls. The The formed withaaprominent prominentlineation lineationthat thatplunges plunges rocksstrike strikenearly nearlyeast-west east-westand anddip dip60-70° 60-70' SSwith rocks 50-60°, 50-60, SS40° 40' W. W. The deposit deposit was was discovered discovered during during an an airborne airborne geophysical geophysical survey survey in in the the 1970's, 1970's, and and The has has been beendrilled. drilled. The Themain mainpart partof ofthe thegeophysical geophysicalanomaly anomalyextends extendsdownstream downstreamwithin within theriver riverchannel channelbetween betweenexposures exposuresof of rhyolite rhyoliteon onthe the north northand andbasaltic basalticrocks rockson onthe the the south. Drill Drillcores cores show show that that the the rhyolite rhyolite consists consists of of coarse coarse fragments fragmentsin inaa relatively relatively south. sulfide-richmatrix, matrix, aa typical typical "stringer "stringer ore". ore". Rhyolite Rhyoliteexposed exposed at at the the falls falls is is very very pitted, pitted, sulfide-rich due due to to breakdown breakdownof of the the sulfide-rich sulfide-rich(pyrrhotite-chalcopyrite) (pyrrhotite-chalcopyrite)matrix matrix material. material. Exposures Exposures 44 �immediately below below the falls, at the stratigraphic stratigraphic level level of the the main main EM EM anomaly, consist consist of immediately approximately approximately18 18m m of of sulfide-bearing sulfide-bearingschist schist and and cherty cherty units. units. The Thesedimentary sedimentaryunit unit isis mainly laminated laminated chloritic chloritic and and sericitic sericitic schist with pyritic lenses lenses up to 3 mm mm thick. mainly Garnets Garnets are are common commonin insome some layers. layers. Sprays Sprays of of black black tourmaline tourmaline are are present present in in the the mafic mafic volcanic volcanic rocks rocks on on the the south south side side of of the the river. river. The The structure structure in inthe the area areaisissomewhat somewhat puzzling. puzzling.Based Basedon on regional regional geology, geology, Dutton Dutton (1971) Bayley and and others others (1966) (1966) concluded concluded that that the the Quinnesec Quinnesec Formation Formationfaces faces (1971) and and Bayley north northnear near the the Niagara NiagaraFault. Fault. Sims Simsand andothers others(1984) (1984) also also reported reportedthat that the the Quinnesec Quinnesec faces DunbarDome, Dome, which which lies lies south south of of "LaSalle "LaSalleFalls". Falls". In In faces northward northwardaway away from from the the Dunbar addition, although although deformation deformationhas hasobscured obscuredfacing facingdirection directionindicators, indicators,such suchas as pillows, pillows, addition, %, NE NE ¼, %, sec. sec. 26, 26, T. T. 34 34N., N., A. R. in most mostareas, areas,north-facing north-facingpillows pillowsare areexposed exposedininthe theSW SW1%, in 17 E., about abouttwo two miles mileswest westof of"LaSalle "LaSalleFalls". Falls".However, However,the thelithologic lithologicsequence sequenceand and 17E., pattern patternof of mineralization mineralizationat at "LaSalle "LaSalleFalls" Falls"suggests suggests that that the the rocks rockshosting hostingthe the mineralization mineralization face face southward. southward. The The zone zone of of "stringer "stringerore" ore" isisnorth northof of the the main mainsulfide sulfide zone, zone, and and the the sequence sequence is is "overlain"(?) "overlain1'(?)by by mafic mafic volcanic volcanic rocks rocksto to the the south. south. SupplementalStop Stop1-10. 1-10.Pine PineRiver RiverPegmatite Peqmatite bodies. (NW NE V*.sec. sec.22, 22, TT .39 .39 Supplemental bodies. (NW ¼,%,NE 1/4, N., figures 3-1 and 3-2 for for location.) location.) N., R.17 R.17 E.) See figures - (WARNING: (WARNING: - Because Because this thisexposure exposureisisnear nearthe thePine PineRiver RiverWILD WILDRIVERS RIVERSAREA. AREA, no no collecting collectinuis ispermitted permittedat at this this locality.) locality.) Dutton Dutton (1971) (1971) reported reportedthe the occurrence occurrenceof of pink pinktourmaline tourmalinein inaa pegmatite pegmatitedike dikefrom fromthis this area. area. The Thepegmatites pegmatitesare arelocated locatedapproximately approximately150 150feet feetwest westof ofthe thePine PineRiver Riverand and approximately approximately300 300feet feetsouth southof of Highway Highway101 101in inFlorence FlorenceCounty County(fig. (fig.3-1). 3-1).The Thelocation location is is about about aa mile milesouth southof of the theNiagara NiagaraFault. Fault. The Thepegmatite pegmatitebodies bodiesare areup upto toaafew fewmeters meterswide wideand andcut cutfelsic felsicvolcanic volcanicrocks rocksof ofthe the Quinnesec QuinnesecFormation. Formation.The Thepegmatites pegmatitesare aresub-parallel sub-parallelto tothe thefoliation foliationininthe thevolcanic volcanic rocks, rocks,strike strikenearly nearlynorth-south, north-south,and anddip dipabout about50 50degrees degreeswest. west. AA number numberof of small small lithium-rich lithium-richpegmatite pegmatitebodies bodiesininthe thearea areacontain containspodumene, spodumene,lepidolite, lepidolite,and andelbaite elbaite tourmaline tourmalineas aswell wellas asquartz, quartz,albite albiteand andmicrocline. microcline.Some Somepegmatites pegmatitescontain containabundant abundant pink tourmaline crystals 0.5 1.0 cm wide and 2.5 5.0 cm long, commonly oriented pink tourmaline crystals 0.5 - 1.0 cm wide and 2.5 - 5.0 cm long, commonly oriented roughly roughlyperpendicular perpendicularto to the theupper uppercontact contactof of the thepegmatite. pegmatite.Some Sometourmalines tourmalinesare arecolor color zoned, zoned, with withaapink pinkcore coreand andblue-green blue-greenrind. rind.The Thepegmatites pegmatitesare arecomposed composeddominantly dominantly of of aplitic apliticquartz-feldspar quartz-feldsparwith withsome somelepidolite. lepidolite.The Thepegmatites pegmatitesrepresent representhighly highlyevolved evolved granitic graniticmelts meltsrelated relatedto tothe thenearby nearbyBush BushLake LakeGranite Graniteassociated associatedwith withthe theDunbar Dunbar gneiss-granitoid gneiss-granitoiddome dome(Sims (Simsand andothers, others,1992). 1992). Supplemental SupplementalStop Stop1-11. 1-11. Metasedimentary MetasedimentarvRocks Rockson on the the Northwest NorthwestSide Sideof ofthe the Dunbar DunbarDome Dome(SW (SW1/4 %, SE SE¼, VA, sec. sec.7,7,T.T.37 37N., N.,R.R.18 18E.) E.) Metasedimentary Metasedimentaryrocks rocksare areexposed exposedintermittently intermittentlyon onthe thewest westside sideof of the theDunbar Dunbardome dome and andadjacent adjacentto toand andnorthwest northwestof ofthe theBush BushLake Lakelobe lobe(Sims (Simsand andSchulz, Schulz,1993). 1993).Although Although included includedby byDutton Dutton(1971) (1971)ininthe theQuinnesec QuinnesecFormation, Formation,subsequent subsequentmapping, mapping, geophysical geophysicaldata, data, and andcore coredrilling drillingby byaaprivate privatecompany companyshow showthat that these these strata strataunderlie underlie the theQuinnesec. Quinnesec.The Theexposed exposedmetasedimentary metasedimentaryrocks rocksare aremetamorphosed metamorphosedatatamphibolite amphibolite grade gradeand andare aremainly mainlyquartz-rich quartz-richschist, schist,impure impuremarble, marble,calc-silicate calc-silicaterocks, rocks,and andbiotite biotite schist. schist.Drilling Drillingand andelectromagnetic electromagneticdata dataindicate indicatethat thataagraphitic graphiticschist schistlies liesalong alongthe the 45 �west west side side of of the the Bush Bush Lake Lake lobe lobe stratigraphically stratigraphically below below the the exposed exposed metasedimentary metasedimentary rocks. rocks. The The large large exposure exposure at at this this stop stop on on the the south south side side of of Macintire MacintireCreek Creek consists consistsof of interbedded interbeddedcaic-silicate calc-silicaterocks rocks and and biotite biotite schist schist that that are are cut cut by by granite granite pegmatite pegmatite and and apilite apilite dikes, dikes, identical identical to to those those exposed exposed within within the the western western part part of the the Dunbar Dunbar dome. dome. At At another SW %, A, sec. VA, SW sec. 11, 11, T. T. 38 38 N., N., R. R.17 17E.), E.), aa another outcrop outcrop area areato tothe thenorth north(NW (NW1/4, succession 100m mthick thick of of marble, marble,calc-silicate calc-silicaterocks, rocks,and andthin thininterbeds interbedsofofbiotite biotite succession at at least least 100 schist schist and and ferruginous ferruginous quartzite quartzite is is exposed. exposed. The The marble marble at at this this location locationhas has structures structures suggestive suggestive of of stromatolites. stromatolites. In In many many respects respects the the metasedimentary metasedimentaryrocks rocks exposed exposed on on the the west west side side of of the the Dunbar Dunbar dome dome resemble resemble those those of of the the Chocalay ChocalayGroup Group of of the the Marquette MarquetteRange Range Supergroup, Supergroup,as as exposed exposed in in the the Menominee Menominee iron iron range range to to the the north north (Bayley (Bayley and and others, others, 1966). 1966). These These metasedimentary metasedimentaryrocks rocks may may compose compose aa tectonic tectonic slice slice of of continental-margin continental-margin rocks rocks that that is is interleaved Dunbar dome dome and and volcanic volcanic rocks rocks of the Quinnesec interleaved with with granitoid granitoid rocks rocks of of the the Dunbar Formation. Formation. Alternatively, Alternatively, they they may may have have been been uplifted uplifted from from beneath beneath the the over-thrust over-thrust Quinnesec Formation during the formation of the Dunbar dome. Quinnesec Formation during the formation of the Dunbar dome. 46 �FIELD FIELD TRIP TRIP 22 MENOMINEE MENOMINEE IRON IRON DISTRICT DISTRICT Gene Gene L. L. LaBerge, LaBerge, University Universityof of Wisconsin-Oshkosh Wisconsin-Oshkosh(retired) (retired)and andUSGS; USGS;John JohnS. S. Kiasner, Klasner, Western WesternIllinois IllinoisUniversity University(retired) (retired)and andUSGS; USGS;William WilliamF. F.Cannon, Cannon, USGS; gas, University USGS; Richard Richard W. W.Ojakan Ojakangas, University of Minnesota Minnesota Duluth Duluth (retired) (retired) The Quinnesec Quinnesec Mine Mine near near Quinnesec, Quinnesec, Michigan Michigan produced produced about 500,000 tons of siliceous siliceous iron iron ore between between 1887 1887 and 1935 1935 from open pits pits and stopes in the Vulcan IronIronformation. The Theworkings workingsseen seenhere hereare areon onthe the overturned overturnednorthern northern limb limb of a syncline. The The locality locality is is also also noted notedfor for the the excellent excellentexposures exposuresof of the thebasal basalCambrian Cambrian unconformity. The The roof roof of of the the working working in in the the upper upper right right is is the base base of the Munising Sandstone, Sandstone, which which lies lies with with an an angular angular unconformity unconformityon onthe theoverturned overturnedVulcan VulcanIronIronPhotographby byElizabeth ElizabethHeinen. Heinen. formation. Photograph �FIELD TRIP TRIP 22 FIELD MENOMINEE MENOMINEE IRON IRONDISTRICT DISTRICT Gene L. LaBerge, University University of Wisconsin-Oshkosh Wisconsin-Oshkosh(retired) (retired)and and USGS; USGS;John John S. S. Gene F. Cannon, Cannon,USGS; USGS; Klasner, Western WesternIllinois IllinoisUniversity University(retired) (retired)and and USGS; USGS;William WilliamF. Kiasner, Richard W. Ojakangas, Ojakangas, University University of of Minnesota MinnesotaDuluth Duluth(retired) (retired) Richard W. 88" 730 87 5700 87 46 30" 11 Fern Creek Fe, 'reek Fm 88 730 22 0 I 55 22 44 I 00 66 88 12 12 10 10 I I 55 I 10 10 15 15 Miles Miles Kilometers Kilometers EXPLANATION EXPLANATION Cambrian Cambrian MunisingSandstone Sandstone Munising North Northof ofNiagara Niagarafault fault Paleoproterozoic Paleoproterozoic . South South of of Niagara Niagara fault fault Paleoproterozoic Paleoproterozoic Metadiabase Metadiabase Hoskins HoskinsLake LakeGranite Granite MichigammeFormation Formation.graywacke Michigamme graywacke Marinette MarinetteQuartz QuartzDionte Diorite Badwater Badwater Greenstone Greenstone Metagabbro Metagabbro Vulcan Vulcan Iron-formation Iron-formation Quinnesec QuinnesecFormation Formation Randville Randville Dolomite Dolomite llh1llJ Sturgeon Sturgeon Quartzite Quartzite Fern Fern Creek Formation Formation - — fault fault Archean Archean Granitic Graniticrocks rocksand andgneiss gneiss Carney CarneyLake LakeGneiss Gneiss Figure 2-1. Geologic Geologic map map of of part part of of the the Menominee MenomineeIron-district Iron-districtshowing showingthe thelocation locationof of Figure field trip trip stops. stops. Geology Geologysimplified simplifiedfrom fromBayley Bayleyand andothers others(1966) (1966)and andSims Simsand andSchulz Schulz field (1993). (1993). 48 �0 5000 I I 1 217 10000 t IUt Kmbeyç 5000 I 1 / 20000 — - M ion 2-4rway I I 30000 I / I 10000 T I 0 I 40000 ciP etQp 'L 15 000 I Stfl —=-- NNii 5J L- r I n ci' ( I I I ,— I 70000 FEET 20 000 METERS 288k _— _- _—''2 g3j I ' c—_? — Figure 2-2. Part of the Escanaba 1:100,000-scale topographic map showing the location of field trip stops 2-1 through 2-5. See figure 3-2 for the location of stop 2-6. 0 —— 1000 700 57 - J" �This This trip trip examines examines rocks rocks of of the the Menominee Menomineeiron-bearing iron-bearingdistrict, district, with with emphasis emphasis on on units units of the the Paleoproterozoic PaleoproterozoicMarquette Marquette Range Range Supergroup. Supergroup.Archean Archean rocks rocks of of the the Carney Carney of Lake Lake Gneiss Gneiss are are seen seen at at two two stops stops and and volcanic volcanic rocks rocks of of the the Wisconsin Wisconsinmagmatic magmatic terranes, intensely intensely deformed deformed in in the the Niagara Niagara fault fault zone, zone, are are also also seen. seen. The The geologic geologic map map terranes, of the the field field trip trip area area isis shown shown in in figure figure 2-1 2-1and and aa road roadmap mapof of the the field fieldtrip tripstops stopsisisinin of figure figure2-2. 2-2. Stop River. (SE 1/4, NE 1/4, Stop2-1. 2-1. Piers PiersGorge Gorgeon onthe theMenominee Menominee River. (SE 114, NE %, sec. sec.24, 24,1. T.39 39N., N.,R. R. 30 30W.) W.) Rocks Rocksexposed exposedalong alongthe the Menominee MenomineeRiver Riverat atPiers PiersGorge Gorgeare arealmost almostcertainly certainlyaabranch branch of of the theNiagara Niagarafault faultzone zoneand andrepresent representone oneof of the thefew fewexposures exposuresof ofthe thefault faultzone. zone.This This location location is is about about one one kilometer kilometer south south of of the the mapped mappedtrace trace of of the the Niagara Niagarafault. fault. The Thehill hill lying lying north north of of the the gorge, gorge, but but still still south south of of the the mapped mappedfault, fault, isisunderlain underlainby bymetagabbro metagabbro that that isismuch muchless lessdeformed deformedthan thanthe therocks rocksininthe thegorge. gorge.These Theserelationships relationshipsindicate indicatethat that strain strain along alongthe the fault fault was was distributed distributedvery very heterogeneously heterogeneouslyand andconcentrated concentratedin indiscrete discrete zones zones of of very very high highstrain strainsurrounding surroundingislands islandsof of weakly weakly deformed deformedrocks. rocks.The Therocks rocksininthe the gorge gorgeare arehighly highlyfoliated foliatedand andlineated lineatedquartz-sericite quartz-sericiteschists schistsand andchloritic chloriticschists, schists, probably probablydeveloped developedfrom from felsic felsic and andmafic maficvolcanic volcanicrocks. rocks.Felsic Felsicand andmafic maficvolcanic volcanicrocks rocks with with only only weak weak foliation, foliation, along along with with mafic maficsills sills with with little little internal internaldeformation, deformation, are are exposed exposed on onboth bothsides sides of of this this strongly stronglyfoliated foliatedzone. zone. Metagraywacke Metagraywackeof of the the Marquette MarquetteRange Range Supergroup Supergroupisisexposed exposedin inNorway, Norway,about about 22miles milesnorth northofofthis thislocality, locality,and andvolcanic volcanicand and plutonic plutonicrocks rocksof of the the Wisconsin Wisconsinmagmatic magmaticterranes terranesare areexposed exposedalong alongthe theMenominee Menominee River Riverininthis thisarea. area. The 80'-85' W W and anddips dips 800850 80'-85' N. N. and and has has aa stretch stretch lineation lineation Thefoliation foliationhere herestrikes strikesNN800850 that plunges 600650, N 85° W. that plunges 60'-65', N 85' W. As As the the recognized recognizedboundary boundarybetween between the the dominantly dominantly sedimentary sedimentary rocks rocks of the the Marquette Marquette Range RangeSupergroup Supergroupto tothe thenorth northand andthe theWisconsin Wisconsinmagmatic magmaticterranes terranesto tothe thesouth, south,the the Niagara Niagarafault faultzone zoneisiscommonly commonlyreferred referredto toas asaasuture. suture.However, However,ititlacks lackssome somefeatures features (such (suchas asaamélange) melange)that thatare aretypical typicalof of suture suturezones. zones.Geophysical Geophysicalevidence evidence(Attoh (Attohand and Klasner, LaBergeand andKlasner, Klasner,2001) 2001) suggests suggeststhat thatthinned thinnedcontinental continentalcrust crust Klasner,1989; 1989;and andLaBerge of of the theSuperior Superiorcraton cratonhas hasbeen beenoverridden overriddenby bythe theWisconsin Wisconsinmagmatic magmaticterranes, terranes,and and extends extendsin inthe the subsurface subsurfacefor for 10-50 10-50miles milessouth southof of the theNiagara Niagarafault faultzone. zone.IfIfthis thisisisthe the case, case, the the Niagara Niagarafault faultzone zonemay maybe bethe the frontal frontalthrust thruston onwhich whichoceanic oceanicrocks rocksof of the the Wisconsin Wisconsinmagmatic magmaticterranes terranesoverrode overrodethe thecontinent continentmargin marginassemblage assemblageof ofthe the Marquette MarquetteRange RangeSupergroup. Supergroup.Continued Continuedcompression compressionof of the thesuture suturezone zoneresulted resultedin inthe the steepening of the thrust surfaces into their present, nearly vertical orientation. steepening of the thrust surfaces into their present, nearly vertical orientation. Stop Stop2: 2: Fern FernCreek Creeklocality locality—-Archean Archean basement. basement.Fern FernCreek Creek Formation, Formation,and and Sturgeon SturqeonQuartzite. Quartzite.(N(N½, Vi,sec. sec.34, 34,T.T.40 40N., N.,R. R.29 29W.). W.). Take TakeCounty CountyRoad Road573 573to tothe thenortheast northeastoff offofofU.S. U.S.2,2,about about11mile milenorthwest northwestofofthe thetown town of ofNorway. Proceedabout about22miles milesand andturn justbeyond beyondthe thebridge overPine Pine Norway.Proceed turnnorth north(left) (left)just bridgeover Creek, milesto toFern FernCreek, Creek,which whichcrosses crosses Creek, onto ontoaasecondary secondaryroad. road.Proceed Proceedabout about1½ 1Vz miles the about 1% thesecondary secondaryroad roadatataasharp sharpbend bendininthe theroad. road.Proceed Proceed about 34 mile milefarther farthertotoaasmall small intermittent intermittentcreek creekthat thatalso alsocrosses crossesthe theroad. road.Park Parknear nearhere. here.See Seefigure figure2-3 2-3for foraa detailed 2-4for foraageneralized generalizedrock rockcolumn. column. detailedmap, map,and andFigure Figure2-4 50 �r OuTCffOP M A P Figure2-3. Location Location map map of of stop stop 2-2. 2-2. (from (fromPettijohn, Pettijohn,1943.) 1943.) Figure2-3. FERN FERN CREEK FM FM M (FERN LOCALITY) (FERN CREEK LOCALITY) 150 100 50 -°.o: • DIAMICTITE DIAMICTITE ARGILLITE ARGILLITE DROPSTONES DROPSTONES 0 CONGLOMERATE CONGLOMERATE Figure2-4. 2-4. Generalized Generalizedstratigraphic stratigraphiccolumn columnat at Fern FernCreek Creeklocality. locality.SQ SQat atthe thetop topof ofthe the Figure column column designates designates the the Sturgeon SturgeonQuartzite. Quartzite. 51 �situated in a NW-trending NW-trendingvalley about 175 175 m m wide, between between two Note that the road is situated prominent topographic highs. The Carney Lake Lake Gneiss Gneiss forms the prominent prominent bluff bluff and and prominent Sturgeon Quartzite Quartzite forms aa prominent prominent upland on the northeast side of the road, and the Sturgeon Fern Creek ridge on the southwest side of the road. The valley is situated on the Fern Formation, Formation, of which the the lowest lowest 60 60 to to 70 70 m mare are somewhat somewhatexposed. exposed. Archean-Paleoproterozoic contact, a major unconformity unconformity representing representing a few The Archean-Paleoproterozoic hundred million years of erosion, is now subvertical, as are the Fern Creek Formation Formation substop is is at this this unconformity, unconformity, and and then we will and the Sturgeon Quartzite. Our first substop move up-section up-section through the subunits of the Fern Fern Creek Formation Formation and end in the Sturgeon Sturgeon Quartzite. Quartzite. Substop subs to^ 1. From the road, move about 100 100 m up the small creek bed bed to the subvertical unconformity, unconformity, where we can can observe observe both both the the Carney CarneyLake LakeGneiss Gneissand andthe theoverlying overlying basal gneiss-fragment (i.e., arkosic) conglomerate conglomerate of the Fern Fern Creek Creek Formation. Formation. Note Note basal the angular to subangular nature nature of the clasts, obviously locally derived. Minor beds of red arkosic sandstone are also present. The entire entire sequence has stratigraphic tops toward the south. Interpretation: High-velocity High-velocityfluvial fluvialand andin-situ in-siturubble/debris rubbleldebrisflow flow deposits. deposits. Substop 2. Reverse Reverse direction and head back toward the road, moving up the stratigraphic section. This poorly exposed subunit, 60-75 feet thick, is a laminated laminated finescattered larger stones. Many of these stones show clear evidence grained argillite with scattered fine-grained sediment from above, causing a bowing of having been "dropped" into the fine-grained penetration of the underlying laminae. Others do not show these downward and/or a penetration relationships relationships and are therefore therefore called called "lonestones" "lonestones"rather rather than than "dropstones" "dropstones"(fig. (fig.2-6). 2-6). Note the E-W E-W slatey slatey cleavage cleavage that that crosses crossesthe thebedding, bedding,which whichstrikes strikesabout aboutNN500 50' W. W. S that the stone has Figure 2-5. Thin-bedded argillite and siltstone with dropstone. Note that both both pierced pierced and and bowed bowed down down the the underlying underlying laminae. laminae. 52 �Figure Figure 2-6. 2-6. Large Large (35 (35 cm) cm) lonestone lonestone (beneath (beneath hammer) hammer) in in vertical vertical laminated laminated siltstone/sandstone siltstone/sandstone beds. beds. Some Some smaller smaller lonestones lonestones are are also also present. present. Interpretation: Deposition Depositionof of fine-grained fine-grainedsediment sedimentin in aa body body of of water water (marine?) (marine?) near near a melting melting glacier, glacier, with with the the larger larger clasts clasts dropped dropped in in from from icebergs icebergs and/or and/or aa floating floating ice ice shelf. shelf. Substop Substop 3. The next next subunit subunit is is a diamictite, diamictite, aa matrix-supported matrix-supported conglomerate conglomerate with with rather rather sparse clasts clasts set in in a massive massive graywacke graywacke matrix matrix (fig. (fig. 2-7). 2-7). Note Note the the total total lack lack of of bedding bedding sparse and and the presence presence of aa crude crude schistosity schistosity that causes causes the the rock rock to to break break into into thick thick slabs. slabs. Interpretation: (i.e.tillite) tillite)or orby bythe the"raining "rainingout" out" Interpretation: Deposition Depositiondirectly directlyby byglacial glacialice ice(i.e. of detritus detritus from from icebergs icebergsor or aa floating floating glacier glacier onto onto aa basin basin floor floor lacking lackingcurrents currentsto to generate (i.e., "rainout "rainouttill"). till"). generate lamination lamination (i.e., Substop Cross the road, road, following following the marked marked trail trail about about 100 100m m across across aa low-lying low-lying Substop 4. Cross area area without outcrops, base of the the ridge ridge that that lies southwest of the road. The first outcrops, to the the base lies southwest of the road. The first low-lying low-lying exposure at the base base of the ridge ridge is is poorly poorly exposed exposed and and is is composed composed of argillite/sericite m thick. thick. The The low-lying low-lyingarea areamay maybe be argillite/sericiteschist schist and and sericitic sericiticquartzite, quartzite, about about 55 m totally totally or in part underlain by by this rock, which is softer softer than than the rocksof substops or in part underlain this sericitic sericitic rock, which is the rocks of substops 1-3 1-3 and and the overlying overlying vitreous quartzite. The The prominent prominent ridge ridge is is composed composed of the the Sturgeon Sturgeon Quartzite Quartzite (fig. (fig. 2-8). 2-8). ItIt is is well well cemented cemented with with silica silica and and is is totally totally recrystallized, recrystallized, making making itit aa very resistant resistant rock rock unit. unit. ItIt is is composed composed almost almost totally totally of of well-sorted, well-sorted, finefine- to to 53 �medium-grained quartz sand; few grains are coarser than 1 mm. Original Original grain boundaries are easily seen boundaries seen only only in in the the sericitic sericitic quartzite. quartzite. Diamictite with scattered scattered stones stones and and crude crude schistosity. schistosity. Figure 2-7. Diamictite Interpretation: The The sericite sericite schist schist is is a paleosol and the sericitic quartzite is aa reworked paleosol that that was developed upon the the Fern Creek Creek Formation during during aa long period of extensive weathering in a subtropical or tropical climate. The overlying period Sturgeon Quartzite is is the product product of the reworking reworking and and sorting sorting of the the weathered weathered detritus detritus Sturgeon by wind and water. The quartzite here is about 325 m thick, and elsewhere in the region region it has a maximum thickness of 600 m (Freedman and others, 1961). It obviously represents the transported resistant quartz sand fraction of a very broad, deeply chemically weathered surface that was largely developed upon granitic rock over a long period period of time. There is is no no diagnostic diagnostic evidence evidence within the quartzite itself of a terrestrial terrestrial (fluvial) versus a marine environment environment of deposition. deposition. However, However, an an apparently apparently conformable conformable relationship with the overlying thick Randville Randville Dolomite Dolomite (300-430 (300-430m), m), which which is is stromatolitic and contains detrital detrital quartz grains and thin interbeds interbeds of quartzite, is strongly marine environment environment for the Sturgeon Sturgeon Quartzite. In In addition, suggestive of a shallow marine high in the formation suggests suggests a gradational gradational contact contact with the diopside-rich quartzite high and others, others, 1961). Detrital quartz grains in the the sericitic Randville Dolomite (Freedman and schist show a marked 1966), indicative of marked elongation and alignment (Bayley and others, 1966), shearing shearing along along this this softer softer zone zone beneath beneaththe the quartzite. quartzite. 54 �Figure Figure 2-8. 2-8. Ripples Ripplesin inSturgeon SturgeonQuartzite Quartzite Stop Stop2-3: 2-3: Sturgeon SturqeonRiver RiverDam Damlocality. locality.Archean Archeanbasement, basement,Fern FernCreek CreekFormation, Formation, and andSturgeon SturgeonQuartzite. Quartzite.(E(E½,95sec. ,sec.8,8,T.T.3939N., N.,R.R.29 29W.). W.). From Stop Stop 2, 2, backtrack backtrack about about 1½ 1V2 miles milesto to County CountyRoad Road573. 573. Turn Turn left left (east) (east) and and From proceed proceedabout about3½ 3%miles. miles.Turn Turnleft left(east) (east)on onSwede SwedeSettlement SettlementRoad, Road,which whichleads leadstoward toward the mile.Turn Turnleft left(north) (north)on onaasmaller smallerroad road thepower powerstation/dam stationldamand andproceed proceedfor for about about1½ 1V2 mile. that V2 mile mileto tolocked lockedgate. gate.Park Parkhere. here.IfIf thatleads leadsto to the thepower powerstation/dam. stationldam. Proceed Proceedabout about½ approaching approachingthis stopfrom fromthe thevillage villageof Loretto,take takeCounty CountyRoad Road573 573north outof thisstop of Loretto, northout of Loretto V2 mile. mile.Then Thenturn turnright right(east) (east)on onSwede SwedeSettlement SettlementRoad, Road,which whichleads leads Lorettofor forabout about½ miles.Turn Turnleft left(north) (north)on onaa towardthe the power powerstation/dam, stationldam,and andproceed proceedfor for about about1½ 1V2 miles. toward smaller smallerroad roadthat that leads leadsto to the the power powerstation/dam. stationldam. Proceed Proceedabout about½ V2 mile mileto to locked lockedgate. gate. Park Parkhere. here.As As of of this thiswriting writinginin2003, 2003, the thedam damisisslated slatedfor forremoval removalininthe therelatively relativelynear near future. future. Here Herethe theSturgeon SturgeonRiver Riverhas hascut cutaadeep deepgorge gorgethrough throughthe theSturgeon SturgeonQuartzite; Quartzite;the the formation formationwas wasnamed namedfor forthis thislocality. locality.This Thissmall smallarea areahas hasbeen beenwell wellstudied, studied,especially especially because Archean-Paleoproterozoiccontact contactat at the thedam. dam.The Thearea area becauseof of the thepresence presenceof of the theArchean-Paleoproterozoic has hasbeen beendescribed describedby byCredner Credner(1869), (1869),Brooks Brooks(1873), (1873),Rominger Rominger(1881), (188l), Irving Irving(1890), (1890), Bayley Bayley(1904), (1904),Lamey Lamey(1937), (1937),Pettijohn Pettijohn(1943), (1943),and andTrow Trow(1948). (1948). SubstoD subs to^ 1. 1. Walk Walk past past the the gate gate to to the the end end of of the the road roadat at the the powerhouse powerhouseand and dam. dam. We We will willtraverse traverseback theroad roadto vehicles,thus thusobserving observingthe therock unitsinin backup upthe tothe thevehicles, rockunits stratigraphic stratigraphicsequence. sequence.The Thedam damwas wasconstructed constructedon onSturgeon SturgeonRiver RiverFalls, Falls,which whichwas was held heldup upby byaa thick thick mafic maficdike dikethat thatcan canbe beseen seenininthe thewoods woodsoff offthe theeast eastend endof ofthe thedam. dam. The ArcheanCarney CarneyLake LakeGneiss Gneissand andthe thePaleoproterozoic Paleoproterozoic Theunconformity unconformitybetween betweenthe theArchean Fern FernCreek CreekFormation Formationcan canbe beseen seenininaasmall smallground-level ground-levelexposure exposureadjacent adjacenttotothe thedam dam 55 �(fig. 2-9). The lowest lowest bed bed in in the the Fern Fern Creek Creek is is aa diamictite diamictite at at this this spot, spot, whereas whereas aa short short distance to the west on the river bottom bottom by by the power power station, station, the the lowest lowest unit unit is is arkosic arkosic sandstone with rare oversized oversized stones. Figure Figure 2-9. Unconformity Unconformity at Sturgeon Sturgeon River River dam. dam. Hammer Hammer head head rests rests on on Archean Archean Carney Lake Lake Gneiss Gneiss and and hammer hammer handle handle is is on on basal basal diamictite diamictite of the the Fern Fern Creek Creek Formation. Formation. Nearby Nearby in river bottom, bottom, the the basal basal unit unit is is arkosic arkosic sandstone sandstone with with rare rare dropstones, illustrated 1. dropstones, illustratedininfigure figure2-1 2-11. FERN FERN CREEK FM FM (STURGEON LOCALITY) (STURGEONRIVER RIVER LOCALITY) M - 75 SANDSTONE ARGILLITE 50 ARKOSE • 25 / 25 M 20 CONGLOMERA GcK' SANDSTONE SANDSTONE SANDSTONE SANDSTONE DROPSTONES L 2Fm5nw DIAMITITE DIAMICTITE DIAMICTITE DIAMICTITE DIAMICTITE DIAMICTITE SANDSTONE SANDSTONE ARGILLITE ARGILLITE 10 CONGLOMERATE GRAYWACKE ARGILLITE DIAMICTITE DIAMICTITE SANDSTONE SANDSTONE DROPSTONES DROPSTONES ONFSTONFS DIAMICTITE DIAMICTITE SANDSTONE SANDSTONE DROPSTONES DROPSTONES 2-10. Figure 2-1 0. Stratigraphic column at Sturgeon Sturgeon River River locality. SQ at the the top of the the column column designates the Sturgeon designates Sturgeon Quartzite. Quartzite. 56 �2-10 Figure 2-1 0 is a measured measured column column of of the the Fern FernCreek Creek Formation. Formation.The Thelower lower25 25m mare arewell well exposed when there is no water in the channel. Note that this portion of the formation consists of five beds of diamictite (matrix-supported (matrix-supportedconglomerate) as thick as 2.5 m, arkosic sandstone sandstone beds beds as as thick as as 2.6 m m with rare rare oversized oversized stones, stones, stacked stacked three arkosic sandstone beds beds with minor intercalated intercalated siltstone and argillite argillite laminae, laminae, an argillite argillite arkosic sandstone 15 cmconglomerate. conglomerate.Because Becausethe thewater waterlevel levelisiscommonly commonlyhigh, high, bed 4.5 cm thick, and a 15cm figures 2-11 2-11 and and 2-12 2-12 are are included included here here to illustrate illustrate some important important features. figures 2-11. stratigraphic column. Note Figure 2-1 1. Granitic dropstone in lowest sandstone of the stratigraphic that the stone has pierced and bowed down the underlying laminations. stone has pierced and bowed down the underlying laminations. the river bottom is not found found on the west Interestingly, the well-exposed section seen in the bank mof of conglomeratic conglomeratic rock is present present there. 1%m there. Apparently Apparently the the more more bank of the the river; river;only only11/2 complete section is preserved preserved in a topographic low on the Archean surface. However, faulting may may be a factor factor as as well, for weathered weathered pyrite pyrite is present along a fault between the Archean basement basement and the Fern Fern Creek west of the powerhouse. The middle middle 25 m m of the Fern Fern Creek Creek Formation Formation is is relatively relatively poorly poorly exposed; figure 2-10 2-10 shows this portion portion consisting of conglomerate, conglomerate, graywacke graywacke sandstone sandstone with oversized stones, stones, and and arkosic arkosic sandstone sandstonewith with oversized oversizedstones. stones. 57 �beds of stratigraphic stratigraphic column column of Figure Figure 2-10. View Figure 2-12. Second, third, and fourth beds is to west. Beds 2 and 4 are diamictites below and above arkosic sandstone with rare dropstones. dropstones. Interpretation: This This isisaa glaciogenic glaciogenic sequence. sequence. The The diamictites may be thin tills suggested by deposited beneath glacial ice, but more likely are debris flow deposits as suggested one diamictite bed that grades upward upward into sandstone. Some of the conglomeratic beds beds are difficult to clearly classify classify as either matrix-supported matrix-supported or clast-supported. One 20 cm bed at the 15 15 m level level in the section is graded from medium medium sand to clay, suggestive of a turbidity current mechanism. Several of the oversized stones in the sandstone and greywacke beds show either a bowing down of the underlying laminae or an actual penetration, indicating indicating that the stones were dropped into the basin from above and are indeed dropstones. Other lonestones lonestones may be dropstones, too, but clear evidence is lacking. The likely likely mechanism mechanism for deposition deposition of dropstones dropstones is release from melting melting icebergs icebergs or from from aa floating floatingglacier. glacier. Substop 2: The 50 and and 75 75 m m on Figure Figure 2-9 is intermittently intermittently The25 25m msection section between between 50 exposed on the west bank of the river, but this area is usually inaccessible because of high water. It includes beds of sericitic sericitic quartzite interbedded interbedded with sericite schist. The sericitic nature of this interval is illustrated by sericitic nature of this interval is illustrated by aa small small road-level road-leveloutcrop outcropbetween betweenthe the the river just just north of the quartzite ridge. This is a sericitic quartz pebble road and the conglomerate with sericite clay chips, some reddish rather than yellow-green in color. Interpretation: This Thissericitic sericitic portion portion of of the the column column is is interpreted interpreted as a reworked reworked paleosol that formed on the Fern Fern Creek Formation Formation during a warm climatic period period that followed glaciation. Trow (1948) first suggested suggested that this might might be be a paleosol. paleosol. Substop 3: Sturgeon SturgeonQuartzite Quartzite ridge. ridge. Note Note that that the the bedding bedding is is slightly overturned towards the south, and that cross-bedding cross-bedding indicates that stratigraphic tops are to the 58 �Cross-bedding is is of both both trough trough and and planar planar types. types. According According to to Trow Trow (1948), (1948), the the south. Cross-bedding general cross-bedding cross-bedding indicates indicates aa paleocurrent paleocurrent trend trend from from the the northwest northwest toward toward the the general southeast. southeast. Interpretation: Abundant Abundantasymmetrical asymmetricalripple ripplemarks markshave havelow lowripple rippleindices indices Interpretation: lengthlripple height) height) indicative indicative of deposition deposition by water rather rather than by wind. The (wave length/ripple beds are generally generally of even thickness, indicative indicative of aa shallow shallow marine marine rather rather than than aa fluvial fluvial beds environment of of deposition. deposition. See See text text for for substop substop 44 of of stop stop 2-2 2-2for for additional additionalinterpretation interpretation environment of the the genesis genesis of of the the Sturgeon SturgeonQuartzite. Quartzite.The TheSturgeon Sturgeonhas haslong longbeen beencorrelated correlatedwith with of the the Mesnard Mesnard Quartzite Quartzite of of the the Marquette MarquetteTrough. Trough. Stop 2-4. Brier Brier Slate Member of the Vulcan Iron-formation inNorway, Norway,MI. MI. (Modified (Modified Stop Iron-formation in fromDutton, Dutton,1958). 1958).(E(E1/2, Vz, SE SE ¼, Vn, sec. sec. 5, 5, T. T. 39 39N., N., R. R. 18 18W.) W.) from Three iron iron mines, mines, the the Aragon, Aragon, Cyclops, Cyclops, and and Norway Norway mines mines were were developed developed in in the the Three northern part part of of the the city city of of Norway, Norway, MI, MI, and and subsidence subsidence of of these these abandoned abandonedmines mines has has northern limited development development in in this this part part of of the the city. city. The The Aragon Aragon mine mine was was the the third third largest largest limited producer of of iron iron ore ore in in the the Menominee Menominee district district (Dutton, (Dutton, 1958), 1958), and and the the head headframe frame of of the the producer mine still still stands stands about about 55 blocks blockseast east of of this this stop. stop. Figure Figure2-13 2-13 shows shows the the location locationof of mine several interesting interestinggeological geological features, features, but but the the trip trip will will visit visit only only the the Brier Brier Slate Slate Member Member several type type locality. locality. According to to Dutton Dutton (1958), (1958), outcrops outcrops along along the the ridge ridgenorth northof of Norway Norwayare are Randville Randville According Dolomite with with aa rather rather extensive, extensive, but but incomplete, incomplete, cover cover of breccia breccia composed composed of of dolomite dolomite Dolomite fragments that that are are slightly slightly to to thoroughly thoroughly silicified. silicified. (This (This breccia breccia is is similar to that at the top fragments of the the correlative correlative Bad Bad River River Dolomite Dolomite in in the the Gogebic Gogebic district, district, some some 120 120miles miles westwestof northwest of here.) here.) The Thesilicification silicificationisisbelieved believedto to be bethe the result result of of surficial surficial weathering weathering in in northwest pre-iron-formationtime. time. The The dolomite dolomite dips dips about about 60 60 degrees degrees southward, southward, post-Randville -- pre-iron-formation post-Randville and the the convexity convexity of stromatolites stromatolites in the dolomite face in the dolomite indicate indicate that that the the beds beds also also face and southward. Small Small outliers outliers of of Cambrian Cambrian sandstone sandstone overlying overlying the the Randville Randville Dolomite Dolomite are are southward. also (e.g., at at Rochon RochonSt. St. and and Curry Curry Lane Lane along along the the ridge). ridge). also present present (e.g., The TheRandville Randvilleisisoverlain overlain by by the the Felch FelchFormation, Formation,the the basal basalunit unit of of the the Menominee Menominee Group. Group. One One of of the the few few exposures exposures of of the the Felch FelchFormation Formationis is at at this this locality locality in in Norway Norway (Dutton, 1958). 1958). The The Felch FelchFormation Formationconsists consists of of sericitic sericitic slate slate and and thin thin layers layers of of (Dutton, quartzite. quartzite. The The uppermost uppermostpart part of of the the formation formation isisaa ferruginous ferruginous quartzite quartzitethat that isisaamarker marker bed bed throughout throughout the the district. district. The The quartzite quartziteis is aa transitional transitional unit unit between betweenthe the dominantly dominantly clastic, clastic, non-ferruginous non-ferruginousstrata strataof of the theFelch FelchFormation Formationand andthe theiron-rich iron-richchemical chemical sediments of the Vulcan Iron-formation (Dutton, 1958). sediments of the Vulcan Iron-formation (Dutton, 1958). Three Threemembers membersof of the theVulcan VulcanIron-formation Iron-formationwere wereexposed exposedininNorway Norwayin in1958, 1958, accordingto to Dutton Dutton(1958). (1958).Exposures Exposuresof of the thebasal, basal,thin-bedded thin-beddedoxide oxidefacies faciesTraders Traders according Member are are present present at at the the northeastern northeastern end end of of aa southwesterly southwesterly trending trending area area of of Member outcrops.The TheTraders Tradersisispresently presentlyexposed exposednorth northof of Sixteenth SixteenthAve. Ave. and andMain MainSt., St., in inthe the outcrops. oldCyclops Cyclopsmine mineworkings, workings, which which isisbeing beingused usedas as aa landfill landfillby by the the city city of of Norway. Norway. The The old Brier Brier Slate Slate Member Member (this (this stop) stop) is is exposed exposed on on an an outcrop outcrop knob knob at at Eleventh Eleventh Ave. and and the the Aragonlocation, location,southwest southwestof of the theTraders TradersMember Memberlocation. location.In Infact, fact, the theBrier BrierSlate Slatewas was Aragon named namedfor for exposures exposureshere. here. Several Several small-scale small-scale folds folds that that plunge plunge about about 45 45 degrees degrees easterlyare areexposed exposedalong alongthe the old oldrailroad railroadcut cut on onthe the north northside sideof of the theoutcrop. outcrop. easterly According theoxide oxidefacies, facies,oolitic ooliticand andgranular granulariron-formation iron-formationof ofthe the Accordingto to Dutton Dutton(1958), (1958),the 59 �Curry member member of of the theVulcan Vulcan Iron-formation Iron-formationwas wasexposed exposedat atthe thesouthwest southwestend endof ofthe the Curry outcroparea. area. outcrop The Michigamme MichigammeFormation Formationunconformably unconformablyoverlies overlies the the iron-formation iron-formationand andisisexposed exposedin in The several low low roadcuts roadcutsalong alongU.S. U.S. Hwy-8 Hwy-8on on the the southern southernoutskirts outskirtsof of Norway. Norway. several 00 I 1I2 112 I I 1 1 1 i Miles Miles Figure 2-13. 2-13. Part Partofofthe theNorway Norway7½' 7 %quadrangle quadrangleshowing showingthe thelocation locationof of stop stop2-4 2-4and and Figure some other features of geologic interest. some other features of geologic interest. Stop 2-5. 2-5. Quinnesec Quinnesec mine, mine, just just northwest northwestof of Quinnesec, Quinnesec,Ml, MI, (Modified (Modifiedfrom fromDutton, Dutton, Stop T. 39 39 N., N., R. R. 30 30W.) W.) 1958)(SW (SW1/4 Y4, SE SE 1/4 34, sec. 34, T. 1958) sec. 34, (NOTE: For this property. property. For For (NOTE; For safety safety reasons reasons aa security security fence fence surrounds surrounds this MI, Ph (906-774-4471). permission to enter contact Joe Massie, Quinnesec, permission to enter contact Joe Massie, Quinnesec, MI, Ph (906-774-4471). The abandoned abandonedworkings workingsof of the theQuinnesec Quinnesecmine mineare aremainly mainlyininthe theTraders Tradersiron-bearing iron-bearing The member of the Vulcan Iron-formation. The mine lies on the overturned north, limbofof aa member of the Vulcan Iron-formation. The mine lies on the overturned north, limb 60 second order syncline (fig. 2-1 4). The Precambrian strata at the mine dip about second order syncline (fig. 2-14). The Precambrian strata at the mine dip about 60 degreeshorth, north,but butface facesouthward, southward,inasmuch inasmuchas asthe theBrier Brierslate slatemember memberofofthe theVulcan Vulcanisis degrees 60 �along the south side side of the the excavated excavated approach approach to to the the mine, mine, and and the the Felch FelchFormation Formationisis along the north north wall of the the workings. Note the Cambrian sandstone north side side of the the sandstone overlying the mine workings along the north hill, providing an unusual view of an unconformity unconformity (fig. 2-15). 2-15). The basal basal portion portion of the the sandstone contains contains numerous numerous angular angular slabs slabs of of oxidized oxidized iron-formation, iron-formation,iron iron ore, ore, and and sandstone slate in a sandy matrix. matrix. Clearly, this area area was a small island island as the Cambrian Cambrian sea sea advanced over the area. The clasts clasts of iron iron ore ore in in the the basal basal conglomerate conalomeratealso also indicate indicate the area. that the ore here here was formed formed before before the the Cambrian Cambrian sea sea covered coveredthe area. R. 30W. R. 30 W. I I 1000 feet 0 <") 1 1 I xmj Xm Michigamme MichigammeFormation Formation Vulcan Iron-formation Iron-formation [&c Curry Curry Member Member Iron-bearing Iron-bearing Member Member Xvb Brier Slate Member Xvt Traders Iron-bearing Iron-bearing Member :1;X 1 1 xfXf 1 1IXrI Xr 1 Open pit ..>.Exposed Exposedbedrock bedrock J<^_) Small Small area of exposed exposed bedrock bedrock $j Strikeanddipof Strike and dip ofbeds beds X 50 Strike Strikeand and dip dip of overturned overturned beds beds Beds dip 70°-80° 70'-80' except Felch Formation Formation --*- Randville Dolomite Randville Dolomite as noted 2-14. Figure 2-1 4. Map Map of of the the Quinnesec Quinnesec mine mine and and vicinity vicinity at stop stop 2-5 showing that the workings were developed developed in the overturned overturned northern northern limb limb of aa small small syncline. syncline. Map Map from from Bayley Bayley (1957). (1957). 61 �5. Abandoned workings at Quinnesec mine. Ore bodies were mined from the Figure 2-1 2-15. Iron-bearingmember member of Vulcan Iron-formation. Iron-formation.View View looking looking west west shows shows beds beds Traders Iron-bearing but facing facing south. Roof Roof of workings workings is is base base of of the the Munising Munising Sandstone Sandstone of of dipping north, but of the the basal Cambrian unconformity. Cambrian age and provides an excellent view of Photo Photo by Elizabeth Elizabeth Heinen. Heinen. Stop 2-6. Marqaret St. on Lake Antoine in Iron Mountain. Mountain, 2-6. Randville Randville Dolomite Dolomite alonq along Margaret in Iron (Modified from 1958) (SE 1/4, MI. (Modified fromDutton, Dutton, 1958) (SE %,NE NE1/4, %, sec. 29, 29, T. T. 39 39 N., N., RR30 30 W.) W.) - This exposure exposure of the Randville Randville Dolomite Dolomite on the south shore of Lake Lake Antoine Antoine was formerly the site of a small small quarry quarry for the the production production of of road road material. material. Operation Operation of of the the quarry was halted during the late 1930's through the influence of geologists and other interested people who wanted the the site preserved for for future geologic examination interested (Dutton, 1958). 1958). The glacially scoured scoured outcrop shows a variety of sedimentary sedimentary and structural structural features. The rocks rocks are are dipping dipping nearly nearly vertically, and face southward. southward. Sedimentary Sedimentary features features preserved here include abundant stromatolites (fig. 2-15), typically 3-4 inches high and preserved 2-15), 4-8 inches inches in diameter. The old old quarry quarry face provided provided a vertical vertical view of the stromatolites now largely obscured by graffiti. Thin layers of quartz sand are interbedded interbedded with the dolomite, and and ripple ripple marks marks and and mud mud cracks cracks are are present present in in places. places. These These features features suggest a very shallow water environment of deposition, and several authors (e.g. Larue, 1981) have suggested that the Randville Dolomite may have been deposited in a paleo paleo sabkha sabkha environment. environment. - 62 �Structural Structural features features exposed exposed here here include include deformed deformed stromatolites. stromatolites. Several Several layers layers of stromatolites that are are strongly strongly skewed skewed in in a right-lateral right-lateraldirection direction are are present. present. The The dolomite dolomite evidently evidently recrystallized recrystallized readily, readily, allowing allowing ductile ductile deformation deformationof of the the stromatolites. stromatolites. Possible fracture is nearly nearly "axial planar" to the deformed deformed Possible fracture cleavage is oriented so that it is stromatolites, and and is is roughly roughly parallel parallel with the regional regional structure. Figure interbedded Randville Dolomite at stop 2-6. Laminated Laminated dolomitic layers layers are interbedded Figure 2-16. 2-16. Randville with with stromatolitic stromatolitic dolomite. dolomite. Stromatolite Stromatolite mounds mounds are are deformed deformed and and show show an an asymmetry asymmetry indicating right lateral lateral sense of shear. View is looking looking down at a horizontal surface and indicating a right beds bedsdip dip vertically. vertically. 63 �FIELD TRIP 3 STRATIGRAPHY AND STRUCTURE OF THE IRON RIVERCRYSTAL FALLS BASIN BASIN William F. Cannon, USGS, USGS, Reston, VA; VA; John John S. S, Kiasner, Klasner, Western WesternIllinois Illinois (retired) and USGS; University (retired) USGS; Gene Gene L. LaBerge, LaBerge, University Universityof of WisconsinWisconsinOshkosh Oshkosh (retired) (retired) and and USGS USGS Tightly folded Riverton Iron-formation Michigan. Tight Iron-formation near Stager Lake, Michigan. complex folds such as these are typical of the structural style of the Iron RiverCrystal Falls Crystal Falls allochthon. allochthon. �FIELD TRIP 3 FIELD BASIN STRATIGRAPHY AND STRUCTURE OF THE IRON RIVER-CRYSTAL FALLS FALLS BASIN William F. Cannon, USGS, Reston, VA; John John S. Klasner, Kiasner, Western Illinois University (retired) and USGS; Gene L. LaBerge, University University of Wisconsin-Oshkosh Wisconsin-Oshkosh (retired) (retired) and (retired) USGS USGS The Iron Iron River-Crystal River-Crystal Falls iron district was mined extensively for high-grade high-grade "soft" iron 1882 until the early 1970's 1970's producing producing more more than than 200 200 million million tons of ore. ores from 1882 Because of of the high economic interest in the region, the availability of many underground mine workings, workings, and extensive diamond drilling, drilling, a detailed detailed stratigraphy stratigraphy and and deciphered in in what what otherwise otherwise would would have havebeen beenaa largely largelyunknown unknownterrane, terrane, structure was deciphered mostly concealed by the nearly continuous cover of glacial deposits. The most detailed region was conducted by a large group of USGS USGS and affiliated affiliated geologists geologists study of the region 1943 and continuing 10 years. Surface exposures exposures were mapped mapped beginning in 1943 continuing for about 10 in detail, as were most underground underground mine workings. Ground magnetic surveys helped delineate the surface surface trace trace of of certain certain units unitsand and the the extensive extensivecollection collectionof of exploration explorationdrill drill core was examined. One of the earliest aeromagnetic surveys was conducted here II.The The work of the the USGS USGS when the technique was still classified shortly after World War II. was aided immeasurably immeasurably by the cooperation of the numerous mining companies active The results results of that that painstaking painstakingwork work were weresummarized summarizedininUSGS USGSProfessional Professional in the area. The Paper 570 (James and others, 1968), which remains the the only comprehensive comprehensive account account of of the geology geology of the region. Much Much of the the descriptive descriptive material material in in this this guide guide is is taken taken from from that that work. Additional detailed studies of the Florence area, Wisconsin were done by Dutton 1971) and also were instrumental (Dutton, 1971) instrumental in determining the geological relationships Iron River-Crystal River-Crystal Falls basin, including areas visited in along the southern extent of the Iron the first three stops of this this trip (figs. (figs. 3-1 3-1 and and 3-2). 3-2). The Iron Iron River-Crystal River-Crystal Falls basin is a triangular structure, with an area of about 300 square miles, underlain underlain by strata of the Paint Paint River Group. It is surrounded, except on part of the eastern side, by volcanic rocks of the Badwater Greenstone. Greenstone. Our current current interpretation of the area is that the Paint Paint River Group, as originally defined, and the Badwater northward during the Badwater Greenstone, are an allochthon, and were thrust northward driven by arc arc collision collision south south of of the the Niagara fault. fault. This trip trip traverses traverses Penokean orogeny, driven from the Niagara fault, as seen at Pine River Flowage in northern Wisconsin, northward Niagara Pine River Flowage northward across the complexly deformed deformed fault panels of the Niagara Niagara suture zone, and onto the structurally simpler rocks rocks north of the Iron Iron River-Crystal River-Crystal Falls Falls allochthon. Most stops examine the lithology and structure of rocks of the Paint Paint River River Group. Principal Principal observations are the preponderance preponderance of steeply plunging folds in all panels of the suture particularly in the Iron Riverzone rocks, the extraordinarily complex fold patterns, particularly Crystal Crystal Falls Falls allochthon, and the contrast with the simpler, gently plunging folds north of the suture suture zone. zone. 65 �EXPLANATION EXPLANATION North North of o f Niagara Niagara fault Tobin Tobin Lake Lake Granite Granite Metagabbro Metagabbro El Paint undivided Paint River Group - undivided Fortune FortuneLake LakeSlate Slate Riverton Riverton Iron-formation Iron-formation Dunn DunnCreek CreekSlate Slate X . " El El Badwater Badwater Greenstone Greenstone Michigamme MichigammeFormation. Formation, graywacke and graywacke and volcanic volcanicrocks rocks Michigamme MichigammeFormation Formation --quartzite quartzite Michigamme MichigammeFormation Formation -- graywacke graywacke Iron-formation Amasa Iron-formation Hemlock Volcanics undivided undivided HemlockVolcanics Randville Dolomite Randville Dolomite Saunders Formation Formation Saunders Dickinson undivided Dickinson Group undivided South o off Niagara fault fault Bush Lakegranite Granite and tonalite Quinnesec Formation Formation Qulnnesec El Metasedimentary rocks Metasedimentary rocks — faults - faults 2 I field field trip tripstops stops NE-Niagara fault fault NF-Niagara SRF-South South Range Range fault fault SRF- 0 I 5 5 • I I 2 4 I I 8 6 I I I I 0 0 Miles 5 5 10 10 Kilometers NRF-North Range fault NRF-North Range fault BF-Badwater fault fault BF-Badwater Riverfault fault PRF-Paint River CS-Commonwealth syncline MA-Mastodon anticline anticline MA-Mastodon Bowers syncline syncline TBS-Tim Bowers Figure 3-1. Geologic map of the eastern eastern part part of of the the Iron Iron River-Crystal River-CrystalFalls Falls basin basin showing the location location of stops stops for for field field trip trip 3. 3. 66 �Figure Figure3-2. 3-2. Part Partof of the theIron IronMountain Mountain1:100,000-scale 1:100,000-scaletopographic topographicmap mapshowing showingthe the location locationof of field fieldtrip tripstops stops3-1 3-1through through3-4 3-4and and1-9, 1-9,1-10. 1-10. Stop3-1. 3-1. Michiqamme MichiciammeFormation FormationatatPine PineRiver RiverFlowage Flowaue(NE (NE1/4, 114,SW SW1/4, 114,sec. sec.28, 28,T. T. Stop 39 39N. N.R. R.18 18E.) E.) This Thisoutcrop outcroplies lieswithin withinthe thePine PineRiver Riverstructural structuralblock blockas asdefined definedby byDutton Dutton(1971). (1971). Large Largeexposures exposuresare arealong alongthe thegorge gorgeof ofthe thePine PineRiver Riverjust justdownstream downstreamfrom fromthe thePine Pine River River dam. dam. The The Pine PineRiver Riverblock blockisiscomposed composedalmost almostentirely entirelyof ofthe theMichigamme Michigamme Formation Formationconsisting consistingmostly mostlyof ofgraywacke graywackeand andlesser lesserquartzite quartziteand andconglomerate conglomerate(stop (stop 3-2). 3-2).Although Althoughpenetrative penetrativedeformation deformationisisintense intenseand andthere thereare aremany manysmall-scale small-scalefolds folds well wellexposed exposedat atthis thisstop, stop,the theoverall overallstructure structureofofthe theblock blockseems seemstotobe beaauniformly uniformly south-facing south-facingsuccession successionas asindicated indicatedby bycross crossbeds bedsand andgraded gradedbeds. beds. The Theoutcrops outcropsseen seenhere hereare arevery veryclose closetotothe theNiagara Niagarafault faultwhose whoselocation locationhere hereisiswell well constrained constrainedby byrather ratherabundant abundantoutcrops outcrops(see (seefig. fig.3-3). 3-3).The Therocks rocksexposed exposedbelow belowthe the dam damare areno nomore morethan than500 500feet feetnortheast northeastof of the thevolcanic volcanic rocks rocksof of the theQuinnesec Quinnesec Formation, Formation,part partof of the theWisconsin WisconsinMagmatic MagmaticTerranes. Terranes.The TheMichigamme MichigammeFormation Formationininthis this vicinity vicinitywas waswell welldescribed describedby byDutton Dutton(1971) (1971)and andthe thefollowing followingisisextracted extractedfrom fromhis his report. report."The "Therock rockisisgray grayand andwell-bedded well-beddedininlayers layersone-fourth one-fourthtotoone-half one-halfinch inchthick. thick.The The percentage percentageofofminerals mineralsininthe therocks rocksisisapproximated approximatedas asquartz quartzfrom from20 20toto50; 50;sericite sericiteand and muscovite to 40. 40. Dark Dark muscovitefrom from20 20to to70; 70;biotite biotitefrom from10 10to to25; 25; and andchlorite, chlorite,ifif present, present,from from55 to red redgarnets garnetsare areabundant abundantininthe theschist schistnear nearthe thequartzite; quartzite;they theylocally locallyare areconcentrated concentratedinin layers layersand andlenses, lenses,but butthey theymay maybe beminor minorororabsent. absent.Some Somegarnets garnetshave havebeen beenrotated rotated 67 �formation as as much much as as 25 25 degrees, degrees,as as shown shownby bythe the angular angulardiscordance discordancebetween between after formation general foliation foliation and and the the layers layers of of very very fine fine opaque opaque grains grainswithin within the the garnets. garnets. Deflected Deflected general foliation at at the the boundary boundarysurfaces surfacesof of the thegarnets garnetsalso alsoindicates indicatesdirection directionbut butnot notamount amount foliation of of rotation." rotation." 'so O EXPLANATION 1T S . n Michigamnie Slate qu.rte slate. Locally includes,nleor conglomerate (cQi) 055. agglomerate and tremoiite orhist nsl qC QUuttottlc cungluma,ate. Locally In. g:un,ritic (at) or magnetuLic cludea (m..t Iron-formation a. oes.tnblage of this units. Locally includes graphitic slat. (gal), guotte. mice slate (Cl). quarts grsywaclte (gw), grunerittc achiot (Qru). and amphi bout. (am) If 1000 FEET A L 0 Outcrop Outcrop or orgroup group of of small smalloutcrops outcrops Metaf,lsic cod q5arcz. Uetafelalczocks.toetazhyoiite? rocks-metarhyolll~? andquartzmuscovite m u s c o v i f achier; schist: locally locsltyincludes tncludos in. intertoiiated fsch) t e r t o f l à § tactual ~schlat(schi Strikeand and dip dipof ofbode beds Strike Direction uf of top tv@ detennined detçwaineby bygraded çrçd Direction bedding bedding z Contact L.ong daubed wfl eta .pprox4mately 1*. hott dashed where Inferred; fl rated: dotted where concealed; queried Øzare Strike and dip of beds beddlu Dlrectinh of top detenulned by cross doubtful 0 Iii UJ 0 Probable Probablefault fault aDotted i?otted wh,re whewcOncealed conceiled II.. U. upthcown tiptftrown U. Q. slde;D. d d à §0 downthrawnalde;querlatd downtbtowniildfquer~~d wftwe where doubtful do~bttui d5 Quinnesec Portnation ...... T s - Vertical Inclined Inclined Vertical Strikeand anddip dipof ofbeds beds Strike Strikeand anddip dipof of foliation foliation Strike *t_ Strike and anddip di ofofbed bedand andplunge plunge Strike o&eatiou of lineation SB Abandonedshaft shaft Abandoned x Test Testpit pit Figure3-3. 3-3. Geologic Geologicmap mapof of part partof of the theFlorence Florencearea, area,Wisconsin Wisconsinshowing showingthe thelocation locationof of Figure 3-2.Map Maporiginally originallypublished publishedby byDutton Dutton(1971, (1971,figure figure3). 3). fieldtrip tripstops stops3-1 3-1and and3-2. field 68 �A STOP 3-1 flU dots- stretched concretions diamonds- mineral mi squares- fold axes Figure 3-4. 3-4. Lower Lower hemisphere hemisphereequal equalarea areastereoplots stereoplotsshowing showingorientation orientationof of fold foldaxes axes Figure and andrelated relatedstructures structuresat atvarious variousstops stopsfor forfield fieldtrip trip3.3. 69 �The rocks rocks near near the dam dam are are aa strongly strongly foliated foliated and and lineated lineated chlorite-garnet chlorite-garnetschist. Primary Primary layering is well preserved preserved but but has has been been transposed transposed parallel parallelto to S1 8, foliation. Dextral drag folds in the transposed layering have steeply plunging axes (see fig. fig. 3-4,A) that are parallel parallel to mineral mineral lineations lineations on foliation foliation surfaces. Axes of maximum maximum elongation in deformed clasts and andconcretions concretionsplunge plunge60 60 degrees degrees toward toward the south, parallel parallel to the in mineral mineral lineation. lineation. These These highly highly strained strainedrocks rockshave havean anS1 S1foliation foliation that that dips dips steeply steeply south south as do the elongation elongation (stretching) (stretching) axes. These These steep, steep, south south plunging plunging structures structures seem seem to be be characteristic characteristic of the the Niagara Niagara fault fault zone. Similarly Similarly oriented, steeply south plunging plunging structures structures occur south south of the the Niagara Niagara Fault Fault zone, probably probably in in splays of the Niagara Niagara fault in in volcanic rocks rocks of the Wisconsin Wisconsin magmatic magmatic terrane terrane that that crop crop out out aa few few miles miles south south of of Pine River River Dam (Sims (Sims and others, 1985). 1985). The structural structural fabric in in this region region reflects reflects the Pine overthrusting--with overthrusting-with a right right -lateral -lateral component--of component--of the Wisconsin Wisconsin magmatic magmatic terrane terrane from from the south onto the continental margin margin and and later steepening steepening of the thrusts thrusts to their their present present orientation. orientation. Stop3-2. Conglomerate of the Michigamme Stops-2. Quartzite and Conqlomerate Michiqamme Formation Formationnear near Pine Pine River sec. 28, T. 39 (NE1/4, %, NW 1/4 %, sec. 39 N., N., R. I?.18 18E.) E.) RiverDam. Dam.(NE The quartzite conglomerate conglomerate exposed exposed here (fig. (fig. 3-3) 3-3) is is the most most prominent prominent and and bestbestexposed andothers (1935)considered considered exposed unit in the Pine River Block (Dutton, 1971).Leith Leithand others(1935) (Dutton,1971). it to be be a separate separate formation (the (the "Breakwater "Breakwater Quartzite"). Quartzite1').However, However, according to Dutton Dutton (1971), (1971), it appears appears to be be a lens lens within the Michigamme Formation Formation and he did not give it a separate separate stratigraphic stratigraphic name. name. Figure Figure 3-5. 3-5. Conglomerate ConglomerateininMichigamme MichigammeFormation Formationatatstop stop3-2 3-2showing showingstrongly strongly stretched figure 5. 5. stretched and and aligned aligned pebbles. pebbles. Photograph Photograph from from Dutton Dutton (1971), (1971), figure 70 70 �The quartzite conglomerate conglomerate is about 700 feet thick and extends northwestward northwestward about 3 near the center of sec. 28, T. 39 39 N., N., R. R. 18 18 E., E., to to the the NE NE corner, corner, sec. sec. 24, 24, T. T. 39 39 miles from near R. 17 17 E., E., (Dutton, (Dutton, 1971). 1971). Cross-bedding Cross-bedding indicates indicates that statigraphic statigraphic tops tops are are toward toward N., A. the southwest. Layers and lenses of quartzite with flat (or flattened and stretched) pebbles and cobbles (fig. 3-5) of recrystallized recrystallized chert and iron-formation iron-formation are the dominant lithology. (Note: This locality is is only only about about one one half half mile mile north northof ofthe theNiagara Niagarafault.) fault.) The matrix within the pebbly units is composed of quartz, fine-grained fine-grained hematite or magnetite, 965), who showed that or both. The unit was studied studied in detail by Nilsen (1 (1965), that it consists of of two conglomeratic subunits separated by a quartzite and pebbly quartzite subunit. His paleocurrent analysis indicates a predominant current flow toward the southeast in a shallow, near-shore near-shore basin. basin. Structurally, there appears appears to to be be aa cleavage-parting cleavage-partingparallel parallelwith with bedding. bedding.Bedding Bedding 65' SW. Stretched Stretched N55OW, parallel parallel to the strike of the Niagara Niagara fault, and and dips 65° strikes N55°W, pebbles pebbles plunge plunge steeply steeply southwest. southwest. Iron-formation and Stop 3-3. Riverton Iron-formation and Wauseca Wauseca Pyritic Pvritic Member Member of Dunn Dunn Creek Slate (SW %, SW SW¼, %, sec. sec.34, 34,T. T. 40 40N., N.,R. R. 18 18 E.) E.) Slate (SW¼, - named by Dutton (1971) and This stop is a'ong along the axis of the Commonwealth Commonwealth syncline named is within Dutton's Dutton's Brule Brule River River block, block, now now included includedas as the the southeastern southeasternextension extensionof of the the River-Crystal Falls allochthon. Roadcuts Iron River-Crystal Roadcuts on Highway Highway N N about about 2 miles miles southeast southeast of of Florence, Wisconsin were made made after Dutton's Dutton's mapping mapping of the area, but but reveal reveal geology geology very much as inferred inferred on his maps. The roadcuts consist of alternating units of black, pyritic slate of the Wauseca Pyritic Member, the uppermost uppermost member of the Dunn Creek iron-formation of the overlying Riverton IronSlate, and cherty carbonate and silicate iron-formation formation. Individual Individual lithologic units of slate and iron-formation iron-formation are generally a few tens of feet thick and and the contact between between them is well exposed in many places along the roadcut. roadcut. The interleaving interleaving of the the two two units units is is probably probably aa result result of of repetition repetitionby by tight tight folding, folding, contacts cannot be traced around fold hinges within the limits of the roadcut. although contacts The lithologies lithologies could could be be stratigraphically stratigraphically interlayered, interlayered, but but such such broad-scale broad-scaleinter-bedding inter-bedding is not known elsewhere in the district where a few feet, feet, at most, of transitional beds occur between workings. We think it more between the two units where exposed in many mine workings. likely that folds with amplitudes greater than the height of the roadcut roadcut cause the repetitions with a geometry like those displayed in parts of figure 7 (p. 19) at the Buck mine in which fold amplitudes are many times greater then fold wave-lengths. Smallscale tight folds showing this type of geometry are common within individual lithologic units in this roadcut. The Wauseca consists consists of multiply multiply deformed, black, ferriginous, and and highly pyritic slate with thin - up to a few inches thick - beds of chert. The Wauseca Wauseca was classified as a sulfide fades iron-formation classified sulfide facies iron-formationby (1954) and and was his principal byJames James (1954) was his principal example for defining defining this facies. Foliation is generally parallel to the bedding and axial Foliation planar folds in N55OW,85°NE, 85ONE, and and is is parallel parallelto to the the trend trend in bedding. bedding. ItIt is is roughly roughly oriented oriented N55°W, of the Brule Brule River River block. block. The The rock rock is is folded foldedisodlinally isoclinally with with axes axes that that plunge plungevariably variablybut but most plunge steeply northwest (see stereoplot on fig. 3-4,B). These small folds are most likely likely parasitic to the Commonwealth Commonwealthsyncline. syncline. The Wauseca Wauseca Pyritic Pyritic Member Member has has some anomalous anomalous chemical characteristics. Ongoing studies by the USGS of the geochemistry geochemistry of black slates in the region found that a roadcut contained contained 1230 1230 parts parts per million composite of 30 feet of black black slate in in this roadcut arsenic and 14 14 parts per million selenium, both values being the highest that we have 71 �detected in northern northern Wisconsin Wisconsin and the upper peninsula detected peninsula of Michigan. Michigan. Studies are continuing Wauseca might might contribute contribute to aa regional regional continuing to determine to what degree the Wauseca arsenic anomaly icial materials. surficial materials. arsenic anomaly in insurf 3-4. Riverton Riverton Iron-formation Iron-formation along Stop 3-4. alonq eastern limb limb of of Iron Iron River-Crystal River-CrystalFalls Falls basin (NW basin (NW¼, ?A,sec. sec.31, 31,T.T.42 42N., N., R. R. 32 32 W.). W.). Typical Riverton Riverton Iron-formation Iron-formationis is exposed exposed in in cuts cuts along along two sub-parallel sub-parallel abandoned abandoned railroad single-track road. The exposures railroad spurs, the northern of which is drivable as a single-track are approximately in the middle of the Riverton, which in this area strikes NNE and dips of the area are steeply to the west into the Iron Iron River-Crystal Falls basin. Detailed maps of Professional Paper 570 by James and others (1 (1968) 968) and detailed included in USGS Professional lithologic descriptions of the Riverton are in the same publication. In general, the Riverton, where unaffected unaffectedby by secondary secondary oxidation, oxidation, which which is is widespread widespreadin inthe the district, district,isis thin-bedded and consists mostly of interbedded chert and siderite. Iron silicate minerals, thin-bedded and consists mostly interbedded and Iron silicate minerals, mostly stilpnomelane, are only locally important. Thin partings of argillaceous and carbonaceous carbonaceous material material are common common and and some pyritic pyritic layers layers are also widespread. Iron-formation at stop 3-4. Figure 3-6. Tight folds in the Riverton Iron-formation The rock seen here here is is generally only weakly oxidized so preserves many of the original sedimentary minerals and structures. Like Like all of the Iron Iron River-Crystal River-Crystal Falls allochthon, 72 I..,— �___ metamorphic metamorphicgrade gradeisisextremely extremelylow lowand andno nometamorphic metamorphiceffects effectsare aredetectable detectableininhand hand specimens. specimens.Small-scale Small-scalefolds foldsare arevery verywell welldeveloped. developed.Most Mostplunge plungegently gentlyto tomoderately moderately toward towardthe thenorth northor orsouth south(fig. (fig.3-4,C). 3-4,C). -N ) ' V — 7 Xr (J'-',p / - - - () v4 / - - La M — u3': 1UO1S - - 1650 iom 5000 reek ., ' I \ 4 -— - - 632W 4Q 1DJO METERS 5000 0 I 0 10000 4O FEET 20000 Figure Figure3-7. 3-7.Part Partofofthe theIron IronRiver River1:100,000-scale 1:100,000-scaletopographic topographicmap mapshowing showingthe thelocation location ofoffield trip stops 3-5 through 3-9. field trip stops 3-5 through 3-9. 73 �Stop 3-5. 3-5. Dunn DunnCreek CreekSlate Slatenear nearAlpha, Alpha,MI MI(NW (NW SE1/4, %, sec. sec. 7, 7,T. T. 42 42 N., N., R. R. 32 32 W). Stop ¼,%,SE The Dunn Creek Slate is the lowermost lowermost unit of the Paint River Group as defined by James (1958). (1958). It lies, probably conformably, on the Badwater Greenstone Greenstone and has a gradational upper contact with the Riverton Iron-formation. Iron-formation. It is a unit of greatly varied Iron River-Crystal River-Crystal Falls basin, and is lithology and thickness considering the entire Iron defined more as a stratigraphic interval interval than by a distinctive lithology. James and others (1968) described the variations in lithology and thickness. The area near the village of (1968) Alpha contains the best exposures and probably the greatest stratigraphic thickness of Dunn Creek. The The detailed detailed mapping mapping of the the area area by by the the USGS USGS as as part part of the the Iron Iron the Dunn 968) also produced a series River-Crystal Falls study presented presented in James and others (1 (1968) of more detailed reports published by the Geological Survey of Michigan. The report on area (Pettijohn (Pettijohn and and others, 1969) 1969) subdivided subdivided the Dunn Dunn Creek into into three the Alpha area mappable units based on a unit of distinctive laminated slate that forms the middle part of the formation and separates upper and lower units of gray to black, cherty, in part part sideritic, slate. The mapping of these units units was very useful in tracing the northward northward extension of the Mastodon Mastodon anticline, but for reasons not known to us these internal internal units on maps in in Professional Professional Paper Paper 570. 570. According to Pettijohn and others were not shown on (1969) (1969) the exposures seen at this stop are in the lower unit of the Dunn Creek Slate and lie about 1500 feet west of the trace of the axial plane of the Mastodon Mastodon anticline. This MI (fig. outcrop is located on the north side of Highway N about one mile east of Alpha, Ml 3-7). The outcrop consists of black ferruginous slate with more massive, massive, openly folded, cherty layers. There is a slatey foliation parallel to bedding in places. Elsewhere foliation N40°W is axial planar to the open open folds folds in in the the massive massivelayers layers and and is is generally generallyoriented orientedN40°W, 75ONE. 75°NE. A stereoplot of of fold fold axes from the broader region in this area (fig.3-4,D) shows that the folds plunge plunge steeply steeply to gently gently north-northwest. north-northwest. Fold Fold axes axes reported reported by Pettijohn Pettijohn plunged from 40-80 degrees northwest. and others (1969) (1969) also plunged northwest. It is clear from the relationships mapped in this area that even regional folds like the Mastodon anticline have steep plunges plunges within the allochthon. allochthon. Iron-formation at Stop 3-6. Riverton Iron-formation at the the Paint Paint River River Dam Dam in in Crystal Crystal Falls. Falls. Mt. MI. (Center, sec. (Center, sec. 20, 20, T. T. 43 43 N., N., R. R. 32 W.) 500-800 feet thick in the Crystal Falls area and is mostly The Riverton Iron-formation Iron-formation is 500-800 inter-laminated siderite Exposures inter-laminatedchert and siderite; Exposuresjust just below belowPaint PaintRiver RiverDam Damprovide providethe the of the the Riverton and of of the best example in the district both of the primary Iithology lithology of characteristic of the Iron extraordinary structural complexity of the deformation characteristic Iron RiverCrystal Falls allochthon. A sketch map of the outcrop, published by James and others (1968), (1968), is reproduced reproduced here to provide a view of the entire outcrop, some of which is flooded periodically, depending on the rate of flow of the Paint River. The exposure chert-siderite lithology, which comprises the bulk of of the Riverton IronIronshows the typical chert-siderite formation throughout the district. district. There are are also good good examples examples of a silicate silicate ironironformation, a lithologic type unique to the Crystal Falls area. These were described by "......at at the the apron apron of of the the Paint PaintRiver River dam dam in in Crystal Crystal James and others (1968) (1968) as follows: ".. Falls the chert-siderite chert-siderite iron-formation iron-formation contains layers that consist dominantly of stilpnomelane. These layers, which are from a fraction of an inch inch to several inches thick, distinguished in the outcrop from the chert and siderite by their inferior are readily distinguished inferior hardness hardness and by by their their peculiar peculiar closely closely spaced spaced cross cross fracture fracture that that resembles resemblesthat that of ofsome some coal." Some Somebeds bedsare arealso alsorelatively relativelyrich richin inpyrite. pyrite. 74 �Inclined Vertical Overturned Strike and dip of bedding . ......... ... .......... .... . Bedding plane Trace of selected bedding plane Traced o r r o ~ nm Daubed w h e w u.vproxiwzlely locnted -.... . t k oulcw~ Fault,showing relative movement Dotted where concealed 1010 I L 00 , , I l 1010 I I 20 20 I I 30 30FOCI FEET I SurveyedbybyH.HL. L. Jamesand and Srveyod James W W 5, S Why,, White 948 1948 I Figure3-8. 3-8.Sketch Sketchmap mapofofRiverton RivertonIron-formation Iron-formationatatPaint PaintRiver RiverDam, Dam,Crystal CrystalFalls, Falls, Figure Michigan. Michigan.(James (Jamesand andothers, others,1968, 1968,fig fig21) 21) showsthat thatmost mostfolds foldsplunge plungesteeply steeplyto tothe thewest west(note (notethat thatnorth northisistoward toward Figure3-4, 3-4,EEshows Figure 55degrees degreestotovertical. vertical.But, But, theupper upperright rightininfigure figure3-8). 3-8).Plunges Plungesare aresteep, steep,varying varyingfrom from55 the onthe themost mostnortheasterly northeasterlypart partofofthe theoutcrop outcropfolds foldsplunge plungetoward towardthe thenorth northatatabout aboutright right on angles anglesto tothe theother otherfolds. folds.These Theseabrupt abruptvariations variationsininfold foldgeometry geometryare arecharacteristic characteristicofof the thedistrict districtas asrevealed revealedininthe themany manyunderground undergroundmines minesthat thatwere weremapped mappedduring duringthe the periodof ofmining miningininthe theearly earlytotomid-i mid-1900's. additiontotothe thefold foldaxes axesthat thatwe wehave have period 900's. InInaddition Larue(1987), (1987),who who measured,this thisexposure exposurewas wasstudied studiedinindetail detailby byUeng Uengand andLarue measured, recognized44 phases phasesof of deformation deformationand andprovide provideaadetailed detaileddiscussion discussionof ofthe thestructural structural recognized history.To Togeneralize generalizetheir theirconclusions, conclusions,the thefolds foldsthat thatplunge plungesteeply steeplytotothe thewest westformed formed history. ininan second aninitial initialmajor majorfolding foldingphase phaseand andthose thosethat thatplunge plungesteeply steeplytotothe thenorth northare areaasecond phase. phase. 75 �Stop 3-7. Badwater Greenstone north northof ofCrystal CrystalFalls Falls(sec. (sec.18, 18,T.T.43 43N., N.,RR32 32 W.) W.) Badwater Greenstone Alloutcrops outcropsininthis thisarea areaare areon onprivate privateland landand andshould shouldnot notbe bevisited visitedwithout without NOTE: All prior permission permission of property property owners. owners. The Badwater Greenstone is a thick succession of submarine mafic volcanic rocks that underlies the Dunn Creek Slate, probably conformably, although evidence is scant. As the Paint River Group because discussed above, we propose to include the Badwater in the structural evidence indicates that it is part of the Iron Thus structural Iron River-Crystal Falls allochthon. Thus it has definable stratigraphic stratigraphic relationships relationships to to the the overlying overlying Paint Paint River River strata stratabut but isisin in fault fault contact with all other surrounding units. James and others (1968) (1968) described rocks of the Badwater as .somewhat varied in detail, but in in general general they they are are massive massivefine-grained fine-grained Badwater as ". ...somewhat dark-greenish-gray rocks that consist chiefly of chlorite, actinolite, hornblende, dark-greenish-gray rocks consist chiefly of chlorite, actinolite, hornblende, albite, albite, clinozoisite-epidote, and carbonate. carbonate. Ellipsoidal Ellipsoidal and and agglomeratic agglomeratic structures structuresare are common. common. fragmental volcanics . The rocks rocksalmost almostcertainly certainly originated originatedas as sub-marine sub-marine flows and fragmental entirely, of of primary primary basaltic basaltic composition." composition." The formation and are dominantly, ifif not entirely, carbonate-rich slate and thin ironcontains minor sedimentary rocks consisting of carbonate-rich formation. James James estimated estimated that the the Badwater Badwater is is as as much much as as 15,000 15,000 feet thick thick in in places, places, but its thickness varies varies greatly greatly within within the the district. district. I'. . . At this stop the Badwater Greenstone is a strongly deformed pillow basalt and pillow stretched volcanic clasts and breccia. The basalt is weakly to strongly foliated, and has stretched N80°W85°NE. 85ONE. Stretched Stretched clasts clasts and and pillows pillows pillows. Foliation is generally oriented oriented N80°W, plunge steeply northwest as shown in figure 3-4, F. These steeply plunging structures, similar to those inherently part of the Niagara suture zone rocks from here southward to the Niagara Niagara fault, are are the the principal principal evidence evidence that the Badwater Greenstone Greenstone is is part part of the Iron Iron River-Crystal River-Crystal Falls Falls allochthon allochthon and and is is structurally structurally detached detachedfrom from more moresimply simply deformed rocks to the north that will be seen at stop 3-8. Stop 3-8. 3-8. Michiciamme Michiqamme Formation Formation north northof of Crystal CrystalFalls, Falls,MI MI(SW (SW¼, %, NE NE ¼, %, sec. 12, T.43N., T. 43 N., R.33W.) R. 33 W.) The Michigamme Formation, a thick sequence of clastic rocks, dominantly graded and bedded graywackes and thinner-bedded siltstones, underlies a very large area north and east of the Iron River-Crystal Falls allochthon. Broad, open folds with steeply dipping, Iron River-Crystal Falls more or less less east-west east-west trending, trending, axial axial planes planes and and shallowly-plunging shallowly-plungingfold foldaxes axes scattered outcrops of varicolored slate are characterize deformation. At this locality scattered located on the north side of the Paint River, about six-tenths six-tenths of a mile west from the Highway-141 this characteristic fold fold geometry. Prominent slatey Highway-141 bridge and display well this cleavage cleavage is oriented oriented N75°W, N75¡W80°NE, 80°NEand and intersections intersections of of bedding beddingand andcleavage cleavageindicate indicate that fold axes plunge 15 15 degrees west. Outcrop-scale Outcrop-scale folds with this orientation are also present here. This nearly horizontal, westerly plunge of the fold axes is characteristic characteristic of fold axes found in the Michigamme Formation Formation along the north and east sides of the Iron Iron River-Crystal Falls district, and contrasts contrasts sharply with the steeply plunging orientation of Greenstone and overlying Paint fold axes and stretch stretch lineations lineations found in the Badwater Greenstone the rocks here and those at River Group strata. This contrast in structural style between the all other stops seen to this point indicate that there is a fundamental structural structural boundary boundary between the Badwater Greenstone and other rocks of the allochthon and the Michigamme Michigamme Formation. The outcrops visible south of the River are Badwater Greenstone, Greenstone, indicating indicating that the boundary boundary lies immediately immediately south of these Michigamme Michigamme 76 �probably beneath beneath the river. We have have proposed proposed the name name Paint Paint River River fault for outcrops, probably structure and interpret interpret itit to be be the basal basal detachment of the allochthon. this structure Stop 3-9. Little Tobin Lake Lake Granite Granite (NE (NE 1/4, %, see. Stop Little Tobin sec.21, 21,T. T. 42 42 N., R. 32 32 W.) A dike-like dike-like body of granite, called called the little Tobin Lake Lake dike dike by James and others others (1968), intrudes the Badwater intrudes Badwater Greenstone Greenstone and forms a body about 2 miles long long and a quarter quarter of a mile wide (fig. 3-1). The The rocks rocksare are gray gray to to reddish reddish gray gray and and finefine- to to medium-grained medium-grained and are composed dominantly of microcline microcline microperthite, microperthite, albite, and mica. In In this area the Badwater Badwater is intensely intensely folded on the Mastodon anticline and the adjacent Tim Bowers syncline, syncline, both both of which which are are traversed traversed by by the the dike, dike, which which maintains maintainsaa nearly nearlystraight straighttrace. trace. emplaced after the major major folding of the Paint Paint River River Group. Group. The dike clearly seems to be emplaced It yielded a U-Pb zircon date of 1833+/-6 Ma (Schneider and others, 2002), which places emplacement of the Iron Iron River-Crystal River-CrystalFalls Falls allochthon. a younger limit on the age of emplacement bodies of granite granite lie between here and Crystal Falls, about 5 miles Several other smaller bodies to the north. north. These, too, are apparently apparently post-tectonic post-tectonic and and occur both both within the allochthon and in the structurally lower Michigamme Formation. James and others (1968) (1 968) states "The granitic granitic bodies bodies all all have have been been sheared sheared to to some some extent, extent, and andall allhave have been metamorphosed". Perhaps these granites record a younger, circa 1650 Ma been record circa 1650 deformation as has been been proposed proposed for this area by Romano Romano and others (2000). (2000). deformation 77 �REFERENCES REFERENCES CITED CITED K., and and Klasner, Klasner, J.S., J.S., 1989, 1989,Tectonic Tectonicimplications implicationsof of metamorphism metamorphismand andgravity gravity Attoh, K., field in in the Penokean Penokean orogen of northern northern Michigan: Tectonics, Tectonics,v. v. 8, 8, p. p. 911-933. 91 1-933. 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Pettijohn, F.J., Gair, Gair, J.E., Weir, Weir, K.L., K.L., and andPrinz, Prinz,W.C., W.C., 1969, 1969,Geology Geologyand andmagnetic magneticdata data Pettijohn, F.J., for for Aipha-Brule Alpha-BruleRiver River and and Panola PanolaPlains Plains areas, areas, Michigan: Michigan: Geological GeologicalSurvey Survey Division, Division, Michigan Michigan Department Department of Natural Natural Resources Resources Report Report of of Investigation Investigation10, 10, 12 12p. p. Prinz, W.C., 1958, 1958, Geology Geology of of the the southern southern part partof of the the Menominee Menomineedistrict, district,Michigan Michiganand and Wisconsin: Wisconsin: U.S. U.S. Geological GeologicalSurvey SurveyOpen-File Open-FileReport Report476, 476, 221 221 p. p. Puffett, Reany Creek Creek Formation, Formation, Marquette Marquette County, Michigan: U.S. U.S. Puffett, W.P., 1969, 1969, The The Reany Geological Geological Survey Survey Bulletin Bulletin 1274-F, 1274-F,p. p. F1-F25. F1-F25. Romano, Holm, D.K., and and Foland, Poland, K.A., K.A., 2000, Determining Determining the the extent extent and and nature nature of Romano, D., HoIm, Mazatzal-related overprinting overprinting of the Penokean Penokean orogenic orogenic belt belt in in the the southern southern Lake Lake Mazatzal-related Superior Superior region, region, north-central north-centralUSA: USA:Precambrian PrecambrianResearch, Research,v. v. 104, 104,p. p.25-46. 25-46. Rominger, C., C., 1881, 1881, Menominee MenomineeIron Iron Region: Region: Michigan Michigan Geological Geological Survey, Survey, v. IV, IV, pp. pp. 190-1 190-192. 92. Roscoe, S.M., and and Card, K.D., K.D., 1993, 1993, The The reappearance reappearanceof of the the Huronian Huronianin in Wyoming: Wyoming: rifting rifting and and drifting drifting of of the the ancient ancient continents: continents:Canadian Canadian Journal Journalof of Earth EarthSciences, Sciences, v. v. 30, 30, p. 2475-2480. p. 2475-2480. Schneider, Schneider, D.A., D.A., Bickford, Bickford,M.E., M.E., Cannon, Cannon,W.F., W.F., Schulz, Schulz,K.J., K.J., and andHamilton, Hamilton,M.A., M.A., 2002, 2002, Age Age of of volcanic volcanic rocks rocksand andsyndepositional syndepositionaliron ironformations, formations,Marquette MarquetteRange Range 82 �Supergroup: implications implications for for tectonic tectonic setting setting of of Paleoproterozoic Paleoproterozoic iron ironformations formationsof of the the Supergroup: Lake Superior region: Canadian Canadian Journal of Earth Earth Sciences, Sciences, v. 39, p. p. 999-1012. 999-1012. Schulz, K.J., 1987, 1987, An Early Early Proterozoic Proterozoicophiolite ophiolite in in the the Penokean Penokean orogen orogen [Abstract]: [Abstract]: Schulz, Geological Society Society of Canada—Mineralogical Canada-Mineralogical Association Association of Canada, Canada, Program Programwith with Geological Abstracts, v. v. 12, p. p.87. 87. Scutter, C.R., Cas, Cas, R.A.F., R.A.F., Moore, Moore, C.L., C.L., and and de de Rita, Rita, D., D., 1998, 1998,Facies Faciesarchitecture architectureand and Scutter, submarine rhyolite rhyolite flow-dome flow-dome complex, complex, Ponza, Ponza, Italy: Italy: Journal Journal of of Geophysical Geophysical origin of submarine Research, v. 103, 103, p. p. 27,551 27,551-27,566. Sedlock, K.L., and Larue, D.K., 1985, 1985, Fold Fold axes axes oblique oblique to the the regional regional plunge plunge and and Proterozoic terrane accretion in the southern southern Lake Lake Superior Superior region: region: Precambrian Precambrian Proterozoic Research, v. 30, p. p. 249-262. 249-262. Shervais, J.W., 2001, Birth, Birth, death, death, and and resurrection: resurrection:the the life life cycle cycle of of suprasubduction suprasubduction zone ophiolites: Geochemistry Geophysics Geosystems, vol.2, Paper ophiolites: Geochemistry Geophysics Geosystems, Paper number number 2000GC000080 http://g2000GC000080 [20,925 [20,925 words, 88 figures, figures, 33 tables]. tables]. On-line On-linepublication publicationat at http:llgcu bed .o rg cubed.org. Shervais, J.W. and and Kimbrough, Kimbrough, D.L., D.L., 1985, 1985, Geochemical Geochemicalevidence evidence for for the the tectonic tectonic setting of the Coast Range Range ophiolite: ophiolite: aa composite composite island island arc-oceanic arc-oceanic crust crust terrane terrane in in western v. 13, 13, p. p. 35-38. 35-38. western California: California: Geology, Geology, v. Sims, P.K., 1976, 1976, Middle Precambrian Precambrian age age of volcanogenic massive massive sulfide sulfide deposits deposits in in northern Wisconsin [Abstract]: [Abstract]: Proceedings Proceedingsof of the the 22nd 22nd Annual Annual Institute Instituteon on Lake Lake Superior Geology, Superior Geology, p. p. 57. 57. Sims, P.K., compiler, compiler, 1990, 1990, Geological Geological map map of of Precambrian Precambrianrocks rocks of of the the Iron IronMountain Mountain X 2' 20quadrangles, quadrangles, northeastern northeastern Wisconsin and northwestern and Escanaba Escanaba 10 1' X northwestern Michigan: Michigan: U.S. Geological Geological Survey Survey Miscellaneous MiscellaneousInvestigations InvestigationsSeries SeriesMap Map1-2056, 1-2056, scale scale 1:250,000. 1:250,000. Sims, P.K., compiler, 1992, 1992, Geologic Geologic map map of Precambrian Precambrian rocks, rocks, southern southern Lake Lake Superior region, Wisconsin Wisconsin and and northern northern Michigan: Michigan: U.S. US. Geological GeologicalSurvey Survey Miscellaneous Miscellaneous Investigations Series 85, scale scale 1 1:500,000. Investigations SeriesMap Map1-21 1-2185, :500,000. Sims, P.K., Peterman, Peterman, Z.E. and and Schulz, Schulz, K.J., K.J., 1984, 1984,Dunbar Dunbar gneiss-granitoid gneiss-granitoiddome: dome: Field Field Trip Guidebook, Guidebook, 30th 30th Annual Institute Instituteon on Lake Lake Superior Superior Geology, Geology, p. p. 1-23. 1-23. Sims, P.K., Peterman, Peterman, Z.E., and and Schulz, Schulz, K.J., K.J., 1985, 1985, The The Dunbar Dunbar Gneiss-granitoid Gneiss-granitoiddomedomeimplications implications for Early Early Proterozoic Proterozoic tectonic tectonic evolution evolution of northern northern Wisconsin: Wisconsin: Geological Geological Society of America Bulletin, Bulletin, v. 96, 96, p. p. 1101-1112. 1101-1112. Sims, P.K., Van Schmus, W.R., W .R., Schulz, Schulz, K.J., K.J., and and Peterman, Peterman,Z.E., Z.E., 1989, 1989,TectonoTectonostratigraphic stratigraphic evolution evolution of the the Early Early Proterozoic Proterozoic Wisconsin Wisconsin magmatic magmatic terranes terranes of of the the 26,p.p.212145-21 45-2158. 58. Penokean Orogen: Orogen: Canadian CanadianJournal JournalofofEarth EarthSciences, Sciences,v.v.26, 83 �Sims, P.K., P.K., Schulz, K.J., and Peterman, Peterman, Z.E., 1992, 1992, Geology Geology and and geochemistry geochemistry of Early Early Proterozoic rocks in the Dunbar area, northeastern northeastern Wisconsin: Wisconsin: U.S. U.S. Geological Geological Survey Survey Professional Professional Paper Paper 1517, 1517, 65 65 p. p. Sims, P.K., and and Schulz, K.J., K.J., 1993, 1993, Geologic Geologic map map of of Precambrian Precambrianrocks rocksin in parts partsof of Iron Iron Escanaba 30' X 60' quadrangles, quadrangles, northeastern northeastern Wisconsin Wisconsin and and adjacent adjacent Mountain and Escanaba Michigan: U.S. Geological Geological Survey Survey Miscellaneous MiscellaneousInvestigations InvestigationsSeries SeriesMap Map1-2356, 1-2356, scale 1:100,000. 1:100,000. Sims, P.K., P.K., Schulz, K.J., DeWitt, DeWitt, E., E., and and Brasaemle, Brasaemle,B., B., 1993, 1993, Petrography Petrographyand and geochemistry geochemistry of Early Early Proterozoic Proterozoicgranitoid granitoid rocks rocks in in Wisconsin Wisconsin magmatic magmaticterranes terranesof of the the Penokean orogen, northern Penokean northern Wisconsin—a Wisconsin-a reconnaissance reconnaissance study: study: U.S. U.S. Geological Geological Survey Bulletin Bulletin 1904-J, lgO4-J, p. p. J1-J31. J1-J31. Trent, V.A., 1976, 1976, The The Emperor Emperor Volcanic Volcanic Complex Complex of of the the eastern easternGogebic Gogebic Range, Range, G.V., and Wright, Wright, W.B.,eds., Changes Changes in in stratigraphic stratigraphic nomenclature nomenclature Michigan, in Cohee, G.V., by the U.S. Geological Geological Survey, 1975: U.S. Geological Survey Bulletin 1422-A, p. 69-74. Geological Bulletin S.B., 1966, The Carney Lake Gneiss, in Bayley, Treves, S.B., Bayley, R.W., R.W., Dutton, Dutton, C.E., C.E., and and Lamey, C.A., C.A., Geology Geology of the Menominee Menominee Iron Iron Bearing Bearing District, District, Dickinson Dickinson County, County, Michigan and Florence and Marinette Counties, Counties, Wisconsin: Wisconsin: US. US. Geological Geological Survey Professional Paper 513, p.20-30. Professional Paper 513, p.20-30. Trow, J.W., 1948, 1948, The The Sturgeon Sturgeon Quartzite Quartzite of of the the Menominee Menomineedistrict, district, Michigan: Michigan: unpublished dissertation, University unpublished Ph.D. dissertation, University of Chicago, Chicago, Illinois, Illinois, 60 p. Tyler, S.A., Barghoorn, Barghoorn, E.S., E.S., and and Barrett, Barrett, L.P., L.P., 1957, 1957, Anthracitic Anthracitic coal coal from from the the Precambrian Precambrian upper Huronian Huronian black shale shale of the the Iron Iron River River district, district, northern northern Michigan: Michigan: Geological Society Geological Society of America America Bulletin, Bulletin,v. v. 68, 68, p. p.1293-1 1293-1304. Ueng, W.L., and Larue, D.K., D.K., 1987, 1987, The The early early Proterozoic Proterozoic structural structural and and tectonic tectonic history history and Larue, of the south central Lake Lake Superior Superior region: region: Tectonics, Tectonics, v. v. 6, 6, p. 369-388. 369-388. Van Hise, C.R., and and Bayley, W.S., 1900, 1900, Description Descriptionof of the the Menominee Menominee special special quadrangle, Michigan: U.S. Geological quadrangle, Geological Survey Survey Geologic Geologic Atlas, Folio Folio 62, 62, 13 13 p., p., 33 maps. maps. Van Hise, C.R., C.R., and Leith, Leith, C.K., 1911, 1911, Geology Geology of the the Lake Lake Superior Superior region: region: U.S. U.S. Geological Survey Monograph Monograph 52, 641p. 641p. Van Schmus, W.R., 1976, 1976, Early Early and and middle middle Proterozoic Proterozoic history history of of the the Great Great Lakes Lakesarea, area, Royal Society Society of London London Philosophical PhilosophicalTransactions, Transactions, Ser. A280, no. no. North America: Royal 1298, p. 605-628. W.R., Van Schmus, W .R., 1980, 1980, Chronology Chronology of of igneous igneous rocks rocksassociated associatedwith with the the Penokean Penokean orogeny in Wisconsin: Geological Society of America Special Paper, 182, p.159-168. in Wisconsin: Geological Society of America Special Paper, 182, p.159-168. Winchell, H.V., 1895, 1895, Historical Historical sketch sketch of of the the discovery discovery of of mineral mineraldeposits depositsin in the the Lake Lake Superior region: Minnesota Minnesota Geological Geological and and Natural Natural History History Survey, Survey, 23rd 23rdAnnual Annual Report, Report, 1984, p. 116-155. 116-155. 84 �Young, G.M., 1970, 1970, An An extensive extensive Early Early Proterozoic Proterozoic glaciation glaciation in in North North America?: America?: Paleooceanography, Paleoclimatology, Paleoclimatology,Paleoecology, Paleoecology, v. 7, 7, p. p. 85-101. 85-101. Paleooceanography, Young, G.M., G.M., 1973, 1973,Tillites Tillitesand andaluminous aluminous quartzites quartzitesas as possible possibletime time markers markersfor for middle middle Young, inYoung, Young, G.M., G.M., ed., ed., Huronian Huronian Precambrian (Aphebian) (Aphebian) rocks rocks of of North NorthAmerica, America, in Precambrian Stratigraphy and and Sedimentation: Sedimentation: Geological Geological Association Association of of Canada Canada Special Special Paper Paper 12, 12, p. p. Stratigraphy 97-127. 97-127. Young, G.M., G.M., 1983, 1983,Tectono-sedimentary Tectono-sedimentaryhistory historyof of early earlyProterozoic Proterozoicrocks rocksof ofthe the Young, northern Great Great Lakes Lakes region, region,in inMedaris Medaris Jr., Jr., L.G. L.G. ed., ed., Early EarlyProterozoic Proterozoicgeology geology of of the the northern Great Lakes Lakes region: region: Geological GeologicalSociety Societyof of America America Memoir Memoir160, 160, p.15-32. p.15-32. Great 85 �FIELD FIELD TRIP TRIP 44 LIFE LIFE CYCLE CYCLE OF OF AN AN IRON IRON DEPOSIT—THE DEPOSIT-THE REPUBLIC REPUBLIC MINE MINE FROM ORE GENESIS GENESIS TO MINE MINE RESTORATION RESTORATION William F. Cannon, USGS, Reston, VA; VA; John G. G. Meier, Cleveland Cleveland Cliffs Cliffs Iron Iron Co., Ishpeming, Ishpeming, MI; Thomas Thomas Waggoner, Waggoner,Geologic Geologic Consultant, Consultant, Negaunee, Negaunee, MI MI Republic pit pit in in 1973, 1973, approximately approximatelyat at mid-point mid-pointof of production. production. The Republic The The Republic RepublicWetlands Wetlands Preserve Preserveinin2002, 2002, constructed constructedon onthe theformer formertailings tailings basin basin of of the the Republic Republicmine. mine. �FIELD FIELDTRIP TRIP 44 LIFE LIFE CYCLE CYCLE OF OF AN AN IRON IRONDEPOSIT—THE DEPOSIT-THE REPUBLIC REPUBLICMINE MINEFROM FROMORE OREGENESIS GENESIS TO TO MINE MINERESTORATION RESTORATION William William F. Cannon, Cannon, USGS, USGS, Reston, Reston, VA; VA;John John G. G.Meier, Meier, Cleveland ClevelandCliffs CliffsIron IronCo., Co., lshpeming, Ishpeming, MI; MI; Thomas ThomasWaggoner, Waggoner,Geologic GeologicConsultant, Consultant,Negaunee, Negaunee, MI MI Brief Brief history historyof of mining miningand andgeologic geologicstudies studies Mmmci: M i n i : The The Republic RepublicMine Minein inwestern westernMarquette MarquetteCounty, County,Michigan Michiganproduced produced more more than than 75 75million milliontons tonsof ofiron ironore oreduring duringtwo twogenerations generationsofofmining. mining.The TheRepublic RepublicIron Iron Company was was organized organized in in Marquette, Marquette, Michigan Michigan in in October October 1870 1870 and and mining mining at at what is is Company now now the the site site of of the theclosed closedRepublic RepublicMine Minebegan beganinin1871. 1871.Operated Operatedby bythe theRepublic RepublicIron Iron Company, Company, the the mine mine was was producing producing 235,000 235,000 tons tons of of hard, hard, specular specular hematite hematite by by 1880. 1880. Mining Mining operations operations were were conducted conducted in in open open pits pits and and 14 14 shafts. shafts. The The greatest greatest shaft shaft depth depth was was 2,910 2,910 feet. feet. The TheCleveland-Cliffs Cleveland-CliffsIron IronCompany Companyacquired acquiredthe the mine minein in1914 1914and and operated operateditituntil until1926 1926when whenititclosed. closed.Shipments Shipmentsfrom from the the stockpile stockpilecontinued continueduntil until1937. 1937. The 937 was Thetotal totalore oreshipped shippedfrom from1872-1 1872-1937 was more morethan than 8.5 8.5 million milliontons. tons.After Afterthat, that,the the mine 950s. minesat satidle idleuntil untilthe theearly early11950s. In In 1952, 1952, construction construction of of the the modern-day modern-dayfacilities facilities began began and and the the open open pit pit mine mine was was in in production productionby by1956, 1956,mining mininglow-grade low-gradeiron ironore ore that that was was concentrated concentratedat at the the mine's mine'splant plant and andpelletized pelletizedat at the the former formerEagle EagleMills Millspellet pelletplant plantin inNegaunee NegauneeTownship. Township. In Inthe the early early 1960's 1960'san anexpansion expansionof of the themine, mine,including includingpelletizing pelletizingand andadditional additionalconcentrating concentrating equipment, equipment, took tookplace. place.This Thisexpansion expansionwas was completed completedin in1964, 1964,at at which which time time Republic Republic was was producing producingmore morethan than22 million milliontons tons of of iron ironore orepellets pelletsannually. annually. AA unique uniquepart partof of the the mining miningoperation operationat at Republic Republicwas was the the in-pit in-pitcrushing crushing system, system, which which performed performedcoarse coarse crushing crushingnear nearthe the floor floor of of the thepit pitand andtransported transportedthe thecrushed crushedore orefrom fromthe thepit pitto tothe the plant, plant,aavertical verticallift liftof of 647 647feet, feet,via viaaa2,814-foot 2,814-footconveyor conveyorininan aninclined inclinedtunnel tunnelin inthe the footwall footwallof of the theore orebody. body. In In1981, 1981,with with the the iron ironore oreand andsteel steelindustry industryin inaa deep deep recession, recession,operations operationsat at Republic Republic were were suspended. suspended. When Whenhope hopefaded fadedthat thatmarket marketconditions conditionswould wouldimprove improveto to allow allowpellet pellet production productionto to resume, resume,Republic Republicwas was permanently permanentlyclosed closed in in early early 1996. 1996. More More than than 45 45 million milliontons tonsof of pellets pelletswere wereproduced producedatatthe theRepublic Republicplant plantbetween between1964 1964and and1981. 1981. Including Includingpellets pelletsproduced producedat at the theEagle EagleMills Millsplant plantand andsome someproduced producedat at the the Humboldt Humboldt plant plantwith withRepublic Republicore, ore,Republic Republicaccounted accountedfor formore morethan than63 63million milliontons tonsofofpellets pelletssince since 1956. 1956.When Whenthe themine mineclosed closedinin1981, 1981,Republic Republicemployed employedmore morethan than700 700people. people. In Inaddition additionto to being beingaamajor majoriron-producing iron-producingarea areafor for more morethan thanaacentury, century,the thegeologic geologic relationships relationshipsshown shownininthe themine minearea areaand andother othernearby nearbyexposures exposureshave haveplayed playedan an important importantrole rolein indeveloping developingconcepts conceptsof of the theorigin originof of iron-formations iron-formationsof ofvarious various sedimentary sedimentaryfacies, facies, their theiralteration alterationthrough throughregional regionalmetamorphism, metamorphism,and andaarather rather complex complex history historyof of iron ironconcentration concentrationthat that led ledto tothe theformation formationof of the thehigh-grade high-gradeores oreson on which whichthe theinitial initialmining miningwas wasbased. based.This Thistrip tripvisits visitssome someof of these theseexposures exposuresand andprovides provides aareview reviewof of the the rich richhistory historyof of studies studiesof of the thegeology geologyof of iron irondeposits depositsininthis thispart partof of the the Lake LakeSuperior Superiorregion. region. 87 �Geologic Geoloqic studies: The Thefirst firstrecorded recordedgeologic geologicobservations observationsat at Republic Republic were made by Government Government Land Office surveyors in 1846 1846 while surveying the township and range grid of the area. They clearly recognized recognized the importance importance of the ore that was readily visible on what was then a prominent prominent hill hill along along the the Michigamme MichigammeRiver. River. Five years later the area was visited by J.W. Foster Foster and and J.D. Whitney, Whitney, U.S. U.S. Government Government Geologists, "The 1851): "The Geologists, who provided provided the the following description description (Foster (Foster and and Whitney, Whitney, 1851): largest mass mass observed by us in this region occurs on the left bank of the Machi-gamig, Machi-gamig, in section section 7, of township township 46, range range 29, 29, and and traces traces of of itit are are to to be be observed observedin in several several of of the the adjoining adjoining sections. sections. ItIt here here rises rises in in aa nearly nearly vertical vertical cliff to to the the height height of of one one hundred hundredand and thirteen thirteen feet, and and is is somewhat somewhat variable variable in in purity. purity. For For the the most most part part itit has has aa slaty slaty cleavage, cleavage, and, on on close close inspection, inspection, is is observed observed to be be composed composed of alternating alternatingbands bands of of micaceous micaceous specular iron and quartz, tinged red by the peroxide of iron: but there are occasional occasional belts belts which display display a granular texture, and and apparently apparently possess possess a greater greater degree degree of purity. purity. These These laminae laminae are are nearly nearly vertical, vertical, exhibiting exhibiting few contortions, contortions, and and range range with so much much uniformity, uniformity, that the observer observer would would be be inclined inclined to refer refer both both the the slates slates and and the iron iron to aa common common origin. origin. Interlaminated Interlaminatedwith with itit is is aa band band of of rock rock composed composedmainly mainly of of white, granular granular quartz, quartz, with with traces traces of feldspar, feldspar, through through which which are are disseminated disseminated particles, particles, as as well as rounded rounded masses, of specular specular iron. iron. ItIt is is difficult difficult to to pronounce pronounce whether whether this this is is aa conglomerate conglomerate or or breccia." breccia." During the ensuing 20 years, the wave of exploration and development along the Marquette Marquette Range reached reached Republic. In 1871, shortly before before mining was initiated, the ore was was described described by by Swineford Swineford (1871). (1871). His His description description of Smith Smith Mountain, Mountain, as as itit was then then called, called, was "The "The mountain mountain rises rises to aa height height of nearly, nearly, ifif not not quite, quite, 1,000 1,000 feet feet above above the the waters waters of the the Michigammi Michigammi River, River, which which runs runs near near its its base, base, and and the explorations explorations made made last summer summer reveal reveal the the existence existence of an an immense immense body body of ore, which can be traced over a last mile mile by by outcrops outcrops alone. The The writer writer visited visited this this mountain mountain last last summer summer and and has has no no hesitation hesitation in in saying saying the he he believes believes itit to be be by by far the the most most valuable valuable property property yet discovered. discovered. The The ore ore is is aa very pure pure magnetic, magnetic, similar similar to to that that of of the the Washington Washington and and Champion. Champion. The The elevation elevation is is such such that the ore ore can can be be mined mined at aa comparatively comparatively trifling trifling cost, cost, and and itit would would be be an an easy easy matter matter to to mine mine and and ship ship aa hundred hundred thousand thousand tons tons in in the the first first year after after commencing commencing operations." He Hefurther furtherstates states"It "It was was originally originally discovered discovered by by S.C. Smith, Smith, Esq., Esq., of of Marquette, Marquette, from from whom whom itit takes takes its its name, name, and and who who threw threw away away an an immense immense fortune fortune in its sale at a nominal price." Apparently Apparently Mr. Mr. Swineford Swineford had had a rather optimistic optimistic eye, both both in in estimating estimating topographic topographic relief relief and and ore ore reserves. reserves. In 1873, 1873, T.M. Brooks Brooks published published aa comprehensive comprehensive survey survey of the iron-bearing iron-bearingrocks rocks of of the the In Marquette range range including including both both descriptions descriptionsand and an an atlas. atlas. At Republic Republic he he determined determined the the Marquette detailed detailed internal internal stratigraphy stratigraphy of of the the iron-formation iron-formationand and produced produced the the first first detailed detailedmap map of of the deposit. deposit. He He recognized recognized the synclinal synclinal character character of the host host rocks rocks and and described described the the several several lithologic lithologic types of iron-formation iron-formation and and ores ores exposed exposed in in the area. Like Like Swineford, Swineford, Brooks was was very very impressed impressed by by the the large large ore ore exposure exposure and and wrote wrote "The "The immense immense mass mass of of Brooks pure Va of of the the pure specular specular ore, which which was was naturally naturally exposed exposed near near the center center of the the north north ½ southeast Sec.7, 7,T. 1. 46, 46, R. R. 29, 29, could could leave leave no no reasonable doubt in the mind mind of the southeast 1/4 % ofofSec. experienced experienced observer, observer, that this deposit of ore ore was one of the the largest, this deposit was one largest, ifif not not the the largest, largest, in in the the Marquette Marquette region. region. This This outcrop, outcrop, the the extent extent of of which which is is shown shown on on the the map map of of the the Republic Mountain, Mountain, being being there marked marked "pure "pure specular specular ore", is, is, so so far far as as II know, know, the the Republic largest outcrop outcrop of of any any equally equally rich rich ore, ore, ever ever found found in in the the United United States." States." largest 88 �more detailed detailed and accurate accurate description of the geology geology was published published in 1897 1897 by Henry A more of the classic USGS Monograph 28 by Van Hise and Bayley Lloyd Smyth as Chapter VI of on the geology geology of the "Marquette "Marquette Iron-bearing Iron-bearingDistrict" District" (Van (Van Hise Hise and Bayley,1897). Bayley,1897). By original mines mines were well established and an understanding understanding of the that time, the original processes of ore-formation ore-formation was being being developed developed by Van Hise and Bayley. Smyth's Republic were an important part of that research research and are discussed in observations at Republic more detail below. Among Smyth's contributions were recognition of the detailed sequence of the Early Early Proterozoic Proterozoic strata, which has has not been been significantly stratigraphic sequence modified since, with with the exception exception of some some changes changes in in nomenclature, nomenclature,and anddelineating delineatingthe the modified With regard regard to to the the ore ore deposits, deposits, Smyth's Smyth's maps maps plunging synclinal geometry of the strata. With were the first (and (and only) ones that showed showed a distinct separation between between dominantly hard specularite ores and dominantly magnetite ores. He correctly surmised that the hard ores formed formed by secondary secondary concentrations concentrations within the iron-formation, iron-formation,in part part through leaching of original original siliceous beds, and also recognized recognized the detrital detrital character of iron iron enrichments Goodrich Quartzite. Quartzite. enrichments in the basal basal conglomerates conglomerates of the Goodrich Much later, in the early 1950's, 1950's, Harold Harold James of the USGS USGS published published two fundamental papers papers in which Republic Republic played a major role. His work on zones of regional metamorphism metamorphism (James, 1955) 1955) was strongly influenced influenced by the Republic Republic area, which was at the center of a zone of sillimanite grade grade metamorphism metamorphism of his his Republic Republic metamorphic metamorphic node. node. The The Republic Republic area area was was the the sole sole locality locality where where several severallithologic lithologictypes typesof of ironironformation were within within this this high-grade high-gradezone zone and and was was the the principal principalexample exampleupon uponwhich whichhe he defined the very high-grade high-grade metamorphic metamorphic effects effects on iron-formation. iron-formation. A A fundamental fundamental outcome outcome of that study was that even very intense metamorphism metamorphism does not change change the majority of the iron, iron, an an idea idea contrary contrary to widely held held concepts original oxidation state of a majority at that time. The relationships relationships upon upon which he based based that conclusion are clearly displayed in a glacially polished outcrop at the Kloman Kloman Mine (fig. 4-1, stop 4-1). James's studies of the importance importance of sedimentary sedimentary conditions in determining the mineralogic mineralogic character of iron-formations, published published as his his famous "Sedimentary "Sedimentary facies of iron-formation" iron-formation" (James, 1954), also drew heavily heavily on relationships relationships seen seen in in the the Republic Republicarea, area, and and again again the the exposures exposures at Kloman Kloman mine mine were were instrumental instrumentalin inshowing showingthe therelationship relationshipbetween between bedding bedding characteristics characteristicsand and mineralogy mineralogyof of iron-bearing iron-bearingphases. phases. The most most recent recent comprehensive comprehensive geologic geologic study of the Republic Republic area area was conducted by Cannon (1975) who mapped mapped the geology of the iron-bearing iron-bearingsequence and and surrounding surrounding Cannon (1975) the rocks, including detailed mapping in the open-pit. Because the pit was developed in the same stratigraphic unit unit that contained contained the high-grade high-gradeores ores mined mined previously previously underground, exposures of those high grade ores and their relationships to host rocks rocks were abundant in the pit and were a fundamental component of Cannon's study of the component Cannon's origin of hard hard iron iron ores of the Marquette Marquette Range Range (Cannon, (Cannon, 1976). 1976). Unfortunately Unfortunately those those exposures exposures are are now now flooded flooded in in the the abandoned abandonedpit. pit. Geology of the Republic Republic area The Republic Republic area area has has many many similarities similarities to the Marquette Marquette Iron Iron Range Range to the north north and and east and and is is generally generally considered considered to be be an extension of it. Paleoproterozoic Paleoproterozoic strata strata of the Marquette Range Supergroup are preserved in a syncline, the Republic trough, between uplifted blocks of Archean gneisses. The stratigraphic units defined in the Marquette Iron Range, can be applied in the Republic trough as well, and to some extent can be traced directly into into the Marquette Marquette Range. Range. The The Paleoproterozoic Paleoproterozoic units units in the Republic Republic area area consist of a basal sequence of the Ajibik Ajibik Quartzite Quartzite and and Siamo Siamo Slate Slate which were basal clastic sequence 89 �mapped mapped as as aa single single unit unit by by Cannon Cannon (1975) (1975) because because very limited limited exposure exposure does does not not allow allow accurate accurate mapping mappingof of aa contact contactbetween betweenthem. them.These Theseare areoverlain overlainby bythe theNegaunee Negaunee Iron-formation, Iron-formation,the the principal principaliron-bearing iron-bearingunit. unit.The TheGoodrich GoodrichQuartzite Quartzitelies liesunconformably unconformably on on the the Negaunee. Negaunee. The The unconformity unconformity was was well well exposed exposed in in the Republic Republic pit pit before before flooding. flooding. The Thebasal basalpart partof of the theGoodrich Goodrichisisaaferruginous ferruginousconglomerate, conglomerate,which whichgrades grades upward into quartzite. The youngest strata are biotite-garnet schist with beds of upward into quartzite. The youngest strata are biotite-garnet schist with beds of impure impure quartzite, quartzite, which which make makeup upthe the Michigamme MichigammeFormation. Formation.The The Michigamme Michigammealso also contains contains aa basal unit unit of silicate-magnetite silicate-magnetiteiron-formation. iron-formation. Several Several sills sills of of diabase diabase were were intruded intrudedinto into basal the the strata, strata, mostly mostlyin inthe the iron-formation, iron-formation,and andhave havebeen beenfolded foldedwith with it. it. The synclineof of The fundamental fundamentalstructure structure of of the thearea areaisisaanarrow, narrow,deep, deep,northwest-plunging northwest-plungingsyncline Paleoproterozoic Paleoproterozoicstrata strata of of the the Marquette MarquetteRange Range Supergroup Supergroupflanked flanked by by Archean Archean gneisses. gneisses.Gravity Gravitystudies studiesindicate indicatethat that along along Highway Highway95, 95, where where the the trough trough is is about about 3,000 3,000 feet feet wide, wide, ititisisabout about5,000 5,000 feet feet deep deep(Klasner (Klasnerand andCannon, Cannon,1974). 1974).Axes Axes of of minor minor folds foldsmeasured measuredin inthe the Republic Republicpit, pit, which which follows follows the the iron-bearing iron-bearingbeds beds around around the the keel keel of to the 45'to the northwest. northwest.The The tight tightcompression compressionwithin within the the of the the trough, trough, plunge plungeabout about45° syncline syncline has hasproduced producedintense intensesmall small scale scale folding folding in in some some units, units, such such as as the the schists schists of of the 4-2(fig. (fig.4-1). 4-1).This Thisdeformation deformationapparently apparentlyalso also the Michigamme MichigammeFormation Formationseen seenat at stop stop4-2 has hascaused causedsevere severeattenuation attenuationof of the the iron-formation iron-formationalong alongthe thelimbs limbsof of the thesyncline synclineas as shown shown by by the the drastic drasticthinning thinningseen seen in in traversing traversingfrom from the the Republic Republicpit pitat at the the keel keelof of the the structure structure along alongstrike strikeonto ontothe thenearly nearlyvertical verticallimbs. limbs. The Thecontact contactbetween betweenthe the Paleoproterozoic Archean basement basement gneisses gneisses is is interpreted interpretedto to be be aa fault fault Paleoproterozoicstrata strata and and Archean along along both bothlimbs limbsof of the the syncline. syncline.The Thefault fault on onthe thesouthwestern southwesternlimb limbwas was well well exposed exposedin in the thepit pitand andcompletely completelytruncates truncatesthe the Negaunee Negauneeand andAjibik/Siamo AjibikISiamostrata strataproducing producingthe the fish-hook fish-hook shape shapeof of the thetaconite taconiteorebody. orebody. Bedding Beddingininthe theiron-formation iron-formationnear nearthe thefault, fault,as as well Archeanrocks rocksalong alongthe thefault, fault,generally generallydips dipsfrom from75°-85° 75'-85' totothe the well as asshear shearfoliation foliationininArchean southwest indicating that the fault is a high-angle reverse fault at the present exposure southwest indicating that the fault is a high-angle reverse fault at the present exposure level. level.The The fault fault along alongthe the northeast northeastlimb limbisisnot notas aswell well documented documentedby byexposures, exposures,but but beds bedson on the the northeast northeastlimb limbare arecommonly commonlyoverturned overturnedand anddip dip about about 85° 85' to to the the northeast northeast suggesting suggestingthat that the thenortheast northeastlimb limbfault faultisisalso alsoaahigh highangle anglereverse reversefault. fault.Thus Thusthe the Republic Republictrough troughhas hasan anunusual unusualgeometry geometryininbeing beingaasyncline synclinethat thatwidens widenssomewhat somewhat below belowthe thepresent presentsurface. surface. The Theintense intensePenokean Penokeandeformation deformationrecorded recordedininthe thePaleoproterozoic Paleoproterozoicstrata strataof of the the Republic Archean gneisses gneisseswhere where intense intense Republictrough troughisisnot notpresent presentwithin withinthe the surrounding surroundingArchean multiple multiplefolding foldingevents eventsdocumented documentedby byTaylor Taylor (1967), (1967),Cannon Cannon(1975), (1975),and andHoffman Hoffman (1987) (1987)appear appearto tobe beentirely entirelyaaresult resultof of late lateArchean Archeantectonism. tectonism.Cannon Cannon(1973) (1973)used used metadiabase metadiabasedikes dikesas asstructural structuralmarkers markersto toindicate indicatethe thegeneral generalabsence absenceof of penetrative penetrative Penokean Archeanrocks. rocks.These Thesedikes, dikes,which whichoccur occurinin Penokeandeformation deformationininthe theareas areasof of Archean northeastnortheast-and andnorthwest-trending northwest-trendingswarms, swarms,are aremetamorphosed metamorphosedby byPenokean Penokean metamorphism metamorphismof of the theRepublic Republicnode nodeand, and,ininplaces, places,have havesheared shearedmargins marginscaused causedby by Penokean Penokeandeformation, deformation,but buteverywhere everywheremaintain maintaintheir their vertical, vertical, planar planardike dikegeometry geometryand and have haveessentially essentiallyno nointernal internaltectonic tectonicfabric. fabric.Relict Relictdiabasic diabasictextures texturesare arecommonly commonlywell well preserved preservedeven evenininthe themost mosthighly highlymetamorphosed metamorphoseddikes. dikes.Thus Thusititappears appearsthat thatPenokean Penokean deformation Archeanbasement basementrocks rockswas waslargely largelyconfined confinedto tozones zonesof of deformationwithin withinareas areasof of Archean shearing, shearing,which whichseparate separatelarger largerblocks blocksof of rock rockthat thatremained remainedrigid. rigid.The TheRepublic Republictrough, trough, therefore, therefore,isisaagraben grabenwith withrespect respecttotoArchean Archeanrocks. rocks.The Thesynclinal synclinalform formofofthe thetrough troughofof Paleoproterozoic strata is a result of those strata being molded around the structural Paleoproterozoic strata is a result of those strata being molded around the structural form formcreated createdby bydifferential differentialmovement movementofofindividual individualdiscrete discreteblocks blocksofofArchean Archeanbasement basement rocks. rocks. 90 �____ EXPLANATION EXPLANATION Mesoproterozoic Mesoproterozoic Diabase Diabase Paleoproterozoic Paleoproterozoic Metadia Metadiabase base E Michigamme Fm. Fm. --Michigamme lower lower slate slate member member MichigammeFm. Fm. Michigamme -- banded banded silicate-magnetite silicate-magnetite iron-formation iron-formation Goodrich GoodrichQuartzite Quartzite ::-: Goodrich Qua rtzite Goodrich Quartzite -- basal conglomerate basal conglomerate Negaunee NegauneeIron-Fm Iron-Fm hematite-richoxide oxidefades facies -- hematite-rich Negaunee NegauneeIron-Fm Iron-Fm -- magnetite-rich magnetite-richoxide oxidefacies facies Negaunee NegauneeIron-Fm Iron-Fm -- iron ironsilicate silicatefacies facies Siamo Slate and Ajibik Siamo Ajibik Quartzite, undifferentiated undifferentiated Quartzite, Archean Archean Granitic Graniticgneiss gneiss Mafic gneiss Lii. Mafic gneiss open open pit pit minedumps dumps EJ mine - — faults faults 8800 1/2 0 ,I 1 I miles miles Figure 4-1. Geologic Geologic map map of of the the Republic Republicarea, area, Michigan Michiganshowing showingthe the location locationof of field field Figure trip trip stops. stops. Geology Geologyfrom from Cannon Cannon(1975). (1975). t 91 �Economic geology geology The iron iron ores produced produced at Republic Republic were of two different different types: early production production was of high-grade high-gradeores ores that occur within the upper upper part part of the the Negaunee Negaunee Iron-formation Iron-formationand and lower lower part part of the overlying overlying Goodrich Goodrich Quartzite. Quartzite. These These are are locally locally known known as "hard ores" because because of their their compact, compact, coarse coarse crystalline crystalline nature. nature. The The more more recent recent production productionof iron iron concentrate concentrate and and pellets pellets was was from from lower-grade lower-gradematerial materialtypical typical of of upper upperstratigraphic stratigraphicunits units of the Negaunee Iron-formation. Negaunee Iron-formation. Concentrating-grade Concentratinci-aradeore (taconite): (taconite): The The modern modern open open pit pit of the the Republic Republic mine mine was developed developed to mine mine the uppermost uppermost parts parts of the Negaunee Negaunee Iron-formation Iron-formationconsisting consisting mostly mostly of hematitic hematitic jasper (fig. (fig. 4-2). 4-2). The The ore ore horizon horizon varied varied from from 400-600 400-600 feet feet thick in in the pit. The ore horizons horizons dip approximately approximately vertically along in the northern part of the pit, which which lies lies along along the the northeast northeast limb limb of the the Republic Republic syncline, syncline, but but dips dips flatten flatten to to about about 45 45 degrees degrees in in the the southern southern part part of of the the pit pit near near the the synclinal synclinal axis. axis. Coarse-grained Coarse-grainedspecular specular hematite hematite was the most important important ore mineral mineral but but some some magnetite magnetite was also produced produced from from aa thin thin unit unit of of banded bandedchert-magnetite chert-magnetiteiron-formation iron-formationthat that formed formedaacontinuous continuousunit unit no no more more than a few tens of feet feet thick stratigraphically stratigraphically below below the hematitic hematitic jasper. Some Some iron Goodrich iron production production also came from highly highly ferruginous conglomerate conglomerate of the Goodrich Quartzite, mostly present as a lens lens as much much as several hundred hundred feet thick in in the southwest part part of the pit. Essentially Essentially all of the iron iron was deposited as primary sedimentary sedimentary accumulations. accumulations. The iron iron content content of the ore ore was no no greater than than typical of the Negaunee Negaunee elsewhere in the region. region. The economics economics of the deposit deposit were controlled controlled principally by the structural structural geometry geometry of the ore ore beds beds and and by by the the oxide oxide mineralogy mineralogy and and grain grain size. Lower Lower units units of the Neguanee, grunerite)- magnetite magnetite iron-formation, iron-formation, Neguanee, never mined, consist of silicate silicate (mostly (mostly grunerite)which which was was not not amenable amenable to to concentration concentration by by the the techniques techniques used used at at Republic. Republic. These These rocks, rocks, along along with three diabase diabase sills, underlie underlie the prominent prominent ridge ridge along the northeast northeast and and southeast flanks flanks of the pit pit where they dip dip nearly nearly vertically. The uppermost uppermost diabase diabase sill the orebody orebody (figs. (figs. 4-3, 4-3, 4-5). 4-5). The The unusual unusual strength strength and and stability stability of sill forms forms the the footwall of the this this metadiabase metadiabaseallowed allowed the the development development of of aa high high footwall footwall of of the the pit, pit, which which stood stood several several hundred hundred feet high high as a vertical vertical to slightly overhanging overhanging rock face. This This situation situation was was an an important important economic economic factor factor in in that that itit eliminated eliminated the the need need for for the the large large amount amount of of waste footwall waste rock rock removal removal that that would would have have been been necessary necessary to to maintain maintain aa more more typical typical footwall pit pit slope. slope. In In addition to the occurrence occurrence of the hematite hematite and and magnetite magnetite iron-formation, iron-formation, other other geologic geologic factors factors were critical critical in in enhancing enhancing the economics economics of the Republic Republic mine. The The geometry geometry of the syncline syncline created created the moderately-dipping moderately-dipping and thick sequence sequence of ironironformation formation along along the the keel, keel, which which was was required required to to extract extract ore ore with with an an economically economically permissible permissible amount amount of of removal removalof of underlying underlyingiron-silicate iron-silicateand anddiabase, diabase,and andoverlying overlying Goodrich Goodrich Quartzite Quartzite waste rock. rock. Also, the the high high degree degree of metamorphic metamorphicrecrystallization recrystallization was was vital vital in in enhancing enhancing the the liberation liberationand and concentration concentration of of iron iron minerals, minerals, in in that that grinding grinding to to very-fine very-finegrain grain size size was was not not required required to to allow allow separation separation of of iron-minerals iron-mineralsfrom from chert chert by by the the flotation flotationprocess processused usedat atRepublic. Republic. 92 �:f • • 4- • - • • • .1: • • • •;r. • :-•'.. :•' - • • -- - •.•• - - • . I— - • • -• • • • - :. Ygure 4-2. Wavy-bedded jaspilite typical of the ore zone for the Republic open pit. .ighter layers are specular hematite and darker layers are lenticular beds of ietamorphosed jasper. �Figure Figure 4-3. 4-3. The The Republic Republicopen open pit pit in in 1973. 1973. View View looking lookingSE SE along along the the NE NE limb limb of of the the Republic Republicsyncline. The high wall on the left side of the pit the upper upper contact contact of syncline. The high wall on the left side of the pit is is the of aa metadiabase metadiabasesill, sill,now nowdipping dippingvertically. vertically. Hard Hardores: ores: The Thefirst firstphase phaseofofmining miningatatRepublic Republicwas wasbased basedon onhigh-grade high-grade(60-65% (60-65%Fe) Fe) concentrations concentrationsof of specular specularhematite hematiteand andmagnetite, magnetite,which whichwere were referred referredto toas ashard hardore ore (fig. (fig.4-4), 4-4),inincontrast contrastto tothe theearthy earthymasses massesof ofiron ironoxides oxidesand andhydroxides, hydroxides,the thesoft softores, ores, widely widely mined minedin inthe the eastern easternparts partsof of the theMarquette Marquettedistrict. district. The Theorigin originof ofhard hardores oreshas has been investigated since the geologic studies of Van Hise and Bayley (1897), who were been investigated since the geologic studies of Van Hise and Bayley (1897), who were the thefirst firstto torecognize recognizeaaconnection connectionbetween betweenthe theNegaunee-Goodrich Negaunee-Goodrichunconformity unconformityand and occurrence occurrenceof of hard hardore. ore.They Theyproposed proposedthat thatoxidation oxidationof of siderite sideriteand andleaching leachingof ofsilica silica from fromthe theiron-formation iron-formationby bygroundwater groundwaterformed formedhematite hematiteconcentrations. concentrations.They They envisioned envisionedtectonism tectonism as as important importantin inproducing producingaapermeable permeablecrushed crushedzone zoneat at the the unconformity unconformityto toaccentuate accentuategroundwater groundwaterflow. flow. Later, Later,Van VanHise Hiseand andLeith Leith(1911) (1911) proposed surficialweathering weatheringand and proposedthe theclassic classictheory theorythat thatthe thehard hardores oresformed formedby bysurticial leaching leachingof of the theNegaunee NegauneeIron-formation, Iron-formation,during duringthe theuplift upliftpreceding precedingdepositon depositonofofthe the Goodrich Quartzite, and are paleosupergene concentrations; this weathered and Goodrich Quartzite, and are paleosupergene concentrations; this weathered and leached leachedmaterial materialwas was later laterdeformed deformedand andmetamorphosed metamorphosedto toproduce producethe thepresent presenthard hard ore. ore. This Thistheory theorywas was widely widely accepted acceptedand andmost mostrecently recentlysupported supportedby byGair Gair(1975) (1975) based based largely largelyon onobservations observationsin inthe the Cliffs CliffsShaft Shaftmine mineininlshpeming. Ishpeming.Others Othershave haveproposed proposedthat that the thehard hardores oresare areat atleast leastpartly partlyhydrothermal hydrothermal(Roberts (Robertsand andBartley, Bartley,1943; 1943;Crump, Crump,1948; 1948; Anderson, Anderson,1968; 1968;Marsden, Marsden,1968). 1968). AAcomprehensive comprehensivereexamination reexaminationof of the the hard hardores oresof of the theMarquette MarquetteRange Rangeby byCannon Cannon (1976), (1976),including includingcritical criticalrelationships relationshipsthen thenexposed exposedin inthe the Republic Republicpit, pit, suggested suggestedthat that there thereare aretwo twotypes typesofofhard hardore, ore,each eachformed formedby byaasubstantially substantiallydifferent differentset setofof processes. processes.He Herecognized recognizedaadistinction distinctionbetween betweendominantly dominantlyspecular specularhematite hematiteores, ores, 94 �possessed Penokean Penokean tectonic fabrics, and dominantly magnetite magnetite ores, which commonly possessed massive and appear to post-date which generally are massive post-date tectonism. The hematite ores show geologic relationships relationships that are fully consistent with the original original paleosupergene paleosupergene origin proposed by Van Hise and Leith and are still believed to record an interval interval of oxidative proposed hiatus between between the Negaunee Negaunee IronIronweathering and leaching during the stratigraphic hiatus Goodrich Quartzite. The geologic geologic map map of the Republic Republic area area published published by by formation and Goodrich (in Van Hise and Bayley, 1897) clearly distinguished distinguished hematite hematite and and magnetite Smyth (in separate ore lenses. lenses. Cannon Cannon types of ores, which occurr in distinct and to some extent separate documented that a similar duality of ore types is widespread in the western Marquette documented range, most particularly at the Greenwood and Champion mines. The magnetite ore is invariably post-tectonic, commonly commonly contain contain euhedral euhedral quartz quartz crystals crystals in vugs, vugs, and and occurs occurs Cannon proposed proposed that the magnetite magnetite ores formed formed from from aa within hematitic host rocks. Cannon hydrothermal fluid that was reducing with respect to the hematitic host rock. He suggested that fluids fluids released released by metamorphic metamorphic devolatilization of the iron-formation, iron-formation, suggested which was rich in both hydrous and carbonate minerals in its primary state, was the origin of the hydrothermal hydrothermal fluids. Metamorphism Metamorphism of the Republic Republic node node is is known known to have origin begun begun during the closing phases phases of Penokean Penokean deformation deformation but but to have have outlasted outlasted and reached its peak after deformation. Thus, the magnetite magnetite hard hard ores were interpreted interpreted to have been deposited shortly after formation of the Republic syncline and to be precipitates from reduced metamorphic fluids carrying ferrous iron from stratigraphically precipitates lower parts of the iron-formation iron-formation to the upper parts where reaction with hematite hematitc beds caused caused precipitation precipitationof magnetite. magnetite. Figure 4-4. Former in Republic pit showing onleft leftand Former exposure exposure in Republic pit showing wavy-bedded wavy-beddedjaspilite jaspilite on and magnetite hard ore on right. The hard truncates jaspilite bedding hard ore truncates bedding at a nearly nearly right angle. Within a few centimeters of the contact, much much of the jasper is is converted converted to to milky milky crystals have quartz and partly removed removed from the rock, creating vugs into into which quartz crystals grown. The loss of volume resulted in brecciation brecciation of the iron-formation, which was later healed and largely obliterated by magnetite deposition. Magnet is about 12 cm long. 95 �pit and mill The Republic pit mill metallurgical In 1947 Cleveland Cliffs Iron Co. began research on devising a viable metallurgical concentrating scheme scheme to upgrade upgrade the banded banded specular specular chert iron-formation iron-formationat at Republic Republic to a suitable grade of concentrate. concentrate. The crude crude ore composition composition was 38% 38% Fe, Fe, 42.5% Si02, SiOa, 0.033% P, P, 0.90% 0.90% MgO, MgO, 0.53% 0.53% CaO, CaO, 0.72% 0.72%A1203, A1203,0.03% Na20, NaaO,and and 0.04% 0.04% K20. K20.AA concentration process process was needed needed to produce a product with roughly 65% Fe and 5% Si02. The Company settled on a hot anionic flotation of the specularite after the crude Si02. was ground to 90% passing passing 325 mesh mesh size. After the board board of directors directors approved approved a new new mining operation at Republic, Republic, the company company teamed teamed with with three three partners partnerswho who would would consume & LL Steel Steel Corp., Corp., and and consume the pellet pellet product (i.e. Wheeling Pittsburgh, Pittsburgh, JJ & International Harvester Harvester Co.). Operations International Operations started started with with site site clearing clearingin in1952 1952foUowed followed by construction construction of a concentrator concentrator capable capable of producing producing an initial initial 600,000 long long tons tons (It) of concentrate. concentrate. Pelletization Pelletization (agglomeration) (agglomeration)was was initially initiallyconducted conductedat at the the Eagle EagleMills Millspellet pellet plant located located southwest of the City of Marquette Marquette starting starting in in 1956. 1956. In In 1962, 1962, a pelletizing facility located at the mine was brought on line with production increasing to 2.6 million It of pellets pellets per per year. year. The open pit was developed by a conventional bench-berm system from surface to an of +940 feet. A unique feature of the Republic ultimate depth of Republic Mine was the development of the vertical footwall on the northeast side of the pit that was vertical to slightly overturned overturned footwall referred required steel mesh screen referred to as the highwall (fig. 4-5). This distinctive design required spalled rock from falling into work areas. held in place by rock bolts to prevent spatted the pit crusher was relocated from from the pit crest to an During the middle 1970s the aditkhamber located located in the high high wall on the +1130 +I130 bench (lower left of fig. 4-5). A 28002800adit/chamber grade had had been been driven from surface surface to the chamber chamber to to house house foot-long gallery at 11% 11% grade foot-long the conveyor used to move the ore from the crusher to the plant plant stockpile (upper (upper right right of fig. 4-5). 4-5). Mining was conducted using conventional shovel-truck shovel-truck equipment equipment to to get the the ore to to the crusher. Initial Initial haulage haulage units units were 34 tons Euclid Euclid trucks. However, by the time the mine was idled idled in in 1981, 1981, 80-ton 80-ton units units were were standard. standard. Republic crude was extremely hard hard and and initial initial production production drilling drilling used used the the Linde LindeJet Jet Piercing machine machine in which fuel oil and spa11the the rock rock and oxygen oxygen were combusted combusted to to heat heat and and spall for drilling. During 970s the the mining equipment industry developed During the the 11970s developed rotary rotary machines machines capable of delivering 90,000 lbs. Ibs. of down pressure to the bit, which allowed the mine to change change over to conventional blast hole drilling. Blasting Blasting required required the use of ammonium nitrate and fuel oil (ANFO) at a high usage exceeding one pound per ton whereas other required results for half the powder factor. mines could get the required dewatering wells wells on on the the pit pit perimeter perimeter that that Rock stability at the mine was enhanced by dewatering drew down water keeping keeping it from the pit faces and and further stabilizing stabilizing the pit pit walls. This was especially important on the northwest side of the pit where the Michigamme Michigamme River had been partly diverted to allow pit development. 96 �Figure 4-5. The highwall at Republic. View is looking southeast. The steep rock face is the upper upper contact of a metamorphosed metamorphosed diabase diabase sill, which was emplaced emplaced at the base base of the oxide-facies ore unit. Other rock faces in upper upper left are underlying beds of magnetitegrunerite grunerite iron-formation, iron-formation,which were removed removed as waste. The units units now now dip vertically or are slightly overturned so that, in in places, places, the highwall highwall projects projects slightly outward over the pit. The two portals portals in in lower center are the access access to the primary primary crusher. Crushed Crushed ore ore was moved through an inclined moved to stockpile (on horizon horizon in upper right) by conveyor conveyor through inclined tunnel. The highwall highwall was about 300 feet high high at the time time of this this photograph photograph near near the the end end of pit pit operation. operation. Crude Crude ore underwent underwent three crushing crushing stages stages to provide provide a product product less less than than ½-inch %-inch size, which was fed to the grinding grinding section. Grinding Grinding consisted consisted of a conventional conventional rod rod mill mill with a steel rod rod charge charge to effect effect size size reduction. reduction. The The crude crude was then then sent sent to to aa ball ball mill mill containing containing small steel steel balls balls to complete complete the grinding grinding to aa size size suitable to liberate liberate ore ore minerals from gangue. The crude crude was discharged at 90% passing passing 325 mesh. Flotation Flotation using a fatty acid reagent separated the iron iron from the gangue gangue by by floating floating the iron iron minerals (anionic). The coarse nature of the product required required further grinding grinding to make make itit suitable for pelletizing. Concentrate Concentrate was reground reground in in ball ball mills, heated, heated, and and the pulp pulp was sent hot hot to roughers, roughers, cleaners cleaners and and scavengers. scavengers. The The final final product product was was dewatered, dewatered, balled, balled, and finally sent to the Allis Chalmers Grate Kiln system to be heated to 2440° F to Chalmers Grate Kiln system be heated 2440' F produce a tough pellet capable of withstanding handling and transport to the furnace. The product product was carried carried to Marquette Marquette by by rail rail and and then shipped shipped via lake lake cargo vessels vessels to steel furnaces in in the lower lower Great Great Lakes. Lakes. Scrubber Scrubber and and electrostatic electrostatic precipitators precipitators 97 �removed removed 99% 99% of the the particulate particulatematter matter from from the the discharge discharge air. air. Tailings Tailings from from the the process process settled in in vast settling settling ponds ponds located located southeast southeast of the the plant plant that that have have subsequently subsequently were settled been been converted converted to to viable viable wetlands. wetlands. From 1956 1956 to 1981 1981 the mine mine produced produced 62 62 million million tons tons of pellets pellets recovered recovered from from 145 145 From million million tons tons of of crude crude ore. ore. Considerable resources resources remain remain in in the ground ground at currently currently subeconomic status. The The Considerable resources, as as estimated estimatedby byCleveland ClevelandCliffs CliffsIron IronCo., Co., include includeboth bothspecular specularhematite hematiteore, ore, resources, the traditional traditionalore ore mined minedat at Republic, Republic,and and magnetite-silicate magnetite-silicateiron-formation, iron-formation,which which was was the not amenable amenable to concentration concentration by by the the process process used used at at Republic. Republic. The The estimated estimated not resources in million of long tons (MLT) of ore are: resources in million long tons (MLT) ore are: Ore Resource Grade SiOsininconc. cone. Oretype type Grade Fe Fe Recovery Recovery Fe Feininconcentrate concentrate Si02 Spec. hematite 63 MLT ML T 38% Fe 44% Spec. hematite 38% Fe 44%Wt. Wt.Rec. Rec. 65.0% 65.0%Fe Fe 5.00% 5.00% Mag. 60 MLT 27% Wt. 27%Fe Fe 33% 33% Wt,Rec. Rec. 67.0% 67.0% Fe Fe 5.80% Mag.-Silicate -Silicate 60 MLT 5.80% Talc is is common common in in footwall footwall ore ore in in the the area areaof of the the axial axial keel keelof of the the syncline. syncline. MgO MgO Talc analyses analyses range range from from 1% 1% to 4.7%. Schistose Schistoseconglomerates conglomeratescontain containsericite, sericite, chlorite, chlorite, epidote and and tourmaline. tourmaline. Most Most ore ore in in the the pit pit contained contained 0.20% 0.20% Ti02 TiOpbut but the the southwest southwestPark Park epidote City area area had had elevated elevated values values in in the the .50 .50 to to 1.65% 1.65%Ti02 Ti02as as rutile rutile associated associated with with hematite hematite City Mine Mine closure closureand and restoration restoration From tailings tailinas basin basin to to the the Republic RepublicWetlands Wetlands Preserve: Preserve:The The Republic RepublicMine Mine in in Marquette Marquette From County, Michigan Michiganceased ceasedoperations operationsinin1981, 1981,but butwas wasnot notofficially officiallyclosed closeduntil until1996. 1996. County, Planswere were made madeto to reclaim reclaimaasubstantial substantialportion portionofofthe theRepublic RepublicMine Mineas asaawetland wetland Plans mitigationproject projectto to serve serve the the needs needsof of the the nearby nearbyEmpire Empireand and Tilden Tilden mines minesand and other other mitigation Cliffs-managedproperties. properties. The The Cleveland-Cliffs Cleveland-CliffsIron IronCompany Companyand and its its partners partnersin in the the Cliffs-managed two active active mines mines agreed agreed to to proceed proceedwith with what what is is called called the the Republic Republic Wetlands Wetlands Preserve Preserve two (RWP). The The Michigan MichiganDepartment Departmentof of Environmental EnvironmentalQuality Quality and and the the U.S. U.S. Environmental Environmental (RWP). Protection Agency were involved from the inception of the project. Protection Agency were involved from the inception of the project. NorthernEcological EcologicalServices, Services,Inc. Inc.(NES) (NES)and andCliffs CliffsMining MiningServices ServicesCompany Company(CMSC) (CMSC) Northern formed the the project project team team involved involvedin in the the planning, planning, design, design, construction, construction, and and monitoring monitoring of of formed createdlrestoredon on iron iron the RWP RWPproject. project. Approximately Approximately650 650 acres acres of of wetlands wetlands were were created/restored the tailingsand andreuse reusewater water basins basinsat atthe theRepublic RepublicMine Mine(fig. (fig.4-6). 4-6). Another Another 1,650 1,650acres acresof of tailings wetlands and and uplands uplands were were included included in in the the RWP, RWP, bringing bringingthe the total total acreage acreage'to 2,300 wetlands to 2,300 acres. acres. Water Water levels levelsin in the the tailings tailings basins basins were were managed managedto to create create optimal optimal conditions conditions for for wetlandvegetation, vegetation, with with dormant dormantseeding seedingand andfertilizing fertilizingbeing beingdone, done, as aswell well as as aerial aerial wetland application application of of seed seed and and fertilizer fertilizer on on less less accessible accessible areas. areas. Over Over 250,000 250,000 trees trees were were planted plantedin inthe the wetlands. wetlands. 98 �Figure 4-6. Wildlife trail in a forested and emergent wetland developed on the former tailings basin tailings basin at the Republic Republic Mine, Mine, now now the the Republic RepublicWetlands WetlandsPreserve. Preserve. RWP completed its fifth, and final, year of monitoring monitoring and and the In 2002, phase I of the RWP wetland plant growth and wildlife use is nothing short of spectacular. Peregrine Peregrine falcons, a federal endangered endangered species, have have been been documented documented on a number of occasions using pair of bald bald eagles eagles nests nests there, as as well as as osprey osprey and and common common loons, loons, all the RWP. A pair state-threatened species. There is also a great blue blue heron heron nesting nesting colony, with approximately 60 nests active each year. Over 100 100 species of birds have been documented documented using the site. The land has been transformed from former mined lands to a diverse component component of the landscape landscape in about five years' time. The wetland credits credits that have not been been used to compensate compensate for unavoidable unavoidable wetland impacts at the Empire Empire and Tilden Tilden mines mines are in the process process of being being placed placed in in a wetland mitigation bank to be used for future mine-related mitigation mine-related projects projects in Marquette Marquette County. In summary, the RWP RWP is is a classic classic example example of how how the mining mining industry industry and and government government a valuable natural regulators can work together to reclaim former mine land and create 9 asset that benefits requirements. benefits area area wildlife and satisfies satisfies the mitigation mitigation requirements. FIELD FIELD TRIP TRIP STOPS STOPS Several Several previous previous ILSG ILSG field field trips trips have have visited visitedthe the Republic Republicarea. area. The Thefollowing following descriptions are taken largely from guidebooks guidebooks prepared prepared by by Cannon Cannon and Klasner Klasner (1972) and Cannon Cannon and and others others (1975). (1975). Stop Iron-formation at Stop 4-1. Negaunee Iron-formation at Kloman Klomanmine. mine. Kloman (also (also known known as Columbia) Columbia) Mine was a small, early mine that produced The Kloman about 95,000 tons of hard hard ore between between 1873 1873 and 1883. 1883. The ore body lay along the 99 �Negaunee Iron-formation Iron-formationand and Goodrich Goodrich Quartzite. Quartzite. The The pit pit is is visible visible contact with the Negaunee inside of the fenced area but is not available for examination because of safety concerns. The principal principal interest interest at this this stop stop is is the the 'arge, large, glacially glacially polished polishedoutcrop outcrop of of the the concerns. Negaunee Iron-formation Iron-formation lying just northeast northeast of the fenced fenced area. Here the Negaunee exhibits a small-scale interbedding of several different lithologic types of iron-formation. iron-formation.A A detailed detailed section section is is shown shown here here as as originally originally presented presented by by Cannon and Klasner Klasner (1972) and much much of the description description is is likewise likewise taken taken from that that Cannon earlier field guide. The outcrop outcrop is is on the northeast northeast limb limb of the the Republic Republic syncline. syncline. Beds Beds earlier N 45°W 45OW and anddip dip vertically. vertically. As As aa point pointof of reference referenceto tothe thedetailed detailedsection sectionthe the strike about N 42 feet feet on on the the section. section. The The large eye-bolt set into into the the outcrop outcrop corresponds corresponds to to the the bed bed at at 42 'arge Negaunee as seen here shows a marked lateral facies change from the Republic Republic open Whereas the the ore ore horizon horizon in in the the pit pit is is entirely entirely pit only about 3,000 feet along strike. Whereas with aa few tens tens of of feet feet of of chert-magnetite chert-magnetiteiron-formation iron-formationat at the the base, base,the the same same jaspilite, with ironchert-magnetite and and chert-magnetite-silicate chert-magnetite-silicate ironhorizon here contains many units of chert-magnetite formation interbedded interbedded with with the the jaspilite. jaspilite. correspondence between characteristics and mineralogy A close correspondence between bedding characteristics mineralogy suggests that the present mineralogy, although a product of sillimanite-grade mineralogy, product sillimanite-grade metamorphism, metamorphism, still still differences in the sediments, particularly reflects original differences particularly in oxidation state of the iron. Hematite-bearing units units are are typically typically wavy-bedded wavy-beddedand and contain contain red redor ormaroon maroonjasper, jasper, Hematite-bearing much much of which has abundant bright red red granules, possibly possibly originally oolites. This is in contrast to the magnetite-rich magnetite-richand and silicate-bearing silicate-bearingunits, units, which are are typically evenbedded. Presumably Presumably the wavy-bedded wavy-bedded hematitic rocks were deposited in relatively shallow water, above wave base, where currents were sufficiently intense to produce disturbed bedding and oolites, and the water was sufficiently sufficiently oxygenated to result in a disturbed even-bedded magnetite and silicate rocks, on the thoroughly oxidized sediment. The even-bedded other hand, are presumed presumed to have have formed formed in in deeper deeper water, below wave base, where a combination of quiet bottom bottom conditions and relatively unoxygenated unoxygenated water allowed the accumulation of ferrous ferrous iron this is is so, so, the rocks rocks here here iron minerals minerals in in uniform uniform layers. layers. IfIf this indicate indicate deposition deposition in progressively shallower shallowerwater severalfluctuations eitherinin in progressively water with with several fluctuationseither depth of water or depth of wave action action to produce produce the interlayering interlayering of lithologic types in in the transition zone between the dominantly ferrous sediments at the base of the section transition between sediments base and the ferric sediments sediments at the top of the section. section. The correspondence correspondence between between bedding bedding characteristics characteristics and mineralogy mineralogy is convincing evidence that these rocks represent represent a primary primary oxide facies facies of iron-formation iron-formationand and that the the specularite and magnetite magnetite are metamorphic metamorphic derivatives derivatives of primary primary hematite hematite and magnetite or some precursor minerals with similar oxidation states. Some minerals that are considered formed by diagenetic or early metamorphic considered primary may have formed metamorphic changes; changes; however, the the oxidation oxidation state state of of each bed must reflect the composition of bottom or interstitial interstitial waters, for adjacent beds beds are are commonly commonly of markedly markedly different different oxidation oxidation states. The relationships relationships shown in this outcrop were instrumental instrumental in the recognition recognition that some iron-formation is a primary facies of iron-formation hematitic iron-formation iron-formation (James, 1954). 1954). 100 �____ _______ feet GoodrichQuartzite Quartzite Goodrich meters . 0 unconformity EXPLANATION EXPLANATION massivespecularite specularite with with variable Hard ore: massive magnetite and amounts of magnetite and little little or or no no chert. chert. 5 :-:: yq ,--- Wavy-bedded discontinuous beds Wavy-bedded jaspillite: discontinuous beds jasper or or jasper-mantled jasper-mantled chert, much much with with of jasper interbedded with with specularite granular texture, interbedded layers with minor to moderate amounts of layers with moderate amounts magnetite. to22 inches inchesthick. thick. magnetite.Beds Beds typically typically11to 10 c 50 --H E Even-bedded chert-magnetite chert-magnetite iron-formation: iron-formation: typically in 1/2 to to 11 inch inch thick. thick. Chert Chert is is typically in beds beds 1/2 white or gray and interbedded magnetite is white interbedded magnetite is fine-grained and fine-grained andmassive. massive. 20 — - —1—— -- Even-bedded chert-magnetite-silicate chert-magnetite-silicate iron-formation: similar similar to to chert-magnetite chert-magnetite iron-formation: iron-formation iron-formationbut butwith withaaselvage selvage of of grunerite grunerite at chert-magnetite chert-magnetite contacts. contacts. 25 1±TTIT chert-silicate iron-formation: iron-formation: Even-bedded chert-silicate chert and and grunerite grunerite with with thin thin interlayered chert of magnetite magnetite within within grunerite grunerite laminae of layers layers / —, , iron-formation: grunerite Silicate-magnetite iron-formation: grunerite with magnetite; with laminae laminae and and disseminations of magnetite; non-cherty. non-cherty. 14, b - - 4 -"i.kLi><-!" covered covered Figure 4-7. Detailed stratigraphic section section across across the outcrop outcrop of Negaunee NegauneeIron-formation Iron-formation at the Kloman Kloman mine mine (from (from Cannon Cannon and and Klasner, Klasner, 1972). 1972). The rocks rocks here here were metamorphosed metamorphosedto to sillimanite sillimanite grade grade during during the the Penokean Penokeanorogeny orogeny (James, 1955), as indicated by the coarse grain size of chert and by mineral (James, 1955), as indicated by the coarse grain size of chert and by mineral assemblages assemblages in in nearby nearby pelitic pelitic and and mafic mafic rocks. rocks. Of particular particular interest interest at at this this outcrop outcrop is is the the 101 �lack of equilibration of oxidation states between between adjacent units units in spite of the intense metamorphism. metamorphism. Dominantly Dominantly hematitic units are in sharp contact with dominantly magnetitic units, and although hematitic beds are nowhere in direct contact with silicatebearing beds, they are separated by only thin beds in many places. These relationships bearing attest to the inability of metamorphic metamorphic fluids to induce induce widespread oxidation or reduction of solid phases and are a classic illustration illustration of rocks in which the chemical potential potential of oxygen during metamorphism metamorphism was buffered buffered by the solid phases. 4-2. Michigamme Michigamme Formation Formation garnet-amphibole garnet-amphibole schist. schist. Stop 4-2. roadcut on the east east side side of Highway Highway 95 95 shows shows tightly tightly folded folded schist of the Michigamme Michigamme A roadcut Formation, which here is mostly an iron-rich meta-argillite, meta-argillite, now consisting of biotitegarnet-amphibole (grunerite) (grunerite) schist containing a few inch-thick inch-thick layers layers of impure quartzite. Although the rock is in the sillimanite zone of metamorphism, metamorphism, sillimanite is not present here because of the lack of appropriately aluminous compositions. Blades and rosettes of light-colored rosettes light-colored iron-amphibole iron-amphibole are common and have been mistaken for sillimanite in field examination of this outcrop by many geologists. It is interesting interesting to note Harold James James who defined defined the Republic Republic metamorphic metamorphic node, and the sillimanite zone that Harold its core, in in his his classic classic paper paper on zones of regional regional metamorphism metamorphism (James, (James, 1955) 1955) never never at its observed sillimanite in outcrops. The only occurrence was in a glacial erratic boulder of of Michigamme-type schist, which he presumed was transported only a short distance. The Michigamme-type zone was defined more on the basis of assemblages in mafic rocks than on the distribution of sillimanite. Later, Later, detailed detailed mapping mapping of the region region (Cannon (Cannon and and Klasner, Klasner, distribution (fibrolite) in in one one outcrop outcrop of of the the Michigamme MichigammeFormation Formationseveral several 1976) did find sillimanite (fibrolite) miles northeast of this locality. An additional sillimanite occurrence was reported in drill core about three miles miles northwest northwest of of here here by by Haase Haase(1979). (1979). This outcrop outcrop is is approximately approximately on the axis of the Republic Republic syncline. Minor folds with attenuated limbs are common amplitudes much greater than wavelengths and greatly attenuated and and reflect reflect the gross gross geometry geometry of of the the Republic Republictrough, trough, which which along along the the highway highway is is estimated estimated to be be about about 5,000 feet feet deep deep by by gravity gravity models models (Klasner (Klasner and and Cannon, 1974) 1974) but is only about 3,000 feet wide. The folding is markedly non-cylindrical at outcrop scale, and domains of homogeneous homogeneous strain are very small, being measurable measurable in a few tens tens of square square feet. feet. Although Although most most minor minor folds folds plunge plunge northwest northwest in in accord accord with with the the plunge of the Republic Republic syncline, the plunge of minor folds vary from about 15 15 to 65 degrees. degrees. The The non-cylindrical non-cylindrical nature nature of the folding folding may may be be aa reflection reflectionof an an earlier earlier folding, which has has been been strongly overprinted overprinted by by the development development of the Republic Republic syncline. The style of folding roadcut folding is is especially especially well shown near the north north end of the roadcut (fig. 4-8) where a 13-inch-thick quartzite bed bed is is repeated repeated many many times by folds. Axes 1- to 3-inch-thick of adjacent folds, only a few inches apart, have plunges that diverge by as much as 60 degrees. degrees. At the next next stop stop (stop (stop 4-3) 4-3) we will see a major major contrast contrast between between the high high degree degree of structures seen here and their near absence in nearby Archean penetrative Penokean Penokean structures basement rocks. basement rocks. 102 �Figure 4-8. Vertical face about 1 1 meter high high showing tight folds in Michigamme Michigamme biotitie-garnet Formation at stop 4-2. Folds are shown by a thin quartzite layer (light) in biotitie-garnet schist. Note Note thickened thickened hinge hinge regions regions and and attenuated attenuated limbs. limbs. Fold Fold at right right is is completely completely dismembered, a geometry geometry reflecting reflecting the the Republic Republic syncline syncline itself. itself. 4-3. Archean Archean gneiss gneiss and younger younger dikes. Stop 4-3. This roadcut roadcut on the west side of Highway Highway 95 near near the intersection intersection with Old Old Highway Highway 95 shows a widespread variety of granitic gneiss, which makes makes up part of the southern southern complex, complex, the basement basement on on which Paleoproterozoic Paleoproterozoic strata strata were deposited. The most most abundant abundant rock type in in the southern southern complex complex is is aa coarsely coarsely megacrystic megacrystic granite, granite, named named the Bell Bell Creek Creek Gneiss Gneiss by Cannon Cannon and and Simmons Simmons (1973). (1973). Rock Rock typical typical of this unit unit is is exposed roadcut. The The rocks rocks near near the the center center exposed in the northern northern and and southern southern parts parts of this this roadcut. are finer-grained finer-grained and and somewhat sheared sheared granite. granite. The The relationships relationships in in the southern southern part part of the outcrop outcrop are are sketched sketched in in figure figure 4-9 4-9 and and illustrate illustratetwo two major major periods periods of of orogeny orogeny that that affected Archean rocks; rocks; the the older older being beingan an intense intense Late Late Archean Archean folding folding and and the the affected the Archean younger being being non-penetrative non-penetrative deformation deformation during during the Penokean Penokean orogeny. orogeny. The The oldest oldest rock is the granitic gneiss, which was deformed and metamorphosed metamorphosed in the Archean event. Northwest-trending Northwest-trendingplanar planar structures structures are are shown shown mostly mostly by by preferred preferred orientation orientation of large microcline megacrysts and, in places, by a subtle compositional layering expressed expressed as variations in in abundance abundance of the megacrysts. megacrysts. A northeast-trending northeast-trending truncates the northwest-trending porphyritic dike of metadiabase truncates northwest-trending structures. The dike trends roadcut exaggerating exaggerating its its true true thickness. thickness. Typically, Typically, such such dikes dikes trends subparallel to the roadcut are no no more more than a few tens tens of feet feet thick. thick. This This dike dike truncates truncatesthe the granite granite foliation, foliation, but butisis metamorphosed to a grade typical of this part of the Republic massive, although metamorphosed Republic 103 �metamorphic metamorphic node. Thus, it must must have have been been emplaced emplaced after after the Archean deformation deformation but before the Penokean Penokean metamorphism. metamorphism. Its Its massive massive nature, nature, including including preserved preserved diabasic texture in thin section, and its straight trace indicates indicates that itit was not not deformed deformed during the Penokean Penokean orogeny, even though its its emplacement emplacement must must predate predate the orogeny. orogeny. Such dikes are very common throughout the southern southern complex complex and consistently show these same relationships of retaining retaining planar planar dike dike geometry geometry and relict diabasic textures. Deformation, Deformation, presumably presumably of Penokean Penokean age, is is seen seen in in some dikes as sheared sheared margins, margins, but even in those cases, the planar dike geometry geometry is not not altered. Good Good examples examples of this can be seen in other roadcuts roadcuts north north of here here heading heading toward Humboldt. Humboldt. The occurrence occurrence of such planar metadiabse metadiabse dikes at literally literally hundreds hundreds of localities throughout the southern complex is prime evidence that the complex was not penetratively deformed basement during the Penokean Penokean orogeny. Rather, Penokean Penokean deformation deformation of the Archean basement appears to have been been accomplished by relative movement between discrete, rigid, faultbounded blocks. This indicates indicates that the intense intense deformation deformation of Paleoproterozic Paleoproterozic strata, does not not extend extend into into the basement. The The fault blocks blocks of such as just seen at stop 4-2, does Archean rocks rocks appear to have have provided a rigid rigid form around around which the Paleoproterozoic Paleoproterozoic strata were molded. In In the case of the Republic Republic trough, the structure with respect respect to Archean rocks is a deep, narrow narrow graben graben between between two high-angle high-angle reverse faults. The syncline developed in the Paleoproterozoic rocks as a result of their compression between the two bounding fault uplifts. Estimates Estimates of metamorphic metamorphic pressures pressures by Hasse Hasse (1979) (1979) and Attoh and Klasner Klasner (1989) (1989) indicate indicate that the Republic Republic area was buried buried beneath kilometers of strata during Penokean roughly eight kilometers Penokean deformation and was heated to 550' to 600° metamorphic conditions, conditions, which slightly postdated postdated temperatures temperatures of 5500 600°C at peak metamorphic the major deformation. Under Under those conditions the Archean granitic gneisses gneisses apparently apparently did not deform plastically, but rather retained retained considerable strength strength so that the shape shape of fault-bounded basement the individual fault-bounded basement blocks blockscontrolled controlledthe the geometry geometryof of structures structuresin inthe the overlying overlying strata. strata. roadcut is is a Mesoproterozoic Mesoproterozoic diabase dike related related to the A final feature in this roadcut Midcontinent rift and part of the Baraga dike swarm of reversed magnetic polarity dikes intruded has a chilled chilled contact against both the granitic intruded at roughly roughly 1.1 Ma. The dike has gneiss and and metadiabase metadiabase dike virtually unmetamorphosed. unmetamorphosed. dike and and is is virtually 104 �or 0 0 0 '0 0 ip 10 feet feet 9O M Figure Figure 4-9. Sketch Sketch map map of southern southern part part of of roadcut roadcut showing showing relationships relationshipsbetween between Archean granitic gneiss and diabase dikes of two different ages. Archean granitic gneiss and diabase dikes of two different ages. Ref erences References Anderson, JJ G, G, 1968, 1968, The The Marquette Marquettedistrict, district, in inRidge, Ridge,J. J. D. D.ed., ed., Ore Oredeposits depositsof of the the United United States States 1933-1967 1933-1967 (Graton-Sales (Graton-SalesVol.): Vol.): New New York, York, American American Institute Instituteof of Mining, Mining, Metallurgical, Metallurgical, and Petroleum Petroleum Engineers, Engineers, v. 1, p.508-517. p. 508-517. K, and and Klasner, Klasner, J.S., J.S., 1989, 1989,Tectonic Tectonicimplications implicationsof of metamorphism metamorphismand andgravity gravity Attoh, K, field in in the the Penokean Penokean orogen orogen of of northern northernMichigan: Michigan:Tectonics, Tectonics,v. v. 8, 8, p. p. 911-933. 91 1-933. field Brooks, T.B., 1873, 1873, Iron-bearing Iron-bearingrocks rocks(economic): (economic):Michigan MichiganGeological GeologicalSurvey, Survey,Upper Upper Brooks, Peninsula, Peninsula, v. v. 1, 1,pt. pt. 1, 1,319 319p. p. Cannon, inYoung, Young, G.M., G.M., ed., ed., Cannon, 1973, 1973, The The Penokean Penokean orogeny orogenyin in northern northernMichigan, Michigan, in Huronian stratigraphy stratigraphy and and sedimentation: sedimentation:Geological GeologicalAssociation Associationof of Canada Canada Special Special Huronian Paper Paper 12, 12, p.251-271. p. 251-271. Cannon, Cannon, W.F., 1975, 1975,Bedrock Bedrockgeologic geologic map mapof ofthe theRepublic Republicquadrangle, quadrangle,Marquette Marquette County, Michigan: Michigan: U.S. U.S. Geological Geological Survey Survey Miscellaneous Miscellaneous Investigations InvestigationsSeries Series map map I-I862, 862, scale scale 1:24,000. 1:24,000. 105 �1976, Hard Hard iron iron ore ore of of the the Marquette MarquetteRange, Range, Michigan: Michigan: Economic Economic Cannon, W.F., 1976, Geology, v. 71, P. p. 1012-1028. 1012-1028. and Klasner, Klasner, J.S., 1972, 1972, Guide Guide to to Penokean Penokean deformational deformationalstyle style and and Cannon, W.F., and regional metamorphism metamorphismof the the western western Marquette MarquetteRange, Range,Michigan: Michigan:Proceedings Proceedingsofof18th 18th Institute on Lake Lake Superior Superior Geology, v. 18, 18, p. B1-B38. B1-B38. Annual Institute and Kiasner, Klasner, J.S., 1976, 1976, Geologic Geologic map map and and geophysical geophysical interpretation interpretationof of Cannon, W.F., and the Witch Witch Lake Lake quadrangle. quadrangle. Marquette, Marquette, Iron, Iron, and and Baraga BaragaCounties, Counties, Michigan: Michigan: U.S. U.S. Geological Survey Miscellaneous 1:62,500. Geological Miscellaneous Investigation InvestigationSeries Series Map Map 1-987, 1-987, Scale 1 :62,500. Cannon, W.F., and and Simmons, Simmons, G. G. C., C., 1973, 1973, Geology Geology of of part part of of the the southern southerncomplex, complex, Marquette district, Michigan: Journal of Research of the U.S. Geological Geological Survey, v. 1, p. 165-172. 165-1 72. Gair, J.E., Kiasner, Klasner, J.S., J.S., and and Boyum, Boyum, B.H., B.H., 1975, 1975, Marquette MarquetteIron IronRange: Range: Cannon, W.F., Gair, Proceedings of 21St Proceedings 21'' Annual Institute on Lake Superior Geology, v. 21,p. p.125-1 125-174. v.21, 74. Crump, R.M., 1948, Ph.D. 1948, Origin Origin of hard hard iron iron ores ores of the the Marquette Marquette district: district: unpublished unpublishedPh.D. dissertation, University of Wisconsin-Madison, Madison, Wisconsin, 87 p. dissertation, University Madison, 87 J.W., and Whitney, J.D., 1851, Foster, J.W., 1851, Report Report on on the the geology geology of of the the Lake Lake Superior Superior land land district, part 2, the iron region, together with the general geology: geology: U.S. U.S. 32nd 32'' Congress, Congress, Special Session, Session, Senate Senate Executive Executive Document, Document,v. v. 3, 3, no. no.4, 4, 406 406p. p. Gair, J.E., J.E., 1975, 1975, Bedrock geology and ore deposits deposits of the Palmer quadrangle, Marquette County, Michigan: U.S. Geological Geological Survey Survey Professional ProfessionalPaper Paper769, 769, 159 159p. p. C.S., 1979, Metamorphic petrology of the Negaunee Iron-formation, Haase, C.S., Iron-formation, Marquette Marquette district, northern northern Michigan: Michigan: Ph. Ph. D. D. Dissertation, Dissertation, Indiana Indiana University, University, 246 p. Hoffman, M.A., M.A., 1987, The southern complex: geology, geochemistry, mineralogy mineralogy and mineral chemistry of selected uranium- and thorium-rich granites: unpub. Ph. D. dissertation, dissertation, Michigan Michigan Technological Technological University, Houghton, Houghton, Michigan, 382 p. James, H.L., 1954, 1954, Sedimentry Sedimentry facies facies of iron-formation: iron-formation:Economic Economic Geology, Geology, v. v. 49, 49, p. p. 235-293. 235-293. James, H.L, H.L., 1955, 1955,Zones Zonesof of regional regionalmetamorphism metamorphismin in the the Precambrian Precambrianof of northern northern Michigan: Geological Geological Society Society of America America Bulletin, Bulletin, v. v. 66, 66, p.1455-1488. p.1455-1488. Kiasner, J.S. and Cannon, W.F., W.F., 1974, Klasner, J.S. 1974, Geologic interpretation interpretation of gravity gravity profiles profiles in in the western Marquette Marquette district, district, northern northern Michigan: Michigan: Geological Geological Society Society of America Bulletin, v. 8. 85, p. p. 213-21 213-218. Marsden, R.W., 1968, 1968, Geology Geology of the iron iron ores ores of the the Lake Lake Superior Superior region region in in the the united united in Ridge, J.D., ed., ed., Ore Ore deposits depositsof of the theUnited UnitedStates States1933-1 1933-1967 967 (Graton-Sales (Graton-Sales States, in Vol.): New York, American Institute Vol.): Institute of Mining, Metallurgical Metallurgical and Petroleum Petroleum Engineers, v. 1, p 489-507. 489-507. 106 �M.W., Roberts, H.M., and Bartley, M.W ., 1943, 1943, Hydrothermal replacement in deep seated iron ore deposits deposits of the Lake Lake Superior region: region: Economic Economic Geology, v. 38, p.1-24. p.1-24. Swineford, A.P., 1871, 1871, Swineford's Swineford's history historyof of the the Lake LakeSuperior Superioriron irondistrict-its district-itsmines minesand and furnaces: Marquette, Michigan, Marquette Marquette Mining Mining Journal, 98 p. Taylor, W.E.G., 1967, 1967, The The geology geology of of the the lower lower Precambrian Precambrianrocks rocks of of the the ChampionChampionRepublic area of upper upper Michigan: Michigan: Northwestern NorthwesternUniversity University Report Report 13, 13, 33 33 p. p. Van Hise, Hise, C.R., and and Bayley, Bayley, W.S., W.S., 1897, 1897,The TheMarquette Marquetteiron-bearing iron-bearingdistrict districtof ofMichigan: Michigan: Geological Survey Monograph U.S. Geological Monograph 28, 608 608 p. p. Van Hise, C.R., and and Leith Leith C.K., 1911, 1911, The geology of the Lake Lake Superior Superior region: region: U.S. U.S. Geological Survey Monograph Geological Monograph 52, 641 p. p. 107 ������Proceedings Volume 49 PARTI - PROGRAMS AND ABSTRACTS �INSTITUTE ON LAKE LAKE SUPERIOR GEOLOGY GEOLOGY 49TH ANNUAL MEETING MAY 7-11, 2003 IRON MOUNTAIN, MICHIGAN HOSTED BY: BY: LAUREL G. G. WOODRUFF AND WILLIAM WILLIAM F. LAUREL WOODRUFF F. CANNON CANNON Co-Chairs Co-Chairs U.S. GEOLOGICAL SURVEY U.S. GEOLOGICAL SURVEY With assistance Technological University assistance from Michigan Michigan Technological University and John John Gartner, Gartner, Coleman Coleman Engineering Engineering Company Company Volume Volume 49 49 —Proceedings Proceedingsand and Abstracts Abstracts Part 1 edited by Compiled and edited by Laurel Laurel Woodruff, Woodruff, U.S. U.S. Geological Geological Survey Survey and and Theodore Bornhorst, Bornhorst, Michigan Michigan Technological Technological University University Cover Photo: Cover Photo:Berkshire BerkshireShaft, Shaft,Menominee Menominee Range, Range, Michigan. Michigan. Photo from the Michigan Technological Technological University University Mining Mining Engineering EngineeringDepartment DepartmentCollection. Collection. �49TH INSTITUTE ON LAKE SUPERIOR GEOLOGY 4gTH GEOLOGY VOLUME VOLUME49 49CONSISTS CONSISTS OF: OF: PART 1 1::PROGRAM AND PART PROGRAM AND ABSTRACTS ABSTRACTS PART 2: 2: FIELD TRIP PART FIELD TRIPGUIDEBOOK GUIDEBOOK OVERVIEW: PALEOZOIC PALEOZOICSTRATIGRAPHY STRATIGRAPHYAND AND TECTONICS TECTONICS ALONG THE NIAGRA SUTURE ZONE, ZONE, MICHIGAN AND WISCONSIN 1: PEMBINE-WAUSAU PEMBINE-WAUSAUMAGMATIC MAGMATICTERRANE TERRANE TRIP 1: MENOMINEEIRON IRONDISTRICT DISTRICT TRIP 2: MENOMINEE TRIP TRIP 3: 3:STRATRIGRAPHY STRATRIGRAPHYAND ANDSTRUCTURE STRUCTUREOF OFTHE THEIRON IRONRIVER RIVER—CRYSTAL CRYSTAL FALLS FALLS BASIN BASIN TRIP TRIP 4: 4: LIFE LIFECYCLE CYCLEOF OFAN ANIRON IRONDEPOST DEPOST— - THE THE REPUBLIC REPUBLIC MINE MINE RESTORATION FROM ORE GENESIS TO MINE RESTORATION to material in Part 1 should should follow follow the Reference to the example example below: below: Rogala, B., Fralick, Fralick, P., and Borradaile, G., 2003, A magnetostratigraphic magnetostratigraphicand and secular secular variation varittion 49th study of the Sibley Sibley Group Group [abstract]; [abstract]; Institute Institute on Lake Lake Superior Geology Geology Proceedings, Proceedings, 49 Annual Annual Meeting, Meeting, Iron Iron Mountain, Mountain, Ml, MI, v. v. 49, 49, part part 1, 1, p. p. 65-66. 65-66. 49th Published by the 4gth Institute on Lake Superior Geology and distributed distributed by Published Institute by the Secretary-Treasurer: ILSG Secretary-Treasurer: Mark Jirsa (through (through 2003) 2003) Minnesota Minnesota Geological Geological Survey 2642 University University Avenue Avenue St. Paul, MN 551 55114-1057 14-1057 USA USA @tc.umn.edu JirsaOOl @tc.umn.edu In 2004 contact: contact: Peter Hollings Hollings Lakehead University University Department Department of Geology Geology Thunder Bay, ON P7B P7B5E1 5E1 CANADA peter.hollinas@ lakeheadu.ca Deter.hollincl@lakeheadu.ca I LSGwebsite: website: httr://www.ilscjeolociy.orci htt~://www.ilsaeoloav.org ILSG ISSN 1042-9964 1042-9964 �CONTENTS CONTENTS PROCEEDINGS VOLUME VOLUME 49 PART 1—PROGRAM I-PROGRAM AND AND ABSTRACTS ABSTRACTS Institutes Institutes on Lake Lake Superior Superior Geology, 1955-2003 1955-2003............................................................ iv Constitution of the Institute Constitution Institute on on Lake Lake Superior Superior Geology Geology ...................................................vi vi By-Laws vii By-Laws of the Institute Institute on Lake Lake Superior Geology Geology ....................................................... ... Membership Criteria Criteria ...................................................................................................... VIII viii ix Goldich Medal Medal Guidelines Guidelines............................................................................................... ix Goldich Medal Committee ............................................................................................... xx Past Goldich Medallists Medallists .................................................................................................. xi Citation xii Citation for 2003 2003 Goldich Goldich Medal Medal Recipient Recipient ..................................................................... xii Eisenbrey Student Eisenbrey Student Travel Travel Awards Awards ................................................................................. xiv Student Student Travel Award Application Application Form Form ....................................................................... xiiv Student Student Paper Paper Awards Awards ................................................................................................... xv Student xv Student Paper Paper Awards Committee Committee ................................................................................ Session Session Chairs Chairs .............................................................................................................. xv Board xvi Board of Directors Directors ........................................................................................................ xvi Local xvi xvi Local Committees Committees......................................................................................................... Banquet xvi Banquet Speaker Speaker.......................................................................................................... xvi Report Report of the Chair of the the 48th 48th Annual Annual Meeting Meeting............................................................ xvii Program xxi xxi Program ....................................................................................................................... List of Contributors xxii Contributors ....................................................................................................... ... Abstracts ................................................................................................................... xxviii Abstracts XXVIII III iii �INSTITUTES LAKE SUPERIOR GEOLOGY INSTITUTES ON LAKE SUPERIOR GEOLOGY # YEAR PLACE PLACE CHAIRS CHAIRS 1 1955 Minneapolis, Minnesota Minnesota C.E. Dutton Dutton 2 1956 Houghton, Michigan Houghton, Michigan A.K. Snelgrove Snelgrove 3 1957 East Lansing, Michigan Michigan B.T. Sandefur 4 1958 Duluth, Minnesota Minnesota R.W. Marsden Marsden 5 1959 Minneapolis, Minnesota G.M. Schwartz & C. Craddock Craddock 6 1960 Wisconsin Madison, Wisconsin Cameron E.N. Cameron 7 1961 Port Arthur, Ontario Ontario E.G. Pye Pye 8 1962 Houghton, Michigan Michigan A.K. Snelgrove A.K. Snelgrove 9 1963 Duluth, Minnesota Minnesota H. Lepp Lepp 10 1964 lshpeming, Michigan Ishpeming, Michigan A.T. Broderick Broderick 11 1965 St. Paul, Paul, Minnesota Minnesota P.K. Sims & R.K. Hogberg Hogberg 12 1966 Sault Ste. Marie, Marie, Michigan Michigan R.W. White 13 1967 Lansing, Michigan East Lansing, Michigan W.J. Hinze Hinze 14 1968 Superior, Superior, Wisconsin Wisconsin A.B. Dickas A.B. Dickas 15 1969 Oshkosh, Wisconsin Wisconsin LaBerge G.L. LaBerge 16 1970 Thunder Bay, Ontario Ontario M.W. Bartley E. Mercy Mercy Bartley & E. 17 1971 Duluth, Minnesota Minnesota D.M. Davidson Davidson 18 1972 Houghton, Houghton, Michigan Michigan J. Kalliokoski Kalliokoski 19 1973 Madison, Wisconsin Wisconsin M.E. Ostrom Ostrom 20 1974 Sault Ste. Marie, Ontario P.E. Giblin Giblin 21 1975 Marquette, Marquette, Michigan Michigan J.D. Hughes Hughes 22 1976 St. Paul, Minnesota Minnesota M. Walton 23 1977 Thunder Bay, Ontario Ontario M.M. Kehlenbeck Kehlenbeck 24 1978 Milwaukee, Wisconsin Wisconsin G. Mursky Mursky 25 1979 Duluth, Duluth, Minnesota Minnesota D.M. Davidson Davidson 26 1980 Eau Claire, Wisconsin Wisconsin P.E. Myers Myers 27 1981 East Lansing, Michigan Michigan W.C. Cambray Cambray 28 1982 International International Falls, Falls, Minnesota Minnesota Southwick D.L. Southwick 29 1983 Houghton, Houghton, Michigan Michigan T.J. Bornhorst Bornhorst iv �30 1984 Wausau, Wausau, Wisconsin Wisconsin G.L. LaBerge LaBerge 31 1985 Kenora, Ontario Ontario G.E. C.E. Blackburn Blackburn 32 1986 Wisconsin Wisconsin Rapids, Rapids, Wisconsin Wisconsin J.K. Greenberg Greenberg 33 1987 Wawa, Wawa,Ontario Ontario 34 1988 Marquette, Michigan Marquette, Michigan J. S. Klasner Klasner 35 1989 Duluth, Minnesota Minnesota J.C. Green Green 36 1990 Thunder Bay, Bay, Ontario Ontario M.M. Kehlenbeck Kehlenbeck 37 1991 Wisconsin Eau Claire, Wisconsin P.E. Myers Myers 38 1992 Hurley, Wisconsin A.B. Dickas A.B. Dickas 39 1993 Eveleth, Minnesota Minnesota D.L. Southwick Southwick 40 1994 Houghton, Michigan Michigan T.J. Bornhorst Bornhorst 41 1995 Marathon, Ontario Ontario M.C. Smyk 42 1996 Cable, Wisconsin Wisconsin L.G. Woodruff 43 1997 Sudbury, Ontario Ontario R.P. Sage & W. Meyer Meyer 44 1998 Minneapolis, Minneapolis, Minnesota Minnesota J.D. Miller Miller & M.A. M.A. Jirsa Jirsa 45 1999 Marquette, Marquette, Michigan Michigan T.J. Bornhorst R.S. Regis Regis Bornhorst & R.S. 46 2000 Thunder Bay, Ontario Ontario S.A. Kissin & P. Fralick Fralick 47 2001 Madison, Wisconsin Wisconsin B.A. Brown Brown M.G. Mudrey, Jr. & B.A. 48 2002 Kenora, Ontario Ontario P. Hinz & R.C. Beard Beard 49 2003 Iron Iron Mountain, Mountain, Michigan Michigan L.G. Woodruff & W.F. Cannon Cannon E.D. Frey & R.P. Sage E.D. V �__________(some CONSTITUTION SUPERIOR GEOLOGY CONSTITUTION OF THE INSTITUTE ON ON LAKE LAKE SUPERIOR GEOLOGY (Last 1997) (Last amended amended by by the the Board—May Board-May 8, 8,1997) Article Article II Article Article IIII Article Ill Article III Name Name The name name of the the organization organization shall shall be be the the "Institute "Instituteon on Lake Lake Superior Geology". Geology". Objectives Objectives The objectives objectives of this this organization organization are: are: A. To Toprovide provideaameans meanswhereby wherebygeologists geologistsin inthe the Great GreatLakes Lakesregion regionmay may exchange exchange ideas ideas and and scientific scientificdata. data. B. To Topromote promotebetter betterunderstanding understandingof of the the geology geology of of the the Lake LakeSuperior Superior region. region. C. To plan and conduct geological field trips. To plan and conduct geological field trips. Status Status No part of the income income of the organization organization shall insure insure to the the benefit benefit of of any any member or individual. individual. In the event of dissolution, dissolution, the assets assets of the the organization organization distributed to shall be distributed (some tax free free organization). organization). (To avoid avoid Federal Federal and State State income income taxes, the organization organization should should be be not not only only "scientific" 'scientific" or "educational, "educational, but but also also "non-profit") "non-profit") Article Article IV IV Article V Article Article VI Article VI Article VII Article VII Article Article VIII VIII Minn. Stat. Anno. 290.01, subd. 44 Minn. Stat. Anno. 290.05(9) 1954 Internal 1954 Internal Revenue RevenueCode Codes.501 s.501(c)(3) (c)(3) Membership Membership The membership membership of the the organization organization shall shall consist consist of of persons personswho who have have registered for an annual meeting meeting within the past three years, and and those those who who indicate interest in being a member according to guidelines guidelines approved approved by by the Board of Directors. Directors. Meetings Meetings The organization organization shall meet once a year. The The place placeand andexact exact date dateof of each each meeting meeting will be designated designated by the Board Board of Directors. Directors. Directors Directors The Board Board of Directors Directors shall shall consist consist of the the Chair, Chair, Secretary-Treasurer, Secretary-Treasurer,and andthe the last three past past Chairs; Chairs; but but if the the board board should should at at any any time time consist consist of of fewer fewer than than five persons, persons, by reason reason of unwillingness unwillingness or inability inability of any any of of the the above above persons persons to serve as directors, directors, the vacancies on the board board may may be be filled filled by by the the Chair Chair so so as as to bring bring the membership membership of the the board board to to five five members. members. Officers Officers The officers of this organization organization shall shall be be aa Chair Chair and and Secretary-Treasurer. Secretary-Treasurer. A. The TheChair Chairshall shallbe beelected electedeach each year year by by the the Board Board of of Directors, Directors, who who shall shall give due consideration consideration to the wishes of any group group that may may be be promoting promoting the the next annual meeting. His/her Hislher term term of of office officeas as Chair Chair will will terminate terminate at at the the close close of of helshe presides, presides, or when his/her hislher successor successor shall shall the annual meeting meeting over which he/she have been appointed. He/she Helshe will will then then serve serve for aa period period of of three three years years as as aa member of the Board Board of Directors. Directors. B. The TheSecretary-Treasurer Secretary-Treasurershall shall be elected at the annual meeting. His/her Hislher term of office shall shall be be four years, or until until his/her hislher successor successor shall shall have have been been appointed. appointed. Amendments Amendments This constitution may be amended amended by a majority majority vote (majority (majority of of those those voting) voting) of of the membership membershipof of the the organization. organization. vi �BY-LAWS OFTHE THE INSTITUTE INSTITUTE ON LAKE LAKESUPERIOR SUPERIOR GEOLOGY BY-LAWS OF GEOLOGY I. Duties I. Duties of the Officers Officers and Directors Directors A. A. It shall be the duty of the Annual Chairman to: 1. Preside Presideat at the theannual annualmeeting. meeting. Appointall allcommittees committeesneeded neededfor for the theorganization organizationof of the theannual annualmeeting. meeting. 2. Appoint 3. Assume Assumecomplete completeresponsibility responsibilityfor for the theorganization organizationand andfinancing financing of of the the annual annual meeting meeting over over which which he/she helshepresides. presides. B. It shall be the duty of the Secretary-Treasurer B. Secretary-Treasurer to: 1. Keep Keepaccurate accurateattendance attendancerecords recordsof of all all annual annual meetings. meetings. 2. Keep accurate records of all meetings of, and correspondence Keep accurate records of all meetings of, and correspondencebetween, between,the the Board of Directors. Directors. 3. Hold Holdall allfunds fundsthat thatmay mayaccrue accrueas asprofits profitsfrom fromannual annualmeetings meetingsor or field fieldtrips trips and to make make these these funds funds available available for the the organization organization and and operation operationof of future meetings meetings as as required. required. C. C. It shall be the duty of the Board Board of Directors Directors to plan plan locations locations of of annual annual meetings meetings and and to advise on the organization organization and and financing financing of of all all meetings. meetings. II. Duties Duties and and Exrenses Expenses A. Regular Regularmembership membershipdues duesof of $5.00 $5.00 or or less lesson on an an annual annual basis basis shall shall be be assessed each member member as determined determined by by the Board Board of Directors.. Directors.. B. Registration be determined Registrationfees feesfor forthe theannual annualmeetings meetingsshall shallbe determinedby by the the Chair Chair in in of Directors. The consultation with the Board of The registration registrationfees fees can can include include expenses to cover operations operations outside of the annual annual meeting meeting as as determined determinedby by the Board Board of Directors. Directors. It is strongly recommended recommended that registration registration fees fees be be kept at a minimum minimum to encourage encourage attendance attendance of students. students. III. Ill. Rules Rules of Order Order The rules contained contained in Robert's Robert's Rules Rules of Order Order shall shall govern govern this this organization organizationin in all all cases to which they they are are applicable. applicable. IV. Amendments Amendments amended by a majority These by-laws by-laws may be amended majority vote (majority (majority of those those voting) voting) of of the the membership of the organization; membership organization; provided provided that such modifications modifications shall shall not not conflict conflict with the constitution constitution as as presently presently adopted adopted or or subsequently subsequentlyamended. amended. Last Last Amended Amended— - May, 1996 1996 vii vii �MEMBERSHIP CRITERIA FOR MEMBERSHIP CRITERIA FOR THE THE INSTITUTE LAKE SUPERIOR GEOLOGY INSTITUTE ON LAKE SUPERIOR GEOLOGY Approved May May 8, 8, 1997 1997 A. Membership Membershipininthe theInstitute Instituteon onLake LakeSuperior SuperiorGeology Geologyrequires requireseither either participation participationin in on aa regular regular basis basis of of interest interest in in the the Institute. Institute. Those Institute activities, or an indication on Those individuals years unless: unless: individuals registering registering for an an annual annual meeting meeting will remain remain as as members membersfor for 44 years 1) they indicate no further interest in the Institute Institute by by responding responding negatively negatively to to the the 2) two two statement on meeting meeting circulars circulars "Remove "Removemy my name name from from the the mailing mailinglist"; list";or or 2) statement successive mailings mailings in different different years are are returned returned by by the the postal postal service service as as address address successive unknown. unknown. Those individuals individuals who have have not not registered registered for for an an annual annual meeting meetingin in the the past past44 years years B. Those must indicate indicate an interest interest in in the the Institute Institute by by postal, postal, electronic, electronic, or or verbal verbal correspondence correspondence Secretary-Treasurer at least once every two two years. years. Such with the Secretary-Treasurer Such individuals individuals will be removed from the membership membership ifif they they indicate indicate no no further further interest interest in in the the Institute Instituteor or two two successive mailing mailing in different different years are are returned returned by by the the postal postal service service as as address address successive unknown. unknown. C. The TheSecretary-Treasurer Secretary-Treasurerwill will maintain maintain a list of current members. The The list list will will include include of returned mail, dates of of last the date of the beginning of continuous membership, dates of contact (expression (expression of interest), interest), and and the the date date membership membership expires, expires, barring barringaa change changeof of status initiated by the member. Those individuals who have become members of ILSO Those individuals who have become members ILSG by by Section B will have an expiration date date listed listed at at 22 years years from from the the upcoming upcoming meeting. meeting. For For example, a member who expresses interest interest in September of 1997 1997 (the (the next next annual annual meeting is May, 1998) will have an expiration date of May, 2000, unless unless the member member contacts the Secretary-Treasurer Secretary-Treasurer or attends attends an an annual annual meeting. meeting. D. "Member "Memberfor for Life" Life"status statusisisgranted grantedto to individuals individualswho who have have been been (nearly) (nearly) continuous continuous 15 years, years, Goldich Goldich Medal Medal recipients, recipients, or or those those who who participants of the ILSG ILSG meetings meetings for 15 have served as meeting chairs. This This status status will will be be further further maintained maintained unless unless the the mailings in in different different years years are are individuals indicate no further interest interest in the Institute, Institute, or 4 mailings returned by the postal service as address unknown, unknown, or they are deceased. deceased. E. All All members memberswill will be be mailed mailedthe the First First Circular Circular for for the the Annual Meeting Meeting and and the ILSG ILSG Newsletter. The The Chair Chair of of the the annual annualmeeting meetingmay may opt opt to to send send the the first first circular circular to to additional individuals. All All returned returned mail mail should should be be reported reported to the Secretary-Treasurer. Secretary-Treasurer. F. The TheSecretary-Treasurer Secretary-Treasurercan candesignate designateany any individual individual who is is on the ILSG ILSG membership list (mailing list) as of January January 1, 1, 1997 1997 as a member for life life based based on on participation participationin in ILSG ILSG activities. activities. G. Members Membersare are strongly stronglyencouraged encouragedto to send send address address corrections corrections to to the the SecretarySecretaryTreasurer Treasurer to avoid avoid unintentional unintentional lapse lapse of of membership. membership. VIII viii �GOLDICH MEDAL GUIDELINES (Adopted by the 1981; amended 1999) 1999) the Board of of Directors, 1981; Preamble Preamble Institute on on Lake Lake Superior Superior Geology Geology was was born born in in 1955, 1955, as as documented documentedby bythe the fact fact that thatthe the27th 27th The Institute annual meeting was held in 1981. The TheInstitute's Institute'scontinuing continuingobjectives objectives are are to to deal deal with with those those aspects of geology geology that are are related related geographically geographicallyto Lake Lake Superior; Superior; to to encourage encourage the the discussion discussionof of aspects subjects and sponsoring field trips that will bring together together geologists geologists from from academia, academia, government and industry; industry; and and to maintain maintain an informal informal but but highly highly effective effective mode modeof of operation. operation. surveys, and During the course of its existence, existence, the membership membership of the the Institute Institute (that (that is, is, those thosegeologists geologistswho who indicate an interest interest in in the objectives objectives of the the ILSG ILSG by by attending) has has become become aware awareof of the the fact fact that that indicate colleagues have have made made particularly particularly noteworthy noteworthyand and meritorious meritorious contributions contributionsto to the the certain of their colleagues understanding understanding of Lake Lake Superior Superior geology geology and and mineral mineral deposits. deposits. The first award award was made made by by ILSG ILSG to to Sam Sam Goldich Goldich in in 1979 1979for for his his many manycontributions contributionsto tothe thegeology geology of the region extending over about 50 years. years. Subsequent Subsequent medallists medallists and and this year's recipient recipient are listed listed in the table table below. below. Guidelines Award Guidelines 1) The The medal medalshall shall be beawarded awardedannually annually by by the ILSG Board of Directors to a geologist whose name is associated with a substantial interest in, and contribution contribution to, the geology of the Lake Lake Superior region. region. 2) The TheBoard Boardof of Directors Directorsshall shall appoint appoint the Goldich Medal Committee. The The initial initial appointment appointment will for two two years, years, and and one one for for one one year. year. The be of three members, one to serve for three years, one for The briefest incumbency shall be chair chair of of the the Nominating Committee. Committee. After member with the briefest After the the first first year, the Board Board of Directors Directors shall shall appoint at each each spring spring meeting meeting one one new new member memberwho who will will serve serve years. In the chair. chair. The Committee for three years. In his/her hislher third third year this member shall be the Committee membership membership geographic distribution distribution of of ILSG membership. membership. The should reflect the main fields of interest and geographic The outoutgoing, senior member member of the the Board Board of of Directors Directorsshall shall act act as as liaison liaisonbetween betweenthe the Board Boardand andthe the Committee for a period Committee period of one one year. 3) By Bythe theend endof of November, November,the the Goldich GoldichMedal Medal Committee Committee shall shall make make its its recommendation recommendationto to the the Chair of the Board Board of Directors, Directors, who will then inform inform the the Board Board of the the nominee. nominee. 4) The TheBoard Boardof of Directors Directorsnormally normallywill will accept accept the the nominee nominee of of the the Committee, Committee, inform informthe the medallist, medallist, medal engraved and have one medal engraved appropriately appropriately for presentation presentation at the next next meeting meetingof of the the Institute. Institute. recommended that the Institute 5) It is recommended Institute set aside aside annually from whatever sources, sources, such such funds funds as as will will required to support the continuing be required continuing costs of this award. award. Nominating Procedures Nominatina Procedures 1) The The deadline deadline for nominations nominations is November 1. The The Goldich Goldich Medal Medal Committee Committee shall shall take time. Committee nominations at any time. Committeemembers membersmay maythemselves themselves nominate nominatecandidates; candidates; however, however, Board members may may not not solicit for or or support support individual individual nominees. nominees. 2) Nominations Nominationsmust mustbe beininwriting writingand andsupported supportedby byappropriate appropriatedocumentation documentationsuch such as as letters letters of of recommendation, lists recommendation, lists of publications, publications, curriculum curriculum vita's, vita's, and and evidence evidenceof of contributions contributionsto to Lake Lake Superior geology and and to the the Institute. Institute. 3) Nominations Nominationsare arenot notrestricted restrictedto to Institute Instituteattendees, attendees, but but are are open open to anyone anyone who has worked on and and contributed contributedto the the understanding understandingof of Lake LakeSuperior Superiorgeology. geology. ix �Selection Guidelines Nomineesare areto tobe beevaluated evaluatedon on the the basis basis of of their their contributions contributionsto to Lake Lake Superior Superior geology geology 1) Nominees (sensu (sensu lato) lato) including: including: a) importance importanceof of relevant relevant publications; publications; b) promotion promotionof of discovery discoveryand andutilization utilization of of natural natural resources; resources; c) contributions contributionsto tounderstanding understandingof of the the natural naturalhistory history and and environment environment of the the region; region; d) generation generationof of new newideas ideasand and concepts; concepts; and and e) contributions contributionsto to the thetraining trainingand and education education of of geoscientists geoscientists and and the the public. public. 2) Nominees Nomineesare areto tobe beevaluated evaluatedon ontheir their contributions contributions to to the the Institute Instituteas as demonstrated demonstratedby by attendance attendance at at Institute Institutemeetings, meetings, presentation presentationof of talks talks and andposters, posters,and andservice serviceon onInstitute Instituteboards, boards, committees, committees, and and field field trips. trips. 3) The Therelative relativeweights weightsgiven givento toeach eachof of the theforegoing foregoing criteria criteria must must remain remainflexible flexible and andat at the the discretion discretion of the the Committee Committeemembers. members. 4) There Thereare areseveral severalpoints pointsto tobe beconsidered consideredby bythe the Goldich Goldich Medal Medal Committee: Committee: Anattempt attemptshould shouldbe bemade madeto tomaintain maintainaabalance balanceof of medal medalrecipients recipientsfrom from each eachof of the the a) An three estates—industry, estates-industry, academia, academia, and and government. government. b) It must be noted noted that industry industry geoscientists are are at aa disadvantage disadvantage in in that that much muchof of their their work in in not not published. published. 5) Lake of LakeSuperior Superiorhas hastwo two sides, sides, one one the U.S., and the other Canada. This This is is undoubtedly undoubtedly one of the Institute's Institute's great great strengths strengths and and should should be be nurtured nurturedby by equitable equitable recognition recognition of of excellence excellenceininboth both countries. countries. GOLDICH GOLDICH MEDAL MEDAL COMMITTEE COMMITTEE Serving through the meeting year shown in parentheses Frank Frank Luther Luther(2003) (2003) University of Wisconsin, Whitewater Ron R o n Sage Sage (2004) (2004) Ontario Geological Geological Survey Survey (retired) (retired) David David Meineke Meineke (2005) (2005) Meriden Engineering, Engineering, Hibbing, Hibbing, Minnesota Minnesota Steve Kissin, Kissin,as asout-going out-goingsenior seniormember member of of Institute Institute Board Board of Directors, Directors, is is liaison between Goldich Medal Committee and the Board through the 2004 meeting meeting x �2003 2003GOLDICH GOLDICHMEDAL MEDALRECIPIENT RECIPIENT Klaus Klaus J. Schulz Schulz U.S. Geological Geological Survey U.S. Reston, Virginia Virginia GOLDICH MEDALISTS MEDALISTS 1979 Samuel SamuelS. S.Goldich Goldich 1991 Hinze 1991 William Hinze 1980 not notawarded a warded 1992 William WilliamF. F.Cannon Cannon 1981 1993 1 993 Donald DonaldW. W. Davis Davis Carl E. Dutton, Dutton, Jr. 1982 Ralph RalphW. W. Marsden Marsden 1994 Cedric CedricIverson Iverson 1983 Burton BurtonBoyum Boyum 1995 Gene GeneLaBerge LaBerge 1984 Richard RichardW. W. Ojakangas Ojakangas 1996 David DavidL. L.Southwick Southwick 1985 1997 Ronald RonaldP. P. Sage Sage Paul K. Sims Sims 1986 G.B. G.B. Morey Morey ZelI Peterman 1998 Peterman 1998 Zell 1987 Henry HenryH. H. Halls Halls 1999 Tsu-Ming Tsu-MingHan Han 1988 Walter Walter S. S. White White 2000 John JohnC. C.Green Green 1989 Jorma JormaKalliokoski Kalliokoski 2001 2001 John S. Klasner Klasner 1990 Kenneth KennethC. C. Card Card 2002 Ernest ErnestK. K.Lehmann Lehmann xi �CITATION Klaus J. Schulz 2003 Goldich Medal Recipient Recipient ning more Klaus Schulz has had had a long long and and productive productivecareer careerspan spanning more than than 30 30 years years as a geologist geologist in in the the Lake Lake Superior Superior region. region. He He was was introduced introducedto to the the geology geologythrough throughhis his education education in the area, he he completed completed graduate graduate studies studies in in the the region, region,performed performedseveral several summers of field work for mining mining companies in a number of different areas, and has conducted extensive extensive research research as aa scientist scientist with the U.S. U.S. Geological Geological Survey. Survey. This This conducted extensive extensive and diverse diverse experience experience has has made made him him a real real authority authority on on the the geology geology of of the the Lake Superior region. Klaus Klaus received received his B.S. degree degree in in geology geology from the the University Universityof of WisconsinWisconsinOshkosh Oshkosh in 1971. 1971. He He completed completed his his Masters Masters degree degree at at the the University Universityof of MinnesotaMinnesotaDuluth Duluth in 1974, with a thesis project project in in the the Vermilion district district of northern northern Minnesota. Minnesota. He He received received his Ph.D. from the University University of Minnesota Minnesota in in 1977 1977 with aa dissertation dissertation on on the the petrology petrology of volcanic rocks rocks in in the Vermilion district. district. Klaus Klaus spent spent the the next next two two years years as as aa National National Research Research Council Council Research Research Associate Associate with NASA at the Johnson Johnson Space Space Center Center in Houston, where he studied basaltic and ultramafic ultramafic magma magma types types as as analogs analogs of of studied Archean basaltic early planetary crust. In 1982, after three years as a faculty faculty member at at Washington Washington University University in St. Louis, Louis, Klaus Klaus resigned resigned his his teaching teaching position position and and joined the the U.S. U.S. Geological Geological Survey in Reston, VA, fulfilling a long-standing long-standing dream dream of of his. his. During During the the next nexttwenty twenty years years with the USGS Klaus Klaus was a research research scientist scientist and administrator administrator with a strong strong interest interestin in the geology of the Lake Lake Superior Superior region. region. The traits that have have made made Klaus Klaus a success success were evident early early in in his his career. career. In In his his undergraduate days at Oshkosh, undergraduate Oshkosh, Klaus Klaus distinguished distinguished himself himself as as an an avid avid reader reader of of the the geological literature. literature. As a junior in in 1970, 1970, he he wrote wrote an an outstanding outstanding research research paper paper discussing the similarities similarities between between Archean greenstone greenstone belts belts and and modern modern island island arcs. arcs. He He worked several summers doing fieldwork fieldwork for Bear Bear Creek Creek Mining Mining Company Company in in central central Wisconsin and northern Michigan, and for U.S. U.S. Steel Corp. Corp. in the the Vermilion Vermilion district district of of northern Minnesota. This combination combination of field work and and a thorough thorough knowledge knowledgeof of the the literature has continued continued to be be a hallmark hallmark of his his professional professional career, career, and and has has led led to to aa number of significant significant contributions contributions to the the geology geology of the the Lake Lake Superior Superiorregion. region. In the summer of 1971, 1971, Klaus Klaus and and William William Spence Spence discovered discovered the the Lake LakeEllen Ellen kimberlite near Crystal Falls, Falls, Michigan, while working as exploration exploration geologists geologists in in the the area. Klaus was very much involved involved in the recognition recognition of the rock rock as as aa kimberlite. kimberlite. This This was the first kimberlite kimberlite discovered discovered in in the the Lake Lake Superior Superior region. region. xl' �Masters thesis involved involved considerable mapping in the Ely Ely greenstone greenstone belt belt in in His Masters considerable mapping studies for his his Ph.D. Ph.D. dissertation dissertation showed showed that that the the Newton Newton Minnesota, and geochemical geochemical studies Lake Formation Formation was a high-magnesium high-magnesium basalt, basalt, similar to komatiites. komatiites. This This was was the the first first documented occurrence occurrence of of komatiitic komatiiticrocks rocksin in the the Lake LakeSuperior Superiorregion. region. documented In the early 1980's, his field mapping mapping and geochemistry geochemistry of rocks rocks in in the the Pembine Pembine area area of the the Wisconsin Wisconsin magmatic magmaticterranes terranesdemonstrated demonstratedthe thepresence presenceof ofophiolitic ophioliticrocks. rocks. Again, this was the the first first documented documentedophiolite ophiolite in in the the Lake Lake Superior Superiorregion, region,and andshowed showed magmatic terranes were, at least in part, an oceanic island arc. His that the Wisconsin magmatic model for the the evolution evolution of of the the Marquette MarquetteRange RangeSupergoup Supergoupon onthe thecontinental continental subsequent model margin during during the the Penokean Penokean orogeny orogeny is is an an extension extension of of his his familiarity familiarity with with the the rocks rocksininthe the margin region combined combined with his his encyclopedic knowledge knowledge of the geologic geologic literature literature on on the the region evolution of continental continental margins. margins. Klaus also contributed contributed to the GLIMPCE GLIMPCE program, program, which ultimately ultimately provided provided significant insight into the structure and origin of the Mid-continent rift, and into its magmatic magmatic origin origin and and metallogeny. metallogeny. He has authored authored and and co-authored co-authored more more than 120 120 publications, publications, maps, maps, abstracts abstracts and field guides, including including field field guides guides for the the 1984, 1984, 1992, 1992, and and 2003 2003 Institute Institutemeetings. meetings. Klaus' contributions contributions have provided provided a better understanding understanding of the the Archean, Archean, the the Early Proterozoic, Proterozoic, the Middle Proterozoic, and the Phanerozoic history history of the the Lake Lake Superior region. And he continues continues to be be an active active contributor contributor on on aa global global stage, stage, taking taking the the knowledge experience that he knowledge and experience he has gained gained in the Lake Lake Superior Superior region region and and applying applying itit to international international projects. projects. Therefore, it is my my distinct distinct pleasure pleasure and and honor honor to present present Klaus Klaus Juergen Juergen Schulz Schulz as as the 2003 recipient recipient of the Goldich Medal "For Outstanding Contributions To The Lake Superior Region". Region". Submitted by Gene Submitted Gene L. L. LaBerge LaBerge xiii �____________________________________________ __________________________ EISENBREY STUDENT TRAVEL AWARDS The 1986 1986 Board Board of of Directors Directors established established the the ILSG ILSG Student Student Travel Travel Awards Awards to to support support student student participation at at the annual meeting of of the the lnstitute. Institute. The The name name "Eisenbrey" "Eisenbrey" was added added to the award in 1998 to honor honor Edward Edward H. Eisenbrey Eisenbrey (1926-1985) (1926-1985) and utilize substantial contributions made to the 1996 Eisenbrey 1996 Institute lnstitute meeting meeting in his name. "Ned" "Nedt1 Eisenbreyisiscredited creditedwith withdiscovery discoveryof of significant scope was was much much significant volcanogenic volcanogenic massive massive sulfide sulfide deposits deposits in in Wisconsin, Wisconsin, but but his scope broader—he broader-he has been described as having having unique unique talents as an an ore ore finder, finder,geologist, geologist,and and teacher. These are intended to help defray teacher. These awards awards are intended to defray some some of the the direct direct travel travel costs costs of of attending Institute lnstitute meetings, and include a waiver waiver of registration fees, but but exclude exclude expenses expenses The annual Chair in for meals, meals, lodging, lodging, and and field field trip trip registration. registration. The in consultation consultation with the the Secretary-Treasurerdetermines determinesthe the number number of of awards will be Secretary-Treasurer awards and and value. value. Recipients Recipients will be announced at the annual banquet. banquet. The annual Chair, who is is responsible responsible for the the selection, selection, will will consider consider the the following following general general criteria: criteria: 1) The Theapplicants applicantsmust musthave haveactive activeresident resident (undergraduate (undergraduateor or graduate) graduate) student student status status at at the time of the the annual annual meeting meetingof of the the Institute, lnstitute,certified certifiedby bythe thedepartment departmenthead. head. 2) Students Studentswho who are arethe the senior senior author author on on either either an an oral oral or or poster poster paper paper will will be be given given favored consideration. consideration. 3) It is desirable for two or more more students students to jointly request request travel travel assistance. assistance. 4) InIngeneral, general,priority prioritywill willbe begiven givento tothose thoseininthe the Institute lnstituteregion regionwho who are are farthest farthest away away from from the meeting meeting location. location. 5) Each Eachtravel travel award award request requestshall shall be be made made in in writing writing to to the the annual annual Chair, and and should should explain other significant details. details. The need, student and author status, and other The form form below below is is optional. optional. Successful applicants will receive receive their awards during the meeting. meeting. n INSTITUTE ONLAKUPERIOAGEOLOGY Application Eisenbrey Student Travel Award Application Student Student Name: Name: Date: Date: Address: email: Department Department Head-Typed Head-Typed Department Educational Status: Educational Status: Department Head-Signature Head-S~gnature Areyou youthe the senior author anororal or paper? poster paper? YES_ Are senior author of anof oral poster YESNO- NO_ Will any other other students students be be traveling travelingwith with you? you? Who? Who? Statement Statement of need need (use (use additional additional page page ifif necessary) necessary) Please Please return return to: to: xiv �STUDENT PAPER AWARDS Each year, the lnstitute student presentations presentations and and honors honors Institute selects the best of the student Funding for for the the award award is is generated generated from from registrations registrations presenters with a monetary award. Funding of the annual meeting. The TheStudent StudentPaper PaperCommittee Committeeisis appointed appointedby by the the annual annual meeting meeting Chair in such a manner as to represent represent aa broad broad range range of of professional professionaland andgeologic geologic expertise. Criteria Criteriafor for best beststudent student paper—last paper-last modified modifiedby by the the Board Board in in 2001—follow: 2001-follow: Thecontribution contributionmust mustbe bedemonstrably demonstrably the the work work of of the the student. student. 1) The 2) The student must present the contribution in-person. The student must present the contribution in-person. TheStudent StudentPaper PaperCommittee Committeeshall shalldecide decidehow how many manyawards awards to to grant, grant, and and whether whether or or 3) The not to give separate awards for poster poster vs. oral presentations. presentations. casesofofmultiple multiplestudent studentauthors, authors,the theaward awardwill will be bemade madeto to the the senior senior author, author,or or 4) InIncases award will be be shared shared equally by by all all authors authors of the the contribution. contribution. the award Thetotal totalamount amountof ofthe theawards awardsisisleft leftto tothe thediscretion discretionof of the themeeting meetingChair Chairand and 5) The Secretary-Treasurer,but but typically typically is is in in the the amount amount of of about about $500 $500 US US (increase (increaseapproved approved Secretary-Treasurer, by Board, Board, 10/01). 10101). TheSecretary-Treasurer Secretary-Treasurermaintains, maintains,and andwill will supply supply to to the the Committee, Committee,aa form form for for the the 6) The numerical ranking of presentations. presentations. This Thisform formwas wascreated createdand andmodified modifiedby byStudent Student numerical Paper Committees over several years in an effort to reduce reduce the difficulties difficultiesthat that may may arise arise from selection by raters of of diverse diverse background. background. The use of the form is not required, but is left to the discretion discretion of the the Committee. Committee. Thenames namesof ofaward awardrecipients recipientsshall shall be beincluded includedas as part part of of the the annual annual Chair's Chair's report report 7) The that appears appears in the next next volume of the the Institute. lnstitute. Student papers papers will be noted noted on the Program. Program. 2003 STUDENT PAPER AWARDS COMMITTEE Theodore TheodoreBornhorst Bornhorst- -Michigan MichiganTechnological TechnologicalUniversity, University, Houghton, Houghton, MI MI -- Chair Kevin WI KevinSikkila Sikkila— - Wisconsin Department of Transportation, Superior, Wl Anne Argast PurdueUniversity University Fort FortWayne, Wayne, Fort Wayne, Wayne, IN Anne Argast— - Indiana Indiana University University — - Purdue Tim Tim Flood Flood— - St. Norbert Norbert College, De De Pere, Pere, WI Wl 2003 SESSION CHAIRS Peter Ontario Geological Survey, Survey, Kenora, Peter Hinz Hinz— - Ontario Kenora, ON ON Eric Jerde Eric Jerde--Morehead MoreheadState StateUniversity, University, Morehead, Morehead, KY KY James James Miller Miller- -Minnesota MinnesotaGeological GeologicalSurvey, Survey,Duluth, Duluth, MN MN Mike Mudrey, Wisconsin Geological Mike Mudrey,Jr. Jr.— -Wisconsin Geological and and Natural Natural History History Survey, Survey, Madison, Madison, WI Wl xv �2003 BOARD OF 2003 BOARD OFDIRECTORS DIRECTORS appointment continues continues through through the the close close of of the the meeting meeting year year shown shown in in parentheses, parentheses, or oruntil untilaa Board appointment successor is selected Laurel Woodruff Woodruff Co-Chair Co-Chair2003 2003meeting meeting(2006) (2006) U.S. Geological Geological Survey, Survey, St. St. Paul, Paul, MN MN Peter Hinz Hinz (2005) (2005) Geological Survey, Kenora, ON Ontario Geological Michael G. Mudrey, Jr. (2004) Michael (2004) Wisconsin Wisconsin Geological Geological and and Natural NaturalHistory HistorySurvey, Survey, Madison, Madison, WI Wl Stephen A. A. Kissin Kissin(2003) (2003) Lakehead University, Lakehead University, Thunder Thunder Bay, Bay, ON ON Hollings-Secretary-Treasurer (2006) Peter Hollings-Secretary-Treasurer (2006) Lakehead University, Thunder Lakehead Thunder Bay, Bay, ON ON A. Jirsa-Secretary-Treasurer-"erneritu~~~ Jirsa-Secretary-Treasu rer-"emeritus" (in (in transition) transition) Mark A. Minnesota Geological Geological Survey, Minnesota Survey, St. Paul, Paul, MN MN 2003 COMMITTEES 2003LOCAL LOCAL COMMITTEES General General Co-Chairs Co-Chairs Laurel Laurel G. G. Woodruff Woodruff—-U.S. U.S.Geological GeologicalSurvey, Survey, St. St. Paul, Paul, MN MN William F. Cannon — U.S. Geological Survey, Reston, VA William F. Cannon - U.S. Geological Survey, Reston, VA and Abstracts Abstracts Editors Program and Laurel Laurel G. G. Woodruff Woodruff----U.S. U.S.Geological GeologicalSurvey, Survey,St. St. Paul, Paul, MN MN Theodore J. Theodore J. Bornhorst Bornhorst—-Michigan MichiganTechnological TechnologicalUniversity, University,Houghton, Houghton,MI MI Field Trip Guidebook Editor Editor William F. William F.Cannon Cannon—- U.S. U.S. Geological GeologicalSurvey, Survey, Reston, Reston, VA VA Acting Local Local Committee, Iron lron Mountain John Gartner John Gartner— - Coleman Coleman Engineering, Engineering, Iron lron Mountain, Mountain, MI MI Connie Dicken Connie Dicken— - U.S. Geological Geological Survey, Reston, Reston, VA Sally LaBerge Sally LaBerge— - Oshkosh, Oshkosh, WI WI 2003 BANQUET SPEAKER Susan Martin Martin Department of Social Department Social Sciences Sciences Michigan Technological University University Houghton, Houghton,Michigan Michigan The indigenous indigenous people of the Lake Superior Superior Basin: Basin: Understanding Understanding the links links among environment, geology and religious religious belief xvi xvi �48TH Report of the Chair of the 4aTHAnnual Meeting Meeting Hinz, Co-Chair Co-Chair ILSG ILSG 2002 2002 Peter Hinz, 48th The 4athAnnual Institute lnstitute on Lake Lake Superior Superior Geology Geology was hosted hosted by by the the Ontario Ontario Geological Geological 9-12, Survey on May 9-1 2, 2001. Principal Principal local committee members members were Peter Peter Hinz Hinz and and Richard C. Beard, co-chairs, co-chairs, Carmen C. Storey, and and Kevin Kevin O'Flaherty OrFIahertyProgram Programco-chairs, co-chairs, Richard McGowan-Hinz,Treasurer, Charles E. Blackburn, Blackburn, Field Field Trip Co-ordinator, Co-ordinator, M. Kathleen Kathleen McGowan-Hinz, and Christine C. Blackburn, Blackburn, Secretary. Secretary. Other principal principal individuals individuals are listed listed in in the the Proceedings ProceedingsVolume. Volume. Attendanceat atILSG ILSG2001 2001 Attendance A total of 97 97 professionals professionalsand and student student professionals professionalsattended attendedthe the meeting, meeting,39 39of of whom whom pre-registered by the April 2, 2001 deadline. A total of 8 students pre-registered students were were registered, registered,77 of of whom requested requested and and received received travel travel assistance. Eisenbrey Student Eisenbrey Student Travel Travel Awards Awards2001 2001 Seven students students requested requested and and received received travel travel assistance assistance from from the the Eisenbrey EisenbreyStudent Student Travel Award Fund Fund established established to support student student participation participation at at the the Annual Annual Institute. lnstitute. Details, including criteria and application application forms, are available available at the ILSG ILSG website. Bogdan Nitescu Bogdan Nitescu Claire Claire Sturm Sturm Elizabeth Fein Elizabeth Fein Justin Johnson Johnson Becky Rogala Rogala William Jahn Jahn Daniela Daniela Vallini Vallini University of Toronto, Toronto, Toronto, ON University ON Oberlin Oberlin College, College, Oberlin, Oberlin, OH OH Oberlin Oberlin College, College, Oberlin, Oberlin, OH OH Lakehead University, Thunder Bay, Lakehead Bay, ON ON Lakehead University, Thunder Bay, Lakehead Bay, ON ON University of Minnesota - Duluth, Duluth, Duluth, Duluth, MN MN of Western Australia, Nedlands, University of Nedlands, WA Meetin Meetin Summary Summary tf The 48 Annual AnnualInstitute lnstituteon onLake LakeSuperior SuperiorGeology GeologyAnnual Annual Meeting Meeting was held held at the Best Western Lakeside Lakeside Inn Inn and and Convention Convention Centre, Centre, the the same same location locationas as the the 1985 1985meeting. meeting. The one-and-a-half days of technical sessions were preceded by: Field Trip 1 — Tanco one-and-a-half days of technical sessions were preceded by: Field Trip 1 - Tanco Rare-Element Rare-Element Pegmatite, Pegmatite, Southeastern Southeastern Manitoba Manitoba led led by by staff of of the the Tantalum TantalumMining Mining Corporation of Canada Quaternary Geology of - Quaternary Corporation Canada Ltd.; Ltd.; followed followedby byField FieldTrip Trip22— Southeastern Manitoba Nielsen and Gaywood Matile Matile (Manitoba (Manitoba Geological Geological Southeastern Manitoba led led by E. Nielsen Survey); and Field Field Trip 3- Structure Structure and and Sedimentology Sedimentology of the the Seine Seine Conglomerate, Conglomerate,Mine Mine Centre Area, Ontario Ontario lead lead by by Dyanna Dyanna Czeck (Department (Department of Geology, Geology, Oberlin Oberlin College) College) and Philip Fralick Fralick (Department (Department of Geology, Geology, Lakehead Lakehead University) University) Due to the small number number of talks talks submitted, submitted, the the Technical Technical Session Session Chairs Chairs were were unable unable to to group talks into into session session themes. The The meeting meeting began began with with an an anecdotal anecdotal history historyof of mining miningin in northwestern Ontario presented O7FIaherty,followed followed by by regional regional scale scale talks talks on on northwestern presented by by Kevin Kevin O'Flaherty, the Western Western Superior Superior Province. Province. The Theremainder remainderof ofthe thetechnical technicalsessions sessionsincluded includedaabroad broad range of talks focusing focusing on on ground ground water, water, petrography, petrography,sedimentology, sedimentology,mineralogy mineralogyand and structural topics. The final final session session ended ended at noon, noon, allowing allowing for an an early early departure departure of of Field Field Trip Trip 66 to to Red RedLake. Lake. Post Postmeeting meetingtrips tripsincluded: included:Field FieldTrip Trip44— - Industrial IndustrialMinerals Mineralsand and Paleozoic Geology 5 - Separation SeparationRapids RapidsRareRareGeology of Southeastern SoutheasternManitoba; Manitoba;Field FieldTrip Trip5— Element Pegmatite Geology of of the Red Lake - Geology Lake Camp. Camp. All All Pegmatite Field, Field, Ontario; Ontario;and andField FieldTrip Trip66— field field trips trips ran ran smoothly smoothly considering considering the the frigid frigid conditions conditionsof of early earlyMay Mayin innorthwestern northwestern Ontario. ILSG Secretary Secretary -Treasurer, -Treasurer, Mark Mark Jirsa Jirsa was the the lone lone participant participant of of Field FieldTrip Trip 66 successful in obtaining samples samples from from Goldcorp's Goldcorp's Red Red Lake Lake Mine Mine in in Red Red Lake. Lake.He Hewas was able to do this this by by cunningly cunningly embedding embedding the the samples samples in in the the back back of of his hisneck. neck.Upon Upon returning to Kenora Kenora the samples were proudly proudly displayed displayed in a baggy baggy kindly kindly supplied supplied by by the the staff of Red Red Lake's Lake's Margaret MargaretCochenour CochenourMemorial MemorialHospital Hospitalemergency emergencyroom. room. xvii xvii �Goldich Award Annual Banquet Banquet and Goldich the Annual Annual Banquet Banquet Ted Ted DeMatties DeMatties presented presentedthe the citation citation for for Ernest ErnestK. K.Lehmann, Lehmann, At the Goldich Medal Medal for 2002 for his his contributions contributions to to the the Institute lnstituteand and Lake Lake recipient of the Goldich Superior Geology. L. Harvey Harvey Thorliefson, Geological Survey of Canada, Canada, provided provided aa Thorliefson, Geological scintillating discussion on The Search Search for Diamonds Diamonds in in Canada Canada for the the after after dinner dinner address. Laurel Woodruff and and Bill Bill Cannon Cannon of the the U.S. U.S. Geological Geological Survey Survey invited invited 49th participants the 4gth Meeting in in Iron Iron Mountain, Mountain, Michigan. Michigan. participants to the Annual Meeting 2002 Best Student Student Paper Awards Awards 1) Becky Becky Rogala Rogala-- Lakehead LakeheadUniversity,Thunder University,Thunder Bay, Bay, Ontario Ontario ($400, oral presentation) presentation) formation from the Sibley New in information Sibley Group Group 2) Elizabeth ElizabethFein Fein-- Oberlin OberlinCollege, College, Oberlin, Oberlin, Ohio Ohio ($50, ($50, poster; poster; Co-authors Co-authors C.L. C.L. Sturm Sturm and D.M. Czeck) Anisotropy of magnetic susceptibility in the Ottertail Ottertailpluton, pluton, Northern Ontario Ontario 3) Claire ClaireSturm Sturm-- Oberlin OberlinCollege, College, Ohio Ohio($50, ($50, oral; oral; Co-authors Co-authorsD.M. D.M. Czeck Czeck and and E. E. Fein) Fein) Petrographic study of the Petrographic the Ottertall Ottertail pluton, Superior Superior Province, Province, Northwestern NorthwesternOntario Ontario Student Travel Awards 2002 Eisenbrey Student Awards 1) Bogdan BogdanNitescu Nitescu-- University Universityof of Toronto, Toronto, Toronto, Toronto, ON ON ($250) ($250) 2) Claire OberlinCollege, College,Oberlin, Oberlin,Ohio Ohio($200) ($200) ClaireSturm Sturm--Oberlin 3) Elizabeth ElizabethFein Fein-- Oberlin OberlinCollege, College,Oberlin, Oberlin, Ohio Ohio ($200) ($200) 4) Justin LakeheadUniversity, University,Thunder Thunder Bay, Bay, ON ON ($150) ($150) JustinJohnson Johnson--Lakehead 5) Becky Rogala Lakehead University, Thunder Bay, ON ($150) 5 ) Becky Rogala - Lakehead University, Thunder Bay, ON ($150) 6) William WilliamJahn Jahn--University Universityof of Minnesota, Minnesota, Duluth, Duluth, MN MN ($150) ($150) 7) Daniela Universityof of Western WesternAustralia, Australia,Nedlands, Nedlands, WA WA ($400) ($400) DanielaVallini Vallini--University 2002 Goldich Goldich Medal Recipient 2002 Ernest K. Lehmann Lehmann Archives Donation MTU Archives Donation A check check for for $100 $100 was was sent sent to to Michigan Michigan Technological TechnologicalUniversity UniversityArchives, Archives, as as required requiredby by Board agreement ($1 per per participant participant per per meeting), meeting), for for maintenance maintenanceof of ILSG ILSGproceedings proceedings archives. archives. Proceedings Proceedings including Part Part 11 (Programs (Programs and and Abstracts) and and Part Part 22 (Field (Field Trip Trip Guidebook) Guidebook) are available available from from the the Institute: lnstitute: Institute on Lake Superior Geology lnstitute do c/o Mark Mark Jirsa, Jirsa, Secretary Secretary -- Treasurer Treasurer Minnesota Minnesota Geological Geological Survey 2642 University University Avenue St. Paul Paul MN MN 55114-1 55114-1057 057 612.627.4539 Phone: 61 2.627.4539 Fax: Fax: 612.627.4778 612.627.4778 e-mail: jirsaOO1 jirsaool @tc.umn.edu @tc.umn.edu xviiiii xvi �48th ANNUAL INSTITUTE ON LAKE SUPERIOR GEOLOGY BOARD OF DIRECTOR'S MEETING Board of Directors Directors Peter Hinz (2002 (2002General GeneralChair) Chair) Michael Mudrey Mudrey (2001 (2001 Co-chair) Co-chair) Steve Kissin Kissin (2000 (2000 Co-chair) Co-chair) Laurel Woodruff: Proxy Proxy for Ted Ted Bornhorst Bornhorst (1999 (1999 Co-chair Co-chairand and liaison liaisonwith with Goldich Goldich committee) committee) Mark Jirsa (Institute (Institute Secretary-Treasurer) Secretary-Treasurer) Guests Guests Phil Fralick Fralick (2000 (2000 Co-chair) Co-chair) Carmen Carmen Storey Storey (2003 (2003 Program ProgramChair) Chair) O'Flaherty (2003 Kevin O'Flaherty (2003 Program ProgramChair) Chair) Bill Cannon (proposed 2003 Co-chairs) (proposed 2003 Co-chairs) Rod Johnson (Goldich (Goldich Committee) Committee) Frank Luther (Goldich (Goldich Committee) The The following following is based on the secretaries' notes notes and and recollection; recollection; any any omissions omissions or or unintentionaL Motions misstatements are unintentional. Motions by by the the Board Board of Directors Directors are are generally paraphrased—"approved" paraphrased-"approved" or or "accepted" "accepted"implying implying that that aa motion motionwas wasmade, made, seconded, and passed unanimously. The Theexpression expression"generally "generallyagreed" agreed" carries carriesless less be pursued. pursued. Some formality, but indicates a directive that will be Some issues issues that that were were conference are are included included here here for for resolved after the Board meeting, but during the conference closure. closure. MINUTES MINUTES 1. Accepted Acceptedreport reportof of the the Chairs Chairsfor for the the 47th 47th ILSG, ILSG, Madison, Madison, Wisconsin; Wisconsin; as as printed printed in in the the Proceeding Volume (Mudrey), and minutes of last Board Board meeting, meeting, May May 10, 10, 2001 (Jirsa) (Jirsa) 2. Received, Received,discussed, discussed, and andaccepted accepted2001-2002 2001-2002 ILSG ILSG Financial FinancialSummary Summary (Jirsa). (Jirsa). (4gth 3. Discussed annual) meeting Discussedand and approved approved 2003 2003 (4gth meeting location—Iron location-Iron Mountain, Mountain, Michigan, Michigan, and tentative co-chairs co-chairs Laurel Laurel Woodruff Woodruff and and Bill Bill Cannon, Cannon,USGS. USGS.As As currently currently envisioned, Ted Bornhorst Bornhorst will handle handle logistics logistics of field field trips. 4. Approved ApprovedPeter PeterHinz Hinzas ason-going on-goingILSG ILSGBoard Boardmember. member. 5. Discussed Discussedreplacing replacingRod RodJohnson Johnsonas as the the "member "member from from industry" industry" on on Goldich Goldich Committee Committee (end (end of term term 2002) 2002) with with several several candidates candidates including including Dave Dave Meineke Meinekeof of Meriden Meriden Engineering, Hibbing, Minnesota. Dave later accepted the position and was welcomed, Engineering, Minnesota. Dave later accepted the position and was welcomed, and Rod was thanked for his service to the Institute, during the the annual banquet. Dave's Dave's term will end after Goldich Goldich selection selection for the the meeting meeting of of 2005. 2005. 6. Discussed Discussedreplacement replacementof of Mark MarkJirsa Jirsaas as ILSG ILSGSecretary-Treasurer Secretary-Treasurer(end (endof of 4-year 4-year term term 2002). A new member member to the the Institute, Institute, Peter Peter Hollings, Hollings,Lakehead LakeheadUniversity Universityin in Thunder Thunder Bay, was installed installed as "Secretary-Treasurer "Secretary-Treasurer in-training," pending pending a vote vote by by the the general general membership membership (as (as required required in in By-Laws). By-Laws). Because Because of his his newness newness to to the the Institute, Institute,the the board board generally agreed that Peter Peter would serve 2 years of the the 4-year term term concurrently concurrentlywith with Mark Mark in a period period of transition. At the the end end of the the 22 years years (following (followingthe the 2004 2004 meeting), meeting),the the finances and records records of the the institute, institute, and and responsibilities responsibilitiesof the the position positionwould would fall fall to to Peter. Peter. was generally generally This was presented to the membership after the Board meeting, and was accepted. accepted. 7. Other Other business: business: a) Discussed Discussed the offer by by Mike Mike Mudrey Mudrey to take take over as as ILSG ILSG webmaster—It webmaster-It was was generally agreed that Mike Mike could could do that, assuming assuming Ted Ted was was busy busy with with other other obligations and probably probably would not mind mind the relief. relief. Subsequent Subsequent discussions discussions indicate indicate xix xix �that Ted Ted would would like like to to continue continuein in this this endeavor, endeavor, and and has has already alreadypaid paidin inadvance advancefor for that 55 years years of of web web service service to to continue. continue. ItIt remains remains in in Ted's Ted's hands. hands. b) Discussed Discussedefforts effortsby by Graham GrahamWilson Wilsonto to list listILSG ILSGpublications publicationsas aspart partofofhis his b) MINLIB project and S t e v e Kissin Kissin volunteered volunteered to and website (www.turnstone.ca) -—Steve contact Graham Graham and and see see ifif there there isis anything anythingthat that the the ILSG ILSGcan canand andshould shoulddo doto to contact assist. assist. Discussedthe the prospect prospect of of extending extending aa "free "free ride" ride" to to annual annual Goldich GoldichMedal Medal c) Discussed recipients. ItIt was was generally generally agreed agreed that that registration registrationcosts costs should shouldbe bepaid paidby bythe the recipients. annual meeting meeting committee, committee, and and that that lodging, lodging, meals, meals, and and travel travel costs costscould couldbe bepaid, paid, annual at the the discretion discretion of of the the annual annual meeting meetingchairs. chairs. Discussed the the ILSG ILSG Newsletter—Peter NewsletterÑPete Hinz Hinz has has offered offeredto to write write ititbeginning beginninginin d) Discussed 2004 or or so. so. He Hecan cancoordinate coordinatewith with Ted TedBornhorst Bornhorstabout aboutthat thattransition. transition.The Thetopic topicofof 2004 whether the the Newsletter Newslettershould shouldremain remainpaper, paper, or or be bechanged changedto toaawholly whollyelectronic electronic whether format was was discussed discussedand andtabled. tabled. Most Mostseemed seemedto to think think we weshould shouldeventually eventuallyswitch switch format to to aa web-based web-basednewsletter, newsletter,perhaps perhapswith with email emailnotification. notification.This Thisraised raisedaafurther further issue that members members must must be be encouraged encouraged to to notify notify the the secretary-treasurer secretary-treasurerof of issue changes in email address or other status. changes in email address or other status. e) Questionable Questionablesampling—An sampling~An issuewas was raised raisedthat that at at least leastone onegroup groupof ofregular regular e) issue meeting meeting participants participants has has aa tradition tradition of of using using guidebooks guidebooksto to locate locateplaces placesfor formassive massive sampling programs. programs. In In this this one one case, case, samples samples are sold sold to Wards Wards or other other rock rock and and sampling 1) some some of of the the localities localitiesdiscussed discussedin in mineral specimen specimen dealers. dealers. The The problems problems are are 1) mineral guidebooksare are on on private privateland land(and (andtherefore thereforetrespassing trespassingisislikely), likely),and and2) 2)taking taking guidebooks large amounts amountsof of sample samplefrom fromsome somelocalities localitieslimits limitsthe theuse useof ofthese thesesites sitestotofuture future large generations.ItItwas was generally generallyagreed agreedthat that ILSG ILSGwould would print printinintheir theirguidebooks guidebooksaa generations. Policy Statement Statement that that warns warns of this this "questionable "questionablesampling sampling practice." practice." Mark Mark Jirsa Jirsa will will Policy create create such such language languagefor for inclusion inclusionin infuture futureguidebooks. guidebooks. f)f) Discussed Hinz abstracts~peter Hinzwarns warns from fromexperience experiencethat that Discusseddigital digitalsubmission submission of of abstracts—Peter this practice practice can can easily easilyturn turn into intoaa nightmare nightmarefor for preparers, preparers,particularly particularlyififthe the this submitters don't don't follow follow (or (or the the host host organization organization doesn't specify) specify) rigid rigid guidelines guidelines for submitters submission formats. Adjournment Adjournment submission formats. This This includes includes both both text and illustration formats. Respectfully Respectfullysubmitted submittedon on January January 27, 27, 2003 2003to to Peter PeterHinz, Hinz,Chair Chair of of the the48th 48thannual annual meeting, meeting, for for incorporation incorporationinto intothe theReport Reportof of the theChair Chairto toappear appearininProceedings ProceedingsVolume Volume 49. 49. Mark Mark Jirsa, Jirsa, Secretary-Treasurer, Secretary-Treasurer,Institute Instituteon onLake LakeSuperior SuperiorGeology Geology xx �0 0 -o PROGRAM >< x xxi �49th The following companies made generous contributions to the 4gth Annual Meeting. Meeting. We thank them and and John John Gartner Gartner of of the the Local Local Committee Committee for for their commitment to the Institute on Lake Superior Geology. For almost commitment the Institute on Lake Superior Geology. For almost 50 years this sustained interests interests of this organization organization has thrived through through the sustained individuals, corporations, universities, and government agencies individuals, corporations, universities, and government agencies in the the international geologic community. community. This This dedication dedicationtoto an an exchange of international geologic exchange of scientific ideas and aa passion for for field field trips trips (even (even in in driving driving rain rain or or snow) snow) has enabled the ILSG to to fulfill fulfill one one of of its its primary primary objectives: objectives: to promote better understanding of the geology in the Lake Superior region. Kleiman Pump & Well Drilling, &Well Drilling, Inc. Inc. P.O. Box Box 704 704 Iron Mountain, Michigan 49801-0704 49801-0704 Prime Meridian Meridian Resources Resources Ltd. Ltd. N7478 Niagara Niagara Lane Lane Fond du Lac, Lac, WI Wl 54935 54935 Coleman Engineering Engineering Company Company 635 Circle Drive Drive Iron Mountain, MI 49801 49801 xxii �+ WEDNESDAY MAY 7, 2003 8:00 a.m. TRIP 8:00 a.m.FIELD FIELD TRIP1:1WISCONSIN : WISCONSINMAGMATIC MAGMATICTERRANE TERRANE (#1 (#I IN IN GUIDEBOOK) GUIDEBOOK) Klaus Schulz, U.S. Geological Geological Survey Oshkosh, emeritus Gene LaBerge, LaBerge, University University of of Wisconsin Wisconsin— - Oshkosh, 2: THE ORE 8:00 a.m. FIELD TRIP 2: THEREPUBLIC REPUBLICMINE MINE— - LIFE LIFE CYCLE CYCLEOF OF AN IRON ORE FROM GENESIS GENESIS TO TO RECLAMATION RECLAMATION(#4 (#4 IN GUIDEBOOK) GUIDEBOOK) DEPOSIT FROM William William Cannon, U.S. US. Geological Geological Survey John Meier, Meier, Cleveland Cleveland Cliffs Cliffs Iron Iron Company Company 6:00 p.m. p.m. Return Return of Trips 11 and 6:00 and 2 4:00 p.m. p.m. -- 8:00 8:00p.m. p.m. Registration Registration 7:00 p.m. p.m. -- 9:00 9:00p.m. p.m. Ice Breaker Social and Poster Setup THURSDAY MAY 8, 2003 8:00 a.m. -- 9:00 9:00a.m. a.m.REGISTRATION REGISTRATION Note: Technical Technical Sessions Sessionsare are in inWhite White Spruce, Spruce, Pine Pine Mountain Mountain Resort Resort + Denotes Denotes Student Presentation Presentation 8:15 a.m. 8:15 a.m. INTRODUCTORY INTRODUCTORYREMARKS REMARKS Laurel Laurel G. Woodruff and and William William F. F. Cannon, Cannon, Co-Chairs Co-Chairs TECHNICAL TECHNICAL SESSION SESSION II Session Chair: Jim Miller, Minnesota Minnesota Geological Survey, Duluth, MN MN - 8:30 a.m. Harold HaroldBernhardt Bernhardt- Menominee MenomineeRange RangeHistorical HistoricalFoundation FoundationMuseum Museum A brief history history of iron iron mining mining on on the the Upper Upper Peninsula's Peninsula's Menominee MenomineeIron Iron Range Range 9:00 a.m. a.m. Cannon, Cannon, W.F., W.F., LaBerge, LaBerge, G.L. GL. and Klasner, Klasner, J.S. J.S. Niagara suture suture zone, northern northern Michigan Michigan and and Wisconsin—tectonics Wisconsin-tectonics in in the the 1.85 1.85 Ma arc-continent arc-continent collisional collisional boundary boundary 9:30 a.m. a.m. Schulz, Schulz, K. 9:30 A Paleoproterozoic Paleoproterozoic suprasubduction suprasubduction zone ophiolite-island ophiolite-island arc arc complex complex in northeastern northeastern Wisconsin Wisconsin 10:00 10:OO a.m. a.m. COFFEE COFFEEBREAK BREAKAND ANDPOSTER POSTER SESSION SESSION 10:40 a.m. a.m.Schneider, Schneider, D.A., D.A., Holm, HoIm, D.K., D.K., O'Boyle, O'Boyle, C., C., Hamilton, Hamilton, M. and Jercinovic, Jercinovic, M. 10:40 M. Paleoproterozoic development of a gneiss dome corridor in the the southern southern Lake Lake Superior region, USA USA 11:00 a.m. Holm, Hoim, D.K., D.K, Van 11:OO a.m. Van Schmus, Schmus, W.R., W.R., MacNeill, MacNeill, L.C., L.C., Boerboom, Boerboom, T.J., T.J., Schweitzer, Schweitzer, D. D. and and Schneider, Schneider,D.A. D.A. Late Paleoproterozoic (1900-1600 (1900- 1600 Ma) tectonic history of the the northern northern midmidU.S.A.: Implicationsfor for crustal crustalstabilization stabilization continent, U. S.A. :Implications 11:20 a.m. Medaris, Medaris, L.G., L.G., Jr. Jr. and Dott, 11 :20 a.m. Dott, R.H., R.H., Jr. The Sioux Quartzite metamorphism, geochemistry Quartzite revisited: sedimentology, metamorphism, geochemistry and and the origin of pipestone 11:40 p.m. Smyk, M.C. 1 1:40 p.m. The planned activities activities and The Lake Lake Nipigon Nipigon Geoscience GeoscienceInitiative Initiative— - planned and objectives objectives xxiii xxiii �12:00 p.m. Lunch Poster Session Session and 12:OO p.m. LunchBreak Break— - Poster and ILSG ILSG Board BoardMeeting Meeting(by (byinvitation) invitation) TECHNICAL SESSION SESSION IIII TECHNICAL Session Chair: Mike MikeMudrey, Mudrey, Jr., Jr., Wisconsin Wisconsin Geological Geological Survey, Survey, Madison, Madison, WI Wl 1:30 p.m. •: Heggie, Heggie, G. G. and and Hollings, Hollings, P. 1 :30 p.m. P. Geochemistry and mineralization of the Seagull Intrusion, Northern Ontario Geochemistry Ontario +:+ p.m.++:•:•Johnson, Johnson,J.R., J.R.,Hollings, Hollings, P. P. and and Kissin, Kissin, S.A. 1:50 p.m. Mineralization Mineralization of the Norton Lake Cu-Ni-PGE Cu-Ni-PGE deposit 2:10 p.m. Miller, J. J. D., Jr. 2:lO Petrology and PGE potential of the Greenwood Lake Intrusion, central central Duluth Complex, Lake County, County, Minnesota Minnesota p.m. •: Joslin, Joslin, G.D., Miller, J.D., Jr. and 2:30 p.m. G.D., Miller, and Rowell, Rowell,W.F. W.F. Stratiform Pd-Pt-Au mineralization in the Sonju Lake Intrusion, Lake County, Minnesota Minnesota +:+ 2:50 p.m. p.m. + + Marma, J., J., Brown, Brown, P. 2:50 P. and and Hauck, Hauck, S. S. Magmatic Magmatic and hydrothermal hydrothermal PGE mineralization mineralization of the Birch Birch Lake Lake Cu-Ni-PGE Cu-Ni-PGE Deposit Deposit in the the South South Kawishiwi, Duluth Complex, northeast northeast Minnesota Minnesota 3:10 p.m. 3:lO p.m.COFFEE COFFEEBREAK BREAKAND ANDPOSTER POSTER SESSION SESSION Waggoner, T. 3:30 p.m. Waggoner, T. Marquette Range perspective A hydrothermal hydrothermal component component of of Iron IronFormations Formations—A -A Marquette p.m. Tsu-Ming Tsu-Ming Han 3:50 p.m. Han Mode of occurrence occurrence of trona trona and and thermonatrite thermonatrite and and their their possible possible origin origin in in the the Iron-Formation of the Marquette Negaunee Iron-Formation Marquette Range, Lake Superior District, District, USA USA 4:10 p.m. Blaske, Blaske, A.R. 4:lO A.R. Mississippi-Valley at Bellevue, Michigan Geology of the MississippiValley type mineralization at 4:20 p.m. p.m. + Larson, Larson, P. 4:20 P. Mean transport length in tills of the southern portion of the Laurentide Laurentide ice sheet: implications for drift exploration exploration in the Lake Superior region +:+ 4:50 p.m. p.m. 63+ Marlow, Marlow, L., Mooers, H. and Larson, Larson, P. 4:50 P. environmental history Glacial Lakes Aitkin and Upham: their origin and environmental 5:10p.m. TrowJ. 5:10 p.m. Trow, J. Five gold possibilities in some wan copper copper sulfides sulfides in Ontario some Keweena Keweenawan Ontario and Michigan Michigan xxiv �6:00 p.m. CASH BAR 6:00 p.m. ICE BREAKER BREAKER— - MIXER — - CASH 7:00 p.m. p.m. ANNUAL ANNUALBANQUET BANQUETAND ANDAWARD AWARDPRESENTATION PRESENTATION 50th • Location Announcement of 5oth Annual Meeting Location • 2003 Klaus Schulz Schulz 2003 Goldich Goldich Award Presentation Presentation to Klaus • 2003 Banquet Banquet Address 2003 Dr. Susan Martin, Martin, Michigan Michigan Technological Technological University University The indigenous indigenous people of the Lake Superior Basin: Understanding Understanding the links links among environment, environment, geology geology and religious religious belief Meeti ng participants Meeting participantswho whoare arenot notregistered registeredfor forthe thebanquet banquetare arewelcome welcometo tothe thebanquet banquetaddress address FRIDAY MAY 9, 2003 TECHNICAL SESSION Ill Ill Session Session Chair: Eric Jerde, Morehead Morehead State State University, Morehead, Morehead, Kentucky Kentucky 8:20 8:20 a.m. a.m. INTRODUCTORY INTRODUCTORYREMARKS REMARKS Laurel G. Woodruff Woodruff and and William William F. F. Cannon, Cannon, Co-chairs Co-chairs Laurel 8:30 a.m. a.m. Hollings, Hollings, P., P., Fralick, Fralick, P. P. and and Kissin, Kissin,S. S. Geochemistiy and of the the1537 1537Ma Ma Redstone Redstone Point Geochemistry and geodynamic geodynamicimplications implications of Point anorogenic anorogenic granite, Ontario, Canada Canada 8:50 a.m. a.m. +*.:. Buttram, P.M. R.M. and and Bjornerud, M. : Buttram, M. Textural constraints in in thethe Wolf River BatBatholith ho 11th Textural constraintson onthe theorigin originofofrapakivi rapakivitextures textures Wolf River 9:10 a.m. a.m. + .:.Sandin, Sandin, N.A. N.A. and and Bornhorst, Bornhorst, T.J. Sequence of Precambrian Precambrian mafic dikes in Marquette County, County, Michigan, Michigan, with with emphasis on the Sugar/oaf Sugarloaf Mountain and Republic areas 9:30 a.m. a.m. Jerde, Jerde, E.A. E.A. Gabbro/granophyre theCrocodile CrocodileLake Lake Intrusion: Intrusion: aa possible possible vent vent Gabbro/granophyre relations relations ofofthe for the Hovland Lavas? for the Hovland Lavas? 9:50 Vislova, 9:50 a.m.. :.:• 0 Vislova, T. T. Evaluation of initial initial magma compositions compositions for the Bald Bald Eagle Eagle Intrusion and Evaluation of magma for the Intrusion and associated rocks rocks associated 10:10 a.m. COFFEE 10:10 COFFEEBREAK BREAKAND AND POSTER POSTER SESSION SESSION 10:30 a.m. a.m. + •:•Charkoudian, Charkoudian, K., K., Tikoff, Tikoff, B. and :+ and Bauer, Bauer, R. R. Stike -slipseparation separationofof Burntside trondhjemite Wakemup Bay Bay Stike-slip thethe Burntside trondhjemite andand thethe Wakemup tonatilte,Northern NorthernMinnesota Minnesota tonatlite, 10:50 1O:5O a.m. <+ .:.Garbowicz, Garbowicz, A. A. and Bjornerud, Bjornerud, M. M. Wisconsin Paleostress inferences from slip vectors the eastern eastern part part of the Paleostress inferences from slip vectors in in the the Wisconsin segment rift segmentofofthe theMidcontinent Midcontinent rift 11:10 Potter, E.G. E.G.and and Mitchell, Mitchell, R.H. 11:10 a.m. +*•:• : Potter, R.H. The and exotic exoticmineralogy mineralogyofofthe the WesternSubcomplex Subcomplex of of the theDeadhorse Deadhorse The rare rare and Western Creek Diatreme, Northwestern Ontario Creek Diatreme, Northwestern Ontario xxv XXV �B.A., Mudrey, Mudrey, M.G., M.G., Jr., Jr., Czechanski, Czechanski, M.L., M.L., Reid, Reid,D.D. D.D. and andHunt, Hunt,T.C. T.C. 11:30 a.m. a.m. Brown, Brown, B.A., Highway construction, construction, mine mine reclamation, reclamation, and and land-use land-use planning planning challenges challenges in in the historic historic Upper Upper Mississippi Mississippi Valley Valley lead-zinc district of southwest southwest Wisconsin Wisconsin 11:50a.m. 11:50 a.m. Wattrus, Wattrus, N. N. High-resolution multibeam bathymetry in Lake Lake Superior Superior High-resolution 12:10 p.m. p.m.LUNCH LUNCHBREAK BREAK—-POSTERS POSTERSREMOVED REMOVEDAFTER AFTER LUNCH LUNCH 12:10 \ TECHNICAL SESSION IV IV TECHNICAL Session Chair: Peter PeterHinz, Hinz,Ontario OntarioGeological GeologicalSurvey, Survey, Kenora, Kenora, ON ON 1:40 a.m. a.m. +*:•:•Vallini, Vallini,D.A., D.A.,McNaughton, McNaughton,N.J., N.J.,Rasmussen, Rasmussen,B., B.,Fletcher, Fletcher, I. I. and and Griffin, Griffin, B.J. B.J. U-Pb geochronology to unravel the history history of Proterozoic Proterozoic Using xenotime U-Pb sedimentary basins: basins: aastudy studyininWestern WesternAustralia Australia and andthe theLake LakeSuperior Superiorregion region sedimentary 2:00 p.m. p.m. Kissin, S.A., S.A., Vallini, D.A., D.A., Addison, W.D. W.D. and Brumpton, Brumpton, G.R. G.R. New zircon ages from from the the Gun Gunflint Ontario flint and and Rove Formations, northwestern northwestern Ontario 2:20 p.m. p.m. +%+ Richardson, Richardson, A., A., Fralick, P. and Hollings, Hollings, P. P. Sibley Basin sediment provenance using zircon and whole rock geochemical methods: Possible Possiblesource sourceareas areasof of the thePass Pass Lake Lake Formation Formation G. 2:40 p.m. p.m. ++:•:•Rogala, Rogala,B., B.,Fralick, Fralick, P. P. and and Borradaile, Borradaile, G. A magnetostratigraphic magnetostratigraphic and secular variation study study of of the Sibley Group 3:00 3:00 p.m. p.m. COFFEE COFFEE BREAK BREAK 3:20 p.m. Argast, A. sediment chemistry tell us about rocks like those from the What does sediment the Fern Fern Creek Formation? 3:40 p.m. p.m. Bartnik, Bartnik, P. J. and and Evans, B. B. W. ford, Michigan area Geology and hydrogeology hydrogeology in in the the Kings Kingsford, 4:00 p.m. p.m. Presentation Presentation of Student 4:00 Student Paper Paper Awards Awards Bornhorst, Michigan Michigan Technological University: University: Student Paper Committee Ted Bornhorst, Committee SATURDAYMAY MAY10,2003 10, 2003 SATURDAY 8:00 a.m. a.m. FIELD FIELDTRIP TRIP3: 3:MENOMINEE MENOMINEEIRON IRONRANGE RANGE (#2 (#2 IN IN GUIDEBOOK) GUIDEBOOK) Gene emeritus Gene LaBerge, LaBerge, University Universityof of Wisconsin Wisconsin— - Oshkosh, emeritus John Klasner, University University of of Western WesternIllinois, Illinois,emeritus emeritus William Cannon, U.S. William U.S. Geological Geological Survey 6:00 p.m. p.m. Return Return of Trip 3 6:00 3 SUNDAYMAY MAY11, 2003 SUNDAY 11,2003 TRIP4:4:Iron Iron River River — Crystal Falls Falls Iron ININGUIDEBOOK) 8:00 a.m. FIELD TRIP - Crystal IronDistrict District(#3 (#3 GUIDEBOOK) Gene Oshkosh, emeritus Gene LaBerge, LaBerge, University Universityof of Wisconsin Wisconsin— - Oshkosh, John Klasner, Klasner, University University of of Western WesternIllinois, Illinois, emeritus emeritus William Cannon, Cannon, U.S. Geological Survey 6:00 p.m. p.m. Return Return of of Trip 4 6:00 xxvi xxvi �POSTER PRESENTATIONS Brown, Czechanski, M.L., Mudrey, MG., M.G., Jr. Jr. and andReid, Reid, D.D. D.D. Brown, B.A., Czechanski, Wisconsin mineral resource GIS GIs and related digital map and database database products Wisconsin —a a progress report Boerboom, Boerboom,T. T. Bedrock geologic geologic maps maps of of Keweena Keweenawan intrusive rocks rocks in in the the Bedrock wan volcanic and intrusive North Shore Shoreof of Lake Lake Lakewood, Lake wood, French French River, River, and and Knife River 7.5' quadrangles, quadrangles, North Superior, Minnesota Easton, Easton, R.M. P.M. Geology and mineral potential of Proterozoic Proterozoic mafic intrusions in the the northern northern Grenville Province of Ontario Grenville Ontario Hart, Hart, T.R. Keweena wan mafic ma f/cand andultramafic ultramaficintrusive intrusive rocks rocks of of the Lake Nipigon Nipigon and Keweenawan and Crystal Crystal Lake areas, areas, northwestern Ontario Ontario S.A., Oreskovich, Oreskovich, J.A. and Hauck, S.A., and Severson, Severson, M.J. M.J. and Beaver Bay Geology, drill holes, mineral leases, and geophysics in the Duluth and Integration of of various various GIS GIs databases databases to to tell tell a Compexes, northeastern Minnesota: Integration story story of the the history history of of past past and and current currentCu-Ni-PGE Cu-Ni-PGEmineral mineral exploration exploration •+ Heiling, C.D. Heiling, :+ C.D. Peperites of the Gafvert Gafvert Lake Volcanic Volcanic Complex, Complex, St. Louis County, County, Minnesota Minnesota Hocker, S.M., S.M., Hudak, Hudak, G.J., G.J., Odette, Odette, J.D. J.D. and Newkirk, ++•:. : Hocker, Newkirk,T.T. T.T. alteration mineral phases at the Five Mile Lake volcanic-hosted Chemistry of alteration volcanic-hosted massive massive sulfide sulfide prospect, NE NE Minnesota Minnesota ++•: Keatts, Keatts,M.J., M.J., Jirsa, Jirsa,M. M. and and Hoim, Holm, D. D. single-grainanalyses analyses of of Precambrian Precambrian mafic intrusions intrusions in Results of 40ArI9Ar ^Ar^Ar single-grain in northern northern and north-central north-central Minnesota Minnesota Mckenzie, M.A., and Jercinovic, Jercinovic, M. ++ McKenzie, M.A., Hoim, Holm, O.K., D.K., Schneider, D.A. and M. Evidence for largeResults of EMP monazite geochronology in in E-C E-C Minnesota: Minnesota: Evidence scale geon 17 17 metamorphism metamorphism associated associated with with post-tectonic post-tectonic plutonism plutonism Metsaranta,R., R.,Fralich, Fralich, P. P. and and Hollings, Hollings, P. ++:•:•Metsaranta, geochemical investigation volcanic and metasedimentary A geochemical investigation of of Mesoarchean Mesoarcheanmeta metavolcanic metasedimentary rocks from Uchi greenstone greenstone belt from the theBirch Birch— - Uchi Nicholson, S. Nicholson, S.W. W. and Cannon, Cannon, W.F. W.F. Stratigraphy wan rocks of the St. Stratigraphy and and structure structure of of Keweena Keweenawan St. Croix Croix horst, horst, northwestern northwestern Wisconsin Wisconsin Stott, G.M., G.M., Davis, D. W., K.J. and Tomlinson, Tomlinson,K. K.Y. Y. Stott, W.,Parker, Parker,J.R., JR., Straub, 1(4. Archean tectonostratigraphic assemblages of eastern Waba goon Subprovince, Archean tectonostratigraphicassemblages of eastern Wabagoon Subprovince, northwestern north western Ontario Ontario xxvii ��ABSTRACTS xxvffl ��WHAT DOES SEDIMENT CHEMISTRY TELL US ABOUT ROCKS LIKE THOSE FROM THE FERN CREEK FORMATION? Department of Geosciences, Indiana University Argast, Anne, Department University Purdue Purdue University University Fort Fort Wayne, Fort Wayne, IN 46805-1499, 46805-1499, Argast@ipfw.edu Argast@ipfw.edu Bulk chemical chemical analyses analyses are are accepted accepted and and powerful powerful tools tools for for the the study study of of metamorphic metamorphicand and igneous rocks. rocks. Bulk Bulk chemical chemical analyses analyses are are less less accepted accepted and and less less widely widely used used for for the the study studyof of sedimentary rocks. This is at least partly the result of a widely-held view that chemical chemical data data are are unreliable indicators indicators of of sedimentary sedimentary events events due due to to the the post-burial post-burial diagenetic diageneticalteration alterationof of the the sediment. In recent years, this view has been reinforced with studies sediment. studies indicating indicating the the potential potential for for extreme extreme diagenetic diagenetic alteration alteration of of sediments, sediments,with with km-scale km-scale transport transport proposed proposedin insome somesystems systems (e.g., Wintsch and Kvale, 1994). 1994). Potassium Potassium is often singled-out singled-out as an especially especially mobile component in diagenetic systems. For example, example, Awwiller (1993), (1993), working working in in the the Gulf Gulf Coast Coast Tertiary, Tertiary,postulates postulatesan an increase increasefrom from2.0 2.0to to 3.8 wt. percent K20 1 0 pore K20in mudrocks, mudrocks, due due significantly to the transport of K as part of i03 volumes of fluid fluid passing through through the the system system in in the the depth depth range range from from 1500 1500to to 4000 4000m mbelow below the the surface. surface. An alternate alternate view (Argast (Argast and and Donnelly, Donnelly, 1987) 1987)maintains maintains K20 K20isis aa generally generallyconservative conservative element in diagenetic settings, and that observed variations in K K20 2 0 content preserve compositional compositional characteristics characteristicspresent present at at and and before before accumulation. accumulation.Depending Dependingon on your your point-ofpoint-ofview, the chemistry chemistry of diagenetically diageneticallyand metamorphically metamorphically altered altered sedimentary sedimentaryrocks rocks may may (or (or may not) provide useful information information about about provenance, provenance, weathering weathering and and other other qualities qualitiesof of the the sedimentary sedimentary system. system. Unconsolidated sediments, Unconsolidated sediments, delivered delivered as turbiditic pulses of siliciclastic debris eroded from the Mountains, accumulated Himalaya Mountains, accumulated on the the Bengal Fan (DSDP (DSDP 218) 218) and and now now at at subbottom subbottomdepths depths from 12 noted in lithified 12 to 729 m, produce chemical trends very similar to those previously noted (and metamorphosed) sedimentary sedimentary rocks. The similarity in chemical trends across across this this broad broad range of conditions conditions and and environments environmentssuggests suggests sedimentary sedimentary chemical chemical trends trends arise arisefrom from fundamental conditions conditions imposed imposed upon upon the the system system before before burial, burial, and and are are not not necessarily necessarily the the result result of extensive extensive diagenetic diagenetic alteration alteration at at depth. depth. The Fern Creek Formation Formation (Early (Early Proterozoic, Proterozoic, Lower Lower Chocolay Chocolay Group) Group) is is well well exposed exposedalong along the the Sturgeon Sturgeon River near the dam dam northeast of of Loretto, Loretto, Michigan. Michigan. These These rocks rocks have have been been interpreted interpreted as glaciogenic in origin, origin, and and the diamictites diamictites they they contain contain used as as evidence evidence for for glacially-derived glacially-derived dropstone dropstone units. units. Others Others have have interpreted interpretedthe the Fern Fern Creek Creek Formation Formation as as nonglaciogenic nonglaciogenicin in origin origin with the sediments accumulated environments grading upward into lagoonal or accumulated in fluviatile environments estuarine environments. environments. Field and textural qualities (to be discussed as part of a post-meeting fieldtrip in the Menominee Menominee Iron Range) Range) support support aa glaciogenic glaciogenic origin. origin. 1 �chemistry also also supports supports aa glaciogenic glaciogenic origin origin (Argast, (Argast, 2002) 2002) . The chemical data, Bulk rock chemistry chemical data, including the absence absenceof of aa correlation correlationbetween betweenK20 K 2 0and andA1203, A1203,show sediments sedimentsfrom from the the Fern Fern including deposited without without extensive extensivesorting sortingor or demixing demixingof of hydraulically hydraulicallycoarsercoarserCreek Formation Formation were deposited + Na ++ K)/A1 and finer-grained finer-grained fractions. Other data, including the Na20/K20 Na20/K20 and (2Ca + K)/Al atomic atomic ratio suggest sediment accumulated with abundant original feldspar. The The chemical index of of 61, similar similar to to the average average CIA CIA of 57 57 in in Gowganda Gowganda diamictite diamictite alteration (CIA) ranges from 50 to 61, matrices. The accessory suite suite is complex and and includes includes poorly poorly rounded rounded zircons. zircons. These These attributes attributes are consistent with an origin as as aa glacial glacial till. till. . Several minerals enriched enriched in rare earth elements elements (REE) (REE) and/or andlor thorium thorium were were identified identifiedin in the the Fern Creek Formation. These include include monazite, huttonite huttonite (monoclinic (monoclinic ThSiO4) ThSi04) and a fluorfluorhydroxy-REE mineral. Th concentrations concentrations as high as 53 53 ppm were noted in one bulk analysis. Efforts Efforts to obtain obtain aa chemical chemical date date on on these these minerals minerals have have so sofar far been been unsuccessful. unsuccessful. The Carney Lake Gneiss Gneiss is aa chemically-compatible chemically-compatible possible possible source source for for the the Fern Fern Creek Creek Formation. Formation. REFERENCES Argast, A., 2002, 2002, The The lower lower Proterozoic Proterozoic Fern Fern Creek Creek Formation, Formation,northern northern Michigan: Michigan:mineral mineraland and bulk geochemical evidence J. Earth Earth Sci., Sci.,v. v. 39, 39,p. p.481481evidence for for its its glaciogenic glaciogenic origin: origin: Can. Can. J. 492. 492. Argast, S. and Donnelly, T.W., 1987, of clastic sedimentary 1987, The chemical discrimination of components: J. Sed. Pet., v. 57, p. 813-823. 813-823. components: Awwiller, D.N., 1993, Illite/smectite formation and potassium mass transfer during burial 1993, Illite/smectite diagenesis Sed. diagenesis of mudrocks: aa study study from from the Texas Texas Gulf Coast Coast Paleocene-Eocene: Paleocene-Eocene:J.J. Sed. Pet., v. 63, p. 501-512. 501-512. of 1994, Differential mobility of elements in burial diagenesis of Wintsch, R. P. and Kvale, C. M., 1994, siliciclastic siliciclastic rocks: J. Sed. Sed. Res., Res., v. v. 64A, 64A, p. p. 349-361. 349-361. 2 �GEOLOGY AND HYDROGEOLOGY IN THE KINGSFORD, MICHIGAN MICHIGAN AREA GEOLOGY BARTNIK, PATRICK J., pbartnik@arcadis-us.com, pbartnik@arcadis-us.com,ARCADIS ARCADIS G&M, G&M, Inc., Inc., Kingsford, Kingsford, Michigan, 49802; and EVANS, ARCADIS G&M, G&M,Inc., Inc.,Milwaukee, Milwaukee, EVANS, BRUCE BRUCE W., W., bevans@arcadis-us.com, bevans @ arcadis-us.com,ARCADIS Wisconsin, 53202 Wisconsin, 53202 Investigations have been undertaken in a portion of the City of Kingsford, Investigations Kingsford, Michigan Michigan and and Breitung Township, Michigan (the study study area) to determine determine the geologic geologic and and of glacial sediments sediments and and bedrock. bedrock. The hydrogeologic characteristics of The study study area is located located south-central Upper Peninsula Peninsula of Michigan. Michigan. in Dickinson County in the south-central The ARCADIS investigations investigations were largely largely completed between April April 1997 1997and and January January 2001, but are continuing. During Duringthe theinvestigations, investigations,over over300 300soil soil borings borings were were completed, along with 47 test pits and 9.5 miles of geophysical study. The The topography topography is is characterized by three distinct characterized distinct landform terraces (Upper, (Upper, Lower, and and Riverside), Riverside), which which range in elevation elevation from from approximately approximately 1,120 1,120feet feet above above mean sea sea level level (ft (ft msl) msl) to to approximately approximately 1,045 1,045 ft msl. The The.Upper Upper Terrace Terrace contains contains several several isolated isolated glacial glacial kettles. kettles. The elevation of the Menominee River is approximately 1,037 ft msl. The Thegeology geology is is 3 �comprised of unconsolidated unconsolidated glaciofluvial and and glaciolacustrine glaciolacustrinedeposits deposits of of clay, clay, silt, silt,sand, sand, and gravel that exhibit complex horizontal and vertical spatial variability. These These the Lower sediments overlie the Middle Precambrian Michigamme Slate and the Precambrian metavolcanic Quinnesec Formation. Formation. Depth to groundwater in the unconsolidated unconsolidated deposits deposits ranges from about 10 10 feet below land surface (bls) near the Menominee bis in the Upper Terrace. Groundwater Menominee River to more than 50 feet bls Groundwaterflow flow follows irregular pathways toward the Menominee River, but generally flows from northeast to southwest. +0.863ft/ft ft/ftin in upland upland southwest. Vertical Verticalhydraulic hydraulicgradients gradientsrange range from from +0.863 ft/ftnear nearthe theMenominee Menominee River. River. Hydraulic areas to —0.012 -0.012 ft/ft Hydraulicconductivities conductivitiesrange rangefrom from1 O3 centimeters per second (cdsec) (cm/sec) to to 10.' 101ccm/sec forcoarser-grain coarser-grainmaterial materialtoto1.03 1.03x x1 i0 d s e c for 0'~ cm/sec " c~d s e cfor forthe thevery veryfine-grain fine-grainsand sand and and sandy sandy silt. The Thebedrock bedrock is c d s e c to 3.94 xx 1i00cm/sec generally considered impermeable. Groundwater Groundwaterflow flow velocities velocities range range from from approximately 3 ft/day to 280 ft/day in coarser-grain units, and from approximately approximately0.1 0.1 approximately ft/day ft/day to 33 ft/day ftlday in in the the very very fine-grain fine-grain sand sandand andsandy sandysilt. silt. To aid in the understanding of the complex geology within the study area, threedimentional dimentional modeling of the geology was undertaken using the topographic surface, bedrock surface, and glacial sediments. sediments. Thirteen geologic units identified from the borehole data were categorized in to 3 units, based on permeability and anticipated effects on groundwater flow. The The modeling modeling and visualization of the geology were completed completed using aa Geostatistical GeostatisticalSoftware SoftwareLibrary Library (GSLIB), (GSLEB), developed developed at at Stanford Stanford University, FORTRAN programs, and and Environmental Environmental Visualization VisualizationSystem System(EVS) (EVS) software developed by the C Tech Development Corporation. Corporation. 4 �GEOLOGY GEOLOGY OF OF THE THE MISSISSIPPI-VALLEY MISSISSIPPI-VALLEYTYPE TYPE MINERALIZATION MINERALIZATIONAT BELLE VUE, MICHIGAN MICHIGAN BELLEVUE, BLASKE, Allan Allan R., R., Blaske BlaskeGeoscience, Geoscience,8313 8313Hartel, Hartel,Grand GrandLedge, Ledge,ME MI 48837 48837 BLASKE, The Bayport Limestone is exposed in quarrying quarrying operations at Bellevue, in southwestern southwestern Eaton Eaton County, Mining has County, Michigan. Michigan. Mining has been been active active around around Bellevue Bellevue since since the mid-1800's. Approximately 25 feet of the Bayport is exposed in the the quarrying quarrying operations, and consists of a gray to buff colored thin-bedded thin-bedded limestone. limestone. Bayport limestone Mississippian in comprises the upper portions portions of The Bayport limestone is is late Mississippian in age, age, and comprises the upper of the Grand Rapids Group. Group. It is underlain underlain by by the the Michigan Michigan Formation, Formation, also of the the Grand Grand Rapids Rapids Group. The Theearly earlyMississippian MississippianMarshall Marshall Sandstone Sandstone and and Coldwater Coldwater Shale Shale lie lie below below the the Grand Grand Rapids Group. The TheBayport Bayportisisoverlain overlainby bythe theearly earlyPennsylvanian PennsylvanianSaginaw SaginawFormation Formation(within (within the Michigan Basin), Basin), but but covered covered only only by by glacial glacial sand sand and and gravel gravel at at the the quarry quarry site. site. Mineralogy of the deposit deposit isis simple, simple,consisting consistingpredominantly predominantly of of pyrite, pyrite, marcasite, marcasite, and and calcite. calcite. Pyrite is most commonly commonly found as encrustations encrustations of cubic cubic crystals, crystals, formed directly directly on limestone. limestone. Marcasite is generally generally lighter lighter in in color color than than the the pyrite, pyrite, and and often often in in iridescent, iridescent,platy platycrystal crystalgroups. groups. Marcasite is by by far the dominant iron sulfide. sulfide. Two of calcite calcite are are observed. observed. Early Marcasite is dominant iron Two generations generations of calcite is found as small coatings. The small crystals crystals lining cavities as drusy drusy coatings. The second second generation generation of calcite is found in large, masses. Trace large, euhedral euhedral crystals and cleavable masses. Trace amounts amountsof of sphalerite, sphalerite, barite, and fluorite are present. Fluorite was the earliest to form, as small brown crystals Fluorite was the earliest to form, as small brown crystalsdirectly directly on the Marcasite and Pyrite was was formed formed in association association with with the the early early calcite. calcite. Marcasite and the limestone. limestone. Pyrite sphalerite sphalerite are later than the early calcite and pyrite. Second Second generation generation calcite calcite began slightly slightly after the marcasite. Tiny Tinycrystals crystalsof of marcasite marcasite can can also alsobe befound foundon onthe thelarge largecalcite, calcite,indicating indicatingthat that Barite appears later than formation of marcasite continued to the end of mineralization. formation the end of mineralization. appears than the the sulfides, sulfides, but before the end end of of the the calcite calcite formation. formation. Mineralization present predominantly predominantlyinin brecciated brecciated zones zones and and vein Mineralization isis present vein structures structures within within the Bayport Limestone. Limestone. The The most most common common type type of of breccia breccia consists consists of of small, small,angular angular clasts clasts surrounded by open-space open-space filling filling of of sulfides sulfides and and calcite. calcite. A second surrounded by second type type of of breccia breccia consists consists of larger, rounded clasts, clasts, with with the interstitial larger, rounded interstitial spaces spaces filled with aa muddy muddy limestone limestone and and pyrite. pyrite. Orientation Orientation and size of the mineralized zones within the limestone is not known, known, due due to to lack lack of of mapping. Fine-grained exposure within the quarry and insufficient historical mapping. Fine-grained iron iron sulfide sulfide is is also also observed as replacement structures, structures, along along apparent apparent solution solution fronts fronts within within the the massive massivelimestone. limestone. 36-element ICP of the sulfides indicates the the simplicity simplicity of of the the mineralization. mineralization. 36-element The geochemistry geochemistry of analysis of pyrite and marcasite separates, as well as as aa composite composite breccia breccia sample, sample, indicate indicate very very low concentrations of of trace elements. Copper, Copper, lead lead and andzinc zinc are arefound foundatatconcentrations concentrationsof of less less than 60 ppm; nickel is less than 30 is than 30 ppm; ppm; and and cadmium cadmium and cobalt less than 5 ppm. Barium is also low, generally less than 20 ppm. Manganese is high in the breccia (385 ppm), and lower in Manganese is high in the breccia (385 pprn), and lower in the sulfide separates (64 to 109 109 ppm), pprn), while chromium is high in the sulfides sulfides (150 (150 ppm) and and low 5 �in the breccia (31 ppm). Arsenic Arsenic isis present present in in the the breccia breccia (7 (7 ppm), ppm), but low in the the iron iron sulfides sulfides at less than 5 ppm. Sulfur isotopic were analyzed on separated Sulfur isotopic compositions compositions were analyzed on separated samples samples of pyrite, pyrite, marcasite, marcasite, and and sphalerite. Sulfur Sulfur isotopic isotopic compositions compositions (34S) (S^S) of of the the sulfide sulfide phases phases from from the Bayport Limestone are 14.5°/ for for for are 14.5°/o forthe the pyrite, pyrite, 12.8°/ 12.8°/o forthe themarcasite, marcasite, and and 19.8°/ 19.8°/o forthe thesphalerite. sphalerite. These compositionsindicate indicatethat that the the mineralizing fluids were were basinal basinal brines compositions mineralizing fluids brines from within within the the surrounding Mississippian Mississippian and and Pennsylvanian Pennsylvanian formations. formations. Unpublished surrounding Unpublished data data obtained obtained from from the the USGS (Westjohn, D. D. B., pers. comm.) as part of the RASA program indicate a large range range of of program indicate 6S ininsamples S^S samplescollected collectedfrom fromthe theunderlying underlyingMarshall MarshallSandstone Sandstoneand andMichigan Michigan Formation, Formation, as well as the overlying Formation and and the the Jurassic Jurassic Red Red Beds. Beds. Pore well overlying Saginaw Formation Porewater, water,whole-rock, whole-rock, sulfide, and sulfate sulfur sulfur isotope isotope compositions compositions for the the underlying underlying formations formations exhibit exhibit average average 345 20°/, while S^S near 20°/oo whilethe the average average S^S within the overlying overlying formations formationsisisnear near170/00. 17 Oleo. Temperature of the mineralization has been determined determined using fluid inclusions in in calcite calcite (Panter, (Panter, K. 5., S., 2001). 2001). Calcite afforded the only only mineral mineral phase phase with with inclusions inclusionsfor formicrothermometric microthermometric study. Temperatures Temperaturesof of homogenization homogenization indicate indicate aa bimodal bimodal distribution, distribution, with with aa low low temperature temperature mean of approximately approximately 58°C, 5g°Cand a high temperature mean of approximately approximately 138°C. 138OC. The Themean mean These temperatures are similar to those temperature temperature of all all inclusions inclusions analyzed analyzed was was 107°C. 107OC. These temperatures are those the Mississippian observed using isotopic compositions in authigenic minerals in observed using isotopic compositions authigenic minerals Mississippian and and Pennsylvanian sandstones (Westjohn, (Westjohn, D. B., Pennsylvanian sandstones B., 1994). 1994). Fluid salinities salinities based based on on freezing freezing point point depression range from 2.6 to 9.5 9.5 equivalent equivalentweight weight percent percent NaCI. NaCl. The quarries at Bellevue are located located within within55 to to 66 miles miles to the north and west of the quarries at Bellevue are the known known northwest end of the northwest the Albion-Scipio Albion-Scipio Oil Oil Field Field Trend. Trend. This oil field field (dolomitized (dolomitized fracture fracture and and solution cavities) structure is located within the Trenton-Black River (Middle Ordovician) rocks, cavities) structure is located within the Trenton-Black River (Middle Ordovician) rocks, some 4,000 feet deeper than the Bayport Limestone. The The structure structureisis related relatedto tofaulting faultingwithin within the basement basement rocks. Evidence Evidenceof ofthe thestructure, structure,however, however, isispresent present ininthe thelower lowerMississippian Mississippian Sunbury Shale Formation, (approximately 3,000 feet higher than the Middle Middle Ordovician Ordovician rocks), rocks), the Coldwater the Coldwater Shale (overlying (overlying the Sunbury), Sunbury), and and the the Marshall Marshall Sandstone Sandstone (overlying (overlying the the are evident Coldwater). Coldwater). If movements movements associated associated with this this structure structure are evident in in the theformations formations immediately below the Bayport Limestone, Limestone, itit seems seems likely likely that that the the Bayport Bayport Formation Formationwould would also also with the the structure. structure. Faulting associated with the Trend is likely be affected by faulting associated with responsible for for small structures responsible structures in the the Bayport, Bayport, allowing allowing for for brecciation, brecciation, subsequent subsequent fluid fluid migration, and precipitation of the the mineralization. mineralization. REFERENCES REFERENCES of Fluid Inclusions in Calcite from 2001. A A Preliminary Preliminary Microtermometric Study of Panter, K. S., 2001. Bellevue, Michigan, unpublished data, Bowling Green State University, OH Westjohn, D. B., 1994, of 1994, Michigan Basin RASA Solid-Phase Solid-Phase Investigation, in Geohydogeology of Carboniferous Aquifers Aquifers of of the Michigan Basin, Great Lakes Carboniferous Lakes Section-SEPM, Section-SEPM, 1994 1994 Fall Fall Field Field Conference, Conference, September September23-24, 23-24, 1994, 1994,Lansing, Lansing, MI MI 6 �WAN VOLCANIC AND INTRUSIVE ROCKS BEDROCK GEOLOGIC GEOLOGICMAPS MAPSOF OFKEWEENA KEWEENAWAN IN THE THE LAKE WOOD, FRENCH RIVER, AND KNIFE RIVER 7.5' QUADRANGLES, QUADRANGLES, NORTH LAKEWOOD, FRENCH KNIFE SHORE OF LAKE LAKE SUPERIOR, SUPERIOR, MINNESOTA MINNESOTA BOERBOOM, Terrence Terrence J., Minnesota Minnesota Geological GeologicalSurvey, Survey, St. St. Paul, Paul, MN, MN, boerb001@umn.edu boerb001 @umn.edu The Minnesota Minnesota Geological Geological Survey, with with partial partial funding funding by by the theU.S. U.S.Geological GeologicalSurvey SurveySTATEMAP STATEMAP geologic mapping program, has recently recently published published detailed detailed bedrock bedrock geologic geologic maps maps of of three three quadrangles quadrangles located along the North Shore of Lake Lake Superior Superior northeast northeast of of Duluth, Duluth, Minnesota Minnesota (Fig. (Fig. 1;1;Boerboom Boerboomand and others, 2002a, was completed completed at at a scale of 1:12,000, at a scale 2002a, b). b). Field mapping mapping was 1:12,000, and compiled compiled at scale of 1:24,000. This mapping has shown that some flow sequences can can be traced inland as far as 1:24,000. as 10 10to to 12 12 kilometers, and has identified hundreds of of individual flows flows within within the the larger larger flow flow units. units. Several mafic to felsic, subcordant subcordant to discordant discordant sills sills and and intrusions intrusionshave have also also been been mapped. mapped. Prior to this mapping, bedrock geologic geologic maps maps for for this this area area were were at at a scale of mapping, the only only published published bedrock 1:200,000 (for example Miller and others, 2001), and other work was concentrated along the shoreline of Lake Superior. Brannon Brannon(1984) (1984)sampled sampled160 160successive successivevolcanic volcanic flows, flows, starting starting above above the the Lester Lester River River sill and ending in Two Two Harbors, Harbors, as part part of of an an exhaustive exhaustive geochemical geochemical study. Green Greenand andothers others(1977) (1977) included this area as part included this part of of aamore morebroad broadcoastal coastalzone zonemanagement managementstudy. study. Schwartz and Sandberg Sandberg (1940) published a paper on the diabase sills near Duluth that included some of the the sills sills mapped mapped during during this study. Sandberg Sandberg(1938) (1938)mapped mapped the the stratigraphy stratigraphyof of the the flows flows exposed exposed at the shoreline shoreline from Duluth to Two Harbors, identifying some 180 180 lava lava flows. flows. Although all of these these studies studies made made some some incursions incursions inland from the shore, shore, none none of them them provided systematic systematicmapping mapping away away from fromthe the shoreline shorelineproper. proper. Bedrock exposure in the map area area varies varies greatly, greatly, from from nearly nearly continuous continuous outcrop outcrop along alongthe the shoreline shoreline and many of the short short streams streams along along the the slope slope into into Lake Lake Superior, Superior, to variably variably abundant abundant outcrop outcrop in the hills inland hills inland from the lakeshore. Throughout Throughout the the map map area, area, there there are are many many closely closely spaced spaced streams that have eroded into the bedrock perpendicular to the strike of the volcanic stratigraphy. Hence, Hence,many many of of the the individual flows flows could could be be traced traced for for a great individual great distance distance along along strike by tying tying them them together together from from one one the shoreline shoreline outcrops. outcrops. In contrast, the more resistant intrusive streamcut to the next, in combination with the rocks are typically typically exposed exposed on on the the tops tops and and slopes slopes of of high high hills. hills. The rocks The northeast northeast part of the the map map area area is is poorly exposed and thus much of the bedrock geology in that area is constrained largely by aeromagnetic aeromagnetic data. data. Green (2002) has proposed a subdivision subdivision of of the the North North Shore Shore Volcanic Volcanic Group Group into intoaa series seriesof of informal informal sequences and formations that are separated sequences and formations that separated by major major lithological lithological and and geochemical geochemical breaks breaks or or by by intrusions. Within intrusions. Within the the area area of of the themaps mapsshown shown here, here, these these include include the the Larsmont Larsmont basalts, Sucker River River basalts, the Lakewood lavas, and and the Lakeside Lakeside lavas lavas (Fig. (Fig. 2). The Thedetailed detailedbedrock bedrockgeologic geologicmaps mapsshown shown here subdivide these informal into multiple layers comprised comprised of of lava flows here subdivide these informal formations formations into multiple layers flows of of similar similar composition and texture in which which multiple multiple flow flow contacts contacts have have been been documented. documented. REFERENCES REFERENCES Boerboom, T.J., Green, J.C., and Jirsa, M.A., 2002a, 2002a, Bedrock Bedrock geology geology of the French French River River and and Lakewood quadrangles, St. Louis County, Minnesota: Minnesota quadrangles, MinnesotaGeological Geological Survey Survey Miscellaneous Miscellaneous Map Map MM128, scale 1:24,000. 1:24,000. 2002b, Bedrock geology of the Knife River River quadrangle, quadrangle, St. St. Louis Louis and and Lake Lake Counties, Counties, Minnesota: Minnesota: Minnesota Minnesota Geological Geological Survey Survey Miscellaneous MiscellaneousMap Map M-129, M- 129,scale scale1:24,000. 1:24,OOO. Brannon, J.C. 1984, 1984, Geochemistry Geochemistry of successive successive lava flows of the the Keweenawan Keweenawan North North Shore Shore Volcanic Volcanic Group: St. St.Louis, Louis,Washington WashingtonUniversity, University,Ph.D. Ph.D. dissertation, dissertation,312 312p. p. 7 �Green, J.C., J.C., 2002, 2002, Volcanic Volcanic and and sedimentary sedimentary rocks rocks of of the theKeweenawan Keweenawan Supergroup Supergroup in in northeastern northeastern Green, Minnesota, Minnesota, Chapter Chapter 55 of of Miller, Miller, J.D., J.D.,Jr., Jr.,Green, Green,J.C., J.C.,Severson, Severson,M.J., M.J.,Chandler, Chandler,V.W., V.W.,Hauck, Hauck,S.A., S.A., and Wahi, Wahl, T.E., T.E., Geology Geology and and mineral mineral potential of of the the Duluth Duluth Complex Complex and and related related Peterson, D.M., and rocks of of northeastern northeastern Minnesota: Minnesota MinnesotaGeological GeologicalSurvey SurveyReport Report of of Investigations Investigations 58, 58, p. p. 9494105. 105. Green, Green, J.C., J.C., Jirsa, Jirsa, M.A., M.A., and and Moss, Moss, C.M., C.M., 1977, 1977,Environmental Environmental geology geology of of the the North North Shore Shoreof ofLake Lake Superior: Superior: Minnesota MinnesotaGeological GeologicalSurvey, Survey,99 99 p.p. Miller, J.D., J.D., Jr., Jr., Green, Green, J.C., J.C., Severson, Severson,M.J., M.J.,Chandler, Chandler,V.W., V.W.,and andPeterson, Peterson,D.M., D.M.,2001, 2001,Geologic Geologicmap map Miller, of of the the Duluth Duluth Complex Complex and and related related rocks, rocks, northeastern northeastern Minnesota: Minnesota: Minnesota MinnesotaGeological GeologicalSurvey Survey Miscellaneous MiscellaneousMap MapM-1 M- 119, 19,scale scale1:200,000. 1:200,000. Sandberg, Sandberg, A.E., 1938, 1938, Section Section across across Keweenawan Keweenawan lava flows flows atatDuluth, Duluth,Minnesota: Minnesota: Geological Geological Society Societyof of America AmericaBulletin, Bulletin,v.v.49, 49, p. p. 795-830. 795-830. Schwartz, G.M., G.M., and Sandberg, Sandberg, A.E., 1940, 1940, Rock Rock series series in in diabase diabase sills sills atatDuluth, Duluth,Minnesota: Minnesota: Schwartz, Geological Geological Society Society of of America America Bulletin, Bulletin, v.51, v. 51, p. p. 1135-1172. 1135-1172. Figure Figure 1.1. Index Index map map showing showing location location of of mapped Work isis currently currently inin mapped quadrangles. quadrangles. Work progress progress on on the the Two Two Harbors Harbors and andCastle Castle Danger Dangerquadrangles. quadrangles. diabase Figure Figure 2.2. Index Index map map showing showing the the relative positions positions of of the the informal informal relative volcanicunits unitsof ofGreen Green(2002). (2002). volcanic - LIII Intrusiverocks rocks Intrusive Volcanicrocks rocks Volcanic Boundary of informal volcanic units 8 �WISCONSIN MINERAL RESOURCE RESOURCE GIS GIs AND RELATED DIGITAL DIGITAL MAP MAP AND DATABASE PRODUCTS RELATED PRODUCTS -A REPORT A PROGRESS REPORT BROWN, BA.1, B:A.', CZECHANSKI, CZECHANSKI,M.L.', M.L.',MUDREY, MUDREY,M.G., M.G.,Jr.1, ~ r . 'and ,andREID, REID,Daniel DanielD.2 D .(1) (1) ~ BROWN, Geological and and Natural Natural History History Survey, Survey, Univ. Univ. of of Wisconsin-Extension, Wisconsin-Extension,3817 3817Mineral Mineral Wisconsin Geological Point Road, Madison, Madison, WI 53705, 53705, babrowni babrownl @facstaff.wisc.edu, @facstaff.wisc.edu,(2) Wisconsin Dept of Transportation, Transportation, 3502 3502 Kinsman Kinsman Blvd, Blvd, Madison, Madison,WI WI 53704-2507 53704-2507 A new Mines, Mines, Pits Pits and and Quarries Quarries (MPQ) (MPQ) database database containing containing information information on on 1,302 1,302 significant nonmetallic mining sites sites throughout Wisconsin has has been been completed completedby by the the Wisconsin Wisconsin Geological and Natural History History Survey Survey (WGNHS) (WGNHS) in cooperation cooperation with with the the U.S U.S Geological Geological Survey (USGS). Locations Locations were digitized digitized from from county-based county-based digital digital orthophotography orthophotographywherever wherever available (MASIMILS available and by site visits. Data Datatables tableswere were linked linked to to existing existingUSGS USGS databases databases(MAS/MILS and MRDS) and to Wisconsin Wisconsin Department Department of of Transportation Transportation(WDOT) (WDOT)aggregate aggregatetest testdata; data;this this linkage linkage of all previous digital digital and and analog analog databases databases is is the the first first updated updated inventory inventorysince since1980. 1980. Future versions will be augmented with current site information, collected under the nonmetallic reclamation program of Wisconsin Department of Natural Resources, and additional historic sets such as the Road Material WGNHSJWDOT. Material Survey Survey sites sites of of the the WGNHS/WDOT. Georeferenced maps layered with digital geology, topography, orthophotography, Georeferenced orthophotography,soil, soil, and so forth provide a valuable land-use planning resource. Concern Concern for for safety safety and and construction construction the historic historic Upper Upper Mississippi problems in the reconstruction of U.S. Highway 151 through the possible the scanning and Valley Base-Metal Mining District, southwest Wisconsin, made possible georeferencing of the Wisconsin Mineral Development Atlas. The The Mineral Mineral Development DevelopmentAtlas Atlas of mine workings, drill-hole is a detailed set of 1,450 1,450 section-scale section-scale maps (1 inch equal 200 feet) of location and ancillary ancillary data dating from from 1900 1900 until mining mining ceased ceased in in 1979. 1979.These These maps maps were were maintained by the WGNHS and and the University University of Wisconsin-Platteville Wisconsin-Plattevilleand and were were scanned scannedby by the the WDOT. WDOT. All Wisconsin water well construction reports for 1936-1988 1936-1988 are now available on CDmapping as well well as environmental environmental and Rom. They provide an extensive data set for geologic mapping water resource analysis. analysis. New WGNHS map products are are being produced produced in in digital digital form form and andaa variety of analog maps including including the 1:24,000 1:24,000USGS geologic geologic quadrangle quadrangle maps maps of of the the lead-zinc lead-zinc district are being converted converted to digital as as resources allow. allow. This presentation will will provide provide an an interactive interactivedemonstration demonstrationof of these thesedata datasets setsand andGIS GIs layers, a review of available available map data such as regional geophysics, and and an update update on on the the status status of of geologic mapping at the WGNHS. WGNHS. 9 �HIGHWAY CONSTRUCTION, CONSTRUCTION,MINE MINERECLAMATION, RECLAMATION,AND ANDLAND-USE LAND-USEPLANNING PLANNING HIGHWAY HISTORIC UPPER UPPER MISSISSIPPI MISSISSIPPIVALLEY VALLEY LEAD-ZINC LEAD-ZINC CHALLENGES IN THE HISTORIC DISTRICT DISTRICT OF OF SOUTHWEST SOUTHWEST WISCONSIN BROWN, B.A.1, B.A.~,MUDREY, MUDREY,M.G., M.G.,Jr.1, ~ r . 'CZECHANSKII, ,CZECHANSKI, M.L.1, M.L.', REID, REID, Daniel DanielD.2, D . ~and and , HUNT, HUNT, BROWN, T.C.3, T.c/, (1) Wisconsin Geological and and Natural History Survey, Survey, Univ. Univ. of Wisconsin-Extension, Wisconsin-Extension, Wisconsin Geological 3817 38 17 Mineral Point Road, Madison, Madison, WI 53705, 53705, babrown1@facstaff.wisc.edu, babrown 1@ facstaff.wisc.edu, mgmudrey@wisc.edu,(2) Wisconsin Dept. of Transportation, 3502 Kinsman Blvd, Madison, WI mgmudrey@wisc.edu, 53704-2507, Wisconsin-Platteville,712 712 Pioneer PioneerTower, Tower, 53704-2507, (3) Reclamation Program, Univ. of Wisconsin-Platteville, Platteville, Platteville, WI 53818 538 18 of Wisconsin, Illinois, Illinois, Iowa, Iowa, and and Minnesota Minnesota The Upper Mississippi Valley Lead-Zinc District of produced nearly 10 million tons of lead-zinc ore from the 1820s until the last mine closed in 1978. The district will probably never be mined again, but problems related related to to mineralization mineralization and and past mining activity pose significant significant problems problems for for highway highway construction construction and and post-mining post-miningland land use. Specific hazards and engineering problems include (1) (1) Highly Highly altered altered and and unstable unstable rock rock and and shallow abandoned leachate from from abandoned mine workings workings encountered encountered during during highway highway construction, construction,(2) (2) leachate from lead-zinc lead-zinc sulfide sulfide mines. mines. As roaster-pile waste and (3) locally degraded groundwater from As rural rural residential development increases, increases, the the abandoned abandoned workings, workings, particularly particularlypoorly poorly sealed sealedshafts, shafts,can can be a hazard. Most low-sulfide low-sulfide waste waste rock rock has has been recycled recycled as as aggregate, aggregate, and and carbonate-rich carbonate-rich tailings overgrown with vegetation make make it difficult difficult to to find find any any surface surface evidence evidenceof of small, small,older older mine sites that may cause problems. problems. in an an High sulfate in groundwater samples was noted in 1978 following closure and flooding in area where large mines had operated for more than 50 drawdown cone cone had had developed developed 50 years and a drawdown over a 20-square mile area. area. A well-replacement well-replacement program program near Shullsburg Shullsburg restored restored potable potable water water supplies. supplies. Onsite Onsite reclamation consisted consisted of of establishing establishingvegetation vegetation on on the the tailings tailingsand andcrushing crushingthe the coarse waste rock for aggregate. Leachate from zinc roaster waste piles produced over 100 years over 100 years resulted in highly acidic and metal-rich surface surface water near Mineral Point. Point. The The roaster roaster piles piles were were successfully reclaimed by surface contouring along with neutralization surface grading and contouring neutralization and and of the cost of of fertilization to allow vegetation to establish. This was accomplished at a fraction of removal of the roaster waste piles. piles. rock alteration exposed exposed during Previously undiscovered sulfide mineralization and associated rock highway construction along U.S. Highway 151 near Mineral Point resulted in the 151 resulted in the unanticipated unanticipated engineering redesign of major roadcuts roadcuts and structures. structures. The The need need to to identify identify areas areas of of need for engineering the Wisconsin Wisconsin Mineral Mineral Development Development Atlas, which potentially unstable slopes slopes led to scanning scanning Of of the of mineralization, mineralization, alteration, alteration, and and abandoned has proven to be invaluable in identifying areas of workings in the path of construction. construction. These These detailed detailed maps maps (1 (1 inch inch to to 200 200 feet) feet) of of mine mineworkings workings exploration drillhole and exploration drillhole locations locations are are now now being being used used by by county county and and regional regional planners planners and and potential mining mining related related hazards hazards into into land-use zoning authorities to identify and incorporate potential planning. 10 �TEXTURAL CONSTRAINTSON ONTHE THEORIGIN ORIGINOF OF RAPAKIVI TETURE IN IN THE THEWOLF WOLF RIVER RiVER TEXTURAL CONSTRAINTS RAPAKIVI TEXTURES BATHOLITH BATHOLITH R. Michele Buttram Buttram and and Marcia Marcia Bjornerud Bjornerud Geology Department, Lawrence University, Appleton, WI 54912 549 12 The Wolf River Batholith of north-central Wisconsin, a 1.47 Ga composite anorogenic pluton, includes some of the world's finest examples of 'rapakivi' granite, granite, in in which which large large potassium feldspar crystals are mantled by plagioclase. Although Although rapakivi rapakivi granites granites have have been described for more than a century, the origin of of this distinctive texture, both in the Wolf River complex and elsewhere, remains remains controversial. controversial. Some workers argue that rapakivi mantles are coronae formed at a peritectic or eutectic point under equilibrium crystallization conditions. Others Others maintain maintain that rapakivi textures record disequilibrium associated with magma mixing and or sudden sudden changes in pressure. While most previous previous investigations investigations have have focused focused on on the the chemistry chemistry of of rapakivi rapakivi granites, granites, specifically, this study study examined examinedthe thephysical physicalcharacter characterofofthe theWolf WolfRiver Riverrocks rocks—- specifically, the feldspar crystals. crystals. Among size, shape, orientation and distribution of the rapakivi-type feldspar Among the most striking characteristics of these rocks is the large size of the feldspars (up size feldspars (up to to 77 cm cm in in length). Statistical Statistical analyses analysesshow show that that there there is is no no significant significant difference difference in in size size or or aspect aspect ratio between crystals with and without the rapakivi mantle. However, However, the the K-feldspar K-feldspar cores of the rapakivi-type rapakivi-type crystals crystals tend to be rounder (less (less euhedral) euhedral) than than non-rapakivi non-rapakivi grains, suggesting that they experienced significant resorption prior to the growth growth of of the the plagioclase mantle. AAweak weakgrain grainshape shapefabric fabric and and random random juxtaposition of of rapakivi rapakiviand and non-rapakivi grains must must also also be be explained explained by by any any viable viable model model for for the the origin origin of of the the texture. Our Ourdata data appear appearto to be be most most consistent consistent with with the the magma magma mixing mixing model, model, which which isis compatible with earlier geochemical studies of the Wolf River River complex. complex. 11 �Niagara suture suture zone, zone, northern northernMichigan Michiganand andWisconsin—tectonics Wisconsin-tectonics in Ma arc-continent arc-continent collisional boundary boundary the 1.85 Ma W.F. Cannon, Cannon,(U.S. (U.S. Geological GeologicalSurvey, Survey, Reston, Reston, VA VA 201 92, wcannon @ usgs.gov) G.L. 20192, wcannon@usgs.gov) G.L. LaBerge, LaBerge, (University of of Wisconsin-Oshkosh Wisconsin-Oshkosh(retired) (retired)and andU.S. US.Geological GeologicalSurvey), Survey),John JohnS. S.Klasner Kiasner illinois University (retired) (retired) and (Western Illinois and U.S. U S . Geological Geological Survey) Survey) The Niagara suture suture zone, as used here, is a belt varying varying in width from from about about 6 km to 40 km lying north of the Niagara Niagara fault. fault. It separates the accreted volcanic arcs of the Wisconsin magmatic terranes (WMT) on the south south from from the autocthonous autocthonous and parautochtonous parautochtonous continental continentalmargin margin rocks on the north. ItItconsists consistsof of Paleoproterozoic Paleoproterozoic metasedimentary metasedimentary and and metavolcanic metavolcanic rocks of epicratonic Marquette Marquette Range Range Supergroup Supergroup and and Archean Archean basement basement rocks rocks upon upon which which they they were were the epicratonic deposited. The TheArchean Archeanrocks rocksconstitiute constitiutethe thesouthern southernmargin margin of of the the Superior Superiorcraton, craton, which which was was deposited. rifted and eventually eventually separated separated during during extensional extensional phases phases of of the the Penokean Penokean orogenic orogeniccycle, cycle,and and during Penokean Penokean convergence. convergence. The The suture suture zone zone is is marked marked by by very very high high then thrust northward during strain and widespread multiple steeply to vertically plunging folds. The suture suture zone zone is is one one of of subdivisions of the Penokean Penokean orogen whose hierarchy of component component parts parts is is shown shown numerous subdivisions below. below. Michigamme subterrane Foreland fold fold and thrust i Foreland Niagara suture \~ i a ~ a suture ra zone /, Park Falls panel Watersmeet Watersmeet panel Beechwood Beechwood panel Iron Iron River River panel Menominee panel Menominee oanel j J / I I/ PENOKEAN OROGEN PENOKEANOROGEN,, \\ \ north north - - -south south- - Niagara fault - - - - Pembine-Wausau (northern part ~ e m b i n e - ~ a u s aterrane u (northern part of of WMT) WMT) Mars hfield terrane (southern (southern part Marshfield part of ofWMT) WMT) -- The map pattern shown here was derived from published detailed detailed maps in the east east (Bayley (Bayley and and Dutton, 1971; others, 1966; 1966; Button, 1971; James and others others 1968; 1968; and James and others, others, 1961) 1961)and and from from our our but access access to to recent recent exploration exploration drill drill recent work in the west, where outcrops are scarce but information as well as proprietary detailed aeromagnetic aeromagnetic and electromagnetic electromagneticdata data have have aided aided in in relationships (Cannon clarifying the geologic relationships (Cannon and others, others, 1998). 1998). Each of the the five five fault fault panels panels of of the the Niagara Niagara suture suture zone zone has has aa unique unique set set of of characteristics. characteristics. Watersmeet panel- Paleoproterozoic Paleoproterozoic strata strata are are mostly mostly pelitic pelitic schists schists and and gneisses gneissescontaining containing near the the base. base. They ferruginous strata and locally dolomite near They were were deposited deposited on on a basement of deformed into into gneiss gneissdomes. domes. High-pressure Archean gneiss. Both Both basement and cover were deformed metamorphism produced kyanite-bearing kyanite-bearing assemblages. assemblages. Park Falls panelpanel- Generally Generally similar similar to to Watersmeet Watersmeet panel panel except except that that metamorphism metamorphismwas was lower lower pressure and sillimanite-bearing sillirnanite-bearing assemblages assemblages are are predominant. predominant. Beechwood Beechwood panelpanel- Consists Consists of of Paleoproterozoic Paleoproterozoic graywacke graywacke and shale shale and and mafic mafic volcanic volcanic rocks rocks in in roughly equal parts. Archean Archean basement basement is not exposed. Folds Folds are are ENE-trending ENE-trending and have subhorizontal axes. Rocks are in greenschist facies. facies. Iron River panelvanel- Rocks are the Paint River Group, Group, including including the the Badwater Badwater Greenstone. Greenstone.Archean Archean basement is not exposed. exposed. Strata Strata are are multiply multiply folded folded creating creating aa complex complex fold fold interference interferencemap map pattern. Most Mostfold foldplunge plunge steeply. steeply. Metamorphosed Metamorphosedto togreenschist greenschist or or sub-greenschist sub-greenschist facies. facies. 12 �88' 9•1 46' c-is tic r-,c V A a V - A r-ic a r-tc A ?AA .A vA I I I a LV" I_VA A 0 A a '- A 7 a Lv A A a Fembine— "ausau terrane ,%/AA — -l LV i_L V IL Vi L) 50 A— cv' 1.-V1 c-is) .1 I_V1 v a LVA cv" 1 50 1KM ai ,a , 1a i 91' 89' 88' 0• Map showing showing the the five five structural structuralpanels panels (Park (ParkFalls, Falls,Watersmeet, Watersmeet,Beechwood, Beechwood,Iron IronRiver, River, Map and Menominee) that that constitute constitute the Niagara Niagara suture zone. Faults Faultsthat thatbound boundthe thepanels panelsare are Flambeau Flambeau Flowage Flowage fault fault (FFF), (FFF), Powell Powell fault (PF), Elmwood fault (EF), Paint River fault (PRF), (PRF), Badwater Badwater fault fault (BF), (BF),North NorthRange Rangefault fault(NRF), (NRF),and andSouth SouthRange Rangefault fault(SRF). (SRF). Menominee Menominee panel- Rocks are entirely of Paleoproterozoic age. No No Archean Archean basement basement is is exposed. exposed. Strain Strain was extreme. Commonly Commonlyall allstructural structuralelements, elements,including including fold fold axes, axes, are are subvertical. subvertical. Metamorphism Metamorphism is is lower lower to to upper upper greenschist greenschist facies faciesand and largely largely post-tectonic. post-tectonic. These These panels, panels, and and the the Niagara Niagara suture suturezone zone that that they they constitute, constitute,differ differfrom fromthe theMichigamme Michigamme subterrane Archeanbasement basementisisin in the the subterrane to the north. There Therewas waslittle littlepenetrative penetrativedeformation deformationof of Archean Michigamme subterrane. Paleoproterozoic deformed. Folds, Paleoproterozoic strata strata were moderately to weakly deformed. for for the the most part, are are simple simpleand and gently gently plunging. plunging. Metamorphic Metamorphic grade grade is is variable variableand and mostly mostly postposttectonic. Thus, Thus,the theNiagara Niagarasuture suturezone zonedocuments documentsaarange range of of tectonic tectonic styles styles unique unique to to the the very very high strains strains in in aa belt belt no no more more than than aa few few tens tens of of kilometers kilometers wide, wide, along along which which differential differential movement between the accreting arcs on the south and the craton margin on the accreting arcs on the south and the craton margin on the north north was was concentrated. concentrated. References References Bayley, Bayley, R.W., R.W., Dutton, Dutton, C.E., C.E.,and and Lamey, Lamey, C.A., C.A., 1966, 1966,Geology Geologyof of the the Menominee Menomineeiron-bearing iron-bearingdistrict, district, Dickinson Dickinson County, County, Michigan Michigan and and Florence Florence and and Marinette MarinetteCounty, County,Wisconsin: Wisconsin:U.S. U.S. Geological GeologicalSurvey Survey Professional 96p. Professional Paper 513, 5 13,96p. Cannon, Cannon, W.F., LaBerge, G.L. Klasner, J.S., and Schulz, K.J., 1998, 1998, Reinterpretation of the Penokean 44th continental margin in part of northern Wisconsin and Michigan (abs.): Proceedings Proceedings of 44thAnnual Annual Institute Institute continental on on Lake Lake Superior Superior Geology, Geology,v. v. 44, 44, p. p. 52-53. 52-53. Dutton, Dutton, C.E., 1971, 1971, Geology Geology of the Florence area, Wisconsin and Michigan: U.S. Geological Survey Professional Paper 633, 633,54 p. Professional 54 p. James, James, H.L., H.L., Clark, Clark, L.D., L.D., Lamey, Lamey, C.A., C.A., and and Pettijohn, Pettijohn,F.J., F.J., 1961, 1961,Geology Geologyof ofCentral CentralDickinson DickinsonCounty, County, Michigan: Michigan: U.S. U.S. geological geological Survey SurveyProfessional ProfessionalPaper Paper 310, 3 10,176 176p.p. James, H.L., Dutton, C.E., C.E., Pettijohn, F.J., F.J., and Weir, K.L., 1968, 1968, Geology and ore deposits of the Iron River Professional Paper Paper 570, 134 p. -- Crystal Crystal Falls district, Iron County, Michigan: U.S. Geological Survey Professional 13 �Strike-slip separation separation of the Burntside trondhjemite and and the Wakemup Wakemup Bay tonalite, Northern Minnesota Northern Minnesota Karoun Charkoudian, Basil Tikoff Department of Geology and Geophysics, Geophysics, University of Wisconsin, Wisconsin, Madison Madison WI, WI, 53706 53706 Robert Bauer Department of Geological Geological Sciences, Sciences, University of Missouri, Missouri, Columbia, Columbia, MO, MO, 65211 6521 1 INTRODUCTION The INTRODUCTION TheVermilion Vermilionfault faultisisaalocal localtectonic tectonicboundary boundary in the southern Canadian Shield juxtaposing the Quetico Quetico subprovince subprovince (granites (granites and and schists) schists) with with the the Wawa Wawa greenstones. greenstones. Burntside trondhjemite The Bumtside trondhjemite and and the Wakemup Bay tonalite tonalite are are small, small, elliptical, elliptical, Archean Archeangranites granites separated km of right lateral offset on the Vermilion Vermilion fault fault in in northern northernMinnesota. Minnesota. The separated by 35 35 krn normal fault, fault, juxtaposing juxtaposing the the shallow Wawa Wawa Vermilion fault is interpreted as initially active as a normal greenstone to the south with the deeper deeper granites granites and and migmatized schists schists to to the the north north (figure (figure1,1, stage 1). fault, separating the the Burntside Burntside trondhjemite trondhjemite from from 1). It was later reactivated as a strike-slip fault, stage Bay tonalite (figure 1, stage 2). 2). Although the Vermilion fault is the regional the Wakemup Bay boundary between between the the Quetico Quetico and and Wawa Wawa subprovinces, subprovinces,the th Haley fault lies to the south of the Vermilion fault and contains Quetico Quetico schists schists that belong belong on the north side of the Vermilion Vermilion fault fault (figure (figure 1). 1). The purpose purpose of this this study study is is to to compare compare emplacement setting, fabrics, composition, composition, and shape shape of the two plutons to determine if they constitute constitute a piercing piercing point on the Vermilion fault. In In addition, addition, we have determined the dip on the Vermilion Vermilion fault, constrained constrained the emplacement emplacement history of the the Wakemup Wakemup tonalite, tonalite, and and determined a potential cause for the isolated fault block that now contains contains the the Wakemup Wakemup Bay pluton (figure (figure 1, 1, stage 2). The Burntside trondhjemite trondhjemite is a small lenticular pluton that intruded the schist that lies to the north of the Burntside Bumtside Lake fault, fault, a continuation continuation of of the the Vermilion Vermilion fault at its eastern end. The The Wakemup Wakemup Bay pluton is a biotite-bearing biotite-bearing tonalite tonalite that that intruded intruded the the schist schistthat that lies lies Figure 11 just to to the the north north of of the the Haley Haley fault. fault. Figure g'one The Anisotropy AMS ANALYSIS ANALYSIS Anisotropy of Magnetic Magnetic Susceptibility Susceptibility (AMS) (AMS) is is aa rapid, rapid, nonnondestructive technique, commonly used in granitic studies to obtain obtain magnetic magnetic fabrics. fabrics. Principle Principle The magnetic foliation is defined as the kmx-kint AMS ellipsoid axes axes are are defined defined as as knBx>kint>kmin. plane, and the magnetic lineation is defined as the orientation of kmx. The bulk susceptibility j.tSI) susceptibilityvaries varies widely widelyin inboth boththe theWakemup WakemupBay Baytonalite tonalite(500-8500 (500-8500pS1) and the Burntside trondhjemite (5OO-350OSI). (500-3500pSI). This This range of susceptibility susceptibility is attributed attributed to the large variation in magnetite content throughout these bodies. The The AMS AMS foliations foliations parallel parallel the the measured field foliation in both plutons. Lineations Lineationsin inthe the Wakemup Wakemup Bay Bay tonalite tonalite dip dip shallowly shallowly to the E and W, and lineations in the Burntside trondhjemite dip shallowly to the ENE and WSW. WSW. Magnetic lineations lineations consistently consistently parallel parallel the the long long axis axis of of the the plutons. plutons. The Burntside GRAVITY GRAVITY STUDY STUDY Burntside and Wakemup plutons were selected selected for for aa gravity gravity study study because they both contain a single surrounding lithology (biotite schist) with a significant and g/cc). In consistent density density contrast contrast (Adensity (Mensity = -0.08 to -0.1 glcc). = -0.08 In addition, addition,the the gravity gravitydata data allows allowsus us to model the dip of the Vermilion Vermilion fault. fault. 14 �meter model model G G was was used usedfor forboth both areas. areas. After A Lacoste and Romberg gravity meter corrections, a forward forward model approach approach was used to interpret interpret the depth of the Burntside Burntside pluton pluton and and Vermilion fault geometry using WinGLink, WinGLink, aa geophysical geophysical interpretation interpretation software software program. program. The km in in thickness. thickness. The Vermilion Vermilion fault is a steeplysteeplyBurntside pluton is a thick body between 2-3 km north dipping to vertically oriented feature. Using Using aa gravimetric gravimetric three-dimensional three-dimensional iterative iterative technique on the Wakemup Bay pluton pluton resulted resulted in in aa good good first-order first-order picture picture of of the the pluton. pluton. Most of the pluton is very thin, less than 0.5km thick. There are are two root zones zones of up to to 4 km depth, thick. There both of which lie on the southern southern portion portion of the the Wakemup pluton, pluton, furthest furthest away away from from the the Vermilion fault. INTERPRETATION We interpret interpret the the Burntside Burntside and and Wakemup Wakemup plutons plutons as as part part of of the the same same on the the Vermilion Vermilion fault. fault. These granitic complex prior to strike-slip faulting on These igneous igneous bodies bodies are are similar in composition and both have undergone solid-state deformation. The plutons have similar structural settings. The TheWakemup WakemupBay Bay pluton pluton intrudes intrudes an an F3 F3 fold hinge hinge and the Burntside Burntside pluton Given the the separation, separation, the the folding folding episodes episodes may may or may has refolded F2 folds at its southern end. Given not correlate correlate exactly. exactly. The gravity gravity inversion inversion and and AMS study study on the Wakemup Wakemup pluton provide provide constraints constraints on on pluton emplacement. The km. Because The pluton pluton has has an average thickness of 0.5 km. Because the pluton contains a roof of wallrock, this estimate reflects the true thickness of the pluton. The TheAMS AMS foliation foliation and and lineation lineation parallel parallel the the fold fold limbs limbs and and fold fold hinge, hinge, respectively, respectively,of of aakm-scale km-scale F3 F3fold. fold. Therefore Therefore we interpret interpret the the Wakemup Wakemup as as syntectonically syntectonicallyintruding intruding an an F3 F3 fold fold hinge. hinge. We use a forward forward gravity model to estimate estimate the dip dip on on the the Vermilion Vermilion fault, fault, which whichdips dips between 70° 70' N and vertical. This Thisinterpretation interpretationrequires requiresthat thatthe the section sectionof of the theVermilion Vermilionfault fault south of the Burntside pluton was not active active as a south-side south-side down normal normal fault. fault. We propose the following tectonic model (figure 1). The The Burntside Burntside pluton pluton and and the the granitic complex. complex. The Wakemup Bay pluton were initially part of the same granitic The Vermilion Vermilion fault fault was was initiated as a normal fault (figure 1, juxtaposed the amphibolite facies Quetico 1, stage 1), I), which juxtaposed with the the greenschist greenschist facies facies Wawa Wawa belt. belt. The Wakemup tonalite, with a thick root on sub-province with its south side, acted as a promontory in the fault system. The The Vermilion Vermilion fault fault was was then then reactivated as a strike-slip fault (figure 1, stage 2), cutting through the the thinnest thinnest (NW) section of of the Wakemup Bay pluton. This This created createdthe the fault-bounded fault-bounded block block that that contains contains the the Wakemup Wakemup Bay pluton. Therefore, Therefore,ititisisevident evidentthat thatthe thepluton plutonshape shapehas has played played aa crucial crucialrole role in in controlling controlling Vermilion Vermilion fault orientation, orientation, both both for for the the early early normal normal faulting faulting and and later later strike-slip strike-slipfaulting. faulting. REFERENCES REFERENCES Minnesota. Minnesota Bauer, R.L., 1985, 1985, Norwegian Bay Quadrangle, St. Louis County, Minnesota. Minnesota Geological Geological Survey, Miscellaneous Miscellaneous Map series, series, Map Map M-59, M-59, 1:24,000. 1:24,000. Bauer, R.L., 1986, 1986, Multiple Multiple folding folding and and pluton pluton emplacement emplacementin in Archean Archeanmigmatites migmatitesof of the the southern southern Vermilion granitic 1753-1764. granitic complex, complex, northeastern northeastern Minnesota. Minnesota. Can. Can. J. J. Earth Earth Sci., Sci., v. v. 23, 23, p. p. 1753-1764. Bauer, R.L., and Bidwell, M.E., 1990, 1990, Contrasts in the response to dextral transpression across the QueticoWawa subprovince boundary in northeastern Minnesota. Can. Can. J. J. Earth Earth Sci., Sci., v. v. 27, 27, p. p. 1521-1535. 1521-1535. Sims, P.K., and Mudrey, M.G., 1972, district, in in Sims, P.K., P.K., et et al., eds., eds., 1972, Burntside granite gneiss, Vermilion district, Geology of Minnesota: Centennial Volume: St. St. Paul, Minnesota Minnesota Geological Geological Survey, Survey, p. p. 98-101. 98-101. Minnesota: A Centennial Vigneresse, Vigneresse, J.L., J.L., 1995, 1995,Control Control of of granite graniteemplacement emplacementby by regional regional deformation: deformation:Tectonophyiscs, Tectonophyiscs,v.v.249, 249,p.p. 173-186. 173-186. 15 �GEOLOGY AND MINERAL MINERAL POTENTIAL POTENTIAL OF PROTEROZOIC PROTEROZOIC MAFIC MAFIC INTRUSIONS IN THE NORTHERN GRENVILLE GRENVILLE PROVINCE PROVINCE OF ONTARIO ONTARIO R.M. EASTON, Ontario Geological Survey, Survey, 933 Ramsey Lake Road, Sudbury, Ontario P3E 6B5, mike.easton@ndm.gov.on.ca rnike.easton@ndm.gov.on.ca Since 1998, 1998, mafic intrusions intrusions near the Grenville Grenville Front in Ontario Ontario have have been been prime prime exploration targets for Cu-Ni-PGE Cu-Ni-PGE mineralization. mineralization.To To assist assist in in this this effort, effort, the the Ontario OntarioGeological Geological Survey has conducted detailed mapping in high potential areas of the Grenville Grenville Province Province between between 1999 and 2002. This poster summarizes the results of these mapping efforts. efforts. East Bull Bull Lake Lakeintrusive intrusivesuite, suite,including includingthe theRiver RiverValley Valleyintrusion: intrusion:Country Countryrocks rockstotoEast East East Bull Lake intrusive intrusive (EBLI) (EBLI) suite suite rocks in the area area are inferred to be mainly Archean Archean in in age, age, and and are grouped into 4 gneiss associations. associations. Metamorphic grade is upper amphibolite amphibolite facies; fades; country country rocks to the mafic intrusions intrusions are are commonly commonly migmatitic. migmatitic. The Paleoproterozoic PaleoproterozoicEBLI EBLI suite suite consists consists of several several mafic mafic layered layered intrusions intrusions emplaced emplaced between 2490 krn, roughly roughly 2490 and and 2468 2468 Ma Ma (James (Jamesetetal. al.2002) 2002)that thatoccur occurover overaadistance distanceofof—250 -250 km, site of Sudbury. Sudbury.The The largest largest of these bodies in in the the Grenville Grenvilleisis the the River River centered on the present site Valley intrusion, intrusion, which underlies roughly 100 of Dana Dana and Crerar townships. Previous maps 100 km2 krn2 of correlated correlated mafic rocks west of Crerar Crerar Township Township with with the the River River Valley Valley intrusion. intrusion.This This study study indicates indicates that at least least 3 separate separate intrusions intrusions are are present, present, each each emplaced emplacedinto into different differentcountry country rocks, and with different different stratigraphy stratigraphy and mineral potential. EBLI suite rock types range in composition from anorthosite anorthosite to melanorite, melanorite, troctolite troctolite and and rarely peridotite; leucogabbronorite leucogabbronorite and gabbronorite dominante. dominante. The The crystallization crystallizationorder order of of olivine orthopyroxene (An80-62), olivine (Fo7659), orthopyroxene primocryst phases phases is is most mostcommonly commonlyplagioclase plagioclase(An8062), (En7558), titanomagnetite,and andclinopyroxene. clinopyroxene. In In Dana Dana Township, Township, the the River River Valley intrusion (En75-58),titanomagnetite, locally exhibits primary mineralogy and well preserved igneous textures. textures. Phase Phase layering layering varies from cm- to m-scale, m-scale, which is discernable discernable in outcrop, outcrop, and and dm dm or or larger, larger, which which is is identified identifiedby by detailed mapping. Isomodal layering is most common; common; mineral and and size size graded graded layers layers are are less less common. Cryptic layering is well documented for the River Valley intrusion. intrusion. Pearce-element Pearce-element ratio and chondrite-normalized chondrite-normalizedREE REE diagrams diagrams illustrate illustrate that that each each body formed formed from from one one or or more more tholeiite composition composition can can explain explain the the cogenetic magmas (James et al. 2002). A high-Al, low-Ti tholeiite al. 2002). 2002). dominant leucocratic rock compositions compositions in the the EBLI EBLI suite suite (James (James et al. Within g/t Pd+Pt+Au) Pd+Pt+Au) Within the the EBLI EBLI suite, suite, contact-type contact-type Cu-Pd-Pt Cu-Pd-Pt mineralization mineralization (1 (1 to to 10 10g/t occurs in the matrix of an inclusion and/or fragment-bearing gabbronorite to leucogabbronorite inclusion fragment-bearing gabbronorite to leucogabbronoriteat at the base or side of the intrusions intrusions where the primary igneous contact is preserved. preserved. A A second, second, m above the the contact. contact. Examples Examples occur occur similar, zone of mineralization may occur 100-200 m throughout the EBLI suite, however, the most consistent consistent grades have been reported reported from from the the % sulfide, sulfide, River Valley intrusion in Dana Township. Township. Chalcopyrite and lesser lesser pyrrhotite pyrrhotite form form 1-3 1-3% disseminated or as local cm-sized either finely disseminated cm-sized patches. PGE mineralization mineralizationis is commonly commonly associated with sulphide. sulphide. Study Study of the East Bull Lake intrusion indicates indicates that that mineralization mineralization originates from the intrusion and subsequent subsequent dynamic dynamic mixing of S-saturated, S-saturated,inclusion-bearing, inclusion-bearing, second-stage (PGE enriched, i.e. 20-100 20-100 ppb PGE) magmas that entered entered the the magma magma chamber chamber second-stage carrying liquid sulfide sulfide droplets droplets (James (James et et al. al. 2002). 2002). Reef-style Reef-style mineralization mineralizationhas has yet yet to to be be documented within the the EBLI EBLI suite. suite. 16 �history between Geological history between Sudbury Sudburyand andRiver RiverValley: Valley:Archean Archeanrocks rocksin in this this area area record a sequence of events similar to that observed in the Levack Gneiss Gneiss complex complex and and high-grade high-grade portions of the Quetico subprovince, subprovince, but unlike the Pontiac subprovince. The following geological history is inferred. inferred. After deposition of greywackes greywackes south south of the the Temagami Temagarni greenstone greenstone belt, invasion by tonalitic tonalitic to to granodioritic granodioritic plutons, plutons, probably probably accompanied accompaniedby by burial, burial, formed formed the the migmatitic gneisses gneisses now represented represented by the Pardo Pardo and and Red Red Cedar Cedar Lake Lake gneiss gneiss associations, associations,likely likely between 2685 and 2675 2675 Ma. Ma. This This was was followed followedby by aa second second period period of of tonalitic tonalitic to to granodioritic granodioritic magmatism, deformation deformation and and metamorphism metamorphism at at mid-crustal mid-crustal levels levels between between 2670 2670and and2660 2660Ma. Ma. The Crerar gneiss association association represents represents the the products products of of this this latter latter activity. activity.Subsequent Subsequentfelsic felsic magmatism at roughly 2640 Ma was accompanied accompanied by emplacement of pegmatite pegmatite veins. veins. A logical extension of this work is to interpret the gneiss associations as a southwarddeepening section of the crust. crust. As interpreted, interpreted, Archean Archean metawackes metawackes exposed exposed immediately immediatelynorth north of the Grenville Grenville Front represent high-levels of the crust. crust. The The Pardo Pardo gneiss, gneiss, immediately immediatelysouth south of of the Grenville Grenville Front, represents represents the middle middle part of aa 10-15 10-15 km km thick thick upper upper crustal crustallayer layer dominated dominated supracrustal and intrusive by supracrustal intrusive rocks. rocks. The Red Cedar Cedar Lake gneiss and and the Street Street gneiss gneiss association association represent the basal portion of this upper crustal crustal layer, layer, with the former former derived derived from from aa metasedimentary rock sequence and the latter latter from a greenstone sequence. sequence. Intrusive Intrusive rocks rocks of of the the km thick middle middle crustal crustal layer. layer. This This crustal crustal section section is is Crerar gneiss association are part of a 10-15 10-15 km roughly equivalent to that observed observed across the Wawa gneiss domain. domain. Emplacement Emplacementof of EBLI EBLI suite suite bodies occurs at several levels levels within this this crustal crustal section. section. Flett Evidence forfora amafic Flett Township Townshipmafic maficIntrusions: Intrusions: Evidence maficand andA-type A-typegranite granitemagmatic magmaticprovince province in the northern Grenville Grenville Province Province was discovered while examining mafic intrusions near Temagami that occur occur in Tomiko Tomiko domain, domain,near near its its contact contact with with the the Grenville GrenvilleFront Fronttectonic tectoniczone. zone. Proterozoic Proterozoic country country rocks consist consist of of gneissic gneissic granite, granite, with with minor minor mafic mafic and andquarztose quarztosegneiss gneissand and metaconglomerate. metaconglomerate. The Fall Lake intrusion consists of little metamorphosed gabbro and leucotroctolite. The Fanny Lake intrusion consists of olivinite leucotroctolite. olivinite and and troctolite. troctolite. Igneous Igneous texture texture is is well preserved, but metamorphic coronas coronas occur occur around around primary olivine olivine and and clinopyroxene. clinopyroxene. Geochemistry indicates that both bodies are slightly alkalic, alkalic, compositionally compositionally similar similar to to the the ±44 Ma, Ma, and and have have affinities affinities to to within-plate within-plate basalts. basalts. Sudbury diabase dike swarm dated at 1238  The Fall Lake intrusion intrusion yielded yielded pristine pristine baddeleyite, baddeleyite, with 33 concordant concordantor or just slightly slightly discordant grains 1235±22Ma. Ma. The The Fanny Fanny Lake sample 2 0 7 ~ b / 2 0age 6age ~ bofof1235 sample discordant grainsgiving givingan anaverage average207Pb/206Pb yielded baddeleyite, with some grains having thin zircon overgrowths, overgrowths, consistent consistent with with the the presence of corona corona textures textures in in the the body. body. Two Two concordant concordant grains grains without without overgrowths overgrowthsgave gavean an average 207Pb/206Pb 1238  ± 2 Ma. 2 0 7 ~ b / 2 0age 6age ~ bofof 1238 average Both intrusions intrusions are are spatially spatially associated associatedwith with the the A-type A-type Mulock Mulock granite, granite,dated dated previously previously at 12444L3 1 2 4 4 1 - Ma. ~ Intrusions of similar age include the Sudbury dike swarm, swarm, Mercer Mercer anorthosite, anorthosite, granitoid plutons. The new age data data provides provides further further evidence evidence and the West Bay and Powassan granitoid for the presence of a bimodal magmatic province active from 1270-1235 1270-1235Ma Ma in in the the Laurentian Laurentian margin of the Grenville Province. The tectonic setting is interpreted as an extensional rift nft that formed formed inboard inboard of aa continental continental arc arc active active on on the the southern southern margin margin of of North North America Americabetween between 1450-1300 1450-1300 Ma. This setting setting resembles that of the Cenozoic Cenozoic Columbia ColumbiaRiver River Basalt BasaltGroup. Group. James, R.S., Easton, Easton, R.M., R.M., Peck, Peck, D.C. D.C. and and Hrominchuk, Hrorninchuk, J.L. J.L. 2002. 2002.The TheEast EastBull BullLake Lakeintrusive intrusivesuite: suite:remnants remnantsof ofaa —2.48Ga Galarge largeigneous igneousand andmetallogenic metallogenicprovince province in in the the Sudbury Sudbury area of the Canadian Canadian Shield; -2.48 Shield; Economic Economic p.1577-1606. Geology, v.97, p. 1577-1606. 17 �PALEOSTRESS INFERENCES PALEOSTRESS INFERENCES FROM FROMFAULT FAULT SLIP SLIP VECTORS VECTORS IN IN THE THE EASTERN EASTERNPART PARTOF OFTHE THE WISCONSIN SEGMENT SEGMENTOF OFTHE i'H MIDCONTINENT Rwr WISCONSIN MIDCONTINENT RIFT Amy Garbowicz, Garbowicz, Marcia Marcia Bjornerud, Bjornemd, Geology Department, Department, Lawrence University, Appleton, WI 54912 549 12 Building accurate accurate models models for for both both the opening Building opening and closing closing of the the Midcontinent Midcontinent Rift Rift requires an an understanding of the evolution requires understanding of evolution of regional regional stresses stresses over over time. time. This This study study paleostress indicators indicators in in the the portion portion of of the Rift exposed near focused on slickenfibers as paleostress of Lake Superior in in northeasternmost northeasternmost Wisconsin. Wisconsin. The orientations of the southern shore of slickenfibers were were used used to determine slickenfibers determine slip vectors vectors on on outcrop-scale outcrop-scale faults within riftspan the the entire range of the related igneous and related and sedimentary sedimentary rocks. rocks. Rocks Rocks sampled sampled span the Keweenawan Supergroup, from the Tyler Formation to the Freda Sandstone, with most of the sampling in the Porcupine Porcupine Volcanics, Volcanics, the Kallander Kallander Creek Creek Volcanics, Volcanics, and and the the Mellen Mellen Gabbro. . The The mineral composition of the slickenfibers was used as a proxy for their age, Gabbro. based on the based the known known regional regional sequence sequence of of secondary secondary mineralization mineralization within within the Rfit. Rfit. Chlorite and epidote slickenfibers were grouped together and considered older since these slickenfibers were among the first were first minerals minerals precipitated precipitated by hydrothermal hydrothermal fluids following the main main of calcite and zeolite were magmatic interval. interval. Slickenfibers of were considered considered to to be be younger. younger. Some individual faults were observed to have have multiple multiple generations generations of of slickenfibers slickenfiberswith with different compositions, indicating indicating either either reactivation reactivation or or continuous continuous slip slip over a protracted protracted different period Fault Kinematics Kinematics (by R. period of time. time. Data Data from the the field field were were analyzed analyzed using using Fault Ailmendinger, Cornell Unviersity), a program that calculates Allmendinger, calculates best-fit best-fit paleostress paleostress tensors tensors from fault slip tensors all indicate normal stress slip information. information. The calculated tensors stress regimes regimes (maximum principal principal stress subvertical), even for the (maximum the latest latest generations generationsof ofslickenfibers. slickenfibers. This This contrasts contrasts with with the results results of of studies studies on on the theKeweenaw Keweenaw Peninsula, Peninsula, which which have have documented two two distinct stress documented stress regimes. regimes. There, There, early early normal normal faulting faulting gives gives way way to reverse faulting, possibly as a response to far-field far-field stresses stresses associated associated with the the Grenville Grenville Orogeny. The Orogeny. The absence absenceof of reverse-slip reverse-slipvectors vectors in in the the northeastern northeastern Wisconsin Wisconsin segment segment of of the Midcontinent Midcontinent Rift may reflect the misorientation of this part of the rift with respect to those far-field far-field stresses. stresses. . 18 �Possible Origin in in the the Negaunee Negaunee Mode of Occurrence of Trona and Thermonatrite and their Possible Iron-Formation of the Marquette Range, Lake Superior District, USA Tsu-Ming Han (Retired) (Retired) Research Laboratory, Cleveland-Cliffs Cleveland-Cliffs Inc. the silicate-bearing Negaunee Negaunee IronIronA white colored substance substance is often seen on the surface of the Formation of low metamorphic Range, Michigan. This substance is metamorphic grade grade on the Marquette Marquette Range% thermonatrite) ItIt occurs mostly of a mixture containing hydrous sodium carbonates (trona and thermonatrite) occurs as as thin coatings along bedding (Figure (Figure 1-A), 1-A), and in fractures cutting cutting across the bedding; as coatings patterns distributed distributed and colloform clusters on bedding planes (Figure 1-B); and as contour patterns Furthermore, nearly pure trona trona was developed quickly quickly between the fractures of bedding surfaces. Furthermore> dendrites and minute dots as dendrites dots on the cut cut surfaces surfaces of of some some hand specimens specimens in in storage storage(Figure (FigureC). C). To the writer's knowledge, knowledge, these these minerals minerals have not been previously reported reported from Precambrian Precambrian BIF B E of of the the equivalent equivalent metamorphic metamorphicgrade grade in in other other districts. districts. ILl Figures - Mode Mode of occurrence occurrence of trona and thermonatrite. thermonatrite. Figures 11— A—As whitecoatings coatingsalong along bedding. bedding. B-As B—Ascolloform colloformclusters clusters on on aa bedding bedding plane. A-As white C— Asdendrites dendriteson onthe thecut cut surface surface of of aa hand hand specimen in storage. C- As storage. The iron-formation is composed composed of of magnetite, magnetite, siderite, siderite, ankerite, ankerite, and and stilpnomelane.. stilpnomelane.. Minnesotaite Minnesotaite is also locally present in noticeable quantities. K is more than Na in these minerals as is the case in nearly nearly all all of of the the Precambrian Precambrian iron-formations iron-formationsof oflow lowmetamorphic metamorphicgrade. grade. As As aa 19 . �general generalrule, rule,stilpnomelane stilpnomelanecontains containsmore moreKKand andNa Nathan thanthe theminnesotaite. rninnesotaite.However, However,the the K20:Na.20 K20:NaZOratio ratioininthese theseminerals mineralsand andinthe in theiron ironformationa formationsmay mayvary varysubstantially.. substantially. . Based Basedon onthe theresults resultsfrom fromthe thehighly highlypurified purifiedwater waterleaching leachingtests testson onmore morethan thantwenty twentydifferent different samples, samples,the theNa Naininthe theiron-formation iron-formationisiswater-soluble water-solublewhereas whereasthe theKKisispractically practicallyinsoluble insoluble (Figures 2Aand andB). B).The TheXRD XRDand andanalytical analyticaldata datashow showaagood goodcorrelation correlationbetween betweenthe theamount amount (Figures2A ofofstilpnomelane Na20, K20 ofNa.20, K20and andA1203 A1203(Figures (Figures3A 3AtotoC). C). stdpnomelaneand andthe theamounts amountsof y = 9.9759x- 0645 0.05 0 0.10 0.15 020 - Figure22 AAand andBB Solubility Solubilityof ofK20 K 2 0and andNa20 Na20 Figure in the silicate-bearing iron-formation with high in the silicate-bearing iron-formation with high andlow low K20:Na20 K20:Na20ratios. ratios. and - 25: R 2O 15 15 - Figure33- AAto to C C Relationship Relationship of of stilpnomelane stilpnomelane Figure toK20, K20,Na20 Na20and andA1203. Al203, to 00 0.5 0.5 1.0 1.O 1.5 1.5 2.0 2.0 2.5 2.5 ItItmay may be be logically logicallyconcluded concludedthat thatmost mostof ofthe thesodium sodiumwas wasderived derivedfrom fromthe the stilpnomelane, stilpnomelane, which ofthe thehydrous hydroussodium sodiumcarbonates carbonateswas was whichwas was leached leachedout outby bymeteoric meteoricwater. water.The Themixture mixtureof then thendeveloped developedthrough throughevaporation evaporationunder underthe theatmospheric atmosphericconditions. conditions. 20 3.0 3.0 �Keweenawan Mafic and and Ultramafic Intrusive Intrusive Rocks of the Lake Nipigon and Ontario Crystal Lake areas, northwestern Ontario Hart, Hart, Thomas ThomasR., R.,Ontario OntarioGeological GeologicalSurvey, Survey,933 933Ramsey RamseyLake LakeRoad, Road, Sudbury, Sudbury, Ontario Ontario P3E 6B5; tom.hart@ndm.gov.on.ca tom.hart @ndm.gov.on.ca The Keweenawan diabase diabase sills sills in the Lake Lake Nipigon and and Crystal Crystal Lake Lake areas, areas, northwest of Lake Superior, Superior, consist consist of of two distinct distinct geochemical geochemical and and geographical geographicalgroups groups with each area also hosting hosting aa number number of unique unique intrusions intrusions that that suggest suggest different differenttectonic tectonic processes. Mapping Mapping by Smith Smith and Sutcliffe Sutcliffe (1987) (1987) in the Crystal Crystal Lake Lake area area identified identified aa series of 6 Logan diabase diabase sills sills >5 >5 m thick that gently gently dip dip to the southwest, southwest,and and intrude intrude into the early Proterozoic Proterozoic Rove Rove Formation. Formation. Northeast Northeast trending trending dykes dykes of of the the Pigeon Pigeon River River swarm range from olivine olivine to quartz diabase diabase in composition, composition,and and include includedykes dykes that that crosscut the Logan sills and dykes that appear to merge with the sills. sills. The The layered layered gabbro gabbro — anorthosite anorthosite -- troctolite troctolite Crystal Lake Gabbro crosscuts and contains inclusions of Pigeon -Tb/Yb - ZrIY Zr/Y River dykes. Samples of the Logan sills sills can be subdivided subdivided into into aa low low TiO2 Ti02 -TWYb group and -Tb/Yb - Zr/Y (OGS2002). 2002).The The high Ti02 group ZrIY group group (OGS high Ti02 group waswas and aa high high TiO2 Ti02 -Th/Yb identified as asbeing beingquartz quartz normative, comparable theLogan type Logan by Sutcliffe normative, andand comparable to thetotype sills bysills Sutcliffe (1991). Samplesidentified identified Pigeon River dykes exhibit high degree of variability (1991). Samples as as Pigeon River dykes exhibit a higha degree of variability suggesting that they represent represent at least three three unrelated unrelated intrusions. intrusions. One One subset subset of of the the comparable to the low TiO2 of Logan Logan sills, and another Ti02 group of Pigeon River dykes is comparable Gabbro. Most subset is comparable to the Crystal Lake Gabbro. Most of of aa third third subset subset of of dyke dyke samples samples contain lower lower trace trace element element are located along Highway 61 close to the Pigeon River, and contain abundances and ratios than the other other intrusions intrusions in the area. area. Gabbro Gabbro samples samplesfrom from the the Crystal Lake Gabbro intrusion display some overlap with the low TiO2 group of Logan Ti02 of Tb/Ta ratios. The Th/Yb, and TWTa sills but also includes samples samples with higher Zr/Y, ZrIY, ThIYb, The Logan diabase sills are confined to the area to the south of Thunder Thunder Bay, with the Nipigon Nipigon diabase sills located to the north. north. The initial initial Keweenawan Keweenawan intrusive intrusive event event in in the the Lake Lake Nipigon Nipigon area area isis probably probably represented by the relatively flat lying to shallowly dipping dipping peridotites peridotites located located in in the the Disraeli, Seagull - Fox Mountain, Hele, and Kitto areas that form intrusions a - Seagull Leckie— Disraeli,Leckie few kilometres in diameter. The peridotites are composed of orthocumulate to mesocumulate textured wehrlite to lherzolite, containing 1 to 2% reddish brown mica and commonly a discontinuous (e.g. Sutcliffe Sutcliffe1987; 1987;Hart Hart et et al. al. discontinuous olivine olivine gabbro gabbro border border phase phase (e.g. 2002). The Disraeli, Seagull Seagull and Hele peridotites are characterized characterized by higher higher MgO and and Zr/Y Tb/Yb values but lower ThITa Th/Ta ratios than the Nipigon diabase sills. A series of of ZrIY and TWYb 0.5 to 3.0 m thick sills are located stratigraphically below below the Nipigon sills, as exposed Tb/Yb and Tb/Ta along Highway 17 at Kama Hill. These sills have MgO, TWYb TWTa values values intermediate Tb/Ta intermediate between the peridotites and Nipigon sills, and subdivided into higher Th/Ta and lower Tb/Ta subgroups may be possible with additional sampling. These sills have TWTa subgroups with additional sampling. These sills have Tb/Ta La/Yb ratios comparable to the high Ti02 group of Logan sills, but generally ThITa and LalYb have lower trace element abundances. The Kitto peridotite also has ThIYb, Th/Yb, Th/Ta Tb/Ta and Zr/Y ratios that overlap with these sills rather than the other peridotites. The olivine ZrIY tholeiite Nipigon diabase sills are up to 200 m thick, and are chilled against the peridotite pendotite intrusions. Previous work indicates that some sills were formed by multiple pulses of of magma (e.g. Sutcliffe, Sutcliffe, 1987; 1987; Hart et al. al. 2002), 2002), but the the chemistry chemistry of of the the sills sills over overthe the 21 �entire Lake Nipigon area area displays displays little little variation. variation. Geochemical Geochemical differences differencesbetween between the the entire peridotites and Nipigon diabase diabase sills are comparable comparable to the variations observed in the peridotites Osier values Osler Group volcanic rocks (e.g. Sutcliffe, 1991). The The Nipigon Nipigon sills sills have have Ti02 Ti02 values Ti02group group of of Logan sills sills but higher higher ThITa Th/Ta and and lower lower La/Yb LdYb comparable to the the low low Ti02 comparable differences in the the geochemistry geochemistry of the the diabase diabase sills sills between between the the Lake Lake Nipigon Nipigon ratios. The differences and Crystal Lake areas is similar to the differences observed in the volcanic volcanic rocks of aa flood basalt provinces provinces (e.g., (e.g., Mantovani Mantovani et et al. al. 1985). 1985).The The regional regionalextent extentof of the the number of flood An initial initial geochemical groups within the Keweenawan intrusions is not known. An examination of troctolites troctolites from from the the Babbit Babbit deposit deposit of of the the Duluth Duluth Complex Complex(Ripley (Ripleyetetal. al. examination 1999) Th/Yb, and and Zr/Y Zr/Y ratios ratios comparable to the Nipigon peridotites peridotites ThITa, Th/Yb, 1999) indicates ThTFa, rather than the intrusions intrusions of the the Crystal Crystal Lake area. area. References References Hart, T.R., terMeer, M., and Jolette, C. 2002. Precambrian geology of Kitto, Eva, Summers, Dorothea and Sandra Townships, Beardmore area, northwestern Ontario; Ontario Geological Survey, Open File Report 6095, 206 206 p. PL.S., de de Sousa, Sousa, M.A., MA., Civetta, L., Atalla, L., and Innocenti, Innocenti, F., F., 1985. Trace Trace element element and and Mantovani, M.S.M., Marques, L.S., strontium isotopic constraints on the origin and evolution of Parana continental flood basalts of of Santa Santa Catarina State (southern Brazil); Journal of Petrology, v. 26, p. 187-209. Ontario Geological Survey, 2002. Proterozoic Volcanic and Intrusive Whole Rock Geochemical Data associated with the Keweenawan Midcontinent Rift, Lake Superior Area, Ontario; Ontario Geological Survey Miscellaneous Release—Data 114. Release-Data 114. EM., Lambert, Pb isotopic isotopic constraints constraints on on mantle mantle and and crustal crustal Ripley, E.M., Lambert, D.D., and Frick, L.R., 1999. 1999. Re-Os, Sm-Nd, and Pb to magmatic magmatic sulfide mineralization in the Duluth Complex; Geochimica et Cosmochimica Acta, contributions to v. V. 62, p.3349-3365. Smith, A.R. and Sutcliffe, R.H. 1987. Keweenawan intrusive rocks rocks of of the the Thunder Bay area; in Summary Sunnnary of Field Work and Other Activities, Ontario Geological Survey Miscellaneous Paper 137, 137, p. 248-255. 1987.Petrology Petrology of of Middle Middle Proterozoic Proterozoic diabase diabase and picrites from Lake Nipigon, Sutcliffe, R.H., RH., 1987. Nipigon, Canada; Canada; Contributions Contributions to Mineralogy and Petrology, v.96, p. 201-211. 201-21 1. Sutcliffe, R.H., RH., 1991. 1991.Proterozoic Proterozoic geology geologyof of the the Lake Lake Superior Superior area; area; in in Geology Geology of of Ontario, Ontario, Ontario Ontario Geological Geological Survey Survey Part Special Volume 4, P art 1, 1, p. 627-658. 22 �GEOLOGY, DRILL HOLES, MINERAL MINERAL LEASES, AND GEOPHYSICS GEOPHYSICS IN THE THE DULUTH DULUTH GEOLOGY, INTEGRATIONOF OF AND BEAVER BAY COMPLEXES, NORTHEASTERN MINNESOTA: INTEGRATION VARIOUS GIS GIs DATABASES DATABASES TO TO TELL TELL A A STORY STORYOF OFTHE THE HISTORY HISTORYOF OFPAST PASTAND AND VARIOUS CURRENTCU-NI-PGE CU-NI-PGEMINERAL MINERALEXPLORATION EXPLORATION CURRENT Steven A. Hauck, Hauck, Julie Julie A. A. Oreskovich, Oreskovich,and and Mark Mark J.J. Severson, Severson,Economic EconomicGeology GeologyGroup, Group, Steven Natural Natural Resources ResourcesResearch Research Institute Institute(NRRI), (NRRI),University University of of Minnesota, Minnesota,Duluth, Duluth,5013 5013 MN 55811-1442, 55811-1442,shauck@nrri.umn.edu shauck@nrri.umn.edu Miller Trunk Trunk Highway, Highway, Duluth, Duluth,MN Miller Mineral exploration exploration in the Duluth Duluth Complex Complex began in 1948 1948 on Spruce Spruce Road when two prospectors found found sulfide sulfide mineralization. mineralization. Subsequent Subsequentcore coredrilling, drilling,geological geologicalmapping, mapping,and andairborne airborneand and ground ground geophysics geophysics by more more than than 28 28 exploration explorationcompanies companies (including (includingthe the NRRI, NRRI,MGS MGS-Minnesota Minnesota Geological Survey, Survey, and and the DNR -- Dept. Dept. of Natural Resources, Resources,Division Divisionof of Lands Lands and and Minerals), over the next 52 years led to the discovery of copper-nickel±platinum-group element Minerals), over the next 52 years led to the discovery of copper-nickelkplatinum-group element (PGE) mineralization mineralization along the basal contact of the Duluth Complex (Fig. 1). Ten Ten Cu-Ni-PGE Cu-Ni-PGE or or PGE-Cu-Ni deposits deposits were were defined by drilling during these years. Over Over2,142 2,142drill drill holes holes have have been drilled drilled into into the Duluth Duluth and and Beaver Bay complexes with 1,666 1,666 of these holes being drilled along along the basal contact. contact. Over Over954,000 954,000ft. ft.of ofdrill drillcore corefrom fromthe thebasal basalcontact contacthas hasbeen beenrelogged relogged by NRRJ, publications. Geophysical NRRI, and their results are discussed in many publications. Geophysical exploration exploration began began as as early as 1956, 1956, by Bear Creek Mining Company, and continues today. The The State State of of Minnesota Minnesota (MGS), with funding funding from from the Legislative Legislative Commission Commission on on Minnesota MinnesotaResources, Resources,flew flewhigh high resolution aeromagnetics aeromagnetics over over this area as well as the rest of the state. The TheMGS MGShas hasalso also collected collected and produced a gravity map covering both complexes. Peak Peakexploration exploration(1966-1978) (1966-1978) began with the leasing leasing of State State of Minnesota Minnesota mineral rights in 1966 1966 (Fig. 1). Exploration Explorationand and development (drilling, bulk sampling, shaft sinking, resource calculations) continued through development (drilling, bulk sampling, shaft sinking, resource calculations) continued through 1978. 1978. In In1998, 1998,State Stateand andFederal Federal mineral mineral leasing leasing and and exploration explorationdrilling drilling began began to to increase increase with with the: 1) 1) rise in in price price of of PGEs; PGEs; 2) 2) possible possible use use of of new new hydrometallurgical hydrometallurgicaltechniques techniquesto tomore more efficiently efficiently recover recover copper copper and and nickel; nickel; and and 3) 3) introduction introductionof of new new PGE PGE exploration explorationmodels models (sulfide i.e.,atatSonju Sonju (sulfide saturation; saturation;Miller Miller et et al., al., 2002) 2002) for for intrusions intrusionsin in the the Beaver BeaverBay BayComplex, Complex,i.e., Lake, and the Duluth Complex, Duluth). The Complex, i.e., Greenwood Lake and Layered Series at Duluth). The maps maps in this this poster poster illustrate illustrate the the relationship relationshipbetween between geology, geology,geophysics, geophysics, drilling, drilling,and andmineral mineral leasing leasing and were produced in ArcView (GIS). (GIs). The Themaps maps were were also alsocompiled compiledby by using using information from: 1) (minarchive.dnr.state.mn.us)attributeattribute-and and GIS-based GIs-based DNR's online online(minarchive.dnr.state.mn.us) 1) the DNR's database database of non-ferrous non-ferrous minerals' minerals' information information and and State State mineral mineral rights rights holdings; holdings; 2) 2) U.S. U.S. Forest Forest Service GIs data dataon onthe thehistory history Service leases, leases, permits, permits, and and applications applicationsdatabase; database; and and3) 3) NRRI NRRI in-house in-houseGIS of Cu-Ni-PGE Cu-Ni-PGE mineralization. mineralization. Using Usingthe theresulting resultingGIS GIsdatabase, database,the thespatial spatialrelationships relationshipsin inthe the changes in drilling, drilling, leasing, leasing, etc. etc. with with time time and and place place were were then then combined combinedwith withgeological geological information information from from Miller Miller et et al. al. (2002) (2002)to to better better understand understandthe the past past and andpresent presentexploration explorationareas areas and and to assist assist in in defining definingnew new areas areas in in which which to to explore explorefor fornon-ferrous non-ferrousminerals. minerals. References References Miller, Jr., Green, Green, J.C., J.C., Severson, Severson, M.J., M.J., Chandler, Chandler, V.W., V.W., Hauck, Hauck, S.A., S.A., Peterson, Peterson, D.M., and Miller, J. D., Jr., Wahl, T.E., 2002, 2002, Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: 207 p. Minnesota: Minnesota MinnesotaGeological Geological Survey SurveyReport Report of of Investigations Investigations58, 58,207 p. 23 �- CU S CU SC INCO 0 CD Creeku Beer Creek I = U (Ii U Ui S Ut S S 0 0 S 0 CU USS S N 1 t S 0 S 0 Dovl Coo Neweori 0 S 10 INCO Beer Creek' I S N 10 S OJ N S 0 N I I I - CU = (S Theel U XX CU 0 CU CU CU CU I CU CU CU a 0' INc0 0' CU 0 S Di 10 I S 0' CU S = 0 - - I PotyMeti 10 0 0 0' CU 0' Xl 0' - CU 0' - - 0 = = CONCENTRATED DRILLING - II IU \felkrdgeU• S 0 Di = SCATTERED DRILL NG Arerieer S/ic d/NICDR CK Lehcerrr I - 0 CU SSS CU 10 N 0 N CU CU S 5555555555555 N !hosorr 1 oxocL1 Cleveerd CliP Fe I • N CD 11111 0 S 55 N Blerkerkerg—Wkiteside U I orrotioc dril eg) I r4 I AMAX - S CU uss •I Devot helps D ooge CD I (costly iroc Beer Creek IIHIIIIHHIII BI I I k/S Moore/A ercerr ShijId Creek I S - sPLie Rood Mokori S ---Beor Ceek •L,L Lrek1 Beer Creeki Bee B Sproce Rood cUr Creek INCO •1US'BM1MGS S CU - Metri = S IIIjIflIII S UCkidere eed Cdhiteeide S CU Figure 1. History of Cu-Ni-PGE exploration in the Duluth Complex (after Miller et at., 2002). GUNFLINT TRAIL DRILLING RECONNAISSANCE OR SCATTERED LONGNOSE (Oul) BIRCH LAKE WATER HEN (OUI) WESTERN MARGIN SO. FILSON CREEK WETLEGS DUNKA PIT WYMAN CREEK (NorthMef) DUNKA ROAD (Minnamax) (Mesaba) BABBITT MATURI SPRUCE RD & Deposit/Area kin.iesolo sow C—Ni nvwol INCO • Duval • Exxon • Other Companies AMAX • Bear Creek !I USS • LEGEND �Geochemistry and Mineralization of the Seagull Seagull Intrusion, Northern Ontario Heggie, P., P., (Department of Geology, Lakehead Heggie,G., G.,and andHollings, Hollings, (Department of Geology, LakeheadUniversity, University,955 955 Oliver Oliver Road, Thunder Bay, On, P7B 5E1, gheggie gheggie@mail.lakeheadu.ca) @ mail.1akeheadu.ca) Platinum Group Elements Elements (PGE-Platinum, (PGE-Platinum, Palladium, Osmium Osmium and Iridium) Indium) have seen seen substantial increases in demand over the last substantial 30 years, as industrial and commercial users have increased their consumption. consumption. Canadian Canadian production of these metals has until recently been limited to by-products from nickel Sudbury). Opening of the copper mines (e.g., Sudbury). Lac des mine in in Ontario demonstrated demonstrated the des Ties Iles mine the potential for economic economic PGE PGE deposits deposits in in Canada. Further work on deposit and exploration exploration models is is essential essential to to identifying identifying new targets and prospective prospective host rocks. rocks. The Seagull Lake intrusion intrusion is found found within within the the Nipigon Embayment, approximately 70km approximately 70km north east of Thunder Thunder Bay, Ontario Ontario (Fig.1). (Fig.1). Relative Relative age dating dating places the age age of the Seagull Lake intrusion to be younger than Map showing location location of Seagull Intrusion Intrusion Figure 1. Map the Sibley Sibley Group sedimentary sedimentary sequence, sequence, and (1339±33 Ma) (Franklin et al., 1980), and regional regionalgeology. geology. (1339k33 1980), as part of the intrusion has been seen to cross cut Sibley Sibley stratigraphy. A chilled chilled margin has been observed between the Seagull Seagull Intrusion and the younger Nipigon Sills defining defining an upper age of approximately 1.1 Ga (Davis (Davis and and Sutcliffe, Sutcliffe,1985). 1985).This This falls falls within within the the time time of of mid-continental mid-continental rifting in the Lake Superior region. Volcanic activity was responsible for production Superior region. Volcanic activity responsible for productionof of thick thick basaltic sequences sequences (Cannon (Cannon et et al., al., 1989) 1989)beneath beneath and and around around the the shores shoresof of Lake LakeSuperior Superiorand andthe the emplacement of numerous mafic to ultramafic complexes (e.g., Duluth complex). complexes (e.g., Duluth complex). The Seagull Seagull Intrusion is currently currently under exploration exploration by East East West West Resource Resource Corporation, Corporation,ItIt isis aa consisting of cumulate layered ultramafic intrusion consisting cumulate olivine, olivine, and oxide oxide minerals minerals with with pyroxene pyroxene oikocrysts and interstitial feldspar. feldspar. Lithological phases include include dunites, dunites, iherzolites, lherzolites, olivine olivine gabbronorites, gabbros, and pryoxenites. A distinctive distinctive olivine olivine gabbronorite gabbronorite is is found found within within the the gabbronorites, intrusion but this exhibits chilled chilled margins and is thought to post date the formation of the rest of the intrusion. intrusion. 25 �DDH WMOO-01 WMOO-01 Depth (m) (m) Depth 375.0 375.0 408.0 572.0 546.0 379.0 Interval Interval (m) (m) 4.0 4.0 4.5 12.0 6.0 8.0 6.0 6.0 6.0 Cu Cu (ppm) (ppm) 269 269 Ni (ppm) Ni ( P P ~ 1160 1160 Pt pt (ppb) (PP~) 1413 1565 779 WM98-02 1180 987 WM98-05 112 1647 569.0 1843 1841 579.0 1220 1455 579.0 1220 1455 Figure Figure 2. Table Table of metal metal contents contents from from assay assay (Caven, (Caven, R., R., 2000) 501 307 307 336 363 535 336 693 458 458 Pd (ppb) pd (PP~) 383 383 418 438 566 393 847 537 Mineralization occurs in the form of PGE minerals associated with disseminated FeNi sulfides (pentlandite). Pentlandite is found in higher abundances at discrete intervals intervals throughout throughout the the intrusion, with a general increase increase towards the base base of of the the intrusion. intrusion. Work is currently being undertaken to understand the the stratigraphy stratigraphy of the the intrusion, intrusion, the the nature nature of of the platinum group mineralization, mineralization, and and formational formational controls controls on on the the mineralized mineralized zones, zones,which whichare are present in the intrusion in order to aid in the development and refinement of exploration techniques, and deposit models. Cannon, W.F., Green, A.G., Hutchinson, D.R., Lee, M., Milkereit, B., B., Behrendt, Behrendt, J.C., J.C., Halls, Halls, H.C., H.C., Green, J.C., Dickas, A.B., Morey, G.B., Sutcliffe, Dickas, Sutcliffe, R., and and Spencer, Spencer, C., C., 1989, 1989,The TheNorth NorthAmerican AmericanMidcontinent Midcontinent rift rift Reflection profiling. profiling. Tectonics, Tectonics, v. 8, p. 305-332. beneath Lake Superior from GLIMPCE Seismic Reflection Caven, R.J., 2000, Progress Caven, R.J., Progress Report Report on on the the Wolf Wolf Mountain Mountain and and Disraeli Disraeli Properties Properties for for East EastWest WestResource Resource Corporation, Corporation, Canadian Canadian Golden Golden Dragon Dragon Resources Resources Ltd. Ltd. and and Avalon Avalon Ventures VenturesLtd. Ltd. Davis, D.W., D.W., and R.H., 1985, Davis, and Sutcliffe, Sutcliffe, R.H., 1985, U-Pb U-Pb ages ages from from the the Nipigon Nipigon plate plate and andNorthern Northern Lake LakeSuperior. Superior. Geological Geological Society Society of American American Bulletin, Bulletin, v.96, v.96, p. 1572-1579. 1572-1579. Franklin, J.M, Mcllwaine, McIlwaine, W.H., Poulsen, K.H., and and Wanless, Wanless, R.K., R.K., 1980, 1980,Stratigraphy Stratigraphy and and depositional depositional setting setting of of the Sibley Group, Thunder Thunder Bay Bay district, district, Ontario, Ontario, Canada. Canada. Canadian Canadian Journal of of Earth Earth Sciences, v.17, p. p. 633-65 633-651. 1. 26 �PEPERITES PEPERITESOF OF THE THEGAFVERT GAFVERTLAKE LAKE VOLCANIC VOLCANIC COMPLEX, COMPLEX,ST. ST. LOUIS COUNTY, MINNESOTA MINNESOTA Heiling, Carrie D., Department of Geological Geological Sciences, Sciences, University University of of Minnesota Minnesota Duluth, Duluth, 1114 1114 Kirby Drive, Duluth, MN, 55812; cheiling@d.umn.edu cheiling@d.umn.edu The Gafvert Lake area, located within the Upper Ely member of the Ely Greenstone Greenstone of the Vermilion District in Northeastern Minnesota (Figure 1) (Card, 1990), forms part part of of a large, Archean, felsic volcanic complex. Morton Morton (personal (personal communication) communication) has has interpreted the complex to be a composite volcano that underwent late stage caldera collapse. This Thisstudy studyhas has focused focused on on aa two two square square mile mile area area in in the the central central part part of of the the complex. Here Herethe thecomplex, complex,from fromthe theoldest oldestto to the the youngest youngest rocks, rocks, is is composed composed of of a) a) coarse, heterolithic breccias (interpreted (interpreted to represent meso-and mega breccias) breccias) (Morton, (Morton, personal communication), b) more than 3000 3000 feet feet of massive massive to to bedded bedded pumice-rich pumice-rich lapilli tuff, c) dacitic lavas and domes, and and d) d) lenses lenses and and beds of of chert chert and and massive massive to to semi-massive pyrite (Figure 2). The The breccias breccias and and lapilli lapilli tuffs tuffs have have been intruded by a swarm of feldspar porphyry dacite dikes that represent feeders to the domes and/or and/or flows. flows. Peperites are rocks formed by the in situ disintegration of magma intruding and and mixing mixing with wet unconsolidated sediment et al., al., 2002). At At Gafvert Gafvert Lake the sediment or or ash (Skilling (Skilling et peperites formed near the top of the complex where dacite dacite porphyry dikes intruded intruded and and unconsolidated pumice-rich pumice-rich lapilli lapillituff. tuff. This mixing led to quenching mixed with wet, unconsolidated and fragmentation of the dacitic magma and disruption and vesiculation vesiculation of of the the lapilli lapilli tuffs. The peperites occur within 100 feet of dike contacts though they form much The peperites occur within 100 feet of dike contacts though they form much more together. Angular and extensive areas where several dikes occur close together. and finger-like blocks of dike material occur within the peperite, locally these are connected to aa nearby nearby dike. dike. Macrotextures in outcrop and and microtextures microtextures in thin thin section section helped helped identify identify and and classify classify the following fragment types and internal structures within the peperites: a) a) blocky blocky jig-saw fit textures, b) platy to ragged juvenile fragments with chilled rims and occasional jig-saw juvenile fragments with curviplanar surfaces and broken gas bubbles, c) ameboid to globular juvenile fragments, d) abundant pumice which exhibits variable variable vesicularity. vesicularity. Most of this pumice is juvenile to the lapilli tuffs but a small percentage contains feldspar crystals identical to those found in the dikes possibly indicating local, local, rapid rapid vesiculation vesiculation of dike material. Close Closeto todike dikemargins marginsfeldspar feldspar crystals crystals are are broken broken and and internally internally fractured with fractures filled by lapilli tuff. Pumice, Pumice, close close to to dike dike contacts, contacts, may may be jigsaw-fit pieces. pieces. Locally the ash matrix to broken or disaggregated into several small jigsaw-fit the lapilli tuffs is amygdaloidal amygdaloidal with amygdules amygdules radiating away from dike margins. margins. References References Card, K.D., 1990, Province of of the the Canadian Canadian Shield, Shield, aa product product of of 1990, A review of the Superior Province Archean accretion: Precambrian Research, v. 48, pp. 99-156. Precambrian Research, v. 48, pp. 99-156. Minnesota, 2003, Mapquest, Ely, Minnesota, Mapquest, www.mapquest.com. www.rnapquest.com. communication, University University of Minnesota-Duluth. Minnesota-Duluth. Morton, R.L., 2003, personal communication, Skilling, Skilling, I., White, J., McPhie, McPhie, J., J., 2002, 2002, Peperite: Peperite: aa review of magma-sediment magma-sediment mingling, mingling, Journal Journal of Volcanology Volcanology and Geothermal Geothermal Research, Research, v. 114, 114, pp 1-17. 1-17. 27 �Figure 1: 1: Location Location of of Gafvert Gafvert Lake Lake complex complex (Mapquest, (Mapquest, 2003). 2003). Explanation Explanation • Peperite samples Fault Zone Contacts / '" ' Railroad grade grade , " Mud Creek Road / Mud Road f' * A/ - Mbas - Metabasalt Metabasalt SIst METERS 200 00 J0 Â¥s 410 400 600 oo 800 Figure Figure 2: Generalized Generalized map of a portion of Gafvert Gafvert Lake volcanic complex. complex. 28 Q Qfo- Qtzfeld - QtzfeldPorphyry Porphyry Black Chert Cht -- Black Cht Diab -- Diabase Diabase Dikes Dikes Dior Dior -- Diorite Diorite Dac -- Dacite Dikes Dikes Dac Tuff -- Lapilli Lapilli Tuff luff Bx Bx -- Breccia Breccia SIst SIst - Siltstone &&Iron Iron Fm Fm �CHEMISTRY OF OF ALTERATION ALTERATION MINERAL MINERAL PHASES PHASES AT THE FIVE MILE MILE LAKE LAKE CHEMISTRY VOLCANIC-HOSTED MASSIVE MASSIVE SULFIDE SULFIDE PROSPECT, PROSPECT,NE NE MINNESOTA MINNESOTA VOLCANIC-HOSTED locker, Hocker, S. S.M., M.,Hudak, Hudak, G. G.J.,J.,Odette, Odette,J.J.D., D.,and andNewkirk, Newkirk,T.T.T., T.,Department Department of ofGeology, Geology, University of Wisconsin Wisconsin Oshkosh, Oshkosh, 800 800 Algoma Algoma Blvd., Oshkosh, WI 54901, hudak@uwosh.edu hudak@uwosh.edu Alteration mineral Lake Prospect Prospect in in the the Vermilion Vermilion Alteration mineral assemblage assemblage mapping mapping at at the the Five Mile Lake identified two distinct types of alteration zones within 2.7 District of northeastern Minnesota has identified year-old volcanic volcanic and and volcaniclastic volcaniclastic rocks rocks associated associated with withvolcanic-hosted volcanic-hosted massive massive billion year-old 2001a,2001b; 2001b;Peterson, Peterson, mineralization (Hudak et a!., al., in in press; press; Odette Odette et et al., al.,2001a, sulfide (VHMS) mineralization 2001). 2001). Regional Regional semi-conformable semi-conformable alteration alterationzones zones are are composed composed of of various various proportions proportions of of quartz quartz ++ epidote epidote ±Â amphibole amphibole ± chlorite chlorite ±Â plagioclase plagioclase feldspar. feldspar. These regional, regional, These are locally locally cross-cut cross-cut by by several severalrelatively relativelynarrow, narrow,northeastnortheastsemiconformable alteration zones are composed of fine-grained fine-grained chlorite chlorite and/or andlor sericite sericite that that trending disconformable alteration zones composed are closely closely associated associated with with synvolcanic synvolcanicfault faultzones. zones. Electron microprobe microprobe analyses the various various alteration alteration mineral mineral phases phases (epidote (epidote group group Electron analyses of of the minerals, chlorite, feldspar) have conducted in an effort effort to to minerals, chlorite, amphibole, amphibole, white white mica, mica, and and feldspar) have been been conducted better understand understand hydrothermal hydrothermal processes processes associated associated with with the the development development of the the semiconformable and Mile Lake Lake prospect. prospect. These These semiconformable and disconformable disconformable alteration alteration zones zones at at the the Five Mile zoisite/clinozoisite to analyses indicate that: a) epidote epidote group group minerals minerals range in composition composition from zoisite/clinozoisite analyses pistacite; b) chlorite chlorite is is dominantly dominantly ripidolite; ripidolite; c) c) amphibole amphibole is is primarily primarily actinolite actinolite and and ferroferropistacite; actinolite, with magnesio-hornblende magnesio-hornblende and is fineheactinolite, and ferro-hornblende ferro-hornblendealso also present; present; d) d) sericite is grained grained muscovite; muscovite; and and e) e) plagioclase plagioclasefeldspar feldsparisisdominantly dominantlyalbite. albite. chemistry at the Five Five Mile Mile Lake Lake Prospect Prospect is is remarkably remarkably similar similar to that that Alteration mineral mineral chemistry VHMS mining mining camp camp of of Canada, Canada,as as well well as as other other VHMS VHMSmining miningcamps campsaround around from the Noranda VHMS the world. world. This alteration alteration mineral mineral chemistry chemistry suggests the presence of aa complex, complex, long-lived long-lived hydrothermal system evolved from from seafloor-proximal seafloor-proximal (hundreds meters) to deeper deeper hydrothermal system that that evolved (hundreds of of meters) subseafloor environments kilometers) as as the the volcanic volcanic rocks rocks were wereburied buriedby byapparently apparently subseafloor environments (-1-3 (1 -3 kilometers) dominantly effusive effusive mafic mafic to to intermediate intermediate volcanism volcanism and and associated associated sedimentation. sedimentation. This This rapid, dominantly addition to the the Five Five Mile Mile Lake Lake Prospect, Prospect, the the uppermost uppermost several several hundred hundred meters meters suggests that in addition Lower Member Member of of the the Ely Ely Greenstone Greenstonealso also has has excellent excellentexploration explorationpotential potential for for VHMS VHMS of the Lower mineral deposits. deposits. References References F., Paradis, Paradis, S., S., Hannington, M., Holk, G., G., Katsube, Katsube, J., J., Paquette, Paquette, F., Galley, A., Bailes, A., Hannington, Galley, A., Bailes, M., Holk, Santaguida, Santaguida, F., F., and and Taylor, Taylor, B., B., 2002, 2002, Database Database for CAMIRO CAMIRO Project Project 94E07: 94E07: Interrelationships between subvolcanic intrusions, intrusions, large-scale large-scale alteration zones, and and VMS VMS Interrelationships between subvolcanic deposits: deposits: Geological Geological Survey Surveyof ofCanada CanadaOpen OpenFile FileReport Report4431 443 1(CD-ROM). (CD-ROM). and Hauck, Hauck, S., S., in in press. press. Comparative Comparativegeology, geology, Newkirk, T., T., Odette, Odette, J., and Heine, J., J., Newkirk, Hudak, G. J., Heine, Mile Lake, Lake, Quartz Quartz Hill, Hill, and and Skeleton SkeletonLake Lake stratigraphy, and lithogeochemistry of the Five Mile VMS VMS occurrences, occurrences, Vermilion Vermilion District, District, NE Minnesota: AAreport reportto tothe theMinerals MineralsCoordinating Coordinating Committee, Minerals Division, State of Minnesota. DNR Minerals Minnesota. Committee, DNIR 29 �Kranidiotis, Kranidiotis, P. P. and and MacLean, MacLean, W. W. H., H., 1987, 1987,Systematics Systematicsof of chlorite chlorite alteration alteration at at the the Phelps Phelps Dodge Dodge Massive Sulfide Sulfide Deposit, Deposit,Matagami, Matagami,Quebec: Quebec:Economic EconomicGeology, Geology,v.v.82, 82, p. p. 1898-1911. 1898-1911. Massive Odette, Odette, J. D., Hudak, Hudak, G. G. J., J., Suszek, Suszek, T., T., and and Hauck, Hauck, S. S.A., A.,2001a, 2001a. Preliminary Preliminary evaluation evaluation of hydrothermal hydrothermal alteration alteration mineral mineral assemblages assemblages and and their their relationship relationship to to VMS-style VMS-style Archean Vermilion Vermilion Greenstone Greenstone Belt, Belt, NE NE mineralization in the Five Mile Mile Lake Lake area area of of the theArchean mineralization 470h Minnesota: Institute Institute on on Lake LakeSuperior SuperiorGeology, Geology, 47thAnnual Meeting, Meeting, Proceedings Proceedings Volume Volume 47, 47, Part Part 1-Program 1-Programand and Abstracts, Abstracts, p. p. 75-76. 75-76. Odette, D., Hudak, Hudak, G. G. J., J., Suszek, Suszek, T., T.,and andHauck, Hauck, S.S.A., A.,2001b, 2001b,Preliminary Preliminary evaluation evaluation of Odette, J. D., hydrothermal hydrothermal alteration alteration mineral mineral assemblages assemblages and and their their relationship relationship to to VMS-style VMS-style mineralization in the Five Five Mile Mile Lake Lake area area of of the theArchean Archean Vermilion Vermilion Greenstone Greenstone Belt, Belt, NE NE mineralization Minnesota: Geological Geological Society Society of of America America Abstracts Abstracts and and Programs Programs Volume Volume 33, 33,No. No. 6,6, p. p. AAMinnesota: 420. 420. Peterson, Peterson, D. M., M., 2001, 2001, Development Development of of Archean Archean lode-gold lode-gold and and massive massive sulfide sulfidedeposit deposit exploration models models using using geographic geographic information information system system applications: applications: targeting targeting mineral mineral exploration exploration in northeastern northeastern Minnesota Minnesota from analysis analysis of of analog analog Canadian Canadian mining mining camps: camps: exploration Duluth, Minnesota, Minnesota, 503 503 p. p. unpublished Ph. Ph. D. D. dissertation, dissertation, University University of of Minnesota, Minnesota, Duluth, unpublished Fe (total) 18 0.2 0.4 0.6 16 14 12 10 0 AlzQ, CORUNDUM CORUNDUM 0.9 0.8 0.7 0.6 La :' 0.5 0.4 0.3 0.2 0.1 I 6.0 6.5  Flm Mite Lake Amphlboto I 7.0 7.0 7.5 8.0 ORTHOCLASE ORTHOCLtSE KAISiiO. KOJSieOe Figure Figure 1.1. Summary Summary of of electron electronmicroprobe microprobeanalyses analysesfor forepidote-group epidote-groupminerals minerals(A), (A),chiorites chlorites (B), (B), amphiboles amphiboles (C), (C), and andwhite whitemicas micas(D) (D)from fromthe theFive FiveMile MileLake LakeProspect Prospectand andselected selectedVHMS VHMS mines. mines.Compositional Compositionalfields fieldsfor forNoranda Norandaminerals mineralsdetermined determinedfrom from Galley Galleyetetal. al.(2002). (2002). 30 30 �GEOCHEMISTRY AND GEODYNAMIC GEODYNAMICIMPLICATIONS IMPLICATIONSOFOFTHE THE 1537 GEOCHEMISTRY AND 1537 MA MA REDSTONE REDSTONE POINT POINT ANOROGENIC ANOROGENICGRANITE, GRANITE,ONTARIO, ONTARIO,CANADA CANADA Hollings, (DepartmentofofGeology, Geology,Lakehead Lakehead University, University, 955 955 Hollings, P., Fralick, Fralick, P.P.and andKissin, Kissin,S.S.(Department Oliver Rd., Rd., Thunder Bay, Bay, Ontario, Ontario, P7B P7B 5E1, 5E1, Canada; Canada; Peter. Peter.Hollines@lakeheadu.ca) Hollinss @ lakeheadu. ca) The Redstone Point Point granite is is aa felsic igneous Mesoproterozoic Mesoproterozoic felsic igneous complex Ma; Davis complex (1537+10/-2 (1537+10/-2 Ma; Davis and and Sutcliffe, 1984) located in the northern northern portion of the Sibley Sibley Basin on the the west west shore of Lake Nipigon (Fig. 1). shore of Lake Nipigon (Fig. 1). It is is unconformably overlain by by arenites unconformably overlain arenites of of the Pass Lake Lake Formation Formation of of the the Sibley Sibley are in turn Group. Group. These These sediments sediments are turn intruded and overlain by an an extensively extensively sills developed developed sequence sequence of diabase diabase sills related to an early stage of the Midrelated to early stage of the Midevent. The entire Continent Continent Rifting Rifting event. entire sequence sequence has been gently folded into aa shallowly, easterly plunging plunging succession succession of open synclines and anticlines, with open synclines and anticlines, with of the the Redstone Point Point Figure 1. Map showing the location of dips usually exceeding 150. Outcrop not usually exceeding 15O. Outcrop granite in relation relation to Proterozoic Proterozoic anorogenic anorogenic granite granite density of of igneous igneous units units is very very good good of North North America. America. Modified after after Anderson (1983) density complexes of Anderson(1983) sedimentary sequences, sequences, which which only only provide provide along the shoreline of Lake Nipigon, in contrast contrast to to sedimentary small, scattered scattered outcrops. outcrops. The igneous rocks of Redstone Point Point have have been been briefly briefly described describedby by Davis Davisand andSutcliffe Sutcliffe(1985), (1985), of Redstone wherein they they emphasised emphasised that that the the rocks are to rhyolites and wherein are anorogenic anorogenic granites gradational gradational to fragmental rhyolites rhyolites and and dacites. dacites. In fact, presently fragmental presently accessible accessible outcrop indicates that extrusive extrusive members dominate dominate the the magmatic magmatic rocks rocks of the area. members area. Porphyritic Porphyritic texture texture with with volcanic volcanic features features including vesicles, vesicles, flow flow structures, units, rubbly rubbly flow tops including structures, agglomeratic agglomeratic units, tops and andsegregation segregation cylinders differentiate differentiate extrusive extrusive rocks rocks from from more limited cylinders limited exposures exposures of of uniformly uniformly textured textured intrusive rocks. As contacts between units are generally unexposed and the base of the section is nowhere exposed, thicknesses thicknesses of of units units and of nowhere exposed, of the the entire entire succession succession are are unknown; unknown; however, however, continuous outcrop in in cliff-forming units indicates indicates that that a minimum continuous outcrop cliff-forming units minimum thickness thickness of of lOOm 100m of of volcanic rock is present in the area. area. The igneous rocks are the dominace of ferric are distinctively distinctively brick brick red, suggesting suggesting the dominace of ferric iron iron in in the the various mineral hosts but especially in trace amounts in feldspars. The intrusive member displays equigranular phaneritic phaneritic texture texture with with most most mineral mineral grains grains 11 to to 5 mm in diameter. equigranular diameter. The The volcanic volcanic rocks are true porphyries porphyries with with phaneritic phaneritic phenocrysts phenocrysts of of alkali alkali feldspar, feldspar,quartz quartzand andhornblende hornblendein in an aphanitic matrix of of the same minerals. Quartz phenocrysts are are euhedral euhedraland and 11 to to 3 mm in an aphanitic matrix Quartz phenocrysts diameter associated with alkali feldspar phenocrysts occasionally exhibiting synneusis twinning as well as as albite-pencline albite-pericline twins twins indicative indicative of of microcline. microcline. Hornblende Hornblende and and magnetite magnetite are are less less 31 �abundant and finer grained than in the the intrusive intrusive rocks. rocks. Near Near flow flow tops tops the the porphyries porphyriesgrade grade into into uniformly textured aphanitic rhyolites with sparse phenocrysts. textured rhyolites with sparse phenocrysts. The samples from the The the Redstone Redstone Point Point intrusive intrusive complex complex are all all characterised characterised by by high high Si02 Si02 contents (73-83 (73-83 wt%) wt%) and elevated contents elevated K20 &O and and Na20 Na,0 abundances abundances (2-7 wt% wt% and and 0.2-3.5 0.2-3.5 wt% wt% respectively). They They are respectively). are typically typically LREE enriched LREE enriched with relatively unfractionated IIREE (La/Sm,, unfractionated HREE (La/Smn = = 2.8-5.1; Gd/Yb,, = = 1.1-1.6; 1.1-1.6; Fig. 2) 2.8-5.1; Gd/Ybn 2) and are are characterised characterised by by elevated elevated Zr, Y Y and and Nb Nbcontents. contents. Samples Samples from the Redstone Redstone Point Point igneous igneous complex fulfil fulfil the complex the detailed detailed trace trace criteria of of Whelan element criteria Whelan et al. al. (1987) for anorogenic anorogenic granites. granites. . f _J Rb BaTii Tb UU Nb La Ce C <b Ba Nb La Pr Pr Sr Nd Sr Nd Zr Ba T La M Zr Hf llf Sn, Sin Eli Ti Gd (a Tb Tb fly W) 7Y Ho 110 P Fr Yb Yb LII 41 V Sc Sc mantle normalised Figure 2. Representative primitive mantle normalised diagram diagram for for samples from the the Redstone Redstone Point Pointigneous igneouscomplex complex Similarities Similarities between between Proterozoic Proterozoic basin sequences sequences (e.g., (e.g., Athabaska, Athabaska, Thelon, Hornby Bay and and Sibley Sibley basin fill fill sequences) sequences) imply that basin genesis genesis and and developmental developmental controls were similar. The setting, architecture, depositional systems and deformational histories of all four basins strongly infer that they are intracratonic, forming as a result of heating cratonic The heating heating event is represented represented in in northern northern Canada by by numerous 1790 to 1730 Ma lithosphere. The anorogenic, syenogranite syenogranite batholiths batholiths and comagmatic ash-flow tuffs occurring west of anorogenic, comagmatic ash-flow of Hudson Hudson Bay. In the the western western Great Great Lakes Lakes region region aa heating heating event event produced produced the the 1537 1537Ma Ma Redstone Redstone Point Point assemblage and and other 1500 records a assemblage 1500 Ma anorogenic anorogenic batholiths. The southern southern mid-continent mid-continent records heating event with anorogenic granite granite production production from from approximately 1480 to to 1320 lithospheric heating Ma (Fig. 1). These events outline a progressive southward displacement of lithospheric heating 1). events outline a progressive southward from a maximum age of approximately 1750 1750 ma in northern Canada to aa minimum minimum age age of of 1310 1310 Ma in the Ma the southwestern southwestern United States. States. As heat heat transfer transfer from from the the asthenosphere asthenosphere is the the only only mechanism for producing extensive lithospheric heating, drift of North America over hotter than average asthenosphere is implied. Using regional ages of heating, drift rates of approximately 1.1 1.1 cm/year are necessary, and agree in magnitude magnitude with present rates. rates. to 1.4 1.4 cmlyear REFERENCES REFERENCES Anderson, J., 1983. Proterozoic anorogenic granite granite plutonism plutonism of of North North America. America. In: In: Medaris Medaris et Anderson, al., (Eds), Proterozoic Proterozoic geology. geology. Geological Society of America Memoir Memoir 161, 161,133-154. 133-154. Davis, and Sutcliffe, Sutcliffe, R., 1985. 1985. U-Pb U-Pb ages ages from from the the Nipigon Nipigon Plate Plate and and Northern Northern Lake Lake Davis, D., and Superior. Superior. Geological Geological Society Society of America Bulletin, Bulletin, 96, 96, 1572-1579. 1572-1579. Whelan, J., Currie, Currie, K., K., and andChappell, Chappell,B., B.,1987. 1987.A-type A-typegranites: granites:geochemical geochemicalcharacteristics, characteristics, Whelan, discrimination and petrogenesis. Contributions to Mineralogy and Petrology, 95, 407-419. discrimination and petrogenesis. Contributions to Mineralogy and Petrology, 95,407-419. 32 �PALEOPROTEROZOIC (1900-1600 (1900-1600 Ma) Ma) TECTONIC TECTONIC HISTORY HISTORY OF OF THE THE NORTHERN NORTHERN LATE PALEOPROTEROZOIC MID-CONTINENT, MID-CONTINENT, U.S.A: IMPLICATIONS IMPLICATIONS FOR CRUSTAL CRUSTAL STABILIZATION STABILIZATION HOLM, D.K., Dept. of Geology, Geology, Kent State State University, Kent, OH OH 44242; 44242; VAN VAN SCHMUS, SCHMUS, W.R., and MacNEILL, L.C., Dept. of Geology, Geology, University of Kansas, Lawrence, Lawrence, KS 66045; 66045; BOERBOOM, T.J., Minnesota Geological Survey, 2642 University University Avenue, Avenue, St. Paul, Paul, MN MN 55114; SCHWEITZER, D., Dept. of Geology, Kent State University, Kent, OH 44242; 551 14; SCHWEITZER, D., Dept. of Geology, State Kent, OH 44242; SCHNEIDER, SCHNEIDER, D.A., Dept. of Geological Geological Sciences, Sciences, Ohio Ohio University, University, Athens, Athens, OH OH 45701 45701 We propose that the late late Paleoproterozoic Paleoproterozoicigneous igneous and and deformational deformationalhistory history preserved preserved in in the the southern Lake Superior southern Lake Superior region region is the the result result ofofnorthwest-directed northwest-directed convergence convergence during during and following geon 18 18 Penokean accretion. accretion. New U-Pb zircon ages indicate that late to post-Penokean magmatism began ca. 1800 1800 Ma Ma and and generally generally migrated migrated southeastward southeastwardacross across the the newly newlyaccreted accreted terrane. Magmatic Magmatic pulses pulses atatca. ca.1800, 1800,1775, 1775,and and1750 1750Ma Mamay maycorrelate correlatewith withnorthwest-directed northwest-directed subduction associated with southward growth of the North American mid-continent. mid-continent. We suggest suggest that geon 17 Yavapai-age slab rollback caused continental arc magmatism to step southeastward 17 continental to step southeastward As the the slab slab steepened, steepened, the reduced compressional compressional stresses between 1800 and 1750 Ma (Fig. 1A). As and increased thermal input allowed allowed for collapse collapse of of the the overthickened overthickenedportions portions of of the the Penokean Penokean In northern collapse involved involved the the formation of gneiss crust. In crust. northern Wisconsin, Wisconsin, collapse formation of gneiss domes domes and and their their within discrete fault-bounded panels panels brought brought up up from depth via tectonic exhumation within tectonic extrusion extrusion (Schneider - and possibly temporary temporary (Schneider et al., al., ILSG, ILSG, 2003). 2003). Collapse of the the Penokean Penokeanorogen orogen— resulted in crustal stabilization stabilization and deposition deposition of of Baraboo Baraboo Interval Interval cessation of of slab slabsubduction subduction—- resulted However, in a long-lived orogen model, renewed quartzites between 1750 and 1650 Ma. quartzites between 1650 Ma. However, in long-lived orogen model, renewed tectonism to to the the south of a Mazatzal tectonism south resulted resulted in the the eventual eventual accretion accretion of Mazatzal arc (Fig. (Fig. 1B) 1B) with with widespread deformation deformation and and mild mild reheating reheating of of Penokean widespread Penokean crust to the the north. north. The age of this this deformation is inferred from conventional Ar/Ar ArIAr step-heating studies on basement rock beneath deformed and undeformed undeformed Baraboo Baraboo Interval Intervalquartzites. quartzites. The 1900 to 1600 Ma tectonic history of of United States, not surprisingly, records the southward the north-central north-central United surprisingly, records southward growth growth and tectonic tectonic development of the southern southern Laurentian margin. New and published mineral agesdelineate delineatethe the northern northern and and western extent of geon ^ ~ r / ^ ~mineral r ages published 40Ar/39Ar 16 crustal deformation. Interestingly, Interestingly, only only lower-grade lower-grade crust intruded by the shallower-level ca. 1750 Ma plutons (and associated rhyolites) were were deformed significantly during during geon geon 16. Deeper Deeper level by the older level collapsed crust and crust crust pervasively pervasively invaded by older magmatic magmatic pulses pulses are are largely largely by Mazatzal deformation and reheating. reheating. We suggest that post-orogenic intrusions and unaffected by crustal thinning was an important step in strengthening strengthening and stabilizing the crust in the the southern southern Lake Superior Superior region. region. Schneider, Holm, O'Boyle, Hamilton, Hamilton, and and Jercinovic, Jercinovic, 2003, 2003, Paleoproterozoic Paleoproterozoic development of a gneiss dome corridor in the southern Lake Superior region, region, USA: USA: Institute Institute on on Lake Lake Superior Superior Geology Abstracts (this (this volume). volume). 33 �subduction (ca' vapai subduction (ca. 1750 1750 Ma) Ma) w —southward pro propagation "Â¥southwar agation of magmatism magmattsm I from to 1750 Ma granites granites and and rhyolites from 1775 1775 Ma Ma ECMB ECMB to 17 0 Ma rhyolites in WI Wl S N NFZ NFZ 2J750-1630 7 5 0 4 6 3 0Ma Maquartzites quartzites /_— ,. ', ,f — — 1...AVRENtIA -I . '- ' '—-— C -' Mazatzal orogen orogeny Mazatzal y (1650-1630 (1650-1630 Ma) Ma) 2 deformation deformation of "post-Penokean ost-Penokean"quartzites quartettes during Mazatzal azatzai accretion accretion A) Subduction Subduction roHback model proposed proposed to to explain magmatic magmatic age Figure 1: 1: A) rollback model progression the Penokean progressionacross across the Penokean orogen, orogen,ca.Yavapai ca.Yavapai convergence, preaccretion. ECMB = East-central East-central Minnesota Minnesota batholith. batholith. B) Mazatzal accretion. ECMB = 6)Mazatzal Mazatzal accretion model (modified after Romano et al , 2000) to explain deformation to explain deformation accretion model (modified after Romano et a]., quartzites and and southward southward growth growth of of Baraboo Interval quartzites of Laurentia, Laurentia, 34 �GABBRO/GRANOPHYRE LAKE INTRUSION: INTRUSION: A GABBROIGRANOPHYRE RELATIONS OF THE CROCODILE LAKE POSSIBLE VENT FOR THE HOVLAND LAVAS? JERDE, Eric A. (e.jerde@morehead-st.edu), (e.jerde@morehead-st.edu),Department Department of Physical Physical Sciences, Sciences,Morehead Morehead State State University, Morehead, KY 40351 4035 1 Morehead, KY One of the notable characteristics characteristics of of the the Midcoritinent Midcontinent Rift is the presence of large large amounts amounts of felsic felsic material. material. Indeed, the nature and origins origins of this this abundant abundant silicious silicious material material has has been been the the source sourceof of numerous numerousstudies studies(e.g., (e.g., Nelson, 1991; a!., 2000; 2000; Sandland et al., 2001). To 1991; Green and Fitz, 1993; 1993; Vervoort and Green, 1997; Kennedy et al., Tothe the south of the Early Gabbro Series is a pronounced ridge composed of of this this felsic felsic material, material, properly properly termed termed aa granophyre. granophyre. The Early Gabbro Gabbro Series Series layers layers are are inclined inclined to to the the south, south,thus thus are are below below the the granophyre granophyre stratigraphically. This This felsic felsic rock rock was was noted noted and and described described by by Nathan Nathan (1969) (1969) as as aa very very late-stage late-stage material, material, and has generally been presumed to have formed significantly after after the the gabbros gabbros in inthe theregion. region. However, several observations indicate that the gabbro was emplaced later than than the the granophyres. granophyres. These These include include gradational gradational contacts, contacts, with some chilling of of the the gabbro. Another Another observation observation is is the the abundance abundance of material described as "intermediate "intermediate rock" by various investigators in the past (e.g., Grout et al., 1959). This This material material isis always always found found between between the gabbro and granophyre, granophyre, and is is presumably presumably the the result result of assimilation assimilationof granophyre granophyre by by an an intruding, intruding,hot hot gabbro. gabbro. Investigations into possible origins origins of the granophyres granophyres (Sandland (Sandland et et al., al., 2001; 2001; Karl Karl Wirth, Wirth,pers. pers.comm.) comm.) included radiometric age determinations, determinations, and revealed revealed that that the the granophyres granophyresadjacent adjacent to to the the Early EarlyGabbro GabbroSeries Seriesare, are, of the rift rift (-1 (-.1107 Ga), and and essentially essentially contemporaneous. contemporaneous. Because like the gabbros, among the earliest rocks of 107 Ga), silicious material generally is a late-stage late-stage product of magma evolution, evolution, the surprising surprising antiquity antiquity of the the granophyres granophyres adds to questions questions surrounding surrounding their their origin. origin. The early age for the granophyres granophyres does, does, however, suggest suggest an origin origin for the the layered layered nature nature of of the the Early Early Gabbro Gabbro Series located below them stratigraphically. Due Due to to their their low low density, density, the granophyric material would have created a barrier that retarded the rise of buoyant gabbroic material coming up up from from below. below. These rising liquids liquids would have layering that that is is observed, observed, and and providing providing aa cap, cap, blocking blocking any any been forced to spread laterally, resulting in the apparent layering further rise of gabbroic gabbroic material. material. of the layered Early Early Gabbro Gabbro Series Series of of Nathan Nathan (1969) (1969) is is another another occurrence occurrence of of Immediately to the east of Ga;Karl KarlWirth, Wirth,pers. pers. comm.). comm.). This granophyre, also determined to be among those those of an early early origin origin (i.e., (i.e., —1107 -1 107 Ga; by Miller Miller et et al. al. (2001), (2001), and and the the rocks rocks are are interpreted interpreted to to be be rock group has been termed the Crocodile Lake Intrusion by sample examinations examinations (Babcock, (Babcock, 1959). Work gabbroic based on geophysical evidence, and a few sample Work done done between between 1913 and 1948 1948 included included the the very very edges edges of of this this intrusion, intrusion, and and indicates indicatesthat that they they are are basalt basalt lavas lavasand andgabbroic gabbroic intrusions, along with "red rock" (Grout (Grout et et al., al., 1959) 1959)that that isis now now known known to to refer refer to to granophyre. granophyre. Like the series mapped by Nathan (1969), there is a body of gabbroic material stratigraphically below the granophyre. granophyre. was made made into into the the Crocodile Crocodile Lake Lake Intrusion Intrusionto to examine examine some someof ofthe therock rock. During the past year, a reconnaissance was relations (Fig. 1). Traverses Traverseswere weregreatly greatly hampered hampered by by forest forest blowdown, blowdown, but the outcrops are are numerous. numerous. Several Several gabbro units are present, as well as a band of "intermediate "intermediate material" material" at the the very very top top of of the the gabbro, below below the the granophyre. Within Within the the granophyre granophyreitself, itself, several several bodies bodies of gabbro gabbro were were found to have actually actually intruded the granophyre. In In the the coarse-grained coarse-grainedinteriors interiorsof of these these bodies, bodies, the the gabbro gabbro is is indistinguishable indistinguishable from from the gabbros gabbros below the granophyre granophyre stratigraphically). stratigraphically). observed further north (i.e., below of the the granophyre granophyre are are prominent prominent knobs knobs and andridges ridgesthat thatare arecomposed composed of ofbasalt. basalt. These Immediately to the south of are mapped as part of the Hovland Hovland Lavas, Lavas, which which are are reversely reversely polarized, polarized, and and were were extruded extrudedduring duringthe theearliest earliest period of the rifting. ItIt isisperhaps perhapspossible possiblethat that the the discontinuous discontinuousbodies bodies (and (and other other stringers stringers and and local local dikes) dikes) within within the granophyre represent the feeder feeder conduits conduits for for the the eruptive eruptive basalts basalts immediately immediatelyto to the the south south (shown (shownschematically schematically in Fig. 2). In In several severalother otherplaces places within within the the granophyre, granophyre,there there are are basaltic basaltic stringers stringers and small dikes. Surrounding Surrounding has been been assimilated assimilated into into the the gabbro. gabbro. In the larger gabbroic bodies are obvious reaction zones where granophyre has immediately to the south, numerous inclusions are present that are pinkish one of the flows immediately pinkish in in color, color, along along with with felsic stringers and irregular masses masses of felsic material. material. Further work is planned to assess assess the relation relation between the gabbros gabbros within within the granophyre granophyre and and the the lavas lavas to to the the south. IfIf this and the the this isis indeed indeedaa feeder feeder system, system, itit might might provide provide insight insight into the mechanism of magma emplacement and "breakthrough" to the eventual "breakthrough" the surface, surface, during during the the onset onset of of rifting. rifting. 35 �References Cited: References Cited: Babcock, R.C., Jr. (1959) M MS. S .thesis, thesis,University Universityof of Wisconsin, Wisconsin,Madison, Madison, 47 47 p. p. Research, 54, 177-196. 177-196. Green, J.C. and Fitz, T.J., 1993, 1993, Journal of Volcanological and Geothermal Research, Grout, SurveyBulletin Bulletin39, 39, 163p. l63p. Grout, F.F., F.F., Sharp, Sharp, R.P., and Schwartz, Schwartz, G.M. 1959 Minnesota Geologica Survey Jerde, E.A. and Kennedy, Kennedy, B.C., B.C., 2000, 2000, American American Geophysical Geophysical Union 2000 2000 Fall Meeting, Meeting, San San Francisco. Francisco. Jerde, Jerde, Salvato, D.J, D.J, Thole, Thole, J., J., and and Wirth, Wirth, K.R. K.R. 2001, 2001, ILSG ILSG 47, 47, 36-37. 36-37. Jerde, E.A., Salvato, Kennedy, B.C., B.C., Wirth, Wirth, K.R., K.R., and and Vervoort, Vervoort,J.D., J.D.,2000, 2000,ILSG ILSG46, 46,29-30. 29-30. Jr., Green, J.C., Severson, Severson, M.J., Chandler, Chandler, V.W., and Peterson, D.M., 2001, Minnesota Minnesota Geological Geological Miller, J.D., Jr., Survey 19. Survey Miscellaneous MiscellaneousMap MapSeries SeriesM-1 M-119. Nathan, H.D., 1969, of Minnesota, Minnesota, Minneapolis, Minneapolis, 198p. l98p. 1969, Ph.D. dissertation, University of Nelson, N. 1991, 1991,M.S. M.S. Thesis, Thesis, University Universityof of Minnesota, Minnesota,Duluth. Duluth. Sandland, TO., Wirth, 47, 85-86. Wirth,K.R., K.R.,Vervoort, Vervoort,J.D., J.D.,Gehrels, Gehrels,G.E., G.E.,Kennedy, Kennedy, B.C., and Harpp, K.S. 2001, ILSG 47,8586. Sandland, T.O., Vervoort, J.D. and Green, J.C., 1997, 34, 521-535. 1997, Canadian Canadian Journal of Earth Sciences, 34,521-535. Fig. 1. Reconnaissance Reconnaissancegeologic geologicmap map of of the the region region just south south of Crocodile Crocodile Lake, Lake, showing showing location location of gabbro bodies in the the granophyre granophyre that forms forms the the cap cap above above the the Crocodile Crocodile Lake LakeIntrusion Intrusiongabbros gabbros and intermediate intermediate rocks. rocks. .4 SchematicN-S N-S cross crosssection sectionof of Fig. Fig. 11showing showing the the possible possible feeder feeder for for the the Hovland Hovland Lavas. Lavas. Fig. 2. Schematic 36 �MINERALIZATION MINERALIZATION OF OF THE THE NORTON NORTON LAKE LAKE Cu-Ni-PGE Cu-Ni-PGEDEPOSIT DEPOSIT JOHNSON, JOHNSON, J.R., J.R.,HOLLINGS, HOLLINGS,P.P.and andKISSIN, KISSIN,S.A. S.A.Department DepartmentofofGeology, Geology,Lakehead Lakehead University, Thunder Bay, ON, P7B 5E1, jrjohnsonca@yahoo.ca 5E1, jrjohnsonca0 yahoo.ca The Norton Lake Cu-Ni-PGE lun northeast northeast of of Fort Fort Cu-Ni-PGE deposit deposit is located located approximately approximately50 50 km Hope, within the Miminiska-Fort Hope Greenstone belt of the Uchi Subprovince, northwest Hope, Miminiska-Fort Hope Greenstone belt of the Uchi Subprovince, northwest Ontario (Figure 1). Only Onlylimited limitedgeological geological investigations investigationshave have been been undertaken undertaken within within the the belt due to both its remote location and sparse outcrop. As As aa result result of of this this the the belt belt has has been been subdivided based on limited structural and regional stratigraphic considerations considerations (Stott (Stott and and Corfu, Corfu, 1991). The The Norton Norton Lake Lake deposit depositis is located located within within an an unnamed assemblage assemblage comprising comprising basaltic flows with magnetite iron formations. formations. ItItisisthought thought that that this this assemblage assemblagecan can be be correlated, correlated, through similar rock types and aeromagnetic trends, with with the the -2900 Ma Northern Pickle Pickle terrane of the Pickle Lake greenstone greenstone belt (Corfu (Corfu and and Stott, Stott, 1996). 1996). Figure Figure1:1:A-Map A-Mapof of Superior SuperiorProvince Provinceshowing showinglocation location of of Uchi Uchi Subprovince. Subprovince. B Simplified Simplified geology geology map of the Miminiska-Fort Miminiska-Fort Hope Hope Greenstone Greenstonebelt belt (after Stott and Corfu, Corfu, 1991). 1991). The Norton Lake area consists consists of massive massive to pillowed pillowed basalts basalts with with rare rare ultramafic ultramaficflows. flows. The deposit itself is hosted within a sheared amphibilite amphibolite with minor gabbroic units. Previous Previous work determined 944 500 500 tonne tonne nickel-copper nickel-copper deposit depositcontaining containing0.72% 0.72%Ni Ni determined the deposit deposit to to be be aa 944 West Resource Resource Corporation, Corporation, 2001). 2001). The and 0.56% Cu with an undefined PGE potential (East West The geological setting, host rock and mineralization of the Norton Lake deposit deposit are are comparable comparableto to that of the Thierry Deposit, Pickle Lake, Ontario. The The Thierry Thierry Mine Mine is is currently currently undergoing renewed exploration exploration to to determine determineits its viability viability as as aa PGE PGE deposit deposit (PGM (PGM Ventures). Ventures). 37 �East West Resource Resource Corporation Corporation (EWR) has undertaken a detailed exploration program of Norton Lake, including an an extensive extensive drilling drilling program. program. Detailed in the vicinity of Detailed examination examination of of being paid paid to to the the mineralized mineralized 'main' zone drill core has been undertaken with special attention being zone to to determine the exact nature of the mineralization. Preliminary Preliminary results results indicate indicate the the deposit deposit consists consists chalcopyrite and and pyrite. pyrite. The platinum group of massive pyrrhotite with pentlandite, magnetite, chalcopyrite element's s) are element's (PGE' (PGE's) are found found forming discrete discrete platinum group minerals and are also believed to form a solid solution solution with the suiphides. sulphides. Results Resultsshow showthat thatin inaddition additionto toprimary primarymineralization mineralization a secondary, hydrothermal, enrichment of PGE's has has taken taken place. place. Mineral Mineral Pyrrhotite Pyrrhotite Pentlandite Pentlandite Pyrite Pyrite Chalcopyrite Chalcopyrite Manganoan Manganoan Illmenite Illmenite Formula Fei-xS FeiS (Fe,Ni)9S8 (Fe,Ni)9S8 FeS2 FeS2 CuFeS2 CuFeS2 (Fe,Mn)Ti03 (Fe,Mn)Ti03 Magnetite Michenerite Michenerite Hessite Hessite Fe304 PdBiTe PdBiTe Ag2Te Ag2Te Minor Elements Elements Ni Co Co Co Ni Notes Main mineral Secondary Secondary Trace Trace, also veins More common common than magnetite, magnetite, easily mistaken for magnetite magnetite in polished section section Sb, Sb, Pt Table Table 1: 1: Summary Summary of the mineralogy mineralogy of Norton Norton Lake Lake deposit. deposit. Corfu F. and Stott G.M. 1996. 1996. Hf isotopic composition and age constraints on the evolution of the Archean Central 78,pp53-63 53-63 Central Uchi Subprovince, Subprovince,Ontario, Ontario,Canada. Canada.Precambrian Precambrian Research, Research,v. v. 78, East West Resources Resources Corporation, Corporation,2001. 2001. Annual Annual Report. Report. PGM Ventures, 2003. 2003. www.pgm-ventures.com www.pgm-ventures.com Stott G. M. and and Corfu Corfu F. F. 1991, 1991,Uchi Uchi Subprovince, Subprovince,in in Geology Geology of of Ontario, Ontario,Ontario OntarioGeological Geological Survey Special Special Volume 4, 4, Part 1. 1. 38 �Pd-Pt-Au MINERALIZATION STRATIFORM Pd-Pt-AU MINERALIZATION IN THE SONJU LAKE INTRUSION, LAKE COUNTY, MINNESOTA JOSLIN, GregoryD. D.Department Departmentof of Geological GeologicalSciences, Sciences,University University of of Minnesota-Duluth, Minnesota-Duluth, 1114 1114 Kirby Kirby JOSLIN, Gregory joslOOl3@d.umn.edu;MILLER, MILLER, James D., Jr., Minnesota Drive, Duluth, MN 55812, email: josl0013@d.umn.edu; Geological Survey, Survey, c/o do NRRI, 5013 Miller Miller Trunk TrunkHwy, Hwy,Duluth, Duluth,MN MN558 55811; andROWELL, ROWELL, 11; and William, F., Franconia Minerals MineralsCorp., Corp.,12 125. S.6th 6"' St., Minneapolis, MN 55402. The Sonju Sonju Lake intrusion intrusion (SLI) (SLI) is a 1200 1200m m thick, closed-system, closed-system, well-differentiated, well-differentiated,tholeiitic, tholeiitic, layered intrusion located within the the Mesoproterozoic Mesoproterozoic Midcontinent Midcontinent Rift-related Rift-relatedBeaver Beaver Bay Bay Complex of of northeastern northeastern Minnesota Minnesota (Miller (Millerand andChandler, Chandler,1997). 1997). In In the the late late 1990's, outcrop sampling by Miller (1999) (1999) indicated the presence of meter-scale meter-scale stratiform stratiformPd-Pt-Au Pd-Pt-Au mineralized mineralized interval (or PGE reef) within the oxide oxide gabbro gabbro unit of the the SLI, SLI, located about about 2/3 213 of of the the way way up up from the basal contact of the intrusion. In In June June of of 2002 2002 Franconia Franconia Minerals Minerals Corp. Corp. conducted conducted exploratory drilling drilling through through the the Pd-Pt-Au Pd-Pt-Au enriched enriched zone. zone. exploratory In hand sample, the mineralized interval appears as a homogeneous oxide gabbro, gabbro, with no visible indication of precious metals enrichment. However, However, geochemically the location location of the mineralization is distinct. Three Threedrill drillcores, cores,spanning spanningaastrike strikelength length of of approximately approximately 800 800m, m, define define and are are correlated correlatedon on the the basis basis of of aa distinctive distinctiveCu-Au Cu-Au break break datum datum(Fig. (Fig.1). 1). With the exception of localized Pt enrichment associated with an interval enriched in olivine about 110 m m below the Cu-Au horizon, horizon, all Pd-Pt-Au enrichment occurs over an interval interval of 0 to to 90 90 m m below the the In general general precious precious metals metals peaks peaks are are stratigraphically stratigraphicallyoffset offset from from one one defined datum (Fig. 2). In Maximumgrades gradesin in 0.3m 0.3m long core another, progressing progressingupward upwardininthe thesuccession successionPd+Pt+Au. Pd-)Pt-Au. Maximum samples are 410 ppb Pd, 275 ppb Pt, and 1080 ppb ppb Au. Au. Above Above the Cu-Au break, all precious metals are very strongly depleted. Strong A1 and and modal modal olivine olivine with Strongcorrelation correlationbetween between Fe, Fe, Al precious metals peaks indicates a possible connection between subtle modal modal layering layering of of plagioclase, plagioclase, oxide, oxide, and olivine olivine with with mineralization. mineralization. The oxide gabbro-hosted gabbro-hosted PGE reef in the Sonju Sonju Lake Lake intrusion intrusion shows shows marked marked similarities, similarities, with some differences, differences, to stratiform stratiformPGE PGE mineralization mineralization in in the the Skaergaard Skaergaard intrusion intrusionof of East East Greenland (Andersen et al., 1998),the the Rincon Rincon del del Tigre Tigre Complex Complex of of Bolivia Bolivia (Prendergast, (Prendergast,2000), 2000), al., 1998), and many other tholeiitic mafic layered intrusions throughout throughout the the world. world. Whole Whole rock rock geochemistry, clinopyroxene clinopyroxene and olivine compositions, and petrographic data data are are consistent consistentwith with an orthomagmatic origin for the mineralization related to the fractional fractional segregation segregation of of sulfide sulfide melt from silicate magma. The The homogeneity homogeneity of the host rock, the thickness of the mineralized interval, interval, and the offset offset of metal metal concentrations concentrationsimply imply that that sulfide sulfide saturation saturationwas was passively passively triggered by fractional crystallization crystallization of the Sonju magma. magma. Mungall (2002) recently argued Mungall(2002) argued that stratigraphic offsets offsets of Pd, Pd, Pt, Pt, Au Au and and Cu Cu peaks common to many PGE reefs can can be satisfactorily satisfactorily explained by a kinetic model of sulfide liquation and settling. The The model model shows shows that that the the degree degree of offset and metal enrichment will be controlled by by kinetic kinetic factors, factors, such as as the the diffusivity diffusivity of of chalcophile elements, the degree of sulfide supersaturation, supersaturation, sulfide droplet size, and its settling chalcophile velocity, which result in variability of the apparent silicatelsulfide silicate/sulfide melt melt ratio ratio (R factor). The The correlation of multiple peaks of PGE with subtle, correlation subtle, broad broad modal variations variations may may be be related related to to by the crystallization crystallization of of magnetite magnetite in in an an environment environment of of repeated convective overturn caused by sulfide over-saturation, as suggested by Prendergast (2000) to explain a similar correlation in sulfide over-saturation, suggested (2000) explain a similar correlation in the the Rincon del Tigre Complex. Some Someevidence evidence of of late-stage late-stage sulfide sulfide dissolution dissolution and remobilization exists, but it appears appears to have little little to to no no effect effect upon upon the the distribution distributionof of precious precious metals. metals. 39 �nrelorl ,bovo *60.0 - - -' Ft!lI1lI—._+50.0 ' +40,0 " *30,0 -. .. +70.0 *70.0 *60.0 *60.0 *50.0 *50.0 *40.0 *40.0 +30.0 *20.0 - -'-' ' ' ';'*. - ,,::-- +10_A 410.0 0.0 -10.0 -10.0 -10.0 .20.0 -20.0 -20.0 -30.0 -30.0 -40.0 .40.0 -500 -50.0 -5 0.0 1AJJ114i1_ -40,0 -70.0 '60,0 -60.0 -70.0 -70.0 -80.0 -60.0 -90.0 -90.0 I—.— " -30.0 1111111— 85000001 5 -40.0 11111111— . , -ao,o 00.0 1+00 -110.0 4 -110.0 '110.0 -120.0 , -120.0 -120.0 -130.0 'break +200 0.0 0.0 Cu-Au *30.0 920.0 *100 5L02-3 SLO2-2 SLO2—1 Cu-Au break +70,0 r -130.0 Fig. 1: Correlation Correlation of of drill cores SLO2-1, SL02-1, SLO2-2, SL02-2, and SLO2-3 showing showing distinctive SL02-3 break. The Cu-Au break. The Cu-Au Cu-Au break is used to provide a datum to which all stratigraphic plots are correlated, and position in stratigraphy is measured as meters above above or or below below Cu-Au Cu-Au break. break. -130.0 0 • Au ppb) Cu (ppm) 5L02-3 SLO2-2 SLO2-1 '*70.0 '+70.0 +60.0 .660.0 —.50.0 *50.0 *40.0 -+40.0 *30.0 +30.0 *20.0 -620.0 *10.0 *10.0 -10.0 -10.0 -20.0 -20.0 -30.0 -30.0 .40.0 '' -40.0 - -50.0 -50.0 -60.0 '60.0 .70.0 -70.0 .80.0 -80.0 -90.0 -90.0 l00.0 -100.0 -510.0 -110.0 '-520.0 -120.0 Fig. 2: Correlation Correlation of Pd Pd and Pt Pt in in drill drillholes holesSLO2SL021, I, SLO2-2, SL02-2, and and SLO2-3. SL02-3. Notice multiplicity of spikes spikes and offset offset between Pt and Pd peaks. -130.0 -130.0 S • Pd)ppb) S Pt(ppb) References: References: 0., Rasmussen, Andersen, J. C. O., Rasmussen, H., H., Nielsen, Nielsen, T. T. F. F. D., Ronsbo, Ronsbo, J. U., G., 1998, 1998, The The Triple Triple Group Group and and the the Platinova Gold and Palladium Reefs in the Skaergaard Intrusion: Stratigraphic and Petrographic Petrographic Relations. Relations. Economic Economic Geology. Geology. Vol. Vol. 93, 93, pp. pp. 488-509. 488-509. Miller, J. D. Jr., 1999, 1999, Geochemical Geochemical Evaluation Evaluation of Platinum Group Element (PGE) Mineralization in the Sonju Lake Intrusion, Finland, Minnesota: Minnesota: Minnesota Minnesota Geological Geological Surv. Surv. Information Information Circular Circular 44, 44, 31 3 1p. p. Miller, J. D., Jr., Jr., and and Chandler, Chandler, V. W., 1997, 1997, Geology, Geology, petrology, and tectonic significance of the Beaver Bay Complex, northeastern Minnesota, in Ojakangas, R. R. W., W., Dickas, A. A. B., B., Green, J. C., eds., Middle Proterozoic Proterozoic to to Cambrian Cambrian Rifling, Rifting, Central CentralNorth North America: America: Geological GeologicalSociety Societyof of America AmericaSpecial SpecialPaper Paper 312, p. 73-96. 73-96. Mungall, Mungall, J. E., 2002, 2002, Kinetic KineticControls Controlson on the thePartitioning Partitioningof of Trace TraceElements ElementsBetween BetweenSilicate Silicateand andSulfide Sulfide Liquids. Journal Journalof of Petrology. Petrology. Vol. Vol. 43, 43, pp. pp. 749-768 749-768 Prendergast, M. D., 2000,Layering 2000,Layering and Precious Metals Mineralization in the Rincon del Tigre Complex, Eastern Bolivia. Bolivia. Economic Economic Geology. Geology.Vol. Vol. 95, 95, pp. pp. 113-130. 113-130. 40 �RESULTS OF OF 40Ar/39Ar 4 0 ~ r / 3 9SINGLE-GRAIN ~r ANALYSES MAFIC RESULTS SINGLE-GRAIN ANALYSESOF OF PRECAMBRIAN PRECAMBRIAN MAFIC INTRUSIONS IN NORTHERN NORTHERN AND EAST-CENTRAL EAST-CENTRAL MINNESOTA MINNESOTA KEAYFS, M.J., Dept. of Geology, Kent State University, Kent, OH 44242; JIRSA, KEATTS, JIRSA, M., Minnesota Geological Geological Survey, 2642 University Avenue West, St. St. Paul, Paul, MN MN 55114-1057; 55114-1057; HOLM, D., Dept. of Geology, Kent State University, Kent, OH 44242 Age information from mafic mafic intrusive intrusive suites suites is is critical of the Age information from critical for proper proper interpretation interpretation of geologic history and for mineral deposit models in the Lake Superior region. As part of an effort to evaluate evaluate PGE potential potential in in mafic mafic intrusions intrusions in in Minnesota, Minnesota, several several plutons plutons have have been been dated dated CO2 laser ArIAr Ar/Ar incremental heating technique at the University University of Wisconsin-Madison using the C 0 2 laser Rare Rare Gas Gas Geochronology Geochronology Laboratory. Laboratory. For late-stage late-stage shallow shallow plutons plutons containing containingprimary primary magmatic hornblende, ArIAr Ar/Ar mineral ages are are likely likely to to closely closely approximate approximate the the crystallization crystallization age. In regions age. regions with with aamore moreprotracted protractedthermal thermalhistory history(i.e., (i.e.,low-grade low-grademetamorphism, metamorphism, slowslowcooling, etc.), the Ar/Ar data provide minimum ages for the mafic plutons. Mafic intrusions ArIAr data minimum ages plutons. intrusions from from Minnesota selected for this study represent a broad range of geologic settings, including 1) small mafic and intrusive mafic plutons plutons emplaced emplaced into into Paleoproterozoic Paleoproterozoic supracrustal supracrustal and intrusive rocks rocks within within the the Penokean orogen (samples 264, R17); and 2) varied, primarily late- to post-tectonic intrusions in rocks of of the Archean Wabigoon Wabigoon (samples (samples Al, Al, B21, UBD) and Wawa (samples K15, supracrustal rocks of Superior Province. Province. We We report report here the initial LP, ANA) subprovinces subprovinces of initial results results from from eight eight separate intrusions (Fig. (Fig. 1). 1). East-central ~Minnesota. A hornblende ~ast-central innesota.A hornblende grain grain (R17) (R17) from from aa sample sampleofofmedium-grained medium-grained homblendite batholith in hornblendite from the Tibbett's Brook intrusion cutting the East-central Minnesota batholith Morrison Co. yields yields aa plateau date of from 44contiguous contiguousincrements increments Morrison Co. plateau date of 1.770 1.770 ±Â 0.006 Ga from constituting 74% of of the the gas released. constituting 74% released. AAbiotite biotitegrain grain(264) (264)from fromaasample sampleofofcoarse-grained coarse-grained biotitic olivine gabbronorite cutting the Little Falls Formation in Morrison Co. Co. yields yields aa plateau plateau date of 1.791 1.791 ± Â0.008 0.008 Ga from 5 contiguous increments constituting constituting 68% of the gas gas released. released. twa Subprovince. A Ahornblende Wawa Subprovince. hornblendegrain grain(ANA) (ANA)from fromaasample sampleofofprismatic prismatichornblende hornblende diorite diorite collected near Red Lake in Beltrami Co. yields a near-plateau date of 2.587 ±0.012 ~ 0 . 0 1Ga 2Ga in in 55 nonnoncontiguous incrementsconstituting constituting50% 50%ofofthe the gas. gas. A A biotite biotite grain grain (K15) (K15) from from a sample contiguous increments sample of collected in in Norman Norman Co. Co. yields yields aa plateau plateau date date of of 2.639  ± 0.007 biotite granodiorite porphyry collected 0.007 Ga from 6 contiguous increments constituting 79% 79% of of the gas released. A biotite grain (LP) from a sample of porphyritic syenite collected at the Wawa-Quetico subprovince boundary in St. Louis Co., from 77contiguous contiguous Co., in the the Linden Linden Pluton, Pluton, yields yields a plateau plateau date date of 2.666 2.666 ±Â 0.006 Ga from increments increments constituting constituting 88% 88% of of the the gas gas released. released. Wabigoon Subprovince. A hornblende grain (B21) from the Oaks intrusion leucodiorite leucodionte sampled 0.008 Ga from 88 near Fault in Roseau 1 ±Â 0.008 near the Vermilion Vermilion Fault Roseau Co. Co. yields yields aa plateau plateau date date of of 2.67 2.671 contiguous increments constituting 75% 75% of of the the total total gas gas released. released. A hornblende grain (Al) (Al)from from 0.0111 Ga from the Black River gabbro, collected in Roseau Co., yields a plateau date from date of of 2.685 2.685 ±Â 0.01 11 contiguous increments increments constituting constituting 90% 90% of of the the total total gas gas released. released. A A hornblende hornblende (UBD) (UBD) from a sample Co. north north of the sample of of hornblende-biotite hornblende-biotite gabbro collected collected in Koochiching Koochiching Co. the Rainy Rainy LakeLakeSeine River Fault Fault yields yields a plateau date of Seine River plateau date of 2.695 2.695 ±  0.007 Ga from from 66 contiguous contiguous increments increments constituting constituting 49% of the gas released. released. 41 �The mineral age data from from mafic mafic plutons plutons from from the the Wabigoon Wabigoon subprovince subprovince are are synchronous synchronous Mafic plutons plutons from from the the with the last deformation event (D2) dated in the range 2.685-2.674 Ga. Mafic Wawa subprovince give give an an 80 m.y. age range from 2.58 to 2.66 Ga. Interestingly, Interestingly,the theLinden Linden Pluton gives a biotite biotite date date concordant concordant (within error) with the youngest youngest mafic pluton from the Pluton Wabigoon subprovince. Wawa are are consistent consistent with with Wabigoon subprovince.The Theyounger youngerspread spreadofof ages ages from from the Wawa southward growth of of the Superior Province Province during during the the latest latest Archean. Archean. Mafic plugs evident from aeromagnetic maps 1.775 Ga Ga EastEastaeromagnetic maps in in east-central Minnesota Minnesota are are comagmatic comagmatic with with the circa 1.775 central Minnesota batholith. Further constraining the mafic intrusions intrusions the temporal framework of mafic may contribute contribute to mineral deposit deposit models these intrusions intrusions and and their their analogs analogs in in may to mineral models for for PGE PGE in these adjacent states states and and provinces. provinces. Fig.i agespectra: spectra:t t, ==plateau plateau age, age, t, tq == total total gas age. Fig.l 40Ar/39Ar "Ar/^Ar age age. 2.0 0 2.0 Ri 7 264 1.5 2:: fl.... 1 1.8 J0.5 3.0 Biotite MSWD MSWD 2.57 2.57 Amphibole MSWD 1.78 Amphibole MSWD 1.78 t, tp== 1.770 1.770±Â0.006 0.006Ga Ga t9= t = 1.653±0.005 1.653 0.005Ga Ga I I 6t ==1.791 1.791±Â0.008 0.008Ga Ga t, = 1.782 1.782±  0.007 0.007Ga Ga 1.5 3.0 ANA B21 —--————ur Ct 0 2.5 1 Amphibole 0.71 Amphibole MSWD MSWD0.71 2.587  ± 0.012 0.012 Ga tp= 2.587 tg 2.550  ± 0.009 0.009 Ga Ga t, == 2.550 2.0 2.0 2.0 3.0 " 3.0 Ki 5 .__.__.__ Ct 0 . Al ti. ...:::.::::::: 2.5 Biotite MSWD Biotite MSWD 2.48 2.48 2.639 ± t, = 2.639  0.007 0.007Ga Ga = 2.640 2.640±Â 0.007 0.007Ga Ga 3.0 3.0 LP t:,Vt:::::::ZV:. : Amphibole MSWD Amphibole MSWD 1.81 1.81 t, = 2.685 2.685 ± tp  0.011 0.011 Ga Ga t9== 2.700 2.700  ± 0.01 0.010 tg 0 Ga Ga 2.0 -. 2.0 UBD —iF :1:2.:, 0 2.5 - Amphibole Amphibole MSWD MSWD 0.30 0.30 t ==2.671 2.671 ±Â0.008 0.008Ga Ga tto==2.705 2.705±Â0.013 0.013Ga Ga 2.5 Biotite MSWD Biotite MSWD1.11 1.I1 t,, = 2.666 2.666±Â 0.006 0.006Ga Ga t9 2.657 ± 6 == 2.657  0.006 0.006 2.0 0 0 10 10 20 20 30 30 40 40 50 50 60 60 700 7 60 80 Cumulative released (%) Cumulative39Ar "Ar released (%) 2.0 900 1100 0 9 00 0 42 , 10 10 Amphibole MSWD MSWD 1.03 1.03 2.695 ± $, = 2.695  0.007 0.007Ga Ga 2.730 ± 6 .= 2.730  0.006 0.006Ga , --"-" *- - , 20 30 30 40 40 50 60 60 700 7 80 80 Cumulative Cumulative 9Ar "Ar released released (%) (%) r-- 90 90 100 100 �New zircon New zircon ages agesfrom fromthe the Gunflint Gunflint and and Rove RoveFormations, Formations,northwestern northwestern Ontario Ontario Kissin, S.A., Department Department of Geology, Geology, Lakehead University, Thunder Thunder Bay, ON, P7B 5E1 5E1 Canada, Canada, stephen.kissin@lakeheadu.ca; stephen.kissin@lakeheadu.ca ;Vallini, Vallini, D.A., D.A.,University University of of Western Western Australia, Australia,35 35 Stirling Hwy, Stirling Hwy, Australia; Addison, W.D., W.D., RR 2, 2, Kakabeka Kakabeka Falls, Falls, ON, ON, POT POT iWO, 1W0,Canada; Canada; Crawley, 6009, W.A., Australia; Brumpton, G.R.., 211 Bmmpton, 21 1 Henry St, Thunder Thunder Bay, ON, P7E 4Y7, Canada. Canada. Previous work based on U-Pb geochronology geochronology from presumed volcanogenic zircons zircons obtained obtained 1878 ±Â 2Ma, from a tuff layer at the lower/upper lowerlupper Gunflint Formation boundary yielded an age of 1878 believed to approximate the age of of deposition of of the unit unit (Fralick et et al., 2002). 2002). This This age age corresponds closely with the age of the correlative correlative Hemlock Formation Formation of Michigan Michigan (1874 (1874 ±: 9Ma; Schneider 9Ma; Schneider et a!., al., 2002). We report report here preliminary preliminary age age determinations determinations based on on SHRIMP SHRIMPanalyses analyses of of zircons zircons Formation, and and two two within within the the extracted from three volcanic ash layers; one lying in the Gunflint Formation, Rove Formation. Formation. The overlying Rove The Gunflint-Rove Gunflint-Rovecontact contact is is an an important important reference reference point. point. Pufahi Pufahl and Fralick (2000) placed it at the top of a sequence sequence of chert-carbonate chert-carbonategrainstones grainstones which which is is of the the Rove Rove Formation. Formation. The Gunflint exposure exposure outcrops outcrops at at overlain by carbonaceous shales of The Gunflint Little Little Falls, Falls, on on the the south southside sideof of the theKakabeka KakabekaFalls FallsGorge, Gorge,--1Om -10m (topographically) (topographically) below below the the Gunflint lapilli tuff dated by Fralick et a!. al. (2002). A Rove volcanic ash exposure exposure at Oliver Oliver Creek Creek is estimated to be -70m (stratigraphically) (stratigraphically) above above the Gunflint-Rove Gunflint-Rove contact. contact. Zircons were also extracted extracted from an ash ash layer layer within within Falconbridge Falconbridge Pine Pine River River (PR98-1) (PR98-1)drillcore drillcore(688.24m (688.24mdown down hole), above the Rove-Gunflint Rove-Gunflint contact. hole), located located —4m -4m above contact. The a mean 207Pb/206Pb of~1821 16 Ma while aa single single ^ ~ b l age ~age~of 1821 ~ b±Â 16 The Oliver OliverCreek Creekzircons zirconsrecorded recorded a mean of 184OMa wasobtained obtained from from the the dnllcore drilicore PR98-1 PR98-1 sample. sample. The The errors errors cited cited are at the one age of 1840Ma was discordant. deviation ((la) standard deviation l o ) and and 95% 95% confidence confidence level and the analyses are less than 5% discordant. There are recorded from from each each locality locality which which are assumed to are also also two two younger younger ages ages of of —1786Ma -1786Ma recorded be outliers. outliers. The Little Falls zircons, zircons, which are are somewhat somewhat rounded rounded and and fractured, fractured, yielded yielded various various ages, all older than 2000Ma. Most Most of of the the ages ages are are more more than 10% 10% discordant, discordant, and and these these samples samples may have suffered lead loss. As As well, well, there thereare are some some indications indicationsof of admixture admixtureof of shalely shalelymaterial material in the ash layer at this locality. Older Olderzircons zirconsfrom from the the lapilli lapilli tuff tuff layer layer at at the the lower/upper lowerlupper Gunflint Gunflint contact contact (Fralick (Fralick et al., al., 2002) 2002) were were also also found found to to be be admixed admixedwith with Paleoproterozoic Paleoproterozoic zircons. zircons. Using the stratigraphic stratigraphic column of Pufahl and Fralick (2000), we estimate that the drillhole lOmabove abovethe theGunflint Gunflint lapilli lapilli tuff layer containing containing the drillhole (PR98-1) (PR98-1)samples samplesare are—1 -1 10m the zircons zircons dated by Fralick Fralick et a!. (2002), while we estimate the Oliver Creek samples to be —150m above al. (2002), while we estimate the Oliver Creek samples to be -150m this same layer. The ages reported reported here indicate indicate that aa slow slow sedimentation sedimentationrate rate must must have have been been required in order to account account for for the age age difference difference between between the lapilli lapilli tuff tuff of of Pufahl Pufahl and andFralick Fralick and the two sets of Rove dates reported here. This slow Rove sedimentation rate is comparable that reported in banded iron iron formations formations of of the the early early Proterozoic Proterozoic Campbell CampbellGroup, Group,Griqualand, Griqualand, West Sequence, Sequence, South South Africa Africa (Barton (Barton et et a!., al., 1994). 1994). 43 �The Oliver Creek ages reported here are in reasonable agreement with the 1833 1833±Â 6Ma undeformed by by age reported by Schneider et al. (2002) for the Tobin Lake Pluton, which is undeformed Penokean deformation and intrudes presumed Hemlock Volcanic However, the the Volcanic equivalents. equivalents. However, zircons from the Rove Formation Formation suggest suggest that volcanic activity activity associated associated with with the the Penokean Penokean continued for for at at least least 40 40 m.y. m.y. Further Orogeny continued Furtherstudies studies are are underway to clarify some of the questions raised questions raised by our our results. results. Barton, E.S., Altermann, W., William, I.S., amd Smith, C.B. 1994. U-Pb U-Pb zircon zircon age age for for aa tuff tuff in in the Campbell Campbell Group, Group, Griqualand GriqualandWest West Sequence, Sequence,South South Africa: Africa: Implications Implicationsfor forEarly Early Proterozoic 22: 343-346. 343-346. Proterozoic rock accumulation accumulation rates. rates. Geology Geology 22: Fralick, P., Davis, D.W., and Kissin, S.A. S.A. 2002. The Theage ageof of the the Gunflint GunflintFormation, Formation,Ontario, Ontario, Canada: single zircon U-Pb age determinations determinations from reworked volcanic ash. ash. Canadian Canadian Journal 1089-1091. Journal of Earth Earth Science Science 39: 39: 1089-1091. Pufahi, Pufahl, P. and and Fralick, Fralick, P. P. 2000. 2000. Fieldtrip Fieldtrip 4: 4: Depositional Depositionalenvironments environmentsof of the thePaleoproterozoic Paleoproterozoic Gunflint 46, pt.2. pt.2. Gunflint Formation. Formation. Proceedings Proceedings of of the the Institute Institute on on Lake Lake Superior Superior Geology, Geology, 46, W.F., Schulz, K.J., K.J., and and Hamilton, Hamilton, M.A. M.A. 2002. Age Schneider, D.A., Bickford, M.E., Cannon, W.F., Age of of volcanic rocks and and syndepositional syndepositional iron iron formations, formations, Marquette Marquette Range Range Supergroup: Supergroup: implications implications for for the the tectonic tectonic setting setting of of Paleoproterozoic Paleoproterozoiciron iron formations formationsof of the theLake Lake Superior Region. Canadian Journal of Earth Science 39: 999-1012. Canadian Journal of Earth Science 39: 999-1012. 44 �THE SOUTHERN PORTION OF OF THE THE LAURENTIDE LAURENTIDE ICE ICE MEAN TRANSPORT LENGTH LENGTH IN TILLS OF THE SHEET: IMPLICATIONS IMPLICATIONS FOR FOR DRIFT DRIFT EXPLORATION EXPLORATION IN THE THE LAKE SUPERIOR SUPERIOR REGION REGION LARSON, Department of Geological Sciences, Sciences, University Minnesota, Duluth, 55812, LARSON, Phillip Phillip C., Department of Geological University of of Minnesota, Duluth, MN MN 55812, plarson2 @d.umn.edu plarson2@d.umn.edu Introduction Introduction Bedrock the Lake Lake Superior Superior region region isistypically typically covered covered by byaamantle mantleofofglacigenic glacigenicsediments sediments—- till, Bedrock in the outwash,and andlacustrine lacustrinesediments sediments — presents presents a significant - that significant challenge to successful successful application application of of surficial surficial outwash, geochemical techniques techniques widely widely used used to help geochemical help generate generate drilling drilling targets. targets. The glacial glacial environment environment is is very very complex, complex, with sediments produced produced by by a range of processes. with processes. Till represents represents the the ideal ideal sampling samplingmedia media ininthese theseenvironments, environments, since a vector (ice flow direction) is attached to the composition composition at any location indicating the direction to the source of any defined anomaly. anomaly. However, recent work work has led led to to recognition recognition that that both both the themagnitude magnitudeof ofaatill tillgeochemical geochemical anomaly and and the the potential potential transport transport distance distance to to its its source source may may have have aa wide wide range range of of values. values. This anomaly This isis aa reflection reflection of of material, and and is related to the the mean mean transport distance of till-forming till-forming material, the fundamental fundamental sediment sediment transport process responsible for forming forming the the till. till. Theory The concentration of an an indicator indicator (a (a distinct distinct lithologic lithologic or or geochemical geochemical component component derived derived from from aa discreet discreet in till till is the direct product of the physical processes processes of of glacial glacial erosion, erosion,transport, transport,and anddeposition. deposition. Indicator Indicator source) in concentration concentration is controlled by a number number of of variables, variables, including including substrate substrate hardness and the the efficacy efficacy of of the theglacial glacial erosional regime. regime. Under Under steady steady state state ice ice flow flow conditions conditions and and uniform uniform bed bed erosion erosion rates, rates, indicator indicator mass mass concentration ci c in till at any transport transport length length TT (1) (1) down-ice of an indicator source source of finite finite flow-line flow-linelength length LL (1) (I) is: cTO — L (1) where For tills down-ice of of the indicator source, under steady state conditions, the where X. is the erosion length length scale scale (1). (1). For decrease decrease in indicator indicator concentration concentration with with increasing increasingtransport transportlength lengthcäc/5T 8c/8T assumes assumes aaquasi-exponential quasi-exponential form. form. Erosion length scale, scale, X, X,isis related related to to the the spatial spatial bed bed erosion erosion rate rate EE (m-1'") (ml3) and and the the thickness thickness of the the debris debris layer layer in in Erosion length transport transportmd md (m12): (m-l'2): (2) E X closely related related to to the the mean transport distance of till-forming material; as ?X increases, X isis closely increases, so so does does the the mean mean transport transport distance. distance. ShortTransport: Examples Short- vs. Long-Distance Transport: Examples Tills in the Lake Superior Superior region region can can be be broadly broadly grouped grouped into into two two categories categories based based on onthe themean meantransport transport length of the till forming forming material. material. Tills characterized by short-distance short-distance mean transport transport length length are are commonly commonly composed composed of of coarse-grained coarse-grained material containing abundant abundant angular angular clasts, and display material containing display rapid decrease decrease in in indicator indicator concentration concentration with with transport transport length. This Thisisisexemplified exemplifiedby bytills tillsoverlying overlyingthe theVermilion Vermiliongreenstone greenstonebelt beltof ofnorthern northernMinnesota, Minnesota,which whichdisplays displays rapid in concentration concentrationof ofnumerous numerousindicator indicatorlithologies; lithologies; range from 2.0 l0m. m. A XX forfor thisthis tilltill range from 1.01.0 to to 2.0-lo3 rapid decrease decrease in dispersal train composed composed of of clasts clasts of of Nipigon dispersal train Nipigon diabase diabase in till till east east of of Lake LakeNipigon, Nipigon, Ontario Ontario displays displays similar similar characteristics. Dispersal for this this Dispersalisischaracterized characterizedby byaasimilar similarrapid rapiddecrease decreaseininindicator indicatorconcentration; concentration;calculated calculatedXX for till range from 5.5-10' 5.5.102 m m over over relatively relatively soft soft greenstone greenstone to to 2.4-lo4 2.4 i04 m m over over hard hard diabase. diabase. Both Both tills tills are areinterpreted interpretedto to have formed by erosion of hard bed by quarrying quarrying and abrasion, abrasion, and and englacial englacial transport. transport. Tills characterized Tills characterized by long-distance long-distance mean transport transport length length are are commonly commonly composed composed of offine-grained fine-grained material with with a relatively material relatively low low abundance abundance of rounded rounded clasts, clasts, and and display display little little apparent apparent decrease decrease in in indicator indicator with transport length. Cretaceous concentration with Cretaceous shale shale grains grains in in Des Moines Lobe tills of the Minnesota Minnesota River valley show relatively little decrease in concentration along the flow axis extending down down the valley valley (Matsch, 1972); the XI.for forthis thistill tillisis5.0-10 5.0 l0 m. m.Carbonate-bearing Carbonate-bearingtills tillsoverlying overlyingthe the Canadian Canadian Shield Shield north of Lake Superior calculated ? (Thorleifson and Kristjansson, 1993) 1993) show show similar similar long-distance long-distance transport transport of of Paleozoic Paleozoic carbonate carbonateand andProterozoic Proterozoic m. Both tills greywacke clasts concentration; the 6.0-10' tills are are greywacke clasts with with little little decrease decrease in in concentration; the calculated calculated XXfor forthis this till till is 6.0 I m. interpreted interpreted to have deposited deposited from from deforming deforming subglacial subglacialsediment sediment layers layers (deformation (deformationtills). tills). 45 �Discussion by A,? that are 10 to 100 The data indicate deformation tills are characterized by 100 times higher than those of thin, by intermediate intermediate values values of of the the erosion erosion length length scale scale 'k? have have not as yet been coarse-grained tills. Tills characterized characterized by recognized in in the Lake Lake Superior Superior region, region, and and perhaps perhaps do do not not exist. exist. This recognized This gap gap in in recognized recognized values values suggest suggest there are entrainment and two main main processes by which bed material two material is is eroded eroded and and entrained, entrained, transported, transported, and and deposited deposited— - entrainment transport by a deforming subglacial layer, and erosion by quarrying and abrasion and transport as an englacial debris load with deposition by lodgement lodgement or meltout. Deformation tills form with little little accompanying accompanying erosion of underlying underlying hard bedrock. bedrock. Their formation formation is is consistent with redistribution of unconsolidated regolith or or sediment derived from consistent with redistribution of unconsolidated regolith sediment derived from aa preglacial preglacial reservoir. reservoir. Consequently, till composition reflects that of distant Consequently, till composition reflects distant (>100 (>I00 km) krn) bedrock. bedrock. Tills characterized characterized by short short mean mean transport length indicate spatially and temporally restricted erosion and entrainment and transport of hard bedrock. Their formation is consistent consistent with with erosion erosion by by quarrying quarrying and and abrasion abrasion of of hard hard bedrock bedrockwith withsubsequent subsequentextensive extensive Till composition closely reflects reflects that textural modification modification during during transport. transport. Till composition closely that of of nearby nearby (—10 (-10 km) krn) bedrock. bedrock. Consequently, Consequently, these tills have enormous enormous potential potential value as geochemical geochemical sampling sampling media. media. Recognition of the process responsible responsible for till till formation formation in aa given given area area is is critical critical for forsuccessful successfulapplication application of surficial surficial geochemical geochemical and boulder tracing tracing exploration exploration techniques in the Lake Lake Superior Superior region. region. Limited scope scope orientation surveys aimed aimed at characterizing the erosion length provide a means orientation surveys characterizing the length scale X A, provide means of of quickly quickly assessing assessing the the potential for successful successful application application of of drift drift exploration explorationtechniques techniqueson on both both regional regionaland andproperty propertyscales. scales. 46 �Glacial Lakes Aitkin Aitkin and and Upham: Upham: their their origin and environmental history Lisa Marlow, Howard Mooers, & Phillip Phillip Larson Geological Sciences, Department of Geological Sciences, University of Minnesota, Minnesota, Duluth, Duluth, Minnesota Minnesota 55812 55812 Aitkin and Upham occupied a basin in north-central Minnesota Glacial Lakes Aitkm Minnesota bounded bounded on on the the north by the Giants Range and to the east, south, south, and west by hummocky moraines moraines of of the the Rainy Rainy The lakes lakes came came into into existence existence with with the the retreat retreat of of the the lobe, Superior lobe, and St. Louis sublobe. The Rainy lobe from the St. St. Croix Croix phase phase sometime sometime after after 15,000 15,000yr yr BP BP (Clayton (Clayton and and Moran, Moran,1982; 1982; Mooers and Lehr, 1997). 1997). The Thebasin basinwas waslater lateroveridden overidden by by the the St. St. Louis Louis sublobe sublobefrom from the the northwest. With sublobe,the the basin basin was was again again occupied occupied by by Withthe thewastage wastageof of the theice iceof of the theSt. St.Louis Louis sublobe, Aitkin/Upham I and the later phase as as Lake Lake lakes; the earlier phase is referred to as Lake AitkinJUpham AitkinflJpham at a few few localities. localities. One Sediment of of the the early early lake phase is preserved at One such AitkinAJpham II. Sediment locality preserves a sequence that helps redefine the glacial chronology. chronology. A sedimentary sedimentary sequence located in the northeast Upharn basin basin reveals reveals sub-aqueously sub-aqueouslydeposited depositedRainy Rainy Lobe Lobe northeast corner comer of the the Upham outwash beneath glaciotectonically glaciotectonicallydeformed deformed fine-grained fine-grained lake lake sediments sediments deposited depositedby by the the St. St. Louis sublobe. This, This,along alongwith with other othergeomorphic geomorphicrelationships relationships(P.C. (P.C. Larson, Larson, unpublished unpublisheddata) data) indicates indicates that the Rainy Lobe ice margin was coincident coincident with the the Giants Giants Range Range when when the the St. St. sublobe advanced across the lake lake basin. Louis sublobe Using a Digital Elevation Elevation Model Model (DEM) (DEM) the elevation elevation of of lake lake basin basin was was adjusted adjustedfor forisostatic isostatic rebound based on the highest lake lake level, level, then then tilted tilted incrementally incrementally through through several severalstages stagesto to assess assess beaches, inlets, and outlets over time. AAseries seriesof of successively successively lower lower outlets outlets draining draining to to the the St. St. and Upham Upham (Hobbs, (Hobbs, 1983; 1983;Farnum, Farnum,1964; 1964; Louis River served as outlets outlets for for Glacial Glacial Lakes Aitkin and Wright, 1972). Meltwater Meltwaterentered enteredthe the lakes lakes from from Glacial Glacial Lake Lake Norwood Norwood through through the the Embarrass Embarrass gap, and later from Glacial Lake Koochiching along the the Prairie Prairie River. River. During During this this time time Aitkin Aitkin and Upham were confluent, confluent, and and the outlet outlet was was established established down down the the modern modem St. St.Louis Louis River. River. The lakes were separated separated by a sill ca 11,500-10,100 11,500-10,100yr BP, after after inflow inflow from from Koochiching Koochiching was was diverted to Glacial Glacial Lake Lake Agassiz. Agassiz. of the lakes. Granulometry Extensive dune fields formed following initiation of drainage of Granulometry 4ip grain grain size size signature signaturecharacterizes characterizesdunes dunesthroughout throughoutthe thebasin. basin. Maximum dune indicates a 4(p amplitude metersand anddune dunemorphologies morphologiesrecord recordprominent prominentnorthwesterly northwesterlywinds. winds. Dunes amplitude is —3 -3 meters overly source areas, which include include an underfiow underflow fan deposited deposited by the Prairie Prairie River River inlet inlet and and the the western margin of Lake Upham. Lake Upham. A sediment core collected collected from from Hay Lake (93°W, (93OW, 52°N), 52ON), located within aa dunefield dunefieldat at the the edge edge of Glacial Lake Upham, records records three prominent peaks in whole-core whole-core magnetic magnetic susceptibility susceptibility between 10,100 10,100 and 6,600 yr BP. No No clastic clasticinput input isis evident evident after after 6,600 6,600 yr yr B.P., B.P., suggesting suggestingdune dune stability. The timing of dunes within the basin has important implications for other dunes throughout Minnesota. Eolian Eolianevents eventsrecorded recorded in in the the core core are are interpreted interpretedas as the the result result of of lake lake drainage and exposure of abundant source material during Late Glacial and Early Holocene exposure source material during Glacial and Early Holocene rather than landscape landscape destabilization destabilization because because of mid-Holocene mid-Holocene aridity aridity (Keen (Keen et et al., al., 1990; 1990;Grigal Grigal et al., 1976; Dean et a!., 1996; Dean, 1997). Additionally, this sediment core places a minimum 1976; al., Additionally, this sediment core places a minimum age on the drainage of of Glacial Lakes Aitkin Aitkin and Upham Upham II. II. Lake Upham must have drained after 47 �Lakes Aitkin Aitkin and and Upham Upham II. II. Lake Upham must have drained after age on the drainage of Glacial Lakes 11,500 yr BP and before 10,100 10,100 yr B.P. and Lake Aitkin may have persisted until ca. 7,000 7,000 yr 11,500 BP. BP. Clayton, L. and Moran, S.R. S.R. 1982, 1982, Chronology Chronology of late late Wisconsinan Wisconsinan glaciation glaciation in in middle middleNorth North America. Quaternary Quaternary Science Science Reviews Reviews 11 (1), (I), 55-82. 55-82. Dean, W.E., Ahlbrandt, Ahlbrandt, T.S., T.S., Anderson, Anderson, R.Y., R.Y., Bradbury, Bradbury, J.P., J.P., 1996. 1996.Regional Regional aridity aridityin in North North America during the middle (2), 145-155. 145-155. middle Holocene. Holocene. The The Holocene Holocene 66 (2), Dean, W.E., 1997. 1997. Rates, Rates, timing, timing, and and cyclity cyclity of Holocene Holocene eolian eolian activity activity in in north-central north-central United United States: States: Evidence from varved lake lake sediments. sediments. Geology Geology 25 25 (4), (4), 331-334. 331-334. Farnham, R.S., McAndrews, J.H., and Wright, H.E., Jr. 1964. 1964. A Late-Wisconsin buried soil near Famham, Aitkin, Minnesota, and 393-4 12. and its its paleobotanical paleobotanical setting. setting. American American Journal Journal of of Science Science262, 262,393-412. History of of the the Lake Lake Superior Superior Farrand, W.R. & Drexler, 1985. 1985. Late-Wisconsinan and Holocene History Basin. In In Karrow, Karrow,Quaternary Quaternaryevolution evolution of of the the Great Great Lakes, Lakes, Geological Geological Association Association of of Canada Canada Special Paper 30, 30, 17-32. 17-32. Grigal, D.F., Severson, Severson, R.C., Golz, Golz, G.E., G.E., 1976. 1976.Evidence Evidence of of eolian eolian activity activity in in north-central north-central 1-1254. Minnesota 8,000 8,000 to 5,000 5,000 yr. ago. ago. Geological GeologicalSociety Societyof ofAmerica AmericaBulletin Bulletin87, 87,125 1251-1254. Upham, and and early early Lake Lake Hobbs, H.C. 1982, 1982, Drainage Drainage relationships relationships of of Glacial Glacial Lakes Lakes Aitkin, Aitlun, Upham, Agassiz in northeastern Minnesota. In Teller, J.T., and Clayton, Lee, eds., Glacial Lake In Teller, J.T., and Clayton, Lee, eds., Glacial Lake Agassiz: Agassiz: 245-259. Geological Association of Canada Canada Special Special Paper Paper 26, 26,245-259. Keen, K.L., Shane, Shane, L.C.K, 1990. 1990. A continuous continuous record of Holocene Holocene eolian eolian activity activity and and vegetation change at Lake Ann, east-central Minnesota. Minnesota. Geological Geological Society Society of America America Bulletin Bulletin 102, 102, 1646-1657. 1646-1657. Mooers, H.D., Lehr, J.D., 1997. 1997.Terrestrial Terrestrial record of Laurentide Laurentide ice ice sheet sheet reorganization reorganization during during Heinrich events. Geology Geology 25 (11), (1 I), 987-990. 987-990. Sims, P.K., P.K., and and Morey, Morey, G.B., G.B.,eds., eds., Wright, H.E. 1972, 1972, Quaternary Quaternary history of Minnesota. Minnesota. In Sims, Geology of of Minnesota: A 15-578. A Centennial CentennialVolume: Volume:Minnesota MinnesotaGeological GeologicalSurvey, Survey,5515-578. 48 �Magmatic and Hydrothermal HydrothermalPGE PGEMineralization Mineralizationof of the the Birch Birch Lake LakeCu-Ni-PGE Cu-Ni-PGEDeposit Intrusion, Duluth Complex, northeast northeast Minnesota in the South Kawishiwi Intrusion, John Marma, Marma, Phil Phil Brown Brown and and Steve Steve Hauck* Hauck* Department Department of Geology Geology and Geophysics, Geophysics, University of Wisconsin, Wisconsin, Madison, Madison, Wisconsin Wisconsin53706, 53706,USA USA *Natural *Natural Resources Research Research Institute, Institute, University of Minnesota, Minnesota, Duluth, Duluth, Minnesota Minnesota 55811, 5581 1,USA USA The Birch Lake Cu-Ni-PGE Cu-Ni-PGE Deposit Deposit is located located 12 12 miles miles south south of of Ely, Ely, MN MN in in the the South South Kawishiwi Intrusion (SKI) of the Duluth Complex (DC). The The SKI SKIis is one one of of two two layered layered mafic mafic intrusions along the basal contact of the DC to host sub-economic Cu-Ni-PGE deposits. intrusions contact of the DC to host sub-economic Cu-Ni-PGE deposits. Mineralization Mineralization is dominantly dominantly hosted hosted by by the the U3 U3 layer, layer, the lower-most lower-most of of three three ultramaficultramafictroctolite troctolite packages characterized characterized as as aa zone zone of alternating alternatingultramafic ultramafic (picrite-peridotite) (picrite-peridotite)and and troctolite (>5%) ultramafic ultramaficand/or andlormassive massiveoxide. oxide. troctolite horizons horizons with lenses lenses and and pods pods of of oxide-bearing oxide-bearing(>5%) The purpose of this study study was to locate, locate, describe, describe, and and characterize characterizethe the textural textural relationships relationships among platinum group group minerals minerals (PUM), (PGM), sulfides, sulfides,and and silicate silicatephases phases to to help help delineate delineatethe the relative significance significance of primary and remobilized remobilized platinum platinum group group element element (PGE) (PGE)concentrations. concentrations. Samples drill holes transecting transecting the the Birch Birch Lake Lake Deposit Deposit were were obtained obtainedfrom fromthe the Samples from 4 drill Natural Resource Research Institute (NRRI) located in in Duluth, Duluth, MN. MN. EMPA EMPA and and detailed detailed petrography were used to locate PGE bearing minerals, minerals, averaging averaging e15pm <15tm in in diameter, diameter,and and to to characterize characterize the host mineral geochemistry. Identifying Identifying the PGM PGM textural textural relationships relationships with with other phases is critical to understanding the mechanism by by which which PGMs PGMs were deposited. Data Data from this study will aid exploration in locating other deposits and guide metallurgists in improving improving recovery techniques. techniques. Ir,Ru, Ru, Au, Au,Ag, Ag, Te, Te, PGEs occur most often often as various various Pd minerals minerals with with associated associatedPt, Pt, Os, Os,Ir, categories of silicate-sulfide-PGM silicate-sulfide-PGMtextural textural Bi minerals and were grouped into into the following following 44 categories relations: 1) PGMs PGMs that occur in "halos" residing most commonly in anorthite-enriched relations: anorthite-enrichedzones zones in in primary plagioclase around either interstitial interstitial sulfide sulfide (dominantly (dominantly chalcopyrite), chalcopyrite),interstitial interstitial sulfide sulfide and silicate silicate (dominantly chalcopyrite, chalcopyrite, clinopyroxene, and hydrous silicates silicates (amphibole (amphiboleand and silicate) (Figure (Figure 1). This This style style of biotite)), or silicate (dominantly clinopyroxene or hydrous silicate) of the the total total PGMs PGMs identified. identified. 2) Remobilized PGMs that occur in mineralization hosted 58% of chlorite, serpentine, or secondary magnetite. This This style style of of mineralization mineralization hosted hosted 21% 21% of of the the total total PGMs identified. 3) 3)Random RandomPGMs PGMsthat thatoccur occurin inpoikilitic poiluliticanorthite-rich anorthite-richplagioclase plagioclase(An (An 75-An 75-An 95) and clinopyroxene clinopyroxene (Wo 30-Wo 50) with PGEs sometimes residing in disseminated but no no association with with "halos". "halos". This style of chalcopyrite or hydrous silicate pockets, but mineralization hosted 11% 4)In In interstitial interstitialsulfides sulfidesor orsilicates silicatesthat that mineralization 11% of the total PGMs identified. 4) include hydrous silicates, (?) textures, textures,or or silicates, chalcopyrite, chalcopyrite, clinopyroxene, sulfides sulfides with symplectite symplectite(?) calcite. This style of mineralization mineralization hosted hosted 10% 10% of the the total total PGMs PGMs identified. identified. of high PGM concentrations in The following is a summarized summarized model for the formation of the Birch Lake deposit. The TheSKI SKIbegins begins as as aa magma magma body body that is replenished replenished with multiple multiple injections of magma, which becomes sulfur saturated. The The magma magma body body is is relatively relatively PGE PGE poor, due to partial loss of sulfides during emplacement leaving the conduits with a PGE enriched segregation of the total sulfide. The Thesulfides sulfidesin in the the magma magma body body scavenge scavengeavailable available PGEs PGEs and and crystallize as disseminated, interstitial sulfide grains. Primary Primary hydrous hydrous phases phases form form at at this this time time 49 �fro:maafluorine-rich, fluorine-rich,deuteric deutenc fluid. fluid. A Cl-, Cu-, PGE-rich, sulfide-poor from sulfide-poor fluid enters the magma faults. The chamber at its base via the original magma conduit(s) and/or faults. The fluid fluid migrates migrates along along grain boundaries, and interacts with the larger interstitial interstitial sulfides. A A dynamic dynamic environment environment is is created in which the fluid, containing a significant concentration of dissolved metals, begins containing significant concentration of dissolved metals, begins to to to produce produce more more sulfides. sulfides. At consume and use sulfur from the larger grains to At the the same same time, time, the the fluid is reacting with neighboring grains, specifically plagioclase, and through aa cation cation exchange exchange them in in calcium. calcium. This reaction, alters the plagioclase rims, enriching them This reaction reaction causes causes aa volume volume (±chlorite) producing producing the the disseminated disseminated loss that is filled with precipitated sulfides and PGMs (±chlorite "halos" around the larger interstitial grains (Figure 1). 1). Finally, Finally, another another fluid fluid migrated migrated through through "halos" the intrusion that remobilized remobilized PGMs on on aa small small scale. scale. Based on evidence evidence solely solely from from the Birch Lake deposit, deposit, PGM PGM mineralization mineralizationappears appears concentrated or "compartmentalized". This Thiscould couldbe be the the result result of of two two possible possiblemechanisms: mechanisms: 1) 1) of high PGM concentrations are dependent on on their their proximal proximal distance distance to to "feeder" "feeder" zones Areas of 2) High PGM PGM concentrations concentrationsare are (i.e. conduits or faults) where fluids can be introduced; introduced; and/or and/or 2) due to structural controls within these these heterogeneous heterogeneous rocks that localize localize fluid fluid movement. movement. This study contributes to the current debate on the roles of of primary vs. remobilized mafic intrusions. intrusions. For (deuteric?) PGE mineralization in layered mafic For the the Birch Lake Lake deposit, deposit, the data data suggest both mechanisms played im of the ore minerals. )ortant roles in the sussest nlaved imiortant orisin ore minerals. Figure Figure 11 locations A-L. A-L. a.) Photomicrograph in a.)~hotomicro~ra~h inplane-polarized plane-polarizedlight lightofofthin thinsection sectionBL BL89-2 89-22516.4 25 16.4—- locations Large interstitial chalcopyrite chalcopynte and pyroxene cross-cut by vertical chlorite veins, all of which are surrounded by a disseminated, disseminated, dominantly chalcopyrite halo that is in An-enriched plagioclase rims. Notice Notice all allPGMs, PGMs,except except one, one, either either occur occur in the halo; in chlorite veins; in interstitial biotite; or in clinopyroxene. clinopyroxene. The Thealtered alteredplagioclase plagioclase and and altered alteredpyroxene pyroxene on on the the left left side side of of the the image are devoid includesareas areaswithin withinthe theoriginal originalhalo. halo. This devoid of any PGM occurrences occurrences —this -this includes suggests a second alteration fluid event that removed PGMs and altered altered the the minerals, minerals,which which itit passed through. Dashed Dashedline linerepresents representsthe theextent extentof of halo halo and andAn-enrichment An-enrichment in in the the adjacent adjacent plagioclase grains. White Cpx=Clinopyroxene, White stars stars represent represent PGM occurrences. occurrences. Cpx=Clinopyroxene, Opx=Orthopyroxene, Bi=Biotite, Bi=Biotite, Chl=Chlorite, Opx=Orthopyroxene, Chl=Chlorite, Cpy=Chalcopyrite, Cpy=Chalcopyrite, Plag=Plagioclase Plag=Plagioclase 50 �RESULTS OF OF EMP EMP MONAZITE MONAZITE GEOCHRONOLOGY GEOCHRONOLOGYIN E-C MINNESOTA: MINNESOTA: EVIDENCE EVIDENCE FOR LARGE-SCALE GEON 17 METAMORPHISM ASSOCIATED WITH POSTLARGE-SCALE GEON 17 METAMORPHISM ASSOCIATED WITH POSTTECTONIC PLUTONISM MCKENZIE, M.A., and HOLM, D.K., D.K., both at at Dept. Dept. of Geology, Geology, Kent State State University, University, Kent, Kent, OH; SCHNEIDER, SCHNEIDER, D.A., D.A., Dept. Dept. of of Geological Geological Sciences, Sciences, Ohio Ohio University, University,Athens, Athens,OH; OH; JERCINOVIC, M., Dept. of Geosciences, Geosciences, University of Massachusetts, Massachusetts,Amherst, Amherst, MA MA Determination of the timing and extent of poly-phase metamorphism is essential in Determination unraveling the tectonic history of a region. The The pattern pattern and and degree degree of of metamorphism metamorphism preserved preserved across the Penokean orogenic belt in the southern Lake Superior Superior region is highly variable. Abundant 40Ar/39Ar dates from east-centralMinnesota Minnesotaindicate indicatewidespread widespread cooling cooling at at -1760 - 1760 Ma ~ r l dates ~ from r east-central shortly after the emplacement emplacementof of the the east-central east-centralMinnesota Minnesotabatholith batholith(ECMB) (ECMB)atat—1775 -1775 Ma (Hoim (Holm et al., 1998; 1998; in review). Yet Yet U-Pb U-PbSHRIMP SHRIMPmonazite monaziteages ages from from three three localities localitiesacross across the the Ma metamorphic MN, northern northern MI, MI, and and northern northern WI) WI)record recordonly onlyaauniform uniform—1830 -1 830 Ma region (e-c MN, episode 1800 Ma thermal thermal pulse pulse linked linkedtotoaarecently recentlyidentified identified—1800 -1800 episode and and aa secondary secondaryyounger younger—1800 Ma magmatic event (Schneider et al., a!., in in review). review). This This study study utilizes utilizes the the total total Pb Pb electron electron microprobe (EMP) monazite age dating technique to better constrain the the extent of of thermal overprinting overprinting surrounding surrounding the batholith. batholith. metamorphic monazite monazite ages ages from from three three For this study study we obtained obtained in situ metamorphic Paleoproterozoic gamet-staurolite schist samples and one garnet-cordierite Paleoproterozoic metasedimentary garnet-staurolite gneiss sample from the plutonic zone of east-central Minnesota (Figure 1). Schist Schistsample sampleAMAM016 contains contains predominantly elongate elongate monazite monazite grains grains displaying displaying aa mottled mottled chemical chemical variation variation in in Y and Th content. This Thissample sampleyielded yieldedaa mean mean age age of of 1746 1746±Â 3 Ma from 79 spots spots on seven grains. Two Maand and1760 1760Ma. Ma. A third less prominent Twoage agedomains domainswere were recognized recognized at at —1738 -1738 Ma —1780 Maage agedomain domainwas wasobtained obtainedon onsome somehigh highYYregions. regions. Schist sample MN-29 contains -1'780 Ma sub-euhedral monazite displaying prominent regions of of high high Th Th content. content. This This sample sampleyielded yielded aa 10 Ma from 92 spots on five grains. A mean age of 1764  10 A single singleprominent prominentage age domain domain was was 1764 ± recognized 1772 Ma. Schist recognizedatat— 1772 Schistsample sampleP-16 P-16contains containsmonazite monazitewith withvery veryirregular irregulargrain grain boundaries, numerous inclusions, and variable Th content. This This sample sampleyielded yielded aa mean mean age age of of 1772 Two age age domains domains are identified: aaprominent prominent age age 1772 ±Â 33 Ma from 92 spots on seven grains. Two domain Maand and aa smaller smaller population population age domain Ma. Lastly, the Sartell domain at at —1770 -1770 Ma domain at at —1800 -1800 Ma. Sartell gneiss, corelrimtextures. textures. gneiss, sample sample S-2, S-2, contains contains euhedral euhedral monazite monazite grains grains displaying displaying distinctive distinctivecore/rim Ma from 102 spots on seven grains. Three This sample yielded a mean age of 1756 ± 3 Three age age sample 1756  3 domains 1750 Ma and 1770 1770 Ma and and aa third third less less domains are are identified: identified:two twoprominent prominentdomains domainsatat— 1750 prominent 1800 Ma on high U U cores. cores. prominent domain domainatat—1800 Our EMP Ma thermal thermal imprint imprint associated with intrusion EMP results results reveal reveal aa profound profound—1770 -1770 Ma intrusion of the 1775 1775 Ma ECMB. The Theconsiderable considerabledistance distanceof of some someof of these thesesamples samplesfrom fromthe the western western edge of the exposed batholith (30-40 km) and the absence of Penokean metamorphic ages exposed batholith (30-40 krn) and the absence of Penokean metamorphic ages suggests that the thermal pulse must have been dramatic. However, However, the the garnet-schist garnet-schistsample sample KKR (east of Mille Mule Lacs) that records only geon 18 SHRIMP SHRiMP ages lies north of the region of thermal influence of the batholith. We Wenote notethat thatsample sampleK-R K-R isislocated locatedjust just north north of of the theMalmo MalmoStructural Structural of it. Our Discontinuity (MSD) and sample AM-016 is located south of Our data datareveal reveal that that the the MSD MSD juxtaposes rocks of different different metamorphic age (geon 18 18 metamorphism to the north from geon - - - - 51 �17 metamorphism to the south). We We propose, propose, therefore, therefore, that the the MSD MSD is is aa geon geon 17 17 structure structure that exhumed the plutonic terrane of east-central Minnesota. West West of of Mille Mille Lacs, Lacs, aa significant significant juxtaposes post-Penokean plutons to the south against older metamorphic portion of the MSD juxtaposes rocks to the north. This Thisclearly clearlysupports supportsour our interpretation interpretationthat that this this structure structurewas was active activewell well after Penokean orogenesis. orogenesis. Hoim, D.K., Darrah, K., and Lux, D., 1998, 60-8 1. 1998,American American Journal Journal of of Science, Science,298, 298,60-8 Holm, T., Schweitzer, D., and Schneider, Holm, D.K., Van Schmus, W.R., MacNeill, L., Boerboom, T., D.A., in review, review, Geological GeologicalSociety Society of of America America Bulletin. Bulletin. Schneider, D.A., Holm, Hoim, D.K., O'Boyle, C., C., Hamilton, Hamilton, M., Jercinovic, Jercinovic, M., in review, review, Geological Society of America Special Volume "Gneiss Domes and Orogeny." Special volume Domes and Orogeny." Composite MN-29 Composite AM-Ol 6 Composite 0 7 0 1700 04 7720 2 1740 7 4 1760 1760 1780 1780 1800 1800 Mo8 More 1700 1700 1720 1720 1740 1740 Age (Ma) AW (Ma) 1760 1760 1780 1780 !do0 1000 More Mo. Age ~ g (Ma) (Ma) e Composite S-2 Composite P-16 Composite 20 - 1520 10- IS U. 5- 1 0 1700 71720 7 4 1740 1760 1760 1780 1780 1800 1000 More Mo,. 1 1 0-' 1700 1700 1720 1720 1740 1740 1760 1760 1780 1780 1800 WOO Mor. More Age (Ma) Ag. (Ma) - Histogram AllSamples Samples - All Ages Histogram All 700 1720 1740 1760 780 1800 1820 Age (Ma) - Figure 1: Histograms Histogramsof of EMP EMP Th-U-total Th-U-total Pb in situ s i t ~monazite spot ages. 52 �THE SIOUX QUARTZITE REVISITED: SEDIMENTOLOGY, SEDIMENTOLOGY, METAMORPHISM, METAMORPHISM, GEOCHEMISTRY GEOCHEMISTRY AND THE THE ORIGIN ORIGIN OF PIPESTONE PIPESTONE Geology & & Geophysics, Geophysics, University University of of MEDARIS, L.G., Jr., and DOTT, R.H., Jr., Dept. of Geology Wisconsin-Madison, Madison,WI, WI,53706; 53706;medaris@geology.wisc.edu; medarisgeology.wisc.edu;rdott@geology.wisc.edu rdottgeoIogy.wisc.edu Wisconsin-Madison, Madison, Red, supermature quartzites of the Baraboo Baraboo Interval were deposited after after 1.75 1.75 Ga Ga on on aa stable stablecraton craton ininthe the presence of free atmospheric oxygen oxygen under under conditions of intense intense chemical chemical weathering. weathering. Some presence Some quartzites quartzites (Baraboo and and Flambeau) Flambeau) were were folded folded and and recrystallized recrystallizedatat 1.63 1.63 Ga Ga (Holm (HoIm et et al., 1998), 1998), and and many many (Baraboo quartzites were were hydrothermally hydrothermallyaltered alteredatat 1.46 1.46 Ga Ga (Medaris (Medaris et et a!., quartzites al., 2002, 2002, in in press), press), presumably presumably in in response to brine migration promoted by by continental continental scale scale A-type A-type magmatism. magmatism. These response migration promoted These discoveries discoveries have have prompted us to reevaluate the sedimentology, metamorphism, and geochemistry of the Sioux Sioux Quartzite. Quartzite. Sedimentolo'y SedimentologvThe TheSioux SiouxQuartzite, Quartzite,which whichisisseveral severalhundred hundredmeters metersthick, thick,isiscomposed composed mostly mostly of quartz sandstone with interstratified interstratified lenses of red red mudstone mudstone (Southwick (Southwick et et al., al., 1986). 1986).Heterogeneous Heterogeneous cobble conglomerate occurs at the base and finer finer pebbly layers layers are are scattered scattered throughout throughout the the lower lower half half or or Sedimentary structures structures in the sandstones so, whereas mudstones mudstones occur chiefly chiefly within within the the upper upper half. half. Sedimentary sandstones include 15 cm in in thickness, thickness, rare rare include predominant predominant festoon-style, festoon-style, nested trough trough cross cross beds bedsaveraging averaging1010—- 15 zones of planar-tabular sets, a few zones planar-tabular sets, few examples examples of of herring herring bone bone cross cross bedding, bedding, and and both bothasymmetric asymmetric and and symmetric ripple marks. The mudstones are mostly massive, but parallel- laminated and ripple-laminated varieties are are also also present. present. In In most cases, quartz quartz silt silt and and fine sand grains are disseminated in a finer varieties most cases, disseminated in finer rare graded graded laminations laminations are are also also present. present. Some mudstones show polygonal polygonal 'mud' cracks, matrix, but rare cracks, and and the overlying sandstones commonly contain intraclasts ripped up from such such cracked cracked beds beds Interpretations of the Sioux depositional environment include shallow marine and and braided fluvial fluvial (Doll, 1983; Southwick et al., a!., 1986). 1986). In the latter latter scenario, scenario, the cross cross bedded bedded sandstones sandstones represent represent river river (Dott, channel deposits, and the mudstones, mudstones, slack slack water water deposits deposits in in ponds ponds between between active activechannels. channels.However, However, this interpretation interpretation is inconsistent with the rarity of scoured channel bases and tabular sets sets of of planar planar cross cross laminations, which would would have have formed formed by laterally bars, and the existence laminations, which laterally migrating migrating bars, existence of of wave wave ripples, ripples, Ojakangas and and Weber Weber (1984) (1984) suggested suggested that that the the which are not expected in the the sands of a braid plain. Ojakangas expected in braid plain. one-third of the Sioux formation was deposited in a shoreline upper one-third shoreline marine marine setting setting with with tidal tidal influences, influences, polygonal desiccation cracks, and accounting for the the herringbone herringbone cross cross bedding, bedding, wave ripples, polygonal and thickness thickness and extent of certain certainmudstone mudstonelayers layers(now (nowpipestone). pipestone). Interpretation Interpretation of of the the Sioux Siouxas asaafluvial-to-marine fluvial-to-marine transgressive succession would conform to the present interpretation of the correlative Baraboo transgressive succession would conform to the present interpretation of the correlative Baraboo Quartzite, which has wave ripples and reactivation surfaces surfaces in in its its upper upper half half (Medaris (Medariseteta!., al.,ininpress). press). Metamorphism Metamorphism Mineral Mineralassemblages assemblages in in finefine55 grained Sioux sedimentary rocks rocks can be 9 expressedininthe the system, system,KASH, KASH,asasportrayed portrayedin inac- 4 _ expressed Figure 1, 1, where rock rock compositions compositions are are projected projected onto the anhydrous onto anhydrous plane, plane, K-Al-Si, K-Al-Si, and and two two 3critical dehydration reactions are plotted. are plotted. Additional phases phases include abundant hematite Additional hematite and 2a Ti02 TiOz phase, phase, either either anatase anatase in in the the Cottonwood Cottonwood (Stelz, 1989), 1989), or rutile in the Basin (CB) Basin (CB) (Stelz, 11 The stable existence Pipestone Basin (PB). Pipestone (PB). The existence of - co~onwood the CB kaolinite in the CB (Stelz, (Stelz, 1989) 1989) requires temperatures —300°C, whereas pyrotemperatures below -300°C pyroI /I I I phyllite ininthe thePB PB isis stable stableabove above-300°C 300°C. The phyllite The 250 300 350 T, OC 53 �quartz-pyrophyllite assemblageininthe thePB PB(0, (0, Fig. 1), quartz-pyrophyllite assemblage I), in in which which vermicular kaolinite has been replaced by by pyrophyllite pyrophyllite (Fig. (Fig. 2A), 2A), most likely most likely represents represents higher higher temperature, temperature, largely largely isochemical isochemical recrystallization of a quartz-kaolinite protolith, like that in the CB CB The occurrence of muscovite in both (0, Fig. 1). 1). The occurrence of muscovite in both basins basins is is attributed to K-metasomatism related to 1.46 attributed K-metasomatism related 1.46 Ga Ga hydrothermal hydrothermal ("I",Fig. Fig. 1)1)isisaametasomatic metasomatic rock composed of activity. Pipestone Pipestone (+, pyrophyllite, muscovite, diaspore, hematite, and and rutile, pyrophyllite, muscovite, diaspore, hematite, rutile, in in which which former quartz grains have been completely former completely replaced by diaspore, diaspore, pyrophyllite, and and muscovite pyrophyllite, muscovite (Fig. (Fig. 2B). 2B). Because Because the Sioux Sioux undeformed and lies north of the extrapolated Quartzite is largely undeformed trend of the 1.63 al., 1998), 1998), 1.63 Ga Ga Mazatzal Mazatzal tectonic tectonic front front (Hoim (Holm et al., we suggest suggest that all all metamorphic metamorphic features features of of the the Sioux Sioux Quartzite Quartzite 1.46Ga Ga hydrothermal hydrothermal activity, activity, rather rather than than aa Mazatzal Mazatzal are due to 1.46 event. event. Geochemistry Where Where unmodified unmodified by by K-metasomatism, K-metasomatism, finegrained grained sedimentary rocks of the Baraboo Interval are remarkably mature, being practically Mg, and and Mn Mn (Fig. (Fig. mature, practically devoid devoid of K, Na, Ca, Mg, 3), and having Critical Critical Index Index of of Alteration Alteration values values of of 97 97 to to 99. 99. In In such rocks rocks the the wide wide range range in in proportion proportion of of Si Si to to A1 Al (Fig. 3) and such quartz to kaolinite, 1), reflects reflects the the original original proportion proportion of quartz to kaolinite in the kaolinite, or or pyrophyllite pyrophyllite (Fig. (Fig. I), protolith sediments. sediments. K-metasomatism has stabilized 200 muscovite in both both the the CB and PB, but but muscovite in .4i the muscovite-bearing rocks in the CB muscovite-bearing rocks CB 150 150 record record a lower lower temperature temperature and and higher higher ratio of Si/Al SiIAl compared to pipestone in the PB (Fig. 1). The Theclassic classicpipestone, pipestone, 5 100 loo .s 8 in addition addition to to substantial substantial KK contents, contents, contains lower Si Si and higher contains lower higher Al A1 than than 8oa.to 50 that in associated associated quartz + + pyrophyllite pyrophyllite fca"!& samples (Figs. 1 & & 3). 3). Assuming samples (Figs. Assuming Zr to ~ g ? be an immobile element, isocon ID calculations indicate the mean indicate that the mean E -50 pipestone composition was produced by 0 removal of 20 to 65% Si02, removal of Si02, 45 to 55% 55% -100 Ti02, Si Fe TI Ti02, 35 to 65% 65% Fe203, Fe203,and addition of K Na Mn Fe Ti Al Si K Na Ca Mg Mg Mn 15 to to 45% 45%A1203 A1203and and—800% -800% 1(20, K20, compared to the average compositions of the two Si-rich Si-rich and two Al-rich Al-rich quartz quartz ++ pyrophyllite samples. samples. The reconstructed composition of of one one pipestone pipestone sample sample (*, (*, Fig. Fig. 1) 1) requires requires removal removal of of 68% 68% Si02 Si02and and reconstructed composition addition during metasomatism. addition of of 50% 50%A1203 A1203during metasomatism. Further Further investigation investigation is underway underway to provide a more more detailed detailed evaluation evaluation of of brine brine compositions compositions and and metasomatic processes involved in this important, important, regional scale, scale, 1.46 1.46 Ga Ga hydrothermal hydrothermal event. event. References Dott, Dott,R.H. R.H.Jr. Jr.(1983) (1983)Geol. Geol.Soc. Soc.Amer. Amer.Memoir Memoir 160, 160,129-141; 129-141;HoIm, Holm, D. eta!. et al.(1998) (1998)Geology, Geology, v. 26, 907-9 10; Medaris, L.G., Jr. eta!. (2002) 48th Inst. Lake Superior Geol., 24-25; Medaris. L.G., Jr. Geol., 24-25; Medaris. L.G., Jr. eta!. et al.(in (in press) press) 907-910; Medaris, L.G., Jr. et al. (2002) 48th Inst. Lake Jour. Geol.; Ojakangas, Ojakangas, R.W. R.W. & & Weber, Weber,R.W. R.W.(1984) (1984)Mimi. Minn. Geol. Geol. Surv., Surv., Rept. Inv. 32, 1-15; 1-15; Southwick, D.L. et a!. al. (1986) (1986) Geol. Soc. Soc. Amer. Bull., v. 97, 1432-1441; 1432-1441;Stelz, Stelz, D.E. D.E. (1989) (1989)M.S. M.S. Thesis, Thesis,Wichita Wichita State StateUniv., Univ.,140 140pp. pp. (, Cd, C.) d . = - r 5 54 �A geochemical investigation investigation of of Mesoarchean Mesoarchean metavolcanic and metasedimentary metasedimentary rocks from the Birch-Uchi Birch-Uchi greenstone belt Metsaranta, Hollings, P.P. (Department of of Geology, Metsaranta,R., R.,Fralick, Fralick,P.P.and and Hollings, (Department Geology,Lakehead LakeheadUniversity, University,Thunder ThunderBay Bay ON CAN, P7B P7B 5E1) 5E1) Most Mesoarchean belts in the Mesoarchean greenstone greenstone belts the Western Western Superior Superior Province Province are are comprised comprised primarily of komatiite-tholeiite komatiite-tholeiite sequences sequences and and associated associated sedimentary sedimentary rocks rocks (Thurston (Thurston and Chivers Chivers 1990). 1990). These These—2.9-3.0 -2.9-3.0 Ga Ga assemblages assemblages have been been interpreted interpreted to to represent represent plume generated generated volcanism volcanism in in oceanic oceanic plateau plateau settings settings(for (forexample, example,Hollings Hollingsetetal. al.1999, 1999, Tomlinson et et al.1999). al.1999). This study study isis aapreliminary preliminary investigation investigation of of metavolcanic metavolcanic and and metasedimentary strata from the metasedimentary strata the Mesoarchean Mesoarchean Balmer Balmer assemblage assemblage of of the theBirch-Uchi Birch-Uchi greenstone belt. belt. Rogers Rogers et al. al.(2000) (2000)have havesuggested suggested that, that,given giventheir theirgeochemical geochemical affinities affinities and and Nd isotopic isotopic evidence evidence for for contamination contamination by by older older crust, crust, volcanic volcanic rocks rocks of of the the Balmer assemblage may represent a continental arc arc setting. This This implies implies that that the the Balmer Balmer Assemblage may represent those proposed proposed for for other other sent aa distinct tectonic tectonic setting from those Mesoarchean Mesoarchean rocks rocks in in the the Superior Superior P \ B,U' Province. Province. Sediment geochemistry geochemistry and and depositional depositional environment environment studies studies along along with igneous igneous geochemistry geochemistry will will be M8 applied to provide provide further further constraint constraint on on the possible tectonic setting of these possible tectonic setting of these — Ss.thviti LSO rocks. rocks. A i:3 I N N The Birch-Uchi Birch-Uchi greenstone greenstone belt belt is is located in the located in the central central portion portion of of the the MNI NM It Uchi (Fig.1). It is is Uchi Subprovince Subprovince (Fig.1). volcanic units three comprised comprised of three volcanic units termed the Balmer, Balmer, Narrow Narrow Lake Lake and and spanning assemblages, Woman Woman assemblages, spanning approximately 250 Ma. Ma. The Balmer Balmer assemblage assemblage is is the the oldest oldest of these these volcanic units and has volcanic units has U-Pb U-Pb zircon zircon ages from felsic felsic volcanic volcanic horizons horizons that that suggest suggest an age age of of ca. ca. 2975-2989 2975-2989 Ma Ma (Rogers et al., 2000). The Thestratigraphy stratigraphy of the the Balmer Balmer assemblage assemblage is is divided divided into four four suites: suites: aalower lowersedimentary sedimentary sequence, sequence, a mafic mafic volcanic volcanic suite suite and and two two petrographically petrographically distinct distinct felsic felsic volcanic volcanic suites suites (Rogers (Rogers et et al. al. 2000). 2000). Samples for this study Samples collected collected for study are are located in the the southern southern portion portion of of the the Figure Figure 11-Location Locationand andgeneralized generalizedgeology geologyof ofstudy study area area and and Birch-Uchi Birch-UchiGreenstone Greenstonebelt belt(modified (modifiedfrom from Balmer assemblage assemblage in in the Woman Woman Stott Stott and and Corfu Corfu 1991) 1991) River/Bear RiverIBear Lake area. These Thesecomprise comprise 16 samples from the 34 samples samplesof of the themafic maficvolcanic volcanic the lower lower sedimentary sedimentary sequence sequence and and 34 suite suite of of Rogers Rogers et et al.(2000). al.(2000). 55 �Field observations suggest that the the Balmer Balmer assemblage assemblage sedimentary sedimentary rocks are are turbiditic. turbiditic. Sediment geochemistry geochemistry will will be be applied applied to constrain the source rocks compositions Sediment compositions for these sediments. As Asno nocontact contactwith with underlying underlying older older rocks rocks has been identified identified this might provide provide valuable valuable information information about about the the preexisting preexisting older older crust. crust. Alternatively, the the sediments may be derived from the Balmer assemblage volcanics and this could support a hypothesis that the Balmer hypothesis that Balmer assemblage assemblage represents represents aa continental continental arc setting setting with with the the sediments deposited in a fore-arc fore-arc trench. Volcanic rock samples appear Volcanic rock appear to fall fall into intotwo twocompositional compositionaltrends. trends. The The first first is aa tholeiitic trend trend comprised comprised of of primarily primarily tholeiitic tholeiitic basalts basalts and and andesites. andesites. The The second second is is aa caic-alkaline trend of andesitic to rhyodacitic compostion. The geochemistry of these calc-alkaline trend of andesitic rhyodacitic geochemistry of samples will will be be applied samples applied to suggest suggest aa possible possible tectonic tectonic setting setting for these these rocks rocks and and implications ten-anes. implications of this this in relation relation to to other other Mesoarchean Mesoarchean terranes. 400 0 300 * 200 0 U II I I 0.01 100 .001 .01 I U 0 0.1 1 0 10 200 100 300 Zr Nb/Y NbN Figure 33- Lithology Lithology Discrimination diagram for Balmer Assemblage volcanics. Circles Circles are Tholeiitic trend squares are calc-alkaline trend. calc-alkaline trend. Figure 3-A plot of V vs Zr showing compostional groups in Balmer Assemblage Assemblage volcanics. volcanics. Circles are Tholeiitic trend squares are calc-alkaline calc-alkaline trend. trend. References: References: Hollings, P., Wyman, D. and Kerrich, R. 1999. Komatiite-basalt-rhyolite Komatiite-basalt-rhyolite volcanic associations associations northern northern Superior Superior Province Province greenstone greenstonebelts: belts: significance significanceof of plume-arc plume-arc interaction in the generation of of the the proto proto continental continentalSuperior SuperiorProvince. Province. Lithos Lithos 46: 137-162. 137-162. Lithogeochernical Rogers, N., McNicoll, V., van Stall, C.R., and Tomlinson, K.Y. 2000. Lithogeochemical studies in the Uchi-Confederation Uchi-Confederation greenstone greenstone belt, belt, northwestern northwestern Ontario: Ontario: implications implicationsfor for Archean Archean Tectonics. Geological 2000-C 16:1lip. GeologicalSurvey Survey of of Canada, Canada, Current Research 2000-C16: lp. Stott, G.M., and Corfu, F. F. 1991. Uchi Uchi Subprovince. Subprovince. In: In:Geology Geology of of Ontario, Ontario, special special volume 4, part 1. 8. 1. Ontario OntarioGeological GeologicalSurvey, Survey,pp pp145-23 145-238. Thurston, P.C. and Chivers, K.M. 1990. 1990. Secular Secularvariations variationsin ingreenstone greenstonesequence sequence development development emphasizing Superior Province, Canada. Precambrian PrecambrianResearch. Research. 46: 46: 21-58 21-58 D.J., Thurston, Thurston,P.C., P.C., and andHall, Hall,R.P. R.P. 1999. 1999. Plume Tomlinson, K.Y., Hughes, D.J., magmatism and crustal growth at 2.9 to 3.0 Ga in the Steeprock Steeprock and and Lumby Lumby Lake Lake area, area, Western Western Superior Province. Province. Lithos 46: 46:103-136. 103-136. 56 �PETROLOGY PETROLOGYAND AND PGE POTENTIAL POTENTIAL OF OF THE THE GREENWOOD GREENWOOD LAKE LAKE INTRUSION, INTRUSION, COMPLEX, LAKE CENTRAL DULUTH COMPLEX, LAKE COUNTY, COUNTY, MINNESOTA MILLER, James, D., Jr., Minnesota Minnesota Geological Geological Survey, Survey, mille066@tc.umn.edu mille066@tc.umn.edu MILLER, This report summarizes the results of a petrologic and metallogenic of drill core and metallogenic study of outcrop samples samples that profile profile the the Greenwood Greenwood Lake Lake intrusion intrusion (GLI) (GLI) of of the the central centralDuluth DuluthComplex Complex (Fig. 1). 1). The Thelittle littlethat thatwas wasknown knownabout aboutthis thisvery verypoorly poorlyexposed exposedlayered layeredmafic maficintrusion intrusionprior priorto to this study came from interpretation interpretation of its its aeromagnetic aeromagnetic signature, seven drill cores, and sparse, sparse, localized outcrop. The Thepurpose purposeof of this thisstudy study was was to to establish establish the the igneous igneous stratigraphy stratigraphy of of the GLI and to evaluate evaluate its its potential potential for for PGE PGEreef reef mineralization. mineralization. The GLI is an an approximately approximately two two kilometer-thick, sheet-like 10') to to the the east east and and covers covers an kilometer-thick, sheet-like intrusion intrusion that that dips dips gently gently (approximately (approximately10°) area of about 300 square kilometers. For For this this study, study,19 19bedrock bedrock drill drill cores cores (20 (20 to to 80' 80'in in length) length) Erie/LTV railroad railroad and powerline were acquired acquired in in early early 2002 2002along alongthe thewest—northwest-trending west-northwest-trending ErieILTV west of Lake 1). Samples Samples from from these these cores and west Lake County County Highway Highway 22 (Fig. (Fig. 1). and from from intermittent intermittent outcrops along the eastern extent of the railroad grade were subjected subjected to petrographic study in transmitted and reflected light, microprobe microprobe analyses analyses of olivine olivine and and pyroxene pyroxene composition, composition, and and whole rock whole rock analyses analyses of of their theirlithogeochemistry, lithogeochemistry, including including platinum, platinum, palladium, palladium, and gold gold concentrations. concentrations. The results of the drilling drilling and and petrographic petrographic study study show show that the the igneous igneous stratigraphy stratigraphy of of the the GLI can be grossly (GLtr, 0-650 0-650meters), meters), composed composed GLI grossly subdivided subdivided into into aa lower lower troctolitic troctolitic zone zone (GLtr, mostly of leucotroctolitic leucotroctolitic cumulates, cumulates, aa medial gabbroic gabbroic zone (GLog, 650-1800 meters), composed of olivine oxide gabbro (GLfg, 1800-2130 1800-2130 meters), gabbro cumulates, cumulates, and and an an upper upper ferrogabbroic ferrogabbroic zone (GLfg, composed largely of magnetite magnetite gabbro gabbro (Fig. (Fig. 2). 2). The troctolitic zone contains contains abundant, abundant, large anorthositic and oxide oxide gabbroic gabbroic inclusions, inclusions, presumably presumably derived derived from from anorthositic anorthositicseries seriescountry country rock. Although Althoughthe theGLI GLIisisaawell-differentiated well-differentiatedintrusion intrusionthat that formed formedas asan anopen openmagma magma system, system, microprobe data show that cryptic cryptic layering layering trends trends (such (such as as Fo Fo in in olivine, olivine, Fig. Fig. 2) 2) are areinconsistent inconsistent with formation by in situ crystallization differentiation. This This and and other other evidence evidence (such as abrupt changes in lithology, lithology, leucocratic leucocratic compositions compositions of of troctolitic troctolitic rocks, rocks, and and suspect suspect cumulus cumulus textures textures of troctolitic rocks) suggest of the GLI was suggest that the the differentiated differentiated character of was probably inherited from a deeper crustal crustal magma magma chamber, chamber, which which was was itself itself undergoing undergoing open opensystem systemdifferentiation. differentiation. The chemostratigraphy of chalcophile chalcophile elements elements through through the GLI GLI are are difficult difficult to to interpret interpretin in such a complex open magma system, system, but suggest that some potential for PGE reef mineralization may occur in the lower part of the gabbroic gabbroic zone (Fig. 2). Below Belowthis thislevel, level,recharging rechargingmagmas magmas appear to have been undersaturated undersaturated in in sulfide, sulfide, and and copper copper and and sulfur sulfur concentrations concentrationshigher higherin inthe the gabbroic zone (above 800 meters) indicate intermittent intermittent saturation. saturation. An unexpected result of this study was the discovery discovery of of aa large, large,sulfide-bearing sulfide-bearingoxide oxide gabbro gabbro inclusion inclusion within within the thetroctolitic troctolitic zone. Aeromagnetic Aeromagneticdata datasuggest suggestthat thatthis thisinclusion inclusionisisaaconformable conformabletabular tabularmass masswith withaastrike strike length of about 8 kilometers. The The magnetic magnetic data data further further suggest suggest that similar similar rock types form part of the footwall to the GLI. The Thepossibility possibilityof of sulfur sulfurcontamination contamination in the contact aureole around this inclusion and along the base of the intrusion of these areas for intrusion warrants further exploration of contact-type contact-type Cu-Ni-PGE Cu-Ni-PGE sulfide sulfide mineralization. mineralization. Funding for this project was provided to the Minnesota Geological Survey by a grant from recommendation of the Minerals Coordinating the Minnesota State Legislature on the recommendation Coordinating Committee. Committee. 57 �Figure 1. 1. Generalized geology of the Greenwood Lake intrusion and the central Duluth Small dots dots denote Complex. Small denote drill drill hole hole and and diamonds diamonds denote outcrop locations locations along ErieILTV ErieILTV railroad tracks. tracks. Long Long dashed dashed lines lines denote denote faults. faults. Intrusive Intrusiveunits units are: are: GLtr—GLI GLtr-GLI troctolitic zone Glog—GLI Glog-GLI gabbroic gabbroic zone zone GLfg—GLI GLfg-GLI ferrogabbroic ferrogabbroic zone MW—Mt. Weber granophyre MW-Mt. granophyre CLLS—Cloquet CLLS-Cloquet Lake Lake layered layered series series BEI—BaId Eagle intrusion BEI-Bald intrusion SKI—South Kawishiwi intrusion SKI-South PRI—Partridge River PRI-Partridge River intrusion intrusion WMI—Western WMI-Western Margin Margin intrusion intrusion Layered Layered Series Series Ferrogabbroic Ferrogabbroic Gabbroic Gabbroic Troclolitlc Troclolillc 3 L: ] Felsic Series k Anorthositic Series North Shore Volcanic Group Group Virginia Formation Biwabik IronFormation Giants Range Granite • • meters 20 Kilometers Kilometers 10 10 0 0 Who rock Whole rockgeochemistry geochemistry OHvine 2000 / 1600 / ,Pt+Pd * re .1 1200 * P. 0 800 400 C -,* I. 0 r— = = • 807060504030 Fo : 0 400 800 1200 Cu (ppm) 0 = •. —— 204060 Pti-Pd PttPd && Au Au (ppb) (ppb) Cu/Pd (xlO Figure Figure 2. Chemostratigraphy Chemostratigraphyof of Fo Fo in in olivine olivine and and of Cu, Pt ++ Pd, and Au concentrations concentrations through through the the Greenwood Lake Stratigraphic locations locationsofof drill drill core (boxes) Lake intrusion. intrusion. Stratigraphic (boxes) and and outcrop outcrop (diamonds) (diamonds) samples and general lithostratigraphy are are shown shown in in the the left columns. columns. Large Large inclusions inclusions of of anorthositic anorthositic series rocks (AS) and oxide gabbro (ox gb) are denoted. Abrupt (arrows) may mark Abrupt increases in Cu/Pd Cu/Pd (arrows) mark increases in sulfide saturation events. The The zone zone found foundmost most favorable favorable to host PGE reef mineralization mineralization is identified. identified. 58 �Stratigraphy and northwestern Stratigraphy andstructure structureof ofKeweenawan Keweenawan rocks rocks of the St. Croix horst, northwestern Wisconsin Wisconsin S.W. Nicholson, and W.F.Cannon, U.S.Geological U.S.Geologica1Survey, Survey, Reston, Reston, VA VA graben of of the Midcontinent Rift System The St. Croix horst is the partially inverted central graben (MRS) that extends southwestward southwestward from western Lake Lake Superior. Superior. ItIt is is bounded bounded by by the the Douglas fault on the northwest and the Atkins Lake fault on the southeast. Both are now reverse faults, but may have been graben-bounding normal faults during rifting and volcanism. The northern northern limit of the horst is White's Ridge, a subsurface basement high volcanism. evident in both seismic and gravity data, which did not subside substantially during rifting and against which rift volcanic and sedimentary rocks pinch out out or or become become much much thinner. thinner. White's Ridge Ridge effectively effectively separates separates the the MRS MRS in in western western Lake Lake Superior Superior from from the the St. St. Croix Croix horst and the volcanic, volcanic, sedimentary, sedimentary, and and structural structural history history of of the the two two rift rift segments segmentsdiffer differin in High-resolution aeromagnetic, gravity, gravity, and and seismic seismic data data permit permit the the tracing tracing of of several aspects. High-resolution flow sequences for long distances and to great depth. This geometry combined with the chemistry of the volcanic rocks allows us to decipher a volcanic volcanic stratigraphy in spite of widespread cover by glacial deposits deposits and and Paleozoic Paleozoic sedimentary sedimentary rocks rocks (Cannon (Cannon etetal., al.,2001). 2001). Our interpretation, interpretation, aided aided by previous gravity gravity and and seismic seismic interpretations interpretations (Chandler (Chandleretetal., al., 1989), is that the original structure of the St. Croix horst was an an asymmetric asymmetric graben, graben, or or possibly a half graben, like like those those of of the the Lake Lake Superior Superior portion portion of of the the MRS. MRS. The TheLake LakeOwen Owen fault was a major growth fault on on the the southeast southeast side side of of the the graben graben and and the the volcanic volcanic fill fill thickened toward and terminated against the fault. The Douglas fault on the northwest side of of the horst is not clearly a growth feature and may be simply a thrust formed during rift rift km or more because it inversion. Thrust displacement on the Douglas fault must be 20 krn juxtaposes of the base of a thick volcanic sequence over the younger Bayfield Group. Cannon et al. (2001) and Nicholson et al. (2001) used chemical and aeromagnetic data to underlying Clam Falls Volcanics, and the Chengwatana define the Minong Volcanics, the underlying Volcanics as three formations making up the graben-filling graben-filling volcanic volcanic sequence. sequence. The The three three three-part have similar chemistry, but were defined by structure and geochronology. The three-part division no longer seems seems justified and and the the Chengwatana Chengwatana and and Clam Clam Falls Falls Volcanics Volcanicsare are combined into a single single unit. The The Chengwatana Chengwatana Volcanics, Volcanics, as as earlier earlier defined, defined, were were restricted restricted to a fault-bounded belt between the Douglas and and Pine Pine faults faults and and their their stratigraphic stratigraphic relationships to other other volcanics were were not known known directly. directly. We We now now believe, believe, based based on on seismic seismic data, that the Pine fault does not extend into the northern northern part of the horst, where the depositional previously defined Chengwatana and Clam Falls units appear to be a continuous depositional sequence of compositionally indistinguishable indistinguishable flows flows that that we we propose propose be be called called entirely entirely Chengwatana. The Minong Volcanics, aa sequence krn thick, thick, low-Ti02basalts basalts about about 33 km sequence of of low-Ti02 overlie the Chengwatana, along along an an apparent apparent low low angle angle disconformity disconfomity based based on on aeromagnetic aeromagnetic disconformity within within the Minong volcanics on the form lines. These form lines also show a disconformity southeast limb of the Ashland syncline. syncline. A A lower lower unit, unit, not not present present on on the the northwest northwestlimb, limb,isis mostly high-Ti02 high-Ti02 basalt. Based on the presence of abundant high-Ti02 basalts and more evolved rocks, we infer infer that aa localized magmatic center center was was active active in in this this area area sometime sometime upper part part of of this sequence. A second, but but before 1095 Ma, the age of a rhyolite flow in the upper apparently older, volcanic center may have existed existed on the the western western margin margin of of the the graben graben near near 59 �______ Volcanics are mostly mostly high-TiOz high-Ti02 basalts, the Amnicon Complex where the Chengwatana Volcanics andesites and rhyolites. rhyolites. Clastic sedimentary rocks of the Oronto Oronto Group overlie overlie the volcanic volcanic rocks. rocks. Only Only the the basal basal unit, the Copper Harbor Conglomerate, Conglomerate, is is preserved in most of of the the St. St. Croix Croix horst horst where where as as krn of sandstone and conglomerate conglomerate lie along along the the axis axis of of the the Ashland Ashland syncline. syncline. The The much as 2 km Copper Harbor thins to only a few tens of meters toward the northern northern end end of of the the horst horst in in the the same areas where the volcanic section also shows substantial substantial thinning. thinning. Apparently Apparently the the area area now comprising the northern part of the St. St. Croix Croix horst did did not not subside subside nearly nearly as as deeply deeply as as parts farther to the southwest. This relatively positive relief persisted throughout throughout volcanic volcanic activity and deposition of the Copper Harbor Conglomerate. Conglomerate. The The overlying overlying Nonesuch Nonesuch Shale Shale Ashland syncline, syncline, maintains a relatively uniform thickness around the northern part of of the the Ashland suggesting that the topographic high was buried by that time. time. 9 I OOO' 92W 47OO 47-00, EXPLANATION EXPLANATION Bayfield Bayfield Group Group and and equivalent sandstones i:: Freda Sandstone Nonesuch Shale Copper Copper Harbor Harbor Conglomerate Conglomerate Gabbro granophyre Gabbro and and granophyre \\\ Minong Volcanicslow-Ti basalts Minong Minong VolcanicsVolcanics- Y11 high.Ti basalts Chengwatana Volcanics Chengwatana Volcanics Kallander Creek Kallander Creek Volcanics Volcanics 1 Siemens Creek Siemens Creek Volcanics Volcanics Archean and Paleoproterozoic Paleoproterozoic rocks 46OO 46'00' 92°OO 0 30 60 90 KM Cannon, W.F., Daniels, D.L., Nicholson, S.W., SW., Phillips, Phillips, J., J., Woodruff, Woodruff, L.G., L.G., Chandler, Chandler, V.W., V.W., Morey, Morey, G.B., G.B., Boerboom, T., Wirth, KR., K.R.,and andMudrey, Mudrey,M.G., M.G., Jr., Jr., 2001, 2001, New New map map reveals reveals origin origin and and geology geology of North American American Midcontinent Midcontinent rift: EOS, EOS, v. v. 82, 82, no. no. 8, 8, pp. pp. 97-101 97-101 Chandler, Chandler, V.W., V.W., McSwiggen, McSwiggen, P.L., P.L., Morey, Morey, G.B., G.B., Hinze, Hinze, W.J., W.J., and and Anderson, Anderson,R.R., R.R.,1989, 1989,Interpretation Interpretationof of seismic seismic reflection, reflection, gravity, gravity, and and magnetic magnetic data data across across Middle Middle Proterozoic Proterozoic Mid-continent Mid-continentRift Rift system, system, northwestern northwestern Wisconsin, Wisconsin, eastern eastern Minnesota, Minnesota, and and central central Iowa: Iowa: American American Association Associationof of Petroleum PetroleumGeologists Geologists Bulletin, Bulletin, v. 73, 73, p. p. 261-275. 261-275. Nicholson, S.W., Boerboom, T., Cannon, Cannon, W.F., W.F., Wirth, Wirth, K. K. and and Isachsen, Isachsen, C.E., C.E., 2001, 2001, A A new new look look at at the the 1.1 1.1Ga Ga Boerboom, T., Chengwatana Chengwatana Volcanics Volcanics in the St. St. Croix Croix horst, horst, Minnesota Minnesota and Wisconsin, Wisconsin, Institute Institute on on Lake Lake Superior Superior Geology, Geology, v. 47, part 1, 1, p. 71-72. 71-72. 60 �of the Western Subcomplex of of the the Deadhorse Deadhorse Creek The Rare and Exotic Mineralogy of Diatreme, Northwestern Ontario. Ontario. Eric G. Potter and Roger Roger H. Mitchell egpotter@mail.lakeheadu.ca egpotter@mail.lakeheadu.ca Dept. of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON. P7B 5E1 5E1 The main mineralized zone of the western western subcomplex of the Deadhorse Creek diatreme exhibits complex involving: first first and second exhibits complex mineralization mineralization involving: second order order transition transition metals metals (specifically Sc, Ti, Ti, V, V, Cr, Mn, Mn, Fe, Fe, Zr Zr and and Nb); Nb); REE; REE; Be; Be; Th; Th; and and U. U. The The mineralization mineralization (specifically Sc, is manifested by the presence of of the the following following minerals: minerals: thortveitiite, thortveitiite, Sc-V-aegirine, Sc-V-aegirine,NbNbV-rutile, V-crichtonite, Ba-Mn-hollandite, zircon, monazite-Ce, monazite-Ce, xenotime-Y, uraninite, tyuyamunite, phenacite, thorite, thorogummite, thorite, thorogummite, barite, barite, barylite, tyuyamunite, phenacite, pyrite, pyrite, hematite, magnetite and several as of yet unnamed mineral species (Platt (Platt and and Mitchell, Mitchell, 1996; 1996;Smyk Smyk et a!., al., 1993; 1993; this study). Of Ofinterest interestininthis thispresentation presentation are: are:Nb-V-rutile, Nb-V-rutile, cnchtonite crichtoniteand and Sc-V-aegirine. Sc-V-aegirine. I I The Nb-V-rutile The Nb-V-rutile is enriched in Cr203, Cr203,with concentrations concentrations reaching Alkaline Igneous Suites 30 30 ,--. The 6.49 wt.%. wt.%. The enrichment enrichment of of &: Cr203 and Cr2O3 and Nb2O5 Nb2O5isis similar similar to that 80 A Xenoliths in Kimberlite of rutile reported in alkaline igneous reported alkaline igneous 20 2o rocks, as illustrated in an atomic rocks, illustrated an atomic t-1+ - percent percentplot plotofofcr3+ Cr3 ++ Nb5* Nb5 + ~Ta5a ~ ^ "0 1991). (Haggerty, vs. Ti4i ~ (Haggerty, vs. ~ * 1991). z+ lo However, the Nb205 However, the Nb205 contents contents are are 3 unusually high compared to alkaline compared Lunar Meteorites igneous igneous rocks in general, general, with 0 0 29.32 90 95 50 55 60 65 75 80 85 50 55 60 65 70 75 80 85 90 95 100 100 concentrations concentrations reaching 29.32 , <.a+ Ti 11 (Atomic (Atomic °') o,/o) wt.%. Such such Nb2O5 m205contents contents are are wt.%. which is is historically historicallyfound foundin in pegmatites. pegmatites. Also similar to those reported in ilmenorutile, which unique to the Deadhorse Creek Creek rutile rutile is the distinct of V203 (up to 10.52 unique distinct enrichment enrichment of V203 (up 10.52 wt.%) and the lack of tantalum. tantalum. I I I I I I I !! Deadhorse Creek contain the The Sc-V-aegirines present at Deadhorse The Sc-V-aegirines the highest reported of Sc203 Sc203 and V203 V203 (16.46 (16.46 and and 11.99 wt.%, wt.%, respectively). respectively). The concentrations of The only only other other have been occurrences of occurrences of VV- and andSc-enriched Sc-enriched aegirine aegirine have been reported reported from from alkaline alkaline a!., 1994; metasomatites associated with iron-ore deposits in Ukraine (Valter et al., 1994; Pavlishin presence thortveitiite (Sc2Si2O7) et al., al., 2000). 2000). OfOfnote noteis isthethe presenceofofboth both thortveitiite (Sc2Si207)and andSc-enriched Sc-enriched within the main main mineralized mineralized zone. zone. Although aegirine within Although the the source source of of the the Sc Sc in in the the aegirine aegirine remains somewhat somewhat conjectural, conjectural,itit appears appearsthat thatthe the Sc, Sc, V V and Na was remains was scavenged scavenged from alteration of the main mineralized mineralized zone by Fe-rich fluids. fluids. analogues of of crichtonite-(Sr) and The V-rich crichtonites are best termed vanadium-rich analogues senaite-(Pb). The Nb2O5in the crichtonites is peculiar, as the presence presence of Theenrichment enrichmentin inNb2O5 Nb has been a distinguishing distinguishing feature feature of of the the mantle-derived mantle-derived end end members members lindsleyite-(Ba) lindsleyite-(Ba) 61 �_____________________ ___________________ __________________ and mathiasite-(K) mathiasite-(K) (LIMA). (LIMA). Interestingly, Interestingly, the the crichtonites crichtonites plot plot in the theupper-mantle upper-mantle LIMA quadrant quadrant of of FeO ++ Fe203 LIMA Fe203++ MgO MgO vs. vs. TiO2 Ti02(Haggerty, (Haggerty, 1991), 1991), near the the LIMA LIMA compositions due to the the replacement replacement of of iron iron by vanadium. vanadium. The Nb-enriched rutiles mtiles and crichtonite crichtonite are believed to to have have Non-Kimberlitic Crichtpnite in formed aa formed relatively early in 30 C,,chtornte (Sr). Davthtc (UREB) Armalcolite Ouadrant multistage-alteration sequence of the the Deadhorse Creek diatreme by Senaite (Pb), 25 of stoichiometric rutile with reaction mtile Lovetingite (Ca). hydrous alkaline solutions hydrous solutions enriched enriched 20 1 These hydrous Nb and V. in Nb and V. hydrous Upper-Mantle LIMA Crichtonite alkaline solutions likely also also altered altered Armalcolite Armalcolite Ouand.rant Ouandrant 15 unnamed hydrated zircon to to an unnamed hydrated -DFIC Crichtomtc calcium zirconosilicate, zirconosilicate, which is is JIMA cric(ttonites__Pptle 10 found in association with the 56 52 56 60 64 68 72 60 64 68 crichtonite and rutile. rutile. Textural and and /o) TiO, (Wt.%) 1102 (Wt. compositional data suggest suggest that imparted the pervasive subsequent alteration alteration formed the Sc-V-aegirines Sc-V-aegirines and imparted pervasive hematitization hematitization to the the main main mineralized mineralized zone. zone. 35 I I I — I I References References Haggerty, S.E. (1991): of the upper (1991): Oxide Oxide mineralogy mineralogy of upper mantle. mantle. In: In: Oxide Oxide Minerals: Minerals: petrologic and 25, Mineral. Mineral.Soc. Soc. and magnetic magnetic significance. significance. Reviews Reviewsin inMineralogy, Mineralogy, 25, Amer., 335-416. 335-416. Platt, R.G. and and Mitchell, Mitchell, R.H. R.H. (1996): (1996): Transition Transition metal metal rutiles mtiles and titanates titanates from from the the Miner. Miner. Deadhorse Creek Diatreme complex, northwestern Ontario, Canada. Deadhorse Diatreme complex, northwestern Ontario, Canada. 403-413. Mag., 60, 60,403-413. Pavlishin, V.I., V.1., Baklan, Baklan, F.G., F.G., Bugaenko, V.M., Voznyak, D.K., Galaburda, Pavlishin, Bugaenko, V.M., Voznyak, D.K., Galaburda, Yu, A., A., Dekhtulins'ky, E.S., Donskey, O.M., Krivdik, S.G., Kulchic'ka, G.O., Mel'nikov, V.S., Radzivill, A, Ya. Ya. And And Zimbal, Zimbal, S.M. S.M. (2000): (2000): Science-based Science-basedperspectives perspectives of of improvement of of mineral mineral resources resourcesor orrare raremetals metalsinin Ukraine. Ukraine. Mineral., Journal, improvement 22, no.1, 1, 5-20. 5-20. (in Russian) Russian) 22, no. Smyk, M.C., Taylor, R.P., Jones, Smyk, M.C., Taylor, R.P., Jones, P.C. and and Kingston, Kingston, D.M. D.M. (1993): (1993): Geology Geology and and geochemistry of the West Dead Dead Horse Horse Creek Creek rare-metal rare-metal occurrence, occurrence, northwestern northwestern Ontario. Explor. Mining. Geol., Ontario. Geol., 2, no. 3, 245-251. 245-251. Sharkin, O.P. and and Yakolev, Yakolev, V.M. V.M. (1994): (1994): AAvanadian vanadian Valter, A.A., Khomenko, V.M., Sharkin, aegirine in alkaline alkaline metasomatites from Zheltye Vody. Dokiady DokladyAkademii Akademii Nauk Ukrainy, No. 3, 110-116. 110-116. (in Russian) 62 �methods: Possible Sibley Basin sediment provenance using zircon and and whole whole rock rock geochemical methods: source areas of the Pass Lake Formation Richardson, A., A., Fralick, Fralick,P., P.,and andHollings, Hollings,P.P.(Department (DepartmentofofGeology, Geology,Lakehead LakeheadUniversity, University, 955 Richardson, SE!, Canada; Oliver Rd., Thunder Bay, Ontario, P7B 5E1, Canada; ajrichar@mail.lakeheadu.ca) airichar@mail.lakeheadu.ca) [CANADA A. +4+ - + + 48 $upfls. + +4+1 + + + +1 USA L. Huron 4__u_,r ' —+ 4 4 +++ +4+ 4 30km 4+ ++4 ÷4+ LSGEND II. Proterozoic 4+4. 1097 Ma V Odor Group 1110 Ma *4 1.. 1 / 1537 EJ GranfteMa and Rhyolit. l800Ma Arilmikie Group AnImikle Group Archean Granific anmitic Rocks R W ~ ^] Metasedimentary Metasedlrnentary a MÇ.VOIC;~, .4* 4+ ++ + epa.s ÷+ >1339Ma Sibley Group 9 + Rocks Rocks 1 .1 Lake Superior The Sibley Group consists of Proterozoic Proterozoic sediments sediments that outcrop discontinuously over a 15000 15000 sq. km region in the area surrounding central and southern Lake Nipigon. Its age is bracketed bracketed by the underlying underlying Redstone Redstone Point Point Complex Complex (1537 (1537 +101-2 Ma; Davis and Sutcliffe, +/- 33 33 Ma Ma +lo/-2 Ma; Sutcliffe, 1985) 1985)and andaa1339 1339+1Rb-Sr age on diagenetically diagenetically altered altered Sibley Sibley sediments sediments(Franklin, (Franklin, 1978). The Sibley Group was divided into three formations: the Kama Hill Formation Formation (top), (top), the the Rossport Rossport Formation, Formation, and and the the Pass Lake Formation (bottom), by Franklin, et al. (1980). The The Kama Hill Formation Formation consists consists of aa laminated laminatedshale shale facies, facies,the the Rossport of mudstone mudstone and stromatolitic stromatolitic facies, facies, and and the the Pass Lake of a conglomeratic conglomeratic facies facies and and aa plane-bedded plane-beddedor or crosscrossbedded sandstone sandstone facies (Cheadle, (Cheadle, 1986). 1986).This This study study investigates investigates the sources sources that fed sediment sediment to to the the Pass Pass Lake Lake Formation in the southern portion of the basin. Formation southern portion basin. Regional granitic granitic sources sources may include: include: the the Mesoproterozoic Redstone Point anorogenic intrusion, Redstone Point anorogenic intrusion, mpjjQfions Neoarchean peraluminous Quetico Quetico granites, granites, and and Redstone Point Granite McKenzie granites. Of Of these, these,the theRedstone RedstonePoint Point isis more more highly evolved than the others others and contains contains abundant abundant Regional Granltes Granites R Regional zircon and a distinct distinct geochemical geochemical signature signaturewith with very very • Pass PassLake LakeFm, elevated values for the high field Fm. field strength strength elements elements (HFSE). (HFSE). Samples Samples were collected collected from from surface surface exposures exposuresat at several several locations l o c a t i o n(Fig. s ( ~ i1). ~ . Representative Representativesamples samplesof of the the Pass Pass Lake Lake Formation of the Sibley Group Group were were taken taken from from aa cliff cliff section section directly across from Pass Lake on Hwy. 587. Individual Individual beds beds were grouped into assemblages consisting of of up up to to 16 beds. beds. Bed thickness total of of thickness became finer and thinner up section. AA total 26 hand samples were obtained from the Pass Lake cliff and consisted of of fine fine to to medium medium grained grained sandstone. sandstone. Two additional additional Pass Lake Formation Formation samples samples were were obtained obtained from from road road cuts cuts further further up-section up-sectionthat that consisted consistedof of medium medium grained grained sandstone. sandstone. Figure Fiigure 1. 1. Regional Regionalgeologic geologicmap mapwith withsample sample locations. locations. Additional granitic granitic samples were obtained from from road cuts cuts along along Hwy Hwy 11/17 11/17 Hwy. 527 527 (Fig. (Fig. 1). One sample was was taken taken from from each each location. location. Samples and Hwy. of Redstone Point sandstones, and granite samples were previously obtained obtained by P. Fralick Fralick from the English Bay region of Lake Nipigon (Fig. (Fig. 1). 1). Figure Fiigure 2. 2. Backscatter Backscatter X-Ray X-Ray SEM-EDS image of a zircon from sample AR-Ol. AR-01. ICP-AES (inductively Coupled Plasma -- Atomic Emission Spectroscopy) ICP-AES (Inductively Spectroscopy) Samples were cut into approximately 4 x 3 x 0.5 cm sections sections and crushed to a fine powder of <30 microns. Chemical Chemical preparation preparationincluded included hydrofluoric acid digestion to remove all silica and allow complete complete solution solution of samples. Prepared Prepared samples samples were were analysed analysed at at the the Lakehead University Instrument Laboratory. 63 �SEM-EDS (Scanning (Scanning Electron Microscope -- Energy Dispersive X-Ray Microanalysis) SEM-EDS X-Ray Microanalysis) Samples were ground to 30 micron thin sections and cut into discs suitable for the SEM stage. Before Before analysis, analysis, samples samples were carbon coated to prevent charge build- up while being analysed. Samples analysed. Samples were were analysed analysed for for 50 50 seconds seconds with with an an accelerating voltage of 20 KeV, and a beam current of 0.475 pA using a JEOL 5900 5900 SEM SEM with a system system resolution resolution of of 139 139 University Instrument Instrument Laboratory. Laboratory. Images eV, at Lakehead University taken using using aa backscatter-electron backscatter-electron detector. detector. Zircons were taken Zircons were analysed for five elements: Zr, Y, Th, U, and Hf. The use of zircons in sediment provenance studies has been 1o*Y+Th+U limited to work done by Owen (1987) (1987) which involved employing hafnium content of detrital zircons in determining Figure 3. SEM-EDS SEM-EDS analyses analyses of zircons zircons from from the source of the upper Jackfork Sandstone and the Parkwood Fm sandstones (points), (points) Redstone Pass Lake Fm Formation. He Hecame came to to the the conclusion conclusion that that hafnium hafnium content content Point sandstones (squares), Redstone Point these zircons agreed with optical and cathodoluminescence of optical and cathodoluminescence (ranites (+), Granites (+),and andregional regional Archean Archean and modal analyses, and is a viable method for provenance Neoarchean granites (triangles), (triangles). determination. determination. This study is the first to use SEM-EDS SEM-EDS methods methods as as well well as as analyses for Y, Th, and U. U. Fig. results. The Fig. 33 shows shows zircon analysis results. The majority majority of of zircons zircons plot at Zr/Hf ZdHf ratio of significant population population show show aa Y, Y, Th, Th, U approximately 40 with relatively low amounts of Y, Th, and U, but a significant enrichment trend. The enrichment trend. The geochemical geochemical signature signature of zircons from both sandstones sandstones and granites show show 3000 of sediment similarity, and indicates local sourcing sourcing sediment A with a possible influence influence of of regional regional Archean Archean and and . Proterozoic felsic igneous intrusives. 2000 2000 0 Whole rock interpretation interpretation of of ICP-AES ICP-AES ppm Zr/%Ti02 geochemistry (Fig. 4) trends agree with SEM 11000 ow elemental distribution within samples. Immobile Immobile element ratios of the the Pass Pass Lake Lake sandstones sandstones tend tend to to enriched and nonfall on a mixing trend between J 0 0 25 50 125 150 175 sources. This O 25 50 ppm 75Nb/%Ti02 loo Iz5 150 175 enriched sourc&. This study study highlights highlights the possible ppm Nb/%T02 usefulness of using SEM-EDS generated data in concert with more traditional chemical chemical analyses analyses in Figure 4. Immobile Immobileelement elementplot plot of of ICP-AES ICP-AESanalyses. analyses, provenance studies. Pass Lake Lake sandstones sandstones (points) (points)plot plot in in similar similarfield field to to sandstones derived from, and overlying overlying Redstone Redstone Point granite (squares). Redstone Redstone point granite (+), and other (triangles) are also shown. granites (triangles) shown. . ' I A I References References Cheadle, Cheadle, B.A. (1986) (1986)Alluvial-playa Alluvial-playa sedimentation sedimentationin in the the lower lower Keweenawan Keweenawan Sibley Sibley Group, Group,Thunder ThunderBay BayDistrict, District, Ontario; Ontario; Canadian Canadian Journal Journal of of Earth Earth Sciences, Sciences,v. v. 23, 23, p. p. 527-542. 527-542. Davis, D., and Sutcliffe, of Sutcliffe, R., (1985) (1985) U-Pb ages from the Nipigon Plate and Northern Lake Superior. Geological Society of America Bulletin, Bulletin, 96, 96, 1572-1579. 1572-1579. Franklin, age studies, Report Report 2, geological Franklin, J.M., (1978) (1978) The Sibley Sibley Group, Group, Ontario; Ontario; in Rubidium-strontium isochron age Survey Survey of Canada, Canada,Paper Paper 77-14, 77-14,p.p.331-34. 1-34. Franklin, J.M., J.M., McIlwaine, Mcllwaine, W.H., Franklin, W.H., Poulsen, Poulsen, K.H. and Wanless, R.K. (1980) (1980) Stratigraphy and depositional setting of the Sibley Sibley Group, Thunder bay District Ontario, Ontario, Canada; Canadian Journal of Earth Sciences, v. 17, p.633-651. Owen, M. (1987) 57, No.5., 1-838. (1987)Hafnium Hafnium in in Detrital Detrital Zircons: Zircons: Journal Journal of of Sedimentary SedimentaryPetrology, Petrology,Vol Vol57, NOS.,1987., 1987.,p.p.83 831-838. 64 �A Magnetostratigraphic Magnetostratigraphic and andSecular SecularVariation VariationStudy Studyof of the theSibley Sibley Group Group Rogala, P. and Borradaile, G. (Department of Geology, Lakehead University, Thunder Rogala,B., B.,Fralick, Fralick, P. and Borradaile, G. (Department of Geology, Lakehead University, ThunderBay, Bay, brogala@lakeheadu.ca) Ontario, P7B 5E1, brogala@lakeheadu.ca) The Sibley Group is aa red red bed bed sequence sequence that that was was deposited deposited in in aasubsiding subsidingintracratonic intracratonic basin (Fralick and Kissin, 1995) 1995)overlying, overlying, in part, part, aa 1537+10-2 1537+10-2Ma Ma (Davis (Davisand andSutcliffe, Sutcliffe,1984) 1984) The Group anorogenic anorogenic granite-rhyolite granite-rhyolite complex. complex. The Group was was previously previously divided divided into into three three main main Formations: Pass Lake, Rossport, and Kama Kama Hill. An An unnamed unnamed Formation Formation and and the the Nipigon Nipigon Bay Bay Formation have recently recently been been added. added. The The Pass PassLake LakeFormation Formation consists consists of of the theconglomeratic conglomeratic Formation have Loon of the Fork Loon Lake Member Member and the the sheet-like sheet-like sandstones sandstones of Fork Bay Bay Member, Member, representing representing a The Rossport braided environment (Cheadle, (Cheadle, 1986). 1986). The Rossport Formation Formation is separated separated into the the braided fluvial environment Channel Island, Middlebrun Bay, and Fire Hill Members. The Channel Island Member is Channel Island, Middlebrun Bay, and Fire Hill Members. The Channel Island Member aa The cyclic unit interpreted interpreted to to be playa cyclic dolomite-shale dolomite-shale unit playa lake lakesediments sediments(Cheadle, (Cheadle, 1986). 1986). The Middlebrun Bay Member, considered a marker bed for the Sibley Sibley Group, Group, isis aastromatolitic stromatoliticunit unit that represents a migrating strandline. The The Fire Fire Hill Hill Member Member consists consists of mudcracked mudcracked red silt with mudchip conglomerates and sand of tectonic tectonic tilting tilting of of the the sand sheet sheet incursions. incursions. It signifies a time of The Kama basin. basin. The Kama Hill Formation Formation is not not subdivided, subdivided, and is is composed composed of of purple purple shales shales and and siltstones interpreted as mud flat deposits (Cheadle, (Cheadle, 1986). The Theunnamed unnamed Formation Formation isis divided divided These represent represent aa deltaic deltaic and and fluvial fluvial environment. environment. The TheNipigon Nipigon into two unnamed Members. Members. These Bay Formation consists of cross-stratified sandstone beds, beds, and and is thought to denote denote an an aeolian aeolian environment. environment. Samples were taken taken from Samples were from the the Pass Pass Lake, Lake, Rossport, Rossport, Kama Kama Hill, Hill, and and Nipigon Nipigon Bay Bay Formations for a paleomagnetic paleomagnetic study. study. The unnamed Formation was not sampled due to the lack of exposure. The ThePass PassLake, Lake,Kama KamaHill, Hill,and andNipigon Nipigon Bay Bay Formation Formation were were used used to to conduct conduct aa preliminary study of of the magnetostratigraphy of the the Sibley Group. The TheRossport RossportFormation Formation was was magnetostratigraphy of sampled from unoriented drill drill core, core, thus thus could could only only be be used used to to study study secular secularvariation. variation. The paleopoles calculated from the Pass Lake, Lake, Kama Kama Hill, Hill, and and Nipigon Nipigon Bay Bay Formations Formations have been plotted along an apparent polar wander path (APWP) defined by Elston et al. (2002) (2002) Elston et al. an apparent polar wander (APWP) defined (Figure 1). 1). The samples samples from from the the Pass Pass Lake Lake Formation Formation have have been been divided dividedinto intosample samplegroups groups corresponding to Quarry Island, Transitional to the Rossport Formation, and and an an outcrop outcrop at at Pass Pass The paleopole of the with a diagenetic Lake. Lake. The paleopole of the Quarry Quarry Island Island Group Group corresponds corresponds with diagenetic event event at 1978), and and the latter 1339±3 Ma (Franklin, (Franklin, 1978), latter two two groups groups have havepaleopoles paleopoles approximately 1339±33 approximately The Kama has an older associated with with an an early earlyKeweenawan Keweenawan overprint. overprint. The Kama Hill Formation Formation has older associated discordant paleopole paleopole and a younger paleopole paleopole that that is is located within within the 1500 Ma section of the that the the Sibley Basin formed formed prior prior to to this, this, as is apparent apparent polar wander wander path. path. This This suggests suggests that Sibley Basin supported by the recent discovery discovery of sedimentary sedimentary xenoliths within the 1537 1537 Ma Ma Redstone Redstone Point Point Formation has has paleopoles paleopoles that that lie lie on on the APWP near 1400 granite. granite. The Nipigon Bay Formation 1400 Ma and and 1100 Ma. The Thefirst firstpaleopole paleopolemay maybe beprimary primary or orrelated related to to the thediagenetic diageneticevent eventthat thataffected affectedthe the Pass Lake samples at 1339 OsierVolcanics. Volcanics. 1339 Ma. The Thelatter latterpaleopole paleopole correlates correlateswith with the the Osler The paleomagnetic study on a 90 90 cm cm core core section section from from the the Rossport Rossport Formation Formationrevealed revealed aa When this curve variation curve. curve. When curve was was compared compared to to typical typical secular secular variation variation curves curves secular variation (Butler, 1998; Tauxe, Tauxe, 1998), the the time-span time-span for for Sibley Sibley deposition depositioncan canbe be estimated. estimated. The 90 (Butler, 90 cm cm section was estimated to represent represent 2500 2500 to to 3000 years. years. This This can can be be extrapolated extrapolated to estimate estimate that the Rossport Formation could potentially represent 75 000 years of deposition. Rossport Formation could potentially represent 75 000 years of deposition. 65 �Figure Figure4.12 4.12 AAwell-defined well-definedProterozoic ProterozoicApparent ApparentPolar PolarWander WanderPaths Paths(APWP) (APWP)isisplotted plotted (after Elston et al., al., 2002). 2002). The ThePass PassLake LakePCA PCAcomponents componentsare aredesignate designatewith with QI, QI,T, T, and indicatethe theQuarry QuarryIsland, Island,Transitional, Transitional, and and Outcrop Groups. The TheKama Kama Hill Hill and 00totoindicate Formation is designate KH and the Nipigon Bay Bay Formation Formation is is NB. NB. The The PCA, PCB, and PCC PCC components components are are denoted denoted respectively respectively by A, B or C after the Formation short short form. Note that NB-C is a reversed pole on the back side of of the the globe. globe. Elston Elston et et al. al. (2002) (2002) has has provided a lower (Si) (Sl)and andupper upper (S2) (S2) Sibley Sibley Group Group pole pole based on data from Robertson (1973), as well as a pole for the Keweenawan Keweenawan Osler Osier Group (Ki) (Kl)and and lower lower Powder Powder Mill Mill Volcanics (K2). (K2). magnetic domains domains to to geological Butler, R.F. R.F. 1998. 1998. Paleomagnetism: Butler, Paleomagnetism: magnetic geological terranes, terranes, Department Department of of Geosciences Geosciences University (originally published published by http://www.~eo.arizona.edu/Paleomae/book/ (originally by Blackwell Blackwell University of of Arizona, Arizona,http://www.geo.arizona.edu/Paleomag/book/ Scientific Publications Publications in in 1992) 1992) Scientific Cheadle, B.A. B.A. 1986. Alluvial-playa Cheadle, Sibley Group, Group, Thunder Thunder Bay Bay District, District, Alluvial-playasedimentation sedimentationin inthe thelower lowerKeweenawan Keweenawan Sibley CanadianJournal JournalofofEarth EarthSciences, Sciences,23, 23,527-542. Ontario. Canadian 527-542. Davis, D.W. and Sutcliffe, R.H. R.H. 1984. Geological 1984.U-Pb U-Pbages agesfrom fromthe theNipigon NipigonPlate Plate and and Northern Northern Lake Lake Superior. Superior. Geological 96, 1572-1579. 1572-1579. Society of America America Bulletin, 96, Elston, D.P., Enkin, R.J., Baker, Baker, J.J. and and Kisilevsky, Kisilevsky, D.K. D.K. (2002). (2002).Tightening Tighteningthe theBelt: Belt:paleomagnetic-stratigraphic paleomagnetic-stratigraphic constraints constraints on deposition, deposition, correlation, correlation, and and deformation deformation of of the the Middle Middle Proterozoic Proterozoic (ca. (ca.1.4 1.4Ga) Ga)Belt-Purcell Belt-Purcell GeologicalSociety Society of of America Bulletin, 114, 114,619-638. Supergroup, United States and Canada. Geological 619-638. basin development in central North America: and Kissin, Kissin, S. S. 1995. Fralick, P. and 1995. Mesoproterozoic Mesoproterozoic basin development in America: implications implications of Sibley Group volcanism volcanism and and sedimentation sedimentation at at Redstone Redstone Point. Point. In: Petrology Petrology and and metallogeny metallogeny of of volcanic volcanic Proceedings of of the the International International Geological Geological and intrusive intrusive rocks rocks of of the themid-continent mid-continent rift rift system, system, Proceedings and Correlation Program, 336. Program, Project Project 336. Franklin, J.M. J.M. 1978. Franklin, in: Wanless, Wanless,R.K. R.K.and andLoveridge, Loveridge,W.D., W.D.,Rubidium-strontium Rubidium-strontium 1978. The Sibley Sibley Group, Group, Ontario, Ontario, in: isotopic age studies, report report 2. Geological Geological Survey Survey of Canada Paper Paper 77-14, 77-14, 31-34. 3 1-34. W.A. (1973a). (1973a). Pole position position from from thermally thermally cleaned Sibley Sibley Group Group sediments sediments and and its its relevance relevance to to Robertson, W.A. Proterozoic magnetic stratigraphy. Canadian Canadian Journal Journal of of Earth Earth Sciences Sciences,10, 10,180-193. 180-193. Tauxe, L. 1998. KluwerAcademic Academic Publishers, Publishers, Netherlands, Netherlands, 299 p. 1998.Paleomagnetic Paleomagneticprinciples principlesand andpractice, practice,Kluwer 66 �Mafic Dikes Dikes in in Marquette Marquette County, County,Michigan Michigan with with Sequence of Precambrian Precambrian Mafic Sugarloaf Mountain Mountain and and Republic Republic Areas emphasis on the Sugarloaf Sandin and and T.J. T.J. Bornhorst Bornhorst (Department of Geological and Mining Engineering and N.A. Sandin Sciences, Sciences,Michigan Michigan Technological TechnologicalUniversity, University,Floughton, Houghton,MI MI49931) 49931) Precambrian Precambrian mafic mafic dikes dikes are arevery very common common throughout throughout Marquette MarquetteCounty, County,Michigan. Michigan.These These 1.1 Ga). dikes Archean(—2.7 (-2.7 Ga) Ga)to tomiddle middleProterozoic Proterozoic(— (-1.1 Ga). Past Past studies studiesby by Kantor Kantor dikes have haveages agesfrom fromArchean (1968), (1968), Gair (1969), (1969), Cannon Cannon (1974), (1974),and and Baxter Baxter and and Bornhorst Bornhorst (1988) (1988)have havesuggested suggestedup upto tosix six different of different mafic dike events in Marquette County. These events were interpreted to consist of (from Archean mafic mafic dikes dikes post-Archean post-Archean volcanism volcanism and and before before Archean Archean (from old to young): young): 1) 1) Archean granitoid intrusions intrusions which cut the Archean volcanic rocks; 2) Archean mafic dikes that cut Archean granitoid intrusions, intrusions, but but are are subjected subjectedto to Archean Archean deformation; deformation;3) 3)Archean Archeanmafic maficdikes dikes that cut Archean basement rocks, but do not cut Early Proterozoic sedimentary rocks of the that cut Archean basement rocks, but do not cut Early Proterozoic sedimentary rocks of the Marquette 4) Early Early Proterozoic Proterozoic mafic mafic dikes dikesthat thatcut cutMarquette MarquetteRange Range Marquette Range Range Supergroup; Supergroup;4) Supergroup Supergroup sedimentary sedimentaryrocks prior prior to to Penokean Penokean metamorphism metamorphism and and deformation; deformation;5-6) 5-6)N-S N-Sand and E-W E-W Keweenawan Keweenawan mafic mafic dikes. dikes. This This study study has confirmed confirmed much of Baxter and Bomhorst Bornhorst (1988), however, however, new new data data indicate indicatesignificant significantmodifications. modifications. This This study study focused focusedon on the the Sugarloaf SugarloafMountain Mountain area areanear near Marquette, Marquette,MI MIbecause becauseof ofthe the excellent excellent exposures exposures on on shore shore and and adjacent adjacent to to Lake Superior, Superior, and previous work by Kantor (1968), (1968), who identified identified mafic mafic dikes dikes of of multiple multiple ages. ages. In In the the Sugarloaf Sugarloaf Mountain Mountainarea areaover over300 300 mafic dikes Archean tonalitic tonalitic basement basement were were identified identifiedand andmapped mappedusing usingaaGPS GPS dikes intruding intruding Archean receiver receiver and and the the compass compass and and pace method. Dikes Dikesidentified identifiedas ascritically criticallyimportant importanttoto understanding the the sequence sequence of of events events were were sampled sampled for for microscopic microscopic and andchemical chemicalstudy. study. Baxter Baxter and and Bornhorst Bornhorst (1988) (1988) interpreted interpreted thin, thin, discontinuous, discontinuous, tabular tabular mafic mafic bodies at Wetmore post-plutoniclpre-deformation Wetmore Landing Landingin in the the Sugarloaf SugarloafMt. Mt.area areaas asbeing beingArchean Archeanpost-plutonic/pre-deformation mafic dikes (number (number 2 above). While this interpretation interpretation is still possible, the favored interpretation interpretation here is that these mafic bodies are xenoliths that were deformed during the Archean along with the the host host plutonic plutonic rocks. rocks. In Marquette Marquette County, County, Baxter Baxter and and Bornhorst Bornhorst (1988) (1988)as as well well as asprevious previousworkers workersrecognized recognized the numerous mafic intrusives intrusives that that cut cut Marquette Marquette Range Range Supergroup Supergroupsedimentary sedimentaryrocks rocks prior prior to to Penokean metamorphism metamorphism and and deformation. deformation.These These were were presumed presumed to to be be of of generally generallythe the same same age. This study indicates indicates that in the the Sugarloaf Sugarloaf Mt. Mt. area, area, three three age age separate separatemafic mafic intrusive intrusiveevents events of this age are present. present. Based Based on on cross-cutting cross-cuttingrelationships, relationships,the the sequence sequenceconsists consistsof of diabase diabase dikes trending N20°E, N60°E, and and diabase diabase dikes dikes trending trending east-west. In N20°Ediabase dikes trending N60°E In addition to cross-cutting cross-cuttingrelationships, relationships,these these groups groups can can be be discriminated discriminatedfrom fromeach eachother otherby by trace elements. elements. trace The of the Early Proterozoic Proterozoic dikes. dikes. In The N20°E N20° diabase dikes are the oldest of In the the Sugarloaf SugarloafMt. Mt. area, these these dikes dikes vary vary in in trend trend from fromN05°E N05OE to to N20°E N 2 0 2 and range in width width from from one one to to25 25feet. feet. Mafic dikes of this age are the most common of the Early Proterozoic dikes in the Sugarloaf Mountain area. These Thesedikes dikesexhibit exhibitaavarying varyingtexture texturefrom fromporphyritic porphyritic to to phaneritic phaneritic from fromthe the dike dike interiors interiors to to the the margins. margins. They They consist consist of of hornblende, hornblende, pyroxenes, pyroxenes, chlorite, chlorite,plagioclase, plagioclase, epidote, epidote, and sericite. sericite. The TheREE REEpatterns patternsare areenriched enrichedin inlight-REE light-REEwith withaamoderate moderateslope. slope. Compared to the REE patterns of the sills from the Marquette Range Supergroup, the Compared REE patterns of the sills from the Marquette Range Supergroup, theN20°E N20° series has a higher concentration concentrationof of light-REE, light-REE,is is less less depleted depletedin in heavy-REE, heavy-REE, and andhas hasaa shallower shallower slope. Thus, Thus,our ourinitial initialinterpretation interpretationisisthat thatthese thesedikes dikesare arenot notrelated relatedto tothe thesills. sills. 67 �The N60°E mafic dikes dikes are intermediate intermediate Early Early Proterozoic Proterozoic age. age. They vary in trend from N60° mafic N45°E N60°E and and range range in in width width from from one one to to 60 60 feet. feet. These These are are the the least least common of the Early N45OE to N60° Proterozoic dikes in the Sugarloaf Mountain area. They generally have They generally have thinly thinly foliated foliated margins margins fine-grained interiors. interiors. These dikes have a phaneritic phaneritic texture texture and and consist consist of of with a massive, fine-grained hornblende, pyroxenes, chlorite, chlorite, plagioclase, plagioclase, epidote, epidote, sericite, sericite,and and minor minoramounts amountsof of carbonate. carbonate. diabase dikes. dikes. REE N20°diabase REE patterns are enriched enriched in light-REE light-REE and have These dikes cross-cut the N20°E N20% series and the sills of the Marquette Range Supergroup, Supergroup, a steep slope. Compared Comparedto to the the N20°E the N60°E dikes are more enriched in light-REE with a steeper N60°dikes steeper slope. The TheN60°E N60°dikes dikesare aremore more depleted in heavy-REE interpretation is that these heavy-REE than the N20°E N20° series. Our initial interpretation these dikes dikes are are aa distinct distinct magmatic event event with with respect respect to to the the earlier earlier N20°E N20° dikes and the mafic sills. sills. of the the Early Early Proterozoic Proterozoic dikes. dikes. They The east-east diabase dikes are the youngest series of They vary in width from from five five to to 75 75 feet feet wide. These dikes generally have thinly foliated foliated margins with aa massive, fine-grained fine-grained interior, interior, although although two two dikes dikes had had porphyritic porphyritic interiors. interiors.They They have have aa phaneritic texture and consist of hornblende, pyroxenes, chlorite, plagioclase, epidote, and sericite. sericite. These Thesedikes dikescross-cut cross-cutthe theN20°E N20Z diabase diabasedikes dikesand andthe theN60°E N60Z diabase diabasedikes. dikes.Compared Compared to the REE patterns of the sills from the Marquette Range Supergroup, the east-west dikes have a higher concentration of light-REE and a steeper slope. The The east-west east-westdikes dikes are are depleted depletedin in the the compared to the N20°E N20% dikes. They are lower in light-REE and have a shallower shallower heavy-REE compared slope than the N60°E interpretation is that these dikes are N60° series. Our initial interpretation are a distinct distinct magmatic magmatic event from the earlier earlier dikes dikes and and the mafic mafic sills. sills. Mt. Area. We propose that these There are three distinct mafic dike events in the Sugarloaf Mt. dikes are not related to the mafic mafic sills sills that that cut cut the the Marquette MarquetteRange Range Supergroup. Supergroup.IfIf true, true,then then and likely likely more, more, Early Proterozoic mafic magmatic magmatic pulses pulses in in the the there must be at least 4, and Marquette area. Marquette area. Two groups groups of unmetamorphosed diabase dikes were identified in the Sugarloaf Mt. area, consistent with Baxter and Bornhorst (1988). These Thesedikes dikesare are Keweenawan Keweenawan in in age age and and consist consist of of diabasictexture texture a north-south trending series and an east-west trending series. Both Both dikes dikes have have aa diabasic and vary from 10 10 to 75 75 feet wide. that some some metamorphosed metamorphosed In the Republic area, Baxter and Bornhorst (1988) suggested that mafic dikes with distinct distinct plagioclase plagioclase phenocrysts phenocrysts are are older older than the the metamorphosed metamorphosedProterozoic Proterozoic dikes in the Sugarloaf area. area. They proposed that these dikes dikes might might correlate correlate with with the the Matechewan Matechewan dike swarm north of Lake Superior Superior in Canada. Canada. We We tested tested this this hypothesis hypothesis by by doing doing chemical chemical analysis of these dikes. The REE data data for for these dikes dikes are are similar similar to to Matechewan Matechewan dikes dikesfrom from elsewhere and support support the hypothesis proposed proposed by Baxter Baxter and and Bornhorst Bornhorst (1988). (1988). References References Baxter, D.A. and Bornhorst, T.J., 1988, 1988, Multiple Discrete Mafic Intrusions of Archean to Keweenawan Age, 2 pp. western Upper Peninsula, Michigan: Michigan: Institute Institute on on Lake Lake Superior Superior Geology Geology Proceedings Proceedings and and Abstracts, Abstracts,v.v.34, 34,2 pp. U.S. Cannon, W.F., 1975, 1975, Bedrock Geological Map of the Republic Quadrangle, Marquette County, MI: U S . Geological Miscellaneous Investigations Survey, Miscellaneous InvestigationsSeries SeriesMap, Map,1-862. 1-862. Gair, J.E. and Thaden, R.E., 1968, 1968, Geology Geology of the Marquette and Sands Quadrangles, Marquette County, MI: U.S. 77 pp. Geological Survey Survey Professional Professional Paper Paper 397, 397,77 pp. Hails, H.C. and Phinney, Halls, Phinney, W.C., 2001, 2001, Petrogenesis Petrogenesisof of the the Early Early Proterozoic ProterozoicMatachewan MatachewanDyke DykeSwarm, Swarm,Canada, Canada,and and Implications for Magma Emplacement 22 38,22 Implications Emplacement and Subsequent Subsequent Deformation: Deformation: Canadian Canadian Journal Journal of of Earth Earth Sciences Sciences38, pp. PP. Kantor, J.A., 1969, 1969, Assimilation Assimilation and and Dike Dike Swarms Swarms in in the the Sugarloaf Sugarloaf Mountain Mountain Area, Area, Marquette MarquetteCounty, County,MI: MI:M.S. M.S. Thesis, Michigan Technological Technological University, University, Houghton, Houghton, MI, MI, 83 83 pp. pp. 68 �PALEOPROTEROZOIC DEVELOPMENT OF A GNEISS DOME CORRIDOR IN THE SOUTHERN LAKE SUPERIOR SUPERIOR REGION, REGION, USA SCHNEIDER, D.A., Dept. of Geological Sciences, Sciences, Ohio Ohio University, University, Athens, Athens, OH OH 45701; 45701; HOLM, D.K., and O'BOYLE, O'BOYLE, C., C., Dept. Dept. of of Geology, Geology, Kent Kent State State University, University, Kent, Kent, OH OH 44242; 44242; HAMILTON, M., Continental Continental Geoscience Division, Geological Survey of Canada, Ottawa, ON Canada; M., Dept. Dept. of of Geosciences, Geosciences, U-Mass, U-Mass, Amherst, Amherst,MA MA 01003 01003 Canada; and JERCINOVIC, JERCINOVIC, M., Paleo-reconstruction of a Paleo-reconstruction of the Penokean orogen at ca. 1750-1700 1750-1700 Ma reveals the presence of narrow corridor corridor of Archean Archean cored cored Paleoproterozoic Paleoproterozoic gneiss gneiss domes domes just north north of of and andparallel parallel to tothe the main suture zone in Minnesota, Wisconsin, and northern Michigan. Penokean Penokean (ca. (ca.1850 1850Ma) Ma) metasedimentary rocks infolded within the domes give predominantly 1750-1700 1750-1700 Ma cooling ages and are overlain depositionally depositionally by ca. ca. 1700 1700Ma Ma Baraboo Baraboo interval interval quartzites. quartzites.We Weconducted conducted U-Pb SHRIMP and total-Pb EMP geochronometry to obtain metamorphic timing constraints on distinct distinct monazite mineral domains domains from from amphibolite amphibolitegrade grade rocks rocks sampled sampledacross acrossthe theentire entirelength length of the gneiss dome corridor. Based Basedon onmetamorphic metamorphicmonazite monazite crystallization crystallizationages, ages,midcrustal midcrustal amphibolite amphibolite facies metamorphism (Ml) (Ml) peaked peaked around around 1830 1830Ma Ma and and was was concurrent concurrentwith with late late Penokean plutonism; subsequent thermal pulses are reliably recorded at ca. 1800 Ma (M2) and again at ca. 1765 1765 Ma (M3), (M3), both also also coeval coeval with with magmatic magmatic activity. activity. Ar-Ar mineral mineral age data, data, which which indicate indicate The youngest monazite ages overlap with abundant Ar-Ar widespread cooling cooling of the gneiss gneiss dome dome corridor corridor immediately immediately following followingM3. M3. We We propose propose that that the the of the tectonically buried gneiss domes formed at this time during structural modification of continental margin rocks. In Inour ourconceptual conceptual model model (Fig. (Fig.1), I), northward northward vertical vertical extrusion extrusionof of aa midcrustal block containing decoupled midcrustal containing the the gneiss gneiss dome dome corridor corridor accommodated accommodatedgravitational gravitational collapse of overthickened crust. Elevated Elevatedcountry country rock rock temperatures temperatures accompanied accompaniedwith with profuse profuse promoted doming of the lower melting (i.e., intrusion of the East-central Minnesota batholith) promoted density Archean basement into the more more dense dense overlying overlying Paleoproterozoic Paleoproterozoicmetasedimentary metasedimentary rocks, ultimately enabling its complete decoupling from the remaining lower crust. This This process, process, redistribution of of crustal crustal mass from thick to primarily driven by buoyancy forces, allows for the redistribution thin regions without significant horizontal crustal extension. Tectonic Tectonicextrusion extrusionand andcrustal crustal thinning at this stage stage may have have been facilitated facilitated by aa decrease decrease in in horizontal horizontal compressive compressivestresses stresses acting on the region from from the south south (i.e., (i.e., Yavapai Yavapai slab slab rollback rollback as as proposed proposed by by Hoim Holm et et a!., al., ILSG, 2003). In In our ourmodel model(Fig. (Fig.1), I),the thefaults faultsbounding bounding the the gneiss gneissdome domecorridor corridorare areca. ca. 1765 1765 Ma structures, structures, although some, some, like like the the Niagara Niagara Fault Fault zone, zone, are are reactivated reactivatedPenokean Penokean structures. structures. of the Malmo Structural We note that in east-central Minnesota, a significant portion of discontinuity juxtaposes post-Penokean plutons to the south against older metamorphic rocks to discontinuity of Mille Mule Lacs). This the north (west of This clearly clearly supports supports our interpretation that this structure (and the Flambeau Flowage fault equivalent in northern Wisconsin) was active well after Penokean orogenesis. orogenesis. Holm, D.K., Van Schmus, W.R., MacNeill, L.C., Boerboom, T.J., Schweitzer, D., and Schneider, Schneider, D.A., 2003, 2003, Late Paleoproterozoic Paleoproterozoic(1900-1600 (1900-1600 Ma) Ma) tectonic tectonic history history of of the the northern northern mid-continent, U.S.A.: Implications for crustal stabilization: Institute on Lake Superior mid-continent, Institute on Lake SuperiorGeology Geology abstracts (this abstracts (this volume). volume). 69 �Penokeanorugm, orogen,Ml: Ml: 1830 Ma to M2: Penohan 1830 Ma M2: 1800 1800 Ma Ma N N s$ warm, Penokean orogen, M3: 1768 Ma N ONEISS GNEISS DOME DOME CORRtDOR CORRIDOR WISCONSIN WISCONSIN MAGMATIC MAGMATIC TERRANE TERRAME (uveniIe Quvimiie island iaiad an;) arc) ARCHEAN GRANITE-GREENSTONE GRAWE-GREENSTONE ARCHEAN GNEISS E PALEOPROTEROZOIC ROCKS (supracrustal) Figure Figure 1. 1. Schematic SchematicN-S N-Scross-sections cross-sectionsatat1830-1800 1830-1800Ma Ma (A) and (B) depicting depictingthe the proposed proposed evolution evolution of ofthe the gneiss gneiss dome 1765 1765 Ma Ma (6) northern Wisconsin. Note corridor in northern Note relative relative locations locationsof of gray gray circles circles that represent represent depth depth of of crustal cruslaf blocks. blocks. 70 s$ �suprasubduction zone A Paleoproterozoic suprasubduction zone ophiolite-island ophiolite-island arc arc complex complex northeastern Wisconsin in northeastern Wisconsin Schulz, Klaus Schulz, Klaus J., J., (U.S. (U.S.Geological GeologicalSurvey, Survey,Reston, Reston,VA VA20192, 20192,kschuiz@usRs.Rov) kschulz@usgs.gov) The Paleoproterozoic Paleoproterozoic volcanic and associated associated intrusive rocks exposed in northeastern Wisconsin are the easternmost easternmost exposures exposures of the Pembine-Wausau terrane, the northernmost of the two Wisconsin magmatic terranes that were accreated to the southern margin of the Archean Archean Superior SuperiorCraton Craton during during the the Penokean Penokean Orogeny Orogeny (Sims (Simsand andothers, others, 1989). The The rocks rocks of of the the Pembine-Wausau Pembine-Wausau terrane are separated separated from the epicratonic epicratonic sedimentary rocks of the Marquette Marquette Range Supergroup sedimentary Supergroup to the north in Michigan Michigan by by the the Niagara fault fault zone. zone. The volcanic rocks of the Pembine-Wausau terrane exposed northeastern Wisconsin, formed synformed at about about 1,870 1,870Ma Ma and and are are cut cut by by aa variety variety of intrusive intrusive rocks rocks ranging ranging from from synvolcanic gabbros, diorites, and tonalities to syn-and post-tectonic granitoids (i.e., Dunbar Gneiss and related rocks). The Thevolcanic volcanicrocks rocksare aredivided divided into into four fourfault-bounded fault-boundedunits, units, the Quinnesec, Quinnesec, McAllister, Beecher, and Pemene formations. These Theseunits units are areinterpreted interpreted to record the evolution evolution of of aa Paleoproterozoic Paleoproterozoicsuprasubduction suprasubductionzone zone ophiolite-island ophiolite-islandarc arc complex, complex, the Pembine Pembine ophiolite-arc ophiolite-arccomplex. complex. The Quinnesec Formation is the oldest volcanic unit and consists predominantly of pillowed basalt flows and massive diabase, but includes andesite and rhyolite lava flows and fragmental rocks locally. Several Severallarge largegabbro gabbro sills sills are are present, present, particularly particularly near near the Niagara fault zone, some with peridotite and pyroxenite layers. In In addition, addition,aa large large serpentinized serpentinized peridotite-gabbro peridotite-gabbrobody that produces produces aa large large positive positive magnetic magneticanomaly anomalyisis exposed Timrns Lake Lake (Morgan (Morgan County County Park) Park) east east of of Pembine, Pembine,Wisconsin. Wisconsin. exposed south south of Timms Serpentinized Serpentinized pendotite peridotite is is dominant dominant in in the the western western part of of this this body where it is locally locally cut by coarse-grained coarse-grained (1-5 cm) dikes of pyroxenite. Layered Layered and massive massive gabbro gabbro and and masses of strongly strongly foliated-lineated foliated-lineatedgabbro gabbro are are dominant dominant in in the the eastern eastern part part of of the the body body where they are cut by numerous mafic dikes dikes with diabasic diabasic to to microdioritic microdioritictextures; textures;some some of the dikes dikes appear appear to be sheeted. sheeted. The rocks of the Quinnesec Quinnesec Formation Formation appear appear to to record record the the birth birth and and youth youth stages stagesof of aa zone ophiolite ophiolite (Shervais, (Shervais, 2001). 2001). Rocks formed fonned during the initial phase suprasubduction zone of ophiolite ophiolite evolution evolution typically typically include include layered layered and and isotropic isotropic plutonic plutonic gabbros, gabbros,sheeted sheeted dikes, and a "lower" volcanic section consisting of low-K tholeiitic basalt and basaltic consisting MORB and primitive arc tholeiite affinities. Gabbros Gabbros formed formed during during this this andesite with MORE stage are often ductilely deformed (foliated or boudinaged) in response response to syn-magmatic syn-magmatic extension. extension. Rocks Rocksformed formedduring duringthe thesecond secondor oryouth youth stage stageof of ophiolite ophioliteformation formationinclude include intrusive intrusive mafic-ultramafic mafic-ultramafic sills and diabase dikes, and an "upper" volcanic volcanic unit characterized characterized by basalt and and andesite andesite with highly highly depleted depleted incompatible incompatibletrace trace element element compositions compositions (i.e., low-Ti basalt, high-Mg andesite and boninite) (Shervais, 2001). Compositionally, Compositionally,the Quinnesec Quinnesec basalts basalts and and gabbros gabbros are are tholeiitic, tholeiitic,with with generally generallylow low Ti02 and TiOz and other other high field field strength strength element abundances, and flat to extremely light REE 71 �depleted patterns (Sims and others, 1989). 1989). In In addition, addition,some some of of the the basalts, basalts,gabbros, gabbros, and and andesites REE abundances, abundances, but but relatively relativelyhigh high Cr Cr and andNi Ni andesites have very low Ti02 TiOzand and REE contents. The Thetrace traceelement elementcharacteristics characteristicsof of the the mafic mafic rocks rocks overlap overlapthose those of of mid-ocean mid-ocean ridge basalts and primitive island-arc tholeiite suites whereas the andesites show fore-arc-relatedboninites. boninites. The presence in the upper part part of of compositional affinities with fore-arc-related Quinnesec Formation Formation of mafic rocks derived the Quinnesec derived from from highly highly refractory mantle mantle is is diagnostic of a relationship particularly diagnostic relationship to the early stages of intraoceanic intraoceanic subduction subductionand and This also also implies implies that that the the Quinnesec Quinnesec formation in a forearc setting (Shervais, 2001). This Formation Formation and associated associated rocks did not form in a back-arc basin near or on the margin of the Superior 1997),but but Superior Craton, Craton, as has recently been proposed (Van Wyck and and Johnson, Johnson, 1997), rather formed as an intraoceanic intraoceanic ophiolite-arc ophiolite-arc system system above a southward southward dipping dipping (in (in coordinates) subduction zone. present coordinates) The McAllister, McAllister, Beecher and Pemene formations consist of volcanic and volcaniclastic rocks ranging from from andesite andesite (McAllister) (McAllister)to to rhyolite rhyolite (Pemene), (Pemene), all all with with caic-alkaline calc-alkaline compositions compositions characteristic characteristic of mature oceanic arcs. These Thesevolcanic volcanic rocks rocksand andassociated associated Twelve Foot Foot Falls Falls Quartz Diorite, Diorite, appear intrusives, such as the Newingham Tonalite and Twelve compatible compatible with the third or maturity stage of suprasubduction zone ophiolite evolution (Shervais, 2001). Characteristic Characteristicof of this this stage stageare are intrusive intrusive rocks, rocks, such such as as hornblende hornblende rocks ranging ranging from from basalt basalt to to diorite, quartz diorite, and tonalite, as well as volcanic rocks rhyolite, all with transitional transitional to calc-alkaline calc-alkaline compositions. Volcanism Volcanismtypically typicallybecomes becomes more silicic with time in these sequences. In In many many cases, cases, rocks of of this this stage stage have have not been considered considered part of the subjacent subjacent ophiolite, but rather have been attributed to postophiolite ophiolite arc volcanism volcanism (Shervais, (Shervais,2001). 2001). It appears likely that growth of the Pembine Pembine ophiolite-arc ophiolite-arccomplex complex was was terminated terminatedby by its its collision obduction onto the passive southern margin of the Superior Craton. collision with and obduction Because subduction appears appears to to be be largely largely driven driven by slab slab pull, pull, the the southward southwardsubduction subduction of oceanic lithosphere lithosphere attached attached to the Superior continental margin would have pulled the continental continental lithosphere lithosphere along with it as it descended into the subduction zone below the ophiolite-arc system. With With detachment detachment of of the the subducting subducting oceanic oceanic lithosphere, lithosphere,the the buoyancy of the continental continental lithosphere lithosphere would have have led led to to its its rapid rapid uplift uplift along alongwith withthe the leading edge of the ophiolite-arc ophiolite-arc complex (Shervais, 2001). This Thisstage stageisisrecorded recordedby by the the deformation of the syn-to to post-tectonic post-tectonic the ophiolite-arc ophiolite-arcsequence sequence and and by by the the intrusion intrusion of of the the synunits of the Dunbar Dunbar dome. dome. Shervais, J.W., 2001, Birth, death, and resurrection: resurrection: the life cycle of suprasubduction zone ophiolites: Geochemistry Paper number number 2000GC000080. 2000GC000080. On-line publication publication at Geochemistry Geophysics Geosystems, vol.2, Paper http://g-cubed.org. http://e-cubed.org. P.K., Van Schmus, W.R., W.R., Schulz, K.J., K.J., and and Peterman, Peterman, Z.E., Z.E., 1989, Tectono-stratigraphic Tectono-stratigraphic evolution evolution of of Sims, P.K., the Early Proterozoic Proterozoic Wisconsin Wisconsinmagmatic magmatic terranes terranesof of the the Penokean PenokeanOrogen: Orogen:Canadian CanadianJournal Journalof ofEarth Earth Sciences, v. 26, 158. Sciences, 26, p. p. 2145-2 2145-2158. Van Wyck, N., and and Johnson, Johnson, C.M., C.M., 1997, 1997,Common Common lead, lead,Sm-Nd, Sm-Nd,and and U-Pb U-Pbconstraints constraintson onpetrogenesis, petrogenesis, crustal architecture, and tectonic setting of the Penokean orogeny (Paleoproterozoic) in Wisconsin: crustal architecture, and tectonic setting of the Penokean orogeny (Paleoproterozoic) in Wisconsin: Geological Geological Society Society of America America Bulletin, Bulletin, v. v. 109, 109,p. p. 799-808. 799-808. 72 �THE THE LAKE NIPIGON GEOSCIENCE GEOSCIENCE INITIATIVE INITIATIVE -- PLANNED ACTIVITIES ACTIVITIES AND AND OBJECT! VES OBJECTIVES SMYK, Mark C., C., Ontario Ontario Geological Geological Survey, Survey, Ministry Ministryof ofNorthern NorthernDevelopment Developmentand andMines, Mines, SMYK, Mark Suite B002,435 B002, 435 James James St. St. South, South, Thunder Thunder Bay, Bay, ON ON P7E P7E 6S7, 6S7,and andmembers membersof ofthe theScientific Scientificand and Implementation Committees, Conmiittees, Lake Lake Nipigon Nipigon Geoscience Geoscience Initiative, Initiative,c/o do Ontario Prospectors Association, 1000 Association, 1000 Alloy Drive, Drive, Thunder Thunder Bay, Bay, ON ON P7B P7B 6A5 6A5 was created created in in 2002 2002 as as aa $7.0 $7.0 M M Cdn. Cdn. project project aimed at The Lake Nipigon Geoscience Initiative (LNGI) was the area area around Lake Lake Nipigon. Nipigon. The The Ontario OntarioProspectors Prospectors Association's Association's attracting mineral investment to the (OPA) portion of the project project is is funded funded through through an an agreement agreementwith with the the Northern Northern Ontario OntarioHeritage HeritageFund. Fund.The The OPA is partnering with the Ontario Ontario Geological Geological Survey Survey (OGS), (OGS), the the Ministry Ministry of Northern Northern Development Developmentand and Mines (MNDM), the Canadian Canadian Mining Mining Industry Research Research Organization Organization (CAMIRO), (CAMIRO),Lakehead Lakehead University, University, as well as with private sector partners and communities communities in the Lake Nipigon area. area. It will will focus focus on on four four key key objectives: objectives: 1. Maintain 1. Maintain and then increase increase mineral mineral investment investmentin in the the Lake Lake Nipigon region region through through collection collectionof of high high quality geological data and provision provision of interpretations interpretationsthat meet meet the the needs needs and and priorities priorities of of the the mineral mineral industry and that maintain or attract mineral investment investment to Ontario; 2. 2. Increase the mineral exploration discovery discovery rate by addressing addressing "masking and deep deep search search challenges challenges and skill gap" in the area; area; 3. Respond nickel3. Respond to, and and evaluate, evaluate, new new and and exciting exciting mineral mineral deposit deposit models models recently recently recognized recognizedfor fornickelcopper, copper, palladium-platinum, palladium-platinum,and and gold-copper gold-coppermineralization mineralizationin in the the region; region; 4. 4. Reinforce Reinforce and and demonstrate demonstrate an an innovative innovativeeconomic economic development developmentmodel model based based on on local localcommunity, community, industry, industry, and government government partnerships partnerships in in geoscience geoscience that result result in in mineral mineral resource resource economic economic development development in the the local local communities, communities, the the region, region, and and Ontario. Ontario. The LNGI is Embayment, which which consists consists predominantly predominantly of of is focused focused on on the the Nipigon Nipigon Basin Basin // Embayment, Mesoproterozoic, Mesoproterozoic, Midcontinent MidcontinentRift-related, Rift-related,ultramafic ultramaficto to mafic mafic intrusions intrusionsthat that have haveintruded intruded Mesoproterozoic Mesoproterozoic Sibley Sibley Group Group sedimentary sedimentaryrocks rocks and and Archean Archean basement basementrocks rocks of of the theQuetico Queticoand and Wabigoon Wabigoon subprovinces. subprovinces. The project will develop a comprehensive will assist in in mineral mineral exploration. exploration. The comprehensive geoscience database that will The LNGI evolved evolved through aa series series of of community community and industry industry consultations consultations that helped define define the the project project parameters. AAthorough thoroughcompilation compilationof of previous previousexploration explorationand and geological geological data data provided provided aa baseline baseline for for the project and identify the geoscience database. The identify potential potential gaps gaps .in in the Themain main components componentsof ofthe theinitiative initiative include: include: • • • • • • • • Detailed geological geological mapping, mapping, undertaken undertaken by by Precambrian Precambrian Section, Section, OGS OGS Airborne Airborne magnetic magnetic survey survey Gravity Gravity survey survey Quaternary Quaternary (surficial) (surficial) case case studies, studies, undertaken undertaken by Sedimentary SedimentaryGeoscience Geoscience Section, Section,OGS OGS Geochronology Geochronology Physical property studies studies Geographic (GIs) compilation compilation Geographic Information Information Systems Systems (GIS) Complementary Complementary research at Lakehead Lakehead University • Sibley Group studies studies (P. (P. Fralick) Fralick) • Nipigon mafic intrusion intrusion studies studies (P. (P. Hollings; Hollings; G. G. Borradaile) Borradaile) • Sulphide Sulphide mineralization mineralization studies studies (S. (S. Kissin) 73 �The Ontario Ontario Geological Geological Survey Survey will will help acquire acquire and and publish publish the the results results of of the the geoscience geosciencestudies studiesas asmaps, maps, reports, and digital data sets. The Theinformation informationwill will then then be be available available over over the the Internet Internet through through the the MNDM's MNDM's ERMES and CLAIMap CLAIMap systems. systems. This valuable valuable information information will be used to globally globally market market the the resource resource potential and investment investment appeal appeal of the the Lake Lake Nipigon region. region. 74 �TECTONOSTRATIGRAPHIC ARCHEAN T E C T O N O S T U T I G M P H I C ASSEMBLAGES ASSEMBLAGES OF O F EASTERN EASTERN WABIGOON SUBPROVINCE, NORTHWESTERN ONTARIO STOTT Greg ONyP3E P3E 6B5 6B5 STOTT GregM., M.,Ontario OntarioGeological Geological Survey, Survey, Sudbury, Sudbury, ON, (greg.stott@ndm.gov.on.ca), DAVIS, Don. W., Department of Geology, University of of (greg.stott@ndm.gov.on.ca)y Toronto, Toronto, Toronto, ON, ONyPARKER, PARKER,Jack JackR., R.,Ontario OntarioGeological GeologicalSurvey, Survey,Sudbury, Sudbury,ON, ON, Toronto, STRAUB, Kristan J., Laurentian University, University, Sudbury, Sudbury, ON ON and andTOMLINSONy TOMLINSON, Kirsty Y., Geological Survey of Canada, Ottawa, ON The Archean Wabigoon Subprovince Subprovince is a complex of volcanic and sedimentary supracrustal assemblages supracrustal assemblages and granitoid suites suites of Mesoarchean Mesoarchean to Neoarchean Neoarchean age. age. The The easternmost Onaman-Tashota greenstone belt easternmost part of this subprovince, subprovince, which includes the Onaman-Tashota east of Lake Nipigon, preserves aa history history of over over 250 250 million million years years of of volcanism. volcanism.This This area has recently been treated to a regional mapping, geochemical and geochronological 11250000 000 compilation compilation synthesis as part of the Western Superior Superior NATMAP project. A 1:250 map (Stott et al. 2002) arising arising from this project illustrates the subdivision of the OnamanTashota (O-T) (Figure 1), based (0-T) greenstone greenstone belt into tectonostratigraphic assemblages (Figure based on on stratigraphic correlationsy correlations, geochronological and geochemical similarities stratigraphic similarities and and contact contact relationships. Figure 2, 2,isisthe the relationships. A more interpretive interpretive component of this map, summarized in Figure delineation of the assemblages assemblages in in terms terms of of the the environment environmentof of crystallization crystallizationof of volcanic volcanic and plutonic rocks and deposition deposition of sedimentary rocks. This is based on lithologic and geophysical characteristics, characteristics, whole-rock whole-rock geochemical geochemical classification, classification,and and where where available, available, Nd isotopic isotopic signatures. signatures. The Onaman-Tashota Onaman-Tashotagreenstone greenstone belt straddles straddles the the width width of of the the eastern easternWabigoon Wabigoon Subprovince Subprovince between the English English River River and and Quetico Queticometasedimentary metasedimentarysubprovinces. subprovinces.ItIt isis mainly dacitic flows, flows, mainly composed composedof ofNeoarchean Neoarchean(dominantly (dominantly2.74 2.74—- 2.72 Ga) basaltic and dacitic autobreccia and autobreccia and pyroclastic pyroclasticrocks. rocks.Mesoarchean Mesoarchean(3.05 (3.05—- 2.92 2.92 Ga) volcanic rocks occur occur in the northwest northwest and and along along the the western western margin margin of of the the belt. belt. Widespread Widespread Nd Nd isotopic isotopicevidence evidence in the northern part of of the the Onaman-Tashota Onaman-Tashota belt belt suggests suggests that that Neoarchean Neoarchean volcanism volcanism erupted erupted through Mesoarchean Mesoarchean basement. basement. Basement Basement in in the the northern northern half half of of the the belt belt contains contains an older older component component than than that that south south of of the the Humboldt Humboldt Bay Bay High High Strain StrainZone. Zone.The The 2.74 Ga Willet Willet assemblage assemblagetholeiitic tholeiitic basalts basalts of of ocean ocean floor floor affinity affinity dominate dominatethe thenorthern northern half of the O-T calc-alkalic 0 - T belt. belt. This This assemblage assemblage is is flanked flanked to to the the north north and and south southby by calc-alkalic assemblages assemblages of continental continental margin margin arc arc affinity affinity that border border metasedimentary metasedimentarysubprovinces subprovinces composed of flysch-like wacke derived from the erosion of the O-T belt and composed flysch-like derived from the erosion of the 0 - T belt and plutons plutons during orogenesis orogenesis at circa circa 2.7 2.7 Ga. Ga. Most Most sedimentary sedimentary units units within within the the O-T 0 - T belt belt form formthe the youngest supracrustal supracrustal assemblages, assemblages, reflecting erosion of the the underlying underlying volcanic volcanic and and plutonic rocks towards towards the the English English River River and and Quetico Queticobasins basins to to the the north northand andsouth. south. Reference Reference Stott, G.M., Davis, D.W., Parker, J.R., J.R., Straub, K.J. and Tomlinson, K.Y. 2002. Geology and Tectonostratigraphic Tectonostratigraphic Assemblages, Assemblages,eastern eastern Wabigoon Wabigoon Subprovince, Subprovince,Ontario; Ontario;Ontario OntarioGeological GeologicalSurvey, Survey, 000. Preliminary Map P.3449, scale 1:250 1~250 Tomlinson, K.Y., Stott, G.M. and Davis, D.W. 2000. Nd isotopes in the eastern Wabigoon subprovince: implications for crustal recycling and correlations with with the the central central Wabigoon; Wabigoon; in in Harrap, Harrap, R.M. R.M. and Helmstaedt, H.H. (eds.), 2000, Western Superior Transect Sixth Annual Workshop, Lithoprobe Report #77, Lithoprobe Secretariat, Secretariat, University University of of British British Columbia, Columbia,p.119-126. p. 119-126. 75 �Figure Figure1. 1. Tectonostratigraphic Tectonostratigraphic assemblages assemblages of of the the Onaman-Tashota Onaman-Tashota greenstone greenstone belt and and Proterozoic diabase dike swarms, Eastern Wabigoon Subprovince. Subprovince. , +•t!4+ •-4..:L--•, .....a.4 ++ ++ + +++ •++ +÷++÷ + + + + .4 P + + + + + .4 + 4 + + ++ 4 + + + + .4 + + + + + + + + + 4 + + + .4 + .4 + + 4. + .4 + + + + 4 +ZP+ .4 + F ( + + + .._+ + ++ +4 + + .4 4+ + + + 4 + + + + + .4 + .4 .4 + + .4 + + + .4 + + + + + + + + + .4 + + + 4* + .4 + + + + Figure 2. Tectonic Figure Tectonic affinities assigned to volcanic and sedimentary sedimentary assemblages and assemblages plutonic suites. suites. + + .4 + +4+ + + + .4 + + 4 .4 + + + .4 ++t#4J + 4+4+ +±+ + + + + + •# 4 .4 + + + r++.— + '4.4 + + + + —-,"+ + +_ + + + 4.' + + + + 4 .t(__... (4. + + + +,I. + +1) + / / Lake Nipigon + + + + + + +c--.4+ + + + Proterozoic Proterozoic [ I- ' I I plume Continental p i m e related related m 4-L 11 - Orogenic plutons plutons orogenic orogenic sediments sedimen& Orogenic Continental arc Contmental Continental rnargm margin arc Cont~nsntal unsubdivided Continental unsubdivided 3 Ocean Ocwan floor floor Oceanic Oceanic unsubdivided unsubdivided Archean Archean Unknown Unknown tectonic tecton~caffinity afflnlty I0 10 0 0 7 Mesoarchean .Mesoarchean ..- - - -.-..- - Kilometres Kllometres ?//////A 76 20 20 ÷ �FIVE GOLD POSSIBILITIES IN SOME KEWEENAWAN COPPER SULFIDES SULFIDES IN IN ONTARIO AND MICHIGAN Trow, Jim, Trow! Jiml Geological Sciences, Sciences! Michigan State State University, University! emeritus, emeritusl 540 540 Lake Lake Avenue *2, #Z1 Hancoek, HancoC!kl Michigan Michigan 49930 49930 gold, Most fire—assayed fire-assayed "invisible" ltinvisiblell goldl from from .12 .12 to to 2.50 2.50 oz oz Au/st, Au/str occurs in minor covellite) in "blue I1bluechalcocite" c h a l ~ o c i t e(with (with ~~ covellite) but but not not in in black chalcocite (with (with no no covellite) covellite) on on the the adit, adit! 1st, lstl 2nd, 2nd1 and and 3rd levels of the Pointl Ontario. Ontario. the Coppercorp Coppercorp mine mine at at Mamainse Mamainse Point, Both occur occur with with specular specular hematite. hematite. Copper mineral zoning zoning exextending from carbonates and oxides through native copper, copper! black chaleocite and specularite, chalcocite specularitel "blue Itbluechalcocite" c h a l ~ o c i t eand ~ ~ specularite, specularitel to to bornite and chalcopyrite is related to nearness to the Keweenaw and related faults faults apparently down which circulated oxidizing solutions The solutions during an an upward-migrating upward-migrating hydrothermal hydrothermal episode. episode. The anomalies! whereas whereas former faults display positive SP electrical anomalies, nearly perpendicular cross faults faults with commercial ores ores display negative SP anomalies of this convective hydrothermal cell cell (Trow). (Trow). Such progressive oxidation of hydrothermal fluids fluids is is suggested suggested for for the Keweenawan of Michigan by the USGS's USGSts Woodruff, Woodrufff Cannon, Cannon! and and Back. Ontario, Trow deduces thermochemical calculations For Ontario! calculations with standard standard free energies and typical activities for constituents constituents (except (except for for oxygen, whose activities oxygenl activities are are the the unknowns). unknowns). These are arrayed arrayed on on a logarithmic scale which mimics the the observed copper copper mineral zones, zonesl AuS- first and in that sequence AUS-I first oxidized oxidized to to deposit deposit gold gold at at the the same oxygen activity at which chalcopyrite first first oxidized oxidized to to covellite and and specularite. specularite. At the present it it is is uncertain if if the the "blue chalcocite" iito chalcoCite and covellite from I1blue c h a l ~ o c i t eexsolved ~~ ivto chalcocite digenite at low temperatures, temperatures, or if if most of original covellite covellite was replaced by 2!500 times times the the oxygen oxygen by late late chalcocite chalcocite at at roughly roughly 2,500 activity at which which covellite covellite originally originally formed. formed. Essentials for gold at Coppercorp Coppercorp include include 1) 1) Keweenawan permeable permeable vesicularbeds conglomeratesl 2) 2) felsite felsite intrusives intrusives basaltic vesicular beds and and conglomerates, with permeable permeable border border breccias breccias as as conduits conduits for for rising rising hydrohydrothermal solutions, solutionsl 3) 3) nearness to to the the Keweenaw Keweenaw and and related related faults faults with positive SP anomalies, anomaliesl 4) 4) mineralized cross cross faults faults with with ores ores yielding negative SP anomalies, anomaliesr and 5) 5) "blue 'Iblue chalcocite". chalc~cite~~. In Michigan! Michigan, field examination of ore ore deposits deposits and and structures structures (Balticl Ashbed, Ashbed! mapped by the USGS shows shows that the the major lodes lodes (Baltic, Isle Royale, Royalel Pewabic, Pewabicl Osceola, Osceola, Calumet conglomerate, conglomeratet and and Kearsarge) Kearsarge) and the Cliff, Cliff! Central, Central! and Delaware Delaware fissure fissure deposits deposits all all display display Keweenaw, Hancock, Mayflowerl Mayflower, and negative SP SP anomalies. anomalies. The Keweenaw! Gratiot—Suffolk faults Gratiot-Suffolk faults all all display positive positive SF SP anomalies, anomaliesl approappropriate for downward oxidative contamination hypogene contamination of of rising rising hypogene (not supergene) supergene) mineralization. mineralization. From southwest southwest to northeast the the best matches to to Canadian Canadian gold gold in in Michigan examined, occur 1) from Mass City to the Indiana so far examinedl Michigan so Indiana mine adjacent to felsite felsite intrusives intrusives and and the the Keweenaw Keweenaw fault fault in in Ontonagon County, Countyl 2) 2) In In Houghton Houghton and and Keweenaw Keweenaw Counties Counties the the Allouez Allouez Gap fault between Copper City and and New Allouez Allouez is is near near the the Copper Copper , 77 �2 City felsite felsite and the the Keweenaw Keweenaw fault. fault. According to to Bornhorst, Bornhorstl page 132, 132! Randy Weege of C & H thought thought that this this fault fault perhaps perhaps was a fluid pathway for 60% of the district's districtls copper copper production. production. Further, it replicates replicates and improves Further! improves upon the the best geophysical geophysical signature Coppercorp, the persistent SB signature at Coppercorpl SB zone, zone! with with flanking flanking negative negative SP anomalies anomalies in in the midst of which which is is aa positive positive SP SP anomaly. llcorell splinters anomaly. In Michigan! Michigan, the positive "core" anomaly splinters westward off the northern northern end of the the negative negative anomalies anomalies in in the the vicinity vicinity of of Ahmeek. Ahmeek. This part of the the district district contains contains arsenic, arsenicl which accompanies accompanies gold in in many western western mining camps. camps. 3) In In 1999 1999 Bornhorst reported on the Maki and Bornhorst the 4½ 4% million million tonnes of chalcocite chalcocite in drilled amygduloids of the Gratiot deposit in in Keweenaw Keweenaw County, Countyl where these these beds are intruded intruded by dacite dacite (felsite). (felsite). This lode This lode appears appears at the intersection intersection with the the southward southward extension extension of of Trow's Trowls negative SP anomaly as as observed at at the the Central Central mine mine and and 2¼ 2% miles miles negative to to the the SSE. SSE. 4) 4) In Keweenaw Keweenaw County, Countyt the the USGS's USGS1s Hank Hank Cornwall Cornwall on on 166-167 describes describes minor traces traces of gold with with mainly mainly chalco— chalcopages 166-167 specularite and some some covellite covellite and and chalcopyrite chalcopyrite in in an an cite and specularite amygduloid amygduloid near near the the top top of of the the Greenstone Greenstone flow. flow. This is is not not near near faultl but it it is is cut cut by aa N.4°E. N.dOE. vertical vertical fault fault with with the Keweenaw fault, negative SP anomaly, anomalyl which which must be intersected intersected at at depth depth by by aa a negative N.4°E., 35°-45°NW. N.4OE.! 35O-45O~W.fault fault with a positive positive SP SP anomaly, anomalyt where where it it is is exposed to to the the east east of of the the vertical vertical fault. fault. There exists exists a possibility for a horizontal horizontal ore ore shoot shoot at at these 5 h b e faults' faults4 intersection. intersection. possibilities are plotted on on the the latest latest geologic geologic map map These four possibilities of the the Keweenaw Keweenaw peninsula, peninsulal by by Cannon Cannon and and Nicholson. Nicholson. Not yet yet reconnoitered possililities possililities may occur occur to to the the northeast northeast of of these. these. Remember, Remember! from from 1849 1849 to to 1961 1961 the the old timers timers all all missed the the Carlin Carlin "invisible" gold. llinvisiblell gold. Nevada is is now now the the biggest biggest gold gold producing producing state state because of the observationst observations, thinkingl thinking, and and Perseverance perseverance of of the the USGS's Ralph Roberts USGS1s Roberts and Mewmont's Newmontls John John Livermore. Livermore. REFERENCES CITED Bornhorst, T. J., Bornhorstl J e t1997, 1997! Tectonic Tectonic context context of of native native copper copper deposits deposits of the North A~erican American Midcontinent Midcontinent Rift Rift System, Systeml in in Geological Geological Society of America Special Special Paper Paper 312, 3121 p. 127—136. 127-136. Cannon, Cannon! W. F. and Nicholson, Nicholsonl S. S. W., W e 12001, 2O0lt Geologic Geologic map map of of the the Keweenaw Peninsula Peninsula and adjacent adjacent area, areal Michigan, Michigan! USGS USGS Geological Geological Investigations Investigations Series Series Map Map 1—2696. 1-2696. Cornwall, Cornwalll H. R., R e 1 1951, 19511 Differentiation Differentiation in in lavas lavas of of the the Keweenawan Keweenawan series and the origin Michigan! origin of the the copper copper deposits deposits of of Michigan, Geological Geological Society Society of of America America Bull. Bull. v. v. 62, 62# no.2, no.2# p. p. 159-201. 159-201. Maki, J. C., Makit C.! 1999, 199g1 The The Gratiot Gratiot chalcocite chalcocite deposit, deposit! Keweenaw Keweenaw Peninsulal Technological University, Universityl M.S. M.S. Peninsula, Michigan! Michigan, Michigan Technological Thesisl 71 p. p. Thesis, 71 Trow, J., Trow, J o t 1992, 1992! Inductive Inductive electrostatic electroskakic gradiometry gradiometry (IESG) (IESG) deciphers Keweenawan Keweenawan copper plumbing system, systeml Soc. SOC. Mining, Miningl Metall. and and Expl. Expl. Phoenix Phoenix Meeting, Meetingl Preprint Preprint 92—32, 92-32! 22 22 p. p. Woodruff, Woodrufft L. L. G., G e lCannon, Cannon! W. W. F., F.! and and Back, Back! J. J. M., M.! 1994, 1994# Chalcocite Chalcocite mineralization mineralization in in the the Portage Portage Lake Lake volcanics, volcanicst Keweenaw Keweenaw Peninsula, Peninsulal Michigan, 40th Michiganl 40th Ann. Inst. Inst. on on Lake Lake Superior Superior Geology, Geologyl Houghton, Houghtonl Abstracts, p . 77—78. 77-78. Abstracts! p. 78 �Using xenotime U-Pb geochronology to to unravel the history U-Pb geochronology history of ofProterozoic Proterozoicsedimentary sedimentary basins: a study basins: study in in Western Western Australia Australia and and the the Lake Lake Superior Superior Region Region McNaughton, N.J., N.J., Rasmussen, Rasmussen, B., B., Fletcher, Fletcher, I., I., Griffin, B.J., Vallini, D.A., dvallini@peol.uwa.edu.au, dvallini@aeol.uwa.edu.au, McNaughton, B.J., University of Western Australia, 35 Stirling Hwy, Crawley, 6009, Australia Diagenetic xenotime (YPO4) is aa trace trace constituent in a wide variety of siliciclastic (YPOJ is siliciclastic sedimentary sedimentary rocks. rocks. It typically forms pyramidal crystals of only aa few few microns microns in in size, size, rarely rarelyexceeding exceeding10 10pm, vm,growing growingon on [isostructural] detrital detrital zircons. zircons. A A recent study by Vallini et al. (2002) [isostructural] (2002) showed showed convincing convincing petrographic petrographic and age relationships that demonstrate demonstrate this U-bearing U-bearing phosphate phosphate could begin begin forming forming in in sediments sediments at at or just below below the sediment-water sediment-water interface, interface, shortly shortly after burial. burial. A few years ago itit was discovered discovered that itit 10 pm is possible to date xenotime crystals 210 vmininsize, size,using usingthe theSHRIMP SHRIMP(Sensitive (SensitiveHigh HighResolution ResolutionIon Ion for its formation, hence an age for early diagenesis and a Microprobe), providing aa robust isotopic age for close proxy for sediment deposition. Xenotime especially useful useful in in that that it has very high U contents Xenotime is especially and remains remains closed closed to toradiogenic radiogenicparent-daughter parent-daughter mobility, mobility, unlike unlike most mostother otherdateable dateablediagenetic diagenetic mineral. Diagenetic Diageneticxenotime xenotimeU-Pb U-Pbgeochronology geochronologyhas hasthe thepotential potentialto tounravel unravelthe thechrono-stratigraphy chrono-stratigraphy of unfossiliferous unfossiliferous sedimentary sedimentary basins, especially especially those sequences sequences devoid devoid of of dateable dateable interlayered interlayered volcanic basins where where aa lack lack of aa reliable volcanic rocks. Its Its main main application application isis in Precambrian Precambrian basins reliable temporal temporal affiliations, framework basin evolution evolution and framework hinders hinders an an understanding understanding of basin and maturation, maturation, tectonic affiliations, metallogeny and value as exploration metallogeny exploration targets. targets. Xenotime also forms during post-diagenetic Xenotime post-diagenetic fluid flow events, events, such such as as alteration, alteration,mineralisation mineralisationand and metamorphism, as as well well as being a magmatic metamorphism, magmatic mineral mineral and aa detrital detrital heavy heavy mineral. mineral. The The exceptional exceptional in situ its excellent excellent properties range of of xenotime, range of formation formation conditions conditions of xenotime, coupled coupled with with its properties for in situ geochronology, provide many many new new opportunities opportunities in in establishing establishing the the timeframe timeframe of of events geochronology, provide events in many many hitherto poorly understood understood sedimentary sedimentary basins. basins. Unusually coarse coarse (up (up to 200 crystals in the metametaUnusually 200 microns) microns) and and abundant abundant diagenetic diagenetic xenotime xenotime crystals sandstones of the greenschist greenschist facies facies Mount Mount Barren Barren Group, Group, southwestern Australia, allow the detailed detailed study of xenotime xenotime and its its host host rock. rock. Xenotime Xenotime occurs occurs within within aa phosphatic phosphatic sandstone sandstone interval interval and is is present in multiple different styles styles - as as cement cement overgrowths overgrowths on on zircons, zircons, pyramidal pyramidal present multiple morphologically morphologically different overgrowths on zircons, cement (no (no zircon) zircon) in in shale shale laminations, laminations, replacement replacement of of shale shale (?) (7)intraclasts intraclasts fluid events and as intraclasts. Multiple Multiple fluid events from from early early diagenetic diagenetic to low low as xenotime xenotime crystals crystals within within intraclasts. temperature/early temperaturelearly hydrothermal, hydrothermal,prior prior to to metamorphism, metamorphism,were were recorded recordedwithin withinsingle singlexenotime xenotimecrystals. crystals. U/Pb geochronology, accompanied by observations of petrographic relationships between SHRIMP UlPb between the various various generations generations of xenotime xenotime and and between between xenotime xenotime and other other diagenetic diagenetic minerals minerals and and pyrobitumen,allowed allowedfor for the the construction of a temporal pyrobitumen, construction of temporal framework framework for the the diagenetic diagenetic and and early early events that that occurred within these rocks; (1) ca 1700 hydrothermal events 1700 Ma: Ma: deposition deposition of of partly partly re-worked re-worked phosphatic siliciclastic siliciclastic sediments sedimentson on the the seafloor seafloor was was followed followed by in-situ of the phosphatic in-situ phosphatisation phosphatisation of sediments and an initial period 1697k 7 Ma), Ma), (2) (2) With With burial, burial, an an period of xenotime xenotime formation (mean age of 1697± early pore-filling pore-fillingcarbonate carbonate cement cement was introduced introduced into into parts parts of of the the interval, interval, as as well well as as early earlydiagenetic diagenetic cuboid pyrite ca 1650 1650Ma: Ma:during duringburial burialdiagenesis, diagenesis, aafluidfluid-movement movementevent eventcaused causedthe the pyrite growth, growth, (3) (3) ca Ma), with with partial dissolution of primary pore space and formation of xenotime (mean age of 1646 1646 ± 88 Ma), accompanying phosphate remobilisation, remobilisation, (4) Oil migration migration event, (5) Several Several silica silica cement cement generations generations ca 1560 1560Ma: Ma: minor minor addition addition of of xenotime xenotimerims rimsto toexisting existingovergrowths, overgrowths, introduced around this time, (6) (6) ca (7) ca 1480 1480 Ma: Ma: addition addition of of xenotime xenotime cement cement (no (no zircon) zircon) in in shale shale interlaminations, interlaminations, (8) (8) ca ca 1200 1200Ma: Ma: peak of metamorphism. metamorphism. Wavelength Wavelength Dispersive Dispersive Spectrometer Spectrometer (WDS) (WDS) microprobe microprobe analysis of each each type type of of xenotime xenotimeshowed showed with time, time. Due to this a gradual gradual change from LREE LREE enrichment enrichment to MREE MREE enrichment, enrichment, with this gradational gradational could not be established. nature, discrete boundaries between generations, based on chemistry, could This study study of of diagenetic diagenetic to to hydrothermal hydrothermalxenotime xenotimedramatically dramaticallyimproved improvedthe the estimated estimatedage agerange rangeof of the Mount Mount Barren Barren Group, Group, which which was was previously previously constrained constrained to to 1200 1200Ma Ma(peak (peakmetamorphism) metamorphism)and and 1790 1790 Ma Ma (youngest (youngest detrital detrital zircon zircon population), population), and and discounted discounted some some previous previous tectonic tectonic models models concerning the timing of of collision collision between between major major cratons cratons within within western western Australia Australia and andthese thesecratons cratons with East East Antarctica. Antarctica. Using the information gleaned from the study of xenotime xenotime in the Mount Mount Barren Barren Group, a similar study basin in in the Lake Superior Region is currently underway on another Proterozoic Proterozoic sediment-dominated basin containing and its equivalents, the North Range, containing the Marquette Marquette Range Supergroup Supergroup and equivalents, the Range, MilIe Mille Lacs Lacs and and Animikie Groups. Groups. The early early Proterôzoic Proterozoic strata strata consist consist of of three three unconformity-bounded unconformity-bounded lithostratigraphic groups consisting consisting of glaciogenics, glaciogenics, quartzites, quartzites, dolomite, dolomite, iron iron formation, formation, greywacke greywacke and and shale shale and and minor minor volcanics. Sedimentation is thought intercalated volcanics. thought to to have havebegun begun—2240 -2240 Ma Ma (correlation (correlationof of Chocolay Chocolay * 79 �Group with Gowganda Gowganda Fm, upper Huronian Huronian Supergroup, Ontario) (Fairbain et al., 1969) 1969) and ceased ceased by —1850Ma Ma(coinciding (coincidingwith with orogen-normal orogen-normalarc arccollision collision along along the the Niagara Fault zone and -1850 and the the Malmo Malmo Discontinuity, during during the the Penoken Penoken Orogeny) Orogeny)(Sims (Simsetetal., al., 1993). 1993). Part Partof of the the study study is to determine if Discontinuity, xenotime-rich horizons, such such as as that in the Mount xenotime-rich horizons, Mount Barren Barren Group, can be be located located in in this this stratigraphy stratigraphy and to document the sedimentological, sedimentological, structural structural or or stratigraphical stratigraphical features features that thatthey theyhave haveinincommon. common. Certain rock rock units from the different Certain different sequences sequences over over the whole region region were targeted targeted for xenotime xenotime analysis using proposed proposed sedimentological controls for xenotime formation that were determined determined from the Mount Mount Barren Barren Group Group study. study. feature favourable favourable to to xenotime xenotime formation formation may may be be the the presence of of large quantities One sedimentary feature of sedimentary sedimentary apatite apatite within the host host rock rock or or adjoining adjoining rocks. rocks. A field field sample sample of of low lowgreenschist greenschist facies facies phosphatic chert-conglomerate, chert-conglomerate,atatthe the base base of of the Baraga Group, phosphatic Group, from a documented documented phosphorite phosphorite locality in the Dead Dead River River Basin, Basin, northern northern Michigan, Michigan, contains contains large large quantities quantities of of xenotime xenotimeranging rangingfrom from <30 <30 pm pm pitted pitted overgrowths overgrowthson on detrital detrital zircons, zircons,to to>100 >I00micron micronxenotime xenotimecements. cements. Other rock units units that that contain contain xenotime xenotime overgrowths overgrowths and cements of appreciable size and quantity, were; were; (i) (i) quartzite quartzite beds beds in inseveral several drillholes drillholes through through the the Mahnomen Mahnomen Formation, Formation, Mille Mille Lacs Lacs Group, Group, Cuyuna Range, contain up to 50 50 xenotime xenotime crystals crystals per per thin thin section, section, some some of ofthese theseup uptoto—60 -60 pm pm in in size, (ii) a sandstone sandstone bed bed within within drillcore drillcore from from the the base baseof of the theBaraga BaragaGroup GroupininDead DeadRiver RiverBasinBasin-its its largest was 60 pm (iii) (iii)a agrit-pebble grit-pebbleconglomerate conglomerateand andvery verycoarse-grained coarse-grained largest xenotime xenotime observed observed was 60 pm sandstone outcrop at Slate River Hill Hill locality, Baraga Basin, which is assumed to lie at the base base of the Baraga xenotime grains grains per thin section which are pm in size, and (iv) Baraga Group, averaged averaged —15 -15 xenotime are up up to to —60 -60 pm the basal basal Baraga Baraga Group Group hematitic hematitic conglomerate conglomerate at at Big Big Eric's Eric's Crossing Crossing locality, locality, Baraga BaragaBasin, Basin,contains contains up to 55 xenotime xenotime grains grains per per thin thin section, section, some some of of these these are are up uptoto—100 -100 pm pm in in size. size. Pokegema Pokegema Quartzite samples, West Mesabi Mesabi Range, Range, showed showed minor <30 pm xenotime overgrowths on zircons. All of of the therock rocksamples samplesdescribed describedabove aboveare arevery verycoarse-grained coarse-grainedsandstone/conglomerate sandstone/conglomerate beds beds which are either which either located located near near aastratigraphic stratigraphic boundary boundary and/or and/or are are interbedded interbedded with with shale shale beds. beds. Xenotime from from these localities were were analysed on the SHRIMP Xenotime SHRIMP and revealed revealed several age groups; groups; (i) xenotime in xenotime in the the Mahnomen MahnomenFormation Formationdrillcore drillcorerevealed revealedages agesofof—1870 -1 870Ma Maand and—1770 -1 770Ma Ma(1760-1 (1760-1790 790 Ma), (ii) One One large large xenotime xenotime overgrowth overgrowth from the Dead Dead River River Basin Basin drillcore, drillcore, gave gave an anage ageofof—2600 -2600 Ma, (iii) xenotime xenotime contained contained within the the Slate Slate River River Hill Hilloutcrop outcropyielded yielded ages agesofof—2500 -2500 Ma, Ma, (iv) (iv) the the samples from from Big BigEric's Eric'sCrossing Crossingcontained containedxenotime xenotimeshowing showingages agesofof—2550 -2550Ma Maand and—1750 -1 750 Ma. Ma.The The sample from the Pokegema Quartzite in the West Mesabi Mesabi Range, Range, contained contained xenotime with an an age age of of -2300 -2300 Ma Maand and—1770 -1 770 Ma. Ma. Xenotime yielding yielding ages ages of of ca 2500 Ma Xenotime Ma or or older older may may be be from from recycled recycleddetrital detrital (magmatic) (magmatic) grains. grains. The younger age age of of —1770 -1770 Ma (1760-1790 Ma), occurs in xenotime from from widespread widespread localities localitiesacross across thermal event event across across the region. The age the Lake Superior Superior Region and may reflect reflect an epigenetic epigenetic thermal anorogenic magmatism, appears to correlate correlate with with an anepisode episodeat at—1760 -1760 Ma of anorogenic magmatism, pluton pluton emplacement emplacementand and It gneissic doming gneissic doming recorded recorded throughout throughout Wisconsin, Wisconsin, northern northern Michigan Michigan and and central central Minnesota. Minnesota. It postdates the Penoken Orogeny and involved partial melting of of crustal rocks as a result result of of continentcontinentcontinent-arc collision to the south of the region (Sims, 1996). This event is approximately continent or continent-arc coeval with the development development of the the Central Central Plains Plains Orogen Orogen (1800-1630 (1800-1630 Ma) Ma) to to the the south south and and may may be be aa consequence of the accretion of this terrane to the North American continent (Sims, 1996). 1996). This study highlights highlights the sensitivity sensitivity of of in-situ in-situxenotime xenotime geochronology geochronology to to identifying identifyingcryptic cryptic fluid fluid flow flow events within basins. This study will be be ongoing in 2003-2004. Fairbairn Fairbairn H.W., Hurley, Hurley,P.M., P.M., Card, Card, K.D. K.D. and and Knight, Knight, C.J., C.J., 1969, 1969,Correlation Correlationand and radiometric radiometricages agesof of Nipissing Nipissing Diabase Diabase and and Huronion Huronion metasediments metasedimentswith with Proterozoic Proterozoicevents events in in Ontario: Ontario: Canadian CanadianJournal Journalof of Earth Earth Sciences, Sciences, v. v. 6, 6, P. p. 489-497. 489-497. Sims, P.K., 1996, in Sims Sims P.K. P.K. and and Carter, Carter, L.M.H., L.M.H., eds., eds., Archean Archean and and 1996, Early Early Proterozoic Proterozoic Penokean Penokean Orogeny, Orogeny, in Late Proterozoic Geology Geology of the Lake Lake Superior Superior Region, Region, U.S.A., 1993: U.S. Geological Geological Survey Professional Professional Paper Paper 1556, 1556, p. p. 28-60. 28-60. Sims, P.K., et al., 1993, in Reed, Reed,J.C., J.C., Jr., Jr., and and 1993, The Lake Lake Superior Superior Region Regionand and Trans-Hudson Trans-HudsonOrogen, Orogen, in others, eds., Precambrian:Conterminous Precambrian:ConterminousU.S.: U.S.: Boulder, Boulder, Colorado, Colorado, Geological GeologicalSociety Society of America, the Geology of North America, v. C-2, p. p. 11-120. 11-120. Vallini, D., Rasmussen, Rasmussen, B., B., Krapez, Krapez, B., B., Fletcher, Fletcher,l.R., I.R., and and McNaughton, McNaughton, N.J., N.J., 2002, 2002, Obtaining Obtainingdiagenetic diagenetic ages from metamorphosed metamorphosedsedimentary sedimentary rocks: rocks: U-Pb U-Pbdating dating of of unusually unusuallycoarse coarse xenotime xenotimecement cementinin phosphatic phosphatic sandstone: sandstone: Geology, Geology, v. 30, 30, p. p. 1083-1086. 1083-1086. 80 �EVALUATION OF INITIAL MAGMA COMPOSITIONS FOR THE BALD EAGLE INTRUSION AND ASSOCIATED ROCKS VISLOVA, Tatiana, Department of Geology and Geophysics, University of Minnesota The funnel-shaped concentrically-zoned Bald Eagle Intrusion in the Duluth Complex is characterized by very restricted mineral compositions, and consists of only two units: an olivine—plagioclase cumulate and an olivine—plagioclase-clinopyroxene cumulate (Weiblen, 1965; Weiblen and Morey, 1980). In terms of differentiated units expected in a typical layered intrusion, the Bald Eagle Intrusion appears to be petrologically incomplete. This has raised the question whether the four-phase (olivineplagioclase-clinopyroxene-oxide) cumulates, assigned to the Greenwood Lake Intrusion (Miller et al., 2002), and granophyre found to the south of the Bald Eagle Intrusion are genetically related to the Bald Eagle Intrusion (Weiblen and Morey, 1980). New petrographic studies and microprobe analyses (Vislova, 2003) make it possible to evaluate parent magma compositions for the Bald Eagle Intrusion, and quantitatively assess possible petrogenetic relationships between the Bald Eagle Intrusion and spatially associated rocks. Computer programs (MELTS, Ghiorso and Sack, 1995; and COMAGMAT, Ariskin et al., 1993) were used to investigate these questions. A primitive North Shore Volcanic Group olivine tholeiite (P-melt) was used as an initial magma composition (Miller and Ripley, 1996). Equilibrium crystallization of P-melt, calculated by MELTS at 1 atm total pressure and oxygen fugacity near or below the quartz-fayalite-magnetite buffer, reproduces the crystallization order and mineral assemblages observed in the Bald Eagle Intrusion. The calculated composition of the first clinopyroxene (mg 81) equals the one observed, however calculated compositions of the first plagioclase and olivine are much higher than those observed. This could be ascribed to the dynamics of crystal-melt segregation in a flowing magma system. Until the crystals suspended in magma grow large enough they might be carried away, erupted, and found as phenocrysts in lavas. At -7 % melt remaining MELTS reproduces the most evolved mineral compositions in the Bald Eagle Intrusion (Fig. 1). This suggests that the Bald Eagle Intrusion might be a complete crystallization sequence with a few percent remaining melt. It leaves unanswered the question of the origin of four-phase cumulate and granophyre. Modeling shows that a more evolved high Ti and high Fe melt (D-melt) is required for crystallization of the evolved units in the Greenwood Lake Intrusion (Fig. 1). This melt can be produced by fractional crystallization of P-melt in an intermediate magma chamber at 2-3 kbar total pressure. Equilibrium crystallization of D-melt at 1 atm reproduces the crystallization order, the appearance of Fe-Ti oxides, and the compositions of most of the units associated with the Bald Eagle Intrusion (Fig. 1). However, the most evolved rocks in the Greenwood Lake Intrusion (ferrogabbro with Fo < 50) and granophyre were not reproduced by equilibrium crystallization. These units might require fractional crystallization or assimilation. 81 �85 . ,. *A G. 0 0 x I- 0. 0 .E 75 0 U+ A tL±&& A , --. 0) 6565 1 # E E 0 o <60 60 BaId Eagle Intrusion AGreenwood LakeIntrusion Intrusion Greenwood Lake A • Calculated 4 Calculated from P-melt P-melt Calculated from D-melt Lsf4. 55 55 30 30 40 50 50 60 Atom % % Mg/ (Mg+Fe) in olivine (Mg+Fe) olivine 70 70 80 80 Mg/(Mg+Fe) variations variations in in coexisting olivine and clinopyroxene. Fig. Fig. 1. Mg/(Mg+Fe) References: References: Ariskin, A.A., Frenkel, M.Y., Barmina, Barmina, G. S., and Nielsen, R. L., 1993, 1993, COMAGMAT; COMAGMAT; aa FORTRAN program to model magma differentiation processes, Computers Computers & Geosciences, Geosciences, 19 19 (8), p. 1155-1170. 1155-1170. IV, A Ghiorso, M. S., and Sack, R.O., 1995, 1995, Chemical Chemical mass transfer in magmatic processes; processes; IV, revised and internally internally consistent consistent thermodynamic model for the interpolation interpolation and and extrapolation extrapolation of liquid-solid equilibria equilibria in magmatic systems at elevated elevated temperatures temperatures and and pressures, pressures, Contributions Contributions to Mineralogy Mineralogy and and Petrology, Petrology, 119 119 (2-3), (2-3), p. 197-212. 197-212. Miller, J.D., Jr., Green Green J.C., J.C., Severson, Severson, M.J., M.J., Chandler, Chandler,V.W., V.W., Hauck, Hauck, S.A., S.A., Peterson, Peterson, D.M., D.M., and and mineral potential potential of of the the Duluth Duluth Complex Complex and and related related rocks rocks of of Wahl, T.E., 2001, Geology and mineral Report of of Investigations Investigations 58,207 58. 207 pp. pp. ++ northeastern Minnesota: Minnesota Geological Survey Report compact disc in back pocket, 2002. 2002. Miller J.D., Jr. and andE.M. E.M. Ripley, Ripley,1996, 1996,Layered Layeredintrusions intrusionsof of the the Duluth Duluth Complex, Complex, Minnesota, In: Cawthorn Cawthorn R.G. R.G. (ed.) (ed.)Layered LayeredIntrusions, Intrusions,531 53 1 pp. USA. In: Vislova, T., 2003, Petrology Petrology of the Bald Eagle Intrusion Intrusion and associated associated rocks rocks and and its relevanceto to its relevance crystallization in dynamic Midcontinent Rift, Ph.D. Dissertation, Dissertation, crystallization dynamic magma chambers in the Midcontinent University of Minnesota, 226 pp. Weiblen, Weiblen, P.W. and and Morey, Morey, G. G. B., B., 1980, 1980,A A summary summary of of the the stratigraphy, stratigraphy,petrology, petrology,and and structure structure of the Duluth Complex. In: In: frying, Irving, A. A. J., J., and and Dungan, Dungan, M. A. A.(ed.), (ed.),1980, 1980,The TheJackson Jacksonvolume, volume, American Journal of Science, Science, Vol. 280-A, Part 1, 1, p. 88-133. 88-133. Weiblen, P.W., 1965, 1965, A funnel-shaped, funnel-shaped, gabbro-troctolite gabbro-troctolite intrusion intrusion in in the the Duluth Duluth Complex, Complex,Lake Lake County, Minnesota, Ph.D. Dissertation, University of Minnesota, 161 pp. University of Minnesota, 161 82 �A Hydrothermal Hydrothermal Component Component of Iron Iron Formations-A Formations-A Marquette Marquette Range Range Perspective Perspective Waggoner, T.D., 141 141 Chippewa, Chippewa,Negaunee, Negaunee, MI MI49866 49866 Waggoner, T.D., The origin of Lake Superior Superior banded iron formations (BIF) has been a contentious contentious issue for at least a century century and aa half. half. Concepts of origin origin include include weathering, weathering, volcanic volcanic and and organic activity activity whereby whereby ions ions are are carried in and organic and precipitated precipitated from from solution. solution. Clear definition of of the source, definition source, mode mode of of transport transport or ordepositional depositional mechanisms mechanisms is is generally generally lacking. This Thispaper paperwill willaddress addressthe thestrong strongevidence evidencefor for aa hydrothermal hydrothermal source source for for "hard "hard ores" found in the upper upper parts parts of of the the Negaunee Negaunee Iron Iron Formation Formation (NIF) (NIF) and and by extension extension aa possible source for the precursor hematite in the BIF portion. Range was formed formed in a portion. The Range tectonically active area believed to be an extensional extensional rifting rifting environment environment not not unlike unlike those those REE) and and some some VHMS deposits deposits found in Fe-Oxide (Cu, U, Au, REE) The Marquette Range portion of of the the Lake Lake Superior SuperiorIron Iron District Districtdisplays displaysmany many features features BJFs found around the world and, thus, making it an similar to other large Lake Superior BIFs excellent study subject subject for for the source excellent study source and and role role played played by by igneous igneous and andsedimentary sedimentary processes. The Goodrich units units exhibit exhibit BIF, BIF, soft soft supergene supergene enriched TheNegaunee Negaunee and and basal basal Goodrich concentrations and "hard concentrations and "hard ores" ores" as as massive massive bodies, bodies, banded banded jaspilites jaspilites and and detrital detrital conglomerates. conglomerates. "Hard "Hard ores" are are generally generally dense silver gray gray to to black black massive massive metallic metallic magnetite or schistose metallic hematite associated with jaspilite jaspilite and contain in excess of 60% iron. of the the origin iron. Discussion Discussion of origin of of the the "hard "hard Ores" Ores" on on the the Marquette Marquette Range Range has has revolved revolved around around supergene supergene enrichment enrichment prior prior to metamorphism metamorphism or or hydrothermal hydrothermal enrichment enrichment associated associated with the the Penokian Penoluan Orogeny. Orogeny. of the field geology geology do do not not support support with witheither eitherof ofthese thesepositions. positions. First, the Many features of Goodrich conglomerate conglomerate show show random random cobble and pebbles of jasper hematite hematite in in the the basal basal Goodrich orientation of the 'schistose' orientation of 'schistose' hematite hematite indicating indicating the schistose schistose nature nature of of the the hematite hematite existed prior to emplacement and not a result of metamorphism. In In addition addition many many of of the the rocks associated with the "hard "hard ores" ores" exhibit exhibithydrothermal hydrothermal minerals minerals including including sericite, sericite, chlorite, chloritoid, chloritoid, high aluminous aluminous silicates, silicates,garnet, garnet, hematite, hematite, magnetite magnetiteand and tourmaline. tourmaline. The lower NIF exhibit exhibit lower Proterozoic Proterozoic Chocolay Chocolay and Menominee Menominee sediments sediments below below the NW multiple examples of of high-grade hematite that that can be interpreted multiple examples high-grade hematite interpreted as vents. vents. Specular, Specular, microplaty and bytroidal bytroidal hematite hematite are are fairly fairly common commoninin many many outcrop outcropareas. areas. Some microplaty and Some of of these have been described described previously in literature while others have not. not. All the sites sites were lgth century and most exhibit exhibit were subject to exploration exploration for iron ore during during the late late 19th shallow shafts. The Themajor majorcomponents componentsare arechert, chert,jasper jasper and andvein veinquartz quartzalong alongwith withcoarse coarse specular, microplaty and and bytrioidal hematite contained contained in in breccia zones specula, microplaty bytrioidal hematite zones that that exhibit exhibit episodic reworking. reworking. There There are are alterations alterations to to the the host hostrock rockas assome someoccurrences occurrencesexhibit exhibit chlorite, silica, silica, k-spar k-spar and aluminous aluminous silicates. silicates. A large area in sections sections 21, 21, 22, 22, and 23, 47-26 contain contain multiple multiple enriched enriched hematite hematite sites sites that form two northwest trending trending breccia zones zones adjacent adjacent to to northwesterly northwesterlytrending trending faults. faults. In addition there isis a In addition to the the silica silicaflooding, flooding, brecciation brecciation and hematite hematite concentration concentration there significant area of of andalusite and chloritoid adjacent to the significant area andalusite cordierite cordierite and chloritoid adjacent the eastern eastern linear linear 83 �breccia zone in section 23. These Theseminerals minerals are are present present in in aa much much broader broader lower lower regional chlorite zone of metamorphism and most likely are a result of the hydrothermal event that impacted impacted the three square square miles referenced above. above. A conglomerate in Sec. 22 and 23, 47-26 has been previously described as "unusual" and 23,47-26 is sandwiched sandwiched between lower lower Chocolay Chocolay argillite argillite units. units. Clasts causing dimpling in the the underlying argillite were described as rafted clasts from a glacial glacial interlude. interlude. It is unlikely unlikely reef growth during the the same period period of of that a glacial event coincided with significant algal reef coarse, tightly tightly packed packed and shows no time. The "unusual" "unusual" conglomerate conglomerate is extremely extremely coarse, sedimentary sedimentary features. features. In addition significant rinds of k-spar have formed formed on on the the granite granite gneiss cobbles. The Thecobbles cobblesand andmatrix matrix contain contain euhedral euhedral magnetite, magnetite, martite and specular specula hematite suggesting this area was tectonically tectonically active and may well have been an an active active vent area over a period vent period starting starting at at the the earliest earliest extensional extensional period period and continued to be resembles some some of of the breccias active beyond beyond the Ajibik Ajibik time. time. The conglomerate conglomerate resembles breccias at at Olympic Dam and could well be a hydrothermal breccia. analysis of of the hematite match quite closely with both REE chondrite normalized analysis hematite vents match "hard ores" the hematite, hematite, magnetite magnetite and and combination combination hematite/magnetite hematitelmagnetite "hard ores" found throughout the the Range. Range. Recent throughout Recent work work on onthe theBrockman Brockrnan microplaty microplaty hematite hematite confirms confirms aa hydrothermal origin due to the recognition of surrounding surrounding alteration alteration to the the host. host. Initially the the vent vent areas areas were were studied studied in in relation relation to to the the "hard "hard Ores" Ores" but but the the fact that that the Initially vents are all hematite suggests they could be the source source for the the precursor precursor hematite hematite seed cores have have cores that Han identified in low low metamorphic metamorphic grade BIF units. These seed cores been been identified identified in in the theNegaunee, Negaunee,Biwabik, Biwabik, Brockman, Brockrnan, Sokoman, Sokoman, Temiscamie Temiscamie and and Kuruman Iron Formations. Formations. It is quite NIF time time quite plausible plausible that that the the extensional extensional phase phase of of rifting rifting ceased ceased near the end end of of NIP and aa reverse and reverse compressional compressional event event started started causing causing faulting faulting that that produced produced erosional erosional material for the basal Goodrich conglomerate. Reference: Reference: Han, T.H., T.H., 1988, 1988, Origin Origin of Magnetite Magnetite in in Precambrian Precambrian Iron Iron Formations Formations of of Low Low Metamorphic Grade, Grade, Proceedings Proceedings of of the Seventh Metamorphic Seventh Quadrennial Quadrennial IAGOD IAGOD Symposium, Symposium, p. 641-656. 641-656. 84 �High-Resolution Multibeam Bathymetry Bathymetry in Lake Superior. Superior. N. N. JJ.. Wattrus Large Lakes 558122 Lakes Observatory, Observatory,University Universityof of Minnesota, Minnesota,Duluth, Duluth,MN MN 5581 Like all large large lakes, lakes, the composition and and shape shapeof of the the lake lakefloor floor of of Lake Superior reflects the processes that that shape shape its itsformation formation today as a s well as in the past. Maps of of the the lake floor floor made made with with traditional traditional echosounders echosounders lack the the resolution resolution to to permit scientists scientists to to read read the the subtle subtle"fingerprint" "fingerprint" of these processes preserved in the lake-floor. The The advent advent of of modem, modem, high-resolution high-resolution multibeam sonar sonar has has revolutionized the the mapping mapping of of the the sea-floor. sea-floor. In a traditional echosounder, the the depth depth to to the the lake-floor lake-floor below the ship ship is measured measured by by timing timing how how long long it takes takes for for an a n acoustic acoustic ping ping to to travel travel to to the lake-floor and back to the ship. lake-floor and ship. longer the the delay, the the deeper the The longer lake floor is. This type of of surveying provides high-resolution bathymetric information along the trackline followed by the the survey boat. Nothing is followed by known about the the lake lake floor floor either side. side. High-resolution multibeams use a fan High-resolution multibeams of acoustic beams 100)to beams (over (over 100) measure the the shape shapeof of the the lake lake floor floor along a "swath". By sailing sailing aa series of "swath". By overlapping swaths, swaths, it is is possible possible to to achieve complete coverage coverage of the lake lake floor aatt high resolution. Backscatter information Backscatter information collected collected with the bathymetric data can be used to create psuedo-sidescan psuedo-sidescan images images of of the These can can be be used used to map lakefloor. These spatial variations in in the the composition composition of of the lake floor. lake floor. Examples, drawn from the the catalog of of multibeam multibeam surveys surveys conducted conducted by the Large Lakes Observatory, are presented. These illustrate the potential of Large Lakes Observatory, are presented. These illustrate of this this for mapping mapping the the subtle subtle signal processes technology for signal of past geologic processes superimposed superimposed on on the the lake lakefloor. floor. 85 � Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology: Proceedings, 2003 Description An account of the resource Institute on Lake Superior Geology. Iron Mountain, Michigan. May 7-11, 2003. Creator An entity primarily responsible for making the resource Institute on Lake Superior Geology Date A point or period of time associated with an event in the lifecycle of the resource 2003 Format The file format, physical medium, or dimensions of the resource PDF Language A language of the resource English https://digitalcollections.lakeheadu.ca/files/original/fbb8f910e05cdd14c47cd729c9a998c9.pdf 7d8d30bac0d37353bfa2cdd70c285dea PDF Text Text 50TH ANNUAL INSTITUTE ON LAKE SUPERIOR GEOLOGY DULUTH, MINNESOTA, MAY 4-9, 2004 �INSTITUTE ON LAKE SUPERIOR GEOLOGY 50TH ANNUAL MEETING MAY 4-9, 2004 DULUTH, MINNESOTA HOSTED BY: STEVEN A. HAUCK AND MARK J. SEVERSON Co-Chairs NATURAL RESOURCES RESEARCH INSTITUTE, UNIVERSITY OF MINNESOTA DULUTH WITH ASSISTANCE FROM THE NRRI ECONOMIC GEOLOGY GROUP, MINNESOTA GEOLOGICAL SURVEY, AND THE DEPARTMENT OF GEOLOGICAL SCIENCES, UMD Volume 50 Part 1 – Proceedings and Abstracts Compiled and edited by Steven A. Hauck, Dean Peterson, and Julie Oreskovich Cover Photos: Upper Left – Soudan Iron Formation, 25th Level West, Soudan Underground Mine State Park, Soudan, MN; Center – ILSG 2003 Quinnesec Mine, WI, Menominee Iron District Field Trip; Lower Left – Natural Ore Auburn Mine, Eveleth MN, Looking Northwest Along the Auburn Fault. 2 �50TH INSTITUTE ON LAKE SUPERIOR GEOLOGY VOLUME 50 CONSISTS OF: PART 1: PROGRAM AND ABSTRACTS PART 2: FIELD TRIP GUIDEBOOK TRIP 1: Volcanic Stratigraphy, Hydrothermal Alteration, and VMS Potential of the Lower Ely Greenstone, Fivemile Lake to Sixmile Lake area. TRIP 2: Geologic Highlights of New Mapping in the Southwestern Sequence of the North Shore Volcanic Group and Beaver Bay Complex. TRIP 3: Late Wisconsinan Superior-lobe Deposits in the Lake Superior Basin Northeast of Duluth. TRIP 4: Geology of the Eastern Mesabi Iron Range, Northeastern Minnesota. TRIP 5: Classic Outcrops of Northeastern Minnesota. TRIP 6: Glacial and Postglacial Landscape Evolution in the Glacial Lake Aitkin and Upham Basin, Northern Minnesota. TRIP 7: Economic Geology of Archean Gold Occurrences in the Vermilion District, Northeast of Soudan, Minnesota. TRIP 8: Geology of the Western Contact of the Duluth Complex, Partridge River and South Kawishiwi Intrusions, Northeastern Minnesota. Reference to material in Part 1 should follow the example below: Holm, D.K., Van Schmus, R.W., and Schneider, D.A., 2004, The Influence of Radiometric Dating for Unraveling the Precambrian Geologic History of the Lake Superior Region [abstract]; Institute on Lake Superior Geology Proceedings, 50th Annual Meeting, Duluth, MN, v. 50, part 1, p. 80-84. Published by the 50th Institute on Lake Superior Geology and distributed by the ILSG Secretary-Treasurer: Peter Hollings Lakehead University Department of Geology Thunder Bay, ON P7B 5E1 CANADA peter.hollings@lakeheadu.ca ILSG website: http://www.lakesuperiorgeology.org ISSN 1042-9964 3 �CONTENTS PROCEEDINGS VOLUME 50 PART 1—PROGRAM AND ABSTRACTS Institutes on Lake Superior Geology, 1955-2004 5 Constitution of the Institute on Lake Superior Geology 7 By-Laws of the Institute on Lake Superior Geology 8 Membership Criteria 9 Goldich Medal Guidelines 10 Goldich Medal Committee 11 Past Goldich Medalists 12 Citation for 2004 Goldich Medal Recipient 13 Eisenbrey Student Travel Awards 14 Student Travel Award Application Form 14 Student Paper Awards 16 Student Paper Awards Committee 16 Session Chairs 16 Board of Directors 17 Local Committees 17 Banquet Speaker 18 Report of the Chair of the 49th Annual Meeting 19 Program 25 List of Contributors 26 Abstracts 37 4 �INSTITUTES ON LAKE SUPERIOR GEOLOGY # YEAR PLACE CHAIRS 1 1955 Minneapolis, Minnesota C.E. Dutton 2 1956 Houghton, Michigan A.K. Snelgrove 3 1957 East Lansing, Michigan B.T. Sandefur 4 1958 Duluth, Minnesota R.W. Marsden 5 1959 Minneapolis, Minnesota G.M. Schwartz & C. Craddock 6 1960 Madison, Wisconsin E.N. Cameron 7 1961 Port Arthur, Ontario E.G. Pye 8 1962 Houghton, Michigan A.K. Snelgrove 9 1963 Duluth, Minnesota H. Lepp 10 1964 Ishpeming, Michigan A.T. Broderick 11 1965 St. Paul, Minnesota P.K. Sims & R.K. Hogberg 12 1966 Sault Ste. Marie, Michigan R.W. White 13 1967 East Lansing, Michigan W.J. Hinze 14 1968 Superior, Wisconsin A.B. Dickas 15 1969 Oshkosh, Wisconsin G.L. LaBerge 16 1970 Thunder Bay, Ontario M.W. Bartley & E. Mercy 17 1971 Duluth, Minnesota D.M. Davidson 18 1972 Houghton, Michigan J. Kalliokoski 19 1973 Madison, Wisconsin M.E. Ostrom 20 1974 Sault Ste. Marie, Ontario P.E. Giblin 21 1975 Marquette, Michigan J.D. Hughes 22 1976 St. Paul, Minnesota M. Walton 23 1977 Thunder Bay, Ontario M.M. Kehlenbeck 24 1978 Milwaukee, Wisconsin G. Mursky 25 1979 Duluth, Minnesota D.M. Davidson 26 1980 Eau Claire, Wisconsin P.E. Myers 27 1981 East Lansing, Michigan W.C. Cambray 28 1982 International Falls, Minnesota D.L. Southwick 5 �29 1983 Houghton, Michigan T.J. Bornhorst 30 1984 Wausau, Wisconsin G.L. LaBerge 31 1985 Kenora, Ontario C.E. Blackburn 32 1986 Wisconsin Rapids, Wisconsin J.K. Greenberg 33 1987 Wawa, Ontario E.D. Frey & R.P. Sage 34 1988 Marquette, Michigan J. S. Klasner 35 1989 Duluth, Minnesota J.C. Green 36 1990 Thunder Bay, Ontario M.M. Kehlenbeck 37 1991 Eau Claire, Wisconsin P.E. Myers 38 1992 Hurley, Wisconsin A.B. Dickas 39 1993 Eveleth, Minnesota D.L. Southwick 40 1994 Houghton, Michigan T.J. Bornhorst 41 1995 Marathon, Ontario M.C. Smyk 42 1996 Cable, Wisconsin L.G. Woodruff 43 1997 Sudbury, Ontario R.P. Sage & W. Meyer 44 1998 Minneapolis, Minnesota J.D. Miller & M.A. Jirsa 45 1999 Marquette, Michigan T.J. Bornhorst & R.S. Regis 46 2000 Thunder Bay, Ontario S.A. Kissin & P. Fralick 47 2001 Madison, Wisconsin M.G. Mudrey, Jr. & B.A. Brown 48 2002 Kenora, Ontario P. Hinz & R.C. Beard 49 2003 Iron Mountain, Michigan L.G. Woodruff & W.F. Cannon 50 2004 Duluth, Minnesota S.A. Hauck & M.J. Severson 6 �CONSTITUTION OF THE INSTITUTE ON LAKE SUPERIOR GEOLOGY (Last amended by the Board—May 8, 1997) Article I Article II Article III Name The name of the organization shall be the "Institute on Lake Superior Geology". Objectives The objectives of this organization are: A. To provide a means whereby geologists in the Great Lakes region may exchange ideas and scientific data. B. To promote better understanding of the geology of the Lake Superior region. C. To plan and conduct geological field trips. Status No part of the income of the organization shall insure to the benefit of any member or individual. In the event of dissolution, the assets of the organization shall be distributed to _________ (some tax free organization). (To avoid Federal and State income taxes, the organization should be not only "scientific" or "educational, but also "non-profit") Article IV Article V Article VI Article VII Article VIII Minn. Stat. Anno. 290.01, subd. 4 Minn. Stat. Anno. 290.05(9) 1954 Internal Revenue Code s.501(c)(3) Membership The membership of the organization shall consist of persons who have registered for an annual meeting within the past three years, and those who indicate interest in being a member according to guidelines approved by the Board of Directors. Meetings The organization shall meet once a year. The place and exact date of each meeting will be designated by the Board of Directors. Directors The Board of Directors shall consist of the Chair, Secretary-Treasurer, and the last three past Chairs; but if the board should at any time consist of fewer than five persons, by reason of unwillingness or inability of any of the above persons to serve as directors, the vacancies on the board may be filled by the Chair so as to bring the membership of the board to five members. Officers The officers of this organization shall be a Chair and Secretary-Treasurer. A. The Chair shall be elected each year by the Board of Directors, who shall give due consideration to the wishes of any group that may be promoting the next annual meeting. His/her term of office as Chair will terminate at the close of the annual meeting over which he/she presides, or when his/her successor shall have been appointed. He/she will then serve for a period of three years as a member of the Board of Directors. B. The Secretary-Treasurer shall be elected at the annual meeting. His/her term of office shall be four years, or until his/her successor shall have been appointed. Amendments This constitution may be amended by a majority vote (majority of those voting) of the membership of the organization. 7 �BY-LAWS OF THE INSTITUTE ON LAKE SUPERIOR GEOLOGY I. Duties of the Officers and Directors A. It shall be the duty of the Annual Chairman to: 1. Preside at the annual meeting. 2. Appoint all committees needed for the organization of the annual meeting. 3. Assume complete responsibility for the organization and financing of the annual meeting over which he/she presides. B. It shall be the duty of the Secretary-Treasurer to: 1. Keep accurate attendance records of all annual meetings. 2. Keep accurate records of all meetings of, and correspondence between, the Board of Directors. 3. Hold all funds that may accrue as profits from annual meetings or field trips and to make these funds available for the organization and operation of future meetings as required. C. It shall be the duty of the Board of Directors to plan locations of annual meetings and to advise on the organization and financing of all meetings. II. Duties and Expenses A. Regular membership dues of $5.00 or less on an annual basis shall be assessed each member as determined by the Board of Directors.. B. Registration fees for the annual meetings shall be determined by the Chair in consultation with the Board of Directors. The registration fees can include expenses to cover operations outside of the annual meeting as determined by the Board of Directors. It is strongly recommended that registration fees be kept at a minimum to encourage attendance of students. III. Rules of Order The rules contained in Robert's Rules of Order shall govern this organization in all cases to which they are applicable. IV. Amendments These by-laws may be amended by a majority vote (majority of those voting) of the membership of the organization; provided that such modifications shall not conflict with the constitution as presently adopted or subsequently amended. Last Amended – May, 1996 8 �MEMBERSHIP CRITERIA FOR THE INSTITUTE ON LAKE SUPERIOR GEOLOGY Approved May 8, 1997 A. Membership in the Institute on Lake Superior Geology requires either participation in Institute activities, or an indication on a regular basis of interest in the Institute. Those individuals registering for an annual meeting will remain as members for 4 years unless: 1) they indicate no further interest in the Institute by responding negatively to the statement on meeting circulars "Remove my name from the mailing list"; or 2) two successive mailings in different years are returned by the postal service as address unknown. B. Those individuals who have not registered for an annual meeting in the past 4 years must indicate an interest in the Institute by postal, electronic, or verbal correspondence with the Secretary-Treasurer at least once every two years. Such individuals will be removed from the membership if they indicate no further interest in the Institute or two successive mailing in different years are returned by the postal service as address unknown. C. The Secretary-Treasurer will maintain a list of current members. The list will include the date of the beginning of continuous membership, dates of returned mail, dates of last contact (expression of interest), and the date membership expires, barring a change of status initiated by the member. Those individuals who have become members of ILSG by Section B will have an expiration date listed at 2 years from the upcoming meeting. For example, a member who expresses interest in September of 1997 (the next annual meeting is May, 1998) will have an expiration date of May, 2000, unless the member contacts the Secretary-Treasurer or attends an annual meeting. D. "Member for Life" status is granted to individuals who have been (nearly) continuous participants of the ILSG meetings for 15 years, Goldich Medal recipients, or those who have served as meeting chairs. This status will be further maintained unless the individuals indicate no further interest in the Institute, or 4 mailings in different years are returned by the postal service as address unknown, or they are deceased. E. All members will be mailed the First Circular for the Annual Meeting and the ILSG Newsletter. The Chair of the annual meeting may opt to send the first circular to additional individuals. All returned mail should be reported to the Secretary-Treasurer. F. The Secretary-Treasurer can designate any individual who is on the ILSG membership list (mailing list) as of January 1, 1997 as a member for life based on participation in ILSG activities. G. Members are strongly encouraged to send address corrections to the Secretary-Treasurer to avoid unintentional lapse of membership. 9 �GOLDICH MEDAL GUIDELINES (Adopted by the Board of Directors, 1981; amended 1999) Preamble The Institute on Lake Superior Geology was born in 1955, as documented by the fact that the 27th annual meeting was held in 1981. The Institute's continuing objectives are to deal with those aspects of geology that are related geographically to Lake Superior; to encourage the discussion of subjects and sponsoring field trips that will bring together geologists from academia, government surveys, and industry; and to maintain an informal but highly effective mode of operation. During the course of its existence, the membership of the Institute (that is, those geologists who indicate an interest in the objectives of the ILSG by attending) has become aware of the fact that certain of their colleagues have made particularly noteworthy and meritorious contributions to the understanding of Lake Superior geology and mineral deposits. The first award was made by ILSG to Sam Goldich in 1979 for his many contributions to the geology of the region extending over about 50 years. Subsequent medallists and this year's recipient are listed in the table below. Award Guidelines 1) The medal shall be awarded annually by the ILSG Board of Directors to a geologist whose name is associated with a substantial interest in, and contribution to, the geology of the Lake Superior region. 2) The Board of Directors shall appoint the Goldich Medal Committee. The initial appointment will be of three members, one to serve for three years, one for two years, and one for one year. The member with the briefest incumbency shall be chair of the Nominating Committee. After the first year, the Board of Directors shall appoint at each spring meeting one new member who will serve for three years. In his/her third year this member shall be the chair. The Committee membership should reflect the main fields of interest and geographic distribution of ILSG membership. The out-going, senior member of the Board of Directors shall act as liaison between the Board and the Committee for a period of one year. 3) By the end of November, the Goldich Medal Committee shall make its recommendation to the Chair of the Board of Directors, who will then inform the Board of the nominee. 4) The Board of Directors normally will accept the nominee of the Committee, inform the medallist, and have one medal engraved appropriately for presentation at the next meeting of the Institute. 5) It is recommended that the Institute set aside annually from whatever sources, such funds as will be required to support the continuing costs of this award. Nominating Procedures 1) The deadline for nominations is November 1. The Goldich Medal Committee shall take nominations at any time. Committee members may themselves nominate candidates; however, Board members may not solicit for or support individual nominees. 2) Nominations must be in writing and supported by appropriate documentation such as letters of recommendation, lists of publications, curriculum vita's, and evidence of contributions to Lake Superior geology and to the Institute. 3) Nominations are not restricted to Institute attendees, but are open to anyone who has worked on and contributed to the understanding of Lake Superior geology. Selection Guidelines 1) Nominees are to be evaluated on the basis of their contributions to Lake Superior geology (sensu lato) including: 10 �a) b) c) d) e) importance of relevant publications; promotion of discovery and utilization of natural resources; contributions to understanding of the natural history and environment of the region; generation of new ideas and concepts; and contributions to the training and education of geoscientists and the public. 2) Nominees are to be evaluated on their contributions to the Institute as demonstrated by attendance at Institute meetings, presentation of talks and posters, and service on Institute boards, committees, and field trips. 3) The relative weights given to each of the foregoing criteria must remain flexible and at the discretion of the Committee members. 4) There are several points to be considered by the Goldich Medal Committee: a) An attempt should be made to maintain a balance of medal recipients from each of the three estates— industry, academia, and government. b) It must be noted that industry geoscientists are at a disadvantage in that much of their work in not published. 5) Lake Superior has two sides, one the U.S., and the other Canada. This is undoubtedly one of the Institute's great strengths and should be nurtured by equitable recognition of excellence in both countries. GOLDICH MEDAL COMMITTEE Serving through the meeting year shown in parentheses George Hudak (2006) University of Wisconsin, Oshkosh Ron Sage (2004) Ontario Geological Survey (retired) David Meineke (2005) Meriden Engineering, Hibbing, Minnesota Steve Kissin, as out-going senior member of Institute Board of Directors, is liaison between Goldich Medal Committee and the Board through the 2004 meeting 11 �2004 GOLDICH MEDAL RECIPIENT Paul W. Weiblen Department of Geology & Geophysics University of Minnesota, Minneapolis GOLDICH MEDALISTS 1979 Samuel S. Goldich 1992 William F. Cannon 1980 not awarded 1993 Donald W. Davis 1981 Carl E. Dutton, Jr. 1994 Cedric Iverson 1982 Ralph W. Marsden 1995 Gene LaBerge 1983 Burton Boyum 1996 David L. Southwick 1984 Richard W. Ojakangas 1997 Ronald P. Sage 1985 Paul K. Sims 1998 Zell Peterman 1986 G.B. Morey 1999 Tsu-Ming Han 1987 Henry H. Halls 2000 John C. Green 1988 Walter S. White 2001 John S. Klasner 1989 Jorma Kalliokoski 2002 Ernest K. Lehmann 1990 Kenneth C. Card 2003 Klaus J. Schultz 1991 William Hinze 2004 Paul W. Weiblen 12 �Citation Paul W. Weiblen 2004 Goldich Medal Recipient Paul W. Weiblen, or P.W., has been a friend and professional colleague for more than 40 years. We have worked together on more projects than I can remember since P.K. Sims selected us to help implement his programs at the Minnesota Geological Survey in the early 1960s. Therefore, it is my distinct honor and privilege to serve as P.W.’s citationist for the 2004 Goldich Medal. P.W. was born and raised in Miller, South Dakota, and after graduation from high school in 1945, he entered the U.S. Army. In the summer of 1945, World War II had ended in Europe, but we were still in combat with Japan. Luckily, before P.W. finished training, the war ended, and he was sent to Germany. After military service P.W. returned to college and earned a B.A. degree at Wartburg College in Waverly, Iowa (1950), and an M.A. in History at the University of Minnesota (1952). P.W. came into geology in sort of a roundabout way. Apparently, he was in Istanbul working as an agent for American Express when he met a geologist who exposed him to the wonders of the profession. Consequently, he returned to the University of Minnesota in 1959, received an M.S. degree (1962) and Ph.D. degree (1965). He stayed at the University in the Geology and Geophysics Department as an Assistant Professor (1965), Associate Professor (1969), Professor (1980), and Professor Emeritus (1997). He was hired specifically to organize and supervise the Department’s Electron Microprobe Laboratory (1965-1980). In the 1960s, electron microprobes were at the cutting edge of modern research, and this facility was one of the first in the country. Along the way he served as Curator of the petrology collection (1970-1997) and supervisor of the scanning electron microscope facility (1970-1997). He took a year off from the University to work at NASA Headquarters in Washington, D.C., and served as Director of the University’s Space Science Center (1985-1990). As an academic, P.W. served on an array of academic, professional, and service committees. That service included the Board of Directors of the Campus Club (1994-1996) and its President (1996-1997). He regularly taught courses in physical geology, igneous and metamorphic petrology, optical mineralogy/electron microprobe techniques, and numerous seminars covering all kinds of geologic topics. He served as mentor for 10 Ph.D. theses—by students including M.G. Mudrey, Jr., K.J. Schulz, R.W. Copper, R. Bauer, W. Day, J.D. Miller, Jr., S.W. Nicholson, and B. Saini-Eidukat—and 13 master’s theses. P.W. was first and foremost an igneous petrologist with eclectic interests, and he could generate ideas faster than anyone I know, which he was always willing to share. Much of the research that I have received credit over the years started out in P.W.’s brain as a throwaway. Unlike many research geologists today who focus on one narrow topic, P.W. concentrated on five separate research topics over much of his career. One subject included activities that focused primarily on the petrogenesis of the Midcontinent Rift System, especially the Duluth Complex. A second subject focused on the origin of copper- and nickel-sulfide mineralization in the Duluth Complex, especially as it relates to metal 13 �recovery from these possible ores. A third topic revolved around the origin of Archean greenstones in northern Minnesota and the high-grade gneisses in the Minnesota River Valley. A fourth subject, which he researched with Ed Roedder of the U.S. Geological Survey, focused on petrologic and geochemical attributes of melt inclusions in lunar samples obtained on various Apollo missions. Lastly, P.W. has been active in research to improve quantitative chemical analyses using electron beam techniques. Lately P.W. has delved into high-voltage electrical pulse methods for disaggregating rocks to produce clean mineral separates. All of these activities produced well over 100 publications including many presented here at the Institute. All are marked by the careful use of data, acquired both in the field and in the laboratory, and a strong intellectual component. Some of his contributions have been controversial, but they have always made us think. That thinking has led us to a better understanding of geologic processes, and for us in the Institute, a better understanding of early earth history in the Lake Superior region. Before I close, I would like to say a few words about P.W. the man. You never really know a person until you have to live with them in a tent camp after five days of rain. P.W. was always an easy-going, personable, considerate guy who was a pleasure to be around. I would go into the bush with him any day. It is my distinct honor to present to the Institute, Paul W. Weiblen as its 2004 recipient of the Goldich Medal for “Outstanding contributions to the geology of the Lake Superior region”. Submitted by G. B. Morey April 2004 14 �EISENBREY STUDENT TRAVEL AWARDS The 1986 Board of Directors established the ILSG Student Travel Awards to support student participation at the annual meeting of the Institute. The name "Eisenbrey" was added to the award in 1998 to honor Edward H. Eisenbrey (1926-1985) and utilize substantial contributions made to the 1996 Institute meeting in his name. "Ned" Eisenbrey is credited with discovery of significant volcanogenic massive sulfide deposits in Wisconsin, but his scope was much broader—he has been described as having unique talents as an ore finder, geologist, and teacher. These awards are intended to help defray some of the direct travel costs of attending Institute meetings, and include a waiver of registration fees, but exclude expenses for meals, lodging, and field trip registration. The annual Chair in consultation with the Secretary-Treasurer determines the number of awards and value. Recipients will be announced at the annual banquet. The annual Chair, who is responsible for the selection, will consider the following general criteria: 1) The applicants must have active resident (undergraduate or graduate) student status at the time of the annual meeting of the Institute, certified by the department head. 2) Students who are the senior author on either an oral or poster paper will be given favored consideration. 3) It is desirable for two or more students to jointly request travel assistance. 4) In general, priority will be given to those in the Institute region who are farthest away from the meeting location. 5) Each travel award request shall be made in writing to the annual Chair, and should explain need, student and author status, and other significant details. The form below is optional. Successful applicants will receive their awards during the meeting. I NSTITUTE ONLAKE SUPERIOR GEOLOGY Eisenbrey Student Travel Award Application Student Name: Date: Address: email: Department Head-Typed Department Head-Signature Educational Status: Are you the senior author of an oral or poster paper? YES Will any other students be traveling with you? NO Who? Statement of need (use additional page if necessary) Please return to: 15 �STUDENT PAPER AWARDS Each year, the Institute selects the best of the student presentations and honors presenters with a monetary award. Funding for the award is generated from registrations of the annual meeting. The Student Paper Committee is appointed by the annual meeting Chair in such a manner as to represent a broad range of professional and geologic expertise. Criteria for best student paper—last modified by the Board in 2001—follow: 1) The contribution must be demonstrably the work of the student. 2) The student must present the contribution in-person. 3) The Student Paper Committee shall decide how many awards to grant, and whether or not to give separate awards for poster vs. oral presentations. 4) In cases of multiple student authors, the award will be made to the senior author, or the award will be shared equally by all authors of the contribution. 5) The total amount of the awards is left to the discretion of the meeting Chair and SecretaryTreasurer, but typically is in the amount of about $500 US (increase approved by Board, 10/01). 6) The Secretary-Treasurer maintains, and will supply to the Committee, a form for the numerical ranking of presentations. This form was created and modified by Student Paper Committees over several years in an effort to reduce the difficulties that may arise from selection by raters of diverse background. The use of the form is not required, but is left to the discretion of the Committee. 7) The names of award recipients shall be included as part of the annual Chair's report that appears in the next volume of the Institute. Student papers will be noted on the Program. 2004 Student Paper Awards Committee Glenn Adams (Chair) – Doe Run Company, Viburnum, MO Angelique Magee – Ontario geological Survey, Thunder Bay, ONT Jim Small – Edward Kraemer and Sons, Burnsville, MN 2004 Session Chairs David Dahl – MN Dept. of Natural Res., Lands & Minerals Div., Hibbing, MN Jayne Englebert – MSA Professional Services, Baraboo, WI Sidney Hemming – Lamont-Doherty Earth Observatory, Palisade, NY Douglas Hunter – Wallbridge Mining Company, Lively, Ont. Jill Peterman – Wisconsin Department of Transportation, Superior, WI Mark Smyk – Ontario Geological Survey, Thunder Bay, Ont. Wanda Taylor – University of Nevada at Las Vegas, Las Vegas, NV Scott Wolter – American Petrographics Service Company, St. Paul, MN 16 �2003 BOARD OF DIRECTORS Board appointment continues through the close of the meeting year shown in parentheses, or until a successor is selected Steven A. Hauck Co-Chair 2004 Meeting (2007) Natural Resources Research Institute, University of Minnesota Duluth, Duluth, MN Laurel Woodruff (2006) U.S. Geological Survey, St. Paul, MN Peter Hinz (2005) Ontario Geological Survey, Kenora, ONT Michael G. Mudrey, Jr. (2004) Wisconsin Geological and Natural History Survey, Madison, WI Peter Hollings-Secretary-Treasurer (2006) Lakehead University, Thunder Bay, ONT 2004 LOCAL COMMITTEES General Co-Chairs Steven A. Hauck – Natural Resources Research Institute, Univ. Minn. Duluth Mark J. Severson – Natural Resources Research Institute, Univ. Minn. Duluth Program and Abstracts Editors Steven A. Hauck -- Natural Resources Research Institute, Univ. Minn. Duluth Dean Peterson -- Natural Resources Research Institute, Univ. Minn. Duluth Julie Oreskovich -- Natural Resources Research Institute, Univ. Minn. Duluth Field Trip Guidebook Editor Mark J. Severson – Natural Resources Research Institute, Univ. Minn. Duluth . Acting Local Committee, Duluth, MN Barbara Hauck – Duluth, MN John Heine – Natural Resources Research Institute, University of Minnesota Duluth Julie Heinz – Natural Resources Research Institute, University of Minnesota Duluth Charles Matsch - Department of Geological Sciences, University of Minnesota Duluth James D. Miller, Jr. – Minnesota Geological Survey, Duluth, MN Penny Morton – Department of Geological Sciences, University of Minnesota Duluth Julie Oreskovich - Natural Resources Research Institute, University of Minnesota Duluth Richard Patelke - Natural Resources Research Institute, University of Minnesota Duluth Dean M. Peterson - Natural Resources Research Institute, University of Minnesota Duluth Lawrence M. Zanko - Natural Resources Research Institute, University of Minnesota Duluth 17 �2004 BANQUET SPEAKER R. H. Dott, Jr. Department of Geology & Geophysics University of Wisconsin, Madison, WI 53706 THE VAN HISE ARMY AND OTHER PIONEERS OF LAKE SUPERIOR GEOLOGY 18 �Report of the Chair of the 49th Annual Meeting REPORT OF THE 49TH ANNUAL MEETING OF THE INSTITUTE ON LAKE SUPERIOR GEOLOGY Iron Mountain, Michigan The U.S. Geological Survey, with assistance from Michigan Technological University, hosted the 49th Annual Institute on Lake Superior Geology on May 7 – 11, 2003 at the Pine Mountain Resort in Iron Mountain, Michigan. The meeting consisted of two days of technical sessions with two pre- and two post-technical session field trips. John Gartner and Ted Bornhorst provided pre-meeting assistance. Sally LaBerge, Gretchen Klasner, and Suzanne Nicholson provided valuable logistical assistance on-site at Pine Mountain. Connie Dicken was media czar for the technical sessions, keeping all presentations on track while ignoring all needless advice. Pre-meeting registration was 104 students and professionals with an additional 61 on-site registrations, for a total of 165 registrants. Proceedings Volume 49 was published in two parts. Part I – Program and Abstracts, edited by Laurel Woodruff and Ted Bornhorst – the volume contains 45 published abstracts, for 33 oral and 12 poster presentations; and Part 2 – Field Trip Guidebook, edited by William Cannon, with assistance from Connie Dicken and Stacy Saari. The 49th meeting marked the first time in its history that an ILSG meeting was held in this part of Michigan. Field trips visited areas new to the ILSG, which may have resulted in the excellent subscription for all the trips. On Wednesday, May 7, Bill Cannon and staff from Cleveland Cliffs Mining lead a field trip to the Republic Mine, where the life cycle of the deposit, from ore genesis to mining and restoration, was covered. Three other field trips were developed to examine exposures along and on both sides of the Niagara suture zone, a major structural and geologic feature that marks the boundary between the Superior craton and the Wisconsin magmatic terranes. Klaus Schulz and Gene LaBerge lead the Wednesday field trip to the Wisconsin magmatic terrane on the southern side of the suture zone. On Saturday and Sunday, May 10 and 11, Bill Cannon, Gene LaBerge, John Klasner, and Dick Ojakangas were co-leaders for successive field trips through the Menominee Iron District (Saturday) and the Iron River – Crystal Falls area (Sunday). The Field Trip Guidebook for these three trips drew on previous studies in the area and the more than 100 cumulative years of Lake Superior geology expertise of the field trip leaders to provide a comprehensive and definitive compilation of the Paleoproterozoic stratigraphy and structure of the Niagara suture zone in this part of Michigan and Wisconsin. One hundred and twenty participants attended the banquet on Thursday night. This year’s banquet speaker was Susan Martin of Michigan Technological University. Dr. Martin is a professor of industrial archeology at Michigan Technological University and the author of the book Wonderful Power: The Story of Ancient Copper Working in the Lake Superior Basin. The title of Dr. Martin’s post-banquet talk was: The indigenous people of the Lake Superior Basin: understanding the links among environment, geology, and religious belief. As always, a highlight of the banquet was the presentation of the 2003 Goldich medal to Klaus Schulz of the U.S. Geological Survey, recognizing his long and productive career as a geologist in the Lake Superior region. 19 �The technical session began with three invited presentations. The first was by Harold Bernhardt of the Menominee Range Historical Foundation Museum on the mining history of the Menominee Iron Range. The following two talks, by Bill Cannon and Klaus Schulz, were on the Paleoproterozic rocks of the Niagara suture zone, established the context for the three field trips on that topic. The student paper committee had a difficult job this year. Twenty of the presentations in the technical sessions were from students – 15 oral and 5 posters. In the end, three awards were given: Best Student Paper ($300) went to Karoun Charkoudian (University of Wisconsin-Madison) for her talk titled: Strike-slip separation of the Burntside trondhjemite and the Wakemup Bay tonalite, Northern Minnesota. In recognition of the large number of excellent student presentations, two additional students were chosen for Honorable Mention ($100 each) - Amy Garbowicz (Lawrence University) and Stephanie Hocker (University of Wisconsin – Oshkosh). The Student Paper Award fund was supplemented by silent auction of an original volume of the classic Butler and Burbank USGS Professional Paper 144 on Copper Deposits of Michigan. Eisenbrey Student Travel Grants were given to 15 students: Greg Joslin and Phillip Larson – University of Minnesota, Duluth; Merida Keatts and Mary McKenzie – Kent State; John Marma and Karoun Charkoudian, – University of Wisconsin, Madison; Amy Garbowicz – Lawrence University; Daniela Vallini – University of Western Australia; Stephanie Hocker – University of Wisconsin-Oshkosh; and Becky Rogola, Geoff Heggie, Justin Johnson, Riku Metsaranta, Eric Potter and Adam Richardson, all from Lakehead University. All awards were presented at the conclusion of the technical sessions. The Institute’s Board of Directors met on May 8, 2003 and a brief overview of the meeting is provided below: 1. Accepted the Report of the Chair for the 48th ILSG from Peter Hinz and minutes of last Board meeting, May 14, 2002 from ILSG secretary-treasurer, Mark Jirsa. 2. Accepted the 2002-2003 ILSG Financial Summary from Mark Jirsa. 3. Approved one co-chair from the 49th meeting, Laurel Woodruff, as on-going board member. 4. Nominated George Hudak of the University of Wisconsin, Oshkosh to replace Frank Luther on the Goldich Committee, a position that George later graciously accepted. 5. Approved Duluth, Minnesota as the location for the 2004 (50th annual) ILSG and cochairs Steve Hauck and Mark Severson. 6. Discussed the transition of the Secretary Treasurer position from Mark Jirsa to Peter Hollings and accepted a proposal that both be appointed co-treasurers during the transition period. 7. Discussed the transition and evolution of the ILSG webpage and procedures required to move the ILSG into the electronic era. th 8. Discussed possible activities related to 50 meeting to commemorate the longevity and impact of the ILSG. The 49th ILSG meeting was a great success and we wish to thank all the people who contributed to that success. The staff of Pine Mountain was professional and responsive to the needs of a large group. Kleiman Pump and Well Drilling, Iron Mountain, MI, Prime Meridian Resources, Ltd., Fond du Lac, WI, and Coleman Engineering Co., Iron Mountain, MI provided generous monetary contributions. The field trips this year had a large number of participants, and thanks are due to field trip leaders, van drivers, and everyone else who stepped up when needed to drive, hand out lunches, unlock gates, or keep the crowds 20 �moving. As always, everyone who attended the 49th ILSG was willing to help as necessary or adapt to any situation that developed. The meeting this year was well attended and we are heartened by the excellent student participation and attendance, a trend we hope continues. Because of the outstanding response to the meeting and field trips, the 49th ILSG generated several thousand dollars for the ILSG general fund. We both are very happy with the outcome of the 49th meeting and hope that others think it was a success. An ILSG meeting requires a lot of work and time for all involved, but the assistance of the larger ILSG community makes the job of the co-chairs almost bearable, and we encourage others to take on the task. Laurel Woodruff and Bill Cannon Co-Chairs, 49 21 �INSTITUTE ON LAKE SUPERIOR GEOLOGY BOARD OF DIRECTORS MEETING 49th Annual Institute Meeting Thursday, May 8, 2003 Iron Mountain, Michigan Board of Directors Laurel Woodruff (2003 General Chair) William Cannon (2003 Co-chair) Peter Hinz (2002 Co-chair) Michael Mudrey (2001 Co-chair) Steve Kissin (2000 Co-chair) Peter Hollings (Institute Secretary-Treasurer) Guests Mark Jirsa (Emeritus Institute Secretary-Treasurer) Steve Hauck (proposed 2004 Co-chair) Mark Severson (proposed 2004 Co-chair) Ted Bornhorst (Communications Coordinator) Ron Sage (Goldich Committee) Frank Luther (Goldich Committee-outgoing member) The following is based on the secretaries' notes and recollection; any omissions or misstatements are unintentional. Motions by the Board of Directors are generally paraphrased—"approved" or "accepted" implying that a motion was made, seconded, and passed unanimously. The expression "generally agreed" carries less formality, but indicates a directive that will be pursued. Some issues that were resolved after the Board meeting, but during the conference are included here for closure. MINUTES 1. Peter Hinz presented his report on the 48th meeting and stressed the importance of ensuring that the audiovisual component of the sessions runs smoothly. Hinz remarked that the folder of advice to meeting chairs has been replaced by an electronic version, which Woodruff agreed to forward to the next Chairs. Jirsa noted that the full minutes of the Board of Directors Meeting is not normally published in the Proceeding Volume. Accepted report of the Chairs for the 48th ILSG, Kenora, Ontario; as printed in the Proceeding Volume (Hinz), and minutes of last Board meeting, May 14, 2002 (Jirsa). 9. Received, discussed, and accepted 2002-2003 ILSG Financial Summary (Jirsa). Bornhorst agreed to close the ILSG Michigan accounts this year. 10. Approved Laurel Woodruff as on-going board member 11. Discussed replacing Frank Luther as “academic member” on Goldich Committee (end of term 2003). George Hudak of the University of Wisconsin, Oshkosh was proposed and accepted by the Board. George later accepted the position and was welcomed. 12. Discussed and approved 2004 (50th annual) meeting location—Duluth, Minnesota, and co-chairs Steve Hauck and Mark Severson. Hauck and Severson presented a list of seven potential field trips proposed for the meeting. Bornhorst advised the co-chairs to keep field trips close to home in order to avoid impacting upon future meetings. 13. Discussed the transition of the Secretary Treasurer position from Jirsa to Hollings. Distribution of publications to be transferred to Hollings this year with transfer of the Archives and funds at the 50th Annual Meeting. Hollings to open Canadian bank accounts and arrange non-profit status for ILSG in Canada. Accepted a proposal that Jirsa and Hollings be appointed co-treasurers during the transition period. 22 �14. Discussed the transition of Webmaster from Bornhorst to Mudrey. It was agreed that the website would be moved from Michigan Tech to a commercial ISP with Mudrey and Bornhorst to coordinate the transition 15. Discussed and accepted transition from a paper to an electronic newsletter. Mudrey suggested establishing an Email list and agreed to moderate this. Woodruff, Jirsa, Mudrey and Hollings to form a committee to investigate the possibility of making the proceedings volumes available online. Committee to report to the board at the 50th annual meeting. 16. Discussed and agreed that long service pins are created for members who have been involved with the Institute for more than 15 years. Woodruff has the attendance lists and will pursue for presentation at the 50th annual meeting. 17. Other business a. Bill Cannon proposed a compilation of 50 years of ILSG photos. This would be prepared as a CD to coincide with the 50th annual meeting. Cannon will be assisted by Gene LaBarge. A request for photographs was presented to the membership at the banquet. b. It was requested by Luther that GPS locations of outcrops be included in future field guides c. Mark Smyk and Pete Hollings offered to organize 51st Annual meeting in Nipigon, Ontario in 2005, to coincide with the culmination of new geoscience initiatives in the region. Adjournment Respectfully submitted on May 13, 2003 to Laurel Woodruff, Co-chair of the 49th annual meeting, for incorporation into the Report of the Chair to appear in Proceedings Volume 50. Pete Hollings and Mark Jirsa Secretary-Treasurer, Institute on Lake Superior Geology 2003 Best Student Paper Awards Best Student Paper ($300): Karoun Charkoudian, University of Wisconsin, Madison; for her presentation co-authored with Basil Tikoff; Strike-slip separation of the Burntside trondhjemite and the Wakemup Bay tonalite, northern Minnesota. Honorable Mentions ($150 each): Amy Garbowicz, Lawrence University, Appleton, Wisconsin; for her presentation co-authored with Marcia Bjornerud; Paleostress inferences from fault slip vectors in the eastern part of the Wisconsin segment of the Midcontinent Rift. Stephanie Hocker, University of Wisconsin, Oshkosh; for her poster co-authored with G. Hudak, J. Odette, and T. Newkirk; Chemistry of alteration mineral phases at the Fivemile Lake volcanic-hosted massive sulfide prospect, northeastern Minnesota. 2003 Eisenbrey Student Travel Awards 1) Geoff Heggie, Lakehead University ($100) 2) Adam Richardson, Lakehead University ($100) 3) Justin Johnson, Lakehead University ($100) 4) Becky Rogala, Lakehead University ($100) 5) Eric Potter, Lakehead University ($100) 6) Riku Metsaranta, Lakehead University ($100) 7) Greg Joslin, University of Minnesota, Duluth ($100) 8) Phillip Larson, University of Minnesota, Duluth ($100) 9) Merida Keatts, Kent State, Ohio ($100) 10) Mary McKenzie, Kent State, Ohio ($100) 11) John Marma, University of Wisconsin, Madison ($100) 12) Karoun Charkoudian, University of Wisconsin, Madison ($100) 23 �13) Amy Garbowicz, Lawrence University, Appleton, Wisconsin ($100) 14) Stephanie Hocker, University of Wisconsin, Oshkosh ($100) 15) Daniela Vallini, University of Western Australia ($200) 2004 Goldich Medal Recipient Paul W. Weiblen, Department of Geology & Geophysics, University of Minnesota MTU Archives Donation Proceedings including Part 1 (Programs and Abstracts) and Part 2 (Field Trip Guidebook) are available from the Institute: Institute on Lake Superior Geology c/o Mark Jirsa, Secretary - Treasurer Minnesota Geological Survey 2642 University Avenue St. Paul MN 55114-1057 Phone: 612.627.4539 Fax: 612.627.4778 e-mail: jirsa001@tc.umn.edu 24 �PROGRAM 25 �The following companies made generous contributions to the 50th Annual Meeting. We thank them for their commitment to the Institute on Lake Superior Geology. For 50 years this organization has thrived through the sustained interests of individuals, corporations, universities, and government agencies in the international geologic community. This dedication to an exchange of scientific ideas and a passion for field trips (even in driving rain or snow) has enabled the ILSG to fulfill one of its primary objectives: to promote better understanding of the geology in the Lake Superior region. Franconia Minerals Corporation, Spokane, WA Idea Drilling Incorporated, Virginia, MN Iron Mining Association, Duluth, MN Lehmann Exploration Management, Minneapolis, MN Meriden Engineering, LLC, Hibbing, MN Minerals Processing Corporation, Duluth, MN Minnesota Exploration Association (MExA), Minneapolis, MN Minnesota Minerals Coordinating Committee, St. Paul, MN Teck Cominco American Incorporated, Spokane, WA Wallbridge Mining Company, Lively, ONT 26 �Tuesday May 4, 2004 7:30 a.m. Field Trip 1: Volcanic stratigraphy, hydrothermal alteration, and VMS potential of the lower Ely Greenstone, Fivemile Lake to Sixmile Lake area. Field Trip Leaders: George Hudak (UWO), John Heine (NRRI), Mark Jirsa (MGS), Dean Peterson (NRRI). 6:00 p.m. Overnight in Tower, MN at Fortune Bay Casino. 8:00 a.m. Field Trip 2: Geologic Highlights of New Mapping in the Southwestern Sequence of the North Shore Volcanic Group and Beaver Bay Complex. Field Trip Leaders: Terry Boerboom (MGS), Jim Miller (MGS), and John Green (UMD – Dept. of Geol. Sci.). 6:00 p.m. Overnight in Duluth on your own. Wednesday May 5, 2004 8:00 a.m. Field Trip 2: Geologic Highlights of New Mapping in the Southwestern Sequence of the North Shore Volcanic Group and Beaver Bay Complex. Field Trip Leaders: Terry Boerboom (MGS), Jim Miller (MGS), and John Green (UMD – Dept. of Geol. Sci.). 8:00 a.m. Field Trip 3: Late Wisconsinan Superior-lobe Deposits in the Lake Superior Basin Northeast of Duluth. Field Trip Leader: H. Hobbs (MGS). 8:00 a.m. Field Trip 4: Geology of the Eastern Mesabi Iron Range, Northeastern Minnesota. Field Trip Leaders: R. Ojakangas (UMD – Dept. of Geol. Sci.), M. Severson (NRRI), P. Jongewaard (United Taconite), D. Halverson (Northshore mining), J. Arola (Inland), J. Evers (Cliffs-Services). 6:00 p.m. - Return of Trips 1, 2, 3, and 4 4:00 p.m. - 8:00 p.m. Registration 7:00 p.m. - 9:00 p.m. Ice Breaker Social and Poster Setup 27 �Thursday May 6, 2004 7:00 a.m. – 4:00 p.m. REGISTRATION 8:00 a.m. – 8:05 a.m. INTRODUCTORY REMARKS Steven A. Hauck and Mark J. Severson, Co-Chairs SPECIAL TECHNICAL SESSION I The History of Geologic Investigations in the Lake Superior Region Session Chairs: Mark Smyk, Ontario Geological Survey, Thunder Bay, ONT Sidney Hemming, Lamont-Doherty Lab., Columbia Univ., NY 8:05 a.m. – Old Prospector “Gold is Where You Find It! So Is Ag and Cu and Fe!” The OLD PROSPECTOR: Gold Rushes and Mineral Prospecting, 1848 to 1900 in Western North America and The Lake Superior Region 8:35 a.m. – Johnson, A.M. Douglass Houghton’s 1840 Field Excursion to Lake Superior 8:55 a.m. – Miller, J.D., Jr. N.H. Winchell's Study of the Keweenawan Supergroup Rocks of Northeastern Minnesota, 1872-1900 9:15 a.m. – Smyk, M.C., and Magee, A. Silver Threads and Golden Needles: Geological Milestones in Northwestern Ontario 9:35 a.m. – Holm, D.K., Van Schmus, R.W., and Schneider, D.A. The Influence of Radiometric Dating for Unraveling the Precambrian Geologic History of the Lake Superior Region 9:55 a.m. - 10:20 a.m. COFFEE BREAK AND POSTER SESSION 10:20 a.m. – Chandler, V.W., Boerboom, T.J., and Jirsa, M.A. Promontory Tectonics of the Penokean Orogen in Minnesota: A Gravity and Magnetic Perspective 10:40 a.m. – Medaris, G., Jr., and Singer, B. Geochronology of Precambrian Rocks in Central Wisconsin: A Review and New 40 Ar/ 39Ar Analyses 11:00 a.m. – Southwick, D.L. Late Paleoproterozoic Rhyolite-Quartzite Sequences in the Southwestern U.S.: Speculative Relationship to Rocks of the Baraboo Interval 11:20 a.m. – Ormand, C.J., and Czeck, D.M. Three-Dimensional Geometry and Strain of the Baraboo Syncline: Kinematic Implications 28 �11:40 a.m. – Fralick, P., and Pufahl, P.K. Oxygenation of the Archean Hydrosphere: Evidence from the Eagle Island Deltaic Complex 12:00 p.m. – 1:10 p.m.–LUNCH BREAK – POSTER SESSION and ILSG BOARD MEETING (by invitation) TECHNICAL SESSION II Session Chairs: Jill Peterman, Wisconsin Department of Transportation, Superior, WI Dave Dahl, MN Dept. Natural Resources, Lands & Minerals Div., Hibbing, MN 1:10 p.m. – Metsaranta, R.T.*, and Fralick, P.W. Geochemistry and Petrography of Altered Basement Rocks Underlying the Middle Proterozoic Sibley Group 1:30 p.m. – Heggie, G.*, and Hollings, P. Multiple Intrusive Stages Associated with Keweenawan Rifting: The Leckie Stock, Seagull Intrusion, and Nipigon Sill. 1:50 p.m. – Richardson, A.*, and Hollings, P. A Geochemical Study of the Sills of the Nipigon Basin, Ontario 2:10 p.m. – Boerboom, T.J. Newly Recognized Diatreme Breccia Dikes on Lake Superior Near Two Harbors, Minnesota 2:30 p.m. – 2:55 p.m. COFFEE BREAK AND POSTER SESSION 2:55 p.m. – Hollings, P. Trace Element Geochemistry of the Osler Group Volcanics – Implications for Mid-Continent Rifting 3:15 p.m. – Hart, T.R. Geochemistry of the Proterozoic Intrusive Rocks of the Nipigon Embayment 3:35 p.m. – MacDonald, C.A. and Tremblay, E. Lake Nipigon Region Geoscience Initiative: Results of Bedrock Mapping in the Northern Part of the Western Nipigon Embayment, Northwestern Ontario, Canada 3:55 p.m. – Schneider, R.V. Depth Migration of Seismic Reflection Data: An Example for Lake Superior Studies Ballroom Must Be Empty by 4:30 p.m. for Banquet Setup. 6:00 p.m. ICE BREAKER – MIXER – CASH BAR 7:00 p.m. ANNUAL BANQUET AND AWARD PRESENTATION • • • Announcement of 51st Annual Meeting Location 2004 Goldich Award Presentation to Paul Weiblen 2004 Banquet Address Meeting participants who are not registered for the banquet are welcome to attend the banquet address. 29 �Friday May 7, 2004 8:00 a.m. – 8:05 a.m. INTRODUCTORY REMARKS Steven A. Hauck and Mark J. Severson, Co-Chairs TECHNICAL SESSION III Session Chairs: Jayne Englebert, MSA Professional Services, Baraboo, WI Douglas Hunter, Wallbridge Mining Company, Lively, Ontario, Canada 8:05 a.m. – Johnson, J.R.,* Hollings, P., and Kissin, S. Regional Geochemistry Surrounding the Norton Lake Cu-Ni-PGE Deposit, Uchi Subprovince, Ontario 8:25 a.m. – Rossell, D.M., and Coombes, S. The Geology of the Eagle Nickel-Copper Deposit: Marquette County, Michigan 8:45 a.m. – Mahin, R.A., Quigley, T.O., and Lynott, J.S. The Discovery and Geology of the L-K Massive Sulfide Deposit, Menominee County, MI 9:05 a.m. – Bornhorst, T.J., and Robinson, G.W. Precambrian Aged Supergene Alteration of Native Copper Deposits in the Keweenaw Peninsula, Michigan 9:25 a.m. – Severson, M.J., and Hauck, S.A. Whatever Happened to Those Cu-Ni Deposits? 9:45 a.m. – Shafer, P.L.,* and Ripley, E.M. Hydrogen Stable Isotopic Evidence for Hydrothermal Alteration and PGE Concentration Involving Meteoric Water in the Birch Lake Area, Duluth Complex, MN 10:05 a.m. - 10:30 a.m. COFFEE BREAK AND POSTER SESSION 10:30 a.m. – Hudak, G.J., Newkirk, T.T., Drexler, H., Odette, J.D., and Hocker, S.M. Neoarchean Peperites in the Vicinity of Fivemile Lake, Vermilion District, NE Minnesota 10:50 a.m. – Han, T.-M. Effect of Mineralogy on Processing of Low Grade Iron Ores From the Negaunee IronFormation on Marquette Range of the Lake Superior District 11:10 a.m. – Fosnacht, D., Iwasaki, I., and Bleifuss, R. Iron Nodule Research at the Natural Resources Research Institute, UMD 11:30 a.m. – Zanko, L.M., Oreskovich, J.A., and Niles, H.B. Taconite Aggregate Potential of Coarse Tailings from the Biwabik Iron Formation, With an Emphasis on Geology, Mineralogy, and Microscopy 11:50 a.m. – Larson, P.C., Regional Till Sampling in the Vermilion Greenstone Belt, Minnesota: Preliminary Results and Interpretations 30 �12:10 p.m. - 1:00 p.m. LUNCH BREAK – POSTERS REMOVED AFTER LUNCH SPECIAL TECHNICAL SESSION IV Department of Geological Sciences, University of Minnesota Duluth Fifty Years of Geological Contributions to Lake Superior Geology and Other Geological Areas 1:00 p.m. – Introductory Remarks on 50 years of Geology at University of Minnesota, Duluth: Penny Morton Session Chairs: Scott Wolter, American Petrographics Service Company, St. Paul, MN Wanda Taylor, University of Nevada at Las Vegas, Las Vegas, NV 1:05 p.m. – Ojakangas, R.W., and Ojakangas, G.W. Deposition of Paleoproterozoic Siliciclastics and Iron-Formation in a Tidally Influenced Shelf Environment, Animikie Basin, Lake Superior Region 1:25 p.m. – Breckenridge, A.* The Lake Superior Varve Stratigraphy and Implications for Eastern Lake Agassiz 14 Outflow From 10,700 to 8,900 YBP (9.5-8.0 C KA) 1:45 p.m. – Syverson, K.M. Origin of Pre-Wisconsinan Glacial Units in Northern Wisconsin Based on Lithologic Characteristics 2:05 p.m. – Jirsa, M.A. Mapping by the Minnesota Geological Survey in Support of Land-use and Water Planning on the Mesabi Iron Range 2:25 p.m. – Brown, T.R. Recent Geophysical and Geochemical Applications to Exploration Activities in Cripple Creek Mining District, Colorado 2:45 p.m. – 3:10 p.m. 3:10 p.m. the COFFEE BREAK AND POSTER SESSION Student Paper and Travel Awards 3:20 p.m. – Vervoort, J.D., and Wirth, K.R. Origin of the Rhyolites and Granophyres of the Midcontinent Rift, Northeast Minnesota 3:40 p.m. – Schmidt, S.Th., and Seifert, K. Ocean-Floor-Type-Alteration of Drilled MRS Volcanic Rocks in Iowa 4:00 p.m. – Davidson, D.M., Jr. Heller, Sims and Marsden: Mentors Extraordinaire 4:20 p.m. – Grant, J.A. Isocon Analysis: How to Make It Work for You 4:40 p.m. – Morton, R. Twenty-One Years in a Caldera: UMD Geology Students and Sturgeon Lake, Ontario 31 �5:30-7:30 p.m. Buffet at the Depot Sponsored by the UMD Dept. of Geological Sciences (All ILSG Registrants are invited) Saturday May 8, 2004 8:00 a.m. Field Trip 5: Classic Outcrops of Northeastern Minnesota. Field Trip Leaders: M. Jirsa (MGS), T. Boerboom (MGS), R. Ojakangas (UMD-Dept. Geol. Sci.), J. Miller (MGS), J. Green (UMD-Dept. Geol. Sci.), G.B. Morey (MGS), D. Peterson (NRRI), M. Severson (NRRI), R. Patelke (NRRI). 6:00 p.m. Overnight in Tower, MN at Fortune Bay Casino. 8:00 a.m. Field Trip 6: Glacial and Postglacial Landscape Evolution in the Glacial Lake Aitkin and Upham Basin, Northern Minnesota. Field Trip Leaders: L. Marlow, P. Larson, H. Mooers (UMD-Dept. Geol. Sci.). 6:00 p.m. Return of Trip to Radisson Hotel, Duluth Minnesota. 7:00 a.m. Field Trip 7: Economic Geology of Archean Gold Occurrences in the Vermilion District, Northeast of Soudan, Minnesota. Field Trip Leaders: D. Peterson (NRRI), R. Patelke (NRRI). 6:00 p.m. Return of Trip to Radisson Hotel, Duluth Minnesota. 8:00 a.m. Field Trip 8: Geology of the Western Contact of the Duluth Complex, Partridge River and South Kawishiwi Intrusions, Northeastern Minnesota. Field Trip Leaders: M. Severson (NRRI), J. Miller, Jr. (MGS). 6:00 p.m. Return of Trip to Radisson Hotel, Duluth Minnesota. Sunday May 9, 2004 8:00 a.m. Field Trip 5: Classic Outcrops of Northeastern Minnesota. Field Trip Leaders: M. Jirsa (MGS), T. Boerboom (MGS), R. Ojakangas (UMD-Dept. Geol. Sci.), J. Miller (MGS), J. Green (UMD-Dept. Geol. Sci.), G.B. Morey (MGS), D. Peterson (NRRI), M. Severson (NRRI), R. Patelke (NRRI). 6:00 p.m. Return of Trip to Radisson Hotel, Duluth Minnesota. 32 �POSTER PRESENTATIONS Boerboom, T.J. Bedrock Geologic Maps of the Two Harbors and Castle Danger 7.5-Minute Quadrangles, North Shore of Lake Superior, Minnesota Buchholz, T.W., Falster, A.U., and Simmons, Wm.B. A Greisen-like Mineral Assemblage from the Nine Mile Pluton, Marathon County, Wisconsin Cordua, W.S. Enigmatic 1300 – 1400 Ma Mafic Pluton from the Koss Pit, Marathon County, WI Drexler, H.L.*, Hudak, G.J., and Peterson, D.M. A Field and Laboratory Study to Evaluate the Genetic Relationships Between the Purvis Pluton and Volcanic Rocks and Volcanic-Associated Mineralization in the Vermilion District of NE Minnesota Erickson, M.L.*, and Barnes, R.J. Late Wisconsin Till and Arsenic Contamination in Upper Midwest Groundwater Fitzpatrick, F.A. Influence of Geologic Setting on Hydrogeomorphic Characteristics of Southern Lake Superior Tributaries Green, J.C., and Miller, J.D., Jr. The Geology of the Duluth Complex and the North Shore Volcanic Group Portrayed in New 7.5' Quadrangle Maps of the Duluth Metropolitan Area Hart, T.R., and Magyarosi, Z. Precambrian Geology and Mineralization of the Northern Black Sturgeon River area, Nipigon Embayment Hemming, S.R., and Roy, M. 40 Ar/39Ar Hornblende Evidence for Provenance of Ice Rafted Detritus in the North Atlantic: Implications for Tracking Past Changes in the Extent and Dynamics of Northern Hemisphere Ice Sheets Hoffman, A.T.*, Peterson, D.M., Patelke, R.L., and Hudak, G.J. Preliminary Petrography and Hydrothermal Alteration of the Soudan Mine Area, Vermilion District, Northeastern Minnesota Jirsa, M.A. Regional Compilations of Bedrock Geology in Northern Minnesota: The Vermilion, Ely, and Basswood Lake Quadrangles Kaukonen, R.J., and Alapieti, T.T. Platinum Mineralization at Drill Hole A4-11 of the Wetlegs Area of the Partridge River Intrusion, Duluth Complex, Northeast Minnesota 33 �Kean, Wm.F. Magnetic Susceptibility Anisotropy and Remanent Magnetism of Quartzite and Phyllite from Baraboo, Wisconsin Klawiter, B. Lithic Materials and Archaeology in the Western Lake Superior Region Larson, P.C., Mooers, H.D., and Marlow, L.M. Early Advance of the St. Louis Sublobe: A Revised Chronology of the Deglaciation of Northeastern Minnesota MacDonald, C.A., and Tremblay, E. Precambrian Geology of the South Armstrong–Gull Bay Area, Nipigon Embayment, Northwestern Ontario, Canada Maes, S., Tikoff, B., Brown, P., and Ferré, E. Magnetic Fabric Constraints on Magmatic Flow: Insizwa Sill, South Africa and the Sonju Lake Intrusion, Minnesota. Magee, A. Mining and Exploration Activities in Northwestern Ontario Mahin, R.A., Quigley, T.O., and Lynott, J.S. The Geology of the L-K Massive Sulfide Deposit, Menominee County, MI McSwiggen, P.L., and Morey, G.B. Mineral Chemistry and Stratigraphy of the Biwabik Iron Formation, Near the Virginia Horn, Mesabi Iron Range, Minnesota Mudrey, M.G., Jr., and Cannon, W.F. Status of Publicly Available Mid-Continent Reflection Seismic Data Patelke, R., and Severson, M. Duluth Complex Bulk Samples Patelke, R., Severson, M., and Peterson, D. Untested Targets in the Duluth Complex Peterson, D.M., and Patelke, R.L. The Proposed National Underground Science and Engineering Laboratory at the Soudan Mine, Northeastern Minnesota: A Geological Site Investigation Piercey, P., Schneider, D.A., and Holm, D.H. Petrotectonic Evolution of Paleoproterozoic Granitic Rocks Across the Central Penokean Orogen, Northern MI and WI Planavsky, N.*, and Bjornerud, M. Blowing in the Wind: The Copper Harbor Stromatolites Revisited Ruhanen, R.W. Geologic Reconnaissance of the Spaulding Mine Area, Cook County, Minnesota 34 �Schneider, R.V. Depth Migration of Seismic Reflection Data: An Example for Lake Superior Studies Stott, G.M. Close Proximity of Kimberlite Pipes to Diabase Dykes: Structural Controls and Predictiveness in the James Bay Lowlands, Ontario Trow, J. Dowsing Employs Classical Mechanics and Static Electricity to Locate SelfPotential Anomalies Inductively and Rapidly NOTE: Asterisk * denotes a student eligible for a Best Student Paper Award. 35 �ABSTRACTS 36 �BEDROCK GEOLOGIC MAPS OF THE TWO HARBORS AND CASTLE DANGER 7.5MINUTE QUADRANGLES, NORTH SHORE OF LAKE SUPERIOR, MINNESOTA BOERBOOM, Terrence J., Minnesota Geological Survey, boerb001@umn.edu Two recent quadrangle-scale geologic maps of an area adjacent to Lake Superior have been published by the Minnesota Geological Survey as part of the U.S. Geological Survey STATEMAP program. This ongoing mapping effort has resulted in five published quadrangle geologic maps in an area stretching from Duluth to Castle Danger (Boerboom and others, 2002a, b, 2003a, b; Fig. 1a). Work is currently in progress on a sixth geologic map (Split Rock Point); and another (Two Harbors NE) will be mapped in the coming year. Field mapping for all these maps was or will be conducted at a scale of 1:12,000, and map compilations are at 1:24,000. The North Shore is experiencing ever-increasing development of both commercial property and private residences, creating a concomitant demand on ground-water resources, which near Lake Superior come mainly from bedrock reservoirs, particularly layered volcanic flows and interflow sedimentary rocks. Development also brings an increased need for quality aggregate materials for both construction and shoreline preservation projects. Although some volcanic flows may be suitable for crushed-rock aggregate, the intrusive rocks are likely to provide the best source of this material. Thus, the goal of this mapping is to refine the stratigraphy of volcanic rocks in the southwest limb of the North Shore Volcanic Group, and to identify the extent, mineralogy, and relationships of intrusive rocks emplaced into the volcanic pile. This mapping will not only lay the groundwork for present and future resource demands, but also further our understanding of the geologic aspects of these rocks. 37 �Green (2002) subdivided volcanic rocks of the Keweenawan North Shore Volcanic Group into a series of informal lithostratigraphic units based on the relative thickness and composition of the volcanic flows or by areas separated by intervening intrusions. Several of these units were extended into the Two Harbors and Castle Danger quadrangles from the southwest (Fig. 1b). Two new lithostratigraphic units were added to those of Green (2002) based on this mapping⎯the Stewart River basalts and the Crow Creek lavas (Fig. 1b). The Stewart River basalts include several flows of diabasic-textured basaltic lavas and an ambiguous unit that has attributes of both a lava flow and an intrusion, interpreted as a shallow-level subvolcanic intrusion that breached the surface to form flows along its upper margin. The Crow Creek lavas are named for olivine tholeiitic and andesitic flows that outcrop mainly along Crow Creek. These lavas contrast with flows east of the Lafayette Bluff diabase and west of the Silver Creek diabase, implying that the intrusions have created a major disruption of the volcanic stratigraphy. Mapping has refined the shape and intrusive relationships of known intrusions, and has identified several new intrusions. Most noteworthy among these is the previously known but poorly understood Two Harbors intrusion, which has been shown to form a small, zoned, east-plunging, synform-shaped body. This intrusion grades from troctolitic diabase through poikilitic olivine gabbro to well-foliated intergranular gabbro, from the lowest exposed to the highest exposed portions of the body. Another smaller body of more massive olivine-rich ophitic olivine diabase may be related to the Two Harbors intrusion, but does not exhibit any consistent modal or textural layering. Of particular interest is the recognition of a diatreme-like breccia that cuts volcanic rocks near the mouth of Crow Creek (Boerboom, 2004). This diatreme contains clasts from less than 1 millimeter to 5 meters in size, in a matrix of zeolite- and chlorite-cemented rock flour. The clasts include a heterogeneous mixture of fine-grained porphyritic basalt, amygdaloidal to ophitic to intergranular basalt, and interflow sedimentary rocks. Fine-grained felty-textured prismatic ferromonzodiorite also occurs as intrusions into basalt adjacent to the diatreme, and as clasts in the diatreme, indicating a possible cogenetic relationship between the two. References Boerboom, T.J., 2004, Newly recognized diatreme breccia dikes on Lake Superior near Two Harbors, Minnesota [abs.]: Institute on Lake Superior Geology (this volume). Boerboom, T.J., Green, J.C., and Jirsa, M.A., 2002a, Bedrock geology of the French River and Lakewood quadrangles, St. Louis County, Minnesota: Minnesota Geological Survey Miscellaneous Map M-128, scale 1:24,000. ———2002b, Bedrock geology of the Knife River quadrangle, St. Louis and Lake Counties, Minnesota: Minnesota Geological Survey Miscellaneous Map M-129, scale 1:24,000. Boerboom T.J., Green, J.C., and Miller, J.D., Jr., 2003a, Bedrock geologic map of the Castle Danger quadrangle, Lake County Minnesota: Minnesota Geological Survey Miscellaneous Map M-140, scale 1:24,000. ———2003b, Bedrock geologic map of the Two Harbors quadrangle, Lake County Minnesota: Minnesota Geological Survey Miscellaneous Map M-139, scale 1:24,000. Green, J.C., 2002, Volcanic and sedimentary rocks of the Keweenawan Supergroup in northeastern Minnesota, Chapter 5 of Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.M., and Wahl, T.E., 2002, Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geological Survey Report of Investigations 58, p. 94-105. Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., and Peterson, D.M., 2002, Geologic map of the Duluth Complex and related rocks, Northeastern Minnesota: Minnesota Geological Survey Miscellaneous Map M-119, scale 1:200,000. Sandberg, A.E., 1938, Section across Keweenawan lava flows at Duluth, Minnesota: Geological Society of America Bulletin, v. 49, p. 795-830. Schwartz, G.M., and Sandberg, A.E., 1940, Rock series in diabase sills at Duluth, Minnesota: Geological Society of America Bulletin, v. 51, p. 1135-1172. 38 �NEWLY RECOGNIZED DIATREME BRECCIA DIKES ON LAKE SUPERIOR NEAR TWO HARBORS, MINNESOTA BOERBOOM, Terrence J., Minnesota Geological Survey, boerb001@umn.edu Recent mapping by the Minnesota Geological Survey during the 2002 and 2003 field seasons (see Boerboom, 2004) has identified two nearly identical diatreme-like breccias located about 15 kilometers apart, both of which intrude volcanic rocks of the North Shore Volcanic Group. One is located near the mouth of Crow Creek, 3 kilometers southwest of Castle Danger, and the other is located near the mouth of the Split Rock River. The diatremes contain clasts that range up to four meters in diameter. The clasts are composed of typical North Shore volcanic group rocks, including intergranular to ophitic basalt, porphyritic basalt, and minor interflow sedimentary rocks, as well as felty-textured prismatic monzodiorite that also occurs as intrusions into basalts adjacent to the diatremes. At both places the clasts are heavily replaced by zeolites (mainly laumontite) and chlorite, and are surrounded by a matrix of zeolites and chlorite mixed with rock flour. Irregularly distributed brown gossans that probably represent zones of pyritic matrix have also been identified at Crow Creek. The Crow Creek diatreme is the best exposed of the two diatremes, but outcrops are mainly accessed by water. Exposures on a shore-parallel high cliff wall and in cliffs of basalt that are cut by the breccia and protrude into the lake show the dike to be approximately 10 meters wide and vertical in orientation, with clasts up to 4 meters in size. At the shore the dike is oriented approximately east–west, but other scattered, low outcrops inland imply that the dike may have an overall 0.25 mile diameter ring-like structure or a very irregular strike direction. Monzodiorite associated with this diatreme is poorly exposed, and where visible contains abundant angular basalt clasts and small amygdules of chlorite and calcite. The same monzodiorite also occurs as clasts in the diatreme, thus it must have preceded the diatreme in timing but may have been closely associated with it. The Split Rock River diatreme is exposed just southeast of the river mouth as a 6-meter wide, northstriking, subvertical dike that truncates amygdule-layered tholeiitic basalts. This dike contains clasts of both intergranular and ophitic basalt up to 20 centimeters in diameter, and small clasts of interflow sedimentary rocks and ferromonzodiorite petrographically identical to that at Crow Creek. The latter also occurs alone in an outcrop near the diatreme. Narrow, dike-parallel, anastamosing zones of brecciation cut both basalt and ferromonzodiorite in the vicinity of the diatreme dike. In both cases, the brecciated texture, multiple clast varieties of North Shore volcanic group-related rocks and sharp cross-cutting contacts with adjacent volcanic rocks imply that the diatremes were emplaced as an explosive, gascharged intrusion that cut across the volcanic strata. The amygdaloidal and deuterically-altered Lafayette Bluff diabase occurs in close proximity to the Crow Creek diatreme (Boerboom and others, 2003), which may have provided a source of volatiles as it cooled. These diatremes are soft and easily eroded and thus unlikely to form significant outcrops inland, and they could easily be misidentified as a volcanic flow breccia. Careful examination of isolated or seemingly out of sequence flow breccias for disparate clast lithologies may reveal more of these diatreme dikes. REFERENCES Boerboom, T.J., 2004, Bedrock geologic maps of the Two Harbors and Castle Danger 7.5-minute quadrangles, North Shore of Lake Superior, Minnesota [abs.]: Institute on Lake Superior Geology (this volume). Boerboom, T.J., Green, J.C., and Miller, J.D., Jr., 2003, Bedrock geology of the Castle Danger quadrangle, Lake County, Minnesota: Minnesota Geological Survey Miscellaneous Map M-140, scale 1:24,000. 39 �Precambrian Aged Supergene Alteration of Native Copper Deposits in the Keweenaw Peninsula, Michigan BORNHORST, Theodore J., and ROBINSON, George W., A.E. Seaman Mineral Museum, Michigan Technological University, Houghton, MI 49931 The Keweenaw Peninsula within the Midcontinent Rift System is host to a world-class native copper mining district from which about 6 billion kg of refined copper was mined from 1845 to 1968. By far the principal economic minerals mined were native copper and native silver. Supergene alteration minerals occur irregularly throughout the district. The most abundant of these are cuprite, tenorite, malachite, and chrysocolla in close association with native copper. Less common are azurite and paramelaconite, though about fifty additional species are known (Table 1). These minerals typically occur as thin coatings on or replacing native copper, irregular thin coatings along fractures, and euhedral crystals in small pockets filling remaining pore space. For the most part supergene minerals are found near the surface (< 300 m) and are well below the current water table. As might be expected, sulfates are conspicuously uncommon, owing to the general absence of sulfide ores in the district. Likewise, the arsenates (e.g., annabergite, tyrolite, erythrite, etc.) are associated with the copper arsenide minerals domeykite and algodonite. One group, however, the chlorides (e.g., paratacamite, calumetite, etc.), may have formed by reaction with fossil brines, rather than strictly supergene means, as these minerals have been found in isolated areas 1375 m below surface with no visible means of connection to downward-moving meteoric waters. The rocks of the Midcontinent Rift System were deposited between roughly 1.1 and 1.0 Ga, and late in the evolution of the rift (about 1.05 Ga) the rift was subjected to regional compression resulting in reverse faulting, uplift, and sediment deposition. During this compression event, native copper was deposited from a large-scale hydrothermal system along with numerous other hydrothermal minerals. Uplift related to reverse faulting likely continued for some time after the formation of the native copper deposits, likely ending about 1.0 Ga. The rift rocks of the Keweenaw Peninsula were subjected to erosion. If enough cover was removed, the native copper could have been sufficiently near the surface to be exposed to oxygenated groundwater, which would form supergene minerals. Erosion ended by the end of Precambrian time, when the rift rocks were unconformably overlain by Paleozoic sediments, which still cover major portions of the rift. These sediments sheltered the rift rocks of the Keweenaw Peninsula from weathering and erosion. The Paleozoic sediments were removed by Pleistocene glaciers, again making supergene alteration possible. Since the last glacial retreat it is probable that the water table has not been significantly lower than it is today. In turn, Pleistocene supergene alteration should be more or less restricted to above the current water table. Because most observed supergene alteration is below the water table, it is unlikely that it formed since the glaciers removed the Paleozoic cover. Thus, supergene alteration must have occurred during/near the end of the protracted period of erosion in late Precambrian time. The copper deposits had to be reasonably near the surface at that time for supergene alteration to occur. While the Pleistocene glaciers removed significant thickness of Paleozoic cover they could not have removed any significant thickness of rift rocks, else the supergene altered zones would have been removed too. Therefore, the current surface of the Keweenaw Peninsula is likely near the same stratigraphic/erosional level that was present when the Keweenawan rocks were buried by Paleozoic sediments. 40 �acanthite annabergite anthonyite ardennite ? atacamite azurite brochantite buttgenbachite calcite calumetite carbonate-cyanotrichite chalconatronite chrysocolla connellite covellite crednerite cuprite dioptase erythrite gerhardtite goethite guerinite gypsum halite humboldtine ? hydromagnesite kaolinite langite lavendulan likasite malachite manganite mirabilite montmorillonite moolooite ? nantokite olivenite paramelaconite paratacamite pharmacolite picropharmacolite plancheite posnjakite pseudomalachite rauenthalite spertiniite tenorite tyrolite vaterite veszelyite vladimirite whewellite ? unknown Ca-Cl mineral unknown Cu-Cl minerals unknown Cu-Ca-Cl mineral Table 1. Supergene minerals found or reported from the Keweenaw native copper deposits. 41 �THE LAKE SUPERIOR VARVE STRATIGRAPHY AND IMPLICATIONS FOR EASTERN LAKE AGASSIZ OUTFLOW FROM 10,700 TO 8,900 CAL YBP (9.5 - 8.0 14C KA) BRECKENRIDGE, Andy, Large Lakes Observatory, University of Minnesota Duluth, 10 University Drive, 109 RLB, Duluth, MN 55812-2496, brec0027@d.umn.edu Glaciolacustrine rhythmites from Lake Superior record the regional recession of the Laurentide Ice Sheet (LIS) from 10,700 to 8,900 cal ybp (ca. 9.5-8.0 14C ka). LIS retreat from Superior opened eastern Lake Agassiz outlets so that the rhythmites reflect the combined impacts of sediment-laden meltwater and Lake Agassiz discharge. Using sediment cores retrieved from Lake Superior, I present rhythmite stratigraphies, a time series analysis of the thickness measurements, and high-resolution inorganic carbonate data to demonstrate that this is an annual record (varved). The varve thickness records primarily document regional ice margin dynamics: thick varve sequences at 9,100 cal ybp (~8.1 14C ka) and 10,400-10,200 cal ybp (~9.29.0 14C ka) record two periods of moraine formation (the Nakina and Nipigon). General varve cessation is associated with the circumvention of Lake Agassiz and glacial meltwater into Lake Ojibway at 9,040 cal ybp (~8.1 14C ka), although adjacent to meltwater inlets, rhythmic sedimentation persisted for another 200 years. Positively identifying Lake Agassiz catastrophic discharge events remains speculative but seems feasible. The initial influx of Lake Agassiz water is expected at around 10,600 cal ybp (~9.4 14C ka), but at this time, most of eastern and northern Lake Superior was covered by ice. Three sets of thick-thin varves in western Lake Superior perhaps record influxes of Lake Agassiz at around 10,630, 10,600, and 10,570 cal ybp (~9.4 14C ka). Varve formation in Superior coincides with high lake levels in Lake Huron, suggesting that high lake levels in Huron correspond to periods of high Agassiz and/or meltwater flow into Lake Superior. 42 �Recent Geophysical and Geochemical Applications to Exploration Activities in the Cripple Creek Mining District, Colorado BROWN, Timothy R., Cripple Creek and Victor Gold Mining Company, P.O. Box 191, Victor, CO 80863, tbrown@anglogoldna.com Approximately 23 million ounces of gold have been produced from underground and surface operations since gold was discovered in the Cripple Creek Mining District in 1891. This total includes nearly 2 million ounces produced by Cripple Creek and Victor Gold Mining Company (CC&V) over the past 10 years. Gold has been produced from numerous veins in a 30 MA alkaline diatreme complex and is strongly associated with intense potassic alteration. Current production is mining low-grade disseminated haloes around the major historic producers from two open pits and the gold is recovered from a heap leach facility. This rate of production, and future production, in a mature mining district could not take place without an aggressive, ongoing exploration program that utilizes every available tool. Airborne geophysical surveys conducted in 1999 included magnetics, resistivity, and radiometrics. The magnetic survey clearly outlines the diatreme and shows numerous geological, structural, and cultural features inside the diatreme while contributing to the understanding of the alteration. Resistivity has given a better understanding of the zones of clay, potassic alteration, and possibly water. All of these suggest the location of structural features. The radiometric survey, and especially K, outlines large zones of alteration, however this is strongly influenced by historical and recent disturbances as well as the leach pad. IP and CSAMT surveys were initiated in 2001 and the results were encouraging enough to launch a much larger survey in 2002 that covered 37 line miles. The density of data allowed us to model and display the information in several different ways that include plans, sections, and 3D shells. CC&V utilizes a survey tool that captures oriented joint and fracture information with a down-hole camera. This tool captures important geotechnical data as well as structural orientations that can be cross-referenced to assay data. The geochemical relationships between Au and Te and the potassic alteration have long been known in the district. Recent studies have shown a consistent relationship between gold and other elements (As, Hg, Sb, and V) and have suggested high-grade gold always occurs with strong potassic alteration although strong potassic alteration does not always indicate zones of high-grade gold. Application of the knowledge gained from a better understanding of the district’s geology, alteration, and structure has led to focused, efficient drilling programs. The results of the exploration programs have continued to add to the operations as mid-year 2003 reserves stand at 4.2 million ounces and production is scheduled to continue through 2013. 43 �A GREISEN-LIKE MINERAL ASSEMBLAGE FROM THE NINE MILE PLUTON, MARATHON COUNTY, WISCONSIN BUCHHOLZ, Thomas W., 1140 12th Street North, Wisconsin Rapids, Wisconsin 54494, FALSTER, Alexander U., and SIMMONS, Wm.B., Department of Geology and Geophysics, University of New Orleans, New Orleans, Louisiana 70148. The Nine Mile pluton is the youngest and most silicic of the four known intrusive centers of the approximately 1.5 Ga Wausau complex, exposed in Marathon County, Wisconsin. Mineralization in this epizonal pluton is often varied and complex, though frequently restricted to small localized environments in pegmatitic veins, aplites, and miarolitic granite. Recently a greisen-like mineral assemblage was identified in the Maguire weathered granite quarry, located approximately ¼ mile west of the Ladick East (or Charneski) quarry in the south central portion of the pluton. Quarrying here has intersected several small aplite-pegmatite dikes ranging in thickness from approximately 2” (5 cm) to about 15” (40 cm). These consist of layered banded aplite, commonly with thin 1-3” thick pegmatitic cores, and locally exhibit a highly peraluminous composition that is unusual in the predominantly alkalic Nine Mile granite. Due to the rapid progress of pit operations and ongoing reclamation work, most dikes could not be studied in their original position, but one briefly exposed gently dipping subhorizontal dike was observed in situ. Piles of discarded aplite-pegmatite boulders indicate the probable locations of additional dikes, possibly as many as three based upon varying characteristics. Most dikes were probably roughly vertically oriented, as adjacent pit walls show no signs of the dikes. Much material was recovered from piles of mingled boulders; sample characteristics generally allow attribution to one of the surmised four dikes. Exotic mineralization is often present in both the aplite and pegmatitic portions, but is concentrated along and adjacent to aplite-pegmatite contacts and along thin discordant mica-rich veinlets cutting across visible banding or layering. Late-stage mineralization in these dikes, generally confined to the vicinity of the pegmatitic cores, shows mineralization indicative of a greisen-like environment. Minerals identified to date include topaz (19-20% F content) as masses intergrown with albite and other minerals and as tiny clear crystals to 0.4 mm. Ferberite/hübnerite is found as well-formed dark red-brown to black crystals to 1.5 mm on and in albite and quartz. Most crystals have a high Mn content, but some portions of crystals are Mn dominant and are therefore hübnerite. Cassiterite, though less common than ferberite/hübnerite, has been found as dark red-brown crystals and masses to 3 mm. It appears that these represent the first reports of topaz, ferberite and hübnerite from in-situ occurrences in the Wausau complex, and indeed from the state of Wisconsin. Cassiterite has been observed in trace amounts from several other sites within the Nine Mile Pluton, but its relative abundance at this site is highly unusual. Monazite- (Ce) is common as clear to translucent yellow-orange crystals up to 4.5 mm in size. Columbite-group minerals are common as well-formed crystals to approximately 4 mm, and are ferrocolumbite with significant Mn and Ta. Some crystals have W-contents of over 12%, making them tungstenian columbite-tantalite. Zircon is locally very abundant, and is Hfrich. Xenotime is sparse, and appears to show significant enrichment in HREE. Other minerals 44 �noted include pyrite, chalcopyrite, barite (excellent clear platelets), an unidentified bright blue Cu sulfide, ilmenite, quartz, microcline, albite, siderophyllite, muscovite and possibly zinnwaldite. Rare phases include tiny inclusions of stolzite/raspite in ferberite, microlite, a Bi mineral forming minute tabular crystals, a beam-sensitive W mineral, an ilmenorutile like mineral, a grayite-like Th phosphate and silvery black crystals of a Nb-dominant ixiolite-like mineral with octahedral morphology. Interestingly, considering the abundance of fluorite at other Nine Mile Pluton sites, fluorite is much less common here. Nonetheless, the high F content of topaz and associated micas indicates that these were very F-rich systems. The association of topaz, cassiterite and ferberite, and the highly peraluminous nature of the mineralization indicate that a greisen-like environment prevailed in the latter stages of crystallization of these dikes. To the knowledge of the authors this is the first observation of such an environment in the Nine Mile pluton, in the Wausau Complex, and probably in the state of Wisconsin. 45 �Promontory Tectonics of the Penokean Orogen in Minnesota: A Gravity and Magnetic Perspective CHANDLER, V.W., BOERBOOM, Terrence J., and JIRSA, Mark A., Minnesota Geological Survey, chand004@umn.edu The sharp southward bend of the Penokean orogen in central Minnesota and possible continuation southwest into Iowa along the Spirit Lake trend (Anderson and Black, 1983; Van Schmus and others, 1993) have been cited as evidence for a promontory in the pre-collisional margin of the Superior Province craton (Schulz and Sims, 1993). Recent geologic investigations in this area have clarified the structure of both the orogen and the proposed promontory. Owing to a lack of bedrock exposure and drill holes, these investigations have relied to varying degrees on geophysical data. Of particular importance have been the high quality gravity and aeromagnetic databases in Minnesota (Chandler, 1991; Chandler and Schaap, 1991) and Wisconsin (Daniels and Snyder, 2002). Gravity and magnetic data have guided recent re-interpretations of the high-grade gneissic rocks in the Minnesota River Valley subprovince, which forms the core of the proposed promontory. On the basis of distinctive anomaly signatures, Southwick and Chandler (1996) divided the subprovince into 4 blocks bounded by three major east- to northeast-striking shear zones, which most likely formed during late Archean accretion of the Minnesota River Valley rocks onto the Superior craton. These shear zones, which are inferred from geophysical models and seismic data to have a moderately steep north dip (Southwick and Chandler, 1996), were likely reactivated during the roughly north–south convergence of the Penokean orogen. On a broad scale, the gravity and magnetic signatures of the Minnesota River Valley subprovince differ significantly from those associated with the Archean gneissic rocks of the Marshfield terrane in Wisconsin. This supports the view that the two Archean gneiss terranes are unrelated, and were most likely brought together during events related to the Penokean orogeny (Schulz and others, 1993). Geologic mapping supported by geophysical data reveals that significant changes in the structure of the Penokean orogen occur near the proposed promontory. Derivative enhanced gravity and magnetic data indicate that the structural grain of the orogen changes from northeast–southwest to nearly north–south (Boerboom and others, 1995; Jirsa and Chandler, 1997), and neither external zone fold-and-thrust belts nor foreland basin deposits appear to extend appreciably south of lat 45°15’N. Part of the internal zone of the orogen includes syntectonic granitic and metavolcanic rocks that are similar to those of the Penokean magmatic terranes of northwestern Wisconsin, but a large proportion of the internal zone is comprised of post-orogenic granitic rocks (Jirsa and Chandler, 1997). In fact, these post-orogenic granites, which are collectively referred to as the east-central Minnesota batholith (Holm and others, in press) are interpreted to constitute the dominant part of the orogen south of lat 45°15’N. In that area, the orogen is characterized by a broad magnetic low, the source of which is tentatively interpreted by geophysical modeling to represent non-magnetic, metasedimentary rocks that occur at a depth of 5 to 15 kilometers beneath Paleoproterozoic granitic rocks. Gravity and magnetic signatures indicate a somewhat complicated structure for the orogen in southern Minnesota. The orogen is truncated near lat 44°45’N. along a fault bounded block of Minnesota River Valley subprovince rocks. Both the orogen and Minnesota River Valley rocks are covered to the east by rocks of the Mesoproterozoic Midcontinent Rift System, but south of lat 44°15’N., rocks of the orogen are interpreted to re-emerge from the Midcontinent Rift System and extend southwest along the Spirit Lake trend. A broad magnetic low, combined with the drilling data from Minnesota (Southwick, 1994) and Iowa (Van Schmus and others, 1993), indicate that the geology along this part of the Spirit Lake trend may be similar to the internal zone of the orogen in east-central Minnesota. In conclusion, recent geologic interpretations in central and south-central Minnesota have helped refine our understanding of the effects of the large continental promontory with the western margin of the 46 �Penokean orogen. In fact, some of the changes in orogen structure may be explainable by the promontory. For example, pre-existing crustal weaknesses within the promontory may have enhanced thrusting, sedimentation, igneous activity, and crustal thickening during Penokean convergence. Increased crustal thickening near the promontory, in the area now occupied by the east-central Minnesota batholith, could have led to the removal of foreland fold-and-thrust belts and foreland basin deposits, and to increased melt generation of the lower crust to produce the post-orogenic granites. REFERENCES Anderson, R.R., and Black, R.A., 1983, Early Proterozoic development of the southern Archean boundary of the Superior Province in the Lake Superior region [abs.]: Geological Society of America Abstracts with Programs, v. 15, no. 6, p. 515. Boerboom, T.J., Setterholm, D.R., and Chandler, V.W., 1995, Bedrock geology, pl. 2 of Meyer, G.N., project manager, Geologic atlas of Stearns County Minnesota: Minnesota Geological Survey County Atlas C-10, scales 1:100,000 and 1:200,000. Chandler, V.W., 1991, Aeromagnetic anomaly map of Minnesota: Minnesota Geological Survey State Map S-17, scale 1:500,000. Chandler, V.W., and Schaap, B.D., 1991, Bouguer gravity anomaly map of Minnesota: Minnesota Geological Survey State Map S-16, scale 1:500,000. Daniels, D.L., and Snyder, S.L., 2002, Wisconsin gravity and aeromagnetic maps and data: A web site for distribution of data: U.S. Geological Survey Open File Report 02-493 <http://pubs.usgs.gov/of/2002/of02-493/index.htm>. Holm D.K., Van Schmus, W.R., MacNeil, L.C., Boerboom, T.J., Schweitzer, D., and Schneider, D., in press, U-Pb zircon geochronology of Paleoproterozoic plutons from the northern mid-continent, U.S.A.: Evidence for subduction flip and continued convergence after geon 18 Penokean orogenesis: Geological Society of America Bulletin. Jirsa, M.A., and Chandler, V.W., 1997, Scientific test drilling and mapping in east-central Minnesota, 1994-1995: Summary of lithologic results: Minnesota Geological Survey Information Circular 42, 105 p. Schulz, K.J., Sims, P.K., and Morey, G.B., 1993, Tectonic synthesis, the Lake Superior region and TransHudson orogen, in Reed, J.C., Jr., Bickford, M.E., Houston, R.S., Link, P.K., Rankin, D.W., Sims, P.K., and Van Schmus, W.R., eds., Geology of North America: Precambrian: Conterminous U.S.: Boulder, Colo., Geological Society of America, v. C-2, p. 60-64. Southwick, D.L., 1994, Assorted geochronologic studies of Precambrian terranes in Minnesota: A potpourri of timely information, in Southwick, D.L., ed., Short contributions to the geology of Minnesota: Minnesota Geological Survey Report of Investigations 43, p. 1-19. Southwick, D.L., and Chandler, V.W., 1996, Block and shear zone architecture of the Minnesota River Valley Subprovince: Implications for late Archean accretionary tectonics: Canadian Journal of Earth Sciences, v. 33, p. 831-847. Van Schmus, W.R., Bickford, M., and Condie K., 1993, Early Proterozoic crustal evolution, in Reed, J.C., Jr., Bickford, M.E., Houston, R.S., Link, P.K., Rankin, D.W., Sims, P.K., and Van Schmus, W.R., eds., Geology of North America: Precambrian: Conterminous U.S.: Boulder, Colo., Geological Society of America, v. C-2, p. 279-281. 47 �Enigmatic 1300 – 1400 Ma Mafic Pluton from the Koss Pit, Marathon County, WI CORDUA, William S., Dept. Plant and Earth Science, University of Wisconsin - River Falls, 10 South Third Street, River Falls, WI 54022, william.s.cordua@uwrf.edu In late Fall, 2002, an unusual body of mela-diorite cutting granite was found by mineral collector Tom Buchholz in a newly excavated area of the Red Rock quarry #510 (A.K.A. Koss Pit), on the east side of County O, SW 1/4 sec 2 T27N R6E, SW of Wausau, Marathon County, WI. The quarry is dominantly in the 1520 –1480 Ma Nine Mile granite (Buchholz, et. al., 2000). The unusual rock is exposed as rubble in a teardrop shaped area approximately 2 meters wide by 15 meters long, trending N55oE. This rock contains spectacular euhedral twinned crystals of plagioclase up to 20 cm. long. It has chilled margins against the granite and contains granite xenoliths. It is cross-cut by thin pinkish diorite dikes less than a centimeter wide. Samples were collected for thin section work, major and minor trace element analysis and radiometric dating. Study of 8 thin sections revealed the pluton is a mela-diorite consisting of 7 – 20 % plagioclase phenocrysts in a matrix of medium to fine -grained hornblende (20- 24%), biotite (27 - 37%), titanite (3-4%) and felsic material (22 -28%). The felsic material is an equigranular mosaic consisting mostly of oligoclase (about An 14). The hornblende and biotite occasionally form clots of coarser crystals. No feldspathoids, olivine or pyroxenes were found. The plagioclase phenocrysts were rounded and embayed, suggestions partial resorption. Their compositions were andesine (about An 43) but showed zoning on the rims and along cleavages to oligoclase. The phenocrysts had numerous inclusions of biotite and hornblende identical to that found in the groundmass. This suggests that the plagioclase crystals are not xenocrysts. Chemical analyses were done of 2 samples: WSC-03-01 was from near the contact with the granite. WCS-O3-02 is from the center of the body. Both samples are nepheline normative (WSC-03-01 ne = 0.63; WSC-03-02 ne = 3.41) and olivine normative (WSC-03-01 ol = 9.16 WSc-03-02 ol = 10.46). The less undersaturated nature of WSC-03-01 may be due to contamination by the granite country rock. These rocks are similar chemically to lamprophyres. Some of their major element trends fall within the shoshonite fields (Joplin, 1968). Mineralogically, however, the rocks are distinct from typical lamprophyres in the conspicuous plagioclase phenocrysts and lack of modal pyroxene or olivine (Rock, 1991). Trace element chemistry of these two samples is consistent with trends from shonshonitic and alkalic rocks formed in mid-plate regions (Pierce, 1982). Spider gram plots show the Koss Pit mela-diorite is enriched in both compatible and incompatible lithophile elements relative to MORB (figure 1). Its pattern resembles that of lamprophyres, such as the calc-alkaline lamprophyre series (Pierce, 1982, Rock, 1991). REE distribution shows a negative slope and enrichment in all REEs relative to Chengwatana volcanic rocks (Wirth, et al., 1997) and local Penokean granites. They are inconsistent with a Keweenawan body contaminated with granitic basement. They are consistent with the partial melting of a metasomatically enriched lithospheric mantle. A possible source of enrichment could be volatile- rich material subducted during the Penokean orogeny. K-Ar ages were determined by Activation Labs Ltd. Argon was determined by isotope dilution procedure on noble gas mass spectrometry. K concentrations were determined by ICP. The whole age measured was 1307.2 +/- 41 Ma. A biotite separate was determined to have an age of 1410.8 +/- 47 Ma. These ages are unique for central Wisconsin, which had shown a gap in igneous activity between the Nine-Mile granite at 1520 - 1480 Ma, and Keweenawan events at 1100 Ma. The ages are consistent with the field data, in that the mela-diorite clearly cross-cuts the Nine-Mile 48 �Granite but is chemically and mineralogically dissimilar to younger Keweenawan bodies. Unless its radiometric age has been reset due to uplift, the Koss Pit mela-diorite pluton represents a newly discovered igneous event in central Wisconsin. Fieldwork by the author has found other thin, highly altered dikes of mafic to lamprophyric character cutting older granites elsewhere in central Wisconsin. Their altered character make petrographic and radiometric work difficult. One may speculate that these are related in age to the Koss Pit mela-diorite. The small size, easily eroded character, and extensive glacial cover make the extent and relationships of such mafic igneous rocks difficult to determine. Trace element Spidergram 1000 100 WSC-03-01 10 WSC-03-02 Av. Calc-Alk Lamp 1 Sr K2O Rb Ba Th Ta Nb Ce P2O5 Zr Hf Sm TiO2 Y Yb Sc Cr 0.1 Trace element Buchholz, T.W., A.U. Falster and W.B. Simmons, 2000, “Ta, Nb, U, Y and REE Minerals of the Koss Quarry, Marathon County, Wisconsin” [abstract], 26th Rochester Mineralogical Symposium, Rocks and Minerals, vol. 75, p. 170-171. Joplin, J.A., 1968, “The Shoshonite Association: a review”, Geological Society of Australia, vol. 15 #2, p. 275-294. Pierce, J.A., 1982, “Trace element characteristics of lavas from destructive plate boundaries” in Andesites: Orogenic andesites and related rocks edited by R.S. Thorpe, John Wiley and Sons Pub., p. 525-548. Rock, N.M.S., 1991, Lamprophyres, New York, Van Nostrand Reinhold, 285 p. Wirth, K; J.D. Vervoort and Z.J. Naiman, 1997, “The Chengwatana Volcanics, Wisconsin and Minnesota: petrogenesis of the southernmost volcanic rocks exposed in the Midcontinent Rift”, Canadian Journal of earth Sciences, vol. 34, p. 536-548. 49 �Heller, Sims and Marsden: Mentors Extraordinaire DAVIDSON, Donald M., Jr., P.O. Box 2571, Tubac, AZ 85646 In considering the measure of my “contributions to the science”, particularly as related to UMD, it became apparent that three men played essential roles in this pilgrim’s progress. Robert L. Heller was instrumental in building an outstanding geoscience department at UMD. Basically he hired people, including myself, who were committed to effective teaching as well as research. But above all, he set the pattern for hiring people largely based on “how well we got along” and then “what they did”. This premise served me well in building departments at both UTEP and NIU. Bob was also highly committed to teaching himself and thus was not afraid to ask considerable of his department. I distinctly remember a 27 contact hour quarter my first year in harness! However, he was equally quick to encourage educational innovation and that led to what I consider an outstanding series of team taught courses and course sequences: Earth Materials (integrated mineralogy-petrology); Earth Structure (integrated sedimentationtectonics); Geology of North America; and Precambrian Geology to name a few. Although more difficult to set in place under the semester system, I tended to encourage such thinking at other schools and the team-teaching process served me well at Exxon Research. Finally, through his activities at AGI Bob helped me truly realize that “the whole was indeed bigger than the sum of the parts” and thus I worked diligently at getting AGU back under the AGI umbrella during my tenure at GSA, I believe for the betterment of the science. Paul K Sims is well known to this Institute. His professional work represents to me the essence of doing “good science”. He was a patient reviewer, particularly for neophytes, and thus influenced my approach to graduate student thesis supervision as well as “in-house” publications at Exxon. It was a real pleasure to work for him summers under the sponsorship of the Minnesota Geological Survey mapping in the Boundary Waters with outstanding assistants and colleagues such as Paul Weiblen. While he demonstrated considerable administrative skill as Survey Head, I was fortunate to see such talent in action again during his tenure as SEG President and President of the Economic Geology Publishing Company. This outstanding role model of organization stood me in good stead both serving as director of graduate studies at UMD, but in preparing for and carrying out the first graduate program review. Ralph W. Marsden, as well as helping found the ILSG, was the consummate Department Chair. He was infinitely patient and delegated frequently and well. More importantly, once he gave you a responsibility, he left you alone to work on it, although always available for advice. He also gave young faculty license to develop and grow - in my case organizing informal departmental sessions in 1969 on a new paradigm for how the earth works: Plate Tectonics. Yet I believe Ralph will best be remembered for his service to the science through society work. He was extremely active both in SEG and AIME and encouraged young faculty to join and participate in societies of their interest. Under his tutelage I became the Business Manager for Economic Geology, and later Treasurer of SEG. I am truly pleased SEG named its distinguished service medal after him. None could be more deserving. Happy Fiftieth Birthday UMD Geology and ILSG! 50 �THE VAN HISE ARMY AND OTHER PIONEERS OF LAKE SUPERIOR GEOLOGY DOTT, R.H., Jr., Department of Geology & Geophysics, University of Wisconsin, Madison, WI 53706 Sir William Logan, first Director of the Canadian Geological Survey (1842-1869), initiated investigations of Precambrian rocks. He coined Laurentian for the complex granitic and gneissic rocks of southern Ontario and adjacent Quebec and he assumed that these represented the original continental crust. The Algoman granite and overlying Huronian sedimentary series were soon named north of Lake Huron. These three names were thought to represent universal Precambrian subdivisions for many years. The first comprehensive surveys south of Lake Superior were initiated by Congress to investigate mineral deposits. The first two were led by David Dale Owen in 1839-40 to the lead mining district of the upper Mississippi Valley and in 1848-49 farther north to Lake Superior. In 1850-51 J.W. Foster and J.D. Whitney surveyed northern Michigan and adjacent Wisconsin, where copper and iron deposits were known. Minnesota, Wisconsin, and Michigan funded state surveys during the 1860s and 1870s, but these were parochial and uneven in quality. Most important were surveys under N.H. Winchell in Minnesota, T.C. Chamberlin in Wisconsin, and T.B. Brooks and R.J. Pumpelly in northern Michigan. For the Tenth National Census of the United States, Congress mandated that the U.S. Geological Survey catalogue the nation’s mineral resources. In 1880 several university geologists, including T.C. Chamberlin and Roland D. Irving of Wisconsin, were recruited to help accomplish this formidable task. At the conclusion of the census, Wisconsin’s Irving suggested that an integrated geological investigation of the Precambrian of the Lake Superior region was needed to facilitate the understanding and exploitation of the iron ranges. U.S. Geological Survey Director J.W. Powell agreed, and in 1882 created a Lake Superior Division to be located at Madison with Irving in charge. The first of nine large USGS Monographs to be published by the Division was Irving’s Copper-Bearing Rocks of Lake Superior (1883). In this he recognized the Lake Superior syncline and presented petrographic analyses of varied Keweenawan rocks, an early application of that important, new technique. In 1888 Irving died suddenly, so his young assistant, Charles R. Van Hise, immediately became both Director of the Division and Professor of Geology. The program went forward at a fast pace with a small army of young geologists fanning out across the different iron ranges. Four more Monographs and two Bulletins appeared during the 1890s. In 1903, Van Hise was chosen President of the University of Wisconsin, so his protégée, Charles K. Leith, took over both of his mentor’s former offices. Leith’s own Mesabi Range Monograph (1903) and four others plus one Bulletin were published under his direction. Besides an overarching synthesis of all of the work of the army, which was published in 1911 (Monograph 52), Van Hise and Leith devoted much attention to the development of universal principles for deciphering complex structures and metamorphism in terms of fundamental mechanics and chemistry. To cope with the scattered nature of rock exposures in a recently glaciated region of complex geology, the USGS group honed techniques for determining the relations between visible outcrop-scale (mesoscopic) and very obscure larger-scale (macroscopic) structures. Slaty cleavage and drag folds were employed early as valuable tools for such analysis, and in 1910 William O. Hotchkiss, a student of the Van Hise-Leith school (and about-to-be Wisconsin State Geologist), recognized the value of cross bedding and graded bedding for determining ‘way up.’ These fundamental insights gained by the Van Hise army and ground-breaking textbooks by Leith of structural geology (1913), metamorphic geology (1915; 51 �with his protégée Warren J. Mead), and economic geology (1921) thrust the University of Wisconsin’s Department of Geology into international prominence. Soon many students came from Canada, China, Japan, and Britain for postgraduate work in the ‘Wisconsin School of Precambrian Geology.’ The largest contingent was from Canada after 1910 when Director R.W. Brock of the Geological Survey of Canada adopted the policy that survey geologists henceforth must have the Ph.D. degree. The flow from the north to Wisconsin and other U.S. universities continued until the 1960s when more Canadian institutions began granting the Ph.D. By 1900 there was a need to reconcile some Canadian and U.S. interpretations of the major divisions of the Precambrian rocks of the Lake Superior region. Of particular note was a discrepancy in the Rainey Lake area along the Minnesota-Ontario border, where Andrew C. Lawson had inferred in 1887 that the oldest rocks were sediments of the Coutchiching formation, but U.S. geologists believed that the Keewatin volcanic complex was older and that the Coutchiching was equivalent to the younger Knife Lake series in Minnesota. In 1905 an international committee of survey geologists from both countries reviewed the evidence and favored the U.S. interpretation. In response, Lawson restudied the area in 1911, but stubbornly reaffirmed his original belief in spite of the fact that his field assistant, J.D. Trueman, a PhD student of Leith’s, showed him the value of cross bedding for determining ‘way up,’ which should have led recalcitrant Lawson to see his error. It was not until 1925 that Frank F. Grout of Minnesota disproved Lawson definitively using both graded bedding and cross bedding. The Minnesota-Ontario border region also became the burial ground of the long-standing dogma that Laurentian granites and gneisses represented the original crust of North America, for here the Keewatin volcanic complex now claimed that title. By this time the commonly accepted divisions of the Lake Superior Precambrian record had become Archean (including Keewatin, Algoman and Huronian) and Proterozoic (including Animikie and Keweenawan). By the mid-twentieth Century, the major stratigraphic divisions of the region were established and isotopic dating was beginning at last to provide a sound basis for long-range correlations. A.O. Nier and S.S. Goldich at the University of Minnesota were especially important in applying the new techniques to the Precambrian of this region, which allowed the recognition and dating of several additional tectonic and metamorphic events. Meanwhile, Francis J. Pettijohn had pioneered the study of Precambrian sedimentary rocks with his classic Archean Sedimentation (1943) and many subsequent studies with his University of Chicago graduate students. Stanley A. Tyler’s discovery in 1953 of the Gunflint fossils on the north shore of Lake Superior and Preston Cloud investigations while at the University of Minnesota in the 1960s revolutionized thinking about Precambrian life. Meanwhile the study of the region was greatly enhanced by the widespread application of geophysical techniques such as aeromagnetic surveys and geochemical studies. Finally, since 1970 plate tectonics has revolutionized our understanding of the evolution of the Lake Superior region. 52 �A Field and Laboratory Study to Evaluate the Genetic Relationships Between the Purvis Pluton and Volcanic Rocks and Volcanic-Associated Mineralization in the Vermilion District of NE Minnesota DREXLER, H.L.*, HUDAK, G.J., Geology Department, University of Wisconsin Oshkosh, 800 Algoma Blvd., Oshkosh, WI 54901; drexlh92@uwosh.edu PETERSON, D.M., Natural Resources Research Institute, 5013 Miller Trunk Highway, Duluth, MN 55811 The Purvis Pluton is an east-west trending, moderate- sized (~3km3), sill-like multiphase dioritic to tonalitic intrusion with a strike length of 5.7 km and a thickness, which ranges from 100-1200 meters (Peterson, 2001). This intrusion occurs near the base of the eastern part of the Ely Greenstone – Lower Member (Peterson and Jirsa, 1999; Jirsa et al., 2001). Peterson (2001) has suggested that the Purvis Pluton represents a felsic, synvolcanic sill-like subvolcanic intrusion that may have been the heat engine, which drove subseafloor hydrothermal activity, which produced VMS-like Cu-Zn mineralization at the Eagles Nest and Purvis Road prospects. The evidence for this interpretation is based on the local presence of an east-west oriented D2 foliation within the Purvis Pluton, the lack of a significant contact aureole adjacent to the intrusion, and the intimate relationship between the uppermost margins of the pluton and intense, semi-conformable quartz + epidote alteration zones. Volcanic rocks in the Ely Greenstone – Lower Member consistently contain the D2 foliation, which is constrained by age dates to have occurred during regional deformation between 2674 and 2683 Ma (Boerboom and Zartman, 1993; Peterson et al., 2001). We have conducted detailed field mapping, petrographic and lithogeochemical studies to further evaluate the spatial, mineralogical, and chemical characteristics of the Purvis Pluton. Field mapping, supported by subsequent petrographic studies, indicates that the Purvis Pluton contains several distinct phases. These include: 1) xenolithic hornblende diorite; 2) xenolithic hornblende tonalite; 3) xenolithic leucotonalite; 4) leucotonalite and trondhjemite; and 5) leucotonalite dikes. Paragenetic relationships between these phases have been determined in the field based and by petrography based on cross-cutting relationships and the xenolith contents of the various phases. Angular, coarse-grained gabbro/diorite lapilli, which have rare earth element characteristics similar to the other phases of the pluton (Figure 1a), are common in the xenolithic hornblende tonalite and xenolithic leucotonalite phases, and appear to represent an early product of Purvis Pluton crystallization. The xenolithic hornblende diorite, xenolithic hornblende tonalite, and xenolithic leucotonalite commonly contain basalt-andesite lapilli, epidote + quartz-altered basalt-andesite lapilli, and lapilli and blocks of oxide facies iron formation. These three xenolithic phases are intruded by the main phase of the intrusion, leucotonalite/trondhjemite. Leucotonalite dikes are a minor phase of the intrusion, and when present, consistently cut through the other plutonic phases. A sample of the hornblende tonalite has been submitted for geochronological analysis, and we are anxiously awaiting the results. Preliminary lithogeochemical evaluations indicate that the Purvis Pluton is calc-alkalic (Figure 1b). The various phases of the intrusion are classified as tonalite and trondhjemite using O’Connors normativebased granitic rock classification scheme (Figure 1c). Based on trace element characteristics, all phases of the Purvis Pluton were formed in a volcanic arc setting (Figure 1d), which is consistent with the geochemical affinity of the volcanic strata in the Ely Greenstone – Lower Member (Hudak et al., 2002). Recent studies by Galley (2003) have evaluated the physical and chemical characteristics of synvolcanic intrusions spatially and temporally associated with VMS deposits in Canada and Scandinavia. We are currently completing our evaluation of the lithogeochemical features of the Purvis Pluton, and our comparison of these features to VMS-associated synvolcanic intrusions associated with VMS deposits. 53 �Figure 1. a) REE diagram illustrating trends associated with Purvis Pluton phases, gabbro/diorite xenoliths, and amphibolite xenoliths; b) normative alkali - total iron – magnesium tertiary plot (after Irvine and Baragar, 1971); c) normative feldspar tertiary plot for the Purvis Pluton (after O’Connor, 1965); d) Rb – (Y + Nb) discriminant diagram for the Purvis Pluton (after Pearce et al., 1984). References Boerboom, T. J., and Zartman, R. E., 1993, Geology, geochemistry, and geochronology of the central Giants Range batholith, northeastern Minnesota: Canadian Journal of Earth Sciences, v. 30, p. 2510-2522.Boerboom and Zartman, 1993 Galley, A., 2003, Composite synvolcanic intrusions associated with Precambrian VMS-related hydrothermal systems: Mineralium Deposita, v. 38, p. 443-473. Hudak, G. J., Heine, J., Newkirk, T., Odette, J., and Hauck, S., 2002. Comparative geology, stratigraphy, and lithogeochemistry of the Five Mile Lake, Quartz Hill, and Skeleton Lake VMS occurrences, Vermilion District, NE Minnesota: NRRI Technical Report NRRI/TR-2002/03, 390 pages. Jirsa, M. A., Boerboom, T. J., and Peterson, D. M., 2001, Bedrock geological map of the Eagles Nest Quadrangle, St. Louis County, Minnesota: Minnesota Geological Survey Misc. Map Series M-114. O’Connor, J. T., 1965, A classification for quartz-rich igneous rocks based on feldspar ratios: USGS Professional Paper 1965:B79-B84. Pearce, J. A., Harris, N. B. W., and Tindle, A. G., 1984, Trace element discrimination diagrams for the tectonic interpretation of granitic rocks: Journal of Petrology, v. 25, p. 956-983. Peterson, D. M., 2001, Development of Archean lode-gold and massive sulfide deposit exploration models using geographic information system applications: targeting mineral exploration in northeastern Minnesota from analysis of analog Canadian mining camps: unpublished Ph. D. dissertation, University of Minnesota, Duluth, Minnesota, 503 p. Peterson, D. M., Gallup, C., Jirsa, M. A., and Davis, D. W., 2001, Correlation of Archean assemblages across the U.S.- Canadian border: Phase I geochronology: 47th Annual Meeting, Institute on Lake Superior Geology, Proceedings Volume 47, Part 1 – Programs and Abstracts, p. 77-78. Peterson, D. M., and Jirsa, M. A., 1999, Bedrock Geological Map and Mineral Exploration Data, Western Vermilion District, St. Louis and Lake Counties, Northeastern Minnesota: Minnesota Geological Survey Miscellaneous Map Series M-98, scale 1:48,000. 54 �Late Wisconsin Till and Arsenic Contamination in Upper Midwest Groundwater ERICKSON, Melinda L.*(Water Resources Sciences), and BARNES, Randal J., (Civil Engineering), University of Minnesota, 500 Pillsbury Dr. SE, Minneapolis, MN 55455, eric0984@umn.edu Exposure to arsenic, a recognized human carcinogen, is a widespread public health problem, with more than 150 million people worldwide estimated to be exposed to unsafe levels of arsenic from their drinking water. To reduce arsenic exposure from drinking water, the US Environmental Protection Agency recently adopted a new, more protective arsenic drinking water standard, 10 µg/l. All public water systems must comply with the new arsenic standard by January 2006. In the upper Midwest, arsenic in ground water is a widespread, naturally occurring contamination problem regionally impacting both public and private drinking water wells. Hundreds of upper Midwest public water systems serving over a million people are affected by the change in the arsenic standard. Additionally, in Minnesota alone, 150,000 – 250,000 people are estimated to obtain drinking water from private wells with arsenic concentrations exceeding 10 µg/l. Private well owners are not forced to comply with federal drinking water standards. Figure 1 illustrates that groundwater arsenic concentrations in excess of 10 µg/l are associated with the lateral extent of northwest-sourced late Wisconsin sediment. Statistical analysis supports the visual observation: 10.7% of public water systems located within the footprint of the Late Wisconsin till exceed 10 µg/l, and only 2% of public water systems outside the footprint exceed 10 µg/l. Our research results indicate that the elevated arsenic concentrations in upper Midwest groundwater are not primarily due to high arsenic concentrations in Late Wisconsin till. Sediment analyses indicate that Late Wisconsin sediment in western Minnesota does not have particularly high arsenic concentrations, and water arsenic and sediment arsenic concentrations measured in our research are not correlated (Figure 2). We hypothesize that the specific physical characteristics of the Late Wisconsin till, such as its fine-grained matrix, entrained organic carbon, and active anaerobic biological activity, create the geochemical environment favorable to a regional-scale mobilization of arsenic via desorption. In western Minnesota aquifer sediments, we measured that 0.5 – 0.7 mg/Kg of the total arsenic is adsorbed arsenic. Adsorbed arsenic is labile and can be readily desorbed, especially in suboxic and reduced aquifers. 55 �# S S # S # # S S # S # S # S # S # S # S # S # # S S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # # S S # S # S # S # S # S # S # S # S # S # S # S # S # S #S S # S# # S # S # S # S # # # S S S # S # S # S # S # S # S # North Dakota S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S# # S S # S # S # # S S # S # S # S # S # # S # S S # # S S # S # S # S # S # S # S # S # S # S # S # S # S # # S S # S # S # # S S # S # S # # S S # # S S # S # S # S # S # S # S # S # S # S # S # S # S # # S S # # S S # S # # S S # S # S # S # S # S # S # S # S # S # SS# # S S # S# # S# # S S# # S S # S# # S # # S S S S# # S S # S # # S S# S# S # S # S# # S# # S# ## S # S# S# S # S S S# S S# # S S# S# # S S # # S S# # S# S# # SS # S S # S# # S # S S S # # S # S # S # S # S # S # S # S # S # S # # S S # S # S # S# # S S # S # S # S # S # S # S # S # S # S # S # S # S # ## S S S # S # S # S # # S S # S # S # S # S # S # S # S # S # 75 0 75 S # S # # S S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # # S S # # S S # S # S # S # # S # S S # S # S # S # # S S # S # S # S # # S # S S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # # S S # S # S # # S S # # S S # S # # S S # S # S # S # S # S # # S S # # S S # S # S # S # S # S # # S S # S # ##S S # S S # S # # S # S # S # S # S S # # S S # S # S # S # S # # S S # S # S # S # S # S # S # S # S # # S S # S # S # S # S # S # S # S # S # S # S # S # S # # S S # # S # S S # S # S # S # S # S # # S # S S # S # S # S # # S # S S # S # S # S# # S S ## S S # S # S # S # S # ##S # S S # S ##S#S S # S S # S # # S S # S # S # S # # S S # S # S # S # S # S # S # S # # S # S # S S # S # S # S # S # S # S # S # # S S # # S S # S # S # S # S # S # S # S # S # S # S # S # # S S # # S # S S # S # S # S # S # # S S # # S # S S # S # S # S # S # # S # S # S # S S # S # S # S # S # S # S # S # S # S # S # S # S # # S # S S # S # # S # S # S S # S # S # S # S # # S S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # # S S # # S S # S # S # S # S # # S S # # S # S S # S # S # S # S # S # S # S # S # S # # S S # S # # S S # S # S # S # S # S # # S # S S # # S S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # # S S # # S S # S # # S S # S # # S ## S S # S # S # S Iowa S # S # S # S # S # S # S # # S S # S # S # S # S # S # S # S # S # S # S # S # S # # S S # S # S # S # # S S # S # S # S # S # S # S # # S S # S # S # S # # S S # S # S # S # S # # S S # S # S # S # S # S # S # # # #S S S # S # #S S S # S # S # S # # S S # S # S # S # S # # S # S S # S # S # # S S # # S # S S # S # S # S # # S S # S # S # # S S # S # S # # S S # # S ## S S S # S # S # # S S # S # S # S # S # S # # S S # S # S # S # S # S # S # S # S # S # S # S # # S S # S # S # S # S ## S S # # S S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # # S S # S # S # S # S # S # # S S # S # S # S # S # S # S # S # S # # S # S # S # #S S #S S # S # # S S # S # S # S # S # S # S # S # S # S # # S S # S ## S S # S # # S S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # # S S #S # S # S # S # S # S # ## S S S # S # S # S# # S S # S # S # S # S # S # S # S # S # S # S # S # # S S # S # S S # # S # S # S # S # S # S # S # S # S # # S S # S # S # S # S # S # S S # # S # S # Arsenic Concentration 0 - 10 ug/l 10 - 20 ug/l # 20 - 50 ug/l S Late Wisconsin Till S # # S S # S # S # S # S # # S S # S# # S# ## 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S S # S # S# # S # S # S S # S # S # # S # S S # S # S # S # S # S # S # S # # S S # S # S # S # S ## S # S ## S S S # ## S S S # S# # S S # S # S ## S # S S # S # S # S # S # # S S # S # S # S # S # S # S # S # # S S # S # S # S # S # S # S# S # # SS S # # S # # S S # S # S # S # S # S # S # S # S # S # S # ## S S S # S # S # S # S # # S S # S # S # S # S # S # # S S # S # S # S # S # # S ##S S # S S # S # S # # S # S S # S # S # S # S # S # S # S # # S S # S # S # S # S # S # S # # S S # S # S # S # S # # S S # S # S# # S S # S# # # S S S # # S S # S # S # S # S # S # S # # S S # S # S # S # S S # # S # S # S # # S S # S # S # S # S # S # S # S # # S S # S # S # S# # S S # S # S # S # # S S # S# # S S # S # # S # S S # S # S # S # S # S # S # S # S # S # S # S # # S S # S # S # S # S # S # S # S # S # S # S # ## S SS # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # # S S # S # S # S # # S # S S# # S S S # S# # S# # S # S S # # S S # # S S # S # S # S # S # S # S # S S # ## S S # S # S # S # S # S # S# # S S # S# # S S S ## S S# # S# S# # S S # S # S # S # S # # S S # S # S # S # # S S # S # S # S # S # # S S # S # S # S # S # S # S # S # S # S # S # # S S # S # S # # S S # S # # S S # Figure 1 – Arsenic concentrations in upper Midwest public water systems Figure 2 – Measured arsenic concentrations in water and sediment collected from western Minnesota. 56 �INFLUENCE OF GEOLOGIC SETTING ON HYDROGEOMORPHIC CHARACTERISTICS OF SOUTHERN LAKE SUPERIOR TRIBUTARIES FITZPATRICK, Faith A., U.S. Geological Survey, 8505 Research Way, Middleton, WI 53562, fafitzpa@usgs.gov The U.S. Geological Survey has conducted assessments of historical and current geomorphic, sediment, and flooding characteristics for several Lake Superior tributaries in the Duluth, Minnesota and Bayfield County, Wisconsin areas. These assessments were done to determine historical natural and human-caused alterations in aquatic habitat and to provide base line data for stream restoration activities. For streams with little or no bedrock control, such as those in the Bayfield County area, historical geomorphic responses to clear-cut logging and burning in the late 1800s and subsequent agriculture in the early 1900s included channel incision and lateral migration in upstream reaches and aggradation, widening, and lateral migration in downstream reaches (Fitzpatrick et al., 1999). However, these geomorphic responses to changes in land cover were dependent on composition of glacial deposits, type of glacial or glaciolacustrine landforms, spatial position within the watershed, and relative timing of large floods. Geomorphic responses to increased runoff and sediment inputs were manifested during extreme floods that occurred one to two decades after maximum agricultural activity (Fitzpatrick and Knox, 2000). For streams with bedrock control, such as those in the Duluth area, the main geomorphic response to land clearing, such as urban sprawl, is channel widening. The major source of sediment in most of the southern Lake Superior tributaries has been and continues to be from channels down-cutting through relict glacial lake shorelines. Longitudinal profiles of the tributaries are a reflection of a combination of glacial and glacio-lacustrine landforms and bedrock topography (fig. 1). Slopes are steep where streams intersect glacial lake shorelines and (or) bedrock near the surface. Longitudinal profiles are a useful reconnaissance tool for identifying stream reaches prone to erosion or sedimentation. Inflection points on the longitudinal profiles represent reaches with slope transitions. These transitional reaches tend to be the most sensitive to changes in water and sediment inputs. 1,500 Miller Creek, Duluth, MN 1,400 ALTITUDE, IN FEET 1,300 1,200 Cranberry River, Herbster, WI 1,100 1,000 900 800 700 North Fish Creek, Ashland, WI 600 25 20 15 10 5 0 RIVER MILE FROM MOUTH Figure 1. Longitudinal profiles for three Lake Superior tributaries. REFERENCES Fitzpatrick, F.A., and Knox, J.C., 2000, Spatial and Temporal Sensitivity of Hydrogeomorphic Response and Recovery to Deforestation, Agriculture, and Floods: Physical Geography 21(2): 89-108. Fitzpatrick, F.A., Knox, J.C., and Whitman, H.E., 1999, Effects of Historical Land-Cover Changes on Flooding and Sedimentation, North Fish Creek, Wisconsin: U.S. Geological Survey Water-Resources Investigations Report 99-4083, 12 p. 57 �IRON NODULE RESEARCH AT THE NATURAL RESOURCES RESEARCH INSTITUTE, UMD FOSNACHT, Donald R., Center for Applied Research and Technology Development, Natural Resources Research Institute, University of Minnesota Duluth, Duluth, MN IWASAKI, Iwao, and BLEIFUSS, Rodney, Coleraine Minerals Research Laboratory, Natural Resources Research Institute, University of Minnesota Duluth, Coleraine, MN Synopsis A program jointly funded by the Economic Development Administration and the University of Minnesota Permanent University Trust Fund has been underway at the Coleraine Minerals Research Laboratory over the last two years. During this period, significant research and development has taken place using a variety of new furnace capabilities to test the responsiveness of iron ore taconite concentrates to reduction and smelting under a variety of test conditions. During the coarse of the investigation, over 1150 tests were undertaken using a laboratory tube furnace, these were supplemented by over 200 tests from a 2-stage laboratory box furnace, and finally by dozens of test from a 3-stage pilot-scale linear hearth furnace. During the program, the conditions for producing satisfactory iron nodules with low sulfur, gangue, and tramp impurity levels were elucidated. A variety of reductants, slag fluidizers, and iron ore mixtures were employed during the test program. Laboratory Tube Furnace Tests The test program was initiated using a tube furnace (see Figure 1) with a 2” dia. x 48” long mullite tube, which takes 1” wide x 4” long and 1” high graphite boat, to screen the test conditions for use in laboratory box and pilot plant linear hearth furnaces. Major parameters investigated included such raw materials as: (1) taconite concentrates with different levels of silica content as well as pellet plant wastes and screened pellet fines, (2) different carbonaceous reductants including Eastern anthracite, low-, medium- and high-volatile bituminous and Western sub-bituminous coals as well as their carbonized char and coke, and (3) different types of additives, such as balling binders and some specific additives for slag fusion temperature reduction and iron nugget sulfur control. Furnace operating conditions included temperature and time at temperature, furnace atmosphere, hearth layer materials, iron nugget and slag chemistries as well as iron nugget size. Environmental issues of concern are slag disposal and/or utilization and effluent emission of mercury, NOX, SOX and particulate matter. Figure 1: Laboratory Tube Furnace In the tube furnace, over 1150 different conditions have been tested. Test results demonstrated that larger-sized iron nuggets can be routinely produced by feeding dry raw material mixtures without prior agglomeration. Taconite concentrates with different levels of silica indicated that magnetic concentrates with 6% SiO2 produced metallic iron nuggets more readily than a more expensively produced super-concentrate of 2% SiO2. Pellet plant wastes and screened pellet fines produced satisfactory iron nuggets, but consisting mainly of hematite, these raw materials appeared to require somewhat different conditions. Laboratory Box Furnace Tests A laboratory, electrically-heated box furnace (see Figure 2), having two 12”x12”x12” heating chambers with the two chambers capable of controlling temperatures up to 1450°C (2642°F) independently, and which accepts a 5” wide x 6” long x 1-1/2” high graphite tray was designed and constructed during the project. Over 220 different conditions have been tested in the box furnace, confirming the results obtained in the tube furnace for both dry balled feed and a feed without prior agglomeration. In the box furnace, a major emphasis was placed in developing 58 �methods to produce larger-sized iron nuggets by feeding dry raw material mixtures in an attempt to circumvent costly balling and drying steps. A series of different size iron nuggets were produced, ranging from 5/15” to 2-1/2” (7 to 65 mm) in size. Figure 2: Laboratory Box Furnace Pilot-plant Linear Hearth Furnace Tests (Rotary Hearth Simulator) The natural gas-fired pilot-scale linear hearth furnace simulator has been installed and commissioned. The furnace is a forty-foot long iron reduction furnace (see Figure 3), consisting of three individual heating zones and a final cooling section. Sample trays are conveyed through the furnace by a hydraulically driven walking beam system. Zones are controlled individually according to temperature, pressure and feed rate, making this furnace capable of simulating several reduced iron processes and operating conditions. An Allen Bradley PLC micro logic controller coupled to an Automation-Direct PLC for the walking beam mechanism controls the furnace through a user-friendly PC interface. The PLC control system regulates individual zone burners to manage zone temperatures. A pair of 450,000 BTU/hr natural gas fired burners heats zones one and two. Zone one is rated for a continuous operating temperature of 2000 oF, while zone two can be continuously operated up to 2400 oF. Zone three is fired by a pair of 1 Million BTU/hr burners that were required to achieve the operating temperatures of 2600 oF in reasonable time to complete testing. Each zone has an individual exhaust duct and control damper to regulate pressure in that zone. A manually controlled exhaust fan damper is also installed to reduce the capacity of the exhaust fan, and allow the individual duct control dampers to manipulate pressures to desired set-points. Reducing the burner air, to operate the burners sub-stoichiometric, and operating zone pressures positive is required to reduce oxygen levels to 0.0% and provide acceptable furnace atmospheres for iron reduction. Figure 3: Pilot Scale Linear Hearth Furnace (Rotary Hearth Simulator) Major differences in the test conditions from laboratory electric furnaces were the low CO/CO2 ratio and high turbulence of the furnace gas and the sample pallets acting as an unexpectedly large heat sink. Research work focused on careful adjustment in the amount of coal addition, and on the type and amount of additives in order to minimize the generation of micro nuggets. The Linear Hearth Furnace has routinely been used to test a variety of the test variables shown to be important from the box furnace and tube furnace tests. It is possible to make various size iron nuggets at this scale that have low amounts of micro-nuggets and which have low levels of undesirable tramp elements. The furnace is extremely useful for testing a multiplicity of test parameters in a very short period of time. 59 �OXYGENATION OF THE ARCHEAN HYDROSPHERE: EVIDENCE FROM THE EAGLE ISLAND DELTAIC COMPLEX FRALICK, Philip, Department of Geology, Lakehead University, Thunder Bay, ON, Canada, P7B 5E1, philip.fralick@lakeheadu.ca PUFAHL, Peir K., Department of Geological Sciences and Geological Engineering, Queens University, Kingston, ON, Canada, K7L 3N6, pufahl@geol.queensu.ca The Eagle Island Group is located in southern Uchi Subprovince, Canadian Shield, at the west end of Lake St. Joseph. It overlies 2713 Ma volcanic rocks and is deformed by an event, which probably occurred prior to 2702 Ma (Stott and Corfu 1991). Outcrop along the shores of Eagle Island is excellent and provides a rare opportunity to document an Early Precambrian depositional system including abundant iron formation. The lowermost 35 m of section contains three coarse-grained parasequences separated by assemblages of graded sandstone beds and iron formation (IF) (Fig. 1). The IF consists of three types: (a) dominantly magnetite-rich sediment with mm- to cm-scale, graded or ungraded siltstone interbeds: (b) cm-scale graded to sharply bounded siltstone layers either contiguous or separated by mm-thick laminae of magnetite-rich sediment: (c) sandstone beds in places separated by magnetiterich intervals. The lowermost coarse-grained parasequence consists of graded sandstones and conglomerates separated by a-type IF units. The second parasequence contains: graded sandstones and conglomerates separated by magnetite; graded sandstone lenses surrounded by magnetite; crossstratified sandstones with magnetite drapes on reactivation surfaces; low angle, laterally accreting sandstone and conglomerate packages with internal magnetite laminae; multistory conglomerates with internal magnetite drapes; and ripple laminated sandstones in a-type IF with mm-thick magnetite laminae draping avalanche surfaces in ripple trains. The upper parasequence is composed of cross-stratified, coarse-grained sandstones interlayered with conglomeratic lenses. This is sharply overlain by 73 m of b-type, gradational to a-type, IF. A 182 m thick succession of graded, medium-grained sandstone beds overly the IF and these are succeeded upwards by 67 m of trough cross-stratified, coarse-grained sandstones. Next is a 60 m thick sandstone-conglomerate assemblage consisting of sharp-sided, nongraded laterally extensive, thin conglomerate lenses interlayered with well-sorted medium-grained sandstones with internal, shallowly inclined pebble stringers. Approximately 50% of the clasts are IF. This assemblage is overlain by a thin package of turbidites capped by a-type IF. 60 �The Eagle Island Group was previously considered a deep-water submarine fan deposit (Meyn and Palonen 1980, Berger 1981). The lithofacies associations present and their architectural organization make this unlikely. The thick succession of graded beds is interpreted as shallow-water storm deposits and the trough cross-stratified sandstones represent either nearshore sands, a distributary mouth bar complex or braided fluvial channels. The overlying conglomerate-sandstone unit consists of layers characteristic of foreshore beach deposits interlayered with fluvial mouth gravel bars. The 400 m thick, coarsening upwards sequence represents a wave modified delta. Thirty-five meters of strata underlying the coarsening-upwards delta is also progradational with three parasequences building from subaqueous (lower) to strandline (middle) to braided fluvial (upper). The parasequences are separated by IF developed on flooding surfaces. The most interesting features exist in the middle (strandline) parasequence. Here, IF was deposited at periods of low stream discharge in the very proximal distributary mouth environment. Magnetite laminae drape all scales of reactivation surfaces developed in this proximal nearshore setting. In contrast the 73 m thick IF was deposited on the delta top during a major flooding event and represents a portion of the transgressive and highstand systems tract (Fig. 2) where chemical sedimentation kept pace with relative sealevel rise. The accumulation of IF only in the nearshore of this depositional system limits possible precipitation mechanisms. IF accumulation models relying on relatively constant Fe precipitation in the world ocean combined with siliciclastic starvation are not compatible with data presented here. Freshwater influx into the nearshore, and resultant increase in pH, may have been responsible for iron precipitation but accompanying fresh water dilution of marine waters makes this unlikely. Photo-synthetically induced oxygenation of the shallow nearshore is the probable cause of iron precipitation. Berger, B.R. 1981. Stratigraphy of the west Lake St. Joseph greenstone terrain, northwestern Ontario. Unpub. M.Sc. Thesis, Lakehead University, 117p. Meyn, H.D. and Palonen, P.A. 1980. Stratigraphy of an Archean submarine fan. Precambrian Research, Vol. 12, 257-285. Stott, G.M. and Corfu, F. 1991. Uchi Subprovince. In: Geology of Ontario. Ont. Geol. Sur. Spec. Vol. 4, Pt. 1, 145-238. 61 �Isocon Analysis: How To Make It Work For You. GRANT, James A., Department of Geological Sciences, University of Minnesota Duluth, MN 55812 Isocon analysis (Grant, 1986) is a simple and effective means of quantitatively estimating changes in mass or volume or concentrations in mass transfer. The method has been applied to such diverse phenomena as hydrothermal alteration, replacement, migmatites, shear zones, paleosols, silcretes, sedimentary exhalative deposits and fumarolic deposits. It may be accomplished graphically by plotting an altered composition (CiA) against an original composition (CiO) with no significant manipulation of the data. Species that have remained immobile in the process define the isocon, which is a straight line through the origin. Data points falling above the isocon represent gain, and those below represent loss, of the corresponding chemical species, and the slope of the isocon gives the mass change in the process. It is critical to obtain as close an approximation to the original rock composition as possible, since that rock no longer exists. Sampling the altered rock is generally less problematic, even if the alteration is zoned. Given zonation or general heterogeneity of the rocks, judicious sampling and averaging of samples is necessary. Fortunately it is painless to try different combinations and arrive at a reasonable compromise. Scaling should be important only in producing a satisfactory isocon diagram to portray the model in question. If the slope of the isocon is based on CiA/CiO values, scaling cannot affect the results because the scale factor cancels out. Scaling can however affect the perception of the results especially if points are crowded close to the origin. The choice of immobile species can be determined by inspection of a well-constructed isocon diagram, by inspection of CiA/CiO values, by statistical methods like that of Baumgartner and Olsen (1995), or by plotting pairs of species like Cail and Cline (2001). In any case the geochemical characteristics of the species and of the process involved need to be considered thoughtfully. Characterization of an isocon based on immobile species (as opposed to constant mass, volume or predetermined species) devolves to defining a slope for the isocon. This may be done graphically, by a least squares method of linear regression, or by averaging the slopes for the immobile species. Commonly there is some range within which reasonable isocons could be chosen. Data points that lie close to a chosen isocon within such ranges would correspond to small gains and losses, and not much significance should be placed on them. Data points far from the range of possible isocons will not be affected significantly by the minutiae of the choice of isocon. Log-log plots do not add anything to the analysis, except the possibility of confusion, and should be avoided. 62 �RFERENCES Baumgartner, L. P. and Olsen, S. N., 1995. A least-squares approach to mass transport calculations using the isocon method. Economic Geology, 90, 1261-1270. Cail, T. L., and Cline, J. S., 2001. Alteration associated with gold deposition at the Getchell Carlin-type gold deposit, North-central Nevada. Economic Geology, 96, 1343-1359. Grant, J. A., 1986. The isocon diagram - a simple solution to Gresens' equation for metasomatic alteration. Economic Geology, 81, 1976-1982. 63 �THE GEOLOGY OF THE DULUTH COMPLEX AND THE NORTH SHORE VOLCANIC GROUP PORTRAYED IN NEW 7.5' QUADRANGLE MAPS OF THE DULUTH METROPOLITAN AREA GREEN, J.C., Department of Geological Sciences, University of Minnesota Duluth, Duluth, MN 55812 (jgreen@d.umn.edu) MILLER, J.D., Jr., Minnesota Geological Survey, c/o Dept. of Geological Sciences, University of Minnesota, Duluth, MN 55812 The most current bedrock geologic map of the Duluth area is the 40 year old map by R.B. Taylor (1964), which was the first detailed-scale (1:24,000), full color map of Precambrian bedrock geology published by the Minnesota Geological Survey. This map, which built on the earlier field studies of Grout (1918) and Schwartz (1949), served as a companion to MGS Bulletin 44 and focussed mainly on the geology and petrology of the Duluth Complex. Taylor's map divided the complex into two major series, the layered series and the anorthositic series, and several subordinate units. The volcanic rocks of the North Shore Volcanic Group (NSVG), which compose the hanging wall and part of the footwall of the Duluth Complex, were not subdivided, but the major mafic sills that intrude the hanging wall volcanic rocks were delineated. The first author (Green) has conducted reconnaissance mapping in the Duluth area since 1960, focussing mostly on the volcanic rocks of the Duluth Quadrangle. In 1992, the Minnesota Geological Survey received funding from the Minnesota Minerals Coordinating Committee to conduct detailed mapping in nine quadrangles encompassing the Duluth metropolitan area. Approximately five months of intense field mapping resulted in the production of a 1:48,000-scale open-file map (Miller, Green and Chandler, 1993). We have conducted intermittent mapping over the past 11 years, which has continued to improve our understanding of the Duluth area. These field data are currently being digitally compiled into new 1:24,000-scale maps for the Duluth, Duluth Heights, and West Duluth quadrangles and parts of the Esko and Adolph quadrangles. Preliminary versions of these maps will be displayed as a poster presentation. One of the most important contributions of these new maps is their detailed delineation of the igneous stratigraphy of the layered series. As Taylor (1964) recognized, the layered series at Duluth (DLS) is a well-differentiated, 3- to 4.5-km-thick, moderately east-dipping, sheet-like mafic layered intrusion. Our mapping has subdivided the DLS into six major zones and various subzones on the basis of dominant cumulate rock types. A 150-300m-thick basal contact zone is composed of coarse-grained, taxitic olivine gabbro and augite troctolite. This is overlain by a 300-600m thick zone of complexly layered cumulate rocks including feldspathic dunite, oxide peridotite, melatroctolite, augite troctolite and olivine gabbro that comprise the melanocratic zone. These rock types commonly define several macrocyclic subzones grading from dunite/melatroctolite upward to augite troctolite/olivine gabbro. However, in many areas, melatroctolite transgresses augite troctolite. The structural complexities of this zone are interpreted to represent the effects of multiple intrusions during the early inflation stage of the DLS magma chamber. The next zone up is the troctolite zone. It is 700-1200m thick, consists mostly of homogeneous foliated troctolitic cumulates. The cyclic zone forms the medial section of the DLS and is characterized by cyclical variations in cumulus mineralogy between troctolitic and gabbroic cumulates. At least four major troctolite-gabbro macrocycle subzones are delineated within the well-exposed southern extent of the cyclic zone. Miller and Ripley (1996) suggested that the cyclicity formed by pressure fluctuations attending magma venting episodes. The persistent occurrence of gabbroic cumulates defines the next unit, the 600m- to 1700m-thick gabbro zone. Gabbroic cumulates, in turn, grade upward into nonfoliated (noncumulate) apatitic quartz ferromonzodiorite, which composes most of the 50m- to 200m-thick upper contact zone. This quartz ferromonzodiorite complexly mixes with a fine-grained biotitic ilmenite ferrodiorite, which forms the "chilled" DLS contact with anorthositic series rocks. A couple of small bodies of melanogranophyre, which irregularly cut through the anorthositic series, probably represent the uppermost differentiate of the DLS. This igneous stratigraphy, which is complimented by cryptic 64 �layering of cumulus mineral compositions, implies that the DLS formed by bottom-up, open-system fractional crystallization of a moderately evolved, olivine tholeiitic magma. The structurally and lithologically complex anorthositic series (AS), which caps the layered series, is subdivided into four units: 1) a troctolitic anorthosite unit that fringes the lower part of the series; 2) an ophitic olivine leucogabbro unit that occurs mainly as inclusions in the upper part of the layered series; 3) a plagioclase-phyric gabbro unit that occurs along the upper contact; and 4) a main unit of undifferentiated gabbroic anorthosite that comprises 90% of the AS. Although their physical relations clearly indicate that the DLS intruded the AS, precise U/Pb zircon dating (Paces and Miller, 1993) shows essentially identical ages of 1099 Ma for both series. A more complete picture of geologic structure in the Duluth area is also portrayed by these new maps. Much of the area is cut by ENE to ESE-trending faults. With the exception of the fault zone exposed in Stewart Creek, these faults are speculative and inferred from topography, aeromagnetic data, and geologic offset. One of the more enigmatic structural features of the area is an antiform-synform duplex defined by foliation and layering in the Spirit Mountain area (West Duluth quadrangle). The limb between these fold structures is near vertical and their N-S fold axes are doubly plunging. By their orientation, they do not appear to be related to faulting. Perhaps the folds developed by collapse of the cumulate pile overlying a feeder zone. Another major improvement on the geologic picture of the Duluth area is in the subdivision of the North Shore Volcanic Group and related hypabyssal intrusions, which form the Duluth Complex hanging wall. The AS intruded normal-polarity lavas of the NSVG, of which approximately 2445 m form the section within the Duluth quadrangle. These volcanic rocks are intruded by the 600m thick Endion sill near the middle, the lensing Northland sheet in the upper part, and the Lester River sill at the top, plus two minor diabasic intrusions along with several late basaltic dikes. Several distinctive and mappable volcanic and sedimentary units within this section include five large felsic flows that constitute 37% of the sequence (Green and Fitz, 1993). From lowest to highest, these are the City Hall icelandite, the Congdon Park rhyolite, the 40th Ave. East icelandite (dated at 1098.4+/-1.9Ma, Davis and Green, 1997), the 42nd Ave. East rhyolite, and the Lester Park icelandite. The lowermost lavas are contact-metamorphosed by the Duluth Complex to pyroxene-hornfels to hornblende-hornfels facies; the remainder show greenschist and laumontite-prehnite-pumpellyite assemblages due to burial metamorphism (Schmidt, 1993). References Davis, D.W, and Green, J.C., 1997, Geochronology of the North American Midcontinent rift in western Lake Superior and implications for its geodynamic evolution. Canadian Journal of Earth Science 34, p. 476-488. Green, J.C., and Fitz, T.J.III, 1993, Extensive felsic lavas and rheoignimbrites in the Keweenawan Midcontinent Rift plateau volcanics, Minnesota: petrographic and field recognition. Journal of Volcanology and Geothermal Research 54, p. 177196. Grout, F.F., 1918, Internal structures of igneous rocks; their significance and origin with special reference to the Duluth Gabbro. Journal of Geology 26, 439-458 Miller, J.D., Jr., Green, J.C., & Chandler, V.W., 1993, Preliminary geologic map of the Duluth area, St. Louis County, Minnesota. Minnesota Geological Survey Open-file Report 93-2, scale 1:48,000 Miller, J.D., Jr., and Ripley, E.M., 1996, Layered intrusions of the Duluth Complex, Minnesota, USA. In Cawthorne, R.G., ed., Layered Intrusions: Amsterdam, Elsevier Science, p. 257-301. Paces, J.B., and Miller, J.D., Jr., 1993, Precise U-Pb ages of Duluth Complex and related mafic intrusions, northeastern Minnesota: Geochronological insights to physical, petrogenetic, paleomagnetic, and tectonomagmatic processes associated with the 1.1 Ga Midcontinent Rift System. Journal of Geophysical Research 98, No B8, p. 13,997-14,013. Schmidt, S.Th., 1993, Regional and local patterns of low-grade metamorphism in the North Shore Volcanic Group, Minnesota, USA. Journal of Metamorphic Geology 11, p. 401-414. Schwartz, G.M., 1949, The geology of the Duluth metropolitan area. Minnesota Geological Survey Bulletin 33, 136p. Taylor, R. B., 1964, Geology of the Duluth Gabbro Complex near Duluth, Minnesota. Minnesota Geological Survey Bulletin 44, 63 pp. 65 �EFFECT OF MINERALOGY ON PROCESSING OF LOW GRADE IRON ORES FROM THE NEGAUNEE IRON-FORMATION ON MARQUETTE RANGE OF THE LAKE SUPERIOR DISTRICT HAN, Tsu-Ming, Senior Research Scientist (Retired), Cleveland-Cliffs Inc., Ishpeming, Michigan, USA During the past half-century, Cleveland-Cliffs has investigated and processed three types of lowgrade iron ores from the Negaunee Iron-Formation on the Marquette Range of the Lake Superior District. The three ore types are: (1) Specular hematite ore of medium metamorphic grade with the principal gangue of chert and locally some sericite, garnet, epidote and grunerite. (2) Magnetite ore of low metamorphic grade with the principal gangue of chert, siderite, ankerite, stilpnomelane, minnesotaite and some clastics. (3) Oxidized magnetite ore in which martite is the principal ore mineral. Ultrafine-grained hematite, microplaty hematite, goethite and earthy hematite are locally present in substantial quantities. Chert and clastic quartz are the principal gangue with local distribution of some gypsum and clay minerals (kaolin, dickite, and montmorillonite). Some minor minerals in the above ores have caused problems in the concentration process and concentrate grade control either in plant practice or from the laboratory testing. Others have caused the physical and chemical quality of the final pellet product. This paper reports the mode of occurrence of these minerals and their negative effects affecting the plant operation and the quality of the pellet product. I – Mineralogy affecting the process of concentration A – Laboratory data showed that gypsum causes total flocculation during desliming due to the release of calcium ions into solution. It prevents the rejection of slimes from flotation feed. This minimizes the selectivity of amine flotation resulting in lower the weight and iron unit recoveries as a result. B– Montmorillonite absorbs amine killing froth leading the catastrophic failure in separating ore minerals from gangue. It can easily be detected by a simple procedure referred to as “Shake Test”, or by analyzing MgO content of the crude ore before processing, i.e., in most cases, the concentrate grade is determined by the MgO content of the crude ore. II – Ore mineralogy that can affect concentrate grade A – Ultrafine-grained magnetite is typically present in the iron-formation adjacent to chloritized basic dikes and sills. The magnetite may represent as much as 40% of the ironformation. However, its grain size is generally finer than 5 microns and it is practically impossible to produce a magnetic concentrate with an acceptable grade by any feasible mechanical means from this type of material. B – Ultrafine-grained hematite occurs as irregular grains and microplates of a few microns or less finely disseminated in chert. Such a hematite and chert relationship has been referred to as 66 �“Hematitic Chert”. It is the host of martite and is considered as waste, which cannot be rejected by the magnetic oxide conversion process (MOC) but can be partially removed by the amine flotation. III – Mineralogy that can affect pellet physical quality Graphite can be rejected during desliming and flotation. However, some of it is entrapped in the magnetic concentrate for palletizing. It causes internal fusion of pellets due to the differential rate of heat transfer and oxygen diffusion toward center of the pellets. This leads to the development of concentric cracking in the pellets and consequently lowers the physical quality of the pellets during transfer. The resulting structural weakness in the pellets can be minimized by the addition of hematite to the balling feed to enhance oxygen availability IV – Mineralogy that can affect pellet chemical quality A – Phosphorus in the oxidized ore occurs as apatite, which has been designated as P1, and as an impurity in goethite and dove-tailed hematite, as P2. Most of the P1 can be rejected during the desliming and flotation stages, whereas the P2 increases with the increase of iron in the concentrate. In order to produce an acceptable final product containing less than 0.03% P from some of the highly oxidized ore, a substantial amount of these ore minerals has to be rejected by high intensity magnetic separation. B – Titanium is present as rutile in some specular hematite. It is practically impossible to separate from its specular hematite host by any known mechanical means. Consequently, it reports with the iron in the concentrate from the specular hematite. C – Stilpnomelane and minnesotaite contain the potassium and sodium. These constituents are a detriment to blast furnace refractory. Most of these minerals are rejected during magnetic separation and flotation. Some of them are entrapped in the magnetite fines as a result of magnetic flocculation. Generally, the alkaline content in the pellets has not been a major problem. Specular hematite concentrate is no longer produced. Oxide ore containing montmorillonite or gypsum remain to be successfully processed. Magnetite iron-formation adjacent to the intrusives has been considered as “waste” and removed by selective mining. 67 �Geochemistry of the Proterozoic Intrusive Rocks of the Nipigon Embayment HART, T.R., Precambrian Geoscience Section, Ontario Geological Survey, 933 Ramsey Lake Road, Sudbury, Ontario P3E 6B5 (tom.hart@ndm.gov.on.ca) The Nipigon Embayment is an approximately 19 000 km2 area of Proterozoic igneous and sedimentary rocks centred on Lake Nipigon, north of Lake Superior and approximately 110 km northeast of Thunder Bay. There are three geochemically distinct Proterozoic intrusive rock types in the Nipigon Embayment, the southern ultramafic intrusions, the Kitto-Jackfish intrusions and the Nipigon Sill Complex (Hart, 2003). The southern ultramafic intrusions consist of the Disraeli, Seagull-Leckie, and Hele intrusions located in the area between Lake Nipigon and Lake Superior. These intrusions are composed of a pyroxene peridotite core with irregular olivine gabbro zones along their margins, and intrude the rocks of the Archean Quetico Subprovince and the Proterozoic Sibley Group. Their present geometry is sill-like, but all three intrusions are composed predominantly of cumulate wehrlite and lherzolite and may represent the remnants of large bodies. The intrusions have calc-alkaline affinities with [La/Yb]mn (mantle normalized) ratios of 5.3 to 18.53 and have weakly depleted Th and little to no Nb anomalies ([Th/Yb]mn ratios of 0.78 to 1.1). The broad range in composition is probably a result of a combination of igneous processes within the magma chambers and assimilation of country rock. Evidence of assimilation is most evident in small, irregular monzogabbro pods that are geochemically similar to the surrounding olivine gabbro but have higher K2O contents. All three intrusions have REE and HFSE ratios that are comparable to ocean island basalts as has been noted for the Seagull Intrusion by Heggie and Hollings (2004). The Kitto-Jackfish intrusions range from an approximately 750 m thick sill-like body at Kitto to an approximately 50 m thick sill at Jackfish Island, English Bay area (MacDonald, 2004), and sills up to a few metres thick at Kama Hill, Nipigon Bay of Lake Superior. The Kitto Intrusion is composed of a pyroxene peridotite core with an irregular olivine gabbro border zone comparable to the southern peridotites (Hart et al., 2002). These sills intrude the rocks of the Quetico and Wabigoon subprovinces and the Sibley Group. The Kitto-Jackfish sills have calc-alkaline affinities with [La/Yb]mn ratios of 6.3 to 10.7, weak Th depletion and weak negative Nb anomalies ([Th/Nb] mn ratios of 1.1 to 1.8) and rare earth element (REE) and high field strength element (HFSE) contents similar to the southern peridotites. The 68 �trend of these intrusions on a La/Sm - Gd/Yb diagram could be a result of either a higher degree of assimilation than the southern peridotites or an indication of a different magma source. The presence of platinum group element (PGE) mineralization in the Seagull and Kitto intrusions suggests that differences in geochemistry are not a useful tool in area selection during exploration. A series of generally flat-lying to shallow-dipping diabase sills of the Nipigon Sill Complex ranging from a few metres to greater than 100 m in thickness intrude both the southern ultramafic intrusions and the Kitto intrusion. The 1109 Ma sills (Davis and Sutcliffe, 1985) have tholeiitic affinities with [La/Yb]mn ratios of 1.61 to 3.29 and moderate negative Nb anomalies ([Th/Nb]mn ratios of 1.8-3.1). Although the sills probably formed as a result of multiple injections of magma, geochemistry suggests that many of the sills are single cooling units. There is geochemical evidence of assimilation of the country rock, which is supported by field relationships. The effects of assimilation are most evident in the chilled margins, although sills of similar thickness appear to display variability in composition suggesting that the degree of assimilation depends on the composition of the country rock. The Nipigon diabase sills have lower TiO2 (0.87 to 2.0 wt.%), Zr/Y (4.2 to 2.7) and [La/Yb]mn ratios than the Logan sills (TiO2: 3.45 to 3.79 wt.%; Zr/Y: 5.5 to 7.7; [La/Yb]mn: 6.5 to 7.8) located south of Thunder Bay. The [La/Yb]mn ratios of the Logan sills are comparable to the ultramafic intrusions of the Nipigon Embayment, but their REE and HFSE contents are about 3 times higher than the intrusions. Similar geographic related geochemical variations have been observed in some flood basalt provinces (e.g., Mantovani et al. 1985). References Davis, D.W. and Sutcliffe, R.H. 1985. U-Pb ages from the Nipigon plate and northern Lake Superior; Geological Society of America Bulletin, v.96, p.1572-1579. Hart, T.R. 2003. Keweenawan Mafic and Ultramafic Intrusive Rocks of the Lake Nipigon and Crystal Lake areas, northwestern Ontario; in Part 1: Programs and Abstracts, Institute on Lake Superior Geology, Proceedings Volume 49; Iron Mountain, Michigan, May 7-11, 2003. Hart, T.R., terMeer, M. and Jolette, C. 2002. Precambrian Geology of Kitto, Eva, Summers, Dorothea and Sandra Townships, Beardmore Area, Northwestern Ontario. Ontario Geological Survey, Open File Report 6095, 206p. MacDonald, C.A. 2004. Precambrian geology of the south Armstrong-Gull Bay area, Nipigon Embayment, northwestern Ontario; Ontario Geological Survey, Open File Report 6136, 42p Mantovani, M.S.M., Marques, L.S., de Sousa, M.A., Civetta, L., Atalla, L. and Innocenti, F. 1985. Trace element and strontium isotopic constraints on the origin and evolution of Parana continental flood basalts of Santa Catarina State (southern Brazil); Journal of Petrology, v.26, p.187-209. 69 �Precambrian Geology and Mineralization of the Northern Black Sturgeon River area, Nipigon Embayment HART, T.R., and MAGYAROSI, Z., Precambrian Geoscience Section, Ontario Geological Survey, 933 Ramsey Lake Road, Sudbury, Ontario P3E 6B5 (tom.hart@ndm.gov.on.ca) A 1:50 000 scale bedrock mapping project in the northern Black Sturgeon River area was conducted to investigate the regional geological setting of the platinum group element (PGE) bearing mafic to ultramafic intrusion in the Seagull Lake area, in the southern portion of the Nipigon Embayment (Hart and Magyarosi, 2004). The Nipigon Embayment is an approximately 19 000 km2 area of Proterozoic igneous and sedimentary rocks centred on Lake Nipigon, north of Lake Superior. This mapping was completed by the Ontario Geological Survey as part of its commitment of in-kind support to the Lake Nipigon Region Geoscience Initiative (LNRGI). The LNRGI is a geoscience-based geological data acquisition and compilation program operated by the Ontario Prospectors Association (OPA) and funded through an agreement with the Northern Ontario Heritage Fund Corporation (NOHFC). The LNRGI also includes partnerships with the private sector, Lakehead University and communities in the Lake Nipigon area. The initiative also includes airborne magnetic and radiometric surveys, ground gravity surveys, and targeted surficial geochemical and geochronological studies. The north Black Sturgeon 280000 River map area is located approximately 110 km northeast of Thunder Bay and south of Lake Nipigon, and is underlain by 5600000 metamorphosed and deformed Archean volcanic and sedimentary rocks of the southern Wabigoon Subprovince and metamorphosed feldspathic wackes and siltstones of the Quetico Subprovince. Sedimentary rocks of the relatively flat-lying 1340 Ma Sibley Group (Franklin, 1978), consisting of conglomerates, sandstones, LLaakkee mudstones, siltstones, limestone, N Niippiiggoonn unconformably overlie the Archean rocks. Proterozoic mafic to ultramafic intrusions in the Disraeli Lake and the Seagull–Leckie lakes areas intrude the Quetico Subprovince and Sibley Group. Both intrusions are composed of a pyroxene peridotite core with an irregular olivine gabbro zones along the margin. A series of undulating, generally flat-lying to shallow-dipping 1109 Ma diabase sills of the Nipigon Sill Complex 5400000 (Davis and Sutcliffe, 1985) intrude 430000 all other rock units in the map area. The Black Sturgeon fault zone consists of a series of north and northwest-trending faults which form an asymmetric basin or half-graben. Less prominent northeasttrending faults probably represent reactivation of structures in the Archean basement rocks. 70 �The location of alteration and the ultramafic intrusions appear to be controlled by the distribution of faults within the Black Sturgeon fault zone. The Disraeli and Seagull ultramafic intrusions occur along the same series of north trending faults that appear to correlate with north-trending structures in the northern Gull Bay area about 80 km to the north. A parallel series of north-trending faults about 25 km to the east hosts the Hele ultramafic intrusion. The most significant PGE mineralization in the Seagull Intrusion was intersected at or near the basal contact of the peridotite with metasedimentary rocks of the Quetico Subprovince. The mineralization is interpreted by Heggie and Hollings (2004) to be magmatic, and formed as a result of sulphur saturation of the magma during initial stages of emplacement. Biotite is ubiquitous in the ultramafic intrusions, but textural evidence is unclear as to whether the biotite is a primary igneous mineral. Preliminary microprobe analyses from the Disraeli Intrusion indicates that biotite may contain up to 5 wt.% Cl, which is similar to the high Cl and F contents reported in biotite associated with PGE minerals in the Coldwell Complex (Watkinson and Ohnenstetter, 1992). The ultramafic intrusions are also cut by the faults suggesting a later reactivation. Metre to tens of metre wide zones of intense hematization occur in along these faults in the Seagull intrusion, and a 40 m thick interval in one drill hole is reported to contain a sylvite rich brine. One sample from the brine rich interval has 3.4 ppm Pt and 1.2 ppm Pd along with elevated Cu and K2O contents suggesting that late fluids could remobilize, and possibly concentrate, the PGEs. Further work is required to understand the timing of this late alteration and the metallogenic significance. References Davis, D.W. and Sutcliffe, R.H. 1985. U-Pb ages from the Nipigon plate and northern Lake Superior; Geological Society of America Bulletin, v.96, p.1572-1579. Franklin, J.M. 1978. The Sibley Group, Ontario; in Rubidium-strontium isochron age studies, report 2; ed. R.K. Wanless and W.D. Loveridge; Geological Survey of Canada, Paper 77-14, p.31-34. Hart, T.R. and Magyarosi, Z. 2004. Precambrian Geology of the northern Black Sturgeon River and Disraeli Lake Area, Nipigon Embayment, northwest Ontario; Ontario Geological Survey Open File Report 6138, 56 p. Heggie, G.J., and Hollings, P., 2004. Controls on PGE Mineralization in the Seagull Intrusion, Northwestern Ontario; Geological Association of Canada-Mineralogical Association of Canada, Joint Annual Meeting, St. Catharines 2004, Program with Abstracts. Watkinson, D.H. and Ohnenstetter, D. 1992. Hydrothermal origin of platinum-group mineralization in the Two Duck Lake Intrusion, Coldwell Complex, northwestern Ontario; Canadian Mineralogist, v.30, p.121-136. 71 �Multiple Intrusive Stages Associated with Keweenawan Rifting: The Leckie Stock, Seagull Intrusion, and Nipigon Sill HEGGIE*, G., and HOLLINGS, P., Department of Geology, Lakehead University, 955 Oliver Rd., Thunder Bay, Ontario, P7B 5E1, Canada; gheggie@lakeheadu.ca Igneous activity related to Keweenawan Rifting (ca. 1108 Ma, Davis and Green, 1997) has produced a wide spectrum of lithologies and numerous igneous suites (Osler Volcanics, North Shore Volcanics, Mamainse Point Volcanics, Duluth Complex, Logan Sills, Nipigon Sills, Coldwell Complex, English Bay Complex, Eva Kitto Intrusion, Seagull Intrusion, Leckie Stock). Found on the periphery of Lake Superior and Lake Nipigon these igneous bodies have been studied for 95 years (first mapped in Canada by Wilson 1910), as work further expands our understanding of the magmatic history, the more complex igneous relationships become. Rift related intrusive rocks outcrop extensively around Lake Nipigon (Figure 1). The most abundant intrusions in this region are the Nipigon Sills (Sutcliffe 1986, Hart and McDonald 2003). Other igneous bodies have been identified in the process of PGE, Cu and Ni exploration. These intrusive bodies, including the Seagull Intrusion and Leckie Stock, although not as expansive as the Nipigon Sills still play an integral part in the development of a unified petrogenetic model. Figure 1. Location of intrusive, extrusive, and sedimentary rocks associated with the Keweenawan Rifting event. Modified after Sutcliffe, 1991. Contact relationships between the three intrusive suites (Leckie Stock, Seagull Intrusion, and Nipigon Sill) are not yet fully understood. Chill margins on a Nipigon Sill crosscutting the Leckie Stock have been identified in drill core, establishing that the Nipigon Sill post dates the 72 �Leckie Stock. Age relationships between the Seagull Intrusion and Nipigon Sill are still unknown, as is the relationship between Seagull Intrusion and Leckie Stock. Lithologically, geochemically, and mineralogically it is possible to distinguish between the three bodies. The Leckie Stock is characterized by ultramafic lithologies, with rare earth element plots similar to ocean island basalt with (La/Sm)n values of 1.01-5.21 and (Gd/Yb)n of 1.82-3.73. Mineralogically the Leckie Stock is dominated by olivine with an average composition of Fo82. This compares to the mafic lithologies (olivine gabbro - gabbro norites) of the Nipigon Sills with (La/Sm)n of 1.51-1.61 and (Gd/Yb)n values of 1.46-1.49. Olivine mineralogy in the Nipigon Sill varies more than in the Leckie Stock, and has an average olivine composition of Fo50. Lithologically the Seagull Intrusion, displays the greatest variation, varying from olivine gabbros, - gabbros to granophyres. Olivine analysed from this body has an average composition of Fo67. Mineralogical and geochemical data suggest that the Leckie Stock is the most primitive magma of the three intrusive suites. The Seagull Intrusion and Nipigon Sills are similar in terms of mineralogy and petrology in that they display overlapping olivine compositions, but REE data (Hart 2002) suggests that the Seagull Intrusion is more closely related to the ultramafic Leckie Stock, if a cross cutting relationship exists between the two (Seagull Intrusion crossing Leckie Stock). The Leckie Stock could not be a feeder zone for either the Seagull Intrusion or the Nipigon Sill, but a feeder for some yet unidentified igneous body higher up in stratigraphy. Davis, D.W., and Green, J.C., 1997. Geochronology of the North American Midcontinent rift in western Lake Superior and implications for its geodynamic evolution. Canadian Journal of Earth Sciences. V.34, p. 476-488. Hart, T., 2002, Keweenawan Mafic and Ultramafic Intrusive Rocks on the Lake Nipigon and Crystal Lake areas, northwestern Ontario, in Institute on Lake Superior Geology, Proceedings Volume 49, Part 1 –Programs and Abstracts, p. 21-22. Hart, T., and MacDonald C.A., 2003. Lake Nipigon Region Geoscience Initiative. Proterozoic and Archean Geology of the South-Central and North areas of the Western Nipigon Embayment. Ontario Geological Survey, Summary of Field work and other Activities. Open File Report 6120. Sutcliffe, R.H., 1991. Proterozoic Geology of the Lake Superior area, in Geology of Ontario, Ontario Geological Survey, Special Volume 4, Part 1, p. 627-658. Sutcliffe, R.H., 1986. The Petrology, Mineral Chemistry and Tectonics of Proterozoic rift-related igneous rocks at Lake Nipigon, Ontario. Unpublished PhD. Thesis, The University of Western Ontario, London, Ontario. Wilson, A.W., 1910, Geology of the Nipigon Basin, Ontario. Memoir No.1 Canada Department of Mines, 152 p. 73 �40 Ar/39Ar Hornblende Evidence for Provenance of Ice Rafted Detritus in the North Atlantic: Implications for Tracking Past Changes in the Extent and Dynamics of Northern Hemisphere Ice Sheets HEMMING, Sidney R., and ROY, Martin, Lamont-Doherty Earth Observatory of Columbia University, Rt. 9W, Palisades, NY 10964 The concentration and provenance of terrigenous sand grains from North Atlantic sediment cores from pelagic environments provide important constrains on the dynamics of former Northern Hemisphere ice sheets. The association of this ice rafted detritus (IRD) signal with proxy records for climate conditions in the North Atlantic, brings additional constraints on ice sheet dynamics in a paleoclimate context. Studies at proximal locations identify configuration changes in specific ice sheets, while studies of cores in the open ocean can yield a history of variation where relative timing of iceberg discharges from different source regions as constrained by simple stratigraphic principles. An example is the case of the Laurentide ice sheet (LIS) where the distinctive variations in age provinces underlying the LIS from south to north can be used to constrain its extent and ice stream activity during the last glacial cycle. 40Ar/39Ar hornblende data from IRD indicate the ice reached Grenville and Appalachian provinces along the margin after about 30 kyr (e.g., and the Last Glacial Maximum, LGM). The increase in flux of IRD at the LGM is coincident with the increase in southeastern LIS provenance components. The last glacial cycle was punctuated by Heinrich events, massive discharges of icebergs from the Hudson Strait ice stream. The general trend in LIS development and identification of major ice stream events that are captured by the marine sediment record show the potential power of this approach for understanding the evolution of ice sheets in general. This approach can be applied throughout the Pleistocene to understand the temporal and spatial development of large volume ice sheets and the transition to the 100-ky cycle that dominated the past 700 kyrs. We seek to expand the studies both geographically and through the Pleistocene. 74 �PRELIMINARY PETROGRAPHY AND HYDROTHERMAL ALTERATION OF THE SOUDAN MINE AREA, VERMILION DISTRICT, NORTHESTERN MINNESOTA HOFFMAN, A.T.*, Dept. of Geosciences, University of Minnesota, Duluth, hoff0578@d.umn.edu PETERSON, D.M., PATELKE, R.L., Economic Geology Group, Natural Resources Research Institute, University of Minnesota Duluth, 5013 Miller Trunk Highway, Duluth MN 55811 HUDAK, G.J., Geology Department, University of Wisconsin Oshkosh, Oshkosh, WI The Ely Greenstone of the western Vermilion District is a lithologic and structurally diverse setting located in the southern extension of the Wawa Subprovince of the Superior Province of the Canadian Shield. The Wawa Subprovince is the host for a significant number of mineral deposits and showings, most notably lode gold and volcanogenic massive sulfides (VMS) (Williams et al., 1991). In the last decade, there has been a significant effort put forth to document areas in the Vermilion district that seem geologically and mineralogically compatible for hosting these deposits. This work includes reconnaissance mapping in the eastern Murray shear zone for mesothermal gold mineralization (Peterson, 2001; Peterson and Patelke, in prep.) and the re-evaluation of the Fivemile Lake, Purvis Lake, Eagles Nest, Skeleton Lake, and Needleboy Lake VMS prospects (Peterson, 2001; Hudak et al., 2002a; Hudak et al., 2002b). In the summer of 2003 mapping for a National Underground Science and Engineering Laboratory (NUSEL) was completed near the Soudan Mine in northeastern Minnesota (Peterson and Patelke, 2003). This work revealed localized lode gold and volcanogenic massive sulfide type alteration. This alteration may correlate with alteration located in the adjacent Fivemile Lake Prospect (Hudak et al., 2002b) and Murray Gold Prospect (Peterson, 2001). The goal of this project is to better understand the volcanic stratigraphy, and syn- and post hydrothermal alteration near the Soudan Mine. Eighty thin sections were analyzed to defined primary syn-volcanic alteration assemblages in the Soudan Mine area. Classification of mineral assemblages followed Hudak et al. (2002b) in effort to establish consistent hydrothermal mineral alteration nomenclature across the Vermilion District. Four mineral assemblages dominate the area: 1) Epidote + Quartz + Actinolite ± Chlorite; 2) Epidote + Quartz ± Chlorite; 3) Mottle Epidote + Quartz ± Actinolite ± Chlorite ± Albite (Epidosites); and 4) localized and lesser amounts of Garnet + Magnetite. Alteration assemblages 1 (Epidote + Quartz + Actinolite ± Chlorite) and 2 (Epidote + Quartz ± Chlorite) likely represent early-formed, semi-conformable zones associated with down welling seawater. Alteration assemblages 3 (Mottle Epidote + Quartz ± Actinolite ± Chlorite ± Albite (Epidosites)) and 4 (localized Garnet + Magnetite) appear to represent hydrothermal fluid upflow zones that may be proximal to syn-volcanic structures (Harper, 1999; Gibson et al., 1999). These results suggest that volcanogenic massive sulfide targets may be present near the Soudan Mine. Petrographic analysis is ongoing and a detailed field analysis during summer 2004 will better constrain these relationships. 75 �References Harper, G.D., 1999, Structural Styles of Hydrothermal Discharge in Ophiolite/Sea-Floor Systems: Reviews in Economic Geology v. 8, p. 53-73 Hovis, S. T., 2001, Physical Volcanology and Hydrothermal Alteration of the Archean Volcanic Rocks at the Eagles Nest Volcanogenic Massive Sulphide Prospect, Northern Minnesota. Unpublished M.S. Thesis, University of Minnesota Duluth, Duluth, Minnesota, 102 p. Hudak, G.J., Heine J., Hocker, S.M., Hauck, S., 2002a, Geologic Mapping of the Needleboy Lake-Six Mile Lake Area, Northeastern Minnesota: A Summary of Volcanogenic Massive Sulfide Potential. Report of Investigations NRRI/RI-2002/14. Hudak, G.J., Heine, J., Newkirk, T., Odette, J., and Hauck, S., 2002b. Comparative Geology, Stratigraphy, and Litho-Geochemistry of the Five Mile Lake, Quartz Hill, and Skeleton Lake VMS Occurrences, Vermilion District, NE Minnesota. NRRI Technical Report NRRI/PR-2002/03, 390 p. Peterson, D.M., Patelke, R. L., 2003 National Underground Science and Engineering Laboratory (NUSEL): Geologic Site Investigation for the Soudan Mine, Northeastern Minnesota. NRRI Technical Report NRRI/TR-2003/29, 87 p. Peterson, D.M., 2001 Development of Archean lode gold and massive sulfide exploration models using Geographic Information System applications: Targeting mineral exploration in northeastern Minnesota from analysis of analog Canadian mining camps: Unpublished Ph.D. dissertation, University of Minnesota Minneapolis, 502 p. Peterson, D.M., and Patelke, R.L., in Prep, Neo-Archean gold mineralization in the Mud Creek area, Northern St. Louis County, Minnesota: Natural Resources Research Institute, University of Minnesota Duluth. Williams, H.R., Stott, G.M., Heather, K.B., Muir, T.L., and Sage, R.P., 1991, Wawa Subprovince, in Thurston, P.C., Williams, H.R., Sutcliffe, R.H., and Stott, G.M., eds., Geology of Ontario: Ontario Geological Survey Special Volume 4, Part 1, p. 485-539. 76 �Trace Element Geochemistry of the Osler Group Volcanics – Implications for Mid-Continent Rifting HOLLINGS, Pete, Department of Geology, Lakehead University, 955 Oliver Rd., Thunder Bay, Ontario, P7B 5E1, Canada; peter.hollings@lakeheadu.ca To date three major Proterozoic events have been recognized on the northern margins of Lake Superior (Sutcliffe, 1991); 1) the Paleoproterozoic Animikie Group sediments (~1.86 Ga), 2) ~1.54 Ga Mesoproterozoic anorogenic granites and Sibley Group sediments, 3) Mesoproterozoic rifting at ~1.11 to 1.09 Ga during which the Keweenawan Supergroup was deposited. The Osler Group volcanics are found towards the base of the Keweenawan Supergroup. The Osler Group volcanics comprise a bimodal sequence of basalts and less abundant rhyolites that are well exposed on a series of islands off Rosport on the northern shore of Lake Superior and also as sparse outcrop on the Slate Islands off Terrace Bay (Fig. 1). The Osler Group has been described as a thick sequence of tholeiitic flood basalts with up to 2800m exposed on the Black Bay Peninsula (Sutcliffe, 1991). The majority of the sequence comprises magnetically reversed Lower Keweenawan flows with a small section of magnetically normal flows at the top of the sequence (Halls, 1974). Davis and Sutcliffe (1985) have reported U-Pb ages of 1107.5 +4/-2 Ma for a rhyolite from the base of the sequence and 1097.6 ±3.7 Ma for a rhyolite towards the top. Figure 1. Location of the intrusive, extrusive and sedimentary rocks associated with Keweenawan Rifting. Modified after Sutcliffe (1991). 77 �Detailed sampling traverses were undertaken on Wilson and Vein islands in the summer of 2002, in order to investigate geochemical variations within the Osler Group. Exposure on the two islands is excellent, beginning at the northern end of the islands with a basal conglomerate that includes abundant clasts of Sibley sediments. The conglomerate is strongly heterolithic and indicates an approximately northward flow direction. The volcanic units comprise numerous flows ranging from a few centimeters to a few metres in thickness. Individual flows are frequently marked by rubbly flow tops or ropey, pahoehoe textures, while thicker flows may show well developed columnar jointing (Fig. 2). Figure 2. A) Columnar basalts within the Osler Group on Simpson Island. B) Pahoehoe texture in thin basalt flow on Wilson Island. SiO2 and MgO contents of the Osler Group volcanics on Wilson and Vein Islands range from 4754 wt. % and 5-15% respectively, consistent with data from earlier studies. Basalts are characterized by weak to moderate LREE enrichment (La/Smn = 1-5), weakly fractionated HREE (Gd/Ybn = 2-4) and display primitive mantle normalised patterns comparable to modern Ocean Island Basalts. Preliminary examination of REE data for the Group suggests that the La/Smn ratio, a good indicator of crustal contamination, increases towards the top of the sequence (ranging from ~1.5 near the base to ~5 near the top). References Davis, D. and Sutcliffe, R., 1985. U-Pb ages from the Nipigon plate and northern Lake Superior. Geological Society of America Bulletin, 96, 1572-1579. Halls, H. C., 1974. A paleomagnetic reversal in the Osler volcanic group, northern Lake Superior. Canadian Journal of Earth Sciences, 11, 1200-1207. Sutcliffe, R., 1991. Proterozoic geology of the Lake Superior area. In. Geology of Ontario, Ontario Geological Survey, Special Volume 4, Part 1. 627-660. 78 �The Influence of Radiometric Dating for Unraveling the Precambrian Geologic History of the Lake Superior Region HOLM, Daniel K., Dept. of Geology, Kent State University, Kent, OH 44242, dholm@kent.edu VAN SCHMUS, R.W., Dept. of Geology, Univ. of Kansas, Lawrence, KS 66045 SCHNEIDER, D.A., Dept. of Geological Sciences, Ohio University, Athens, OH 44701 The paucity of biostratigraphic controls in Precambrian rocks has long made radiometric dating of primary importance for unraveling the complex geologic history in the Lake Superior region (Goldich, 1968). The first ILSG meeting in 1955 occurred very close to the time of initiation of a radiometric dating program by S.S. Goldich and A.O Nier in 1956. Subsequent geochronologic investigations laid the theoretical and practical foundation upon which current radiometric studies are expanding. Thus the tremendous influence of geochronologic studies in this region, only briefly summarized here, has been well chronicled at ILSG meetings over the past 50 years. Pioneering studies in K-Ar, Rb-Sr, and U-Pb dating. Early studies by Goldich et al. (1961, 1970), Aldrich et al. (1965), and Peterman (1966) were pioneering applications of radiometric dating which provided a broad-brush means of correlation (based on age rather than on degree of metamorphism and deformation) and contributed to the formation of a world-wide time scale for the Precambrian (Goldich, 1968), flatteringly referred to by some as “Goldich’s time scale”. The early results represented a significant breakthrough in Precambrian geology, yet proved difficult to interpret as co-existing minerals gave different ages for the same decay scheme. Comparative mineral studies by Aldrich et al. (1965) and Hanson and Gast (1967) led to the recognition of metamorphic effects on various isotopic systems (especially K-Ar and Rb-Sr). For instance, Van Schmus et al. (1975a) used Rb/Sr age data to document the existence of a widespread but poorly understood low-grade 1650 Ma metamorphic event in Wisconsin. The 2700 Ma age of the greenstone-granite belts in Minnesota and southern Ontario was documented early on by whole-rock Rb-Sr ages (Jahn and Murthy, 1975), K-Ar, and U-Pb ages (Peterman et al., 1972). The large analytical uncertainty in these data, however, precluded their use for resolving the time of important, short-lived, geological events (plutonism, deformations) that occurred during their formation and accretion. Likewise, the errors on initial U-Pb zircon ages on Mid-continent Rift rocks would not allow for differentiation of magmatic pulses, but simply suggested “a sharp pulse of igneous activity” at 1115+15 Ma (Silver and Green, 1963). Early U-Pb zircon work in the Minnesota River Valley temporarily produced some of the world’s oldest dated rocks (Catanzaro, 1963; Goldich et al., 1970; Goldich and Hedge, 1974), and clearly demonstrated that K-Ar and Rb-Sr mineral ages reflect younger metamorphic events. For instance, using K-Ar and Rb-Sr ages, Goldich et al. (1961) first bracketed the Penokean orogeny as having occurred between 1800 and 1600 Ma. Similarly young K-Ar biotite ages (compared to U-Pb zircon ages from the same rock) from Precambrian shields and orogenic belts worldwide led to the concept of a ‘metamorphic veil’ (Armstrong, 1966) which reflects younger overprinting events (common in Precambrian terranes) and obscures the age of older rock forming events. Modern U-Pb dating (bulk aliquot to single crystal to spot dating). In the 1970’s and 1980’s, advances in conventional U-Pb zircon analyses (Krogh, 1973, 1982) allowed geochronologists to see through the ‘metamorphic veil’ and firmly established the age spectrum of igneous activity in the region (Silver and Green, 1972; Van Schmus et al., 1975b; Van Schmus, 1976; 1980). For instance, the age of the Penokean orogeny was definitively bracketed between 1870-1830 Ma by Van Schmus (1976, 1980), who first invoked the plate tectonic concept of a southern magmatic arc colliding with a northern passive margin. Archean age gneisses (Marshfield terrane) were dated south of the Penokean magmatic arc rocks (Van Schmus and Anderson, 1977) and Early Archean ages were obtained from gneiss dome rocks north of the Niagara fault zone (Peterman et al., 1980). The greatly improved accuracy in U-Pb zircon dating has allowed researchers (Boerboom and Zartman, 1993; Corfu and Stott, 1986, 1998; Davis et al., 1989; Zaleski et al., 1999; Ayers et al., 2002) to document the time of short-lived magmatic and deformational events commonly involved in the 79 �formation of late Archean granite–greenstone belts. Enough high precision data has been obtained (i.e., Shebandowan, Vermillion, and Manitouwadge) to suggest correlation of volcanic assemblages and their subsequent deformation along 600 km of strike (Peterson et al., 2001). New U-Pb single-crystal zircon studies continue to refine the earlier plutonic ages, which were obtained on mg-size fractions. Post-Penokean “1760 Ma” plutonism (Sims et al., 1989) is now known to have actually occurred in southeast younging pulses at ca. 1800, 1775, and 1750 Ma, possibly reflecting flip in subduction polarity and slab-rollback of Yavapai-age oceanic lithosphere (Holm et al., 2004). The Penokean orogenic belt twice formed the source region for generation of crustal-melt batholiths; first at ca. 1775 Ma with intrusion of the East-central Minnesota batholith (Holm et al., 2004) and again at 1470 Ma in central Wisconsin (Van Schmus et al., 1975). The new single crystal results show that the Eastcentral Minnesota batholith was emplaced over a ca. 20 m.y. duration (Holm et al., 2004; Keatts et al., 2004), whereas Wolf River magmatism was relatively short-lived (with nine different phases intruded over a <5 m.y. interval; Dewane and Van Schmus, 2003). New important U-Pb zircon results have also been obtained from volcanic rocks over the last decade or so (see Van Schmus and Hinze, 1985 for a summary of earlier age data). High precision U-Pb geochronology on the volcanics of the Midcontinent Rift system indicate rapid eruption over a relatively short duration (1108-1094 Ma; Davis and Paces, 1990; Paces and Miller, 1993; Davis and Green, 1997; Zartman et al., 1997). The great volume of magma generated over this short time span and the limited degree of extension strongly favor a mantle plume origin. New geochronology on interlayered volcanics within the Marquette Range Supergroup yield essentially identical ages of 1875 Ma for the Hemlock (Schneider et al., 2002) and Gunflint (Fralick et al., 2002) Formations. The revised age constraining the timing of deposition of the Supergroup across the Animikie basin suggests these sediments were laid down coeval with arc formation to the south and supports the hypothesis of Hoffman (1987), who first proposed that the iron formations are syn-Penokean foredeep deposits. Conventional single crystal and SHRIMP U-Pb dating of detrital zircons (Van Wyck, 1995; Holm et al., 1998; Medaris et al., 2003) finally resolved the controversy over the maximum age of the Baraboo Quartzite and similar red quartzites in northern Wisconsin and Minnesota (<1750 Ma). A completely different geochronologic database described next provided firm constraints on the minimum age of quartzite deposition (>1650 Ma). Ar-Ar thermochronology. Thermochronology (the study of the time-temperature evolution of rocks) was born from the realization than many mineral-isotopic systems could be used to determine the time of cooling of a terrain through a series of predictable temperatures. Because an intimate relationship exists between the thermal and tectonic processes operating during orogenesis and subsequent events, reconstructing the thermal history of a region ultimately aids in constraining its tectonic evolution. Thermochronologic data from the region was initially dominated by the Rb/Sr method on biotite (Peterman and Sims, 1988). In the 1990’s, application of the Ar-Ar incremental heating technique on hornblende, muscovite, and biotite yielded considerable information bearing on the Proterozoic thermal history of the southern Lake Superior region (Holm and Lux, 1996, 1998; Schneider et al., 1996; Holm et al., 1998a; Romano et al., 2000). Especially important was the proposal that a separate, perhaps widespread, geon 17 amphibolite facies metamorphic event occurred after the Penokean orogeny, followed by an episode of rapid crustal exhumation (Holm et al., 1998a). Additionally, the widespread geographic distribution of basement Ar/Ar biotite ages serendipitously provided a key minimum age constraint (ca. 1630 Ma) on the overlying Proterozoic red quartzites and led to a greater appreciation of the role of younger tectonism (i.e. Mazatzal deformation) in the region (Holm et al., 1998b). New minerals, new techniques, and new directions. Rapid advances in technology (allowing higher precision, increased mass and spatial resolution, and in situ capabilities) and the application of new chronometers (monazite and xenotime) are now verifying earlier hypotheses for widespread tectonothermal events following Penokean orogenesis. Recent results of U-Pb monazite dating using both ion and electron microprobe techniques now recognize distinct metamorphic pulses at 1835, 1800, and 1770 Ma – thermal pulses which can be directly tied to known magmatic events (Schneider et al., 2004). New U-Pb xenotime ages (Vanelli et al., 2003, and in progress) from coarse sandstone/conglomerate beds 80 �of the Marquette Range Supergroup identify a widespread 1790-1760 Ma fluid flow event – an event that may have been ultimately responsible for forming the high-grade iron ore deposits in this region (Morey, 1999). Recent single crystal Ar-Ar dating of fine-grained, low-temperature minerals within the Baraboo Interval Quartzites (Medaris et al., 2003) has documented a much younger, but apparently even more vast hydrothermal fluid system generated by Mesoproterozoic plutonism. Finally, Ar-ion laser dating results reveal significant age gradients (~200 to 500 m.y.) in coarse muscovite from basement samples interpreted to signify partial resetting by fluids during both Mazatzal orogenesis and Mesoproterozoic magmatism (McKenzie, 2004; Rose, 2004). In addition to direct dating of metamorphism and fluid flow, geochronology is now being used to put absolute time (not simply time constraints) on important deformation features in the Lake Superior region. The age of the Mazatzal tectonic front in northwest Wisconsin is dated by the age of reset biotite in basement beneath the deformed quartzites. The Malmo Structural discontinuity in Minnesota and the Flambeau Flow fault in Wisconsin are now thought to be geon 17 exhumation structures (formed during orogenic collapse) on the basis of differing metamorphic ages across these faults (McKenzie et al., 2003; Schneider et al., 2004). Current monazite work on tectonite samples of the Niagara fault zone (Rose, 2004) and the Eau Pleine shear zone (Loofboro, 2005) should provide direct time constraints on their formation (Williams and Jercinovic, 2002). Beginning this month we anticipate the publication of a large number of new ages in just the next few years – in terms of raw numbers, perhaps more than has been published in the Lake Superior region in the entire past 50 years! For instance, Van Wyck and Martin (2004) exploit the relatively fast LA-ICP-MS technique to report over 200 U-Pb detrital zircon ages from the Baraboo and Hamilton Mounds quartzites. Geochronologic work accepted and in progress (Schneider et al., 2004, 2005; Holm et al., 2004, 2005; Vanelli et al., in progress; Bickford et al., in progress; McKenzie, 2004; Rose, 2004; Keatts, 2004, Loofboro, 2005; Schmitz and Bowring, in progress; Medaris et al., in progress; et al. that we may be unaware of) will certainly continue to refine our knowledge of the Precambrian geologic history of this region and very likely yield surprises which can not be anticipated. Aldrich, L.T., Davis, G.L., and James, H.L., 1965, Ages of some minerals from metamorphic and igneous rocks near Iron Mountain, Michigan: Journal of Petrology, 6, 445-472. Armstrong, R.L., 1966, K-Ar dating of plutonic and volcanic rocks in orogenic belts. In O.A. Schaeffer and J. Zahringer, eds., Potassium argon dating, 117-131. Springer-Verlag, New York, 234 p. Ayer, J., Amelin, Y., Corfu, F., Kamo, S., Ketchum, J., Kwok, K., and Marquis, R., 2002: Evolution of the southern Abitibi greenstone belt based on U-Pb geochronology; autochthonous volcanic construction followed by plutonism, regional deformation and sedimentation: Precambrian Research, 115, 63-95. Boerboom, T.J., and Zartman, R.E., 1993, Geology, geochemistry, and geochronology of the central Giants Range batholith, northeastern Minnesota: Canadian Journal of Earth Sciences, v. 30, p. 2510-2522. Catanzaro, E.J., 1963, Zircon ages in southwestern Minnesota: Journal of Geophysical Research, v. 68, p. 20452048. Corfu, F., and Stott, G.M., 1986, U-Pb age for late magmatism and regional deformation in the Shebandowan Belt, Superior Province, Canada: Canadian Journal of Earth Sciences, 23, 1075-1082. Corfu, F., and Stott, G.M., 1998, Shebandowan greenstone belt, western Superior Province: U-Pb ages, tectonic implications, and correlations: Geological Society of America Bulletin, 110, 1467-1484. Davis, D.W., and Green, J.C. 1997, Geochronology of the North American Midcontinent Rift in western Lake Superior and implications for its geodynamic evolution: Canadian Journal of Earth Sciences, 34, 476-488. Davis, D.W., and Paces, 1990, Time resolution of geologic events on the Keweenaw Peninsula and implications for the development of the Midcontinent rift system: Earth and Planetary Science Letters, 97, 54-64. Davis, D.W., Poulsen, K.H., and Kamo, S.L., 1989, New insights into Archean crustal development from geochronology in the Rainy Lake area, Superior Province, Canada: Journal of Geology, 97, 379-398. Dewane, T.J., and Van Schmus, W.R., 2003, Detailed U-Pb geochronology of the Wolf River batholith, northcentral Wisconsin: Evidence for a short-lived magmatic event ca. 1470 Ma: Geological Society of America Abstracts, 37, 92. Fralick, P., Davis, D., and Kissin, S., 2002, The age of the Gunflint Formation, Ontario, Canada: single zircon U-Pb age determinations from reworked volcanic ash: Canadian Journal of Earth Sciences, 39, 1085-1091. Goldich, S.S., Nier, A.O., Baadsgaard, H., Hoffman, J.H., Krueger, H.W., 1961, The Precambrian Geology and geochronology of Minnesota, Minnesota Geology Survey Bulletin 41, 193 p. 81 �Goldich, S.S., Hedge, C.E., and Stern, T.W., 1970, Age of the Morton and Montevideo gneisses and related rocks, southwestern Minnesota: Geological Society of America Bulletin, 81, 3671-3696. Goldich, S.S., 1968, Geochronology in the Lake Superior region: Canadian Journal of Earth Sciences, 5, 715-724. Goldich, S.S., and Hedge, C.E., 1974, 3800-Myr granitic gneiss in south-western Minnesota: Nature. 252, 467-468. Hanson, G.N., and Gast, P.W., 1967, Kinetic studies in contact metamorphic zones: Geochim. Cosmochim. Acta, 31, 1119-1153. Hoffman, P., 1987, Early Proterozoic foredeeps, foredeep magmatism and Superior-type iron formations of the Canadian shield. In Proterozoic lithospheric evolution. Edited by A. Kroner, American Geophysical Union Geodynamics Series, 17, 85-98. Holm, D.K., and Lux, D., 1996, Core complex model proposed for gneiss dome development during collapse of the Paleoproterozoic Penokean orogen, Minnesota: Geology, 24, 343-346. Holm, D.K, and Lux, D., 1998, Depth of emplacement and tilting of the Middle Proterozoic (1470 Ma) Wolf River batholith, Wisconsin. Ar-Ar thermochronologic constraints: Canadian Journal of Earth Science, 35, 1143-1151. Holm, D., Darrah, K., and Lux, D., 1998a, Evidence for widespread ~1760 Ma metamorphism and rapid crustal stabilization of the Early Proterozoic Penokean orogen, Minnesota: American Journal of Science, 298, 60-81. Holm, D., Schneider, D., and Coath, C., 1998b, Age and deformation of Early Proterozoic quartzites in the southern Lake Superior region: Implications for extent of foreland deformation during final assembly of Laurentia: Geology, 26, 907-910. Holm, D.K., Van Schmus, W.R., MacNeill, L., Boerboom, T., Schweitzer, D., and Schneider, D., 2004, U-Pb zircon geochronology of Paleoproterozoic plutons from the northern mid-continent, U.S.A.: Evidence for subduction flip and continued convergence after geon 18 Penokean orogenesis: Geological Society of America Bulletin, in press. Jahn, B.M., and Murthy, V.R., 1975, Rb-Sr ages of the Archean rocks from the Vermilion district, northeastern Minnesota: Geochimica et Cosmochimica Acta, 39, 1679-1689. Keatts, M., Holm, D., Jirsa, M., and Boerboom, T., 2004, Generation of a 1790-1770 Ma continental arc batholith in east-central Minnesota: Compass, v. 82 (in press). Krogh, T. E., 1973, A low contamination method for hydrothermal decomposition of zircon and extraction of U and Pb for isotopic age determinations: Geochimica et Cosmochimica Acta, 37, 485-494. Krogh, T.E., 1982, Improved accuracy of U-Pb zircon ages by the creation of more concordant systems using an air abrasion technique: Geochim. Cosmochim. Acta, 46, 637-649. Loofboro, J., 2005, Timing and nature of Proterozoic poly-metamorphism in central Wisconsin: M.S. thesis, Kent State University, Kent, OH (in progress). McKenzie, M.M., 2004, Age pattern and nature of Paleoproterozoic metamorphism of the internal zone of the Penokean orogen, east-central Minnesota: M.S. thesis, Kent State University, Kent, OH, 105 p. McKenzie, M.M., Holm, D.K., Schneider, D.A., and Jercinovic, M.J., 2003, Results and implications of monazite geochronology from the western Penokean orogen, Minnesota: Geological Society of America Abstracts with Programs, 35, 272. Medaris, G., Singer, B.S., Dott, R.H., Naymark, A., Johnson, C.M., and Schott, R.C., 2002, Late Paleoproterozoic climate, tectonics and metamorphism in the southern Lake Superior region and Proto-North America: Evidence from Baraboo interval quartzite: Journal of Geology, 111, 243-257. Morey, G.B., 1999, High-grade iron ore deposits of the Mesabi Range, Minnesota – Product of a continental scale Proterozoic ground-water flow system: Economic Geology, 94, 133-142. Paces, J.B., and Miller, J.D., Jr., 1993, Precise U-Pb ages of Duluth Complex and related mafic intrusions, northeastern Minnesota: Geochronological insights to physical, petrogenetic, paleomagnetic and tectonomagmatic processes associated with the 1.1. Ga Midcontinent Rift system: Journal of Geophysical Research, 98, 13997-14013. Peterman, Z.E., 1966, Rb-Sr dating of Middle Precambrian metasedimentary rocks, Minnesota: Bulletin of the Geological Society of America, 77, 1031-1044. Peterman, Z.E., and Goldich, S.S., Hedge, C.E., and Yardley, D.H., 1972, Geochronology of the Rainy Lake region, Minnesota-Ontario, in Doe, B.R., and Smith, D.K., eds., Studies in Mineralogy and Precambrian Geology (John W. Gruner Volume): Geological Society of America Memoir 135, 193-215. Peterman, Z.E., and Sims, P.K., 1988, The Goodman Swell: A lithospheric flexure caused by crustal loading along the Midcontinent Rift System: Tectonics, 7, 1077-1090. Peterman, Z., Zartman, R., and Sims, P., 1980, Tonalitic gneiss of early Archean age from northern Michigan: Geological Society of America Special Paper 182, 125-138. Peterson, D.M., Gallup, C., Jirsa, M.A., and Davis, D., 2001, Correlation of Archean assemblages across the U.S.Canadian border: Phase I Geochronology: ILSG Abstracts, 47, 77-78. 82 �Romano, D., Holm, D., and Foland, K., 2000, Determining the extent and nature of Mazatzal-related overprinting of the Penokean orogenic belt in the southern Lake Superior region, north-central USA: Precambrian Research, 104, 25-46. Rose, S., 2004, The age and extent of metamorphism within the Paleoproterozoic Penokean orogen, northern Wisconsin and Michigan: M.S. thesis, Ohio University, Athens, OH, 105 p. Schneider, D., Holm, D., and Lux, D., 1996, On the origin of Early Proterozoic gneiss domes and metamorphic nodes, northern Michigan: Canadian Journal of Earth Sciences, 33, 1053-1063. Schneider, D., Bickford, M., Cannon, W., Shulz, K., and Hamilton, M., 2002, Age of volcanic rocks and syndepositional iron formations, Marquette Range Supergroup: implications for the tectonic setting of Paleoproterozoic iron formations of the Lake Superior region: Canadian Journal of Earth Sciences, 39, 999-1012. Schneider, D., Holm, D.K., O’Boyle, C., Hamilton, M., and Jercinovic, M., 2004, Paleoproterozoic development of a gneiss dome corridor in the southern Lake Superior region, U.S.A.: Geological Society of America Special Paper (in press). Silver, L.T., and Green, J.C., 1963, Zircon ages for Middle Precambrian rocks of the Lake Superior region: Trans. Am. Geophysical Union, 44, 107. Sims, P.K., Van Schmus, W.R., Schulz, K.J., and Peterman, Z.E., 1989, Tectonostratigraphic evolution of the Early Proterozoic Wisconsin Magmatic Terranes of the Penokean orogen: Canadian Journal of Earth Sciences, 26, 2145-2158. Vallini, D.A., et al., 2003, Using xenotime U-Pb geochronology to unravel the history of Proterozoic sedimentary basins: a study in Western Australia and the Lake Superior Region: ILSG abstr, 49, 79-80. Van Schmus, W.R., 1976, Early and Middle Proterozoic history of the Great Lakes area, North America. Royal Society of London Philosophical Transactions, 280, 605-628. Van Schmus, W.R., 1980, Chronology of igneous rocks associated with the Penokean orogeny in WI. In Morey, G., and Hanson, G., (eds.) Selected studies of Archean gneisses and lower Proterozoic rocks, southern Canadian Shield. Geological Society of America Special Paper, 182, 159-168. Van Schmus, W.R., Thurman, E.M., and Peterman, Z.E., 1975a, Geology and Rb-Sr chronology of middle Precambrian rocks in eastern and central Wisconsin: Geol. Society of America Bulletin, 86, 1255-1265. Van Schmus, W.R., Medaris, L.G., and Banks, P.O., 1975b, Geology and age of the Wolf River batholith, Wisconsin: Geological Society of America Bulletin, 86, 907-914. Van Schmus, W.R., and Anderson, J.L., 1977, Gneiss and migmatite of Archean age in the Precambrian basement of central Wisconsin: Geology, 5, 45-48. Van Schmus, W. R., and Hinze, W. J., 1985, The Midcontinent Rift System. Annual Reviews Earth Planetary Sciences, 13, 345-383. Van Wyck, N., 1995, Major and trace element, common Pb, Sm-Nd, and zircon geochronology constraints on petrogenesis and tectonic setting of pre- and Early Proterozoic rocks in Wisconsin: Ph.D. thesis, University of Wisconsin-Madison, 280 p. Van Wyck, N., and Martin, N., 2004, Detrital zircon ages from Paleoproterozoic Quartzites: Measured by laser – ablation ICP-MS: Journal of Geology (in press). Williams, M.L. and Jercinovic, M.J., 2002, Microprobe monazite geochronology: putting absolute time into microstructural analysis. Journal of Structural Geology, 24, 1013-1028. Zaleski, E., van Breemen, O., and Peterson, V.L., 1999, Geological evolution of the Manitouwadge greenstone belt and Wawa-Quetico subprovince boundary, Superior Province, Ontario, constrained by U-Pb zircon dates of supracrustal and plutonic rocks: Canadian Journal of Earth Sciences, 36, 945-966. Zartman, R.E., Nicholoson, S.W., Cannon, W.F., and Morey, G.B., 1997, U-Th-Pb zircon ages of some Keweenawan Supergroup rocks from the southern shore of Lake Superior: Canadian Journal of Earth Sciences, 34, 549-561. 83 �NEOARCHEAN PEPERITES IN THE VICINITY OF FIVEMILE LAKE, VERMILION DISTRICT, NE MINNESOTA HUDAK, G. J., NEWKIRK, T. T., DREXLER, H., ODETTE, J. D., and HOCKER, S. M., Department of Geology, University of Wisconsin Oshkosh, 800 Algoma Blvd., Oshkosh, WI 54901, hudak@uwosh.edu The Ely Greenstone – Lower Member in the vicinity of Fivemile Lake contains a well-studied sequence of ~2722 Ma, more or less east-west striking, steeply north-dipping and north-facing, arc-associated submarine basalt-andesite pillow lavas, tuffs and lapilli tuffs, rhyodacite to rhyolite tuffs, lapilli tuffs, and lavas, and synvolcanic diabase and diorite sills and dikes (Peterson et al., 2001; Hudak et al., 2002). Volcanic facies mapping at 1:5000 scale, supported by grants from the Natural Resources Research Institute (University of Minnesota Duluth), the University of Wisconsin Oshkosh, and the Minerals Diversification Plan of the Minnesota State Legislature, identified a northeast- trending zone of basalt - andesite tuffs, lapilli tuffs, and tuff-breccias which contain zones of northeast-trending coherent facies andesite–dacite that display pillowed, amoeboid, and commonly, jigsaw-puzzle fit morphologies. The southern extent of this zone occurs along a peninsula on the southwestern shoreline of Fivemile Lake. The northern extent of this zone extends approximately 150 meters north of the north-central shoreline of Fivemile Lake. Over the past year, extremely detailed volcanic facies mapping (1:50-1:100 scale), as well as petrographic and lithogeochemical studies, have been performed in an attempt to determine the genesis of these apparently genetically related coherentand volcaniclastic facies-bearing, northeast-trending zones. On the south shoreline of Fivemile Lake, the volcaniclastic facies is typically matrix-supported. The matrix consists of non-bedded basalt-andesite tuff, and locally contains amoeboid-shaped zones that contain up to 60% <1 mm quartz amygdules. Localized zones adjacent to northeast-striking coherent facies andesite–dacite comprise lapilli tuffs and tuff-breccias. Clasts in these deposits are commonly curviplanar to angular and jigsaw puzzle-fit in shape (Figure 1a), are moderately to highly vesicular (10-50%), have extremely finegrained margins, and locally contain <1-2 cm wide, apparently contact metamorphosed zones adjacent to their margins. These blocks and lapilli are compositionally similar to, more common adjacent to, and have their long axes aligned with, northeast-trending, sheet-like to pillow-shaped coherent facies andesite-dacite. The deposits on the north-central shoreline of Fivemile Lake have slightly different morphologies. A sharp, northsouth contact occurs between east-west striking, steeply north-dipping, bun- to mattress-shaped pillowed basalt-andesite and basalt-andesite tuffs and lapilli tuffs that comprise the base of a 700 meter wide, 200 meter thick, shallow submarine tuff cone (Hudak et al., in prep.). The tuffs and lapilli tuffs at this location are intruded by northeast-trending, synvolcanic, amoeboid (Figure 1b), pillow-like, and lobe-like zones of coherent-facies andesite-dacite which appear to have locally undergone in-situ fragmentation to produce localized lapilli tuffs and tuff-breccias that contain moderately to highly vesicular curviplanar, ameoboid, and blocky jigsaw puzzle-fit lapilli and blocks. These fragments have very fine-grained margins that are also rimmed by fine-grained, apparently contact metamorphosed zones. Locally, numerous thin (up to 1cm wide), parallel, highly vesicular zones occur within the volcaniclastic deposits that more or less mimic the orientations the margins of amoeboid coherent facies andesite–dacite or the margins of andesite-dacite blocks. The northeast-trending zones containing coherent and volcaniclastic facies are interpreted to comprise peperites with associated synvolcanic dikes. Peperites are fragmental rocks that form from the intimate mixing of hot magma with unconsolidated, typically wet sediments (Batiza and White, 2000; Schmidt and Schminke, 2000). They are common in submarine arc-related volcano-sedimentary sequences (Skilling et al., 2002), and commonly occur in proximity to synvolcanic fault zones. The interpretation that the volcaniclastic deposits represent peperites is based on the morphology of the lapilli and blocks which comprise the deposits, the consistent grain size variations from the margins to the centers of the lapilli and blocks, their spatial relationships adjacent to, and orientations parallel to, synvolcanic sheet-like to pillowed coherent facies andesite-dacite, and the presence of amygdules within the fine-grained tuffaceous matrix. The interpretation that the sheet-like to pillowed, coherent facies andesite-dacite represents dikes rather than lava flows is based on the northeast strike of the pillowed dikes (which is parallel to synvolcanic structures mapped in the Fivemile Lake region (Hudak et al., 2002)), the lack of consistent facing directions in the pillowed coherent 84 �facies, and the similarity in the chemical compositions of the coherent domains and the lapilli and blocks within the lapilli tuffs and tuff-breccias. The differences in the morphologies of the peperites on the southwestern and north-central shorelines of Fivemile Lake may represent their different levels of formation in the shallow sub-seafloor. Blocky peperites south of Fivemile Lake appear to have formed in lava flowconfined submarine aquifers located several hundred meters below the paleoseafloor, whereas more amoeboid peperites found north of Fivemile Lake appear to have formed near the seafloor within water-saturated submarine vent-fill deposits associated with a shallow submarine tuff cone volcano. Discordant zones of peperites can be used to identify synvolcanic fault zones that may be important in the exploration for volcanichosted massive sulfide deposits. Figure 1. Photographs of peperites deposits south (A) and north (B) of Fivemile Lake. References Batiza, R., and White, J. D. L., 2000, Submarine Lavas and Hyaloclastite, in Sigurdsson, H., 2000, Encyclopedia of Volcanoes: Academic Press, San Diego, California, p. 361-381. Hudak, G. J., Heine, J., Hocker, S., and Hauck, S. A., 2002, Geological mapping of the Needleboy Lake – Six Mile Lake area, northeastern Minnesota: a summary of volcanogenic massive sulfide potential: Natural Resources Research Institute Report of Investigation NRRI/RI – 2002/14, 16 p. Hudak, G. J., Heine, J., Newkirk, T. T., and Hocker, S. M., in prep. Comparative geology, stratigraphy, and lithogeochemistry of the Needleboy Lake to Sixmile Lake area, Vermilion District, NE Minnesota: University of Minnesota Permanent University Trust Fund Research Report. Peterson, D. M., 2001, Development of Archean lode-gold and massive sulfide deposit exploration models using geographic information system applications: targeting mineral exploration in northeastern Minnesota from analysis of analog Canadian Mining camps: unpublished Ph. D. dissertation, University of Minnesota, Duluth, Minnesota, 503 p. Peterson, D. M., Gallup, C., Jirsa, M. A., and Davis, D. W., Correlation of Archean assemblages across the U.S. – Canadian Border: phase I geochronology: ILSG Proceedings Volume 47, p. 77-78. Schmidt, R., and Schminke, H.-U., 2000, Seamounts and Island Building, in Sigurdsson, H., 2000, Encyclopedia of Volcanoes: Academic Press, San Diego, California, p. 383-402. Skilling, I. P., White, J. D. L., and McPhie, J., 2002, Peperite: a review of magma-sediment mingling: Journal of Volcanology and Geothermal Research, v. 114, p. 1-17. 85 �MAPPING BY THE MINNESOTA GEOLOGICAL SURVEY IN SUPPORT OF LANDUSE AND WATER PLANNING ON THE MESABI IRON RANGE JIRSA, Mark A., Minnesota Geological Survey, jirsa001@umn.edu This presentation describes two mapping projects on the Mesabi Iron Range underway by the Minnesota Geological Survey: 1) Hydrogeologic base maps of the Mesabi Iron Range—Funded by the Legislative Commission on Minnesota Resources; and 2) Bedrock and Quaternary geologic maps of the Mesabi Iron Range—Funded by the Minnesota Minerals Coordinating Committee. In recent years, a number of mining companies have been working with the University of Minnesota’s Department of Landscape Architecture on development concepts for the Mesabi Iron Range. Although not directly a part of those efforts, the Minnesota Geological Survey’s mapping projects are designed to provide technical data to address issues—primarily related to water—that require consideration prior to development. When mining and associated pumping ceases in any particular locality, water enters the mine from a variety of sources: rain and runoff, percolation through porous unconsolidated sediment layers, and flow through fractures and weathered zones in bedrock. The movement of ground water through these diverse materials is complex and poorly understood; however, it is clear that the boundary between the bedrock and overlying unconsolidated sediments is the single most important interface in the movement of ground water. Project 1, the hydrogeologic maps, addresses the location and shape of this hydrologically important interface. Some of this information for the western half of the Mesabi Range has already been published (Fig. 1; Jirsa and others, 2002; Lively and others, 2002). Project 2 will provide details about the lithologic content, structure, and water-bearing characteristics of the materials above and below that interface. Among other objectives, the bedrock geologic mapping is designed to identify other structures that may be important hydrologic interfaces, such as fractures, faults, and bedding surfaces. The bedrock mapping will modify and digitize the most recent existing map of the Mesabi Range compiled by a large group of scientists (Meineke and others, 1999). This will be compiled with other archived maps and new fieldwork. These efforts are reconnaissance in scope, and considerably more detail could be extracted from mine exposures, mining company records, and surface geologic mapping. Nevertheless, these projects will produce framework geologic maps and site-specific data in digital format that will be useful in resolving issues related to ground and surface waters, aggregate resource management, and land-use planning. They are intended to lay the groundwork for more detailed investigations into specific areas of future concern. 86 �Figure 1. Generalized geologic map of the Mesabi Iron Range showing the subcrop location of Paleoproterozoic Biwabik Iron Formation (gray) and Mesoproterozoic Duluth Complex, and various taconite mining operations (black; asterisk indicates inactive mine). Dashed polygons outline western and eastern map areas. None of these projects would be possible without the contributions of drill hole and geologic data and insight from members of the Minnesota Department of Natural Resources, the Natural Resources Research Institute, and most particularly, the mining companies. REFERENCES Jirsa, M.A., Setterholm, D.R., Bloomgren, B.A., and Lively, R.S., 2002, Bedrock topographic and depth to bedrock maps of the western half of the Mesabi Iron Range, northern Minnesota: Minnesota Geological Survey Miscellaneous Map M-126, scale 1:100,000. Lively, R.S., Morey, G.B., and Bauer, E.J., 2002, One hundred years of mining: Alterations to the physical and cultural geography of the western half of the Mesabi Iron Range, northern Minnesota: Minnesota Geological Survey Miscellaneous Map M-118; 4 pls., scale 1:100,000. Meineke, D.G., Buchheit, R.L., Dahlberg, E.H., Morey, G.B., and Warren, L.E., comps., 1999 Geologic map of the Mesabi Iron Range, Minnesota (2nd ed.): Hibbing, Minn., Mesabi Range Geological Society, scale 1:62,500. 87 �REGIONAL COMPILATIONS OF BEDROCK GEOLOGY IN NORTHEASTERN MINNESOTA: THE VERMILION, ELY, AND BASSWOOD LAKE QUADRANGLES JIRSA, Mark A., Minnesota Geological Survey, jirsa001@umn.edu Compilations of geologic mapping, although not typically portraying new field information in great detail, serve the important functions of providing regional mapping consistency, framework for various lithologic and geochemical surveys, and regional guidance for exploration and environmental issues. With this in mind, several regional bedrock geologic compilations at scale 1:100,000 have recently been completed and are underway for areas in northeastern Minnesota (Fig. 1), funded in large part by the STATEMAP program of the U.S. Geological Survey. The Vermilion Lake 30' x 60' quadrangle was completed in 2003 (Jirsa and Boerboom, 2003), and is displayed and described here. The adjacent Ely and U.S. portion of Basswood Lake 30' x 60' quadrangles are slated for completion in 2004. Perhaps as important as the functions listed above, the new compilations integrate disparate analog mapping, including some from the late 1800s and early 1900s, into a lithostratigraphically consistent, digital (GIS) format. Figure 1. Generalized geologic map of northeastern Minnesota showing the location of 30' x 60' quadrangles. The Vermilion Lake quadrangle covers most of the exposed area of the Vermilion district of northeastern Minnesota that lies outside of the Boundary Waters Canoe Area Wilderness (BWCAW), and is thus open in the broad sense to mineral exploration and mining. The Vermilion district is a local designation applied to the portion of the Neoarchean Vermilion greenstone belt in which there has been mining activity in the past, primarily of highgrade iron ore. The quadrangle also covers the east end of the Mesabi Iron Range, a world-class mining district in Paleoproterozoic strata that for decades has been the principal economic force in this part of Minnesota. Nevertheless, there is interest in finding alternative industries for cities along the iron range and alternative commodities for a diversified mining industry. At the present time there are encouraging indications that one or more viable polymetallic mines may develop in the basal contact zone of the Mesoproterozoic Duluth Complex. The zone of prime exploration and development interest in this regard crosses the southeastern corner of the Vermilion Lake map sheet. For the first time, the Vermilion Lake compilation merges well-exposed Neoarchean rocks of the Tower–Soudan area with more scattered exposures to the west into a single structural and lithologic interpretation. The map highlights a large, regional anticlinal structure that is cored by the steeply dipping Tower–Soudan anticline (Fig. 2). Tracing this fold structure westward into sedimentary rocks of the Lake Vermilion Formation, it becomes a broad and complex nappe structure that extends at least 30 kilometers. The Vermilion greenstone belt is comparable in its geologic attributes to other Neoarchean belts in Canada that contains significant deposits of gold, copper, zinc, and other metals. This general analogy has been recognized for decades and has stimulated several waves of mineral 88 �exploration in the area. Although past exploration efforts have been substantial, they have not been exhaustive. A major handicap was the lack of detailed geologic mapping, a situation that has been largely corrected over the past 25 years through the efforts of the Minnesota Geological Survey and the exploration industry itself. Figure 2. Schematic geologic map of the Vermilion Lake 30' x 60' quadrangle (Jirsa and Boerboom, 2003) showing folds, faults, and stratigraphic facing in Archean strata. Archean intrusions are gray. Three generations of folds are shown: F1—solid; F2—long dash; F3—short dash. Open arrows indicate horizontal facing directions in steeply dipping supracrustal sequences and upright facing directions where bedding is shallower. Solid arrows indicate the direction of downward stratigraphic facing. The bold fault line is the boundary between the Quetico subprovince to the north, and the Wawa subprovince to the south. The regional compilations of mapping data at scale 1:100,000 may prove to be valuable tools for the next cycle of exploration, which inevitably will come when local and global economic conditions appropriately conjoin. The compilation effort underway for the Ely and U.S. portions of the Basswood Lake 30' x 60' quadrangles involves many of the same geologic entities as depicted on the Vermilion Lake sheet. Because it includes areas within the BWCAW, which are generally protected from exploration and mining, this map will serve a slightly different purpose from that of the Vermilion Lake map, by providing regional geologic context. This effort will pave the way for such products as a “geologic user-guide” to the BWCAW, which has been initiated by the Minnesota Geological Survey. REFERENCE Jirsa, M.A., and Boerboom, T.J., 2003, Bedrock geology of the Vermilion Lake 30’ x 60’ quadrangle, northeast Minnesota: Minnesota Geological Survey Miscellaneous Map M-141, scale 1:100,000. 89 �Douglass Houghton’s 1840 Field Excursion to Lake Superior JOHNSON, Allan M., Professor Emeritus, Department of Geological & Mining Engineering & Sciences, Michigan Technological University, Houghton, Michigan 49931 In 1840, as the first State Geologist of Michigan, Douglass Houghton was near the end of the first geological survey of the state. In 1837, when Michigan entered statehood, and 1838 he was involved with and oversaw the reconnaissance surveys of the southern peninsula. In 1839 the survey work progressed to the northern peninsula, along the north shores of Lakes Huron and Michigan. For 1840, the program was much more ambitious, for he and his assistants (Bela Hubbard and C.C. Douglass) would undertake a survey along the south shore of Lake Superior. Detroit businessman, Charles Penny, also accompanied the group. The party left Detroit near the end of May 1840 and through the spring and early summer made their way from Sault Ste. Marie westward along the south shore of Lake Superior in their flotilla consisting of a larger Mackinaw boat and several smaller craft. This survey resulted in the mapping of rocks exposed adjacent to the shore, with more detailed mapping along major streams and rivers flowing into the big lake. Special attention was given to mapping volcanic and sedimentary rocks of the Keweenaw Peninsula and taking samples of previously known locations of copper bearing rocks (copper oxide at Copper Harbor and native metal from the copper boulder on the Ontonagon River). At the end of July, Houghton’s assistants returned to southern Michigan to continue with ongoing geologic investigations including boring a salt well at Grand Rapids. Houghton remained on Lake Superior with his voyageurs through mid-September. When his assistants left from LaPointe on Madeline Island, Houghton continued reconnaissance mapping at Isle Royale, the Porcupine Mountains and Portage Lake, and more detailed work on the Black and Eagle Rivers. It was on the Eagle River where Houghton discovered numerous occurrences of native copper and the presence of native silver which gave him the confidence to express cautious optimism in the potential for copper mining on the Keweenaw Peninsula in his early 1841 report to the Michigan Legislature. Word of this report soon led to the US War Department establishing a land office at Copper Harbor to issue mining permits and ultimately to the first great mining rush in the United States. For 150 years the native copper district of the Keweenaw was active in supplying a growing nation and the world with high purity lake copper with over 12 billion pounds of production. It is of interest to note that the 1840 journals of Hubbard and Penny have been published, while Houghton’s 1840 geologic notes became ‘lost’. It was not until 1978 that the notes appeared at public auction in New York City advertised as “the notes of an early Michigan surveyor”, where they became the property of Central Michigan University. 90 �References Carter, James L., and Rankin, Ernest H., eds., 1970, North to Lake Superior, The Journal of Charles W. Penny, 1840, The John M. Longyear Research Library, Marquette, Michigan, 84 p. Cummings, John, September 14, 2000, personal communication in Mount Pleasant, Michigan regarding purchase of Douglass Houghton’s 1840 field notes in New York City in 1978. Fuller, George N., 1928, Geological Reports of Douglass Houghton, Michigan Historical Commission, Lansing, 700 p. Houghton, Douglass, 1840, Original Field Notes of 1840 Expedition to Lake Superior, Clarke Library, Central Michigan Univ., Mount Pleasant, MI 325 p. Merrill, George P, 1906, Contributions to the History of American Geology, Smithsonian Institution, Washington Government Printing Office, 733 p. Mason, Philip, P., ed., 1958, Schoolcraft’s 1832 Expedition to Lake Itasca, Michigan State University Press, East Lansing, 390 p. Peters, Bernard C., ed., 1983, Lake Superior Journal, Bela Hubbard’s Account of the 1840 Houghton Expedition, Northern Michigan University Press, Marquette, 113 p. Rintila, Edsel K., 1954, Douglass Houghton, Michigan’s Pioneer Geologist, Wayne University Press, Detroit, 119 p. Williams, Mentor L., ed., Schoolcraft’s 1821 Narrative Journal of Travels through the Northwestern Regions of the United States, Michigan State University Press, East Lansing, 1992, 520 p. 91 �REGIONAL GEOCHEMISTRY SURROUNDING THE NORTON LAKE Cu-Ni-PGE DEPOSIT, UCHI SUBPROVINCE, ONTARIO JOHNSON, J.R.*, HOLLINGS, P., and KISSIN, S., Department of Geology, Lakehead University, Thunder Bay, ON, P7B 5E1 The Norton Lake Cu-Ni-PGE deposit (delineated in 1981 as 944,000 tonnes at 0.72% Ni and 0.56% Cu) is located within the northern most unnamed assemblage of the Miminiska-Fort Hope Greenstone Belt approximately 50 km northeast of Fort Hope, northwest Ontario (Fig. 1). The deposit is hosted within a sheared amphibolite unit that is typically found at, but not limited to, the contact between an upper basalt and a lower sedimentary unit. Due to both its remote location and sparse outcrop only limited geological investigations have been undertaken within the belt to date. Consequently, the belt has been subdivided based on available regional stratigraphic and structural interpretations. Previous work by the Ontario Geological Survey has tentatively correlated the unnamed assemblage with the McGruer assemblage of the North Caribou, located to the west, based on a similarity of rock types and a pervasive aeromagnetic anomaly that extends between the two (Stott and Corfu, 1991). Figure 1: Uchi Subprovince, northwestern Ontario, study area outlined. A 12 km east-west by 6 km north-south block surrounding the Norton Lake Deposit has been mapped and sampled to allow characterization of the volcanic assemblages. In addition to field mapping, drill core left in the area by previous exploration programs was examined (covering an area of 20 km by 10 km). Samples selected to provide good areal coverage and minimal alteration were analysed by XRF and ICP-MS. Three suites can be recognised within the assemblage; Suite I basalts are characterized by weakly depleted to enriched LREE [(La/Sm)n=0.9-1.2] and weakly fractionated HREE [(Gb/Yb)n=1.1-1.2] in conjunction with flat to positive Nb anomalies. Suite II is characterized by LREE depleted basalts [(La/Sm)n=0.4-0.7] and unfractionated HREE [(Gb/Yb)n=0.9-1.1], while Suite III consists of basalts to dacites that are LREE enriched with weakly fractionated HREE [(La/Sm)n=2.6-4.7, (Gd/Yb)n=0.8-2.4] and pronounced negative Nb anomalies. 92 �Sm-Nd isotope work on select samples is currently underway with initial epsilon-Nd values indicating little to no crustal contamination of the samples. Preliminary interpretations suggest the Norton Lake area formed in a tectonic setting analogous to modern island arcs and is best compared with the Northern Pickle Assemblage of the Pickle Lake belt rather than the McGruer assemblage of the North Caribou terrane. Hollings P., 1998, Geochemistry of the Uchi Subprovince, northern Superior Province: and evaluation of the geodynamic evolution of the northern margin of the Superior Province ocean basin; Ph. D. thesis, University of Saskatchewan, Saskatoon, Saskatchewan, 229 p. Stott G. M. and Corfu F., 1991, Uchi Subprovince, in Geology of Ontario, Ontario Geological Survey Special Volume 4, Part 1. Young, M. D., 2003. New structural, geochronological, and geochemical constraints on the tectonic assembly of the Archean Pickle Lake greenstone belt, Uchi subprovince, western Superior Province; M. Sc. thesis, Queen’s University, Kingston, Ontario, 182 p. 93 �Platinum Mineralization at Drill Hole A4-11 of the Wetlegs Area of the Partridge River Intrusion, Duluth Complex, Northeast Minnesota KAUKONEN, R.J., and ALAPIETI, T.T., University of Oulu, Oulu, Finland Apparently either the more peculiar PGE mineralization at DU-15 of the South Kawishiwi Intrusion or the vast but low grade deposits of disseminated base-metal sulfides may have been the reason why, as it seems, the platinum potential of the Wetlegs area of the Partridge River Intrusion has almost been forgotten. The whole rock analyses from drill core A4-11 (Gladen, 1990) show significant PGE values (Fig.1), which undoubtedly warrant an investigation. Unfortunately the best parts of the drill core A4-11 have been previously used for analysis and hence material for thin sections was not available throughout the core. The cryptic variation paths of the main rock-forming silicates are presented in Fig. 1. Compared to data from drill core A4-14 (Kaukonen, 1994), the crystallization paths of olivine and pyroxenes show distinctive similarities. The general upward increasing trend of primitiveness is particularly evident in olivine. Plagioclase compositions on the other hand don’t really present any clear trends aside from perhaps a slight decrease down the drill hole in the lower half of the core. Dpeth in Drill Hole (m) 0 0 100 100 200 200 300 300 400 400 500 500 600 600 700 40 50 60 70 80 30 40 50 60 70 40 50 60 70 80 30 Plag An% Oliv Fo% Opx En% 40 Cpx En% 50 0 700 4000 8000 Pt+Pd+Au ppb Fig.1. Cryptic variation paths of the main rock-forming silicates and the determined concentrations of noble metals in drill hole A4-11. The best PGE values are found between 70-80 meters down hole. The highest Pt+Pd+Au value is about 7 ppm, of which Au accounts for just less than 1 ppm (Fig.1). Petrologically the mineralization seems to be a somewhat ’classic’ base-metal sulfide type PGE mineralization where the platinum-group minerals are mainly associated with disseminated base-metal sulfides, mainly chalcopyrite and pyrrhotite. The most significant difference to the classic or often referred to as orthomagmatic PGE mineralizations is probably related to the silicates rather than sulfides, as the host rock is essentially troctolite rather than orthopyroxenite. This feature, however, is well in accordance with the crystal mush theory of Miller and Weiblen (1990). 94 �As each mineralization has some unique characteristics, the A4-11 PGE mineralization also has its own signature. One of the most striking features is the notable occurrence of native copper, which is widely present. The grain size of copper is usually very small, yet it is readily distinguishable with an ore microscope using a relatively small magnification. The presence of native copper is possibly an indication of a reducing environment, and here it is mainly an alteration product of chalcopyrite. The platinum-group mineralogy of the A4-11 mineralization is also rather special. The most common PGMs are stannides and plumbates (Komppa, 1998). The minerals identified and analyzed for this investigation included atokite (Pd2Sn) and zvyagintsevite (Pd3Pb). However, as pointed out by Cawthorn et al. (2002), the platinum-group mineralogy of any given deposit is not necessarily a reflection of the primary processes involved in the enrichment of the PGE in a stratiform deposit, but rather a product of secondary processes that shape the mineralization further. References: Cawthorn, R.G., Lee, C.A., Schouwstra, R.P., and Mellowship, P., 2002, Relationship between PGE and PGM in the Bushveld Complex: Can. Min., Vol. 40, pp. 311-328. Gladen, L., 1990, Duluth Project: Report on 1989 Exploration in the Duluth Complex, Minnesota Department of Natural Resources, Internal Report, 11 p. Kaukonen, R.J., 1994, Cryptic Variation in the Partridge River Intrusion of the Duluth Complex, unpublished M.Sc thesis, University of Oulu, Finland, 66 p., (in Finnish with English Abstract). Komppa, U.E., 1998, Oxide, Sulphide and Platinum Mineralogy of the South Kawishiwi and Partridge River Intrusions of the Duluth Layered Intrusion Complex, Minnesota, U.S.A. M.Sc. thesis, University of Oulu, Finland, 104 p., (in Finnish), English version published as Natural Resources Research Institute, Report of Investigation in 2002, NRRI-RI2002/02 Miller, J.D., Jr., and Weiblen, P.W., 1990, Anorthositic Rocks of the Duluth Complex: Examples of Rocks Formed from Plagioclase Crystal Mush: Jour. Petrol., Vol. 31, Part 2, pp. 295339. 95 �MAGNETIC SUSCEPTIBILITY ANISOTROPY AND REMANENT MAGNETISM OF QUARTZITE AND PHYLLITE FROM BARABOO WISCONSIN KEAN, William F., Department of Geosciences, UW-Milwaukee, P.O. Box 413, Milwaukee WI 53201, wkean@uwm.edu The Baraboo quartzite and the associated phyllite are paleomagnetically well behaved rocks, with single magnetic directions carried by hematite, but with different directions for the rock types. The magnetism in the quartzite is associated with single domain hematite grains, is pre-folding, and is consistent with an ~1760 Ma age of formation (Kean and Mercer, 1986). The phyllite is dominated by multidomain hematite, (Kelly and Kean 2001), and although the magnetic directions are internally consistent at any one site, they are scattered between sites both before and after a fold test. The presence of multidomain hematite grains (up to 100 µ) suggests the remanence was developed during metamorphism which could have occurred during the folding in the region, or from fluids introduced at a later time when the radiometric ages were reset (Medaris et al., 2003). In an attempt to better understand these differences in magnetic directions, a large block of rock that contained quartzite and phyllite was collected from the south limb of the Baraboo Syncline at an outcrop on Highway 12. A series of 2.5-cm. diameter cores were drilled from each of the layers for thermal demagnetization and anisotropy of magnetic susceptibility (AMS) measurements. The results presented in Figure 1 show that the phyllite has higher values of AMS (P`) than the quartzite, and the magnetic grains are more oblate in shape (T) compared to the quartzite. 1 0.8 Quartzite 0.6 SHAPE T Phyllite 0.4 0.2 0 1 1.1 1.2 1.3 1.4 1.5 -0.2 -0.4 Anisotropy P' Figure 1. Anisotropy of Magnetic Susceptibility (AMS) P’ versus shape factor T for samples from the Baraboo region. T values below zero are prolate in shape, values near zero are neutral, and values above zero are oblate in shape. The phyllite shows large values of anisotropy and is strongly oblate. The quartzite is close to neutral in shape and of lower anisotropy. 96 �The comparison between remanent magnetic directions and AMS ellipsoid axes for the two rock types is presented in Figure 2. The magnetic direction for the phyllite is perpendicular to the minimum anisotropy axis and is rotated toward the bedding plane. The magnetic direction for the quartzite appears to be unrelated to the susceptibility anisotropy. Figure 2: Comparison of paleomagnetic directions and AMS for quartzite and phyllite samples at Highway 12 outcrop. It appears that the disparity in the magnetic directions in the phyllite zone is related to the amount of local strain that occurred during the metamorphism and formation of the Baraboo syncline, and not related to later fluid injection associated with the Wolf River batholith intrusive event. Limited AMS results from other phyllite sites in the region provide similar results. References: Kean, W.F. and Mercer, D., 1986, Preliminary Paleomagnetic Study of the Baraboo Quartzite, Wisconsin, Geoscience Wisconsin, Vol. 10, p 46-53. Kean, W.F. and Kelly, C., 2001, Rock Magnetic Studies of Phyllite Layers from Baraboo Interval Rocks in Wisconsin, Abstracts with Programs, GSA Annual Meeting, 33, p. A143. Medaris, L.G., Singer, B.S., Dott, Jr., R.H., Naymark, A., Johnson, C.M., and Schott, R.C., 2003, Late Paleoproterozoic Climate and Tectonics in Southern Lake Superior Region and Proto-North America: Evidence from Baraboo Interval Quartzites. Journal of Geology, Vol. 111, p 243-257. 97 �Lithic Materials and Archaeology in the Western Lake Superior Region KLAWITER, Brian, Superior National Forest, Duluth, Minnesota 55808, USA The most prevalent and enduring artifacts left behind by prehistoric cultures are those made of stone. The study of these cultural material remains shows that prehistoric people were very perceptive and discerning “geologists” when it came to choosing the stone materials that they would use. The available lithic materials had a broad range of useful properties that were to be carefully considered when choosing the raw material for a particular tool or purpose. Prehistoric people developed a keen eye for the desired physical properties of stone, and they knew where to get it. One enduring legacy from these prehistoric and early historic people is in the form of place-names such as Gunflint Lake and Knife Lake; both translated from the earlier Ojibwe names (Upham, 1920), and both of which have subsequently passed their names along to major geologic features: the Gunflint Iron Formation and the Knife Lake Group metavolcanics and metasediments. This presentation will display and describe many of the geologic materials and their prehistoric uses as discovered by archaeologists working in the western Lake Superior region. In addition to studying prehistoric cultures, it is often equally interesting to study the modern archaeologists who study the ancient cultures. Most archaeologists are students of the Social Sciences whose occupation requires them to acquire a basic working knowledge of geology. This geologic education is often acquired in the field or in the lab by peer interaction, rather than in the classroom by professional geologists. As a result, the field of archaeology has in many ways developed its own sub-culture of geology, which is best displayed in the naming systems applied to the geologic materials recovered from archaeological excavations. This ad hoc (and locally territorial) nomenclature is often confusing to archaeologists and bewildering to geologists. For those interested in the overlap of geology and archaeology, this presentation outlines the archaeological nomenclature applied to geologic formations and materials in the western Lake Superior region, and highlights the efforts of this author and others to “reform” the archaeological nomenclature to more generally conform to geologic standards. Keywords: archaeology, lithics, Gunflint, Knife Lake, nomenclature, Superior National Forest Upham, W., 1920, Minnesota Geographic Names: Their Origin and Historic Significance: Minnesota Historical Society, Saint Paul, Minnesota. 98 �REGIONAL TILL SAMPLING IN THE VERMILION GREENSTONE BELT, MINNESOTA: PRELIMINARY RESULTS AND INTERPRETATIONS LARSON, P.C., Department of Geological Sciences, University of Minnesota, Duluth, MN 55812, plarson2@d.umn.edu A project of regional till sampling was undertaken over the Vermilion greenstone belt during the 2001 and 2003 field seasons. Samples were collected to provide coverage of ~1 sample per 3 km2 in the area between Tower and Ely, MN. The silt+clay fraction (<63µm) of B- and Chorizon till samples was analyzed for gold, platinum, and palladium by fire assay and for 47 major- and trace-elements by ICP-MS and ICP-AES. The regional sampling program provides information on the background concentrations of various elements in till, as well as the presence and location of anomalous gold and base metal indicator concentrations. In conjunction with the sampling, observations on the character and distribution of glacial sediments overlying the greenstone belt were made to provide a framework for interpretation of the till geochemical data set. The study area records a simple record of Quaternary events. Till cover is generally thin and discontinuous over the study area, with the exception of the Vermilion and Wahlsten moraines. Essentially all glacial sediments were deposited during the final retreat of ice from the study area (the Vermilion Phase). Moraine orientations indicate ice was flowing at 180° to the Wahlsten moraine, and 195° to the slightly younger Vermilion moraine. Elements in till associated with the granite-gneiss Vermilion granitic complex to the north of the greenstone belt (Be, K, Rb, Ba) show a systematic decrease in concentration along glacial flowlines south of the Vermilion fault (the contact between the granitic complex and greenstone belt). This demonstrates that the composition of till overlying the greenstone is overwhelmingly a function of physical transport by glacial action. Till overlying the greenstone is therefore a mixture of material transported from the Vermilion granitic complex and material eroded from the greenstone belt; quantification of this mixing trend indicates a mean transport length of ~8 km for till-forming material. The till sampling program has clearly identified a number of areas with anomalous precious and base metal concentrations. Highlights include a number of samples with >50 ppb gold, including one sample with 940 ppb gold. Copper values up to 314 ppm, zinc values up to 368 ppm, and platinum values of up to 8 ppb were also reported. Higher density follow-up sampling is recommended to more clearly define and determine the significance of anomalies, as well as determine the location of potential source rocks. The results of this survey clearly demonstrate the utility of till compositional surveys for exploration in the Vermilion greenstone belt, as well as nearby areas with a similar glacial geological setting. 99 �EARLY ADVANCE OF THE ST. LOUIS SUBLOBE: A REVISED CHRONOLOGY OF THE DEGLACIATION OF NORTHEASTERN MINNESOTA LARSON, P.C., MOOERS, H.D., and MARLOW, L.M., Department of Geological Sciences, University of Minnesota, Duluth, MN 55812, plarson2@d.umn.edu The general retreat of the Laurentide Ice Sheet (LIS) from Minnesota during the Late Wisconsin was characterized by a fluctuating boundary between ice originating in the Keewatin and Labradoran accumulation centers. Recent investigations into the glacial geology of central and northern Minnesota have newly recognized stratigraphic relationships prompting revision of the deglacial chronology. A prominent event during deglaciation was the advance of the Keewatin-provenance St. Louis Sublobe (Alborn phase) from the Red River Valley across northern Minnesota into the area vacated by the retreating Labradoran-provenance Rainy Lobe. This advance has previously assigned a relatively late age (~12 ka) and correlated with post-Vermilion phase margins of the Rainy Lobe, implying that ice-free conditions existed between the Rainy Lobe and the Giant’s Range at the time of the advance. However, the absence of Keewatin till north of the Giant's Range despite the presence of such till at the crest of the Giant's Range, combined with evidence for abundant stagnant Rainy Lobe ice south of the Giant’s Range, suggests the St. Louis Sublobe advance occurred while active Rainy Lobe ice was present immediately north of the Giant's Range. A newly identified Rainy Lobe ice margin confirms this relationship; this margin is probably correlative with the previously described Allen moraine. We refer to the phase of the Rainy Lobe responsible for its formation as the Northofnashwauk phase. The relationships above indicate that the Alborn phase occurred significantly earlier than previously believed. Retreat of the St. Louis Sublobe from its Alborn phase maximum occurred by stagnation and wastage of a large mass of ice. The next prominent recessional moraine formed at the suture between active and stagnant ice in southern Beltrami County (Rabideau moraine). Here, ice-contact meltwater channels emanated outward from the ice margin, terminating as outwash fans at the edge of the stagnant ice. The Rabideau moraine is correlative with the post-Northofnashwauk Big Rice phase and moraine of the Rainy Lobe. Much of the meltwater drained into Glacial Lake Sucre, a western extension of Glacial Lakes Aitkin and Upham II. Sucre drained after abandonment of the Hellwig Creek outlet of Upham II. The Rainy and St. Louis Sublobes retreated from the Big Rice and Rabideau moraines to the Vermilion and Big Stone(?) moraines, respectively. These newly recognized stratigraphic relationships indicate that the St. Louis Sublobe advanced immediately after recession of the Rainy Lobe. Consequently, Glacial Lakes Aitkin-Upham I were much shorter lived, and Aitkin and Upham II much longer lived, than previously believed. 100 �Figure 1. Ice margin positions during Northofnashwauk and Alborn Phases, ca. 13.5 kyr BP. Figure 2. Ice margin positions during Big Rice Phase, ca. 13 kyr BP. Figure 3. Ice margin positions during Vermilion Phase, ca. 12.5 kyr BP. 101 �Lake Nipigon Region Geoscience Initiative: Results of Bedrock Mapping in the Northern Part of the Western Nipigon Embayment, Northwestern Ontario, Canada MacDONALD, C.A. and TREMLAY, E. Precambrian Geoscience Section, Ontario Geological Survey, Ministry of Northern Development and Mines, Sudbury, Ontario, Canada, P3E 6B5 As part of the Lake Nipigon Regional Geoscience Initiative (LNRGI), a 2-year, 1:50 000 scale mapping program was begun in 2003 to better understand the geology of the western Nipigon Embayment. This talk presents results based on mapping of an area of roughly 1200 km2 during the 2003 field season, and focuses on Proterozoic rocks in the map area (Fig. 1) (MacDonald 2004; MacDonald et al. 2004a, 2004b). This project is funded by the Northern Ontario Heritage Fund Corporation (NOHFC) through the Ontario Prospectors Association. The 2003 map area is located approximately 190 km north of Thunder Bay and is partially bordered by the shore of Lake Nipigon on its east margin (Fig. 1, 2). Rocks of both the Superior and Southern provinces occur in the map area and include Archean rocks of the volcanic-plutonic central Wabigoon Subprovince that are disconformably overlain by Mesoproterozoic sedimentary rocks of the Sibley Group, all of which are intruded by Mesoproterozoic mafic igneous rocks. Archean rocks consist dominantly of variably foliated to gneissic, felsic to intermediate, granitoid rocks and mafic to felsic metavolcanic rocks. A 30 metre thick flatlying succession of mafic volcanic pillowed flows occurs in the northern part of the map area. The undeformed nature of these flows, combined with the well-preserved Figure 1. Key map showing the location of the nature of relatively delicate features such as hyaloclastite 2003 map area. suggests that these rocks may be Proterozoic rather than Archean in age. If so, they would be the first Proterozoic mafic volcanic rocks reported this far north of the Midcontinent Rift. Alternatively, they could represent a previously unrecognized greenstone belt within the Superior Province. Pre-Keweenawan rocks include anorogenic igneous and associated metavolcanic rocks of the English Bay complex and sedimentary rocks of the Sibley Group. The English Bay complex (~1540 Ma, Davis and Sutcliffe 1985) consists mainly of massive quartz and feldspar crystal tuffs with a variety of fragment types suggesting an extrusive, volcanic origin for most of the complex. Sibley Group clastic and chemical sedimentary rocks are few, and a combination of paleotopography on the Archean surface as well as the erosional level of the overlying diabase sills probably controls their distribution. At least one olivine gabbro sill, referred to as the Jackfish Island sill, intrudes the English Bay complex. The Jackfish Island sill has geochemical affinities (Fig. 3) with the Kitto peridotite intrusion located on the east side of the Nipigon Embayment (Hart 2003). Both the Jackfish Island and Kitto peridotite are roughly coeval with the ~1110 Ma Nipigon diabase sills (Davis and Sutcliffe, 1985). Nipigon diabase sills intrude and overlie all previously noted rock types and are part of the 1.11 to 1.09 billionyear-old Midcontinent Rift. The Nipigon sills are generally massive, medium-grained, intergranular-textured and gabbroic in composition with local variations including ophitic and poikilitic textured diabase, oikocrystic diabase and magnetite-rich to locally glomeroporphyritic magnetite phases. Coarse-grained pods, veins and/or monzogabbroic phases also occur within the diabase and could represent a late magmatic phases of the sills or assimilation of Sibley Group sedimentary rocks, or both (Hart and Magyarosi, 2004). The diabase sills intrude and typically overlie all other rock units. The diabase sills are tentatively subdivided into 2 types: 1) The Inspiration sills, located in the northern part of the map area, which have higher trace element ratios (Fig. 3) and normal 102 �remnant magnetization than typical Nipigon sills, and 2) Sills located in the southern part of the map area that are reversely polarized and which have lower trace element ratios. This southern group of sills are grouped with the typical Nipigon sills (Fig. 3). A series of subparallel north- and northwest-trending faults that occur within the Black Sturgeon Fault corridor may be related to the formation of the Midcontinent Rift (Fig. 2). These faults may also in part control the location of Archean inliers. Vertical displacement on both fault sets range from 200 to 400 metres. Figure 2. General geology of south Armstrong–Gull Bay area Figure 3. La/Sm versus Gd/Yb for samples from the English Bay complex, Jackfish Island Sill, Nipigon sills and Inspiration sills. References Davis, D.W. and Sutcliffe, R.H. 1985. U-Pb ages from the Nipigon plate and northern Lake Superior; Geological Society of America Bulletin, v.96, p.1572-1579. Hart, T.R. 2003. Keweenawan mafic and ultramafic intrusive rocks of the Lake Nipigon and Crystal Lake areas, northwestern Ontario; in Programs and Abstracts, Proceedings of the Institute on Lake Superior Geology, v.49, Pt. 1, p.21-22. Hart, T.R. and Magyarosi, Z. 2004. Precambrian geology of the northern Black Sturgeon River and Disraeli Lake area, Nipigon Embayment, northwestern Ontario; Ontario Geological Survey, Open File Report 6138, 56p. MacDonald, C.A. 2004. Precambrian Geology of the South Armstrong-Gull Bay area, Nipigon Embayment, Northwestern Ontario; Ontario Geological Survey Open File Report 6136, 42p. MacDonald, C.A., terMeer, M., Lepage, L., Prefontaine, S. and Tremblay, E. 2004a. Precambrian Geology of the Waweig-Wabinosh Lakes area, western Nipigon Embayment, Northwestern Ontario; Ontario Geological Survey, Preliminary Map P.3536, scale 1:50,000. MacDonald, C.A., terMeer, Lepage, L., Prefontaine, S. and Tremblay, E. 2004b. Precambrian Geology of the English Bay-Havoc Lake area, western Nipigon Embayment, Northwestern Ontario; Ontario Geological Survey, Preliminary Map P.3537, scale 1:50,000. 103 �Precambrian Geology of the South Armstrong–Gull Bay Area, Nipigon Embayment, Northwestern Ontario, Canada MacDONALD, C.A., and TREMBLAY, E. Precambrian Geoscience Section, Ontario Geological Survey Ministry of Northern Development and Mines, Sudbury, Ontario, Canada, P3E 6B5 As part of the Lake Nipigon Regional Geoscience Initiative (LNRGI), a 2-year, 1:50 000 scale mapping program was begun in 2003 to better understand the geology of the western Nipigon Embayment. As part of LNRGI, this study mapped an area of roughly 1200 km2 during the 2003 field season (MacDonald 2004; MacDonald et al. 2004a, 2004b). The project was funded by the Northern Ontario Heritage Fund Corporation (NOHFC) through the Ontario Prospectors Association. This poster presentation highlights some of the preliminary results of this mapping program. The south Armstrong–Gull Bay area is located approximately 190 km north of Thunder Bay and is partially bordered by the shore of Lake Nipigon on its east margin. Rocks of both the Superior and Southern provinces occur in the map area and include Archean rocks of the volcanic-plutonic central Wabigoon Subprovince that are disconformably overlain by Mesoproterozoic sedimentary rocks of the Sibley Group, all of which are intruded by Mesoproterozoic mafic igneous rocks. Archean rocks consist dominantly of variably foliated to gneissic, felsic to intermediate, granitoid rocks and mafic to felsic metavolcanic rocks. A 30 metre thick flat-lying succession of mafic volcanic pillowed flows occurs in the northern part of the map area. The undeformed nature of these flows, combined with the well-preserved nature of relatively delicate features such as hyaloclastite suggests that these rocks may be Proterozoic rather than Archean in age. If so, they would be the first Proterozoic mafic volcanic rocks reported this far north of the Midcontinent Rift. Alternatively, they could represent a previously unrecognized greenstone belt within the Superior Province. Pre-Keweenawan rocks include anorogenic igneous and associated metavolcanic rocks of the English Bay complex and sedimentary rocks of the Sibley Group. The English Bay complex (~1540 Ma, Davis and Sutcliffe 1985) consists mainly of massive quartz and feldspar crystal tuffs with a variety of fragment types suggesting an extrusive, volcanic origin for most of the complex. Sibley Group clastic and chemical sedimentary rocks are few, and their distribution is controlled by a combination of original paleotopography on the Archean basement as well as the erosional level of the overlying diabase sills. At least one olivine gabbro sill, referred as the Jackfish Island sill, intrudes the English Bay complex. The Jackfish Island sill has geochemical affinities with the Kitto peridotite intrusion located on the east side of the Nipigon Embayment (Hart 2003). Both the Jackfish Island and Kitto peridotite are roughly coeval with the ~1110 Ma Nipigon diabase sills. Nipigon diabase sills intrude and overlie all previously noted rock types and are part of the 1.11 to 1.09 billion-year-old Midcontinent Rift. The Nipigon sills are generally massive, medium-grained, intergranular-textured and gabbroic in composition with local variations including ophitic and poikilitic textured diabase, oikocrystic diabase and magnetite-rich to locally glomeroporphyritic magnetite phases. Coarse-grained pods, veins and/or monzogabbroic phases also occur within the diabase and could represent a late magmatic phases of the sills or assimilation of Sibley Group sedimentary rocks, or both (Hart and Magyarosi, 2004). The diabase sills intrude and typically overlie all other rock units. The diabase sills are tentatively subdivided into 2 types: 1) The Inspiration sills, located in the northern part of the map area, which have higher trace element ratios and normal remnant magnetization than typical Nipigon sills, and 2) Sills located in the southern part of the map area that are reversely polarized and which have lower trace element ratios. This southern group of sills are grouped with the typical Nipigon sills. 104 �Several northwest- and north-trending faults are prominent in the map area. Northwest-trending faults appear to correlate with structures in the central Wabigoon Subprovince and may control the distribution of Archean inliers. A series of subparallel north-trending faults, which include the Black Sturgeon fault, may represent faults related to the formation of the Midcontinent Rift. The vertical displacement on both fault sets range from 200 to 400 metres. The mineral potential within the map area consists of previously undocumented areas of Archean mafic and felsic metavolcanic rocks that may hold potential for Pb-Cu-Zn volcanogenic massive sulphide (VMS) deposits. Two areas of brittle-ductile deformation within the metavolcanic rocks may also hold potential for shear zone-hosted gold deposits. The mafic to ultramafic portions of an Archean sanukitoid multiphase intrusion should be investigated for platinum group element potential, since gabbroic to pyroxenitic rocks with disseminated chalcopyrite, pyrrhotite and pyrite from the Roaring River Complex have returned assays up to 2.1 g/t Pt+Pd+Au (Schnieders et al. 2002). Cr-Ni-Cu-platinum group element potential exists in both Proterozoic mafic to ultramafic bodies within the map area. Several areas within the Nipigon Embayment may have potential to host iron oxide-copper-gold (i.e., Olympic Dam type) deposits, particularly the English Bay complex. References Davis, D.W. and Sutcliffe, R.H. 1985. U-Pb ages from the Nipigon plate and northern Lake Superior; Geological Society of America Bulletin, v.96, p.1572-1579. Hart, T.R. 2003. Keweenawan mafic and ultramafic intrusive rocks of the Lake Nipigon and Crystal Lake areas, northwestern Ontario; in Programs and Abstracts, Proceedings of the Institute on Lake Superior Geology, v.49, Pt. 1, p.21-22. Hart, T.R. and Magyarosi, Z. 2004. Precambrian geology of the northern Black Sturgeon River and Disraeli Lake area, Nipigon Embayment, northwestern Ontario; Ontario Geological Survey, Open File Report 6138, 56p. MacDonald, C.A. 2004. Precambrian Geology of the South Armstrong-Gull Bay area, Nipigon Embayment, Northwestern Ontario; Ontario Geological Survey Open File Report 6136, 42p. MacDonald, C.A., terMeer, M., Lepage, L., Prefontaine, S. and Tremblay, E. 2004a. Precambrian Geology of the Waweig-Wabinosh Lakes area, western Nipigon Embayment, Northwestern Ontario; Ontario Geological Survey, Preliminary Map P.3536, scale 1:50,000. MacDonald, C.A., terMeer, Lepage, L., Prefontaine, S. and Tremblay, E. 2004b. Precambrian Geology of the English Bay-Havoc Lake area, western Nipigon Embayment, Northwestern Ontario; Ontario Geological Survey, Preliminary Map P.3537, scale 1:50,000. Schnieders, B.R., Scott, J.F., Smyk, M.C., Parker, D.P. and O’Brien, M.S. 2002. Report of Activities 2001, Resident Geologist Program, Thunder Bay South Regional Resident Geologist Report: Thunder Bay South District; Ontario Geological Survey, Open File Report 6081, 45p. 105 �Magnetic Fabric Constraints on Magmatic Flow: Insizwa Sill, South Africa and the Sonju Lake Intrusion, Minnesota. MAES, Stephanie, TIKOFF, Basil, BROWN, Phil, Department of Geology and Geophysics, University of Wisconsin, Madison, Wisconsin 53706 FERRÉ, Eric, Department of Geology, Southern Illinois University, Carbondale, Illinois 62901 Layered mafic intrusions are an important aspect of plate tectonics, commonly related to large igneous provinces and potentially acting as conduits for continental flood basalts. Although they are the source for most of the world’s PGE (platinum group element) and Cr deposits, as well as important Cu and Ni reserves, a large degree of uncertainty exists as to how these systems evolve. Magnetic fabrics, which often closely relate to the mineral fabric, allow us to document magmatic structures and map out the direction(s) of flow within an intrusion. Magnetic fabrics are determined using anisotropy of magnetic susceptibility (AMS) analysis. Knowledge of flow direction may permit us to place constraints on the location of ore deposits as well as to test models of magmatic sulfide deposition. In addition, these techniques allow us to evaluate the importance of vertical cumulus processes versus horizontal particle flow in the formation of layering. When applied to the study of mafic intrusions, standard magnetic techniques such as low field AMS do not always provide a fabric that reflects the flow field. This is due to the complex magnetic behavior of mafic rocks. Mafic rocks often contain multiple magnetic carriers and magnetite domain sizes, which can negatively affect the AMS fabric. A new magnetic approach, high field AMS (HFAMS), has been used to separate these complex anisotropies. By applying HFAMS techniques the ferromagnetic (magnetite) and paramagnetic (mafic silicate) components of the anisotropy can be separated. Hysteresis properties are then used to identify the domain size of magnetite. Our approach is to compare magmatic fabrics in two intrusions with contrasting genetic histories. In a closed system, such as the Sonju Lake intrusion, petrologic models predict a gradual upward increase in magnetite. In contrast, open systems, where episodes of recharge, eruption and/or assimilation can occur, result in a multimodal distribution of magnetite. The Insizwa sill, South Africa represents a system open to magmatic recharge. Bulk magnetic susceptibility measurements of closely spaced samples from vertical borehole cores within the Insizwa sill have documented significant variation in magnetic properties with depth (see attached figure). Variation in the magnetic signal results from changes in concentration and/or mineralogy of the magnetic material. In addition, a strong correlation exists between petrology of particular layers and bulk susceptibility. Preliminary high field results show consistent high field slopes, indicating a constant paramagnetic contribution to the susceptibility. Hysteresis ratios indicate magnetite is predominantly pseudosingle- and multi-domain, with minor single domain in the lower portions of the sill. In the Sonju Lake intrusion, a more straightforward interpretation of the magnetic properties is expected due to the nearly closed system nature. The same magnetic techniques will be applied to Sonju Lake to fully describe the intrusion and provide insight to the various fabric forming processes occurring in open and closed systems. 106 �107 �MINING AND EXPLORATION ACTIVITY IN NORTHWESTERN ONTARIO MAGEE, Angelique, Ontario Geological Survey, Ministry of Northern Development and Mines, Suite B002, 435 James St. South, Thunder Bay, ON P7E 6S7 CANADA Northwestern Ontario experienced a significant upswing in mining and mineral exploration in 2003. Six mines produced a total of 1.8 million ounces of gold in 2003, approximately 70% of Ontario’s total. Gold producers included: Campbell Mine (Placer Dome (CLA) Ltd.); David Bell Mine (Teck Cominco Limited and Barrick Gold Corporation); Golden Giant Mine (Newmont Canada Limited); Musselwhite Mine (Placer Dome (CLA) Limited / Kinross Gold Corporation); Red Lake Mine (Goldcorp Inc.); and Williams Mine (Teck Cominco Limited and Barrick Gold Corporation). North American Palladium Ltd. produced 288 000 ounces of palladium and 24 000 ounces of platinum at its Lac des Iles Mine and recently announced it would develop an underground operation below its open pit mine. There are approximately 300 active exploration projects in the northwest, the vast majority of which are focused on gold. Areas receiving the most interest from exploration companies were, the Red Lake greenstone belt, Shoal Lake area, Dogpaw Lake area, Shebandowan greenstone belt, Fort Hope greenstone belt, Onaman-Tashota belt and the Pickle Lake greenstone belt. If metal prices continue their upward trend, northwestern Ontario may well experience levels of exploration activity not seen since the mid-1980s. 108 �THE DISCOVERY AND GEOLOGY OF THE L-K MASSIVE SULFIDE DEPOSIT, MENOMINEE COUNTY, MI MAHIN, Robert A., Aquila Resources Corp., Duluth, MN QUIGLEY, Thomas O., Minerals Processing Corp, Duluth, MN LYNOTT, Jeffrey S., Environmental Compliance Consultants, Inc, Rhinelander, WI. The L-K zinc-gold deposit is a recently discovered major volcanogenic massive sulfide (VMS) of the early Proterozoic Penokean volcanic belt. Several events led to the eventual drilling of discovery hole LK-2, (37 meters @ 9.15 % Zn and 5.85 grams/tonne Au) none of which were part of earlier Wisconsin VMS exploration efforts. Prior AEM surveys to the north, west, and south failed to include the immediate deposit area, an area that despite proximal outcrops of intensely sericite-pyrite-altered rhyolite tuffs, was mapped as granitic intrusives and Paleozoic cover. The discovery of L-K can be attributed to Harry Kleiman, a water-well driller who deepened a local camp owner’s water well, his partner Rich Lassen, who recognized the sphalerite-rich drill cuttings as a potential VMS and subsequently located a nearby outcropping gold-rich gossan, and Tom Quigley, who later joined the partnership and pinpointed a significant gravity/conductive anomaly that turned out to be the L-K deposit. A joint venture with American Copper and Nickel Company, a subsidiary of Inco, resulted in a drilling program of 71 holes for over 20,000 meters of core. The drilling outlined a significant zinc-gold rich VMS within what appears to be a major felsic center. Significant portions of the deposit contain consistent grades of zinc in excess of 10 percent. Gold grades with in massive sulfides range from 1 to 6 grams/tonne. Significant gold mineralization also occurs in proximal sulfide stringers (e.g., 4.2 meters @ 25.3 grams/tonne Au), and in peripheral silicified zones in rhyolite tuffs and QFP dikes (e.g., 12.5 meters @ 7.4 grams/tonne Au). The near-surface gossan averages approximately 16 grams/tonne Au. The stratigraphy of the deposit consists of a stacked series of quartz-feldspar rhyolite crystal tuffs, locally fragmental, with intermittent fine-grained ash-tuff horizons, overlain by fine-grained, laminated tuffaceous sediments. Mineralization occurs as massive (>80%) pyrite+sphalerite+chalopyrite+galena localized at two stratigraphic contacts. Host rocks are intensely sericitized and silicified quartz-feldspar rhyolite tuffs and tuffaceous sediments. Structurally, the deposit and surrounding host rocks have been folded into a west/southwest-plunging, south verging antiform. The deposit is located both in the core of the antiform, where it appears to be tectonically thickened, and in the limbs of the fold. Numerous syntectonic, west-southwest-striking, steeply dipping, quartz-feldspar dikes intrude proximal to the deposit. Drilling has traced the massive sulfide along strike for 700 meters with vertical projections of the deposit ranging from 50 to 270 meters wide. Down-plunge, massive mineralization has been traced to a depth of approximately 300 meters. The deposit remains open at depth and along strike. 109 �THE GEOLOGY OF THE L-K MASSIVE SULFIDE DEPOSIT, MENOMINEE COUNTY, MI MAHIN Robert A., Aquila Resources Corp., Duluth, MN QUIGLEY, Thomas O., Minerals Processing Corp, Duluth, MN LYNOTT, Jeffrey S., Environmental Compliance Consultants, Inc., Rhinelander, WI The L-K deposit is a newly discovered volcanogenic massive sulfide deposit in the Beecher Formation of the early Proterozoic Penokean volcanic belt in western Menominee County, Michigan. The major economic metals at L-K are zinc and gold, with locally high concentrations of silver and copper. Mineralization occurs as massive (greater than 80%) pyrite+sphalerite+chalcopyrite+galena localized at two stratigraphic contacts within a large felsic volcanic center dominated by intensely sericitized and silicified quartz-feldspar, rhyolite tuffs. The stratigraphy consists of a stacked series of quartz-feldspar rhyolite crystal tuffs, locally fragmental, with intermittent fine-grained ash-tuff horizons, overlain by fine-grained, laminated tuffaceous sediments. The main-zone massive sulfide is hosted by altered hanging wall and footwall rhyolite crystal tuffs that are visually identical, but readily distinguishable by wholerock geochemistry, e.g., ratios of Al and Ti (Cattalani 2003, Shriver 2003). Tuffaceous zone mineralization, while not as consistently thick as the main zone, contains impressive grades and is focused along the contact between hanging wall rhyolites and overlying tuffaceous sediments. The massive sulfides are enveloped by gold (and to a lesser extent copper) bearing stockwork stringer sulfides (10 to 80% sulfide) that grade into a disseminated pyritic halo. Structurally, the deposit and surrounding host rocks have been folded into a west-southwest-striking, southwest plunging, south verging asymmetric antiform. Tectonically thickened portions of the deposit occupy the core of the fold and large sections of mineralization are also found in the limbs. Numerous syntectonic, west-southwest striking, steeply dipping quartz-feldspar dikes intrude the deposit. Faulting has resulted in minor offsets. Drilling has traced the massive sulfide along strike for 700 meters, with vertical projections of the deposit ranging from 50 to 270 meters wide. Portions of the deposit sub-crop at the east-northeast up-plunge extension where an ironoxide-rich, precious metal-enriched gossan has developed. Down-plunge, massive mineralization has been traced to a vertical depth of approximately 300 meters and remains open. Gradewise, drill intercepts of greater than 10 meters in excess of 10% zinc are common. Gold grades with in massive sulfides range from 1 to 6 grams. Significant gold mineralization also occurs in proximal sulfide stringers (e.g., 4.2 meters @ 25.3 grams/tonne Au), and in peripheral silicified zones in rhyolite tuffs and QFP dikes (e.g., 12.5 meters @ 7.4 grams/tonne Au). Intercepts of the near-surface gossan average 3 meters thick and approximately 16 grams/tonne Au. References Cattalani, S., 2003, Lithochemistry and chemostratigraphy, Back 40 project, Michigan: INCO private report, 7p. Shriver, N.A., 2003, Michigan: Back 40 chemostratigraphy and deformation of the Back 40 massive sulfide based upon lithogeochemical interpretation: INCO private report, 6p. 110 �MINERAL CHEMISTRY AND STRATIGRAPHY OF THE BIWABIK IRON FORMATION, NEAR THE VIRGINIA HORN, MESABI IRON RANGE, MINNESOTA McSWIGGEN, Peter L., and MOREY, G.B., McSwiggen & Associates, P.A., 2855 Anthony Lane South, Suite B1, St. Anthony, MN (PMcS@McSwiggenAssoc.com; morey001@umn.edu) The mineralogy of the Biwabik Iron Formation changes dramatically from west to east as the formation nears the basal contact of the Duluth Complex. This reflects a contact metamorphism that took place with the emplacement of the igneous Duluth Complex. However, the mineralogy of the Biwabik Iron Formation also varies vertically through the stratigraphy of the unit. A number of detailed studies have been done on the mineralogy of the contact zone (Bonnichsen, 1975). This investigation focused on the western Mesabi Range through the characterization of the E.J. Longyear Drill Hole #1 (S1/2, SW1/4, SE1/4, sec 23, T57N, R18W), which penetrated the entire section of the Biwabik Iron Formation from the overlying Virginia Formation through to the underlying Pokegama Quartzite (Fig.1). The iron-formation has been subdivided into four broad stratigraphic units (Fig. 2), lower cherty, lower slaty, upper cherty, and upper slaty, and into four lateral mineralogical zones (1-4) that reflect the zonation resulting from the contact metamorphism. Zone 1, the westernmost zone and the one in which the Longyear drill hole is located, is characterized by quartz, magnetite, hematite, carbonates, talc, chamosite, greenalite, minnesotaite and stilpnomelane. Talc and minnesotaite are the Mg- and Fe-end members, respectively, of the talc group [Mg6Si8O20(OH)4 – Fe6Si8O20(OH)4]. Minnesotaite is the dominant end member in the Biwabik Iron Formation, and typically occurs as a fibrous mineral, though it is also found in a tabular form. Chamosite is a Fe-chlorite [(Fe, Al)6 (Si, Al)4O10(OH)8], and generally occurs as a platy mineral in granules within the iron-formation. These granules are sand size gains that are the result of the reworking of previously deposited materials, which reflects a higher energy depositional environment. Greenalite is the iron serpentine [Fe6Si4O10(OH)8]. It is reported generally as having a platy, lizardite-like structure, and in the iron-formation occurs in granules. Stilpnomelane [(K, Na, Ca)0.6(Fe, Mg)6Si8Al (O,OH)27.2-4H2O] is a sheet silicate, but in the iron-formation it can occur in either a platy form or in a fibrous form. The above-described silicates do not occur uniformly throughout the stratigraphic section. In the Longyear core, chamosite + stilpnomelane is the dominant silicate assemblage in the upper three units (upper slaty, upper cherty and lower slaty). The assemblages minnesotaite + stilpnomelane, minnesotaite + talc, and greenalite + minnesotaite are the main silicate assemblages in the lower cherty. The carbonates have a wide range of compositions and include calcite, dolomite-ankerite, siderite, and kutnahorite, the manganese equivalent of ankerite. Carbonates occur throughout the Biwabik Iron Formation, but become more dominant in the lower cherty interval. The manganese component of the carbonates varies greatly, ranging from less than 1 mole% MnCO3 to as much as 25 mole percent. Bonnichsen, B., 1975, Geology of the Biwabik Iron Formation, Dunka River area, Minnesota: Economic Geology, v. 70, no 2, p319-340. Jirsa, M.A., Miller, J.D., Jr., and Morey, G.B., 2004, Geology of the Biwabik Iron Formation and Duluth Complex, (in press). 111 �Figure 1. Simplified geologic map of the Mesabi Iron Range. The main geologic unit is the Biwabik Iron Formation (shown in gray) (after Jirsa and others (in press)). Figure 2. Stratigraphic section and mineralogy of the E.J Longyear Drill Hole #1. It was drilled as a long stratigraphic test hole and was finished in 1910. 112 �GEOCHRONOLOGY OF PRECAMBRIAN ROCKS IN CENTRAL WISCONSIN: A REVIEW AND NEW 40Ar/39Ar ANALYSES MEDARIS, Jr., Gordon and SINGER, Brad, Dept. of Geology & Geophysics, Univ. Wisconsin Madison, Madison, WI, 53706; medaris@geology.wisc.edu; bsinger@geology.wisc.edu Most of the Precambrian evolution of Wisconsin is recorded by basement rocks in the central part of the state. Reported here are new 40Ar/39Ar analyses of hornblende, muscovite, and microcline, which bear on the thermal history of this important area. Because this is the 50th meeting of ILSG, it seems appropriate to provide a selective review of geochronological investigations over the past half century and to place our new results in a historical context. Review The framework for a modern classification of Precambrian rocks in the Great Lakes region was provided in 1961 by the seminal work of Goldich et al., who analyzed a large number of igneous and metamorphic rocks by the K/Ar and Rb/Sr methods. A three-fold Precambrian division was proposed, with boundaries at 2.5 and 1.7 Ga, corresponding to the Algoman and Penokean orogenies, although it was recognized that the apparent K/Ar and Rb/Sr ages might reflect subsequent metamorphism. In 1975 Van Schmus et al. established an apparent Rb/Sr age of ~1.65 Ga for a wide variety of igneous and metamorphic rocks in central and eastern Wisconsin. The few available U/Pb zircon data for such rocks yielded protolith ages of 1.8-1.9 Ga, and it was suggested that a widespread, low-grade metamorphic event, whose origin was poorly understood, affected the western Great Lakes region at 1.65 Ga. Also in 1975, Van Schmus et al. recognized the Wolf River batholith as a major igneous component in Wisconsin and determined equivalent Rb/Sr whole rock and U/Pb zircon ages of 1468±34 and 1485±15 Ma, respectively. With the continued acquisition of U/Pb analyses of zircon in the 1970s and 1980s, the Marshfield terrane in central Wisconsin was shown to consist of Archean gneiss (2.87-2.52 Ga) intruded and overlain by a wide variety of granitic rocks and associated felsic to intermediate volcanic rocks, ranging in age from 1.89 to 1.82 Ga (Sims et al., 1989). In 1983 Dott suggested that folding of Baraboo Interval quartzites was the result of plate collision to the south at ~1.65 Ga, and this concept was expanded by Van Schmus et al. (1993), who related 1.65 Ga deformation and low-grade metamorphism in the Great Lakes region to emplacement of the Outer Tectonic Belt onto the southern margin of Laurentia during the Mazatzal Orogeny. In 1998 Holm et al. located the tectonic and thermal front of Mazatzal deformation in northern Wisconsin, based on the distribution of folded and flat-lying quartzites and cooling ages (Rb/Sr, K/Ar, 40Ar/39Ar) of mica in basement rocks, e.g. 1.75-1.70 Ga vs. <1.63 Ga. The position and nature of the Mazatzal front in Wisconsin was confirmed by Romano et al. (2000), who showed that micas from basement rocks outside the front yield 40Ar/39Ar plateau ages of 1.76-1.75 Ga, and those inside, 1.61-1.58 Ga. Five samples of hornblende from Archean and Paleoproterozoic rocks within the front yield 40Ar/39Ar plateau ages of 1830, 1796, 1782, 1733, and 1638 Ma, which are thought to represent partial to complete Mazatzal resetting. Subsequently, 40Ar/39Ar ages of 1.45-1.47 Ga were obtained for muscovite in Baraboo Interval quartzites, reflecting widespread, but stratigraphically localized, hydrothermal activity related to Wolf River magmatism (Medaris et al., 2003). Central Wisconsin Precambrian Rocks The basement in NE Wood and NW Portage counties consists predominantly of Archean gneiss and a variety of Paleoproterozoic igneous rocks, including tonalite, granodiorite, granite, and associated felsic to intermediate volcanic rocks. U/Pb zircon ages are ~2780 Ma for migmatitic gneiss at Linwood Quarry, and 1892, 1851, 1841, and 1824 Ma for different varieties of tonalite along the Wisconsin River (Sims et al., 1989; Van Wyck, 1995). Many of the igneous rocks have been deformed and recrystallized, exhibiting a range of foliated and lineated fabrics. Amphibolite layers in Archean gneiss at Conants Rapids yield a temperature of 665 ºC and amphibolite (metadiabase) dikes cutting foliated tonalite at Biron Dam give 700 ºC (calculated by the hornblendeplagioclase geothermometer at P = 4 kbar; Holland & Blundy, 1994). Laser step-heating yields plateau ages of 1672 Ma for hornblende in metadiabase at Biron Dam (Fig. 1), 1516 and 1533 Ma for hornblende from two samples of amphibolite at Conants Rapids (Fig. 2), 113 �1530 Ma for muscovite in low-grade schist from the Eau Pleine shear zone (Fig. 3), and 981 Ma for microcline in Wolf River granite and 897 Ma for microcline in Baxter Hollow granite (located in the Baraboo Range) (Fig. 4). We interpret the ages of hornblende at Conants Rapids, 3.9 miles from the Wolf River batholith, and muscovite in the EPSZ, 6.7 miles from the batholith, to represent partial resetting by the Wolf River thermal pulse. The age of hornblende at Biron Dam, 13.3 miles from the batholith, may also reflect partial resetting by Wolf River heating, although this age lies within the range for hornblende reported by Romano et al. (2000) and ascribed by them to Mazatzal disturbance. The closure temperature of microcline can be as low as 150 oC, thereby allowing for the possibility that it may record cooling of the craton in central Wisconsin, following 1.1-1.0 Ga Keweenawan rifting and magmatism. The Ar ages reported here seem to result from degassing in response to regional thermal events, rather than pervasive internal deformation and recrystallization (also observed by Romano et al., 2000). Thus, although there has been widespread disturbance of Rb/Sr and Ar isotopic systems in Wisconsin Precambrian basement, many rocks in central Wisconsin have preserved their Penokean structures, textures, and mineralogical compositions. References Dott (1983) GSA Mem. 160 129-141; Goldich et al. (1961) Minn. Geol. Sur. Bull 41; Holland & Blundy (1994) Contrib. Mineral. Petrol. 116 433-447; Holm, D. et al. (1998) Geology 26 907-910; Medaris et al. (2003) J. Geol. 111 243-257; Romano et al. (2000) Precam. Res. 104 25-46; Sims et al. (1989) Can. J. Earth Sci. 26 2145-2158; Van Schmus et al. (1975a) GSA Bull. 86 1255-1265; Van Schmus et al. (1975b) GSA Bull. 86 907-914; Van Schmus et al. (1993) GSA Geology of North America C-2 270-281; Van Wyck (1995) Ph.D. thesis, Univ. Wis.-Madison, 280 pp. 114 �Geochemistry and Petrography of Altered Basement Rocks Underlying the Middle Proterozoic Sibley Group METSARANTA, R.T.*, and FRALICK, P.W., Department of Geology, Lakehead University, 955 Oliver Rd., Thunder Bay, Ontario, P7B 5E1, Canada. rtmetsar@lakeheadu.ca The Sibley Group is a relatively thin Mesoproterozoic mixed carbonate-clastic succession deposited, at least in part, under restricted shallow marine or lacustrine conditions (Franklin et al., 1980; Cheadle, 1986; Rogala 2003). A diverse mixture of genetically distinct chemical sediments is preserved within various depositional sub-environments of the Sibley Basin. These include: dolomitic mudstones and sandstones, stromatolitic carbonates, calcretes and nodular gypsum/anhydrite. In addition to these chemical sediments, possible weathering profiles are developed in various underlying basement lithologies and at higher stratigraphic positions. Given the diversity of chemical sediment types and possible weathering profiles within the basin the potential exists to develop a detailed model for the hydrology and paleoenvironmental evolution of the Sibley Group. Weathering is an important control on the composition of clastic sediments (e.g. Nesbitt et al., 1996) and also restricted basin brines (Rosen, 1994). As a first step towards an overall model of the hydrology of the Sibley Basin, the purpose of this paper is to investigate the petrography and geochemistry of altered rocks at the unconformity between the Sibley Group and underlying Archean and Paleoproterozoic lithologies. To be of value in an analysis of overall basin hydrology and paleoenvironmental conditions, the relative importance of primary pedogenically induced geochemical changes vs. later diagenetic or hydrothermal effects must be addressed. Suites of samples were collected from 4 areas spanning a variety of basement lithological types. Altered Quetico sandstones and Neoarchean granite were sampled from drill core at variable depths below the Sibley Group contact. Altered and unaltered Paleoproterozoic black shales of the Rove Formation were sampled from outcrop as were Mesoproterozoic anorogenic granites and overlying pebbly sandstones. Polished thin sections were cut from each sample and analysed petrographically using both optical and scanning electron microscopy. Whole rock powders were obtained for each sample and were analysed at Lakehead University via ICP-AES for a selection of major and trace elements. Petrographic evidence such as the progressive destruction of plagioclase upwards in the profiles coupled with extensive development of Fe-Ti oxides along grain boundaries and cleavage planes in biotite and chlorite is consistent with a weathering profile. Pebbles in sandstones associated with the anorogenic granite profile are also strongly iron enriched containing up to 30% total iron suggesting a highly oxidizing atmosphere during pre-Sibley Group weathering conditions. The presence of authigenic, euhedral potassium feldspars and the potassium-rich nature of clay minerals in the altered horizons suggest secondary potassium enrichment. The presence of barite, flourite and sulphide mineralization near the Sibley Group contact with Neoarchean granites also reveals that hydrothermal alteration may have had an influence on primary pedogenic geochemical signatures. Preliminary assessment of the major element geochemistry using a simple alteration index (CIA) and A-CN-K diagrams (e.g. Nesbitt and Young, 1982; Nesbitt et al., 1996) also suggests that primary pedogenic alteration may have been effected by later potassium enrichment. 115 �Al2O3 0 10 100 90 20 80 30 70 40 60 50 50 60 40 70 30 Granite Quetico Rove 80 90 20 10 100 CaO+Na2O 0 0 10 20 30 40 50 60 70 80 90 100 K2O Figure 1. Altered samples from three profiles plotted as molar proportions Al, Ca+Na, and K (as oxides), showing divergence of observed alteration trends towards more K-rich compositions from those expected due to weathering (arrow) Figure 2. Left: backscatter SEM image of K-rich clay mineral aggregates associated with altered primary K-feldspar (altered Neoarchean granite) and right: altered biotite grains with extensive development of Fe and Ti oxides (altered Quetico metasediment). References Cheadle, B.A., 1986. Alluvial and playa sedimentation in the lower Keweenawan Sibley Group, Thunder Bay District, Ontario. Canadian Journal of Earth Sciences. 23:527-542 Franklin, J.M., McIlwaine, W.H., Poulsen, K.H., and Wanless, R.K., 1980. Stratigraphy and depositional setting of the Sibley Group, Thunder Bay District, Ontario, Canada. Canadian Journal of Earth Sciences. 17:633-651. Nesbitt, H.W., Young, G.M., 1982. Early Proterozoic climates and plate motions inferred from major element chemistry of lutuites. Nature. 299:715-717. Nesbitt, H.W., Young, G.M., McLennan, S.M. and Keays, R.R. 1996. Effects of chemical weathering and sorting on the petrogenesis of siliciclastic sediments, with implications for provenance studies. Journal of Geology. 104:525-542. Rogala, B., 2003. The Sibley Group: A lithostratigraphic, geochemical and Paleomagnetic study. Unpublished M.Sc. thesis, Lakehead University, Thunder Bay, Ontario, Canada. 254p. Rosen, M.R. 1994. The importance of groundwater in playas: a review of playa classification and the sedimentology of playas. In Rosen, M.R., ed., Paleoclimate and Basin Evolution of playa systems. Geological Society of America Special Paper 289. Boulder Co. pp. 1-18. 116 �N.H. Winchell's Study of the Keweenawan Supergroup Rocks of Northeastern Minnesota, 1872-1900 MILLER, James D., Jr., Minnesota Geological Survey, mille066@umn.edu In 1872, fourteen years after its admission to the Union, the state legislature of Minnesota granted an initial appropriation of one thousand dollars per year to the University of Minnesota to create a geological and natural history survey of the state. In July of that year, Newton Horace Winchell (1839-1914) was hired to lead this survey, a task to which he committed 28 years of his life. There is little debate as to the contribution Winchell made to our understanding of diverse aspects of Minnesota's geology, especially in light of the paucity of existing research. Much of the survey's observations and interpretations of Paleozoic and Quaternary geology still stand up today. However, Winchell's greatest difficulties came in his attempts to make sense of the "crystalline rocks" of northeast Minnesota. As he stated in the preface to the fourth volume of the Final Report: "Here [among the crystalline rocks] the geologist is deprived of his usual guides and guys, and finds himself floundering in a muddy sea of innumerable conflicting currents" (Winchell, 1899, p. xiv). This talk focuses on Winchell's struggles with deciphering the complexities of what is now known as the Keweenawan Supergroup in northeastern Minnesota. My interest in this topic began while researching the history of geologic mapping in the Duluth Complex (Miller and others, 2002). My characterizations and interpretations of Winchell's ideas in this presentation come from selected readings of the 24 annual reports of the Geological and Natural History Survey of Minnesota, the final two volumes of the Final Report (Winchell, 1899, 1900), and Geological and Natural History Survey of Minnesota Bulletins 1, 2, 6, and 8. From this still incomplete sampling of Winchell's prolific writing on the Keweenawan rocks, I have come to conclude that most of his ideas, which seem unconventional by today's standards, were based on paradigms that were accepted by many, if not a majority, of geologists in his day. In the late nineteenth century the principles of stratigraphy and sedimentology were already well established, but the science of igneous and metamorphic petrology was in its infancy. At that time, numerous different ideas about the origin of magmas, volcanic processes, and progressive metamorphism existed. There was no accepted conventional wisdom. Winchell took advantage of the new field of petrographic petrology, which saw widespread use and growth in the last half of the century. He did not, however, have the benefit of the knowledge gained by experimental petrology, which started to emerge just as the survey was being completed. At the beginning of the Survey, Winchell's views of Lake Superior geology were strongly influenced by the early federal survey of the region (Owen, 1852), as well as by reports of Canadian geologists and the preeminent geologists from his native New York and New England. Over the course of the survey, Winchell regularly compared his observations to those of his U.S. and Canadian colleagues, but more often than not, he seemed to go his own way with many of his interpretations of the crystalline rocks. In fact, he often disagreed with the interpretations of his fellow survey colleagues, many of whom were his relatives—Alexander Winchell (his brother), Horace V. Winchell (his son), and U.S. Grant (his brother-in-law). One of his more curious relationships was with R.D. Irving of the University of Wisconsin, and later of the U.S. Geological Survey. Following up on his work on the Keweenawan rocks of northwestern Wisconsin for the third Wisconsin Geological and Natural History Survey report in 1880, Irving published the first thorough summary of Keweenawan geology in the Lake Superior region in a U.S. Geological Survey monograph (Irving, 1883). In this report, Irving recognized the volcanic character of the mafic and felsic rocks of the shore, and the intrusive nature of the gabbros. It included a remarkably accurate map of the geology of Minnesota's North Shore, in which Irving estimated that the volcanic pile from Duluth to the Temperance River exceeds 18,000 feet, relatively close to current estimates of 28,000 feet (Miller and others, 2002, Chapter 5). Irving relied exclusively on his own fieldwork and gave only passing acknowledgement to the work of the Minnesota survey, then in its tenth year. Winchell, for his part, rarely cited Irving's work, and then commonly only to point out some inconsistency or discrepancy with his observations or interpretations. Winchell's displeasure with the growing influence and overreaching of the U.S. Geological Survey, perhaps triggered by Irving's work, prompted him to found and edit the American Geologist journal in 1888. The journal was meant to highlight North American geology, but regularly featured articles critical of the U.S. Geological Survey (Bain, 1916). Irving died the year of its initial publication. Some of the major aspects of Keweenawan geology that Winchell wrestled with over the course of the state survey were: Potsdam Sandstone—With a strong belief in uniformitarianism, Winchell consistently held to the notion that all red quartzose sandstone lying unconformably on crystalline rocks was equivalent to the Lower Cambrian Potsdam Sandstone of upstate New York and New England, which has a similar lithology and unconformable geologic setting. Despite their lack of fossils, Winchell opined that the Puckwunge, Nopeming, Fond du Lac, and Hinckley sandstones, and even the Sioux Quartzite, were all Potsdam equivalents. He suggested that the lava 117 �flows overlying the Puckwunge and Nopeming Sandstones and interlayered with stratigraphically higher sandstone units (such as the sandstone at Cutface Creek near Grand Marais) represented localized volcanism during what was generally a time of sandstone deposition, a period he termed the Manitou epoch. Red rock—Winchell also consistently believed that all granophyre and rhyolite were metamorphosed and fused sedimentary rocks—a commonly held view at the time. He considered the "granophyre range," which arcs through the central part of the Duluth Complex, as a raised ridge of Animikie sediments that were fused by the eruption of the gabbro (see below). Rhyolites were thought to be strongly metamorphosed sandstone interleaved with and metamorphosed by basalt flows. In Winchell's view, quartz and feldspar-phyric flows represented less severe metamorphism because phenocrysts of quartz and feldspar were considered to be preserved detrital grains. Basaltic lava flows—Early on, Winchell adopted J.G. Norwood's interpretation (Owen, 1952) that the basaltic lava flows of the North Shore were intrusive sills (flow interiors) into volcanic sediments (amygdaloidal tops). Midway through the survey he recognized the physical stratigraphy of basalt flows. Duluth Complex—Winchell variably characterized the Duluth Complex as "the great gabbro flood," "the crowning overflow," "the great gabbro outflow," "a basic eruptive," and "the gabbro eruption." From these and other descriptors, it is clear that he saw the gabbro as a great volcanic outpouring of basic magma that spilled out over (and metamorphosed) its Animikie rampart and flowed downslope toward the Lake Superior basin. The Beaver Bay Complex and the gabbros at Duluth represent the distal part of this great gabbro flood. Anorthosite inclusions—Winchell interpreted the anorthosite inclusions of the Beaver Bay Complex as earlierformed feldspathic gabbros of the Duluth Complex (what is now termed the anorthositic series) that were picked up by later surges of the great gabbro eruptive. This idea was commonly accepted until recently (see Stop 24 in Field Trip 5). Age of the gabbro—Winchell changed his interpretation of the age of the gabbro many times over the course of the survey. Based on interlayering of the gabbro with rocks he thought to be upper Keewatin (the Pewabik Quartzite, which is actually metamorphosed Pokegama Quartzite), he saw the gabbro as syn-Keewatin to preAnimikie. Until Lawson (1893) showed that the Logan sills are intrusive into the Animikie Group rocks, rather than interlayered as lava flows, Winchell saw them and the gabbro as syn-Animikie. In the end, Winchell believed the gabbro to be younger than the Animikie Group and older than the Puckwunge Sandstone and overlying lavas. Winchell called the epoch during which the gabbros were emplaced the Norian and later the Cabotian. Origin of the gabbro magma—Winchell does not speculate on the origin of the basic magma that formed the gabbro and related volcanics until the Final Report. Following the idea that all granites formed by fusion of siliceous sediments, he concluded that the gabbro formed by the melting of greenstone-derived sediments. He considered hornfels basalt inclusions, which he called muscavodyte, an intermediate stage of metamorphism of these basic sediments. In the end, Winchell was certain that the facts of his numerous and well recorded observations would hold up to future scrutiny, and unquestionably most have. He was admittedly less confident in his interpretations, however and speculated that new facts, "…not included in our field of observation, will in the future place different interpretations on those which we have attempted..." (Winchell, 1899, p. xiv). This has happened to be sure, but more significantly, the changes in our geologic paradigms and our greater understanding of igneous petrology have allowed those of us who follow in his footsteps to reinterpret his many detailed observations. REFERENCES Bain, H.F., 1916, N.H. Winchell and the American Geologist: Economic Geology, p. 51-62. Irving, R.D., 1883, The copper-bearing rocks of Lake Superior: U.S. Geological Survey Monograph 5, 464 p. Lawson, A.C., 1893, The laccolithic sills of the north-west coast of Lake Superior: Geological and Natural History Survey of Minnesota Bulletin 8, p. 24-48. Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.M., and Wahl, T.E., 2002, Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geological Survey Report of Investigations 58, 207 p. Owen, D.D., 1852, Report on a geological survey of Wisconsin, Iowa, and Minnesota: Philadelphia, U.S. Department of the Treasury, 638 p. Winchell, N.H., 1899, The geology of Minnesota: Geological and Natural History Survey of Minnesota, Final Report, v. 4, 629 p., 100 pls. ———1900, The geology of Minnesota: Geological and Natural History Survey of Minnesota, Final Report, v. 5, 1025 p., 6 pls. 118 �Twenty-One Years in a Caldera: UMD Geology Students and Sturgeon Lake, Ontario MORTON, Ron, Department of Geological Sciences, University of Minnesota Duluth Over the past 20 years students from the University of Minnesota-Duluth have been involved in studying the Sturgeon Lake Caldera Complex located in northwestern Ontario. Their work, based on outcrop mapping, logging of more than 200,000 meters of diamond drill core, thin section, microprobe, and chemical studies has shown that the complex comprises a wellpreserved, north facing, moderately to steeply north-dipping, homoclinal sequence that is up to 30 km in strike length and contains more than 3000 m of subaerially and subaqueously deposited volcanic and sedimentary strata. The Pre-Caldera Sequence, worked on by Groves (1984)*and Heine (1985) contains subaerial to shallow subaqueous basalt to andesite lava flows, tuffs and lapilli-tuffs with subordinate rhyolite lava flows. The Early-caldera Sequence, worked on by Walker (1993), Hudak (1989), Beszenek (1992) and Drews (1990) along with Rog (1991) and Murphy (1994) comprises subaerially to subaqueously deposited polymict breccias and aphyric or quartz-phyric, rhyodacite-rhyolite tuffs with reworked tuffs and minor andesite to rhyolite lava flows. The Latecaldera Sequence, worked on by Jongewaard (1989) and Hudak (1996) contains syn-eruptive and reworked quartz- and plagioclase-phyric rhyodacite to rhyolite tuffs and lapilli-tuffs, and subaqueous basalt-andesite to rhyolite lava flows, domes, and cryptodomes. Volcanic-hosted massive sulfide deposits occur in the Early and Late Caldera Sequences. Synthesizing these studies, with additional contributions by Peterson (2001), provides a detailed picture of caldera development and associated massive sulfide formation and hydrothermal alteration. The work of the above geologist’s shows this piecemeal caldera complex formed upon a subaerial to shallow subaqueous basalt -andesite shield volcano with associated local fields of scoria and tuff cones. Caldera formation was characterized by more than a kilometer of collapse accompanied by voluminous pyroclastic eruptions, deposition of laterally extensive meso-and mega-breccias, minor effusive volcanism, and massive sulfide ore deposition with widespread hydrothermal alteration. Later stages of caldera development were dominated by submarine effusive volcanism and intracaldera sedimentation, with subordinate pyroclastic volcanism and ore deposition. The lithostratigraphic sequence, caldera diameter, and estimated eruption volumes, determined from geographic information system (GIS) analysis, are consistent with those that characterize modern ash flow caldera complexes. The Sturgeon Lake Caldera Complex illustrates the following: 1) The necessity of long term, detailed field and laboratory studies to understand complex volcanic systems. 2) That evolutionary and mineralizing processes associated with "ash flow" calderas have been remarkably similar since at least Neoarchean time. 3) That good students do very good work and make professors look great! * Year of degree or year started work at Sturgeon Lake 119 �Status of Publicly Available Mid-Continent Reflection Seismic Data MUDREY, M.G., Jr. Wisconsin Geological and Natural History Survey, 3817 Mineral Point Road, Madison, WI 53705 mgmudrey@wisc.edu CANNON, W.F., U.S. Geological Survey, National Center MS 954, Reston, VA 22092 wcannon@usgs.gov The Midcontinent Rift was identified by potential field and geologic studies in the 1950s. However, much of the detail of this largely buried structure was obscure except in a few well exposed areas around Lake Superior. Interest in frontier petroleum rose in the 1970s and the recognition of documented petroleum seeps from the Nonesuch Formation led to the acquisition and evaluation of industrial reflection seismic data. From 1978 to 1986 a series of seismic reflection profiles was obtained by a combination of academic (COCORP), petroleum company, and government (GLIMPCE) research programs. The U.S. Geological Survey and the Geological Survey of Canada acquired 536 line-km in Lake Superior, L.D. McGinnis and Argonne National Laboratory purchased 1,816 line-km in Lake Superior from Grant-Norpac, and A.B. Dickas University of Wisconsin-Superior negotiated the release of 1,336 line-km from Lake Superior south to Iowa from Petty-Ray/Geosource. The profiles provided access to a wealth of new details of the lateral and vertical extent of the rift and its internal structure and stratigraphy from Lake Superior to Kansas. Some of the major findings include: • Particularly significant was the discovery that 20 to 30 km of basalt flows and secondary syn-rift volcaniclastic and post-basalt sedimentary rock produced exceptionally strong and coherent reflections that enabled accurate estimates of the volumes of basalt. • Moho reflections recorded in Lake Superior over the rift range from 46 to 58 km in contrast to 36 to 42 km beneath the surrounding Great Lakes. This provides evidence of magmatic underplating and intrusions within the lower crust and upper mantle contemporaneous with crustal extension...in effect, the mantle was changed into crust by a decrease in seismic velocity. ! Individual basins within the Midcontinent Rift have been delineated by long reflection lines and cross lines -- clearly developed unconformities remove ambiguities of correlation among various volcanic sections within and between basins. Although individual seismic lines have been interpreted, the entire collection of lines, along with newly acquired gravity and magnetic data, have yet to be collectively used to refine our understanding of this 2,000 km rift. GLIMPCE data are available from Robin R. Warnken, National Geophysical Data Center, NOAA/NESDIS/NGDC/ Mail Code E/GC3, 325 Broadway, Boulder, CO USA 80305-3328. phone: (303) 4976338. Email: Robin.R.Warnken@noaa.gov Argonne National Laboratory/Grant-Norpac data are available from 120 �McGinnis, L.D., and Mudrey, M.G., Jr., 2003, Seismic reflection profiling and tectonic evolution of the Midcontinent Rift in Lake Superior: Wisconsin Geological and Natural History Survey Miscellaneous Report MP 91-2, 15 pl. 1 CD-ROM. Files on CD are in PDF format. Petty-Ray/Geosource data are available from Dickas, A.B. and Mudrey, M.G., Jr., 2002, Regional-Scale Geologic Interpretation of Seismic Reflection, Gravity, and Magnetic Profiles Collected along the Western Arm of the Midcontinent Rift System, Upper Peninsula of Michigan, Wisconsin, Minnesota and Iowa: Wisconsin Geological and Natural History Survey Open-file Report 2002-01, 1 CD-ROM. Files on CD are in HTML format. 1. Location of reflection seismic lines: GLIMPCE, heavy dashed; Grant/Norpac, light solid; Petty-Ray/Geosource, medium solid. 121 �“GOLD IS WHERE YOU FIND IT! SO IS Ag AND Cu AND Fe!” (THE OLD PROSPECTOR: GOLD RUSHES AND MINERAL PROSPECTING, 1848 TO 1900 IN WESTERN NORTH AMERICA AND THE LAKE SUPERIOR REGION) OJAKANGAS, Richard W. (a.k.a., The Old Prospector), University of Minnesota Duluth, Duluth, MN 55812, rojakang@d.umn.edu Gold has long provided a chance for the “little guy” to make a fortune. The “Old Prospector” relates factual and illustrative information about several gold rushes, beginning with the 1849 gold rush in California. (“Go West, young man, go West!”) Where did the 200,000 “Forty-Niners” come from? From everywhere! Because it took 11 months for the news to reach the east coast, people living on the Pacific Rim got there first. How many (how few?) hit it rich? Even Captain Sutter and the actual discoverer, Jim Marshall, died poor. The Comstock Lode in Nevada, “discovered” by Henry Comstock, led to the gold and silver rush of 1860, with many of the fortune-seekers crossing back over the Sierra Nevada with their donkeys. A group on their way to California in 1850 found gold in Colorado at the present site of Denver, but the rich California gold was their goal. There were several discoveries in Colorado over the next decades. It was the gold at Cripple Creek, discovered by cowboy Bob Womack in 1890 that really brought the prospectors in. Bob sold his claim for $300 -- $ 7.5 million came out of that gulch! Looking for gold, silver and copper in Montana attracted many prospectors in the ’60s and ‘70s. The copper ore was “so pure that it could be shipped to hell and back for smelting and still make a profit.” (Swansea, Wales, was the location of early smelters.) In 1876, the rush was to the Black hills for gold. General Custer had gone in to look for gold in 1874, and his favorable report brought in a horde of prospectors. All of this was in violation of the Fort Laramie Treaty of 1868. The Indians were embittered, and Custer was sent in to force the Indians back onto their reservations. In the Battle of the Little Big Horn on June 25, 1876, all 265 soldiers in 5 companies of the U.S. 7th Cavalry died. Meanwhile, what was going on in the Lake Superior region? Native copper had been discovered on the Keweenaw Peninsula by Douglas Houghton in 1840, and mining began soon after. Actually, the copper was “rediscovered” by Houghton, as “Old Copper Complex Indians” had been mining it from numerous pits on the peninsula and on Isle Royale for the previous 6000 years. Iron ore was discovered in the UP near Negaunee in 1844 by William Burt and his party, who were running a line using a sun compass and noticed a large deviation in the magnetic azimuth of a regular compass. Chief Manjekijik of the Chippewas led explorers to another outcrop of iron-formation the next year. Major ore shipments began in 1855. Ore in the Menominee district was discovered in 1845 and began production in 1877. Iron ore on the Gogebic was first noted in 1848 by A. Randall and Charles Whittlesey, but production didn’t begin until 1884. 122 �Gold was mined from the Ropes Mine in the Marquette area in the early 1880s, and sporadically thereafter. In 1868, silver was discovered at Silver Islet near Thunder Bay. In 1865, there was a gold rush to Lake Vermilion. Had it been based on a valid discovery of gold, or was the rush a ruse to generate business for suppliers? (There are no authentic reports of gold having been found there.) The gold activity in the Lake Vermilion area led to the 1865 discovery of iron ore by George Stuntz, a surveyor who had a trading post on Minnesota Point in Duluth... The first ore was shipped in 1882 from Soudan, and in a few years 5 mines were producing at Ely. Many men had crossed the Giants Range (the “Mis-sa-be” hills of the Indians) on their trek to Lake Vermilion (Ona-ma-sa-ga-i-gan, or the “lake of the beautiful sunset”) along the 84mileVermilion Trail surveyed and built by Stuntz Some, including Lewis Merritt of Duluth, noted the presence of magnetic iron-formation in the eastern Mesabi. However, it was not until 1890 when the Merritt brothers (sons of Lewis Merritt), with their concept of ore “basins” within the iron-formation, discovered the first high-grade soft ore at Mountain Iron and a year later at Biwabik. (These discoveries ended the fledging developments on the Gunflint Range.) Within a few years, mines reached 30 miles westward, discovered by Frank Hibbing, Archibald Chisholm, Erwin Eveleth, and others. Within 20 years, most of the Mesabi ore had been located. In 1904, 58 Mesabi mines produced more ore than the 78 mines of Michigan and the Vermilion district. Meanwhile, the search for gold continued wherever bedrock was exposed. In 1894, it was discovered in a quartz vein on Little American Island on Rainy Lake, and a mine was developed to a depth of 212 ft. This was Minnesota’s only legitimate gold mine, having produced $5600 worth of gold. Rainy Lake City grew quickly, and died almost as quickly. There were not many steam gravels on bedrock to be panned in this glaciated country, although to the north in Ontario, prospecting continued unabated. The news of the discovery of the Klondike gold in the Yukon Territory by George Carmack in 1896 reached the West Coast, and even Minnesota, in 1897. Several Minnesota prospectors joined 100,000 others who were heading north to Dawson. Perhaps 40,000 made it, maybe 20,000 prospected, and about 4,000 shared in the total gold find of $10,000,000 (an average of $2,500 each). The good stream beds were rapidly staked. In 1898 near Nome, Alaska, on the Bering Sea, 1,000 miles west of the Klondike, gold was discovered by the “Three Lucky Swedes” (Jafet Lindeberg, Eric Lindblom, and John Brynteson, all greenhorns) who became millionaires. Then gold was found in the beach sands of Nome in 1900, and the beaches became “the poor man’s paradise”. More than 23,000 people sailed there from Seattle, Portland, and San Francisco to get rich in one way or another. At the height of beach mining, 2,000 men, women and children were at work, and produced about $2,000,000 worth of gold, an average of $1,000 each. This was the last great placer gold stampede in North America, and really lasted for only the 3 months of the summer of 1900. 123 �Selected References Boyum, Burton H., 1977, The Saga of Iron Mining in Michigan’s Upper Peninsula: John M. Longyear Research Library, Marquette, Michigan, 48 p. Campbell, L.T., 1992, Skagway—a Legacy of Gold: Alaska Geographic, v. 19, # 1, 96 p. Clark, Henry W., 1930, History of Alaska: the Macmillan Company, N.Y., 208 p. Cole, Terrance, 1984, Nome: “City of the Golden Beaches”: Alaska Geographic, v. 11, #1, 183. Davis, E.W., 1964, Pioneering With Taconite: Minnesota Historical Society, 246 p. DeKruif, Paul, 1929, Seven Iron Men: Harcourt, Brace and Company, N.Y., 241 p. Emanuel, Richard P., 1997, The Golden Gamble: Alaska Geographic, v. 24, #2, 96 p. Green, William, 1963, The Bonanza West: University of Oklahoma Press, 430 p. McCourt, Edward, 1969, The Yukon and Northwest Territories: St. Martin Press, N.Y., 236 p. Morgan, Murray and Hegg, E.A., 1967, one Man’s Gold Rush—A Klondike Album: University of Washington Press, 213 p. Oliver Iron mining Company, 1912, Iron Industry of Minnesota: 48 p. Van Barnevald, Charles E., 1912, Iron mining in Minnesota: University of Minnesota, School of Mines Experiment Station, Bulletin No. 1, 214 p. Walker, David A., 1974, Lake Vermilion Gold Rush: Minnesota History, Minnesota Historical Society, Summer 1974, p. 43-54. Walker, David A., 1979, Iron Frontier—The Discovery and Early Development of Minnesota’s Three Ranges: Minnesota Historical Society Press, 315 p. Welbanks, Wallace P. and Woodbridge, Dwight E., 1905, Minnesota Iron Mines: Welbanks, Crandall and Co., Duluth, Minnesota, 46 p. 124 �DEPOSITION OF PALEOPROTEROZOIC SILICICLASTICS AND IRONFORMATION IN A TIDALLY INFLUENCED SHELF ENVIRONMENT, ANIMIKIE BASIN, LAKE SUPERIOR REGION OJAKANGAS, Richard W., University of Minnesota Duluth, Duluth, MN 55812, rojakang@d.umn.edu; OJAKANGAS, Gregory W., Drury University, Springfield, MO 65802; gojakang@drury.edu The Paleoproterozoic Animikie Basin is interpreted as a northward-migrating foreland basin situated north of the Penokean orogen. Basal siliciclastic units are the Pokegama Formation on the Mesabi Range in Minnesota, the Palms Formation on the Gogebic Range in Michigan-Wisconsin, and the Kakabeka Quartzite on the Gunflint Range in Ontario and adjacent Minnesota. The overlying iron-formations are the Biwabik, Ironwood, and Gunflint, respectively. The iron-formations of these three ranges are in turn overlain by the Virginia Formation, the Tyler and Copps Formations, and the Rove Formation, respectively. We interpret the siliciclastics and the iron-formations to have been deposited on the northern edge (i.e., the peripheral bulge and foreland) of the basin about 1900 Ma. Those in Michigan-Wisconsin were likely continuous with those farther north in Minnesota and Ontario prior to their separation by the development of the Mesoproterozoic Midcontinent Rift System at 1100 Ma. However, the units are likely diachronous, with those in Michigan and Wisconsin somewhat older than those in Minnesota and Ontario. They are interpreted to have been deposited on a shelf near a peneplaned surface on Archean rocks. The siliciclastics were deposited near shore and the iron-formations were deposited farther seaward. As the sea transgressed northward, the iron-formations were deposited upon the siliciclastics. Walther’s Law applies, with the vertical facies indicating the lateral facies. The siliciclastic formations consist of lower argillaceous members, middle members of argillite, siltstone, and sandstone, and upper members of mature sandstone (all gradational), interpreted to have been deposited, respectively, in upper tidal, middle tidal, and lower tidal (subtidal?) environments in a transgressing sea. The well-exposed Palms Formation exhibits abundant tidal evidence including bimodal-bipolar paleocurrent plots (N=250) for the formation as a whole and also for specific localities, tidal bedding (lenticular, wavy, and flaser), and minor flat-topped ripple marks and mudcracks. Also present in the middle member are thin sandtextured beds composed of iron silicates that were apparently transported shoreward into the siliciclastic zone. The Pokegama Formation is poorly exposed, but tidal evidence can be interpreted from limited exposures and a few drill cores. Sequences of thicker and thinner laminae in siltstone beds of the lower member are interpreted as evidence of the diurnal inequality that is an alternation in the heights of successive high tides in a twice-daily tidal environment. The diurnal inequality occurs when the moon is above or below Earth’s equatorial plane, because under these conditions any non-equatorial location will pass through different parts of the tidal deformation ellipsoid during each successive high tide. Ideally the diurnal equality disappears at the equator, and therefore our data are suggestive of a non-equatorial depositional location. Poorly exposed packets of progressively thinner and progressively thicker laminae may indicate neap and spring tidal cycles. These investigations are continuing, with a search for longer sequences of laminae in drill cores. 125 �The iron-formations have thick-bedded and granular (‘sandy”) members and thin-bedded and fine-grained (“muddy”) members. The former make up the lower and upper “cherty” members and the latter comprise the lower and upper “slaty” members. The granules are composed of iron oxides, iron silicates, iron carbonates, and chert, whereas the fine-grained units are made up largely of iron silicates and iron carbonates. It has commonly been thought that fine-grained precipitates of silica, iron carbonates and/or iron silicates formed on the shelf edge where upwelling waters supplied the iron and silica to a location below wave-base. Reworking of these fine-grained precipitates by tidal and/or storm currrents resulted in the formation of the granules. The granules were then transported into higher energy locations shoreward of the deeper shelf. The Ironwood Iron Formation is poorly exposed, whereas the Biwabik and Gunflint are well exposed. The Biwabik is exceptionally well exposed in taconite pits. In the Minorca Mine just northeast of Virginia, a paleocurrent plot of 102 cross-beds, including rare herringbone cross-beds, is strongly unimodal to the NNE, perpendicular to the paleogeographically determined shoreline, but with 10 % of the readings in the opposite sense. Therefore, flood tides were dominant. In some pits, such as at Minntac, channels of granular iron-formation are cut into the fine-grained and thinly bedded iron-formation. These are as wide as 1 km and 25 m deep, are oriented perpendicular to the paleogeographically determined shoreline, and are interpreted to be tidal channels in which granular sediment was transported seaward into the realm of fine-grained precipitates. A shallow water environment for the deposition of the granular members is supported by the rounded nature of the grains, the cross-bedding, and two major stromatolite horizons. Stromatolite columns in a bed that has since been mined away were all inclined at 30 degrees to the vertical, suggestive of an environment of deposition at about 30 degrees latitude. The vertical sequence of members—lower cherty, lower slaty, upper cherty, and upper slaty—is due to transgression, regression, and transgression. Selected References Morey, G.B., 2003, Paleoproterozoic Animikie Group, related rocks and associated iron-ore deposits in the Virginia Horn: in Jirsa, M.A. and Morey, G.B., eds., Contributions to the geology of the Virginia Horn Area, St. Louis County, Minnesota, Minnesota Geological Survey, Report of Investigations 53, p. 74-102. Ojakangas, R.W., 1983, Tidal deposits in the early Proterozoic basin of the Lake Superior region—The Palms and the Pokegama Formations: Evidence for subtidal-shelf deposition of Superior-type banded iron-formation: in Medaris, L.G., ed. Early Proterozoic geology of the Great Lakes region: Geological Society of America Memoir 160, p. 49-66. 126 �THREE-DIMENSIONAL GEOMETRY AND STRAIN OF THE BARABOO SYNCLINE: KINEMATIC IMPLICATIONS ORMAND, Carol J., Department of Geology, Wittenberg University, Springfield OH 45501, cormand@wittenberg.edu CZECK, Dyanna M., Department of Geosciences, University of Wisconsin - Milwaukee, P.O. Box 413, Milwaukee, WI 53201 INTRODUCTION Paleoproterozoic sedimentary rocks of the “Baraboo interval” were deposited on a stable cratonic margin with subdued topography, in a warm, humid climate, as evidenced by their physical and chemical maturity (e.g. Dott, 1983; Medaris et al., 2003). The southern deposits, including the Baraboo Quartzite and associated rocks, subsequently underwent both low-grade thermal metamorphism and simultaneous deformation during the Mazatzal orogeny, ~1650-1630 Ma (Holm et al., 1998; Romano et al., 2000). During this collisional event, the flat-lying marginal sediments were crumpled into tight, asymmetric, southward-verging folds. The foreland fold-and-thrust belt of this collision is preserved in isolated outcrops in southern Wisconsin including the Baraboo hills (LaBerge and Klasner, 1986). Approximately 40 kilometers long by 15 kilometers wide, the geometry of the Baraboo Syncline is strikingly three-dimensional. The fold axis trends approximately N80E. The northern limb of the fold is subvertical to slightly overturned; the southern limb dips around 35 degrees northward; the eastern termination plunges approximately 35 degrees westward; and the western termination plunges approximately 25 degrees eastward. DEFORMATION FEATURES Strain on the limbs of the Baraboo Syncline is as three-dimensional as the fold itself. Both limbs of the fold have axial planar phyllitic cleavage, refracted into quartzitic strata. Within the southern limb, however, where quartzite beds are sandwiched within phyllitic layers, threedimensional pinch-and-swell structures (“chocolate tablet boudinage”) show extension parallel to layering, both along strike and down dip. Strain data from quartz grain shapes also indicate three-dimensional strain, with extension either layer-parallel or layer-normal (McKiernan, 2002; Craddock, pers. comm.). In addition, slickensides on the southern limb of the fold indicate one paleostress direction, while slickensides on the northern limb show multiple paleostress solutions (Kirschner et al., 1989). KINEMATIC MODEL Both the fold geometry and the extension within the gently dipping limb of the syncline are consistent with formation in a top-to-the-south simple shear environment (Cambray, 1987). In such an environment, non-cylindrical fold trains would verge southward. In this model, the longer, north-dipping fold limbs are favorably oriented for localized layer-parallel extension, while the shorter, steeply dipping limbs rotate and shorten during deformation. The location of boudinage exclusively on the south limb is consistent with this model. The multiple paleostress directions on the north limb, inferred from slickensides (Kirschner et al., 1989), are also consistent with the rotation of the north limb explicit in the model. Therefore, this simple shear 127 �model is consistent with the majority of the field data. However, it is a two-dimensional model that does not account for the strongly three-dimensional fold geometry. To account for the threedimensional shape of the syncline, we invoke a component of non-plane strain. We envision a variation in degree of shearing along strike, resulting in extension along the fold axis. This model therefore could explain both the strong change in plunge along trend and the threedimensional boudinage within the southern limb of the Baraboo Syncline. We are analyzing microstructural data on both limbs of the fold to further evaluate this kinematic model. REFERENCES Cambray, F. W., 1987. The Baraboo syncline; the shape and refolding explained as a result of superposition of simple shear on a pre-existing fold. GSA Abstracts with Programs, 192. Dott, R. H., Jr., 1983. The Proterozoic red quartzite enigma in the north-central United States: resolved by plate collision? GSA Memoir 160, 129-141. Holm, D., Schneider, D., Coath, C. D., 1998. Age and deformation of Early Proterozoic quartzites in the southern Lake Superior region: implications for extent of foreland deformation during final assembly of Laurentia. Geology 26, 907–910. Kirschner, D., Pershing, J., Teyssier, C., 1989. Nature and kinematics of fault surfaces in the Baraboo Syncline (WI). GSA Abstracts with Programs, 17. LaBerge, G. L., Klasner, J. S.., 1986, Evidence for a major south-directed early Proterozoic thrust sheet in south central Wisconsin. GSA Abstracts with Programs, 664. McKiernan, A., 2002. Stress-strain analysis in Precambrian quartzites from Wisconsin: evidence for eastward continuation of the ca. 1650 Ma Mazatzal and Central Plains orogenies. Honors Paper, Macalester College, MN. Medaris, L. G. , Jr., Singer, B. S. , Dott, R. H. , Jr., Naymark, A., Johnson, C. M., Schott, R. C. , 2003. Late Paleoproterozoic Climate, Tectonics, and Metamorphism in the Southern Lake Superior Region and Proto–North America: Evidence from Baraboo Interval Quartzites. Journal of Geology 111, 243–257. Romano, D., Holm, D. K., Foland, K. A., 2000. Determining the extent and nature of Mazatzalrelated overprinting of the Penokean orogenic belt in the southern Lake Superior region, north-central USA. Precambrian Research 104, 25–46. 128 �DULUTH COMPLEX BULK SAMPLES PATELKE, Richard, and SEVERSON, Mark, Natural Resource Research Institute, University of Minnesota Duluth, Duluth Minnesota 55811, rpatelke@nrri.umn.edu, mseverso@nrri.umn.edu Since copper-nickel exploration began in the Duluth Complex with the first drill hole in 1951, there have been numerous bulk samples taken for metallurgical testing. Our current project is an attempt to develop a narrative history of this work. The project began for two reasons: 1) there is a perception that these bulk samples have usually returned metal grades lower than was expected by the pre-excavation testing; and 2) there was no consolidated listing for where data about these projects might be found. The issue of sample head grade in bulk samples being lower than expected is real, but is not documented well enough to discern an overall cause or to propose a solution beyond more rigorous outcrop stripping, mapping, and drilling before choosing a test site. Most sites have been located based on a single drill hole and as far as we can tell no sites have been rejected after a location has been chosen or once excavation began. There is probably a distinction between a geologist’s definition of a successful test and a metallurgist’s definition. A geologist’s definition of a successful bulk sample is one where the overall grade and mineralogy is what was predicted from assaying, drilling, or outcrop study (a “scientific success”). A metallurgist might define a successful bulk sample as one that represents, or is typical of, the bulk composition and the mineralogical ratios of the deposit, and therefore allows reliable conclusions to be drawn from testing certain steps in the beneficiation process (a “practical success”). The primary source of information for this study is company publications and records in files at MDNR and NRRI, little was found at other locations. Samples in the South Kawishiwi intrusion include: a small outcrop (and drill core?) sample by the USBM; two surface bulk samples from the INCO Spruce Road deposit; samples from the shaft and drift of the INCO Maturi deposit; large excavation and incidental exposure at the Dunka Pit iron mine (over 14 million tons of Duluth Complex material on the surface); and a small incidental exposure of Duluth Complex rock and massive-sulfide related to iron-formation stripping in the Peter Mitchell Taconite mine. Bulk samples at the Babbitt (Mesaba) deposit in the Partridge River intrusion include: work on one surface sample pit and multiple samples from the shaft and drift by AMAX; numerous drill core composites by AMAX; two test pits by Arimetco in the 1990s; one test pit by Teck Cominco in 2001; and a 50,000 ton pit planned by Teck Cominco in the near future. The Dunka Road (NorthMet) deposit had three samples at two locations by USS; a 1991 drill core composite from new large diameter holes by Nerco and Fleck; and a large pilot plant sample by PolyMet from reverse-circulation cuttings. The Longnose Oxide-bearing Ultramafic intrusion (OUI) had two samples taken from pits for beneficiation and process testing of the Fe-Ti oxides. Two complete drill holes in the Water Hen OUI were taken for process testing by the USBM in the 1980s. The major bulk samples are listed in Table 1, the references for the table are in the document below. Reference: Patelke R., and Severson M.J., in prep., 2004, A history of copper-nickel and titanium-oxide test pits, bulk samples, and related metallurgical testing in the Keweenawan Duluth Complex, northeastern Minnesota, Natural Resources Research Institute, Technical Report NRRI/TR-XX, ~100 pages. 129 �SOUTH KAWISHIWI INTRUSION Project Responsible party Year(s) Tonnage Comment Spruce Road USBM Source of sample uncertain, test work done uncertain No data found. INCO 3 holes drilled in 1953, report issued in 1955 19661,150 tons 1967 1974 10,000 tons INCO 1968 INCO 1968? Serpentine Reserve Mining Dunka Pit Erie / LTV Before 1989 1975 to 1998? Comment Grades Reference 0.43% Cu, Ni est. at 0.12% Various files in AMAX archive at MDNR INCO Maturi Grades Lab / bench tests on composite from 3 drill cores and / or outcrop samples? Reported head grade of 0.38% Cu, 0.14% Ni, 0.88% S Reference Grosh et al., 1955, USBM Report 5177 1974 INCO project description on file at MDNR in AMAX archive Pit along south side of Spruce Road, processed by INCO at Sudbury (?) Reported head grade of 0.47% Cu, 0.15% Ni, 1.08% S 1974 INCO project description on file at MDNR in AMAX archive 700 tons (?) Shaft at Maturi, sample sent to INCO lab at Sudbury (?) No data found. 1974 INCO project description on file at MDNR in AMAX archive Drift at Maturi, some drilling done from drift, but little information in NRRI or MDNR files. Assume some material must have been sent for Misc. files at MDNR and NRRI metallurgical tests. Uncertain tonnage An exposure of massive sulfide assumed to be similar or related to the Serpentine deposit is seen in the Peter Mitchell Mine. Exposed during Ruhanen, 2001 for MDNR, Severson South iron-formation stripping. Assayed by MDNR in 2001. Kawishiwi report 14-20 million tons in Stockpiles at Dunka Pit represent Duluth Complex material removed for 0.23% Cu, 0.09% Ni, 2.20% S are the approximate values from Files at MDNR, and Ron Graber at CCI stockpiles? iron ore mine development. exploration drilling. MDNR reports values of 0.29% CuO, 0.10% NiO (uncertain about whether these are oxide or sulfide assays) PARTRIDGE RIVER INTRUSION Project Responsible party Babbitt (Mesaba) AMAX At B1-341 deposit AMAX Dunka Road (NorthMet) deposit Year(s) Tonnage 1978 1976 AMAX 1976 Arimetco at B1-374 1994 Arimetco at B1-411 19951996 Teck Cominco at B1-321 2001 150 tons sent to CMRL, January 1996 5,000 Teck Cominco at B1-321 USS Bulk No 1 Future 50,000 1971(?) unknown tonnage, but small USS Bulk No 2 1971 USS Bulk No 3 1971 Fleck / Nerco 1990 PolyMet Composite 19982000 Longnose Fe-Ti oxide (OUI) 1,150 ton Surface pit in NE corner of deposit. Sample may actually have been taken excavation, 560 tons in South Kawishiwi intrusion, not Partridge River intrusion sent as sample Shaft samples listed as "disseminated." Some of this material used by MDNR for various leaching and ARD tests Drift samples listed as "massive" or "semi-massive." Some of this material used by MDNR for various leaching and ARD tests 200 tons excavated, Surface pit. Sample probably in weakly mineralized pegmatitic zone of sample split to 85 Unit 3 and 115 ton portions American Shield 1984 American Shield 1999 Surface pit. Sample in western part of deposit, location in Unit 1. Reasonably typical material. Various disseminated ore samples. Est. at 0.43% Cu, 0.13% Ni, Various files in AMAX archive at MDNR S unknown Various massive and semi-massive sulfide samples. Various files in AMAX archive at MDNR 0.22% Cu, 0.06% Ni, 0.52% S from blast holes; sorted sample had head grade of 0.30% Cu, 0.08% Ni, 0.63% S 0.61% Cu, 0.12% Ni, 1.02% S from blast holes; CMRL reports MDNR and NRRI files head grade at 0.36% Cu, 0.08% Ni, 0.76% S Surface pit at location drilled by Severson for Arimetco, in Unit 1, near Severson estimate 460 tons at 0.62% Cu center of north edge of deposit. Very typical material. Final sample much larger than 460 tons outlined by Severson. Planned surface pit, at same location as Teck Cominco B1-321. EAW approved by State in 2003 Surface pit near drill hole (26058) with mineralization only in top few feet. Drill hole 26058 grade from 8 to 20 ft. was 0.82% Cu, 0.20% Small pit, found by Zanko and Severson in 1995, no definitive records Ni, 1.21% S; below that hole is not mineralized for hundreds of available. feet; head grade of bulk sample was 0.39% Cu, 0.14% Ni, 0.50% S 300 tons Surface pit near drill hole 26105. Intended to intercept mineralization seen Expected grade based on ddh 26105 was 0.77% Cu, 0.28% in that hole. Material contaminated with hornfels, poor grade. Ni(?), 1.23% S; head grade of sample was 0.40% Cu, 0.13% Ni, 0.97% S 20 tons Surface pit near drill hole 26105. Re-entry of Bulk No 2 site to get material Expected grade based on ddh 26105 was 0.77% Cu, 0.28% not contaminated with hornfels Ni(?), 1.23% S; head grade of sample was 0.58% Cu, 0.22% Ni, 0.98% S 2 large diameter core PolyMet report states they have no records for this work, other than that it No data holes was done in 1991. Two large diameter holes and two smaller twins for submission to state. Nerco holes twin two existing USS drill holes. At least 37 tons Reverse circulation drilling composite from about 55(?) reverse circulation Head grade in 1999 SME/AIME presentation is 0.43% Cu, shipped to testing holes. Data not available on how many or which holes constituted the bulk 0.12% Ni, PolyMet has not published sulfur numbers. laboratory sample. Surface pit, sample sent to CMRL for process testing Surface pit, sample sent to CMRL for process testing Drilled in about 400 ft. of drill USBM samples to test reduction processes on Fe-Ti ore; with goal of Water Hen Fe-Ti Water Hen drill holes SL-27 and SL- 1975 core producing saleable or processable titanium slag product. Work done in Oxide (OUI) 28 1985 Table 1. Major bulk samples of the Duluth Complex. 130 MDNR and NRRI files Study concluded that a high TiO2 product could be made, but that concentration that removes iron is important. High MgO content is not mentioned as a processing issue Communication w/Teck Cominco and NRRI files EAW approved June, 2003 NRRI files NRRI files NRRI files PolyMet January 2000 Prospectus PolyMet press releases and 2001 pre-feasibility study CMRL reports in 1990s on projects, but ore sources for individual projects uncertain CMRL reports in 1990s on projects, but ore sources for individual projects uncertain Nafziger and Elger, 1987, USBM report; drill hole desc. in Ross, 1985 �UNTESTED TARGETS IN THE DULUTH COMPLEX PATELKE, Richard, SEVERSON, Mark, and PETERSON, Dean, Natural Resource Research Institute, University of Minnesota Duluth, 5013 Miller Trunk Highway, Duluth Minnesota 55811, rpatelke@nrri.umn.edu, mseverso@nrri.umn.edu, and dpeters1@nrri.umn.edu Introduction There are at least four, presently sub-economic, large, copper-nickel +/-PGE deposits defined along the western and northwestern margins of the Duluth Complex, as well as many smaller prospects (more than ten?). Development has been slowed by metallurgical problems and the perception of permitting difficulty. Exploration for more of the large low-grade deposits would seem unneeded until some of the known ones are developed. We present a few exploration ideas for less studied areas of the Duluth and Beaver Bay Complexes (with profuse apologies to all who may also have mentioned these in the past). We hope to encourage a discussion about smaller, higher-grade, targets. All of the targets listed here point to the need for detailed field mapping and integration of geophysical data. Remember that five hundred million tons of rock forms a cube about 1,750 feet on a side, and that a viable deposit can be relatively small. The Schroeder-Forest Center crustal ridge target Regional aeromagnetics and gravity data indicate that the Duluth Complex occupies two deep “bowls”. The separation between these two deep zones is the Schroeder-Forest Center crustal ridge. In general, mapped surface geology does not carry across this basement ridge zone. The presence of inclusions of Archean supracrustal rocks could also indicate that the base of the Complex is closer to the surface in this area (Boerboom, 1994). The Wilder Lake intrusion strikes parallel to the ridge and dips northeastward, also indicating that there is a division in the Complex. So, if one applies a model based on what we see for the copper-nickel-PGE deposits, i.e., most economic mineralization is close to the basal contact, then determining the depth to the basal contact along the ridge could open up areas for future exploration if the footwall is found to be reasonably shallow. Also, this area would probably be a much different structural regime than we see at the western and northwestern margins of the Duluth Complex. Cloquet Lake layered series This area is grossly similar to the situation along the Schroeder-Forest Center crustal ridge or the western margin. A group of three holes, drilled near the edge of this funnel-shaped intrusion in 1982, went to the base of the intrusion and hit massive sulfide with magnetite. PGE values were low and no further drilling was done, but scattered thin intercepts (3 ft.?) of about 0.5% copper were seen within 250 ft. of the surface. In this case, the footwall was older Keweenawan intrusive rocks. PGE in oxide-rich, pegmatitic, and other rocks The highest average value in the Complex for combined Pt + Pd + Au are in oxide-rich rocks, particularly those at Birch Lake. Pegmatitic zones, massive sulfides, anorthositic rocks, and massive chlorite also return higher than average values. However, these five rock types represent a small percentage of the over 52,000 feet of sample in Severson and Hauck (2003) that record assays for all three metals (troctolitic rocks make up about three fourths of the Pt + Pd + Au assay footage). These five minor rock types are not often mineralized, but when they are, the mineralization is copper-rich. Could there be larger, near-surface zones of these anomalous rock types? So far, localized copper-rich, semi-massive sulfides (some with high PGE values) have been documented at Dunka Road, Dunka Pit, on trend with the Siphon Fault, and at Skibo. Oxide-bearing Ultramafic Intrusions (OUIs) While the Fe-Ti rich OUIs are not particularly rich in sulfides or PGE, and have not yet proved economic as sources of titanium, they are still viable targets for chromium and vanadium oxides. There is less than 1,000 feet of assaying for these oxides available for the Complex (Severson and Hauck, 2003; Patelke 2003). The 131 �OUIs are enigmatic: the alignments of large OUIs in the Western Margin and Partridge River intrusions would indicate a relation to large and as yet undefined faults. Similar fault-related OUI are present south of the Babbitt deposit. However, information from drill core gives no hint to the geometric nature of the root zones of the OUIs. As the OUI formed along fault zones, often late in the crystallization history of the various intrusions, they may mark zones with potentially vigorous hydrothermal activity that could have concentrated PGE. For example, drill hole SL-19A at the Water Hen OUI has a thin platinum and chrome-bearing horizon that has had no known follow up work for PGE, or chromium and vanadium. INCO records show extremely high copper and nickel values, with widely varying ratios, near the Skibo OUI; again with little follow up. Overall, the OUIs may be worthy of attention beyond being a source of titanium. Structural targets Two major scissor-like faults that lie perpendicular to the line of the basal contact near the Babbitt and Dunka Road deposits are relatively unexplored. The Siphon Fault begins in the former LTV pit and extends some distance into the Complex. The Grano Fault starts in the Peter Mitchell taconite pit, passes through the Serpentine deposit, and lies along the border between the Partridge River intrusion and the South Kawishiwi intrusion. The Grano Fault is inferred to be a vent that formed high-grade copper-PGE enriched massive sulfide at the Local Boy ore zone of the Babbitt deposit. Both of these large faults cross mapped W-NW to E-SE trending faults passing though the Dunka Road deposit and to the south of the Babbitt deposit. The intersection of these systems has not been well examined either by drilling or field mapping. These fault intersections could present two types of targets: 1) massive sulfide in locations where space opened during faulting, especially in the footwall (a Sudbury or Local Boy model); and 2) pathways for altering and /or mineralizing fluids to intersect particular geologic horizons (such as in the Birch Lake model or postulated for the top of Unit 1 in the Partridge River intrusion). Voisey’s Bay targets Peterson’s discussion of “feeder zone sulfide mineralization” in Miller et al., (2002), points to the possibility of such a target south of the Spruce Road area (a.k.a. the Highway One Corridor area). There, the potential conduit between the Bald Eagle intrusion and the South Kawishiwi intrusion is confined below a large raft or pillar of earlier formed anorthosite. Mapping of assay copper-nickel grade and ratios indicates that the highest grades in the area may be in a channel or vent area below this raft. References Boerboom, T.J., 1994, Archean crustal xenoliths in a Keweenawan hypabyssal sill, northeastern Minnesota. White was right!: Institute on lake Superior Geology, 40th Annual Meeting, Houghton Mich., Proceedings, v. 40, Program and Abstracts, pt. 1, p. 5-6. Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., Peterson, D.M., and Wahl, T.E., 2002, Geology and Mineral Potential of the Duluth Complex and related rocks of northeastern Minnesota, Minnesota Geological Survey Report of Investigations 58, 207 p., one CD-ROM. Patelke, R.L., 2003, Exploration drill hole lithology, geologic unit, copper-nickel assay, and location database for the Keweenawan Duluth Complex, northeastern Minnesota: Natural Resources Research Institute, University of Minnesota Duluth, Tech. Report., NRRI/TR-2003/21, 97 pages, 1 CD-ROM. Severson, M.J., and Hauck, S.A., 2003, Platinum-group elements (PGEs) and platinum-group minerals (PGMs) in the Duluth Complex: Natural Resources Research Institute, University of Minnesota Duluth, Technical Report, NRRI/TR-2003/37, 296 pages, 1 CD-ROM. 132 �THE PROPOSED NATIONAL UNDERGROUND SCIENCE AND ENGINEERINGLABORATORY AT THE SOUDAN MINE, NORTHEASTERN MINNESOTA: A GEOLOGICAL SITE INVESTIGATION PETERSON, Dean M., Natural Resources Research Institute, Duluth, MN, dpeters1@nrri.umn.edu PATELKE, Richard L., Natural Resources Research Institute, Duluth, MN, patelke@nrri.umn.edu In 2000, the National Science Foundation (NSF) convened a committee chaired by Dr. John Bahcall, of Princeton University, to evaluate the scientific justification for a national facility for deep underground science. The committee's charge was to evaluate the potential physics research that could b completed by the next generation of solar neutrino, double beta decay, proton decay, dark matter, and related background-sensitive experiments. In addition, the committee considered the possible relevance of such a facility to other disciplines, including geoscience, microbiology, materials development and technology, and monitoring nuclear tests. NSF received the recommendations of the Bahcall Report titled “Underground Science” in 2001 (available online at http://www.sns.ias.edu/~jnb/Laboratory/science.pdf). One of the results of the NSF-sponsored workshop on Neutrinos and Subterranean Science in 2002 was the June 2003 publication “EarthLab, A Subterranean Laboratory and Observatory to Study Microbial Life, Fluid Flow, and Rock Deformation”. This publication sets a framework for geological research that could be undertaken in a deep underground setting, and is available online at http://www.earthlab.org. Three unsolicited proposals to fund (each ~ $275 million) the development of a National Underground Science and Engineering Laboratory (NUSEL) were submitted to NSF, and include the University of Washington's Homestake Mine site at Lead, South Dakota (http://mocha.phys.washington. edu/nusel/proposal.html), the University of Minnesota's Soudan Mine site in northeastern Minnesota (http://www.sudan.umn.edu/NUSEL/), and the University of California Irvine's Mt. San Jacinto site near Palm Springs, California (http://www.ps.uci.edu/~SJNUSL/). On May 28, 2003, a NSF site panel report on developing a NUSEL concluded that the Homestake Mine in South Dakota was the most favorable. In addition, the panel considered the Soudan Mine a possible back up site for NUSEL, and that the San Jacinto site is not a viable NUSEL candidate. The evaluation criteria of each of the sites were partitioned into two broad categories: (1) geological suitability; and (2) relative costs. Geological suitability issues for the Soudan Mine included the uncertainty of the geology and rock mass conditions at depth. On June 2, 2003, Barrick Mining Company, the owner of the Homestake Mine, turned off the pumps and began flooding the deep portions of the Homestake mine, which consequently jeopardizes most of the earth science initiatives outlined in EarthLab. On February 6, 2004, the NSF returned without prejudice all of the unsolicited NUSEL proposals, and will soon publish a three-stage request for new NUSEL proposals, which will include: Stage 1 - develop a preliminary plan of research activities requiring deep underground access, to aggregate the plans into science modules, and to define the physical requirements needed for each module; Stage 2 - fund grants for conceptual planning of infrastructure as related to each site; and Stage 3 - fund grants for technical designs for the underground infrastructure, detailed geological characterization and environmental permitting, and development of management plans. Between mid April and mid June of 2003, geologists from the Economic Geology Group of the Natural Resources Research Institute, University of Minnesota Duluth completed detailed geologic mapping around the proposed NUSEL site at the Soudan Mine, and wrote a detailed report describing the results of the study (Peterson and Patelke, 2003). The geological report, geological maps, and GIS data files generated as a result of this work are available online at http://www.nrri.umn.edu/egg/. This poster presents the three plates that accompany the geological site investigation report of the suitability of the Soudan Mine area for hosting a NUSEL. Based on this recent detailed field mapping and interpretation, the geological and structural setting of the Soudan Mine is perfectly suited for hosting a NUSEL. In addition, a Soudan Mine NUSEL contains all the requirements outlined in the EarthLab document. The report by 133 �Peterson and Patelke (2003) outlines the geological suitability of the area for a NUSEL and an integrated EarthLab. The five major themes addressed in this report include: Bedrock Geology - The Neo-Archean bedrock geology of the Soudan Mine area is divided into five major lithostratigraphic units. These units include the: (1) Fivemile Lake sequence, moderate to shallowwater bimodal volcanic rocks; (2) Central Basalt sequence, deep-water tholeiitic basalts; (3) Upper Sequence, Algoma-type iron formation, tuff, and epiclastic rocks; (4) intrusive rocks, felsic porphyries, granodiorite, diorite, gabbro, and lamprophyre; and (5) sheared rocks, distinct curvilinear zones of chloritesericite-ankerite-pyrite schists. Structural Geology - The field area is divided into four main structural domains that include: (1) the Murray shear zone; (2) the Mine Trend shear zone; (3) the Linking Zone; and (4) the Collapsed Hinge Zone. These domains appear to be internally structurally coherent, and are separated from each other either by areas of relatively undeformed rocks or discrete sheared boundaries. NUSEL Site Selection - The geological criteria deemed most important for NUSEL construction include: (1) definition of a competent rock mass for excavation of large caverns at depths of 1,450 m and 2,500 m; (2) minimizing the occurrence of major lithologic contacts that would be encountered during construction of shafts, drifts, and the helical decline; and (3) minimizing the occurrence of major structural features in the area proposed for construction of the helical decline. A large area in the competent pillowed basalts of the Central Basalt sequence appears to meet all of the criteria for construction of the helical decline, and large laboratories could probably be excavated out of a large, highly indurated dioritic sill. Compatibility with EarthLab - One of the main requirements for EarthLab is a very large, instrumented rock volume and access to great depths. At Soudan, the volume of rock that can be reasonably be accessed for EarthLab research in a Soudan Mine NUSEL is approximately 30 km3. The scientific themes of study proposed for EarthLab are: (1) microbial life at depth; (2) the hydrologic cycle; (3) rock fracture and fluid flow; (4) rock-water chemistry; (5) deep seismic studies; and (6) geophysical imaging. The compatibility of a Soudan Mine NUSEL with each of these themes is favorable. Although the conceptual design plans for the Soudan Mine NUSEL requires relatively high-cost new construction at depth, the pristine nature of this geological environment is highly desirable for EarthLab research. In addition, the close proximity of the Soudan Mine NUSEL to the four structural domains minimizes the cost of drilling and drifting into these structural settings for EarthLab research. Outstanding Geological Research Opportunities - The geological and structural setting of the rocks within and adjacent to the proposed Soudan Mine NUSEL provides a unique opportunity for advances in several areas of earth science. Ideas on outstanding research opportunities include: (1) the structural control of lode-gold mineralization; (2) the hydrothermal alteration of subaqueous volcanic rocks and associated massive sulfide copper-zinc mineralization; (3) the origin of massive hematite ore bodies within Algomatype iron-formation; (4) the genetic evolution and temporal development of a Neo-Archean volcanic arc; (5) the Neo-Archean tectonic architecture of the southern Laurentian margin; (6) the Pleistocene hydrogeology of the Superior Craton; and (7) the permeability of crystalline bedrock. References Peterson, D.M and Patelke, R.L., 2003, National Underground Science and Engineering Laboratory (NUSEL); Geological site investigation for the Soudan Mine, northeastern Minnesota: Natural Resources Research Institute, Technical Report NRRI/TR-2003/29, 97 p., 3 plates, 1 cd-rom. 134 �PETROTECTONIC EVOLUTION OF PALEOPROTEROZOIC GRANITIC ROCKS ACROSS THE CENTRAL PENOKEAN OROGEN, NORTHERN MI & WI PIERCEY, P., SCHNEIDER, D.A., Department of Geological Sciences, Ohio University, Athens, OH 45701 USA HOLM, D.H., Department of Geology, Kent State University, Kent, OH 44242 USA Recent U-Pb single-crystal zircon geochronology of Paleoproterozoic post-Penokean granitic rocks of northern Michigan and Wisconsin, historically interpreted as an "anorogenic suite," has revealed a distinct age trend: magmatic pulses apparently migrated southward from ca. 1800 to 1750 Ma, after cessation of Penokean orogenesis (Holm et al., 2004; figure1). Yavapai-aged subduction slab rollback has recently been hypothesized to explain this magmatic pattern. In this model, the subducting slab steepens, and while depth of melting remains static, the locus of melting migrates trenchward (figure 2). Granitoid bodies intrude the Archean gneissic basement, Proterozoic metasedimentary marginal sequences, and an accreted juvenile arc (the Wisconsin Magmatic Terrane) across the breadth of the orogen. Nine samples from eight localities (the Radisson granite was sampled twice due to significant differences in mineralogy) were analyzed petrologically and geochemically, using major-, trace-, and rare-earth element analysis, to discriminate the tectonic setting into which emplacement occurred. Petrologic analysis shows a grain size decrease to the east. Major-element classification (K2O vs. SiO2) indicates a calcalkaline to shoshonite trend for all localities sampled, indicating subduction-related genesis. Nevertheless, trace element tectonic discrimination diagrams after Pearce et al. (1984) indicate a correlation to the local lithology rather than tectonic setting. That is, Humboldt and Montello granites, which intrude Archean gneissic basement, are classified as within-plate granite (WPG); Park Falls syenite, intruding Proterozoic metasedimentary sequences, is categorized as a collisional granite (COLG); and the remaining granites (Radisson, Lugerville, Jennings, Amberg, and Chequamegon) that intrude the accreted arc of the Wisconsin Magmatic Terrane are classed as volcanic arc granites (VAG). For this reason, caution must be exercised in using this technique, especially on single localities. A rare-earth element spidergram, normalized to chondritic values, shows the COLG as the most evolved and WPG as the least, highlighting relative continental crust evolution/component. In this study, the geochemical analyses did not illustrate an age or geographical trend as expected, but rather a correlation of source area and/or relative crustal contribution. Therefore, the suite is interpreted as products of subduction-induced melting across a variable source terrane. References: Holm, D.K., Van Schmus, W.R., MacNeill, L.C., Boerboom, T.J., Schweitzer, D. and Schneider, D.A., 2004, U-Pb zircon geochronology of Paleoproterozoic plutons from the northern midcontinent, U.S.A.: evidence for subduction flip and continued convergence after Geon 18 Penokean orogenesis: Geological Society of America Bulletin, in press. Pearce, J.A., Harris, N.B.W., Tindle, A.G., 1984, Trace Element Discrimination Diagrams for the Tectonic Interpretation of Granitic Rocks: Journal of Petrology, 25, 956983. 135 �136 �BLOWING IN THE WIND: THE COPPER HARBOR STROMATOLITES REVISITED PLANAVSKY*, Noah, and BJORNERUD, Marcia, Geology Department, Lawrence University, Appleton WI 54912 The Copper Harbor Conglomerate is the basal unit of the sediment-dominated upper part of the Keweenawan series. This sedimentary sequence was deposited within the central basin of the Mid-Continent Rift System beginning at approximately 1087 Ma. The coarse grain size, rounded clast shapes, low clay content, large trough cross bed sets, channel structures and ripple marks observed in the conglomerate are best explained by a prograding alluvial fan complex in an arid environment with rugged topography (Daniels, 1982). Locally, the conglomerate is matrixsupported, indicating that debris flows also contributed to its formation. The clasts within the conglomerate appear to have been derived exclusively from the underlying volcanic flows of the rift, suggesting that the basin was only a relative low within a topographically elevated region – probably a thermally supported high similar to the modern East African rift. All sedimentary features of the Copper Harbor Conglomerate point to a nonmarine depositional setting. In the upper part of the Copper Harbor Conglomerate, thin, discontinuous clayey layers with dessication cracks are interbedded with the coarse and thick conglomerate strata, suggesting that ephemeral playa lakes existed within the basin. Some of the lacustrine deposits contain finely laminated, hemispherical, calcareous stromatolitic structures. In many cases these structures drape around the upper surfaces of large boulders that are partly enclosed by clayey sediment. The external shape and fine internal layering of the hemispheroids as well as the presence of detrital sand grains on their inclined sides (indicating organic stabilization) all suggest that they are true biogenic stromatolites (Elmore, 1983). That is, the Copper Harbor stromatolites appear to record a terrestrial microbial community that established itself far above sea level in middle Proterozoic time. Most modern microbial lithifying mats are complex symbiotic communities of more than one genus (Stal, 2000), although there are modern freshwater stromatolites formed exclusively by cyanobacterial activity (Eggleston and Dean, 1976). The current paradigm is that the mucilaginous sheath excreted by cyanobacteria is essential for formation and lithification of the stromatolite structure. Whether the Copper Harbor stromatolites represent multi-genus communities or simple cyanobacterial mats, it is interesting to consider how the microbes might have found their way into a high, dry basin that had previously been the filled with ponded basaltic lavas. Most modern and fossil stromatolites occur in near-shore marine settings, and migrate over time via aqueous fragmentation or microbial mat bifurcation. The Copper Harbor Basin, however, was clearly high and isolated. The nearest continental margin is thought to have lain some 800 km away, beyond the Grenville front (Ojakangas et al., 2001). Paleocurrent directions and the absence of extrabasinal clasts in the conglomerate, furthermore, exclude the possibility of fluvial transport from another terrestrial community. We suggest, therefore, that the basin was colonized through akinete anemochory, or wind transport of dormant reproductive structures. Colonization by this means is consistent with the inferred wind patterns of the time. Localized aeolian dune deposits within the Copper Harbor 137 �conglomerate indicate paleo-wind directions that were orthogonal to fluvial paleocurrent directions, blowing from the southeast (in paleogeographic coordinates), the azimuth of the Grenville coast. This is consistent with the inferred paleolatitude of 20° N, which would have placed the Copper Harbor basin in a paleo-trade wind region (Taylor and Middleton, 1990), with prevailing winds blowing from the Grenville coast toward the Lake Superior region. Long distance aeolian transport of akinete structures is physically plausible. Cyanobacterial akinetes are on the order of 10µm in diameter (Lang and Whitton, 1973), and several recent studies have documented regional and even transoceanic transport of fine sediment (Prospero, 1999), spores (Josefsson, 2002) and pollen grains (Rogers and Levetin, 1998) of similar size. The stromatolites of the Copper Harbor Conglomerate are among the few known records of Proterozoic terrestrial biological communities, and they provide a glimpse of the early stages of colonization of a barren landscape. Aeolian dissemination of microbes and nutrients may have been an important mechanism of dispersal in the Precambrian biosphere. References: Daniels, P. A., Jr., 1982. Proterozoic sedimentary rocks: Oronto Group, Michigan- Wisconsin, in Wold, R.J., and Hinze W.J., eds., Geology and Tectonics of the Lake Superior Basin, GSA Memoir 156, 107-133. Elmore, R.D., 1983. Precambrian non-marine stromatolites in alluvial fan deposits, the Copper Harbor Conglomerate, Upper Michigan. Sedimentology, 30, 829-842. Eggleston, J.R. and Dean, W.E., 1976. Freshwater stromatolitic bioherms in Green Lake, New York. In Walter, M.R., ed., Stromatolites. Developments in Sedimentology, 20, Elsevier, Amsterdam, 479-488. Josefsson, H., 2002. Long Distance Dispersal in Wood Decaying Basidiomycetes (MS Thesis}: University of Umeå University, Umeå, Sweden. Lang, N.J. and Whitton B.A. 1973. Arrangement and structure of thylakoids. In Carr, N.G. and Whitton, B.A., eds., The Biology of Blue Green Algae. University of California Press: Berkeley, 66-79. Ojakangas, R., Morey, G.B., and Green, J.C., 2001. The Mesoproterozoic Mid-continent Rift System, Lake Superior region, USA. Sedimentary Geology, 141-142, 421-442. Prospero, J. 1999. Long range transport of mineral dust in the global atmosphere: Impacts of African dust on the environment of the Southeastern United States. Proceedings of the. National Academy of Sciences, 96, 3396-3403. Rogers, C.A. and Levetin, E.1998. Evidence of long distance transport of mountain cedar pollen into Tulsa, Oklahoma. International Journal of Biometeorology, 42, 65-72. Stal, L.J., 2000 Cyanobacterial mats and stromatolites, in Whitton, B.A., ed., Ecology of Cyanobacteria. Kluwer Academic, Dordrecht, 61-120. Taylor, I. and Middleton, G., 1990. Aeolian sandstones in the Copper Harbor Fm., late Proterozoic, Lake Superior basin, Canadian Journal of Earth Sciences, 27, 1339-47. 138 �A Geochemical Study of the Sills of the Nipigon Basin, Ontario RICHARDSON*, A., and HOLLINGS, P., Department of Geology, Lakehead University, 955 Oliver Rd., Thunder Bay, Ontario, P7B 5E1, Canada; ajrichar@lakeheadu.ca Introduction and Background The Nipigon Sills are a relatively flat-lying Proterozoic tholeiitic diabase sequence associated with the Keweenawan midcontinent rift (MCR) event centered on the Lake Superior region. The MCR was active 5540000N approximately 1100 My ago with published U-Pb zircon ages of 1108.8 +4/-2 Ma, and 1097.6 ±3.7 Ma from the Nipigon Sills and Osler volcanics respectively (Davis and Lake Sutcliffe, 1985). These sills are up to 200 5510000N Nipigon metres in thickness and dominate the geology of the Lake Nipigon basin. The sills currently cover an area of approximately 11 000 km2 representing a minimum volume of 10 000 km3 (Fig. 1; Sutcliffe, 1986). 5480000N Although related to the Logan sills found further south in the Thunder Bay area, they differ in rare earth geochemistry, mineralogy, and paleomagnetic character. First noted by Sir William Logan in a report to the crown in 5450000N 1863, the region was not mapped in detail until the first decade of the twentieth century Legend by Andrew Wilson, whose report in 1910 Nipigon Sill Metavolcanic Drill Hole Collar Scale N included an accurate representation of the Sibley Sediments Granitic Sample 0 20 Km English Bay sills, in addition to Archean geology and Metasediment 1537 +10/-2 Ma (Davis & Sutcliffe, 1985) limited geology of the Sibley Group Figure 1. Regional Geology with sample sediments. Wilson’s report brought to light locations the question of whether the sills represent an extrusive flood basalt sequence or a hypabyssal intrusive unit. The lack of extrusive characteristics (pillows, ropy flow tops, vesicles, etc.) as well as a billion years of erosion makes interpretation based on direct observation difficult. 6 7 10 11 4 15 3 18 1 19 22 24 25 167 149 62 160 119 120 42 124 63 70 89 35 134 90 92 85 31 77 97 69 10 Project This research is funded as part of the Lake Nipigon Regional Geoscience Initiative (LNRGI) in order to further the understanding of the Lake Nipigon region and promote mineral exploration and development through a public/private sector partnership. The aim of this study is to develop a model of sill emplacement using detailed whole rock and isotope geochemical data, as well as petrographic work and mineral chemistry, to develop a formational model for the sills. 139 �Preliminary Results Of the 170 outcrop samples and 530 diamond drill hole (DDH) samples collected in 2003 (Fig. 1), 80 have been analyzed using XRF, ICP-AES, and ICP-MS methods. Preliminary results point to the sills being a remarkably uniform sequence of olivine basalt with a pronounced negative Nb anomaly, 100 LREE enrichment and slightly fractionated HREE (Fig. 2, grey) with La/Smcn and Gd/Ybcn ratios for normal sill averaging 1.74 and 1.40 respectively. A geochemically distinct sill with Gd/Ybcn and La/Smcn 10 ratios of 3.64 and 13.3 respectively was found in the top of drill holes DDH3 and DDH5. This distinct signature was also observed in the basal unit of drill hole SW08-5 1 Th Nb La Ce Pr Nd Zr Hf Sm Eu Ti Gd Tb Dy Y Ho Er Tm Yb Lu Sc of the ultramafic portion of the Seagull intrusion. A Figure 2. REE profiles of three sill types. Type 1 (black), Type 2 third distinct sill, identified in (grey), Type 3 (dashed) two outcrop samples along the southern shore of Lake Nipigon was found to have La/Smcn, and Gd/Ybcn ratios of 6.46 and 1.89 respectively. Though petrographically indistinguishable, this geochemistry indicates that there were at least three magma sources for the sill complex, of which all possess a significant Nb anomaly indicative of crustal contamination References Davis, D.W., Sutcliffe, R.H. (1985) U-Pub ages from the Nipigon plate and northern Lake Superior. Geological Society of America Bulletin, v. 96, p.1572-1579. Heggie, G., Hollings, P. (2003) Lake Nipigon Region Geoscience Initiative. Petrology and Mineral Chemistry of the Seagull Intrusion. In: Summary of Fieldwork and other Activities 2003, Ontario Geological Survey, Open File Report 6120, p. 48-1 to 48-5. Sutcliffe, R.H (1986) The Petrology, Mineral Chemistry and Tectonics of Proterozoic Rift-Related Igneous Rocks at Lake Nipigon, Ontario. Unpublished PhD thesis, University of Western Ontario, Ontario. 253p. Wilson, A (1910) Geology of the Nipigon Basin, Ontario. Canada Department of Mines Geological Survey Branch Memoir No.1. 152p. 140 �THE GEOLOGY OF THE EAGLE NICKEL-COPPER DEPOSIT: MARQUETTE COUNTY, MICHIGAN ROSSELL, D.M., dean.rossell@kennecott.com, Kennecott Exploration Company, 10861 N. Mavinee Dr. #141, Oro Valley, AZ. 85737 COOMBES, S., Kennecott Canada Exploration Inc., #354-200 Granville Street, Vancouver, B.C. V6C 1S4 Kennecott’s discovery of the Eagle nickel-copper deposit in 2002 marked the culmination of more than a decade of exploration work by Kennecott in the Baraga Paleoproterozoic sedimentary basin. The discovery hole, YD02-02 completed in July 2002, intersected 84.2m of massive sulfide mineralization averaging 6.3% Ni and 4.0% Cu. The resource estimate for the Eagle deposit at the end of 2003 was 5 million tonnes at 3.68% Ni, 3.06% Cu and 0.1% Co. The Eagle deposit is hosted in the westernmost of two small peridotite bodies historically referred to as the Yellow Dog Peridotite. The Yellow Dog intrusions, which lack penetrative foliations and truncate Penokean tectonic fabrics in the surrounding meta-sediments, are believed to be Keweenawan in age (Klasner, et. al., 1979). The intrusions are mainly comprised of coarsegrained, variably serpentinized peridotite and feldspathic peridotite. A fine-grained, olivine poor phase is found along the margins of the intrusions and as xenoliths within the peridotite. Possible amygdules in the olivine poor phase(s) suggest a shallow level of intrusion. Three principal types of sulfide mineralization are recognized in the Eagle deposit: disseminated (blebby), semi-massive (matrix) and massive. Although the nickel contents of massive sulfides are relatively uniform throughout the deposit, copper contents vary significantly. Platinum group metals (PGM) and gold values are significantly higher in the copper rich massive sulfides. Copper rich veins and disseminations, with significant PGM and gold, in the surrounding meta-sediments may constitute a fourth type of ore. Massive and semi-massive sulfide ore types in the Eagle deposit are irregularly distributed. The contacts between different ore types are sharp and show little evidence of the gradation that might be expected if gravity driven accumulation of sulfides from an overlying, sulfide saturated, silicate magma was the principle mechanism of ore formation. Sequential emplacement of various mixtures of silicate and sulfide magma and cumulus minerals, derived from a lower stratified magma chamber, may provide a better model. Klasner, J.S., Snider, D.W., Cannon, W.F., and Slack, J.F., 1979. The Yellow Dog Peridotite and a possible buried igneous complex of lower Keweenawan age in the northern peninsula of Michigan. Geologic Survey of Michigan DNR report of investigation 24, 31 pp. 141 �Geologic Reconnaissance of the Spaulding Mine Area, Cook County, Minnesota RUHANEN, Richard W., 1746 Janet Park Dr. Hibbing, MN 55746 The Spaulding “mine” consists of a series of exploration pits, shafts and trenches constructed on a fissure vein at the east end of Spaulding Lake in northeastern Cook County, MN. William P. Spaulding prospected this area during the late 19th century, from about 1875 until the summer of 1897, with the target being silver mineralization. Silver was being mined in Ontario during this time at the Rabbit Mountain and Silver Mountain areas just 20 miles to the northeast in veins of a similar nature, as well as at Silver Islet near the Sibley Peninsula on the shore of Lake Superior. The Spaulding Lake area is remote, accessible only by canoe, and consists of east-striking hills and ridges with predominantly north-facing cliffs. Porphyritic diabase sills cap the hills and ridges while the low lands are occupied by lakes and swamps underlain by Rove formation sediments. The rocks dip 12 – 16 degrees to the south. Spaulding’s work exposed an east-striking vein on the south side of Spaulding Lake at the base of a north-facing cliff of Rove formation capped by a sill. The vein consists of a breccia of angular, porphyritic sill and Rove formation fragments cemented by quartz, calcite and perhaps barite. Vugs lined with drusy quartz crystals are common. Sparse pyrite is the only mineralization seen, no silver or other sulfide minerals have been observed. Dump piles near the pits and shafts consist almost entirely of this breccia vein material. During October 2003, two traverses were made by the author on the north shore of Crystal Lake 1 and 2 miles, respectively, to the west of the exposed vein. Rocks noted are porphyritic sill(s), granophyre, small basaltic dike-like bodies, and hornfelsed to partially melted blocks of Rove sediments. At each traverse location, a north-south fracture forms a 1 to 2 foot scarp facing west. Gabbroic rocks of the Duluth Complex crop out ¼ mile to the south, forming a low ridge on the southwest shore of Crystal Lake. Inclusions of Rove formation are common in the sills and in the Duluth Complex rocks. At the Spaulding “mine” location, the sill – Rove formation contact rises in elevation to the east while curving towards the south. The exploration pits were sunk along depressions thought at the time to be ancient mining features created by copper culture peoples. At the shaft, the last excavation on the vein to the east, the breccia consists entirely of Rove formation. On one dump pile near the eastern extent of the working, granophyre with quartz-filled fractures was found, indicating that granophyre occurs at some depth beneath the sill. The vein disappears under glacial drift to the east of the main shaft. At the southwest end of Spaulding Lake, an east-west striking zone of fault gouge cuts a sill of porphyritic diabase, indicating that the vein structure may continue further west of the Spaulding workings. 142 �OCEAN-FLOOR-TYPE-ALTERATION OF DRILLED MRS VOLCANIC ROCKS IN IOWA SCHMIDT, Susanne Th., Département de Minéralogie, Rue des Maraîchers 13, CH1205 Genève, Switzerland, susanne.schmidt@terre.unige.ch SEIFERT, Karl, Department of Geological & Atmospheric Sciences, Iowa State University, Ames, IA 50011, kseifert@iastate.edu In the Thor Group of the buried Midcontinent Rift System of Iowa well cuttings and cores of basalts and diabases were studied from five sites to determine the alteration pattern within a subsurface zone of 250 x 50 km (Fig. 1). The metamorphic assemblage, metamorphic grade, bulk rock composition and the chemical composition of the metamorphic minerals such as actinolite, pumpellyite, epidote, and chlorite, were determined. Equilibrium phase diagrams were calculated using the DOMINO-Theriak Software. At sites 3 and 5 the metamorphic assemblage is similar and contains the minerals pumpellyiteactinolite-chlorite (Mg-rich)-albite. However, at site 3 this assemblages is restricted to a thin alteration band along a vein and the rock is unaltered away from the vein, whereas at site 5 the rock is almost totally altered and relicts of the primary magmatic minerals, such as clinopyroxene or Ca-rich feldspar, are rarely observed in thin section. At site 4 the assemblage pumpellyitealbite-chlorite is present in a diabase. For site 2 chlorite and epidote were determined. Chlorites are present at all sites showing a wide range of composition (Fig. 2). Chlorite is Mg-rich in less altered units and Fe-rich in strongly altered units. Cross cutting relationships between these minerals imply the later formation of Fe-chlorites. The observed pattern points to greenschistfacies conditions in site 3 and 5 and probably higher temperatures at sites 2 and 1 (epidote-chlorite assemblage). The fact that alteration is focused along veins such as observed at site 3 indicates that infiltration of a fluid at elevated temperatures occurred at some stage of the alteration. For the assemblage amphibole-chlorite-albite-epidote in the pervasively altered diabase of site 5, an equilibrium diagram was calculated using the DOMINO-THERIAK Software (de Capitani. 1994). It restricts the assemblage to a temperature interval between 210 to 260 °C and a pressure of up to 3 kbar. The presence of pumpellyite in another sample of the same site and its upper temperature stability limit of ca. 250 °C at 3 kbar (Potel et al., 2002) are in agreement with the calculated P-T field. References Seifert, K.E. & Anderson, R.E. (1996) Geochemistry of buried Midcontinent Rift Volcanic rocks in Iowa, Data from well samples. Jour. Iowa Acad. Sci. 103, 63-73. De Capitani, C. (1994) Gleichgewichtsphasendiagramme: Theorie und Software: Beihefte zum European Journal of Mineralogy, v. 72. Jahestagung der Deutschen Mineralogischen Gesellschaft, p. 48. 143 �Potel, S., Schmidt, S.Th. & de Capitani, C. (2002) Composition of pumpellyite, epidote and chlorite from New Caledonia – How important are metamorphic grade and whole rock composition? Schweiz. Miner. Petrograph. Mitt. 82, 229-252 Fig. 1 Location of drill sites in the buried MCR in Iowa (after Seifert & Anderson, 1996) Fig. 2 Composition of chlorite in the Thor group 144 �Depth Migration of Seismic Reflection Data: An Example for Lake Superior Studies SCHNEIDER, Robert V., Energy Institute, University of Louisiana at Lafayette, P.O. Box 43612, Lafayette, LA 70504-3612, USA 2-D reflection seismic profiling is a useful geophysical tool for understanding geologic structures in the Earth’s subsurface. For example, it is the primary tool of choice in hydrocarbon exploration on a worldwide basis. A seismic section, under the right circumstances, can provide an accurate picture of geologic formations. The main differences between seismic and geologic sections are: 1) Seismic data are recorded and displayed in time, not depth; 2) Seismic reflections are caused primarily by velocity changes, which may (or may not) be coincident with geologic boundaries. A key process in preparing a seismic data set for interpretation is application of a concept called migration. This step moves seismic amplitudes from where they are recorded (i.e. at the receiver) to where the reflection actually occurs in the geologic section (Gray et al., 2001). Because seismic data are recorded in time, this requires an accurate understanding of the velocities in the subsurface, which carries the seismic wave field. Seismic time processing, however, depends on several assumptions to simplify complexities in seismic wave theory. Of fundamental importance is the presence of a horizontally smoothly varying velocity field. Where this assumption is violated, the resulting seismic profile is degraded below the velocity change. In other words, structures displayed in such sections are misplaced both horizontally and vertically. In extreme cases, even the edges of bodies with significantly different velocities than the surrounding country rock are difficult to image (Larner et al., 1989). Near-surface velocity changes, especially in marine acquisition, are relatively rare examples of this problem. Where they occur, the subsurface image may be degraded even after the application of time migration. An example is found at the Florida Escarpment in the eastern Gulf of Mexico (Figure 1a). Here the steep slope of the ocean bottom, which approaches 45◦ over a 2000m interval, creates a lateral step change in velocity from water to a rock column. To minimize the effects of this problem, careful velocity analysis must be performed in depth (Schneider et al., 2000). The resulting velocity model was used to migrate the seismic data, which were recorded in time, and to output an image in depth (Figure 1b). The depth of the water bottom of Lake Superior varies over short horizontal distances (Figure 2). This similarity to the Florida Escarpment indicates that a reflection profile acquired over portions of the lake may suffer similar effects. Resulting interpretations may therefore be error-prone. Experience suggests that careful velocity modeling in depth followed by depth migration will be required to maximize our understanding of crustal structure in this region. 145 �a b Figure 1. (a) Example of seismic mis-imaging due to time processing across the abrupt Florida Escarpment velocity change. (b) Depth migrated data showing improvement in the image of the water bottom and the subsurface image (data courtesy of TGS-NOPEC). Figure 2. Bathymetric profile showing rapid variation in depth in Lake Superior (Natural Resources Research Institute, 1998). Gray, S. H., Etgen, J., Dellinger, J, and Whitmore, D., 2001, Seismic migration problems and solutions: Geophysics, 66, 1622-1640. Larner, K., Beasley, C. J., and Lynn, W., 1989, In quest of the flank: Geophysics, 54, 701-717. Natural Resources Research Institute, 1998, Lake Superior Bathymetry Map, http://oden.nrri.umn.edu/lsgis/bathy.htm, accessed April 9, 2004. Schneider, R. V., Gordon, M. K., Sempere, J., Willacy, C., Hightower, S., and Scholz, S.F., 2000, Prestack depth imaging in the eastern Gulf of Mexico: The Leading Edge, 19, 1340-1343. 146 �WHATEVER HAPPENED TO THOSE Cu-Ni DEPOSITS? SEVERSON, Mark J., and HAUCK, Steven A., Natural Resources Research Institute, University of Minnesota Duluth Large resources of low-grade copper-nickel sulfide ore that locally contain anomalous Platinum Group Element (PGE) concentrations are well documented by drilling in the basal zones of the Partridge River and South Kawishiwi intrusions. At least nine subeconomic deposits have been delineated in the basal 100 to 300 meters of both intrusions. The mineralization consists predominantly of disseminated sulfides that collectively constitute over 4.4 billion tons of material averaging 0.66% Cu and 0.20% Ni (Listerud and Meineke, 1977). Serious exploration for Cu-Ni deposits at the base of the Duluth Complex (Complex) began about 13 km (8 miles) to the southeast of Ely, MN, in 1948, when strongly mineralized rocks were uncovered in an excavation used to build a forest service road (Spruce Road). Local prospector Fred S. Childers of Ely noted copper stains in the material and he, along with Roger V. Whiteside of Duluth, began searching along the basal contact in the vicinity of the Kawishiwi River. In 1951, they diamond drilled a 57 meter (188 feet) deep hole and intersected mineralized gabbro that averaged 0.36% Cu and 0.13% Ni. In 1952, both Bear Creek Mining Company (BMC) and the International Nickel Company (INCO) began intensive exploration efforts along a 61 km-long zone (38 miles) that coincided with the basal contact. INCO eventually picked up the Childers-Whiteside properties (Spruce Road and Maturi deposits); whereas, BMC concentrated most of their effort near the town of Babbitt (Babbitt and Serpentine deposits). By 1960, these exploration efforts indicated that large tonnages of disseminated Cu-Ni deposits were present along the basal contact. However, the low-grade nature of the deposits and the unavailability of state-owned mineral lands led to suspension of activities. In 1966, state mineral leases were offered by the Minnesota Department of Natural Resources (DNR) and were awarded to successful bidders. Since 1966, over 20 companies have been actively involved in exploration for Cu-Ni and Fe-Ti-V deposits along the basal contact of the Complex and over 1,700 holes totaling over 1.5 million feet of core have been drilled. During the early 1970s, the Spruce Road and Babbitt deposits came the closest to development. Mining plans were submitted, test shafts were sunk (one each at the Maturi and Babbitt deposits), surface bulk samples were collected (3 deposits), and various landuse and water-use permits were requested from State and Federal agencies. Many of these activities drew strong opposition from environmental groups and some state legislators. In 1974, the Environmental Quality Board required that a regional Environmental Impact Statement (EIS) be conducted prior to acceptance of any site-specific EIS mining-related proposals. The DNR discontinued lease sales of State lands (1974-1982) until completion of the regional EIS. However, by the time the regional EIS was submitted in 1979, development of the Cu-Ni deposits was put on hold by the mining industry due to weakened copper and nickel markets, smelter-related problems with cubanite in the copper concentrate, and other financial reasons. Enter the “PGE era.” During the early period of drilling (prior to 1980), all of the exploration companies recognized that the Cu-Ni deposits had some potential for hosting PGEs. Based on very limited sampling, the companies assumed that the typical Cu-Ni ore contained no more that a few hundred parts per billion (ppb) combined platinum and palladium. In 1985, the DNR and Minerals Resource Research Center (MRRC of the U of M) conducted a geochemical evaluation of portions of drill hole Du-15, from the Birch Lake area, and found significant values of 9 parts per million (ppm) combined Pt and Pd (Sabelin and Iwasaki, 1986). A short time later, Morton and Hauck (1987) compiled all of the known PGE data for the Complex and reported the presence of anomalous PGE values, often associated with high Cu values, at several other Cu-Ni deposits. These discoveries sparked renewed interest in the Cu-Ni deposits as potential polymetallic deposits (Miller et al., 2002; and references therein). Additional drill holes were sampled and 147 �analyzed for PGEs throughout the Duluth Complex, and as a result, significant PGEs were found at several more deposits. Some of the PGE-enriched zones were found to be “stratabound” in that they are correlative with certain units of the igneous stratigraphy as determined by Severson and Hauck (1990), for the Partridge River intrusion, and Severson (1994), for the South Kawishiwi intrusion. Still other PGE-enriched zones were found to be related to either localized structural conditions (Local Boy massive sulfide zone of the Babbitt deposit; Severson and Barnes, 1991) and/or combinations of stratigraphy and structure (Birch Lake). For example, four stratabound horizons, each containing generally 1.0 ppm Pd, have been documented at the Dunka Road deposit (Geerts, 1991) and appear to be related to magma mixing. A single stratabound PGE-enriched horizon is present at the Birch Lake PGE prospect and also appears to be related to magma mixing (albeit, the PGE-mineralization is also related to variably digested iron-formation inclusions). However, the PGE-horizon at Birch Lake is quite variable (thickness and PGE contents) and cases can also be made that favor a late hydrothermal origin and redistribution of PGE along a fault zone or an early magmatic origin based on proximity to a feeder zone along the same fault. At present, close proximity to a vent, along with local magma mixing, appears to have been the major factor in controlling the PGE tenor in the above cases (Hauck et al., in prep). Localized modification of the PGE content by a later hydrothermal event, while not ruled out, appears to have been of lesser importance. Enter the “hydromet era.” In the mid to late 1990s, the potential of developing the Cu-Ni deposits using hydrometallurgical techniques has once again sparked renewed interest in the Duluth Complex. PolyMet Mining Corporation has acquired the Dunka Road deposit (NorthMet deposit) and plans to use its patented PlatSol technique to recover Cu, Ni, Co, and PGE. Teck Cominco has leased the Babbitt deposit (Mesaba deposit) and plans to use its patented CESL (Cominco Engineering Services Laboratory) process to recover the same metals. If it can be proven that these processes are feasible and economical, the next phase (the “permitting era”) in developing the low-grade Cu-Ni deposits of Minnesota could begin in the near future. The “permitting era” is anticipated to span at least a 2.5-3.0-year interval wherein an Environmental Assessment Worksheet (EAW), EIS, and applications for eight mining-related permits would be submitted. References: Geerts, S.G., 1991, Geology, stratigraphy, and mineralization of the Dunka Road Cu-Ni prospect, northeastern Minnesota: Natural Resources Research Institute, University of Minnesota Duluth, Technical Report NRRI/TR-91-14, 63 p. Listerud, W.H., and Meineke, D.G., 1977, Mineral resources of a portion of the Duluth Complex and adjacent rocks in St. Louis and Lake Counties, northeastern Minnesota: Minnesota Department of Natural Resources, Hibbing, MN, Division of Minerals Report 93, 74 p. Morton, P., and Hauck, S.A., 1987, PGE, Au, and Ag contents of Cu-Ni sulfides found at the base of the Duluth Complex, northeastern Minnesota: Natural Resources Research Institute, University of Minnesota Duluth, Technical Report NRRI/GMIN-TR-87-04, 81 p. Miller, J.M., Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., and Peterson, D.M., 2002, Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geological Survey Report of Investigations 58, 207 p. Sabelin, T., and Iwasaki, I., 1986, Evaluation of platinum group metal occurrence in Duval 15 drill core from the Duluth Complex: Internal report, Minerals Resource Research Center, University of Minnesota, Minneapolis, MN, 23 p. 148 �Hydrogen Stable Isotopic Evidence for Hydrothermal Alteration and PGE Concentration Involving Meteoric Water in the Birch Lake Area, Duluth Complex, MN SHAFER, Paula L., and RIPLEY, Edward M., Department of Geological Sciences, Indiana University, Bloomington, IN 47405 The Birch Lake prospect, located along the western margin of the 1.1 Ga Duluth Complex, contains local concentrations of platinum group elements (PGEs) of up to 8 ppm. Footwall rocks in the area are the Early Proterozoic Biwabik Iron Formation and the Archean Giant's Range Batholith. Petrographic analyses indicate minor to extensive late stage alteration of the troctolitic sequence of the Duluth Complex in the Birch Lake area. Olivine has been converted to serpentine, plagioclase has been locally replaced by chlorite, albite and sericite, and pyroxene has been partially converted to a mixture of chlorite and amphibole. Previous studies, both mineralogic and isotopic, (Sabelin and Iwasaki, 1986, Sabelin, 1987, Marma, et al., 2002, Shafer and Ripley, 2002, and Shafer, et al., 2003) have noted that: 1) some, but not all, PGE mineralization is associated with Cr enrichment, 2) oxygen isotopic studies are not supportive of Biwabik Iron Formation assimilation as a major control on either Cr or PGE enrichment, and 3) Re-Os isotopic studies clearly indicate extensive involvement of crustally derived Os in the ore forming process. Due to the intensity of alteration in the Birch Lake area, hydrothermal fluid transport or concentration of PGEs has been proposed as a key factor in the enrichment process. Hydrogen isotopic studies (primarily involving serpentine and sericite) have been undertaken to accompany our previous isotopic measurements, and to aid in the assessment of the alteration process. δD values of serpentine (-87‰ to -96‰) and sericite (-79‰ to -84‰) are very similar to whole rock values found previously (-78‰ to -98‰). Assuming a temperature between ~200º and 350ºC for serpentinization (Allen and Seyfried, 2003), computed δD values for the water in equilibrium with serpentine and sericite range from -75‰ to -84‰. When compared to the average δD of fluid liberated from the Virginia Formation (-53‰), an origin from a metamorphic fluid is unlikely. Considering the suspected low temperature nature of the extensive alteration and low δ18O values of serpentinized oxide melatroctolites (2.35‰ to 3.39‰), a magmatic fluid is also considered unlikely. The δ18O value of water in equilibrium with serpentine at temperatures between 200º and 300ºC is in the range of -3‰ to 3‰. Although mixing between a magmatic fluid and a variously evolved meteoric water can not be ruled out (Fig. 1), we suggest that the geologic and isotopic data are more consistent with hydrothermal alteration involving a fluid of primarily meteoric origin. The water isotopic compositions could be attained via exchange with either igneous rocks of the Complex or with country rocks. The lack of evidence for widespread 18 O exchange in the country rocks suggests that fluid flow was dominantly via fractures, and that isotopic exchange occurred over long path lengths at low time-integrated water/rock ratios. This fluid has locally concentrated PGEs, possibly as a result of differential solubility and removal of previously present sulfide minerals. 149 �References 10 0 -10 -20 -30 -40 -50 -60 -70 -80 -90 W at e rL in e SMOW M et eo ric δ D(o/oo VSM OW ) Allen, Douglas E., and Seyfried Jr., W. E., 2003, Compositional controls on vent fluids from ultramafic hosted hydrothermal systems at mid-ocean ridges: An experimental study at 400ºC, 500 bars, Geochimica et Cosmochimica Acta, vol. 67, no. 8, p. 1531-1542. Marma, John C., Brown, Phil E., Hauck, Steve A., 2002, Magmatic and hydrothermal PGE mineralization of the Birch Lake Cu-Ni-PGE deposit of the South Kawishiwi Intrusion, Duluth Complex, Northeast MN, In: Geological Society of America, 2002 Annual Meeting: Abstracts with Programs, vol. 34, no. 6, p. 112. Sabelin, T., and Iwasaki, I., 1986, Evaluation of platinum group metal occurrence in Duval 15 drill core from the Duluth Complex: Minneapolis, Minerals Resources Research Center, University of Minnesota, Internal Report. Sabelin, T., 1987, Association of platinum deposits with chromium occurrences: An overview with Implications for the Duluth Complex, Skillings Mining Review, p. 4-7. Shafer, Paula L., and Ripley, Edward M., 2002, Stable isotopic studies of PGE mineralization in the Birch Lake area, South Kawishiwi Intrusion, Duluth Complex, MN, In: Geological Society of America, 2002 Annual Meeting: Abstracts with Programs, vol. 34, no. 6, p. 112. Shafer, Paula L., Ripley, Edward M., Li, Chusi, and Hauck, Steve A., 2003, Re-Os isotope Characteristics of PGE mineralization in the Birch Lake Area, South Kawishiwi Intrusion, Duluth Complex, MN, In: Geological Society of America, 2003 Annual Meeting: Abstracts with Programs, vol. 35, no. 6, p. 230. Magmatic Water Mafic igneous rocks H2O in equilibrium with serpentine mixing exchange -15 -10 -5 0 5 18 δ Ο (o/oo VSMOW) Figure 1. Isotopic values of water in equilibrium with serpentine in the Birch lake area, and potential mixing/exchange paths. 150 10 �SILVER THREADS AND GOLDEN NEEDLES: GEOLOGICAL MILESTONES IN NORTHWESTERN ONTARIO SMYK, Mark C., and MAGEE, Angelique, Ontario Geological Survey, Ministry of Northern Development and Mines, Suite B002, 435 James St. South, Thunder Bay, ON P7E 6S7 CANADA Northwestern Ontario has been the focus of much geological inquiry and noteworthy discoveries over the past 140 years, many of which put the region in a worldwide limelight and continue to attract research interest. Some of these geological milestones form the basis of this retrospective paper. Reconnaissance mapping and seminal works by pioneers such as Logan, Macfarlane, Bell, McKellar, Lawson and Wilson helped establish the fledgling Geological Survey of Canada in the mid- to late-19th and early 20th centuries and developed the basic framework of local Precambrian geology. The area was part of the first geologic map in Canada, Geology of Canada, in 1869. Earlier mineral discoveries in the lake Superior area, followed by the discovery of silver in the Thunder Bay District in 1868 at Silver Islet prompted not only detailed surveys of mineral deposits, but also the development of rudimentary mineral policy in Canada. Despite its short-lived (1870-1884) history, Silver Islet was host to a number of “firsts”, including the introduction of the Burleigh drill, the Frue Vanner and the first use in Canada of the diamond drill. The Thunder Bay silver district also attracted E.D. Ingall and Ontario Bureau of Mines geologists, including a young N.L. Bowen, who later attained international fame as an igneous petrologist and geochemist. Evidence of the oldest life in North America is found in the ca. 3.0 Ga stromatolites at Steep Rock Lake. The discovery by Lawson in 1911 of the pseudofossil Atikokania there, named and described by Walcott (1912), led to its hailing as the world’s oldest fossil. Despite the ensuing debate and the widespread ascription of Atikokania to inorganic processes, the region once again gained notoriety when Tyler and Barghoorn (1954) discovered the best-preserved and most diverse microfossil assemblage in North America in the Paleoproterozoic Gunflint Formation. At the time of their discovery, they were deemed the world’s oldest fossils, "the oldest structurally preserved organisms that clearly exhibit cellular differentiation and original carbon complexes which have yet been discovered in pre-Cambrian sediments". It was a benchmark, a monumental "first" in global paleontology. Several minerals’ type localities are found in northwestern Ontario. The discovery of the Hemlo gold deposit in the 1980’s near Marathon yielded criddleite (TlAg2Au3Sb10S10), vaughnite (TlHgSb4S7) and hemloite ((As,Sb)2(Ti,V,Fe,Al)12O23OH). Nisbite (NiSb2); and paracostibite (CoSbS) were discovered in drill core at Trout Bay, Mulcahy Township, in the Red Lake area. The secondary minerals romarchite (SnO) and hydroromarchite (Sn2+3O2(OH)2) were discovered on tin pannikins lost from a voyageur’s overturned canoe between 1801 and 1821, and found 4.5 m below the surface of the Winnipeg River at Boundary Falls, near Kenora. The discovery of these tin-bearing minerals is fittingly related to northwestern Ontario’s colourful history. REFERENCES Tyler, S.A. and Barghoorn, E.S. 1954. Occurrence of structurally preserved plants in Precambrian rocks of the Canadian Shield; Science, v.119, no.3096, p.606-608. Walcott, C.D. 1912. Notes on fossils from limestone of Steeprock series, Ontario, Canada; Geological Survey of Canada, Memoir 28, p.16-23. 151 �Late Paleoproterozoic Rhyolite-Quartzite Sequences in the Southwestern U.S.: Speculative Relationship to Rocks of the Baraboo Interval SOUTHWICK, D.L. (Minnesota Geological Survey (retired); davidsouthwick@earthlink.net) At least 14 mappable units of supermature quartz arenite (quartzite) occur within Proterozoic terranes in the southwestern U.S. These typically are hundreds of meters in thickness, contain relatively minor interbeds of conglomerate and aluminous shale, and quasi-conformably overlie thick sections of high-silica rhyolite and/or rhyolitic pyroclastic rocks (Williams, 2003). Sedimentological features suggest that the quartzite units were deposited in fluvial to shallow marine environments, the latter having been influenced locally by tidal currents (Soegaard and Eriksson, 1985). The rhyolitic rocks beneath the quartzites range in age between 1665 and 1710 Ma. Extrusion apparently peaked in two pulses, the earlier one about 1700 Ma and the later one about 1670 Ma. Various geochronological methods indicate that several of the quartzite units are no more than a few million years younger than the subjacent rhyolite. Thus, as a first approximation, the quartzite formations were deposited in the interval 1700-1660 Ma with apparent depositional maxima around 1700-1695 Ma and 1665-1660 Ma. The rhyolite-quartzite sequences rest unconformably on previously deformed and metamorphosed Proterozoic rocks, and have themselves been involved in one to three fabric-forming tectonothermal events (Williams, 1991; Williams and others, 1999). Although the deformational history of the rhyolite-quartzite sequences is complex and is the topic of continuing research and debate, there is general agreement that the older fabrics were imposed during one or more pulses of the ~1650 Ma Mazatzal orogeny. Some of the younger fabrics and preserved metamorphic assemblages developed during a 1400 Ma tectonothermal event that probably was related to "anorogenic" plutonism in the southern Rocky Mountains and the southern Midcontinent. There are striking temporal, sedimentological, geochemical, and petrographic similarities between the 1700-1660 Ma quartzites in the Southwest and the 1712-1630 Ma quartzites of the Baraboo interval in the Upper Midwest (Medaris and others, 2003). However, the nearly ubiquitous stratigraphic association of quartzite with slightly older rhyolite observed in the Southwest has not been documented in the Upper Midwest. Essentially undeformed rhyolite occurs locally below the basal unconformity of the Baraboo Quartzite in Wisconsin. Clasts of undeformed porphyritic rhyolite and devitrified rhyolite tuff occur very locally in the Sioux Quartzite of extreme southwestern Minnesota (Southwick and others, 1986; Southwick, 1994), and 19th-century drilling in northwestern Iowa reportedly intersected rhyolite units (poorly described) interbedded with and beneath basal strata of the Sioux (Beyer, 1893; 1897). These rhyolites are texturally pristine and unmetamorphosed, although the Sioux occurrences are metasomatically altered. They have been interpreted as extrusive equivalents of late Penokean plutonism (ca. 1770 Ma) (Southwick, 1994), but as yet there is no solid geochronological evidence of their actual crystallization age. If future geochronological investigations should demonstrate a significantly post-Penokean age for the rhyolites beneath quartzites of the Baraboo interval (say 1715-1700 Ma), the possibility of a tectonic connection between the Baraboo rocks and the rhyolite-quartzite sequences of the southwest would become much more tenable. Specifically, the documentation of pre-quartzite rhyolitic volcanism would strengthen the speculative hypothesis that the Sioux Quartzite and related units were deposited in fault-bounded depressions (Southwick and others, 1986) that 152 �originally were volcanically active graben-like basins. Such basins may have been a far-field response to crustal stretching associated with Yavapai and transitional Yavapai-Mazatzal tectonism. References cited: Beyer, S.W., 1893, Ancient lava flows in northwestern Iowa: Iowa Geological Survey Annual Report, v. 1, p. 163-169. Beyer, S.W., 1897, The Sioux Quartzite and certain associated rocks: Iowa Geological Survey Reports and Papers, v. 6, p. 69-112. Medaris, L.G., Singer, B.S., Dott, R.H., Jr., Naymark, A., Johnson, C.M., and Schott, R.C., 2003, Late Paleoproterozoic climate, tectonics, and metamorphism in the southern Lake Superior region and proto-North America: Evidence from Baraboo interval quartzites: Journal of Geology, v. 111, p. 243-257. Soegaard, K., and Eriksson, K.A., 1985, Evidence of tide, storm, and wave interaction on a Precambrian siliciclastic shelf: The 1,700 M.Y. Ortega Group, New Mexico: Journal of Sedimentary Petrology, v. 55, p. 672-684. Southwick, D.L., Morey, G.B., and Mossler, J.H., 1986, Fluvial origin of the Lower Proterozoic Sioux Quartzite, southwestern Minnesota: Geological Society of America Bulletin, v. 97, p. 1432-1441. Southwick, D.L., 1994, Assorted geochronologic studies of Precambrian terranes in Minnesota: A potpourri of timely information [with data contributed by Z.E. Peterman, L.W. Snee, and W.R. an Schmus], in Southwick, D.L. ed., Short contributions to the geology of Minnesota, 1994: Minnesota Geological Survey Report of Investigations 43, p. 1-19. Williams, M.L., 1991, Heterogeneous deformation in a ductile fold-thrust belt: The Proterozoic structural history of the Tusas Mountains, New Mexico: Geological Society of America Bulletin, v. 103, p. 171-188. Williams, M.L., 2003 [abs.], Proterozoic rhyolite-quartzite sequences of the Southwest: Syntectonic "cover" and stratigraphic breaks (~1695 and ~1660 Ma) between orogenic pulses: Geological Society of America Abstracts with Programs, v. 35, no.5, p. 42. Williams, M.L., Karlstrom, K.E., Lanzirotti, A., Read, A.S., Bishop, J.L., Lombardi, C.E., Pedrick, J.N., and Wingsted, M.B., 1999, New Mexico middle-crustal cross sections: 1.65-Ga macroscopic geometry, 1.4-Ga thermal structure, and continued problems in understanding crustal evolution: Rocky Mountain Geology, v. 34, p. 53-66. 153 �Close Proximity of Kimberlite Pipes to Diabase Dykes: Structural Controls and Predictiveness in the James Bay Lowlands, Ontario STOTT, G.M., Ontario Geological Survey, Sudbury, ON P3E 6B5, greg.stott@ndm.gov.on.ca In the northernmost region of Ontario, Paleozoic sedimentary rocks (Figure 1a) cover the Archean Superior Province to form the Hudson Bay (HBL) and James Bay (JBL) lowlands. Diamondiferous kimberlite pipes of Early Jurassic (circa 190 Ma) and Mesoproterozoic (circa 1100 Ma) age occur in two separate clusters in the James Bay Lowlands (Figure 1b). Early Jurassic kimberlite pipes, including the Victor diamond deposit of De Beers Canada Exploration Inc., are close to the Winisk Fault, a major Archean dextral transpressive fault. However, the presence of this fault by itself does not adequately account for the linear chain of these kimberlite pipes near the Victor deposit nor the more scattered distribution, farther west, of Mesoproterozoic “Kyle” kimberlite pipes. Not all of the Kyle intrusions lie close to the Winisk Fault. More significantly, there is a spatial association between Proterozoic diabase dykes and these two clusters of kimberlite pipes. A geological interpretation of regional aeromagnetic maps is being completed of the Precambrian basement underlying the Phanerozoic cover in the JBL and HBL. This analysis includes identification of the various Proterozoic diabase dyke swarms in that region. From this there is reason to suspect a correspondence between both of these kimberlite pipe clusters and two diabase dyke swarms. The Early Jurassic pipes mainly lie in a linear northwestward trend close to a Matachewan (ca. 2446 Ma) diabase dyke. This dyke is part of a parallel bundle of northwest-striking dykes across an approximately 20-kilometre width near the Winisk Fault. It is suggested here that deep crustal fractures associated with these dykes, arising from the giant Matachewan magmatic event, were reopened during subsequent episodes of displacement along the Winisk Fault. Over 90 km farther west, the 2121 Ma Marathon swarm forms an approx. 20 kilometre wide bundle of dykes trending northwards in the vicinity of the Kyle kimberlite pipes. Individual pipes lie close to aeromagnetic traces of the dykes. The occurrence of both sets of kimberlite pipes, close to but generally not on the Winisk Fault, implies the possibility that dyke-associated fracture swarms served as secondorder, extensional “splays” near this major fault and provided preferred emplacement pathways for pipe intrusions at least in the middle to upper crust. Similar observations have been made in the Lac de Gras area in the Slave Province where Paleoproterozoic dykes show a moderate to strong spatial association with kimberlite pipes (Wilkinson et al., 2001). In the context of this model, other areas of potential exploration interest include: 1) an area approximately 60-80 km farther east of the Victor deposit, where there is an overlap of another set of Matachewan and Marathon dyke bundles (see Figure 2) transected by the east-trending Winisk Fault, and where no kimberlite pipes have as yet been discovered; 2) 2) a set of aeromagnetic anomalies near a Marathon dyke 120 km east of Fort Hope; and 3) 3) an area that straddles the Manitoba – Ontario border near the Hudson Bay Lowlands where a set of reversely magnetised, north-striking dykes (and fractures?) occurs between the North Kenyon fault and the Winisk fault. This dyke swarm lies “up-ice” from an area of glacially deposited kimberlite indicator minerals found to the southwest (Stone 2001). These three areas might serve as exploration tests of this empirically apparent correlation between pipe intrusions and dyke and fracture swarms, especially in proximity to the Winisk Fault. It is to be expected that the aeromagnetic expression of the pipes might be masked by the presence of these dykes. This is a testable hypothesis and further research requires dating fracture materials in these diabase/fracture swarms and episodic movement along the Winisk Fault. The apparent correspondence between kimberlite pipe emplacements and geophysically traceable bundles of diabase dykes and accompanying fractures provides a potentially important structural control, especially where subjected to reactivated tensile stress near major transcurrent faults. 154 �References: Stone, D. 2001. A study of indicator minerals for kimberlite, base metals and gold: northern Superior Province of Ontario; Ontario Geological Survey, Open File Report 6066, 140p. Wilkinson, L., Kjarsgaard, B.A., LeCheminant, A.N. and Harris, J. 2001. Diabase dyke swarms in the Lac de Gras area, Northwest Territories, and their significance to kimberlite exploration: initial results; Geological Survey of Canada, Current Research 2001-C8, 17p. Figure 1a. Map of Ontario showing the location of the James Bay and Hudson Bay lowlands. The Winisk fault underlies the Phanerozoic cover rocks of the lowlands. Location of Figure 1b is outlined. ! 1a 1b Figure 1b. A map highlighting the distribution of diabase dyke swarms under the Phanerozoic cover rocks of the James Bay Lowlands and the spatial correlation with kimberlite pipes. Early Jurassic kimberlite pipes, the Attawapiskat kimberlites, including the Victor diamond deposit, concentrate in a train parallel to one dyke in a group of northwest-striking Matachewan (2446 Ma) diabase dykes and inferred associated fractures. Mesoproterozoic (1100 Ma) kimberlite pipes, the Kyle kimberlites, are spatially concentrated near individual diabase dykes in a group of north-striking Marathon (2110-2121 Ma) dykes. 155 �ORIGIN OF PRE-WISCONSINAN GLACIAL UNITS IN NORTHERN WISCONSIN BASED ON LITHOLOGIC CHARACTERISTICS SYVERSON, Kent M., syverskm@uwec.edu, Dept. of Geology, University of Wisconsin, Eau Claire, WI 54702 Till and outwash units deposited before the Wisconsinan Glaciation are found in weathered, isolated erosional remnants that are difficult to interpret. The goal of this paper is to obtain input from other geologists with regards to lithologic trends observed in pre-Wisconsinan till and outwash units in northern Wisconsin (Syverson and Johnson, 2001; Syverson, 2004). Pierce Fm. till in western Wisconsin and till of the Medford Mbr. (Marathon Fm.) in central Wisconsin are very dark gray to yellowish brown, silty, and calcareous (Table 1, Fig. 1). Potential carbonate sources include Hudson Bay and the Winnipeg Lowland. Baker and others (1987) proposed that a pre-Illinoian Des Moines Lobe from the Winnipeg Lowland deposited the Pierce and Medford tills during the same glacial event based on lithologic similarities, reversed paleomagnetic signatures, and boulder trains and till fabrics suggesting ice flow from the NW/NNW. Thornburg and others (2000) noted much higher kaolinite concentrations in Pierce till than in Marathon Fm. till (Table 1). Syverson and Johnson (2001) proposed four possible origins for the Pierce and Medford tills and suggested that the Pierce and Medford tills may have been deposited synchronously by different lobes flowing from the northwest. Question: Do different source areas exist along reasonable flow lines that would cause higher kaolinite values in western Wisconsin than in central Wisconsin? Table 1. Data for calcareous, pre-Wisconsinan till units (from Thornburg and others, 2000; Syverson, 2004). Mean values are reported (the carbonate ratio is for weight % coarse silt fraction; K=kaolinite; V=vermiculite; number of samples in parentheses). Till Unit Pierce Fm. Snd:Slt:Cl % 39:37:24 (41) Calc:Dolo 4.2:1 (5) %K 23.3 (13) V/K 0.5 (13) Edgar Mbr., Mar. Fm. 40:40:20 (283) 0.7:1 (105) 6.8 (3) 2.4 (3) Medford Mbr., Mar. Fm. 35:46:19 (26) 0.3:1 (8) 5.7 (2) 3.2 (2) The easterly extent of the Superior Lobe before the Wisconsinan Glaciation is also uncertain. Reddishbrown, sandy till of the River Falls Fm. unconformably overlies the Pierce till in western Wisconsin (Fig. 1). Syverson (2004) has mapped River Falls Fm. outwash in western Chippewa County that is up to 35 m thick, extremely eroded, and enriched in pedogenic clay to depths of 5 m below the land surface. This outwash commonly contains ice-proximal cobbles, boulders, and large Lake Superior agates. Late Wisconsinan outwash of the Chippewa Lobe in adjacent areas is cobble poor, is relatively unweathered, and rarely contains Lake Superior agates. It is proposed that the Superior Lobe flowed into western Chippewa County (farther east than previously thought) before the Wisconsinan Glaciation. Agate-rich outwash of the River Falls Fm. in Chippewa County might have been deposited in the interlobate junction between the Superior and Chippewa Lobes as the ice wasted from its maximum extent. Baker, R.W., Attig, J.W., Mode, W.N., Johnson, M.D., and Clayton, L., 1987, A major advance of the preIllinoian Des Moines Lobe: Geological Society of America Abstracts with Programs, v. 19, no. 4, p. 187. Clayton, L., Attig, J.W., Mickelson, D.M., and Johnson, M.D., 1992 (revised), Glaciation of Wisconsin: Wisconsin Geological and Natural History Survey Educational Series 36, 4 p. Syverson, K.M., and Johnson, M.D., 2001, Origin of the calcareous, pre-Wisconsinan Pierce and Marathon Formations, Wisconsin: Geological Society of America Abstracts with Programs, v. 33, no. 4, p. A18. 156 �Syverson, K.M., and Colgan, P.M., 2004, The Quaternary of Wisconsin: a review of stratigraphy and glaciation history, in Ehlers, J. and Gibbard, P.L., eds., Quaternary Glaciations -- Extent and Chronology, Part II: North America: Amsterdam, Elsevier Publishing, in press. Syverson, K.M., 2004, Pleistocene geology of Chippewa County, Wisconsin: Wisconsin Geological and Natural History Survey Bulletin, accepted pending revisions. Thornburg, K.L., Syverson, K.M., and Hooper, R.L., 2000, Clay mineralogy of till units in western Wisconsin: Geological Society of America Abstracts with Programs, v. 32, no. 7, p. A270. Figure 1. Pleistocene lithostratigraphic units of Wisconsin (from Syverson and Colgan, 2004; modified from Clayton and others, 1992). The Medford Mbr. of the Marathon Fm. (mentioned in the text) is found in the subsurface below the Edgar Mbr. of the Marathon Fm. 157 �DOWSING EMPLOYS CLASSICAL MECHANICS AND STATIC ELECTRICITY TO LOCATE SELF-POTENTIAL ANOMALIES INDUCTIVELY AND RAPIDLY TROW, Jim, Geological Sciences, Michigan State University, emeritus, 540 Lake Avenue #2, Hancock, Michigan 49930 At last year’s 49th ILSG I showed where dowsing in the Michigan Copper Country could identify SP anomalies associated with four new (-) ore targets, seven historic (-) lodes, three historic (-) fissure veins, and four major (+) faults. A few of these examples are here illustrated as geophysical profiles. This year I should like to show you how this procedures works. A French physicist and a Czech physicist independently ascribed the dowsing phenomenon to magnetics, but this cannot be because famous Russian dowsers could not detect the enormous Kursk magnetic anomaly. The two physicists elicited human dowsing response in magnetic fields caused by direct electric currents. The Czech thought that a head shield of low-reluctance metal blocked the magnetic stimulus (but of course the metal was also a low-resistance Faraday cage which blocked electrical fields.) My experiments with permanent magnets have drawn a complete blank; experiments with static electric charge producers have been a big success. Try them! Over the past 31 years of examining ore deposits in six states and one province, one learns that the linear forces of static electric attraction and repulsion interact with mechanical lever arm, torque, moment of inertia, kinetic energy, and power factors to enable many humans to detect SP anomalies with 3/16” – diameter bare low-fuming bronze welding rods of two configurations: for me, 1) the more sensitive two-pronged 6” x 15” U-rods identify gentle SP gradients (dv/dx), and repeated turnings indicate the electrical magnitude of the SP anomaly by iteration of gradients, whereas 2) the less sensitive single-pronged 6”x15” L-rods identify steep or vertical SP gradients over target boundaries. One needs both kinds. At waist level, with elbows against your sides hold the short rod segments vertically in your hands with the long segments horizontal at essentially the same altitude, free to rotate in horizontal planes, immune from gravity. Extend your hands in front of you at body width with your lower arms horizontal, and aim the horizontal long rod segments away from you, parallel to each other. This is the standard state. Point the horizontal segments in the direction of your traverse, which should be at more than 45 degrees mapwise from the strike of an anticipated anomalous mass. As you walk, keep your eyes straight ahead on the traverse. Head orientation is critical, as the twin sensors are in the brain, as demonstrated by Faraday cage masking of the head and by family members’ CT Scans after strokes which obliterated their dowsing skills. For most people, as a negative SP anomaly is approached each rod swings inwards 90 degrees. At each such turning record location, reset the rods to the standard state, and walk on, recording every 90 degree turning and its location. If you do not reset the rods to the standard state the rods cannot indicate additional small dv/dx increments, and you will forfeit valuable information concerning the magnitude of the anomaly. Eventually, you will encounter a point where each rod swings outwardly 90 degrees for a positive dv/dx, as you finally climb out of the SP electrical “valley”. These readings may spread over hundreds of feet with these rods, which through the sum of incremental gradients, give you an idea of total SP voltage (v) magnitude. 158 �To determine the precise location and width of the shallow part of the anomalous mass, rerun the traverse with the 6” x 15” L-shaped rods. The 90 degrees inward turning of each rod indicates the steep to vertical negative dv/dx of the negative SP anomaly. Record data, reset the rods to the standard state and traverse a short distance more until the 90 degree outward turning of each rod at the positive steep to vertical dv/dx, as you start to climb out of the negative SP “valley”. Trench or drill between these – and + turnings. While one’s head contains the twin “sensors”, one’s rods, arms, body, and legs constitute the “meter”. The static charge to attract or repel the positively charged rods’ ends emanates from one’s body and legs, when activated by an involuntary signal from one’s brain. These conclusions follow from Faraday cage masking of one’s waist-to-knee area (which kills the “meter”), or by one’s shuffling side-ways along a traverse while one’s “eyes-right” head aims along the traverse as usual. Mechanics calculations show that the two sensitive 6” x 15” U-rods require a power of .0064 Watts for both to turn 90 degrees, whereas the less sensitive 6” x 15” L-rods require a power of only .0008 Watts for the same 90 degrees rotation. However, these sensitive U-rods each rotate 90 degrees with half of the torque-causing linear electric force per prong that it takes to rotate each less sensitive L-rod 90 degrees. For simplest interpretation start every traverse on electrically neutral ground. It would take a week to illustrate all the caveats and intricacies of the dowsing art, and then no two of us would react in the same way unless we individually standardized the lengths of our rods over known SP anomalies. To avoid physiological differences among humans, eliminate the human components of the electrical circuits: Connect a 13- or 20-cm.-diameter hollow spun aluminum sphere (the sensor) to an insulated rod handle, and connect the sphere by a wire to a rugged battery-powered center-zero electrometer (the meter) which records millivolts, and observe induced total SP millivoltage, v (not dv/dx). However, use station spacing of five feet or less if you want to make a second derivative (curvature, d2v/dx2) analysis of the data. Thusly, an ancient art transfigures into science. Trow, J., 1992, Inductive electrostatic gradiometry (IESG) deciphers Keweenawan copper plumbing system, Soc. Mining, Metall. and Expl. Phoenix Meeting, Preprint 92-32, 22 p. 159 �ORIGIN OF THE RHYOLITES AND GRANOPHYRES OF THE MIDCONTINENT RIFT, NORTHEAST MINNESOTA VERVOORT, J.D., Department of Geology, Washington State University, Pullman, WA, 99164, vervoort@wsu.edu WIRTH, K.R., Geology Department, Macalester College, St. Paul, MN, 55105. Throughout Earth history continental rift volcanism has been characterized by voluminous outpourings of dominantly mafic magma. Rhyolites and other evolved compositions are occasionally erupted in these rifts—sometimes in significant proportions (e.g., Paraná)—but in most cases they are subordinate or absent. This is also generally true in the Midcontinent Rift (MCR) where magmatism is predominantly basaltic with subordinate rhyolite; intermediate compositions are rare. An exception to this is in northeastern Minnesota where the rhyolite and granitic intrusive complexes comprise a significant percentage (10-25%; Green and Fitz, 1993) of the magmatic volume. Here we present U-Pb zircon and Nd isotopic data from the rhyolites and granitic complexes in order to constrain the timing and origin of these evolved magmas in the context of the dynamic evolution of the rift. Most of the rhyolitic volcanism is contained within a few exceptionally large eruptive units, the largest of which may have exceeded 600 km3 in volume (Green and Fitz, 1993). The rhyolites are characterized by having large areal extent, large aspect ratios, and high-T mineral assemblages all indicative of high-temperature, superliquidus eruptions (Green and Fitz, 1993). The granitic complexes are of comparable size to the rhyolites and occur as tabular concordant intrusions (up to 35 km by 1 km) that consist dominantly of granitic compositions and exhibit granophyric textures. Although dominantly silicic, the granophyres have a wide range of compositions (e.g., 47-76 wt. % SiO2) and plot along linear trends on many variation diagrams. The major and trace element compositions of the rhyolites and the silicic portions of the granophyre complexes are broadly similar and it is tempting to think of the granophyres as rhyolite magmas that never reached the surface. The magmatism of the MCR in the northeast limb of the rift is divided into two main magmatic stages (Miller and Vervoort, 1996): an “early stage” (1108-1105 Ma) of reversed magnetic polarity and a “main stage” (1100-1094 Ma) of normal magnetic polarity (Figure 1). Previous U-Pb zircon geochronology on the rhyolites (Davis and Green, 1997) indicate that whereas a few rhyolitic units were erupted during the early magmatic stage, the majority of these units, including the most voluminous rhyolites, were erupted at 11001097 Ma during the later main stage Figure 1. Plot of U-Pb zircon ages of granophyres compared with stages of rift evolution proposed by Miller and of magmatism. The granophyre Vervoort (1996). complexes we examined have U-Pb 160 �zircon ages consistent with this chronology. The stratigraphically lowest complexes (Misquah Hills, Greenwood Lake) have reversed magnetic polarity and ages of 1106±6 Ma and 1106±3 Ma (2 σ SE), respectively. The southern complexes (Eagle Mountain, Pine Mountain) are higher in the stratigraphy, have normal magnetic polarity, and have ages of 1098±4 to 1095±4 Ma. The rhyolites and granophyres have Nd isotopic compositions that are related to magmatic stage (Figure 2). Compared with the mafic volcanic rocks of the rift (epsilon Nd 0 ± 2), the early-stage rhyolites have slightly negative initial epsilon Nd values (~-4) and the younger, more voluminous, main-stage rhyolites have highly negative initial epsilon Nd values (-10 to -15). A similar correlation exists in the granophyres. The early stage granophyres are isotopically homogenous with epsilon Nd values of 0 to –2. In contrast, the main stage granophyres have more highly negative initial epsilon Nd values (-3 to -8). Thus for both the rhyolites and the granophyres, the main stage magmas appear to have been more highly contaminated by older evolved crust than those of the early stage. Based on these isotopic and age constraints we propose the following scenario for the magmatic evolution of the MCR. During the early stage, rapid extension allowed magmas to migrate through the crust with minimal interaction. Melting higher Sm/Nd (mafic) material in the lower crust at this time may have generated small volumes of silicic melts with slightly negative epsilon Nd values. Reduced extension during the latent stage (1105-1100 Ma) led to ponding of magma near the base of the crust and widespread crustal heating. Renewed extension during the main magmatic stage led to increased magma migration through the crust; prolonged crustal heating and magma flux resulted in increased crustal melting. The main stage rhyolites and granophyres represent melts of older, more evolved (lower Sm/Nd) compositions (Animikie sediments, Archean TTGs) perhaps at Figure 2. Plot of epsilon Nd versus SiO2 values for North mid- to upper-crustal levels. Shore Volcanic Group (NSVG) rhyolites, and granophyres. tholeiites, References Cited Davis, D.W., and Green, J.C., 1997, Geochronology of the North American Midcontinent rift in western Lake Superior and implications for its geodynamic evolution: Can. J. Earth Sci., v. 34, p. 476-488. Green, J.C., and Fitz, T.J., 1993, Extensive felsic lavas and rheoignimbrites in the Keweenawan Midcontinent Rift plateau volcanics, Minnesota: petrographic and field recognition: Journal of Volcanology and Geothermal Res., v. 54, p. 177-196. Miller, J.D., Jr., and Vervoort, J.D., 1996, The latent magmatic stage of the Midcontinent Rift: A period of magmatic underplating and melting of the lower crust: 42nd Annual Meeting of the Institute of Lake Superior Geology, Cable, WI, Proceedings v. 42, p. 33-35. Vervoort, J. D., and Green, J.C., 1997, Origin of evolved magmas in the Midcontinent Rift system, northeast Minnesota: Nd-isotope evidence for melting of Archean crust: Can. J. Earth Sciences, v. 34, p. 521-535. 161 �Taconite Aggregate Potential of Coarse Tailings from the Biwabik Iron Formation, With an Emphasis on Geology, Mineralogy, and Microscopy ZANKO, Lawrence M., ORESKOVICH, Julie A., Economic Geology Group, Center for Applied Research and Technology Development, Natural Resources Research Institute, 5013 Miller Trunk Highway, Duluth, MN 55811 NILES, Harlan B.(retired), Coleraine Minerals Research Laboratory, Natural Resources Research Institute, One Gayley Avenue, Box 188,Coleraine MN 55722 A study by Zanko et al. (2003) was undertaken to assemble a body of technical data that could be used to better assess the potential of using a crushed taconite mining byproduct (coarse tailings) for more widespread construction aggregate purposes, primarily in roads and highways. Fundamental to the project was the collection and generation of geological and mineralogical data. The major Biwabik Iron Formation stratigraphic units from which the samples were derived were identified, and their relative contribution to each sample was quantified. An important part of the mineralogical assessment included X-ray diffraction (XRD) analyses and microscopic evaluation of the size and shape (morphological) characteristics of potentially respirable microscopic mineral particles and fragments. Quantitative mineralogy, based on XRD analyses, showed that the dominant mineral in all samples was quartz (55 to 60 percent), followed by much smaller amounts of iron oxides, carbonates, and silicates. Specialized microscopic analyses and testing performed by the RJ Lee Group, Monroeville, PA, on both pulverized (-200 mesh, or 0.075 mm) and as-is sample composites showed that no regulated asbestos minerals or amphibole minerals were present. A very small number of mineral cleavage fragments/mineral fibers were detected, but these were mostly minnesotaite, a silicate mineral common to the Biwabik Iron Formation. Amphibole minerals, absent in coarse tailings samples from the five western Mesabi Range taconite operations, were present in a single eastern Biwabik Iron Formation sample collected in 2003 for Lake County from the Cliffs Northshore operation in Silver Bay, MN. Importantly, the Superfund Method for the Determination of Releasable Asbestos in Soils and Bulk Materials, EPA 540-R-97-028 (1997), as modified by Berman and Kolk (2000) failed to generate any protocol fibers, i.e., fibers longer than 5mm and thinner than 0.5mm, from either the western coarse tailings samples or the single eastern Biwabik Iron Formation sample. The combined findings suggest coarse tailings and other taconite mining byproducts should be treated with the same common sense safety and industrial hygiene approach practiced for all mineralbased materials that have the potential to generate respirable dust. Overall, the project showed that: 1) a significant source of fine aggregate (coarse taconite tailings) is available at each of the taconite operations studied; 2) coarse tailings are suitable for use in road construction applications; 3) no amphibole or asbestos minerals were present in any of the tailings samples evaluated; and 4) low-cost transportation, workable distribution logistics, and market acceptance will be key for expanded use of taconite mining byproducts like coarse tailings in markets beyond northeastern Minnesota. Furthermore, making greater use of materials that have long been considered “waste” byproducts makes environmental sense, because doing so maximizes the utilization of resources that have already been mined and crushed, and it could reduce pressures to expand existing, or develop new, “natural” aggregate sources. Permitting of pit and quarry expansions and new aggregate mines has become more complex and difficult, both 162 �environmentally and socially, given the growing “not in my back yard” (NIMBY) reaction to such projects. Future research initiatives intend to compile and generate baseline technical information on the quality of potential higher-value aggregate products (e.g., Class A-type aggregate, concrete aggregate, railroad ballast) derived from the major stratigraphic units within the Biwabik Iron Formation. Because the Biwabik Iron Formation is not monolithic, the goal will be to map and identify units that are the best potential sources for various construction aggregate applications. Its major cherty and slaty members, while laterally persistent, have variable geological, mineralogical, physical, and chemical characteristics across the Mesabi Range. Reference: Zanko, L.M., Niles, H.B., and Oreskovich, J.A., 2004, Properties and Aggregate Potential of Coarse Taconite Tailings from Five Minnesota Taconite Operations: Minnesota Local Road Research Board, Final Report 2004-06, 294 p., and Natural Resources Research Institute, University of Minnesota Duluth, Technical Report, TR-20003/44/ 163 � https://digitalcollections.lakeheadu.ca/files/original/bf5768fd04d62dab530753479e5e73ac.pdf b74b74f1c621be41df0769e12085bd07 PDF Text Text 50TH ANNUAL MEETING INSTITUTE ON LAKE SUPERIOR GEOLOGY 50TH ANNUAL MEETING MAY 4-9, 2004 DULUTH, MINNESOTA HOSTED BY: STEVEN A. HAUCK AND MARK J. SEVERSON Co-Chairs NATURAL RESOURCES RESEARCH INSTITUTE Proceedings Volume 50 Part 2 – Field Trip Guidebook Compiled and edited by Mark J. Severson and Julie Heinz, NRRI Cover Photos: Upper left – Dick Ojakangas, Peter Jongewaard and John Arola at the United Taconite mine, MN (2004); Upper right – Hal James and trip participants near Champion, MI (1964); Bottom – Carl Dutton, Bill Cannon and Matt Walton, Northern MI (1975). �CONTENTS PROCEEDINGS VOLUME 50 PART 2—FIELD TRIPS Trip 1. Volcanic Stratigraphy, Hydrothermal Alteration, and VMS Potential of the Lower Ely Greenstone, Fivemile Lake to Sixmile Lake Area................................................ 1 Trip 2. Geologic Highlights of New Mapping in the Southwestern Sequence of the North Shore Volcanic Group and in the Beaver Bay Complex .......................................... 46 Trip 3. Late Wisconsinan Superior-Lobe Deposits in the Lake Superior Basin northeast of Duluth ...................................................................................................................... 86 Trip 4. Geology of the Eastern Mesabi Iron Range, Northeastern Minnesota ............... 99 Trip 5. Classic Outcrops of Northeastern Minnesota ................................................... 129 Trip 6. Glacial and Postglacial Landscape Evolution in the Glacial Lake Aitkin and Upham Basin, Northern Minnesota .................................................................................... 170 Trip 7. Economic Geology of Archean Gold Occurrences in the Vermilion District, northeast of Soudan, Minnesota............................................................................ 200 Trip 8. Geology and Mineralization of the Western Contact of the Duluth Complex, Partridge River and South Kawishiwi Intrusions, Northeastern Minnesota............ 227 �FIELD TRIP 1 VOLCANIC STRATIGRAPHY, HYDROTHERMAL ALTERATION, AND VMS POTENTIAL OF THE LOWER ELY GREENSTONE, FIVEMILE LAKE TO SIXMILE LAKE AREA George J. Hudak1, John Heine2, Mark Jirsa3 and Dean Peterson2 1 2 University of Wisconsin Oshkosh Natural Resources Research Institute, University of Minnesota – Duluth 3 Minnesota Geological Survey INTRODUCTION The Vermilion district of northeastern Minnesota contains one of the classic granite-greenstone terranes in the United States. This district comprises the south-central part of the Wawa subprovince of the Superior Province of the Canadian Shield, and has been broadly correlated with the Saganagons Assemblage of the Wawa subprovince in northwestern Ontario (Peterson et al., 2001; Peterson and Patelke, 2003). In Canada, the Wawa subprovince hosts numerous lode gold (e.g. the Hemlo and Renabie districts) and volcanic-hosted massive sulfide (VMS) orebodies (e.g. the Winston Lake, Willroy, Big Nama Creek, Willecho, and Geco deposits; Fyon et al., 1992). The Vermilion district is well known for its numerous, previously mined, massive hematitic iron ore deposits. These iron deposits were discovered in the early 1880s, and virtually all subsequent exploration efforts in the region were targeted on similar iron-formation hosted hematite deposits. However, the discovery of world-class ore deposits in Ontario (the Kidd Creek VMS deposit in 1964 and the Hemlo gold deposit in 1980) led to short periods of both base-metal and gold mineral exploration in the Vermilion district. To date, no lode gold and/or VMS orebodies have been discovered in the Vermilion district to date. This field trip will explore recent research efforts to better understand the potential for VMS mineralization within the Lower member of the Ely Greenstone (Lower Ely). Peterson and Patelke (2004, this volume) explore new research regarding the potential for lode gold mineralization within the Newton Belt of the Vermilion district. Since the mid-1990’s, the Minnesota Department of Natural Resources, the Minerals Diversification Plan of the Minnesota Legislature through the Minerals Coordinating Committee, the Permanent University Trust Fund of the University of Minnesota, the Natural Resources Research Institute (University of Minnesota Duluth), and the University of Wisconsin Oshkosh have provided funding to support a series of systematic studies to better understand the VMS potential of the Lower Ely. Early studies (Lawler and Riihilouma (1997); Hudak and Morton, 1999; Peterson and Jirsa, 1999a; Peterson, 2001) showed that numerous base metal anomalies suggestive of localized VMS mineralization exist in the soils, rocks, and lake sediments within this region. Beginning in 2000, research efforts regarding the VMS potential of the Lower Ely have been focused in an approximately two mile wide corridor extending from Skeleton Lake northward to Sixmile Lake (Newkirk et al., 2001a, 2001b; Odette et al., 2001a, 2001b; Hudak et al., 2002a, 2002b; Hocker et al., 2003; Hudak et al., in prep. a). This area was chosen for several reasons: 1) there are historical base metal prospects in the area; 2) lake sediment samples in the area contain the highest copper (Sixmile Lake) and zinc (Fivemile Lake) anomalies within the state of Minnesota (Peterson, 2001); 3) the area contains several untested geophysical anomalies that have orientations roughly parallel to the strike of the strata (Peterson, 2001); 4) the area contains regionally extensive quartz-epidote alteration zones (Peterson and Jirsa, 1999a, 1999b; Peterson, 2001) that may be related to ancient, potentially VMS depositproducing hydrothermal systems; and 5) the bulk of the area comprises state land, and is easily accessible. These research efforts have been multidisciplinary, and have included several months of detailed field Page 1 1 �mapping (1:50 to 1:5000 scale) for stratigraphy and metamorphosed hydrothermal alteration mineral assemblages, petrographic studies (600+ thin sections), whole rock lithogeochemical analyses (~200), xray diffraction studies and electron microprobe investigations. REGIONAL GEOLOGICAL SETTING Supracrustal rocks in the Vermilion district consist of volcanic-dominated stratigraphic sequences of the Wawa subprovince of the Superior Province of the Canadian Shield. Rocks of the Wawa subprovince in northern Minnesota are divided on the basis of stratigraphic and structural setting into: (1) the Soudan belt, to the south, and (2) the Newton belt, to the north (Jirsa et al., 1992; Southwick et al., 1998). The boundary between these contrasting structural panels can be traced geophysically across the width of Minnesota, and was designated informally as the Leech Lake structural discontinuity (Jirsa et al., 1992). In the region west and north of the Soudan Mine, the Leech Lake structural discontinuity occurs along the Mud Creek shear zone (Hudleston et al., 1988), small segments of the Vermilion and Wolf Lake faults (Sims and Southwick, 1985), and the Bear River fault (Jirsa et al., 1992). The Soudan belt contains large, broad folds involving calc-alkalic and tholeiitic volcanic strata overlain by, and locally interdigitated with, turbiditic rocks. In contrast, the Newton belt consists of elongate, northeast-trending, and mostly northward-younging volcanic and volcaniclastic sequences. Volcanic rocks of the Newton belt differ from those of the Soudan belt in containing locally abundant komatiitic flows and peridotitic sills. The two belts are fault-bounded, and the relationship between stratigraphic units within each belt is largely conformable, although faults obscure contacts locally. In its eastern extension, the Soudan belt is continuous with the Saganagons assemblage in Ontario and terminates against the Saganaga pluton and Northern Light Gneiss. The Newton belt extends discontinuously eastward into the Shebandowan District of Ontario to form the Greenwater and Burchell assemblages. Intrusive rocks in both belts vary from gabbroic and felsic porphyries demonstrably related to volcanism, to large plutons emplaced posttectonically. Both districts contain unconformable, Timiskaming-type sequences composed of calc-alkalic volcanic rocks, conglomerates, and finer grained sedimentary rocks. A simplified regional geological map of the Neo-Archean terranes of northeastern Minnesota and adjacent Ontario is presented in Figure 1-1. Figure 1-1. Simplified correlation map of Neoarchean assemblages across the U.S. - Canada border (modified from Peterson et al., 2001). Inset illustrates major subprovinces of the southwestern Superior Province. Page 2 2 �Lithostratigraphic units in the western Vermilion district include: (1) the Lower member, Soudan Iron Formation member, and Upper member (Upper Ely) of the Ely Greenstone, the Lake Vermilion Formation (including the informally named Britt and Gafvert Lake sequences), and the Knife Lake Group of the Soudan belt; (2) the Bass Lake sequence (Peterson and Jirsa, 1999a) and the Newton Lake Formation of the Newton belt; and, (3) syn- to post-tectonic granitoid intrusions of the Giants Range batholith, and a suite of post-tectonic alkalic stocks and plutons (Fig. 1-2). Contacts between the different units are typically conformable, although considerable overlap in time and space is documented between volcanic and sedimentary sequences (Southwick, 1993). Rock types associated with the lithostratigraphic units in the area are presented in Table 1-1. 560001 + + 4+4 —t++'.'+— 4 • • 4 + 4-- 4 + + + RI .4• - + + - 4 •t 4 + + + I 4: -Fl. -±---± "-4-+4, +)+ + * * + + + + '+ + +,--) * + + /+ + + * I 4 * + * + + + + -F' + +2+ *+ — + +-r. +j* + * + + -F' 4 1 * 4 -J-/ * + * -t>. 4 -•+ + ' + ++ -+ + .4 Late Intrusive Rocks * VMS Prospect in Lower Ely) t' + '' Newton Lake Formation + +y+ + t(+ + + 4., + + + -# + + + + + + • * 590000 -P 4 + SBG000 , KnifeLakeGroup LakeVermilion Formation :: Britt Sequence Gafvert Lake Sequence :' Upper Member Ely Greenstone , Vermilion Granitic Complex + Giants Range Batholith 550000 + 570000 yt Duluth complex (1.1 Ga) + 4 + +,I + + + , • + + +. 4 4 ÷ +-_ ,- + +/ + + +* +*++ + 4- +\. 7+ -r *t_[+- *4 * I- +•+ + + IA Soudan Iron Formation A Lower Member Ely Greenstone -- -- C Bass Lake Sequence Figure 1-2. Simplified lithostratigraphic map of the western Vermilion district with the location of VMS prospects in the Lower Member of the Ely Greenstone (from Peterson and Jirsa, 1998a). UTM Nad 83 coordinates in meters. Page 3 3 �Table 1-1. Lithostratigraphic units within the western Vermilion district (from Peterson and Jirsa, 1998a). Intrusive Rocks Late Intrusions Vermilion Granitic Complex Giants Range Batholith Supracrustal Rocks Newton Belt Newton Lake Formation Bass Lake Sequence Soudan Belt Knife Lake Group Lake Vermilion Formation Gafvert Lake Sequence Britt Sequence Upper Ely Greenstone Soudan Iron Formation Lower Ely Greenstone Plutons and stocks of syenite, monzonite, diorite, and lamprophyre Granite, schist, amphibolite, and schist-rich migmatite Granite, granodiorite, monzodiorite, and schist-rich migmatite Tholeiitic and komatiitic basalt flows, intrusions, and clastic strata Tholeiitic basalt lava flows, iron-formation, and felsic porphyries Graywacke, slate, conglomerate, & sheared equivalents Graywacke, slate, dacitic tuff, and minor conglomerate Dacitic to trachyandesitic lava flows, tuffs, and intrusions Tholeiitic basalt lava flows Tholeiitic basalt lava flows and iron-formation Layered cherty iron-formation, epiclastic rocks, and tuff Calc-alkalic & tholeiitic basalt-rhyolite lava flows, tuffs, epiclastic rocks and minor iron-formations The Lower Ely and Soudan Iron Formation (Morey et al., 1970) comprise the strata we will be investigating during this field trip. The Lower Ely consists dominantly of volcanic arc-associated, pillowed and massive basalt and andesite flows of calc-alkalic and tholeiitic composition (Fig. 1-3). Hypabyssal diabase, gabbro, and diorite sills, isolated dacite, rhyodacite, and rhyolite lava flows, domes, tuffs, lapilli tuffs, and tuff-breccias (Fig. 1-4), and local chemical, volcaniclastic and epiclastic sedimentary strata occur throughout the sequence. Pillows in the lowermost sections of the Lower Ely (named the Fivemile Lake sequence by Peterson and Patelke, 2003) typically are irregularly shaped and well-vesiculated, indicating moderate to shallow water depths of formation (Schulz, 1982; Peterson, 2001; Hudak et al., 2002a). Rare massive to bedded scoria deposits interstratified with the pillowed flows also indicate shallow subaqueous, and perhaps locally, subaerial volcanism (Peterson, 2001; Hudak et al., 2002a). Pillowed basaltic rocks in the uppermost parts of the Lower Ely (named the Central Basalt Sequence by Peterson and Patelke, 2003) typically are sparsely-vesiculated and ovoid, and scoria-rich primary and reworked volcaniclastic rocks are not prevalent (Hudak et al., 2002b; Hudak et al., in prep. a; Peterson and Patelke, 2003). These features suggest a general subsidence of the volcanic pile to deepwater conditions. Stratigraphically overlying the volcanic rocks of the Lower Ely is the Soudan Iron Formation, which consists dominantly of laminated Algoma-type iron-formation, with interstratified basalt lava flows and volcaniclastic and epiclastic strata derived from basaltic to dacitic volcanic strata. A gradational contact over several tens to hundreds of meters occurs between the underlying Lower Ely and the overlying Soudan Iron Formation member of the Ely Greenstone (Hudak et al., 2002b; Peterson and Patelke, 2003). In general, the exhalative nature of many of the rocks within the Soudan Iron Formation member represent deep-water chemical deposition throughout a period of quiescence, which began during the latest stages of volcanism associated with the Lower Ely. The stratigraphic thickness of the Soudan Iron Formation member varies from 50 to 3,000 meters, and averages approximately 700 meters. The thickest sections occur in the nose of the Tower-Soudan anticline (Fig. 1-2), and probably represent stratigraphy that is repeated by shearing and thickened by folding. The time period over which the Soudan Iron Formation was deposited is poorly constrained, as geochronological data is very limited within the Ely Greenstone. Nevertheless, the size (thickness and strike length) of the Soudan Iron Formation is much larger than typical Algoma-type iron-formations, and Page 4 4 �its formation probably occurred during a profound break in effusive and pyroclastic volcanism that was accompanied by a long-lived period of hydrothermal activity. The upper contact of the Soudan Iron Formation is more diverse; overlying stratigraphic units along strike include the Upper Ely, the Gafvert Lake Sequence, and the Lake Vermilion Formation. Figure 1-3. Lithogeochemical characteristics of mafic and intermediate volcanic and volcaniclastic rocks of the Lower member of the Ely Greenstone in the vicinities of Fivemile, Needleboy, and Sixmile Lakes (after Hudak et al., 2002a; Hudak et al., in prep. a). Figures 1-4a and 1-4b modified from Winchester and Floyd (1977), Figure 1-4c modified from Wood (1980), Figure 1-4d modified from Winchester and Floyd (1976), Figures 1-4e and 1-4f modified from Barrett and MacLean (1999). Page 5 5 �Figure 1-4. Lithogeochemical characteristics of felsic volcanic and volcaniclastic rocks of the Lower member of the Ely Greenstone in the vicinities of Fivemile, Needleboy, and Sixmile Lakes (after Hudak et al., 2002a; Hudak et al., in prep. a). Figures 1-5a and 1-5b modified from Winchester and Floyd (1977), Figure 1-5c modified from Pearce et al. (1984), Figure 1-5d modified from Harris et al. (1986), Figures 1-5e and 1-5f modified from Piercey et al. (2001) and Barrie et al. (1993). STRUCTURAL GEOLOGY Periods of generally N-S directed compression resulted in three major regional deformation events in the Neoarchean terranes of northern Minnesota. The earliest deformation event (D1) produced broad, locally recumbent folds within the Soudan belt and major fault zones throughout the region. In the Page 6 6 �Newton belt, D1 was accommodated by thrust imbrication of large crustal blocks, resulting in mainly northward stratigraphic facing. Field relationships indicate that uplift, faulting, and the deposition of Timiskaming-type clastic sedimentary sequences in local fault-bounded basins occurred late in D1 deformation (Jirsa, 2000). A large, map-scale structure related to D1 deformation in the western Vermilion district is the Tower-Soudan Anticline, which is a west-plunging anticline within which the axis and plunge changes orientation along strike from nearly vertical in basalts to shallow NE plunging in the western sedimentary rocks. Axial-planar cleavage associated with this early fold typically is lacking, although Bauer (1985), Hooper and Ojakangas (1971), Hudleston (1976), and Jirsa et al. (1992) have described early cleavage (S1) locally. A second deformation event (D2) associated with synchronous regional metamorphism resulted in foliation development and structures having largely dextral asymmetry. D2 is constrained in the Vermilion district to the time period 2674 to 2685 Ma (Boerboom and Zartman, 1993), and between about 2680 and 2685 Ma in the Shebandowan (Corfu and Stott, 1998). Because D2 deformation affected all of the supracrustal rocks in the area and is reasonably constrained by geochronology, the regional foliation (S2) can be used in the field to temporally relate other structural, intrusive, and deformation events. The relationship between S2 fabric and shear structures indicates that most shearing occurred relatively late in the D2 event. Major shearing that produced the Mud Creek and related shear zones (e.g., the Murray shear zone, Fig. 1-1) is attributed to the late stages of D2 dextral transpression. Structures related to the third deformation event (D3) include abundant NE- and NW-trending faults that dissect the stratigraphic assemblages. Named structures related to D3 include the NE-trending Waasa and Camp Rivard faults east of the Soudan Mine area, and the WNW-trending, crustal-scale Vermilion and related faults that form the Wawa-Quetico Subprovince boundary north of the Soudan Mine (Fig. 1-1). VMS PROSPECTS AND EXPLORATION HISTORY Since the mid-1860’s, numerous mineral exploration programs have been conducted in the Vermilion district. Most of these exploration programs focused on identifying minable deposits of massive hematitic iron ores, such as those mined between 1883 and 1962 in the Soudan Iron Formation member at the Soudan Mine. During the 1980s and early 1990s, subeconomic lode gold mineralization was discovered in close proximity to the east-west-trending Murray shear zone, which dissects the Lower Ely, and in close proximity to the Mud Creek shear zone, which separates the Soudan belt from the Newton belt to the north (Fig. 1-1). Four VMS prospects (Fig. 1-2) occur within the Lower Ely, and occur in close proximity upsection from a semiconformable quartz-epidote alteration zone that extends for at least 19km along strike in the north limb of the Tower-Soudan Anticline (Peterson, 2001). These include the Skeleton Lake prospect (drilled by Exxon, 1972), the Eagles Nest prospect (drilled by Newmont, 1988), the Fivemile Lake prospect (drilled by Teck, 1994), and the Purvis Road prospect (drilled by Rendrag, 1999). The stratigraphy, hydrothermal alteration, and mineralization associated with the Skeleton Lake, Eagles Nest, and Fivemile Lake prospects have been recently re-evaluated (Hudak and Morton, 1999; Hudak et al., 2002a; Hudak et al., 2002b). Detailed studies of the Fivemile Lake, Eagles Nest, and Skeleton Lake prospects indicate that copper-zinc sulfide mineralization occurs proximal to relatively narrow (10s of meters wide) disconformable zones of chlorite and/or sericite alteration within felsic volcanic and volcaniclastic rocks (Hudak and Morton, 1999; Hudak et al., 2003). Hudak et al. (2003) and Hudak et al., (in prep. a) have noted that these disconformable alteration zones and associated mineralization commonly occur near high concentrations of synvolcanic mafic dikes, suggesting that these disconformable hydrothermal alteration zones occurred within synvolcanic fault zones (c.f. Gibson et al., 1999). Page 7 7 �VMS-ASSOCIATED VOLCANIC AND HYDROTHERMAL ALTERATION PROCESSES The composition and distribution of hydrothermal alteration mineral assemblages in the Lower Ely (Table 1-2) is similar to that described in major lava-flow dominated VMS mining districts worldwide (e.g. the Noranda Camp, Quebec; Morton and Franklin, 1987; Franklin, 1996; Gibson et al., 1999; Hudak and Morton, 1999; Peterson, 2001; Hudak et al., 2002; Hudak et al., in prep. a). Results of recent studies (Peterson, 2001; Odette et al., 2001a, 2001b; Hudak et al., 2002; Hudak et al., in prep. a) indicate that not only are the compositions and geometries of the regional alteration mineral assemblages identical to those present in many lava flow dominated massive sulfide mining districts, but that detailed alteration mineral chemistries (Hocker et al., 2003) are also consistent with those associated with the VMS ore deposits in these mining camps. These two observations suggest that the processes that formed the alteration mineral assemblages in the Lower Ely were similar to those that formed equivalent alteration zones in well-established VMS mining camps. Thus, one of the most perplexing questions plaguing economic geologists is why economically significant VMS deposits have not yet been discovered in the Vermilion district. A general genetic model for the formation of VMS deposits and associated hydrothermal alteration zone, as recently presented by Franklin et al. (1998), requires convective metalliferous hydrothermal fluid generation in the sub-seafloor environment via heating of down-welling seawater and leaching of metals from the enclosing volcanic and sedimentary strata (Fig. 1-5). The size of a convective hydrothermal system is a function of the abundance of heat in the upper two kilometers of the sub-seafloor crust (Franklin, 1996; Franklin et al., 1998). The intrusion of hypabyssal synvolcanic dikes and/or sills into the shallow sub-seafloor may vigorously enhance the dynamics of convective hydrothermal cells (Campbell et al., 1981). On reaching a critical reaction temperature of ~ 350°C, sustained acid pH in the hydrothermal fluid (evolved fluid) is achieved, and metals are leached from the rocks into the evolved fluid via primary mineral breakdown by calcium metasomatism, silicification, and hydrolysis reactions (Seyfried et al., 1999). In basalt-dominated systems (such as that in the Lower Ely), leaching-related alteration of mafic "source" zones (lower semi-conformable alteration) forms a mineral assemblage composed of albite-epidote-zoisite/clinozoisite-actinolite-quartz. These zones are variably metal-depleted, and are characterized by patchy silicification and epidotization associated with areas metasomatically enriched in silica and calcium. In lava flow-dominated stratigraphic sequences, regionally confined discordant “pipe-like”, and more regionally extensive “semiconformable” alteration zones are present (Morton and Franklin, 1987). The “pipe-like” semi-conformable alteration zones are closely associated with zones of cross-stratal permeability (e.g. synvolcanic fault zones), and are characterized by well-defined vertically extensive alteration zones containing anomalous abundances of sericite, chlorite (both Fe- and Mg-rich varieties), actinolite/ferroactinolite, quartz, pyrite, and locally, chalcopyrite and/or pyrrhotite. Semiconformable alteration zones extend for several kilometers to tens of kilometers in the rocks stratigraphically beneath and adjacent to VMS mineralized horizons (Santaguida et al., 2002a; Santaguida et al., 2002b). In maficdominated volcanic environments, such alteration typically is associated with regional zones of spilitization (an alteration assemblage composed of albite + quartz + Mg-rich chlorite ± sericite), silicification (quartz ± albite), and epidote-quartz alteration (epidote + quartz ± actinolite ± carbonate) (Morton and Franklin, 1987; Gibson et al., 1999; Santaguida et al.. 2002a, Santaguida et al., 2002b). Regional semiconformable alteration zones in felsic rocks in VMS producing camps such as Noranda (Quebec) or Sturgeon Lake (Ontario), typically comprise extensive zones of spilitization, silicification, and sericitization (sericite + quartz ± Mg-rich chlorite) (Morton and Franklin, 1987; Gibson et al., 1999). Page 8 8 �Table 1-2. Greenschist facies metamorphosed hydrothermal alteration mineral assemblages recognized in the Lower Ely (Hudak et al., 2002a; Hudak et al., in prep. a). Alteration Mineral Assemblage No apparent alteration (unaltered) Least altered Silicified Quartz + Chlorite (chlorite > 15%) Epidote + Quartz ± Chlorite ± Albite (Epidote >10%, Actinolite <10%, Fe-chlorite <20%) Epidote + Quartz ± Chlorite ± Albite (Epidote >10%, Actinolite <10%, Fe-chlorite >20%) Albite + Quartz ± Epidote ± Actinolite ± Chlorite (Epidote <10%, Actionlite <10%) Secondary Plagioclase Feldspar ± Chlorite ± Sericite Biotite ± Fe-/Mg-chlorite Iron Oxide / Hematite Geometry Not applicable Semiconformable Semiconformable Semiconformable Semiconformable Semiconformable Semiconformable Semiconformable Semiconformable Epidote + Quartz + Actinolite ± Chlorite ± Albite (Epidote>10%, Actinolite > 10%) Mottled Epidote +Quartz ± Actinolite ± Chlorite ± Albite (“Patchy Epidosite”) Semi/Disconformable Semi/Disconformable Actinolite ± Fe-chlorite ± Epidote ± Albite (Actinolite >10%, Epidote <10%) Fe-chlorite + Sericite (Fe-chlorite >10%, Sericite >5%) Fe-chlorite + Sericite (Fe-chlorite >10%, Sericite<10%, Fe-chlorite + Sericite >25%) Fe-chlorite + Fe-carbonate ± Sericite ± Actinolite ± Epidote (Fe-chlorite >25%, Fecarbonate > 5%, Actinolite, Sericite, Epidote < 5%) Sericite ± Pyrophyllite ± Fe-chlorite (Sericite/Pyrophyllite >5%, Fe-chlorite <10%) Sericite ± Carbonate (Sericite > 5%, calcite/dolomite/ankerite present, chlorite absent) Mg-chlorite ± sericite Disconformable Disconformable Disconformable Disconformable Disconformable Disconformable Disconformable Chlorite + Fe-carbonate ± Sericite Sericite + Green Mica ± Carbonate Shear Zones Shear Zones Tourmaline + quartz Veins Semiconformable Both discordant and semiconformable alteration zones have been discovered in the Vermilion district (Table 1-2), and have been described by Hudak and Morton (1999), Peterson (2001), Odette et al. (2001), and Hudak et al. (2002a). Semiconformable alteration zones in the Lower Ely are dominated by mineral assemblages containing various proportions of quartz, epidote, zoisite/clinozoisite, Fe-chlorite, Mg-chlorite, actinolite, ferroactinolite, sericite/pyrophyllite, and albite. Odette et al. (2001a, 2001b) and Hudak et al. (2002a) have shown via mass balance analysis that semiconformable quartz + epidote ± actinolite ± albite ± chlorite alteration mineral assemblages in the Fivemile Lake area are metasomatically enriched in calcium and silica, and depleted in base metals (copper and zinc) by 50-90%. Pipe-like, northeast-trending disconformable alteration zones in the Lower Ely are largely composed of Fe-rich chlorite, sericite/pyrophyllite, actinolite and/or ferroactinolite. Pipe-like alteration zones that have been mapped up-section have, to date, not led to the discovery of economically significant VMS deposits, but have been instrumental in locating potential base metal sulfide-bearing stratigraphic horizons and localized chemical exhalites. The mineralogy (Hudak et al., 2002a) and chemical compositions of the alteration minerals (Hocker et al., 2003) in these disconformable zones are consistent with those associated with economic VMS deposits in both Canada and Scandinavia (c.f. Hannington et al., 2002). Page 9 9 �VMS Deposit Impermeable Silicified Zone Zoisite/Ciinozoisite 7/ \ ha/fr - Magmatic Water ' ÷ Si - Cu +ca -Zn +Na - Fe Reservoir Zone Epk Quartz —400t-— \ / / .\ / / — \Subvolcanic Intrusion — — - Figure 1-5. Simplified schematic model of a convective hydrothermal system associated with the formation of Noranda–type (Morton and Franklin, 1987) or lava-flow dominated-type (Gibson et al., 1999) VMS deposits (modified from Franklin , 1996). Water depth controls aspects of associated VMS deposit composition (metal ratios) and alteration assemblages (Franklin, 1986; Franklin, 1993). Field evidence from the recent studies in the Lower Ely (Peterson, 2001; Hudak et al., in press) indicates that both shallow and deep subaqueous environments were present in the Vermilion district, as represented by the Fivemile Lake and Central Basalt Sequences. Such environments are not only associated with base-metal VMS deposits, but may also contain precious metal-rich (e.g. gold, silver) VMS deposits (Sillitoe et al., 1996; Hannington et al., 1999). Thus, the Vermilion district, and in particular, the rocks within a few kilometers of the area we are investigating on this field trip, may contain VMS deposits with a variety of base- and precious metal compositions. Page 10 10 �FIELD TRIP STOPS – DAY 1 (MAY 4) Figure 1-6. Field trip stop locations for Day 1 of the Vermilion district field trip. STOP 1-1: ARCHEAN SOUDAN IRON FORMATION No hammering please! Location: (NE, NE, S. 27, T.62N., R.15W., NAD83 UTM 557120E, 5296660N) Description: This classic exposure of the Soudan Iron Formation (Fig. 1-6) lies on the north limb of the Tower-Soudan anticline, and at the stratigraphic top of the volcanic sequences known collectively as the Lower member of the Ely Greenstone—the focus of this field trip. In a general way, the upper kilometer or so of strata assigned to the Lower Ely displays an upward-stratigraphic progression marked by increasing numbers and thicknesses of iron-formation-rich units, and corresponding decreasing thicknesses of the interleaved basaltic flows. This represents a general cessation of volcanism over some time. It is interesting to observe the rhythmic microlaminations (1 mm or so thick) in various cherty beds exposed here and speculate about the paleoenvironment—that is, whether these represent daily heating/cooling cycles, tidal, climatic, annual, or some other repetitive influence in the depositional environment. What is known about iron-formation units in the Ely Greenstone is that their deposition occurred during periods of relative volcanic and tectonic quiescence by the slow, subaqueous “rain” of chemical precipitates. The Soudan marks an abrupt transition from a period of waning basaltic volcanism in comparatively deep water, to one of mixed, locally emergent volcanic and associated sedimentary deposition represented by exposures to the north and west. The latter include dacitic rocks of what is Page 11 11 �known as the Gafvert Lake sequence, and tuffaceous graywacke of the Lake Vermilion Formation. Whether the transition at Soudan represents a significant unconformity remains enigmatic. This exposure also represents a microcosm of deformation in the western Vermilion district. It displays two generations of close folding in delicate laminations of chert (creamy white), chert-hematite jasper (red), and magnetite-chert (black to silver-colored). The second generation of folds (F2) is tectonic in origin, having subvertical axial surfaces that trend eastward, and steeply plunging axes. Most display Z-asymmetry. The earlier folds (F0-1) appear to have been sharply refolded to produce complex interference patterns. Lundy (1985) studied folding at this locality and concluded that some of the apparent interference structures are the product of early-formed sheath folds that did not involve refolding by D2. The F1 structures are predominantly intrafolial, and exhibit a great variety of style and orientation; implying they formed by layer-parallel, soft-sediment slumping. Work by Hudleston (1976) and Jirsa et al. (1992) extends these types of observations to regional tectonic interpretations of an early (D1) folding that included the development of the Tower-Soudan anticline in rigid volcanic complexes, and associated large nappe structures in the more ductile marginal sedimentary strata, followed by D2 metamorphism and transpressive deformation. STOP 1-2: XENOLITHIC HORNBLENDE DIORITE, PURVIS PLUTON Location: (NE, SW, S. 30, T.62N., R.13W., NAD 83 UTM 571755E, 5296815N) Description: The Purvis pluton is an east-west trending, moderate-sized (~3km3), sill-like multiphase dioritic to tonalitic intrusion with a strike length of 5.7 km and a thickness that ranges from 100-1200 meters (Peterson, 2001). This intrusion occurs in the lower stratigraphic section of the north limb of the Tower-Soudan anticline (Peterson and Jirsa, 1999a; Jirsa et al., 2001). Recent work by Drexler and Hudak (in press) indicate that the intrusion has several phases, including 1) xenolithic hornblende diorite; 2) xenolithic hornblende tonalite; 3) xenolithic leucotonalite; 4) leucotonalite and trondhjemite; and 5) leucotonalite dikes (Fig. 1-7). Detailed field mapping by Peterson (2001), Hovis (2001) and Drexler and Hudak (in press) suggest that the Purvis pluton is a synvolcanic intrusion based on the following characteristics: 1) it lacks a contact metamorphic aureole; 2) its uppermost contact is proximally associated with intense, semiconformable quartz + epidote alteration zones; 3) D2 deformation fabrics occur in both the intrusion and the surrounding country rocks; and 4) early xenolithic diorite phases are cross-cut by thin, commonly D2-deformed dikes of younger tonalite and trondhjemite phases. Galley (2002) and Galley (2003) have indicated that these characteristics are key features of synvolcanic intrusions temporally associated with the genesis of many Precambrian VMS deposits. Peterson (2001) has suggested that the Purvis pluton may have been the heat source that drove hydrothermal systems that produced the Eagles Nest and Purvis Road VMS prospects. A sample of the leucotonalite phase has been submitted to Dr. Daniel Holm (Kent State University) for geochronological analysis, but this age is not yet available. Based on the presence of the D2 fabric in the pluton, U/Pb zircon dates of a D2-deformed felsic lava dome in the Lower Ely in the Fivemile Lake area, and a post-D2 quartz feldspar porphyry in the Newton Belt (Peterson et al., 2001), we anticipate an age between 2683 Ma and 2722 Ma for the Purvis pluton. This locale offers an opportunity to investigate the xenolithic hornblende diorite phase of the Purvis pluton. The outcrop adjacent to the road predominately contains four types of xenoliths: 1) dark green xenoliths of amygdaloidal (5-8%) basalt-andesite pillow lavas which locally have preserved selvedges and interpillow hyaloclastite, and are locally contact metamorphosed along their margins; 2) pale green epidote + quartz-altered basalt-andesite lava xenoliths that are up to 15 cm in diameter; 3) rare coarse-grained gabbro/diorite xenoliths up to 3 cm in diameter; and 4) rare <1-2 cm diameter subangular Page 12 12 �chert xenoliths. Large iron-formation xenoliths up to several meters in diameter, and amphibolite xenoliths up to several centimeters in diameter, may be observed in xenolithic hornblende tonalite outcrops located east of this location (NAD 83 UTM coordinates 0574920E, 5297600N). * - Hr ft / Figure 1-7. Xenolithic hornblende tonalite (A) and xenolithic hornblende diorite (B) phases of the Purvis pluton.. Iron-formation xenoliths present in outcrops east of here were likely derived from iron-formation horizons that occur immediately southwest of Purvis Lake. Basalt and altered basalt fragments also were derived from the surrounding Lower Ely. The presence of epidote-quartz altered mafic xenoliths suggests that this phase of the Purvis pluton stoped its way upward into an earlier-formed proximal zone of quartzepidote alteration formed from high temperature seawater-rock interaction (e.g. Galley, 2003). Amphibolite xenoliths are believed to represent contact metamorphosed basalt fragments based on petrographic similarities (Drexler and Hudak, in press). Drexler and Hudak (in press) have shown that coarse-grained gabbro/diorite fragments likely represent xenoliths of the earliest phases of the pluton. A short hike up to the top of the ridge allows investigation of a newly exposed (and still lichenfree) outcrop displaying the complicated textures between medium-grained diorite and xenoliths in this phase of the Purvis pluton. Collectively, six types of xenoliths comprise 55-60% of the intrusion at this location. These xenolith types include: 1) pale to dark green, subangular to subround basalt-andesite clasts which range from 1-30cm in diameter and make up 15-20% of the rock; 2) subround to subangular, coarse-grained gabbro/diorite xenoliths that are up to 20cm in diameter; 3) subround epidote + quartzaltered basalt-andesite xenoliths that are up to 20cm in diameter; 4) subangular, dark grayish-green finegrained diorite xenoliths which are up to 10cm in diameter; 5) dark green, coarse-grained, actinolite-rich amphibolite fragments up to 12 cm in diameter; and 6) rare pillow selvedge / interpillow hyaloclastite xenoliths which locally make up 1% of the unit. Moving east along the ridge, one can observe additional outcrops of xenolithic diorite. Locally (UTM NAD 83 coordinate 571798E, 5296828N) ENE-trending 10-20cm wide leucotonalite dikes cut through the xenolithic diorite phase, indicating the age relationships between these two units. Farther east along the ridge, the complicated contact between the Purvis pluton and the overlying basalt-andesite pillow lavas can be observed. These lavas contain patchy epidote + quartz alteration, and have locally been metamorphosed to hornfels immediately adjacent to the intrusion. Studies of ancient VMS deposits has documented that the deposits commonly occur in depressions on the paleo-seafloor (3rd-order basins) while modern deposits on the seafloor are found on high-standing structures, such as ridges. These differences are probably more apparent than real, in that Page 13 13 �the modern deposits are generally confined to the axial graben, or depression, of what are otherwise are high-standing structures. In addition, both ancient and modern deposits occur in areas of anomalously high heat flow, generally linked to synvolcanic intrusions beneath the hydrothermal systems. The recent mapping in the Purvis Road area has shown the presence of all of the attributes of typical VMS-forming hydrothermal systems. These attributes include a synvolcanic intrusive heat source (the Purvis pluton), a paleotopographic high-standing structure, VMS-style alteration mineral assemblages, and the presence of Cu and Zn-rich massive sulfide (recent logging in this area has exposed numerous angular boulders of massive sulfide in the basal till). The VMS attributes of the Purvis road area are presented in Figure 1-8. Figure 1-8. Simplified regional lithostratigraphic sequences draped on a 3D view (looking due west) of a residual field aeromagnetic anomaly map of the western Vermilion district (same map area as Fig. 1-2). The lower two photographs show massive sulfide boulders exposed north of the Purvis pluton. Page 14 14 �STOP 1-3. SILICIFIED FIVEMILE LAKE SEQUENCE PILLOW LAVAS / REGIONAL SEMICONFORMABLE ALTERATION Location: (SE, SE, S. 22, T.62N., R.14W., NAD83 UTM 567818E, 5297818N) Description: Geological mapping by Peterson (2001) has indicated the presence of a regional semiconformable quartz-epidote alteration zone extending for at least 19 km along strike within the Lower Ely along the north limb of the Tower-Soudan anticline. This type of alteration is a common feature in many Archean VMS camps (e.g. Noranda (Gibson, 1989) and Snow Lake (Skirrow and Franklin, 1994)), and is attributed to silica- and calcium-dumping that occurs in the deep, sub-seafloor as downwelling hydrothermal fluids are heated to temperatures in excess of 350°C (Franklin, 1986; Franklin, 1993). Semiconformable alteration zones associated with VMS systems are generally much larger in area than their associated mineralization, and therefore provide exploration geologists regional areas in which to concentrate more detailed, follow-up field mapping, geochemical studies, and geophysical surveys for identifying VMS targets. Semiconformable quartz-epidote alteration zones are commonly metal-depleted. Peterson (2001), Odette et al. (2001a, 2001b), and Hudak et al. (2002a) have shown that that quartz-epidote altered basalts within this regional semiconformable alteration zone are commonly depleted in base metals by as much as 50-90%. Peterson (2001) completed simplified mass balance calculations of Cu and Zn depletion associated with the regionally extensive quartz-epidote alteration zone within the Lower Ely, and this work reveals the importance of this alteration to the VMS mineral potential of this area. The surface expression of the quartz-epidote alteration is approximately 19 kilometers long, and averages approximately 1.5 kilometers in width. The area of this zone is on the order of 30.5 km2. Using Southwick et al. (1998) values for Cu and Zn of 59 and 115 ppm, and a mean density of the rocks equal to 2.8g/cm3, the total contained copper and zinc for various volumes of rock from the Lower Ely are calculated, and presented in the top of Table 1-3. These volumes include a 1.0 kilometer thick slab (volume = 30.5 km3), a 2.0 kilometer thick slab (volume = 61 km3), a 3.0 kilometer thick slab (volume = 91.5 km3), and a circular disk (volume = 591 km3). Percent leaching calculations for copper and zinc are presented in the lower portions of Table 1-3, with values of metal leached (in pounds) from the four calculated volumes of rock. The leaching models include 5%, 10%, 25%, and 50% metal leached from the rock. Given the patchy nature of this alteration, an estimate of somewhere between 10 and 25% metal leaching seems reasonable, as does the 91.5 km3 volume. Therefore, one can estimate that many billions of pounds of copper and zinc have been leached out of the volcanic rocks in the north limb of the Lower Ely Greenstone. The concentration of these base metals into specific sites by a VMS style hydrothermal system would have created significant tonnages of massive sulfide. This data clearly reflects the potential for VMS mineralization up-section from this zone. At this location we can observe part of the regionally extensive quartz-epidote alteration zone. The outcrop contains relatively undeformed bun- and mattress-shaped pillows. Interpillow hyaloclastite zones are generally pale to dark green in color, and are chlorite and/or actinolite-rich. Minor red-brown staining locally occurs in these zones, and is indicative of the presence of trace amounts of pyrite and/or chalcopyrite. Pillow selvedges commonly contain up to 10% round to oval, pipe-like quartz-epidote and/or actinolite chlorite amygdules. The cores of the pillows are typically pale green gray in color due to nearly wholesale replacement of the original igneous minerals by quartz and epidote. Page 15 15 �Table 1-3. Copper and zinc leaching calculations for the regional semiconformable quartz-epidote alteration zone present in the Lower Ely (Peterson, 2001). Volume (Km3) 30.5 11,084,920,000 21,606,200,000 61 22,169,840,000 43,212,400,000 91.5 33,254,760,000 64,818,600,000 591 214,793,040,000 418,664,400,000 5% Cu leached (lbs) 10% Cu leached (lbs) 25% Cu leached (lbs) 50% Cu leached (lbs) 554,246,000 1,108,492,000 2,771,230,000 5,542,460,000 1,108,492,000 2,216,984,000 5,542,460,000 11,084,920,000 1,662,738,000 3,325,476,000 8,313,690,000 16,627,380,000 10,739,652,000 21,479,304,000 53,698,260,000 107,396,520,000 5% Zn leached (lbs) 10% Zn leached (lbs) 25% Zn leached (lbs) 50% Zn leached (lbs) 1,080,310,000 2,160,620,000 5,401,550,000 10,803,100,000 2,160,620,000 4,321,240,000 10,803,100,000 21,606,200,000 3,240,930,000 6,481,860,000 16,204,650,000 32,409,300,000 20,933,220,000 41,866,440,000 104,666,100,000 209,322,200,000 Total Contained Cu (lbs) Total Contained Zn (lbs) STOP 1-4: WELL-PRESERVED PILLOW LAVAS, SOUTH OF FIVEMILE LAKE Location: (NE, SW, S. 29, T.62N., R.14W., NAD83 UTM 563195E, 5296660N) Description: The Fivemile Lake Prospect comprises an extremely well preserved, and locally intensely hydrothermally altered, bimodal package of basalt-andesite pillows lavas and coherant and volcaniclastic facies felsic metavolcanic strata which have locally been intruded by diabase and/or diorite synvolcanic sills and dikes. Recent geological studies at the Fivemile Lake Prospect have included detailed field mapping, diamond drill core relogging, petrographic, lithogeochemical, x-ray diffraction, and electron microprobe investigations (Hudak and Morton, 1999; Peterson, 2001; Newkirk et al., 200a, 2001b; Odette et al., 2001a, 2001b; Hudak et al., 2002a; Hocker et al. 2003). These studies were undertaken to a) better understand the geology (in particular, the physical volcanology) of these regions; b) identify and evaluate the composition and distribution of the metamorphosed hydrothermal alteration mineral assemblages that occur at each of these prospects; c) evaluate and compare the chemical compositions of alteration mineral phases to those found at VMS deposits of various ages in various parts of the Canadian Shield, the western United States, and Tasmania; and d) evaluate the potential for VMS and/or lode gold mineralization in these areas. Field mapping and diamond drill core relogging indicate that the Fivemile Lake Prospect comprises thirteen distinct types of strata that can be subdivided into six major lithological classes (Hudak et al., 2002a). These include: a) calc-alkaline and tholeiitic, mafic to intermediate volcanic and volcaniclastic rocks (basalt-andesite pillow lavas, pillow breccias, hyaloclastite deposits, tuff, lapilli tuffs and monogenic volcanic breccias); b) chemical sedimentary rocks (semi-massive sulfide exhalite); c) calc-alkaline to tholeiitic, intermediate to felsic volcanic and volcaniclastic rocks (massive quartz-phyric rhyodacite lava flows, and aphyric to quartz-phyric rhyodacite to rhyolite tuff, lapilli tuff, and tuff breccia deposits); d) synvolcanic intrusive rocks (diorite sill-dike complexes, diabase dikes and sills, and massive gabbro); e) post-volcanic intrusive rocks (feldspar porphyry dikes and sills and quartz-feldspar porphyry dikes); and f) post-volcanic schists (quartz-sericite-ankerite schists, chlorite schists, and quartz-sericite- Page 16 16 �Figure 1.9. Generalized geological map of the Fivemile Lake – Needleboy Lake – Sixmile Lake area (modified after Peterson and Jirsa (1999a), Jirsa et al. (2001), and Hudak et al. (2002a). Page 17 17 �green mica schists). Supracrustal strata at the prospect vary from east-west to southeast-northwest striking, dip steeply to the north, and are north-facing. Because of this geometry, the geological map of the area (Figure 1-9) essentially illustrates a cross-section through the Lower Ely crust. Volcanic and volcaniclastic rocks have chemical characteristics indicative of formation with an ancient volcanic arc setting (Hudak and Morton, 1999; Hudak et al., 2002a). At this stop, we will investigate two outcrops that display exceptionally well-preserved pillow lavas and associated interpillow hyaloclastite. Detailed analysis (Newkirk et al., 2001a, 2001b; Hudak et al., 2002a) indicates that the pillow lavas at Fivemile Lake are morphologically similarly to undeformed Cenozoic pillow lavas from New Zealand and Tasmania. The nomenclature used for describing pillow lavas is illustrated in Figure 1-10). Intiapiliow Cavity r— Coiicentdc Cooling Joints Piliow Bud (non sight to loft) Blocky HyalocJastite — Radial Cooling Joint Neck and Knob Pipe .anygdule — Seivedge Formerly Glassy L Amygdulssririow — inteipiliow Pillow Margin -lyaloclastito Figure 1-10. Nomenclature used for describing pillow lavas and associated hyaloclastite (after Cas and Wright (1987) and Gibson (1989). At the first outcrop (NAD 83 UTM coordinates 563195E, 5296660N) we can observe bun- to lens-shaped, quartz-epidote-actinolite-chlorite-altered basalt-andesite pillow lavas. The general strike of the pillow lavas at this location is approximately 085°. The pillows vary from 0.4-1.0 meters high and 0.5-2.0 meters long, and contain zones of chlorite-rich interpillow hyaloclastite that ranges from 0.5-3.0 cm wide. Local brownish-red sulfide staining may be observed in the interpillow hyaloclastite zones. The pillow selvedges vary from 1-6 cm wide, are commonly silicified and/or quartz-epidote altered, and contain approximately 15% 1-5mm oval to round, quartz ± carbonate filled amygdules. Multiple pillow rinds, separated by 4-8 cm, may be locally observed. Reentrant pillow crusts and apparent pillow buds at this location suggest flow from west to east. The cores of the pillows at this location contain up to 5% 25 cm rounded quartz amygdules, and are faintly plagioclase feldspar-phyric (3-5% <1mm tabular phenocrysts). Faint radial cooling joints are also locally preserved in several of the pillows on this outcrop. Page 18 18 �A second spectacular outcrop of well-preserved, east-west striking pillow lavas occurs approximately 30 meters to the east (NAD 83 UTM coordinates 563225E, 5296660N). Pillow morphology at this outcrop is quite variable, ranging from bun- and mattress-shaped to amoeboid. Topping directions based on pillow morphology are consistently north. Pillows here are typically 50-80 cm in height and 1-1.5 meters long, although pillows as small as 30 cm in diameter and up to 2 meters long may be locally observed. Pillows selvedges vary from 1-7 cm wide (average 2-3 cm) and contain 15-20%, 3-7 mm diameter, round quartz amygdules. Pillow cores generally contain 5-10% 3-10mm round to oval quartz amygdules. Several pillows on the western and north-central part of the outcrop contain multiple pillow rinds. A pillow displaying exceptionally well-preserved “neck and knob” structure may be observed on the east central part of the outcrop. Vesicle size and abundance in pillow lavas (McPhie et al., 1993 and references therein) may be used with other textural and facies characteristics to interpret pillow lavas erupted in “relatively shallow” (<1-2 km water depth) versus “relatively deep” (>2 km water depth) submarine environments. Multiple pillow rind structures are common features in pillow lavas erupted in “relatively shallow” water environments, and are formed by implosion and rupture of the pillow skin resulting from condensation of exsolved gases concentrated in the upper part of the pillow (Kawachi and Pringle, 1988). The relatively high vesicularity of the pillows, coupled with the presence of multiple rind structures, indicate that the pillow lavas in this part of the Lower Ely were formed in a relatively shallow submarine environment. STOP 1-5: PILLOW DIKES, PEPERITES, AND SYNVOLCANIC FAULTS, SW SHORE, FIVEMILE LAKE No hammering please! Location: (SW, NW, S29, T. 62N, R. 14W; NAD 83 UTM Coordinates 0563305E, 5296955N) Description: Peperite is defined as a breccia-like volcaniclastic deposit formed from the in-situ disintegration and mixing of magma or lava with wet, poorly consolidated sediment (Batiza and White, 2000; Schmidt and Schminke, 2000). Peperite is a common rock type in submarine arc-related volcano sedimentary sequences, and is associated with synvolcanic intrusions. It often occurs in phreatomagmatic vent filling deposits and along contacts between synvolcanic intrusions and poorly consolidated, wet, submarine sediments. Key components that aid in the identification of peperites include evidence of partial fluidization of the sediment, local contact metamorphism, and in some rare instances, vesiculation within the sediment (Skilling et al., 2002). In general, the end-member compositions in a peperite are the igneous component (a synvolcanic dike or sill, or a lava flow) and the sedimentary component. Igneous component domains in peperites vary from tabular to lobe-like to pillow-like, and can commonly have irregular, interconnected, fold like, and pod-like shapes. Mixing that results from quenching, hydromagmatic explosions, magma-sediment density contrasts, and/or mechanical stresses resulting from inflation or movement of magma, causes the igneous component of the peperite to be broken-up and dispersed within the wet sediment to form peperite (McPhie and Orth, 1999). Factors such as magma and/or sediment composition, magma injection velocities, volatile content of the magma, relative volumes of sediment and magma involved, volume of pore water heated, presence or absence of shock waves, and the confining pressure all influence the morphology of peperites. Juvenile clast morphologies in peperite are extremely variable, and may be irregular, blocky (commonly jigsaw-fit), pumiceous or scoriaceous, irregular (amoeboid), elongate, wispy, platy or ragged. It is not uncommon for mixed juvenile clast populations to be present within a single peperite deposit (Skilling et al., 2002). This stop encompasses a series of outcrops that occur on a small peninsula along the southwest shoreline of Fivemile Lake (informally and affectionately known as “Pike Point” based on the exceptionally large pike that can be caught from this shoreline). A generalized geological map of this Page 19 19 �area, which points out key features to observe at this stop, is illustrated in Figure 1-11. The area was originally mapped as a series of pillow lavas and felsic debris flow deposits (Peterson and Jirsa, 1999a; Peterson, 2001); however, if the pillow lavas represent a stratigraphic unit, they are oriented approximately 40°-45° to the strike of the supracrustal strata in this area, suggesting that either a) deposition occurred upon a steep paleotopographic surface (which is not supported by the pillow morphology); or b) a fold may be locally present (evidence for this is not currently supported by regional mapping). (V >1. Fivemile — 5296925 Coherant Pillows Fades Volcanidas& Fades LapllI T'fl / TuIT-Br.a X H1cooct 1ñofSonoatI Difiusa planar stralitpcaton a! deals SVl Ice and topping direct on of pillow SyTwolcanlc Fault 0 I cL) I Oulciop Locaa' 7] Approximate Trail 5 10 20 Lobon Figure 1-11. Geological map of the southwestern shoreline (“Pike Point”) area of Fivemile Lake (after Hudak et al., in prep. b). Detailed facies mapping at scales ranging from 1:50 to 1:100 by Hudak et al. (2002a) and Hudak et al. (in press) now suggests that “Pike Point” comprises a series of northeast-trending andesite-dacite pillowed dikes that intruded a deposit of wet, unconsolidated basalt-andesite tuff to form blocky to amoeboid peperites. Pillows may be formed within water-saturated hyaloclastite or sediments within the sub-seafloor provided that the adjacent rocks are poorly consolidated and saturated with water (McPhie et al., 1993 and references therein). The interpretation that the pillowed igneous domains represent dikes rather than pillowed lava flows is based on several characteristics of the dikes: 1) the strike of the pillows is northeast, whereas the strike of the volcanic strata in this region is general more or less east-west; 2) topping directions measured in the pillows are variable and inconsistent; 3) on the most northwestern of the outcrops, apophyses of the dikes can clearly be seen propagating to the north-northwest; 4) the Page 20 20 �chemical compositions of the pillow domains and the blocky to amoeboid fragments in the volcaniclastic deposits is identical; and 5) the trend of the pillows, when followed to the north shore of Fivemile Lake, leads to a series of chemically identical, northeast-trending dikes and associated peperite deposits that occur within the vent facies of a 700 meter (strike length) by 200 meter (height) tuff cone volcano mapped by Hudak et al. (in prep. a, in prep. b; we will see these deposits tomorrow at field trip Stop 1-9). The interpretation that the volcaniclastic deposits represent peperites is based on the following characteristics of the deposits: 1) at several locations along the point, blocky jigsaw puzzle-fit fragments indicate in-situ fragmentation of northeast-trending pillow dikes; 2) fragment shapes in the volcaniclastic deposits vary from amoeboid to blocky and jigsaw puzzle-fit, to curviplanar, all indicative of fragmentation by external water; 3) vesiculation is common within the matrix of the volcaniclastic deposits; and 4) the general trend of the long directions of the fragments is consistent with the strikes of the pillow dikes. Several textures that may be observed at this location are illustrated in Figure 1-12. Figure 1-12. Textures within peperites that occur along the southwestern shoreline of Fivemile Lake: 1-12a) northeast-trending pillow dikes on the northwest side of the point; 1-12b) fractured synvolcanic dike, west-central part of point; 1-12c) synvolcanic dike and blocky peperite with vesiculated sediments, south-central part of point; 112d) northeast-trending pillowed dikes, east-central part of point. Page 21 21 �STOP 1-6: EPIDOTE – QUARTZ ALTERED DIABASE SILLS AND DIKES, SOUTH OF FIVEMILE LAKE Location: (NE, SW, S.29, T.62N., R.14W., NAD 83 UTM 563515E, 5296610N) Description: This stop affords us the opportunity to investigate both synvolcanic and post-volcanic hydrothermal alteration in the Fivemile Lake area. Field, petrographic, x-ray diffraction, electron microprobe, and lithogeochemical studies have identified twelve distinctive metamorphosed hydrothermal alteration mineral assemblages at the Fivemile Lake prospect (Odette et al., 2001a, 2001b; Hudak et al., 2002a; Hocker et al., 2003). Synvolcanic, semi-conformable alteration assemblages have been identified on the basis of the modal percentages of the minerals epidote, actinolite, and chlorite, and include: a) epidote + quartz ± chlorite ± albite; b) epidote + quartz + chlorite ± albite; c) epidote + quartz + actinolite ± chlorite ± albite; and d) albite + quartz ± epidote ± actinolite. These quartz + epidote-rich alteration assemblages are interpreted to represent “seafloor metasomatism” associated with hydrothermal fluids that moved through the subaqueous rocks. Detailed mapping and lithogeochemical trends suggest that early, relatively low temperature hydrothermal alteration mineral assemblages (e.g., albite + quartz ± epidote ± actinolite) were later overprinted by higher temperature alteration mineral assemblages (e.g., epidote + quartz ± chlorite ± albite, epidote + quartz + chlorite ± albite, epidote + quartz + actinolite ± chlorite ± albite) as the volcanic pile was progressively buried by volcanism deeper below the seafloor. It is believed that the early, near seafloor environment, which was locally base metal enriched by VMSstyle mineralization, eventually became depleted in base metals from later, deep-seated hydrothermal circulation. Synvolcanic, disconformable alteration mineral assemblages are commonly proximal to synvolcanic fault zones and VMS-style mineralization at the prospect, and include: a) actinolite + quartz ± iron-rich chlorite ± epidote ± albite; b) iron-rich chlorite + sericite; c) iron-rich chlorite ± sericite ± ironcarbonate ± actinolite ± epidote; d) sericite ± iron-rich chlorite; and e) mottled epidote + quartz ± actinolite. These alteration zones likely represent areas of high temperature metasomatism associated with VMS mineralization at, or up-section from, the Fivemile Lake prospect. Post-volcanic hydrothermal alteration mineral assemblages at the prospect include: a) carbonate (ankerite, dolomite, or calcite); b) sericite + carbonate (ankerite, dolomite, or calcite); and c) sericite + green mica ± carbonate (ankerite, dolomite or calcite). The close association of structurally deformed rocks and these alteration zones, and the orientations of these alteration zones, suggest that these alteration mineral assemblages resulted from syntectonic hydrothermal processes associated with the D2 Murray Shear Zone deformational event. A hydrothermal alteration mineral assemblage map for the Fivemile Lake prospect is illustrated in Figure 113. The mottled epidote + quartz + actinolite alteration zone present in this outcrop is confined to a diabase dike-sill complex located immediately south of Fivemile Lake. This 400 meter by 500 meter disconformable to semiconformable alteration zone locally cross-cuts semiconformable alteration zones comprising the epidote + quartz ± chlorite ± secondary feldspar assemblage, the epidote + quartz + ironchlorite ± secondary feldspar assemblage, and the secondary feldspar + quartz ± epidote ± actinolite alteration assemblage. The mottled epidote + quartz + actinolite alteration zone is characterized by a dark green, actinolite and epidote-rich groundmass that contains discrete 0.1-2.0 meter diameter, round to lens-shaped masses containing anhedral granular epidote and zoisite intergrown with polygonal quartz. These rounded epidote-quartz-rich masses can easily be mistaken for individual pillow lavas; however, they can be distinguished from pillows because they lack abundant amygdules, and have no discrete pillow selvedges or interpillow hyaloclastite zones. Petrographic, x-ray, and electron microprobe analyses indicate the presence of iron-rich chlorite, iron-rich actinolite, pistacite and zoisite/clinozoisite within this assemblage (Odette et al., 2001a, 2001b; Hocker et al., 2003). Page 22 22 �Figure 1-13. Hydrothermal alteration map of the Fivemile Lake prospect (after Hudak et al., 2002a). The mineralogy, mineral compositions, and pronounced calcium enrichment and copper depletion that characterize the mottled epidote + quartz + actinolite alteration assemblage (Odette et al, 2001a, 2001b) are consistent with formation within a synvolcanic, semiconformable to disconformable alteration zone formed by high temperature, high fluid to rock ratio interactions deep within a sub-seafloor hydrothermal cell (c. f. Franklin, 1996; Seyfried et al., 1999). Similar clinozoisite/zoisite bearing alteration mineral assemblages described by Hannington et al. (2002) that occur down-section from the world-class VMS deposits in the Noranda mining camp in Quebec are believed to have formed from anomalous fluid flow at high water to rock ratios and high temperatures. Large zones of base metaldepleted epidosite in the Troodos ophiolite complex have been interpreted by Richardson et al. (1987) to represent deep-seated fossil reaction zones where metaliferous “black smoker” fluids are produced. The confinement of this alteration assemblage to the base of the northeast-trending diabase dike / sill complex, indicates that this alteration assemblage occurs in the deep subsurface of a northeast-trending synvolcanic structure that focused magma, as well as high temperature, perhaps base metal-rich hydrothermal fluids, up-section. Base metal exhalite mineralization occurs approximately 1 km northeast of this location (Stops 1-7 and 1-8, Fig. 1-13). An east-southeast – west-northwest-trending post-volcanic shear zone is present in the northcentral part of the outcrop. The orientation of the schistosity adjacent to the shear zone indicates a dextral sense of shear. The shear zone is one of several east-southeast – west-northwest trending shears that occur at the Fivemile Lake Prospect. Similarly oriented dextral shear zones mapped by Peterson and Patelke (2003) between the north edge of the Murray Shear Zone and the Mine Shear zone in Sections 25 and 26, T. 62N, R. 15 W (approximately 2-3 km west of this location) have been interpreted as Reidel R structures. Page 23 23 �STOP 1-7: ALTERED PILLOW LAVAS, BASE METAL EXHALITE, SCORIA LAPILLI TUFF, NE OF FIVEMILE LAKE Location: (SE, NE, S.29, T.62N., R.14W., NAD83 UTM 564260E, 5297175N) Description: A 40-meter long, east-northeast striking, <1-2 meter thick chert-pyrite-chlorite ± chalcopyrite exhalite horizon crops out at this location approximately 50 meters northeast of Fivemile Lake (Fig. 1-13). Although this unit is volumetrically insignificant within the Fivemile Lake area, its presence is extremely important in terms of understanding potentially ore forming processes in this part of the Lower Ely. A lithogeochemical analysis of the exhalite (Hudak et al., 2002a) indicates slightly elevated Cu (470-509 ppm) and Zn (66-135 ppm). D. M. Peterson (personal communication, 1996) first recognized this unit as being similar in morphology and mineralogy to chemical exhalites associated with, and locally hosting, volcanic-associated massive sulfide mineralization in the Archean basalts and andesites within the Noranda mining camp of Quebec, Canada. This exhalite horizon appears to be on strike with the northernmost of four untested EM conductors that have been identified within Fivemile Lake (Peterson, 2001; Peterson and Jirsa, 1999a). Extremely detailed mapping (1:120 scale) has been completed (Hudak et al., 2002a) to further evaluate the stratigraphic and structural controls on this exhalite deposit (Fig. 1-14). The exhalite is relatively easy to distinguish from its surrounding rocks based on its deep red-brown, gossan-like, sulfidestained weathered surface. When broken, the exhalite is composed of vaguely banded chert and chlorite that contains 5-10% disseminated to banded pyrite. Locally, anhedral disseminated chalcopyrite and locally, extremely fine-grained, finely-disseminated, honey- to dark brown sphalerite grains are present in trace amounts. Detailed mapping indicates that the exhalite unit separates epidote + quartz + actinolite ± chlorite-altered basalt-andesite pillow lavas from overlying iron-chlorite- and sericite-altered basaltandesite tuffs and lapilli tuffs. The exhalite unit stratigraphically and conformably overlies the pillow lavas, and its sharp basal contact illustrates how the chemical sediment draped over the pillow-dominated paleotopography of the seafloor at the time of deposition. Locally, exhalite deposits cut through the pillow lavas, replacing the formerly glassy interpillow hyaloclastite. This relationship suggests that the interpillow hyaloclastite zones represented areas of high permeability, and these regions were the locus of hydrothermal fluid migration upward to, and possibly within the immediate subsurface, of the paleoseafloor. These features are consistent with the interpretation that this represents a region of ventproximal hydrothermal activity. The contact between the exhalite and the overlying lapilli tuff and tuff deposits is also sharp. It is important to note that fragments of the exhalite have not been observed in the basal parts of these overlying volcaniclastic deposits. In addition, no soft sediment deformation has been observed that could be attributed to syndepositional tractional deformation associated with the sedimentation of the overlying volcaniclastic rocks. This suggests that the exhalite deposit was relatively competent at the time the overlying tuffs were deposited. Such field evidence, combined with the lack of intense quartz-epidoteactinolite ± chlorite alteration (which is so prevalent in the footwall pillow basalts) in the overlying tuffs, also suggests that the hydrothermal activity associated with the genesis of the exhalite deposit was completed by the time that the overlying volcaniclastic rocks were deposited. Three other features suggest that exhalite deposition was closely associated with a volcanic ventproximal setting. First, property-scale mapping at Fivemile Lake (Hudak et al., 2002a) clearly illustrates that a synvolcanic diabase intrusion (observed in the previous stop) cross-cuts the stratigraphy in the immediate vicinity of the exhalite deposit (Fig. 1-9). Second, detailed mapping clearly indicates that the exhalite unit is abruptly terminated at its western margin by north to northwest striking pillow dikes that are similar in morphology to the pillow dikes that occur in close association with the peperite deposits Page 24 24 �Figure 1-14. Detailed geological map of exhalite mineralization northeast of Fivemile Lake (from Hudak et al., 2002a). Note the stratigraphic positioning of the chemical exhalite horizon between basalt-andesite pillow lava and basalt-andesite lapilli tuff, suggesting a period of volcanic quiescence which was accompanied by hydrothermal activity prior to the deposition of the lapilli tuff unit. Also note that the western part of the exhalite is cut by pillowed basalt-andesite dikes, once again suggesting the presence of a synvolcanic structure. Note that all UTM coordinates on the diagram are in NAD 27 coordinates. Page 25 25 �along the southwestern shoreline of Fivemile Lake. The location of both the diabase dikes and the pillow dikes clearly indicates the local presence of a major synvolcanic structure. Thick interpillow hyaloclastite deposits in the vicinity of the exhalite also suggest a vent-proximal environment (Newkirk et al., 2001a, 2001b; Hudak et al., 2002a). In summary, semi-massive sulfide exhalite associated mineralization clearly has formed within a volcanic (and associated hydrothermal) vent-proximal environment at Fivemile Lake. Such environments are commonly associated with economic massive sulfide mineralization in other lava-flow dominated Archean volcanic settings, such as at Noranda, Quebec (Gibson, 1989; Gibson et al., 1999; Santaguida, 1999). STOP 1-8: RHYOLITE LAVA DOME, NE OF FIVEMILE LAKE Location: (SE, NE, S.29, T.62N., R.14W., NAD 83 UTM 564440E, 5297160N) Description: The first day of our field trip concludes with an investigation of a 200-meter long, 15-45 meter thick unit of quartz-phyric, rhyodacitic to rhyolite lava that occurs approximately 200 meters northeast of Fivemile Lake (Figs. 1-9 and 1-13). The U/Pb zircon age for this unit is 2722.6 ± 0.9 Ma (Peterson et al., 2001). This biscuit shaped, relatively thick mass of rhyodacitic to rhyolitic is morphologically similar to felsic lava domes and cryptodomes (Cas and Wright, 1987). The presence of quartz-phyric lava fragments in epiclastic deposits approximately 400 meters southeast of this location, as well as the presence of the overlying subaqueously deposited volcaniclastic sediments and intermediate to mafic lava flows, suggests that this unit was at least partially emergent as a subaqueous lava dome. The uppermost contact of this unit is not exposed in outcrop. Where exposed in drill core (diamond drill hole SXL-2), the massive quartz-phyric lava is interstratified with, and overlain by, 1-2 meters of felsic tuff, which in turn, is overlain by pillowed basalt-andesite lava flows. The basal contact of the unit is exposed in diamond drill holes SXL-2 and SXL-3. In SXL-2, the basal contact is represented by a 3-meter thick shear zone composed of quartz, carbonate, sericite and green mica (fuchsite or mariposite?). In SXL-3, the base of the unit is immediately underlain by a 2-meter thick quartz-feldspar porphyry dike that, in turn, is underlain by a shear zone composed of quartz-carbonate-sericite-chlorite schist. This shear zone locally contains 1-3%, 1-8mm wide veins and bands composed of red to redbrown sphalerite. The presence of this shear zone conservatively precludes using the pillowed basaltandesite lava flows as reliable bounding facies; however, the lack of evidence for extensive structural transposition of the felsic lavas, the extremely thick sequence of pillow lavas stratigraphically below the felsic lavas, and the presence to the southeast of submarine epiclastic strata with felsic lava fragments identical in composition to the dome, suggest that the felsic lavas were formed in a subaqueous environment. Drill core intersections of these lava flows are characterized by a massive to faintly amygdaloidal (up to 1% 5mm rounded gray quartz amygdules), pale gray (least altered) to pale yellow green (sericitized) aphanitic groundmass which contains 1-4% grayish blue to colorless euhedral square to subhedral, locally broken, <1-2.5mm quartz phenocrysts (Fig. 1-15a). Pale brown to honey-colored sphalerite veins are locally present in the drill core and range from <1-8cm in width (Figure 1-15b). Two types of veins are present: a) sphalerite veins which are associated with quartz and/or chlorite, which display no indication of structural deformation (in-situ sphalerite veins); and b) sphalerite veins which are associated with local domains of quartz-sericite-ankerite schist, and appear to have been formed by local remobilization of sphalerite along post-volcanic shear zones (structurally remobilized sphalerite veins). The presence of in-situ sphalerite veins, and the abundance of structurally remobilized sphalerite veins in the quartz-phyric lava flows relative to other units in the Fivemile Lake area, suggests that the original Page 26 26 �sphalerite mineralization in the Fivemile Lake area was synvolcanic and associated with the massive quartz-phyric lava flows. As seen in this exposure, the quartz-phyric lava flows contain an aphanitic, pale-gray to gray groundmass. These lavas contain 5-8% 1-3mm diameter subhedral to square euhedral quartz phenocrysts that are locally broken. Rare, 2-4mm subhedral to euhedral quartz glomerocrysts are locally present. Faint, pale gray <1-2mm tabular feldspar phenocrysts are also present, and comprise up to 5% of the rock. Petrographic analysis indicates that lavas are characterized by a very fine-grained quartzo-feldspathic groundmass and subhedral to euhedral quartz phenocrysts that are locally resorbed. Alteration minerals that occur in the groundmass include sericite, epidote, clinozoisite/zoisite, iron-carbonate and iron-rich chlorite. Also present on the southwest side of this outcrop is a northeast-trending, post-volcanic quartzfeldspar porphyry dike. Figure 1-15. Drill core appearance of unmineralized, sericite-altered rhyolite dome (A, SXL-2-173’) and silicified rhyolite dome cut by red-brown “honey-colored” sphalerite veins (B, SXL-2-280’). Modified from Hudak et al. (2002a). Scale bar units are centimeters. FIELD TRIP STOPS – DAY 2 (MAY 5) Figure 1-16. Locations of field trip stops 1-9, 1-10, and 1-11. Page 27 27 �STOP 1-9: FLUIDAL PEPERITES AND VOLCANIC VENT FACIES, NORTH SHORELINE OF FIVEMILE LAKE No hammering please! Location: (SE, NW, S.29, T.62N., R.14W., NAD 83 UTM 563590E, 5297215N). Note: We will be accessing the north side of Fivemile Lake via a private road. Obtain permission to use this road from people in the first house on the right as you turn onto this private drive. Description: Detailed (1:100 to 1:2000 scale) mapping by Hudak et al. (in prep. a) and Hudak (in press) has identified the paleo-vent of a relative small basalt-andesite tuff cone volcano immediately north of this location (Figs. 1-9 and 1-16). The vent facies here is well exposed in a series of outcrops along the north shoreline of Fivemile Lake, and is extremely complicated (Fig. 1-17a). At this location, we can observe steeply north dipping, east-west striking basalt-andesite pillow lavas that occur stratigraphically beneath the tuff cone, and the contact between the pillowed flows and the vent fill deposits, which are composed of exceptionally well preserved blocky to amoeboid peperites (Figs. 1-17 b and 1-17c). The contact between the pillowed flows and the vent fill deposits is interpreted to be a synvolcanic fault zone. Several jigsaw puzzle-fit, more or less in-situ pillow lava blocks can be observed within the vent fill deposits near this contact. The basalt-andesite tuff comprising the vent fill deposits have been intruded by a series of andesite-dacite dikes that are compositionally identical to the pillowed dikes observed southwest of this location at “Pike Point”. Interactions between the wet, unconsolidated vent fill tuffs and the dikes have produced an absolutely spectacular deposit of amoeboid peperite. The peperite displays several exceptional textures, including: 1) jigsaw puzzle-fit igneous component lapilli and blocks indicative of in-situ fragmentation; 2) local contact metamorphosed zones ranging from a few millimeters to a few centimeters in width immediately adjacent to the lapilli and blocks within the peperite; 3) local chilled margins on the peperite fragments indicating rapid quenching and fragmentation of the igneous component from interactions with relatively cool, wet, unconsolidated vent fill tuffs; 4) complicated amoeboid peperite, especially on the northeastern part of the outcrop; and 5) the presence of 1-2mm diameter, commonly quartz-filled vesicles within the vent fill tuffs. Also note the presence of 1-2 cm wide, several centimeter-long parallel bands containing high abundances (20-50%) of 1-2mm vesicles. These bands are generally more or less parallel with the margins of adjacent igneous component peperite fragments, but locally, these bands show no preferred directional relationship with the margins of nearby clasts. Petrographic observations (Hudak et al., in press; Hudak et al., in prep. b) indicate that these zones are composed of parallel bands of highly vesicular, locally plagioclase-phyric fragments that are compositionally identical to the coherent igneous component clasts in the peperite. The origin of the bands may be related to collapse of thin vapor films along the margin of the hot igneous component as occurs in laboratory simulations of peperite formation (Zimanowski and Buttner, 2002). The parallel bands in this outcrop may have formed when the margins of the igneous component shattered due to explosive interaction with external water as the vapor film collapsed. The parallel bands observed on the outcrop indicate that this process occurred periodically and continually. Such a process is consistent with the pulsating phreatic and phreatomagmatic eruptions that occur in shallow submarine environments. Page 28 28 �DETAILED GEOLOGICAL MAP FIVE MILE LAKE PEPERITE Pillow Basalt - Andesite Pepe.te - Igneous Componenl [j] Pepenle - Sedimentary Component [] Miyg.iuie-ric, banding Figure 1-17. Detailed geological map (A) of the vent facies of a basalt-andesite tuff cone volcano located north of Fivemile Lake. Photographs A and B illustrate amoeboid peperite and parallel, vesicle-rich zones present in the outcrop. Page 29 29 �STOP 1-10: CENTRAL BASALT SEQUENCE SHEET FLOWS, PILLOW LAVAS, AND PERLITIC HYALOCLASTITE, HILLTOP S OF HIGHWAY 169 Location: (SE, SW, S.19, T.62N., R.14W., NAD 83 UTM 56200E, 5297800N) Description: The Central Basalt sequence (Peterson and Patelke, 2003) comprises a steeply northdipping (75°- vertical), north-facing sequence of sparsely amygdaloidal pillowed and massive lava flows of basalt composition that are believed to be correlative with the tholeiitic Armstrong Lake volcanic sequence mapped by Jirsa et al. (2001) in the Eagles Nest quadrangle. Relative to massive and pillowed basalt and andesite flows in the Fivemile Lake sequence, Central Basalt sequence lavas flows are notably less amygdaloidal, and lack multiple pillow rind structures. In addition, the Central Basalt sequence lacks the thick sequences of scoriaceous basalt-andesite lapilli tuffs that are commonly interstratified with lava flows in the Fivemile Lake sequence. These characteristics of the Central Basalt sequence indicate eruption and deposition in a deeper submarine environment than the stratigraphically older Fivemile Lake sequence, and suggest overall increasing water depth during the temporal development of the Lower Ely. This field trip stop (Fig. 1-16) displays exceptional preservation of fine-scale volcanic textures that are characteristic of the Central Basalt sequence. The outcrop comprises two east-southeast striking massive basalt flows, ranging from at least five to nine meters in thickness, that are separated by a ten meter thick flow unit comprising pillows and pillow lobes (Fig. 1-18). Flow 1, at the southern part of the outcrop, is composed of a pale- to dark green, faintly feldspar-phyric (~10% 0.5-1 mm laths), sparsely amygdaloidal, basalt sheet flow that locally exhibits tortoise-shell jointing formed in response to contraction during cooling. The uppermost 10-40 cm of the coherent part of Flow 1 is generally silicified and epidotized. Petrographic observations (Hudak et al., in prep. b) indicate that this section of the flow also contains up to 70% <0.1 cm round spherulites. An irregular contact occurs between the coherent basalt flow and an overlying one- to two meter thick unit of dark green, exceptionally well-preserved perlitic in-situ hyaloclastite and associate self-peperite (c.f. Batiza and White, 2000). The hyaloclastite formed from non-explosive fracturing of the basalt glass developed on the flow top due to quenching by water, whereas the perlite formed following deposition by hydration of volcanic glass. Irregular, amoeboid apophyses of coherent basalt, ranging from 10-30 cm thick and up to 1.5 meters long, locally intrude into the hyaloclastite. These apophyses have pale green cores and dark green, very fine-grained margins which grade outward into in-situ, jigsaw-fit perlitic hyaloclastite. An irregular contact occurs between the hyaloclastite and Flow 2, which is composed of north-facing mattress- to bun-shaped pillow lavas and pillow lobes with numerous “neck and knob” structures. Individual pillows have well developed perlitic hyaloclastite margins that range from 1-4 cm in width. Pillow buds indicate propagation from east to west, suggesting the volcanic vent was located east of this location. A NNE-trending D3 normal fault with a 10 cm displacement occurs in this unit on the southeastern part of the outcrop. The coherent pillows and lobes are overlain by up to 2.5 meters of hyaloclastite breccia that contains 20-40% subround to subangular pale gray green basalt lapilli in a jigsaw puzzle-fit dark green perlitic hyaloclastite matrix. The upper contact of Flow 2 and the overlying basalt sheet flow (Flow 3) is irregular, and is marked by thin (1-8 cm thick), sheet-like basalt fragments that are up to 1.6 meters in length. These fragments locally appear to be isoclinally folded about an east-west-trending fold hinge. Although the genesis of this structure is currently not well understood, it may be due to syneruptive deformation of either thin slabs of hot, basal flow margin crust from the overlying flow, or thin injections of basalt magma into the hyaloclastite from either the underlying pillows or the overlying sheet flow. Flow 3 comprises an at least ten-meter thick pale green-gray, slightly feldspar-phyric, sparsely amygdaloidal sheet flow. Steep, NNE-trending west dipping D3 joints are well developed in this unit, as are lensshaped psuedo-pillows that are up to 50 cm in diameter. Page 30 30 �Figure 1-18. Detailed geological map of sheet flows, pillow lavas, hyaloclastite, and associated “self-peperite” at field trip stop 1-10. Page 31 31 �STOP 1-11: Location: RHYODACITE BLOCK AND ASH FLOW DEPOSITS, NORTH SHORELINE OF NEEDLEBOY LAKE (Optional Stop, NW, SW, S.21, T.62N., R.14W., NAD 83 UTM 564855E, 5298360N) This is an optional stop, as it requires a moderately strenuous 20-30 minute hike along a poorly maintained, easy to trip upon trail. Be extremely careful and watch for protruding boulders and cobbles on the trail. Description: A 50 meter-thick unit of quartz-phyric rhyolite tuff and lapilli tuff crops out in a series of several closely-spaced outcrops along the northwestern shoreline of Needleboy Lake (Fig. 1-16). The pale gray to green-gray matrix contains 1-3% subhedral to angular quartz phenocrysts and crystal shards, 10-15% 1-5 cm subangular to subround pumice lapilli, and locally, up to 15% 2-11 cm subround to subangular quartz-phyric rhyolite lava lapilli and bombs which locally display a crude jigsaw puzzle-fit. Both the pumice and the felsic lava clasts are locally crudely stratified. Pumice lapilli altered by epidote commonly occur as pale green fragments within the deposits. The central part of the outcrop contains a well-developed, east-west trending, steeply north-dipping shear zone. Boudinage locally occur in the rhyolite deposits within this shear zone. A 30-35 meter thick gabbro sill occurs immediately north of this shear zone. This unit is characterized by an equigranular medium gray-green to brownish-green color, with pale green epidote altered feldspar and deep green actinolite present. Petrographic observations (Hudak et al., in prep. a) indicate that the gabbro has a sub-ophitic texture. Locally, steeply-dipping northsouth-trending D3 joints and east-west-trending quartz epidote veins may be observed on the northeastern side of this series of outcrops. A large east-west-trending outcrop occurs approximately 40 meters northwest of the northern-most outcrop near the shoreline. The southernmost 5 meters of the outcrop comprises a mafic lapilli tuff that contains up to 5% subangular silicified mafic lapilli. This tuff is progressively deformed into dark green chlorite schist with a well-developed, east-west-trending, steeply south-dipping S2 foliation. The chlorite schist is in sharp contact with a massive, 3-12 meter-thick quartzand feldspar-phyric rhyodacite sill. The sill is overlain by a mafic lapilli tuff containing up to 25% subangular to subround, locally jigsaw puzzle-fit amygdaloidal basalt and scoria clasts. Felsic volcaniclastic deposits at the base of the outcrop are interpreted to represent a sequence of variably-altered rhyolite block and ash flow deposits. Block and ash flow deposits are unsorted deposits with an ash-matrix containing both monolithologic, low-vesicularity lava lapilli and blocks and pumice lapilli and blocks that may display normal or reverse coarse-tail grading (Cas and Wright, 1987; Freundt et al., 2000). The deposits are formed during the explosive collapse of lava domes, and can occur in either the subaerial or submarine environment (Hudak, 1996; Gibson et al., 1999). These deposits are believed to have been initially overlain by a series of basalt-andesite tuffs and lapilli tuffs; however, a synvolcanic gabbro sill intruded the contact between and subsequently separated these two units. Deformation that is concentrated along the contacts between this sill and the adjacent supracrustal strata is attributed to strain partitioning due to different rheological properties during the D2 deformation event. STOP 1-12: SYNVOLCANIC FAULT ZONE WITH “EPIDOSITES”, SE OF SIXMILE LAKE Location: (NE, SE, S.21, T.62N., R.14W., NAD 83 UTM 565860E, 5298535N) During the hike through the swampy area, be extremely careful to walk on areas with vegetation and tree roots. Description: The next several stops (1-12 through 1-17) represent a north-south section through the uppermost parts of the Lower Ely (Fig. 1-19). Basalt-andesite lava flows in this region vary from massive to pillowed, and are generally less amygdaloidal than the basalt-andesite lava flows in the southern part of the section near Fivemile Lake. The purpose of this stop is to investigate a northeast-trending, Page 32 32 �disconformable zone of intensely quartz + epidote ± actinolite altered pillow lavas (epidosites), and to observe one of the post-volcanic quartz-feldspar porphyry dikes that commonly occur within the volcanic sequence. Figure 1-19. Detailed geological map of the area southeast of Sixmile Lake. Field trip stop locations 1-12 through 1-17 are also illustrated on the map. The first outcrop we will investigate is composed of intensely epidote + quartz-altered basaltandesite pillows and lobes. The pillows may be difficult to see due to the intense synvolcanic hydrothermal alteration, but may be located by identifying actinolite ± chlorite-altered zones of interpillow hyaloclastite, as well as concentrated zones of 0.1-1.0 cm round to oval actinolite ± chloritefilled amygdules which occur within the pillow selvedges. Dark gray veins of magnetite with minor amounts of pyrite, chalcopyrite, and malachite up to 1 cm in width occur primarily within interpillow hyaloclastite zones, but also locally cross-cut the interiors of the pillows and lobes. The patchy pistachiogreen and pink coloration on the outcrop is due to the presence of pistacite epidote (green) and zoisiteclinozoisite (pale pink). Santaguida et al. (2002a, 2002b) indicate that epidote compositions within epidote-quartz altered rocks in the Noranda Volcanic Complex of Quebec are sensitive to small-scale variations in fluid:rock ratios within individual alteration zones due to primary permeability differences in the strata being altered. Hannington et al. (2002) note that within the Proterozoic Kristineberg VMS district of Sweden, zoisite-clinozoisite formation occurs in regions altered at relatively high fluid:rock ratios and high temperatures by acidic hydrothermal fluids. The second outcrop to observe occurs approximately 20 meters to the southwest of the first outcrop, and is located near the top of a prominent east-northeast trending ridge. Compositionally, the rock is identical to the first outcrop. Texturally, the rock is intensely brecciated, containing lapilli- to block-sized subangular fragments of intensely epidote + quartz-altered amygdaloidal basalt. Disconformable zones containing jigsaw-fit epidosite occur proximal to synvolcanic fault zones in the Josephine Ophiolite (Harper, 1999). Based on detailed mapping, the northeast trending, disconformable Page 33 33 �zone of epidosite comprising these outcrops has been interpreted by Hudak et al. (2002b) as being proximal to a synvolcanic fault zone. The third outcrop occurs approximately due north of the second outcrop visited. Exposed at this location is a relatively fresh quartz-feldspar porphyry dike. Similar quartz-feldspar porphyry dikes commonly occur throughout this region of the Vermilion district. Figure 1-20. Photographs of intensely epidote-altered basalt-andesite lava flows from the area southeast of Sixmile Lake. Photo A shows brecciated, intensely epidote–quartz altered pillow lavas. Photo B shows typical relationships between intense epidote alteration, stockwork quartz veins, magnetite stringers, and sulfide staining. STOP 1-13: SYNVOLCANIC FAULT ZONE WITH EPIDOSITES, SE OF SIXMILE LAKE Location: (SE, NE, S.21, T.62N., R.14W., NAD 83 UTM 565850E, 5298560N) Description: Several relatively small outcrops of intensely epidote + quartz ± actinolite ± chloritealtered basalt-andesite pillow lavas occur along the southwestern part of an east-west trending ridge at this location. These rocks are mineralogically similar to those investigated at the previous outcrop; however, despite their intense alteration, they display extremely well preserved primary volcanic textures. The pillows at this location are primarily bun-shaped, and range from approximately 1-1.5 meters in diameter. Interpillow hyaloclastite zones range from <1-2 cm in width, and are pale yellow green in color due to the presence of epidote, actinolite, and chlorite. Pillow selvedges and cores once again vary in color from pale green to pinkish green due to variable percentages of pistacite and clinozoisite/zoisite. The pillow selvedges and cores contain up to 5% 0.1-1.0 cm diameter round to oval quartz ± actinolite ± chlorite-filled amygdules. STOP 1-14: STRINGER COPPER MINERALIZATION IN BASALT–ANDESITE, AND SERICITE-ALTERED RHYODACITE TUFFS, SE OF SIXMILE LAKE Location: (SE, NE, S.21, T.62N., R.14W., NAD 83 UTM 565910E, 5298655N) Description: Another small group of outcrops occurs approximately 50 meters northeast of the previous outcrops investigated (Fig. 1-19). Stratigraphically, we are now positioned within the uppermost 100 to 200 meters of the Lower Ely. At this location, we can observe locally brecciated, strongly epidote + quartz-altered, and locally actinolite- and chlorite-altered basalt-andesite pillow lavas. Pillows at this outcrop are significantly smaller (generally approximately 0.5 meters in diameter) than those observed in the previous outcrop, suggesting that we are positioned near the top of a pillowed flow unit (Dimroth et Page 34 34 �al. (1978) and Staudigal and Schminke (1984) have shown that pillows generally decrease in size near the top of a pillowed unit, and attribute this to change in pillow size due to decreasing effusion rates as an individual eruption comes to an end). The outcrop also locally contains up to 5% dark gray stringer-like veins of magnetite with associated pyrite, chalcopyrite, malachite, and associated deep-red sulfide burn (Fig. 1-21a). These veinlets are commonly parallel to a series of quartz stockwork veins that trend approximately 080° and dip steeply to the north. Both the magnetite-sulfide veins and quartz stockwork veins are consistent with cooling of a high temperature, iron-, base metal-, and silica-rich hydrothermal fluid in the shallow seafloor, and may represent stringer mineralization that occurs immediately downsection from massive sulfide mineralization. Figure 1-21. Mineralization and alteration present at field trip stop 1-14. Photograph A illustrates malachite mineralization associated with magnetite-chalcopyrite veins within altered basalt pillow lavas. B shows sericitealtered felsic tuff deposits located immediately up-section from the altered mafic lavas in photograph A. The pillow lavas are immediately overlain by a 10-20 meter thick sequence of sparsely quartz and plagioclase-phyric rhyodacite tuffs (Fig. 21b). These volcaniclastic deposits typically are brownish-gray in color due to the presence of abundant sericite. Close-up observation of the unit reveals the presence of approximately 1% sub-1mm anhedral to subhedral quartz crystal chips, and 2-3% 1 mm diameter subhedral to euhedral tabular plagioclase feldspar phenocrysts. Rare pumice lapilli can be observed with careful observation; they range from 1-2 cm in diameter, are generally rounded to oval in shape, and contain up to 50% sub-1mm round vesicles (originally carbonate vesicles which have now been weathered-out based on petrographic observations by Hudak et al. (in prep. a)). The pumice lapilli in this outcrop increase in abundance from the base toward the top of the unit. Such inverse grading of pumice may be the result of subaerially erupted air fall deposited into the ocean, as large, cold pumice generally take longer to saturate and sink than small, cold pumice (Whitham and Sparks, 1986). This tuff horizon may represent the distal equivalent to fragmental felsic rocks mapped by Peterson and Patelke (2003) that occur stratigraphically below the Ely Greenstone – Soudan Member and the Soudan Mine. Sericite alteration is common in submarine felsic volcaniclastic rocks (Gibson et al., 1989), and may be associated with either vertically extensive discordant pipe-like alteration zones associated with hydrothermal upflow zones, or laterally extensive semiconformable alteration zones formed by extensive shallow seafloor seawater metasomatism of permeable submarine strata. More detailed mapping will be necessary to determine with certainty the geometry of the sericite alteration zone observed in this outcrop. The exposures of felsic tuff dive abruptly to the north into a swamp. Hand auger boring in this swamp (NAD 83 UTM location 0565938E, 5298671N) yielded dark black, organic-rich soil containing abundant, angular, variably weathered clasts of massive pyrite. Stringer copper mineralization, sericitealtered felsic tuffs, the presence of massive sulfide in the subsurface beneath the swamp, and the presence Page 35 35 �of rare sulfide lapilli in volcaniclastic deposits north of the swamp (seen in the next outcrop) suggests that the swamp may be underlain by a massive sulfide horizon (Hudak et al., 2002b). STOP 1-15: CHERT, BANDED IRON-FORMATION AND SEDIMENTARY STRATA, SE OF SIXMILE LAKE ASSOCIATED CLASTIC Location: (SE, NE, S.21, T.62 N., R.14W., NAD 83 UTM 565840E, 5298760N) Be extremely careful when walking on these outcrop surfaces and descending into the small valley immediately south of these outcrops, as they are commonly very slippery. Description: North of the swamp, a southeast-trending outcrop ridge comprises a sequence of volcaniclastic and chemical sedimentary strata (Fig. 1-19). The purpose of this stop is to observe the variety of interflow sedimentary rocks that occur in this upper part of the Lower Ely, to observe wellpreserved sedimentary textures, and to see further evidence for the presence of ancient hydrothermal activity southeast of Sixmile Lake. A series of small outcrops occurring along the northwestern part of the outcrop ridge comprises west-northwest striking, steeply north dipping 10-30 cm thick chemical exhalites (chert horizons and banded magnetite + chert ± jasper banded iron-formation horizons) are that are interbedded with 10-60 cm thick, well-stratified horizons of mudstones, wackes, and matrix-supported polymict diamictites. The base of the stratigraphic sequence occurs at the southern end of the outcrop, where a 20 cm thick magnetite-rich horizon of banded iron-formation occurs (Fig. 1-22a). This chemical sedimentary horizon is overlain by a several meter-thick sequence of interbedded, laminated to bedded, commonly normally graded, mudstones and polymict diamictites interpreted to have formed from submarine debris flows and turbidity currents. The polymict diamictites range from 10-60 cm thick, and contain up to 5% 1-2.5 cm lens-shaped chert clasts, 10-40% silicified or chlorite-altered mafic volcanic lapilli, and rare (<1%) <12cm sulfide-rich fragments. Locally, clasts within the diamictites are vaguely imbricated. Interbedded mudstone horizons are up to 40 cm thick, and locally display cross beds that indicate stratigraphic topping direction is north. Iron-formation horizons at this location indicate that low temperature exhalative hydrothermal activity was occurring in this part of the Lower Ely. It is interesting to note that these iron-formation horizons occur in the area that would be immediately up-section from the possible sulfide horizon within the swamp to the south. Several VMS horizons in similar volcanic sequences within Canada are capped by iron-formations (Franklin et al., 1981). Debris flows and/or turbidity currents that deposited the polymict diamictites and associated mudstones periodically interrupted periods of relatively lowtemperature hydrothermal activity that formed the iron-formation horizons. Clasts within the diamictites are similar to the volcanic and chemical sedimentary strata that occur in the immediate vicinity of the deposits based on detailed mapping (Hudak et al., in prep. a). Chert clasts within the diamictites were likely derived from the iron-formation horizons. Rare sulfide-rich fragments locally contained within the diamictites also suggest that VMS mineralization occurred on or near the paleoseafloor in the vicinity of the area that is now Sixmile Lake, supporting the interpretation that a massive sulfide horizon may be present beneath the swamp which separates the outcrops at this stop from those at the previous stop. Page 36 36 �_J\ \.a' Figure 1-22. Sedimentary strata in the uppermost parts of the Lower Ely. Figure 1-22a shows oxide-facies banded iron-formation and chert exhalites located up-section from altered tuffs seen at field trip stop 1-15. Figure 1-22b shows diamictites that contain abundant chert clasts and rare sulfide clasts. These deposits are also located stratigraphically up-section from the felsic tuffs seen at the previous stop. STOP 1-16: EPIDOTE-QUARTZ ALTERATION ON MARGIN OF DIABASE/GABBRO SILL, SE OF SIXMILE LAKE Location: (SE, NE, S.21, T.62N., R.14., NAD 83 UTM 565960E, 5298790N) The outcrop at this stop occurs on the Sixmile Lake Road. Be attentive for automobiles, trucks, and four-wheelers when observing this outcrop. Description: This relatively small (3 meters by 5 meters) outcrop exposes the northern margin of the east-west-trending gabbro-diorite sill that extends from at least southwest of Sixmile Lake to at least 500 meters east of this location (Fig. 1-19; Hudak et al., in prep.). The southern two-thirds of the outcrop comprises pale green to pinkish green, epidote- and clinozoisite/zoisite-altered subophitic gabbro which contains up to 1% 1-5mm quartz- and/or actinolite-filled amygdules. Numerous north-northeast, steeply west-dipping joints were developed during the regional D3 deformation. The uppermost one-third of the outcrop contains an in-situ hyaloclastite breccia comprising pale-to dark green angular, locally jigsaw puzzle-fit, iron chlorite + actinolite + zoisite/clinozoisite-altered gabbro lapilli in a pinkish-tan to tan matrix of sericite, epidote, and zoisite/clinozoisite (Fig. 1-23a). This outcrop illustrates, in part, the difficulty in determining hydrothermally altered flow top breccias from hyaloclastite breccias and/or peperites that form from intrusion of sills into wet, relatively unconsolidated sediments that occur within the shallow seafloor. The intense hydrothermal alteration that occurs at the northern margin of the outcrop indicates that high water:rock ratio, high temperature (>300°C) hydrothermal alteration was focused in the permeable, sill-marginal, hyaloclastite or peperite (?) breccia. Synvolcanic sills intruded into the shallow seafloor are important heat sources that drive hydrothermal circulation cells that form VMS deposits (Campbell et al., 1981). Sills such as the one exposed at this outcrop may have contributed significantly to the formation of Algoma-type banded ironformation within the overlying Soudan Iron Formation, and perhaps, VMS mineralization within the uppermost parts of the Lower Ely or the Soudan Iron Formation. Page 37 37 �Figure 1-23. Synvolcanic sills and banded iron-formations in the uppermost parts of the Lower Ely. Figure 1-23a shows the uppermost margin of a gabbro/diorite sill that has apparently intruded wet, unconsolidated sediments to form an extremely altered hyaloclastite or sill-marginal peperite. Figure 1-23b shows folded banded iron-formation horizons that occur approximately 10 meters up-section from the sill. STOP 1-17: BANDED IRON-FORMATION, NORTH OF SIXMILE LAKE ROAD Location: (SE, NE, S.21, T.62N., R.14W., NAD 83 UTM 565953E, 5298823N) Description: Two outcrops located within 30 meters north of the Sixmile Lake Road contain relatively thin (<1-2 meters) magnetite-chert horizons interbedded with mafic to intermediate composition volcaniclastic sedimentary strata, and illustrate features common to the uppermost parts of the Lower Ely. Based on detailed mapping by Peterson and Patelke (2003) and Hudak et al. (in prep. a), the uppermost 50-200 meters of the Lower Ely is transitional into the base of the Soudan Iron Formation. Basalt – andesite lava flows and volcaniclastic sedimentary strata decrease in abundance, whereas Algoma-type banded iron-formations become more common as one progresses up-section through this transition zone. This sequence records the change from a period of dominantly effusive submarine volcanism to a period marked by chemical sedimentation. The outcrop closest to the road contains strongly deformed magnetite-chert iron-formation interbedded with iron-chlorite-rich metasediments. Microfossils are locally present in the Vermilion district iron-formations, and hint at a biogenic contribution to their formation (LaBerge, 1973). Deformation has produced more or less north-northwest striking, steeply dipping beds. The deformation that occurs in these strata is not regionally observed, and is believed to be a local phenomenon resulting from syndepositional intrusion of the mafic sill observed in the previous stop. A later quartz-feldspar porphyry dike occurs at the northeastern end of this small outcrop. Note the localized sulfide staining along the contact between the intrusion and adjacent banded iron-formation. The second outcrop, located approximately 30 meters north of the road, contains poorly exposed mafic – intermediate volcaniclastic strata and associated banded iron-formation horizons. The best exposures of the banded iron-formations occur at the southeastern edge of the outcrop. At this location, the magnetite-chert iron-formations locally contain veins and stringers of malachite, along with up to 20% pale brown garnet (x-ray analysis performed by Hudak et al. (in prep. a) indicate that the garnet is andradite). The presence of malachite is indicative of the local high concentrations of copper within the uppermost parts of the Lower Ely. Similar copper mineralization within the Soudan Iron Formation has been identified by Peterson and Patelke (2003) in the 12th level of the Soudan Mine. Page 38 38 �REFERENCES Barrett, T. J. and MacLean, W. H., 1994, Chemostratigraphy and hydrothermal alteration in exploration for VHMS deposits in greenstones and younger volcanic rocks: in Lentz, D. R. (ed.), Alteration and alteration processes associated with ore-forming systems: Geological Association of Canada Short Course Notes, v. 11, p. 433-467. Barrett, T. J., and MacLean, W. 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D., 2002a, Part I: Semiconformable epidote-quartz hydrothermal alteration in the central Noranda Complex, Canada: relationship to volcanic activity and VMS mineralization, in Galley, A., Bailes, A., Hannington, M., Holk, G., Katsube, J., Paquette, F., Paradis, S., Santaguida, F., and Taylor, B., eds., Database for CAMIRO Project 94E07: Interrelationships between subvolcanic intrusions, large-scale alteration, and VMS Deposits: Geological Survey of Canada Open File Report 4431, p. 139-180. Schmidt, R., and Schminke, H.U-., 2000, Seamounts and island building: in Sigurdsson, H. (ed.), Encyclopedia of Volcanoes: Academic Press, p. 361-381. Schulz, K. J., 1982, Magnesian basalts from the Archean terranes of northern Minnesota: in Arndt, N. T., and Nesbitt, E. G. (eds.), Komatiites: Allen and Unwin, London, p. 171-186. Page 43 43 �Seyfried, W. E. Jr., Ding, K., Berndt, M. 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L., (compiler), 1993, Bedrock geologic map of the Soudan-Bigfork area, northern Minnesota: Minnesota Geological Survey, Miscellaneous Map M-79, scale 1:100,000. Southwick, D. L., Boerboom, T. J., and Jirsa, M. A., 1998, Geologic setting and descriptive geochemistry of Archean supracrustal and hypabyssal rocks, Soudan-Bigfork area, northern Minnesota: implications for metallic mineral exploration: Minnesota Geological Survey, Report of Investigations 51, 69 p. Staudigal, H., and Schminke, H.U.-, 1984, The Pliocene seamount series of La Palma, Canary Islands: Journal of Geophysical Research, v. 89, p. 11195-11215. Whitham, A. G., and Sparks, R. S. J., 1986, Pumice: Bulletin of Volcanology, v.48 , p. 209-223. Williams, H. R., Stott, G. M., Heather, K. B., Muir, T. L., and Sage, R. P., 1991, Wawa Subprovince: in Thurston, P. C., Williams, H. R., Sutcliffe, R. H., and Stott, G. M. (eds.), Geology of Ontario, Special Volume 4, Part 1, p. 485-542. Winchester, J. A., and Floyd, P. A., 1976, Geochemical magma type discrimination; application to altered and metamorphosed basic igneous rocks: Earth and Planetary Science Letters, v. 28, p. 459-469. Winchester, J. A., and Floyd, P. A., 1977, Geochemical discrimination of different magma series and the differentiation products using immobile elements: Chemical Geology, v. 20, p. 325-343. Wood, D. A., 1980, The application of a Th-Hf-Ta diagram to problems of tectonomagmatic classification and to establishing the nature of crustal contamination of basaltic lavas of the British Tertiary Volcanic Province: Earth and Planetary Science Letters, v. 50, p. 11-30. Zimanowski, B., and Buttner, R., 2002, Dynamic mingling of magma and liquefied sediments: Journal of Volcanology and Geothermal Research, v. 114, p. 37-44. Page 44 44 �FIELD TRIP 2 Geologic Highlights of New Mapping in the Southwestern Sequence of the North Shore Volcanic Group and in the Beaver Bay Complex Leaders: Terry Boerboom, Jim Miller Minnesota Geological Survey, University of Minnesota and John Green Department of Geological Sciences, University of Minnesota Duluth Split Rock Lighthouse Point (Stop 2-8) Page 45 �FIELD TRIP 2 Geologic Highlights of New Mapping in the Southwestern Sequence of the North Shore Volcanic Group and in the Beaver Bay Complex by Terry Boerboom, Jim Miller Minnesota Geological Survey, University of Minnesota and John Green Department of Geological Sciences, University of Minnesota Duluth INTRODUCTION For the past 20 years, the Minnesota Geological Survey has been actively conducting detailed (1:24,000-scale) bedrock geologic mapping in northeastern Minnesota. This effort began in 1985 with a mapping project focussed on delineating the geology of the intrusive Beaver Bay Complex in the central part of the region. This eight-year project was partially supported by USGS's COGEOMAP program and resulted in publication of five geologic maps (Miller, 1988; Miller and others, 1989, 1993, 1994; Boerboom and Miller, 1994) covering ten 7.5' quadrangles (Fig. 1). More recently, geologic mapping in northeastern Minnesota has focused on the shoreline quadrangles between Duluth and Split Rock Point, where it joins up with the Beaver Bay mapping. This ongoing mapping, which began in 2001, has produced four geologic maps (Boerboom and others, 2002a, 2002b, 2003a, 2003b) with another currently in production (Fig. 2-1). Mapping in the Two Harbors NE quadrangle, scheduled for this coming field season, and publication of quadrangles of the Duluth metropolitan area later this year, will result in 1:24,000-scale coverage of mappable bedrock along the North Shore from Duluth to Tofte. Long-range plans are to continue to systematically map the shore to the Canadian border. Figure 2-1. North Shore 7.5' quadrangles mapped by the MGS through the USGS-sponsored COGEOMAP and STATEMAP programs. Page 46 �This field trip is intended to highlight some of the characteristic and more intriguing aspects of North Shore geology that the recent detailed mapping has revealed. Of course, there are many more interesting geologic features than we can show in two days. Hopefully, you will take advantage of the geologic maps that have been produced of this area to explore more of its fascinating geology. VOLCANIC AND SEDIMENTARY ROCKS OF THE SOUTHWESTERN SEQUENCE OF THE NORTH SHORE VOLCANIC GROUP by John Green This overview is a slightly modified expert from the MGS Report of Investigations 58, Chapter 5: "Volcanic and sedimentary rocks of the Keweenawan Supergroup in Northeastern Minnesota" by John Green (Miller and others, 2002). Magmatic activity related to the 1.1 Ga Midcontinent Rift produced a more than 10 kilometer thick edifice of lava flows and subvolcanic intrusions that are exposed along Minnesota's north shore of Lake Superior. The lava flows and minor sedimentary rocks are referred to as the North Shore Volcanic Group (Goldich and others, 1961) and the intrusive rocks are variably assigned to the Duluth Complex, the Beaver Bay Complex, and miscellaneous intrusions of the Midcontinent Rift Intrusive Supersuite (Fig. 2-3). Although previous publications have subdivided the North Shore Volcanic Group into informal volcanic suites and distinctive flows (Green, 1972, 1982), the 1:200,000-scale map (M-119, Miller and others, 2001) accompanying the report of Miller and others (2002) represents the first time that the North Shore Volcanic Group has been subdivided into formational entities on a geologic map. A brief description of the North Shore Volcanic Group and associated sedimentary formations of the Keweenawan Supergroup is given here to supplement the information provided on the geologic map about these formational units. Rock classification, recognition, and textures As a coherent tholeiitic compositional suite, the volcanic rocks of the North Shore Volcanic Group can be described using only a few rock names (Fig. 2-2). The most primitive rocks are olivine tholeiites, which form an iron-enrichment trend with further evolution. They display ophitic textures and pahoehoe surfaces nearly everywhere. Most olivine tholeiites are aphyric, but those that are porphyritic contain dominantly plagioclase phenocrysts, less commonly olivine. Transitional basalts contain somewhat higher alkalies and other incompatible elements than the olivine tholeiites, but generally not enough to classify them as alkalic. Their texture is typically intergranular and fine- to medium-grained. Porphyritic varieties generally contain small phenocrysts of plagioclase, olivine, clinopyroxene, and magnetite. The reversed-polarity Hovland lavas, however, are characterized by transitional basalts (grading to basaltic andesites) that contain abundant, large, tabular, plagioclase phenocrysts. Transitional basalt flow surfaces are generally smooth (pahoehoe), although a few show breccia tops. The basaltic andesites and andesites (greater than 52 percent SiO2) are tholeiitic, rather than calcalkaline; they show iron enrichment and contain only anhydrous ferromagnesian minerals. These rocks are typically fine-grained and intergranular to felty or pilotaxitic, and many contain small phenocrysts of plagioclase, olivine, clinopyroxene, and magnetite. The andesites generally weather to a red-brown color, and have flow-brecciated (aa) tops but not bases. Many flows that contain 50 to 55 percent silica show millimeter-scale oxidation lamination (Green, 1989) parallel to the base. A few highly iron-enriched flows, separable only by chemical analysis, can be called ferroandesites. Page 47 �Figure 2-2. AFM compositional diagram for the lavas of the North Shore Volcanic Group (modified from Green, 1982). The boundary between tholeiitic and calc-alkalic rocks is modified from Irvine and Barager (1971). FeO* = FeO + 0.9Fe2O3. Carmichael (1964) first used the name "icelandite" for rocks intermediate in character between andesites and rhyolites in the Tertiary lavas of eastern Iceland. They might be considered the tholeiitic equivalent of calc-alkaline dacite in orogenic suites. Other examples of these rocks have been described from the Galapagos (McBirney and Williams, 1969) and the Miocene of Nevada–Oregon (Wallace and others, 1980). Very large flows of similar composition in the Etendeka volcanics of Namibia have been referred to as quartz latites by Milner and others (1992). Icelandites in the North Shore Volcanic Group (Green and Fitz, 1993) are characterized chemically by SiO2 contents ranging from 60 to 68 percent, high FeO* (averaging 7 percent), K2O + Na2O values between 6.5 and 9 percent, a potassium/sodium atomic ratio of about 0.9, and an Mg number [Mg/(Mg + Fe) atomic] averaging 0.14. Petrographically, North Shore Volcanic Group icelandites grade continuously from the andesites to somewhat paler colors (brown or tan), but have a similar phenocryst assemblage. Quartz and alkali feldspar are common in the groundmass but never occur as phenocrysts. Flowtop features (crusty to coarsely brecciated) indicate that the icelandites were erupted as lavas. The rhyolites have higher silica and total alkali contents, and lower FeO than the icelandites. They are generally light gray, pink, or red. Several are very thick, extensive, and voluminous (up to several hundred cubic kilometers; Green and Fitz, 1993). Although most rhyolites are porphyritic (phenocrysts of quartz, alkali feldspar ± plagioclase, and altered fayalite ± ferroaugite), some lack quartz and alkali feldspar phenocrysts, and rare flows are aphyric. Groundmass textures are fine-grained holocrystalline, typically with a meshwork of platy quartz paramorphs after primary tridymite, which may show a flow structure. A "snowflake" texture is common, in which poikilitic quartz patches (coalesced ex-tridymite grains) enclose small, dusty alkali-feldspar grains. In quartz-phyric flows, these poikilitic quartz patches are in optical continuity with adjacent quartz phenocrysts (Green, 1990). Outcrop-scale flow structure, including folding, is common near flow tops and bases. In some flows, distinct fiamme, deformed to varying degrees, are recognizable. These imply an explosive eruption that produced a pyroclastic flow, which welded and remobilized to produce a rheoignimbrite. Page 48 �Structure and Lithostratigraphy As stated above, the Midcontinent Rift volcanic rocks and interbedded redbeds in northeastern Minnesota comprise the North Shore Volcanic Group (Goldich and others, 1961; Green, 1972, 1977, 1982; Basaltic Volcanism Study Project, 1981). In general, these rocks form an arcuate stack that is slightly tilted toward the southeast and forms the roof rocks into and under which the Duluth Complex and associated hypabyssal intrusions were emplaced. At the southwest end of the North Shore Volcanic Group near Duluth, the volcanic rocks strike north with a 10º to 20º easterly dip; at the northeast end at Grand Portage, the flows strike east–west with a 10º southerly dip (Fig. 2-3). Thus, traveling northeast along the Lake Superior shore northeast from Duluth, and southwest from Grand Portage, one encounters successively higher flows in the volcanic stratigraphy until the Tofte–Lutsen area in southern Cook County, where the highest exposed flows crop out. Exposure is generally excellent along the eroding lakeshore, and along the lower, high-gradient stretches of tributary streams, providing good control on the stratigraphy. A stack of volcanic rocks approximately 9.7-kilometers-thick has been measured in the “southwest limb” (Table 2.1), and another stack of volcanic rocks about 7.2-kilometers-thick has been measured in the “northeast limb” (See Table 5.2, Miller and others, 2002). This implies nearly continuous subsidence during the rifting process. The difference in stratigraphic thickness between the two limbs reflects major complications in the central area, which appears not to have subsided at the same rate as in the limbs, and into which many of the subvolcanic intrusions were emplaced. Except for the capping Schroeder–Lutsen basalt sequence, no stratigraphic unit can be traced from one limb to the other; each limb has its own stratigraphic column. To aid in the correlation of intrusive and extrusive rock units throughout the Midcontinent Rift system, their paleomagnetic polarity has been used. Nearly all of the igneous and sedimentary rocks associated with the Midcontinent Rift were formed either during an earlier, reversed-polarity interval or a succeeding normal-polarity interval. Thus, in each limb of the North Shore Volcanic Group, the lower stratigraphic units show reversed polarity, and the upper sequences show normal polarity. This polarity reversal forms the basis for distinguishing upper and lower sequences in the northeast and southwest limbs. U-Pb zircon dates demonstrate that the reversed-polarity magmatism occurred mainly in the time interval from 1108 to 1107 Ma; whereas, around the Lake Superior basin, normal-polarity magmatism occurred mainly in the interval from 1099 to 1094 Ma (for example Davis and Paces, 1990; Paces and Miller, 1993; Davis and Green, 1997). These two pulses were separated by a magmatically inactive time (at least in the upper crust), which appears to be expressed as a slight unconformity in the volcanic sequence on the north shore. However, because intrusions subsequently penetrated along this horizon in the North Shore Volcanic Group, this unconformity has not been recognized in outcrop. The Duluth Complex separates the upper and lower sequences of the southeastern limb (Fig. 2-3, Table 2.1). One other significant gap in the stratigraphic continuity of the North Shore Volcanic Group occurs near the stratigraphic top, where the Schroeder–Lutsen sequence overlies the upper units of the northeast and southwest sequences (Fig. 2-3, Table 2.1). In the northeast limb, southwest of Grand Marais (Fig. 2-3), the basal flow of the Schroeder–Lutsen sequence (Terrace Point basalt member) overlies a thick sandstone and siltstone unit (Cut Face Creek sandstone), which in turn conformably overlies the Good Harbor Bay andesites of the upper northeast sequence (See Table 5.2, Miller and others, 2002 for details). However, in the southwest limb, the basal Schroeder–Lutsen sequence flow overlies a thinner sandstone and conglomerate unit (the Little Marais conglomerate) that in turn rests in sharp angular unconformity atop structurally disturbed flows of the Belmore Bay lavas near Little Marais (Table 2.1). Furthermore, the gently dipping Schroeder basalts have not been penetrated by the abundant hypabyssal intrusions of the Beaver Bay Complex that complicate the underlying volcanic sequence in this mid-shore area (Green, 1992; Miller and others, 1993). Attempts to date the Schroeder–Lutsen sequence have been unsuccessful to this point. Page 49 �______ ______ ' MESOPROTEROZOIC (KEWEENAWAN) INTRUSIVE ROCKS Beaver Bay Complex and Felsic rocks Misc. Intrusions 1 Mafic rocks Mafic rocks Duluth Felsic rocks Complex NoRTH SHORE VOLCANIC GROUP PALEOPROTERZOIC RoveNirgin ia/Thomson Fm ' General attitude of volcanic rocks Gunflint/Bwabik Fe-Fm ARCH EAN I Southwest Lower 0 10 20 30 40 50 Kilometers Granitoid Intrusions Supracrustal Rocks Sequence Figure 2-3. Simplified geology of northeastern Minnesota showing the major sequences of the North Shore Volcanic Group (taken from Fig. 5.2, Miller and others, 2002). The five lithostratigraphic sequences comprising the two limbs of the North Shore Volcanic Group are further subdivided into informal formational units (Table 2-1). Some formational units are individual flows of distinctive lithology and/or substantial thickness and lateral extent. Most are suites of lava flows that have distinct lithologic characteristics or that are separated by intrusions. Some lava formations contain distinct flows or sedimentary rock units within an otherwise homogeneous package of lavas. Such units are given informal member rank (such as the Silver Beaver rhyolite member of the Baptism River lavas, Manitou transitional basalt member of the Schroeder basalts, and Indian Camp sandstone member of the Lutsen basalts). See Chapter 1 of RI58 (Miller and others, 2002) for more details on stratigraphic nomenclature of Keweenwan rock in northeastern Minnesota. Page 50 �Table 2-1. Generalized stratigraphy of the southwest limb of the North Shore Volcanic Group showing U/Pb ages (Davis and Green, 1997; Green and others, 2001). Positions of intrusions denote approximate stratigraphic level affected and not age of emplacement. Thickness(m) Lithostratigraphic units Lithologic character U/Pb ages 9735 945 Total section 150 Carlton Quarry lavas (fault bounded) Schroeder–Lutsen sequence (normal polarity) 900 Schroeder basalts <45 Little Marais conglomerate basalt, andesite, and rhyolite flows 1094.3±2.0 ophitic olivine tholeiite basalt flows; includes Manitou transitional basalt and Pork Bay breccia polymict volcanic conglomerate and sandstone angular unconformity 8275 Upper southwest sequence (normal polarity) 565 Bell Harbor lavas 100 Palisade Head rhyolite mostly quartz tholeiite basalt and basaltic andesite flows gray-pink, porphyritic rhyolite flow ~1096 Beaver Bay Complex 700 1096.6±1.7 mixed lavas, mostly basalt; includes Baptism River lavas 165-meter-thick Silver Beaver rhyolite 20 Silver Bay porphyritic basalt ophitic basalt flow with abundant large plagioclase phenocrysts 730 Gooseberry River basalts mixed basalt flows, mostly ophitic Lafayette Bluff, Silver Creek diabase intrusions 315 Two Harbors basalts 550 Larsmont basalts mixed aphyric basalt flows; quartz tholeiite flows at base ophitic olivine tholeiite flows Stony Point–Knife Island diabase sheet 1500 Sucker River basalts mixed basalt flows, mostly ophitic 1350 Lakewood lavas mostly basalt flows; rhyolite, icelandite, and ferroandesite at base Lester River diabase sill 1285 mixed basalt, andesite, icelandite, and rhyolite flows Lakeside lavas 1098.4±1.9 Endion diabase sill 1160 mixed basalts, andesites Leif Erickson Park lavas ~1099 Duluth Complex 370 Lower southwest sequence (reversed polarity) 370 Ely's Peak basalts porphyritic, diabasic, and ophitic basalts; pillowed and pyx-phyric basal flow >8 Nopeming Sandstone white to tan quartzite and conglomerate angular unconformity Thomson Formation (Paleoproterozoic) Page 51 �Physical volcanology The volcanic rocks of the Midcontinent Rift, including the North Shore Volcanic Group, represent one of the world’s oldest and best-preserved examples of plateau lavas. However, they are not typical in that they contain a greater thickness of flows and, in the North Shore Volcanic Group, a higher proportion of evolved compositions. They are similar physically and chemically to the Tertiary lavas that make up eastern Iceland (Sigvaldason, 1974; Green, 1977; Wood, 1978), and they formed similarly over a plume at another major rift. The rocks also resemble the late Tertiary and Quaternary volcanic rocks of the southern Snake River Plain, Idaho and southeastern Oregon, because of their interbedded basalts and large rhyolites (e.g., Bonnichsen and Kauffman, 1987; Manley, 1996). The basalts range in character from typical flood flows as voluminous as tens of cubic kilometers to more modest, “plains-type” flows (Greeley, 1982) and thin flow units less than a meter thick. At Duluth in the southwest limb, and Grand Portage in the northeast limb, the lowest flows in the volcanic sequences are pillowed, and thus inferred to have erupted subaqueously; however, nearly all of the other flows were erupted subaerially. The flows show different physical characteristics, closely tied to their chemical compositions and viscosities (Green, 1989; Green and Fitz, 1993). Olivine tholeiites, which dominate the North Shore Volcanic Group, all have pahoehoe surfaces, with or without ropy structures. Other physical characteristics of the various rock types were previously discussed in the “Rock classification, recognition, and textures” section of this guide. All of the flows ranging in composition from basalts to icelandites were erupted as lavas. The rhyolites are notable in their abundance relative to other plateau-lava sequences, their size (up to several hundred cubic kilometers), and extent (Green and Fitz, 1993). Several rhyolites show textural evidence of rheomorphic flow after eruption as ash-flow tuffs, though some were lavas. One of the largest, the Devil Track rhyolite in Cook County, which is as thick as 250 meters and can be traced for 40 kilometers along strike, has ambiguous features that make its mode of eruption difficult to discern; it may be a lava flow. Nearly all the icelandites and rhyolites show evidence of an unusually high temperature of eruption, such as magmatically crystallized groundmass tridymite. The evidently low viscosity of these large rhyolites is attributed to their high temperature, high iron and fluorine contents, and low oxidation state (Green and Fitz, 1993). Geochemistry and chemostratigraphy The North Shore Volcanic Group constitutes a subalkalic, tholeiitic suite that ranges continuously from rather primitive olivine tholeiite to rhyolite, and shows a strong iron-enrichment trend (Fig. 2-2; also Basaltic Volcanism Study Project, 1981; Brannon, 1984). However, relative abundances are strongly bimodal; basalts are greatly predominant, but rhyolites make up 10 to 25 percent of the section. The basalts show trace element and isotopic evidence of derivation mostly from a mantle plume (Nicholson and others, 1997), whereas most of the rhyolites include major contributions from partial melting of the Archean basement (Vervoort and Green, 1997). The most common basalt type, ophitic olivine tholeiite, is generally aluminum-rich (16 to 18 percent Al2O3); the most primitive flows have Mg numbers of about 0.65 to 0.68. The basal few flows in both limbs of the North Shore Volcanic Group have a unique geochemical and petrographic character. Typically they contain augite phenocrysts, are aluminum-poor, and are rich in both compatible (chromium and nickel) and incompatible elements (titanium, phosphorus, and lanthanum) with steep chondrite-normalized lanthanum/ytterbium ratios. This suggests derivation by relatively small-fraction melting of the initial plume head (Nicholson and others, 1997; Green, 1995). In general, there is little stratigraphic regularity of compositional change within the North Shore Volcanic Group, with the following exceptions. In the middle of the upper southwest sequence, there is a marked upward progression toward more primitive compositions through a 3.4-kilometer section from rhyolite east of the Lester River into a thick group of primitive olivine tholeiites in the Knife River–Two Page 52 �Harbors area (Brannon, 1984). This includes the Lakewood lavas, the Sucker River basalts, and the Larsmont basalts. In contrast, in the lower northeast sequence, the approximately 1-kilometer-thick basal Grand Portage lavas progress upsection from basalt to increasingly evolved compositions, ending with Red Rock rhyolite (Green, 1995). As mentioned above, the Schroeder–Lutsen sequence, the youngest in the North Shore Volcanic Group, is composed almost entirely of olivine tholeiites. All of the North Shore Volcanic Group has been affected to some degree by hydrothermal/burial metamorphism. The more permeable (fractured, vesicular) tops and bases of the flows have undergone considerable mineralogical change (deposition of amygdule minerals, alteration of primary minerals), but in many cases the massive flow interiors are remarkably little-altered. Where alteration has approached equilibrium, mineral assemblages range from lower greenschist facies at the base of the North Shore Volcanic Group to zeolite facies at the top (Schmidt, 1993; Schmidt and Robinson, 1997). Interflow Sedimentary Rocks Clastic redbed strata occur at many horizons within the North Shore Volcanic Group (Jirsa, 1984). They are lenticular and range in thickness from a few centimeters to about 100 meters. As these rocks are relatively soft and erodable compared to the adjacent volcanic flows, they are mostly covered and are exposed only along actively eroding sites such as streambeds and the lakeshore. They are predominantly red to brown, well sorted sandstone, with minor conglomerate, siltstone, and shale. Conglomerate beds are most abundant in the midshore area from Little Marais to Lutsen. Compositionally, these redbeds are mainly immature lithic arkose and feldspathic lithic arenite (see Fig. 5.3, Miller and others, 2002). The angular to subrounded clasts are mainly plagioclase, mafic to felsic volcanic rock fragments, clinopyroxene, and Fe-Ti oxides; devitrified or replaced volcanic ash particles and shards are present in a few beds. Quartz is uncommon to absent. The framework grains have been variably cemented with hematite, calcite, prehnite, and a variety of zeolites, depending on the local hydrothermal/burial/contact metamorphic conditions. In some places hydrothermal minerals have replaced many or most of the clasts. A few of these redbed units have thicknesses in excess of 25 meters. These include a crossbedded sandstone in Leif Erickson Park in Duluth (35 meters), which disconformably overlies an eroded basalt flow; the Little Marais conglomerate (and sandstone) exposed in the Manitou River area near Little Marais (as thick as 45 meters); the Indian Camp sandstone (68 meters) northeast of Lutsen; and the Cut Face Creek sandstone southwest of Grand Marais (100 meters), which can be traced for at least 4 kilometers along strike. Of these, the Little Marais and Cut Face Creek units occur at the base of the Schroeder–Lutsen basalt sequence. The sandstone in these units is typically planar- or cross-bedded, and some beds are ripple-marked or mud-cracked. The rocks are inferred to be dominantly fluvial, deposited by moderate-gradient, east- to southwest-flowing streams from sources nearly entirely within the subsiding Midcontinent rift basin (Jirsa, 1984). Many flow-top breccias of andesite and basalt with aa structure contain laminated red sandstone as a matrix because sand filtered down from the flow surface. Similarly, red, laminated sandstone and siltstone form clastic dikes or crevice-fillings a few centimeters wide in the upper parts of some lava flows. Page 53 �INTRUSIONS WITHIN THE SOUTHWESTERN SEQUENCE OF THE NSVG by Terry Boerboom Several hypabyssal sills and sheet-like intrusions occur within the 7-kilometer-thick section of volcanic rocks exposed near Lake Superior from Duluth to Split Rock Point. From lowest to highest in the section, these consist of the Endion Sill, the Northland sill, the Lester River Sill, the Stony Point diabase, the Silver Creek diabase, the Lafayette Bluff diabase, and the Split Rock intrusive felsite. These sheets are variably concordant to the volcanic strata, range from tens of meters to over 500 meters thick, and generally dip gently eastward. They commonly form prominent topographic highlands. This discussion will be limited to those sills visited by this field trip. For a more complete description, see Miller and others, 2002. Lester River diabase The Lester River diabase is named for the outcrops adjacent to the mouth of the Lester River (Schwartz and Sandberg, 1940). The diabase forms a prominent topographic high (Moose Mountain) that extends about 6.5 miles inland from Lake Superior. The diabase forms a sill some 400-m (1,300-feet) thick that dips an average of 19 degrees southeast, conformable with the dip of the surrounding volcanic rocks. The sill generally overlies basalt and underlies rhyolite, the latter of which probably formed a density barrier that trapped the more dense mafic magma as it was emplaced. The upper margin of the sill contains abundant intermediate to felsic rocks that form a continuous, mappable unit along the entire mapped length of the sill, in contrast to the lower margin which contains only thin and discontinuous pods of the hybrid felsic rocks. The hybrid cap rocks likely formed by partial melting of the overlying rhyolite by heat generated from the underplating sill. Jerde (1991) provides considerable geochemical, petrographic, and petrologic data on the Lester River Sill. Most of the Lester River diabase is a dark grayish-black, medium- to coarse-grained, massive to locally weakly porphyritic diabase. The diabase contains approximately 60% plagioclase, 13% intergranular to subophitic augite, 12% variably altered olivine, 0-4% orthopyroxene, 1-4% Fe-Ti oxides, 0-10% quartz and granophyre, 1-3% diktytaxitic chlorite, and less than one percent apatite, biotite, and hornblende. Quartz commonly contains wispy rutile needles. Silver Creek diabase The Silver Creek diabase is named for proximity to Silver Creek, near Silver Cliff, located about 4 miles northeast of Two Harbors. This diabase is composed almost entirely of medium-grained ophitic olivine diabase, with augite oikocrysts from 1 to 5 centimeters in diameter. It forms a subcordant to discordant subhorizontal sill-like intrusion, at least 60-m (200-feet thick), that forms a prominent highland projecting inland about 7 kilometers north-northeast of Lake Superior, and is exposed intermittently for several kilometers beyond. In the area of the Encampment River, the diabase is in subvertical contact with surrounding volcanic and earlier intrusive rocks; this may have been the feeder to the sill. The diabase is free of anorthosite inclusions and contains only rare xenoliths of volcanic rocks. Pope (1976) studied the Silver Creek diabase as part of Master’s thesis under the guidance of Dr. John Green. The Silver Cliff tunnel on Highway 61 cuts through this diabase. Lafayette Bluff diabase The Lafayette Bluff diabase is named after prominent exposures at Lafayette Bluff, forms prominent knobs near Lake Superior but is typically weathered to a brown crumbly grus. It is an irregular, discordant, sheet- to dike-like body composed of soft, dark greenish-black, amygdaloidal and porphyritic Page 54 �diabase, with local differentiated masses of coarse-grained to pegmatitic ferromonzodiorite inland from Lake Superior. The diabase contains locally abundant 1-3 meter diameter inclusions of anorthosite and scattered xenoliths of metamorphosed basalt. The Lafayette Tunnel on Highway 61 has been excavated through the Lafayette Bluff diabase. Split Rock intrusive felsite The Split Rock intrusive felsite (rhyolite) is a pink, fine-grained, weakly porphyritic, flow-banded rock that has a prominent shingle parting to it (hence: Split Rock River). At a given location, the rhyolite has all the physical attributes of a rhyolite flow, but detailed mapping shows that it forms a north-striking, 3.4 kilometer wide body that cuts north from Lake Superior across the stratigraphy of the surrounding volcanic rocks. The felsite forms cliffs on Lake Superior south of the Split Rock River, and in the Split Rock River and nearby streams forms small, sharp canyons. Field measurements show that the flow banding dips gently towards the central axis of the body, implying that the intrusion is keel-shaped. The lower margins of the rhyolite are intruded by, or commingled with, a dark gray, fine-grained ferrodiabase or ferrodiorite. This felsite is exposed in the Split Rock Point 7.5’ quadrangle, which is currently a work in progress as one of the USGS STATEMAP series of geologic maps to be published by the Minnesota Geological Survey. Due to time constraints, this field trip will not visit this unit. GEOLOGY OF THE BEAVER BAY COMPLEX by Jim Miller The Beaver Bay Complex (BBC) is a hypabyssal, multiple-intrusive igneous complex that was emplaced into the upper part of the NSVG over a 600-km2 area in northeastern Minnesota (Fig. 2-4). Much of this area was the focus of detailed bedrock mapping by the Minnesota Geological Survey between 1985 and 1992 (Miller, 1988; Miller and others, 1989, 1993a, 1994; Boerboom and Miller, 1994). Three general areas of the BBC, southern, northern, and eastern, are distinguished on the basis of distinctive rock types and intrusion form (Miller and Chandler, 1997). This field trip will investigate some of the units composing the southern and northern BBC. The relationship of BBC intrusions to other subvolcanic intrusions within the NSVG (Fig. 2-4) is unclear, because of poor exposure to the southwest and insufficient mapping to the northeast. Within the mapped area of the BBC, thirteen intrusive units have been identified that represent at least six major intrusive events (Miller and Chandler, 1997). Most intrusive activity forming the BBC occurred around 1096 Ma based on U-Pb dates of 1095.8±1.2 Ma for a Silver Bay intrusion, the youngest unit of the BBC, and 1096.1±0.8 Ma for the Sonju Lake intrusion (Paces and Miller, 1993). Whether activity overlapped the main stage of Duluth Complex magmatism at 1099 Ma is unknown, because attempts to date the oldest component of the BBC were not successful (Paces and Miller, 1993). The boundary between the BBC and Duluth Complex is generally marked by a northeast-trending keel-shaped intrusion in the northern BBC (Houghtaling Creek troctolite) that separates largely dike and sill intrusions of the BBC to the southeast from massive granophyric granite and extensive areas of structurally complex gabbroic anorthosite to the northwest that are typical of the roof zone of the Duluth Complex. Page 55 �Figure 2-4: Geology of the southern and northern Beaver Bay Complex showing the locations of Stops 8-16. After Miller and others (2001). Units labels are: nsb - NSVG basaltic volcanics; nss - NSVG Schroeder basalts; nsf - NSVG felsic volcanics; asa anorthositic series of the Duluth Complex; fs - felsic series of the Duluth Complex ; slid - Shoepack Lake inclusion-rich diorite; ccpd - Cabin Creek porphyritic diorite; hct Houghtaling Creek trocolite; blg - Blesner Lake diorite; llg - Lax Lake gabbro; frg Finland granophyre; frqm - Finland qtz ferromonzonite; slt - Sonju Lake troctolitic zone; slg - Sonju Lake gabbroic zone; brd Beaver River diabase; sbi - Silver Bay intrusions. See map M-119 for more details. Page 56 �The range of BBC parent magma compositions is similar to the olivine tholeiite and transitional basalt compositions that dominate the NSVG (Fig. 2-5A). Moreover, like the NSVG, the sequence of intrusion of BBC magmas generally involved progressively more primitive compositions. Compositional variations within the various intrusive units developed as a result of in situ magmatic differentiation (Fig. 2-5B), assimilation of footwall rocks, and/or composite intrusions of evolved magma from deeper staging chambers (Fig. 2-5C). The tightly clustered trend of BBC parent magma compositions evident on an AFM diagram (Fig. 2-5A) and the systematic variation of other elemental abundances suggest that all mafic BBC magmas evolved from a common olivine tholeiitic primary magma type. Such a primary composition, which is approximated by the most primitive, high-Al olivine tholeiites of the NSVG, is thought to have given rise to most Midcontinent Rift magmas, especially in later stages of magmatism (Green, 1982; Miller and Weiblen, 1990; Klewin and Shirey, 1992). That even the most primitive of the BBC intrusions, the Beaver River diabase, is significantly evolved from a primitive olivine tholeiite composition (Fig. 2-5A) indicates that all BBC parent magmas were generated in turn by magmatic differentiation of such a primary composition in deeper staging chambers. Petrologic modeling of some BBC intrusions and other hypabyssal bodies that intruded the NSVG (Jerde, 1991) suggests that most magmas experienced multistage, polybaric fractionation between their extraction from the mantle and their subvolcanic emplacement. Although the available radiometric ages do not indicate an overlap of magmatic activity between the BBC and the Duluth Complex (Paces and Miller, 1993), additional dates and more detailed petrologic studies may show that some Duluth Complex intrusions acted as the final levels of staging and differentiation of some BBC-bound magmas. The focus of emplacement of BBC intrusions appears to have migrated toward the rift axis and toward higher stratigraphic levels with time, perhaps reflecting plate drift and thickening of the volcanic pile. Over the exposed extent of the BBC, intrusion shapes appear to have been controlled by a shallow crustal ridge (Schroeder Forest Center Crustal Ridge) which trends northwest across the BBC. The presence of this buried crustal ridge is indicated by a pronounced saddle in the gravity high over northeastern Minnesota (Chandler, 1990) and the presence of Archean-like inclusions of granitic gneiss, biotite schist, metagraywacke and granodiorite in early intrusions over the gravity low (Boerboom, 1994). The broad network of dikes and sheets, characterizing the southern BBC, becomes tightly focused into a narrow zone of subparallel dikes in the northern portion of the BBC that is situated over the gravity minimum. The eastern BBC opens up again into thick sheet intrusions. Based on geologic, geochronologic, geophysical, and geochemical evidence, Miller and Chandler (1997) suggested that the BBC, particularly the youngest Beaver River diabase dike and sheet network, acted as a magma conduit and structural boundary to the formation and infilling of the western end of the Portage Lake Volcanic basin during the main to late stages of rift volcanism and graben formation. On the North Shore, these volcanic rocks are represented by the Schroeder-Lutsen basalts (Fig. 2-2). The second half of this trip will investigate seven intrusive units of the BBC and rocks comprising the roof zone of the Duluth Complex (Fig. 2-4). Stops 2-8 through 2-11 will investigate the composite intrusions of the Beaver River diabase and Silver Bay intrusions, the youngest intrusive components of the BBC. Stop 2-12 will visit typical exposures of the Finland granophyre. Stop 2-13 will traverse the middle cumulate units of the Sonju Lake Intrusion - the most completely differentiated intrusion exposed in the Midcontinent Rift system. Stop 2-14 will examine the units defining the northeastern limit of the Beaver Bay Complex - the Houghtaling Creek troctolite, Dam Five gabbronorite, and the Shoepack Lake inclusion-rich diorite. Finally, Stops 2-15 and 2-16 will view anorthositic and granophyric rocks that comprise the roof zone of the Duluth Complex. For more detailed information on these and other units of the Beaver Bay Complex see reports by Miller and Chandler (1997) and Miller and others (2002, Chapter 7) and the 1:24,000 bedrock geologic maps of the area (Miller, 1988; Miller and others, 1989, 1993, 1994; Boerboom and Miller, 1994). Page 57 �Figure 2-5: AFM diagrams of Beaver Bay Complex (BBC) intrusions. A) Plot of estimated parental magma compositions to various BBC intrusions (see Miller and Chandler, 1997 for unit descriptions and details) compared to major NSVG lava compositions: OT - olivine tholeiite, TB - transitional basalt, A - andesite, FA - ferroandesite, I icelandite, R - rhyolite (after Green, 1983). NSVG-pot is a primitive NSVG olivine tholeiite composition. B) Calculated liquid line of descent of the Sonju Lake intrusion through troctolitic (slmt-sld-slt), gabbroic (slg-slfgslad) and monzodiorite (slmd) intervals of the layered sequence. Also plotted are whole rock compositions of Finland granite (frg) and quartz ferromonzodiorite (frpm). C) Whole rock composition plots of Beaver River diabase (brd; ophitic margins and coarse subophitic interiors distinguished), Silver Bay intrusions (sbi; coarse marginal facies, layered ferrogabbroic cumulate interiors, and granophyric compositions distinguished), and composite intrusions from the northern BBC (nbbc). Sonju Lake intrusion differentiation trend (dashed line) is also shown. Page 58 �FIELD TRIP STOPS The general locations of all stops are shown on Figure 2-6. A more detailed location map is included with each stop description, with a base made at an appropriate scale from a portion of a 7.5’ quadrangle map. This should allow the return visitor to accurately locate the features for each stop. The UTM coordinates listed are all given in NAD83, with easting listed first and northing listed second. The regional setting of the various rock units are given in the introduction, and only facts pertinent to each specific stop are given below. Figure 2-6. Location map for stops for Field Trip 2. Page 59 �DAY 1 GEOLOGY OF THE SOUTHWESTERN SEQUENCE OF THE NORTH SHORE VOLCANIC GROUP FROM DULUTH TO SPLIT ROCK POINT Drive Highway 61 from Duluth towards Two Harbors. At the outskirts of Duluth, just after the Lester River, veer right onto Scenic Highway 61 for approximately one-half mile, then turn right on the road that leads to Kitchee Gammee park. Park near the northeast end of this road and walk down to lakeshore. STOP 2-1: Lester River Diabase in contact with rhyolite T.50N., R.13W., Sec. 4 Lakewood 7.5’ quadrangle. UTM: Start at SW 576929E, 5188417N; End at NE 577390E, 5188981N Highlights: Lester River diabase (lrd), hybrid granophyric rocks (lrf, lrg), rhyolite (nr); late diabase dikes (bdb), glacial striations. The Lester River Diabase is named for the outcrops adjacent to the mouth of the Lester River (Schwartz and Sandberg, 1940). The diabase sill dips southwest about 19 degrees, and forms a prominent topographic high (Moose Mountain) that extends about 6.5 miles inland from Lake Superior. This diabase is mostly intergranular in texture, but does locally exhibit ophitic textures, as seen at this stop. The dominantly intergranular to relatively fine ophitic (less than 1.5 centimeter) texture of the Lester River diabase contrasts with most other Keweenawan diabases such as the Silver Cliff and Beaver River diabases, which typically exhibit a coarse ophitic texture. This stop starts in the massive interior of the sill, and traverses northeast through increasingly felsic rocks that range from ferrodiorite to granite, and ends at a cliff of fine-grained chilled diabase that cuts the preceding rocks. Felsic rocks first show up as irregular felsic dikes that cut the diabase, but moving northeast towards the rhyolite not only do the felsic dikes increase in abundance, but the diabase itself becomes increasingly granophyric until it grades into dioritic rocks. The granophyric felsic hybrids blend into pink, fine-grained, porphyritic rhyolite, informally termed the Lakewood rhyolite (Green and Fitz, 1993). The exact contact between rhyolite and granophyric hybrid rocks is hard to pinpoint due to the similarity in textures between recrystallized rhyolite and the hybrid melts. Note the thin, shorelineparallel, fine-grained diabase dikes that cut the Lester River diabase. On a map scale, the granophyric hybrid phases of the diabase are thick and continuous along the upper sill margin, in contrast with the sill base where hybrid rocks are thin and discontinuous. The diabase sill is sandwiched between rhyolite in the hanging wall, and basalt in the footwall, indicating that the rhyolite acted as a density barrier that trapped and ponded the mafic magma beneath it. The irregularly mixed and distributed hybrid rocks that cap the Lester River diabase are likely due to mixing of remobilized (melted) rhyolite with the underlying mafic magma. Page 60 �--- Climb up bank , walk NE approximately 0.1 mile to small clearing on east side of highway, climb down to lakeshore, stop 2.2. Alternatively walk to next wayside, climb down, and walk back southwest along lakeshore to basalt/rhyolite contact.--- STOP 2-2: Flow contact between rhyolite and basalt of olivine tholeiite composition T.51N., R.13W., Sec. 34 Lakewood 7.51 quadrangle UTM: 577648E, 5189145N Highlights: Contorted flow-banded rhyolite (nr), contact of basalt (nbd) with rhyolite, ropy flow tops in olivine tholeiite (‘diabasic’-textured) basalt flows. Walk southeast to the pink-tinted rhyolite flow that exhibits highly contorted and folded flow banding. Rhyolite contains amygdules of calcite and minor fluorite (please don’t beat up the outcrop with hammers), and is capped by a thin, discontinuous layer of fine-grained sandstone. Move northeast over four basalt flows and observe flow features such as (bottom to top of flow) pipe vesicles, vesicle cylinders, amygdaloidal zones in upper portions of flows, and ropy pahoehoe upper surfaces. The pahoehoe crusts indicate a general southeast flow direction. These basalt flows are of weakly alkaline, transitional intermediate olivine tholeiite composition, with typical olivine tholeiite flow structures. On recently published 1:24,000 scale geologic maps (e.g. Boerboom and others, 2002a), flows such as this are classified as “diabasic-textured’ flows. Our field mapping has shown that the textures of the basalt flows does not always correspond to a unique chemical composition, but rather may have more to do with the amount of volatiles in the lava. Some of the most coarse-grained diabasic-textured basalt flows were mapped in the Sucker River upstream from Old North Shore Road. Despite being relatively thin flows (1-3 meters thick), they exhibit a medium- to coarsegrained felty diabasic texture. Some have a coarse pegmatitic zone located about one-third of the way down from the top of the flow, attributed to the concentration of volatiles in this part of the flow as cooling proceeded from the top and bottom. These coarse-grained flows also have unusually prominent pipe amygdules at the base overlain by a zone of bead-like pipe amygdules/vesicle cylinders up to one meter in length that end below the most coarse-grained part of the flow. The flows observed in stops 2-1 through 2-3 are all part of the Lakewood lavas (see table in introduction), a lithostratigraphic unit characterized by multiple thin ‘packages’ that range in composition from rhyolite to olivine tholeiite. Each flow package consists of multiple flows of like composition. Brannon (1984) conducted a detailed sampling regime of virtually every flow exposed along the shoreline from Duluth to Two Harbors, starting with the rhyolite just above the Lester River diabase. Her geochemistry verifies the mapped stratigraphy and shows that the most silicic rocks (rhyolite, icelandite, and andesite) occur at the base of the Lakewood lava sequence, whereas primitive tholeiitic basalts dominate the top of the sequence. Vervoort and Green (1997) have shown, through Nd isotopic studies, that the NSVG rhyolites are the product of melting of Archean crustal rocks by underplated primitive Page 61 �mafic magma, whereas icelandite and basalt are most likely derived directly from a mantle plume source. In their model, early volcanism (lower reversed sequence) tapped a mantle source with little crustal contamination. This was followed by a period of volcanic quiescence (the 1107 -1100 Ma ‘volcanic hiatus’) during which a mantle plume underplated and melted the crust, generating superheated felsic melts that were later erupted as rhyolite flows and rheoignimbrites, of unusually wide areal extent given their thickness (Green and Fitz, 1993) --- Climb up bank of small stream to wayside rest (this is the third wayside rest on Scenic Highway 61 past Brighton Beach Road), drive or walk approximately 0.3 miles northeast to wayside rest directly across from Old North Shore Cottages Resort, stop 2-3.--- STOP 2-3: Interflow sandstone between icelandite and basaltic andesite flows T.51N., R.13W., Sec. 34 Lakewood 7.51 quadrangle UTM: 578287E; 5189652N Highlights: Interflow sandstone above icelandite (ni), below basaltic andesite (nba) Park at wayside across from North Shore Cottages Area A: Slide down the embankment below wayside (over red clays of Glacial Lake Duluth) to outcrop of sandstone on shoreline. Sandstone is approximately 12-15 feet thick, planar and trough crossbedded. Jirsa (1984) reports the sand grains are predominantly plagioclase (44-53%) and mafic volcanic rock fragments (18-25%), with lesser quartz (1-4), opaques (2-8%), K-spar, felsic volcanic rock fragments, and mudstone fragments (less than 2% each). The sandstone is cemented by a matrix of calcite, zeolite, K-feldspar, and quartz. Crossbedding indicates an easterly paleocurrent direction. The sandstone overlies an icelandite flow which has a rubbly flow top (and unfortunately is quite inaccessible due to cliffs), and is overlain by a flow of basaltic andesite composition (will examine these at site B). The upper surface of this interflow sandstone contains curly molds and bits of baked-on chilled basalt from the overlying flow, which is now eroded away. This sandstone is typical of the thin interflow sedimentary rocks, which are immature, fine- to coarsegrained sediments that were deposited in fluvial and lacustrine environments, although some may interpret some of the bedding in this outcrop to be aeolian in nature. Overall, paleocurrent indicators for the interflow sandstones indicate a transport direction generally toward Lake Superior from a highland of both Keweenawan and pre-Keweenawan sources (Jirsa, 1984). ---Climb back up bank and move about 100 feet northeast and return to lakeshore outcrops.--- Page 62 �Area B: Basaltic andesite. The base of this flow sequence was observed at stop 2-3a. These flows are dark purplish-gray, fine-grained basaltic andesite, which locally have pipe vesicles at the base, and amygdule-rich upper parts that exhibit billowy lobate forms defined by concentrated amygdules. Flow tops are commonly brecciated. Some flows contain sparse plagioclase phenocrysts. Due to time constraints we will not progress up the shore, but the return visitor is encouraged to walk up the shore over at least eight more flows of this composition that are in turn overlain by ophitic basalt flows. Brannon (1984) classifies these flows as ferroandesite, but they are morphologically more similar to flows of basaltic composition. ---- Climb back up back to highway, back to cars. Continue northeast on the Scenic Highway approximately 8 miles to Alseth Road. Turn right on Alseth Road and follow around to a point where shoreline is close to road, park and walk to shore (Stony Point).--- STOP 2-4: Stony Point diabase T.51N., R.12W., Sec. 1 Knife River 7.5’ quadrangle UTM: 590326E; 5197791N Highlights: Stony Point diabase (spd) The diabase forms a sheet-like intrusion that strikes parallel to the shoreline and is discordant to the surrounding volcanic rocks. It is exposed in numerous outcrops between here and Knife River, a distance of nearly 3 miles. It is composed of coarsely ophitic olivine diabase with scattered thin pegmatitic segregations. The island offshore from Knife River forms the northeast-most outcrop of this diabase. The exposed thickness is estimated to be about 100 meters, but a positive aeromagnetic anomaly that corresponds to this diabase, and continues offshore beneath Lake Superior, implies that this diabase is part of a larger intrusive mass whose thickness is unknown. The diabase is chilled against adjacent volcanic rocks, and contains rare small anorthosite xenoliths (neither visible at this stop). ---Continue northeast on the Scenic Highway, to Highway 61 into Two Harbors. Continue through Two Harbors to the northern outskirts of town, turn right on 1st Street and continue south to the crest of the hill adjacent to the picnic grounds. Walk east to lakeshore.--- Page 63 �STOP 2-5: Two Harbors Town Park T.52N., R.10W., Sec. 6 Two Harbors 7.5’ quadrangle UTM: 601851E, 5208673N to 601847E, 5208132N Highlights: Basaltic flows of quartz tholeiite composition (ntb), rubbly flow tops, diabase dike (db) This sequence of quartz tholeiite basalt flows is part of the Two Harbors lithostratigraphic unit (see table in introduction). These flows are comprised of brownish-gray to gray, fine-grained, fresh intergranular basalt that typically exhibits a pronounced oxidation-lamination type of flow banding. The upper parts of these flows typically contain a thick and irregular section of rubbly aa rich in laumontite, and red siltstone is common as thin lenses and crack fillings in the flow tops. Walk south along the shoreline and observe the transition from massive, oxidation-laminated flow interiors to amygdaloidal, rubbly flow tops, and contacts with overlying flows. Each rocky point along this traverse corresponds to the massive, more resistant base of a flow that is underlain by a rubbly, easily eroded, flow top. A N28W striking diabase dike that cuts the basalt can be seen along the south side of the north-most point, but due to time constraints this field trip will not go there. This dike follows a set of closely spaced brittle fractures in the basalt. :---Continue northeast of Two Harbors on Highway 61 approximately 4.5 miles to the Silver Cliff Tunnel. Just past tunnel turn right into an informal parking area adjacent to highway.--*** TRAFFIC HAZARD ****** USE EXTREME CAUTION *** Page 64 �STOP 2-6: Silver Cliff Tunnel T.53N., R.10W., Sec. 15 and 22 Castle Danger 7.5’ quadrangle UTM: 606993E, 5213952N (park area) to 606755E, 5213486N Highlights: Silver Creek diabase (scd), contact with subjacent andesitic flows nca), fault The Silver Creek diabase forms an irregular subcordant, subhorizontal intrusion that is at least 60-m (200ft) thick. The diabase forms a prominent highland that projects inland several miles from Silver Cliff at Lake Superior. The Highway 61 tunnel has created excellent exposures of the contact between both the top and bottom of the diabase with adjacent volcanic rocks, and has exposed a north striking, 55-degree east dipping brittle fault that cuts the base of the diabase. The andesitic flows beneath the sill show a very irregular, rather chaotic flow contact, a rubbly flow top, vuggy quartz-lined stretched vesicles and amygdules of gray agates that have been recrystallized by contact metamorphism from the adjacent diabase. The margins of the diabase are marked by a mix of commingled fine-grained, strongly magnetic, dark gray diabasic rock and pink granophyre. The lower diabase contact strike is approximately N-S, 70 degrees west, and the upper contact (exposed at the south end of the tunnel) strikes N40E, 70 degrees northwest. Out on the old roadbed the diabase displays prominent columnar joints that plunge approximately 60-65 degrees east. From the north edge of the old roadbed, one can look back across the highway down the length of the brittle fault that cuts the base of the diabase. The fault is about 3 meters thick and filled with a mixture of pink zeolites and calcite-filled voids around altered diabase gouge. The old roadbed affords an excellent view of the Lake Superior coast. ----Continue northeast up Highway 61. The next tunnel is cut through the Lafayette Bluff diabase, a dark green, porphyritic, amygdaloidal diabase which bears distinct textural differences from the Silver Creek diabase, and that locally contains anorthosite inclusions up to 2 meters in diameter. As we drive past, note the thick cap of reddish-brown grus developed on top of the Lafayette Bluff diabase, which is typical of this unit. Just inland, the grus was formerly excavated for use as road surfacing material. We will see blocks of the Lafayette Bluff diabase at the parking area for the next stop.-----Continue past Lafayette tunnel, pass Gooseberry State Park. From Gooseberry River, go 3.8 miles to a pulloff on lakeward side of road approximately 300 feet before crossing the Split Rock River. Park at wayside and walk northeast to small point to gain access to shore.--- Page 65 �STOP 2-7: Split Rock River diatreme T.54N., R.8W., Sec. 7 Split Rock Point 7.5’ quadrangle UTM: 620570E, 5226484N to 620497 E, 5226229N Highlights: Split Rock Creek diatreme breccia (dt), brecciated basalt, basalt/andesite flow contact Outcrops to north along shore are of weakly to moderately brecciated ophitic basalt that has apparently been slightly disrupted by the adjacent diatreme dike. A small “island” just offshore of the biggest point here consists of dark gray, medium-grained, ferromonzodiorite that petrographically consists of felty plagioclase, prismatic augite, skeletal to blocky oxides, and red-dusted anhedralinterstitial K-feldspar. The edge of this outcrop nearest the shore shows a dramatic brecciated texture due to diatreme emplacement, and the cliff adjacent to that exposes the diatreme breccia proper. The best diatreme breccia is exposed further south along the beach. Here, the breccia contains visible clasts of massive ophitic basalt, fine-grained amygdaloidal basalt, and intergranular basalt that vary from sub-millimeter to 25 cm in diameter and are all strongly magnetic. Small fragments of interflow sedimentary rocks in the breccia have been noted in thin section. The zeolitic matrix varies from pink to pale green in color. At the south end of this breccia, one can observe the sharp, vertical contact between the diatreme and layered amygdaloidal basalt. The contact here strikes approximately N20W, but overall it is more or less north-south. A short distance south of the diatreme contact are vertical cliffs formed by the blocky jointing pattern of a dark gray, very hard and fresh basaltic andesite flow which overlies amygdule-layered flows that exhibit minor low-amplitude billowy surfaces. The flow contact can be observed southward down the beach to the next point out into the lake. The top of this andesitic flow may be visible across the bay to the north, where it is overlain by massive ophitic basalt. The diatreme exposed here is identical to another to the south, near Crow Creek, that is much more well exposed but unfortunately only accessible by water. There, the diatreme forms a vertical, 10 meter thick, cliff-forming dike that cuts ophitic basalt flows along the shoreline, and in some small outcrops inland. Although poorly constrained, the Crow Creek diatreme may be in the form of a ¼ mile diameter ring-like circular dike, or it may simply have an irregular, ‘wandering’ strike direction that varies from north-south to east-west. The Crow Creek diatreme is a heterolithic breccia that contains clasts of basalt up to 3 meters in length, and has irregular pyrite-rich pods within it. Like the diatreme here, the Crow Creek diatreme has prismatic ferromonzodiorite associated with it, which is itself brecciated, and contains varied types of basalt clasts as well as clasts of interflow sedimentary rocks. Page 66 �The varied types of basalts coupled with interflow sedimentary rocks that make up the clasts in both places demonstrate that the diatreme has cut through a substantial thickness of volcanic strata. Although prismatic ferromonzodiorite is apparently associated with the diatreme in some fashion, the abundance of zeolitic matrix mixed with rock flour implies that the diatremes were formed by a gaseous, volatile-rich explosive mechanism. The volatile source is not known. However, the Lafayette Bluff diabase, which occurs in close proximity to the Crow Creek diatreme, contains abundant zeolitic (mainly laumontite) amygdules, and overall exhibits moderately strong deuteric alteration, which implies that the magma may have been gas-charged. Thus in a highly speculative scenario, degassing during cooling of the Lafayette Bluff diabase may have led to a buildup of volatiles that subsequently exploded through the overlying basalts. Both the Crow Creek and Split Rock river diatreme form vertical dike-like bodies which brecciated clasts of ophitic and intergranular basalt, interflow sandstone and siltstone, fine-medium-grained ferrodioritic rocks, isolated unit quartz grains, and isolated blocky plagioclase grains. At this locality the clasts are generally less than 0.5 meter in diameter, but the diatreme near Crow Creek contains clasts of basalt as large as 4 meters in length. Medium-grained prismatic ferrodioritic intrusive rocks are exposed next to the diatreme dikes at both localities, and at both places the ferrodiorites are cut by and hence predate the diatreme dikes. ---Continue up Highway 61 to Split Rock Lighthouse State Park and History Center. Pull into park lot for the History center and proceed toward the lighthouse. To visit the lighthouse area (Area A) during summer tourist season, the history center charges a fee or requires Historical Society membership. A fee is not required to access the shoreline exposures at Area B. STOP 2-8: Split Rock Lighthouse Anorthosite T.55N., R.8W., Secs. 32 and 33 Split Rock Point NE 7.5' quadrangle UTM: Area A- 623690E, 5228678N Area B - 623625E, 5228665N Highlights: large anorthosite inclusions (anor), ophitic diabase (brd), lower chill, basaltic lava flow with silt-filled aa flow top (nsb) The Beaver River diabase is the most areally extensive intrusive phase of the entire BBC and is found in contact with most other BBC units. In the southern BBC (Fig. 2-4), it occurs as a series of dikes, sills, and sheets of ophitic olivine gabbro that grades into coarser and more subophitic to intergranular gabbro in the medial portions of thicker sheets. One of the most distinctive characteristics of the diabase is that it commonly hosts large (as much as several hundred meters in diameter), rounded to angular inclusions of nearly pure anorthosite. These inclusions, which locally are brecciated and recrystallized (Morrison and others, 1983), are particularly common in the upper and lower margins of the larger diabase sheets. As reported for this stop in the Field Trip 7 guidebook (stop 24), these inclusions have been the focus of geologic debate for over 150 years. Page 67 �Area A: Outcrops of fine-grained, ophitic olivine diabase with centimeter-wide augite oikocrysts, typical of the margins of Beaver River diabase, are exposed just northeast of the lighthouse atop a sheer 30-mhigh sea cliff. The diabase here forms a sill that dips gently (<15°) into the lake and whose basal contact with a basalt flow top is exposed around the base of the point. The diabase is a moderately evolved (mg#=57), high-Al, olivine tholeiite (Fig. 2-5, BRD margin). The prominent point just to the northeast (Rusty Point) is held up by a very large (~200 m) inclusion of medium-grained leucogranite lying at the base of the sill. The diabase is slightly chilled around this granite inclusion, which closely resembles the Finland granophyre (see Stop 2-12). Most of Split Rock Lighthouse Point is held up by a single, large (>35m), layered anorthosite inclusion. Around the base of the lighthouse, the inclusion displays meter-scale modal layering of coarsegrained granular (cataclastic?) anorthosite (>99% Pl) and noritic anorthosite (20% Opx [En70Fs28Wo2], 80% Pl [An60-80]; Morrison and others, 1983). The steeply dipping layers are cut by thin dikes of mediumgrained, granular augite leuconorite (An56, En73Fs24Wo3, En46Fs12Wo41, Morrison and others, 1983). .....Head down the tramway path to the lakeshore and carefully make way along base of slope toward lighthouse. PROCEED CAREFULLY OVER LARGE TALUS BOULDERS. Area B: To the southwest of the lighthouse along the shore, the base of the sill can be observed to conformably overlie an amygdaloidal flow top breccia with a matrix of horizontally bedded siltstone. The diabase here contains a variety of types and sizes of anorthosite inclusions, which stand out as unjointed masses within the highly jointed fine-grained diabase. The diabase is strongly chilled at the basal contact against the flow top, but shows no sign of chilling against the anorthosite inclusions. The variety of anorthosite types can be viewed in the talus blocks at the base of the slope. Looking to the northeast, one can see that the near-vertically, layered inclusion beneath the lighthouse extends to lake-level. This and other anorthosite inclusions hosted in the Beaver River diabase are different from most anorthositic series rocks of the Duluth Complex; the latter are rarely layered, more compositionally evolved, and rarely contain cumulus hypersthene. These inclusions are similar, however, to some anorthosite inclusions within the anorthositic series. The highly disordered structural state of plagioclase (Miller, unpublished data) and the absence of any discernable chill of the diabase against the anorthosite indicate that these inclusions were derived from a middle to lower crustal source. Isotopic and traceelement compositions of these crustal xenoliths suggest that they may be pre-Keweenawan in age (Morrison and others, 1983), but the data are overall ambiguous. Alternatively, if a plagioclase crystal mush origin for Duluth Complex anorthositic rocks is correct (Miller and Weiblen, 1990), a corollary of such a model is that significant amounts of Keweenawan anorthosite, generated by plagioclase flotation under high pressure, should have formed in the deep crust prior to BBC magmatism at 1096 Ma. Under deep crustal conditions, such plagioclase cumulates would probably be distinctive in texture and composition from their shallow crustal counterparts. Moreover, the ambiguous isotopic compositions of the inclusions may indicate that anorthosite-forming Keweenawan magmas were contaminated by older crust, rather than older anorthosite being contaminated by interaction with Keweewawan magmas, as concluded by Morrison and others (1983). Return to Duluth for the evening. DAY 2 Page 68 �GEOLOGY OF THE BEAVER BAY COMPLEX Drive Highway 61 from Duluth past Two Harbors to Lake Co Rd 3. Take this road approximately 25 miles. About 2.3 miles past the Silver Bay municipal airport, look for a prominent roadcut on the adjacent railroad grade on the north side of the road. Pull into gravel pullout. STOP 2-9: Fanning columnar joints in Beaver River Diabase dike/sill T.55N., R.8W., Sec. 9 Silver Bay 7.5' quadrangle UTM: 624590E, 5235865N Highlights: columnar jointed ophitic diabase (brd), anorthosite inclusion (dark outcrops), contaminated and inclusion-rich diabase (brid) The western and northern extent of the Beaver River diabase is marked by a 110-km-long dike (or dike set) across which vertical downward displacement of 1.5 to as much as 6 km on the southeastern (rift axis) side is indicated by offset of older geologic units (Miller and Chandler, 1997). The feature, which has a very pronounced aeromagnetic expression (Fig. 2-7), is termed the Finland tectono-magmatic discontinuity (FTMD). Within the concavity of the FTMD are several other large, often bifurcating dikes and large sheets dipping gently southeast. The complex of sheets holding up tabletop highlands in the southern BBC (Figs. 2-4 and 2-7) may be part of an originally continuous, nearly horizontal sheet that has locally been eroded through to expose volcanic rocks forming the footwall. The fanning pattern of columnar jointing exposed in this railroad cut through Bear Lake Ridge reflects the transition from the FTMD dike into a thick sub-horizontal sheet developed to the east of the ridge. The rock type here is a fine-grained ophitic olivine diabase. The larger railroad cut 150 m to the east reveals the irregular, well-jointed base of the diabase sheet overlying the amygdaloidal flow-top breccia of a deeply weathered ophitic basalt (CAUTION, OUTCROP FACE IS UNSTABLE). Abundant weathered anorthosite inclusions and some quartz- and feldspar-phyric rhyolite inclusions (similar to the Palisade rhyolite) are evident in the diabase. This diabase sheet holds up the extensive 120-m cliff visible across (south of) the Beaver River. Page 69 �Figure 2-7. Shaded-relief image of total magnetic field data over the southern Beaver Bay Complex and generalized geologic map (from M-119) at the same scale. FTMD - Finland Tectono-magmatic Discontinuity. Labelled intrusions of the BBC include the Cloquet Lake layered series (CLLS), the Houghtaling Creek troctolite (HCT), the Sonju Lake intrusion (SLI), the Finland granophyre (FG), the Lax Lake gabbro (LLG), the Blesner Lake diorite (BLD), the Beaver River diabase (BRD), and the Silver Bay intrusions (SBI). Duluth complex units include the anorthositic series (AS), the felsic series (FS), the Osier Lake intrusion (OLI), and the Greenwood Lake intrusion (GLI). North Shore Volcanic Group rocks are denoted as V. Solid lines denote inter-intrusion contacts; dashed lines denote intra-intrusion contacts. See M-119 for subunits of intrusions. Number labels are NAD 83-based UTM grid coordinates. Modified from Figure 7-2 (Miller and others, 2002). Two types of diabase are exposed in the upper ledge of the railroad cut, which is accessible from the east. The more common type is the same fine-grained ophitic olivine diabase observed in the dike, but here it contains scattered anorthosite inclusions and locally is plagioclase porphyritic. Exposed across the eastern half of the ledge, the ophitic diabase is found in sharp contact with a dense, black, aphanitic, intersertal diabase rich in quartz and feldspar xenocrysts, numerous small, blebby inclusions of felsite, and larger inclusions of anorthosite. The presence of anorthosite inclusions argues against it being a volcanic xenolith. Near the western end of the ledge, the ophitic diabase is noticeably chilled and columnar jointed adjacent to a sharp steep contact with the inclusion-rich diabase. In the center of the ledge, the inclusion-rich diabase is itself pseudo-columnar jointed near its sharp contact with the ophitic diabase, but the joints are not orthogonal to the contact. This inclusion-rich diabase is locally observed near the margins of Beaver River diabase intrusions throughout the BBC. It probably represents strong contamination of early anorthosite-bearing Beaver River magmas as they were emplaced into brittle conduits in the volcanic edifice. .... Continue east on Lake Co Rd 3 to junction with Co Rd 4. Turn right and proceed to junction with Highway 61 in Beaver Bay. Turn left, after crossing the Beaver River pull on to shoulder. Walk paths down to beach. Page 70 �STOP 2-10: Inclusion-rich Contact Zone of the Beaver River Diabase at Beaver Bay T.55N., R.8W., Sec. 12 Silver Bay 7.5' quadrangle UTM: 629115E, 5235420N to 629240E, 5235770N Stop 10 Highlights: porphyritic rhyolite inclusion (nspr), hornfels basalt inclusions (nsb) with granophyre ring dikes, anorthosite inclusions (black masses), inclusion-rich Beaver River diabase (brid) Exposed along shoreline outcrops on the north side of the Beaver River is a complex mix of rock types that is commonly characteristic of the basal? contact of the Beaver River diabase. Exposed in the first outcrop north of the Beaver River sandbar is hybrid mix of fine-grained diabase and irregular masses of felsic rock in various stages of mixing and assimilation. While some of the felsic material looks to be rounded irregular melted? inclusions in the diabase, some felsic material clearly occurs as dikes in the diabase. At the steep north end of this exposure is a quartz-feldspar porphyritic rhyolite inclusion that is very similar to the Palisade rhyolite, which holds up Palisade Head and Shovel Point several miles to the north of here. At the base of the steep north side of the outcrop, the rhyolite is in sharp contact with a fine-grained diabase. The diabase is riddled with irregular blebs of felsic material that likely represent melted fragments of the rhyolite inclusion. Wieland Island in the northern part of Beaver Bay is composed of a similar porphyritic rhyolite. The cliffs across the mouth of the river to the southeast are held up by large inclusions of aphyric flow-banded rhyolite and medium-grained massive granophyre. Progressing to the northeast, we cross a small outcrop of an epidotized fragmental mafic rock. This rock may be contaminated diabase or, as implied by similar appearing rock we shall see to the north, it is more likely a metamorphosed flow top breccia of a basalt inclusion. The next low outcrop is of deeply altered (green-tinted) diabase charged with similarly altered anorthosite inclusions. Coming on to a large expanse of semi-continuous outcrop along the rest of the shore, several rock types are observed. Most of the exposure is altered fine-grained diabase. Another prominent rock type is granophyre, which occurs both as irregular masses of varied size and as a curving network of granophyre dikes ranging from several centimeters to over a meter wide. In some areas, granophyre composes almost half of the exposure. In several locations, granophyre dikes are found encircling blocks of dense basaltic hornfels. The basaltic hornfels are sometimes only distinguishable from the fine-grained diabase by a very prominent orthogonal joint pattern and the ring of granophyre. In one block occupying a low spot in the outcrop, a flow contact between massive and flow breccia basalt is recognizable. These blocks of basalt flows were likely thermally metamorphosed by their incorporation into the diabase. The origin of the granophyre is less clear, however. Melting of rhyolite inclusions has evidently given rise to at least some of the granophyre, especially that occurring as irregular masses. However, given the mantling relationship of some granophyre ring dikes to the basaltic hornfels inclusions, is seems possible that some of the granophyre dikes were "sweated out" from the originally hydrated basalt inclusion. Alternatively, the Page 71 �ring dikes may represent rhyolite-derived felsic melts that simply intruded along contacts between the diabase and the basalt inclusions. Following the exposure further to the north, this mix of diabase, granophyre, and basalt abruptly gives way to diabase heavily charged with large anorthosite inclusions. At this point, we will head upslope to the highway and back toward the river. .... Continue along Highway 61 to the northeast past the North Shore taconite plant at Silver Bay and Tettegouche State Park. Proceed 3.0 miles past the junction with MN Highway 1 to a road cut across from (NW of) a Jehovah's Witness Hall. STOP 2-11: Silver Bay intrusions, Beaver River diabase and overturned lava flows T.56N., R.7W., Sec. 1 Illgen City 7.5' quadrangle UTM: A: 637890E, 5247155N B: 638150E, 5247660N C: 638421E, 5247980N Highlights: ferrogabbro layered intrusion (sblg), overturned lava flows (nsb), monzodioritic composite intrusions (sbg) into diabase (brd) containing anorthosite and basalt inclusions The Illgen City quadrangle contains some of the most complex geology exposed in the Keweenawan system (Miller and others, 1988). This complexity resulted from block faulting, displacement, and rotation of volcanic rocks caused by the dike and sill emplacement of the Beaver River diabase and later composite intrusions. The present level of exposure along the shoreline has cut into the footwall of a major sheet-like intrusion of diabase that holds up the highlands about 2 km in from the shore. This is perhaps the same sheet that the Beaver River cuts through near Stop 2-9. The footwall is highly dissected with a complex network of large (50-800m wide) diabase dikes and the intervening blocks of volcanic rocks have been significantly displaced and rotated into a variety of orientations (Fig. 2-8). Multiple composite intrusions of ferrogabbroic to intermediate magmas have produced a variety of intrusions hosted by the Beaver River diabase, which are collectively termed the Silver Bay intrusions. Three roadcuts along a one-and-a-half kilometer stretch of the northwest side of Highway 61 display some of the best examples of the magmatic and structural complexities attending the emplacement of the Beaver Bay Complex. Roadcut A: This roadcut exposes a nearly complete cross section through a small, zoned Silver Bay intrusion. This intrusion, termed the Jehovah body, has the form of a broad, shallow asymmetric synform that plunges about 15° to the east and measures about 450 m north to south. It is intrusive into the axial portion of a Beaver River diabase dike about 500-600 m wide, which dips steeply to the north. Page 72 �A B C Figure 2-8. Geologic cross-section of the shoreline in the vicinity of STOP 2-11 (From Miller and others, 1989). Unit abbreviations are: nsb- basalt; nsob-ophitic basalt; nsr2-rhyolite; nss-interflow sandstone; brd-Beaver River diabase; sbg-Silver Bay gabbro; sblg-Silver Bay laminated gabbro;, gr-granite inclusion. Approximate locations of roadcuts A, B, and C projected to the shore are shown. No vertical exaggeration. On either end of the roadcut west of Highway 61 (Fig. 2-9A) are exposures of a deeply weathered, varitextured but generally coarse-grained, nonfoliated apatitic ferromonzodiorite forming the margins of the intrusion (unit sbg). Toward the center of the roadcut, the ferromonzodiorite abruptly grades into a medium-grained, well-foliated, subprismatic ferrodiorite with poikilitic olivine (unit sbpg). The poikilitic olivine ferrodiorite is composed of about 50-55% prismatic to lath-shaped plagioclase (An45-15); 15-20% subprismatic and partially uralitized augite (En'62-42) and minor intergrowths of pigeonite (En'42-36); 5-10% subequant to bladed iron-titanium oxide; 5-15% subpoikilitic to poikilitic, altered olivine (<Fo48); and 515% granophryic mesostasis and trace apatite. Olivine is almost completely altered to a glassy black sheet silicate mineral, called hisingerite by Gehman (1957). In thin section, the hisingerite appears to be mostly dark-greenish-brown biotite, partially altered to chlorite, with some serpentine and iron oxide. The oxidation of iron oxide, which is especially common around the outer margin of the olivine oikocrysts, produces purplish coronas around the black clots. Variations in the abundance and size of olivine oikocrysts (1.5 to 5 cm across) impart a layering to the gabbro, which is parallel to plagioclase foliation. This internal structure defines an asymmetric synform whose axial plane projects through the northern half of the intrusion. Cryptic variations in the Mg/Mg+Fe contents of mafic minerals are noted upward through this sequence (Fig. 2-9A) and are suggestive of magmatic differentiation. We interpret the coarse-grained, decussate rocks to represent equilibrium crystallization of a stagnant margin around a small convecting core of fractionally crystallizing magma, which formed the foliated gabbro. The dikes of coarse-grained nonfoliated ferrodiorite internal to the zoned intrusion may represent late intrusions of parental magma or remobilization of interstitial magma from within the intrusion. Roadcut B: The next major roadcut to the northeast displays a series of basalt lava flow that have been overturned 125° from horizontal. These flows are part of a large (~1 x 2 km) block of basalts that is completely enclosed by intrusive rocks of the Beaver River diabase and Silver Bay intrusions (Fig. 2-8 and location map). The basalts range in composition from olivine tholeiites, with smooth upper flow surfaces and pipe amygdules at their base, to quartz tholeiites with brecciated (aa) flow tops (Fig. 2-9B). They range in thickness from 3 to 20 meters. Two interflow sandstone units occur beneath the thickest lava flows. These flows are part of the Bell Harbor lavas, the uppermost unit of the upper southwestern sequence of the NSVG (Table 2.1). Page 73 �Figure 2-9. Schematic diagrams of geologic relationships exposed along three roadcuts at STOP 2-11. Foliation development portrayed in Roadcut A by alignment of tick marks. Units: sbg- nonfoliated ferromonzodiorite; sbpg - foliated poikilitic olivine ferrodiorite; sblg - foliated intergranular (non-poikilitic) olivine ferrodiorite. Compositions of augite shown by En content (= MgO/(MgO+FeO), mole %). Roadcut C: A complex sequence of intrusive and inclusive relationships are exposed in the 70' high roadcut to the northeast (Fig. 2-9C). This is one of the tallest roadcuts on the North Shore. Starting at the southernmost end of the roadcut (0 pace) is a medium-grained ophitic olivine diabase with 2cm augite oikocrysts. The diabase gradually coarsens to where by 80 paces the oikocrysts are about 5 cm across. Beginning at about 40 paces, the diabase displays an intense oxidation/alteration that persists through about 200 paces, where it is in contact with hornfels basalt. Between 90 and 130 paces is a coarse grained anorthosite inclusion, which has near vertical contacts with the enclosing diabase and displays the same alteration as the diabase. This anorthosite holds up the crest of the hill that the roadcut traverses. On the north side of the inclusion, the diabase is more subophitic, with coarse (~2 cm) augite clots, and it contains granophyric patches. Progressing north, augite gradually becomes more prismatic and granophyre content increases to where by 160 paces the rock is a ferromonzodiorite typical of the marginal phases of Silver Bay intrusions. Between 170 and 175 paces, two steeply inclined granophyre dikes cut the ferromonzodiorite. At about 180 paces, a subophitic texture again is evident. This rock gradually becomes more ophitic and more medium-grained as a contact with hornfels basalt is approached at 200 paces. The northern contact of this basalt inclusion occurs at a low spot in the roadcut and is obscured by two shallow dipping granophyre dikes. The larger dike (~1.5m wide) cuts medium finegrained ophitic diabase, which contains another hornfels basalt inclusion near the base of the exposure at about 240 paces. The exposure from 240 to 295 paces is relatively unaltered, medium fine to mediumgrained diabase. Between 270 to 280 paces, near the base of the roadcut, is an odd inclusion of coarse- Page 74 �grained, subophitic leucogabbro with apophyses? of very fine, amygdaloidal mafic rock. The inclusion is mantled on its upper right margin by a thin rind of granophyre. The northernmost 10 paces of the roadcut leading to a gravel side road is hornfels basalt. The sequence of events interpreted from the rocks displayed in Roadcut C are: 1) intrusion of diabase dike carrying anorthosite inclusions from depth and incorporating locally derived basaltic wall rock. 2) composite intrusion of ferromonzodioritic magma into the semi-molten core of the diabase dike; 3) injection of granophyre dikes (possibly auto-intrusions of late-stage felsic melt segregated from the ferromonzodiorite); and 4) fluxing of oxidizing fluids through the central zone of the dike. --- Continue on Hwy. 61 to Lake Co. Hwy 6. Take this to MN Hwy 1 in Finland. Turn right on Hwy 1 about 1/4 mile and turn right on to Lake Co Hwy 7. Just past the Finland Community Center and just before the pavement ends, turn left on to the road to the site of a former USAF Radar Station. Take the paved road about 1.4 miles to gravel road crossing and pull out to the left.--- STOP 2-12: Finland granophyre and hybrid dikes T.57N., R.7W., Sec. 4 Finland 7.5' quadrangle UTM: 633180E, 5256520N Highlights: granophyric leucogranite (frg), locally with blebby mafic enclaves (dgh) Exposed over a 40-km2 area in the northwestern part of the southern BBC (Fig. 2-4) is a distinctive, oval-shaped mass of leucogranite and quartz ferromonzodiorite, termed the Finland Radar Station granophyre by Miller and others (1993), or simply the Finland granophyre. The main phase of the granophyre is a homogeneous, salmon-colored, micrographic leucogranite with abundant miarolitic cavities (unit frg, Fig. 2-4). To the north, this granitic phase, which contains less than 5% Fe-silicates and oxides, abruptly grades to a quartz ferromonzodiorite characterized by 5-20% prismatic, iron-rich pyroxene, amphibole, and locally olivine and 20-40% micrographic felsic matrix (unit frqm, Fig. 2-10). At this stop, the typical character of the granitic phase of the Finland granophyre is displayed in the roadcuts on either side of the road to the former USAF radar station (decommissioned in 1974). In the rubble at the north end of this roadcut, the medium-grained granophyre contains small rounded enclaves of fine-grained mafic rock. Some of the rubble blocks are composed completely of this finegrained rock. This occurrence is in line with what to the north is clearly a mafic to intermediate dike cutting the granophyre. This dike is part of an orthogonal network of dikes composed of heterogenous mixtures of mafic and felsic rock types that cut the Finland granite and the underlying Sonju Lake Page 75 �intrusion and that parallel the adjacent Beaver River diabase (BRD-SLI hybrid dikes in Fig. 2-10A). Miller and Chandler (1997) have interpreted these dikes to represent hybrid mixtures of Beaver River diabase magma and residual felsic melts in the upper Sonju Lake intrusion and overlying Finland granophyre. The textures displayed here are consistent with those resulting from two magma mixing, which would imply that the Finland granite was significantly molten when the mafic dikes were emplaced. The intrusive relationships of the Finland granophyre, the older Lax Lake gabbro along its southern margin (llg, Fig. 2-4), and the younger Beaver River diabase at its eastern extent (unit brd, Figs. 2-4 & 210A) are well-established by several exposures of sharp contacts. However the genetic relationship of the granophyre to the underlying Sonju Lake layered intrusion (Stop 13) along its gradational northern boundary (Fig. 2-10A) is more problematic. Stevenson (1974), noting aplitic granophyric dikes cutting the mafic cumulates of the Sonju Lake intrusion, considered the Finland granophyre to be intrusive into the Sonju Lake intrusion. However, mapping by Miller and others (1993) showed these aplite dikes to be phases of the late hybrid dikes (Fig. 2-10A) that cut the Sonju Lake intrusion and both phases of the Finland granophyre. Again, the termination of such a dike is thought to give rise to the mafic enclaves at this stop. Given the parallel zonation of the two phases of the granophyre with the strike of Sonju Lake cumulate units (Figs. 2-4 & 2-10; also see Miller and others, 1993a) and the generally smooth compositional variations across these units (Fig. 2-10B), a comagmatic relationship resulting from crystallization differentiation could be envisioned. Such an interpretation is inconsistent, however, with the large amount of granophyre relative to the layered mafic rocks apparent from the geologic map (Figs. 2-4 & 2-10). Modeling of gravity and aeromagnetic data across these units (Miller and others, 1990) confirms that the volume of granophyre is at least as great as the volume of underlying mafic rocks and thus is too great to be a differentiate of the layered intrusion. Also, preliminary Sm-Nd and Rb-Sr data indicate that the granophyre has an isotopic composition distinct from the mafic rocks and therefore could not have evolved from them (J. Vervoort, unpublished data). A more plausible interpretation is that the mafic magma that produced the Sonju Lake intrusion was trapped by the less dense Finland granophyre. This underplating resulted in partial melting of the granophyre and the development of a diffuse contact. Petrologic and isotopic studies are needed to determine the extent of assimilation across the mafic-felsic contact. --- Return downhill to Lake Co Rd 7. Turn left and proceed 5.6 miles to Sonju Lake Forest Rd. Take the Sonju Rd 3.1 miles to an area of low outcrops--- Page 76 �STOP 2-13: Sonju Lake Intrusion T.58N., R.7W., Sec. 27 Finland 7.5' quadrangle UTM: 633180E, 5256520N Highlights: cumulate stratigraphy of Pl+Ol (slt) -> Pl+Cpx+Ol (slg) -> Pl+Cpx+Ox+/-Ol (slfg) The Sonju Lake intrusion (SLI) is the most completely differentiated mafic layered intrusion recognized in the Keweenawan magmatic system (Stevenson, 1974; Weiblen, 1982). The intrusion occurs as a shallow (15-30°) south- to southeast-dipping, 1200-m-thick sheet of mafic cumulates underlying a granophyre body of similar thickness. The SLI is exposed in outcrop over an area extending about 3 km west from its abrupt truncation by the Beaver River diabase dike of the FTMD structure (Fig. 2-10A). However, it can be traced beneath glacial cover by its aeromagnetic signature for a strike length of about 15 km (Fig. 2-4). Above a basal contact zone of fine- to medium-grained melatroctolite (unit slmt) intrusive into older gabbroic rock, five map units within the SLI are distinguished by their cumulus mineralogy: Ol (unit sld)→ Pl+Ol (unit slt)→Pl+Cpx+Ol (unit slg)→Pl+Cpx+Ox±Ol (unit slfg)→Pl+Cpx+Ox+Ol+Ap (unit slad). At the upper contact with the overlying Finland granophyre is a complex unit (slmd) characterized by a cyclical loss of igneous lamination and enrichment in granophyre at the expense of mafic phases. This igneous stratigraphy is complimented by a smooth, logarithmic cryptic layering exemplified by decreasing mg# values of pyroxene (Fig. 2-10B). Taken together, these characteristics are consistent with formation of the SLI by closed-system fractional crystallization of a moderately evolved tholeiitic basaltic magma (Miller and Ripley, 1996). At this stop, we will traverse the middle cumulate units of the SLI. Exposed in low outcrops on either side of the Sonju Lake Forest Road is coarse-grained, moderately foliated, ophitic olivine gabbro typical of the upper part of the slt unit. It is a Pl+Ol cumulate with abundant (15-20%) augite oikocrysts up to 8 cm in diameter. Downsection from here, the slt unit is more typically an ophitic augite troctolite with olivine about twice as abundant as augite and the latter as 1- to 3-cm oikocrysts (Fig. 2-11). The increased mode and size of augite oikocrysts here presages the approaching cumulus arrival of augite. The arrival of cumulus augite is indicated by its abrupt change from ophitic to granular habit (Fig. 211), which can be observed in intermittent outcrop about 70 paces (~100 m) south of the road. This singular cumulus phase transition from Pl+Ol to Pl+Cpx+Ol is traceable across the entire exposure area of the SLI and serves as a useful datum horizon (Fig. 2-10). The slg unit is about 70 m thick and is typically composed of 60-65% plagioclase, 28-32% subprismatic augite, 5-10% olivine and 1% interstitial Fe-Ti oxide. The rock is consistently coarse-grained and well foliated. Continuing south across a low swamp, the outcrop exposed on the north face and crest of the next ridgeline is medium-grained, well foliated oxide gabbro of the slfg unit. The cumulus arrival of Fe-Ti oxide is defined by its granular habit and its increase in modal abundance to 7-10% (Fig. 2-11). Olivine is commonly in low abundance or absent. This unit is over 200m thick, and in its medial section hosts an 80m-thick interval enriched in PGE (Miller, 1999; Joslin, 2004). Page 77 �91 12 10" — • sidp SNA-li Slit //> Map Units 91 10 4730 sig —. frqm ? :-: e:itei//ii e METER frg . 800 slmd r l1d 600 . 400 n 200 -3- c: x+ slig fl Beaver River diahase -3- /23 BRD-Sl I hybrid dikes Slid • Augile Arrival Soaju Lake Intrusion 7i0hç sirwt olivine lerromonzodiorite si3d apatite ol dionte (PAP05543 slig oxide gtihhio (PAS) sig gahhro sit nociohte P0) slit dumte (0) stmt line inelutioctoliie (OP) Os < -200 (PA 0) —47 27' 30' sld t micrc,graphic granite quaii lenoinoniodiotile -400 8CpxQpx Finland Oranophyrc slmt S + . Western Profiles (1 &2) Eastern Profiles (3-5) harahboiclc 501) oaks SLI-1 Drill Core -600 Volcanic rocks 100 80 60 40 20 MgO/(MgO+FeO) Figure 2-10. A) Geology of the Sonju Lake intrusion and overlying Finland granophyre showing the locations of stops 12 and 13 (modified from Miller and others, 1993). B) Cryptic variation of MgO/(MgO+FeO) (mole %) in augite and orthopyroxene through the Sonju Lake intrusion and Finland granophyre (from Miller and Ripley, 1997). Figure 2-11. Textures of cumulate rocks from the middle units of the Sonju Lake intrusion. Light phase is plagioclase, outlined light gray phase is olivine, dark gray phase is augite, and black phase is Fe-Ti oxide. Page 78 0 �--- Return to Co Rd 7. Proceed north (to left) about 10 miles to Hoist Lake Rd (just past the Trestle Inn on Crooked Lake). Turn left and go 4.5 miles to junction with snowmobile trail heading to right-- STOP 2-14: Northwestern margin of the Beaver Bay Complex T.59N., R.7W., Sec. 12 Cabin Lake 7.5' quadrangle UTM: A - 638780E, 5274975N B - 638460E, 5274980N C - 638125E, 5274755N D - 638565E, 5275005N Highlights: Houghtaling Creek trocolite (hct); Dam Five gabbronorite (dfgn) with inclusion of gabbroic anorthosite (klga); Shoepack Lake inclusion-rich diorite (slid) This sequence of four exposure areas shows the major units comprising the northwestern margin of the Beaver Bay Complex (Miller and others, 1994). The first exposure (Area A) is of a medium coarsegrained, moderately foliated, layered troctolite cumulate with minor (<5%) ophitic augite forming the northwestern margin of the Houghtaling Creek troctolite (Figs. 2-4 and 2-12). The layering and foliation dip about 30º SE. This intrusion occurs as a 2-5 km wide, northeast-trending macrodike, which can be traced along strike for over 50 kilometer in outcrop and by its distinctive aeromagnetic signature (Fig. 27). Intermittent olivine layering and foliation define an asymmetric trough structure with its shorter limb on the southeast side (Fig. 2-12B). ---Following the snowmobile trail north of the road for about 100 m to a junction of trails. Take the trail to the left (west)--Pavement outcrops on the snowmobile trail and in a larger outcrop on the north side of the trail (Area B) are medium-grained, well-foliated gabbronorite cumulates of the Dam Five gabbronorite (dfgn, Fig. 2-12). The rock is composed of 50% plagioclase, 20% subprismatic augite, 27% subprismatic inverted pigeonite, and 3% subpoikilitic Fe-Ti oxide and would classify as a Pl+Cpx+IPig cumulate. Well-developed foliation dips about 27º SSE. This unit occurs exclusively along the northwestern margin of the Houghtaling Creek troctolite. Although a contact between the troctolite and gabbronorite is not observed and their internal structures are nearly conformable, the troctolite does show some fining toward the inferred contact and is considered to be intrusive into the gabbronorite. The very different cumulus mineralogies of the two units, which cannot be related by any conventional differentiation mechanism, is further evidence that these are distinct intrusive units (Fig. 2-12B). --- Continue along snowmobile trail to junction with main road. After crossing Hoist Creek bridge and meeting another road junction, take the trail to the south and then follow flags to Area C outcrops --- Page 79 �The exposures at the crest of this hill (Area C) are of coarse-grained gabbroic anorthosite, which occurs as a large inclusion in the Dam Five gabbronorite. The inclusion is composed of 90-95% moderately foliated plagioclase, 3-7% ophitic augite and 2-3% poikilitic oxide. It is mapped as an inclusion of the Katydid Lake gabbroic anorthosite (klga; see location map), but its ophitic texture is more like lithologies found in the deeper anorthositic series of the Duluth Complex (as, Fig. 2-4; olga, Fig. 212A). --- Return to road junction and continue west and north to clear cut area. Follow flags over crest of hill to outcrop ledge--NW SE A. B. Figure 2-12. Interpretive cross-sections through the northern Beaver Bay Complex from geologic maps by A) Boerboom and Miller (1994) and B) Miller and others (1994). Map unit abbreviations: nsf- felsic volcanics, slid - Shoepack Lake inclusion-rich diorite; slAi - Archean inclusions in slid; whgg - Whitefish Lake granophyre, felsic series of the Duluth Complex; olga, sclg, klga - phases of the anorthositic series of the Duluth Complex; dfgn - Dam Five gabbronorite; hct - Houghtaling Creek troctolite. Exposed in a 5' ledge on the western crest of the clear cut hill (Area D) is a typical exposure of the earliest intrusive unit of the Beaver Bay Complex - the Shoepack Lake inclusion-rich diorite (slid). This unit is composed of a heterogenous mix of aphanitic to very fine-grained diorite and inclusions of predominantly felsite (icelandite and K-feldspar-phyric rhyolite) and lesser amounts of basalt, gabbroic anorthosite, gabbro and locally Archean? rock types (granite, gneiss, amphibolite, schist). Inclusions range in size from single quartz xenocryts to blocks 50m across. Most inclusions exposed here are of felsic volcanic rocks irregularly mixed and partially assimilated into the diorite. However, at the northern end of the exposure, is an inclusion of a cross-bedded interflow sediment hornfels. The rock is composed of a granular mix of plagioclase, oxide and minor pyroxene, with oxide and plagioclase defining graded bedding. The pyroxene (~5-10%) is probably metamorphic. To the north, Archean xenoliths are common. These occurrence reinforce the interpretation that the pronounced gravity low in this area reflects a prominent southeast-trending basement ridge that divides the Keweenawan igneous rocks into two basins of intrusion and eruption (Boerboom, 1994; Miller and Chandler, 1997). The Shoepack Lake diorite is interpreted to be an intrusive breccia that was the first northeast-trending intrusion to breach this crustal ridge and intrude into a rhyolite-dominated part of the NSVG overlying the ridge (Fig. 2-12). This breach of the crustal ridge paved the way for subsequent intrusions, including the Dam Five gabbronorite and the Houghtaling Creek troctolite, to intrude along this same crustal weakness and thus create the composite grouping of northeast-trending intrusions that form the northern BBC (Figs. 2-4, 2-12). The presence of anorthositic series inclusions in this rock suggests that it may be one of the older intrusions in the greater Duluth Complex, perhaps correlative with early stage magmatism. Page 80 �--- Return east to Co Rd 7. Turn left and proceed to Forest Rd 170. Go right about 2.5 miles and then turn left on the secondary forest road to Whitefish Lake. Proceed about 4.0 miles to a gravel pit road on left. Park and walk about 150m south on the forest road to low outcrops on the west side of the road. --- STOP 2-15: Contact between the felsic and anorthositic series of the Duluth Complex T.60N., R.6W., Sec. 2 Wilson Lake 7.5' quadrangle UTM: 646050E, 5286185N Highlights: Contact between Whitefish Lake granophyre (whgg) and flow foliated gabbbroic anorthosite of the Outlaw Lake gabbroic anorthosite (olga). In the low pavement exposure, a sharp contact can be observed between medium-grained pink granophyre and medium-grained, swirly foliated, ophitic oxide gabbroic anorthosite. A bimodal grain size of well-foliated plagioclase in the gabbroic anorthosite imparts a trachytic texture to the rock. Several lines of evidence indicate that the gabbroic anorthosite is intrusive into the granophyre: 1) the gabbroic anorthosite shows a subtle decrease in grain size toward the contact whereas the granophyre show no change in texture as the contact is approached. Gabbroic anorthosite in gravel pit outcrops, about 50 m to the north, is considerably coarser grained than observed here. Two miles to the northeast, a more obvious chill of the gabbroic anorthosite against granophyre is noted. 2) well-developed, but irregular-oriented foliation in the gabbroic anorthosite appears to be concordant within a meter of the contact; and 3) felsic apophyses emanate from the granophyre and into the gabbroic anorthosite, probably caused by back veining of partially melted granophyre at the intrusive contact. This exposure provides rare field evidence for the older age of the felsic series relative to the anorthositic series of the Duluth Complex. This inferred age relationship was reaffirmed by U-Pb zircon dating, which yielded an 1109.6±4 Ma age for the Whitefish granophyre (Sandland and others, 2001) compared to 1099.1 and 1099.6 Ma ages for two anorthositic series samples dated by Paces and Miller (1993). The early stage emplacement of felsic magmas, which gave rise to the granophyre bodies that now dot the upper contact of the Duluth Complex (Fig. 2-3), helps explain the subsequent emplacement of the multiple, massive intrusions of the Duluth Complex. These felsic bodies probably acted as density traps that triggered underplating of first, ferrogabbroic magmas, creating the early gabbro series, then, plagioclase-phyric magmas (Fig.2-12A), creating the anorthositic series, and finally, aphyric mafic magmas, creating the many discreet intrusions of the layered series (Miller and Ripley, 1996; Miller and others, 2002). In the gravel pit are exposures of several varieties of anorthositic rocks that show complex lithologic and structural relationships typical of the anorthositic series (Miller and Weiblen, 1990). Page 81 �--- Return south to Forest Rd 170, then west to Co Rd 7. Proceed north (right) to Forest Rd 172 (Wanless Rd). Go left on 172 for about 2.9 miles to quarry entrance on right side of road (Permission required for entry) --- STOP 2-16: "Lake Superior Green"quarry T.60N., R.7W., Sec. 35 Silver Island Lake 7.5' quadrangle UTM: 636532E, 5278342N Highlights: Dimension stone quarry; porphyritic gabbroic anorthosite (klga); diabase dikes. Cold Spring Granite Company has been operating a dimension stone quarry at this site for the past 10 years. The rock is marketed as Lake Superior Green, but its geologic name is the Katydid Lake gabbroic anorthosite (Boerboom and Miller, 1994; Miller and others, 1994). The rock is a coarse-grained plagioclase-porphyritic gabbroic anorthosite and is composed of 60-70% large (2-3 cm), blocky plagioclase phenocrysts in a medium-grained matrix composed 65% plagioclase, 20% prismatic augite, 510% granophyric mesostasis, and 5-10% accessory phases of oxide, biotite, amphibole, and apatite. The subequant habit of the plagioclase precludes the development of a foliation, though locally some hint of alignment is evident. Locally, small (10-30 cm) rounded inclusions of coarse-grained anorthosite or granulated (tectonized) anorthosite are found. Along the southern wall of the quarry, the gabbroic anorthosite is cut by 10- to 50-m-wide dikes of fine-grained diabase. This rock is thought to be an offshoot from the main mass of the anorthositic series, which occurs about 2 kilometers to the north and is locally called the Outlaw Lake gabbroic anorthosite (Boerboom and Miller, 1994). END OF TRIP ---Continue west along Forest Road 172 10 miles to MN Hwy. 1. Turn left and proceed southeast to Hwy 61 at the shore. Turn right and proceed to Duluth--- Page 82 �References Basaltic Volcanism Study Project (BVSP), 1981, Pre-Tertiary continental flood basalts: in Basaltic Volcanism on the Terrestrial Planets, NY, Pergamon Press, p. 30-77 Boerboom, T.B., 1994, Archean crustal xenoliths in a Keweenawan hypabyssal sill, northeastern Minnesota. White was right!: Proceedings of the 40th Annual Institute on Lake Superior Geology, Program and Abstracts, p. 5-6. Boerboom, T.J., Green, J.C., and Jirsa, M.A., 2002a, Geologic map of the French River and Lakewood quadrangles, St. Louis County, Minnesota: Minnesota Geological Survey Miscellaneous Map M-128, scale 1:24,000 Boerboom, T.J., Green, J.C., and Jirsa, M.A., 2002b, Geologic map of the Knife River quadrangle, St. Louis and Lake Counties, Minnesota: Minnesota Geological Survey Miscellaneous Map M-129, scale 1:24,000 Boerboom, T.J., Green, J.C., and Miller, J.D., Jr., 2003a, Geologic map of the Two Harbors quadrangle, Lake County, Minnesota: Minnesota Geological Survey Miscellaneous Map M-139, scale 1:24,000 Boerboom, T.J., Green, J.C., and Miller, J.D., Jr., 2003b, Geologic map of the Castle Danger quadrangle, Lake County, Minnesota: Minnesota Geological Survey Miscellaneous Map M-140, scale 1:24,000 Boerboom, T. J., and Miller, J. D., Jr., 1994, Geologic map of the Silver Island Lake, Wilson lake, and western Toohey Lake quadrangles, Cook and Lake Counties, Minnesota: Minnesota Geological Survey Miscellaneous Map Series, Map M-81, scale 1:24,000 Bonnichsen, B. and Kauffman, 1987, Physical features of rhyolite lava flows in the Snake River Plain volcanic province, southwestern Idaho: in The Emplacement of Silicic Domes and Lava Flows, J. H. Fink, ed., p. 119145 Brannon, J.C., 1984. Geochemistry of successive lava flows of Keweenawan North Shore Volcanic Group: unpublished Ph.D. Thesis, Washington University, St. Louis, 212pp. Carmichael, I.S.E., 1964, The petrology of Thingmuli, a Tertiary volcano in eastern Iceland: Jounral of Petrology, v. 5, p. 435-460 Chandler, V.W., 1990, Geologic interpretation of gravity and magnetic data over the central part of the Duluth Complex, northeastern Minnesota: Economic Geology, v. 85, p. 816-829 Davis, D.W., and Green, J.C., 1997, Geochronology of the North American Midcontinent rift in western Lake Superior and implications for its geodynamic evolution: Canadian Journal of Earth Science, v. 34, p. 476-488 Davis, D. W. and Paces, J. B., 1990, Time resolution of geologic events on the Keweenaw Peninsula and implications for development of the Midcontinent Rift system: Earth and Planetary Science Letters, v. 97, p. 54-64. Gehman, H.M., Jr., 1957, The petrology of the Beaver Bay Complex, Lake County, Minnesota: unpublished Ph.D. dissertation, University of Minnesota, Minneapolis, 300p. Goldich, S.S., Nier, A.O., Baadsgaard, H., Hoffman, J.H., and Krueger, H.W., 1961, The Precambrian geology and geochronology of Minnesota: Minnesota Geological Survey Bulletin 41, 193 p. Greeley, R., 1982, The Snake River Plain, Idaho: Representative of a new category of volcanism: Journal of Geophysical Research, v. 87, N. B4, p. 2705-2712. Green, J.C., 1972, North Shore Volcanic Group, in Sims, P.K., and Morey, G.B., eds., Geology of Minnesota—A centennial volume: Minnesota Geological Survey, p. 294-332. Green, J.C., 1977, Keweenawan plateau volcanism in the Lake Superior region, in Baragar, W. R. A., Coleman, L. C., and Hall, J. M., eds., Geological Association of Canada Special Paper 16, p. 407-422. Green, J.C., 1982, Geologic Map Atlas of Minnesota, Two Harbors Sheet: Minnesota Geological Survey, scale 1:250,000. Green, J.C., 1989, Physical volcanology of mid-Proterozoic plateau lavas: The Keweenawan North Shore Volcanic Group, Minnesota: Geological Society of America Bulletin, v. 101, p. 486-500. Green, J.C., 1990, Primary tridymite crystallization and inferences from tridymite and quartz textures in high-T Keweenawan rhyolites and granophyres, Minnesota (abs.): Geological Soc. of America, Abstracts with Programs, v. 22, p. A289-290. Green, J.C., 1992, Geologic map of the north shore of Lake Superior, Lake and Cook Counties, Minnesota: Part 1. Little Marias to Tofte: Minnesota Geological Survey Miscellaneous Map Series, M-71, scale 1:24,000 Page 83 �Green, J.C., 1995, Chemostratigraphy at the fringe of the Midcontinent Rift System: The northeast limb of the North Shore Volcanic Group, Minnesota (ext. abs.): in Proceedings, Petrology and Metallogeny of Volcanic and Intrusive Rocks of the Midcontinent Rift System: 1995 IGCP Project 336 Field Conference and Symposium, p. 55-56 Green, J. C. and Fitz, T. J. III, 1993, Extensive felsic lavas and rheoignimbrites in the Keweenawan Midcontinent Rift plateau volcanics, Minnesota: petrographic and field recognition: Journal of Volcanology and Geothermal Research, v. 54, p. 177-196 Green, J.C., Davis, D.W., and Schmitz, M.D., 2001, Three new zircon dates for the Midcontinent Rift, North Shore, Minnesota: more data, more questions: 47th Institute on Lake Superior Geology, Proceedings and Abstracts, p. 29. Irvine, T.N., and Baragar, W.R.A., 1971, A guide to the chemical classification of the common volcanic rocks: Canadian Journal of Earth Sciences, v. 8, no. 5, p. 523-548. Jerde, E.A., 1991, Geochemistry and petrology of hypabyssal rocks associated with the Midcontinent rift, northeastern Minnesota: unpublished Ph.D. Thesis, University of California, Los Angeles, 305 pp. Jirsa, M. A., 1984, Interflow sedimentary rocks in the Keweenawan North Shore Volcanic Group, northeastern Minnesota: Minnesota Geological Survey Report of Investigations 30, 20 pp. Joslin, G.A., 2004, Stratiform Pd-Pt-Au mineralization in the Sonju Lake intrusion near Finland, Minnesota: unpublished M.S. Thesis, University of Minnesota Duluth. Klewin, K.W., and Shirey, S.B., 1992, The igneous petrology and magmatic evolution of the Midcontinent Rift System: Tectonophysics, v. 213, p. 33-40. Manley, C. R., 1996, Physical volcanology of a voluminous rhyolite lava flow: The Badlands lava, Owyhee Plateau, southwestern Idaho: Journal of Volcanology and Geothermal Research, v. 71, p. 129-153 McBirney, A.R., and Williams, H., 1969, Geology and petrology of the Galapagos Islands: Geological Soc. of America Memoir 118, 197p. Miller, J.D., Jr., 1988, Geologic map of the Split Rock Point NE and Silver Bay quadrangles, Lake County, Minnesota: Minnesota Geological Survey, Miscellaneous Map Series, M-68, scale 1:24,000 Miller, J.D., Jr., 1999, Geochemical evaluation of platinum group element (PGE) mineralization in the Sonju Lake intrusion, Finland, Minnesota: Minnesota Geological Survey Information Circular 44, 32 p. Miller, J.D., Jr., and Chandler, V.W., 1997, Geology, petrology, and tectonic significance of the Beaver Bay Complex, northeastern Minnesota: in Ojakangas, R.J., Dickas, A.B., Green, J.C., (eds.) Middle Proterozoic to Cambrian Rifting, Central North America: Geological Society of America Special Paper 312, p. 73-96. Miller, J.D., Jr., and Ripley, E.M., 1996, Layered intrusions of the Duluth Complex, Minnesota, USA: in Cawthorne, R.G., ed., Layered Intrusions: Amsterdam, Elsevier Science, p. 257-301 Miller, J.D., Jr., and Vervoort, J.D., 1996, The latent magmatic stage of the Midcontinent Rift: a period of magmatic underplating and melting of the lower crust: 42nd Annual Institute on Lake Superior Geology, Proceedings Volume, Part I – Program and Abstracts, p. 33-35 Miller, J.D., Jr. and Weiblen, P.W., 1990, Anorthositic rocks of the Duluth Complex: Examples of rocks formed from plagioclase crystal mush: Journal of Petrology 31, p. 295-339 Miller, J.D., Jr., Boerboom, T.B., and Jerde, E.A., 1994, Bedrock geologic map of the Cabin Lake and Cramer 7.5minute quadrangles, Lake and Cook Counties, Minnesota: Minnesota Geological Survey Miscellaneous Map Series, M-82, scale 1:24,000 Miller, J.D., Jr., Green, J.C., Boerboom, T.B., & Chandler, V.W., 1993, Geology of the Doyle Lake and Finland quadrangles, Lake County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series M-72, scale 1:24,000 Miller, J.D., Jr., Green, J.C., and Boerboom, T.B., 1989, Geology of the Illgen City quadrangle, Lake County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series, M-69, scale 1:24,000 Miller, J.D, Jr., Schaap, B.D., and Chandler, V.W., 1990, The Sonju Lake intrusion and associated Keweenawan rocks: geochemical and geophysical evidence of petrogenetic relationships: Proceedings of the 36th Annual Institute on Lake Superior Geology, Part 1. Abstracts, p. 66-68 Page 84 �Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., and Peterson, D.E., 2001, Geologic map of the Duluth Complex and related rocks, northeastern Minnesota: Minnesota Geological Survey Miscellaneous Map Series, M-119, scale 1:200,000 Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.M., Wahl, T.E., 2002, Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geological Survey Report of Investigations 58, 207 p. Milner, S.C., Duncan, A.R., and Ewart, A. , 1992, Quartz latite rheoignibrite flows of the Etendeka Formation, northwestern Namibia: Bulletin of Volcanology, v. 54, p. 200-219 Morrison, D.A., Ashwal, L.D., Phinney, W.C., Shih, C., and Wooden, J.L., 1983, Pre-Keweenawan anorthosite inclusions in the Keweenawan Beaver Bay and Duluth Complexes, northeastern Minnesota: Geological Society of America Bulletin, v. 94, p. 206-221. Nicholson, S. W., Shirey, S. B., Schulz, K. J., and Green, J. C., 1997, Rift-wide correlation of 1.1 Ga Midcontinent rift system basalts: implications for multiple mantle sources during rift development: Canadian Journal of Earth Science, v. 34, p. 504-520. Paces, J.B., and Miller, J.D., Jr., 1993, Precise U-Pb ages of Duluth Complex and related mafic intrusions, northeastern Minnesota: geochonological insights to physical, petrogenetic, paleomagnetic and tectonomagmatic processes associated with the 1.1 Ga Midcontinent rift system: Journal of Geophysical Research, v. 98, no.B8, p. 13,997-14,013. Pope, N.M., 1976, Petrology and structure of the Late Precambrian mafic sills east of Silver Creek, Lake County, Minnesota: Unpublished M.S. thesis, University of Minnesota Duluth, 157 p. Sandland, T.O., Wirth, K.R., Vervoort, J.D., Gehrels, G.E., Kennedy, B.C., and Harpp, K.S., 2001, Roles of fractional crystallization and assimilation in the production of Midcontinent rift granophyres: 47th Institute on Lake Superior Geology, Abstracts and Proceedings, p. 85-86. Schmidt, S. Th., 1993, Regional and local patterns of low-grade metamorphism in the North Shore Volcanic Group, Minnesota, USA: Journal of Metamorphic Geology, v. 11, p. 401-414. Schmidt, S. Th. And Robinson, D., 1997, Metamorphic grade and porosity and permeability controls on mafic phyllosilicate distributions in a regional zeolite to greenschist facies transition on the North Shore Volcanic Group, Minnesota: Geological Society of America Bulletin, v. 109, No. 6, p. 683-697. Schwartz, G.M., and Sandberg, A.E., 1940, Rock series in diabase sills at Duluth, Minnesota: Geological Society of America Bulletin, v. 51, p. 1135-1172. Sigvaldason, G. E., 1974, Basalts from the centre of the assumed Icelandic mantle plume: Journal of Petrology, v. 15, p. 497-524. Stevenson, R.J., 1974, A mafic layered intrusion of Keweenawan age near Finland, Minnesota: unpublished M.S. thesis, University of Minnesota Duluth, 160 pp. Vervoort, J. D. and Green, J. C., 1997, Origin of evolved magmas in the Midcontinent Rift System, northeast Minnesota: Nd-isotope evidence for melting of Archean crust: Canadian Journal of Earth Sciences, v. 34, p. 521-535. Wallace, A.B., Drexter, J.W., Grant, N.K., and Noble, D.C., 1980, Icelandite and aenigmatite-bearing pantellerite from the McDermitt caldera complex, Nevada-Oregon: Geology, v. 8, p. 380-384. Weiblen, P.W., 1982. Keweenawan intrusive igneous rocks. In: Wold, R.J. & Hinze, W.J. (eds.) Geology and tectonics of the Lake Superior Basin: Geological Society of America Memoir 156, 57-82. Wood, D. A., 1978, Major and trace element variations in the Tertiary lavas of eastern Iceland and their significance with respect to the Iceland geochemical anomaly: Journal of Petrology, v. 19, p. 393-436 Page 85 �FIELD TRIP 3 LATE WISCONSINAN SUPERIOR-LOBE DEPOSITS IN THE SUPERIOR BASIN NORTHEAST OF DULUTH Howard Hobbs Minnesota Geological Survey INTRODUCTION The recession of the Superior lobe was marked by many readvances. We will examine glacial striations and tills of the Superior lobe along the scenic North Shore of Lake Superior from Duluth to Two Harbors. The stratigraphy in this area will be tied to ice margins and events elsewhere, leading to a consistent glacial history. Each time the ice lobe retreated into the Superior basin, it created a glacial lake, and each subsequent readvance incorporated reddish-brown lake sediment. Each of these lakes is here considered to be a phase of glacial Lake Superior in a broad sense. Each readvance covered a smaller area and reached a lower elevation than the last, and deposited a finer-grained till than the previous one. STRATIGRAPHY Cromwell Formation The oldest glacial sediment exposed in these stops is a rocky, gravelly, reddish-brown (5YR 4/4) till. Its matrix texture is typically a loam, but some samples are extremely low in clay (Fig. 3-1). It is non-to-slightly calcareous, and is deeply leached where exposed at the surface, even where calcareous at depth. This till and associated meltwater sediment has been named the Cromwell Formation (Wright and others, 1970). The characteristic reddish color and abundant red and black clasts, which define the Superior provenance, are derived from the passage of the ice through the Superior basin, and are presumably absent from deposits up-ice from the basin. The till matrix derived its reddish color and some of its red clasts from pulverized red sedimentary rock from the floor of the basin: sandstone, siltstone, and shale. The other red and black rock fragments are primarily extrusive and intrusive igneous rocks from the northwestern lake shore: basalt, rhyolite, diabase, and gabbro. Black slate fragments from the Proterozoic Rove Formation are locally present. In most of its extent (down-ice from the basin), the Cromwell Formation contains many clasts other than those from the Superior basin: gray to black Proterozoic metamorphic clasts, Archean granite and greenstone, and Paleozoic carbonate. These clasts are mainly derived from older Quaternary sediment and local bedrock. Within the Superior basin, the clast assemblage in the Cromwell Formation in many places is dominated by Superior basin clasts. I call this assemblage a “hyper-Superior” assemblage, as opposed to the mixed Superior assemblage down-ice. Inasmuch as the ice itself did not originate inside the Superior basin, the scarcity of clasts from up-ice of the basin implies that the rate of bedrock erosion is much higher within the basin than outside, which explains why the basin formed in the first place. The standard grain size for clast counting and determining provenance is 1-2 millimeters (very coarse-grained sand), and this size is used unless otherwise specified. Barnum Formation The remainder of the glacial and glaciolacustrine sediment in the area is referred to as the Barnum Formation (Fig. 3-2). The Barnum Formation was originally defined to include clayey Page 86 �till overlying the Cromwell Formation (Wright and others, 1970). Hobbs (2002, 2003a, b) recognized three texturally distinct tills above the Cromwell Formation. The tills are overlain and separated in places by reddish lake and deltaic sediment, which ranges in texture from silt to clay (Fig. 3-1), and contains very little sand and almost no coarse-grained fragments. This sediment ranges from massive to laminated, and from grayish-brown to reddish-brown. More detail is provided in the stop descriptions. The individual tills contain inclusions of similar lake sediment in places. These tills and lacustrine sediments are to be included in a modified Barnum Formation (Johnson and others, unpub. data). Until this work is published, the member names will be used informally. Lakewood member The lowest till in the Barnum Formation is the Lakewood member. It is typically reddishbrown (5YR 4/3 to 3/4) noncalcareous silt loam (Fig. 3-1). Its average clay content is only a little more than that of the Cromwell Formation. Its proportion of coarse-grained fragments is typically less than that of the Cromwell till, but more than that of the other tills of the Barnum Formation. In contrast to the Cromwell Formation, large pebbles and boulders are not common. Most samples have a hyper-Superior clast assemblage in the 1-2 millimeter size fraction. A few samples have a small number of Paleozoic carbonate clasts. The till is relatively thin and patchy in the field trip area; it probably averages less than 2 meters in thickness. Moose Lake member The middle till in the Barnum Formation is the Moose Lake member. It is typically reddishbrown (5 to 2.5YR 5/3 to 4/4) calcareous clay loam to silty clay loam (Fig. 3-1). Coarse-grained fragments are sparser than in the brown silty till of the Lakewood member, and large rocks are rare. The typical clast assemblage is Superior, but most are not hyper-Superior. The Moose Lake Page 87 �member contains more clasts from outside the Superior basin (mostly granitic) than does the Lakewood member. Many samples contain secondary carbonate precipitated by soil processes, but fewer than half of the samples have primary carbonate clasts. The number of primary carbonate clasts is small, even in samples that appear to be quite calcareous, which suggests that the bulk of carbonate is in the finer grain sizes. The average thickness of the till in the field trip area is about 3 meters. Page 88 �Wrenshall member Laminated red and gray lacustrine silt and clay in the Nemadji lake plain was named the Wrenshall Formation by Wright and others (1970). It is stratigraphically above the Moose Lake member, and behind the Nickerson moraine at the margin of the Moose Lake member. I am redefining the Wrenshall Formation as a member of the Barnum Formation, because it is a part of the same stratigraphic package of glacial lake sediment and lake-influenced tills. This member has not been observed in the field trip area. Knife River member The upper till of the Barnum Formation is informally named the Knife River member. It is typically reddish-brown (2.5 YR 5/3 to 4/4) calcareous clay (Fig. 3-1), containing few coarsegrained fragments or large rocks. A rough field criterion for differentiating the Moose Lake and Knife River tills is the sound they give off to a shovel. The Moose Lake member goes “clink,” but the Knife River goes “clunk.” That is, almost every shovel thrust into the Moose Lake till encounters a pebble, but most shovel thrusts into the Knife River do not. Where the Knife River till overlies the till of the Moose Lake member, its color is commonly on the same color chip, but a little lighter and redder. The clast assemblage is commonly Superior, rarely hyper-Superior, and occasionally a mixed Superior-Winnipeg assemblage (the Winnipeg assemblage is found in tills deposited by ice that advanced through the Winnipeg lowland. It is characterized by common to abundant Paleozoic carbonate, granitic grains that are more common than dark grains, few red grains, and little to no Cretaceous shale). More than half of the samples contain primary carbonate grains, and many samples contain common to abundant secondary carbonate. Its average thickness is 3 to 4 meters, but appears to be absent in many places close to Lake Superior. Its clay texture is atypical for glacial till, which led Moss (1977) and Gross (1982) to interpret this till as massive glacial lake clay. GLACIAL HISTORY During the most recent glaciation, the field trip area was dominated by the activities of the Superior lobe, which advanced through the axis of the Superior basin from northeast to southwest as far south as the St. Croix moraine (Wright, 1972). The advance to the maximum, and the earlier retreatal phases, deposited till and associated sediments of the Cromwell Formation. Many retreatal or readvance ice margins have been recognized between the St. Croix moraine and the Superior basin (Mooers, 1988; Patterson, 2001), but only the most recent ones are discussed here. As long as the ice did not front a substantial glacial lake between advances, the texture of the till remained more or less the same: rocky loam to sandy loam. The first retreat into the Superior basin did not create a large continuous lake, but rather a set of small ice-marginal lakes trapped between the ice and the sides of the basin. Meltwater escaped by a series of ice-marginal channels, building ice-contact deltas of sand and gravel into ponds on its way out of the basin to the southwest (Stop 3-8). The next advance of the Superior lobe overrode these deltas and deposited a loamy Cromwell till that ends at the Mille Lacs–Highland moraine. The texture of this till does not reflect any large amount of overridden lacustrine sediment. An outwash plain graded to the Highland moraine buries the adjacent part of the Toimi drumlin field. It is graded to a set of channels that seems to end at the St. Louis River. The present day St. Louis River flows east and south, into the Superior basin, so meltwater flow from the Highland moraine maximum must have flowed west (reversed) into glacial Lake Upham I. The flowpath out of Lake Upham I is unknown; deposits of the St. Louis sublobe later covered this whole area (Fig. 3-2). As the ice receded from the Highland moraine, a set of meltwater channels was formed between the receding ice in the basin, and the higher ground to the northwest. These channels are discontinuous, and must have partly formed on the ice itself. Their general flow direction is to Page 89 �the southwest, and they probably exited the Superior basin at its southwestern corner, where bedrock elevations are the lowest. Deposits of subsequent readvances have since covered this area, but the bottom of the most recently used lake spillway is about 1,050 feet in elevation. Gravel in these channels is somewhat younger than the gravel in the ice-contact deltas, but is also included in the Cromwell Formation. Further retreat opened up a lake in the southwestern corner of the Superior basin that extended about as far northeast as Two Harbors. The advance that followed was named the Split Rock phase by Wright (1972). It spilled out of the basin and deposited a thin, reddish-brown (5YR) till, which I have correlated to the Lakewood member of the Barnum Formation. This advance extended well out of the Superior basin deep into Pine County, where it apparently formed the Split Rock, Askov-Lookout Tower, and Kerrick ice margins. This correlation is uncertain, and the till texture is not exactly the same in Pine County as in the field trip area, but till behind this set of margins is finer-grained than that of the earlier phases (Patterson, 2001). The Lakewood member does not exhibit a morphologically distinct ice margin in the field trip area. Its margin is recognized only by the change in texture of the surface till, from that typical of the Cromwell Formation to that typical of the Barnum Formation. This change occurs fairly consistently between 1,150 and 1,200 feet in elevation. All three till members of the Barnum Formation have fairly consistent margin elevations in the field trip area. The ice was confined in a steep-sided basin, so their margins are similar to bathtub rings. Theoretically, one would expect the margins to rise to the northeast, both to account for the down-ice slope of the glacier, and differential postglacial rebound. There is little evidence of this rise in the margin of the Lakewood member. This is partly because the data points on which it is based are widely separated, and partly because the mapped margin of the member is not necessarily the actual ice margin, but the margin of preserved deposits. A thin patchy till deposited on a sloping surface is subject to extensive erosion, both by waves and by sheet runoff. In addition, the area mapped so far is rather small, and the expected rise in ice margin is not great. Another retreat opened up a larger portion of glacial Lake Superior; it is unclear how much of the basin was ice-free at this time. Apparently, this lake accumulated a considerable amount of clay, because the advance that followed deposited the clayey Moose Lake member and built the Nickerson moraine at the southwestern margin of the basin. In the field trip area, the Moose Lake member was deposited up to about 1,100 to 1,150 feet in elevation. If the glacier were merely recycling its own clay, it is puzzling why the Moose Lake member is so much more clayey than the Lakewood member. Another puzzle is why the Moose Lake member is so calcareous compared to the Lakewood member. One possibility is a change in the nature of debris carried into the Superior basin from the central Laurentide Ice Sheet. For example, Mooers and Lehr (1997) proposed that the change from calcareous to noncalcareous debris carried by the Rainy lobe (in a broad sense) was correlated to the migration of the ice divide south of the Hudson Bay lowland, so that the Paleozoic carbonates in the lowland could no longer be transported south. The eastern boundary of these lowlands is up-ice from the Superior basin, and could have supplied some carbonate. In fact, a small number of Paleozoic dolomite grains are sporadically present in the otherwise noncalcareous Cromwell Formation and Lakewood member tills, so a Hudson Bay component is quite possible. However, the mechanics of how a reverse transition from noncalcareous to calcareous would work in this setting is difficult to envision. Another possibility is that glacial Lake Upham I drained into Lake Superior between the Split Rock and Nickerson advances. This lake may have been fed by meltwater from the calcareous Itasca or Koochiching lobes to the north, or even the incipient St. Louis sublobe. If so, some of the meltwater probably funneled down the St. Louis River to glacial Lake Superior. Fine-grained calcareous lake sediment was thus potentially available for entrainment into the Moose Lake till. Although the sediment carried down the St. Louis River should have been mostly brown and Page 90 �gray, there was abundant red clay deposited in the lake from the melting Superior lobe, and the red color tends to dominate in mixtures (the gray color is not the result of a pigment, but the lack of pigment). The later advance of the St. Louis sublobe filled the Aitkin-Upham basin and buried the deposits of Lake Upham I. This advance coincided more or less with the Nickerson moraine, as shown by diversion channels around the moraine that can be traced upstream to the St. Louis River (Wright and others, 1970). There is no way to be sure which of these channels were fed by outwash from the St. Louis sublobe, and which were fed by overflow from glacial Lake AitkinUpham. The general pattern is that the earlier, higher diversion channels are narrower, and presumably carried less water than the later, lower ones. One might speculate that the earlier diversion channels carried meltwater from the ice margin, and that the later ones carried water from the glacial lake. At times, Lake Aitkin-Upham was fed by overflow from Lake Koochiching (the eastern arm of Lake Agassiz) at its highest levels (Hobbs, 1983). Thus, a much larger flow of water was available than could have been generated by local melting in the St. Louis River watershed. The succeeding retreat first opened up a small ice-marginal lake in the southwestern corner of the basin, called glacial Lake Nemadji by Winchell (1899). This lake appears to have been small and fronted by ice at first—the rapid decline of its depositional surface from the proximal edge of the Nickerson moraine to Duluth can be most easily explained if the Superior lobe still occupied most of the basin. The outlet of Lake Nemadji was the Portage River channel, which cuts through the Nickerson moraine and joins a diversion channel of the St. Louis River at the town of Moose Lake. The bottom of the Portage River outlet is about 1,050 feet in elevation, but lakesmoothed topography and minor beach ridges can be recognized up to 1,080 feet. This does not mean that the water in the outlet channel was necessarily 30 feet deep; the higher wave-washed topography could have formed while the channel was being deepened. A great thickness of laminated glacial clay and silt (Wrenshall member) accumulated at this time, between the proximal side of the Nickerson moraine and the ice remaining in the basin. Further retreat of the ice uncovered the lower Brule outlet in Wisconsin, which lowered the level of the lake to about 1,020 feet and dried up the Portage outlet (Leverett, 1932). This lower level of the lake has been called glacial Lake Duluth, but I prefer to use Duluth and Nemadji as levels or phases of glacial Lake Superior, rather than giving every phase a separate lake name. A large outflow from glacial Lake Aitkin-Upham II came down the St. Louis River after the Superior lobe melted back from the Nickerson moraine. It washed a large area (about 10 square miles) down to bedrock, upstream from the Nemadji lake plain. Although this scouring event was extremely widespread, it did not expose any substantial area of bedrock below 1,050 feet in elevation, suggesting that the local base level was no lower than 1,050 at the time of the event. Subsequent stream erosion has incised a narrow valley into the bedrock below 950 feet in elevation, but this cutting seems to have been accomplished by long erosion of a much smaller stream graded to a much lower base level. Thus, the last major meltwater flood down the St. Louis River seems to have come during the short time that glacial Lake Superior stood at the Nemadji level. As the Superior lobe retreated, the lake declined in stages as lower outlets were uncovered, until the entire lake basin was ice-free. The water level was much lower than it is today (at least in the field trip area). As the ice retreated north of Lake Superior, Lake Agassiz spilled east into Lake Nipigon, into Lake Superior, and then out into Lake Huron (Clayton, 1983). A considerable amount of suspended sediment from glacial Lake Agassiz was deposited in the basin. Combined with the earlier sediment from Lake Aitkin-Upham, the Superior basin now had a considerable amount of clay-rich calcareous sediment to be entrained by the next glacial advance. Lake Superior remained ice-free for about 1,000 years, from 1100 to 1000 B.P. (Drexler and others, 1983). Page 91 �The last, or Marquette advance, filled the basin with ice again, depositing a stone-poor, very clayey 2.5YR till, which I have included in the Knife River member of the Barnum Formation. This till is much more clayey on average than the clayey Moose Lake member (Fig. 3-1); its matrix is more calcareous, and it contains more actual carbonate clasts. These differences are attributed mainly to incorporation of sediment from Lakes Agassiz and Aitkin-Upham. In the field trip area, the Knife River member extends up to a little less than 1,050 feet in elevation; it extends laterally over the Wrenshall member as far as the proximal side of the Nickerson moraine (Howard Mooers, unpub. data). The field trip takes place in the area mapped in the French River, Lakewood, Knife River, Two Harbors, and Castle Danger quadrangles (Fig. 3-3). I have recently mapped the surficial deposits of the first 4 quadrangles under the USGS STATEMAP program (Hobbs, 2002, 2003a, b). A surficial map of the Castle Danger quadrangle is in progress. I combined the knowledge gained from this mapping with earlier mapping and interpretations in the region to create this new interpretation of Superior lobe recessional history. Page 92 �FIELD TRIP STOPS 8:00 a.m. Leave Radisson Hotel, Duluth; get onto I-35/ Highway 61, heading northeast through Duluth. Just beyond the where US Highway 61 becomes a 4-lane expressway, turn right onto the Scenic North Shore Drive (County Road 61). Continue about 6 miles to the village of French River. Stop 3-1. Superior bluff cut at French River. French River Quad., T.51N., R. 12 W., Sec 19 NE corner; 583815E, 5194075N NAD83 UTMs This wave-cut bluff exposes about 25 feet of glacial sediment above basalt bedrock. The top of the bedrock is 3 to 4 feet above lake level. It has been broken up by weathering and wave action in most places, but there are some surfaces that have been smoothed by glacial abrasion. Striations strike northeast, almost parallel to the shore but a little more westerly. These striations were carved by the main flow of the Superior lobe through the Superior basin. Wherever striations are exposed by wave action along the shoreline, they have a similar orientation. The lower third of the cut is the Lakewood member of the Barnum Formation [(5) 28 63 9] over till of the Cromwell Formation [(18) 52 42 6]. Numbers in brackets represent individual texture samples: (gravel) sand, silt, and clay. Sand, silt, and clay are considered matrix, and their numbers add to 100 percent, rounded to the nearest 1 percent. The number in parentheses is the total of all fragments larger than 2 millimeters, and is expressed as a percent of the original sample: matrix plus coarse-grained fragments. Large rocks were avoided for the texture samples. Both tills are firm but not hard, with a platy structure parallel to the bluff face. The platy structure is probably a weathering phenomenon. These tills are a little less red than they usually appear—7.5 YR rather than 5YR. The Cromwell sample has a hyper-Superior grain assemblage; the Lakewood sample has a Superior assemblage with a trace of carbonate. The middle third of the bluff cut is red clayey till of the Moose Lake member: [(1) 13 29 58 and [(2) 19 30 51]. Its lower contact is flat and sharp. Both the middle and upper tills are fairly firm, with a strong angular blocky structure. The structure and consistency are the best way to distinguish in-place till from the soft structureless mudflow sediment that coats the bluff. Two samples of the Moose Lake member have Superior grain assemblage with a trace of carbonate. The upper third of the bluff is red clay till of the Knife River member: [(0.1) 4 22 74] and [(0.2) 2 26 72]. Both tills are reddish-brown (2.5 YR 5/4); the contact between them was not observed, but inferred from the texture of the samples. This till contains irregular, subhorizontal clay inclusions, which look grayish by contrast to the red till. They range from pinkish gray (7.5YR 6/2) to light brown (7.5YR 6/3). These inclusions are more calcareous than the red till; they are interpreted as incompletely digested clumps of Lake Agassiz-derived clay. The texture samples from this interval contain some of these clay inclusions, which explains why they are so poor in gravel and sand, and so rich in clay, even for till of the Knife River member. Each sample contains only a tiny number of 1-2 millimeter grains—one is a Winnipeg assemblage, one is a Superior-Winnipeg mixture. NEXT: Continue along the scenic drive about 4.5 miles. Turn right on a road leading to a scenic overlook. Park and scramble down the bluff. Notice how the stone wall has broken and dropped down in places where the red clay till it was built on has slumped. This bluff is actively retreating by wave action and slope failure. The stop is the bluff just across a small gully west of the scenic overlook. Page 93 �Stop 3-2. Superior bluff by overlook. Knife River Quad., T.51N., R.12W., Sec 2, SE of SW of NW; 5888830E, 5197725N NAD 83 UTMs The bluff is about 28 feet high, but only the upper half is exposed (unless a storm has washed the lower part since I saw it). The top of the exposed bluff is not at its original elevation. There are a few slump scarps hidden in the trees above the top, and their combined offset makes the top of the bluff 10 to 12 feet lower than the original surface. No bedrock is exposed at this site. The lowest part of the exposure is 4 feet of till of the Cromwell Formation [(5) 36 59 5] and [(14) 43 48 9]. This till is unusually low in clay, even for Cromwell till. It is calcareous, apparently protected from leaching by the calcareous tills above it. Both samples of Cromwell till contain hyper-Superior grain assemblages. Above the Cromwell till is a layer of brown silty till [(11) 17 59 24] only 2 feet thick, overlain by a foot of massive silt [(1) 7 73 20], which is similar in color and texture to the silty till, but lacks stones. The brown silty till is included in the Lakewood member of the Barnum Formation; the brown silt could also be included in the Lakewood member. It could either be massive lacustrine, or actual till that happens to contain few coarse-grained fragments. More digging may help us figure this out. Both are calcareous. Both contain hyper-Superior grain assemblages with no carbonate grains, but the 1-2 millimeter sample from the massive silt is very small. Above the massive silt is a layer of interbedded red (2.5YR 4/4) and gray (10YR 4/1) calcareous lacustrine clay just over a foot thick. A sample of the red clay is slightly more clayey [0.2) 3 15 82] than the gray [(0.7) 3 18 79], but the gray clay feels more slippery, possibly because of a higher content of expandable clays. Both have very small Superior grain assemblages. The main red and gray beds are about an inch thick, but there are red and gray micro-laminations within each bed. There are no silt beds, making it unlikely that these beds represent varves. The bedding is more or less horizontal, but not very regular. Beds pinch and swell; is this original, or were the beds deformed by overriding of the next glacier? There are numerous slickensided joints running through both the brown silt and the red and gray clay. Were these caused by overriding ice, or by modern slumping? The uppermost layer exposed in this bluff is about 2 feet of calcareous red clayey till [(5) 31 25 44] of the Moose Lake member. It is weak red (2.5 YR 4/2) in the lower part, and reddish brown (2.5YR 4/4) in the upper part. A sample has a Superior grain assemblage. Red clay till of the Knife River member is not exposed here, but it may be present at the site. The top of the exposure is at least 3 feet below the bluff top, and the bluff top itself is probably not the original surface. A soil boring taken along the Scenic Highway from a bluff about 20 feet higher than this one penetrated about 16 feet of red clay till over lacustrine sediment, with an intercalated contact. NEXT: Leave the parking lot, drive back north to the Scenic Highway, turn left and go west about 0.5 mile to the intersection of Homestead Road (County Road 42). Turn right and go north across the railroad tracks and the Highway 61 expressway, and turn left on Old North Shore Road (County Road 290). Proceed about 0.5 mile west into the Big Sucker Creek valley, and park by the bridge. Walk upstream about 0.3 mile, following the trail on the northeast side of the stream. The trail traverses a slumped stream cut. Stop 3-3. Big Sucker Creek cut. Knife River Quad., T.51N., R. 12W., Sec 4, SW of NE of SE of NE; 586740E, 5198325N NAD83 UTMs The creek has cut down to and into bedrock in this whole area. At this site the basal 2 feet of the stream cut is basalt. The base of the drift section is rocky till of the Cromwell Formation, about 4 feet thick. Color is dark reddish brown (5YR 3/2). The lower sample is gravelly and very low in clay [(31) 48 46 6]. The upper sample is more clayey but even more gravelly than the Page 94 �lower one [(37) 34 47 19]. The till is slightly calcareous, but the grain assemblage is hyperSuperior with no carbonate grains. The Cromwell till is overlain by about 9 feet of the Lakewood member; this is one of the thickest Lakewood layers exposed in the field trip area. The trail crosses the cut at the level of the Lakewood member. This till is dark reddish brown (5YR 3/2), slightly to moderately calcareous, less rocky than the Cromwell till, but much more so than the overlying till. The results of three texture samples were: [(12) 24 54 22], [(6) 16 50 34], and [(11) 17 51 32]. All samples have a hyper-Superior grain assemblage, with only one carbonate grain among the three. The Moose Lake member is not exposed here. The contact between the Lakewood member and the overlying Knife River member is above the trail, and is not exposed. What appears to be the contact is actually a slump plane. Till of the Knife River member is not well exposed, and only one sample was obtained [(1) 7 22 71]. It is reddish brown (2.5YR 5/4), slightly to moderately calcareous, and has a Superior grain assemblage, with a few carbonate grains and one grain of spherical frosted quartz derived from Paleozoic sandstone. NEXT: Return to the road, drive back to Homestead Road, and turn right (south). Turn left (east) on the Highway 61 expressway, and drive about 3.5 miles, cross the Knife River, and turn right at the first street leading to the town of Knife River. Turn right at the first intersection, by the school, and head back south and west to a rest area overlooking the river. Stop 3-4. Lunch at Knife River rest area. Knife River Quad, T.52N., R.11W., Sec 31, NE of SW of NW; 591965E, 5200050N NAD83 UTMs While eating lunch, we can observe the erosion along the Knife River, and attempts to control it. This stream, like most streams emptying into Lake Superior, is fairly short, with a steep gradient and a greatly fluctuating discharge. Most of the time the level is low, but snowmelt and big rainfalls can turn it into a roaring torrent. NEXT: Return to Highway 61. Drive about 10 miles northeast, through Two Harbors. About 2.5 miles northeast of Two Harbors turn left (northwest) onto Lake County Road 3. Drive up the hill, and down into the Silver Creek valley. Park where the road crosses Silver Creek, about 2 miles from the turnoff. Walk 200 feet west (upstream) along the creek to a cut on the outside of a meander bend. Stop 3-5. Silver Creek cut. Castle Danger Quad., T.53N., R. 10W., Sec 16, SE of NW of SW of SW; 604470E, 5213910N NAD83 UTMs This is a small cut, but it illustrates some things rarely seen in this area: a good exposure of postglacial stream channel and overbank sediment, and a layer of artificial fill. The total height of the cut is about 7 feet. It exposes a foot of red clay fill over alluvium (overbank and channel sediment) over red clay till of the Knife River member, over brown sandy till of the Cromwell Formation. The Cromwell till is reddish-brown (5YR 4/3), moderately calcareous, fairly hard and very pebbly [(10) 30 44 26]. Its grain assemblage is Superior, with some calcite rhombs. They all appear to be amygdule fragments. Even though the stream has only cut a few feet into the till, it has developed a continuous pavement of rocks on its bottom. The Cromwell till is overlain by clay till [(2) 10 27 63] of the Knife River member, with a fairly sharp, wavy contact. This till is reddish-brown, on the same color chip but slightly lighter and redder than the color of the Cromwell Formation. It is calcareous, with a medium to coarsegrained blocky structure. Its grain assemblage is hyper-Superior, with no carbonate grains. Page 95 �The overlying alluvium was not sampled. It consists of 1.5 to 2 feet of dirty gravel channel sediment, overlain by 1 to 1.5 feet of silt and fine-grained sand overbank sediment. There is a thin soil (possibly truncated) in the top of the overbank sediment. The gravel contains clasts up to boulder size, but large clasts are much less common in this layer than in the currently active channel sediment. The difference is that at the time the gravel was being deposited, the stream was still eroding the clay till; the stream is now eroding till of the Cromwell Formation, which has more and larger coarse-grained clasts. The uppermost layer here is a layer of red clay that I interpret as recent fill dirt. It is texturally indistinguishable from the red clay till of the Knife River member, and was probably excavated from that material. But it is out of order stratigraphically, being underlain by a soil developed in recent alluvium, and not itself having a soil cover. NEXT: Return to vehicle. Drive 0.75 mile uphill (northeast) and down into another small valley, a tributary of Silver Creek. Park near the double culvert. Walk downstream (south) 200 feet to a slumped cut on the outside of a meander. Stop 3-6. Tributary to Silver Creek. Castle Danger Quad., T.53N., R.10W., Sec 16, SW of SW of NE; 605195E, 5214645N NAD83 UTMs This cut is a series of slumps, so there is a mixture of good exposure and covered areas. The description is sort of a composite section. The base of the exposure is loamy till of the Cromwell Formation [(16) 39 34 27]. This texture is somewhat clayey for Cromwell till, but not unusual for inside the Superior basin. The till is slightly calcareous, unoxidized brown (7.5YR 4/2); it is not especially pebbly for Cromwell till. The grain assemblage is hyper-Superior; a large 1-2 millimeter sample contains one grain of dolomite. The Cromwell till is overlain by red clay till of the Knife River member. There is an inclusion of Cromwell till near the base of the clay till, and another several feet higher. It is not clear whether the inclusions were created by glacial erosion, or if they are related to slumping on this slope. Elsewhere on the slope is a contact between red clayey till of the Moose Lake member [(13) 31 30 39] and red clay till of the Knife River member, at a higher elevation than the contact between the Cromwell Formation and the Knife Lake member. This suggests that some or all of the contacts may have shifted downslope. Near the top of the cut is a contact between red clay till of the Knife River member and red lacustrine clay. Here the texture of the till is [(3) 4 24 72] and the lacustrine texture is [(0) 3 18 79]. There is scarcely any difference other than the number of coarse-grained fragments. The till has a small hyper-Superior grain assemblage with no carbonate rock fragments, though there is a grain of secondary carbonate. The lacustrine sample has a very small 1-2 millimeter fraction, half of which is secondary carbonate. NEXT: Return to vehicle and continue northeast and north about 2 miles to the intersection of Gun Club Road. Turn left (west) and follow the road 1 mile west, 0.5 mile south, and 2 more miles west. Turn right on Lake County 2; go 0.5 mile north and turn left (west) on Reeves Road, an unpaved township road. Go up the hill, under a power line, and stop in a clearing about 0.5 mile west of County Road 2. Stop 3-7. Striated bedrock. Two Harbors Quad., T.53N., R.11W., Sec 1, SE of SE of NW; 600155E, 5217670N NAD83 UTMs There are several small hornfels outcrops here, flush with the ground. They are striated northwest–southeast, about 90 degrees from the striations we saw earlier today. These striations reflect ice moving up and out of the Superior basin toward the Highland moraine. This is the Page 96 �general orientation of striations in the highlands. The landforms in this area tend to be streamlined northwest–southeast parallel to the striations. These landforms, mainly bedrock, but partly mantled by drift, are named the Highland Flutes (Wright, 1970). This hill is one of the southwesternmost of the flutes. They extend northeast inside the Highland moraine as far as Lutsen. NEXT: Return to Lake County Road 2; turn right and go south 2.5 miles; turn right on Lake County 12, and go 3 miles west and 2.5 miles south where it dead-ends on Lake County 11 (Valley Road). Turn right and travel 7 miles south and west to Homestead Road. Lake County Road 12 becomes St. Louis County Road 41 at the county line. County Road 41 ends at County Road 42, which turns and becomes Homestead Road. Turn left on Homestead Road, travel 1.3 miles south to the entrance of a gravel pit. Stop 3-8. Peterson Gravel Pit. Knife River Quad., T. 52N., R.12W., Sec 15, NE of SE of NW; 587620E, 5204895N NAD83 UTMs This pit is developed in one of the many ice-contact deltas that developed between the Superior lobe and the North Shore highlands. They occur in the elevation range of 1,150 to 1,210 feet, and have been mapped in the French River, Knife River, and Two Harbors quadrangles. They have not been observed east of the Two Harbors quadrangle. The general stratigraphy here is sand and gravel of the Cromwell Formation overlain by till of the Cromwell Formation overlain by till of the Lakewood member of the Barnum Formation. The section currently exposed may be different from the one described, due to continued mining. The stratified unit ranges from fine-grained sand to medium-grained gravel, moderately well sorted. The color of the finer beds is dark reddish-brown (5YR 3/4); the color of the coarsegrained beds is dominated by the individual rock particles, which are mostly red, black, and dark gray. The unit is non-to-slightly calcareous; two samples show hyper-Superior grain assemblages with no carbonate grains. The gravel is subangular to subrounded; it is more rounded than the gravel in the till, but not much. It does not appear to have been transported far by water. Depending on the orientation of the cut, the sand and gravel appears to have either trough crossbedding, or foreset bedding. On the south side of the pit, the foreset beds dip west. The sand and gravel delta is overlain by a noncalcareous loamy till [(4) 39 52 9], ranging from 3 to 8 feet thick. The contact is sharp and wavy. Some of the underlying beds are deformed and truncated, and there is a considerable amount of sand and gravel incorporated into the till in places. The till is hard and dense, with a coarse-grained platy structure; this suggests that it was deposited under the weight of a glacier, rather than as a mudflow deposit. The grain assemblage is hyper-Superior, with no carbonate grains. On the east side, there is a discontinuous layer of silty till [(3) 20 71 9] that overlies the Cromwell till and the delta sand and gravel. Where sampled, it overlies the delta with no intervening till. This is interpreted as till of the Lakewood member. It is noncalcareous, reddishbrown (5YR 4/4), and contains only a few stones. Its grain assemblage is hyper-Superior, with no carbonate grains. The uppermost layer at this site is a sandy, silty, unbedded deposit, 1 to 2 feet thick, interpreted as colluvium. It does not bury a soil, but the surface soil is developed in it. It must therefore not be much younger than the glacial sediments. NEXT: Return to Homestead Road, turn right, and travel south to the Highway 61 expressway; turn right to return south to Duluth. Page 97 �REFERENCES Clayton, L., 1983, Chronology of overflow to Lake Superior, in Teller, J.T., and Clayton, L., eds., Glacial Lake Agassiz: Geological Association of Canada, Special Paper 26, p. 291-307. Drexler, C.W., Farrand, W.R., and Hughes, J.D., 1983, Correlation of glacial lakes in the Superior basin with eastward discharge events from Lake Agassiz, in Teller, J.T., and Clayton, L., eds., Glacial Lake Agassiz: Geological Association of Canada, Special Paper 26, p. 309-329. Gross, L.B., 1982, The stratigraphy and lithology of the glaciogenic sediments of the Two Harbors area, northeastern Minnesota: Minneapolis, Minn., University of Minnesota, M.S. thesis, 151 p. Hobbs, H.C., 1983, Drainage relationships of glacial Lakes Aitkin and Upham and early Lake Agassiz in northeastern Minnesota, in Teller, J.T., and Clayton, L., eds., Glacial Lake Agassiz: Geological Association of Canada, Special Paper 26, p. 245-259. ———2002, Surficial geology of the French River and Lakewood quadrangles, St. Louis County, Minnesota: Minnesota Geological Survey Miscellaneous Map M-127, scale 1:24,000. ———2003a, Surficial geology of the Knife River quadrangle, St. Louis and Lake Counties, Minnesota: Minnesota Geological Survey Miscellaneous Map M-137, scale 1:24,000. ———2003b, Surficial geology of the Two Harbors quadrangle, Lake County, Minnesota: Minnesota Geological Survey Miscellaneous Map M-138, scale 1:24,000. Leverett, F., 1932, Quaternary geology of Minnesota and parts of adjacent states: U.S. Geological Survey Professional Paper 161, 149 p., 5 pls. Mooers, H.D., 1988, Quaternary history and ice dynamics of the late Wisconsin Rainy and Superior lobes, central Minnesota: Minneapolis, Minn., University of Minnesota, Ph.D. dissertation, 200 p. Mooers, H.D., and Lehr, J.D., 1997, Terrestrial record of Laurentide Ice Sheet reorganization during Heinrich events: Geology, v. 25, no. 11, p. 987-990. Moss, C.M., 1977, The surficial and environmental geology of the French River quadrangle, St. Louis County: University of Minnesota Duluth, M.S. thesis, 69 p. Patterson, C.J., 2001, Surficial geology, pl. 4 of Boerboom, T.J., project manager, Geologic atlas of Pine County, Minnesota: Minnesota Geological Survey County Atlas C-13, 7 pls., scale 1:100,000. Winchell, N.H., 1899, The geology of Carlton County, chapter 1 of Final report of the Minnesota Geological and Natural History Survey: Minnesota Geological and Natural History Survey, v. 4, 528 p. Wright, H.E., 1972, Quaternary history of Minnesota, in Sims, P.K., and Morey, G.B., eds., Geology of Minnesota: A centennial volume: Minnesota Geological Survey, p. 515-547. Wright, H.E., Mattson, L.A., and Thomas, J.A., 1970, Geology of the Cloquet quadrangle; Minnesota Geological Survey Geologic Map GM-3, 30 p., 1 pl. Page 98 �FIELD TRIP 4 Geology of the Eastern Mesabi Iron Range, Northeastern Minnesota by Richard W. Ojakangas Emeritus, Department of Geological Sciences, University of Minnesota Duluth Mark J. Severson Natural Resources Research Institute, University of Minnesota Duluth Peter K. Jongewaard United Taconite Mining Company John L. Arola Ispat Inland Mining Company Joel Thomas Evers Retired – LTV Mining Company and Douglas G. Halverson Northshore Mining SCALE N 0 5 0 10 10 20 15 20 MILES 30 40 KILOMETERS Babbitt Mesaba Location Map Virginia Mt. Iron ITASCA CO. Buhl Biwabik Virginia Horn Coleraine Grand Rapids ST. LOUIS CO. LAKE CO. Hibbing Keewatin Nashwauk Duluth C omplex Eveleth CASS CO. AITKIN CO. DULUTH CARLTON CO. ke ior La per Su Generalized map of the Mesabi Range (cross-hatched). Note Duluth Complex at east end. Page 99 �INTRODUCTION Iron-formation was described as early as 1866 by Henry Eames for rocks on what was to become the Mesabi Range. Several attempts were made by individuals on their way to the iron mines of the Vermilion Range to find ore on the Mesabi. However, it was not until November 16, 1890, that the first rich iron ore on the Mesabi Range was discovered by the Merritt brothers near what is now Mountain Iron, MN. In 1892, the first shipment from this mine was 4245 tons (White, 1954). Exploration for iron ore ensued and within the next few years, most of the productive parts of the Mesabi Range were discovered. The Mesabi Range is the largest iron range in the United States and is one of the largest in the world. It is 0.25 - 3.0 miles wide and 120 miles long (cover page). The Biwabik Iron Formation, as thick as 750 ft, in general dips gently to the southeast at an angle of about 7-15 degrees. The iron-formation, called taconite, typically contains 30-40% iron and 40-50% SiO2, plus other components (Morey, 1992). In numerous places along the length of the range, silica was leached out, thereby enriching the iron content to 55%+. These pockets became the highgrade natural ore mines; there were more than 500 individual mines prior to merging into larger mines as the ore between adjacent properties was removed. These were very important in making the United States an industrial giant, and were instrumental in providing raw material for WWI and WWII. As the high-grade ore was becoming depleted, the taconite process was developed. In 1967, taconite production exceeded natural ore production. Currently, six taconite plants are in production (Fig. 4-1). Babbitt Chisholm N Ispat-Inland Hibbing Taconite Minorca Pit Buhl 5 4 Keewatin Nashwauk MDDP-7 Butler Taconite (closed) 3 2 Calumet Cliffs-Erie Site (LTV closed) 6 Keewatin Tac Dunka Pit Iron Mine (closed) 12 Minntac 8 Ispat-Inland Virginia MDDP-5 Laurentian Pit United Taconite Evel 7 Hibbing 11 Biwabik eth Aurora MDDP-2 Coleraine Northshore 10 9 Hoyt Lakes 1 MDDP-8 Grand Rapids 0 187000E 192000E 600 197000E 6 202000E 12 18 207000E 24 Miles 217000E 212000E 222000E 227000E 232000E 600 Looking North 8 500 500 U.S. Mined Taconite Intervals 5 4 300 2 1 U.S. U.S. 200 100 10 6 400 Upper Slaty U.S. 3 Slaty Upper U.S. 400 U.S. U.S. U.C. U.S. U.C. Upper Slaty U.C. U.C. L.S. Lower Slaty U.C. L.S. 100 L.S. Lower Slaty L.S. L.S. 0 L.C. L.C. -100 Lower Cherty Lower Cherty L.C. L.C. L.C. Lower Cherty L.C. 300 U.S. 200 U.C. L.S. 12 U.S. U.C. Cherty Upper U.C. 11 U.C. Upper Cherty U.S. U.C. 0 9 7 L.C. L.C. L.C. -100 L.C. -200 -200 -300 Virginia Horn Area -300 Generalized Stratigraphic Sections -400 187000E 192000E 197000E 202000E 207000E ILSG - May, 1993 212000E 217000E 222000E 227000E -400 232000E Figure 4-1: Generalized map of the Mesabi Range showing taconite pits (black) and cities, and in the lower half, a longitudinal section of the Biwabik Iron Formation (compiled by Henry Djerlev, 1993). Mined taconite intervals are shown as black columns adjacent to sections. Modified from Morey, 1993. Page 100 �The name of Biwabik Iron Formation was chosen by Van Hise and Leith (1901, p. 356), “because the word Biwabik is the Chippewa word for a piece or fragment of iron.” The word taconite is also used in discussions pertaining to hard, unoxidized portions of the iron-formation. H.V. Winchell (1882, p. 135) originally called portions of the BIF “taconyte” because he thought the rocks correlated with lower Cambrian rocks in the Taconic Mountains in northern New England. Since that time, many geologists have used taconite in their descriptions of the ironformation and it has thus become firmly established. Perhaps a more proper definition for “taconite” is an economic term for iron-formation from which iron can be profitably extracted after fine-grinding, followed by magnetic separation and pelletizing (Morey, 1993). REGIONAL GEOLOGY The peneplaned Archean craton in the Lake Superior region formed a platform upon which a Paleoproterozoic continental margin assemblage was deposited in Minnesota, Michigan, and Wisconsin. Extension resulted in localized rifts that received thicker accumulations of sediments and volcanic rocks than did adjacent parts of the platform. Seas transgressed onto the continent one or more times and an ocean basin opened south of present-day Lake Superior. Island arcs that formed during southward subduction collided with the craton margin to the north as the ocean basin closed. A remnant of this oceanic crust is poorly preserved as a dismembered ophiolite sequence in Wisconsin (Schulz, 1987, 2003). The arc volcanics are preserved as the Wisconsin magmatic terranes. The collision resulted in a fold-and-thrust belt known as the Penokean orogen. To the north of the fold-and-thrust belt, a northward-migrating foreland basin—the Animikie basin—developed as the stacked thrusts weighed down the crust (Fig. 4-2). Thick turbidite successions were deposited along the basin axis, and terrigenous clastics and Lake Superior-type iron-formation were deposited on the shelf along the northern margin (i.e., the foreland or peripheral bulge) of the basin. See Ojakangas et al. (2001) and Severson et al. (2003) for more detailed summaries on Paleoproterozoic basin development in the Lake Superior region. The development of the Midcontinent Rift System (MRS) at 1.1 Ga severed the basin into northwestern and southeastern segments (Fig. 4-2). If the MRS rocks are removed from the geologic map, the different portions of the Animikie basin become contiguous and the fold-andthrust belt rocks of Minnesota and Wisconsin-Michigan become continuous (Fig. 4-3). Figure 4-4 is an interpretive cross-section of the Animikie basin during its formative stages, with sediments derived from the Archean basement to the north and from the fold-and-thrust belt to the south. The Paleoproterozoic supracrustal rocks in the northwestern segment, including east-central and northeastern Minnesota and the adjoining part of Ontario, are for the most part poorly exposed. However, mining of iron ore on the Mesabi and Cuyuna Ranges and continued mining of taconite on the Mesabi Range have resulted in excellent artificial exposures and an abundance of drill hole information. Geophysical surveys and stratigraphic test drilling by the Minnesota Geological Survey also have been major sources of information (e.g., Southwick et al., 1988). Animikie Group The Animikie Group unconformably overlies the Mille Lacs and North Range Groups to the south and the Archean basement to the north (Southwick and Morey, 1991). Magnetic data show North Range structures are present beneath Animikie strata to the east of the exposed North Range Group (Chandler, 1993). Page 101 �Figure 4-2: Generalized geologic map showing the distribution of Precambrian rocks and structural elements of the Lake Superior region, modified from Ojakangas 1994 and references therein (from Ojakangas, et al., 2001). The group consists of three conformable major formations on both the Mesabi and Gunflint ranges. The respective units on the two ranges are the Pokegama Formation and the Kakabeka Quartzite (the lowest units), the Biwabik and Gunflint Iron Formations (the middle units) and the Virginia and Rove Formations (the upper units, composed of graywacke and shale). The Thomson Formation in the northern part of east-central Minnesota is correlative with the Virginia and Rove Formations. The Biwabik and Gunflint are on strike with each other and were probably continuous prior to the intrusion of the Duluth Complex at about 1100 Ma. The Animikie Group in Minnesota-Ontario on the Mesabi and Gunflint ranges and the Baraga Group of Michigan-Wisconsin on the Gogebic range were both deposited in the Animikie foreland basin. The basal units comprised of siliciclastic sediment derived from the Archean basement, and the overlying iron-formation, were deposited in a shallow sea on the northern edge (i.e., the peripheral bulge or foreland) of the northward-migrating Animikie basin (e.g., Ojakangas, 1994). Additional details are provided below in the section titled, Environments of Deposition. The siliciclastic and iron-formation units are exposed on the Gogebic Range of northern Michigan and Wisconsin (Palms Quartzite and Ironwood Iron Formation), on the Mesabi Range of northern Minnesota (the Pokegama Formation and the Biwabik Iron Formation), and on the Gunflint range of northeasternmost Minnesota and Ontario (the Kakabeka Quartzite and the Page 102 �Figure 4-3: Schematic hypothesized paleogeography at the time of sedimentation of the Animikie Group turbidites that overlie shelf deposits in the Animikie basin. The rocks of the 1.1 Ga Midcontinent Rift System have been removed from the map, and Michigan and Wisconsin are thus positioned 60 miles closer to Minnesota-Ontario than they were after the formation of the Midcontinent Rift System. Arrows denote generalized transportation directions of sediment from major source areas. Compare with Fig. 4-2. Modified from Ojakangas (1994) and references included therein (from Ojakangas, et al., 2001). Gunflint Iron Formation), and are lithostratigraphic equivalents. They probably were continuous from south to north prior to development of the Midcontinent Rift System in Mesoproterozoic time. A consequence of this model is that they are diachronous, with the units in MichiganWisconsin (located about 60 miles to the south of the Mesabi range during deposition) thus somewhat older than those in Minnesota-Ontario. The thickest and uppermost units in the basin, essentially lithostratigraphic correlatives but probably differing somewhat in age, are the Michigamme, Tyler, and Copps Formations of the southeastern segment and the Thomson, Virginia and Rove Formations of the northwestern segment. These are typical turbidite-shale (flysch) sequences, with graded beds and intercalated muddy “rain-out” sediment. Figure 4-5 is a regional correlation chart of the aforementioned units, as well as others that cannot be discussed herein. Page 103 �Figure 4-4: Schematic cross-section depicting deposition of the Animikie Group turbidites that overlie shelf deposits in the Animikie basin, with sediment derived from both the north south. The southern area, the fold-and-thrust belt, comprises a complex assemblage including: 1. accreted Paleoproterozoic volcanic and plutonic rocks and volcanic rocks of the Wisconsin magmatic terranes; 2. accreted Archean miniplate terranes; 3. older Paleoproterozoic passive-margin sedimentary rocks and volcanic rocks produced during initial rifting of the continental margin, both scraped off the southward-subducting Archean Superior craton; and 4. recycled initial foredeep deposits, possibly including basal shallow water sandstones deposited in the transgressing sea of the northward-migrating foreland basin. The peripheral bulge comprises a source-rock assemblage of Archan granitic rocks and Archean volcanic-sedimentary (greenstone) belts. Scale is approximate. Compare with Fig. 4-3. Modified from Ojakangas (1994) and references included therein (from Ojakangas, et al., 2001). Ages Along the Mesabi Range, the Pokegama Formation rests unconformably on diabase dikes of the Kenora-Kabetogama dike swarm that gives a Rb-Sr isochron age of 2125 +/- 45 Ma (Southwick and Day, 1983; Beck, 1988) and this provides a maximum age for deposition of the Pokegama. A minimum age of 1930 +/- 25 Ma (Pb/Pb) for the Pokegama was obtained by Hemming et al. (1990) from quartz veins that cut the Pokegama. A U/Pb age on euhedral zircons from an ash layer in the lower Gunflint Iron Formation of Ontario is 1878 +/- 2 Ma (Fralich and Kissin, 1998; Fralick et al., 2002). A similar age of 1874 +/- 9 Ma was obtained on zircon from rhyolite in the Hemlock Formation that is adjacent to (and is interlayered with?) the Negaunee Iron Formation in the Marquette Range Supergroup of Michigan (Schneider et al., 2002). A zircon age from an ash layer near the base of the Virginia Formation is 1850 Ma (Hemming et al., 1996), and an age of 1821 +/-16 Ma has been obtained from an ash layer in the Rove Formation about 70 m above the Gunflint Iron Formation (Kissen et al., 2003). Several of these ages are shown on Figure 4-5. Page 104 �Gunflint Range Mesabi Range Cuyuna Range Emily District Gogebic Range Gogebic Range Menominee Range Baraga Basin Pokegama Fm Michigamme Fm Ironwood IF unconformity Palms Fm North Range Group Rabbit L. Fm 2125-1930 Ma deposition occurred sometime during this range Bijiki IF unconformity Goodrich Qtzite unconformity unconformity Negaunee IF Fence River IF Vulcan IF Siamo Slate Hemlock Volc. Hemlock Volc. unconformity Trout Lake Dolo Bad River Dolo Mille Lacs Group Sunday Qtzite Michigamme Fm Marquette District Lower Member Emperor Volcs Trommald IF Iron River/Crystal Falls Michigami Fm Mahnomen Fm 1870 Ma unconformity Mille Lacs Group Greenwood Iron-fm Thomson Fm unconformity Ajibik Qtzite Wewe Slate Kona Dolomite unconformity unconformity 1874 Ma Randville Dolo. Mesnard Qtzite Sturgeon Qtzite (Glen Township) (Denham Fm) Fern Creek Fm. Baraga Group Menominee Group Menominee Range Iron River/Crystal Falls Fortune Lake Slate Michigami Fm Stambaugh Fm Michigami Fm Goodrich Quartzite unconformity Negaunee Iron-fm Amasa Iron-fm Hemlock Volcs 1874 Ma unconformity Vulcan Iron-fm Siamo Slate Felch Fm Ajibik Quartzite Chocolay Group Marquette Range Supergroup Marquette Range unconformity Paint River Group Iron-formation Copps Fm Menominee Group Biwabik Fm Pokegama Qtzite Upper Member Tyler Fm Chocolay Group Gunflint IF 1878 Ma Kekabeka Qtzite 1850 Ma Virginia Fm Animikie Group 1821 Ma Virginia Fm Animikie Group Rove Fm Animikie Group Animikie Group Michigami Fm Hiawatha Graywacke unconformity Riverton Iron-fm Dunn Creek Slate Badwater Grnstn unconformity Randville Dolomite Kona Dolomite Sturgeon Quartzite Mesnard Quartzite Enchantment L. Fm Fern Creek Fm. detachment surface Figure 4-5: Generalized correlation chart of Paleoproterozoic strata in the Lake Superior region (after Morey and Southwick, 1995). Note that recently obtained age dates are shown for the Gunflint Formation, Mahnomen Formation, Hemlock Volcanics, and the Rove and Virginia Formations (see text for references). Also included is a correlation chart (lower right corner) of strata in Menominee, Iron RiverCrystal Falls and surrounding terranes (LaBerge et al., 2003 – includes changes to previous usage not yet officially adopted by the USGS). Iron-formations are shaded. Modified from Severson et al., 2003. Pokegama Formation The formation has long been called the Pokegama Quartzite, but because it contains appreciable argillite and siltstone, the name Pokegama Formation is more appropriate. It has been studied by several workers since it was named by Winchell (1893) for exposures at the western end of the Mesabi Range. Much of the previous work has been summarized by Morey (1972, 1973, 2003). Few natural exposures exist, as thick glacial drift generally covers the formation. Outcrops, roadcuts, and mine cuts occur at a few places along the length of the range, but most exposures are in the central portion of the range. A few drill holes have penetrated the entire formation. One is located just south of Eveleth (NE 1/4, NE1/4, Section 5, T. 57 N., R. 17 W.) and another is southwest of Mountain Iron (SE 1/4, SE 1/4, Section 8, T. 58 N., R. 18 W.); the thicknesses are 167 ft and 85 ft, respectively (Fig. 4-6). Three other drill cores, recently rediscovered, have not yet been studied. Numerous drill holes have penetrated only the upper few feet of the formation, as the drilling was generally undertaken in relation to iron ore exploration and development. The Pokegama is thin at the eastern end of the range and thickens to the western end where it may be more than 300 ft thick. The formation is composed of three main rock types—argillite, siltstone, and quartzite. The latter is generally a silica-cemented quartz sandstone, and is therefore an orthoquartzite rather than a metaquartzite. (Morey, 2003, has determined that mineralogical changes in the Pokegama and the Biwabik are the result of diagenesis rather than metamorphism, except at the eastern end of the range adjacent to the Duluth Complex.) These three rock types make up three gradational Page 105 �members-- lower, middle, and upper-- respectively, as shown in Figure 4-6. Minor thin conglomerates occur at the base of the formation, and seem to represent a weathered residuum on the surface of Archean rocks, perhaps reworked by fluvial processes. The Pokegama unconformably overlies Archean metavolcanic, metasedimentary, and plutonic rocks. There may be as much as 100 ft of relief on the Archean surface (Grout and Broderick, 1919), but the surface was, nevertheless, essentially a peneplain. Some Archean “knobs” were islands when the Pokegama was being deposited, and are present in the wooded areas between Eveleth and Virginia. The Pokegama-Biwabik contact is gradational, with some cherty horizons in the upper Pokegama. Various geologists have placed the contact at different stratigraphic levels. EVELETH M BIF 50 SANDSTONE, FINE-MEDIUM, SHALE MINOR CHERT SANDSTONE, COARSE 40 UPPER MEMBER SANDSTONE, FINE, SHALE MINOR MOUNTAIN IRON SANDSTONE, FINE M 30 SILTSTONE, SHALE 30 BIF SANDSTONES, THIN SANDSTONE, COARSE SANDSTONE, FINE MIDDLE MEMBER 20 20 SILTSTONE, SILTSTONE, SHALE SHALE SANDSTONES, THIN 10 10 SHALE, SILTSTONE, MINOR SHALE, SILTSTONE CONGLOMERATE 0 Archean 0 LOWER MEMBER Archean Figure 4-6: Measured sections from two drill holes that penetrate the entire Pokegama Formation. Dark shading represents shale, thin blank units represent siltstone, and dotted pattern represents sandstone. BIF is Biwabik Iron Formation. After Ojakangas (1983). Biwabik Iron Formation This is one of the world’s major iron-formations, and the largest in the United States. The formation is 200 to 750 ft thick and consists of four divisions as defined by Wolff (1917). These lithostratigraphic units, now informal members, are from the bottom up, the Lower Cherty, the Lower Slaty, the Upper Cherty, and the Upper Slaty. (These are miners’ terms, and do not indicate metamorphism.) The cherty members are dominantly granular (i.e., sand-textured), thick-bedded (several inches to several feet), and are largely composed of chert and iron oxides. The slaty members are dominantly fine-grained (i.e., mud-textured), thin-bedded (<1 inch) and Page 106 �composed mostly of iron silicate and iron carbonate with local chert beds. However, these two rock types are interbedded on all scales and are generally gradational. They contain about the same high quantities of silica, 42-47% (Morey, 1992). The Lower Slaty is not present at the far western end of the range. There are some diagnostic marker units within the formation. Two stromatolite-bearing intervals several feet thick are present, one at the base of the Lower Cherty and the other in the middle of the Upper Cherty. The black “intermediate slate” at the base of the Lower Slaty is reportedly an ash-fall tuff containing about 4 to 5.5 % aluminum oxide (Morey, 1992). At the top of the Upper Slaty are several feet of limestone and dolomite. Most of these marker units pinch out to zero in the vicinity of Nashwauk, about 40 mi from the west end of the range (Morey, 1992). Virginia Formation There are rare exposures of the Virginia in mines at the east end of the Mesabi Range where it has been metamorphosed by the mafic intrusions of the Mesoproterozoic Duluth Complex. Several holes drilled south of the range to study the underlying iron-formation have been drilled through the Pleistocene cover and have intersected as much as 1443-ft of the preserved lower part of the formation (Lucente and Morey, 1983). The lower portion of the formation in the drill holes is dominantly black shale. The upper portion of the drill core, while still dominantly shale, contains beds of siltstone and finegrained feldspathic graywacke comprising thickening- and coarsening-upward turbidite sequences. Ash-fall tuffs, cherty sideritic iron-formation, chert, and limestone are minor rock types low in the formation. The contact with the underlying Biwabik Iron Formation is gradational. The clastic rocks were largely derived from the Archean rocks to the north, with some contributions from lower Proterozoic rocks to the south (Lucente and Morey, 1983). The Virginia Formation is correlated with the Thomson Formation (Morey and Ojakangas, 1970) that is exposed 60 miles to the south in the vicinity of Carlton and Cloquet, and also with the Rove Formation in northeasternmost Minnesota and adjacent Ontario (Morey, 1967). Environments of Deposition, Animikie Group The Pokegama is interpreted to have been deposited in a tidally influenced shallow marine setting near the shoreline, having received clastics from the Archean basement to the north (Ojakangas, 1983). In this model of a transgressing sea, the lower (argillaceous) member was deposited at the shoreline in the upper tidal flat, the middle member of intercalated argillaceous and silty sediment was deposited seaward in the middle tidal flat, and the upper member of quartz sand was deposited still further seaward in a lower tidal flat/subtidal environment. This is illustrated in Figure 4-7. Walther’s Law is applicable here, with the vertical facies showing the relationships of the lateral facies. The lowermost Pokegama contains siltstone beds that contain alternating thicker and thinner laminae that have been interpreted as evidence of the diurnal inequality, and are being investigated further for possible clues to the Paleoproterozoic lunar orbit (G. Ojakangas, 1996). Page 107 �la nep e P in High Tide Low Tide NT ME DI SE Mud, Mud Silt, Sand Sand Sand Upper FA ES CI Middle Lower Shoal or Barriers TIDAL FLAT SUBTIDAL "Cherty" ironformation "Slaty" ironformation Shallower Pelagic mud, Turbidites Deeper SHELF SLOPE Figure 4-7: Sedimentation model showing lateral relationships of the siliciclastic tidal facies of the Pokegama Formation, the two main facies of the Biwabik Iron Formation, and the Virginia Formation (on the slope?). Thicknesses and geography not to scale. From Ojakangas (1983). The Biwabik is interpreted to have been deposited seaward of the Pokegama on a shallow marine, tidally dominated shelf (Figure 4-7). Precipitation of iron minerals including iron carbonate, iron silicate, silica, and perhaps some hematite, occurred on the outer shelf in waters below wave base, giving rise to the mud-textured (slaty) iron-formation. These minerals were likely related to upwelling waters from the deeper part of the basin. The two sand-textured members (Lower Cherty and Upper Cherty) formed in a shallowwater, high-energy environment, as indicated by stromatolites, cross-bedding, and rounded (locally oolitic) grains of iron minerals and chert. Shoreward-moving tidal currents (i.e., flood tides) and/or storms may have disrupted the mud-textured sediment (i.e., precipitates) and transported sand-sized aggregates into shallower water where they were altered by seafloor processes and early diagenetic processes. Thus these granules are “intraclasts” derived from within the basin. Shallow channels up to a mile wide and tens of feet deep were cut into the Lower Slaty member and filled with sand-textured grains of iron minerals and chert at Minntac where this is called the IBC unit (interbedded chert unit). These grains apparently were derived from shallow water and carried seaward into the deeper water environment in which the iron minerals were precipitating. Ebb-flow tidal currents are interpreted as the erosion and transportation agent. A plot of 102 cross-bed measurements in the Minorca Mine on the northeast edge of the Virginia Horn (Fig. 4-2) shows 90 % of the readings making a very prominent mode to the northnortheast and a minor broader mode to the south (Fig. 4-8). This distribution is interpreted as the product of a strong flood tide toward the paleogeographically determined northern shoreline and a much weaker ebb tide. Page 108 �N Figure 4-8: Paleocurrent rose diagram of 102 cross-bedded measurements from the Lower Cherty in the Minorca Mine. 102 A study of the orientations of stromatolite mounds in the stromatolite horizon within the Upper cherty member was conducted by Kevin Boerst (1999) as a UROP (Undergraduate Research Opportunity Project) at the University of Minnesota Duluth. His map is presented here as Fig. 4-9). A paleocurrent plot of mound elongation (Fig. 4- 9) is interpreted as the result of shore-normal tidal currents and shore-parallel longshore currents in shallow water. The repetition of the cherty and slaty members has long been interpreted as the result of transgression and regression (White, 1954). The Virginia Formation was deposited seaward of the iron-formation, probably in a slope-type environment (Fig. 4-7) where episodic turbidity currents deposited graded beds. Some volcanic ash falls evidently settled into the basin forming graded beds with a totally volcanic composition. The dominance of black, fissile shale suggests the “raining out” of clay (i.e., settling through the water column) and deposition in deep, anoxic water below wave base. Minor thin sandstone lenses were deposited by bottom currents (Lucente and Morey, 1983). N 12 N 10 50 70 70 Feet 50 12 90 90 110 110 130 130 10 10 12 10 12 K. Boerst, 1999 (Unpub.) K. Boerst, 1999 (Unpub.) 150 150 20 0 20 40 80 60 100 120 140 160 180 Feet Figure 4-9: Mapped stromatolite mounds in the Algal submember (I submember) in the Upper Cherty of the LTV 2E pit. The rose diagram represents the elongation of the mounds. From unpublished work by Kevin Boerst (1999). Page 109 �Mineralogy The detailed origins of the iron minerals are exceedingly complex and are beyond the scope of this introduction. It has to suffice here to say that Eh and pH are major controls on the stability of the various iron minerals in both the depositional and diagenetic environments, and in the easternmost Mesabi, in the metamorphic environment as well. Recrystallization and replacement of the granules during diagenesis has been extensive, and probably consisted of a number of discrete events. Earlier work on the oxidized taconites of the western Mesabi was accomplished by Bleifuss (1964). He showed that late hematite was developed by the oxidation and pseudomorphic replacement of magnetite octahedra, that layers of goethite were precipitated from solutions likely derived from the oxidation of siderite, and that some goethite formed by the oxidation of acicular iron silicate minerals. Some of the hematite inclusions and crystals in magnetite are similar to those illustrated by Han (1982). He proposed that much of the magnetite formed by the replacement of, and overgrowth on, pre-existing hematite that served as nuclei. Han further suggested that ionic diffusion of ferrous iron was a key process in the formation of the magnetite. Organic carbon may have acted as a reductant in this process. The nature of the major hydrologic events that removed the 40-60% of the silica and oxidized the iron minerals, thus forming the high-grade (natural) ore bodies, has long been debated. Were they descending cool meteoric waters or ascending hydrothermal waters related to igneous activity? Did this occur during the Cretaceous (the age of conglomerates composed of clasts of high-grade hematite), or prior to that time? Morey (1999) provided an excellent review of the arguments. He then proposed that a large-scale topography-driven hydrothermal groundwater system moved waters northward through the sands of the underlying Pokegama Formation, from the vicinity of the regional Penokean orogenic uplift in northern Wisconsin and east-central Minnesota, 40 to 80 miles to the south. Production Figures – Iron Ore and Taconite The annual amount of direct shipped and taconite produced are shown in Figure 4-10. Production and shipping of direct ore started in 1892 and rose steadily until 1953 when a maximum 76 million tons were produced in one year (note the precipitous drop in direct ore production corresponding to the Depression). At around 1955, there was a dramatic decrease in the amount of direct ore as the various mines became depleted. This also corresponds to the initial start-up of taconite mining, using a concentrating and pelletizing method developed by E.W. Davis of the University of Minnesota. Reserve Mining opened the first taconite operations in 1955 (Peter Mitchell Mine) and was shortly followed by Erie Mining in 1957 (old LTV site). Six more taconite operations were added in the 1960s, and by 1967, annual taconite production exceeded direct ore production. The mid-1980s marked a serious depression in the iron ore and steel industry that resulted in the closure of one operation (Butler Taconite) and the bankruptcies of two other taconite producers. More recently, LTV Steel and Eveleth Taconite have closed; Evtac has since reopened as United Taconite. Page 110 �80,000,000 TACONITE DIRECT ORE 70,000,000 TONS PRODUCED 60,000,000 50,000,000 40,000,000 30,000,000 20,000,000 10,000,000 18 92 18 97 19 02 19 07 19 12 19 17 19 22 19 27 19 32 19 37 19 42 19 47 19 52 19 57 19 62 19 67 19 72 19 77 19 82 19 87 19 92 19 97 20 02 0 YEAR Figure 4-10: Annual production figures for direct ore (includes all forms of direct ore) and taconite for the period 1892-2003. Data and graph from James Sellner, Minnesota Department of Natural Resources, Lands and Minerals Division, Hibbing, MN. What’s in a Name? (Those confusing iron-formation submembers!) As early as 1917, the Biwabik Iron Formation (BIF) has been informally broken down into four major lithostratigraphic members, or subdivisions, known as (from bottom to top): Lower Cherty, Lower Slaty, Upper Cherty, and Upper Slaty (Wolff, 1917). The cherty iron-formation members are generally thick-bedded and contain round grains (0.5-2.0 mm) of chert that are referred to as granules. These “cherty” members typically contain higher percentages of iron oxides (magnetite, hematite and/or goethite). In contrast, the “slaty” members are thin-bedded (0.5-3.0 mm thick beds) and very fine-grained. They are composed mostly of Fe-silicates and Fecarbonates. Both cherty and slaty iron-formation types are interlayered at all scales. However, one rock type often predominates in each of the four lithostratigraphic members, and are sonamed due to this dominance, i.e., thick-bedded cherty iron-formation is dominant in the cherty members, whereas thin-bedded iron-formation is dominant in the slaty members. The 1917 four-fold stratigraphy of a Lower and Upper Cherty and a Lower and Upper Slaty members is still used at each of currently operating (and inactive) taconite mines on the Mesabi Range. However, each of the mining companies further subdivides the BIF into several Page 111 �submembers based on bedding types (Fig. 4-11) and mineral assemblages. It is at this point that the BIF stratigraphy becomes very complicated and at times confusing. This is mainly due to the following reasons: • There are localized lateral facies changes between mines (and even within a single mine). Some mines reconcile these differences by splitting out numerous submembers (each with a distinct bedding type, texture, ore grade, and/or mineral assemblage), whereas, other mines lump many of these same differences within a single submember. • There are significant lateral facies changes over several miles between mines. For example a particular horizon may be massive-bedded at one location but is regularbedded a few miles away. This is particularly troublesome within the Upper Cherty member in the western 2/3rds of the Mesabi Range. • Not all mines use the same numbering system – some use abbreviations (LC for Lower Cherty, etc.) followed by a number (as in LC-5 at the top of the Lower Cherty). However, other mines use an alphabet system, devised by Gundersen and Schwartz (1962), starting with the A submember at the top of the Upper Slaty (in this system the top of the Lower Cherty corresponds to the R submember). And further still, another mine refers to the Lower Cherty as the number 1 unit and subdivides it into eight submembers, with 1-8 at the top of the Lower Cherty. • Some mines label downward in their numbering system, whereas other mines label upward in their numbering system. Textures associated with granular rocks Disseminated Diffuse Granules Mottled Patches Textures associated with laminated rocks Regular, Sharp Wavy/Irregular, Sharp Regular, Diffuse Wavy/Irregular, Diffuse Shaly Thin-bedded Figure 4-11: Textural characteristics of the Biwabik Iron Formation (modified from Pfleider et al., 1968, from a classification scheme developed by geologists of the Hanna Mining Company). Page 112 �The submember nomenclature that is used at each of the mines is summarized in Figure 412. It can readily be seen on this summary that any particular submember name changes nomenclature from one mine to the next. This is because there are few good marker horizons within the BIF, and even these can exhibit gradual lateral facies changes or pinch-and-swell relationships to each other. A few of the potential marker horizons within the BIF are presented below. • Top contact of the BIF with the Virginia Formation – In the eastern half of the Mesabi Range a carbonate horizon is present at the very top of the Upper Slaty and the contact between the BIF and Virginia Formation is easily recognized (Gruner, 1924). However to the west of Hibbing, the carbonate layer is absent and lenses of thin-bedded Fe-carbonate iron-formation are present in the Virginia Formation, and the top of the BIF is not easily discerned. • Submember I/Algal unit – a thin unit containing algal stromatolites and jasper-bearing intraformational conglomerate is present near the top of the Upper Cherty. This submember is easily recognized but is not present to the west of Hibbing. • Lower Slaty member – The Lower Slaty has a very-well defined “Intermediate Slate” (also referred to as the Q submember or Paint Rock in Fig. 4-12) at its base which is characterized by a black, carbon-rich, thin-bedded, slaty unit that commonly contains pyrite. This unit is readily evident at all of mines on the range. However, the upper contact of the Lower Slaty is “indefinite and a gradual change to other slaty phases takes place” … [which makes] … the dividing line between the two [Upper Cherty and Lower Slaty] somewhat arbitrary” (Gruner, 1924, p.20). The upper contact of the Lower Slaty is particularly troublesome in the Virginia Horn area. Gruner (1946, p. 45) included lenses of cherty and wavy-bedded taconite (referred to as the Interbedded Chert -- IBC unit at Minntac; Fig. 4-12) in the Lower Slaty, whereas White (1954) included these same units in the overlying Upper Cherty member. • Base of the BIF - The base of the Lower Cherty is generally characterized by thin-bedded iron-formation (also called the “red basal unit”) with localized algal stromatolite and basal conglomerate horizons. However, at many localities the base of the BIF exhibits a gradational contact with the underlying Pokegama Formation. In the Virginia Horn area the base of the BIF contains an iron-bearing sandstone (White, 1954) that some mines include with the iron-formation, whereas others lump this type of material with the Pokegama Formation. From the above description it is readily evident that there are few good marker horizons within the iron-formation; even the upper and lower contacts of the iron-formation are gradational and subject to various interpretations. The “Intermediate Slate” and the algal horizon in the Upper Cherty are the only easily recognizable marker units. However, even using these horizons as markers, one can see from Fig.4-12 that there are problems. Clearly, much additional work needs to be done in understanding how submembers at one mine correlate with submembers at an adjacent mine. These types of studies could inevitably be important in determining why ore grades, and waste rock characteristics, change between mines and even within a single mine. For example, some of the best ore-grade taconite corresponds to the wavy-bedded or irregular-bedded taconite present in both the Lower or Upper Cherty members. The sedimentary environment that produced this type of taconite ore has not been fully documented nor have any recent detailed sedimentological studies been attempted. A better understanding of the various sedimentological textures in the BIF could ultimately lead to an increased ability to better predict changes in ore grades as they relate to facies changes. Page 113 �"Eastern" Mesabi Range LTV (Cliffs-Erie) EVTAC (T-Bird So.) Upper Cherty United (T-Bird No.) LC-5 mass-bdd? LC-4 wavy-bdd LC-3 straight-bdd LC-2 thin-bdd LC-1 ss & congl Lower Cherty Ispat Inland (Minorca pit) upper Upper Cherty Algal MUC waste - mixed-bdd MUC - thick/mass-bdd LUC waste - thin-bdd LUC waste - irreg/mass-bdd LUC - reg-bdd LUC - wavy/thin-bdd LUC - slaty LS - thin-bdd er Intermediate Slate Low TLC - thick/thin-bdd TLC - mass congl TLC - irreg/mass-bdd BLC - wavy-bdd Footwall Lean - thin-bdd Footwall Ore - reg-bdd Footwall Taconite Slaty y Slat US-2 slaty pper US-1 thick & thin-bdd U UC-7 thick-bdd UC-6 thick-bdd Ispat Inland UC-5 algal/congl (Laurentian pit) UC-4 thick & thin-bdd US UC-3 mass-bdd UC-3 thick-bdd UC-2 thick & thin-bdd UC-2 reg/thin-bdd UC-1 thin-bdd UC-1 reg-bdd LS-3 slaty IF LS LS-2 thin-bdd Intermediate Slate Intermediate Slate LC-5B transition zone LC-6 thick-bdd LC-5A mass-bdd LC-5 irreg-bdd LC-4 wavy-bdd LC-4 wavy-bdd LC-3 reg-bdd LC-3 reg-bdd LC-2 thin-bdd LC-2 thin-bdd LC-1 sandstone & congl LC-1 ferrug seds & congl A - limestone B - vague-bdd C - thin-bdd D - thin & wavy-bdd E - mass-bdd F - thin-bdd G - wavy/reg-bdd H - thin/wavy-bdd I - algal/congl J - wavy-bdd K - wide-spaced wavy-bdd L - wavy-bdd M - reg-bdd O - wide-spaced wavy-bdd P - thin-bdd Q - carbonaceous IF R - thick-bdd S - vauge wavy-bdd T - wavy-bdd J - thin/reg-bdd V - thin-bdd W - algal/congl Northshore A - chert & marble B - chert & diopside C - thin-bdd Upper Slaty D - thin & wavy-bdd E - mass-bdd F - thin & wavy-bdd G - mass-bdd H- wavy-bdd I - algal/congl J - thick-bdd K- wavy-bdd L - wavy-bdd Upper Cherty M - thin/reg-bdd N - ?-bdd O - ?-bdd P - thick-bdd Lower Slaty Q - graph. arg. IF R - thick-bdd S - ?-bdd T - ?-bdd Lower Cherty U - reg-bdd? V - w/congl MUC = middle Upper Cherty LUC = lower Upper Cherty TLC = top of Lower Cherty BLC = bottom of Lower Cherty Virginia Horn Upper Cherty Upper Slaty Keewatin (National) & Butler US UC LS paint rock LC-1 thick-bdd LC-2 irreg-bdd LC-3 thick-bdd LC-4A Upper irreg-bdd LC-4A Middle thick-bdd LC-4A Lower wavy/thick/thin-bdd LC-4B irreg-bdd LC-4C Upper irreg-bdd LC-4C Middle laminated LC-4C Lower reg/thin-bdd LC-5A reg/thin-bdd LC-5B thin-bdd LC-6 ?-bdd jasp & thin-bdd thin-bdd congl UC Slaty Lower2-1 thin-bdd 1-8 reg-bdd 1-7 wavy-bdd 1-6 wavy-bdd 1-5 wavy-bdd 1-4 thin/wavy-bdd 1-3 even-bdd 8-3 slaty 1-2 mass-bdd 1-0 red basal slaty algal/congl Hibbtac (Buhl-Kinney) 2-1 thn-bdd 1-8 mass-bdd 1-7 wavy-bdd 1-6 wavy-bdd 1-5 mass-bdd 1-4 slaty 1-3 even-bdd 1-2 slaty 1-1 thin-bdd 1-0 congl Red Basal Lower Cherty Lower Slaty US UC-3 thick-bdd UC-2 thick-bdd UC-1 reg-bdd LS paint rock LC-5 thick/reg-bdd LC-4 wavy-bdd LC-4 wavy-bdd LC-2 reg-bdd LC-1B thin-bdd jasper thin-bdd algal/congl LC-1A Upper Slaty Coleraine area (USS) Dolomite Upper Slaty Upper Slaty - thin-bdd UC-16 - irreg/thin-bdd UC-15 algal/congl y er t UC-14 slaty Ch UC-13 thin-bdd r e p UC-12 congl w/algal frags Up UC-11 slaty LS-10 even-bdd LS-9 even/thick/mass-bdd LS-8 slaty LS-7 (IBC) wavy-bdd Ispat Inland LS-6 thin-bdd (Minorca pit) carbonaceous IF LC-5B thick-bdd LC-5 mass-bdd? LC-5A mass-bdd LC-4 wavy-bdd LC-4 wavy-bdd L LC-3 straight-bdd ow LC-3 wavy-bdd LC-2 thin-bdd er LC-2 even-bdd Ch LC-1 ss & congl LC-2 thick/mass-bdd ert LC-1 arg IF y LC-1 thin-bdd LC-1 algal/congl we rS lat y Hibbtac (Hibbing) Minntac Lo "Western" Mesabi Range Figure 4-12: Correlation chart of submembers, at each of the mines/areas within the Biwabik Iron Formation, as deciphered from published descriptions and mine handouts (modified from Zanko et al., 2001). All columns are hung on the base of the Lower Slaty (“Intermediate Slate”). It is important to note that this summary is preliminary as it has not been field-checked. No scale is implied and the true thickness of each submember is not portrayed. Bars to the left of the columns indicate mined taconite ore zones. Note that there are several consistent submembers within the Lower Cherty as opposed to very few laterally persistent submembers in the Upper Cherty. Page 114 �STOP DESCRIPTIONS Babbitt Hywy 21 Minn 135 Minn 169 4-10 4-11 Du nka Ro ad Embarrass Hywy 21 U.S. 53 nn Mi Minn 135 169 Du n oad ka R 4-9 T59N lut u R13W D Biwabik 4-6 Minn 169 T58N T57N Virginia 4-5 Gilbert 4-4A 4-4C 4-1 4-4B 4-2 Eveleth 4-3 Aurora 4-7 nn Mi lex p om C h R12W Hywy 110 135 Hoyt Lakes Minn 37 U.S. 53 R18W R17W R16W R15W R14W Figure 4-13: Locations of stops that will be visited on the field trip. Generalized contacts of the Biwabik Iron Formation, for the eastern half of the Mesabi Range, and Duluth Complex are shown. Note that in order to observe Stops 4-1, 4-2, and 4-3 in stratigraphic order, we will be back-tracking for short distances. STOP 4-1A: Pokegama/Archean Unconformity Location: On U.S. 53 in Eveleth, there is a stoplight at Grant/Industrial Avenue. Proceed northward past the stoplight for about 0.4 mi north to a gas station. Turn right and immediately turn left on the frontage road (Midway Drive). Drive past a church and past the first street on the right (Mesabi Drive). Watch for a low, small outcrop in the trees on the right, just a few feet off the road. Eveleth 7.5’ quadrangle, T.58N., R.17W, Sec 20, NW of SW of SE; 535445E, 5259520N (NAD83 UTMs) Description: This is the only easily accessible exposure of the unconformity. A close examination will reveal the presence of a thin smear of Pokegama conglomerate composed of schist and vein quartz clasts upon Archean schist with a near-vertical foliation. Note that the foliated schist clasts are flat and have a subhorizontal orientation. NO HAMMERING, PLEASE! STOP 4-1B: Argillaceous lower member of the Pokegama Formation Location: From stop 4-1A, CAREFULLY walk across both lanes of U.S. 53 to the long, roadcut on west side of highway. Eveleth 7.5’ quadrangle, T.58N., R.17W, Sec 20, NW of SW of SE; 535380E, 5259555N (NAD83 UTMs) Page 115 �Description: This cut is about 500 feet long and 5-10 ft high, and is the only exposure of this member. It consists largely of shale and siltstone with minor fine-grained sandstone. It has been interpreted as having been deposited in a low-energy upper tidal flat environment in a sea that transgressed onto the peneplaned surface of Archean rocks (Ojakangas, 1983). Minor channeling is common at the bases of the thicker sandstone beds, and at one spot, 0.5 m of section has been eroded. Small-scale cross-bedding is present in some siltstone beds, and elongated sole marks are visible on the bottoms of some sandstone and siltstone beds. A total of 57 of these paleocurrent indicators show that the currents were generally oriented in a north-south direction. A few concretions as large as 6 inches in diameter are present, as is soft-sediment deformation. Hemming et al. (1991) illustrated the soft-sediment folding and interpreted it as evidence that the Animikie basin was tectonically active during deposition of the Pokegama and the Biwabik. Alternatively, it is interpreted herein as soft-sediment slumping into tidal channels. Fine laminations and sequences of laminations in the sandstone beds have been interpreted as tidal rhythmites (G. Ojakangas, 1996). STOP 4-1C: Jaspillite on Archean metaconglomerate Location: From stop 4-1A, drive a long block northward to Merritt Drive. Turn right. About 100 ft farther, turn right on Mesabi Avenue. About another 100 ft farther, there is a fork in the street. Park on Mesabi Avenue and walk uphill on the street on the left to a broad, flat rock exposure in a driveway. Eveleth 7.5’ quadrangle, T.58N., R.17W., Sec 20, SW of SE (No. 7 Mesabi Lane); 535610E, 5259460N (NAD83 UTMs) Description: This is an excellent exposure of jaspillite resting unconformably on subvertical volcanogenic metaconglomerate. The outcrop is an estimated 30 to 40 ft topographically higher than stop 4-1A, and was also that much higher when the jaspillite was deposited. Is it an erosional remnant of basal Biwabik Iron Formation or is it a local chemical precipitate at the base of the Pokegama Formation? NO HAMMERING, PLEASE! NOTE: The middle member of the Pokegama Formation (a unit of intercalated beds of sandstone, siltstone, and argillite) was once poorly exposed in the flat area across the highway from the U.S. Hockey Hall of Fame. However, the best part of the poor exposure has since been covered by a frontage road. STOP 4-2: Upper member of the Pokegama Formation Location: Continue driving down Mesabi Drive to the frontage road (Midway Drive). Turn right and drive past stop 4-1A to a stop sign. Turn left to intersection with U.S. 53. Cross the northbound lane and turn left on highway. Drive south past stop 4-1B, past the Grant/Industrial Avenue stoplight, past the U.S. Hockey Hall of Fame on the right, and stop at the Rustic Rock Inn (you are now opposite a large roadcut on the east side of the highway). Walk across the highway, CAREFULLY! Eveleth 7.5’ quadrangle, T.58N., R.17W, Sec 32, E ½ of SE; 536055E, 5256775N (NAD83 UTMs) Description: This is the upper member of the Pokegama Formation, composed of silicacemented quartz sandstone. It is the rock type found immediately beneath the Biwabik Iron Formation; it was penetrated by countless drill holes during mining and exploration, and resulted in the formation being originally named the Pokegama Quartzite. Note the massive beds Page 116 �separated by thin beds of shale or siltstone. Silica cementation likely obscured some original cross-bedding. This member was interpreted by Ojakangas (1983) as having been deposited in a high-energy, lower tidal or subtidal environment. STOP 4-3: Lower Cherty member, Biwabik Iron Formation Location: Proceed south on U.S. 53 for 0.6 miles (passing over an overpass and past a long roadcut of iron-formation on the right ), to a left-turn lane. Make a U-turn and proceed north on the highway for about 0.2 miles to roadcut on the off-ramp for Highway 37 (right side of highway). Eveleth 7.5’ quadrangle, T.57N., R.17W, Sec 5, E ½ of NE of NE; 536230E, 5256285N (NAD83 UTMs) Description: Note that this member of the Biwabik Iron Formation overlies the sandy member of the Pokegama Formation of the last stop, and that both units dip gently to the southeast. Observe the thick wavy bedding, the trough cross-bedding, and the sandy texture of the iron minerals and chert grains. The cross-beds are best observed in this eastern roadcut, but are also present at both ends of the longer cut on the other side of the highway. Cross-bedding measurements (Fig. 4-16), although not definitive, are suggestive of a tidally-influenced marine environment. This, coupled with Walther’s law of succession of sedimentary facies (i.e., the facies observed vertically are also similarly related laterally), places the deposition of the ironformation seaward of the Pokegama Formation. N Figure 4-14: Rose diagram of 28 crossbedded measurements, mostly from the roadcut of Stop 4-3. 28 Stops 4-4A through 4-4C are at the Thunderbird North Mine of United Taconite (formerly Eveleth Taconite, or Evtac). Simplified stratigraphic submembers of the iron-formation, according to terminology used by Evtac, are portrayed in Figure 4-15. From stop 4-3, proceed north along U.S. 53 past stop 4-2 to the stoplight at Grant/Industrial Avenue. Turn left on Grant Avenue and follow the road past a cemetery to the entrance road to United Taconite (U-Tac). Page 117 �EVTAC Thunderbird South Virgina Fm 50' Upper Slaty US-2 Slaty silicates +/- irreg chert North 20' US-1 thick + thin BDD thick chert + thin BDD, cherty zones 20' Upper Upper Cherty UC-7 Thick BDD +/- slaty UC-6 170' UC-3 thick-BDD salt & pepper textu re 45' slaty wavy BDD salt & pepper 12' Footwall Lea n thin-B DD +/- graphite Footwall Ore Lower Cherty BLC 45' Bottom Lower Cherty LS-2 Thin BDD sil IF Vertical scale (in feet) K Lower Slaty LS Keweenawan Sill wide sp aced thin wavy beds L Wavy BDD w/ intraform rip ups M Reg BDD w/ wavy-straight BDD Thin BDD Silicate IF Algal 20-35' wide spaced wavy beds 85-130 ' 60' LC-4 wavy BDD cherty IF 55' LC-3 Reg BDD = thin beds o f chert & sil P LC-5A massive BDD w/ mottles +/- th in MGT bnds 45' LC-1 Ferrug con gl Graywacke ss = qtz grns in chl matrix Keweenawan Sill 10-14' carb clasts 65' Thin BDD Non-mag D Wavy BDD 2-6' E Sep t. cracks 13-20' F Thin & wavy-BDD Sep t. crack s 25-34' Mass BDD, MGT G granules, +/- thin BDD zones 10-20' Q carbonaceous 10-20' salt & pepper BDD w/ R Thick poor BDD/mott 45-95' LC-4 wavy BDD 4 5' LC-3 straight BDD w/ 4-5" chert bnds S Vague, wavy BDD +/- mott T Wavy BDD +/- mott Wavy BDD +/- congl K 30 -60' Thin-reg (Hem) LC-2 Thin BDD Silicate IF W LC-1 10' SS +/-congl+/-CH Upper Cherty Wav y BDD Silicate IF (minor congl) 9-2 0' Abundant MGT M (thin + reg BDD) 2-5' N Fayalite-q tz 6-17' O Fayalite-q tz Low MGT content MGT g ran ules 60' Thick BDD, green, MGT poor Lower Slaty P The most metamorph & reconstituted U Thin-reg BDD Thick + thin BDD 25-45' Thin BDD V + Arg + jasp 30' Algal / co ngl Upper Slaty 8-11' H Wavy BDD 2-5 ' I Algal/congl 15-24' Granule w/ pebb les J Thick BDD 30-47' L 50' LC-2 thin BDD = jasp, hem slaty, green silicates 26-43' C Thin BDD 40-80' 60' some salt & p epper not drilled or mined 130' 3-5 ' Int slate 15' LC-5B Transition Zone 50 Keweenawan? Sill 3-6 ' A Chert/marble 13-20' B Chert + Diop <5' Algal congl I Virginia Formation Virgina Fm 15-40' LC-5 irreg BDD cherty silicate IF Footwall Taconite Northshore H Thin wavy BDD O 145' thin-BDD rose-colored with depth Mass w/ septarian cracks, High Pho s (congl @ base) 30' Reg-BDD salt & pepper thin-BDD gray-colored Thin&wavy BDD +/- chert pebbles E J Wavy BDD, mott UC-1 Mott, Reg-BDD LS-3 Dark, slaty silicate IF int slate - graphite 25' LC-6 Thick BDD w/ irreg green sil layers 65' TACONITE ORE ir reg- to mass-BDD +/- mott 30' Lower Cherty Lower Slaty Mass congl pinkish TACONITE ORE Lower Cherty Thick- to thin- BDD 15' 0 UC-2 Reg/thin-BDD 30' 45' TLC Top Lower Cherty D 40-75' Green, wavy reg G BDD UC-2 Thick + th in BDD (ch ert/silicate) 40' Lower Slaty 0 50' 35' UC-1 Thin BDD Thin chert layers w/ MGT beds + wavy bnds LS thin-BDD black to dark green Thin BDD Thin BDD F +/- gran jasp beds TAC ORE wavy- to thin-BDD 90' C 25 35-50' TAC ORE Reg-BDD w/congl Vagu e BDD, Chert + Fe Sil 25-55' TAC ORE 40' Lo wer Upper Cherty UC-3 mass BDD, mott w/ irreg beds TACONITE ORE irreg- to massBDD & mott Upper Cherty Lower Upper Cherty Waste 50' ) Algal unit present but not described B 50 TACONITE ORE Thick + thin BDD cherty silicate thick- to mass-BDD congl w/dissem magnetite ( TAC ORE UC-4 35-55' A Limestone TACONITE ORE 50' Middle Upper Cherty thin-B DD slaty TACONITE Upper Cherty TACONITE ORE 65' 45' Virgina Fm UC-5 jasp/ algal/ congl w/ Mn! Mixed : thin-BDD, congl, mass BDD Vertical Scale (in feet) Middle Upper Cherty Waste Thick BDD pink carb motts Cliffs-Erie Site (old LTV) US Green to Grey Thin BDD, Non-magnetic SIPHON FAULT Algal <20' Upper Cherty 15' 25 Upper Slaty 50' TACONITE ORE TACONITE ORE >50' Ispat Inland Laurentian Pit TACONITE ORE EVTAC Thunderbird 26' GraphiticArgillaceous IF Q 11' R Thick BDD 8' S ? BDD/MGT rich 5' T Granules 10' U Reg BDD? 3' V Granule/qtzose Congl Lower Cherty Figure 4-15: Submember nomenclature for the Biwabik Iron Formation as used at the various taconite mines and idled mine sites that will be visited on this field trip. Modified from Plate II in Zanko et al., 2001. Note that the nomenclature portrayed here is preliminary in nature and future correlations of the various submembers used at the different mines still needs field checking and modification. The stratigraphic nomenclature is currently being revised for the Thunderbird North Mine by United Taconite, but was unavailable at press time and is not included in this figure. Page 118 �STOP 4-4A: Submember LC-3 near the base of the Lower Cherty Location: Northeast end of the Thunderbird North Mine. Eveleth 7.5’ quadrangle T.58N., R.17W., Sec 20 , 535290E, 5260255N (NAD 83) Description: The LC-3 submember (new U-Tac terminology or “footwall ore” in the old Evtac terminology) is characterized by regular-bedded and wavy-bedded cherty taconite wherein most of the magnetite is contained within dark-colored wavy (irregular) beds that locally show good pinch-and-swell relationships. This is the lowest unit mined at the Thunderbird North Mine and contains approximately 20-23% mag-Fe and 2% Davis Tube Si. The Auburn Fault can also be viewed at this locality. The fault zone consists of a 20-footwide zone of broken-up and highly oxidized vuggy rock. To the northwest is the Auburn Mine that was originally developed as an underground mine by the Minnesota Iron Company in 18941902. It was reopened as an open pit by the Oliver Iron Mining Company (a subsidiary of United States Steel Corp.) in 1951 and was essentially closed in the 1960s; scram operations continued intermittently until 1999. The mined ore was located along a main ore trough that coincides with the northwest-trending Auburn Fault. Mined material was obtained from the Upper Cherty and Lower Slaty members, and locally from the Lower Cherty member. Jim Small (personal communication to Peter Jongewaard) reported that he was unable to find any evidence of the Auburn Fault anywhere on the newly-scraped floor of the mine during the final scramming period prior to allowing the pit to fill with water. United Taconite is currently filling the mine with waste rock. STOP 4-4B: Submember LC-4 near the base of the Lower Cherty Location: Thunderbird North Mine. Eveleth 7.5’ quadrangle T.58N., R.17W., Sec 31, 533930E, 5257810N (NAD 83) Description: The LC-4 submember (BLC submember – old Evtac terminology) exemplifies typical wavy-bedded taconite (see Fig. 4-16 at end of this report) that is characteristic of much of the taconite ore of the Mesabi Range. This unit varies from the previous stop in that wavy-beds are dominant and locally exhibit cross-bedded features. At this stop the magnetite is in: 1. the wavy beds; 2. disseminated throughout the cherty bands; and 3. within mottles that are generally less than 1 cm in diameter and cored by Fe-carbonate (siderite). The LC-4 is 40-45-ft-thick and contains 20-27% mag-Fe and 1-2% Davis Tube Si. The LC-4 submember is easily recognized in drill core due to the wavy beds and a salt-and-pepper texture that is defined by disseminated magnetite. STOP 4-4C: Submembers LC-5 through LC-8 near the top of the Lower Cherty Location: Thunderbird North Mine. Eveleth 7.5’ quadrangle T.58N., R.17W., Sec 19; 533915E, 5259540N (NAD 83) Description: At the base of this pit exposure is the LC-5 submember (according to the new UTac terminology, or TLC in the old Evtac terminology) characterized by thick- and irregularbedded, mottled chert with 23-28% mag-Fe. The mottles are pink Fe-carbonate; they are not readily evident on fresh break but appear after a short weathering period. Overlying the LC-5 is a 1-1.5-ft-thick intraformational conglomerate (LC-6 submember – U-Tac terminology) that Page 119 �contains abundant red jasper fragments. The next overlying unit is the LC-7 submember (U-Tac terminology) characterized by thin magnetite-rich beds interlayered with green, Fe-silicatebearing, chert beds (1-6 inches thick). The LC-7 is ore with 20-25% mag-Fe, whereas, the overlying LC-8 submember, which physically resembles the LC-7, is waste as the thin beds no longer contain magnetite – rather Fe-silicates are dominant in the thin beds. This is the last stop at the Thunderbird North Mine. From here we will return to U.S. 53 and proceed about 1.5 miles north to a gated entrance on the right to the “Mineview in the Sky” – follow the signs to the overlook on top of a waste rock dump. Stop 4-5: Mineview in the Sky Overlook Location: Eveleth 7.5’ quadrangle, T.58N., R.17W., Sec 17, Ne of NW of SE; 535710E, 5261650N (NAD83 UTMs). Description: The open pit immediately below the overlook is the Rouchleau Mine that connects northward with additional mines. The Rouchleau Mine produced 52,000,000 tons of high-grade “natural ore” containing 50-55% iron from 1920 to 1976. This area is located on the “Virginia Horn” which is a large Z-shaped bend in the otherwise straight ENE trend of the Mesabi Range. Six miles to the northwest on the horizon is United States Steel’s Minntac taconite operation. Steam visible to the northeast of Virginia is from the Ispat Inland taconite plant. From the Mine View we will proceed to the Minorca Mine of Ispat Inland by driving north on U.S. 53 approximately 4 miles around the town of Virginia. About 1 mile north of the third stop light, make a right turn onto an unnamed gravel road (this road is 0.4 miles north of the road to the golf course). Follow this gravel road across 9th Ave. (paved) and continue to the Minorca mine gate. Stop 4-6 (optional): Cross-bedded LC-3 submember, Minorca Mine Location: Near the northwest of the Minorca Mine (UTM coordinates will be acquired at the site during the field trip). Description: Cross-bedding is exceptionally well exposed at one spot in this mine. Most crossbeds here indicate paleocurrents to the north, but one excellent herringbone cross-bedded set shows paleocurrent trends toward azimuth 0º (lower cross-bed) and azimuth 220º (upper crossbed). See Figure 4-8. If stop 4-6 is visited, we will cross the Laurentian Divide on private company mining roads to gain access to the Laurentian Mine of Ispat Inland. If stop 4-6 is not visited we will proceed southward on U.S. 53 from the Mineview overlook and head west on Highway 135 (about 3 miles) to the town of Gilbert. At the junction of highways135and 37, turn left and proceed north to the gated entrance to the Laurentian Mine of Ispat Inland. The stratigraphic terminology of submembers in the Laurentian Mine is portrayed on Figure 4-15. Page 120 �STOP 4-7A: Submember LC-5a and LC-5b at the top of the Lower Cherty Location: Laurentian Mine. Gilbert 7.5’ quadrangle T.58N., R.17W., Sec 24, NW of SE of NE; 542240E, 5260650N (NAD 83) Description: This stop picks up where we left off at the last stop at the Thunderbird North Mine. At this locality the LC-5a and LC-5b submembers are exposed. The LC-5a is a wavy-bedded taconite ore unit with pink to brown Fe-carbonate patches, lenses, and mottles. The overlying LC-5b submember consists of green Fe-silicate-rich granular cherty material. STOP 4-7B: Lower Slaty member and the “Intermediate Slate” at the base of the Lower Slaty Location: Laurentian Mine. Gilbert 7.5’ quadrangle T.58N., R.17W., Sec 24, NW of SE of NE; traverse from 542315E, 5260700N to 542485E, 5260840N (NAD 83) Description: The entire stratigraphic section of the Lower Slaty (130-ft-thick) can be viewed at this impressive face within the Laurentian Mine. The “Intermediate Slate,” at the base of the Lower Slaty, is characterized by thin-bedded, black, organic-rich slate that locally exhibits bright shiny graphitic surfaces with bedding-parallel slickensides. Pyrite is common to this submember and is present as both disseminated fine- to medium-grained cubes and as thin disks (marcasite?) along bedding planes. All of the Lower Slaty constitutes waste rock. Note that the name “slate” has been applied to all thin-bedded rocks in the BIF, but nearly every writer has pointed out that this term is a misnomer, because these rocks do not have the cleavage of a true slate but merely a parting parallel to bedding (White, 1954). Morey (1993) reported that the “Intermediate Slate” is an ash-fall tuff; however the senior author has yet to see any evidence of volcanic shards in thin-sections collected from this unit. STOP 4-7C: UC-1 submember at the base of the Upper Cherty Location: Laurentian Mine. Gilbert 7.5’ quadrangle T.58N., R.17W., Sec 24, SE of NE of NE; 542545E, 5260790N (NAD 83) Description: The UC-1 submember is a regular-bedded (1-2 inches straight/even beds) granular cherty unit that contains magnetite-rich bands, pink Fe-carbonate mottles, and local crossbedding. This material constitutes ore in that it contains 16-40% mag-Fe (the mag-Fe is extremely variable in this unit) and 2-4% Davis Tube Si. Localized pods of very Fe-rich taconite (up to 40% mag-Fe!) were present but have been recently mined out. STOP 4-7D: UC-2 submember at the middle of the Upper Cherty Location: Laurentian Mine. Gilbert 7.5’ quadrangle T.58N., R.17W., Sec 24, SE of NE of NE; 542590E, 5260740N (NAD 83) Description: This submember exhibits both regular-bedded and thin-bedded features and consists of both taconite ore zones (18-20% mag-Fe) and waste zones. At this particular site are numerous bedding-parallel quartz-carbonate veins with chlorite slickensides. The chlorite is Page 121 �particularly troublesome as it cannot be selectively removed from the blasted ore and ultimately “gums up” in the concentrator. Interbedded with the magnetite-rich beds are red-colored hematite-rich beds that appear to be primary – they are pervasive throughout the area and do not appear to be related to a later oxidation event that produced the direct shipping ores of the Mesabi Range. Also at this locality are localized red jasper beds and intraformational conglomerate lenses with red jasper fragments. STOP 4-7E: UC-3 submember at the top of the Upper Cherty Location: Laurentian Mine. Gilbert 7.5’ quadrangle T.58N., R.17W., Sec 24, SE of NE of NE; 542635E, 5260785N (NAD 83) Description: At this locality is a mixed thick-bedded, regular-bedded, and diffuse-bedded granular cherty unit with brown Fe-carbonate-rich beds (irregular to wispy appearance) as well as Fe-carbonate mottles and patches. The basal contact with the UC-2 submember is present at this locality and is marked by a seven-foot-thick thin-bedded zone at the base of the UC-3. A quartzcarbonate vein marks the contact. Within the middle of this unit is an algal stromatolite-bearing unit which will be viewed at the next stop in LTV pit 2E. This is the last stop at the Laurentian Mine. Return to the mine entrance (the one that is north of Gilbert), turn left on State Highway 135 and proceed west to Biwabik and Aurora. Follow Co. Road 110 through Aurora to Hoyt Lakes. At the stop sign in Hoyt Lakes take a left and proceed down Co. Road 666 (approximately 4-5 miles) to the guard shack near the office buildings at the Cliffs-Erie site (old LTV offices). After getting permission to enter the property, take an immediate right and follow this road about 3.5 miles to a T-intersection with the Dunka Road (private company road). Follow this road about 2 miles through a sharp right turn and a sharp left turn. At the last turn is a series of closed mining roads angling off to the south – walk down the road that points in the more southerly direction to the brow of the hill (Stop 4-8). The stratigraphic nomenclature for the Cliffs-Erie site is presented in Figure 4-15. STOP 4-8: Algal submember (I submember) near the top of the Upper Cherty Location: Cliffs-Erie site (old LTV Pit 2E). Allen 7.5’ quadrangle T.59N., R.14W., Sec 23, N1/2 of NW; 568202E, 5270625N (NAD 83). Access to this site is via the Dunka Road which is a private mining company road. Description: Algal structures were first described by Leith (1903) as “contorted bedding.” Grout and Broderick (1919) are the first who assigned an organic origin to them. The algal submember within the Upper Cherty consists of mounds of fossilized algal colonies that are separated by jasper-bearing intraformational conglomerate; both the algal and conglomerate units exhibit a combined thickness of 2-20 feet. This horizon occurs only in the eastern half of the range (not present west of Hibbing) and is only sporadically present between Hibbing and Chisholm. This locality is an excellent place to view a nearly horizontal portion of the iron-formation that contains abundant individual mounds of algal stromatolite. Stripping of glacial overburden in this area has revealed a dip slope the size of a football field that contains the stromatolite mounds Page 122 �(Graber, 1993). See Figure 4-9 in this field guide. Internally, the mounds are characterized by many individual columnar finger-like structures that are convex upward. The mounds protrude up through a thin veneer of the overlying thin- to wavy-bedded H submember. Studies of a nearby zone in this horizon showed that all the columnar stromatolites were inclined at 30 degrees to the vertical. Unfortunately, this particular site has been removed by mining activities. Stromatolite samples will be able to be collected at the extreme eastern edge of this exposure. Also at this locality the J and H submembers locally contain anthraxolite, which is an organic bitumen containing 95% or more carbon that is black with a vitreous luster and conchoidal fracture and resembles obsidian (Morey, 1994). Morey (1994) reported that anthraxolite is present throughout the iron-formation but it is most common beneath the carbon-rich “Intermediate Slate.” Furthermore, he suggested it formed via a mechanism of concentrating carbon from a mass-kill phenomenon, followed by later migration of a carbon-rich liquid to form the anthraxolite. After this site we will proceed eastward about 14 miles down the Dunka Road (through a locked gate) to the Peter Mitchell mine operated by Northshore Mining. The stratigraphic nomenclature of submembers in the Peter Mitchell Mine is portrayed on Figure 4-15. STOP 4-9: Submembers C, D (Upper Slaty), F, G, H, and I (Upper Cherty) Location: Peter Mitchell Mine. Babbitt 7.5’ quadrangle T.60N., R.13W., Sec 26, S ½ of NE. Several subunits of the Upper Cherty can be viewed within short walking distances at this particular stop (see descriptions and locations listed below). Description: Submembers G (wavy-bedded taconite ore), H (wavy-bedded taconite ore) and I (stromatolitebearing unit) at 578533E, 5278414N (NAD83 UTMs). The I submember is present at the base of the exposure and is overlain by the wavy-bedded H submember. At the top of H is a 0.5-1.0-ft-thick intraformational conglomerate that separates the H and G subunits (the G submember is actually present within the Lower Slaty according to Gundersen and Schwartz, 1962). Note that both H and G subunits constitute taconite ore and both are characterized by wavy-bedding. This same bedding characteristic was also evident in the taconite ore zones within the Lower Cherty that were seen earlier on this field trip. Note also that the bedding characteristics of the Upper Cherty at this locality are wavy rather than massive as at the Laurentian pit. Submember F with small septaria-like structures at 578857E, 5278440N (NAD83 UTMs) The F submember is thin- to wavy-bedded and locally contains small septaria-like structures that consist of whitish quartz-filled subvertical fractures in the granular cherty layers. Even though the F submember contains appreciable magnetite, it is classed as waste material because the magnetite is too fine to be economically concentrated. Submember F with small septaria-like structures and submember G with minor garnets (optional) at 578841E, 5278503N (NAD83 UTMs) Submember C (with Keweenawan sill) and submember D (optional) at 578730E, 5278206N (NAD83 UTMs) Both the C and D submembers are thin-bedded units of the Upper Slaty, however, the D submember is different in that it contains slightly thicker beds and lenses of chert. The “contact” Page 123 �between the C and D is exposed on this bench. A 20-ft-thick Keweenawan sill is also present at this stop (see below for geologic description) and is positioned about in the middle of the C submember. STOP 4-10: Submembers A, B, and C within the Upper Slaty, partially-melted Virginia Formation, two Keweenawan sills, and base of the Duluth Complex Location: Peter Mitchell Mine. Babbitt 7.5’ quadrangle T.60N., R.12W., Sec 16, W ½ of SW. Several subunits of the Upper Slaty, the Virginia Formation, and three Keweenawan intrusive rocks can be viewed within short walking distances at this particular stop (see descriptions and locations listed below). Description: Keweenawan Sill (“BIFSill”) within the C submember at 584051, 5280958N (NAD83 UTMs) A 2-18-ft-thick sill is present in the middle of the C submember at the Peter Mitchell and Dunka Pit areas, and within the J submember in the LTV 2E pit. The sill is generally fine- to mediumgrained with locally very coarse-grained plagioclase phenocrysts and polygonal jointing. A granoblastic texture is evident in thin-section indicating that the sill was emplaced in the early Keweenawan and was later metamorphosed by intrusion of the Duluth Complex. Hauck et al. (1997) noted that this sill is chemically similar to the Logan sills, to the northeast in the Rove Formation, and have informally called this sill a “Logan-type” sill. C submember at 584129E, 5281033N (NAD83 UTMs) The C submember is dominated by well-laminated, thin-bedded, slaty iron-formation containing magnetite, fayalite, ferrohypersthene, and chert. Submembers A, B, and C at 584193E, 5281142N (NAD83 UTMs) At the very top of the BIF is a 2-6-ft-thick chert and marble unit (A submember) that corresponds to the carbonate horizon that is present in only the eastern half of the Mesabi Range. This unit is locally absent in some areas (non-depositional unconformity) and extremely thick in other areas. The B submember is characterized by alternating chert and diopside bands up to one foot thick; marble layers are locally present. In some areas at this stop, pink granophyric veins locally cut the B submember. These veins exhibit pinch-and-swell relationships in that the veins thicken within the diopside bands and pinch in the chert bands. Keweenawan Sill (“VIRGSill”) at base of the Virginia Formation at 584226E, 5281159N (NAD83 UTMs) At the very base of the Virginia Formation is a 2-100-ft-thick sill that consists of a fine-grained, granoblastic, rock with varying amounts of plagioclase, clinopyroxene, orthopyroxene, hornblende, olivine, and biotite. The informal term of “Cr-bearing sill” was first used by Hauck et al. (1997) to highlight the relatively high chromium contents (600-1,200 ppm) that are characteristic of this sill. This sill exhibits two varieties: 1. a fine-grained, massive, gray-colored unit (this exposure) that is extremely difficult to distinguish from the hornfelsed Virginia Formation; and 2. a medium- to coarse-grained unit that is olivine- and/or hornblende-rich and is easily recognized. Page 124 �Partially-melted Virginia Formation in close proximity to the Duluth Complex at 584261E, 5280908N (NAD83 UTMs) In close proximity to the Duluth Complex, the well-bedded sediments of the Virginia Formation are typically transformed into a rock that at first appearance looks like an intrusive rock due to the presence of randomly oriented biotite. This rock is informally referred to as the “recrystallized unit” but is more properly classed as a diatexite (Sawyer, 1999). During emplacement of the Duluth Complex, the sediments of the Virginia Formation were heated, generating 20-40% pervasive partial melts, that literally enabled these rocks to flow in response to stresses that were applied during emplacement. All bedding planes are obliterated and what remains is a mediumgrained recrystallized rock that contains plagioclase, cordierite, orthopyroxene, and decussate biotite. Within this recrystallized matrix are blocks/boudins of more structurally competent siltstone and calc-silicate hornfels (originally limey layers). Basal contact of the Duluth Complex at 584266E, 5280874N (NAD83 UTMs) At this locality the basal contact of the South Kawishiwi intrusion (SKI) is irregular with localized “fingers” of the footwall Virginia Formation protruding upward into the intrusive rocks. Rocks of the SKI consist of weakly to moderately mineralized, fine- to medium-grained, ophitic augite troctolite to olivine gabbro. Cu-Ni values are unknown for this exposure. Return to Duluth via highways 21, 169, and U.S. 53. References Beck, J.W., 1988, Implications for Early Proterozoic tectonics and the origin of continental flood basalts, based on combined trace element and neodymium/strontium isotopic studies of mafic igneous rocks of the Penokean Lake Superior belt, Minnesota, Wisconsin, and Michigan: Unpublished Ph.D. Dissertation, University of Minnesota, 262 p. Boerst, K., 1999, Stromatolites in the LTV 2E pit, Mesabi Range, northeastern Minnesota: unpublished Undergraduate Research Opportunity Project (UROP), University of Minnesota Duluth. Chandler, V.W., 1993, Geophysical characteristics: in Sims, P.K., ed. (chap. 2) The Lake Superior region and TransHudson Orogen, Precambrian: Conterminous US: Geologic Society of America: The Geology of North America, v. C-2, p. 81-89. 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Page 125 �Gruner, J.W., 1946, The mineralogy and geology of the taconites and iron ores of the Mesabi Range, Minnesota: Office of the Commissioner Iron Range Resources and Rehabilitation, St. Paul, in cooperation with the Minnesota Geological Survey, 127 p. Gundersen, J.N., and Schwartz, G.M., 1962, The geology of the metamorphosed Biwabik Iron Formation, eastern Mesabi district, Minnesota: Minnesota Geological Survey Bulletin 43, 139 p. Han, T.M., 1982, Iron formations of Precambrian age: Hematite-magnetite relationships in some Proterozoic iron deposits – microscopic observations, in G.C. Amstutz and others, eds., Ore Genesis: The State of the Art: New York, Springer-Verlag, p. 451-459. Hauck, S.A., Severson, M.J., Zanko, L.M., Barnes, S.-J., Morton, P., Alminas, H., Foord, E.E., and Dahlberg, E.H., 1997, An overview of the geology and oxide, sulfide, and platinum-group element mineralization along the western and northern contacts of the Duluth Complex; in Ojakangas, R.W., Dickas, A.B., and Green, J.C., eds., Middle Proterozoic to Cambrian rifting, central North America: Boulder, Geological Society of America Special Paper 312, p. 137-185. Hemming S., McLennan, S.M., Hanson, G.N., and Krogstad, K.M., 1990, Pb isotope systematics in quartz [abs]: Eos (American Geophysical Union Transactions), v. 71, no. 17, p. 654-655. Hemming, S., Hanson, G.N., McLennan, S.M., and Sharp, W.D., 1991, Isoclinal slump-folds in the lower Pokegama Quartzite: Evidence for seismicity and slope instability during deposition of the Animikie Group [abs]: Abstracts and Proceedings, 37th Annual Institute on Lake Superior Geology, Eau Claire, WI, 1991, v. 37, part 1, p. 56-58. Hemming, S.R., McLennan, S.M., and Hanson, G.N., 1996, Geochemical source characteristics and diagenetic trends of the Virginia Formation, Mesabi Iron Range, Minnesota [abs]: 42nd Annual Institute of Lake Superior Geology, Cable, WI, 1996, v. 42, part 1, p. 13. Kissin, S.A., Vallini, D.A., Addison, W.D., and Brumpton, G.R., 2003, New zircon ages from the Gunflint and Rove Formations, northwestern Ontario [abs.]: 39th Annual Institute on Lake Superior Geology, Iron Mountain, MI, 2003, v. 49, part 1, p. 43-44. LaBerge, G.L., Cannon, W.F., Schulz, K.J., Klasner, J.S., and Ojakangas, R.W., 2003, Paleoproterozoic stratigraphy and tectonics along the Niagara suture zone, Michigan and Wisconsin: Field Trip Guidebook, 49th Annual Institute on Lake Superior Geology, Iron Mountain, MI, 2003, v. 49, part 2, p. 1-32. Leith, C.K., 1903, The Mesabi iron-bearing district of Minnesota: United States Geological Survey Monograph 43, 3163 p. Lucente, M.E., and Morey, G.B., 1983, Stratigraphy and sedimentology of the Lower Proterozoic Virginia Formation, northern Minnesota: Minnesota Geological Survey Report of Investigations 28, 28 p. Morey, G.B., 1967, Stratigraphy and sedimentology of the Middle Precambrian Rove formation in northwestern Minnesota: Journal of Sedimentary Petrology, v. 37, no. 4, p. 1154-1162. Morey, G.B., 1972, Mesabi Range: in Sims, P.K. and Morey, G.B., eds., Geology of Minnesota: A Centennial Volume, Minnesota Geological Survey, p. 204-217. Morey, G.B., 1973, Stratigraphic framework of Middle Precambrian rocks in Minnesota: in Young, G.M., ed., Huronian stratigraphy and sedimentation, Geological Association of Canada, Special Paper 12, p. 211-249. Morey, G.B., 1992, Chemical composition of the eastern Biwabik Iron Formation (Early Proterozoic), Mesabi Range, Minnesota: Economic Geology, v. 87, p. 1649-1658. Morey, G.B., 1993, Geology of the Mesabi Range: Field trip guidebook (Trip 1): 39th Annual Institute on Lake Superior Geology, Eveleth, MN, 1993, v. 39, part 2, p. 1-18. Morey, G.B., 1994, Anthraxolite in the Early Proterozoic Biwabik Iron Formation, Mesabi Range, northern Minnesota: in Southwick, D.L., ed.: Short Contributions to the Geology of Minnesota, 1994: Minnesota Geological Survey Report of Investigations 43, p. 39-47. Page 126 �Morey, G.B., 1999, High-grade iron ore deposits of the Mesabi range, Minnesota – Product of a continental-scale Proterozoic ground-water flow system: Economic Geology, v. 94, 133-142. Morey, G.B., 2003, Paleoproterozoic Animikie Group, related rocks, and associated iron-ore deposits in the Virginia horn: in Jirsa, M.A. and Morey, G.B., eds., Contributions to the geology of the Virginia horn area, St. Louis county, Minnesota: Minnesota Geological Survey Report of Investigations 53, p. 74-102. Morey, G.B., and Ojakangas, R.W., 1970, Sedimentology of the Middle Precambrian Thomson Formation, east-central Minnesota: Minnesota Geological Survey Report of Investigations 13, 32 p. Ojakangas, G.W., 1996, Cyclic tidal laminations in the Early Proterozoic Pokegama Formation: digital image analysis and computer modeling [abs.]: 42nd Annual Institute on Lake Superior Geology, 1996, v. 42, part 1, p. 44-45. Ojakangas, R.W., 1983, Tidal deposits in the early Proterozoic basin of the Lake Superior region – The Palms and the Pokegama Formations: Evidence for subtidal-shelf deposition of Superior-type banded iron-formation; in Medaris, L.D., Jr., ed., Early Proterozoic geology of the Great Lakes region: Geological Society of America, Memoir 160, p. 49-66. Ojakangas, R.W., 1994, Sedimentology and provenance of the Early Proterozoic Michigamme Formation and the Goodrich Quartzite, northern Michigan: Regional stratigraphic implications and suggest correlations: United States Geological Survey Bulletin 1904, 31 p. Ojakangas, R.W., Morey, G.B., and Southwick, D.L., 2001, Paleoproterozoic basin development and sedimentation in the Lake Superior region, North America: Sedimentary Geology 141-142, Elsevier Science B.V., p. 319-341. Pfleider, E.C., Morey, G.B., and Bleifuss, R.L., 1968, Mesabi deep drilling project: Progress report no. 1: in Mining Symposium, 29th Annual, and American Institute of Mining and Metallurgical Engineers, Minnesota Section, 41st Annual Meeting, Duluth, 1968 [Proceedings]: University of Minnesota, p. 52-92. Sawyer, E.W., 1997, Criteria for the recognition of partial melting: Physics and Chemistry of the Earth, v. 24, p. 269279. Schneider, D.A., Bickford, M.E., Cannon, W.F., Schulz, K.J., and Hamilton, M.A., 2002, Age of volcanic rocks and syndepositional iron formations, Marquette Range Supergroup: Implications for tectonic setting of Paleoproterozoic iron formations of the Lake Superior Region: Canadian Journal of Earth Sciences, v. 39, p. 999-1012. Schulz, K.J., 1987, An Early Proterozoic ophiolite in the Penokean orogen [abs]: Geologic Association of Canada Program Abstracts 12, p. 87. Schulz, K.J., 2003, A Paleoproterozoic suprasubduction zone ophiolite-island arc complex in northeastern Wisconsin [abs]: 49th Annual Institute on Lake Superior Geology, Iron Mountain, MI, 2003, v. 49, part 1, p. 71-72. Severson, M.J., Zanko, L.M., Hauck, S.A., and Oreskovich, J.A., 2003, Geology and SEDEX potential of Early Proterozoic rocks, east-central Minnesota; Natural Resources Research Institute, University of Minnesota Duluth, Technical Report NRRI/TR-2003/35, 160 p. Southwick, D.L., Morey, G.B., and McSwiggen, P.L., 1988, Geologic map (scale 1:250,000) of the Penokean orogen, central and eastern Minnesota, and accompanying text: Minnesota Geological Survey Report of Investigations 37, 25 p. Van Hise, C.R., and Leith, C.K., 1901, The Mesabi district: United States Geological Survey Annual Report, v. 21, part 3, p. 351-370. White, D.A., 1954, The stratigraphy and structure of the Mesabi Range, Minnesota: Minnesota Geological Survey Bulletin 38, 92 p. Winchell, N.H., 1882, The Potsdam sandstone: Minnesota Geological Survey Annual Report, v. 10, p. 123-136. Winchell, N.H., 1893, Twentieth annual report for the year 1891: Minnesota Geological Natural History Survey, 344 p. Page 127 �Wolff, J.F., 1917, Recent geologic developments on the Mesabi Iron Range, Minnesota: American Institute of Mining and Metallurgical Engineers, Transactions, v. 56, p. 229-257. Zanko, L.M., Severson, M.J., Oreskovich, J.A., Heine, J.H., Hauck, S.A., and Ojakangas, R.W., 2003, Oxidized taconite geological resources for a portion of the western Mesabi Range (west half of the Arcturus Mine to the east half of the Canisteo Mine), Itasca County, Minnesota – A GIS-based resource analysis for land-use planning: Natural Resources Research Institute, University of Minnesota Duluth, Technical Report NRRI/TR-2001/40, 85 p. Figure 4-16: Typical example of wavy-bedded taconite (also referred to as irregular-bedded taconite) that is mined from both the Upper Cherty and Lower Cherty members of the Biwabik Iron Formation on the Mesabi Range of northeastern Minnesota. Photo of the LC-4 submember at the Thunderbird North Mine of United Taconite. Page 128 �FIELD TRIP 5--CLASSIC OUTCROPS OF NORTHEASTERN MINNESOTA Co-leaders and contributors: Mark A. Jirsa1, Terrence J. Boerboom1, John C. Green2, James D. Miller, Jr. 1, G.B. Morey1, Richard W. Ojakangas2, and Dean M. Peterson3 1 Minnesota Geological Survey (staff and emeritus) University of Minnesota-Duluth (emeritus) 3 Natural Resources Research Institute 2 This manuscript has not been reviewed to conform to editorial standards of the Minnesota Geological Survey. INTRODUCTION We propose on this field trip to present the great diversity of Precambrian rock types in northeastern Minnesota using some of the most illustrative and accessible outcrops. We also hope to portray our present understanding of that geologic framework in the context of history— highlighting contrasts between “what we thought we knew” in the earlier years of geologic study, with “what we think we know now.” The trip will revisit many of the outcrops on which so many historical discussions, many of them heated, occurred. Geologic controversies were begun, and in some cases resolved, through discussions of the geologic relationships exhibited at some of these localities. This approach is not intended to criticize earlier works or workers; as each has contributed incrementally to the overall geologic picture. Geologic knowledge, like that in most sciences, is “vertical”— that is, each new iteration stands on the shoulders of previous efforts. Obviously, the trip visits only those places that are reasonably accessible from Duluth and from larger roads (Fig. 5.1). Figure 5.1 Generalized geologic map of northeastern Minnesota showing route of Field Trip #5. Page 129 �Although we consider many of these to be classic outcrops, it should also be noted that— because of access and location—these are only some of the classics; some of the most instructive outcrops are comparatively less accessible. It is likely that we have omitted some favorites, for which we apologize; however, many other fine outcrops can be found in field trip guide books in the accompanying references, including guides of the Institute on Lake Superior Geology, the Minnesota Geological Survey Guidebook Series, and the Geological Society of America (e.g., Biggs, 1987). HISTORICAL CONTEXT The Institute on Lake Superior Geology (ILSG) originated in Minnesota—as the “Lake Superior Institute”—with its first meeting held in Minneapolis in 1955, and has met the past 50 years in a variety of localities around the Great Lakes. Including this year’s meeting, the Institute has met in Minnesota a total of 13 times; five meetings were headquartered in the Twin Cities, six were held in Duluth, and two in more remote localities such as International Falls and Eveleth. Many, though apparently not all of those meetings, involved field trips demonstrating the diverse geology and mineral resources of the state. Unfortunately, the records of field trips during the earliest meetings are scant. Nevertheless, archives show that field trip topics fall into the following general geologic categories, listed geochronologically (with Minnesota meeting years in parentheses) below: Quaternary <1.75 Ma Twin Cities area (1976, 1998) Northern Minnesota (1979, 2004) Paleozoic <545 Ma Southeastern Minnesota (1976, 1979, 1998) Mesoproterozoic—1600-900 Ma North Shore Volcanic Group/Midcontinent rift (1971, 1979, 1989, 1998, 2004) Duluth Complex (1965, 1971, 1979, 1993, 2004) Paleoproterozoic—2500-1600 Ma St. Cloud “granite” district/Penokean orogen (1965, 1979, 1989, 1998) Virginia/Rove Formations (1963, 1971, 2004) Mesabi Iron Range/Biwabik Iron Formation (1963, 1971, 1979, 1993, 2004) Gunflint Iron Formation (1971) Archean—Neoarchean 2900-2500 Ma and Mesoarchean 3400-2900 Ma Vermilion and other greenstone belts (1971, 1979, 1989, 1993, 2004) Vermilion Granitic Complex/Quetico subprovince (1982, 2004) Minnesota River Valley gneiss terrane (1976, 1998) Although there apparently was no companion field trip for the first ILSG meeting in 1955; the “hot topics,” as judged from printed abstracts of that meeting, included the following: • Age, stratigraphic setting, origin, and lithologic subdivision of Lake Superior ironformation (presentations by Harold James, Robert Schmidt, and J.F. Wolff) • Composition and origin of iron ores; addressing competing theories on the creation of “natural ores” by rising hydrothermal vs. descending meteoric waters (Burton Boyum, Stanley Tyler, David White, N.K. Huber, Alan Broderick, Henry Lepp, Tsu Ming Han) • Geophysical characteristics of the “Lake Superior Syncline” (Edward Thiel, Harold Mooney, George Wollard, Lloyal Bacon) • Copper-nickel potential in Duluth gabbro, discovered in 1948 (Gerald Anderson and Donald Yardley) Not surprisingly, most of these same geological topics are important today, and are featured in the various presentations and field trips of this 50th anniversary meeting. Some of the more Page 130 �significant are represented by this field trip to the “classic” outcrops of northeastern Minnesota geology. Many of the controversies during early geologic study of the Lake Superior region involve attempts to correlate geologic units, and the consequent application of stratigraphic nomenclature. In Minnesota and adjacent Ontario, this type of controversy is typified by the debate surrounding the “Couchiching series” in the Rainy Lake area, and the stratigraphic and temporal relationships of Precambrian units in the region. Based on field work, largely in Canada, Lawson (1887) proposed that there was a sequence of sedimentary rocks older than Keewatin greenstone, and named these rocks the Couchiching series. Other workers, notably those with the U.S. Geological Survey, strongly disagreed. The question was so fundamental to understanding the regional geology that it was addressed by a special committee of geologists from the U.S. Geological Survey, Geological Survey of Canada, Michigan Geological Survey, and the Ontario Department of Mines, who visited field localities on both sides of the border. The result was a new model which was applied, rightly or wrongly, to the geology of other greenstone sequences. In retrospect, the controversies arose from the fact that geologists working in different areas observe or infer field relationships and attempt (as a natural consequence of human endeavor) to apply those relationships everywhere, including far afield from their immediate areas of knowledge— the “blind man and the elephant” syndrome. Furthermore, lacking geochronologic markers and dating methods, early workers applied temporal constraints based on generalized rock types; for example, implying that all volcanic rocks are “Keewatin,” all sedimentary rocks are either Seine (younger than Keewatin greenstone) or Couchiching (older than Keewatin greenstone), and recognizing two distinct periods of granitic rock emplacement as either Laurentian (pre-Seine) or Algoman (post-Seine). We now know from additional field work, advances in the understanding of depositional environments, and geochronologic studies that many periods of volcanism, sedimentation, and pluton emplacement have occurred. Much of what was known about the Precambrian geology in Minnesota by the 1950s—when the Institute on Lake Superior Geology (ILSG) was in its infancy—was the result of efforts by F.F. Grout, J.W. Gruner, G.M. Schwartz, and G.A. Thiel, all professors of geology at the University of Minnesota, and their graduate students working for the Minnesota Geological Survey. They summarized their views in a long paper entitled “Precambrian stratigraphy of Minnesota” (Grout and others, 1951). This publication served as backbone for a Geological Society of America field trip in 1956 (Schwartz and others, 1956). This was pioneering work that described major rock units and formalized their classification. It divided Precambrian time into a general three-fold classification scheme of Earlier, Medial, and Later—each Era separated by “great unconformities” in the rocks. The efforts established a geologic framework that was tested and refined by many workers, commonly reporting their results during the early years of the ILSG. For example, geochronologic and geologic studies in the 1960s established the Laurentian as an orogenic event, and much of the evidence for that was presented at ILSG meetings and field trips. FIELD TRIP FACTS The field stops include rocks ranging in age from Archean to Mesoproterozoic (Fig. 5.1 and Table 5-1). For the most part, the stops are presented in geographic rather than geochronologic order. Temporal settings can be deduced from the various figures, descriptions, and Table 5-1. Five topical areas are represented: 1) Virginia Horn—Archean and Paleoproterozoic (Stops 5-1 to 5-7) 2) Tower-Soudan—Archean (Stops 5-8 to 5-14) 3) Ely area—Archean (Stops 5-15 to 5-18) 4) North Shore and Duluth—Mesoproterozoic (Stops 5-19 to 5-25) 5) West of Duluth—Paleoproterozoic and Mesoproterozoic (5-26 to 5-28) Page 131 �TABLE 5-1—Stratigraphic and temporal classification of Precambrian rocks in northern Minnesota (after Sims, 1972). Ages are referenced in the text. Circles enclose the approximate chronostratigraphic position of field trip #5 stops. Considerably more stops appear in this guide than can reasonably be visited during a two-day trip. Other stops are included to provide a more thorough view of the regional geology and to facilitate self-led excursions. Some of these stops, as noted below, are described in detail in other field trip texts in this guide book, and not repeated here. Furthermore, much of the regional geologic framework is covered in the introductory material of those field trips, as noted below. The descriptions of many of the stops contain brief discussions of the “Historical Perspective,” providing the contrast between early and modern understanding of the geology. All UTM locations are given in NAD 83, Zone 15 coordinates. Section subdivisions read from smallest to largest quarter; e.g., “NW, SE, SW” should be read “NW quarter of the SE quarter of the SW quarter.” The small map insets showing stop locations are taken from USGS 7.5-minute topographic quadrangles listed with each stop. Page 132 �FIELD TRIP STOPS Virginia Horn area: Stops 5-1 to 5-7 (refer to Figures 5.2 and 5.3) Regional geologic context of the Archean and Paleoproterozoic rocks described below is reviewed in the introductory material to Field Trips#1, #4, and #7. Figure 5.2 Simplified geologic map of Vermilion Lake 30X60’ quadrangle (modified from Jirsa and Boerboom, 2003) showing location of field stops 5-1 to 5-13. Insets show location of additional figures. Page 133 �Figure 5.3 Generalized geologic map of the Virginia horn area (modified from Jirsa and others, 1998) showing details of field trip stops 5-1 to 5-7. Page 134 �STOP 5-1. Archean pillowed and massive greenstone Location: T.58N., R.17W., sec.23 NW, SE, SW; north edge of athletic fields, Gilbert Junior High School. Gilbert 7.5-minute quadrangle UTM: 539,820E/5,259,750N DESCRIPTION: Outcrop of pillowed and massive basalt is part of the Archean Mud Lake sequence, metamorphosed to low greenschist-grade. Pillow shapes indicate stratigraphic facing is to the northwest, which places this outcrop on the south side of a major D1 structure known as the Mud Lake syncline. Note also the presence locally of fractures filled with reddish jasper, presumably deposited in depressions on the rock surface by overstepping of Paleoproterozoic seas during deposition of the Biwabik Iron Formation. HISTORICAL PERSPECTIVE STOP 5-1: Early work by Gruner (1941) and later by Sutton (1963) interpreted the Archean volcanic and graywacke succession now known as the Mud Lake sequence as an anticline, based on few stratigraphic top indicators, and perhaps influenced by early discussions of the “Couchiching series” (Lawson, 1887) that placed metasedimentary rocks of the Rainy Lake area stratigraphically beneath “Keewatin” volcanic rocks. Detailed structural study by Jirsa and others, (1998) and Jirsa and Boerboom (2003) demonstrate that tholeiitic (Stop 5-1) and calcalkalic volcanic rocks and tholeiitic intrusions are conformably overlain by graywacke and slate (Stop 5-2). In detail, the succession forms a broad, twice-deformed syncline that has been segmented by faults of several generations. NEXT: Return to Highway 37, travel northwest to Hwy 135, thence west on 135 approximately 2.5 miles to Bourgin Road. Turn left (south) on Bourgin Road and continue about 0.4 mile to large cut on left (east) side of road (Stop 5-2). Page 135 �STOP 5-2. Archean graywacke and slate, intruded by quartzofeldspathic porphyry. Location: T.58N., R.17W., sec.21 SW, SW; road cuts on east side of the Bourgin Road. Eveleth 7.5-minute quadrangle. UTM: 536,311E/5,260,659N DESCRIPTION: Outcrops along this side of the road expose quartzofeldspathic porphyry (QFP) intruded into variably deformed graywacke, siltstone, and slate of the Mud Lake sequence. The sedimentary rocks here are moderately deformed, but much of that deformation is inferred to predate the main cleavage-forming event D2, and some may be soft-sediment in origin. The QFP is large and continuous to the east, but at this locality it appears to be segmented into a zone of multiple dikes. Both graywacke and QFP are intensely altered to some combination of iron-carbonate minerals (ankerite, ferroan dolomite) and sericite. Regionally, this style of alteration is commonly, though not always associated with QFP intrusions—presumably because the QFP remained more structurally rigid than the enclosing sedimentary rocks during the shear-related deformation event that accompanied alteration late in D2. Most gold mineralization in the area is closely allied to this alteration, yet this outcrop is surprisingly barren. HISTORICAL PERSPECTIVE STOP 5.2: One of the earliest gold discoveries in Minnesota was made by J.W. Gruner (in Grout, 1937) in a railroad cut not far from stop 5-2. The cut exposes graywacke intruded by quartzofeldspathic porphyry, having visible gold associated with small quartz veins. Despite several episodes of prospecting and systematic study of the region, no economic gold deposits have been discovered. NEXT: Follow Bourgin road to the south and west to a frontage road on the east side of Hwy 53. Turn north (right) on the frontage road and travel about 0.2 miles to first road to right, turn up-hill and continue to #7 Mesabi Lane (Stop 5-3). Page 136 �STOP 5-3. Archean conglomerate Private driveway! Location: T.58N., R.17W., sec.20 SW, SE, No. 7 Mesabi Lane; village of Midway. Eveleth 7.5-minute quadrangle. UTM: 535,713E/5,259,459N DESCRIPTION: Archean conglomerate and lithic sandstone that form the driveway here are part of the northeast-trending Midway sequence, containing these strata types locally interbedded with subaerially deposited, calc-alkalic (trachyandesitic) volcanic rocks. The sequence is inferred to have formed after earliest deformation (D1) of the enclosing graywacke and basaltic rocks of the Mud Lake sequence, but before the cleavage-forming D2 deformation that affected both sequences. The conglomerate contains clasts of basalt, graywacke, porphyritic trachyandesite, and quartzofeldspathic porphyry (QFP). This provenance indicates that the older Archean rocks of the Mud Lake sequence were intruded by QFP, deformed, and uplifted, to provide detritus to what was probably a successor or “pull-apart” basin developed along a major structure now occupied by the Pike River fault zone. Note also the presence of remnant skins of red jasper as at stop 5-1. HISTORICAL PERSPECTIVE: Midway sequence conglomerate has previously been interpreted as a basal sediment (Sutton, 1963), and as a proximal turbidite fan deposit (Levy, 1991), depositionally transitional with graywacke and slate of the Mud Lake sequence. Subsequent work (Jirsa, 2000) indicates that the conglomerate is part of a Timiskaming-type clastic and volcanic sequence that unconformably overlies the older volcanic strata. Deposition of the Midway sequence required uplift, subaerial erosion, continental volcanism, and deposition in isolated basins along a major structural break. Interestingly, Gruner, 1941, inferred that these rocks may be equivalent to similar units of conglomerate in the Knife Lake area, which are now also considered to represent successor-basin deposits that post-date deposition of older greenstone by as much as 30 million years (Corfu and Stott, 1998). The rocks share many attributes with other Timiskaming-type sequences, including the namesake Timiskaming Group near Timmins and the Shebandowan Group near Thunder Bay. The use of the term Timiskaming in this context is quite different from that of Goldich and others, (1961), who applied it as a system term to include the Seine, Knife Lake, and other presumably correlative metasedimentary units. NEXT: Return to frontage road paralleling Hwy 53; turn north and travel about 500 feet to low outcrop (Stop 5-4) on east side of road. Page 137 �STOP 5-4. Archean/Paleoproterozoic unconformity Location: T.58N., R.17W., sec.20 SW, SE; hamlet of Midway. Eveleth 7.5-minute quadrangle. UTM: 535,441E/5,259,520N DESCRIPTION: In the brush on the east side of the frontage road is a small exposure of Archean conglomerate, having nearly vertical foliation and lithologic content much like that at stop 5-3, capped by a thin and discontinuous “skin” of subhorizontally foliated conglomerate. The latter represents basal deposition of the Paleoproterozoic Pokegama Quartzite, the lowest of a tripartite sequence of formations that constitute the Animikie Group. Age dates are somewhat speculative; however, the Archean rocks of the various sequences in the horn probably are about 2.7 Ga (e.g., Peterson and others, 2001). Based on a date from tuffaceous rocks in the Gunflint Iron Formation (Fralick and others, 2002) that is inferred to be equivalent to the Biwabik Iron Formation, the Animikie Group is about 1.8 Ga. Thus, this unconformity represents a geological hiatus of approximately 900 million-years— almost twice that of all Phanerozoic time. The large road cut visible on the west side of Highway 53 is the argillaceous lower member of the Pokegama Quartzite. The road cut exposes shale, siltstone, and minor sandstone, which has been interpreted by Ojakangas (1993) as having been deposited in a low-energy upper tidal flat environment in a sea that peneplaned the Archean surface. Minor channeling is common at the base of thicker sandstone beds, and small-scale cross-bedding occurs in some siltstone beds. Local soft-sediment deformation may be the product of syn-depositional tectonism, or alternatively may represent localized collapse near tidal channels. HISTORICAL PERSPECTIVE STOPS 5-4 and 5-5: The Paleoproterozoic rocks exposed at Stops 5-4 and 5-5, and discussed at stop 5-6 were previously considered to represent a “geosynclinal sequence,” the precise tectonic mechanism of which was poorly understood. Work by a number of authors (notably Southwick and others, 1988) demonstrates that the tripartite formations exposed along the Mesabi Iron Range were deposited along the leading edge of a foredeep known as the Animikie basin that transgressed north over the Archean craton during the Penokean orogeny. In detail, deposition of basal quartzite (Pokegama), medial chemical (Biwabik) and upper turbiditic (Virginia) sediments represents a transgression of near-shore, shelf, and slope environments, respectively (Ojakangas, 1993). NEXT: Travel south on Highway 53 about 1.5 miles; stop on Highway and cross (carefully) to large outcrop on east side (Stop 5-5A) Page 138 �STOP 5-5. Paleoproterozoic Pokegama Quartzite (A) and Biwabik Iron Formation (B) Location: T.58N., R.17W., sec.32 SE, SE, and adjacent, junction of Highways 37 and 53. Eveleth 7.5-minute quadrangle. UTM: scattered outcrops extend from 535,956E/5,256,913N on the north (stop 5-5A), to 536,263E/5,256,200N on the south (stop 5-5B). DESCRIPTION 5-5A: This is the sandy, upper member of the Pokegama Quartzite. It is characterized by coarse grain size and massive beds as thick as 1.5 m. Massive beds are separated by thin beds of shale and siltstone. Ojakangas (1993) interpreted the deposition of this facies as within high-energy, lower tidal or subtidal environment. NEXT: Return to vehicle and continue south on Highway 53 past Highway 37. Turn around and drive north to large outcrop near the SE corner of the junction of Highways 53 and 37 (stop 5-5B) DESCRIPTION 5-5B: This exposure of gently southeast-dipping strata is part of the Lower Cherty member of the Biwabik Iron Formation. It overlies and is generally in transition with the Pokegama Quartzite at stop 5-5A. Notice that both formations have sandy textures and cross-bedding, implying a moderately high-energy depositional environment. The most significant difference between these two units is the abrupt change in sediment source from the extrabasinal quartz grains in the Pokegama, to recycled, chemically precipitated chert in the Biwabik. Measurements of crossbedding in the iron-formation are bimodal, implying deposition in a tidally influenced marine environment (Ojakangas, 1993). NEXT: Drive north on Highway 53 approximately 4 miles to entrance to Mineview overlook (Stop 56), just northwest of the junction of Highways 53 and 135. Follow driveway to overlook. Page 139 �STOP 5-6. Mineview in the Sky Overlook Location: T.58N., R.17W., sec.17 NW SE. North of the junction of hwys 53 and 135. Virginia 7.5-minute quadrangle. UTM: Top of overlook approximately 535,710E/5,261,650N DESCRIPTION: North from this overlook is a 3-mile-long complex of abandoned mining properties, known collectively as the Rouchleau mine; all developed within the Paleoproterozoic Biwabik Iron Formation. In detail, there were some 15 separately named mines within view that shipped ore during the period 1893-1986. All of them, and nearly 500 more along the 150-mile long Mesabi Iron Range, extracted oxidized (hematite- or goethite-rich) and leached iron-formation generally referred to as “natural ore.” The ore deposits here are localized along a set of fault zones (Fig. 5.3) that presumably provided the plumbing system for fluids that first oxidized the formation and produced permeability; and secondly, leached silica from the porous zones. Natural ores contained as much as 50 percent iron and less than 10 percent silica. By contrast, the mammoth open-pit mine in the distance to the northwest, US Steel’s Minntac mine, is developed in unoxidized, magnetite-rich ore containing about 30 percent iron, and 50 percent silica. This type of ore at Minntac, and five other open-pit mines currently in operation along the range, is the source of the iron concentrate known as taconite. The name taconite has also been applied generally to unoxidized iron-formation containing sufficient iron to be mined for a profit using today’s technology. HISTORICAL PERSPECTIVE STOP 5-6: Nearly 70 percent of the 3.6 billion metric tons of iron ore produced on the Mesabi range was extracted as natural ores. Although it has been generally accepted that these ores formed along fracture, bedding, and fault planes by processes of oxidation and leaching, the source of altering solutions has been the subject of considerable debate among economic geologists for nearly 70 years. Much of the literature and geologic observations on the issue are reviewed in Morey (2003). Many writers support the concept of descending meteoric waters to account for the dissolution of silica and oxidation of iron minerals. Others, including Gruner (1930) believed the geologic features were better explained by ascending hydrothermal solutions. Gruner’s theory failed to gain common acceptance, in part because no driving mechanism for such a hydrothermal system could be envisioned. The integration of Animikie Group strata into the tectonic context of the Penokean orogen in east-central Minnesota revived the theory of hydrothermal fluid flow within the Pokegama Quartzite and ultimately the iron-formation, as part of a continent-scale, gravity-driven ground-water system (Morey, 1999). The debate continues, fueled in part by the observation that no oxidized (and subsequently metamorphosed) iron-formation exists on the eastern-most Mesabi range that was metamorphosed by the Mesoproterozoic Duluth Complex. The precise origin of the structural bend in iron-formation known as the Virginia horn has long been questioned. It has been explained as a paired anticline-syncline vs. a modified warping Page 140 �around a fault-bounded horst (Morey, 2003). A topic for future research involves the extent to which vertical tectonism may have been operating during deposition of the iron-formation. NEXT: Return to Highway 53 and travel north through the city of Virginia to wayside rest approximately 2 miles north of town (Stop 5-7). STOP 5-7. Archean Giants Range batholith at “Confusion Hill,” Laurentian Divide Location: T.59N., R.17W., sec.19 SE, SE; wayside off Highway 53. Virginia 7.5-minute quadrangle. UTM: 534,337E/5,269,458N DESCRIPTION: Exposed near this wayside and in road cuts on both sides of the highway is an array of variably layered intrusions having both tonalitic (white) and dioritic (black) compositions. A cursory look shows intrusive relationships that conclusively demonstrate that diorite was emplaced into tonalite at one locality, and at another, tonalite was emplaced into diorite. In detail, all compositions intermediate between the two end members are also present locally. Although the dioritic component is abundant here, the bulk of the mapped unit is tonalitic. Emplacement of this unit, now known as the Lookout Mountain tonalite, probably involved some degree of magma mingling. Dikes of tonalite that cut the adjacent high-grade supracrustal rocks of the Minntac sequence contain metamorphic fabrics, yet little evidence of metamorphic origin can be seen in the interior of the body, implying it is syntectonic with respect to D2 deformation. U-Pb zircon dates (Boerboom and Zartman, 1993) of two components of the batholith exposed to the north bracket the age of D2 deformation between about 2674 and 2682 Ma. Exposures at Confusion Hill are a small part of the Giants Range batholith, which forms the core bedrock of the Laurentian (drainage) divide. The batholith is a 40-mile wide belt of intrusions that can be traced on geophysical maps and outcrop east to the Mesoproterozoic Duluth Complex, and west beyond the western border of Minnesota. It separates Archean supracrustal sequences in the Virginia horn from those of the Tower-Soudan area—making stratigraphic correlation between the two districts speculative. HISTORICAL PERSPECTIVE STOP 5-7: It was once generally thought that the mafic component of this mixed rock represented xenolithic rafts of subjacent amphibolite grade metavolcanic strata. In the process of regional mapping, Boerboom (in Jirsa and Boerboom, 2003) recognized that the intimately co-mingled tonalitic and mafic phases are the product of magma mixing. NEXT: Travel north on Highway 53 to the junction with Highway 169; follow the latter to the northeast approximately 28 miles to County Road 77. Turn left (northwest) on 77 and proceed about 0.5 mi. to Pike River (Stop 5.8). Page 141 �Tower-Soudan Area (refer to Figures 5.2 and 5.4) Much of the framework geology for the Vermilion district, including the Tower-Soudan area, is covered in the prelude to other trips in this guide, notably field trips #1 and #7. HISTORICAL PERSPECTIVE STOPS 5-8 to 5-18; Vermilion District The application of formal stratigraphic nomenclature to these rocks that are poorly constrained by incomplete exposure and scant geochronologic control produced considerable confusion over the years. For example, early workers considered all metabasaltic sequences equivalent, and applied the name “Ely Greenstone” to similar rocks exposed in a broad area of northern Minnesota. It is clear from subsequent mapping and geochronologic work that “greenstones” in the various belts are not all correlative. Use of the term continues today; but is restricted to specific sequences in the Ely and Tower-Soudan areas. In his publication of 1887, Lawson interpreted rocks of the International Falls area as depicting “Couchiching” sedimentary rocks to lie beneath “Keewatin” volcanic strata, and these interpretations were extended to explain stratigraphic relationships throughout much of Minnesota including the Vermilion district. Lawson’s criteria were based in part on stratigraphic younging in metasedimentary sequences. More recent interpretations recognize that these strata have experienced at least two deformations, the earliest of which (D1) involved inversion of large packages of strata into great thrust nappes prior to the major cleavage-forming event (D2). Thus, the boundaries between sedimentary and volcanic rocks in many areas are more structural than stratigraphic, and the use of younging directions in these strata requires detailed structural mapping to fully evaluate. In addition to structural mapping (e.g., Hudleston, 1976; Hudleston and others, 1988; Schultz-Ela and Hudleston, 1991; Jirsa and others, 1992), the study of geochemistry (e.g., Arth and Hanson, 1975; Schulz, 1980; Southwick and others, 1998) and U-Pb zircon geochronology (e.g., Boerboom and Zartman, 1993; Peterson and others, 2001) in Minnesota and adjacent Ontario are beginning to resolve some of the temporal issues. Much work remains in this endeavor. STOP 5-8. Archean graywacke at Pike River Dam Location: T.61N., R.16W., sec. 3, NW, SW; west side of County Road 77, on N side of river. [Note that Fortune Bay Casino—the overnight hotel—lies to the north off of CR 77]. Tower 7.5-minute quadrangle. UTM: 547,300E/5,293,340N HISTORICAL PERSPECTIVE STOP 5-8 Prior to about the 1950s, no depositional mechanism could satisfactorily explain the coincidence in graywacke of 1) coarse sand derived from a source many kilometers distant and having an altered clayey matrix; 2) interbedded black slate; and 3) the lack of evidence for reworking in shallow water (indicative of deposition below wave base). This was changed when the concept of turbidity currents was introduced to the geological profession by Kuenen and Migliorini (1950). Despite widespread publication on turbidites in more modern geologic Page 142 �Figure 5.4. Geologic map of the western Vermilion district (modified from Peterson and Jirsa, 1999), including the Tower-Soudan and Ely areas, and showing details of field trip stops 58 to 5-18. Page 143 �settings through the 1950s and 1960s, the facies model was not refined and applied to Archean and Proterozoic strata in the Lake Superior region until somewhat later (Morey, 1965; Ojakangas, 1966). NEXT: Return to Highway 169 and turn east. Continue approximately 1.7 miles to just east of the junction of 169 and County Road 526. STOP 5-9. Multiply folded Archean graywacke Location: T.61N., R.16W., sec. 2, NE, NE; south side of Highway 169 just east of CR 526. Tower 7.5-minute quadrangle. UTM: 550,050E/5,294,000N DESCRIPTION: This outcrop at the road and several smaller ones in the bush nearby show the superposition of two generations of folds in thin-bedded, well-graded graywacke of the Lake Vermilion Formation. The second-generation folds (F2) are associated with a regional axial plane cleavage in which sedimentary clasts are flattened. The earlier F1 folds have no associated cleavage and tend to be erratic in form, trend, and distribution. Folds display “eye” and “mushroom” shapes that locally are interpreted to be sheath folds (Hudleston and others, 1987). These characteristics are consistent with deformation of poorly lithified sediment. The superposition of deformation events is manifest in the transection of F1 folds by cleavage related to D2. In this area and to the west, one can find anticlinal synclines and synclinal anticlines, indicating stratigraphic inversion prior to D2 folding. HISTORICAL PERSPECTIVE STOP 5-9: The complex structure of the Vermilion district was poorly understood in the 1950s and 1960s. Subsequent structural study (Hooper and Ojakangas, 1971; Hudleston, 1976; Ojakangas and others, 1978; Jirsa and others, 1993) demonstrated two distinct periods of deformation: D1 that includes largely soft-sediment deformation represented at Stop 5-9, and a D2 transpressive deformation and metamorphic cleavage-forming event. Much of the early D1 deformation produced broad areas of down-facing strata formed by large thrust nappes (Jirsa and others, 1992). Prior to this recognition, there was much confusion about temporal relationships of individual volcanic and clastic sequences. NEXT: Continue east on Highway 169 approximately 0.7 mile to Stop5-10. Page 144 �STOP 5-10. [Optional] Archean dacitic tuff/ Paleoproterozoic or Mesoproterozoic diabase dike Location: T.61N., R.16W., sec. 1, SW, NE; Highway 169 road cut. Tower 7.5-minute quadrangle. UTM: 551,160E/5,293,960N DESCRIPTION: These road cuts expose outcrops of white, dacitic tuffaceous sedimentary rock, a component of the Lake Vermilion Formation. Regionally, the formation consists of all compositional gradations between what appears to be first-cycle tuff, tuffaceous graywacke, and mixed-source graywacke, interbedded on all scales. In a general way, the tuffaceous component increases in proportion to the east toward the Tower-Soudan anticline, presumably the source region of volcanic detritus. Rocks of the anticline are interpreted to represent the core of what was probably a large, composite volcanic shield complex, bordered by irregular basins composed of detritus shed from the shield, now represented by the Lake Vermilion Formation. The northeast-trending, steeply dipping, seven-meter wide diabase dike that cuts tuffaceous rocks has been the source of considerable debate. It’s petrographic (olivine-bearing) and geochemical (silica undersaturated) composition is similar to Mesoproterozoic dikes (Schmitz, 1994); yet it lies nearly along strike with, though east of, dikes of the Paleoproterozoic Kenora-Kabetogama dike swarm. NEXT: Continue east on Highway 169 approximately 1.5 miles to road cut. STOP 5-11 [Optional] Archean fragmental volcanic rocks Location: T.62N., R.15W., sec. 32, SW, SW; Highway 169 road cut, west edge village of Tower. Tower 7.5-minute quadrangle. UTM: 553,380E/5,294,430N DESCRIPTION: This outcrop consists of fragmental, variably reworked volcanic conglomerate and tuffaceous rocks of the Lake Vermilion Formation. The presence of this rock type demonstrates the Page 145 �eastward-increasing proximity to volcanic source rocks from stop 5-9 to here. The rock is composed of about 85-95 percent dacitic detritus, 3-5 percent gray clasts of graywacke, slate, and basaltic andesite, and a small percentage of magnetic and sulfidic fragments. Fragments range in size from a few millimeters to 20 cm. The generally poorly developed sorting and bedding, together with varied clast composition, implies a debris-flow origin. NEXT: Travel east on Highway 169, through the city of Tower, and continue east to the village of Soudan. Follow signs to left toward Soudan State Park for approximately 0.3 mi.—at this point, State Park entrance is on your left. Do not follow Park signs, but rather, continue on roadway turning toward the right and follow the road to junction about 0.4 mi. to east. Turn left and follow road for about 0.5 mi., more or less straight past the Soudan Fire Station and up the hill toward the back side of the Park and Stuntz Bay. Disembark at mine buildings and walk about 150 feet north and up-hill to outcrop on the right. STOP 5-12. Archean Soudan iron-formation member of Ely Greenstone No hammering please! Location: T.62N., R.15W., sec. 27, NE, NE; Soudan Mine State Park. Soudan 7.5-minute quadrangle. UTM: 557,120E/5,296,660N DESCRIPTION: This classic exposure of the Soudan iron-formation member of the Ely Greenstone lies on the north limb of the Tower-Soudan anticline, and at the stratigraphic top of the volcanic sequences known collectively as the Lower member of the Ely Greenstone. The outcrop displays two generations of tight folding in delicate laminae of chert (creamy white), chert-hematite jasper (red), and magnetite-chert (black to silver-colored). The second generation of folds (F2) is tectonic in origin, having subvertical axial surfaces that trend east, and steeply plunging axes. Most display Z-asymmetry. The earlier folds (F0-1) appear to have been sharply refolded to produce complex interference patterns. Lundy (1985) studied folding at this locality and concluded that some of the apparent interference structures are the product of early-formed sheath folds that did not involve refolding by D2. The F1 structures are predominantly intrafolial, and exhibit a great variety of style and orientation; implying they formed by layer-parallel, softsediment slumping. It is interesting to observe the rhythmic microlaminae (1 mm or so thick) in various cherty beds exposed here and speculate about the paleoenvironment—that is, whether these represent daily heating/cooling, tidal, climatic, annual, or some other repetitive influence in the depositional environment. What is known about units of iron-formation in the Ely Greenstone, of which there are many, is that deposition occurred during periods of relative volcanic and tectonic quiescence by the slow subaqueous “rain” of chemical precipitates. The deep excavations in this area are the early workings of the Soudan iron mine, the first in Minnesota. The mine produced about 16 mt of high-grade hematite ore (60-63 percent iron Page 146 �content) from 1884 until 1962, when the land was deeded to the State of Minnesota and converted to a park. Most of the production came from underground workings that began here in 1900, and which now can be visited on guided tours. The mine also houses an underground physics research facility at 2340 feet below the surface. A massive expansion of that facility is under consideration to create a national underground laboratory at considerably greater depths (Peterson and Patelke, 2003). NEXT: Return to vehicle and drive to east-bound Highway 169. Travel east approximately 7.5 miles to Mud Creek Road. The Highway follows in a general way, the strike of the nearly vertically dipping Soudan iron-formation, bounded by volcanic rocks of the upper member of the Ely Greenstone on the north and the lower member on the south. Turn north (left) on the Mud Creek Road and follow it for approximately 3.8 miles to Mud Creek. STOP 5-13. Mud Creek shear zone Location: T.62N., R.14W., sec. 5, SE, SE; Outcrop just northwest of Mud Creek near road. Chad Lake 7.5-minute quadrangle. UTM: 564,230E/5,302,800N DESCRIPTION: This outcrop shows highly strained rocks in the Mud Creek shear zone. The rock type is a quartz-iron carbonate-sericite schist, having quartz and tourmaline knots, abundant pyrite, and trace amounts of gold. Its protolith is unknown, because of the intense deformation, but could be any of several rock types in the region, including quartzofeldspathic porphyry, metavolcanic rock, or graywacke. The shear fabric trends east-northeast, and lineations plunge at shallow angles to the east. Development of this shear zone, which occupies most of the valley of Mud Creek, is a product of largely dextral transpressive deformation that has been partitioned into discrete zones, presumably late in D2 deformation. It is generally believed that gold-bearing mineralization was introduced during these later deformation events, and the Mud Creek shear zone and environs continue to attract considerable attention as a gold target (Field Trip #7). The Mud Creek shear is a broad, anastomosing zone that forms the boundary between rocks of the Ely Greenstone and Lake Vermilion Formation on the south, and volcanic and ironformation-bearing rocks known informally as the Bass Lake sequence on the north. The Bass Lake rocks may be equivalent to parts of the Newton Lake Formation exposed north of Ely, but a complex series of faults in the intervening area makes this correlation speculative. NEXT: Drive south from Mud Creek to first gravel trail to the west; turn west and proceed about 0.2 to gravel pit. Page 147 �STOP 5-14 [Optional] Archean graywacke and slate of Lake Vermilion Formation Location: T.62N., R.14W., sec. 8, NW, NE. Soudan 7.5-minute quadrangle. UTM: 564,100E/5,302,400N DESCRIPTION: This recently exposed outcrop of the Lake Vermilion Formation consists of siliceous slate having a 1-2 meter thick unit of thinly bedded tuffaceous graywacke. The slate carries a strong S2 cleavage and is cut by shears; both trending to the east-northeast. Bedding in the graywacke trends north-northeast and youngs to the southeast—this contrasts with the regional setting which strikes to the east-northeast and youngs to the north. The reason for this anomalous trend is unclear, but it may be the result of fault drag along the Mud Creek shear zone. NEXT: Return to Mud Creek Road, travel south to Highway 169, then turn east on 169 and continue approximately 12 miles to Ely. [Enroute, one might visit a spectacular outcrop of pillowed flows described in Field Trip #1, Stop 1-13; Hilltop S of Highway 169 at SE, SW, S19, T.62N, R14W, UTM:056200E, 5297800N]. Pass through Ely and continue approximately 1 mile east of town to junction of 169 and County Road 88; then proceed 0.3 mi farther east to road cut. Ely and vicinity (refer to Figures 5.1 and 5.4) Geologic framework and outcrop locations are shown on Green and Schulz, 1982. STOP 5-15 [Optional] Archean Ely Greenstone Location: T.63N., R.12W., sec. 25, NW, SW; Road cut on Highway 169 approximately 0.3 mi. east of junction with County Road 88 (Spaulding Bay Road). Ely 7.5-minute quadrangle. UTM: 588,350E/5,307,075N Page 148 �DESCRIPTION Pillowed basalt of the Ely Greenstone. Flows and complexly interlayered iron-formation in the area are strongly folded into anticlines and synclines striking predominantly to the northeast. Stratigraphic younging to the north can be established from pillow structures in this exposure and others nearby. High grade iron ore was mined from iron-formations in the noses or axes of many of the folds. The productive Zenith, Chandler, and Pioneer mines, for example, lie within one such fold, the Ely trough. NEXT: Return to junction of Highway 169 and County Road 88, turn north on 88 and travel approximately 2 miles to Echo Trail. Turn right (north) on Echo trail and proceed 0.3 miles north to road cut on west side of the road. STOP 5-16. Archean Newton Lake Formation— variolitic basalt Location: T.63N., R.12W., sec. 22, NE, NW; road cut on Echo Trail. Ely 7.5-minute quadrangle. UTM: 585,750E/5,309,360N DESCRIPTION This road cut exposes pillowed and variolitic flows and hyaloclastite breccia of the Newton Lake Formation. Pillow sizes vary from “normal” to “mattress”, and their shapes indicate younging to the south. The Newton Lake is separated from the Ely Greenstone to the south by a complex zone of faulting (Shagawa Lake and Sibley faults) developed within sedimentary rocks of the Knife Lake Group. Although relatively undeformed conglomerate and sedimentary rocks of the Knife Lake Group are exposed just a few miles to the east, they are typically so sheared and altered in this area as to obscure lithologic and sedimentary interpretations, and will not be visited. The Newton Lake Formation differs from the Ely in that the former contains abundant diabasic sills and rare iron-formation. In addition, lava flows of the Newton Lake typically have larger MgO and incompatible element contents than those of the Ely, and some are classed as komatiites (Schulz, 1980). The Newton Lake Formation (and possibly equivalent Bass Lake sequence) is considered to be the youngest Archean supracrustal sequence in the Vermilion district. Rocks having nearly identical composition and stratigraphic/structural setting occur in Itasca County some 80 kilometers to the west (Jirsa, 1990). NEXT: Continue north on Echo Trail for 0.8 mi. to outcrop 5-17A on left (west) side of road. Page 149 �STOP 5-17. Archean Newton Lake Formation— felsic tuff and mafic intrusions Location: T.63N., R.12W., sec. 15 SE, NW; road cuts on Echo Trail. Ely 7.5-minute quadrangle. UTM: 5-17A=585,775E/5,310,660N 5-17B=585,810E/5,310,700N DESCRIPTION: 5-17A Road cut on west side of Echo Trail is aphanitic to fine-grained, laminated, light-colored felsic tuff and probable reworked tuff, cut by diabase. Minor graded bedding indicates younging to the north; however, it is likely at this exposure that the tuff represents a large block rafted in diabasic intrusions. NEXT: Cross to east side of Echo Trail and walk north to first outcrop. 5-17B This road cut exposes layered metadiabase intruded by hornblende-pyroxene-biotitebearing lamprophyre; both presumed to be intrusions of the Newton Lake Formation. Lamprophyre is coarse- to very coarse-grained and contains inclusions of the diabase. Overall, the Newton Lake Formation contains abundant and locally differentiated sills that vary in composition from diabase and gabbro to pyroxenite and peridotite. NEXT: Return to vehicle and proceed north on Echo Trail, which winds around the east shore of Burntside Lake, for approximately 3.5 miles to junction with County road 803 (Passi Road). Turn left (west) on Passi Road and stop. STOP 5-18. Archean schist (metagraywacke), paragneiss, and trondhjemite of Quetico subprovince Location: T.63N., R.12W., sec. 8, SW, NW; outcrop in SW corner of junction Echo Trail and Passi road. Shagawa Lake 7.5-minute quadrangle. UTM: 581,660E/5,312,050N Page 150 �DESCRIPTION: About one mile south en route to this stop, we passed a sharp, narrow valley that marks the trace of the Burntside Lake Fault. The fault forms a major boundary between comparatively low (greenschist) grade rocks on the south that typify the Wawa subprovince, with high-grade (amphibolite) rocks of the Quetico subprovince on the north. Rocks here include biotite schist, migmatite, and trondhjemite, a part of what’s known as the Vermilion Granitic Complex. Despite the amphibolite grade of metamorphism, the schist retains some evidence of graded bedding locally that indicates a graywacke protolith. NEXT: Backtrack to Highway 169, turn west and continue to Highway 1 at the east side of Ely. Follow Highway 1 south toward the north shore of Lake Superior, a distance of approximately 50 miles. Though we will not stop, this route takes us out of the Archean Wawa subprovince, crossing parts of the Mesoproterozoic Keweenawan Supergroup, including the Duluth Complex and North Shore Volcanic Group. North shore and Duluth areas (refer to Figures 5.1 and 5.5) For regional framework geology, refer to introductory material to Field Trip #2. Figure 5.5 Geologic map of northeastern Minnesota (modified from Miller and others, 2002) showing field trip stops on the north shore and in the Duluth area Page 151 �STOP 5-19. Top of Mesoproterozoic Palisade rhyolite flow (North Shore Volcanic Group) Location: T.56N., R.7W., SEC. 11 NE, SW; 0.2 mi. N of Highway 61 on Highway 1. Illgen City 7.5-minute quadrangle. UTM: 636,600E/5,245,850N DESCRIPTION: Road cuts on the west side of Highway 1 expose the upper zone of the Palisade rhyolite, a very large and extensive porphyritic flow that is part of the North Shore Volcanic Group. Much of the flow, seen for example on Palisade Head and Shovel Point and the Hwy 61 cut just SW of the junction with Hwy 1, has the appearance of a normal lava flow, but features here at the top, and at the inaccessible base of the cliff at Palisade head, show clearly that it was erupted explosively. The hot ash settled and welded, and the resulting hot, devolatilized mass flowed as “reconstituted lava” or ignimbrite. This road cut in the rapidly chilled top part of the flow is made of flow-brecciated, flow-folded welded tuff; microscopic study reveals deformed shards and fiamme in some samples. (More obvious shards are visible in the chilled welded tuff at the flow’s base). Pneumatolytic or hydrothermal alteration has kaolinitized much of the feldspar at this locality. In exposures of the massive flow interior, flow-aligned platy quartz after tridymite is abundant in the groundmass, implying a high temperature of eruption (greater than 870 degrees C). HISTORICAL PERSPECTIVE: Little attention was paid to the rhyolites in the North Shore Volcanic Group (NSVG) in earlier accounts. In some of the earliest work by N.H. Winchell (1889), he attributed all north shore rhyolites to the process of melting of sedimentary rocks. More recently, the importance of these rocks has been increasingly recognized because of their abundance compared to intermediate compositions (bimodal tholeiitic suite), and the opportunity they present for geochronologic study (Goldich and others, 1961). With the further development of the U/Pb zircon dating method, many rhyolites have provided age calibration for magmatism in the Midcontinent Rift. The Palisade rhyolite, for instance, has been dated at 1098.6 ±1.7 my (Davis and Green, 1997). Whereas Bowen-inspired models of magma evolution assumed rhyolites such as this to be the products of crystal fractionation of the dominant basaltic parent, their high abundance in the NSVG, along with recent Sm/Nd analyses (Vervoort and Green, 1997) show this and many others to have been produced by partial melting of Archean basement. Finally, detailed field studies and petrography of this and other large felsic flows has indicated that they were erupted at unusually high temperatures for rhyolite, were highly mobile, and that many are rheoignimbrites (Green and Fitz, 1993). NEXT: Continue south on Highway 1 for 0.3 miles to Highway 61. Turn northeast (left) on 61 and drive approximately 14 miles to turn-in for Sugarloaf Cove Scientific and Natural Area. Walk to Sugarloaf Point. Page 152 �STOP 5-20. Mesoproterozoic basalt at Sugarloaf Cove Location: T.58N., R.5W., Sec. 29 NE, NE. Little Marais 7.5-minute quadrangle. UTM: outcrops begin about 652,020E/5,261,120N Note: This stop is on property of the Sugarloaf interpretive Center Association (a non-profit) and the State of Minnesota. The beach and point are part of a Scientific and Natural Area (MN DNR) established to preserve these features. Please do not use hammers, or remove samples of any kind. Thanks. DESCRIPTION: Many of the attributes of olivine tholeiite lava flows of the NSVG are well exposed on the beach, mainland shore, and point. Flows range from several tens of meters thick to thin flow units of a meter or less. Massive interiors are ophitic: each “ophite” is made of an augite oikocryst enclosing many small plagioclase tablets. Abundant tiny olivine grains occur with plagioclase between the ophites. Pipe amygdules are common in the flow bases, and small vesicle cylinders occur in the massive interior of the thick flow that caps the sequence (The Sugarloaf) on the point. These are all pahoehoe flows, and ropy surfaces are well displayed, as are columnar joints and red sandstone-filled fractures (“clastic dikes”) in the interiors. HISTORICAL PERSPECTIVE: Although considerable petrographic study and some geochemistry had been done on the basalts of the Cu-bearing lavas of the Keweenaw Peninsula of Michigan (e.g., Butler and Burbank, 1929; Broderick, 1935), the NSVG only began to receive such attention in the mid- to late 1960’s, as the world geochemistry community began to focus on basalts and their parent ultramafic rocks of the mantle. The evolution of the plate-tectonics paradigm soon provided a model for the raison d’etre for these basalts, and the concept of the Midcontinent Rift System was born (White, 1972; Green, 1977). Further attention to the physical and geochemical aspects of basalts has generated more data on the NSVG (e.g., Nicholson and others, 1997, and references therein); olivine tholeiites such as these are interpreted as being mainly mantle plume melts, with minor lithospheric contribution. NEXT: Return southwest on Highway 61 approximately 28 miles to entrance to Split Rock Lighthouse State Park. Turn into the park and walk to base of lighthouse. Note that traveling southwest through this western limb of the NSVG, one traverses down-stratigraphic section. Page 153 �STOP 5-21. Mesoproterozoic (Keweenawan) Beaver Bay Complex at Split Rock Lighthouse Location: T.55N., R.8W., Sec. 33 SW, SW; Split Rock Point Lighthouse History Center and State Park. Split Rock Point NE 7.5-minute quadrangle. UTM: base of lighthouse at 623,640E/5,228,680N DESCRIPTION 5-21A: Outcrops of fine-grained, ophitic olivine diabase of the Beaver River diabase, which hosts the majority of anorthosite inclusions from Split Rock to Grand Marais, are exposed just northeast of the lighthouse atop a sheer 30-m-high sea cliff. The centimeter-wide augite oikocrysts obvious here are typical of the lower portion of this diabase sill, which dips gently (<15°) into the lake. The prominent point just to the northeast (Rusty Point) is held up by a very large (>200 m) inclusion of medium-grained granophyric granite lying at the base of the sill. Making our way to the base of the lighthouse, we come upon a good exposure of the large anorthosite inclusion that holds up the entire sea cliff. The coarse-grained (± 1 cm) anorthosite here displays meter-scale modal layering of noritic anorthosite (20% En72 hypersthene; 80% An6080 plagioclase) and anorthosite (>99% Pl). This inclusion is altered and displays a moderate to severe cataclastic and granulated texture indicative of having been recrystallized and/or tectonized (Morrison and others, 1983). The steeply dipping layers are cut by thin dikes of medium-grained, igneous-textured augite leuconorite (Pl-An56; Opx-En75; Cpx-En66). DESCRIPTION 5-21B. Follow the footpath south to the pump house on the lakeshore. Here, black, massive basalt is exposed in wave-washed outcrops. The brecciated, amygdaloidal AA flow top of this basalt flow is exposed in the rubbly bluff to the northeast in the direction of the lighthouse. Laminated siltstone in the matrix of the flowtop dips gently toward the lake. Carefully scrambling over some large boulders of ophitic diabase to the northeast, we can view the inclusion-rich base of the diabase sill conformably overlying the basalt flow top. Note that the diabase is chilled against the basalt, but not against the anorthosite inclusions. The abundance of inclusions disrupts the development of regular columnar jointing in the diabase. Numerous boulders along the base of the cliff display the textural and mineralogical varieties of anorthosite inclusions present. Looking to the northeast, one can see that the vertically layered inclusion beneath the lighthouse extends to lake level. HISTORICAL PERSPECTIVE: Occurrences of nearly pure plagioclase rock holding up various points of high ground between Split Rock Point and Grand Marias have held a special fascination for Lake Superior geologists for over 150 years. These enormous (up to 500m across), light-colored, coarse-grained masses embedded in dark, fine-grained mafic rock were first recognized by J.G. Norwood in 1849 (Fig. 5.6) as part of the federal Owen Survey (Owen, 1852). During the initial state geological survey (1872-1900), N.H. Winchell noted the resemblance of the feldspar rock, as he called it, to the Rice Point Gabbro at Duluth (anorthositic series of the Duluth Complex). He Page 154 �suggested that the anorthosites were blocks plucked from older feldspathic phases of the Duluth gabbro by later outpourings of the great gabbro flood, which he fancied to be an enormous extrusive (Winchell, 1900). He noted that the inclusions became somewhat smaller and more dispersed toward the lake and interpreted this to be due to disaggregation as they traveled downslope (to the southeast) from their source. Midway through the survey, Winchell commissioned A.C. Lawson to study these masses in greater detail. Lawson (1893) concluded that the anorthosite represented the peaks of Archean mountains that where inundated and largely buried by outpouring of the great gabbro flood. The inclusion character evident in most anorthosite occurrences was attributed to spalling of large blocks from the mountain peaks. In the early 1900's the search for corundum in northern Minnesota followed well publicized discoveries of the mineral in Canada. In 1902, several property owners mistook the light green anorthosite on the north shore for corundum. They formed a company, attracted investors, built a plant complete with processing, storage, and transport facilities---all without ever analyzing the rock! The first sale of mined product was also the last when it was discovered that the rock contained no corundum, but instead is composed of the much softer feldspar mineral, anorthite. Despite the setback, the investors regrouped and eventually formed the Minnesota Mining and Manufacturing company (now 3M); demonstrating an important principal: the lessons learned from small failures sometimes lead to great successes. One of the most complete inventories of the anorthosites of the North Shore was reported in Minnesota Geological Survey Bulletin 28 "The Geology of the Anorthosites of the Minnesota Coast of Lake Superior" by Grout and Schwartz (1939). In this report, they documented most known anorthosite occurrences in six 1:31,500 scale township maps. In considering the origin of the anorthosite, Grout and Schwartz clearly recognized them as inclusions of pre-existing rock that were derived from a deeper crustal source and were transported in diabasic magma to their present site by virtue of their low density. They agreed with Winchell that the source was likely the anorthositic rocks of the Duluth Complex. Figure 5.6—Woodcut print from J.G. Norwood’s report on the geology of the north shore (in Owen 1852) showing massive anorthosite inclusions in well-jointed basalt. More detailed considerations of the petrographic and geochemical attributes of the anorthosite inclusions led Phinney (1968) to conclude that Duluth Complex gabbroic anorthosites were an unlikely source for the inclusions. The inclusions, which commonly show signs of severe recrystallization and cataclasis, were more plagioclase-rich (>95%), coarser-grained, and more anorthitic (An70-75) than most anorthositic rocks of the complex. Also, Duluth Complex anorthositic rocks rarely contain orthopyroxene, a common mafic phase in the inclusions. Instead, he speculated that the inclusions were derived from deeper anorthosite bodies in the mid- Page 155 �to lower crust. Proterozoic anorthosite massifs seemed an attractive source by their depth of occurrence and plagioclase-rich composition; however, massif anorthosites are less anorthitic (An45-55) than even Duluth Complex anorthositic rocks (An55-65) and tend to be strongly recrystallized. In a follow-up petrographic and geochemical (Rb-Sr, Sm-Nd, and REE) study (Morrison and others, 1983), Phinney and his coworkers uncovered evidence of crustal contamination in the anorthositic inclusions from a source older than 1.9 Ma. They concluded from this that the inclusions were derived from early Proterozoic anorthosite bodies formed in the lower crust. As a corollary to a model for the origin of plagioclase crystal mush parent magmas to the anorthositic series of the Duluth Complex, Miller and Weiblen (1990) speculated that these anorthositic inclusions may be Keweenawan in age. In their model, plagioclase-rich magmas were generated by ponding of mantle-derived melts in the lower crust. Under the high pressures of the lower crust, plagioclase will be significantly buoyant in mafic magma and would likely give rise to anorthositic cupolas in these deep magma chambers. Early lower crustal intrusions may have been contaminated by interaction with the lower crust and thus imparted crustal isotopic signatures into these early anorthosite cumulates. As rifting progressed and the lower crust was displaced, later magmas may have picked up these early Keweenawan anorthosites and transported them tens of kilometers into subvolcanic dikes and sills of the Beaver River diabase. NEXT: Return to Highway 61, turn southwest (left), and continue approximately 15 miles to Silver Cliff Tunnel. STOP 5-22 [Optional] Mesoproterozoic (Keweenawan) diabase and andesitic flows at Silver Cliff Tunnel Location: T.53N., R.10W., Sec. 15 and 22. Castle Danger 7.5-minute quadrangle. UTM: 606,993E/5,213,952N to 606,755E/5,213,486N; DESCRIPTION: The Silver Creek diabase is an irregular, subcordant, subhorizontal intrusion at least 200 feet thick. The diabase forms a prominent highland that projects inland several miles from Silver Cliff at Lake Superior. Excavation of the Highway 61 tunnel has created excellent exposures of the contact between volcanic rocks and both the top and bottom of the diabase, and has exposed a north striking, 55-degree east-dipping fault that cuts the base of the diabase. Refer to Field Trip 2, stop 2-6 for details. NEXT: Continue southwest on Highway 61 for approximately 30 miles. Near the City of Duluth, 61 becomes London Road; continue southwest on London Road. Do NOT turn onto Interstate Highway 35, but clock mileage and continue approximately 1.25 miles to parking area for Leif Ericson City Park near 12th Avenue East. Walk into the park and to the shoreline. Page 156 �STOP 5-23. Mesoproterozoic (Keweenawan) interflow sedimentary rocks at Leif Ericson Park Location: T.50N., R.14W., Sec. 23 SW. Duluth 7.5-Minute quadrangle. UTM: 570,050E/5,182,950N DESCRIPTION: This classic location exposes an estimated 110’ of interflow sandstone, lying on slightly eroded amygdaloidal flows of the gently southeast-dipping Leif Ericson Park Lavas. The sandstone is fine- to medium-grained, moderately well sorted, and derived almost totally from underlying and adjacent lava flows—no extrabasinal detritus has been detected (Jirsa, 1984). The presence of planar-tabular and trough cross-bedding, together with the lenticular distribution of interflow units, implies the strata represent occasional stream flow deposits in depressions on the evolving lava surfaces. This is one of many interflow sedimentary units, most of which are “red bed-like” in appearance (considered in the early works of N.H. Winchell as Potsdam equivalents). By contrast, this interflow lies very near the base of the North Shore Volcanic Group, and thus has experienced burial metamorphism to nearly greenschist facies—though it is possible that some of the elevated grade above the zeolite facies typical of rocks just to the northeast, is an aureole effect of the adjacent Endion and other diabasic sills. NEXT: Follow signs to Interstate Highway 35, travel southwest on 35 to Highway 53. Turn north on 53 (Piedmont Avenue) and continue about 1 mile to intersection with Skyline Parkway. Turn left (southwest) on Skyline and travel 0.4 mi. to crossing with 28th Ave. W; continue 0.2 mi. farther southwest on Skyline to outcrop on the right (Stop 5-24). STOP 5-24 [Optional] Mesoproterozoic Duluth Complex; Layered Series “Chill”, Granophyre, and Anorthositic Series Location : T50N, R14W, Sec 32, SW, SW; Skyline Parkway about 1/5 mi. SW of 28th Ave. Duluth Heights 7.5-minute quadrangle. UTM: 564,880E/5,179,640N Page 157 �DESCRIPTION: Exposed at the northeast end of this roadcut is a fine-grained mafic rock with intermingled granophyre that together cut coarse-grained olivine gabbroic anorthosite of the anorthositic series. Since early mapping by Grout in the Duluth area in 1911, the relationships displayed by this rather innocuous exposure has been critical in interpreting the intrusive history of the Duluth Complex. The distinct, but complexly intermingled domains of mafic and felsic rock displayed here and commonly observed in the upper parts of the Duluth Complex led Grout (1918c) to the interpretation that these rocks formed by liquid immiscibility. This idea was strongly at odds with the newly introduced notion of granite formation by differentiation of mafic magma touted by Bowen (1928) and led to years of debate between these two famous petrologists. Another significant feature displayed by this exposure of more parochial interest is the relationship between the fine-grained gabbro and the coarse-grained olivine gabbroic anorthosite. The fine-grained mafic rock exposed here can be traced up over the ridge to the west where it merges into the upper contact zone of Layered Series (see Green and Miller, this abstract volume for general summary of the geology of the Duluth Complex at Duluth). Grout and later Taylor (1964) saw this exposure as evidence that the anorthositic series had cooled considerably when the layered series was intruded and thus was considerably older. This paradigm was accepted by all subsequent workers on the Duluth Complex up through the 1990’s. Consequently, it came as something of a shock when high resolution U-Pb ages (Paces and Miller, 1993) revealed that the anorthositic series and the layered series were virtually identical in age (within 0.5 Ma relative to the 22 Ma span of Midcontinent Rift magmatic activity). This precipitated a major paradigm shift in the perception of the intrusive relationships between these two series here and throughout the Duluth Complex (Miller, 1992). A closer look at this layered series “chill” reveals that it is not a thermal quench of the Layered Series at Duluth (DLS) at all. This rock type is found at the contact with the anorthositic series throughout most of this area and has a remarkably homogeneous, evolved composition (MgO/(MgO+FeO) = ~ 37; see Table 2 in Miller and Ripley, 1996). In thin section, it is a subprismatic biotitic oxide ferrodiorite. Phase equilibrium modeling of its composition indicates that it should be in equilibrium with evolved compositions of augite, ilmenite and plagioclase phases that comprise gabbroic cumulates found in the cyclic zone and gabbro zones of the DLS. In sum, this rock is much too evolved to have produced the entire cumulate pile of the layered series. Rather than this being a thermal quench, Miller and Ripley (1996) have interpreted this rock to represent decompression quenching of an evolved, water-saturated layered series magma during venting at a time when the cyclic zone was crystallizing. Whereas decompression of less than water saturated magma will result in superheating and a suspension of crystallization (or at least a significant change in phase equilibrium), decompression under water saturated conditions will cause supercooling and quenching. This model fits nicely with the explanation for the cyclic zone with which this composition is apparently comagmatic (see Stop 25). How does this scenario of venting of a hydrous magma relate to the mafic-felsic duplex which so intrigued Grout? The lobate contacts between the irregular masses of medium-grained granophyre and the fine-grained ferrodiorite host clearly gives the appearance of two-magma mixing. Grout's (1918a) idea that these two magmas formed by silicate liquid immiscibility of hydrous mafic magmas in the roof zone of the Duluth Complex seems more plausible today than when he first proposed it. The concept of silicate liquid immiscibility, which had come to be totally disparaged during Grout's day, has regained some respectability as an plausible, albeit, uncommon petrologic process (Roedder, 1979). However, alternative explanations for this compositional dichotomy can be suggested. One is that these felsic magmas were derived from anatectic melting of various inclusions carried into the layered series chamber. Because of their high silica content and low density, these felsic melts did not readily mix or assimilate with mafic melt, but rather rose to the roof zone where they ponded beneath the anorthositic series cupola. Page 158 �During magma venting from the chamber, the felsic melts became entrained and irregularly mixed with the mafic magmas. While decompression under water saturated conditions caused rapid crystallization of the mafic magma, the felsic melt became irregularly entrapped in the quenched mafic host and cooled more slowly to a medium-grained texture. NEXT: Return to Interstate 35 and follow it southwest to exit for Thomson Hill Rest Area near the crest of the hill west of Duluth. Follow signs to rest area (Stop 5-25). STOP 5-25. Mesoproterozoic (Keweenawan) Duluth Complex at Duluth Location: T.49N., R.15W., Sec. 14 SE, NW; Thomson Hill Rest Area on Skyline Parkway. West Duluth 7.5-minute quadrangle. UTM: 560,700E/5,175,380N HISTORICAL PERSPECTIVE: The well-exposed gabbroic rocks forming the escarpment above the city of Duluth have long been recognized as the type section of the Duluth Complex. While early surveys recognized the presence of two distinct rock types in the Duluth area - normal gabbros and feldspathic gabbros (Winchell, 1900), Grout was the first to interpret the layered gabbros as a product of convection and magma differentiation (Grout, 1918a-c). Taylor (1964) produced the first detailed-scale (1:24,000) geologic map of the complex in the Duluth area wherein he distinguished the stratiform gabbro cumulates of the layered series from the older, structurally complex anorthositic series. He interpreted the structural complexity of the anorthositic series as having formed from multiple injections of a plagioclase crystal mush. He also recognized the differentiated character of the layered series and pointed out that it was basically similar to the Skaergaard Intrusion - the classic example of a well-differentiated intrusion formed by fractional crystallization of a tholeiitic magma (Wager and Deer, 1939). More recently, detailed mapping and petrologic studies in the Duluth area have added substantially to our understanding of emplacement and crystallization history of these two series here at the type locality of the Duluth Complex (Miller and Ripley, 1996, Miller and others, 2002). See Green and Miller, this abstract volume for more details about the geology of the Duluth Complex at Duluth. In two outcrops at this stop, we can view exposures that demonstrate the inclusion/intrusion relationship between of anorthositic and layered series rocks and that illustrate the macrocyclic nature of the cumulate rocks which characterizes the medial part of the Layered Series at Duluth, an interval called the cyclic zone. DESCRIPTION 5-25A: In the road cut north of the rest area parking lot is an exposure of intermittently layered, wellfoliated olivine oxide gabbro hosting several meter-sized, flattened inclusions of olivine anorthosite. The gabbro is a four-phase cumulate of plagioclase, augite, Fe-Ti oxide and olivine. This cumulate typifies the upper gabbroic parts of troctolite→gabbro macrocycles that Page 159 �characterize the cyclic zone. In this three-dimensional view, the anorthositic inclusions clearly have a pancake shape that is conformable to layering and foliation in the host gabbro. This may represent the original shape of the inclusions or it may indicate compaction of a partially molten blocks. In either case, the occurrence of anorthositic inclusions in layered series rocks is a ubiquitous feature throughout the Duluth Complex. That the opposite relationship is rarely if ever observed has reinforced the longstanding interpretation that the anorthositic series is older than layered series. However, high precision U-Pb dating (Paces and Miller, 1993) has shown that this age difference is less than 1 million years relative to the 20-million year magmatic history of the Midcontinent Rift. DESCRIPTION 5-25B: In the layered sequence of gabbroic rocks exposed along the 200-m-long roadcut below the observation deck (Fig. 5.7), changes in cumulus mineral assemblages are displayed that characterize the cyclic zone of the Duluth Layered Series. The approximately 1-km thick cyclic zone is composed of five to six major macrocycles, ranging in thickness between 50 and 200 m, within which troctolitic cumulates (Pl+Ol) grade upward into gabbroic cumulates (Pl+Cpx+FeOx±Ol). Boundaries between the macrocycles are defined by abrupt cumulate reversals back to troctolitic cumulates and are also commonly marked by the occurrence of anorthosite inclusions and microgabbro cumulates at the top of the lower cycle. This exposure traverses the boundary between the third and fourth macrocycles. Figure 5.7. Geology and cryptic variation of mineral compositions along Skyline Parkway road cut near Thompson Hill rest area, Stop 5-24B. View is to the north. Dip of lamination and layering is exaggerated; it averages about 20° to the east. Contacts between units are gradational over thicknesses of 10 cm to 1 m. Mineral composition parameters are An in plagioclase = CaO/(CaO+Na2O+K2O), En' in augite and Fo in olivine = MgO/(MgO+FeO); all in mole%. Cumulus rock codes (in parentheses) list abbreviations of minerals in decreasing order of abundance (P/p plagioclase, C/c-clinopyroxene (augite); F/f -Fe-Ti oxide; O/o-olivine; upper case = cumulus mineral, lower case = intercumulus minerals). Samples A-G are noted by dark circles. The west end of the road cut begins with a coarse-grained, moderately laminated, subophitic olivine gabbro, which locally is intergranular and elsewhere is leucocratic (sample A, Fig. 5.7). Augite in this rock is marginally cumulus but becomes definitely so quickly up-section where it consistently has an anhedral granular to subprismatic habit (sample B). Beyond a poorly exposed interval, this gabbro is interlayered with a 2-m-thick interval in which minor olivine becomes Page 160 �subpoikilitic and concentrated in layers (sample C). Another 3 m above this, the coarse gabbro passes into a medium- to medium fine-grained, well-laminated, subpoikilitic olivine-bearing oxide gabbro (samples D and D') which displays layering of olivine oikocryst concentration and elsewhere isomodal layers rich in Fe-Ti oxide and pyroxene. The very strong foliation and subhedral to euhedral habit of cumulus phases (plagioclase, pyroxene, and ilmenite) impart an adcumulate texture to this rock. Over a poorly exposed interval about 15 m long is an altered, coarse-grained, ophitic gabbroic anorthosite (sample E) that is texturally and mineralogically identical to rocks in the anorthositic series. At the beginning of the next well-exposed section of roadcut, several similar gabbroic anorthosite inclusions are found in a coarse-grained, subophitic to intergranular olivine gabbro (sample F), which gradually grades upward into a more consistently subophitic texture over the remainder of the roadcut (sample G). This rock type closely resembles that at the west end of the roadcut and indicates a downgrading in the cumulus status of pyroxene (and oxide?) and a reemergence of cumulus olivine. At first glance, a sensible interpretation of this cumulus regression is that it represents a magma recharge event. However, the cryptic variation of En and Fo across the cumulus regression exposed in this section (Fig. 5.7) is the reverse of what would be expected from magma recharge. An alternative explanation to magma recharge is that the textural and compositional variations across this interval reflect decompression of the chamber due to eruption to the surface. Decompression of a volatile-enriched magma would cause supercooling and multiple saturation of the magma and thereby explain the abrupt decreases in grain size and cumulus phase changes without much compositional variation. Magma expulsion through the roof of the layered series would also explain the occurrence of a gabbroic anorthosite inclusion at the cumulate regression. The hydrothermally altered nature of the gabbroic anorthosite is consistent with a volatile-rich environment in the anorthositic series cupola of the Duluth Layered Series magma chamber. The cumulus reversal to ophitic olivine gabbro without an increase in mg# could be explained by repressurization of a devolatilized magma. NEXT: Return to Interstate Highway 35 and travel southwest on it approximately 3 miles to Highway 13, Midway Road. Turn north on Midway road and continue 0.8 mi. to road to east (right). Follow road east approximately 0.2 mi., disembark and walk east to base of bluff. West of Duluth (refer to Figures 5.1, 5.5, and 5.8) STOP 5-26. Mesoproterozoic /Paleoproterozoic Unconformity Location: T.49N., R.15W., Sec. 17 SE, SW. Esko 7.5-minute quadrangle. UTM: 555,580E/5,174,380N Page 161 �DESCRIPTION: This area contains exposures showing the unconformable relationship between the Paleoproterozoic Thomson Formation (will see at Stop 5-28), and the Mesoproterozoic (Keweenawan) North Shore Volcanic Group. Scattered outcrops in the brushy area just west of the bluff are folded and metamorphosed feldspathic graywacke, siltstone, and mudstone or slate of the Thomson Formation. By contrast the Keweenawan strata exposed in the bluff have been little affected by tectonism since deposition, as shown by their gentle dip to the east. The unconformity is not exposed; but, inferring from the two sets of exposures, it appears to be an angular one. It represents a nearly 800 million-year hiatus, based on an age of roughly 1100 Ma for the Lakeside lavas northeast of here (Davis and Green, 1997) versus a date of about 1880 Ma for tuff interbedded with iron-formation in the Animikie Group, of which the Thomson Formation is a component (Fralick and others, 2002). The Keweenawan stratigraphic section in this area consists of a basal sedimentary unit, the Nopeming formation, which is conformably overlain by the Ely’s Peak basalts. The latter comprise the oldest volcanic strata of the North Shore Volcanic Group. The Nopeming consists of approximately 30 feet (10 m) of interbedded conglomerate and quartz arenite, with minor siltstone beds in the uppermost part of the unit. Much of the sandstone is medium- to coarsegrained, well-sorted and rounded, quartz arenite. The uppermost, silty parts of the Nopeming contain load casts and other structures indicating soft-sediment deformation. The overlying Ely’s Peak basalts locally contain well-developed pillow structures, indicating subaqueous deposition. Interestingly, a nearly identical stratigraphic sequence—the Puckwunge Formation—is observed 150 miles to the northeast near Grand Portage (Mattis, 1972). Together, these sequences are representative of a broad depositional and tectonic setting at the onset of Keweenawan deposition in this part of the Lake Superior region. HISTORICAL PERSPECTIVE: The stratigraphic position of the Nopeming at the base of the Keweenawan and its correlation with the Puckwunge Formation in the Grand Portage area and the Bessemer Quartzite in the Ironwood, Michigan area has long been accepted. However, a curious complication to this simple interpretation is presented by the 120 year old record of a 500' deep well drilled about 3 miles due south of this site. The log of the Short Line Park well, as reported by Winchell (1889), encountered sandstone and conglomerate, very similar in appearance to the Nopeming that was interlayered with basaltic lavas over a 140-foot interval. Surprisingly, the lower 84 feet of the hole is reported to be exclusively in basalt. This has led some to question whether the Nopeming is the basal unit or just an interflow sedimentary unit (Mattis, 1972). If the latter is correct, it implies a north-south fault along the base of the slope at this site. NEXT: Return to Interstate 35, cross under it, and continue south to join with Becks Road. Follow Becks Road southeast to Gary-New Duluth and Highway 23. Turn south (right) on 23 and follow it through town. At the south edge of town, Highway 23 turns west (right); follow it into the city of Fond du Lac. At the west edge of Fond du Lac, turn northwest (right) on Highway 210 and turn very shortly afterward to left into city park. Walk to northwest to edge of bluff (Stop 5-27). Page 162 �Figure 5.8. Geology and location of stops in the Fond du Lac, Jay Cooke State Park, and Thomson Dam areas (modified from Jirsa and Morey, 1987). STOP 5-27. Mesoproterozoic Fond du Lac Formation Location: T.48N., R.15W., sec. 6 SE, SE along west shore of St. Louis River near city park. Esko 7.5-minute quadrangle (SEE Fig. 5.8 for location). UTM: 554,670E/5,168,140N DESCRIPTION: The Fond du Lac Formation consists of nearly 800 feet of red sandstone, shale, and minor basal conglomerate, of which only a few hundred feet are exposed here along the lower St. Louis River. It is part of an accumulation of as much as 20,000 feet of clastic sediments deposited in the central Lake Superior region following Keweenawan volcanism. The Fond du Lac is considered to be equivalent to the Orienta Sandstone of the Bayfield Group in Wisconsin, and the Jacobsville Sandstone of Michigan. In this area (though not visible at this locality), the Fond du Lac unconformably overlies rocks of the Paleoproterozoic Thomson Formation, Ely’s Peak basalts, and the Duluth Complex (Morey, 1967). The sandstone consists of poorly sorted arkose to subarkose, containing clasts of quartz, chert, microcline, micas, and minor volcanic rocks; implying a source from both outside and inside of the Midcontinent rift. Depositional structures, primarily trough cross-bedding, indicate transport to the east. These sedimentary structures, and the presence of mud cracks and rain imprints, indicate fluvial deposition by streams that meandered across a broad alluvial plane (Morey and Ojakangas, 1982; see also papers in Ojakangas and others, 1997). NEXT: Return to northwest-bound Highway 210 and follow it for about 5 miles to entrance to Jay Cooke State Park. Turn in at main park building area on south side of 210. Page 163 �STOP 5-28, Location 5-28A Paleoproterozoic Thomson Formation Location 5-28A: T.48N., R.16W., Sec. 9 NW, SE; Jay Cooke State Park. Esko 7.5-minute quadrangle. UTM: 548,120/5,166,870N DESCRIPTION 5-28A: The walking bridge over the river south of the park buildings provides a vantage (depending on river level) to steeply dipping beds of graywacke, siltstone, and slate of the Thomson Formation. The Thomson is the upper stratigraphic unit of the Paleoproterozoic Animikie Group, presumably equivalent with the Virginia Formation discussed in stops 5-4, 5, and 6. Geochronologic data indicate that the Thomson Formation was deposited approximately 18701880 Ma (broadly inferred from Fralick and others, 2002), and was deformed during the Penokean orogeny (ca 1850 Ma). As shown in Figure 5.8, the Thomson is unconformably overlain by redbeds of the Mesoproterozoic Fond du Lac Formation, exposed discontinuously just to the east. The unconformity, marked by a basal quartz-pebble conglomerate, can be seen with some difficulty in the river bed at low-water levels near Oldenburg Point, and up Little Creek to the northeast. HISTORICAL PERSPECTIVE In the 1950’s, the Thomson Formation was considered by a number of authors (e.g., Grout and others, 1951) to be equivalent with the Knife Lake Group in northeastern Minnesota. The two rocks share many of the same structural and lithologic attributes. Most significantly, they both unconformably overlie Archean granitic rocks locally. For clarity, the Thomson is part of a sequence of strata that was known to unconformably overlie “Laurentian” granite. As compelling as this argument is, the Thomson is broadly equivalent to the Virginia Formation on the Mesabi range, with which continuity can now be established from geophysical and drill hole data, and the Rove Formation in the Gunflint district. Thus, the Thomson is now assigned as the uppermost unit of the Paleoproterozoic Animikie Group; and the Knife Lake is interpreted to have formed in part within a successor basin during latest Archean. NEXT: Return to Highway 210 and continue northwest approximately 1.6 miles to Thomson Dam. Page 164 �STOP 5-28, Location 5-28B Paleoproterozoic Thomson Formation T.48N., R.16W., sec. 5 SW SW; Thomson Dam. Cloquet 7.5-minute quadrangle. UTM: 545,610E/5,168,100N DESCRIPTION 5-28B: This is the type locality of the Thomson Formation, represented by approximately 650 feet of strata exposed between the dam north of Highway 210 and the RR bridge south of it. These exposures contain about equal proportions of graywacke, siltstone, and slate; metamorphosed to the greenschist facies. The formation contains abundant carbonate-rich concretions that locally are useful for delineating bedding in otherwise massive graywacke beds. Graywacke units range in thickness from 1 inch (2 cm) to 14 feet (4 m), and commonly display sedimentary structures indicative of turbidite deposition, included graded bedding, cross-bedding, sole marks, flute casts, and flame and ball structures (Morey and Ojakangas, 1970). Cross-bedding indicates flow to the south; though the trends of other structures imply more diverse and localized paleoslope directions. Structures in the Thomson Formation include gentle to open folds on varied scales, presumably related to deformation during the Penokean orogeny (Holst, 1984). The fold axes trend east, have vertical to steep south dips, and plunge gently east and west. A well developed axial-planar cleavage is present in the slaty beds, and concretions and mud chips are flattened in the plane of cleavage. The cleavage is deformed locally by kink bands. Quartz veins ranging in width from several cm to 3 meters are common in the formation. One of the largest, just north of the Highway 210 bridge, occupies an extensional fracture near the crest of a large anticline. Smaller veins are more contorted, and presumably were folded along with the adjacent rock. Several dikes of ophitic microgabbro ranging in width from a few inches to 200 feet (65 m) form a dike swarm that occupies northeast-trending joints (Fig. 5.8). The dikes are generally fine- to medium-grained and have chilled, fine-grained margins. Their precise age is unknown; however, their northeast trend and composition implies that they are related to the Mesoproterozoic Midcontinent rift. END OF TRIP REFERENCES Arth, J.G., and Hanson, G.N., 1975, Geochemistry and origin of the early Precambrian crust of northeastern Minnesota: Geochimica et Cosmochimica Acta 39:325-362. Biggs, D.L., ed., 1987, Centennial Field Guide Volume 3, Geological Society of America, North-Central Section; pages 47-73 describe various localities in Minnesota. Boerboom, T.J., and Zartman, R.E., 1993, Geology, geochemistry, and geochronology of the central Giants Range batholith, northeastern Minnesota: Canadian Journal of Earth Sciences 30:2510-2522. Page 165 �Borradaile, G.J., 1982, Tectonically deformed pillow lava as an indicator of bedding and way-up: Journal of Structural Geology, 4:469-479. Bowen, N.L., 1928, The evolution of igneous rocks: Princeton University Press, Princeton, 334 p. Broderick, T.M., 1935, Differentiation in lavas of the Michigan Keweenawan: Geological Society of America Bulletin 46:503-558. Butler, B.S., and Burbank, W.S., 1929, The copper deposits of Michigan: U.S. Geological Survey Professional Paper 144, 238 p. Cannon, W.F., 1992, The Midcontinent rift in the Lake Superior region with emphasis on its geodynamic evolution: Tectonophysics 213:41-48. Corfu, F., and Stott, G.M., 1998, Shebandowan greenstone belt, western Superior Province: U-Pb ages, tectonic implications, and correlations: Geological Society of America Bulletin 110:1467-1484. Davis, D.W., and Green, J.C., 1997, Geochronology of the North American Midcontinent rift in western Lake Superior and implications for its geodynamic evolution: Canadian Journal of Earth Sciences 34:476-488. Fralick, P., Davis, D.W., and Kissin, S.A., 2002, The age of the Gunflint Formation, Ontario, Canada: single zircon U-Pb age determinations from reworked volcanic ash: Canadian Journal of Earth Sciences 39:1085-1091. Goldich, S.S., Nier, A.O., Baadsgaard, H., Hoffman, J.H., and Krueger, H.W., 1961, The Precambrian geology and geochronology of Minnesota: Minnesota Geological Survey Bulletin 41, 193p. Green, J.C., 1977, Keweenawan plateau volcanism in the Lake Superior region, in Baragar, W.R.A.,ed., Volcanic regimes in Canada: Geological Association of Canada Special Paper 16, p. 407-422. Green, J.C., 1982, Geology of Keweenawan extrusive rocks, in Wold, R.J. & Hinze, W.J. (eds.) Geology and tectonics of the Lake Superior Basin. Geological Society of America Memoir 156, p. 47-56. Green, J.C., and Fitz, T.J.III., 1993, Extensive felsic lavas and rheoignimbrites in the Keweenawan Midcontinent Rift plateau volcanics, Minnesota: petrographic and field recognition: Journal of Volcanology and Geothermal Research 54:177-196. Green, J.C., and Miller, J.D., Jr., 2004, Geology of the Duluth Complex at Duluth as portrayed in new 7.5' geological maps. Abstracts and Proceedings of the 50th Institute on Lake Superior Geology. Green, J.C., and Schulz, K.J., 1982, Geologic map of the Ely quadrangle, St. Louis and Lake Counties, Minnesota: Minnesota Geological Survey Miscellaneous Map M-50, scale 1:24,000. Grout, F.F., 1918a, Internal structures of igneous rocks; their significance and origin with special reference to the Duluth Gabbro: Journal of Geology 26:439-458. Grout, F.F., 1918b, Two-phase convection in igneous magmas: Journal of Geology 26:481-499 Grout, F.F., 1918c, A type of igneous differentiation: Journal of Geology 26:626-58. Grout, F.F., 1937, Petrographic study of gold prospects of Minnesota: Economic Geology 37:56-68. Grout, F.F., Gruner, J.W., Schwartz, G.M., and Thiel, G.A., 1951, Precambrian stratigraphy of Minnesota: Geological Society of America Bulletin 62:1017-1078. Grout, F.F., and Schwartz, G.M, 1939, The geology of anorthosites of the Minnesota coast of Lake Superior: Minnesota Geological Survey Bulletin 28, 119 p. Gruner, J.W., 1930, Hydrothermal oxidation and leaching experiments; their bearing on the origin of Lake Superior hematite-limonite ores: Economic Geology 21:697-719, 837-867. Gruner, J.W., 1941, Structural geology of the Knife Lake area of northeastern Minnesota: Geological Society of America Bulletin 52:1577-1642. Holst, T.B., 1984, Evidence for nappe development during the Early Proterozoic Penokean orogeny in Minnesota: Geology 12:135-138. Hooper, P.R., and Ojakangas, R.W., 1971, Multiple deformation in Archean rocks of the Vermilion district, northeastern Minnesota: Canadian Journal of Earth Sciences 8:423-434. Hudleston, P.J., 1976, Early deformational history of Archean rocks in the Vermilion district, northeastern Minnesota: Canadian Journal of Earth Sciences 13:579-592. Hudleston, P.J., Bauer, R.L., Southwick, D.L., Schultz-Ela, D.D., and Bidwell, M.E., 1987, Structural geology of the boundary between Archean terranes of low-grade and high-grade rocks, northern Minnesota: in Balaban, N.H., ed., Field trip guidebook for selected areas in Precambrian geology of northeastern Minnesota, Geological Society of America north-central section meeting, St. Paul, Minnesota; Minnesota Geological Survey Guidebook Series n.17, p. 1-42. Page 166 �Hudleston, P.J., Schultz-Ela, D.D., and Southwick, D.L., 1988, Transpression in an Archean greenstone belt, northern Minnesota: Canadian Journal of Earth Sciences 25:1060-1068. Jirsa, M.A., 1984, Interflow sedimentary rocks in the Keweenawan North Shore Volcanic Group, northeastern Minnesota: MGS Report of Investigations 30, 20 p. Jirsa, M.A., 1990, Bedrock geologic map of northeastern Itasca County, Minnesota: Minnesota Geological Survey, Miscellaneous Map M-68, scale 1:48 000. Jirsa, M.A., 2000, The Midway sequence: A Timiskaming-type, pull-apart basin deposit in the western Wawa subprovince, Minnesota: Canadian Journal of Earth Sciences 37:1-15. Jirsa, M.A., and Boerboom, T.J., 2003, Bedrock geology of the Vermilion Lake 30’X60’ quadrangle, northeastern Minnesota: Minnesota Geological Survey, Miscellaneous Map M-141, scale 1:100,000. Jirsa, M.A., and Boerboom, T.J., 2003, Geology and mineralization of Archean bedrock in the Virginia horn: in Jirsa, M.A., and Morey, G.B., eds., Contributions to the geology of the Virginia horn area, St. Louis County, Minnesota: Minnesota Geological Survey Report of Investigations 53, p. 74-102. Jirsa, M.A., Boerboom, T.J., and McSwiggen, P.L., 1993, Geology of Archean greenstone-granite terrane in the Cook to Side Lake area: Field Trip 3 in Institute on Lake Superior Geology Proceedings, 39th Annual Meeting, Eveleth Minnesota, v. 39, Part 2, p. 97-128. Jirsa, M.A., Boerboom, T.J., and Morey, G.B., 1998, Bedrock geologic map of the Virginia Horn, Mesabi Iron Range, St. Louis County, Minnesota: Minnesota Geological Survey, Miscellaneous Map Series M-85, scale 1:48 000. Jirsa, M.A., and Boerboom, T.J., and Peterson, D.M., 2001, Bedrock Geologic Map of the Eagles Nest Quadrangle, St. Louis County, Minnesota: Minnesota Geological Survey Miscellaneous Map M-114, scale 1:24,000. Jirsa, M.A., and Morey, G.B., 1987, Jay Cooke State Park and Grandview areas: Evidence for a major Early Proterozoic-Middle Proterozoic unconformity in Minnesota: in Biggs, D.L., ed., Centennial Field Guide Volume 3, Geological Society of America, North-central Section; p.6772. Jirsa, M.A., Southwick, D.L., and Boerboom, T.J., 1992, Structural evolution of Archean rocks in the western Wawa subprovince, Minnesota: refolding of precleavage nappes during D2 transpression: Canadian Journal of Earth Sciences 29:2146-2155. Kuenen, P.H., and Migliorini, C., 1950, Turbidity currents as a cause of graded bedding: Journal of Geology, 58:91-127. Lawson, A.C., 1887, Geology of the Rainy Lake region: American Journal of Science 33:473-480. Lawson, A.C., 1893, The anorthosytes of the Minnesota coast of Lake Superior: Geological and Natural History Survey of Minnesota Bulletin 8, p. 1-23. Levy, E.R., 1991, The geology and sedimentology of the Archean metasedimentary rocks of the Virginia horn area, northeastern Minnesota: Unpublished M.S. thesis, University of Minnesota, Duluth, 199 p. Lundy, J.R., 1985, Clues to structural history in the minor folds of the Soudan Iron Formation, northeastern Minnesota: Unpublished M.S. thesis, University of Minnesota, Minneapolis, 144p. Mattis, A.F., 1972, Puckwunge Formation of northeastern Minnesota, in Sims, P.K., and Morey, G.B., eds., Geology of Minnesota: A centennial volume: Minnesota Geological Survey, p. 412-415. Miller, J.D., Jr., 1992, The need for a new paradigm regarding the petrogenesis of the Duluth Complex: Proceedings, 40th Institute on Lake Superior Geology. Hurley, WI, p. 65-67. Miller, J.D., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.M., and Wahl, T.E., 2002, Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geological Survey Report of Investigations 58, 207 p. Miller, J.D., Jr., and Ripley, E.M., 1996, Layered intrusions of the Duluth Complex, Minnesota, USA: in Cawthorne, R.G., ed., Layered Intrusions: Amsterdam, Elsevier Science, p. 257-301. Miller, J.D., Jr. and Weiblen, P.W., 1990, Anorthositic rocks of the Duluth Complex: Examples of rocks formed from plagioclase crystal mush: Journal of Petrology 31:295-339 Morrison, D.A., Ashwal, L.D., Phinney, W.C., Shih, C., and Wooden, J.L., 1983, Pre-Keweenawan anorthosite inclusions in the Keweenawan Beaver Bay and Duluth Complexes, northeastern Minnesota: Geological Society of America Bulletin, 94:206-221. Page 167 �Morey, G.B., 1965, The sedimentology of the Precambrian Rove Formation in northeastern Minnesota (abs.): Institute on Lake Superior Geology, 11th Annual Meeting, St. Paul, Minnesota, Proceedings p. 25-26. Morey, G.B., 1967, Stratigraphy and petrology of the type Fond du Lac Formation, Duluth, Minnesota: Minnesota Geological Survey Report of Investigations 7, 35 p. Morey, G.B., 1999, High-grade iron ore deposits of the Mesabi range, Minnesota—Product of a continental-scale Proterozoic ground-water flow system: Economic Geology 94:133-142. Morey, G.B., 2003, Paleoproterozoic Animikie Group, related rocks, and associated iron-ore deposits in the Virginia horn: in Jirsa, M.A., and Morey, G.B., eds., Contributions to the geology of the Virginia horn area, St. Louis County, Minnesota: Minnesota Geological Survey Report of Investigations 53, p. 74-102. Morey, G.B., and Ojakangas, R.W., 1970, Sedimentology of the Middle Precambrian Thomson Formation, east-central Minnesota: Minnesota Geological Survey Report of Investigations 13, 32 p. Morey, G.B., and Ojakangas, R.W., 1982, Keweenawan sedimentary rocks of eastern Minnesota and northwestern Wisconsin, in Wold, R.J., and Hinze, W.J., eds., Geology and tectonics of the Lake Superior basin: Geological Society of America Memoir 156, p. 135-146. Nicholson, S.W., Shirey, S.B., Schulz, K.J., and Green, J.C., 1997, Rift-wide correlation of 1.1 Ga Midcontinent rift system basalts: implications for multiple mantle sources during rift development; in Bornhorst, T.J., ed., Petrology and metallogeny of intraplate mafic and ultramafic magmatism: Canadian Journal of Earth Sciences 34:504-520. Ojakangas, R.W., 1966, Precambrian stratigraphy and structure of the Tower, Minnesota quadrangle (abs): Institute on Lake Superior Geology Proceedings, 12th Annual Meeting, Sault Ste. Marie, Michigan, Proceedings p. 17. Ojakangas, R.W., 1993, Pokegama Quartzite: in Institute on Lake Superior Geology Proceedings, 39th Annual Meeting, Eveleth Minnesota, v. 39, Part 2, p.19-21 and 46-48. Ojakangas, R.W., Dickas, A.B., and Green, J.C., 1997, Middle Proterozoic to Cambrian rifting, central north America: Geological Society of America Special Paper 312, 322 p. Ojakangas, R.W., Sims, P.K., and Hooper, P.R., 1978, Geologic map of the Tower quadrangle, St. Louis County, Minnesota: U.S. Geological Survey Geologic Quadrangle Map CQ-1457, scale 1:24,000. Owen, D.D., 1852, Report on a geological survey of Wisconsin, Iowa, and Minnesota: U.S. Department of the Treasury, Philadelphia, 638 p. Paces, J.B., and Miller, J.D., Jr., 1993, Precise U-Pb ages of Duluth Complex and related mafic intrusions, northeastern Minnesota: Geochonological insights to physical, petrogenetic, paleomagnetic and tectono-magmatic processes associated with the 1.1 Ga Midcontinent rift system: Journal of Geophysical Research 98:13,997-14,013. Peterson, D.M., Gallup, C., Jirsa, M.A., and Davis, D.W., 2001, Correlation of Archean assemblages across the U.S.-Canadian border: Phase I geochronology (abs): Institute on Lake Superior Geology, 47th Annual Meeting, Madison, Wisconsin, Proceedings v. 47, Part 1, p. 77-78. Peterson, D.M., and Jirsa, M.A., compilers, 1999, Bedrock geologic map and mineral exploration data, western Vermilion district, St. Louis and Lake Counties, northeastern Minnesota: Minnesota Geological Survey Miscellaneous Map M-98, scale 1:48,000. Peterson, D.M., and Patelke, R.L., 2003, National underground science and engineering laboratory (NUSEL): geological site investigation for the Soudan Mine, northeastern Minnesota: University of Minnesota Natural Resources Research Institute, Technical Report NRRI/TR-2003/29, 88p. Phinney, W.C., 1968, Anorthosite occurrences in Keweenawan rocks of northeastern Minnesota: in Isachsen, Y.W., ed., Origin of anorthosite and related rocks: Albany, New York. New York State Museum and Science Service, Memoir 18, p. 135-147. Roedder, E., 1979, Silicate liquid immiscibility in magmas. In Yoder, H.S., Jr., ed., The evolution of the igneous rocks. Princeton, New Jersey. Princeton University Press, p. 15-57. Schmitz, M.D., 1994, Origin and petrogenesis of the Early Proterozoic Kenora-Kabetogama mafic dike swarm: unpublished seniors thesis, Macalester College, 110 p. Schultz-Ela, D.D., and Hudleston, P.J., 1991, Strain in an Archean greenstone belt of Minnesota: Tectonophysics 190:233-268. Page 168 �Schulz, K.J., 1980, The magmatic evolution of the Vermilion greenstone belt, NE Minnesota: Precambrian Research 11:215-245. Schwartz, G.M., Goldich, S.S., Marsden, R.W., eds., 1956, Field Trip 1: Precambrian of northeastern Minnesota: GSA Guidebook Series, 235 p. Sims, P.K., 1972, History of geologic investigations: in Sims, P.K., and Morey, G.B., eds., Geology of Minnesota: A centennial volume: Minnesota Geological Survey, p. 21-26. Southwick, D.L., Boerboom, T.J., and Jirsa, M.A., 1998, Geologic setting and descriptive geochemistry of Archean supracrustal and hypabyssal rocks, Soudan-Bigfork area, northern Minnesota: Minnesota Geological Survey Report of Investigations 51, 69p. Southwick, D.L., Morey, G.B., and McSwiggen, P.L., 1988, Geologic map of the Penokean orogen, central and eastern Minnesota, and accompanying text: Minnesota Geological Survey Report of Investigations 37, 25p. Sutton, T.C., 1963, Geology of the Virginia horn area: Minneapolis, University of Minnesota, M.S. thesis, 97 p. Taylor, R. B., 1964, Geology of the Duluth Gabbro Complex near Duluth, Minnesota. Minnesota Geological Survey Bulletin 44, 63 p. Vervoort, J.D., and Green, J.C., 1997, Origin of evolved magmas in the Midcontinent rift system, northeast Minnesota: Nd-isotope evidence for melting of Archean crust: Canadian Journal of Earth Sciences 34:521-535. Wager, L.R., and Deer, W.A., 1939, Geological investigations in East Greenland, Part III. The petrology of the Skaergaard intrusion, Kangerdlugssuaq, East Greenland. Meddr. Gronland. 105, 352 p. Winchell, N.H., 1889, 18th Annual Report of the Minnesota Geological and Natural History Survey. Winchell, N.H., 1899, The Geology of Minnesota. Geological and Natural History Survey of Minnesota, Final Report v. 4, 629 p. Winchell, N.H., 1900, The Geology of Minnesota, Geological and Natural History Survey of Minnesota, Final Report v. 5, 1025 p. White, W.S., 1972, The base of the Upper Keweenawan, Michigan and Wisconsin: U.S. Geological Survey Bulletin 1354-F, 23 p. Page 169 �FIELD TRIP 6 GLACIAL AND POSTGLACIAL LANDSCAPE EVOLUTION IN THE GLACIAL LAKE AITKIN AND UPHAM BASIN, NORTHERN MINNESOTA Lisa Marlow; Phil Larson; Howard Mooers, Department of Geological Sciences, University of Minnesota - Duluth Glacial Lake Aitkin and Upham Road Log with numbered field trip stops, geographic markers and discussion sites: F: Farnum site, B: Baker Spider Creek site, and Hay Lake. Image from 30 meter Digital Elevation Model of Minnesota Page 170 �GLACIAL AND POSTGLACIAL LANDSCAPE EVOLUTION IN THE GLACIAL LAKE AITKIN AND UPHAM BASIN, NORTHERN MINNESOTA Lisa Marlow; Phil Larson; Howard Mooers, Department of Geological Sciences, University of Minnesota – Duluth INTRODUCTION Recent investigations into the glacial geology of northeastern Minnesota have considerably revised the glacial chronology. This field trip examines features (landforms and stratigraphy) of the Glacial Lakes Aitkin and Upham basins that record a complex sequence of events related to advance and retreat of the Laurentide Ice Sheet. Deglaciation and formation of glacial Lakes Aitkin and Upham were followed by drainage of the lakes and eolian activity on exposed lacustrine sediments. Glacial Lakes Aitkin and Upham occupied a basin bounded by the Giants Range to the north and moraines of the Superior and Rainy Lobes and St. Louis Sublobe to the south, east, and west; respectively. As the Rainy Lobe retreated during the Late Wisconsin, meltwater ponded in front of the ice forming proglacial Lakes Aitkin and Upham I. As the Rainy Lobe continued to retreat northeastward across the basin to a margin roughly coincident with the Giants Range, the St. Louis sublobe advanced into the basin (Fig. 6-1). Stagnation and melting of the St. Louis sublobe resulted in formation of Glacial Lakes Aitkin and Upham II (Hobbs 1983). Continued retreat of the Rainy Lobe northward from the Giants Range resulted in formation of Glacial Lakes Norwood and Koochiching, both of which drained southward into Aitkin and Upham II. The evolution of the lakes by a combination of downcutting of the outlets and isostatic rebound is recorded by a series of beaches, wave-cut scarps, and multiple outlets. Dune formation in the Glacial Lakes Aitkin and Upham II basin was strongly controlled by sediment availability. Dune clusters show a strong correlation with the presence of underlying fine-grained nearshore lacustrine sand. Dune formation was likely episodic, coinciding with periods of rapid lake level lowering and exposure of nearshore sands. Cessation of eolian activity resulted from more gradual stabilization of dunefields. Peaks in eolian activity are indicated at 9.8, 9.3 and 7.4 kyr BP (all dates Page 171 �given in C14 years BP) by the magnetic susceptibility record of Hay Lake, a small lake near the Glacial Lake Upham II shoreline (Figs. 6-2 and 6-3). These peaks in eolian activity may coincide with episodes of drainage of Upham II and Aitkin II. Figure 6-1. The Glacial Lake Aitkin and Upham basin. Beaches are noted by shaded polygons; dunes appear as black dots in the basin; boundaries and channels noted as black lines. Field trip stops will illustrate examples of the following: beach, dune, underflow fan, rhythmites, alluvial fan, Lake Upham sediments in Rainy lobe outwash and a wave-washed esker. Map is modified from Hobbs (1983). Page 172 �Figure 6-2. Location of Hay Lake. The lined region indicates the Lake Upham upland area; white polygons are dunes. (Vanduse 7.5’ USGS quadrangle) (gyttja) (marl) C14 Figure 6-3. Hay Lake sediment core (8.57m): 1. Lithology 2. Age-depth model 3. Magnetic susceptibility 4. Composition 5. Mass accumulation rate. Page 173 �The dunefields in the Glacial Lakes Aitkin and Upham II basin display similarity to other Quaternary eolian deposits throughout Minnesota and the Midwest (Figs. 6-4 and 6-5). Despite well over a century of research on Minnesota's glacial landscape, these paleodunefields have received relatively little attention. Although mentioned by Upham (1896), Winchell (1896), Hall and Sardeson (1898), Elftman (1898), Leverett and Sardeson (1919) and Leverett (1932), the first comprehensive work on Minnesota's eolian landscape was that of Cooper (1935, 1938). Figure 6-4. North American dune and loess distribution. (Brady and Weil, 2000) 96° 94° 92° 90° Conifer/ Hardwood 48° 4 Lake Aitkin and Upham basin 3 1 46° 2 Prairie Deciduous 44° 0 100km Figure 6-5. Location of sites in Minnesota associated with eolian sedimentation referenced in the text. 1) Hay Lake 2) Lake Ann 3) Elk Lake 4) Lake Winnibigoshish. Filled areas are prominent dune colonies. (Modified from Keen and Shane 1990) Page 174 �More recently, Grigal and others (1976) examined a dunefield exposed along the southeast shore of Lake Winnibigoshish (Fig. 6-5). They described several buried soil horizons, which yielded radiocarbon dates in the range of 7910 to at least 5040 yr BP suggesting episodic dune formation during the mid-Holocene The formation of these dunes was related to lake-level instability and episodic exposure of littoral areas to wind erosion during the mid-Holocene (Larson and Mooers 2003). The mid-Holocene climate in the upper Midwest was considerably warmer and drier than the modern climate (Webb et al. 1983; Bartlein et al. 1984; Dean et al. 1984; COHMAP 1988; Dean et al. 1996). The warmer, drier conditions resulted in an eastward shift of prairie ecotone (Fig. 6-5). Lower groundwater levels were accompanied by lake level lowering, particularly in groundwater-dominant closed-basin lakes and ponds, and increased fire frequency and vegetational stress locally resulted in landscape destabilization. Grigal and others (1976) speculated that the dunes of the Anoka Sand Plain may be midHolocene in age, an idea Keen and Shane (1990) explored in detail in their paleoenvironmental investigation of Ann Lake (Fig. 6-5). They concluded that there was a relatively continuous record of eolian activity throughout the mid-Holocene, a reversal of the ideas of earlier workers (Hall and Sardeson 1898; Cooper 1935). Keen and Shane’s (1990) whole-core magnetic susceptibility record from Ann Lake records increased silt and fine sand input to the lake from about 8.0 until 4.5 kyr BP with discrete peaks in clastic input at ca. 7.4, 5.8, and 4.9 kyr BP. However, it is difficult to distinguish whether this sediment was exported to the lake directly from active dunes on the shore, eroded from the shoreline due to lake level instability, or deposited suspended airborne sediment. Despite these ambiguities, they concluded that the increased clastic input to Ann Lake was due to landscape destabilization and widespread dune activity on the Anoka Sand Plain. The distinction between lake level lowering and landscape destabilization as triggers of eolian activity is subtle but important. Although the prairie ecotone shifted eastward during the mid-Holocene, this in itself did not create an environment conducive to widespread landscape destabilization. At present in the central Dakotas there are numerous locations with ample sediment availability, but no widespread dune formation because of the stabilizing vegetation of the prairie environment. Landscape destabilization and dune formation in the existing prairie Page 175 �environment occurs by excessive drying such as in the Nebraska Sand Hills (Winspear and Pye 1996). Viewed in this context, while Grigal and others (1976) obtained dates from paleosols buried by eolian events, the Keen and Shane (1990) and Dean (1997) magnetic susceptibility records may merely be a record of dryer, dustier conditions during the mid-Holocene rather than pervasive local eolian activity. Today the Glacial Lakes Aitkin and Upham basin is patchwork of small ‘islands’ occupied by upland vegetation formed on stabilized dunes. The ‘islands’ are interrupted by vast areas of peatland developed on the poorly drained, low-relief lakebed composed of fine-grained lacustrine sediment. Locally, scattered topographic highs are underlain by older glacial sediments predating formation of the lakes. STOP 6 – 1. Glacial Lake Duluth underflow fan. (NW¼ SW¼ Sec. 30, T48N, R15W, Carlton County, Frogner 7.5’ USGS quadrangle; 553728E, 5161950N, NAD83) This location preserves the record of several glacial events that affected the western Lake Superior region and provides insight into their relative chronologies. Here, red Glacial Lake Duluth clays are overlain by a subaqueous fan deposited by meltwater from Glacial Lake Aitkin and Upham II (Fig. 6-6). These in turn are overlain by Superior Lobe till deposited by of the Marquette advance. These relationships indicate meltwater drained from Glacial Lake Upham into Glacial Lake Duluth (Table 1) (Mooers and Lehr 1997; Clayton and Moran 1982; Wright 1972; Wright et al. 1973). During the last phase of Glacial Lakes Aitkin and Upham II, the two lakes were thought to have separated and drained through different outlets; Upham through the St. Louis River and Aitkin through the Mississippi River (Hobbs 1983). A digital elevation model (DEM) reconstruction correcting the basin for isostatic rebound confirms the separation of the two lakes (Marlow 2004). The Mississippi River outlet to Aitkin is a relatively narrow passage through collapsed ice-cored terrain. It is unlikely this underdeveloped outlet carried significant meltwater. Final drainage of Aitkin therefore occurred after significant meltwater contributions to the lake basins had ceased. Page 176 �Figure 6-6. Stop 6-1. Underflow fan at the mouth of the St. Louis River drainage in Glacial Lake Duluth basin. The sediments were deposited by meltwater from Glacial Lake Upham II. Page 177 �Table 6-1. The glacial chronology of northeastern Minnesota outlined resulted from a compilation of several resources (Mooers & Lehr, 1997; Larson (unpublished); Marlow, 2004; Bjork, 1990; Wright, 1972; Clayton & Moran, 1982; Hobbs, 1983; Fenton, 1983; Clayton, 1983; Lehr & Hobbs, 1992) Phase Moraine Lobe Glacial Lakes Other events C14 Date (kyr BP) drainage of Aitkin II Marquette Superior Vermilion Vermilion Big Rice Big Rice/ Wampus 9.9 Rainy Rainy Aitkin II Upham II Allen Rainy Alborn Culver St. Louis Split Rock Cloquet Superior ~11.7 12.0 St. Louis River outlet Inflow from Embarrass gap Us-Kab-WanKa/Chicken/ Hellwig/Birch/Spider outlets formation of Goodland esker Nickerson Nickerson Superior North of Nashwau k Duluth drainage of Upham II cessation of meltwater inflow to Aitkin and Upham II L. Duluth underflow fan Inflow from Prairie River ~13.0 Aitkin Upham I I Automba Sandy Lake Outing St. Croix St. Croix Rainy Rainy ~15.516 Rainy Page 178 �STOP 6 – 2. Hellwig Creek outlet and Glacial Lake Upham II basin overlook. (SE ¼ SE ¼ Sec. 34, T53N, R17W, St. Louis County, Canyon 7.5’ USGS quadrangle; 540174E, 5207618N, NAD83) This stop is near beaches and outlets that existed during the earliest phases of Glacial Lake Upham II, and provides a good overlook from the Upham shore into the basin (Fig. 6-7). Figure 6-7. Stop 6-2. Location of early outlets to Lake Upham; Triangle marks location of core taken by Baker (1965) containing marl. Circle indicates stop 2 site. Page 179 �Following stagnation of St. Louis Sublobe ice meltwater flowed through a combination of surface and englacial channels toward the ice margin then along the ice margin through a chain of small proglacial lakes. Among the earliest and highest outlets in the Upham basin from which these small lakes drained were the Us-Kab-Wan-Ka River (427 m; 1400’) and Chicken Creek (434 m; 1425’) channels. These channels drained to the southeast along the margin of the Superior Lobe eventually reaching the St. Croix River (Hobbs, 1983). Proglacial lakes and subglacial meltwater in the Aitkin basin drained southward through the Snake channel (381 m; 1250’) to the St. Croix River and perhaps southwest to the Mississippi River (378 m; 1240’) (Hobbs 1983). All of these outlets lie above the highest main beaches of Glacial Lakes Aitkin and Upham II, and the DEM correction for isostatic rebound indicates ice must have been present in the basin for them to function (Marlow 2004). Therefore they were likely associated with small proglacial lakes rather than the main stages of the lakes. After the St. Louis Sublobe advance, the margin of the Rainy Lobe retreated north of the Giants Range resulting in the ponding of water between the retreating ice and the Giants Range, forming Glacial Lake Norwood (Fig. 6-8) (Winchell 1901; Hobbs 1983). Glacial Lake Norwood drained south through the Embarrass Gap entering Glacial Lake Upham II at an elevation of 436 m (1450’) (Hobbs 1983). This was the first major meltwater inlet to Glacial Lakes Aitkin and Upham II. During this time a significant amount of stagnant ice remained in the basin, and it is possible that Aitkin and Upham were separated by stagnant ice along the Swan River Sill (Fig. 68). As the Rainy Lobe continued to retreat northward, Glacial Lake Norwood increased in extent. Downcutting in the Embarrass Gap lowered the level first to 436 m (1430’) and eventually to 427 m (1400’). This lower stage is known as Glacial Lake Koochiching (Nikiforoff 1947; Hobbs 1983; Lehr and Hobbs 1992; Leverett 1932). While Glacial Lake Norwood was in existence, the upper strandlines (411 to 421 m; 1350-1380’) in the extreme northeastern part of Glacial Lake Upham II formed. These levels of Upham likely correspond to outlets at Hellwig Creek (409 m; 1330’), Birch (406 m; 1320’), and Spider Creek (400 m; 1300’). A combination of isostatic rebound and downcutting eventually led Page 180 �to successive abandonment of the Hellwig Creek, Birch, and Spider Creek outlets and lake level drop. Baker (1965) reported a bulk radiocarbon date of 13,000±400 (W-1234) yr B.P from a sequence of lacustrine marl within the Spider Creek outlet (Fig. 6-7; and “s” in Fig. 6-8). The marl must post-date the cessation of drainage through the channel since marl formation requires shallow still water. Baker (1965) expressed concern that this date was too old due to possible contamination by lignite. However, this date is consistent with the other evidence presented in this guide. The Spider Creek date places the minimum age of Glacial Lakes Aitkin and Upham II, and therefore the maximum limit of the St. Louis Sublobe, prior to 13.0 kyr B.P. Figure 6-8. Significant features associated with the recession of the Rainy Lobe after the St. Louis sublobe advance. Glacial Lake Norwood (LN) extent is indicated by a dashed line. Page 181 �STOP 6 – 3. Wave-washed esker island (NE¼ NE¼ Sec. 1, T54N, R20W, St. Louis County, Toivola 7.5’ USGS quadrangle, 514367E, 5226982N, NAD83). There are many landforms within the Glacial Lakes Aitkin and Upham II basins that predate development of the lakes. This exposure is an example of an esker deposited during retreat of the Rainy Lobe that was later modified by waves. (Fig. 6-9). This esker and others like it became wave-washed “islands” once Glacial Lakes Aitkin and Upham I and II formed. Exposed at the base of the sequence are coarse gravels. The gravels contain abundant northeast-provenance material and locally derived mudstone and shale from the Paleoproterozoic Virginia Formation and perhaps younger Cretaceous strata. They were deposited by a beaded esker system during retreat of the Rainy Lobe. Overlying the gravels is a fine-grained till deposited by the St. Louis Sublobe. On top of the till is a sequence of nearshore sands and gravels. These presumably eroded from that portion of the esker rising above the level of Glacial Lake Upham II; the strandline formed at about 397 m (1200’) elevation. The uppermost portion of the sequence is a blanket of eolian sediment exported from the surrounding lake plain to the upland after final drainage of Upham. Figure 6-9. Stop 6-3. Wave-washed esker island in Glacial Lake Upham II. Page 182 �STOP 6 – 4. Rainy Lobe collapsed outwash, Glacial Lake Upham I sediments, and St. Louis Sublobe till (NE¼ SE¼ Sec. 15, T57N, R19W, St. Louis County, Kirk 7.5’ USGS quadrangle; 520373E, 5251907N, NAD83) The Glacial Lakes Aitkin and Upham basins were occupied by glacial lakes on two separate occasions during the Late Wisconsin glaciation. The retreat of the Rainy and Superior Lobes from their maximum positions formed a series of ice-cored recessional moraines bordering the lake basins to the south and west leading to ponding of water and formation of Glacial Lakes Aitkin and Upham I (Wright 1972; Hobbs 1983; Lehr and Hobbs 1992). The initial retreat of the Rainy Lobe from the Mille Lacs and Outing moraines (Mooers 1988) led to the formation of Aitkin I (Fig. 6-10). Continued retreat of the Rainy Lobe from the Sandy Lake moraine led to the formation of Upham I. Although the exact time of formation of Aitkin and Upham I is not known, it occurred after the Rainy Lobe retreated from the St. Croix moraine about 15.5 kyr B.P. (Clayton and Moran 1982; Mooers and Lehr 1997), and before the St. Louis Sublobe advanced into the Aitkin and Upham basins from the northwest. Exposed at the base of the sequence are steeply south-dipping foreset beds of a subaqueously deposited fan. These sediments are Rainy Lobe provenance, deposited along the southern margin of stagnant ice lying on the southern slope of the Giant’s Range, an area now characterized by collapsed ice-cored topography. The upper portion of the sequence is a St. Louis Sublobe till. Between the fan sediments and till are a number of elongate slabs of fine-grained lacustrine sediment derived from Glacial Lake Upham I. This lacustrine sediment was eroded from deeper water and thrust onto the fan during the advance of the St. Louis Sublobe (Figs. 6-11 and 6-12). The relationships visible in this exposure indicate that the St. Louis Sublobe advanced while a substantial amount of Rainy Lobe ice was still present south of the Giant’s Range. Page 183 �Figure 6-10. Prominent moraines in the Glacial Lakes Aitkin and Upham basin with some modern features included. Modified from Mooers (1988) and Lehr and Hobbs (1992). Figure 6-11. Subaqueously deposited Rainy Lobe outwash exposed in a gravel pit near Cherry, MN. Page 184 �Figure 6-12. St. Louis sublobe till containing Glacial Lake Upham I sediment. STOP 6 – 5. Eolian dunes. (SW¼ SW ¼ Sec. 31, T56N, R20W, St. Louis County, Riley 7.5’ USGS quadrangle, 504626E, 5237098N, NAD83) Exposed in the sand pit is a small (<2 m) dune composed of characteristic 4-φ sand (62.5 microns or 230 mesh). Most of the sand exposed in the pit is massive and structureless. However, relict cross bedding can be observed in small patches near the base of the sequence. Primary sedimentary structures in the upper portion of the exposure have been obliterated by bio and crioturbation. Clusters of eolian dunes are widely distributed throughout the Glacial Lakes Aitkin and Upham basins. They formed as Glacial Lakes Aitkin and Upham II incrementally drained, exposing areas of littoral sediment to wind erosion. Dune heights range from 1 to 5 meters and are composed of a characteristic fine to very fine grained sand (Marlow 2004). They occur as parabolic or longitudinal dunes, but are commonly distorted in morphology as a result of forming in a variable hydrologic environment (Fig. 6-13) (Bagnold 1941) There are a large number of longitudinal dunes and elongate clusters of dunes oriented in a NW-SE direction, suggesting they formed under prevailing NW winds (Fig. 14). Bagnold’s (1941) description of dunes indicates that they “tend to occur in belts or chains, whose direction coincides with that of the resultant long-period Page 185 �sand vector Q”, with Q being sand flow because of the sum of the strong and gentle wind directions, and the width of the belt at right angles to Q.” Figure 6-13. Distorted dunes in the northern end of Lake Aitkin basin showing a combination of crescentic and longitudinal dunes (Jacobson and Split Hand 7.5 minute USGS quadrangles). Page 186 �Figure 6-14. Trend in dune clusters indicating a NW-SE wind direction. Elongate polygons are beaches. Circles indicate stops 8, 9, and 10 sites. STOP 6 – 6. Glacial Lake Upham II Shoreline. (SE¼ SE¼ Sec. 33, T56N, R21W, St. Louis County, Silica 7.5’ USGS quadrangle, 499595E, 5236726N, NAD83) After the time of upper beach formation, Lake Upham experienced a drop in water level documented at Townline beach (411 m; 1350’) through the use of Ground Penetrating Radar (Fig. 6-15). The GPR results are interpreted as a down stepping of shoreline deposits (Fig. 6-16). The progradation of the bedform resulted in a constructional shoreline and indicates regression of the lake. The regression is an example of Forced Regression as discussed by Posamentier and Allen (1999). Forced regression takes place when there is a relative sea-level fall that progressively exposes the sea (or lake) floor, thereby causing the shoreline to migrate seaward (Posamentier and Allen 1999). Page 187 �Figure 6-15. Shoreline of Glacial Lake Upham II along Townline Road south of Hibbing. Shoreline indicated by black dotted line. Heavy solid line indicates where GPR data was collected (Silica & Riley 7.5 ‘ USGS quadrangle). Figure 6-16. Ground Penetrating Radar profile of Glacial Lake Upham II shoreline. Vertical data represents ~9 meter; horizontal data ~121 meters. (Data processing courtesy of Nigel Wattrus at Large Lakes Observatory, University of Minnesota-Duluth) Page 188 �STOP 6 – 7. Goodland Esker. (SE¼ NE¼ Sec. 9, T55N, R23W, Itasca County, Calumet 7.5’ USGS quadrangle, 479736E, 5234496N, NAD83) This stop is a gravel pit exposure in the Goodland esker, one of the most prominent glacial landforms in Itasca County. The location of the esker and details of its formation place important constraints on the timing of the St. Louis Sublobe advance and deposition of the Alborn drift (Fig. 6-17). The morphology of the esker system is best described by the model of Shreve (1985). The proximal (with respect to the Giants Range) segment is an ~600 m wide, 10 km long channel incised into bedrock and older drift (tunnel valley), occupied by a multicrested esker. The medial segment consists of a single broad-crested esker segment deposited in a 10 km long channel incised into the overlying ice. The distal component of the esker system is a supraglacial fan complex. The base of the subglacial channel drops from 428 m at the crest of the Giant’s Range to <400 m at a point 6 km downstream, then rising to 409 m over the next 4 km. Downstream of this point, the elevation of the top of the broad-crested esker rises 19 m over 10 km, from 434 to 455 m at the apex of the distal fan. The transition from a multi-crested to broad-crested esker is apparently triggered by an increase in the adverse slope up which the subglacial channel flowed. Multi-crested eskers form in areas of moderate upgradient flow as the channel tends to migrate laterally, rather than upward, in the ice, while broad-crested eskers form in areas of steeper upgradient flow due to freezing of the tunnel walls, a process favoring low, wide tunnel geometries (Shreve 1985). Lithologies in the multi-crested segment esker are dominantly Archean granitics and greenstones transported from north of the Giant’s Range. The broad-crested segment has a markedly different lithologic assemblage than the multi-crested segment. In addition to Archean lithologies, an abundance of Paleoproterozoic Animikie Basin lithologies are present including quartzite, sulfidic mudstones, iron formation, and pisolitic lateritic iron formation. The abundance of locally-derived Animikean lithologies indicates material in the broad-crested segment was deposited as incision of the Nchannel was taking place upstream. The broad-crested segment is therefore older than the multi-crested segment. The wide range of mean sizes of the various Animikean Page 189 �lithologies attests to the strong control of primary rock characteristics on particle size. Note in particular the abundance of Pokegama quartzite as large (~1 m) boulders and its paucity in smaller size fractions. Deposition of the distal fan occurred throughout the period the esker system was active. The initial phase was characterized by deposition of a supraglacial fan on stagnant ice. Fan deposits are identifiable up to 10 km from the fan apex, and cover in excess of 100 km2. The maximum elevation on the fan complex is about 477 m, indicating at least 20 m of ice was present above the outlet of the englacial channel at the fan apex. During later stages of the esker system, underlying stagnant ice melted collapsing the earliest deposited fan sediment. The final phase of fan formation was characterized by incision of the collapsed fan head down to about 450 m. The subglacial drainage system that deposited the esker was probably short-lived. It could not have formed before the St. Louis Sublobe occupied the area around Grand Rapids, blocking the natural southwesterly flow of meltwater from the watershed of the present-day Prairie River. Similarly, it could not have persisted after St. Louis Sublobe ice wasted away and the Prairie River began flowing into Glacial Lake Aitkin II. Consequently, it may have been active a few hundred years at most. Despite its short duration, the large size of the esker system – it is one of the largest in Minnesota – attests to an enormous discharge of meltwater responsible for its formation. Meltwater was gathered from an interlobate zone that existed to the north of the Giant’s Range between the St. Louis Sublobe and the Rainy Lobe. This interlobate area may have drained well in excess of 1000 km2 of the ice sheet. The elevation difference between the fan apex (477 m) and the surrounding (subglacial) landscape (400 m) indicate that a continuous cover of Rainy Lobe ice at least 75 m thick was present south of the Giant’s Range at the time of esker formation and advance of the St. Louis Sublobe. This argues strongly against the postulated ice-free zone between the northern margin of the St. Louis Sublobe and the southern margin of the Rainy Lobe. Previous workers have postulated a relatively late advance for the St. Louis Sublobe (ca. 11.7 kyr BP), correlating it with the Vermilion Phase (ca. 12.0 kyr BP), or an even later phase, of the Rainy Lobe (Wright 1972; Hobbs 1983). However, the presence of active Rainy Lobe ice just north of the Giant’s Range at the time of the Page 190 �advance of the St. Louis Sublobe and esker formation provides further evidence that the St. Louis Sublobe advanced at a significantly earlier date. Figure 6-17. Main features of the Goodland esker system. (Image from 30 m DEM of MN) Page 191 �STOP 6 – 8. Prairie River underflow fan in Glacial Lake Aitkin II. (NE¼ NE¼ Sec. 35, T53N, R24W, Itasca County, Jacobson 7.5’ USGS quadrangle, 473831E, 5209702N, NAD83) Glacial Lakes Aitkin and Upham II had two successive major meltwater inlets, the Embarrass Gap and the Prairie River. Inflow of the Prairie River into Aitkin II resulted in deposition of a large underflow fan extending 50 km from its apex (Hobbs 1983). The fan was later incised by the Mississippi River. The cutbank at this stop exposes ~9 m of sandy underflow fan sediments overlying a lacustrine clay (Fig. 6-18). Visible within the underflow sediments are several Bouma sequences. Outflow from Glacial Lake Koochiching into Glacial Lake Upham II through the Embarrass Gap ceased when a lower outlet opened north of Grand Rapids because of collapse of stagnant ice-cored terrain. Meltwater then flowed down the Prairie River into Glacial Lake Aitkin II, lowering the level of Koochiching from 427 to 411 m (1400’ to 1350’) (Hobbs 1983) The Prairie River entered Glacial Lake Aitkin II at an elevation of 396 m (1300’) (Fig. 6-1). Although it is not known exactly when the Embarrass Gap was abandoned, it must have occurred prior to a 10.2 kyr B.P. transition from predominantly clastic to organic lake sedimentation in Sabin Lake located in the Embarrass Gap (Björk 1988; Lehr and Hobbs 1992). Fenton (1983) suggests that the Prairie River outlet was initiated between 12.3 and 10.8 kyr B.P. based on the chronology of the Lake Agassiz basin to the west. Clayton (1983) suggests 11.5 kyr B.P. for the inception of the Prairie River outlet. The Prairie River inlet was abandoned once Glacial Lake Koochiching began to flow into Glacial Lake Climax. However, Glacial Lakes Aitkin and Upham II persisted after abandonment on the last meltwater inlet. Upham finally drained as the St. Louis River outlet was incised to its modern level. Hobbs (1983) indicates that water could have still been flowing into Aitkin after drainage of Upham by way of the Mississippi River, which has a prominent terrace at 389 m (1275’). Farnham and others (1964) obtained a date of 11,635 yr BP on a paleosol in the southwestern part of the Glacial Lake Aitkin II basin. This paleosol is overlain by a thin Page 192 �marl and 3 feet of clay, indicating Aitkin was in existence well after this time. A radiocarbon date of 10,000 yr BP was obtained from a marl from the Ball Bluff quadrangle in the northeastern part of the Aitkin basin (Hobbs 1983) suggests the lake persisted at least until this time. There are two alternative explanations explaining how the southwestern portion of the Glacial Lake Aitkin II basin was successively drained and re-inundated. The close similarity between the presumed initiation of the Prairie River inlet at 11.5 kyr BP (Clayton 1983) and the re-inundation of the southwestern part of the Glacial Lake Aitkin II basin at 11,635 yr BP suggests a causal relationship. Glacial Lake Aitkin II may have assumed a lower, stable level by ca. 11.6 kyr BP. The relatively rapid lowering of Glacial Lake Koochiching would have sent a surge of meltwater into the lake, resulting in a rise in lake level even if only temporarily. The lacustrine sediment overlying the Farnham (1964) paleosol was thus derived from the Prairie River inlet. The alternate explanation is that much of the Aitkin basin was drained through Glacial Lake Upham II by downcutting of the St. Louis River outlet. The Swan River Sill controlled the water level in the remaining lake. As isostatic rebound of the basin progressed, the sill on the northeastern side of the basin rose at a higher rate than the southwestern portion of the basin. Lake level transgression resulted in re-inundation of lake bottom exposed by drainage of Upham. Page 193 �Figure 6-18. Stop 6-8. Exposure of the Prairie River underflow fan at a Mississippi River cutbank. STOP 6 – 9. Rhythmite in Glacial Lake Aitkin II. (SW¼ NW¼ Sec. 4, T52N, R23W, Aitkin County, Jacobson 7.5’ USGS quadrangle, 479288E, 5207689N, NAD83) A rhythmite is overlain by cross-bedded sands, which is then overlain by underflow sediments. This is an interesting site that may document a dramatic inception of the Prairie River inlet into a basin that was previously depositing clays. Page 194 �STOP 6 – 10. Eolian dunes in Glacial Lake Upham II basin. (NW¼ NE¼ Sec. 18, T 52 N R 22 W, Aitkin County, Vanduse Lake 7.5’ USGS quadrangle, 486857E, 5204509N, NAD83) Exposed at this site are eolian sand dunes overlying lacustrine clay of Glacial Lake Upham II (Fig 6-19). Dunes in the Glacial Lakes Aitkin and Upham basins are most densely concentrated near the inlet of the Prairie River to the Aitkin basin and east of the Swan River Sill in the Upham basin (Fig. 6-14). The 4-φ mean grain size of the dunes sampled in these areas in the basin match that of sediments from the underflow fan sediments seen at Stop 8. These sediments have been mapped in the Aitkin and Itasca Soil Surveys (Nyberg 1987, 1999) as the Zimmerman, Cowhorn, and Wawina soil series, all composed of fine to very fine grained sands of lacustrine origin. From the inception of Glacial Lakes Aitkin and Upham II net flow of water was from Aitkin to Upham. The initiation of the Prairie River inlet greatly increased this discharge. The flow of water from Aitkin to Upham resulted in formation the Ball Bluff Spit (Marlow, 2004) extending eastward from the Swan River Sill (Fig. 6-19). The dense dune clusters east of the Swan River Sill in the Upham basin formed from sediment transported from the underflow fan. Page 195 �Figure 6-19. Location of the Ball Bluff spit located within the dashed line. Circle at stop 10 indicates where lacustrine clay is overlain by 1 m. of sand derived from the underflow fan. Triangle indicates site where lake clay is exposed at the surface. REFERENCES Arbogast, A.F., Wintle, A.G., Packman, S.C. 2001. Widespread middle Holocene dune formation in the eastern Upper Peninsula of Michigan and the relationship to climate and outlet-controlled lake level. Geology 30, 55-58. Bagnold, R.A. 1941. The Physics of Blown Sand and Desert Dunes. William Morrow & Company, 265 p. Baker, R.G. 1965. Late-glacial pollen and plant macrofossils from Spider Creek, So. St. Louis County, MN. Geological Society of America Bulletin 45, 645-665. Ballantine, J.W. 1991. Late-Wisconsin Stratigraphy and Glacial History of Southwestern St. Louis County, Minnesota. Unpublished Masters Thesis, University of Minnesota Duluth. 154 p. Bartlein, P.J., Webb, T., and Fleri, E. 1984. Holocene climatic change in the northern Midwest: Pollen derived estimates. Quaternary Research 22, 361-374 Björk, S. 1990. Late-Wisconsinan history North of Giants Range, northern Minnesota, inferred from complex stratigraphy. Quaternary Research 33, 18-36. Brady, N.C. and Weil, R.R., 2000, Elements of the Nature and Properties of Soils. 12th Edition, Prentice Hall, New Jersey. P.558. Page 196 �Clayton, L. 1983. Chronology of Lake Agassiz drainage to Lake Superior, in Teller, J.T., and Clayton, Lee, eds., Glacial Lake Agassiz: Geological Association of Canada Special Paper 26, 291-307. Clayton, L. and Moran, S.R. 1982. Chronology of late Wisconsinan glaciation in middle North America: Quaternary Science Reviews 1, 55-82. COHMAP Members. 1988. Climatic changes of the last 18,000 years: Observations and model simulations. Science 241,1043-1052. Cooper, W.S. 1935. The history of the Upper Mississippi River in late-Wisconsinan and post-glacial time. Minnesota Geological Survey Bulletin 26. Cooper, W.S., 1938, Ancient dunes of the upper Mississippi Valley as possible climatic indicators. American Meteorological Society Bulletin, v. 19, p. 193-204. Dean, W.E. 1997. Rates, timing, and cyclity of Holocene eolian activity in north-central United States: Evidence from varved lake sediments. Geology 25, 331-334. Dean, W.E. 1999. The carbon cycle and biogeochemical dynamics in lake sediments. Journal of Paleolimnology 21, 375-393. Dean, W.E., Ahlbrandt, T.S., Anderson, R.Y., Bradbury, J.P., 1996, Regional aridity in North America during the middle Holocene. The Holocene 6, 145-155. Dean, W. E., Bradbury, J.P., Anderson, R.Y., Barnosky, C.W. 1984. The variability of Holocene climate change: Evidence from varved lake sediments. Science 226, 1191-1194. Elftman,, A.H. 1898. The Geology of the Keweenawan area in northeastern Minnesota. The American Geologist 18, 225-226. Farnham, R.S., McAndrews, J.H., and Wright, H.E. 1964. A Late-Wisconsin buried soil near Aitkin, Minnesota, and its paleobotanical setting. American Journal of Science 262, 393-412. Farrand, W.R. and Drexler. 1985. Late-Wisconsinan and Holocene History of the Lake Superior Basin. in Karrow, Quaternary evolution of the Great Lakes. Geological Association of Canada Special Paper 30, 17-32. Fenton, M.M.(Moran, S.R.; Teller, J.T.; Clayton, Lee). 1983. Quaternary stratigraphy and history in the southern part of the Lake Agassiz basin, in Teller, J.T., and Clayton, Lee, eds., Glacial Lake Agassiz. Geological Association of Canada Special Paper 26, 49-74. Grigal, D.F., Severson, R.C., Golz, G.E. 1976. Evidence of eolian activity in north-central Minnesota 8,000 to 5,000 yr. ago. Geological Society of America Bulletin 87, 1251-1254. Hall and Sardeson, 1898, Eolian deposits of Eastern Minnesota. Bulletin of the Geological Society of America, v. 10, p. 349-360. Hobbs, H.C. 1983. Drainage relationships of Glacial Lakes Aitkin and Upham and early Lake Agassiz in northeastern Minnesota. In Teller, J.T. and Clayton, L., eds., Glacial Lake Agassiz. Geological Association of Canada Special Paper 26, 245-259. Keen, K.L., Shane, L.C.K, 1990, A continuous record of Holocene eolian activity and vegetation change at Lake Ann, east-central Minnesota. Geological Society of America Bulletin 102, 1646-1657. Larson, P.C., and Mooers, H.D. 2003. Holocene drainage evolution of the Mississippi Headwaters, Minnesota: implications for mid-Holocene eolian activity in the North American midcontinent. GSA Abstracts with Programs 35 (7). Lehr, J.D. and Hobbs, H. 1992. Field Trip Guidebook for the Glacial Geology of the Laurentian Divide Area, St. Louis and Lake Counties, Minnesota. Minnesota Geological Survey Guidebook Series 18. Leverett, F., with contributions by Sardeson, F.W. 1932. Quaternary Geology of Minnesota and Parts of Adjacent States. United States Geological Survey Professional Paper 161. Leverett, F. and Sardeson, F.W. 1919. Surface formations and agricultural conditions of north-eastern Minnesota. Minnesota Geological Survey Bulletin 13. Marlow, L. 2004. Late Glacial and Early Holocene history of the Glacial Lakes Aitkin and Upham basin, NorthCentral Minnesota: Implications for the timing of post-glacial eolian activity. Unpublished Masters Thesis, University of Minnesota Duluth, 82 p. Mooers, H.D. and Lehr, J.D. 1997. Terrestrial record of Laurentide ice sheet reorganization during Heinrich events. Geology 25, 987-990. Mooers, H.D. 1988. Quaternary history and ice dynamics of the late Wisconsin Rainy and Superior lobes, central Minnesota. Unpublished Doctoral Thesis, University of Minnesota, 200 p. Nikiforoff, C. 1947. The life history of Lake Agassiz: alternative interpretation. American Journal of Science 245, 205-239. Page 197 �Nyberg, P.R. 1985. Soil Survey of Itasca County, Minnesota. United States Department of Agriculture Soil Conservation Service, 197 p. Nyberg, P.R. 1999. Soil Survey of Aitkin County, Minnesota. United States Department of Agriculture Natural Resource Conservation Service. 163 p. Ojakangas, R.W. and Matsch, C.L., 1982, Minnesota’s Geology. University of Minnesota Press, 255 p. Posamentier, H.W. and Allen, G.P. 1999. Siliciclastic Sequence Stratigraphy-Concepts and Applications. SEPM Concepts in Sedimentology and Paleontology 7. Seppala, M. 1993. Climbing and falling sand dunes in Finnish Lapland. in Pye, K., ed., The Dynamics and Environmental Context of Aeolian Sedimentary Systems. Geological Society London. Shreve, R.L. 1985. Esker characteristics in terms of glacier physics, Katahdin esker system, Maine. Geological Society of America Bulletin 96, 639-646. Upham, W. 1896-1899. The geology of Aitkin County. in Minnesota Geological and Natural History Survey Final Report: The Geology of Minnesota 4, 25-54. Webb, T. III, Cushing, E.J., and Wright Jr., H.E., 1983, Holocene changes in the vegetation of the Midwest. In Wright Jr., H.E., ed., Late-Quaternary Environments of the United States: the Holocene, 109-127. Winchell, N.W. 1901. Glacial Lakes of Minnesota. Geological Society of America Bulletin 12, 109-128. Winchell, N.W. 1896-1899. The geology of the northern part of St. Louis County. in Minnesota Geological and Natural History Survey Final Report: The Geology of Minnesota 4, 222-265. Winspear, N.R. and Pye, K. 1996. Textural, geochemical and mineralogical evidence for the sources of Aeolian sand in central and southwestern Nebraska, U.S.A. Sedimentary Geology 101, 85-98. Wright, H.E. 1972. Quaternary history of Minnesota. in Sims, P.K., and Morey, G.B., eds., Geology of Minnesota: A Centennial Volume. Minnesota Geological Survey. Wright, H.E., Matsch, C.L. and Cushing, E.J. 1973. Superior and Des Moines Lobes. in Black, R.F., Goldthwait, R,P., and Willman, H.B., eds., The Wisconsinan Stage. Geological Society of America Memoir 136, 153-185. Road Log MILES Begin in Duluth, MN. 0-3 Interstate 35 south to Grand Avenue/MN 23 exit. 3-22 14 miles to County 4. East (left) 2 miles to railroad crossing. Stop 1 Glacial Lake Duluth underflow fan. 22-39 County 4 west ~12 miles to Interstate 35. 39-49 North on Interstate 35 ~12miles to MN 33. 49-69 North on MN 33 through Cloquet for ~18 miles to US 53. 69-73 North on US 53 for ~4 miles. Cloquet River Hellwig Creek Stop 2 Hellwig Creek outlet and Glacial Lake Upham II basin overlook. 73-84 North on US 53 for ~12 miles to County 52 (Arkola Road) at Cotton, MN. 84-99 West on County 52 for ~16 miles to County 5. 99-101 North on County 5 for ~2 miles to gravel pit on left. Stop 3 Wave-washed esker island (Toivola, MN). 101-115 North on County 5 from previous stop for ~13 miles to MN 37. 115-119 East on MN 37 for ~3.5 miles to County 25. 119-120 North on County 25 for ~1.5 miles to the gravel pit on the left. Stop 4 Rainy Lobe collapsed outwash, Glacial Lake Upham I sediments, and St. Louis Sublobe till (Cherry, MN). 120-121 South on County 25 to MN 37. 121-135 West on MN 37 for ~9.5 miles to County 57 (1st Avenue). Page 198 �135-140 South on County 57 for ~8 miles. Stop just before County 442 (Townline Road South). Stop 5 Eolian dunes. 140-140 South on County 57 to County 442 (Townline Road South). 140-144 West on County 442 for ~3 miles. Stop at gravel pit access road on north side. Stop 6 Glacial Lake Upham II Shoreline. 144-145 West on County 442 (Townline Road South) to MN 73. 145-145 North on MN 73 for ~0.5 miles to County 16. 145-156 West on County 16 for ~11 miles to County 20. 156-164 West on County 20 for 0.25 miles to MN 65. 164-168 North on MN 65 for ~8.5 miles to County 12. 168-169 West on County 12 for ~3.5 miles to County 70. 169-170 South on County 70 for ~1 mile to County 434. 170-170 South on County 434 for ~1 mile to gravel pit on right Stop 7 Goodland Esker. 170-175 Continue south on County 434 for ~5 miles to County 10. 175-176 West on County 10 for ~1.5 miles to County 71. 176-181 South on County 71 for ~4 miles to US 2. 181-181 Cross US 2 and continue for ~ 0.25 miles to County 441 (Bluebird Lane) 181-184 West on County 441 for ~2.5 miles to County 3 (River Road). 184-197 South on County 3 for ~2.5 miles to cutbank along Mississippi River on left. Stop 8 Prairie River Underflow fan in Glacial Lake Aitkin II. 197-200 Continue southbound on County 3 (River Road) for ~2 miles to Mississippi River Boat Landing Road on the left Stop 9 Rhythmite in Glacial Lake Aitkin II. 200-201 Return to County 3. 201-202 South on County 3 to MN 200. 202-203 East on MN 200 for 1.5 miles to MN 65. 203-203 North on MN 65 for 0.5 miles to MN 200. 203-208 East on MN 200 for ~4 miles to 154th Avenue. 208-208 North on 154th Avenue. Stop 10 Eolian dunes in Glacial Lake Upham II basin. To return to Duluth: 208-208 Return to MN 200. 208-213 East on MN 200 to US 2. 213-262 East on US 2 to Interstate 35. North on Interstate 35 to start point. Page 199 �FIELD TRIP 7 ECONOMIC GEOLOGY OF ARCHEAN GOLD OCCURRENCES IN THE VERMILION DISTRICT, NORTHEAST OF SOUDAN, MINNESOTA By Dean M. Peterson and Richard L. Patelke Natural Resources Research Institute, University of Minnesota Duluth INTRODUCTION A widespread area of gold mineralization occurs in numerous prospects east of Lake Vermilion, within the Vermilion greenstone belt of northeastern Minnesota. The mineralization occurs in rocks of the Neoarchean (~2.7 Ga) Bass Lake sequence (Peterson and Jirsa, 1999a) of the Wawa subprovince of the Canadian Shield. This zone of abundant gold mineralization is bounded to the south by the Mud Creek shear zone and to the north by the Vermilion fault (Fig. 7-1). The main access to these prospects is along the Mud Creek road (St. Louis County 38). A brief period of mineral exploration for lode-gold deposits in this immediate area of the Vermilion district occurred in the mid 1980s to early 1990s. These programs typically consisted of grid-based geologic mapping, bedrock sampling, ground geophysics, and the completion of soil geochemical surveys. Conversations with many of the people involved in gold exploration programs in the immediate field trip area (centered on Section 6, Township 62 North, Range 14 West), and compilation of all exploration data from the district as a whole (data from the terminated lease files of the Minnesota DNR), has led to the conclusion that interpretation of linear structural elements exposed in outcrops were not used in designing exploratory drilling plans in the field trip area. Therefore, many of the prospects discovered as a result of these exploration programs remain untested by drilling. The authors recently completed a project (Peterson and Patelke, 2004) consisting of detailed geologic and structural mapping within many of the known gold prospects in the field trip area, and will highlight some of the insights generated as a result of this work during the course of this one day field trip. This recent work consisted of detailed outcrop mapping (at scales ranging from 1:1,000 to 1:3,000) that was focused on structural (i.e. shear zones, lineations, intersecting foliations and small-scale folds), geological (i.e. contact relationships, competence contrasts), geochemical (i.e. gold assays, trace element characteristics), and alteration (Fe-bearing carbonate, sericite, pyrite, silicificaton) features within and around areas of gold mineralized exposures. Based on detailed mapping of features described above, the project's goal was to try to determine the location and orientation at depth of gold-rich mineralized zones exposed in outcrops. The mapped geological features were incorporated into the data compiled and described by Peterson (2001), and are available online as a 1:12,000 scale geological map, as well as ArcView GIS data files at http://www.nrri.umn.edu/egg/. Page 200 �Figure 7-1. Location, simplified geology, lode gold prospects, and field trip #7 stops. LODE GOLD ORE DEPOSIT MODEL The brief description of Archean lode-gold deposits that follows is presented as both a basic reference and also to highlight the important features of the model that will be seen during the field trip. Archean lode-gold deposits are one category of ore deposit classified as mesothermal lode-gold deposits (Hodgson, 1993). This deposit type has also been called orogenic gold (Groves et al., 2000), greenstone gold (Robert et al., 1991), Archean lode-gold, mesothermal gold-quartz veins, shear-hosted gold, low-sulfide gold-quartz veins (Berger, 1986b), lode-gold, Mother Lode veins (Bohlke and Kistler, 1986), and iron formation-hosted gold deposits (Berger, 1986a; McMillan, 1996; Rye and Rye, 1974; Fripp, 1976; Kerswill, 1993; Thorpe and Franklin, 1984; and Vielreicher et al., 1994). Whatever the name, they are a widespread group of epigenetic ore deposits that have formed in similar settings throughout geologic time. In general, the deposits form during Page 201 �compressional or transpressional deformation at convergent plate margins in accretionary or collisional orogens (Fig.7-2). They form over a large crustal-depth range (2 to 20 km) from deep-seated, low-salinity H2O-CO2 ± CH4 ± N2 ore fluids, with Au transported as reduced sulfur complexes. The ore fluids are believed to be generated during lower crustal metamorphism from dehydration reactions. Regional structures provide the main control on distribution of lode gold deposits and mining camps. In many terranes, first-order faults or shear zones appear to have controlled regional fluid flow, with greatest ore-fluid fluxes in, and adjacent to, subsidiary faults, shear zones and/or large folds. Highly competent and/or chemically reactive rocks are the most common hosts to the larger deposits. Gold deposition occurs late during the evolutionary history of the host terranes, normally within D3 or D4 in a D1-D4 deformation sequence. Absolute ages of mineralization support their late-kinematic timing, and, in general, suggest that deposits formed diachronously towards the end of the evolutionary history of hosting orogens. Figure 7-2. Generalized tectonic model for the formation of mesothermal gold deposits, after Groves et al., 2000. The late timing of lode-gold deposits is critical to geology-based exploration methods, and hence mineral potential evaluations for these deposits. The late timing is critical because of the present structural geometry of: (1) the deposits, (2) the mining camps, and (3) the enclosing geologic terranes are essentially all similar to the structural geometry during gold mineralization. Therefore the interpretation of bedrock geological maps and cross-sections can be used to discern the physical conditions that existed at the time of ore deposition. Exploration for mesothermal lode-gold deposits should incorporate various aspects of the ore deposit model into criteria that can vector into the most favorable areas for hosting such mineralization. The most fundamental characteristic of this class of deposit is the spatial association of the deposits to regional structures in metamorphic terranes. Zones of widespread carbonate alteration (adjacent to regional structures) should be identified and used to focus subsequent exploration. Within carbonate alteration zones, gold is typically only in areas containing quartz veins, silicification, and/or sericite alteration (with or without sulfides). Two general structural controls on the orientation of lode gold ore shoots include deflections and curvatures of shear zones, and where high strain zones intersect favorable geological elements (Poulsen and Robert, 1989). A generalized sequence of events associated with the formation of typical Archean mesothermal gold deposits is outlined below. The possible observation of many of these important features during the course of the field trip is highlighted by the bracketed, bold text. Page 202 �I. Deposition of Archean Supracrustal Strata (Volcanic and Sedimentary Rocks) II. Compression and Early (D1) Folding [stop 7-1] III. Renewed Compression and Transpression (D2) 1) Development of Major Crustal Shear Zones [stop 7-1] 2) Strain Partitioning and Development of Subsidiary Shear Zones [stop 7-3, 7-4] 3) Intrusion of Porphyritic Intrusions, Continued D2 Deformation [stop 7-2, 7-3 and 7-5] -Local Development of Competency Contrasts [stop 7-3, 7-5] 4) Lower Crustal Dehydration and Fluxing of CO2-rich Fluids up Structures - Fe-Carbonate Alteration Zones [stop 7-3, 7-4, 7-5] 5) Fluxing of Gold-Bearing Hydrothermal Fluids up Subsidiary Shear Zones, Latest D2 - Gold Mineralization Associated with: A) Quartz-Pyrite Veins [stop 7-2, 7-3, 7-4, and 7-5] B) Silicification with Pyrite [stop 7-4] C) Pyritization of Fe-Rich Host Rocks [stop 7-5] D) Quartz-Sericite-Pyrite Schists/Phyllites [stop 7-4] E) Contact Zones Between Rock Units [stop 7-5] IV. Erosion REGIONAL GEOLOGIC SETTING OF THE VERMILION DISTRICT The field trip area is located in the Neo-Archean (~2.7 Ga) Vermilion Greenstone Belt of the classic Vermilion district of northeastern Minnesota. Supracrustal rocks in the Vermilion district consist of volcanic-dominated stratigraphic sequences of the Wawa subprovince of the Superior Province of the Canadian Shield. Rocks of the Wawa subprovince in northern Minnesota are divided on the basis of stratigraphic and structural setting into: (1) the southern Soudan belt and (2) the northern Newton belt (Jirsa et al., 1992; Southwick et al., 1998). The boundary between these contrasting structural panels can be traced geophysically across the width of Minnesota, and was designated informally as the Leech Lake structural discontinuity (Jirsa et al., 1992). In the Vermilion district, the Leech Lake structural discontinuity occurs along the Mud Creek shear zone (Hudleston et al., 1988), small segments of the Vermilion and Wolf Lake faults (Sims and Southwick, 1985), and the Bear River fault (Jirsa et al., 1992). The Soudan belt contains large, broad folds involving calc-alkalic and tholeiitic volcanic strata overlain by, and locally interdigitated with, turbiditic rocks. In contrast, the Newton belt consists of elongate, east- to northeast-trending, and mostly northward-younging volcanic and volcaniclastic sequences. Volcanic rocks of the Newton belt differ from those of the Soudan belt in containing locally abundant komatiitic flows and peridotitic sills. The two belts are faultbounded, and the relationship between stratigraphic units within each belt is largely conformable, although faults obscure contacts locally. In its eastern extension, the Soudan belt is continuous with the Saganagons assemblage in Ontario and terminates against the Saganaga pluton and Northern Light Gneiss. The Newton belt extends discontinuously eastward into the Shebandowan District of Ontario to form the Greenwater and Burchell assemblages. Intrusive rocks in both belts vary from gabbroic and felsic porphyries, demonstrably related to volcanism, to large plutons emplaced post-tectonically. Both districts contain unconformable, Timiskaming- Page 203 �type sequences composed of calc-alkalic volcanic rocks, conglomerates, and finer grained sedimentary rocks. A simplified regional geological map of the Neo-Archean terranes of northeastern Minnesota and adjacent Ontario is presented in Figure 7-3. Figure 7-3. Simplified correlation map of Neo-Archean assemblages across the U.S. - Canada border, modified from Peterson et al. (2001). Inset shows major subprovinces of the southwestern Superior Province. Periods of generally N-S directed compression resulted in three major regional deformation events in the Neoarchean terranes of northern Minnesota. The earliest deformation event (D1) produced broad, locally recumbent folds within the Soudan belt and major fault zones throughout the region. In the Newton belt, D1 was accommodated by thrust imbrication of large crustal blocks, resulting in mainly northward stratigraphic facing. Field relationships indicate that uplift, faulting, and the deposition of Timiskaming-type clastic sequences in local faultbounded basins occurred late in D1 deformation (Jirsa, 2000). A large, map-scale structure related to D1 deformation in the Soudan Mine area is the Tower-Soudan Anticline, which is a west-plunging anticline within which the axis and plunge changes orientation along strike from nearly vertical in basalts to shallow NE plunging in the western sedimentary rocks. Axial-planar cleavage associated with this early fold typically is lacking, although Bauer (1985), Hooper and Ojakangas (1971), Hudleston (1976), and Jirsa et al. (1992) have described early cleavage (S1) locally. A second deformation event (D2) associated with synchronous regional metamorphism resulted in foliation development and structures having largely dextral asymmetry. D2 is constrained in the Vermilion district to the time period 2674 to 2685 Ma (Boerboom and Zartman, 1993), and between about 2680 and 2685 Ma in the Shebandowan (Corfu and Stott, 1998). Because D2 deformation affected all of the supracrustal rocks in the area and is reasonably constrained by geochronology, the regional foliation (S2) can be used in the field to Page 204 �relate other structural, intrusive, and deformation events. The relationship between S2 fabric and shear structures indicates that most shearing occurred relatively late in the D2 event. Major shearing that produced the Mud Creek and related shear zones is attributed to the late stages of D2 dextral transpression. Regional D2 strain patterns in the Vermilion district vary from north to south in the belt (Hudleston et al., 1988; Schultz-Ela and Hudleston, 1991). These patterns include flattening strains that occur to the north, near the present Vermilion Fault and constrictional strains to the south. The field trip area lies within the zone of flattening strain of Hudleston et al. (1988) and Schultz-Ela and Hudleston (1991). Schultz-Ela and Hudleston (1991) mathematically modeled the observed strain patterns as deformation paths that produced flattening strains (west plunging ë 1 axes) by dextral shear of the pre-existing constrictional strains (east plunging ë 1 axes). The linear E-W strain patterns and minor rotations about horizontal axes during the deformation, as described by Hudleston et al. (1988) and Schultz-Ela and Hudleston (1991), preclude early theories (Hooper and Ojakangas, 1971; and Hudleston, 1976) on the structural setting of the greenstone belt by infolding and shear off the flanks of rising granitic diapirs. An estimate of 50% north-south shortening across the belt is proposed by Schultz-Ela and Hudleston (1991), and their model favors an origin of the Vermilion district rocks at a convergent margin, most likely as a N-dipping subduction complex with shallow slab dip. The origin of the southern constrictional strains remains enigmatic. Structures related to the third deformation event (D3) include abundant NE- and NWtrending faults that dissect the stratigraphic assemblages. Named structures related to D3 include the NE-trending Waasa and Camp Rivard faults SSE of Ely, and the WNW-trending, crustalscale Vermilion and related faults that form the Wawa-Quetico Subprovince boundary immediately north of the field trip area. LOCAL GEOLOGIC SETTING; THE BASS LAKE SEQUENCE The informally named Bass Lake sequence (Peterson and Jirsa, 1999a) of the Newton Belt occurs in an east-west trending, fault-bounded panel that widens to the west. The sequence is bounded by segments of the Mud Creek and Shagawa shear zones, and the Bear River, Haley, Vermilion, Burntside Lake, and Wolf Lake faults (Fig. 7-4). The continuation of stratigraphic and geophysical trends from areas of excellent exposure (east of Lake Vermilion) into poorly exposed areas (to the west) have led to the inclusion of rocks of the informally named Cook sequence (Southwick, 1993; Southwick et al., 1998) in the Bass Lake sequence. The western portion of the sequence consists of linear belts of graywacke and basalt cut by late felsic to intermediate composition intrusions. East of Lake Vermilion, the geology of the Bass Lake sequence is dominated by six basic rock types, which include: (1) Tholeiitic pillowed basalt flows (see Fig. 7-4) interpreted to have formed in a deep-water setting based on volcanic textures; (2) Gabbro sills interpreted as synvolcanic in age due to their stratigraphic continuity and similar deformation as the enclosing pillowed basalts; (3) Felsic porphyries (feldspar porphyry and quartz-feldspar porphyry) interpreted to have intruded during late stages of D2 deformation based on field relationships and geochronology (quartz-feldspar porphyry from the Pac Man Pond prospect returned a 207Pb/206Pb age of 2683.0 +/- 1.4 Ma (Peterson et al., 2001)); (4) Algoma-type iron-formation; (5) Thinly- Page 205 �bedded argillite and siltstone; and (6) Sheared rocks, which are dominated by chlorite-rich schist, phyllite, and phyllonite. In addition, localized areas of fragmental felsic volcanic rocks occur stratigraphically below distinct iron-formation horizons. Regional and detailed geologic maps of the sequence are presented in Fig. 7-5. In the last twenty years, numerous gold prospects have been discovered in the eastern portion of the sequence (Fig. 7-1). These prospects generally fall into one of three categories; (1) auriferous quartz-carbonate-pyrite veins and sulfidized zones in iron-formation; (2) auriferous quartz-sericite-ankerite-pyrite schists; and (3) felsic intrusive-hosted auriferous quartz veins and stockworks. All of the prospects are found within areas of moderate to strong ironcarbonate alteration, with the best mineralization commonly found within sericitic alteration zones. Numerous equigranular and porphyritic felsic intrusions occur within the areas of alteration and gold mineralization, and are a good guide for locating mineralized structures. The gold mineralization is generally related to deformation in subsidiary structures associated with movement along the D2 Mud Creek shear zone. Geological and geochemical descriptions of many of the gold prospects have previously been given by Peterson and Jirsa (1999b). Figure 7-4. Jensen cation plot (Jensen, 1976) for volcanic and hypabyssal rocks of the Bass Lake sequence. Undoubtedly the most striking structural feature in the immediate study area is the juxtaposition of the Soudan and Newton belts along the Mud Creek shear zone. In addition, numerous east-northeast trending highly strained zones occur to the north of the Mud Creek shear zone and these subsidiary sheared zones host a majority of the gold prospects in this area of the Bass Lake sequence. A stereonet projection of planar and linear structural features within the field trip #7 area of the Bass Lake sequence is shown in Fig. 7-6. Page 206 �Figure 7-5. Regional to detailed geological maps of the Bass Lake sequence. Geology simplified from Peterson (2001) and Peterson and Patelke (2004). Page 207 �Figure 7-6. Stereonet projections of planar and linear structural features from the field trip area. GLACIAL HISTORY Late Pleistocene glacial deposits (Late-Wisconsin glaciation) in the Vermilion district are associated with the stepwise retreat of the Rainy Lobe of the Laurentide Ice Sheet, approximately 14,000 to 12,000 years ago. The repeated glaciations of the Pleistocene epoch modified the preexisting topography of northeastern Minnesota, i.e. the surface was scoured by glacial ice, exposing fresh bedrock, and new surficial materials were deposited following the retreat of the glaciers. During the retreat of the glacier, the margin of the ice-sheet blocked the natural drainage to the north, and pro-glacial lakes formed in front of this barrier (glacial lakes Norwood, Koochiching, and Agassiz). Subglacial streams left sinuous ridges of sorted sand and gravel (eskers), and delta/fan complexes formed where these streams exited the ice margin and entered the pro-glacial lakes. The field trip area lies within the scoured bedrock terrain of northeastern Minnesota, immediately north of the Vermilion Moraine. Regional and detailed digital elevation maps of the area are presented in Figure 7-7, and display the pronounced recent bedrock scouring of the area, especially along the Vermilion fault and Mud Creek shear zone. Late Pleistocene surficial deposits and landforms that unconformably overlie the Neoarchean bedrock in the field trip area include a thin veneer of basal till, local outwash deposits, and sinuous eskers. Page 208 �Figure 7.7. Regional (top) and local (bottom) digital elevation maps depicting superimposed glacial landforms. Grid values in the detailed map are UTM coordinates, in meters. The locations of field trip stops are depicted in the lower image. FIELD TRIP STOPS Stops for this field trip will include short traverses that visit a number of outcrops for each locality. The traverses are designed to highlight specific features of the lode-gold ore deposit model for each area, though time constraints preclude detailed analysis of each area. Page 209 �STOP 7-1: MUD CREEK SHEAR ZONE Location: (SE, SE, Sec. 5, T.62N., R.14W., NAD83 UTM 564200E, 5302800N) Description: The regional scale Mud Creek shear zone occupies the east-northeast trending valley of Mud Creek, which is clearly visible at this location. This shear zone separates rocks of the Newton Belt (here the Bass Lake sequence) to the north and rocks of the Soudan Belt (Gafvert Lake sequence and the Upper Ely Greenstone Formation) to the south. The Mud Creek shear zone is analogous with major faults (Destor-Porcupine fault) and “breaks” (Cadillac-Larder Lake break) of major lode-gold mining districts in Canada. Historic gold assays taken from rocks of the shear zone itself are essentially devoid of gold, as is the case for most major structures within Archean lode-gold mining camps. The short field trip traverse is located within the northern margin of the internal highly strained zone of the shear, and will visit outcrops of: (1) ankerite-sericite-quartz-green mica-pyrite schist with quartz and tourmaline knots, and (2) highly folded and compositionally banded phyllites with quartz veins (Fig. 7-8). A simplified geology and field trip traverse map of stop 7-1 is presented in Figure 7-9. Figure 7-8. Outcrop photographs of ankerite-sericite-quartz-green mica-pyrite schist (A and B), and highly folded and compositionally banded phyllites (C and D). Page 210 �Figure 7-9. Simplified geology and field trip traverse map of stop 7-1, geology from Peterson and Patelke, 2004. STOP 7-2: SECTION 6 GOLD PROSPECT Location: (SE, SE, Sec. 6, T.62N., R.14W., NAD83 UTM 561895E, 5303410N) Description: Gold mineralization within the Section 6 prospect is dominantly associated with quartz-pyrite veins and stockworks in feldspar ± quartz porphyry. The stockwork-style mineralization is generally not apparent until one takes a detailed look at the outcrops, then the thin quartz veining is seen seemingly everywhere. The highest-grade gold assays taken to date occur in areas of strong sulfide oxidation (sulfide burn) in the porphyry. Mineralization also occurs in sulfidized and epidote-altered basaltic rocks adjacent to the felsic porphyries, and may Page 211 �locally contain significant quantities of chalcopyrite. In addition to porphyry and basaltic rocks, thin dikes and sills of diorite, peridotite, and inclusion-rich lamprophyre will be observed during the field trip stop. Outcrop photographs and a geologic map of the Section 6 prospect area are presented in Figures 7-10 and 7-11, and a table of anomalous gold assays is given in Table 7-1. Figure 7-10. Outcrop photographs from the Section 6 prospect. (A) 1 Ft.-wide dioritic dike near a basalt-porphyry (with BIF inclusion) contact; (B) strong sulfide burn within feldspar porphyry; (C) stockwork quartzveins (black lines) and local sulfide burn within feldspar porphyry; and (D) sulfide burn in quartzveined feldspar porphyry. Table 7-1. Anomalous gold assays from the Section 6 prospect. Description Au (ppb) Siliceous, sericitic and strongly pyritic basalt Carbonatized quartz-feldspar porphyry with pyrite Carbonatized feldspar porphyry with pyrite and quartz veins Pyritic selvage about a 3' quartz vein in basalt Pyritic selvage about a 3' quartz vein in basalt Epidote-silica-hematite altered feldspar porphyry with pyrite-rich veins Page 212 1,460 1,360 1,135 940 890 860 �Description Au (ppb) Pyritic quartz-feldspar porphyry Chloritic feldspar porphyry with ankerite, 1-3% pyrite, and quartz-ankerite-pyrite veins Chlorite-epidote-silica altered basalt with pyrite and chalcopyrite Carbonatized quartz-feldspar porphyry with pyrite Chlorite-epidote-silica altered basalt with pyrite and chalcopyrite 855 840 790 780 445 Figure 7-11. Simplified geology and field trip traverse map of stop 7-2, geology from Peterson and Patelke, 2004. Page 213 �STOP 7-3: SECTION 6 EAST GOLD PROSPECT Location: (SE, SE, Sec. 6, T.62N., R.14W., NAD83 UTM 562840E, 5302905N) Description: Three features of the generalized Archean lode-gold ore deposit model are beautifully exposed along the traverse of this stop. These features include: (1) intense ironcarbonate alteration, (2) competency contrast and associated auriferous quartz veining, and (3) shear deformation. Anomalous gold was first discovered by soil sampling in this locality in 1988 by Chevron Resources. Gold mineralization is generally confined to quartz-pyrite veins in ironcarbonate altered feldspar-porphyry intrusive rocks. The felsic porphyries are generally located adjacent to linear zones of intense strain, and are believed to have intruded along these structural breaks during D2 deformation. Outcrop photographs and a geologic map of the Section 6 East prospect area are presented in Figures 7-12 and 7-13, and anomalous gold assays in Table 7-2. Figure 7-12. Outcrop photographs from the Section 6 East prospect. (A) iron-carbonate altered feldspar porphyry with quartz veins; (B) close-up of quartz-pyrite veins in carbonatized feldspar porphyry; (C) large quartz ± pyrite ± galena veins in massive iron carbonate; and (D) intense iron carbonate alteration Page 214 �Table 7-2. Anomalous gold assays from the Section 6 East prospect. Description 6" quartz vein in sheared feldspar porphyry Feldspar porphyry with ankerite, pyrite, and quartz veins Feldspar porphyry with ankerite, pyrite, and quartz veins Feldspar porphyry with local fracturing and shearing Feldspar porphyry with a rusty quartz-pyrite vein Ankerite-sericite altered basalt with pyrite Au (ppb) Description Au (ppb) 6,210 2,400 1,760 1,380 1,130 1,030 Soil Sample Soil Sample Soil Sample Soil Sample Soil Sample Soil Sample 1,520 622 615 294 170 101 Figure 7-13. Simplified geology and field trip traverse map of stop 7-3, geology from Peterson and Patelke, 2004. Page 215 �STOP 7-4: KERR MCGEE GOLD PROSPECT Location: (SE, SE, Sec. 31, T.63N., R.14W., NAD83 UTM 562480E, 5304310N) Description: The Kerr McGee gold prospect is hosted within an extensive zone of highly strained rocks, interpreted to be a subsidiary structure associated with the Mud Creek shear zone. Moderate to high-grade gold mineralization at the Kerr McGee prospect occurs within multiple thin (0.2 – 2.0 meter) zones of quartz-sericite-ankerite-pyrite ± green mica ± tourmaline schist hosted by an extensive zone of essentially gold-barren chlorite-rich schist. Thin and probably boudined iron-formation horizons occur locally in the chlorite-rich schist, and locally are strongly mineralized in this area. Mineralized zones locally contain extensive foliation and shear parallel quartz, ankerite, and/or quartz-ankerite veins, and may widen in zones of silicification. The style of gold mineralization exposed in the Kerr McGee prospect is similar to both the Clear Cut (~½ mile west) and Railroad Zone (1½ miles east) prospects. In fact, the sericitic zone that hosts the mineralization may have continuity to both of these other prospects. The field trip traverse will take us to numerous outcrops in the prospect area, and discussions will highlight the style of mineralization, possible continuity to other prospects, and shear deformation. Anomalous gold assays from the Kerr McGee prospect are presented in Table 7-3. Outcrop photographs and a geologic map of the prospect area are presented in Figures 7-14 and 7-15 respectively. Table 7-3. Anomalous gold assays from the Kerr McGee prospect. Description 1' wide zone of quartz-sericite-ankerite-pyrite tourmaline schist Quartz-pyrite-sericite knot in ankeritic sericite schist 1' wide zone of quartz-sericite-ankerite-pyrite tourmaline schist 1' wide zone of sericite-quartz-pyrite-ankerite phyllite 1' wide zone of sericite-quartz-pyrite-ankerite phyllite 1' wide zone of sericite-quartz-pyrite-ankerite phyllite 0.5' wide zone of sericite-quartz-pyrite-ankerite phyllite 1.1' wide zone of sericite-quartz-pyrite-ankerite phyllite Sericite-chlorite schist 0.33' wide zone of sericite-quartz-ankerite-pyrite phyllite 0.5' wide zone of sericite-quartz-ankerite-pyrite phyllite Road cut of siliceous quartz-sericite-pyrite schist with quartz veins 1.2' wide zone of sericite-quartz-pyrite-ankerite phyllite 1.1' wide zone of sericite-quartz-pyrite-ankerite phyllite Chlorite-sericite-ankerite schist with 10-20% pyrite Chlorite-quartz-ankerite schist with pyritic (5-10%) siliceous zones Chlorite-quartz-ankerite schist with pyritic (5-10%) siliceous zones Chlorite-quartz-ankerite schist with 1-5% pyrite Quartz-ankerite-pyrite (2-3%) vein Chlorite-sericite-ankerite schist with 10-20% pyrite Brecciated ankeritic-pyritic (5-10%) chert Sericite-chlorite-quartz-ankerite-pyrite (5%) schist Page 216 Au (ppb) DDH 8,010 5,030 4,991 4,660 4,410 3,430 2,770 2,740 2,440 1,675 1,140 980 695 685 1,980 RC-3 1,470 RC-3 1,040 RC-3 620 RC-3 613 RC-3 448 RC-3 369 RC-3 334 RC-3 Interval (ft) 1.0 3.0 4.0 7.0 1.0 1.0 2.0 2.0 �Figure 7-14. Outcrop photographs from the Kerr McGee prospect. (A) Road side outcrop of quartz-sericite-pyriteankerite-green mica-tourmaline schist; (B) minor northwest-trending fault offsetting highly foliated chloritic and sericitic schists; (C) close-up view of complex deformation fabric associated with quartz veins and knots; (D) channel samples taken across thin mineralized zones; (E) more channel samples taken across thin mineralized zones in sericite-rich schist, barren chlorite-rich schist at the bottom of the photograph; and (F) dextral shear fabrics in a mineralized zone at the east end of the outcrop area. Page 217 �Figure 7-15. Simplified geology and field trip traverse map of stop 7-4, geology from Peterson and Patelke, 2004. Three-dimensional visualization (Fig. 7-16) of the detailed lithological and structural mapping by Peterson and Patelke, (2004) within the Kerr McGee prospect area reveals important information that can be used to design drilling plans that significantly increase the chance of intersecting gold mineralization exposed in outcrop at the surface. For example, drill hole RC-3, which is located 100 meters east of the map presented in Figure 7-15, was drilled due north (at a dip of 45º) and targeted to intersect the mineralization exposed in outcrop at the Kerr McGee Page 218 �showing. Chevron Resources drilled this hole in 1987, at the western boundary of their lease property (the Kerr McGee prospect was then held by Kerr McGee). Detailed structural mapping in these outcrops reveals that the rocks within the mineralized zone have moderate to strong elongation and intersection (foliation and shear planes) lineations trending 60º and dipping northeast at 72º. The best interpretation of the down-dip orientation of the mineralized zone is this lineation trend and plunge, and drill hole RC-3 never intersected the mineralized zone. A three-dimensional view of these relationships is given in Figure 7-16. Figure 7-16. Three-dimensional view of the relationship between structural boundaries, the mineralized zone exposed on the surface at the Kerr McGee prospect, and drill hole RC-3. Upward extension to the surface of the two anomalous zones (> 1,000 ppb gold) intersected in hole RC-3 would place these zones in the black spruce and cedar swamp located south-southeast of the prospect. As briefly described in the preceding paragraphs, the sericitic-schist hosted style of mineralization at the Kerr McGee prospect is similar to the mineralization at both the Clear Cut and Railroad Zone prospects, and may form a continuous zone of anomalous to ore-grade gold mineralization over a strike length of > 2.5 miles. By analogy to many Canadian and Australian Archean lode gold deposits, the odds of discovering high-grade zones within this trend would be significantly increased if possible future exploration ventures utilized structural elements exposed in outcrop. A simplified geologic map encompassing the highly strained zone that hosts the Clear Cut, Kerr McGee, and Railroad Zone gold prospects is presented in Figure 7-17. In addition, the trend and plunge of measured lineations are given in both the plan map of Figure 717 and the underlying east-west cross-section. Although the field trip will not visit the Clear Cut Page 219 �and Railroad Zone prospects, anomalous gold assays from these associated prospects are presented in Tables 7-4 and 7-5 respectively. Figure 7-17. Highly simplified geologic and lineation map of the highly strained shear zone that hosts the Clear Cut, Kerr McGee, and Railroad Zone gold prospects. The total lengths of the lineation arrows depicted in the east-west cross-section (lower portion of the figure) are all equal, their apparent difference in length is related to their individual trend and plunge in relation to the view angle of the section. Table 7-4. Anomalous gold assays from the Clear Cut prospect. Description Au (ppb) DDH Interval (ft) 2' wide zone of quartz-sericite-pyrite schist Quartz-sericite-pyrite schist Quartz-sericite-pyrite schist Quartz-sericite-pyrite schist Quartz-ankerite vein with a sericite-pyrite selvage in ankerite-chlorite schist 1/2" wide quartz-pyrite-arsenopyrite vein Oxide-facies iron-formation with 10% pyrite and 5% arsenopyrite Quartz-sericite-pyrite schist Oxide-facies iron-formation with 1-2% cross-cutting pyrite Oxide-facies iron-formation with 10% pyrite and 5% arsenopyrite Chlorite-pyrite phyllite with quartz veining and 5% pyrite Chlorite phyllite with trace to 5% pyrite Chlorite-pyrite phyllite with quartz veining and 1% pyrite Chlorite-pyrite phyllite with quartz veining and 5% pyrite Chlorite-pyrite phyllite with quartz veining and 5% pyrite Page 220 22,000 7,100 2,940 1,970 1,970 598 590 568 440 402 1,510 715 550 528 471 RC-5 RC-5 RC-5 RC-5 RC-5 2.0 2.0 5.0 2.0 2.0 �Table 7-5. Anomalous gold assays from the Railroad Zone prospect. Description Grab sample from large boulder encased in the roots of an overturned tree Sericite-chlorite-ankerite-pyrite phyllite with quartz-ankerite-pyrite veins 4' Sericite-chlorite-ankerite-pyrite phyllite with quartz-ankerite-pyrite veins Chlorite-ankerite-pyrite schist Siliceous sericite schist with ankerite and pyrite Chlorites schist with pyrite and sericite Chlorite schist with pyrite and sericite Chlorite schist with pyrite and sericite Siliceous chlorite schist with pyrite Massive pyrite lens in sericite-ankerite schist Chlorite-ankerite-pyrite phyllite with quartz-ankerite veins Chlorite schist with pyrite and sericite 5' zone of very rusty and weathered, sericite-ankerite-quartz-pyrite phyllite Chlorite-ankerite phyllite with numerous quartz-ankerite-pyrite veins Sericite-quartz-ankerite-pyrite phyllite with 5% pyrite Quartz-sericite-pyrite phyllite with 5% pyrite Au (ppb) DDH Interval (ft) 37,324 17,100 13,623 3,110 2,670 2,410 970 960 960 776 705 620 605 777 V-2 1.5 713 V-4 1.8 591 V-1 6.0 STOP 7-5: PACMAN POND GOLD PROSPECT Location: (SE, SE, Sec. 6, T.62N., R.14W., NAD83 UTM 562225E, 5303780N) Description: The Pac Man Pond gold prospect was explored by Noranda in the late 1980s and early 1990s. They completed a quite extensive program that consisted of geological mapping, bedrock sampling, till prospecting, trenching, and drilling. Three rock types, that include pillowed basalt, iron-formation, and intrusions of feldspar ± quartz porphyry, dominate the prospect geology, with rare occurrences of argillaceous and graphitic sediments as well as carbonate facies iron-formation intersected in drill holes. Gold mineralization is hosted dominantly by brecciated, quartz-pyrite veined, and sulfidized iron-formation, with locally anomalous gold in sulfidized basalt and porphyry. The outcrops observed on the field trip will be concentrated on one of the areas that Noranda completed extensive trenching and gold assaying. Outcrop photographs from the Pac Man Pond gold prospect are given in Figure 7-18, and a geologic map of the Pac Man Pond gold prospect area is presented in Figure 7-19. Anomalous gold assays from bedrock exposures and Noranda drill hole intersections are given in Table 7-6. Page 221 �Figure 7-18. Outcrop photographs from the Pac Man Pond prospect. (A) Strongly iron-carbonate altered quartzfeldspar porphyry; (B) sheared and laminated pyrite and chlorite at the contact between foliated to sheared basalt and sulfidized iron-formation; (C) irregular contact between carbonatized quartzfeldspar porphyry and sulfidized iron-formation; and (D) strong sulfide burn on highly sulfidized iron-formation. Sawed samples of gold-bearing, pyritic iron-formation reveal one of the classic chemical models for the precipitation of gold out of hydrothermal solutions. In this model, when silica-saturated auriferous hydrothermal fluids (with gold carried by HS complexes) encounter a rock rich in FeO (such as Fe-tholeiitic basalt or iron-formation), quartz, pyrite and gold are precipitated out of solution. The classic chemical equation is presented below. 2HAu(HS)2 aq + ½O2 aq + 2FeO s ↔ Page 222 2FeS2 s + 2Au s + 3H2O aq �Figure 7-19. Simplified geology and field trip traverse map of stop 7-5, geology from Peterson and Patelke, 2004. Page 223 �Table 7-6. Anomalous gold assays from the Pac Man Pond prospect. Description Large angular boulder of pyrite-rich (5-50%) iron-formation Sulfidized iron-formation Siliceous, pyritic iron-formation with limonite Siliceous, pyritic iron-formation Sulfidized iron-formation Sulfidized iron-formation with quartz-pyrite veins Limonite stained iron-formation with 5-15% pyrite Sulfidized (10-70% pyrite) basalt Siliceous, pyritic iron-formation Iron-formation with trace pyrite Sulfidized iron-formation Laminated iron-formation with trace pyrite Pyrite-rich iron-formation near contact with quartz-feldspar porphyry Sulfidized iron-formation with 5-20% pyrite Quartz-feldspar porphyry with a rusty quartz vein Laminated iron-formation with trace pyrite 6' wide zone of sulfidized iron-formation with 3-10% pyrite Mineralized iron-formation with 5-15% pyrite Brecciated iron-formation with 5-20% pyrite in matrix Brecciated iron-formation with 5-20% pyrite in matrix Iron-formation with 1-2% pyrite in veinlets Brecciated iron-formation with quartz-pyrite (10%) veins Thin-bedded and folded iron-formation with 3% pyrite Brecciated iron-formation with chlorite-quartz-pyrite (5-10%) matrix Brecciated iron-formation with 5-10% pyrite Weakly brecciated iron-formation with 3-10% pyrite in matrix Fine-grained mafic dike Quartz-feldspar porphyry with trace pyrite Oxidized iron-formation with ankerite and 8% pyrite Au (ppb) 2,701 2,190 1,940 1,810 1,750 1,635 1,020 985 890 875 845 845 815 811 785 760 758 705 28,460 5,400 4,212 2,255 1,747 1,364 1,026 960 856 856 782 DDH Interval (ft) V89-1 V89-1 V-90-2 V-90-4 V-90-7 V-90-2 V-90-3 V89-1 V-90-4 V-90-4 V-90-3 2.0 1.7 3.5 2.0 1.8 1.8 3.2 2.3 1.0 1.0 2.0 REFERENCES Bauer, R.L., 1985, Correlation of early recumbent and younger upright folding across the boundary between an Archean gneiss belt and greenstone terrane, northeastern Minnesota: Geology, v. 13, p. 657-660. Berger, B. R., 1986a, Descriptive model of Homestake Au: in Mineral Deposit Models, Cox, D.P. and Singer, D.A., eds., U.S. Geological Survey, Bulletin 1693, p. 245- 247. Berger, B. R., 1986b, Descriptive model of low-sulphide Au-Quartz veins: in Mineral Deposit Models, Cox, D.P. and Singer, D.A., eds., U.S. Geological Survey, Bulletin 1693, p. 239-243. Page 224 �Boerboom, T.J., and Zartman, R.E., 1993, Geology, geochemistry, and geochronology of the central Giants Range batholith, northeastern Minnesota: Canadian Journal of Earth Sciences v. 30, p. 2510-2522. Bohlke, J.K. and Kistler, R.W., 1986, Rb-Sr, K-Ar and stable isotope evidence for the ages and sources of fluid components of gold-bearing quartz veins in the Northern Sierra Nevada Foothills Metamorphic Belt, Economic Geology, v. 81, p. 296- 422. Corfu, F., and Stott, G.M., 1998, Shebandowan greenstone belt, western Superior Province: U-Pb ages, tectonic implications, and correlations: Geological Society of America Bulletin, v. 110, p. 1467-1484. Fripp, R.E.P., 1976, Stratabound gold deposits in Archean banded iron-formation, Rhodesia; Economic Geology, v. 71, p. 58-75. Groves, D.I., Goldfarb, R.J., Knox-Robinson, C.M., Ojala, J., Gardoll, S., Yun, G.Y., and Holyland, P., 2000, Late-kinematic timing of lode-gold deposits and significance for computer-based exploration techniques with emphasis on the Yilgarn Block, Western Australia; Ore Geology Reviews, v. 17, Issues 1-2, Pages 1-38. Hodgson, C.J., 1993, Mesothermal lode-gold deposits: in Mineral Deposit Modeling. Kirkham, R.V., Sinclair, W.D., Thorpe, R.I., Duke, J.M. eds., Geological Survey of Canada, Special Paper 40, p. 635–678. Hooper, P., and Ojakangas, R., 1971, Multiple deformation in the Vermilion district, Minnesota: Canadian Journal of Earth Sciences, v. 8, p. 423-434. Hudleston, P.J. 1976, Early deformational history of Archean rocks in the Vermilion district, northeastern Minnesota: Canadian Journal of Earth Sciences, v. 13, p. 579-592. Hudleston, P.J., Schultz-Ela, D., and Southwick, D.L., 1988, Transpression in an Archean greenstone belt, northern Minnesota: Canadian Journal of Earth Sciences, v. 25, p. 1060-1068. Jensen, L.S., 1976, A new cation plot for classifying subalkalic volcanic rocks: Ontario Department of Mines, Miscellaneous Paper 66. Jirsa, M.A., 2000, The Midway sequence: a Timiskaming-type pull-apart basin deposit in the western Wawa subprovince, Minnesota: Canadian Journal of Earth Science, v. 37. p. 1-15. Jirsa, M.A., Southwick, D.L., and Boerboom, T.J., 1992, Structural evolution of Archean rocks in the western Wawa subprovince, Minnesota: Refolding of pre-cleavage nappes during D2 transpression: Canadian Journal of earth Sciences, v. 29, p. 2146-2155. Kerswill, J.A., 1993, Models for Iron-formation-hosted Gold Deposits: in Mineral Deposit Modeling, Kirkham, R.V., Sinclair, W.D., Thorpe, R.I. and Duke, J.M., eds., Geological Association of Canada, Special Paper 40, p. 171-200. McMillan, R.H., 1996, Iron formation-hosted Au, in Lefebure, D.V. and Hoy, T, eds., Selected British Columbia Mineral Deposit Profiles, Volume 2 - Metallic Deposits, British Columbia Ministry of Employment and Investment, Open File 1996-13, p. 63-66. Page 225 �Peterson, D.M., 2001, Development of Archean lode-gold and massive sulfide deposit exploration models using geographic information system applications: Targeting mineral exploration in northeastern Minnesota from analysis of analog Canadian mining camps: Unpublished Ph.D. thesis, University of Minnesota, 503 p. Peterson, D. M., and Jirsa, M.A., 1999a, Bedrock geologic map and mineral exploration data, western Vermilion district, St. Louis and Lake Counties, northeastern Minnesota: MGS Miscellaneous Map M-98, scale 1:48,000. Peterson, D. M., and Jirsa, M.A., 1999b, Lode gold and massive sulfide prospects in the Archean western Vermilion district; Minnesota Exploration Conference 1999, Field trip guidebook, 10 maps, 30 p. Peterson, D.M and Patelke, R.L., 2004, Bedrock Geology and Lode Gold Prospect Data Map of the Mud Creek Road Area, Northern St. Louis County, Minnesota: Natural Resources Research Institute, Map Series NRRI/MAP-2004-01. Peterson, D.M., Gallup, C., Jirsa, M.A., and Davis, D.W., 2001, Correlation of Archean assemblages across the U.S. - Canadian border; Phase I geochronology, abstract and oral presentation, Institute on Lake Superior Geology, 47th Annual Meeting, Thunder Bay, Ontario, v. 47. Poulsen, K.H., and Robert, F., 1989, Shear zones and gold: Practical examples from the southern Canadian Shield: in Bursnall, J.T. ed., Mineralization and Shear Zones, Geological Association of Canada, Short Course Notes Volume 6, p. 239-266. Robert, F., Sheahan, P.A., and Green, S.B., eds., 1991, Greenstone gold and crustal evolution: NUNA Conference Volume, Geological Association of Canada, Mineral Deposits Division. Rye, D. M. and Rye, R. O., 1974, Homestake Gold Mine, South Dakota: I. Stable Isotope Studies; Economic Geology, v. 69, p. 293-317. S Schulz-Ela, D.D., and Hudleston, P.J., 1991, Strain in an Archean greenstone belt of Minnesota: Tectonophysics, v. 190, p. 233-268. Sims, P.K., and Southwick, D.L., 1985, Geologic map of Archean rocks, western Vermilion district, northern Minnesota: U.S. Geological Survey, Miscellaneous Investigations Map I-1527, scale 1:48,000. Southwick, D.L., Boerboom, T.J., and Jirsa, M.A., 1998, Geologic setting and descriptive geochemistry of Archean supracrustal and hypabyssal rocks, Soudan-Bigfork area, northern Minnesota: Implications for metallic mineral exploration: Minnesota Geological Survey Report of Investigations 51, 69 p. Southwick, D.L., compiler, 1993, Bedrock geologic map of the Soudan-Bigfork area, northern Minnesota: Minnesota Geological Survey Miscellaneous Map M-79, scale 1:100,000. Thorpe, R.I and Franklin, J.M., 1984, Chemical-sediment-hosted Gold: in Canadian Mineral Deposit Types: A Geological Synopsis, Eckstrand, O.R., Editor, Economic Geology Report 36, Geological Survey of Canada, p. 29. Vielreicher, R.M., Groves, D.I., Ridley, J.R. and McNaughton, N.J., 1994, A Replacement Origin for the BIF-hosted Gold Deposit at Mt. Morgans, Yilgarn Block, W.A; Ore Geology Reviews, v. 9, p. 325-347. Page 226 �FIELD TRIP 8 Geology and Mineralization of the Western Contact of the Duluth Complex, Partridge River and South Kawishiwi Intrusions, Northeastern Minnesota Leaders: Mark Severson Natural Resources Research Institute, University of Minnesota Duluth and Jim Miller Minnesota Geological Survey, University of Minnesota Railroad-cut exposures in the Wetlegs area of the Partridge River Intrusion (Stop 8-3) Page 227 �FIELD TRIP 8 Geology and mineralization of the western contact of the Duluth Complex, Partridge River and South Kawishiwi intrusions, northeastern Minnesota by Mark Severson Natural Resources Research Institute, University of Minnesota Duluth and Jim Miller Minnesota Geological Survey, University of Minnesota INTRODUCTION The Duluth Complex and associated Keweenawan intrusions in northeastern Minnesota constitute one of the largest mafic intrusive complexes in the world, second only to the Bushveld 2 Complex of South Africa. These rocks cover an arcuate area of over 5,000 km (Fig. 8-1) and give rise to two strong gravity anomalies (+50 & +70 mgal) that imply intrusive roots to more than 13 kilometers depth (Allen and others, 1997). The intrusive rocks of northeastern Minnesota were emplaced into a comagmatic volcanic edifice during formation of the Midcontinent Rift between 1108 and 1086 Ma. The Keweenawan geology of northeastern Minnesota has been recently reinterpreted in a 1:200,000 scale geologic map (MGS Miscellaneous Map, M-119; Miller and others, 2001) and in a comprehensive report that focused on the geology and mineral potential of the Duluth Complex (MGS Report of Investigation, RI-58; Miller and others, 2002). The general description of the Duluth Complex given below is largely excerpted from that report. The Duluth Complex is physically defined as a more or less continuous mass of mafic to felsic plutonic rocks that extends in an arcuate fashion from Duluth to nearly Grand Portage (Fig. 8-1). It is bounded by a footwall of predominantly Paleoproterozoic and Archean rocks, a hanging wall of largely mafic volcanic rocks and hypabyssal intrusions, and internally, it contains scattered bodies of strongly recrystallized mafic volcanic and sedimentary hornfels. Defining the Duluth Complex more genetically, it is composed of multiple discrete intrusions of mafic to felsic tholeiitic magmas that were episodically emplaced into the base of a comagmatic volcanic edifice in two general stages - an early stage at about 1108 Ma and an main stage at 1099 Ma. Within the Duluth Complex, four general rock series are distinguished on the basis of age, dominant lithology, internal structure, and structural position. These are: Felsic Series – massive granophyric granite and smaller amounts of intermediate rock that occurs as a semicontinous mass of intrusions strung along the eastern and central roof zone of the complex and was emplaced during early stage magmatism (~1108 Ma). Early Gabbro Series – layered sequences of dominantly gabbroic cumulates that occur in two major intrusions along the northeastern contact of the Duluth Complex and were also emplaced during early stage magmatism (~1108 Ma) Anorthositic Series – a structurally complex suite of foliated, but rarely layered, plagioclase-rich gabbroic cumulates that was emplaced throughout the complex during main stage magmatism (~ 1099 Ma). Layered Series – a suite of stratiform troctolitic to ferrogabboic cumulates that comprise at least 11 variably-differentiated mafic layered intrusions and occurs mostly along the base of the Page 228 �Duluth Complex. These intrusions were emplaced during main stage magmatism, but generally after the anorthositic series. The Partridge River (PRI) and South Kawishiwi (SKI) intrusions, the focus of this field trip, occur along the northwestern margin of the Duluth Complex (Fig. 8-1) and are two of the earliestformed intrusions of the Layered Series. These two intrusions are most renown for hosting the largest tonnage of Cu-Ni sulfide ore in the world (Naldrett, 1997). The bedrock geology of the northwestern margin of the Duluth Complex, as portrayed on the regional geologic map of northeastern Minnesota (M-119; Miller and others, 2001), is shown in Figure 8-2. Detailed mapping in the Babbitt NE, Babbitt SE, and Babbitt SW quadrangles, which was conducted since the publication of M-119 (Miller, Severson, and Foose, 2002; and unpublished field mapping, Miller, 2002-03), has revealed a somewhat different picture of the geology that has yet to be incorporated into the regional map. Mapping in this area will continue this summer with the objective of producing 1:24,000-scale maps of all four of the Babbitt 7.5' quadrangles in the summer of 2005. Figure 8-1. Generalized geology of northeastern Minnesota showing the field trip area. Partridge River intrusion (PRI) and South Kawishiwi intrusion (SKI) are labeled. Page 229 �Figure 8-2. Geology of the northwestern margin of the Duluth Complex showing field stops for Field Trip 8 (see Miller and others, 2001 for more details). Geologic units: Agr - Giants Range granite, PbifBiwabik Iron Fm, Pvf - Virginia Fm, pri - Partridge R. intr., ski-South Kawishiwi intr., bei-Bald Eagle intr.; gltr-Greenwood Lake intr-troctolitic zone; glgb-Greenwood Lake intr, gabbroic zone; glfgGreenwood Lake intr, ferrogabbroic zone; mwgp-Mount Weber granophyre, asau-anorthositic series undifferentiated; asa-anorthositic series, anorthositic rocks; asg-anorthositic series, gabbroic rocks; nsv-NSVG undifferentiated; nsbh-NSVG basaltic hornfels; nssh-NSVG sedimentary hornfels. Page 230 �Figure 8-3. Location of copper-nickel sulfide deposits, Fe-Ti±V deposits, and other exploration areas along the western base of the Duluth Complex (from Miller and others, 2002; Fig. 2.3) Page 231 �Partridge River Intrusion The Partridge River intrusion (PRI, Bonnichsen, 1974) is perhaps one of the most well studied intrusions of the Duluth Complex because it hosts at least four sub-economic Cu-Ni deposits and at least seven potential Fe-Ti deposits. The PRI consists mainly of troctolitic cumulates that are exposed in an arc-shaped area that extends from the Waterhen Fe-Ti deposit area (T.57N., R.14W., Sec. 27) to the Babbitt Cu-Ni deposit (T.60N., R.12W., Secs 31-33; Fig. 83). Miller and Ripley (1996) estimate that the PRI is 2.5 km thick. The footwall of the PRI includes the Paleoproterozoic Virginia Formation (slate and graywacke), and to a lesser extent, the Biwabik Iron Formation. The top of the PRI is in complex contact with anorthositic rocks (unit asa, Fig. 8-2), gabbroic rocks (unit asg), mafic volcanic hornfels (unit nshb), and in one location, an unusual fine-grained and cross-bedded sedimentary hornfels (Colvin Creek hornfels of Bonnichsen, 1972; Patelke, 1996; unit nssh). This assemblage of anorthositic, gabbroic, and hornfelsic rocks are also present as large inclusions within the interior of the PRI (Severson and Miller, 1999) and are thought to represent earlier roof zone septa that were overplated and isolated by later injections of PRI magma. The lower 900 meters of the PRI is known in great detail from the abundance of exploration drill core. This marginal zone, consisting of varied troctolitic and gabbroic rock types, is subdivided into seven stratigraphic units (Severson, 1991; 1994; Severson and Hauck, 1990; 1997; Geerts, 1991) that can be correlated over a strike length of 24 km (Fig. 8-4). All of these igneous units generally exhibit shallow dips (10-20°) to the southeast. The stratigraphy shown in Figure 8-4 is based on the relogging of almost 700 drill holes, or 672,000 feet of drill core. The units of the Partridge River marginal zone are briefly described below starting at the base of the PRI. Units I and III will be viewed at Stop 8-3. For locations of ore deposit areas mentioned below, see Figure 8-3. Unit I - This lower unit consists of a heterogeneous mixture of ophitic troctolitic to gabbroic rocks that contain abundant inclusions of hornfelsic sedimentary footwall rocks and minor thin, discontinuous layers of melatroctolite and peridotite. It is the dominant sulfide-bearing member of the PRI. Noritic rocks are common at the basal contact and peripheral to sedimentary hornfels inclusions, probably due to contamination of the magma. An ultramafic interval, consisting of an oxide-bearing peridotite overlying an oxide-bearing pyroxenite, is present at the base of Unit I in areas where the PRI is in direct contact with the Biwabik Iron Formation (extreme southeastern portion of the Babbitt deposit). Unit II - This unit exhibits considerable variation from one Cu-Ni deposit to the next. At the Dunka Road and Babbitt deposits, Unit II consists of homogenous troctolitic rocks, with minor sulfide mineralization, and a fairly persistent basal ultramafic layer that separates Unit II from Unit I. The upper contact with Unit III is gradational at the Dunka Road deposit. At the Wetlegs deposit, Unit II is either 1) a single ultramafic layer immediately beneath Unit III, and/or 2) the Wetlegs Layered Interval (Fig. 8-3) which consists of repeated, thin cyclic units that internally grade upward from ultramafic rock to troctolite. Still farther to the west at the Wyman Creek deposit, Unit II consists of a single ultramafic horizon that separates sulfide-bearing and heterogeneous troctolitic rocks of Unit I from homogenous troctolitic rocks of Unit IV. Unit III – This unit consists of poikilitic leucotroctolite that commonly grades into poikilitic/ophitic augite troctolite. Because of its fine-grained texture and distinctive olivine oikocrysts that impart a mottled appearance, Unit III is a useful marker horizon, although it is absent from the Wyman Creek deposit and portions of the Babbitt deposit. Hornfelsed basalt inclusions, or roof rocks, are commonly associated with Unit III at the Dunka Road deposit. This relationship, and the highly gradational contact of Unit III Page 232 �with Unit II, suggest that Unit III may have formed as a roof cumulate during crystallization of Unit II. Unit IV - Homogenous ophitic augite troctolite that contains a local basal unit of ultramafic rock. Unit IV typically exhibits a highly gradational upper contact with Unit V. A cyclic sequence of alternating troctolitic and ultramafic layers, termed the Bathtub Layered Interval, is present in the Bathtub ore zone of the Babbitt deposit. Unit V – Unit V consists of homogenous, coarse-grained leucotroctolite. At the Wyman Creek deposit, drill hole relationships suggest that Unit V cuts downward into the lower units (Fig. 8-3). Units VI and VII - Each of these units consist of homogenous leucotroctolite that locally grades into ophitic augite troctolite; both also contain a fairly persistent ultramafic base. Several other similar units (Unit VIII and up) are present above these two units but they are not defined by drilling. OUI - Several late-stage pegmatitic plugs and vertical lenses of oxide-bearing ultramafic intrusions (OUI) intrude the troctolitic rocks of the PRI. The acronym OUI was first used by Severson and Hauck (1990) to designate cross-cutting pegmatitic bodies of peridotite, melatroctolite, melagabbro, and clinopyroxenite that contain a high percentage of coarsegrained oxides (15-100%). Known OUI bodies in the PRI, with Fe-Ti potential, occur at Section 17, Longnose, Longear, Wyman Creek, Section 22, Skibo, Skibo-South, and Waterhen. In all instances, the OUI are spatially arranged along linear trends suggesting that structural control was important to their genesis. In addition, the Longnose-LongearSection 17 group of OUIs are positioned over a window in the basal contact where the Biwabik Iron-Formation is the footwall rock. This relationship suggests a genetic link between assimilation of iron-formation at the basal contact and formation of OUI along a coincident fault zone. Figure 8-4. Generalized stratigraphy of the marginal zone of the Partridge River intrusion (modified from Severson, 1994). Stratigraphic relationships for the area between the Wyman Creek deposit and the Water Hen deposit are poorly understood and are not portrayed (taken from RI-58 figure 6.10). Page 233 �Attributes of the more heterogeneous units (I and II) near the base of the PRI are interpreted to indicate rapid magma replenishment in a progressively developing magma chamber, and magmatic contamination from assimilated footwall rocks. The more homogeneous upper units (IV-VII), each floored by a persistent ultramafic layer, were probably emplaced later in a welldeveloped magma chamber. The ultramafic layers in the upper units, and abundant ultramafic layers of the Wetlegs and Bathtub layered intervals, probably represent the inception of episodic magma injection that crystallized more primitive ultramafic layers before mixing with the resident magma. The contact zone between the PRI and South Kawishiwi Intrusion (SKI) is poorly exposed and poorly drilled (Fig. 8-2). All of the units of the PRI marginal zone, including even the upper units, become unrecognizable in the contact zone with the SKI and correlation of igneous units in one drill hole to a nearby drill hole is tenuous at best. Severson (1994) reasoned that the heterogeneous contact zone of the PRI originally formed at the contact with pre-Keweenawan footwall rocks, and that later emplacement of the SKI effectively removed the footwall portion and positioned SKI intrusive rocks up against PRI intrusive rocks. Complicating the picture of the contact zone is the coincident Grano fault (Fig. 8-16), which appears to have been repeatedly reactivated during, and after, emplacement of the PRI and SKI. The Grano fault may have also served as a feeder channel to the massive sulfides of the Local Boy ore zone at the Babbitt deposit (Fig. 8-15). More petrologic studies have been conducted on the PRI than any other intrusion in the Duluth Complex. These studies pertain to drill holes in the Babbitt deposit (see Miller and others, 2002 for references). Most of the studies pertain to a stratigraphic section intersected in one or a few drill holes that represent only a small fraction of the PRI magmatic system. Divergent progenetic interpretations have resulted from these studies and have been summarized by Miller and Ripley (1996). South Kawishiwi Intrusion The South Kawishiwi intrusion (SKI) of Green and others (1966) hosts at least five subeconomic Cu-Ni deposits and a potential PGE-Cu-Ni deposit (Fig. 8-2). The SKI is dominantly composed of troctolitic cumulates that are exposed in an 8- x 32-kilometer arcuate band. Footwall rocks include the Virginia Formation, in the Serpentine and Dunka Pit deposits, the Biwabik Iron Formation in the Dunka Pit and Birch Lake deposits, and the Archean Giants Range Batholith in the Dunka Pit deposit north to the Spruce Road deposit. The presence of Biwabik Iron Formation as inclusions as far north as the Spruce Road deposit indicates that the majority of Paleoproterozoic units were assimilated and removed from the footwall during emplacement of the SKI leaving the Giants Range Batholith as the dominant footwall rock type. Also present as inclusions in the Dunka Pit and Serpentine deposits, are mafic volcanic hornfels (probable North Shore Volcanic Group) and quartz sandstone hornfels (probably either the Puckwunge or Nopeming sandstones. Anorthositic rocks (unit asau Fig. 8-2) abut the SKI on the northeast and enclose a possible SKI feeder dike that extends farther northeast. To the east, the SKI is inferred to be in semi-conformable contact with the Bald Eagle Intrusion (unit bei; Fig. 8-2). However, based on their relative cross cutting relationships to the Greenwood Lake intrusion, it is clear that the Bald Eagle is younger than the SKI (Chandler, 1990; Miller and others, 2002). On the regional Duluth Complex map (M-119, Miller and others, 2001), the SKI is subdivided into five major map units. These are, from the base upward, 1. a basal contact zone that is a heterogeneous mix of sulfide-bearing troctolitic, gabbroic, and noritic rocks with abundant hornfels inclusions (unit skcz, M-119); 2. a thick unit of subophitic to ophitic augite troctolite (unit skat, M-119) that contains an internal ophitic olivine gabbro unit (unit skog, M-119) ; Page 234 �3. discontinuous and localized layers of poikilitic leucotroctolite (unit skpt, M-119); 4. a thick homogeneous sequence of ophitic troctolite (unit sktr, M-119) ; and 5. an uppermost thick sequence of homogeneous troctolite (unit skta, M-119) that contains numerous anorthositic layers (designated as unit skan where anorthositic layers are mapped in detail). Detailed mapping of the Babbitt NE 7.5' quadrangle in 2001 and 2002 and published as an MGS open-file map (Miller, Severson and Foose, 2002), has subdivided the southern part of the SKI into eight stratiform map units. Most of these units show considerable variation in thickness and some pinch out altogether. Moreover, whereas Foose and Cooper (1978) have interpreted most of the anorthositic rock occurrences in the upper part of the SKI to be conformable plagioclase cumulate layers interleaved within the largely troctolitic cumulates, we have interpreted most anorthositic rock occurrences to be inclusions. Severson (1994) and Zanko and others (1994) further subdivide the marginal zone of the SKI (including units skcz, skat, skog, and sktr, Fig. 8-2) into 17 different lithostratigraphic units (Fig. 8-5) that are present in over 180 drill holes over a strike length of 31 kilometers. Sulfide mineralization is confined to the BH, BAN, UW, and U3 units, and to a lesser extent the U1 and U2 units. Major marker horizons that are correlated in drill hole include three horizons with abundant cyclic ultramafic layers (U1, U2, and U3 units) and a pegmatite-bearing unit (PEG Unit) that was initially recognized by Foose (1984). A large anorthositic inclusion (> 1 km thick) is intersected in six deep drill holes in the Highway One corridor area (AN-G Unit in Fig. 8-5). Figure 8-5. Generalized stratigraphy of the marginal zone of the South Kawishiwi intrusion (modified from Severson, 1994; taken from RI-58 figure 6.12). The lowest units of the marginal zone (Fig. 8-5) are the most varied with respect to textures, rock types, and sulfide content. They are very unevenly distributed along the strike length of the SKI in a “compartmentalized” fashion, suggesting a complicated intrusive history. The lowest units were emplaced early into several restricted magma chambers via repeated and close-spaced magmatic pulses. The U1, U2, and U3 units (Fig. 8-5) represent periods of rapid and continuous magma injection that crystallized more primitive ultramafic layers before mixing with the resident magma. The U3 Unit is unique among the lower units in that it contains several massive Page 235 �oxide pods (titanomagnetite-rich) along its entire length. A spatial correspondence between the U3 Unit and footwall iron-formation suggests that most of the massive oxide pods are iron-rich “restite” produced by the magmatic digestion of iron-formation. The U3 Unit also contains the majority of the high PGE values that have been sampled to date within the SKI. The upper units of the SKI are more laterally continuous throughout the intrusion, though recent mapping in the Babbitt NE quadrangle shows significant variation in thickness and, in some cases, pinching out of these upper troctolitic units. Still, the generally greater lateral continuity and monotonous troctolitic composition of these upper units suggest that they crystallized in a more quiescent and open magmatic system characterized by widely-spaced, large volume magmatic pulses. The occurrence of abundant anorthositic inclusions, especially in the uppermost unit (Fig 8-2), implies that much of the growth of the SKI chamber was accomplished by emplacement of new magma between previous troctolitic injections and an anorthositic series hanging wall. Disseminated Cu-Ni Sulfide Mineralization in the PRI and SKI Large resources of low-grade copper-nickel sulfide ore that locally contain anomalous PGE concentrations are well documented by drilling in the basal zones of the Partridge River and South Kawishiwi intrusions. At least nine subeconomic deposits (Fig. 8-3) have been delineated in the basal 100 to 300 meters of both intrusions. The mineralization consists predominantly of disseminated sulfides that collectively constitute over 4.4 billion tons of material averaging 0.66% Cu and 0.20% Ni (Listerud and Meineke, 1977). Overall, the copper to nickel ratio averages 3.3:1; however, there are wide variations in the Cu:Ni ratio from one Cu-Ni deposit to another deposit and also internally within each of the deposits. PGE concentrations average about 10 ppm Pt+Pd (recalculated to 100% sulfide), but may range as high as 50 ppm Pt+Pd (recalculated to 100% sulfide) in associated stratabound zones, such as at the Dunka Road and Birch Lake deposits. The disseminated sulfide deposits are hosted by taxitic troctolitic to gabbroic rocks that contain abundant inclusions of footwall rock types. Within the Partridge River intrusion, the basal unit - Unit I of Severson and Hauck (1990; Fig. 8-4) hosts the vast majority of the disseminated sulfides. Similarly, mineralization within the South Kawishiwi intrusion is confined to the bottom-most units (Fig. 8-5) that include the following units: BH (basal heterogeneous); BAN (basal augite troctolite and norite); three ultramafic units (U1, U2, and U3); and UW (updip wedge). The disseminated sulfide minerals (dominantly pyrrhotite, chalcopyrite, cubanite, and pentlandite) occur as interstitial grains that make up between trace amounts and 10% of the rock by volume (visual estimation). Pyrrhotite is generally the dominant sulfide, especially closer to the basal contact. Although this mineralization type is categorized as being present within the basal portions of the intrusions, it is important to stress that the rock units do not always contain sulfides throughout their entire vertical section. Mineralized zones are independent of rock type and are typically extremely erratic in their spatial extent and ore grades. Zones that are barren of sulfides commonly “interfinger” with mineralized zones in a random pattern. This erratic pattern of mineralization, in part, mirrors the lithologic heterogeneity of the basal units. The only exception to this random mineralization pattern is the Maturi deposit, and its downdip extension (Maturi Extension; Peterson, 2001), where the uppermost portions of the BH Unit generally exhibit copper values in excess of 1.0% that gradually decrease with depth toward the basal contact. Contrasting with this heterogeneity of rock types and mineralization, some internal PGEbearing sulfide zones within the lower units of both intrusions exhibit a stratabound relationship to the igneous stratigraphic section. Examples include Dunka Road and Birch Lake (see discussion below). Page 236 �Basal Cu-Ni Massive Sulfide Mineralization In a few localized areas along the basal zones of the SKI and PRI, semi-massive to massive sulfide mineralization is present at the basal contact. In most cases, the massive to semi-massive sulfide is proximal to either sulfide-rich footwall rocks or structures such as faults and preComplex folds. Massive sulfide zones that are spatially related to sulfide-rich footwall rocks are intersected in scattered drill holes in the Babbitt, Serpentine, and Dunka Pit deposits (Severson and others (1994), Zanko and others (1994), and Severson (1994), respectively). All of these massive sulfides are pyrrhotite-rich (with generally <2% Cu) and are present at, or slightly above, the basal contact. In all cases, a pyrrhotite-rich member of the footwall Virginia Formation (BDD PO unit) is located at the basal contact and is situated up-dip of the massive sulfide occurrences. This relationship suggests that the BDD PO unit acted as a local sulfur source, which generated a Cu-poor, sulfide-rich melt that was gravitationally concentrated downdip, along the basal contact. The massive sulfide occurrence in the Local Boy ore zone of the Babbitt deposit (Fig. 8-3) is clearly structurally controlled. At this locality, the massive sulfide zones are Cu-rich (generally 5-25% Cu) and are situated along the axis of an anticline defined by the footwall rock units. The highest PGE values (11 ppm Pd and 8 ppm Pt) yet found within the Duluth Complex are associated with these structurally-controlled Cu-rich massive sulfides. The massive sulfides are almost exclusively hosted by the Virginia Formation, present as both inclusions above the basal contact and in the footwall rocks below the basal contact. These relationships, plus sulfide textures that are indicative of structural preparation, suggest that the massive sulfides were "injected" into the footwall rocks. Ripley (1986) and Severson and Barnes (1991) propose that an immiscible sulfide melt, formed in an auxiliary magma chamber at depth, was injected into structurally prepared zones in the footwall rocks along the anticline to form the Local Boy ores. Late movement of Cl-rich fluids, along the axis of the anticline, further redistributed and concentrated the PGEs (Severson and Barnes, 1991). Recent studies (Severson and Zanko, in prep.) indicate that there is an overall increase in the Cu-PGE content of the massive sulfide in an east-to-west direction (Fig. 8-6); this is perhaps the result of fractional crystallization of immiscible sulfide melt as it migrated into the footwall rocks. In this scenario, the north-south trending Grano fault is inferred to be a potential feeder zone. Structurally-controlled veins and irregular pods of massive sulfide are locally present within granitic footwall rocks immediately beneath the SKI. These occurrences are intersected in scattered holes that outline two northeast-trending belts (Fig. 8-7). The linearity of the belts suggests they are fault controlled. One of these belts crudely aligns with the Birch Lake fault zone which trends through the Birch Lake PGE prospect. The veins are moderately Cu-enriched due to fractional crystallization of the sulfide melt as it moved down through the footwall rocks (Bonnichsen and others, 1980; Severson, 1994). The occurrence of local massive sulfide veins near and below the basal contact of the Duluth Complex is an indication that larger, potentially economic footwall massive sulfide deposits may yet be found. In the Sudbury Complex, pooling of a monosulfide solid solution (mss) melt at the basal contact appears to be an important prerequisite to the injection of fractionated sulfide melts (Naldrett, 1997). Stratabound PGE Mineralization Most of the PGE-enriched zones in both the Partridge River and South Kawishiwi intrusions are stratabound in nature. These stratabound PGE horizons are intimately associated with the CuNi sulfide mineralization and with one or more ultramafic layers that indicate magma recharge events. Thus, their PGE-enrichment appears to be related to magma mixing. However, there are also indications that the PGE content in some of these horizons was locally modified by later Clrich hydrothermal solutions. Page 237 �231 2400E 3600E 2800E 3200E 4400E 4000E 211 3600S 3600S Axis of Bathtub Syncline 189 149 253 ? 197 4000S 144 217 4000S 230 ? ? ? A-4 228 ? 234 156 A-3 4400S A-5 121 4400S 154 A-6 ? A-2 158 ? U D ? SHAFT A-1 4800S 162 146 138 142 4800S 152 ? ? Axis of Local Boy Anticline ? ? ? 159 143 ? ? 124 B-3 5200S 133 U D B-1 161 132 B-1A B-2 B-2A 136 5200S B-5 B-4 B-3A 150 C-0 Kulas Fault ? ? ? C-1 119 C-1A ? ? D-5 D-4 D-3 D-2 130 160 ? C-2A 116 5600S ? C-2 D-1 ? ? 148 ? 5600S C-3 139 105 127 135 C-3A U C-4 D C-4A 2400E C-5 C-5A C-6 153 6000S 141 134 129 131 137 ? ? Grano Fault 163 140 120 N MASSIVE SULFIDE TYPES: 1. SEMI-CONTINUOUS MASSIVE SULFIDE HORIZONS ASSOCIATED WITH FOOTWALL ROCKS OR HORNFELS INCLUSIONS ABOVE THE BASAL CONTACT Pyrrhotite Dominant i.e. >85% of sulf. is PO Pyrrhotite Rich i.e. 50-85% of sulf. is PO Copper Rich (i.e. PO is 50%, Cu minerals 50%) SCALE 0 100 FT. 2. WIDELY SCATTERED PODS OF SEMIMASSIVE TO MASSIVE SULFIDE ASSOCIATED WITH EITHER HORNFELS INCLUSIONS OR TROCTOLITIC ROCKS (Pyrrhotite Dominant, >85% PO) UNDERGROUND DRIFT B-5 UNDERGROUND DRILL FAN 153 SURFACE DRILL HOLE Figure 8-6. Potential distribution of semi-massive to massive sulfide types, relative to the Grano fault and Local Boy anticlinal axis, at the Local Boy ore zone of the Babbitt deposit (from Severson and Zanko, in prep.; taken from RI-58, Figure 8.4). Page 238 �Figure 8-7. Linear distribution of massive sulfide, disseminated sulfide, and copper-rich veins in the Giants Range granitic footwall rocks beneath the South Kawishiwi intrusion (after Severson, 1994; taken from RI58, Figure 8.5). Stratabound PGE-enriched horizons, with low to moderate sulfide concentrations (0.05-1.0 wt.% S), are commonly associated with ultramafic layers in the Dunka Road, Babbitt, Wetlegs, and Birch Lake deposits (Fig. 8-3). Elevated Cu and PGE concentrations at the Dunka Road deposit occur at the extreme top of Unit I immediately beneath a laterally persistent ultramafic layer. This stratabound horizon (red horizon of Geerts, 1991, 1994) averages about 10 meters thick and contains an average of 1.0 ppm Pd+Pt. Recent work by Theriault and others (1997, 2000) suggests that the sulfur was largely derived from the mafic magma and that this stratabound horizon was formed as a result of magma mixing. Two similar stratabound PGEenriched horizons, related to laterally discontinuous ultramafic layers, occur toward the middle of Unit I at Dunka Road (orange and yellow horizons of Geerts, 1991; 1994). To the west of Dunka Road, the stratabound PGE horizon at the top of Unit I is also present at the Wetlegs and Wyman Creek deposits. There however, the overall Pd content in this horizon exhibits a definite decrease in an east-to-west direction suggesting that as the magma was intruded it became progressively impoverished with respect to PGE (Severson and Hauck, 1997; Theriault and others, 1997). Additional PGE-enriched stratabound horizons are situated much further above the basal contact in the Partridge River intrusion. These horizons are also often associated with ultramafic layers and are present at Dunka Road, Wetlegs and the Fish Lake area (Sassani, 1992; Severson, 1995). Another example of a PGE stratabound horizon is at the Birch Lake PGE prospect within the SKI. There, PGE contents as high as 9 ppm Pd+Pt, and Cr2O3 contents locally as high as 10 wt.%, are associated with a wide variety of rock types within the U3 Unit. The U3 Unit consists of alternating troctolitic and ultramafic layers in which variably sulfide-mineralized zones and discontinuous pods of Cr-bearing massive oxide both occur (Severson, 1994). The massive oxide pods are interpreted, based on empirical relationships, to have been produced by assimilation and partial melting of the Biwabik Iron Formation. This oxide-rich partial melt may have initially acted as a trap that concentrated Cr and Ti, and through further assimilation and contamination of Page 239 �the magma, may have led to precipitation of PGE. However, because the ultramafic layers of the U3 Unit are interpreted to record new influxes of more primitive magma (Severson, 1994), magma mixing may have played a more significant role in PGE mineralization. Furthermore, the presence of Cl-rich drops on the surface of drill core from the Birch Lake area suggests that a hydrothermal model of concentrating the PGE could also be invoked. A model involving ascending Cl-rich hydrothermal fluids, depicted in Figure 8-8 and similar to a model proposed by Boudreau and McCallum (1992), may have remobilized and further concentrated the PGE along the northeast-trending Birch Lake fault (Severson, 1994). A possible explanation, depicted in Figure 8-8, is that the Birch Lake area represents an area where there was a local increase in the amount of upward-moving Cl-rich solutions that were concentrated, or funneled, along the Birch Lake fault. Another explanation is that the Birch Lake Fault may have initially served as a subsidiary feeder zone to the South Kawishiwi intrusion (Hauck and others, in prep.). According to their model, uncontaminated PGE-enriched magmas were vented in the Birch Lake area. Upon mixing with the resident magma, which was contaminated due to interaction with the Biwabik Iron Formation, PGEs were deposited in zones of turbulent mixing. As the magma migrated away from the vent area (up-dip?) it became progressively impoverished with respect to PGEs. Figure 8-8. Schematic diagram showing the possible role that the Birch Lake fault may have played in funneling upward-moving Cl-rich solutions and the resultant reconcentration of significant magmatic PGE within the U3 Unit at the Birch Lake PGE area (modified from Severson, 1994; taken from RI-58, Figure 8.8). Page 240 �Oxide Ultramafic Intrusions (OUI) Exploratory drilling for Cu-Ni mineralization encountered several OUIs, that later were evaluated for their Fe-Ti±V potential. Many of the OUIs are expressed as aeromagnetic highs, commonly with an associated electromagnetic conductor, and thus they were initially drilled in search of conductive sulfide mineralization. At least thirteen OUIs have been intersected in drill holes along the basal contact of the Duluth Complex (Fig. 8-3). The OUIs are plugs or pipe-like bodies that commonly have irregular apophyses. They intrude troctolitic rocks of the Partridge River, Western Margin, and Boulder Lake intrusions. The Waterhen OUI appears to be rootless as defined by detailed drilling; the three-dimensional configurations of the other OUIs are unknown due to insufficient drilling. In general, the OUIs are spatially arranged along linear trends suggesting that structural control was important to their genesis. Almost all of the OUIs are cross-cutting. Rock types include coarse-grained to pegmatitic clinopyroxenite, dunite, peridotite, melatroctolite, and minor melagabbro; all rock types are oxide-bearing. Some OUIs exhibit a crude zonation from an olivine-rich core (dunite, peridotite, melatroctolite) to an outer clinopyroxenite margin, whereas others consist of only one dominant rock type. Oxide content is variable in the OUIs and ranges from 15% in disseminated zones to 100% in localized massive oxide zones. OUIs that contain thick intervals of massive oxide include Longnose (up to 30 meters thick; Linscheid, 1991) and Section 34 (up to 40 m thick; Severson, 1995). Titanomagnetite is dominant in some of the OUIs whereas ilmenite is dominant in other OUIs. Sulfide minerals (predominantly pyrrhotite) are ubiquitous in all the OUIs and range from trace amounts to 5% in disseminated zones to >70% in localized net-textured and massive sulfide zones, as at Waterhen, Boulder Lake South, and Fish Lake (Fig. 8-3). Page 241 �STOP DESCRIPTIONS STOP 8-1: Hornfels Virginia Formation, Duluth Complex Footwall at Linwood Lake Location: Approximately 300 feet southwest of the intersection of Linwood Lake Road and Reagan Road. Harris Lake 7.5’ quadrangle, T.56N., R.14W., Sec 22, NE of SE of SW; 567865E, 52407780N (NAD83) Description: Outcrops of the Virginia Formation at Linwood Lake show a progressive increase in the amount of deformation, metamorphism, and degree of partial melting towards the basal contact of the Duluth Complex (which based on limited drilling in this area exhibits a nearvertical cliff-like geometry). In this area, outcrops furthest from the contact consist of interbedded argillite and fine-grained distal graywacke (Bouma sequence B with minor C) that exhibit shallow dips of 15° to the southeast. However, slightly closer to the Complex, bedding dips increase to 40-60° to the northeast and small wisps (<5 mm) of partial melt are present along bedding planes. Still closer to the Complex (Stop 8-1) the grade of metamorphism and associated deformation progressively increase and at least two metamorphic varieties are superimposed on the original sedimentary package. Both of these metamorphic varieties are visible at this stop and consist of the following: Disrupted unit (Fig. 8-9)– In close proximity to the Complex the well-bedded sediments of the Virginia Formation are typically transformed into a highly deformed rock or metatexite (Sawyer, 1999). Textures that characterize this rock are bedding planes that are extremely chaotic and random in orientation due to small-scale folding, faulting, and brecciation. Superimposed on this chaotic pattern are abundant zones of partial melt that are also chaotic and folded. Albeit differences in grain size, the partial melts are composed of the same mineralogy as the surrounding sedimentary rocks (feldspar, quartz, orthopyroxene, cordierite, and biotite). Figure 8-9. Photograph of disrupted unit of metamorphosed and folded Virginia Formation with abundant partial melt lenses and wisps in close proximity to the Duluth Complex at Linwood Lake, MN. Page 242 �Recrystallized unit (Fig. 8-10)– This unit is a higher-grade metamorphic equivalent of the disrupted unit, and is properly classed as a diatexite (Sawyer, 1999). By contrast, rocks of the recrystallized unit were heated, generating 20-40% pervasive partial melts, which literally enabled the rocks to flow in response to stresses that were applied during emplacement of the Duluth Complex. All bedding planes are obliterated and what remains is a medium-grained recrystallized rock that contains plagioclase, cordierite, orthopyroxene, and decussate biotite. Within this recrystallized matrix are blocks/boudins of more structurally competent siltstone and calc-silicate hornfels (originally limey layers). Figure 8-10: Photograph of recrystallized unit of metamorphosed Virginia Formation in close proximity to the Duluth Complex at Linwood Lake, MN. STOP 8-2: Anorthositic Series, Duluth Complex, Skibo Vista Location: Roadcut on St. Louis Co. Highway 110, approx. 3.7 mi. north of Co. Hwy 16. Bird Lake 7.5' quadrangle; T.57N., R.13W., Sec 17, SE of NW; 574140E 5252806N (NAD83) Description: Exposed in roadcuts on either side of Co. 110 is an exposure of poikilitic troctolitic anorthosite that is a common lithology of the anorthositic series of the Duluth Complex. The rock varies in texture and modal mineralogy on a meter scale over the outcrops ranging from medium- to medium coarse in grain size, from subpoikilitic to poikilitic in olivine habit, and from 80 to 90% in plagioclase mode. Plagioclase foliation is locally well developed and varies in orientation throughout the exposures. Olivine oikocrysts range from 1 to 5 cm in diameter (larger ones on north side of exposures) and are locally concentrated in planes that parallel foliation and thereby impart a subtle layering. This exposure lies at the western extent of a large expanse of plagioclase-rich gabbroic rocks that comprise the Anorthositic Series of the Duluth Complex (Fig. 8-1). West of here, troctolitic cumulates of the Western Margin intrusion are inferred to occur (Figs. 8-1, 8-2). The lithologic characteristics and structural complexities of the Anorthositic Series, which are hinted at here, Page 243 �have lead many workers (Grout, 1918; Taylor, 1964; Miller and Weiblen, 1990) to conclude that these rocks formed from multiple injections of plagioclase-enriched mafic magmas (plagioclase crystal mushes) early in the main stage of Duluth Complex magmatism. The effectively identical ages of Anorthositic and Layered Series rocks (~1099Ma, Paces and Miller, 1993) implies that intrusion of normal (crystal-free) magmas, forming layered series intrusions like the Western Margin Intrusion, followed soon after the emplacement and crystallization of the Anorthositic Series. STOP 8-3a: Erie hornfels – mafic volcanic unit overlying the Virginia Formation, Erie/LTV RR Grade Location: Access via Dunka Road (private mining company road) approximately 5 miles west the LTV guard shack. Allen 7.5’ quadrangle, T.59N., R.13W., Sec 18, NW of SE of SW; 571815E 5271060N (NAD83) Description: Inclusions of hornfelsed basalt bodies are documented in many places in the Duluth Complex – the earliest detailed studies were by Kilburg (1972), for the Ely’s Peak basalts beneath the Complex near Duluth, MN, and Bonnichsen (1972), for several inclusions located within the Complex. The Erie hornfels was originally discussed by Bonnichsen (1972), and was more recently described in detail by Tyson (1976). Detailed geologic mapping of the area (Severson and Miller, 1999) indicates that the Erie hornfels directly overlies the Virginia Formation (based on the geology in surrounding drill holes) and is most likely situated near the base of the North Shore Volcanic Group in this area. The Erie hornfels is exposed in a 215-meter long railroad cut. Tyson (1976) described it as a series of basalt flows, dipping 55° to the southeast, that are characterized by fine-grained granoblastic rocks containing variable amounts of plagioclase, olivine, and augite with lesser amounts of hornblende and biotite. Relict amygdules (plagioclase- or augite-filled ovoids) and relict plagioclase phenocrysts are present in many places in the railroad cut but the amygdules appear to be more concentrated in the upper portions of the flows. Tyson (1976) also reports that biotite-rich layers are locally present in the exposures and that these layers were probably formed from either interflow sedimentary rocks or altered flow tops. STOP 8-3b: Marginal Units of the Partridge River Intrusion, Erie/LTV RR Grade Location: Erie/LTV RR Grade approximately 1/3 mile east of previous stop. Allen 7.5’ quadrangle, T.59N., R.13W., Sec 18, SE of SE, and Sec 19, SW of SW; traverse from 572430E, 5271135N to 572960E, 5271205N (NAD83) Description: The Wetlegs Cu-Ni deposit was initially drilled by Bear Creek Mining in 19581960 (referred to as the A4 grid) on the basis of electromagnetic conductors which were found to be related to graphitic portions of the footwall Virginia Formation. Some indications of Cu-Ni mineralization were found in the initial drilling and after acquiring state mineral leases in 1966 Bear Creek conducted some follow-up drilling. However, the Cu-Ni grades were low overall, and Bear Creek abandoned further mineral exploration endeavors in this area of the Complex. Exxon Minerals Corp. leased the same lands and conducted drilling programs in 1975 and 1979. They estimated that the Wetlegs deposit contained 38 million tons of material grading 0.29% Cu and 0.10% Ni using a cutoff grade of 0.15% Cu. Recent sampling for platinum group elements (PGE) indicates that there are at least three potential stratabound zones at Wetlegs (Severson and Hauck, 2003). Page 244 �A west to east traverse along the LTV railroad tracks will be conducted at this field stop. The traverse, approximately 0.5 miles long, will start at the basal contact and progress upwards through the igneous stratigraphy of the Partridge River intrusion. Outcrops of Units I and III will be viewed. A generalized geologic map of the Wetlegs area is shown in Figure 8-11. Detailed descriptions of the railroad cuts are listed below. Oxide Ultramafic Intrusion Troctolite (V) Augite Troctolite(IV) PoikiliticTroctolite (III) Melatroctolite (II) Contact Zone (I) EEl MaficVolanic H or n fe Is Virginia Formation Figure 8-11: Geology of the Partridge River Intrusion in the vicinity of the Wetlegs Cu-Ni deposit showing stops 8-3a (Erie hornfels) and 8-3b (Wetlegs deposit). Roman numerals correspond to stratigraphic units of Severson and Hauck (1990). Modified from Severson and Miller (1999). Feet (to the East) Description 0 ft. 100-110 ft. (left side of tracks) Railroad culvert – Longnose Creek Sulfide/gossan-cemented till consisting mostly of large boulders of a pyrrhotite- and graphite-rich member of the Virginia Formation (the BDD PO unit). Note the rounded granite cobbles beneath the BDD PO boulders. Drilling indicates that the BDD PO subcrops very close to this location. 337-355 ft. Very fine-grained (chilled) gabbronorite at basal contact with 60% (left side of tracks) plagioclase, 30% orthopyroxene, 5 % clinopyroxene, trace to 2 % biotite, and 3 % oxides. The outcrop contains well-assimilated “streaks” of Virginia Formation inclusions (very hard to see). 400-1025 ft. Unit I – taxitic medium- to coarse-grained ophitic augite troctolite (POcf) to (both sides of olivine gabbro (PcOf) with patches and lenses of augite-rich pegmatite. tracks) The outcrops are sulfide bearing with the sulfides unevenly distributed along patches, spots, lenses, and joint faces (at numerous orientations!). Trace amounts to 5% uralite due to pervasive deuteric alteration is present. 1245-1355 ft. Unit I – taxitic, pervasively uralitized, medium- to coarse-grained ophitic (two outcrops on olivine gabbro (PcOf) to augite troctolite (POcf) with trace to 1% sulfides. left side of tracks) Also present within these outcrops are very coarse-grained anorthosite inclusions that vary from 10 cm across to 3x5 meter blocks. The edges of the inclusions are sharp, and straight to highly lobate. Within the inclusions, the plagioclase foliation is subvertical and highly variable. Unit I – taxitic, medium- to coarse-grained ophitic augitic troctolite (POcf) 1385-1685 ft. (many outcrops on with abundant irregular pegmatitic patches and lenses. The pegmatites Page 245 �both sides tracks) of contain variable amounts of saussurized plagioclase, clinopyroxene, oxides, biotite, and uralite, with minor quartz, k-feldspar, graphic granite, and sulfides. Most of the sulfides are either within or adjacent to the pegmatites. Uralite is common throughout the outcrops as pervasive replacement products in irregular patches and along joints. 2355-2480 ft. Unit III – mottled, poikilitic/ophitic troctolite (Po(cf)) to augite troctolite (four outcrops on (Pocf). This is a major marker bed due to the presence of medium- to highboth sides of the density olivine oikocrysts up to 10 cm across. This unit is easily recognized tracks) in drill core due to the mottled texture, and the relatively finer-grained plagioclase (1-5 mm). Dunka Road and Babbitt Deposits As the bus proceeds to the next field stop, we will traverse across the southernmost limits of the Dunka Road deposit (originally drilled by United States Steel, now held by PolyMet Mining and called the NorthMet deposit) and the Babbitt deposit (originally drilled by Bear Creek and Amax, now held by Teck Cominco and called the Mesaba deposit). There is very little to see in the way of mineralized exposures for either of these deposits. However, one point to keep in mind is the immense size of these low-grade deposits. The Dunka Road/NorthMet deposit extends for about 3 miles along the road, and the Babbitt/Mesaba deposit extends for another 2.5 miles along the road (the deposits are separated by undrilled and untested area that is about 1 mile wide). STOP 8-4: Hornfels mafic volcanic inclusion and oxide ultramafic intrusions near the margin of the Partridge River Intrusion Location: Erie/LTV RR Grade approximately 2 miles west of the locked gate on the Dunka Road. Babbitt NE 7.5’ quadrangle T. 60N., R.13W., Sec 33.; 584510E, 5275715N (NAD83) Description: The Dunka Railroad hornfels is a large mafic volcanic inclusion located near the eastern margin of the Partridge River intrusion and just south of the Babbitt/Mesaba deposit. Outcrops and drill hole information indicates that the inclusion is about 900 x 1,500 meters across. The hornfels is a fine-grained, granoblastic to poikiloblastic basaltic hornfels that contains variable amounts of plagioclase, augite, hypersthene, inverted pigeonite, and olivine. Both massive and meta-amydaloidal varieties are present. In addition to the basaltic hornfels, several small bodies of late intrusive OUIs are also present in the railroad cuts. The abundance of OUIs at this locality is related to proximity to the north-trending Grano Fault, which may have served as a feeder vent for the massive sulfides at the Local Boy area of the Babbitt/Mesaba deposit. These OUIs, and a wide variety of granitic rocks, occur as lenses and bodies that cut the troctolitic rocks of the Partridge River intrusion. They are extremely common in drill holes, within a 400-580 meter wide zone, on the west side of the Grano Fault. The OUIs at this locality are characterized by medium- to coarse-grained clinopyroxenite with 75-80% augite, 0-15% olivine, <5% interstitial plagioclase, and 5-15% oxides (ilmenite is dominant). Contact relationships with the basaltic hornfels are sharp but highly irregular and lobate. Granitic dikes and veins, present within a NNE-trending zone, are also evident in the exposures. They are rarely observed in close proximity to the OUI, but where they are, the dikes crosscut the OUI. Page 246 �STOP 8-5: Cu-Ni sulfide mineralization, Spruce Road deposit, South Kawishiwi Intrusion Location: Barrow pit 200 feet west of Spruce Road (Forest Rd 181) approximately 3.5 miles north of Highway One. Bogberry Lake 7.5’ quadrangle, T.62N., R.11W., Sec 24, NE of SE of SW; 599295E, 5299010N (NAD83) Description: It all started here! Serious exploration for Cu-Ni deposits at the base of the Duluth Complex began somewhere near this site in 1948 when strongly mineralized and gossanous rocks were uncovered in an excavation into weathered gabbro rubble used to build the Spruce Road. Local prospector Fred S. Childers of Ely, MN, noted copper stains in the material and began searching the outcrops along the basal contact in the vicinity of the Kawishiwi River. Roger V. Whiteside of Duluth, MN, later joined him in further exploration. In 1951, they diamond drilled a 57 meter (188 feet) deep hole located several miles to the southwest of this site in what is now referred to as the Maturi deposit (Fig. 8-3). This hole intersected disseminated sulfides in gabbroic rock that averaged 0.36% Cu and 0.13% Ni (Watowich and others, 1981). In 1952, both Bear Creek Mining Company and the International Nickel Company (INCO) began intensive exploration efforts along a 61 km-long zone (38 miles) that coincided with the basal contact (an area stretching from south of the town of Hoyt Lakes northeastward to this site at Spruce Road). INCO eventually picked up the Childers-Whiteside properties and began drilling activities in 1954; whereas, Bear Creek concentrated most of their effort near the town of Babbitt and drilled several properties during 1957-1960. During this same period, the Minnesota Geological Survey, in cooperation with the U.S. Bureau of Mines, began an examination of the Maturi-Spruce Road area that culminated in the drilling of three holes in 1953. All of these exploration efforts indicated that large tonnage, but low grade, disseminated Cu-Ni deposits were present along the basal contact. In 1966, the Minnesota Department of Conservation adopted rules for state mineral leases. The leases were offered through the Department of Natural Resources (DNR) and were awarded to successful bidders. Since 1966, over 20 companies have been actively involved in exploration for Cu-Ni and Fe-Ti-V deposits along the basal contact of the Complex and over 1.700 holes totaling over 1.5 million feet of core have been drilled. In regards to the Spruce Road deposit, INCO drilled over 150 holes on the property at approximately 200-foot spacings. They calculated that the deposit contains 248 million tons of low-grade material averaging 0.46% Cu and 0.17% Ni. INCO collected at least two bulk samples from Spruce Road: 1) a 1,150 ton sample from several unknown small pits in 1966-67 (Watowich and others, 1981) and; 2) a 10,000 ton sample in 1974. It is uncertain if this barrow pit is one of the 1966-67 INCO bulk sample sites or if this pit was only used for road material. Rock types at this pit consist of sulfide-bearing, taxitic augite troctolite and olivine gabbro of the BH Unit (Basal Heterogeneous Unit) in the Marginal Zone of the South Kawishiwi intrusion. The BH Unit is approximately 360 meters thick (1,200 feet) at Spruce Road. Footwall rocks at Spruce Road consist dominantly of granitic rocks of the Archean Giants Range batholith; but in local areas the Biwabik Iron Formation is still preserved beneath the basal contact. At this locale, the pit is situated about 150 meters (480 feet) above the basal contact. Cu-Ni grades in the top of a nearby drill hole (34801) are 0.51-0.63% Cu and 0.15-0.17% Ni. Page 247 �Figure 8-12: Location map of Stop 8-5 at the Spruce Road Cu-Ni deposit. Two possible INCO bulk sample sites are shown (this stop included), as well as, the reclaimed location of a 10,000 ton bulk sample collected by INCO in 1974. STOP 8-6: Quarry exposure of homogeneous troctolite, South Kawishiwi Intrusion Location: Approx. 3.0 miles south of Spruce Road on Highway 1; follow old access road to abandoned quarry about 800m S of Highway 1. Bogberry Lake 7.5' quad; T.61N., R.11W., Sec 11, SE of NW; 597940E 5293550W (NAD83) Description: This abandoned quarry provides a 3-dimensional look at the remarkable homogeneity of troctolitic cumulates, which compose most the South Kawishiwi (and Partridge River) intrusion. The average rock type is a medium-grained, moderately foliated, augite-poor troctolite. Moderately aligned, cumulus plagioclase makes up 70-75% of the rock, though locally up to 80% plagioclase occurs and the rock would be classified as a leucotroctolite. Subhedral granular olivine composes 20-25% of the average rock, though here too, local olivine enrichment Page 248 �of up to 30% occurs in thin layers parallel to foliation. Interstitial augite and Fe-Ti oxide compose 3-7% of the rock. The textural and mineralogic homogeneity of the troctolitic cumulates composing the middle to upper sections of the SKI (and PRI), and displayed here, are matched by limited variation in mineral compositions of plagioclase, olivine and pyroxene. Phinney (1969) reported a range of plagioclase and olivine compositions in troctolitic units of SKI (excluding the marginal rocks) of An57-72 and Fo50-63, respectively. Similar results are displayed by unpublished microprobe data from troctolitic rocks from the Babbitt NE quadrangle, which indicate olivine compositions of Fo50-65 and augite compositions of En68-75. Many see this lithologic homogeneity and lack of significant cryptic variation as indicating that the SKI and PRI magma systems were open to frequent recharge of relatively primitive magma (Severson, 1994; Lee and Ripley, 1996; Miller and Ripley, 1996). New recharge events are commonly marked by the intermittent occurrences of olivine-rich (melatroctolite) intervals separating otherwise subtle differences in troctolite texture and mode (Severson, 1994). These melatroctolite intervals show a significant increase in An, Fo, and En compositions consistent with the recharge of a more primitive magma. An alternative explanation has been put forth by Chalokwu (Chalokwu and Grant, 1990; Chalokwu and others, 1993). He has suggested that this homogeneity is indicative of single stage emplacement of a viscous semi-crystallized olivine-plagioclase mush that experienced little in situ differentiation. However, his sampling routinely overlooked the melatroctolite intervals. STOP 8-7: Anorthositic inclusion? in troctolite, South Kawishiwi Intrusion Location: Road cut on Tomahawk Rd. about 7.5 miles west of Highway 1; Babbitt NE 7.5' quad; T.60N., R.11W., Sec 18, center of NE; 592355E 5282430W (NAD83) Description: Scattered throughout the entire sequence of monotonous troctolitic cumulates are masses of coarse- to medium-grained troctolitic anorthosite to olivine gabbroic anorthosite. In recent mapping of the Babbitt NE quadrangle (Miller, Severson and Foose, 2002), we (Miller and Severson) have consistently interpreted these anorthositic masses as inclusions. We believe that they represent pieces of the anorthositic series roof pendant that was upwardly displaced and dissaggregated with successive recharge events into the SKI magma chamber. In an earlier detailed field mapping of this mixed anorthosite-troctolite terrane, mainly to the northeast of here, Foose and Cooper (1978, 1981) interpreted many of these anorthositic bodies as comagmatic layers within the troctolite. They saw their elongate and conformable relationship with the internal structure of the enclosing troctolite as evidence that most of these bodies are lensoidal plagioclase cumulate intervals intermittently layered within the troctolite. They used the apparent offset of these layers as evidence for a complex fault pattern (Fig. 8-13). This stop along the Tomahawk road will examine one such example of a troctolitic anorthosite body within leucotroctolitic cumulates. This interval of the SKI is particularly rich in anorthosite bodies. The relationships displayed here are difficult to reconcile with either the Miller and others (2002) or the Foose and Cooper (1978,1981) interpretations. An irregularly shaped anorthosite block projects into the leucotroctolite and appears to cut across the foliation in the enclosing leucotroctolite (Fig. 8-13). Page 249 �Figure 8-13. Geologic map of the Harris Lake area, which is northeast of Stop 8-7 from by Foose and Cooper (1981). Map units are: 1 - augite troctolite, 2 - troctolite, 2a - ophitic augite troctolite layer, 2b - traceable anorthosite layer, 3 - anorthosite haAn.csgaLx ,!aa 44/ •i Page 250 Figure 8-14. Photo of road cut at Stop 8-7 showing an irregularly shaped troctolitic anorthosite mass in leucotroctolite. Foliation in the leucotroctolite is approximately parallel to the hammer. �STOP 8-8: Grano fault zone, Partridge River Intrusion (optional) Location: Small exposure north of Forest Rd 113, 100m east ; Babbitt SE 7.5' quad; T.59N., R.12W., Sec 16, NW of NW; 584486E 5272519W (NAD83) Description: The Grano fault is a major N-S trending structure that was first recognized in the subsurface by compiling exploration drill hole data related to the Babbitt and Serpentine deposit areas (Severson, 1994). As shown in Fig. 8-15, the fault creates a significant (~60m) displacement (down to the east) of the Duluth Complex footwall and affects both the Partridge River and the South Kawishiwi Intrusions. Drill holes into the fault zone in the southeastern portion of the Babbitt deposit shows that to the immediate west of the fault is a 400-580 meterwide (up to 2,000 feet) zone that contains abundant subvertical lenses of granophyre and OUI. To the north along the fault trace the zone with late subvertical lenses pinches down appreciably (not present at Serpentine) and the amount of motion along the fault diminishes as well, indicating that the fault is a “scissors-type” fault. The massive sulfides of the Local Boy ore zone of the Babbitt deposit have been inferred to have been “vented” from the Grano fault. The surface expression of the fault is a prominent linear valley that can be traced over 10 kilometers south from the basal contact (Fig. 8-16). This stop is in the axis of that valley. The rock types exposed on the margins of this valley are a complex mixture of augite leucotroctolite with poikilitic olivine, ophitic augite troctolite to olivine gabbro, poikilitic olivine gabbroic anorthosite, and locally granophyric gabbroic pegmatite. This assemblage represents a complex mix of anorthositic series inclusions and unmineralized marginal zone rocks of the Partridge River Intrusion. The granophyric gabbroic pegmatite is evidently related to a late, volatile-rich mafic magma intruded into Grano fault zone. In the small pavement exposure near the road, a medium-grained poikilitic augite leucotroctolite is cut by gabbroic pegmatite. Figure 8-15. Grayscale image of the footwall topography of the Partridge River and South Kawishiwi intrusions near their contact showing the subsurface expression of the Grano Fault (image generated by Dean Peterson) Page 251 �I•i-_• -- '-.• 2000 m Figure 8-16. Grayscale image of surface topography of the Partridge River and South Kawishiwi intrusions near their contact showing the surface expression of the Grano Fault. Lines show geologic contacts and faults taken from MGS Misc. Map M-119. SKI - South Kawishiwi intrusion; PRI - Partridge River intrusion; AS - anorthositic series; PG - "Powerline Gabbro" of Bonnichsen (1974); HB hornfels basalt; BIF - Biwabik Iron-formation; VF - Virginia Formation. STOP 8-9: Volcanic hornfels and plagioclase-phyric leucogabbro, Partridge River Intrusion Location: Roadcuts along Erie/LTV RR grade approx 1km west of crossing by Forest Rd 113 Babbitt SW 7.5' quad; T.59N., R.12W., Sec 18, SW of SW; 581500E 5271020W (NAD83) Description: Scattered throughout the homogeneous troctolites of the Partridge River intrusion are kilometer-scale inclusions that are composed of various mixtures of anorthositic rocks, volcanic hornfels and oxide gabbro (Fig. 8-2). This assemblage is thought to represent initial intrusions of anorthositic series magmas into the base of the volcanic pile during the main stage of Duluth Complex magmatism (~1099 Ma). The anorthositic rock lithologies, which dominate the Anorthositic Series throughout the Duluth Complex, are interpreted to have crystallized from plagioclase crystal mushes (evolved basaltic melts laden with suspended plagioclase crystals). However, in this area, these anorthositic rocks are locally intruded by oxide gabbro. Bonnichsen (1974b) speculated that the oxide gabbro, which he termed the Powerline Gabbro, may be a differentiate of the Partridge River intrusion. However, detailed mapping in the Allen quadrangle to the west (Severson and Miller, 1999) demonstrated that this gabbro is intruded by troctolitic rocks of the PRI. We consider the gabbro to be a plagioclase-poor component of the Anorthositic Series. In a roadside exposure on the forest road, we can see a typical example of the "Powerline Gabbro". It is a coarse-grained, non-foliated, subophitic olivine oxide gabbro composed of 55% plagioclase, 20% granular olivine, 15% subophitic augite, 10%anhedral Fe-Ti oxide, and trace amounts of hornblende and biotite. The mafic phases locally show incipient granoblastic Page 252 �recrystallization textures and plagioclase and augite display moderate clouding by the exsolution of oxide needles. Both features are indicative of thermal metamorphism. About 200 m through the woods to the north of the forest road, we come upon the Erie/LTV railroad grade. Exposed along a 500m-long road cut on both sides of the grade is a complex mixture of two general rock types. Most of the exposure is a very fine-grained, dense mafic volcanic hornfels that is sparsely plagioclase porphyritic. In thin section, the hornfels displays a perfect granoblastic texture (Fig. 8-17). In sharp to abrupt contact with this volcanic hornfels is a medium grained plagioclase porphyritic leucogabbro. In thin section (Fig. 8-17), the leucogabbro is composed of strongly zoned plagioclase phenocrysts, which show deep clouding by oxide needles, in a matrix of granoblastic-textured oxide melagabbro (40%Pl, 30% Aug, 30% Fe-Ti Ox). At the contacts between the hornfels and the leucogabbro (best seen on the upper surface of the northern roadcut), the plagioclase phenocrysts in the leucogabbro show a strong alignment parallel to a steep contact with the hornfels. One possible interpretation of this leucogabbro is that it represents a chilled intrusion of plagioclase crystal mush that was emplaced into the volcanic pile early in the emplacement of the anorthositic series. Subsequent intrusions of more voluminous anorthositic series magma and later layered series magma forming the Partridge River intrusion produced the strong thermal metamorphism now evident in both the mafic volcanic and the porphyritic leucogabbro. Figure 8-17. Photomicrographs of mafic volcanic hornfels and plagioclase-phyric leucogabbro observed at Stop 8-9. Page 253 �STOP 8-10: Cross-bedded Colvin Creek hornfels, Partridge River Intrusion Location: Approximately 0.5 miles south of where Forest Rd 113 crosses the south branch of the Partridge River and 520 meters (1700 feet) east along a flagged trail. Babbitt SW 7.5' quadrangle; T.59N., R.13W., Sec 25, NE of SW of SE; 580255E 5267943N (NAD83) Description: Magnetic basalt hornfels inclusions (most notably the Colvin Creek hornfels) within the Duluth Complex were first observed by Bonnichsen (1972) and first described in some detail by Tyson (1976). According to Tyson, the magnetic basalt flows of the Colvin Creek hornfels are characterized by fine-grained granoblastic rocks composed of plagioclase, augite, and magnetite with relict plagioclase- and augite-filled amygdules. Tyson (1976) noted a major difference between the magnetic basalts and the more common non-magnetic basalt inclusions (as in stops 8-3a and 8-4) and theorized that the magnetic basalts were derived from weathered and oxidized basalt flows that were subsequently metamorphosed by the Complex. Tyson’s work was of a reconnaissance nature and much more detailed work has since been completed on the Northern Colvin Creek Body by Patelke (1996). The Northern Colvin Creek Body is a large inclusion (2,500 X 800 meters), associated with a magnetic high, that has been rotated to near vertical and exhibits excellent stratigraphic tops to the northwest. Patelke (1996) subdivided the northern Colvin Creek Body into five mappeable units including two metavolcanic units, two intrusive gabbroic sill units, and a cross-bedded sedimentary unit. Within the metavolcanic units, each consisting of multiple basalt flows, Patelke (1996) identified several volcanic features that include: pipe amygdules, sheeted amygdules, and local convoluted flow bases. Figure 8-18. Photograph of cross-bedded “microgabbroic” sediment (X-BDD Unit) of the Northern Colvin Creek Body. Enigmatic cross-bedded sedimentary rocks, with near-vertical dips, were first discovered in the Northern Colvin Creek hornfels and reported by Severson and Hauck (1990). They mapped a X-BDD Unit that exhibited a strike length of over 1,600 meters and an exposed thickness of over 240 meters. Overall the cross-bedded rocks are gabbroic in composition and consist of very finegrained granoblastic concentrations of plagioclase, diopsidic augite, and magnetite with lessor amounts of ilmenite, orthopyroxene, and poikiloblastic pyroxene – the rocks do NOT contain Page 254 �quartz or biotite, nor a basal conglomeratic unit where it overlies magnetic basalt (Patelke, 1996). Because sedimentary-like features were found in the cross-bedded rocks, and in the overlying gabbroic rocks (both with near-vertical dips), Severson and Hauck (1990) postulated that the cross-bedded rocks were deposited via magmatic density currents (a concept they no longer believe in). More recent studies by Patelke (1996) suggests that these cross-bedded rocks were sedimentary and were most likely deposited in a restricted basin as an eolian sediment that was derived from a strictly basaltic terrain (thus no quartz). Whatever their origin, these rocks exhibit beautiful sedimentary structures that include: bedding, cross-bedding, density-graded modal layering, and scour and fill structures (Patelke, 1996). Patelke (1996) suggests that the crossbedded rocks of the Colvin Creek hornfels are similar to a thin sandstone unit near Phantom Lake, north of Two Harbors, MN. However, he also cautions that neither of these sedimentary units are analogous to any of the typical interflow sandstones of the North Shore Volcanic Group as described by Jirsa (1984). Cross-bedded sediments with a gabbroic composition have been found at six locations within the Duluth Complex (most recently shown on a map in Severson, 1995; and in Severson and Miller, 1999). The last stop of this field trip is at one of the more “easily” accessible crossbedded localities. At this locale, the cross-bedded sediments exhibit shallow dips to the northeast and only about 10 feet of stratigraphic section are exposed. The best exposures are present in the northern Colvin Creek Body which is located about two miles to the west of this stop in sections 27, 33, and 34, T.59N., R.13W. A more complete description of the units exposed at the northern body, and a thorough examination of their origin, can be found in Patelke (1996). Figure 8-19: Scanned polished thin section image and photomicrographs of X-BDD Unit of the Northern Colvin Creek Body. Page 255 �References Allen, D.J., Hinze, W.J., Dickas, A.B., and Mudrey, M.G., Jr., 1997, Integrated geophysical modeling of the North American Midcontinent Rift System: New interpretations for western Lake Superior, northwestern Wisconsin, and eastern Minnesota. In: Ojakangas, R.J., Dickas, A.B., Green, J.C., (eds.) Middle Proterozoic to Cambrian Rifting, Central North America: Geological Society of America Special Paper 312, p.47-72. Bonnichsen, B., 1972, Southern part of the Duluth Complex. In: Sims, P.K. & Morey, G.B. (eds.) Geology of Minnesota - A centennial volume. Minnesota Geological Survey, p. 361-388 Bonnichsen, B., 1974a. Copper and nickel resources in the Duluth Complex, northeastern Minnesota. Minnesota Geological Survey Information Circular 10, 24p. Bonnichsen, B., 1974b. Geology of the Ely-Hoyt Lakes district, northeastern Minnesota. Unpublished report by the Minnesota Geological Survey for the Minnesota Department of Natural Resources, 30 p. Bonnichsen, B., Fukui, L.M., and Chang, L.L.Y., 1980, Geologic setting, mineralogy, and geochemistry of magmatic sulfides, South Kawishiwi Intrusion, Duluth Complex, Minnesota: Proceedings, IAGOD Symposium, 5th Stuttgart, Germany, p. 545-565. Boudreau, A.E., and McCallum, I.S., 1992, Concentration of platinum group-elements by magmatic fluids in layered intrusions: Economic Geology, v. 87, p.1830-1848 Chalokwu, C.I. & Grant, N.K., 1990. Petrology of the Partridge River intrusion, Duluth Complex, Minnesota: I. Relationships between mineral compositions, density, and trapped liquid abundance. J. Petrology 31, 265-93. Chalokwu, C.I. & Grant, N.K., Ariskin, A.A., & Barmina, G.S., 1993., Simulation of primary phase relations and mineral compositions in the Partridge River intrusion, Duluth Complex, Minnesota: implications for the parent magma composition. Contrib. Mineral. Petrol. 114, 539-49. Chandler, V.W., 1990, Geologic interpretation of gravity and magnetic data over the central part of the Duluth Complex, northeastern Minnesota. Economic Geology, v. 85, p. 816-829 Foose, M.P., 1984, Drill logs and correlations within part of the South Kawishiwi intrusion, Duluth, Minnesota. U.S. Geological Survey Open-file Report 84-14, 230p. Foose, M.P., and Cooper, R.W., 1978, Preliminary geologic report on the Harris Lake area, northeastern Minnesota. U.S. Geological Survey Open-file Report 78-385, 24pp. with map, scale 1:12,000 Foose, M.P., and Cooper, R.W., 1981, Faulting and fracturing in part of the Duluth Complex, northeastern Minnesota. Canadian Journal of Earth Science, v. 18, p. 810-814. Geerts, S.D., 1991. Geology, stratigraphy, and mineralization of the Dunka Road Cu-Ni prospect, northeastern Minnesota. Natural Resources Research Institute, University of Minnesota, Duluth, Technical Report, NRRI/TR-91-14, 63p. Geerts, S.D., 1994, Petrography and geochemistry of a platinum group element-bearing horizon in the Dunka Road prospect, (Keweenawan) Duluth Complex, northeastern Minnesota: Unpubl. M.S. thesis, Univ. Minn., Duluth, 155 p. Green, J.C., Phinney, W.C., & Weiblen, P.W., 1966. Gabbro Lake quadrangle, Lake County, Minnesota. Minnesota Geological Survey Miscellaneous Map M-2, scale 1:24,000 Grout, F.F., 1918, Internal structures of igneous rocks; their significance and origin with special reference to the Duluth Gabbro. Journal of Geology 26, 439-458 Hauck, S.A., Miller, J.D., Jr., Severson, M.J., in prep., Petrographic, geochemical, and oxide, sulfide, and platinum-group mineral relationships between the PEG and U3 Units, Birch Lake area, South Kawishiwi intrusion, Duluth Complex, Minnesota: Natural Resources Research Institute, University of Minnesota, Duluth, Technical Report. Jirsa, M. A., 1984, Interflow sedimentary rocks in the Keweenawan North Shore Volcanic Group, northeastern Minnesota. Minnesota Geological Survey Report of Investigations 30, 20 pp. Kilburg, J.A., 1972, Petrology, structure, and correlation of the Upper Precambrian Ely's Peak basalt. Unpublished M.S. thesis, University of Minnesota, Duluth, 97 p. Lee, I., and Ripley, E.M., 1996, Mineralogic and stable isotopic studies of the South Kawishiwi intrusion, Spruce Road Area, Duluth Complex, Minnesota. Journal of Petrology, v. 37, p. 1437-1461. Page 256 �Linscheid, E.K., 1991, The petrology of the Longnose peridotite deposit and its relationship to the Duluth Complex: Unpubl. M.S. thesis, Univ. Minn., Duluth, 121 p. Listerud, W.H., and Meineke, D.G.., 1977, Mineral resources of a portion of the Duluth Complex and adjacent rocks in St. Louis and Lake Counties, northeastern Minnesota: Minnesota Department of Natural Resources, Division of Minerals, Report 93, 74p. Miller, J.D., Jr., and Ripley, E.M., 1996, Layered intrusions of the Duluth Complex, Minnesota, USA. In Cawthorne, R.G., ed., Layered Intrusions: Amsterdam, Elsevier Science, p. 257-301. Miller, J.D., Jr. and Weiblen, P.W., 1990, Anorthositic rocks of the Duluth Complex: Examples of rocks formed from plagioclase crystal mush. Journal of Petrology 31, p. 295-339. Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., and Peterson, D.E., 2001, Geologic map of the Duluth Complex and related rocks, northeastern Minnesota. Minnesota Geological Survey Miscellaneous Map Series, M-119, scale 1:200,000 Miller, J.D., Jr., Severson, M.J., and Foose, M.P., 2002, Bedrock geologic map of the Babbitt NE 7.5' quadrangle, St. Louis and Lake Counties, Minnesota. Minnesota Geological Survey Open-file Map, scale 1:24,000 Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.M., Wahl, T.E., 2002, Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota. Minnesota Geological Survey Report of Investigations 58, 207 p. Naldrett, A.J., 1997, Key factors in the genesis of Noril’sk, Sudbury, Jinchuan, Voisey’s Bay, and other world class Ni-Cu-PGE deposits: implications for exploration: Australian Journal of Earth Sciences, v. 44, p. 283-315. Paces, J.B., and Miller, J.D., Jr., 1993, Precise U-Pb ages of Duluth Complex and related mafic intrusions, northeastern Minnesota: geochonological insights to physical, petrogenetic, paleomagnetic and tectono-magmatic processes associated with the 1.1 Ga Midcontinent rift system: Journal of Geophysical Research, v. 98, no.B8, p. 13,997-14,013. Patelke, R.L., 1996, The Colvin Creek body, a metavolcanic and metasedimentary mafic inclusion in the Keweenawan Duluth Complex, northeastern Minnesota: Unpublished M.S. thesis, University of Minnesota, Duluth, 232 p. Peterson, D.M., 2001, Development of a conceptual model of Cu-Ni-PGE mineralization in a portion of the South Kawishiwi Intrusion, Duluth Complex, Minnesota: Laurentian University – Society of Economic Geologists, Second Annual PGE Workshop, Sudbury, Ontario. Phinney, W.C., 1969, The Duluth Complex in the Gabbro Lake quadrangle, Minnesota. Minnesota Geological Survey Report of Investigation 9, 20 p. Ripley, E.M., 1986, Origin and concentration mechanisms of copper and nickel in Duluth Complex sulfide zones – a dilemma: Economic Geology, v. 81, p. 974-978. Sassani, D.C., 1992, Petrologic and thermodynamic investigation of the aqueous transport of platinumgroup elements during alteration of mafic intrusive rocks: Unpubl. Ph.D. thesis, Washington Univ., St. Louis, MO, 2 vols., 952 p. Sawyer, E.W., 1996, Melt segregation and magma flow in migmatite: Implications for the generation of granite magmas: Earth Sciences, v.87, p. 85-94. Severson, M.J., 1991, Geology, mineralization, and geostatistics of the Minnamax/Babbitt Cu-Ni deposit (Local Boy area), Minnesota, Part I: Geology: Natural Resources Research Institute, University of Minnesota, Duluth, Technical Report NRRI/TR-91/13a, 96 p. (with plates) Severson, M.J., 1994, Igneous stratigraphy of the South Kawishiwi intrusion, Duluth Complex, northeastern Minnesota: Natural Resources Research Institute, University of Minnesota, Duluth, Technical Report NRRI/TR 93/34, 210 p. (with plates) Severson, M.J., 1995, Geology of the southern portion of the Duluth Complex: Natural Resources Research Institute, University of Minnesota-Duluth, Technical Report NRRI/TR 95/26, 185p. (with plates) Severson, M.J., and Barnes, R.J., 1991, Geology, mineralization, and geostatistics of the Minnamax/Babbitt Cu-Ni deposit (Local Boy area), Minnesota, Part II: Mineralization and geostatistics: Natural Resources Research Institute, University of Minnesota, Duluth, Technical Report NRRI/TR-91/13b, 221 p. Page 257 �Severson, M.J., and Hauck, S.A., 1990, Geology, geochemistry, and stratigraphy of a portion of the Partridge River intrusion: Natural Resources Research Institute, University of Minnesota-Duluth, Technical Report, NRRI/GMIN-TR-89-11, 236p. (with plates). Severson, M.J., and Hauck, S.A., 1997, Igneous stratigraphy and mineralization in the basal portion of the Partridge River intrusion, Duluth Complex, Allen Quadrangle, Minnesota: Natural Resources Research Institute, Univ. Minn., Duluth, Tech. Rept. NRRI/TR-97/19, 102 p. Severson, M.J., and Hauck, S.A., 2003, Platinum group elements (PGEs) and platinum group minerals (PGMs) in the Duluth Complex: Natural Resources Research Institute, Univ. Minn., Duluth, Tech. Rept. NRRI/TR-2003/37, 296p. Severson, M.J., and Miller, J.D., Jr., 1999, Bedrock geologic map of Allen quadrangle, St. Louis County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series, M-91, scale 1:24,000 Severson, M.J., and Zanko, L.M., in prep., The Babbitt Cu-Ni deposit, Part D: Footwall hosted massive sulfide at the Local Boy ore zone (revisited): Natural Resources Research Institute, University of Minnesota, Duluth, Technical Report Severson, M.J., Patelke, R.L., Hauck, S.A., and Zanko, L.M., 1994, The Babbit copper-nickel deposit, Part B: structural datums: Natural Resources Research Institute, University of Minnesota, Duluth, Technical Report, NRRI/TR-94/21b, 48p. (with plates) Taylor, R. B., 1964. Geology of the Duluth Gabbro Complex near Duluth, Minnesota. Minnesota Geological Survey Bulletin 44, 63 pp. Theriault, R.D., Barnes, S.-J., and Severson, M.J., 1997, The influence of country-rock assimilation and silicate to sulfide ratios (R factor) on the genesis of the Dunka Road Cu-Ni-platinum-group element deposit, Duluth Complex, Minnesota: Canadian Journal of Earth Science, v. 34, p. 375-389. Theriault, R.D., Barnes, S.-J., and Severson, M.J., 2000, Origin of Cu-Ni-PGE sulfide mineralization in the Partridge River intrusion, Duluth Complex, Minnesota: Economic Geology, v. 95, p. 929-943. Tyson, R.M., 1976, The mineralogy and petrology of the Partridge River troctolite in the Babbitt-Hoyt Lakes region of the Duluth Complex, northeastern Minnesota. Unpublished Ph.D. dissertation, Cornell University, 179p. Watowich, S.N., Malcolm, J.B., and Parker, P.D., 1981, A review of the Duluth Gabbro Complex as a domestic source of critical and strategic metals. Society of Mining Engineers of AIME, preprint 81351, 9 p. Zanko, L.M., Severson, M.J., and Ripley, E.M., 1994, Geology and mineralization of the Serpentine copper-nickel deposit, Duluth Complex, Minnesota. Natural Resources Research Institute, University of Minnesota, Duluth, Technical Report, NRRI/GMIN-TR-93-52, 90p. Page 258 � Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology: Proceedings, 2004 Description An account of the resource Institute on Lake Superior Geology. Duluth, Minnesota. May 4-9, 2004. Creator An entity primarily responsible for making the resource Institute on Lake Superior Geology Date A point or period of time associated with an event in the lifecycle of the resource 2004 Format The file format, physical medium, or dimensions of the resource PDF Language A language of the resource English https://digitalcollections.lakeheadu.ca/files/original/22858aa3bc9f4b3560ee5a63f7e169ab.pdf ad8628deb5689276c27baa82945c876c PDF Text Text 51st ANNUAL MEETING Nipigon, Ontario - May 24-28, 2005 INSTITUTE ON LAKE SUPERIOR GEOLOGY e- <(j. . I Part 1 – Proceedings and Abstracts 51st ILSG Nipigon 2005 wwwIakesuperiorgeology.org �51st ANNUAL MEETING INSTITUTE ON LAKE SUPERIOR GEOLOGY May 24-28, 2005 Nipigon, Ontario HOSTED BY: Mark Smyk and Pete Hollings Co-Chairs Ontario Geological Survey and Lakehead University Proceedings - Volume 51 Part 1 – Proceedings and Abstracts Edited by Mike Easton (Ontario Geological Survey) & Pete Hollings (Lakehead University) Cover Photos: Left - pahoehoe texture in basalts of the Osler volcanic group, Wilson Island, Middle - diabase sills on the shores of Lake Nipigon, Right - No. 1 Shaft headframe, MacLeod-Cockshutt Mine, Geraldton. �Proceedings of the 51st ILSG Annual Meeting - Part 1 51ST INSTITUTE ON LAKE SUPERIOR GEOLOGY VOLUME 51 CONSISTS OF: PART 1: PROGRAM AND ABSTRACTS PART 2: FIELD TRIP GUIDEBOOK TRIP 1: GEOLOGY AND FOLD MINERALISATION OF THE BEARDMORE-GERALDTON GREENSTONE BELT TRIP 2: QUATERNARY GEOLOGY OF THE BEARDMORE – NIPIGON AREA TRIPS 3 & 6: A STRATIGRAPHIC TRANSECT ACROSS THE NORTHERN FLANK OF THE MIDCONTINENT RIFT NEAR ROSSPORT TRIP 4: GEOLOGY AND RARE ELEMENT PEGMATITES OF THE QUETICO SUBPROVINCE NEAR NIPIGON TRIP 5: GEOLOGY OF THE BLACK STURGEON AREA Reference to material in Part 1 should follow the example below: Albers, P.B., and Miller, J.D.., 2005. The Geology and Petrology of the Leveaux Porphyritic Diorite, Cook County, MN: Investigating Possible Magmatic Relationships to the Anorthositic Series of the Duluth Complex. In; Easton, M. and Hollings, P. (Eds.), Institute on Lake Superior Geology Proceedings, 51st Annual Meeting, Nipigon, Ontario, Part 1 - Proceedings and Abstracts, v.51, part 1, 3-4. Published by the 51st Institute on Lake Superior Geology and distributed by the ILSG Secretary: Pete Hollings - ILSG Secretary Department of Geology Lakehead University 955 Oliver Road Thunder Bay, ON P7B 5E1 Canada Email: peter.hollings@lakeheadu.ca ILSG website: www.lakesuperiorgeology.org ISSN 1042-9964 -i- �Proceedings of the 51st ILSG Annual Meeting - Part 1 Table of Contents Institutes on Lake Superior Geology, 1955-2005 ............................................................. iii Constitution of the Institute on Lake Superior Geology .....................................................v By-Laws of the Institute on Lake Superior Geology ....................................................... vii Goldich Medal Guidelines .............................................................................................. viii Goldich Medallists ............................................................................................................ ix Goldich Medal Committee .................................................................................................x Citation for Goldich Medal Recipient.................................................................................x Eisenbrey Student Travel Awards ..................................................................................... xi Eisenbrey Student Travel Award Application .................................................................. xii Student Paper Awards ..................................................................................................... xiii Student Paper Awards Committee................................................................................... xiii Session Chairs ................................................................................................................. xiii Membership Criteria for the Institute on Lake Superior Geology .................................. xiv Board of Directors.............................................................................................................xv Local Committee...............................................................................................................xv Banquet Speaker ...............................................................................................................xv Report of the Chair of the 50th Annual Meeting ........................................................... xvi Acknowledgements ....................................................................................................... xviii Program ........................................................................................................................... xix Abstracts .............................................................................................................................1 Author Index .....................................................................................................................70 - ii - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Institutes on Lake Superior Geology, 1955-2005 # Date Place Chairs 1 1955 Minneapolis, Minnesota C.E. Dutton 2 1956 Houghton, Michigan A.K. Snelgrove 3 1957 East Lansing, Michigan B.T. Sandefur 4 1958 Duluth, Minnesota R.W. Marsden 5 1959 Minneapolis, Minnesota G.M. Schwartz & C. Craddock 6 1960 Madison, Wisconsin E.N. Cameron 7 1961 Port Arthur, Ontario E.G. Pye 8 1962 Houghton, Michigan A.K. Snelgrove 9 1963 Duluth, Minnesota H. Lepp 10 1964 Ishpeming, Michigan A.T. Broderick 11 1965 St. Paul, Minnesota P.K. Sims & R.K. Hogberg 12 1966 Sault Ste. Marie, Michigan R.W. White 13 1967 East Lansing, Michigan W.J. Hinze 14 1968 Superior, Wisconsin A.B. Dickas 15 1969 Oshkosh, Wisconsin G.L. LaBerge 16 1970 Thunder Bay, Ontario M.W. Bartley & E. Mercy 17 1971 Duluth, Minnesota D.M. Davidson 18 1972 Houghton, Michigan J. Kalliokoski 19 1973 Madison, Wisconsin M.E. Ostrom 20 1974 Sault Ste. Marie, Ontario P.E. Giblin 21 1975 Marquette, Michigan J.D. Hughes 22 1976 St. Paul, Minnesota M. Walton 23 1977 Thunder Bay, Ontario M.M. Kehlenbeck 24 1978 Milwaukee, Wisconsin G. Mursky 25 1979 Duluth, Minnesota D.M. Davidson 26 1980 Eau Claire, Wisconsin P.E. Myers 27 1981 East Lansing, Michigan W.C. Cambray 28 1982 International Falls, Minnesota D.L. Southwick 29 1983 Houghton, Michigan T.J. Bornhorst 30 1984 Wausau, Wisconsin G.L. LaBerge 31 1985 Kenora, Ontario C.E. Blackburn 32 1986 Wisconsin Rapids, Wisconsin J.K. Greenberg 33 1987 Wawa, Ontario E.D. Frey & R.P. Sage - iii - �Proceedings of the 51st ILSG Annual Meeting - Part 1 # Date Place Chairs 34 1988 Marquette, Michigan J. S. Klasner 35 1989 Duluth, Minnesota J.C. Green 36 1990 Thunder Bay, Ontario M.M. Kehlenbeck 37 1991 Eau Claire, Wisconsin P.E. Myers 38 1992 Hurley, Wisconsin A.B. Dickas 39 1993 Eveleth, Minnesota D.L. Southwick 40 1994 Houghton, Michigan T.J. Bornhorst 41 1995 Marathon, Ontario M.C. Smyk 42 1996 Cable, Wisconsin L.G. Woodruff 43 1997 Sudbury, Ontario R.P. Sage & W. Meyer 44 1998 Minneapolis, Minnesota J.D. Miller & M.A. Jirsa 45 1999 Marquette, Michigan T.J. Bornhorst & R.S. Regis 46 2000 Thunder Bay, Ontario S.A. Kissin & P. Fralick 47 2001 Madison, Wisconsin M.G. Mudrey & Jr., B.A. Brown 48 2002 Kenora, Ontario P. Hinz & R.C. Beard 49 2003 Iron Mountain, Michigan L. Woodruff & W.F. Cannon 50 2004 Duluth, Minnesota S. Hauck & M. Severson 51 2005 Nipigon, Ontario M. Smyk & P. Hollings - iv - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Constitution of the Institute on Lake Superior Geology (Last amended by the Board—May 6, 2004) Article I - Name The name of the organization shall be the “Institute on Lake Superior Geology”. Article II - Objectives The objectives of this organization are: A. To provide a means whereby geologists in the Great Lakes region may exchange ideas and scientific data. B. To promote better understanding of the geology of the Lake Superior region. C. To plan and conduct geological field trips. Article III - Status No part of the income of the organization shall insure to the benefit of any member or individual. In the event of dissolution, the assets of the organization shall be distributed to _________ (some tax free organization). (To avoid Federal and State income taxes, the organization should be not only “scientific” or “educational”, but also “non-profit”) Minn. Stat. Anno. 290.01, subd. 4 Minn. Stat. Anno. 290.05(9) 1954 Internal Revenue Code s.501(c)(3) Article IV - Membership The membership of the organization shall consist of persons who have registered for an annual meeting within the past three years, and those who indicate interest in being a member according to guidelines approved by the Board of Directors. Article V - Meetings The organization shall meet once a year. The place and exact date of each meeting will be designated by the Board of Directors. Article VI - Directors The Board of Directors shall consist of the Chair, Secretary, Treasurer, and the last three past Chairs; but if the board should at any time consist of fewer than six persons, by reason of unwillingness or inability of any of the above persons to serve as directors, the vacancies on the board may be filled by the Chair so as to bring the membership of the board to six members. Article VII - Officers The officers of this organization shall be a Chair, a Secretary and a Treasurer. A. The Chair shall be elected each year by the Board of Directors, who shall give due consideration to the wishes of any group that may be promoting the next annual meeting. His/her term of office as Chair will terminate at the close of the annual meeting over which he/she presides, or when his/her successor shall have been appointed. He/she will then serve for a period of three years as a member of the Board of Directors. -v- �Proceedings of the 51st ILSG Annual Meeting - Part 1 B. The Secretary shall be elected at the annual meeting. His/her term of office shall be four years, or until his/her successor shall have been appointed. C. The Treasurer shall be elected at the annual meeting. His/her term of office shall be four years, or until his/her successor shall have been appointed. The terms of the Secretary and Treasurer shall be staggered so that there will always be a two year overlap between the two. Article VIII - Amendments This constitution may be amended by a majority vote (majority of those voting) of the membership of the organization. - vi - �Proceedings of the 51st ILSG Annual Meeting - Part 1 By-Laws of the Institute on Lake Superior Geology (Last amended by the Board—May 6, 2004) I. Duties of the Officers and Directors A. It shall be the duty of the Annual Chairman to: 1. Preside at the annual meeting. 2. Appoint all committees needed for the organization of the annual meeting. 3. Assume complete responsibility for the organization and financing of the annual meeting over which he/she presides. B. It shall be the duty of the Secretary to: 1. Keep accurate attendance records of all annual meetings. 2. Keep accurate records of all meetings of, and correspondence between, the Board of Directors. 3. Maintain an up-to-date mailing list 4. Act as the first point of contact for all enquires about the Institute 5. Respond to requests for back issues of the Proceedings of the Institute C. It shall be the duty of the Treasurer to: 1. Hold all funds that may accrue as profits from annual meetings or field trips and to make these funds available for the organization and operation of future meetings as required. 2. Store the Goldich medals and each year ensure one is engraved with the name of that years winner D. It shall be the duty of the Board of Directors to plan locations of annual meetings and to advise on the organization and financing of all meetings. II. Duties and Expenses A. Regular membership dues of $5.00 or less on an annual basis shall be assessed each member as determined by the Board of Directors.. B. Registration fees for the annual meetings shall be determined by the Chair in consultation with the Board of Directors. The registration fees can include expenses to cover operations outside of the annual meeting as determined by the Board of Directors. It is strongly recommended that registration fees be kept at a minimum to encourage attendance of students. III. Rules of Order The rules contained in Robert’s Rules of Order shall govern this organization in all cases to which they are applicable. IV. Amendments These by-laws may be amended by a majority vote (majority of those voting) of the membership of the organization; provided that such modifications shall not conflict with the constitution as presently adopted or subsequently amended. - vii - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Goldich Medal Guidelines (Adopted by the Board of Directors, 1981; amended 1999) Preamble The Institute on Lake Superior Geology was born in 1955, as documented by the fact that the 27th annual meeting was held in 1981. The Institute’s continuing objectives are to deal with those aspects of geology that are related geographically to Lake Superior; to encourage the discussion of subjects and sponsoring field trips that will bring together geologists from academia, government surveys, and industry; and to maintain an informal but highly effective mode of operation. During the course of its existence, the membership of the Institute (that is, those geologists who indicate an interest in the objectives of the ILSG by attending) has become aware of the fact that certain of their colleagues have made particularly noteworthy and meritorious contributions to the understanding of Lake Superior geology and mineral deposits. The first award was made by ILSG to Sam Goldich in 1979 for his many contributions to the geology of the region extending over about 50 years. Subsequent medallists and this year’s recipient are listed in the table below. Award Guidelines 1) The medal shall be awarded annually by the ILSG Board of Directors to a geologist whose name is associated with a substantial interest in, and contribution to, the geology of the Lake Superior region. 2) The Board of Directors shall appoint the Goldich Medal Committee. The initial appointment will be of three members, one to serve for three years, one for two years, and one for one year. The member with the briefest incumbency shall be chair of the Nominating Committee. After the first year, the Board of Directors shall appoint at each spring meeting one new member who will serve for three years. In his/her third year this member shall be the chair. The Committee membership should reflect the main fields of interest and geographic distribution of ILSG membership. The out-going, senior member of the Board of Directors shall act as liaison between the Board and the Committee for a period of one year. 3) By the end of November, the Goldich Medal Committee shall make its recommendation to the Chair of the Board of Directors, who will then inform the Board of the nominee. 4) The Board of Directors normally will accept the nominee of the Committee, inform the medallist, and have one medal engraved appropriately for presentation at the next meeting of the Institute. 5) It is recommended that the Institute set aside annually from whatever sources, such funds as will be required to support the continuing costs of this award. Nominating Procedures 1) The deadline for nominations is November 1. Nominations shall be taken at any time by the Goldich Medal Committee. Committee members may themselves nominate candidates; however, Board members may not solicit for or support individual nominees. 2) Nominations must be in writing and supported by appropriate documentation such as letters of recommendation, lists of publications, curriculum vita’s, and evidence of contributions to Lake Superior geology and to the Institute. 3) Nominations are not restricted to Institute attendees, but are open to anyone who has worked on and contributed to the understanding of Lake Superior geology. - viii - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Selection Guidelines 1) Nominees are to be evaluated on the basis of their contributions to Lake Superior geology (sensu lato) including: a) importance of relevant publications; b) promotion of discovery and utilization of natural resources; c) contributions to understanding of the natural history and environment of the region; d) generation of new ideas and concepts; and e) contributions to the training and education of geoscientists and the public. 2) Nominees are to be evaluated on their contributions to the Institute as demonstrated by attendance at Institute meetings, presentation of talks and posters, and service on Institute boards, committees, and field trips. 3) The relative weights given to each of the foregoing criteria must remain flexible and at the discretion of the Committee members. 4) There are several points to be considered by the Goldich Medal Committee: a) An attempt should be made to maintain a balance of medal recipients from each of the three estates— industry, academia, and government. b) It must be noted that industry geoscientists are at a disadvantage in that much of their work in not published. 5) Lake Superior has two sides, one the U.S., and the other Canada. This is undoubtedly one of the Institute’s great strengths and should be nurtured by equitable recognition of excellence in both countries. Goldich Medalists 1979 Samuel S. Goldich 1992 William F. Cannon 1980 not awarded 1993 Donald W. Davis 1981 Carl E. Dutton, Jr. 1994 Cedric Iverson 1982 Ralph W. Marsden 1995 Gene LaBerge 1983 Burton Boyum 1996 David L. Southwick 1984 Richard W. Ojakangas 1997 Ronald P. Sage 1985 Paul K. Sims 1998 Zell Peterman 1986 G.B. Morey 1999 Tsu-Ming Han 1987 Henry H. Halls 2000 John C. Green 1988 Walter S. White 2001 John S. Klasner 1989 Jorma Kalliokoski 2002 Ernest K. Lehmann 1990 Kenneth C. Card 2003 Klaus J. Schulz 1991 William Hinze 2004 Paul Wieblen 2005 Goldich Medal Recipient Mark Smyk Ontario Geological Survey, Thunder Bay, Ontario - ix - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Goldich Medal Committee Serving through the meeting year shown in parentheses David Meineke (2005) Meriden Engineering, Hibbing, Minnesota George Hudak (2006) University of Wisconsin, Oshkosh Tom Hart (2007) Ontario Geological Survey David Meineke, as out-going senior member of Institute Board of Directors, is liaison between Goldich Medal Committee and the Board through the 2005 meeting. Citation for Goldich Medal Recipient Mark Smyk It is a pleasure to acknowledge the many contributions of Mark C. Smyk to the understanding of the geology of the Lake Superior region at the 51st annual meeting of the Lake superior Institute on Lake Superior Geology. This is the 26th Goldich medal awarded by the Institute to individuals that have made significant contributions to Lake Superior geology. Mark was born in Dryden, Ontario, in 1961 and received his Honors Bachelor of Science degree in Geology from Lakehead University in 1984 upon completion of a thesis titled “A comparative study of silver occurrences, Island Belt Silver Region, Thunder Bay District, Ontario”. From Lakehead University Mark entered Carleton University where he received his Masters of Science degree in geology upon completion of a thesis on silver veins in the Cobalt silver area in 1987. The thesis was titled “Geology of Archean Interflow Sedimentary Rocks and their relationship to Ag-Bi-Co-Ni-As Veins, Cobalt Silver Area, Ontario”. Mark completed contract geological mapping in the Schreiber-Hemlo and Swayze greenstone belts in 1984. He has also completed contracts with David Bell Geological Services Inc. and Saarberg-Interplan Ltd. From 1998 to the present Mark has served as guest lecturer at Lakehead University, Thunder Bay. He has also lectured and prepared course curriculum in geology for the Qikiqtaaluk Corporation, Iqalut, Northern Territory. Mark has long been active in the Institute on Lake Superior Geology and co-hosted the 41st and 51st annual meetings. He is a Registered Professional Geologist in the Province of Ontario, fellow in the Geological Association of Canada, member of the Northwest Prospectors Association and guest editor for “Exploration and Mining Geology”, Canadian Institute of Mining and Metallurgy. Since 1987 Mark has worked with the Ontario Geological Survey. During this period he has authored or co-authored 7 reports, 14 abstracts and 7 guidebooks. The Ontario Geological Survey library lists 62 items as authored or co-authored by Mark. Since working with the Ontario Geological Survey Mark has advanced to become Regional Resident Geologist, Thunder Bay North. His work entails geology related publications, land use planning and interaction with the public, prospectors and industry. Submitted by Ron Sage -x- �Proceedings of the 51st ILSG Annual Meeting - Part 1 Eisenbrey Student Travel Awards The 1986 Board of Directors established the ILSG Student Travel Awards to support student participation at the annual meeting of the Institute. The name “Eisenbrey” was added to the award in 1998 to honor Edward H. Eisenbrey (1926-1985) and utilize substantial contributions made to the 1996 Institute meeting in his name. “Ned” Eisenbrey is credited with discovery of significant volcanogenic massive sulfide deposits in Wisconsin, but his scope was much broader—he has been described as having unique talents as an ore finder, geologist, and teacher. These awards are intended to help defray some of the direct travel costs of attending Institute meetings, and include a waiver of registration fees, but exclude expenses for meals, lodging, and field trip registration. The number of awards and value are determined by the annual Chair in consultation with the Secretary and Treasurer. Recipients will be announced at the annual banquet. The following general criteria will be considered by the annual Chair, who is responsible for the selection: 1) The applicants must have active resident (undergraduate or graduate) student status at the time of the annual meeting of the Institute, certified by the department head. 2) Students who are the senior author on either an oral or poster paper will be given favored consideration. 3) It is desirable for two or more students to jointly request travel assistance. 4) In general, priority will be given to those in the Institute region who are farthest away from the meeting location. 5) Each travel award request shall be made in writing to the annual Chair, and should explain need, student and author status, and other significant details. The form below is optional. Successful applicants will receive their awards during the meeting. - xi - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Eisenbrey Student Travel Award Application Student Name : __________________________________ Address: Date: ____________ __________________________________________________________ __________________________________________________________ __________________________________________________________ __________________________________________________________ email: __________________________________________________________ Educational status: _____________________________________________________ Are you the senior author of an oral presentation or poster? Yes ____ No _____ Will any other students be traveling with you? Yes ____ No _____ If yes, then who? ___________________________________________________ ___________________________________________________ Statement of need (use additional page if necessary): __________________________ _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ Signature: ____________________________________________________ Department Head: ____________________________________________________ - xii - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Student Paper Awards Each year, the Institute selects the best of the student presentations and honors presenters with a monetary award. Funding for the award is generated from registrations of the annual meeting. The Student Paper Committee is appointed by the annual meeting Chair in such a manner as to represent a broad range of professional and geologic expertise. Criteria for best student paper—last modified by the Board in 2001—follow: 1) The contribution must be demonstrably the work of the student. 2) The student must present the contribution in-person. 3) The Student Paper Committee shall decide how many awards to grant, and whether or not to give separate awards for poster vs. oral presentations. 4) In cases of multiple student authors, the award will be made to the senior author, or the award will be shared equally by all authors of the contribution. 5) The total amount of the awards is left to the discretion of the meeting Chair in conjunction with the Secretary, but typically is in the amount of about $500 US (increase approved by Board, 10/01). 6) The Secretary maintains, and will supply to the Committee, a form for the numerical ranking of presentations. This form was created and modified by Student Paper Committees over several years in an effort to reduce the difficulties that may arise from selection by raters of diverse background. The use of the form is not required, but is left to the discretion of the Committee. 7) The names of award recipients shall be included as part of the annual Chair’s report that appears in the next volume of the Institute. Student papers will be noted on the Program. Student Paper Awards Committee Penelope Morton, University of Minnesota Duluth Greg Stott - Ontario Geological Survey, Sudbury, Ontario Wally Rayner - Toronto, Ontario Session Chairs Charlie Blackburn, Blackburn Geological Services Terry Boerboom, Minnesota Geological Survey William Cannon, United States Geological Survey Mike Easton, Ontario Geological Survey Mary Louise Hill, Lakehead University Peter Hinz, Ontario Geological Survey Tom Lane, CAMIRO Mark Severson, Natural Resources Research Institute - xiii - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Membership Criteria for the Institute on Lake Superior Geology Approved May 8, 1997. Amended by the Board—May 6, 2004 A. Membership in the Institute on Lake Superior Geology requires either participation in Institute activities, or an indication on a regular basis of interest in the Institute. Those individuals registering for an annual meeting will remain as members for 4 years unless: 1) they indicate no further interest in the Institute by responding negatively to the statement on meeting circulars “Remove my name from the mailing list”; or 2) two successive mailings in different years are returned by the postal service as address unknown. B. Those individuals who have not registered for an annual meeting in the past 4 years must indicate an interest in the Institute by postal, electronic, or verbal correspondence with the Secretary at least once every two years. Such individuals will be removed from the membership if they indicate no further interest in the Institute or two successive mailing in different years are returned by the postal service as address unknown. C. The Secretary will maintain a list of current members. The list will include the date of the beginning of continuous membership, dates of returned mail, dates of last contact (expression of interest), and the date membership expires, barring a change of status initiated by the member. Those individuals who have become members of ILSG by Section B will have an expiration date listed at 2 years from the upcoming meeting. For example, a member who expresses interest in September of 1997 (the next annual meeting is May, 1998) will have an expiration date of May, 2000, unless the member contacts the Secretary or attends an annual meeting. D. “Member for Life” status is granted to individuals who have been (nearly) continuous participants of the ILSG meetings for 15 years, Goldich Medal recipients, or those who have served as meeting chairs. This status will be further maintained unless the individuals indicate no further interest in the Institute, or 4 mailings in different years are returned by the postal service as address unknown, or they are deceased. E. All members will be mailed the First Circular for the Annual Meeting and the ILSG Newsletter. The Chair of the annual meeting may opt to send the first circular to additional individuals. All returned mail should be reported to the Secretary. F. The Secretary can designate any individual who is on the ILSG membership list (mailing list) as of January 1, 1997 as a member for life based on participation in ILSG activities. G. Members are strongly encouraged to send address corrections to the Secretary to avoid unintentional lapse of membership. - xiv - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Board of Directors Board appointment continues through the close of the meeting year shown in parentheses, or until a successor is selected Mark Smyk/Pete Hollings - General Chair 2005 meeting (2008) - Ontario Geological Survey/Lakehead University Steve Hauck (2007) - University of Minnesota, Duluth Laurel Woodruff (2006) - U.S. Geological Survey Peter Hinz (2005) - Ontario Geological Survey Peter Hollings - Secretary (2006) - Lakehead University, Thunder Bay, Ontario Mark A. Jirsa - Treasurer (2007) - Minnesota Geological Survey Local Committee Co-Chairs Mark Smyk - Ontario Geological Survey, Thunder Bay, Ontario Pete Hollings - Lakehead University, Thunder Bay, Ontario Program and Abstracts Editors Mike Easton - Ontario Geological Survey, Sudbury, Ontario Pete Hollings - Lakehead University, Thunder Bay, Ontario Field Trip Guidebook Editor Pete Hollings - Lakehead University, Thunder Bay, Ontario Organising Committee Mary Louise Hill - Lakehead University, Thunder Bay, Ontario Phil Fralick - Lakehead University, Thunder Bay, Ontario Bill Addison - Thunder Bay, Ontario Ryan Tuomi - Ontario Geological Survey, Thunder Bay, Ontario Levina Collins - Township of Nipigon Banquet Speaker Jim Franklin, Franklin Geosciences Ltd. Mineral Resources for the Future: The Resource Potential of Northern Lake Superior (See abstract on page 19) - xv - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Report of the Chair of the 50th Annual Meeting Duluth, Minnesota The Natural Resources Research Institute (NRRI) of the University of Minnesota Duluth hosted the 50th Annual Institute on Lake Superior Geology on May 4 – 9, 2004 at the Radisson Hotel Duluth-Harborview in Duluth, Minnesota. The meeting consisted of two days of technical sessions with four pre- and four posttechnical session field trips. Julie Ann Heinz, Barbara Hauck, and Jeanne Lukkarila provided excellent on-site assistance. John Heine was responsible for: the 50th ILSG website and online registration; AV expertise that kept the sessions flowing on time; and worked with Mark Severson on logistics for the field trips. Julie Oreskovich was the poster session czar. Richard Patelke was invaluable as the data base manager and the designer of the 50th ILSG T-shirt. Bill Cannon and Gene LaBerge compiled a CD of photographs of past ILSG members and field trips. Other members of the local committee included: Charlie Matsch, Jim Miller, Penny Morton, Dean Peterson, and Larry Zanko – all of whom provided much need expertise. Pre-meeting registration was 275 students and professionals, with an additional 24 on-site registrations, for a total of 299 registrants. This was the first time that on-line registration was offered, and 222 individuals registered on-line. Proceedings Volume 50 was published in two parts: Part 1 – Program and Abstracts, edited by Steven Hauck, with 69 published abstracts, for 39 oral and 30 poster presentations; and Part 2 – Field Trip Guidebook, edited by Mark Severson. The 50th meeting marked the sixth time (1958, 1963, 1971, 1979, 1989, 2004) the ILSG annual meeting was held in Duluth, Minnesota. The nine field trips (one trip with two sections) were well attended with 315 total participants attending one or two field trips. On Tuesday, May 4th, marked the beginning of two two-day field trips: George Hudak and co-leaders lead a field trip to examine the volcanic stratigraphy, hydrothermal alteration, and VMS potential of the Lower Ely Greenstone; and Terry Boerboom and co-leaders lead a trip to visit newly mapped outcrops in the Southwestern Sequence of the North Shore Volcanic Group and in the Beaver Bay Complex. On Wednesday, May 5th, there were two one-day field trips: Howard Hobbs led a trip to look at the Late Wisconsinan Superior-lobe deposits northeast of Duluth; and Richard Ojakangas, Mark Severson, and co-leaders examined the geology of the eastern Mesabi Range. On Saturday, May 10th, there were four field trips: Mark Jirsa, Terry Boerboom, and co-leaders led two sections on a two-day investigation of the classic outcrops of northeastern Minnesota; Lisa Marlow and co-leaders examined the glacial and post-glacial landscape evolution in the Glacial Lake Aitkin and Upham basin; Dean Peterson and Richard Patelke led a field trip to the Vermilion District, northeast of Soudan, MN, to look at the economic geology of Archean gold occurrences; and Mark Severson and Jim Miller visited outcrops along the western contact of the Duluth Complex to illustrate the geology and Cu-Ni mineralization. One hundred and sixty-three participants attended the annual banquet on Thursday night. This year’s banquet speaker was Bob Dott of the University of Wisconsin, Madison. Dr. Dott is professor emeritus from the Department of Geology and Geophysics. Dr. Dott’s post-banquet presentation was: The Van Hise army and other pioneers of Lake Superior geology. The other highlight of the evening was the presentation of the 2004 Goldich medal to Paul Weiblen of the Department of Geology and Geophysics at the University of Minnesota, Minneapolis, which recognized his efforts in many areas in the Lake Superior region, especially in the Duluth Complex. The technical sessions began with a special session on “The History of Geologic Investigations in the Lake Superior Region”. The “Old Prospector” (Richard Ojakangas) gave a brief historical overview of geology between 1848 and 1900 that was followed by four other presentations on the geology, mineralization, and geochronology of the Lake Superior region. As always, the student paper committee had a difficult time of picking a winner. There were six oral presentations and four poster presentations. The winners were: 1) Andy Breckenridge – Large Lakes Observatory ($250, Winner, best oral presentation) - xvi - �Proceedings of the 51st ILSG Annual Meeting - Part 1 2) Heidi Drexler – University of Wisconsin Oshkosh ($125, Winner, best poster presentation) 3) Justin Johnson, Lakehead University ($125, Honourable mention) 4) Adam Hoffman - University of Minnesota ($125, Honourable mention, best poster presentation) In addition five Eisenbrey Student Travel awards were presented in 2004, they winners were: Paula Shafer, Indiana University ($250); Justin Johnson - Lakehead, Thunder Bay ($125); Geoff Heggie - Lakehead, Thunder Bay ($125); Adam Richardson - Lakehead, Thunder Bay ($125); and Riku Metsaranta - - Lakehead, Thunder Bay ($125) The Institute’s Board of Directors met on May 6, 2004, and brief summary of the meeting follows: 1. Accepted the Report of the Chair for the 49th ILSG from Laurel Woodruff, and the minutes of the last Board meeting, May 8, 2003 from Pete Hollings. 2. Accepted the 2003-2004 ILSG Financial Summary from Mark Jirsa. 3. Approved Steven Hauck to continue on as a Board member. 4. Approved Nipigon, Ontario as the 51st annual meeting location with Mark Smyk and Pete Hollings as co-chairs. 5. Replaced Ron Sage, who was on the Goldich Committee with Tom Hart, who had agreed to serve. 6. Discussed splitting the Secretary-Treasurer position. Pete Hollings was nominated and elected secretary by the ILSG members, and Mark Jirsa was nominated and elected treasurer. The 50th ILSG meeting was a great success, and we wish to thank all of the individuals who contributed to this success. The staffs of the Radisson Hotel Duluth-Harborview, UMD Food Service, and the Fortune Bay Casino (field trips) were always professional and responsive to our evolving needs to handle a very large group. The following organizations provided generous monetary contributions: Department of Geosciences, University of Minnesota Duluth, MN; Franconia Minerals Corporation, Spokane, WA; Idea Drilling Incorporated, Virginia, MN; Iron Mining Association, Duluth, MN; Lehmann Exploration Management, Minneapolis, MN; Meriden Engineering, LLC, Hibbing, MN; Minerals Processing Corporation, Duluth, MN; Minnesota Exploration Association (MExA), Minneapolis, MN; Minnesota Minerals Coordinating Committee, St. Paul, MN; Teck Cominco American Incorporated, Spokane, WA; and Wallbridge Mining Company, Lively, ONT. The field trips were very well attended, and our appreciation is due to the field trip leaders, van drivers, and everyone else who contributed to the success of the 50th ILSG meeting. Because of the large attendance for the meeting and field trips, the 50th ILSG meeting generated several thousand dollars to the ILSG general fund. The 50th ILSG meeting also coincided with the 50th anniversary of the Department of Geosciences at the University of Minnesota Duluth (UMD). A special session on Friday afternoon featured papers by UMD faculty, students, and alumni. A banquet celebrating this occasion was held at the Duluth Depot on Friday evening after the ILSG meeting, which was attended by many UMD alumni, faculty, and students, and ILSG members. Both of us are very happy with the results of 50th meeting, and we hope that future ILSG meetings can be as well attended as this meeting. We also hope that the attendees were as happy with the meeting as the local committee was. This meeting, like every ILSG meeting requires a lot of work and time on behalf of the cochairs and the local committee. The additional assistance of ILSG members at-large helped make the meeting a success. Respectively Submitted, Steven Hauck and Mark Severson Co-Chairs, 50th ILSG meeting - xvii - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Acknowledgements The following organizations made generous contributions to the 51st Annual Meeting. We thank them for their commitment to the Institute on Lake Superior Geology. For the past 50 years this organization has thrived as a result of the interest of individuals, corporations, universities and government agencies. The dedication to an exchange of scientific ideas and a passion for field trips has enabled the institute to provide one of it’s primary objectives – to promote better understanding of the geology of the Lake Superior Region. North Western Ontario Prospectors Association Lake Nipigon Region Geoscience Initiative Ontario Prospectors Association Ministry of Northern Development and Mines – Ontario Geological Survey Township of Nipigon Municipality of Greenstone Canadian Institute of Mining and Metallurgy, Thunder Bay Branch Department of Geology, Lakehead University David Malouf, Roxmark Mines Limited Chaltrek Geological Supplies Inc. - xviii - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Program Tuesday May 24 8:00 a.m. Field Trip 1: Geology and gold mineralization of the Beardmore-Geraldton greenstone belt Leaders: Mark Smyk & Tom Hart (OGS) and Phil Fralick (Lakehead University) 6:00 p.m. Overnight in Beardmore Wednesday May 25 8:00 a.m. Field Trip 1: Geology and gold mineralization of the Beardmore-Geraldton greenstone belt Leaders: Mark Smyk & Tom Hart (OGS) and Phil Fralick (Lakehead University) 8:00 a.m. Field Trip 2: Quaternary Geology of the Beardmore – Nipigon area Leader: Peter Barnett (OGS) 8:00 a.m. Field Trip 3: A Stratigraphic transect across the Northern flank of the Midcontinent Rift near Rossport (I) Leaders: Peter Hollings and Phil Fralick (Lakehead University) 6:00 p.m. Return of Trips 1,2 and 3 4:00 p.m. - 8.00 p.m. Registration (Nipigon Community Centre) 7:00 p.m. - 9.00 p.m. Ice Breaker Social (Nipigon Curl-a-Drome) and Poster Setup (Nipigon Community Centre) Thursday May 26 8:00 a.m. - 4:00 p.m. Registration (Nipigon Community Centre) 9:00a.m. - 9:05 a.m. Introductory Remarks - Mark Smyk and Peter Hollings, Co-Chairs Technical Session I Session Chairs: T. Boerboom (Minnesota Geological Survey), W. Cannon (US Geological Survey) 9:05 a.m. Addison, W., Brumpton, G., Vallini, D., McNaughton, N., Davis, D., Kissin, S., Fralick, P. and Hammond, A. Discovery of distal ejecta from the 1850 Ma Sudbury impact 9:25 a.m. Schnieders, B. and Scott, J. Mining and exploration activity in the Thunder Bay South District 9:45 a.m. Hill, M.L. and Smyk, M. Penokean fold-and-thrust deformation of the Paleoproterozoic Gunflint Formation near Thunder Bay, Ontario 10:05 a.m. Middleton, R. and Heggie, G. Seagull Intrusion; A unique PGE-Ni-Cu Setting. 10:25 a.m. - 11:00 a.m. Coffee Break and Poster Session 11:00 a.m. Maric, M.*, and Fralick, P. Sedimentology of the Rove and Virginia Formations and their tectonic significance - xix - �Proceedings of the 51st ILSG Annual Meeting - Part 1 11:20 a.m. Easton, R.M. The Grenvillian Tomiko quartzites of Ontario: correlatives of the Baraboo quartzites of Wisconsin, the Mazatzal orogen of New Mexico, or unique? Implications for the tectonic architecture of Laurentia in the Great Lakes region. 11:40 a.m. Medaris, G. and Dott, B. Riddle of the sands (Proterozoic) solved by quartzites at Hamilton Mounds Wisconsin 12:00 p.m. - 1:30 p.m. Lunch Break and Poster Session (ILSG Board Meeting by invitation) Technical Session II Special Session - Lake Nipigon Regional Geoscience Initiative Session Chairs: Tom Lane (CAMIRO), Mark Severson (Natural Resources Research Institute) 1:30 p.m. Heaman, L. and Easton, R.M. Proterozoic history of the Lake Nipigon area, Ontario: Constraints from U-Pb zircon and baddeleyite dating 1:50 p.m. Smyk, M. Mineral deposits and metallogeny of the Midcontinent Rift in Ontario 2:10 p.m. Fralick, P., Metsaranta, R., and Rogala, B. Stratigraphy of the Mesoproterozoic Sibley Group and Nipigon Sills 2:30 p.m Hart, T. and MacDonald, C. Mesoproterozoic diabase sills of the Nipigon Embayment, northwest Ontario 2:30 p.m. - 3:00 p.m. Coffee Break and Poster Session 3:00 p.m. Richardson, A.* and Hollings, P. Geochemical variation within the Mesoproterozoic Nipigon diabase sills 3:20 p.m. Metsaranta, R.* and Fralick, P. Depositional setting of the Pass Lake and Rossport Formations (Sibley Group) inferred from a combined sedimentologic/geochemical approach 3:40 p.m. Laarman, J.* and Hollings, P. Petrogenesis and PGE mineralization of the Eva Kitto Intrusion, Northern Ontario 4:00 p.m. Magee, A.*, Hollings, P. and Fralick, P. Preliminary stratigraphy and geochemistry of the Mesoproterozoic Pillar Lake volcanics, Wabigoon subprovince, Superior Province, Armstrong, Ontario, Canada 6:00 p.m Annual Banquet and Award Presentation (Royal Canadian Legion) Announcement of 52nd Annual Meeting Location 2005 Goldich Award Presentation to Mark Smyk 2005 Banquet Address - Dr. J. Franklin Meeting participants not registered for the banquet are welcome to attend the address - xx - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Friday May 27 9:00 a.m. - 12:00 p.m. Registration Technical Session III Session Chairs: Mary Louise Hill (Lakehead University), Mike Easton (Ontario Geological Survey) 9:00 a.m. Holm, D., Cannon, W., Chandler, V., Schneider, D., Schulz, K., & Van Schmus, W. The incredible shrinking Penokean orogen: a new look at the accretionary history of the southern Lake Superior region. 9:20 a.m. Vallini, D.*, Cannon, W. and Schulz, K. New age data for the Chocolay Group, Marquette Range Supergroup: implications for the Paleoproterozoic evolution of the Lake Superior and Lake Huron regions 9:40 a.m. Knudson, D.*, Saini-Eidukat, B., Miller, J., and Daniels, P. Structural state of plagioclase phenocrysts in porphyritic rocks of the Midcontinent rift, northeastern Minnesota 10:00 a.m. Albers, P.* and Miller, J. The geology and petrology of the Leveaux porphyritic diorite, Cook County, MN: Investigating possible magmatic relationships to the Anorthositic Series of the Duluth Complex 10:20 a.m. - 11:00 a.m. Coffee Break and Poster Session 11:00 a.m. Conly, A. and MacDonald, J. Origin of high-sulphate waters of the Hogarth Pit Lake, Steep Rock iron mine, Atikokan, Ontario. 11:20 a.m. Breckenridge, A. and Johnson, T. Lake Superior’s oxygen isotope record suggests overflow to Lake Ojibway between 10,000 and 9,400 CAL BP (~8.9-8.4 14C KA) 11:40 a.m. Blackburn, C. and Kor, P. Control of Quaternary erosional and depositional landforms at the Eastern outlet of glacial Lake Agassiz by Precambrian bedrock and structure, Ottertooth-Pantagreul Lakes area, northwestern Ontario 12:00 p.m. - 1:30 p.m. Lunch Break Technical Session IV Session Chairs: Peter Hinz (Ontario Geological Survey), Charlie Blackburn (Blackburn Geological Services) 1:30 p.m. Weiblen, P., Peterson, D., Vislova, T. Implications of Midcontinent Rift and oceanic ridges analogies and 3D interpretations of the subsurface structure of the Bald Eagle Intrusion in the Duluth Complex and the East Pacific Rise 1:50 p.m. Halls, H. Dyke swarms around the Lake Superior Region define Proterozoic (~2 Ga) deformation of the Archean Superior Province associated with evolution of the Kapuskasing Zone 2:10 p.m. Dahl, D. Gold grains, pathfinder elements, and till clast composition in a portion of the Vermilion Greenstone Belt, Northeastern Minnesota 2:30 p.m Miller, J. and Jirsa, M. - xxi - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Geology, geochemistry and PGE potential of mafic and ultramafic intrusions in Minnesota, excluding the Duluth Complex 2:50 p.m Presentation of Best Student Paper Award and Eisenbrey Awards 3:00 p.m. - 3:30 p.m. Coffee Break and Poster Session (Posters removed after the break) 7:00 p.m Mixer - Cash Bar (Nipigon Curl-a-Drome) Poster Presentations Bucholz, T., Falster, A., and Simmons, W. Mineralogy of pegmatites and spatially associated metasomatized zones, Michels Materials Quarry, Waterloo, WI Cannon, W., Schulz, K., Daniels, D., Anderson, R., Chandler, V., Holm, D., Schneider, D., Van Schmus, R. Geology of Precambrian basement rocks in Iowa and the southern parts of Wisconsin and Minnesota Jirsa, M. and Miller, J. Geologic implications of bedrock mapping in the Ely and Basswood Lake quadrangles, Northeast Minnesota Lane, C.* and Hollings P. Geochemistry and petrography of the Rabbit Islands Breccia, North Central Lake Nipigon MacTavish, A. MetalCORP Ltd. Big Lake Ni-Cu-PGE, Cu-Zn-Ag, and Mo Property Magee, A. Mining and exploration activity in northwestern Ontario Planavsky, N.* and Murphy, J.* New thoughts on old circles: A reexamination of spheroidal Gunflint taxa Rossell, D. and Coombes, S. The geology of the Eagle Lake Nickel-Copper deposit: Marquette County, Michigan Severson, M. PGE and gold potential of the Archean Deer Lake Complex, Minnesota, USA Shareef, S. and Craven, J. A view into the Nipigon Embayment: preliminary results of the largest magnetotelluric study ever in Ontario. Stott, G. and Rayner, N. Discrimination of Archean terranes in the Sachigo subprovince and relevance to volcanogenic masive sulphide exploration Trow, J. and Young, C. Correlation between self-potential and dowsing (IESG) at the Quincy Mine and at the Calumet and Hecla Mine, Michigan - xxii - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Weiblen, P., Peterson, D., and Vislova, T. Implications of Midcontinent Rift and oceanic ridges analogies and 3D interpretations of the subsurface structure of the Bald Eagle Intrusion in the Duluth Complex and the East Pacific Rise Zieg, M., Forsha, C., and Habarka, J. Textural examination of Kama Point diabase sill, Nipigon, Ontario NOTE: Asterisk * denotes a student eligible for a Best Student Paper Award Saturday May 28 8:00 a.m. Field Trip 4: Geology and Rare Element-bearing pegmatites of the Quetico Subprovince Leaders: Mark Smyk (OGS) and Steve Kissin (Lakehead University) 8:00 a.m. Field Trip 5: Geology of the Black Sturgeon Area Leader: Tom Hart (OGS) 8:00 a.m. Field Trip 6: A Stratigraphic Transect Across the Northern Flank of the Midcontinent Rift near Rossport (II) Leaders: Peter Hollings and Phil Fralick (Lakehead University) 6.00 p.m. Return of Trips 4, 5 and 6 - xxiii - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Discovery of Distal Ejecta from the 1850 Ma Sudbury Impact ADDISON, W.D., R.R. 2, Kakabeka Falls, Ontario, P0T 1W0, Canada; BRUMPTON, Gregory, R., 211 Henry Street, Thunder Bay, Ontario, P7E 4Y7, Canada; VALLINI, Daniela A, McNAUGHTON, Neal, J., Centre for Global Metallogeny, School of Earth and Geographic Sciences, University of Western Australia, Nedlands, Western Australia, 6009, Australia; DAVIS, Don W., Department of Geology, Earth Sciences Centre, University of Toronto, 22 Russell Street, Toronto, Ontario, M5S 3B1, Canada; KISSIN, Stephen A., FRALICK, Philip W., HAMMOND, Anne L., Department of Geology, Lakehead University, Thunder Bay, Ontario, P7B 5E1, Canada A 25–70-cm-thick, laterally correlative layer near the contact between the Paleoproterozoic sedimentary Gunflint Iron Formation and overlying Rove Formation and between the Biwabik Iron Formation and overlying Virginia Formation, western Lake Superior region, contains shocked quartz and feldspar grains found within accretionary lapilli, accreted-grain clusters, and spherule masses, demonstrating that the layer contains hypervelocity impact ejecta. Smectite replaced microtektites, spherules and crushed spherules, and sphere-insphere features make up the bulk of the ejecta. Accretionary lapilli appear near the middle of the ejecta column in the Ontario Gunflint Formation cores but they are not present in the Minnesota Virginia Formation cores. Zircon geochronologic data from tuffaceous horizons bracketing the layer reveal that it formed between circa 1878 Ma (Fralick et al., 2002) and 1836 Ma. The Sudbury impact event, which occurred 650–875 km to the east at 1850±1 Ma (Krogh et al., 1984), is therefore the likely ejecta source, making these the oldest ejecta linked to a specific impact. Shock features, particularly planar deformation features (French, 1998) are remarkably well preserved in localized zones within the ejecta, whereas in other zones, mineral replacement, primarily carbonate, has significantly altered or destroyed ejecta features. LEGEND racks younger than Paleoproterozoic Paleoproterozoic rocks, BIF in black Archean greenstone granite terrane Figure 1. Location of drill holes in relation to the Sudbury structure. The Sudbury impact is the only known impact location close enough to have produced a craton-sourced (quartz and feldspar grains), westward-thinning ejecta layer (70 cm thinning to 25 cm) this thick, in these locations, given the time constraints established by zircon dating. References Addison, W.D., Brumpton, G.R., Vallini, D.A., McNaughton, N J., Davis, D.W., Kissin, S.A., Fralick, P.W. and Hammond, A.L. 2005. Discovery of distal ejecta from the 1850 Ma Sudbury impact: Geology, 33, 193-196. Fralick, P., Davis, D.W. and Kissin, S.A. 2002. The age of the Gunflint Formation, Ontario, Canada: Single zircon U-Pb age determinations from reworked volcanic ash: Canadian Journal of Earth Sciences, 39, 1085–1091. French, B.M. 1998. Traces of catastrophe: Lunar and Planetary Institute Contribution 954, 120p. Krogh, T.E., Davis, D.W. and Corfu, F. 1984. Precise U-Pb zircon and baddeleyite ages for the Sudbury area, in Pye, E.G., Naldrett, A.J., and Giblin, P.E., eds., The geology and ore deposits of the Sudbury structure: Ontario Geological Survey Special Volume 1, 431–446. -1- �Proceedings of the 51st ILSG Annual Meeting - Part 1 Figure 2. Overview of the Gunflint and Rove Formations based on drill core 89MC-1s. The 1878 Ma tuff age is from Fralick et al (2002). Detail from column 1 showing the Gunflint-Rove boundary zone and the relationship of the ejecta to the dated Rove tuff. The Minnesota cores and other Ontario cores show similar features in this zone. Detail of the ejecta layer in drill core BP99-2, the most complete and least altered ejecta section in all the cores. The possible carbonate-replaced fireball layer fades into the overlying carbonate. Minnesota ejecta layers are thinner and lack accretionary lapilli. LEGEND C Possible fireball rayer. carbonate replaced Acoretionary lapilli Ejecta P Possible ejecla. carbonate replaced S [] Inaclastjo Grainstone — Ei Fine - grained chemical sedimentwy rooks Siliclaslic Sandstone [II — Fine-grained siliastics Recrystalljzed and silioffied carbonates -2- �Proceedings of the 51st ILSG Annual Meeting - Part 1 The Geology and Petrology of the Leveaux Porphyritic Diorite, Cook County, MN: Investigating Possible Magmatic Relationships to the Anorthositic Series of the Duluth Complex ALBERS, Paul B., Department of Geological Sciences, University of Minnesota-Duluth, Duluth, MN 55812, albe0167@d.umn.edu and MILLER, James D., Jr., Minnesota Geological Survey, c/o NRRI, 5013 Miller Trunk Hwy, Duluth, MN 55811, mille066@umn.edu The Leveaux porphyritic diorite (LPD) is a 72-m thick, discontinuous, hypabyssal, sheet-like intrusion located within the Mesoproterozoic (1.1 Ga) Midcontinent Rift-related Beaver Bay Complex of northeastern Minnesota (Miller and Chandler, 1997). The LPD makes up five prominent, cuesta-like ridges (Figure 1) that trend parallel to Lake Superior over a 20 km distance. In the Caribou Lake region, the LPD makes up smaller knob-like outcrops that dip in multiple directions and are segmented by the younger Beaver River diabase. The LPD is composed of an upper porphyritic zone, which contains 40-50% of 1-4 cm labradorite megacrysts in a fine-grained ferrodiorite matrix (Figure 2), and a lower aphyric zone composed of fine-grained ferrodiorite with rare megacrysts. The aphyric and porphyritic zones are separated by a 20-30 cm thick gradational transition. The orientation of this planar transition and sheet joints from the aphyric section indicate a gentle (10-20 degrees) southeast dip. The upper contact of the LPD is fault bounded. In two localities, the lower contact is observed to be chilled against a thin arkosic sandstone unit, which in turn is underlain by an ophitic basalt. Few outcrops of a sparsely porphyritic (5-20% plagioclase megacrysts) ferrodiorite off the trend of the main sheet are found and may represent feeder zones to the LPD. Figure 1. Cuesta-like ridge of LPD at Moose Mountain dipping gently toward Lake Superior (looking southeast). The ridge to the right is the Beaver River diabase. Figure 2. Nonfoliated, lath-shaped labradorite megacrysts (1-4 cm) in a dark, fine-grained ferrodiorite matrix at Oberg Mountain. Geochemical and petrographic characteristics of the aphyric zone and the matrix material of the porphyritic zone are nearly identical vertically and laterally throughout the sheet. Mineralogically, the ferrodiorite is composed of plagioclase, augite, K-feldspar, Fe-Ti oxides, and minor accessory minerals of quartz, inverted pigeonite, calcite, and apatite, which collectively display an intergranular, nonfoliated texture. Modally, the ferrodiorite varies between diorite and quartz monzonite. XRF analyses indicate a normative An (An/An+Ab+Or) matrix composition of 39, while microprobe analyses range from 19-64. Matrix augite contains a constant composition (mg # 55-61) throughout the intrusion. Microprobe traverses and Nomarski DIC microscopy of plagioclase megacrysts indicate subtle zoned cores (~An 65) and strongly zoned rims with An compositions comparable to matrix plagioclase. XRD analyses indicate that the plagioclase megacrysts have an intermediate ordered structural state (see Knudsen and others, this volume). Based on these observations, we conclude that the LPD formed by the shallow emplacement of a single plagioclase-phyric ferrodiorite magma. Density calculations indicate that the upper porphyritic section may have -3- �Proceedings of the 51st ILSG Annual Meeting - Part 1 formed by plagioclase megacryst flotation in the host ferrodiorite magma. The average composition, zonation patterns, and structural state of LPD plagioclase megacrysts are similar to cumulus plagioclase of the anorthositic series of the Duluth Complex. Also, the evolved compositions of the LPD ferrodiorite are similar to the estimated anorthositic series trapped liquid compositions (Miller and Weiblen, 1990). With these petrogenetic links, we conclude the LPD is a hypabyssal equivalent to the anorthositic series, which is thought to have formed from plagioclase crystal mushes derived from deep crustal magma chambers. References Miller, J. D., Jr., and Chandler, V. W. 1997. Geology, petrology, and tectonic significance of the Beaver Bay Complex, northeastern Minnesota, in Ojakangas, R.W., Dickas, A.B., Green, J.C., eds., Middle Proterozoic to Cambrian Rifting, Central North America: Geological Society of America Special Paper 312, 73-96. Miller, J. D., Jr., and Wieblen, P. W. 1990. Anorthositic rocks of the Duluth Complex, Minnesota: Examples of rocks formed from plagioclase crystal mush: Journal of Petrology, 31, 295-339. -4- �Proceedings of the 51st ILSG Annual Meeting - Part 1 Control of Quaternary Erosional and Depositional Landforms at the eastern outlet of Glacial Lake Agassiz by Precambrian Bedrock and Structure, Ottertooth-Pantafreul Lakes area, Northwestern Ontario BLACKBURN, C.E., Blackburn Geological Services, Victoria, B.C. and KOR, P.S.G., Ontario Parks, Peterborough, Ontario During late Wisconsinan/early Holocene time, glacial Lake Agassiz debouched into the Nipigon basin in catastrophic bursts at the continental divide through a number of spillway channels. Two protected areas managed by MNR Ontario Parks, Ottertooth Conservation Reserve and Pantagreul Creek Provincial Nature Reserve, encompass the eastern portion of the 15-km-wide Kaiashk Spillway corridor, the southern-most of five major channel complexes (Figure 1). The power of the bursts can be appreciated by the fact that water flowing through the outlet would have fallen from the 457 m elevation of the divide to about the 320 m level of Lake Nipigon - a drop of 137 m over a distance of 20 km. 44 1# C, C, t I. to Erosional features resulting from this outflow include plunge basins that formed at the break in slope from the Archean gneissic basement into the Nipigon lowlands. Two plunge basins are present. The more spectacular of the two, Devils Crater, has walls on the order of 100 m high down which today two water falls cascade. The second un-named plunge basin lies some 1.5 km to the southwest. Streams drain to the southeast from both plunge basins, the one from Devilʼs Canyon being through a deeply incised, narrow gorge. Similar though less prominent features suggesting plunge of water over a pre-existing bedrock “lip” can be seen: a) at the north end of Rabelais Lake; b) about 3 km to the northeast of Devils Crater, and c) near the west shore of Pantagruel Creek. Notably, all of these features lie along a NE-trending line over a distance of ~20 km. Depositional features resulting from the outflow include an alluvial fan at the mouth of the more southerly of the channel-ways exiting from the two plunge basins, which unites with the main spillway channel through which Pantagruel Creek now flows. An “island” of braided eskers forms an erosional remnant in the centre of the channel. Figure 1. Location of the Kaiashk Spillway system, and its relation to four other systems that linked Lake Agassiz with Lake Nipigon during the Nipigon Phase (from Teller and Thorleifson, GAC Special Paper 26, 1983, Fig. 7). Underlying Archean and Proterozoic bedrock geology had a profound influence on development of the above Quaternary features. To the northwest of the northeasterly-trending line joining points of plunge of Agassiz lake waters, Archean gneissic bedrock was peneplained prior to deposition of Proterozoic Sibley Group sediments, and exhumed during the time of outlet from Lake Agassiz. The result is a broad, level plain. Flat-lying Sibley sedimentary rocks were intruded by Nipigon diabase sills, the erosional remnants of which form numerous scarps and cuestas. Evidence of regional scale faulting post dating intrusion of the sills is seen at Devilʼs Crater, where Archean gneisses exposed on its northwest face are uplifted relative to Proterozoic diabase that is exposed throughout the rest of Devilʼs Crater and its gorge. Faulting is probably present at the smaller plunge basin, and at the north end of Rabelais Lake where there is also transition from peneplained Archean gneiss to the incised channel-way of Rabelais Creek. However this latter channel has eroded completely through the diabase “cap” into the gneisses below. Interpretation of -5- �Proceedings of the 51st ILSG Annual Meeting - Part 1 such a regional scale fault is supported by presence of similar NE to NNE-trending regional-scale faults that have been mapped throughout the region. Projection of the regional-scale NE-trending fault is coincident with the five identified points of plunge of Agassiz lake waters during late Wisconsinan/early Holocene time. Other such fault structures, as yet undetected, may have had considerable influence on development of landform features at the eastern exit from glacial lake Agassiz, possibly along the entire length of the spillway channels northwest of present Lake Nipigon (Figure 1). References Teller, J.T. and Thorleifson, L.H. 1983. The Lake Agassiz–Superior connection; in Glacial Lake Agassiz, Geological Association of Canada, Special Paper 26, p.61-290. -6- �Proceedings of the 51st ILSG Annual Meeting - Part 1 Lake Superiorʼs Oxygen Isotope Record Suggests Overflow to Lake Ojibway between 10,000 and 9,400 CAL BP [~8.9-8.4 14C ka] BRECKENRIDGE, Andy and JOHNSON, Thomas C., Large Lakes Observatory and Dept. of Geological Sciences, University of Minnesota Duluth, Duluth, MN 55812 Late glacial ostracode and bivalve records from Lakes Huron and Michigan are characterized by extreme δ18O variations, ranging from values that reflect a source that is primarily glacial in origin (~–20 ‰ PDB) to much heavier values characteristic of a regional meteoric source (~–5 ‰ PDB). In contrast, a coeval record from benthic ostracodes (Candona subtriangulata) from a varve sequence in Lake Superior is consistently depleted in 18 O, ranging from –18 to –22 ‰ PDB. Between 10,000 and 9,400 cal BP [8.9-8.4 14C ka], high δ18O values in Huron strongly contrast with much lower values in Superior, which suggests Lake Superior overflow circumvented Lake Huron, and discharged through the Pic-White Otter River Valley en route to Glacial Lake Ojibway. The northern outlet may have opened when lake levels fell from an upper post-Minong water plane at around 10,000 cal BP [~8.9 14C ka]. Thereafter, lake levels probably transgressed towards the post-Minong III shoreline. Because deltaic deposits in the Pic River Valley dated around 9,200 cal BP [8.2 14C ka] record glacial meltwater flow towards Lake Superior (Bajc et al., 1997), northern drainage of Lake Superior must have been blocked by ice advance at around 9,400 cal BP [8.4 14C ka], rather than by differential uplift of a northern outlet. Negative δ18O excursions in Lakes Huron and Michigan between 9,400 and 9,000 cal BP [8.4-8.1 14C ka] record the return of Lake Superior overflow to the upper Great Lakes. In Huron the negative δ18O excursion was previously ascribed to the Late Stanley lowstand (Rea et al., 1994), and in Michigan the event was attributed to Lake Agassiz overflow and labeled ʻA2ʼ (Colman et al., 1994). During this entire period, both Lake Agassiz and glacial meltwater discharged into Lake Superior via the Nipigon inlets. Lake Agassiz and glacial meltwater fluxes into Lake Superior diminished to zero between 9,040 and 8,840 cal BP [~8.1-7.9 14C ka]. References Bajc, A.F., Morgan, A.V., and Warner, B.G. 1997. Age and paleoecological significance of an early Postglacial fossil assemblage near Marathon, Ontario, Canada. Canadian Journal of Earth Sciences. 34, 687-698. Colman, S.M., Keigwin, L.D., and Forester, R.M. 1994. Two episodes of meltwater influx from glacial Lake Agassiz and their climatic contrast. Geology 22, 547-550. Rea, D.K., Moore, T.C., Jr., Anderson, T.W., Lewis, C.F.M., Dobson, D.M., Dettman, D.L., Smith, A.J., Mayer, L.A. 1994. Great Lakes paleohydrology: complex interplay of glacial meltwater, lake levels, and sill depths. Geology 22, 10591062. -7- �Proceedings of the 51st ILSG Annual Meeting - Part 1 Mineralogy of Pegmatites and Spatially Associated Metasomatized Zones, Michels Materials Quarry, Waterloo, WI BUCHOLZ, Thomas W., 1140 12th Street North, Wisconsin Rapids, Wisconsin 54494, FALSTER, Alexander. U. and SIMMONS, Wm. B., Department of Geology and Geophysics, University of New Orleans, New Orleans, Louisiana 70148. The Michels Materials Waterloo Quarry exposes Baraboo Interval quartzites and metapelites that have been intruded by pegmatites of Wolf River age, approximately 1,440 Ma (Aldrich, 1959; cited in Brown, 1986). A pegmatite outcropping nearby in the Crawfish River has been long known, but pegmatite exposures in the quarry itself were first noted during an ILSG field trip in 2001. Samples of pegmatites and associated rocks were collected at that time and during a number of subsequent visits. The pegmatites are highly weathered and in hand specimen consist of kaolinite, quartz and muscovite; virtually all feldspars have altered to kaolinite. The dikes are small, discontinuous, apparently randomly oriented and frequently crosscut bedding in the metasediments. Despite the extensive alteration of feldspars, associated muscovite is quite fresh and a number of accessory phases persist. Small miaroles appear to be present, but are obscured by kaolinite. Most pegmatites exposed to date are in the upper level of the quarry, although recent operations (2004) have exposed one pegmatite in the lower level. Accessory minerals identified in dikes in the upper level of the quarry include dark green gahnite as small aggregates to approximately 2 mm and as well-formed octahedral crystals to approximately 500 µm: MnO content ranges from 0.4 to 1.1 wt. % and FeO content ranges from 1.5 to 2.9 wt. %. Columbite-tantalite group minerals are not uncommon but are inconspicuous due to their small grain size. They form prismatic crystals up to about 900x50 µm. Mn enrichment is pronounced and Ta-enrichment is modest, with compositions falling into the manganocolumbite and manganotantalite fields; there are two distinct composition trends present in these small dikes, one clustering around a Nb-dominant composition and the other clustered around a Ta-dominant composition. TiO2 contents range from 0.4 to 0.9 wt. %. Sn, Bi, and Sb were not detected; the absence of Bi is interesting in light of recent finds described below. Additional minerals in the upper level dikes include apatite, sprays of acicular goethite crystals, ilmenite as crude hexagonal platelets, small grains of a LREE-Ca phosphate (probably monazite-(Ce) or rhabdophane-(Ce)), zircons giving evidence of minor Hf enrichment and a grayitelike Th-phosphate associated with gahnite. Despite the intense alteration of pegmatitic feldspars, associated muscovite is well preserved and retains a substantial Li2O content of up to 0.98 wt %. In September 2004 a somewhat less weathered pegmatite was discovered in the lower level of the quarry. Similar to the upper level dikes, this pegmatite appears to cut across the bedding and terminates before intersecting the upper bench level. Remnants of partially altered feldspars are present in this pegmatite, and the overall mineralogy is somewhat more complex than in the upper level dikes. Some micas in this pegmatite have a pronounced dark pink color and contain substantial Li2O enrichment up to 2.05 wt %. Some additional minerals found include manganotantalite, goethite replacements of pyrite and chalcopyrite crystals, thin tabular crystals of hematite and sparse apatite. Noteworthy are small grains of bismutomicrolite {(Bi,Ca) (Ta,Nb)2 O6 (OH)} to approximately 0.4 mm embedded in feldspar. EMP analysis indicates Ta>Nb, and Bi>Ca+Na. Pale greenish elongated crystals forming small branching groups in vugs and voids, usually along surfaces of quartz grains, are a mixture of kettnerite {(Ca,Bi) (CO3) O, F} and other Bi secondary minerals as indicated by x-ray diffraction study. Kettnerite is a typical secondary Bi mineral probably derived from alteration of a primary Bi mineral such as bismuthinite, although the primary Bi mineral has not yet been found. The Waterloo occurrence represents the first find of gahnite from a pegmatite in the state and the overall composition of these small pegmatites indicates highly peraluminous composition, even though such common peraluminous species such as garnet and tourmaline appear to be absent in them. These are also the first finds of kettnerite and bismutomicrolite for Wisconsin. -8- �Proceedings of the 51st ILSG Annual Meeting - Part 1 Spatially and perhaps genetically related alteration or metasomatic vein-like bodies have been found near the pegmatites. These range from quartz-muscovite±andalusite veins to massive fine-grained lithian muscovite (0.6 wt % Li2O), the two phases interfingering and grading into each other laterally. Considering the close spatial association of these bodies to pegmatites, it seems likely that the fluids responsible for the alteration may have been derived either from the pegmatites or the source igneous body, although mobilization from the metasediments cannot be excluded based on the present evidence. Schorl tourmaline is locally common in the quartz-muscovite+-andalusite veins in small vugs and adjacent pelitic schists. Colors range from black to olive green, brown and pinkish brown. Microprobe analysis indicates FeO contents of 8-9%, reflecting an overall schorl composition, though site occupancy considerations indicate a significant Li content. Hence, as the Fe-content of these tourmaline crystals is only slightly on the schorl side of the line between elbaite and schorl they might be considered elbaitic or lithian schorl. Apatite is common in small vugs near tourmaline, and ranges from dark red tabular hexagonal crystals to short visually zoned yellow-brown hexagonal prisms. Additional accessory minerals noted in vuggy portions of the veins include Ti-oxides, ilmenite-hematite, pyrite, chalcopyrite (both sulfides usually replaced by goethite), calcite and sparse REE-phosphate grains. Accessory minerals noted in the massive fine-grained muscovite-rich portions include ilmenite-hematite grains, enigmatic muscovite replacements of an undetermined mineral, and sparse zircon. The zircons may be detrital and predate metasomatism. Hence it is likely that the highly evolved pegmatites formed in conjunction with regionally widespread Wolf River age magmatism (Medaris et al., 2003). Elevated Ta and Mn levels evidenced in the pegmatites, and B and Li in the metasomatic units are anomalous for Wolf River age intrusions in Wisconsin. However, it appears likely that fluids either from these pegmatites or more directly from the parent intrusion may have had a strong, localized metasomatic effect on adjacent rocks, introducing P, B, Li and perhaps other elements to favorable areas in the host rocks. An intriguing question is what influence assimilation of peraluminous metasediments may have had on the source magma, and subsequently on pegmatite composition. Could this have influenced the mineralogy of the pegmatites and other features, or was the source magma already evolved, having originated by partial melting of different source rocks than typical Wolf River intrusions? References Brown, Bruce A. 1986. Baraboo Interval in Wisconsin, In Proterozoic Baraboo Interval in Wisconsin, Geoscience Wisconsin 10, 1-14. Medaris, L.G. Jr., Singer, B.S., Dott, R H. Jr., Naymark, A., Johnson, C.M. and Schott R.C. 2003. Late Paleoproterozoic Climate, Tectonics, and Metamorphism in the Southern Lake Superior Region and Proto–North America: Evidence from Baraboo Interval Quartzites. Journal of Geology, 111, 243-257. -9- �Proceedings of the 51st ILSG Annual Meeting - Part 1 Geology of Precambrian basement rocks in Iowa and the southern parts of Wisconsin and Minnesota CANNON, W.F., SCHULZ, Klaus J., and DANIELS, David L.,U.S. Geological Survey, Reston, VA, ANDERSON, Raymond, Iowa Geological Survey, Iowa City, IA; CHANDLER, Val, Minnesota Geological Survey, St. Paul, MN, HOLM, Daniel, Kent State University, Kent, OH, SCHNEIDER, David, Ohio University, Athens, OH, and VAN SCHMUS, W.R., Kansas University, Lareence, KS The well-known Precambrian rocks of the Lake Superior region record geologic events in the evolution of Laurentia from 3.6 Ga to 1.1 Ga. Similar rocks extend southward as the crystalline basement beneath Paleozoic and Cretaceous strata in southern Minnesota and Wisconsin and throughout Iowa. Using aeromagnetic and gravity maps of the region, and several hundred basement-penetrating drill holes, we have produced a geologic map and preliminary tectonic interpretation of the Precambrian subcrop for this region (Figure 1). Our interpretation extends the well known Superior Province (Archean) and Penokean orogenic belt (Paleoproterozoic) of the Lake Superior region southward to a prominent geophysical discontinuity, shown clearly by aeromagnetic patterns, named the Spirit Lake-Trempealeau discontinuity (SLTD). This discontinuity separates fundamentally different terranes; rocks to the south are part of the Yavapai Province and those to the north are part of the Penokean Province. Although rocks of Yavapai age, represented by extensive ultra-mature sediments, such as the Baraboo and Sioux Quartzites, and granitic plutons, extend north of the discontinuity, there are as yet no firmly identified Penokean or older rocks south of the discontinuity, although data are sparse. Thus we suggest the possibility that the discontinuity marks the southern limit of the preserved Penokean orogen and its Archean basement. South of the SLTD, rocks at the subcrop include subaerial potassic rhyolite and epizonal granite, formed at about 1.75 Ga, and ultra-mature quartzite, such as the Baraboo Quartzite, which lies unconformably on them. An orogenic complex of gneisses and mafic volcanic rocks, probably basement rocks on which the rhyolites were deposited and from which they formed by partial melting, are inferred from gravity and magnetic patterns to be at subcrop throughout much of this terrane, but its lithology and age are not well constrained. All of these rocks were strongly deformed at about 1.63 Ga during the Mazatzal orogeny, whose northeastern limit may underlie the extreme southeastern part of the map area. The nature of the SLTD is controversial with possibilities including a north-directed Yavapai subduction zone or a major strike-slip fault zone. Major granitic plutons were emplaced into the Yavapai terrane in the interval 1.50-1.43 Ga and some are stitching plutons of the SLTD providing an upper age limit on deformation within it. The final major Precambrian event in the region was formation of the Midcontinent Rift at 1.1 Ga. The rift transects older terranes at a high angle and consists of more than 10 km of flood basalts accumulated in and near a series of grabens, and slightly younger clastic rocks. Its location in the subsurface is clearly shown by very pronounced geophysical anomalies. A major igneous complex in NE Iowa and SE Minnesota is also interpreted to be part of the Midcontinent Rift. - 10 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 ______ _______ _______ 98L0.W _______ 94.0YW 88 OOW SBOW go•do•w EXPLANATION Ma Manson impact structure Red clastic sedimentary rocks Vo Mafic intrusive rocks Penokean Orogen Arthean basement granitic rocks Archean Granite and tonalite Pembine-Wausau Terrane Gnelss of Jeffers Block Mafic volcanic rocks LAng 1 Granite,anorthosite,norite Gneiss of Montevideo Block Mazatzal Orogen Felsic volcanic rocks Gneissof Morton Block Granitic plutons Rhyoliteandgranlte Baraga Group Orogenic complex Late- and post-tectonic granite Flood basalt, minor rtiyolite Anorogenic Plutons Foreland basin Mat Ic volcanic rocks Midcontinent Rift MRs Penokean Orogen (cont.) Yavapai Orogen (cont.) Bimodal volcanic rocks Units of undetermined age ç:: Gneiss — Yavapai Orogen Marshfleld Terrane Mafic gneiss Quartzlte (BarabooSioux, etc) Mt Yr iF Rhyolite and granite ftiOIIl]I]OI Felsic yneiss Iron-formation — Diabase dikes (mostly MCR related) Mafic and untramafic rocks 11111111 Mylonite and sheared granite Mafic volcanic rocks Volcanic rocks Figure 1. Geological Map of Precambrian basement rocks in Iowa and the southern parts of Minnesota and Wisconsin inferred from aeromagnetic and gravity data and drill holes. - 11 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Origin of High-Sulphate Waters of the Hogarth Pit Lake, Steep Rock iron Mine, Atikokan, Ontario CONLY, A.G., and MacDONALD, J.C., Department of Geology, Lakehead University, Thunder Bay, On, P7B 5E1, andrew.conly@lakeheadu.ca Hogarth Pit Lake was formed from the continual flooding of the main open pit iron mine at Steep Rock Lake near Atikokan, Ontario. Hogarth Pit Lake is located in the former middle arm of Steep Rock Lake and was mined for itʼs predominantly goethite iron ores from 1954 to 1977. During this time over 25 million tons of ore were mined [1]. Water within the pit lake is characterized by near neutral pH and extraordinarily high SO42- concentrations (1000 to 3000 ppm; [2], this study). Consequently, the aim of the research is to determine the source(s) of the high SO42- levels. Possible sources of sulphate that are evaluated include: i) oxidation and leaching of finely disseminated sulphides within hanging wall and footwall rock units by groundwater; ii) oxidation and leaching of pyritic portions of the ore zone by groundwater; and, iii) oxidation and leaching of sulphides/sulphates contained in waste rock dumps and tailings piles by surface waters Hogarth Pit Lake is approximately 160 m deep, with a surface area of approximately 100 hectares and is elongated along a north-south axis. The lake is steep sided, well-sheltered by high rock walls. Footwall rocks, east of the Jolliffe ore zone, consist of (in ascending stratigraphic order) the Marmion Geniss Complex, metasedimentary rocks of the Wagita Formation and the Mosher Carbonate [3]. To the west, hanging wall rocks include ultramafic-mafic volcanic and volcaniclastic rocks of the Dismal Ashrock Formation, which is overlain by mafic to felsic volcanic rocks of the Witch Bay Formation [3]. Since cessation of mining activities the water level within the pit continues to increase (~ 3 m/year) from surface runoff and groundwater seepage. Environmental assessment studies completed shortly after closure of the mine indicate that approximately 85% of the water entering the pit is groundwater (R. Bernachez, personal communication, 2003). The lower contribution of surface water is consistent with the limited number of tributaries that currently flow into the pit lake. Water from Hogarth Pit Lake sampled for this study has a pH of ~7.6, contains 1340 to 1381 ppm of SO42, only trace levels (0.1-0.2 ppm) of Fe and Mn, and no detectable deleterious metals (As, Cd, Pb). Pit water has δ13C and δ34S values that range from 0.8 to 0.9‰ and -2.7 to -2.9‰, respectively. Surface waters that are following into the pit are characterized by a wide range in pH (2.8 to 8.1) and SO42- content (734 and 2580 ppm), with δ13C and δ34S values ranging from -20.3 to 1.5‰ and –3.6 to -0.4‰, respectively. The composition of surface and pit waters is comparable with the data of [2], although there is a trend of decreasing SO42- between 1998/99 and 2003. The variation in the chemistry of surface water reflects differences in wall-rock interactions. Surface waters with low pH and δ13C values reflect reactions with footwall and hanging wall volcanic and genesis units and organic matter. Water with near neutral pH values and δ13C values approaching 0‰ indicate buffering by the Mosher Carbonate. Groundwater was not sample as part of this study. However, regional groundwaters in the Atikokan area are generally enriched in Cl- with substantially lower SO42- contents than observed at Hogarth Pit [4]. However, some regional groundwaters have SO42- contents that approach the lower values for surface waters [4]. Tailings Lillill Pyric waste rock Ore zone Hanging wall C Hogarlh pit waters C Hanging wall tributaries Footwall tributaries P 6'$ (permil) Figure 1. Histogram showing the distribution -of12 sulphur isotope data for Hogarth Pit Lake waters The sulphur isotope composition of the pit and tributaries waters (Figure 1) is consistent with oxidation and dissolution of pyrite from the ore zone, waste rock dumps and the hangingwall. The data for the pyritic waste rock dumps is also used to better constrain the composition of pyritic Figure 1. Histogram showing the distribution of sulphur isotope data for Hogarth Pit Lake waters and lithological units �Proceedings of the 51st ILSG Annual Meeting - Part 1 portions of the ore zone, since only one in situ ore sample was obtained for the study. The shift to slightly more depleted δ34S values for the water samples relative to pyrite-bearing rocks is within the range anticipated for pyrite oxidation [5]. However, sulphur isotopes cannot be used in isolation to discriminate potential sources of SO42-. It is necessary to consider both the total sulphur content of the potential source material and water flow pathways. Consequently, the most likely source of SO42- is the oxidation and dissolution of pyrite contained with the Jolliffe ore zone by groundwater. The high bulk sulphur content (3.5-43.8 wt% as FeS2) of pyritic ore and waste rocks indicate that there is an amply supply of pyrite. Conversely, only very minor amounts of SO42- could have been derived from the hanging wall units and tailings, since these lithogies are characterized by low sulphur contents (<2 wt% and < 0.06 wt%, respectively). Furthermore, the substantially higher proportion of groundwater to surface water filling the pit indicates that water-rock interactions with exposed lithologies contribute a subordinate amount of SO42-. References [1] Taylor, B. 1978. Steep Rock The Men and the Mines. Quetico Publishing: Atikokan, Ontario, 114 p. [2] McNaughton, K.A. 2001. The limnology of two proximal pit lakes after twenty years of intense flooding. M.Sc. Thesis, Lakehead University, Thunder Bay, Ontario, 85p. [3] Stone, D., Kamineni, D.C. and Jackson, M.C. 1992. Precambrian geology of the Atikokan area, northwestern Ontario. Geological Survey of Canada, Bulletin 405, 106p. [4] Ophori, D.U. 1996. Regional groundwater flow in the Atikokan research area: spatial variable density and viscosity. Atomic Energy of Canada Limited Report AECL-11082, COG-93-184, 44 p. [5] Ohmoto, H. and Rye, R.O. 1979. Isotopes of sulfur and carbon. in Barnes, H.L., ed., Geochemistry of Hydrothermal Ore Deposits, 2nd edition: New York, John Wiley and Sons, p. 509-567. - 13 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Gold Grains, Pathfinder Elements, and Till Clast Composition in a Portion of the Vermilion Greenstone Belt, Northeastern Minnesota DAHL, David A., Lands and Minerals Division, Minnesota Department of Natural Resources, 1525 3rd Avenue East, Hibbing, MN 55746, dave.dahl@dnr.state.mn.us. An orientation project to determine variability of glacial till in the Mud Creek area of northeastern Minnesotaʼs Vermilion Greenstone Belt demonstrates that gold grains and pathfinder elements are present in basal till, and that anomalies stand out in contrast to regional background levels. Clastic and chemical variations within the till sample set are sufficient to consider using the basal till as sampling media for gold dispersal mapping. Of the thirty-two till samples analyzed, four were highly anomalous for gold, with counts of 88-1,282 gold grains per 10 kg of –2mm table sample, and pristine gold grain proportions up to 98%; up to 8,050 ppb gold in HMC (nonmagnetic heavy mineral concentrates); and up to 1,050 ppb gold in the -63µm silt/clay fraction of till. A suit of bedrock grab samples collected as reference mineralization returned assays up to 12,247 ppb Au, and silver concentrations up to 42,500 ppb. Analytical results for the till samples support a hypothesis that clastic dispersal trains of mineralized material exist in tills in the area. Within the project area, particulate gold is more anomalous in basal till samples than in the thin drape of overlying melt-out till. The gold grain counts and gold grain morphology add a transport distance value to chemical measurements of gold in soils and till, and suggest that the gold in the samples is locally derived. Pebble counts and pebble morphology similarly suggest a local derivation for the particulate gold. Analytical results for the present study are comparable to larger, more extensive regional evaluations conducted in neighboring Ontario and further confirm anomalous soil and fine fraction gold values reported in earlier Vermilion Greenstone Belt studies. Analysis of basal till in this portion of the Vermilion Greenstone Belt, particularly for gold grains, offers a capacity to positively detect local, previously unrecognized mineralization both inside and outside of areas of detailed bedrock geologic mapping. - 14 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 The Grenvillian Tomiko quartzites of Ontario: correlatives of the Baraboo quartzites of Wisconsin, the Mazatzal orogen of New Mexico, or unique? Implications for the tectonic architecture of Laurentia in the Great Lakes region EASTON, R.M., Precambrian Geoscience Section, Ontario Geological Survey, 933 Ramsey Lake Road, Sudbury, Ontario P3E 6B5, mike.easton@ndm.gov.on.ca The Baraboo and 6 other correlative red quartz arenite sequences in the southern Lake Superior region were deposited between 1710 and 1630 Ma, and locally were affected by regional deformation and metamorphism at about 1630 Ma, as well as by potassium metasomatism at 1465 Ma (Medaris et al., 2003). The Baraboo and related quartz arenites may be correlative with quartz arenites in the Athabasca and Thelon basins (Medaris et al. 2003), indicating widespread sedimentation across much of North America in the late Paleoproterozoic. Key features of the Baraboo and Athabasca basin quartz arenites, include the local presence of well-developed paleosols, a high-degree of supermaturity of the quartz arenites, and chemical index of alteration (CIA) values of 95-99. These three features have been interpreted to indicate that profound chemical weathering occurred across much of North America between 1750 and 1630 Ma (e.g., Medaris et al., 2003). A thick sequence of paragneiss, including quartz arenite and iron formation (hereafter referred to as the Tomiko supracrustal rocks), occurs within the Tomiko terrane of the northern Central Gneiss Belt of the Grenville Province in Ontario. Holmden and Dickin (1995), based on Nd-Sm model age data and regional tectonics, suggested that the Tomiko quartz arenites were possible correlatives of the Baraboo and related quartz arenite sequences. If valid, then this correlation indicates that sedimentation and weathering between 1710 and 1630 Ma occurred east of Lake Superior, and that rocks of the Penokean orogen may underlie large parts of the northern Central Gneiss Belt in Ontario. Testing this correlation has been problematic, as until recently, only limited mapping and geochronological data were available for Tomiko terrane. Mapping during the 2003 field season (Easton, 2003) provides additional constraints for regional correlation, as follows: • Discovery of B-rich (>15% tourmaline) and Mn-rich rocks in association with magnetite-chert iron formation in northern Tomiko terrane, as well as epidote-rich (>20%) rocks in association with rusty gneisses near Crocan Lake and chemically unusual (high TiO2 & P2O5) calc-silicate rocks). • The recognition of possible dacitic metavolcanic rocks and sills interlayered with the Tomiko supracrustal rocks, along with amphibolites that may represent sills or flows. • The possibility that many of the muscovitic and feldspathic units within the Tomiko supracrustal sequence may represent hydrothermally altered rocks (CIA of 60 to 64). • The recognition of possible Archean “basement” rocks infolded with the Tomiko supracrustal rocks. This basement is likely structural, not depositional. • The identification of at least 3 metamorphic events affecting the Tomiko supracrustal rocks, with the intensity of the 2 latest events increasing to the southeast. M2 forms a migmatite front between northern and southern Tomiko terrane, with lower grade rocks to the north. M3 is a regional hydrothermal event, and results in the replacement of blue kyanite and granite leucosome formed during M2 by pale-green kyanite and quartz segregations. • Stratigraphic correlation with metasedimentary rocks of the Huronian Supergroup, exposed immediately to the north, is unlikely, based on geochemistry and Nd/Sm ages. On the basis on the new data, correlation of the Tomiko rocks with the Baraboo and related quartz arenites is still compelling. The Tomiko quartz arenites appear to be mature rather than supermature (CIA of 69 to 78 for quartzites), even though they are associated with metamorphosed volcanic and plutonic rocks and minor amounts of iron formation. It can be argued that kyanite-bearing and muscovite-rich gneisses in Tomiko terrane represent a combination of metamorphosed paleosols and potassium metasomatized quartz-rich sedimentary rocks, respectively. U/Pb zircon ages on detrital zircons from quartz arenite at two localities at different metamorphic grades give a maximum depositional age of 1687 Ma (Krogh, 1989; Easton and Kamo, 2004). Minimum - 15 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 depositional age is 1250±10 Ma, based on the age of grey gneisses interlayered with the quartz arenite; identical within error to the age of several A-type granite plutons that intrude the Tomiko supracrustal rocks (1257+4/-2 and 1244+4/-3 Ma; Easton and Kamo, 2004; Lumbers et al., 1991, respectively). If the correlation with the Baraboo is valid, then the Tomiko supracrustal rocks likely have limited mineral potential. Equally compelling, however, is that the Tomiko supracrustal rocks have many lithologic, stratigraphic and geochronologic similarities to Geon 17 rocks of the Mazatzal orogen exposed in Arizona and New Mexico (Tonto Basin Supergroup and Hondo Group, respectively). For example, the Hondo Group overlies a sequence of older greenstones, and consists of a thin basal unit of quartz pebble conglomerate, over 1 km of quartz arenite, and a capping sequence of muscovite schist and arenite, metarhyolite, phyllite and calc-silicate rocks (Robertson et al., 1993). A manganese-rich horizon, possibly a paleosol, perhaps lateritic, occurs at the top of the greenstones. Furthermore, some of the muscovite schists in the Hondo Group represented hydrothermally altered felsic volcanics. Rocks of the Hondo Group were deposited between 1700 and 1644 Ma (Robertson et al., 1993), and contain detrital zircons ranging in age from 1850 to 1700 Ma. A key difference is that plutons intruded the Hondo Group between 1680 and 1650 Ma, not at circa 1250 Ma. The rock types, stratigraphy, thickness, alteration history and age of the Tonto Basin Supergroup and the Hondo Group is similar to that observed in the Tomiko supracrustal rocks, even though the Mazatzal rocks in New Mexico are now some 3500 km distant. If correlation with the Mazatzal Orogen is valid, then the iron formation in northern Tomiko terrane may be equivalent to the manganese-rich unit at the base of the Tonto Basin Supergroup and the Hondo Group, and may have served as the decollément surface along which northward-directed thrusting occurred. If the Tomiko supracrustal rocks do represent a sliver of the Mazatzal Orogen, which at one time must have stretched from Arizona to Labrador, then the eastern end of the Penokean Orogen has been truncated in Ontario. A third possibility, that the Tomiko quartz arenites were deposited close to 1270 Ma, similar in age to the grey gneiss units interlayered with them, would mean that the sequence is unique in the Lake Superior region, and further begs the question of what happens to the Penokean Orogen east of the Grenville Front? References Easton, R.M. 2003. Reconnaissance study of the geology and mineral potential of the eastern Tomiko terrane, Grenville Province; in Ontario Geological Survey Open File Report 6120, p. 16-1 to 16-25. Easton, R.M. and Kamo, S.L. 2004. The Grenvillian Tomiko quartzites of Ontario: correlative with the Baraboo quartzites of Wisconsin or the Mazatzal orogen of New Mexico? Implications for the tectonic architecture of Laurentia in the Great Lakes region; Geological Society of America, Abstracts with Program, 36, no.5, p.A-459. Holmden, C. and Dickin, A.P. 1995. Paleoproterozoic crustal history of the southwestern Grenville Province; Canadian Journal of Earth Sciences, 32, 472-485. Krogh, T.E. 1989. Provenance and metamorphic ages in the Grenville (NW); in Lithoprobe Abitibi-Grenville Project Workshop, March 1989, p. 5-7. Lumbers, S.B., Wu, T-W, Heaman, L.M., Vertolli, V.M., and MacRae, N.D. 1991. Petrology and age of the A-type Mulock granite batholith, northern Grenville Province, Ontario; Precambrian Research, 53, 199-231. Medaris, L.G., Jr., Singer, B.S., Dott, R.H., Jr., Naymark, A., Johnson, C.M. and Schott, R.C. 2003. Late Paleoproterozoic climate, tectonics, and metamorphism in the southern Lake Superior region and proto-North America: evidence from Baraboo interval quartzites; Journal of Geology, 111, 243-257. Robertson, J.M., Grambling, J.A., Mawer, C.K., Bowring, S.A., Williams, M.L., Bauer, P.W. and Silver, L.T. 1993. Precambrian geology of New Mexico; in Precambrian Conterminous US, Geological Society of America, The Geology of North America, Volume C-2, p. 228-238. - 16 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Stratigraphy of the Mesoproterozoic Sibley Group and Nipigon Sills FRALICK, Philip, METSARANTA, Riku and ROGALA, Becky, Department of Geology, Lakehead University, Thunder Bay, ON, Canada, P7B 5E1 This paper is designed to complement papers on the geochemistry of the Sibley Group (Metsaranta and Fralick, this volume) and Nipigon Sills (Richardson and Hollings, this volume). It will review the existing nomenclature and highlight recent advances made in deciphering the stratigraphy of the Sibley Basin during field investigations forming a portion of the Lake Nipigon Region Geoscience Initiative. Rocks correlated with the Sibley Group are present primarily to the south and west of Lake Nipigon. They are divided into five Formations (Franklin et al., 1980; Cheadle, 1986; Fralick et al., 2000; Rogala, 2003) encompassing, at their thickest, approximately nine hundred meters of stratigraphic section. The basal Pass Lake Formation lies on metamorphosed igneous and sedimentary rocks, except for a limited area in the southwest where it overlies Paleoproterozoic strata of the Rove Formation. Evidence for the existence of small hills and valleys on the paleosurface is common. Age of the Pass Lake Formation is constrained by a Rb-Sr isochron on overlying dolomitic mudstones of 1339±33 Ma (Franklin, 1978), probably representing a diagenetic reset, and detrital zircon geochronology on a Pass Lake sandstone giving a concordant youngest zircon age of approximately 1650 Ma (see Heaman and Easton, this volume). The conglomerates and sandstones of this unit represent a number of depositional environments ranging from Scott-type and South Saskatchewan-type braided streams to cobble beaches, offshore storm sand sheets and possible aeolian dunes. The subaerial environments, where present, are situated at the base of the section, overlain by a flooding event. As the water depth increased the nearshore, storm deposited, sand sheets were succeeded upwards by laminated and massive silt (Rossport Formation). Delta progradation reversed this trend in areas proximal to sediment entry points. Caliche developed in underlying fluvial, off-channel deposits attests to a semi-arid climate and this caused the water body to become saline. Cyclically banded red and cream coloured dolomitic siltstone accumulated as salinity increased. The millimeterto decimeter-scale banding probably reflects periodic movement of the redox front from the water mass to beneath the sediment surface, possibly driven by organic loading. The central portions of thicker, light coloured layers contain bladed crystals and small nodules of gypsum attesting to peak aridity during oxygen minima in the bottom sediments. As the water mass contracted sand, possibly derived from the south, formed extensive sheet deposits in most areas. The strandline is marked by the sporadic development of algal flat deposits with carbonate silt storm layers. Though most of this one to seven meter thick calcareous unit is dominated by smooth mat and breccia small panicles and more classic stromatolites are present. Tepees and gypsum filling stromatactis are also encountered, though more rarely. The upper few centimeters of the carbonates is intensely weathered and commonly overlain by either an excellently developed terra rosa or subareal debris-flows. Regional computer modeling of thickness data indicates that this interval may represent initiation of north-south orientated, half graben formation in the area. The succeeding silt-dominated mudflat assemblage contains abundant nodular and vein gypsum, attesting to the continuation of dry conditions, albeit with a high water table. The transition from the Rossport to Kama Hill Formation is marked by a flooding event and re-establishment of open water conditions, though no evaporite minerals were precipitated from this water mass. Purple shales dominate the lower portion of the succession with thin ripple laminated, fine-grained sandstones, which become more numerous, thicker and coarser grained upward. Trough and hummocky cross-stratified , medium-grained sandstones (Outan Island Formation) appear and then dominate the section, continuing the coarsening upward trend. Slump scars and large areas of chaotic slide-block deposits attest to the periodic failure of, in places, extensive portions of the outbuilding delta complex. The delta top deposits consist of fining and thinning upwards sandstone successions meters to a few tens of meters thick (channels) interbedded with fine-grained sediments (floodplains). The floodplains are mostly composed of friable mudstone with soil horizons representing subareal accumulation but parallel laminated shales deposited in floodplain ponds are also present. The upper deltaic and fluvial deposits of the Outan Island Formation are only present in a core from a hole drilled in Nipigon Bay. - 17 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Detrital zircon geochronology performed on this unit gave a maximum age of deposition of approximately 1470 Ma (see Heaman and Easton, this volume). The Outan Island Formation is erosively truncated and overlain by the Nipigon Bay Formation. Planar cross-sets up to 17 meters thick indicate that the winds which deposited this sea of aeolian sand had a predominant direction from the northeast. The medium-grained sandstones of the Nipigon Bay Formation comprise the uppermost 400 meters of the Sibley Group with conglomerates of the basal Osler Group unconformably overlying them. At approximately 1109 Ma, diabase sills were intruded into the Sibley sedimentary rocks as the Midcontinent Rift established itself in the area. The southernmost region underlying Nipigon Bay has the most complete section of Sibley strata, but only one thin sill near the base of the assemblage. Sibley on the adjacent mainland contains a thick sill in the basal Kama Hill Formation, which is traceable across the southern portion of the basin. Thus, the thick sills are confined to north of Nipigon Bay with a fundamental divide running down this topographic feature. Moving from the southern to central basin a second, lower sill appears in the stratigraphy and sill width increases (toward Muskrat Lake) where their composite thickness is approximately 300 meters. Rossport Formation strata between the two thick sills in the central basin have been extensively thermally metamorphosed reaching temperatures of 600°C locally and 300-400°C tens of meters from the sills. The sills are commonly locally discordant, up- and down-ramping through the stratigraphy. They also appear to be vertically offset by faults, in places defined by surface lineaments. Toward the northern portion of the basin the erosion level cuts downward through the stratigraphy, with only the lower sill and a thin layer of underlying, Sibley-like sediment preserved. In the northernmost region this sill directly overlies a Mesoproterozoic volcanic-intrusive complex. To the east of the basin marginal Black Sturgeon Fault a sill overlies a thin, sporadically present, baked sedimentary layer lying on basement; a scenario similar to the northern area west of the Fault. Generally sill thickness decreases to the west of the central basin and the sills may bifurcate in this direction as well. An anomalous area exists in the western region, northwest of Muskrat Lake where a drillhole encountered a thick (500 m) sequence of diabase. The significance of this occurrence is unknown. Regional sill stratigraphy implies the feeder area was probably in the vicinity of Muskrat Lake and possibly related to the Black Sturgeon Fault system. References Cheadle, B.A., 1986. Alluvial-playa sedimentation in the lower Keweenawan Sibley Group, Thunder Bay District, Ontario. Canadian Journal of Earth Sciences, vol. 23, p. 527-542. Fralick, P.W., Smyk, M. and Mailman, M., 2000. Geology and stratigraphy of the Mesoproterozoic Sibley Group. Institute on Lake Superior Geology, Proceedings Volume46, Part 2: Fieldtrip Guidebook, yellow section. Franklin, J.M., 1978. The Sibley Group, Ontario. In, Rubidium-strontium isochron age studies, Report 2, Geological Survey of Canada, Paper 77-14, p. 31-34. Franklin, J.M., Mcilwaine, W.H., Poulsen, K.H. and Wanless, R.K., 1980. Stratigraphy and depositional setting of the Sibley Group, Thunder Bay District, Ontario, Canada Canadian Journal of Earth Sciences, vol. 17, p. 633-651. Rogala, B., 2003. The Sibley Group: a lithostratigraphic, geochemical, and paleomagnetic study. Unpubl. M.Sc. Thesis, Lakehead University, Thunder Bay, Ontario, 254p. - 18 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Mineral Resources for the Future: The Resource Potential of Northern Lake Superior FRANKLIN, James M., Franklin Geosciences Ltd., 24 Commanche Drive, Nepean, Ontario, K2E 6E9 The demand per person in the industrialized nations for primary “old - economy” metals has increased tenfold since 1900. Globalization in recent years has increased competitiveness and access to highly productive regions such as South America, Africa and Oceana, leading to an overall decline in metal prices of 40%. The demand for “new - economy” metals has increased 5 to 100 times since 1985; their prices have increased significantly. In the past five years, industrial expansion in China and India has created an almost unprecedented need for new metal resources. Even with recycling, vast quantities of new resources, particularly for “green” products, must be found, mined and processed in environmentally sustainable ways. The geological attributes of the Northern Lake Superior region provide an outstanding opportunity for exploration for an exceptional range of commodities and ore deposit types. Archean volcano-sedimentary strata with much additional potential include the historically productive Abitibi – Wawa and Wabigoon belts, with world – class volcanogenic massive sulfide deposits (Manitouwadge district), orogenic gold deposits (Geraldton) and a porphyry/epithermal district (Hemlo). Significant pegmatite bodies (Georgia Lake) and Algoma iron formations (Wawa) are also present. Early Proterozoic strata are comprised of an extensional and miogeoclinal sequence that includes the giant Gunflint Superior-type iron formation. .Mid Proterozoic intrusions associated with magmatic plumes (~1500-1600Ma) and the Keweenawan plume-intracratonic rift assemblage (1130-1090Ma) have proven potential for Cu-Ni-PGE deposits, Nb-REE in carbonatite, as well as potential for IOCG (REE-Zr) mineralization. Vein deposits that are contemporaneous with Keweenawan magmatism include Pb-Zn-Ba unconformity-vein deposits, two generations of Ag veins, and unconformity-style uranium occurrences. Epithermal and breccia – pipe copper deposits occur near Batchawana Bay. Redbed copper occurrences are associated with interflow sedimentary strata in the Osler volcanic sequence. Future discoveries require innovative exploration. Quantitative estimation of key geological attributes (e.g. magma fluid contents, paleo- permeability, and fault dynamics) must be applied vigorously in the search for ore. Geophysical techniques normally used for deep imaging of the earth (seismic, magnetotelluric) are being adapted for exploring shallow crystalline terrains. Developing quantitative models of ore forming processes that can be applied at all scales will ensure the supply of metals needed for the rapidly developing nations, and for improved quality of life everywhere. - 19 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Dyke swarms around the Lake Superior Region define ~2 Ga Proterozoic deformation of the Archean Superior Province associated with evolution of the Kapuskasing Zone HALLS, H.C., Department of Geology, University of Toronto, hhalls@utm.utoronto.ca Proterozoic dyke swarms in the Archean Superior Province straddle a ~2 Ga crustal dislocation, the Kapuskasing Zone (KZ), that extends for more than 500 km from James Bay to Lake Superior and divides the Archean Superior Province into eastern and western halves. About ten swarms are present ranging in age from the Matachewan, (2446-2473 Ma) to the Keweenawan (1108-1094 Ma). Nearly all the swarms include at least one reversal of the earthʼs magnetic field. During the Matachewan igneous episode only one reversal, from R to N, occurred at 2446 Ma. Within the KZ only N dykes are found, but outside, R dykes outnumber N by more than 4 to 1. N dykes are relatively common in the focal region of the swarm north of Lake Huron, but gradually disappear to the northwest with only R dykes being found in the Lake Nipigon region. The KZ can be divided up into a number of domains based on Matachewan dyke polarity, the boundaries between R and N domains being major faults that define the crustal uplift along the KZ. N polarity dykes within the KZ represent the roots of R dykes that have been remagnetized at depth before being uplifted. Using the polarity domain concept, major faults have been mapped that define a new segment of the KZ at its southwestern end (the Pineal Lake block), which is offset about 20 km sinistrally from the main Chapleau block to the NE1. The pattern of polarity domains follows exactly that produced by variations in feldspar clouding intensity within the dykes, whereby the clouding, (caused by exsolution of magnetite and other minerals in groundmass feldspars) occurs only in N polarity domains. The secondary magnetite, thought to be caused by slow cooling of the dykes at large crustal depths, increases in concentration with increasing depth. The growth of this magnetite led to the secondary N magnetization. Only a few dykes within the KZ show vestiges of their formerly R polarity1. When Matachewan dyke populations in regions of about 100 km in lateral dimensions were compared, a positive correlation was found 2 between mean Matachewan dyke trend and magnetization declination, a correlation that was absent between dykes within the areas. This result suggested that the variation in mean dyke trend was a product of differential crustal rotation about vertical axes, and therefore that the broad Z-shaped trend of the dykes across the southwestern end of the KZ was the result of the deformation of an originally more linear dyke swarm2. Further work on Matachewan dykes3, and also on younger swarms that have equivalents on both sides of the KZ (2170 Ma Biscotasing4, 2076 Ma Fort Frances/2069 Ma Lac Esprit5), confirm that the western half of the Superior Province has rotated counterclockwise about 20° with respect to the eastern half. More recent work on 2101-2121 Ma Marathon dykes6 suggests that the western half of the Superior Province has not rotated as a single unit because the KZ as a fault zone extends for more than 100 km to the northwest of the main zone of crustal uplift. The age of the rotation is thought to be about 2 Ga old and related to the deformation along the KZ. A possible consequence of the rotation is that a rift in the Superior province opened beneath Hudson Bay and remained a region of crustal weakness to be subsequently exploited in later Phanerozoic subsidence that formed the Hudson Bay basin and associated lowlands4. References [1] Halls, H. C., Zhang, B. 2003. Crustal uplift in the southern Superior Province, Canada, revealed by paleomagnetism. Tectonophysics 362, 123-136. [2] Bates, M. P., Halls, H .C. 1991. Broad-scale Proterozoic deformation of the central Superior Province revealed by paleomagnetism of the 2.45 Ga Matachewan dyke swarm. Canadian Journal of Earth Sciences 28, 1780-1796. [3] Halls, H. C., Stott, G. 2003. Paleomagnetic studies of mafic dykes in the vicinity of Lake Nipigon, northwestern Ontario. - 20 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 in Summary of Fieldwork and Other Activities 2003, Ontario Geological Survey, Open File Report 6120, p. 11-1 to 117. [4] Halls, H.C., Davis, D.W. 2004. Paleomagnetism and U-Pb geochronology of the 2.17 Ga Biscotasing dyke swarm, Ontario, Canada: evidence for vertical-axis crustal rotation across the Kapuskasing Zone. Canadian Journal of Earth Sciences 41, 255-269. [5] Buchan, K.L., Goutier, J., Hamilton, M., Ernst, R E., Mathews, W. 2004. Paleomagnetism of the Lac Esprit dykes and implications for crustal rotation of the Canadian Shield. AGU-CGU Meeting, Montreal [Abstract]. [6] Halls, H.C., Davis, D.W., Stott, G. 2005. Paleomagnetism and U-Pb dating of Proterozoic dykes: a new radiating swarm and an increase in post-Archean crustal rotation westwards from the Kapuskasing Zone, Ontario. GAC-MAC Halifax, Meeting, [Abstract]. - 21 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Mesoproterozoic Diabase Sills of the Nipigon Embayment, northwest Ontario HART, T.R., Precambrian Geoscience Section, Ontario Geological Survey, 933 Ramsey Lake Road, Sudbury, Ontario P3E 6B5 (tom.hart@ndm.gov.on.ca) and MacDONALD, C.A., Caracle Creek International Consulting Inc., Sudbury, Ontario, P3E 2V7 The Nipigon Embayment is an approximately 19,000 km2 area of Mesoproterozoic igneous and sedimentary rocks centred on Lake Nipigon, north of Lake Superior. The current extent of the Embayment is defined by the sporadic occurrence of diabase sills suggesting that the Embayment was probably more extensive prior to erosion. A series of shallow dipping, 1110 - 1113 Ma, diabase sills of the Nipigon Sill Complex (Heaman et al., 2005) intrude all Archean and Proterozoic rocks in the Embayment. The Nipigon Sill Complex is composed of at least 3 major, and a series of minor, diabase sills ranging in thickness from a few tens of metres to >200 m. The diabase is commonly sub-ophitic to ophitic, massive, medium- to coarse-grained feldspar and pyroxene with trace olivine and 1 to 2% magnetite. Medium- to coarsegrained diabase forming the majority of the sills should properly be classified as gabbro, but to avoid confusion with other intrusions in the area the diabase classification has been applied to all rocks associated with the sills. The sills are generally shallow dipping occasionally forming broad, up to 6 km wide, saucers with interiors occupied by older rock types (Figure 1). Figure 1. First vertical derivative of the total field magnetic data (Ontario Geological Survey, 2004) showing an inward dipping saucer shaped diabase sill in the southern Black Sturgeon River map area (Hart, 2005). The geometry of these saucers is in part evident from contact relationships observed during bedrock mapping (e.g. Hart, 2005), and from the magnetic patterns on the first vertical derivative of the total field airborne magnetic data (e.g. Ontario Geological Survey, 2004). Modelling a saucer located in the Garden Lake greenstone belt of the Wabigoon Subprovince, west of Lake Nipigon, produced two possible models (J. Rudd, Ontario Geological Survey, personal communication, 2000). However, the model of a shallow dipping thin sill best agreed with field observations, and is the best fit between field observations and geophysical data for similar structures in the Beardmore area (Hart et al., 2000) and in the southern Black Sturgeon River area (Hart, 2005). Similar sill geometry has been suggested for other mafic sill complexes (e.g., Thomson and Hutton, 2004), although the saucers and nested saucers structures in those areas are generally smaller in diameter. One reason for the large scale of the saucers in the Nipigon Embayment may be the more competent nature of the Archean basement rocks compared to the host rocks of other sill complexes. - 22 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 The majority of the sills have relatively uniform mineralogy and geochemistry complicating the correlation of the sills within the Embayment. Minor stratigraphic variations within some sills suggest that they may have formed by injection of multiple pulses of magma. However, a thickening of some sills to 250 and 400 m in thickness in the Muskrat and Gieke lakes area, in combination with geochemical trends, suggests that these thicker sills are single cooling units, and may also indicate proximity to the feeders for the sills. The exception to the uniform geochemistry of the Nipigon sills is the >1120 Ma Inspiration Sill (Heaman and Easton, 2005) in the northwest portion of the Nipigon Embayment, which has higher La/Yb and Zr/Y ratios and a positive magnetic polarity in contrast to the negative polarity of the Nipigon sills (MacDonald, Tremblay and Easton, 2005). Another significant geochemical change occurs around Thunder Bay (Hart, 2004) where the majority of the sills located to the south of the city are part of the 1115±1 Ma Logan Sill Complex (Heaman and Easton, 2005). Similar geochemical variations related to geographic position have been observed in some flood basalt provinces (e.g., Mantovani et al., 1985) suggesting that the Logan and Nipigon sills may be part of a single intrusive event. References Hart, T.R. 2004. Geochemistry of the Proterozoic intrusive rocks of the Nipigon Embayment; abstract in Institute on Lake Superior Geology, Proceedings, 50th Annual Meeting, Duluth, Mn, v.50, pt.1, p.68-69. Hart, T.R. 2005. Precambrian Geology of the South Black Sturgeon River and Seagull Lake Area, Northwestern Ontario; Ontario Geological Survey, Open File Report 6165, 63p. Hart, T.R., terMeer, M. and Jolette, C. 2002. Precambrian Geology of Kitto, Eva, Summers, Dorothea and Sandra Townships, Beardmore Area, Northwest Ontario. Ontario Geological Survey, OFR 6095, 206p. Heaman, L.M. and Easton, R.M. 2005. Proterozoic history of the Lake Nipigon area, Ontario: Constraints from U-Pb zircon and baddeleyite dating; abstract in Institute on Lake Superior Geology, Proceedings, 51ts Annual Meeting, Nipigon, Ontario, v.51, pt.1. Heaman, L.M., Easton, R.M., Hart, T.R., MacDonald, C.A., Fralick, P., and Hollings, P., 2005. Proterozoic history of the Lake Nipigon area, Ontario: Constraints from U-Pb zircon and baddeleyite dating; Canadian Institute of Mining and Metallurgy Annual Meeting, Toronto 2005, Program with Abstracts. MacDonald, C.A., Tremblay, E., and Easton, R.M. 2005. Precambrian Geology of the west-central map area, Nipigon Embayment, northwestern Ontario; Ontario Geological Survey, Open File Report 6164, 48p. Ontario Geological Survey 2004. Ontario airborne geophysical surveys, magnetic and gamma-ray spectrometer data, grid and vector data, ASCII format, Lake Nipigon Embayment Area; Geophysical Data Set 1047a. Thomson K. and Hutton, D. 2004. Geometry and growth of sill complexes: insights using 3D seismic from the North Rockall Trough; Bulletin of Volcanology 66, 364-375. - 23 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Proterozoic history of the Lake Nipigon area, Ontario: Constraints from U-Pb zircon and baddeleyite dating HEAMAN, L.M., Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, T6G 2E3 larry.heaman@ualberta.ca and EASTON, R.M., Precambrian Geoscience Section, Ontario Geological Survey, Sudbury, Ontario P3E 6B5 The Lake Nipigon Region Geoscience Initiative, a 2-year, science-focused, Industry-Government-University collaborative geological study, collected new geoscience data in the Lake Nipigon region of Ontario between May 2003 and May 2005. As part of this study, over 40 new U-Pb baddeleyite and zircon age determinations were acquired from predominantly mafic rocks of the Lake Nipigon region. Preliminary results of this study are reported herein. William Logan first described the copper-bearing rocks of the Midcontinent Rift (MCR) in Ontario in 1863. Subsequent studies by many researchers indicate that the MCR in Ontario comprises 3 main tectonic elements: 1) the volcanic and sedimentary rocks in and along the margin of Lake Superior, 2) associated alkalic complexes, 3), the mafic and ultramafic intrusions centred on Lake Nipigon, and whether they represent a “failed arm” of the main rift. Work in Ontario has tended to focus on the Lake Nipigon area, because of accessibility and mineral potential. The best estimate for a maximum depositional age of a greywacke from the upper Rover Formation is provided by a 1777 Ma concordant detrital zircon grain, as the youngest grain in the sample, at 1731 Ma, is 5% discordant. This interpretation is supported by the abundance of grains (n=10) between 1796 to 1777 Ma, and is consistent with U-Pb ash bed ages of 1836±5 and 1832±3 Ma from the basal Rove Formation (Addison et al., 2005). Detrital zircon ages from sandstones of the lower (Pass Lake Formation) and upper (Nipigon Bay Formation) Sibley Group indicate maximum depositional ages of 1634 and 1670 Ma, respectively, with a predominance of Geon 17 and 18, not Archean, detritus. Data from the middle to upper (Outan Island Formation) Sibley Group indicates a maximum depositional age of 1450 Ma, as well as Geon 15 and 17 detritus, but no Geon 18 grains. The Geon 15 detritus may be locally derived, e.g. from the 1547±4 Ma English Bay complex volcanic rocks, the 1590±1 Ma Pillar Lake felsic intrusion, and the 1599±1 Ma Pillar Lake gabbro. In addition, sediment interbedded with flat-lying mafic volcanic rocks south of Armstrong are younger than 1514 Ma, based on detrital zircons in interbedded sediments, but older than overlying, circa 1159 Ma (minimum age of 1120±1 Ma), Inspiration diabase sills. The abundance of Geon 15 ages in the western Nipigon Embayment is impressive, as the period between 1600 and 1520 Ma in eastern North America has been recognized for some time as a period of quiescence throughout Laurentia (Gower et al., 1990). Furthermore, there are no dike swarms or reliable paleopoles for the period 1700 to 1500 Ma. The only other reliable Geon 15 age in North America is the 1576+/-13 Ma upper intercept age from the Priest River metamorphic complex in Idaho, which served as basement to the circa 1450 Ma Belt and Purcell Supergroups (Evans and Fischer, 1986). Gower (1996) suggested that, at least in the eastern Grenville Province, this quiescence might be linked with the development of a passive continental margin, however, such a setting does not explain the felsic magmatism present in the Nipigon area. The new U-Pb data obtained in this study suggest the presence of an earlier period of magnetically normal, localized alkalic (lamprophyre dikes, Queen et al., 1996) and mafic magmatism (Inspiration sills and the Pigeon River dikes (1141±20 Ma)) between 1150 and 1135 Ma in the MCR in Ontario, including the possibility of localized volcanism and sedimentation in the Pillar Lake area south of Armstrong. Baddeleyite data from the 4 ultramafic intrusions in the region shows some scatter within individual - 24 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 intrusions (e.g. from 1113 to 1124 Ma in the Seagull intrusion), which could indicate a protracted (up to 10 m.y.) crystallization history for some of these bodies. In addition, the suite of ultramafic intrusions has a range of ages from circa 1118 to 1107 Ma, however, the aforementioned scatter makes it difficult to ascertain the significance of this range – does it reflect protracted cooling or different emplacement times, or both? A related ultramafic body, the Jackfish Island sill, cuts the English Bay complex and is geochemically similar to the Kitto intrusion, although it appears to be slightly younger in age (1112±3 Ma versus 1118±2 Ma, respectively). In contrast, baddeleyite ages from 4 different Nipigon diabase sills analyzed so far cluster between 1110 and 1114 Ma. Although geochemically distinct from the Nipigon sills (Hart, 2003), a sample from the Logan sill at Mount McKay in Thunder Bay, is similar in age, at 1115±1 Ma. Thus, most mafic and ultramafic rocks in the Lake Nipigon and Superior areas, including the Nipigon and Logan sills, appear to have been emplaced in a short, magnetically reversed, internal between 1115 and 1100 Ma. Emplacement of alkalic intrusions, such as the Coldwell complex, as well as filling of much of the submerged part of the rift in Lake Superior, also occurred in this period. This was followed by a period of magnetically normal, waning mafic and felsic magmatism, between 1096 and 1085 Ma, that is preserved mainly along the Lake Superior shore by units such as the Crystal Lake (1099±1 Ma), Moss Lake (1095±2 Ma) and Blake Lake (1095±2 Ma) gabbros, and the Arrowhead dike (1093±3 Ma). References Addison, W.D., Brumpton, G.R., Vallini, D.A., McNaughton, N.J. Davis, D.W. Kissin, S.A. Fralick, P.W. and Hammond A.L. 2005. Discovery of detrital ejecta from the 1850 Ma Sudbury impact event; Geology, 33, 193-196. Evans, K.V. and Fischer, L.B. 1986. U-Pb geochronology of two augen gneiss terranes, Idaho – new data and tectonic implications; Canadian Journal of Earth Sciences, 23, 1919-1927. Hart, T.R. 2003. Keweenawan mafic and ultramafic intrusive rocks of the Lake Nipigon and Crystal Lake areas, northwest Ontario; Institute on Lake Superior Geology, Proceedings, 49, pt.1, Programs and Abstracts, 21-22. Gower, C.F. 1996. The evolution of the Grenville Province in eastern Labrador Canada; in Precambrian crustal evolution in the North Atlantic Region, Geological Society of London, Special Publication 112, 197-218. Gower, C.F., Ryan, A.B. and Rivers, T. 1990. Mid-ProterozoicLaurentia-Baltica: an overview of its geological evolution and a summary of the contributions made by this volume, in Mid-Proterozoic Laurentia-Baltica, Geological Association of Canada, Special Paper 38, 1-20. Queen, M., Heaman, L.M., Hanes, J.A., Archibald, D.A. and Farrar, E. 1996. 40Ar/39Ar phlogopite and U-Pb perovskite dating of lamprophyre dikes from the eastern Lake Superior region: evidence for a 1.14 Ga magmatic precursor to Midcontinent Rift volcanism; Canadian Journal of Earth Sciences, 33, 958-965. - 25 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Penokean Fold-and-thrust Deformation of the Paleoproterozoic Gunflint Formation near Thunder Bay, Ontario HILL, Mary Louise, Department of Geology, Lakehead University, Thunder Bay ON P7B 5E1 CANADA and SMYK, Mark C., Ontario Geological Survey, Ministry of Northern Development and Mines, Suite B002, 435 James St. South, Thunder Bay, ON P7E 6S7 CANADA Flat-lying, Paleoproterozoic Gunflint Formation chemical and clastic sedimentary rocks unconformably overlie Archean basement rocks near Thunder Bay. Folds have long been recognized in the Gunflint Formation near Pass Lake, 40 km northeast of Thunder Bay, but no tectonic explanation has previously been demonstrated. The recent recognition of thrust faults in this area appears to link this folding to Penokean compression. Although Penokean orogenic events (circa 1875 to 1835 Ma) and resultant deformation have been well established south and west of Lake Superior, no structures ascribed to Penokean deformation have ever been described north of Lake Superior. Recent geochronologic data show that the Gunflint Formation (circa 1878 Ma) predates the earliest Penokean thrusting and thrust-loading caused by the collision of the Pembine-Wausau terrane (circa 1860-1850 Ma). This prompted Fralick et al. (2002) to contend that the Gunflint formed in a back-arc extensional setting, rather than in a foredeep (foreland basin) and also suggests that Penokean structures should exist in these rocks. Recent examination of the Gunflint Formation near Pass Lake has led to the recognition of structures typical of fold-and-thrust belt deformation. Discrete bedding-plane faults with locally developed gouge and breccia can be traced laterally into horizontal, hanging-wall ramps with associated fault-bend folding. Fold-and-thrust belt deformation is caused by regional compression. Previous workers had ascribed the folds to syn-sedimentary slumping and Keweenawan diabase sill emplacement and thought that they were attributable to local, rather than regional-scale, deformation. Displacement in fold-and-thrust belts tends to be localized along discrete bedding planes and not easily recognized. This may account for the perceived lack or absence of structures elsewhere in the Gunflint Formation. Penokean structures on the northern side of Lake Superior represent the northward migration of thrust faults into the foreland (passive margin Archean basement + Gunflint Formation) caused by hinterland collision to the south. Future work will focus on recognizing other deformed locales and quantifying fault displacements based on stratigraphic correlation of sedimentary units. References Fralick, P., Davis, D.W. and Kissin, S.A. 2002. The age of the Gunflint Formation, Ontario, Canada: single zircon U–Pb age determinations from reworked volcanic ash; Canadian Journal of Earth Science, v.39, p.1085–1091. - 26 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 The incredible shrinking Penokean orogen: a new look at the accretionary history of the southern Lake Superior region HOLM, D.K., Dept. of Geology, Kent State University, Kent, OH; CANNON, W.F., USGS, Reston, VA; CHANDLER, V.W., MGS, Mnpls, MN; SCHNEIDER, D.A., Ohio University, Athens, OH; SCHULZ, K., USGS, Reston, VA; and VAN SCHMUS, W.R., University of Kansas, Lawrence, KS. The Penokean orogen in the stable continental interior of the upper Great Lakes region records the initiation of rapid late Paleoproterozoic (1.9-1.6 Ga) southward growth of Laurentia. The Penokean orogeny is generally accepted to have been a period of southward subduction from about 1900 to 1850 Ma during which juvenile Paleoproterozoic terranes were accreted to the southern edge of the Superior craton (Archean) and foreland sedimentation and deformation occurred on the craton margin. Historically, juvenile Penokean crust was interpreted to extend southward beneath Paleozoic and Cretaceous strata into southern Iowa, southern Wisconsin, and northern Illinois. However, a growing body of geochronologic data suggests that there may be no rocks older than 1.75 Ga in those areas and that the current Penokean orogen has a much more restricted geographic distribution than heretofore inferred. We have identified a fundamental ENE-trending boundary, the Spirit Lake-Trempealeau discontinuity CB: Cheyenre Belt GFTZ: Great F alIsTectoniozone (SLTD), using a combination of potential field geophysical GLTZ: Great LakesTecton Zore data and drill hole information (see Figure 1), which marks MSM: Mojave-Sonora Megashear NFZ: Niagara F ault Zone the southern limit of Archean and known Penokean rocks. SLTD: Spir Lake-Trempealeau South of this boundary rocks at the subcrop are dominated Dbonnuii 300 km by subaerial potassic rhyolite and epizonal granite, formed at about 1.75 Ga, and ultra-mature quartzite, such as the Baraboo Quartzite, which lies unconformably on them. Gneisses and mafic volcanic rocks, probably basement rocks from which the rhyolites formed by partial melting, are inferred from gravity and magnetic highs to be at subcrop in several areas. Because the SLTD transects geon 18 Penokean structures, it appears to be a post-Penokean feature perhaps marking the northern margin of a Yavapai age accreted terrane. It lies directly above an abrupt offset in Moho depth (deeper to the south) beneath Lake Michigan identified in GLIMPCE deep seismic surveys. The offset was previously interpreted as a north-dipping Penokean subduction zone, but it now seems more likely to be a Yavapai feature. We interpret the SLTD to be a fundamental Yavapai-age Proterozoic boundary, equivalent to the Cheyenne belt paleosubduction zone in N southern Wyoming. The Cheyenne belt juxtaposes geon Figure 1. Map showing major Precambrian crustal 17 Yavapai orogen crust on the south against the Archean Map showing major Precambrian crustal *je provinces age provinces and tectonic boundaries in the western Wyoming craton, and transects geon 18 (Trans-Hudson) and tectonic Ixundaries in thewestem and central and central United Stated United States. structures in southern South Dakota. The interpretation shown here suggests that the Cheyenne suture extends eastward, joins with the Spirit Lake trend in Iowa, and continues east across the Midcontinent Rift as the Trempealeau discontinuity in Wisconsin. Geon 17 plutonic rocks north of the SLTD, and a north-dipping crustal reflection across it, are consistent with intervals of north-directed subduction during Yavapai accretion. Younger, Mazatzal-age compression deformed quartzites that overlie the Yavapai, Marshfield, and Penokean terrane rocks indicating that geon 16 deformation extended far north of the still imprecisely defined Yavapai-Mazatzal terrane boundary. Barring major strike-slip motion, our revised tectonic map suggests progressive accretion of juvenile arc terranes every 100 million years or so during the late Paleoproterozoic (circa 1.85, 1.75, and 1.65 Ga). - 27 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Geologic Implications of Bedrock Mapping in the Ely and Basswood Lake Quadrangles, Northeast Minnesota JIRSA, Mark A. and MILLER, James D., Jr., Minnesota Geological Survey (jirsa001@umn.edu and mille066@umn.edu) Two regional bedrock geologic compilation maps of northeast Minnesota at scale 1:100,000 have recently been completed by the Minnesota Geological Survey (Figure 1), funded in large part by the STATEMAP program of the U.S. Geological Survey. The most current—described here—covers the Ely and U.S. portion of the Basswood Lake 30ʼ x 60ʼ quadrangles (Jirsa and Miller, 2004). The compilation integrates new mapping, unpublished work contained in 7 theses, and the classic Knife Lake maps of Gruner (1941), into a lithostratigraphically consistent, digital (GIS) format. Much of the map depiction of Paleoproterozoic (Animikie Group) and Mesoproterozoic rocks (Duluth Complex and North Shore Volcanic Group) is not changed substantively from recent works by Miller and others (2001). By contrast, the Archean geology is considerably modified from earlier geologic compilations at 1:250,000 scale (Green, 1982). The area encloses the central part of the Boundary Waters Canoe Area Wilderness (Figure 2), and is the first new publication of mapping of that area in many decades. . 9 7?:. -d _______ International Falls -r—,,,•,•.-,,,,,,,•,fl,, - • VERMILION LAKE Quadrangle - . - 92 7, ELY-BASSWOOD LAKE Quadrangles MESOPROTEROZOIC Duluth Complex and North Shore Volcanic Group PALEOPROTEROZOIC Virginia-Thomson Formations Biwabik Iron Formation NEOARCHEAN Granitoid batholiths and plutons I Schist of sedimentary protolith Metavolcanic and metasedimentary rocks Duluth Figure 1. Generalized geologic map of northeastern Minnesota showing the location of recently mapped 30ʼ x 60ʼ quadrangles; Jirsa and Boerboom, 2003, and Jirsa and Miller, 2004. Exposures provide what is likely the most complete view of Archean geology anywhere in Minnesota. The Neoarchean bedrock lies within the Wawa and Quetico subprovinces of the Superior Province (Figure 3). A third, distinctive sequence of rocks—the Knife Lake Group—is interpreted to have been deposited in a complex array of successor basins developed along early-formed faults near the boundary between the two subprovinces. The Knife Lake strata include hornblende-bearing alkalic and calc-alkalic volcanic flows and unusual hornblenderich tuff, conglomerate derived from multiple sources including the 2,689 Ma Saganaga Tonalite, and feldspathic graywacke and mudstone. Much of the temporal distinction between various geologic elements of the Archean bedrock is based on fabrics that resulted from three major phases of deformation, denoted D1, D2, and D3. All three deformation events are the result of north–south- to northwest–southeast-directed compression. The timing of D1 deformation is bracketed between deposition of the volcanic and clastic rocks of the Wawa subprovince at about 2,722 Ma (Peterson and others, 2001), and emplacement of the Saganaga Tonalite at about 2,689 Ma (Corfu and Stott, 1998). Folding in volcanic rocks of the Ely Greenstone attributed to D1 deformation is truncated by Knife Lake strata, indicating that the latter is synchronous with or post-dates deposition and early deformation of the Ely Greenstone. As such, the Knife Lake Group is inferred to be a Timiskaming-type sequence temporally - 28 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 equivalent to the Shebandowan assemblage exposed in adjacent parts of Ontario (Corfu and Stott, 1998). D2 deformation affects all of the Archean supracrustal units and is bracketed by U-Pb dates of intrusions in the Giants Range batholith that place the regional deformation and metamorphic event between about 2,674 Ma and 2,685 Ma (Boerboom and Zartman, 1993). D3 deformation produced faults in the low-grade supracrustal and intrusive rocks of the Wawa subprovince, and folding of granitic and migmatitic rocks in the Quetico subprovince. Saganta Tonallie lUSCr L9' BWCA DULUTH COMPLEX 4sorthosItIc sMn Figure 2. Schematic geologic map of the Ely and Basswood Lake 30ʼ x 60ʼ quadrangles showing major lithologic components, including subdivisions of rock units in the Archean bedrock, Paleoproterozoic rocks of the Mesabi Iron Range, and intrusions of the Mesoproterozoic Duluth Complex. f_i / / South KawlshIw- // BIdEgI C--;' jMASI IRON RANGE - - - - - - /'vr > Partndge F Greenwood , / Beaver I Comple: ,--4' / SHORE Ey-Bassswoad L Th:, SO. x 60 qJ3]. SHEBANOOWAN %. GRtENSIONE BELT _RMPLION (!It!pI@ VERMIUON SHEBANOOWAN T!k!!Q.Lypth&& I(NIFE LME SHEBANOOWN qees GROUP ASSEMBLAGE ft]'Y NEWTOn LAKE GREEN WATERJBURCHELL SobprQWEs M@ISi L—J Ok.-thd€ih-kIIi& Ia 1114 oy& {271&27M} ELYGREENSTONE M) Figure 3. Generalized geologic map showing potential stratigraphic correlation between Archean rocks of the Vermilion district in Minnesota and those of the Shebandowan greenstone belt in Ontario. Shaded areas are lakes. Abbreviated References Boerboom, T.J., and Zartman, R.E. 1993. Canadian Journal of Earth Sciences 30:2510-2522. Corfu, F., and Stott, G.M. 1998. Geological Society of America Bulletin 110:1467-1484. Green, J.C. 1982. Minnesota Geological Survey, Two Harbors Sheet; scale 1:250,000. Gruner, J.W. 1941. Geological Society of America Bulletin 52: 1577-1642. Jirsa, M.A. and Boerboom, T.J. 2003. Minnesota Geological Survey Map M-141, scale 1:100,000. Jirsa, M.A., and Miller, J.D., Jr., 2004, Minnesota Geological Survey Map M-148, scale 1:100,000. Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W. and Peterson, D.M. 2001. MGS Map M-119, scale 1:200,000. Peterson, D.P., Gallup, C., Jirsa, M.A. and Davis, D.W. 2001. ILSG, 47th Annual Meeting, Proceedings, p. 77-78. - 29 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Structural State of Plagioclase Phenocrysts in Porphyritic rocks of the Midcontinent Rift, Northeastern Minnesota KNUDSEN, Damion, Department of Geosciences, North Dakota State University, Fargo, ND 58105, damion.knudsen@ndsu.edu; SAINI-EIDUKAT, Bernhardt, Department of Geosciences, North Dakota State University, Fargo, ND 58105, MILLER, James D., Jr., Minnesota Geological Survey, University of Minnesota, c/o Natural Resources Research Inst., 5013 Miller Trunk Hwy, Duluth MN 55811, and DANIELS, Peter, Markstrasse 123, 44803 Bochum, Germany. The structural states of plagioclase phenocrysts from porphyritic rocks of the Midcontinent Rift in northeastern Minnesota were analyzed to determine the origin of the plagioclase and to evaluate possible petrogenetic relationships between rock units. As a general rule, higher states of structural disorder in the crystal lattice of plagioclase reflect higher temperatures of equilibration. The units studied include hypabyssal sills of the Beaver Bay Complex (BBC), anorthositic rocks of the Duluth Complex, and porphyritic basalts of the North Shore Volcanic Group (NSVG). The NSVG is a 7-10 kilometer thick volcanic edifice composed of flood basalts and minor felsic volcanics, intermediate lavas, and interflow sandstones (Green, 1972). The BBC is composed of at least thirteen hypabyssal intrusions, with widely varying compositions, that were emplaced in the medial section of the NSVG (Miller and Chandler, 1997; Miller et al., 2002). The chemistry and texture of plagioclase phenocrysts in some porphyritic NSVG basaltic flows and BBC intrusives suggest they are related to plagioclase-rich magmas (mushes) that are thought to have produced the anorthositic series rocks of the stratigraphically deeper, more plutonic Duluth Complex (Miller and Weiblen, 1990). Plagioclase phenocrysts were separated for XRD using common magnetic and heavy liquid methods. Patterns were collected in 0.02° step scan mode on a Philips Xʼpert PW 3040-MPD diffractometer using 2θ from 18° to 55°, 5 sec/step, and λ =1.54178 Å. Refined unit-cell parameters were used to approximate plagioclase Al, Si distributions using Kroll and Ribbeʼs (1980) γ method, and a derivative method using t10-<t1m> vs. anorthite content (An). An contents were determined by electron microprobe at the Department of Geology and Geophysics, University of Minnesota-Twin Cities. The rock types analyzed for this study are: (a) plagioclase porphyritic, ophitic basalts from near Silver Bay and Croftville, (b) plagioclase porphyritic ferrodiorites of the Cabin Creek and the Leveaux Porphyry intrusions (early intrusions of the BBC), (c) porphyritic leucogabbros and gabbroic anorthosites of the Scott Creek and Katydid Lake intrusions that occur near the transition between the BBC and Duluth Complex (both are thought to represent offshoots of the anorthositic series; Boerboom and Miller, 1994); and (d) a porphyritic diabase dike, possibly an offshoot of the main Scott Creek intrusion. The phenocrysts in these rock types are compared with anorthosite xenoliths occurring in the Beaver River diabase (Knudsen et al., 2005) and with anorthositic series rocks of the Duluth Complex (Bandli and Saini-Eidukat, 2000). XRD results (Figure 1) show the Croftville basalt phenocrysts, along with those of the porphyritic dike, have intermediate to high disorder, plotting near the curve for disordered plagioclase. In contrast, the Silver Bay basalt phenocryst has intermediate to low structure. The Leveaux Porphyry phenocrysts have intermediate structures and plot near Duluth Complex anorthositic series samples, while the Cabin Creek phenocrysts have distinctly more disordered structures. Plagioclase phenocrysts from the Scott Creek leucogabbro and the Katydid Lake gabbroic anorthosite are intermediate to ordered and plot within the field of Duluth Complex anorthositic series samples. Anorthosite xenoliths in the BBC are intermediate to ordered. Plagioclase separated from the matrix of selected samples all have lower An contents than the phenocrysts - 30 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 they host, and show intermediate to high disorder. The relationship of the matrix t10-<t1m> values to those of the phenocrysts they host reverses in a manner similar to that noted by Hoffer (1968) for the Rock Creek porphyritic basalt flow of Idaho. Figure 1. Order distributions of measured plagioclases between the various rock types. A. γ vs. An, after Kroll and Ribbe (1980). B. derivative plot of t10-<t1m> vs. An. Abbreviated References Albers, P. and Miller, J.D., Jr.,2005. (this volume). Bandli, B. and Saini-Eidukat, B. 2000. Proc. ILSG, v. 46, part 1: 4-5. Boerboom, T.J., and Miller, J.D., Jr. 1994. MGS Misc. Map M-81. Knudsen, D., Saini-Eidukat, B., Miller, J.D., Jr., Daniels, P. 2005. GSA–N. Centr, Abstr., Mpls, MN, May 19-20. Green, J.C. 1972. in Geology of Minnesota – A centennial volume: 294-332. Kroll, H. and Ribbe, P.H. 1980. Am. Min. 65: 449-457. Hoffer, J.M. 1968. Am. Min. 53: 908-916. Miller, J.D. Jr. and Chandler, V.W. 1997. in GSA Sp. Pap. 312: 73-96. Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.M. and Wahl, T.E. 2002. MGS RI 58, 207 p. - 31 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Petrogenesis and PGE mineralization of the Eva-Kitto Intrusion, northern Ontario LAARMAN, J. and HOLLINGS, P., Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, On, P7B 5E1, jlaarman@lakeheadu.ca The Eva-Kitto Intrusion is a Mesoproterozoic semicircular peridotitic ring complex situated in the Nipigon Embayment on the eastern shore of Lake Nipigon near Beardmore, Ontario. The age of the intrusion is 1117.7±1.8 Ma (L.Heaman, University of Alberta, personal communication, 2005) and has been interpreted to be emplaced as the feeder conduit to a more evolved cone sheet during initial asthenospheric upwelling of a mantle plume in the Midcontinent rift system (MCR) (Sutcliffe, 1986). The intrusion cross-cuts both the Archean Southern Sedimentary Belt (SSB) and the Southern Volcanic Belts (SVB) of the Beardmore-Geraldton Greenstone belt, dated at circa 2696-2691Ma, of the Wabigoon Subprovince (Lafrance et al., 2004). Within the SVB are sulphidized magnetite-chert iron formations that run parallel to the strike of the main structural trends of the greenstone belt and are intercalated with mafic volcanic units. In drill core, the intrusion cross-cuts calcareous sandstones and mudstones interpreted to be the Sibley metasediments and is cross-cut by Nipigon gabbro sills which are also found to crop out as horizontal circular ridges around the intrusion. The Eva-Kitto peridotite has high TiO2, K2O and P2O5 and is LREE-enriched similar to the chemistry of the Fe-Ti Osler Volcanics of the MCR (Hart et al., 2002). The sills, on the other hand, have lower TiO2, K2O and P2O5 than the ferropicritic peridotites, and are similar in chemistry to the olivine tholeiitic flood basalts of the MCR. Therefore the Nipigon sills originated from a more fractionated underplated basaltic magma later in the Keewanawan event circa 1109 Ma (Sutcliffe, 1987). j... The petrography and geochemistry of the lithologies within the Eva-Kitto Intrusion are being studied from drill core held by East West Resources Inc. Four holes were drilled by Kennecott Canada Exploration Inc. in 2002 Lake N4Igon based on airborne MegaTEM anomalies surveyed by Fugro Airborne Surveys that same year. Hole EK-2 intersected disseminated po-cpy mineralization and was the only hole to intersect magnetiferous pyritic metasediments of the Archean basement. Assays values up to 0.28%Ni, 0.13%Cu, and 563ppb Pt+Pd in a 1.22m interval have been reported (Rossell, 2003). Lithologies within EK-2 from the top to the end of hole at 345m at depth comprise lherzolite, olivine websterite, sulphidized olivine websterite, vari-textured pyroxenite, spotted pyroxenite, pyroxenite, melanogabbro, magnetiferous pyritic metasediment, and mafic volcanics. EK-2 is the only hole to intersect the more fractionated varitextured pyroxenite to orthocumulate pyroxenite lithologies that contain PGE mineralization. .::L: Figure 1: Geological map ofthe Eva Kitto Ring Complex with its Figure 1. Geological map of the Eva Kitto Ring location circled on the Lake Nipigon inset. The four ER drill holes Complex. four EK drillofthe holes are located at the are located atThe the southern extent peridotitic intrusion. southern end of the peridotitic intrusion Based on preliminary interpretation of the petrography the petrogenesis of the Eva-Kitto Intrusion can be characterized by two magmatic pulses. A first pulse of magma is suggested by the pyroxenite and melanogabbro units of EK-2. The pyroxenite has an orthocumulate texture with cumulate pyroxenes enclosed in a groundmass of lathy plagioclase. The melanogabbro has a poikilitic texture with cumulate pyroxene crystals housed in plagioclase oikocrysts. A second pulse of magma is suggested by the incipient break up of cumulate - 32 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 pyroxene and growth of clusters of secondary pyroxene in the vari-textured and spotted pyroxenites. The patchy texture within these units can be attributed to the churning up of the pyroxenite by a second influx of magma. Lherzolite is the dominant lithology in all the drill holes and may represent the primary cumulate fractionation of an enriched ferropicritic magma (Sutcliffe, 1986). Lherzolite displays a mesocumulate texture of ovoidal cumulate olivine grains and larger cumulate clinopyroxene within poikilitic orthopyroxene and plagioclase. The olivine websterite is darker due to fractionation of pockety pyroxenes with breakdown of olivine. The olivine websterite possibly formed by flow differentiation of a more fractionated magma from the central lherzolite flow zone. Both the lherzolite and olivine websterite contain large amounts of primary biotite minerals. PGE minerals are associated with disseminated sulphides in the sulphidized olivine websterite, vari-textured pyroxenite, pyroxenite and melanogabbro units of drill hole EK-2. Within the olivine websterite and pyroxenite, disseminated pyrrhotite-pentlandite-chalcopyrite sulphides are found interstitial to cumulate olivine and pyroxene minerals. Blebby pyrrhotite-chalcopyrite sulphides, possibly a result of liquid immiscibility, are locally found in the sulphidized olivine websterite/melanogabbro units at the contact with the wallrock metasediments. Ongoing studies are focusing on petrogenesis and mineralization of the intrusion. Scanning electron microscope (SEM) discs made from drill core in Holes EK-1, 3 and 4 are being analysed under the petrographic microscope to compare lithologies in these holes with the fractionated suite of EK-2. The interstitial sulphides will be examined by SEM to identify the PGE minerals. Mineral chemistries within the various lithologies will also be identified using SEM. Bulk rock chemistries, S/Se elemental ratios, trace elements, εNd and Sr patterns, and Ni abundances of olivine will be analysed to further understand the petrogenesis of source magmas, and crustal contamination involved with mineralization. References Hart, T.R., terMeer, M. and Jolette, C. 2002. Precambrian geology of Kitto, Eva, Summers, Dorothea and Sandra townships, northwestern Ontario: Phoenix Bedrock Mapping Project. Ontario Geological Survey, Open File Report 6095, 206p. Lafrance, B., DeWolfe, J.C., Stott, G.M. 2004. A structural reappraisal of the Beardmore-Geraldton Belt at the southern boundary of the Wabigoon subprovince, Ontario, and implications for gold mineralization. Canadian Journal of Earth Sciences 41, 217-235. Rossell, D. 2003. November 2003 Report on Diamond Core Drilling on the Eva Kitto Property. Kennecott Canada Exploration Inc. Sutcliffe, R.H. 1986. Proterozoic rift-related igneous rocks at Lake Nipigon, Ontario. unpublished PhD thesis, University of Western Ontario, London, Ontario, 325p. Sutcliffe, R.H. 1987. Petrology of Middle Proterozoic diabases and picrites from Lake Nipigon, Canada. Contributions to Mineralogy and Petrology, 96, 201-211. - 33 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Geochemistry and Petrography of the Rabbit Islands Breccia, North Central Lake Nipigon LANE, C. J. and HOLLINGS, P., Department of Geology, Lakehead University, 955 Oliver Rd., Thunder Bay, Ontario, P7B 5E1, Canada. cjlane@lakeheadu.ca The Rabbit Islands breccia is a dyke-like intrusion, rich in calcite and dolomite, which intrudes the Nipigon diabase on the southern most island of the Rabbit Islands group in north central Lake Nipigon (Figure 1). Geological relationships indicate that the breccia must represent one of the youngest events associated with the Midcontinent rift. In the past this breccia has been referred to as a carbonatite diatreme (Sutcliffe and Greenwood, 1982), and one of the objectives of this study was to evaluate this model and to compare the breccia to the carbonatites of the Coldwell complex. Whole-rock geochemistry, petrography and scanning electron microscope (SEM) data, indicate that the groundmass material of this breccia is not of carbonatite affinity. The low abundance of trace elements such as Sr, Mn and Ba in SEM semi-quantitative scans of the carbonates and feldspars combined with the low abundances of Sr, Ba, V, and rare earth elements (REEʼs) determined by XRF and ICP-MS, do not satisfy the requirements of Samoilov (1991). Samoilov suggested that in order to be considered a carbonatite the following criteria must be met: enrichment in Sr (>700 ppm), Ba (>250 ppm), V (>20 ppm) and REE (including Y; >500 ppm). The samples analyzed from the Rabbit Islands breccia groundmass contained 191 ppm Sr, 286 ppm V, 115 ppm total REEʼs and low amounts of barium. Consequently, the breccia does not meet the limits set by Samoilov (1991) and therefore is not considered to be of carbonatitic affinity, but rather a carbonate-rich breccia. Evidence for changes in fluid composition has been observed within the groundmass material. For example, in many instances calcite has been shown to brecciate and replace dolomite crystals present in the breccia, however, quartz and feldspar brecciate and replace the dolomite. Other textures, such as calcite replacing feldspar emphasize the fact that there is an overlap of fluid phases, which may be controlled by fluid source or contamination by the (0 •0 (a C 2. a 0= = -, @02 EQ (a e. 0 = o (0 0 -o 'C 2o o = —. == OW0 ao = (a0 (a 0 'C wmz The geochemistry of the clasts and the country rock associated with the breccia has been compared with the different suites of Nipigon diabase: the Inspiration suite, the Jackfish suite, the McIntyre suite and the Main Nipigon suite (MacDonald and Tremblay, 2004; Rabbit Islands Richardson and Hollings, 2004), showing that the breccia intruded into the Main Nipigon suite (Figure 2). Clasts of diabase found within the breccia have undergone extensive alteration. Many of the clasts display consecutive zones of alteration which can be seen in both hand sample and thin section. Elements which have become mobile within the altered clasts, and show variation between each alteration zone include: potassium, calcium, magnesium, strontium, europium, yttrium, lanthanum, and neodymium. Depletion of calcium, strontium, europium, yttrium, lanthanum and neodymium can be seen from the centre of the clast outwards. Whereas, potassium, magnesium and loss on ignition (due to the presence of chlorite) increase from the centre of the clast outwards. Figure 1. Location of the Rabbit Islands (Modified from Hollings et al. 2004). - 34 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Rabbit Islands i><+><i Main Nipigon Suite RbThUNbKTaLaCePrSrNdZrHfSmEuTGdThDyYHoErTmYbALuVSc Figure 2. Primitive mantle multi-element plot to show the similarities between the Rabbit Islands Diabase and the Main Nipigon suite. country rock. References Hollings, P., Fralick, P., Kissin, S. 2004. Geochemistry and Geodynamic Implications of the Mesoproterozoic English Bay Granite-Rhyolite Complex, Northwestern Ontario. Canadian Journal of Earth Sciences, 41, 1329-1338. MacDonald, C.A. and Tremblay, E. 2004. Lake Nipigon Region Geoscience Initiative: Results of bedrock mapping in the northern part of the western Nipigon Embayment, northwestern Ontario, Canada; oral presentation abstract, Institute on Lake Superior Geology, Proceedings, v. 50, pt. 1, Programs and Abstracts p.102-103. Richardson, A.J. and Hollings, P. 2004. Lake Nipigon Region Geoscience Initiative: Geochemistry and Mineralogy of the Nipigon Diabase Sills. Summary of Field Work and Other Activities 2004. Ontario Geological Survey, Open File Report 6145, p.51-1 to 51-5. Samoilov, V.S. 1991. The Main Geochemical Features of Carbonatites. Journal of Geochemical Exploration 40, 251-262. Sutcliffe, R.H. and Greenwood, R.C. 1982. Geology of the Lake Nipigon Area. In Summary of Field Work 1982. Ontario Geological Survey, Miscellaneous Paper 106, 19-23. - 35 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 MetalCORP Ltd. Big Lake Ni-Cu-PGE, Cu-Zn-Ag, and Mo Property MACTAVISH, Allan, MetalCORP Ltd., 309 South Court Street, Thunder Bay, ON, P7B 2Y1, Canada. amactavish@metalcorp.com The Big Lake Property of MetalCORP Ltd. of Thunder Bay, Ontario comprises 33 claims (365 units totaling 5840 hectares) and is located approximately 230 km east-northeast of the City of Thunder Bay and 18 km southeast of the Town of Marathon in Northern Ontario, Canada. Work completed by MetalCORP since early 2004 includs a MegaTEM airborne survey, detailed prospecting (677 samples), linecutting, ground pulse-EM surveys, and 2 phases of diamond drilling, totaling 22 diamond holes (5672 m). This work resulted in the discovery of 4 previously unknown mineralized zones that represent 3 separate and distinct mineralization styles. The mineralized zones include: the J4 and J5 Pt-Pd reefs within the Big Lake Ultramafic Complex; the A2 Ni-Cu Zone within the Gus Creek Mafic Intrusion; and the BL14 Cu-ZnAg Zone within strongly altered mafic metavolcanic flows and associated metasedimentary rocks. The A2 and BL14 zones are not exposed on surface and are buried beneath 10 to 75 m of glacial drift. The property is also host to the historic Playter Cu-Pb-Mo-Ag Prospect which has yet to be evaluated by MetalCORP. The Big Lake Property is located near the southern margins of the eastern portion of the Archean-age Schreiber-Hemlo greenstone belt of the eastern Wawa Subprovince of the Canadian Shield. The greenstone belt is split into distinct eastern and western segments by the 1108 Ma Mesoproterozoic Coldwell Alkalic Complex. The eastern part of the belt is subdivided into the Hemlo-Black River assemblage (2.77 Ma) to the north and the Heron Bay (2.70 Ma) assemblage to the south, both of which are primarily affected by amphibolite-facies regional metamorphism. The western portions of both assemblages are lower in grade and exhibit upper greenschist facies regional metamorphism. The Big Lake Property occurs within the Heron Bay Assemblage which is intruded by the granitic to granodioritic Heron Bay Batholith, the recently recognized mafic to ultramafic Gus Creek Intrusion, the Bellʼs Lake Ultramafic Intrusion, and the Big Lake Ultramafic Complex. The J4 and J5 Pt-Pd Reefs consist of narrow, apparently stratabound intervals hosted within thick peridotite units contained within the upper and central intrusive cycles of the eastern portion of the sill-like Big Lake Ultramafic Complex. The complex is not layered. The two host intrusive cycles dip approximately 43° north and are very similar in their appearance, progression of rock units, and apparent thickness. The observed mineralization consists of low amounts (trace to ~1%) of very finely disseminated pyrrhotite and chalcopyrite within serpentinized to locally talcose, fine-grained peridotite. The J4 Reef varies between 0.58 and 2.11 m in thickness, occurs within the basal peridotite unit of the uppermost intrusive cycle of the Big Lake Complex and is usually directly adjacent to the contact with an overlying feldspathic pyroxenite unit. The J5 Reef is identical in appearance to the J4 Reef, varies between 0.75 and 3.00 m in thickness, and occurs within the basal peridotite of the central intrusive cycle of the complex near, but not adjacent to, the upper contact of the host unit with an overlying feldspathic pyroxenite. The J4 Reef has been traced for 1.60 km and contains up to 0.70 gpt Pt and 0.79 gpt Pd (1.49 gpt PGE)/1.67 m. The J5 reef has been traced for a similar distance and contains up to 0.81 gpt Pt, 0.85 gpt Pd (1.86 gpt 2PGE)/0.75 m. The A2 Ni-Cu Zone occurs near the base of the discordant Gus Creek Mafic Intrusion and consists of disseminated, blebby, and stringered, locally semi-massive pyrrhotite, chalcopyrite, and possibly pentlandite hosted within a 2 to 20 m thick host sequence. The host sequence is a complex interval of variably mineralized (1 to 30% sulphides), varitextured, inclusion-rich, gabbroic to melagabbroic intrusive rocks overlain by unmineralized, medium- to coarse-grained gabbro and quartz leucogabbro and underlain by occasionally mineralized, pyroxenephyric melagabbro and feldspathic pyroxenite. The strongest mineralization occurs near the base of the host sequence, comprises the A2 Ni-Cu Zone, and includes 1.66% Ni and 0.20% Cu/0.30 m, 1.00% Cu and 0.80% - 36 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Ni/0.40 m, 1.40% Cu and 0.27% Ni/0.77 m, and 0.98% Cu and 0.29% Ni/1.40 m. The geometry of the A2 mineralized zone remains uncertain and may be more complex that initially thought, but within the area drilled appears to strike between 125 and 150° and dip southwest at between 40 and 60°. The BL14 Cu-Zn-Ag Zone is located near the east end and stratigraphically below the sill-like Big Lake Ultramafic Complex, approximately 800 metres south of the A2 zone. The zone closely resembles high temperature Cu-rich VMS stringer mineralization and is composed of an intensely biotitized and strongly chloritized breccia containing up to 30% bands, veins, stringers and pods of chalcopyrite and pyrite with up to 5% disseminated to streaked sphalerite and minor galena. The mineralized zone occurs within a strongly to intensely K- and Mg altered, Na2O-depleted package of mafic metavolcanic rocks, minor associated interflow clastic metasedimentary rocks, and deformed bands of chert. Mineralization intersected to date includes 2.56% Cu, 1.00% Zn, 46.0 gpt Ag, 1.60 gpt Au, and 0.10% Pb/0.93 m; 0.80% Cu, 0.50% Zn, and 21.3 gpt Ag/2.62 m; and 0.74% Cu, 0.23% Zn, and 9.0 gpt Ag/0.75 m. - 37 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Mining and Exploration Activity in Northwestern Ontario MAGEE, M. Angelique, Ministry of Northern Development and Mines, Resident Geologist Program, Thunder Bay, Ontario, Canada, P7B 6S7 Northwestern Ontario experienced a significant upswing in mining and mineral exploration in 2004. Six mines produced a total of 1.7 million ounces of gold in 2004, approximately 70% of the Ontario total. Gold producers included: Campbell Mine (Placer Dome Inc.); David Bell Mine (Teck Cominco Limited and Barrick Gold Corporation); Golden Giant Mine (Newmont Canada Limited); Musselwhite Mine (Placer Dome Inc./Kinross Gold Corporation); Red Lake Mine (Goldcorp Inc.); Williams Mine (Teck Cominco Limited and Barrick Gold Corporation). North American Palladium Ltd. produced 308,931 ounces of palladium and 25,128 ounces of platinum at its Lac des Iles Mine and has begun the development of an underground operation below its open pit mine. There are approximately 300 active exploration projects in the northwest, the vast majority of which are focused on gold. Areas receiving the most interest from exploration companies were the Red Lake greenstone belt, Shoal Lake area, Dogpaw Lake area, Shebandowan greenstone belt, Fort Hope greenstone belt, OnamanTashota belt and the Pickle Lake greenstone belt. Elevated mineral commodity prices are contributing to levels of exploration activity in northwestern Ontario not seen since the mid-1980s. - 38 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Preliminary stratigraphy and geochemistry of the Mesoproterozoic Pillar Lake Volcanics, Wabigoon Subprovince, Superior Province, Armstrong, Ontario, Canada MAGEE, M. Angelique, Ministry of Northern Development and Mines, Resident Geologist Program, Thunder Bay, Ontario, Canada, P7B 6S7, HOLLINGS, P. and FRALICK, Philip, Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, Ontario, Canada, P7B 5E1 The Mesoproterozoic Pillar Lake Volcanics (PLV); an approximately 50 m thick series of flat-lying, greenschist facies undeformed massive and pillowed basalt flows; are located 15 km southeast of Armstrong, Ontario. The Pillar Lake Area was originally mapped as part of a geological compilation series with the dominant lithology in the area comprising Nipigon diabase sills formed during the Keweenawan mid-continent rift system of North America (Breaks, 1980; Davies et al., 1970). Mapping completed at a scale of 1: 50,000 by MacDonald et al. in 2003 resulted in the discovery of previously unmapped lithologies which include the Pillar Lake Volcanics (MacDonald, 2004). The PLV are part of a suite of Mesoproterozoic rocks that unconformably overlie the Archean basement of the central Wabigoon Subprovince, Superior Province. The PLV have not yet been successfully dated by geochronological methods. Magnetic remanence studies of the PLV places them at 1140 Ma but there is a significant error associated with this date (C. Hercun, pers. comm., 2005). Fifty detrital zircons from an interflow lithic arenite bounded at its base and top by PLV were dated and the youngest detrital zircon was dated at 1514 Ma (Heaman et al., 2005). The dominant populations of zircons from the interflow lithic arenite fall within 1780 to 1880 Ma and 1900 to 1950 Ma (Heaman et al., 2005). The PLV unconformably overlie a 1599±1 Ma layered gabbroic intrusion but no contact has yet been found except in drill core (Heaman et al., 2005; Middleton, 2005). A nearby 1590±1 Ma syenite may also underlie the PLV (Heaman et al., 2005). The PLV are overlain by Inspiration diabase sills dated at 1120±1 Ma (Heaman et al., 2005). The Inspiration diabase sills are geochemically distinct from the Nipigon sills and hence have been given a new designation. The Inspiration diabase sills extend the timing of North American mid-continent rift-related intrusive activity. Based on these age dates, the PLV are Mesoproterozoic in age, and were erupted between 1514 Ma and 1120 Ma. The PLV have been metamorphosed to greenschist facies and are extensively chloritized with accessory epidote and sericite. The PLV are hematite and actinolite altered with intense alteration found at the top of each flow. The individual massive basalt flows are approximately 0.2 – 1 m thick and each flow is capped with a 5 cm thick flow top breccia with actinolite rims and hematite cores forming the breccia fragments (Figure 1). The pillowed basalt flows tend to be pervasively altered and exhibit autobrecciation throughout the flow layer, perhaps due to higher permeability. Near the base of the volcanic pile, the pillowed basalt flows are rarely unaltered, unbrecciated and preserve primary volcanic features such as pillow selvages, interpillow hyaloclastite, and degassing structures (Figure 2). Associated with the massive and pillowed basalt flows are numerous breccias including an undeformed volcaniclastic breccia that contains fragments of gabbro, amygdaloidal basalt with clay minerals infilling the amygdules, and possibly Figure 1. Flow top breccia capping massive basalt flow with actinolite rims and recessively weathered hematite cores - 39 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 granitoid. Field relationships between the PLV and the breccias are not always discernible due to poor outcrop control, but in several instances the breccias occur as dikes found within the PLV. Based on preliminary geochemistry, the PLV may be the result of a mantle plume and possibly predate the main North American mid-continent rift event. It is hoped that detailed mapping of the PLV in conjunction with additional geochemical analyses will determine the source and cause of the volcanism that resulted in this suite of volcanic rocks. Figure 2. Undeformed chlorite-altered pillowed basalt with preserved selvages and interpillow hyaloclastite. References Breaks, F.W. 1980. Sioux Lookout - Armstrong Sheet, Geological Compilation Series, Ontario Geological Survey, Final Map 2442. Davies, J.C., Pryslak, A.P., Pye, E.P. 1970. Sioux Lookout - Armstrong Sheet, Geological Compilation Series, Ontario Geological Survey, Final Map 2169. Heaman, L.M., Easton, R.M., Hart, T.R., MacDonald, C.A., Fralick, P., and Hollings, P. 2005. Proterozoic history of the Lake Nipigon area, Ontario: Constrains from U-Pb zircon and baddeleyite dating; Canadian Institute of Mining and Metallurgy, 2005 Annual Meeting, April 24-27, Toronto, Ontario. Macdonald, C.A. 2004. Precambrian geology of the south Armstrong-Gull Bay area, Nipigon Embayment, northwestern Ontario; Ontario Geological Survey, Open File Report 6136, 42p. Middleton, R.S. 2005. Diamond Drilling on Red Granite Property, Pillar Lake Sheet, Armstrong, ON, 52I03NW, Resident Geologist Program Thunder Bay North Assessment Files, 55p. - 40 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Sedimentology of the Rove and Virginia Formations and their Tectonic Significance MARIC, Mike and FRALICK, Philip, Department of Geology, Lakehead University, Thunder Bay, ON, P7B 5E1, philip.fralick@lakeheadu.ca The Paleoproterozoic Rove and Virginia Formations form the upper portion of strata in the Animikie Basin. They are lithostratigraphically and chronostratigraphically correlative units overlying iron formation of the Biwabik and Gunflint Formations. Tuffaceous layers very near the base of the Rove and Virginia Formations gave U-Pb zircon ages of approximately 1835 Ma (Addison et al., 2005), whereas a sandstone sample collected by R.M. Easton (O.G.S.), approximately 400 m higher in the stratigraphy, had a U-Pb minimum detrital zircon age of 1780 Ma (see Heaman and Easton, this volume). These age constraints place sedimentation commencing during the final stages of Penokean igneous activity and stretching over an extended time span. The Rove and Virginia Formations would have been in the appropriate spatial and temporal setting to represent deposition in a foredeep transitional into a foreland basin. This study investigates the internal stratigraphy and sedimentology within the Rove and Virginia to ascertain what the lithofacies present and their stacking order can reveal about the dynamics of the tectonic setting. The data set consists of detailed logging of twelve continuously cored drill holes extending from south of Duluth to south of Thunder Bay. In total 3200 meters of core were examined. This approach is similar to that of Lucente and Morey (1983), though with advances in depositional models and process sedimentology, since their classic work, more information may be able to be extracted. Cl y - § . § - ..— The Rove and Virginia Formations overlie an intensely altered zone in the upper Gunflint and Biwabik Formations. Silicification, formation of neomorphic carbonate spar, pyrite growth in strataform bands and pisoliths all occur in the upper few meters of these underlying units. This is sharply overlain by the basal Rove and Virginia consisting of black, carbonaceous shale with thin interbeds of siltstone and very fine-grained sandstone. Green, friable tuffaceous layers are common in this assemblage, especially in the northern drillholes. - 41 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Approximately five meters above the base the amount of siltstone and sandstone interlayers gradually begins to decrease upward until they are relatively scarce. The next approximately one hundred to one hundred and fifty meters is almost completely dominated by fissile black shale. Microscopic examination reveals the presence of very thin laminae composed of angular silt grains and other microlayers consisting of carbon. The shale dominated succession is interrupted midway towards its top by a siltstone and very fine-grained sandstonerich interval traceable across the basin. This coarser unit is thicker near both the northern and southern basin margins. As well, the black shale dominated interval, in general, contains more coarse-grained interbeds in the southern one third of the basin. A gradational contact exists at the top of the shale assemblage with the overlying sandstone-shale unit. The gradation occurs over eighty meters in the north but thins to the south. The overlying sandstone-shale succession is up to 350 meters thick and consists of a stacked assemblage of over one hundred individually coarsening-upwards parasequences. Internally they consist of graded, commonly massive, finegrained sandstones separated by mmʼs to cmʼs of shale. The shales between parasequences are decimeters to one or two meters thick. This sandstone-shale assemblage fines appreciably to the south. The uppermost unit overlying the graded sandstone packages at approximately 500 meters above the base of the section is dominated by black shale with thin rippled sandstones. Both current and wave ripples are present. The upper Gunflint represents a subareally weathered surface exposed during upbuckling concurrent with the Penokean Orogeny. This is highlighted by the 1878 Ma age of the Upper Gunflint (immediately pre-Penokean, Fralick et al., 2002) and the 1835 Ma age of the Rove (Addison et al., 2005) immediately overlying the weathered surface. Upramped orogenic load probably resulted in resubmergence of the area and a slightly coarser unit was deposited forming the basal Rove and Virginia as the flooding surface migrated across the basin. Eruptions at this time in the core of the Penokean deformed terrain may have supplied the volcanic ash. With increased water depth a sediment starved, condensed sequence developed with anoxia probably caused by high organic loading in the bottom sediments. A slight coarsening of detritus in the southern area probably reflects a source in the Penokean deformation belt, but the general scarcity of sediment influx from this area strongly implies only minor upraising of an orogenic zone during the Penokean. The condensed sequence is further condensed by the lack of biogenic sediments other than carbon resulting in tens of millions of years being represented by only a hundred meters of black shale. Progradation of a turbiditic to shelf system from the north ends the sediment starvation with a flood of detritus from this direction. Detrital zircon geochronology and paleocurrent directions (Morey, 1973) are consistent with derivation of the sediment influx from the TransHudson Orogenic zone to the northwest. References Addison, W.D., Brumpton, G.R., Vallini, D.A., McNaughton, N.J., Davis, D.W., Kissin, S.A., Fralick, P.W. and Hammond, A.L., 2005. Discovery of distal ejecta from the 1850 Ma Sudbury impact event. Geology, vol. 33, p. 193-196. Fralick, P.W., Davis, D.W. and Kissin, S.A., 2002. The age of the Gunflint Formation, Ontario, Canada: single zircon U-Pb age determinations from reworked volcanic ash. Canadian Journal of Earth Sciences, vol. 39, p. 1085-1091. Lucente, M.E. and Morey, G.B., Stratigraphy and sedimentology of the lower Proterozoic Virginia Formation, northern Minnesota. Minnesota Geological Survey, Report of Investigations 28, 28p. Morey, G.B., 1973. Stratigraphic framework of Middle Precambrian rocks in Minnesota. In, ed G.M. Young, Symposium on Huronian Sedimentation, Geological Association of Canada, Special Paper 12, p. 211-249. - 42 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Riddle of the Sands (Proterozoic) solved by Quartzites at Hamilton Mounds, Central Wisconsin MEDARIS, L.G., Jr and DOTT, R.H., Jr, Department of Geology and Geophysics, University of WisconsinMadison, Madison, WI 53706, medaris@geology.wisc.edu, rdott@geology.wisc. edu Quartzite inliers in central Wisconsin have long been enigmatic with regard to the Proterozoic evolution of the southern Lake Superior region. At Hamilton Mounds the intrusion of quartzite by 1.76 Ga granite has led some to suggest that Baraboo Interval sedimentation (1.75-1.63 Ga) was spatially and temporally associated with Geon 17 magmatism. Re-examination of the Hamilton Mounds area reveals the presence of two different quartzites, an older one intruded by granite, which we designate the Hamilton Mounds Quartzite (HMQ), and a younger one resembling the Baraboo Quartzite, which we call the Seven Sisters Quartzite (SSQ), from the present name of the quarry. Quarrying operations have largely removed the HMQ and intrusive granite, although numerous displaced blocks remain for examination. The HMQ consists of gray, laminated quartzite (Fig. 1), which commonly contains calc-silicate (qtz+ep±am) layers and ellipsoidal domains, perhaps former concretions (Fig. 2; cs, calcsilicate). The HMQ is a metamorphosed immature wacke or arenite, which contains biotite, muscovite, and substantial amounts of microcline (mc) and plagioclase (pl) (Fig. 3). The HMQ is intruded by granite (gr) (Fig. 4), which yields a U-Pb zircon age of 1763 ± 7 Ma (Van Schmus, cited in LaBerge et al., 1991). Contact metamorphic effects of the granite on the HMQ include metasomatic growth of alkali feldspar, but the coarsegrained, poikiloblastic texture of such feldspar (Fig. 5) allows easy distinction between it and fine-grained, recrystallized detrital feldspar (Fig. 3). The SSQ, which crops out in the hills surrounding the quarry, consists of pink, massive to cross-bedded supermature quartzite (Fig. 6), resembling Baraboo Interval quartzites elsewhere, except for a greater degree of recrystallization and the presence of muscovite, rather than pyrophyllite. Such features and extensive brecciation probably reflect the influence of the nearby Wolf River batholith, with which are associated wide-spread Geon - 43 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 14 hydrothermal alteration and K-metasomatism as documented in the Baraboo and Sioux quartzites (Medaris et al., 2003). p r JM1 -- S —S Geon 17 detrital zircons have recently been recognized in a single sample of the “Hamilton Mounds Quartzite” by Van Wyck and Norman (2004). However, the analyzed sample, described as supermature quartzite, was collected several hundred yards from granite and is likely to be from the SSQ, which we correlate with other supermature quartzites of the Baraboo Interval. Thus, it is not surprising that this sample should contain Geon 17 detrital zircons and display a spectrum of detrital zircon ages similar to that from the Baraboo Quartzite. Although the contact between the HMQ and SSQ is not exposed, we interpret the HMQ and intrusive granite to represent basement upon which the SSQ was deposited unconformably. Thus, like the Baraboo Quartzite, the SSQ appears to be younger than, and rests upon, Geon 17 basement. The results of this investigation are consistent with our view of the Baraboo Interval (1.75-1.63 Ga) as representing an episode of extensive weathering and widespread deposition of supermature quartz arenite on a stabilized craton in the interval following cessation of Geon 17 magmatism and prior to Geon 16 Mazatzal deformation (Medaris et al., 2003). Abbreviated References LaBerge, G. L. et al. 1991. U.S. Geol. Surv. Bull. 1904B: B1-B18. Medaris, L. G., Jr et al. 2003. J. Geol. 111: 243-257. Van Wyck, N. and Norman, M. 2004. J. Geol. 112: 305-315. - 44 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Depositional setting of the Pass Lake and Rossport Formations (Sibley Group) inferred from a combined sedimentologic/geochemical approach METSARANTA, R.T. and FRALICK, P.W., Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, Ontario, P7B 5E1, Canada, rtmetsar@lakeheadu.ca The depositional setting of the lower portions of the Mesoproterozoic Sibley Group have been investigated through detailed lithofacies analysis, stable isotope and trace element geochemistry. The portion of the Sibley Group studied is comprised of a lower clastic assemblage, middle mixed carbonate-clastic assemblage and an upper clastic dominated, evaporite rich assemblage. The thickness of each unit varies but is in general about 20-80m. The lower clastic section was deposited in a variety of depositional environments including: poorly developed, coarse-grained alluvial fans; braided, sand rich fluvial systems; fluvial/wave dominated deltaic settings; and nearshore/beach environments. The middle mixed carbonate clastic succession represents lacustrine or shallow marine deposition in a semi-arid environment. The upper red siltstone, evaporite-rich section has been interpreted to represent a saline mudflat environment or sahbka-like setting. Numerous chemical sediment types are preserved within the three distinct successions. Calcrete soils are developed in coarse-grained alluvial fan conglomerates in the lower clastic dominated package as well as higher in the stratigraphy within the upper package. Dolomitic mudstones are found in a cyclic dolomite-siltstone lithofacies association of the middle assemblage. Stromatolitic carbonates are preserved in the transition from the middle to upper succession along with evidence of subaerial exposure of this strand-proximal lithofacies. Sulfate minerals occur as fine nodules within dolomites of the middle mixed carbonate-clastic assemblage and as coarser nodules in the upper evaporitic succession. Carbon and oxygen stable isotopic compositions, Sr isotopic compositions and trace element compositions were determined for various chemical sediment types to study facies (process) dependent and stratigraphic variations in isotopic compositions and to evaluate the degree of post depositional diagenetic alteration. Stable isotopic compositions for dolomitic mudstones range from –3 0/00 (PDB) to about +1 0/00 (PDB) for δ13C and span δ18O compositions from –4 to –8 0/00 (PDB). Stromatolitic carbonates have δ13C from 0 to 2 0/00 and δ18O from –3 to -14 0/00 (PDB). Soil related carbonates vary over δ13C compositions from –1.5 to 1.50/00 (PDB) and δ18O varies from -2 to -6 0/00 (PDB). Sampling through a stratigraphic thickness of about 45m of the middle carbonate clastic succession shows a good correlation between increasing stratigraphic height and heavier stable isotopic composition (Figure 1). Petrographic analysis and trace element geochemistry suggests little diagenetic effect on stable isotopic compositions. A • • • A S l S •• • U• A • • • •A.• . A A. I —a S S S dolomitic mudstone stromatolite soil carbonate • S U 0 1 .4 2 -3 -2 -1 0 81 3c (PDB) dolomitic mudstone 313C (PDB) Figure 1. Stable isotopic compostions of various Sibley Group carbonates (left), and stratigraphic variation in δ13C for the middle dolomite/siltsone lithofacies association (right) - 45 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Sr isotopic compositions were also determined for a variety of carbonate and sulfate samples. Dolomitic mudstones (n=4) display 87Sr/86Sr ratios varying from 0.70846 to 0.71249±1. Soil carbonates appear to have variable compositions from a calcrete low in the stratigraphy at 0.70478±1 (n=1) to weathered stromatolitic material higher in the stratigraphy at a value of 0.70737±1 (n=1). One unaltered stromatolite sample has a value of 0.71234±1. Sulfates (n=8) from the upper evaporitic package vary through a fairly narrow range of compositions between 0.70647±3 to 0.708836±1. A single sample of sulfate from the middle succession in a dolomitic layer yielded a fairly high value of 0.71090±1. Further analyses have been undertaken to strengthen the validity of the apparent facies related variation. Rare earth element geochemistry of carbonate samples was also undertaken using an acetic acid dissolution method to avoid clastic contamination of primary carbonate geochemical signatures. Figure 2 shows Post Archean Australian Shale normalized REE diagram for Sibley Group carbonates. Stromatolites and soil carbonates generally show a distinct negative Ce anomaly, and overall relatively flat slope with a slightly negative HREE slope. Dolomitic mudstones exhibit an enrichment in MREE, with little or no negative Ce anomaly. Positive Gd anomalies are present in most samples of all carbonate types. Stromatolite Together lithofacies analysis and geochemical data provide insight into the depositional environments of the Sibley Group. C and O isotopic compositions fall within the realm of values determined for other Mesoproterozoic carbonate rocks deposited in marine systems. Upward stratigraphic changes in carbon and oxygen isotopic compostions may have been brought about through global scale changes, with a positive shift in C values attributable to increased rates of light (organic carbon) burial. Alternatively the carbon La Ce Pr Nd Sm Eu Tb Gd Dy Ho Er Tm Yb Lu isotopic shift maybe have resulted from processes Dolomitic rnudstone occurring within a restricted lacustrine basin with the upward shift corresponding to increasingly restricted depositional conditions near the top of the sampled succession. 87Sr/86Sr values also lie within the realm of possible seawater Sr values for the Proterozoic. The lowest values in calcrete from near the bottom of the lower succession suggests that the Sr source for these carbonates may have been marine. Relatively high Sr values in the middle succession may imply a lacustrine system. Diagenetic sulfate in the upper succession 0.001 also falls within “marine” values for the Proterozoic La Ce Pr Nd SmEu TbGd Dy Ho Er TmYb Lu and is generally lower than carbonates in the middle Soil Carbonate succession. REE element patterns have a signature consistent with oxygenated seawater; however, little comparative data is available for modern of ancient carbonates deposited in lacustrine or evaporitic environments. 0.001 Pr Nd SmEu Tb Gd Dy Ho Er TmYb Lu Figure 2. Post Archean average shale normalized rare earth element diagrams for various Sibley Group carbonate types. La Ce - 46 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Seagull Intrusion, Ontario: A Unique PGE-Ni-Cu Setting MIDDLETON, R.S., P.Eng. and HEGGIE, G., Thunder Bay, Ontario The Seagull ultramafic (Proterozoic) intrusion is an unique setting when compared to other mafic intrusion in the Lake Superior region and for that matter the rest of the world. A dunite layer exceeding 400 metres in thickness with pristine olivine (unaltered) having a magnesium oxide content exceeding 33% average olivine composition of Fo86 occurs with a 50-100 metre thick gabbro-pyroxenite (lherzolite) basal unit. Overlying is a peridotite 200-250 metres thick and an upper granophyric gabbro-gabbro pyroxenite-gabbro norite (50-100 metres thick). The intrusion has been age dated as 1112 Ma although older dates of 1124 Ma are appearing in other phases (see Heaman and Easton, this volume). The intrusion is adjacent to a failed arm of the Midcontinent Rift that extends north into the Nipigon Embayment and is cut by Nipigon olivine gabbro (1108 Ma) that forms extensive sills in the region. Other olivine-picrite bodies occur in the area such as Eva-Kitto and Hele, but are not known to contain the extensive ultramafic observed at Seagull. Sulphides occur in the basal lherzolite with zoned blebs containing cubanite tops and pyrrhotite-pentlandite bottoms. Assays up to 3.58g Pt +Pd have been obtained with % S and a 1 to 1.1 ratio of Pt and Pd. Nickel values up to 0.25% and Cu up to 0.35% have been observed. Horizons of fine disseminated sulphide occur in the dunite up to 5 metres in thickness containing up to 5.5g PGE, 0.68% Cu and 0.36% Ni in the 470 to 500 metre depth area. Two distinct layers have been identified so far, with highly anomalous zones in between the layers. The horizons are hosted in a medium to coarse dunite above a very coarse dunite. Fine-grained chalcopyrite with minor pyrrhotite-pentlandite occur in these layers. There is no development of feldspar or oxide mineral (magnetite or chromite) layering, however there appears to be a change in the pyroxene composition from both orthopyroxene and clinopyroxene below the mineralized horizons to just clinopyroxene above the mineralized horizon, which occurs in conjunction with a shift to slightly more primitive olivine compositions. Magnetic inversion modeling has been the most useful tool for mapping the shape of the intrusion. Down hole EM has identified conductors within the basal lherzolite which will be followed up for stringer-massive sulphide accumulations. Gravity surveying has helped identify a trough extending east and a scarn zone in the Sibley Group Metasediments. Most of the basement rocks beneath the areas containing values are Archean Quetico Metasediments. Sulphur isotope work suggests the Quetico sulphides acted as a sulphur source. Minerals identified in the basal zone are merenskyite, michnerite, sperrylite, a palladium-copper alloy, native copper, native iron-bismuth, chalcopyrite, cubanite, pyrrhotite and pentlandite. Where the horizons are dominated by bravolite, polarite, sperrylite, telluropalladinite and copper. - 47 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Geology, Geochemistry and PGE Potential of Mafic and Ultramafic Intrusions in Minnesota, excluding the Duluth Complex MILLER, James D. Jr., and JIRSA, Mark A., Minnesota Geological Survey, mille066@umn.edu and jirsa001@umn.edu Exploration for polymetallic deposits, including platinum group elements (PGE), has traditionally focused on mafic intrusions of the Mesoproterozoic Duluth Complex for obvious reasons—the complex is relatively well exposed and contains occurrences of PGE associated with basal Cu-Ni deposits that were discovered decades ago. By contrast, mafic intrusions in the Archean and Paleoproterozoic terranes of the remaining four-fifths of the state have received only cursory investigations by exploration companies and the state. This stems in large part from thick glacial cover over all but the northeast and southeast parts of the state; there is Paleozoic cover in the southeast. The completion of a high-resolution aeromagnetic survey of the state between 1982 and 1990 and the increased demand for PGE has spurred more exploration activity targeting ultramafic and mafic intrusions over the past two decades. Because of the requirement that drill core and related geochemical and geophysical data be turned over to the state after mineral leases expire, a sizeable public database of geologic, geochemical, and geophysical information exists on mafic intrusions in the state. However, this database has not been consistently evaluated for what it implies about the potential for these intrusions to host PGE deposits. Beginning in 2001, a four-year, two-phase study was funded by the state for the Minnesota Geological Survey (MGS) and the Natural Resources Research Institute (NRRI) to inventory the public database and acquire new data for the purpose of evaluating the potential for PGE deposits in mafic intrusions outside the Duluth Complex. Phase 1 of this study, which was completed in July of 2003 and published by the MGS as an interim openfile report, took inventory and digitally compiled basic geologic, lithologic, and geochemical attributes of over 150 individual mafic intrusions studied from field exposures or from drill core. Information was also collected on some of the thousands of diabasic dikes and other geophysical anomalies inferred to be mafic intrusions. The project work conducted on each intrusion varied with the level of detail that existed, but included some combination of geophysical (aeromagnetic and gravity) delineation, outcrop mapping, drill core examination, petrography, and geochemical analysis. This compilation of pre-existing data was augmented with geophysical modeling, relogging of drill core, outcrop mapping and sampling in well exposed areas of north-central Minnesota, petrographic studies of over 100 thin sections from 53 intrusions, and geochemical analyses and assays of 83 samples from 58 intrusions. Also, eight new 40Ar/39Ar analyses of magmatic hornblende and biotite separates were acquired to better constrain the general temporal framework of some intrusions. Phase 2 of this study, which is currently underway, sets out to conduct a more detailed evaluation of the geologic setting, igneous stratigraphy, and geochemistry of 15 specific intrusions for which there is sufficient sampling by drill core. The petrographic and geochemical results of this study phase were not ready at the time of this writing, but will be presented at the meeting. Another component of this study, which is being conducted by the NRRI, is a detailed mapping and chemostratigraphic analysis of the well-exposed Deer Lake layered mafic complex of north-central Minnesota. Based on an evaluation of this database, mafic intrusions can be grouped into several major types according to age and type of host rock, timing of emplacement relative to tectonism and metamorphism, and general lithologic attributes. The attributes of these intrusion types and some examples are summarized in Table 1. Phase 1 of this study acquired 75 new lithogeochemical and assay analyses from more than 50 intrusions. In addition, over 400 publicly available geochemical analyses have been compiled from exploration company files and data previously acquired by various state and federal agencies. For Phase 2, an additional 85 analyses are - 48 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Table 1. Classification of mafic intrusions outside the Duluth Complex in Minnesota. Tecto-metamorphism ithologic attributes Examples (Fig. 1) Subvolcanic maficultramafic sills Intrusion type Age Ar Archean greenstone belts Host rock Pre- to syn-tectonic; low to moderate metamorphic grade Tholeiitic-komatiitic; peridotite, pyroxenite, gabbro, & diorite Deer Lake Complex, Winterfire intr, KIB intrusions, Warroad intrusion Amphibolite sills & dikes Ar Archean highgrade gneiss terrains Pre-tectonic; high metamorphic grade Schistose amphibolite (metagabbro) Small amphibolite bodies in Quetico Subprovince Lamproid dikes & plugs Ar Archean greenstone belts Syn- to post tectonic; low metamorphic grade Hbld-, oxide-, & biobearing ultramafic rocks Lamprophyre plugs in Wawa Subprovince Gabbroic to intermediate intrusions Ar Archean granitegreenstone terrains Post-tectonic; low metamorphic grade Gabbro/diorite, tonalite, monzonite w/mafic enclaves Linden pluton, Oaks intrusion, Grygla pluton Gabbroic anorthosite massifs p< Archean granitegreenstone terrains Syn- to post-tectonic; moderate metamorphic grade Coarse gabbroic anorthosite Mentor intrusive complex Layered gabbronorite intrusions p< Penokean orogen & MN River Valley Subprovince Syn- to post-tectonic; low to moderate metamorphic grade Modally & texturally layered gabbronorite, gabbro, & pyroxenite Lake Washington, LL, BKV & Providence intrs. Pyroxenite & peridotite plugs p< Penokean orogen Post-tectonic; low metamorphic grade Alkaline ultramafic rocks Small plugs in central Minnesota Hornblendic maficintermediate complex p< Penokean orogen Syn- to post-tectonic; low to moderate metamorphic grade Hornblendite, diorite, granodiorite Tibbets Brook intrusion Olivine gabbro intrusions m< Unknown Syn-continental rifting Layered oxide olivine gabbro Fillmore County intrusions currently being acquired from 15 intrusions to more completely characterize the chemostratigraphy of these select intrusions. Currently, the highest PGE values have been found in sulfidic basal contact zones of the Winterfire subvolcanic sills in north-central Minnesota with Pd, Pt, and Au values in 1-3 meter intervals in the range of 5001,000, 100-400, and 50-200 parts per billion, respectively. The discovery of anomalous PGE values is not the primary objective of this study, however. Rather, the goal is to assess the overall potential of particular intrusion types to host either contact-type or reef-type PGE deposits. Working toward this goal, geochemical data will be used to interpret the composition and PGE tenor of the parent magmas, determine the degree to which the intrusions display internal differentiation, and evaluate conditions of sulfide saturation through the intrusions. A draft report on this study will be completed in the summer of 2005 with a final report to follow. In addition to digitally compiling pertinent geologic, geophysical, petrographic, and geochemical data, this report will evaluate the data to assess the PGE potential of the various intrusion types and make recommendations as to what intrusion types merit further study and exploration. - 49 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 New thoughts on old circles: A reexamination of spheroidal Gunflint taxa PLANAVSKY, N.J. and MURPHY, J., Department of Geology, Lawrence University, Appleton WI The first systematic description of the Gunflint biota (Barghoorn and Tyler, 1965) is a seminal work in Precambrian paleontology. The Gunflint biota are represented by an assemblage of incredibly well-preserved microfossils contained within the approximately two billion year old Gunflint Iron Range of the Animikie Basin. Because the Gunflint microfossils were the first definitive and widely publicized evidence of Precambrian life, they have become one of the most famous fossil assemblages in the world. The high level of preservation and diversity of the biota has led to the microfossils becoming a benchmark against which early records of life are compared. The Gunflint and other Animikie basin microfossils are also significant because they provide the most complete record of the biosphere during a dramatic change in Earthʼs history—the initial oxygenation of the atmosphere and shallow seas and the sequestration of the iron from the oceans. The importance of the Animikie microfossils is based upon a clear understanding of the diversity and structure of the Gunflint community. Most Gunflint genera are determined based on obvious morphological characteristics such as having a filamentous or spheroid morphology. However, species boundaries within broad morphological groups such as the spheroids are more difficult to define. Diameter distribution (i.e. unimodal or polymodal) is one factor that has previously been used to determine species composition of a spheroidal population (Strother and Tobin, 1987; Knoll et al., 1978; Knoll and Simonson, 1981). Although the range of species distributions has always been noted, it has been afforded little attention in the evaluation of specific taxon (Knoll et al., 1978). We conducted a study to determine the diameter distribution within a population of the coccoid cyanobacteria Microcystis aeruginosa and a survey of previously recorded diameter ranges for 100 extant coccoid cyanobacteria taxa. Our goal is to determine the validity of Gunflint and other Animkie basin species classifications. Previous studies that have noted a polymodal diameter distribution in a microfossil population have asserted that this indicates the presence of multiple taxa. This was most notably assumed for the genus Huroniospora, one of the most abundant biota of Gunflint. Strother and Tobin (1987) recorded a skewed bimodal size-frequency distribution (modes at 4 and 7 µm) for Huroniospora microfossils found within the Schreiber Beach assemblage. Huroniospora specimens from the Sokoman Iron Formation have a size-frequency distribution with modes at 3, 5, and 10µm (Knoll and Simonson, 1981), in accordance with previous findings from the Schreiber locality. We obtained a diameter distribution for a cultured population (from Lake Winnebago, Appleton, WI) of Microcystis aeruginosa through measurements with an optical micrometer. We made wet mount preparations and recorded the diameter of all specimens within a single optical plane. The diameters were placed into .5μm groups, which is the level of accuracy of the optical micrometer used. In all, 425 individuals were measured. The population showed a distinct bimodal pattern with modes at 6 and 8.5 μm. Although previous algae populations have been shown to have a broadly unimodal diameter distribution, our results show that a monospecific cyanobacteria population can contain a biomodal diameter distribution. Polymodal distributions of cyanobacteria have also been recorded (Prescott, 1951). These results strongly suggest that polymodal diameter distributions with closely spaced modes in a microfossil population, specifically those described by Strother and Tobin (1989) and Knoll and Simonson (1981), in the definitively cyanobacteria taxon of Huroniospora are very unlikely to indicate multiple taxa. At minimum the observations show that a polymodal diameter distribution within a microfossil population cannot be used as a diagnostic indication of multiple taxa. We also conducted an evaluation of the range of diameter distributions in extant cyanobacterial species. Our goal was to see if the reported diameter ranges for fossil taxon are consistent with the ranges seen for extant species. We surveyed 100 marine and freshwater cocciod cyanobacteria species from previous reports. The average diameter distribution was 3.0, with a standard deviation of 3.1. We found a range of 0.2 to 24 μm, with only two species with a reported range greater than 10μm. This observation is relevant because many of the Gunflint taxon have very large ranges that are skewed left. A notable example is Leptoteichos golubicci. The - 50 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 taxon is broadly unimodal with a mean cell diameter of 13.5μm. However, the reported range for the species is 26μm (5-31μm), larger than any of the species we encountered and far above the mean for diameter range. Less than the five percent of Leptoteichos golubicci have a diameter above 24 μm (Knoll et al., 1978). The diameter distribution of and range reported for Leptoteichos golubicci is inconsistent with monospecfic classification of the population. We believe that the cells above 24 microns in the Leptoteichos golubicci population are most likely akinetes, which are large reproductive structures (analogous in many ways to spores) produced by filamentous cyanobacteria. Although akinetes have previously been shown to be present to present in the Gunflint (Licira and Cloud, 1968), no estimate of their abundance was made. Our reinterpretation of the Leptoteichos golubicci population could be used to determine an estimate of akinete abundance. Although the spheroidal Gunflint microfossils are exceptionally well preserved, we need to remember that we are still looking at a collection of circles, from which little morphological variation can be gathered. Therefore information on population structure from modern analogues needs to be used to help ascertain the basic ecology of the Gunflint community. Selected References Braghoorn, E.S. and Tyler, S.A. 1965. Microorganisms form the Gunflint chert. Science, 147, 563-577. Licari, G.R., and Cloud, P.E., Jr., 1968. Reproductive structures and taxonomic affinities of some nannofossils from the Gunflint Iron Formation. Proceedings of the National Academy of Sciences, 59, 1053-1060. Knoll, A.H. Barghoorn, E.S., Awramik, S.W. 1978. New microorganisms from the Aphebeian Gunflint Iron Formation, Ontario. J. Paleontology 52, 976-992. Knoll, A.H. and Simonson, B. 1981. Early Proterozoic microfossils and pencontemporaneous quartz sedimentation in the Sokoman Iron Formation, Canada. Science. 211, 478-480. Prescott, G.W. 1951. Algae of the Western Great Lakes Area. Cranbrook Institute of Science: Bloomfield Hills, Michigan. Strother, P.K. and Tobin, K. 1987. Observations on the genus Huroniospora braghoorn: implications for paleoecology of the gunflint microbiota. Precambrian Research 36, 323-333 - 51 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Geochemical Variation within the Mesoproterozoic Nipigon Diabase Sills RICHARDSON, Adam and HOLLINGS, Pete, Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, Ontario, P7B 5E1, Canada, ajrichar@lakeheadu.ca The Nipigon sills are related to the Mesoproterozoic Keweenawan mid-continent (MCR) rift event. Centered on what is now the Lake Superior basin, the igneous activity is interpreted to be the result of a mantle plume interacting with the continent as far to the southwest as Kansas, USA, and to the southeast as far as the Grenville front in southern Ontario, Canada (Davis and Green, 1997). The system operated for approximately 20 Ma producing a volume of intrusive and extrusive igneous rock estimated to be in excess of 1.5 million km3 (Klewin and Shirey, 1992). The Nipigon sills are thick, relatively flat lying hypabyssal, medium-grained subophitic to ophitic olivine gabbronorite with U-Pb ages ranging from 1107 Ma to 1112 Ma (Heaman and Easton, this volume). The goals of this project are to evaluate geochemical variations within the Nipigon sills, compare them to other MCR rocks and generate an emplacement model which identifies and explains any identified intrasill relationships, magma sources. The project area (Figure 1) covers approximately 15,000 km2 of the Nipigon embayment. As part of this project a detailed sampling program was undertaken in the summer of 2003 and 2004 resulting in 250 shoreline samples being collected from Lake Nipigon (Figure 1) and 530 samples from diamond drill core in the southern Lake Nipigon basin near Black Sturgeon and Shillabeer lakes. Selected samples were analysed for major, trace and rare earth elements with a representative subset analysed for Rb-Sr, Sm-Nd and Pb-Pb isotopes. Three distinct sill suites were identified within the basin. Two additional suites have been identified by T. Hart and C.A. MacDonald (Ontario Geological Survey) as part of their mapping projects in the Nipigon Embayment (MacDonald, 2004; Hart and Magyarosi, 2004). The various suites were defined, in part, using rare earth element data (Figure 2). The most spatially abundant suite has been termed the Main Nipigon suite. This suite outcrops over much of the Nipigon embayment having La/Smcn and Gd/Ybcn ratios of 1.4-1.8 and 1.3-1.5 respectively (Figure 2). Mga s s.ys.dns El Meta,oeic • The Main Nipigon and Inspiration suites share similar Gd/Ybcn (Figure 2), TiO2 and MgO values. The Inspiration suite has a higher La/Smcn ratio, but these data lie on a possible trend from the Main Nipigon suite and the whole rock data suggest that the Inspiration and the Main Nipigon suites may be related. OS' Hole CdL3! 9Spl {U Scale 0 102O Km Figure 1. Map showing sample locations, drill holes, geology and project area. - 52 - Other suites in the Nipigon Embayment include the McIntyre and Jackfish suites. These two suites share similar Gd/Ybcn and La/Smcn ratios and lie on a fractionation trend (Figure 2) with the Jackfish �Proceedings of the 51st ILSG Annual Meeting - Part 1 possibly being the source of the McIntyre. The Shillabeer suite is geochemically similar to the Disraeli ultramafic intrusion using Gd/ Ybcn, La/Smcn and whole rock geochemistry suggesting the possibility that this suite is not part of the Nipigon sills but is related to the ultramafic intrusions of the southern Lake Nipigon embayment. 4.5 • • .1;I •a . A U Isotope data (Figure 3) reveals differences in the degree of crustal contamination of the ShiIlaber • Distoeli sill suites. The Main Nipigon and McIntyre •Jockfish suites are the least contaminated (εNd ~0) o o. 1.5 2 2.5 4 4.5 3 3.5 with the Main Nipigon data trending toward La IS men Sibley group sediments suggesting that these Figure 2. Chondrite normalized rare earth data defining sill suites in sediments may be the dominant contaminant. Shillabeer and Inspiration sill suites are the the Nipigon embayment. most contaminated (εNd -5.5 to -7) but do not 0.705 0.110 OIlS 0.720 0.850 0.900 0.950 trend towards Sibley data implying a distinct contaminant. . Moin Nipigon a £Kama Hill • McIntyre .—- 1 p £ £ References Klewin, K.K, and Shirey, S.B. 1992. The igneous petrology and magmatic evolution of the -4 0 Midcontinent rift system. Tectonophysics 213, 33-40. Davis, D.W. and Green, J.C. 1997. Geochronolgy •. •Jadcfih of the North American Midcontinent rift in AMoin Mpon A Shillthter western Lake Superior and implications for its •MCIMyT •Sibky geodynamic evolution. Canadian Journal of Earth Sciences 34, 476-488. 875r/86Sr Hart, T.R. and Magyarosi, Z. 2004. Precambrian Figure 3. Nd and Sm isotope data for the Nipigon sill suites Geology of the northern Black Sturgeon River compared with data from the Sibley sediments and Quetico basement and Disraeli Lake Area, Nipigon Embayment, rocks. northwest Ontario; Ontario Geological Survey Open File Report 6138, 56 p. MacDonald, C.A. 2004. Precambrian geology of the south Armstrong-Gull Bay area, Nipigon Embayment, northwestern Ontario; Ontario Geological Survey, Open File Report 6136, 42p. -'.0 £ •/ - 53 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 The Geology of the Eagle Nickel-Copper Deposit: Marquette County, Michigan ROSSELL, D. M., dean.rossell@kennecott.com, Kennecott Exploration Company, 10861 N. Mavinee Dr. #141, Oro Valley, AZ. 85737 and COOMBES, S., Kennecott Canada Exploration Inc., #354-200 Granville Street, Vancouver, B.C. V6C 1S4 Kennecottʼs discovery of the Eagle nickel-copper deposit in 2002 marked the culmination of more than a decade of exploration work by Kennecott in the Paleoproterozoic in age Baraga sedimentary basin. The discovery hole, YD02-02 completed in July 2002, intersected 84.2m of massive sulfide mineralization averaging 6.3% Ni and 4.0% Cu. The resource estimate for the Eagle deposit at the end of 2003 was 5 million tonnes at 3.68% Ni, 3.06%Cu and 0.1% Co. The Eagle deposit is hosted in the westernmost of two small peridotite bodies historically referred to as the Yellow Dog Peridotite. The Yellow Dog intrusions, which lack penetrative foliations and truncate Penokean tectonic fabrics in the surrounding meta-sediments, are believed to be Keweenawan in age (Klasner et al., 1979). The intrusions are mainly comprised of coarse-grained, variably serpentinized peridotite and feldspathic peridotite. A fine-grained, olivine poor phase is found along the margins of the intrusions and as xenoliths within the peridotite. Possible amygdules in the olivine poor phase(s) suggest a shallow level of intrusion. Three principal types of sulfide mineralization are recognized in the Eagle deposit: disseminated (blebby), semi-massive (matrix) and massive. Although the nickel contents of semi-massive and massive sulfides are relatively uniform through out the deposit, copper contents vary significantly. Platinum group metals (PGM) and gold values are significantly higher in the copper rich massive sulfides. Copper rich veins and disseminations, with significant PGM and gold, in the surrounding meta-sediments may constitute a fourth type of ore. Massive and semi-massive sulfide ore types in the Eagle deposit are irregularly distributed. The contacts between different ore types are sharp and show little evidence of the gradation or layering that might be expected if gravity driven accumulation of sulfides from an overlying, sulfide saturated, silicate magma was the principle mechanism of ore formation. Sequential emplacement of various mixtures of silicate and sulfide magma and cumulus minerals, derived from a lower stratified magma chamber, may provide a better model. Reference Klasner, J.S., Snider, D.W., Cannon, W.F. and Slack, J.F. 1979. The Yellow Dog Peridotite and a possible buried igneous complex of lower Keweenawan age in the northern peninsula of Michigan. Geologic Survey of Michigan DNR report of investigation 24, 31p. - 54 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Mining and Exploration Activity in the Thunder Bay South District SCHNIEDERS, B.R. and SCOTT, J.F., Ontario Geological Survey, Ministry of Northern Development and Mines, Suite B002, 435 James St. South, Thunder Bay, ON P7E 6S7 CANADA The Thunder Bay South District covers an area from west of Atikokan, east to White River and north to Armstrong from the United States border. There were four producing precious metal and base metal mines, as well as 15 seasonal amethyst producers in 2004. The three Hemlo gold mines produced 655,000 ounces of gold in 2004, and more than 18 million ounces of gold has been produced from the Hemlo deposit since mining began in 1985. The Lac des Iles mine produced a record 308,931 ounces of palladium from the open pit, as well as platinum, gold, nickel and copper. Underground development was initiated in 2004 and underground mining is scheduled to begin in 2006. Exploration for gold, base metals, platinum group metals, uranium, molybdenum, diamonds and industrial minerals were all active in 2004. Highlights include a new base metal discovery by Freewest Resources Canada Inc. in early 2005. The Sungold Occurrence in the Western Shebandowan greenstone belt (Wawa Subprovince) is located 120 km west of Thunder Bay. Grab samples assayed 33.2 % zinc and 12.5 % Cu, discovered after a V-TEM (Dreamcatcher) and winter beep-mat geophysical surveys were conducted. Gold highlights include the Tower Mountain Property of Valgold Resources Ltd. who conducted more than 10,000 m of diamond drilling with results of up to 164.7 g/t gold over 1.5 m. Further drilling is ongoing. In the Hemlo area Navasota Resources Ltd. intersected 18.28 g/t gold and 47.18 g/t silver over 4 m in diamond drilling. North American Palladium Ltd. continued deep diamond drilling on the High-Grade Offset Zone, including intersecting 5.25 g/t palladium and 0.515 g/t platinum over 3.1 m. Exploration is ongoing on the mine block, and at the Shebandowan project. East West Resource Corporation and Canadian Golden Dragon Resources Ltd. entered into a $7.5 million joint venture with Platinum Group Metals Ltd. on the Seagull-Disraeli property. Several new reef horizons were discovered, assisted by the Ontario Geological Survey, the Lake Nipigon Region Geoscience Initiative and Lakehead University. Assays of up to 7.90 g/t platinum + palladium + gold over 0.44 m have been intersected with a 1:1 Pt:Pd ratio. Rampart Ventures Ltd., in joint venture with New Shoshoni Vcentures Ltd., have staked more than 2000 claims in the Sibley Basin for uranium. Airborne geophysical surveys have been flown and diamond drilling is planned. R ipple Lake Diamonds Inc. discovered three micro-diamonds in a small 3 by 3 m diatreme associated with a calc-alkalic (minette) dike, located on the Trans-Canada Highway approximately 30 km west of Marathon. More than 2200 claim units have been acquired. Exploration is active for a variety of commodities in the Thunder Bay South District. We invite you to come and explore in Northwestern Ontario! - 55 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 PGE and gold potential of the Archean Deer Lake Complex, Minnesota, USA SEVERSON, Mark J., Natural Resources Research Institute, University of Minnesota, Duluth The precious metal potential of the Deer Lake Complex (DLC) has recently been addressed by detailed geologic mapping (2004) and two detailed sampling campaigns (2002 and 2004). The DLC is Archean in age, is located in northeastern Itasca County, Minnesota, and consists of a series of multiple sills, each of which are variably differentiated and steeply inclined. Sampling results, along several traverses across the DLC, suggest that PGE contents are low with maximums of only 84 ppb Pt and 52 ppb Pd. Gold contents, on the other hand, are more encouraging and values up to 2.4 ppm are associated with a shear zone in one of the basal DLC sills. The Deer Lake Complex (DLC) consists of a series of steeply-dipping, tholeiitic, mafic-ultramafic sills that are variably differentiated and complexly interfingered. Where differentiation is complete, the sills exhibit a layering sequence consisting of a basal chilled margin grading upwards through peridotite, pyroxenite, melagabbro, melagabbro-porphyritic gabbro, gabbro, quartz gabbro (or diorite), and in some rare localities, an upper chilled margin. The sills were mostly emplaced into calc-alkaline volcanic and sedimentary rocks of the Joy Lake Volcanic Sequence (Jirsa, 1990). The bottom-most portion of the DLC was initially intruded into highMg tholeiitic basalt and basaltic komatiite; these volcanic rocks are assumed to be the early extrusive equivalent of the DLC (Jirsa, 1990; Englebert and Hauck, 1991). All of the sills are metamorphosed to greenschist facies. The DLC was explored for base metal deposits in the 1970s; however, no significant indications of Ni-Cu mineralization were found. Initial geologic mapping of the DLC (Berkley, 1972; Ripley, 1973) suggested that only 2-3 completely differentiated sills were present, and that repetitions of specific rock types were related to tight isoclinal folds. Later review (Severson, 1987; Jirsa, 1990; this study) suggests that the DLC formed from the emplacement of numerous interrupted magmatic pulses with partial to complete fractional crystallization occurring between the pulses. Thus, not all of the sills contain a peridotite base or grade completely upwards into quartz gabbro. Stratigraphic Order As can be seen on a plot of Pt+Pd versus stratigraphic position (Figure 1), most of the individual DLC sills contain extremely low Pt+Pd contents except for weakly anomalous samples in central portions of several sills. The overall patterns for two of these sills in particular consist of low Pt+Pd values that gradually increase with stratigraphic height to noticeable peaks, followed by dramatic dropoffs to virtually no PGE in the upper portions of the sills. It may be possible that this dramatic changeover in the Pt+Pd content may be related to a Sulfide Saturation??? 7 sulfide saturation event; whereby, PGE was scavenged from the melt. 6 A similar pattern has been reported for the Mesoproterozoic Sonju Lake Sulfide Saturation? intrusion, where a PGE-bearing reef 5 with up to 320 ppb Pd and 66 ppb Pt has been identified by Miller (1999). 4 3 However, detailed follow-up, close2 1 spaced, sampling at one of the “sulfide 0 50 100 150 saturation horizons” during 2004 failed Pt+Pd (ppb) to produce any significant PGE in the DLC. Figure 1. Vertical changes in Pt+Pd (ppb) contents with stratigraphic height in the various sills (numbered in boxes) of the Deer Lake Complex. Shaded areas are peridotite horizons. - 56 - Several anomalous gold values �Proceedings of the 51st ILSG Annual Meeting - Part 1 (>100 ppb) were obtained in samples collected from a drill hole (26508) near the base of the DLC. These values (up to 2,399 ppm Au) are from a well-foliated/sheared gabbro that contains 1-5% pyrite and pyrrhotite, and contains some of the highest Cu values (up to 0.07% only) in the DLC. In summary, the PGE potential of the Deer Lake Complex appears to be low but there is an outside possibility that some of the sills may contain PGE reefs that are associated with sulfide saturation events. On the other hand, Au potential appears to be better, especially in the sheared rocks in drill hole 26508. REFERENCES Berkley, J.L. 1972. The geology of the Deer Lake gabbro-peridotite complex, Itasca County, Minnesota. University of Missouri, M.S. thesis, 107p., 2 pls. Englebert, J.A. and Hauck, S.A. 1991. Bedrock geochemistry of Archean rocks in Minnesota. Natural Resources Research Institute, Technical Report NRRI/TR-91-12, 200 p. Jirsa, M.A. 1990. Bedrock Geologic Map of northeastern Itasca County, Minnesota. Minnesota Geological Survey Miscellaneous Map Series M-68, scale 1:48,000. Miller, J.D. 1999. Geochemical evaluation of platinum group element (PGE) mineralization in the Sonju Lake intrusion, Finland, Minnesota. Minnesota Geological Survey Information Circular 44, 32 p. Ripley, E.M. 1973. The ore petrology and structural geology of the lower Precambrian Deer Lake mafic-ultramafic complex, Effie, Itasca County, Minnesota. Duluth, Minnesota, University of Minnesota Duluth, M.S. thesis, 143p., 2 pls. Severson, M.J. 1986. Summary report on exploration activities in the Deer Lake Area, Itasca County, Minnesota. Santa Fe Minerals Exploration report, Minnesota DNR open files. - 57 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 A view into the Nipigon Embayment: preliminary results of the largest magnetotelluric study ever in Ontario. SHAREEF, S., and CRAVEN, J.A., Geological Survey of Canada, Ottawa, ON K1A 0E9. Over 800 audiomagnetotelluric and combined audiomagnetotelluric and magnetotelluric (A/MT) sites were surveyed along five transects (Figure 1) during the period of late Aug to mid October 2004. The primary goal of the survey was to investigate the deep structures of the region to complement the studies that comprise the Lake Nipigon Regional Geoscience Initiative (LNRGI). The LNRGI was created to provide a comprehensive 4-D geoscience knowledge base in order to facilitate proper land-use decisions and to reduce the risk in mineral exploration of the region. The studies conducted as part of the Initiative are comprised of surface geological and stratigraphic investigation, surface and airborne geophysical surveys and other studies such as geochronological or paleomagnetic studies. Missing from these studies is an investigation of the deep geological structure. The A/MT survey conducted last year was designed to provide subsurface information down to mantle depths and beyond along five major transects. I\ I \. I B38AMTstes orna MT/MJT sites PR OT E H OZO IC Felsic to interrnethale metavolcanic rocks I relsic 0 Intern,edsIe fliwsive rocks —, Coarse dastic Melsedimentaly rocks ARCH EA N Felsic granitic rocks Melsedime ntaiy rocks Massive granodiorite '0 grane 20km Disraeli Lk 3-D • Larger 3-D • Figure 1. Site Locations for 2004 A/MT Nipigon Survey. Possible plan view locations for future 3-D model studies are also shown by red boxes. The magnetotelluric technique is a geophysical technique for imaging the deep structure of the Earth. It images conductors in the Earth and as such will be useful to delineate contacts between crystalline and sedimentary units at depth as they have dramatically different electrical characteristics. The novel feature of A/MT is that it uses the Earthʼs natural magnetic field as a transmitter of energy into the earth. This enables almost an infinite depth of investigation; however in practice the depth of investigation is limited to the asthenosphere. Preliminary results of the Nipigon A/MT survey will be presented with a view to providing a clue to the origin of the enigmatic sedimentary and intrusive units in this area. - 58 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Mineral Deposits and Metallogeny of the Midcontinent Rift in Ontario SMYK, Mark C., Ontario Geological Survey, Ministry of Northern Development and Mines, Suite B002, 435 James St. South, Thunder Bay, ON P7E 6S7 CANADA A variety of metallic and non-metallic mineral deposits are associated with Mesoproterozoic (Keweenawan) Midcontinent Rift (MCR)-related magmatism and associated hydrothermal activity in Ontario. Orthomagmatic deposits of copper, nickel and platinum group elements (PGE) are associated with early ultramafic to mafic intrusive rocks. Stratabound, “reef-style” Cu-Ni-PGE mineralization occurs in peridotite in the Seagull layered intrusion; within massive, cumulate Ti-Fe-oxide layers in the border gabbro of the Coldwell Complex; and with Cr-bearing spinel in cyclic anorthosite-gabbro units above the Great Lakes Nickel deposit in the Crystal Lake Gabbro. Varied-textured to pegmatitic gabbro at the base of the Crystal Lake Gabbro hosts the Great Lakes Nickel deposit (~ 41.4 mT @ 0.334% Cu, 0.183% Ni, 0.69 g/t Pd, 0.21 g/t Pt). Similar rocks of the Two Duck Lake gabbro in the Coldwell Complex host the Marathon deposit (24.4 mt @ 1.22 g/t Pd, 0.31 g/t Pt, 0.37% Cu). Late-stage, magmatic deposits of pyrochlore-hosted, U-Nb-mineralization have been investigated in the Prairie Lake Carbonatite Complex and in syenites of the Coldwell Complex (Nb-Zr-Th-U-Ce). Rare metal (Be-Zr-U-Th-) mineralized structures crosscut the Dead Horse Creek carbonatitic diatremes. Epigenetic deposits associated with MCR structures and hydrothermal activity are hosted in a variety Neoarchean, Paleoproterozoic and Mesoproterozoic rocks. Native copper and Cu-sulphides occur in Keweenawan basalt and interflow sedimentary rocks at Mamainse Point (e.g., Coppercorp Mine), on Michipicoten Island (e.g., Quebec Mine) and in Osler Group rocks in western Lake Superior. Hydrothermal fluids ascribed to MCR magmatism are believed to have produced Cu-Mo-Ag-Pb-mineralized breccia pipes in Archean country rocks at the Tribag Mine and Cu-Mo-mineralization at the Jogran porphyry deposit. Copper-mineralized carbonate units of the Mesoproterozoic Sibley Group are found near the contacts with Nipigon diabase sills. Silver-bearing, carbonate-quartz veins near Thunder Bay have been subdivided into two groups: the Mainland Belt (Ag + Zn-Cu-Pb-bearing veins in Paleoproterozoic Animikie Group sedimentary rocks and Logan diabase); and the Island Belt (Ag-Bi-Co-Ni-As-bearing veins in Keweenawan diabase/gabbro; e.g., Silver Islet Mine). Lead-zinc-barite veins occupy structures near the unconformity between Neoarchean basement and Mesoproterozoic Sibley Group sedimentary rocks. Some of these structures also host amethyst deposits. Silver-bearing, lead-zinc veins occur within and proximal to the Coldwell Complex. All of these veins are likely contemporaneous but were generated by different fluid sources. Uraniferous, hematite-rich veins and breccias that cut Neoarchean basement and locally contain Proterozoic xenoliths may also be related to MCR hydrothermal activity, which remobilized uranium from granitic pegmatites in the basement. - 59 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Discrimmination of Archean Terranes in the Sachigo Subprovince and relevance to Volcanogenic Massive Sulphide Exploration STOTT, G. M., Ontario Geological Survey, Sudbury, ON, P3E 6B5 (greg.stott@ndm.gov.on.ca), and RAYNER, N., Geological Survey of Canada, Ottawa, ON Geological and geophysical constraints on the mechanisms for tectonic growth of the Superior Province are progressively summarized in Williams et al., 1992; Stott, 1997; and Percival, 2005, the latter based on Lithoprobe and Natmap research activities in northwestern Ontario and Manitoba since 1997. This increased knowledge permits greater confidence in the subdivision of the Superior Province into terranes and superterranes of contrasting tectonostratigraphic history. Prior to this, for many years, the broad region of Archean crust north of Uchi Subprovince in Ontario was more commonly called the Sachigo Subprovince (e.g., Card and Ciesielski, 1986). A reconnaissance geochronological and Hf isotopic study was initiated on a transect of granitoid and felsic volcanic samples just west of the James Bay Lowlands (Figure 1) to complement the Natmap studies previously conducted near the Ontario - Manitoba border (e.g., Stone, 2005). Thus far, results from U-Pb geochronology in eastern Sachigo Subprovince further confirm that this subprovince can be subdivided into several terranes. Most notably, a broad Neoarchean, Oxford-Stull terrane contains crustal magmatic ages ranging from 2690 to 2730 Ma, and separates two major superterranes to the north and south, each of which contains evidence of > 3.0 Ga history. Since virtually all significant Cu-Zn volcanogenic massive sulphide (VMS) deposits in the Superior Province are Neoarchean in age, the presence of Oxford-Stull terrane bears some importance for exploration. A Neoarchean age of 2737±7 Ma has been determined for a felsic volcanic host of recently discovered VMS deposits in the McFaulds Lake area, inside the edge of the James Bay Lowlands (Figure 2). This argues for the potential of a chain of VMS occurrences to be explored along a Neoarchean tectonic assemblage extending from Kasabonika to the McFaulds Lake area, comparable to the VMS-rich assemblage along the discontinuous Shebandowan-Terrace Bay-Manitouwadge greenstone belt (Figure 2). Assean Lake On Lake Block Bloc SpFit Lake B!ock / / J&nes Bay Lowlands Greenstone Belt a Sedimentary Belt () Pltjtonjc Rocks Figure 1. A subdivision of NW Superior Province into terranes (modified from Stone, 2005). Uncertainty of boundaries increases eastwards reflecting current distribution of geochronological and isotopic data. - 60 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 _______ Kasabonika-MeFaulds greenslone bell I 0 100 I Olher greenstone bells A MFaulds Lake Cu-Zn * Community massive sulpriide deposits (Spider Resou rcesIKWG Resources exploration) 200 K Ia metres 100 200 — Shebandowan-Terrace Bay-Michipicoten band aigreenstone belts Other greenstone bells Kilometres a Cu-Zn massive sulphide deposits Community Figure 2. (a). The discontinuous Neoarchean Kasabonika-McFaulds greenstone belt is the focus of new Cu-Zn VMS exploration, and may resemble (b) the Shebandowan-Terrace BayManitouwadge belt, a bandlike tectonic assemblage, which contains several VMS deposits and mines along its length. References Card, K.D. and Ciesielski, A. 1986. Subdivisions of the Superior Province of the Canadian Shield. Geoscience Canada, 13, 5-13. Percival, J.A. 2005. The Ancient Earth: Development of the Canadian Shield, the oldest parts of North America – building a continental foundation from 4000 to 2500 million years ago. In LITHOPROBE Celebratory Conference Oral and Poster Presentations, R.M. Clowes and C. Li (compilers). Published by the LITHOPROBE Secretariat, University of British Columbia, E-Publication No. 5, v.1. Stone, D. 2005. Geology of the Northern Superior Area, Ontario. Ontario Geological Survey, Open File Report 6140, 94p. Stott, G.M. 1997. The Superior Province, Canada. In Greenstone Belts; Edited by M.J. de Wit and L.D. Ashwal. Oxford Monograph on Geology and Geophysics 35, Oxford Clarendon, p.480-507. Williams, H.R., Stott, G.M. and Thurston, P.C. 1992. Tectonic Evolution of Ontario: Summary and Synthesis, Part 1: Revolution in the Superior Province. In Geology of Ontario; Ontario Geological Survey, Special Volume 4, Part 2, pp.1256-1294. - 61 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Correlation Between Self-Potential and Dowsing (IESG) at the Quincy Mine and at the Calumet and Hecla Mine, Michigan TROW, Jim, Geological Sciences, Michigan State University, emeritus, 540 Lake Avenue #2, Hancock, Michigan 49930, and YOUNG, Charles, T., Geological Engineering and Sciences, Michigan Technological University, 1400 Townsend Drive, Houghton, Michigan 49931. The focus of this paper is an 11/24/04 joint traverse over and mapwise perpendicular to the strike of the native copper Calumet Conglomerate Lode at the Hecla #1 Mine, Calumet Michigan, with self-potential (Young) and dowsing (Trow) observations plotted in profile on Figure 1. According to Young (Young and Trow, 2005), “The self-potential apparatus consists of hand-made non-polarizing copper copper-sulfate electrodes, a reel containing AWG 20 stranded, tinned copper wire, patch cords and clip leads and a high-input-impedance digital voltmeter. The reference electrode is installed at a convenient sheltered position along the line where the wire can be tied to a sturdy object. The wire is clipped to the reference electrode and is unwound from the reel to the observation points. The electrodes are installed in shallow holes made with a garden trowel, which is used to remove loose and dry surface material so that the electrode is in contact with moist soil. The repeatability of these self-potential measurements is typically about five millivolts.” Horizontal distances are measured with fiberglass surveyorʼs tape. ! 9 U,. I I .4. •tlfl I 2. . .: I Ic_ — 14• 1994 99 99 —l—m ••91 •U!14944 .9*9 .11 — 191 •tt3flt 1194•t IOU . .tn. T: t 93 9 —Ic 9• 9• 4 .99919,.., ••—1199,• .9ltI. ••!T499 •tC •34 .9 fl I • ni. it/c oIl •!lM •_• 'ull •;3• •Tt •.94 l.!fl.. •4n•9& .4..•.9nulb. .9 9* 9 ' 9 ct .•9t m .mE .9 — •n..Tp •9c! .l l •jj•9] 4!—.. •.—_. •93 •lS.9 •fl9 .9!,; fl .9I •.jfl.9 1.991 •9Iul97_ •3'9n• .9—. .4,3. 9 .9 •..I—_,.9— . . *1 £9 c_fl.... ••..!p •!! 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Figure 1 shows dowsing by Trow with sensitive 3/16”-diameter 6” x 15” iron welding rods, bent into a Ushape, and with less sensitive 6” x 15” L-rods of the same diameter and material. Dowsing with U-rods reset to the standard state after each episode of turning (CCʼ) detects a broader and lower anomaly than Youngʼs –350 mV SP anomaly (AAʼ), but both appear to be distributed symmetrically about the SP negative center, as though dowsing implies an upward-continuation curve for the SP. After all, SP is a conductive method, whereas dowsing is an inductive “airborne” method, involuntarily detected by oneʼs brain (Faraday cage shielding experiments; - 62 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Trow, 1992). BBʼ shows the less sensitive L-rod dowsing results. When favorable weather returns, Trow would like to reverse by traversing SE, to see if the L-rods define the NW edge of the subcrop of the ore body as BBʼ defines its SE edge when traversing NW. DDʼ represents Trowʼs dowsing with the sensitive U-rods, when they are not reset to the standard state after turning, in conformity with most dowsersʼ procedure, which ignores important data compared to CCʼ. They have run out of lever arm (physics terminology). Figure 2 (not shown, please see poster) suggests three hypotheses for a mathematical function to reconcile SP and dowsing: The geologically most plausible is that the square root of the number (x) of Urod convergences over the total –SP anomaly is proportional to the –SP readings of its negative center. An even better fit than Figure 2 results when one considers the total SP relief on Figure 1, from +15 mV at Aʼ to the –350 mV negative center = -365 mV. Calculating for Quincy [(√7/√13)(-365mV)] yields – 268mV, 99% of Youngʼs observed – 270 mV there. Two points, Quincy and C & H, do not define a curve, but they hint at one. Figure 3 (not shown, please see poster) symbolically illustrates (some elements displaced to avoid overlap) characteristic dowsersʼ rod movements, which Trowʼs (1992) experiments with permanent magnets, low-voltage static electric charges, and Faraday cages demonstrated are related to electrostatics, not magnetostatics. REFERENCES Trow, J. 1992. Inductive electrostatic gradiometry (IESG) deciphers Keweenanwan copper plumbing system, Soc. Mining, Metall. And Expl., Phoenix, Preprint 92-32, 22p. Trow, J. 2004. Dowsing employs classical mechanics and static electricity to locate self-potential anomalies inductively and rapidly, 50th Institute on Lake Superior Geology, Duluth, Proceedings, v.50, pt 1., 158-159. Young, C.T. and Trow, J. 2005. Human-sensed fields?: Does dowsing response correlate with self-potential or conductivity anomalies?, 18th Annual Symposium on the Application of Geophysics in Engineering and Environmental Problems, Atlanta, 9p. - 63 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 New age data for the Chocolay Group, Marquette Range Supergroup: implications for the Paleoproterozoic evolution of the Lake Superior and Lake Huron regions VALLINI, Daniela A., School of Earth and Geographical Sciences, University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia, CANNON, William F., and SCHULZ, Klaus J., U.S. Geological Survey, 954 National Center, Reston, VA 20192 A geochronological study of the Chocolay Group at the base of the Paleoproterozoic Marquette Range Supergroup in Michigan, Lake Superior region, is attempted for the first time. Age data from detrital zircon grains and hydrothermal xenotime from the basal glaciogenic formation, the Enchantment Lake Formation, and the overlying Sturgeon Quartzite and its equivalent, the Sunday Quartzite, provide maximum and minimum age constraints for the Chocolay Group. The youngest detrital zircon population in the Enchantment Lake Formation is 2317±6 Ma, in the Sturgeon Quartzite it is 2306±9 Ma and in the Sunday Quartzite it is 2647±5 Ma. The oldest hydrothermal xenotime in the Enchantment Lake Formation is 2133±11, in the Sturgeon Quartzite it is 2115±5 Ma and in the Sunday Quartzite it is 2207±5 Ma. The depositional age of the Chocolay Group is constrained to 2300-2200 Ma and the radiometric age data proves it is correlative with part of the Huronian Supergroup in the Lake Huron region, Ontario and reveals that the unconformity which separates the Chocolay Group from the overlying Menominee Group is up to 350 m.y. in duration. The detrital zircon suite in the Sunday Quartzite and part of those in the Enchantment Lake Formation and Sturgeon Quartzite were derived from the underlying Archean basement rocks. The source of the ~2300 Ma detrital zircon populations in the Enchantment Lake Formation and Sturgeon Quartzite remains an enigma as there is no known terrane of this age in the Michigan area. It is speculated that there was once a widespread volcano-sedimentary cover sequence in Michigan which was removed or concealed prior to Chocolay Group deposition. The hydrothermal xenotime ages are coeval with the 2219±4 Ma Nipissing Diabase in the Lake Huron region and the 2200-2100 Ma Kenora-Kabetogama mafic dyke swarm in Minnesota and adjacent Ontario, Lake Superior region. - 64 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Implications of Midcontinent Rift and Oceanic Ridges Analogies and 3DInterpretations of the Subsurface Structure of the Bald Eagle Intrusion in the Duluth Complex and the East Pacific Rise WEIBLEN, Paul, Department of Geology & Geophysics, University of Minnesota, PETERSEN, Dean, Natural Resources Research Institute, University of Minnesota Duluth, and VISLOVA, Tatiana, Department of Geology University of St. Thomas The geometry of the gravity and magnetic anomalies of the Bald Eagle intrusion and the Duluth Complex to the south, as well as the overall Midcontinent Rift, is very similar to the pattern of the seismic reflections profiles of ridge systems (Vislova, 2003). In detail, the geophysical expressions of the Bald Eagle intrusion have the same shape and dimensions as the “bulls eye” pattern of low velocity seismic reflection anomalies along the East Pacific Rise. These anomalies are interpreted to define regions of melt concentrations, i.e., active magma chambers. This suggests that the Bald Eagle intrusion could be a “frozen” dynamic magma chamber. In support of this analogy we note that the magmatic systems of mid-ocean ridges, extensional regimes in back-arc environments, and ophiolites have a common characteristic: the emplacement of magma in extensional environments, and the common products in all four are varieties of layered intrusions, dikes and sills, and overlying volcanic rocks. There is a long history of comparative studies of ophiolites and oceanic magmatic environments (See web sites for ophiolites and the East Pacific Rise). This suggests that it would be profitable to add the studies of the igneous systems of intracontinental rifts to the current interdisciplinary, studies of ophiolites and the oceanic environments. In the accompanying poster session we explore aspects of this approach using 3D modeling of surface mapping, geophysical, and drill hole data of the basal mineralized zone of the Duluth Complex, the Bald Eagle intrusion and seismic reflection data of the East Pacific Rise. Since its discovery in 1961, the Bald Eagle intrusion has posed unresolved questions concerning its origin and magmatic significance (Weiblen, 1965; Weiblen and Morey, 1980; Miller and others, 2002). A number of its characteristics contrast markedly with those of the other mapped intrusions in the Midcontinent Rift: (1) It has a well-defined intrusive contact in anorthositic gabbros around its perimeter except for its southern extension; (2) There is a subtle, but recognizable metamorphic contact effect on the anorthositic gabbros; (3) A primary magmatic foliation is well defined by mineral orientation and discoid segregation of plagioclase from mafic phases; (4) Over 150 measurements of the foliation define a steeply-dipping asymmetric funnel with the foliation paralleling the contact and grading from steep to horizontal inward; (5) The intrusion consists of two cumulus units, a plagioclase-olivine outer cumulate (troctolite) and an inner plagioclase-olivine-clinopyroxene (olivine gabbro) cumulate; and (6) There is only minor (< a few %) intercumulus material in the cumulates which is found as the expected lower temperature minerals in the crystallization sequence, i.e. clinopyroxene and iron oxides in the plagioclase-olivine. Recent petrologic studies include over 2000 electron microprobe analyses and interpretations using the computer-based mineral-melt equilibria routines MELTS and COMAGMAT (Vislova, 2003). These studies indicate that the following scenario could account for the characteristics of the Bald Eagle intrusion listed above. As up-welling magma streams through a dynamic (expanding) funnel-shaped feeder, a constant temperature appropriate to plagioclase-olivine crystallization is maintained by a balance between the heat content of the incoming magma plus the heat of crystallization and the heat loss through the chamber walls. Plagioclase and olivine are left behind and oriented/segregated on the walls of the expanding chamber. At some point conditions change and a temperature appropriate to plagioclase-olivine-clinopyroxene crystallization is established and maintained to form the inner olivine gabbro of the intrusion. This scenario could also produce the plagioclaseolivine-clinopyroxene-iron oxide cumulates found to the south of the mapped expression of the Baled Eagle intrusion. A dynamic, flow-through magmatic system raises questions about mineralization. We note that there are active “smokers” above the proposed magma chambers on the East Pacific Rise. In view of this and assuming - 65 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 the analogy suggested above is appropriate, comparative studies of the smokers and the sulfide mineralization in the intrusions adjacent to the Bald Eagle intrusion may be profitable. Applying this approach, some of the attributes of the Duluth Complex copper-nickel-PGE sulfide deposits resemble those of deposits at Norilʼsk, Russia and Voiseyʼs Bay, Canada that are associated with sulfide mineralization in intrusive feeder zones. The common attributes include occurrence in shallow tholeiitic intrusions associated with plateau basalt volcanism, an external sedimentary source of sulfur, and openness to repeated magma influx and expulsion. A critical attribute of the high-grade Norilʼsk-Talnakh and Voiseyʼs Bay deposits, not yet positively identified in the Duluth Complex deposits, is the location of a magma conduit. A conduit that experienced repeated influxes of magma appears to be key to the formation of high-grade copper-nickel-PGE deposits (Naldrett, 1997). One of the difficulties in evaluating the potential for feeder zone mineralization in the Duluth Complex is determining whether intrusions were fed one-by-one by local magma conduits or by master conduits that sequentially fed several intrusions. Another long-standing question is the source of sulfur. In the case of the Norilʼsk deposits, Jurassic sediments are obvious candidates as are the Virginia and Biwabik Iron formations for the mineralization at the base of the Duluth Complex. However, designation of these sources begs the question of how, when, and where the sulfur is delivered to the magmatic system. This is more straightforward with regard to smokers: circulating hydrothermal solutions deliver sulfur from sea water directly to magma chamber vents. We suggest that consideration of this mechanism in the Midcontinent Rift magmatic systems could provide new insights for exploration targets. Field mapping (Green and others, 1966; Foose and Cooper, 1978) showed the Bald Eagle and the South Kawishiwi intrusions to be in direct contact. Petrologic observations and geophysical interpretations (Chandler, 1990; Chandler and Ferderer, 1989) suggest that the Bald Eagle and South Kawishiwi intrusions were emplaced by successive overplating of magmas from a common feeder centered on the northern Bald Eagle intrusion and extending along the trace of the macrodike that links the Bald Eagle and South Kawishiwi intrusions. Another model relating the emplacement of the two intrusions to a single magma feeder has been presented by Miller and Severson (2002; their Fig. 6.15). This model further speculates that the Bald Eagle intrusion had a more complete differentiation sequence (this contrasts with the dynamic flow model of Vislova (2003) outlined above) that has been eroded away and that this magma system may have fed surface eruptions. At a more detailed level, Peterson (2001) interpreted the copper-PGE mineralization in the Maturi deposit and its extension to the east (Maturi Extension deposit) as indicative of magma input from the northwest via the arcing macrodike that connects the Bald Eagle and South Kawishiwi intrusions. Peterson (2001) envisioned a confined magma flow model that invokes a change from laminar to turbulent flow beneath a pillar of older anorthositic series rocks, increasing the R-factor of the entrained sulfides, and resulting in higher metal contents of the exited (Maturi deposit) and remaining (Maturi Extension deposit) sulfide fraction. If correct, this model predicts that a Voiseyʼs Bay-type copper-nickel-PGE massive sulfide body may exist in an area south of the Spruce Road deposit. References Chandler, V.W. 1990. Geologic interpretation of gravity and magnetic data over the central part of the Duluth Complex, northeastern Minnesota. Economic Geology, v. 85, no. 4, p. 816-829. Chandler, V.W. and Ferderer, R.J., 1989, Copper-nickel mineralization of the Duluth Complex, Minnesota-A gravity and magnetic perspective: Economic Geology 84, 1690-1696. Foose, M.P. and Cooper, R.W. 1978. Preliminary geologic report on the Harris Lake area, northeastern Minnesota. U.S. Geological Survey Open-File Report 78-385, 24p., 1 pl., scale 1:12,000. Green, J.C., Phinney, W.C. and Weiblen, P.W. 1966. Gabbro Lake quadrangle, Lake County, Minnesota. Minnesota Geological Survey Miscellaneous Map M-2, scale 1:31,680. Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.M. and Wahl, T.E. 2002. Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota. Minnesota Geological Survey Report of Investigations 58, 207p. Miller, J.D. and Severson, M.J. 2002. Chapter 6: Geology of the Duluth Complex, in Miller, J.D., Jr., Green, J.C., Severson, - 66 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 M.J., Chandler, V.W., Hauck, S.A., Peterson, D.M. and Wahl, T.E. 2002. Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota. Minnesota Geological Survey Report of Investigations 58, 207p. Naldrett, A.J. 1997. Key factors in the genesis of Norilʼsk, Sudbury, Jinchuan, Voiseyʼs Bay and other world-class Ni-CuPGE deposits: Implications for exploration. Australian Journal of Earth Sciences, 44, no. 3, 283-315. Peterson, D.M. 2001. Development of a conceptual model of Cu-Ni-PGE mineralization in a portion of the South Kawishiwi intrusion, Duluth Complex, Minnesota. Society of Economic Geologists, Second Annual PGE Workshop, Sudbury, Ontario, 3 pages. Vislova, Tatiana. 2003. Petrology of the Bald Eagle Intrusion and associated rocks and its relevance to crystallization in dynamic magma chambers in the Midcontinent Rift, unpublished PhD. Thesis, University of Minnesota. Weiblen, Paul W. 1965. A funnel-shaped, gabbro-troctolite intrusion in the Duluth Complex, Lake County Minnesota, unpublished. PhD. Thesis, University of Minnesota. Weiblen, P. W. and Morey, G. B. 1980. A summary of the stratigraphy, petrology, and structure of the Duluth Complex. American Journal of Science, 280A, Part I, 88-133. - 67 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Textural Examination of Kama Point Diabase Sill, Nipigon, Ontario ZIEG, Michael J., FORSHA, Clinton J., and HABARKA, Joseph D., Department of Geography, Geology, and the Environment, Slippery Rock University, 1 Morrow Way, Slippery Rock, PA, 16057, USA, michael. zieg@sru.edu The texture of an igneous rock provides valuable constraints on the cooling history of the magma from which it crystallized. When combined with mineralogical data, the sizes and abundances of crystals in a rock reflect the interactions between the thermal and chemical evolution of the parent magma. In order to avoid uncertainty relating to the absolute magnitude of kinetic parameters such as nucleation and growth rates, this study focuses on textural gradients. These gradients reflect differences in crystallization history and magma composition, and are related to differences in the thermal and compositional state of the magma. The lowermost 35 meters of the 1100 Ma olivine diabase sill at Kama Point, approximately 25 km east of Nipigon, Ontario, has been sampled at ~3 meter intervals from the basal contact upwards. For each sample, textures were quantified using crystal size distributions (CSDs) as well as measurement of mean crystal size. As expected, the CSDs exhibit a clear coarsening-inwards trend through the lowermost 25 meters of the sill. However, above 25 meters, the texture becomes finer-grained. Above this, the coarsening-inwards trend continues, but with a lesser gradient (Figure 1). In this study, analysis of plagioclase CSD data is shown to provide a method for quantitatively evaluating hypotheses for magmatic processes as well as important constraints on crystallization kinetics. CSD Intercept Variation CSD Slope Vailation . . . . . . . 20 . 20 . . . . . . 0 9 10 . . 11 . -14 12 Intercept, In(ni Imm41 -12 -10 -O -S -4 -2 Slope Imm11 Figure 1. Variations in CSD intercept and slope. Intercept is related to the number of crystals per unit volume; slope is related to the mean length of the crystals in the sample. With increasing height above the base of the sill, the number density decreases and the mean length increases. These trends are consistent with increasing cooling durations. These The measured textural gradients have been compared to calculated magmatic cooling durations, based on conductive cooling of an instantaneously injected 200 meter thick diabase sill. The initial conditions for the cooling model are an initial magma temperature of 1200°C and an initial country rock temperature of 100°C; far-field temperature is held at 100°C. The model is used to calculate the length of time required for the magma to cool to a temperature of 1000°C, which is the approximate solidus of the Nipigon diabase. Measured textures were compared to the calculated cooling durations (Figure 2), which yields a bulk growth rate of 1.8×10-10 mm/s (R2 = 0.925). The total number of crystals decreases with cooling duration, consistent with a nucleation rate controlled by the degree of undercooling, which is maximized during rapid cooling. The break in textural gradient at 25 meters is attributed to a recharge event in which a second pulse of - 68 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Mean Length vs Cooling Duration Total Number vs Cooling Duration 5T0 — 4T0 020 E E E 3T0 . C 0,15 a C -I . C 2T0 z 0.10 ITO . . 0 10 20 0 40 10 20 30 40 Cooling Duration yrJ Coo Iiruj Duration [yrj Figure 2. Variations in textural parameters obtained from CSD intercept and slope. Textural parameters are related to calculated cooling durations. 25-m break in texture corresponds to 22-yr cooling duration. magma is injected into the magma chamber, which had at that point crystallized a thickness of 25 meters. This event is believed to have taken place approximately 22-25 years after the initial injection. Cooling of the new batch of magma was slower than cooling in the initial injection because of the cooler, previously crystallized diabase located between the fresh magma and the cold country rock. This new cooling environment qualitatively explains the change in textural gradient above the 25-m discontinuity. Additional modeling will focus on relating this discontinuity to a specific set of recharge conditions. Further field investigation of the sills in the Nipigon area is planned. This work will extend the sampling transect higher into the sill and will include samples from higher in the sill. Goals of this research include a more thorough understanding of the injection history and original dimensions of this intrusion, as well as empirical relationships between intrusion dimensions, initial and boundary conditions, and textural profiles. - 69 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Author Index Addison, W. 1 MacDonald, C.A. 22 Albers, P. 3 MacDonald, J. 12 Anderson, R. 10 MacTavish, A. 36 Blackburn, C. 5 Magee, M.A. Breckenridge, A. 7 Maric, M. Brumpton, G. 1 McNaughton, N. Bucholz, T. 8 Medaris, L. Cannon, W. 10, 27, 64 Metsaranta, R. 17, 45 Chandler, V. 10, 27 Middleton, R. 47 38, 39 41 1 43 Conly, A. 12 Miller, J. Coombes, S. 54 Murphy, J. 50 Craven, J. 58 Petersen, D. 65 Dahl, D. 14 Planavsky, N. 50 Daniels, D 10 Rayner, N. 60 Daniels, P. 30 Richardson, A. 52 Davis, D. 1 Rogala, B. 17 43 Rossell, D. 54 Saini-Eidukat, B. 30 Dott, R. Easton, R.M. 15, 24 Falster, A. 8 Schneider, D. Forsha, C. 68 Schnieders, B. Fralick, P. 1, 17, 39, 41, 45 Schulz, K. 3, 28, 30, 48 10, 27 55 10, 27, 64 Franklin, J. 19 Scott, J. 55 Habarka, J. 68 Severson, M. 56 Halls, H. 20 Shareef, S. 58 Hammond, A. 1 Simmons, W. 8 Hart, T. 22 Smyk, M. Heaman, L. 24 Stott, G. 60 Heggie, G. 47 Trow, J. 62 Hill, M.L. 26 Vallini, D. Hollings, P. 32, 34, 39, 52 Van Schmus, W. 26, 59 1, 64 10, 27 Holm, D. 10, 27 Vislova, T. 65 Jirsa, M. 28, 48 Weiblen, P. 65 Johnson, T. 7 Young, C. 62 Kissin, S. 1 Zieg, M. 68 Knudsen, D. 30 Laarman, J. 32 Lane, C. 34 - 70 - � https://digitalcollections.lakeheadu.ca/files/original/942b8efd07100ae7b5e8a86a8ac19475.pdf 2e91b73833d712df8bd61840ebb527f9 PDF Text Text 51st ANNUAL MEETING Nipigon, Ontario - May 24-28, 2005 INSTITUTE ON LAKE SUPERIOR GEOLOGY Part 2 – Field Trip Guidebook 51st ILSG Nipigon 2005 wwwIakesuperiorgeology.org �51st ANNUAL MEETING INSTITUTE ON LAKE SUPERIOR GEOLOGY May 24-28, 2005 Nipigon, Ontario HOSTED BY: Mark Smyk and Pete Hollings Co-Chairs Ontario Geological Survey and Lakehead University Proceedings - Volume 51 Part 2 – Field Trip Guidebook Compiled and edited by Pete Hollings, Lakehead University Cover Photos: Left - basaltic dyke cutting Osler volcanics on Wilson Island, Middle - Aerial view of the Black Sturgeon fault, Right - Ruby Lake near Nipigon, Northern Ontario. �51ST INSTITUTE ON LAKE SUPERIOR GEOLOGY VOLUME 51 CONSISTS OF: PART 1: PROGRAM AND ABSTRACTS PART 2: FIELD TRIP GUIDEBOOK TRIP 1: GEOLOGY AND FOLD MINERALISATION OF THE BEARDMORE-GERALDTON GREENSTONE BELT TRIP 2: QUATERNARY GEOLOGY OF THE BEARDMORE – NIPIGON AREA TRIPS 3 & 6: A STRATIGRAPHIC TRANSECT ACROSS THE NORTHERN FLANK OF THE MIDCONTINENT RIFT NEAR ROSSPORT TRIP 4: GEOLOGY AND RARE ELEMENT PEGMATITES OF THE QUETICO SUBPROVINCE NEAR NIPIGON TRIP 5: GEOLOGY OF THE BLACK STURGEON AREA Reference to material in Part 2 should follow the example below: Hart, T.R., 2005. Geology of the Black Sturgeon Area. In; Hollings, P. (Ed.), Institute on Lake Superior Geology Proceedings, 51st Annual Meeting, Nipigon, Ontario, Part 2 - Field trip guidebook, v.51, part 2, 2-39. Published by the 51st Institute on Lake Superior Geology and distributed by the ILSG Secretary: Pete Hollings - ILSG Secretary Department of Geology Lakehead University 955 Oliver Road Thunder Bay, ON P7B 5E1 Canada Email: peter.hollings@lakeheadu.ca ILSG website: www.lakesuperiorgeology.org ISSN 1042-9964 �Proceedings of the 51st ILSG Annual Meeting - Part 2 Table of Contents Introduction, safety considerations and acknowledgements ...............................................1 Fieldtrip 1 - Geology and gold mineralisation of the Beardmore-Geraldton greenstone belt ..............................................................................................................................3 Fieldtrip 2 – Quaternary geology of the Beardmore – Nipigon area ................................41 Fieldtrips 3 and 6 - A stratigraphic transect across the Northern flank of the Midcontinent Rift near Rossport .....................................................................................................57 Fieldtrip 4 - Geology and rare element pegmatites of the Quetico Subprovince near Nipigon .....................................................................................................................71 Fieldtrip 5 - Geology of the Black Sturgeon Area .........................................................85 -i- �Proceedings of the 51st ILSG Annual Meeting - Part 2 Introduction, safety considerations and acknowledgements Pete Hollings Department of Geology, Lakehead University, Thunder Bay, Ontario, P7B 5E1, Canada and Mark Smyk Resident Geologistʼs Office, Ontario Geological Survey, Ministry of Northern Development and Mines, Thunder Bay, Ontario, P7E 6S7, Canada This volume is intended to serve not only as a guide for 51st ILSG field trip participants but also as a reference for those planning to revisit these areas at a later date. Consequently we have included UTM coordinates in the NAD 83 datum for stops, as well as instructions on how to reach them. As some of the stops are on private and staked land, particularly on Trip 1, please be sure to obtain the land owners’ permission before entering their land. In what is perhaps a first for the Institute this year we are offering a fieldtrip onto Lake Superior. This creates a number of unique safety issues. Please exercise caution when getting in and out of the boats as the outcrops are often extremely slippery. Life jackets must be worn in the boats at all times. If you are planning to revisit these sites please be very careful as Lake Superior is a dangerous lake, waves can often be many metres high and even in mid summer fog can appear very quickly. major highways or busy logging roads. Please take care when crossing or parking along these roads. We would like to thank all the other authors who contributed to this field guide (Peter Barnett, Tom Hart, Phil Fralick and Steve Kissin) and also all those who provided comments and assisted with the running of the field trips themselves. We appreciate the assistance and cooperation of the exploration and mining companies in providing us access and information concerning their properties, particularly David Malouf, Roxmark Mines Limited. The other fieldtrips will be visiting stops along either LAKE r _____9— NIPIGON 0 ta Figure 1. Map showing the location of the five field trips. -1- �Proceedings of the 51st ILSG Annual Meeting - Part 2 -2- �Proceedings of the 51st ILSG Annual Meeting - Part 2 Fieldtrip 1 - Geology and gold mineralisation of the Beardmore-Geraldton greenstone belt Mark Smyk Resident Geologistʼs Office, Ontario Geological Survey, Ministry of Northern Development and Mines, Thunder Bay, Ontario, P7E 6S7, Canada Philip Fralick Department of Geology, Lakehead University, Thunder Bay, Ontario, P7B 5E1, Canada Thomas R. Hart Ontario Geological Survey, Ministry of Northern Development and Mines, Sudbury, Ontario, P3E 6B5, Canada Introduction The Beardmore-Geraldton belt (BGB) is a Neoarchean metavolcanic-metasedimentary terrane at the boundary between the Quetico Subprovince and the eastern Wabigoon Subprovince of the Superior Province (Fig. 1a). The belt can be subdivided into six east-striking sub-belts, all of greenschist facies metamorphic grade (Fig. 1b; Devaney and Williams, 1989). They are: the northern metasedimentary subbelt (NMB), northern volcanic sub-belt (NVB), central metasedimentary sub-belt (CMB), central volcanic sub-belt (CVB), southern metasedimentary sub-belt (SMB) and southern volcanic sub-belt (SVB) (Fig. 2). The northern units are not as laterally continuous as the southern units. Previous studies have shown that although these belts are fault-bounded (Fig. 3), mainly along the southern boundaries of the volcanic sub-belts, they probably reflect an original sedimentary assemblage deposited on a cratonic margin in environments ranging from alluvial fan-braid plain in the NMB, through fan delta-braid delta in the CMB to a submarine fan/ramp in the SMB (Devaney and Fralick, 1985; Barrett and Fralick, 1985; Devaney, 1987; Barrett and Fralick, 1989). Original continuity of this succession prior to tectonic disruption cannot be proven, but is supported by consistent stratigraphic trends and sedimentary structures that mostly young to the north. Although isoclinal folds are present on varying scales, particularly in the southeast part of the terrane near Geraldton, the overall structure of the belt appears to be initially one of stacked, imbricate, internally northward-younging sheets which have been interpreted as the product of accretionary wedge tectonics (Williams, 1986, 1987; Devaney and Williams, 1989; Williams and Stott, V (,n Hudson I et_w. N LEGEND A Paleozoic Cover Prolerozoie Covet Grenville Provincc — •Abitpbi Churchili Province Supenor Province Uciti — Areas: E'iglish River ?ontic Winnipeg LANADA 1 U.SA -Wabigeon' rnIoAea Metasedimentary Belts Cranitic Areas Volcanic Sedimentary Belts Cneissic Terranes -'-—-Quetico kiii Wawa Figure 1a. Location of the Beardmore-Geraldton greenstone belt, Superior Province. -3- �- Proceedings of the 51st ILSG Annual Meeting - Part 2 15 /a..aaa..N?LTnsl r4 I Garuiue—gncnMone suhpnwincc BanMto,e-CcSiion Deli 8GB Aras: I RSy Ltt di*ICL 2 SticN.id.iwan. 3 Manitounfrj 4 tolhioo thNuicl Tnsuc Am" Beardmore-Geraldton Belt belts 0 parsoihests $1 -i Luio 3M IO knu NY iw :2 -r 01 lnieatd II) PROTEINOZOIC 7/4 —— ARCHEAN Da,d.a-G.nIdtoc Bell Eli couuw - kin C rn Md C a_-it ia Pset.poducloIdrrnne; ItAilch. 3 M.ctcodccxbhutt Met.moiplmaad seethtooe & s1flhit ease mdtvokiSv ivck. MaBe In • 2 BaikThdd. • -_ - Real siguiada. number • moqawI saooe. tgdlnc. polymjdk iIIoma*t ta.2à3 — Road and b#way n-ri PWd TrW IbuuaJIe In Sk Geld pefl I' L::: SmedlNemMtmeIndc.rncTocks: o P18nMIS _____ Sn arc cavok reds ___ ______________ Figure 1b. Regional geology and field trip stop locations modified after Lafrance et al. (2004); see text for sub-belt index. 86aow • + * * • * • * • * • • * * • • * • * * • . * • • * •*.,* + * • * * • * 50N • • • * * + ..:wabi9oonsubProvince:.::4:.:.:3> nCCMB t :/. * * Bea rdmore— Geraldton Belt ., 'I, SVB r?ij::::::::::Quetit •:i• _.r.Tir2_!_._&r / I . 10 krii *.] Granitoids Metnodiriiontary rooks I • Felsic 10 irlterrMdiate rrmtavolcanic rocks :E:E:E.j Quetico Subprovince sedimenlary rocks Mafic metavolcanic rocks Figure 2. Generalized geology of the Beardmore-Geraldton belt; see text for sub-belt index. -4- �Proceedings of the 51st ILSG Annual Meeting - Part 2 minor interflow chert-magnetite iron formation. The northern metavolcanic sub-belt (NVB) is subdivided into the northern Bish Bay assemblage (BBA) and the southern Poplar Point assemblage (PPA) (Hart et al., 2002; Fig. 4). The BBA is composed of eaststriking, mafic pillowed to massive flows and rare tuffs resembling the SVB. The PPA consists of northweststriking, intermediate flows, tuff-breccias and tuffs resembling the CVB, with subordinate mafic massive and pillowed flows. Qu.tIo&Wabgoon SVB OVB NVB B,! p LS Z w Figure 4. Schematic cross-section of the southern half of the eastern Wabigoon Subprovince showing imbrication of fault-bounded metavolcanic (VB’s) and metasedimentary (MB’s) sub-belts (modified after Stott and Davis, 1999); S = Southern; C = Central; N = Northern; PLSZ - Paint Lake Shear Zone; B - Blackwater Fault; P - Princess Lake Fault; W - Standingstone - Watson Lake Fault. 1991). Thus, the steeply dipping sheets probably represent the telescoped version of an original cratonic margin assemblage. South of the Beardmore-Geraldton belt, more highly metamorphosed, turbiditic sediments of the submarine margin and trench-fill are present in the Quetico Subprovince (Williams, 1986; Fralick et al., 1993). A number of rock types intrude the supracrustal rocks of the BGB, including a series of mafic to ultramafic, synvolcanic, intermediate to felsic synvolcanic, mafic post-tectonic intrusive rocks and diabase dikes (Hart et al., 2002). The synvolcanic gabbroic rocks form thin sills, subparallel to the strike of the mafic metavolcanic rocks of the SVB and the BBA. A large intrusive body located in the BBA is a composite intrusion with gabbroic and peridotitic phases in the southern and northern parts of the intrusion, respectively. A series of intermediate to felsic dikes and sills ranging from massive granodiorite to quartz-porphyritic, feldspar- N The presence of the Onaman-Tashota volcanic arc terrane (ca. 2740 Ma; Stott and Davis, 1999) to the immediate north, the imbricate thrust structure of the Quetico metasedimentary rocks to the south (Williams, 1986, 1987), and the style of sedimentary depositional systems in the region strongly suggests that the Beardmore-Geraldton belt represents a forearc assemblage or a cross section of a complete island arc system (see “Depositional Environments”) (Fig. 3). The mostly mafic volcanic rocks which form the basement to the sedimentary rocks are probably slivers of oceanic volcanic terranes which were accreted prior to development of the clastic wedge. Similar depositional and tectonic evolutionary trends in the Great Basin of California have been described by Dickinson and Seely (1979). 0 1 Dorothea Sandra 2 Km BBA NVB CSB PPA CVB Lake Nipigon Eva Summers Eva Lake Hwy 11 Hwy 580 SSB SVB Igneous rocks Mafic metavolcanic rocks of the southern metavolcanic sub-belt (SVB) consist of massive and pillowed flows with minor tuffs, lapilli tuffs, and tuff breccias and interflow chert-magnetite iron formations. The central metavolcanic sub-belt (CVB) consists of intermediate massive and pillowed flows with significant tuffs, lapilli tuffs and tuff breccias, and ELMHIRST -RICKABY Camp 72 Road 8..,dmor.-G.raldton R CN Beardmore w rail ay Mafic Intrusive rocks Ultramafic Intrusive rocks Mafic Metavolcanic rocks Intermediate Metavolcanic rocks H Kitto QUETICO SIBLEY Felsic Metavolcanic rocks Metasedimentary rocks Sibley Group sedimentary rocks Post-tectonic Intrusive rocks Synvolcanic mafic intrusive rocks Synvolcanic felsic intrusive rocks Figure 4.of theGeology ofArea,the Beardmore area (HartNVBet al., General geology Beardmore-Geraldton Kitto, Eva, Summers, Dorothea and Sandra townships. Northern Metavolcanic sub-belt, CVB-Central Metavolcanic sub-belt, SVB-Southern Metavolcanic sub-belt, SSBSouthern Sedimentary sub-belt, CSB-Central Sedimentary sub-belt, PPA-Poplar Point Assemblage (part of the NVB), 2002); BBA Bish Bay Assemblage; PPAPoplar Point BBA-Bish Bay Assemblage (part of NVB). Assemblage. -5- �Proceedings of the 51st ILSG Annual Meeting - Part 2 20 Ti/Zr 15 10 5 0 0 5 10 15 La/Yb Figure 4. Zr/Ti versus La/Yb diagram for the metavolcanic rocks of the Beardmore area. Circles - SVB, open and filled triangles - CVB, squares - NVB. Data from Hart et al. (2002) and Tomlinson (1996). porphyritic and feldspar-quartz-porphyritic phases occur within the metavolcanic rocks of the PPA. These units appear to have been emplaced along the regional foliation, although some bodies are subhorizontal in orientation. A feldspar-porphyritic granodiorite dike intrudes the mafic flows of the SVB and resembles the dikes of the PPA. Late, post-tectonic diorite sills predominantly occur within the metasedimentary and metavolcanic rocks along the contact between the SSB and CVB. Additional intrusions located along the northern and southern contacts of the PPA are generally undeformed diorite sills that have chilled contacts with the metasedimentary rocks. A swarm of narrow, generally north-striking diabase dikes intrude the supracrustal rocks and appear to be predominantly Paleoproterozoic in age. A series of Mesoproterozoic diabase sills of the Nipigon Sill Complex intrude all other supracrustal rocks of the BGB. Geochemical groupings in the metavolcanic rocks span the three belts within the western portion of the BGB. The mafic metavolcanic rocks of the SVB and NVB are predominantly basalt with high Ti/ Zr and low Zr/Y, and negative slopes on mantlenormalized, extended element diagrams characteristic of mid-ocean ridge basalt (Fig. 5; MORB; Tomlinson, 1996; Hart et al., 2002). A subset of basalt samples have a mixture of elemental abundances and ratios characteristic of subduction-related volcanic rocks which led Tomlinson (1996) to suggest that these rocks formed in a back-arc basin, similar to the Lau Basin, where there are volcanic rocks with both arc and/or MORB geochemical characteristics. The intermediate metavolcanic rocks are generally confined to the CVB and PPA of the NVB and are predominantly andesite. The andesites have low Ti/Zr, high Zr/Y, and positive slopes with prominent negative niobium and tantalum anomalies on a mantle-normalized, extended element diagram, that are generally characteristic of calcalkaline andesite formed in an island-arc environment (Fig. 5; Tomlinson, 1996; Hart et al., 2002). Two felsic metavolcanic rocks from the CVB and the PPA were analysed by Hart et al. (2002) , and have low (La/Yb)n ratios with moderately high ytterbium values characteristic of FII felsic metavolcanic rocks (Lesher et al., 1986), indicative of a potential to host volcanogenic massive sulphide mineralization. Both samples have elemental abundances and ratios characteristic of active continental margin and/or the destructive plate margin felsic volcanic rocks (e.g., Wood, 1980). A sample of the flow from the PPA has a U/Pb age from zircons of 2724.9 ± 1.1 Ma and the flow from the CVB has a U/Pb age from zircons of 2724.9±1.2 Ma (Hart et al., 2002). The geochemistry of the intrusive rocks of the western portion of the BGB was characterised by Hart et al. (2002). The mafic intrusive rocks have elemental abundances and ratios that generally reflect the composition of the metavolcanic rocks that they intrude. Unfortunately these similarities also mean that the geochemistry does not aid in differentiating coarse-grained, gabbroictextured flows from subvolcanic, gabbroic intrusions. Both the southern mafic and northern ultramafic phases of the composite intrusion in the BBA have similar tholeiitic geochemical characteristics. A few analyses that are available for the intermediate to felsic dikes and bodies indicate that these units have high Zr/Y and (La/Yb)n ratios characteristic of alkalic rocks, which may be interpreted to be the products of either an early Archean, post-tectonic intrusive event or part of a Proterozoic magmatic event related to the initial rifting during formation of the Nipigon Embayment. A number of post-tectonic, mafic intrusions located along the contact between the SSB and the CVB have Ti/Zr, Zr/Y and (La/Yb)n ratios similar to the andesite of the CVB, and generally characteristic of calc-alkaline rocks. The overlap in the geochemistry of these intrusions and the CVB andesite is interpreted to indicate that these two -6- �Proceedings of the 51st ILSG Annual Meeting - Part 2 A B north. . FlAt The turbiditic association of the SMB can be divided into a clastic association and a chemical association, with a high proportion of oxide-facies, banded iron formation (BIF) layers. In the chemical association, clastic interbeds are generally less than several centimetres thick, and range in grain-size from silt to coarse sand. Upward-thickening and -coarsening trends over several metres are locally present, as at Solomon’s Pillars and the Leitch Mine near Beardmore (Fig. 6; Barrett and Fralick, 1985). Within the overall upward trend, oscillations between silts, sands, and iron formation occur. Depending on the relations between these types of beds, four iron formation lithofacies associations (IFLA) can be defined: magn at! le, •d Iat FLAt interlaminated manstit. asp.,. ilbtL and &ate San 2mCn,, E. IFLAc sadtn; siltstone. Slate and mantit IFLAb :—t iItstoe, slot, and • IFLA-a consists of dominantly magnetite-rich sediment with millimetre- to centimetre-scale, graded or ungraded silt interbeds (Fig. 7). Figure 6. Stratigraphic sections of iron formation bearing units. A) Outcrop section measured at the Leitch Mine (for detailed location see Barrett and Fralick, 1985). B) Outcrop section measured at Solomon’s Pillars (for detailed location see Fralick 1987). From Fralick and Barrett (1995). • IFLA-b comprises centimetre-scale graded to sharply bounded silt beds, either contiguous or separated by millimetre-thick laminae of magnetiterich sediment. rock types formed by similar petrogenetic processes, but the occurrence of intrusions in both metavolcanic rocks and metasedimentary rocks indicates that some of these intrusions are younger. • IFLA-c represents sand-rich composite units up to about 1m thick, generally consisting of thin, stacked, ungraded, laminated sand beds. These composite units consist mainly of mediumto coarse-grained sand. They are separated by intervals of IFLA-b up to 15cm thick. Sedimentary rocks Lithofacies Associations The NMB, the northern (uppermost) third of the CMB and the northernmost portion of the SMB are dominated by a conglomeratic assemblage with minor amounts of sandstone. The clast-supported conglomerates are poorly to moderately sorted, and almost always non-graded with a poorly to moderately sorted sand matrix. Bedding is defined by variation in average or maximum clast size between units, but it is commonly indistinct. Scouring is locally preserved, but most other primary features such as imbrication have been destroyed by deformation. Sandstones interbedded with the conglomerates commonly appear massive, but in some outcrops planar lamination and cross-stratification are present. They form lenses in conglomeratic beds; thin, irregular sheets blanketing conglomeratic beds; wedges abutting conglomeratic beds; and thick units separating conglomerates. Clast types in the conglomerates are almost exclusively igneous, representing a suite of rocks similar to those present in the Onaman-Tashota volcanic terrane to the • IFLA-d consists of framework-supported, polymictic conglomeratic beds up to a few metres thick, interbedded with sandstone and minor iron formation, or interbedded with fairly thick iron formation and thin-bedded sands. Conglomerate contain mainly mafic to felsic volcanic and granitic clasts. Although flattened clasts indicate that IFLA-d outcrops are tectonically thinned, their associations are primary, with the conglomeratic units erosively cutting down into BIF-sandstone packages. Transitions between IFLA-a, -b and -c can be gradual or abrupt. Some silt-sand successions containing iron formation exhibit intervals of thicker and wellgraded clastic beds. They form structured sections up to several metres thick within successions that are otherwise generally disorganized. Clastic units in the lower two thirds of the CMB and the SMB are divisible into three lithofacies associations: a thin-bedded, turbidite-dominated -7- �till HUIIIILiIllP EThJliJIIi p p p 11L Proceedings of the 51st ILSG Annual Meeting - Part 2 Figure 7. Photomicrographs of siltstone associated with laminated, magnetite-rich sediment (IFLA-a). Logs to right of photos outline layer contacts and schematically illustrate grainsize variations. Dots and dashes represent siliciclastic and magnetite-rich components, respectively. Scale bars: A.) 5.0 mm; B.) 2.0 mm; C.) 3.0 mm; D.) 2.0 mm. All images are in crossed nicols. From Barrett and Fralick (1985). association (LA2); a medium-bedded, turbiditedominated association (LA3); and a thick-bedded association (LA4). LA2 consists mostly of graded, <10 cm thick siltstone and/or sandstone beds that are either unstructured or thin and fine upwards over 1 to 3 m. LA3 is divisible into two types. LA3a consists of medium- to coarse-grained sandstones with sharp bottom and top contacts. Parallel lamination is present near the tops of some of the otherwise massive beds. These successions are unstructured. LA3b is similar to LA3a except these beds are organized into either upward-thickening or upward-thinning trends. Thick, poorly graded sandstones dominate LA4. The beds usually have a coarse sand or pebbly base, grading into a thick, poorly sorted, massive central area. They are often abruptly capped by thin, fine-grained sandstone. Irregular, erosional bases and scattered rip-up clasts are common. Structured, thinning- and fining-upward sequences, metres to tens of metres thick, are present in the area south of Beardmore. The successions are topped by CDE and/or DE turbidites which are abruptly overlain by massive grain flows/high-density turbidites with internal inverse- to normal-graded, conglomeratic bands. Pebbles present in the conglomerates are mainly felsic igneous rocks (extrusive and intrusive), while rip-up clasts are not the expected mudstone or siltstone, but rather clay- and silt-rich, fine-grained sandstone. Load structures are ubiquitous throughout the area. Commonly, the base of one unit sags into the underlying beds. Locally, multiple internal loads are developed, usually in the B division. These loads sag into the A division, in places extending into the underlying beds. Depositional Environments Deposition of the clastic and chemical sediments in the Beardmore-Geraldton belt occurred within a narrow time frame. Crosscutting felsic intrusive rocks provide a minimum age for the sediments of 2691+3/-2 Ma (Anglin, 1987; Anglin et al., 1988). UPb detrital zircon geochronology on sandstone samples from the region consistently give youngest zircon ages in the range 2701±1 Ma to 2696±2 Ma (Hart et al., 2002; Don Davis, unpublished data, personal communication, 2004). These ages are very similar to -8- �Proceedings of the 51st ILSG Annual Meeting - Part 2 Subprovince on Lake Superior (Purdon, 1995). In addition to geochronology, lithofacies associations and geochemistry indicate that the depositional system probably extended from the cratonic margin (i.e., Wabigoon Subprovince), across the arc-trench gap and trench (i.e., Quetico Subprovince) to the outboard ocean floor (i.e., Wawa Subprovince) immediately prior to the commencement of collision of an island arc system to the south. The NMB is represented by sandstone and conglomerate deposited in a fluvial fan, and/or braid Figure 8a. 2700 Ma paleogeography of the region surrounding plain environment (Devaney and Fralick, 1985). The Beardmore-Geraldton. The island arc in the south represents the Schreiber-Hemlo area of Wawa Subprovince. The trench CMB displays this type of assemblage in its northern system to the north of this arc is now metaturbidites of third, with successions in the southern portion similar the Quetico Subprovince. The sediments feeding from the to those described above for the SMB. Thus, the southern flank of the Wabigoon cratonic arc in the north paleo-shoreline exposed by the present level of erosion formed the Beardmore-Geraldton succession. From Fralick lies within the CMB. All of the sediments in the SMB and Barrett (1991). are marine, except for rare conglomerates at the top ages of detrital zircons from the Quetico Subprovince of the succession and throughout the section in the and a more distal turbiditic assemblage, the McKellar highly deformed Geraldton area. The turbidites were Harbour Formation, outcropping in the northern Wawa deposited in fore-arc environments ranging from deltafront to submarine fan or ramp. Many ______________________ Onommi—Tas bota VoIcoo!c Terrc!t, unstructured sandstone sequences are -. D L ItbfdIfl Aoc!ot!on interpreted as having accumulated on LIt 24 a submarine ramp that was locally Br WI transected by channels feeding deeper fan lobes (Barrett and Fralick, 1989). / An interpretive view of the overall paleoenvironment is shown in Figures 4c' 8a and b. I r - / rTsTL I 0-- (z) c - H--H— zoooe ond — -— -I(onFormacr, 0 !1 seofloor Sediments deposited in the Quetico trench to the south are similar in many respects to the submarine ramp deposits of the adjacent fore-arc basin, but there are differences. AB and ABC Figure 8b. Suggested depositional environments for sedimentary rocks in the Beardmore-Geraldton belt. The subaqueous portion of this diagram is depicting environments mostly present in the Southern Metasedimentary Belt. The Central Metasedimentary Belt represents mainly fan-delta environments, and the Northern Metasedimentary belt a braidplain environment. The entire sedimentary assemblage in all three belts forms a large-scale progradational system. In the hinterland is the Onaman-Tashota volcanic arc terrain which supplied the detritus. This sediment was transported across braidplains to distributary mouth bars. Because of rapid accumulation rates and the tectonically active environment, gravity flows and slumps were frequently triggered. The resultant grain-flows produced the vertically unstructured, poorly graded, mud-poor, medium- to coarse-grained deposits of the ramp succession. More structured sandstone sections formed on locally developed fan lobes and thick grain-flows filled the submarine feeder channels. Iron formation was deposited immediately offshore from the channel mouths on braid deltas during periods of low sediment influx. The meter-scale coarsening-upwards cycles present in the iron formation record the effects of delta lobe outbuilding. Progradational events may result in the stacking of coarsening-upwards iron formation-bearing, subaqueous parasequences or the delta top braid system may erode through the uppermost chemical sediments during maximum outbuilding. The iron formation is limited to the shore-proximal zone indicating a precipitation model where nutrients delivered to the system by rivers caused heightened cyanobacterial growth rates in the near-shore, similar to modern systems. The oxygen produced by photosynthesis caused precipitation of iron oxides in these oxygen oases. From Barrett and Fralick (1989) -9- �Proceedings of the 51st ILSG Annual Meeting - Part 2 turbidites dominate the trench fill, whereas grainflows and A turbidites characterize the more proximal, fore-arc environment. Iron formation is present in the fore-arc basin associated with thinner-bedded, finer-grained sediments deposited in the shallow near-shore portions of the deltas. Iron formation accumulated when transgressions caused intervals of clastic sediment starvation in the proximal pro-delta and distributary bar environments. Iron formation is absent from sediments deposited further offshore in the Beardmore-Geraldton belt and the Quetico trench deposits. The limited distribution of oxide-facies iron formation (occurring only in shore-proximal areas) indicates that biological activity thrived in the nearshore waters where the commonly limiting nutrients, nitrogen and phosphorous, are delivered at higher levels due to river input. It is difficult to prove that iron compound precipitation was driven by free oxygen liberated during photosynthesis by cyanobacteria, but this would be a logical assumption. Rare successions that coarsen and thicken upward may represent limited establishment of discrete fan lobes in the distal fore-arc basin and Quetico trench CVB *CA CYB CA *•J3 OTT The discussion of the structural development is taken largely from recent work and synthesis conducted by Lafrance et al. (2004). Their treatment of the topic is the most recent structural study, outlining three phases of structural overprinting of the volcano-sedimentary assemblage and ascertaining the timing of each. CA I Sediment supplied to the fore-arc basin and the trench was derived from erosion of felsic and intermediate igneous material, with a large contribution probably coming from resedimentation of unconsolidated volcaniclastic material. Clast compositions indicate that the Onaman-Tashota arc terrane to the north was the source (Devaney, 1987; Devaney and Williams, 1989). Grain size and lithofacies trends also strongly support a source to the north. In addition, whole rock geochemistry conducted on sandstone samples from the Beardmore-Geraldton area and the Quetico Subprovince further confirm that the Onaman-Tashota terrane is the only igneous complex in the area which could have supplied the majority of the detritus (Figs. 9 and 10; Fralick and Kronberg, 1997; Fralick, 2003) The ease with which volcaniclastic material can be physically eroded from subaerially erupted debris could have led to short subaerial residency periods, and thus lowered the extent of chemical weathering and production of clays. This is probably the major factor contributing to the rarity of E division fines in the deposits. Structure * OTT 0 floor, but for the most part the trench fill was dominated by overlapping, interfering fans fed by multiple channels extending from the shallow marine portion of the fore-arc basin. One of these channels outcrops south of Beardmore (Stop 1-2). The channel is filled with thick, upward-thinning and -fining successions that grade from conglomerate and coarse sandstone grainflows or turbidites deposited from high density currents near channel bases, to CDE and DE, lowdensity current turbidites that fill abandoned channels near their tops. 2 3 Ti02/(Nb xl,000) Figure 9. Ratio plot of chemically immobile elements in sandstones showing derivation of the sands deposited in the Beardmore-Geraldton terrain and the Quetico Subprovince from the Onaman-Tashota calc-alkaline volcanic arc. From Fralick and Kronberg (1997); Fralick (2003); igneous data from Tomlinson et al. (1993) and Kresz and Zayachivsky (1989). After deposition of the clastic succession the area was tectonised by thrust faulting, regional folding and dextral shearing. Thrust faulting resulted in the regional volcanic and sedimentary packages being vertically juxtaposed in an imbricated thrust stack (Devaney and Williams, 1989). This represents D1 and may be related to uncommon, early, F1 folds. The youngest detrital - 10 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 * on NVB CVB SVB CA 160 Kresz and Zayachivsky, 1989 Tomlinson et al, 1993 mafic to intermediate volcanic Onaman - Tashota Terranc rocks Northern Volcanic Belt Central Volcanic Belt Southern Volcanic Belt Gale - Alkaline o mafic volcanic rocks A intermediate volcanic rocks • intermediate to felsic sub - volcanic intrusive rocks intermediate to felsic intrusive rocks intermediate to mafic intrusive rocks A • *CVB CA Tholeiitic volcanic CYB CA / —— a— and intrusive rocks SVBh 0 ULTRAMAFIC Onaman - Tashp calc - alkaline volcanic and intrusive rocks p I 20 40 60 p p p 00 80 20 p 40 160 p 80 p 200 hO2! Zr Figure 10. A ratio plot similar to Figure 9 using Zr. From Fralick and Kronberg (1997); Fralick (2003); igneous data from Tomlinson et al. (1993) and Kresz and Zayachivsky (1989). zircon recovered from the sedimentary units is 2696±2 Ma (Hart et al., 2002) and this places a maximum age on the thrusting event. The D2 event is recorded by the development of tight to isoclinal folds and a flattening strain fabric responsible for transposed bedding and flattening of clasts and pillows. A homoclinal, north-younging panel of regional extent developed at this time and represents the sheared-off southern limb of a larger syncline. D2 deformation affects altered and gold-mineralized porphyry dykes in the syn-tectonic Croll Lake stock which U-Pb geochronology indicates are 2691+3/-2 Ma (Anglin, 1987; Anglin et al., 1988). An age of 2699±1 Ma for a Au-mineralized feldspar porphyry dyke at the Hard Rock Mine and identical ages of 2690±1 Ma for two phases of the Croll Lake stock put constraints on the timing of major deformation and hydrothermal activity in the belt (Corfu, 2000). The final event, D3, occurred as regional transpression developed in the compressive framework of the area. Vertical bed orientations developed during D2 did not refold but rather were overprinted by a steeply dipping, regional cleavage. Partitioning of the strain, during east-west dextral shear, between less competent argillites and more competent sandstones and porphyries resulted in cleavage refraction near lithologic contacts. The pervasive cleavage developed in the Paint Lake shear zone at this time shows a progressive rotation towards the orientation of the zone (Lafrance et al., 2004; DeWolfe et al., 2000). This is in contrast with the Barton Bay Lithotectonic Zone (BBLZ) where the S2 fabric was reactivated to accommodate the D3 shear. Some folds were generated during this interval but they tend to be smaller Z-folds, overprinting limbs of regional F2 folds. Shear zones active at this time were dextral with nearly horizontal displacements. To sum up the results of Lafrance et al. (2004): • Thrusting along the southern margin of Wabigoon Subprovince in the Beardmore-Geraldton area between 2696 Ma and 2691 Ma resulted in the largescale assemblage of what was to become the belt structure. • The rocks were then modified by a compressive event (D2), probably between 2692 Ma and 2686 Ma, which steepened the beds to a near-vertical position, caused extensive flattening and formed large-scale fold structures. • D3 is more poorly constrained in time but may be synchronous with 2670 to 2650 Ma peak metamorphic conditions in Quetico Subprovince. It is characterized by the development of dextral shear zones and small-scale Z-folding. Gold mineralisation Overview Gold was first discovered in the Beardmore-Geraldton Greenstone Belt in 1925 (Mason and MacConnell, - 11 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Table 1 - Gold mineralisation in the Beardmore-Geraldton area. From Mason and McConnell (1983). Mine Years of production Ore milled (tons) Gold produced (Oz) Average grade (Oz/ton) Silver produced (Oz) Bankfield 1937-42, 1944-47 231,009 66, 417 0.29 7,590 Brengold 1941, 1949 46 134 2.91 Crooked Green Creek 1980-1984 1,455 471 Hard Rock 1938-1951 1,458,375 269,081 0.18 Jellicoe 1939-1941 10,620 4,238 0.40 145 Leitch 1936-1968 920,745 847, 690 0.92 31,802 9,009 Little Long Lac 1934-54, 1956 1,780,516 605,499 0.34 52,750 MacLeod-Cockshutt 1938-1968 10,337,229 1,475,728 0.14 101,388 Magnet Consolidated 1938-43, 1946-52 359,912 152,089 0.42 16,879 Maloney Sturgeon 1937 1 73 73 16 Maylac 1946-1947 1,518 792 0.52 46 Mosher Long Lac 1962-1966 2,710,657 330,265 0.12 34,604 Northern Empire 1934-41, 1949 425,866 149, 493 0.35 19,803 Orphan (Dikdik) 1934-1935 3,525 2,460 0.70 1,558 Sand River 1937-1942 157,870 50,065 0.32 3,628 Sturgeon River 1936-1942 141,123 73,438 0.51 15. 922 Talmora-Long Lac 1942, 1948 6,634 1,417 0.21 36 Tashota-Nipigon 1935, 1938 51,200 12,356 0.24 14, 527 Theresa 1935-38, 1941-1943, 26,120 4,785 0.18 202 69, 120 0.36 1945, 1950-53, 1955 Tombill 1938-42, 1955 190, 622 Total 4,115,611 1983). The Beardmore-Geraldton greenstone belt has produced approximately 4.1 million ounces of gold and over 300,000 ounces of silver. Most of this production came from two distinct gold camps at Beardmore and Geraldton, respectively. The balance of gold production came from several small mines scattered 30 km northeast of Beardmore and one mine south of Longlac. Table 1 (Mason and McConnell, 1983) is a production summary of all past gold producers. Gold in the field trip area occurs dominantly in quartz-carbonate veins and sulphide replacement zones cutting all the major Neoarchean rock types of the area. The vein systems are most typically hosted by: 1) clastic sedimentary rocks; 2) contacts between clastic sedimentary rocks and albite porphyry; 3) interbedded iron formation and clastic sedimentary rocks (Macdonald, 1984); 4) interflow iron formation and mafic metavolcanic rocks; and 5) post-tectonic mafic intrusive rocks (Hart et al., 2002). Structures Related to Mineralization The fine-grained sedimentary rocks in the southern portion of the area are isoclinally folded with a well- 8,595 318,500 developed, sub-vertical, east-striking cleavage. The fold axes vary in trend and plunge: in the west, the fold axes plunge west at moderate angles (Hart et al., 2002; Lafrance et al., 2004); near Jellicoe fold axes are vertical; and as we move progressively to the east the fold axes return to a westward plunge (Kehlenbeck, 1983). Shear discontinuities along axial planar surfaces also become more prevalent to the east (Kehlenbeck, 1983). Earlier work also outlined a later deformational event. It formed chevron folds with cleavage co-planar to the axial surfaces (Kehlenbeck, 1983; Anglin and Franklin, 1985). The second deformational event of Kehlenbeck (1983) is probably spatially and genetically related to regional, east-trending fault zones and is thus at least partially the same event as D3 of Lafrance et al. (2004). Geraldton Camp Gold mineralization in the belt has resulted from the introduction of hydrothermal fluids in zones of high crustal permeability. Permeability was generated by prolonged, multiple periods of deformation which focused not only fluids, but magmatic activity and intrusions. In the Geraldton camp (Fig. 11), a major zone - 12 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 ______ Vv ..VvV . VV - VVVVVV.vVq Vd •''VVVVv.vvvvvvvvv VVVVVVVVy vvvvvyvv i'i. VYVVVvJvvv,..' V-/VQAflqVI ./1IVJ7\#vvvvvvvvvvvvvvvvvv VVVVVVVVV 4vV.#V,..e' :.VQV.vI;--Ivvvvvv-.v., vvv1vvvyy.g Fvv-v vvvvvvv ..rnv--, vv•,v i-# iv., -VJ,V .--.r-ty,v /vVvv vVYgvvvv ,VJVV vvvv vqyyvy.g. 1'. :.v-IV.dVVvVvvvv.,vv..,vv! V vv.vvNv1vvvvv ivvvvvv . V ,.VVv VV/ VVVVVVVVVV Ivivvvv yVjVy1Vv V' .j•. I ;I.vvV;vVvvvvvvv vvvvvjvv Ra,tonliay --IIv,vvvvvv VVVVVVJ I VVVVv . -- IThH S V — Uthotectonjc Zone V 'I GERALOTQn A. - - :: - .... - . -&/ -. 4 + .. — '.IV . I— - — -,.—--._c-_ .- --———-- - . VVv'- .- — — t + 4 ...jCy. .t+ — .—-———.— --- --— —.0 I - _-—: - — - 2 + ÷ 4 4 4 + 4 + + 4 4 + .V.yvVVVVV 4 3 SMile, 6 It iti Porphyfflic lelsic intrusion Equigranular teisic intrusion Matic intruson A S Clastic rnetp$edln,ents iron tormation Gold Occurrences Major producing mines Fault ILITJ Matic to intermeiaIe metavolcamcs Figure 11. Spatial association of gold deposits to the Barton Bay lithotectonic zone north of the Bankfield-Tombill Shear Zone. Modified after Macdonald (1985). of deformation in which the gold mines are located has been alternatively termed: the Bankfield-Tombill Fault Zone (Pye, 1952; Horwood and Pye, 1955); TombillBankfield Deformation Zone (Lafrance et al., 2004); 2 — and Barton Bay Deformation Zone (Williams and Stott, 1991). In acknowledging not only the deformation within this zone, but also its lithologic variety, we will use the term Barton Bay Lithotectonic Zone (BBLZ) (Fig. 12). N Pt (/ —— N _* FFpa,tw Horwood and Pye (1955) noted that the area adjacent to and north of the BBLZ not only has S-folds related to the regional folding pattern but also has Z-folds. The Zfolds in this area only occur along the north side of and proximal to the BBLZ. This led Pye (1952), Horwood Figure 12. Structure and Stop locations within the Barton Bay Lithotectonic Zone. After Lafrance et al. (2004). 'l*ir1JS - 13 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 -- SFABRIC.. north of, and genetically linked to, the Barton Bay Deformation Zone (BBDZ) (Fig. 13). This zone of deformation varies from 1000 to 3000 m in total width (Lavigne, 1983; in press), while the crush zone of the Bankfield-Tombill Fault proper ranges from meters to hundreds of meters in width. The crush zone has been intensely silicified (Pye, 1952), carbonatized (Anglin and Franklin, 1985) and contains minor amounts of gold (Pye, 1952). - — - BANKEICLO — MAGNET MrNE Malic sIotayoicanlcs Malic Introsives _J Mastive Aikose Felsic Dike H Greywacke ore Figure 13. Fault-related fabric development in the Barton Bay Lithotectonic Zone near Geraldton. Modified after Colvine et al. (1984); from Scott (1985). and Pye (1955) and Buck and Williams (1984) to hypothesize that the second deformational event was related to the development of east-trending faults. This was confirmed by Lafrance et al. (2004) (their D3) and the faulting was attributed to transpression causing dextral shear throughout the region. Most mineralized occurrences in the Geraldton camp lie in a zone of deformation to the immediate Numerous Z-folds on various scales were formed in the deformation zone. Auriferous vein systems in the MacLeod-Cockshutt and Hard Rock mines are hosted by one of the Z-folds. This structure plunges shallowly west (Fig. 14; Horwood and Pye, 1955) and is mimicked by minor parasitic folds in the BIF. The parasitic Z-folds are generally less than 5 m in amplitude (Macdonald, 1982, 1983a) and are commonly cut by quartz-carbonate veins subparallel to axial traces. Displacement zones, also subparallel to axial traces, developed at lithologic contacts on isoclinal fold limbs, also host auriferous veins. Veins parallel to axial planes of folds are relatively straight and non-sinuous whereas those orientated at an angle to axial planes of Z-folds are highly ptygmatically folded. Beardmore Camp Gold mineralization in the Beardmore area is hosted by quartz-carbonate and sulphide-bearing quartz veins located in foliation-parallel, carbonatized, brittle-ductile shear zones in either clastic metasedimentary rocks of the SMB or in mafic metavolcanic rocks, proximal to NORTH ZONE F2 S Figure 14. Block diagram of the North Zone at the MacLeod-Cockshutt and Hard Rock mines drawn by Lafrance et al. (2004) using level mine plans published in Horwood and Pye (1955). Diagram shows the overprinting of a S F2 fold by a Z F3 fold on the north limb of the Hard Rock anticline. Ore pods are shown in black. - 14 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 T-e- '293 Figure 15. Map showing the surface trace of mineralized quartz veins on the Sand River - Leitch properties. After Ferguson (1967a). BIF of the SVB (Hart et al., 2002). Gold occurrences are also hosted by quartz veins in post-tectonic intrusive rocks and disseminated sulphides in carbonatized shears in metavolcanic and metasedimentary rocks associated with intrusive rocks. Regional mapping by Hart et al. (2002) and a composite longitudinal section of the Sand River Mine and Leitch Mine vein system based on that of Ferguson (1967a,b; Fig. 15) indicates that: 1) the Leitch Mine No. 2 vein and the Sand River Mine vein are the same vein; 2) the vein is foliation-parallel and is at a low angle to the regional fold axis at a strike of 240° and a dip of 65°; 3) the vein contains multiple ore shoots that are a series of dilatant zones controlled by the intersection of two regional fabrics such that the ore shoots plunge moderately steeply at about 320° and have good down-plunge continuity. The veins are hosted by the ductile-brittle shear system which is part of the Standingstone Lake fault (Hart et al., 2002; a.k.a. Standingstone Lake – Watson Lake fault of Bruce and Laird, 1937; Mackasey, 1975; Shanks, 1993), the western extension of the Oxaline Lake – Watson Lake fault (e.g., Williams, 1986). This fault trends about 075° northeast, and is topographically expressed by a swampy valley that includes Standingstone Lake and its associated tributaries, north of Highway 580, east of the Leitch Mine. The fault is manifested by the development of numerous small, northeast-striking, carbonatized shear zones. Lafrance et al. (2004) defined the structure hosting this Sand River vein as being parallel to the axial plane of this fold and S3. The Standingstone Fault has been interpreted to be a normal, south-side down fault (Carter, 1987) that has experienced multiphase deformation from dip-slip to dextral strike-slip (Williams, 1987). The Northern Empire Mine is hosted by a series of en échelon and composite quartz veins containing differing amounts of pyrite, arsenopyrite, pyrrhotite and chalcopyrite, hosted by carbonatized, sheared metavolcanic rocks and interflow iron formation of the SVB near the Empire fault and the SVB–SMB contact. The quartz veining occurs in brittle features of the deformed iron formation. The Empire Fault trends approximately 060° northeast, is steeply south dipping (but slightly shallower than the average dip of the overturned metavolcanic flows) and is marked by a thin fault gouge of black graphite bordered by a few feet of sheared rock (Benedict and Titcomb, 1948). Previous interpretations (Benedict and Titcomb, 1948; Shanks, 1993) placed the Empire Fault adjacent to the SVB-SMB contact and Shanks (1993) interpreted kinematic indictors to suggest a south side-up sense of motion. Hart et al. (2002) suggested that there are 2 distinct, subparallel faults: the Empire Fault to the east, and the Princess Lake Fault to the west, splaying off the SVB-SMB boundary. Both of these resulted in the formation of zones of brittle deformation in interflow iron formations in mafic metavolcanic rocks of the SVB that host auriferous quartz stringers containing arsenopyrite, pyrite, pyrrhotite and chalcopyrite (Hart et al., 2002). Co-incidence of structures related to the fault separating the BBA and the PPA northwest of - 15 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 _________ Beardmore, and the location of post-tectonic, mafic intrusion appear to have controlled the emplacement of quartz veins hosting gold, copper and/or molybdenum mineralization (Hart et al., 2002). The Tyson #1 gold occurrence (Mason and White, 1986) and the Tyson #2 molybdenum occurrence (Hart et al., 2002; Mackasey 1975) are hosted by quartz veins in the post-tectonic intrusion. Copper mineralization is hosted by sheared, carbonatized lapilli tuff within the fault (Hart et al., 2002). The fault trends approximately 075° to 080°, is marked by intense carbonatization and weak to moderate ankerite and calcite-flooding, and may have controlled the emplacement of the intrusion. Vein Systems Auriferous veins are typically 2 to 10 cm wide and primarily composed of quartz plus ankerite, dolomite and other carbonate species. Pyrite, arsenopyrite, pyrrhotite, sphalerite and chalcopyrite are locally present. The gold occurs as small blebs and fracture fills within individual quartz and sulphide crystals and in voids between crystals. It has also been observed as fine threads extending along cleavage planes in carbonate minerals (Armstrong, 1943). The auriferous veins often show evidence of brecciation and multiple generations of quartz. The brecciated zones are richer in gold that non-brecciated areas (Bruce, 1935, 1937a,b; Horwood and Pye, 1955). Crack-seal-textured ore veins, which predominate at the Leitch and Sand River mines, are indicative of successive periods of dilation and fluid influx. These veins contained minor and varied amounts of pyrite, arsenopyrite, chalcopyrite and tetrahedrite, scheelite ——— a —- - VEPN — Ii] Li .— 1: LY! JHT•::7 Figure 17. Plan of the North Zone (No. 30 Vein system), 250-foot level, Hard Rock Mine. After Horwood and Pye (1955). with septa of sericitic and carbonatized wall rocks. Alteration Halos Substantial alteration halos surround auriferous veins (Fig. 16). Carbonatization (ankeritic dolomite) dominates in host lithologies with low concentrations of iron-bearing minerals (e.g., clastic sedimentary rocks) and fracture zones associated with both major and minor faults. Siltstones have been found to contain up to 20% CO2 (Anglin and Macdonald, 1984). Carbonatization is ubiquitous in the clastic sedimentary rocks at the MacLeod-Cockshutt and Hard Rock mines. However, 300 m along strike away from this structure, carbonate alteration is rare (Macdonald, 1983a). Siderite supercedes ankeritic dolomite as the major carbonate phase within 5 cm of the vein margin. In addition to carbonatization, pyritization occurs in iron-rich rocks (e.g., BIF) adjacent to veins. These pyritic halos not only replace the magnetite/hematite laminae in BIF but also sandstone and siltstone laminae in the Geraldton area. The pyritization generally extends only centimetres to tens of centimetres away 3JN. RELATER MINERALOGY I CLI ——- IJI UAI'ON CL!AST1 4Hr / HOST BOCLM(N &A Lt Figure 16. Host rock, alteration halo and vein mineralogy for veins in iron formation-siliciclastic assemblages. From Scott (1985). - 16 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 1976; Mason and McConnell, 1983; Scott, 1985). Concentrations of gold may reach in excess of 15,000 ppb. In contrast to this, the surrounding, sulphide-free BIF consistently contains less than 20 ppb gold (Anglin and Macdonald, 1984; Fig. 19). E? TREMC CAR 0 OlAt 2 AT ION Generation of Hydrothermal Fluids and Timing of Mineralization S U LPHI OAflOti HALO Figure 18. Schematic representation of vein-related alteration, Hard Rock Mine (Scott, 1985); carbonatized zone is approximately 1 m wide. from individual veins. However, where densely packed systems of veins exist sulphide bodies meters by tens of meters in size have developed (e.g., MacLeodCockshutt-Hard Rock mines; Horwood and Pye, 1955; Figs. 17 and 18). Concentration ofOold ( ppb S r An alternative model to those outlined above is the genesis of the fluids through metamorphic de-watering. Studies conducted by Horwood and Pye (1955) indicated the fluids which deposited the auriferous veins contained Ag, As, Au, C, Ca, Co, Hg, Mg, Pb, S, Si, Te, W, Zn and possibly Fe and Ti. This assemblage is similar to the elements enriched in and near auriferous veins studied by Phillips and Groves (1983), Kerrich and Hodder (1982) and Kerrich (1981). These authors interpreted the fluids which formed those deposits to have been generated during burial and metamorphic de-watering of a volcanic pile. Cuncent,alio'i of Arsi'iic (ppm) Concentration of Arsenic ( ppm) ft Concentration ofOold ( ppb The vast majority of the gold associated with the vein systems in BIF is contained within sulphides in alteration halos. The gold generally occurs as submicroscopic particles and minute blebs within the pyrite (Pye, 1952; Horwood and Pye, 1955; Mackasey, Past studies have suggested the gold-bearing fluids in the Geraldton camp were derived either from a granodioritic body (Croll Lake Stock) lying to the immediate east of the area (Horwood and Pye, 1955; Macdonald, 1983b) or by metamorphic redistribution of gold contained in the banded iron formation (Boyle, 1976; Mackasey, 1975). Arguments can be made against gold originating in the BIF. Sulphide-free BIF in the area generally contains less than 20 ppb gold (Anglin and Macdonald, 1984). This amount is much lower than the average values of 61 ppb Au for highFe tholeiites and 78 ppb Au for volcanic-associated BIF which were obtained from a volcanic pile lying in Superior Province northwest of the BeardmoreGeraldton greenstone belt (Cowan and Crockett, 1980). Obviously, if volcanic and related chemical sedimentary rocks in the map area have similar gold contents (and sparse data indicate they do) they would provide a more adequate source of gold. In addition, alteration halos around the veins indicate the fluids were in marked disequilibrium, both in composition and Eh, with the BIF signifying that they were derived from an external source. Figure 19. Gold and arsenic distribution associated with veins at MacLeod-Cockshutt (MC) and Solomon’s Pillars (SP). From Scott (1985). The spatial association between the east-striking fracture systems and the gold mineralization has led previous authors to believe that the faults and related minor structures served as passageways along which the fluids moved (Horwood and Pye, 1955; Macdonald, 1984; Anglin and Franklin, 1985). Structural control of - 17 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 vein systems, and by inference, fluid migration paths, is common in the Superior Province (Rigg and Helmstaedt, 1981; Fryer et al., 1979; Stott and Schnieders, 1983; Poulsen, 1983; Durocher and Hugon, 1983; Hodgson, 1983). The fluids were probably generated at depth and utilized the permeability developed in deformed zones to gain access to higher crustal levels. Permeable fault zones provide conduits to the fluids allowing them to move upwards towards lower-pressure/temperature zones. Episodic and protracted development of dilatant zones and shearing is indicated by the open spacefilling nature of the veins and their brecciation. Fluids migrated towards dilation zones enveloping fault zones, driven by a pressure gradient and seismic pumping (Sibson et al., 1975). The same model has been put forward to explain structurally controlled gold veins in the western Wabigoon Subprovince (Poulsen, 1983). The development of abundant carbonate and iron sulphide in the reaction halos around veins indicates the fluid could not have been very acid (Krauskoph, 1967; Boyle, 1969) and was probably near neutral (Phillips and Groves, 1983). The formation of massive amounts of carbonate minerals also denotes a very CO2rich fluid. This conforms to data from other studies which advance H2O- CO2-dominated fluids as of major importance in the formation of Archean lode gold deposits (Kerrich, 1981; Kerrich and Hodder, 1982). Fluid inclusion studies of the MacLeod-Cockshutt deposit also indicated a CO2-rich fluid at a temperature in excess of 380°C was involved in forming at least some of the veins (e.g., Macdonald, pers. comm., 1985). Recent research has suggested that major gold deposits may be the result of prolonged mixing of two fluids: a hot, reducing, orogenic fluid that migrates along major regional fault structures; and a colder, oxidized, goldand base metal-enriched, magmatic fluid associated with late, mantle-derived plutons (c.f. Neumayr et al., 2004; Rogers et al., 2004). Gold mineralization in the Beardmore area has not been studied to the same degree as in the Geraldton area, but much of the preceding discussion is probably applicable there as well. However, there are a number of points specific to the Beardmore area. An intermediate to felsic intrusion comparable to the Croll Lake Stock has not been identified. There are a number of posttectonic mafic intrusions, and one of these intrusions along the faulted BBA-PPA boundary hosts gold and molybdenite mineralization (Tyson occurrences) and there is copper mineralization in the adjacent fault (Hart et al., 2002). Mafic intrusions of comparable composition are located to the west of the Sand River Mine, south of the interpreted trace of the Standingstone Lake Fault, but intrusions are not spatially associated with mineralization in either the Leitch or Sand River mines. The Standingstone Lake Fault may be part of the fault system separating the SMB and CVB as it, in part, parallels this contact, but it also is the approximate location of the reversal in top directions in the metasedimentary rocks of the SMB (Hart et al., 2002). Axinite, a borosilicate similar to tourmaline, is located in brittle structures in mafic metavolcanic rocks along strike with the Leitch – Sand River vein to the east, and along the BBA-PPA contact to the north (Hart et al., 2002). Although axinite is not associated with gold mineralization, as in the Chibougamau (Dubé and Guha, 1993) and Cadillac areas (Bardoux et al., 1990), it may be a late-stage mineral associated with an intrusion that may have supplied hydrothermal fluids during the gold-mineralizing event. However, the axinite may be unrelated to the gold mineralizing event and reflecting an unexposed pegmatite or the intrusion of the diabase sill (e.g., Ozaki, 1972). Gold mineralization at the Leitch and MacLeodCockshutt mines, the two largest, past-producing gold mines in the Beardmore–Geraldton Belt, is associated with D3 brittle shear zones and folds, overprinting regional F2 folds (e.g., Lafrance et al., 2004). The plunge of the ore zones is parallel to F3 fold axes and to the intersection of D3 shear zones with F2 and F3 folds. At the deposit scale, the plunge of the ore zones is similar in orientation to the plunge of F3 axes (e.g., North Zone, MacLeod-Cockshutt Mine) and to the intersection of D3 shear zones with F3 folds (e.g., Leitch Mine; Hart et al., 2002; Lafrance et al., 2004) and F2 folds (e.g., F Zone, MacLeod-Cockshutt Mine; Horwood and Pye, 1955). Lafrance et al. (2004) provided an estimate for the age of syn-D3 gold mineralization ranging from 2686 Ma (i.e., the age of the pre-D3 Ottertail granite in the Rainy Lake district) to 2640 Ma (i.e. the minimum age for the formation of D3 structures and high-temperature metamorphism in the Quetico subprovince; Zaleski et al., 1999). On a subprovince scale, regional folds that are cut by D3 dextral shear zones are promising targets for discovering the next generation of large gold deposits (ibid). - 18 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Stops Geraldton gold camp Junction Hwys 11 & 584 Preface 2-1 MacLeod - Cockshutt headframe The Beardmore-Geraldton area has been the subject of numerous field trips and published and unpublished field trip guide books, including those by Mason et al. (1985), Fralick and Barrett (1991) and Williams and Stott (1991). Bear in mind that when visiting active exploration or mine properties, permission must be granted by the property owner. Current ownership information can be obtained from the Resident Geologist’s Office in Thunder Bay. Please exercise caution along highway, road right-of-ways and lake shores. 2-2 Porphyry Hill km Beardmore - South Intersection of Hwy. 11 & CNR (Beardmore) 0 1-3 Blackwater fault/pillowed volcanic rocks 0.0 2-3 Glory Hole BIF Field Trip Road Log Stop Locality 0.0 1.3 Junction Hwys 11 & 584 0.0 Ashmore/Errington boundary 0.5 Mosher Mine turn-off 1.3 2-4 Conglomerate 1.3 2-5 Gabbro 1.6 Gravel Pit turn-off 2.3 Powerline crossing 3.8 Wintering Road turn-off 4.7 Powerline crossing 5.9 McClellan Strip 6.4 Magnet Mine turn-off 8.5 Bankfield Mine turn-off 10.6 Key Lake/Malouf Vein 11.8 Wild Goose Lake Park 27.0 1-2 Quetico metasedimentary rocks 5.7 Jellicoe-Kinghorn Road 1-1 Sibley-Quetico unconformity 13.6 2-8 Southern Sedimentary belt sandstone Jellicoe Post Office Beardmore - West 2-9 Pillowed mafic flows 39.6 47 47.2 Junction of Hwy. 11 & 580 0 East Leitch turn-off 5.8 Junction Hwy 11 & Kinghorn Road 0 Longlac Superior strips 3.7 Ara Lake sign 1.7 72 Road turn-off 5.3 CNR tracks 2.9 7.1 Sturgeon River Bridge 5.3 Sand River Mine turn-off 7.4 6 km Road turn-off 6.0 1-8 Sand River 16 Vein Zone 8.3 2-6 Missing Link extension 7.2 1-7 Leitch BIF 8.4 Missing Link turn-off 7.7 1-6 Eva Creek: conglomerate/BIF 9.3 Missing Link parking spot +0.4 1-5 Eva Creek: pyroclastic rocks 9.6 2-7 Missing Link (NE corner) +0.2 1-9 Leitch mine turn-off Million Dollar Road turn-off 10.4 High Hill Harbour turn-off 12.0 1-4 Poplar Lodge pillowed flows, G & G Goodman driveway (access to lakeshore) 14.0 Stop 1-1 - Archean – Proterozoic Unconformity UTM coordinates - 0423431E 5483881N The angular unconformity between east-trending, steeply dipping Neoarchean Quetico metasedimentary rocks and flat-lying Mesoproterozoic Sibley Group - 19 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Figure 21. Stratigraphic column of Quetico metaturbidites, Stop 1-2 (Fralick et al., 1992) t..& Figure 20. Angular unconformity between steeply dipping Quetico metaturbidites and overlying Sibley Group sandstone-carbonate units, Highway 11 (Stop 1-1). sedimentary rocks is exposed on both sides of Highway 11. The phyllitic Quetico rocks display layer-parallel slickensides (bedding-parallel fault?). This outcrop consists of metasedimentary rocks unconformably overlain by a thin layer of Sibley Group sandstone (Fig. 20). The underlying rocks were folded to a near vertical position during the Kenoran Orogeny, prior to Mesoproterozoic deposition of the Sibley sedimentary units. The Sibley here consists of an upper, 30 cm thick, orthoquartzite separated from the Archean basement by 0 to 30 cm of silicified carbonate. The carbonate occurs in fractures in the substrate up to 60 cm below the paleosurface. This gives the distinct impression that the carbonate is a well-developed, caliche zone. Caliche soils form in semi-arid settings where the evaporation rate exceeds the precipitation rate. Irregular laminations in this layer are up-buckled in places, forming domical structures, probably the result of expansion during crystallization. It is difficult to say much about the overlying orthoquartzite. It is not even known if it is wind- or water-lain. Similar, thin sandstone layers occur both to the east and west of Lake Nipigon, separating basement from Mesoproterozoic (Keweenwan) diabase sills. They are correlated with the Sibley simply on the basis of their probable Mesoproterozoic age. Stop 1-2 - Quetico Metasedimentary Rocks UTM coordinates - 0428530E 5489570N Most of the Quetico consists of an unstructured submarine ramp system (i.e., an environment where large, long-lasting channels do not exist and sediment delivery routes switch rapidly, producing a chaotic structuring to the deposits). This outcrop provides one of the few examples of structured channel outbuilding in this ramp setting. A thinningand fining-upward succession of Bouma ADE, CDE and DE turbidites is sharply overlain by a thick-bedded unit of ABCD turbidites with abundant clay rip-ups and extra-basinal pebbles (Fig. 21). The lower, 8 m-thick, fine-grained assemblage is the upper portion of a subaqueous channel feeding sediment from the Beardmore-Geraldton forearc basin to the Quetico trench. It is Orn erosively overlain by the basal section of the next channel complex. The phyllitic rocks that occur at the northern margin of the Quetico Subprovince south of Beardmore are strongly sheared, exhibiting rust-spot elongation, quartz fibre lineation, and crinkles, some of which can be attributed to non-parallel,cleavage-bedding and cleavage-vein intersections (Williams and Stott, 1991). None of these intersections appear to have reliable or consistent tectonic significance. Stop 1-3 - Blackwater Fault / Pillowed Metavolcanic Rocks UTM coordinates - 0430174E 5493162N Deformed, pillowed mafic metavolcanic rocks of the Southern Volcanic Sub-belt are exposed along the sides of Highway 11 just south of Beardmore. They form the southern margin of the Beardmore-Geraldton greenstone belt and the eastern Wabigoon Subprovince in this area, where it is in fault-bounded contact with Quetico metasedimentary rocks to the south. The Blackwater River Fault (BRF) is a regional-scale structure which separates the two subprovinces. It is likely a dextral, transcurrent fault, similar to other subbelt-bounding structures in the Beardmore-Geraldton belt. It has been traced for over 100 km and underlies the valley occupied by Camproad Creek, a few hundred - 20 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 intermediate to mafic metavolcanic rocks of the Central Volcanic Sub-belt are exposed along the shoreline of Lake Nipigon, just north of Poplar Lodge. They have been described by numerous workers, including Coleman (1907), Laird (1937), O’Brien (1985), Tomlinson (1996) and Hart et al. (2002). Figure 22. Deformed pillow basalt near Blackwater Fault, Highway 11 (Stop #1-3). metres south of this field trip stop. Immediately north of the BRF, Shanks (1993) noted a series of sub-vertical, non-penetrative planar surfaces, interpreted to be either slip surfaces or C-planes. Estimation of the finite strain ellipsoid shape by Williams and Stott (1991) using pillow shapes and selvage thickness indicated plane strain. Localized, small-scale c-s fabrics were noted by Williams and Stott (1991) in schistose selvages, indicating a north over south sense of displacement. Light grey-green weathering basalt flows are foliated at approximately 240°/80° north. They are locally phyllitic, friable and gossanous. Despite flattening and aspect ratios of 3:1 to 10:1, pillows give northward top directions. Pillows range in size from 0.5 to 1.0 m and have recessively weathered selvages (Fig. 22). They contain flattened vesicles near their rims and are locally variolitic. Isolated patches of autoclastic(?) breccia contains 0.10 to 0.25 m-sized, lenticular fragments. The flows are more massive to the south and are crosscut by rusty, “bull-white” quartz veins. Joint surfaces are coated with quartz and calcite. A rusty quartz vein occupying the sheared contact between basalt and sandstone to the north was sampled but returned no gold nor silver. A 3m wide diabase dyke trends ~155° and cuts the supracrustal rocks. Massive, vesicular/amygdaloidal and/or pillowed basalt/andesite, heterolithic breccias, as well as porphyritic and flow-banded units were described by O’Brien (1985). Individual flows in the CVB are commonly a few metres thick, with a few examples that are tens of metres thick with pillows that are generally weakly deformed to undeformed. In this outcrop, the pillows may be greater than 1 m in diameter (Fig. 23). Pillow selvages are in these flows are generally thicker than in the mafic metavolcanic rocks. Internal features include amygdaloidal and vesicular flow laminae textures and feldspar phenocrysts, with some pillows in flows to the south containing concentric flow laminae defined by the alignment of vesicles approximately 1 mm in diameter. Pillow breccias occur as a transition from the pillowed flows to the hyaloclastite-rich, pillow fragment breccias, and are exposed on the western end of the outcrop as it dips into the lake. The pillow breccia portion of the units is generally 1 m wide, with the pillows very abruptly diminishing in size away from the main body of the flow. A good example of this type of breccia is exposed in the bay to the south, ~150m from this location, but is only accessible when the water levels are low. Pillow fragment breccia forms the major part of the unit, and is generally a few metres in width adjacent to the flow front. This transition is visible near the waterline. The breccia thickens to the Srop 1-4 - Poplar Lodge Pillowed Metavolcanic Rocks Private property access to lake shore; please ask permission from cottage owners UTM coordinates - 0421034E 5499814N (northern end of section), 0421089E 5499665N (southern end of section) Weakly deformed, northward-younging, pillowed Figure 23. Pillow basalt, Poplar Lodge, Lake Nipigon (Stop 1-4). - 21 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 west, which may suggest that the direction of flow was towards the west. Fragments generally consist of sub-angular parts of pillows and rounded fragments or amoeboid fragments and small pillows in hyaloclastite matrix. Although the pillow breccias have been classified as volcaniclastic rocks, a portion of the units could be classified as hyaloclastite or rocks that are the result of fragmentation of the magma due to interaction with water (Cas and Wright, 1987; Batiza and White, 2000). There is no depth restriction to the formation of the pillow breccias, as the breccias have been observed in both shallow water and on seafloor sites. Heterolithic breccias consist of predominantly 0.5 to 10 cm sub-angular to angular clasts of intermediate metavolcanic rocks, but minor clasts of feldsparporphyritic, felsic metavolcanic rocks, cherts and mafic metavolcanic rocks overlie pillow breccias in a few locations along the lake shore. Generally, the breccias lack any internal structure. Although some areas appear to have bedding structures, they generally cannot be traced for any distance. The matrix is generally highly chloritized and some of the smaller, more porous clasts may have been totally chloritized; reaction rims along clasts margins may be developed Major element and trace element analysis indicated that the pillowed flows are tholeiitic basalt and andesite (O’Brien, 1985), characteristic of mid-ocean ridge basalt (Tomlinson, 1996; Hart et al., 2002). Stop 1-5 - Eva Creek Pyroclastic and Sedimentary Rocks UTM coordinates - 0424037E 5498780N Dacitic pyroclastic rocks along the southern margin of the CVB are exposed on a small ridge 50 m east of Highway 580 (Fig. 28). A massive, aphyric, monomictic tuff-breccia has an iron-carbonatized matrix and is cut by epidote veins (Fig. 24). It is foliated at 065°/78° south and hosts narrow shear zones with crenulated fabrics and quartz veins. There is a suggestion of flattened pillows at the northern edge of the outcrop. A north-trending diabase dyke cuts the supracrustal rocks. An outcrop on the west side of Highway 580, across from Stop #1-5, exposes typical, medium-bedded turbidites of the SMB. These consist predomintly of A, B, D and very minor E divisions. The minor E division reflects very little clay in the sediment transport system, a logical consequence of sediment feed being mostly derived from active subareal, pyroclastic volcanism to the north in the Onaman-Tashota terrain (Fralick, 2003). The interval of time between eruption, erosion and sedimentation was not sufficient to produce significant amounts of chemical weathering (i.e., clay). The relative lack of C (rippled) divisions is more difficult to account for. One possibility is that higher ocean temperatures resulted in a thinner, laminar sublayer which fine sand was capable of disrupting and, since ripples need a laminar sublayer to form, the C division was suppressed. The non-regular bed thickness variation up through this section reflects rapid changes of the position of sediment feed areas into the subaqueous portions of the basin, resulting in a somewhat chaotic interlayering of thin and thick turbidites. Stop 1-6 - Eva Creek Conglomerate / Banded Iron Formation •L. $ t , , UTM coordinates - 0423925E 5498505N Figure 24. Deformed felsic pyroclastic breccia, Highway 580 (Stop 1-5). Exploration of the local iron ranges began in earnest around 1900 when several claims in what was to become the Sand River and Leitch mines area were staked and surveyed (Laird, 1937). In 1919, the Lake Superior Ore Company was formed to option and drill a portion of the Central range. At this time, the Kokoko camps on claim A.L. 413 were built. Drilling carried on until 1920. Coleman (1907) reported that drilling returned assays in the 40 to 50% Fe range, with negligible amounts of deleterious sulphur and phosphorus. Laird (1937) characterized the iron deposits of the Central range: - 22 - “The beds have been crumpled and intricately folded, �-2. ___2 FIuviaI _________ a Proceedings of the 51st ILSG Annual Meeting - Part 2 ______ •Ly€€d. 40 Dominated .—'--.. .1 .! b SnhiIar. orMagnelite 3 -r and Sirisiclasb. ThinLayered. LI- SiIisidasbc . I 2 0 with Thin MagnetLte t.aminae 20 / No fl(ilrCp Sandstone Lense In Conglomerate 1J1/ IT Trcijh Crot Stfat,flcabnn 0m — Flooding r — Surface Figure 25. Stratigraphy, Eva Creek section (Stop 1-6). vi . ab IF. Im on tSandstV with the result that they have the appearance of great thickness in places. The formation is well-banded and consists of thin layers of red jasper alternating with steelgrey hematite, the whole being irregularly interbedded with dark slaty bands. The jasper-hematite bands are not commonly more than a few feet in width and in places they occur as mere streaks in the slaty material. Magnetite is not a prominent constituent of this band, but in places it is present in quantities sufficient to cause considerable magnetic disturbance.” This is one of the best examples of the stratigraphic position of BIF in the belt. The lower portion of the section consists of interlayered magnetite, siltstone r • • • Figure 26. Conglomerate associated with BIF, Eva Creek (Stop 1-6). and sandstone in various ratios from chemical sediment-dominated to clastic-dominated (Fig. 25). This is erosively truncated by a conglomerate. The clast-supported, polymictic conglomerate (Fig. 26) contains diffuse zones of larger and smaller clasts, lenses of medium-grained sandstone and crescent scours and sand shadows along internal surfaces where sandstone lenses are concentrated. This is overlain by a thick, medium-grained sandstone, which is, in turn, succeeded by a clastic-BIF succession. A coarsegrained sandstone-conglomerate assemblage erosively cuts down into this package, removing the upper metre in places. This outcrop highlights the juxtaposition of BIF and erosively based, fluvial channels. The fluvial system was periodically extending into a nearshore dominated by thin, graded siltstones and sandstones with iron oxide laminae. The depositional architecture is composed of a series of prograding parasequences in the littoral zone with BIF developed on flooding surfaces. Commonly iron-rich BIF directly overlies the flooding surface and is upwardly transitional into more clastic-dominated units. These may be erosively incised by the next channel assemblage. Off-delta, deeper-water turbidite units do not contain BIF. This strongly indicates that BIF deposition was the result of photosynthetic oxidation in the nearshore, producing an oxygenated oasis near nutrient-rich waters emanating from river mouths. - 23 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Stop 1-8 - Sand River Mines Property - #16 Vein system UTM coordinates - 0424339E 5497563N The following history has been modified from a summary reports prepared by Bevan (2004) for Roxmark Mines Limited (who are the current property owners) and Mason and White (1986). Laird (1937) provided the first geological description of the property and its exploration activity. History of the Sand River and Leitch Mines Figure 27. Deformed BIF, Highway 580 (Stop 1-7). Stop 1-7 - Leitch Banded Iron Formation UTM coordinates 0424600E 5497867N This is an excellent exposure of oxide-facies BIF interlayered with siliciclastic layers. Unlike many BIF’S in this region, hematite, not magnetite, dominates this exposure. At first glance the rocks give the impression that there is intense shearing along the cleavage direction disrupting the layering (Fig. 27). Closer examination may lead to a different conclusion. The sharp-sided, clay-poor, medium-grained sandstones form lenses (averaging 1 cm thick), cutting saucershaped scours into the 1 mm-thick, stacked iron oxide laminae. That is, the sandstones are lenticular to flaserbedded. Some lenses appear to be ripple-laminated, although the cleavage cutting through at a similar angle makes identification problematic. This type of siliciclastic depositional system is typical of distal portions of distributary mouth bars in deltaic settings. The interlayering of iron formation indicates relatively rapid precipitation of the chemical sediments in a nearshore environment. The first discovery of gold in the Standingstone River area west of Beardmore was made on what became the Sand River Mine property in June, 1934 by Russell Cryderman (Laird, 1937). The Discovery or Number 1 vein averaged only 13 inches in width, but was remarkably persistent along strike, being exposed in cross-trenches and surface pits over a length of 2100 feet. The vein was traced to the northeast onto the Leitch Mine in 1935 (Fig. 28). Stripping and trenching were carried out by the newly incorporated Sand River Gold Mining Company on the Number 1 vein. Servicing was by two three-compartment shafts, 2950 feet apart in a northeasterly direction: the westerly Sand River shaft to 2656 feet and the Leitch main shaft to 3006 feet, with a winze from the 19th (2875-foot) level to the 30th (4525-foot) level. A road connecting the two properties with Beardmore was constructed in 1936. Production started with milling facilities at 75 tons per day at both properties in 1937. Between 1937 and 1942, Sand River processed 157,870 tons at a recovery grade of 0.32 ounce gold per ton, yielding 50,065 ounces gold, to their 9th (1150-foot) level. Mining was by conventional shrinkage stoping with an approximate width of 3.0 feet. The Sand River operation ceased in 1942. Leitch Mines processed 80 tons per day, milling 176,535 tons at a recovery grade of 0.71 ounce gold per ton in the same period with a high-grade resuing approach. Northern Empire Mines Limited, a subsidiary of Newmont Mining Corp., began milling operations in Beardmore in 1934 with a maximum of 186 tons per day in 1939 and ceased in 1941. They then acquired the Sand River property on a ten-year lease. They drifted out on two of the three un-mined Sand River levels above the diabase sill. Sufficient encouragement led them to collar a winze on the Sand River 11th or 1450-foot level, designed to sink through the diabase. - 24 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 The winze was stopped at 1506 feet when war broke out in Europe. Northern Empire resumed operations at Sand River in 1944 as the Undersill Gold Mining Company. Undersill de-watered and deepened the Sand River shaft through the flat-lying, 600-foot thick diabase sill to 2180 feet. The shaft was bottomed at 2656 feet. They explored the 17th or 2610-foot level with a shaft crosscut of approximately 450 feet and 2340 feet of drifting, of which 1300 feet was eastward to the Leitch boundary, and 1040 feet was westward. The 1947 Leitch Gold Mines Limited Annual Report stated that Northern Empire’s Undersill Gold Mining Company had encountered “good ore” on the Sand River 17th (2610-foot) level, an extension of the highgrade Leitch Number 2 vein. Northern Empire, unable to capitalize on its 1944 to 1947 development with gold at US$35 per ounce, closed down the Undersill Gold Mining operation in 1954 and concentrated development at its Magnet Mine, near Geraldton. Leitch acquired the Sand River property in the same year to protect the depth extension of their steeply westplunging Number 2 vein. Leitch developed the Sand River property by an internal winze at the Leitch Mine from their 19th (2875-foot) level to the 30th (4525foot) level. Leitch’s production from the Sand River - Undersill Gold Mining Company’s holdings totalled 376 283 tons at a recovery grade of 1.11 ounces gold per ton from 1955 to mine closure in 1965. This was an appreciable improvement of grade with depth from 530 112 tons at 0.79 ounce gold per ton from 1938 to 1954 above the 19th level. Environmental requirements were completed on the Leitch Mine and the Sand River property was returned to the original owners in 1968. In 1940, exploration by Halport Gold Mines outlined a parallel vein system, named the #16 vein system. The following table covers the history of this discovery. Historical Summary - #16 Vein System 1940 - Halport Gold Mines located a number of surface veins on Claim AL 415 at the eastern end to the boundary with Sand River Claim TB 12944. 1944 - Leitch acquired the Halport holdings. 1945 - 22 diamond drill holes totalling 16 510 feet were drilled. 1950 - A long crosscut on the 8th level was initiated because surface drilling had indicated ore at this horizon. 1951 - Further crosscutting and some drilling was carried out. 1952 - Further crosscutting and drifting to open up the #16 vein. Drilling of 51 underground holes totalling 11,116 feet, mainly on the 8th level. 1953 - Further drilling and stoping on the #16 vein – production and milling of 3,598 tons grading 0.56 oz. Au/ton. 1969 - Teck Corporation (together with HighlandBell Limited and F.E. Hall) purchased control of the Leitch Gold Mines property. 1971 - The entire Leitch assets were acquired by Teck Corporation. 1946 - Teck Corporation carried out exploration work until 1981 over the Leitch property, including line cutting, VLF-EM magnetometer surveys, 393 holes of overburden drill sampling and 29 short diamond drill holes totalling 3,454 feet and covering a strike length of 1,400 feet. One hole was abandoned. 1987 - Sand River-Cryderman diamond drilling program. 10 holes totalling 2,030 feet were drilled on Claim TB 12944 covering the #16 vein to the east. 1988 - Teck Corporation drilling program in joint venture with San Paulo Explorations Inc. 24 diamond drill holes totalling 22,098 feet, of which three were abandoned, one hit the open stope and one intersected the vertical diabase dyke and was terminated. Airborne and ground geophysical surveys and reverse circulation drilling were also undertaken until 1989. 2003 - Acquisition of Sand River property by Roxmark Mines Limited; Advanced Exploration project initiated. In 2003, Roxmark Mines Limited acquired the Sand River, Leitch and East Leitch properties from Rio Fortuna, Teck Cominco and Kinross, respectively (News Release, Roxmark Mines Limited, November 28, 2003) and began an Advanced Exploration program. In 2004, two vein systems were exposed by trenching, stripping, channel sampling and stockpiling at the East Leitch and Sand River #16 zones, respectively. Stripping of these areas had been recommended in preparation for taking bulk samples (ibid). Geological mapping and sampling was planned to gain a better understanding of the mode of occurrence and identify other possible veins and extensions in the #16 vein system. Further exploration in the form of diamond drilling in five areas was recommended as follow- - 25 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Sand River - Lelich Mines (altar Ferguson (1967a) 0 results confirm the historical sampling data completed by Teck Corporation. Furthermore, the current assay results confirm the historical drill holes that intersected the #16-1 Vein. Recent Roxmark assays of the #16-5 West vein were 1.058 ounces Au per ton over a 26.5 foot strike length. The #16-5 East Vein returned 1.094 ounces Au per ton over a strike length of 35.0 feet. I loOm Geology of the #16 Vein Area Figure 28. Geology of the Sand River-Leitch mines area after Ferguson (1967a), showing Stop locations. up work in preparation for underground exploration and development. Bulk samples will be processed at Roxmark’s 200 ton-per-day Northern Empire Mill in Beardmore. Roxmark reported the results of samples taken from the #16-vein system on the Sand River Mine property (ibid, December 16, 2004). The Company is continuing with its plan for the surface bulk sample program and in particular, the in-fill sampling on the #16-1 and #16-5 veins. Channel samples were cut every three feet. All composite assays used a minimum width of 1.0 feet. The #16-1 West Vein returned an average assay of 1.737 ounces Au per ton over a strike length of 42 feet. The #16-1 East Vein returned 0.347 ounce Au per ton over 37 feet. The recent assay 16 Vein Zone Sand River Mine The Leitch-Sand River area (Figs. 15 and 28), most recently mapped by Hart et al. (2002), is predominantly underlain by clastic metasedimentary rocks of the SMB. Thickly bedded, southward-younging, overturned, feldspathic sandstone and siltstone have been wellexposed by recent stripping operations in the vicinity of the #16 North zone (Fig. 29). Bedding has eastnortheasterly to east-southeasterly orientations with steep, northerly dips. Sub-parallel shear zones are distinguished by well-developed, slaty S3 cleavage, as well as local sericitization and carbonatization. Oblique cleavage – bedding relationships are locally exposed. Cleavage is refracted through interbedded siltstone and sandstone beds (Fig. 30). A number of auriferous quartz veins have been exposed in the immediate area (Figs. 28 & 29). Visible gold occurs within fractures in quartz veins and most commonly is found along thin seams or septa of sericite and chlorite, along with arsenopyrite, pyrite and ankerite in composite, “crack-seal”-textured veins. Tetrahedrite and sphalerite are also reported to occur in these fractures. Pink-beige scheelite is relatively abundant. :i The Leitch Mine No. 2 vein and the Sand River Mine vein are the same vein striking southwest parallel to a N Figure 29. Map of stripping and veins, Sand River 16 Vein zone (Stop 1-8). Figure 30. Refraction of cleavage in sandstone/siltsone, Sand River 16 Vein zone (Stop 1-8). - 26 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 subvertical to vertical tectonic fabric which is subparallel to the original bedding in the clastic metasedimentary rocks (Hart et al., 2002). The vein contains multiple oreshoots that plunge at about 320° in the plane of the 65° northwest dipping vein, and these shoots are controlled by the intersection of the first fabric with a second regional tectonic fabric. Lafrance et al. (2004) defined the strike of the vein as 240° and dip 65°, with plunges to the oreshoots being consistent with the F3 fold axes, parallel to the Standingstone - Watson Lake Fault which was likely developed in two parallel, D3, dextral shear zones. Ore zone plunges are consistent with F3 fold axes. Hart et al. (2002) identified that the potential for additional gold mineralization would be very high in structures subparallel to parallel to the Sand River Mine – Leitch Mine No. 2 vein, such as the No.16 veins that are exposed north of the Sand River Mine shaft. Stop 1-9 - Leitch Mine UTM coordinates - 0425302E 5497484N This stop allows trip participants to scour Leitch Mine dump piles for representative samples of goldbearing quartz veins and clastic sedimentary rocks. Visible gold occurs within fractures in quartz veins and most commonly is found along thin seams or septa of sericite and chlorite, along with arsenopyrite, pyrite and ankerite in composite, “crack-seal”-textured veins. Tetrahedrite and sphalerite are also reported to occur in these fractures. Pink-beige scheelite is relatively abundant. Leitch Mine operated continuously from 1937 to 1965. Mill clean-up ensued between 1966 and 1968. One of the highest-grade gold mines in Canada during its lifetime, the mine yielded 861,981 ounces of fl• gold from 1,022,360 milled tons at a recovered grade of 0.915 ounce gold per ton. 31,802 ounces of silver were also produced at a grade of 0.035 ounce silver per ton. 64.3 tons of tungsten ore were produced at a average grade of 3.95% WO3. Figure 31 gives us a glimpse of the property at the time of its discovery. Day Two - Geraldton Area Past-producing gold mines in the Geraldton camp (~ 3.1 million ounces) are confined to the northern part of the SMB and are localized by the Barton Bay Deformation Zone (BBDZ). The BBDZ (Fig. 12) is a 1 km wide, high-strain zone that extends from the Hard Rock and MacLeod-Cockshutt mines west along Highway 11 to the Bankfield Mine (Pye, 1952; Lavigne, in press; Lafrance et al., 2004). Shear is distributed heterogeneously across the BBDZ. In contrast to the mafic metavolcanic/mafic intrusive rocks to the north, and the monotonous sequence of sandstone to the south, the rocks within the BBDZ (mafic to intermediate intrusive rocks, felsic porphyry dykes and sills, sandstone/pelite, conglomerate and BIF) are complexly intercalated and have high competency contrasts. As noted by Lavigne (in press), only a very few areas in the belt have such lithologic heterogeneity. All of the aforementioned rocks in the BBDZ, except for the conglomerates, host gold. Although Geraldton is often considered to be a BIF-hosted gold camp, it is important to note that less than 30% of the gold came from BIF-hosted ore (Macdonald, 1982). Most of the production came from clastic sedimentary rocks, particularly at or near the contact with feldspar porphyry intrusions. Anglin and Franklin (1985) cited a more consistent spatial relationship between gold mineralization, deformation zones and felsic intrusive rocks. Stop 2-1 - MacLeod-Cockshutt Mine UTM coordinates - 0504200E 5502770N 2 4 Figure 31. No. 1 vein, Leitch Mine, October 1935 (Laird 1937; Stop #1-9). The history of the MacLeod-Cockshutt Mine (Fig. 32) has been summarized by Horwood and Pye (1955) and Mason and White (1985). In 1931, following the discovery of gold by W.W. Smith on Discovery Point, Kenogamisis Lake, F. MacLeod and A. Cockshutt staked the ground adjoining the Hard Rock Gold Mines Limited property to the west. Surface exploration led to the discovery of gold-bearing quartz veins in 1931. The discovery of larger mineralized zones in 1933 - 27 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 'j ____ I Figure 32. Map of the MacLeod-Cockshutt – Hardrock mines area (modified after Ferguson 1967). Pf = albite porphyry; IF = banded iron formation; sandstone is unlabelled and unhatched. led to the organization of a new company, MacLeodCockshutt Gold Mines Limited. In 1934, shaft-sinking began here at the Number 1 shaft; the Number 2 shaft, 600 m to the southeast, was sunk in 1936. The MacLeod-Cockshutt Mine became the fifth producing gold mine in the Little Long Lac area on April 19, 1938 when a mill with a rated capacity of 600 tons per day was brought into operation. In 1967, MacLeodCockshutt Gold Mines Limited, Consolidated Mosher Mines Limited and Hard Rock Gold Mines Limited were amalgamated to form MacLeod Mosher Gold Mines Limited. Underground operations continued until July, 1970. The mine had produced 1,546,980 ounces of gold at an average grade of approximately 0.14 ounces of gold per ton. This total accounts for about half of all the gold produced by the 10 mines in the Geraldton gold camp between 1934 and 1970. In the 1980’s, Lac Minerals Ltd. (now Barrick Gold Corporation) undertook studies of existing underground reserves at the MacLeod-Cockshutt and neighbouring Hardrock mines and carried out lithogeochemical sampling (Gray, 1993). Starting in 1987, Lac conducted ground geophysical surveys, followed by 77 diamond drill holes, totaling approximately 50,000 feet. Targets, especially those with open pit potential, were investigated (e.g., Hardrock D and F; North and South Porphyry; Porphyry Hill zones). In 1992, Asarco Exploration Company of Canada Limited entered into a 5-year earn-in agreement with Lac and in 1993 carried out a program of reverse circulation overburden drilling and diamond drilling, the latter largely focusing on the near-surface portion of the F-zone and targets along the plunging nose of the albite porphyry (ibid). As a result of this work, a geological resource was calculated by Horvath (1993) for the Porphyry Hill, West and East pits: Pit Resource: 1,920,000 tons grading 0.079 ounce gold per ton (with strip ratio, including overburden, of 4.76 to 1) Ramp Resource: 1,160,000 tons grading 0.127 ounce gold per ton Asarco continued their exploration campaign into 1994, completing reverse circulation holes in overburden; sonic holes in tailings; and an additional 40,000 feet of diamond drilling, mostly on the aforementioned targets (Gray, 1994). Cyprus Canada Inc. assumed Asarco’s role in the Lac Minerals agreement in 1996 and drilled 24 holes, leading to the discovery of the B-zone (Mason and White, 1997). The agreement ended in 1997 and the properties have been dormant since then. Lac Minerals Properties Inc. began a rehabilitation program in 1996 that is still active. In the first description of the fledgling Little Long Lac (i.e. Geraldton) camp, Bruce (1935) proposed three types of gold deposits: (1) Shear zones in sedimentary rocks, along which narrow, but closely spaced quartz veins occur in parallel planes; (2) Irregular veins of quartz accompanied by pyrite, filling fractures in iron formation; and (3) Zones of pyritization and silicification in both in sedimentary and intrusive rocks. - 28 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 the porphyry and the adjacent metasedimentary rocks. The second ore type is exemplified by the North, North Limb, South Limb and West zones (Fig. 33). Ore consists of masses, lenses and tongues of almost massive sulphides that replace folded wall rock, particularly laminae of incompetent, sheared sandstone and magnetite beds in banded iron formation. Sulphides, principally pyrite and arsenopyrite, also replace rocks adjacent to steeply dipping, east-trending shear fractures or shallowly west-dipping, north-trending tension fractures. Fractures are commonly healed by late(r)stage quartz. It was reported that replacement-type ore averaged approximately 0.5 ounce gold per ton, while fracture-filling ore typically returned between 1 and 3 ounces gold per ton. p West -. BodieE!\._____ ....-. ... - VS Replacement Systems Albite Porphyry = Iron Sandstone— Argillite 9 6pm Figure 33. Vertical north-south section, MacLeod-Cockshutt Mine. After Horwood and Pye (1955). The third ore type, best exemplified by the Fzone, consists of numerous quartz veins and stringers localized within a wide zone of shearing and fracturing along the northern side of the albite porphyry. The veins and stringers range in width from < 1 to 10 cm. They appear to be largely confined to sandstone and lean BIF nearer to surface and occur in altered quartz diorite at lower levels to the west. Generally, quartz stringers are bordered by buff-coloured, sericitized and carbonatized sandstone with fine- to coarse-grained pyrite and subordinate arsenopyrite. In iron-bearing wall rock, sulphides are more predominant and may Six main ore zones were exploited at the MacLeodCockshutt Mine. Horwood and Pye (1955) classified the MacLeod-Cockshutt orebodies and zones of mineralization into three distinct types: (1) Quartz veins and mineralized zones (2) Irregular, massive sulphide-quartz lenses in a folded series of sandstone and banded iron formation (3) Quartz stringer zones in sandstone and subordinate quartz diorite. The first ore type, exemplified by the South, No. 210 Quartz Vein, No.’s 516, 517 and 519 Drift zones, is manifested by numerous but small lenses of semimassive sulphides (coarse pyrite + arsenopyrite, sphalerite and chalcopyrite) scattered along the contacts of intrusions and metasedimentary rocks; by disseminated, auriferous sulphides in porphyry; and by minor quartz-carbonate veins and stringers cutting both Figure 34. No. 1 Shaft headframe, MacLeod-Cockshutt Mine (Stop 2-1). - 29 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 secondary pyrite in association with iron carbonatization and quartz veining. Transpressive/oblique compression has produced open Z-folds and localized S-folds, with evidence of re-folding and late(r) transposition (Figs. 35 and 36). Consistently shallow (15° to 25°) plunges are observed on fold hinges. Lafrance et al. (2004) noted several, small isoclinal F1folds that were folded by parasitic F2 S-folds, resulting in a strong, rod-like, coaxial lineation that plunges 20° to 40° west, subparallel to the strike of the S2 axial-planar cleavage (100°/85° southwest) of regional folds. Figure 35. Deformed magnetite BIF, MacLeod-Cockshutt Mine (Stop 2-1; coin is 2 cm in diameter). The BIF consists of a finely parallel-laminated sequence of alternating, magnetite-rich layers and silt- to very fine sand-rich layers. The sand layers rarely show grading due to deformation, although some graded examples show increasing magnetite content in their clay-rich tops, which are gradational to the overlying, pure chemical sediment layer. On the microscopic scale, the magnetite layers have small amounts of chert and detrital minerals, but are otherwise mineralogically pure. Although deformation limits the depositional interpretations that can be drawn in this area, other exposures near Beardmore (e.g., Stop 1-6) indicate that the iron-rich sediments accumulated in the shore-proximal positions near the mouths of rivers in braid-delta complexes. Stop 2-2 - Porphyry Hill, Hard Rock Gold Mines Limited 4(j UTM coordinates - 0504810E 5502850N Figure 36. West-plunging folds in BIF, MacLeod-Cockshutt Mine (Stop #2-1). form small replacement lenses. Although the quartz veinlets contain visible gold, much gold is also associated with replacement pyrite and arsenopyrite. The recently exposed outcrop at the base of the No.1 shaft headframe (Fig. 34) displays highly deformed BIF and sandstone, part of a band that extends 600 m east onto the Hard Rock Mine property and hosts the North ore zone there (Stop 2-3). The steeply dipping BIF contains primary magnetite, but displays some In 1935, Hard Rock Gold Mines Limited began surface work and diamond drilling on the “Porphyry Hill” section of the property and discovered goldmineralized zones in and along the northern tongue of porphyry (Horwood and Pye, 1955). The discovery of scattered, finely disseminated pyrite and numerous, small quartz stringers prompted underground followup, resulting in the definition of the No. 1 and No. 2 vein systems. In addition, a small, high-grade shoot outcropped near the western property boundary. The X-vein system was defined underground (3.5 feet wide, 110 feet long, with an average grade of 0.049 ounce gold per ton), within an inclusion of sandstone and lean BIF in albite porphyry (ibid). Asarco’s proposed Porphyry Hill open pit was designed to recover shallow ore from several zones, including the F-zone (sandstone-BIF-hosted; northern porphyry contact-hosted); Porphyry zone (sheared - 30 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Stop 2-3 - Glory Hole BIF, Hard Rock Gold Mines Limited UTM coordinates - 0504862E 5503143N Deformed, gold-bearing BIF is exposed along the edges of a fenced-off, water-filled glory hole, just south of Highway 11 that represents the mined, near-surface portion of the Hard Rock North ore zone. BIF occurs with siltstone/sandstone, porphyry sills and diorite sills and dykes. This orebody consisted of numerous, irregularly shaped replacement lenses of sulphides distributed in an east-trending zone along the northern limb and in the trough of a minor synform having a plunge of approximately 30° west (Horwood and Pye, 1955). The open stope mimics the shape of a F3 Z-fold defined by folded BIF horizons (Macdonald, 1988). Individual lenses were not of great lateral and vertical extent, but had lengths of up to 300 feet and widths up to 30 feet. Collectively, these sulphide lenses formed an ore zone which persisted and plunged onto the Figure 37. Contact between albite porphyry and sandstone/ BIF, Porphyry Hill (Stop 2-2). porphyry-hosted); South Porphyry zone (southern porphyry contact-hosted) and the Southern Iron Formation zone. Horvath’s (1993) geological resource estimate for the Porphyry Hill open pit included an estimated high-grade resource of 1.02 million tons at a grade of 0.100 ounce gold per ton, and a low-grade resource of 0.48 million tons at a grade of 0.034 ounce gold per ton (for a total of 1.5 million tons grading 0.079 ounce gold per ton). Deformed porphyry dykes and sills intruding sandstone and BIF are exposed at this site (Fig. 37). Crowded albite and minor quartz phenocrysts occur in a fine-grained matrix of quartz, feldspar, sericite, calcite and chlorite. An age of 2698.6±1.3 Ma was obtained on a Au-mineralized “crowded albite” feldspar porphyry dyke at the Hard Rock Mine (Corfu, 2000). Figure 38. Oxide-facies banded iron formation (BIF) cut by massive sulphide (MS)-rich alteration envelope surrounding quatz-carbonate vein (QV), Hard Rock Mine (Stop 2-3). MacLeod-Cockshutt property to the west. This is the same BIF that is exposed below the No. 1 headframe (Stop 2-1), approximately 600 m west of this location. The so-called massive sulphide lenses contain up to 65% combined pyrite, arsenopyrite and pyrrhotite, the remainder being quartz. Minor sphalerite, chalcopyrite and galena, as well as accessory ankerite, calcite and tourmaline has also been noted. Replacement, controlled by quartz-filled fractures and bedding planes, is locally pervasive, destroying primary lithologic features (Fig. 38). Pervasive carbonatization accompanies replacement and gold mineralization. Replacement lenses are localized in zones of intense deformation - 31 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 W1iItFigure 39. Pre- and post-rehabilitation views of the MacLeodCockshutt tailings at the junction of Highways 11 and 584, showing new Heritage Interpretative Centre. along contacts between sandstone and BIF, and in the noses of small, asymmetric folds in an orientation consistent with the axial-planar cleavage in these folds (Anglin and Franklin, 1985). North zone ore mined to the mid-1940’s averaged approximately 0.25 ounce gold per ton. Stop (optional) - Geraldton Reclamation Project In the late 1990’s, Barrick Gold Corporation, in conjunction with the Town of Geraldton and government agencies embarked on a tailings reclamation project. This Tourism Development Project led to the development of the Heritage Interpretative Centre and a 9-hole expansion of the Kenogamisis Golf Course. The following description has been gleaned from the website of one of the designers (http://www.gsd. harvard.edu/people/faculty/schwartz/projects.html). In the Geraldton recalmation project, mine tailings were reshaped for both aesthetic and economic reasons (Fig. 39). 14 million tons of tailings from the mines covered 170 acres to a depth of 27 feet. In order to spur economic redevelopment, the Town made the decision to make something of the tailings by improving their appearance and adding opportunities for visitors. Design alternatives involved sculpting the flat pile into compelling sculptural landforms which serve as a dynamic roadway edge and a gateway to the town. Trails invite one to walk, bird watch, mountain bike, snow board, sled or snowmobile. Technical constraints were key to the final form of the earthwork. The different types and sizes of earth-moving equipment and their turning radii provided guidelines for the grading plan. A primary objective in the project was to balance cut and fill, and to maintain a maximum total earth moving of 150 000 cubic meters. Cut is kept to a minimum as arsenic levels are higher toward the bottom of the pile. There is a cap at the bottom of the pile of tailings, and there is a maximum of an additional 5 m that can occur on top. Standing water has been considered as a design element, but the water table has been respected by the re-grading. Storm drainage is maintained and the proposed earthwork does not impede sight lines for traffic safety. Six to twelve inches of peat topsoil were added to disturbed areas to aid in revegetation. A planting plan for the project focuses primarily on native grasses, especially those golden in color. The Geraldton project reveals the power of design to remake a wasteland into a new landscape. Even more than an earthwork, this landform is also a cultural artifact, highlighting the location and role of mining in the life of the town. Stop 2-4 - Conglomerate, Highway 11 UTM coordinates - 0502906E 5502908N Low outcrops on the south side of Highway 11 opposite the turnoff for the Consolidated Mosher Mine display deformed conglomerate, sandstone and BIF of the SMB within the BBLZ. As noted by Williams and Stott (1991), strain is exhibited by differentiated layering in pelitic rocks and elongated clasts in the conglomerates, especially in less-competent, mafic clasts (Fig. 40). Abundant, shallowly west- Figure 40. Sheared conglomerate in BBLZ, Highway 11 (Stop 2-4; Pen is 15 cm long.) - 32 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 plunging, Z-folds with attendant axial-planar fabrics, back-rotated pebbles, pressure fringes and intraclast fractures support the contention that the BBLZ is a major dextral, ductile shear zone (Williams and Stott, 1991). Lafrance et al. (2004) noted that the long axes of deformed granitic clasts in polymictic conglomerate are parallel to the axial plane of F3 folds. S2 strikes 100° in the sandstone matrix away from the clasts, but as it wraps around the clasts, it changes orientation to that of the elongated granitic clasts, resulting in a geometry similar to dextral, asymmetrical strain shadows around rigid objects (ibid). Polymictic, clast-supported, cobble-pebble conglomerate with deformed, lenticular lenses of deformed sandstone is interbedded with thicker, medium-grained sandstone sheets. They collectively represent a gravelly, braided fluvial system with sanddominated channels, separated by gravelly longitudinal bar complexes. The bars were active during flood events with small sand channels within the bars developing as chutes during falling stage. In these types of braided systems, more massive, coarser-grained gravels developed in the bar-head region of the bars. Bar-tails are often composed of interlayered sand and thin gravel layers, dissected by secondary channels. Stop 2-5 - Gabbro, Highway 11 UTM coordinates - 0502530E 5502860N Approximately 300 m west of Stop 2-4, outcrops of deformed gabbro lie on the south side of Highway 11, just east of a small pond. The amphibolitized, medium-grained gabbro is cut by a network of dark green, chloritic zones < 1 cm wide in which hornblende megacrysts are elongated. These anastomosing shear bands separate lozenges of relatively undeformed gabbro, imparting an disconcerting impression of a pillowed basalt flow. With increasing degrees of deformation, the shear bands become sub-parallel, forming a C fabric (Macdonald et al., 1990). In the most deformed rock, a third (C’) fabric develops at 45° to the C fabric. The northwestern orientation of C’ is indicative of dextral shear. Elsewhere on the outcrop, sharply bounded, metre-wide zones of fine-grained greenschist represent mylonitized gabbro (Williams and Stott, 1991). Buck and Williams (1984) considered such variations in the preservation of primary textures, versus the development of shear fabrics, to be a function of the inhomogeneity of strain and not of the protolith. Stop 2-6 - Missing Link Extension, Kinghorn Road UTM coordinates - 0466744E 5511220N The Missing Link Extension occurrence was discovered by Beardmore prospector Myron Nelson in 1986 and optioned to Freewest Resources Inc. in 1990 (Resident Geologist’s Files, Thunder Bay North District, Thunder Bay). It is situated on the west side of the Kinghorn Road, just north of the Paint Lake Deformaion Zone (PLDZ). The PLDZ is a 500 m-wide high-strain zone at or near the northern boundary of the Beardmore-Geraldton belt (Mackasey, 1976; Reilly, 1988; DeWolfe, 2002; Lafrance et al., 2004). The shear zone extends for over 40 km, from Lake Nipigon to the late, sinistral Jellicoe Fault, where it is offset by 500 m. The shear zone can be traced for ~ 9 km on the east side of the fault before it is lost under overburden. At the Missing Link property (1 km to the northeast; Stop 2-7), a northern splay/extension of the PLSZ has been exposed by mechanized stripping. At Paint Lake (DeWolfe, 2002), the PLSZ is parallel to lithological contacts between polymictic conglomerate (NMB) and volcanic rocks (NVB), and forms the boundary with the Onaman–Tashota belt to the north. Shear sense indicators are observed in both conglomerate and volcanic rocks. Several granitic clasts in conglomerate are bounded by dextral, asymmetrical strain shadows filled with quartz fibers. Steeply dipping dextral shear bands cut across S3 (Lafrance et al., 2004). The property straddles intermediate to mafic metavolcanic rocks on the north and clastic metasedimentary rocks to the south, separated by a major, east-striking structure interpreted to be the PLSZ (Resident Geologist’s Files, Thunder Bay North District, Thunder Bay). Stripping and trenching exposed two sub-parallel zones of alteration and mineralization within sheared and isoclinally folded andesite. The orientation of the major fold axis is generally subparallel to the shear fabric, which strikes between 080° and 090° and dips steeply south to vertically. The fold is believed to plunge shallowly to the east. The Main Zone was exposed over a strike length of 40 m and a width of 12 m. Sub-zones were recognized on the basis of variations in the relative amounts of chloritization, carbonatization, silicification, quartz-carbonate veining and sulphide minerals. Finely disseminated arsenopyrite, pyrite and chalcopyrite were noted. Values of up to 0.20 ounce gold per ton were returned - 33 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 variety of surveys were undertaken on the property by Beardmore prospector, Nolan Cox ca. 19861988. Older, undocumented trenches (ca. 1950’s) were discovered during the course of this work. The property straddles intermediate to mafic metavolcanic rocks on the north and clastic metasedimentary rocks to the south, separated by a major structure (at 080°) interpreted to be an extension or splay of the Paint Lake Shear Zone (PLSZ) (Lafrance et al., 2004). At the Missing Link, phyllonites (mafic metavolcanic protolith?) display intense microfolding of S3 on millimetre-scale, producing a fine L3’ crenulation lineation along S3 (ibid) (Figs. 41a,b ). Figure 41a. Crenulated and altered metavolcanic phyllonite with quartz-carbonate veins, Paint Lake Shear Zone, Missing Link property (Stop 2-7). IZ Figure 41b. Crenulated and altered metavolcanic phyllonite, Paint Lake Shear Zone, Missing Link property (Stop 2-7). in sulphide-rich grab samples. Stop 2-7 - Missing Link Property / Paint Lake Shear Zone UTM coordinates - 0467535E 5511479N The Missing Link occurrence was described by Mason et al. (1989). Stripping, prospecting and a DeWolfe et al. (2000) stated that gold mineralization and associated hydrothermal alteration were focused along second- and third-order fault splays off of the PLDZ. Second-order splays generally occur south of the PLDZ, often following lithologic contacts and refracting into an orientation sub-parallel to the PLDZ. Third-order splays are concentrated within the Northern Volcanic Sub-belt, where they are oriented both to the northeast and southeast. Gold mineralization at the Missing Link property is associated with sheared and altered mafic metavolcanic rocks. The deformation zone has been exposed over a width of 60 to 90 m by stripping. Disseminated, foliation-parallel, euhedral pyrite and arsenopyrite within zones of both shearing and quartz-carbonate veining, host gold. Pervasive carbonatization and more localized, erratically distributed silicification and hematitization have also been noted (Mason et al., 1989). Three zones (North Zone; Shaft Pit and Baseline Trench) were sampled by N. Cox. The North Zone strikes 085° and dips 61° south and has returned assay values of up to 0.256 ounce gold per ton over 1.8 m. Grab samples collected by Mason et al. (1989) from all the zones returned up to 0.14 ounce gold per ton. The property was optioned by Homestake Mineral Development Company in 1989, who conducted a number of geochemical, geological and geophysical surveys. Only low gold values (e.g. 2.06 g/t Au over 1.0 m) were returned and in 1990 the property was returned to the vendor (Resident Geologist’s Files, Thunder Bay North District, Thunder Bay). Placer Dome Inc. optioned the property in 1991 and completed just over 1000 m of drilling in four holes. Despite delineating a zone approximately 175 m wide within which intensely sheared and altered rocks occurred, no intersections of economic significance were encountered (ibid). - 34 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Stop 2-8 - Sandstone, Southern Sedimentary Subbelt, Highway 11 UTM coordinates - 0468062E 55005080N Thickly bedded, north-younging, overturned turbidites are exposed on the south side of Highway 11 opposite the Kinghorn Road turn-off . They display a variety of well-preserved sedimentary structures, including graded bedding, ball-and-pillow structures, flames and scours (Fig. 42). Some sandstone beds display a weakly developed cleavage. @) I © 1) 04 0) 0 N) :9 Figure 43. Stratigraphic column of sandstone outcrop (Stop 2-9; scale in metres). N) ri © a 01 ® a G 01 me ICr N) a I) C N) -— 0) 04 0) e melerS 0) ____ __ it — Figure 42. Load casts at base of sandstone (arrows), Highway 11 (Stop 2-8). This section of turbidites contains graded AE and AA beds up to 8m thick (Figs. 43 and 44). Most grading is in the basal few centimeters where the amount of very coarse sand grains decreases rapidly and in a zone approximately three-quarters of the way up through the bed where there is a rapid decrease from medium- to 0 - COG® - Figure 44. Measured sections of the turbidite assemblage in the Southern Metasedimentary Belt between Jelicoe and Geraldton. Note the extreme thickness of some of the beds. These units can be divided into a thin-bedded association, a medium-bedded association and a thick-bedded association. From Barrett and Fralick (1989). - 35 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 fine-grained sand. This results in beds that are quite monotonous, with little change in their grain size through most of their thickness. These sediments were transported by typical, high-density turbidity currents and grainflows which deposit sediment in sourceproximal locations. The fine-grained sand tops on the massive beds were possibly the result of resultant turbulence after the main turbidity current swept through the area. Currents moving water in behind the tail of the turbidity current would sweep sand from the surrounding bottom onto the coarser turbidity current deposits. Stop 2-9 - Pillowed Mafic Metavolcanic Rocks, Highway 11 UTM coordinates - 0460740E 5503325N Pillowed and massive, greenschist-facies, mafic metavolcanic rocks occur on the south side of Highway 11, 200 m west of the Jellicoe post office. They occur in a small, fault-bounded enclave of metavolcanic rocks with clastic metasedimentary rocks of the Southern Sedimentary Sub-belt (Bruce, 1937; Mackasey, 1976). They display a non-penetrative schistosity, on which there is a steeply plunging mineral and stretching lineation. Parallel to the schistosity are shear zones in which a non-penetrative fabric is forming at an angle to the zone, becoming parallel to it and more intense near the axis of the zone, characteristic of S-C fabric relationships (Williams and Stott, 1991). The shear sense based on this local relationship suggests south side-down. In graphitic metapelites and BIF exposed in road cuts 200 m west, the stretching lineation is well-developed. Acknowledgements The authors would like to thank John Mason and Gerry White (Resident Geologist’s Program, OGS, Thunder Bay) for providing property information and historical data. Greg Stott (Precambrian Geoscience, OGS, Sudbury) provided useful structural information and insights on the local tectonic framework. We would also like to acknowledge the support of the Municipality of Greenstone and local exploration companies, especially David Malouf (Roxmark Mines Limited). References Anglin, C.D., 1987. 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A structural reappraisal of the Beardmore-Geraldton belt at the southern boundary of the Wabigoon Subprovince, Ontario, and implications for gold mineralization; Canadian Journal of Earth Sciences, v.41, p.217235. Laird, H.C. 1937. The western part of the Sturgeon River area (Sturgeon River-Beardmore section); Ontario Department of Mines, Forty-fifth annual report, v. XLV, pt.II, p.60-117. Hodgson, C.J., 1983. The structure and geological development of the Porcupine camp - a re-evalutation; in The Geology of Gold in Ontario, Ontario Geological Survey, Miscellaneous Paper 110, p.211-225. Lavigne, M.J. (in press). Structural geology and gold mineralization in the Geraldton area, BeardmoreGeraldton greenstone belt; Ontario Geological Survey, Open File Report. Hodgson, C.J., and MacGeehan, P.J., 1982. Geological characteristics of gold deposits in the Superior Province of the Canadian Shield; in Geology of Canadian Gold Deposits, Canadian Institute of Mining and Metallurgy, Special Volume 24, p.211229. Lavigne, M.J. 1983. Gold Deposits of the Geraldton area; in Summary of Field Work 1983, Ontario Geological Survey, Miscellaneous Paper 116, p.198-200. Horvath, A.S. 1993. Ore reserve estimate, Geraldton Project, 1993, Asarco Exploration Company of Canada Limited; unpublished OMIP report, Assessment Files, Thunder Bay North District, Thunder Bay, 7p. Horwood, H.C., and Pye, E.G., 1955. Geology of Ashmore Township, Little Long Lac area, Thunder Bay District; Ontario Department of Mines, Annual Report, v.60, pt.5, 105p. Kehlenbeck, M.M., 1983. Structural studies in the Beardmore- Lesher, C.M., Goodwin, A.M., Campbell, I.H. and Gorton, M.P. 1986. Trace-element geochemistry of oreassociated and barren, felsic metavolcanic rocks in the Superior Province, Canada; Canadian Journal of Earth Sciences, v.23, p.222-237. Macdonald, A.J., 1982. The MacLeod-Cockshutt and Hard Rock mines, Geraldton: examples of an iron formation-related gold deposit; in Summary of Field Work 1982, Ontario Geological Survey, Miscellaneous Paper 106, p.188-191. Macdonald, A.J., 1983a. The iron formation-gold association evidence from Geraldton area; in The Geology - 38 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 of Gold in Ontario, Ontario Geological Survey, Miscellaneous Paper 110, p.75-83. Australia; Geological Society of Australia Journal, v.30, p.25-39. Macdonald, A.J., 1983b. A re-appraisal of the Geraldton gold camp; in Summary of Field Work 1983, Ontario Geological Survey, Miscellaneous Paper 116, p.194197. Pirie, J. and Mackasey, W.O. 1978: Preliminary examination of regional metamorphism in parts of Quetico metasedimentary belt, Superior Province, Ontario; in Metamorphism in the Canadian Shield, Geological Survey of Canada, Paper 78-10, p. 37-48. Macdonald, A.J. 1984. Gold mineralization in Ontario, I: The role of banded iron formation; Canadian Institute of Mining and Metallurgy, Special Volume 34, p.412430. MacDonald, R.D. 1940. Geology of the Kenogamisis River area; Ontario Department of Mines, Annual Report 1940, v.49, pt.7, p.1-17. Mackasey, W.O. 1975. Geology of Dorothea, Sandra and Irwin Townships, District of Thunder Bay; Ontario Division of Mines, Geological Report 122, 83p. Mackasey, W.O. 1976. Geology of Walters and Leduc Townships, District of Thunder Bay. Ontario Division of Mines, Geological Report 149. 58p. Mackasey, W.O., Edwards, G.R. and Cape, D.F. 1976. Legault Township, District of Thunder Bay; Ontario Division of Mines, Preliminary Map P.1191, scale 1:15 840. Mason, J.K. and McConnell, C.D. 1983. Gold mineralization in the Beardmore-Geraldton area; in The geology of gold in Ontario, Ontario Geological Survey, Miscellaneous Paper 110, p.84-97. Mason, J.K. and White, G.D. 1986. Gold occurrences, prospects and deposits of the Beardmore-Geraldton area; Ontario Geological Survey, Open File Report 5630, 680p. Mason, J.K. and White, G.D. 1997. Beardmore-Geraldton Resident Geologist’s District 1996; Ontario Geological Survey, Open File Report 5958, p.1-1 to 1-22. Mason, J.K., White, G.D. and McConnell, C. 1985. Field guide to the Beardmore-Geraldton metasedimentary – metavolcanic belt; Ontario Geological Survey, Open File Report 5538, 73p Neumayr, P., Walshe, J., Connors, K., Cox, S., Morrison, R. S., and Stolz, E. 2004. Gold mineralization in the St Ives camp near Kambalda; Geological Survey of Western Australia, Record 2004/16, p.23-45. O’Brien, M.S. 1985. A detailed geological study of the mafic metavolcanic rocks north of Poplar Lodge, Ontario; unpublished H.B.Sc. thesis, Lakehead University, Thunder Bay, Ontario, 76p. Poulsen, K.H., 1983. Structural setting of vein-type gold mineralization in the Mine Centre-Fort Frances Area: implications for the Wabigoon Subprovince; in The Geology of Gold in Ontario, Ontario Geological Survey, Miscellaneous Paper 110, p.174-180. Purdon, R.H. 1995. Lithostratigraphy and provenance of the Neoarchean McKellar Harbour sequence, Superior Province, Ontario, Canada; unpublished M.Sc. thesis, Lakehead University, Thunder Bay, Ontario, 172p. Pye, E.G. 1952. Geology of Errington Township, Little Long Lac area; Ontario Department of Mines, Annual Report, v.60, pt.6, 140 p. Reilly, B.A. 1988. Structural analysis of the Paint Lake Deformation Zone, northern Ontario. M.Sc. thesis, Brock University, St. Catharines, Ontario. Rigg, D.M., and Helmstaedt, H., 1981. Relationships between structure and gold mineralization in Campbell Red Lake and Dickenson Mines, Red Lake Area, Ontario; in Genesis of Archean, Volcanic Hosted Gold Deposits, Ontario Geological Survey, Miscellaneous Paper 97, p.111-127. Rogers J. R., Joyce, K. A., Masterman, G. J., Halley, S. W., and Walshe, J. L. 2004. Kanowna Belle gold mine; Geological Survey of Western Australia, Record 2004/16, p.47-51. Scott, B.M., 1985. A study of the auriferous quartz veins found within the oxide facies banded iron formations of the Beardmore-Geraldton area of northern Ontario; Unpublished H.B.Sc. thesis, Lakehead University, Thunder Bay, Ontario, 96 p. Shanks, W.S. 1993. Geology of Eva and Summer Townships, District of Thunder Bay. Ontario Geological Survey, Open File Report 5821. Sibson, R.H., Moore, J.M., and Rankin, A.H., 1975. Seismic pumping - a hydrothermal fluid transport mechanism; Journal of the Geological Society of London, v.131, p.191-213. Ozaki, M. 1972. Chemical composition and occurrence of axinite; Kumamoto Journal of Scientific Geology, v.9, p.1-34. Stott, G.M., and Davis, D. 1999. Contributions to the tectonostratigraphic analysis of the Onaman–Tashota greenstone belt, eastern Wabigoon Subprovince; in 1999 Western Superior Transect 5th Annual Workshop. Report 70, Lithoprobe Secretariat, The University of British Columbia, Vancouver, B.C., p.122–123. Phillips, G.N., and Groves, D.I., 1983. The nature of Archean gold fluids as deduced from gold deposits in Western Stott, G.M., and Schneiders, B.R., 1983. Gold mineralization in the Shebandowan Belt and in relation to regional - 39 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 deformation patterns; in The Geology of Gold in Ontario, Ontario Geological Survey, Miscellaneous Paper 110, p.181-193. Tomlinson, K.Y. 1996. The geochemistry and tectonic setting of Early Precambrian greenstone belts, northern Ontario, Canada; unpublished Ph.D. thesis, University of Portsmouth, Portsmouth, United Kingdom, 287p. Tomlinson, K.Y., Hall, R.P, Hughes, D.J. and Thurston, P.C. 1993. Mafic metavolcanic rocks of the BeardmoreGeraldton greenstone belt: Preliminary geochemical findings; in Summary of Field Work and Other Activities, Ontario Geological Survey, Miscellaneous Paper 162, p.54-58. Williams, H.R., 1986. Structural studies in the BeardmoreGeraldton belt, northern Ontario; in Summary of Field Work 1986, Ontario Geological Survey, Miscellaneous Paper 130, p.138-146. Williams, H.R., 1987. Structural studies in the BeardmoreGeraldton belt and in the Quetico and Wawa subprovinces; in Summary of Field Work 1987, Ontario Geological Survey, Miscellaneous Paper 137, p.90-92. Williams, H.R. and Stott, G.M., 1991. Subprovince accretion in the southern Superior Province. G.A.C.-M.A.C. annual Meeting, Toronto, Field trip B6, Guidebook, 26 p. Wood, D.A. 1980. The application of a Th–Hf–Ta diagram to problems of tectonomagmatic classification and to establishing the nature of crustal contamination of basaltic lavas of the British Tertiary volcanic province; Earth and Planetary Science Letters, v.50, p.11-30. - 40 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Fieldtrip 2 – Quaternary geology of the Beardmore – Nipigon area Peter J. Barnett Ontario Geological Survey, Ministry of Northern Development and Mines, Sudbury, Ontario, P3E 6B5, Canada Introduction then returns to Nipigon along the same route (Fig. 1). In association with the International Quaternary Association in 1987 (INQUA’87) an excursion quidebook C-12: Quaternary features and scenery along the North Shore of Lake Superior was created (Geddes et al., 1987). Unfortunately the trip was never run. This current field trip and field trip guide utilizes much of the information presented in the INQUA’87 field trip guide particularly as it pertains to the Beardmore – Nipigon area. The route follows the east branch of the Nipigon River, initially across the Nipigon lowlands then along the Pijitawabik Canyon cut through Nipigon Sills and onto more-typical Canadian Shield terrain north of Beardmore. Although this part of Ontario is pivotal to many large-scale glacial histories and theories of the Great Lakes Region, North America and global climate change, very little if any detailed mapping of the Quaternary geology has been done. One exception is the work of Thorleifson and Kristjansson (1993) in the very northern part of the field trip area (scale 1:100,000). In addition, Mollard and Mollard (1981) undertook a terrain analysis (scale 1:100,000) of a large part of the field trip area and produced a map of some of the major landforms and material types that occur there. The field trip area has been displayed on several regional maps including Zoltai (1965a), Sado and Carswell (1987), Barnett et al. (1991) and Sado et al. (1994). In addition, regional studies on selected aspects of interest to Quaternary researchers, particularly associated with ancestral lake features and deposits (e.g., Farrand, 1960; Teller and Thorleifson, 1983, 1987; Teller and Mahnic, 1987) have been done. Zoltai (1965b) described and discussed the formation of some of the major landforms and deposits of the Nipigon area. Lake Nipigon During the trip we will visit several sites and landforms that display clues to the glacial history of the area and hopefully discussions on how they relate, support or conflict, with several of the regional histories proposed in the literature will ensue. The fieldtrip route basically follows Highway 11 north from Nipigon to Beardmore and then northwest on Highway 580 to Poplar Lodge on Lake Nipigon and Figure 1. Location, route and stops. Rectangle is for reference between the various regional figures. - 41 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Regional geological setting During the fieldtrip we will be travelling between Lake Superior and Lake Nipigon following a now abandoned drainage way used during deglaciation between these two lakes. The present drainage divide is located between McKirdy and Shamrock Lake within the Pijitawabik Canyon. Keemle Creek via a series of small lakes drains northward into Pijitawabik Bay of Lake Nipigon (approximately 260 m or 852 ft asl). An unnamed east branch of Cash Creek drains southward from McKirdy Lake into Cash Creek and onto Helen Lake and then into the Nipigon River to Lake Superior (approximately 183 m or 600 ft asl). The main branch of the Nipigon River is the present-day outlet of Lake Nipigon. The fieldtrip area, located on the Canadian Shield, is an irregular, subdued, gently rolling, bedrockdominated terrain underlain by Precambrian igneous and metamorphic rocks (Figs. 2, 3). The area immediately south of Lake Nipigon, however, is not typical shield terrain. It has been referred to as the Nipigon Plain by Bostock (1970) and “is formed on nearly flat-lying Proterozoic gabbro sills and sediments surrounding Lake Nipigon” (Bostock, 1970, p.16). Combined past tectonic activity, differential weathering and glacial/ glacial meltwater erosion of the Proterozoic sills and Sibley Group metasedimentary rock surfaces has produced a unique landscape of buttes and mesas and steep-walled valleys along the margins of the Nipigon Plain. To the east of Lake Nipigon, beyond the extent of the sills, in the area underlain by Archean igneous Nipigon diabase sills Sibley Op. j1 nepheline syenite suite III1 granodiorite to granite 2' granitic rock Metasedinientary rocks Felsic metavolcanic rocks Mafic metavolcanic rocks Figure 2. Fused digital elevation model (DEM) and hillshaded DEM. Rectangle is for reference between the various regional figures. Figure 3. Bedrock geology of the fieldtrip area (OGS 2003). - 42 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 and metamorphic rocks, more typical Shield terrain is prevalent. Farrand (1960) has referred to the southern part of the fieldtrip area as the Nipigon Lowlands. Here, the irregular surface of the Canadian Shield has been subdued and partially infilled by sand, silt and clay of glaciolacustrine origin producing a broad dissected plain. Farrand (1960) recognized at least 4 terraces or steps within this lowland at various elevations (692, 800, 907-912, 950-952 and 980 ft or about 211, 244, 276-278, 290, 299 m) that he related to ancestral lake levels in the Lake Superior basin. Relief over the entire region exceeds 350 m (about 1150 ft). Local relief in areas of Archean rocks is commonly between 15 and 50 m (50 and 165 ft). Along the eroded edges of sills that surround the Nipigon Plain it can exceed 100 m (about 328 ft). Local relief of over 100 m (about 328 ft) also exists along the walls of the Pijitawabik Canyon. Bedrock geology The study area contains rocks of the Superior Craton and the Southern Province (Fig. 3). Major geological subdivisions include the Wabigoon and Quetico subprovinces (Blackburn et al., 1991; Williams, 1991), the Proterozoic Sibley Group and Nipigon Diabase Sills (Sutcliffe, 1991). Metasedimentary and granitic rocks dominate the rocks of the Quetico Subprovince in the southern part of the fieldtrip area. Mafic metavolcanic rocks dominate the Wabigoon Subprovince with granodiorite to granite intrusions forming the inter-greenstone belt parts. Diabase/gabbo in the form of sills and dikes is the dominant rock type in the Nipigon embayment or the north-central part of the fieldtrip area (Fig. 3). Nipigon sills are massive bodies that vary in thickness from 150 to 200 m. Internal textural zoning indicates that the sills are single cooling bodies (Blackburn et al., 1991). The Nipigon sills in the area are very similar texturally and monotonous in appearance. They overlie and interfinger with Sibley Group sedimentary rocks that outcrop or subcrop within the southern part of the Nipigon embayment. The Nipigon sills formed contemporaneous with the initiation of the Lake Superior Midcontinent rift age-dated at approximately 1108 Ma (Blackburn et al., 1991). With the possible exceptions of some lateral and vertical outliers the age of the Sibley is constrained between the youngest detrital zircon in a sample from the Kama Hill Formation (1420 Ma; L. Heaman pers. comm. 2004) and a Rb-Sr isochron from authogenic minerals in the Rossport and Kama Hill Formations (1339 Ma; Franklin, 1978). The Sibley consists of unmetamorphosed red-bed sequences consisting of conglomerates, sandstones and shales. They are believed to have accumulated in a subsiding graben prior to the formation of the midcontinent rift (Sutcliffe, 1991). Greenstone belts east of Lake Nipigon are part of the Eastern Wabigoon Subprovince and have been traced beneath the diabase and sedimentary units (Blackburn, 1991; Smyk et al., 2005). Quaternary geology The surficial (Quaternary) geology of the field trip area south and east of Lake Nipigon has been investigated only at a regional scale despite its importance to the overall understanding of the glacial history of Ontario and in particular to mineral exploration using unconsolidated sediments (drift exploration). Regional maps at various scales that cover the area include Zoltai (1965a), Sado and Carswell (1987), Barnett et al. (1991) and Sado et al. (1994). Zoltai (1965b) described and discussed the formation of some of the major landforms and deposits of the QueticoNipigon area. Mollard and Mollard (1981) undertook a terrain analysis of the study area and produced maps of some of the major landforms and material types that occur there (Fig. 4). Several theses and papers have been written on significant aspects or features of Quaternary age from within the area of interest (Farrand, 1960; Teller and Thorleifson, 1983, 1987; Teller and Mahnic, 1987). Zoltai (1965a, 1965b), Elson (1967), Prest (1970), Dredge and Cowan (1989) Barnett (1991) and Sado et al. (1994) have summarized regional concepts of the glacial history surrounding Lake Nipigon. There is very little agreement as to the details of the sequence and timing of events. The deposits in the area probably represent ice movement during the Michigan Subepisode (Late Wisconsinan), however; it may reflect ice cover during the entire Wisconsin Episode or approximately the last 100,000 years. The Laurentide Ice Sheet flowed southwestward during its maximum extent about 20,000 years - 43 - �r re-advance to produce most of the major moraines in northwestern Ontario. The Marks, Mackenzie and Dog Lake moraines have been suggested to have formed at about the same time, about 10,000 years ago (Burwasser, 1977; Drexler et al., 1983) and are probably correlative to the Grand Marais Moraines that formed during the Marquette advance (Drexler et al., 1983). With further ice margin recession and a suggested major re-advance, the Lac Seul moraine formed west of the fieldtrip area (Prest 1970). Zoltai (1965b) suggested that this ice margin formed the northern part of the Kaiashk Interlobate Moraine (Fig. 5). — — a 4. ,m Proceedings of the 51st ILSG Annual Meeting - Part 2 Prest (1970) suggested that with further ice margin retreat, the Sioux Lookout and Whitewater moraines formed west of Lake Nipigon. These events were followed by a westward advance in the Lake Nipigon basin that produced the Nipigon Moraine (Stop 1) along the west and southern side of the lake (Zoltai, 1965a, b). Prest (1970) suggests that only the northern part of this moraine formed following the Whitewater Moraine and that the southern part is older. Zoltai (1965a, b) identified several other moraines east of the Nipigon Moraine in the Lake Nipigon basin that appear to be younger than the Nipigon Moraine. The Nakina Moraines that are located along the northeastern edge of the Lake Nipigon basin mark the last significant ice margin positions to directly affect the region. To complicate the history even more is the suggestion that glacial Lake Agassiz waters drained catastrophically into the Nipigon/Superior basin several Figure 4. Surficial sediments in the fieldtrip area (modified from Mollard and Mollard, 1981). ago. During deglaciation and possibly during initial ice movement into the area, ice flow direction was controlled on a regional scale by topography. During these times ice flow followed broad topographic lows, especially the Great Lakes basins and most likely Lake Nipigon, producing lobate ice margins that could have possibly behaved somewhat independently (Fig. 5). Prest (1970), following the work of Zoltai (1965b), suggested that an active ice margin in northwestern Ontario fronted glacial Lake Agassiz during ice margin retreat. They (Zoltai, 1965b; Prest, 1970) postulated several alternating stages of ice margin retreat and NbItt 2. . It k BI Ck M@! 3. IC k Mk d Lk gk!& ph d 3.4.7.8 k Kbk LIttbfl 9. k Figure 5. Ice flow direction and ice margins in the Lake Nipigon area (Sado et al., 1994). - 44 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 _________ ________ ______________ _____________ __________ AC4SSIZ EASTERN LAKE SUPERIOR OUTLETS PHASES LAKE MICHIGAN ID LAKE HURON - EARLY NIPISSING (rising): SAULT STRAIT: EARLY NIPISSING 7000 4 SAULT RAPIDS HOUGHTON — (bedrock) CHIPPEWA- STANLEY 8000 8000 I ION PROTO- (toIling) P11<1.— ST MARYS RIVER riS,rI --b (downcutting through drift) KAI. — rising MINONG POST )..::Y.cM!l!f.::: 0 8 I AU TRAIN - • ESCANADA GRAND MARAIS 10,000 PROTOST MARYS -—b RIVER MLNONQI 10,000 CIIIPPEWA / EARLY — — UPPER GROUP STANLEY SHEGUIANDAH SKORAM PAYETTE through 2 ALGONQUIN 1!000 '' 9000 PoRTAGE1jJ::RCuPINE 11,000 Tt?'"-.,, fl750 Figure 6. Chart of ice-sheet fluctuations and glacial lake phases (from Farrand and Drexler, 1985). times during the deglaciation of the Lake Nipigon and Superior basins. Farrand and Drexler (1985) produced a chart summarizing some of the ice sheet fluctuations and their relation to glacial lake phases in northwestern Ontario and the upper Great Lakes basins (Fig. 6). Agassiz drainage into the Nipigon and Superior basins occurred both before and after the Marquette maximum (Farrand and Drexler, 1985). A network of 5 groups of channels (Teller and Thorleifson, 1983, 1987) carried drainage from glacial Lake Agassiz following the Marquette advance (Fig. 7). And “as postulated by Teller and Thorleifson (1983), the outflow through the Nipigon channels must have been catastrophic” (Farrand and Drexler, 1985, p.23). These “catastrophic discharges from Lake Agassiz were responsible for cutting down the Lake Minong sill” (Farrand and Drexler, 1985, p.17). The Nipigon Moraine marks a former ice margin located along the north shore of Lake Superior and has been traced along the southern and western side of Lake Nipigon (Fig. 5). It is commonly associated with ancestral glacial Lake Minong and as such believed Figure 7. Eastern outlets of Lake Agassiz. Outlet number 26 is the Pijitawabik Canyon (from Teller and Thorleifson, 1983). - 45 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 to have formed about 9500 years ago (Farrand and Drexler, 1985). Farrand’s regional work on abandoned shoreline features along the north shore of Lake Superior (Farrand, 1960) is the most comprehensive field study on shorelines to date in the area (Figure 8). Farrand (1960) identified evidence of glacial Lake Minong about 25 km north of the Nipigon Moraine (Stop 3). And recently Slattery (2003) identified deltas that formed along the Nakina Moraines built to Minong III (?) or post-Minong levels at elevations of 360 m in the Longlac area. If catastrophic Agassiz drainage carved the outlet at St Marys River, why do shore features of Minong or fairly high Post Minong levels exist well behind the Nipigon Moraine, a feature that in part post-dates the opening or use of many of the earlier eastern outlets? The proposed eastern outlet events and features were based on several coarse-grained deposits associated with the large deep steep-walled valleys that occur throughout the region (Teller and Thorleifson, 1983, 1987). The Pijitawabik Canyon (Stop 3, Outlet number 26, Fig. 7) was proposed as one of the passageways for this meltwater (Teller and Thorleifson, 1983, 1987) and the Pijitawabik delta (Stop 2) was inferred to have formed during Agassiz drainage events (Geddes et al., 1987). An alternative explanation for the development of most, if not all, the steep-walled valleys (e.g., Pijitawabik Canyon) is that they were initially formed in part by differential erosion along fractures and faults within the diabase/gabbro sills prior to glaciation. The valleys were then subsequently carved by subglacial meltwater floods based on the similarities of this channelled landscape to that of the Channelled Scablands of eastern Washington State (Baker et al., 1987). Many of these valleys contain glacial sediments indicating that the bedrock channels themselves were not carved by postglacial Lake Agassiz drainage events, but already existed. Agassiz outflow may have reoccupied some of these steep-walled channels that were carved by the subglacial meltwater flood(s). The complex and still poorly understood deglacial history of the Nipigon basin does not make drift exploration easy. The events and features discussed above could have resulted in many areas having multiple directions of ice movement due to the shifting of ice centers through time that will require linking evidence of ice movement to specific tills (Fig. 4). Determining the history of meltwater and ancestral lakes in the study area is also very important to interpret the results of drift exploration programs. Studies of tills deposited in the BeardmoreGeraldton area (Thorleifson and Kristjansson, 1993) and in the Hemlo-Matheson-White River area (Geddes and Kristjansson, 1986; Karrow and Geddes, 1987; Hicock, 1988) have brought to light another factor that affects mineral exploration using till (drift exploration) in the fieldtrip area. The till in the Beardmore area appears for the most part to have been deposited subglacially by an actively flowing glacier (ideal for mineral exploration using till). However, where the till is thick it is commonly rich in clasts of Paleozoic carbonate and Proterozoic metasedimentary rocks derived from Hudson Bay Lowlands some 100 km or greater away (Stop 6). The content of carbonate in the till matrix is also high (up to 50%; Thorleifson and Kristjansson, 1993). This “exotic” till tends to grade into locally derived till (Stop 4) at surface in the area surrounding the community of Beardmore. Thorleifson and Kristjansson (1993) attribute the abundance of far-traveled debris to: 1) the high susceptibility of carbonate rocks to erosion 2) low erodibility of granitic rock which outcrop between the Beardmore area and the Hudson Bay Lowlands 3) the short distance of transport over the erodible greenstone belt rocks and 4) a zone of vigorous ice flow (ice streaming). Thorleifson and Kristjansson (1993) suggested that an ice stream would be required to produce and distribute the carbonate-rich or exotic till to the Beardmore-Geraldton area based in part on the regional distribution in a radiating tongue parallel to ice flow. They also suggest that the exotic till was transported beneath the glacier by pervasive shear (deformable bed). They argued that “deposition by lodgement of basal debris seem contradicted by carbonate clast preservation” (Thorleifson and Kristjansson, 1993, p.57). Hicock (1988), however, records evidence within exotic till exposures of discrete not pervasive shear that would suggest lodgement as the process of deposition, although, he also suggested ice streaming to produce the exotic till. - 46 - Geddes and Kristjansson (1986) suggested that �Proceedings of the 51st ILSG Annual Meeting - Part 2 Stops englacial transport of the carbonate debris over long distances was required. Hicock (1986) suggested that englacial load under dynamic ice conditions can be transferred to the glacier bed in the leeside of upland settings. Stop 1 – Nipigon Moraine (J. Nichols Sand and Gravel, Nipigon) UTM coordinates – 0406748E 5430513N Regardless of the mechanism of entrainment, transport and deposition of the carbonate-rich till, it has been suggested that the “standard rules of glacial comminution as applied in mineral exploration may not be applicable” (Karrow and Geddes, 1987, p.368). And that care be taken to recognize and avoid this exotic till when sampling for mineral exploration purposes (Thorleifson and Kristjansson, 1993). Note - permission from the land owner is required for access to this stop. The Nipigon Moraine marks a former ice margin of the Laurentide Ice Sheet that fronted a large glacial lake. It is generally accepted that this lake was glacial Lake Minong and that its formation occurred about 9500 yr BP (Farrand and Drexler, 1985). In the Nipigon area it is a discontinuous ridge that rises more than 40 m above the surrounding lake plains. Elevation at the site is approximately 280 m asl. The ridge rises to the west where it becomes flat-topped at an elevation of about 295 - 296 m asl. The form here does not appear to be primary (i.e. delta) but the result of subsequent erosion and deposition (wave cut and wave built platform). A lower plain occurs to the north of the moraine here at about 290 m asl. These features raise the question of what was the actual water level in the Superior basin during the formation of the Nipigon Moraine. The field trip stops were chosen to give the participants an overview of the Quaternary geology of the area between Lake Superior and Lake Nipigon and to introduce them to some remarkable landforms and scenery that exists in this part of Ontario. For the INQUA’87 fieldtrip guide, Geddes et al. (1987) prepared a figure showing the profiles of water planes and shoreline features recognized by Farrand (1960) in the Lake Superior basin between Thunder Bay and Kama Bay. This figure appears as Figure 9 with a few modifications for reference purposes. An ice marginal feature built along the Nipigon NE SW KILOMETRES NORTH OE AGASSIZ OUTLET -J 200 taJ —300 .J C Iii -250 In 0, —200 I-ISO 400 — L.a 0 a Figure 8. Shoreline-relation diagrams and isobases of glacioisostatic rebound (modified 8. Shoreline-relation and general isobases of from Figure Farrand and Drexler, diagrams 1985). A) glacioisostatic rebound (modified from Farrand and diagram emphasizing glacial retreat (Farrand, Drexler 1985) A) general diagram emphasizing retreat 1969). B) Diagram showingglacial relationships (Farrand 1969) between Lakes Superior and Nipigon (Teller B) diagram showing lakes Superior-Nipigon and relationships Thorleifson, 1983). C) Isobases of (Teller and Thorleifson 1983) C) isobases of rebound glacioisostatic for Washburn glacioisostatic forrebound Washburn and and Minong (Fanand 1960). Minong (Farand, 1960) - 47 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 PROFILES OF FORMER WATER PLANES BETWEEN THUNDER BAY AND NIPIGON (after Parrand 1960) io o I m S 80 60 40 20 0 e I I 50 40 30 20 I I It I I a Ifl •er t 0 S . r —1 S — — — —— 5— - 0 'r-• — =i_— I * • * 0 -. -c r0 I II — • r •—• I S I I 0 •V 0 r m r r 5• 5— • C t 5• - ———... —S •lI* i 00 C I 0 -I In In , S jI 0r I 0 •j_ UGHT0N.- •p_• — 'NIPISSIN0 ALGOMA S .—4.-—--.———— I 3 •___.____1_._. ——4- — . —.—-— •—• • S ION i—-— 55 5 • —e — —;__• S 5•— a, — • -T--I I r m fli I z 0 0 — * S C S In 0 z Figure 9. Profiles of former water planes in the Lake Superior basin from Thunder Bay and Kama Bay (after Farrand, 1960; Geddes et al., 1987). Figure 9. Profiles of former water planes in the Lake Superior basin from Thuder Hay and Kama Bay (after Farrand 1960, Geddes et al. 1987) Moraine south east of Black Sturgeon Lake that has a delta form occurs at an elevation of between 300 and 304 m. This feature would fall near to the isobase passing through Nipigon, essentially making the diagram one isobar off (Fig. 8C). Shore features identified as ice-marginal deltas at Marathon and Wawa, that were reported to be built into glacial Lake Minong (Geddes et al., 1987), may not be deltas at all, but subaqueous fans later planed off by waves of a subsequent lake. This is the case at Wawa (S. Slattery, personal communications, 2003). The sedimentology of the feature at Marathon remains to be studied in detail before such a conclusion can also be made but preliminary observations are not consistent with a deltaic origin. The Jim Nichols Trucking Limited sand and gravel pit provides a look at the internal structures of the sediments making up the Nipigon Moraine. In general he ridge here is made up of a fining upward and outward sequence of stratified sand and gravel. The lower part or core is composed of steeply dipping beds of gravel and sand (Fig. 10). These avalanche or foreset beds probably formed during rapid growth by sediment gravity flow processes. The beds generally dip to the south or across the ridge form. Trough cross-bedded (dunes) sand, pebbly sand and pebble gravel overlie the steeply dipping beds. The aforementioned sediment sequence is common in subaqueous fans (Shaw, 1985). Near the top and along the ice-proximal side (northside) minor flowtill lenses are present. Faults and postdepositional slumping are common along the proximal side of the ridge likely formed following the removal of the glacier support and sediment supply. The sequence is interpreted as an ice marginal subaqueous fan. If so, it only places a minimum (280 m asl) for the lake that existed during its formation. As mentioned previously, the water here during formation could be up to 20 m deep. A shorebluff and bouldery gravel beach bar of a younger lake occurs along the roadway at the entrance of the pit at approximately 250 m asl (Figs. 10 and 11). * Alternative stop - look-out over community of Nipigon from the Nipigon Moraine (UTM coordinates – 0407343E 5430274N). - 48 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 w Profile 260 broad shallow channel /sulting shoreline a £ C' = 240 0 a fining 220 •I.. N 4flntng 260 decrease dip of beds 200 U a i6oo 500 0 C a isào Distance (ni) D N —4 — - H -- im -- -I--'_- B Figure 10. Internal structures of the Nipigon Moraine, Jim Nichols Trucking Ltd sand and gravel pit: A – longitudinal view, B – steeply dipping avalanche bedding, C – faulting and slumping along northern slope of the ridge and D – schematic crosssection of the Nipigon Moraine at this site. Stop 2 – Pijitawabik delta UTM coordinates – 041382E 545403N The route from Stop 1 to Stop 2 crosses the Nipigon lowlands along the east side of Helen Lake. Glaciolacustrine fine-grained rhythmites occur beneath the surface of the plain until just north of Mignet Creek. At the break in slope about 400 m north of the creek, Farrand identified a Dorion shore feature at 800 ft asl or about 244 m (Figs. 11 and 12). Another prominent bluff, a Post-Minong shoreline, occurs at an elevation of between 907 and 912 ft (276-278 m) asl (Farrand, 1960). Farrand (1960, p.194) suggested that the “very extensive, near level surface of sand and some rather heavy gravel” about 25 km north of Nipigon is also a — Like Profile: Lake Superior to Pijitawabik Bay /elth? ]UL}\ B shore bluff Nipigon Moraine shore bluff?—) Pijitawabik Ray V/ H Helen Lake IOD I 0 Approximate utbem limit of the Pthtawabik Canyon ::::: I Distate (m) frail LAe Stwia Figure 11. Profile from Lake Superior to Pijitawabik Bay of Lake Nipigon following the Pijitawabik Canyon. Stop 1 is on the crest of the Nipigon Moraine, Stop 2 at the Pijitawabik delta and stop 3 within the Pijitawabik Canyon. Several features controlled by ancestral water levels in the Superior basin can be seen. - 49 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Figure 12. A fused DEM and hillshade of the Pijitawabik delta and the southern part of the Pijitawabik Canyon. product of glacial Lake Minong (Figs. 11 and 12). This feature has an elevation of 950-952 ft or about 290 m asl. At 980 ft or 299 m asl, Farrand (1960, p.194) identified another possible Minong feature within the Pijitawabik Canyon. Both these features are well inside or north of the Nipigon Moraine. The following quote from Farrand (1960) sums up some of his conclusions as to the origin and relationships that these deposits have with lakes in the Superior and Nipigon basins. “The 980-foot gravel accumulation lies nearly on the isobase of Terrace Bay where the Minong beaches lie at 950 to 1000 feet. Moreover, the abundance of sediment in the Pijitawabik Bay canyon implies the presence of the ice sheet either in Pijitawabik Bay itself or at least in the Lake Nipigon basin”. “These facts lead to the conclusion that the Nipigon basin was ice-filled (or nearly so) in Minong time. As lake levels fell in the Superior basin and the ice sheet retreated from the Lake Nipigon basin these basins apparently contained separate water bodies connected only by the ancestral courses of the Nipigon and Black Sturgeon rivers. In other words, there appears to be no validity in the assumption generally made (Stanley, 1932, p.135; Leverett, 1927, p. l.7) that the Nipigon and Superior basins were occupied at any time by one continuous lake” (Farrand, 1960, p.82). be sound, however convincing evidence that Stanley’s or Leverett’s assumptions had “no validity” was not substantiated. Down-cutting of the delta to the 290 m level may have been caused by a drop in base level as water in the Superior basin lowered in response to erosion at the outlet sill at Sault Ste Marie. The delta may have been fed from the glacier margin that remained within the Pijitawabik Canyon farther up the valley. Alternatively, during basin (Nipigon/Superior) separation, as an outlet stream from the lake Nipigon basin, or possibly built by an Agassiz drainage event through the valley. Stop 3 – Pijitawabik Canyon UTM coordinates – 0420816E 5462100N This stop may be a series of stops along the route The Pijitawabik Canyon is a deep, steep-walled valley cut into Nipigon diabase/gabbro sills. It is about 20 km long with a prominent near right-angle bend about 11 km from its south end. It can be considered to be made up of two segments; a southern segment that is about 2 km wide and a northern segment that is only about 1 km wide (Figs. 13 and 14). Local relief along the valley walls of both segments can exceed 150 m. The initial conclusions made by Farrand appear to - 50 - The valley is controlled by major structures in the �r Proceedings of the 51st ILSG Annual Meeting - Part 2 scJLT Pijitaabik Canyon ProfileA C 0— 285 m 292 m 10000 5000 UsIam(n4 Profile B Profile C 500 500 400 rTh 450 400 i 350 - 350 E30 250 10 E ooo 250 K4onielocs iS 20% 200 3000 4000 5000 20 0 1Q00 Profile D Profile E 450 350- 3000 4000 5000 Profile F - 450- Th 2000 (m) (m) 00 / E 350300 I I 500 1000 (m) 1500 I I 2000 2500 0 lOU 20(X) 310 4000 (m) 1000 2 3000 4 (m) Figure 13. Selected profiles across the Pijitawabik Canyon. bedrock. In addition, the rock characteristics of the Nipigon sills are such that weathering can be intense along fractures, producing grus-like residual soils that are easily removed by erosion. The Pijitawabik delta emerges from the southern end of the canyon and deltaic sediments occur within the canyon walls (Figs. 11, 12 & 13). Remnant terraces of sand and gravel (Figs. 12 & 13) fall from an elevation of about 305 m at the canyon bend to the delta surface at about 299 m at the mouth of the canyon. The ice margin feeding this system must have been just north of the bend as suggested by Farrand (1960), in the vicinity of this stop. The side canyon, Profile D (Fig. 13) is cut down to the level of the terraces only, so major down-cutting here, must have occurred during delta building. Stop 4 – Sand River Property, Roxmark Mines Limited UTM coordinates – 0424364E 5497579N Note - permission from the land owner is required for access to this stop. In 2003, Roxmark Mines Limited acquired the Sand River, Leitch and East Leitch properties from Rio Fortuna, Teck Cominco and Kinross, respectively (News Release, Roxmark Mines Limited, November 28, 2003) and began an advanced exploration program. In 2004, trenching and stripping at Sand River exposed vein systems filled with quartz. Visible gold occurs within fractures in quartz veins and most commonly is found along thin seams or septa of sericite and chlorite, along with arsenopyrite, pyrite and ankerite. Channel samples were cut every three feet. All composite assays used a minimum width of 1.0 feet. The #16-1 West Vein returned an average assay of 1.737 ounces Au per ton over a strike length of 42 feet. The #16-1 East Vein returned 0.347 ounce Au per ton over 37 feet. - Figure 14. View of the Pijitawabik Canyon looking north toward Lake Nipigon (photo courtesy of P. Kor, OMNR). The Sand River area was most recently mapped by Hart et al. (2002) and is underlain predominantly by clastic metasedimentary rocks. Thickly bedded, southward-younging, feldspathic wacke, sandstone, - 51 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 I i. :1 The rock surface has been polished and striated with at least two ice flow directions indicated (Fig. 15). The oldest set is oriented between 265-270°N and the youngest set at 290°N. Minor meltwater erosion forms are also present. The edges of the stripped area and trenches expose up to 2 m of glacial and postglacial sediments. II In one section, an interesting sequence of sediments is exposed. Resting directly on the bedrock surface is a thin (30 to 50 cm) layer of stony, slightly silty, sand diamicton interpreted as subglacial till (Fig. 16). Clast content is high and is composed of many locallyderived lithologies, dominated by metasedimentary rock fragments. The till is typical of areas of thin till within the Canadian Shield allowing for variations in lithology and stoniness. It has less than 2% carbonate content in the silt/clay fraction of the matrix. A sample of till from this layer contained 69% sand, 28% silt and 3% clay-sized particles (2 microns). In comparison a sample of till from Stop 6 contained 45% sand, 49% silt and 6% clay and had a matrix carbonate content of about 27%. Figure 15. Polished bedrock surface containing 2 sets of striations (photo courtesy of M. Smyk, OGS). and argillaceous rocks have been well exposed by the recent stripping operations. Bedding has eastnortheasterly to east-southeasterly orientations with steep, northerly dips. Well-developed, slatey cleavage, as well as local sericitization and carbonatization distinguish sub-parallel shear zones. Oblique cleavage - bedding relationships are locally exposed. Cleavage is refracted through interbedded siltstone and sandstone beds. Overlying the till layer is 30 to 50 cm of rhythmically bedded silt and clay. These sediments were deposited in an ancestral lake located in at least the Lake Nipigon basin during ice marginal recession. A channel sample of the rhythmites contained 7% sand, 77% silt and 16% clay and its carbonate content was about 30%. The rhythmites have been overturned toward the northwest (fold axis ~30°N) and are overlain by a thin layer of diamicton containing sorted lenses and wisps I Figure 16. “Local till” overlain by silt and clay rhythmites at the Sand River Property (photo courtesy of M. Smyk, OGS). Figure 17. Overturned rhythmites and overlying diamicton (photo courtesy of M. Smyk, OGS). - 52 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 till, overlies the bedrock surface. The bedrock surface is striated with the striations oriented at 245°N. The bedrock directly below the till is composed of fine to very fine grained sandstone, 20 to 30 cm thick, resting on Quetico metasedimentary rocks. The Archean - Proterozoic unconformity, an angular unconformity between the flat-lying Sibley Group sedimentary rocks and east-trending steeply dipping Quetico metasedimentary rocks, is exposed on both sides of Highway 11 (UTM 0423431E 5483881N). Figure 18. Transverse dune at stop 5. Inset shows internal structure (photos courtesy of M. Smyk, OGS). of sand and gravelly sand (Fig. 17). The diamicton may represent a minor fluctuation of the ice margin; overriding the glaciolacustrine rhythmites followed by ice marginal debris flow sedimentation (flowtills) or may be debris flows related to slumping or flowing of debris downslope; a post glacial event. The direction of overturning of the rhythmites, however, is similar to the direction of the younger set of striations and perpendicular to the slope direction at the site. Stop 5 – Nipigon dunes UTM coordinates – 0421102E 5497420N At this stop a road cut exposes the internal structures of a transverse dune that probably formed along the shore of an ancestral lake in the Nipigon basin (Fig. 18). Thorleifson and Kristjansson (1983) mapped out a shorebluff several metres above present-day Lake Nipigon. The till fizzes when a weak solution of hydrochloric acid (~10%) is applied. It contains about 27% carbonate (11% calcite and 16% dolomite) in the silt/ clay fraction (Chittick analysis). The till at Sand River (Stop 4) contained less than 2% carbonate. A sample of till from this site contained 45% sand, 49% silt and 6% clay-sized particles (2 microns). In comparison a sample of till from the Sand River Property (Stop 4) contained 69% sand, 28% silt and 3% clay. This exposure of thick, carbonate-rich “exotic till” is one of the furthest west exposures. This exotic till is best exposed farther to the east, between Geraldton and Jellico especially around Wildgoose Lake. It underlies a spectacular streamlined surface (Fig. 19) where “drumlins up to 1 km in length are scattered across the area, but the principal morphological feature on thick till deposits are flutes about 1 km wide and over 20 km long” (Thorleifson and Kristjannsson, 1983, p.54). Zoltai (1965a) noted that the Nipigon Moraine, west of Lake Nipigon, corresponded to the extent of carbonate-rich till and Karrow and Geddes (1986) used the extent of carbonate-rich till to suggest that the The second part of the stop is to a small field of dunes developed on an abandoned lake bottom that presently has an elevation of about 296 m asl. The route follows along the shorebluff mapped by Thorleifson and Kristjansson (1983), then crossing a subtle break in slope at about 178 m asl on our way to the dune field. Stop 6 – Carbonate till UTM coordinates – 0423418E 5483886N In the 7 m exposure on the west side of Highway 11, approximately 6 m of light brownish grey, massive to fissile, slightly silty to silty, very fine to coarse grained sand with pebbles, cobbles and boulders, interpreted as Tn 1 Figure 19. Streamlined till surface near Wildgoose Lake (photo courtesy of OMNR 74-4929-18-156). - 53 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Nipigon Moraine was the correlative of the Chapleau Moraine north of Lake Huron. Stop 7 – Nipigon Lowland Rhythmites UTM coordinates – 0407200E 5437660N This stop is to look at some of the sediments that underlie the Nipigon lowland. At this site, 0.6 to 1 m of apparently massive slightly clayey silt to silt containing freshwater molluscs overlies greater than 14 m of silt and clay rhythmites (Fig. 20). The upper 6 m of the section is well exposed in a drainage ditch just east of Highway 11. The rhythmites are slightly contorted at the top of the unit then become horizontally bedded with depth. Concretions have formed around rootlets below 4.5 m. In the woods to the east, a small abandoned sand and gravel pit occurs at the base of the bedrock-controlled hill and may possibly mark the position of the shore and level of the lake that the molluscs inhabited. Farrand (1960) identified a couple of terraces east of the highway and Helen Lake that he attributed to Nipissing/Algoma. Acknowledgements The writer would like to thank Mr James Nichols, Jim Nichols Trucking Ltd., Nipigon and Mr. Dave Malouf, Roxmark Mines Ltd., for kindly giving permission to visit their properties. Mr Phil Kor (OMNR) and Mark Smyk (OGS) provided most of the photographs used in the fieldguide and Mark kindly provided text on the bedrock geology for several of the field trip stops. Roxanne Corcoran (OGS) produced some of the line illustrations. All of their help is much appreciated. In addition, the indirect contributions of the contributors to the 1987 INQUA fieldtrip guide (Geddes et al., 1987) and Dr. W.R. Farrand (1960) for his work on ancestral lake levels in the Superior basin improved the content of this field guide. This fieldtrip guide was produced with the permission of the Director, Ontario Geological Survey. References Baker, V.R., Greely, R., Komar, P.D., Swanson, D.A., and Waitt Jr., R.B., 1987. Columbia and Snake river plains; in Geomorphic Systems of North America, Geological Society of America, Centennial Special Volume 2, p.403-468. Barnett, P.J. 1991. Quaternary geology of Ontario; in Geology of Ontario, Ontario Geological Survey, Special Volume 4, Part 2, p.1011-1088. Barnett, P.J., Henry, A.P. and Babuin, D. 1991. Quaternary geology of Ontario, west-central sheet; Ontario Geologic Survey, Map 2554, scale 1:1 000 000. Blackburn, C.E., Johns, G.W., Ayer, J.A. and Davis, D.W. 1991. Wabigoon Subprovince; in Geology of Ontario, Ontario Geological Survey, Special Volume 4, Part 1, p.303-382. Bostock, H.S. 1970. Physiographic subdivisions of Canada; in Geology and Economic Minerals of Canada, Geological Survey of Canada, Economic Geology Report no.1, p.10-30. Burwasser, G.J. 1979. Quaternary geology of the city of Thunder Bay and vicinity, District of Thunder Bay; Ontario Geological Survey, Geological Report 164, 70p. Dredge, L.A. and Cowan, W.R. 1989. Quaternary geology of the southwestern Canadian Shield; in Quaternary geology of Canada and Greenland, Geological Survey of Canada, Geology of Canada, no.1, p.214-249. Dreimanis, A. 1989. Genetic classification of tills; in Genetic Classification of Glacigenic Deposits, Balkema, Rotterdam, p.17-84. Drexler, C. W., Farrand, W. R. and Hughes, J. D., 1983. Correlation of glacial lakes in the Superior Basin with eastward discharge events from Lake Agassiz; in Glacial Lake Agassiz, J. T. Teller and L. Clayton (eds), Geological Association of Canada Special Paper 26, p.261-290. Figure 20. Example of fresh water molluscs collected from Stop 7. Elson, J.A. 1967. Geology of glacial Lake Agassiz; in Life, land and water, University of Manitoba Press, - 54 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Winnipeg, Manitoba, p.36-95. Farrand, W.R., 1960. Former shorelines in western and northern Lake Superior basin; unpublished Ph.D. thesis, University of Michigan, Ann Arbor, Michigan, 226p. Farrand, W.R. and Drexler, C.W., 1985. Late Wisconsin and Holocene history of the Lake Superior basin; in Quaternary Evolution of the Great Lakes, P.F. Karrow and P.E. Calkin, eds., Geological Association of Canada, Special Paper 30, p.17-32. Franklin, J.M., 1978. The Sibley Group, Ontario. In, Rubidium-strontium isochron age studies, Report 2, Geological Survey of Canada, Paper 77-14, p. 31-34 Geddes, R.S. and Kristjansson, F.J. 1986. Quaternary geology of the Hemlo area: Constraints on mineral exploration; Canadian Geology Journal of the Canadian Institute of Mining and Metallurgy, v.1, no.1, p.5-8. Geddes, R.S., Kristjansson, F.J. and Teller, J.T., 1987. Quaternary features and scenery along the north shore of Lake Superior; International Quaternary Association, INQUA’87, XII International Congress, Excursion Guide Book C-12, 62p. Hart, T.R., terMeer, M. and Jolette, C., 2002. Precambrian geology of Kitto, Eva, Summers, Dorothea and Sandra Townships, northwestern Ontario : Phoenix Bedrock Mapping Project; Ontario Geological Survey, Open File Report 6095, 206p. Hicock, S.R. 1988. Calcareous till facies north of Lake Superior, Ontario: Implications for Laurentide ice streaming; Géographie Physique et Quaternaire, v.42, p.121-135. Karrow, P.F. and Geddes, R.S. 1987. Drift carbonate on the Canadian Shield; Canadian Journal of Earth Sciences, v.24, p.365-369. Leverett, F., 1929. Moraines and shorelines of the Lake Superior basin; United States Geological Survey, Professional Paper, 154-A, 72p. Mollard, D.G. and Mollard, J.D. 1981. Frazer Lake area (NTS 52H/SE), District of Thunder Bay; Ontario Geological Survey, Northern Ontario Engineering Geology Study 42, 32p. Accompanied by Map 5052, scale 1:100 000. Ontario Geological Survey 2003. 1:250,000 scale bedrock geology of Ontario; Ontario Geological Survey, Miscellaneous Release – Data 126. Prest, V.K. 1970. Quaternary geology of Canada; in Geology and economic minerals of Canada, 5th edition Geological Survey of Canada, Economic Geology Report 1, p.676-764. Sado, E.V., Fullerton, D.S. and Farrand, W.R. 1994. Quaternary Geological Map of the Lake Nipigon 40X60 Quadrangle, United States and Canada; U.S. Geological Survey, Miscellaneous Investigations Series, Map I-1420 (NM-16), scale 1:1 000 000. Shaw, J. 1985. Subglacial and ice marginal environments; in Glacial Sedimentary Environments, Society of Economic Paleontologists and Mineralogists, Short Course no. 16, p.7-84. Slattery, S.R., 2003. Sedimentary architecture and Quaternary geology of the Longlac area, northcentral Ontario, Canada; unpublished MSc thesis, Laurentian University, Sudbury, Ontario, 125p. Smyk, M., Fralick, P.W., and Hart, T., 2005. The geology and gold mineralisation of the Bearmore-Geraldton greenstone belt. Institute on Lake Superior Geology 51st Annual Meeting, Proceedings Volume 51, Part 2 – Field trip guide, this volume. Sutcliffe, R.H. 1991. Proterozoic Geology of the Lake Superior Area. in Geology of Ontario, Ontario Geological Survey, Special Volume 4, Part 1, p.627661. Teller, J.T. and Mahnic, P. 1987. History of sedimentation in the northwestern Lake Superior basin and its relation to Lake Agassiz overflow; Canadian Journal of Earth Sciences, v.25, p1660-1673. Teller, J.T. and Thorleifson, L.H. 1983. The Lake Agassiz– Superior connection; in Glacial Lake Agassiz, Geological Association of Canada, Special Paper 26, p.61-290. Teller, J.T. and Thorleifson, L.H., 1987. Catastrophic flooding into the Great Lakes from Lake Agassiz; in Catastrophic flooding, Allen & Unwin, Boston, Massachusetts, p.121-138. Thorleifson, L.H. and Kristjansson, F.J., 1993. Quaternary geology and drift prospecting, Beardmore-Geraldton area, Ontario, Geological Survey of Canada, Memoir 435, 146p. Williams, H.R. 1991. Quetico Subprovince; in Geology of Ontario, Ontario Geological Survey, Special Volume 4, Part 1, p.383-405. Zoltai, S.C. 1965a. Glacial features of the Quetico–Nipigon area, Ontario; Canadian Journal of Earth Sciences, v.2, p.247-269. Zoltai, S.C. 1965b. Surficial geology, Thunder Bay; Ontario Department of Lands and Forests, Map S265, scale 1:506 880. Sado, E.V. and Carswell, B.F. 1987. Surficial geology of northern Ontario; Ontario Geological Survey, Map 2518, scale 1:1 200 000. - 55 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 - 56 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Fieldtrips 3 and 6 - A stratigraphic transect across the Northern flank of the Midcontinent Rift near Rossport Pete Hollings and Philip Fralick Department of Geology, Lakehead University, Thunder Bay, Ontario, P7B 5E1, Canada Safety As this trip will be taking place on Lake Superior it will be weather dependant and could be cancelled or curtailed at very short notice. An alternative trip to the Winston Lake mine will be undertaken in case of bad weather. Please exercise caution when getting in and out of the boats as the outcrops are often extremely slippery. Life jackets must be worn in the boats at all times. It will probably be very cold out on the lake so please dress warmly. Regional geology Archean granites, outcropping along the shoreline near Rossport, are unconformably overlain by strata of the Gunflint Formation (Fig. 1). These sediments were deposited on a south facing shelf at approximately 1878 Ma (Fralick et al., 2002). The Formation consists of a Lower Member composed of basal stromatolitic bioherms overlain by ankeritic, interclastic grainstones. A regressive, karstified surface caps the northern portion of this assemblage (Fralick and Barrett, 1995) and is succeeded by the Upper Member. It begins with a repetition of the underlying lithologies to which, higher in the succession, are added carbonaceous shales, tuffs and rarely mafic volcanic rocks. These chemical and fine-grained siliciclastic sediments record parasequence development on a storm-dominated shelf (Pufahl and Fralick, 2004) forming the relatively stable portion of a back-arc basin (Kissin and Fralick, 1994; Hemming et al., 1995) prior to compressive northward thrusting of the arc at approximately 1860 to 1835 Ma. As the compression of the Penokean Orogeny waned the sea again transgressed over the area depositing black, carbonaceous shales of the Rove Formation. This depositional cycle lasted from 1835 Ma (Kissin et al., 2003; Addison et al., 2005) till less than 1780 Ma ago (new detrital zircon laser ablation ICP MS data). The lower portion of the Gunflint Formation in the Rossport area is poorly exposed. The limited outcrop of the Lower Gunflint is similar to the succession comprising the thin, basal Kakabeka Conglomerate and overlying interclastic grainstones present in exposures to the west near Thunder Bay. Good exposure of the Upper Gunflint exists on Quarry Island and consists of possible basaltic flow rocks with associated stromatolites, overlain by a succession of mediumto coarse-grained, graded, sandstone beds. The geochemistry of the sandstones is similar to Archean rocks to the north indicating probable derivation from this source. Black shales, lithically correlative with the Rove Formation, outcrop on an island approximately 5 km to the west. The shales do not overly the turbidite succession on Quarry Island where arenites of the Sibley Group disconformably rest on an erosion surface at the top of the Gunflint sandstones. The basal unit of the Sibley Group is the Pass Lake Formation. It is composed of the conglomeratic Loon Lake Member and the overlying sandstones of the Fork Bay Member (Cheadle, 1986). Where the basal conglomerates are present they either represent: 1) large channel fills cutting down into sandstones to shales with abundant caliche zones, or; 2) more laterally continuous conglomerates interbedded and overlain by parallel laminated, medium-grained sandstones. The former represent channel gravels in braided fluvial systems and the latter coarse-grained strandline deposits. The overlying Fork Bay sandstones likewise record both braided fluvial deposition and subaqueous, strand proximal sand-sheet development. In addition to upward thinning and fining successions developed during transgressive systems tract formation, other sandstone assemblages thicken and coarsen upwards representing progradational, delta lobe outbuilding. It is not known whether the water body was lacustrine or marine, though preliminary isotopic analyses (C, O, S and Sr) are more in agreement with a hypersaline lacustrine setting for at least the later history of the water mass. This is consistent with Cheadle’s (1986) findings - 57 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 A B Figure 1b Osler Group 1108 Ma Nipigon diabase Stop 8 Sibley Group Lake Superior C 48°30ʼ a Animikie Group Figure 1c 1105 Ma Lake Superior 200 km Rossport N Stop 7 Quarry Island Stop 6 Simpson Island Stop 3 Vein Island Stop 2 Archean rocks a 15 km Stop 5 Stop 4 Stop 1 Channel Island Wilson Island 48°30ʼ Keweenawan intrusive rocks Osler Group volcanics Keweenawan sediments Sibley Group sediments Gunflint Formation Archean basement Copper Island 48°45ʼ 87°30ʼ 87°45ʼ 48°45ʼ 1 km Figure. 1. Map showing the location of the fieldtrip area. B) Regional geology map showing the extent of the Osler Volcanic Group. Age data from Davis and Sutcliffe (1985) and Davis and Green (1997). Modified after Sutcliffe (1986). C) Geological map of the Osler Volcanic Group showing sample locations. Modified after Giguerre (1975). based on regional paleogeography. The increasing salinity of the water resulted in dolomite, minor gypsum and rarer barite and celestite precipitation mixed with mud deposition. A red and green banding developed in this assemblage due to periodic anoxia of the bottom sediments. The final desiccation of the lacustrine basin is recorded by the development of strandline microbial bioherms (stromatolites) which are overlain by either a terra rosa (red, wind-delivered soil) or subareal, intraformational, mass-flow conglomerates. This is succeeded upwards by mudstones with abundant gypsum nodules representing mudflats formed in an arid climatic setting where hypersaline groundwaters precipitated gypsum. Together all these fine-grained sediments comprise the Rossport Formation. It is overlain by the Kama Hill Formation; a coarsening upwards deltaic succession recording flooding of the basin and development of a more humid climate. The age of the Pass Lake and Rossport Formations can be bracketed between laser ablation MS youngest ages on detrital zircons of 1600 Ma and a Rb-Sr isochron of 1339 Ma (Franklin, 1978). Youngest detrital zircons in the Kama Hill Formation are 1420 Ma. The Sibley Group is very well exposed along the shorelines of the islands off Rossport. The basal disconformity can be seen about two thirds of the way up the cliff face on the western side of Quarry Island where it overlies graded sandstone beds of the Gunflint Formation. Blocks of medium-grained sandstone were extracted from the cliff face on the island for use in the construction of buildings in Thunder Bay. These sandstones are medium- and large-scale planar crossstratified and may represent a sandflat composed of transverse bars in a braided stream or small barchan, eolian dunes. Rare pebbles indicate the former may be the case but this evidence is not conclusive. Channel and Copper Islands contain excellent exposures of the lacustrine rocks with outbuilding of channel systems along the paleolake margins. One of the best outcrops of the strandline stromatolitic carbonates occurs on Channel Island and will be visited during this fieldtrip. On Copper Island the Rossport Formation is overlain - 58 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Logan Sills (1109+4/-2 Ma, Davis and Sutcliffe, 1985), the lower portion of the North Shore Volcanics (1108±2 Ma, Davis and Green, 1997), the Swamper Lake Gabbro and Nathan’s Series intrusives (1107 Ma, Paces and Miller, 1993) and the lower portion of the Powder Mill Group (1107±2 Ma, Davis and Green, 1997). These older units outcrop on the rift flanks where erosion has removed the younger rift sequence or, in the case of the The oldest rift-related rocks on which U-Pb age Coldwell and Logan Sills, are intruded into older rocks determinations have been performed lie along the immediately north of the rift. northwestern portion of the rift. These include, from NE The Osler Volcanic Group comprise a ~3km thick to SW, the alkaline intrusives of the Coldwell Complex (1108±1 Ma, Heaman and Machado, 1992), the lower sequence (Cannon et al., 1989) lying unconformably Osler Group volcanics (1108+4/-2 to 1105±2 Ma, above Sibley Group metasediments (Fig. 2). The Davis and Sutcliffe, 1985; Davis and Green, 1997), the volcanic sequence is overlain and intruded by the disconformably by pebbly, fluvial conglomerates of the basal Osler. Thirty kilometers to the west the uppermost unit of the Sibley, the Nipigon Bay Formation, underlies the same disconformity. This highlights the fact that approximately 600 m of erosive downcutting occurred in the Rossport area before the basal Osler was deposited. V 1095 N Copper Harbour Conglmerate Portage lake Volcanics IV Portage Lake IV Volcanics Group 7 V (Group 8) III Group 6 Upper Suite R R Kallander Creek Volcanics Siemens Creek Volcanics Beaver Bay Complex Mostly basalt units Osler Group 1105 Schroeder Basalts V Duluth Complex Great Conglomerate and Group 5 Central Suite III II I Bessemer Quartzite 1110 Lower Suite I Simpson Isl Cgl Nipigon Sills Groups 3,4 Group 2 Group 1 IV IV IV 1100 NE Minnesota SW limb III II I IV North Shore Volcanic Group Copper Harbour Conglomerate 1090 Mamainse Point Michipicoten Island Michipicoten Island Formation Isle Royale 1085 Age (Ma) Isle Royale Black Bay Peninsula Lake Nipigon Upper Michigan NW Wisconsin Elyʼs Peak Basalts I, II, III Nopeming sandstone Archean Basement Figure 2. Schematic correlations of volcanic rocks of the Midcontinent Rift based on the stratigraphic position of distinctive basalt sequences, magnetic polarity and absolute age where possible. Modified after Nicholson et al. (1997). Dashed lines in Upper Michigan section separate lower and upper members of Kallander Creek and Siemens Creek volcanics. Left hand column shows magnetic polarity. Roman numerals I-IV refer to five distinctive laterally extensive basalt compositions identified on the south shore of western Lake Superior. Where equivalent basalt compositions occur in other stratigraphic successions, the appropriate Roman numeral is noted (see Nicholson et al., 1997 for data sources). Shaded regions represent intervals in which contacts are covered or obscured by plutonic rocks. - 59 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 St. Ignace Island Volcanic-Plutonic Complex an intercalated sequence of basaltic rocks and rhyolitic flows (Sutcliffe and Smith, 1988). Detailed descriptions of the Osler Group have been provided by McIlwaine and Wallace (1976) and Lightfoot et al. (1991). Generally the mafic flows of the Osler Group consist of massive to amygdaloidal flows, with locally developed ropey tops and pahoehoe textures (Sutcliffe and Smith, 1988). The flows range in thickness from 5cm to 30m (Lightfoot et al., 1991) with a regional dip of ~6-15° S (Giguerre, 1975; Lightfoot et al. 1991; Hollings et al., 2005). The majority of the exposed section is magnetically reversed with only the upper 100m displaying a normal polarity (Halls, 1974). The contact between the two units is marked by the presence of the Puff Island conglomerate and a discordance between the basalt flows above and below the contact. This has been interpreted as representing a significant break in the eruption history. A felsic porphyry near the base of the Osler Group has yielded an age of 1107.5+4/-2 Ma (Davis and Sutcliffe, 1985) whereas zircons from the Agate Point rhyolite towards the top of the reversely magnetized sequence have yielded an age of 1105±2 Ma (Fig. 1b; Davis and Green, 1997). Within the Osler Group interflow sediments are typically thin and of limited extent. Field descriptions of the sedimentary successions appear in Giguere (1975) and McIlwaine and Wallace (1976). They show that there are two main zones of sedimentary rocks within the Osler Group. One occurs near the base of the volcanic pile. The other is present approximately 2700 meters higher in the succession marking the paleomagnetic reversal. Lightfoot et al. (1991) in a study of the Osler Volcanic Group exposed along the shores of the Black Bay Peninsula to the west of the field trip location proposed that the major and trace element geochemical data could be used to subdivide the flows into an Upper, Central and Lower Suite although the boundaries between the suites were not clear cut. While the geochemical compositions of the Central (750900m) and Upper suites (1900-3000m) overlap their Lower Suite (0-750m) is distinguished by elevated Mg numbers (0.55-0.7 versus 0.3-0.6), lower Al2O3 (8-12 wt% versus 13-17wt%), lower Th/Nb ratios (0.090.70 versus 0.3-0.6) but higher Gd/Ybn ratios (3.5-4.5 versus 1.6-2.6). Nicholson et al. (1997) concluded that there were five geochemically distinct flood-basalt compositions within the Mid-continent rift that are common to most sections and appear in approximately the same stratigraphic order (Fig. 2) They recognized a lower suite in the Siemens Creek Volcanics (Basalt Type 1; Fig. 2), which they suggest is analogous to the Lower Suite of Lightfoot et al. (1991). In the United States this unit is less than 100m thick whereas the Lower Suite of Lightfoot et al. (1991) is ~750m thick. They report a narrow range of εNd(1100) values for the Siemens Creek Volcanics of -0.7 to +0.7. Nicholson et al. (1997) further suggest that there is a broadly recognizable suite of basalts above this (Basalt type II) which includes the upper Siemens Creek Volcanics and in the upper part of the Grand Portage lavas (Fig. 2). The suite is characterized by slight negative Nb anomalies and a range of εNd(1100) values of -1.4 to -6.9. They suggest that this may be analogous to the most primitive members of the Central Suite of Lightfoot et al. (1991). The volcanic flows of the Osler Group on Wilson Island are all basalts or basaltic andesites (SiO2 = 4756 wt%; MgO = 5-16 wt%; Hollings et al., 2005). The basalts are characterized by LREE enrichment (La/Smn = 1.5-3.9) in conjunction with moderately fractionated HREE (Gd/Ybn = 1.5-3.7) and slight positive to moderately negative Nb anomalies (Nb/ Nb* = 0.56-1.13; Hollings et al., 2005; Fig. 3). Major and trace element data show trends of increasing SiO2 and decreasing MgO and display strong positive correlations between La/Smn, Th/La and Th/Nb with height (Fig. 4). This correlation is most pronounced above 400m with samples from the base of the stratigraphy displaying more or less constant values of these ratios (Fig. 4). Measured 143Nd/144Nd ratios for the seven Osler basalts analysed range from 0.5118570.512286 with εNd(t=1106Ma) of +0.3 to -5.3 (Hollings et al., 2005). The high incompatible element abundances, in conjunction with LREE enrichment and strongly fractionated HREE are comparable to modern OIB, albeit at lower absolute abundances (Fig. 3). When compared to other flood basalt sequences the more primitive basalts from this study closely resemble basalts from the Parana-Etendeka flood basalt sequence (Fig. 4; Gibson et al., 2000). The εNd data from the most primitive members of the Osler Group is consistent with an enriched mantle plume rather than a contaminated depleted mantle source, given the lack of trace element evidence for contamination in these samples. Depleted mantle at 1100 Ma would have had a positive εNd perhaps as high as +6 whereas an - 60 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 by an older lithospheric component characterized by pronounced LREE enrichment, high Th abundances but generally unfractionated HREE (Hollings et al., 2005). 100 The sedimentary successions near the base of the Osler Group constitute the Simpson Island Formation and have recently been described in detail by Hollings et al. (2005). They are composed of a Lower Member dominated by trough cross-stratified, medium-grained sandstones directly overlying basement and an Upper Member with a greater variety of siliclastic units. The Lower Member sits on an irregular, erosional surface cut into the underlying quartz arenites of the Nipigon Bay Formation, Sibley Group. A massive pebble-cobble conglomerate overlies the unconformable surface and is in turn overlain by decameter-scale layers of coarse-grained and pebbly sandstone (Fig. 5, Section 1). Sandstones are parallel laminated, commonly have cross-stratified tops, more rarely contain pebbly transverse ribs and chute and pool-like structures. The central portion of the succession is composed of trough cross-stratified, medium-grained sandstone organized into a stacked assemblage of lenses. Pebble stringers and pebbly sandstones commonly occur on the deeper portions of curving set boundaries (Fig. 5, Section 2). Massive pebble-cobble conglomerates sharply overly the sandstone succession (Fig. 5, Sections 2 and 4). The conglomerates contain trough cross-stratified, mediumgrained sandstone lenses; decameter- to meter-scale wedges of planar cross-stratified sandstone and are interbedded with assemblages of trough cross-stratified sandstones up to one meter thick. Another assemblage of trough cross-stratified sandstone, similar to the one in the central portion of the succession, caps the basal sedimentary assemblage (Fig. 5, Sections 3 and 4). Clast lithologies in the pebble-cobble population are dominated by quartz, chert, various types of volcanic 10 Hawaiian OIB Deccan Traps CFB Parana-Etendeka CFB 1 Th Nb La Ce Pr Nd Zr Hf Sm Eu Ti Gd Tb Dy Y Ho Er Tm Yb Lu Al V Sc Rock/Primitive Mantle 100 10 Type 1 Lower Suite 1 Th Nb La Ce Pr Nd Zr Hf Sm Eu Ti Gd Tb Dy Y Ho Er Tm Yb Lu Al V Sc 100 10 Type 2 1 Central Suite Th Nb La Ce Pr Nd Zr Hf Sm Eu Ti Gd Tb Dy Y Ho Er Tm Yb Lu Al V Sc Figure 3. Comparison of primitive mantle normalized plots from the Osler Group with A) Phanerozoic OIB and Continental Flood Basalts (CFB) and B & C) the Lower and Central suites of Lightfoot et al. (1991). From Hollings et al. (2005). enriched plume source would have εNd ~0 (Nicholson and Shirey, 1990; Shirey et al., 1994). Up sequence the basalts are characterized by higher SiO2, Th and La/Smn abundances in conjunction with increasingly negative Nb and Ti anomalies and εNd(1106) values of -4 to -5. This is consistent with contamination of these basalts 900 800 SiO2 MgO Fe2O3 La/Smn Th Gd/Ybn Th/La εNd Th/Nb 700 600 500 400 300 200 100 0 40 50 60 5 10 15 10 15 1 2 3 4 1 2 3 4 1 2 3 4 0.05 0.10 0.15 0.20 0.10 0.15 0.20 -6 -4 -2 0 Figure 4. Geochemical stratigraphy of the Osler Group on Vein and Wilson Islands. The stratigraphic position of the samples has been calculated assuming a dip of 10° parallel to the section. From Hollings et al. (2005). - 61 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 V V V V V V V V V 6 No Longitudinal 24 Complex V V V V V Major 12 V V V V 6 V V V V V V 18 V 3 0 Section 5. V V No O/C V V V V V Nipigon Bay V V V Ripples Pebbles Hummocky Cross-Strat. V V V V V No O/C ? m. m V V V V V Complex No O/C ? m. Sandy Sheetfloods 6 Sheetfloods and Debris flows 3 V V V V V V V V V V Conglomerate Pebbly V. Coarse Medium V. Fine Siltstone Shale Sst. V V V V V V 7 15 V V V 8 m. No O/C 12 21 18 15 12 Bar V 0 Massflow 3 m. No O/C V V V V V V V V V V V V V 5 6 V V V V V V 15 V V V V V V V 10 km Older Units Section Locations Section 7. V V V V V V V V V V Sand Channels with Small Gravel Longitudinal Bars Stacked Channels 9 Sheetflood Sands with Channels 6 Small Stacked Channels Distal 6 V 4 2 1 3 VV V Sedimentary Rocks Igneous Rocks 1 27 Sandy Channels to Distributary Mouth Bar 9 V m Gravel Channels 26 V V V V V Osler Group 24 29 23 V V Section 6. 32 Longitudinal No O/C Trough Cross-Strat. Small Irregular Lenses Paleocurrent Direction V V V V V V V V V V V V V V V V V V V V V V V V V V Bar 0 0 Complex Rhyolite V V V V =318o 15 9 Longitudinal Parallel Lamination V V V V Bar Tail Sand Sheet Longitudinal Bar Nipigon Bay V Sandy Channel 3 Bar Sandy Channel V Section 4. m Basalt INTERFLOW SEDIMENTS m V Channel O/C =268o V 9 3 0 V l e g e n d 18 No 6 V V Sst. V 9 V Major Sandy Channel 0 Bar V m V 3 O/C 15 Section 1. V V No O/C V n = 38 V V 21 o Section 3. m V = 265 27 Section 2. Conglomerate Pebbly V. Coarse Medium V. Fine Siltstone Shale N m V SIMPSON ISLAND FORMATION ( Basal Sediments ) 3 0 Massflow Sheetflood Sands with Channels Figure 5. Sections of sedimentary rocks in the Osler Group. Sections 1 through 4 are the basal sedimentary succession of the Lower Member, Simpson Island Formation, at different locations (see inset map). Sections 5 and 6 are of the Upper Member, Simpson Island Formation, interlayered with basal basalt flows. Section 7 is the sedimentary assemblage near the top of the Osler Group on Puff Island. From Hollings et al. (2005). - 62 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 rocks, red siltstone, metamorphosed granite and at the east end of the outcrop belt, on Copper Island, a higher proportion of unmetamorphosed red granite. Current indicators show flow was to the west, averaging 265°. A sedimentary assemblage also occurs near the upper limit of outcrop of the Osler Formation, at the top of the magnetically reversed interval (Fig. 5, Section 7). These interflow sediments are located on Puff Island and overly a felsic porphyry with a U-Pb age of 1105 Ma (Davis and Green, 1997). They contain: sharp sided assemblages of laterally continuous, pebbly, coarsegrained sandstone beds with caliche horizons which are scoured into by small lenses of conglomerate; large scours filled with trough cross-sets over a meter thick; stacked assemblages of irregular lenses filled by trough cross-stratified, coarse-grained, pebbly sandstone; and, poorly sorted, disorganized, massive boulder-cobble conglomerate. Clasts are all volcanic, ranging from quartz-feldspar porphyries to mafic compositions. Paleocurrents on large-scale sedimentary structures consistently show flow to the southeast. Island. Stop 1 – Osler volcanics, Wilson Island UTM coordinates – 0462794E 5402095N Flows of the Osler Group on Wilson Island are typically >1m thick, frequently amygdaloidal towards the top and bottom of the flow with a massive core and rarely displayed a pahoehoe texture on the flow surface. The basalts are characterized by clinopyroxene and plagioclase phenocrysts in a groundmass of plagioclase, augite and Fe-Ti oxides. Rarely, basalts from the base of the sequence contained pseudomorphed olivine phenocrysts. The basalts have all been subjected to low-grade metamorphism ranging from zeolite to prehnite-pumpellyite facies (McIlwaine and Wallace, 1976). At this stop, approximately 500m above the basal conglomerates, are exposed a sequence of thin rubbly basalt flows ~50cm thick with rare massive flows ~2-4m thick and thin interflow sediments (Fig. 6). Geochemically basalts at this outcrop are similar to the Central Suite of Lightfoot et al. (1991). Sedimentary units are predominantly quartz sandstones with thin shale partings. These are best interpreted as sands washing into small hollows on the surface of the flow units with a mud drape settling out towards the top of the layer. In places units that appear to have been deposited on the surface of basalt flows connect into sub-vertical cooling cracks in the flows. Sediment filled cooling cracks can be up to 2m deep (Fig. 7). cr1 The Simpson Island Formation is composed of a laterally continuous sedimentary succession up to 25 meters thick and discontinuous sedimentary units up to 30 meters thick interlayered with the basal basalt flows. The lowest sedimentary beds fill channelways cut into the underlying sandstones of the Nipigon Bay Formation. The channel fills and overlying sedimentary assemblage represent a braided stream system, similar to the South Saskatchawan model (Miall, 1978), where dunes composed of coarse-grained sand migrated Stop 2 – Osler Volcanics, North end of Wilson down the channels and gravelly longitudinal bars with Island chute channels and bar edge sand wedges form the UTM coordinates – 0461810E 5403438N higher relief areas (Fig. 5). The fluvial interpretation Exposed at this outcrop, are the lower flows of the is consistent with Tanton (1931) and McIlwaine and Osler Volcanic Group ~300m above the conglomerates Wallace (1976). Clast lithologies indicate debris was mainly derived from erosion of local lithologies. C I Stops The trip will depart from the public dock at Rosport and will undertake a traverse through the stratigraphy of the Midcontinent Rift, starting with the youngest rocks of the Osler Volcanic Group, proceeding through the Sibley and Gunflint Formations and finishing with a look at the granites of the Archean basement (Fig. 1). In order to make the most of the calmer weather typical of early mornings we will first travel for approximately Figure 6. Thin interflow sediments between basaltic flows, 30 minutes to the most southerly outcrop on Wilson Stop 1 on Wilson Island. - 63 - �I ') i 1 Proceedings of the 51st ILSG Annual Meeting - Part 2 Figure 8. Pahoehoe texture at Stop 2, Wilson Island. Figure 7. Approximately 2m thick mafic flow cut by sediment filled cooling crack. Stop 1, Wilson Island. of the Upper Simpson Island Formation. The basaltic flows are generally massive, ranging in thickness from 1-3m. Flow tops vary from rubbly to well-developed pahoehoe textures (Fig. 8). The basalts are vesicular and amygdaloidal and in places the vesicles are elongated giving them almost a pipe-like appearance. Geochemically basalts at this outcrop are similar to the Central Suite of Lightfoot et al. (1991). In some areas the basalt flows incorporate xenoliths of a more vesicular material which have both sharp and diffuse contacts (Fig. 9) At the north end of the outcrop the flows are cut by a 2-3m wide mafic dyke. This dyke is geochemically distinct from the flows but comparable to the older diabase intrusions in the vicinity of Lake Nipigon. tel Figure 9. Amygdaloidal xenoliths in basalt flows at Stop 2, Wilson Island. sandstone beds (Figs. 10, 11). A covered interval separates this assemblage from overlying mediumto large-scale, trough cross-stratified, coarse-grained sandstones to conglomerates (Fig. 12). Paleocurrent indicators show flow to the west, though with a higher variance than other sections. Clast lithologies Stop 3 – Upper Simpson Island Formation, Daylight are probably locally derived from both Archean and Point, Wilson Island Proterozoic sources. The fine-grained sandstone near the base of the section represents a wave modified deltaUTM coordinates – 0461450E 5404650N front (ie., a distributary mouth bar of a small delta). A sedimentary assemblage of the Upper Member occurs on Wilson Island, overlying approximately The presence of small, dish-shaped scours suggests a 50 meters of basal basalt. This coarsening upwards shallow water environment with no large channels. The succession has oscillation rippled, very fine-grained upper part of the sequence represents badly organized sandstones at its base (Fig. 5, Section 6). These coarsen river deposits with gravelly, longitudinal bar forms and upwards by the addition of increasing amounts of channels filled with sand. The planar cross sets at the medium-grained, parallel laminated to hummocky base of the cliff were formed by transverse bars, while cross-stratified to oscillation rippled, decimeter-scale the trough cross beds represent migrating dunes. - 64 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 red siltstones outcrop on the shoreline here (Fig. 13). These are interpreted to be part of the cyclic facies (Channel Island Member) of the Rossport Formation as exposed at Kama Hill. The dolomite and minor gypsum indicate a hypersaline environment interpreted to be lacustrine because of the multiple deltas and sand sheets building in from a variety of directions. The cyclic facies is overlain by stromatolites and interbedded thin, sandy, carbonate storm layers (Fig. 14) of the Middlebrun Bay Member. This meter thick Figure 10. Fine-grained red sandstones with thin shale assemblage is similar to Recent sabkha deposits on the partings forming the base of the deltaic deposits at Stop 3. south shore of the Persian Gulf, and in particular the open water ponds on this sabkha where sandy storm layers are well preserved. The upper few centimeters of the stromatolitic unit is altered to a grey-green layer that represents a weathered horizon interpreted to ., 4r 17 ; Figure 11. Oscilation ripples with overlying hummocky, medium-grained sandstone showing the effects of wave reworking on the sediments forming the delta front at Stop 3. Figure 12. Cross-stratified sandstone and massive conglomerate forming the upper portion of the prograding deltaic succession present in the Upper Member of the Simpson Island Formation present at Stop 3. These sediments represent a longitudinal bar-channel complex of a braided stream. Stop 4 – Sibley Group, Mary Ann Bay, Channel Island UTM coordinates – 0462831E 5405801N Note - this is a very small outcrop and access to the rocks may be limited if lake levels are high. Please take care moving from the landing point to the outcrop. A succession of grey dolomites interbedded with Figure 13. Interlayered red siltstones and dolostones (lower unit underlying the more massive strandline carbonate with overlying mass-flow deposits) were deposited in a saline lake away either temporally or spatially from areas of coarse sand influx. The colour banding reflects the position of the redox boundary as the sediments accumulated. The grey layers commonly have slightly higher dolomite contents possibly reflecting higher organic productivity leading to more photosynthetically mitigated carbonate precipitation (higher dolomite content) and heavier organic loading to the sediment (redox boundary moving upward to at or above sediment water interface). Stop 4, Mary Ann Bay. - 65 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 ________ U t1 ii' IIIIII!! CENTIMETRE Figure 14. Stromatolitic layering (smooth mat with small pinnacles) with interbedded coarse silt to very fine-grained sand storm layers (white). This is typical of strandline to sabkha environments and especially open water ponds on the sabkha. Some of the storm layers were remobilized in the form of clastic dykes and sills. www. ]akcu pC 1 recj Ic IgyUIr have formed as the sequence became subaerial and the stromatolites weathered in situ. This weathered zone is traceable throughout the basin with the strandline deposits below it commonly containing well developed tepee structures. The carbonates are overlain by a massflow unit with intraformational clasts of red siltstone, sandstone and dolostone up to boulder size. Although the contact between the carbonates and the mass flow units is locally obscured by the intrusion of a sill, the transition is interpreted to represent a minor time gap based on the weathered zone which expands to thick terra rosa (soil) deposits at other locations. Stop 5 – Gunflint Formation, Quarry Island UTM coordinates – 0462305E 5406730N A succession of sandstones and mafic volcanic rocks outcrop on the south shore of Quarry Island. On the northeastern end of the outcrop area a gabbro, probably related to the Midcontinental Rift, is exposed. Next to this is a small outcrop of stromatolites with a box-like appearance (Fig. 15). The rectangular to square outline of the mounds contrasts with the round to oval appearance of classic stromatolites, though their organic origin is exemplified by the high angle layering, which, when projected into the area now eroded, can be seen to form mounded structures. Areas between the stromatolites are infilled with coarser siliciclastic sandstones and cherty clasts. The next outcrop of Figure 15. Odd shaped structures of probable stromatolitic origin. within the Gunflint Formation. Stop 5, Quarry Island Gunflint volcanic rocks is problematic. Mafic volcanic flow rocks occur interbedded with Upper Gunflint lithologies southwest of Thunder Bay. These are also associated with stromatolites that developed on the firm substrate of the flow tops. Thus, the igneous rocks in the Gunflint assemblage on Quarry Island could be correlative to the other flow rocks, but it is difficult to conclusively show that these rocks are extrusive. Possible flow banding is present, as are areas of finer and coarser material in individual units. The igneous rocks are overlain by medium- to coarse-grained sandstones with bed thicknesses averaging approximately 30 cm. The sandstones are dark in appearance giving the impression they were derived from mafic detritus, but their geochemistry indicates an intermediate source similar in composition to the Archean crust to the north. The layers are excellently graded (Fig. 16) with the only sedimentary structure being sporadically developed parallel lamination. Beds such as these are commonly thought of as typical turbidites and the deposits ascribed to reasonably deep water. However, it must be remembered that graded bedding simply means a decelerating flow deposited the bed, which can occur in any water depth. These beds may be tempastites, ie. beds formed by storm events, in this case in water deeper than storm wave base but certainly not anything approaching abyssal depths. Or they may have formed - 66 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Figure 16. Well graded sandstone layers. Though these are probably turbidites they were not deposited in excessively deep water and may, in fact, represent distal tempestites. Stratigraphic position is problematic as these beds may belong to either the Upper Gunflint or Rove Formations. Stop 5, Quarry Island. from inter- or overflow sediment-water plumes off river mouths, though lack of current reworking of bed tops makes this unlikely. Alternatively they may represent prodelta deposits formed by slumping of the delta front. All of these environments are relatively shallow which agrees with the presence of stromatolites not far stratigraphically below the graded beds. Another Figure 18. A second example of unusual marking observed at Stop 6. The origin of these markings is unclear. Stop 5, Quarry Island interesting point concerning these clastic units is that although such sandstones are common in the upper Rove Formation they are not present in the Gunflint at any other location. Thus, their stratigraphic position is debatable. The third unusual attribute is the presence of difficult to interpret structures on some bedding planes. Series of enechelon small crack-fill like features cut across bedding planes (Fig. 17). In addition a jellyfishlike impression was found on a bedding plane (Fig. 18). This feature had five-fold symmetry, similar to echinoderms, but the age of the rocks and its presence in sandstone leads to the distinct possibility that it was manufactured. Stop 6 – Pass Lake Formation, Quarry Island A 50 meters inland from the last shore outcrop (Stop 5) where the iron train rail lies across the cobble-boulder beach - this stop will be time dependant. This outcrop consists of a cliff-face in sandstones, which were quarried, and the blocks produced used in the construction of buildings in Thunder Bay (Fralick et al., 2000). Here we see the basal sediments of the Sibley Group, the Pass Lake Formation. The Pass Lake forms a diverse group of basal coarse clastic deposits representing environments ranging from braided fluvial through to subaqueous sand sheets. The mediumgrained sandstones present in this cliff are organized into a series of large-scale planar cross-stratified sets with normal to low dip angles. Sorting is fairly good and only one pebble has been found in the succession. Assemblages such as this pose a dilemma in formulating Figure 17. Unusual markings on the bedding planes of the an interpretation of their depositional environment. graded sandstones. Stop 5, Quarry Island. Both aeolian sand dunes and sandflats composed of - 67 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 i —I • , • —l •d• Figure 19. Basal conglomerate of the Gunflint exposed at Gut Point. Photo courtesy of Mark Smyk. transverse bars in braided rivers are capable of creating such an organization of lithofacies. The presence of the pebble is significant as only freak wind-storms, such as tornadoes, can move material of this size, and these do not form sand dunes. So, it is more likely that these deposits are subaqueous but this rests only on a slim piece of evidence. Figure 21. Black chert with gossan within the basal conglomerate of the Gunflint Formation at Gut Point. Photo courtesy of Mark Smyk. is matrix-supported, with subangular to rounded pebbles of white, sugary quartz, lesser cherty and lithic fragments and minor jasper in a medium-grained, sandy matrix (Fig. 20). A black, pyritiferous chert breccia is marked by a conspicuous gossan (Fig. 21). Sulphide mineralization may be related to a persistent, parallel fracture set at 115°. Fractures may host quartz-calcitebarite veins ranging from < 1 to 20 cm wide as well Stop 7 – Basal conglomerate of the Gunflint, Gut as vein breccia. A conjugate fracture set at right angles to the first is locally developed. Silicification adjacent Point to the veins has preserved a thin (1 to 5 cm) veneer of UTM coordinates – 0461610E 5408587N Gunflint from being eroded (especially the carbonate The unconformity and basal Gunflint are exposed units). A 2m wide diabase dyke strikes at 115° through at Gut Point as a thin, discontinuous veneer along the the outcrop. lakeshore on top of Archean basement (Fig. 19). The basement is a medium-grained, equigranular granite, Stop 8 – Archean basement, Selim Point which has been altered (sausseritized/chloritized) UTM coordinates – 0469219E 5409146N beneath the basal Gunflint. The basal conglomerate is up to 30 cm thick and occupies depressions in the paleoFrom the dock in Rosport return to Highway 17 and erosion surface in the basement. The conglomerate head east for ~5 km. Turn right on to Lakeshore Drive just west of Whitesand Provincial Park. Follow the dirt road to a parking spot opposite a small tombola (Fig. 22). The porpyritic granite exposed here is Archean in age and part of the Wawa Subprovince. The area was mapped by Carter (1988) who described the rocks as porphyritic pink, hornblende + biotite alkali feldspar granite, a phase of the Whitesand Lake Batholith. The porphyritic “facies” is surrounded by massive pink and grey phases of alkali feldspar granite that is not exposed at this locality. The batholith is about 8 x 16 km in size and intrudes the Schreiber greenstone belt; no radiometric date has been generated. Feldspar Figure 20. Matrix supported basal conglomerate of the phenocrysts are typically 3-4 cm across (Fig. 23), Gunflint Formation containing rounded pebbles of quartz, subhedral to euhedral and in places appear to display chert and lithic fragments. Photo courtesy of Mark Smyk. localized alignment suggestive of flow banding. - 68 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Morey, G., Sutcliffe, R., and Spencer, C., 1989. The North American Midcontinent Rift beneath lake Superior from GLIMPCE seismic reflection profiling. Tectonics, 8, 305-332. Carter, M.W. 1988. Geology of the Schreiber-Terrace Bay area, District of Thunder Bay; Ontario Geological Survey, Open File Report 5692, 287p. Cheadle, B.A., 1986. alluvial-playa sedimentation in the lower Keweenawan Sibley Group, Thunder Bay District, Ontario. Canadian Journal of Earth Sciences, v. 23, p. 527-542. Davis, D.W., and Green, J.C., 1997. Geochronology of the North American Midcontinent rift in western Lake Superior and implications for its geodynamic evolution. Canadian Journal of Earth Sciences, 34, 476-488. Davis, D.W. and Sutcliffe, R., H., 1985. U-Pb ages from the Nipigon plate and northern Lake Superior. Geological Society of America Bulletin, 96, 1572-1579.Davis, D.W., and Green, J.C., 1997. Geochronology of the North American Midcontinent rift in western Lake Superior and implications for its geodynamic evolution. Canadian Journal of Earth Sciences, 34, 476-488. Figure 22. Porphyritic Archean granite, Selim Point. Fralick, P.W. and Barrett, T.J., 1995. Depositional controls on iron formation associations in Canada. In, ed by A.G. Plint, Sedimentary Facies Analysis, Special Publication of the International Association of Sedimentologists, v. 22, p. 137-156. Fralick, P.W., Kissin, S.A. and Davis , D.W., 2002. The age of the Gunflint Formation, Ontario, Canada: single zircon U-Pb age determinations from reworked volcanic ash. Canadian Journal of Earth Sciences, v. 39, p. 1085-1091. Figure 23. Feldspar phenocrysts in Archean porphyritic granite at Selim Point. Acknowledgements We would like to thank Mark Smyk and John Scott for their help and advice in the preparation of this field guide. In particular Mark Smyk provided text and photographs for Stop 7. References Addison, W.D., Brumpton, G.R., Vallini, D.A., Davis, D.W., Kissin, S., Fralick, P.W., McNaughton, N.J., and Hammond, A., 2005. Discovery of distal ejecta from the 1850 Ma Sudbury impact event. Geology, 33, 193-196. Cannon, W., Green, A., Hutchinson, D., Lee, M., Milkereit, B., Behrendt, J., Halls, H., Green, J., Dickas, A., Fralick, P.W., Smyk, M. and Mailman, M., 2000. Geology and stratigraphy of the Mesoproterozoic Sibley Group. In, ed. by P. Fralick, Fieldtrip Guide Books, Forty-Sixth Annual Meeting, Institute of Lake Superior Geology. p. 7-42. Franklin, J.M., 1978. The Sibley Group, Ontario. In, Rubidium-strontium isochron age studies, Report 2, Geological Survey of Canada, Paper 77-14, p. 31-34. Giguere, J.F., 1975. Geology of St. Ignace Island and adjacent islands, District of Thunder Bay. Ontario Ministry of natural Resources, Geological Report 118, 35p. Halls, H.C., 1974. A paleomagnetic reversal in the Osler Volcanic Group, Northern Lake Superior. Canadian Journal of Earth Sciences, 11, 1200-1207/ Heaman, L.M., and Machado, N., 1992. Timing and origin of the Midcontinent Rift alkaline magmatism, North America: evidence from the Coldwell Complex. Contributions to Mineralogy and Petrology, 110, 289303. - 69 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Hemming, S.R., McLennan, S.M. and Hanson, G.N., 1995. Geochemical and Nd/Pb isotopic evinence for the provinance of the early Proterozoic verginia Formation, Minnisota. Implications for the tectonic setting of the Animikie Basin. Journal of Geology, v. 103, p. 147-168. Hollings, P., Fralick, P. and Cousens, B., 2005. Early History of the Mid-Continental Rift Inferred from Geochemistry and Sedimentology of the Proterozoic Osler Group, Northwestern Ontario. Submitted to GSA Bulletin. Kissin, S.A. and Fralick, P.W., 1994. Early Proterozoic volcanics of the Animikie Group, Ontario and Michigan, and their tectonic significance. Proceedings Institute of Lake Superior Geology, v. 40, p. 18-19. Sutcliffe, R. H., 1986. The petrology, mineral chemistry and tectonics of Proterozoic rift-related igneous rocks at Lake Nipigon, Ontario. Unpublished Ph.D. thesis, University of Western Ontario, London, 325p. Sutcliffe, R.H., and Smith, A.R., 1988. Project number 8717. Geology of the St. Ignace Island volcanic-plutonic complex. Summary of Fieldwork and Other Activities 1988. Ontario Geological Survey Miscellaneous Paper 141, 368-371. Tanton, T.L., 1931. Fort William and Port Arthur, and Thunder Cape map areas, Thunder Bay District, Ontario. Geological survey of Canada Memoir 167, 222p. Kissin, S.A., Vallina, D.A., Addison,W,D. and Brumpton, G.R., 2003. New zircon ages from the Gunflint and Rove Formations, northwestern Ontario. Proceedings Institute of lake Superior Geology, Lightfoot, P., Sutcliffe, R., and Doherty, W., 1991. Crustal contamination identified in Keweenawan Osler Group tholeiites, Ontario: A trace element perspective. Journal of Geology, 99, 739-760. McIlwaine, W.H., and Wallace, H., 1976. Geology of the Black Bay Peninsula Area, District of Thunder Bay, Accompanied by Map 2304, scale 1 inch to 1 mile. Ontario Division of Mines, GR133, 54p. Miall, A.D., 1978. Lithofacies types and vertical profile models in braided river deposits: A summary. In ed. A.D. Miall, Fluvial Sedimentology, Canadian Society of Petroleum Geologists Memoir 5, 597-604. Nicholson, S.W., Shirey, S., Schulz, K., Green. J., 1997. Riftwide correlation of 1.1 Ga Midcontinent rift system basalts: implications for multiple mantle sources during rift development. Canadian Journal of Earth Sciences, 34, 504-520. Paces, J.B., and Miller, J.D, Jr., 1993. Precise U-Pb ages of Duluth Complex and related mafic intrusions, northeastern Minnesota; geochronological insights to physical, petrogenetic, paleomagnetic, and tectonomagnetic processes associated with the 1.1 Ga Midcontinent Rift System. Journal of Geophysical Research, B, Solid Earth and Planets, vol.98, no.8, pp.13,997-14,013. Pufahl, P.K. and Fralick, P.W., 2004. Depositional controls on paleoproterozoic shallow-water iron formation accumulation, Gogebic Range, Wisconsin, U.S.A. Sedimentology, v. 54, p. 791-808. Shirey, S., Lewin, K., Berg, J., and Carlson, R., 1994. Temporal changes in the sources of flood basalts: Isotopic and trace element evidence from the 1100 Ma old Keweenawan Mamainse Point Formation, Ontario, Canada. Geochimica et Cosmochimica Acta, 58, 4475-4490. - 70 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Fieldtrip 4 - Geology and rare element pegmatites of the Quetico Subprovince near Nipigon Mark Smyk Ontario Geological Survey, Resident Geologist’s Program, Thunder Bay, Ontario, Canada Stephen Kissin Department of Geology, Lakehead University, Thunder Bay, Ontario, P7B 5E1, Canada Overview The Quetico Subprovince of the Superior Province is situated between the Wabigoon and Wawa volcanoplutonic subprovinces that bound the Quetico on its northern and southern margins, respectively. This easttrending subprovince has a fairly consistent width of 70 km and consists predominantly of metasedimentary rocks and their migmatitic and anatectic derivatives (Williams, 1991). In general, the boundaries of the Quetico, whether or not they may be primary and/or tectonic, have been mapped as steeply dipping surfaces across which there is commonly a distinct contrast in lithology. In northwestern Ontario, the Quetico has been defined to exclude mafic metavolcanic rocks and derived migmatites; these rocks are instead assigned to the bounding Wawa and Wabigoon subprovinces. In contrast, Seemayer (1992) also described an asymmetric metamorphic grade distribution that had metamorphic grade increasing from south to north across the Quetico, southwest of Lake Nipigon. The southern margin was characterized by greenschist-facies rocks, the central portions were at amphibolite facies and the northern margin displayed an abrupt decrease in grade adjacent to the Wabigoon Subprovince to the north. The Quetico Fault, which is normally situated at the northern margin of the Quetico, lies well within the Quetico southwest of Lake Nipigon (Seemayer, 1992). Seemayer (1992) determined temperatures based on garnet-biotite thermometry ranging from 526°C at the southern margin of the Quetico Subprovince, increasing asymmetrically to a maximum of 714°C, then falling sharply at the northern margin to 517°C. An overview of the lithologic, metamorphic, structural and tectonic characteristics of the Quetico Subprovince has most recently been provided by Easton (2000): “The intensity of metamorphism varies within the subprovince, such that rocks marginal to the subprovince tend to be at lower grade than in the interior. The lowest metamorphic grade is found along the northern boundary with the Wabigoon subprovince (Pirie and Mackasey, 1978). Locally, subgreenschistto greenschist-facies rocks occur along the southern boundary (Borradaile, 1982), but typically, there is a rapid rise in metamorphic grade north of the Wawa subprovince, especially north of Manitouwadge, where a belt of metasedimentary granulites occurs within the Quetico subprovince close to, and parallel with, the northern margin of the Wawa subprovince (Coates, 1968; Williams and Breaks, 1989, 1990; Pan et al., 1994). As a result, grade distribution is asymmetrical, with the maximum in temperature and pressure occurring south of the central Quetico, locally coincident with the southern margin.” 4 Figure 1. Field trip stop locations. - 71 - I �Proceedings of the 51st ILSG Annual Meeting - Part 2 _______ ________ GeneaI geology of the Quetico Subprovince near Nipigon (after Williams 1991) Key to Locations: 7. Jean lake B. Lake HSen II. Georgia Lake 50km ._— Alkalic rrptexes Y to Paleoproterozoic ver and intrusions; Pharierozoic ver Moso Trerd of regional S fabric Sttatigrapliic fadng Fault arrows iricate MusooVe-beaiing granffio raCKs (peralumir,ous). may indude slip sense, baths on upthrow &de. if mown biothe granite S yncline, Massive granadiodte to granite; unsubdivided, mainly biotfte beang antdiie Gneis&c and foliated tonalite wiles Metasedinientary rocks: wacke, paragneiss, granulite; minor mac units Wabigoon, Wawa and Abitibi subprovincos Figure 2. Generalized regional geology (after Williams, 1991). Pressure calculated at near peak temperature was 5 ± 1.5 kbar. Easton (2000) described the regional metamorphic conditions and metamorphic history: “P–T conditions increase from west to east, for example, 500ºC and 2.5 kbar at the Minnesota border west of Thunder Bay (Percival, 1989), to 700–780ºC and 5.4–6.1 kbar adjacent to the Kapuskasing structural zone (Percival and McGrath, 1986; Percival, 1989). Typical conditions in the central region are on the order of 620ºC and 3.3 kbar (Percival, 1989). Granulites north of Manitouwadge yield 680–770ºC and 4.4–6.4 kbar (Pan et al., 1994; Percival, 1989). The regional variation in P–T can be ascribed to a relatively shallow level of erosion in the west (<10 km) and a deeper level in the east (>12 km) (Percival et al., 1985). Rocks located east of the Kapuskasing structural zone are believed to be generally at upper-amphibolite-facies conditions (Williams, 1991).” “Evidence for an earlier, medium-pressure, low-temperature, pre-tectonic or early syntectonic metamorphism comes from four areas within the subprovince. In the Atikokan region, and in northern Minnesota, both at the northern margin of the Quetico subprovince, an early M1 metamorphic peak between D1 and D2 produced Ky–St–Bt assemblages (Ayres, 1978; Tabor et al., 1989). Kyanite inclusions in plagioclase within Grt–Sil–Bt–Pl–Qtz schist near Raith, north of Thunder Bay, have been reported by Percival et al. (1985). Kehlenbeck (1976) also presented textural evidence for a polymetamorphic history along the northern margin of the Quetico subprovince north of Thunder Bay. Again, along the northern margin of the subprovince, in the Beardmore–Geraldton area (Williams, 1989), amphibolite-facies conditions were attained prior to D2 deformation, with the distribution of facies being structurally controlled within thrustbounded panels (Williams, 1991).” “In contrast, the main phase of regional metamorphism (M2), which produced the observed - 72 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 map-pattern (Fig. 2), occurred late syntectonically (Sawyer, 1983; Williams, 1991). The general sequence of isograds, based on the appearance of diagnostic assemblages in pelites, is Chl–Ms–Bt, Grt+And+Sil, Grt+Crd+Sil, in situ granitic leucosome, and Opx (Pirie and Mackasey,1978; Percival and Stern, 1984). The common occurrence of Grt–And in metapelites in the western Quetico subprovince is diagnostic of bathozone 2 (<3.4 kbar; Carmichael, 1978), whereas the presence of Sil–St in the eastern Quetico is diagnostic of bathozones 3 and 4 (3.4–5.5 kbar).” “As noted by Williams (1991), tectonic thickening of the sedimentary pile and intrusion of minor Itype granitic rocks occurred prior to the thermal acme. Most of the large pre- to syntectonic granitic bodies are peraluminous and have sedimentary sources but display little evidence of thermal contact metamorphism; one exception is the South Beatty Lake pluton in the northern Quetico subprovince (Pirie & Mackasey, 1978). Steeply dipping thermal gradients, local increases in temperature around large plutons, and the general association of the highest-grade rocks with abundant generation of leucosome, indicate that the source of heat was a combination of burial, upward magmatic transport, and tectonism.” “In the northern Quetico, M1 metamorphism is estimated to have occurred between 2698 Ma, the maximum age of sedimentation (Davis et al., 1990) and 2688+4 Ma, the age of emplacement of the late syntectonic Blalock pluton (Davis et al., 1990). In the southern Quetico, M1 occurred after 2690 Ma, the maximum age of sedimentation (Zaleski et al., 1999). The timing of M2 metamorphism is less well constrained, and may have been protracted. In the Manitouwadge area, Zaleski et al. (1999) constrained regional D2 deformation to 2680–2677 Ma, and suggested that migmatization in both the northern Wawa and southern Quetico subprovinces occurred after 2679 Ma, broadly coincident with D3 deformation. This inference is consistent with observations elsewhere in the Quetico that contact aureoles around late plutons, dated at 2671+2 to 2665+2 Ma, as well as late granitic pegmatites dated at 2653+4 Ma, overprint regional metamorphic fabrics (Percival and Sullivan, 1988; Percival, 1989). North of Manitouwadge, Pan et al. (1998) reported a U–Pb zircon age of 2666+1 Ma from a granitic pegmatite concordant with respect to the D3 fabric, and suggested that the regional amphibolitefacies metamorphism occurred between 2671 and 2665 Ma, consistent with the ages cited above. The timing of peak granulite-facies metamorphism north of Manitouwadge appears to be some 15 Ma younger, on the basis of U–Pb zircon ages of 2650+ and 2651+3 Ma from a mafic granulite and a tonalitic leucosome, respectively (Pan et al., 1998). Zaleski & van Breemen (1997) reported that titanite ages young with increasing metamorphic grade, ranging from ~2686 Ma in the southern Manitouwadge greenstone belt to ~2640 Ma in the southern Quetico, suggesting that the thermal effects of regional metamorphism may have lasted over ~30 million years, from 2677 to 2640 Ma, in the higher-grade parts of the Wawa and Quetico subprovinces. On the basis of their regional geological and geochronological studies, Zaleski et al. (1999) concluded that “M2 metamorphism occurred after the tectonic juxtaposition of the Quetico and Wawa subprovinces.” This field trip will cover the southern two-thirds of the Quetico Subprovince, from south of Beardmore to Nipigon (Fig. 1). As mentioned above, there is an asymmetric distribution in metamorphic grade, with a gradual progression from greenschist-facies, clastic metasedimentary rocks near the northern contact with the Wabigoon Subprovince; to lower amphibolite-facies schists and gneisses; through to upper amphibolitefacies migmatites and derived granitic rocks near the southern contact with the Wawa Subprovince near Nipigon. Thermal and pressure maximum occurs south of the center of the Quetico. The metamorphic character is of high-temperature/low-pressure (Abukuma-type) metamorphism, associated with the abundant intrusion of granitoid rocks and the regional distribution of migmatites derived from the metasedimentary rocks (Kamineni et al., 1988; Percival and McGrath, 1986; Percival, 1989; Williams, 1989). Peak metamorphic assemblages in the field trip area suggest conditions >650ºC and 5 kbar, corresponding to bathozones 4 to 5 (14 to 15 km deep in the crust) (Easton, 2000; Percival, 1989). The nomenclature of Mehnert (1968) has been used in describing migmatites in this field guide. Overview of the Georgia Lake Pegmatite Field The Georgia Lake pegmatite field, as described by Pye (1965) and Mulligan (1965) represents the largest concentration of rare element mineralization in the - 73 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Table 1. Estimated reserves for the Georgia Lake pegmatite field. Deposit Estimated reserves (tons) Average grade (% Li2O) 759,475 1.65 96,000 1.50 Conway 1,830,000 0.96 Jean Lake No. 4 (Parole) 1,689,000 1.30 261,000 1.03 Nama Creek North Zone 2,784,000 1.11 Nama Creek South Zone 1,508,332 0.96 Jackpot 2,000,000 1.09 750,000 1.38 11,677,807 1.14 Aumacho No. 1 (Brink) Aumacho No. 2 Aumacho No. 3 McVittie Vegan No. 2 (Newkirk) Total Superior Province of Ontario (Breaks et al., 2003a). It comprises 38 rare element occurrences and 10 spodumene-bearing, pegmatite deposits (total resource of 11.7 million tons grading an average of 1.14% Li2O; Pye, 1965; Table 1; Fig. 3) were discovered during an exploration rush in the 1950’s (Pye, 1956). As delineated by Breaks et al. (2003a), the field covers approximately 1200 km2 (32 km by 37 km) and is hosted by upper greenschist- to lower amphibolitefacies, clastic metasedimentary country rocks. Four types of rare element pegmatites (using the classification of Cerný (1991a) have been identified locally; Breaks +:duaoeke aatholth OpSb1::*:..:::.,11rr._ + + L Diebase dikes and Rare V.] (co"pIredfoundbype5entsu'vey) Slb1o GrOtJP. xnsobdivlded ,. be beryp ,.I,wrtonaI,t and m!gn,arnic m.tas.diftl.ntary col -tent endaver GPaOer Lake eatholith Li coumbite-t.ntaILte Lithium Peraluminous S-type graniuc rocks, ssIte,1t pi-nt grantte and pota5sc pegTlaUte r1 clastic maaethn,entary rocks io and medium maçnec oxLdeminera grade metawacke and m*tap.IIte Pet spod petahte spodvnw,.e Maflc to int.rmslate metavorcan,c rocks tour tourmalint bçOOn subprwMce Figure 3. Generalized regional geology (after Breaks et al., 2003a). - 74 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Table 2. Subdivisions of the Georgia Lake pegmatite field, after Breaks et al. (2003a) Pegmatite group Postagoni River Lake Jean Barbara Lake Forgan Lake Type Albite-spodumene Albite-spodumene Albite-spodumene Albite-spodumene Examples McVittie; Dunning; Nama Creek Foster; Giles; Trans Vegan; Newkirk; Point; Jackpot Forgan; Lucky Lake (Lun-Echo) Cosgrave Lake Beryl±petalite sub-type Swanson; MNW Gathering Lake Albite Newly discovered, unnamed biotite-muscovite and muscovite-biotite granite and granodiorite. et al., 2003a): 1) Beryl-type 2) Albite-spodumene-type 3) Complex-type/petalite-subtype 4) Albite-type The Georgia Lake pegmatite field was initially subdivided by Milne (1962) and then Zayachivsky (1985) and Kissin and Zayachivsky (1985) into the Northern, Central and Southern groups. More recent data and mapping conducted by Breaks et al. (2003a) has led to a revised, six-fold subdivision (Table 2). The two pegmatites that will be visited on the field trip, the Dunning pegmatite (Stop 2) and the Foster pegmatite (Stop 4), are examples of the albitespodumene-type. The Glacier Lake Batholith (GLB) is a large mass of medium- to coarse-grained, S-type granite and pegmatitic granite situated immediately south of the northern metasedimentary part of the Quetico Subprovince, extending east of Highway 11 at least 50 km. It dominantly consists of grey, medium- to coarse-grained biotite and muscovite-biotite granite, which locally contains minor tonalitic and migmatitic metasedimentary enclaves. Generally massive, this rock may display a local, weak foliation developed in 1 to 2 cm-thick, shallowly lineated veins of fibrolite, muscovite and quartz that appear to have formed during aluminous hydrothermal alteration controlled by late shearing (Breaks et al., 2003a). Gradation into __ Zayachivsky (1985) divided the felsic plutonic rocks in the Georgia Lake area into three groups (Fig. 4): —— SoRe (1) Two-mica leucogranites, occurring as a large plutonic mass south of the pegmatite field (cf. Glacier Lake Batholith) and as smaller, satellite intrusions; * T—sqs — (2) Kilgour Lake Group granitoids centered around a small gabbroic unit near Kilgour Lake; and I mon I Isiopil — 2t (3) Tonalitic sills, distributed throughout the pegmatite field. / 7 .7./ - [.L Zayachivsky (1985) suggested that the two-mica leucogranites and tonalitic sills were derived from the partial melting of metapelites and metawacke, respectively (i.e. S-type granitoids). The Kilgour Lake rocks were presumed to be the products of the fractional crystallization of a mafic magma generated in the upper mantle/lower crust (i.e. I-type granitoids). Breaks et al. (2003a) described two distinct, fertile, peraluminous, S-type granite plutons (MNW and Barbara Lake stocks) that occur within metasedimentary country rocks. They consist of virtually identical, grey, medium-grained, generally homogeneous, __ __ — — —— Kikwtk. S,S. CR -— MMflOak Figure 4. Plutonic rocks in the Georgia Lake area (after Pye, 1965; Zayachivsky, 1985; Breaks et al., 2003a). - 75 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 consanguineous pods of tourmaline-muscovite potassic pegmatite up to 1 m in diameter occurs locally. There are several episodes of S-type granitic magmatism in the GLB. Undeformed, tourmaline-cordierite or tourmuscovite dikes are the youngest and most chemically evolved units (F. Breaks, Ontario Geological Survey, personal communication, 2004). can be pursued by the identification of dispersion haloes of rare alkali metals (Li, Rb, Cs) in country rocks and some other elements such as Be, Sn and Cu. Hidden pegmatite targets in promising terrain may thus be located. Pye (1965) undertook the first lithogeochemical survey for rare elements in the aforementioned felsic plutonic rocks in the Georgia Lake area. Many anomalies, ranging from 100 to 400 ppm Li, were noted. Uranium-bearing pegmatite dykes are associated with felsic plutonic rocks in this part of the Quetico Subprovince. Several occurrences have been documented by Scott (1987), including some in the vicinity of Nipigon. White, albite-rich, biotitequartz+muscovite+apatite pegmatite dykes up to 30 m wide intrude gneissic and migmatitic rocks. Yellow and yellow green stain (secondary uranophane (Ca(UO2)2Si2O7 · 6H2O?)) characterize many of the uraniferous dykes. Uraninite appears to be the primary uranium-bearing mineral (Franklin, 1978). The Lake Helen occurrence is situated on the western shore of Lake Helen, opposite the roadside rest area on Highway 11 (Stop 7). Trenching and sampling of this occurrence in 1967 by Aggressive Mining Limited yielded values up to 0.135% U3O8 (ibid). Such dykes are considered to be the primary source of uranium that occurs in structures at or near the Archean / Proterozoic unconformity in the Nipigon Basin. Two modes of source fluid derivation may have operated during the emplacement of the Georgia Lake pegmatites (Zayachivsky, 1985). Pegmatites of the Central Group were derived by igneous fractionation of a granitic parent near Barbara Lake and emplaced as low-viscosity, volatile-rich melts in a direction away from the source area. This is supported by a fractionation trend defined most clearly by the concentration of Rb and Cs in perthitic microcline and muscovite across the Central Group. The pegmatites of the Northern Group may have been the result of direct anatexis of lithium-rich Quetico metasedimentary rocks. Breaks et al. (2003a) described the MNW pegmatite (petalite sub-type) as the most evolved of the local pegmatites, based on its geochemistry, mineralogy, including the occurrence of manganotantalite. A general rule for rare element-bearing Ontario pegmatites is that tantalum mineralization is associated with lithium mineralization in the form of spodumene and/or petalite (ibid). Prediction of the source of lithium deposits in “fertile” parental granites is a somewhat uncertain art. The essential parental granite is a siliceous, peralkaline leucogranite, but the lithium content of a prospective parental granite is not highly indicative of the presence of lithium-rich pegmatites (Cerný, 1991a). Data summarized by Stewart (1978) and more recently by Cerný (1991a) indicate that Li-contents in parental granites are typically <100 ppm. Moreover, analyses of bulk compositions of spodumene- and petalitebearing pegmatites yield a closely grouped mean of 1.53 weight % Li2O (Stewart, 1978) and range between 1.5 and 2.0 weight % Li2O (Cerný, 1991b). These data indicate that differences between spodumene- and petalite-bearing pegmatites are caused by pressure and temperature differences during crystallization rather than differences in bulk composition. Cerný (1991b) suggested that exploration for rare-element pegmatites Uraniferous Dykes Stops Field Trip Road Log Stop Locality Lake Jean stops Intersection of Hwy.’s 11 &17 in the Town of Nipigon Take Hwy 11 north Gorge Creek turn-off - head east Reset Postagoni River Bridge 1 Postagoni Stock 2 Dunning pegmatite Powerline crossing 3 Quetico metasedimentary rocks 4 Foster pegmatite Lake Helen stops Intersection of Hwy.’s 11 &17 in the Town of Nipigon Take Hwy 11 north 5 Glacier Lake Batholith - 76 - km 0 37.4 12.4 14.4 18.4 19.1 28 28 0 17.1 �Proceedings of the 51st ILSG Annual Meeting - Part 2 6 7 8 9 10 11 Biotite leucogranite Migmatite - roadside rest area Pull-off area Pegmatitic granite Pegmatitic granite Pegmatites in migmatite Intersection of Hwy.’s 11 &17 in the Town of Nipigon Take Hwy 17 east Migmatites (optional) rocks and display no foliation nor significant internal variation. An U-Pb zircon age of 2684.8 + 2.1 Ma was determined for the Postagoni Stock (D. Davis, University of Toronto, unpublished data). 10.8 7.4 5.9 5.2 4.85 4.6 Stop 2 - Dunning Pegmatite UTM coordinates - 0427823E 5474882N 0 18.4 Stop 1 - Postagoni Stock UTM coordinates - 0425041E 5472869N Large, locally derived boulders of leucotonalite of the Postagoni Stock (a.k.a. Postagoni Lake sill) occur on the eastern side of the forest access road. Pye’s (1965) mapping identified a porphyritic granitic intrusion approximately 1.6 km long and 700 m wide in metasedimentary country rocks. The McVittie pegmatite cuts the stock north of Dive Lake; it was visited during an ILSG field trip in 1990 (Kissin, 1990). Both the McVittie and Dunning (Stop 2) pegmatites are part of the Northern Group (Zayachivsky, 1985) or Postagoni Lake Group (Breaks et al., 2003a), comprising unzoned albite-spodumene-type pegmatites. This small roadside outcrop exposes the Dunning pegmatite dyke, which has an estimated width of 2 to 3 m at this location and intrudes Quetico metasedimentary rocks. Woolverton (1956) noted that pegmatite dykes on the Dunning property were exposed intermittently in a zone extending over 600 m in length. This dyke is part of Zayachivsky’s (1985) Northern Group, which are characterized by medium- to very coarse-grained, perthitic microcline and spodumene in a fine-grained matrix of quartz, muscovite and albite, with trace amounts of apatite and garnet. They are typically unzoned to poorly zoned and commonly contain aplitic stringers, bands and pods, within and parallel to dyke contacts. This dyke is a typical albite-spodumene-type pegmatite (Fig. 5). Green spodumene crystals, up to 4.5 cm long, are aligned perpendicular to the dyke Zayachivsky (1985) described this sill as a coarsegrained tonalite with closely packed crystals of plagioclase up to 1.5 cm long, locally imparting a porphyritic texture. This texture occurs throughout the sill except near its contacts with the metasedimentary country rocks where a several metre-wide chilled margin has developed. The chilled margin texture resembles that of the porphyry dykes described by Pye (1965). A modal analysis of the Postagoni Stock was given by Zayachivsky (1985; Table 3). Table 3. Modal analysis of the Postagoni Stock. Mineral Plagioclase K-feldspar Quartz Muscovite Biotite Epidote Zircon Modal abundance(%) 66.6 5.6 25.7 0.4 1.6 0.1 Trace The Postagoni Stock and the other local tonalitic sills are invariably in sharp contact with the country Figure 5. Drusy spodumene, Dunning pegmatite (Stop 2). - 77 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 ___ ______________________ ____________ -2 _______________ Logging Camp (abandonedi - Sketch Map of the Foster Pegniatite area Amieno Resourc.s Inc. 19B7) STOP 3 nd // / •/./ /// STOP4 of the submarine margin and trench-fill are present in the Quetico (Fralick et al., 1992; Williams, 1989). 10 to 20 cm thick sandstone-siltstone beds strike 100º and likely dip steeply to the south, based on observations in the area by Pye (1965). Graded bedding suggest tops to the south (Fig. 7). Pye (1965) noted that biotitequartz-feldspar schist such as this are most commonly granoblastic; garnet porpyhryoblasts have also been noted. Some lenticular pods of quartzo-feldspathic leucosome are present. Stop 4 - Foster Pegmatite 9 a 100ut1 UTM coordinates - 0436379E 5473665N .---—----—Metasedimentary rocks Figure 6. Sketch map of Foster pegmatite area (Stops 3 & 4). contact. This phenocrystic spodumene typically contains several percent iron and is commonly altered to a dark green to black alteration product of chlorite and sericite (aka “rotten spodumene”). Some altered, brown spodumene is also present, along with quartz, albite (+cleavelandite), beige alkali feldspar, green muscovite and minor oxides. Pye (1965) reported that the lithia contents of the two dykes on the Dunning property were low and of no economic importance. Stop 3 - Quetico Metasedimentary Rocks UTM coordinates - 0436345E 5473931N Medium-grade clastic metasedimentary rocks typical of the north-central part of the Quetico Subprovince are exposed in a small outcrop north of the Foster pegmatite (Fig. 6). South of the Beardmore-Geraldton belt, more highly metamorphosed, turbiditic sediments A series of stripped and washed outcrops reveals a swarm of narrow pegmatite dykes cutting an easttrending sill of massive, biotite tonalite, termed the Parole Lake sill by Zayachivsky (1985; Fig. 8). The Foster deposit was discovered in 1956 and was tested by diamond drilling later that year by Towagmac Exploration Co. Ltd. (Walter, 1957; Pye 1965; Table 4). This work indicated that the lithium deposit was too low-grade and the property was allowed to lapse. Armeno Resources Inc. explored the property (including the neighbouring Lew deposit) from 1986 to 1989, carrying out stripping, geological, magnetometer and electromagnetic surveys, as well as diamond drilling (Assessment Files, Thunder Bay North District, Thunder Bay). The lone drill hole intersected biotite tonalite with numerous quartz-rich, pegmatitic dykes < 1m thick with up to 5% spodumene; no chemical analyses were reported (ibid). The main Foster dyke strikes approximately 085º and dips 80º to 85º south. It is exposed continuously 1 F? 'r--- r-- r - t'r Figure 7. Graded bedding in metaturbidites, Foster area (Stop 3). Figure 8. Pegmatite dykes in tonalite, Foster (Stop 4). - 78 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Table 4. Results of drilling at the Foster deposit (Walter, 1957). Drill Hole Number 7 6 9 Location (feet) 3+50 west 4+50 west 6+50 west 10 7+50 west Core length (feet) 29.0 30.0 31.3 20.0 32.7 over 75 m and averages 9 m in thickness. To the west, it splits into a number of thin, parallel dykes. In the most westerly drill hole (T-10), 7 individual dykes, 0.3 to 2.4 m wide were intersected, separated by sections of host tonalite a few centimetres to a metre wide (Pye, 1965). The deposit contains 10 to 15% spodumene, which is for the most part, unaltered. Similar to other deposits in the Lake Jean Group, crystals are prismatic and perpendicular to dyke contacts and may extend across the entire dyke (ibid). Milne (1962) listed the Foster pegmatite in the Jean Lake sub-group (with the Giles, Camp and Lew pegmatites) on the basis of welldeveloped spodumene orientation and the occurrence of narrow, branching and continuous, longitudinal aplite dykes. Lithia Content (%) 0.26 0.55 1.04 0.58 0.58 Large road cuts on both sides of Highway 11 provide excellent exposures of the Glacier Lake Batholith (GLB). At this location, it consists of white, massive to locally foliated, muscovite < biotite, medium- to coarsegrained granite. Localized pods of tourmaline-biotitemuscovite-potassium feldspar pegmatite occur within the granite. These pods may reach 1 m in diameter and locally contain tourmaline-quartz intergrowths. Numerous, curvate, fibrolite - muscovite + tourmaline veins up to 1 cm thick also occur in the host granite. Purple fluorite is exposed on a fractured outcrop face on the west side of the highway (Fig. 10). Stop 6 - Glacier Lake Batholith Leucogranite/ M igmatite UTM coordinates - 0407163E 5440408N This is another example of the southern margin of the Glacier Lake Batholith where it is in contact with metasedimentary migmatite (Fig. 11). White, biotite-muscovite leucogranite contains rare, bluegreen apatite. Foliation is developed in feldspathic segregations and biotitic seams. Leucogranite dykes and migmatite locally exhibit a lit-par-lit structure. Tight to isoclinal folds have developed in the quartzbiotite-feldspar schist (Fig. 12). All rocks display boudinage and folding. Folded leucosome suggests a Figure 9. Drusy spoduemene, Foster pegmatite (Stop 4). These are classic albite-spodumene-type pegmatites. Fresh green to dark green-black, altered spodumene crystals may reach lengths of 8 cm (Fig. 9). Other constituent minerals include K-feldspar, albite (+ cleavelandite) and quartz, minor apatite and garnet, and tiny, sparsely distributed, black tantalum-niobiumbearing oxide minerals (F. Breaks, OGS, personal communication, 2004). A locally developed border zone (< 5 cm) consists of quartz + feldspar. Stop 5 - Glacier Lake Batholith Leucogranite UTM coordinates - 0409911E 5445897N Figure 10. Fluorite on fracture surface in sericite- and fibrolite-bearing Leucogranite (Stop 5). - 79 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 r '2..—:, 4 A- I.— A Figure 11. Contact between folded metasedimentary migmatite and Leucogranite (Stop 6). Figure 13. Folded metasedimentary schist, incipient leucosome development (Stop 6). an early and protracted deformation history (Fig. 13). Metasedimentary migmatite enclaves are common in the white, S-type pegmatitic granites along the highway. Note that less evolved S-type granite may contain biotite as the only mica. whaleback outcrop behind the highway rest stop/ pull-off area. There is quite a bit of variation in the relative amounts of leucosome and restite (Fig. 14). Boudinaged and pytgmatically folded leucosome pods and veins occur in a quartzo-feldspathic with narrow, biotitic restite septa. Narrow diabase dykes cut the country rocks on the western side of the highway. Stop 8 - Pegmatitic Granite UTM coordinates - 0407272E 5435208N Stop 7 - Migmatite UTM coordinates - 0406986E 5437331N Migmatites are exposed on the shore and islands of Lake Helen and on a large, glacially streamlined, Figure 12. Folded metasedimentary migmatite (Stop 6). This outcrop shows a transition in magmatism from S-type (i.e. Glacier Lake Batholith) to I-type granitoids. Pink, coarse-grained, biotite granite here consists of pink, coarse-grained, perthitic K-feldspar, quartz, coarse-grained biotite, and brown, altered plagioclase (up to 4 cm long). These weakly peraluminous, pegmatitic, biotite granites are relatively primitive and are younger than the white, two-mica leucogranites. Such rocks are probably of I-type origin and typically are metasedimentary enclave-free. Bulk rock levels of Figure 14. Lit-par-lit migmatites with large metasedimentary inclusions (schollen) (Stop 7 area). - 80 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 • [j\ ti'a( Figure 15. Pegmatitic K-spar - quartz – biotite granite (Stop 8). rare-elements are very low: 128 ppm Rb, 2.63 ppm Cs, 1.8 ppm Nb, 0.39 ppm Ta (F. Breaks, OGS, personal communication, 2004). A K-feldspar-megacrystic, pink biotite granite (Fig. 15) with a shallowly east-dipping foliation and metasedimentary xenoliths is intruded by a more massive granite at this locality. The intrusive contact is embayed and scalloped, suggestive of co-mingling magmatic textures (Fig. 16). The megacrystic, foliated granite may represent an earlier, xenolith-bearing phase that was intruded by the massive granite while still relatively warm and plastic. Stop 9 - Pegmatitic Granite UTM coordinates - 0407458E 5434906N feldspar megacrysts may attain lengths of over 60 cm. Stop 10 - Pegmatite Dykes in Migmatite UTM coordinates - 0407587E 5434690N Approximately 200 m south of the massive pink granite, white pegmatite dykes intrude fine-grained metasedimentary schist. This dark, fine-grained, feldspathic, biotite schist displays a well-developed foliation and minor folds, indicative of a long, protracted ductile deformation history, perhaps coeval with dyke emplacement. Note the sub-horizontal mineral lineation. Flattening of feldspars, producing small-scale, augen structures, is also indicative of hightemperature (>500º C) deformation. The boudinaged and annealed, sericite- and biotitebearing dykes range from a few centimetres (lit-parlit structured) to several metres in thickness. They are mineralogically and texturally interesting, containing Lt- ! H -v A large, bare, whaleback outcrop on the east side of the highway consists of coarse-grained to pegmatitic, massive, homogeneous pink granite (i.e. the younger granitic unit at Stop 8; Fig. 17). Crystals or interstitial patches of quartz, biotite and locally sericitized, Kfeldspar average 2 to 3 cm in size (Fig. 18). Individual Figure 17. “Whaleback” outcrop of coarse-grained to pegmatitic granite; note large (>60 cm) feldspar megacryst (circled) (Stop 9). Figure 16. Scalloped contact (arrow) between foliated biotite granite (top) and pegmatitic granite (bottom) (Stop 8). Figure 18. Coarse-grained to pegmatitic granite (Stop 9). - 81 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 both cordierite and quartz-tourmaline intergrowths with alkali (locally sericitized) feldspar. Garnet is conspicuous by its absence. metamorphic rocks. John Scott (Ontario Geological Survey) generated maps for this report and provided information on local uranium mineralization. I 1 Sf C Breaks et al. (2003b) described potassic, biotite pegmatite that grades into a medium-grained, biotite granite near this stop (UTM: Easting 408339; Northing 5433016). The pegmatite contains coarse, euhedral K-spar; quartz; prismatic, medium- to coarse-grained, black tourmaline (schorl-dravite; Fig. 19); and finegrained, green and blue fluor-apatite (0.3 to 0.9 weight % MnO). A biotite-rich, metasomatic contact occurs between the biotite granite and diabase. Graphic, coarse-grained cordierite (< 2 cm) and tourmaline occur in the granite near a diabase contact. Figure 20. Schollen structure in migmatite (Stop 11). References Ayres, L.D., 1969. Metamorphism in the Superior Province of northwestern Ontario and its relationship to crustal development; in Metamorphism in the Canadian Shield; Geological Survey of Canada, Paper 78-10, p.25-36. Ayres, L.D. 1978. Metamorphism in the Superior Province of northwestern Ontario and its relationship to crustal development; in Metamorphism in the Canadian Shield, Geological Survey of Canada, Paper 78-10, p.25-36. Figure 19. Quartz-tourmaline intergrowth in pegmatite dyke (Stop 10). Stop 11 - Migmatites (Optional) UTM coordinates - 5429614N 0425006E This optional stop displays schollen (raft)-structured migmatites in which folded and schlieric, mafic, paleosome xenoliths have been highly deformed and disrupted (Fig. 20). There is a relatively high proportion of quartzo-feldspathic neosome matrix to the xenoliths, suggesting high(er) degrees of partial melting. Patches and dykelets of coarse-grained, biotite-quartz-feldspar neosome contain garnet, cordierite and large (>10 cm), euhedral green apatite crystals. Acknowledgements Dr. Fred Breaks (Ontario Geological Survey) is thanked for the provision of unpublished data on pegmatites and host rocks. Dr. Mary Louise Hill (Lakehead University) provided useful insight into the metamorphic and structural history of the high-grade Borradaile, G.J. 1982. Comparison of Archean structural styles in two belts of the Canadian Superior Province; Precambrian Research 19, p.179-189. Breaks, F.W., Selway, J.B. and Tindle, A.G., 2003a. Fertile peraluminous granites and related rare-element pegmatite mineralization, Barbara-Gathering-Barbaro lakes area, north-central Ontario; Summary of Field Work and Other Activities, 2003, Ontario Geological Survey, Open File Report 6120, p.14-1 to 14-13. Breaks, F.W., Selway, J.B. and Tindle, A.G., 2003b. Fertile peraluminous granites and related rare-element mineralization in pegmatites, Superior Province, northwest and northeast Ontario: Operation Treasure Hunt; , Ontario Geological Survey, Open File Report 6099, 179p. Carmichael, D.M., 1978. Metamorphic bathozones and bathograds: a measure of the depth of postmetamorphic uplift and erosion on the regional scale; American Journal of Science, 278, p.769-797. Cerný, P., 1991a. Rare-element granitic pegmatites, part I. Anatomy and internal evolution of pegmatite deposits; Geoscience Canada, v.18, p.49-67. Cerný, P 1991b. Rare-element granitic pegmatites. Part II: - 82 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Regional to global environments and petrogenesis. Geoscience Canada v.18, p.68-81. north of Manitouwadge, Ontario; Canadian Journal of Earth Sciences, v.31, p.1427-1439. Coates, M.E., 1968. Stevens–Kagiano Lake area; Ontario Department of Mines, Geological Report 68, p. Pan, Y., Fleet, M.E. and Heaman, L.M., 1998. Thermotectonic evolution of an Archean accretionary complex: U–Pb geochronological constraints on granulites from the Quetico subprovince, Ontario, Canada; Precambrian Research,v.92, p.117-128. Davis, D.W., Pezzutto, F. & Ojakangas, R.W., 1990. The age and provenance of metasedimentary rocks in the Quetico Subprovince, Ontario, from single zircon analyses: implications for Archean sedimentation and tectonics in the Superior Province; Earth and Planetary Science Letters,v.99, p.195-205. Easton, R.M. 2000. Metamorphism of the Canadian Shield, Ontario, Canada I: The Superior Province; The Canadian Mineralogist, v.38, p.287-317. Fralick, P., Wu, J., and Williams, H.R., 1992. Trench and slope basin deposits in an Archean metasedimentary belt, Superior Province, Canadian Shield. Canadian Journal of Earth Sciences, 29: 2551–2557. Franklin, J.M., 1978. Uranium mineralization in the Nipigon area, Thunder Bay District, Ontario; Current Research, Part A, Geological Survey of Canada, Paper 78-1A, p.275-282. Kamineni, D.C., Stone, D. and Johnston, P.J. 1988. Metamorphism of Quetico sedimentary rocks near Atikokan, Ontario; in Program with abstracts, Geological Association of Canada-Mineralogical Association of Canada-Canadian Society of Petroleum Geologists Annual Meeting, v.13, p.A63. Percival, J.A and Stern, R.A., 1984. Geological synthesis in the western Superior Province, Ontario; in Current Research, Part A, Geological Survey of Canada, Paper 84-1A, p.397-407. Percival, J.A and Sullivan, R.W., 1988. Age constraints on the evolution of the Quetico belt, Superior Province, Ontario; in Radiogenic Age and Isotopic Studies: Report 2, Geological Survey of Canada, Paper 88-2, p.97-108. Percival, J.A., 1989. A regional perspective of the Quetico metasedimentary belt, Superior Province, Canada; Canadian Journal of Earth Sciences,v.26, p.677-693. Percival, J.A. and McGrath, P.H., 1986. Deep crustal structure and tectonic history of the northern Kapuskasing uplift of Ontario: an integrated petrological-geophysical study; Tectonics,v.5, p.553-572. Percival, J.A., Stern, R.A. and Digel, M.R., 1985. Regional geological synthesis of western Superior Province, Ontario; in Current Research, Part A, Geological Survey of Canada, Paper 85-1A, p.385-397. Kehlenbeck, M.M., 1976. Nature of the Quetico–Wabigoon boundary in the De Courcey – Smiley Lakes area, northwestern Ontario; Canadian Journal of Earth Sciences, v.13, p.737-748. Pirie, J. and Mackasey, W.O., 1978. Preliminary examination of regional metamorphism in parts of Quetico metasedimentary belt, Superior Province, Ontario; Geological Survey of Canada, Paper 78-10, p.37-48. Kissin, S.A., 1990. Granitoid-related mineral deposits of the western Lake Superior region; 36th annual Institute on Lake Superior Geology, Thunder Bay, Field Trip Guidebook, p.52-66. Pye, E.G., 1956. Lithium in northern Ontario; Canadian Mining Journal, v.77, p.73-75. Kissin, S.A. and Zayachivsky, B., 1985. Genesis of pegmatites in the Quetcio gneiss belt of northwestern Ontario: rare-element pegmatites and associated granitoids of the Georgia Lake pegmatite field; in Geoscience Research Grant Program, Summary of Research 1984-1985, Ontario Geological Survey, Miscellaneous Paper 127, p.186-199. Mehnert, K.R., 1968. Migmatites and the origin of granitic rocks. Elsevier. 405p. Milne, V.G., 1962. The petrography and alteration of some spodumene pegmatites near Beardmore, Ontario; unpublished Ph.D. thesis, University of Toronto, Toronto, Ontario, 242p. Mulligan, R., 1965. Geology of Canadian lithium deposits; Geological Survey of Canada; Economic Geology Report 21, 131p. Pan, Y., Fleet, M.E. and Williams, H.R., 1994. Granulitefacies metamorphism in the Quetico subprovince, Pye, E.G., 1965. Geology and lithium deposits of the Georgia Lake area, District of Thunder Bay; Ontario Department of Mines, Report 31, 113p. Sawyer, E.W., 1983. The structural history of a part of the Archean Quetico metasedimentary belt, Superior Province, Canada; Precambrian Research,v.22, p.271-294. Scott, J.F., 1987. Uranium occurrences of the Thunder Bay – Nipigon – Marathon area; Ontario Geological Survey, Open File Report 5634, 158p. Seemayer, B.E., 1992. Variations in metamorphic grade in metapelites in transects across the Quetico Subprovince north of Thunder Bay, Ontario; unpublished M.Sc. thesis, Lakehead University, Thunder Bay, 163p. Stewart, D.B., 1978. Petrogenesis of lithium-rich pegmatites. American Mineralogist, v.63, p.970-980. Tabor, J.R., Hudleston, P.J. and Magloughlin, J., 1989. Metamorphism of the Quetico supracrustals north of - 83 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 the Vermillion granitic complex, northern Minnesota; Geological Association of Canada – Mineralogical Association of Canada, Program with Abstracts, v.14, p.A38. Walter, J.P. 1957. Towagmac Explorations Ltd., Reoprt on exploration program, 1956 season; unpublished report, Assessment Files, Thunder Bay South District, Thunder Bay, 3p. Williams, H.R., 1989. Geological studies in the Wabigoon, Quetico, and Abitibi–Wawa subprovinces, Superior Province of Ontario, with emphasis on the structural development of the Beardmore–Geraldton belt; Ontario Geological Survey, Open File Report 5724, p. Williams, H.R., 1991. Quetico subprovince; in Geology of Ontario, Ontario Geological Survey, Special Volume 4, v.1, p.383-403. Williams, H.R. and Breaks, F.W., 1989. Geological studies in the Manitouwadge – Hornepayne area; Summary of Field Work and Other Activities 1989, Ontario Geological Survey, Miscellaneous Paper 146, p.7991. Williams, H.R. and Breaks, F.W., 1990. Geological studies in the Manitouwadge – Hornepayne area; Summary of Field Work and Other Activities 1990, Ontario Geological Survey, Miscellaneous Paper 151, p.4751. Woolverton, R.S. 1956. Report on McVittie lithium option, Noranda Exploration Co. Ltd.; unpublished report, Assessment Files, Thunder Bay South District, Thunder Bay, 6p. Zaleski, E. and van Breemen, O., 1997. Age constraints on plutonism, metamorphism and deformation across the Wawa–Quetico subprovince boundary near the Manitouwadge greenstone belt, northeastern Ontario; Institute on Lake Superior Geology, Program with Abstracts, v.43, p.67-68. Zaleski, E., van Breemen, O. and Peterson, V.L., 1999. Geological evolution of the Manitouwadge greenstone belt and Wawa–Quetico subprovince boundary, Superior Province, Ontario, constrained by U–Pb zircon dates of supracrustal and plutonic rocks; Canadian Journal of Earth Sciences, v.36, p.945-966. Zayachivsky, B., 1985. Granitoids and rare-element pegmatites of the Georgia Lake area, northwestern Ontario; unpublished M.Sc. thesis, Lakehead University, Thunder Bay, Ontario, 234p. - 84 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Fieldtrip 5 - Geology of the Black Sturgeon Area Thomas R. Hart Ontario Geological Survey, Ministry of Northern Development and Mines, Sudbury, Ontario, P3E 6B5, Canada Introduction The Nipigon Embayment is an approximately 19,000 km2 area of Mesoproterozoic igneous and sedimentary rocks centred on Lake Nipigon, north of Lake Superior, approximately 100 km northeast of Thunder Bay, Ontario (Fig. 1). The Embayment is underlain by Archean rocks of the Wabigoon Subprovince to the north, and the Quetico Subprovince to the south. This field trip will be concentrating on the Proterozoic and underlying Archean rocks of the southern portion of the Embayment. u nI_u • nil nuN UU The first geological work in the Lake Nipigon area was conducted by Bell (1870) for the Geological Survey of Canada. Portions of the area were examined by McInnes (1896), Parks (1901), Coleman (1909), and Wilson (1910). Geological mapping by Coates (1972) covered the area south of Lake Nipigon at a scale of 1:63,360. The adjacent area to the south was mapped Figure 1. Mafic and ultramafic intrusions of the Nipigon Embayment. by McIlwaine and Tihor (1975a,b) at 1:15,840 scale. The area surrounding Lake Nipigon, and extending to the southern edge of Black Sturgeon Lake, was mapped at a scale of 1:50 000 by Sutcliffe (1982a,b; 1985a,b,c). Sutcliffe (1986) completed a doctoral thesis on the intrusive rocks of the Lake Nipigon area, including the Nipigon diabase sills and ultramafic intrusions of the Disraeli and Seagull-Leckie Lakes areas. Franklin et al. (1980), Cheadle (1986), and Rogala (2003) have carried out detailed investigations of the stratigraphy of the Sibley Group. Bedrock mapping and research studies were undertaken between 2003 and 2005 as part of the Lake Nipigon Region Geoscience Initiative (LNRGI). The LNRGI is a geoscience-based geological data acquisition and compilation program managed by the Ontario Prospectors Association (OPA) and financially supported by the Northern Ontario Heritage Fund Corporation (NOHFC). The major goals of the initiative are to conduct research and collect new geoscience data that will help to explain the geological history of the area, and identify areas of high mineral potential that will attract exploration to the region. The initiative included bedrock mapping projects (MacDonald, 2004; MacDonald and Tremblay, 2005), airborne magnetic and radiometric geophysical surveys (Ontario Geological Survey, 2004a), ground gravity surveys (Ontario Geological Survey, 2004b), and targeted surficial geochemical and lineament studies (e.g., Barnett, 2004; Dyer, 2004), a paleomagnetic study (e.g., Ernst et al., 2005) and geochronological studies (e.g., Heaman et al., 2005). Mapping of the southern portion of the Nipigon Embayment was completed by the Ontario Geological Survey as part of its commitment of in-kind support to the LNRGI (Hart and Magyarosi, 2004; Hart, 2005a). Lakehead University, as a partner in the Initiative, contributed a detailed sedimentary basin reconstruction (e.g., Metsaranta and Fralick, 2004) and petrochemical characterization of the mafic and ultramafic intrusions (e.g., Heggie and Hollings, 2004; Richardson and Hollings, 2005). - 85 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Mineral exploration and prospecting in the area has been sporadic with the first recorded activity being the exploration for iron ore in the early 1900s (Coates, 1972). Exploration in the early 1900s also resulted in discovery of lead and zinc mineralization to the southwest, and limited production (Shklanka, 1969). The discovery of sporadic copper mineralization near Disraeli Lake in 1965 resulted in a surge in copper and the other base metals in the late 1960s (Coates, 1972). Exploration for uranium has been cyclical, with the greatest activity occurring in the middle to late 1970s coinciding with the discovery of large unconformity related deposits in other locations including the Wollaston Basin of Saskatchewan (Scott, 1987). A uranium occurrence was discovered by R.H. Sutcliffe during the 1981 field season and subsequently acquired and explored by Uranerz Exploration and Mining Corporation Limited until 1985 (Scott, 1987). A number of companies are actively exploring for uranium mineralization at the time this guide was being prepared. Several companies investigated the potential of sedimentary-hosted base metal mineralization in the Sibley Group in the early 1990s, but other than an airborne geophysical survey completed by Cominco in 1993, little of the work was filed for assessment. Exploration for PGE mineralization intensified after the 1998 discovery of mineralization in the Seagull/Leckie Lake intrusion (Osmani and Rees, 1998). Initial results of diamond drilling indicated the presence of a basal pyroxenite hosting up to 1.71 ppm Pt and 1.87 ppm Pd over 2.1 m (Durham, 2000). Resampling completed in early 2004 defined two zones higher in the intrusion with up to 1.52 ppm Pt and 1.78 ppm Pd over 3.0 m and 1.56 ppm Pt and 1.87 ppm Pd over 3 m in the upper and lower zones respectively (East West Resource Corporation, 2004a). Drilling in late 2004 intersected a mineralized zone consisting of 2.38 g/t Pt, 2.65 g/t Pd, 0.19 g/t Au, 0.18 g/t Rh, 0.39 g/t Os, 0.34 g/t Ir and 0.08 g/t Ru (Pd:Pt ratio of 1.11) associated 0.19% Cu and 0.20% Ni over 3.21 metre (Platinum Group Metals Ltd., 2005). General Geology The southern portion of the Nipigon Embayment is underlain by Archean rocks of the Quetico subprovince that are unconformably overlain by Proterozoic sedimentary rocks of the Sibley Group (Fig. 2). Rocks of the Quetico Subprovince and the Sibley Group are io a Kilometres C N!p!g&,] UIt,amafi cIrtr(]iOns s'brey Group eotite Granite [3] Muscovite Gran!Ie FJ Faults Roads Outco Metaseth,]&nta Rocks Transmission Line Gas Pipel,ne Figure 2. Geology of the southern portion of the Nipigon Embayment. - 86 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 intruded by Proterozoic mafic to ultramafic intrusive rocks in the Disraeli and Seagull Lakes, and Hele and Kitto township areas (Fig. 2). Mafic sills of the Nipigon Diabase Sill Complex intrude all other rock types in the map area. Rocks of the Quetico Subprovince consist of biotite and/or andalusite schists that are gradually replaced towards the south by amphibolites (Hart, 2005a). The rocks to the north have a schistosity oriented at approximately 050° to 060°, with near vertical dips. In the south, leucogranites intrude the amphibolite or biotite gneisses and form complex mixtures that have been classified as migmatite. The airborne magnetic survey indicates a regional fabric with about an 080° to 090° orientation in the migmatitic rocks (Ontario Geological Survey, 2004). A number of areas east of the Black Sturgeon River contain a high percentage of coarse grained to pegmatitic irregular bodies and dykes of the muscovite granite suite. Irregular bodies of medium to coarse-grained rocks of the biotite granite suite intrude the amphibolites and leucocratic phases form the leucosome of the migmatitic rocks. Widely spaced, weakly deformed and metamorphosed, north trending diabase dikes intrude the Archean rocks of the Quetico Subprovince. A paleomagnetic study of similar dykes located to the west, in the area of Highway 527, by Ernst et al. (2005) has interpreted these dykes to be part of the early Proterozoic 2121 – 2101 Ma Marathon dyke swarm. The Proterozoic Sibley Group sedimentary rocks unconformably overlie the Archean rocks of the Quetico Subprovince, and are relatively flat-lying, with dips of less than 5° and strikes that are usually difficult to determine. Rocks within this area have been subdivided in to three relatively flat-lying formations, the lower Pass Lake, the middle Rossport, and the upper Kama Hill (e.g., Cheadle, 1986; Fig. 3). The Pass Lake Formation in this area is best observed in drill core, and consists of conglomerates, quartz arenites, and minor dolomites, with rare outcrops of the upper sandstone members. A sandstone of the Pass Lake Formation has a youngest detrital zircon grain with a U-Pb age of 1670 Ma (Heaman et al., 2005). The Rossport Formation consists of mudstone, dolomite and siltstones of the lower Channel Island Member, limestone and stromatolitic limestone of the Middlebrun Bay Member, and predominantly muddy variably carbonatized siltstones and mudstones of the upper Fire Hill Member (see Fig. 3). The Kama Hill Noo Diabass $11 Oonlsx, 1110le1113 S 2005) C,ta,I laed Fom,ton atleasI3silsha,ebeeelifle, d450 Ma Hea!,sa,s $1 at.. 2005) dykes may e*1ea let tholt dlanc.s from the sills .olpresenlirlmap area -1339 M -d€it!!]l dQslt tFrakls]197fl) __— Kane ll FoIrnallo,, mar!csl, one lntades lw el!lts!0 I Pi!a H!ll Mmb. —1ROSSPO,tFOlmalior. SwI. Hale. Dali t,ln,SlOflS. 1106— 1124 Ma (ReAmanetal.2005) Legend Dabs.. s!lI 4e H Mdlethn Say Membr Rosspefl Fomlaijo,, •P PsssLa&sF Ch..,! Id Mrsb., Met JCeflsls!]dMemt.r 1ROSSPOflFO!n,akn,. fl,eH!llMembe[ 0_ Ks H!II Flm!.reks Gs... I Pass Lake Fo,aleUon 2121-2101 MaMaralliorl dib dyks mscoe grane and I lO70Ma O*aman.tal.2005) I blotice andior andalusile ___-I hbls mpl]ibd. blSbs Oa salt.. Figure 3. Generalized stratigraphic column for the southern portion of the Nipigon Embayment. Formation is the uppermost formation of the Sibley Group exposed in the map area and consists of laminated mudstones and siltstones with sandstone prominent in the upper section. A sandstone of the Outan Island Formation, which overlies the Kama Hill Formation has a youngest detrital zircon with a U-Pb age of 1450 Ma (Heaman et al., 2005). A sandstone of the Nipigon Bay Formation, which overlies the Outan Island Formation, has a youngest detrital zircon grain with a U-Pb age of 1657 Ma (Heaman et al., 2005). A Rb/Sr age of 1339 +/- 33 Ma based on analyses of a combination of Rossport and Kama Hill formation samples by Franklin (1978), probably denotes a post-depositional diagenetic event. Cheadle (1986) estimated up to 200 m of topographic relief on the Archean paleosurface at the time of deposition of the Sibley Group. Although the bedding is generally flat-lying on an outcrop scale, these extremes in topographic relief probably controlled sedimentation processes on a local scale complicating stratigraphic correlations (P. Fralick and B. Rogala, Lakehead University, personal communication, 2003). Regionally, the Sibley Group thickens to the south under Lake Superior with two additional formations - 87 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 overlying the Kama Hill formation (Rogala, 2003), and thins to the north and west (e.g., Cheadle, 1986). may be difficult to distinguish from the diabase sills without petrography and lithogeochemistry. Mafic to ultramafic intrusions occur in the Disraeli and Seagull-Leckie Lakes, Hele and Kitto township areas (Fig. 1), and all four are composed of a pyroxene peridotite core with an irregular olivine gabbro zone along the margin (e.g., Hart et al., 2002b; Hart and Magyarosi, 2004). The Seagull Intrusion has been studied in the greatest detail, and petrography indicates that it consists predominately of cumulate textured dunites, lherzolites, and olivine websterite, with minor olivine- hornblende pyroxenite (Heggie and Hollings, 2004). An olivine gabbro forms an irregular border zone to the ultramafic rocks, and is the unit most commonly exposed in outcrop. All of the intrusions have a monzogabbro, or granophyre, unit occurring as irregular pods or bands in the olivine gabbro, but are distinguished from it by the presence of abundant pink feldspar and minor amphibole. An olivine gabbro from the Hele Intrusion has a U-Pb baddeleyite age of 1106+/-1.5 Ma with one fraction yielding a 1119.4 Ma age (L. Heaman, University of Alberta, personal communication, 2004). Samples from the Seagull Intrusion have U-Pb ages from zircon and baddeleyite of between 1105 and 1124 Ma (L. Heaman, personal communication, 2004). Samples from the Disraeli Intrusion have U-Pb ages from baddeleyite and zircon of 1111 Ma (L. Heaman, personal communication, 2004). Samples of an olivine gabbro from the Kitto Intrusion has a U-Pb baddeleyite age of 1117 Ma, but has baddeleyite fractions with a range of ages comparable to the other intrusions (L. Heaman, personal communication, 2004). Diabase sills have chill contacts with olivine gabbro of the Seagull, Hele and Kitto intrusions. A series of shallow-dipping Proterozoic diabase sills intrude all other rock units in the map area. The sills display a range of U-Pb zircon ages from 1110 to 1113 Ma from sills located west of Lake Nipigon (Heaman et al., 2005). The diabase sills have variable internal subdivisions, a generalized section from top to bottom would contain: 1) chill, 2) fine-grained variably amygduloidal, 3) magnetite-rich, 4) medium to coarse-grained, 5) coarse to very coarse-grained, 6) fine-grained, variable amygduloidal, and 7) chill (Hart and Magyarosi, 2004). These subdivisions resemble those proposed by Sutcliffe (1986). Massive, mediumto coarse-grained feldspar and pyroxene diabase is the most common rock type and generally lacks a sub-ophitic or diabasic texture. Although these rocks should properly be classified as gabbro the diabase classification has been applied to all rocks associated with the sills to avoid confusion with other intrusions in the area. The sills range in thickness from less than 5 m to greater than 180 m and there appears to be at least two major sills greater than 100 m in thickness. However, erosion and block faulting hinders the correlation of the sills and thus the determination of the original number of sills. A group of fine grained, massive mafic sills with a higher olivine content may intrude the Sibley Group sedimentary rocks. Some of these sills are spatially associated with the ultramafic intrusions, as in the Moraine Lake and Hele Township areas, and probably represent extensions of the intrusions (Hart, 2005). In the Shillabeer Lake and Kama Hill areas, similar sills are not associated with known ultramafic intrusions (e.g., Richardson and Hollings, 2005; Hart, 2004) and may be comparable to the Jackfish Sills described by MacDonald (2004) in the English Bay area. One of these sills intrudes the Disraeli Intrusion suggesting that some examples of this group post-date the larger ultramafic intrusions (Hart, 2005). This group of sills Quaternary ice-marginal deltaic deposits of the Nipigon Moraine form an extensive cover along the Black Sturgeon River and the area immediately to the west of the river (Barnett, 2004). The rest of the map area further from the river is covered by differing thickness of till that is reported by Barnett (2004) to be mainly locally derived. Regional metamorphism of the Archean rocks of the Quetico Subprovince varies from upper greenschist to lower amphibolite facies, with limited areas of migmatite. Hornfels metamorphism, in zones up to 10 m wide, is variably developed in the Sibley Group in contact with the peridotites, and occasionally the diabase sills. Hematite and specular hematite is associated with some late north-trending fracturing in both Archean and Proterozoic rocks, and appears to be a useful indicator of the north-trending fault systems in this area, where most structures are hidden by talus and glacial deposits (Hart and Magyarosi, 2004). Metasedimentary rocks of the Quetico Subprovince have experienced multiple phases of deformation. Schistosity is well developed along the bedding planes - 88 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 obscuring primary structures but the rocks appear to be the metamorphosed equivalents of the greywackes and siltstones present in the Beardmore-Geraldton greenstone belt (Hart et al., 2002a). North- and northwest-trending regional fault trends have displaced the Proterozoic Sibley Group, and a northeast-trend may only represent minor late movement along previous Archean structures (Hart and Magyarosi, 2004; MacDonald, 2004). The north and northwest trending faults, define the Black Sturgeon Fault Zone, which has been interpreted to be en échelon resulting in the formation of a block faulted asymmetric basin or graben (e.g., Coates, 1972). There appears to have been little lateral displacement based on the correlation of the iron formations across Black Sturgeon Lake in the area north of the current map area (Hart and Magyarosi, 2004). The north trending faults are interpreted to be regionally continuous structures that may correlate with north-trending structures in the Gull Bay area, approximately 150 km to the north. The northwesttrending faults display vertical movement with no apparent horizontal displacement, and may be traced to the west into the adjacent Wabigoon Subprovince. Accurate determination of displacements on these structures is not possible as they have been obscured by talus slopes and glacial deposits. north-trending and centred on Lake Nipigon. The paleomagnetic and geochemical study of the northtrending dykes located to the west of the Nipigon Embayment by Ernst et al. (2005) suggests that they are part of the 2101 to 2121 Ma Marathon dyke swarm, not Keeweenawan. If the identification of these dykes as part of the Marathon swarm is indeed correct, then combined with the lack of a north trending dyke swarm north or east of the Nipigon Embayment, a key element of the failed arm model appears to be absent. The timing of the different faults is not known, and it is probable that they have been reactivated. There have been several proposals made to account for the structural history of the area, with most focussing on the Sibley Group sedimentary rocks. Coates (1972) proposed a half graben with the Black Sturgeon River as the eastern boundary that also served as the main control on the deposition of the Sibley Group. Franklin et al. (1980) proposed that a failed arm of the Mesoproterozoic Keweenawan Midcontinent Rift created a basin, which included the half graben. Sutcliffe (1986) further developed the failed arm model as a major control on the emplacement of the peridotite intrusions and the diabase sills, and highlighted the existence of a subparallel fault system along the Nipigon River to the east. Alternatively, Fralick and Kissin (1995) and Hollings et al. (2004) proposed that the thermal upwelling that formed the English Bay Complex was followed by relaxation and formation of an intracratonic basin in which the Sibley Group was deposited. Timing and magnitude of displacement along faults related to the emplacement of either the ultramafic intrusions or diabase sills is also difficult to determine. The high degree of fracturing and hematite alteration in some of the diamond drill core from the 1106-1124 Ma Seagull Intrusion (Heaman et al., 2005) suggests post emplacement fault activity. Extensive hematite and the occurrence of uranium mineralization in north trending structures has been dated at 1090±20 Ma (Ruzicka and LeCheminant, 1984). Current interpretations suggest that faulting has resulted in displacement of igneous horizons within the Seagull intrusion, but it is just as possible that the intrusion was emplaced into a preexisting structure to produce its current form. Preexisting fault structures probably, in part, controlled the emplacement of the 1111-1113 Ma Nipigon sills (Heaman et al., 2005), as has occurred in other mafic sill complexes (e.g., Leaman, 1975). However, due to the lack of a distinctive marker in the area, it is difficult to determine if the apparently disjointed nature of many of the sills is the result of the sills ramping upward along pre-existing structures and subsequently being eroded away, or if it is a product of late post- The failed arm model predicts the presence of a dike swarm (e.g., Ernst and Buchan, 2003), in this case, The presence of Pass Lake Formation rocks at the base of a number of diamond-drill holes in the map area suggests that the half graben was present before, or was formed during, the deposition of the Sibley Group, sometime between 1670 and 1339 Ma (Heaman et al., 2005; Franklin, 1978). The occurrence of Pass Lake Formation at the base of Moseau Mountain, on the east side of the Black Sturgeon River, suggests that fault activity continued throughout the entire period of sedimentation, or that there has been significant postsedimentation fault re-activation. At this time, there is no definitive means of distinguishing between these models although, the location of Pass Lake Formation rocks at higher elevations to the east could mean that sedimentation originally extended across much of the Nipigon Embayment, consistent with the first model. - 89 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 emplacement fault displacement. 9 Geochemistry 8 Central suite Osler Group 7 Hele Intrusion 6 The Hele Intrusion may be subdivided into olivine gabbro, olivine melagabbro, and peridotite based on their MgO contents, with the peridotite having between 19 - 29 wt.% MgO and the olivine gabbro having MgO contents of less than 11 wt.% MgO. Elemental variations in diamond drill core samples appear to indicate that the Hele Intrusion is composed of more than one pulse of magma, resulting in an interlayering of the peridotite and melagabbro. Pink feldspar may occur as a minor mineral phase in the olivine gabbro, and is a major constituent of the monzogabbro, or granophyre. The presence of pink feldspar usually correlates with higher K2O content that may be up to 7.42 wt.%, with no appreciable change in the Fe2O3 contents compared to the other olivine gabbro. Although the high K2O is possibly related to late stage igneous fractionation, it is considered more likely that these values are a result of assimilation of Sibley Group rocks and that the pink colour is a result of potassic alteration rather than hematization. Total rare earth element (REE) and high field strength (HFSE) concentrations in the peridotite 100 I x-1r o osa S a m p le /p rim itiv e m a n tle DI 1000 10 1 0.5 Th Ta Ce Nd Hf Eu Gd Dy Ho Tm Lu Sc Nb La Pr Zr Sm Ti Tb Y Er Yb V Figure 4. Mantle-normalized extended element diagrams of the olivine gabbro, melagabbro and peridotite of the Hele Intrusion (normalizing factors from Sun and McDonough, 1989). L a /S m Mafic to Ultramafic Intrusive Rocks 5 4 Lower suite Osler Group 3 Upper suite Osler Group 2 1 1.0 1.8 - 2.7 3.5 4.3 5.2 6.0 Gd/Yb Figure 5. Gd/Yb versus La/Sm diagram of the mafic to ultramafic intrusive rocks and the Nipigon diabase sill complex with fields for the volcanic rocks of the Osler Group (after Lightfoot et al., 1991). Legend for symbols, see Figure 4. samples are generally lower than in the olivine gabbro or olivine melagabbro (Fig. 4). All of the rocks have weak negative TiO2 and Zr anomalies on the mantle normalized extended element diagram (see Fig. 4), and these anomalies are more pronounced than negative Nb-Ta anomalies for the Seagull and Disraeli intrusions (Hart and Magyarosi, 2004) although all three intrusion have flattening of the trends between Th and La. All three units have [La/Yb]mn (mantle normalized) and Gd/Yb ratios which are distinctly higher than the diabase sills, and plot toward the high end of the field defined by the Lower Suite of the Osler Group volcanic rocks (Hart, 2005; Fig. 5). Rocks of the Hele Intrusion have a wider range of Gd/Yb ratios than the rocks from the Disraeli and Seagull intrusions, but overlap with the values for the other intrusions. The peridotite and olivine gabbro have REE and HFSE ratios that are comparable to ocean island basalts and most of the rocks from the Disraeli and Seagull intrusions in contrast to the diabase sills (Fig. 6). The peridotite and gabbro are not enriched in nickel, and the peridotites follow an olivine-controlled igneous fractionation trend, which would be consistent with a parental MgO concentration of approximately 13 wt.% (Fig. 7). Both units have a flat positive slope on a primitive mantle normalized PGE diagram with enrichment in Ir and Pt (Fig. 8), compared to the trends for the diabase sills that follow a trend similar to that of - 90 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 (Fig. 9), but the [La/Yb]mn are distinct from those of the rocks of the mafic to ultramafic intrusions. Most of the diabase samples have weak negative TiO2 anomalies, but no apparent Zr anomalies, on the mantle normalized extended element diagram (see Fig. 9), and pronounced negative Nb-Ta anomalies. Some HFSE and REE ratios for the diabase show some similarities to the Central and Upper Suite volcanic rocks of the Osler Group as has been suggested by Sutcliffe (1987) (see Fig. 5). most continental flood basalts (Hart et al., 2002a). 20 Upper Suite Central Suite 10 Upper Crust SZB T h /Ta Logan sills Primitive Mantle The Nipigon diabase sills are not enriched in nickel, and follow an igneous fractionation trend (see Fig. 7). PGE contents of the diabase are similar to the trend followed by most continental flood basalts, and comparable to the PGE contents and trends noted for the diabase sills from the Beardmore area by Hart et al. (2002a; Fig. 10). OIB OPB 1 NMORB Lower Suite MORB 0.5 0.8 1 10 50 La/Yb Figure 6. La/Yb versus Th/Ta diagram of the mafic to ultramafic intrusive rocks and the Nipigon diabase sill complex with fields for the volcanic rocks of the Osler Group and the Logan diabase sills south of Thunder Bay (after Condie et al., 2002). Legend for symbols, see Figure 4. 2000 Within plate basalt Ni (ppm) 1000 Upper crust 100 Mapping and geochemical sampling by Sutcliffe (1986) and Hart et al. (2002a) suggested that the sills do not represent single cooling units. However, the results of detailed sampling of a number of continuous sill sections from diamond drill core suggest that some sills may be single cooling units. Whereas others have chemical variations suggesting a more complex history of emplacement that may include multiple pulses of magma that did not re-equilibrate before cooling. However, the variations are subtle and generally not distinctive enough to be evident in a group of outcrop samples. The diabase sills have HFSE and REE ratios with some similarities to subduction zone basalts and the Central and Upper Suites of the Osler Group (see Fig. 6). 1000 10 0 10 20 30 40 MgO (wt. %) Figure 7. MgO versus nickel diagram of the mafic to ultramafic intrusive rocks and the Nipigon sill complex (after Lightfoot et al., 2001). Legend for symbols, see Figure 4. S a m p le /M a n tle P G E 100 10 1 0.1 0.01 Nipigon Diabase Sill Complex The diabase sills plot along a line indicative of fractionation controlled mainly by plagioclase and clinopyroxene, as suggested by Sutcliffe (1987). The diabase sills display a range in REE and HFSE concentrations that will require further investigation .001 Ni Ir Ru Rh Pt Pd Au Ag Cu Figure 8. Primitive mantle-normalized platinum group element diagrams of the olivine gabbro, melagabbro and peridotite of the Hele Intrusion (normalizing factors from Barnes et al., 1987). Legend for symbols, see Figure 4. - 91 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 and low TiO2 rocks is similar to regional distributions observed in some flood basalt provinces suggesting that the Logan and Nipigon sills may be part of a single intrusive event. S a m p le /p rim itiv e m a n tle 1000 100 10 1 0.5 Th Ta Ce Nd Hf Eu Gd Dy Ho Tm Lu Sc Nb La Pr Zr Sm Ti Tb Y Er Yb V Figure 9. Mantle-normalized extended element diagrams of the Nipigon Diabase Sill Complex (normalizing factors from Sun and McDonough, 1989). Legend for symbols, see Figure 4. 1000 S a m p le /M a n tle P G E 100 10 1 The presence of variants of the sills that have specific geochemical and/or geographic distribution has resulted in the introduction of a variety of new, informal, names for the diabase sills in the past few years, such as the distinction between the Logan sills in the Thunder Bay area and the Nipigon sills around Lake Nipigon. In addition, new sill types, such as the Inspiration sills have also been identified (e.g., MacDonald, 2004; Richardson and Hollings, 2005). This does cause a problem, however, in that the historical term Logan Sills has been used exclusively in the paleomagnetic literature, and in much of the international literature, for all of the diabase sills north of Lake Superior. Until these terminology issues are resolved, it is recommended that the historical nomenclature of Logan Sills should be applied to all the diabase sills in the area north of Lake Superior, with subdivision into the informal terms, Logan sills in the south and Nipigon sills to the north. The Nipigon sills may be further subdivided into several chemical variants, such as the normal Nipigon, Inspiration, and Jackfish-like sills. Clarification of this nomenclature is contingent on further geochemical, petrological and geochronological work. Mineralization 0.1 Platinum Group Elements Hele Intrusion 0.01 .001 Ni Ir Ru Rh Pt Pd Au Ag Cu Figure 10. Primitive mantle-normalized platinum group element diagrams of Nipigon Diabase Sill Complex (normalizing factors from Barnes et al., 1987). Legend for symbols, see Figure 4. The Nipigon diabase sills have lower TiO2, Zr/Y, La/ Yb values than the Logan diabase sills located to the south of Thunder Bay (see Fig. 6) and have a regional distribution that extends south to at least the north side of the City of Thunder Bay (Hart, 2004). Several samples collected from the area south of Thunder Bay have geochemical characteristics comparable to the Nipigon diabase sills (see Fig. 6), but further investigation is required. This regional geographic distribution of high Mineralization has not been located in either of the two diamond drill holes or in outcrop of the Hele Intrusion. The peridotite and olivine gabbro have similar PGE contents with Pt/Pd ratios of 1.03 to 4.63 with the highest values being 75.87 ppb Pt, 50.24 ppb Pd and 24.07 ppb Au in an olivine gabbro (Hart, 2005). The peridotite and olivine gabbro also have similar base metal contents with 18 to 1320 ppm Ni, 17 to 459 ppm Cu, and 72 to 280 ppm Zn. Seagull Intrusion Mineralization in the southern portion of the Nipigon Embayment occurs as three reef style zones hosted by the Seagull Intrusion. The upper zone contains up to 1.52 ppm Pt, 1.78 ppm Pd, 0.34 ppm Os, 0.11 ppm Rh, 0.23 ppm Ir, 0.15 ppm Au, 0.58% Cu, and - 92 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 0.34% Ni over 3 m (East West Resource Corporation 2004a,b). The lower zone contains up to 1.56 ppm Pt, 1.87 ppm Pd, 0.20 ppm Os, 0.10 ppm Rh, 0.18 ppm Ir, 0.15 ppm Au, 0.55% Cu, and 0.35% Ni over 3 m (East West Resource Corporation 2004a,b). The best intersection in the basal zone, close to the intrusion Quetico contact, is 1.71 ppm Pt and 1.87 ppm Pd over 2.1 m associated with 10% disseminated to net textured sulphide minerals including pyrite, pyrrhotite and minor chalcopyrite (Durham, 2000). Diamonddrill holes completed in 2004 intersected a zone near the base of the intrusion consisting of 2.38 g/t Pt, 2.65 g/t Pd, 0.19 g/t Au, 0.18 g/t Rh, 0.39 g/t Os, 0.34 g/t Ir, 0.08 g/t Ru, 0.19% Cu and 0.20% Ni over 3.21 metre (Platinum Group Metals Ltd., 2005). Mineralization in the basal zone is present near the lower contact of the intrusion, and is interpreted to be caused by sulphur saturation of the magma during initial stages of emplacement (Heggie and Hollings, 2004). The upper and lower zones are associated with more primitive whole rock geochemistry and mineralogy possibly reflecting an influx of less evolved magma. The mineralogy has been characterized by Heggie and Hollings (2004) and the following details are from that source. Nickel and copper sulphide minerals occur as disseminated bravoite and chalcopyrite respectively, found interstitial to olivine. Copper is also present as veinlets of native copper cross cutting sulphide minerals. Platinum group minerals are located at the grain boundaries of NiFe sulphide minerals, and are dominated by sperrylite, keithconnite, stibiopalladinite, and a copper palladium alloy. Stops Field Trip Road Log Stop 1a 1b Locality Junction of Highway 585 and Highway 11/17 in the Town of Nipigon Red Rock Black Sturgeon River Rock quarry north of Big Squaw Creek Road cut Big Squaw Creek Road Cut Junction of Highway 11/17 and the Stewart Lake Road Reset Junction of the Stewart Lake Road and Fowlkes Lakes roads km 0 5.5 16 19 19.5 26.5 5.9 Stop 2 3 4 5 6 7a 7b Locality km Reset Hele Intrusion peridotite 2.9 Junction of the Fowlkes Lakes and Driftstone Lake roads 6.3 Reset Hele Intrusion olivine gabbro 3.9 Return to Stewart Lake Road Reset Junction of Stewart Lake and Black Sturgeon roads 3.8 Reset Black Sturgeon Road 3.3 Eagle Mountain 20.3 Junction of Black Sturgeon and Camp 42 roads 22.5 Reset Junction of Camp 42 and Church Road 14.1 Reset Andalusite schist 7 Reset Return to south to junction of Camp 42 and Church Road 7 Reset Road to left 0.75 Granite-diabase contact. South on trail 0.2 Pegmatite 12 Trip starts at the junction of Highway 585 and 11/17 in Nipigon. Proceed southwest on Highway 11/17. 5.5 km - Rossport Formation Oxidized units of the Rossport Formation of the Sibley Group are exposed in the hillside on the east side of the highway. These rocks unconformably overlie Archean rocks of the Quetico Subprovince, and are intruded at the top of the hill by a diabase sill of the Nipigon Sill Complex 16 km - Black Sturgeon River (Photo 1) There are two major regional fault trends, north and northwest, that define the structural zone referred to as the Black Sturgeon Fault Zone. The combination of these two trends has been interpreted to be en échelon resulting in the formation of an asymmetric basin or graben, as originally proposed by Coates (1972). A series of subparallel north-trending faults are interpreted to be regionally continuous structures that can be traced - 93 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 by geophysics and topography for over 150 km north into the area of the English Bay Complex. A series of northwest-trending faults can be traced for similar distances to the west into the Wabigoon Subprovince. Apparent flexures in the north-trending faults probably represent intersections with northwest-trending faults. Vertical displacements on these faults are varied (may be up to 350 m) and are visible along the Black Sturgeon River, interpreted by Coates (1972) to be the halfgraben hinge. There appears to have been little lateral displacement on these faults based on the correlation of the Archean iron formations that extend across Black Sturgeon Lake (Hart and Magyarosi, 2004). Corridors of north-trending faults cut, and may have controlled the emplacement of, the Seagull and Disraeli peridotite intrusions, and possibly the Hele peridotite intrusion. 19 km - turn right onto natural gas pipeline access road on the north side of the highway and immediately turn left and follow trail along the north side of the highway. Proceed 200 m to the edge of the gravel pit * r 4 • 1. 'r 1 *: and park vehicles. Walk for 115 m along trail heading north and skirting the edge of the gravel pit. Stop 1a - Rock Quarry UTM coordinates – 0396320E 5416664N This stop is located on the south side of a rock quarry in a diabase sill which produced fill for highway construction. This outcrop is the upper contact zone of the diabase sill that extends to the south into the Big Squaw Creek road cut. This contact zone is a relatively rare variant of the typical chilled contact consisting of pebble sized, rounded xenoliths of quartz and feldspar in a biotite-rich fine-grained diabase. Contact relationships observed to the south, in the next stop, indicate that the diabase is intruding sedimentary rocks of the Sibley Group. The composition of the country rock probably influences the formation of this relatively rare contact type, as the diabase sills commonly have glassy chilled contacts. This type of contact zone has been interpreted to be a result of the partial melting and assimilation of country rock xenoliths resulting in a quartz-feldspar restite and a biotite-rich diabase. Similar quartz-feldspar xenoliths in a hornfels carbonate-rich sediment are observed along some contacts in the road cut to the south suggesting that the presence of a carbonate-rich unit may be important to the formation of this contact style. However, a similar contact zone is present in a diabase sill intruding rocks of the Quetico Subprovince without any obvious evidence of Sibley sediments, about 10 km southwest of Beardmore, near the Warnesford Quarry —. ' __ 7 4- fl'_. 4. 'C Photo 1. The erosional valley of the Black Sturgeon River following a northwest fault, with the Archean to the right (east) and Hele Intrusion to the left (west). The valley is approximately 800 m wide. Photo courtesy of J. Scott. Stop 1a. Fine-grained, massive, biotitic diabase matrix containing angular to subrounded pebble-sized xenoliths of coarse-grained quartz and white to pinkish feldspar. Markings on scale card are at 1 cm intervals. - 94 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 SeE! •% "tt' Stop 1b. The east block of hornfels units of the Rossport Formation surrounded by diabase sill along the north side of Highway 11/17. on the CNR railway. In other areas, intrusion of diabase and ultramafic sills into carbonate-rich Sibley sediment has resulted in the formation of variolitic sills with a chilled margin composed in part by densely packed fine varioles and a core of widely spaced large varioles. Variolitic textures have been observed southwest of Moraine Lake, in a sill cut by diamond drill hole NI92-7 located west of Disraeli Lake, and in and drill hole HE02-01 below the Hele Intrusion. Return to the highway and turn right. Proceed west for 600 m, driving past the outcrop, located on the north side of the highway, and turn left into the entrance to a private dump to park. Stop 1b - Big Squaw Creek road cut UTM coordinates – 0396114E 5416374N A diabase sill which has intruded and metamorphosed blocks of Rossport Formation mudstone and dolostone is exposed in a rockcut along the north side of Highway 11/17, in east Stirling Township. The eastern block appears to have been lifted, resulting in a westerly dip, with diabase intruding underneath and from the east (see photo above). However, the block is abruptly truncated by a vertical contact to the west, and a less then 25 cm thick sill intrudes along the bedding eastward from this contact. The ambiguity of the injection direction of the diabase may be a result of the rock cut being oblique to the injection direction, and the block may not be totally separated from the surrounding Sibley Group. The western block is composed of sedimentary rocks comparable in composition to those in the east block but has a chaotic structure with no preservation of bedding, and is probably a detached, deformed and metamorphosed xenolith Metamorphism in the sedimentary rocks decreases away from the contact, and the sedimentary rocks close to the contact contain an assemblage of diopside, phlogopite, clinochlore, pargasite, and calcite (Hart, 2005). In some areas along the contact there are irregular zones to isolated pebble sized clasts of quartz and microcline within the calc-silicate hornfels. The quartz and microcline may represent a partial melt of the sedimentary rocks by the diabase, and may be equivalent to the xenoliths observed at the previous stop. A calc-silicate hornfels or skarn (Meinert, 1992) may be developed along the contact between the diabase sills and the Sibley Group sedimentary rocks, but is ubiquitous along the contacts between the peridotites and Sibley Group contacts (Hart, 2005). Where present, the hornfels is varied in width, extending up to 10 m into the sedimentary rocks, although an approximately 60 m thick layer of hornfels occurs in the Rossport Formation between the two diabase sills in diamonddrill hole ST02-01 (Hart and Magyarosi, 2004). The effects of metamorphism are most pronounced in the Rossport Formation, with the mudstone and siltstones beds commonly developing a light green, mauve, reddish gray to light gray banding. Dolomite-rich beds form light green, white to buff marbles dependent on the original mud content. Radial texture in 3 to 8 cm wide zones occur along some sill contacts, with radially oriented amphibole of the 0.2 to 0.5 cm in diameter. - 95 - During his regional study of the Nipigon diabas sills, �Proceedings of the 51st ILSG Annual Meeting - Part 2 Sutcliffe (1986) identified two common assemblages of fine-grained, calc-silicate minerals, calcite-tremoliteforsterite (clinochlore) and calcite-diopside-fosterite at lower and higher metamorphic grades, respectively. The forsterite is altered to serpentine. No dolomite and only minor quartz were observed. Rogala (2001) completed a study of interbedded mudstones and dolomites of the cyclic facies of the Channel Island Member and lower Middlebrun Bay Member of the Rossport Formation, intersected by diamond drill hole NI92-5 located west of Disraeli Lake. These rocks contained a mineral assemblage of pargasite, tremolite, talc, clinochlore, calcite and barite with varying amounts of hematite and apatite, halite, sylvite, gypsum and dolomite. The halite and sylvite were interpreted to be a result of evaporite minerals present in the Sibley Group and the presence of these minerals probably controlled the formation of pargasite (Rogala, 2001). The barite was interpreted to have been introduced into the Sibley Group by later fluid associated with the lead-zinc veining. Rogala (2001) noted that one sill contact had a thin band of pyrite with chalcopyrite, magnetite, ilmenite, galena, and rare gold and silver. High gold has been noted in a sample from the McVicars Resources Muskrat Lake diamond drill hole ML01-3 at 239.8 m and also in samples from along the shore of Lake Nipigon (A. Richardson, MSc student, Lakehead University, personal communication, 2004). Past workers have suggested that the copper mineralization may be related to the intrusion of the diabase or ultramafic intrusions (e.g., Coates, 1972). However, it is not clear if the metals are igneous or sedimentary in origin. The diabase is commonly medium brown to brownish grey, massive, medium- to coarse-grained feldspar and pyroxene with trace to 3% olivine and 1 to 2% magnetite. Medium- to coarse-grained diabase forms the majority of the sills and generally lacks a sub-ophitic or diabasic texture and should properly be classified as gabbro, but to avoid confusion with other intrusions in the area the diabase classification has been applied to all rocks associated with the sills. Portions of this outcrop contain ophitic textures consisting of pyroxene oikocrysts up to 2 cm in diameter, and in diamond drill hole the coarse-grained ophitic textured diabase is often interlayered with sub-ophitic diabase. Calculations by Sutcliffe (1986) suggested that the diabase sills were emplaced at shallow crustal levels, at a lithostatic load of between 0.06 and 0.43 kilobars. The sills were estimated to have crystallized over a temperature range of 1100° to 800°C, adjacent wall rocks were estimated to have reached 665°C, and the metamorphic mineral assemblages suggest temperatures of above 540°C. 26.5 km - west along Highway 11/17, and turn right on to the Stewart Lake Road - reset. 5.9 km follow the Stewart Lake Road for 4.2 km north and then west after a right-angled turn for an additional 1.7 km. Turn right onto the Fowlkes Lakes Road - reset. 2.9 km – north on the Fowlkes Lakes Road to where a skidder trail departs to the west. Stop 2 - Hele Peridotite UTM coordinates – 0386762E 5420119N Park the trucks and proceed by foot along the skidder trail for 500 m to the outcrop. Located in the southeast corner of Hele Township, the Hele Intrusion covers approximately 40 km2 and has a maximum thickness of 130 m, as defined by diamond drilling Peridotite is very poorly exposed in the central portion of the body, east of Driftstone Lake, and is interlayered with olivine gabbro and feldspathic peridotite in diamond drill core. The peridotite is best exposed in an area of clear cut logging located on the northern edge of Stirling Township as a highly weathered and serpentinized rock with a high magnetic susceptibility overlain by a diabase sill. The peridotite is dark green, medium to coarse-grained, massive, variably ophitic, and composed of olivine and pyroxene with variable plagioclase and trace to 1% fine-grained reddish brown mica (Hart, 2005). Numerous, subparallel serpentine and chlorite-rich fractures result in a banded or layered appearance to the otherwise massive peridotite. Contacts between the peridotite, feldspathic peridotite and olivine gabbro observed in diamond drill core are generally gradual transitions in grain size and olivine and plagioclase content. There appears to be more than one band of peridotite interlayered with the melagabbro and feldspathic peridotite suggesting internal subdivisions similar to the Seagull Intrusion. The Hele Intrusion appears to be a relatively flatlying sill intruding the Sibley Group with no known feeder zone, although there are fewer diamond drill holes with which to constrain the geometry of the intrusion. Geophysical inversion modelling of the airborne magnetic data by Desmond Rainsford - 96 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 grained, reddish brown mica is common in the olivine melagabbro in the central areas of the intrusion. An absence or very low abundance of mica and a lower concentration of olivine in the olivine gabbro which forms the northern portion and margins of the intrusion makes it difficult to distinguish these rocks from the olivine-bearing diabase sills. The gabbro contains pods of monzogabbro in outcrop (see photo below) and is interbanded with monzogabbro in the upper portion of the diamond drill holes. - It Stop 2. Example of parallel fractures filled with serpentine and minor biotite in the peridotite of the Disraeli Intrusion. Pencil is approximately 15 cm long. (geophysicist, Ontario Geological Survey) supports this interpretation (Hart, 2005). Inversion modelling of the ground gravity data indicated the presence of an anomaly located along the western edge of the surface expression of the intrusion and extending to the southwest. This anomaly may represent an extension of the ultramafic intrusion, but recent diamond drill results in the area of the Seagull Intrusion intersected a magnetite rich skarn associated with a similar gravity anomaly. The Seagull Intrusion hosts PGE mineralization and differs from the Hele Intrusion in having a lopolithic shape, and intruding along or close to the contact between the Quetico Subprovince and the Sibley Group. The central portion of the intrusion appears to be saucer-shaped, but this may be a product of post-intrusion faulting, and a flat-lying sill along the periphery of the body. An underlying, or peripheral, feeder structure for the Seagull Intrusion has not yet been identified. The monzogabbro, or granophyre, is generally medium to coarse-grained, and massive with a variable amphibole content and distinctive pink feldspar or groundmass that occur in irregular shaped pods up to a metre in diameter. These pods are best exposed along the logging trails in the northeast portion of the intrusion, and are hosted by medium to fine-grained olivine gabbro containing minor pink feldspar. Less commonly, the monzogabbro is fine-grained, and massive reddish pink with minor quartz. This type of monzogabbro is present in irregular pods and dykelets with sharp contacts, with and without the coarser grained monzogabbro, in the western portion of the intrusion. Some of the best examples are in the central part of the intrusion, close to a small exposure of Sibley Group sedimentary rock that is in contact with the upper portion of the intrusion. In diamond drill core, the two types of monzogabbro are generally closely associated and in some examples the finer grained variety is rimmed by the coarser grained rock. The abundance of pink feldspar correlates with elevated K2O values, and has been interpreted to be the result of the assimilation of 6.3 km – junction with the Driftstone Road and turn right - reset. 3.9 km – east along the Driftstone Road. Stop 3 - Hele Intrusion - olivine gabbro UTM coordinates – 0390358E 5424134N The olivine gabbro to olivine melagabbro forms much of the exposed area of the Hele Intrusion. The gabbro is fine to medium-grained, medium to dark green, massive and composed of pyroxene, up to 30% plagioclase, and up to 40% olivine. Trace to 1% fine •i :• Stop 3. Contact between olivine gabbro and an irregular pod of monzogabbro / granophyre, in the western portion of the Hele Intrusion. The pen magnet is approximately 16 cm long. - 97 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Sibley Group sedimentary rocks (Hart and Magyarosi, 2004). The finer grained monzogabbro may be a poorly assimilated sediment xenolith and the coarser grained monzogabbro may be the result of assimilation of the volatile rich sedimentary rocks. Turn vehicles around and return by the same route to the corner of the Driftstone Road and the Stewart Lake Road 10.2 km to the south. Turn right onto the Stewart Lake Road. 3.8 km - Stewart Lake Road ends at the Black Sturgeon Road. Turn right onto the Black Sturgeon Road and drive 3.3 km north. contact of the sill is evident, which appears to be a result of later movement along a northwest-trending structure. Joints are rare in the outcrops forming the interior of the sills, and when present are generally linear in nature. The column-like jointing is probably a result of late stage fracturing of the cooling magma combined with later fault activity rather than a true columnar jointing. 20.3 km – this portion of the Black Sturgeon Road is underlain by units of the Sibley Group and outcrops are rare. Stop 5 - Eagle Mountain UTM coordinates – 0385387E 5433471N Stop at the 20.3 km in the open space on the east side of the road for a view of Eagle Mountain. Stop 4 - Black Sturgeon Road UTM coordinates – 0385313E 5418737N Stop at kilometer 9 of the Black Sturgeon Road and look to the east across the clear cut, a diabase sill intrudes the Rossport Formation of the Sibley Group. Jointing in the diabase sills may be either arcuate or linear. The arcuate joints formed by cooling of the magma vary in strike direction by up to 70° and are commonly observed on horizontal surfaces. Jointing is best exposed in an area to the north, of the clear cut within the 1999 burn. Jointing located close to the sill contacts forms crude polygon joint patterns with “T” or flattened “Y” junctions, and the interiors of the sills lack the “Y” junctions commonly present in polygonal jointed unit (e.g., Aydin and DeGraff, 1988). This lack of “Y” junctions suggests a slow cooling rate which would be in agreement with the 315 to 560 year cooling times for the sills estimated by Sutcliffe (1986). Looking to the northeast, an offset in the basal Eagle Mountain is an approximately 10 km long ridge located west of the Black Sturgeon River and is the first of a series of north-trending fault bounded ridges located further west. The ridges are commonly capped by a diabase sill that has intruded units of the Kama Hill Formation. The lower portions of the ridges are generally poorly exposed but appear to be composed of units of the Rossport Formation. In some areas, as in Eagle Mountain, a second diabase sill intrudes the lower portions of the ridge. There were three diamond drill holes that were completed on the lower slope of Eagle Mountain that provide a stratigraphic section though the lower portion of the Sibley Group. Two of these holes intersected a mafic dyke, which is also exposed in a ravine on the central portion of the ridge. The 5 to 10 m wide fine-grained massive dyke dips approximately 50° south. This dyke appears to be related to a series of fine grained mafic sills with a higher olivine content. Although, these sills Stop 4. Nipigon diabase sill intruding units of the Rossport Formation located along the Black Sturgeon Road. Photo is looking to the southeast. - 98 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 within the wackes (Hart and Magyarosi, 2004). The presence of andalusite, with minor cordierite, suggests a low pressure, high temperature metamorphic event possibly related to the emplacement of the felsic intrusive rocks. The andalusite pseudomorphs are partially replaced by a mica indicating some degree of retrograde metamorphism. Stop 5. Looking north from the top of Eagle Mountain, with units of the Kama Hill Formation intruded, overlain, by a diabase sill. The sill – sediment contact is approximately along the upper tree line in the cliff. are geochemically similar to the ultramafic intrusions, they do not appear to be associated with any known ultramafic intrusion (Hart, 2005; Richardson and Hollings, 2005). 22.5 km – (km 26 on the Black Sturgeon Road) turn right at the load levelers, on to the Camp 42 Road reset. 14.1 km - continue east and then north on the Camp 42 Road until the junction with the Church Road reset. These metasedimentary rocks closely resemble the clastic metasedimentary rocks of the Quetico and Wabigoon subprovinces in the Beardmore area, east of Lake Nipigon (Hart et al., 2002a). For this reason, these rocks are interpreted to have accumulated in a trench environment of an accretionary complex comparable to the depositional environment of the Beardmore-Geraldton greenstone belt (e.g., Williams, 1989). The transition from the southern Wabigoon to Quetico subprovince is located approximately 25 km to the north, where a series of iron formations appear to correlate with the iron formations of the BeardmoreGeraldton belt (e.g., Sutcliffe, 1986). However, the iron formation exposed on the east side of Black Sturgeon Lake (Hart and Magyarosi, 2004) is comparable to the chert-magnetite iron formation in the Mawn Lake area (Coates, 1972) rather than the chert poor units of the Beardmore area. Turn vehicles around 7 km - proceed back along the Church Road to the junction with the Camp 42 Road 7 km – north along the Church Road. Stop 6 - Andalusite schist 0.750 km – trail to left, proceed south up hill along the trail These rocks consist of metamorphosed feldspathic wacke, lithic wacke, and siltstone with metamorphic grade increasing progressively toward the south (Hart 2005). The metawackes and metasiltstones are light grayish brown to medium gray, with 3 to 30 cm thick beds, and a schistosity parallel to bedding resulting in the variable preservation of original textures. These rocks commonly contain 5 to 10% biotite, trace to 1% very fine-grained garnet and, a trace subhedral mineral tentatively identified as cordierite. Specific bands are composed of individual to multiple beds containing 10 to 15% coarse- to very coarse-grained, subhedral to euhedral andalusite. The andalusite occurs disseminated through some beds and occasionally concentrated along parallel bands that appear to represent the original bedding plane. This distribution probably reflects the distribution of clay minerals, with higher Al2O3 content, 0.200 km – park and walk right, to the west, for ~10 m S UTM coordinates – 0388613E 5449432N Stop 6. Coarse-grained andalusite pseudomorphs in metamorphosed feldspathic wackes and siltstones of the Quetico Subprovince. - 99 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Stop 7a - Granite – Diabase Contact UTM coordinates – 0390536 E 5442913N The contact between the leucocratic muscovite granite of the Quetico Subprovince and the Mesoproterozoic diabase sill of the Nipigon Sill Complex is exposed in this flat-lying outcrop. The medium to coarse-grained leucocratic muscovite granite occurs are rounded xenoliths up to 20 cm in length in the chill zone of the diabase sill (see photo below). The sills is composed of a fine to very fine-grained, glomeroporphyritic diabase with polygonal jointing and some area have the glassy chill zone preserved. This outcrop is interpreted to be the upper contact of the diabase sill which is dipping shallowly to the north, and the hill to the southwest is capped to the same diabase sill. This same sill caps the hill above the outcrop at stop 7b. About 20 m to the right or east of the trail, an outcrop of the Quetico Subprovince granite is cut by a northeast-trending, approximately 8 cm wide, aphanitic diabase dyke. Dykes may occur close to the diabase sill contacts generally extending for less than a few metres, but may in some cases extend for a few tens of metres. This dyke is though to be injected from the diabase sill which is located beneath this outcrop. 1.60 km – return to the Camp 42 Road and continue back to the gravel pits. Stop 7b - Pegmatite and Nipigon Sill UTM coordinates – 0389764E 5442111N The west dipping outcrop consists of a muscovite granite dyke intruding biotite schists, and the hill above is capped by a diabase sill. Muscovite granite dykes in this area are generally a few metres in width but may be up to a few tens of metres wide, and are usually irregular in form and orientation although generally following the fabric of the surrounding country rock. This outcrop is composed of pegmatitic quartz, potassium feldspar, and plagioclase with graphic intergrowths of quartz and feldspar. Black, coarse-grained to pegmatitic tourmaline occurs as tabular grains and as graphic intergrowths with quartz. Fine grained subhedral, blue-green apatite occurs as an accessory mineral located along the contacts with the biotite schist country rock. This dyke is one of a number of dykes intruding the metamorphic rocks adjacent to the leucocratic muscovite granite. A body of leucocratic muscovite granite occurs to the north and northwest and is light gray, pinkish gray, to white, massive, and medium to very coarsegrained with occasional pegmatitic sections, composed of quartz and potassium feldspar, and plagioclase with typically less than 5% muscovite (Hart, 2005). Irregular to tabular shaped xenoliths of metasedimentary and schistose rocks up to a few metres in diameter are present through out, but appear to be more common to wards the margins of the body. Numerous muscovite granite dykes intrude the massive body and may constitute up to 30% of the outcrop. These dykes are commonly a few metres in width, but may be up to a The outcrop is located approximately 300 m east of the road, and is accessible by a trail leading from the back of the small gravel pit. Stop 7a. Xenoliths of Quetico granite in the glassy chill zone of a Nipigon diabase sill. Stop 7b. Black tourmaline in the leucocratic muscovite granite dyke. - 100 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 dykes contain intergrowths of tourmaline and quartz, and muscovite and quartz commonly observed in rocks that have been classified as fertile granites, or granites that have the potential to host rare-element mineralization (Breaks et al., 2003). Additional work is on-going to characterise these bodies. The muscovite leucogranite occurs along the regional trend extending from Georgia-Barbara lakes area, about 60 km to the northeast, to the DeCourcey Lake and Onion Lake areas, approximately 45 km to the southwest (Breaks et al., 2003), and this area may represent a similar concentration of fertile granites. Stop 7b. Northeast dipping diabase sill observed to the west of the pegmatitic dyke outcrop. few tens of metres. The granite dykes are composed of pegmatitic quartz, potassium feldspar, and plagioclase and frequently contain graphic intergrowths of quartz and feldspar, black, coarse-grained to pegmatitic, tabular tourmaline usually graphically intergrown with quartz and plumose intergrowths of muscovite-quartz. Trace to 1% fine-grained, disseminated, subhedral, reddish brown garnet, fine grained light green mica or rosy quartz was identified in a few of the dykes. Contact relationships suggest that the leucocratic muscovite granite and pegmatite dykes intruded the country rocks rather than being products of partial melting of the host country rocks. However, the initial mode of formation is not known and these rocks may have formed by partial melting of sedimentary rocks at depth. The pegmatitic, muscovite leucogranite body and Looking west across the valley, a diabase sill can be observed to dip shallowly to the northeast, whereas along the Black Sturgeon River the same sill can be observed to dip shallowly east (see Photo below). The shallow inward dipping orientation of this sill, forming a broad saucer with a width of about 5 km, is also visible in the airborne magnetic pattern (Fig. 11) (Ontario Geological Survey 2004). A number of these saucer-shaped sills, with widths of up to 6 km and interiors occupied by older rock types, are evident from airborne magnetic surveys both in this area and in sills intruding Archean rocks of the Wabigoon Subprovince to the northeast and northwest. Variation in sill orientation over short distances is common in other mafic sill complexes (e.g., Tasmania: Leaman, 1975; Karoo: Chevailler and Woodford, 1999). This change in orientation may be due to a number of factors including a change in orientation of the principal stress direction (Gretener, 1969) during sill emplacement, and the sill may follow pre-existing structures such as faults. There appear to be at least three major sills (100 to ~250 m), and possibly a number of thinner (5 to 25 m) sills in the area, based on a combination of outcrop and diamond drill hole information. However, correlation between outcrops and diamond-drill holes is complicated by the saucer shape of some sills, the presence of block faulting that may post-date sill emplacement, and the lack of distinctive geochemical differences between the sills. 12 km – back to the junction of the Camp 42 and Black Sturgeon roads 26 km – south to Highway 11/17 along the Black Sturgeon Road – bypassing the Stewart Lake Road Figure 11. First vertical derivative of the total field magnetic data the inward dipping saucer shaped diabase sill. Star marks the location of field trip Stop 7. - 101 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 References Office, assessment file 52A15-2.21989. Aydin, A. and DeGraff, J.M., 1988. Evolution of polygonal fracture patterns in lava flows; Science, v. 239, p. 471-476. Barnes, S.J., Boyd, R., Korneliussen, A., Nilsson, L-P., Often, M., Pedersen, R.B. and Robins, B., 1987. 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Electron microprobe and bulk analyses of fertile peraluminous granites and related rare-element pegmatites: Superior Province, northwest and northeast Ontario: Operation Treasure Hunt; Ontario Geological Survey, Open File Report 6099, 179p. Cheadle, B.A., 1986. Alluvial-playa sedimentation in the lower Keweenawan Sibley Group, Thunder Bay District, Ontario. Canadian Journal of Earth Science, v. 23, p. 527-542. Chevallier, L., and Woodford, A., 1999. Morpho-tectonic and mechanism of emplacement of the dolerite rings and sills of the western Karoo, South Africa; South African Journal of Geology, v.102, p. 43-54. Coates, M.E., 1972. Geology of the Black Sturgeon River Area, District of Thunder Bay; Ontario Department of Mines and Northern Affairs, Geoscience Report 98, 41p. Condie, K.C., Frey, B.A., and Kerrich, R., 2002. The 1.75Ga Iron King volcanics in west-central Arizona: a remnant of an accreted oceanic plateau derived from a mantle plume with a deep depleted component; Lithos, v. 64, p. 49-62. Coleman, A.P., 1909. Black Sturgeon iron region; Ontario Bureau of Mines, v. 18, part 1, p.163-179. Durham, B. 2000. Log for diamond drill hole WM-00-05 – Wolf Mountain Joint Venture, East West Resource Corporation; Thunder Bay Resident Geologist’s Dyer, R.D., 2004. Lake Nipigon Region Geoscience Initiative. Surficial Geochemistry and Mapping Case Studies Project: Update and Preliminary Results; Ontario Geological Survey, Open File Report 6145, p.52-1 to 53-6. East West Resource Corporation, 2004a. East West Resources Corporation website; News Release: New Platinum Horizons Extended – Seagull Intrusion Thunder Bay, Ontario: dated June 9, 2004; http://www.eastwestres. com/main.asp?section=news&page=20040609 [accessed June 10, 2004] East West Resource Corporation. 2004b, East West Resources Corporation website; News Release: Highly elevated Platinum Group Elements discovered in new horizons at the Seagull Intrusion: dated July 29, 2004; http:// www.eastwestres.com/main.asp?section=news&pag e=20040729 [accessed July 29, 2004] Ernst, R.E. and Buchan K.L., 2003. Recognizing Mantle Plumes in the Geological Record; Annual Review of Earth and Planetary Sciences, v.31, pp.469-523. Ernst, R.E., Buchan, K.L., Hart, T.R., and Morgan, J., 2005. North-trending diabase dykes west of the Nipigon Embayment: paleomagnetism, geochemistry and correlation with known magmatic events; Geological Association of Canada-Mineralogical Association of Canada, Joint Annual Meeting, Halifax 2005, Program with Abstracts. Fralick, P. and Kissin, S.A., 1995. Mid-Proterozoic basin development in central North America: Implications of Sibley Group volcanism and sedimentation; in Proceedings, 1995 International Geological Correlation Program, Project 336, Petrology and metallogeny of volcanic and intrusive rocks of the Midcontinental Rift System, p.51-52. Fralick, P., Smyk, M., and Mailman, M., 2000. Geology and stratigraphy of the Mesoproterozoic Sibley Group: in Part 2: Field Trip Guide Books, Institute on Lake Superior Geology, Proceedings Volume 46; Thunder Bay, Ontario, May 8-13, 2000. Franklin, J.M., 1978. The Sibley Group, Ontario; in Rubidium-strontium isochron age studies, report 2; ed. R.K. Wanless and W.D. Loveridge; Geological Survey of Canada, Paper 77-14, p.31-34. Franklin, J.M., McIlwaine, W.H., Poulsen, K.H. and Wanless, R.K., 1980. Stratigraphy and depositional setting of the Sibley Group, Thunder Bay District, Ontario, Canada; Canadian Journal of Earth Sciences, v.17, p.633-651. Gretener, P.E., 1969. On the mechanics of the intrusion of sills; Canadian Journal of Earth Sciences, v.6, p. 1415-1419. Hart, T.R., 2004. Geochemistry of the Proterozoic intrusive - 102 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 rocks of the Nipigon Embayment; abstract in Institute on Lake Superior Geology, Proceedings, 50th Annual Meeting, Duluth, Minnesota, v.50, pt.1, p.68-69. evolution and origin of nickel sulphide mineralization in the Sudbury Igneous Complex, Ontario, Canada; Economic Geology, v.96, p.1855-1875. Hart, T.R., 2005a. Precambrian Geology of the South Black Sturgeon River and Seagull Lake Area, Northwestern Ontario; Ontario Geological Survey, Open File Report 6165, 63p. MacDonald, C.A., 2004. Precambrian geology of the south Armstrong – Gull Bay area, Nipigon Embayment, northwestern Ontario. Ontario Geological Survey, Open File Report 6136, 42p. Hart, T.R., 2005b. South Black Sturgeon River–Seagull Lake Area, Nipigon Embayment, Northwest Ontario: Lithogeochemical, Assay and Compilation Data. Ontario Geological Survey, Miscellaneous Release of Data 147 MacDonald, C.A. and Tremblay, E., 2005. Precambrian Geology of the west-central map area, Nipigon Embayment, northwestern Ontario; Ontario Geological Survey, Open File Report 6164, XXp Hart, T.R. and Magyarosi, Z., 2004. Precambrian Geology of the Northern Black Sturgeon River and Disraeli Lake Area, Northwestern Ontario; Ontario Geological Survey, Open File Report 6138, 56p. Hart, T.R., terMeer, M. and Jolette, C., 2002a. Precambrian Geology of Kitto, Eva, Summers, Dorothea and Sandra Townships, Beardmore Area, Northwestern Ontario. Ontario Geological Survey, Open File Report 6095, 206p. Hart T.R., terMeer M., Jolette C., and Duggan B.M., 2002b. Proterozoic Ultramafic-Mafic Intrusions of the Nipigon Plate, Beardmore, Ontario; 9th International Platinum Symposium, Abstract with Program, 21-25 July, Billings, Montana; http://www.env.duke.edu/ people/faculty/boudreau/IPS_Abstracts.htm Heaman, L.M., Easton, R.M., Hart, T.R., MacDonald, C.A., Fralick, P., and Hollings, P., 2005. Proterozoic history of the Lake Nipigon area, Ontario: Constraints from U-Pb zircon and baddeleyite dating; Canadian Institute of Mining and Metallurgy Annual Meeting, Toronto 2005, Program with Abstracts. Heggie, G.J., and Hollings, P., 2004. Controls on PGE Mineralization in the Seagull Intrusion, Northwestern Ontario; Geological Association of Canada-Mineralogical Association of Canada, Joint Annual Meeting, St. Catharines 2004, Program with Abstracts. Hollings, P., Fralick, P. and Kissin, S., 2004. Geochemistry and geodynamic implications of the Mesoproterozoic English Bay Granite-Rhyolite complex, northwestern Ontario. Canadian Journal of Earth Sciences, 41, 1329-1338. Leaman, D.E., 1975. Form, mechanism, and control of dolerite intrusion near Hobart, Tasmania; Journal of the Geological Society of Australia, v.22, p. 175186. Lightfoot, P.C., Sutcliffe, R.H. and Doherty, W., 1991. Crustal contamination identified in Keweenawan Osler Group tholeiites, Ontario: a trace element perspective; Journal of Geology, v.99, p.739-760. Lightfoot, P.C., Keays, R.R. and Doherty, W., 2001. Chemical Meinert, L.D., 1992. Skarns and skarn deposits; Geoscience Canada, v. 19, p. 145-162. Metsaranta, R. and Fralick, P., 2004. Lake Nipigon Region Geoscience Initiative: Stratigraphy of the Sibley Group and its Relationship to Sedimentary Umits of the Basal Osler Group and the Nipigon Sills: Ontario Geological Survey, Open File Report 6145, p. 50-1 – 50-5. McIlwaine, W.H. and Tihor, L.A., 1975a. Dorion-Wolf Lake Area (Western Part), District of Thunder Bay; Ontario Division of Mines, Preliminary Map P.994, Geological Series, scale 1 inch to 1⁄4 mile or 1:15 840. Geology 1972. McIlwaine, W.H. and Tihor, L.A., 1975b. Dorion-Wolf Lake Area (Eastern Part), District of Thunder Bay; Ontario Division of Mines, Preliminary Map P.995, Geological Series, scale 1 inch to 1⁄4 mile or 1:15 840. Geology 1972. McInnes, W., 1896. Summary report on the survey of Lake Nipigon; Geological Survey of Canada, Report for 1894, New Series, v.7, part A, p.48-51. Ontario Geological Survey, 2004. Ontario airborne geophysical surveys, magnetic and gamma-ray spectrometer data, grid and vector data, ASCII format, Lake Nipigon Embayment Area; Geophysical Data Set 1047a. Ontario Geological Survey, 2004b. Ontario geophysical surveys, gravity data, grid and point data (ASCII and Geosoft® formats) and vector data, Lake Nipigon Embayment area; Ontario Geological Survey, Geophysical Data Set 1052. Osmani, I.A. and Rees, K., 1998. Report on the Phase I Exploration Program, Wolf Mountain Property, north of Lake Superior, District of Thunder Bay, Leckie Lake Area G-67; Thunder Bay Resident Geologist’s Office, assessment file 52H02SW2001 – 2.19142. Parks, W.A., 1901. The country east of Nipigon Lake and River; Geological Survey of Canada, Summary Report 1901, partA, p.105A-109A. Platinum Group Metals Limited, 2005. Platinum Group Metals Limited website: News Release: New Platinum Reef Discovery in Canada, dated January - 103 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 19, 2005; http://www.platinumgroupmetals.net/s/ NewsReleases.asp?ReportID=98017&_Type=NewsReleases&_Title=New-Platinum-Reef-Discovery-inCanada Richardson, A. and Hollings, P., 2005. Rare Earth and Isotopic Geochemistry of the Keweenawan Nipigon Sills; Geological Association of CanadaMineralogical Association of Canada, Joint Annual Meeting, Halifax 2005, Program with Abstracts. Rogala, B., 2001. A Metamorphosed Evaporite Section from the Sibley Basin, northwestern Ontario; Unpublished BSc thesis, Lakehead University, 38p. Rogala, B., 2003. The Sibley Group: a lithostratigraphic, geochemical and paleomagnetic study. Unpublished MSc thesis, Lakehead University, 206p. Ruzicka, V. and LeCheminant, G.M., 1984. Uranium deposit research, 1983; in Current Research, Part A, Geological Survey of Canada, Paper 84-1a, p. 39-51. geology, Lake Nipigon area, Kelvin Island sheet, District of Thunder Bay; Ontario Geological Survey, Preliminary Map P.2838, scale 1:50 000. Sutcliffe, R.H. and Greenwood, R.C., 1985c. Precambrian geology of the Lake Nipigon area, Livingstone Point sheet, District of Thunder Bay; Ontario Geological Survey, Preliminary Map P.2839, scale 1:50 000. Williams, H.R., 1989. Geological studies in the Wabigoon, Quetico, and Abitibi–Wawa -----subprovinces, Superior Province of Ontario, with an emphasis on the structural development of the Beardmore–Geraldton belt; Ontario Geological Survey, Open File Report 5724, 189p Wilson, A.W.G., 1910. Geology of the Nipigon Basin, Ontario; Canada Department of Mines, Geological Survey Branch, Memoir 1, 152p. Scott, J.F., 1987. Uranium occurrences of the Thunder BayNipigon-Marathon area; Ontario Geological Survey, Open File Report 5634, 158p. Shklanka, R., 1969. Copper, Nickel, Lead, and Zinc Deposits of Ontario; Ontario Department of Mines Mineral Resource Circular 12, 394 p. Sun, S.-S. and McDonough, W.F., 1989. Chemical and isotopic systematics of oceanic basalts: implications for mantle compositions and processes; in Magmatism in the ocean basins, Geological Society, Special Publication No. 42, p.313-345. Sutcliffe, R.H., 1982a. Precambrian geology of the Wabigoon-Quetico Subprovince Boundary, Orient Bay Sheet, Thunder Bay District; Ontario Geological Survey, Map P.2530, Geological Series Preliminary Map, scale 1:50 000. Sutcliffe, R.H., 1982b. Precambrian geology of the Wabigoon-Quetico Subprovince Boundary, Orient Bay Sheet, Thunder Bay District; Ontario Geological Survey, Map P.2531, Geological Series Preliminary Map, scale 1:50 000. Sutcliffe, R.H., 1986. Proterozoic rift related igneous rocks at Lake Nipigon, Ontario; unpublished PhD thesis, University of Western Ontario, London, Ontario, 325p. Sutcliffe, R.H., 1987. Petrology of Middle Proterozoic diabase and picrites from Lake Nipigon, Canada; Contributions to Mineralogy and Petrology, v.96, p. 201-211. Sutcliffe, R.H. and Greenwood, R.C., 1985a. Precambrian geology, Lake Nipigon area, Castle Lake–Pikitigushi Lake sheet, District of Thunder Bay; Ontario Geological Survey, Preliminary Map P.2836, scale 1:50 000. Sutcliffe, R.H. and Greenwood, R.C., 1985b. Precambrian - 104 - � Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology: Proceedings, 2005 Description An account of the resource Institute on Lake Superior Geology. Nipigon, Ontario. May 24-28, 2005. Creator An entity primarily responsible for making the resource Institute on Lake Superior Geology Date A point or period of time associated with an event in the lifecycle of the resource 2005 Format The file format, physical medium, or dimensions of the resource PDF Language A language of the resource English https://digitalcollections.lakeheadu.ca/files/original/12d45312aebf45dc0fad52fdffb93c7a.pdf 4b91a4fa095c82217c8aef3e3a361ccb PDF Text Text 52nd Annual Meeting Sault Ste Marie, Ontario – May 8 – 12, 2006 Institute on Lake Superior Geology Part 1 – Proceedings and Abstracts Volume 52 part 1 – Proceedings and Abstracts �Proceedings of the 52nd ILSG Annual Meeting – Part 1 52nd Annual Meeting INSTITUTE ON LAKE SUPERIOR GEOLOGY May 8 – 12, 2006 Sault Ste Marie, Ontario Hosted by R. P. Sage and A. C. Wilson Co-chairs Volume 52 Part 1 – Proceedings and Abstracts Edited by A. C. Wilson (Ontario Geological Survey) Cover Photos (clockwise from upper left) – Generalized geology of the Sault Ste Marie, Ontario area (Ontario Geological Survey Map 2543); Stone quarry in nodular anorthosite of the Agnew Lake Intrusion (photo courtesy of M. Easton OGS); Arch Rock, Mackinac Island, Michigan;Gowganda Formation, Hwy. 108 Elliot Lake area; Nicholson outcrop, Arctic Star Diamond Corp. property, Menzies Township; Chippewa Falls unconformity, Hwy.17N. i �Proceedings of the 52nd ILSG Annual Meeting – Part 1 52nd INSTITUTE ON LAKE SUPERIOR GEOLOGY VOLUME 52 CONSISTS OF: PART 1: PROGRAM AND ABSTRACTS PART 2: GLACIAL LAKES ALGONQUIN AND NIPISSING SHORELINE BEDROCK FEATURES: MACKINAC ISLAND, MICHIGAN - FIELD TRIP GUIDEBOOK PART 3: UNUSUAL DIAMOND-BEARING BRECCIAS OF THE WAWA AREA FIELD TRIP GUIDEBOOK PART 4: THE HURONIAN SUPERGROUP BETWEEN SAULT STE MARIE AND ELLIOT LAKE - FIELD TRIP GUIDEBOOK PART 5: KEWEENAWAN ROCKS OF THE MAMAINSE POINT AREA FIELD TRIP GUIDEBOOK PART 6: GEOLOGICAL GUIDEBOOK TO THE PALEOPROTEROZOIC EAST BULL LAKE INTRUSIVE SUITE PLUTONS AT EAST BULL LAKE, AGNEW LAKE AND RIVER VALLEY, ONTARIO - FIELD TRIP GUIDEBOOK Reference to material in Part 1 should follow the example below: Brown, B. A., Czechanski, M. L., Reid, D. D. and Mudrey, M. G. Jr. 2006. New evidence for syn-depositional subsidence in the Middle Ordocivician rocks of southwest Wisconsin; in Wilson A. C. (ed.), Proceedings and Abstracts, Institute on Lake Superior Geology, 52nd Annual Meeting, Sault Ste Marie, Ontario, v. 52 pt 1, p. 7. Published by the 52nd Institute on Lake Superior Geology and distributed by the ILSG Secretary: Pete Hollings - ILSG Secretary Department of Geology - Lakehead University 955 Oliver Road Thunder Bay, ON P7B 5E1 Canada Email: peter.hollings@lakeheadu.ca ILSG website: www.lakesuperiorgeology.org ii �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Table of Contents Institutes on Lake Superior Geology, 1955-2006 ............................................................. iv Constitution of the Institute on Lake Superior Geology................................................... vi By-Laws of the Institute on Lake Superior Geology ....................................................... vii Membership Criteria for the Institute on Lake Superior Geology.................................. viii Goldich Medal Guidelines ................................................................................................ ix Goldich Medallists ........................................................................................................... xi Goldich Medal Committee ............................................................................................... xi Citation for Goldich Medal Recipient.............................................................................. xii ILSG Student Research Fund ......................................................................................... xiv Eisenbrey Student Travel Awards ....................................................................................xv Eisenbrey Student Travel Award Application ................................................................ xvi Student Paper and Poster Awards .................................................................................. xvii Student Paper and Poster Awards Committee ............................................................... xvii Report of the Chairs of the 51st Annual Meeting ......................................................... xviii Board of Directors.............................................................................................................xx Session Chairs...................................................................................................................xx Local Committee...............................................................................................................xx Banquet Speaker ............................................................................................................. xxi Acknowledgements......................................................................................................... xxi Program.......................................................................................................................... xxii Abstracts .............................................................................................................................1 Author Index .....................................................................................................................72 iii �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Institutes on Lake Superior Geology, 1955-2005 # Date Place Chairs 1 1955 Minneapolis, Minnesota C.E. Dutton 2 1956 Houghton, Michigan A.K. Snelgrove 3 1957 East Lansing, Michigan B.T. Sandefur 4 1958 Duluth, Minnesota R.W. Marsden 5 1959 Minneapolis, Minnesota G.M. Schwartz & C. Craddock 6 1960 Madison, Wisconsin E.N. Cameron 7 1961 Port Arthur, Ontario E.G. Pye 8 1962 Houghton, Michigan A.K. Snelgrove 9 1963 Duluth, Minnesota H. Lepp 10 1964 Ishpeming, Michigan A.T. Broderick 11 1965 St. Paul, Minnesota P.K. Sims & R.K. Hogberg 12 1966 Sault Ste. Marie, Michigan R.W. White 13 1967 East Lansing, Michigan W.J. Hinze 14 1968 Superior, Wisconsin A.B. Dickas 15 1969 Oshkosh, Wisconsin G.L. LaBerge 16 1970 Thunder Bay, Ontario M.W. Bartley & E. Mercy 17 1971 Duluth, Minnesota D.M. Davidson 18 1972 Houghton, Michigan J. Kalliokoski 19 1973 Madison, Wisconsin M.E. Ostrom 20 1974 Sault Ste. Marie, Ontario P.E. Giblin 21 1975 Marquette, Michigan J.D. Hughes 22 1976 St. Paul, Minnesota M. Walton 23 1977 Thunder Bay, Ontario M.M. Kehlenbeck 24 1978 Milwaukee, Wisconsin G. Mursky 25 1979 Duluth, Minnesota D.M. Davidson 26 1980 Eau Claire, Wisconsin P.E. Myers 27 1981 East Lansing, Michigan W.C. Cambray 28 1982 International Falls, Minnesota D.L. Southwick 29 1983 Houghton, Michigan T.J. Bornhorst 30 1984 Wausau, Wisconsin G.L. LaBerge 31 1985 Kenora, Ontario C.E. Blackburn iv �Proceedings of the 52nd ILSG Annual Meeting – Part 1 # Date Place Chairs 32 1986 Wisconsin Rapids, Wisconsin J.K. Greenberg 33 1987 Wawa, Ontario E.D. Frey & R.P. Sage 34 1988 Marquette, Michigan J. S. Klasner 35 1989 Duluth, Minnesota J.C. Green 36 1990 Thunder Bay, Ontario M.M. Kehlenbeck 37 1991 Eau Claire, Wisconsin P.E. Myers 38 1992 Hurley, Wisconsin A.B. Dickas 39 1993 Eveleth, Minnesota D.L. Southwick 40 1994 Houghton, Michigan T.J. Bornhorst 41 1995 Marathon, Ontario M.C. Smyk 42 1996 Cable, Wisconsin L.G. Woodruff 43 1997 Sudbury, Ontario R.P. Sage & W. Meyer 44 1998 Minneapolis, Minnesota J.D. Miller & M.A. Jirsa 45 1999 Marquette, Michigan T.J. Bornhorst & R.S. Regis 46 2000 Thunder Bay, Ontario S.A. Kissin & P. Fralick 47 2001 Madison, Wisconsin M.G. Mudrey & Jr., B.A. Brown 48 2002 Kenora, Ontario P. Hinz & R.C. Beard 49 2003 Iron Mountain, Michigan L. Woodruff & W.F. Cannon 50 2004 Duluth, Minnesota S. Hauck & M. Severson 51 2005 Nipigon, Ontario M. Smyk & P. Hollings 52 2006 Sault Ste Marie, Ontario R. P. Sage & A. C. Wilson v �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Constitution of the Institute on Lake Superior Geology (Last amended by the Board—May 6, 2004) Article I - Name The name of the organization shall be the “Institute on Lake Superior Geology”. Article II - Objectives The objectives of this organization are: A. To provide a means whereby geologists in the Great Lakes region may exchange ideas and scientific data. B. To promote better understanding of the geology of the Lake Superior region. C. To plan and conduct geological field trips. Article III - Status No part of the income of the organization shall insure to the benefit of any member or individual. In the event of dissolution, the assets of the organization shall be distributed to _________ (some tax free organization). (To avoid Federal and State income taxes, the organization should be not only “scientific” or “educational”, but also “non-profit”) Minn. Stat. Anno. 290.01, subd. 4 Minn. Stat. Anno. 290.05(9) 1954 Internal Revenue Code s.501(c)(3) Article IV - Membership The membership of the organization shall consist of persons who have registered for an annual meeting within the past three years, and those who indicate interest in being a member according to guidelines approved by the Board of Directors. Article V - Meetings The organization shall meet once a year. The place and exact date of each meeting will be designated by the Board of Directors. Article VI - Directors The Board of Directors shall consist of the Chair, Secretary, Treasurer, and the last three past Chairs; but if the board should at any time consist of fewer than six persons, by reason of unwillingness or inability of any of the above persons to serve as directors, the vacancies on the board may be filled by the Chair so as to bring the membership of the board to six members. Article VII - Officers The officers of this organization shall be a Chair, a Secretary and a Treasurer. A. The Chair shall be elected each year by the Board of Directors, who shall give due consideration to the wishes of any group that may be promoting the next annual meeting. His/her term of office as Chair will terminate at the close of the annual meeting over which he/she presides, or when his/her successor shall have been appointed. He/she will then serve for a period of three years as a member of the Board of Directors. B. The Secretary shall be elected at the annual meeting. His/her term of office shall be four vi �Proceedings of the 52nd ILSG Annual Meeting – Part 1 years, or until his/her successor shall have been appointed. C. The Treasurer shall be elected at the annual meeting. His/her term of office shall be four years, or until his/her successor shall have been appointed. The terms of the Secretary and Treasurer shall be staggered so that there will always be a two year overlap between the two. Article VIII - Amendments This constitution may be amended by a majority vote (majority of those voting) of the membership of the organization. By-Laws of the Institute on Lake Superior Geology (Last amended by the Board—May 6, 2004) The by-laws of the Institute on Lake Superior Geology are in revision and will be posted on the ILSG website when completed and approved. Please visit www.lakesuperiorgeology.org vii �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Membership Criteria for the Institute on Lake Superior Geology Approved May 8, 1997. Amended by the Board—May 6, 2004 A. Membership in the Institute on Lake Superior Geology requires either participation in Institute activities, or an indication on a regular basis of interest in the Institute. Those individuals registering for an annual meeting will remain as members for 4 years unless: 1) they indicate no further interest in the Institute by responding negatively to the statement on meeting circulars “Remove my name from the mailing list”; or 2) two successive mailings in different years are returned by the postal service as address unknown. B. Those individuals who have not registered for an annual meeting in the past 4 years must indicate an interest in the Institute by postal, electronic, or verbal correspondence with the Secretary at least once every two years. Such individuals will be removed from the membership if they indicate no further interest in the Institute or two successive mailing in different years are returned by the postal service as address unknown. C. The Secretary will maintain a list of current members. The list will include the date of the beginning of continuous membership, dates of returned mail, dates of last contact (expression of interest), and the date membership expires, barring a change of status initiated by the member. Those individuals who have become members of ILSG by Section B will have an expiration date listed at 2 years from the upcoming meeting. For example, a member who expresses interest in September of 1997 (the next annual meeting is May, 1998) will have an expiration date of May, 2000, unless the member contacts the Secretary or attends an annual meeting. D. “Member for Life” status is granted to individuals who have been (nearly) continuous participants of the ILSG meetings for 15 years, Goldich Medal recipients, or those who have served as meeting chairs. This status will be further maintained unless the individuals indicate no further interest in the Institute, or 4 mailings in different years are returned by the postal service as address unknown, or they are deceased. E. All members will be mailed the First Circular for the Annual Meeting and the ILSG Newsletter. The Chair of the annual meeting may opt to send the first circular to additional individuals. All returned mail should be reported to the Secretary. F. The Secretary can designate any individual who is on the ILSG membership list (mailing list) as of January 1, 1997 as a member for life based on participation in ILSG activities. G. Members are strongly encouraged to send address corrections to the Secretary to avoid unintentional lapse of membership. viii �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Goldich Medal Guidelines (Adopted by the Board of Directors, 1981; amended 1999) Preamble The Institute on Lake Superior Geology was born in 1955, as documented by the fact that the 27th annual meeting was held in 1981. The Institute’s continuing objectives are to deal with those aspects of geology that are related geographically to Lake Superior; to encourage the discussion of subjects and sponsoring field trips that will bring together geologists from academia, government surveys, and industry; and to maintain an informal but highly effective mode of operation. During the course of its existence, the membership of the Institute (that is, those geologists who indicate an interest in the objectives of the ILSG by attending) has become aware of the fact that certain of their colleagues have made particularly noteworthy and meritorious contributions to the understanding of Lake Superior geology and mineral deposits. The first award was made by ILSG to Sam Goldich in 1979 for his many contributions to the geology of the region extending over about 50 years. Subsequent medallists and this year’s recipient are listed in the table below. Award Guidelines 1) The medal shall be awarded annually by the ILSG Board of Directors to a geologist whose name is associated with a substantial interest in, and contribution to, the geology of the Lake Superior region. 2) The Board of Directors shall appoint the Goldich Medal Committee. The initial appointment will be of three members, one to serve for three years, one for two years, and one for one year. The member with the briefest incumbency shall be chair of the Nominating Committee. After the first year, the Board of Directors shall appoint at each spring meeting one new member who will serve for three years. In his/her third year this member shall be the chair. The Committee membership should reflect the main fields of interest and geographic distribution of ILSG membership. 3) By the end of November, the Goldich Medal Committee shall make its recommendation to the Chair of the Board of Directors, who will then inform the Board of the nominee. 4) The Board of Directors normally will accept the nominee of the Committee, inform the medallist, and have one medal engraved appropriately for presentation at the next meeting of the Institute. 5) It is recommended that the Institute set aside annually from whatever sources, such funds as will be required to support the continuing costs of this award. Nominating Procedures 1) The deadline for nominations is November 1. Nominations shall be taken at any time by the Goldich Medal Committee. Committee members may themselves nominate candidates; however, Board members may not solicit for or support individual nominees. 2) Nominations must be in writing and supported by appropriate documentation such as letters of recommendation, lists of publications, curriculum vita’s, and evidence of contributions to Lake Superior geology and to the Institute. 3) Nominations are not restricted to Institute attendees, but are open to anyone who has ix �Proceedings of the 52nd ILSG Annual Meeting – Part 1 worked on and contributed to the understanding of Lake Superior geology. Selection Guidelines 1) Nominees are to be evaluated on the basis of their contributions to Lake Superior geology (sensu lato) including: a) importance of relevant publications; b) promotion of discovery and utilization of natural resources; c) contributions to understanding of the natural history and environment of the region; d) generation of new ideas and concepts; and e) contributions to the training and education of geoscientists and the public. 2) Nominees are to be evaluated on their contributions to the Institute as demonstrated by attendance at Institute meetings, presentation of talks and posters, and service on Institute boards, committees, and field trips. 3) The relative weights given to each of the foregoing criteria must remain flexible and at the discretion of the Committee members. 4) There are several points to be considered by the Goldich Medal Committee: a) An attempt should be made to maintain a balance of medal recipients from each of the three estates—industry, academia, and government. b) It must be noted that industry geoscientists are at a disadvantage in that much of their work in not published. 5) Lake Superior has two sides, one the U.S., and the other Canada. This is undoubtedly one of the Institute’s great strengths and should be nurtured by equitable recognition of excellence in both countries. x �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Goldich Medallists 1979 Samuel S. Goldich 1993 Donald W. Davis 1980 not awarded 1994 Cedric Iverson 1981 Carl E. Dutton, Jr. 1995 Gene LaBerge 1982 Ralph W. Marsden 1996 David L. Southwick 1983 Burton Boyum 1997 Ronald P. Sage 1984 Richard W. Ojakangas 1998 Zell Peterman 1985 Paul K. Sims 1999 Tsu-Ming Han 1986 G.B. Morey 2000 John C. Green 1987 Henry H. Halls 2001 John S. Klasner 1988 Walter S. White 2002 Ernest K. Lehmann 1989 Jorma Kalliokoski 2003 Klaus J. Schulz 1990 Kenneth C. Card 2004 Paul Wieblen 1991 William Hinze 2005 Mark Smyk 1992 William F. Cannon 2006 Goldich Medal Recipient Michael G. Mudrey Jr Mount Horeb, Wisconsin Goldich Medal Committee Serving through the meeting year shown in parentheses. George Hudak (2006) University of Wisconsin, Oshkosh Tom Hart (2007) Ontario Geological Survey Doug Duskin (2008) Member from Industry xi �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Citation for Goldich Medal Recipient Michael G. Mudrey, Jr. 2006 Goldich Medal Recipient It is my pleasure to acknowledge the many contributions of Mike Mudrey on the occasion of his being awarded the 27th Goldich Medal for “Outstanding Contributions to the Geology of the Lake Superior Region”. Over the past 30 years, Mike has produced an extensive list of papers, maps, and abstracts that have significantly contributed to our understanding of regional geology. He has also compiled an outstanding record of service to the Institute on Lake Superior Geology. Mike Mudrey began his education at South Dakota School of Mines in 1963. In 1964 he transferred to Princeton, where he graduated with an A.B. degree in Geology and Geochemistry in 1967. At Princeton he had the opportunity to study with some of the pioneers of modern geology, and received a strong background in geology and chemistry. He began his association with economic geology working at Homestake in the summer of 1965. After working briefly with vertebrate paleontology in the summer of 1966, he began his long career in the Lake Superior Precambrian working as an assistant to Sam Goldich in the summer of 1967. Mike was a graduate student at SUNY Stony Brook in 1968, then moved to Northern Illinois University, where he completed his M.S. in 1969. His thesis topic was the petrology of the Northern Light Gneiss, completed under Sam Goldich's supervision. During the summers he assisted Sam at the Bureau of Standards and in the field in Minnesota and Ontario. In 1969 Mike moved on to the University of Minnesota, where he graduated with a Ph.D. in geology and analytical chemistry in 1973. His thesis research was a petrologic study of the Pigeon Point Sill, with Paul Weiblen as his advisor. While at Minnesota, Mike worked as a geologist for the Minnesota Geological Survey where he gained additional experience in field mapping and geochemical studies. After graduation, Mike returned to Northern Illinois University to work two years as a Scientist and Project Manager for the Dry Valley Drilling Project of the NSF Antarctic Program. In 1976 Mike joined the Wisconsin Geological and Natural History Survey, were he worked until retirement in 2005. At the WGNHS Mike's first job was to start up a Precambrian mapping program, necessitated by the discovery of volcanogenic massive sulfide deposits in the north. This work led to the first state bedrock map to subdivide the Precambrian, published in 1984. Mike was the driving force behind the long effort to complete gravity and aeromagnetic surveys of the state. Those who have been regulars at the ILSG are familiar with Mike's many contributions to the Precambrian geology of Wisconsin, but working at a small state survey requires one to wear more than one hat. Over his career at WGNHS, Mike ably served as expert on such diverse topics as earthquakes and seismicity, radioactive waste management, mineral and water resources, oil exploration, regional stratigraphy, and radon in the environment. Mike has always had a strong commitment to public service and education as well as scientific research. He was never too busy to answer a question on Wisconsin geology, whether from a legislator or a K-12 science student. In retirement he remains active, continuing to collaborate with his Survey and agency colleagues, and serving as consultant on radon to the Wisconsin Department of Health. xii �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Mike has been an active contributor to the Institute on Lake Superior Geology since the early 1970s, when he first met Sam Goldich. He has served as co-chair, field trip chair, board member, member of the Goldich medal committee, Secretary-Treasurer (1990 to 1994), and field trip leader and session chair numerous times. Mike has nearly always contributed an abstract or two, and in the true spirit of Sam, he has never been at a loss for some good critical discussion. It is my pleasure to present the 2006 Goldich Medal to my friend and colleague of many years Mike Mudrey, in recognition of his many contributions to regional geology and service to the Institute. Submitted by B. A. Brown xiii �Proceedings of the 52nd ILSG Annual Meeting – Part 1 ILSG Student Research Fund The 2005 Board of Directors established the ILSG Student Research Fund with US$10,000 from the Institute’s general fund to encourage student research on the geology of the Lake Superior region. A minimum of two awards of US$500 each for research expenses (but not travel expenses) will be made each year. Students are expected to present their research orally or during a poster session at an ILSG meeting. The award winners will also be automatically eligible for the Eisenbrey Travel Awards. To allow the fund to grow, the Fund will receive one-half of any additional proceeds from each annual meeting, after all other commitments and expenses are covered. • • • • • • The Board of Directors will be responsible for selecting a minimum of two awards. The ILSG Treasurer will issue the awards. The ILSG Student Research Fund is available for undergraduate or graduate students working on geology in the Lake Superior region. The applications are due to the ILSG Secretary by August 31st each year. Awards will be made by October 1st of each year. Names of the award recipients will be announced at the next annual meeting and posted on the ILSG website. The proposal application should be at least 500 words, and should have a statement of the research project, background information, a map of the research area, research steps necessary to complete the research, figures (if needed), references, and a list of research expenses. The proposal should also include a proposed end date for the research. The proposal will need to be signed by the researcher’s supervisor. xiv �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Eisenbrey Student Travel Awards The 1986 Board of Directors established the ILSG Student Travel Awards to support student participation at the annual meeting of the Institute. The name “Eisenbrey” was added to the award in 1998 to honor Edward H. Eisenbrey (1926-1985) and utilize substantial contributions made to the 1996 Institute meeting in his name. “Ned” Eisenbrey is credited with discovery of significant volcanogenic massive sulfide deposits in Wisconsin, but his scope was much broader—he has been described as having unique talents as an ore finder, geologist, and teacher. These awards are intended to help defray some of the direct travel costs of attending Institute meetings, and include a waiver of registration fees, but exclude expenses for meals, lodging, and field trip registration. The number of awards and value are determined by the annual Chair in consultation with the Secretary and Treasurer. Recipients will be announced at the annual banquet. The following general criteria will be considered by the annual Chair, who is responsible for the selection: 1) The applicants must have active resident (undergraduate or graduate) student status at the time of the annual meeting of the Institute, certified by the department head. 2) Students who are the senior author on either an oral or poster paper will be given favored consideration. 3) It is desirable for two or more students to jointly request travel assistance. 4) In general, priority will be given to those in the Institute region who are farthest away from the meeting location. 5) Each travel award request shall be made in writing to the annual Chair, and should explain need, student and author status, and other significant details. The form below is optional. Successful applicants will receive their awards during the meeting. xv �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Eisenbrey Student Travel Award Application Student Name : __________________________________ Address: Date: ____________ __________________________________________________________ __________________________________________________________ __________________________________________________________ __________________________________________________________ email: __________________________________________________________ Educational status: _____________________________________________________ Are you the senior author of an oral presentation or poster? Yes ____ No _____ Will any other students be traveling with you?Yes ____ No _____ If yes, then who? ___________________________________________________ ___________________________________________________ Statement of need (use additional page if necessary): __________________________ _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ Signature: ____________________________________________________ Department Head: ____________________________________________________ xvi �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Student Paper and Poster Awards Each year, the Institute selects the best of the student presentations and honors presenters with a monetary award. Funding for the award is generated from registrations of the annual meeting. The Student Paper and Poster Committee is appointed by the annual meeting Chair in such a manner as to represent a broad range of professional and geologic expertise. Criteria for best student paper—last modified by the Board in 2001—follow: 1) The contribution must be demonstrably the work of the student. 2) The student must present the contribution in person. 3) The Student Paper and Poster Committee shall decide how many awards to grant, and whether or not to give separate awards for poster vs. oral presentations. 4) In cases of multiple student authors, the award will be made to the senior author, or the award will be shared equally by all authors of the contribution. 5) The total amount of the awards is left to the discretion of the meeting Chair in conjunction with the Secretary, but typically is in the amount of about $500 US (increase approved by Board, 10/01). 6) The Secretary maintains, and will supply to the Committee, a form for the numerical ranking of presentations. This form was created and modified by Student Paper and Poster Committees over several years in an effort to reduce the difficulties that may arise from selection by raters of diverse background. The use of the form is not required, but is left to the discretion of the Committee. 7) The names of award recipients shall be included as part of the annual Chair’s report that appears in the next volume of the Institute. Student papers and posters will be noted on the Program. Student Paper and Poster Awards Committee Dan England - Eveleth Fee Office Inc. John Klasner -Western Illinois University (Retired) Norm Trowell - Ontario Geological Survey xvii �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Report of the Chairs of the 51st Annual Meeting REPORT OF THE 51st ANNUAL MEETING OF THE INSTITUTE ON LAKE SUPERIOR GEOLOGY Nipigon, Ontario The Ontario Geological Survey and Lakehead University co-hosted the 51st Annual Institute on Lake Superior Geology meeting on May 24-28 in Nipigon, Ontario. The meeting consisted of two days of technical sessions with three pre-meeting and three post-meeting field trips. Ryan Tuomi provided excellent on-site AV assistance that kept the sessions running on schedule and also designed the Meeting web site. Bill Addison, Peter Hinz, Bernie Schnieders, John Scott, Mary Louise Hill and Mike Easton provided invaluable assistance with the field trips. Levina Collins of the Nipigon Economic Development Office acted as a liaison with the Town of Nipigon. Total registration for the meeting was 127 students and professionals. Proceedings Volume 51 was published in two parts: Part 1 – Program and Abstracts, edited by Mike Easton and Pete Hollings, with published abstracts for 26 oral and 14 poster presentations; and Part 2 – Field Trip Guidebook, edited by Pete Hollings. The 51st meeting marked the first time the ILSG Annual Meeting was held in Nipigon, enabling the organization of excellent field trips. On Tuesday, May 24, Tom Hart, Phil Fralick and Mark Smyk co-led a two-day trip to examine the geology and gold mineralization of the BeardmoreGeraldton greenstone belt. The following day, Peter Barnett led a small but dedicated group to view the Quaternary geology of the Beardmore-Nipigon area and, in a first for the Institute; Pete Hollings led a flotilla of small boats out on to Lake Superior to examine the Mesoproterozoic Midcontinent Rift (MCR) stratigraphy near Rossport. On May 28, three trips set out from Nipigon: Pete Hollings and Phil Fralick reprised the Rossport Trip; Tom Hart led a group to examine the geology of the Black Sturgeon area, focusing mainly on the diabase sills and ultramafic intrusions associated with the MCR; and Mark Smyk led a trip to look at the pegmatites and high-grade metamorphic rocks of the Quetico subprovince. One hundred and ten participants attended the Annual Banquet. Dr. Jim Franklin provided the after-dinner presentation, entitled “Mineral Resources for the Future: The Resource Potential of Northern Lake Superior”. Peter Hinz had the privilege of presenting the 2005 Goldich Medal to Co-Chair Mark Smyk of the Ontario Geological Survey. Mark has worked tirelessly for the Institute over the last 17 years and has also made significant contributions to the understanding of Lake Superior geology. The student paper committee (Penelope Morton, Greg Stott and Wally Rayner) were faced with the usual dilemma when it came to picking a winner from the eight talks and two posters. The winners were: 2005 Best Student Paper Awards 1) Daniela Vallini – University of Western Australia ($300, Winner, best oral presentation) 2) Noah Planavsky and Jennifer Murphy – Lawrence University ($200, Winners, best poster presentation) 3) Angelique Magee, OGS/Lakehead University ($100, Honourable mention, oral presentation) In addition, Eisenbrey travel awards in varying amounts were presented to students from: • Lawrence University (Noah Planavsky and Jennifer Murphy); xviii �Proceedings of the 52nd ILSG Annual Meeting – Part 1 • • North Dakota State University (Damion Knudsen); Lakehead University (Adam Richardson, Riku Metsaranta, Dawn-Ann Trebilcock, Chris Lane, Mike Maric, Jordan Laarman, and Bjarne Almqvist). The Institute’s Board of Directors met on May 26, 2005, and brief summary of the meeting follows: 1. 2. 3. 4. 5. 6. 7. Accepted report of the Chairs for the 50th ILSG, Duluth, Minnesota Received, discussed, and accepted 2004-2005 ILSG Financial Summary from ILSG Treasurer Mark Jirsa. Approved Mark Smyk as on-going ILSG Board member Approved 2006 (52nd annual) meeting location—Sault Ste Marie, Ontario, and co-chairs Ron Sage (OGS - retired) and Ann Wilson (OGS). Replaced David Meineke as the “member from industry” on Goldich Committee with Doug Duskin. Amended the Institute’s by-laws in order to qualify for 501c3 status with the IRS. Established the ILSG Student Research Fund with US$10,000 from the Institute’s general fund to encourage student research on the geology of the Lake Superior region. The 51st ILSG meeting was a great success and we would like to thank all the individuals who contributed to this success, including the people and businesses of Nipigon. The following organizations are thanked for their sponsorship of the meeting: Ontario Geological Survey, Lakehead University, Lake Nipigon Region Geoscience Initiative, Ontario Prospectors Association, Canadian Institute of Mining and Metallurgy (Thunder Bay Branch), Northwestern Ontario Prospectors Association, and Chaltrek Geological Supplies Inc. The field trips were well-attended and we would like to extend our thanks to the trip leaders and all those who found themselves with keys to rental cars thrust into their hands at short notice. The Municipality of Greenstone and Roxmark Mines Limited provided generous in-kind support for the BeardmoreGeraldton trip. We would also like to thank all those attendees who pitched in to help move poster boards, chairs and dining tables without having to be asked. The members of the Institute never cease to impress. Both of us were very pleased with the 51st meeting and thankful that it was not marred by accident, injury or inclement weather. We appreciated all the positive feedback from delegates, who enjoyed the small-town setting, meeting venues and varied field trips. Logistical arrangements, although much more daunting in a small community, did not prove to be insurmountable. It bodes well for those considering hosting the Annual Meeting in a smaller town. It was a thoroughly enjoyable and rewarding experience. Respectfully submitted Pete Hollings and Mark Smyk Co-Chairs, 51st ILSG Meeting xix �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Board of Directors Board appointment continues through the close of the meeting year shown in parentheses, or until a successor is selected Ron Sage/Ann Wilson - General Chair 2006 meeting (2009) - Ontario Geological Survey Mark Smyk (2008) - Ontario Geological Survey Steve Hauck (2007) - University of Minnesota, Duluth Mark A. Jirsa - Treasurer (2007) - Minnesota Geological Survey Laurel Woodruff (2006) - U.S. Geological Survey Peter Hollings - Secretary (2006) - Lakehead University, Thunder Bay, Ontario Session Chairs Theodore Bornhorst – Michigan Technological University Peter Hinz – Ontario Geological Survey George Hudak – University of Wisconsin – Oshkosh Helene Lukey – Cleveland Cliffs Inc. Joseph Mancuso – Bowling Green University James Miller – Minnesota Geological Survey - Duluth Richard Ojakangas – University of Minnesota - Duluth Laurel Woodruff – United States Geological Survey Local Committee Co-Chairs R. P. Sage - Ontario Geological Survey (retired), Sault Ste Marie, Ontario Ann Wilson - Ontario Geological Survey, Timmins, Ontario Program and Abstracts Editor Ann Wilson - Ontario Geological Survey, Timmins, Ontario Field Trip Guidebooks Editor R. P. Sage - Ontario Geological Survey (retired), Sault Ste Marie, Ontario Organizing Committee Nora Simm – Chartwells Dining Services Lisa Bagnall - Sault College of Applied Science and Technology Banquet Speaker Dr. Ed Walker, Petrologic Ltd. Exploring for Diamonds in Unconventional Rocks xx �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Acknowledgements Thank you to the following individuals, groups and organizations who contributed to making the 52nd Annual Meeting of the Institute on Lake Superior Geology a success. Ontario Prospectors Association Ministry of Northern Development and Mines – Ontario Geological Survey Minuteman Press - Timmins Volunteer Field Trip Leaders Volunteer Van Drivers xxi �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Program Monday May 8 8:00 a.m. Field Trip 1: Classic Stratigraphy of the Huronian Supergroup – Elliot Lake Transect Leaders: Gerry Bennett (OGS-retired) and Mike Hailstone (OGS) 6:00 p.m. Return to Sault Ste Marie Tuesday May 9 8:00 a.m. Field Trip 1: Classic Stratigraphy of the Huronian Supergroup – Searchmount Transect Leaders: Gerry Bennett (OGS-retired) and Mike Hailstone (OGS) 8:00 a.m. Field Trip 2: Unusual Archean Diamond-bearing rocks of the Wawa Area Leader: Ann Wilson (OGS) 6:00 p.m. Conclusion of Trips 1 and 2 6:00 p.m. - 8.00 p.m. Registration (Sault College)) 6:30 p.m. - 9.00 p.m. Ice Breaker Social (Sault College Cafeteria) and Poster Setup (Sault College) Wednesday May 10 8:00 a.m. - 4:00 p.m. Registration (Sault College) 9:00a.m. - 9:10 a.m. Introductory Remarks – Ron Sage and Ann Wilson, Co-Chairs Technical Session I Session Chairs: Peter Hinz (Ontario Geological Survey), Helene Lukey (Cleveland Cliffs Inc.) 9:10 a.m. Hailstone, Mike An overview of geology of the Sault Ste Marie area 9:35 a.m. 10:00 a.m. Rainbird, Robert H. and Davis, William J. Detrital zircon geochronology of the western Huronian Basin Bennett, Gerry The “Kona Dolomite” of Ontario 10:25 a.m. – 10:45 a.m. Coffee Break and Poster Session Moran, Patrick*, Fralick, Philip and Hollings, Pete Geochemical constraints on the deposition of Mesoarchean banded iron formation at the Musselwhite Mine, North Caribou greenstone belt, Superior Province. 11:10 a.m. Fralick, Philip Iron formation in Neoarchean deltaic successions; Layering styles developed during siliciclastic and chemical sediment deposition, Superior Province, Canada. 11:35 a.m. Jirsa, Mark. A. and Chandler, Val W. Structure of the Biwabik Iron Formation, Mesabi Iron Range, Minnesota 10:45 a.m. 12:00 p.m. – 1:30 p.m. Lunch Break and Poster Session (ILSG Board Meeting by invitation) xxii �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Technical Session II Session Chairs: Joseph Mancuso (Bowling Green University), Jim Miller (Minnesota Geological Survey – Duluth) 1:30 p.m. 1:55 p.m. Grabowski, Gary Sampling lamprophyre dikes for diamonds – Discover Abitibi Initiative Shute, Amy* and Hollings, Pete Geology and alteration associated with VMS mineralization in the Hamlin Lake area, Northwestern Ontario 2:20 p.m. – 2:45 p.m. Coffee Break and Poster Session 2:45 p.m. Cannon, William F., Horton, J. Wright Jr., and Kring, David A. The Sudbury impact layer in the Marquette Range Supergroup of Michigan 3:10 p.m. Hollings, Pete and Wyman, Derek Geochemistry of the ~2.7 Ga Blake River Group and Confederation Assemblages: Implications for supra-subduction zone volcanism in the Superior Province 3:35 p.m. Holm, Daniel K., Anderson, R., Boerboom, Terrence J., Cannon, William F., Chandler, Val, Jirsa, Mark, Miller, James, Schneider, D. A., Schultz, Klaus and Van Schmus, W. Randy Continental growth and evolution of the northern interior of the conterminous U. S. 3:55 p.m. Announcements 6:00 p. m. – 7:00 p.m. Cash Bar (Sault College Cafeteria) Annual Banquet and Award Presentation (Sault College Cafeteria) 7:00 p.m. Announcement of 53rd Annual Meeting Location 2006 Goldich Award Presentation to M.G. Mudrey Jr. 2006 Banquet Address - Dr. E. C. Walker Meeting participants not registered for the banquet are welcome to attend the address. Thursday May 11 9:00 a.m. – 10:30 a.m. Registration Technical Session III 9:00a.m. - 9:05 a.m. Announcements Session Chairs: George Hudak (University of Wisconsin-Oshkosh), Laurel Woodruff (United States Geological Survey) 9:05 a.m. 9:30 a.m. Planavsky, Noah*, Knudsen, Andrew and Shapiro, Russell Evidence for widespread distribution of iron dependent metabolisms in Precambrian oceans Waggoner, Thomas D. Sulphur Isotopes from pyrite in the Negaunee Iron Formation xxiii �Proceedings of the 52nd ILSG Annual Meeting – Part 1 9:55 a.m. Mudrey, Michael G. Jr. Statistical analysis of indoor radon data and relationships to geology in Wisconsin 10:20 a.m. – 10:45 a.m. Coffee Break and Poster Session 10:45 a.m. 11:10 a.m. Magee, M. Angelique*, Hollings, Pete and Fralick, Philip W. Geology and geochemistry of the Chimney Lake volcaniclastic breccia near Armstrong, Ontario Miller, James D. Jr. and Peterson, Dean M. The Precambrian Research Center – A new initiative to promote Precambrian field studies at the University of Minnesota Duluth 11:35 a.m. - 1:30 p.m. Lunch Break and Poster Session (Posters removed after lunch) Technical Session IV Session Chairs: Theodore Bornhorst (Michigan Technological University), Richard Ojakangas (University of Minnesota - Duluth) 1:30 p.m. 1:55 p.m. 2:20 p.m. Smyk, Mark C., Hollings, Pete and Heaman, Larry M. Preliminary investigations of the petrology, geochemistry and geochronology of the St. Ignace Island Complex, Midcontinent Rift, northern Lake Superior, Ontario Halls, Henry C., Stott, Greg M., Ernst, R. E., and Davis, Donald W. A Paleoproterozoic mantle plume beneath the Lake Superior region Vallini, Daniela A., Cannon, William F., Schultz, Klaus J., and McNaughton, Neal J. The thermal history of low metamorphic grade Paleoproterozoic metasedimentary rocks of the Penokean orogen, Lake Superior Region: Recognizing a widespread 1786 Ma overprint using xenotime geochronology 2:45 p.m. Presentation of Best Student Paper and Poster Awards and Eisenbrey Awards 3:10 p.m. - 3:35 p.m. Coffee Break NOTE: Asterisk * denotes a student eligible for a Best Student Paper Award 3:30 p.m. Field Trip 6 – Geology of the Paleoproterozoic East Bull Lake Intrusion departs Sault College, Sault Ste Marie for East Bull Lake; overnight at East Bull Lake Lodge 4:00 p. m. Field Trip 4 – Unusual Archean Diamond-bearing rocks of the Wawa Area, participants make their own way to Wawa; overnight in Wawa xxiv �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Poster Presentations Bartingale, R. J.* and Shaw, C. A. Defining flow patterns: Paleomagnetic characteristics of the Wissota Dike Boerboom, T. J. Bedrock geological maps of the Split Rock Point and Two Harbors Northeast 7.5’ quadrangles, north shore of Lake Superior, Minnesota Brown, B. A., Czechanski, M. L., Reid, D. D. and Mudrey, M. G. Jr. New evidence for syn-depositional subsidence in the Middle Ordovician rocks of southwest Wisconsin Buchholz, T.W., Falster, A, U. and Simmons, Wm. B. Some accessory minerals of the Cary Mound granite/granophyre complex, Wood County, Wisconsin Cote, V. The Sault and District Prospectors Association Craddock, J. P., Patel, D., Porter, R., and Wirth, K. Anisotropy of magnetic susceptibility (AMS) analysis of Keweenaw Rift rhyolites, Minnesota Easton, R. M. Complex folding and faulting history in Huronian Supergroup rocks located north of the Murray fault zone, Southern Province, Ontario Gross, A.* and Holm, D. K. Kinematic analysis and monazite geochronology of the Eau Pleine and Niagara shear zones, Wisconsin Hudak, G. J., Hocker-Finamore, S. M. and Heine, J. Field distribution, petrography and lithogeochemistry of epidosites in the vicinities of Fivemile, Needleboy and Sixmile Lakes, Vermilion District, NE Minnesota Jirsa, M. A. New geological mapping of the Mesabi Iron Range Juda, N.*, Wirth, K., Craddock, J., Vervoort, J. and Andring, M. Petrogenesis of a granite xenolith in the 1.1 Ga Midcontinent Rift at Silver Bay, MN Kissin, S. A., Heggie, G. J., Franklin, J. M., Karimzadeh Somarin A. Sulphide saturation mechanisms in gabbroic intrusions in the Nipigon Embayment MacTavish, A. MetalCORP Ltd. Big Lake Cu-Zn-Ag-Au-Co, Ni-Cu-PGE and Mo Property Magee, A. Mining and exploration activity in northwestern Ontario Miller, J. D., Jr. and Severson, M. J. Geology of the Duluth Complex in the four Babbitt 7.5’ quadrangles, northeast Minnesota Mudrey, M. G., Jr. Statistical analysis of indoor radon data and relationships to geology in Wisconsin Peterson, D. M. 3D visualization of mafic intrusions in the Duluth Complex, northeastern Minnesota xxv �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Rousell, D. H. Unresolved problems and the evolution of Sudbury geology Stonier, P.*, Holm, D. K., Medaris, L. G., Jr. and Schneider, D. Characterizing the monazite fingerprint of Paleoproterozoic (Statherian) metasedimentary sequences in central Wisconsin Wirth, K. R., Vervoort, J., Craddock, J. P., Davison, C., Finley-Blasi, L., Kerber, L., Lundquist, R., Vorhies, S. and Walker, E. Source rock ages and patterns of sedimentation in the Lake Superior region: Results of preliminary U-Pb detrital zircon studies NOTE: Asterisk * denotes a student eligible for a Best Student Poster Award Friday May 12 8:00 a.m. Field Trip 3: Keweenawan Rocks of the Point Mamainse Area Leaders: Tom Hart and Anthony Pace (OGS) 8:00 a.m. Field Trip 4: Unusual Archean Diamond-bearing rocks of the Wawa Area Leader: Ann Wilson (OGS) 8:00 a.m. Field Trip 5: Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features – Mackinac Island, Michigan Leader: Ron Sage (retired-OGS) We will be meeting at the Arnold Transit Co. boat dock in St. Ignace MI no later than 9:15 a.m. to catch the 9:30 a.m. ferry 8:00 a. m. Field Trip 6: Geology of the Paleoproterozoic East Bull Lake Intrusion Leaders: Mike Easton (OGS) and R. S. James (Laurentian University) 6.00 p.m. Return of Trips 3 and 6 to Sault Ste Marie, Ontario Field Trip 4 concludes in Wawa Field Trip 5 concludes at ferry dock on Mackinac Island, Michigan xxvi �Proceedings of the 52nd ILSG Annual Meeting – Part 1 DEFINING FLOW PATTERNS: PALEOMAGNETIC CHARACTERISTICS OF THE WISSOTA DIKE. BARTINGALE, R.J. and SHAW, C.A., Department of Geology, University of Wisconsin – Eau Claire, Eau Claire, WI 54702-4004 We analyzed a gabbro dike intruding Precambrian granite below the Lake Wissota Dam in western Wisconsin. Data consisted of alternating field demagnetization and anisotropy of magnetic susceptibility measurements (AMS). Chan (1991) interpreted previous results as consistent with a Keeweenawan age (1.1 Ga) for the dike. However, research done by Macouin et. al. (2003) show similar dikes in the upper midwest and above Lake Superior have been reinterpreted to be related to the 2.07 Ga Kenora-Kabetogama Dike swarm based on moderately SE-plunging paleomagnetic directions. This study was designed to test the age interpretation of the Wissota dike and magma flow patterns. AMS data taken with respect to the major mineral axis indicates a north-east trending, horizontal flow pattern within 4 meters of the north contact and vertical flow in the center. This suggests the concentrations of feldspar phenocrysts on the northern contact were formed near the present level, possibly being fed by the vertical flowing magma. The poles have strong correlation in the center, but weaken within 4 meters of the contact. When fit to a girdle, many samples show a strong foliation. Paleomagnetic poles in several gabbro sites have a characteristic remnant magnetization plunging between 28° and 289° in a WNW direction. Samples have an N-directed overprint we interpret as recent, and record one episode of magnetism. Plotted on an apparent polar wander path for North America, the poles plot near 24° north and 176° west, which is consistent with ages of approximately 1.1 Ga. We conclude that the Wissota dike is probably Keeweenawan in age (Figure 1). References Chan, Lung, 1991, Paleomagnetism of central Wisconsin dike swarm; constraints on thermomechanical model of Midcontinent Rift: Institute on Lake Superior Geology Proceedings and Abstracts, v.37, Part1, p.23. Macouin, M., Valet, J.P., Besse, J., Buchan, K., Ernst, R., LeGoff, M., and Scharer, U., 2003, Low paleointensities recorded in 1 to 2.4 Ga Proterozoic dykes, Superior Province, Canada: Earth and Planetary Science Letters, v. 213, p. 79-95. 1 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Figure 1. Wissota dike virtual geomagnetic poles (VGPs). 2 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 THE "KONA DOLOMITE" OF ONTARIO BENNETT, GERALD, 123 LaSalle Court, Sault Ste Marie, ON The stratigraphic similarity between the Chocolay Group of the Marquette Range Supergroup of Michigan and the lower part of the Cobalt Group of the Huronian Supergroup of Ontario has been recognized for some time. Young (1983) and others accepted the correlation but at that time there were also doubters. Both Chocolay Group and the Cobalt Group lie (at least in part) on Archean basement rocks. The lowermost formations of both groups contain rocks generally considered to be glaciogenic, which are overlain by formations dominated by quartz-arenite. But there the lithologic similarity seemed to end. The Mesnard Formation of the Chocolay Group is overlain by the Kona Formation which contains a thick sequence of dolostone, whereas the (proposed equivalent) Gordon Lake Formation is predominantly a siltstone/sandstone sequence. There have however been reports of thin beds and nodules of dolostone within the Gordon Lake formation by Hoffman et al. (1980) and Jackson (1994). In 1986 Peter Born of the Ontario Geological Survey called the writers attention to a previously unmapped dolostone unit apparently overlying the Lorrain Formation in Fenwick Township, northwest of Sault Ste Marie, Ontario. Subsequent more detailed mapping by the writer revealed that the unit is comprised of at least 10 m of laminated dolostone and chert with clastic dolostone and oolitic dolostone. The writer correlated the dolostone of Fenwick Township with the Gordon Lake Formation of the Huronian Supergroup. Mr. Ken Hatfield, then of Lake Superior State University, pointed out the similarity to the Kona Formation of the Marquette area (Bennett et al., 1989, Born, 1988). No stromatolitic structures comparable to the "big cusp" dolomite of the Kona Formation were noted, but that some thinly laminated units are probably stromatolitic structures or algal mats (Personal communication, Dr. Hans Hoffman, 1990). The occurrence in Fenwick Township is strikingly similar to the dolostone of the Kona Formation of Michigan. Given the recent geochronological studies of Vallini et al. (2005), there is now little doubt that the Cobalt Group of Huronian Supergroup may be correlated with the Chocolay Group of the Marquette Range Supergroup. References Bennett, G., Leahy, E.J, Melisek, J. Born, P. and Hatfield, K. 1989. Sault Ste. Marie Resident Geologists District–1988; in Report of Activities 1988, Resident Geologists, Ontario Geological Survey, Miscellaneous Paper 142, p. 207-217. Born, Peter, 1987. Geology of the Havilland Bay – Goulais Bay Area District of Algoma; Ontario Geological Survey, Open File Report 5602, 114 p with map at a scale of 1:15 840 (1 inch to ¼ mile) Hoffman, H. J., Pearson, D.A.B. and Wilson, B.H., 1980. Stromatolites and fenestral fabric in Early Proterozoic Huronian Supergroup, Ontario; Canadian Journal of Earth Sciences, v.17, p.1351-1357. Jackson, S. L., 1994.Geology of the Aberdeen area; Ontario Geological Survey, Open File Report, 5903, 69p Vallini, Daniela, A., Cannon, William, F, and Schulz, Klaus J., 2005. New age data for the Chocolay Group, Marquette Range Supergroup: Implications for the Paleoproterozoic Evolution the Lake Superior and Lake Huron regions. Institute on Lake Superior Geology Proceedings, 51st Annual Meeting, Nipigon, Ontario, Part I – Proceeding and Abstracts, v.51 part 1 Young, G.M. 1983. Tectono-sedimentary history of early Proterozoic rocks of the northern Great Lakes; in Early Proterozoic Geology of the Great Lakes Region, Geological Society of America Memoir, v.160, p.15-32. 3 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 BEDROCK GEOLOGIC MAPS OF THE SPLIT ROCK POINT AND TWO HARBORS NORTHEAST 7.5-MINUTE QUADRANGLES, NORTH SHORE OF LAKE SUPERIOR, MINNESOTA BOERBOOM, TERRENCE J., Minnesota Geological Survey, boerb001@umn.edu The Minnesota Geological Survey is continuing quadrangle-scale geologic mapping of 7.5' quadrangles adjacent to Lake Superior as part of the U.S. Geological Survey STATEMAP program. This mapping effort has resulted in seven published geologic maps in an area from Duluth to Split Rock Point (Fig. 1A). Work is currently in progress on the Little Marais, Schroeder, and Tofte quadrangles, and Lutsen will be mapped in the coming year. Field mapping is at scale 1:12,000, and map compilations are at 1:24,000. The North Shore is experiencing burgeoning development, creating a growing need for understanding bedrock aquifers and for identifying construction resources. Nearly all water wells near Lake Superior are finished in bedrock aquifers, and saline brines are commonly encountered. Refining the volcanic stratigraphy is the first step in understanding where these brines originate. Identification of intrusive rocks is the first step in locating sources for crushed-rock aggregate. Also, mapping has identified potential sources of paving stone, for which new sources are being pursued by landscaping companies. Thus, the goal of this mapping is to refine the knowledge of the volcanic and intrusive rocks for societal needs, as well as to provide a geologic framework for ongoing studies of the geochemical evolution of the Keweenawan Midcontinent rift system, through collaborative studies with staff from Macalester College. TWO HARBORS NE Prior to this study, no mapping had been done in this quadrangle. Although parts of the quadrangle contain few bedrock outcrops, areas underlain by intrusive rocks are generally well exposed. The newly recognized, informally named, London intrusion is a crudely layered or composite intrusion with a basal laminated ferrogabbro, an intermediate granophyric ophitic gabbro, and a cap of ophitic diabase. Coarse-grained felsicintermediate granophyric rocks form an irregular layer between the ophitic gabbro and the upper ophitic diabase, and also lenses within the other units. Fine-grained ferromonzodiorite occurs locally at the base of the intrusion, likely as a hybridized partial melt of adjacent andesite. Other intrusions include the northward extension of the Silver Creek diabase and Lafayette Bluff diabase, and other poorly exposed units whose distributions are based largely on geophysical data. Sporadically exposed volcanic rocks include the southwestern portion of the Gooseberry River lavas (Green, 2002), here composed of olivine tholeiite in the upper part and transitional basalt to andesite in the lower part, and a newly recognized sequence termed the Gustafson Hill lavas composed of variably porphyritic ferroandesitic to basaltic rocks, separated from the Gooseberry River lavas by the Silver Creek diabase. 4 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 SPLIT ROCK POINT The Split Rock Point quadrangle is dominated by volcanic rocks of the Gooseberry River lavas, with subordinate mafic to felsic intrusions that include the Silver Creek and Beaver River diabase, the Split Rock intrusion, and a narrow multilithic breccia dike (Boerboom, 2004; Boerboom and others, 2004). The upper Gooseberry River lavas include a thick porphyritic basalt flow and a faultsliced sequence of andesite sandwiched between ophitic olivine tholeiite flows. The lower Gooseberry River lavas are poorly exposed, but available outcrops indicate they are composed dominantly of andesitic rocks continuous with those mapped in the Two Harbors Northeast quadrangle. The Split Rock intrusion is a hypabyssal, north–south elongate body with the form of a south-plunging syncline that has a thin, lower mafic phase coeval with the dominant phase of pink, flow-banded, weakly porphyritic felsite that contains scattered but ubiquitous small mafic enclaves. References Boerboom, T.J., 2004, Newly recognized diatreme breccia dikes on Lake Superior near Two Harbors, Minnesota [abs.]: Institute on Lake Superior Geology, 50th Annual Meeting, Duluth, Minn., Proceedings, v. 50, p. 39. Boerboom, T.J., and Green, J.C., 2004, Bedrock geology of the Split Rock Point quadrangle, Lake County, Minnesota: Minnesota Geological Survey Miscellaneous Map M-147, scale 1:24,000. ———2005, Bedrock geology of the Two Harbors NE quadrangle, Lake County, Minnesota: Minnesota Geological Survey Miscellaneous Map M-155, scale 1:24,000. Boerboom, T.J., Green, J.C., and Jirsa, M.A., 2002a, Bedrock geology of the French River and Lakewood quadrangles, St. Louis County, Minnesota: Minnesota Geological Survey Miscellaneous Map M-128, scale 1:24,000. ———2002b, Bedrock geology of the Knife River quadrangle, St. Louis and Lake Counties, Minnesota: Minnesota Geological Survey Miscellaneous Map M-129, scale 1:24,000. Boerboom T.J., Green, J.C., and Miller, J.D., Jr., 2003a, Bedrock geologic map of the Castle Danger quadrangle, Lake County Minnesota: Minnesota Geological Survey Miscellaneous Map M-140, scale 1:24,000. ———2003b, Bedrock geologic map of the Two Harbors quadrangle, Lake County Minnesota: Minnesota Geological Survey Miscellaneous Map M-139, scale 1:24,000. Boerboom, T.J., Miller, J.D., Jr., and Green, J.C., 2004, Geologic highlights of new mapping in the southwestern sequence of the North Shore Volcanic Group and Beaver Bay Complex: Institute on Lake Superior Geology, 50th Annual Meeting, Duluth, Minn., Proceedings, v. 50, pt. 2, Field trip guidebook, p. 46-85. Green, J.C., 2002, Volcanic and sedimentary rocks of the Keweenawan Supergroup in northeastern Minnesota, chapter 5 of Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.M., and Wahl, T.E., 2002, Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geological Survey Report of Investigations 58, p. 94-105. 5 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Figure 1. A. Index map showing the location of mapped quadrangles along the North Shore of Lake Superior. M-128–Boerboom and others (2002a); M-129–Boerboom and others (2002b); M-139– Boerboom and others (2003b); M-140–Boerboom and others (2003a); M-147–Boerboom and Green (2004); M-155– Boerboom and Green (2005). Little Marais, Schroeder, and Tofte to be published in 2006; Lutsen in 2007. 6 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 NEW EVIDENCE FOR SYN-DEPOSITIONAL SUBSIDENCE IN THE MIDDLE ORDOVICIAN ROCKS OF SOUTHWEST WISCONSIN BROWN, B.A., CZECHANSKI, M.L. Wisconsin Geological and Natural History Survey, 3817 Mineral Point Road, Madison WI 53705 REID, DANIEL D. Wisconsin Dept. Transportation, 3502 Kinsman Blvd., Madison, WI 53704 MUDREY, M.G. JR.† 106 Ravine Road, Mount Horeb, WI 53572 Extensive new rock cuts and exposures were created during the rebuilding of U.S. Highway 151 into a modern 4-lane highway through the Driftless Area of southwest Wisconsin. These cuts, some exceeding 100 feet high, provide a unique cross section of the Middle Ordovician rock of the historic Upper Mississippi Valley Base Metal District of zinc and lead. The new cuts provide a detailed view of the stratigraphy, and expose some structures not previously described in the region. Examples of collapse structures and local block faulting have been recognized throughout the mining district for many years. These structures could be seen in older road cuts, and they were described in many early reports on the mining district. Structures of this type have traditionally been interpreted as pitch-and- flat structures, which resulted from solution and collapse related to the formation of the zinc-lead deposits. The oldest collapse features observed in the new cuts formed in Early Middle Ordovician time, during deposition of the St. Peter Sandstone. The youngest known at this time formed in Late Middle Ordovician time, during deposition of the carbonate of the Galena Formation. These features formed as much as 200 million years earlier than the zinc-lead mineralization, which has been dated as Early Permian in age. The early syn-depositional collapse structures are interpreted to be the result of local collapse of paleokarst features developed in the underlying carbonate of the Early Ordovician Prairie du Chien Group. Extensive karst is known to have formed during the interval of aerial exposure following lithification of the Prairie du Chien rock and prior to deposition of the St. Peter Sandstone. This interval contains a major regional unconformity which marks the SaukTippecanoe sequence boundary throughout the region. Collapse occurred as overlying sediment accumulated and compacted, prior to complete lithification. The role of these syn-depositional features in controlling the path of mineralizing fluids is unknown. The few examples known at this time contain no significant mineralization, although mineralized pitch and flat structures and gash vein lead deposits are known to occur nearby. It is possible that the paleokarst features were small and localized, and were not important as conduits for mineralizing fluids, which migrated along regional tectonic features. In contrast, areas of sulfide mineralization were typically associated with extensive rock alteration and deep weathering, which required modifications to the design of cuts and structures, and use of alternative slope stabilization methods during construction. 7 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 SOME ACCESSORY MINERALS OF THE CARY MOUND GRANITE/GRANOPHYRE COMPLEX, WOOD COUNTY, WISCONSIN. BUCHHOLZ, THOMAS W.†1, FALSTER, ALEXANDER U.2, and SIMMONS, WM. B. 2, 1 1140 12th Street North, Wisconsin Rapids, Wisconsin 54494, 2 Department of Geology and Geophysics, University of New Orleans, New Orleans, Louisiana 70148. Early Proterozoic (1,833 ± 4 Ma) granite, granophyre and comagmatic rhyolite outcrop on Cary Mound in western Wood County, WI, and are exploited in several quarries. All phases of the complex are cut by numerous faults and fractures that served as avenues for fluid transport, resulting in widespread chloritization of the granite and granophyre and development of thin hydrothermal veins. Several studies (Sims, 1990; Bruesewitz and Cordua, 2003) postulate that the complex may be a collapsed caldera and indicate that the complex may be of anorogenic or late orogenic origin. Granophyric phases are locally miarolitic, particularly in the Haske quarry, and host a complex mineralogy ranging from simple magmatic through pegmatitic to hydrothermal mineralization, even though no pegmatites sensu stricto have yet been found on Cary Mound. Miaroles may be either simple vugs lined with crystals of quartz, microcline +biotite, probably formed as a result of local volatile saturation, or may have marginal pegmatitic facies marking the transition from granophyre to miarole, the latter primarily noted in the Haske quarry. In areas where such miaroles are in close proximity, pegmatitic margins may merge and form larger areas of pegmatitic texture. These pegmatitic facies may represent pods of pegmatitic melt generated by fractionation of the crystallizing granophyre; if so, the melt was probably enriched in volatiles and incompatible elements. Typical NYF (niobium-yttrium-fluorine = typical A-type granitic association as opposed to LCT (lithium-cesium-tantalum = typical S/I-type granitic association) pegmatite mineralization is present in these pegmatitic phases; quartz, microcline, fluorite, allanite(Ce) and zircon. Locally ferrocolumbite, samarskite-(Y) and thorite have been identified; a Ti-rich Y-Nb oxide mineral has been noted as well and may be polycrase-(Y), but confirmation is required, and a number of additional phases await further study. Pegmatite-bordered miaroles may be quartz cored or filled with quartz + late chlorite, sulfides, fluorite, siderite and calcite; these are interpreted as products of a pervasive late hydrothermal phase introduced along networks of thin fractures. The most abundant sulfides are pyrite, chalcopyrite and pyrrhotite, though sphalerite, galena, marcasite, arsenopyrite and rarely molybdenite may be present. Unusual acicular sulfide crystal morphologies are sometimes present (Buchholz et al, 1997). Small silvery-colored grains of Cu-Co-Ni sulfides have been noted; no further identification work has been done due to paucity of material. Barite is common in small amounts but is usually inconspicuous. Gypsum of secondary, weathering origin may be locally common in small amounts. Cassiterite is uncommon but has been identified from thin fissures in several sites (Cepress and County quarries), and appears to have formed early, probably as a highertemperature hydrothermal phase perhaps transitional from magmatic/pegmatitic to 8 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 hydrothermal/pneumatolytic. Xenotime-(Y) from a thin fissure in the Haske quarry may be of similar origin. Hydrothermal alteration and chloritization is widespread throughout the complex, and associated mineralized veins and fissures in general reflect the late hydrothermal mineralization noted in miaroles. Although most are rich in chlorite, one vein system in the Cary-Rock Road quarry is mineralized with fine-grained lithian muscovite (1.1 wt. % Li2O) associated with pyrite, chalcopyrite, siderite, apatite and barite. Pyrrhotite is generally absent from vein mineralization, whereas the Fe-S paramorphs pyrite and/or marcasite are usually common. Tiny grains of molybdenite are often common in metasomatized granite/granophyre in the Haske quarry. Sparse millerite (NiS) has recently been identified from a hydrothermal vein in the Haske quarry. Rutile is uncommon but has been noted from the Cepress and Haske quarries. Small late-formed crystals and grains of a LREE-phosphate (probably either monazite-(Ce) or rhabdophane(Ce) have been found on chlorite and pyrite from the Haske and Cepress quarries. The mineralization present in pegmatitic miarole margins may indicate the parent magma had locally evolved to a Nb, Y and F-enriched phase. Abundant fluorite and the existence of lithium-bearing muscovite veins suggest the possibility of pneumatolytic or greisen-type mineralization within the complex. The pervasive chloritic hydrothermal alteration and sulfide mineralization suggest that small vein-type sulfide deposits may be present. However locating these, if they exist, may be challenging due to extensive forests and remnant Cambrian sandstone cover. References Bruesewitz, Jeff and Cordua, W.S., 2003, The Cary Mound Granite: A mineralized collapsed caldera in Wood County, Wisconsin, abstract, Geological Society of America, Abstracts and Programs - North Central Section annual meeting, vol. 35 #2, St. Louis, Mo, p.45 Buchholz, Thomas W., 1997, Apatite and Lithium Bearing Muscovite from Central Wisconsin: Mineral News, June 1997, p.6. Buchholz, T. W., Falster, A. U., and Simmons, Wm. B., 1997, An Unusual Miarolitic Mineral Assemblage From Central Wisconsin, abstract, Rochester Mineralogical Symposium, Program and Abstracts Volume, p. 8. Sims, P.K., 1990, Geologic Map of Precambrian Rocks, Eau Claire and Green Bay 1º x 2º Quads, Central Wisconsin, U.S. Geological Survey Map I-1925 9 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 THE SUDBURY IMPACT LAYER IN THE MARQUETTE RANGE SUPERGROUP OF MICHIGAN CANNON, WILLIAM F.† and HORTON, J. WRIGHT, JR., U.S. Geological Survey, Reston VA 20192 KRING, DAVID A., Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ The 1850 Ma meteorite impact at Sudbury, Ontario created a crater estimated to be 180 km in diameter (Abramov and Kring, 2004). A layer of material formed by the Sudbury impact has been well documented in northwestern Ontario and northern Minnesota (Fig. 1) (Addison and others, 2005). In Michigan, only about 500 km from the center of impact, marine sediments of the Marquette Range Supergroup were being deposited and should record the impact. One possible record of an impact is addition to local sediments of material excavated from the crater. This material may vary from coarse fragments of the target rock from the ejecta curtain to finer particles from the impact-generated dust cloud, including accretionary lapilli and mineral grains bearing shock metamorphic features. At the time of impact at least parts of the Michigan sedimentary basin were in shallow-water suggesting the likelihood of major tsunami-related deposits. We are investigating possible impact generated rocks at five sites in northern Michigan (fig.1), which are at a comparable stratigraphic horizon to the Ontario ejecta and are similar petrographically. All localities are at or within a few hundred meters above the base of the Baraga Group and may record both airborne and tsunami deposition. Baraga Basin- layer 1-15 m thick in lower part of Michigamme Formation. Well developed accretionary lapilli (fig. 2) and planar deformation features (fig. 3). West Dead River- isolated outcrop of bedded to massive lapilli-rich material (fig. 4). Contains angular chert fragments to about 1 m diameter. Strong carbonate replacement. At least 2 m thick and probably at least 100 m above base of Michigamme Formation. East Dead River- bed of breccia about 30 m thick (fig. 5, 6). Sparse lapilli. Crudely graded with coarsest clasts at base. Underlain by banded iron-formation and overlain by black pyritic slate. Contains numerous clasts of chert. About 300 m above base of Michigamme Formation. Marenisco- bed about 2 m thick near base of Copps Formation. Coarse sand to conglomerate containing many clasts of underlying Archean granite. Also contains slabs of chert to about 2 m diameter and quartz grains with possible relict planar deformation features. Strong carbonate replacement. Republic- numerous boulders of lapilli-rich breccia in gravel pit, possibly locally derived. Our study of these localities and the search for additional sites is in its early stages. We hope that calling attention to these likely impact-related rocks will encourage additional searches and discoveries. We suspect that most new “discoveries” will result from recognizing telltale signs of impact processes within already known “unusual” breccias or “volcanic” units within this narrow stratigraphic interval. References Abramov, O, and Kring, D.A., 2004, Numerical modeling of an impact-induced hydrothermal system at the Sudbury crater: J. Geophys. Res., v. 109, p.1-16. Addison, W.D., Brumpton, G.R., Vallini, D.A., McNaughton, N.J., Davis, D.W., Kissin, S.A., Fralick, P.W., and Hammond, A.L., 2005, Discovery of distal ejecta from the 1850 Ma Sudbury impact: Geology, v. 33, p. 193-19 10 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Figure 1 Location of impact layer sites Figure 4. West Dead River. Bedded accretionary lapilli unit unconformably overlain by massive breccia with angular chert. Card 8 cm long. Figure 2. Baraga Basin. Accretionary lapilli in drill core. Figure 5. East Dead River. Photomicrograph of breccia with “volcanic” shards and rounded quartz grains. Figure 3. Baraga Basin. Quartz grain with two sets of relict planar deformation features. Figure 6 East Dead River. Multi-lithic breccia of chert and a variety of “volcanic” fragments in finer breccia groundmass. 11 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 THE SAULT AND DISTRICT PROSPECTORS ASSOCIATION COTE, VIVIENNE, President, Sault and District Prospectors’ Association, Sault Ste Marie, Ontario The Sault and District Prospectors Association (SDPA) has been in existence since the early 1970’s. The purpose of the association is to promote mineral exploration in the area and raise awareness of the role mineral development plays in the economy of the region and the north in general. Although the group is relatively small it is quite active with speakers from various backgrounds and interests presenting a diverse array of topics at the monthly meetings. The highlight of the year is the SDPA annual field trip. The trips proved so popular that in 2005 a fall field trip was added. The latest trips have included the Archean diamond bearing rocks in the Wawa area, the Keweenawan rocks of the Mamainse Point area, Huronian stratigraphy of the Elliott Lake area as well as the Eagle River Mine in the Wawa district. The poster is a visual overview of the various fieldtrips undertaken by the group against a backdrop of the general geology of the area. A wide variety of participants have attended including prospectors, geologists, students, rock hounds and of course, our mascot, the dog “Chloe”. The SDPA is an associate member of the Ontario Prospectors Association. 12 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 ANISOTROPY OF MAGNETIC SUSCEPTIBILITY (AMS) ANALYSIS OF KEWEENAW RIFT RHYOLITES, MINNESOTA CRADDOCK, JOHN P. and PATEL, DHIREN, Geology Dept., Macalester College, St. Paul, MN 55105; PORTER, RYAN, Geology Dept., Whitman College, Walla Walla, WA; and WIRTH, KARL, Geology Dept., Macalester College, St. Paul, MN 55105 The North Shore Volcanic Group (NSVG) of the 1.1 Ga Midcontinent Rift System (MRS) in Minnesota is dominated by basalt, with approximately 10–25% of the bi-modal igneous suite being composed of felsic flows (rhyolites and icelandites). Several of the rhyolite flows may be rheomorphic ignimbrites due to their vast expanse and presence of tridymite paramorphs and local exposures of flow banding, laminations and lineations (Green and Fitz, 1993). In this study we identified four well-exposed rhyolite flows between Grand Portage and Duluth where the over and underlying mafic flows clearly distinguish the rhyolite flow thickness and outcrop character. Oriented samples were collected from the bottom, middle and top of each of the four flows and oriented cores (or equant cubes) were analyzed at the Institute for Rock Magnetism, University of Minnesota using the “RolyPoly”, which is an alternating current (AC) susceptibility bridge for determining anisotropy of low-field magnetic susceptibility. An alternating current in the external "drive" coils produces an alternating magnetic field in the sample space with a frequency of 680 Hz and amplitude of up to 1 mT. The induced magnetization of a sample is detected by a pair of "pickup" coils, with a sensitivity of 1.2*10-6 SI volume units. For anisotropy determination, a sample is rotated about three orthogonal axes, and susceptibility is measured at 1.8° intervals in each of the three measurement planes. The susceptibility tensor is computed by least squares from the resulting 600 directional measurements. The output is a trend and plunge for each of the principal susceptibility tensors (i.e. Kmax, Kint, Kmin), mean susceptibility, and three axial ratios L=Kmax/Kint, F=Kint/Kmin, and P=Kmax/Kmin (lineation, foliation, and degree of anisotropy respectively). Principal tensors are plotted on lower hemisphere steroenet projections. AMS is, thus, a magnetic proxy for interpreting magmatic flow in the rhyolites. From north to south, we sampled the Kimball Creek (near Hovland, n=56, 366 m thick), Devil Track (near Grand Marais, n=61; 250 m thick), Palisade Head (near Silver Bay, n=62, 100 m thick), and Lakewood (north of Duluth, n=58, 78 m thick) rhyolites. Despite an average anisotropy for the sample suite (n=237) of 7.5%, only the Devils Track (base and top) and Palisade rhyolites preserve a layer-parallel Kmax grouping that is interpretable as rift-normal, suggesting northwestward eruption from the rift axis (Fig. 1). References Green, J.C. and Fitz, T. J. III, 1992, Extensive felsic lavas and rheoignimbrites in the Keweenawan Midcontinent Rift plateau volcanics, Minnesota; petrographic and field recognition: Journal of Volcanology and Geothermal Research, v. 54, 177-196. Rochette, P., Jackson, M., Aubourg, C., 1992, Rock magnetism and the interpretation of anisotropy of magnetic susceptibility: Reviews of Geophysics, v. 30, p. 209-226. 13 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 14 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 COMPLEX FOLDING AND FAULTING HISTORY IN HURONIAN SUPERGROUP ROCKS LOCATED NORTH OF THE MURRAY FAULT ZONE, SOUTHERN PROVINCE, ONTARIO EASTON, R.M., Precambrian Geoscience Section, Ontario Geological Survey, 933 Ramsey Lake Road, Sudbury, Ontario P3E 6B5, mike.easton@ndm.gov.on.ca Huronian Supergroup strata located north of the Murray fault zone are generally thought to record a relatively simple structural history of broad folding and faulting, related primarily to distal effects of the ~1835 Ma Penokean orogeny. Detailed mapping by the Ontario Geological Survey in Porter and Vernon townships (Easton 2005, 2006), northeast of Agnew Lake indicates that, at least in the area immediately west of Sudbury, this view is incorrect. At least 2 periods of folding are present, roughly orthogonal to one another - the resulting interference forms a dome and basin pattern (Figure 1). F1 folds Nipissing gabbro intrusions present in the lowermost part of the stratigraphy, whereas Nipissing gabbro appears to be emplaced along fractures related to F2 axial planes. This suggests either multiple periods of gabbro emplacement, or that gabbro emplacement occurred synfolding. In either case, folding cannot be significantly younger than 2210 Ma. The map pattern is also affected by at least 5 major fault sets, 4 of which are post-folding. The earliest faults are north-trending, and juxtapose Archean granitic basement against Huronian Supergroup strata. These faults appear to have been fluid conduits, as indicated by the presence of large quartz vein systems and microbrecciation in Archean basement, and hydrothermal annealing of quartz in sedimentary rocks, adjacent to the faults. Eastnortheast faults also juxtapose Huronian strata against basement rocks, but are post- F1 folding, with both vertical and lateral movement. They may be associated with a set of north to northeast, dominantly normal faults, which may have an older thrust component. Most significant in terms of map pattern, at least in the southern part of the map area closest to the Murray fault system, are east to east-northeast normal faults, across which major changes in stratigraphic level occur. There may be a thrust component to these faults, but if so, it has been obscured by subsequent vertical movement, and the fact that the east to east-northeast faults are the loci for the development of extensive zones of Sudbury breccia. The localization of Sudbury breccia along this fault set suggests that it may have developed at ~1850 Ma, due to the Sudbury impact or the peak of the Penokean orogeny, or both. Finally, significant vertical displacement, occurs along a major set of closely spaced northwest-trending faults. Some of these faults are the loci of Sudbury swarm diabase dikes (~1240 Ma). The dikes are undeformed, which suggest that this fault set formed between 1850 and 1240 Ma. The complex history of the area provides new evidence for an earlier orogenic event in the region (“Blezardian?”), and has major implications for detailed stratigraphic correlation of Huronian Supergroup strata and mineralized Nipissing gabbro intrusions. 15 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Figure 1. Simplified geological map of the northeast shore of Agnew Lake, showing the distribution of fold styles within Porter and southern Vernon townships. The contact between the Mississagi and Bruce formations has been highlighted to illustrate the fold pattern, and units stratigraphically above the Bruce Formation are shown by a pattern. Between the Cameron Creek and Midport faults, the area is dominated by a dome and basin geometry, indicating the presence of two fold generations, with approximately perpendicular axial planes. North of the Midport fault, the early, north-oriented fold style (F1) dominates. Abbreviations: BB = Big Swan basin, CB = Cygnet Lake basin, HB = Hunter basin, PB = Porter basin, SB = Sutherland basin, VS = Vernon syncline. References Easton, R.M. 2005. Geology of Porter and Vernon townships, Southern Province; in Summary of Field Work and Other Activities, 2005, Ontario Geological Survey Open File Report 6172, p.13-1 to 1320. Easton, R.M. 2006a. Geology of Porter and Vernon townships; Ontario Geological Survey, Preliminary Map P.2845. Scale 1:20 000. Colour, with Marginal Notes. 16 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 IRON FORMATION IN NEOARCHEAN DELTAIC SUCCESSIONS: LAYERING STYLES DEVELOPED DURING SILICICLASTIC AND CHEMICAL SEDIMENT DEPOSITION, SUPERIOR PROVINCE, CANADA FRALICK, PHILIP, Department of Geology, Lakehead University, Thunder Bay, Ontario, Canada (philip.fralick@lakeheadu.ca) Neoarchean iron formations (IFs) developed in volcanically quiescent, shallow marine settings consist of magnetite- and/or hematite-rich chemical sediments interbedded with siltstones and slates. The mechanism responsible for depositing such successions contrasts with the two principal models for iron hydroxide or oxyhydroxide precipitation from early Precambrian seawater. The deposition of large Paleoproterozoic iron formations through the mixing of Fe+2 enriched, deep ocean waters with the oxygenated waters on shelves is generally accepted (Cloud 1973, Holland 1973, Pufahl and Fralick 2004). In contrast, many Archean IFs appear to have formed through the venting of hydrothermal fluids associated with volcanically active terrains (Fralick and Barrett 1995). The latter model is not applicable to rocks formed in shallow, volcanically inactive areas and the former has only been applied to Paleoproterozoic shelfs where precipitation was occurring during the oxygenation of the Earth’s atmosphere. The shallow water, Neoarchean iron formations form a unique class of IFs where precipitation was driven by factors other than upwelling or hydrothermal venting. This IF type was examined in the Beardmore-Geraldton area of Wabigoon Subprovince and in the Eagle Island Group of Uchi Subprovince. The shallow water Neoarchean iron formations described here were primarily deposited on flooding surfaces overlying fluvial channel and shore-proximal braid delta deposits. Magnetite and/or hematite laminae are also interbedded with some distributary mouth sediments draping reactivation surfaces on barforms to ripples. Additionally, the iron oxides are present as: disseminated detritus in the upper portion of thin graded siliciclastic layers; intervals of finely parallel laminated hematite and jasper, or magnetite and magnetite+chert, separating clastic layers; and, micro-ripple laminated jasper with hematite drapes. The chemical sediments precipitated in the water column of the nearshore deltaic environment and accumulated during periods of lower current activity and siliciclastic supply. The ferric compounds were redistributed during intervals of river plume outflow, especially accumulating in association with fine-grained detritus in event layers formed where the plume lost contact with the bottom (Fig. 1). Offshore equivalents of these assemblages do not contain IF. The model presented for IF deposition relies on an elevated nutrient flux (N, P) in the near-shore that stimulated microbially induced oxidation of Fe+2. This implies the existence of thriving microbial communities in Neoarchean, near-shore settings; communities of organisms that were able to produce their energy by photosynthesis, oxidize iron either intra- or extra-cellularly, and generate thick successions of IF (Fralick and Pufahl in press). 17 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 References Cloud, P.E., 1973. Paleoecological significance of banded iron-formation. Econ. Geol., v. 68, p. 1135-1143. Fralick, P.W. and Barrett, P.J., 1995. Depositional controls on iron formation associations in Canada; in, G. Plint (ed), Facies Analysis. Int. Ass. of Sedimen., Spec. Pub 22, p. 137-156. Fralick, P.W. and Pufahl, P.K., in press. Iron formation in Neoarchean deltaic successions and the microbially mediated deposition of transgressive systems tracts. Jour. of Sed. Res. Holland, H.D., 1973. The oceans: a possible source of iron in iron formations. Econ. Geol., v. 68, p.1169-1172. Pufahl, P.K. and Fralick, P.F., 2004. Depositional controls on Paleoproterozoic iron formation accumulation, Gogebic Range, Lake Superior region, USA. Sedimentology, v.51, p.791-808. Fig. 1. Schematic representation of the processes responsible for depositing iron formation in shallow Neoarchean settings. 18 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 SAMPLING LAMPROPHYRE DIKES FOR DIAMONDS; DISCOVER ABITIBI INITIATIVE GRABOWSKI, GARY, District Geologist, Ontario Geological Survey, Kirkland Lake, Ontario gary.grabowski@ndm.gov.on.ca The Discover Abitibi Initiative is funded by the private sector and, the federal and provincial governments (respectively Industry Canada through FedNor and the Ontario Ministry of Northern Development and Mines through the Northern Ontario Heritage Fund). The program is designed to stimulate mineral exploration in the Ontario portion of the Abitibi greenstone belt. A project to sample lamprophyre dikes, in the Kirkland Lake – Cobalt area, was approved by the Discover Abitibi program in July, 2004. Forty-five samples, each weighing 24 kg, were submitted to SGS Lakefield Research Ltd. in Lakefield, Ontario for litho-geochemical analysis and diamond extraction, selection and description. Six of the forty-five samples submitted returned diamonds. Samples GGDA0402 and GGDA0432 each returned one microdiamond. Samples GGDA0433, GGDA0435 and GGDA0441 returned 5, 3 and 23 microdiamonds respectively. Sample GGDA0410 contained one 0.011 carat (2.214 mg) macrodiamond. The results of this project demonstrate that diamonds occur in the lamprophyric rock from the Kirkland Lake – Cobalt area. • A 25 kg sample represents about one cubic foot of rock. Although every attempt was made to collect as representative a sample as possible from each exposure, the relatively small volume sampled may have easily missed a diamond. Therefore, sample locations that did not return a diamond should not be considered to barren. • Further study is needed to determine where the diamonds are located within the dikes. Most dikes sampled that returned diamonds contained xenoliths. Spider Resources Ltd. has recently postulated that the diamonds are found in the xenoliths on their Wawa property. • A variety of rock types host lamprophyre dikes and breccia, including all types of metavolcanic and metasedimentary rocks, as well as felsic intrusive rocks including granodiorite, granite and syenite. No preference is apparent for those that contain diamonds. • There are numerous lamprophyre locations that were not sampled in this project. Published Ontario Geological Survey (and its predecessors) can be used to locate these exposures. • The Kirkland Lake – Cobalt area hosts more than 30 kimberlite pipes, over half of which are diamondiferous. There are many targets being tested for potential kimberlite. In May 2005, Tres-Or Resources Ltd. discovered a kimberlite pipe on its Temagami North property, Lapointe 1 target, located 16 km northwest of sample GGDA0402. 19 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 KINEMATIC ANALYSIS AND MONAZITE GEOCHRONOLOGY OF THE EAU PLEINE AND NIAGARA SHEAR ZONES, WISCONSIN GROSS, AMANDA, and HOLM, D.K., Geology, Kent State University, Kent OH, agross4@kent.edu; SCHNEIDER, D.A., Geological Sciences, Ohio University, Athens, OH Introduction. The southern margin of Laurentia experienced several episodes of arc accretion that account for the growth of new continental crust during the late Paleoproterozoic, 1900-1600 Ma. The Niagara fault zone and the Eau Pleine shear zone are structural remnants of an ancient arc-continent collision that occurred during the Penokean orogeny (1870-1830 Ma). As Laurentia continued to grow, these sutures likely persisted as zones of weakness; recent studies have proposed that the Niagara Fault zone may have been reactivated during gneiss dome exhumation (Schneider et al., 2004). Other structural discontinuities in the area show evidence of long-lived reactivation including the Great Lakes tectonic zone to the north. Our purpose is to evaluate the importance of tectonic heredity on the geologic history of the central Penokean orogen. Kinematic analysis. Tectonites from both shear zones contain steep penetrative foliations and dominantly down-dip stretching lineations. Oriented samples of these tectonites exhibit kinematic indicators suggestive of a multi-stage displacement history. Samples of the Niagara Fault zone from Pier’s gorge show spectacular quartz-filled strain shadows (Fig. 1a) that show south-side up relative motion, along with asymmetrical tails on feldspar grains (Fig. 1b) that show south-side down movement. Mesoscopic field indicators from a large outcrop on the north side of Highway 101 preserve definitive south-side down movement as seen in sigma structures and near isoclinal folding of late veins. Oriented samples of the Eau Pleine shear zone at March Rapids display south-side down movement in rotated feldspar grains. Sheared quartzofeldspathic gneiss at the south end of Dancy Quarry exhibits variable grain-size reduction – from coarse gneissic fabric to strongly sheared ultramylonitic fabric all of which are subparallel. The coarse gneiss samples show south-side up movement seen in rotated feldspars. In contrast, the finer-grained mylonites contain bent mica and fish structures which show south-side down relative motion. Multiple episodes of movement are evident from the kinematic work described here. Monazite geochronology. U-Pb dating of monazite has proved useful for determining the timing of formation and reactivation of large shear zones in the western U.S. (McCoy et al. 2005). Initial monazite microprobe work on a single coarse-grained sample of the Eau Pleine shear zone produced a tight cluster of monazite total-Pb ages at ~1846 Ma (Loofboro et al., 2004). This age is consistent with the timing of formation of the shear zone based on cross-cutting relations (Sims et al., 1989). Dating of monazite from the finer-grained ultramylonites is currently in progress. Medium-grained monazite spot dating on two samples of the Niagara fault zone yielded two age populations: the older age of ~1628 Ma is about the same age as the Mazatzal orogeny, whereas the younger ca. 1496 Ma date is about the time that the Wolf River batholith intruded the area (Rose, 2004). Both of these dates, although preliminary, suggest that the Niagara fault zone was influenced by younger tectonic stresses and fluid channelization events. Textural work shows that some monazite grains at the Pier’s gorge site are sheared (Fig. 1c), providing opportunity for constraining maximum age of deformation. Meso- and microscopic kinematic studies combined with total-Pb ages of metamorphic monazite hold great potential for constraining the timing and sense of relative motion along these shear zones. 20 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 21 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 AN OVERVIEW OF GEOLOGY OF THE SAULT STE MARIE AREA HAILSTONE, M. P.GEO, Ministry of Northern Development and Mines, Ontario Geological Survey, Resident Geologist Program, District Geologist, Sault Ste. Marie This presentation will provide an overview of the geology of the Sault Ste. Marie area. The variety of lithologies in the area span the ages from Archean to Paleo- and Mesoproterozoic. The Archean Batchawana Greenstone Belt is part of the Abitibi subprovince and is typical of many Archean Greenstone Belts within the Superior province dominated by metabasalts with intercalated calc alkaline to felsic metavolcanics and metasediments. This belt has been divided into an older Eastern Domain and a Western domain by Grunsky (1991) along a plate-plate collision boundary. The Batchawana Greenstone belt is surrounded on three sides by younger Archean gneisses of the Chapleau, Algoma and Ramsey Lake Gneiss domains. The belt of Paleoproterozoic Huronian rocks between Sudbury and Sault Ste. Marie are part of the Southern structural province. Standard nomenclature for the division of the sedimentary groups within the Huronian utilize a model of four glacial cycles from conglomeratic diamitictite base formations through deep marine formations to deltaic shallow marine formations. Although these environments of deposition work for the sedimentary groups, they mask the genesis of the Huronian basin which is now thought to be an early Proterozoic, active then passive rift system. (Bennett, G. 2006.) Recent studies into the earth’s early atmosphere have revealed that at approximately 2.35 Ga, oxygen made its appearance in the earth’s atmosphere. Huronian sedimentary rocks in the Elliot Lake area preserve that event. Lake Superior is the one of the longest and deepest continental rift systems on the face of the planet and is approximately 1 Ga years old. The rift is also known as the Mid Continental Rift (MCR). In the Mamainse Point Formation Keweenawan subaerial, alkaline basaltic flows, intercalated with conglomerates and intruded by felsic potassic keratophyres of the MCR are exposed on the west side of the Batchawana Greenstone Belt approximately 60 kilometers north of Sault Ste. Marie. Studies of these alkaline basalts demonstrate a geomagnetic reversal separated by a conglomerate unit with the older eastern alkaline basalt flows being reversely polarized. (Hart, T., R. and Pace, A. 2006) References Bennett, G., 2006: The Huronian Supergroup between Sault Ste. Marie and Elliot Lake-Field Trip Guidebook Institute on Lake Superior Geology, 52nd Annual Meeting, Volume 52, Part 4, 65p. Grunsky, E.,C., 1991: Geology of the Batchawana Area, District of Algoma; Ontario Geological Survey, Open File Report 5791, 214p. Hart, T., R. and Pace, A., 2006: Middle Keweenawan Rocks of the Mamainse Point Area - Field Trip Guidebook Institute on Lake Superior Geology, 52nd Annual Meeting, Volume 52, Part 5, 28p. 22 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 A PALEOPROTEROZOIC MANTLE PLUME BENEATH THE LAKE SUPERIOR REGION HALLS, H.C.†, University of Toronto at Mississauga, Mississauga, Ontario L5L IC6, hhalls@utm.utoronto.ca, STOTT , G.M., Ontario Geological Survey, Sudbury, Ontario, ERNST, R.E ., Ernst Geosciences, 43 Margrave Avenue, Ottawa, and DAVIS, D.W., Department of Geology, University of Toronto, Toronto, Ontario. New paleomagnetic and radiometric U-Pb age data on baddeleyite1 show that the 2101 to 2126 Ma Marathon dyke swarm radiates from a region, approximately in east-central Wisconsin (after closure of the 1.0 Ga Mid-Continent Rift), suggesting that these dykes are associated with a possible plume centre that lies off the southern margin of the Superior Province rather than on the northern side within the Hudson bay embayment. The period of magmatic activity includes a reversal of the magnetic field from moderately steep negatively inclined remanences (R polarity) with southeasterly declination to approximately antipodal ones (N polarity)1,2. The Marathon swarm was originally defined on the basis of a set of north-trending dykes in the general area of Marathon, but the new age data show that NE-trending dykes east of Wawa also belong to this swarm. These NE-trending dykes occur within a swarm of NE to ENE-trending “Kapuskasing” dykes that give similar paleomagnetic directions to the Marathon dykes, but with steeper positive and negative inclinations. Outside the Kapuskasing Zone (KZ) both polarities are observed for which positive baked contact tests exist1 . Inside the KZ, only Kapuskasing dykes of R polarity occur. Since feldspar clouding and negative baked contact tests are associated with Kapuskasing dykes lying in the high grade eastern part of the Chapleau Block, thereby demonstrating a secondary, magnetization3, it is possible that all Kapuskasing dykes within the southern KZ have been remagnetized, either during the uplift or as a consequence of slow cooling at depth.. By analogy with the 2.45 Ga Matachewan dykes which show the same phenomenon4, the R magnetization in Kapuskasing dykes is younger than N, which is the same age relation deduced on the basis of U-Pb ages for the Marathon swarm1. The remanence inclination of Kapuskasing dykes, whether R or N, is steeper than the average value for Marathon dykes. A few dykes with comparably steep inclinations are present in the Marathon swarm as originally defined, and one of these dykes (of N polarity) gives a U-Pb age of 2125.7 ±1.2 Ma1. Another, with a relatively shallow N inclination compared to the mean, gives an age of 2121+14/-7 Ma2, so steep inclinations may be a reflection of secular variation rather than apparent polar wander. Geochemically, Kapuskasing dykes cannot be distinguished from Marathon ones, so we provisionally place the Kapuskasing dykes with the Marathon swarm, thus defining a radiating swarm with a fan angle of about 70°, with N to ENE trends. The Fort Frances dyke swarm has R polarity5 and an age of 2076 Ma2 and trends NW, and together with the recently dated 2067± 1 Ma R polarity Franklin dyke6,7 that trends WNW in the Minnesota River valley, forms a broadly radiating swarm that converges to a focal region approximately in central-southern Wisconsin. Taken as a whole, the Marathon and Fort Frances dykes define a radiating swarm with a fan angle of about 140° and a plume centre approximately in Wisconsin. The plume had a life span of about 60 My (from 2126 to 2067 Ma), comparable to that of the older 2.45 Ga Matachewan plume to the east, which had a longevity of at least 50 million years. 23 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Geochemical data from dyke chilled margins show that the Fort Frances dykes have flatter REE patterns compared to Marathon dykes but that one or two dykes within the Marathon and Fort Frances swarms may belong to the other one, which would indicate a radial stress pattern. Alternatively, the noticeable dyke-free gap between the Marathon and Fort Frances swarms may arise if intrusion of the N-NE trending Marathon dykes changed the orientation of the maximum principal stress to favour NW-WNW intrusion of the later Fort Frances dykes. References 1 – Halls, H.C. et al., 2005. Ontario Geological Survey Open File Report 6171, 59 p; 2 - Buchan et al., 1996. Can. J. Earth Sci. 33: 1583-1795; 3 - Halls, H. C. et al., 1994. Can. J. Earth Sci. 31:1182-1196; 4 Halls, H.C. & Zhang, B., 2003. Tectonophysics 362: 123-136; 5 - Halls, H.C. 1986. Can... J. Earth Sci. 23:142-157; 6 – Schmitz, M.D. et al. 2006. GSA Bull. 118: 82-93; 7 - Cavanaugh, M.D. 1983. Unpublished Ph.D.Thesis, University of South Carolina, 79 p. 24 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 GEOCHEMISTRY OF THE ~2.7 GA BLAKE RIVER GROUP AND CONFEDERATION ASSEMBLAGES: IMPLICATIONS FOR SUPRASUBDUCTION ZONE VOLCANISM IN THE SUPERIOR PROVINCE HOLLINGS, PETE† Department of Geology, Lakehead University, Thunder Bay, ON, P7B 5E1 Canada, peter.hollings@lakeheadu.ca and WYMAN, DEREK School of Geosciences, University of Sydney, NSW, 2006 Australia The broadly coeval Blake River Group (BRG) of the southern Abitibi Belt and the Confederation Assemblage of the Birch-Uchi greenstone belt, have been interpreted as subduction-related volcanic assemblages generated in oceanic and continental margins respectively (Hollings and Kerrich, 2000; Péloquin et al., 1996). Both greenstone terranes contain a range of mafic rocks types (i.e., variably tholeiitic to calc alkaline) and host volcanogenic massive sulfide (VMS) deposits. The Blake River Group is host to numerous VMS deposits, ranging from the Horne Mine (55 Mt massive sulphide ore mined, total tonnage ~144 Mt), to the Quemont Mine along the southern margin of the Sequence (16 Mt), and relatively small ore bodies common in the Noranda Mine Sequence (e.g., 1-5 Mt; Gibson and Watkinson, 1990). The Confederation assemblage is host to the past-producing South Bay VMS mine which produced 1.6 million tons of ore with an average grade of 11% Zn, 2% Cu and 2.12 ounces Ag per ton (Atkinson et al., 1990). Differences in the proportions and types of rocks in the two areas suggest they represent end-members in a range of subduction settings present during the late Archean. The BRG was erupted over a short period between about 2703 - 2698 Ma and is one of the youngest pre-orogenic volcanic suites in the southern Abitibi belt. Plume-associated komatiites are inter-layered with arc-type rocks in the south-central part of the Abitibi Subprovince that contains the BRG. The ~2725-2745 Ma Confederation assemblage was not associated with plume volcanism and was situated at the margin of a proto-continent containing rocks that date back to ~3 Ga. Despite the important differences in their settings, the major element trends of tholeiitic rocks from the two areas resemble each other, and Phanerozoic arcs, more than tholeiites from continental rift settings that may be analogues for some Archean greenstone belts. Rhyolites in both areas are interpreted to be the fractionation products of mantle-derived melts. In addition to documenting variable crustal contamination, the trace elements systematics of the rhyolites provide evidence of zircon fractionation events that occurred without significant changes in major element compositions. These results are probably attributable to extraction of rhyolitic liquids from crystal mush zones that was accompanied by preferential entrainment of zircon crystals, leading to Zr fractionation. The BRG suite includes magmas generated in relict plume asthenosphere but the chemical trends also provide evidence of slab melt metasomatism (Wyman and Hollings, 2005). Primitive rocks in the Confederation assemblage define trace element trends that are analogous to typical modern arcs with no indication that melt-mobilized elements such as Zr and Nb have been introduced in significant amounts. Adakite-like rocks were formed as a result of local events such as arc rifting in the South Bay area, or as an indirect consequence of larger events such as global-scale mantle-plume episodes that 25 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 strongly influenced the southern Abitibi subprovince and the BRG (Wyman and Hollings, 2005). Niobium enriched basalts associated with crustally contaminated rhyolites in the southwest of the South Bay study area are most plausibly linked to rifting of the Uchi Subprovince proto-continent margin. The lack of evidence for HFSE metasomatism in the sources of tholeiitic and calk alkaline mafic rocks, indicates that metasomatism of the sub-arc mantle was dominated by hydrous fluids. Therefore, slab melting occurred not in response to a pervasive steep geotherm but to specific geodynamic events, which in this case were probably linked to the early phases of arc rifting along the continental margin. References Atkinson, B.T., Parker, J.R. and Storey, C.C., 1990, Red Lake Resident Geologist’s District-1990; In Report of Activities 1990, Resident Geologists, Ontario Geological Survey, Miscellaneous Paper 152, 31-66. Gibson, H.L. and Watkinson, D.H., 1990, Volcanogenic massive sulphide deposits of the Noranda shield volcano and cauldron, Quebec. In, Rive, M., Verpaelst, P., Gagnon, Y., Lulin, J.M., Riverin, G. and Simard, A.(eds.), The northwestern Quebec polymetallic belt, The Canadian Institute of Mining and Metallurgy, Special Volume 43, 119-132. Hollings, P. and Kerrich, R., 2000. An Archean arc basalt - Nb-enriched basalt - adakite association: The 2.7 Ga Confederation assemblage of the Birch-Uchi greenstone belt, Superior Province. Contributions to Mineralogy and Petrology 139, 208-226. Peloquin, S., Potvin, R., Paradis, S., Lafleche, M., Verpaelst, P., Gibson, H., 1990. The Blake River Group, Rouyn-Noranda area, Quebec; a stratigraphic synthesis. In: Rive, M., Verpaelst, P., Gagnon, Y., Lulin, J-M., Riverin, G., Simard, A. (Eds.). The northwestern Quebec polymetallic belt; a summary of 60 years of mining exploration. Special Volume - Canadian Institute of Mining and Metallurgy, vol.43, pp.107-118. Wyman, D.A. and Hollings, P., 2005. Late Archean convergent margin volcanism in the Superior Province: A comparison of the Blake River Group and Confederation Assemblage. In: Archean Geodynamics and Environments, AGU Geophysical Monograph Series, 164, 215-237. 26 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 CONTINENTAL GROWTH AND EVOLUTION OF THE NORTHERN INTERIOR OF THE CONTERMINOUS U.S. NICE (Northern Interior Continental Evolution) Working Group HOLM, D.K.† (corresponding author), Kent State University; ANDERSON, R., IGS; BOERBOOM, T.J., MGS; CANNON, W.F., USGS; CHANDLER, V., JIRSA, M. and MILLER, J., MGS; SCHNEIDER, D.A., Ohio University; SCHULZ, K., USGS; VAN SCHMUS, W.R., University of Kansas. www.geo.umn.edu/mgs/index_wNICE.html The Penokean orogeny, long considered the dominant Paleoproterozoic event in the Lake Superior region, has been extrapolated to much of the buried basement of WI, IA, NE, and MI. In contrast, geon 17 crust (Yavapai orogen) is dominant south of the Archean Wyoming craton requiring a 100 m.y. age difference of juvenile crust along strike of the Transcontinental Proterozoic Province. We reconcile this problem with a revised history of the growth and evolution of continental crust in the northern mid-continent based on integration of modern geochronology and regional aeromagnetic data. A new aeromagnetic compilation of the region documents a complex terrane of 3.5-1.0 Ga rocks. In MN, the Archean craton is subdivided by the Great Lakes tectonic zone, a late Neoarchean suture. Paleo-proterozoic rifting of the craton created an irregular continental margin consisting of the Becker embayment and the MRV promontory. Bordering the embayment and onlapping the craton are a Paleoproterozoic fold-thrust belt and foreland basin. Within the Becker embayment are calc-alkaline volcanic and granitoid arc rocks formed by Penokean subduction and suturing of the arc with the craton. A sharp post-Penokean aeromagnetic discontinuity, the Spirit Lake tectonic zone (SLtz), extends eastward from NW Iowa, where it is defined by an abrupt southeastward decrease in the magnetic anomaly, through NC Wisconsin, where it is expressed by a sharp truncation of linear patterns in the Archean gneisses of the Marshfield terrane. The SLtz marks the northern extent of juvenile Yavapai age crust and the southern extent of Archean and Penokean crust. The nature of the SLtz is enigmatic and as yet poorly imaged in the third dimension by geophysical data. However, the preservation of Penokean juvenile crust only in an embayment along the southern rifted margin of the Superior craton suggests the SLtz formed initially as a major strike-slip fault zone responsible for margin truncation. South of the SLtz, the structural grain is subparallel to the SLtz and to axial traces of Mazatzal-age folds (Baraboo syncline). Gneisses and mafic volcanic rocks, probably basement rocks from which 1750 Ma rhyolites formed, are inferred from gravity-magnetic highs to be at subcrop in several areas. The Yavapai terrane is marked by abundant geon 14 granites identifiable by a generally smooth aeromagnetic pattern and low gravitational attraction. Additional high-relief, highintensity circular magnetic anomalies within the Yavapai terrane of Iowa delineate related granites. In southern WI and NE Iowa the aeromagnetic pattern reveals an extensive area of folded basement and Baraboo Interval quartzites beneath thin Paleozoic cover. The YavapaiMazatzal terrane boundary must be southeast of these deformed rocks. This area is bordered to the SE by large irregular shaped magnetic highs making up the Green Island 27 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 plutonic belt (GIPB) along the north edge of the Eastern Granite-Rhyolite province. We interpret the GIPB as having intruded into dominantly Mazatzal age crust. Geochronologic and thermochronologic data corroborate the new tectonic province map and provide important time constraints on the evolution of southern Laurentia during the late Paleoproterozoic. Basement crystallization ages. U-Pb zircon crystallization ages for juvenile crust exposed in WI definitively bracket the Penokean orogeny between 1880 & 1830 Ma. In contrast, U-Pb zircon ages from basement drill hole samples in NE are dominantly 1800 Ma or younger and probably represent eastward extension of the Yavapai age basement of CO. However, very few samples were available from the basement of Iowa, southern MN, SE South Dakota, and southern WI, with the result that many late-20th century interpretations for the buried basement in this region consisted of south-westward extension of the Penokean terrane. For example, the Precambrian basement of NW Iowa was commonly shown as Penokean crust abutting Archean crust, similar to the situation in northern WI along the Niagara Fault Zone. New basement ages appear representative of a growing UPb database for the region south of the SLtz. Key points relevant to this summary are as follows: (a) single-crystal TIMS U-Pb zircon data yield Yavapai-interval crystallization ages (1740 & 1760 Ma); (b) ion-probe analysis of zircons confirm these data; (c) zircons separated from gneissic xenoliths from the Manson impact structure in Iowa confirm the existence of igneous activity ca. 1760 Ma. These data yield no Penokean interval ages. Thus, all presently available U-Pb data support our interpretation that Yavapai orogenic crust extends eastward from NE into Iowa and southern WI, and that Penokean crust may be entirely absent from Iowa and SE Minnesota. Metamorphic & igneous ages. Metamorphism along the southern margin of the Superior province has been historically attributed to Penokean orogenesis. Indeed, a narrow window of amphibolite-facies rocks north of the Niagara Fault zone does record 1.831.80 Ga monazite U-Pb metamorphic ages. Peak metamorphic conditions with attendant magmatism likely mark the culmination of arc accretion. However, the dominant metamorphic and igneous imprint on the Precambrian basement is a regional Yavapai age tectonothermal event dated at ca. 1.76 Ga. Yavapai convergence led to weakening of the mid-crust and generation of the classic gneiss domes now exposed in northern MI and WI. Geon 17 metamorphism extends eastward into the Lake Huron region, where geon 18 metamorphic or magmatic activity is largely absent. Cooling/resetting ages. The results of over 100 modern Ar/Ar mineral ages from basement rocks of the NC midcontinent have allowed detailed characterization of its Proterozoic thermal history. Penokean biotite cooling ages are preserved only in lowgrade arc rocks in EC Minnesota; a few hornblende ages also record Penokean cooling in the metasedimentary rocks of the orogen. Elsewhere throughout the Superior and Huron region, hornblende and mica Ar/Ar ages are predominantly 1.76-1.75 Ga or somewhat younger, reflecting rapid, widespread cooling and orogenic collapse following the aforementioned Yavapai amphibolite-facies metamorphism and magmatism. Geon 17 rapid cooling of Archean and Paleoproterozoic metamorphic rocks of the gneiss dome corridor was caused by their exhumation and regionally was followed by a period of 28 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 tectonic quiescence, crustal stabilization, and deposition of the supermature Baraboo Interval (1730-1630 Ma) quartzites. Across much of the WI bedrock, low-temperature reheating was responsible for resetting mica cooling ages caused by geon 16 Mazatzal collision and foreland deformation. Interestingly, the northern limit of Mazatzal deformation and reheating is approximately located along the Niagara Fault zone in northern WI and upper MI. However, in MN the deformational front must bend south of the MRV promontory as those rocks are not thermally/isotopically reset and are overlain by flat-lying Baraboo Interval quartzites (Sioux quartzite). In the Huron region, Mazatzal heating is recorded only locally along the north shore of Lake Huron. Intrusion of the Wolf River batholith and associated geon 14 A-type plutons across the continental margin had a limited thermal effect on the country rock, in part reflecting their rapid emplacement at shallow levels. However, hydrothermal alteration along the Paleoproterozoic basement/cover contact occurred at considerable distances from the batholith. In summary, the continental interior straddles several terrane boundaries, including the transition from Archean tectosphere to Paleoproterozoic lithosphere. The SLtz is a fundamental Yavapai-age Proterozoic boundary, equivalent to, and possibly a direct extension of the Cheyenne belt suture zone, which also juxtaposes Yavapai orogen crust on the south against the Archean craton, and transects geon 18 (Trans-Hudson) structures in southern South Dakota. The Cheyenne-Spirit Lake structure is a fundamental feature in the evolution of the southern margin of Laurentia, the North American craton. Our new interpretation of the Paleoproterozoic continental growth and evolution of the northern interior of the North American craton suggests greater correspondence to that of the Rocky Mountains than previously thought. Although that region has structurally and magmatically modified during Cenozoic and older tectonism, relatively little tectonism has occurred in the cratonic interior in the last one billion years, providing us a uniquely unaltered perspective into Precambrian evolution of the North American continental lithosphere. 29 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 FIELD DISTRIBUTION, PETROGRAPHY, AND LITHOGEOCHEMISTRY OF EPIDOSITES IN THE VICINITIES OF FIVEMILE, NEEDLEBOY AND SIXMILE LAKES, VERMILION DISTRICT, NE MINNESOTA HUDAK, G. J., HOCKER-FINAMORE, S. M., Department of Geology, University of Wisconsin Oshkosh, Oshkosh, WI 54901, hudak@uwosh.edu HEINE, J., Natural Resources Research Institute, University of Minnesota – Duluth, Duluth, MN 55811 The Lower Member of the Ely Greenstone Formation (LMEG) contains a well-studied, more or less east-west striking, steeply north-dipping and north-facing sequence of Neoarchean submarine volcanic, volcaniclastic, chemical sedimentary, and intrusive strata in the vicinity of Fivemile Lake, Needleboy Lake, and Sixmile Lake in the Vermilion District of northeastern Minnesota. Primary volcanological features of these rocks are generally well-preserved despite syn- to post-volcanic hydrothermal alteration (quartz, epidote, chlorite, sericite, actinolite, albite, iron carbonate, dolomite and calcite) and subsequent greenschist facies metamorphism. Based on field characteristics, the LMEG has been subdivided into an older Fivemile Lake Sequence (FLS) and a younger Central Basalt Sequence (CBS; Peterson and Patelke, 2003). Volcanogenic massive sulfide prospects have been identified in near Fivemile Lake, Skeleton Lake, Needleboy Lake and Eagles Nest Lake #4 (Giagrande, 1981; Peterson and Jirsa, 1999; Hudak and Morton, 1999; Peterson, 2001; Hovis, 2001; Hudak et al., 2002; Hudak et al., 2003). Epidosites (granular to granoblastic, high varience mineral assemblages comprising epidote + quartz ± chlorite ± actinolite) have been identified in several locations within the LMEG. South of Fivemile Lake, epidosites occurs as discrete 0.1-2m diameter round- to lens-shaped pale yellow green masses within a 400m by 500m discordant alteration zone located in an actinolite-epidote-quartz altered diabase dike – sill complex that intrudes the FLS. Petrographic and electron microprobe studies (Hocker et al., 2003) indicate the presence of both pistacite and zoisite within this alteration zone. Epidosites also occur within a 300m by 200m, northeast-trending disconformable alteration zone approximately 500-700m east- southeast of Sixmile Lake. At this location, CBS pillow lavas and lobes are intensely altered to a mineral assemblage composed of pistacite, zoisite, quartz, actinolite, Fe-chlorite, Mg-chlorite, magnetite, chalco-pyrite (locally altered to malachite) and minor sphalerite within approximately 200m of a synvolcanic gabbro sill-dike complex. Isocon analysis (Grant, 1986) has been used to evaluate metasomatism during the genesis of the epidosite alteration zones. Least-altered compositions were selected based on boxplot analysis (Large et al., 2001) and petrographic observations. The LMEG epidosites have specific gravities 5-10% greater than least-altered samples, consistent with observations from epidosites in the Josephine Ophiolite (Harper, 1999). Variation diagrams indicate that Zr and Hf are the least mobile elements during metasomatism in both the diabase intrusion and the CBS lava flows; the best-fit line for these two elements on an isocon diagram defines the isocon. Relative to least-altered diabase, epidosite masses are enriched in Ca, Al, Si, and Sr, and depleted in Fe, K, Na, Mn, Mg, Cu, Zn and Eu. These geochemical variations are consistent with alteration in a high temperature (>350°C), high water:rock ratio reaction zone deep within a synvolcanic submarine 30 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 hydrothermal system (e.g. Harper, 1999) capable of producing volcanogenic massive sulfide mineralization up-section. Variations in chemical behavior within the CBS pillowed flows with increasing distance from the gabbro sill-dike complex suggest that epidosite zones at this locality formed within localized high temperature hydrothermal zones driven by heat derived from the gabbro. Further work is needed to evaluate the extent of epidosites in the CBS, and potential VMS mineralization up-section in the Upper Member of the Ely Greenstone Formation. References Giagrande, P., 1981, Geology and sulphide mineralization of the Skeleton Lake Prospect: unpublished M. S. thesis, University of Minnesota-Duluth, 118 p. Grant, J. A., 1986, The isocon diagram – a simple solution to Gresen’s equation for metasomatic alteration: Economic Geology, v. 81, p. 1976-1982. Harper, G. D., 1999, Structural styles of hydrothermal discharge in ophiolite / sea-floor systems: Reviews in Economic Geology, v. 8, p. 53-73. Hocker, S. M., Hudak, G. J., and Heine, J., 2003, Electron microprobe analysis of alteration mineralogy at the Archean Fivemile Lake volcanic-associated massive sulfide mineral prospect in the Vermilion District of northeastern Minnesota: Natural Resources Research Institute Report of Investigations NRRI/RI-2003/17, 49 p. Hovis, S. T., 2001, Physical volcanology and hydrothermal alteration of the Archean volcanic rocks at the Eagles Nest volcanogenic massive sulfide prospect, northern Minnesota: unpublished M. S. thesis, University of Minnesota – Duluth, Duluth, Minnesota, 137 p. Hudak, G. J., Heine, J., Newkirk, T., Odette, J., and Hauck, S., 2002, Comparative geology, stratigraphy, and lithogeochemistry of the Five Mile Lake, Quartz Hill, and Skeleton Lake VMS occurrences, Vermilion District, NE Minnesota: A report to the Minerals Coordinating Committee, DNR, Minerals Division, State of Minnesota: Natural Resources Research Institute Technical Report NRRI/TR2002/03, 390 pages. Hudak, G. J., Heine, J., Hocker, S. M., and Hauck, S., 2003, Needleboy Lake – Sixmile Lake Geological Mapping Progress Report: June 2003: Natural Resources Research Institute Report of Investigations NRRI/RI-2003/18, 22 p. Hudak, G. J., and Morton, R. L., 1999, Mineral Potential Study, Minnesota Department of Natural Resources Project 326, Bedrock and Glacial Drift Mapping for VMS and Lode Gold Alteration in the Vermilion–Big Fork Greenstone Belt, Part A: Discussion of Lithology, Alteration, and Geochemistry at the Fivemile Lake, Eagles Nest, and Quartz Hill Prospects: Minnesota DNR Division of Minerals Project 326 Report, 136 p. Large, R. R., Gemmell, J. B., Paulick, H., and Huston, D. L., 2001, The alteration box plot: a simple approach to understanding the relationships between alteration mineralogy and lithogeochemistry associated with volcanic-hosted massive sulfide deposits: Economic Geology, v. 96, p. 957-971. Peterson, D. M., 2001, Development of Archean lode-gold and massive sulfide deposit exploration models using geographic information system applications: targeting mineral exploration in northeastern Minnesota from analysis of analogy Canadian Mining Camps: unpublished Ph. D. dissertation, University of Minnesota, Duluth, MN, 503. Peterson, D. M., and Jirsa, M. A., 1999, Bedrock geological map and mineral exploration data, western Vermilion District, St. Louis and Lake Counties, northeastern Minnesota: Minnesota Geological Survey Miscellaneous Map Series M-98, scale 1:48,000. 31 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 STRUCTURE OF THE BIWABIK IRON FORMATION, MESABI IRON RANGE, MINNESOTA JIRSA, MARK A.†, and CHANDLER, VAL W., Minnesota Geological Survey (jirsa001@umn.edu; chand004@umn.edu) Six years of mapping by the Minnesota Geological Survey along the Mesabi Iron Range generated a variety of new maps, and considerable data regarding the structure of the Paleoproterozoic Biwabik Iron Formation and adjacent units (Fig. 1). Bedrock mapping utilized GIS to integrate data sources that included archived geologic and structure contour maps created by industry and government organizations, test pit and drill hole records, digital bedrock topography, and several iterations of aeromagnetic data. The aeromagnetic data delineate oxidation zones along faults, folds, and fractures in otherwise strongly magnetic iron-formation. These data and field work in more than 400 mines created a mass of structural observations that provide a context for understanding the deformation history of the range. Figure 1. Generalized geologic map of the Mesabi Iron Range showing structures in and along the subcrop of Paleoproterozoic Biwabik Iron Formation (gray). Bedrock north of the Biwabik Iron Formation is largely Archean in age; south of the Biwabik Iron Formation is Paleoproterozoic Virginia Formation bedrock; and to the southeast is the Mesoproterozoic Duluth Complex. The Biwabik Iron Formation is part of the Paleoproterozoic Animikie Group—a sequence of quartzose sandstone, iron-formation, and mudstone—that was deposited unconformably on a relatively stable shelf composed of Archean granite and greenstone. Depositional ages of the Animikie Group vary from 1,878 to 1,777 Ma (Fralick and others, 2002; Addison and others, 2005; Heaman and Easton, 2005). This broad temporal span indicates a protracted history of deposition, and probably also deformation. Much of the Biwabik Iron Formation forms a south-dipping homocline that contains little evidence of disruption, with the exception of locally well developed deformation structures. Although the precise ages of various structural elements on the Mesabi range are nearly impossible to ascertain, a relative chronology has been established from crosscutting relationships. Assigning deformation events to specific structures is probably premature; however, "D0, D1, D2…" nomenclature is applied here to refer to suites of 32 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 apparently related structures. The oldest are those presumably related to soft-sediment deformation (D0), including slumps, sedimentary breccias, and structures that appear to be the result of differential compaction. The earliest "regional" deformation (D1) is manifest in localized, small-scale rotational structures, bedding-parallel slickensides, and larger nappe and sheath folds. The structures commonly lie along boundaries between units having strong rheologic contrast, such as the contacts between thick sequences dominated by mudstone and siliceous grainstone. Nearly all of these structures display asymmetry that indicates south-over-north tectonism. This northward vergence, and the apparent timing relative to later structures, is consistent with compressional deformation—potentially related to the Penokean orogen. One of the long-standing controversies in iron-ore genesis is the question of whether the oxidation and leaching of iron-formation that formed the high-grade hematite ores occurred by supergene or hypogene processes. Although not conclusive, the observation of several early-formed, south-dipping thrust faults with folded, mineralized wall rocks, and bedding-parallel slickensides that host abundant secondary iron and silica implies that at least some of the mineralization was coincident with compressional deformation, perhaps during Penokean orogenesis. This is consistent with the hypogene model proposed by Morey (1999) that attributes oxidation and leaching to ground-water flow driven northward from uplift in the Penokean fold and thrust belt. A second regional suite of structures (D2) is largely extensional. These are monoclines and normal faults that are mutually transgressive; that is, faults that have sympathetically folded wall rocks, and folds that pass laterally to faults. These are some of the major structures along which oxidation and leaching has occurred, and the focus of most hematite ore mining. Veins, vugs, and other secondary mineralization features are abundant. D2 structures likely formed as localized responses to regional tilting. The most recent deformation effects (D3) are trough-like collapse structures, presumably related to post-leaching subsidence. The collapse, and associated oxidation and weathering, are best developed in the uppermost subcrop of iron-formation, implying supergene alteration played a significant role. Thus, the answer to the supergene vs. hypogene debate appears to be that both processes were significant, perhaps at different times. The Virginia horn—where the subcrop extent of iron-formation makes a hookshaped bend—is a complex horst, bounded by faults in the subjacent Archean rocks along which vertical movements have occurred during the entire temporal spectrum from early deposition to latest crustal accommodation. Lacking finite ages, the structures may record Penokean (Geon 18), Yavapai (Geon 17), Mazatzal (Geon 16), and/or Keweenawan (Geon 11) deformation events. The presence of diabase dikes cutting ironformation as far west as Keewatin raises the possibility that at least some of the later phases of deformation on the Mesabi range are related to development of the 1,100 Ma Midcontinent Rift. Funding for Minnesota Geological Survey mapping on the Mesabi range was provided by the Minnesota Legislature on recommendation of the Minerals Coordinating Committee, the Environmental Trust Fund administered by the Legislative Commission on Minnesota Resources, the EDMAP program of the U.S. Geological Survey, and the State Special appropriation to the Minnesota Geological Survey. Products can be digitally downloaded or 33 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 ordered on paper through the Minnesota Geological Survey website (www.geo.umn.edu/mgs). Available products include maps of bedrock topography and depth to bedrock (Miscellaneous maps M-126, M-158), historic 1899 land-surface topography, hydrology, and infrastructure (M-118, M-157), bedrock geology (M-163), and Quaternary geology (M-164). REFERENCES Addison, W.D., Brumpton, G.R., Vallini, D.A., McNaughton, N.J., Davis, D.W., Kissin, S.A., Fralick, P.W., and Hammond, A.L., 2005, Discovery of distal ejecta from the 1850 Ma Sudbury impact event: Geology, v. 33, p. 193-196. Fralick, P.W., Davis, D.W., and Kissin, S.A., 2002, The age of the Gunflint Formation, Ontario, Canada: Single zircon U-Pb age determinations from reworded volcanic ash: Canadian Journal of Earth Sciences, v. 39, p. 1085-1091. Heaman, L.M., and Easton, R.M., 2005, Proterozoic history of the Lake Nipigon area, Ontario: Constraints from U-Pb zircon and baddeleyite dating, in Easton, M., and Hollings, P., eds., Institute on Lake Superior Geology Proceedings, 51st Annual Meeting, Nipigon, Ontario, Program and Abstracts, v. 51, pt. 1, p. 24-25. Morey, G.B., 1999, High-grade iron ore deposits of the Mesabi range, Minnesota—product of a continental-scale Proterozoic ground-water flow system: Economic Geology, v. 94, p. 133-142. 34 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 PETROGENESIS OF A GRANITE XENOLITH IN THE 1.1 GA MIDCONTINENT RIFT AT SILVER BAY, MN JUDA, NATALIE, WIRTH, KARL, CRADDOCK, JOHN, Geology Department, Macalester College, St. Paul, MN, 55105; VERVOORT, JEFF, Dept. Geological Sciences, Washington State University, Pullman, WA 99164; ANDRING, MATT, Whitman College, Walla Walla, WA This study examined a well-known granitic xenolith locality within the hypabyssal Beaver Bay Complex of the Midcontinent Rift System (MRS). The xenolith is exposed along the shore of Lake Superior at Silver Bay, MN. Our goal was to constrain the origin of the granite using U-Pb zircon geochronology, whole-rock and trace element geochemistry, and anisotropy of magnetic susceptibility. Previous researchers have interpreted the origin of granite xenoliths contained within MRS rocks as either Archean crustal fragments or MRS felsic plutons. The granite xenolith (~ 50 meters in diameter) occurs within Beaver River diabase, and is crosscut by a mafic dike. The rock consists primarily of quartz, albite, and orthoclase. Granophyric intergrowths of quartz and feldspar are common. In addition, accessory minerals including sphene, apatite, and zircon are present. At the macroscopic level, the xenolith exhibits no indications of magmatic flow or foliation, and our study of anisotropy of magnetic susceptibility (AMS) as a proxy for magmatic flow confirms this (Figure 1). U-Pb analyses of zircons from the xenolith yield an age of 1094 ± 11 Ma on a concordia diagram. This is within error of the age of the youngest dated MRS granophyres. The geochemistry of the granite is similar to other MRS granophyres (e.g., Eagle Mountain, Finland Granite), except that the granite xenolith has higher concentrations of silica and sodium and very low potassium and other alkali elements (e.g., Rb, Ba; Figure 2). The apparent alkali mobility may have resulted from fluid infiltration during late-stage cooling. The compositions of several granitic dikes at Beaver Bay are similar to the Silver Bay xenolith. The Silver Bay xenolith and Beaver Bay dikes share “within plate” and A-type granite major and trace element compositions with MRS granophyres. Geochemical data from granite xenoliths at Split Rock are significantly different from those at Silver Bay and from other MRS granophyres by having volcanic arc characteristics. This suggests that the Split Rock xenoliths might have a different origin from those at Silver Bay. Figure 1. Equal area lower hemisphere projections of AMS results for samples granite xenolith (KP05-45B) and a cross-cutting mafic dike (KP05-45I). 35 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 The Keweenawan age and A-type granite characteristics of the granite xenolith at Silver Bay suggest a greater distribution of MRS granophyres than previously thought. These granophyre bodies may also underlie volcanic flows in the more central portions of the rift. Further isotopic analysis of the granite xenoliths, such as with the Sm-Nd system, would help constrain the petrogenesis of the MRS granophyres. REFERENCES Kennedy, B.C., Wirth, K., Vervoort, J.D. 2000. Petrogenesis of the Midcontinent Rift Granophyric Complexes of Northern Minnesota: Proceedings and Abstracts Institute on Lake Superior Geology, vol. 46, p. 29-30. Miller, J., Chandler, V. W. 1997. Geology, petrology, and tectonic significance of the Beaver Bay Complex, northeastern Minnesota: Geological Society of America Special Paper 312, p. 73-96. Vervoort, J.D., Wirth, K. 2004. Origin of the Rhyolites and Granophyres of the Midcontinent Rift, northeastern Minnesota: Proceedings and Abstracts Institute on Lake Superior Geology, vol.50, p. 158-159. Figure 2. 36 Harker diagrams showing the concentrations of Na2O, K2O, and Al2O3 in the granite xenolith compared to data from other MRS granophyres (Wirth and Vervoort, in prep.). Symbols: filled inverted triangles = Silver Bay granite xenolith; open inverted triangles = Split Rock granite xenoliths; other symbols are MRS granophyre bodies. �Proceedings of the 52nd ILSG Annual Meeting – Part 1 SULPHIDE SATURATION MECHANISMS IN GABBROIC INTRUSIONS IN THE NIPIGON EMBAYMENT KISSIN, S.A., Department of Geology, Lakehead University, Thunder Bay, ON, P7B 5E1 sakissin@lakeheadu.ca, HEGGIE, G.J., East West Resource Corporation, 1158 Russell Street, Thunder Bay, ON, P7B 5N2, FRANKLIN, J.M., Franklin Geosciences, 24 Comanche Drive, Nepean, ON, K2E 6E9, KARIMZADEH SOMARIN, A., Department of Geology, Faculty of Natural Sciences, University of Tabriz, Tabriz, Iran Interest in the Nipigon Embayment as favourable exploration target for platinum-group element (PGE) deposits was stimulated by the suggestion by Naldrett (1992) that the area is a likely geological setting for the development of nickel-copper-PGE deposits based on criteria established in studies of the Noril’sk deposit in Siberia. Proterozoic gabbroic intrusions in the Nipigon Embayment of northwestern Ontario were studied with the aim of discerning the mechanism of sulphide saturation leading to (PGE) concentrations recently discovered. Two intrusions, the Seagull intrusion (1116.2±9.2 Ma), south of Lake Nipigon, and the Kitto intrusion (1117±1.8 Ma), on the eastern shore of Lake Nipigon, were the subject of the study, as they contain potentially economically significant PGE concentrations. Most of the study was carried out on the Seagull intrusion, as only limited samples were available from the Kitto intrusion. Neither intrusion is well exposed, and most samples were taken from drill-core. Profiles of sulfur, copper, nickel, gold, palladium and platinum as a function of depth in drillholes reveal that sulphur saturation occurred at the base of the Seagull intrusion, where a zone of sulfide mineralisation is developed. However, sulphur saturation was noted at higher levels in the intrusion, notably in the high-grade RGB zone. These observations suggest the operation of different processes in formations of the mineral occurrences – a Noril’sk-type process involving assimilation of sulphur for the basal zone and a reef-type process for the higher zones (Naldrett 1993). Olivine compositions were determined in both intrusions, and in both cases, the compositions indicate that the parental magmas were undersaturated with respect to sulphur. Thus, according to theories of PGE deposit formation, both intrusions have potential for PGE concentration. Contamination of the parental magma, either through assimilation of country rock or magma mixing, has been ascribed a crucial role in the formation of an immiscible melt (Irvine 1975; Naldrett 1989). Neodymium (Nd)-samarium(Sm) isotopes provide a means of testing for contamination. Heggie (2005) reported data on Nd/Sm isotopic studies on Seagull intrusion samples for a range of depths in several drill-holes, as well results from underlying Quetico Subprovince metasedimentary rocks and Sibley Group sedimentary rocks. The calculation of εNd for theses samples yielded values of -0.2 t0 -4.0 (±0.5) for the Seagull intrusion, but -16 to -23 for the Quetico metasediments and a mean of -5 for Sibley Group sediments. Rb/Sr isotopic studies on the same samples were used in a comparison of 143Nd/144Nd vs. 87 Sr/86Sr. Sibley Group sediments differ markedly from the Seagull intrusion in both factors, whereas the Quetico metasediments have lower 143Nd/144Nd ratios and similar 87Sr/86Sr ratios. As the 87Sr/86Sr ratios for the Seagull samples trend to somewhat higher values than those found in the Quetico metasediments, some assimilation of the high 87Sr/86Sr from Sibley material must have occurred. However, since 143Nd/144Nd in the Seagull samples decreases with depth, trending toward lower Quetico metasediment values, assimilation of Quetico material is also likely. 37 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 The source of sulfur in the Seagull intrusion was investigated through the study of sulphur isotopes and selenium/sulfur ratios. Sulphur isotopic compositions of samples from the base of the Seagull intrusion were compared with those from Quetico metapelites and evaporites from the Sibley Group. Sulphides from the Seagull intrusion had ∂34S ranging from –2.3 per mil (‰) to +2.6‰ with a mean value of –0.9‰. Sulphides in Quetico Subprovince metapelites has ∂34S ranging from –2.3‰ to +1.1‰ with a mean value –0.8‰. Finally, Sibley Group evaporites had ∂34S ranging from +7.7‰ to +9.0‰. According to Franklin and Mitchell (1977), these correspond to a mean H2S composition of –4.2‰, based on prior knowledge of temperatures of formation of the sulphate minerals (barite, anhydrite and gypsum) in the Sibley Group. These data provide plausible evidence for incorporation of Quetico sulphide in the sulphide zone at the base of the Seagull intrusion. The source of sulphur in higher zones remains to be determined. Comparison of ∂34S with Se/S x 106 in Seagull sulphides and Quetico metapelites revealed that the Quetico samples lie well outside the region for mantle sulphur, whereas the Seagull samples show considerable scatter in Se/S x 106. Together with their negative ∂34S values, it is evident that assimilation of Quetico sulphide is the explanation for these data. Although the Nipigon Embayment has a number of features that seem to provide for a Noril’sk model setting, continental rifting, evaporites in the section, voluminous basaltic eruption among others, there are problems in its application to the two cases studied here. The Seagull and Kitto intrusions are among the earliest igneous events associated with the Mid-Continent Rift (Davis and Green 1997). Although some contribution from Quetico metapelite sulphide seems likely, the Sibley Group evaporites do not seem to be likely sulphur sources. Rather a reef-type of process seems to be responsible for zones of PGE enrichment at higher levels in both intrusions. References Davis, D.W. and Green, J.C. 1997. Geochronology of the North American Midcontinent Rift in western Lake Superior and implications for its geodynamic evolution. Canadian Journal of Earth Sciences, 34: 476-488. Franklin, J.M., and Mitchell, R.M. 1977. Lead-zinc-barite veins of the Dorion Area, Thunder Bay District, Ontario; Canadian Journal of Earth Sciences, 14: 1963-1979. Heggie, G.J. 2005. Whole rock geochemistry, mineral chemistry, petrology and Pt, Pd mineralization of the Seagull Intrusion, northwestern Ontario; unpublished MSc thesis, Lakehead University, Thunder Bay, Ontario, 156p. Irvine, T.N. 1975. Crystallization sequences of the Muskox intrusion and other layered intrusions-II. Origin of chromitite layers and similar deposits of magmatic ores; Geochimica et Cosmochimica Acta, 39: 991-1020. Naldrett, A.J. 1989. Magmatic sulfide deposits; Oxford University Press, New York, 186p. Naldrett, A.J. 1992. A model for the Ni-Cu-PGE ores of the Noril’sk region and its application to other areas of flood basalt; Economic Geology, 87: 1945-1962. Naldrett, A.J. 1993. Models for the formation of strata-bound concentrations of platinum-group elements in layered intrusions; in Mineral deposit modeling; Geological Association of Canada, Special Paper 40: 373-387. 38 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 MetalCORP LTD. BIG LAKE Cu-Zn-Ag-Au-Co, Ni-Cu-PGE, AND Mo PROPERTY MACTAVISH, ALLAN, MetalCORP Ltd., 309 South Court Street, Thunder Bay, ON, P7B 2Y1, Canada The Big Lake Property of MetalCORP Ltd. of Thunder Bay, Ontario comprises 33 claims (365 units totalling 5840 hectares) and is located approximately 230 km east-northeast of the city of Thunder Bay and 18 km southeast of the town of Marathon in Northern Ontario, Canada. Work completed by MetalCORP since early 2004 includes a MegaTEM airborne time-domain EM and magnetometer survey (2004), a detailed helicopter-borne ATEM III time-domain EM and magnetometer survey, detailed and reconnaissance prospecting (918 samples), linecutting, surface and down-hole pulse-EM surveys, geological mapping, and 3 phases of diamond drilling, totaling 31 diamond holes (8300 m). This work resulted in the discovery of 4 previously unknown mineralized zones that represent 3 separate and distinct mineralization styles. The mineralized zones include: the J4 and J5 Pt-Pd reefs within the Big Lake Ultramafic Complex; the A2 Ni-Cu Zone within the Gus Creek Mafic Intrusion; and the BL14 Cu-Zn-Ag-Au-Co Zone within strongly altered ultramafic metavolcanic flows and associated metasedimentary rocks. The A2 and BL14 zones are not exposed on surface and are buried beneath 10 to 75 m of glacial drift. The property is also host to the historic Playter Mo-Cu-Pb-Ag Prospect which has yet to be fully evaluated by MetalCORP. The Big Lake Property is located near the southern margins of the eastern portion of the Archeanage Schreiber-Hemlo greenstone belt of the eastern Wawa Subprovince of the Canadian Shield. The greenstone belt is split into distinct eastern and western segments by the 1108 Ma Mesoproterozoic Coldwell Alkalic Complex. The eastern part of the belt is subdivided into the Hemlo-Black River assemblage (2.77 Ma) to the north and the Heron Bay (2.70 Ma) assemblage to the south, both of which are primarily affected by amphibolite-facies regional metamorphism. The western portions of both assemblages are lower in grade and exhibit upper greenschist facies regional metamorphism. The Big Lake Property occurs within the Heron Bay Assemblage which is intruded by the granitic to granodioritic Heron Bay Batholith, the recently recognized mafic Gus Creek Intrusion, the Bell’s Lake Ultramafic Intrusion, and the >30 km long Big Lake Ultramafic Complex. The BL14 Cu-Zn-Ag Zone is located stratigraphically below the eastern end of the sill-like Big Lake Ultramafic Complex, approximately 800 metres south of the A2 zone (see below). The south-facing, high temperature, mafic dominated Cu-rich VMS zone is overturned, dips at ~25o to the north, and consists of: 1. A stringer zone of intensely biotitized and strongly chloritized and talcose breccia containing up to 30% bands, veins, stringers and pods of chalcopyrite and pyrite with up to 5% disseminated to streaked sphalerite and minor galena; 2. A semi-massive to near-massive zone of chalcopyrite, pyrrhotite, sphalerite, and minor galena underlain by a strongly to intensely biotitic, chloritic, and talcose footwall alteration zone that locally contains anthophyllite and sillimanite. Trace element lithogeochemistry shows that the footwall metavolcanic rocks contain unusually high amounts of Ni, Cr, TiO2, and Pd that suggests that the host-rocks were originally komatiitic basalts or possibly ferro-picrites. The mineralized zone is capped by relatively unaltered clastic and chemical metasedimentary rocks. The thickest BL14 Zone intersection (DDH BL06-24) to date (April 6, 2006) contained 5.57% Cu, 103.9 grams/tonne (gpt) Ag, 6.74 gpt Au, 1.66% Zn, 39 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 689 ppm Co, and 0.15% Pb over 5.38 m (17.65 ft). This interval included 7.45% Cu, 137.8 gpt Ag, 9.18 gpt Au, 2.24% Zn, 891 ppm Co, and 0.21% Pb over 3.95 m (12.96 ft). Stringer zone mineralization intersected to date contained up to 2.56% Cu, 1.00% Zn, 46.0 gpt Ag, 1.60 gpt Au, and 0.10% Pb over 0.93 m (3.05 ft). The J4 and J5 Pt-Pd Reefs consist of narrow, apparently stratabound intervals hosted within thick peridotite units contained within the upper and central intrusive cycles of the eastern portion of the north-facing, well-differentiated, unlayered, sill-like Big Lake Ultramafic Complex. The ultramafic complex is presently undated; however, it is thought to be younger than most of the supracrustal rocks observed within this portion of the Schreiber-Hemlo Greenstone Belts. Diamond drilling shows that the central portions of the complex dip to the north at ~45o, whereas the eastern portions of the complex dip to the north at ~25o. Geological mapping suggests that the western portions of the complex exhibit a steeper northerly dip. The two host intrusive cycles and are very similar in appearance, progression of rock units, and apparent thickness. The observed mineralization consists of trace amounts of very finely disseminated pyrrhotite and chalcopyrite within serpentinized to locally talcose, fine-grained, pyroxene-oikocrystic peridotite. The J4 Reef varies between 0.58 and 2.11 m in thickness, occurs within the basal peridotite unit of the uppermost (J4) intrusive cycle of the Big Lake Complex and is usually directly adjacent to the contact with an overlying olivine-bearing pyroxenite unit. The J5 Reef is identical in appearance to the J4 Reef, varies between 0.75 and 3.00 m in thickness, and occurs within the basal peridotite of the central (J5) intrusive cycle of the complex near, but not adjacent to, the upper contact of the host unit with an overlying olivine-bearing pyroxenite. The J4 Reef has been traced for 2.20 km (1.37 mi) and contains up to 0.70 gpt Pt and 0.79 gpt Pd (1.49 gpt 2PGE)/1.67 m. The J5 Reef has been traced for a similar distance and contains up to 0.81 gpt Pt, 0.85 gpt Pd (1.86 gpt 2PGE)/0.75 m. It is interesting to note that both reefs were intersected while drill testing the BL14 Zone described above. The A2 Ni-Cu Zone occurs near the base of the discordant Gus Creek Mafic Intrusion (2669.3 ± 1.8 Ma., Jack Satterly Geochronology Laboratory, University of Toronto, 2005) and consists of disseminated, blebby, and stringered, locally semi-massive pyrrhotite, chalcopyrite, and pentlandite hosted within the 2 to 20 m thick, conduit-like, A2 host intrusion breccia sequence. The A2 intrusion breccia is a complex interval of variably mineralized (1 to 30% sulphides), varitextured, inclusion-rich, gabbroic to melagabbroic intrusive rocks overlain by unmineralized, medium- to coarse-grained gabbro and quartz leucogabbro and underlain by occasionally mineralized, pyroxene-phyric melagabbro and feldspathic pyroxenite. Inclusion/fragment types include a variety of gabbros, ultramafic intrusive rocks, and clastic and chemical metasediments. The strongest mineralization occurs near the base of the intrusion breccia, comprises the A2 NiCu Zone, and includes 1.66% Ni and 0.20% Cu/0.30 m, 1.00% Cu and 0.80% Ni/0.40 m, 1.40% Cu and 0.27% Ni/0.77 m, and 0.98% Cu and 0.29% Ni/1.40 m. The geometry of the A2 mineralized zone remains uncertain and may be more complex that initially thought, but within the area drilled appears to strike ~140o and dip southwest between 40 and 60o. It is presently thought that much of the observed sulphide mineralization within the A2 Zone consists of fragments from a high R-factor, massive, Ni-Cu sulphide body located somewhere at depth. 40 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 MINING AND EXPLORATION ACTIVITY IN NORTHWESTERN ONTARIO MAGEE, ANGELIQUE, Ontario Geological Survey, Ministry of Northern Development and Mines, Suite B002, 435 James St. South, Thunder Bay, ON P7E 6S7 CANADA Northwestern Ontario continued to see a significant increase in mining and mineral exploration in 2005. Six mines produced a total of 1.5 million ounces of gold in 2005, approximately 70% of Ontario’s total. Gold producers included: Campbell Mine (Placer Dome Inc.); David Bell Mine (Teck Cominco Limited and Barrick Gold Corporation); Golden Giant Mine (Newmont Canada Limited); Musselwhite Mine (Placer Dome Inc./Kinross Gold Corporation); Red Lake Mine (Goldcorp Inc.); and Williams Mine (Teck Cominco Limited and Barrick Gold Corporation). North American Palladium Ltd. produced 177 167 ounces of palladium and 18 833 ounces of platinum at its Lac des Iles Mine and development of the underground operation below its open pit mine continues. The Golden Giant Mine closed its mining operation in December of 2005 and will be decommissioning the mine site in the first half of 2006. There are approximately 400 active exploration projects in the northwest, the vast majority of which are focused on gold. Areas receiving the most interest from exploration companies were the Red Lake greenstone belt, Shoal Lake area, Dogpaw Lake area, Shebandowan greenstone belt, Fort Hope greenstone belt, Onaman-Tashota belt and the Pickle Lake greenstone belt. Elevated mineral commodity prices are contributing to levels of exploration activity in northwestern Ontario not seen since the mid-1980’s. Exploration in northwestern Ontario continues for the following mineral deposit types: diamonds, uranium, copper-nickel-platinum group elements, volcanic hosted massive sulphides, rare earth elements in pegmatites, iron-oxide-copper-gold, coppermolybdenum-gold porphyry, and lode gold. 41 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 GEOLOGY AND GEOCHEMISTRY OF THE CHIMNEY LAKE VOLCANICLASTIC BRECCIA NEAR ARMSTRONG, ONTARIO MAGEE, M. ANGELIQUE† 1, 2, HOLLINGS, PETE 2, FRALICK, PHILIP W. 2 Ontario Geological Survey, Resident Geologist Program, Suite B002, 435 James St. S., Thunder Bay, Ontario, Canada, P7E 6S7, 2 Lakehead University, 955 Oliver Road, Thunder Bay, Ontario, Canada, P7B 5E1 1 The Chimney Lake volcaniclastic breccia (CLVB) is part of a group of Mesoproterozoic rocks that unconformably overlie the Archean basement of the central Wabigoon Subprovince near the northwestern end of Lake Nipigon. Mapping in 2003 resulted in the discovery of a number of previously unmapped, Mesoproterozoic units, including the Badwater layered gabbro intrusion (Middleton 2005), the Pillar Lake volcanic assemblage (PLV), and an undeformed volcaniclastic breccia located on the north shore of Chimney Lake (MacDonald 2004). The Badwater layered gabbro (1599 Ma; Heaman and Easton 2006) intrusion has anorthositic to gabbroic layers. The PLV are a series of flat-lying, greenschist-facies, undeformed, massive and pillowed basalt flows. A robust age date of the PLV has not yet been obtained, but dates obtained by geochronological methods indicate that the PLV were erupted between 1514 Ma and 1120 Ma (Heaman and Easton 2006). The CLVB was originally mapped as a conglomerate within the Pass Lake Formation of the Sibley Group and has not been dated by geochronological methods (MacDonald 2004). Field relationships between the CLVB and surrounding lithologic units, such as the PLV, are not discernible due to poor outcrop control. CLVB contains fragments of gabbro, basalt, amygdaloidal basalt, and fragments tentatively described as granitoid and sedimentary rocks. Fragments range in size from 0.2 cm to 30 cm in diameter. Gabbroic fragments are angular with sharp edges, displaying no evidence of assimilation. Basalt fragments sometimes exhibit assimilation features and have angular to subrounded and ameboidal shapes. Amygdules within basalt fragments are infilled with chlorite and locally clay minerals. Alteration envelopes of chlorite, hematite and clay minerals surround the amygdules. The breccia matrix consists of very fine- to coarse-grained, angular to sub-rounded, hematite- and actinolite-altered volcanic and igneous fragments. Preliminary geochemical results indicate that the CLVB contains fragments of basalt that are similar in composition to the alkaline PLV basalt. The gabbroic clasts vary in composition but it appears that they are similar to the Badwater layered gabbroic intrusion. The Badwater layered gabbroic intrusion and the PLV are geochemically dissimilar. The similarity between the fragment composition in the CLVB and nearby lithologic units suggests a local source. The CLVB could be a reworked autoclastic breccia related to Pillar Lake volcanism, or alternately it may represent a diatreme breccia dike that was emplaced after Pillar Lake volcanism. Ongoing detailed mapping of the volcaniclastic breccia in conjunction with additional geochemical analyses will determine the source and cause of the volcanism that resulted in this breccia unit, as well as other local volcanic rocks. 42 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 REFERENCES Macdonald, C.A. 2004. Precambrian geology of the south Armstrong-Gull Bay area, Nipigon Embayment, northwestern Ontario; Ontario Geological Survey, Open File Report 6136, 42p. Middleton, R.S. 2005. Diamond Drilling on Red Granite Property, Pillar Lake Sheet, Armstrong, ON, 52I03NW, Resident Geologist Program Thunder Bay North Assessment Files, 55p. Heaman, L.M. and Easton, R.M. 2006. Preliminary U/Pb geochronology results: Lake Nipigon Region Geoscience Initiative; Ontario Geological Survey, Miscellaneous Release of Data 191, 86p. 43 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 THE PRECAMBRIAN RESEARCH CENTER—A NEW INITIATIVE TO PROMOTE PRECAMBRIAN FIELD STUDIES AT THE UNIVERSITY OF MINNESOTA DULUTH MILLER, JAMES D., JR.†, Minnesota Geological Survey (mille066@umn.edu) and PETERSON, D.M., Natural Resources Research Institute (dpeters1@nrri.umn.edu) As the minerals industry enters an anticipated period of protracted expansion, a major impediment to this growth is a scarcity of new geoscientists adequately trained in basic field methods. This is especially true in field studies of Precambrian terrains, which host much of the world's ore deposits. The Precambrian Research Center (PRC) is being created at the University of Minnesota Duluth (UMD) with the primary goal of satisfying this new demand for field geologists by providing training and support to upper-level undergraduate students, graduate students, and professional geologists in modern methods of geologic mapping in glaciated Precambrian terrains. A secondary goal of this center is to attract exceptional students, who have an interest in conducting field-oriented thesis research, to the University of Minnesota Duluth's graduate program. Ultimately, it is our hope that the PRC will sustain and enhance the reputation the geology department at UMD has developed over the past 50 years for producing well-trained field geologists. The PRC will have five programmatic components: 1. Summer geology field camp in northeastern Minnesota Beginning in 2007, the PRC will offer a summer field camp focused on the unique aspects of field mapping in Precambrian terrains of the southern Canadian Shield. The field camp will be a course accredited through the College of Science and Engineering at UMD. It will be open to undergraduate and graduate students from throughout the U.S., Canada, and abroad. It will be staffed by 4 to 6 professional field geologists contracted with the PRC. Field camp highlights • Introduction to basic field methods in glaciated Precambrian shield terrains. • Overview of the Precambrian geology of the southern Canadian Shield. • Week-long capstone field mapping projects with small field excursions supervised by professional field geologists; Boundary Waters Canoe Area Wilderness option. • GIS compilation of field data and digital geologic map-making. • Integrating structure, drill core, and geophysics into 3-D geologic interpretations. 2. Graduate assistantships and grants The PRC will offer several yearly research assistantships for students accepted into the UMD graduate program who wish to pursue field-based research projects focused on the Precambrian geology of the Lake Superior region. Small research grants will also be available to undergraduate and graduate students to assist in various aspects of fieldbased studies of Precambrian geology. Undergraduate or graduate students may apply for these grants, but preference will be given to students from UMD and those who have attended the PRC field camp. 3. Professional workshops/field experiences The PRC's goal of providing advanced training to professional geologists, as well as students, will be achieved by sponsoring a regular series of workshops and field 44 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 experiences (at least two per year) on various topics related to field mapping of Precambrian shield geology. In addition, customized geologic mapping experiences for groups of industry geologists and/or the geologic staff of individual companies can be arranged. The PRC will work with UMD to ensure that these programs meet the continuing professional development requirements of geologic licensing boards. Industry members of the PRC will receive registration waivers to these advanced training sessions depending on their level of support. 4. Advanced geology courses at UMD Three new field-based courses will be offered for upper level undergraduates and graduate students within the Department of Geological Sciences. These courses include Advanced Field Methods and Geological Maps; Geology in Three-Dimensions; and Geologic Problem Solving Using Digital Methods. 5. Outreach, field trip offerings, and career planning for students The PRC will offer outreach education to K-12 students and the general public on the geology and mineral resources of the Lake Superior region. It will also offer to lead field trips on the Precambrian geology of the area for UMD students and students visiting the area from other colleges and universities. Finally, the PRC will serve as a clearinghouse for students to find job opportunities in the public or private sector that require field mapping skills. PRC activities and finances will be overseen by the heads/directors of three principal organizing institutions within the University of Minnesota: the Natural Resources Research Institute (NRRI) at UMD, the Department of Geological Sciences at UMD, and the Minnesota Geological Survey (MGS) at the University of Minnesota Twin Cities campus. The NRRI will oversee the business activities of the PRC; the geology department will oversee the PRC's educational programs; and MGS will provide guidance on geologic mapping projects. The PRC will employ 3 to 4 permanent staff and will contract with many field-experienced academic and professional geologists from throughout the Lake Superior region for its various programs and activities. A Board of Advisors consisting of preeminent geologists from industry and academia will be established to provide advice and oversight of PRC activities and programs. Funding for the PRC will come from many sources. Base funding will be sought from the State of Minnesota to support administrative expenses, from the University of Minnesota in the form of tuition deferments, and from students and professional geologists by tuition and fees paid for summer field camp, academic courses, and workshops/field experiences. Funding for particular research projects will be sought from the U.S. Geological Survey through their EDMAP program and from the minerals industry for sponsorship of industry-generated thesis projects. The minerals industry, which stands to be a major beneficiary of the PRC by its professional workshops and by having access to well-trained students, will also be requested to serve as a key benefactor of the PRC. 45 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 GEOLOGY OF THE DULUTH COMPLEX IN THE FOUR BABBITT 7.5' QUADRANGLES, NORTHEAST MINNESOTA MILLER, JAMES D., JR., Minnesota Geological Survey (mille066@umn.edu) and SEVERSON, MARK J., Natural Resources Research Institute (mseverso@nrri.umn.edu) The Minnesota Geological Survey published 1:24,000-scale digital geologic maps of the Babbitt, Babbitt Northeast, Babbitt Southwest, and Babbitt Southeast 7.5' quadrangles in 2005 (Severson and Miller, 2005; Miller and others, 2005; Miller and Severson, 2005; and Miller, 2005, respectively). The area was originally reconnaissance mapped by Bonnichsen (1970a-d). The Precambrian rocks in the four Babbitt quadrangles are best known for hosting the easternmost Mesabi Iron Range taconite mines and some of the Cu-Ni-PGE deposits that occur along the northwestern margin of the Duluth Complex. The maps, which include three cross sections, will be on display as a poster presentation. Trending northeasterly through the map area, the Duluth Complex is in intrusive contact with Paleoproterozoic rocks of the Animikie Group and Archean granitic rocks of the Giants Range batholith. The Animikie Group units include the basal Pokegama Quartzite, the overlying Biwabik Iron Formation, and the Virginia Formation. The Peter Mitchell (Northshore Mining) and Dunka Pit (closed) taconite mines occur in the map area. The northwestern margin of the Duluth Complex is exposed in the Babbitt quadrangles. The Duluth Complex is the largest exposed intrusive component of the Mesoproterozoic (1.1 Ga) Midcontinent Rift. It was emplaced as multiple intrusions into the lower section of comagmatic volcanic rocks of the North Shore Volcanic Group, which is evident from field relationships in the map area. The oldest Mesoproterozoic rock units in the Babbitt quadrangles are several types of mafic hornfels inclusions, which represent thermally metamorphosed remnants of the North Shore Volcanic Group. The most common type is basaltic hornfels, which by the common occurrence of meta-amygdaloidal textures, are clearly thermally metamorphosed mafic volcanic lava flows. Magnetic, nonmagnetic, and plagioclase porphyritic varieties of basalt hornfels are recognized, with the magnetic types usually occupying a stratigraphically lower position. An interesting hornfels type is a well crossbedded mafic hornfels that is interpreted to be a strongly metamorphosed volcanogenic interflow sandstone unit (Patelke, 1996). An enigmatic hornfels type is a medium- to fine-grained, equigranular oxide olivine gabbro that displays a domainal distribution of granular mafic phases possibly representing granoblastic recrystallization of an originally ophitic texture. The thickness, homogeneity, and average medium grain size imply that the unit may be a metamorphosed subvolcanic sill or a large lava flow. The oldest intrusive rocks of the Duluth Complex in the map area are gabbroic to anorthositic rocks of the early anorthositic series. The anorthositic series consists of a structurally complex suite of plagioclase-rich gabbroic rocks that cover large expanses of the upper reaches of the Duluth Complex (Miller and others, 2002). Over most of the map area, anorthositic rock types, with plagioclase ranging from 75 to 95 percent, typically occur as meter- to decameter-sized inclusions in troctolitic rocks. However, in 46 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 the Babbitt Southwest quadrangle, several varieties of anorthositic-series rocks occur over large areas. In addition to standard plagioclase-rich anorthositic-series lithologies, typically with poikilitic to intergranular olivine, these large areas of anorthositic-series rocks also contain a distinct olivine oxide gabbro lithology that only locally is leucocratic. This rock type was referred to as the Powerline gabbro by Bonnichsen (1972), who suggested it was an upper differentiate of the Partridge River intrusion. Paces and Miller (1993) acquired a U-Pb age of 1,098.6 ± 0.5 Ma for this unit, also considering it part of the Partridge River intrusion. However, this mapping has clearly shown this gabbro to be in gradational contact with other anorthositic-series rocks and to be crosscut by troctolitic rocks of the Partridge River intrusion. The main Duluth Complex units in the Babbitt quadrangles are various types of troctolitic (Ol + Pl) cumulates of the layered series. The layered series is the youngest component of the Duluth Complex and is composed of a suite of discrete layered mafic intrusions that show variable degrees of internal differentiation (Miller and others, 2002). The Babbitt quadrangles include parts of four major layered series intrusions: the Partridge River intrusion, the South Kawishiwi intrusion, the Greenwood Lake intrusion, and the Bald Eagle intrusion. Only the Partridge River and the South Kawishiwi intrusions are sufficiently exposed to subdivide their igneous stratigraphies into map units; both intrusions are composed mostly of olivine-plagioclase cumulates, and different map units are distinguished on the basis of subtle differences in the amount of interstitial augite and Fe-Ti oxide, the occurrence of melatroctolitic intervals, and on variable concentrations of anorthositic-series inclusions. The igneous layering in the lower 500 meters of both intrusions is better known because of the high density of exploration drill core. Several of the major Cu-Ni-PGE sulfide deposits that occur along the base of the Duluth Complex in this area were first discovered in the early 1960s. From southwest to northeast, these include the Northmet (formerly Dunka Road), the Mesaba (formerly Babbitt), the Serpentine, the Dunka Pit, and the Birch Lake deposits. Polymet is in the final stages of permitting the Northmet deposit, and if successful, is scheduled to begin development in 2008. Assessment activity on the other deposits has increased as well, particularly on the Birch Lake deposit. The geologic picture portrayed in these maps and cross sections provide new insights and important constraints on models for the emplacement, crystallization, and mineralization histories of the Partridge River and South Kawishiwi intrusions. The potential development of Cu-Ni-PGE deposits will provide further insights into the detailed geology and mineralization of this economically important part of the Duluth Complex. REFERENCES Bonnichsen, B., 1970a, Reconnaissance geologic map of Babbitt quadrangle: Minnesota Geological Survey Open-File Map, scale 1:24,000. ———1970b, Reconnaissance geologic map of Babbitt NE quadrangle: Minnesota Geological Survey Open-File Map, scale 1:24,000. ———1970c, Reconnaissance geologic map of Babbitt SE quadrangle: Minnesota Geological Survey Open-File Map, scale 1:24,000. ———1970d, Reconnaissance geologic map of Babbitt SW quadrangle: Minnesota Geological Survey Open-File Map, scale 1:24,000. ———1972, Southern part of the Duluth Complex, in Sims, P.K., and Morey, G.B., eds., Geology of Minnesota: A centennial volume: Minnesota Geological Survey, p. 361-388. 47 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Miller, J.D., Jr., 2005, Bedrock geology of the Babbitt Southeast quadrangle, St. Louis and Lake Counties, Minnesota: Minnesota Geological Survey Miscellaneous Map M-162, scale 1:24,000. Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.E., and Wahl, T.E., 2002, Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geological Survey Report of Investigations 58, 207 p. Miller, J.D., Jr., and Severson, M.J., 2005, Bedrock geology of the Babbitt Southwest quadrangle, St. Louis County, Minnesota: Minnesota Geological Survey Miscellaneous Map M-161, scale 1:24,000. Miller, J.D., Jr., Severson, M.J., and Foose, M.P., 2005, Bedrock geology of the Babbitt Northeast quadrangle, St. Louis and Lake Counties, Minnesota: Minnesota Geological Survey Miscellaneous Map M-160, scale 1:24,000. Paces, J.B., and Miller, J.D., Jr., 1993, Precise U-Pb ages of Duluth Complex and related mafic intrusions, northeastern Minnesota: Geochronological insights to physical, petrogenetic, paleomagnetic and tectono-magmatic processes associated with the 1.1 Ga Midcontinent rift system: Journal of Geophysical Research, v. 98, no. B8, p. 13,997-14,013. Patelke, R.L., 1996, The Colvin Creek body, a metavolcanic and metasedimentary mafic inclusion in the Keweenawan Duluth Complex, northeastern Minnesota: Duluth, Minn., University of Minnesota Duluth, M.S. thesis, 232 p. Severson, M.J., and Miller, J.D., Jr., 2005, Bedrock geology of the Babbitt quadrangle, St. Louis County, Minnesota: Minnesota Geological Survey Miscellaneous Map M-159, scale 1:24,000. 48 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 GEOCHEMICAL CONSTRAINTS ON THE DEPOSITION OF MESOARCHEAN BANDED IRON FORMATION AT THE MUSSELWHITE MINE, NORTH CARIBOU GREENSTONE BELT, SUPERIOR PROVINCE MORAN, PATRICK†, FRALICK, PHILIP and HOLLINGS, PETE, Department of Geology, Lakehead University, Thunder Bay, Ontario, Canada, P7B 5E1, pcmoran@lakeheadu.ca Iron formations (IFs), chemical sedimentary rocks containing greater than 15% iron (James 1954), can be broadly divided into Superior- and Algoma-type. Superior-type IF are laterally extensive, associated with sedimentary rocks deposited in shallow water settings, and are generally Paleoproterozoic in age. They formed from the upwelling of oxygen-deficient, Fe+2bearing ocean water onto shallow shelves where oxygen was present (Cloud 1973). In contrast, Algoma-type IFs are laterally and vertically limited, associated with volcanic and sedimentary rocks deposited in deep-water, and mostly Archean in age (Gross 1996). They are commonly considered to have formed by precipitation from venting hydrothermal fluids, although shallowwater deposits of Algoma-type are present and probably represent microbially induced precipitation (Fralick, this conference). This study utilized banded chert-magnetite iron formation present in surface exposures at the Musselwhite Mine. The site sampled is an unmineralized area of the gold-bearing horizon. Sixteen samples were collected, from which monomineralic layers were separated and analyzed using XRF and ICP-MS. This is an amphibolite facies Algoma-type IF that overlies a thick mafic metavolcanic succession with apparent conformity. Millimeter to approximately one centimeter thick layers of chert and magnetite alternate in the lower half of the IF. These layers contain very small amounts of siliciclastic material. Higher in the IF there is a gradational increase in the amount of siliciclastic debris intercalated with the chemical sediment layers, until the succession is dominated by siliciclastics. All samples came from the lower, relatively siliciclastic free zone. Concentrations of most major and trace elements are relatively low in the magnetite and chert samples. Exceptions to this are Si in the cherts and Fe, Mn and P in the magnetite layers. The Si, Fe and Mn are self-explanatory. The slightly elevated phosphorous values indicate possible scavenging of PO4-2 from seawater by iron hydroxides or oxyhydroxides during precipitation or microbial activity in the sediment. Trace element abundances normalized to chondrite (Fig.1a,b) indicate the fluid that precipitated the IF was depleted in Ni, Cr, Zn, Co, Cu, Ti and K; and enriched in Sc, Y, W and Cs, relative to chondrites. Figure 1C shows the chert layers have less admixed siliciclastic material and also less Ni, Cr, Zn, Co and Cu, possibly denoting the iron compounds precipitated from higher temperature fluids. Rare earth element plots portray similar patterns to REE plots of recent metalliferous sediment and vent water from the TAG field (Atlantic) and the Atlantis II Deep (Red Sea) (Peter 2003). The geochemical data indicate both the Si- and Fe-rich layers precipitated from vented fluids in an environment where there was sufficient oxygen to form iron hydroxides or oxyhydroxides. References Peter, J.M., 2003, Ancient iron formations: their genesis and use in the exploration for stratiform base metal sulfide deposits, with examples from the Bathurst Mining Camp, in Lentz, D.R., Ed., Geochemistry of Sediments and Sedimentary Rocks: Evolutionary Considerations to Mineral Deposit-Forming Environments: Geological Association of Canada, GeoText 4, p. 145-176 James, H.L., 1954, Sedimentary Facies of iron-formation: Economic Geology, v. 49, p. 29-44 Cloud, P.E., 1973, Paleoecological significance of banded iron-formation: Economic Geology, v. 68, p. 1135-1143 Gross, G.A., 1996, Stratiform Iron: Lake Superior-type iron-formation, Algoma-type iron-formation, Ironstone, in Eckstrand, O.E. Sinclair, W. D., And Thorpe, R.I., Eds., Geology of Canadian Mineral Deposit Types, The Geology of North America, n. 8: Geological Survey of Canada, p. 41-80 49 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Figure 1. (a) Logarithmic plot of trace element values from magnetite layers taken from the BIF at Musselwhite mine. Trace element values normalized to CI Carbonaceous chondrite values of McDonough and Sun (1994). (b) Logarithmic plot of trace element values from chert layers taken from the BIF at Musselwhite mine. Trace element values normalized to CI Carbonaceous chondrite values of McDonough and Sun (1994). (c) Chert values normalized to magnetite values from the present study. (d) REE plot normalized against CI chondrite. 50 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 STATISTICAL ANALYSIS OF INDOOR RADON DATA AND RELATIONSHIPS TO GEOLOGY IN WISCONSIN MUDREY, M.G. JR., 106 Ravine Road, Mount Horeb, WI 53572 (mgmudrey@mhtc.net) In the 1986 with the discovery of exceeding high radon values in Pennsylvania, the US Environmental Protection Agency initiated a nation-wide study to determine the population and geographical risk associated with radon. Because Wisconsin has a strong state radiation protection program, and known occurrences of uranium and other radionuclides, it was one of 10 initial states to be analyzed. Since then, 84,262 residential indoor radon in air covering all areas has been collected. These data are not randomly distributed and reflect collection by interested by home owners. Nearly 50% of the analyses have been accurately located and digitized permitting geologic analysis; the remainder has only zipcode locational information. This study compares the EPA radon survey with the more extensive Wisconsin data base and evaluates the data with respect to various geologic attributes in order to define those areas and geologic units in Wisconsin where radon may pose a higher risk. The original EPA survey of 1194 homes is considered the only statistically useful survey for evaluating average values of indoor radon in Wisconsin: the mean of 3.4, with a Q1 of 1.2 and a Q3 of 4.1. The data are highly skewed. The highest value found in Wisconsin is 938 near Hudson. Elevated indoor radon is found in all areas of Wisconsin as is predicted by geological analysis. Soil derived from granite and carbonate rock that is the principal geological factor leading to elevated indoor radon in Wisconsin. Because radon can migrate only a few meters over the 3.8 day half-life, soil chemistry and soil texture principally influence elevated indoor radon. Because elevated radon is found in all areas of Wisconsin, and because do-ityourself radon testing is inexpensive, it is highly recommended that all houses in Wisconsin should be tested for radon. This study was funded by the U.S. Environmental Protection Agency to the Radiation Protection Section, Wisconsin Division of Public Health, Wisconsin Department of Health and Family Services, and to the Wisconsin Geological and 51 Natural History Survey, University of Wisconsin Extension, while Mudrey was with the WGNHS. �Proceedings of the 52nd ILSG Annual Meeting – Part 1 3D VISUALIZATIONS OF MAFIC INTRUSIONS IN THE DULUTH COMPLEX, NORTHEASTERN MINNESOTA PETERSON, DEAN M., Natural Resources Research Institute, Duluth, MN, dpeters1@nrri.umn.edu) One of the main hallmarks of science is that it allows one to imagine reality. In the geosciences, one of the main realities that geologists try imagine is the geometry and structural juxtaposition of geological units and/or mineralized zones in the subsurface. This is especially true for geologists that work in the mineral exploration and mining industries, earthquake monitoring and hazard assessment agencies, and at contaminated groundwater sites. The advent of 3D geological computer programs has brought about a revolution in the understanding of the Earth in three dimensions. This digital poster presentation using the computer program gOcad (by Earth Decision Science) will display geological features of numerous troctolitic intrusions within the Duluth Complex, and will highlight how such visualizations advance our understanding of geological processes that ultimately led to the formation of billions of tonnes of Cu-Ni-PGE mineralized rocks. An image of the basal surface of a portion of the Partridge River Intrusion is presented in Figure 1. The bowl-shaped depression hosts the Babbitt deposit, currently held by Teck-Cominco. Figure 1. 3D view of the basal surface of a portion of the Partridge River Intrusion. View looking due west and down 10º. Grid lines are UTM coordinates, in meters. 52 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 EVIDENCE FOR WIDESPREAD DISTRIBUTION OF IRON DEPENDENT METABOLISMS IN PRECAMBRIAN OCEANS PLANAVSKY, NOAH †, KNUDSEN, ANDREW, Lawrence University Appleton, WI SHAPIRO, RUSSELL, Gustavus Adolphus College Saint Peter, MN noah.j.planavsky@lawrence.edu Starting with the first detailed descriptions of Precambrian microfossils (Barghorn and Tyler, 1965), the dominant view has been that cyanobacteria were the primary producers in Paleoproterozoic, and more generally in all early Precambrian, ecosystems (Awramik, 1992). An increased understanding about the chemical evolution of the ocean atmosphere system lead some to question the theoretical and observational foundation of this dogma (Blank, 2004) Currently, there are little constraints on many of the basic attributes of most pre1.8ga ecosystems. For instance, the diversity of bacterial metabolism in early environments in still debated and distribution of bacterial metabolisms in early earth’s history is poorly constrained. Although the atmosphere became oxygenated approximately 2.5Ga widespread marine anoxia and sulfate limitation resulted in pre1.8Ga oceans with significant quantities of dissolved iron. There is strong theoretical support from ecological modeling that these iron rich oceans supported an abundant iron dependent microbial community (Konhauser et al., 2002). In our analysis of the Paleoproterozoic Animikie basin we found empirical evidence for the widespread distribution of a microbial community with iron dependent metabolisms that thrived at a characterizeable oxygenic chemocline. There are two distinct stromatolites Animikie Basin; large, hemispheroidal, calcitic, peritidal stromatolites, and iron rich, subtidal, digitate stromatolites. Based on the morphology of the stromatolites, the inferred primary silica composition (Barghoorn and Tyler, 1965), and the microstructure the iron rich stromatolites were proposed as having formed as either sinter deposits or in the hot spring apron (Walter, 1972). The stromatolites proposed to be sinter deposits are defined by the presence of thin laminations (thinner in width than the majority of Gunflint microfossils) with distinct boundaries between the laminations (Hoffman, 1969; Walter, 1972). The spring origin of the stromatolites has been widely accepted, (Sommers and Awramik, 2002; Siminson, 1987) and even popularized to general audiences (Knoll, 2003). Field and microscopy work revealed that the ‘abiogenic’ stromatolites formed under a strong microbial influence. Field observation demonstrated that the stromatolites represent one facies in a well preserved sedimentary package. The iron rich stromatolites also display mesostructural features that cannot be readily explained by abiotic processes. For instance, the columnar sections of the stromatolites display crestal thickening, often with tuffs similar those seen in modern microbial mats and laminations that are far above angle of repose. The presence of alternating residual organic rich and residual organic poor lamination couplets and total organic carbon values of up to 2.5% also strongly suggests microbial mediation. In the least metamorphically altered stromatolites the thinner organic rich laminations are composed predominately of hematite in siliceous cement. The organic poor laminations and the surrounding siliceous material contain very low concentrations of iron. The iron distribution cannot be explained by secondary 53 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 oxidation and therefore necessities a means for iron disproportionation within the organic rich laminae. The rare earth element (REE) pattern and sedimentological characterization suggest that the iron rich stromatolites formed at an oxygenic chemocline. Ce is the only REE that can be oxidized in surface aqueous solutions, which results redox reactions controlling the element’s cycling and abundance. The stromatolites display a negative cerium anomaly, which indicates stromatolite formation in aerobic conditions or at an oxygenic mixing zone. The stromatolites occur at a transition out a zone with regular authogenic iron deposition, which implies formation at an oxygenic chemocline. In modern environments, iron oxidizing β proteobacterium dominate at oxygen mixing zones or in microaerophilic conditions where there are similar or even significantly lower ferric iron concentrations than the predicted (Holland, 1984,) values for the Paleoproterozoic oceans (Emerson and Revsesbach, 1994). Iron oxidizing β proteobacterium induce iron precipitation on average 60 times faster than abiotic reactions providing a means for the observed iron disproportionatation in the organic rich stromatolite laminae. The stromatolites also display a positive Gd anomaly, which serves as independent biogenicity proxy. Abiogenic iron precipitates, because of the lanthanide tetrade effect, display a negative Gd anomaly (Bau, 1999). Microbial communities slightly preferential or proportionately adsorb Gd compared to Tb, and Dy (Anderson and Pedersen, 2003). Modern ocean water has positive Gd anomalies that are mirrored by negative Gd anomalies in the largely abiogenic ferromanganese pavements. The stromatolites have similar or even more pronounced Gd anomalies than modern oceans. The stromatolites positive Gd anomaly necessitates biogenic precipitation of iron oxides within the stromatolites. References: Anderson, C. R. & Pedersen, K. 2003. In situ growth of Gallionella biofilms and partitioning of lanthanides and actinides between biological material and ferric oxyhydroxides. Geobiology 1 (2), 169-178. Barghoorn, E.S. and Tyler S.A., 1965. Microorganisms from the Gunflint Chert. Science 147 (3658), 563577. Bau M., and Dulski P. (1999) Comparing yttrium and rare earths inhydrothermal fluids from the MidAtlantic Ridge: Implications for Yand REE behaviour during near-vent mixing and for the Y/Ho ratio of Proterozoic seawater. Chem. Geol. 155, 77–79. Emerson, D., and N. P. Revsbech. 1994. Investigation of an iron-oxidizing microbial mat community located near Aarhus, Denmark: field studies. Applied Environmental Microbiology 60:4022-4031 Holland, H. D., 1984. The Chemistry of the Atmosphere and Oceans. Wiley, New York. Konhauser, K.O., Hamade, T., Morris, R.C., Ferris, F.G., Southam, G., Raiswell, R., and Canfield, D., 2002. Could bacteria have formed the Precambrian banded iron formations? Geology, 30:1079-1082. Simonson B. M. n, 1985, Sedimentological constraints on the origins of Precambrian iron-formations, GSA Bulletin, 96: 244-252. Walter, M.R. 1972. A hot spring analog for the depositional environment of Precambrian iron formations of the Lake Superior region. Economic Geology, 67: 965-972. 54 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 DETRITAL ZIRCON GEOCHRONOLOGY OF THE WESTERN HURONIAN BASIN RAINBIRD, ROBERT H.† and DAVIS, WILLIAM J. Geological Survey of Canada, 601 Booth St, Ottawa, Ontario, K1A 0E8 rrainbir@nrcan.gc.ca The Paleoproterozoic Huronian basin hosts an up to 12 km-thick succession of mainly siliciclastic sedimentary rocks deposited along the southern margin of the Superior Province (Huronian Supergroup). Paleocurrent data from crossbedding in fluvial sandstones throughout the succession suggest provenance from the west and northwest. U-Pb SHRIMP analysis of detrital zircon from six sandstone samples from the western part of the Huronian basin indicates provenance mainly from Neoarchean sources with prominent modes ca. 2.67 and 2.72 Ga. A sample from the stratigraphically lowest unit (Livingstone Creek Formation-Elliot Lake Group) contains zircon ranging in age from 2.90 - 2.65 Ga, with one ca. 2.50 Ga grain (weighted mean 207Pb/206Pb age of 2497 ± 10 Ma). This grain probably was derived from co-eval volcanic rocks erupted during rifting and initiation of the Huronian basin and provides a maximum age for deposition of the Huronian. A sample from the overlying Matinenda Formation has a unimodal zircon age population at ca. 2.67 Ga. The overlying Mississagi Formation (Hough Lake Group) has a polymodal zircon population varying in age from 3.62 - 2.45 Ga. Given the easterly paleocurrent indicators at the sampling locality, the pre-3.0 Ga grains could have derived from gneisses of the Minnesota River Valley terrane, southwest of the Huronian basin. The two youngest grains from the Mississagi Formation (weighted mean 207Pb/206Pb ages-2445± 9 Ma and 2451 ± 6 Ma), likely were eroded from volcanic rocks (or their intrusive equivalents) in the unconformably underlying Elliot Lake Group. Samples from succeeding thick fluvial quartz arenites of the Serpent (Quirke Lake Group) and Lorrain (Cobalt Group) formations show similar detrital zircon age profiles with a range of ages from 2.88-2.68 Ga, and a significant ca. 2.72 Ga population. A marine quartzarenite from the uppermost unit of the Huronian (Bar River Formation-Cobalt Group) has a generally similar population to that of the Serpent and Lorrain formations but with a broader range of ages, including 3 grains at ca. 2.53 Ga of unknown provenance, and 4 grains at ca. 3.00 Ga. Overall, detrital zircon geochronology and sedimentology of the western Huronian basin is compatible with provenance from the Wawa Subprovince of the western Superior craton with contributions from adjacent older gneissic terranes. 55 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 56 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 UNRESOLVED PROBLEMS AND THE EVOLUTION OF SUDBURY GEOLOGY ROUSELL, D. H. Department of Earth Sciences, Laurentian University, Sudbury, ON, P3E 2S4, drousell@laurenian.ca. Geological events, which shaped the area now occupied by the Sudbury Basin and surrounding footwall, span at least 1470 Ma from 2711 Ma, the minimum age of the Levack Gneiss Complex, to 1238 Ma, the age of olivine diabase dikes. The area has undergone several tectonic, magmatic, metamorphic and mineralization events which have been largely overshadowed by the Sudbury Event at 1850 Ma. Ascribing the Sudbury Event to meteorite impact is an entrenched paradigm; several unresolved problems are largely ignored. The aim of this abstract is to outline the geological evolution of the area and to identify certain outstanding problems. The events which have affected the area may be grouped as follows: doming, Sudbury Event and post Sudbury Event. Early workers recognized that the Paleoproterozoic rocks of the Sudbury Basin were superimposed on an Archean dome, with the NW boundary extending to the Huronian outliers and the other boundaries obscured by later deformation. Evidence for the dome is as follows: 1) in the South and East Ranges the footwall rocks become progressively younger away from the dome; 2) rocks of the Levack Gneiss Complex, metamorphosed to the granulite facies at depths of up to 28 km and uplifted to depths of 5 to 11 km, possibly during emplacement of the Cartier batholith, suggests that the complex cored a paleodome; 3) mafic dikes in the footwall of the North and East Ranges, located between 10 and 15 km from the outer margin of the Sudbury Igneous Complex ( SIC) , are oriented normal to the adjacent margin of the SIC which is consistent with dike emplacement during local magmatic doming; 4) the Nipissing gabbro, which has an affinity for rocks of the Huronian Supergroup, is absent between the NW edge of the SIC and the Huronian outliers, which suggests little or no deposition of Huronian sediments or their complete erosion, implying that the site of the basin was a topographic high; and 5) three felsic plutons , viz., Murray, Creighton and Skead intrude the area of the inferred paleodome. Prior to the Sudbury Event, rocks of the Southern Province and the Huronian outliers were folded about EW- to NE-trending axes and locally about NNE-to NNW-trending axes. The Sudbury Event is ascribed, without question, to meteorite impact by virtually all investigators. The bolide coincidentally struck a local dome which was presumably pregnant with sufficient Ni-Cu-PGE- Zn-Pb-Cu-Ag-Au mineralization to form one of the world’s largest mining camps. The notion that the ores are of cosmic origin has received little support. Features formed by the event include: Footwall Breccia and Sudbury Breccia (SB); shatter cones; planar deformation features; Onaping Formation; and SIC. SB in the granitic rocks of the North Range footwall consists of pseudotachylite, a rock considered to have formed by frictional melting in dry rocks during high-speed slip along large faults concentric with the outer margin of the North Range SIC. Field work has led others to question the existence of the faults. In contrast to the North Range SB, clastic SB occurs in the footwall of the South and East Ranges. It apparently formed by explosive dilation, fluidization and flowage into extension fractures. Clastic SB occurs at Lake Temagami, 80 km NE of the outer margin of the SIC and the most distant locality reported to date. Clasts in breccias surrounding impact craters tend to increase in size with distance from impact. Clasts at Lake Temagami are smaller than those in some bodies near the SIC. Curved and striated fracture surfaces known as shatter cones are supposedly unique to impact craters. At Sudbury, they are scarce in the North Range footwall but are common in the South and East Range footwall where they tend to occur in clusters. Most fractures are in the shape of curved surfaces rather than cones. In attempts to define the size of the Sudbury crater, numerous authors have subscribed to the thesis, first proposed in 1970, that the Huronian outliers 57 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 were preserved in down-faulted blocks formed by the simultaneous collapse of the central uplift and the emplacement of the SIC. Despite convincing evidence that the sediments were deposited in a rim syncline around a domal structure long before the Sudbury Event, relating the preservation of the outliers to impact still persists The origin of the Onaping Formation has been interpreted as follows: volcanic, fall-back breccia from meteorite impact, impact-induced volcanism or hyaloclasis. A discontinuous breccia at the base of the formation contains fragments as much as 79 x 23 km which reflect the lithology of the adjacent country rocks. The clasts were possibly emplaced by sliding down a submarine slope (crater wall?). Above the basal unit the formation consists of a series of breccias. Possibly melt, continuously fed from below, underwent passive to explosive fragmentation and rapid cooling on interaction with water. The “Onaping melt” may represent an impact melt, a hypabyssal intrusion or some combination. Diamonds in the Onaping Formation are referred to, by their discoverers, as “impact diamonds” without even considering alternatives such as a diatreme origin. Based largely on geochemical data, most investigators interpret the SIC to be an impact melt sheet. However, the granophyre, the upper unit of the SIC, intrudes the Onaping Formation. This implies that the SIC crystallized from a magma, possibly impact-triggered. An alternate scenario is that the granophyre represents a later intrusion and that the bulk of the SIC was emplaced before the Onaping, perhaps as a melt sheet. This means that the Onaping “melt” was injected through the SIC before fragmenting into a hyaloclastite. Structural data suggests that the SIC was emplaced in approximately the present disposition of the North Range i.e., dipping inward at 420. Evidence is as follows. Igneous layering in the norite dips less than the dip of the base of the SIC. Folding of a horizontal melt sheet would produce a foliation with a steeper dip than the basal contact and a strain in possible hinge zones such as the lobes located at both ends of the East Range. A plagioclase lineation in the north lobe is orthogonal to the base of the SIC. The lineation is attributed to crystal growth in a magma chamber. Apparently, even minor deformation will destroy the orthogonality. Thus the mineral fabric and the low overall strain in the North Lobe preclude a fold origin for the present shape of the SIC. The Sudbury Event was followed by differentiation of the units of the SIC (1850 Ma), formation of the Ni-Cu-PGE deposits, deposition of the Vermilion, Onwatin and Chelmsford Formations, hydrothermal alteration and formation of Zn-Pb-Cu deposits inside the basin. NW-directed compression and a weaker SW-directed compression (Penokean Orogeny, 1900 to 1700 Ma) folded the rocks of the basin about NW-trending, doubly-plunging fold axes and developed a prominent cleavage in them. Deformation died out to the NW as the North Range Onaping Formation and SIC have undergone only local and mild ductile deformation. In the South Range, the Onaping Formation and the SIC are steepened to the NW and displaced by a SE-dipping zone of reverse shear. On the outcrop-scale, the SIC displays anastomosing conjugate shear zones. A revised model is required which embraces all aspects of Sudbury Geology. Many features are either force-fitted into an impact model or ignored. Some, but not all, elements are better explained by endogenic models such as diatremes and plumes. Because of the time span of Sudbury geology, perhaps too many pieces of the puzzle have been lost, thus precluding an unequivocal history of Sudbury Geology. 58 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 GEOLOGY AND ALTERATION ASSOCIATED WITH VMS MINERALIZATION IN THE HAMLIN LAKE AREA, NORTHWESTERN ONTARIO SHUTE, AMY† and HOLLINGS, PETE, Department of Geology, Lakehead University, 955 Oliver Rd., Thunder Bay, Ontario, P7B 5E1, Canada; alshute@lakeheadu.ca, The Shebandowan Greenstone Belt is located within the Wawa Subprovince of the Superior Province and has been the target of numerous exploration efforts over the last century. The belt is host to many different precious and base metal mineral deposits. Past producers include the Shebandowan Ni-Cu PGE mine, the North Coldstream Cu-AuAg mine, and the Ardeen Au mine, Northern Ontario’s first gold producing mine. With a renewed interest in the belt by many exploration companies, the potential for new discoveries is growing. The Hamlin Lake area is within the Shebandowan Greenstone belt and located approximately 120 km west of Thunder Bay, Ontario. This project will be looking at the alteration and tectonic setting that is associated with the Hamlin Lake volcanogenic massive sulfide (VMS) system. The VMS system was first recognized when massive sulfides were found while surface sampling during the 2005 field season. The mineralization includes pyrite, chalcopyrite and pyrrhotite all at the surface with stringer mineralization found in some areas as well. Trenching and drilling followed during the fall and winter of 2005/06 and revealed significant additional mineralization. The copper values as high as 1.49% have been found in surface samples with 4.88g of gold, and the Ray Smith showing is part of the Hamlin Lake property with copper values as high as 6% and 6.0g of gold. A B Figure 1. (a) Rhyolite showing round amygdules; (b) Felsic fragmental showing partial alignment of the clasts (magnet for scale is approximately 12.0cm long) The Hamlin Lake area consists of four major units; felsic volcanic rocks; mafic volcanic rocks; pink brecciated rock; and banded iron formation. The smallest unit is the banded iron formation, found in one larger outcrop, but also found sporadically throughout the Hamlin Lake area in small (1-2m) lense-like showings. The mineralized pink breccia is the second least abundant unit and is the focus of a currently ongoing drilling project because of its higher grade mineralization. 59 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 The second major unit is the mafic volcanic rocks. This unit consists of a fragmental unit and can vary in groundmass from being chlorite rich to magnetite rich in the field. Geochemically the mafic volcanic rocks have SiO2 values that range from 51.1 to 64.4 wt.%, TiO2 from 0.08 to 0.61 wt.%, and Fe2O3 from 7.0 to 34.3 wt.%. The most abundant rock type and the focus of this study are the felsic volcanic rocks. In the field, the felsic volcanics contain quartz-eyes, amygdules, and fragments with a fine-grained groundmass and overall light grey colour (Fig.1a and b). These rhyolites vary in colour, texture and in geochemistry with preliminary work showing at least two distinct felsic suites when the REE’s are plotted (Fig.2b). The major elements in the felsic volcanic rocks show SiO2 values that range from 76 to 89 wt.%, TiO2 from 0.2 to 0.8 wt.%, and Fe2O3 from 0.9 to 8.1 wt.%. Although the groundmass is different between the felsic and mafic fragmental volcanic rocks, their fragments are both lenticular-shaped and chert-like in appearance. These fragments have yet to be studied to distinguish their origin, but they are likely either pumaceous clasts or fragmented chert layers. 7000 100.00 6000 10.00 4000 REE/PM Ti (ppm) 5000 3000 AS-05-012 AS-05-015 1.00 AS-05-029 AS-05-057 2000 0.10 1000 0 0.01 0 50 100 150 200 250 300 350 400 Th Nb La Ce Pr Nd Zr Hf Sm Eu Ti Gd Tb Dy Y Ho Er Tm Yb Lu Al Zr (ppm) Figure 2. (a) Preliminary Zr vs. Ti (ppm) plot showing two distinct groupings of samples. (b) PM normalized REE plot showing two distinctly different rhyolites suites. Felsic volcanic rocks associated with VMS systems have been the subject of considerable study over the past two decades. Several classifications been created in an attempt to characterize felsic volcanic rocks that are associated with VMS deposits, and that are barren of VMS deposits. Lesher et al. (1986) classified felsic volcanic rocks as being FI, FII and FIII’s with distinctions between their REE patterns, Zr/Y ratios and abundances in high field strength elements. Preliminary work has shown that there are at least two distinct felsic volcanic suites, suite I having flat REE patterns and positive Zr and Hf anomalies and suite II having more fractionated REE and lacking positive anomalies. When comparing Lesher’s classification with the felsic volcanic rocks in the Hamlin Lake area, suite I most closely resemble FII whereas suite II is similar to the FI group. References Lesher, C.M., Goodwin, A.M., Campbell, I.H., and Gorton, M.P., 1986, Trace-element geochemistry of ore associated and barren, felsic metavolcanic rocks in the Superior Province, Canada: Canadian Journal of Earth Sciences, v. 23, p. 222-237. 60 V Sc �Proceedings of the 52nd ILSG Annual Meeting – Part 1 PRELIMINARY INVESTIGATIONS OF THE PETROLOGY, GEOCHEMISTRY AND GEOCHRONOLOGY OF THE ST. IGNACE ISLAND COMPLEX, MIDCONTINENT RIFT, NORTHERN LAKE SUPERIOR, ONTARIO SMYK, MARK C.†, Ontario Geological Survey, Ministry of Northern Development and Mines, Suite B002, 435 James St. South, Thunder Bay, ON P7E 6S7 CANADA, HOLLINGS, PETER, Department of Geology, Lakehead University, 955 Oliver Rd., Thunder Bay, Ontario, P7B 5E1, Canada and HEAMAN, LARRY M., Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, T6G 2E3. As part of the Lake Nipigon Region Geoscience Initiative, a helicopter traverse was undertaken in 2005 in northern Lake Superior, in order to sample igneous rocks associated with the Mesoproterozoic Midcontinent Rift (MCR), including the St. Ignace Island Complex (SIC; Fig. 1). The SIC intruded the upper portions of MCR-related, Osler Group volcanic rocks (ca.1008 Ma; Davis and Sutcliffe 1985). It consists of a gabbroic to anorthositic ring dyke, which encloses quartz-feldspar porphyritic volcanic rocks (Sutcliffe and Smith 1988; Giguere 1975). Sutcliffe and Smith (1988) described the volcanic component of the SIC as intercalated plagioclase-glomeroporphyritic basaltic rocks, quartz-feldspar-phyric rhyolite flows and fragmental rocks. The pink to grey, rhyolitic rocks in the core of the SIC are dominantly quartz-phyric, with rare pyroxene and feldspar phenocrysts set in a fine-grained to glassy groundmass. They commonly contain wispy to amoeboid, mafic (basaltic?) inclusions, which are typically plagioclasephyric. Geochemically, the quartz-feldspar-phyric rocks from the core of the SIC are dacites and rhyolites (62 to 74 wt% SiO2) with elevated K2O contents (2.3 to 4.8 wt%). The lower silica contents within the core of the complex are apparently associated with small mafic inclusions within the more felsic units. The sampled mafic intrusive rocks from the ring dyke are plagioclase- and pyroxene-phyric, coarse- to fine-grained gabbros to monzogabbros (53 to 58 wt% SiO2). Figure. 1. A) Map of upper Great Lakes showing the location of the study area. B) Regional geology map showing the extent of the exposed portion of the Osler Group and the location of the St. Ignace Island Complex. The SIC samples yielded both zircon and baddeleyite. Subhedral baddeleyite grains from a rhyolite in the core of the SIC yielded a 207Pb/206Pb age of 1107.2±2.4 Ma, whereas zircons recovered from the rhyolite yielded a 207Pb/206Pb age of 1124 Ma. The zircons are large and show signs of resorption; they are also characterized by high Th/U contents that are typical of zircons derived from a mafic source. The fact that this latter age is much older than that of the Osler Group basalts that the SIC has intruded, combined with a lack 61 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 of euhedral zircon and baddeleyite grains, suggests that these grains may be of xenocrystic origin. The presence of baddeleyite in a rhyolite is also somewhat unusual as these are more commonly associated with syenites found in alkalic complexes (e.g. Coldwell Complex). Gabbro from the margin of the SIC yielded a small number of zircon grains with baddeleyite cores. These grains yielded a 207Pb/206Pb age of 1089.2±3.2 Ma. The growth of euhedral zircon on baddeleyite cores is occasionally observed in mafic rocks and is interpreted to indicate increasing silica activity conditions during magma crystallization. This is consistent with field relationships, which suggest that rhyolitic and gabbroic magmas may have intermingled during emplacement. Consequently, the 1089 Ma age may represent the emplacement age of both the rhyolite and the gabbro and suggests that all dates obtained from the rhyolite are xenocrystic. This is consistent with textures observed by Sutcliffe and Smith (1988) who also reported evidence for localized magma mixing and the presence of vesicular basalt fragments in felsic, welded tuffs. This age is similar to that of other MCR-related intrusions in the area (e.g. Crystal Lake, Blake and Moss Lake gabbros; Arrow River dyke) that have intruded Paleoproterozoic rocks, older MCR intrusions and/or Osler Group volcanic rocks during the late stages of MCR magmatism (Heaman and Easton 2006). Further field, petrographic and geochemical studies will seek to better determine the relationships between the various volcanic and intrusive rocks in order to understand the development of the SIC within the Midcontinent Rift. References Davis, D.W. and Sutcliffe, R.H. 1985. U-Pb ages from the Nipigon plate and northern Lake Superior; Geological Society of America Bulletin, v.96, p.1572-1579. Davis, D.W. and Green, J.C., 1997. Geochronology of the North American Midcontinent rift in western Lake Superior and implications for its geodynamic evolution; Canadian Journal of Earth Sciences, 34, p.476-488. Giguere, J.F. 1975. Geology of St. Ignace Island and adjacent islands, District of Thunder Bay; Ontario Division of Mines, Geological Report 118, 35p. Heaman, L.M. and Easton, R.M. 2006. Preliminary U/Pb geochronology results: Lake Nipigon Region Geoscience Initiative; Ontario Geological Survey, Miscellaneous Release of Data 191, 86p. Sutcliffe, R.H. and Smith, A.R. 1988. Geology of the St. Ignace Island volcanic-plutonic complex; Summary of Field Work and Other Activities, Ontario Geological Survey, Miscellaneous Paper 141, p.368-371. 62 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 CHARACTERIZING THE MONAZITE FINGERPRINT OF PALEOPROTEROZOIC (STATHERIAN) METASEDIMENTARY SEQUENCES IN CENTRAL WISCONSIN STONIER, PEGGY, and HOLM, D.K., Kent State University, Kent, OH, peshelma@kent.edu; MEDARIS, L.G., JR, Univ. of Wisconsin-Madison, Madison, WI; SCHNEIDER, D., Ohio University, Athens, OH. Introduction: Supermature siliciclastic rocks of the 1750-1630 Ma Baraboo Interval are widespread in the southern Lake Superior region and signify a period of crustal stability following the Penokean Orogeny and subsequent geon 17 magmatism (Medaris et al., 2003). Although the chronology and tectonic significance of Baraboo Interval sedimentation has been firmly established, certain quartzite inliers in central Wisconsin remain enigmatic. At Hamilton Mounds Baraboo Interval quartzite is reported to be intruded by geon 17 granite (Greenberg, 1986; Van Wyck and Norman, 2004). This proposed cross-cutting relation conflicts with recent detrital zircon age data showing all Baraboo Interval quartzites to be younger than 1750 Ma (Holm et al., 1998; Medaris et al., 2003). Van Wyck and Norman (2004) propose that early onset of Baraboo Interval quartzite deposition was synchronous with magmatism, an interpretation that is unusual for this rock type and in disagreement with deposition on a recently stabilized craton (Dott, 1983). Instead, Medaris et al. (in review) demonstrate that the metasedimentary rocks at Hamilton Mounds consist of two Paleoproterozoic sedimentary sequences: an older meta-arkose intruded by geon 17 granite, and a younger, overlying supermature quartzite, which is likely correlative with Baraboo Interval quartzite elsewhere. Our purpose here is to characterize and date monazite grains in these two units. Recently, studies of detrital zircons in Baraboo Interval rocks have been invaluable for establishing their maximum age and identifying their source terrane. Assessing the monazite "fingerprint" in these rocks may allow us to differentiate between the two depositional interpretations and to better establish sedimentalogical aspects of post-Penokean crustal stabilization. Monazite Textures: The quartzite unit contains only tiny (10-40 micron diameter), rounded to subrounded monazite grains that show simple chemical zonation (rims/cores). Many have cores that are high in both Yttrium and Uranium (Fig. 1a). In contrast, the meta-arkose contains a larger monazite grain-size variance (10-90 micron diameters) and more variable morphology. A few grains are rounded and chemically simple, but many have embayed grain boundaries and are complexly zoned (Fig. 1b). Some grains are irregular and some have a bladed elongate morphology that are concordant to pre-existing textural features (Fig. 1c, d). Monazite Geochronology: Electron microprobe total-Pb analyses of the fine-grained detrital monazite from the quartzite unit yield spot ages ranging from ~2050 Ma to ~1750 Ma, with dominant peaks at ~1800 Ma and ~1860 Ma (Fig. 2a; composite mean age of 1837 ±23 Ma). Similar analyses on the meta-arkose unit yields ages ranging from ~1900 Ma to ~1730 Ma, with a single dominant age peak at 1850 Ma (Fig. 2b; composite mean age of 1849 ±7 Ma). Interpretation: Both rock units at Hamilton Mounds contain abundant Penokean age 63 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 detrital grains, consistent with their having been deposited after the Penokean orogeny. Both rock units also contain some geon 17 ages. The geon 17 ages within the quartzite unit are clearly from detrital grains. However, the geon 17 ages in the meta-arkose are unlikely to be detrital considering that this unit is cut by a ca. 1761 Ma granite dike. We interpret these ages to instead reflect metamorphism associated with dike intrusion. This interpretation is most consistent with the varied monazite morphology in this unit. Our results show that monazite geochronology of metasedimentary units is a powerful tool when combined with detailed in situ textural analysis aided by a comprehensive understanding of the area's geologic context. Figure 1a: Figure 1b: Figure 1c & 1d: In-situ BSE image of meta-Meta-arkose monazite In-situ BSE images of meta-arkose monazite grains quartzite monazite grain grain mapped for Y References Dott, R.H., Jr., 1983. The Proterozoic red quartzite enigma in the north central United States: resolved by plate collision?; Geological Society of America Memoir, v. 160, p. 129-141. Greenberg, J.K., 1986. Magmatism and the Baraboo Interval: breccia metasomatism and intrusion; Geoscience Wisconsin 10, 96-112. Holm, D.K., Schneider, D., and Coath, C., 1998b. Age and deformation of Early Proterozoic quartzites in the southern Lake Superior region: Implications for extent of foreland deformation during final assembly of Laurentia; Geology, v. 26, p. 907-910. Medaris, L.G., Singer, B.S., Dott, R.H., Naymark, A., Johnson, C.M., and Schott, R.C., 2003. Late Paleoproterozoic climate, tectonics, and metamorphism in the southern Lake Superior region and protoNorth America: Evidence from Baraboo interval quartzites; The Journal of Geology, v. 111, p. 243-247. Van Wyck, N., and Norman, M., 2004. Detrital zircon ages from Early Proterozoic quartzites, Wisconsin, support rapid weathering and deposition of mature quartz arenites; The Journal of Geology, v. 112, p. 305315. 64 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 THE THERMAL HISTORY OF LOW METAMORPHIC GRADE PALEOPROTEROZOIC METASEDIMENTARY ROCKS OF THE PENOKEAN OROGEN, LAKE SUPERIOR REGION: RECOGNIZING A WIDESPREAD 1786 MA OVERPRINT USING XENOTIME GEOCHRONOLOGY VALLINI, DANIELA A. University of Western Australia, 35 Stirling Highway, Crawley, Western Australia 6009 CANNON, WILLIAM F.†, SCHULZ, KLAUS J. U.S. Geological Survey, MS 954, Reston, VA 20192 MCNAUGHTON, NEAL J. University of Western Australia, 35 Stirling Highway, Crawley, Western Australia 6009 Paleoproterozoic strata in northern Michigan, Wisconsin, and Minnesota were deposited between 2.3 and 1.75 Ga within the Penokean foreland. These strata were metamorphosed by multipleevents, all previously attributed to the Penokean orogeny (1875-1830 Ma). We sampled 10 localities (Fig. 1) in the Marquette Range Supergroup in Michigan and the Animikie, Mille Lacs, and North Range Groups in Minnesota that contain xenotime suitable for in situ SHRIMP U-Pb geochronology and where the metamorphic grade is greenschist to sub-greenschist. The units sampled are Enchantment Lake Formation (sample 1), Sunday Quartzite (sample 5), Ajibik Quartzite (sample 7), and Michigamme Formation (samples 8, 9, 10) in Michigan, and the Mille Lacs Group (sample 2, 3), the Mahnomen Formation (sample 4), and Pokegema Quartzite (sample 6) in Minnesota. Thirty-two U-Pb ages of xenotime in these samples give a population at 1786 ± 4 Ma and 9 ages give a population at 1861 ± 10 Ma. Both populations are contained in samples from the Chocolay Group in Michigan and the Mille Lacs and North Range Groups in Minnesota (Fig. 1) and thus record a region-wide 1860 Ma low-temperature thermal event that is slightly older than the basal units of the Baraga Group in Michigan and the Rove Formation in Minnesota and Ontario. This event coincides with regional uplift that led to the unconformity between the Baraga and Menominee Groups in Michigan, hence xenotime growth must have occurred at shallow depths. Younger units, including the Animikie Group in Minnesota and the Baraga Group in Michigan, record only the 1786 Ma event. Amphibolite-granulite facies rocks within a gneiss dome corridor in the southern part of the foreland, south of our sample sites, show an 1800-1790 Ma monazite population that overprints 1830 Ma Penokean metamorphism (Schneider and others, 2004). These high grade rocks are adjacent to gneiss domes and early geon 17 post-Penokean granite plutons. Our samples are 50 to 150 km away from these features so the 1786 Ma xenotime ages do not appear to reflect local thermal imprints from plutonism and gneiss dome formation. Several sample sites in Michigan are within the low temperature zones of the Republic metamorphic node where metamorphic monazite has been dated at 1760 ± 5 Ma (Rose and others, 2003). Thus, most of our xenotime ages are significantly older than the Republic metamorphism, which does not appear to have been a significant xenotime-forming event at our sample sites. The geographic extent of the 1786 Ma xenotime growth event suggests it was a basin-wide subtle thermal pulse. We suggest two possible causes for this event. First, all of our age localities lie north of a corridor of gneiss domes and granitic plutons that formed in the interval 1800-1765 Ma, during a period of gravitational collapse of overthickened crust of the Penokean orogen (Schneider and others, 2004). This period of 65 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 gravity-driven tectonism and coincident heating may have driven a northward flow of hydrothermal fluids which subtly but pervasively altered the northern parts of the Penokean foreland and resulted in growth of xenotime. Alternatively the xenotime ages may record very distal effects of events within the Yavapai orogen, which truncated the southern part of the Penokean orogen on the south in central Wisconsin and southeastern Minnesota, about 200 km south of our sample sites. This early geon 17 crust-forming event occurred across the central and southwestern U.S. and may, in some as yet poorly understood manner, have caused widespread subtle heating across a broad foreland on its north. References Rose, S., Schneider, D.A., Loofboro, J., and Holm, D.K., 2003, Results and implications of monazite geochronology from the central Penokean orogen, WI & MI (abs): Geological Society of America Abstracts with Programs, v, 35, no.6, p. 505. Schneider, D.A., Holm, D.K., O’Boyle, C.O., Hamilton, M., and Jercinovic, M., 2004, Paleoproterozoic development of a gneiss dome corridor in the southern Lake Superior region, USA: in Whitney, D.L., Teyssier, C., and Siddoway, C.S., eds., Gneiss domes in orogeny: Geological Society of America Special Paper 380, p. 339-357. Figure 1. Map of the western Lake Superior region showing sample locations in relation to major tectonic and stratigraphic units. Inset shows density functions of xenotime ages divided into older and younger stratigraphic groups. 66 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 SULFUR ISOTOPES FROM PYRITE IN THE NEGAUNEE IRON FORMATION WAGGONER, T.D., Negaunee, MI, USA 49866-1007 In 2003 evidence was presented at the ILSG on hydrothermal venting systems preserved in the sediments older than the Negaunee iron formation. Rare earth elements patterns for the hard ore and vent hematite suggested a commonality for the iron source. During the study it was noted that within the hard iron oxide deposits on the Marquette Range there are numerous occurrences of veins, disseminated and massive sulfides. Pyrite is the common sulfide but both chalcopyrite and bornite can be present. Nine pyrite samples associated with hard ores from geographically diverse locations on the Marquette Range were submitted (Geochron Labs) for sulfur isotope analysis. The isotope ratios were determined by using the Canon Diablo troilite (CDT) standard. The physical relation of the oxides to sulfides indicated either a syndeposition or post replacement of the chert and iron oxides by pyrite. Fig. 1 Location of sulfur isotopes from the Marquette Iron Range Sulfur associated with sedimentary processes reflect the composition of biogenic sulfide produced by bacterial reduction of marine sulfate and is likely to result in δ34S values. Sulfur associated with igneous rocks is isotopically similar to that of meteorites and have δ34S values close to 0%o. Further variations are due to complex and interactive chemistry of the fluids and host. 67 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 The values obtained range from + 1%o to +6.8%o with the mean value of 1.9%o The narrow low positive values would suggest that the sulfur is attributable to a hydrothermal source thus supporting the earlier conclusion based on the REE data. Since a hydrothermal component has been shown to exist for portions of the Brockman (Hagemann et al, 1999) Caue (Rosiere et al, 2004) and Carajas (Guedes et al, 2002), it is logical to project an igneous-hydrothermal source be considered for the formation of BIFs in general. It is further suggested that water deposited BIFs could be a natural end product of hydrothermal IOCG type mineralization. Many features (e.g. age, extensional cratonic or continental margin setting, not easily related to igneous activity, mineral assemblage and alteration patterns) common to Iron Oxide deposits (IOCG) are also common to banded iron formations. The existence of end member BIFs in IOCG deposits (e.g. Pilot Knob, MO and Olympic Dam, South Australia) supports the hypothesis. References Guedes, S.C. et al, 2002, Carbonate Alteration Associated with the Carajas High- Grade Hematite Deposits, Brazil. Proceedings: AusIMM Iron Ore 2002, p. 63-66. Hagemann, S.G. et al, 1999, A Hydrothermal Origin for the Giant BIF-Hosted Tom Price Iron Ore Deposit. In: Stanley et al. (eds), Mineral Deposits: Processes to Processing, Balkema, Rotterdam, p. 41-44. Rosiere, C.A. et al, 2004, The Origin of Hematite in High-Grade Iron Ores Based on Infrared Microscopy and Fluid Inclusion Studies: The Example of the Conceicao Mine, Quadrilatero Ferrifero, Economic Geology v. 90 p. 611-624. 68 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 SOURCE ROCK AGES AND PATTERNS OF SEDIMENTATION IN THE LAKE SUPERIOR REGION: RESULTS OF PRELIMINARY U-PB DETRITAL ZIRCON STUDIES WIRTH, K.R.1, VERVOORT, J.2, CRADDOCK, J.P.1, DAVIDSON, C.3, FINLEY-BLASI, L.3, KERBER, L.4, LUNDQUIST, R.3, VORHIES, S.5, WALKER, E.6 1 Geology Department, Macalester College, St. Paul, MN 55105 (wirth@macalester.edu) 2 Department of Geological Sciences, Washington State University, Pullman, WA 99164 3 Department of Geology, Carleton College, Northfield, MN 55057 4 Geology Department, Pomona College, Claremont, CA 91711 5 Department of Geology, Smith College, Northampton, MA 01063 6 Department of Geology, Allegheny College, Meadville, PA 16335 U-Pb age analysis of detrital zircons provides information about source region ages and patterns of sedimentation. Although most commonly applied to orogenic belts and accreted terranes, this technique also has great potential for illuminating the evolution of cratonic regions. Here we report preliminary results of U-Pb analyses of detrital zircons from Paleoproterozoic (Denham Formation, Pokegama Quartzite, Palms Formation, Rove Formation, Thomson Formation), Neoproterozoic (Puckwunge Sandstone, Nopeming Sandstone, Rift Interflow sediments, Fond du Lac Sandstone, and Hinckley Sandstone), and early Paleozoic (St. Peter Sandstone) rocks from Minnesota and Wisconsin. U-Pb analyses of detrital zircons were conducted using laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) at Washington State University. Approximately 120 grains were analyzed from each sample. All reported ages are 207 Pb/206Pb ages. Only those grains that are <10% discordant, based on comparison of 206 Pb/238U and 207Pb/206Pb ages, are presented. Arkosic conglomerate and quartz arenite of the Denham Formation are the oldest rocks that we examined in this study. The Denham samples contain zircons with ages between 3.6 and 2.1 Ga, however most grains fall into two age ranges: 3.5 – 3.4 Ga and 2.8 – 2.5 Ga. The youngest grains observed in the Denham Formation are 2.07 Ga. Basal Sandstones of the Animikie (Pokegama) and Marquette (Palms) Supergroups in Minnesota and Wisconsin, respectively, contain mostly Neoarchean zircons with similar age distributions (2.9 to 2.6 Ga). Both formations contain scattered grains of Mesoarchean (Pokegama) and Paleoarchean age (Palms), but neither contains grains with ages < 2.6 Ga. Fine-grained sandstones from upper Rove Formation (NE Minnesota), Thomson Formation (E. Central Minnesota), and Tyler Formation (NW WI) were deposited in a migrating foredeep north of the Penokean orogen. Most zircon grains from these three formations have ages between 2.05 and 1.80 Ga (Fig. 1). All three formations also contain some Paleoproterozoic to Paleoarchean grains, but these ages are relatively few in number. The zircon age histograms also lack the major peak at 2.7 Ga that occurs in 69 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 the basal Pokegama and Palms Formations. Zircons from sandstones immediately below basal volcanics of the Keweenawan Midcontinent Rift (1.1 Ga) of east-central (Nopeming Sandstone) and NE Minnesota (Puckwunge Sandstone) have age distributions that are strikingly different. Zircon ages from the Nopeming Sandstone form three groups: 2.8 – 2.5 Ga, 2.1 - 1.8 Ga, and 1.2 - 1.1 Ga (Fig. 1). A few grains also have ages from 3.3 to 2.8 Ga and 2.4 to 2.2 Ga. Puckwunge zircons have a similar age distribution except that no Mesoproterozoic ages (1.2 – 1.1 Ga) are present. Zircons from interflow sediments of the SW limb of the North Shore Volcanic Group have a dominant age peak at 1.15 to 1.0 Ga, and scattered ages in the range of 2.7 to 2.3 Ga. Neoproterozoic Fond du Lac and Hinckley Sandstones were deposited after the main pulse of rift-related magmatism. Fond du Lac zircons have ages that range from 1.5 to 1.0 Ga; a few grains have older ages at 2.9, 2.5, and 1.9 to 1.6 Ga. Hinckley zircons have similar age distributions, but with many more ages from 3.1 – 2.7 Ga and 2.1 – 1. 5 Ga (Fig. 1). Zircons from the Middle Ordovician St. Peter Sandstone have two age populations: 2.8-2.6 Ga and 1.5-1.0 Ga (Fig. 1). Only three grains have ages between 2.5 and 1.5 Ga. Most of the observed zircon ages can be correlated with known source rock ages in the Lake Superior region (Fig. 1). Some ages, however, have no obvious local sources (e.g., 2.5 – 2.1 Ga, 1.6 – 1.5 Ga, and 1.4 – 1.1 Ga) and must have been derived from more distal sources (Van Wyck and Norman, 2004) or from regional sources with unrecognized multicyclic components. In particular, all Neoproterozoic and Paleozoic sediments that we studied have abundant ages between 1.5 and 1.1 Ga that might have been derived from Grenville sources (e.g., Rainbird et al., 1992; Johnson and Winter, 1999). Figure 1. 70 Histograms of 207Pb/206Pb ages from detrital zircons in Thomson Formation, Nopeming Sandstone, Hinckley Sandstone, and St. Peter Sandstone. Shaded bands indicate possible source region ages in Lake Superior Region. �Proceedings of the 52nd ILSG Annual Meeting – Part 1 References Johnson, C.M. and Winter, B.L, 1999, Provenance analysis of Lower Paleozoic cratonic quartz arenites of the northern Midcontinent region: U-Pb and Sm-Nd isotope geochemistry: Geological Society of America Bulletin, v. 111, 1723-1738. Rainbird, R., Heaman, L., and Young, G., 1992, Sampling Laurentia: Detrital zircon geo-chronology offers evidence for an extensive Neoproterozoic river system originating from the Grenville orogen: Geology, v. 20, p.351-354. Van Wyck, N. and Norman, M., 2004, Detrital zircon ages from Early Proterozoic quartzites, Wisconsin, support rapid weathering and deposition of mature quartz arenites: Journal of Geology, v. 112, p. 305315. 71 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Author Index Magee, M.A. McNaugton, N.J. Medaris, L.G. Jr. Miller, J.D. Jr.27, Moran, P. Mudrey, M.G. Jr. Patel, D. Peterson, D.M. Planavsky, N. Porter, R. Rainbird, R.H. Reid, D.D. Rousell, D.H. Schneider, D.A. Schultz, K.J. Severson, M.J. Shapiro, R. Shaw, C.A. Shute, A. Simmons, W.B. Smyk, M.C. Stonier, P. Stott, G. M. Vallini, D. A. Van Schmus, W.R. Vervoort, J. Vorhies, S. Wagonner, T.D. Walker, E. Wirth, K.R. Wyman, D. Anderson, D.K. 27 Andring, M. 35 Bartingale, R J. 1 Bennett, G. 3 Boerboom, T.J. 4, 27 Brown, B.A. 7 Buchholz, T.W. 8 Cannon, W.F. 10, 27, 65 Chandler, V.W. 27, 32 Cote, V. 12 Craddock, J.P. 13, 35, 69 Czechanski, M.L. 7 Davidson, C. 69 Davis, D.W. 23 Davis, W. J. 55 Easton, R.M. 15 Ernst, R.E. 23 Falster, A.U. 8 Finley-Blasi, L. 69 Fralick, P. 17, 42, 49 Franklin, J.M. 37 Grabowski, G. 19 Gross, A. 20 Hailstone, M. 22 Halls, H.C. 23 Heaman, L.M. 61 Heine, J. 30 Heggie, G.J. 37 Hocker-Finamore, S.M. 30 Hollings, P. 25, 42, 49, 59, 61 Holm, D.K. 20, 27, 63 Horton, J.W. Jr. 10 Hudak, G.J. 30 Jirsa, M.A. 27, 32 Juda, N. 35 Karimzadeh Somarin, A. 37 Kerber, L. 69 Kissin, S.A. 37 Knudsen, A. 53 Kring, D.A. 10 Lunquist, R. 69 MacTavish, A. 39 72 41, 42 65 63 44, 46 49 7, 51 13 44, 52 53 13 55 7 57 20, 27 27, 65 46 53 1 59 8 61 63 23 65 27 35, 69 69 67 69 13, 35, 69 25 � https://digitalcollections.lakeheadu.ca/files/original/2494bf3ae2892ac712fe458477742e86.PDF 843d0608ade7fa8cb4884c9ec370ae36 PDF Text Text Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan i Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan t ) ). By: U Ronald P. Sage, PhD Victoria L. Sage, BSc 2006 -- Field Trip Guidebook, Volume 52, Part 2 Institute on Lake Superior Geology ti. Sault Ste. Marie, Ontario PDF compression, OCR, web-optimization with CVISION's PdfCompressor �ii Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan iii Glacial Lakes Algonquin and Glacial Lakes Algonquin and Nipissing Nipissing Shoreline Bedrock Features: Shoreline Bedrock Features: Mackinac Island, Michigan Mackinac Island, Michigan By: Ronald P. Sage Victoria L. Sage 2006 52nd Field Trip Guidebook for the Institute on Lake Superior Geology Sault Ste. Marie, Ontario By: Ronald P. Sage, PhD Victoria L. Sage, BSc 2006 Field Trip Guidebook, Volume 52, Part 2 Institute on Lake Superior Geology Sault Ste. Marie, Ontario On the cover: Lithograph of Arch Rock on east shore of Mackinac Island. See page 16-17. Colorization: V.Sage. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �iv Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan v Acknowledgements This one-day trip is designed to visit all the prominent rock exposures carved by glacial Lakes Algonquin and Nipissing into the limestones exposed on Mackinac Island. The senior author has more than 30 years of geological field experience working in the Canadian Shield. The bedrock exposures on Mackinac Island are of interest to the author as they relate to the island economy. Victoria L. Sage, BSc, has worked in scientific and technical communication for the medical field. She has provided the computer programming skills required to put this guidebook together and has provided editorial assistance with text and guide format. Both authors have worked on Mackinac Island as Guest Service Representatives for Mackinac State Historic Parks. During this period of employment Mr. Greg Hokans, Marketing; Mr. Phil Porter, Museum Programs and Dr. David Armour, Deputy Director offered encouragement and support to the staff working on the island and assisted in locating various published articles describing the features of Mackinac Island. Mr. Steve Brisson, Curator of Collections, provided copies of most of the historical lithographs of rock formations used in this guide that have made the Island famous. Carol R. Sage, MS, has provided editorial assistance in preparing the Guidebook. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �vi Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan vii Table of Contents Introduction.....................................................................................................................................................1 Silurian–Devonian............................................................................................................................................2 Point aux Chenes Formation.................................................................................................................2 Bois Blanc Formation............................................................................................................................2 Detroit River Formation.......................................................................................................................5 Mackinac Breccia.............................................................................................................................................5 Breccia Origin......................................................................................................................................7 Age of Brecciation.................................................................................................................................7 I. Stop Descriptions................................................................................ 13 Stop 1: Robinson’s Folly............................................................ 14 Stop 2: Arch Rock..................................................................... 17 Stop 3: Eagle Point Cave........................................................... 18 Optional.................................................................................... 18 Stop 4: Pulpit Rock................................................................... 19 Stop 5: Chimney Rock.............................................................. 20 Stop 6: Devil’s Kitchen and Lover’s Leap.................................. 21 Stop 7: Sugar Loaf..................................................................... 22 Stop 8: Skull Cave..................................................................... 24 Stop 9: Crack-in-the-Island and Cave-of-the-Woods................... 24 Additional Suggestions.............................................................. 25 iii r jirA iil!!Y..:. In Recent...............................................................................................................................................................8 Upper Algonquin.................................................................................................................................9 Lower Algonquin..................................................................................................................................9 Submerged (Buried) Stream Valley of the Straits of Mackinac.............................................................. 10 Nipissing Shorelines.............................................................................................................................. 11 Post-Nipissing......................................................................................................................................... 11 Lithograph of ramp into Fort Mackinac and “Gibraltar Rock,” which forms the foundation of the Fort above Lake Huron (Allen 1891, p.185). Gibraltar Rock is part of a sea cliff formed by glacial Lake Nipissing. Bibliography.............................................27 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �viii Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan 1 M ackinac Island occurs between Lakes Huron and Michigan and just south of Lake Superior. It lies on the northeast rim of the Michigan Basin, a basin structure that underlies the state of Michigan and portions of the states of Wisconsin, Illinois, Indiana, Ohio and the province of Ontario, Canada (Figures 1 and 2). It is a continental scale basin that is generally poorly exposed except in scattered locations along its rim where the older rocks forming the basin are sometimes exposed. Within central Michigan and in the center of the basin, the rocks exposed along the rim are buried beneath many thousands of feet of younger rocks. Mackinac Island represents one of the best exposures of rocks of Silurian (440-395 million years) and Devonian (395-345 million years) age along the basin perimeter, and these Silurian and Devonian rocks project above the highest water level of the older glacial Lake Algonquin. The effects of coastline erosion are well recorded in the wave cut cliffs and abandoned beaches found on the island. The island offers an excellent opportunity to examine shoreline features related to the glacial Lakes Algonquin and Nipissing formed at the edge of the receding continental ice sheets of 10,000 to 12,000 years ago. Since shoreline features are commonly in unconsolidated Riyer material only the latest events N are recorded in these materi5Mb Boypal Slim Michigan als, the earlier features being SMncn •Mo obliterated by later events. It is SMbb Bno.Bedford SMC ElbwocTh-&a,im anticipated that the actual Mü.ü1t. events are much more complex SMrYc Mtthu than presented here. The SD? Traven. City present island contains a surSüd Thridss SOdr DMrcitR$vw face area of 2,221 acres SDbb Bali Blanc — — (Russell, 1905, p. 56). 0-Sn. MOdSuIc -S — Sand Pdrt St S. — — Oi.ia Erqra&. ajif Sme 0. RId.susid Oc 0? TrSan Cnn £j CrnnbIIOn MILES Figure 1: Bedrock geology of the state of Michigan (Dorr and Eschman, 1970). PDF compression, OCR, web-optimization with CVISION's PdfCompressor �2 Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan FNWQOTU — Figure 2: Outline map of the distribution of Silurian age rocks around the Michigan basin (Dorr and Eschman, 1970, p. 104). The line of section passes very close to Mackinac Island near point A. Silurian – Devonian Mackinac Island is composed of rocks of Silurian and Devonian age. The lower Silurian stratigraphy consists of the Point aux Chenes or Salina formation and the upper Devonian consists of the Bois Blanc and Detroit River formations (Figures 3, 4 and 5). Point aux Chenes formation The Silurian Point aux Chenes formation consists of upper beds of variegated shale and thin brown dolomites, which overlie a lowersalt series that can contain 1600 feet of salt in a number of beds (Landes et al., 1945, p. 159). The salt beds are thin along the northeast and northwest flanks of the Michigan Basin and the upper salt beds have been leached from the formation in upper Michigan ( Landes et al., 1945, p. 159160). Landes et al., (1945, p. 161) report that shale beds commonly separate salt beds in the upper half of the formation and dolomite in the lower half. A basal dolomite occurs everywhere below the lowest salt and, where unleached, the Point aux Chenes formation in northeastern Michigan has a thickness of 1175 to 2886 feet (Landes et al, 1945, p.161). Bois Blanc formation Landes et al., (1945, p. 163) report that the Bois Blanc formation contains Onondaga fossils of lower middle Devonian age. The formation consists of limestone and dolomite and ranges from 165 to 1000 feet in thickness in upper Michigan (Landes et al., 1945, p. 165-166). PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan 3 IEGENO SAEI$?011 lvii. r—I .vp._ L____J —NtS*flI fl SALT SItar Figure 3: Generalized columnar section of the region around the Mackinac Straits (Landes et al., 1945, p. 154). PDF compression, OCR, web-optimization with CVISION's PdfCompressor �4 Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan CLASSIFICATION CF ROCKS IN STRAITS THE MACKINAC STRAITS AREA CITY '0. 4 TO. ii:; ROflRC CITY LIMESTONE CflA N OVIA Iii (it______ DUNDEE LIMESTONE T Figure 4: Generalized stratigraphic column at the Straits of Mackinac showing the distribution of Mackinac breccia across the Silurian and Devonian time periods (Sheldon, 1959, p. 12). It C Cr 0 I- 4 a V Eu U I- C, to -J 0015 BLANC FM. PARI( 400' IS. F GARDEN 250•. ST. IGNACE DOLOMITE C, C z C z 'C U 600 —IC POINTE AUX CHENES 511. S IN INDICATE STRATIC ENGADINE DOLOMITE AR MX I V t et. 7 ci 4- ROGERS CITY ORMAIION GARDEN IS. FORMATION DUNDEE FORMATION St IGNACE DOLOMITE DETROIT RIVER GROUP POINTE AUX CHENES C BLANC FORMATION SHALE MACKINAC BRECCIA Figure 5: Generalized surface geologic map of the Mackinac Straits area that displays the distribution of the Mackinac breccia (Sheldon, 1959, p. 3). PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan 5 Detroit River Formation This formation is not exposed in the area of Mackinac Island. The lower part of the formation consists of limestone and dolomite that is difficult to separate from the Bois Blanc formation (Landes et al., 1945, p. 174). Above the basal limestone of the Detroit River formation, there is an evaporate series consisting of dolomite, anhydrite and salt that in the center of the Michigan Basin may be 1145 feet in thickness (Landes et. al., 1945, p. 174). The evaporate thickness ranges up to 600 feet along the periphery of the basin, but the evaporate series is absent along the rim in the vicinity of Mackinac Island (Landes et al., 1945, p. 174175). Mackinac Breccia Mackinac breccia is the most prominent rock type found on Mackinac Island when one examines the rock formations created by the interaction of glacial Lakes Algonquin and Nipissing. All the rock formations that have received special attention on Mackinac Island are composed of this rock unit. The term “Mackinac breccia” was originally applied by Douglas Houghton, first state geologist for the state of Michigan (Shelden, 1959, p. 19). The Mackinac breccia is an indurated breccia although, in the region of Mackinac Island, non-indurated breccia is the dominate lithology (Landes et al.; 1945, p. 135). Mackinac Island and the St. Ignace peninsula are composed of a non-indurated megabreccia and a small amount of transformational breccia (Landes et al., 1945, p. 134-135). Landes et al. (1945, p. 133) report that the cement to the indurated breccias is carbonate. The indurated Mackinac breccia is easy to recognize in an outcrop by its pock-marked surface that is caused by differential weathering of the breccia fragments. The more soluble fragments are removed leaving large cavities in the breccia units giving rise to the well-developed pock-marked surface. The fragments are angular and up to 10 feet or more in maximum length and consist of a mixture of rock fragments from units higher in the stratigraphy. The resistance to weathering of the indurated breccia suggests that some silica also serves as a cementing agent for the breccia fragments. If all the cement was carbonate, it would be dissolved upon exposure to weathering or solution and the Mackinac breccia would collapse and become indistinguishable from the regional brecciation. Erosion of the regional limestones of Silurian and Devonian age by the glacial Lakes Algonquin and Nipissing has removed the softer, less indurated material enclosing the indurated breccia masses. This has resulted in the prominent rock formations now preserved along the former coastlines of the glacial lakes. Mackinac Island occurs in a broad zone of brecciation found at the northeast corner of the Michigan Basin (Figure 6). Salt decreases in thickness from 1200 feet to 0 feet northwest of Alpena County and north of Cheboygan County, Michigan, and the zone of brecciation corresponds with the disappearance of salt in the stratigraphy. This salt has a blunt edge suggesting this boundary may be a leached rather than a natural, depositional edge (Figure 7) (Landes et. al., 1945, p. 146). PDF compression, OCR, web-optimization with CVISION's PdfCompressor �6 Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan c SR*NO I £ L CO 14* KALKAflA TN*YflSE SUROCE LYIDOIG! OF COLLAPSI OF' .COLLAPSt WITH 0 (VIQCHCC OF Figure 6: Map of the Mackinac Straits area showing the region of solution collapse and Mackinac breccia distribution (Landes et al., 1945, p. 175). Figure 7: Isopach map showing combined thickness of Salina salt (Landes et al., 1945, p. 146). The salt disappears abruptly just southeast of Mackinac Island and outcroppings of Mackinac breccia. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan 7 Breccia Origin The origin of the Mackinac breccia has been discussed by Stanley (1945), Landes et al. (1945), and Shelden (1959). The most complete discussion of the origin is that of Landes et al. (1945), who completed a detailed summary of all previous work in determining breccia development. Landes et al., (1945, p.142) attribute the development of a modern model for the formation of the Mackinac breccias to Mr. Henry H. Hindshaw, former assistant to the state geologist for New York. A brief summary of the model for the formation of the Mackinac breccia is as follows (Landes et al., 1945, p. 143-145) and has been previously summarized by Shelden (1959). The land surface is floored with rocks of Niagara age (Silurian) and was submerged beneath the Pointe aux Chenes Sea. During this time, several hundred feet of shale, dolomite, salt and gypsum were deposited. Post-deposition, emergence followed along the rim of the Michigan Basin and percolating ground waters leached salt from the rim of the salt-bearing rocks. Caves were produced when the salt in the Pointe aux Chenes was removed from rocks lying above the ground water table. The solution of the salt created caverns that became unstable and collapsed. There were two types of collapse: regional and local. The collapse was probably sudden and the overlying rocks broke into angular fragments of all sizes. This probably created localized sink holes above the collapse, and the larger areas of regionalized collapse created tilted stratigraphy in the region. The period of collapse took place during emergence following Detroit River deposition and preceding Dundee deposition. This collapse took place over a time interval rather than at a specific time. Shelden (1959, p. 23) reports slickensides and normal faults within larger blocks of collapsed rock. The Mackinac breccias occur in columns, and some clasts may represent down drop of 600 to 750 feet (Landes et al., 1945, p. 129). After collapse, erosion of the surface developed a peneplane and surface irregularities (such as sinkholes) were filled. The limestones of the Dundee formation were deposited on this peneplane surface. Percolating ground waters gradually cemented the collapse breccias. Recent emergence has brought the rocks to surface where differential erosion has completed sculpting the breccias into the forms observed on the Island. Age of brecciation Landes et al., (1945, p.137) interpret the age brecciation as likely post-Detroit and pre-Dundee. The brecciation likely took place over an extended period of time. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �8 Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan Recent Mackinac Island has been affected by recent glaciation of the Wisconsin period and glacial Lakes Algonquin and Nipissing following retreat of the glaciers. Glacial Lake Algonquin represents an age of 11,000 to 12,000 years ago, and glacial Lake Nipissing represents an age of approximately 4,000 years ago. Viewing of the island from St. Ignace and Mackinaw City presents a profile of an island with a hump in the middle with two relatively flat planes separated by a steep slope (Figures 19 and 20). The present mean level of Lake Huron is 580.37 feet above sea level and the highest point on the island is at Fort Holmes, 900.5 to 904.1 feet above sea level (Stanley, 1945, p. 65, 72-73). The highest level reached by glacial Lake Algonquin is 809 feet above sea level (Taylor, 1915, p. 69), which is the highest peneplane observed in the profile of the island. This former high water level is approximately 229 feet above the present lake level (Taylor, 1915, p. 69). That portion of Mackinac Island lying above this high water mark is referred to as the “Ancient Island”. The “Ancient Island” part of Mackinac Island was glaciated during the Wisconsin period and a few glaciated boulders, cobbles and pebbles displaying glacial striations can be found in the area (Stanley, 1945, p. 13). The ice moved in a southwest direction and left a thin covering of glacial till on the “Ancient Island” (Stanley, 1945, p. 12-13). Glacial ice retreated northeastward in a series of retreats and advances with the retreats exceeding the advances (Landes et al., 1945, p. 10). The process of retreat-and-advance by glacial ice will destroy evidence left in unconsolidated materials from earlier (older) retreats and advances and only the last event is preserved in the geological record. _____________________ U sot SOUTH URE HURON ISO ALSn#01 LIWL soJJlHt4sr NAUlNO LEVEL LAKE NUAON 550 Figure 8: Profile across Mackinac Island showing former shore levels looking east from St. Ignace (Stanley, 1945, p. 22). Glacial LoRe Nipissiog water plane Ancient Island" I Fort Holmes atop GIOCICI Luk. àiqooQuir. wave cut cliff w°ve Lake Nipássing wove duff Glacial UII4 AIUOMU1O watt, PWM Glacial LaKe Nipissing woterplone I I I MdernIoke$evht I Figure 9: Profile of Mackinac Island looking eastward from St. Igance with the various glacial lake levels noted (Dorr and Eschman, 1970, p. 177). PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan Upper Algonquin The Lake Algonquin shoreline is a well-developed major feature in the region of the Straits of Mackinac (Landes et al., 1945, p. 11) (Figures 10 and 11). Landes et al. (1945, p. 11-12) indicate that drainage from this glacial lake was eastward towards North Bay and Mattawa, Ontario. But, as the ice sheet withdrew, isostatic rebound caused uplift to the northeast until the waters overflowed to the south at Port Huron and Chicago. This is a three-stage outlet for Lake Algonquin. As isostatic uplift continued in response to the retreating ice sheets, the lake level was stabilized by the outflow through Port Huron and Chicago and the eastward drainage abandoned (two stage outlet) (Landes et al., 1945, p. 12). As uplift continued, the lake level continued to recede and gradually erosion along the Port Huron drainage caused the abandonment of the Chicago outlet (Landes et al., 1945, p. 12). Beach lines associated with the Upper Algonquin of glacial Lake Algonquin lie between 799 and 759 feet above sea level and the type section is the Short Rifle Range lying between Fort Mackinac and Fort Holmes (Stanley, 1945, p. 31-32). Taylor (1915, p. 69) reports that the highest beach level is 809 feet above sea level or approximately 229 feet above the present level of Lake Huron. Glacial straie have not been observed on glacial boulders below the level of 205 feet above lake level (Taylor, 1892, p. 212-213). 9 1' Figure 10: High water level for glacial Lake Algonquin approximately 11,000 years ago. This illustration indicates most of the region was under water. This view is oriented north looking south (Porter and Nelhiebel, 1984, p. 11). Lower Algonquin Below the Upper Algonquin is a zone relatively free of beach development, which is thought to represent a urrLa Iauw SNORE period of relatively rapid falling of the lake level 10.000 Years Ago 116 Abow tflc flunn todsy (Stanley, 1945, p.32). This zone lies between approximately 762 and 635 feet above sea level (Stanley, 1945, Figure 11: Upper glacial Lake Algonquin shoreline approximately 10,000 years ago (Porter and Nelhiebel, 1984, p. 47). p. 32-36). The best-described beach line in the Lower Algonquin is the “Battlefield Beach” at an elevation of 718 feet above present sea level located in the north central part of the island (Stanley, 1945, p. 33-34; Landes et al., 1945, p. 13). Stanley (1945, p. 35-36) and Landes et al. (1945, p. 13-14) describe other locations as examples of beaches within the Lower Algonquin. The Lower Algonquin beaches are characteristically less well developed than the Upper Algonquin. The quick lowering of the lake level is in response to the opening of discharge channels to the east as the ice sheet retreats (Landes et al., 1945, p. 13). The lowering probably extended to much lower levels than present lake levels, and a buried river valley between PDF compression, OCR, web-optimization with CVISION's PdfCompressor �10 Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan Lake Huron and Lake Michigan was probably reactivated (Landes et al., 1945, p. 14). At this low water stage the present islands within the Straits did not exist but were part of what is now the mainland. Landes et al. (1945, p. 14) indicate that the lake levels ceased falling when the lowest outlet from the Huron Basin, the Mattawa Valley (east of North Bay), was freed of ice and unobstructed flow eastward was established. The removal of glacial obstruction left only isostatic rebound as a mechanism of changing lake water flow. The Mattawa Valley is located to the east and northeast of Mackinac Island where this continental uplift would be most effective (Landes et al., 1945, p.1 4). Continental uplift to the east generated continually rising lake levels until southern flow was reestablished, and then eastward flow through the Mattawa Valley was terminated (Landes et al., 1945, p. 14). As the lake waters rose, topographic features created in the unconsolidated glacial deposits as the lake level dropped were obliterated or buried beneath the rising waters. Submerged (buried) stream valley of the Straits of Mackinac The presence of a submerged valley in the Straits of Mackinac was disclosed by soundings between 1918 and 1924 (Stanley, 1938, p. 966) (Figure 12). This valley loops around Mackinac Island to the north and is likely the result of Pleistocene or earlier erosion (Stanley, 1938, p. 966, 974). This valley was likely filled with glacial drift during the Wisconsin ice advance and cleaned out during the low water level between the glacial Lakes Algonquin and Nipissing (Stanley, 1938, p. 974). Stanley (1938, p. 966) and Sheldon (1959 p. 59) suggest flow along this valley was towards to the east. This valley lies 150 to 250 feet below the present level of Lake Huron with the greatest depths being recorded through the Straits near Mackinaw City (Stanley, 1938, p. 968). Stanley (1938, p. 968) reports the greatest depth to be 289 feet below present lake level. This valley exceeds 70 miles in length (Stanley, 1938, p. 966). LAKE M'GHIGAN 7 Figure 12: Submerged valley through the Straits of Mackinac (Stanley, 1938, p. 967). At this time, the water level of Lake Huron was approximately 120 feet lower than it is today. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan 11 Nipissing Shorelines As isostatic rebound continued, the level of Lake Huron returned to a Scott's Cavg level 50 feet above its present level Croglian Water (Stanley, 1945, p. 39) (Figure 13). Stanley (1945, p. 39) suggests the process of uplift and higher water levels took place slowly over a period of several thousand years and the previous geological features in unconsolidated glacial material were de- Devil's stroyed. Nipissing beaches reached elevations of 629 to 635 feet above sea level (Stanley, 1945, p. 42). At Robinson's rally abmt Laict Huron the time of Nipissing beach developlakE Huron rreseifl ShorelIne ment, 85% of the isostatic rebound had been completed (Shelden, 1959, Figure 13: Modified oblique view of Mackinac Island showing the shoreline of glacial Lake Nipissing with scenic rock formations noted (Porter and Nelhiebel, 1984, p. 13). Friendship p. 59; Landes et al., 1945, p. 14). Nipissing beaches do not completely Altar and Pulpit Rock are the same feature. This shoreline is approximately 4,000 years old. enclose Mackinac Island but obliquely transect the lower Algonquin beaches (Stanley, 1945, p.39, 41). Nipissing beaches are preserved on the northwest and southeast portions of Mackinac Island; the City of Mackinac is built on Nipissing beaches (Landes et al., 1945, p. 15; Stanley, 1945, p. 44; Leverett and Taylor, 1915, p. 452). Post-Nipissing During post-Nipissing time, Lake Huron drains south past Port Huron and is approximately 55 feet lower than the highest level of glacial Lake Nipissing (Sheldon, 1959, p. 63). PDF compression, OCR, web-optimization with CVISION's PdfCompressor �12 Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan 13 Stop Descriptions T he Mackinac Island field trip is a one-day trip that is designed to visit all the major bedrock outcrops that contribute to the scenery of the island. These outcrops have been described in previous technical and non-technical literature for more than 150 years. These scenic bedrock exposures have contributed much to the island economy over the years. Figure 14 is a simple sketch of Mackinac Island showing the location of the sites on this tour and those geographic features mentioned in this text. The island’s major bedrock outcrops are the direct product of the interaction between glacial lakes and bedrock during the retreat of the Wisconsin ice sheets. Upon arrival, one disembarks on the south side of Mackinac Island where the glacial Lake Nipissing beach deposits are best developed. Fort Mackinac is built on the Lake Nipissing sea cliffs. Below this cliff lie the deposits of glacial Lake Nipissing. These beach deposits are largely obscured in most areas by subsequent construction. The sea cliff below Fort Mackinac has a prominent rock formation called Gibraltar Rock that projects out from the cliff but is not detached. Continued erosion around this prominent rock could have caused its detachment from the sea cliff to create a sea stack (see the Table of Contents for a lithograph of Gibraltar Rock). The cliff face on which Fort Mackinac is built is fenced off from the public and, if time permits, a closer examination can be made after completion of the field trip. The field trip consists of two parts. The first part is a trip around the island on Shoreline Road to examine the erosive activities of glacial Lake Nipissing. The second part is an interior tour of the island to examine the erosive activities of glacial Lake Algonquin. Cave Cake For Fairy Arch hore Former ScoWs Cave Foot N CQve cc the Woods N A 1000 Feet & trn]k HerLot Kitchen Rock Figure 14: Sketch map indicating the sites of rock outcroppings to visit and those geographic features needed to locate them. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �14 Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan Stop1: Robinson’s Folly Moving eastward along Shoreline Road (through the settlement) approximately one mile from the landing on Mackinac Island, the first outcrop one encounters is a prominent cliff face of Mackinac breccia known as Robinson’s Folly (Figure 15). This prominent headland is reportedly named after Captain Daniel Robertson of the British 84th Regiment who supposedly built a summer house on the promontory, which subsequently collapsed into Lake Huron (Wood, 1918, p. 584-585). The name “Robinson” is a corruption from the French addressing him as “Robinçon” (Wood, 1918, p. 584-585). Meade (1986 [1897], p. 165-170) presents five stories on the naming of Robinson’s Folly. The reader should refer to Meade’s lengthy discussion of the name if interested in the origin. Robinson’s Folly is 127 feet (Van Fleet, 1970, p. 147; 1882, p. 24) or 128 feet (Winchell, 1861, p. 210) above the present level of Lake Huron. It represents a prominent portion of the sea cliffs formed along the eastern side of Mackinac Island through the erosive action of glacial Lake Nipissing. A short distance north of Robinson’s Folly is a second outcropping of breccia that was the former site of a sea arch known as Fairy Arch (Figure 16). This arch was destroyed during road construction around the island, but a photograph was published by an anonymous source (1899, p. 5). Ak Figure 15: Lithograph of Robinson’s Folly, a glacial Lake Nipissing sea cliff (Woolson, 1894, p. 289). PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan 15 fr Figure 16: Lithograph of Fairy Arch, a glacial Lake Nipissing shoreline feature (Woolson, 1894, p. 285). This arch was destroyed during road building along the east side of Mackinac Island. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan .1 16 Figure 17: Lithograph of Arch Rock from the shore of Lake Huron (Disturnell, 1875). This is a glacial Lake Nipissing feature. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan 17 Stop 2: Arch Rock Arch Rock is located on the east side of Mackinac Island and is the best known rock formation on the island (Figures 17 and 18). The limestone arch is butted against seacliffs of glacial Lake Nipissing to the north and a promontory of Mackinac breccia that projects out from the cliff face but remains attached. The top of the arch is approximately 140 feet above Lake Huron and the top of the buttress that projects out from the cliff is 105 feet above present-day Lake Huron (Winchell, 1861, p. 210; Van Fleet, 1970, p. 147; 1882, p. 24). Arch Rock has a span of 40 to 50 feet (Van Fleet, 1882, p. 20). Figure 19: Lithograph looking down through Arch Rock toward Lake Huron. This view is from the top of the glacial Lake Nipissing sea cliffs (Woolson, 1894, p. 281). Arch Rock is composed of highly fractured limestone. Many of the fractures have been sealed with man-made cement. The northern abutment has been reinforced with man-made cement and rock. In the spring, loose fragments that have fallen from the arch commonly lie below the structure. The highly fractured appearance of the limestone prompted McKenny (1959 [1827], p. 390) and Foster and Whitney (1851, p. 163) to predict a very short life for the arch. In terms of geologic time, this will likely be true. I At the base of the promontory which forms the south abutment to Arch Rock, there is a small arch known as the Sannillac Arch. This arch is named after an Indian warrior named Sannillac, who is the subject of a poem by Henry Whiting that was published in 1831 (Wood, 1918, p. 588-589). In front of Arch Rock on the lake side of the shoreline road there is a large boulder of Mackinac breccia that has broken free of the promontory. This boulder has been referred to as Gitchie Manitou (Stanley, 1945, p. 51). Figure 18: Lithograph of Arch Rock in the moonlight looking up from Lake Huron through the arch that forms part of the glacial Lake Nipissing sea cliffs (Woolson, 1894, p. 279). PDF compression, OCR, web-optimization with CVISION's PdfCompressor �18 Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan Stop 3: Eagle Point Cave Eagle Point Cave is located approximately two miles northwest beyond Arch Rock. Take Lake Shore Road to Scott’s Shore Road; turn inland approximately 500 feet to Scott’s Road. The cave is situated approximately 1000 feet south of the junction of Scott’s Shore Road and Scott’s Road on the west side of the Scott’s Road. The site of this cave was once used for some unknown purpose. The remains of concrete steps are found in front of the cave and the floor of the cave has been leveled using logs and dirt fill. Eagle Point Cave occurs in Mackinac breccia and is represented by a large amphitheater-type opening in the breccia. Eagle Point Cave is typical of most of the shoreline features described on the island as caves. These caves are commonly amphitheater-type openings of very limited depth that have resulted from lakeshore weathering-erosion processes. Eagle Point Cave is a glacial Lake Nipissing shoreline feature. Optional After the visit to Eagle Point Cave one can return to the junction of Scott’s Shore Road and Scott’s Road and continue north for 200-300 feet where approximately 100-150 feet west of the road is a small promontory of bedrock in which the former Scott’s Cave was displayed. This cave has either caved-in or become filled-in and relatively little remains to view. The site warrants some restoration effort. Stanley (1945, p. 47) published a good photograph of the cave when it was exposed for viewing. This cave occurred in the same glacial Lake Nipissing shoreline bluff as Eagle Point Cave. One can walk a trail along the top of the bluff from one cave to the other. Scott’s Cave was named after Captain Thomas Scott of the British 53rd Regiment who was stationed at Fort Mackinac in 1787 (Wood, 1918, p.591). Scott’s Road occurs along a flat land surface representing the action of glacial Lake Nipissing. Scott’s Road continues around the northern portion of Mackinac Island but remains relatively unscenic. Return to the Lake Shore Road and continue north around the island to British Landing. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan 19 Stop 4: Pulpit Rock Figure 20: Pulpit Rock, a glacial Lake Nipissing shoreline feature on the northwest corner of Mackinac Island. This sea stack occurs 50 to 75 feet west of the wave-cut bench, representing beach erosion of unconsolidated material of glacial Lake Nipissing (Allen, 1891). Pulpit Rock (Figure 7), at present better known as Friendship Altar, is located a short distance northeast of British Landing along the western end of Scott’s Road. The vertical standing rock formation consists of Mackinac breccia standing approximately 10 feet in front of the bluff formed by glacial Lake Nipissing. While the most commonly used name for this rock unit is Friendship Altar, the outcrop is vertical standing like a pulpit and not horizontal lying as an altar would be. The author prefers the term “Pulpit Rock” because it best describes the outcrop shape or form. Wood (1918, p.536-537) mentions that both names, Pulpit Rock and Friendship Altar, have been applied to this outcropping near British Landing. But, he also suggests the term “Pulpit Rock” may have been applied to another exposure known as Vista Rock in the area of Sugar Loaf. Vista Rock, as indicated on the map of Wood (1918), is a poorly exposed outcrop that does not resemble a pulpit and, perhaps, is an outcrop of no particular note. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �20 Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan Stop 5: Chimney Rock Continuing along the shoreline road for approximately two miles one passes numerous raised beaches of former glacial Lake Nipissing and of present-day Lake Huron (Figure 21). Bluffs and sea cliffs of the former glacial Lake Nipissing begin to appear south of Heriot Point (Wood, 1918). A prominent vertical rock formation, sometimes referred to as Sunset Rock, occurs approximately ¼ mile from where these bluffs begin to appear along the shoreline road. The name “Chimney Rock” is the original name of this shoreline exposure and the name “Sunset Rock” appears to be of recent origin, perhaps generated by whomever built the platform on the top of the formation. The name “Sunset Rock” appears on this platform outlined by dark pebbles set in concrete used in building the structure. The author uses the term “Chimney Rock” since it is of historical usage and is a very good description of the appearance of the exposure. This feature has only recently become part of the Mackinac State Historical Park holdings and accessible to the public. This was largely due to the efforts of David Armour, PhD, former deputy director of Mackinac State Historic Parks. Van Fleet (1870, p. 144; 1882, p. 24) and Wood (1918, p. 521) have cited Alexander Winchell, professor of geology at the University of MichiFigure 21: Lithograph of Chimney Rock, a glacial Lake Nipissing shoreline gan (Winchell, 1870) and a former state geologist feature. The viewing platform that has been constructed on top of this formation may have damaged the original profile as shown in this lithograph for the State of Michigan (Winchell, 1861), as de- (Woolson, 1894, p. 283). scribing this rock exposure as “one of the most remarkable masses of rock in this or any other state”. The author has failed to identify the original source for this comment. Chimney Rock is a promontory of Mackinac breccia developed by selective erosion and removal of less resistant limestone enveloping the more indurated breccia forming the vertical column of rock making up Chimney Rock by glacial Lake Nipissing. The breccia column remains attached to the headlands, so Chimney Rock is not a sea stack. The top of Chimney Rock was originally 131 feet above the level of Lake Huron (Van Fleet, 1870, p. 147, 1882, p. 24; Winchell, 1861, p. 210). It is unknown whether those that built the platform on Chimney Rock vandalized the upper portion of the chimney-like rock outcrop thereby altering the original profile. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan 21 Stop 6: Devil’s Kitchen and Lover’s Leap 1 Continuing along the shoreline road for ½ to ¾ mile, one comes to the Devil’s Kitchen. Devil’s Kitchen represents an amphitheaterlike opening in a large outcrop of Mackinac breccia. This feature is the product of recent interaction of the shoreline of Mackinac Island and the waters of Lake Huron. Earlier erosion by glacial Lake Nipissing may have also had an influence. 4 3: I IL a w Lover’s Leap occurs on the wave-cut bluff behind and slightly west of Devil’s Kitchen. It remains private property and, thus, is unavailable for examination (Figure 22). Lover’s Leap is composed of Mackinac breccia and is comparable to Chimney Rock in appearance and origin. The top of Lover’s Leap is 145 feet above the present level of Lake Huron (Van Fleet, 1870, p. 147; 1882, p. 24; Winchell, 1861, p. 210). - Figure 22: Lithograph of Lover’s Leap, a glacial Lake Nipissing shoreline feature (Woolson, 1894, p. 288). I nner Island Tour Upon completion of the visit at Devil’s Kitchen, one continues back towards the village on Mackinac Island. As we approach the village, turn left onto Mahoney Ave. and continue to Cadotte Ave. Turn left on Cadotte Ave. and travel to Huron Road passed the Grand Hotel. Turn right on Huron Road and continue to the eastern side of Mackinac Island to continue an inner island tour dominated by features related to glacial Lake Algonguin. Huron Road passes along the top of east bluff that represents the effects of glacial Lake Nipissing. Along Huron Road, the first stop will be Robinson’s Folly (Stop 1) where participants can view Lake Huron from the top of the rock formation. From Robinson’s Folly, the group will continue to Arch Rock (Stop 2). The revisit of Stop 2 allows participants to view Arch Rock from above. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �22 Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan Stop 7: Sugar Loaf From the revisit of Arch Rock, one continues into the interior of Mackinac Island along Rifle Range Road to Sugar Loaf Road (Figures 23 and 24). Continue along Sugar Loaf Road to Sugar Loaf. Sugar Loaf is a sea stack that is the product of glacial Lake Algonquin. This sea stack is composed of Mackinac breccia and is separated from the headlands of the former glacial Lake Algonquin by approximately 300 feet. The sea cliffs to the west are composed of breccia. The wooden stairs from the former terrace of glacial Lake Algonquin allow great access to the cliffs where the details of the breccia can be closely examined. The stairs continue to the top of the sea cliff known as Point Lookout where an excellent overall view of Sugar Loaf and Lake Huron is possible. The Lake Algonquin terrace around the Sugar Loaf is approximately 140-150 feet above the present Lake Huron. The top of the Sugar Loaf is 855.81 feet above sea level or 274.94 feet above the present level of Lake Huron (Stanley, 1945, p. 65, 72). The base of Sugar Loaf is approximately 134 feet above Lake Huron (Van Fleet, 1870, p. 20-21; 1882, p. 140). Wood (1918, p. 594-595) indicates the Sugar Loaf is 79 feet high using the road as a base. The conical shaped sea stack reportedly received its name from honey bees that constructed a hive in the formation (Wood, 1918, p. 595). t Figure 23: Lithograph of Sugar Loaf, a glacial Lake Algonquin shoreline feature. View is of the west side (Woolson, 1894, p. 287). PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan 23 Figure 24: Lithograph of Sugar Loaf, a glacial Lake Algonquin shoreline feature. View is of the east side (Woolson, 1894, p. 286). PDF compression, OCR, web-optimization with CVISION's PdfCompressor �24 Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan Stop 7: Skull Cave From Sugar Loaf, we continue along Sugar Loaf Road to Rifle Range Road and east to the junction of Rifle Range Road and Garrison Road. Skull Cave is a crumbling sea stack that has a large amphitheater-like opening. This feature is composed of Mackinac breccia and is the result of glacial Lake Algonquin. The sea stack has a summit of 831 feet above sea level, is between 15 and 25 feet high, and is separated from the headland of the former Lake Algonquin sea cliff and bluff by 50 feet (Stanley, 1945, p.26-27). Skull Cave is named after an experience by Alexander Henry, a fur trader who survived the massacre in 1763 of the British garrison at Fort Michilmackinac in Mackinaw City. Henry was sequestered in this cave by Wawatam, a Chippewa Indian, thus saving him from massacre at the Fort. Henry found the cave full of human bones and skulls and was very happy to leave his place of refuge at his earliest opportunity. It is unknown whether the bones are the result of burial or ceremonial sacrifice. Wood (1918, p. 592) stated that Alexander Henry was of the opinion that the bones were from prisoners devoured at war feasts, but there is no evidence as to which interpretation is most probable. Slight variations in this story occur in almost all discussions of Skull Cave. Stop 8: Crack-in-the-Island and Cave-of-the-Woods From Skull Cave, continue north along Garrison Road to State Road and turn left. Continue along this State Road until you reach Island Trail that leads from the road to the west. This trail is accessible with a trail bike but is very rough; caution is recommended. Crack-in-the-Island and Cave-of-the-Woods lie approximately ¼ mile west of State Road and at the edge of Mackinac Island Airport. They are separated by only a few tens of feet. Crack-in-the-Island is a solution crack in the limestone that, at this location, is on the order of one foot wide and 1-2 feet deep. Wood (1918, p. 523) described the crack as a deep fissure several feet wide, but the crack as seen today does not fit that description. Wood (1918) indicates these solution cracks occur in several other places on the island. Cave-in-the Woods is an amphitheater-shaped opening or sea cave in Mackinac breccia formed by glacial Lake Algonquin that sits approximately 140 feet above the level of present-day Lake Huron (Porter and Nelhiebel, 1984, p. 47). Upon completion of the examination of this site, return along the trail to State Road. Even though it is longer in distance, the easiest way back to town and the ferry docks is to continue north along State Road, which rejoins with Garrison Road and continues on towards British Landing. This route is down hill through the center of the island and, then, relatively flat along the perimeter of the island, Lake Shore Road. At British Landing, turn left on to Lake Shore Road and return to the ferry docks, a distance of several miles that passes previous stops illustrating glacial Lake Nipissing shore features. If time permits, it is suggested to tour Fort Mackinac, a Revolutionary War fort. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan 25 Additional Suggestions The tour of Mackinac Island is designed to visit all prominent glacial-lake modified bedrock outcroppings on the island in a single day. At the visitor’s discretion and convenience, some outcroppings in and near the city of St. Ignace on the mainland of the Upper Peninsula are worth visiting. St. Anthony’s Rock occurs in a parkette behind the business establishments lining the main street of the city. This is a sea stack of Mackinac breccia formed by glacial Lake Nipissing. It sits several yards in front of a Lake Nipissing cut headland. Castle Rock located just north of St. Ignace is a commercial property that permits tourists of the area to obtain a view from its high vantage point. Castle Rock is a prominent promontory that remains attached to the glacial Lake Nipissing headlands and is, thus, not a true sea stack. Landes et. al. (1945, p. 136) classify Castle Rock as being an indurated transformational breccia, which lies immediately east of megabreccia that is regional in extent. Landes et al. (1945, p. 125) report that the best place to see the various styles of regional brecciation is from the road cut leading to the north entrance of the Mackinac Bridge. Landes et. al. (1945) provides the most complete description of the geology of Mackinac Island and the surrounding area. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �26 Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan 27 Bibliography Allen, E.A.; 1891. A Jolly trip; or, where we went and what we saw last summer; Central Publishing House, Atlanta, Georgia, 266p. Anonymous, 1899. The Standard Guide, Mackinac Island and Northern Lake Resorts; Foster and Reynolds, 88 p. Disturnell, J., 1875. Island of Mackinac; Philadelphia, 96 p. Dorr Jr., John A. and Eschman, Donald F., 1970. p. 176-217; in Geology of Michigan; University of Michigan Press, Ann Arbor, Michigan; 475 p. Foster, J. W. and Whitney, J.D., 1851, p. 163-166; in Report on the Geology of Lake Superior Land district; A. Boyd Hamilton, Washington D.C., 456p. with illustrations Landes, K. K., Ehlers, G. M. and Stanley, G. M., 1945. Geology of the Mackinac Straits Region and subsurface geology of Northern southern Peninsula; State of Michigan Department of Conservation, Geological Survey Division; pub. 44, Geological Serial 37, 204 p. Leverett, F. and Taylor, F.B., 1915. p. 452-453; in The Pleistocene of Indiana and Michigan and the History of the Great Lakes; United States Geological Survey Monograph, v. 53 McKenny, Thomas L., 1827. Sketches of a tour to the Lakes; Ross and Haines Inc., Minneapolis, Minnesota, 1959, 493 p. Meade, William C., 1897[1986]. Early Mackinac. A sketch Historical and Descriptive with Introductory essay by Larry Massie; Republished 1986 by Avery Color Studios, Au Train, Michigan, 184 p. NOTE: Early Mackinac was first published in 1897 and then revised and published in 1901 and 1912 Porter, Phil and Nelhiebel, Victor, R., 1984. The Wonder of Mackinac, Mackinac Island State Park Commission, Pendall Printing Inc., 52 p. Russell, Israel C., 1905, p. 44-45, 55-57, 102-104; in A geological reconnaissance along the north shore of Lakes Huron and Michigan; Report of the State Board of Geological Survey of Michigan for the year 1904, Wynkoop Hallenbeck Crawford Co., Lansing, Michigan Sheldon, Frances D., 1959. Geology of Mackinac Island and lower and middle Devonian south of the Straits of Mackinac, Michigan; Michigan Basin Geological Society Guide Book; 63 p. Stanley, G.M., 1938. The submerged Valley through Mackinac Straits; Journal of Geology, v. 46, n. 7, p. 966-974 Stanley, George M., 1945. Pre-Historic Mackinac Island; State of Michigan Department of Conservation, Geological Survey Division, Publication 43, Geological Series 36, 74p. Taylor, F.B., 1892. The highest old shoreline on Mackinac Island; American Journal of Science, v. 43, p. 210-218 Taylor, Frank B., 1915. Old shorelines of Mackinac Island and their relations to the Lake History; Geological Society America Bulletin (abst), v. 26, p. 68-70 Van Fleet, J. A., 1870. Old and New Mackinac; Courier Steam Printing-house, Ann Arbor; 176 p. Van Fleet, J. A.; 1882. Mackinaw Region and Adjacent Localities; Lever Print, Detroit, Michigan, 49 p. Winchell, A., 1861. First Biennial Report of the Progress of the Geological Survey of Michigan Geology, Zoology and Botany of the lower Peninsula; Geological survey of Michigan, 339 p. Winchell, Alexander, 1870. p. 247-245; in Sketches of Creation; Harper and Brothers, New York, 459 p. Wood, Edwin O., 1918. Historic Mackinac; The Historical, Picturesque and Legendary Features of the Mackinac Country; The MacMillan Company, New York, 540 p. Woolson, Constance F., 1894. Mackinac, p. 279-291; in Picturesque America, No. 5, March 17, 1894, (previously published 1872), Artist F. T. Woodward; D. Appleton, Publishers PDF compression, OCR, web-optimization with CVISION's PdfCompressor �28 Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan References not cited Schoolcraft, Henry R. 1832 [1953]. Narrative Journal of travels through the northwestern regions of the United States extending from Detroit through the great chain of American lakes to the sources of the Mississippi River in the year 1820; Republished Michigan State College Press, 1953, edited by Mentor L. Williams, 520 p. Strickland, W.P. 1860. Old Mackinac or the Fortress of the Lakes; James Challen and Son, Philadelphia, 404 p. PDF compression, OCR, web-optimization with CVISION's PdfCompressor � https://digitalcollections.lakeheadu.ca/files/original/0c4e5c4f4cf1d00d709139b4dc0e22f7.pdf 28c0b52caa344bdb859324c05ee86abc PDF Text Text Unusual -bearing Rocks of Unusual Diamond Diamond-bearing the Wawa Area - - . : - £- a — L . Institute on Lake Superior Geology 52nd Annual Meeting Sault Ste Marie, Ontario Volume Part 3 – Field Trip Guidebook PDF compression, OCR,52web-optimization with CVISION's PdfCompressor �Unusual Archean Diamond-bearing rocks of the Wawa Area by A. C. Wilson Ministry of Northern Development and Mines, Resident Geologist’s Program, Ontario Geological Survey, Timmins, Ontario On the cover (clockwise from top): Giant lower crustal to upper mantle xenoliths, Enigma Property, Menzies Township, Oasis Diamond Corporation Inc.; Diamonds from the Festival Property (photo courtesy of Pele Mountain Resources Inc.); Sandor Diamond Occurrence, Highway 17, Spider Resources Inc. & KWG Resources Inc.; Heterolithic diamond-bearing breccia, Engagement Zone, Northern Sierra Minerals Corporation PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Table of Contents Introduction .................................................................................................................................................... 3 Geological Overview of the Michipicoten Greenstone Belt........................................................................... 3 Quaternary Geology ....................................................................................................................................... 6 Structural Geology of the Diamond-bearing Rocks ....................................................................................... 6 Description of the Diamond-bearing Rocks ................................................................................................... 7 Relationship between diamond content and lithology .................................................................................. 10 Geochronology ............................................................................................................................................. 11 Geochemistry of the Diamond-bearing Rocks ............................................................................................. 11 Diamond characteristics ............................................................................................................................... 11 Origin of the Diamond Deposits................................................................................................................... 12 Field Trip Road Log ..................................................................................................................................... 14 STOP 1 - GQ Diamond Discovery Site ................................................................................................... 14 STOP 2 - Northern Sierra Minerals Corporation Area B......................................................................... 17 STOP 3 - Northern Sierra Minerals Corporation Engagement Zone ....................................................... 21 STOP 4: - Moet Occurrence, Festival Property ....................................................................................... 23 STOP 5: - Sandor Diamond Occurrence.................................................................................................. 25 STOP 6: - Dubreuilville Dike - Xenolith-rich lamprophyre .................................................................... 27 STOP 7: - Monchiquite Dike ................................................................................................................... 28 STOP 8: - Contemplation of the rocks on the fireplace at the Wawa Motor Hotel ................................. 28 Bibliography................................................................................................................................................. 29 Figures 1. Generalized geological map of the Michpicoten greenstone belt ............................................................. 4 2. Composite structural section through the central part of the Michipicoten greenstone belt. .................... 7 3. Detailed geology of the southwestern corner of the Festival Property ..................................................... 8 4. Geological compilation of the GQ Property ........................................................................................... 15 5. Occurrences of diamondiferous bedrock on the GQ Property................................................................ 16 6. Drill hole sections GQ-00-01, GQ-00-02 and GQ-00-03, GQ Property.................................................. 18 7. Northern Sierra Minerals Corporation Area B ........................................................................................ 20 8. Geology and sample locations at the Engagement Zone ......................................................................... 22 9. Simplified cross section through the Engagement Zone looking northwest........................................... 23 10. Detailed geology of the Moet Occurrence.............................................................................................. 24 10. Detailed geology of the Sandor Occurrence ........................................................................................... 26 11. Generalized geology of the Wawa Project ............................................................................................. 26 12. Lower crustal to upper mantle xenolith, Dubreuilville dike ................................................................... 27 Tables 1. Diamond recovery results from 2000 Band-Ore Resources Ltd. diamond-drilling program ................. 17 2. Diamond results from Engagement Zone bulk samples (2001). ............................................................. 21 3. Summary of the diamond results from the 2001-02 sampling of the Moet Occurrence......................... 25 2 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Unusual Diamond-bearing rocks of the Wawa Area Introduction In 1991, local prospector C. “Mickey” Clement recovered at least three alluvial diamonds from the Michipicoten River. Two of the stones were sent to the Department of Mineralogy, Royal Ontario Museum, where they were identified as industrial-grade diamonds with weights of 1.05 and 1.13 carats. Both stones were described as frosted and graphite-inclusion riddled. In 1995, prospector Sandor Surmacz and geologist Marcelle Hauseux recovered diamonds from a bedrock occurrence on the Trans Canada Highway, approximately 20 km north of the town of Wawa. An 18.1-kg bulk sample of a xenolith-rich lamprophyre yielded 1 macrodiamond and 5 microdiamonds. All but one was gem quality. Since then, more than 50 occurrences of diamondiferous bedrock have been reported in an area of approximately 30 km2 in size, centred approximately 20 km north of the town of Wawa. The occurrences are hosted within a sequence of unusual, Archean-aged, heterolithic breccias. Historically, this sequence of rocks has received little exploration interest and was considered to have little economic significance. This field trip will focus on exposures of a series of foliated lamprophyre dikes and associated heterolithic breccias outcropping in Lalibert, Leclaire, Menzies and Musquash townships. This field trip guide represents a summary of information available at the time of writing and should not be considered the final analysis of these rock types. Much more research is required on these rocks. Active exploration and research is still underway on the properties included in this field guide. Given the limitations of time, the field trip will visit only some of the more accessible properties. Bear in mind that when visiting active exploration or mine properties, permission must be granted by the property owner. Current ownership information can be obtained from the Resident Geologist’s Office in Timmins, or the District Geologist’s Office in Sault Ste Marie, Ontario. Geological Overview of the Michipicoten Greenstone Belt The Wawa region lies within the Wawa subprovince of the Canadian Shield. The Michipicoten greenstone belt extends inland for approximately 150 km from the Lake Superior shore and has an average width of 38 km. The greenstone belt consists of supracrustal rocks of Archean age. Younger Archean granitic rocks surround the greenstone belt. Figure 1 shows a generalized geological map of the Michipicoten greenstone belt. The oldest volcanic cycle is approximately 2900 Ma and is of limited distribution. This cycle is best developed in Esquega Township along the southern flank of the supracrustal assemblage. Portions of this metavolcanic cycle extend into eastern McMurray and western Lastheels townships. The base of this volcanic cycle consists of massive to pillowed komatiitic flows intruded by mafic sills. The ultramafic rocks are overlain by intermediate to felsic tuffs capped by thinly bedded chert-magnetite-sulphide iron formation (Judith-Kathleen Iron Formation). Intermediate to felsic metavolcanic tuffs below the Judith iron formation have been dated at 2889 ± 9 Ma (Turek et al. 1992). Overlying the 2900 Ma cycle is a 2750 Ma volcanic cycle. This volcanic cycle is predominately composed of intermediate to felsic tuffs, breccias and flows. Porphyritic and spherulitic flows are not common and most of the intermediate to felsic metavolcanic rock is fragmental. The base of this cycle consists of high magnesium and high iron tholeiitic massive and pillowed flows. It lies conformably atop the JudithKathleen iron formation at the east end of Wawa Lake, but the basal unit is poorly exposed elsewhere. Overlying the mafic metavolcanic rocks is a sequence of heterolithic, intermediate to mafic breccia that has been traced for a distance of over 13 kilometers. This unit is in turn, overlain by a thick section of intermediate to felsic tuffs, breccias and massive flows that reaches a maximum thickness of approximately 2000 m below the Helen Iron Formation. The upper part of the intermediate to felsic metavolcanic rocks has been dated at 2749 ± 2 Ma (Turek et al. 1992). The Michipicoten (Helen) iron formation caps this 3 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �— 10 I —— Lake Superioi Creek Fault an I I I I . \ Diamond Occurr ences in Archean Heterol ithic Breccia Faults Mafic to fic Intrusive Rocks Rocks I Chemical Clastic Metasedi mentary Rocks I Felsic Metavolca nic Rocks Mafic Metavolcai riic Rocks I Carbonatite Intr usive Rocks Alkalic Intrusion S Sanukitoid Suite Intrusions I Granitoids Figure 1. Generalized geological map of the Michipicoten greenstone belt showing some of the diamond occurrences (modified after Stott et al. 2002). N ickenson Lake Stock 1J Lalibert Lec / Discussed in 1 Enigma Prop erty 2 Sandor Occu rrence 3 Cristal Occur rence 4 Engagement Zone 5 GQ Occurren ce 6 Leadbetter P roperty Diamond Pro pe1 rties \\\\ PDF compression, OCR, web-optimization with CVISION's PdfCompressor �this volcanic cycle. It commonly exceeds 100 m in thickness and, in the vicinity of the past-producing Helen Mine; it has been tectonically thickened to over 300 m. This iron formation was the source of virtually all commercial iron ore production in the Wawa area from 1898 to 1998. The youngest metavolcanic rocks in the area are those of the 2700 Ma volcanic cycle. These rocks underlie approximately fifty-percent of the Michipicoten greenstone belt and are found in the central and northern parts of the belt. The basal unit is composed of massive and pillowed mafic amygdaloidal flows that attain a maximum thickness of 5.5 km in the Goudreau area. This unit is overlain by intermediate to felsic metavolcanic rocks, or their stratigraphic equivalent, the Doré metasedimentary rocks. These metavolcanic rocks are typically composed of tuffs and monolithic and heterolithic breccias. Quartz + feldspar crystal tuff and an intermediate tuff from this volcanic cycle returned a U-Pb zircon age of 2701 ± 8 Ma (Turek et al. 1992) and 2701.4 ± 2.1 Ma (Ayer et al. 2003). The intermediate to felsic tuffs interdigitate with clastic metasedimentary sequences that include cross-bedded sandstone and a tonalite cobble conglomerate (Doré conglomerate). Corfu and Sage (1987, 1992) reported an age of 2698 ± 2 Ma for a tonalite clast in the Doré conglomerate and maximum ages of 2680 ± 3 and 2682 ± 3 Ma for sedimentary sequences in northern and central parts of the Michipicoten greenstone belt. Geochronological and structural evidence indicates that sedimentation continued after cycle 3 volcanism and predated a major folding and faulting event. Arias (1996) noted that the rocks comprising cycle 3 in the central part of the Michipicoten greenstone belt are upside down and represent the overturned limb of a belt-scale recumbent nappe fold. This inverted limb has been refolded and imbricated by subsequent southverging thrust faults, which caused local repetition of the stratigraphic sequence (Wilson 2004). Felsic plutonism occurred synchronous with all of the major volcanic cycles and continued after volcanism ceased at Wawa. Plutonic rocks associated with cycle 1 volcanism include the Murray-Algoma porphyry (2881± 6 Ma) and the Regnery biotite granite of the Hawk Lake granitic complex (2888 ± 2 Ma). Both intrusions are situated in Esquega Township. The Jubilee granitic stock, located in McMurray Township, was dated at 2745 ± 3 Ma and is coeval with cycle 2 volcanism. Plutons associated with cycle 3 volcanism range in composition from tonalite through granodiorite and granite and have ages ranging from 2698 to 2693 Ma. These plutons are located south and west of the Michipicoten greenstone belt (Stone and Semenyna 2004). The Kapuskasing Structural Zone extends east from the shore of Lake Superior, northeast through Kapuskasing and into the Hudson Bay Lowland. Local features interpreted to be associated with it include northeast-striking Proterozoic lamprophyre dikes (Sage 1994; Morris 1999). Lamprophyre dikes of middle Proterozoic age are common in the region south of the Wawa-HawkManitowik Lake Fault and rare to non-existent north of the fault. They are carbonate, biotite, and sometimes olivine-rich and usually less than 1.0 m in width. The dikes generally strike northeast. Dikes exposed in McMurray Township commonly have blue to blue-green sodic amphibole developed in their wall rocks. This mineral has been interpreted to be a product of fenitization. These dikes are likely spatially and temporally related to the emplacement of the Keewenawan-age Firesand River Carbonatite (Sage 1994). North of the Wawa-Hawk-Manitowik Lake Fault, in the area extending west from the former Magpie Mine (Leclaire Township), to the east side of the Dickenson Lake Stock (Lalibert Township), there are a series of what appear to be narrow, biotite-amphibole-rich dikes that have been interpreted as Archean lamprophyres. These dikes commonly have large, rounded inclusions (lower crust to upper mantle-derived xenoliths) up to 3.0 m in size, the centres of which are often completely altered to radiating clusters of actinolite crystals. Titanite from the matrix of one of these dikes returned an age of 2703 ± 42 Ma (Sage 2000). The date is interpreted to be a minimum age of intrusion. Subsequent dating of a zircon from a gneissic xenolith from the same dike returned an age of 2684.9 ± 1.4 Ma (Ketchum, Kamo and Davis 2003). 5 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Several narrow (<10 m) kimberlite dikes also intrude the Michipicoten greenstone belt. These dikes are largely restricted to the eastern part of the greenstone belt and are spatially related to the northeast-trending Kapuskasing Structural Zone. Several of these dikes have been dated. Intrusions K115 and K121, located in Isaac Township, returned an average 87Rb/86Sr age of 1097 ± 7 Ma (Kaminsky et al. 2002). A kimberlite dike intersected in Pele Mountain Resources Inc.’s drill hole 97-34, drilled in Riggs Township, returned a 207 Pb/206Pb age of 1197 ± 24 Ma (R. P. Sage, Ontario Geological Survey, unpublished, 2000; Wilson 2004). Quaternary Geology All Quaternary deposits within the Wawa area were deposited during the Late Wisconsin by the Labrador sector of the Laurentide Ice Sheet. Peat, recovered from a bog located within the surface of a terrace associated with the highest glacial lake in the Lake Superior basin, was radiocarbon dated at 9759 ± 170 BP. A caribou antler, recovered from near the surface of the delta of glacial lake Minong III, yielded a radiocarbon date of 8820 ± 80 BP (Morris 2001). Bedrock striae indicate that there were two prominent ice flow directions. The oldest and most pervasive ice flow was south to southwest (159° - 240°). A later, weaker ice flow was to the southwest and west (220° - 290°). The younger set of striae was formed during the latter stages of glaciation as the ice sheet began to thin and bedrock topography began to influence the direction of ice flow. Much of the overburden consists of a thin (< 1 m), discontinuous till veneer draped over the bedrock. Most of the area's thicker surficial deposits are located within bedrock controlled valleys where glaciolacustrine waters from the Lake Superior basin covered the area. In several of these valleys, the glaciofluvial material reaches thicknesses up to 32 m (Morris 2001). Structural Geology of the Diamond-bearing Rocks The greatest concentration of diamond-bearing rocks in the Michipicoten greenstone belt is constrained to a roughly 30 km2 block of land coinciding with a D1 recumbent nappe identified by Arias (1996). The nappe lies immediately north and adjacent to the Kapuskasing Structural Zone and has affected the two youngest rock sequences in the greenstone belt. In cross section, a generalized structural section across the greenstone belt would be upside down and represents the overturned limb of a belt-scale recumbent fold. This inverted limb of the nappe fold has been refolded and imbricated by subsequent south-verging thrust faults. The result is that the geometry of the rocks hosting the diamondiferous bedrock is that of an inverted anticline (Arias and Helmsteadt 1990). The effect is to create a tectonic repetition of the diamond-bearing sequence of rocks. In simplified terms, the rocks hosting the diamonds in the Wawa area can be considered as a single, overturned fold limb that has been faulted along a series of thrust zones. As a result, the diamond-bearing breccias, the lamprophyres associated with them and the surface onto which these metavolcanic rocks were deposited has been repeated at least four times (Walker 2002). The western limit of the nappe is likely the Dickenson Lake fault that passes along the west side of the Dickenson Lake stock. Extrusive lithologies similar to those hosting diamonds were recognized to the east of the stock by Sage (1993) during reconnaissance mapping. To date, no diamondiferous occurrences have been discovered to the west of the Dickenson lake fault. The eastern limit of the nappe is the Marsden Lake fault. Prior to 2002, rocks favourable to hosting diamonds had not been observed east of this structure (Wilson 2004). However, in 2002, Oasis Diamond Exploration Inc. made a discovery of diamonds on the east shore of the Magpie River and in 2004, diamondiferous occurrences of bedrock were found on the west shore of the Magpie River in Chabanel Township. These discoveries suggest that the potential for these host rocks extends farther eastward than previously believed. Figure 2 shows a graphic representation of the nappe structure. 6 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �500 F oslon Figure 2. Composite structural section through the central part of the Michipicoten greenstone belt. Section X-Y from south-central Corbiere Tp. (Josephine Iron Range) to Andre Lake central Corbiere Tp. Section Y-Z is a schematic section from west central Lalibert Tp. The sketch in the lower left explains the present configuration of the belt as a regional nappe fold (F1) refolded about F2. Imbricate thrusts are considered related to Fs (Arias 1996). Description of the Diamond-bearing Rocks The diamond-bearing breccia and associated lamprophyre is broadly distributed throughout Lalibert, Leclaire, Menzies and Musquash townships. On-going mapping by Pele Mountain Resources Inc. on the Festival property and by Nathalie Lefebvre on the GQ Property has helped to refine the classification of these rocks. Systematic exploration and sampling suggest that the individual diamond occurrences are part of a much larger suite of rocks and that diamonds occur primarily within discrete layers at the base of diamond-bearing zones. North-northwest-trending diamond-bearing zones of breccia and lamprophyre are up to 1500 m in length and up to 800 m in width (Pele Mountain Resources Inc., press release, January 18, 2005). The breccia forms thick units (maximum true thickness is approximately 110 m) dipping to the northeast 30°. The lateral extent and thickness of the breccia unit is not well constrained, owing to the large-scale regional folding and thrusting (Lefebvre 2004). Figure 3 provides a detailed map of the southwest corner of the Festival Property showing a recent interpretation of these diamond-bearing zones. The diamond-bearing rocks can be visually subdivided into two classes, lamprophyre (dikes and bodies of indeterminate morphology) and heterolithic or polymict breccias. It is often difficult to differentiate between the two classes since the lamprophyre dikes frequently contain an assortment of inclusions that 7 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �8 PDF compression, OCR, web-optimization with CVISION's PdfCompressor Genesis West II I Geri Mumm ---: I I Rosigtj Bollinger Lake Perth Per Dom .Perlgnon Musquash Tp. Leclaire Tp. it East Perch Salon Salon Cristal Taft • Other • iyric Suite I km • South Zor • North Zor Leger * Diamond I Occurrences F' Figure 3. Detailed geology of the southwestern corner of the Festival Property (modified from Pele Mountain Resources, Inc. press release, January 18, 2005). The units shown in green consist of mafic to intermediate massive and pillowed flows, breccias and tuffs (Vaillancourt 2005b, Sage et al. 1982) Veuv e iot Menzies Tp. —— — — — I_ �give them the appearance of breccia. The lamprophyre dikes cut the breccia units. Both lithologies have been metamorphosed to upper greenschist facies. The breccia primarily consists of angular, pebble-sized, lithic fragments, mainly of volcanic composition, contained within a green to grey fine-grained matrix. The matrix grain size ranges from < 2 mm to 1 mm. At least eleven distinctive types of lithic fragments have been observed in the breccia and the fragments are irregularly distributed throughout the breccia. The clast population is primarily derived from rocks with which the breccia is intercalated. Most typically these clasts are mafic and felsic metavolcanic rocks and intermediate to mafic intrusive rocks. Other clast types include fragments of clast-supported breccia within matrix supported breccia, fragments of earlier matrix-supported breccia with fewer than 5% fragments and coated lithic fragments (Lefebvre 2004). The breccia is characteristically massive, unstratified and poorly sorted with clast size ranging from sand to boulders up to 9 m. Primary sedimentary structures such as bedding and crude grading are rare (Lefebvre 2004). Petrographic work by Lefebvre (2004) on the breccias identified a typical fragmental texture within a mineralogically variable matrix. Typically the groundmass is dominated by actinolite, but chlorite and biotite dominated groundmass are locally predominant. Juvenile magmatic material also was observed as discrete fragments and rims on other clasts in the breccia. Its petrography is distinct from the breccia groundmass. The juvenile magmatic material contains more abundant actinolite grains; fewer epidote and fine-grained plagioclase grains and more fine-grained oligoclase and muscovite; and more oscillatoryzoned hornblende. Most exploration efforts over the past few years have concentrated on the breccias because they are considered to have the best potential for hosting commercial diamond deposits. Over the course of the last few years, explorationists have subdivided the diamond-bearing breccias into three separate facies. These facies are volcanic (pyroclastic), subvolcanic/intrusive breccia and hypabyssal facies. A variable, but distinctive proportion and composition of fragments and/or xenoliths characterizes each facies (Wilson 2004). The volcanic facies contains breccia, lapilli- and ash sized fragments and consists of medium to thickly bedded pyroclastic air-fall deposits. They are characterized by angular to sub-angular Archean supracrustal fragments, some hypabyssal fragments and rare lower crustal to upper mantle xenoliths. The subvolcanic/intrusive breccia facies are the most variable in texture and appearance. The rock is characterized by observed intrusive relationships, a high proportion of fragments and a close proximity to the volcanic facies. The fragment characterization is variable. The facies can contain all or some of the following fragment types: supracrustal fragments, crustal fragments and lower crustal to upper mantle xenoliths. This facies was once included with the lamprophyres but is now interpreted by various exploration companies as a debris flow. The hypabyssal facies hosts variable proportions (<25%), of sub-rounded to rounded mantle xenoliths, as well as minor proportions of gneiss and/or trondhjemite fragments. This facies also was originally classified as lamprophyre but is now considered to be part of the debris flows (Lefebvre et al. 2003). The brecciated unit(s) could also be a single or multiple diatreme(s) localized within Lalibert, Leclaire, Menzies and Musquash townships. A tectonic repetition of the diamond-bearing diatreme(s) has been achieved through the regional deformation events described previously. This regional deformation may help to explain the layered appearance observed at some of the diamond occurrences (J. Ayer, Ontario Geological Survey, personal communication 2006). Lamprophyre occurs as narrow dike-like intrusions or in bodies of indeterminate morphology cross-cutting or intercalated with local country rocks. Dikes range in width from 50 cm to 2 m and may display 2-3 cm offshoots in some locations. Contacts with the host rocks vary from sharp and straight to highly irregular. No variation in grain size within the dikes has been observed and the no variation in colour or mineralogy 9 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �has been observed within country rock adjacent to the dikes. The lamprophyre is fine-grained, grey in colour and contains approximately 5-10% subrounded to subangular fragments. The fragment population is dominated by actinolite-rich monomineralic rocks, or by biotite-rich greenstone and hornblende-rich ultramafic rocks (Lefebvre 2004). Lamprophyre is differentiated from the breccia by: a lower clast content; a predominance of highly altered, coarse-grained actinolite fragments; scarcity of wall rock fragments; the rounded shape of the xenoliths; and the presence of a weaker fabric (Lefebvre 2004). The lamprophyre dikes post-date all other lithologies. The lamprophyre with indeterminate morphology predates most of the host lithologies since fragments of each are found within these lamprophyres. However, in some locations, lamprophyre fragments have been observed within intermediate and mafic intrusive rocks near the contact with lamprophyre (Lefebvre 2004). Fragments within lamprophyre dikes commonly have a biotite-rich rim enclosing the xenoliths and the fragments found in the lamprophyres of indeterminate morphology do not. The lamprophyres of indeterminate morphology also show positive relief of less severely weathered xenoliths more so than the lamprophyre dikes. Lastly, the lamprophyre dikes have a less variable fragment lithology (Lefebvre 2004). The lamprophyre is petrographically distinct from the breccia. It contains a lower abundance of clasts and fewer clast types. Unlike the breccia, the lamprophyre contains no juvenile magmatic material and oscillatory-zoned hornblende grains are rare. Detailed petrographic descriptions of the lamprophyre can be found in Lefebvre (2004). Relationship between diamond content and lithology Microdiamonds have been recovered from a wide array of breccias and lamprophyres in the Wawa area. Between 2003 and 2004, the Ontario Geological Survey (OGS) investigated a number of diamond occurrences during a mapping program conducted in Menzies and Musquash townships. Closer look at locations where bulk samples had been collected suggested that the bulk samples probably included more than one rock type. Diamonds were recovered from each of these bulk sample sites, but it would be difficult to establish from which of the rock types the diamonds were recovered. The OGS undertook a limited sampling program to further investigate the diamond content of specific lithologies (Vaillancourt et al. 2005a). The OGS collected three small samples (95.21 kg total weight) from three different lithological units at the Cristal and Genesis diamond occurrences. These occurrences are on the Festival Diamond Property owned by Pele Mountain Resources Inc. Results from this limited sampling program reinforce the observation that microdiamonds are not restricted to a single unit. Microdiamonds were recovered from heterolithic breccia, both with and without ultramafic magma pockets, and from a fragment-free ultramafic dike. There is the possibility, however, that the diamonds recovered from the ultramafic dike are xenocrysts derived from diamond-bearing host breccia (Vaillancourt et al. 2005a). The OGS concluded that the results from only three samples are not sufficient to draw irrefutable conclusions regarding the location of the microdiamonds. Collection and analysis of more, well constrained samples is necessary to further refine the diamond potential of specific host rocks. In 2005, Spider Resources Inc. and KWG Resources Inc. conducted a similar bedrock sampling program to investigate the diamond-bearing potential of specific lithologies. Separate representative samples (approximate weight 16 kg each) of the matrix, xenoliths and run of mill (ROM) portions of the bedrock were collected from the Wawa Diamond Project and were sent for caustic dissolution. The results are as follows: matrix sample returned 67 diamonds (0.008 total ct), xenolith sample returned 244 diamonds (0.051 total ct) and the ROM sample returned 86 diamonds (0.006 total ct) (Spider Resources Inc., press release, February 20, 2006). 10 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Geochronology In conjunction with recent geological mapping in the area, the Ontario Geological Survey has been conducting geochronological work to help understand the nature and timing of the diamondiferous units and their host rocks within the Michipicoten greenstone belt. A felsic volcanic horizon hosting diamondiferous units returned a 207Pb/206Pb age of 2701.4 ± 2.1 Ma. Maximum 207Pb/206Pb ages of 2685.1 ± 1.0 Ma and 2684.9 ± 1.4 Ma have been returned for diamondiferous lamprophyre dikes cutting the Catfish assemblage (2.7 Ga) intermediate to felsic metavolcanic rocks in Lalibert and Menzies townships (Ayer et al. 2003). A second sample of felsic lapilli tuff, part of the Catfish assemblage, adjacent to the Moet Occurrence contains zircons that returned a 207Pb/206Pb age of 2698.7 ± 1.1 Ma (Vaillancourt et al. 2004). A sample was collected from the diamondiferous breccia at the Moet Occurrence in order to determine the age of brecciation. Five zircons were analyzed. The three oldest ages are 2687 ± 2 Ma, 2683 ± 2 Ma and 2681 ± 2 Ma. The two youngest zircons cave results that precisely overlapped one another at 2679.2 ± 2.1 Ma (Vaillancourt et al. 2004). Since this is the youngest zircon age obtained from the breccia, it represents either the time of cystallization or emplacement of the body if the zircons are magmatic, or a maximum time of emplacement if the zircons are xenocrystic (Vaillancourt et al. 2005a). If the zircons are xenocrystic, this age must still be close to the time of breccia emplacement since a lamprophyre dike from the GQ property returned a date of 2673 ± 8 Ma from titanite (R. P. Sage, Ontario Geological Survey, unpublished 2000). The data indicate that the felsic metavolcanic rocks hosting the diamondiferous breccias are part of the Catfish assemblage. The maximum age for the diamondiferous breccias and the associated dikes is less than 2680 Ma. These absolute age constraints indicate that the breccias are not volcaniclastic units belonging to the Catfish assemblage (Vaillancourt et al. 2005a). Zircons from a sample of felsic lapilli tuff from northwestern Menzies township returned an age of 2736.0 ± 0.8 Ma which is taken to represent the age of eruption and crystallization of the tuff. This age clearly indicates that the volcanic package underlying the iron formation in the western part of Menzies Township is part of the Wawa assemblage (2.75 Ga) and brackets the uppermost part of the assemblage at 2736 Ma (Vaillancourt et al. 2005a). Geochemistry of the Diamond-bearing Rocks Both Williams (2002) and Lefebvre (2004) conclude that the whole rock major element geochemistry is consistent with a calc-alkaline classification for both the lamprophyres and the associated breccia. Both authors also noted that the compositions of chromite in the Wawa metavolcanic rocks are in the range typical for lamprophyres and dissimilar to those in kimberlites and lamproites. Whole rock geochemistry for the diamond-bearing rocks is tabulated in Sage (2000), Williams (2002), Lefebvre (2004), Stone and Semenyna (2004) and Vaillancourt et al. (2005c). Sage and Williams’ work is specific to the diamond-bearing and non diamond-bearing lamprophyres. Work by the other authors relates to both the breccia and the lamprophyres. Whole rock geochemistry for kimberlites of the Wawa area can be found in Kaminsky et al. (2002). Diamond characteristics Lefebvre (2004) undertook a study examining a parcel of 80 macrodiamonds recovered from the volcaniclastic breccia on the GQ Property. Results from this work are summarized below and in De Stefano et al. (2006). Additional work on the morphology of the Wawa diamonds can be found in Stone and Semenyna (2004). Stachel et al. (2004) summarize results of analysis conducted on diamonds from the Genesis and Cristal diamond occurrences presently held by Pele Mountain Resources Inc. 11 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Shape: The diamonds display a highly variable primary growth form. Most of the diamonds are either octahedral aggregates (44% of the population) or single octahedral crystals (26%). Single cubic and cubicoctahedral crystals and their aggregates, as well as macles form the remainder of the population. Fortyeight percent of the diamond population is single crystals and only 28 diamonds could be evaluated for crystal regularity. The majority of the diamonds also display some degree of distortion. Colour and transparency: The diamonds included in this study are classified into colourless, brown, grey, black, yellow and white. No pink, green, violet or blue diamonds were observed. The colour distribution within the population is: colourless (48%), heterogeneous (24%), yellow (11%), black (3%), brown (10%) and grey (3%). The heterogeneity in colour is observed only in aggregates. The diamond population consists of 48% transparent crystals, 25% translucent crystals, 14% opaque crystals and 14% is a combination of opaque and translucent crystals. Transparent crystals are typically colourless and also comprise a few yellow octahedral single crystal and coarse aggregates as well as macles. Translucent crystals comprise all possible primary crystal forms and colours. Opaque crystals are mostly fine-grained aggregates which have black body colouring. Resorption: Generally speaking, the diamonds have experienced low degrees of resorption. Only 21% of the diamond population displays extensive resorption. Some crystals (14%) exhibit non-uniform resorption where one part of the crystal is more strongly resorbed than another. Inclusions: Mineral inclusions were identified in 58% of the diamonds. Both primary and secondary inclusions were observed. The mineralogy of the recovered primary inclusions is listed in descending order of abundance: olivine (Fo92 and Fo89), clinopyroxene (omphacite), plagioclase (albite and An-rich), orthopyroxene (En93) and Fe-Ni sulphide (pentlandite). Cathodoluminescence: The relative abundances of cathodoluminscence (CL) colours for the Wawa diamonds are: orange-red (46%), yellow (28%), orange-green (10%), green (6%), and other non-uniform colours (10%). None of the 69 diamond examined displayed the more common blue CL. Impurities: Fournier Transform Infrared (FTR) spectrometry was used to investigate the nitrogen and aggregation states for 41 diamonds. The majority of the diamonds have low nitrogen contents, < 300 ppm. The diamonds show two modes of nitrogen aggregation suggesting mantle storage at 1100 - 1170° C. Diamonds from the Genesis occurrence are almost exclusively cubes including some fragmented, twinned and moderately resorbed cubes. Most of the crystals contain clouds. Fully transparent stones are dominantly brown although colourless stones also are common; one diamond was yellow in colour. Nitrogen concentrations range from below detection (<10 ppm) to 600 atomic ppm. Nitrogen aggregation is very low (Stachel et al. 2004). Diamonds from the Cristal occurrence range from un-resorbed octahedra to highly resorbed dodecahedra. Octahedral and weakly resorbed octahedral stones dominate the population. About 25% of the population are irregular crystals, macles are common (15%) and about 5% of the diamonds show cubo-octahedral growth. The stones fall into two dominant colour classifications, colourless and a range of brown colouration. Nitrogen contents range from <10 – 560 ppm, but with only one exception nitrogen is ≤170 ppm. Nitrogen aggregation varies between 0 and 97% B-centre. Olivine is the most common mineral inclusion, followed by pyrope garnet and Mg-chromite (Stachel et al. 2004). Origin of the Diamond Deposits Based on published data on the diamond-bearing rocks at Wawa and Cobalt, Wyman et al. (in press) suggest that the tectonic setting of the deposits and nature of the host rocks indicate that the diamonds may be derived from the asthenospheric wedge and subducted slab at shallow depths (100 – 160 km) rather than the deep keels of Archean cratons associated with traditional diamond deposit types. Models of lowtemperature Phanerozoic diamond formation in active subduction zones, or rapid uplift and emplacement of peridotite massif occurrences, can be adapted to the Archean deposits. The stability field of 12 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �diamonds in most Phanerozoic scenarios may be too deep to be accessed by the lamprophyric magmas. Shallow subduction, as proposed for these occurrences of adakitic-type rocks in the Wawa subprovince, could generate two different diamond stability windows at sufficiently shallow depths to account for their presence in lamprophyric magmas. Wyman et al. (in press) states that any tectonic model for these Archean diamond occurrences must address several requirements. These requirements include 1. a deep source for oxidized metasomatic fluids that is activated prior to lamprophyre emplacement 2. a mechanism to isolate this isotopically aged and depleted source for tens or hundreds of millions of years until it is heated in the mantle during orogeny 3. a hybridized mantle source for primitive, hydrous, shoshonitic lamprophyres 4. sustained cold finer effect in the mantle to establish a shallow-mantle diamond stability window Two theories of diamond origin are postulated by De Stefano et al. (2006). Both a cratonic and orogenic model of diamond formation are discussed in an effort to rationalize the observed diamond characteristics. The authors conclude that neither model fully explains all of the observed characteristics. 13 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Field Trip Stops Field Trip Road Log Stop 1 2 3 4 5 6 7 8 Locality Intersection of Hwy 101 and Hwy 17 Take Highway 17 north km 0 Catfish Road forestry road – turn east 19.2 GQ Diamond Discovery Northern Sierra Minerals Corporation Area B Northern Sierra Minerals Corporation Engagement Zone 23.3 23.9 29.1 Return to Highway 17 reset odometer Drive north to access road – turn east Park Walk eastward along trail Moet Occurrence, Festival Property 0 6.2 6.6 7.6 7.6 Return to Highway 17 reset odometer Drive north, park on shoulder of highway Sandor Diamond Occurrence Continue north on Highway 17 to intersection of Highway 519, turn right 0 4.3 4.3 12.6 Safely turn in parking area and return to Intersection of Hwy 17 and 519 reset odometer Drive south on Highway 17, park on shoulder of road Dubreuilville Dike Continue south on Highway 17 Turn left into access road to gravel pit, park Walk south approximately 150 m Monchiquite Dike Drive north on Hwy 17 to Wawa Wawa Motor Hotel 0 3.1 3.1 44.9 45 53.2 STOP 1 - GQ Diamond Discovery Site Northern Sierra Minerals Corporation Area A UTM co-ordinates – 0665570E 5333291N NAD83 Several outcrops of diamondiferous breccia outcrop on the west side of a forestry road. This exposure is an example of the hypabyssal facies of the three identified diamond-bearing units. The rock cut displays the apparently conformable nature of these “lamprophyre” dikes. The most notable features of these outcrops are the actinolite-rich nature of the matrix and the presence of biotite-rich reaction rims around the xenoliths. It is frequently difficult to distinguish these dikes from the mafic to intermediate agglomeratic and tuffaceous host rocks. In the vicinity of the discovery area, located at the south end of Area A, the diamondiferous breccias are arrayed linearly along the logging road where the topography indicates a 5 – 10 m thick, northwest-trending dike (Cavey 2002). A compilation of the geology of the GQ Property is shown in figure 4. Figure 5 provides a compilation of the diamond occurrences on the GQ Property. 14 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �GQ PROPERTY I / I AR! ' I GEOLOGY REVISED FROM CANADA EXPLORATION INC. GABBRO LII METASEDIMENTS GO PROPERlY INTERMEDIATE TO FELSIC METAVOLCANICS L CALCALKALINE IIVB AND LAMPROPIIYRE LOCATIONS ARE APPROXIMATE PROPERTY GEOLOGY MAP Sault Ste. Mane Mining Division Ontario Atter K. KM (2003), Kennecott Figure 4. Geological compilation map of the GQ Property, Musquash Township, Northern Sierra Minerals Corporation (Cavey 2004). 15 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �_____ ____________ iT ___________ a § Drill Hde Occurrences ci In&usive and/or Exinisive flreccias KEY • drilled in 2002 I 2003 KEY D;scovemdprlorto Opthn during a GO PROPERTY COMPILATION MAP drilled in 2000 logging road. 2004 MusquashTownship It Ste. Marie Mirlfng Division Ontario Figure 5. Occurrences of diamondiferous bedrock on the GQ Property, Musquash Township, Northern Sierra Minerals Corporation (Cavey 2004). 16 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �A relatively precise 207Pb/206Pb age of 2674 ± 8 Ma was returned from a sample collected in 2000 by R. P. Sage (R. P. Sage, Ontario Geological Survey, unpublished 2000). The sample analysed was titanite. The titanite grains are considered to be a primary mineral and not xenocrystic. In thin section, the rock is characterized by a green, medium-grained, granoblastic to decussate groundmass of actinolite, biotite, chlorite, plagioclase and accessory minerals. Amphibole or biotite macrocrysts up to 1 mm are common (Stone and Semenyna 2004). Local prospectors T. Nicholson, J. Robert and M. Tremblay made the discovery in the fall of 1999. The first two bedrock samples (63.4 kg and 70.5 kg) collected were processed by Kennecott Canada Exploration Inc. in their Thunder Bay laboratory. According to Kennecott’s report, the 63.4 kg sample yielded 45 diamonds, of which 10 were macro diamonds and 35 were microdiamonds. One of the macro diamonds measured 1.01 mm in one dimension. The 70.5 kg sample yielded 9 microdiamonds. All stones were white in colour and transparent in clarity. Duplicate samples were collected by Band-Ore Resources Ltd. in early 2000 and were processed at SGS Lakefield Research Limited. A 54.6 kg sample yielded 98 microdiamonds. A confirmation sample from the same area yielded 98 microdiamonds from a 54.6 kg sample. In 2000, Band-Ore Resources drilled 3 short holes (75 m total) at the discovery site. Table 1 details the diamond recovery results from the drill program. Only partial intervals from drill hole DDH GQ-00-3 were submitted for microdiamond recovery since portions of the core were used for thin sections, microprobe analysis and display purposes. In total 5 diamonds were recovered from DDH GQ-00-3, including one champagne coloured macro diamond and one white microdiamond from a sample weighing 7.5 kg. Drill sections for DD GQ-00-01 through 03 are shown on figure 6. Table 1. Diamond recovery results from 2000 Band-Ore Resources Ltd. diamond-drilling program Drill Hole DDH GQ-00-1 DDH GQ-00-2 Sample No. Sample Size (kg) No. Macro Diamonds No. Micro Diamonds Sample 1A 63.35 1 Sample 1B 30.17 10 Sample 1C 28.51 6 Sample 2A 37.52 1 30 Sample 2B 30.97 434 Sample 2C 28.51 30 results compiled from Band-Ore Resources Ltd. press releases 2000 To date, the discovery site has yielded 746 diamonds, including 15 macro diamonds, from sample material weighing 785 kg. The largest diamond recovered exceeds 1.0 mm in size and the majority of the stones are gem quality, white, clear and transparent. STOP 2 - Northern Sierra Minerals Corporation Area B UTM co-ordinates – 0665425E 5334748N NAD83 The exposure on the east side of the forestry road provides an excellent exposure of the subvolcanic/intrusive breccia facies. Subrounded to rounded xenoliths dominate the vertical exposure (Figure 7). Field relationships between the intrusive breccia and other heterolithic breccias can be observed in several outcrops along the road. Texturally, the subvolcanic/intrusive facies may resemble both the hypabyssal facies and the intrusive heterolithic breccias. The facies consists of mica and amphibole phenocrysts (<2mm) in a groundmass of mica, actinolitic amphibole and lesser albite, carbonate, sphene and oxides. Alteration includes variably chloritized mica while the other phenocrysts have been extensively altered to varying proportions of mica, albite and actinolitic amphibole. 17 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �z Q 000 / MAFIC VOLCAXICS A S\\IPI I lB -, / MARC VOLCANICS Band-Ore Resources Ltd. GQ PROPERTY Drill Section 3Dm Musquash Township Ste. Marie Mining Division Ontario Drill Hole GQ-OO-O1 Section Looking West Figure 6. Drill hole sections GQ-00-01, GQ-00-02 and GQ-00-03, GQ Property, Musquash Township, Northern Sierra Minerals Corporation (Cavey 2002). 18 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �SAM PLI- XENOLITHIC microdinmonds A (33.81kg) 3 macrodiamonds DIATREME 00) 2A (3 7.52kg) / XYN(YLITIIIC DI.\I I 2B 7 GQ-OO-03 S\\IPLI 2€ 4, Band-Ore Resources Ltd. GQ-OO-02 27m GQ PROPERTY Drill Section Musquash Township Sault Ste. Marie Mining Division Ontario Drill Holes G-OO-2 and GQ-OO-03 Section Looking West Figure 6 cont’d. Drill hole sections GQ-00-01, GQ-00-02 and GQ-00-03, GQ Property, Musquash Township, Northern Sierra Minerals Corporation (Cavey 2002). 19 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �I- - . —I —.-..' - ½ 4 - ( "- Ip .t •q Figure 7. Stop 2 - Northern Sierra Minerals Corporation Area B – rounded lower crustal to upper mantle xenoliths Stone and Semenyna (2004) completed a petrographic examination of one of the ultramafic xenoliths from this site. The sample was dominated by a coarse, radiating to decussate, clear to pale green amphibole of tremolitic to magnesium-rich actinolite composition. Carbonate occurs locally and biotite is concentrated at the rims of the xenolith. Band-Ore Resources Ltd. discovered area B in 2000. Thirty-three (33) reconnaissance samples were collected from this area and a total of 273 diamonds was recovered from 352 kg of material (Cavey 2002). One 24 kg sample returned 126 microdiamonds (1.37 mg total weight). No macro diamonds were recovered from Area B. Band-Ore Resources Ltd. completed only a reconnaissance sampling program, minor stripping and trenching in this area. The Barnett Zone lies approximately 1.6 km to the northwest. It was discovered in the fall of 2001 by Kennecott Canada Exploration Inc. who completed a limited program of mechanical stripping, channel sampling and washing of outcrops over the area. A total of 27 outcrop channel samples (270 kg) were collected. The channel samples returned 330 microdiamonds and 3 macro diamonds. A single 24 kg sample of heterolithic breccia from the Barnett Zone returned 3 macrodiamonds and 123 microdiamonds. A total of 273 diamonds (261 microdiamonds and 12 macrodiamonds) were recovered from 34 samples (352 kg) collected between September 2000 and July 2001 by Kennecott Canada Exploration Inc. in Area B (including the Barnett Zone). 20 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �STOP 3 - Northern Sierra Minerals Corporation Engagement Zone Bulk Sample Site UTM co-ordinates – 0667760E 5336073N NAD83 The main outcrop exposure consists of medium to thickly bedded pyroclastic air-fall deposits and debris flows described as heterolithic tuff-breccias that grade upwards to lapilli-tuff and tuff. The groundmass of the extrusive phase is fine-grained with variable proportions of relatively small (<2mm) altered phenocrysts, including chloritized mica. The groundmass consists primarily of actinolitic amphibole with rare to minor mica or granular albite. The pyroclastic rocks are mineralogically and compositionally similar to intrusive varieties, but have a significantly higher proportion of mica phenocrysts. Both diamond-bearing intrusive and extrusive rocks host significant proportions of fragments derived from the local country rock. Rare to minor, deep crustal and upper mantle xenoliths, such as banded gneiss and extensively altered talcose ultramafic xenoliths are present. Fresh mantle rocks, such as lhertzolite, harzburgite and eclogite have not been identified. The matrix material is typically fine-grained, green, weakly foliated actinolite schist. Albite is present, although it is less abundant (<10%) than in the lamprophyre dikes and implies a more ultramafic composition for the breccia matrix than for the lamprophyre dikes. Titanite is fairly abundant and calcite, epidote, apatite and sulphide minerals occur locally. Macrocrysts of actinolite are commonly observed. Rare macrocrysts of amphibole also are observed and are frequently altered to actinolite. The actinolite macocrysts are probably metamorphic in origin, whereas the amphibole macrocrysts may represent accidental or cognate crystals (Stone and Semenyna 2004). The Engagement Zone has a minimum strike length of 335 m and a horizontal width in excess of 75m. The zone strikes northwesterly and has a shallow dip to the northeast. This zone may represent the southeast extension of Pele Mountain Resources Inc.’s Cristal diamond occurrence located approximately 2 km to the northwest. In 2003, a 0.72 carat macrodiamond was recovered from a bulk sample collected from the Cristal. The geology of the Engagement Zone and sample locations are shown in Figure 8. A simplified cross section of the Engagement Zone is shown in Figure 9. Band-Ore Resources Ltd. discovered this zone in January 2001. A 16 kg sample from a single angular boulder of diatreme breccia returned 128 microdiamonds. Four subsequent samples (96 kg total weight) returned 5045 microdiamonds and 65 macrodiamonds. In 2001 a mini-bulk sample weighing 12.5 tonnes was collected under the supervision of Kennecott Canada Exploration Inc. and shipped to the Saskatchewan Research Council. The largest diamond recovered from this sample was a 0.254 carat, broken, white octahedral stone. Two additional bulk samples were collected in 2003. A 22 tonne sample tested an area where 6 channel samples (63 kg total) recovered 1752 stones. A 20 tonne sample tested an area where 5 channel samples weighing 41.8 kg returned 552 stones. The results from these bulk samples are shown in Table 2. Table 2. Diamond results from Engagement Zone bulk samples (2001) Sample Weight Sieve +1mm Sieve +2mm Sieve +3mm Sieve +5mm Sieve +6mm Total Diamonds Total Carat Weight Engagement Zone East 22.1 tonnes 1 4 4 2 1 12 0.375 Engagement Zone West 20.4 tonnes 2 3 3 8 0.155 Occurrence results compiled from Band-Ore Resources Ltd. press releases 2004 To date, exploration on the Engagement Zone has included an orientation geochemical survey, geological mapping, channel sampling, trenching, bulk sampling and a 9-hole (1775 m) diamond drilling program designed to test the strike continuation of the zone. The drilling program demonstrated that thick diamondiferous breccia deposits can feather out and thin to a few centimeters thickness (Cavey 2003). 21 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Engagement Zone // N 1/ II / APPARENT OUTLINE OF // ii 0 / // MICROS 73 MACROS FROM 481.83 I - excludes the resuj from the Kenneec channel sampling / II V C \ I Mt.51262 12 tonne Exploration Sample hz' - -- I y '.APPROX. DII' - -. O.2o4CaratWhrtsOctahedral / LOG A1FONØ ARE APP ROXIM so icc Engagement Zone • OCCURRENCES OF BRECCIA large angular slabs and/or Duicrop) * SAMPLE PROCESSED FOR MICRODIAMOND RECOVERY (caustic fusion) Figure 8. Stop 5 – Geology and sample locations at the Engagement Zone, GQ Property, Northern Sierra Minerals Corporation (Cavey 2002) 22 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �2001 MINI-DULK SAMPLE A SIRIFFLNG N N VOLCAN N4AFLC VOLCANICS AND INIERELOW N HETEKOLtIHIC MEGALITH VOLCAN[CLAST:C I CC IA — VOLGAN[C •I "NN 105, N I HETEWLFIHIC N N N LOCATIONS SKECCIA ArFROX!MATE "N Figure 9. Simplified cross section through the Engagement Zone looking northwest (Cavey 2004). STOP 4: - Moet Occurrence, Festival Property Pele Mountain Resources Inc. UTM co-ordinates – 0662709E 5338009N NAD83 The Moet occurrence is a large stripped outcrop that extends in a north-south orientation across the forestry road. The outcrop displays all three facies of diamond-bearing bedrock exposed over an area 500 m by 300 m. It is hosted within fine- to medium-grained mafic metavolcanic rocks with closely associated intermediate to felsic metavolcanic rocks and metasediments. The volcanic facies was found concentrated within a series of outcrop exposures along the west side of a 5-8 m north-trending ridge and the subvolcanic/intrusive breccias and hypabyssal rocks are present in several outcrops east of the volcanic facies. The subvolcanic/intrusive breccias are hosted in the metavolcanics and the volcanic facies appears to overlie these rocks, and are in turn overlain by intermediate to felsic metavolcanic rocks. The fragments 23 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �within the volcanic facies consist of country rock fragments with lesser crustal fragments and lower crust to upper mantle xenoliths. The hypabssyal facies displays primarily mantle xenoliths (Walker 2003). A detailed geological map of the Moet locality is shown in Figure 10. ' Stop4 t Moot 5—-/ — I I 1YPE \">" — — — M.lic rock. + an&'o reck, tomainjin mamle xpnnMha cpsTs 0 Figure 10. Detailed geological map of the Moet Occurrence, Festival Property, Pele Mountain Resources Inc. (Kjarsgaard et al. 2003). The access trail to the outcrop passes through a sequence of intermediate to felsic tuffs and tuffaceous breccias. The composition of the fragments and that of the matrix are highly variable from felsic to intermediate and a combination of felsic matrix with intermediate fragments and vice versa is not uncommon. At a distance of approximately 1km, the trail passes through a sequence of mafic to intermediate tuffs and lapilli tuffs (Vaillancourt et al. 2005b). An age date of 2698 ±1 Ma was returned from sample of the felsic metavolcanic rocks at the occurrence. A sample of the breccia returned an age date of 2680 Ma (J. Ayer, Ontario Geological Survey, personal communication, 2004). Discovered by Pele Mountain Resources Inc. in 2001, the occurrence initially gained interest because the breccia has a size distribution of diamonds that includes coarser sized diamonds from relatively small samples. For example, an 8 kg sample collected in 2001 recovered a total of 9 diamonds, 4 of which were in the +600 mesh fraction. Diamonds are consistently recovered from both the volcanic and subvolcanic facies at this showing. A summary of the results of the 2001 and 2002 sampling of the occurrence by Pele Mountain Resources Inc. is found in Table 3. 24 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Table 3. Summary of the diamond results from the 2001-02 sampling of the Moet Occurrence Facies Weight (kg) <425 mesh >425 mesh volcanic volcanic hypabyssal hypabyssal volcanic volcanic hypabyssal subvolcanic breccia 117 16 24 8.6 9.3 14.1 9.3 32 48 2 0 0 7 12 0 30 0 1 0 0 0 5 0 1 total diamonds 48 3 0 0 7 17 0 31 results compiled from Pele Mountain Resources Inc. press releases 2002 Further exploration was conducted at the occurrence in 2003 when DeBeers Canada Exploration Inc. completed a detailed airborne geophysical survey, stripping, mapping and sampling program. A 47.8tonne bulk sample from the site returned 5 diamonds with a total carat weight of 0.13. All diamonds were recovered from the +1 to +3 sieve class screens (Pele Mountain Resources Inc., press release, March 17, 2004). STOP 5: - Sandor Diamond Occurrence Spider Resources Inc. & KWG Resources Inc. UTM co-ordinates – 0659805E 5342191N NAD83 The Sandor occurrence the first confirmed occurrence of diamonds in bedrock in the Wawa area The occurrence is located in a 4 m high road cut on the east side of the Trans Canada Highway (Highway 17). The dike is approximately 5 m wide, steeply dipping and strikes roughly parallel to the regional schistosity at 120°. The dark, greenish-grey rock weathers olive grey, is highly fractured, moderately carbonatized and is non-magnetic. It is composed of up to 40% actinolite replaced mantle xenoliths and supracrustal xenoliths. Towards the margins of the dike xenoliths are less common and the rock grades into an adjacent micaceous dike. Only remnants of the dike remain in situ. The dike is hosted by gabbros and intermediate to felsic crystal tuffs. A (308.6 kg) sample of the dike, collected by Spider Resources Inc. in 1997, returned a total of 97 diamonds comprising 1 commercial stone, 13 macrodiamonds and 83 microdiamonds. A short walk into the forest from the top of the outcrop leads to a larger stripped area where field relationships between the host gabbro and the dike can be observed. A second, xenolith-bearing dike (occurrence LAL-3) is located at the north end of the outcrop. This dike is 2 m wide and closely resembles the Sandor occurrence. A 34.6 kg sample of this dike contained 1 microdiamond. A detailed geological map of the Sandor occurrence is shown in Figure 11. A compilation map of the geology of the Spider Resources Inc. and KWG Resources Inc. property is shown in Figure 12. Using normative mineralogy, Sage (2000) concluded that this dike should be classified as a spessartite. A spessartite is defined as a lamprophyre composed of phenocrysts of green hornblende or clinopyroxene in a groundmass of sodic plagioclase with accessory olivine, biotite, apatite and opaque oxides. Titanite and rutile from the matrix of the Sandor dike returned an age of 2703 ± 42 Ma (Sage 2000). The date is interpreted to be a minimum age of intrusion. Subsequent dating of a zircon from a gneissic xenolith from the Sandor dike returned a 207Pb/206Pb age of 2684.9 ± 1.4 Ma (Ketchum, Kamo and Davis 2003). Spider Resources Inc. and joint venture partner KWG Resources Inc. have taken 3 mini-bulk samples along the 1 km strike extent of the dike since the fall of 2001. The three bulk samples had a combined weight of 7.61 tonnes and returned 11 commercial stones and 9 macro diamonds. These samples were tested only for macro diamond and commercial diamond content (Spider Resources Inc., press releases, February and March 2002). 25 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �0 ' - — M*VSc.iic — tbcoveryOflop ——-Opel— 2__21' S Stops Figure 11. Detailed geology of the Sandor Occurrence, Spider Resources Inc. & KWG Resources Inc. (Kjarsgaard et al. 2003). Figure 12. Generalized geology of Wawa Project, Spider Resources Inc. and KWG Resources Inc. (Spider Resources Inc. and KWG Resources Inc., CD-ROM presentation, update February 24, 2004) 26 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �STOP 6: - Dubreuilville Dike - Xenolith-rich lamprophyre Spider Resources Inc. & KWG Resources Inc. UTM co-ordinates – 0657251E 5347023N NAD83 The dike outcrops in a 1.5 m high exposure on the west side of the Trans-Canada Highway (Highway 17). In the area between the former Magpie Iron Mine in Leclaire Township to approximately the eastern contact of the Dickenson Lake Stock, there are a number of exposures of unusual dike rocks. These rocks are characterized by prominent round, to elliptical inclusions of actinolite or actinolite plus talc. Xenoliths are altered to fine-grained actinolite with or without talc, and some display zoning from talc core to an actinolite rim. The actinolite inclusions may consist of prismatic green crystals as large as 8 cm in length, which may be randomly oriented or radiating inward towards the core. The inclusions containing talc consist of a talc core with the prismatic to acicular actinolite projecting radially inward towards the core. The xenoliths are believed to represent at least two original mafic compositions which are likely to be originally of lower crust origin or deeper. A weakly developed regional schistosity crosses the dike implying an Archean age (Sage 1993, 2000). An example of one of the lower crustal to upper mantle xenoliths is shown in Figure 13. Figure 13. Lower crustal to upper mantle xenolith, Dubreuilville Dike Stop 6, Highway 17 North. This dike may be the one described by Higgins (1986). He reported that the dike consists of 60% euhedral amphibole, 20% biotite replacing amphibole and 15%plagioclase. Minor sphene and opaque minerals are present and chromite is reported from the core of the talc-bearing clasts. The bulk composition of the nodules is reportedly pyroxenite, but Higgins did not indicate whether their source was mantle or crust. The rounded outline of the clasts may reflect magmatic erosion during transport and the present mineralogy is the product of regional metamorphism (Higgins 1986). 27 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �STOP 7: - Monchiquite Dike Michipicoten Post Provincial Park UTM co-ordinates – 0663600E 5309223N NAD83 On the west side of Trans Canada Highway (Highway 17) two, 1-metre wide and one, 3-metre wide monchiquite dikes outcrop at the base of a 3 m high outcrop. The dikes are parallel, strike 060° and crosscut a portion of the Mission Stock. A 4 m wide, east-trending microsyenite dike also can be observed cutting the granodiorite and the monchiquite at this location. The north-northwest-trending Trembley Fault cuts across the north end of the outcrop (Massey 1985). These lamprophyres are most commonly seen in road cuts along the highway and along the shore of Lake Superior. Typically, they are black when fresh and weather to an orange-brown colour. The dikes are usually narrow and are typically less than 1 m wide. Biotite-phyric and olivine-phyric varieties are most common, but pyroxene and feldspar phenocrysts also are observed. Often the dikes show evidence of multiple and composite intrusion, sometimes with tin screens of country rock trapped within the dikes. Lamprophyre dikes south of the Michipicoten River typically have a northeast trend (Massey 1985). Lamprophyre dikes are commonly found cross cutting all lithologies south of the Wawa – Hawk – Manitowik Lakes Fault. Because of their ease of weathering they are infrequently seen in outcrop. However, the dikes are frequently found in underground mine workings. These dikes have a number of macrocrystic resemblances to the dikes seen here. As early as 1927, geologists working in the area had noted a similarity between these lamprophyre dikes and kimberlite (Gledhill 1927). Recent petrographic and mineralogical studies on several dikes in both McMurray and Lendrum townships have suggested that some of the geochemistry falls within the classification of type II kimberlites (orangeites) (Barnett 2001). To date, no diamonds have been recovered from these dikes. These lamprophyre dikes are probably Proterozoic in age and are interpreted to represent Proterozoic alkalic magma emplacement into structures related to the Kapuskasing Structural Zone, perhaps consanguineous with the nearby Firesand Creek carbonatite complex. Rocks from the Firesand Creek carbonatite complex have a U-Pb date of 1078 ± 2.4 Ma (Sage 2000). STOP 8: - Contemplation of the rocks on the fireplace at the Wawa Motor Hotel Acknowledgments The author would like to thank Wayne O’Connor, Northern Sierra Minerals Corporation, Al Shefsky, President, Pele Mountain Resources Inc. and Neil Novak, Vice-President of Exploration, Spider Resources Inc. for permission to access properties described in the field guide. Editorial comments by R. P. Sage also were appreciated during the preparation of the field guide. My thanks also go to Glenn Seim for his technical assistance in the preparation of the guidebook. 28 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Bibliography Arias, Z. G. 1996. Structural evolution of the central Michipicoten greenstone belt, Superior Province, Wawa, Ontario, Canada; unpublished MSc thesis, Queen’s University, Kingston, Ontario, 140p. Arias, Z. G. and Helmstaedt, H. 1990. Structural evolution of the Michipicoten (Wawa) greenstone belt, Superior Province: Evidence for an Archean fold and thrust belt; in Geoscience Research Grant Program, Summary of Research 1989-1990, Ontario Geological Survey, Miscellaneous Paper 150, p. 107-114. Ayer, J. A., Conceição, R. V., Ketchum, J. W. F., Sage, R. P., Semenyna, L. and Wyman, D. A. 2003. The timing and petrogenesis of diamondiferous lamprophyres in the Michipicoten and Abitibi greenstone belts; in Summary of Field Work and Other Activities 2003, Ontario Geological Survey, Open File Report 6120, p. 10-1 to 10-9. Barnett, R. L. 2001. Fletch Kimberlite correspondence, Report of Work, Matchinameigus and Fletch Properties, Dolson & Echum Townships, Sault Ste Marie Mining Division, Northern Ontario; Timmins Resident Geologist’s Office, Dolson Township, assessment file WP–Dolson–4, unpaginated. Cavey, G. 2002. Geological report on the Wawa properties for Band-Ore Resources Ltd. Sault Ste Marie Mining Division, Ontario; unpublished NI 43-101 report, 50p. Cavey, G. 2003. Updated geological report on the Wawa property for Band-Ore Resources Ltd. Sault Ste Marie Mining Division, Ontario; unpublished NI 43-101 report, 35p. Cavey, G. 2004. Summary geological report on the GQ Property for Band-Ore Resources Ltd. Sault Ste Marie Mining Division, Ontario; unpublished NI 43-101 report, 39p. Corfu, F. and Sage R. 1987. A precise U-Pb zircon age for a trondhjemite clast in Doré conglomerate, Wawa, Ontario; in Proceedings and Abstracts, Institute on Lake Superior Geology Annual Meeting, v. 33, p. 18. Corfu, F. and Sage, R. 1992. U-Pb age constraints for deposition of clastic metasedimentary rocks and late-tectonic plutonism, Michipicoten belt, Superior Province; Canadian Journal of Earth Sciences, v. 29, p. 1640-1651. De Stefano, A., Lefebvre, N. and Kopylova, M. 2006. Enigmatic diamonds in Archean calc-alkaline lamprophyres of Wawa, southern Ontario, Canada; Contributions to Mineralogy and Petrology, on-line edition, January 5, 2006. Gledhill, T. L. 1927. Michipicoten Gold Area, District of Algoma; Ontario Department of Mines, Annual Report, 1927, v. 36, pt. 2, p. 1-49. Higgins, M. D. 1986. Nodule-bearing spessartite (lamprophyre) dike near Wawa, Northern Ontario; in Program with Abstracts GACMAC-GCU-AGC-AMC-UCG, vol. 11, p. 81. Kaminsky, F. V., Sablukov, S. M., Sablukova, L. I, Shchukin, V. S. and Canil, D. 2002. Kimberlites from the Wawa area; Canadian Journal of Earth Sciences, v. 39, p. 1819-1838. Ketchum, J., Kamo, S. and Davis, D. 2003. U-Pb ages from the Superior and Grenville Provinces of Ontario; unpublished report of the Jack Satterly Geochronology Laboratory, Toronto. Kjarsgaard, B. A., McClenaghan, M. B., Boucher, D. R. and Kivi, K. 2003. Kimberlites and ultrabasic rocks of the Wawa, Chapleau, Kirkland Lake and Lake Timiskaming Areas; in VIIIth International Kimberlite Conference, Northern Ontario Field Trip Guidebook, B. A. Kjarsgaard (ed) p. 1-37. Lefebvre, N. S. 2004. Petrology, volcanology, and diamonds of Archean calc-alkaline lamprophyres, Wawa, Ontario, Canada; unpublished MSc thesis, The University of British Columbia, Vancouver, British Columbia, 265 p. Lefebvre, N., Kopylova, M., Kivi, K. and Barnett, R. 2003. Diamondiferous volcaniclastic debris flows of Wawa, Ontario Canada; long abstract prepared for VIIIth International Kimberlite Conference, Victoria, British Columbia, Canada. Massey, N. W. D.1985. Geology of the Mishewawa Lake Area, District of Algoma; Ontario Geological Survey, Open File Report 5532, 167p. Morris, T. F. 1999. Overburden as a media for kimberlite, base metal and gold exploration, Wawa region, northeastern Ontario; Geological Association of Canada – Mineralogical Association of Canada, Joint Annual Meeting, GAC-MAC Sudbury 1999, Field Trip B6 guidebook, 67p. Morris, T. F. 2001. Quaternary geology of the Wawa Area, Northeastern Ontario; Ontario Geological Survey, Open File Report 6055, 67p. 29 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Sage, R. P. 1993. Geology of Killins, Knicely and Lalibert townships, District of Algoma; Ontario Geological Survey, Open File Report 5589, 141p. Sage, R. P. 1994. Geology of the Michipicoten greenstone belt; Ontario Geological Survey, Open File Report 5888, 592p. Sage, R. P. 2000. The “Sandor” diamond occurrence, Michipicoten greenstone belt, Wawa, Ontario: A preliminary study; Ontario Geological Survey, Open File Report 6016, 49p. Sage, R. P., England, D., Calvert, T., Oudkerk, G., Worona, R. and Kusciusko, R. 1982. Precambrian geology of Musquash Township, Algoma District; Ontario Geological Survey, Map P.2556, scale 1:15 840. Stachel, T., Blackburn, L., Kurszlaukis, S., Barton, E. and Walker, E. C. 2004. Diamonds from the Cristal and Genesis volcanics, Wawa area, Ontario; in Abstracts of Talks & Posters, 32nd Annual Yellowknife Geoscience Forum, 16-18 November 2004, p. 74-75. Stone, D. and Semenyna, L. 2004. Petrography, chemistry and diamond characteristics of heterolithic breccia and lamprophyre dikes at Wawa, Ontario; Ontario Geological Survey, Open File Report 6134, 39p. Stott, G. M., Ayer, J. A., Wilson A. C. and Grabowski, G. P. B. 2002. Are Neoarchean diamond-bearing breccias in the Wawa area related to late-orogenic alkalic and “sanukitoid” intrusions?; in Summary of Field Work and Other Activities 2002, Ontario Geological Survey, Open File Report 6100, p. 9-1 to 9-10. Turek, A., Sage, R. P. and Van Schmus, W. R. 1992. Advances in the U-Pb ziron geochronology of the Michipicoten greenstone belt near Wawa, Ontario; Canadian Journal of Earth Sciences, v. 27, p. 649-656. Vaillancourt, C., Ayer, J. A., Zubowski, S. M. and Kamo, S. L. 2004. Synthesis and timing of Archean geology and diamond-bearing rocks in the Michipicoten Greenstone Belt: Menzies and Musquash townships; in Summary of Field Work and Other Activities 2004, Ontario Geological Survey, Open File Report 6145, p. 6-1 to 6-9. Vaillancourt, C., Ayer, J. A. and Hamilton, M. A. 2005a. Synthesis of Archean geology and diamond-bearing rocks in the Michipicoten greenstone belt: Results from microdiamond extraction and geochronological analyses; in Summary of Field Work and Other Activities 2005, Ontario Geological Survey, Open File Report 6172, p. 8-1 to 8-11. Vaillancourt, C., Dessureau, G. R. and Zubowski, S. M. 2005b. Precambrian geology of Menzies Township; Ontario Geological Survey, Preliminary Map P.3366, scale 1:20 000 Vaillancourt, C., Zubowski, S. M. and Dessureau, G. R. 2005c. Lithogeochemical data and field photographs for the Wawa area: Menzies and Musquash Townships; Ontario Geological Survey, Miscellaneous Release – Data 151. Walker, E. C. 2002. Diamond deposits of the Festival Property, Wawa, Ontario; Report prepared for Pele Mountain Resources Inc. under National Instrument 43-101, 41p. Walker, E. C. 2003. Diamond deposits of the Festival Property, Wawa, Ontario; Report prepared for Pele Mountain Resources Inc. under National Instrument 43-101, 39p. Williams, F. 2002. Diamonds in late Archean calc-alkaline lamprophyres Ontario, Canada: Origins and implications; unpublished BSc thesis, University of Sydney, Sydney, Australia, 82 p. Wilson, A. C. 2004. Diamond exploration targets, Michipicoten greenstone belt; Canadian Institute of Mining Bulletin, v. 97, no. 1077, p. 41-46. Wyman, D. A., Ayer, J. A., Conceição, R. V., and Sage, R. P. in press. Mantle processes in an Archean orogen: evidence from 2.67 Ga diamond-bearing lamprophyres and xenoliths, Lithos. 30 PDF compression, OCR, web-optimization with CVISION's PdfCompressor � https://digitalcollections.lakeheadu.ca/files/original/97cacb27af6036606c9fbddb715df7e4.PDF 878291d60fa2702c5fc3825c929f6be8 PDF Text Text tFu3 iitiitiuitui blflv..a sri .aridi 7 — - IL - ' r4' -4 V 1. - I -4 'I / Gerald Bennett 2006 Field Trip Guidebook, Volume 52, Part 4 Institute on Lake Superior Geology Sault Ste Marie, Ontario — PDF compression, OCR, web-optimization with CVISION's PdfCompressor �PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Acknowledgements I would like to acknowledge the rocks of the Huronian Supergroup for providing me with so many years stimulating and rewarding work and for teaching me so much, only to eventually show me how much more there was to learn. I would like to thank my wife and children for their love and support during my long absences while a field geologist for the Ontario Geological Survey. Dr. Ron Sage edited an early version of this manuscript and made many useful suggestions. Cover and page layout by Victoria L. Sage, BSc. Cover Photo: Ripple marks in the lower red quartzite member of the Lorrain Formation at Stop S2.2, Highway 17, west of the Town of Desbarats. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Contents IlSG Field Trip No 1- Day 1 ........................................................................................ 1 Part 1 - Overview of the Huronian Supergroup ....................................................... 1 Introduction ................................................................................................................. 1 The Elliot Lake Group ................................................................................................ 4 The Livingstone Creek Formation ................................................................................ 4 Huronian Magmatism ................................................................................................. 7 Introduction ................................................................................................................... 7 Basement Dikes ............................................................................................................. 7 Layered Gabbro/Anorthosite Complexes ...................................................................... 8 Huronian Volcanic Rocks of the Sudbury Area ............................................................ 8 The Huronian Volcanic Rocks of the Sault Ste. Marie-Elliot Lake area ...................... 8 Sedimentary Rocks Associated with the Thessalon Formation .................................. 14 The Matinenda Formation ........................................................................................... 15 The McKim Formation ................................................................................................ 16 Stratigraphic Relationships within the Elliot Lake Group of the Sault Ste. MarieElliot Lake Area. .................................................................................................... 17 Hough Lake Group .................................................................................................... 17 Introduction ................................................................................................................. 17 Ramsay Lake Formation.............................................................................................. 17 Pecors Formation ......................................................................................................... 18 Mississagi Formation .................................................................................................. 18 Aweres Formation........................................................................................................ 19 Quirke Lake Group ................................................................................................... 19 Bruce Formation .......................................................................................................... 19 Espanola Formation ..................................................................................................... 20 Serpent Formation ....................................................................................................... 20 Cobalt Group ............................................................................................................. 20 Gowganda Formation .................................................................................................. 20 Lorrain Formation ....................................................................................................... 21 Gordon Lake Formation .............................................................................................. 22 Bar River Formation .................................................................................................... 22 Nipissing Intrusions ................................................................................................... 21 Huronian Paleosols and Evidence for the Accumulation of Oxygen in the Huronian Atmospher.............................................................................................23 Regional Tectonic Patterns and Metamorphism .................................................... 25 Tectonic Models for the Development of the Huronian Basin .............................. 29 ILSG Field Trip No. 1 - Day 1 .................................................................................. 29 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �The Elliot Lake Transect ........................................................................................... 29 Stop Descriptions and Road Log .............................................................................. 30 STOP 1.1: McKim Formation and Nipissing Diabase ............................................... 30 STOP 1.2: Radioactive quartz-pebble conglomerate of the Matinenda Formation. .. 30 STOP 1.3: Mississagi Formation, Hough Lake Group. ............................................. 35 STOP 1.4: Nipissing Diabase (Gabbro) altered Mississagi Formation, Bruce Formation. ......................................................................................................................... 35 STOP 1.5: Espanola Formation and Nipissing diabase sills. ..................................... 36 STOP 1.6: Stratified Gowganda Formation. .............................................................. 36 Stop 1.7: Laminated varvite? ..................................................................................... 37 Stop 1.8: Gowganda Formation Serpent disconformity............................................. 38 Stop 1.9: Espanola Limestone Member of the Espanola Formation........................... 38 Stop 1.10: Ramsay Lake Formation overlain by Pecors Formation. .......................... 38 STOP 1.11: Huronian volcanic rocks of the Thessalon Formation (Dollyberry Volcanics). ....................................................................................................................... 38 STOP 1.12. Archean metavolanics with pillow structures. ......................................... 39 STOP 1.13: Bar River Formation. ............................................................................... 40 STOP 1.14: Red beds of the Gordon Lake Formation ................................................ 40 STOP 1.15: Gordon Lake Formation. ......................................................................... 40 STOP 1.16: Lorrain Formation.................................................................................... 40 Highway 556 Transect ............................................................................................... 41 Road Log and stop Descriptions .....................................................................................................40 ILSG Field Trip No. 1 -......................................................................................................................40 Day 2 - Part 1............................................................................................... ...................................40 Huronian Stratigraphy along Highway 556 and correlation with the Chocolay Group of the Marquette area. ............................................................................. 40 STOP 2.1: Island Lake Fault Zone ............................................................................41 STOP 2.2: Gowganda Formation unconformably overlying Conglomerate of the Aweres Formation. ................................................................................................. 45 STOP 2.3: Drop-stones of the Gowganda Formation ............................................... 45 STOP 2.4: Contact between Gowganda Formation and Huronian metabasalt of the Thessalon Formation .............................................................................................. 46 STOP 2.5: Archean granitic rocks and Gowganda Formation. ................................. 46 STOP 2: 6: Jasper Pebble Conglomerate Member of the Lorrain Formation. .......... 46 STOP 2.7: Outcrop of Archean metavolcanic rocks. ................................................ 46 STOP 2.8: Outcrops of Dolostone at the Dolostone Knob. ...................................... 48 STOP 2.9: This stop will be visited as time allows. ................................................... 48 Geological Features and Correlation of Dolostone Unit in Fenwick Township Northeast of Sault Ste Marie ............................................................................... 49 Correlation with the Gordon Lake Formation............................................................. 50 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Correlation with the Chocolay Group of Michigan .................................................... 50 Road Log and Outcrop Descriptions. ...................................................................... 51 STOP S1-1: Lowermost Gordon Lake Formatiom......................................................................51 Supplimentary Road Log for Highway 638 49 STOP 2.9: Dolostone nodules and discontinuous dolostone beds in the Gordon Lake Formation. .............................................................................................................. 52 STOP S1-2: Mudstone overlying matrix-supported conglomerate (diamictite) of the Gowganda Formation. ............................................................................................ 52 STOP S1-3: Dropstones in laminated siltstone and mudstone of the Gowganda Formation. .............................................................................................................. 53 STOP S1-4: Diamictite of the Gowganda Formation. ............................................... 53 STOP S1-5: Jasper pebble conglomerate of the Lorrain Formation (“Pudding Stone”). .............................................................................. 53 Supplimentary Road Log for the area between Sault Ste Marie and Highway 108 ....................................................................................................... 54 STOP S2.1: Sub-Jacobsville unconformity ................................................................ 54 STOP S2.2: Lower red quartzite member of the Lorrain Formation. ......................... 54 STOP S2.3: Purple siltstone member of the Lorrain Formation. ................................ 54 STOP S2.4: Basal Arkose Member of Lorrain Formation. ......................................... 54 STOP S2.5: Bruce Mines Copper Vein. ...................................................................... 54 STOP S2.8: Outcrop of Matinenda Formation on east side of Highway 129. ........... 55 STOP S2.9: Laminated siltstone of the Gowganda Formation with dropstones. ....... 55 STOP S2.10: Rhyolite of the Thessalon Formation. .................................................. 55 STOP S3.1: Mineralized quartz vein breccia. ............................................................ 56 STOP S3.3: McKim Formation - Staurolite (pseudomorph) schist. ........................... 58 STOP S3.2: Pronto Mine Location............................................................................. 58 Publications Cited or Consulted .............................................................................. 59 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �List of Figures Figure 1.1: The Distribution of the Huronian Supergroup in Ontario…………… 1 Figure 1.2: Rock- Time chart for southeast Lake Superior region……………… 2 Figure 1.3: Stratigraphic column in the Elliot Lake area…………………….... 3 Figure 1.4: The cyclicity of Huronian Formations………………………….…… 4 Figure 1.5: Paleocurrent directions in the Huronian Supergroup…………......... 5 Figure 1.6: The distribution of some formations of the Elliot Lake Group…… 6 Figure 1.7: Stratigraphic relationships within the Elliot Lake Group…………… 9 Figure 1.8: Photo of anorthosite of the East Bull Lake Suite, Foul Bight ………… 10 Figure 1.9: Thessalon basalt overlying Livingstone Creek Fm ……………….… 10 Figure 1.10: Internal stratigraphy of the Thessalon Formation………………… 11 . Figure 1.11: Discrimination diagrams for volcanic rocks of the Thessalon Fm…… 13 Figure 1.12: Photo of radioactive quartz-pebble conglomerate and grit overlying the Livingstone Creek Formation……………………………………………… 14 Figure 1.13: Quartz pebble size vs. pyrite size in Matinenda ore beds……..... 15 Figure 1.14: Diamictite of the Bruce Formation, Highway 108…………… 19 Figure 1.15: Concentration ratios for sub-Thessalon, sub-Matinenda and sub-Lorrain Fm. paleosols………………………………………………… 24 Figure 1.16: Geological cross-section of the Blind River and Sudbury-Manitoulin areas. ……………………………………………… 27 Figure 1.17: Metamorphism of the Huronian Supergroup…………………… 28 Figure 1.18: Index maps for geological maps and some areas referred to in the text………………………………………………………… 31 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Figure 1.19: Geological Map of Quirke Lake Syncline ……………………... 33 Figure 1.20: Legend for Figure1.19 and 1.21……………….......................... 32 Figure 1.21: Geological maps of the Flack Lake area………………………... 34 Figure 1.22: Trough cross-bedding in the Mississagi Fm……………………... 33 Figure 1.23: Metamorphosed Espanola Fm………………………………….... 36 Figure 1.24: Stratified conglomerate of the Gowganda Fm. ……………............ 37 Figure 1.25: Dropstone in laminated siltstone of the Pecors Fm………….......... 39 Figure1.26: Pillow Structure in Archean metavolcanics ……………….......... 40 Figure 2.1: Geological map of the Highway 556 transect. ………………....... 42 Figure 2.2: Stratigraphic column for the Highway 556 transect. ………..…..... 43 Figure 2.3: A diagrammatic cross-section along the Highway 556 transect...... 44 Figure 2.4: Gowganda Formation unconformably overlying Aweres Formation. 44 Figure 2.5: Photo of a “drop-pebble” in laminated Gowganda argillite. ……. 44 Figure 2.6: Polymictic conglomerate of the Aweres Formation……………… 45 Figure 2.7: Dolostone of the Kona Formation intruded by a mafic dike. …… 47 Figure 2.8: Dolostone and chert, Fenwick Township………………………..... 47 Figure 2.9: Dolostone nodules in the Gordon Lake Formation ………………… 47 Figure 2.10: Jasper-pebble conglomerate of the Lorrain Fm…………………… 47 Figure 2.11: Map of Dolostone are in Fenwick Township …………………… 49 Figure 2.12: Correlation of the dolostone occurrences of Ontario and the Marquette area, Michigan ……………………………………………... 50 Figure 2.13: Geology of the Pronto Mine……………………………................. 57 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �1 ILSG Field Trip Number 1 - Day 1 Part 1 - Overview of the Huronian Supergroup These first pages are in tended to give participants new to the Huronian Supergroup of Ontario a summary of what we think we knowof these ancient rocks. Much of the material is borrowed from an earlier ILSG guidebook (Bennett et al , 1997). The important ontributions to that guidebook by K. D. Card and Kirsty Tomlinson are acknowledged with thanks. The Huronian Supergroup is one of the Earth’s most studied sequences of rocks. Since the turn of the century the results of hundreds of studies of Huronian rocks have been published in scientific journals and government publications. These studies have led geoscientists, to present evidence for the Earth’s earliest glacial periods, the development of free oxygen in the atmosphere of the early Earth, the deposition of paleoplacer deposits of uranium, and evidence for plate tectonic activity during the Paleoproterozoic. Much of the evidence presented is based on rock exposures, which will be visited during this field trip. The 2219 Ma radiometric age of the Nipissing intrusions places an upper limit on the age of the Huronian Supergroup, while the Copper Cliff Ryolite (2450 Ma) is probably close to the date of initial Huronian deposition in the Sudbury area. The Huronian Supergroup consists of four groups, which in ascending stratigraphic order are: The Elliot Lake Group, Hough Lake Group, PROTEROZOIC Mid-Late Early Archean Hudson Bay Pr ov inc e Introduction nv i lle e North The Huronian Supergroup, of the James Bay Su Southern Tectonic Province of the peri c or Provin Canadian Shield, is a sequence of Range Paleoproterozoic sedimentary and minor Marquette Supergroup re G volcanic rocks lying unconformably 250 km e c Huronian n i v upon Archean rocks of the Superior o Southern Pr Supergroup Province of the Canadian Shield. The Huronian rocks extend eastward from Lake Superior, along the north shore of Figure 1.1: Distribution of Early Proterozoic Rocks Lake Huron to Sudbury and then in the Great Lakes region. northward to the Noranda area of Quebec, a distance of about 450 km Quirke Lake Group and Cobalt Group (Figure (270 miles) (Figures 1.1, 1.2). The Huronian 1.3). Formations of the three upper groups, with Supergroup attains its greatest thickness of the exception of the Serpent Formation of the 12,000 meters (40,000 feet) southeast of Quirke Lake Group, show stratigraphic Sudbury. The sequence thins northward due to continuity, and display a remarkable cyclicity of the wedging out of basal units, thinning of lithological units. Each cycle begins with clastic units, and erosion within the sequence matrix-supported conglomerate (Roscoe, 1969; Frarey and Roscoe, 1970). (diamictite/mixtite), followed by mudstone, gb 2006 (after Jackson, 2001) PDF compression, OCR, web-optimization with CVISION's PdfCompressor �2 G Erosion Erosion LEGEND 200 500 Michigan 600 700 800 900 Erosion 1000 1900 2000 2100 2200 2300 2400 2500 2600 2700 2800 Continental glaciation Radiometric age determination Penokean Orogeny Chocolay Group Folding, faulting, metamorphism Erosion Nipissing Intrusions G G Huronian volcanics G Layered gabbro Dike Swarms Kenoran Orogeny Elliot Lake, Ont. Sault Ste. Marie, Ont. Erosion Free oxygen accumulating in the atmosphere 1800 G G Huronian Supergroup Rifting Erosion folding faulting metamorphism methane-rich atmosphere (no oxygen) 1700 Mainly granite Erosion East Lake Superior, Ont. 1600 Michigan 1500 Marquette Range Supergroup 1400 Thousands of years before the present Proterozoic 1300 Mainly gabbro Keweenawan Mid-continent Rifting Supergroup 1100 1200 Volcanic Rocks Elliot Lake, Ont. 400 Sedimentary Rocks Sault Ste Marie, Ont. 300 East Lake Superior, Ont. Michigan Basin Phanerozoic 100 Archean Pleistocene Glaciation 2900 3000 G. Bennett, 1994-2005 Figure 1.2: A rock-time chart for the southeast Lake Superior region. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �3 Nipissing Diabase (2.22Ga) Gabbro, diabase, granophyre Stop Number Quartz arenite, subarkose minor mudstone, siltstone. 300 m 1.14, 1.15 Gordon Lake Fm. 500 m 1.16 Siltstone, mudstone chert, sandstone Cobalt Group Bar River Fm. 1.13 Quartz arenite, quartzpebble conglomerate Arkose, subarkose Lorrain Fm. 2500 m Mudstone, siltstone 1.6, 1.7 Gowganda Fm. Arkose, siltstone 600 m (top missing) Diamictite, polymictic conglomerate Subarkose, wacke 250m 1.5, 1.9 Espanola Fm. 200 m Diamictite, sandstone siltstone Bruce Fm. 1.4 Dolostone , siltstone, wacke limestone 50 m Subarkose, arkose Mississagi Fm. quartz-pebble 1.3 1.10 450m conglomerate Pecors Fm. Mudstone, siltstone 127 m Ramsay Lake Fm. Diamictite Disconformity Hough Lake Group Serpent Fm. Quirke Lake Group Unconformity 1.8 20 m McKim Fm 1.2 Matinenda Fm 0 - 80 m U, Th 1.11 East Bull Lake Suite 2.48 Ga Gabbro anorthosite Arkose, subarkose quartz-pebble conglomerate Disconformity 180 m Thessalon Fm. Disconformity 0 - 100 m? Livingstone Creek Fm. 0 - 10 m? Archean Basement 1.12 Metavolcanics, metasediments, granitic rocks Stop Numbers Mafic volcanics Minor qtz. peb. cong. Elliot Lake Group U, Th U, Th Mudstone, siltstone wacke 1.1 Subarkose, conglomerate Unconformity The thickness of formations was determined from three drill holes drilled near the transect. gb 1996, 2006 Figure 1.3: A Stratigraphic column for the Elliot Lake transect PDF compression, OCR, web-optimization with CVISION's PdfCompressor �4 siltstone or limestone and capped by a thick sequence of coarse, cross-bedded sandstone( Figure 1.5). Many paleocurrent studies have shown paleocurrent flowed south to southeast, with southeast being the predominant direction (Figure 1.5). The Elliot Lake Group The Elliot Lake Group differs from the overlying Huronian groups in that: (1) Its internal stratigraphy is generally discontinuous and less extensive. (2) It does not show the diamictite-mudstone-sandstone sequence of the overlying groups. (3) It contains the only important uranium deposits and the only volcanic rocks of the supergroup. (4) Most formations have disconformable surfaces. The Livingstone Creek Formation Clast-supported, polymictic conglomerate is the prominent rock type in the lower sections of the Livingstone Creek Formation of most areas. Cobble- to boulder-sized clasts generally of grey granitic rocks and minor mafic plutonic and metamorphic rock clasts are set in a sparse matrix of coarse, grey arkose or arkosic grit. The writer has not observed clasts of Huronian volcanic rocks in these conglomerates. Thin units of cross-bedded, grey arkose are locally interbedded with the conglomerate (Frarey, 1977, Bennett et al., 1991). The granitic mega-clasts of the conglomerate member are predominately pale grey in contrast with the predominantly reddish hues of the underlying Archean basement rocks. A few of the granitic Main environment of deposition COBALT GROUP Main rock types Arkose, subarkose, quartz arenite quartz-pebble conglomerate Disconformity Cycle 2 QUIRKE LAKE GROUP Cycle 3 Bar River Formation Gordon Lake Formation Lorrain Formation Gowganda Formation Serpent Formation Espanola Formation Bruce Formation Fluvial Mudstone, siltstone, carbonate (Espanola Fm.) Marine HOUGH LAKE GROUP Diamictite, mudstone, siltstone clast supported conglomerate Mississagi Formation Pecors Formation Ramsay Lake Formation Elliot Lake Group Cycle 1 Reducing Atmosphere inferred from grey-hued sandstones. Oxidizing atmosphere inferred from presence of red sandstones The conglomerates and sandstones of the Livingstone Creek Formation (Frarey, 1967, 1977) form the lowermost Huronian formation. The Livingstone Creek Formation is at least 1200 feet (400 m) thick in the Sault Ste Marie area and between 300 feet (100 m) and 1000 feet (300 m) thick in the Thessalon area. It consists of two distinctive rock types: an upper, well-sorted, grey sandstone and clast-supported polymictic conglomerate (Bennett et al, 1991), (Figures 1.3, 1.6, 1.7). Glaciogenic Volcanic rocks, marine and fluvial deposits. Paleoplacer uranium deposits Note: Where present, the upper part of the Gowganda Formation is predominantly mudstone. Figure 1.4 The cyclicity of Huronian rocks PDF compression, OCR, web-optimization with CVISION's PdfCompressor �km Elliot Lake Paleocurrents - Matinenta Fm. Lake Huron 50 Huronian Supergroup Figure 1.15: Paleocurrents in the Mississagi and Matinenda Formations N Sault Ste Marie 0 Post Huronian rocks LEGEND Paleocurrents - Mississagi Fm. Archean rocks Some modifications made for display purposes. Data from Long, 1977 and McDowell, 1957. Sudbury Igneous Complex Sudbury 5 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �No subsurface information No subsurface information Murray Fault City of Pecors Lake Elliot Lake Area East Bull Lake Complex No Huronian volcanic rocks (from Drilling Dolleyberry lake area Archean rocks Fault Gabbro, anorthosite East Bull Lake Suite Sandstone , polymictic conglomerate (from outcrop data). Livingstone Creek Fm. The areal extent of some units are exaggerated Crazy Lake Area No Huronian volcanic rocks (from drill hole data) Lake Huron Foul Bight No Huronian volcanic rocks (from drill hole data)) Mafic dikes of Thessalon Fm. intrude Livingstone Creek Fm. East Bull Lake Suite present (From drill hole data) No subsurface information elt eB ak rL No subsurface information Quartz pebble conglomerate at base of Thessalon flows. (locally radioactive) Haughton Tp. area pe Co o Thessalon ) ke ke La La ss en Ba rde rea e a b (A Figure 1.6: The distribution of some formations in the Elliot Lake Group Sault Ste. Marie No subsurface information Duncan Volcanic belt Basalt, andesite, rhyolite etc. (From outcrop and drill hole data) Thessalon Formation Upper Huronian rocks Meso-Proterozoic and Phanerozoic rocks LEGEND 6 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �7 megaclasts in the predominately grey conglomerate near the south end of Pine Ridge Road show the distinct texture of the typical Archean, massive, pink, k-spar megacrystic granite – but with only a hint of the pink color in the phenocrysts. The grey colour of the Livingstone Creek Formation conglomerates appear to be due to the reduction of ferric iron in the feldspars of the granitic clasts, and not a result of differing provenance as some authors have suggested. This conclusion is supported by the writer’s observation that granitic rocks in a “paleosol zone” a few meters to a few tens of meters below the base of the Livingstone Creek Formation commonly display a grey colour as well. conglomerate member occurs as far east as Pecors Lake (Figure 1.6) on the south limb of the Quirke Lake Syncline (Jensen, 1990) and below Huronian volcanic rocks south of Stinson Lake in the Elliot Lake area. The Livingstone Creek Formation has not been recognized east of the Quirke Lake Syncline. The clast size, local source and low stratigraphic position of the Livingstone Creek conglomerates are consistent with deposition on an alluvial fan. The uniform, fine- to medium-grained sand of the trough cross-bedded sandstone member suggests a different, although probably related, more distal depositional environment than that of the conglomerate. The sandstone member may represent deposition by median streams flowing in a fault-bounded valley with walls of Archean rocks partly covered by alluvial fans (Bennett et al., 1991). The well-sorted nature of the sandstone suggests an aeolian component or even aeolian deposition as proposed by Meyer (1983). The grey, sandstone member of the Livingstone Creek Formation may be distinguished from most Huronian sandstones by the uniform grain size (fine- to medium-sand) of the former. Mudstone and pebbly units are lacking within the sandstone member of the Livingstone Creek Formation. Another characteristic feature of the sandstone units is the presence of carbonate along the foreset beds of the commonly Huronian Magmatism well-developed trough cross-beds. Introduction At Maud Lake in Duncan Township near Sault Ste Marie fine to medium-grained, pale-grey quartz-arenite forms the upper few meters of the Livingstone Creek Formation, suggesting a more prolonged period of weathering in that area. Four distinct more-or-less coeval (ca. 2450 Ma), post-Kenoran igneous rock sequences are associated with the Huronian Supergroup: (1) Mafic dike swarms in the basement rocks but not found cutting the Huronian Supergroup. In addition to the well-known exposures in the (2) Igneous complexes of layered Thessalon and Sault Ste Marie areas the gabbro/anorthosite. (3) Mafic to felsic volcanic Livingstone Creek Formation has been flows. (4) 2.48 Ga. felsic plutons in the Sudbury correlated with grey sandstone and conglomerate area (Krogh et al., 1996). near Crazy Lake in Nicholas Township (Bennett, The widespread intrusion of post-Huronian (Ca 1978; Bottrill, 1971) (Figure 1.6). The writer 2200 Ma) Nipissing gabbro/diabase dikes and correlated a basal grey sandstone unit directly sills have yet to be placed in a plate tectonic underlying the Matinenda Formation in context. Haughton Township (Figure 1.6) with the Livingstone Creek Formation (Bennett et Basement Dikes al.1991). An area of clast-supported, grey granite-cobble conglomerate near Samried Lake The north to northwest trending Matachewan-Hearst dike swarm is the second reported by Jackson (2001) is probably an largest dike swarm of the Canadian Shield. The erosional remnant of the Livingstone Creek 2.45 Ga age of the swarm (Heaman, 1988) is Formation. Other probable remnants of the PDF compression, OCR, web-optimization with CVISION's PdfCompressor �8 essentially the same as that of the East Bull Lake thick), and Stobie Formations (1500 m thick) and the felsic, Copper Cliff Formation (760 m Suite and the Huronian volcanic rocks of the thick) . Krogh et al. (1984) obtained and age of Copper Cliff Formation. 2450+25-10 Ma (U-Pb zircon) age for rhyolite of the Layered Gabbro/Anorthosite Complexes Huronian Copper Cliff Formation; the only Huronian supracrustal rock for which there is an Several petrographically distinctive, mafic to absolute age. The volcanic rocks of the Sudbury ultramafic sills and cone-shaped bodies have been recognized between the eastern end of the area show evidence of submarine eruption from fault controlled vents along the edge of a Sudbury Igneous Complex and the nose of the depositional basin into which arkosic sandstones Quirke Lake Syncline. The intrusions are were being transported from the Archean characterized by the presence of anorthositic phases, and locally by a well-developed, primary granitic terrain to the north, while turbidites rhythmic layering of alternating anorthositic and were deposited from sediment from both the volcanic and rejuvenated basement marginal to gabbroic layers. Segregations, dikes and the basin (Card, 1978). sheet-like bodies of granophyre are locally present (Card and Palonen, 1976; Peck et al., The volcanic rocks of the Sudbury differ in 1995). Major intrusions of gabbro-anorthosite terms of internal stratigraphy, overall thickness are found at Agnew Lake (Card and Palonen, and depositional environment from the 1976) and at East Bull Lake (Born and James, Huronian volcanic rocks in the Sault Ste 1978; Kamineni et al., 1984) Marie-Elliot Lake area. Krogh et al. (1984) reported a U-Pb age The Huronian Volcanic Rocks of the Sault 2491+5-5 Ma for zircons from the Ste Marie-Elliot Lake area gabbro-anorthosite intrusion at Agnew Lake and 2480+10-5 for the East Bull Lake Frarey (1967) named the Huronian volcanic gabbro-anorthosite body. The similarity of these rocks in the Thessalon and Sault Ste Marie ages to the 2450+25-10 Ma (U-Pb zircon) age areas that overlie the Livingstone Creek of rhyolite of the Huronian Copper Cliff Formation as the “Thessalon Formation” Formation (Krogh et al., 1984) suggests that (Figures 1.3, 1.6). these gabbro-anorthosite intrusions form part of This writer has examined all known exposures of the early Huronian volcanic events. Bennett et Huronian volcanic rocks as well as all available al. (1991) report two additional bodies between drill-core and drill-hole logs reporting volcanic Blind River and Thessalon. Layered anorthosite rocks in the Elliot Lake–Sault Ste Marie area. It and associated coarse-grained gabbroic rocks was concluded that there is no credible evidence form a sill-like body about 5 km long at Foul for more than one period of Huronian volcanism Bight on the North Shore of Lake Huron in the area of the study and that all the (Figure 1.8). A drill-hole drilled from the ice Huronian volcanic rocks west of the nose of the south of the Town of Thessalon intersected Quirke Lake Syncline are stratigraphically layered gabbroic rocks (Figure 1.6). All correlative with the Thessalon Formation gabbro-anorthosite bodies found to date have (Figure 1.6)(Bennett 1978, Bennett et al 1991). been emplaced in the Archean basement at, or just below, the Archean-Huronian boundary. Unfortunately, none of the many attempts to obtain an absolute age determination from rocks Huronian Volcanic Rocks of the Sudbury of the Thessalon Formation have been Area successful. However, the spatial association of The Huronian bimodal volcanic sequence in the the East Bull Lake intrusions with the Huronian volcanic rocks of the Quirke Lake Syncline Sudbury area has been subdivided in the suggests Huronian volcanism in that area predominately mafic Elsie Mountain (1000 m PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Th,U,py THESSALON AREA py HAUGHTON-OTTER TWPS Feeder dike of Thessalon Fm. Th, U, py Fault U,Th, py U,Th,py ELLIOT LAKE AREA Unconformity or disconformity Quartz-pebble conglomerate Granite, greenstone ARCHEAN BASEMENT Grey arkose, subarkose Polymictic conglomerate LIVINGSTONE CREEK FM. Basalt, andesite, rhyolite flows basalt dike THESSALON FM. CRAZY LAKE AREA NICHOLAS TWP. Paleosol on Livingstone Cr. Fm and basaltic dike (Thessalon Fm.) The thickness of some units is exaggerated purposes of visualization Figure 1.7: Stratigraphic relationships in the Elliot Lake Group. METERS SAULT STE MARIE AREA 1000 LEGEND Arkose, grit quartz-pebble conglomerate MATINENDA FM. ELLIOT LAKE GROUP HOUGH LAKE GROUP 9 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �10 - -4 JjJ622910LJ EW Figure 1.8: Layered anorthositic gabbro. Foul Bight, Lake Huron. Figure 1.9: Stratigraphic relationships at Cullis Lake, Thessalon area. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �11 Thessalon Basalt Upper Island Lake Bass Lake 500 m 1200 m 650-850 m Unit6 Unit 1 Unit 6 Unit 6 Rhyolite Unit 5 Basalt to andesite cycles Mg-lava Radioactive, Quartz-pebble conglomerate Livingstone Creek Fm. Unit 4 Unit 3 Unit 3 Unit 2 Unit 1 Unit 1 Livingstone Creek Fm. I'll Unit 7 ? Livingstone Creek Fm. From Tomllinson, 1996 Figure 1.10: Internal Stratigraphy of the Thessalon Formation in the Sault Ste Marie Area occurred about the same time as the emplacement of the East Bull Lake intrusions (2480 Ma). Given the potential error for the relevant age dates (Krogh et al., 1984), the age of the Thessalon Formation may not differ significantly from that of the Copper Cliff Formation of the Sudbury area (2450 Ma). The maximum thickness of the Thessalon Formation in the Sault Ste Marie area is approximately 650 m (2100 feet) and 820 m (2700 feet) (Frarey, 1977). Diamond drilling has indicated at least 670 m (2200 ft) of Thessalon volcanics under Lake Huron south of the town of Thessalon. The Thessalon Formation may be as much as 1080 m (3500 feet) thick north of Bass Lake in Aberdeen Township. in the Dollyberry Lake, Pecors Lake and Thessalon areas. The lower flow sequences show much lower concentrations of Ni, Cr, and contain higher amounts of Ti and P than the upper flows. In the Dollyberry Lake area the upper basalts appear to be missing. This is assumed to be due to erosion (unpublished analyses in the writers files). Tomlinson (1996), completed a comprehensive analysis the geochemistry of the Huronian volcanic rocks between Sault Ste Marie and Thessalon, concluded that the lavas of the Thessalon Formation are divisible into 7 distinct units based on mapping, petrography and major and trace element geochemistry (Figures 1.10, 1.11). The 7 units were grouped into two “lava In the Sault Ste Marie, Thessalon and Aberdeen series”. The upper lava series (units 6) of Lake areas the Huronian volcanics rocks can be Tomlinson, 1996) is equivalent to the upper tholeiitic basalt sequence of Bennett et al. subdivided into upper tholeiitic basalt unit and lower complex or “mixed member”(Bennett et al (1991). The lower lava series (units 1-5, of Tomlinson, 1996) consists mainly of basaltic 1991) which includes more fractionated rocks andesite with subordinate, local rhyolite, including basaltic andesite, tholeiitic andesite, mugearite, andesite and high magnesium basalt mugearite, hawaiite, rhyolite, basalt. flows; and corresponds to the “diverse member” Magnesium-rich basalts with some of the chemical characteristics of komatiites are present of Bennett et al. (1991). PDF compression, OCR, web-optimization with CVISION's PdfCompressor �12 Bennett et al. (1991) proposed that the upper, basaltic flows of the Thessalon Formation (upper lava series) probably represent part of a continental flood basalt sequence, while the diverse member (lower lava series) appears to have erupted from central vents. The Huronian volcanic rocks of the Quirke Lake Syncline display lithological and geochemical similarities to the lower lava series of the Thessalon Formation west of the Quirke Lake Syncline (G. Bennett, unpublished data). The upper, tholeiitic basalt flows of the Thessalon Formation (upper lava series of Tomlinson, 1996) are almost uniformly greenish-grey fine- to medium-grained tholeiitic metabasalt. The essential minerals are albite, actinolite, chlorite, clinozoisite, epidote and Fe-Ti oxides. Primary clinopyroxene is present in only a few samples of basalt from the Sault Ste Marie area. The andesitic rocks of the lower lava series of are generally darker coloured and commonly contain stilpnomelane and biotite with green pleochroism (Fe+3 rich?) in addition to albite and actinolite (Bennett et al. 1991). Tomlinson (1996) reported the presence of tremolite in some flows. Quartz is a minor component of basaltic and andesitic types. In most areas the metamorphic grade of the Thessalon Formation flows is lower greenschist facies although the presence of albite and primary pyroxene along with elevated soda indicates sub-greenschist grades at the northern end of the Duncan volcanic belt near Sault Ste Marie. “The field and geochemical data indicate that the lavas are comparable to modern day continental basalt and andesites but that their source was similar to that of island arc basalt. Figure 1.11 shows the lavas plotted on a variety of discrimination diagrams where they are most comparable to modern within-plate basalt. Their arc-basalt geochemical characteristics result from their derivation from an upper mantle that had been metasomatically enriched during subduction in the Kenoran orogeny. Units within the sequence were generated from different pulses of magma, each of which stemmed from a different region of the relatively homogenous Huronian mantle. Only the unit 2 lavas require generation from part of the mantle with slightly different characteristics to a source similar to that required to generate island arc basalt ID16 (a primitive basalt used by Tomlinson (1996) in making calculations. gb). Different degrees of partial melting of this source that contains garnet in the residue (at less than 29.8% partial melting) are responsible for the REE characteristics of each unit. In the generation of the lower lava series, different batches of magma underwent crustal contamination to a greater or lesser extent within the lower or upper crust (or both) prior to eruption. The upper lavas series rocks are not contaminated. In all but the unit 2 lavas crystal fractionation occurred both early, as magma ascended to shallower depths, and later, in shallow level sills or magma chambers. Early fractional crystallization was responsible for the low Mg, Ni and Cr values in most units, while later stage crystal fractionation was responsible for the major element and compatible element trends within single units, as shown on bi-element plots”(Tomlinson, 1996). Amygdules of epidote, chlorite, calcite, quartz and stilpnomelane, in complex zonal arrangements, are common. Flattened chlorite-filled amygdules a centimeter or less across is a distinctive feature of most mafic flows of the Thessalon Formation. Pillow structures are rare but were observed in most areas. Scoriaceous flow-tops and crosscutting breccias On the geochemistry and tectonic setting of the are commonly filled with a fine-grained mixture Thessalon volcanic rocks Tomlinson (1996) states: of quartz and grey to red secondary albite. Tomlinson (1996) states: PDF compression, OCR, web-optimization with CVISION's PdfCompressor �13 V - M!1PIU B- pA .100 JO kiP S .1 - MbV : VI VII 3- E 2- Mbk: VII C ki AVB: CD - B D A Figure 1.11: Discrimination diagrams for the Thessalon Formation in the Sault Ste Marie area. From Tomlinson, 1996. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �14 “Various lines of evidence from the Elliot Lake Group as a whole can be used to characterize the Huronian rifting event. Active rifting involves deep mantle upwelling or plume magmatism, which splits the continental crust, whereas passive rifting involves mantle diapirism and adiabatic upwelling induced by differential stresses and stretching in the lithosphere....…..The geochemical evidence from the Thessalon Volcanic Formation indicates that the source of the lavas was metasomatised upper mantle rather than a deep mantle or plume component. Structural subsidence patterns in the Archean basement (Zolnai et al., 1984) are thought to be responsible for lithospheric stretching, in-turn causing mantle upwelling, episodic partial melting and volcanism. This is supported by the presence of syndepositional faults in up to Archean basement. These features indicate that initially volcanism was a consequence of rifting. In active rifts just one uplift and melting event occurs as a plume impacts the lithosphere, but in passive rifts uplift and melting are episodic ........... Multiple erosional surfaces within the Elliot Lake Group indicate that numerous episodes of uplift occurred. Based on these lines of evidence the Huronian rifting event can best be characterized as a typical passive rifting event” (Tomlinson, 1996). Jolly (1987) concluded the Thessalon Formation is a continental flood basalt sequence associated with continental rifting. Sedimentary Rocks Associated with the Thessalon Formation Some early reports refer to the presence quartz-pebble conglomerate within the Livingstone Creek Formation. The writer has examined most of the known occurrences of the Livingstone Creek Formation between Sault Ste Marie and the Quirke Lake Syncline and found no interbedded quartz –pebble conglomerate. However at many locations a thin unit (< 1 m) of radioactive, pyritic, quartz-pebble conglomerate overlain by a few meters of coarse, arkose was found to lie upon the Archean Figure 1.12: Sandstone of the Livingstone Creek Fm. Overlain by basement or directly upon radioactive quartz-pebble conglomerate and grit of the Thessalon the Livingstone Creek Fm. Thessalon Township. Formation where the latter is present. In the absence of the Livingstone Creek Formation the 400 meters of Huronian sedimentary rocks quartz-pebble conglomerate lies directly upon the below the volcanics. G. Bennett (personal basement rocks. In Duncan Township of the communication, 1996) has also described Sault Ste Marie and Thessalon areas this syndepositional features (at 2 locations near quartz-pebble conglomerate-arkose sequence is Elliot Lake and Sault Ste Marie) where found within the lower flows of the Thessalon sedimentary rocks of the Livingstone Creek Formation. (Hay, 1963; Bennett et al., 1978; Formation infill fractures in the underlying Meyer, 1983) (Figures 1.6, 1.7, 1.9, 1.12). PDF compression, OCR, web-optimization with CVISION's PdfCompressor �15 The Matinenda Formation Size of pyrite grains (mm) Bennett et al. (1991) proposed that the conglomerate-arkose units indicate the presence of a disconformity between the Huronian volcanics of the Thessalon Formation and the Livingstone Creek Formation. The wide distribution of these units (Figure 1.6) suggests that they reflect early Huronian erosional period of regional extent. The resistate nature of the mineral assemblage in the quartz-pebble conglomerate (assuming an oxygen deficient atmosphere) points to a period of extreme weathering. Some of this 2.4 quartz-rich regolith may still be visible as a quartz breccia upon the granitic basement west of 1.8 Highway 639 (Stop 1-11of this guide). mudstones of the McKim Formation (Card, 1978). The most abundant rock type of the Matinenda Formation in the Elliot Lake area is generally described as medium- to coarse-grained subarkoses, arkoses and grits consisting of poorly sorted quartz and feldspar grains set in a matrix of sericite and comminuted rock and mineral Correlation Coefficient = 0.93 1.2 The Matinenda Formation of 0.6 the Elliot Lake Group is a sequence of arenites and From Theis, 1976 intercalated quartz-pebble conglomerates which host the 7 14 21 28 35 42 49 56 63 once important uranium Pebble size (mm) deposits of the Elliot Lake Figure 1.13: Quartz pebble size vs. pyrite grain size area. The Matinenda in the ore beds of the Matinenda Formation. Formation is up to 180 m (600 feet) in the Elliot Lake area where it lies on Huronian volcanic rocks and fragments. The ratio of K-spar to sodic feldspar Archean basement (Roscoe, 1969; Robertson, is about 8:1. Minor constituents are pyrite, 1968,1976). In the Thessalon and Sault Ste calcite, chlorite, zircon and rarely, leucoxene Marie areas the Matinenda Formation consists coated iron oxides and monazite. Variable predominantly of fine-to medium-grained, amounts of sericite produce what has various subarkose to subwacke and is probably less than been described as a green, apple green or 50 m (150 ft) thick (Bennett et al., 1978). In greenish-yellow colour. Well-sorted Haughton Township the Matinenda Formation quartz-pebble conglomerate beds, containing lies upon grey sandstones that the writer equates well-rounded pebbles and cobbles of quartz and with the Livingstone Creek Formation (Figure chert, and pebbly subarkose units are scattered 1.7). throughout the coarse subarkose units of the In the Sudbury area clastic units correlated with Matinenda Formation, but are more common the Matinenda Formation are as much as 600 m near the base. (Robertson, 1968; Pienaar, 1963). (2000 ft) thick but they thin rapidly eastward and is intercalated with the mainly metavolcanic Trough cross-bedding, scour and fill structures rocks of the Stobie Formation and the are common in arkosic units (Robertson, 1968; Roscoe, 1969). Paleocurrent studies by PDF compression, OCR, web-optimization with CVISION's PdfCompressor �16 McDowell (1957) and Long (1977) have established a northwest source area for the sediment of the Matinenda Formation (Figure 1.5). Fralick and Miall (1989) suggest the Matinenda Formation was deposited from shallow braided streams flowing down a south dipping paleoslope which underwent tilting to the southeast during deposition. Kimberly et al. (1980) reported that the uraniferous conglomerates contained almost no magnetite-ilmenite and had very high potash/soda ratios. These are also features of Huronian paleosols, and suggest the sediment of the Matinenda Formation was formed by the intense weathering of a granitic source terrain as proposed by Roscoe (1969). Two southeast trending ore zones were recognized since the early days of uranium mining in the Elliot Lake camp. The Nordic zone, east of the City of Elliot Lake is about 1 mile (1.6 km) wide and 3.6 miles (5.6 km) long. The Quirke Zone, in the Quirke Lake area, is about two miles (3.2 km) wide and six miles (9 km) long. Paleotopographic features of the basement are thought to have had a determining influence on the position and orientation of the zones. Ore grade (circa 0.1 % U3O8) conglomerate occurs as persistent lenses with individual units up to 15 feet (4.5 m) thick. The uraniferous quartz-pebble conglomerates are commonly well developed at the base of the Matinenda Formation but also occur within the arkose up to 150 feet (45 m) above the base (Roscoe, 1969, Robertson, 1968). modified paleoplacer origin of the ores as outlined by Roscoe (1969). Advocates of this hypothesis propose that prior to the accumulation of significant free oxygen in the Earth’s atmosphere, southeastward flowing streams carried quartz, pyrite and uraniferous minerals released by the extensive weathering of the Archean granitic terrain, and deposited them as southeast trending units determined by basement topography. Evidence supporting the paleoplacer hypothesis was provided by Theis (1979) who demonstrated that a direct relationship exists between the size of quartz pebbles and pyrite grains (Figure 1.13), and the concentration of many other components of the ore zones. The McKim Formation The McKim Formation is the uppermost formation of the Elliot Lake Group. It is mainly a turbidite sequence of generally dark grey, subarkosic wackes, mudstones, subarkoses, lithwacke and litharenite that extends from the Blind River area to the Grenville Front. Graded beds, parallel laminations, ripple marks, ripple-drift cross-laminations and Bouma cycles, indicative of deposition by submarine turbidity currents are reported (Parvianen, 1973; Card et al., 1977). Robertson (1968) gives a thickness of 0-380 feet (0-100 m) for the McKim Formation on the south limb of the Quirke Lake syncline. It is missing on the north limb. The McKim Formation is thickest in the Sudbury area, where The quartz-pebble conglomerate consists mainly it is up to 2400 meters (8000 feet) thick. of well rounded pale-grey to dark-grey quartz The Murray Fault appears to have exerted an and chert pebbles set in matrix of pyrite with important influence on the deposition of the some quartz/feldspar grit and sericite. Pyrite McKim sediments. North of the Murray Fault commonly forms from about 15% of minable the McKim rarely exceeds a few hundred meters units. Radioactive minerals include uraninite, in thickness, whereas south of the fault the brannerite, and uranothorite (Roscoe, 1968). thickness of the McKim Formations is at least Monazite and zircon are characteristic heavy 2400 meters. Card (1978) noted the change minerals. from laminated siltstone in the west to more The sedimentological and mineralogical features wacke in the east indicated a change from more of the uranium bearing zones of the Elliot Lake distal to proximal facies, in turn suggesting more camp are now generally believed to support a tectonic activity and possibly a source for the PDF compression, OCR, web-optimization with CVISION's PdfCompressor �17 McKim sediments in the east. Fralick and Miall (1989) concluded that the McKim Formation of the Elliot Lake area represents a marine transgression that gradually drowned the Matinenda fluvial plain. extended to the Quirke Lake Syncline as well. The outcrop pattern of the Huronian volcanic rocks on geological maps also suggests that the volcanic rocks are erosional remnants preserved in basement depressions. Stratigraphic Relationships within the Elliot The Thessalon Formation may once have extended beyond the limit suggested from its Lake Group of the Sault Ste Marie-Elliot present outcrop distribution. Its present Lake Area. The stratigraphic relationship between the Matinenda Formation, Thessalon Formation and Livingstone Creek Formation is revealed on a rock face near the northern boundary of Haughton Township about 30 km (18 miles) north of the town of Thessalon (Figures 1.6, 1.7). At this location pyritic quartz-pebble conglomerate of the Matinenda Formation (Chandler, 1976) directly overlies an apple-green paleosol on grey, fine-grained sandstone, which the writer correlated with the Livingstone Creek Formation (Bennett et al.1990). About 600 meters (380 feet) northwest of the above occurrence arkose and quartz-pebble conglomerate of the Matinenda Formation disconformably overlies a steeply dipping, east-striking, mafic dike; the upper few meters of which is a sericite-leucoxene paleosol. The dike intrudes grey sandstone and apple-green paleosol of the Livingstone Creek Formation (Bennett et al. 1990, Bennett et al, 1991; Sutton and Maynard, 1993). Less than 2 km south of the above location Chandler (1976) identified a fault-bounded block of Thessalon Formation volcanic rocks with a minimum thickness of approximately 500 m. The mafic dike referred to above was a feeder for Thessalon flows, since the Thessalon Formation is the only known igneous activity at this stratigraphic position The above observations show clearly and unequivocally that there was a period of volcanic activity, as well as a significant period of erosion, separating the Matinenda Formation and the Livingstone Creek Formation. Since paleosols are well known upon Huronian flows in the Elliot Lake area, the sub-Matinenda unconformity seen in Haughton Township distribution may be but erosional remnants of a once extensive continental flood basalt sequence. This is to be expected on consideration of Macdougall’s (1988) statement “Since many CFB provinces have been uplifted and occur as elevated plateaus, dissection and removal is rapid”. He notes as well that the general scarcity of pre-Cretaceous Continental flood basalt provinces is probably due to their erosion. Hough Lake Group Introduction The Hough Lake Group (Robertson, et al., 1969; Roscoe, 1969) is lowest of the three Huronian groups that display the cyclic deposition of diamictite followed by a mudstone, siltstone, and turbidite or carbonate sequence; and overlain by a cross-bedded arenite unit. Each cycle is generally thought to represent a sequence of glaciogenic - marine - fluvial and/or shallow marine deposition (Figure 1.4). Ramsay Lake Formation The Ramsay Lake Formation is the lowermost formation of the Hough Lake Group and is the oldest of three such conglomerate units that define the base of Hough Lake, Quirke Lake and Cobalt Groups (Roscoe, 1969; Pienaar, 1963) (Figures 1.3, 1.4). The Ramsay Lake Formation is a widespread, relatively thin unit, which is up to 41 m thick in the Bass Lake area (Chandler, 1973). In the Elliot Lake area the thickness of the Ramsay Lake Formation is from 0 to just over 30 m. (Diamond drill logs Assessment files, Sault Ste Marie District Geologist’s Office). The Ramsay PDF compression, OCR, web-optimization with CVISION's PdfCompressor �18 Lake Formation is from 70 to 170 m thick in the common (Card, 1978; Robertson, 1976). The Pecors Formation is the result of transgressive Sudbury-Manitoulin area (Card 1978). units formed in deep water by turbidity currents Matrix supported polymictic conglomerate (Card, 1978). The presence of drop stones is (diamictite) is the most abundant rock type in evidence of a cold paleoclimate, and provides the Ramsay Lake Formation, especially near the supporting evidence for the glaciogenic origin of base. The lowermost few meters of the the underlying Ramsay Lake Formation. formation usually reflects the underlying rock type (Robertson, 1968; Parviainen, 1973). Mississagi Formation Minor amounts of mudstone, wacke and arenite The Mississagi Formation is a thick sequence of are locally present. predominantly grey, arenitic rocks extending Subrounded to well rounded pebbles and throughout most of the length of the Huronian cobbles of grey granitic rocks and angular to outcrop belt. Within the Quirke Lake syncline rounded clasts of very dark green to black the Mississagi Formation is from 344 to 704 m volcanic rocks generally form less than 30 thick. South of the Murray Fault the formation percent of the total volume of the diamictite. is notably thicker, being more than 3000 m in The dark matrix consists of quartz, feldspars, the Sudbury area (Card, 1978; Long, 1978). By chlorite, muscovite-sericite-illite and pyrite far the dominant rock type in the Mississagi (Parvianen, 1973). Formation is moderately well sorted, medium to coarse-grained subarkose and arkose. Small to Although some writers have argued for a debris medium quartz/chert pebble conglomerate is a flow origin, most writers now accept the Ramsay minor component of the formation, but is more Lake Formation as having a significant common in the western and northeastern parts glaciogenic component (Roscoe, 1969; of the Huronian belt. Fine-grained pyrite along Robertson, 1976). Fralick and Maill (1989) forsets commonly results in rusty staining of identified an ice proximal association of pebbly outcrops. Greenish, sericitic units form sandstone and diamictite; subaqueous gravity relatively thin planar-bedded units between flows and ice rainout deposits; and ice-proximal, cross-bedded sandstones. Palonen (1973) fluvial outwash deposits. provided evidence supporting a marine origin for the Mississagi Formation; however, Long (1978) Pecors Formation argued that the abundance of mud-grade matrix The Ramsay Lake Formation is conformably in the immature arenites, and the predominance overlain by a sequence of generally dark, bedded of unimodal paleocurrent directions, and the and laminated wackes, mudstone, siltstones and lack of quartz arenites argued against a marine sandstones of the Pecors Formation (Roscoe, environment for the Mississagi Formation. He 1969). The Pecors Formation is 30 m thick at concluded that the Mississagi Formation was Quirke Lake (Robertson, 1968) but is as much deposited from braided streams with low to as 900 m thick south of the Murray Fault in the intermediate sinuosity and high width to depth Sudbury area (Card, 1978). It was not identified ratios. in the area between Thessalon and Sault Ste Bedding units are commonly about a meter thick Marie (Frarey, 1977). Ripple marks, graded bedding, cross-laminations parallel laminations, but range from a few centimeters to over four meters. Trough cross-stratification and ripple ball and pillow structures, clastic dikes and cross-stratification are common sedimentary slumpage features have been reported in the structures (Long, 1978). Individual formation. The basal part of the formation is commonly laminated resembling varves and, in cross-stratified beds may show grain size gradation (McDowell, 1957). places, has dropstones (Robertson, 1968; Parvianen, 1973). Partial Bouma sequences are PDF compression, OCR, web-optimization with CVISION's PdfCompressor �19 Long (1978) measured over 2500 cross-stratified units in the Mississagi Formation (Figure 1.5). Two major stream systems were recognized: a stream system flowing southeast to easterly from the Sault Ste Marie area joined a stream system flowing southwestward from the Cobalt Plain to form southward flowing system southwest of the Sudbury area. These observations suggest that the area now occupied by the Sudbury Igneous Complex was elevated during the period of Mississagi deposition(Long, 1978). Aweres Formation fan system that extended in a more or less northeast direction beyond the present northern limit of the groups below the Cobalt Group. The Mississagi Formation may represent a more distal depositional environment than that of the Aweres Formation. Quirke Lake Group Bruce Formation The Bruce Formation extends from the Garden River Indian Reserve near Sault Ste Marie to about 70 km northeast of Sudbury. It consists mainly of matrix supported and minor clast supported conglomerate. Pebbly wacke, arkose, wacke and siltstone are locally present. Robertson (1968) reports the Bruce Formation is from 79 m (260 feet) to 12 m (40 feet) thick in the Elliot Lake area. It is 26 m (85 feet) to 37 m (120 feet) thick under the greater part of the Quirke Lake Figure 1.14: Diamictite of the Bruce Fm. Syncline. Highway 108, Elliot Lake area. In the Sault Ste Marie area, the 1700 m thick sequence of conglomerate and sandstone of the Aweres Formation (McConnell ,1927) unconformably overlies the mafic volcanics of the Thessalon Formation (Figures 2.1, 2.2). The internal stratigraphy and lithologies of the Aweres Formation is consistent with deposition in an alluvial fan environment. The base of the formation is almost entirely of mafic volcanic clasts while higher levels show a progressive increase in granitic components. The uppermost rocks of the Aweres Formation south of Aweres Lake are mainly arkose with thin pebble conglomerate beds. The lithological variation with stratigraphic height indicates the continual erosion of an uplifted fault-bounded plateau of Huronian volcanic rocks. Pebble- to boulder-sized, angular to subrounded clasts generally consist of pale-grey granitic rocks, Archean supracrustal rocks and fine-grained mafic clasts. The upper parts of the formation may contain up to 5% carbonate (Robertson, 1968). The Bruce Formation is generally interpreted as a tillite with minor beds and lenses of glacially derived sandstone. Dropstones have been observed in laminated units (Robertson, 1968). Casshyap (1969) concluded the formation was The distinct lithology of the Aweres Formation deposited from terrestrial wet-base glaciers. prevents its direct correlation with other However Sims et al. (1981) proposed that the Huronian rocks. The upper surface is partly fault Bruce Formation is an accumulation of debris bounded but is unconformably overlain by the flows released as a result of normal faulting, a Gowganda Formation on Highway 556 (Figure sudden increase in paleoslope and a sudden 2.3). It is possible that the Aweres Formation is increase in water depth. an erosional remnant of a once extensive alluvial PDF compression, OCR, web-optimization with CVISION's PdfCompressor �20 Espanola Formation The Espanola Formation is the only widespread carbonate unit of the Huronian Supergroup. It is a present from the Sault Ste Marie area to the Maple Mountain area about 70 km northeast of Sudbury. Its widespread distribution and distinctive lithology make it the most useful stratigraphic marker unit of the Huronian Supergroup. In the Elliot Lake area the Espanola Formation can be subdivided into three parts: a lower limestone member (the Bruce Limestone), a middle siltstone, wake, arenite member (the Espanola Greywacke) and an upper dolomite member (The Espanola Limestone) (Robertson, 1968). The latter generally contains 3 - 4% total iron which gives a distinctly brownish colour to weathered surfaces. All 3 members are generally thinly bedded to laminated. The threefold subdivision is not so well developed south of the Murray Fault (Young, 1982). Contacts between members tend to be gradational. Intraformational breccias, mud cracks, ripple-marks, flame structures and ball-and-pillow structures are common sedimentary features. Hofmann et al. (1980) described stromatolites in the Espanola Formation on Quirke Lake. These features suggest deposition in a quiet shallow water environment with carbonate deposition being interrupted by influx of fine-grained sediment. Young (1973) proposed that the relatively sharp change from the diamictites of the Bruce Formation suggests warm climatic conditions following a glacial advance. However, the recognition of carbonate deposition at high latitudes (Williams, 1975) and the association of detrital uranium with intense chemical weathering (Maynard et al, 1991) adds uncertainty to attempts to model Huronian paleoclimate. activity preceding deposition of the Gowganda Formation of the Cobalt Group. Reported thickness estimates of the Serpent Formation range from 150 up to 1500 m (Bennett et al., 1991). Robertson (1968) states that nowhere in the Blind River-Elliot Lake area is there any evidence that the total thickness of the Serpent Formation has been preserved. The Serpent Formation is mainly fine to medium-grained, quartz arenite and arkose. Conglomeratic units have been noted, especially near the base of the Formation. Carbonate is a significant component near the base of the formation in the Elliot Lake area (Robertson, 1968). Planar cross-bedding, festoon cross-bedding, rip-up clasts, fine-laminations and mud cracks have been reported. Long (1976) proposed that the Serpent Formation was deposited in a distal braided stream environment with calcareous units representing a sabkha environment. Young (1982) noted that the presence of very large cross-beds and a bi-modal size distribution suggest aeolian processes may have been active at least locally. Cobalt Group Gowganda Formation The Gowganda Formation is a complex sequence of conglomerates, sandstones, siltstones and mudstones, which comprise the lowermost formation of the Cobalt Group. Its thickness ranges from 1070 m in the Sault Ste Marie area; from 970 to 1150 m around Whitefish Falls, on the North Shore of Lake Huron; and from 950 to 2700 m near Sudbury. Near Dunlop Lake, in the Elliot Lake area, the Gowganda Formation is about 600 m thick. Matrix-supported conglomerates are common, especially in the lower parts of the formation. However, these are commonly intercalated with clast-supported conglomerates, sandstone, Serpent Formation wacke units. Laminated mudstones and siltstone are especially prominent in the upper parts of The Serpent Formation is found throughout the Gowganda Formation. Many occurrences of much of the Huronian belt; however, it is locally ice-rafted drop-stones have been reported in removed by erosion during a period of tectonic laminated mudstone/siltstone units. Individual PDF compression, OCR, web-optimization with CVISION's PdfCompressor �21 units are generally relatively thin and discontinuous making subdivision of the Gowganda formation difficult except in well-exposed areas. Most granitic clasts of Gowganda conglomerates have a distinctly pinkish or reddish hue, in comparison to the grey, granitic clasts in the matrix-supported conglomerates of the stratigraphically inferior Ramsay Lake and Bruce Formations. Also, pink and red hued sandstones first make their appearance in the Gowganda Formation. Roscoe (1969) pointed out the appearance of red coloration (i.e. ferric iron) just above the basal units if the Gowganda Formation, and argued that it represents the appearance of free oxygen in the Earth’s atmosphere, and a change from the previously reducing atmospheric conditions that allowed the accumulation of readily oxidized minerals such as pyrite and uraninite in a fluvial environment. Lorrain Formation The Lorrain Formation is generally well exposed throughout most of the Huronian belt, where it commonly forms a background to some of the most scenic views in northern Ontario. It is overwhelmingly an arenitic sequence, with local siltstone units present in lower parts of the section. The formation is up to 2500 m thick near Sault Ste Marie and in the LaCloche Syncline, southwest of Sudbury. It is up to 2300 m thick in the Cobalt Embayment. In general, the lower part of the Lorrain Formation is dominated by pink, arkosic sandstones; the middle by hematite-rich subarkose and quartz-arenite; and the upper portion by pale grey to white mature, quartz-arenite. A distinctive jasper-pebble conglomerate found in the Sault Ste Marie area is a popular decorative stone, known locally as “pudding stone”. The presence of aluminous minerals is a characteristic feature of the upper, quartz-arenites of the Lorrain Formation. Diaspore and kaolinite The depositional environment of the are common in the quartz-arenite of the Sault Ste diamictites in the Gowganda Formation have Marie area and north of Elliot Lake (Wood, 1973) been the subject of discussion since Coleman while kyanite, andalusite and kaolinite are present (1905) proposed a glacial origin for the matrix as metamorphosed equivalents in the LaCloche supported conglomerates. Many later writers Lake-Kilarney area (Card, 1978). Young (1973) including Ovenshine (1965); Casshyap (1969); and Wood (1973) interpret the presence of Lindsay (1971); Young and Nesbitt (1985), diaspore and kaolinite as the result of and others have also supported a glacial, post-depositional in-situ alteration of feldspar glacial-marine or glaciolacustrine origin for the under hot and humid climatic conditions. Gowganda Formation. The presence of abundant detrital hematite in the Card (1968) concluded that, although Lorrain Formation and the occurrence of glaciation may have supplied coarse detritus to thorium-rich, monazite-bearing, quartz-pebble the basin initially, debris flows and turbidity conglomerate north of Elliot Lake has been currents, released by vertical tectonic interpreted by Frarey and Roscoe (1970) as movement, may better explain the thickness indicating an oxidizing environment. variations, rock associations and distribution he observed it the Sudbury Manitoulin area. Planar and trough cross-bedding are common, as Roscoe (1969) also emphasized that glaciation are ripple marks and other primary depositional is only one of the several processes responsible structures. There is no consensus as to the for the deposition of Gowganda sediments. depositional environment of the Lorrain PDF compression, OCR, web-optimization with CVISION's PdfCompressor �22 Formation. Most of the sedimentary structures of the Lorrain Formation can be found in either shallow marine or fluvial environments. Wood (1973), Young (1973) and Frarey (1977) favored a fluviatile model while Pettijohn (1970) supported a marine environment for the Lorrain Formation. Card (1976) proposed that the Lorrain Formation represents near-shore coastal shelf deposition during episodic marine transgression and regression. Gordon Lake Formation The Gordon Lake Formation displays a gradational contact with the underlying Lorrain Formation. It is made up predominantly of variegated mudstone, siltstone, chert and minor fine-grained sandstone. Robertson (1986) subdivided the Gordon Lake Formation of the Flack Lake area into a lower member of reddish arenite, siltstone, and chert with anhydrite and gypsum nodules; a middle member of green siltstone and mudstone; and an upper reddish mudstone, siltstone and chert. Abundant sedimentary features include small-scale cross-bedding, ripple marks and desiccation cracks. Some of the features of the Gordon Lake Formation are unique for the Huronian Supergroup. Wood (1973) noted on the abundance of feldspar in the formation, a marked contrast to rocks of the immediately underlying Lorrain Formation. He also described hematite ooliths and the abundance of grains in the 0.02 to 0.05 mm range, a relatively uncommon grain size in sedimentary rocks. Since this size is found in loess deposits, Wood (1973) proposed that the quartz silt of the Gordon Lake Formation was formed by glacial action and carried by the wind and deposited in a tidal flat environment. Bar River Formation The Bar River Formation is the uppermost formation of the Huronian Supergroup. It is characterized by quartz-arenite with minor ferruginous arenite and siltstone. It is approximately 300 m thick in the Flack Lake area, north of Elliot Lake. Wright and Rust (1985) concluded that the Bar River Formation was deposited in a tidal environment. Nipissing Intrusions Dikes, sills, and cone sheets of gabbro, diabase and granophyre, commonly referred to, as “Nipissing diabase” are the most widespread igneous rocks associated with the Huronian Supergroup. Baddelyite from Nipissing gabbro sills in the Gowganda area has been dated at 2219 Ma; the minimum age for the Huronian Supergroup (Corfu and Andrews, 1986). Buchan and Card (1985) report that paleomagnetic data suggests at least two periods of Nipissing intrusive activity. Olivine-bearing hypersthene gabbro, gabbro, feldspathic pyroxenite, two-pyroxene quartz gabbro, hornblende gabbro, granophyric gabbro and granophyre have been identified in Nipissing intrusions. Many of the Nipissing sills are characterized by chilled margins 50 cm to 5 m wide, overlain by 10-20 m of quartz gabbro, then 100-500 m of hypersthene-poor gabbro-norite and vari-textured diabase (Lightfoot and Naldredt, 1996) Nipissing intrusions are widely and evenly distributed throughout the Huronian belt but, with few exceptions, are not recognized within the Archean terrain. Individual intrusions may be up to several hundred meters thick and extend over an area of several hundred square kilometers. The form and orientation of Nipissing intrusions indicate that their emplacement may be controlled by older faults, folds and competency of the enclosing rocks (Card and Pattison, 1973). Lightfoot and Naldredt (1996) discuss the geochemical characteristics of the Nipissing magmas and the potential for platinum group metal deposits. They conclude that the Nipissing magmas were emplaced into the Huronian sedimentary sequence over a period of less than 10 million years. Parental magmas of remarkably uniform composition underwent PDF compression, OCR, web-optimization with CVISION's PdfCompressor �23 in-situ contamination and differentiation within the intrusions Lightfoot and Naldredt (1996). A spatial association between Nipissing intrusions and both magmatic and vein-type mineralization have long been recognized. Huronian Paleosols and Evidence for the Accumulation of Oxygen in the Huronian Atmosphere It has long been recognized that the study of Huronian paleosols (ancient soil profiles) could provide information pertaining to the development the Earth’s atmosphere and climate during the Proterozoic. Since iron is much less soluble in the ferric state than when in the ferrous state, the behavior of iron in paleosols should provide some indication of the oxygen partial pressure of the environment. Many of the best descriptions of Precambrian paleosols have been from those associated Huronian unconformities (Gall, 1992). Grandstaff et al. (1986) identified 8 features of paleosols; most of which have been described in Huronian paleosols. These features are: 1. Stratiform 2. Relatively thin (<20 m) Roscoe, 1970; Gay and Grandstaff, 1980; Kimberly et al, 1984; G-Farrow and Mossman, 1988; Prasad and Roscoe, 1991, Sutton and Maynard, 1992, 1993). Bennett et al. (1991) have proposed that there are three disconformities or unconformities within the Elliot Lake Group, which have, potential for paleosol development (Figure 1.7). These are in descending stratigraphic order the sub-Matinenda disconformity, the sub-Thessalon Formation disconformity, and sub-Livingstone Creek Formation unconformity. The sub-Livingstone Creek Formation unconformity is the basal unconformity of the Huronian Supergroup and is the only entirely sub-Huronian unconformity (Figure 1.3, 1.7). This unconformity is exposed in the Thessalon area, where the upper few meters of the Archean granitic rocks can be seen to progress from angular, slightly rotated blocks, separated by grey grit and fine-grained sandstone, upward, to more rounded boulders with a higher proportion of finer clastic material (Collins, 1925). This zone may be termed a “paleo-regolith”, since there is little or no obvious development of the yellow, sericitic paleosol commonly found in the younger, sub-Matinenda paleosols. 3. Transitional lower boundary-sharp upper boundary Prasad and Roscoe (1996) described two paleosols in the same drill core from the Denison Mine at Elliot Lake. A paleosol was found 4. Colour variations above Huronian volcanic rocks and another, less 5. Destruction of primary rock textures well-developed paleosol, was found upon accompanied by the development of soil textures Archean tonalite below a short section of quartz-pebble conglomerate and grit (Prasad, 6. Destruction of primary minerals with personal communication, 1997) below the 9 m formation of clay minerals or metamorphic thick volcanic unit (Figure 1.15). equivalents 7. Dikes of material from overlying sediment washed down into desiccation cracks in the soil 8. Rip-up clasts of overlying sediments Well-preserved paleosols below the Matinenda Formation in the Elliot Lake area have been described by many workers (Roscoe, 1969; Pienaar, 1963; Robertson, 1968; Frarey and The best developed, and most studied, Huronian paleosols have been found directly below the Matinenda Formation. On mafic rocks, the sub-Matinenda paleosols can generally be recognized by the presence of an upper, distinctly apple-green to yellowish, sericitic zone which grades downward, over few centimeters to several meters, to an underlying, PDF compression, OCR, web-optimization with CVISION's PdfCompressor �24 granitic rocks is generally lacking or relatively thin. black, fine-grained, chlorite-rich eluvial zone up to several meters thick. Abundant pseudomorphs of titanium oxides after ilmenite are a feature of paleosols on mafic igneous rocks. Rip-up clast of sericitic paleosol (the “argillite scraps” of mine geologists) are commonly found in the lower few meters of the overlying Matinenda Formation. Prasad and Roscoe (1996) report significant amounts of carbonate and pyrite in sub-Matinenda paleosols in the Elliot Lake area. Paleosol Denison Drill hole S-62 Sub-Matinenda Formation Paleosol K K 0 C CO2 Sub-Matinenda Paleosol -178 Sub-Lorrain Formation Paleosol Massive Basalt -1336 1850 -1950 Fe2 Fe3 Ca Mg Fe2 K Fe3 Paleosol [( Na liii Ville Marie granite Transition zone -899 500 Metabasalt-increasingly weathered upward Fet Mg Ca Sub-Thessalon Fm. Paleosol Chlorite, sericite rich bands in paleosol "Argillite" (paleosol) NaCa Mg Fe2 Fe3 Coglomerate Grit 0 Tonalite - increasingly weathered upward cm Hematitc breccia Lorrain Formation Sub-Matinenda paleosols commonly show the pronounced loss of soda found in most paleosols. Lime, and magnesia are also depleted, but there is generally a large increase in potash content (Gay and Grandstaff, 1980) (Figure 1.15). In most cases, iron and manganese are depleted in upper parts of the paleosol. This is held to provide good evidence of weathering in a reducing environment. However, Gay and Grandstaff (1980) noted an upward increase in The uppermost sections of sub-Matinenda Formation paleosols on Archean granitic rocks total iron in paleosol from the Pronto Mine area. After Prasad and Roscoe(1996), Rainbird et al. (1990). Figure 1.15: A comparison of sub-Matinenda Fm. and sub-Lorrain Fm. Paleosols and arkosic sedimentary rocks, is generally apple-green to yellowish rock composed mainly of quartz and sericite (Robertson, 1968; Gay and Grandstaff (1980); Sutton and Maynard (1992). Where the texture of the protolith is well preserved, but the original mineralogy is replaced, the paleosol may be termed a saprolith (Rainbird et al, 1990). The chlorite-rich eluvial zone of paleosols on They concluded this upward increase in iron content indicated the presence of free oxygen in early Huronian atmosphere although at approximately 1% of the present level (Gay and Grandstaff, 1980). They suggested that the loss of iron shown in most Huronian paleosols could be due to local reducing environments. Some writers have concluded that the increase in potash (as sericite) in Huronian paleosols is largely due largely to diagenetic and PDF compression, OCR, web-optimization with CVISION's PdfCompressor �25 metamorphic processes which may mask the environmental and hydrologic conditions operative during paleosol development (Gay and Grandstaff (1980); G-Farrow and Mossman (1988). purple siltstone member of the Lorrain Formation near Sault Ste Marie (Frarey, 1977). Not all workers accept the above explanation for the preservation of uraninite and pyrite, and the observed change of colour with stratigraphy. The mineralogy and geochemistry of Sub-Lorrain Ohmoto (1996) has stated “ the loss of total iron in paleosols of all ages is not due to a Formation paleosols have been described by Rainbird et al. (1990) and Sutton and Maynard reducing atmosphere but to the reductive (1992). In contrast to the older, sub-Matinenda dissolution of ferric hydroxides under an oxic paleosols. Sub-Lorrain paleosols commonly show atmosphere”. an enrichment of Fe+3 at the expense of Fe+2 Regional Tectonic Patterns and without a significant loss of total iron. Hematite Metamorphism is a common mineral in the upper parts of sub-Lorrain paleosols, in contrast to the presence Major structures of the Huronian belt follow two of pyrite in sub-Matinenda paleosols. In this main trends: 1) west-northwest trending folds respect the sub-Lorrain paleosols resemble many and faults of the Sault Ste Marie-Elliot Lake post-Huronian paleosols and are consistent with area; and 2) east to northeast striking folds and weathering in an oxidizing atmosphere (Prasad faults of the Sudbury-Manitoulin area (Figure and Roscoe, 1996; Rainbird et al., 1990). 1.16). These two structural orientations are associated with differing fold styles, Since pyrite and uraninite are unstable under metamorphic grade and metamorphic fabric. oxidizing conditions, the abundance of detrital Changes are most notable across the east-west pyrite and uraninite in the paleoplacer uranium faults of the Murray fault system. This northeast ore zones in the Matinenda Formation provide striking fault zone (Figure 1.16) is the most good evidence for an oxygen deficient significant structural feature of the Huronian atmosphere during weathering, transport and belt. Since many formations show a significant deposition of early Huronian sediments. increase in thickness south of the fault, it is In marked contrast to the common red beds of generally interpreted as an inverted growth fault. younger clastic sequences, sandstones (and most (i.e. an early listric normal fault, active during granitic clasts in conglomerates) below the sedimention; which, during a later compressive Cobalt Group are almost all drab coloured in regime, was converted to a thrust or reverse spite of the abundance of red and pink granitic fault (Card, 1978; Jackson (2001). rocks in the continental source area (Roscoe The rocks of the Huronian Supergroup have 1969, 1973). Frarey and Roscoe (1970) noted been subjected to several deformational events. the above and proposed that the drab colour of This is particularly evident south of the Murray lower Huronian clastic rocks is due to the lack of Fault zone. In the Whitefish Falls area (south of free oxygen in the atmosphere during the period the Murray Fault) Young and Nesbitt (1985) of deposition of lower Huronian rocks. conclude that some large-scale folding is related Red hued, hematite bearing rocks, which Roscoe to syn-depositional and/or post-depositional (1969) proposed mark the presence of an deformation of unconsolidated sediment. Early oxidizing atmosphere, make an appearance with syndepositional deformation is also indicated by the Gowganda Formation of the Cobalt Group, an important unconformity below the and are important in parts of the Lorrain and Gowganda Formation of the Cobalt Group; and Gordon Lake Formations of that group. What the presence of ragged, slumped contacts and may be the first true red-beds are found in the large slump blocks along major faults (Card, 1978; Young, 1983). PDF compression, OCR, web-optimization with CVISION's PdfCompressor �26 Convincing evidence of at least one important pre-Nipissing (2.2 Ga) deformational event, assigned to the Blezardian Orogeny (Stockwell, 1982), is provided by the observation that Nipissing bodies in the Sudbury-Whitefish Falls area transects axial surfaces of major folds (Card, 1978). Jackson (2001) did not observe such relationships in the areas north of the Murray Fault. However, Jackson (2001) points out, as earlier noted by Robertson (1964), that the tendency of Nipissing dikes to occupy structures parallel to the axial plane of the Chiblow Anticline, suggests pre-Nipissing folding. Jackson (2001) also noted evidence of pre-Nipissing faults north of the Murray Fault zone. Following emplacement of the Nipissing intrusions but prior to the emplacement of the Sudbury Igneous Complex there was further deformation and regional metamorphism. Rb-Sr isotope studies of Huronian metasediments indicate that prograde metamorphism occurred at about 1.90-1.85 Ga (Fairbairn et al., 1969). This event is probably correlative with the Penokean Orogeny of Michigan-Minnesota (Sims et al., 1989). After the emplacement of the SIC and the deposition of the Whitewater Group, there is evidence of further deformation and low-grade metamorphism, followed by intrusion of granite plutons at about 1.75 and 1.5 Ga. The post-SIC deformation and retrograde metamorphism was found mainly south of the Murray Fault, especially in the area between the SIC and the Grenville Front. Shanks and Schwerdtner(1991) report that this deformation is characterized by south-dipping thrust faults or deformation zones with northeast-trending foliation and southeast-plunging lineation. The last recognized deformational feature affecting Huronian rocks is the movement along faults, which post-date the diabase dikes of the 1.25 Ga Sudbury swarm (Jackson, 2001). A study of magnetic fabrics, strain patterns, and microstructures in granitoid rocks of the Creighton and Murray plutons and their Huronian host rocks by Riller (1996) have provided information on the pre-2.2 Ga “Blezardian Orogeny” (Stockwell, 1982). Riller concluded that major folding and amphibolite facies regional metamorphism in the Sudbury area was coeval with the emplacement of the Creighton Pluton and Murray Plutons, which were dated at 2.3 Ga by Frarey et al (1982) and Krogh et al. (1984) and more recently at 2477+/- 9 Ma by Krogh et al. (1996). Large-scale dome-and-basin structures in Archean basement and Huronian cover rocks represent major south-vergent nappes. The Sudbury Structure, a deformed relic of an astrobleme, is superimposed on a major antiformal dome cored by Archean granulite and granitoid rocks and flanked by overturned Huronian strata on the south and several remnant rim synclines of Huronian rocks on the north (Riller, 1996; Bennett et al, 1997). In the Sault Ste Marie-Elliot Lake area fault and fold structures trend in general west-northwest to northwest direction. Folds are generally upright, open concentric structures with gentle, variably plunging hinges (Figure 1.16). There is only a weak development of minor tectonic structures. The metamorphic grade is subgreenschist (Card, 1978b). The major structural features of the Elliot Lake area include a gently south-dipping homocline south of the Flack Lake Fault, the open fold of the Quirke Lake Syncline and the Chiblow Anticline between the Quirke Lake Syncline and the Murray Fault. Jackson (2001) found no evidence of detachment at or near the basement-cover interface in the Elliot Lake area. He also proposed that the “inverted growth-fault” model as applied by Zolnai et al. (1984) to structural-stratigraphic relationships in the Huronian may, in some cases at least, be interpreted as thrust faults with flats following depositional boundaries, and ramps that cut up through the stratigraphic section. Given the data available, neither model could be rejected for major northwest trending faults in the Sault Ste Marie area (Jackson, 2001). Jackson (1997) points out that the curvature of the Flack Lake PDF compression, OCR, web-optimization with CVISION's PdfCompressor �27 M q V ±PLfl2I 4011f4' Mall uqicc1G blOCK roK6 gOflil ?b04 fl CCLGUA!IIG bLOAIUCG) QOMU4JLOMU ?!qG c°p°ii CLOnb L0CK2 EQflI4 (Op?GLAGq — bk6-COPOU 43LOflb LOCK2 qsLIAGq 410w VLCIJ6OU pO2GWGU4 I2lauq ELOU4 IGC4OU!C LOCI(2 (3flbGLiOL bLOAIUCa) E0fl14 wE2 VU4!Cl!LJ& bIG—CopoK Eafl14 (4LOC6OpJ6 OU 2flL4CC6 4L0C60P16 22 wee IU E r000iIou 241nc4n16 El r1uuaw6q 5 U0141J 04 Figure 1.16: Main structural features of the Huronian Supergroup. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �28 Fault is in the opposite direction to that expected if it is a thrust fault as proposed by Zolnai et al. (1984). In the Sault Ste Marie area the presence of trust faults, as suggested by out-of-sequence units, has been confirmed, in at least one location, by diamond drilling (Assessment files, Sault Ste Marie Resident Geologist’s Office). The Murray fault zone separates the moderately developed. More than one age of major and minor structures can be discerned south of the Murray Fault (Jackson, 2001). Metamorphism south of the Murray fault ranges from low greenschist to low amphibolite facies (Figure 1.17). Rocks of higher metamorphic grade occur in two zones or nodes, one along the Murray Fault zone and another northwest of the Grenville Front. Both zones coincide with major r.ø.uq 4dCIIU LJ rUM 1 w!qqç dLG6L)UCIJ!24 WQ OLGUAIIIG LLOU4 EIUIC IU4LUUIOUU bLOA!UCG Figure J1.17: Metamorphism of the Huronian Supergroup deformed, low metamorphic grade rocks to the north from the multi-deformed, higher-grade rocks of the Sudbury-Manitoulin area to the south. In the Sudbury-Manitoulin area is characterized by open to sub-isoclinal, flattened buckle folds with upright to northward overturned axial surfaces. Elongate domes and basin are formed by reversals in plunge. Penetrative axial place cleavage and steeply plunging rodding or mineral lineations are well anticlinoria, although in detail, metamorphic isograds transect fold axes (Jackson, 2001). Higher-grade metamorphic nodes do not coincide with the few granitic intrusions that intrude the Huronian rocks south of the Murray fault. The 1.850-1.900 Ga age of metamorphism is much younger than the Creighton-Murray granite (2.45 Ga) and older than the PDF compression, OCR, web-optimization with CVISION's PdfCompressor �29 1.700-1.750 Ga of the Chief Lake and Cutler granites. Jackson (2001) considers the origin of the high-grade staurolite-biotite assemblages of the McKim Formation in the hanging wall of the Murray Fault as one of the most enigmatic aspects of the tectonic history of the Southern Province. He concludes that geobarometry indicates a relatively low- pressure metamorphism (2-3 kbar) at high temperature. These conditions differ significantly from the 6-7 kbar pressures estimated for the Penokean metamorphism in Minnesota as determined by Holm and Silverstone (1990). He concluded that the high-temperature metamorphism was at or below pressure corresponding to the thickness of the Huronian rock column, thereby precluding crustal thickening as the origin of the metamorphism. Jackson (2001) concluded that a high heat flow regime as developed in areas of crustal extension and related mantle upwelling Jackson (2001). Such a model is compatible with Card’s (1964) view that the high-grade metamorphism may be the result of rapid, focused heat flow. Tectonic Models for the Development of the Huronian Basin There have been various tectonic models proposed for the early development and later deformation of the Huronian basin. Many reconstructions are essentially modifications of the model put forth by Deitz and Holden(1966) which stated that the Huronian Supergroup represents a rift and passive margin sequence that was compressed, partly tectonically buried and metamorphosed during a collision with the Superior craton and another mass which overrode its southern edge. Zolnai et al. (1984), Bennett et al. (1991) accept the essential aspects of the Dietz and Holden (1966) model. They propose that rifting and continental break-up was coeval with Huronian volcanism (2.45 Ga) and that the much later deformation was equivalent to the Penokean Orogeny (1.860-1.835 Ga). This model does not attempt to account for the multiple deformation events affecting Huronian rocks and the origin of the Nipissing magmatic event. Young (1982) proposed that the Huronian Supergroup was deposited in an aulocogen, an easterly trending fault bounded trough, which opened towards an ocean in the area now occupied by the Grenville Province. Sims et al. (1980, 1981) concluded that the Huronian Supergroup, the Marquette Range Supergroup and Animikie rocks were deposited as intra-continental, fault controlled basins developed along a major, late Archean structure, the Great Lakes Tectonic Zone. More recently, Roscoe and Card (1992), noting the close stratigraphic correlation between the Early Proterozoic sequences of the Wyoming craton and the Huronian Supergroup, proposed that the Superior and Wyoming cratons are rifted portions of what was once a single continental land mass. They suggest the direction of the Matachewan-Hearst dike swarm (2.45 Ga) indicates an east-west tensional regime, which resulted in, a Huronian basin elongated in a north-south direction. On this larger craton the Huronian sediment was deposited in a southward deepening intracratonic basin. Roscoe and Card (1992) propose that it was during the Nipissing igneous event (2.2 Ga) that successful rifting of the Superior Province took place with the eventual drifting of part of the missing Superior Province to its present location as Wyoming craton. They attribute pre-Nipissing folding to the Blezardian Orogeny of Stockwell (1982) and the later more important deformation to be coeval with the Penokean Orogeny of Michigan, Wisconsin and Minnesota (Roscoe and Card, 1992). Jackson (2001) supports the model of Roscoe and Card (1992) since the high heat flow, which he considers necessary to give the observed features of the high-grade metamorphic rocks, would be a necessary effect of mantle upwelling during continental break-up. He also interprets some of the early high-strain deformation as being consistent with a Nipissing age break-up of the Superior craton. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �PDF compression, OCR, web-optimization with CVISION's PdfCompressor �30 ILSG Field Trip No. 1 - Day 1 The Elliot Lake Transect On the first day of this field trip we will examine many of the excellent rock exposures of one of the Earth’s most well-known and most completely studied stratigraphic sequences. The rocks of the Elliot Lake transect display clearly visible evidence of the nature of the climate and atmosphere of the Earth more than 2 billion years – of course not all Earth scientists agree as to what this evidence tells us. Stop Descriptions and Road Log From Sault Ste Marie, Ontario proceed east on Highway 17 for 162 km (101 mi) to the intersection of Highway 17 and Highway 108, then 29 km (18 mi) north on Highway 108 to Elliot Lake. At Elliot Lake continue north past Hillside Drive South and the first few stoplights to Hillside Drive North. Turn left (west) onto Hillside Drive North. Continue west on Hillside Drive North for about 1 km (0.6 mi) to Spruce Street. Turn right (North) on Spruce Street to Valley Crescent continue on Valley Crescent to Balsam Place and stop at the cull-de-sac. When collecting samples please exercise care to avoid damaging any vehicles parked nearby. cul-de-sac. Note the deflection of the axial plane cleavage in the mudstone units. The movement of adjacent beds inferred from the deflection of the cleavage indicates the south limb of a syncline. A gabbroic dike is separated from the sedimentary rocks by a zone of sheared and fractured rocks. The McKim Formation is missing on the north limb of the syncline. Return to Hillside Drive North and continue west for about 1 km to Spine Road. Turn right (west) onto Spine Road to Lawerence Avenue at the west end of Spine Road. 14.9 km = 9.3 mi. 17-370462E, 5137991N STOP 1.2: Radioactive quartz-pebble conglomerate of the Matinenda Formation. The low outcrops on north side of Spine Road are grey, buff and dark-grey arkose and radioactive, pyritic, quartz-pebble conglomerate of the Matinenda Formation. Pebble units are about 20-30 cm thick and dip about 10 degrees to the north. Pebbles in this outcrop are about 1-2 cm across and are generally much smaller than the typical pebbles in the ore zones of the Elliot Lake mines. Ruzicka and LeCheminant, (1984) report the radioactive conglomerate contains “rare-earth-element-bearing uranothorite (?), large zircons, a Ti-U-Si-Fe phase (brannerite?), 17 – 372751E, 5138695N (All coordinates in chalcopyrite and chromite. The distribution of the Elliot Lake are within UTM zone 17; NAD radioactive minerals in the conglomerate 83 which is essentially equivalent to NAD displays layering thus indicating a detrital WGS84). origin of these grains” STOP 1.1: McKim Formation and Nipissing Diabase The dark-grey areas in the radioactive beds are due to the presence of minor amounts Outcrops are argillite/slate and grey sandstone radioactive carbon generally known in the of the Mckim Formation on the east side of the Elliot Lake area as “thucolite” but also referred PDF compression, OCR, web-optimization with CVISION's PdfCompressor �North 2.1 Hy. 17 2.7 Hy. 556 Sault Ste. Marie Area of Figure 2.1 2.8 2.9 1.12 Hy. 17 from drill hole Hy. 128 LAKE HURON Thessalon Bruce Mines Hy. 638 Foul Bight Highway 556 transect Elliot Lake transect Field trip stop Faults Cutler Granite Haughton Twp. area. Gordon Lake area Hy. 17 Hy. 556 Fenwick Township Hy. 546 Area of Figure 1.21 Hy. 108 50 km City of Elliot Lake cline Hy. 108 Area of Figure 1.19 e syn e Lak Quirk Hy. 639 Flack Lake area Archean rocks East Bull Lake Suite Gabbro anorthosite Elliot Lake, Hough Lake and Quirke Lake Groups Cobalt Group Gordon Lake Fm. Huronian Supergroup Murray Fault LEGEND Figure 1.18: Index map for included geological maps and some areas mentioned in the text. Lake Superior Paleozoic and Keweenawan rocks 31 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �32 LEGEND POST-HURONIAN MAFIC INTRUSIVE ROCKS NIPISSING DIABASE HOUGH LAKE GROUP Subarkose, arkose, conglomerate Diabase, gabbro, metagabbro, granophyre 16 HURONIAN SUPERGROUP COBALT GROUP Bar River Formation 15 Pecors Formation (4) Argillite siltstone 8 Ramsay Lake Formation Quartz arenite Gordon Lake Formation 14 Siltstone, argillite, subarkose Lorrain Formation 13 Quartz arenite, subarkose, arkose, conglomerate, thorium-bearing conglomerate Gowganda Formation 12 Conglomerate, argillite, wacke, subarkose, siltstone DISCONFORMITY QUIRKE LAKE GROUP Serpent Formation 11 Subarkose, conglomerate Espanola Formation 10 Limestone, dolomite, calcareous siltstone Bruce Formation Diamictite Not Shown Mississagi Formation Fault 3 9 7 Diamictite ELLIOT LAKE GROUP McKim Formation 6 Siltstone, argillite, wacke Matinenda Formation 5 Subarkose, arkose, conglomerate, uranium-bearing conglomerate DISCONFORMITY Thessalon Formation 4 Basalt, andesite, mugearite-hawaiite, minor basal quartz-pebble conglomerate 4 DISCONFORMITY Livingstone Creek Formation 3 Sandstone, polymictic conglomerate UNCONFORMITY ARCHEAN 2 ARCHEAN Plutonic granitic rocks, gneisses, INTRUSIVE CONTACT Mine (Past Producer) 1.2 Field trip stop 1 Felsic to mafic metavolcanic rocks, metasedimentary rocks Sources of Information Giblin and Leahy(1979) Figure 1.20: Legend for figures 1.19 and 1.21. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �7 8 9 10 2 12 12 7 8 12 9 5 8 7 10 12 14 2 13 9 16 9 16 5 2 5 Elliot Lake 1.2 10 12 13 11 4 6 1.1 1.3 2 2 16 12 City of Elliot Lake 1.4 1.5 1.6 Hy 108 1.11 Hy 639 9 16 9 12 1.7 1.8 1.9 9 10 2 16 5 8 16 6 11 5 12 11 Qirke Lake 12 7 6 1.10 16 10 2 Figure 1.19: Geological map of the Quirke Lake Syncline. 5 9 10 11 Dunlop Lake 4 9 2 7 4 1 11 4 9 7 10 2 2 14 1 11 10 3 rs 1 co Pe 16 12 ke 6 12 1.1 La 9 16 2 16 5 10 2 1.1 9 16 0 1 9 1 9 2 2 9 16 3 2 1 6 1 N 1 4 miles km 6 2 Whiskey Lake 2 33 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �2 12 13 2 13 12 12 12 3 14 Hy 546 13 12 16 12 9 13 1.14 Ten Mile Lake 4 9 4 3 Flack Lake 1.15 Hy 639 16 11 15 10 14 1.16 12 2 14 2 15 1.13 14 9 15 Hy 639 1.12 13 Mount Lake Figure 1.21: A geological map of the Flack Lake area. 10 14 16 16 12 2 14 1.11 16 1 16 151 16 2 14 1 16 Rawhide Lake 4 10 Semiwite L Lake 16 13 12 2 2 0 16 1 3 2 2 13 16 12 2 1 2 North 4 miles km 6 2 34 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �35 to a hydrocarbon kerogen. Ruzicka and LeCheminant, (1983) note that several generations of carbon occur in the conglomerates of the Matinenda Formation. The earliest generation occurs as layers concordant with the bedding or a component of the matrix and appears to have been deposited in areas of quiescent sedimentation during the last phase of an upward fining sedimentary cycle. Diamond drilling has indicated that there are no ore-grade units in this area. Grab samples collected by the author in 1982 returned up to 0.80 lbs U3O8 /ton and 0.78 lbs ThO2/ton. A continuous chip sample returned 0.31 lbs U3O8/ton and 0.53 lbs ThO2/ton. Return to Highway 108 and proceed north on Highway 108. Reset odometer at Stanleigh Road. About 1.5 km north of Hillside Drive North). 1.0 km = 0.63 mi. 17 - 371717E, 5140426N STOP 1.3: Mississagi Formation, Hough Lake Group. One-meter thick beds of grey subarkose of the Mississagi Formation on the west side of the highway display the rusty staining on the face of the outcrop reflecting the minor pyrite content along the foreset beds of trough cross-beds (Figure 1.22). The paleocurrent direction (from the west) can be best observed on the upper surface of the outcrop. – Please Figure 1.22: Trough cross-bedding in the Mississagi Fm. exercise caution when walking on smooth, wet rock surfaces - The grey Stop 1.3. colour of these sandstones and the presence of apparently detrital pyrite Later generations are probably remobilized is held by most geoscientists to indicate of the phases of the first generation of carbon. The very low partial pressure of free oxygen of the carbonaceous matter in the Elliot Lake ores is atmosphere during the deposition of the comparable in occurrence and composition to Mississagi Formation. similar hydrocarbon in the Witwatersrand gold 1.4 km - 0.9 mi. 17-371652E, 5140948N reefs; interestingly Ruzicka and LeCheminant (1984) report elevated gold content (1000 STOP 1.4: Nipissing Diabase (Gabbro) 2000 ppb) in the carbonaceous matter of the altered Mississagi Formation, Bruce Elliot Lake ore beds. The radioactive carbon at Formation. this site is reported to be auriferous, although A sill-like body of Nipissing gabbro/diabase is the gold content is not available. exposed on the east side of Highway 108. In 1955 Rio Algom Mines Limited completed a Rhythmic, compositional layering is visible on diamond drill hole about 30 m south of this the vertical face of the road-cut. Near the north location. The logs of this hole indicate that the end of the Nipissing outcrop face note the radioactive beds exposed here are about 35 relatively planar, striated, surface is truncated by meters above the Archean basement rocks. more irregular surface that has been interpreted PDF compression, OCR, web-optimization with CVISION's PdfCompressor �36 as the result of erosion by high-pressure, waterborne sediment, presumably as a result of melting of an adjacent Pleistocene ice sheet. North of gabbro sill, the upper portion of the Mississagi Formation is exposed along the east side of the highway. The distinctly pinkish hue of the sandstone is probably due to the formation of albite by hydrothermal fluids activated by the adjacent intrusion. Further evidence of hydrothermal activity is seen by dark-green to black chlorite deposited along fractures. below the north-dipping diabase sill near the north end of the exposure, where it occurs as sub-parallel groups of pale-grey to white prismatic crystals about one mm wide and up to a centimeter long. X-ray diffraction analysis has determined that the pink coating on joint surface is apophyllite (KFCa4[Si8O20]8H20) an uncommon mineral, sometimes found in amygdules in basalts, but is also associated with calc-silicates (Figure 1.23). Young (1991) states Continue northward on foot for a few tens of meters. Here (17-371668, 5141245) the Mississagi Formation is overlain by diamictite of the Bruce Formation at the base of the Quirke Lake Group. The dispersed megaclasts of the Bruce Formation are predominantly grey granitic rocks with smaller mafic clasts of predominantly Huronian volcanic rocks of the Thessalon Formation. The abundant matrix of the conglomerate is dark-grey to black. Sand-sized quartz grains have a glassy, black appearance, a reflection of the dark matrix behind the Figure 1.23: Espanola Formation with calc-silicate minerals. Stop 1.5 clear quartz. There is no evidence of significant disconformity at the base of the Bruce Formation. that the small scale thrust faults and folds in the limestone on the west side of the highway is Proceed by vehicle to near the top of the hill. probably the result of slumping during early 1.8 km = 1.4 mi. 17-371621E, 5142684N tectonic activity. STOP 1.5: Espanola Formation and Nipissing diabase sills. The upper, ferruginous dolostone-bearing member of the Espanola Formation and the The base of the Espanola Formation is a green, overlying Serpent Formation are not present at this location but are well represented on the laminated unit about a meter or so thick. Laminated silty limestones and minor thin, chert north limb of the syncline. Since the Serpent beds of the Bruce Limestone member, Espanola Formation is missing we can infer that the ferruginous dolostone was removed during a Formation, overlie this unit. At this location the proximity of Nipissing gabbro intrusions has period of pre-Gowganda erosion. led to the development of calc-silicate minerals Continuing north the basal units of the including: grossularite garnet, idocrase Gowganda Formation are visible in road-cuts (vesuvianite), diopside, and wollastonite typical along the highway. of a skarn (Robertson, 1968). Wollastonite 2.0-2.5 km = 1.3-1.6 mile. Gowganda (identified by X-ray diffraction) is found just Formation diamictite. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �37 Megaclasts of pink granite, grey granite and granitic gneiss and mafic rocks are widely distributed in a dark green matrix. Most geologists now consider at least some of the diamictites in the Gowganda Formation to be tillites, although a debris-flow origin, either glaciogenic or as submarine debris flows is a more reasonable interpretation at specific localities. Roscoe, (1969) places the appearance of free oxygen in the atmosphere (“oxyatmoversion”) as coinciding with the appearance of the reddish hue of hematite just above the base of the Gowganda Formation a short distance north of STOP 1-5. (Figure 1.24). The mineral assemblage and metamorphic grade of the few of the mafic clasts examined by the writer many years ago indicated that the clasts were probably from Huronian basaltic flows. 4.7 km. = 2.9 mi. Pink sandstones and diamictite of the Gowganda Formation. 6.7 km = 4.2 mi. Diamictite with large boulder, Gowganda Formation. t —-.1 3.8 km. = 2.4 mi. 17- 371841E, 5143180N STOP 1.6: Stratified Gowganda Formation. Park just south of the rock-cut on east side of the highway. This is an impressive exposure though a stratified sequence of diamictites, Figure 1.24: Crudely stratified conglomerate of the clast-supported conglomerates and sandstones of the Gowganda Formation. Gowganda Fm. with mafic volcanic clasts. Stop 6. The base of the sequence at the extreme southern end of the exposure on the west 8.2 km. = 5.0 mi. Stanrock Mine Road. Reset side of Highway 108 consists of massive odometer. Turn east onto Stanrock Road diamictite overlain by a thin unit of laminated Optional Stop mudstone-siltstone with a few small dropstones. 2.2 km = 1.4 mi. 17-374911E, 5147007N About a meter of stratified Gowganda overlies the siltstone. Distinct beds of diamictite, Stop 1.7: Laminated varvite? sandstone, pebbly sandstone and clast supported Laminated siltstone/mudstone of the Gowganda polymictic conglomerate are present. Some Formation on the north side of the road. This conglomerate units display normal and reverse unit has been interpreted as varves found as grading suggestive of debris flows. The rocks deposits from Pleistocene glacial lakes. Return to displayed here may be interpreted to represent a Highway 108. depositional environment proximal to retreating glacial margin. Set odometer to 0 at Stanleigh Road and Highway 108. Note the predominance of red and pink granitic clasts, in marked contrast to the pale grey clasts 3 km = 1.9 mi. Denison Mine Road - Turn east. of the Bruce and Livingstone Creek Formations Reset odometer to 000 km. seen earlier. There is also a significant 1.5 km. = mi. 17 -375102E, 5150499N proportion of black pebble to cobble-sized clasts PDF compression, OCR, web-optimization with CVISION's PdfCompressor �38 Stop 1.8: Gowganda Formation Serpent disconformity Stop 1.10: Ramsay Lake Formation overlain by Pecors Formation. The Serpent Formation is not present in the south limb of the Quirke Lake Syncline and the Blind River Sault Ste Marie area where it was probably removed during a period of pre-Gowganda erosion. At this location, on the south side of the road, well-sorted sandstone of the Serpent Formation is overlain by polymictic conglomerate of the Gowganda Formation. The contact is sharp but irregular. Evidence of a sub-Gowganda disconformity at this location is given by the presence of pebble and cobbles of the Serpent Formation near the base of the Gowganda Formation. Diamictites of the Ramsay Lake Formation contain cobbles of grey granitic rocks, mafic clasts of Huronian volcanic rocks and Archean felsic volcanic clasts in an abundant dark-grey to black sandy matrix. The Ramsay Lake Formation is overlain by dark laminated siltstone and mudstone (argillite) of the Pecors Formation. The latter contains a few dropstones (Figure 1.25). Note: the Matinenda Formation of the Elliot Lake Group, expected between the basement and the Ramsay Lake Formation, is truncated by the Ramsay Lake Formation in this area. The Matinenda Formation does occur in the mine workings down-dip from this location. Return to Highway 108. Reset odometer at Highway 108 and Denison Mine Road. Continue east to the end of the Panel Mine road. This is the rehabilitated area of the former 0.5 km = 0.3 mi. Disseminated carbonate in Panel Mine. There is little evidence of the sandstone of the Serpent Formation on east side uranium mine and mill complex that was on this of Highway 108. site until 1993. 1.1 km = 0.68 mi. Road to Quirke Lake and Return to Highway 108. Reset Odometer. former Panel Mine. Reset odometer to 0. Turn Continue north on Highway 108. Tailings dam east onto Panel Mine road. of the Quirke Mine is visible west of the 1.5 km = 0.9 mi. 17-375258E, 51511331N Highway. Stop 1.9: Espanola Limestone Member of the Espanola Formation. Rock-cut in ferruginous dolomite and siltstone of the Espanola Limestone Member of the Espanola Formation. The Espanola Limestone Member is the uppermost of the thee members of the Espanola Formation recognized in the Elliot Lake area (Robertson, 1968). It is characterized by intercalated siltstone and reddish-brown weathering, ferruginous dolostone beds containing 3-4% FeO. Intraformational breccia, ripple marks, small-scale cross-bedding, and various soft sediment features are present but faintly visible on the south outcrop. Near the east end of the outcrop a grey clastic dike crosses the stratification at a high angle. 4.1 km = 2.5 mi. 17 – 377379E, 5152019N Highway 108 ends and Highway 639 begins. 1.0 km = 0.6 mi. Diamictite of the Bruce Formation is exposed west of the Highway. 1.5 km = 0.9 mi. Outcrops of Mississagi Formation are exposed along Highway 639. Note the yellowish colour characteristic of the Mississagi Formation where it lies directly on the Archean granitic basement (Robertson, 1968). 2.5 km = 1.6 mi. 17 – 371508E, 5152302N STOP 1.11: Huronian volcanic rocks of the Thessalon Formation (Dollyberry belt). A gated road leads west from Highway 639 leads to one of the Quirke Mine tailings dams. Park near the gate and walk a short distance along a rough road from the gate to the base of the tailings dam. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �39 Note the very dark-green to black, flattened, chlorite amygdules characteristic of the Huronian mafic volcanic rocks between Sault Ste. Marie and the Elliot Lake area. Cross the stream and proceeding northward a short distance along a rough road to the crest to the low hill. The Huronian volcanics at this location (17 – 371428E, 5152342N) include transitional observed identical scattered, quartz pebbles were found along the contact of the Huronian volcanic rocks where they overlie Archean mafic volcanic rocks in the haulage drift of the Stanleigh Mine on the south limb of the Quirke Lake Syncline. The writer proposes that these quartz cobbles are lag deposits left behind while the finer sediment was washed off the surface. A few kilometers west of this location, occurrences of this conglomerate unit contain more rounded quartz, and are commonly are overlain by a thin arkosic unit. Quartz-pebble conglomerate, which locally contain significant pyrite as well as uranium, occur sporadically at the base of the Huronian volcanic sequence of the Quirke Lake Syncline and westward to the Sault Ste. Marie area. The conglomerate at this location occupies the same stratigraphic position as the radioactive, quartz-pebble conglomerate. Figure 1.25: Dropstone in laminated argillite of the Pecors Formation. Stop 10. alkalic types, hawaiite and mugearite (unpublished analyses in the authors files). The eastward trending, south dipping unconformity between the Archean granitic basement rocks and Huronian volcanic is visible near the crest of the hill. There appears to be no visible paleosol development at this location. Near the west end of the outcrop, a thin, quartz-pebble conglomerate/breccia unit, comprised mainly of angular, quartz-clasts, overlies the granitic rocks at the base of the volcanic unit. Scattered, isolated, mainly cobble-sized clasts of quartz can also be found along the unconformity. Some visitors to this site have proposed that the contact between the Archean and Huronian volcanics is not an unconformity, but a fault contact. However, during a visit to the Stanleigh Mine in 1990, the writer along with Rob Henderson, Mine Geologist and Dr. Larry Jensen of the Ontario Geological Survey Return to vehicle, set odometer to 0 and continue north on Highway 639. 0.2 km = 0.1 mi. Archean outcrops cut by diabase outcrops along the highway. 6.8 km = 4.3 mi. 17 – 368530E, 515773N STOP 1.12. Archean metavolanics with pillow structures. Archean mafic metavolcanics displaying well-developed pillow structures are exposed, on north sloping outcrop, the east side of highway (Figure 1.25). Pillows are deformed, but facing directions can easily be determined. Small amygdules are concentrated near the upper surface of many pillows. 7.7 km =4.9 mi. Flack Lake fault occupies a valley near this point. 8.2 km = 5.2 mi. Outcrops of hematite-stained sandstone of the Bar River Formation. 8.8 km = 5.6 mi. Entrance to Mississagi Provincial Park. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �40 9.1 km = 5.7 mi. Pale grey sandstone of the Bar River Formation with herringbone cross-bedding. to suggest that these features are the result of the transportation of consolidated desiccation fracture fillings. 9.8 km = 6.2 mi. Christman Lake. 13.6 km = 8.6 mi. 17 – 364395E, 5162758N 17 – 366863E, 5160228N STOP 1.14: Red beds of the Gordon Lake Formation 10.2 km - 6.5 mi. Laminated, maroon buff siltstone and chert with reduction spots in the upper part of the Gordon Lake Formation. This is another red-bed occurrence within the Cobalt Group. 17 –366742E, 5160652N STOP 1.13: Bar River Formation. 14.9 km = 9.4 mi. 17 – 363062E, 5163194N STOP 1.15: Gordon Lake Formation. — - 'U: __t - - -. Siltstones and sandstones of the Gordon Lake Formation display ripple marks, desiccation cracks, cross bedding and a late cleavage . Note: the presence of pyrite in contrast to hematitic nature of the Gordon Lake Formation near the top of the formation. 19.3 km = 12.20 mi. 17 – 361771E, 5166967N STOP 1.16: Lorrain Formation White to pale pink quartz arenite of the upper portion of the Lorrain Formation is exposed on the east side of the highway. Figure 1.26: Pillow structures in mafic metavolanics. Stop 1.12. 21.0 km = 13.3 mi. At Little White River Road (Highway 546) – End of Field Trip Return to Sault Ste Marie. Sandstones of the Bar River Formation display ripple marks, mud cracks and sinuous structures, 22.8 km 17.8 mi. Junction with Little White which have been described as possible worm River road (Highway 546.) casts. Comparison with desiccation structures in End of Day one field trip. the Gordon Lake Formation led Young (1969) PDF compression, OCR, web-optimization with CVISION's PdfCompressor �41 ILSG Field Trip No. 1 Day 2 - Part 1 Huronian Stratigraphy along Highway 556 and correlation with the Chocolay Group of the Marquette area. On Day 2 we will examine some of the Huronian rocks of the area north of Sault Ste Marie where we will see important differences in the stratigraphy and structural relationships when compared to the sequence in the Quirke Lake Syncline of Day 1. The transect along Highway 556 will show that all three formations of the Quirke Lake Group are missing and correlation with formations of the Hough Lake Group is problematical. Also, there is a significant unconformity at the base of the Gowganda Formation. We will spend part of Day 2 examining a dolostone unit in Fenwick Township near the north shore of Goulais Bay, and a dolostone-bearing unit in the Gordon Lake Formation, southeast of Sault Ste Marie. Those familiar with the Kona Formation of the Chocolay Group in the Marquette Range Supergoup will notice the correspondence in lithology and stratigraphy. Road Log and Stop Descriptions (Refer to Figures 2.1 and 2.2) Island Lake fault in this area are locally chloritized and albitized micro-breccia. The lack of such alteration in the Huronian rocks south of the fault suggests that the alteration pre-dates Huronian sedimentation (Delio Tortosa personal communication, 2005). IMPORTANT: When observing roadside rock-cuts, be especially careful to park your vehicle well away from the pavement and exercise the necessary caution when crossing STOP 2.1: Island Lake Fault Zone the road. Where stops are located on or near blind turns, park in a safe place and walk to the Rocks of the Island Lake Fault Zone separating rock exposure. the Archean rocks on the north from Huronian rocks on the south are exposed along Highway GPS datum is NAD 83 =WGS 84. UTM 638 near Lower Island Lake. Rocks within the Zone17. fault zone are highly sheared and brecciated. Pink rock in the shear zone is a Keweenawan 000 km- Second Line and Highway 17 N. felsic intrusion (felsite). Continuing north on Highway 17. 13.3 km. = 8.2 mi. Intersection of Highway 17 6.4 km. = 4.9 mi. - Railroad crossing and Highway 566. Reset odometer to 0. 8.1 km. = 5.0 mi. 16-709788E, 5175568N Proceed east on Highway 556. 5.6 km. =3.5mi. Fractured altered Archean rocks. The Archean granitic rocks north of the Park on shoulder just west the outcrops. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �42 1a 6 1a 1a Hy 556 2.7 9 7 9 9 2.4 2.3 6 2.6 7 1a 2.2 2b 6 7 2.5 ACR 2b Hy 552 2.1 4 2a 2a o -50 25 Jarvis Lake 2b 20 o 4 5 2a Reserve Lake o -45 5 15 Aweres Lake N 3 -45 Hy 5565 Island Lake Upper Island Lake LEGEND 0 2 1 Keweenawan Supergroup 9 1 Jacobsville Fm. Red Sandstone, conglomerate 2 Gabbro, diabase Huronian Supergroup Mafic flows Livingstone Creek Fm 3 Sandstone, conglomerate Archean 2b. Granitic gneiss 2a. Massive granitic rocks 2a, 2b 7 Lorrain Fm. Quartz arenite, quartz-pebble conglomerate. 6 1 Mafic metavolcanics Gowganda Fm Mudstone, siltstone, diamictite 5 km Thessalon Fm 4 Nipissing 8 3 miles Aweres Fm Sandstone, polymictic conglomerate 2.4 Field trip stop. (Day2) Fault (inferred or assumed) Figure 2.1: Geology of part of Highway 566. (Day 2) From Bennett et al, 1975 Frarey, 1977, unpublished maps and notes of P.E. Giblin and E. J. Leahy. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Jacobsville Fm. Red sandstone, conglomerate Gordon Lake Fm. Lorrain Fm. Keweenawan Mudstone, sandstone, minor dolostone in the Gordon Lake area and Fenwick Twp. only. Quartz arenite, quartzpebble conglomerate Cobalt Group (2.21 Ga) Gabbro, diabase, granophyre 43 Mudstone, siltstone Nipissing Diaabse Gowganda Fm Arkose, siltstone Diamictite, polymictic conglomerate Unconformity Hough Lake Group ? Mainly arkose, subarkose & pebble conglomerate Aweres Fm. ( = Ramsay Lake / Mississagi Fm. ? ) Mainly subarkose, granite & volc. cobble conglomerate Thessalon Fm. U, Th Disconformity Metabasalt, basaltic andesite Minor qtz. peb. cong. near base Disconformity Livingstone Creek Fm. Archean Subarkose, conglomerate Unconformity Elliot Lake Group Mainly volcanic cobble conglomerate Metavolcanics, metasediments, granitic rocks Figure 2.2: A stratigraphic column for the Highway 556 transect PDF compression, OCR, web-optimization with CVISION's PdfCompressor �44 Symbols Fault Unconformity Gowganda Formation Stop 2.7 Archean metabasalt Belleview Fault Stop 2.5 Stop 2.6 Gowganda Fm. Stop 2.2 Stop 2.4 Stop 2.3 Stop 6 Lorrain Formation Archean Archean Metabasalt syenite Nipissing Gabbro Island Lake Fault Aweres Formation Huronian Metabasalt Stop 2.1 Hydrothermally altered Archean granitic rocks Livingstone Creek Fm. (Not exposed on Highway 556) Figure 2.3: A diagramatic cross-section along the Highway 556 transect. I' i1'jq G. Bennett, 2005 Figure 2.4: Gowganda Formation. (Behind figure) unconformably overlying conglomerate of the Aweres Formation. Stop 2.2. Figure 2.5: Small dropstone in laminated siltstone of the Gowganda Formation. Stop 2.3. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �45 STOP 2.2: Gowganda Formation unconformably overlying Conglomerate of the Aweres Formation. On south side of highway an almost black, volcanic pebble to cobble, clast-supported conglomerate of the Aweres Formation is overlain by massive, grey, pebbly wacke of the Gowganda Formation. The contact is abrupt and visible in outcrops along both sides of the The Aweres Formation seems to have been deposited as alluvial fans along the valley walls of a down-dropping fault block (rift valley?). The surrounding plateau of Huronian volcanic rocks were eventually eroded to the underlying Archean basement. The Aweres Formation is probably stratigraphically equivalent to the Mississagi Formation. At the north boundary of the Garden River Indian Reserve, about 12 km southeast of this area, the writer noted a southward transition from Aweres-type conglomerate to the sandstones of the Mississagi Formation (Bennett and Sawiuk, 1979). 9.2 km = 5.6 mi. 16 - 710458E; 5176230N STOP 2.3: Drop-stones of the Gowganda Formation The vertical face of an outcrop of laminated argillite of the Gowganda Formation on the Figure 2.6: Polymictic conglomerate of the Aweres south side of the highway contains a few Formation, Aweres Twp. pebble-size dropstones. (Figure 2.5). The juxtaposition of pebble to boulder-sized clasts, normally indicative of a high-energy highway at this location (Figure 2.4). The entire environment, with laminated mudstone of a low Quirke Lake Group is missing in this area energy environment is unusual. Dropstones are (Figures 1.3, 2.2). It is possible that tectonic the important indicators of paloclimate because, activity north of this area, such as that which in Precambrian rocks, dropstones can only be produced the Kapuskasing structure may be responsible for the uplift and erosion of some of explained as a rain of coarse sediment from floating ice onto silt and mud of a relatively still the sub-Gowganda rocks in this area. lake bottom or sea floor. It is the presence of dropstones at within stratified, fine-grained The lithology of the Aweres Formation seen metasediment of Gowganda Formation (also the here is identical to the lowermost part of the Aweres Formation as seen directly overlying the Bruce and Ramsay Lake Formations) which points to the glaciogenic origin of Huronian Thessalon Formation in the Jarvis Township a matrix supported conglomerates. A very few kilometres southeast of this point. The proportion of volcanic clasts within the Aweres impressive occurrence of dropstones is found in formation decreases as the proportion of granitic a cliff of Gowganda Formation about 50 meters clasts and sandstone increases with stratigraphic west of Highway 129 about 30 km north of the intersection of Highway 129 and Highway17. height (Figure 1.14) (Bennett et al., 1975). PDF compression, OCR, web-optimization with CVISION's PdfCompressor �46 9.5 km. = 6.9 mi. 16 - 710962 E, 5176196 N granitic rocks. About 15 meters of overburden separates the granitic rocks from an outcrop of STOP 2.4: Contact between Gowganda grey pebbly wacke assigned to the Gowganda Formation and Huronian metabasalt of the Formation. The east end of the outcrop (nearest the syenitic rocks) contains angular Thessalon Formation blocks of reddish the syenite/syenodiorite suggesting an unconformable relationship Park on south shoulder just west of outcrop. Pebbly wacke and matrix supported, polymictic rather than a fault. The unconformity crosses pre-Gowganda rocks that dip from 10o to 30o conglomerate with abundant granite clasts west. (Gowganda Formation) show an abrupt contact with the dark-grey Huronian basalt 12.6 km. - 7.9 mi - Quartzite of the Lorrain flows to the east. The contact between the Formation. Fractured reddish quartzite volcanics and the Gowganda conglomerate strikes 135 dips 65 W. Faint bedding (?) in Gowganda Formation is essentially parallel to 14.9 km. = 9.2 mi. 16- 714951E, 5177887N the contact with the volcanic rocks. There is minor fracturing parallel to the contact in the STOP 2: 6: Jasper Pebble Conglomerate Member of the Lorrain Formation. volcanics but no significant deformation is noticeable in the conglomerate near the This striking rock is a pebbly sandstone and contact. The writer (eventually) concluded quartz pebble conglomerate known locally as that the contact is an unconformity (Figure “puddingstone”. The abundant pebbles of 2.3). variously coloured jasper presumably had a provenance in Archean iron formation, which The Huronian volcanics (Thessalon Formation) contains clots of altered plagioclase is well represented in the Superior Province to the north. This member is in about the crystals (glomeroporphyritic texture). Whole rock chemical analyses of the basaltic flows in stratigraphic middle of the Lorrain Formation this area show the elevated soda similar to that (Frarey 1977). At this point the dip of the bedding here is vertical or nearly so. A possible of spilite. Albite, clinozoizite, actinolite and some (original) clinopyroxene are the essential axial plane cleavage is visible, suggesting the rocks lie on the south limb of an anticlinal minerals. structure. 10.8 km. - Railroad trestle. 16.4 km.= 10.25 mi - Northland Lake Road. 11.3 km. = 7.2 mi. 16- 712648E, 5176124N. Sharp Corner – watch for traffic. You may park 17.6 km. = 10.9 mi (Optional) on the level area south of the Highway if your vehicle has a relatively high road clearance. STOP 2.7: Outcrop of Archean metavolcanic rocks. STOP 2.5: Archean granitic rocks and Gowganda Formation. The easternmost, high rock-cut is of pink, coarse-grained Archean syenite to syenodiorite. Two mafic dikes intrude the Fine to medium-grained amphibolite is part of the hornfels zone near the granitic plutons. End of road log for Highway 556. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Figure 2.7: Dolostone of the Kona Formation. Highway 63, Marquette area, MI. Figure 2.8: Dolostone and chert. Fenwick Township, ON. 47 0 .:Y3 Figure 2.10: Jasper-pebble conglomerate member of the Lorrain Formation. Stop 2.6. Figure 2.9: Dolostone nodules and lenses (dark areas) in the Gordon Lake Formation. Near Highway 638, Gordon Lake area, ON. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �48 Return to Highway 17. At intersection of Highway 17 and Highway 556 reset odometer to 0. Continue north on Highway 17 to Highway 552 (about 13 km). Turn left (west) onto Highway 552. 5.5 km = 3.4 mi. Goulais Mission Road. Turn right onto Goulais Mission Road. 11.4 km = 7.1 mi. Private road of Case Construction Company. Permission must be obtained from Case Construction before continuing. We will continue to the end of the road as indicated by a locked gate. A short walk (> 100m) in a southwest direction will bring us to a small hill we refer to as the “dolostone knob”. STOP 2.8: Outcrops of Dolostone at the Dolostone Knob. Thinly bedded and laminated dolostone and chert are found along the steep sides of an outcrop knob. Some laminated silty sections may be stromatolitic bedding. STOP 2.9: This stop will be visited as time allows. This stop is located near Highway 638. Directions and a stop description are included in the supplementary road log for Highway 638. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �49 Geological Features and Correlation of a Dolostone Unit in Fenwick Township, Northeast of Sault Ste Marie, Ontario millimeters to at least 1 m thick. The metachert beds have a similar thickness range but form less than 25 percent of the visible section (Figure 2.8). No clearly identifiable stromatolitic structures were observed but irregular and wavy, closely spaced laminae of brownish chert, suggestive of algal mats, were noted. Pale grey, clastic dolostone and oolitic dolostone appear to be the main rock types in the upper part of the carbonate unit. A few lenses, up to 50 cm wide, of coarse, pink barite are found within dolostone on the south side of the knob. Born (1987) described, a previously unrecognized occurrence of dolostone and associated clastic rocks in Fenwick Township, approximately 20 miles (30 km) northwest of Sault Ste. In the spring and fall of 1988 the writer mapped the known extent of the dolostone, and some of the surrounding rocks, was mapped by at a scale of about 1:8000 (Bennett et al, 1990). Mr. Paul Morra of Sault Ste Marie provided a stadia survey of selected areas. 1 1 1 1 1 1 1 1 1 3 Gravel Pit 1 4c 1 4c 70 4c,b 3 3 4c,b 1 1 1 3 3 1 1 75 4b 3 4b 75 4b 3 3 Dolostone knob 4b 3 4b 4a 35 65 4b 3 No outcrop 4c 2 3 Sand and gravel 2 Mi ss North 0 100 200 Meters 300 Legend Gordon Lake Fm 4a. Red Sandstone, siltstone 2 2 3 4c 4b 4b 4b 70 io n 4a Ro ad 4b. Dolostone, chert 4c. Siltstone, mudstone Lorrain Fm 3. Quartz arenite Outcrop with code Fault Gowganda Fm 2. Mudstone, sandstone Archean Figure 2.11: Geology of dolostone occurrence in Fenwick Township. 1. Metavolcanics Geology by G. Bennett, 1988 The general geological setting of the dolostone units in Fenwick Township is shown on Figure 2.11. The best exposures of the dolostone unit are found in the area indicated on the figure as the “dolostone knob”. In the area of the dolostone knob, the lowermost third of the dolostone sequence consists mainly of very fine-grained, pale pink to reddish pink dolostone with intercalated grey to pink metachert. Individual dolostone beds are from a few At the western side of the knob, the clastic dolostone is directly overlain by deep-red sandstone, and red to pink laminated siltstone. The dolostone unit is underlain by green and maroon siltstone and argillite. The southern contact between the Lorrain quartz arenites and the dolostone is not exposed but lies within an east-trending, narrow, linear, PDF compression, OCR, web-optimization with CVISION's PdfCompressor �50 recessive zone about 10 m wide, and is inferred to be a fault. lithological and stratigraphic similarities of the Chocolay Group (and equivalent groups further west) and the Cobalt Group have been pointed Correlation with the Gordon Lake out by many authors (Figure 2.12). At the time, Formation this correlation was not widely accepted because previous radiometric dating of rocks considered Red siltstone and sandstone, purple and green to underlie the Chocolay Group was seen as siltstones, chert and dolostone of Fenwick providing evidence that the Chocolay Group is Township have been assigned to the Gordon significantly younger than the Huronian Lake Formation of the Huronian Supergroup Supergroup of Ontario (Young, 1983). (Bennett et al, 1990). This correlation is However, Vallini et al (2005) have successfully indicated by the juxtaposition of the dated hydrothermal xenotime in the Kona dolostone and associated rocks with the Formation and detrital zircons in the underlying Lorrain Formation (Although this is likely a Mesnard Formation thereby constraining the fault contact). The reddish hues of the associated sandstones Gordon Lake area Flack Lake Area Marquette Area Fenwick Twp indicate they are Sault Ste Marie Bar River Fm. Wewe Slate Ontario stratigraphically superior Kona Gordon Lake Fm. Cu Cu Fault Fm. to the Serpent Formation Mesnard Fm. Lorrain Fm Lorrain Fm (Roscoe (1969). Most Gowganda Enchantment L. Fm. Fm importantly, nodules and Gowganda Fm. Archean Archean continuous beds of 250 km dolostone have been Lower Huronian Groups Dolostone identified in the Gordon Lake Formation of the Diamictite 130 km Gordon Lake area SIltstone, sandstone (Jackson, 2001; Hofmann Slate, argillite et al 1980). Quartz arenite Figure 2.12: Proposed stratigraphic relationships between the During a discussion (in Lower Proterozoic rocks of Ontario and Michigan. 1988) of the problematic correlation of the timing of deposition of the Chocolay Group dolostone units with Mr. Ken Hatfield of between 2300-2200 Ma thus proving a Lake Superior State University, the latter correlation with the Huronian Supergroup of suggested a correlation with the Kona Ontario. Dolomite of the Marquette, Michigan area (Figure 2.7). There seems litte doubt now that the Cobalt Correlation with the Chocolay Group of Group rocks of the Huronian Supergroup are Michigan stratigraphically equivalent to the Chocolay Group of the Marquette Range Supergroup of There has been a long-standing debate as to Michigan and to correlative units further west. stratigraphic and temporal equivalence of the Huronian rocks of Ontario and the Middle Precambrian rocks of the Lake Superior region of Michigan (Young, 1983) The PDF compression, OCR, web-optimization with CVISION's PdfCompressor �51 Supplimentary Road Log for Highway 638 Highway 638 extends north from Highway 17 at A few small, ground-level rock exposures on the Echo Bay and continues in a broad loop to rejoin north side of the road near the western limit of Highway 17 at Bruce Mines (Figure 1.18). the northern rock-cut exhibit ancient mud-cracks as polygonal patterns of apple-green Only STOP 2-9 of this road log is part of the mudstone and pink sandstone. Day 2 sequence. Stops preceded by “S” are supplementary or optional stops. A few of these Ripple-marks and laminated, red siltstone can be stops may be made if time allows. The seen at about midpoint on the north outcrop. remaining outcrop descriptions are abstracted Small-scale (less than 20 cm) cross-bedding is from a previous guidebook (Bennett et al, 1997) well developed in pale grey and pink sandstone for the general interest of the reader. on the eastern portion of the southern road-cut but can be observed in most of the sandstone of the road-cut. Road Log and Outcrop Descriptions. 0 km = 0 mi. The Husky Service Station on Highway 17 near the eastern limits of the City of Sault Ste. Marie. 17.8 km =11.1 mi. Junction of Highway 638 and Highway 17 in the Town of Echo Bay. Reset odometer. Continue along Highway 638. 5.4 km = 3.4 mi. Outcrops of pink weathering sandstone of the Lorrain Formation. 10.6 km = 6.6 mi. Large road cuts on both sides of highway. The west limit of the outcrop area is at NAD 83 (WGS 84) 17-273278E, 5015292N. STOP S1-1: Lowermost Gordon Lake Formation The sandstones and minor mudstone exposed in the rock cuts are either the lower units of the Gordon Lake Formation or the uppermost rocks of the underlying Lorrain Formation. A large portion the sandstone is “bleached” almost white in contrast to the pink colour dominant in much of the outcrop. Note that the boundary between the pink and pale-grey sandstone clearly crosses the stratification in the rocks. That the bleaching (reduction) is the later event can be seen near the eastern limit of the road-cut. There the pale-grey sandstone is cut by dark red, hematite staining for several meters on both sides of steeply dipping, northeast striking fractures in the sandstone. A wide zone of reduction can be seen on both sides of the oxidized fractures. A hypothesis consistent with these observations is that chemically active fluids passing upward though the fractures first reduced the red hematite in the pink feldspar of the sandstone over a broad area along the fracture and in doing so became enriched in ferrous iron. Some time later, the system became oxidizing resulting in the precipitation of the much less soluble ferric iron as hematite adjacent to the fracture zone. These later fluids may have been oxygenated groundwater percolating downward from the surface encountering the ascending Fe+2 bearing reducing fluids that had scavenged iron from the underlying rocks. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �52 A mafic dike about 10 cm wide intrudes the sandstone near the fracture zone. The dike occupies a fracture with a strike (direction) and dip similar to that of the hematite-filled (reddened) fractures. This suggests that hot magma was in the vicinity when the fractures existed and thus may have provided the thermal energy to drive the fluids through the fracture system. 10.9 km = 6.7 mi. East end of outcrop at 17-273416E, 5150243N. 11.6 km = 7.2 mi 17 – 274165E, 5150594N STOP 2.9: Dolostone nodules and discontinuous dolostone beds in the Gordon Lake Formation. NOTE: The outcrop at this location is on private property. To protect the rights of the property owner the exact location of the outcrop will not be provided here. Outcrops of buff coloured siltstone and sandstone contain deeply recessed, discontinuous beds and scattered dark-brown to black dolostone. Jackson (1994) states: “Buff brown to black weathering siliceous dolostone nodules are common throughout the lowermost Gordon Lake Formation. The nodules are commonly amoeboid and concentrically zoned with carbonate-rich core and dark hematitic-rich rinds. A large number of the amoeboid forms are, however, asymmetrically zoned with the hematitic rind being concentrated, or better developed, towards the stratigraphic top of the amoeboid forms suggesting at least local upward flow of fluids. In red to deep maroon siltstones, the carbonate-rich nodules are very pale pink to white, reflecting the widespread reduction and/or removal of iron from these volumes of the rock” Return to Highway 638. 23.2 km = 14.4 MI. Village of Leeburn Corner and general store. 25.0 km = 15.3 mi. East side of road. Dark grey-green mudstone and siltstone of the Gowganda Formation. Thin-bedded to very thick bedded with some thin beds of pink siltstone. 25.3 km = 15.8 mi. Dark grey siltstone with smooth, striated outcrop surface due to most recent (Pleistocene) glaciation. 26.4 km = 16.4 mi. Nipissing granophyre and diabase, fractured and cut by calcite and iron carbonate veinlets. 30.7 km = 19.1 mi. Stop Sign at the village of Ophir. 32.7 km = 19.7 mi. West side of highway. STOP S1-2: Mudstone overlying matrix-supported conglomerate (diamictite) of the Gowganda Formation. Matrix supported conglomerate consist of pebbles and bounders dispersed in a fine matrix so that the pebble and boulder sized clasts are supported by the matrix, not each other. Such conglomerates cannot be formed by the transport and deposition of clay to boulder-sized clasts by water or wind. Matrix supported conglomerates usually imply deposition from mudflows or directly from ice as glacial till. 32.9 km = 20.4 mi. West side of Highway. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �53 STOP S1-3: Dropstones in laminated siltstone and mudstone e of the Gowganda Formation. STOP S1-5: Jasper pebble conglomerate of the Lorrain Formation (“Pudding Stone”). A few granitic pebbles lie within dark grey laminated siltstone. It is generally agreed that the only process that can allow the juxtaposition of pebbles or boulder-sized clasts in stratified, fine-grained sediment of is the release of stones by melting icebergs onto the sea bed or lake bottom. Outcrops of pale-grey quartz arenite and jasper-quartz pebble conglomerate are found at east side of the highway. It is assumed that the jasper pebbles were derived from Archean banded iron formation, which is well represented in the Batchawana greenstone belt about 130 km northwest of these outcrops. The jasper-pebble conglomerate member (Frarey, 1977) of the Lorrain Formation is a minor but persistent unit. It has been found near the middle of the Lorrain Formation from the Sault Ste Marie area in the west to the Quebec border in the east. Although the high concentration of red, maroon and pink jasper presents a striking appearance it has a limited commercial use as a decorative stone because of its extreme hardness. 33.6 km = 20.9 mi. Large gravel pit with high rock face on east side of the highway. A short unsurfaced road leads to the gravel pit. STOP S1-4: Diamictite of the Gowganda Formation. The high rock face is worn smooth by rushing water charged with abrasive sand etc. Vertical channels in the rock face suggest the erosive effect of sand-charged water under the extreme pressure of an overlying ice sheet. The northern section of the outcrop is matrix-supported conglomerate of the Gowganda Formation. A diabase dike (probably of Nipissing age) intrudes the southern part of the outcrop. The dike is about 20 meters wide (estimated). 42.8 km =26.6 Outcrops of Lorrain Formation sandstone on west side of highway. At least two faults show hematization in the fracture zone. A pebble unit is seen to be displaced along one of the fault. 44.1 km = 27.4 mi. Bridge at Rydal Bank. 36.2 km = 22.5 miles Rock Lake Road. 52.7 km = 32.7 mi Junction of Highway 628 and Highway 17 at Bruce Mines. 39.8 km = 24.7 mi. End of road log for Highway 638. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �54 Supplimentary Road Log for parts of Highway 17 and Highway 129 between Sault Ste Marie and Highway 108 Most of the outcrops included in this section are not included in the ILSG 2006 field trip. Stops indicated as supplementary are preceded by the letter “S”. They are included to allow those interested to visit them if traveling in the area at some future date. UTM coordinates have been provided for some locations. Note: the differing ripple directions and local interference ripples. This impressive outcrop is known locally as the ”Ripple Rock”. 26.8 km = 16.6 mi. STOP S2.3: Purple siltstone member of the Lorrain Formation. Dark red to purple siltstone and sandstone Proceed east along Highway 17 from Sault Ste. constitute true red-beds in the Huronian (Frarey, 1977). These hematite-rich rocks Marie. display the reduction along fractures typical of The road log starts at the intersection of red beds, and a good indicator of deposition in Highway 638 and Highway 17 East in the an oxidizing environment. The bedding is Town of Echo Bay faint, but locally distinct, striking 140o and dipping 10o south. 0.0 km = 0.0 miles - Highway 638 at Echo Bay - continue east on Highway 17. 27.7 km = 17.2 mi. Lake Huron Drive in the town of Desbarats. 8.8 km = 5.5 miles - At sign for Calabogie Road UTM 16- 725322E,5144204 N,. 28.0 km = 17.4 mi. 17- 5136434N, 275510E, STOP S2.1: Sub-Jacobsville unconformity STOP S2.4: Basal Arkose Member of Lorrain Formation. Outcrop on west side of Highway 17. The most southerly outcrop is white and grey quartzite (quartz arenite) of the white orthoquartzite member (Frarey, 1977) of the Lorrain Formation quartzite is overlain by a sedimentary breccia of the Keweenawan, Jacobsville Formation. The conglomerate is comprised of angular clasts of Lorrain quartzite up to 60 cm across in a red siltstone matrix. The contact is irregular. No stratification was observed. Large outcrops of pink, medium-grained, rather massive arkose beds with hematite-rich spots about one centimeter across. Pale laminations in outcrop on south side of the road indicate shallow dips of about 10o west. The unit is about 1700 feet thick in this area (Frarey, 1977). 40.5 km = 25.0 mi. STOP S2.5: Bruce Mines Copper Vein. 25.0 km = 15.5 mi. In a rock-cut on the north side of Highway 17, in the town of Bruce Mines, is a 3 m wide STOP S2.2: Lower red quartzite member quartz vein in Nipissing diabase. The vein contains small amounts of chalcopyrite, of the Lorrain Formation. chalcocite, bornite and malachite. It is one of On the north side of Highway 17 steeply the few remaining remnants of the copper dipping beds of the lower red siltstone member mineralization that once made this town of the Lorrain Formation reveal well preserved known throughout the mining world. Mining oscillation ripples an a large south facing began here in 1846 and continued to 1875, surface. making Bruce Mines the first mining town in Canada. Please do not take samples from this outcrop. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �55 About 75 meters north of this outcrop there is a reconstruction of a copper mining operation of that period built on one of the original mine workings, known as the “Simpson Shaft”. 42.6 km = 26.5 mi. Entrance to the quarry of Ontario Trap Rock (R.W. Tomlinson Limited). The quarrying of trap rock (Nipissing diabase) at Bruce Mines resumed in 1990. A predecessor near Lake Huron operated during the First World War. chlorite. The relatively small, monomineralic chlorite amygdules (“chlorite buttons”) are characteristic of the mafic volcanics of the Thessalon Formation between Sault Ste. Marie and Elliot Lake. Glacial striae and chatter marks are well developed on the outcrop surface. Proceeding north on Highway 129. Reset odometer to 00 at Intersection of Highway 17 and Highway 129. 45.3 km = 28.1 mi. 17-289512E, 5129999N 1.0 km = 0.62 mi. STOP S2.6: Low outcrops of Mississagi Formation. STOP S2.8: Outcrop of Matinenda Formation on east side of Highway 129. Medium-sand to grit-sized, grey, subarkose with thin quartz-pebble beds and prominent planar cross-bedding are well exposed along the north side of the highway. Rusty staining on the outcrop reflects minor detrital (?) pyrite along the foreset beds. Black chert pebbles in pebbly beds, minor pyrite and meter scale bedding are common features of the Mississagi Formation. Roscoe, (1969) concluded that the drab colour and presence of detrital pyrite in Huronian sandstones and conglomerates is evidence of the reducing nature of the early Huronian atmosphere. 45.4 km = 28.2 mi. Waltonen Road near east end of the outcrop area. This fine-grained, pale pink to greenish grey, subarkose which directly overlie the Thessalon Formation in the Thessalon area has been correlated with the Matinenda Formation of the Elliot Lake Group (Bennett et al., 1991). Faint outlines of trough cross-bedding, visible on the upper surface of the outcrop, indicate paleocurrent directions from the northwest. The greenish (sericitic) units and the trough cross-beds are typical of the Matinenda Formation, but the fine grain size is not. At Thessalon Point about 4 km southwest of this location these rocks contain thin quartz pebble conglomerate units, but the pyrite and radioactivity, characteristic of the Matinenda Formation of the Elliot Lake area, is lacking. 60.6 km = 37.6 mi. 17 - 303650E, 5126650N 23.0 km = 14.9 miles The village of Warncliffe Intersection of Highway 17 and Highway 129. 29.1 km = 18.4 miles north end of Appleby Lake. Stop S2.7: Thessalon Formation tholeiitic basalt. Dark green, fine to medium grained basalt is exposed on the east side of Highway 129 just north of the intersection with Highway 17. This tholeiitic basalt forms the top of the Thessalon Formation in the Thessalon area and is typical of the thicker tholiitic sequences which make up the upper parts of the Thessalon Formation in the Sault Ste. Marie and Aberdeen Township areas. The rocks consist of albite, chlorite, epidote, clinozoisite, leucoxene and minor quartz and oxides. The metabasalt contains amygdules of albite, quartz, epidote, calcite and 30.3 km = 19.4 miles UTM 17, 5 145 009N, 320 904E STOP S2.9: Laminated siltstone of the Gowganda Formation with dropstones. West of Highway 129. Proceed up steep slope to cliff face located a few tens of meters into the bush. Vertical joint faces in laminated siltstones-mudstone of the Gowganda Formation form an impressive cliff just west of Highway 129. The bedding comprises regular and remarkably continuous mudstone-siltstone PDF compression, OCR, web-optimization with CVISION's PdfCompressor �56 couplets from 0.5 to 2 cm thick. The grey siltstone unit at the base of each couplet grades up to a darker mudstone which is in sharp contact with the siltstone base of the overlying couplet. The sequence closely resembles varves of Pleistocene glacial lakes. A few pink, discontinuous, fine, sandstone beds are also present. Widely scattered trough the sequence are pebble to boulder-sized “drop stones”, predominantly of Archean granitic rocks. In the absence of volcanic activity within the Gowganda Formation, the only plausible source of the drop stones is the presence of floating ice which released its rock load during melting. This outcrop, and many others of a less spectacular nature, provide some of the best evidence for a cool climate during Gowganda deposition more than 2 Ga.. Return to Highway 17 Reset Odometer. 3 km = 1.9 Pine Ridge Road Proceeding east on Highway 17 from Pine Ridge Road Reset odometer to 0. 13.5 km = 8.4 mi. Melwell Road 14.1 km = 8.7 mi. 17-321874E, 5129338N – Outcrop with prominent white quartz veins near crest of hill . STOP S3.1: Mineralized quartz vein breccia. Quartz-carbonate veins and vein breccia cut pebbly siltstone of the Gowganda Formation Church and Young, (1972) interpret the on the south side of Highway 17. The vein laminated unit as a varved sequence. They and mineral assemblage also note the presence of small sedimentary (quartz-carbonate-pyriteclasts, lithologically similar to the enclosing rocks. They interpret the clasts of sedimentary chalcopyrite-hematite) are typical of many in rocks as having been derived from the ablation such occurrences the area between Sault Ste. of sediment on the upper surface of the glacier. Marie and Sudbury. The extensive brecciation suggests surface venting with resulting 30.9 km = 19.5 miles. Intersection with hydrothermal boiling, adiabatic cooling with Highway 554. Return to Thessalon along coincident brecciation and mineral deposition. Highway 129. Some sections of the vein system show well-developed mineral zoning and pre-vein 30.9 km = 19.5 miles. Intersection of alteration. Minerals identified include: quartz, Highway 17 and Highway 129 at Thessalon. iron-carbonate (ankerite?), pink calcite, Reset odometer to 0.0 chalcopyrite and specular hematite. The lack 7.3 km - 4.6 miles. 17 - 306085E, 5126967N. of pyrite and the late deposition of hematite, indicates a sulfur deficient hydrothermal fluid. Green Lane. Cross the railway tracks (with care) and immediately turn left then south to short section of unpaved road between private 47.6 km = 29.6 mi. Town of Blind River properties. Outcrops of Thessalon Formation 66.7 km = 41.4 mi. Pronto Road. 17 – felsic metavoclanics lie at the end of the road. 367425E, 5117509N STOP S2.10: Rhyolite of the Thessalon Formation. Access to granitic paleosol and a remnant of radioactive quartz-pebble conglomerate of the former Pronto Mine. Volcanics are fine grained, grey to pink and maroon rhyolite consisting of a fine grained mosaic of albite, k-feldspar, and quartz with At the time of writing (2006) the Pronto Mine minor green pleochroic biotite. Amygdules are site is currently not open to the public. Those filled with quartz, biotite and stilpnomelane. planning to visit this site should contact the Collapsed vesicules are also present. Local iron staining and darker chloritic areas indicate the presence of iron sulfides. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �57 A S3.2 Former mine site B Pr on to Th ru st LEGEND Fa ul Diabase t HURONIAN Mine workings Higher Formations Matinenda Fm. Ore bed Paleosol ARCHEAN After Robertson (1970). Granitic rocks N A S3.2 Former mine site Since rehabilitated rust Fault nto Th Pro Hy dro El ec tri c Line Pond Pond B nto Pro d Roa l Fau r r ay Mu t ay ghw 17 Hi Lake Huron 1 km After Robertson (1970). HURONIAN SYMBOLS Formations above the Matinenda Matinenda Formatioin Pater Formation ARCHEAN Granitic rocks S3.2 Field trip stop Fault Projection of original ore body Figure 2.13. Plan and section of the geology of the (former) Pronto Mine area. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �58 displayed as dikes and irregular bodies of aplite and pegmatite, can be recognized in the paleosol (saprolith). Continuing up the southward sloping outcrop the yellowish colour of the paleosol can be seen to take on a pinkish and reddish hue normal to the Archean granitic rocks of the area. Continuing north Turn north onto the Pronto road and continue and west to near the highest point on the until prevented by a gravel barrier from outcrop area one can discern the faint outline proceeding further. Continue on foot past the of core-stones in the original granitic rocks. barrier to the west end f a beaver pond just north of the road. Proceed along a trail across 11.6 km = 7.2 mi Highway 108 to Elliot Lake the west outlet of the pond, then around the Return to Highway 17. Continue east along north end of the pond and up a steep hill. Turn left (to the north) on a gravel road at the Highway 17 to Highway 108 to Elliot Lake. top of the hill. Continue to an area of outcrop Continue eastward for a few hundred meters and gravel which marks the former open stope past Highway 108 to an outcrop just south of of the Pronto mine at 17-367804, 5119019. If Highway 17. not accompanied by a guide, please do not Continue for about 100 m past the intersection attempt to access this site without a GPS of Highway 17 and Highway 108. receiver or compass as well as suitable footwear and clothing. Low outcrops a few tens of meters south of Highway 17. (Refer to Figure XXX). District Geologist office of the Ministry of Northern Development and Mines at Sault Ste Marie, Ontario (705 - 945-6931) to learn of the site status and the phone number of a contact person with the authority to grant permission to access the property. STOP S3.2: Pronto Mine Location This property is currently (2006) undergoing restoration to a condition as closely as possible to it’s original state. STOP S3.3: McKim Formation Staurolite (pseudomorph) schist. This outcrop, which lies a few hundred meters south of the Murray Fault Zone, displays the The Pronto Mine of Rio Algom Limited, the higher metamorphic grade and deformation first operating uranium mine of the Algoma common to rocks south of the fault. At this District, produced 2.1 million tones of ore location argillaceous units of the McKim between 1955 and 1960. (Figure 2.13. Formation contain pale prismatic crystals of staurolite that have undergone retrograde This is one of the few locations where metamorphism to sericite-quartz aggregates. A exposures of uraniferous quartz pebble conglomerate of the Matinenda Formation can few pseudomorphs show the characteristic cruciform twinning of staurolite. The growth of be found on surface in the Elliot Lake-Blind large staurolite crystals in the originally more River area. In the depression south of the argillaceous upper portions of the beds has road, a few remnants of rusty-weathering resulted in “reverse graded bedding” in a few outcrops of radioactive, pyritic, quartz-pebble places. conglomerate of the Matinenda Formation mark the base of the ore bed of the Pronto Return to the intersection of Highway 17 and Mine. The conglomerate lies directly upon Highway 108. Proceed north along Highway yellowish, sericitic paleosol developed on 108 to the Town of Elliot Lake. Archean granitic basement. Visitors are requested not to take samples of the Continue north on Highway 108 to the City of conglomerate. Elliot Lake. Proceeding in a general northerly direction, some features of the original granitic rocks, PDF compression, OCR, web-optimization with CVISION's PdfCompressor �59 Publications Cited or Consulted Bennett, G. 1978. Huronian volcanism, districts of Algoma and Sudbury; in Summary of Field Work, 1978; Ontario Geological Survey, Miscellaneous Paper 82, p.105-111. Bennett, G. 1982. Geology of the Two Horse Lake Area, District of Algoma; Ontario Geological Survey, Geological Report 210, 63p. Bennett, G., Hillier, R. D., Nentwich, F., Dupuis, C. P. and Pucovsky, M.., 1975. Jarvis Lake-Garden River area, District of Algoma, Ontario Division of Mines; Preliminary Geological map, P.1064, 1 inch to ¼ mile or 1:15,840. Bennett, G and Sawiuk, Myron. 1979, Jarvis Lake-Garden River area (Southern Part), District of Algoma, Ontario Division of Mines (Now Ontario Geological Survey); Preliminary Geological map, P.2241, 1 inch to ¼ mile or 1:15,840. Born, Peter, 1987. Geology of the Havilland Bay – Goulais Bay Area District of Algoma; Ontario Geological Survey, Open File Report 5602, 114 p with map at a scale of 1:15 840 (1 inch to ¼ mile) Born, P. and James, R.S. 1978. Geology of the East Bull Lake anorthosite intrusion, District of Algoma, Ontario; in Proceedings and Abstracts, Geological Association of CanadaMineralogical Association of Canada, Joint Annual Meeting, v.3, p.369. Born, P., Worona, R., and Stephenson C., 1986. Precambrian Geology of the Haviland-Goulais Area. District of Algoma; Ontario Geological Survey, Map P2959. Geological Series-Preliminary Map. Scale 1:15 840 or 1 inch to ¼ mile. Geology 1985. Bottrill, T. J., 1971. Uraniferous conglomerates of the Canadian Shield; in Report of Activities, Part A: April to October, 1970, Geological Bennett, G., Leahy, E. J, Melisek, J. Born, P. and Survey of Canada, Paper 71-7, p.77-83. Hatfield, K. 1989. Sault Ste. Marie Resident Buchan, K. L. and Card, K.D., 1985. Preliminary Geologists District—1988; in Report of comparison of petrographic and paleomagnetic Activities 1988, Resident Geologists, Ontario Geological Survey, Miscellaneous Paper 142, p. characteristics of Nipissing diabase intrusions in northern Ontario; in current Research, Part A, 207-217. Geological Survey of Canada, Paper 85-1A, Bennett, G., Leahy, E. J. and Walmsley, J. 1990. p.131-140. The Sault Ste. Marie Resident Geologist’s District-1989; in Report of Activities, Regional Cannon,W. F., 1962. Plutonic evolution of the and Resident Geologists, Ontario Geological Cutler area, Ontario; unpublished PhD thesis, Syracuse University, Syracuse, New York, 105p. Survey, Miscellaneous Paper 147, p.205-215. Bennett, G., Dressler, B.O. and Robertson, J.A. Card, K. D., 1964. Metamorphism in the Agnew 1991. The Huronian Supergroup and associated Lake area, District of Sudbury, Ontario, Canada; intrusive rocks; in Geology of Ontario, Ontario Geological Society of America Bulletin, v.75, no. Geological Survey, Special Volume 4, Part 1, p. 10, p.1011-1030. 549-592. Bennett, G., Card K. D. and Tomlinson, K. Y. 1997. The Huronian Supergroup between Sault Ste Marie and Elliot Lake; Institute on Lake Superior Geology, 43rd Annual Meeting, May 6, 11, 1997, Sudbury, Ontario. Card, K. D., 1976. Geology of the MacGregor Bay – Bay of Island Area, Districts of Sudbury and Manitoulin, Ontario Division of Mines Geoscience Report, 131, 63p. Card, K. D., 1978. Geology of the Sudbury-Manitoulin area, districts of Sudbury PDF compression, OCR, web-optimization with CVISION's PdfCompressor �60 and Manitoulin; Ontario Geological Survey, Report 166, 238p. Card, K. D., Church, W.R., Franklin, J.M., Frarey, M.J., Robertson, J.A., West, G.F. and Young, G.M. 1972. The Southern Province; in Variations in Tectonic Styles in Canada, Geological Association of Canada, Special Paper 11, p.335-380. Card, K. D., Innes, D.G. and Debicki, R.L., 1977. Stratigraphy, sedimentology, and petrology of the Huronian Supergroup in the Sudbury-Espanola Area; Ontario Division of Mines, Geoscience Study 16, 99 p. Chandler, F.W., Young, G.M. and Wood, J., 1969. Diaspore in early Proterozoic quartzite (Lorrain Formation) of Ontario; Canadian Journal of Earth Sciences, v.6, p.337-340. Church, W. R. and Young, G. M., 1972. Precambrian geology of the southern Canadian Shield with emphasis on the lower Proterozoic (Huronian) of the North Shore of Lake Huron; International Geological Congress 24th, Montreal, Guidebook to field excursion A36-C36, 65 p. Coleman, A. P., 1905. The Lower Huronian ice age; Journal of Geology, v.16, no. 2, p.149-158. Card, K. D. and Pattison, E. F., 1973. Nipissing diabase of the Southern Province; in Huronian Collins, W. H., 1925. North shore of Lake Huron; Geological Survey of Canada, Memoir Stratigraphy and Sedimentation, Geological 143, 160p. Association of Canada, Special Paper 12, p.7-30. Corfu, F., and Andrews, A., 1986. A U-Pb age Card, K. D. and Palonen, P.A., 1976. Geology for mineralized Nipissing diabase, Gowganda, Ontario; Canadian Journal of Earth Sciences, of the Dunlop-Shakespeare area, District of v.23, p.107-112. Sudbury; Ontario Division of Mines, Geoscience Report 139, 52p. Dietz, R. S. and Holden, J.C, 1966. Card, K. D. and Jackson, S.L., 1995. Tectonics Miogeosynclines in space and time; Journal of Geology, 74, p. 566-583. and metallogeny of the Early Proterozoic Huronian fold belt and the Sudbury Structure Dyer, B. D., Krumbein, W.E. and Mossman, of the Canadian Shield; Field Trip Guidebook, D.J., 1988. Nature and origin of stratiform Precambrian 95, Geological Survey of Canada kerogen seams in lower Proterozoic Open File 3139. Witwatersrand-type placers-the case for Casshyap, S. M., 1969. Petrology of the Bruce biogenicity; Geomicrobiology Journal, v.6, p.33-47. and Gowganda formations and its bearing on Huronian Sedimentation in the Eisbacher, G. H., and Bielenstein, H.U.,1969. Espanola-Willisville area, Ontario, Canada; The Flack Lake Depression, Elliot Lake area, Paeleogeography, Paleoclimatology and Ontario (41J/10); in Report of Activities, Part Paleoecology, v.6, p5-36. B, Geological Survey of Canada, paper 69-1B, Chandler, F. W., 1973. Geology of McMahon p.58-60. and Morin Townships, District of Algoma, Fahrig, W. F., 1987. The tectonic setting of Ontario Division of Mines, Geoscience report continental mafic dike swarms: failed arms and 112, 77p. early passive margins; in Mafic Dike Swarms, Geological Association of Canada, Special Chandler, F. W., 1976. Geology of the Paper 34, p.331-348. Saunders Lake area, District of Algoma, Ontario Division of Mines, Geoscience report Fralick, P. W. and Miall, A.D., 1989. 155, 46p. Sedimentology of the lower Huronian Supergroup (early Proterozoic), Elliot Lake PDF compression, OCR, web-optimization with CVISION's PdfCompressor �61 area, Ontario, Canada; Sedimentary Geology, v.63, p.127-153. Frarey, M J., 1967. Three new Huronian names; Geological Survey of Canada, Paper 67-6, 3p. Frarey, M. J., 1977. Geology of the Huronian belt between Sault Ste. Marie and Blind River, Ontario; Geological Survey of Canada, Memoir 383, 87p. Frarey, M. J. and Roscoe, S.M., 1970. The Huronian Supergroup north of Lake Huron; in Symposium on Basins and Geosynclines of the Canadian Shield; Geological Survey of Canada, 70-40, p. 143-158. Frarey, M. J., Loveridge, W.D. and Sullivan, R.W., 1982. A U-Pb zircon age for the Creighton granite, Ontario; in Rb-Sr and U-Pb Isotopic Age Studies, Report 5, Current Research, Part C, Geological Survey of Canada, Paper 82-1C, p.129-132. Frarey, M. J. and Krogh, T.E., 1986. U-Pb zircon ages of late internal plutons of the Abitibi and eastern Wawa subprovinces, Ontario and Quebec; Geological Survey of Canada, paper 86-1A, p.43-48. Gall, Q., 1992. Precambrian peleosols in Canada; Canadian Journal of Earth Sciences, Vol. 29, p. 2530-2536. paleosols at the base of the Dominion and Pongola groups (Transvaal, South Africa); Precambrian Research, v.32, p97-131. Hay, R.E. 1963. The Geology of the Sault Ste. Marie map area; unpublished PhD thesis, McGill University, Montreal, Quebec, 325p. Heaman, L. M., 1988. A precise U-Pb zircon age for a Hurst dike; in Program with Abstracts, Annual Meeting Geological Association of Canada-Mineralogical Association of Canada, v.13, p. A53. Hoffman, H. J., Pearson, D.A.B. and Wilson, B.H., 1980. Stromatolites and fenestral fabric in Early Proterozoic Huronian Supergroup, Ontario; Canadian Journal of Earth Sciences, v.17, p.1351-1357. Holm, D. K. and Silverstone, J., 1990. Rapid growth and strain rates inferred from synkinematic garnets, Penokean Orogeny, Minnesota; Geology, 18(1). Innes, D. G., 1977. Proterozoic volcanism in the Southern Province of the Canadian Shield; unpublished MSc thesis, Laurentian University, Sudbury, Ontario, 150p. Innes, D. G. and Colvine, A.C., 1979. Metallogenetic development of the eastern part of the Southern Province of Ontario; in Summary of Field Work, 1979, Ontario Geological Survey, Miscellaneous Paper 90, p.184-189. G-Farrow, C. E. and Mossman, D.J. 1988. Geology of Precambrian paleosols at the base of the Huronian Supergroup, Elliot Lake, Ontario, Canada; Precambrian Research, v.42, p.107-139. Jackson, S. L., 1994.Geology of the Aberdeen area; Ontario Geological Survey, Open File Gay. A. L. and Grandstaff, D.E., 1980. Report, 5903, 69p. Chemistry and mineralogy of Precambrian Jackson, S. L., 2001. On the structural geology paleosols at Elliot Lake, Ontario; Precambrian of the Southern Province between Sault Ste Research, v.12, p.349-373. Marie and Espanola, Ontario; Ontario Grandstaff, D. E., 1980. Origin of uraniferous Geological Survey, Open File Report 5995, 55p. conglomerates at Elliot Lake, Canada and Jensen, L. S, 1990. Geology of the Whiskey Lake Witwatersrand, South Africa: implications for greenstone belt, districts of Algoma and oxygen in the Precambrian atmosphere; Sudbury; in Summary of Field Work and other Precambrian Research, v. 13, p. 1-26. Activities; Ontario Geological Survey, Grandstaff, D. E., Edelman, M. J., Foster, R. W., Miscellaneous Paper 151, p.53-58. Zbinden, E, and Kimberly M. M.,1986. Chemistry and mineralogy of Precambrian PDF compression, OCR, web-optimization with CVISION's PdfCompressor �62 Jolly, W. T., 1987. Lithophile elements in Huronian low-Ti continental tholeiites from Canada and evolution of the Precambrian mantle; Earth and Planetary Science Letters, v.85, p.401-415. Junnila, W. T., 1987. A bibliography of the Huronian Supergroup; 1921-1987; Ontario Geological Survey, Open File Report 5651, 71p. igneous province; Ontario Geological Survey Study 58, 80p. Lindsay, D.A., 1971. Glacial marine sediments in Precambrian Gowganda Formation at Whitefish Falls, Ontario; Paeleogeography, Paleoclimatology and Paleoecology, v.9, p7-25. Long, D. G. F., 1976. Stratigraphy and sedimentology of the Huronian (Lower Aphebian) Mississagi and Serpent Formations; Kamineni, D. C., McCrank, G.F.D., Stone, D., unpublished PhD thesis, University of Western Ejeckman, R.B., Flindall, R. and Sikorsky, R., Ontario, London, Ontario, 291 p. 1984. Geology of the central plateau of the Long, D. G. F., 1978. Depositional East Bull Lake pluton, northeastern Ontario; environments of a thick Proterozoic sandstone, in Current Research, Part B, Geological Survey the (Huronian) Mississagi Formation of of Canada, Paper 84-1B, p.75-83. Ontario, Canada; Canadian Journal of Earth Kimberly, M. M., Tanaka, R.T and Farr, M. R Sciences, v.15, p.190-206. .,1980. Composition of middle Precambrian Maynard, J. B, Rigger, S. D and Sutton, S. J., uraniferous conglomerates in the Elliot 1991. Geochemistry of sands from the modern Lake-Agnew Lake area of Canada; Indus River and the Archean of Precambrian Research, v.12, 375-392. Witwatersrand Basin: implications for the composition of the Archean atmosphere; Kimberly, M. M., Grandstaff, D. E. and Geology, v19, p265-268. Tanaka, R. T., 1984. Topographic control on Precambrian weathering in the Elliot Lake McConnell, R. G. 1927. Sault Ste. Marie area, uranium district, Canada; Journal of the District of Algoma; Ontario Department of Geological Society of London, 141, 229-233. Mines, v.35, pt.2, p.1-52. Krogh, T. E., Davis, D. W. and Corfu, F., McDowell, J. P., 1957. The sedimentary 1984. Precise U-Pb zircon and baddeleyite ages petrology of the Mississagi quartzite in the for the Sudbury area; in The Geology and Ore Blind River area; Ontario Department of Deposits of the Sudbury Structure, Ontario Mines, Geological Circular 6, 31p. Geological Survey, Special Volume 1, p.431-446. Meyer, W. 1983. Lower Huronian gold; an investigation of quartz-clast conglomerates Krough, T. E. Kamo, S. L. and Bohor, B.F., between Sault Ste. Marie and Elliot Lake; in 1996. Shock metamorphosed zircons with Summary of Field Work, 1983; Ontario correlated U-Pb discordance and melt rocks Geological Survey, Miscellaneous Paper 116, with concordant protolith ages indicate an p.259-262. impact origin for the Sudbury Structure; in Earth Processes- Reading the isotopic code, Miall, A. D.,1985. Sedimentology of an early American Geophysical Union, Geophysical Proterozoic continental margin under glacial Monograph, 95, p. 343-353. influence: the Gowganda Formation (Huronian), Elliot Lake area, Ontario, Lightfoot, P. C. and Naldtrett, A.J. 1996. Canada; Sedimentology, v.32, no.6,p.763-788. Petrology and geochemistry of the Nipissing Gabbro: Exploration strategies for nickel, Ohmoto, Hiroshi., 1996. Evidence in pre-2.2 copper and platinum group elements in a large Ga paleosols for early evolution of atmospheric PDF compression, OCR, web-optimization with CVISION's PdfCompressor �63 oxygen and terrestrial biota; Geology, December, v. 24; no. 12, p.1135-1138. Research Part C, Geological Survey of Canada paper 91-1C, p. 43-54. Ojakangas, R. W. and Morey, G. B., 1982. Keweenawan sedimentary rocks of the Lake Superior Region: A summary; Geological Society of America Memoir 156, p. 157-164. Prasad, N., and Roscoe, S. M., 1996. Evidence for anoxic to oxic atmospheric change during 2.45-2.22 Ga from lower and upper sub-Huronian paleosols, Canada; Catena 27, p. 105- 121. Ovenshine, A. T., 1965. Sedimentary structures in part of the Gowganda Formation, north shore of Lake Huron, Canada; unpublished PhD. thesis, University of California, Los Angeles, California, 213p. Palonen, P. A., 1973. Paleogeography of the Mississagi Formation and lower Huronian cyclicity; in Huronian Stratigraphy and Sedimentation, Geological Association of Canada, Special Paper 12, p.157-168. Parviainen, E. A. U., 1973. The sedimentology of the Huronian Ramsay Lake and Bruce formations, north shore of Lake Huron; unpublished PhD thesis, University of Western Ontario, London, Ontario. Peck, D. C., James, R. S., and Chubb, P. T. , Previc, S. A. and Keays, R. R., 1995. Geology, metallogeny and petrogenesis of the East Bull Lake Intrusion, Ontario; Ontario Geological Survey, Open File Report 5923, 117p. Pettijohn, F. J., 1970. The Canadian Shield: a status report; in Symposium on Basins and Geosynclines of the Canadian Shield, Geological Survey of Canada, Paper 70-40, p.329355. Pettijohn, F. J., 1975, Sedimentary Rocks, Harper and Rowe, 628p. Pienaar, P. J., 1963. Stratigraphy, petrography and genesis of the Elliot Group, Blind River, Ontario, including the uraniferous conglomerate; Geological Survey of Canada, Bulletin 83, 140p. Prasad, N. and Roscoe, S M. ,1991. Profiles of altered zones at ca 2.45 Ga unconformaties beneath Huronian strata, Elliot Lake Ontario: evidence for early Aphebian weathering under anoxic conditions; in Current Pretorius, D.A. 1981. Gold and uranium in quartz-pebble conglomerate; Economic Geology, 75th Anniversary Volume, p.117-138. Rainbird, R. H., Nesbit, H. W. and Donaldson, J. A., 1990. Formation and diagenesis of sub-Huronian saprolith: comparison with a modern weathering profile; Journal of Geology, 98, p. 801-822. Rice, R. J., 1991. Regional sedimentology and paleoplacer gold potential of the Lorrain Formation, Huronian Supergroup, in the Cobalt plain; Ontario Geological Survey, Open File Report 5761. Riller, U. P. ,1996. Tectonometamorphic episodes affecting the southern footwall of the Sudbury Basin and their significance for the origin of the Sudbury Igneous Complex, Central Ontario, Canada; unpublished Ph.D. thesis, University of Toronto, 135 p. Robertson, J. A., 1961. Geology of townships 143 and 144; Ontario Department of Mines, Geological Report 4, 66p. Robertson, J. A., 1962. Geology of Townships 137 and 138; Ontario Department of Mines, Geological Report 10, 94p. Robertson, J. A.,1968. Geology of Township 149 and Township 150, District of Algoma; Ontario Department of Mines, Geological Report 57, 162p. Robertson, J. A., 1970. Geology of the Spragge area, District of Algoma; Ontario Department of Mines, Geological Report Number 76, 109p. Robertson, J. A., 1976. The Blind River uranium deposits; the ores and their setting, Ontario Division of Mines, Miscellaneous Paper 65, 45p. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �64 Robertson, J. A., 1986. Huronian Geology and the Blind River (Elliot Lake) uranium deposits, 7-31, the Pronto Mine, in Uranium Deposits of Canada, Canadian Institute of Mining and Metallurgy, Special Paper 33, p. 46-43. Sutton, S.J. and Maynard, J.B. 1992. Multiple alteration events in the history of a sub-Huronian regolith at Lauzon Bay, Ontario, Canada; Canadian Journal of Earth Sciences, vol. 29, p. 432-445. Robertson, J. A., Frarey, M .J. and Card, K. D., 1969. The Federal-Provincial Committee on Huronian Stratigraphy: Progress Report; Canadian Journal of Earth Sciences, v.6, p.335-336. Sutton, S.J. and Maynard, J.B. 1993. Sediment and basalt hosted regoliths in the Huronian supergroup: role of parent lithology in middle Precambrian weathering profiles; Canadian Journal of Earth Sciences, vol. 30, p. 60-76. Roscoe, S. M., 1969. Huronian rocks and uraniferous conglomerates; Geological Survey of Canada, Paper 68-40, 205p. Theis, N.J. 1979. Uranium-bearing and associated minerals and their geochemical and sedimentological context, Elliot Lake, Ontario; Geological Survey of Canada, Bulletin 304, 50p. Roscoe, S. M., 1981. Temporal and other factors affecting deposition of uraniferous conglomerates; in Genesis of Uranium- and Gold-Bearing Precambrian Quartz-Pebble Conglomerates; United States Geological Survey, Professional Paper 1161-A-BB, p.W1W7. Tomlinson, K.Y. (1996). The geochemistry and tectonic setting of early Precambrian greenstone belts, Northern Ontario, Canada; unpublished Ph.D. Thesis, University of Portsmouth, 278p. Vallini, Daniela, A., Connon, William, F, and Schulz, Kalus J., 2005. New age data for the Chocolay Group, Marquette Range Supergroup: Implications for the Paleoproterozoic Evolution the Lake Superior Ruzicka, V. and LeCheminant, G. M., 1984. and Lake Huron regions. Institute on Lake Uranium deposit research 1983; in Geological Superior Geology Proceedings, 51st Annual Survey of Canada paper 84-1A, p. 39-44. Meeting, Nipigon, Ontatio, Part I – Proceeding Shanks, W. S. and Schwerdtner, W. M., 1991. and Abstracts, v.51 part 1 Structural analysis of the central and Williams, G. E. 1975. Late Precambrian glacial southwestern Sudbury Structure, Southern climate and the Earth’s obliquity; Geological Province, Canadian Shield; Canadian Journal Magazine, v112, p.441-465. of Earth Sciences, vol. 28, p. 411-430. Williamson, M. and Keen, C.E., 1995. How Sims. P. K., Card, K. D. and Lumbers, S. active are passive margins? Modern analogues B.,1981. Evolution of early Proterozoic basins of magmatism in rifts; the Canadian of the Great Lakes Region; in Proterozoic Mineralogist, vol. 33, p. 943-944. Basins of Canada, Geological Survey of Wright, D. J. and Rust, B. R., 1985. Canada, paper 81-10, p. 379-397. Preliminary report on the stratigraphy and Stockwell, C. H.,1982. Proposals for time sedimentology of the Bar River Formation; in classification and correlation of Precambrian Geoscience Research Grant Program, rocks in Canada and adjacent areas of the Summary of Research 1994-1995, Ontario Canadian Shield: part 1- a time classification Geological Survey, Miscellaneous Paper, 127, of Precambrian rocks and events; Geological p. 119-123. Survey of Canada Paper 80-19, p690-698. Roscoe, S.M. and Card, K.D., 1992. Early Proterozoic tectonics and metallogeny of the Lake Huron region of the Canadian Shield; Precambrian Research, V. 58, 9. 99-119. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �65 Wood, J., 1973.Stratigraphy and sedimentation in Upper Huronian rocks of the Rawhide Lake-Flack Lake area; in Huronian Stratigraphy and Sedimention, Geological Association of Canada Special Paper 12, p73-95. Young, G. M., 1983. Tectono-sedimentary history of early Proterozoic rocks of the northern Great Lakes; in Early Proterozoic Geology of the Great Lakes Region, Geological Society of America Memoir, v.160, p.15-32. Young, G. M., 1973. Tillites and aluminous quartzites as possible time markers for Middle Precambrian (Aphebian) rocks of North America; in Huronian Stratigraphy and Sedimentation, edited by G. M. Young; Geological Association of Canada Special Paper 12, p. 97-127. Young, G. M. and Nesbitt, H. W., 1985. The Gowganda Formation in the southern part of the Huronian outcrop belt, Ontario, Canada; Precambrian Research, v.29, p.265-301. Young, G. M., 1991. Stratigraphy, sedimentology and tectonic setting of the Huronian Supergroup; Field Trip B5:Guidebook; Young, G. M., 1982. Field excursion guide book; Geological Association of Canada and excursion 13B: Depositional environments and Mineralogical Association of Canada, Society of Economic Geologists Joint Annual Meeting, tectonic setting of the early Proterozoic Huronian Supergroup; International Association Toronto 1991., 34 p. of Sedimentologists, Eleventh International Zolnai, A. I., Price, R. A. and Helmstaedt, H., Congress on Sedimentology, McMaster 1984. Regional cross-section of the Southern University, Hamilton, Ontario, Canada. Province adjacent to Lake Huron, Ontario: implications for the tectonic significance of the Murray fault zone; Canadian Journal of Earth Sciences, v.21, p.447-456. PDF compression, OCR, web-optimization with CVISION's PdfCompressor � https://digitalcollections.lakeheadu.ca/files/original/7f3aee4f6a39e32e2c43f4b4a0ddd107.pdf 0babd4791a930a720e5fac1812b24e76 PDF Text Text Keweenawan Rocks of the Mamainse Point Area Field Guide for the 52nd Annual Institute on Lake Superior Geology Vol. 52, Part 5 U . -J • S U By: Thomas R. Hart, Ontario Geological Survey, Ministry of Northern Development and Mines Antonio Pace, Resident Geologist Program, Sault Ste. Marie District, Ministry of Northern Development and Mines PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Contents Page INTRODUCTION ............................................................................................................ 1 REGIONAL GEOLOGY.................................................................................................. 1 GENERAL GEOLOGY.................................................................................................... 2 Mafic Volcanic Rocks........................................................................................... 3 Felsic Rocks .......................................................................................................... 4 Clastic Sedimentary Rocks ................................................................................... 5 Younger Clastic Sedimentary Rocks .................................................................... 6 ALTERATION ................................................................................................................. 7 STRUCTURAL GEOLOGY ............................................................................................ 7 LITHOGEOCHEMISTRY ............................................................................................... 9 ECONOMIC GEOLOGY................................................................................................. 10 Mamainse Mine .................................................................................................... 10 Coppercorp Mine .................................................................................................. 11 GEOCHRONOLOGY ...................................................................................................... 15 PALEOMAGNETISM ..................................................................................................... 15 REFERENCES ................................................................................................................. 15 FIELD TRIP GUIDE ........................................................................................................ 18 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide 1989; Nicholson et al. 1997). Although there is only very limited geochronology in the MPF, recent work in the Lake Nipigon area indicates that rift related magmatic activity began at ~1114 Ma (Heaman and Easton 2005) which extends the period of magmatic activity from the ~22 Ma by Davis and Green (1997) to ~28 Ma. This extended period of time is much longer than the 1 to 5 Ma of volcanic activity proposed for more Phanerozoic continental flood basalt provinces by Jerram and Widdowson (2005), but is comparable to the time periods proposed for Archean age magmatic events possibly related to mantle plumes. This trip will hipiccten provide an opportunity to reIs Ia rid examine many of the same Ontado stops described by Giblin (1974) and Annells (1973). MIDDLE KEWEENAWAN ROCKS OF THE MAMAINSE POINT AREA INTRODUCTION Mid-Keweenawan mafic volcanic flows, felsic intrusive to extrusive rocks, and clastic sedimentary rocks of the Mamainse Point Formation (MPF) are located about 64 km north of Sault Ste. Marie, along the east shore of Lake Superior (Fig. 1). This area was the subject of a Nipigon Emb ayment—.5. M anitoba Say -Jr to z Minnesota ir. - REGIONAL GEOLOGY The ~1.1 Ga Midcontinent Rift extends for over 2000 km and is interpreted to be an aborted Wisconsin continental rift (Fig. 1) (e.g. Van Schmus and Hinze, 'C 1985). Seismic profiling of Michigan Lake Superior indicates that -J the rift consists of a series of a series of asymmetric grabens 'S.. separated by accommodation zones filled with up to 30 km Inferred extent of Keweenawan and sediments of volcanic and sedimentary Iowa rocks (e.g. Cannon et al., Upper sediments 1989). Initiation of rifting has KeweenaA'an igneous been related to a mantle KeweenaNan extrusive rocks plume, or hot spot (e.g. sediments Kansas Hutchinson et al. 1990; Figure 1. The Midcontinent Rift System and some of the Nicholson and Shirey 1990), major regional geological features from Lightfoot et al. which resulted in the production of < 1 500 000 (1999). km3 of volcanic and intrusive rocks (Klewin and Shirey 1992). Rift related volcanic and field trip by Giblin (1974) during the Annual sedimentary rocks of the Keweenawan Meeting of the Institute of Lake Superior Supergroup are generally exposed along the Geology. Since that time, there has been various margins of the rift, with the greatest volumes studies completed on the MPF (e.g. Massey 1980; exposed west of Lake Superior (Fig. 2) Klewin and Berg 1991; Shirey et al. 1994; (Nicholson et al. 1997). Limited exposures are Lightfoot et al. 1999; Walker et al. 2002) and on present along the east shore of Lake Superior at the rocks in other parts of the Keweenawan Mamainse Point, Cape Gargantua south of Wawa, Midcontinent Rift (e.g. Green 1983; Cannon et al. and Michipicoten Island with a more extensive Penr Ia \VVVV. S v'.vv 1 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide 1114 r 1 Seagull intrusion 1117 Ma Kltto Intrusion 1114 r 1 Disraeli intrusion Ca. 1,1 Ga Pigeon River Arr: call 1540 Ma English eay Grante ca. :.i GaKenora-Fortrrances GENERAL GEOLOGY The Keweenawan rocks are underlain by Archean rocks of the Batchawana Greenstone Belt that consists of mafic to intermediate metavolcanic and minor felsic metavolcanic rocks, and Algoman-type iron formation (Giblin 1974). The Archean rocks strike east, have been metamorphosed to amphibolite facies, and deformed resulting in northeast 01.12 TtCtOflICZOflt trending isoclinal North Bhoro folds and penetrative Pr fabric with steep dips. MW. MichipicoMn Mand Faue The Mamainse Point Formation (MPF) unconformably Archean overlies the Archean rocks, and consists of a sequence * alkalic complexes — International boundary estimated to be Faults between 4200 and ..A.._A.. Thrustfauks 6000 m thick Dikes consisting of subaerial mafic flows, intercalated clastic sedimentary and felsic igneous rocks (Fig. 4 and 5). The mafic flows can be subdivided into lower olivine- and plagioclasebearing basalts and upper plagioclase-bearing basalts (Annells 1973), which corresponds to a major change in the chemistry of the basalts (e.g. Klewin and Berg 1991; Lightfoot et al. 1999). Clastic sedimentary rocks consist of predominately conglomerates, and the 550m thick Great Conglomerate horizon marks the break between the lower and upper basalts. Minor felsites, quartz porphyry and flowNCF KF OF Paleozoic Illinois and Michigan basins lesoproterozoic Midcontinent Rift CIa stic sedimentary rocks AnorogenicfPost-orogenic suites Sibley Group-sandstone, - >134 Ga) Granitoid rocks (1.48.1.77 Ga) Paleoproterozoic Penokean Orogen and Related Rocks Granitoid and volcanic arc rocks (1 .89 - 1.84 G a) Intrusive rocks (1 11-1 Group Marquette Sup ergroup (ca, 2.1 1,85 Ga) Figure 2. Geological map of the Lake Superior basin after Lightfoot et al. (1999). and modified based on the recent mapping in the Nipigon Embayment (Hart and MacDonald in press). sequence of volcanic rocks present on Simpson and St. Ignace islands and Black Peninsula near Nipigon (Fig. 2). A rift-wide correlation of the volcanic rocks was presented by Nicholson et al. (1997) (Fig. 3), but the position of the MPF could only be estimated using paleomagnetic data due to a lack of detailed geochronology. C FauC —— 2 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide — (b) (a) a (d) (c) n Upper Michigan NWWisocnsin I 085 —0.- NE MInnesota OWlimb NE Minnesota NElirtib (e) Isle Royale Black Bay Peninsula a LakeNipigon o Lake Silo,. Trspsk*, 3 Man,ainse Point Michipicolen Island echipicoten and Forniator logo 1095 1100—s1105 1110 —0APPROX. THICKNESS OF VOLC. SECTION: Isle Royele: Oaler Group: Figure 3. Schematic correlation of MRS volcanic rocks in western and eastern Lake Superior from Nicholson et al. 1997. 3.5 lust 3 lust Midi. Island: Manieinse Pt: Along the north side, the Mamainse Point Formation is unconformably overlain by the Mica Bay Formation which is considered to be the equivalent of the Freda Formation south of Lake Superior (Fig. 4) (e.g. Annells 1973; Giblin and Armsburst 1969). To the south, the Mamainse Point Formation is in fault contact with a red sandstone interpreted to be part of the Jacobsville Formation. Paleomagnetic age estimates by Halls and Pesonen (1982) suggest that both of these units are late Keweenawan. 4 lan 5 km Mafic Volcanic Rocks The MPF consists of 300 to 350 individual mafic flows that commonly range in thickness from 1.5 to 9.0 m, with some flows being up to 30m thick and other as thin as 0.15 m (Annells 1973). Many of the flows have upper and lower vesicular zones, with the lower zone vesicles often pipe-like and bent in the direction of flow. Most of the flows have ropy pahoehoe surfaces, but some have clinkery scoriaceous flow tops, and Annells (1973) noted that the olivinerich flows commonly have scoriaceous rather than pahoehoe flow tops. Prismatic jointing is also common in the olivine-rich flows, but also observed in the finer grained flows. The mafic flows were subdivided into a Lower and Upper Division by Annells (1973) with the polymicitic conglomerates of the Great Conglomerate forming the break between the divisions (Fig.5). The Lower Division, located along the north side of the MPF, is about 1700 m thick and consists of about 20% flows with olivine phenocrysts and 80% flows with plagioclase and minor olivine and pyroxene phenocrysts. Two volcanic conglomerates and two thin felsic horizons are intercalated with the flows. The Upper Division, located to the south, consists of banded rhyolites occur as intrusive and possibly extrusive units within the MPF. Felsic dykes, porphyries, and breccias also intrude the Archean rocks to the east, and are considered to be related to the felsic volcanic and intrusive rocks occurring within the MPF. Some of the intrusions host mineralization, with the best examples being the ~1055 Ma breccia pipe hosting Cu-Au-Ag at the past-producing Tribag Mine (Wanless et al. 1968) and several Cu-Mo prospects at the Jogran Porphyry (Tortosa and Moss 2004). The volcanic rocks generally strike to north to northwest with homoclinal dips of 15o-60o to the southwest and are cut by northwest- and northeast-trending faults. The mafic flows of the Alona Bay area form an about 1300 m thick sequence of basaltic flows with less lithological variation than the MPF, and lacking intercalated sedimentary and felsic igneous rocks (Annells 1973). Located approximately 4 km to the north of the MPF, the Alona Bay flows consist of olivine- and plagioclase-bearing basalts that have been proposed to be equivalent to the lower division of the MPF (Walker et al. 2002; Annells 1973). 3 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide 2 ki Iometree KINCAID + RYAN about 65 km to the north. These flows also resemble a plagioclase-rich ponded flow in the Osler Volcanic Group volcanic rocks which is exposed on the southeast corner of St. Ignace Island, in the Thunder Bay area (Sutcliffe and Smith 1988). The top of that flow has a texture resembling the glomeroporphyritic “Daisey Stone” while the core of the flow consists of plates of feldspar up to 5 cm in width. Felsic Rocks Generally rhyolitic in composition, the felsic igneous rocks in the MPF have been classified as felsite, quartz porphyry and flowbanded rhyolite. Many of these rocks occur as plugs, Cong dykes, and sheets intruding Basalt the mafic flows commonly Baserrent with auto-brecciated zones Ro Ia centimeters to tens of centimeters in width containing clasts of basalt and Dip orb felsite and occasional agglomeratic zones suggesting there may be eroded rhyolitic domes (e.g. Annells 1973; Giblin 1974). The depth of intrusion is not known, or whether some of these units may have formed high level cryptodomes that may have breached surface. These rocks are commonly reddish brown to pink to grey-white, and vary from fine-grained and massive to quartz and/or feldspar porphyritic, with the groundmass and massive portions composed of quartz, altered feldspar, mica, chlorite, and epidote. Some units may be flow banded with the best exposed examples located along the margins of one body exposed at Cottrell Bay (Fig, 3). This flow banding shows tight asymmetric isoclinal folds with bands that are generally parallel to the contact. The quartz porphyries commonly form small plugs or thin intrusive sheets with paramorphed b-quartz Figure 4. Geological map of the Mamainse Point Formation. Based on OGS Map 2251 (Giblin and Armburst 1969) and modified after Lightfoot et al. (1999). about 3000m of fine-grained to aphanitic, subophitic to ophitic flows. There are a number of clastic sedimentary and felsic horizons intercalated with the Upper Division flows. The plagioclase glomeroporphyritic “daisy stone” flow is a distinctive unit towards the base of the Lower Division and contains radiating calcic plagioclase laths, up to 5 cm across (Annells 1973). Annells (1973) also described a flow in the Mamainse Point area, near the base of the Upper Division, as containing a 3.25 m zone of large euhedral plagioclase up to 10 cm long. Annells (1973) also notes that there is a flow with similar textures in the Cape Gargantua sequence 4 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide it P Mafic Intrusive Rocks Mafic intrusive rocks are relatively rare in the MPF and typically consist of narrow diabase and fine-grained dykes cutting the flows. Annells (1973) describes the dykes as typically less than 3 m, east or north striking with steep dips, and varying degrees of alteration. No connection was observed between the dykes and flows. I rhyolite magmaltsm Diab ass V rhyolite magmaltsm to ophitic tholeiite rhyolite magmatism Diab ass to fine-grained aphyric tholeiite — VI Cong 10 m crate A — rhyolite magmatism Ophftio basalt III — rhyolite magmatism Oltvine-phyric basalt basalt Fbgioclase-oliv basalt basalt spherulite daisystone 1 Oltvine-phyric pioritic basalt Figure 5. Petrographic and geochemical variations through the Mamainse Point Formation with the stratigraphic subdivisions of Annells (1973), Massey (1983) and Klewin and Berg (1991) modified by Lightfoot et al. (1999). Magnetic polarity is based on Halls and Pesonen (1982). phenocrysts up to 8 mm in size and altered euhedral feldspar (Annells 1973). In one location Annells (1973) described a composite sheet of felsic and mafic material which was interpreted to indicate that the felsic and mafic lavas were erupted simultaneously. Felsite dykes cut, and are cut by, structures hosting the copper mineralization at the Coppercorp and Mamainse mines, and are considered to have been emplaced contemporaneously with fault movement, brecciation and sulphide deposition (Heslop 1970). Keweenawan quartz-feldspar porphyries and breccia pipes hosting copper and molybdenum mineralization intrude the Archean rocks to the east of the MPF (e.g. Tribag Mine), and may be related to the felsite dykes. Clastic Sedimentary Rocks Clastic sedimentary rocks occur at a number of levels within the stratigraphy of the MPF and consist of volcanic conglomerates in the Lower Division and polymicitic conglomerates in the Upper Division (Fig. 5). The volcanic conglomerate was described by Annells (1973) as being composed of poorly sorted, angular to subangular clasts of fine- to coarse-grained Keweenawan basalt in a silty basaltic matrix. Some of the clasts are irregular, rounded, amoeboid shaped which was interpreted to be a result of deposition while still hot. Rare red siltstone, but no basement rocks or felsic volcanic, clasts were observed. It was noted that the horizons become finer grained and well laminated upwards. There are a number of horizons of clastic sedimentary rocks intercalated with the mafic flows of the Upper Division, but the largest volume occurs in the Great Conglomerate which is located approximately in the middle of the Formation between the Lower and Upper Divisions. The Great Conglomerate is a sequence of predominantly polymictic boulder conglomerates containing thin lenses of red to grey sandstone, siltstone, shale and granite, and is interbedded with two basalt flows of the Upper Division. The base of the conglomerate is described by Annells (1973) as a thin layer of red sandstone, overlying a scoriaceous flow top. The conglomerates are poorly sorted, with indistinct bedding containing well rounded to sub-rounded, pebble to boulder sized clasts of granite and basalt, with minor amounts of diabase, felsite, and vein quartz. Conglomerates exposed along strike to the south of the highway occur as 10 to 20 cm thick, graded beds with some beds displaying weakly developed cross bedding suggesting that the trough in which the Great Conglomerate was deposited is deeper to the north-northwest. Some of the well laminated sandstone lenses in the 5 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide with fresh feldspar. Plagioclase is variably zeolitised and some have patches of albite and epidote. Augite is partially altered to clinoamphibole and chlorite. Opaque minerals are commonly hematized, with titanium-rich phases altered to leucoxene and sphene. In the felsic rocks, the feldspar is kaolinized and quartz fills cavities. The felsic rocks at Cotrell Cove are mottled light reddish brown to beige, and also contain irregular patches or funnel-shaped features. The mottling resembles the variations observed in the oxidized Sibley Group sedimentary rocks of the Nipigon area, suggesting that this variation may be in part a result of possibly deuteric reduction of an originally oxidized rock. However, the funnel-shaped features may be a result of alteration during degassing of the flows. All vesicles within the mafic flows are always filled, and the composition of the amygdules is variable. Annells (1973) describes some fillings as an outer zone of chlorite with a chalcedony rim, and a core of colourless quartz and zeolite but some have yellow epidote, colourless prehnite and calcite. Similar mineralogy may also occur as fracture fillings. Lightfoot et al. (1999) suggest that there was a stratigraphic variation with zeolites, including heulandite and stilbite, common in the Upper Division flows and prehnite and pumpellyite common in the Lower Division flows. Epidote alteration is also more common in the Lower Division flows (e.g. Annells 1973) and, along with hematite and specular hematite, in major crosscutting veins and fissures associated with mineralization at the Coppercorp and Mamainse Mines (e.g. Richards and Spooner, 1989). Calcite is also present in fractures cutting the clastic sedimentary units interbedded with the Lower Division and in the Great Conglomerate. highway section are cross-bedded. Annells (1973) suggested that the conglomerates were deposited in an alluvial fan environment. The presence of granite clasts suggests a reduction in the rate of, or possible hiatus in, volcanism so that material could be transported from the adjacent Archean terrains or a more extended period of time than previous interpreted. Regionally, the Great Conglomerate resembles the conglomerate exposed on Puff Island, near the top of the exposed portion of the Osler Group which is located near the top of the ~3 km of exposed volcanic rocks. Younger Clastic Sedimentary Rocks The Mica Bay Formation is an approximately 61 m thick sequence of clastic sedimentary rocks unconformably overlying the volcanic rocks of the MPF along the north side of the formation (Giblin 1974). An up to 30 cm thick, polymictic, matrix supported conglomerate forms the basal member of the formation. Most of the formation is composed of grey-brown siltstones, arkoses, and minor immature quartz pebble conglomerates with siltstones forming approximately 70% of the section. Giblin (1974) interpreted these rocks to have been deposited in a shallow water environment based on the presence of ripple marks, graded bedding, cross-lamination, flame structures, and ball-and-pillow structures with flute casts and clastic dikes indicating current flow towards the north. The Mica Bay Formation is considered to be the equivalent of the Freda Formation (e.g. Annells 1973; Giblin and Armburst 1969). To the south, the Mamainse Point Formation is in fault contact with sandstones displaying the typical red and white mottling of the Jacobsville Formation. The Jacobsville Formation is interpreted to form the floor of much of the east part of Lake Superior (e.g. Giblin 1974). Paleomagnetic age estimates by Halls and Pesonen (1982) suggest that both of these younger sedimentary units are late Keweenawan. STRUCTURAL GEOLOGY The flows of the MPF vary in strike from south to north from northwest to north with dips decreasing from east to west from 55o to 15o west defining a broad antiform that gently plunges to the west (Annells 1973). In the area of Pancake Point and Cottrell Cove, the strike of the flows is highly variable and some flows appear to be overturned which has been interpreted to be a ALTERATION All of the mafic flows appear to have undergone low-grade hydrothermal metamorphism, and some alteration appears to be deuteric. Annells (1973) noted that olivine is replaced by saponite and hematite even in basalts 6 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide Major faults (upthrown - side indicated) I - L 0 100km Cross-structure faults Extent of MCR rocks the flows. These fractures are up to 30 cm wide and commonly filled with silt and other material similar to the matrix of the conglomerates. Annells (1973) speculated that these fracture fillings were clastic dykes formed as a result of minor adjustment of the flows during deposition. There are 2 major regional faults, Mamainse Point and Montreal River faults, that bound the MPF to the north and south (Fig. 4 and 6) (Manson and Halls 1997). To the south, the northeast-trending Mamainse Point Fault juxtaposes the flows of the MPF against red sandstones of the Jacobsville Formation. This fault is a re-activated ductile shear zone that forms the southern margin of the Batchawana greenstone belt to the east and may correlate with faults in northern Michigan and Wisconsin. To the north, the Montreal River Fault forms the northern boundary of the MPF and is the western boundary of the Abitibi and Wawa subprovinces, and the southwest extension of the Ivanhoe Lake fault which is the eastern boundary of the Kapuskasing Structural Zone. This fault may correspond to the Keweenaw Fault to the west (Fig. 6). Both of these faults have been interpreted to be major reverse faults associated with late compression related to the Grenville Orogen in the late Keweenawan. Two major faults, Mamainse Lake and Hibbard Bay faults, that transect the MPF and offset or truncate stratigraphy (Fig. 4 and 6). The Mamainse Lake Fault trends northeast, has a variable sinistral displacement, and appears to Figure 6. Major faults of the Lake Superior portion of the Midcontinent Rift. From Manson and Halls (1997) result of the intrusion of a number of felsic bodies. Rocks of the Mica Bay Formation, that overlie the flows of the MPF to the north, strike 065o, are gently folded, and dip 15o to 30o north or south (Giblin 1974). The Jacobsville sandstones, which overlie the flow to the south, strike 335o to 320o and dip 15o to 25o west. The Alona Bay flows strike north and dip 45o to 49o west with the basal unit unconformably overlying Archean felsic plutonic rocks (Annells 1973). This section terminates against a northwest-trending fault filled by a diabase dyke. The orientation of these flows is comparable to the northern flows of the MPF. A large number of northeast- and northwest-trending normal faults with limited vertical displacements cut the flows of the MPF (Annells 1973). However, Annells (1973) considered the ~6000 m of stratigraphy within the MPF to be in part a result of possible fault repetition. A series of apparently radially distributed faults, with a focal point roughly near the Coppercorp Mine, offset stratigraphy (Tortosa and Moss 2004). The focal point corresponds to a magnetic high and an adjacent large felsite body located about 4 km east of the Coppercorp Mine. A large number of fractures of variable orientation and no discernable displacement cut 7 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide converge with the Mamainse Point Fault under Pancake Bay. The Hibbard Bay Fault is northwest-trending, subparallel to the rift axis located to the west under Lake Superior, and truncates stratigraphy at an acute angle. Cu in some flows; other flows have high La/Sm and SiO2 suggesting crustal contamination. Height in section (m) LITHOGEOCHEMISTRY Geochemical studies by Massey (1980) and Klewin and Berg (1991) have characterised the flows of Mamainse Point Formation, and have identified geochemical variations correlating with stratigraphic position within in the MPF. Massey (1983) identified five major groups of flows with the break between the Lower and Upper Division occurring stratigraphically at the break between Groups III and IV, across the Great Conglomerate (Fig. 5). Klewin and Berg (1991) subdivided the MPF into seven groups, with Groups 1 to 5 corresponding to Groups I to III of Massey (1983) (Fig. 5). Lightfoot et al. (1999) proposed a subdivision of the groups of Klewin and Berg (1991) (Fig. 5). The Lower Division contains olivine-rich flows (Annells 1973), and some of these flows were identified by Klewin and Berg (1991) to be picritic in composition. Based on the presence of skeletal olivine and the fact that the flow composition did not fall along an olivine control line, Berg and Klewin (1990) concluded that these flows were not olivine cumulates but rather formed as a result of melting at deeper levels in the mantle. Overall the Lower Division flows, Groups 1 to 5, consist of basal picrites to picritic basalts overlain by the “daisy stone”, and then a series of tholeiitic to high Mg-tholeiitic basalts. Some of the main characteristics of the different groups were summarized by Lightfoot et al. (1999) (Fig. 5, 7, and 8a, b): Figure 7. Variations in selected elements with stratigraphic position after Klewin and Berg (1991) and arrows indicating the general direction of geochemical evolution. GC - Great conglomerate; BCC - basalt clast conglomerate. Sample position in metres above the base (from Lightfoot et al. 1999). Most of the variation in the Upper Division flows is a difference in the absolute trace element abundances rather than incompatible element ratios. Some of the main characteristics of the different groups were summarized by Lightfoot et al. (1999) (Fig. 5, 7, and 8a, b): Group 6 - high Yb (2.3-5.7 ppm), highest TiO2 (1.8-3.4 wt.%) and Mg-number (0.30-0.48); subgroups 6a, 6b, and 6c defined by varying Ce, Yb, and Zr contents and may reflect interdigitate throughout the stratigraphy, possibly in part due to local fault repetition Group 7 – more primitive than Group 6 with elevated Ni (>75 ppm), low Cu (dominantly less than 100ppm), and TiO2 (~0.9 wt.%); subgroup 7a has elevated Mg-number (0.62-0.70), low TiO2 (0.9-1.2 wt.%), and low incompatible elements abundances (e.g. Ce=15-20 ppm); subgroup 7b has lower Mg-number (dominantly 0.52-0.60), Group 1 and 2a - high Mg-numbers (0.640.70), TiO2 (1.1-2.0 wt.%) and Gd/Yb, with low CaO and Al2O3. Groups 2b and 3 - lower MgO, TiO2, and Gd/Yb compared with Groups 1 and 2a, but high La/Sm, Al2O3, and CaO. Group 4 – higher MgO (9.0-10.5 wt.%) but low P, Zr and Hf compared to other groups Group 5 - quite low TiO2 (<1.5 wt.%), a wide range of MgO and La/Sm with low Gd/Yb, with very low Ni and moderate 8 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide but higher TiO2 (1.3-1.9 wt.%) and incompatible element abundances (Ce=20-25 ppm). Group 8 – are interspersed with Groups 6 and 7; elevated incompatible elements and LREE (e.g. Ce: 120-130ppm) and high TiO2 (3.6-3.7 wt.%) Group 9 - elevated TiO2, Ce, Yb, Zr, La/Sm, LILE, and low Mg-number, Al2O3, and Ni 1973; Lightfoot et al. 1999). These rocks have Zr/Y ratios of 1.9 to 7.3, 10 to 53 ppm Y, (La/Yb)cn ratios of 2.67 to 8.92 with (Yb)cn values of 4.55 to 25. They also have moderately positive Nb+Ta, and negative P, Ti, and Eu on a chrondrite normalized extended element diagram (Lightfoot et al. 1999). Lightfoot et al. (1999) compared these rocks to sialic volcanic rocks found in other CFB such as the Salsette Island suite north of Bombay, India, in the Deccan Trap, and based on this similarity suggested that these rocks formed by a similar mechanism of partial melting of basaltic crust. These rocks could also be classified as FII type felsic volcanic rocks which suggests that they may have the potential to host volcancogenic massive sulphides mineralization, and that they formed by partial melting of a basaltic source at depths >10 km (e.g. Hart et al. 2004). The model proposed by Klewin and Berg (1991) to explain the upward geochemical variations in the MPF was partial melting of different sources at decreasing depths followed by fractional crystallization replaced by periodic replenishment, tapping, and fractionation in magma chambers combined with assimilation and fractional crystallization, and finally simple fractional crystallization at two broad levels in the crust. This model was modified by Shirey et al. (1994) based on isotopic data to reflect interaction with old subcontinental lithospheric mantle in the lower flows, crustal assimilation in the middle flows, and a mixture of plume and depleted mantle comparable to Phanerozoic mid-ocean ridge basalt (MORB). A study of the picritic basalts by Shirey (1997) suggested a mixed source of enriched mantle plume and unradiogenic continental lithosphere, and possible involvement of recycled slab from late Archean subduction. Lightfoot et al. (1999) compared the MPF with the Osler Group volcanic rocks and observed that Groups 1 to 3c, except for the possible lithospheric interaction, resemble but are not identical to the Lower Formation of the Osler Group. Picritic basalts are also present in the Lower Formation but are not the lowermost flows and are underlain by tholeiites. The Osler Group lacks flows comparable to Group 5. Groups 6 and 7 broadly resemble the least contaminated basalts of the Central formation of the Osler Group. 4.6 Gd/Yb I I Osler Group 'Upper Formation 'Central Form A Lower Fom,ation Mamainse Point Group 09 • 7a-d • Ga-c 0 5ac ft 2o A l,2a,2c, 1000 La/Lu Figure 8a. Variation in La/Sm versus Gd/Yb with symbols representing samples from different groups in the MPF and Osler Group volcanic rocks; Figure 8b. Variation in Ce/Nb versus La/Lu in MPF and Osler Group flows. (from Lightfoot et al. 1999) The felsic rocks have SiO2 of 72-81 wt.% with moderate Al2O3 (11-15 wt.%) (e.g. Annells, 9 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide There are only a couple of picritic basalt flows in the Osler Group, and although they are located near the base of the Group but are not the lowest flows (Hart 2002). ECONOMIC GEOLOGY Copper deposits of Keweenawan age are of economic importance in the area and consist of 3 types: (i) fissure filling carbonate vein breccias which carry chalcocite, bornite, chalcopyrite, specular hematite, and very minor amounts of native copper (ii) porphyry copper type deposits, in which chalcopyrite, chalcocite, molybdenite and pyrite are disseminated in quartzfeldspar porphyry (iii) breccia pipe deposits in which wallrock fragments are set in a matrix of quartz, carbonate and minor fluorite. Chalcopyrite and pyrite, with minor molybdenite, sphalerite, galena, tetrahedrite, stibnite and scheelite occur in the matrix of the breccia. Photo a: Mamainse copper mill (Coleman 1899) The Tribag Mine is the best example of the breccia pipe deposits but will not be visited during this field trip. The deposit is located 16.1 km north of Batchawana Bay, and past production included copper and minor gold and silver from a breccia pipe during the period 1967 to 1974. Milling rate at these mines averaged 400 to 500 tons per day of ore grading approximate 2% Cu with minor values in gold and silver. Photo b: Shaft and engine house at Mamainse (Coleman 1899) of 1890, the deepest workings were on the 300foot horizon at the Copper Creek shaft. There is no record that any significant amount of copper was produced until recent years when two mines, the Coppercorp and the Tribag came into production in the Batchawana Bay area. There are three shafts at the Mamainse Mine with 4 levels established. Work began by the Lake Superior Native Copper Company in 1882. The shafts are about 300 m apart on the vein. Amongst the companies engaged in early exploration were the Montreal Mining Company in 1856-57, the Ontario Mineral Lands Company in 1871, the Silver Islet Consolidated Mining and Lands Company in 1882-84, the Canada Lands Purchase Company in 1890, and the Nipigon Mining Company, probably about 1892. From 1906 to 1908, the property was optioned by the Mamainse Mine The existence of copper at Mamainse Point was known in very remote times and the socalled “Indian diggings” near Hibbard Bay attest to the mining activities of the natives in this region. The similarity of the geology at Mamainse Point to that of the Michigan copper district attracted the attention of mining interests when the Michigan camp was being developed. There are sketchy records of intermittent mining activity from 1842 until 1894 (Photo a and b). During this period shafts were put down at several places in the area: the best known developments are those at the Mamainse mine and the Copper Creek and Silver Creek shafts. According to the report of the Royal Commission 10 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide mineralization consists of disseminated native copper in amygdules, and veins and vein hosted copper sulphides that occur in fault-related breccia zones that transect flood basalts, conglomerates, and felsic intrusive and volcanic rocks of the Keweenawan-age Mamainse Point Formation (Fig. 11). Mineralization consists of chalcocite with minor malachite and chalcopyrite associated with pyrite and hematite. Regional westward folding of the Mamainse Point Formation combined with possible contemporaneous radial faulting may have provided the structural conduits for the mineralizing fluids in the Coppercorp Mine and elsewhere on the property (Fig. 12). The presence of a high area of magnetic intensity in the focal area of the radial fault system combined with associated felsic intrusive and extrusive activity in the lower volcanic sequence suggests the presence of a volcanic or intrusive centre in the area. Calumet and Hecla Mining Company of Michigan. Mineralization at the Mamainse Mine is hosted by a quartz-carbonate vein / fracture fill, striking 334o dipping 50o east and varying from 0.46 to 4.0 m wide. Tracable for 457 m, the vein and an associated felsite intrusion pinch and swell along strike in the northwest-trending fracture which cross-cuts the north striking mafic volcanic flows at an oblique angle. Mineralization in these veins consists of hematite and chalcocite in veins with minor chalcopyrite and native copper along with malachite and azurite as weathering products. The structure associated with the mineralization at the Mamainse Mine is similar to the main NNW trending, east dipping structures seen at the Coppercorp (Tortosa and Moss 2004). The main vein can be examined along the shoreline (see Stop 6), and it was along this vein where three shafts were sunk to 18.3, 85.3 and 97.6 m with four levels established. The vein cuts flows of the Upper Division and hosts minor native copper and finely disseminated chalcocite in cross fractures. The basalts are thin flows of diabasic and ophitic olivine tholeiite with amygdules containing quartz, agate, calcite, copper sulphides and native copper. Annells (1973) reported that specks of native copper are common in the vesicles and flow tops of the Upper Division flows and that a 67 kg piece of native copper was discovered during highway construction in the area south of the Mamainse Mine. History Exploration on the property probably began at the same time as the work on the nearby Mamainse Mine, and much of the early work appears to have concentrated on the native copper mineralization in amygdules and fractures (Table 1). Between 1948 and 1952, work by Macassa Mines, and C.C. Houston and Associates examined the old copper showings and drilled 10,180 m outlining several mineralized zones, including the C Zone, D Zone, SB Zone and Silver Creek Zone (Fig. 11 and 13). Table 1: History of Ownership of Montreal Mining Sand Bay Location Years Ownership Coppercorp Mine The Coppercorp Mine is a past producer located in the Mamainse Point area, about 80 km north of Sault Ste. Marie, Ontario. The mine produced 1.021 million tons grading 1.16% Cu, along with 237,603 oz Ag and 1,964 oz Au from a number of veins between 1965 and 1972 (Tortosa and Moss 2004). Nikos Explorations Ltd. currently has an agreement to acquire Amerigo Resources Ltd. interest in the property. The following information has been extracted from the technical report of Nikos Explorations Ltd. completed by Tortosa and Moss (2004). The Coppercorp deposit is hosted by volcanic and intrusive rocks situated on the eastern edge of the Midproterozoic Midcontinental Rift system. Copper 1856-1857 1871 1882-1884 1890 1892 1906-1908 1948 1951 1955 1964 2002 Montreal Mining Co. Ontario Mineral Lands Co. Silver Islet Consolidated Mining and Lands Co. Canada Lands Purchase Synd. Nipigon Mining Co. Calumet and Hecla Co. Macassa Mines Ltd. C.C. Huston and Associates Coppercorp Ltd. leased by Vauze Mines Ltd North Canadian Enterprises Ltd. Terry Nicholson & William Gibbs from Tortosa and Moss (2004) 11 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide Figure 10: Detailed Geology around the Coppercorp Mine area showing the location of some of the surface and projected mineralized zones (Giblin, 1973). Blue outline shows the C Zone, L Zone, Lutz Vein and Mamainse Vein. J — — — Oldest Figure 11: Distribution of faults and simplified regional geology of the Mamainse area with the outline of the Coppercorp Property shown in yellow (after Giblin, 1973; Richards, 1995) from Tortosa and Moss (2004). Figure 12: Mineralized structures in the Coppercorp deposit (Heslop, 1970) from Tortosa and Moss (2004) 12 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide Coppercorp Ltd. was a new company created in 1954 to sink a shaft, to 168 m with levels at 76, 114, and 152 m (Tortosa and Moss 2004). During the underground development, 4,267 m of lateral development were completed and 60,000 tons of ore were stockpiled. Operations ceased in 1957 due to falling copper prices. Vauze Mines Ltd. (controlled by Sheridan Geophysics Limited) leased the mine in 1963 and completed additional drilling along with a surface exploration program which included geophysical surveys and geological and geochemical examinations. In 1965, a decision was made to bring the Coppercorp deposit into production and the original shaft was dewatered and deepened to 192 m. The operation produced 500 tons per day processed into a 50% copper concentrate at a recovery rate of <90%. The Coppercorp Mine produced 1, 021,358 tons grading 1.16% Cu plus approximately 237,603 ounces of silver and 1,964 ounces of gold from such veins between 1965 and 1972. Some of the available historical statistics on underground development, drilling, pre-production ore reserve estimates and production figures are provided in Table 2. Recent exploration on portions of the property was completed by J.F. Paquette and Cominco Ltd.. J.F. Paquette held a property covering the Lutz vein and L zone in 1991-92, and a self-potential survey along with prospecting and sampling programs were conducted (Fig. 11). Cominco Ltd. optioned this property in 1993 and completed geological mapping, surficial geochemistry, electromagnetic (UTEM) and magnetic surveys. Mineralization Copper mineralization occurs as disseminated native copper in amygdule and veins, and vein-hosted copper sulphide deposits. Production on the property has concentrated on the copper sulphides mineralization. The copper sulphide veins occur in faultrelated breccia zones that generally display gradual transitions from high grade sulphide veins to barren oxide cement. The copper sulphides are dominantly chalcocite, with lesser chalcopyrite and bornite, rarely native copper and are usually accompanied by specular hematite. Massive chalcocite veins, 20 to 25 cm wide, were found at numerous localities within the deposit. Richards (1985) recognized four stages of mineralization, which were: 1) pyrite-chalcopyrite, 2) chalcopyrite-bornite, 3). chalcocite-hematite, 4). native copper, native silver, copper arsenides, malachite and hematite. The third stage was the most important source of copper producing rich veins of chalcocite and replacing earlier sulphides. The veins and breccias consist of quartz and carbonate with subordinate laumontite and fluorite. Large vugs of varying size are lined with quartz, calcite, and sulphides and were commonly found throughout the deposit, suggesting a shallow ‘open space filling’ type of mineralizing process (Heslop, 1970). The wallrock is commonly chloritized and sericitized and may contain epidote. The faults hosting mineralization cut Keweenawan basalt flows and conglomerates. The width of the fault zones varies along strike from shears less than 1 m to disrupted lenses up to 12 m across (Richards, 1985). The faults have two orientations, northeast and northwest. Northnortheast trending faults dip 60o to 65o east and host the Copper Creek Zone, Silver Creek Zone and `G`, `H`, and `F` Zones. North-northwest Table 2: Historical statistics on underground development and drilling at the Coppercorp Mine (Tortosa and Moss 2004). ______________________________ _______________________ ________ ltI,naitlo. Sourer FsplunIIun Activist I Indcrgruund flnckçrnenl DnAuig. 34.8*2 [ccl SMS1R (1150X57 Cronuculs 3,62* (cci L)nllang StutTacc. 16,000 [cci ShOOK 000152 20,000 eel j • SMDR 000152 - Iable 4 H,stcrral Pre'Production Ore Reserve Eshma4cC at the Coppertoip Muir Mi•erslIrS Zone Ij,toemitiss Source Tore Maceve Rationale C Zone red C7.apa Soisth" 400000 ions® 23% Cu ShOOK 000852; Cuppntcoqi L Silta Creek ScoUt Zone 490,000 ices a I 9%Cu SB and Silsti Creek North 630,000 Ions Tolal Ore Runurce 1,540000 loot I I I%Cu I 965 SMOR '100852, Coppeccoep SMOR 000852, Coppccccep Report lee 1983 • Cu SMOR 000*52, Cnppeeroqi Repoil (or 1965, Northern MIner 1961 Iv,, Year •J9570 1965 1966 1967 Au(Os) US$2 2*519 3*6 115,848 1146,601 149691 Mills 1971 tT.thr" Ag(ck) ('uØI*) 411000 - 1 I 146,441 142,9*6 — 1969 1970 (Source 5M014000*52L toot Hoisted 3,716.325 290 I - 268 33,622 3,175,730 231 1,785 33,570 3,109,73* t(pj,1181 141,955 111,811 83.5/') 156111 84.492 , ni on 440 '1 I 7 23,782,128 13 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide trending faults that dip 50o to 70o east and almost parallel the volcanic and sedimentary rocks (Fig. 12 and 13). These faults are the most productive structures, hosting the C Zone, SB Zone, D Zone and B Zone. Where northwest faults intersect the Great Conglomerate, the fault narrows and the sulphide mineral content decreases possibly due to the lower competency of the conglomerate compared to the mafic volcanic rocks (Heslop 1970). Mineralized structures also cut, and are cut by, felsite and diabase dykes. Both the diabase and felsite intrusions are considered to have been emplaced contemporaneously with fault movement, brecciation and sulphide deposition. Heslop (1970) defined four major stages of fault development based on the crosscutting relationships of the faults (Table 3) with an apparent younging in the faults from south to north, but mineralogical changes in the ore or other characteristics associated with this relative structural timing have not been documented. Some zones, including the C, SB, and L zones, Lutz and Mamainse veins, display an apparent stratigraphic control. Mineralization in these zones occurs primarily where the structure is hosted by basalts of the upper section of the Mamainse Point Formation, 75 to 150 m above the Great Conglomerate (Fig. 11). Table 4: Copper deposits in the Mamainse PointBatchawana Area (Tortosa and Moss 2004) Strike Dip SB Zone Copper Creek Zone C Zone Silver Creek Zone D Zone B Zone F Zone G Zone H Zone 342-335 020 East 55-60 E 1 2 345 010 55-68 E 50-65 E 3 4 300 345 030 020 020 45 NE East Southeast East East 4 4 4 4 4 Production Rarnts 1965to in 102 M cons 3 4 Copperguatlz YCLO iBSZco 1884 3 3 2 Ratio I%7ro1973 2% Co 4CM tons oboes 300ns 125M tons (410.13% I Deposit Typo Coppacorp CoppoT. Mancciose Thus8 qon coin Sours . a • I 16% Co ii I6S%Cu Pipes Jlrrson lien cIa &sss8rrccso 3 CootcdoO4%MoS2 F Wnsltrrrcaa F 0 RI tons a 0.6 to I IO%WO, Jogs00 porphyry I8Mtomta#0.19% N6 SoorcesiRoport, 1%?; 2 Moore 1426, 3 I Cuond080%Mo5 1 M&R. t989, 4 SMDR 000852 Deposit Model Tortosa and Moss (2004) have proposed that the Coppercorp deposit is an iron-oxide copper-gold (IOCG) type deposit similar to Olympic Dam based on: 1. A continental rift-related tectonic setting on the eastern margin of the Midcontinent Rift system. 2. The Keweenawan basalts represent a significant volume of potential copper source rocks with an estimated thickness of 4,300 to 6,000 m. 3. The presence of a massive magnetite vein grading 3.9% Cu over 1.05 m at the Jogran porphyry and fluorite associated with the Breton Breccia at Tribag and with Coppercorp ore. 4. The presence of numerous faults some of which are splays off major crustal faults including the Mamainse Point Fault to the south of the property. 5. The apparent high level emplacement of the felsic intrusives 6. The presence of dilational sites along active structures. 7. The presence of a high temperature saline brine (350o to 450oC) 15-20 eq. wt.% CaCl2 believed to be magmatic in origin, and a lower temperature fluid (<100o to 350oC, 0 to 15 eq. wt. %) believed to be a mixture of magmatic and meteoric fluid (Richards, 1985). 8. The occurrence of widespread Cu mineralization in the area as both low tonnage medium grade deposits (e.g. Coppercorp) and high tonnage low grade Table 3: Relative age of fault zones based on crosscutting relationships with 1= oldest and 4=youngest (Heslop, 1970) from Tortosa and Moss (2004). Mineralized (Fault) Zone Production Yonn Deposit Relative Age There are several other deposits in the Batchawana area and these are summarized in Table 4. 14 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide PALEOMAGNETISM Paleomagnetic studies of Keweenawan rocks of the Midcontinent Rift (e.g. Portage Lake volcanic rocks, Michigan; North Shore volcanic and intrusive rocks, Minnesota; various dykes south of Thunder Bay; Logan sills, Thunder Bay; Osler Group volcanic rocks, Nipigon; Cape Gargantua volcanic rocks, Wawa) indicate that the older rocks have reverse magnetic polarity and the younger rocks have normal polarity (e.g. Halls and Pesonen 1982; Nicholson et al. 1997). The volcanic rocks of the MPF have more complex pattern with 2 zones of reversed polarity overlain by zones of normal polarity with the polarity changes coinciding with the base of clastic sedimentary units (Fig. 5) (Palmer 1970). It was originally proposed that this repetition was the result of a strike fault located at the boundary between the lower normal and upper reverse zones, but the lithogeochemical stratigraphy indicates another explanation is required. deposits (e.g. East Breccia zone of Tribag mines). 9. The presence of a broad, regional aeromagnetic anomaly over the property and the presence of several gravity anomalies. 10. The production of limited amounts of gold and silver along with the copper at the Coppercorp Mine and the anomalous concentrations of gold and silver found in the outlying copper occurrences. GEOCHRONOLOGY There is a limited amount of geochronology completed in the Mamainse Point area and correlations with other areas in the Midcontinent Rift are often based on paleomagnetic data (e.g. Nicholson et al, 1997). A number of K/Ar ages have been determined but are regarded as minimum ages due to the probability of Ar loss (Wanless et al, 1966, 1967, 1968). These ages include the basalt exposed at Chippewa Falls (Stop 11), south of Pancake Bay and the main portion of the Mamainse Point Formation, which have an age of 915 +/-140 Ma, and samples of the Tribag Mine breccia pipe which returned ages of 785 +/- 103 Ma to 1055 +/- 35 Ma. A more accurate age of volcanism is the 1070 +/- 50 Ma Rb/Sr age for a felsites in the Mamainse Point area (Van Schmus, 1971). A U/Pb age of 1096.2 +/- 1.9 Ma for a felsic unit in the area of the volcanic conglomerate of the Lower Division is very similar to the Rb/Sr age but younger than the 1105 +/-4 Ma felsic volcanism in the Osler Group (Davis et al. 1995). A Re-Os age of 1128 +/- 54 Ma for the basal picrite volcanic rocks (Shirey 1997) probably represents a maximum age for the Mamainse Point Formation, and is comparable to the 1124 to 1114 Ma age for the ultramafic Seagull Intrusion of the Lake Nipigon area (Heaman and Easton 2005). However, if the picritic flows of Mamainse Point are equivalent to the Seagull Intrusion, the basal portion of the MPF is much older previous interpretations, and 1096 Ma felsic age of Davis et al. (1995) means that the sediments of the Great Conglomerate may represent a longer hiatus in volcanic activity. REFERENCES Annells, R. N. 1973. Proterozoic Flood Basalts of Eastern Lake Superior: the Keweenawan Volcanic Rocks of the Mamainse Point Area, Ontario; Geological Survey of Canada, Paper 72-10. 51 p. Cannon, W.F., Green, A.G., Hutchinson, D.R., 1989. The North American Midcontinent rift beneath Lake Superior from GLIMPCE seismic reflection profiling. Tectonics, v. 8, p. 305-322. Coleman, P.A., 1899. Copper Regions of the Upper Lakes, Report of the Bureau of Mines Vol. VIII Part 2, p.152 Davis, D.W. and Green, J.C. 1997. Geochronology of the North American Midcontinent Rift in western Lake Superior and implications for its geodynamic evolution; Canadian Journal of Earth Sciences, v.33, p.476-488. Davis, D.W., Green, J.C., and Manson, M., 1995. Geochronology of the 1.1. Ga North American Mid-continent Rift; Institute of Lake Superior Geology, 41st Annual Meeting Proceedings Volume 41, Part 1: Abstracts, p. 9-10. Giblin, P. E. 1974. Middle Keweenawan Rocks of the Batachawana-Mamainse Point Area. Institute of Lake Superior Geology 20th Annual Meeting, Nipigon, Ontario, Program, Abstracts, and Field Guides, v. 20, p. 39-67. Giblin, P. E., and Leahy, E. J. 1967. Sault Ste. Marie - Elliot Lake Sheet, Districts of Algoma, Manitoulin, and Sudbury. Ontario Department of Mines, Geological Compilation Series, Map 2108. Scale 1 inch to 4 miles. Giblin, P.E. and Armsburst, G.A. 1969. Batchawana, Ontario Department of Mines. Map 2251, scale 1:63 360. 15 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide Gmitro, T.T., 1990. Chemostratigraphy and petrogenesis of Keweenawan lavas at Alona Bay, Ontario. unpub. M.S. thesis, Northern Illinois University, DeKalb, Ill. Green, J.C., 1983. Geological and geochemical evidence for the nature and development of the Middle Proterozoic (Keweenawan) midcontinent rift of North America: Tectonophysics, v. 94, p.413-437. Halls, H.C. and Pesonen, L.J., 1982. Paleomagnetism of Keweenawan rocks; in Geology and Tectonics of the Lake Superior Basin; ed. R.J. Wold and W.J. Hinze; Geological Society of America, Memoir 156. p.173-201. Hart, T.R. 2002. Proterozoic Volcanic and Intrusive Whole Rock Geochemical Data Associated with the Keweenawan Midcontinent Rift, Lake Superior Area, Ontario; Ontario Geological Survey, Miscellaneous Release Data 114. Hart, T.R. and MacDonald, C.A., submitted, Proterozoic and Archean Geology of the Nipigon Embayment: Implications for Emplacement of the diabase sills and PGE-enriched mafic to ultramafic intrusions; Canadian Journal of Earth Sciences, Hart, T.R., Gibson, H.L., and Lesher, C.M., 2004. Trace Element Geochemistry and Petrogenesis of Felsic Volcanic Rocks Associated with Volcanogenic Massive Cu-Zn-Pb Sulfide Deposits; Economic Geology, v. 99, p. 1003-1013. Heaman, L.M., and Easton, R.M., 2005. Proterozoic history of the Lake Nipigon area, Ontario: Constraints from U-Pb zircon and baddeleyite dating. ; Institute of Lake Superior Geology, 51st Annual Meeting, Nipigon, Ontario, Part 1 – Proceeding and Abstracts, v. 51, part 1, 24-25. Heslop, J.B., 1970, Geology, Mineralogy and textural relationships of the Coppercorp Deposit, Mamainse Point area, Ontario. unpublished M.Sc. Thesis, Department of Geology, Carleton University, Ottawa, 95 p. Hutchinson, D.R., White, R.W., Cannon, W.F., and Schulz, K.J., 1990. Keweenaw hot spot: geophysical evidence for a 1.1 Ga mantle plume beneath the Midcontinent rift system in western Lake Superior; Journal of Geophysical Research, v. 95, p. 10 86910 884. Jerram, D.A. and Widdowson, M. 2005. The anatomy of Continental Flood Basalt Provinces: geological constraints on the processes and products of flood volcanism; Lithos, v. 79, p. 385– 405 Klewin, K.W. and Berg, J.H. 1990. Geochemistry of the Mamainse Point volcanics, Ontario, and implications for the Keweenawan paleomagnetic record; Canadian Journal of Earth Sciences, v. 27, p. 1194-1199. Klewin, K.W., and Berg, J.H. 1991. Petrology of the Keweenawan Mamainse Point Lavas, Ontario: Petrogenesis and continental rift evolution. Journal of Geophysical Research, v. 96, p. 457-474. Klewin, K.W. and Shirey, S.B. 1992. The igneous petrology and magmatic evolution of the Midcontinent rift system; Tectonophysics, v. 213, p.33-40. Lightfoot, P.C., Sage R.P., Doherty W., Naldrett A.J., and Sutcliffe, R.H. 1999. Mineral potential of Proterozoic Keweenawan intrusions: implications of major and trace element geochemical data from bimodal felsic and volcanic sequences of Mamainse Point and the Black Bay Peninsula, Ontario; Ontario Geological Survey OFR 5998, 91p. Manson, M.L. and Halls, H.C. 1997. Proterozoic reactivation of the southern Superior Province and its role in the evolution of the Midcontinent Rift. Canadian Journal of Earth Sciences, v. 34, p. 562-575. Massey, N.W.D. (1980). The geochemistry of some Keweenawan metabasites from Mamainse Point, Ontario. Ph.D. thesis. McMaster University, Hamilton, Ontario. 352p. Massey, N.W.D. 1983. Magma genesis in a late Proterozoic proto-oceanic rift: REE and other trace-element data from the Keweenawan Mamainse Point Formation, Ontario, Canada. Precambrian Research, v. 21, p. 81-100. Nicholson, S.W., and Shirey, S.B., 1990. Evidence for a Precambrian mantle plume: a Sr, Nd, and Pb isotopic study of the Midcontinent Rift System in the Lake Superior region; Journal of Geophysical Research, v. 95, p. 10 851-10 868. Nicholson, S.W., Shirey, S.B., Schulz, K. and Green, J.C., 1997. Rift-wide correlation of 1.1 Ga Midcontinent rift system basalts: implications for multiple mantle sources during rift development. Canadian Journal of Earth Sciences, v. 34, p. 504-520. Palmer, H. C.1970. Paleomagnetism and Correlation of some Middle Keweenawan Rocks, Lake Superior. Canadian Journal of Earth Sciences, v. 7, p. 14101436. Ojakangas, R.W., and Morey, G.B. 1982. Keweenawan sedimentary rocks of the Lake Superior region: a summary. In Geology and tectonics of the Lake Superior basin. Edited by R.J. Wold and W.J. Hinze. Geological Society of America, Memoir 156, pp. 157–164. Richards, J.P., 1985, A fluid inclusion and stable isotope study of Keweenawan fissure-vein hosted copper sulphide mineralization, Mamainse Point, Ontario. Unpublished M.Sc. thesis, Department of Geology, University of Toronto, 290p. Richards, J.P. and Spooner, E.T.C. 1989. Evidence for Cu(Ag) Mineralization by Magmatic-Meteoric Mixing in Keweenawan Fissure Veins, Mamainse Point, Ontario. Economic Geology, v. 84, pp. 360385. Shirey, S.B., 1997. Re-Os isotopic compositions of Midcontinent rift system picrites: implications for plume – lithosphere interaction and enriched mantle sources. Canadian Journal of Earth Sciences, v. 34, p. 489-503. Shirey, S.B., Klewin, K.W., Berg, J.H. and Carlson, R.W., 1994.Temporal changes in the sources of flood basalts: Isotopic and trace element evidence from the 1100 Ma old Keewenawan Mamainse Point Formation, Ontario, Canada; Geochemica et Cosmochimica Acta, v. 58, pp. 4475-4490. 16 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide Sutcliffe, R.H. and Smith, A.R. 1988. Project Number 87-17. Geology of the St. Ignace Island Volcanic-Plutonic Complex; in Summary of Field Work and Other Activities, Ontario Geological Survey Miscellaneous Paper 141, p. 368-371. Tortosa, D. and Moss, R. 2004. Geology and Exploration of the Coppercorp Property, Sault Ste. Marie Mining Division, Ontario; March 23, 2004, prepared for: Nikos Explorations Ltd., 158p.; filed with SEDAR May 11, 2004; available from http://nikosexplorations.com/_resources/Coppercor p2004Report.PDF Van Schmus, W. R. 1971. Rb-Sr Age of Middle Keweenawan Rocks, Mamainse Point and Vicinity, Ontario, Canada. Geological Society of America, Bulletin 82, p. 3221-3226. Van Schmus, W.R. and Hinze, W.J. 1985. The mid-continent rift system. Annual Review of Earth and Planetary Sciences, v. 13, p.345-384. Walker, J.A., Gmitro T.T. and Berg J.H. 2002. Chemostratigraphy of the Neoproterozoic Alona Bay lavas, Ontario; Canadian Journal of Earth Sciences, v. 39, p.1127-1142 Wanless, R. K. , Stevens, R.D., Lachance, G.R., and Rimsaite, J. Y. H. 1966. Age Determinations and Geological Studies, K-Ar Isotopic Ages, Report 6. Geological Survey of Canada, Paper 65-17, p. 58. Wanless, R. K., Stevens, R. D., Lachance, G. R. and Edmonds, C. M. 1967. Age Determinations and Geological Studies, K-Ar Isotopic Ages, Report 7. Geological Survey of Canada, Paper 66-17, p. 8485. Wanless, R. K., Stevens, R. D., Lachance, G. R. and Edmonds, C. M.1968. Age Determinations and Geological Studies, K-Ar Isotopic Ages, Report 8. Geological Survey of Canada, Paper 67-2, Part A, p. 95-96. 17 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide ITINERARY Stop Locality Km (south) Km (north) 1 Alona Bay 0 105.7 2 Unconformity: Mica Bay Formation Mamainse Point Formation 8.0 97.7 3 Glomeroporphyritic Basalt "Daisy Stone" 8.5 97.2 4 Ropy Flow Top 9.2 96.5 5 Volcanic Conglomerate 10.7 or 11.3 10.7 95.0 6 Mamainse Mine 14.2 91.5 entrance to road 15.0 90.7 7 Ubetuwantit (Coppercorp Property) 2.3 8 Coppercorp Mine 4.6 9 Interbedded Conglomerate and Basalt 21.8 83.9 10a Domeykite Occurrence 25.4 80.3 10b Pseudotechylite 25.9 79.8 10c Cottrell Cove Felsic Intrusion 26.1 79.6 Pancake Provincial Park 32.9 72.8 Pancake River 34.6 71.1 Mamainse Point Fault (approx.) 35.5 70.2 Road to the Tribag Mine Site 40.1 65.6 Batchawana River 45.3 60.4 Chippewa Falls 54 51.7 Keweenawan basalt flows 65.2 40.5 Jacobsville Formation sandstone 69.0 36.7 Intersection at Highway 17 and 556 Water Tower Inn 105.7 0 11 the 1840's. A sample of pitchblende was described by Le Conte in the American Journal of Science in 1847 with the location given as a vein 5.1 cm wide located about 112.6 km north of Sault Ste. Marie, but subsequent papers gave the location as between 64.4 to 144.8 km. In 1948, Robert Campbell of Toronto found pitchblende at the northwest end of Theano Point, which can be seen north across the bay. Subsequently prospectors found several other pitchblende deposits inland, to the north and east. Point Aux Mines on the south side of Alona Bay, was the site of the earliest organized mining venture in Ontario. In 1772-73, a mining company formed by Alexander Henry, the noted fur-trader, worked a copper deposit and shipped a small amount of ore to England. Theano Point to the north and Point Aux Mines to the west southwest consists of Archean felsic plutonic rocks cut by diabase dikes. The Alona Bay volcanic rocks are an approximately 1200 m thick, southward younging sequence of Keweenawan basalt flows unconformably overlying the Archean rocks (Annells 1973; Gmitro 1990). These flows are unconformably overlain by siltstones and sandstones similar to the Mica Bay Formation, and possibly comparable to the post-magmatic Freda Sandstone (e.g. Annells 1973; Ojakangas and Morey 1982). The Alona Bay flows have been correlated with the Lower Division flows of the Mamainse Point Formation (MPF) (Walker et al. 2002; Annells 1973). The Alona Bay flows are described by Annells (1973) and Gmitro (1990) as being composed of about 107 mafic flows averaging 6.8 m in thickness. Most of the flows have pahoehoe surfaces and vesicular-amygdular flow tops, and pipe vesicles are relatively common just above flow bases. Gmitro (1990) subdivided these flows into olivine phyric, plagioclase phyric, olivineplagioclase phyric, and aphyric groups. Two basic dikes cut the section, one about 500 m from the base of the section and one at the top of the section in fault contact with Archean rocks. Clastic dikes are common throughout the Alona Bay section. All of the Alona Bay lava flows have undergone some low-grade burial metamorphism to prehnite-pumpellyite facies and deuteric alteration, This field guide is an amalgamation of the field trip guides of Annells (1973) and Giblin (1974). All coordinates are in UTM (Universal Transverse Mercator), Zone 16, with a NAD83 Canada datum. STOP 1 Alona Bay Scenic Lookout UTM coordinates - 673636E 5219651N The following historical description is from Giblin (1974). An historical plaque commemorates the first discovery of uranium in Canada, made nearby in 18 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide _______ General Geology and Field Trip Stop Locations Map-Batchawana-Mamainse Point Area N 'b in ,C ii Stop Stop Stop Stop Stop lOb Stop I 200 0 200 400 Meters Legend Jacobsville Formation Archean Rocks Granitic Sandstone, Siltstone Mets Mafic Intrusive ROCkS 1 Feisk Diabase, Gabbro Keweenawan Felsic Intrusive and Volcanic ROCkS Felsite, quartz-feldspar porphyry Cong Iomer2te Mat ic r Basalt Figure 14. Field trip stops geology from Giblin and Leahy (1967) 19 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide shallowly west dipping outcrop located along the edge of the water. similar to that observed in the MPF (e.g. Massey 1980). Walker et al. (2002) identified 4 geochemical groups based on decreasing TiO2 Ni and MgO contents that reflect an upwards progression through the volcanic sequence. Although proposed to be equivalents to the flows of the Lower Division of the MPF, the majority of the Alona Bay flows have MgO concentrations between 6 and 10 wt.%, and none have MgO > 15.0 wt.%. BE CAUTIOUS AS THE OUTCROP WILL BE SLIPPERY IF WET The underlying Keweenawan basalts of the MPF strike N 30° W and dip 45° to 50° W. The basalts are thin olivine tholeiitic flows near the base of the MPF (Annells 1973), with upper vesicular zones exposed in this outcrop. Unconformably overlying the basalts is an up to 30 cm thick polymictic, matrix supported conglomerate of the Mica Bay Formation (Photo 1a). Very little of the basal conglomerate remains, please do not collect samples or deface the exposure The Mica Bay Formation has a total thickness of approximately 61 m, but only the lower 18.3 m are exposed along the beach. This section includes the basal conglomerate, which is overlain by grey-brown siltstones, arkoses, and minor immature quartz pebble conglomerates. Siltstones constitute approximately 70% of the section. Sedimentary structures include ripple marks, graded bedding, cross-lamination, flame structures, and ball-and-pillow structures indicative of a shallow water environment (Photo 1b and 1c). In other exposures, north of the section examined, flute casts and clastic dikes indicate a northerly direction of current flow. The sedimentary rocks strike N 65° E, and in the section examined, dip 15° N. Steeply-dipping faults cut the sedimentary rocks with offsets of up to 1.8 m. These rocks lie on the south limb of a regional syncline, with the north limb exposed between the creek and the small granite headland at the north end of the beach. The north limb is in fault contact with the underlying Archean granite, and a thin basal breccia occurs on the granite north of the fault. Similar basal breccias are found overlying the granite, at several points along the shore of Mica Bay. The age and correlation of these rocks are uncertain. The unconformity observed at this stop indicates that they are post-Middle Keweenawan, but they are lithologically and structurally different than the rocks of the Jacobsville Formation. The Mica Bay Formation appears to underlie the Jacobsville, which forms nearby islands in Lake Superior and much of the floor of Proceed for ~8 km to the south 0 – 2 km: Alona Bay flows 2 – 6.6 km: Archean felsic plutonic rocks and diabase dykes unconformably underlying the Alona Bay and MPF 6.6 – 8 km: Mica Bay Formation STOP 2 Unconformity: Mica Bay Formation and Mamainse Point Formation UTM coordinates – 673691E 521891N Park in the east side of the highway on a portion of the old highway roadbed. BE CAUTIOUS AS THE HIGHWAY IS BUSY AND HEAVILY TRAVELLED BY TRACTOR TRAILERS Cross the highway and proceed north to the culvert where the stream can be followed down to the beach. BE CAUTIOUS AS THE MUDDY SILTSTONES CAN BE SLIPPERY, ESPECIALLY IF WET Fine-grained siltstone and graphitic siltstone with trace sulphides are exposed in the section cut by the stream. Proceed about 200 m to the south along the cobble beach, passing exposures of the Mica Bay Formation. An angular unconformity between the clastic sedimentary rocks of the Mica Bay Formation and the underlying Keweenawan basalt of the Mamainse Point Volcanic Group is exposed in a 20 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide the eastern part of the lake. The age of the Jacobsville Formation is uncertain and has been variously assigned to the Lower and Middle Cambrian, and to the Upper Keweenawan. Thus, the Mica Bay Formation may be either Upper Keweenawan or Lower Cambrian. A thinner section of similar clastic sedimentary rocks occurs in Alona Bay, 6.4 km north, and may also be part of the Mica Bay Formation. Proceed south for ~1.0 km STOP 3 BE CAUTIOUS AS THE HIGHWAY IS BUSY AND HEAVILY TRAVELLED BY TRACTOR TRAILERS Glomeroporphyritic "Daisy Stone" flows of the Lower Division of the MPF STOP 4 Ropy Flow Top UTM coordinates – 673127E 5217816N Park in the drive about 200 m south of the rock cut Basalt UTM coordinates – 673437E 5218459N Cross the highway and walk north along the shoulder of the highway to the south side of the rockcut at the top of the hill. Proceed south along the highway for about 400 m from the same parking site as used for Stop 2. BE CAUTIOUS AS THE HIGHWAY IS BUSY AND HEAVILY TRAVELLED BY TRACTOR TRAILERS A ropy flow top is exposed on the south face of the outcrop located on the east side of the highway (Photo 3). The pahoehoe marks the top of a 6.5 m thick olivine tholeiite flow (Annells, 1973). Proceeding to the north through the rock cut, there are several flows with thin basal pipe vesicle zones, thicker amygdaloidal zones in the upper parts of the flows, and ropy flow tops. The vesicles and amygdules are filled with calcite, celadonite, and a green mica. About 150 m further north, on the east side of the highway, a northwest trending, 26o E dipping diabase dyke crosscuts the flows located near the top of the olivine phyric group (Annells, 1973). Scoriaceous flowtops probably on olivine-rich flows, are reported to occur in the west side of the rock cut in the area of the dyke. The distinctive glomeroporphyritic “daisy stone flows” are located interbedded with olivine tholeiite flows of the Lower Division of Annells (1973), towards the north side of the MPF. The flow is exposed in the rock cut located on the east side of the highway, and can also be observed along the shoreline to the northwest. This flow strikes north to northwest and dips about 50° W which is steeper than the dips present in subsequent stops higher in the sequence. Giblin (1974) reported that the flow can be traced intermittently along strike for 11.3 km, and a new exposure located about 4.3 km south of this location provides an excellent exposure of these flows. This flow varies from massive, fine-grained basalt to glomeroporphyritic, with radiating calcic plagioclase laths, up to 5 cm across (Photo 2). Annells (1973) reported that the massive portions of the flows are non-porphyritic containing abundant pseudomorphs after small olivine crystals. The plagioclase is variably epidotized or hematitized in different portions of the flow. A plagioclase-rich ponded flow in the Osler Volcanic Group volcanic rocks is exposed on the southeast corner of St. Ignace Island, in the Thunder Bay area (Sutcliffe and Smith 1988). The top of that flow has a texture resembling the glomeroporphyritic “daisy stone” texture. Proceed south for 1.5 km 0 – 1.0 km flows of the Lower Division of the MPF 1.0 – 1.5 km clastic sediments (volcanic conglomerate horizon) STOP 5 Volcanic Conglomerate UTM coordinates – 670790E 5214845N Parking is along the shoulder of the road. 21 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide Photo 1a: Polymictic conglomerate of the Mica Bay Formation unconformably overlying a MPF flow. Photo 2: Glomeroporphyritic Basalt (daisy stone) with a distinctive radiating calcic-plagioclase in a fine grained to aphanitic basaltic matrix (new location to the south of the highway). Photo 1b: Ball and Pillow Structures within the siltstone of the Mica Bay Formation. Photo 3: Ropy flow top on the south face of the outcrop, east of the highway. Photo 1c: Cross-laminations in the siltstones and arkoses of the Mica Bay Formation. Photo 4: Mamainse Vein looking east. Fracture fill carbonate-quartz vein hosted by the basaltic flows of the Upper Division. 22 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide BE CAUTIOUS AS THE HIGHWAY IS BUSY AND HEAVILY TRAVELLED BY TRACTOR TRAILERS mining operation, containing abundant specular hematite with lesser chalcocite. The area is underlain by thin, dark green, amygduloidal olivine tholeiite flows (Annells 1973), that strike north and dip 30o to 40o west. Amygdules filled with quartz, agate, calcite, chlorite and / or epidote, and carbonate and quartz-carbonate fracture fillings are common in the flows. The Mamainse Vein is described as striking o 335 and dipping 50o east, similar to the C Zone on the Coppercorp property (Tortosa and Moss 2004). The vein was traced for 457 m, pinching and swelling along strike varying in width from 0.46 to 4.0 m. This vein appears to be exposed along the shore to the south, and there are a number of smaller fractures containing mineralization including one north of the parking site. Interbedded with the basalt flows in the Mamainse Point Group are volcanic conglomerates composed entirely of basalt clasts. The conglomerate was described by Annells (1973) as poorly sorted, angular to subrounded clasts in a matrix of fine basaltic debris with some silty material. The clasts are massive to vesicular, up to 0.30 m in diameter, and some clasts are irregular shaped which Annells (1973) interpreted to be a result of emplacement while still semisolidified. Annells (1973) also reported that this unit has a fine-grained, well laminated upper zone resembling a hyaloclastite or peperite. Although there are clasts and lenses of occasional crossbedded red-brown silt there are no clasts of Archean basement rocks. The conglomerate is cut by carbonate veins which carry minor amounts of quartz and laumontite. BE CAUTIOUS AS THE OUTCROP WILL BE SLIPPERY IF WET Proceed south for 3.7 km Approximately 60 m south of the parking area on the lake shore, a west northwest trending fracture hosts an about 10 cm wide felsite dyke and the quartz-carbonate Mamainse vein fracture filling (Photo 4). Very fine fractures, filled with native copper and calcite, are oriented at right angles to the main northwest-trending fracture. The carbonate-quartz fracture filling and veining contains disseminated chalcocite, chalcopyrite, lesser native copper, with malachite and azurite weathering products, and associated specular hematite. North of the parking area, a subparallel 5 to 10 cm wide fracture filled with carbonate and quartz also hosts chalcocite and chalcopyrite, with malachite and azurite. Along the shore to the north, and in the rock cut on the east side of the highway, a polymictic conglomerate is interbedded with the flows. This conglomerate is located stratigraphically above the “Great Conglomerate” (Giblin and Armburst 1969). 0 – 1.8 km: Flows of the upper portion of the Lower Division 600 m: distinctive hematite alteration along fractures in the flows - east side of the highway 1.8 – 2.6 km: clastic sediment, mainly polymicitic conglomerates, of the “Great Conglomerate” 2.6 – STOP 6 3.5 km: mafic flows and intercalated conglomerates of the Upper Division of the MPF Mamainse Mine UTM coordinates – 669939E 5214040N Exit to the west onto the bush road, proceed about 100 m to the second opening and park. The Mamainse Mine consists of 3 shafts that were put into production between 1842 and 1894, along with a stamp mill, service facilities and a village were constructed by the Lake Superior Native Copper Company. The area to the south of the parking site consists of rubble from the Proceed south for ~875 m Turn east on the bush road Proceed south for 2.3 km 23 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide STOP 7 Ubetuwantit Property) Vein (Coppercorp Proceed south for ~5.8 km UTM coordinates – 670432E 5211228N 0 – 5.8 km: flows and intercalated clastic sediments and felsic rocks of the lower portion of the Upper Division, MPF Park the vehicles along the bush road and proceed about 300 m to the east along the trail to the pits. This vein is exposed in two shallow pits on the west side of the powerline. Sampling by Nikos Explorations (Tortosa and Moss 2004) indicates that the 5 to 10 centimetre wide vein is composed of quartz and carbonate hosted by fracturing and minor brecciation within a basalt flow. The veins contain chalcocite and chalcopyrite with abundant specular hematite. The flow is amygduloidal with calcite veinlets and trace malachite staining. Some portions of the flow are pervasively hematitized with up to 1% specular hematite. 4.3 km: south entrance to the Coppercorp Mine. STOP 9 UTM coordinates – 66836E7 5207904N Park in the area near the top of the rise on the west side of the highway BE CAUTIOUS AS THE HIGHWAY IS BUSY AND HEAVILY TRAVELLED BY TRACTOR TRAILERS Continue south for ~ 2.3 km Walk north along the highway to the rock cut and cross with caution. Passing through the area of the main shaft and waste pile for the Coppercorp Mine. STOP 8 Interbedded Conglomerate and basalt At this location, two polymictic conglomerates are separated by a basalt flow. There are a number of conglomerate horizons interbedded with the flows the MPF, with thicker concentration occurring in the “Great Conglomerate” located to the north and stratigraphically below this stop, and this conglomerate is typical of the polymicitic conglomerates of the MPF (e.g. Annells 1973; Giblin 1974). The conglomerates generally contain subrounded, up to 0.61 m clasts of predominantly Archean granitic rocks, with minor amounts of Archean mafic metavolcanic rocks, iron formation, and Keweenawan basalt (Fig. 5). In some locations, well laminated to cross-bedded sandstone is interbedded with the conglomerates (Annells 1973). The basalt flow separating the conglomerate beds was described by Giblin (1974) as having a narrow amygdaloidal zone, with a few pipe vesicles, at the base, rapidly coarsening upwards through the flow, and a thicker amygdaloidal zone near the flow top. Minor faulting, ranging from a few to 30 cm, occurred at the base of the flow, and has been filled by carbonate. Carbonate occurs in fractures cutting the conglomerate and also in the matrix. Coppercorp Mine UTM coordinates – 671103E 5209365N In the open cut, the C Zone can be observed in a north-northwest trending fault. This is one of a set of north-trending, generally 50o-70o east dipping, faults that are almost parallel to the strike of the flows. The veins and breccia fillings consist of quartz and carbonate with subordinate laumontite and fluorite. The width of the fault zones varies along strike from shears less than 1 metre to disrupted lenses up to 12 metres across (Richards, 1985). The wallrock is commonly chloritized and sericitized and may contain epidote. Mineralization consists of predominantly copper sulphides, chalcocite with lesser chalcopyrite and bornite, usually accompanied by specular hematite. Large vugs of varying size are lined with quartz, calcite and sulphides and were commonly found throughout the deposit. Samples may be obtained from the blocks piled to the south of the open cut. The mine may be exited either by returning north for ~4.6 km along the bush road to the highway, or if the roads are passable proceed south and west for ~2.3 km to the highway Proceed south for ~3.6 km. 24 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide felsic magma, perhaps concentrated in late stage magmatic fluids migrating along zones of shearing and deformation during the last stages of crystallization of the magma. 0 – 3.6 km: flows and intercalated clastic sediments and felsic rocks of the Upper Division ~2.6 km: hyaloclastite intercalated with the massive flows – west side of highway STOP 10a Return to the vehicles Domeykite Occurrence Proceed south for ~800 m UTM coordinates – 669783E 5204844N STOP 10b Park in the lane on the west side of the highway, south of the outcrop Cottrell Cove Felsic Intrusion UTM coordinates – 670390E 5204502N Park in the bush road on the east side of the highway and proceed about 60 m north to the outcrop on the west side. BE CAUTIOUS AS THE HIGHWAY IS BUSY AND HEAVILY TRAVELLED BY TRACTOR TRAILERS BE CAUTIOUS AS THE HIGHWAY IS BUSY AND HEAVILY TRAVELLED BY TRACTOR TRAILERS Walk north to the large sloping outcrop located on the east side of the highway and cross the highway with caution. The rock cut consists of a rock of rhyolitic composition (Annells 1973; Lightfoot et al. 1999), and is part of an approximately 250 m wide body that includes the pseudotachylite stop to the north (Stop 10c). The domeykite outcrop has been interpreted to be the northwest branch of this unit (Giblin and Armburst, 1969). This unit is also exposed along the shore of Cottrell Cove to the west. The felsic unit is fine-grained, reddish brown, and varies from massive in the core to quartz porphyritic and flow banded towards the margins. The flow bands forms tight asymmetric isoclinal folds that are generally parallel to the contacts (Photo 6a). The contact consists of a breccia ranging from centimetres to tens of centimetres, and containing subangular to subrounded, pebble to cobble sized fragments of this unit, basalt, and granite in a felsic matrix (Fig. 6b). Dykes of felsite breccia reportedly intrude the adjacent basalts (Giblin 1974). The massive core contains light brown to beige lenses and irregular patches or funnels Domeykite (Cu3As) is isometric, hextetrahedral, gray to yellow-brown to white, with a brittle fracture and habits that include botryoidal, reniform, and massive uniformly indistinguishable crystals. It has a hardness of 3 to 3.5, metallic luster, and a black gray streak (http://webmineral.com /data/Domeykite.shtml). Domeykite mineralization is restricted to a pink to red, fine-grained felsic rock of rhyolitic composition that dips from 30o to 40o northwest with an overall thickness from 10 to 15m. The lowermost 3 to 4 m of the unit is strongly flow banded with tight asymmetric isoclinal folds. The lower contact with the basalt is sharp and has an aphanitic, 2 to 10 cm chilled zone that contains subangular to subrounded fragments of basalt. Spherical calcite-filled amygdules occur within the felsic unit for up to a few centimetres from the basalt. The basalt is a medium-grained, diabasic, dark green amygduloidal olivine tholeiite. This felsic unit is interpreted to be an intrusion into the older basaltic flows (e.g. Annells 1973). Domeykite is a copper arsenide concentrated in green to grey patches best developed in the flow banded portions of the lower and central parts of the unit. The domeykite has not been observed in the basalts stratigraphically above or below. The restriction of the copper arsenide mineralization to the felsic unit suggests that the mineralization may be genetically related to the 25 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide Photo 5: Polymicitic conglomerate containing clasts of Archean felsic plutonic rocks intercalated with basaltic flows. Photo 6c: Alteration along vertical fractures in rhyolite possibly related to magmatic degassing. Photo 6a: Flow-banded rhyolite near the contact, with pink, grey to white alteration. Photo 7: Pseudotachylite consisting of an aphanitic, matrix containing fragments of quartz, felsite, and basalt — -— -k.- i Photo 6b: Autobrecciated felsite in contact with diabasic basalt. Photo 8: Keweenawan basalt unconformity overlying Archean felsic plutonic rocks. (http:// www.start.ca /users/ mharris/ waterfalls/ chippewa-falls.html) 26 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide are set in a matrix of finely comminuted fragments of the above-noted materials and larger, irregular grains of carbonate and chlorite. An overall pinkish colour was interpreted to be a result of finely disseminated hematite. Giblin (1974) tentatively interpreted the pseudotachylite to be a result of gas escaping from the underlying felsic magma streamed upwards, under high pressure, through fractures in the overlying rocks. Fragments of the wallrocks, detached by erosive power of the gas and structural dislocation, were transported in a fluidized gas-solid system, which further eroded the wallrocks and the entrained particles. The fluidized material filled the fractures, and upon the eventual decrease in gas pressure, remained as veins of very fine-grained rock powder. (Photo 6c). Giblin (1974) reported that this change was a result of alteration and extensive development of kaolin. The orientation of the funnel suggests that this alteration may be a result of post-depositional degassing and possibly related to the pseudotachylite of the previous stop. This irregular colour variation also resembles variations observed in the oxidized Sibley Group sedimentary rocks of the Nipigon area, suggesting that some of this variation may be in part a result of reduction of an originally oxidized rock. These felsic units or felsites have been interpreted to be intrusive in nature (e.g. Annells 1973; Giblin 1974) based primarily on the brecciated nature of the exposed contacts. The depth of intrusion is not known, or whether some of these units may have formed high level cryptodomes that may have breached surface. The only known geochronology for the Mamainse Point area is a U/Pb zircon age of 1096+/-2.1 Ma (D. Davis, personal communication, 2005) for a unit with similar characteristics located north of the Mamainse Mine. However, the contact relationships at that location are ambiguous. Proceed south for ~27.7 km 6.6 km: Pancake Bay Provincial Park UTM coordinates – 675727E 5204458N 8.3 km: Pancake River UTM coordinates – 678422E 5203459N Proceed for ~200 m north ~8.8 km: Mamainse Point Fault separates the MPF to the north from the Jacobsville Formation sediments to the south. outcrop located on the west side STOP 10c Pseudotachylite 8.8 – 21.8 km: Jacobsville Formation UTM coordinates-670190 5204595 ~13.8 km: the road to the Tribag Mine exits on the east side of the highway Please do not collect samples from the outcrop, as there are abundant fragments in the rubble used as fill for the road bed. ~19.0 km: Batchawana River UTM coordinates – 688231E 5200663N The pseudotachylite was originally described by Giblin (1974) and occurs as narrow branching veins, ranging in thickness from a few centimetres to thin films on fracture surfaces, cutting both the massive fine-grained felsic rock and the wallrock basalts (Photo 7). Veins are composed of a dark grey, black, to dark brown, aphanitic, matrix, that often breaks with a sub-conchoidal fracture, containing fragments of quartz, felsite, and basalt. Thin-sections material show the veins to be a microbreccia consisting of angular fragments of quartz (free of strain shadows), feldspar, devitrified glass, quartz-feldspar porphyry with a matrix of devitrified glass, opaque minerals, felsite, and basalt (Giblin 1974). These fragments ~21.8 – 27.7 km: Archean felsic plutonic rocks The highway crosses over the Harmony River The entrance to the parking lot is to the south side of the river on the east side of the highway. STOP 11 Chippewa Falls UTM coordinates – 695982E 5200414N 27 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide A monument in the small roadside park marks the approximate mid-point of the Trans-Canada Highway. Follow the trails for approximately 90 m to the top of the lower falls, where there is an excellent exposure of Mamainse Point flows unconformably overlying Archean trondhjemite and diabase dykes. The best exposure is on the small island which is accessible only during periods of low water (Fig. 8). The contact on the island and the east side of the river can be viewed from the trail. Giblin (1974) described the unconformity as generally 10° - 30° southwest dipping, but is nearly vertical on the east bank of the river. The basalt flow has a chilled lower contact with rare pipe vesicles, and near the east side of the island, a few pillows ranging in length from 0.3 to 1.2 m occur in the lower 3 m of the flow. These are the only pillows found to date in the Keweenawan lavas of the east shore of Lake Superior. 25.0 – 32..1 km: Jacobsville Formation in the Goulais River valley. 32.1 – 45.9 km: Archean granitic gneisses and migmatites cut by diabase dikes with Huronian sediments to the east along Highway 550. 45.9 - 51.7 km: Jacobsville Formation to the Water Tower Inn Proceed 51.7 km south to Sault Ste. Marie 0 – 9.8 km: Archean felsic plutonic rocks 9.8 - 13.1km: Keweenawan mafic flows 11.2 km: Optional Stop, Keweenawan Basalt Optional Stop – Keweenawan Basalt UTM coordinates –700261E 5192354N Good cross-sections of the basaltic flows are exposed in the rock cut on the eats and southeast side of the highway, and include thin basal amygdaloidal zones, thicker upper amygdaloidal zones, and common reddish, ropy flow tops. 13.1 – 15.2 km: Jacobsville Formation 15.2 – 17.4 km: Archean felsic plutonic rocks 17.4 – 19.9 km Archean felsic to intermediate metavolcanic rocks with the hill top to the west being a Nipissing sill. 19.9 – 25.0 km: sedimentary rocks of the Gowganda Formation of the Southern Province, intruded by gabbro sills of the Nipissing diabase to the northeast. 28 PDF compression, OCR, web-optimization with CVISION's PdfCompressor � https://digitalcollections.lakeheadu.ca/files/original/22ea3b680000e76ba99ce28da77c8e43.PDF 384ef9ee0a2ad25d4df809f0ebf0283f PDF Text Text Ontario Ontario Geological Survey Open File Report 6135 Geological Guidebook to the Paleoproterozoic East Bull Lake Intrusive Suite Plutons at East Bull Lake, Agnew Lake and River Valley, Ontario 2004 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �PDF compression, OCR, web-optimization with CVISION's PdfCompressor �ONTARIO GEOLOGICAL SURVEY Open File Report 6135 Geological Guidebook to the Paleoproterozoic East Bull Lake Intrusive Suite Plutons at East Bull Lake, Agnew Lake and River Valley, Ontario by R.M. Easton, L.S. Jobin--Bevans and R.S. James 2004 Parts of this publication may be quoted if credit is given. It is recommended that reference to this publication be made in the following form: Easton, R.M., Jobin--Bevans, L.S. and James, R.S. 2004. Geological guidebook to the Paleoproterozoic East Bull Lake intrusive suite plutons at East Bull Lake, Agnew Lake and River Valley, Ontario; Ontario Geological Survey, Open File Report 6135, 84p. e Queen’s Printer for Ontario, 2004 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �PDF compression, OCR, web-optimization with CVISION's PdfCompressor �e Queen’s Printer for Ontario, 2004. Open File Reports of the Ontario Geological Survey are available for viewing at the Mines Library in Sudbury, at the Mines and Minerals Information Centre in Toronto, and at the regional Mines and Minerals office whose district includes the area covered by the report (see below). Copies can be purchased at Publication Sales and the office whose district includes the area covered by the report. Although a particular report may not be in stock at locations other than the Publication Sales office in Sudbury, they can generally be obtained within 3 working days. All telephone, fax, mail and e-mail orders should be directed to the Publication Sales office in Sudbury. Use of VISA or MasterCard ensures the fastest possible service. Cheques or money orders should be made payable to the Minister of Finance. Mines and Minerals Information Centre (MMIC) Macdonald Block, Room M2-17 900 Bay St. Toronto, Ontario M7A 1C3 Tel: (416) 314-3800 Mines Library 933 Ramsey Lake Road, Level A3 Sudbury, Ontario P3E 6B5 Tel: (705) 670-5615 Publication Sales 933 Ramsey Lake Rd., Level A3 Sudbury, Ontario P3E 6B5 Tel: (705) 670-5691(local) 1-888-415-9845(toll-free) (705) 670-5770 pubsales@ndm.gov.on.ca Fax: E-mail: Regional Mines and Minerals Offices: Kenora - Suite 104, 810 Robertson St., Kenora P9N 4J2 Kirkland Lake - 10 Government Rd. E., Kirkland Lake P2N 1A8 Red Lake - Box 324, Ontario Government Building, Red Lake P0V 2M0 Sault Ste. Marie - 70 Foster Dr., Ste. 200, Sault Ste. Marie P6A 6V8 Southern Ontario - P.O. Bag Service 43, 126 Old Troy Rd., Tweed K0K 3J0 Sudbury - Level B3, 933 Ramsey Lake Rd., Sudbury P3E 6B5 Thunder Bay - Suite B002, 435 James St. S., Thunder Bay P7E 6S7 Timmins - Ontario Government Complex, P.O. Bag 3060, Hwy. 101 East, South Porcupine P0N 1H0 Toronto - MMIC, Macdonald Block, Room M2-17, 900 Bay St., Toronto M7A 1C3 This report has not received a technical edit. Discrepancies may occur for which the Ontario Ministry of Northern Development and Mines does not assume any liability. Source references are included in the report and users are urged to verify critical information. Recommendations and statements of opinions expressed are those of the author or authors and are not to be construed as statements of government policy. If you wish to reproduce any of the text, tables or illustrations in this report, please write for permission to the Team Leader, Publication Services, Ministry of Northern Development and Mines, 933 Ramsey Lake Road, Level B4, Sudbury, Ontario P3E 6B5. Cette publication est disponible en anglais seulement. Parts of this report may be quoted if credit is given. It is recommended that reference be made in the following form: Easton, R.M., Jobin-- Bevans, L.S. and James, R.S. 2004. Geological guidebook to the Paleoproterozoic East Bull Lake intrusive suite plutons at East Bull Lake, Agnew Lake and River Valley, Ontario; Ontario Geological Survey, Open File Report 6135, 84p. iii PDF compression, OCR, web-optimization with CVISION's PdfCompressor �PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Contents Abstract ...........................................................................................................................................................xiii Introduction........................................................................................................................................................ 1 Safety......................................................................................................................................................... 1 Acknowledgements ................................................................................................................................... 2 East Bull Lake Intrusive Suite............................................................................................................................ 2 Nomenclature ............................................................................................................................................ 2 Regional Geological Setting...................................................................................................................... 3 Overview of East Bull Lake Intrusive Suite .............................................................................................. 9 Depth and Mechanism of Emplacement.................................................................................................. 10 Magma Composition and its Relationship to Mineralization .................................................................. 10 Orthopyroxene Hornblendite Bodies....................................................................................................... 12 Geochemistry .................................................................................................................................. 14 Field Trip Guidebook....................................................................................................................................... 16 East Bull Lake Intrusion—Overview ...................................................................................................... 16 Country Rocks, Structure and Geometry of the Intrusion............................................................... 16 Stratigraphy and Petrography.......................................................................................................... 17 Marginal Series ...................................................................................................................... 18 Lower Series........................................................................................................................... 18 Main Series............................................................................................................................. 18 Upper Series ........................................................................................................................... 20 Geochemistry .................................................................................................................................. 21 Mineralization................................................................................................................................. 26 Road Log, Day 1, East Bull Lake Intrusion............................................................................................. 27 Stop 1. Paleoproterozoic Metagabbro............................................................................................. 28 Stop 2. Neoarchean Parisien Lake Syenite ..................................................................................... 28 Stop 3. Southern Margin of the East Bull Lake Intrusion ............................................................... 28 Stop 4. Country Rocks .................................................................................................................... 29 Stop 5. Rhythmically Layered Zone ............................................................................................... 29 Stop 6. Folson Lake Deformation Zone.......................................................................................... 30 Stop 7. Dendritic Texture in the Varitextured Gabbronorite Zone ................................................. 30 Stop 8. Layered Gabbronorite Zone................................................................................................ 30 Stop 9. Olivine Gabbronorite Zone................................................................................................. 31 Stop 10. Rhythmically Layered Zone, East Bull Lake Intrusion .................................................... 31 Stop 11. Moon Lake Traverse......................................................................................................... 31 Stop 12A. Parisien Lake Traverse................................................................................................... 33 Stop 12B. Parisien Lake Traverse................................................................................................... 34 Agnew Lake Intrusion—Overview.......................................................................................................... 35 Country Rocks, Structure, and Geometry of the Intrusion.............................................................. 35 Stratigraphy and Petrography.......................................................................................................... 35 Marginal Series ...................................................................................................................... 35 Lower Series........................................................................................................................... 37 Upper Series ........................................................................................................................... 39 Geochemistry .................................................................................................................................. 39 v PDF compression, OCR, web-optimization with CVISION's PdfCompressor �PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Mineralization................................................................................................................................. 40 Road Log, Day 2, Agnew Lake Intrusion................................................................................................ 42 Stop 1. Inclusion-Bearing Unit ....................................................................................................... 43 Stop 2. Nodular Anorthosite Unit, Marginal Leucogabbronorite Zone .......................................... 44 Stop 3. Inclusion-Bearing Unit, Lower Gabbronorite Zone............................................................ 44 Stop 4. Inclusion-Bearing Unit, Lower Gabbronorite Zone............................................................ 46 Stop 5. Sudbury-Type Breccia........................................................................................................ 47 Stop 6. Varitextured Unit, Marginal Leucogabbronorite Zone ....................................................... 47 Stop 7. Dendrite Unit, Lower Gabbronorite Zone .......................................................................... 48 River Valley Intrusion, Overview............................................................................................................ 49 Country Rocks, Structure and Geometry ........................................................................................ 49 Structural State and Degree of Preservation........................................................................... 50 Contacts.................................................................................................................................. 52 Stratigraphy and Petrography.......................................................................................................... 55 North of the Sturgeon River ................................................................................................... 55 Marginal Zone ............................................................................................................... 55 Inclusion and/or Fragment-Bearing Zone ...................................................................... 56 Olivine Gabbronorite Zone............................................................................................ 57 Gabbronorite Zone......................................................................................................... 57 Leucogabbronorite Zone................................................................................................ 57 South of the Sturgeon River ................................................................................................... 57 Stratigraphic Comparison with other East Bull Lake Suite Intrusions ................................... 59 Geochemistry .................................................................................................................................. 60 Road Log, Day 3, River Valley Intrusion................................................................................................ 64 Stop 1. Shear-Zone Hosted Orthopyroxene Hornblendite Body..................................................... 64 Stop 2. Rocks of the Crerar Gneiss Association ............................................................................. 65 Optional Stop. Crerar Gneiss Association ...................................................................................... 65 Optional Stop. Red Cedar Lake Gneiss........................................................................................... 65 Stop 3. Varied Degrees of Preservation in Rocks of the River Valley Intrusion ............................ 66 Stop 4. Autolith Fragments and Layering, Layered Gabbronorite Zone......................................... 66 Stop 5. Layered Olivine Gabbronorite............................................................................................ 67 Stop 6. Footwall Alkali Feldspar Granite and Sudbury Diabase Dike............................................ 67 Stop 7. Azen Creek Copper-Nickel-PGE Occurrence .................................................................... 68 Optional Stop. Huronian Metavolcanic Rocks................................................................................ 69 Optional Stop. Variably Preserved Nipissing Gabbro .................................................................... 69 Stop 8. Mylonitized Anorthosite of the River Valley Intrusion ...................................................... 70 Optional Stop. Mississagi Formation and Mylonitic Contact with the River Valley Intrusion....... 70 Stop 9. Dana Lake Copper-Nickel-PGE Occurrence ...................................................................... 71 Stop 9A. L6+00N, Contact environment................................................................................ 74 Stop 9B. L7+00N, Contact environment................................................................................ 74 Stop 9C. Road Zone ............................................................................................................... 74 Stop 9D. Central Zone............................................................................................................ 75 Stop 9E. Trench Zone............................................................................................................. 76 Stop 9F. South Zone............................................................................................................... 76 References........................................................................................................................................................ 77 vii PDF compression, OCR, web-optimization with CVISION's PdfCompressor �PDF compression, OCR, web-optimization with CVISION's PdfCompressor �FIGURES 1. Distribution of U-Pb zircon ages from Neoarchean and Paleoproterozoic rocks in the Sudbury area, as well as the distribution of East Bull Lake intrusive suite bodies................................................................. 4 2. Geologic map of East Bull Lake Intrusion .................................................................................................. 16 3. Stratigraphic sections for the East Bull Lake and Agnew Lake intrusions.................................................. 17 4. Pearce-element ratio diagrams for the East Bull Lake, Agnew Lake and River Valley intrusions ............. 22 5. Chondrite-normalized rare earth element plots for the East Bull Lake and Agnew Lake intrusions........... 23 6. Geological map of the Moon Lake area, East Bull Lake Intrusion, showing the route of the traverse undertaken during Stop 11........................................................................................................................... 32 7. Geology of the Agnew Lake Intrusion ........................................................................................................ 36 8. Map showing the degree of primary textural preservation within rocks of Crerar and Dana townships..... 51 9. Interpreted stratigraphic sequence and distribution of primary mineral phases and compositions within the River Valley intrusion north of the Sturgeon River..................................................................................... 56 10. Generalized cross-sections for the Street metagabbro and the River Valley, East Bull Lake and Agnew Lake intrusions of the East Bull Lake intrusive suite .................................................................................. 59 11. Chondrite-normalized rare earth element plots for the River Valley intrusion for rocks north of the Sturgeon River............................................................................................................................................. 61 12. Map of the Dana North property of Pacific North West Capital Corporation, showing the location of stops 9A through 9F, as well as the extent of the drilling program undertaken between 1999 and 2002............. 73 PHOTOS Frontispiece. Typical copper-nickel-PGE mineralization from the River Valley intrusion ................................. xv 1. Orthopyroxene phenocrysts in matrix-rich orthopyroxene hornblendite in Henry Township..................... 13 2. Igneous layering in the East Bull Lake Intrusion ........................................................................................ 19 3. Dendritic texture within the Upper Series of the East Bull Lake Intrusion ................................................. 20 4. Centimetre- to decametre-scale igneous layering within rocks of the Agnew Lake Intrusion .................... 38 5. Dendrite texture developed with the Lower Gabbronorite Zone of the Agnew Lake Intrusion .................. 38 6. Stop 1, Day 2. Large pyroxenite pod hosted by rocks of the Inclusion-Bearing unit, which at this locality are hosted in a predominantly leucogabbronoritic matrix ........................................................................... 43 Stop 2, Day 2. a) Stone quarry in Nodular Anorthosite unit. b) Close-up of nodular anorthosite, showing closely packed plagioclase glomerophenocrysts with minor interstitial, altered, pyroxene ........................ 45 Nodular anorthosite from the Inclusion-Bearing Gabbronorite Zone, Lower Series, of the Agnew Lake Intrusion ...................................................................................................................................................... 46 Diabase dike containing large, rounded plagioclase crystals, cutting across rocks of the Inclusion-Bearing Gabbronorite Zone, Lower Series, of the Agnew Lake Intrusion................................................................ 47 7. 8. 9. ix PDF compression, OCR, web-optimization with CVISION's PdfCompressor �PDF compression, OCR, web-optimization with CVISION's PdfCompressor �PHOTOS Cont'd 10. Progressive textural changes. a) Igneous-texture in medium-grained norite of the River Valley intrusion, Crerar Township. b) Partly recrystallized rock of the River Valley intrusion, preserving igneous-texture. c) Recrystallized but massive leucogabbronorite from leucogabbronorite zone of the River Valley intrusion, Crerar Township. d) Typical undeformed but recrystallized leucogabbronorite from the Positano Quarry, Loughrin Township ......................................................................................................... 52 11. Layering in the River Valley intrusion. a) Igneous layering within the norite zone. b) Igneous layering near the top of the anorthosite zone and the base of the olivine gabbronorite zone. c) Layering within leucogabbronoritic rocks immediately above the heterogeneous basal zone. d) Large-scale igneous layering within leucogabbronoritic rocks exposed in the Dana Quarry....................................................... 53 12. a) Typical leucogabbroic gneiss of an East Bull Lake intrusive suite intrusion in Henry and Loughrin townships. b) Protomylonitic to mylonitic textures present in a high-strain zone in the River Valley intrusion. c) Strain gradient within rocks of the River Valley intrusion, northern Crerar Township. d) Epidote-clot developed in leucogabbro gneiss of the River Valley intrusion, Crerar Township................. 54 13. Photographs illustrating the textural variation observed in the Inclusion-Bearing unit in the Dana North area .............................................................................................................................................................. 75 TABLES 1. Summary of stratigraphic, mineralogical and geochemical data for the East Bull Lake intrusive suite...... 5 2. Summary of geochronology on rocks of the East Bull Lake intrusive suite................................................ 7 3. Timing of major geological events and summary of age constraints on the main rock units present in the study area..................................................................................................................................................... 8 Representative and average anhydrous whole-rock geochemical analyses from the East Bull Lake and Agnew Lake intrusions................................................................................................................................ 24 Representative chemical analyses from the River Valley intrusion ............................................................ 62 4. 5. xi PDF compression, OCR, web-optimization with CVISION's PdfCompressor �PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Abstract Intrusions of the ~2480 million-year-old East Bull Lake intrusive suite occur as an east-northeast-trending belt along the boundary of the Archean Superior and the Proterozoic Southern provinces in Ontario, Canada. The East Bull Lake intrusive suite is part of a regional Paleoproterozoic, bimodal magmatic event resulting from a mantle-plume driven, intracontinental rifting event that formed a major basin, filled by sedimentary and igneous rocks of the Huronian Supergroup. Intrusions of similar age and composition in Finland and Wyoming were once contiguous with the East Bull Lake intrusive suite prior to tectonic dispersion during the Proterozoic. Field and geochemical evidence indicates that the 3 largest intrusions (East Bull, Agnew, River Valley) crystallized from similar, low-Ti, high-Al, PGE-rich, tholeiitic parent magmas that originated in deeper, more mafic chambers. The primary magmas were likely second-stage melts derived from a sublithospheric depleted mantle source modified by Neoarchean subduction and accretion events. Petrologically, the intrusions reflect plagioclase-dominated fractional crystallization that generated a pronounced Fe-enrichment trend in the residual magmas. Olivine and orthopyroxene occur as cumulus phases in only the interpreted lower parts of the stratigraphy. Rhythmically and irregularly modally layered leucogabbronorite and gabbronorite make up much of the middle part of the stratigraphy. Ferrogabbro and ferrosyenite occur only in the uppermost Agnew Lake Intrusion. A marginal facies, comprising brecciated and, locally, thermally recrystallized and/or partially melted footwall rocks, is common and is testimony to a high-energy flow regime during initial emplacement. This marginal unit typically grades into a heterolithic, inclusion-rich gabbronorite with erratically distributed leucocratic to melanocratic autoliths and footwall xenoliths and eventually into a thick interval of undifferentiated, plagioclase-rich cumulates, locally with spectacular glomerocrystic textures. Disseminated, bleb and interstitial magmatic chalcopyrite-pyrrhotite-pentlandite mineralization (1 to 5% sulphide minerals, up to 10 g/t Pt+Pd) occurs in autolith-rich gabbronorite breccia within 5 to 50 m of the contact of the intrusions. The mineralization is commonly spatially associated with pyroxenite cumulates and autoliths that are otherwise poorly represented in the stratigraphy. Vigorous convection and explosive breccia-producing emplacement of sulphide-saturated magma formed PGE-rich zones at the margins (sidewall or floor?). The magmatic sulphide zones are overprinted and enclosed by a broader envelope of metamorphic and/or hydrothermal sulphide mineralization of similar mineralogy (± pyrite) that extends into adjacent leucogabbronoritic to anorthositic units, and less so into the country rocks. The hydrothermally enriched zones contain 2 to 10% sulphide minerals with 1 to 10 g/t Pt+Pd, and represent the main exploration target. The high-grade zones are enclosed by broader, lower grade mineralization with Pt+Pd levels in the background range of 20 to 50 ppb. Day long field excursions to each of the East Bull Lake, Agnew Lake, and River Valley intrusions are also included in this report. The combination of road accessible and walk-in stops is designed to illustrate important aspects of the geology, stratigraphy and mineralization found in each of the intrusions. xiii PDF compression, OCR, web-optimization with CVISION's PdfCompressor �PDF compression, OCR, web-optimization with CVISION's PdfCompressor �S Frontispiece. Typical copper-nickel-PGE mineralization from the River Valley intrusion. a) Typical outcrop exposure of mineralized olivine gabbronorite from the River Valley intrusion in the vicinity of the Jackson’s Flats occurrence. Note sulphide “burns” to the right of the 8.5 cm long scale card. b) Drill core from drill hole DL-14 in the Dana North area (Stop 9C, Day 3), showing the typical low tenor of sulphide and the presence of blue-grey quartz. A sample like this typically contains greater than 2 g/t of Pt and Pd mineralization. c) Close-up of mineralization from the Inclusion-Bearing unit at Dana South (Stop 9F, Day 3). All photographs courtesy of S. Jobin-Bevans. xv PDF compression, OCR, web-optimization with CVISION's PdfCompressor �PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Geological Guidebook to the Paleoproterozoic East Bull Lake Intrusive Suite Plutons at East Bull Lake, Agnew Lake and River Valley, Ontario R.M. Easton1, L.S. Jobin-Bevans2 and R.S. James3 Ontario Geological Survey Open File Report 6135 2004 1 Precambrian Geoscience Section, Ontario Geological Survey Ministry of Northern Development and Mines, Sudbury, Ontario, Canada P3E 6B5 2 Caracle Creek International Consulting Inc., Sudbury, Ontario, Canada P3E 2V7 3 Department of Earth Sciences, Laurentian University, Sudbury, Ontario, Canada P3E 2C6 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Introduction Since the summer of 1998, mafic intrusions of the East Bull Lake intrusive suite, specifically the East Bull Lake, Agnew Lake and the River Valley intrusions, have been the subject of ongoing copper-nickelplatinum group element (PGE) mineral exploration. This increased economic interest in these intrusions has led to renewed interest in the geology, stratigraphy and geochemistry of the East Bull Lake intrusive suite, as summarized by James et al. (2002a, 2002b) and Easton (2003). After some 3 decades of study there are only a very few field trip guides to any of these intrusions. Peck et al. (2000) and Brisbin et al. (2001) produced guidebooks for the East Bull Lake Intrusion for PGE Exploration Workshops at Laurentian University in 2000 and 2001. A field guide for the Agnew Lake Intrusion has never been prepared. Hrominchuk and Jobin-Bevans (2000) prepared a guidebook for the River Valley intrusion to compliment the first PGE Exploration Workshop at Laurentian University in 2000. Earlier guides by Lumbers (1978) and Davidson (1986) included a few stops in the River Valley intrusion, but were prepared prior to detailed mapping of the intrusion. The geology of the East Bull Lake suite intrusions is a significant topic in understanding the earliest evolution of the Southern Province in central Ontario. There are now numerous professional publications that describe and discuss the geology, geochemistry and mineral deposit potential of these intrusions (Table 1, and references in James et al. 2002a, 2002b). This integrated field guidebook to the East Bull Lake, Agnew Lake and River Valley intrusions is intended to compliment those studies. The field trip stops have been selected to show typical stratigraphic units in each intusion, as well as the geology of mineralized zones near the contacts of all 3 intrusions. The stops are also designed to compare and contrast the geological characteristics of the 3 major intrusions. The field trip uses a combination of road accessible outcrops, as well as some stops that require some short traverses through the bush, generally utilizing trails of varied condition. Although most of the stops can be accessed using a two-wheel drive vehicle, use of a vehicle with high ground clearance and fourwheel drive capability is recommended for both the Agnew Lake and River Valley intrusion stops, especially in the spring and fall. The structure of the guidebook is as follows: following the introduction, a brief overview of the East Bull Lake intrusive suite is provided. This is followed by descriptions of each one-day field trip to the East Bull Lake, Agnew Lake and River Valley intrusions. These descriptions include an overview of the geology, stratigraphy, geochemistry and mineralization in each intrusion, as well as a detailed road log and stop description. Day 1, the East Bull Lake Intrusion, was prepared mainly by R.S. James. Day 2, the Agnew Lake Intrusion, was prepared mainly by S. Jobin-Bevans and R.S. James. Day 3, the River Valley intrusion, was prepared mainly by R.M. Easton and S. Jobin-Bevans. The guidebook was compiled and edited by R.M. Easton. SAFETY For users of this guidebook, please bear in mind that some of the stops listed in this guidebook involve hiking in the bush. Therefore, standard bush safety practices should be followed by users of this guidebook. Such practices include travelling in pairs; advising others of your starting time and location and your expected return time; carrying sufficient water for the trip; being prepared for sudden changes in the weather; and carrying the appropriate emergency and safety gear. Most of the trip routes are on Crown land, but access is on or near private property for some routes. As in all such situations, please respect the property rights of others, so that future access for other geologists is not adversely affected. 1 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �ACKNOWLEDGEMENTS The authors acknowledge the contributions of information kindly made available to them by Mustang Minerals Corporation with regard to access and information on the geology and mineralization of the East Bull Lake Intrusion. Pacific North West Capital Corporation and its partner Anglo American Platinum Corporation Limited are also thanked for similar information on the Agnew Lake and River Valley intrusions. Some of the material on the River Valley intrusion is related to work conducted as part of Ontario Geological Survey bedrock mapping projects PU99-007, 99-019, 01-009 and 02-008, as described in Easton (2003). This guidebook was prepared initially for use with a field trip held in conjunction with the Geological Association of Canada–Mineralogical Association of Canada joint annual meeting in St. Catharines, May 12 to 14, 2004. East Bull Lake Intrusive Suite NOMENCLATURE The term East Bull Lake intrusive suite was only recently introduced by Easton (1999), and is similar to the term, East Bull Lake suite, used by Vogel et al. (1999). Prior to 1991, rocks of this suite were generally only described as individual intrusions, or as gabbro-anorthosite bodies that were intrusive into the lower part of the Elliot Lake Group (e.g., Bennett, Dressler and Robertson 1991). Prevec et al. (1995) referred to rocks of the East Bull Lake intrusive suite as “trans-Huronian” intrusions. Keays et al. (1995) and Peck et al. (1995) grouped them as part of the Huronian–Nipissing Magmatic Province, Domain or Belt. Nomenclature of individual intrusions within the East Bull Lake intrusive suite has been inconsistent over the years; however, James et al. (2002a) attempted to standardize the nomenclature for the intrusions within the suite, and to make this terminology consistent with the North American Stratigraphic Code (NACSN 1983). For example, the intrusion located west of Agnew Lake has been termed the Shakespeare–Dunlop intrusion, the Shakespeare–Dunlop gabbro-anorthosite intrusion, the Agnew Lake Intrusion and the Agnew intrusion, which was standardized in James et al. (2002a) as the Agnew Lake Intrusion. The standardization of nomenclature recommended by James et al. (2002a) is used herein, and is summarized in Table 1. Table 1 also includes the location of reference sections for the better-studied intrusions, details on contact relationships, and brief descriptions of the various intrusions. Recent studies on the East Bull Lake intrusive suite have used a three-tiered classification scheme to describe the rocks of the suite (e.g., Chubb 1994; Vogel 1996; James et al. 2002a), which has been used in this report. The first-tier differentiates rock types based on colour index and variation in modal abundance of mafic minerals and plagioclase. The second-tier groups individual rock layers into mappable series, zones, subzones and units (Irvine 1982). It should be noted that units outlined by this second-tier classification are not formal stratigraphic units as defined by the North American Stratigraphic Code (NACSN 1983), although some units, in particular the zones, are roughly equivalent to lithodemic units. The third-tier is based on petrography or distinctive textures, but also uses the IUGS classification scheme for plutonic rocks (Le Maitre 1989) where primary mineralogy or whole-rock geochemistry is present. Most primary igneous mafic minerals are replaced by amphibole, chlorite and epidote group minerals (e.g., Kamineni et al. 1985; Kamineni 1986), which reflects regional metamorphism in East Bull Lake intrusive suite bodies in the Southern Province. Plagioclase may be unaltered or partially to completely 2 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �saussuritized. Rocks of the East Bull Lake intrusive suite within the Grenville Province can range from almost wholly unaltered to completely recrystallized to an upper amphibolite facies, amphiboleplagioclase assemblage (e.g., Easton and Hrominchuk 1999; Tettelaar 2000; Easton 2002). In either geological province where metamorphic alteration is pervasive, rock nomenclature is based on geochemically determined CIPW-normative mineralogy combined with petrographic data. Unless otherwise stated, all UTM co-ordinates are in Zone 17, datum NAD 83. Unless otherwise noted, all analytical work in this report was completed by the Geoscience Laboratories, Geoservices Centre, Sudbury. REGIONAL GEOLOGICAL SETTING The Paleoproterozoic East Bull Lake intrusive suite (Easton 1999; James et al. 2002a) consists of several bodies of dominantly gabbronorite to gabbroic anorthosite that occur in both the Southern and Grenville provinces between Elliot Lake and the Ottawa River (Figure 1). The 3 largest bodies are the East Bull Lake and Agnew Lake intrusions (Southern Province) and the River Valley intrusion (Grenville Province) (see Table 1). All were emplaced between ~2491 and 2475 Ma (Table 2). Smaller bodies include the intrusions in Drury, Falconbridge, May, Street and Wisner townships (see Table 1, see Figure 1). Most bodies of the East Bull Lake intrusive suite were emplaced into Archean rocks present in either the Superior or Grenville provinces. Table 3 summarizes the main geological events that have occurred in this part of the Canadian Shield during the Precambrian. The distribution of the bodies of the East Bull Lake intrusive suite approximates the base of the Huronian Supergroup in the Southern Province (see Figure 1). Contact relationships between rocks of the East Bull Lake intrusive suite and the Huronian Supergroup are either faulted or equivocal. Consequently, it is not known for certain if the East Bull Lake intrusive suite intruded the Huronian Supergroup, or was unconformably overlain by it, or both (e.g., Card 1978; McCrank et al. 1989; Peck, James and Chubb 1993). Emplacement of the bodies of the East Bull Lake intrusive suite, subsequent eruption of volcanic rocks of the Huronian Supergroup, and formation of the depositional basin filled by Huronian Supergroup sediments is attributed by most authors (e.g., Card et al. 1972; Young 1983; Fahrig 1987; Hoffman 1989; Bennett, Dressler and Robertson 1991; Heaman 1997; Card and Poulsen 2000; Ernst and Buchan 2001) to an intracontinental rifting event resulting from a mantle-plume centred near Sudbury. A suite of igneous rocks (hereafter called the “rifting suite”) records the initial trace of this plume-induced rifting event; from oldest to youngest, they include: 1. The East Bull Lake intrusive suite (~2490 to 2470 Ma). 2. Elliot Lake Group metavolcanic and minor plutonic rocks, for example, the Elsie Mountain, Stobie and Copper Cliff formations, May Township metavolcanic rocks, and minor synvolcanic mafic intrusions (~2490 to 2450 Ma). Metavolcanic rocks of the Elliot Lake Group constitute the lowermost of 4 stratigraphic groups in the Huronian Supergroup. 3. Matachewan (2473 Ma) and Hearst (2446 Ma) mafic dike swarms (Heaman 1997). 4. Granitic intrusions within the Elliot Lake Group and the Superior Province (~2385 to 2460 Ma), including the Creighton and Murray plutons and the Street Township granitic plutons that may be coeval with Elliot Lake Group felsic volcanism. 3 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x < x 2473 >Matachewan x x x x x x Hearst x x x x -x Province x x X X x X x x x x x X x x x /x x )< / x x xI x A x,'x x I x X )< x X x x X x X x X x x 2681 ±13 x x X / x + )( + )< x + < > x + + ++ Province i- + > + + )< ) x X x +25 + -r —In. - + + + + + + + + + + + + + + - + + - + + + + + + +++ + + + + + + + + + + + + + + + + + + 2475±2+ + • •+ + + - + + - Lake Nipissing + 1+ East Bull Lake intrusion tonalite gneiss Agnew Lake intrusion Archean basement Falconbridge Twp. intrusion ® MayTwp. River Valley intrusion ® Levackgneiss Sudbury Igneous Complex intrusion plutonism + metamorphism Copper Cliff rhyolite 8 (fl\ Drury Twp. '—' intrusion 1 Temagami Island Gabbro + + Flett Twp. intrusions Grenville Province rocks Wisnerlwp. Huronian metasedimentary rocks intrusion Huronian volcanic rocks southern Wanapitei complex Street Twp. intrusion EBLI suite rocks x Superior Province rocks Figure 1. Distribution of U-Pb zircon ages from Neoarchean and Paleoproterozoic rocks in the Sudbury area, as well as the distribution of East Bull Lake intrusive suite bodies. Dashed circle indicates 125 km radius from a point central to the Sudbury Structure; most intrusions lie within this radius. Sources used: 1, 2, 8, 9) Krogh, Davis and Corfu 1984; 4) Wodicka and Card 1995; Krogh, Davis and Corfu 1984; 5) Chen, Krogh and Lumbers 1995; 6 and 10) Prevec 1993; 7) Heaman in Easton, Davidson and Murphy 1999; below and above circle 16, Corfu and Easton (2000). Age of Hearst and Matachewan dikes from Heaman (1997). Abbreviations: GFTZ = Grenville Front tectonic zone; EBLI = East Bull Lake intrusive suite. Current geochronological data suggests 2 main pulses in magmatism. 1. A predominantly mafic pulse at ~2475 Ma, represented by the East Bull Lake intrusive suite, Matachewan dikes, and the mafic part of the Elliot Lake Group. 2. A felsic-dominated pulse at ~2450 Ma, represented by felsic metavolcanic rocks of the Elliot Lake Group, related granitic intrusions, and Hearst dikes. The rifting event produced an east-trending, southward-deepening basin that likely resulted in formation of oceanic crust (e.g., Card et al. 1972; Young 1983; Fahrig 1987; Hoffman 1989; Bennett, Dressler and Robertson 1991). The geometry of the Matachewan and Hearst dikes and their distribution with respect to the metavolcanic rocks of the Elliot Lake Group is consistent with the north-northwest trend of the dikes reflecting the orientation of a failed arm of the rift (e.g., Fahrig 1987; Ernst and Buchan 2001). If this geometry is correct, then the east-trending East Bull Lake intrusive suite bodies not only occur along the continent-side of the rift basin, but also within a 125 km radius of a plume centered on Sudbury (see Figure 1). At present, it is uncertain if the rocks of East Bull Lake intrusive suite occur directly along a cryptic structure, or structures, related to this rift basin, or if the present distribution of the bodies reflects some other emplacement mechanism (James et al. 2002a). 4 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �5 PDF compression, OCR, web-optimization with CVISION's PdfCompressor Size (km2) Dominant Rock Types 15 ~5 May Twp. or Salmay Lake Wisner Twp. 43 East Bull Lake, includes outliers at Splake and Tee Lakes 1-2 8-10 Drury Twp. or Worthington Falconbridge Twp. or Norduna 50 Agnew Lake or Shakespeare –Dunlop Leucogabbronorite Leucogabbro, gabbro, melanogabbro. Leucogabbronorite gabbronorite. Leucogabbronorite gabbronorite, olivine gabbronorite. Leucogabbro, gabbro. Leucogabbronorite gabbronorite, olivine gabbronorite. Southern Province hosted intrusions Intrusion Shock metamorphosed, upper greenschist? Lower to upper greenschist Shock metamorphosed, upper greenschist Lower to upper greenschist Shock metamorphosed, locally upper greenschist Upper greenschist to lower amphibolite Metamorphic Grade n = 8 major and trace element analyses, REE and isotope data also available n = ~15 major and trace element analyses, REE and isotope data also available n = ~5 major and trace element analyses, REE and isotope data also available n = ~200 major and trace element analyses, REE and isotope data also available n = ~100 major and trace element analyses, REE and isotope data also available Geochemisty 5 No data No data Plag = An60-80 Rhythmically layered zone: ol = Fo59-65, opx = En64-66Fs30-32Wo4, cpx = En41-45Fs15-16 Wo39-44 Olivine gabbronorite zone: ol = Fo65-74, opx = En67-706Fs26-29Wo4, cpx = En41-43Fs14-15 Wo43-44 Plag = An60-76 No data Plag = An2-79 Primary Mineral Chemistry Unknown Contacts typically zones rich in Sudbury breccia. Intrusion in contact aureole of the Sudbury Igneous Complex. Melanogabbro better preserved than leucocratic rocks. Ramsey-Algoma granitoid complex rocks in footwall, diking and brecciation in footwall at contacts, to east, possible unconformable contact with Huronian Sgp. metasedimentary rocks. Contacts typically zones rich in Sudbury breccia. Intrusion in contact aureole of the Sudbury Igneous Complex. Parisien Lake syenite, Whiskey Lake greenstone belt, RamseyAlgoma granitoid complex rock, diking and brecciation in footwall at contacts, possible unconformable contact with Huronian Supergroup metasediments to the north. Contact relationships Unknown Unknown Not defined Not defined Not defined Not defined ~850 m Unknown Not defined Not defined Location of Reference Sections, UTM Zone 17, NAD 83 Unknown at least 1000m, as much as 2100 m in some interpretations Stratigraphic Thickness Table 1. Summary of stratigraphic, mineralogical and geochemical data for the East Bull Lake intrusive suite (modified from James et al. 2002a; Easton 2003). Vogel, James and Keays 1998 Cape 1973, Robertson 1976, Prevec 1993 DeGagne 1982, Prevec 1993 Born 1979; James and Born 1985; McCrank et al. 1989; Peck, James and Chubb 1993; Peck et al. 1995; Prevec 1993, Vogel , James and Keays 1998 Brons 1984; Prevec 1993 Vogel 1996; Vogel, James and Keays 1998; Vogel et al. 1999 References �6 PDF compression, OCR, web-optimization with CVISION's PdfCompressor Size (km2) Dominant Rock Types ~25 ~5 ~5 Contact Third Concession ~100 Loughrin Lake River Valley Gneissic leucogabbronorite, gabbronorite, associated orthopyroxene hornblendite. Gneissic, leucogabbronorite, gabbronorite, associated orthopyroxene hornblendite. Gneissic leucogabbronorite, gabbronorite, associated orthopyroxene hornblendite. Leucogabbronorite gabbronorite, olivine gabbronorite. Grenville Province hosted intrusions Intrusion Table 1. continued Upper amphibolite Upper amphibolite Upper amphibolite Greenschist to upper amphibolite, with some areas of primary mineralogy preserved, mainly in Dana Township. Metamorphic Grade n = 1 major and trace element and REE analyses No data n = 12 major and trace element analyses, REE data also available n = ~70 major and trace element analyses, REE data also available Geochemisty 6 No data No data No data Marginal zone: Plag = An79 (core); An68, (intercumulate) Olivine gabbronorite zone: ol = Fo74 plag = An70-75 opx = En60-80 Primary Mineral Chemistry Primary Contacts: Dana Twp.: Sharp contact with paragneiss or alkali feldspar granite, no dikes in footwall, marginal zone contains footwall xenoliths. Fault contacts with Huronian Sgp. metasedimentary and metavolcanic rocks, equivocal contact with Nipissing gabbro. Crerar Twp: Sharp contact with alkali feldspar granite, alteration of granite in footwall, fine-grained phase at contact, no dikes in footwall, marginal zone contains footwall xenoliths. Hosted by Crerar gneiss association. North contact mainly tectonic. South contact sharp, with scattered orthopyroxene hornblendite bodies. Hosted by Street gneiss association. North contact locally cut by Geon 24 to 23 felsic intrusions. South contact sharp, with scattered orthopyroxene hornblendite bodies. Hosted by Street and Crerar gneiss association. Same as Contact intrusion. Both contacts locally intrude migmatitic garnet amphibolite. Hosted by Street gneiss association. Contact relationships ~500 m less than 500 m ~500 to 1000m ~1000 m Stratigraphic Thickness Not defined Not defined Marginal zone: 555310E, 5172625N; 556610E, 5174225N to 556510E, 5171225N; 558760E, 5167825N to 558510E, 5167475N. Olivine Gabbronorite zone: 559260E, 516825N; 563260E, 5166225N; 560410E, 5167825N. Gabbronorite zone: 558510E, 5165575N Not defined Location of Reference Sections, UTM Zone 17, NAD 83 Easton 2003 Easton 2003 Easton 2003 Easton and Hrominchuk 1999; Hrominchuk 1999, 2000; Easton 2003 References �7 PDF compression, OCR, web-optimization with CVISION's PdfCompressor ~5 ~10 ~5 Street metagabbro Red Deer Lake southern Wanapitei complex Gneissic leucogabbro, gabbro, norite. Gneissic leucogabbronorite, gabbronorite. Leucogabbronorite gabbronorite, norite, associated orthopyroxene hornblendite. Dominant Rock Types Upper amphibolite Upper amphibolite Upper amphibolite Metamorphic Grade n = 8 major element analyses n = 1 major element analysis n = 25 major, trace element and REE analyses Geochemisty Coronitic olivine present No data Coronitic orthopyroxene present Primary Mineral Chemistry Unknown Relative Ages River Valley Street Twp. River Valley Drury Twp. Falconbridge Twp. Intrusion Absolute Ages Agnew Lake East Bull Lake Rb/Sr whole-rock >2446±3 >2460±20, 2475+25/-10 Ashwal and Wooden (1989) Pb/Pb whole-rock Nd/Sm whole-rock and mineral 2562±165 2377±68 2165±130 2185±105, 1960±100 2468±5 U/Pb baddeleyite and zircon U/Pb zircon 7 Heaman (1997) Corfu and Easton (2000), Easton and Hrominchuk (1999) Corfu and Easton (2000) Krogh, Davis and Corfu (1984) Krogh, Davis and Corfu (1984) McCrank et al. (1989) Kamineni (1986) McCrank et al. (1989) McCrank et al. (1989) Prevec (1993) Prevec (1993) Prevec (1993) Heaman, unpublished in Easton, Davidson and Murphy (1999) Ashwal and Wooden (1989) Ashwal and Wooden (1989) U/Pb zircon U/Pb baddeleyite and zircon Nd/Sm whole-rock and mineral Ar/Ar hornblende K/Ar whole rock K/Ar whole rock U/Pb zircon U/Pb zircon U/Pb zircon U/Pb baddeleyite and zircon 2491±5 2480+10/-5 2472±76 1859±5 1725±18 1155±14 1855±10 2441±3 ~1850 2475+2/-1 U/Pb zircon Reference Method Age (in Ma) Unknown Unknown less than 500 m Stratigraphic Thickness Not defined Not defined Location of Reference Sections, UTM Zone 17, NAD 83 530510E, 5155225 to 5155725N Rousell and Trevisiol 1988 Lumbers 1975 Easton 1999, 2003; Easton and Murphy 2002 References Dikes of the Matachewan dike swarm cut the River Valley intrusion. Felsic dikes likely related to felsic intrusions coeval with Huronian Stobie Formation cut the River Valley intrusion. age from orthopyroxene hornblendite body thought to be genetically related to East Bull Lake intrusive suite. n=4, second age is resetting event. lower intercept, dates Sudbury event. from layered gabbronorite zone, either in Erana or Dana quarries, north of Sturgeon River. n=6, igneous textured samples only. n=4, igneous textured samples only, second age is resetting event. from alkali feldspar granite that may not be part of the intrusion. from olivine gabbronorite zone, few grain, hence high error. from olivine gabbronorite zone. no details on location. from olivine gabbronorite zone. from olivine gabbronorite zone. shocked zircons, dates Sudbury event. Comment Basal contact intrudes migmatitic garnet amphibolite, ~2475 Ma granites contain fragments of intrusion. Hosted by Street gneiss association. Unknown Contact relationships Table 2. Summary of geochronology on rocks of the East Bull Lake intrusive suite (from Easton 2003). Size (km2) Intrusion Table 1. continued �Table 3. Timing of major geological events and summary of age constraints on the main rock units present in the study area (from Easton 2003). Event and/or Map Unit Age Constraint (in Ma) Comment and/or Source Grenville dike swarm 586±4 Kamo, Krogh and Kumarapeli (1995) Pegmatite vein emplacement 989±2 Corfu and Easton (2000) Age of peak metamorphism in the hanging wall of the Grenville Front tectonic zone 1000 to 990 Corfu and Easton (2000) Age of peak Grenvillian metamorphism in Crerar and Dana townships 1040 and 1030 Sudbury dike swarm 1238±4 emplaced in or along northwest-trending faults in the Southern Province. Deformed and metamorphosed within the Grenville Province. Krogh et al. (1987). Killarney magmatic belt second-stage magmatism, coincident with magmatism in the Eastern Granite Rhyolite Province and in the Central Gneiss Belt 1471±3 van Breemen and Davidson (1988) Regional albitization metasomatic event 1701±4 U/Pb monazite, Schandl, Gorton and Davis (1994); fluid focussed along northwesttrending faults Killarney magmatic belt volcanism and high-level plutonism 1740, 1747±3, 1749±12 van Breemen and Davidson (1988); Sullivan and Davidson (1993); Davidson and van Breemen (1994) Northwest-trending regional faults Pre-1700, post-1850 Penokean Orogeny (folding and metamorphism of Huronian Supergroup rocks) ~1835 Peak metamorphism. Holm et al. (2001). Impact event and formation of Sudbury breccia 1850±1 Krogh, Davis and Corfu (1984) Thrust faulting post-F2 pre-regional faulting Sudbury breccia localized along these faults, suggesting they are pre-Sudbury Structure F2 folding post-2200, pre-1700, pre 1850? Pre-regional faulting, Nipissing sills folded, relationship to Sudbury Structure uncertain F1 folding Pre-2200 Emplacement of Nipissing gabbro sills 2219±4 to 2210±4 Huronian sedimentation >2220 but <2460 Huronian felsic volcanism and related plutonic rocks ~2477 to 2375 (2450±25, 2460±20, 2477±9, 2415±5, 2376±2) Emplacement of East Bull Lake intrusive suite rocks 2475±2 see also Table 2 Emplacement of orthopyroxene hornblendite bodies 2468±5 Emplacement of alkali feldspar granite bodies in Crerar and Dana townships 2640 High-grade Archean metamorphism and migmatization 2647±4 Emplacement of megacrystic granodiorite bodies in Crerar gneiss association 2660 Emplacement ages of Archean units in the Sudbury area 2711±7 to 2642±1 this study; see also Carr et al. (2000) Faults cut Sudbury Structure Nipissing sills in Street Township appear unaffected by this folding Corfu and Andrews (1986); Noble and Lightfoot (1992) Krogh, Davis and Corfu (1984), Corfu and Easton (2000), Krogh, Kamo and Bohor (1996), Smith (2002) Heaman (geochronologist, University of Alberta, personal communication, 1999) Corfu and Easton (2000) Bodies intrude Crerar and Pardo gneiss, this study Krogh, Davis and Corfu (1984); Wodicka and Card (1995) this study Krogh, Davis and Corfu (1984); Wodicka and Card (1995); Chen, Krogh and Lumbers (1995); Meldrum et al. (1997) 8 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Within the Southern Province, rocks of the East Bull Lake intrusive suite are affected by folding and upper greenschist to lower amphibolite facies metamorphism associated with the Penokean Orogeny, likely around 1835 Ma (Holm et al. 2001; see also Table 3). Within the Grenville Province, rocks of the East Bull Lake intrusive suite may have been locally affected by metamorphism and deformation at ~1700 Ma and ~1450 Ma, in addition to the main pulse of Grenville orogenesis at ~1070 to 1040 Ma (Corfu and Easton 2000). OVERVIEW OF EAST BULL LAKE INTRUSIVE SUITE Table 1 lists all known major members of the East Bull Lake intrusive suite along with notable features common to all and a listing of all studies performed to date. The distribution of the bodies is illustrated in Figure 1. Table 2 summarizes the geochronology of the intrusive suite. The 3 largest intrusions are the East Bull Lake, Agnew Lake, and River Valley intrusions, all of which have been mapped in the last 25 years at either 1:10 000 or 1:20 000 scale in whole or in part (Born 1979; Kamineni et al. 1984; Chubb 1994; Peck et al. 1995; Vogel 1996; Easton and Hrominchuk 2001a, 2001b), with accompanying petrological, geochemical or economic studies (James and Born 1985; McCrank et al. 1989; Chubb 1994; Peck, James and Chubb 1993; Peck et al.1995; Vogel 1996; Vogel, James and Keays 1998; Vogel et al. 1998, 1999; Easton and Hrominchuk 1999; Hrominchuk 1999, 2000; Peck et al. 2001; James et al. 2002a, 2002b). The preserved size of the intrusions varies from 1 to ~100 km2 (Table 1). The most completely preserved body is the Agnew Lake Intrusion with approximately 2 km of stratigraphic section; both the East Bull Lake Intrusion and River Valley intrusion contain roughly 1 km of section. The East Bull Lake, Agnew Lake and River Valley intrusions are sporadically layered at centimetre- to decametre-scale throughout their stratigraphy. Leucogabbronorite and gabbronorite are the dominant rock types in the lower and middle parts of most bodies of the East Bull Lake intrusive suite. Olivine gabbronorites and leucogabbronorites are common in the Main Series of the East Bull Lake intrusion and the Lower Series of the Agnew Lake Intrusion. Melagabbronorites and troctolites are common in the lower parts of the River Valley intrusion. Ferrogabbros, ferrosyenites and alkali granites form the top 150 m of the Agnew Lake Intrusion. The significant volume of melanocratic norites and troctolites recognized in the River Valley Intrusion is not present in those intrusions west of the Grenville Front, and may indicate that the former represent a deeper part of the stratigraphy of this intrusive suite. The crystallization order of primocryst phases in most of the intrusions is plagioclase, olivine, orthopyroxene, clinopyroxene and titanomagnetite. The attitude of metre-scale phase layering and stratigraphic units in the East Bull Lake and Agnew Lake intrusions suggest that they represent lopoliths joined by dike-like units (James et al. 2002a). Original geometry of the bodies present in the Grenville Province cannot be reliably ascertained, although the northeast part of the River Valley intrusion appears to be a shallowly-dipping (10 to 30°) sheet. Without exception, breccias containing footwall and mafic cognate xenoliths in a gabbronorite matrix occur at the base of all intrusions, where the footwall contact is preserved. The thickness and abundance of these breccia units varies considerably. Also near the contact, footwall breccias and zones of extensive footwall dikes may be present. Disseminated chalcopyrite and pyrrhotite, typically in modal amounts from 0.5 to 2.0%, occur in the matrix of the marginal and brecciated rocks. This sulphide mineralization commonly contains between 1 and 5 g/t of combined Pt+Pd+Au, and is the focus for current mineral exploration (James et al. 2002a, 2002b). 9 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �DEPTH AND MECHANISM OF EMPLACEMENT Researchers studying East Bull Lake suite intrusions in the Southern Province have suggested shallow depths of emplacement (<8 km), possibly at the contact between the Archean basement and overlying supracrustal rocks of the Huronian Supergroup and its precursors (e.g., Card 1978; Peck et al. 1995; Vogel et al. 1998, 1999). In contrast, researchers studying the River Valley intrusion have favoured moderate to deeper depths of emplacement (8 to 12 km), either at the boundary between Archean plutonic rocks and greenstone belts or at the boundary between gneisses and a plutonic rock-dominated midcrustal layer (e.g., Easton 2000; J.L. Hrominchuk, personal communication, 2000). Easton (2002) went further, suggesting that several depths of emplacement are recorded within the East Bull Lake intrusive suite, based on the fact that the intrusive bodies display subtly different features depending on which country rock gneiss association they are in contact with. Two models have been proposed with respect to the emplacement mechanism of the East Bull Lake intrusive suite; both are discussed in James et al. (2002a), and are summarized here. The first model has been proposed specifically for the area extending from Elliot Lake to Sudbury. Based on geologic reconstruction and “unfolding” of the effects of Penokean deformation on the East Bull Lake and the Agnew Lake intrusions, and their similar stratigraphy, it has been proposed that the middle and upper parts of both these intrusions were part of an extensive, subhorizontal sheet. This sheet may have been approximately 2 km thick, representing the upper 75% of a lopolith that had horizontal dimensions of 30 to 50 km in a northerly direction and greater than 100 km in an easterly direction (James et al. 2002a; Vogel et al. 1999). In this model the lower and mineralized sections of the intrusions occupied rift faultrelated structures or embayments that controlled magma intrusion, replenishment, and mineralization, and thus, may not have been as widespread as the main mass of the lopolith. Furthermore, the mineralized zones would not have been connected. This model has been expanded to include the possibility that the East Bull Lake, Agnew Lake, and River Valley intrusions, as well as the intervening smaller intrusions (see Figure 1, Table 1), are the remnants of one large interconnected Great Dike-like lopolith that can be traced for at least 300 km across central Ontario (see Figure 1) (James et al. 2002a). If so, then small size is not a concern in terms of modelling the potential size of mineralized sections, and the area of potential interest for exploration is much larger. The second model proposes that the East Bull Lake intrusive suite was emplaced as a group of smaller, separate bodies, rather than as one or two large, now dismembered, sill complexes (James et al. 2002a). One consequence of this model is that the various bodies could have been emplaced at different crustal levels. This model would also provide an explanation for the differences in stratigraphy between the various intrusions, as not every intrusion would differentiate to the same degree. Finally, an important implication of this model is that mineralization style and grade will vary between intrusions. MAGMA COMPOSITION AND ITS RELATIONSHIP TO MINERALIZATION Detailed studies (Peck et al. 1995, 2001; Vogel et al., 1999; James et al. 2002a, 2002b) demonstrate that the magma(s) that formed the East Bull Lake and the Agnew Lake intrusions were high-Al, low-Ti (Al2O3 ~17.5 wt %; TiO2 ~0.5 wt %) tholeiites. In these 2 intrusions, the main fractionation trend formed by leucogabbronorite and norites is interrupted by emplacement of more primitive (less fractionated), olivine-rich rocks (the Olivine Gabbronorite zones) which form a distinctive geochemical pattern not easily attributed to the same magma source. 10 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �For the River Valley intrusion, at least north of the Sturgeon River, its stratigraphy, including the Olivine Gabbronorite, Gabbronorite and Leucogabbronorite zones, could be attributed to a magnesian variant (8 to 10 % MgO) of the high-Al, low-Ti, tholeiite (James et al. 2002a). Such magma would have olivine followed by plagioclase on the liquidus, which satisfies the petrographic data for these rocks. The magma giving rise to the sporadically mineralized orthopyroxene-phyric norite and gabbronorite that is the main component of the Marginal Zone of the River Valley intrusion is part of an earlier magmatic sequence: its pyroxene-phyric, olivine-absent character and disseminated PGE mineralization distinguish it from norites and gabbronorites higher in the stratigraphy (James et al. 2002a). The magmatic evolution of River Valley intrusion, however, is not as well constrained as it is for the East Bull Lake and Agnew Lake intrusions. As noted by Hrominchuk in James et al. (2002a), a unit of massive, medium-grained, orthopyroxene-phyric norite with a boninitic composition (MgO ~11.5 wt %, Al2O3 ~13.5 wt. %, SiO2 ~51.5 wt %), located near the southeast margin of the River Valley intrusion, could represent the parent magma of the intrusion. Furthermore, Hrominchuk argued that the Olivine Gabbronorite, Gabbronorite and Leucogabbronorite zones were derived from this boninitic parental magma by a combination of fractional crystallization and mixing of variably fractionated magma batches within the magma chamber. With the available analytical data, it is equally plausible, however, that the Olivine Gabbronorite, Gabbronorite and Leucogabbronorite zones of the River Valley intrusion formed from a high-Al, low-Ti magma similar to that which produced the East Bull Lake and the Agnew Lake intrusions. The boninitic magma may have only been injected near the base of the intrusion to form the Marginal and Inclusion and/or Fragment-bearing zones (James et al. 2002a). An argument against a boninitic parental magma is that such magmas are currently found only in ocean island arc settings (e.g., Bonin islands, Japan, Papau New Guinea), unlike the inferred continentalrift plume-setting for the East Bull Lake intrusive suite. Nonetheless, boninites are thought to form from a relatively high degree of partial melting of previously melted metasomatized lithospheric mantle (Hamlyn et al. 1985). Metasomatism is generally thought to occur from fluids derived from dehydration of a subducted slab of oceanic crust (Hickey and Frey 1982) and can produce light rare earth element (LREE) enrichment in the resulting melt. Alternatively, crustal contamination of a second-stage mantle melt can produce LREE enrichment in an otherwise primitive melt (Vogel et al. 1999). Vogel et al. (1999) found crustal contamination to be an unlikely mechanism for the uniformly elevated LREE in the Agnew Lake Intrusion, and suggested that the parent magma must have come from a partial mantle melt enriched by LREE-rich fluids. In the case of the River Valley intrusion, the LREE and trace element patterns are much less uniform and contamination is an important process, especially in the Marginal and Inclusion and/or Fragment-bearing zones (Hrominchuk 2000), which is one reason to favour a boninitic parental magma (James et al. 2002a). Regardless, both the proposed high-Al, low-Ti and boninitic parental magmas are second-stage, mantlederived magmas as described by (Hamlyn and Keays 1986). Hamlyn and Keays (1986) argued that second-stage magmas are critical to the formation of PGE deposits, a view also favoured in James et al. (2002a, 2002b). These fertile magmas arise from previously depleted mantle, typically in areas underlying Precambrian cratons or island arcs. Both the interpreted high-Al and boninitic parental magmas of the East Bull Lake intrusive suite are PGE fertile (Pt+Pd ~20 to 50 ppb) and sulphur-poor (~100 to 200 ppm), and together with their continental rift-margin tectonic setting, are good examples of second-stage magmas. Their low sulphur-content likely reflects prior extraction of MORB-type basalt liquids from the mantle, which removed most of the mantle sulphide phase, but left a small amount of a PGE-rich residual sulphide phase (Keays 1995). Pd and Se data from barren cumulates and possible feeder dikes from the East Bull Lake Intrusion (Peck et al. 2001) illustrates the PGE-rich nature of the parental magmas compared to first stage (MORB-type) magmas derived from fertile mantle, which have significantly higher S(Se) and lower PGE/S(Se) ratios. 11 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �In terms of crust-mantle scale processes, James et al. (2002a) suggested that fractional crystallization of a mantle-derived, second-stage magma in the lower to middle crust generated PGE-enriched, sulphurundersaturated leuconorite or norite magmas in the upper part of a zoned ultramafic to mafic magma chamber. During ascent and intrusion into the upper crust, this buoyant residual magma (parent to much of the marginal parts of the East Bull Lake and Agnew Lake intrusions) became sulphur-saturated, and apparently escaped from the more primitive subchamber without the loss of the sulphide phase. In this model, rocks of the more primitive Olivine Gabbronorite zone in the East Bull Lake and Agnew Lake intrusions represent influxes of magma from the more mafic parts of the same magmatic system. They should be relatively barren in PGE, except where mixing with fractionated liquids from the high-Al magmas may have generated reef-type deposits. Similar layered olivine-rich rocks in the River Valley intrusion appear to be part of a fractionating system that forms norites and leucogabbros in the upper part of this body (James et al. 2002a). Detailed studies on PGE mineralization have been done only on the East Bull Lake Intrusion. Many fine-grained gabbros in the Marginal Series and feeder-type plagioclase-phyric dikes in the footwall, have 20 to 100 ppb Pt+Pd (Pd/Pt = 2:1 to 1:1) and contain small (<1%) amounts of fine-grained disseminated sulphide minerals (pyrite ± pyrrhotite ± chalcopyrite). These PGE and sulphide contents are interpreted as evidence that early magma pulses were sulphur-saturated when they entered the magma chamber (James et al. 2002a). The PGE values in these rocks far exceed those expected for basaltic liquids, even those derived from depleted mantle sources (Hamlyn and Keays 1986; Keays 1995), which is further evidence for sulphur saturation. Vigorous convection within the magma chamber, as evidenced by the Inclusion-bearing and Anorthosite zones at the East Bull Lake Intrusion, produced high silicate/sulphide liquid mass ratios that allowed growing sulphide liquid droplets to attain the observed high PGE contents (1 to 10 g/t) (James et al. 2002a). ORTHOPYROXENE HORNBLENDITE BODIES Bodies of orthopyroxene hornblendite consist of orthopyroxene (bronzite) phenocrysts, ranging from 0.5 to 5 cm in length, hosted in a fine-grained amphibole-plagioclase matrix (Photo 1). These bodies occur throughout Awrey, Dryden, Henry, Loughrin and Street townships (Easton 1998, 1999, 2002, 2003; Easton and Murphy 2002; Davidson 1998). Large glacial erratics of orthopyroxene hornblendite, some up to 6 m across, are common in Henry, Loughrin and Street townships. The erratics are generally found within several hundred metres of observed bodies in outcrop, or in areas where bodies may be hidden beneath overburden. In most of these aforementioned townships, the orthopyroxene hornblendite bodies are found in close proximity to mafic intrusive rocks of the East Bull Lake intrusive suite. Zircon from an orthopyroxene hornblendite body in Street Township has been dated at 2468±5 Ma (Corfu and Easton 2000), similar in age to other intrusive rocks of the East Bull Lake intrusive suite (James et al. 2002a). Even though some orthopyroxene hornblendite bodies occur within 100 m of the Grenville Front (Easton and Murphy 2002; Hubbard 1998), equivalent rocks to these bodies have not been reported in conjunction with East Bull Lake intrusive suite bodies located within the Southern Province. No orthopyroxene hornblendite bodies have been found in association with the River Valley intrusion, although some ultramafic layers present within the River Valley intrusion in Crerar Township are chemically similar to rocks of the orthopyroxene hornblendite suite, even though they are texturally and mineralogically distinct. Easton (2002) suggested that the orthopyroxene hornblendite bodies are restricted to country rocks of the Street gneiss association, although some bodies do occur in association with other East Bull Lake intrusive suite rocks along the boundary between the Street and Crerar gneiss associations. This would explain the absence of these rocks in the Southern Province and in association with the River Valley intrusion, as well as their abundance in Awrey, Dryden, Henry, Loughrin and Street townships, all of which are underlain by rocks of the Street gneiss association. 12 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �—I-', Photo 1. Orthopyroxene phenocrysts in matrix-rich orthopyroxene hornblendite in Henry Township. Hammer handle is 30 cm long. UTM 547314E 5158815N. Easton and Murphy (2002) suggested that these bodies exhibited only 2 types of contact relationships; however, work by Easton (2003) suggests that there might be 4 types of contact relationships. These contact types are summarized below: 1. The first type occur near sharp, possibly modified igneous contacts between metagabbroic and metaleucogabbroic rocks of the East Bull Lake intrusive suite and their host gneisses (Street gneiss association). Bodies of this type appear to have been emplaced into pre-existing embayments along the metagabbro-gneiss contact. All of the Street Township bodies are of this type. 2. The second type occurs within deformation zones characterized by flattened gneissic rocks (e.g., the body exposed on the south side of Highway 17 in Awrey Township, Stop 1, Day 3). 3. Some bodies in Loughrin Township, show a linear distribution and may represent boudinaged dikes. It is possible that types 2 and 3 are related, with type 2 representing a more deformed version of type 3. 4. Many of the larger bodies found in Henry and Loughrin townships, are not clearly associated with the contact of any of the East Bull Lake intrusive suite bodies. Some are located over 1 km from the contact. The field term for these rocks, orthopyroxene hornblendite, is based on the dominant mineral phases observed in hand sample. Some of the better-preserved bodies contain relict olivine grains. Mesonorm calculations suggest a metamorphic mineralogy dominated by pyroxene and amphibole consistent with the observed mineralogy in the field. CIPW normative calculations for these rocks suggest a primary mineral assemblage dominated by olivine (4 to 22%), hypersthene (32 to 62%), diopside (3 to 22%) and calcic plagioclase (13 to 21%) (Easton and Murphy 2002). The CIPW normative mineralogy suggests 13 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �that these rocks were originally olivine gabbronorites or olivine websterites, depending on whether they contained moderate or only minor amounts of plagioclase, respectively. Orthopyroxene hornblendite bodies located near the Grenville Front contain euhedral bronzite grains up to 5 cm long. The bronzite grains become subrounded to rounded and overall grain size decreases with increasing distance from the Grenville Front. Similar observations can be observed from the margin to the core in some of the smaller hornblendite bodies. This change in bronzite form with increasing distance into the Grenville Province can be attributed to greater degrees of metamorphic recrystallization toward the interior of the Grenville Province. Mineralogically, the orthopyroxene crystals in the bodies are complex and represent primary phenocrysts that recrystallized during regional metamorphism. Evidence that they were primary phenocrysts comes from the presence of zircon and chromite inclusions within the bronzite grains. Both types of inclusions contain well-developed cores and rims. In the case of the zircon grains, subrounded cores are surrounded by euhedral fractured rims. The cores yielded the aforementioned U/Pb age of 2468±5 Ma, whereas the rims yielded an age of 1471±10 Ma (Corfu and Easton 2000). Chromite cores are rimmed by iron chromite that typically forms only during regional metamorphism (Deer, Howie and Zussman 1966). Small euhedral zircons are also present in the fine-grained amphibole matrix and yielded an age of 1052±19Ma (Corfu and Easton 2000). Mineral chemistry data on the matrix amphiboles indicates that they are magnesium hornblende, tremolite, cummingtonite and magnesium cummingtonite. The amphiboles occur both as individual grains and complexly exsolved and intergrown grains. Olivine grains from a body located near the Grenville Front have compositions of Fo73. Orthopyroxene compositions lie in the bronzite field and are magnesium-rich (En75-80) (Easton and Murphy 2002; Hubbard 1998). Geochemistry Analyses from 11 separate bodies show only a limited compositional range in major element chemistry (Easton 2003). The SiO2 content of the unaltered orthopyroxene hornblendites ranges from 46.0 to 50.5 weight percent which is unusually high given the corresponding MgO content of 21.0 to 27.0 weight percent. On a Jensen diagram the data from the orthopyroxene hornblendites plot as a cluster in the peridotitic komatiite field although there is no evidence to suggest that these rocks were extrusive. Plots of Ti versus Cr and Cr-Ni are consistent with these rocks being crystallized from layered mafic sills. Trace element data from orthopyroxene hornblendite samples plot in 2 distinct groups on chondritenormalized rare earth element (REE) diagrams (Easton 2003). One group has a relatively flat, 10 times chondrite pattern with a negative Eu anomaly and total REE less than 35 ppm, whereas the second group has moderate light REE enrichment (to 60 times chondrite), no Eu anomaly, and total REE greater than 80 ppm. The REE content and high silica content of the orthopyroxene hornblendites suggests that they do not represent the most primitive rocks of the East Bull Lake intrusive suite. The first group, characterized by low total REE contents and magnesium contents greater than 20 weight percent, occur as isolated bodies, near, but not intimately associated with other East Bull Lake intrusive suite rocks. In contrast, the second group with the higher REE contents and lower magnesium contents (<19.0 wt. %) occur at primary intrusive contacts, or as layers within East Bull Lake intrusive suite bodies. For example, hornblendite samples from the Street metagabbro plot in the LREE enriched group, as do olivine gabbronorites and pyroxenites from the River Valley intrusion. It can be argued that the two groups are related to one another, with the lower magnesium group being a more fractionated differentiate of the magma that produces these unusual rocks. 14 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �All unaltered orthopyroxene hornblendite samples contain from 2900 to 4700 ppm Cr and 750 to 1000 ppm Ni. Copper content is generally low (Easton 2003). Chromium is present as chromium-spinel, iron-chromite and chromite and is concentrated within olivine and orthopyroxene crystals. Platinum group element content in all of the analyzed orthopyroxene hornblendite samples is low (generally at or below detection limits), regardless of degree of alteration. The orthopyroxene hornblendites differ from other rocks of the East Bull Lake intrusive suite in their high chromium and nickel contents, as listed above. One explanation for this difference is that precipitation and removal of pyroxene from the primary magma (possibly by crystal settling) increases the silica content of the melt. The increase in silica would move the composition of the melt away from the olivine-chromite cotectic into the field of chromite and lead to the co-precipitation of bronzite and chromite. This process, in conjunction with olivine precipitation, would effectively strip the primary magma of much of its chromium and nickel, leaving rocks of the East Bull Lake intrusive suite with low chromium and nickel contents. Chromium numbers of the orthopyroxene hornblendites are 20 to 38 and magnesium numbers are 78 to 82. The bodies have chemical affinities to high-aluminium chromitite from ophiolitic complexes, most notably, chromium number less than 50, TiO2 greater than 0.2 weight percent, CaO greater than 1.5 weight percent, and Al2O3 greater than 1.2 weight percent. High aluminium chromitites are found in back-arc spreading and rifting tectonic environments that are consistent with previous suggestions of a rift-setting for the lower Huronian Supergroup (e.g., Bennett, Dressler and Robertson 1991). 15 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Field Trip Guidebook EAST BULL LAKE INTRUSION—OVERVIEW Country Rocks, Structure and Geometry of the Intrusion The East Bull Lake Intrusion, located roughly 80 km west of Sudbury (see Figure 1), is the type example of the East Bull Lake intrusive suite. The intrusion is hosted by a Neoarchean granite-greenstone belt to the southwest, but principally by tonalitic gneiss and syenite (James and Born 1985). The geometry of the intrusion (Figure 2) has been interpreted as primary (Peck, James and Chubb 1993; Born 1979) or as a result of polyphase open folding related to Penokean deformation (Vogel, James and Keays 1998). The East Bull Lake Intrusion consists of 2 interconnected magma chambers that locally exceed 1 km in thickness (see Figure 2). The eastern end of the East Bull Lake Intrusion is connected to the western end of the Agnew Lake Intrusion by the Streich dike (see Figure 2). The dike-like conduit that connects the eastern and western lobes of the East Bull Lake Intrusion (see Figure 2) consists of the lowermost stratigraphic units of the intrusion, and may represent part of the feeder dike to the magma chambers. These stratigraphically lowest parts of the intrusion are significant, as they are proximal to mineralized zones and feeder dikes that are largely obscured by the main mass of the lopolith. An unknown thickness of the upper part of the intrusion apparently has been eroded away. The East Bull Lake Intrusion was metamorphosed at the greenschist to lower amphibolite facies (Card 1978; Kamineni 1986; Chubb 1994). Igneous textures are typically well preserved throughout the intrusion, although primary pyroxene and olivine are commonly pseudomorphed by secondary amphiboles. Primary mineralogy has been deduced mainly from detailed textural and geochemical studies (e.g., James and Born 1985; Chubb 1994). EAST BULL LAKE SCALE 1 20000 0 05 1 2 25 Kflometres Figure 2. Geologic map of East Bull Lake Intrusion. Numbered units within the intrusion correspond to stratigraphic zones shown in Figure 3. Units 1 to 3 are Archean strata; units 14 to 17 are post-intrusion Paleoproterozoic units; for details see Peck et al. (1995). 16 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �w C a >< 12 Agnew Intrusion .2 (U 5: Transition unit ö o 0 a E .2 ö 1= _2000 _1 800 _1 600 (0 w w a 0) An<3 Ferrosyenite unit Fe-Ti Oxide Zone Leucogabbro unit G) a Porphyritic unit w 0. 0. Upper .1400 Gabbronorite ( Zone _1200 Pod-bearing unit Porphyritic unit East Bull Lake Intrusion Ce w Mixed unit Dendrite unit Layered unit Massive Gabbronorite Zone _1 000 Olivine Gabbronorite•- _800 Layered Gabbronorite Zone Olivine Gabbronorite Zone — Dendrite unit Rhythmically layered Zone Lower Gabbronorlte Zone Van-textured Gabbrononite Massive unit Leucogabbronorite Zone 0 Anorthosite Zone ow .E 0)01 Inclusion-bearing Zone Gabbronorite Zone Border Zone _600 Inclusion-bearing Gabbronorite Zone Marginal Leucogabbronorite Zone 5 Marginal Gabbronorite Zone Figure 3. Stratigraphic sections for the East Bull Lake and Agnew Lake intrusions. Mineral distribution and plagioclase compositions are from Vogel et al. (1999). Unit numbers correlate with geologic maps shown in Figures 2 and 6a. Stratigraphy and Petrography The type stratigraphic section for the East Bull Lake Intrusion is shown in Figure 3. Three mappable subdivisions are recognized: the Lower, Main and Upper Series. 17 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �MARGINAL SERIES The contact between the East Bull Lake Intrusion and Archean basement is represented by the Border and Gabbronorite zones of the Marginal Series. The Border Zone is commonly several tens of metres thick, and comprises varied proportions of gabbroic and anorthositic veins and locally derived basement xenoliths. The relative proportion of footwall xenoliths and gabbroic veins and the size of the basement xenoliths increase away from the intrusion and into the undisturbed basement. The basement xenoliths are commonly thermally recrystallized to hornfels and locally display evidence of in situ melting. Adjacent gabbroic veins commonly contain several percent quartz and/or patchy, granophyric-textured leucotonalite and leucogranite. The Border Zone is locally separated from the main mass of the intrusion by the Gabbronorite Zone, a 1 to 50 m thick texturally and modally banded gabbronorite. Chubb (1994) suggested that the Gabbronorite Zone, as exposed in the conduit linking the western and eastern lobes of the East Bull Lake Intrusion, probably developed from late injections of mafic magma that were unable to penetrate the overlying Lower Series rocks. Elsewhere in the East Bull Lake Intrusion, the Gabbronorite Zone is clearly gradational into the overlying Inclusion-Bearing Zone and underlying Border Zone, and likely represents an irregular chilled margin. LOWER SERIES The Lower Series hosts most of the known PGE mineralization in the East Bull Lake Intrusion, and comprises a lower inclusion-rich unit (Inclusion-Bearing Zone) and an upper anorthositic unit (Anorthosite Zone). The Inclusion-Bearing Zone consists of a matrix of medium- to coarse-grained gabbronorite and/or leucogabbronorite and a heterolithic inclusion suite. The inclusions typically represent only 1 to 10% of the unit and include basement xenoliths, anorthosite and leucogabbro autoliths and discontinuous, podiform pyroxenite and melagabbro bands and pods. The Anorthosite Zone consists of monotonous, coarse-grained anorthosite and leucogabbronorite. Locally, these anorthositic rocks are brecciated by crosscutting gabbroic vein networks. Spectacular “nodular” (glomerocrystic) textures occur in anorthositic rocks in the lower parts of the Anorthosite Zone. These occurrences are largely restricted to the conduit linking the west and east lobes and to the easternmost part of the east lobe. The nodular textures consist of tightly packed centimetre-size spheroidal aggregates of coarse-grained plagioclase in a matrix of coarse-grained pyroxenite and/or finer grained gabbronorite. The textures are believed to have formed due to enhanced cooling rates in the narrow parts of the intrusion (Chubb et al. 1995). MAIN SERIES The Main Series incorporates the Leucogabbronorite Zone, the Rhythmically Layered Zone and the Olivine Gabbronorite Zone (see Figure 3). Metre-scale, isomodal, phase layering becomes increasingly better developed upward through the Main Series (Photo 2). Igneous layering becomes better developed upward through the Main Series. In the Leucogabbronorite Zone, most of the layering reflects modal variations of plagioclase and pyroxene. Phase layering in the Rhythmically Layered Zone is typically caused by variations in olivine and plagioclase and like the underlying zone, overall, these rocks are leucocratic. The Olivine Gabbronorite Zone is distinct in having a high proportion of olivine-rich layers throughout its 50 to 70 m stratigraphy, relative to the underlying layered zones. Throughout the Main Series, modally graded layers typically become more plagioclase-rich upward. They are believed to have formed by separation of high-temperature, buoyant, cumulus plagioclase from dense, Fe- and Mg-rich residual magma ± olivine. 18 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �—. .' 4 •! .,. Photo 2. Igneous layering in the East Bull Lake Intrusion. a) Well-developed layering between plagioclase-rich and plagioclasepoor units. Hammer handle is approximately 30 cm long. b) More typical, subtle layering within gabbronoritic rocks of the Rhythmically Layered Zone, Main Series. 19 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �UPPER SERIES The most chemically evolved rock types in the East Bull Lake Intrusion occur in the Upper Series, but they are less evolved than rocks present in the Upper Series of the Agnew Lake Intrusion (see Figure 3). Accordingly, it is believed that the uppermost parts of the East Bull Lake Intrusion have been eroded away. The Upper Series comprises the Layered Gabbronorite Zone and the Massive Gabbronorite Zone. The Layered Gabbronorite Zone consists of planar or disrupted, irregular, texturally and modally layered gabbronorite. The Massive Gabbronorite Zone comprises planar, fine- to medium-grained gabbronorite and ferrogabbronorite (Peck et al. 1995). Varitextured gabbronorite occurs throughout the East Bull Lake Intrusion, forming irregular, metresized veins, pods and discontinuous layers that commonly display dendritic textures and have highly varied grain sizes (Photo 3). The thickest band occurs in the Layered Gabbronorite Zone to the west of Moon Lake (see Figure 2). Born (1979) mapped these rocks as a stratigraphic unit in the central plateau area of the East Bull Lake Intrusion. In the eastern part of the East Bull Lake Intrusion, these same rocks are not apparently stratigraphically controlled (Chubb 1994) leading to the schematic interpretation in Figure 3 of these rocks transecting stratigraphy. At the Agnew Lake Intrusion these same rocks form a distinctive stratigraphic unit as indicated in the stratigraphic section in Figure 3 (Vogel 1996). The crystallization order throughout the East Bull Lake Intrusion is plagioclase ± olivine → orthopyroxene → clinopyroxene + magnetite → sodic myrmekite + apatite. Core plagioclase compositions vary from An70-80 in the Anorthosite Zone to near An60 at the top of the stratigraphy. Analyses of preserved igneous plagioclase, olivine and 2 pyroxenes in small areas of the Rhythmically Layered and the Olivine Gabbronorite zones (see Table 1) show Fo60-74 compositions for primocryst olivine. This suggests that the Olivine Gabbronorite Zone formed from a more primitive magma composition than the underlying Rhythmically Layered Zone. Where present, cumulus olivine appears to have co-precipitated with plagioclase and is commonly mantled and partly resorbed by orthopyroxene. Orthopyroxene includes hypersthene and iron-rich bronzite, whereas the dominant clinopyroxene is augite. Disseminated, chromium-rich spinel and clotty, coarse-grained magnetite locally occur within the Gabbronorite Zone (Marginal Series) in the southwestern part of the East Bull Lake Intrusion. '. Photo 3. Dendritic texture within the Upper Series of the East Bull Lake Intrusion. a) Frond-like pyroxene dendrites. Hammer handle is approximately 30 cm long. b) Large, pyroxene-dendrites present just to the right of the 5.5 cm lens cap. 20 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Geochemistry A comprehensive geochemical database for the East Bull Lake Intrusion can be extracted from Peck et al. (1995) and Chubb (1994), which includes several hundred, whole-rock major and minor element analyses and selected trace and rare-earth element analyses. Additional, relevant geochemical data for the East Bull Lake Intrusion is contained in several Atomic Energy of Canada Ltd. publications (e.g., Ejeckam et al. 1990) and previous academic studies (Born 1979; James and Born 1985). Selected representative and average geochemical compositions for the major stratigraphic units in the East Bull Lake Intrusion are presented in Table 4 and illustrated in Figures 4 and 5. Included in Table 4 are 2 estimates of the parent liquid composition for the majority of the rocks in the East Bull Lake Intrusion. These are represented by a probable feeder dike (sample 91DCP291; Chubb 1994) from the eastern lobe of the East Bull Lake Intrusion, and a weighted bulk composition for the entire intrusion (Average WEBLI). The bulk composition was determined using the calculated volumes of all of the major stratigraphic units and their average chemical compositions. Both estimates give similar, Al-rich, low-Ti leucogabbroic magma compositions. The East Bull Lake Intrusion is dominated by rocks having leucogabbronorite compositions that are olivine-normative in parts of the Lower Series and much of the Main Series. Detailed geochemical studies of surface and diamond-drill core samples from both the eastern and western lobes of the intrusion (e.g., James and Born 1985; Ejeckam et al. 1990; Chubb 1994; Peck et al. 1995) have shown that welldefined chemical fractionation trends are absent through the East Bull Lake Intrusion stratigraphy. This likely reflects the fact that 1. the early crystallization of the parent magmas was dominated by plagioclase with intermediate to calcic compositions that, as calculated by James et al. (2002a), were near neutrally buoyant in their Al-rich parent magmas. 2. the chamber was fed by multiple injections of chemically similar magma that became mixed with the resident magma, thereby precluding the development of chemical fractionation trends. Nonetheless, some geochemical trends are present in the intrusion, a few of which are illustrated on Pearce-element ratio diagrams (see Figure 4) and chondrite-normalized rare-earth element plots (see Figure 5). In Figure 4a, Olivine Gabbronorite Zone rocks from the East Bull Lake and Agnew Lake intrusions (olivine-plagioclase cumulates) clearly form a more olivine-controlled group than do the rest of the samples from these intrusions. These data support the interpretation that intrusion of the Olivine Gabbronorite Zone magma represents a separate and more primitive magma pulse into the chamber. Figure 4b shows that plagioclase is the only significant calcium-bearing phase involved in the fractionation of the East Bull Lake Intrusion magma. This is consistent with petrography and rules out clinopyroxene as a mineral phase influencing fractional crystallization for this intrusion. In both Figures 4a and 4b, Upper Series rocks plot closer to the origin of the diagram reflecting their more fractionated compositions. Many of the mafic veins in the Border Zone are quartz normative and display field and geochemical evidence of significant assimilation of basement-derived, siliceous partial melts. Nevertheless, there is no evidence for significant contamination of the main body of magma in the East Bull Lake Intrusion through partial or total assimilation of country rock inclusions. The majority of the rocks in the lower parts of the East Bull Lake Intrusion are enriched in Al and depleted in incompatible trace elements in comparison to the proposed parent magma estimates (see Table 4, Figures 4a and 4b). The Main Series contains, on average, the most magnesian rocks in the East Bull Lake Intrusion, with olivine-plagioclase cumulates occurring throughout the Olivine Gabbronorite Zone and in parts of the underlying Rhythmically Layered Zone. The Upper Series contains the most evolved rocks in the East Bull Lake Intrusion, and on most chemical fractionation plots, fall at the evolved end of the fractionation trends (e.g., see Figure 4b). 21 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �a) 300 0I X Opx Cpx 250 • Lower and Main Series A Upper Series, EBLI X OGNZ, EBLI • Marginal and Lower Series, AGI 0 Upper Series, AGI X OGNZ,AGI • Feeder Dyke, EBLI • Streich Dyke, AGI ——Linear (OGNZ, EBLI) Linear (Lower and Main Series, EBLI( Plag .X 200 • . •U 0, XX. U- U U • 150 y=O.3085x-O.1328 R2= 0.7853 • 100 • U U 0 0 0 100 200 400 300 500 600 S11T1 140 120 80 60 20 200 100 300 Al/Ti Figure 4. Pearce-element ratio diagrams for the East Bull Lake, Agnew Lake and River Valley intrusions. a) Fe+Mg/Ti versus Si/Ti plot for East Bull Lake and Agnew Lake intrusions. b) Ca/Ti versus Al/Ti plot for East Bull Lake and Agnew Lake intrusions. c) Ca/Ti versus Al/Ti plot for the River Valley intrusion. In figures 4a and 4b, solid symbols are for lower and middle portions of the stratigraphy, with open symbols for the upper part. The Olivine Gabbronorite Zone is shown separately as are the estimated parent magma compositions for the 2 intrusions. Solid lines are fitted curves for the East Bull Lake Intrusion, as examples. Mineral vectors indicate how the data should vary if trends are due to that phase only. The data in figures 4a and 4b show that the 2 intrusions exhibit identical chemical variations. In Figure 4a, Olivine Gabbronorite Zone rocks are clearly more mafic (primitive) in composition than the remainder of each intrusion which fall on a single trend with a much shallower (plagioclase-enriched) slope. The positions of the parent magma composition are consistent with the 2 groups being derived from cogenetic magmas. In Figure 4b, data for all samples fall on a linear trend with a slope in the range 0.40 to 0.45 indicating that calcic plagioclase feldspar mainly controls this trend, and that clinopyroxene is not a major modal phase in these rocks. In the lower left of 4a, samples of Upper Series rocks deviate from the trend near the origin of the diagram because they have primocryst ilmenomagnetite and thereby, Ti is no longer a conserved element. In figure 4c, rocks from the River Valley intrusion display a similar plagioclase-controlled trend, but with a greater proportion of samples lying closer to the origin, reflecting a slightly more mafic bulk composition of the samples. 22 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �1000 a) C C 0 C.) 0 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu —4-- Ferrosyenite Subzone (7b) b) —0— Leucogabbro Subzone (7a) —U— Porphyritic unit (6c) —X— Olivine Gabbronorite Zone (5) unit (4b) —0—Inclusion-bearing and Massive units (3+4a) —4— Marginal Leucogabbronorite Zone (2) 100 ci) •0 C 0 0 C.) 0 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Figure 5. Chondrite-normalized rare earth element plots for the East Bull Lake and Agnew Lake intrusions. a) East Bull Lake Intrusion. b) Agnew Lake Intrusion. Data are from Peck et al. (1995) and Vogel (1996). Normalizing values are from Sun and McDonough (1989). In both diagrams, all data are the average for 2 or more samples. “Feeder dike” in Figure 5a and “Streich dike” in 5b are estimates of parent magma compositions to the East Bull Lake Intrusion and the Agnew Lake Intrusion, respectively. In 5a, note the U-shaped boninite-like pattern for pyroxenite layers from the Inclusion-Bearing Zone. Figure 5a shows that all major rock types in the intrusion are enriched in light REE; this is typical for the East Bull Lake intrusive suite as a whole. The plagioclase-rich and cumulate character of rocks from the Lower and the Main Series in the East Bull Lake Intrusion is illustrated by their depleted total REE relative to the parent magma composition (“feeder dike”), and strong to moderate Eu anomaly. 23 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Table 4. Representative and average anhydrous whole-rock geochemical analyses from the East Bull Lake and Agnew Lake intrusions (from James et al. 2002a). Analysis Series Zone Rock Type N SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 Mg # Cr Ni Co Sc V Cu Zn Rb Ba Sr Nb Zr Y Analysis Series Zone Rock Type 1 Basement Footwall Archean tonalite 11 69.2 0.09 17.5 1.70 0.02 0.44 2.89 6.94 1.21 0.04 33.75 17 61.8 0.54 17.8 4.19 0.09 1.16 2.48 5.36 6.22 0.27 35.46 1 50.9 0.09 25.7 2.74 0.04 2.24 13.3 4.28 0.56 0.08 61.82 1 52.9 0.09 22.2 4.34 0.08 4.88 11.1 3.80 0.52 0.08 69.02 2 51.8 0.21 13.8 9.72 0.20 11.9 10.4 1.71 0.22 0.08 70.86 6 Marginal Border Quartz-rich gabbronorite vein 8 56.1 0.50 17.1 8.17 0.13 5.33 9.13 2.72 0.74 0.09 56.38 <10 <5 20 1.8 16 12 31 23 784 378 8.8 74.6 13.6 <10 8 7 4.0 53 17 77 201 1252 723 32.4 550.6 41.6 38 51 28 <1.0 32 377 31 27 225 607 4.9 37.8 7.6 113 107 41 <1.0 39 88 50 24 195 675 4.1 37.5 7.3 682 392 61 <1.0 131 922 112 20 103 259 2.6 24.4 8.6 138 192 42 15.2 153 214 70 27 370 367 7.0 73.7 15.9 12 Lower Anorthosite Anorthosite 14 Main LGN Leucogabbro norite 42 50.5 0.34 20.3 8.09 0.13 6.46 11.1 2.44 0.60 0.05 61.28 72 49.6 0.42 21.0 8.28 0.12 6.16 11.1 2.58 0.59 0.06 59.55 25 46.9 0.30 14.6 13.4 0.18 14.5 8.19 1.56 0.27 0.04 68.22 155 194 41 17.2 100 95 71 26 166 271 6.2 46.9 11.1 81 195 40 17.2 127 80 72 24 201 295 6.0 48.6 12.1 230 505 90 13.1 104 91 91 15 118 183 6.1 41.5 14.4 9 Lower IBZ Pyroxenite pods, EEBLI 2 Basement Footwall Parisien Lake syenite 10 Lower IBZ Pyroxenite layers, WEBLI 3 Marginal Border Anorthosite vein 11 Lower IBZ Chlorite schist, WEBLI 4 Marginal Border Leucogabbro vein 5 Marginal Border Melagabbro norite vein N SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 Mg # 10 53.8 0.38 6.28 13.5 0.23 17.0 8.12 0.52 0.11 0.05 71.37 2 52.71 0.17 4.59 14.15 0.21 23.76 4.29 0.05 0.06 0.01 76.88 1 38.55 0.12 16.96 20.74 0.26 17.99 4.77 0.56 0.03 0.02 63.21 88 49.7 0.36 21.9 7.67 0.12 5.50 11.6 2.52 0.56 0.05 58.71 13 Lower Anorthosite Matrix to nodular anorthosite 10 49.2 0.84 13.9 13.1 0.19 10.0 10.8 1.49 0.31 0.12 60.19 Cr Ni Co Sc V Cu Zn Rb Ba Sr Nb Zr Y 1360 383 87 <1.0 196 80 116 15 51 31 6.7 57.7 17.3 87 403 59 47 152 1010 95 <2 <3 4 <1.0 11 2 998 2670 152 12 100 2000 365 <2 8 59 <1.0 21 3 116 289 40 14.1 120 338 69 21 177 300 6.9 40.5 11.2 289 332 96 <1.0 296 72 97 19 76 117 9.9 70.4 24.3 7 Marginal GN Gabbronorite EEBLI 8 Lower IBZ Matrix, EEBLI 4 50.7 0.56 15.0 13.6 0.19 7.55 9.63 2.21 0.52 0.06 52.38 9 51.5 0.57 15.3 12.2 0.19 7.67 9.79 2.24 0.53 0.06 55.41 264 201 79 39.1 340 188 99 19 113 333 4.1 62.7 16.7 210 244 43 19.4 127 209 67 16 140 253 <1.0 34.9 7.8 15 Main RL Leucogabbro norite 16 Main OGN Olivine gabbronorite 24 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Table 4. continued. Analysis Series Zone Rock Type N SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 Mg # Cr Ni Co Sc V Cu Zn Rb Ba Sr Nb Zr Y 17 Main LGN Gabbronorite, Folson Lake 18 Main LGN Gabbronorite, Bull Lake 191 Upper GN Varitextured gabbronorite 20 Upper GN Massive gabbronorite 21 Dikes 22 Dikes Plagioclasephyric dikes, WEBLI 8 51.3 0.80 15.0 12.7 0.20 6.36 10.7 2.04 0.73 0.10 49.74 89 85 47 37.3 250 119 97 35 224 230 <1.0 67.9 18.8 24 51.2 0.52 18.1 10.0 0.16 6.24 10.6 2.37 0.76 0.07 55.25 30 52.2 0.69 18.1 10.5 0.16 4.79 9.73 2.67 1.07 0.09 47.48 29 51.7 0.65 15.6 11.5 0.18 6.81 10.4 2.32 0.71 0.08 53.90 32 52.9 0.90 15.3 13.3 0.22 5.59 7.79 2.80 1.13 0.12 45.46 Gabbronorite dike, Folson Lake 6 51.0 0.80 15.8 12.7 0.19 6.30 10.2 2.17 0.74 0.11 49.60 138 121 43 25.3 167 90 85 27 258 261 6.3 60.2 14.2 72 92 37 25.6 200 182 85 42 328 255 <1.0 66.8 15.9 184 113 51 30.6 220 121 90 27 245 223 6.4 68.3 16.6 63 67 43 32.1 264 138 125 46 392 193 7.4 100.8 20.5 136 116 45 30.8 235 138 97 34 260 210 <1.0 80.5 18.5 23 Dikes Aphyric diabase dikes, WEBLI 24 Parent Average WEBLI 25 Parent 11 41.0 1.22 14.1 15.2 0.22 5.37 9.01 2.86 0.89 0.16 41.13 342 50.2 0.45 19.5 9.31 0.14 6.60 10.6 2.44 0.65 0.06 58.41 Feeder 91DCP291, Streich dike 1 50.8 0.54 19.6 9.25 0.16 5.47 10.8 2.96 0.30 0.07 53.94 69 65 44 33.7 310 153 120 44 312 239 <1.0 116.6 32.6 125 224 45 18.9 142 179 80 26 211 267 6.5 52.5 13.1 153 140 33 <1.0 171 66 70 8 93 261 2.5 65.0 14.6 Notes: Major element oxides are in wt. %, trace element data are in ppm; Mg number = atomic Mg/Mg + Fe, where Fe = total Fe expressed as ferrous iron. All samples from Peck et al. (1995), except analyses 7, 8, and 13, which are from Chubb (1994). Abbreviations: EEBLI = eastern East Bull Lake Intrusion; GN = Gabbronorite Zone; IBZ = Inclusion-Bearing Zone; LGN = Layered Gabbronorite Zone; OGN = Olivine Gabbronorite Zone; RL = Rhythmically Layered Zone; WEBLI = western East Bull Lake Intrusion. Thermodynamic modelling of the crystallization of the East Bull Lake Intrusion based on the “feeder dike” parent magma (see Table 4), using the program COMAGMAT (Ariskan et al. 1993), suggests that plagioclase was the sole liquidus mineral over the temperature range 1300 to 1200°C, followed by olivine and then orthopyroxene (1176°C) (James et al. 2002a). The most robust model is in agreement with the predictions made by James and Born (1985), using Pearce-element ratio diagrams, who indicated that plagioclase accounts for nearly 85% of the phase volume of the fractionating assemblage in the Lower Series and 77% in the Upper Series. Orthopyroxene and clinopyroxene represent only about 20% of the phase volume throughout most of the East Bull Lake Intrusion, and olivine control is only evident in the middle to upper parts of the Main Series. The average Mg-number for the parent magmas is about 73, and the average, theoretical, plagioclase composition is about An73. Our calculations also suggest that most of the pyroxenite pods in the Inclusion-Bearing Zone represent dense, fractionated, Mg- and Feenriched residual magma that originated in enclosing or overlying strata. Their REE-enriched character (see Figure 5a), relative to the main mass of Anorthosite Zone rocks, is consistent with this interpretation. There are, however, discontinuous, metre-scale, cumulate layers in the PGE-rich, basal part of the Inclusion-Bearing Zone along the southwest margin of East Bull Lake Intrusion (Bullfrog zone, Mustang Minerals Corporation) that have U-shaped REE profiles (see Figure 5a) indicating that they formed from boninitic magma. The origin and distribution of these atypical rocks requires further study, as they suggest that 2 magma types were present in the early (mineralized) stages of formation of this intrusion. 25 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Mineralization Mineral exploration in the East Bull Lake Intrusion began with prospectors who gained access along newly completed logging roads in the early 20th century. They focused their attention on gold and sulphide mineral prospects. Several companies, including Noranda Mines Ltd., El Pen-Rey Oil-Gas and Mines Ltd., Sylvanite Gold Mines Ltd. and the Mining Corporation of Canada Ltd., investigated the base metal potential of previously discovered sulphide mineral occurrences in the 1950s and 1960s. The PGE potential of the East Bull Lake Intrusion was first recognized by independent prospectors E. Gallo, M. Hauseux and S. Surmacz, as well as by BP Minerals. Their observations were corroborated by an extensive study of the mineral potential of the East Bull Lake Intrusion carried out by researchers from Laurentian University and the Ontario Geological Survey in the early 1990s (Chubb 1994; Peck, James and Chubb 1993; Peck et al. 1995). Both Inco and WMC International undertook small exploration programs on the East Bull Lake Intrusion, but sustained exploration did not take place due to low palladium prices and the unfamiliar mineralization style. Renewed exploration of the East Bull Lake Intrusion began in 1998, led by Mustang Minerals Corporation, whose property covers about 90% of the intrusion. The current exploration target is a bulk, near surface, low grade (e.g., 2.5 g/t Pd+Pt) PGEcopper-nickel resource suitable for open pit mining (Brisbin et al. 2001). The geological environments for PGE-copper-nickel mineralization in the East Bull Lake Intrusion were first described in detail by Peck, James and Chubb (1993). They recommended the non-genetic term “contact sulphide mineralization” (now “contact-type”) be used to describe all of the PGE-rich disseminated sulphide mineralization occurring in the Marginal and Lower Series of the intrusion. Sulphide mineralization is present throughout the Lower and underlying Marginal Series but is best developed within the Inclusion-Bearing Zone, within a few tens of metres of the footwall or sidewall contact. Recent diamond drilling by Mustang Minerals Corporation and Freewest Resources Canada Inc. has proven that this zone is typically 20 to 50 m thick and, despite the nugget-type distribution of the sulphide minerals, consistently displays an average sulphide mineral content of 1 to 2%. The sulphide minerals typically consist of approximately equal amounts of coarse chalcopyrite and pyrrhotite, and subordinate, finer-grained pentlandite. Where their original textures are preserved, the sulphide minerals in the Inclusion-Bearing Zone form coarse-grained blebs up to 3 cm in diameter. Sulphide minerals (<1 to 10%) also commonly occur in the Border Zone and the Gabbronorite Zone of the Marginal Series, but these occurrences are enriched in pyrrhotite and/or pyrite relative to the chalcopyrite-rich mineralization in the Inclusion-Bearing Zone. Sulphide minerals rarely occur in the Main and Upper Series and, where present, are principally composed of pyrrhotite and appear to have formed as late-crystallizing grains from trapped intercumulus liquids. In Olivine Gabbronorite Zone samples, however, disseminated finegrained chalcopyrite and magnetite have been reported in the most altered rocks (James et al. 2002a) and may account for Pd+Pt+Au assays in the 0.5 to 1.0 g/t range from this unit (Peck et al. 1995, 2001). Disseminated to massive sulphide minerals occur within the Parisien Lake deformation zone (PLDZ on Figure 2), a major east-trending structure. Most of the known mineralization is exposed in trenches located about 1 km east of East Bull Lake (see Figure 2). The sulphide minerals occur in metre-size, ovoid zones that are elongate parallel to the strike of the primary shear fabric in the Parisien Lake deformation zone. The sulphide minerals consist of pyrrhotite and lesser chalcopyrite, pyrite and pentlandite and are intergrown with quartz, amphibole, chlorite and magnetite. Field and geochemical data suggest that hydrous fluids permeating the Parisien Lake deformation zone leached chalcophile metals from pre-existing, magmatic sulphide mineralization in the deformed Lower Series rocks and redeposited them within dilatant zones. The geochemistry and genesis of the contact-type PGE-copper-nickel mineralization in the East Bull Lake Intrusion is discussed in detail by Peck et al. (2001). Based on all available data, the mineralization 26 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �is characterized by highly varied whole-rock PGE grades that reflect the erratic distribution of the sulphide minerals on a hand sample scale. Diamond drilling confirms, however, that the average PGE grade of the Inclusion-Bearing Zone in the southwestern part of the East Bull Lake Intrusion is roughly 0.5 to 2.5 ppm combined Pd+Pt+Au. Analyses from grab samples range from typical values of 1 to 2 ppm to 43 ppm combined Pd+Pt+Au (sample collected by Freewest Resources Canada Inc. from a trench on their Folson Lake property). Data presented by Peck et al. (1995) show that 20% of all of the reconnaissance samples collected from the Lower Series in the western lobe of the intrusion contain greater than 1 ppm combined Pd+Pt+Au, with an average Pd content of approximately 200 ppb. These values represent highly anomalous background PGE contents in comparison to most other mafic or ultramafic intrusions. For comparison, barren norites, pyroxenites and harzburgites from the Lower, Critical and Main Zones of the Bushveld Complex have values ranging from 13 to 478 ppb Pt+Pd (Barnes and Maier 1999). Average Pd/Pt ratios are highest in the Anorthosite Zone (3.2 to 4.3) and lowest in the pyroxenitic pods of the Inclusion-Bearing Zone (1.56) and the Border Zone (2.0). Average Cu:Ni ratios range from 0.6 in samples from the matrix in Inclusion-Bearing Zone to 4.8 in the pyroxenitic pods; however, most samples have values between 2 and 3. Border Zone sulphide occurrences have the lowest average PGE grades (at similar total sulphide mineral abundance), but show a considerable range in abundance. Average S/Se ratios for the PGE-rich sulphide mineralization range from 1240 to 2840, which are within the empirically-determined range for mantle-derived sulphide mineralization (Eckstrand et al. 1989), and are indicative of a magmatic origin for the contact-type PGE mineralization. Copper displays a strong positive correlation with both S and Se. Pd and Pt display significant, positive, correlation with each other, with Cu and, to a lesser degree, with S. As is typical of most mafic-hosted magmatic sulphide mineralization, the contact-type sulphide mineralization in the East Bull Lake Intrusion shows strong enrichment of the low-melting point PGE (e.g., Pd, Pt) relative to the refractory PGE (Ir, Rh, Ru) on a chondrite-normalized chalcophile metal plot (James et al. 2002a). The structurally-controlled sulphide mineralization from the Parisien Lake deformation zone show lower PGE contents per unit sulphide in comparison to the contact-type mineralization. The former sulphide mineralization type also have much higher Pd:Ir ratios, a feature consistent with the known differences in degree of mobility of Pd versus Ir during hydrothermal alteration of mafic and ultramafic rocks (Keays et al. 1982). ROAD LOG, DAY 1, EAST BULL LAKE INTRUSION Note: Highway 533 is used regularly by logging trucks. Consequently, extreme caution should be taken when parking vehicles on the shoulder and when examining outcrops along Highway 533. Geological map reference: Chubb, Hannila and Peck (1994). 0.0 km Start at the junction of Highway 69S and the Highway 17W bypass in Sudbury. Set odometer to zero. Drive west on Highway 17 to Massey. 0.4 km Junction of Highway 17W bypass and Regional Road 80 (Long Lake Road), continue west on Highway 17 bypass. 70.9 km Junction of Highway 17 and 6, just north of Espanola. Continue west on Highway 17. 97.0 km Junction of Highway 17 and 553 in Massey. Turn right (north) onto Highway 553 toward East Bull Lake. 119.9 km Pull over on shoulder of Highway 533 at the top of a high ridge. Examine cleaned outcrops on both sides of the highway. 27 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Stop 1. Paleoproterozoic Metagabbro The high ridge consists of a linear(?) body of massive, medium-grained, metagabbro which may represent part of a feeder system to other East Bull Lake intrusive suite sills that have been eroded from the surrounding terrain. Prevec (1993) suggested that the rocks from this stop have a suitable parent composition for the East Bull Lake Intrusion. Alternatively, these rocks could be part of a large Matachewan diabase dike, but unlike most of the Matachewan dikes, it is not plagioclase-phyric. Return to vehicles and continue north on Highway 533. UTM 408177E, 5133860N. 122.4 km Power line crosses the Highway, continue north. 124.8 km Approximate southern margin of the Neoarchean Parisien Lake syenite (2665±2; Krogh, Davis and Corfu 1984). 125.3 km Junction of Highway 533 and logging road to the east. Park vehicles and walk a short distance east along the logging road to examine outcrops on the south and north sides of the logging road. Stop 2. Neoarchean Parisien Lake Syenite At this locality we see small amounts of coarse-grained syenite showing a well-developed igneous flow fabric on a local scale, as well as more abundant exposures of massive, medium-grained syenite. Regional mapping indicates that several types of medium-grained syenite intrude the coarser-grained variety; the latter is the more abundant rock type in the body. The Neoarchean age of the syenite is distinct from the 2470 to 2490 Ma age range of the East Bull Lake intrusive suite (see Table 2). These rocks form the footwall to the East Bull Lake Intrusion at Stop 12b. Return to vehicles and continue north on Highway 533. 126.0 km Low outcrops on the east side of Highway 533 consist of a plagioclase-phyric diabase dike typical of the Matachewan dike swarm. The dike contains greenish altered plagioclase phenocrysts in a fine-grained metadiabase matrix. Mafic diabase dikes, some plagioclasephyric, but many not, intrude both the Archean basement and the Parisien Lake syenite. Many of the same dikes also intrude the East Bull Lake Intrusion. A few very large dikes do not cut the intrusion, and are interpreted to be potential feeders to the sills to the East Bull Lake intrusive suite bodies in this area and elsewhere in the Southern Province. Typically, the dikes strike to the northwest, but they also strike west to west-northwest when they are proximal to the Southern–Superior province boundary. 126.5 km Outcrop of coarse-grained syenite, in the central part of the Parsien Lake syenite. The outcrop is intruded on its south side by a 10 to 20 m wide, northwest-trending, Matachewan diabase dike. 127.6 km Pull off to shoulder just past the curve in road where the highway passes between 2 large outcrop ridges. Leave vehicles and work your way up to the top of the large ridge on the west side of the road. Stop 3. Southern Margin of the East Bull Lake Intrusion This area of the East Bull Lake Intrusion forms a high ridge of very coarse-grained, massive varitextured leucogabbronorite. At its southern extremity, it is inclusion-bearing and sparsely mineralized. The outcrop area represents both the Inclusion-Bearing and Anorthosite zones, of the Lower Series, inward of 28 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �the intrusion contact. At this locality, the contact with the Parisien Lake syenite is sharp and most likely is a fault. Return to vehicles and continue north on Highway 533. 128.0 km Folson Lake deformation zone. At this location Highway 553 is “offset” to the east due to this major shear zone that strikes northwest across the entire intrusion (see Figure 2). 128.7 km Pull off to the shoulder and examine low outcrops on both sides of the road exposed on a curve as it passes over a small rise. Stop 4. Country Rocks These outcrops consist of country rocks of the Neoarchean Ramsay–Algoma gneiss complex. The Ramsay–Algoma gneiss complex (Jackson and Fyon 1991) is intruded by both the Parisien Lake syenite and the East Bull Lake Intrusion. Typically these gneisses are mixtures of strongly foliated granitic gneiss and metatextite to diatextite that enclose strongly deformed enclaves of mafic gneiss that likely represent remnants of greenstone belt metavolcanic rocks and/or diabase dikes. UTM 408300E, 5141425N. 128.8 km Parisien Lake road leads to mineralized zones along southern contact of the East Bull Lake Intrusion. 129.3 km Junction to the west between Highway 533 and the AECL (Atomic Energy of Canada Limited) road. The AECL road leads to Moon Lake, Central Plateau and the Folson Lake deformation zone. 129.8 km Outcrops present in this sandpit, 30 m east of Highway 533, expose metre-scale isomodal phase layering in the Rhythmically Layered Zone, Main Series, East Bull Lake Intrusion. The layering consists of thin leucogabbronorite layers (with scarce olivine primocrysts) that pinch and swell in thickness and continuity, and metre-thick isomodal layeres of olivine melagabbronorite. Boundaries between the layers are sharp (similar to Photo 2a). Outcrops at the road at this locality show graded layers, as well as layers with more gradational boundaries (similar to Photo 2b). UTM 409022E, 5142098N. 130.5 km East Bull Lake Lodge just to the west of Highway 533. Lodge is owned by Gerry Vauteur. 130.7 km Pull over and park where space is available. Examine vertical faces in outcrop on the west side of the road. Stop 5. Rhythmically Layered Zone This stop is located within the Rhythmically Layered Zone of the Main Series of the East Bull Lake Intrusion. On the west side of the road, vertical faces in outcrop exhibit several 0.3 to 1 m thick layers that dip west into the outcrop at 20 to 30°. The layers contain primocrysts of olivine and plagioclase enclosed in a matrix of orthopyroxene and clinopyroxene. Typically within the East Bull Lake Intrusion, all of the mafic minerals are replaced by secondary amphiboles, but plagioclase is commonly preserved. In this outcrop, however, well-formed outlines of olivine are easily observed; their proportion changes abruptly across layer boundaries. Layer compositions vary from olivine leucogabbronorite to olivine melagabbonorite; in all cases the layer boundaries are typically sharp and evidence of grading is very sparse. In this outcrop, evidence of a gravity-driven convection system is negligible. Return to vehicles, turn around and retrace route south to the junction of Highway 533 and the AECL road. Turn on to the AECL route and follow it to the end. Park vehicles in the pullout area (UTM 409405E, 5142725N), and walk south along the trail to a series of stripped outcrops. 29 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Stop 6. Folson Lake Deformation Zone Be careful of your footing on these smooth outcrops if the surfaces are wet. At this locality the Folson Lake deformation zone intersects the Layered Gabbronorite Zone of the Upper Series of the East Bull Lake Intrusion (see Figure 2). Kamineni et al. (1984) refer to this altered gabbronorite that extends as a narrow zone along the northwest-trending Folson Lake fault as the “Stockwork Gabbro”. It contains a complex network of at least 2 generations of quartz veins as well as feldspar veins (both plagioclase and adularia); that together comprise up to 30 volume per cent of the rock. Diabase dikes transect the altered gabbro stockwork both parallel and perpendicular to the northwest-striking zone of deformation and faulting. Petrological studies of the gabbros in this and related areas by Kamineni et al. (1985) indicate that the regional greenschist facies to epidote-amphibolite facies mineralogy is altered on a local scale to minerals characteristic of prehnite-pumpellyite facies and zeolite facies (e.g., laumontite, analcite, calcite). UTM location at parking area is 406330E, 5142108N. Return to vehicles. 0.0 km Reset odometer to zero and head east toward Highway 533. 0.8 km Park at edge of road by large outcrop. Stop 7. Dendritic Texture in the Varitextured Gabbronorite Zone This outcrop is located in the Varitextured Gabbronorite Zone, Upper Series, East Bull Lake Intrusion. Pods and lenses, 0.5 to 1.0 m in maximum dimension, of coarse-grained, pegmatitic and dendritic gabbronorite occur sporadically in the massive, medium-grained gabbronorite host rock. The rapid textural variation is diagnostic of this zone, as is the appearance of pyroxene dendrites forming frond-like feather patterns in a plagioclase matrix (see Photos 3a, 3b). Rarely, the feldspar also exhibits this growth pattern. At this locality the dendritic pegmatites occur as pods; elsewhere (e.g., Agnew Lake Intrusion) they have been observed in layer-like zones, dendrites pointing upward, reminiscent of crescumulate textures. In the East Bull Lake Intrusion, rocks hosting these textural varieties of gabbronorite have been mapped by Born (1979) as a conformable stratigraphic zone of the Upper Series of the intrusion. More recently, D Peck and P. Chubb have interpreted these rocks as late diapiric melts that were intrusive into the pre-existing stratigraphy. UTM 406755E, 5142610N. Return to vehicles, continue east. 1.1 km Park at edge of road by large outcrop. Stop 8. Layered Gabbronorite Zone This outcrop exposes delicate small-scale, centimetre- to decimetre-scale layering in the Layered Gabbronorite Zone, Upper Series. Unlike the layering observed so far today, there is the suggestion here of current action and erosion of earlier formed cumulates suggestive of either active convection or slumping and/or soft sediment deformation of units. Rock compositions in the lower part of the outcrop are medium-grained gabbronorite, whereas at the top of the outcrop it is a coarse-grained leucogabbronorite. UTM 406900E, 5142790N. Return to vehicles, continue east. 2.2 km Park at edge of road in pullout area. To reach this stop, head downhill toward the north end of Moon Lake. Cross the beaver dam at the end of the lake and follow the slope roughly ~200 m to the south to an area where scattered low-lying outcrops occur on the west side of Moon Lake at the base of the central plateau. This traverse should not be made alone and should be only conducted using appropriate footwear and field equipment. 30 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Stop 9. Olivine Gabbronorite Zone This stop is located in the Olivine Gabbronorite Zone, within the uppermost part of the Main Series of the East Bull Lake Intrusion. Rocks of this zone occur as metre-scale layered sequences with the more mafic olivine-rich cumulates forming the dominant rock types interspersed with thin, leucocratic plagioclasedominant layers. In these rocks olivine, plagioclase and occasionally orthopyroxene are primocryst phases. Olivine and plagioclase compositions are more primitive than in zones above and below this sequence. The Olivine Gabbronorite Zone is interpreted to represent an influx of more primitive magma into the magma chamber which interrupted the fractional crystallization process that links the Rhythmically Layered Zone of the Main Series with the Layered Gabbronorite Zone of the Upper Series. UTM 407370E, 5142942N. Return to vehicles, continue east. 2.6 km Park alongside the road where possible. Stop 10 is located on an outcrop about 20 m west of the AECL road. Stop 10. Rhythmically Layered Zone, East Bull Lake Intrusion This outcrop shows metre-scale layering in rocks of the Rhythmically Layered Zone, Main Series, that have a typical shallow dip to the west. At the road, these rocks are cut by a 5 to 10 m wide, northwesttrending diabase dike. As previously noted, such dikes are abundant throughout the intrusion. UTM 407585E, 5142708N. Return to vehicles, continue east. 3.3 km Park on the west side of the road in small pullout area. This stop is a kilometre-long walk through the units that form the margin (bottom or sidewall) of the East Bull Lake Intrusion in the Moon Lake area. The trace of the traverse is shown in the accompanying map of this area (Figure 6) that was prepared by D. Peck and P. Chubb. This traverse should not be made alone and should be only conducted using appropriate footwear and field equipment. Stop 11. Moon Lake Traverse Stop 11 is a traverse through the Lower Series and the Border Zone of the intrusion, which are both areas where disseminated copper-PGE mineralization is common. The Border Zone (see Figure 6, units 4a to 4e) represents footwall and/or sidewall lithologies to the main intrusion. It consists of Archean gneissic and metaplutonic rocks that are intruded by, or which contain, irregular to linear zones of anorthosite, leucogabbnorite and gabbronorite that appear to be related to the East Bull Lake Intrusion. The Border Zone is in abrupt contact with the Lower Series, which at this locality consists of the Inclusion-Bearing Zone (see Figure 6, units 6c and 6d) and the Anorthosite Zone (see Figure 6, unit 7a and 7b). Sulphide mineralization occurs in units 6c and 6d (Inclusion-Bearing Zone) proximal to the margin of the intrusion; leucogabbronorites in these units are typically very coarse grained, commonly varitextured, variably inclusion bearing, and show no evidence of layering. Xenoliths and autoliths vary from abundant to scarce to absent; they are typically 1 to 10 cm in maximum dimension. Autoliths vary in composition from pyroxenite to anorthosite. 31 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �LEGEND Aphyric and PlagioclasePhyric Diabase Dikes GABBROIC ANORTHOSITE SUBZONE 1 rn-Scale Layered Unit Leacogabbronorite ANORTHOSITE SUBZONE C Leacogabbrononte to Pyroxenite (pods) BORDER ZONE to Mediurn-Grained Gabbronorite and Anorthosite Camulaten Gabbroic and Anorthositic Veins and Anorthositic Veins and Granitoids and Anorthositic Veins, Sabordinate Granitoids ARCH EAN GRANITOIDS Tonalite, Tonalitic Gneiss and Granite SYMBOLS Geological Boandary Observed Geological Boundary Approximate Fault Traverse Route C:.) Outcrop Main Zone ot PGE-Cu-Ni Mineralization Figure 6. Geological map of the Moon Lake area, East Bull Lake Intrusion, showing the route of the traverse (heavy line) undertaken during Stop 11. Shaded areas indicate areas of disseminated sulphide mineralization. D.C. Peck and P.C. Chubb originally prepared the map in 1991 for the Department of Earth Sciences, Laurentian University. Inset map in the upper right indicates in detail the route of the traverse, as well as outcrop areas crossed during the traverse. Numerical rock codes in the legend correspond to those used in Figures 2 and 3, and on the map of Chubb, Hannila and Peck (1994). Note that UTM coordinates in this figure are in Zone 17, datum NAD 27. Trace to 3% disseminated chalcopyrite, pyrrhotite ± pentlandite and pyrite ocurs in these units. Typically the sulphide minerals are medium- to coarse-grained aggregates in an epidote (saussurite)-rich zone of alteration in plagioclase, and also as much finer grained xenoblasts in the same silicate minerals. The sulphide mineral assemblage appears to have both a magmatic and metamorphic and/or hydrothermal paragenesis. In these rocks Cu+Ni = 0.1 to 0.5 wt %, with Cu>Ni, and Pd+Pt = 0.5 to 10g/t and Pd/Pt = 1 to 5. The Anorthosite Zone (see Figure 6, units 7a and 7b) consists of coarse- to very coarse-grained massive anorthosite and leucogabbronorite. Altered pyroxenes locally occur as coarse-grained interstitial anhedral to subhedral crystals and also as 1 to 10 cm zones (aggregates of crystals) that are interpreted to have formed from fractionated interstitial liquids due to prolonged early crystallization of plagioclase feldspar. As illustrated in the inset map in Figure 6, the traverse route encounters the following sequence 32 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �of rock types: units 7a and 7b (Anorthosite Zone); unit 6c (Inclusion-Bearing Zone), unit 4a (Border Zone), units 6d and 6c (Inclusion-Bearing Zone). The shaded area in Figure 6 shows the general distribution of the PGE mineralization, which may be associated with disseminated sulphide minerals. Some more specific data on the mineralization in the Moon Lake area is given below, summarized from detailed information presented in Peck et al. (1995, Appendix 5, Part A). Border Zone (unit 4) rocks related to the East Bull Lake Intrusion include gabbro, quartz gabbro and leucogabbro. Unlike the sulphide mineral assemblage in the Lower Series of the intrusion, pyrite is particularly abundant. In this zone Pd+Pt normally does not exceed 700 ppb with Pd/Pt ≈ 2. Cu+Ni is in the range 80 to 1300 ppm and typically Cu ≈ Ni. Mineralization in the Inclusion-Bearing Zone shows a range in Pd+Pt = 150 to 3000 ppb and Pd/Pt = 2 to 6; with Cu+Ni = 700 to 7000 ppm and Cu/Ni = 2 to 4. The data that is reported by Peck et al. (1995) indicates that the range and ratios for the PGE, Cu and Ni for the Anorthosite Zone are very similar to those for the Inclusion-Bearing Zone. Unpublished semiquantitative data collected by D. Peck and B. Jago on the platinum group minerals in 2 samples from the Inclusion-Bearing Zone in the Moon Lake area indicate that the PGE in the samples comes from 1 to 10 micron-size grains of sperrylite, moncheite, Bi-kotulskite and platarsite/hollingsworthite. These phases are normally found at grain boundaries of one or more of chalcopyrite, actinolite, epidote, albite or quartz, or within the silicate minerals in fractures or cleavage planes and much less commonly in chalcopyrite. Return to vehicles, continue east on AECL road to Highway 533. 4.0 km Junction of the AECL road and Highway 533. Turn south (right) onto Highway 553. 4.5 km Junction, Highway 533 and Parisien Lake trail on the east side of the highway. The trail is poorly marked and partly overgrown by saplings. To reach the key outcrop areas of Stops 12A and 12B, walk east on the trail for 20 to 30 minutes to a stripped north-trending ouctrop area (Stop 12A). Following examination of these stripped outcrops, continue east to a large east-trending stripped area (Stop 12B). The 2 stripped areas expose rocks of the Marginal and Lower Series, and associated mineralization along the south side of the East Bull Lake Intrusion, respectively. This traverse should not be made alone, and should be only conducted using appropriate footwear and field equipment. Stop 12A. Parisien Lake Traverse This series of north-trending, stripped outcrops lies perpendicular to the contact of the East Bull Lake Intrusion, and consists of rocks of the Border Zone (footwall lithologies) and mineralized leucocratic rocks of the Lower Series. Walking north from the southernmost outcrops in the Border Zone, one observes layered Archean metasedimentary gneisses intruded by Parisien Lake syenite. Both rock types are in sharp contact on the north side of the outcrop with a Matachewan plagioclase-phyric diabase dike; an alternative interpretation is that the diabase represents a unit of the Gabbronorite Zone in the Marginal Series. The north side of the diabase is in sharp fault (?) contact with a 3 to 5 m thick, lens-shaped zone of sulphide-rich metapyroxenite to peridotite. The silicate minerals in this zone consist of pyroxene and/or olivine replaced by medium-grained calcic amphibole (tremolite?) enclosed and intergrown with fine- to medium-grained magnetite-chalcopyrite-pentlandite. The opaque-rich material in the zone is typically a foliated, medium- to coarse-grained, calcic amphibole-magnetite-pyrite-pyrrhotite assemblage. To the north of this mafic autolithic zone is a 20 m thick unit of varitextured leucogabbronorite which has abundant disseminated interstitial magmatic pyrrhotite-chalcopyrite-pentlandite and minor pyrite mineralization. Locally the silicate mineral assemblage has been altered to rusty zones of fine- to medium-grained biotite-actinolite-chlorite-feldspar and up to 5% idioblastic tourmaline (no sulphide 33 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �minerals). Platinum group element content in grab samples range from 1.4 to 2.4 g/t; channel samples have analysed 0.84 g/t over 21 m, including 1.36g/t over 6 m. This rock becomes “flinty” in the northernmost part of the stripped outcrop, probably reflecting mylonite development proximal to the Parisien Lake deformation zone. Continue to walk east along trail. Roughly 3 minutes to the east, you will encounter a long series of stripped outcrop that are oriented subparallel to the trail. Stop 12B. Parisien Lake Traverse The dominant rock type is coarse-grained leucogabbronorite, with lesser amounts of melagabbronorite, at the west and north ends of this large stripped outcrop. Some of the melagabbronorite could be interpreted as disrupted dikes. In a few places discontinuous, sinuous, steeply dipping, centimetre- to metre-scale layering is observed in the leucogabbronorites. The orientation of the layering is subparallel to the margin of the intrusion. Both rock types host disseminated sulphide minerals. Convoluted boundaries with reaction zones indicate interaction between the leucogabbronorite and the melagabbronorite, suggesting that magma mixing occurred between these phases. At the east end of the stripped outcrop area, leucogabbronorite is in sharp contact with Parisien Lake syenite. This contact represents the boundary between the Lower and Marginal Series of the intrusion. UTM co-ordinates at the east end of the stripped area are 409788E, 5141490N. This concludes Day 1. Retrace route back to Highway 533. 34 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �AGNEW LAKE INTRUSION—OVERVIEW Country Rocks, Structure, and Geometry of the Intrusion This section is largely based on data reported in Vogel (1996), Vogel, James and Keays (1998) and Vogel et al. (1999). The main mass of the Agnew Lake Intrusion is located on the west side of Agnew Lake, roughly 65 km southwest of Sudbury (see Figure 1). It is similar in age and size to the East Bull Lake Intrusion (2491 versus 2480 Ma and 50 km2 versus 43 km2, respectively, see Table 2). It is linked to the East Bull Lake Intrusion, on its northwest side, by the Streich dike, a 200 to 300 m wide composite body with a strike length of approximately 10 km. Pink to white, coarse-grained granites and orthogneisses of the Neoarchean Ramsay–Algoma gneiss complex (Jackson and Fyon 1991) form the footwall rocks to the intrusion around its north, west, and south sides (Figure 7a). There are only rare occurrences of footwall breccia zones at the intrusion's contact with the granitic rocks equivalent to the Border Zone of the East Bull Lake Intrusion. More commonly there is an abundance of felsic dikes (thermal aureole partial melts) intruding older granitic basement rock, particularly where mafic (feeder) dikes are abundant near the intrusion contact. The east margin of the intrusion is in fault contact with, or is apparently unconformably overlain by, McKim Formation metasedimentary rocks of the Huronian Supergroup (see Figure 7a). The longitudinal axis of the Agnew Lake Intrusion is 110 to 120° (see Figures 7a and 7b), which is parallel to the axis of the East Bull Lake Intrusion, the Streich dike and Matachewan diabase dikes in the southernmost Superior Province (Halls and Bates 1990). This trend is thought to reflect the orientation of the rift structure that permitted magma intrusion. The attitude of igneous layering in the intrusion is illustrated in Figure 7b. It shows that the body has an asymmetric (synclinal) shape with the axis of the structure plunging gently northeast. Its axial plane is parallel to that of Penokean folds in younger Huronian Supergroup strata. This fact and its high angle of intersection with the longitudinal axis of the intrusion and Streich dike, suggests that the present geometry of the Agnew Lake Intrusion is due to Penokean deformation of a lopolith. Stratigraphy and Petrography Figure 3 presents a composite stratigraphic section for the Agnew Lake Intrusion. It consists of a Marginal, Lower and Upper Series which correlate, as illustrated, with the Lower, Main and Upper Series of the East Bull Lake Intrusion. Mafic mineralogy is replaced by calcic-amphiboles formed during lower amphibolite facies, Penokean-age metamorphism. Compositions of plagioclase cores commonly appear to be near original magmatic values. Primary igneous textures are preserved, but less clearly than in the East Bull Lake Intrusion. MARGINAL SERIES The Marginal Gabbronorite Zone (unit 1, see Figures 3 and 7a) is characterized by a massive, mediumgrained, commonly olivine-normative, gabbronorite. It occurs as units up to 200 m thick with chilled outer contacts against the footwall and encloses large in situ remnants of granitic footwall. Sills or dikes, 1 to 20 m thick with chilled margins, commonly occur at the intrusion contact, and may be related to young dike sequences that transect the entire intrusion. 35 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Figure 7. Geology of the Agnew Lake Intrusion. a) Geologic map showing main stratigraphic units; unit numbers correspond to those in Figure 3 and in the text. b) Structural map showing attitude of centimetre- and metre-scale layering. The strike and plunge of the axial plane of the synclinal, lopolithic structure is the same as that in the Huronian Supergroup supracrustal rocks on the east side of Agnew Lake. Both figures modified from Vogel (1996). 36 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �The Marginal Leucogabbronorite Zone (unit 2, see Figures 3 and 7a) averages 200 m thick and occurs along most margins of the intrusion, where it either overlies the younger, Marginal Gabbronorite Zone or is in direct contact with the footwall. It consists of 3 distinctive units, namely, the Varitextured, Mottled and Nodular units. The Varitextured Unit occurs in the west and south margins of the intrusion, where it is dominated by leucocratic gabbronorites; mafic layers or lenses are common as are felsic differentiates. Grain size is highly varied on a centimetre-scale, from medium-grained to pegmatitic. These rocks are very similar to varitextured gabbronorite pods in the Anorthosite Zone in the East Bull Lake Intrusion. The equivalent, or somewhat younger, unit in the north-central margin of the Agnew Lake Intrusion is the Mottled Unit, which is characterized by large amphibole oikocrysts (after pyroxene) in a plagioclase-rich matrix. As with the Varitextured Unit, smaller but significant gabbronorite, melanogabbronorite and anorthosite also occur. Layering in the Mottled Unit is irregular and discontinuous. Of great significance in both units is the presence of centimetre- to metre-scale inclusions and pods of melanogabbronorite, footwall granite and, rarely, massive quartz where the unit is adjacent to the footwall. Disseminated PGE-rich chalcopyrite and pyrrhotite are common in the inclusion-bearing parts. The Nodular Unit forms large lens-like areas in the northwest part of the intrusion. It is distinguished by the dominance (70 to 90 modal percent) of 6 cm diameter, well-sorted and closely packed plagioclase nodules in a coarse-grained amphibolitized pyroxenitic matrix. Disseminated PGEbearing sulphide minerals sporadically occur in the matrix. Whole-rock Mg-number indicates this unit is the most fractionated of the Marginal Series. All of the units in this series have significant proportions of rock with normative olivine in the 1 to 10% range and supporting textural evidence for the presence of modal olivine. Plagioclase core compositions range from An77 to An32. LOWER SERIES The base of this series is the Inclusion-Bearing Gabbronorite Zone (unit 3, see Figures 3 and 7a). The zone is about 180 m thick and hosts footwall and cognate xenoliths; associated copper-nickel-PGE mineralization; and is limited in outcrop to the northwest margin of the Agnew Lake Intrusion where it leads to the Streich dike. Inclusions of Marginal Series rocks are common—especially of the Nodular Unit—in a gabbronorite matrix (An79-52). Similar rocks form the Inclusion-Bearing Zone at the base of the Lower Series in the eastern lobe of the East Bull Lake Intrusion (Chubb 1994). The overlying 350 to 400 m of the Lower Series is represented by the Lower Gabbronorite Zone, consisting of a Massive (4a) and a Lower Layered Unit (4b) (Photo 4), that are separated by a younger conformable phase of the Dendrite Unit (8) (Photo 5). The Massive Unit consists of massive, medium- to coarse-grained gabbronorite and leucogabbronorite (An79-61) and rare melanocratic inclusions. Unit 4b (see Figures 3 and 7a) is a layered (centimetre- to metre-scale), medium- to coarse-grained gabbronorite or leucogabbronorite (An65-53). Rocks in both units are commonly olivine-normative. These 2 units are similar to rocks in the Anorthosite and Rhythmically Layered zones, respectively, in the East Bull Lake Intrusion. Dendrite Unit rocks are coarse-grained to pegmatitic gabbronorites, commonly weakly layered, and typically varitextured, with a diagnostic feature being the common occurrence of delicate, branching, pyroxene crystals several centimetres long with interstitial plagioclase, titanomagnetite and granophyre. This unit also occurs higher in the stratigraphy and formed from a more evolved magma than its host rocks. 37 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Photo 4. Centimetre- to decametre-scale igneous layering within rocks of the Agnew Lake Intrusion. V Photo 5. Dendrite texture developed with the Lower Gabbronorite Zone of the Agnew Lake Intrusion. Compare with Photo 3a and 3b. Similar rocks will be observed at Stop 7, Day 2. 38 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �UPPER SERIES Most of the rocks in this series have not been recognized in other intrusions of the East Bull Lake intrusive suite; either they were never deposited or they were eroded away. The Olivine Gabbronorite Zone (unit 5, see Figures 3 and 7a) is notably more mafic than other units in the Agnew Lake Intrusion. It consists of centimetre- to metre-scale, layered olivine gabbronorite and leucogabbronorite. Disseminated sulphide minerals are common and PGE contents can be as high as those present in the inclusion-bearing rocks in the Marginal and Lower Series. Similar strata, 50 to 60 m thick, occur at the same stratigraphic level in the East Bull Lake Intrusion. The Upper Gabbronorite Zone (units 6a to 6e, see Figure 3) forms about 1000 m of the stratigraphy of the Agnew Lake Intrusion. Vogel, James and Keays (1998) and Vogel et al. (1999) divided it into 5 units. Olivine is absent from rocks in the Upper Gabbronorite Zone. Plagioclase and orthopyroxene are the primocryst (cumulate) phases, with plagioclase compositions being mostly in the range An65-40. The upper Layered (6a) and Mixed units (6b) underlie the Porphyritic Unit (6c), which forms the main mass of this part of the stratigraphy (see Figures 3 and 7a). The Porphyritic Unit is a dominantly quartz-normative rock unit characterized by varied amounts of plagioclase phenocrysts and glomerophenocrysts set in a gabbronorite matrix. Diffuse, macrorhythmic, decametre-scale layering of plagioclase-phyric leucogabbronorite and nonporphyritic gabbronorite is typical of this unit. The Fe-Ti Oxide Zone (units 7a and 7b, see Figure 3) forms the upper 200 to 400 m of the Agnew Lake Intrusion; its base is marked by the first appearance of titanomagnetite primocrysts. Primary igneous plagioclase is totally altered in this zone and magmatic textures are very poorly preserved. The Leucogabbro Unit (unit 7a, see Figure 3) forms the lower part of this zone. It consists of massive, coarsegrained leucogabbro (normative clinopyroxene greater than orthopyroxene) that has a distinctive clotty texture as a result of metamorphic biotite growth around altered primocryst titanomagnetite. Metamorphic garnet occurs sporadically in these iron-rich rocks. Of particular interest is the presence of disseminated sulphide minerals (with anomalous PGE), most commonly at the top of the unit. The basal contact of the overlying Ferrosyenite Unit (unit 7b, see Figure 3) is not exposed, but has been drilled by New Millenium Minerals Corporation (which became Platinum Group Metals Limited in October 2001 as the result of a merger). In this unit, ferrosyenite grades upward into alkali feldspar granite. The ferrosyenite consists of relatively scarce phenocrysts of albite (An3) and as much as 25 modal percent primocryst magnetite in a recrystallized feldspar-quartz matrix. The magnetite-rich phase of this unit is steel-grey, massive and fine grained. The overlying granite is a white, recrystallized quartz-feldspar assemblage with less than 5% magnetite. Normative calculations suggest that the igneous assemblage had as much as 30% potassium feldspar. Geochemistry A comprehensive geochemical database for the Agnew Lake Intrusion can be extracted from Vogel (1996) and a representative suite is presented in Vogel et al. (1999). Representative analyses for units from the Upper Series (not represented in the East Bull Lake Intrusion) and the Streich dike are presented in Table 4. The latter is an estimate of the parent magma composition for all units in the intrusion except for the Olivine Gabbronorite Zone (see Figures 3 and 7a). As with the East Bull Lake Intrusion, it is a high-Al, low-Ti, tholeiitic composition. Rocks in the Marginal and Lower Series of the Agnew Lake Intrusion (except for unit 8, the Dendrite Unit) are olivine-normative. Olivine leucogabbronorite compositions dominate in the Marginal Series (Mg-number 65-76), whereas olivine gabbronorite is dominant in the Lower Series (Mg-number 39 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �55-74). In the lower two-thirds of the Upper Series, gabbronorite and their leucocratic equivalents (Mgnumber 63-31) dominate. Ferrogabbro, ferrosyenite, and alkali granite compositions (Mg-number 46-12) characterize the Fe-Ti- Oxide Zone. Detailed stratigraphic studies in the Marginal and Lower Series show that these rocks formed in an open system where frequent magma pulses produced a rather monotonous sequence of mafic rocks with thin irregular intervals of fractionated material (Vogel 1996). Pearceelement ratio plots in Figures 4a and 4b show that the main chemical variation is due to plagioclase and olivine for the Marginal Series, and plagioclase, olivine and orthopyroxene for the Lower Series. All units in the Agnew Lake Intrusion show flat heavy rare earth element (HREE) patterns (10 to 40 times chondrite) and light REE enrichment (6 to 100 times chondrite for La) (see Figure 5b). REE element concentrations for the Marginal (unit 2) and Lower Series (units 3, 4a, 4b) are the most primitive of the suite (see Figure 5b). Both show positive Eu anomalies and both are more primitive in composition than the Streich dike parent magma composition. Vogel et al. (1999) illustrated, using incompatible elements such as Y, Zr, and Nb, that rocks from the Upper Series of the Agnew Lake Intrusion ultimately fractionate to form iron-rich differentiates, namely, the Fe-Ti Oxide Zone of associated ferrogabbros and oxide-rich ferrosyenites. There is no physical or geochemical evidence that these differentiates are due to magmatic assimilation of country rock. Rather, this part of the intrusion, at least in its upper portion, has apparently become a closed system, i.e., Skaergaard-like in terms of magmatic process (McBirney 1996). Pearce-element ratio diagrams (see Figure 4) and petrography indicate that plagioclase and orthopyroxene are the major phases that account for the chemical variation of rocks in the Upper Gabbronorite Zone. REE data for this zone (unit 6c) are consistent with it being less primitive than the underlying Marginal and Lower Series rocks (see Figure 5b). Rocks in the Leucogabbro subzone (unit 7a) near the top of the stratigraphy are classified as iron-rich gabbronorites by Vogel (1996). Here titanomagnetite ± clinopyroxene join plagioclase and replace orthopyroxene as the dominant mafic phases. Titanium is not a conserved element in rocks of the Fe-Ti Oxide Zone; consequently, as seen in Figure 4b, rocks from this zone are distinct from all previously described units, and their trend can not be ascribed to fractionation by a specific phase or phases. The REE pattern for this subzone (see Figure 5b) mimics the previous patterns but has substantially higher total REEs, consistent with these rocks being comagmatic with the underlying part of the Upper Series. In the Ferrosyenite subzone (unit 7b) at the roof of the intrusion, titanium is not conserved in the Pearce-element ratio plots. The trend of the data for these rocks extends from near the origin parallel to the Al/Ti axis, reflecting the varied proportion of alkali feldspar in these calcium-poor rocks. The slope of the data in Figure 5b for this subzone, and their increased total REE abundance is consistent with it being a fractionated part of the intrusive sequence. The negative Eu anomaly is due to the absence of plagioclase. Although rocks of the Olivine Gabbronorite Zone occur in the central part of the Agnew Lake Intrusion stratigraphy (see Figure 3), the trend they define in the Pearce-element ratio plot (see Figure 4a) shows that they are, on average, more mafic than strata above and below. Their trend intersects that defined by the rest of the rocks in the Agnew Lake Intrusion near the estimated parent magma composition (Streich dike) indicating that the 2 groups are comagmatic. These data are consistent with previously discussed mineralogical data. In Figure 5b, the REE pattern for rocks from the Olivine Gabbronorite Zone has a slope that is similar to other Agnew Lake Intrusion rocks, supporting the comagmatic nature of the 2 groups. The low total REE abundance is indicative of the relatively primitive and mafic character of this zone. Mineralization Finely disseminated and erratically distributed, blebby (up to 3 cm in diameter) chalcopyrite and pyrrhotite, with minor to trace pentlandite, in modal amounts from 0.5 to 2.0%, typically characterize 40 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �mineralized localities in the Agnew Lake Intrusion. Typically Cu is greater than Ni and combined Cu+Ni is approximately 1 to 1.5 wt %. In a high proportion of sulphide-rich samples, values of Pt+Pd = 0.5 to 3.0 g/t are not uncommon and values as high as 10 g/t do occur, with Pd/Pt typically 1 to 3 (data from Assessment File Reports filed by BP Resources Canada Limited, Resident Geologist’s office, Sudbury). Although there is often a positive correlation between PGE, Cu and total sulphide contents, some of the highest PGE samples have less than 100 ppm S. As with the East Bull Lake Intrusion, the greatest sulphide mineralization and highest PGE values have been found in inclusion-bearing units, with the sulphide minerals normally occurring in the matrix. Mineralization occurs in the Marginal Leucogabbronorite Zone at the stratigraphic base of the Agnew Lake Intrusion, specifically along its west margin. The varitextured Leucogabbronorite unit (unit 2a, see Figure 7a) is highly prospective within 50 to 100 m of the contact, as is the Mottled Leucogabbronorite (unit 2b) along the north margin of the body. The rocks that host this mineralization are very similar in mineralogy, texture and stratigraphic position to those that host sulphide mineralization in the Anorthosite Zone in the western East Bull Lake Intrusion (i.e., Moon Lake area). Mineralized, fragment-rich rocks also occur in the Inclusion-Bearing Gabbronorite Zone (unit 3, see Figures 3 and 7a), which forms the lowest subdivision of the Lower Series. The mineralogy and textural distribution of the silicate and sulphide mineral phases is the same as in the Marginal Series. These inclusion-bearing rocks intrude units of the Marginal Series, in particular the Nodular Unit, and clearly represent a major phase of magma injection into the magma chamber. The similarity in rock types, mineralization and field relationships with those at the east and west ends of the eastern lobe of the East Bull Lake Intrusion, suggest that the Lower Series, Inclusion-Bearing Zone, in the Agnew Lake Intrusion is correlative with the Lower Series, Inclusion-Beaing zone, in the East Bull Lake Intrusion (see Figure 3). A third significant mineralized zone occurs near the top of the intrusion in the uppermost part of the Leucogabbro subzone and immediately below the Ferrosyenite subzone (see Figure 3). The mineralization was predicted to be present by Vogel (1996), forming as a result of closed-system fractionation of a sulphur-undersaturated, PGE-rich magma. This type of occurrence is characterized by disseminated chalcopyrite and pyrrhotite, in amounts of 1 to 2 modal percent, that increase in abundance toward the contact with the overlying Ferrosyenite subzone. Mapping and trenching by New Millenium Minerals Corporation has identified several occurrences of this type of mineralization near this contact along the eastern side of Agnew Lake. A fourth environment merits mention although mineralization has yet to be recognized. The Olivine Gabbronorite Zone at the base of the Upper Series is believed to represent an influx of primitive magma (Mg-number ~70) into a chamber dominated by fractionated liquids (Mg-number ~55). Thus, the opportunity may have existed for the formation of reef-type mineralization due to magma mixing. This idea is supported by the presence of Pt+Pd values as high as 0.5 to 1.0 g/t in rocks of the same type and stratigraphic level in the East Bull Lake Intrusion (Peck et al. 2001). Vogel (1996) analyzed 22 barren samples from the Agnew Lake Intrusion, sampling all stratigraphic units shown in Figure 3. Since the PGEs partition strongly into sulphide liquid, their abundance in a silicate rock is an excellent indicator of the status of sulphur-saturation of the magma from which the sample crystallized (Hamlyn and Keays 1986; Vogel and Keays 1997). In the barren samples, Pt varies from 0.7 to 62 ppb, with most samples in the 10 to 30 ppb range; and Pd varies from 0.2 to 58 ppb, with most samples ranging from 10 to 40 ppb. These data mimic those for the East Bull Lake Intrusion (Peck et al. 1995) and are very similar to most, non-reef, silicate mineral assemblages from the Lower, Critical and lower Main Zones of the Bushveld Complex (Barnes and Maier 1999). When plotted against stratigraphic position, Pt and Pd data show peaks: 1. at the base of the Marginal Series 2. at the base of the Inclusion-Bearing Zone, Lower Series 41 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �3. near the estimated position of the Olivine Gabbronorite Zone, and 4. at the base and the top of the Leucogabbro subzone near the roof of the intrusion. All correspond to areas of known or suspected mineralization. Mantle-normalized PGE distribution for barren rocks from the Agnew Lake Intrusion is similar to that for rocks of the East Bull Lake Intrusion. The Pt-Pd-Au-Cu part of the figure is elevated (1 to 10 times mantle abundance except for Au) relative to the Ni-Ir-Ru part (<0.01 to 0.5 times mantle values) (James et al. 2002a). The anomalous PGE concentrations, together with typically very low sulphur contents (≤150 ppm), of barren samples from the Agnew Lake Intrusion indicate that the main mass of Agnew Lake Intrusion magma did not experience sulphide saturation prior to emplacement into the chamber. All samples plot in the sulphur-undersaturated area in the Cu-Pd diagram of Vogel and Keays (1997). Peach, Mathez and Keays (1990) estimated that saturation for mantle-derived magmas is about 800 ppm, well above what is observed in these rocks. Peck et al. (2001) argue that contact-type mineralization in the Marginal and Lower Series of the East Bull Lake Intrusion formed from magmas that were sulphur-saturated prior to intrusion. Mineralization was further upgraded in PGE content by prolonged high temperature mixing of very small volumes of sulphide phases with high volumes of silicate liquid upon entering the magma chamber. The complex, chaotic, varitextured nature of the rocks that host mineralization in the Marginal and Lower Series of the Agnew Lake Intrusion, and the evidence that the magma system was “open” during formation of this part of the intrusion is consistent with the model of Peck et al. (2001). The mechanism to explain mineralization near the roof of the intrusion, i.e., sulphide saturation due to fractionation in a closed system, is also consistent with a PGE-rich, sulphur-undersaturated, parent magma. It also allows for the possibility of reef-type mineralization at the base of the Upper Series due to magma mixing between the sulphur-undersaturated, parent magma and the sulphur-saturated, fractionated magma present within the evolving magma chamber (e.g., the Olivine Gabbronorite Zone, unit 5, see Figure 3.) ROAD LOG, DAY 2, AGNEW LAKE INTRUSION Note: Most of the roads used during day 2 are used regularly by logging trucks. Consequently, extreme caution should be taken when parking vehicles on the shoulder and when examining outcrops during day 2. Geological map reference: Vogel (1996), Card and Palonen (1976). 0.0 km Start at the junction of Highway 69S and the Highway 17W bypass. Set odometer to zero. Head west on Highway 17 to Massey. 0.4 km Junction of Highway 17W bypass and Regional Road 80 (Long Lake Road). Continue west on Highway 17 bypass. 70.9 km Junction of Highway 17 and 6, just north of Espanola. Continue west on Highway 17. 76.4 km Junction of Highway 17 and Agnew Lake Lodge Road on the west side of the hamlet of Webbwood. Turn right (north), and follow Agnew Lake Lodge Road north to the intersection with West Branch Road. 82.4 km Turn left onto West Branch Road and drive north for 17.0 km to the intersection with Power Line Road. 42 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �99.4 km Junction between West Branch Road and Power Line Road, turn left and drive west on Power Line Road. 100.8 km Junction between Power Line Road and Quarry Road. Park vehicles and walk south and west from junction onto a large outcrop. Stop 1. Inclusion-Bearing Unit UTM 4226691E, 5137144N. This outcrop is within the Inclusion-Bearing unit, which occurs at the base of the Lower Gabbronorite Zone (Unit 3, Vogel (1996) map, outcrop #153). On the west side of this outcrop the Inclusion-Bearing unit is in contact with the rocks of the Neoarchean Ramsay–Algoma gneiss complex. This stop shows the following features. 3 1. Overall the unit consists of metre-scale, pod-like autoliths of nodular anorthosite and pyroxenite in a medium grained, massive leucogabbronorite to melagabbronorite matrix (Photo 6). The outcrop is cut by plagioclase-phyric Matachewan diabase dikes with chilled margins. The dikes strike 340°, and exhibit fractures and veins that trend 250° and which show sinstral displacement. Photo 6. Stop 1, Day 2. Large pyroxenite pod in centre of photograph is hosted by rocks of the Inclusion-Bearing unit, shown in the foreground, which at this locality are hosted in a predominantly leucogabbronoritic matrix. Scale card near contact between the pod and the leucogabbronorite is 8.5 cm long. 43 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �2. PGE concentration for disseminated chalcopyrite-pyrrhotite-pentlandite mineralization averages 107 ppb Pd+Pt based on 723 analyses; the range is 1 to 1873 ppb, Pd/Pt = 2.3 and Cu/Ni = 2.1. 3. In part of the outcrop, pyroxenite (assemblage blue-green hornblende+quartz+magnetite in one sample) occurs as discontinuous lenses, or pods, that are 30 to 40 m in length and 5 m in width, strike 290° (see Photo 6), and which contain trace disseminated copper-nickel-PGE mineralization. 4. Assay data for the pyroxenite pod shown in Photo 6. Sample 154547, at the edge of pyroxenite pod has Pd+Pt = 264 ppb, Pd/Pt = 4.6, Ni+Cu = 969ppm, Cu/Ni = 4.2, S = 1800 ppm. Sample 154533 has Pd+Pt = 37 ppb, Pd/Pt = 1.5; Cu+Ni = 220 ppm, Cu/Ni = 1.3, S = 200ppm. Samples 154607 and 154739, middle of pyroxenite pod, have PGE at or below detection limits. 5. Granitic xenoliths are common in the Inclusion-Bearing unit at the contact between the intrusion and the older granitic gneisses at the west margin of the outcrop. Return to the Power Line Road and walk north for about 200 m along the Quarry Road to Stop 2. Stop 2. Nodular Anorthosite Unit, Marginal Leucogabbronorite Zone The quarry outcrop (Photo 7a) is an excellent example of the Nodular unit, the youngest unit of the Marginal Leucogabbronorite Zone, Agnew Lake Intrusion (Unit 2c, Vogel (1996) map, outcrop #126). This is the only area of the intrusion where this unit is observed and its age relationship relative to the previous outcrop area is clear as it occurs as autoliths in the Inclusion-Bearing unit. It occurs in a similar stratigraphic and marginal position at the east margin and the central narrow part of the East Bull Lake Intrusion (Nodular Anorthosite unit, Anorthosite Zone; Chubb, Hannila and Peck (1994)). At this locality the principal rock type is a very coarse-grained, nodular, glomeroporphyritic anorthosite (Photo 7b). It consists of glomerophenocrysts of plagioclase (1 to 14 cm in diameter) that form 80 to 90% of the rock and interstitial altered pyroxene. One to 3% disseminated chalcopyrite and pyrrhotite occur in the matrix to the nodules. Peck et al. (1995, Appendix 5, Part A) reported Pd+Pt in the range 35 to 470 ppb, Pd>Pt, Cu+Ni = 244 to 2138 ppm and Cu>Ni from similar sulphide mineral-bearing rocks from the East Bull Lake Intrusion. Chubb et al. (1995) suggested that the nodules formed due to elevated degrees of undercooling within dike-like constrictions at the margin of the intrusion. Nodule formation may have resulted from feldspar growth on pre-existing plagioclase phenocrysts to form primary nodules, followed by agglomeration processes forming larger secondary and tertiary nodules. Northwest-trending plagioclase-phyric Matachewan diabase dikes intrude the Nodular unit in this outcrop (see Photo 7a). UTM 426718E, 5137364N. Return along the Quarry Road for about 30 m and walk onto the outcrop immediately east of the road. Stop 3. Inclusion-Bearing Unit, Lower Gabbronorite Zone In walking from Stop 2 to Stop 3 you have traversed into the Inclusion-Bearing unit of the Lower Gabbronorite Zone (Vogel (1996) map, outcrops #192 to 195). The main phase of the Inclusion-Bearing unit consists of metre-scale zones or autoliths of nodular anorthosite enclosed in a coarse-grained matrix 44 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Photo 7. Stop 2, Day 2. a) Stone quarry in Nodular Anorthosite unit. Author R.S. James is standing on nodular anorthosite; dark rocks in foreground belong to a Matachewan swarm diabase dike that intrudes the Nodular Anorthosite unit. b) Close-up of nodular anorthosite, showing closely packed plagioclase glomerophenocrysts with minor interstitial, altered, pyroxene. Scale card is 8.5 cm long. 45 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �of gabbronorite to leucogabbronorite. Two several metre-wide Matachewan diabase dikes, striking 290°, intrude the unit. A younger, Mesoproterozoic, 10 to 20 cm wide Sudbury swarm diabase dike (~1240 Ma) strikes 300° and intrudes all rock units. The Sudbury dike has narrow chilled margins, cooling fractures perpendicular to its margin, and contains trace disseminated sulphide minerals. It also contains small pyroxenitic and granitic xenoliths, and weathers a rusty brown colour, in contrast to the older Matachewan swarm dikes. At this locality Pd+Pt are as high as 2155 ppb and average 178 ppb (based on 154 analyses), with Pd/Pt = 3.1 and Cu/Ni = 1.4. UTM 426870E, 5137333N. Return to vehicles, and head east from the Quarry Road–Power Line Road junction. Reset odometer to zero. 0.2 km Park by side of road and walk to outcrop beside the road. (Vogel (1996) map, outcrop #215.) Stop 4. Inclusion-Bearing Unit, Lower Gabbronorite Zone This is an additional stripped outcrop area illustrating the Inclusion-Bearing unit. It contains autoliths of pyroxenite and nodular anorthosite in a matrix of leuco- to melagabbronorite (Photo 8). Disseminated sulphide minerals occur in the matrix assemblage and not in the autoliths and range from 1 to 5 modal percent. Assay values for this mineralization are Pd+Pt average 180 ppb (based on 451 analyses), ranging from 2 to 2012 ppb, with Pd/Pt = 3.1 and Cu/Ni = 1.5. Note the 2 to 5 cm wide plagioclase-phyric dike that is locally choked with euhedral plagioclase phenocrysts where the dike width narrows (Photo 9). UTM 427074E, 5137348N. Return to vehicles and head east on Power Line Road. 0.6 km Park by side of road and examine outcrop on the north side of the road (Vogel (1996) map, outcrops #216 and 217). Photo 8. Nodular anorthosite from the Inclusion-Bearing Gabbronorite Zone, Lower Series, of the Agnew Lake Intrusion, roughly 200 m south of the nodular anorthosite quarry, in the vicinity of Stop 4, Day 2. Note the pyroxenite fragment in the centre of the photograph. 46 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Photo 9. Diabase dike containing large, rounded plagioclase crystals, similar to the nodules of the Nodular Anorthosite unit, cutting across rocks of the Inclusion-Bearing Gabbronorite Zone, Lower Series, of the Agnew Lake Intrusion. Stop 5. Sudbury-Type Breccia In this outcrop, Sudbury-type breccia cuts through a Matachewan diabase dike that strikes 230°. The dike intruded rocks of the Ramsay–Algoma gneiss complex. The breccia consists of xenoliths of plagioclase, diabase, and granitic material, ranging from millimetre- to centimetre-sized, in a foliated, fine-grained matrix that, in part, has a distinctive flow-like fabric. Chubb et al. (1995) provide detailed descriptions of several similar dikes and argue that they represent pseudotachlyte that formed as a result of the Sudbury impact event at 1850 Ma. UTM 427504E, 5137368N. Return to vehicles and head east on Power Line Road. 1.4 km Junction with West Branch Road, turn right and head south on West Branch Road. 7.1 km Pull over and park vehicles, examine outcrops on both sides of the road (Unit 2a, Vogel (1996) map, outcrops #326 (east) and #329 (west side of road)). Stop 6. Varitextured Unit, Marginal Leucogabbronorite Zone This outcrop lies within the Varitextured Unit of the Marginal Leucogabbronorite Zone, Marginal Series. Both outcrop areas (AZ 1 – east side of road, and AZ 2 – west side of road) consist dominantly of gabbronorite to melagabbronorite and minor leucogabbronorite. Inclusions of pyroxenite and scarce nodular anorthosite are scattered throughout the matrix assemblage, indicating that at least part of the 47 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �outcrop area may belong to the Inclusion-Bearing unit. Disseminated sulphide mineralization occurs mainly in the gabbronorite matrix. Over 200 analyses from the 2 outcrop areas average 232 ppb Pd+Pt with individual analyses as high as 2375 ppb; Pd/Pt is about 2 as is Cu/Ni. Later quartz veins strike at 220° and are offset by fractures and/or faults that strike 290°. UTM 425257E, 5133932N. Return to vehicles and continue south on West Branch Road. 8.0 km Junction with logging road. Turn left (east) and drive east on the logging road for ~200 m to red flagging on road side, located at UTM 425917E, 5133294N. Follow the flagged line through bush, walking north at about 334° for approximately 440 m. On the route, you will cross Gridline 3-5, 275E of Pacific North West Capital Corporation. This traverse should not be made alone, and should be only conducted using appropriate footwear and field equipment. Stop 7. Dendrite Unit, Lower Gabbronorite Zone UTM 425744E, 5133665N. The outcrop is the bottom of a cliff face that shows extensive excellent exposure of the dendritic unit of the Lower Gabbronorite Zone. This unit is recognized as a distinct stratigraphic unit in the Agnew Lake Intrusion where it is well-exposed along the west and southwest margin of the plateau area that forms the central part of the intrusion. The textures and mineralogy are the same as observed the previous day at East Bull Lake Intrusion (Stop 7, Day 1), but here the dendritic texture is much more extensively developed. The texture occurs either as large irregular masses, sometimes almost layer-like, or as pod-like zones, particularly where this unit is gradational into the overlying layered gabbronorites. At this locality the dendritic habit of the altered pyroxenes can be rapidly replaced by coarse-grained gabbroic pegmatite assemblages, so that the rocks become quite varitextured. Examine the outcrop at the base of the cliff face and then work your way up the ridge, all the time observing the variation in rock types or the lack of variation. Having climbed the cliff face we will traverse east through the Dendritic Unit and into the overlying Layered unit of the Lower Gabbronorite Zone. In doing so we are walking through outcrop #333 into #334 (Vogel (1996) map). At the top of the cliff face (in outcrop #333) the dendritic gabbronorite is more typically restricted to local pod-like areas and enclosed in a coarse-grained to almost pegmatitic gabbroic rock. In the western part of outcrop #334 metre-scale isomodal phase layering is present, identical in general features to that observed in the East Bull Lake Intrusion. At this locality it dips approximately 30° to the east. Locally, pods of dendritic gabbronorite are observed in the Layered unit. The stratigraphic units that overlie the Layered unit to the east exhibit decametre or thicker scale layering. As a result, shallow dipping surfaces consisting of individual layers form extensive outcrops of apparently only one rock type. Return to the vehicles by walking southward (~160°) across the top of the plateau to intersect the logging road, then follow the road southwest to the parked vehicles at the base of the plateau. This ends Day 2. Retrace route to West Branch Road, Agnew Lake Lodge Road, and Highway 17. 48 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �RIVER VALLEY INTRUSION, OVERVIEW Country Rocks, Structure and Geometry The River Valley intrusion is located about 65 km east-northeast of Sudbury (see Figure 1) near the community of River Valley. The intrusion underlies parts of Crerar, Dana, Henry, Janes, McWilliams and Pardo townships, but is best exposed in Dana Township, where it locally contains well preserved primary mineralogy and textures (Hrominchuk 1999, 2000; Easton and Hrominchuk 1999, 2001a, 2001b; Easton 2001, 2003). Although recently mapped (Easton and Hrominchuk 1999, 2001a, 2001b; Easton 2003; Easton and terMeer 2004; Hrominchuk 1999, 2000), studies of the stratigraphy, geochemistry and mineralization present within the River Valley intrusion are limited in comparison to similar studies on the East Bull Lake and Agnew Lake intrusions. Of all the East Bull Lake intrusive suite bodies studied to date, the River Valley intrusion exhibits the clearest relationships with other members of the Paleoproterozoic rifting suite described earlier. It is cut by mafic dikes geochemically correlated with the Matachewan and Hearst dike swarms, as well as by felsic intrusive rocks coeval with the Huronian Supergroup metavolcanic rocks (Easton and Hrominchuk 1999, 2001b). Furthermore, in Street Township, granitic rocks dated at 2460±20 Ma, contain inclusions of East Bull Lake intrusive suite leucogabbroic and anorthositic rocks (Easton and Murphy 2002). Easton (2003) has reported similar inclusions in granitic plutons in Henry and Loughrin townships. Unfortunately, the relationship between the River Valley intrusion and the Huronian Supergroup is obscured by tectonism along the Grenville Front. Along the Grenville Front, the River Valley intrusion is either in thrust contact with quartzite of the Mississagi Formation (Davidson 1986) or is in an unknown contact relationship with mafic and felsic metavolcanic rocks of the lower Huronian Supergroup (Easton and Hrominchuk 1999). On the basis of geochemical correlation, however, these metavolcanic rocks are likely ~2460 Ma in age (Easton and Hrominchuk 1999), and consequently, most likely were deposited after emplacement of the River Valley intrusion. The River Valley intrusion is also cut by olivine diabase dikes of the 1238 Ma Sudbury dike swarm (Easton 2000) and the Grenville dike swarm (590 Ma). Intrusive contacts between the River Valley intrusion and the Archean basement are exposed throughout the eastern half of the intrusion, and are generally sharp. Zones of intense diking, equivalent to the Border Zone in the East Bull Lake Intrusion, have not been observed. In fact, in both Dana and Crerar townships, Archean para- and orthogneisses adjacent to the intrusion are devoid of any mafic dikes, apart from a few Matachewan–Hearst swarm diabase dikes. In some parts of Dana Township, a fine- to medium-grained gabbro is present at the contact, and may represent a chilled facies of the intrusion. In many areas, however, an inclusion and/or fragment-bearing facies occurs proximal to the contact, making it difficult to ascertain the true extent and significance of this finer grained facies. As in the case of the East Bull Lake Intrusion, a “syenite” (actually an alkali feldspar granite) body in Dana Township was originally interpreted as a felsic differentiate of the River Valley intrusion (e.g., Card and Lumbers 1975). On the basis of contact relationships and geochemistry, Easton (2000) interpreted it as Archean in age, an interpretation that has been recently confirmed by U/Pb zircon dating, which yielded a preliminary age of ~2660 Ma (Easton and Kamo 2003). 49 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Previous maps of the River Valley intrusion (e.g., Lumbers 1973; Card and Lumbers 1975) indicated that the intrusion extended west into Henry and Loughrin townships as a series of “tails” surrounded by gneissic country rocks. This map pattern was interpreted to be the result of either 1. a pattern of complex interference folding during Penokean and Grenvillian orogenesis, or 2. a system of feeders to the main body of the intrusion located in Crerar and Dana townships. Furthermore, it was also speculated that other isolated bodies of East Bull Lake intrusive suite rocks in the Grenville Province, west of the River Valley intrusion (e.g., Red Deer Lake, Street Twp., southern Wanapitei complex, Table 1), represented parts of the River Valley intrusion now isolated from the main mass through erosion and tectonism. Work conducted by Easton (2003) suggests otherwise. East Bull Lake intrusive suite rocks west of Crerar Township are different in form, stratigraphy, and composition, and are intruded into distinctively different country rocks than those found in the River Valley intrusion. Consequently, East Bull Lake intrusive suite rocks located west of Crerar Township likely represent part of one or more intrusions that were emplaced separately from the River Valley intrusion. STRUCTURAL STATE AND DEGREE OF PRESERVATION Figure 8 outlines the general structural state of the River Valley intrusion and its country rocks in Dana and Crerar townships, as well as the degree of textural preservation within the intrusion and the country rocks. Six main preservation regimes can be recognized, which are described by Easton (2003), these form the basis of the divisions shown in Figure 8. The Sturgeon River, and the Neoproterozoic to early Paleozoic fault that extends along it, is an important structural feature within the River Valley intrusion (see Figure 8). North of the Sturgeon River, the River Valley intrusion contains large areas of preserved or partly preserved primary mineralogy. Deformation is concentrated along discrete shear zones, both vertical and subhorizontal, that cut the intrusion (Easton and Hrominchuk 1999, 2001b; Hrominchuk 2000). Grenvillian deformation is largely concentrated near the Grenville Front, along the west side of the intrusion (see Figure 8). North of the Sturgeon River, Grenvillian metamorphism results in the incipient development of corona textures in rocks of the River Valley intrusion and the Sudbury diabase dike swarm, with metamorphic conditions estimated at 5 to 7 kb and 625°C (Tettelaar 2000; Easton and Hrominchuk 1999). An empirical observation is that rocks of leucogabbroic composition are most commonly recrystallized, whereas orthopyroxene-bearing rocks are generally more resistant to metamorphic recrystallization. The geometry of the intrusion north of the Sturgeon River appears to be largely sheet-like, with igneous layering generally dipping shallowly (20 to 30°) to the south and southeast. This has resulted in exposure of the basal contact (margin or sidewall?) of the River Valley intrusion for 8 to 10 km along its north and east margins, providing an extensive target area for mineral exploration. In contrast, south of the Sturgeon River, pockets of preserved mineralogy and texture (Photo 10) are less abundant, occurring mainly in eastern Crerar Township (see Figure 8). Igneous layering at several scales is preserved within this area of the intrusion (Photo 11) (Easton 2003). Where present, igneous layering dips moderately (40° to 60°), and stratigraphy is apparently folded (Easton and Hrominchuk 2001a). This folding results in a variety of trends for the basal, mineralized contact of the intrusion, making mineral exploration more difficult than in the north. Penetrative fabrics of Grenvillian-age are common (Photo 12), with much of the River Valley intrusion consisting of gneissic rocks of gabbroic and leucogabbroic composition. Grenvillian metamorphic effects result in more complete development of corona textures in rocks of both the River Valley intrusion and the Sudbury diabase dike swarm in Crerar Township, with metamorphic conditions estimated at 7 to 9 kb and 650°C (Tettelaar 2000; Easton and Hrominchuk 1999). 50 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �5174225 N C) drift covered Structural and Preservation Regimes Regime I Regime2a Regime 2b Regime2c Regime3a Regime 3b Regime4a Regime 4b Regime 5 Regime 6 • Regime 7 ,,' geological contact road ,)( railway (abandoned) ,V township boundary 5153225 N Figure 8. Map showing the degree of primary textural preservation within rocks of Crerar and Dana townships (from Easton 2003). 51 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Photo 10. Progressive textural changes. a) Igneous-texture in medium-grained norite from leucogabbronorite zone of the River Valley intrusion, Crerar Township. Note dark colour of feldspars. UTM 556115E 5171466N. b) Partly recrystallized rock of the River Valley intrusion, preserving igneous-texture in medium-grained gabbronorite from leucogabbronorite zone of the River Valley intrusion, Crerar Township. Mafic minerals have thick, green-weathering amphibole coronas rimming dark orthopyroxene grains. UTM 556886E 5161009N. c) Recrystallized but massive leucogabbronorite from leucogabbronorite zone of the River Valley intrusion, Crerar Township. This rock no longer preserves primary mineralogy, but is relatively undeformed. Note white colour of feldspars, and lack of well defined crystal shapes. Mafic mineral clots are aggregates of fine-grained amphibole. UTM 557400E 5160225N. d) Typical undeformed but recrystallized leucogabbronorite from the Positano Quarry, East Bull Lake intrusive suite, Loughrin Township. UTM 547000E 5160600N. Hammer handle in all photos is 30 cm long. CONTACTS Copper, nickel and platinum group element exploration in the River Valley intrusion has focussed on its contacts, as outlined by Lumbers (1973). This is only an effective exploration tool if the contacts of the intrusion are primary and not tectonic. Easton (2003) noted 3 main contact types that are present between the River Valley intrusion and other rock units in the map area: 1. preserved intrusive contacts, 2. deformed and disrupted intrusive contacts, and 3. wholly tectonic (fault) contacts. Near the northwest edge of Dana Township, the western contact of the intrusion is tectonic, as originally described by Davidson (1986). The northern and eastern contacts of the intrusion are primary, although they are locally cut, with minimal lateral offset, by mylonite of the Southern–Grenville province boundary zone. Mineralization in northwestern Dana Township (Dana North and South zones of Pacific North West Capital Corporation) is spatially associated with preserved intrusive contacts. Much of the 52 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Photo 11. a) Igneous layering within the norite zone, River Valley intrusion, Crerar Township. UTM 556769E 5159980N. b) Igneous layering within the River Valley intrusion in Crerar Township, near the top of the anorthosite zone and the base of the olivine gabbronorite zone. Dark layers are melanorites, some of which are olivine-bearing. UTM 557390E 5160360N. c) Layering within leucogabbronoritic rocks immediately above the heterogeneous basal zone of River Valley intrusion, Crerar Township. UTM 557585E 5159590N. d) Large-scale igneous layering within leucogabbronoritic rocks of the River Valley intrusion exposed in the Dana Quarry, Dana Township. UTM 558630E 5165385N. Hammer handle in all photos is 30 cm long. northern contact of the intrusion in Dana Township is likely primary, and both the Lismer’s Ridge and Azen Creek zones occur near this contact. It should be noted that based on surface exposure, the Azen Creek zone is apparently located near, but not at the contact. Recent drilling by Pacific North West Capital Corporation has shown that the contact in the Azen Creek zone dips generally shallowly to the south and in fact the Azen Creek zone is proximal to the contact – the difference is that the mineralization is hosted in “inclusion-bearing” or fragment-bearing units rather than the breccias observed in the Dana North area. Also, recent drilling at Jackson’s Flats shows the contact is oriented in a northwest direction and is steeply dipping, with mineralization occurring in fragment-bearing units rather than “breccia”, much like the situation at the Azen Creek zone. In Crerar Township, a primary intrusive contact is preserved in the area that includes the Tomrose occurrence, and Crerar 2 to 4. To date, most of the mineralization identified in Crerar Township lies near this contact. 53 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �a. rN Photo 12. a) Typical leucogabbroic gneiss of an East Bull Lake intrusive suite intrusion in Henry and Loughrin townships. Note the lack of a prominent leucosome phase. UTM 548443E 5159789N. b) Protomylonitic to mylonitic textures present in a highstrain zone in the River Valley intrusion, Crerar Township. UTM 559751E 5163890N. c) Strain gradient within rocks of the River Valley intrusion, northern Crerar Township. Foliated to gneissic leucogabbro on top, straight and irregularly layered gneiss on bottom. UTM 557225E 5162512N. d) Epidote-clot developed just right of the hammer handle in leucogabbro gneiss of the River Valley intrusion, Crerar Township. Such epidote clots are common in gneissic leucogabbro throughout the study area. UTM 555495E 5163580N. Hammer handle in all photos is 30 cm long. In contrast, most of the southern contact of the intrusion in Crerar Township, although poorly exposed, is a major shear zone, which is discordant to internal contacts in the intrusion. It is likely that the eastern contact of the River Valley intrusion north of the Sturgeon River, is also a tectonic contact. Examples of tectonized intrusive contacts may be present in 3 areas. 1. The western contact of the intrusion along Highway 805, particularly in the southern half of Dana Township. However, it is possible that this contact represents an upper, rather than a basal, contact of the intrusion. 2. The other 2 examples occur in Crerar Township, but represent areas where zones of mafic straight gneisses are in contact with other highly deformed gneisses or different units within the intrusion. Similar rocks occur just south of Glen Afton, south of the Sturgeon River; however, to date no mineralization has been reported from the latter area. 54 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Hrominchuk (1999, 2000) recognized 2 types of mafic rock associations near areas of primary contacts. These are a marginal zone and an inclusion-bearing zone. There is considerable variation in thickness of both these zones along the primary contacts of the intrusion, ranging from 50 m thick to almost 500 m thick in the Dana North area. To date, mineralized zones along the contact occur where these units are relatively thick (e.g., Dana North, Varley, Azen, Tomrose). Stratigraphy and Petrography Although historically referred to as an anorthosite to anorthositic gabbro intrusion, the River Valley intrusion is dominated (~60% of surface area) by gabbro, norite, gabbronorite, leucogabbronorite and leuconorite, with some units containing modal olivine. True anorthosite makes up less than 10% of the surface area of the intrusion, but even in these areas, the distribution of mafic minerals is irregular, so that the areas of anorthositic rock are closer to noritic anorthosite or gabbroic anorthosite in composition. Noritic and gabbroic anorthosite probably underlie about 30% of the surface area of the intrusion. Observed stratigraphy in the intrusion differs north and south of the Sturgeon River. NORTH OF THE STURGEON RIVER Low regional dip and poor exposure in key areas in Dana Township has hampered stratigraphic analysis, despite the textural and mineralogical preservation of the rocks. Despite these difficulties, Hrominchuk (2000), as presented in Figure 9, developed an interpretive stratigraphic section for the River Valley intrusion located north of the Sturgeon River. The intrusion north of the Sturgeon River has been subdivided into 5 zones (Hrominchuk 2000; James et al. 2002a), as follows: 1. Marginal zone 2. Inclusion and/or Fragment-Bearing zone 3. layered Olivine Gabbronorite zone 4. Layered Gabbronorite zone, and 5. massive, locally varitextured, Leucogabbronorite zone. Marginal Zone The Marginal zone is characterized by a fine- to medium-grained, black plagioclase-phyric, quartzphenocryst-bearing, gabbronorite that is partly chilled and is typically strongly mineralized at the contact with granitic plutonic and gneissic rocks. This contact is commonly sheared and normally consists of a complex chaotic mixture of footwall blocks and fine- to medium-grained gabbronoritic rocks. Because rocks immediately adjacent to the contact usually contain large blocks and screens of footwall material, exact definition of the contact in some areas is problematic. The Marginal zone consists mostly of medium-grained norite to gabbronorite and is typically well preserved. In thin section, the rock consists of dark-brown to tan cumulate (An79 core; An74 rim) to intercumulate (An68) plagioclase, woody-brown cumulate orthopyroxene (En44-76), interstitial clinopyroxene, with accessory magnetite, quartz, biotite, sulphide minerals, and calcic myrmekite. Fragments are commonly quartzofeldspathic gneiss, alkalifeldspar granite and amphibolite gneiss. Granitic fragments contain quartz with a small amount of alkali feldspar and biotite or amphibole. Quartzofeldspathic gneiss fragments commonly have a preserved gneissic texture and are mostly quartz with a small amount of biotite. Amphibolite gneiss fragments are a fine-grained, commonly foliated, matte of blue-green amphibole and epidote group minerals. 55 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �______ 0) tic. U C.0.2 a -s 900— vv vvv vv vvv vv a a u 9 a ._a I En% Fo°k An°/o 800— v v v a I I I I Leucogebbronorite . zone 56-63 700— v"v"v vvv vv vvv 56-69 I I - - - - - - t - 600— : I 500— 57-66 5875 Gebbronorite zone : I ' • . : 300— . olivine Gebbronorite zone 200— -. . es;-:->: Inclusion and/or Fragment Bearing -zone Ivierginel zone 67-78 72-76 ' . 170-76 ' I • • 68-73 1 68 155-69 I • 39-81 I :44-76 1 1 : 1 • • I 1 :I 1 1 1 I I (meters) Primocryst mineral Interstitial mineral Figure 9. Interpreted stratigraphic sequence and distribution of primary mineral phases and compositions within the River Valley intrusion north of the Sturgeon River. From Easton (2003). Inclusion and/or Fragment-Bearing Zone This zone is commonly in gradational contact with the underlying Marginal zone, but its appearance is marked by a sharp increase in the size and amount of felsic fragments and the appearance of compositionally and texturally varied mafic to ultramafic inclusions or xenoliths. Inclusions consist of 3 main types: alkali feldspar granite and other granitic rocks, amphibolite, and olivine-rich rocks (troctolites, peridotites). Granitic, gneissic and amphibolite fragments are similar to those described above for the Marginal zone, but they are commonly surrounded by pods or lenses of a very coarsegrained to pegmatitic assemblage of biotite, muscovite, amphibole, plagioclase, quartz and alkali feldspar (Hrominchuk 2000). It is important to note, however, that in all of the mineralized areas explored by Pacifc North West Capital Corporation to date, there have been less than 1% exotic fragments in the Breccia and Inclusion-Bearing units, which is their equivalent to this zone. Autoliths are abundant (typically up to 90%) and locally (e.g., outcrop scale) may constitute up to 100% of the rock. Troctolite or peridotite autoliths are similar in all respects to the olivine norite to melatroctolite and peridotite from the Olivine Gabbronorite zone described below; most are olivine-bearing gabbronorite to peridotite in composition, with subordinate leucogabbronorite. They are usually well rounded and weakly to very strongly mineralized. The host matrix to the autoliths and fragments is typically a fine- to mediumgrained, green to black gabbro or olivine-bearing gabbro to gabbronorite, often with disseminated to blebby sulphide mineralization. 56 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Olivine Gabbronorite Zone The Olivine Gabbronorite zone is composed of 0.3 to 1.5 m thick layers that are olivine-rich at the base, and erosive into underlying layers, and which grade upward into olivine gabbronorite or troctolite to slightly more plagioclase-rich end members. Layers are deeply weathered due to the presence of serpentine minerals and fine-grained magnetite after olivine. This zone is well exposed in only a few places, but is normally 50 to 150 m thick, and grades into the overlying, olivine-poor layered Gabbronorite zone. The Olivine Gabbronorite zone contains a wide range of rock types; however, olivine is present in most, with troctolite and olivine norite predominant. Olivine is generally medium- to coarsegrained, Mg-rich (Fo72-76), coronitic, and variably serpentinized. Plagioclase is calcic (An72-76), darkcolored, and inclusion rich. Orthopyroxene (En70-76) occurs in 2 forms, as euhedral woody prisms, and as large (up to 10 cm) oikocrysts enclosing olivine and plagioclase. Clinopyroxene is found only as interstitial anhedral crystals. Gabbronorite Zone The Gabbronorite zone is composed of metre-scale, modally layered rocks with a maximum continuous thickness of about 200 m. This represents a minimum stratigraphic thickness, since faults or zones of intense deformation always truncate the top of the section. The unit typically consists of alternating metre-thick layers of norite and gabbronorite (±olivine) and leucogabbronorite. Locally, this unit is very coarse-grained, with zones rich in felsic fragments surrounded by pegmatitic pods containing quartz, epidote, magnetite, alkali feldspar, plagioclase, amphibole, pyroxene, biotite and garnet. The Gabbronorite zone consists of layers that begin as olivine-bearing norite and fractionate to gabbronorite. Coarse-grained black norite dominates; it consists of large (2 cm) orthopyroxene (En57-66) and brown plagioclase (up to 3 cm)(An58-75) primocrysts, interstitial clinopyroxene and inverted pigeonite, and rare olivine. Metamorphosed equivalents contain altered plagioclase, amphiboles (fine-grained actinolite and coarse-grained hornblende), small garnets and quartz. Leucogabbronorite Zone This zone consists of non-layered, massive, medium- to coarse-grained rocks ranging in composition from leucogabbronorite to anorthosite. It forms the highest stratigraphic levels of the intrusion (generally the tops of topographic highs) in Dana Township. They are commonly highly deformed or faulted; a complete section of these rocks has not yet been found in Dana Township; however, a more complete section may be present in Crerar Township (Easton and Hrominchuk 1999). These rocks consist mainly of dark plagioclase (An56-63) with subordinate interstitial and primocryst clinopyroxene and inverted pigeonite (En56-69). Where metamorphosed, the more leucocratic rocks form very white-weathering rocks with small pods of amphibole-rich material dispersed throughout. SOUTH OF THE STURGEON RIVER As previously noted, establishing a stratigraphic column for the River Valley intrusion has been difficult, despite the state of preservation of the rocks. South of the Sturgeon River, steeper regional dips have made the task somewhat easier; however, the section there may contain more layer-parallel faulting, and units are only well preserved in the southwestern part of the intrusion. Easton and Hrominchuk (1999) originally suggested that a section up to 3.5 km thick could be identified; however, this interpretation assumed that the units were dipping uniformly to the west at roughly 50°, that the section was not folded, 57 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �and that there were no layer-parallel faults present. Further analysis indicated that the section was indeed folded (Easton and Hrominchuk 2001a). Unlike the area north of the Sturgeon River, this section has not been formally divided into stratigraphic zones; for reference, however, informal zone names are given in italics to facilitate comparison with the Dana Township section. The Crerar Township section has a maximum thickness of 1.4 km, and from base upward, consists of: • 150 to 200 m of gabbro and melanogabbro, forming a heterogeneous Marginal zone that contains fragment-rich zones, disrupted layering, and considerable diking. Sulphide minerals are common in this zone. This zone grades upward into leucogabbro and anorthositic gabbro. This zone corresponds roughly to the Marginal and Inclusion and/or Fragment-Bearing zones present in the intrusion north of the Sturgeon River. • 400 to 500 m of gabbroic anorthosite to anorthosite (Anorthosite zone and thin Olivine Gabbronorite zone). Melanogabbro and olivine melanogabbronorite layers up to 2 m thick occur near the top of this zone. This zone apparently does not have an equivalent unit north of he Sturgeon River, but a lowermost gabbroic anorthosite to anorthosite is present in most other East Bull Lake suite intrusions (Figure 10). It is possible that the melanocratic layers in the upper part of this zone correspond to the Olivine Gabbronorite zone present north of the Sturgeon River. • 350 m of norite and gabbronorite (Gabbronorite zone), commonly layered on the scale of centimetres to decametres, with layers having pyroxenite bases and gabbroic anorthosite tops (Photo 10b). Layering on the scale of metres to decametres is also inferred to be present in this zone. This zone may be equivalent to the Gabbronorite Zone present north of the Sturgeon River. • 200 m of leucogabbro to gabbroic anorthosite (Leucogabbronorite zone). This zone may be equivalent to the Leucogabbronorite Zone present north of the Sturgeon River. • 200 m or more of oxide-bearing leucogabbro to gabbroic anorthosite (Oxide-Bearing Leucogabbronorite zone). A regional magnetic anomaly is associated with this unit. Rocks within this zone show a marked decrease in Mg-number (less than 50) compared to other stratigraphic units in the River Valley intrusion (typically greater than 57). • The top of the section is not observed due to intense deformation in the rocks above this stratigraphic level. The most significant difference between the Dana and Crerar township sections is the presence of a thick unit of leucogabbro to anorthosite between the Marginal zone and the Olivine Gabbronorite and Gabbronorite zones in the Crerar Township section. In one locality, a few outcrops of nodular anorthosite are present near the base of the leucogabbro to anorthosite unit. At another locality, this unit contains a decametre-thick layer of megacrystic anorthosite, with 1 to 5 cm equant, blocky plagioclase crystals partly separated by clotty, coarse-grained and weakly poikilitic, amphibolitized pyroxene. In addition, the varitextured to massive uppermost Oxide-Bearing Leucogabbronorite zone is not observed in Dana Township, and is the only unit that contains both Mg-number and normative An less than 55. 58 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �____________ ______________ ___________ __________ , +. __________ __________ well mineralized zones Fe-Ti oxide zone e layered zones C leucogabbronorite C gabbronorite, norite Agnew Lake Intrusion EliEl gabbroic anorthosite a ] melatroctoiite, 0 livine nonte and 2250 gabbronorite 2000 olivine gabbronorite inciusion - bearing zone 1750 border zone Upper Series 1500 River Valley Intrusion (south) East Bull Lake Intrusion / V VVVV — 1250 River Valley Intrusion 1 (noah) Norite-gebbro1 norite zone Upper Series Street Intrusion 1000 750 — norite zone1 Main Olivine 500 Series zone £ s SASS a a aS 1 Inclusion! Fragment Bearing zone tLeU000abbro Contact 250 H ii —in— ni Lower -S-if. 0 Marginal Series (metres) Figure 10. Generalized cross-sections for the Street metagabbro and the River Valley, East Bull Lake and Agnew Lake intrusions of the East Bull Lake intrusive suite. East Bull Lake section modified from Peck et al. (1995), Agnew Lake Intrusion section modified from Vogel (1996), River Valley data from Easton (2003). STRATIGRAPHIC COMPARISON WITH OTHER EAST BULL LAKE SUITE INTRUSIONS As shown in Figure 10, the composite stratigraphic section from the best preserved enclave of the River Valley intrusion in Crerar Township contains elements common to both the northern River Valley, the East Bull Lake, and the Agnew Lake intrusions. If the Crerar Township stratigraphy is a true section through the intrusion, then it is similar in thickness to other East Bull Lake intrusions (see Table 1). The exception is the Agnew Lake Intrusion (∼2.3 km thick), which unlike the other intrusions, includes felsic differentiates near the top. The Crerar Township section also suggests that the River Valley intrusion locally includes an anorthositic zone near the base of the body, similar to most other East Bull Lake intrusions (see Figure 10). 59 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Geochemistry Geochemical data from the intrusion can be found in Easton and Hrominchuk (2002) and Easton (2003). Overall, the chemistry of the rocks are similar to that of other intrusions of the East Bull Lake suite. Silica is generally less than 50 weight percent. Vogel et al. (1999) noted that in the case of the Agnew Lake Intrusion, SiO2 less than 50 weight percent is characteristic of the stratigraphically lower part of the intrusion. The same effect may be present in the River Valley intrusion, as most of the samples reported to date are from the lower part of the intrusion (below the Oxide-Bearing Leucogabbronorite zone). A few samples, located stratigraphically higher in the River Valley intrusion in Crerar Township show a marked decrease in Mg-number (from typically greater than 57 to less than 43). These rocks are generally of gabbroic anorthosite composition and oxide mineral-bearing. Representative analyses of each stratigraphic unit present in the River Valley intrusion north of the Sturgeon River are listed in Table 5, as well as in Hrominchuk (2000) and James et al. (2002a). Rocks of the Marginal zone are CIPW quartz-normative; Olivine Gabbronorite zone rocks are olivine-normative, whereas most rocks in the upper units are only silica saturated. Samples from the Marginal zone (see Table 5, analyses 1 and 2) show much higher Ti, Fe, K, Na, P, Ba, Zr and Y contents than do most other River Valley intrusion samples. Hrominchuk (2000) interpreted these elemental variations as a possible contamination signature. The high concentration of Ti and Fe in some footwall phases (see Table 5, analysis 12), relative to typical River Valley intrusion rocks, is consistent with some of the Ti and Fe contents in sample CZ7-1000 resulting from wall rock assimilation (James et al. 2002a). The matrix of the Inclusion and/or Fragment-Bearing zone is high in Mg, Ca, Al, and Fe, consistent with its plagioclase, orthopyroxene and olivine-dominated mineralogy. The fragments, inclusions and xenoliths of this zone are extremely varied in composition, partially resorbed, and commonly mineralized. The felsic fragments show similarities to the footwall alkali feldspar granite (James et al. 2002a). Mafic inclusions are similar to the rocks in the lowest part of the layered Olivine Gabbronorite zone. The Olivine Gabbronorite zone is characterized by rocks high in MgO (15 to 24 wt.%) and low in SiO2 (38 to 45 wt.%), with anomalous Ni contents attributed to the predominance of primocryst olivine (see Table 5, analyses 5 and 6). Rocks from the layered Gabbronorite zone show gradual fractionation to more Si-, Al-, Ca-, Na-, K-rich and Mg-poor compositions (see Table 5, analysis 9), a trend also observed in the uppermost stratigraphic unit. A sample of magnetite gabbro from the uppermost part of the stratigraphy in Crerar Township (Easton 2003) may be similar to the lowermost ferrogabbros from the Agnew Lake Intrusion, and is the most evolved sample found to date in the River Valley intrusion. Pearce element ratio diagrams for the River Valley intrusion (James et al. 2002) and detailed petrography indicate that major and trace element trends are controlled by plagioclase, olivine and orthopyroxene, with very little contribution from clinopyroxene (James et al. 2002a). Samples from the Olivine Gabbronorite zone show a variation from olivine + plagioclase near the base to plagioclase + orthopyroxene controlled geochemistry near the top of the zone (James et al. 2002a). Plagioclase is commonly a phenocryst phase within marginal and fine-grained rocks indicating very early crystallization, and most likely quickly joined by olivine as the first cumulate phases. Fractionation and cooling of the magma caused orthopyroxene to replace olivine as a cumulate phase during crystallization of the marginal rocks and the Olivine Gabbronorite zone. This is indicated by orthopyroxene rims on olivine and large orthopyroxene oikocrysts enclosing olivine and plagioclase near the top of this zone (James et al. 2002a). Rocks above this point show trends that indicate fractionation of magma due to plagioclase and orthopyroxene toward more Fe- and alkali-rich compositions. Chondrite-normalized rare earth element patterns for samples north and south of the Sturgeon River are shown in Figure 11. Most samples show total REE contents and patterns similar to those found in the 60 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �East Bull Lake and the Agnew Lake intrusions. In Figure 11, REE patterns for the proposed Dana Township stratigraphy show a slight fractionation trend with stratigraphic height and the influence of cumulate plagioclase (reflected in the magnitude of the negative Eu anomaly). The contaminated contact sample shows significant light REE enrichment over other marginal samples and rocks of all other zones. There are 2 groups of melanocratic rocks (Easton 2003). One with patterns and REE contents similar to other River Valley intrusion rocks; the other with high total REE and La/Yb. This second group has higher normative-orthopyroxene and lower-normative olivine contents than the first. A similar two-fold grouping occurs in the orthopyroxene hornblendite samples, perhaps suggesting a genetic linkage between the River Valley intrusion and these bodies. A sample of a primitive dike or layer from the Marginal zone exhibits a flat (~10 times chondrite) to slightly U-shaped pattern (see Figure 11), and is also characterized by relatively high silica and MgO, and low Ti and alkalis. It may represent a boninite-like parent magma composition for the River Valley intrusion (James et al. 2002a). The significance of such a parental magma has already been discussed in the section “Magma Composition and its Relationship to Mineralization”. —4— Contaminated Contact —0—Marginal Zone —A—Inclusion and\or Fragment Bearing Zone —X—Olivine Gabbronorite Zone —— Gabbronorite Zone —0-— Leucogabbronorite Zone —0-— Primitive dike/layer La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Figure 11. Chondrite-normalized rare earth element plots for the River Valley intrusion for rocks north of the Sturgeon River. Sample from the “primitive dike or layer” may represent the parent magma composition to the mineralized parts of the intrusion in Dana Township. 61 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Table 5. Representative chemical analyses from the River Valley intrusion (data from Easton and Hrominchuk 2002). Analysis Sample No. 1 CZ7-500 2 CZ7-1000 3 99RME-2242 4 99RME-2243 Zone Rock Type MZ fg gabbrodiabase MZ fg gabbrodiabase IFBZ Stop 7, cg troctolite inclusion 559150 5167678 IFBZ Stop 7, mg olivine gabbronorite 559149 5167677 Easting Northing wt. % SiO2 TiO2 Al2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O P2O5 CO2 S LOI Total Mg # Ppm Cr Ni Co Sc V Cu Zn Rb Ba Sr Nb Zr Y Zr/Y Ppb Au Pt Pd Pt/Pd Total (Au+Pt+Pd) 558431 5168104 558431 5168604 5 99RME2371A OLGN cg troctolite 559597 5166341 6 JLHS1-S3 7 JLHS2-S1 8 JLHS2-S4 OLGN cg melatroctolite GN cg gabbronorite GN cg norite 559657 5166341 558643 5165245 558644 5165393 52.51 0.93 14.17 13.40 N/A 0.17 5.16 8.79 2.16 0.85 0.09 0.26 0.024 1.36 99.59 40.9 51.85 0.91 14.36 13.09 N/A 0.18 5.48 8.96 2.25 0.94 0.10 0.22 <0.005 1.60 99.72 43.0 44.14 0.16 12.79 2.57 13.21 0.20 15.67 7.18 1.18 0.20 0.01 <0.03 0.67 0.01 97.32 62.1 48.42 0.21 18.81 1.51 8.38 0.14 10.16 9.82 1.87 0.30 0.03 00.07 0.18 0.25 99.90 62.9 43.32 0.10 18.30 9.34 N/A 0.12 14.06 8.37 1.34 0.28 0.03 0..22 <0.005 4.51 99.77 73.0 38.30 0.14 7.85 15.10 N/A 0.20 23.72 3.70 0.70 0.12 0.04 0..22 <0.005 10.12 99.99 73.9 50.21 0.13 19.95 8.23 N/A 0.13 6.90 10.32 2.09 0.21 0.03 0..18 <0.005 1.99 100.19 60.2 50.81 0.18 18.43 8.92 N/A 0.14 7.46 11.19 1.87 0.20 0.03 0.15 <0.005 0.43 99.66 60.1 90 57 50 29 284 171 109 37 341 255 2.9 86 16 5.5 115 75 39 30 287 401 108 46 251 193 0.9 80 18 4.3 203 1840 119 15 93 4512 122 3 81 151 0.4 12 4 3.0 250 623 65 14 91 1320 84 5 110 219 0.7 21 6 3.5 136 595 78 11 43 16 92 4 93 235 0.3 7 4 1.7 159 1360 155 17 48 162 111 1 79 94 0.9 17 8 2.2 73 97 41 20 71 18 56 16 139 238 0.2 3 0.7 4.7 90 92 38 28 109 94 66 10 121 220 0.2 4 2 2.4 <5 18 19 0.95 36 6 18 25 0.72 49 145 576 1980 0.30 2701 59 106 293 0.36 458 <5 <8 <8 <5 10 11 0.95 21 <5 <8 19 <5 11 29 0.38 40 <21 19 Abbreviations: cg = coarse-grained, fg = fine-grained; mg = medium-grained; N/A = not analyzed; IBZ = inclusion-bearing zone, GN = gabbronorite zone, LGN = leucogabbronorite zone, MZ = marginal zone, OLGN = olivine gabbronorite zone. Easting and Northings are in UTM Zone 17, datum NAD 83. 62 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Table 5. continued. Analysis Sample No. Zone Rock Type Easting Northing wt. % SiO2 TiO2 Al2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O P2O5 CO2 S LOI Total Mg # Ppm Cr Ni Co Sc V Cu Zn Rb Ba Sr Nb Zr Y Zr/Y Ppb Au Pt Pd Pt/Pd Total (Au+Pt+Pd) 9 99RME-2116 LGN cg gabbronoritic anorthosite 556346 5166811 10 99RME-2244 LGN cg gabbronoritic anorthosite 558522 5165429 11 99RME-2139 Footwall Stop 6, alkalifeldspar granite 559785 5167228 12 99RME-0375 Footwall biotite diorite gneiss 49.59 0.16 25.52 1.99 2.55 0.07 3.17 13.65 2.41 0.30 0.01 0.03 <0.01 0.98 100.40 54.2 51.07 0.10 22.84 1.08 4.47 0.11 5.38 10.91 3.03 0.22 0.01 0.08 <0.01 0.72 99.94 61.6 72.26 0.10 15.21 0.16 0.44 0.01 0.28 1.14 4.02 5.52 0.03 0.20 <0.01 0.61 99.78 46.1 55.29 1.57 13.8 3.37 10.02 0.17 3.35 6.73 2.58 1.94 0.19 0.11 0.04 0.76 99.77 31.4 63.76 0.58 15.62 2.09 3.11 0.06 3.08 3.23 3.63 2.97 0.13 0.12 0.09 1.18 99.44 52.4 50.36 1.15 14.07 3.09 11.11 0.22 5.89 10.02 1.97 0.68 0.12 <0.03 0.12 0.68 99.36 43.0 45.50 2.97 15.60 2.16 13.60 0.23 5.21 8.21 3.26 1.45 0.75 0.12 0.12 0.09 99.03 37.4 138 44 13 13 75 46 36 4 85 258 0.4 9 5 1.8 86 83 28 11 55 18 51 3 89 246 0.1 3 2 1.5 20 <5 <5 <1 11 <5 15 106 1748 950 1.2 93 1 93 30 57 41 26 330 92 142 75 593 157 12.5 166 47 3.5 183 64 19 9 17 10 94 139 670 464 6.8 117 8 14.6 90 81 57 37 319 278 67 13 91 145 4.8 92 31 3.0 37 68 57 26 204 71 190 41 673 250 21.6 348 59 5.9 5 15 19 0.79 39 <5 <8 <8 <5 <8 <8 <5 <8 <8 <21 7 38 72 0.53 117 <5 <8 <8 <21 9 6 14 0.43 29 558780 5159433 13 99RME-0025 Footwall Migmatitic metwacke, Pardo gneiss 560005 5173855 <21 14 99RME-0031 Dike Matachewan diabase dike 15 99RME-0137 Dike Sudbury diabase dike 559913 5174428 561255 5169258 <21 Abbreviations: cg = coarse-grained, fg = fine-grained; mg = medium-grained; N/A = not analyzed; IBZ = inclusion-bearing zone, GN = gabbronorite zone, LGN = leucogabbronorite zone, MZ = marginal zone, OLGN = olivine gabbronorite zone. Easting and Northings are in UTM Zone 17, datum NAD 83. 63 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �ROAD LOG, DAY 3, RIVER VALLEY INTRUSION Note: Highway 805 is used regularly by logging trucks. Consequently, extreme caution should be taken when parking vehicles on the shoulder of the highway and when examining outcrops located along Highway 805. Geological map reference: Lumbers (1973); Card and Lumbers (1975); Easton and Hrominchuk (2001a, 2001b), Easton (2001). 0.0 km Start at the junction of Highway 69S and the Highway 17 southeast bypass. Get on the bypass heading east toward Coniston and North Bay. 11.8 km Junction Highway 17 and southeast bypass, turn right onto Highway 17 and head east toward North Bay. 20.0 km Bridge over the Wanapitei River. The river lies along the trace of the Grenville Front. 30.0 km Pull off onto a pullout located on the south side of Highway 17. Stop 1. Shear-Zone Hosted Orthopyroxene Hornblendite Body Examine the outcrop and large blasted boulders present on the west side of the pullout. They belong to an orthopyroxene hornblendite body that is present within a high-strain zone that extends subparallel to the highway. Examples of these highly strained felsic gneisses can be examined in outcrops at the base of the hill east of the pullout. The top of the ridge south of the road and above the stop consists of layered leucogabbronorite of the East Bull Lake intrusive suite. Thus, although proximal to rocks of the suite here, the orthopyroxenite body is not directly in contact with the suite, although that relationship has been observed elsewhere in Street and Awrey townships (Easton and Murphy 2002). Note the large, equant, orthopyroxene crystals, and the fine-grained amphibole matrix. Return to vehicles and head east on Highway 17. 60.6 km Junction of Highway 17 and 539 in Warren. Turn north onto Highway 539, reset odometer to zero. 0.0 km Junction, Highway 17 and 539 in Warren, turn north onto Highway 539 and proceed toward River Valley. After heading 200 m north on Highway 539, turn right, then after going 100 m east, turn left and continue north on Highway 539. 4.8 km Coloured Aggregates Limited/Crea-Mac quarry operation is located east of the highway. This operation is one of several quarries in Ratter Township which produces pink aggregate from syenite pegmatite of probable Archean age that occur within this part of the Grenville Province. 8.3 km Junction Highway 539 and Kipling Road West, continue north on Highway 539. 12.5 km Outcrops on both sides of the road expose typical rocks of the Crerar gneiss association, which consists of granodiorite, tonalite, and amphibolite rocks (2680 to 2660 Ma), commonly migmatized. Tight to isoclinal folds are common in the outcrop on the east side of the road. 14.5 km Pull over onto right shoulder and park by outcrops at crest of hill. 64 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Stop 2. Rocks of the Crerar Gneiss Association This outcrop illustrates typical mafic and intermediate composition gneisses of the Crerar gneiss association (Easton 2003), including tonalite, 2 phases of granodiorite, amphibolite layers and pods, and minor calc-silicate rock. Rocks of the Crerar gneiss association form the country rocks to the southern margin of the River Valley intrusion. A sample from this outcrop yielded a Nd/Sm model age of 2710 Ma (Dickin 1998). Return to vehicles and continue northward on Highway 539. 21.6 Park on shoulder of road by large, isolated outcrop knob on east side of the highway. Optional Stop. Crerar Gneiss Association UTM 561947E, 5157087N. The outcrop on the east side of the road exposes a megacrystic granodiorite, which is a late intrusion into the Crerar gneiss association. This rock cuts earlier migmatitic fabrics, but is itself locally strongly deformed. A sample from this outcrop has yielded a U/Pb zircon age of 2663±4 Ma, with titanite dating the waning of Grenville metamorphism at 974±12 Ma (Easton and Kamo 2003). Return to vehicles and continue northward on Highway 539. 23.1 km Bridge over the Sturgeon River. 23.6 km Junction Highway 539, which heads east toward Field, and Highway 539A, which heads west-northwest toward River Valley. Turn left. Immediately pull over and park for optional stop. Optional Stop. Red Cedar Lake Gneiss This is Stop 2.3 of Lumbers (1978). Outcrops north of the highway at and just west of the junction expose typical granular gneiss of the Red Cedar Lake gneiss. The Red Cedar Lake gneiss is a widespread unit that extends from its western terminus here, at the Sturgeon River, east to the Ottawa River, a distance of over 80 km. It is thought to be a diatextite derived from Archean metasedimentary and intermediate composition intrusive rocks (Easton 2003), and forms the country rock along the eastern side of the River Valley intrusion north of the Sturgeon River. Much of the texture in the gneiss is probably Archean, rather than Grenvillian, in age. Return to vehicles and continue northward on Highway 539. 24.9 km Dupras Store in "downtown" River Valley. 26.2 km Bridge across the Temagami River. 27.6 km Junction Highway 539A and Erana Mines Road. The Erana Mines Road leads to Erana building stone quarry and Crea-Mac aggregate quarry, both are hosted in well-preserved rocks of the Gabbronorite Zone of the River Valley intrusion. 29.5 km End Highway 539A, start Highway 805. End of continuously paved road. 29.55 to 29.9 Pull over and park by the shoulder at roughly 29.55 km. The 350 m long section of outcrop on the north side of the road constitutes Stop 3. 65 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Stop 3. Varied Degrees of Preservation in Rocks of the River Valley Intrusion This is Stop 2.4 of Lumbers (1978). UTM at the west end of the outcrop is 559691E, 5163308N. This 350 m long section along the north side of the highway exposes rocks of the River Valley intrusion exhibiting a wide variety of textures and deformation. The east end of the outcrop consists of dark-grey to grey weathering gabbronorite and leucogabbronorite, with well-preserved plagioclase laths and only incipient corona development around mafic minerals. As one proceeds east, the rocks become paler, largely due to whitening of the plagioclase grains during recrystallization. Note the presence of many small, thin, anastomosing shear zones. Heading west, the outcrop becomes a black and white leucogabbronorite, then a foliated equivalent of the same, finally becoming protomylonitic to gneissic at the west end of the outcrop where the road curves slightly. The variation seen in this outcrop is typical of the variation in the degree of preservation and deformation observed throughout the River Valley intrusion north of the Sturgeon River. In general, well-preserved, little-deformed igneous rocks form the core of topographic highs, whereas valleys and low areas are generally underlain by gneissic rocks similar to those observed in the west end of this outcrop. South of the Sturgeon River, recrystallized to gneissic varieties of gabbronorite and leucogabbronorite predominate. 30.2 km Junction with Giroux road. Turn right (north) and proceed north on gravel road. Note that the road is gated. Seek permission (and key) for entry from Mr. Giroux, who lives in the house just east of the gate. 31.3 km Road crosses Azen Creek, a major north-northeast-trending deformation zone within the River Valley intrusion. Most of the copper-nickel-PGE mineralization in the River Valley intrusion lies to the west of the Azen Creek zone, even though similar rock units occur along the contact of the intrusion to the east of the Azen Creek shear zone. 31.8 km Junction, stay to the left and follow side road roughly 600 m to the first of several quarry pits present in the Dana quarry. Park at base of hill by first large cliff face, just past the large quarry building. Proceed uphill along the sideroad to the west (right) to large open area of outcrop. Stop 4. Autolith Fragments and Layering, Layered Gabbronorite Zone This is the southernmost quarry face of a large building stone quarry, which has been exploited at various times over the last 30 years. Stone from this quarry face was used as floor tiles in the Willet Green Miller Centre, Ministry of Northern Developments and Mines, at Laurentian University in Sudbury. The rocks exposed here span nearly the entire Layered Gabbronorite zone, ranging from olivine-bearing norite in the south part of the exposure to a pigeonite gabbro in the northwest part. The dominant rock type here is a very coarse-grained, black norite to gabbronorite with varied amounts of oxide minerals in addition to metamorphic amphibole and garnet. Layering is almost horizontal in this area (~15 to 25°). Layering is subtle, and commonly disrupted or convoluted. Pods, dikes and irregular layers of pegmatitic and granophyric material are found in the more central region of this outcrop area. Pegmatite and granophyre commonly occur together and in association with mafic cognate inclusions and felsic fragments. The development of very coarse grains of pyroxene, plagioclase, amphibole and magnetite around these inclusions led Hrominchuk (2000) to conclude that these pods and dikes were not formed simply by differentiating magma. He suggested that they may have formed by contamination of a relatively unfractionated magma (olivine norite) by cooler, compositionally different, footwall and cognate xenoliths. These pegmatitic patches are very similar to those developed in the Inclusion-Bearing zone, which we will see subsequently at Stop 7. 66 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �If one continues to follow the road north, it passes by an abandoned quarry pit at 400 m and ends above another 20 m high quarry face at 600 m. In this quarry, subtle, shallow dipping (~15 to 25°) layering is present (see Photo 11d) as are pegmatitic veins and pods. Unlike the main stop, mafic cognate inclusions and disrupted layering are not observed. Turn vehicles around and retrace road back to main gravel road. 32.5 km Junction, continue north on main road. 34.4 km Pull off onto small road to the left (west) of the road and park. Follow old skidder trail through the bush west for approximately 200 m to the top of a cliff face, then work your way down safely along the side of the cliff. Watch for loose rocks on the slope and falling rocks. Stop 5. Layered Olivine Gabbronorite This stop displays olivine-rich rocks of the Olivine Gabbronorite zone, which may be stratigraphically equivalent to the Olivine Gabbronorite zones in both the East Bull Lake and Agnew Lake intrusions (see Table 5, analyses 5 and 6). The sequence of olivine-bearing rocks here is roughly 30 m thick. The section grades upward from melatroctolite to orthopyroxene oikocrystic melatroctolite near the base to a clinopyroxene-bearing orthopyroxene melatroctolite near the top. Layering is well developed in the lower portion of the cliff face and contains erosive scours that grade from plagioclase-poor olivine cumulates to more plagioclase-rich troctolites. All olivine in the section is partially to extensively serpentinized, creating a fractured and rubbly appearance. Plagioclase in this zone is generally interstitial with some euhedral primocrysts. Mineral compositions obtained by Hrominchuk (2000) are plagioclase, An70-76; olivine, Fo72; orthopyroxene forms oikocrystic or euhedral primocrysts at En70 and clinopyroxene is either absent or interstitial. Low-temperature alteration of the olivine-rich rocks produces serpentine, carbonate and magnetite. Return to vehicles, continue north on gravel road. 35.5 km Junction with side road to Azen Creek occurrence to the left (west), continue on main road. 36.3 km Park vehicles and walk south to clean outcrop exposures. Stop 6. Footwall Alkali Feldspar Granite and Sudbury Diabase Dike This quick stop shows a few outcrops of a weakly foliated alkali granite, which is the main footwall rock type to the River Valley intrusion north of the Sturgeon River in Dana Township. This granite cuts the migmatitic, metasedimentary-derived Pardo gneiss. Lumbers (1973) classified these granitoid rocks as monzonite to quartz syenite; however, geochemistry (Easton 2003; see also Table 5, analysis 11) suggests that these rocks are more appropriately termed alkali feldspar granite using the normative classification of Streckeisen (1976). The alkali feldspar granites are peraluminous, with low total REE (less than 50 ppm), positive Eu anomalies (Eu/Eu* 7.9), and are corundum-normative, suggesting an S-type (sedimentaryderived) affinity. Breaks and Moore (1992) reported similar REE patterns from their Type 2 phase of the Neoarchean, peraluminous, S-type Ghost Lake batholith in northwest Ontario, and suggested that the unusual chemistry of these rocks resulted from feldspar accumulation. U/Pb zircon geochronology from this outcrop yielded an age of 2660 Ma (Easton and Kamo 2003). Although we are at least 300 m north of the contact with the River Valley intrusion at this point, lead loss related to emplacement of the intrusion at 2475 Ma has affected this granite, and prevents determination of a more precise age. Titanite from this outcrop gives an age of 966±6Ma (Easton and Kamo 2003), confirming that these rocks, as well as the River Valley intrusion, have been affected by Grenvillian metamorphism. As was the case for the Parisien Lake syenite adjacent to the East Bull Lake Intrusion, this granite was originally thought to be a felsic differentiate that was coeval with the River Valley intrusion (e.g., Card and Lumbers 1975). 67 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Also present in the outcrop is a Sudbury swarm diabase dike. In the Southern Province, the Sudbury swarm dikes trend northwesterly, and consist of titaniferous (2.5 to 3.5 wt. % TiO2; see also Table 5, analysis 15) olivine-biotite-magnetite-gabbros. In Dana Township, Sudbury swarm dikes generally show northwest or northerly trends, and can be traced for several hundreds of metres along strike. Dikes range in size from a few centimetres in width to greater than 100 m wide. Olivine in the Sudbury swarm dikes north of the Sturgeon River commonly show incipient corona formation, similar to that observed in the River Valley intrusion, suggesting that coronas in both rock types developed in response to Grenvillian metamorphism. Sudbury swarm dikes south of the Sturgeon River are more extensively metamorphosed, exhibiting nearly complete replacement of olivine by orthopyroxene, spinel and garnet, and dike segments are shorter and more randomly orientated. UTM 559785E, 5167228N. Return to vehicles, turn around and retrace route. 37.1 km Sideroad to Azen Creek occurrence, turn right (west) and proceed roughly 500 metres to stripped outcrops. Stop 7. Azen Creek Copper-Nickel-PGE Occurrence The Azen Creek zone was one of the initial discovery sites for PGE mineralization in the River Valley intrusion in the fall of 1998. Its location along the north contact of the intrusion, some 6 km from Dana Lake, was the first indication that PGE mineralization might occur discontinuously to continuously over a considerable strike length. The stripped area south of the road contains inclusions of medium-grained gabbronorite, troctolite, amphibolite, alkali feldspar granite and other felsic material within a mediumgrained, grey-green olivine gabbronorite. Inclusions are more abundant on the west side of the stripped area, and vary from centimetre- to metre-scale. Mineralization consists of finely disseminated chalcopyrite and pyrrhotite in the matrix, as well as some sulphide-rich inclusions. Hrominchuk and Jobin-Bevans (2000) suggested that the inclusion-bearing zone might be intrusive into the contact zone of the intrusion, roughly coincident with the boundary between the fine-grained contact zone rocks and overlying olivine-rich cumulates of the Olivine Gabbronorite zone (e.g., Stop 5). The stripped area north of the road contains less mineralization, but shows a variety of pegmatitic veins and pods. The pegmatitic pods are commonly cored by inclusions of felsic material, likely incorporated from the footwall of the intrusion. Return to vehicles, turn around, and retrace route to Highway 805. 0.0 km Junction with Highway 805, reset odometer to zero. Proceed west on Highway 805. 0.3 km Highway bends south, side road heads west to Upper Canada Stone Company and Giroux gravel pit. Note piles of crushed, black aggregate derived from the well-preserved gabbronorite of the River Valley intrusion. From this point westward, the road parallels the Sturgeon River on the south, as well as winding its way back and forth across an abandoned Canadian National Railways right-of-way. 1.4 to 1.1 km High ridges to the north of the road are well preserved gabbronorite of the River Valley intrusion. 5.4 km Outcrop ridge north of the road and the right-of-way consists of gabbronorite of the River Valley intrusion which contains 2 fabrics, the later fabric being parallel to the Sturgeon River. Rocks similar to these were previously mapped as being deformed equivalents of Nipissing gabbro (Dressler 1979), however, chemically and petrographically they belong to the River Valley intrusion. 68 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �6.8 km Road turns north at Glen Afton. Glen Afton lies near the eastern margin of the 1 to 2 km wide boundary zone that marks the Grenville–Southern Province boundary in this area. For the next 20 km, Highway 805 weaves back and forth across this boundary zone, exposing zones of intense mylonization separated by lenses of relatively well-preserved rocks of the Huronian Supergroup, the River Valley intrusion, and the Nipissing intrusive suite. Note that Highway 805 was re-aligned between 1988 and 1989; consequently, users of Davidson's (1986) guidebook for the Grenville Front in this area are cautioned that the distances and stop locations listed in that guide are not always easily determined. Where possible, this guide notes the location of some of Davidson's stops. 7.0 km Park vehicles just past old railroad right-of-way and examine outcrops on west side of Highway. Optional Stop. Huronian Metavolcanic Rocks This is Stop 2.6 of Lumbers (1978). UTM for the south outcrop 554000E, 5166220N; for the north outcrop 554005E 5166267N. The outcrop just north and west of the abandoned right-of-way consists of medium-layered dacite, which may be part of the Stobie Formation of the Huronian Supergroup. The low outcrop beside the road (100 m to the north) consists of felsic to intermediate pyroclastic rocks assigned to the Huronian Supergroup. These outcrops represent the easternmost extent of Huronian felsic metavolcanic rocks that have been recognized to date. It is difficult to differentiate fine-grained metavolcanic rocks from mylonitic rocks along the Grenville Front, due to the intensity of the deformation. Return to vehicles and continue north on Highway 805. 8.0-8.2 km Outcrops west of the highway consist of fine-grained amphibolite, possibly representing Huronian Supergroup metavolcanic rocks, whereas outcrops east of the highway, generally covered by vegetation, consist of deformed leucogabbronorite of the River Valley intrusion. 9.5 km Low outcrops of gneissic leucogabbronorite of the River Valley intrusion occur on both sides of the road. 9.8 km ATV trail to Lismer's Ridge area on property of Pacific North West Capital Corporation on the right (east). 10.1 to 10.6 km Outcrops in this stretch of road, particularly those on the north side of the road, represent an optional stop. Optional Stop. Variably Preserved Nipissing Gabbro This location approximates Stop 2.7 of Lumbers (1978). The highway transects a body of Nipissing gabbro that has intruded the River Valley intrusion. Outcrop exposures on the north side of the road, in the vicinity of UTM 554777E 5168511N, exhibit varied degrees of preservation of the Nipissing rocks, similar to that observed in rocks of the River Valley intrusion. Dark green to almost black-weathering rocks are Nipissing gabbro retaining primary orthopyroxene and minor olivine grains, whereas greenweathering outcrops consist of typical, greenschist-grade, amphibolitized Nipissing gabbro, typical of what most of these rocks look like in the Southern Province. Note that plagioclase is much less abundant in the Nipissing rocks than in the River Valley intrusion. Return to vehicles and continue north on Highway 805. 10.7 km Outcrops north of road consist of protomylonitic gabbronorite of the River Valley intrusion and ultramylonite of unknown protolith. 69 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �10.9 to 11.1 km Outcrops east of the road consist of foliated to protomylonitic leucogabbronorite of the River Valley intrusion, intruded by deformed, pink felsite veins that are chemically similar to Huronian Supergroup felsic metavolcanic rocks (Easton 2003). UTM 554415E, 5168855N. 11.2 km Outcrop east of the road consists of varitextured gabbro of the River Valley intrusion. UTM 554500E, 5169105N. 11.8 km Park vehicles and examine outcrops on east side of road. Stop 8. Mylonitized Anorthosite of the River Valley Intrusion This is Stop R6a of Davidson (1986) and Stop 2.8 of Lumbers (1978). Outcrops west of the road consist of ultramylonite derived from rocks of the River Valley intrusion. Please note a few large blocks in the area, in which mylonitic texture is developed in rocks that compositionally are anorthosite. Return to vehicles and continue north on Highway 805. 12.0-12.2 km Road extends uphill and around a curve, passing through outcrops of greenish weathering Nipissing gabbro. This curve is the location of stop R5 of Davidson (1986). 12.3 km Outcrops on both sides of the road consist of black, flinty, ultramylonite of unknown protolith, but which may be derived, in part, from Huronian felsic metavolcanic rocks. This outcrop approximates stop R4 of Davidson (1986). 12.5 km Outcrop west of road consists of boulder conglomerate, likely belonging to the Gowganda Formation, in contact with weathered granite of unknown age. Outcrops east of the road are a black, flinty ultramylonite (UTM 554057E, 5170254N). The section of road between kilometre 11.8 and 12.5 illustrates the rapid transition between weakly deformed and intensely deformed rocks within the Southern–Grenville province boundary. Metasedimentary and metavolcanic rocks of the Huronian Supergroup occur in close proximity to rocks of the River Valley intrusion in this area, but deformation has obscured primary contacts. 13.3 km Outcrops on curve consist of felsite and black, flinty ultramylonite. 15.1 km Pull over and park, examine outcrops on the east side of Highway 805. Optional Stop. Mississagi Formation and Mylonitic Contact with the River Valley Intrusion This is Stop R3a and R3c of Davidson (1986). Outcrops on the east side of Highway 805 consist of deformed quartz arenite of the Mississagi Formation. UTM of road outcrop is 554413E, 5172600N. Proceed east through the bush for approximately 100 m to a cliff face. Black, flinty ultramylonite occurs at the base of the cliff, above quartz arenite of the Mississagi Formation. The mylonite is graditional upward, over about 1 to 3 m, into foliated leucogabbronorite of the River Valley intrusion. The Dana North zone, at the next field trip stop, occurs about 50 m stratigraphically above this location, and about 400 m east of this stop. Return to vehicles and continue north on Highway 805. 15.5 km Turnoff on right (east) leads to Pacific North West Capital Corporation Dana North exploration area. 16.3 km Junction, continue south roughly 500 m on road and park in flat area opposite large stripped outcrop. 70 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Stop 9. Dana Lake Copper-Nickel-PGE Occurrence Prospectors L. Luhta, R. Bailey and R. Orchard first reported promising platinum-group element assay values from rocks of the River Valley intrusion near Dana Lake and Azen Creek in November 1998. Subsequently, Pacific North West Capital Corporation optioned and staked most of the north contact of the River Valley intrusion in Dana Township (inset map in Figure 12). Mustang Minerals Corporation optioned and staked most of the River Valley intrusion in Crerar Township, as well as part of the north contact near the Dana–McWilliams Township boundary. As of this writing, Pacific North West Capital Corporation has identified a dozen exploration targets along this northern contact. These are, from northwest to southeast, the Pardo, Dana North, Dana South, Banshee, Lismer's Ridge (Lismer's North and Lismer's South), Macdonald, Varley, Azen, Jackson's Flats, and Razor zones. The Thomson zone is located more centrally in the intrusion. Between 2000 and spring 2003, Pacific North West Capital Corporation, in conjunction with their partner, Anglo American Platinum Corporation Limited, had completed 5 diamond-drill programs. Approximately 288 diamond-drill holes totalling 58 000 m have been completed on their River Valley intrusion property in Dana Township. Diamond drilling was focussed almost entirely on the Dana North (see Figure 12) and Lismer's Ridge targets. A 6th diamond-drill program, aiming for a potential total of another 40 000 m of diamond-drill core, is underway, also focussing mainly on the Dana North and Lismer's Ridge zones (Pacific North West Capital Corporation, Press Release, April 10, 2003; http://www.pfncapital.com/s/NewsReleases). The scope of these diamond drilling programs is too large to effectively summarize herein; consequently, the reader is referred to the comprehensive assessment file reports (Resident Geologist’s office, Sudbury) related to these programs. Pacific North West Capital Corporation has released 2 Mineral Resource estimates for the Dana North zone, based on the Phase 1 to 5 drilling programs. In both cases, Derry, Michener, Booth and Wahl Consultants Limited conducted the resource studies. The first in situ resource estimate (Pacific North West Capital Corporation, Press Release, October 16, 2001) was a total measured, identified and inferred resource of 593 000 ounces palladium, platinum and gold at Dana North, Dana South and Lismer's Ridge. This estimate used a 0.7 g/t Pt+Pd cut-off grade, and can also be expressed as 12.7 million tonnes at 1.46 g/t Pt+Pd+Au. This can be broken down into 7.74 million tonnes at 1.60 g/t Pt+Pd+Au at Dana North and South, and 4.97 million tonnes at 1.24 g/t Pt+Pd+Au at Lismer's Ridge. The second Mineral Resource estimate (Pacific North West Capital Corporation, Press Release, October 17, 2002) was a total measured and indicated resource of 825 900 ounces palladium, platinum and gold. There were inferred resources of 200 600 ounces palladium, platinum and gold, yielding a total of 1 026 500 ounces at Dana North and Lismer's Ridge. This estimate used a 0.7 g/t Pt+Pd cut-off grade, and can also be expressed as 18.1 million tonnes of measured and indicated at 1.36 g/t Pt+Pd and 5.4 million tonnes inferred at 1.11 g/t Pt+Pd. Roughly 60% of this resource is in the Dana North area. A description of the Dana North zone has been published by S. Jobin-Bevans, Projects Manager for Pacific North West Capital Corporation, in James et al. (2002b). The description below, in a slightly smaller font, is derived from that published report. The distribution of the marginal Inclusion-bearing/Breccia zone (roughly equivalent to the Inclusion- and/or Fragment-bearing zone of Hrominchuk (2000) and James et al. (2002a)), the loci of mineralization, is in abrupt, intrusive contact to the east with the Neoarchean-age Pardo gneiss, and to the west these zones are replaced by a weakly layered to massive leucogabbro-gabbro-melagabbro sequence. Small pod-like, alkalic intrusions of unknown age are observed at two places along the intrusive contact where they displace the Inclusionbearing/Breccia Zone. As well, faults related to the Grenville Front and a 1238 Ma Sudbury swarm olivinemagnetite gabbro dyke cut or displace this zone. Silicate assemblages that host the mineralization in this part of the 71 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �River Valley intrusion exhibit upper greenschist facies mineralogy in contrast to the upper amphibolite facies and preserved magmatic assemblages that dominate the remainder of the intrusion. The stratigraphy of the Inclusionbearing/Breccia Zone is normally ~100 m wide in plan view. From the footwall Pardo gneiss, westward into the intrusion, the sequence and character of the distinguishable units are: 1. Footwall Breccia Unit: typically 5 to 15 m wide, but may be absent. It consists of partly rounded to angular, centimetre- to decimetre-size fragments of country rock (~75%; Pardo gneiss, Archean gabbro, diabase, diorite, minor Huronian Supergroup metasedimentary rocks) and River Valley intrusion material (~25%; chilled gabbro and medium-grained melagabbro) in a matrix of finer grained rock of similar composition and (or) an aplitic to granitic matrix. A narrow zone of migmatite at the contact of the intrusion is probably due to contact metamorphism, and granitic veins can be traced from this unit into the footwall at Lismer’s Ridge. Sulphide minerals are dominantly pyrite and pyrrhotite with local areas of trace to 1 volume percent chalcopyrite + pyrrhotite. Platinum group element concentrations are normally less than 25 ppb. 2. Boundary Unit: 5 to 20 m wide, but may be absent. It contains partly rounded to subangular, centimetre- and decimetre-size fragments of country rock (typically 10-25%), and cognate xenoliths of melagabbro, gabbro and less commonly leucogabbro to anorthosite in a matrix of gabbro to melagabbro ± aplite/granite, as in the Footwall Breccia. Sulphide minerals are mainly pyrite and pyrrhotite, locally up to 3 volume percent chalcopyrite + pyrrhotite occur; platinum group element contents are typically <75 ppb with local concentrations greater than 1000 ppb. 3. Breccia Unit: 20 m wide to greater than 100 m. It contains as much as 95% dominantly cognate xenoliths of gabbro to melagabbro and subordinate leucogabbro in a medium-grained matrix of similar composition; fragments are partly rounded to round probably due to partial assimilation, and centimetre to decimetre in size. Those greater than a metre are mainly footwall compositions (including Huronian Supergroup metasedimentary rocks) and tend to be larger with increasing proximity to the intrusive contact. Sulphide minerals (1 to 5 volume percent pyrrhotite + chalcopyrite) occur as both bleb and disseminated types; platinum group element contents are highly varied, but most values range from 500-6000 ppb with local concentrations greater than 10,000 ppb. 4. Inclusion-bearing Unit: 10 to 50 m wide. It contains >90% autoliths of leucogabbro, subordinate gabbro and less melagabbro in a matrix of either medium-grained leucogabbro or gabbro; the leucogabbro xenoliths are subangular to partly rounded, dominantly decimetre to metre in scale, and appear to be stoped inclusions from the adjacent (overlying) Leucogabbronorite zone. Sulphide minerals include trace to 3 volume percent pyrrhotite + chalcopyrite; platinum group element contents range from 100-500 ppb with local concentrations greater than 1000 ppb. Interestingly, the Breccia Unit, which shows the highest and most persistent sulphide-associated platinum group element mineralization, has the smallest proportion of footwall inclusions (<1%); perhaps an indication that chemical contamination from footwall lithologies is not a major controlling factor on mineralization. Fine-grained gabbro and diabase dikes cut all of the above units as well as the Leucogabbronorite zone in the main part of the intrusion. These dikes are metamorphosed at a grade similar to the intrusion in the Dana North area and are distinct from younger dikes of the Sudbury swarm. Drill hole data suggest that the dip of the contact of the Inclusion-bearing/Breccia zone with the footwall is at least 45° to 65° and toward the intrusion, whereas the apparent dip of the surface between the Inclusion-bearing Unit and Breccia Unit is steeper (~70° ) and toward the intrusive contact. The attitude of metre-scale layering in the Leucogabbronorite zone adjacent to the mineralized Inclusion-bearing/Breccia zone is poorly constrained but is estimated to be near-vertical (~70° to 90° west and east) and possibly shallowing (i.e. less than 70° west dip) westward into the intrusion. S/Se ratios for 13 of 15 samples (mineralized and barren) from the Dana North and Lismer’s Ridge areas have values from 500 to 2120, which are well within the magmatic range indicted by the Merensky and J-M reefs, and Konttijarvi-Portimo contact-type mineralization (i.e., 700-800 S/Se). Footwall rocks have low Pt, Pd and Au, and Pd/Pt and Cu/Ni are less than 1; these metal values indicate that the Archean footwall rocks are not genetically related to the platinum group element mineralizing event(s). Estimates of the metal values for barren magma that forms large parts of the intrusion or which are feeders, have less than 100 ppm Cu and anomalous Pt + Pd 72 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �concentrations averaging 32 to 35 ppb, but their metal ratios, Pd/Pt and Cu/Ni, are both less than 1, which is unlike mineralized samples (see below). Analytical data for sulphide-bearing felsic and mafic dikes and the Boundary, Breccia and Inclusion-bearing Units all show moderate to high average Pt+Pd (peaking in the Breccia Unit, as expected), and all show Pd/Pt and Cu/Ni ratios based on metal averages in the range 1.1-4.8, distinct from the low sulphide, Cu-poor assemblages. The mineralized zones at the Bull Frog Zone of the East Bull Lake Intrusion show the same geochemical features. River Valley Project -2004 • Dr,II Hole Locetono EHilIholeTrecen — — - lntrno,oe Content — — - Faults — Roads Trenches Lnke Figure 12. Map of the Dana North property of Pacific North West Capital Corporation, showing the location of stops 9A through 9F, as well as the extent of the drilling program undertaken between 1999 and 2002. 73 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Stop 9 is located on the Pacific North West Capital Corporation property, and consists of an examination of several outcrop areas that were exposed between 1999 and 2001 (see Figure 12). These outcrops provide an excellent opportunity to examine the contact of the River Valley intrusion, the mineralized zones and the character of the inclusion-bearing unit (Photo 13). Stop 9, Day 3, makes for an interesting comparison with Stop 11, Day 1, the traverse across the East Bull Lake mineralized contact in the Moon Lake area. STOP 9A. L6+00N, CONTACT ENVIRONMENT The line 6+00N clearing to the west of the road was completed in May 2000 in order to expose and sample the up-dip geology and mineralization intersected in diamond-drill hole RV00-08. This clearing provides excellent exposure of the stratigraphy in the contact environment. Walk to the western edge of the cleared area, and then walk back past diamond-drill hole collar RV00-08 toward the road. This will bring you from massive gabbronorite through the inclusion-bearing zone at the west end of the clearing through to the footwall Archean migmatite, paragneiss and gabbroic rocks at the east end of the clearing. The highest assay value from samples collected in this area is 10.1 g/t Au+Pd+Pt (sample L6N-09). Note that the footwall gneisses are pale green and washed out, reflecting a regional chloritic alteration zone, first noted by Lumbers (1973), which affects all rock units in northwest Dana Township and southwest Pardo Township. Walk north and across the access road for about 100 m to the next clearing to the east that intersects the road, this is grid line 7+00N. STOP 9B. L7+00N, CONTACT ENVIRONMENT The line 7+00N clearing was completed in May to June 2000 in order to expose and sample an extensive induced polarization chargeability anomaly outlined in this area. This clearing offers excellent exposure of the stratigraphy in the contact environment including numerous complex structural features that crosscut both country rocks and intrusive rocks. As on line 6+00N, walk to the western edge of the cleared area, then walk back toward the road. Diamond-drill hole collars RV00-23 and 24 are located at the north edge of the clearing, on your left. Pay special attention to the extensive east-northeast trending mylonite zone that cuts across the exposure. The highest assay value from samples collected in this area is 8.6 g/t Au+Pd+Pt (sample L7N-164). Walk south along the road for about 150 m to north end of large stripped outcrop ridge. Start by the rusty weathering part of outcrop closest to the road. STOP 9C. ROAD ZONE The Road zone exposes several different rock types of the River Valley intrusion, including massive gabbroic anorthosite and gabbro that “overlie” the inclusion-bearing and breccia units. A large, faulted felsic dike cuts the breccia zone and appears to originate from the footwall contact that is exposed toward the north end of the clearing. This area is one of the original discovery zones from the fall of 1998, with the remnants of the original exposed pit located at the extreme northeast corner of the clearing, alongside the road. Another cleared area, referred to as the Road zone east, is located immediately east of the road. The highest assay value from samples collected in this area is 13.4 g/t Au+Pd+Pt (sample RZ-361). 74 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Follow the ATV trail situated on the east side of the road by the Road zone and walk southeast. On the way you will pass by North zones 1, 2 and 3 on your left. After about 200 m (about grid line 3+00N), follow a drill road north (toward the left) for about 100 m (near diamond-drill hole collar RV00-05). The Central zone is located on a north-trending outcrop ridge. STOP 9D. CENTRAL ZONE The Central zone exposes the inclusion-bearing unit (northwest part of clearing) (Photo 13b, 13d) and the mafic breccia unit (southeast part of clearing). Note the large fragments in the outcrop at the northeast edge of the clearing and note the leucocratic nature of the matrix (see Photo 13b) for comparison to the melanocratic matrix you will see in the South zone (see Photo 13c). The highest assay value for samples collected from this zone is 9.1 g/t Au+Pd+Pt+Rh (sample CZ-54). r Retrace your route back to the ATV trail and continue south for roughly 250 m. • .-.. ..•, • Photo 13. Photographs illustrating the textural variation observed in the Inclusion-Bearing unit in the Dana North area (Day 3, Stop 9). a) Large, leucogabbronorite fragment hosted in a gabbronorite to pyroxenite matrix. Scale card is approximately 8.5 cm long. b) Gabbronorite to pyroxenite fragments hosted in a leucogabbronorite matrix in stripped outcrop at the central zone (stop 9D, Day 3). c) Mafic fragment and matrix-dominated portion of the Inclusion-Bearing unit at Dana South (stop 9F, Day 3). Hammer handle is approximately 35 cm long. d) Gabbronorite to pyroxenite fragments hosted in a leucogabbronorite matrix in stripped outcrop at the Central zone (stop 9D, Day 3). Marker is approximately 14 cm long. 75 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �STOP 9E. TRENCH ZONE The Trench zone is one of the original discovery areas from the fall of 1998, with one of the original trenches (circa 1969) still visible along the west edge of the clearing. This zone exposes the mafic breccia unit, which is best developed along the western edge of the clearing (Photo 13c). Note the “shotgunpattern” of sulphide mineralization “bleeding” in outcrop. A plagioclase-phyric dike (Matachewan swarm?) occurs in the sheared contact with footwall migmatitic paragneiss (altered Pardo gneiss) and is in sharp contact with rocks of the intrusion along the southern edge of the clearing. Samples from the dike have analyzed up to 818 ppb Au+Pd+Pt+Rh and 448 ppm Cu (sample TZ-03); finely disseminated sulphide mineralization is visible. Take time to look at the “fault breccia” located at the east edge of the clearing, just north of the dike. What is this breccia reminiscent of? The highest assay value from samples collected from this zone is 13.2 g/t Au+Pd+Pt+Rh (sample TZ-07). Keep walking south along the trail down the hill from the Trench zone and cross the stream (beaver dam) that connects Platadium Pond on the right with Dana Lake on the left. A worn bush trail will take you to the South zone clearing. Total distance from the Road zone to the South zone is about 650 m. STOP 9F. SOUTH ZONE The South zone is one of the original discovery areas from the fall of 1998. Initially, several small pits along the eastern and northwestern edges of the clearing were the only exposed areas of mineralized outcrop. It is in this area that one grab sample assayed greater than 12 g/t Au+Pd+Pt. This clearing exposes the mafic mineralized breccia unit along the east half of the outcrop and the Inclusion-Bearing zone along the western half. Note the presence of several northeast-trending diabase dikes that cut across stratigraphy. The highest assay value to date from samples collected in this area is 16.4 g/t Au+Pd+Pt+Rh (sample SZ-219). Note the presence of blue-grey quartz grains in the mafic rocks and its general association with sulphide mineralization, particularly in the area of the large blast pit. This ends Day 3. Return to vehicles and retrace route to Highway 805, head south on Highway 805 to River Valley, Warren and Highway 17. 76 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �References Ariskin, A.A., Frankel, M.Ya., Barmina, G.S., and Nielsen, R. 1993. COMAGMAT: A Fortran program to model magma differentiation processes; Computers and Geoscience, v.19, p.1155-1170. Ashwal, L.D. and Wooden, J.L. 1989. River Valley pluton, Ontario: A late-Archean/early Proterozoic anorthositic intrusion in the Grenville Province; Geochimica et Cosmochimica Acta, v.53, p.633- 641. Barnes, S.-J. and Maier, W.D. 1999. The fractionation of Ni, Cu and the noble metals in silicate and sulfide liquids; in Dynamic Processes in Magmatic Ore Deposits and their Application to Mineral Exploration; Geological Association of Canada, Short Course Notes, v.13, p.69-106. Bennett, G., Dressler, B.O., and Robertson, J.A. 1991. The Huronian Supergroup and associated intrusive rocks; in Geology of Ontario, Ontario Geological Survey, Special Volume 4, Part 1, p.549-591. Born, P. 1979. Geology of the East Bull Lake mafic intrusion, District of Algoma, Ontario; unpublished MSc thesis, Laurentian University, Sudbury, Ontario, 147p. Breaks, F.W. and Moore, J.M. 1992. The Ghost Lake batholith, Superior Province of northwestern Ontario: a fertile, S-type, peraluminous granite-rare-element pegmatite system; Canadian Mineralogist, v.30, p.835-875. Brisbin, D., Wood, P., Kleinboeck, J. and Lapierre, K. 2001. Geology of the East Bull Lake Intrusion and its contact-style PGE-Cu-Ni mineralization; Laurentian University SEG Student Chapter PGM Exploration Short Course Field Trip Guidebook, October 28, 2001, 25p. Brons, D. 1984. Geology of the Drury gabbro-anorthosite intrusion; unpublished MSc thesis, Laurentian University, Sudbury, Ontario. Cape, D.F. 1973. A petrologic and geochemical study of a gabbro, anorthositic gabbro intrusion and neighbouring volcanics, northwest corner of May Township; unpublished BSc thesis, University of Windsor, Windsor, Ontario, 62p. Card, K.D. 1978. Geology of the Sudbury–Manitoulin area, districts of Sudbury and Manitoulin; Ontario Geological Survey, Geological Report 166, 238p. Card, K.D. and Lumbers, S.B. 1975. Sudbury–Cobalt, geological compilation series, Algoma, Manitoulin, Nipissing, Parry Sound, Sudbury and Timiskaming districts; Ontario Geological Survey, Map 2361, scale 1:253 440. Card, K.D. and Palonen, P.A. 1976. Dunlop and Shakespeare townships, Sudbury District; Ontario Division of Mines, Map 2313, scale 1:31 680. Card, K.D. and Poulsen, K.H. 2000. Archean and Paleoproterozoic geology and metallogeny of the southern Canadian Shield; Exploration and Mining Geology, v.7, p.181-215. Card, K.D., Church, W.R., Franklin, J.M., Frarey, M.J., Robertson, J.A., West, G.F. and Young, G.M. 1972. The Southern Province; in Variations in Tectonic Styles in Canada, Geological Association of Canada, Special Paper 11, p. 335-380. Carr, S.D., Easton, R.M., Jamieson, R.A., and Culshaw, N.G. 2000. Geologic transect across the Grenville Orogen of Ontario and New York; Canadian Journal of Earth Sciences, v.37, p.193-216. 77 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Chen,Y.D., Krogh, T.E. and Lumbers, S.B. 1995. Neoarchean trondhjemitic and tonalitic orthogneiss identified within the northern Grenville Province in Ontario by precise U-Pb dating and petrologic studies; Precambrian Research, v.72, p.263-281. Chubb, P.T. 1994. Petrogenesis of the eastern portion of the Early Proterozoic East Bull Lake Gabbro-Anorthosite Intrusion, District of Sudbury/Algoma, Ontario; unpublished MSc thesis, Sudbury, Ontario, Laurentian University, 230p. Chubb, P.T., Hannila, K.K. and Peck, D.C. 1994. East Bull Lake gabbro anorthosite intrusion; Ontario Geological Survey, Preliminary Map P.3274, scale 1:20 000. Chubb, P.T., Peck, D.C., James, R.S. and Ercit, T.S. 1995. Nature and origin of nodular textures in anorthositic cumulates from the East Bull Lake Intrusion, Ontario; Mineralogy and Petrology, v.54, p.93-103. Corfu, F. and Andrews, A.J. 1986. A U-Pb age for mineralized Nipissing diabase, Gowganda, Ontario; Canadian Journal of Earth Sciences, v.23, p.107-109. Corfu, F. and Easton, R.M. 2000. 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Tectono-sedimentary history of early Proterozoic rocks of the northern Great Lakes area; in Early Proterozoic Geology of the Great Lakes Region, Geological Society of America, Memoir 160, p.15-32. 83 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Metric Conversion Table Conversion from SI to Imperial Conversion from Imperial to SI SI Unit Multiplied by Gives Imperial Unit 1 mm 1 cm 1m 1m 1 km 0.039 37 0.393 70 3.280 84 0.049 709 0.621 371 LENGTH inches 1 inch 25.4 inches 1 inch 2.54 feet 1 foot 0.304 8 chains 1 chain 20.116 8 miles (statute) 1 mile (statute) 1.609 344 mm cm m m km 1 cm@ 1 m@ 1 km@ 1 ha 0.155 0 10.763 9 0.386 10 2.471 054 AREA square inches 1 square inch square feet 1 square foot square miles 1 square mile acres 1 acre 6.451 6 0.092 903 04 2.589 988 0.404 685 6 cm@ m@ km@ ha 1 cm# 1 m# 1 m# 0.061 023 35.314 7 1.307 951 VOLUME cubic inches 1 cubic inch cubic feet 1 cubic foot cubic yards 1 cubic yard 16.387 064 0.028 316 85 0.764 554 86 cm# m# m# CAPACITY 1 pint 1 quart 1 gallon Multiplied by 1L 1L 1L 1.759 755 0.879 877 0.219 969 pints quarts gallons 1g 1g 1 kg 1 kg 1t 1 kg 1t 0.035 273 962 0.032 150 747 2.204 622 6 0.001 102 3 1.102 311 3 0.000 984 21 0.984 206 5 MASS ounces (avdp) 1 ounce (avdp) 28.349 523 ounces (troy) 1 ounce (troy) 31.103 476 8 pounds (avdp) 1 pound (avdp) 0.453 592 37 tons (short) 1 ton (short) 907.184 74 tons (short) 1 ton (short) 0.907 184 74 tons (long) 1 ton (long) 1016.046 908 8 tons (long) 1 ton (long) 1.016 046 90 1 g/t 0.029 166 6 1 g/t 0.583 333 33 CONCENTRATION ounce (troy)/ 1 ounce (troy)/ ton (short) ton (short) pennyweights/ 1 pennyweight/ ton (short) ton (short) Gives 0.568 261 1.136 522 4.546 090 L L L g g kg kg t kg t 34.285 714 2 g/t 1.714 285 7 g/t OTHER USEFUL CONVERSION FACTORS 1 ounce (troy) per ton (short) 1 gram per ton (short) 1 ounce (troy) per ton (short) 1 pennyweight per ton (short) Multiplied by 31.103 477 grams per ton (short) 0.032 151 ounces (troy) per ton (short) 20.0 pennyweights per ton (short) 0.05 ounces (troy) per ton (short) Note: Conversion factors which are in bold type are exact. The conversion factors have been taken from or have been derived from factors given in the Metric Practice Guide for the Canadian Mining and Metallurgical Industries, published by the Mining Association of Canada in co-operation with the Coal Association of Canada. 84 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �PDF compression, OCR, web-optimization with CVISION's PdfCompressor �ISSN 0826--9580 ISBN 0--7794--5906--7 PDF compression, OCR, web-optimization with CVISION's PdfCompressor � Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology: Proceedings, 2006 Description An account of the resource Institute on Lake Superior Geology. Sault Ste Marie, Ontario. May 8-12, 2006. Creator An entity primarily responsible for making the resource Institute on Lake Superior Geology Date A point or period of time associated with an event in the lifecycle of the resource 2006 Format The file format, physical medium, or dimensions of the resource PDF Language A language of the resource English https://digitalcollections.lakeheadu.ca/files/original/30b3fdc2335802234c13370066ca4fc8.pdf d8abacfe9bcc9e8d2baa6532033dc6cb PDF Text Text 53 ANNUAL MEETING INSTITUTE ON LAKE SUPERIOR GEOLOGY LUTSEN, MINNESOTA MAY 8 — 13,2007 PROCEEDINGS VOLUME 53 PART 1- PROGRAM AND ABSTRACTS �INSTITUTE ON LAKE SUPERIOR GEOLOGY 53RD ANNUAL MEETING MAY 8-13, 2007 LUTSEN, MINNESOTA HOSTED BY: LAUREL G. WOODRUFF AND JAMES D. MILLER, JR. Co-Chairs U.S. Geological Survey Minnesota Geological Survey Volume 53 Part 1 – Proceedings and Abstracts Edited by Laurel Woodruff, U.S. Geological Survey Cover Photos: Top - Lake Superior shoreline at Lutsen Resort; Right - Grand Portage National Monument stockade and Hay Bay; Left - High Falls on Pigeon River; Bottom - Suzie Islands (photographs courtesy of Bill Cannon) �53RD INSTITUTE ON LAKE SUPERIOR GEOLOGY VOLUME 53 CONSISTS OF: PART 1: PROGRAM AND ABSTRACTS PART 2: FIELD TRIP GUIDEBOOK TRIP 1: IGNEOUS STRATIGRAPHY OF THE POPLAR LAKE INTRUSION (FORMERLY NATHAN’S LAYERED SERIES) TRIP 2: GEOLOGIC AND CULTURAL HISTORY OF GRAND PORTAGE NATIONAL MONUMENT TRIP 3: MIDCONTINENT RIFT-RELATED MAFIC INTRUSIONS NORTH OF THE INTERNATIONAL BORDER TRIP 4: GEOLOGY AND CU-NI-PGES MINERALIZATION – NICKEL LAKE MACRODIKE, SOUTH KAWISHIWI INTRUSION TRIP 5: GEOLOGY OF THE NORTH SHORE FROM LITTLE MARAIS TO GRAND MARAIS TRIP 6: GEOLOGY OF THE GUNFLINT TRAIL Reference to material in Part 1 should follow the example below: Hart, T.R. and MacDonald, C.A., 2007, Emplacement of the Nipigon Sill Complex and mafic to ultramafic intrusions of the Nipigon Embayment [abstract]; Institute on Lake Superior Geology Proceedings, 53rd Annual Meeting, Lutsen, MN, v. 53, part 1, p. 36-37. Published by the 53rd Institute on Lake Superior Geology and distributed by the ILSG Secretary: Peter Hollings Department of Geology Lakehead University Thunder Bay, ON P7B 5E1 CANADA peter.hollings@lakeheadu.ca ILSG website: http://www.lakesuperiorgeology.org ISSN 1042-9964 ii �TABLE OF CONTENTS PROCEEDINGS VOLUME 53 PART 1—PROGRAM AND ABSTRACTS Institutes on Lake Superior Geology, 1955-2007.............................................................. iv Goldich Medal Committee................................................................................................. vi Past Goldich Medalists ...................................................................................................... vi Citation for 2007 Goldich Medal Recipient ..................................................................... vii Sam Goldich and the Goldich Medal............................................................................... viii ILSG Student Research Fund ..............................................................................................x Student Paper Awards........................................................................................................ xi Eisenbrey Student Travel Awards .................................................................................... xii Report of the Chair of the 52nd Annual Meeting ............................................................ xiii 2007 Board of Directors ....................................................................................................xv 2007 Session Chairs...........................................................................................................xv 2007 Student Paper Awards Committee ............................................................................xv 2007 Local Committees .....................................................................................................xv 2007 Banquet Speaker ..................................................................................................... xvi Program........................................................................................................................... xvii Poster Presentations ....................................................................................................... xxiii Abstracts ..........................................................................................................................xxv iii �PREVIOUS INSTITUTES ON LAKE SUPERIOR GEOLOGY ILSG YEAR PLACE CHAIRS 1 1955 Minneapolis, Minnesota C.E. Dutton 2 1956 Houghton, Michigan A.K. Snelgrove 3 1957 East Lansing, Michigan B.T. Sandefur 4 1958 Duluth, Minnesota R.W. Marsden 5 1959 Minneapolis, Minnesota G.M. Schwartz and C. Craddock 6 1960 Madison, Wisconsin E.N. Cameron 7 1961 Port Arthur, Ontario E.G. Pye 8 1962 Houghton, Michigan A.K. Snelgrove 9 1963 Duluth, Minnesota H. Lepp 10 1964 Ishpeming, Michigan A.T. Broderick 11 1965 St. Paul, Minnesota P.K. Sims and R.K. Hogberg 12 1966 Sault Ste. Marie, Michigan R.W. White 13 1967 East Lansing, Michigan W.J. Hinze 14 1968 Superior, Wisconsin A.B. Dickas 15 1969 Oshkosh, Wisconsin G.L. LaBerge 16 1970 Thunder Bay, Ontario M.W. Bartley and E. Mercy 17 1971 Duluth, Minnesota D.M. Davidson 18 1972 Houghton, Michigan J. Kalliokoski 19 1973 Madison, Wisconsin M.E. Ostrom 20 1974 Sault Ste. Marie, Ontario P.E. Giblin 21 1975 Marquette, Michigan J.D. Hughes 22 1976 St. Paul, Minnesota M. Walton 23 1977 Thunder Bay, Ontario M.M. Kehlenbeck 24 1978 Milwaukee, Wisconsin G. Mursky 25 1979 Duluth, Minnesota D.M. Davidson 26 1980 Eau Claire, Wisconsin P.E. Myers 27 1981 East Lansing, Michigan W.C. Cambray 28 1982 International Falls, Minnesota D.L. Southwick 29 1983 Houghton, Michigan T.J. Bornhorst iv �30 1984 Wausau, Wisconsin G.L. La Berge 31 1985 Kenora, Ontario C.E. Blackburn 32 1986 Wisconsin Rapids, Wisconsin J.K. Greenberg 33 1987 Wawa, Ontario E.D. Frey and R.P. Sage 34 1988 Marquette, Michigan J. S. Klasner 35 1989 Duluth, Minnesota J.C. Green 36 1990 Thunder Bay, Ontario M.M. Kehlenbeck 37 1991 Eau Claire, Wisconsin P.E. Myers 38 1992 Hurley, Wisconsin A.B. Dickas 39 1993 Eveleth, Minnesota D.L. Southwick 40 1994 Houghton, Michigan T.J. Bornhorst 41 1995 Marathon, Ontario M.C. Smyk 42 1996 Cable, Wisconsin L.G. Woodruff 43 1997 Sudbury, Ontario R.P. Sage and W. Meyer 44 1998 Minneapolis, Minnesota J.D. Miller, Jr. and M.A. Jirsa 45 1999 Marquette, Michigan T.J. Bornhorst and R.S. Regis 46 2000 Thunder Bay, Ontario S.A. Kissin and P. Fralick 47 2001 Madison, Wisconsin M.G. Mudrey, Jr. and B.A. Brown 48 2002 Kenora, Ontario P. Hinz and R.C. Beard 49 2003 Iron Mountain, Michigan L.G. Woodruff and W.F. Cannon 50 2004 Duluth, Minnesota S.A. Hauck and M. Severson 51 2005 Nipigon, Ontario P. Hollings and M.C. Smyk 52 2006 Sault Ste. Marie, Ontario R.P. Sage and A.C. Wilson 53 2007 Lutsen, Minnesota L.G. Woodruff and J.D. Miller, Jr. v �PAST GOLDICH MEDALISTS 1979 Samuel S. Goldich 1993 Donald W. Davis 1980 not awarded 1994 Cedric Iverson 1981 Carl E. Dutton, Jr. 1995 Gene La Berge 1982 Ralph W. Marsden 1996 David L. Southwick 1983 Burton Boyum 1997 Ronald P. Sage 1984 Richard W. Ojakangas 1998 Zell Peterman 1985 Paul K. Sims 1999 Tsu-Ming Han 1986 G.B. Morey 2000 John C. Green 1987 Henry H. Halls 2001 John S. Klasner 1988 Walter S. White 2002 Ernest K. Lehmann 1989 Jorma Kalliokoski 2003 Klaus J. Schulz 1990 Kenneth C. Card 2004 Paul Weiblen 1991 William Hinze 2005 Mark Smyk 1992 William F. Cannon 2006 Michael G. Mudrey 2007 GOLDICH MEDAL RECIPIENT Joseph Mancuso Bowling Green State University Bowling Green, Ohio GOLDICH MEDAL COMMITTEE Serving for the meeting year shown in parentheses Tom Hart (2004-2007) Doug Duskin (2005-2008) Richard Ojakangas (2006-2009) vi Government representative Industry representative Academic representative �CITATION FOR GOLDICH MEDAL RECIPIENT Joseph Mancuso, 2007 Goldich Medal Recipient Membership in the Institute of Lake Superior Geology is composed of government personnel who produce maps and other essential services; exploration geologists who walk the bush looking for ore deposits, and university professor who teach geology to future government personnel, exploration geologists, and professor. Membership also includes students who are learning the dimensions of the profession. In addition to teaching, Joe was an active exploration geologist during the 40 years he taught at Bowling Green State University. Joe has been an active member of ILSG for over 50 years. He attended the first meeting when he was an undergraduate student at Carleton College. During the past 50 years he has contributed more than 20 oral presentations and posters to ILSG meetings, plus 42 published papers and abstracts on the geology of the Lake Superior region. He also direct 39 MS theses on the geology of the Lake Superior region, as well as conducting annual field trips for his students to examine Precambrian geology in the field and in mines in Michigan, Wisconsin, and Ontario. Joe has produced some outstanding students. One brought two gold mines into production; another induced Freeport to drill Grasberg Mountain (4100 meters elevation) in Irian Jaya, Indonesia. Grasberg is now the largest single gold producer in the world in terms of ounces and revenue per year. It is also a high profitable porphyry copper mine (225,000 tons per day milled) that has produced the cash to Freeport to purchase PhelpsDodge. Another student recognized the alteration associated with a blind VMS deposit in Manitoba and directed a successful drilling program. He was recognized for this achievement by being named "Prospector of the Year" by the Prospectors and Developers Association of Canada. Another student collected the grab samples on the J-M platinum/palladium layer in the Stillwater layered Igneous Complex in Montana. This layer now supports two mines: Stillwater and Boulder. Finally, two of his students did the grab and development sampling at the Jarrett Canyon gold mine prior to being brought into production. Joe, you earned the Goldich award. Submitted by Ron Seavoy vii �SAM GOLDICH AND THE GOLDICH MEDAL Sam Goldich received an AB from the University of Minnesota in 1929, a M.A. from Syracuse University in 1930, and a Ph.D. from Minnesota in 1936. During World War II Sam worked for the U.S. Geological Survey in mineral exploration. In 1948, Sam returned to the University of Minnesota, and became Professor and Director of the Rock Analysis Laboratory the following year. He rejoined the U.S. Geological Survey in 1959 and was appointed as the first Branch Chief of the Branch of Isotope Geology. Sam returned to academia in 1964 when he went to Pennsylvania State University. He left PSU in 1965 and moved to the State University of New York at Stony Brook, where he stayed for 3 years. Restless yet again, he moved to Northern Illinois University in 1968 where he was a professor until his retirement in 1977. Sam’s final move was to Denver where he became an emeritus at the Colorado School of Mines. Sam died in 2000, less than a month before his 92nd birthday. In the late 1970’s, Geological Society of America Special Paper 182, which included seminal geochronological studies by Sam Goldich and coworkers on the Archean rocks of the Minnesota River Valley, was nearing completion. At this time various ILSG regulars began discussing the possibility of recognizing Sam for his pioneering work on the resolution of age relationships and thus the geology of Precambrian rocks in the Lake Superior region. Three members, R.W. Ojakangas, J.O. Kalliokoski and G.B. Morey, presented the idea to the ILSG Board of Directors in 1978. The Board approved the creation of an award, provided funding could be obtained. It was suggested that collecting one or two dollars at registration for a dedicated account would provide resources for striking the medal. A general request was made to the ILSG membership for donations and Sam himself offered a challenge grant to match the contributions. In total $4,000 was collected and thus began the work of creating the Goldich Medal. The initial Goldich Award was presented to Sam by G.B. Morey in 1979 and consisted of a large paper proclamation. For the actual medal, G.B. Morey consulted with the foundry on production details, while Dick Ojakangas and Jorma Kalliokoski worked on the design of the award, suggesting that it be given for “outstanding contributions to the geology of the Lake Superior region.” Simultaneously, a committee of J.O. Kalliokosi, W.F. Cannon, M.M Kehlenbeck, G.B. Morey, and G. Mursky developed the Award Guidelines that were approved by the ILSG Board. By 1981 all the elements of the Goldich Award had come together, and the second recipient, Carl E. Dutton, Jr., received viii �the Goldich Medal for 50 years of significant contributions to the understanding of the geology of the Lake Superior region. Since the beginning, the Awards Committee has consisted of individuals representing industry, government and academia, with each member of the Committee serving for three years. The medal is now awarded every year at the annual ILSG meeting. Reference: Morey, G.B. and Hanson, G.N. (editors). 1980. Selected studies of Archean gneisses and Lower Proterozoic rocks, southern Canadian Shield. Geological Society of America, Special Paper 182, 175 p. Prepared by various Goldich Medal Awardees, 2007 INSTITUTE ON LAKE SUPERIOR GEOLOGY GOLDICH MEDAL ix �ILSG STUDENT RESEARCH FUND The 2005 Board of Directors established the ILSG Student Research Funs with $10,000 US from the Institute’s general fund to encourage student research on the geology of the Lake Superior region. A minimum of two awards of $500 US each for research expenses (but not travel expenses) will be made each year. Students are expected to present their research orally or during a poster session at an ILSG meeting. The award winners will also be automatically eligible for the Eisenbrey Travel Awards. To allow the fund to grow, the Fund will receive one-half of any additional proceeds from each annual meeting, after all other commitments and expenses are covered. • The ILSG Board of Directors will be responsible for selecting a minimum of two awards each year. The ILSG Treasurer will issue the awards. • The ILSG Student Research Fund is available for undergraduate or graduate students working on geology in the Lake Superior region. • The applications are due to the ILSG Secretary by August 31st of each year. Awards will be made by October 1st of each year. • Names of the award recipients will be announced at the next annual meeting and posted on the ILSG website. • The proposal application should be at least 500 words, and should have a statement of the research project, background information, a map of the research area, research steps necessary to complete the research, figures (if needed) , references, and a list of research expenses. The proposal should also include a proposed end date for the research. • The proposal will need to be signed by researcher’s supervisor. In 2006 the ILSG Board of Governors awarded the first three $500 awards from the Student Research Fund. This year awards were made to: Cole Edwards (University of Wisconsin - Oshkosh) - Controls on the formation of the earliest marine phosphate deposits, Marquette Supergroup, Michigan Noah Planavsky (Rosenstiel School of Marine and Atmospheric Sciences Marine Geology and Geophysics, Miami) - Iron isotopes as oceanographic tracers in Animikie Basin Iron Formations Michael Taylor (University of Minnesota - Duluth) - Pleistocene glaciation as a mechanism for emplacement of high-salinity groundwater at anomalously shallow depths in the Lake Superior basin x �STUDENT PAPER AWARDS Each year, the Institute selects the best of the student presentations and honors presenters with a monetary award. Funding for the award is generated from registrations of the annual meeting. The Student Paper Committee is appointed by the annual meeting Chair in such a manner as to represent a broad range of professional and geologic expertise. Criteria for best student paper—last modified by the Board in 2001—follow: • The contribution must be demonstrably the work of the student. • The student must present the contribution in-person. • The Student Paper and Poster Committee shall decide how many awards to grant, and whether or not to give separate awards for poster vs. oral presentations. • In cases of multiple student authors, the award will be made to the senior author, or the award will be shared equally by all authors of the contribution. • The total amount of the awards is left to the discretion of the meeting Chair in conjunction with the Secretary, but typically is in the amount of about $500 US (increase approved by Board, 10/01). • The Secretary maintains, and will supply to the Committee, a form for the numerical ranking of presentations. This form was created and modified by Student Paper and Poster Committees over several years in an effort to reduce the difficulties that may arise from selection by raters of diverse background. The use of the form is not required, but is left to the discretion of the Committee. • The names of award recipients shall be included as part of the annual Chair's report that appears in the next volume of the Institute. Student papers are noted on the Program. In 2006 the ILSG Student Paper Committee presented four awards from the ILSG Student Paper Fund. The awards were made to: Patrick Moran -- Lakehead University, Thunder Bay, ON (best oral presentation) -- $250 Noah Planavsky -- Lawrence University, Appleton, WI (oral presentation, honorable mention) -- $50 Amanda Gross -- Kent State University, Kent, OH (best poster) -- $250 Ryan Bartingale -- University of Wisconsin - Eau Claire, Eau Claire, WI (poster, honorable mention) -- $50 xi �EISENBREY STUDENT TRAVEL AWARDS The 1986 Board of Directors established the ILSG Student Travel Awards to support student participation at the annual meeting of the Institute. The name "Eisenbrey" was added to the award in 1998 to honor Edward H. Eisenbrey (1926-1985) and utilize substantial contributions made to the 1996 Institute meeting in his name. "Ned" Eisenbrey is credited with discovery of significant volcanogenic massive sulfide deposits in Wisconsin, but his scope was much broader—he has been described as having unique talents as an ore finder, geologist, and teacher. These awards are intended to help defray some of the direct travel costs of attending Institute meetings, and include a waiver of registration fees, but exclude expenses for meals, lodging, and field trip registration. The annual Chair in consultation with the Secretary-Treasurer determines the number of awards and value. Recipients will be announced at the annual banquet. The student travel award application is available on the ILSG website. The following general criteria will be considered by the annual Chair, who is responsible for the selection: • The applicants must have active resident (undergraduate or graduate) student status at the time of the annual meeting of the Institute, certified by the department head. • Students who are the senior author on either an oral or poster paper will be given favored consideration. • It is desirable for two or more students to jointly request travel assistance. • In general, priority will be given to those in the Institute region who are farthest away from the meeting location. • Each travel award request shall be made in writing to the annual Chair, and should explain need, student and author status, and other significant details. • Successful applicants will receive their awards during the meeting. In 2006 the ILSG awarded six travel awards from the ILSG Eisenbrey Student Travel Fund. The awards were made to: Peggy Stonier -- Kent State University, Kent, Ohio Amanda Gross -- Kent State University, Kent, Ohio Jenny Murphy -- Lawrence University, Appleton WI Noah Planavsky -- Lawrence University, Appleton WI Clare Stielstra -- Lawrence University, Appleton WI Davis Voights -- Lawrence University, Appleton WI xii �REPORT OF THE CHAIRS OF THE 52ND ANNUAL MEETING INSTITUTE ON LAKE SUPERIOR GEOLOGY SAULT STE MARIE, ONTARIO The Ontario Geological Survey hosted the 52nd Annual Institute on Lake Superior Geology meeting on May 10-11 2006 in Sault Ste Marie, Ontario. The meeting consisted of two days of technical sessions with two pre-meeting and four post-meeting field trips. Pete Hollings and Mike Mudrey provided invaluable technical assistance with the web site and Anthony Pace provided on site assistance with the audio-visual component of the program. Gerry Bennett, Mike Easton, Mike Hailstone and Anthony Pace provided assistance with field trip preparation. Vivienne Coté and Roger Poulin (Ontario Prospectors Association) and Marc Gaudreau and Anthony Pace (Ontario Geological Survey) provided aid with the loan, retrieval, delivery and set-up of the poster boards for the poster session. Lisa Bagnall, Scheduling Office, was the on site co-ordinator with the Sault College of Applied Arts and Technology. Total registration for the meeting was 110 students and professionals. Proceedings Volume 52 was published in six parts: Part 1 – Proceedings and Abstracts, edited by A. C. Wilson contained abstracts for 19 oral presentations and 21 posters; Part 2 – Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan – Field Trip Guidebook; Part 3 – Unusual Diamond-bearing breccias of the Wawa Area – Field Trip Guidebook; Part 4 – The Huronian Supergroup between Sault Ste Marie and Elliot Lake – Field Trip Guidebook; Part 5 – Keweenawan Rocks of the Mamainse Point Area – Field Trip Guidebook and Part 6 – Geological Guidebook to the Paleoproterozoic East Bull Lake Intrusive Suite Plutons at East Bull Lake, Agnew Lake and River Valley, Ontario – Field Trip Guidebook. All guidebook volumes were edited by R. P. Sage. Last held in Sault Ste Marie, Ontario in 1974, a return visit to the area allowed participants to take part in a smorgasbord of new field trips. On Monday, May 8, Gerry Bennett, assisted by Mike Hailstone, led a two-day field trip to examine the classic stratigraphy of the Huronian Supergroup in the Elliot Lake and Searchmont areas. The next day, Ann Wilson introduced a rain-soaked but enthusiastic crowd of participants to the unusual diamond-bearing breccias of the Wawa area. Following the conference four field trips were held: Ann Wilson reprised the Wawa field trip; Tom Hart and Anthony Pace’s trip examined the Keweenawan rocks along the Lake Superior shoreline; Ron Sage led a small group of bicyclists to Mackinac Island to examine bedrock exposures there; and Mike Easton and Dick James (Laurentian University) took a group east of the Sault to investigate a suite of intrusive rocks along the boundary between the Archean Superior and the Proterozoic Southern provinces. Eighty-five participants attended the Annual Banquet. Dr. Ed Walker gave the after dinner key note address providing his insight into the “Emplacement and Geochemistry of Archean-aged diamondiferous rocks of the Wawa Area”. Jim Miller presented the 2006 Goldich Medal to Mike Mudrey for his 30 years of contributions to the understanding of the regional geology and for his service to the Institute. The student paper committee consisted of Dan England, John Klasner and Norm Trowell all of whom conscientiously deliberated over the four oral presentations and four poster presentations. The winners were: xiii �2006 Best Student Paper Awards 1) Patrick Moran – Lakehead University ($250, winner best oral presentation) 2) Amanda Gross – Kent State University ($250, winner best poster presentation) 3) Noah Planavsky – Lawrence University ($50, honourable mention oral presentation) 4) Ryan Bartingale – University of Wisconsin ($50, honourable mention poster presentation) In addition, Eisenbrey travel awards were presented to: 1) 2) 3) 4) 5) 6) Noah Planavsky – Lawrence University ($150) Jenny Murphy – Lawrence University ($150) Clare Stielstra – Lawrence University ($150) Davis Voights – Lawrence University ($150) Penny Stonier – Kent State University ($200) Amanda Gross – Kent State University ($200) The Institute’s Board of Directors met on May 10, 2006 and a brief summary of the meeting follows. 1. Accepted the Report of the Chairs for the 51st ILSG from Mark Smyk and Pete Hollings, and the minutes of the last Board meeting, May 26, 2006 from Pete Hollings. 2. Received, discussed and accepted the 2005-06 ILSG Financial Summary from ILSG Treasurer, Mark Jirsa. 3. Approved Ann Wilson as the on-going board member. 4. Approved 2007 (53rd annual) meeting location as Lutsen, Minnesota with Laurel Woodruff as chair. 5. Replaced George Hudak, academic member, on the Goldich Committee with Dick Ojakangas. The 52nd meeting was not without its challenges, including the last minute resolution of a strike at Ontario’s Colleges. However, no challenges were insurmountable and the meeting and field trips went off without a hitch. We thank all individuals who contributed to the success of the meeting, including the staff of Sault College, the bus drivers from AJ Bus Lines Limited and Reid Bus Line, volunteer field trip leaders and drivers and the Ontario Geological Survey. The field trips were astonishingly well attended and we are grateful for the kind feedback that we received from the field trip participants. Both of us were extremely satisfied with the 52nd meeting. We appreciated all of the positive feedback from the delegates who enjoyed a return visit to the Sault. As always, we wouldn’t have succeeded without the continuing support of the ILSG members. Respectfully submitted Ron Sage and Ann Wilson Co-Chairs, 52nd ILSG Meeting xiv �2007 BOARD OF DIRECTORS Board appointment continues through the close of the last meeting year, or until a successor is selected Laurel Woodruff and Jim Miller, Co-Chairs 53rd meeting (2010) U.S. Geological Survey, St. Paul, MN Minnesota Geological Survey, Duluth, MN Ann Wilson (2009) Ontario Geological Survey, South Porcupine, ON Mark Smyk (2008) Ministry of Northern Development and Mines, Thunder Bay, ON Steve Hauck (2007) Natural Resources Research Institute, University of Minnesota – Duluth, Duluth, MN Peter Hollings – Secretary (2007) Lakehead University, Thunder Bay, ON Mark A. Jirsa – Treasurer (2009) Minnesota Geological Survey, St. Paul, MN 2007 SESSION CHAIRS Ted Bornhorst, Michigan Technological Survey, Houghton, MI Mary Louise Hill, Lakehead University, Thunder Bay, ON Gordon Medaris, Jr., University of Wisconsin, Madison, WI Klaus Schulz, U.S. Geological Survey, Reston, VA Thomas Waggoner, retired, Cliffs Mining Services Co., Ishpeming, MI Ann Wilson, Ontario Geological Survey, South Porcupine, ON 2007 STUDENT PAPER COMMITTEE Marcia Bjornerud (Chair), Lawrence University, Appleton, WI Daniela Vallini, Woodside Energy, Ltd., Perth, Western Australia Graham Wilson, Magma Metals, Campbellford, ON 2007 LOCAL COMMITTEES General Co-Chairs Laurel G. Woodruff – U.S. Geological Survey, St. Paul, MN James D. Miller, Jr. – Minnesota Geological Survey, St. Paul, MN Program and Abstracts Editor Laurel G. Woodruff -- U.S. Geological Survey, St. Paul, MN Field Trip Guidebook Editor James D. Miller, Jr. – Minnesota Geological Survey, St. Paul, MN Acting Local Committee, Lutsen, MN Gretchen Klasner – Marquette, MI xv �2007 BANQUET SPEAKER Don Hunter PolyMet Mining, Inc. Hoyt Lakes, MN PolyMet’s NorthMet Project – the long road from exploration to production xvi �PROGRAM xvii �TUESDAY MAY 8, 2007 8:00 a.m. FIELD TRIP 1: IGNEOUS STRATIGRAPHY OF THE POPLAR LAKE INTRUSION (FORMERLY NATHAN’S LAYERED SERIES) Jim Miller, Minnesota Geological Survey Eric Jerde, Morehead State University WEDNESDAY MAY 9, 2007 8:00 a.m. FIELD TRIP 2: GEOLOGIC AND CULTURAL HISTORY OF GRAND PORTAGE NATIONAL MONUMENT Bill Cannon, U.S. Geological Survey Brian Phillips, Lakehead University David Cooper, National Park Service 8:00 a.m. FIELD TRIP 3: MIDCONTINENT RIFT-RELATED INTRUSIONS NORTH OF THE INTERNATIONAL BORDER Mark Smyk, Ministry of Northern Development and Mines Peter Hollings, Lakehead University 6:00 p.m. Return of Trips 1, 2 and 3 4:00 p.m. - 8:00 p.m. Registration 7:00 p.m. - 9:00 p.m. Ice Breaker Social and Poster Setup THURSDAY MAY 10, 2007 Note: Asterisk * denotes a student eligible for Best Student Paper Award Presenter underlined 8:00 a.m. - 9:00 a.m. REGISTRATION 8:25 a.m. INTRODUCTORY REMARKS Laurel Woodruff and Jim Miller, Co-Chairs, 2007 ILSG TECHNICAL SESSION I Session Chair: Klaus Schulz, U.S. Geological Survey 8:30 a.m. Addison, William D., Cannon, William F. and Brumpton, Gregory R. How to identify Sudbury impact ejecta in the Lake Superior Region 8:50 a.m. Cannon, William F. and Addison, William D. The Sudbury impact layer in the Lake Superior iron ranges: A time-line from the heavens 9:10 a.m. Burton, Justin and Fralick, Philip Deposition and cementation of Paleoproterozoic Gunflint Formation carbonate: Implications for early hydrosphere chemistry xviii �9:30 a.m. Planavsky, Noah* and Murphy, Jennifer Rare earth element patterns in Steep Rock Carbonates 9:50 a.m. - 12:00 p.m. COFFEE BREAK AND EXTENDED POSTER SESSION 12:00 p.m. Lunch Break – 2007 ILSG Board Meeting (by invitation) TECHNICAL SESSION II Session Chairs: Mary Louise Hill, Lakehead University Gordon Medaris, Jr., University of Wisconsin, Madison 1:30 p.m. Green, John C. and Slade, Andrew A State Scientific and Natural Area and a North Shore non-profit association 1:50 p.m. Boerboom, Terry J. Newly recognized thick interflow sandstones in the upper northeast limb of the North Shore Volcanic Group, Minnesota 2:10 p.m. Mudrey, Michael G., Jr. and Wooden, Joseph L. A chemical and Sr isotopic study of the Pigeon Point Sill, Cook County, Minnesota 2:30 p.m. Bjornerud, Marcia Evidence for paleoseismic events during closure of the Midcontinent Rift, Atkins Lake-Marenisco Fault, N. Wisconsin 2:50 p.m. Hudak, George J., Hoffman, Adam T., Peterson, Dean M. and Heine, John Recent developments understanding the volcanic, magmatic, tectonic, and metallogenic evolution of the Ely Greenstone Formation, Vermilion District, NE Minnesota 3:10 p.m. COFFEE BREAK AND POSTER SESSION 3:30 p.m. Puumala, Mark A. New insights into the metallogeny of the eastern portion of the Archean Uchi Domain, Superior Province, Ontario 3:50 p.m. Alexander, Malcolm* and Mitchell, Roger H. Rare metal mineralization in pegmatites of the Coldwell Alkaline Complex 4:10 p.m. Good, David and Walford, Phillip PGE-rich mineralization at the Marathon Deposit, Coldwell Alkaline complex, Ontario xix �4:30 p.m. Brown, Alex C. Deposition of native copper lodes on the Keweenaw Peninsula, northern Michigan, from a gravity-driven evolved meteoric brine 6:00 p.m. ICE BREAKER – MIXER – CASH BAR 7:00 p.m. ANNUAL BANQUET AND AWARD PRESENTATION • Announcement of 54th Annual Meeting Location • 2007 Goldich Award Presentation to Joseph Mancuso • 2007 Banquet Address Meeting participants who are not registered for the banquet are welcome to the banquet address FRIDAY MAY 11, 2007 8:25 a.m. INTRODUCTORY REMARKS Laurel Woodruff and Jim Miller, Co-Chairs TECHNICAL SESSION III Session Chairs: Thomas Waggoner, retired, Cliffs Mining Services Co. Ann Wilson, Ontario Geological Survey 8:30 p.m. Student, James J., Wark, David A., Mutchler, Scott R. and Bodnar, Robert J. Thermal evolution of Proterozoic (>1Ga) rhyolite magma based on analysis of melt inclusions and trace elements in quartz from the Keweenaw Peninsula of Michigan 8:50 p.m. Hart, Tom R. and MacDonald, Carol Anne Emplacement of the Nipigon Sill Complex and mafic to ultramafic intrusions of the Nipigon Embayment 9:10 p.m. Hollings, Pete, Smyk, Mark C. and Hart, Tom Geochemistry of Midcontinent Rift-related mafic dykes and sills near Thunder Bay: new insights into geographic distribution and the geochemical affinities of Nipigon and Logan sills and Pigeon River and other dykes 9:30 p.m. Forsha, Clint J.* and Zieg, Michael J. Textural stratigraphy of Nipigon diabase sills: A tool for correlation and petrologic interpretation 9:50 p.m. Zieg, Michael J. and Forsha, Clint J. A comparison of textural profiles in diabase sills from the Midcontinent and Transantactic rift 10:10 a.m. COFFEE BREAK AND POSTER SESSION xx �10:30 p.m. Cockerton, Sarah, Conly, Andrew G. and Lee, Peter An experimental water-rock interaction study into the origin of high-sulfate waters associated with the Steep Rock Mines, Atikokan, Ontario 10:50 p.m. Stevens, Larissa B.* and Fralick, Philip Investigation of ferromanganese nodule precipitation and arsenic uptake in modern lacustrine biochemical sediments 11:10 a.m. Rodengen, Tommy*, Theissen, Kevin and Sugita, Sugita Elemental and isotopic shallow lake proxies of landscape changes in the Prairie Pothole region of Minnesota 11:30 a.m. Diedrich, Tamara* and Sharp, Thomas G. The effect of H2O on olivine to ringwoodite transformation: Implications for subduction zone dynamics and the deep earth water cycle 11:50 a.m. Thorliefson, Harvey Potential use of the Midcontinent Rift for CO2 sequestration 12:10 p.m. LUNCH BREAK TECHNICAL SESSION IV Session Chair: Ted Bornhorst, Michigan Technological University 1:40 a.m. McSwiggen, Peter L. Trace element analyses: Avoiding data distortion 2:00 p.m. Waggoner, Tom D. Definition of the Proterozoic terrain under the Paleozoic – Central U.P., Michigan 2:00 p.m. Oreskovich, Julie Documenting underground mine workings on the Mesabi Iron range in GIS format 2:40 p.m. Metsaranta, Riku T., Fralick, Philip W. and Bowdidge, Colin Metamorphosed halite-dominated evaporates of the Lower Sibley Group 3:00 p.m. Presentation of Student Paper Awards Marcia Bjornerud, Lawrence University: Student Paper Committee 3:10 a.m. FINAL COFFEE BREAK AND POSTER SESSION – POSTERS TO BE REMOVED AFTER THE BREAK xxi �SATURDAY MAY 12, 2007 8:00 a.m. FIELD TRIP 4: GEOLOGY AND CU-NI-PGES MINERALIZATION – NICKEL LAKE MACRODIKE, SOUTH KAWISHIWI INTRUSION Dean Peterson, Natural Resources Research Institute Paul Albers, Duluth Metals 8:00 a.m. FIELD TRIP 5: GEOLOGY OF THE NORTH SHORE FROM LITTLE MARAIS TO GRAND MARAIS Terry Boerboom, Minnesota Geological Survey John Green, University of Minnesota - Duluth Jim Miller, Minnesota Geological Survey 8:00 a.m. FIELD TRIP 6: GEOLOGY ALONG THE GUNFLINT TRAIL Mark Jirsa, Minnesota Geological Survey Paul Weiblen, University of Minnesota – Twin Cities 6:00 p.m. Return of Trips 5 & 6 SUNDAY MAY 13, 2007 6:00 p.m. Return of Trip 4 xxii �POSTER PRESENTATIONS Behling, Stuart J. Mercury in fish: a geologist’s perspective Boerboom, Terry J., Green, John C., Albers, Paul and Miller, James D., Jr. Bedrock geologic map of the Little Marais, Schroeder, and Tofte 7.5 minute quadrangles, North Shore of Lake Superior, Minnesota Boisjoli, Troy* and Flood, Timothy P. Petrographic differentiation of the five phases of the Lower Cretaceous Star Kimberlite, Saskatchewan, Canada Buchholz, Thomas W., Falster, Alexander U. and Simmons, William B. A novel locality for pseudobrookite – the Nine Mile-Pluton, Marathon County, Wisconsin Costello, Daniel E.*, Flood, Timothy P. and Thole, Jeffrey T. Origin of a mafic pegmatite within the Duluth Complex, Northern Minnesota Dean, J. Frederick and Phillips, Brian A.M. Distribution of certain ichthyofauna in relation to eastern outlets of Lake Agassiz, with emphasis on the Gunflint-Arrow Lakes corridor and the Keating Complex Elsenheimer, Don, Frey, Barry A. and Hudak, Joe N. Gold mineralization in the Virginia Horn Greenstone Terrain, St. Louis County, Minnesota: A prospect re-visited Hogan, Amanda*, Jacobs, Travis* and Theissen, Kevin Lacustrine sedimentary organic matter proxies of recent lake state changes and climatic conditions on Christina and Morrison Lakes of western Minnesota Hudak, Joe N. and Frey, Barry A. Evaluation of mineral exploration drill cuttings in the Rice River area, east central Minnesota Jasinevicius, Renata R.* and Gordon, Elizabeth A. Paleoenvironmental interpretation of a Lower Paleozoic stromatolite reef, northeastern Wisconsin Jirsa, Mark A., Boerboom, Terry J., Chandler, Val W., Lively, Richard S., Miller, James D., Jr., Mossler, John H., Runkel, Anthony C., Setterholm, Dale R. and Wahl, Timothy E. Proposed new bedrock geologic map of Minnesota Medaris, L. Gordon, Jr., Fournelle, John H. and Guggenheim, Stephen J. An occurrence of agrellite in the Wausau Alkaline Igneous Complex, Marathon County, Wisconsin xxiii �Nicholas, Sarah L.*, Wirth, Karl R., Engstrom, Jennifer and Lapakko, Kim A. Investigations of sulfide minerals leached in the presence of alkaline solids Quigley, Patrick O.* Michigan kimberlites revisited: New mineral, chemical and petrographic analyses Rymaszewski, Jody A.*, Friedrich, Jason L. and Czeck, Dyanna M. Bedrock fractures in southeastern Wisconsin: Paleostress estimates and relationships to the Waukesha Fault Saxton, Samantha* and Cordu, William Precambrian geology of the Opelt Quarry, Neillsville, Wisconsin Severson, Mark J. and Heine, John Revised stratigraphy of the Biwabik Iron Formation, Mesabi Range, Minnesota – developing the “Rosetta Stone” Stott, Greg, Corkery, Tim, LeClair, Alain, Boily, Michel and Percival, John A revised terrane map for the Superior Province as interpreted from aeromagnetic data Taylor, Michael L.* and Swenson, John B. Pleistocene glaciation as a mechanism for emplacement of high-salinity groundwater at anomalously shallow depths in the Lake Superior basin Theriault, Stephanie A.* and Hickson, Thomas A. Possible influence of a buried fault on elevated indoor radon levels, south Washington County, Minnesota Thorliefson, L. Harvey, Harris, Kenneth L., Hobbs, Howard C., Jennings, Carrie E., Knaeble, Alan R., Lively, Richard S., Lusardi, Barbara A. and Meyer, Gary N. Till geochemical and indicator mineral reconnaissance of Minnesota xxiv �ABSTRACTS xxv ��HOW TO IDENTIFY SUDBURY IMPACT EJECTA IN THE LAKE SUPERIOR REGION Addison, William D., R.R. 2, Kakabeka Falls, Ontario, POT 1W0, Canada Cannon, William F., U.S. Geological Survey, MS 954, Reston VA 20192, USA Brumpton, Gregory R., 211 Henry St., Thunder Bay, Ontario, P7E 4Y7, Canada For at least 15 years geologists looked for ejecta from the 1850 Ma Sudbury impact in sedimentary rocks of the Lake Superior region. Addison and others (2005) first reported its occurrence but more recent work has shown that “volcaniclastic breccia” or “submarine slump deposits” described and mapped as early as the 1940’s are, in fact, ejecta deposits and other impact-related rocks (Cannon and Addison, 2007). To date, Sudbury ejecta deposits have been identified in six areas in the western Lake Superior region and provide a precise regional stratigraphic marker (Cannon and Addison, 2007). It seems nearly certain that other sites will be discovered. Toward that end we pass on some of our observations and experience in the commonly frustrating search for the impact layer in hopes that others will make additional discoveries of this important chrono-stratigraphic marker bed. Ejecta, the material thrown upward and outward from the Sudbury impact, arrived in the Lake Superior region in two ways. First, was coarse to fine material, primarily melted and solid target rock, that followed a ballistic trajectory to about five times the final crater radius (5r), most recently estimated at 130 km (Spray and others, 2004). Thus, ballistic ejecta should not be expected beyond 650 km from the impact center. Five of the six known occurrences lie within the 5r distance. At those sites ballistic ejecta arrived with horizontal velocities as high as a few km/sec, and were capable of causing extreme disruption and erosion of surface sediments. The result was breccia deposits of ejecta and local country rock. Ejecta inside the 5r ballistic zone include devitrified glass in vesicular blobs (both streamlined and irregular shapes) and microtektites and spherules (frozen melt droplets), both singly and in clusters. Other features include accretionary lapilli, and angular grains of quartz, a small percentage of which contain relict planar deformation features, the only proof that the ejecta are of impact origin (French, 1998). Second, a huge super-heated expanding cloud of vapor, sub-mm melt droplets, and sub-mm rock shards reached hundreds of km above Earth. Condensed and solidified melt droplets and tiny rock shards settled over a period of days to more than a year covering all of Earth. These ejecta do not substantially alter the surface on which they are deposited and do not form the breccias typical of ballistic ejecta. Microtektites and spherules and quartz and feldspar grains, some showing shock metamorphic features, from the cloud have been found at sites near Hibbing, Minnesota, beyond the range of ballistic ejecta. Finally, most of the ballistic deposits and perhaps some of the earlier vapor cloud deposits have been reworked by impact-induced tsunamis resulting in polymict breccias of local rocks and ejecta (Cannon and Addison, 2007). 1 �So, how and where should you look for a Sudbury ejecta deposit and how will you recognize it? Known sites are near the top of the principal iron-bearing units (Gunflint, Biwabik, Ironwood, etc.) in the region to about 300 m farther up section and new discoveries will most likely be within or not far from this interval. Within the 5r zone, easily seen macroscopic features include breccia, especially black chert breccia, mm-cm size blobs of devitrified vesicular glass, and clusters or beds of accretionary lapilli. Smaller-scale features include 0.5-2 mm gray ovoid, round, or irregular microtektites and spherules that may also be replaced by green clay minerals. Similar sized rounded quartz and feldspar grains occur along with sub-mm angular quartz and feldspar grains, a few of which contain one or more sets of thin parallel lamellae, the diagnostic shock deformation features. Note that these lamellae are very rare and commonly require hours or days of careful petrographic examination under high magnification to find. Beyond the 650 km ballistic zone, represented by only the sites near Hibbing, MN, there are no characteristic macroscopic features. Ejecta at the top of the Biwabik Iron-formation are within a siliceous, light gray coarsely recrystallized carbonate layer. Within the light carbonate there is a darker gray reticulated pattern giving a vague appearance of 2-4 mm circles. Thin sections from the reticulated zone reveal microtektites, spherules and vesicular glass. Rounded quartz grains are present as are angular quartz and feldspar. Shocked quartz and feldspar grains with the relict lamellae are exceedingly rare. All known localities are in pristine to weakly metamorphosed rocks. Unfortunately, the considerable regions of higher grade metamorphism in the Lake Superior region may never yield any of the features described here, particularly the diagnostic shock lamellae in quartz, because of thorough recrystallization. Certainly this is true at Peter Mitchell Mine, Babbitt, MN. Searches in weakly metamorphosed sequences are most likely to succeed. References: Addison, W. D., Brumpton, G.R., Vallini, D.A., McNaughton, N.J., Kissin, S.A., Fralick, P.W., and Hammond, A.L., 2005, Discovery of distal ejecta from the 1850 Ma Sudbury impact event: Geology, v. 33, p. 193-196. Cannon, W. F. and Addison, W.D., 2007, The Sudbury impact layer in the Lake Superior iron ranges: a time-line from the heavens: Institute on Lake Superior Geology Proceedings, Vol. 53, Part 1. French, B.M., 1998, Traces of catastrophe: Lunar Planetary Inst. Contrib. 954, 120 p. Spray, J. G., Butler, H. R., and Thompson, L. M., 2004, Tectonic influences on the morphometry of the Sudbury impact structure: Implications for terrestrial cratering and modeling: Meteoritics & Planetary Sci., V. 39, p. 287-301. 2 �Rare Metal Mineralization in Pegmatites of the Coldwell Alkaline Complex Malcolm Alexander and Roger H. Mitchell Department of Geology, Lakehead University, Thunder bay, Ontario ABSTRACT The Coldwell Complex (northwestern Ontario), is a multiphase alkaline intrusion that is host to rare earth element, actinide and other high field strength element mineralization. Preliminary studies have shown that these minerals are concentrated in pegmatites associated with Center 1 ferrorichterite-ferroaugite syenites. These syenites exhibit the geochemical characteristics of A-type granitoid magmatism. The focus of this study is the distribution, nature and petrochemical controls of rare metal mineralization within these pegmatitic bodies. Within the Center 1 formation there are two subunits; the border gabbro, a massive to layered intrusion ranging in composition from wehrlite to diorite and locally to anorthosite; and oversaturated massive to locally layered ferroaugite syenites. Both subunits host pegmatitic quartz bearing residua characterized by cumulus perthitic alkali feldspar, hedenbergite-aegirine pyroxenes, with interstitial quartz, calcite, and calcic-tosodic-calcic-to-sodic amphiboles (Fig. 1). The pegmatitic residua are of the niobiumyttrium-fluorine (NYF) type, and characterized by the enrichment of Nb-Ti-Ta oxides, with niobium predominant over tantalum. Back-scattered electron petrography and quantitative X-ray spectrometry have been used to characterize the mineral paragenesis. Pegmatitic syenitic residua emplaced in, but not derived from, the border gabbro contain a wide range of rare element minerals which include: chevkinite; fergusonite; monazite; allanite; kainosite; xenotime; REE fluorocarbonates: bastnaesite, synchysite and parisite. Other rare element enriched minerals include fersmite, apatite, zirconolite, U-Th-Sipyrochlore, Nb-rutile and Nb-bearing ilmenite. Early-formed rare element minerals such as chevkinite and pyrochlore are commonly replaced by complex aggregates of laterforming phases. REE-fluorocarbonates commonly exhibit syntaxial intergrowths. Other ferroactinolite-quartz bearing pegmatitic residua, occurring in the upper series of a differentiated aenigmatite-ferroaugite syenite unit, contain a more limited range of rare element minerals, including zircon, xenotime, monazite and the fluorocarbonates together with REE bearing apatite and pyrochlore. The differences in rare element mineralization between the two pegmatites examined to date suggest that they were derived from different batches of ferroaugite syenite magma. Initial data indicate that different pegmatitic residua in the Center 1 subunits also evolved differently. Intensive parameters have been estimated using amphibole mineralogy and Fe-Ti-oxide compositions, as seen in Figure 2. These parameters indicate the pegmatites intruding the border gabbro and syenite residua had equilibration temperatures of 450 ºC at oxygen fugacities of +0.5-1.0 and +2.0-2.2, relative to QFM, respectively. 3 �Calcic Amphiboles 1.0 Tremolite 0.8 Actinolite Magnesiohornblende Magnesiotschmakite 0.6 0.4 0.2 FerroFerrohornblende Ferrotschermakite actinolite Mg/(Mg + Fe2+) Mg/(Mg + Fe2+) 1.0 Sodic-Calcic Amphiboles 0.0 0.8 Richerite Magnesiokatotophorite Magnesiotaramite 0.6 0.4 Ferro0.2 richterite Katophorite Taramite 0.0 8.0 7.5 7.0 6.5 6.0 5.5 8.0 Si in Formula 7.5 7.0 6.5 6.0 5.5 Si in Formula A B Figure 1: Stability fields of calcic (A) and sodic-calcic (B) amphiboles. Each point represents a mean value of 5 to 9 point analyses. Circles represent the residua injected into the border gabbro, while triangles represent residua injected into the saturated ferroaugite syenite. A B Figure 2: Oxygen fugacity diagrams, using the quartz-fayalite-magnetite (QFM) oxygen buffer. Diagram A contains two points, the right hand value is representative of the ferroaugite syenite and the left hand value is representative of the pegmatitic residua (Mitchell and Platt, 1977). Diagram B is a Fe – Ti oxide geothermometer and oxygen barometer. ILM is given in mol% ilmenite, USP is mol% ulvöspinel. Triangles are saturated ferroaugite syenite residua, circles are border gabbro residua. Adapted from Ghiorso and Sack (1991). REFERENCES Ghiorso, M.S. and Sack, O.S. 1991, Fe – Ti oxide geothermometry: thermodynamic formulation and the estimation of intensive variables in silicic magmas: Contributions to Mineralogy and Petrology, v.108, p. 485. Mitchell, R.H. and Platt, R.G. 1977, Mafic mineralogy of ferroaugite syenite from the Coldwell Alkaline Complex, Ontario, Canada: Journal of Petrology, v. 19, p. 627. 4 �MERCURY IN FISH: A GEOLOGIST’S PERSPECTIVE BEHLING, Stuart James, Retired, Superior National Forest Geologist 4712 Anderson Rd., Duluth, MN 55811 USA stu-ball@mchsi.com Mercury contamination of fish has been a health concern in Northeastern Minnesota since testing began over 30 years ago. In spite of this concern, less than 15% of the lakes in this area have been tested. The data show that fish mercury contamination varies considerably between lakes but no pattern of distribution has been recognized. The data also show that fish mercury contamination varies between species and increases with length of fish. For example, walleyes are more contaminated than northern pike of the same size, from the same lake, and longer fish have more mercury than shorter ones. However, there is not enough data available from most lakes to plot these contamination trends. A model that could predict fish mercury contamination levels by species and length of fish would be a great asset in understanding the extent and distribution of fish mercury in this area. In this study, Northeastern Minnesota was divided into groups of geologically related lakes termed lake provinces. Ten lake provinces were identified in two distinct geomorphic areas; seven are in an area of bedrock controlled terrain and three are in an area of glacial deposition. In the bedrock controlled terrain, the size shape and orientation of the lakes are directly related to structural or compositional weakness in the underlying bedrock. Weathering and glacial erosion exploited these weaknesses to produce distinct patterns of lakes that were used to identify the provinces. In the areas of glacial deposition, the lake provinces were associated with the deposits left by three distinct glacial advances. When fish mercury was plotted against length of fish by province, the correlation coefficients increased significantly. The R squared values for the provinces ranged from .58 to .81 as compared to an R squared value of .32 for all samples from the study area plotted collectively. A map showing the distribution of mercury contamination in walleyes and northern pike was produced using the results from this ongoing study. 5 �EVIDENCE FOR PALEOSEISMIC EVENTS DURING CLOSURE OF THE MIDCONTINENT RIFT, ATKINS LAKE-MARENISCO FAULT, N. WISCONSIN Bjornerud, Marcia, Geology Department, Lawrence University, 115 S. Drew St. Appleton, Wisconsin 54911 USA. bjornerm@lawrence.edu The closure, or inversion, of the Midcontinent Rift at ca. 1.0 Ga remains one of the least well understood chapters in the Precambrian tectonic history of the Lake Superior region. Rift-bounding normal faults were reactivated as major reverse faults, some with estimated net reverse displacements of several kilometers. One such structure is the north-dipping Atkins Lake-Marenisco fault in Bayfield, Ashland and Iron Counties of northern Wisconsin. According to the interpretation of Cannon et al. (1993), the Atkins Lake-Marenisco fault is a crustal-scale listric fault zone along which lower Proterozoic metasedimentary rocks and middle-Proterozoic rift-related volcanic rocks have been thrust southward over Archean gneisses (the Proterozoic sequences had originally occupied a structurally lower position in the axial graben of the rift). The Atkins Lake-Marenisco Fault is well exposed near the falls of the Marengo River in eastern Bayfield County. There the fault zone is ca. 100 m wide and cuts through the Archean (ca. 2.7 Ga) Puritan Batholith, a granitic body with a weak gneissic fabric. In a downstream transect beginning just below the falls, the rocks record extreme strain gradients. Over a distance of < 20 m, pristine gneiss gives way first to cataclasite and then to dark, extremely fine-grained phyllonites, all heavily veined with K-feldspar, chlorite and zeolite. In thin section, cataclastic textures can be seen to overprint quasiductilely deformed feldspars, indicating that displacement along the fault zone may have begun at temperatures close to 450°C and continued as the rocks cooled. Most of the cataclasites have a crudely fractal (log-linear) clast size distribution, with the average grain size decreasing progressively with strain intensity, as expected when cataclasis is the dominant deformation mechanism. Samples from one outcrop near the base of Marengo Falls, however, have textures suggestive of devitrified pseudotachylyte. Dark-colored veins and lenses, now chloritized, contain relatively large (mm-sized) clasts of feldspar, but finer-grained clasts are notably absent. Many of the 6 �larger clasts, moreover, show unusual microstructural features. Some have embayments and concave edges – shapes not typical of ordinary cataclasites, in which grains are commonly equant and polygonal to rounded. Some of the clasts also display mosaicism, a texture that can form as a result of rapid volumetric changes. The bimodal grain size distribution and embayed and mosaicized grains could be explained by the presence of a seismically generated frictional melt, which could have preferentially assimilated the finest grains, partly resorbed larger ones, and created a short-lived pressure pulse as a result of expansion upon melting. If this interpretation is correct, we should begin to consider the role seismicity may have played in facilitating transient fluid flow and consequent mineralization during the closure of the Midcontinent Rift. Reference cited: Cannon, W., Z. Peterman, and P.K. Sims, 1993. Crustal-scale thrusting and origin of the Montreal River monocline: A 35-km thick cross section of the Midcontinent Rift in Northern Michigan and Wisconsin. Tectonics v. 12, p. 728-744. 7 �NEWLY RECOGNIZED THICK INTERFLOW SANDSTONES IN THE UPPER NORTHEAST LIMB OF THE NORTH SHORE VOLCANIC GROUP, MINNESOTA BOERBOOM, Terrence J., Minnesota Geological Survey, boerb001@umn.edu New mapping, funded in part by the USGS STATEMAP program, has identified several major units of interflow sandstone between lava flows in the upper Northeast Sequence of the North Shore Volcanic Group (NSVG), which are as much as 200-300 feet (60-90 meters) thick. The recognition of these sandstones approximately triples the total thickness of known interflow sandstones in that area. These sandstones were intersected in several water well cutting sets collected by McKeever Well Drilling of Schroeder, Minnesota. Although the wells intersect only partial sections of the sandstone units, these intersections project into prominent northeast-trending topographic lows which correspond with linear low aeromagnetic anomalies. These lie between and parallel to ridges held up by basalt that correspond to linear positive magnetic anomalies. The eastern extent of these sandstones has yet to be determined, but will be examined in the coming field season. The sandstones identified in water well cuttings are pinkish-tan, medium-grained, and similar in composition and appearance to the well-known Cutface Creek sandstone (Fig. 1). In the northern-most sandstone, a well just south of the Monker Lake diabase (Fig. 1) intersected 80 feet (25 meters) of sandstone above the Devil’s Track rhyolite. However, a well located 8 miles along strike to the southwest intersected only 10 feet of sandstone, below basalt and above the Devil’s Track rhyolite, indicating either that it pinches out to the southwest or that the thickness varies due to deposition on an irregular surface. The middle sandstone unit shown on Fig. 1 was intersected in only one well, which penetrated 15 feet (5 meters) of sandstone beneath basalt. The color, composition, and grain size is consistent with it being the uppermost part of a thick interflow sediment, quite distinct from the typical thin interflow sediments that are found throughout lava flows in the NSVG. The southern-most of the three sandstone units is inferred entirely on the basis of topographic and aeromagnetic features that mimic those of the middle unit. Sandstone of substantial thickness was also intersected in water wells near the top of Eagle Mountain at the Lutsen Ski Resort (Fig. 1), which penetrated 15–60 feet (4-20 meters) of tan, medium-grained sandstone. This sandstone lies below the Leveaux ferrodiorite and above the Eagle Mountain basalt, and likely acted as an anisotropic plane of weakness that controlled emplacement of the Leveaux ferrodiorite sill. Its irregular thickness may be the product of having been variably cut out by the intrusion. Two thick interflow sandstone units – the Cut Face and Indian Camp Creek sandstones, had long been recognized in the upper NE limb of the NSVG from outcrops near Lake Superior. Both of these are soft and easily eroded relative to the surrounding volcanic rocks, hence are exposed only near the shore in actively eroding stream channels. Based on inferences from paleocurrent measurements, the interflow sandstones in the NSVG were deposited in two separate depositional basins that correspond to the SW and NE limbs of the NSVG (Jirsa, 1984). Sedimentary rocks between the SW and NE limbs, in the area from Illgen City to Lutsen (Fig. 1), are composed mainly of conglomerate and fragmental volcanic rocks, in contrast to sandstone that dominates on either side. This area coincides with a negative gravity anomaly known as “White’s Ridge” (White, 1966), that represents a block of uplifted crust (Boerboom, 1994). To the southwest of White’s ridge (SW Limb NSVG), the stratigraphically lower lavas dip more steeply towards the center of the rift than higher lavas, implying greater subsidence towards the rift axis during volcanism. To the NE of White’s ridge (NE Limb NSVG), the volcanic rocks throughout the stratigraphy show no appreciable change in dip, implying that subsidence during volcanism was uniform, and that tilting towards the rift axis occurred after volcanism. (Green, 1972). The 250 foot (76 meter) thick Cut Face Creek sandstone, exposed at Terrace Point (Fig. 1), is dominantly a reddish-brown, medium-grained, well-sorted, lithic arkose, but nearly 30% of the section is composed of thinly bedded, graded layers of fine-grained sand, silt, and clay, with lithologic and 8 �sedimentary features indicative of deposition into a fluvial-lacustrine environment, such as streams flowing into ponded water (Jirsa, 1984). The lacustrine aspect is speculated to be caused by blockage of flowing water against an obstruction, perhaps related to deformation and magmatism along White’s ridge. The recognition of several thick interflow sandstones indicates that clastic sedimentation in the NE limb of the NSVG played a more significant role than in the SW limb, but whether this is a result of a higher rate of clastic input, more efficient trapping of sediment against a topographic impediment such as White’s ridge, a higher rate of subsidence, or longer periods of volcanic quiescence is not clear. The inferred fluvial-lacustrine depositional environment of the Cut Face Creek sandstone implies that ponding and sediment-trapping processes were more operative than in the SW limb, where dominantly fluvial sedimentary features indicate sediment flow-through, and deposition closer to the axis of the rift. From a practical standpoint, these sandstones may be important aquifers, however, little is presently known about their water-yielding characteristics. Boerboom, T. J., 1994, Archean crustal xenoliths in a Keweenawan hypabyssal sill, northeastern Minnesota. White was right!: Institute on Lake Superior Geology 40th Annual Meeting, Houghton, MI: Proceedings v. 40, pt. 1 Abstracts, p. 5-6. Green, J.C., 1972, North Shore Volcanic Group, in Sims, P.K., and Morey, G.B., eds., Geology of Minnesota: A Centennial Volume, p. 294-332 Jirsa, M.A., 1984, Interflow sedimentary rocks in the Keweenawan North Shore Volcanic Group, northeastern Minnesota: Minnesota Geological Survey Report of Investigations 30, 20 p. White, W.S., 1966, Tectonics of the Keweenawan basin, western Lake Superior region: U.S. Geological Survey Professional Paper 524-E, 23 p. 9 �BEDROCK GEOLOGIC MAP OF THE LITTLE MARAIS, SCHROEDER, AND TOFTE 7.5MINUTE QUADRANGLES, NORTH SHORE OF LAKE SUPERIOR, MINNESOTA BOERBOOM, Terrence J., Minnesota Geological Survey, boerb001@umn.edu GREEN, John C., University of Minnesota-Duluth, jgreen@d.umn.edu ALBERS, Paul, Duluth Metals, palbers@duluthmetals.com MILLER, James D., Jr., Minnesota Geological Survey, mille066@umn.edu The Minnesota Geological Survey is continuing to map the bedrock geology of 7.5’ quadrangles near Lake Superior as part of the USGS STATEMAP program, resulting to date in ten published 1:24,000 scale maps from Duluth to Tofte, in addition to 10 quadrangles already published under the former USGS COGEOMAP program. The Lutsen quadrangle will be published in July 2007; the Deer Yard Lake and Good Harbor Bay quadrangles will be mapped in the 2007 and published in 2008 (Fig. 1A). All the maps in this series are available as printed maps, or as PDF and ArcView export files at the MGS website (http://www.geo.umn.edu/mgs/). Field trip 5 of this meeting will begin in the Little Marais quadrangle at the southwest, and end in the Good Harbor Bay quadrangle at the northeast. With the exception of a strip near the shore of Lake Superior (Green, 1992), parts of the Little Marais quadrangle that were mapped but not published, and thesis work by Albers (UMD M.S. 2007), much of the area covered by these maps had never been mapped. The combined area of these three maps straddles the transition across the uppermost Southwest to the uppermost Northeast sequences of the North Shore Volcanic Group (NSVG), as well as the SchroederLutsen sequence, which unconformably overlies the Southwest and Northeast sequences (Fig. 1B). Together, these maps cover the stratigraphically highest portion of the NSVG in Minnesota. In addition, components of the Beaver Bay Complex, including multiple phases of the Beaver River diabase, the Blesner Lake diorite, and the Leveaux ferrodiorite are present in the area of these maps. The new mapping has delineated a series of dominantly felsic to intermediate composition volcanic rocks (rhyolite, icelandite, and andesite) beneath the Schroeder-Lutsen basalts, informally termed the Onion River lavas, which include some tholeiitic basalt flows. The strike of these flows at a high angle to that of the overlying Schroeder-Lutsen basalts, which are nearly parallel to the Lake Superior shoreline (Fig 1B). The basal contact of the Schroeder-Lutsen sequence is repeated by several shore-parallel reverse faults. Poorly exposed units of rhyolite, volcanic breccia, and conglomeratic to sandy sedimentary rocks, which are likely components of older volcanic and interflow sedimentary rocks such as the Onion River lavas, are exposed within the Schroeder-Lutsen sequence by normal and reverse faults. The Carlton quarry lavas (Fig. 1B) form a northwest-dipping group of lavas composed of rhyolite at the base, andesite, and ophitic basalt at the top. The position of these lavas relative to the surrounding lavas is poorly constrained due to lack of outcrop, but they are assumed to be in fault contact, given the anomalous northwest dip. The rhyolite here has been dated at 1094.3±2.0, the youngest age obtained to date from the NSVG in Minnesota, but the surrounding volcanic rocks have not been dated. Intrusive rocks of the Beaver Bay Complex are most abundant and diverse in the Little Marais quadrangle, where they are composed of multiple phases of the Beaver River diabase and the slightly older, multi-phase Blesner Lake diorite. Elsewhere, the Beaver River diabase is restricted to erosional remnants of a larger, generally southeast-dipping sheet that commonly contains large anorthosite inclusions, such as at Carlton Peak. The porphyritic Leveaux ferrodiorite occurs mainly in the Tofte quadrangle as a subvolcanic sill emplaced into volcanic rocks, and as inclusions in the Beaver River diabase. 10 �REFERENCES Green, J.C., 1992, Geologic map of the North Shore of Lake Superior, Lake and Cook Counties, Minnesota: Part 1. Little Marais to Tofte: Minnesota Geological Survey Miscellaneous Map M-71, scale 1:24,000. Figure 1. A. Index map showing the location of mapped 7.5’ quadrangles along the North Shore of Lake Superior. M numbers refer to MGS Miscellaneous maps. Visit the MGS website for references to individual maps. Lutsen will be published in 2007; Deer Yard Lake and Good Harbor Bay in 2008. B. Index map showing the locations of the major units mentioned in the abstract. 11 �PETROGRAPHIC DIFFERENTIATION OF THE FIVE PHASES OF THE LOWER CRETACEOUS STAR KIMBERLITE, SASKATCHEWAN, CANADA BOISJOLI, Troy and FLOOD, Timothy P., Department of Geology, Saint Norbert College, DePere, WI 54115; troy.boisjoli@snc.edu Geologic Setting The Star Kimberlite in the Fort a la Corne region, east-central Saskatchewan, is a large lithologically complex structure that preserves both intra-crater and extra-crater kimberlite (Zonneveld et al. 2004). The Star Kimberlite occurs on the eastern rim of the North American Platform. The oldest rocks in the area are believed to be part of the Archean Sask craton, which are overlain by other Precambrian rocks similar to those exposed to the north in the Glennie Domain. These Precambrian rocks are successively overlain by 300-500m of mixed siliciclasticcarbonate sediments that range in age from Cambrian to Devonian; these sediments are overlain by approximately 150m of Lower Cretaceous sediments; which are overlain by 85 to 115m of glacial deposits. The Star Kimberlite itself is intermittent and interstratified with Lower Cretaceous rocks that include the Cantuar Formation inferred as coastal plain facies, the Pense Formation inferred as shoreface facies, and the Joli Fou and Viking Formations inferred to be proximal offshore to shoreface facies. Geology of the Study Area Five different phases of the Star Kimberlite have been recognized (Zonneveld et al. 2004). In stratigraphic order from oldest to youngest they are; Cantuar Kimberlite (CK), Pense Kimberlite (PK), Early Joli Fou Kimberlite (EJFK), Middle Joli Fou Kimberlite (MJFK) and Late Joli Fou Kimberlite (LJFK). The purpose of this study was to determine if petrography and geochemistry could be used as tools to differentiate these different phases of the Star Kimberlite. The CK is the oldest kimberlite in the Star body. In drill core, kimberlite intersections in this unit range in thicknesses from 0.3 to 11.8m and are bedded at the 0.2 to 9.0m scale. The beds within the kimberlites have sharp contacts and are usually massive or fining upwards. The CK is interpreted as medial to distal primary pyroclastic airfall material, with some reworked sedimentary equivalents. The location of the feeder vent for this event has not been determined. The PK overlies CK and drill core intersections range in thickness from approximately 2 to 20m. The individual beds are highly variable, with massive, normal graded and reverse graded beds. Sharp contacts are typical and in some beds the upper 0.1 to 1m show evidence of marine reworking. PK is interpreted as proximal to distal, primary volcaniclastic material. The EJFK overlies the PK and is volumetrically the most important phase of the Star Kimberlite. The EJFK deposits are 1 to 25 m thick, with bedding ranging from 1 to 10 m. Individual beds have sharp to diffuse contacts and are typically massive or normally bedded. Reverse bedding is rare. The EJFK is interpreted as primary subaerial and distal marine fall material. The MJFK overlies the EJFK and until recently was lumped with the upper portion of the EJFK (Kjarsgaard 2007). 12 �The LJFK overlies the MJFK The thickness of this unit is unknown but bedding within the unit occurs on a scale of 0.5 to 30m, with beds of 10 to 20m the most common. The LJFK is interpreted primarily as pyroclastic subaerial fall material to pyroclastic flow material (Zonneveld et al. 2004). Analysis Petrographic analysis was performed on 23 thins sections using an average of 600 points per thin section. For all units, altered olivine comprised from 40-60% of the sections. Juvenile clasts comprised from 5-30% of the sections and opaques ranged from 1-4%. Late alteration calcite and serpentine accounted for between 0.5-6% of the sections. Quantitative petrographic analysis was not useful for distinguishing the different phases of the Star Kimberlite. An alternate petrographic method based on textural and mineralogical criteria of the juvenile clasts (Webb 2006) was performed. In this analysis, ten criteria are the basis for distinguishing various mechanisms of emplacement for kimberlites. These criteria include; sphericity, roundness, irregularity, internal structure, vesicularity, clast-host relationship, primary phenocrysts, groundmass crystallinity, mineralogy and size range/modal average. A preliminary compilation of these criteria suggest that some units are distinguishable from other units relative to emplacement mechanisms. For example, vesicularity in the CK is high relative to other phases and consistent with the original interpretation as distal primary pyroclastic airfall. Geochemical analysis was preformed on the primary olivine clasts using an SEM-EDS. The initial results of this analysis were inconclusive due the highly altered nature of the grains but work is on going. References: Kjarsgaard, B.A., 2007, personal communication. Webb, K.J., 2006, Juvenile clasts in kimberlites: Standardized comprehensive description towards unraveling emplacement mechanisms: in Kimberlite Emplacement Workshop, DeBeers Canada Inc, p. 1-5. Zonneveld, J.P., Kjarsgaard, B.A., Harvey. S.E., Heaman, L.M., McNeil. D.H., and Marcia. K.Y., 2004, Sedimentologic and stratigraphic constraints on emplacement of the Star Kimberlite, east-central Saskatchewan: Lithose 76 115-138. 1: Juvenile clast with primary olivine and vesicles infilled with calcite. Field of view 10mm. 13 �DEPOSITION OF NATIVE COPPER LODES ON THE KEWEENAW PENINSULA, NORTHERN MICHIGAN, FROM A GRAVITY-DRIVEN EVOLVED METEORIC BRINE BROWN, Alex C., Dept. of Civil, Geol. and Min. Eng., École Polytechnique de Montréal P.O. Box 6079, Sta. Centre-Ville, Montreal, QC, Canada H3C 3A7, acbrown@polymtl.ca The Keweenaw Peninsula of northern Michigan is the site of famous native copper lodes hosted by flow-top breccias and amygdaloidal portions of flood basalts and interbedded conglomerates of the Portage Lake Volcanics (PLV). This mineralization is generally attributed to an up-dip flow of a cupriferous metamorphogenic fluid generated during deep burial metamorphism of the host strata in axial portions of the Midcontinent Rift (Stoiber & Davidson 1959; White 1968; Jolly 1974; Livnat et al. 1983). In an alternative model proposed here, copper is leached from immature rift sediments and porous portions of the PLV by deeply circulating, gravity-driven meteoric water. The meteoric water is initially cool, fresh, slightly acid and oxygen-rich. At depth, it becomes warmer, highly saline by the assimilation of evaporites or evaporitic brine, near-neutral in pH by equilibration with silicates and carbonates, and oxygen-deficient by reddening of its aquifers (hematitization of ferrous iron in abundant labile mafic mineral constituents of the rift sediments and mafic constituents of the PLV basalts). At first, the Eh of the brine evolves progressively toward moderately oxidizing conditions at which it becomes capable of leaching and transporting trace amounts of copper from its aquifers. Metamorphogenic water and copper may have been incorporated during circulation at deep burial levels. Highland recharge eventually drives the hybrid cupriferous brine up-dip along PLV aquifers. With continued hematitization of its aquifers, the brine becomes highly reducing by oxygen depletion, and native copper is deposited (in the absence of sulfide). Coincidentally, the same highly reducing conditions favor the solution of hematite, a feature readily visible as locally bleached rock (hematitic pigment removed) where native copper mineralization occurs on the Keweenaw Peninsula. This conceptual model provides 1) an explanation for the highly saline brine needed to mobilize copper, and 2) a means to drive dense brines up-dip from axial portions of the rift basin. It also explains the pervasive hematitic reddening of the PLV, and ties the timing of copper leaching to that diagenetic reddening event. The overall timing of native copper mineralization may be linked to the Grenvillian compressional event which tilted PLV strata to deep levels along the rift axis and probably raised highlands around the rift basin. References: Brown, A.C. (2006) Genesis of native copper lodes in the Keweenaw district, northern Michigan: a hybrid evolved meteoric and metamorphogenic model: Economic Geology, v. 101, p. 1437-1444. Jolly, W.T. (1974) Behavior of Cu, Zn and Ni during prehnite-pumpellyite rank metamorphism of the Keweenawan basalts, northern Michigan: Economic Geology, v. 69, p. 1118-1125. Livnat, A., Kelly, W.C., Essene, E.J., and Rye, R.O. (1983) P-T-X conditions of sub-greenschist burial metamorphism and copper mineralization, Keweenaw Peninsula, northern Michigan: Geological Society of America Abstracts, v. 15, p. 629. Stoiber, R.E. and Davidson, E.S. (1959) Amygdule mineral zoning in the Portage Lake Lava Series, Michigan copper district; Part I, Part II: Economic Geology, v. 54, p. 1250-1460, 1444-1460. White, W.S. (1968) The native-copper deposits of northern Michigan, in Ridge, J.D., ed., Ore Deposits of the United States, 1933-1967: Amer. Inst. of Mining, Metallurgical and Petroleum Eng., GratonSales Volume, p. 303-326. 14 �p. 2 Fig. 1 Schematic illustration of the geologic setting of native copper lodes (from Brown 2006) NW tion a r d Hy / ll y it e al t Ba s l Cg 0 Cu S an H2 O H o 2+ Cu Conditions favorable for significant copper solubilities in redbeds Cu S Cu 2O an H2 O H o Cu H 2 Meteoric Water 2CuCl 3 + CuCl 2 CuO Ground Water o Cu Conditions favorable for significant copper solubilities in redbeds CuO Cu 2S dj O2 O Cu 2O Ground Water 2 Cu 2S 5 6 ppm O H2 2 O Cu 2S -0.4 25 C Cl= 0.5 M Contours of copper solubility 64 ppm Cu4 (OH)6Cl 2 -4 10 (6 ppm) -3 10 mg/l (64 ppm) Cl=0.5 M Meteoric Water Water dj ult o s 2 3 oC 25 25 -4 C a =10 mg/l 2CuCl 3 + CuCl 2 Eh (V) lt sa Ba Cu4 (OH)6Cl 2 o 25 C +0.4 t B) System Cu-O-H-Cl A) System Cu-O-H-S-Cl 2+ Cu l sa Ba lt sa Ba n w Fa ratio d y h De een a pe Pum ns it ion Tr a e t o id Ke w Fig. 2 Eh-pH stability diagrams (below) for A) the transport of copper, and B) the deposition of native copper (from Brown 2006) +0.8 In In Flow-Top Breccias Conglomerates and Amygdaloids Pr esent-day Er osion Sur face Ep Contours of copper solubility SE Native Copper Lodes Cu 2S 7 pH 9 7 5 3 11 pH 9 11 I ON IPITAT PREC Fig. 3 Schematic illustration of the deep flow of meteoric and metamorphogenic waters, to form native copper lodes (from Brown 2006) Hi g h la nd s Evapo rites Meteoric water o +Cl - Cu 2- - CuCl 3 + CuCl2 Progressive reddening of aquifers and leaching of copper Assimilation of metamorphogenic water and copper Mantle heat 15 Deposition of native copper �A NOVEL LOCALITY FOR PSEUDOBROOKITE - THE NINE MILE PLUTON, MARATHON COUNTY, WISCONSIN T.W. Buchholz1, A.U. Falster2, Wm. B. Simmons2. 11140 12th St. N., Wisconsin Rapids, WI 54494; 2Department of Earth and Environmental Sciences, University of New Orleans, New Orleans, LA 70148. Pseudobrookite, (Fe3+, Fe2+)2(Ti,Fe3+)O5, is a typical pneumatolytic mineral found in lithophysae in extrusive Ti-rich rocks such as andesite and rhyolites (Anthony et al., 1997). Recent work in the Ladick Trucking & Excavating weathered granite quarry has exposed a novel occurrence of pseudobrookite within the Proterozoic Nine Mile Pluton, the youngest and most silicic of four plutons comprising the Wausau Complex. The Ladick quarry is located approximately ¼ mile east of STH 107 in the southwest portion of the Nine Mile Pluton. Black to dark grey elongated crystals of pseudobrookite up to approximately 1.5 mm in length and clustered in radiating sheaves and as single crystals were found in an alteration assemblage associated with a thin quartz-pyritefluorite vein. The material was recovered from a pile of shot rock derived from blasting large boulders removed from the grus in the northern portion of the operation. The vein was emplaced in a fracture cutting the granite, and it appears fluids circulating along the fracture preferentially removed quartz and perhaps other minerals from adjacent granite to a distance of up to about 12 cm from the fracture. Subsequently K-feldspar, biotite, minor quartz and a series of associated minerals described below were deposited. EMP analysis of clean grains of pseudobrookite shows no significant components other than Ti and Fe and minor amounts of Mn and Nb, and XRD analysis yielded excellent structural agreement. Morphology is that of ideal, short-prismatic pseudobrookite. Associated minerals other than K-feldspar, biotite and quartz include ilmenite, rutile, anatase, sphalerite, chalcopyrite, pyrite, arsenopyrite or marcasite (now represented by goethite pseudomorphs), molybdenite, cassiterite, fluorapatite, fluorite, monazite, zircon and a LREE-carbonate mineral, probably bastnaesite-(Ce). Secondary minerals include small but attractive sprays of gypsum crystals; the Ca was probably derived from weathering fluorite, and the S from weathering sulfides. Interestingly, the cassiterite crystals often contain inclusions of fluorapatite and monazite; all monazite identified so far has been as inclusions in cassiterite. The monazite and fluorapatite inclusions may represent an earlier, otherwise transient paragenesis that was preserved by inclusion in cassiterite; the LREE released by breakdown of early-formed monazite may have contributed to the later formation of the LREE-carbonate mineral. Although pseudobrookite typically is found in lithophysae in certain eruptive rocks and this would seem to be a very different environment, the Nine Mile pseudobrookite probably formed in an environment that was not inconsistent with its usual environment of formation. As indicated by the association with cassiterite, molybdenite (this is the first report of molybdenite for the Nine Mile granite), fluorite and other Ti-bearing oxides the conditions were probably high-temperature, volatile- and Ti-rich. Conditions were likely close to a pneumatolytic environment – consistent in these respects with typical 16 �pseudobrookite occurrences. The principal variant may have been ambient pressure, although it is believed that the Nine Mile granite was intruded at a shallow, albeit as yet undefined level in the crust. Hence the environment of formation probably had at least some features in common with the usual volatile-rich, high temperature andesites and rhyolites that are the normal hosts for pseudobrookite. Reference cited: Anthony, J.W., Bideaux, R.A., Bladh, K.W. and Nichols, Monte C., 1997, Handbook of Mineralogy, V. III, p. 453. 17 �Deposition and Cementation of Paleoproterozoic Gunflint Formation Carbonate: Implications for Early Hydrosphere Chemistry Justin Burton and Philip Fralick, Department of Geology, Lakehead University, Thunder Bay, Ontario, Canada, philip.fralick@lakeheadu.ca The Gunflint Formation extends 190 km towards the northeast from an intrusive contact with the Duluth Complex. Surface exposures form a narrow band of outcrops with basal siliciclastics and underlying Neoarchean rocks to the west and overlying black shales of the conformable Rove Formation to the east. Erosional remnants 98 km northeast of Thunder Bay at Schreiber, Ontario, suggest a pre-erosional continuation of the chemical sedimentary rocks to the east. Rock comprising the Gunflint Formation primarily consists of an assemblage of chemical sediments including chert, iron oxides and carbonates. Volcanogenic shales and similar debris within the chemical sediments is also present. In 1956 the Formation was divided by Goodwin into six major sedimentary facies representing two depositional cycles and four members: 1) Basal Conglomerate, 2) Lower Gunflint, 3) Upper Gunflint, and 4) Upper Limestone. These units were deposited on a wave and tide influenced broad shelf deepening to the south (Fralick and Barrett 1995, Pufahl and Fralick 2004). Storm events eroded the fine-grained chemical sediments abrading them into sand-sized fragments that accumulated as cross-stratified layers and lenses (Fralick 1988). In this study we examine the limestone unit at the top of the Gunflint Formation and compare its petrology and geochemistry to ferronian dolomitic (ankeritic) grainstones common in the shoreproximal exposures near Thunder Bay. The stratigraphy of the limestones is consistent in the outcrops studied, even though some are tens of kilometers apart. The unit is conformable with underlying silicified ankeritic grainstones. The boundary between the two units forms a replacement front in places, with microscopic examination clearly revealing replacement of calcite by ankerite. The limestone consists of a very coarse sand to granule grainstone layer overlain by cabbage sized and shaped stromatolites. Layers of both fine-grained and very coarsegrained calcareous sand are banked up against and overlie the stromatolites. This assemblage is less than one meter thick. It is sharply overlain by a pebble to boulder conglomerate with some tabular clasts up to four meters long. Fragments of the underlying ankerite and limestone layers form most of the clasts. Ankerites from lower in the succession are composed of neomorphic spar with none of the original texture preserved. Geochemically they have high concentrations of Ca, Fe, Mg and Mn with relatively low abundances of most other elements. Their REE curves are LREE enriched with positive La and, to a lesser extent, Ce anomalies. Eu is flat to positively enriched. The limestones have clasts composed of Fe-rich chlorite, consistently with V contents of 1-2 percent. They are surrounded by blocky calcite cement that in places overgrows the grains. Samples from higher in the grainstone assemblage have more pronounced dissolution pathways along calcite crystal boundaries. These dissolution channels are filled with either calcite or quartz. The stromatolites have alternating layers of Fe-rich chlorite and blocky calcite cement similar in composition to the adjacent grainstones. The grains in some grainstones are pieces of stromatolite. The limestones are very geochemically different from the ankerites. The limestones have higher values of most elements, but especially U (5 to 10 times more abundant), REE’s (10 to 100 times more abundant) and V (100 times more abundant). Their REE curves are LREE enriched and show distinct Ce and Eu depletion. 18 �New U-Pb geochronological data (Don Davis, Pers. Comm) indicates that the pebble to boulder conglomerate is the tsunami deposit that resulted from the Sudbury impact. This reinforces previous interpretations that a significant nondepositonal interval with subaerial exposure existed between deposition of the Gunflint and Rove Formations in the study area (Addison et al. 2005). The blocky calcite cements and Fe chlorites would have formed in the phreatic zone during this interval. The extreme V-U enrichments indicate oxidized fluids leaching V and U from the subaerial environment and precipitating these redox sensitive elements upon encountering the organic-rich sediments. The negative Ce anomalies of the REE curves for the calcite cements agree with this interpretation. The ankeritic sediments deposited in the marine environment have REE abundances typical of precipitation from oxygen deficient water with abundant dissolved Fe and Mn. This data points to an imbalance in oxygen levels at this time with a relatively oxic atmosphere and anoxic oceans. This agrees with findings from other geochemical investigations of the Gunflint and Rove Formations (Poulton et al. 2004). Addison, W. et al., 2005. Geology, vol. 33, p. 193-196. Fralick,P.F. 1988. Memoir 13, Can. Soc. of Petroleum Geologists, p. 24-29. Fralick, P.W. and Barrett, T.J., 1995. Spec. Publ.22, Inter. Ass. of Sedimentologists, p. 137-156. Poulton, S.W., Fralick, P.W. and Canfield, D.E., 2005. Nature, vol. 431, p. 173-177. Pufahl, P.K. and Fralick, P.W., 2004. Sedimentology, vol. 54, p. 791-808. Figure 1. Archean mudstone normalized REE curves for ankeritic samples (lower 5 curves) and limestone samples (upper 3 curves). 19 �THE SUDBURY IMPACT LAYER IN THE LAKE SUPERIOR IRON RANGES: A TIME-LINE FROM THE HEAVENS Cannon, William F., U.S. Geological Survey, MS 954, Reston VA 20192 Addison, William D., R.R. 2, Kakabeka Falls, Ontario, POT 1W0, Canada A large meteorite impact near Sudbury, Ontario at 1,850 Ma produced a geologically instantaneous regional, and probably global, catastrophe leaving a unique imprint within the classic iron ranges of the Lake Superior region 500 to 900 kilometers from the impact site. We have now identified the impact layer in or near five of the iron ranges. The layer is a unique and ultra-precise time line, having formed over a time span of days or less across the region, and allows an unequivocal temporal correlation of the stratigraphic units with which it is interbedded. The impact layer consists of various rock types and may have formed by processes including direct deposition of airborne ejecta either from a high energy ejecta curtain traveling at ballistic velocities, or from a turbulent cloud of finer ejecta. More commonly the layer consists of polymict marine breccias of reworked ejecta and local sediments possibly formed by 1) “ballistic erosion” and mixing of sediment caused by high energy ejecta deposition, 2) erosion and redeposition caused by mega-tsunamis generated by the impact, 3) resurge of ocean water toward the “hole in the bottom of the sea”, or possibly 4) submarine slumping triggered by impact generated earthquakes. Regardless of diverse lithology and depositional processes, occurrences of the impact layer are linked by the presence of quartz grains exhibiting relict shock metamorphic features, which form only under the extreme pressures produced by hypervelocity impacts. 20 �The following are brief summaries of the characteristics each site: Gunflint- At top of Gunflint Iron-formation; nearly pure ejecta deposits of devitrified vesicular glass, microtectites, and spherules; accretionary lapilli and shocked quartz; apparently not reworked by tsunamis. Thunder Bay- Tsunami debrisite at top of Gunflint Iron-formation containing reworked, locally derived breccia fragments plus ejecta similar to the Gunflint ejecta; shocked quartz known from one locality. Mesabi- Poorly known unit at top of Biwabik Iron-formation; contains devitrified spherules, microtectites, and vesicular glass; shock metamorphosed quartz. Puritan- Known from a few thin sections of drill core from 1960’s study. “Volcaniclastic breccia” in Tyler Formation about 100 m above Ironwood Ironformation; rare shock metamorphosed quartz. Marenisco- Possible tsunami deposit at base of Copps Formation lying on Archean granite. Shock metamorphic features not well documented at this time. Huron River- Breccia and fine “volcaniclastic rocks”; near base of Michigamme Formation a few meters above contact with Archean granite. Abundant shock metamorphosed quartz grains. Connors Creek- Breccia, greywacke, and accretionary lapilli beds; erosional contact on slightly ferruginous chert about 150 m above base of Michigamme Formation; shock metamorphosed quartz. McClure- Breccia and greywacke about 300 m above base of Michigamme Formation; erosional contact on carbonate iron-formation; shock metamorphosed quartz; grades up to black slate. Iron River- Breccia and greywacke forming lower part of Hiawatha Graywacke; shock metamorphosed quartz; known best from thin section collection from 1940’s; erosional contact on Riverton Iron-formation. The diagram below shows the precise temporal correlation of stratigraphic units in the five areas where we have identified the Sudbury impact layer. . 21 �AN EXPERIMENTAL WATER-ROCK INTERACTION STUDY INTO THE ORIGIN OF HIGH-SULFATE WATERS ASSOCIATED WITH THE STEEP ROCK IRON MINES, ATIKOKAN, ONTARIO. COCKERTON, S., and CONLY, A.G., Department of Geology, Lakehead University, Thunder Bay, ON, P7B 5E1 andrew.conly@lakeheadu.ca LEE, P., Department of Biology, Lakehead University, Thunder Bay, ON, P7B 5E1 Continual flooding of the two primary open pits of the Steep Rock iron mines near Atikokan, Ontario, has lead to the formation of Hogarth and Caland pit lakes, which are located in the former middle and eastern arms of Steep Rock Lake, respectively. At present both lakes are ~200 m deep and display distinct differences in water chemistry [1]. Hogarth pit lake is characterized by non-stratified to weakly stratified and oxygenated water column (4-12 mg/L O2), near neutral pH (6.4-8.0), extraordinarily high SO42- concentrations (1200-2000 mg/L), lower alkalinity (50-125 mg/L), increased hardness (1200-1800 mg CaCO3/L) and is chronically toxic to aquatic fauna. On the other hand, Caland pit lake is well stratified with an upper oxygenated fresh water lens (~20 m in depth and 4-12 mg/L O2) that is not toxic, which overlies an anoxic (<1 mg/L O2) and moderately saline (200-500 mg/L SO42-) water column. The pH of Caland pit water is similar to that of Hogarth, but is more alkaline (100-200 mg/L) and has a lower hardness (400-1000 mgCaCO3/L). The concentration of metals in both lakes is negligible. With the pits being filled at a rate of ~3 m/year [2] and ground water presumed to be the largest input of water, the differences in water chemistry between the two pit lakes likely reflect differences in the nature of water-rock interactions between ground water and bedrock. Previous stable sulfur isotope studies have concluded that the dissolved sulfate of the pit waters is derived from oxidation and leaching of pyrite-bearing units of the Jolliffe ore zone by either ground water or due to pit water-wall rock interaction [1,3]. The geology of two pits is largely similar and includes [4,5]: 1) footwall rocks that consist of the Marmion Geniss Complex, Wagita Formation and Mosher Carbonate; 2) the Joliffe ore zone, which is comprised of the Manganiferous Paint, Goethite and Pyrite members; and, 3) hanging wall rocks that include the Dismal Ashrock Formation and the Witch Bay Formation. In order to better determine the nature of ground water-rock interactions a series of batchreaction column leaching experiments were performed. A series of four PVC columns (4 inches in diameter and 24 inches in length) were loaded with crushed (1-2 mm) pyrite and goethitehematite waste rock, representing the Goethite and Pyrite members. Two of the columns also contained 2 inches of crushed Mosher Carbonate at the base. Columns were filled with Atikokan ground water ensuring that the crush rock was completely submerged. In addition, another three columns were loaded with crushed goethite-hematite tailings (jit reject), goethite-hematite waste rock and pyritic waste rock, and filled with rain water in order to assess the contribution provided from surface water. Approximately 30 mL of water was extracted and filtered at 3 hr, 24 hr, 3 days, 5 days, 7 days and then weekly for a total of 14 weeks. Cations and metals were determined by ICP-AES, anions (SO42-, Cl-) were determined by ion chromatography, and pH was measured using an Accumet AB15 pH meter. Results from the column leaching experiments indicate that the high SO42-(aq) concentration of the pit lakes could only be produced from ground water oxidation of pyritic portions of the Joliffe ore zone (Fig. 1a). 22 �Figure 1. Normalized plots, to the average composition of Hogarth pit lake, showing the (A) relative concentration of pH, anions and cations from goethite-hematite and pyrite waste rock experiments, and (B) results of mixing between leachates derived from goethite-hematite and pyrite waste rock. To generate the near neutral pH of either pit lake requires subsequent interaction with Mosher Carbonate (Fig. 1a). Figure 1b shows the results of mixing the leachates derived from goethitehematite and pyrite waste rocks. Consequently, the pit waters are likely produced from the mixing of ground waters derived from both iron oxide and pyritic portions of the ore zone that were subsequently buffered by carbonate wall rock.. However, discrepancies between experimental waters and measured Hogarth pit lake water (e.g., Mg) likely reflect interaction of pit lake water with other wall rock lithologies and/or other ground water contributions. Experiments are currently underway to assess both the interaction of ground water with different lithologies and the effect of varying water-rock ratios. Initial results from these experiments will be presented at the meeting. Notwithstanding these discrepancies, the results from batchreaction column experiments supports stable isotope evidence [1,3] that the sulfate in the two lakes is derived from ground water leaching of pyritic portions of the Joliffe ore zone. In addition, the differences in water chemistry between Hogarth and Caland pit lakes primarily reflects differences in the relative pyrite content of the Joliffe ore zone, with the ore zone beneath Hogarth pit lake containing a higher abundance of pyrite. References: [1] Goold, A., Conly, A.G., and Lee, P., 2006. Integrated uses of water chemistry, stable isotope geochemistry and tie studies in identification of metal toxicity in pit lakes: A case study at the Steep Rock iron mine, Atikokan, Ontario: GAC-MAC Program with Abstracts, v. 31, p. 57-58. [2] McNaughton, K.A., 2001. The limnology of two proximal pit lakes after twenty years of intense flooding. M.Sc. Thesis, Lakehead University, Thunder Bay, Ontario, 85 p. [3] Conly, A.G. and MacDonald, J.C., 2005. An Investigation into Potential Sulphate Sources of the Hogarth Pit Lake, Steep Rock Iron Mine, Atikokan. ILSG 51st Annual Meeting, Proceedings Volume 51. [4] Joliffe, A.W. 1966. Stratigraphy of the Steeprock Group, Steep Rock Lake, Ontario. GAC Special Paper No. 3, p. 75-96. [5] Stone, D., Kamineni, D.C., and Jackson, M.C., 1992. Precambrian geology of the Atikokan area, northwestern Ontario. Geological Survey of Canada, v. 405, 106 p. 23 �ORIGIN OF A MAFIC PEGMATITE WITHIN THE DULUTH COMPLEX, NORTHERN MINNESOTA COSTELLO, Daniel E.1, FLOOD, Timothy P.1, and THOLE, Jeffrey T.2 Department of Geology, Saint Norbert College, De Pere, WI 54115, dan.costello@snc.edu, 2 Department of Geology, Macalester College, St Paul, MN 55105 1 Geologic Setting The Duluth Complex of Northern Minnesota is a layered igneous intrusion that formed due to rifting of continental North America approximately 1.1 billion years ago. The Complex is composed of numerous layered plutonic bodies, which formed through multiple intrusions of mafic to felsic tholeiitic magmas. These layered series are typically thick and exhibit foliation and layering that dips to the southeast, towards the axis of the Keweenawan Rift (Miller et. al. 2002). This study focuses on units within the South Kawishiwi intrusion (SKI). Near the base of this intrusion, two similar troctolitic units are separated by a pegmatitic unit. The petrogenetic relationship between the overlying and underlying troctolitic units is uncertain, as is the origin of the pegmatite. This project aims to determine the origin of and relationship between these three units using petrographic and geochemical data. Igneous Stratigraphy The pegmatitic unit, hereafter referred to as the PEG, was first noted by Foose and Weiblen (1984). This unit is used as a marker bed to differentiate the surrounding troctolites: The rock below the PEG is classified as the ultramafic three unit (U3), and the rock above the PEG is classified as part of the ultramafic two unit (U2). In drillholes where there is no PEG present, it is impossible to differentiate between these two troctolitic units. The U3 is the lowermost ultramafic unit within the SKI. The thickness of this unit averages approximately 100 feet, and ranges from 3 to 410 feet. (Severson 1994). The U3 is characterized by massive oxide horizons as well as mineralization of sulfides and platinum group elements. The PEG overlies the U3 unit and has an average thickness of approximately 95 feet. The pegmatite is used as a marker boundary between a sulfide-rich zone below and a sulfidepoor zone above (Foose and Weiblen 1986). The pegmatite is not always present in drill core. The U2 overlies the PEG and is petrologically similar to the U3 unit, with the exception that the U2 lacks massive oxides and sulfide-rich horizons (Severson 1994). This unit averages about 90 feet in thickness. Analysis Petrographic study revealed visible variations among the three units. The U2 and U3 appear to be similar; however, multiple differences occur within the pegmatite. The presence of minerals such as amphibole and biotite in the PEG suggests that this unit formed at a later stage of crystallization and/or within higher P(H20) conditions than its surrounding units. The PEG also contains significant amounts of a plagioclase symplectite texture (Figure 1). This texture may indicate the presence of a late stage magmatic fluid which would promote the growth of a pegmatitic unit. This symplectitic texture is rare below the pegmatitic unit, but is present in small amounts in all units above the PEG (Severson 1994). 24 �Geochemical studies focused on the changing concentrations of elements, notably the different behaviors of compatible and incompatible trace elements. From the U3 to the U2, not including the PEG, the concentrations of both Ni and Co decrease, suggesting olivine fractionation. In a similar fashion, the concentration of Ba increases from the U3 to the U2, suggesting plagioclase fractionation. This is consistent with a model of Ba as an incompatible element during early stages of crystallization. Preliminary mineral chemistry was obtained on plagioclase and olivine grains from all units. The two troctolitic units contain olivine of Fo63-65 and plagioclase of composition An75-77. In comparison, the pegmatite contains olivine with Fo47-60 and plagioclase with An59-63. These results are consistent with a model of fractional crystallization between the U3 and the U2 and with elevated P(H20) conditions within the pegmatite. Using the preliminary results outlined above, we infer that the three units are syngenetic and the U2 and U3 units formed through fractional crystallization. The pegmatite formed as a water rich phase concentrated within the magma chamber, with some water being derived from the footwall. As normal fractionation occurred from the U3 to the U2, a concentration of water resulted in the formation of a pegmatitic horizon between these two units. References: Foose, M.P. and Weiblen, P.W., 1986, The physical and chemical setting and textural and compositional characteristics of sulfide ores from the South Kawishiwi Intrusion, Duluth Complex, Minnesota, USA: in 27th Int. Geol. Congress (Moscow), Special Copper Symposium: Springer-Verlag, New York, p. 8-24. Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.M., and Wahl, T.E., 2002, Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geological Survey Report of Investigations 58, 207 p. Severson, M.J., 1994, Igneous stratigraphy of the South Kawishiwi intrusion, Duluth Complex, northeastern Minnesota: Natural Resources Research Institute, University of Minnesota, Duluth, Technical Report NRRI/TR 93/34, 210 p. Figure 1: Plagioclase symplectite texture found in the pegmatitic unit. Field of view 1.5mm 25 �Distribution of Certain Ichthyofauna in relation to Eastern Outlets of Lake Agassiz, with emphasis on the Gunflint-Arrow Lakes Corridor and the Keating Complex J. Frederick Dean and Brian A. M. Phillips, Lakehead University, Thunder Bay, ON Canada. Based on a retrodeformed elevation model (Phillips, 1997) of the Upper Herman stage of Lake Agassiz, Phillips and Hill (2004) proposed that the lake discharged via the Gunflint-Arrow Lakes Corridor into Lake Superior down the Arrow valley. Fisheries work in Minnesota and Ontario reveals a distribution of fish species that corroborates a flow of Agassiz water into the Arrow, Whitefish and lower Kaministiquia watersheds. There are two sub-species of the temperate Johnny Darter (Etheostoma nigrum eulepis and Etheostoma nigrum nigrum), a small benthic fish unable to migrate up rapids or falls. The former, the earlier of the two, is found along the trend of the Herman shoreline and into the Gunflint-Arrow Lakes Corridor (Figure 1). The later sub-species relates to the Campbell shoreline, appearing in the Matawin and Shebandowan rivers. Intergrades developed where the two sub-species mixed (Underhill, 1963; Schmidt, 1993; Momot and Stephenson, 2007). An Arctic marine glacial relict, Deepwater Sculpin also inhabits Gunflint-Arrow Corridor lakes. Dean (2006, pers. comm.) found Pontoporeia affinis (Lindstrom), another Arctic marine relict amphipod, in bottom waters of Arrow Lake. Both migrated along the northern, icebound margin of Lake Agassiz, as flow was funneled eastwards through the corridor. The topographically rugged west-east trend of the borderlands, the “Keating Complex”, likely caused ice margin retreat to be intricate in detail (Figure 2). During and after the Steep Rock phase of Rainy Lobe retreat (circa 11,000 B.P.), Dean suggests successive spillways directed Agassiz waters towards Lake Superior. While Superior Lobe ice blocked the ArrowPigeon valley, water ran through the Stump and Swamp valleys and along the northern edge of coastal ice, first exiting via the Brule and Kimball valleys. Underscored is the potential for people to have migrated eastwards into the corridor at this time. Further ice retreat later deflected flow down the Whitefish-Kaministiquia valleys, before the Matawin exit opened. Momot, W.T. and Stephenson S.A., 2007, Atlas of the Distribution of Fishes within the Canadian Tributaries of Western Lake Superior: Lakehead University, Thunder Bay, 383p. Phillips, B.A.M., 1997, Retro-deformation of the D.E.M. of Northern Minnesota as a Tool for Reconstruction of Former Glacial Lakes, in Proceedings of the 9th Minnesota GIS/LIS Consortium Conference, St. Cloud, Mn. Phillips, B.A.M. and Hill C.L., 2004, Deglaciation History and Geomorphological Character of the Region Between the Agassiz and Superior Basins, Associated with the ‘Interlakes Composite' of Minnesota and Ontario, in L.J. Jackson and A. Hinshelwood, Eds., The Late Palaeo-Indian Great Lakes - Geological and Archaeological Investigations of Late Pleistocene and early Holocene Environments, Mercury Series, Archaeology Paper 165, Canadian Museum of Civilization, Chapter 10, p. 275-301. Schmidt, K.P., 1993, Minnesota Parks Fish Species Lists: http://www.nativefish.org, 67p. Underhill, J.L., 1963, Distribution in Minnesota of the Sub-species of Percid Fish Etheostoma nigrum, and their Intergrades: American Midland Naturalist, v.70, p. 470-478. 26 2 �Figure 1 – Distribution of Johnny Darters and Flow from Lake Agassiz to Lake Superior. Figure 2 – Flow from Lake Agassiz to Lake Superior and the Keating Complex, MN-ON. 27 3 �The effect of H2O on olivine to ringwoodite transformation: Implications for subduction zone geodynamics and the deep earth water cycle Tamara Diedrich * † Thomas G. Sharp School of Earth and Space Exploration, Arizona State University, Tempe, Arizona 85287 As the most abundant minerals in the earth’s upper mantle and transition zone, phase transformations between olivine and its high-pressure polymorphs, wadsleyite and ringwoodite, have major consequences for mantle geodynamics. Based on the thermal profile of subducting lithospheric slabs, it has been proposed that metastable olivine could persist into the mantle transition zone. Its presence would decrease negative buoyancy of the slab and slow subduction. In addition, the rapid transformation of metastable olivine has been identified as a potential initiating mechanism for deep-focus earthquakes. Several lines of evidence suggest that mantle olivine may contain significant H2O in the form of hydroxyl point defects. While previous studies have shown that H2O enhances olivine to wadsleyite transformation rates, under sufficiently cold conditions, olivine would transform directly to ringwoodite. Our research measures the effect of H2O on olivine to ringwoodite transformation rates and shows that even modest hydration of olivine results in near equilibrium transformation of olivine under subduction zone conditions and no metastable wedge of olivine below 400 km. For this study, a novel experimental approach was developed that consisted of two stages: hydration of mantle derived olivine in a piston-cylinder apparatus, and partial transformation of * Present address: Economic Geology Group, Natural Resources Research Institute, University of Minnesota, 5013 Miller Trunk Hwy, Duluth, MN 55811 † Corresponding author e-mail: tdiedric@nrri.umn.edu 28 �these spheroids in a multi-anvil device. After hydration, the olivine contained 298 (±29) ppm by weight H2O. The multi-anvil experiments were conducted for varying run durations at 18 GPa and 700°C, 900°C, and 1100°C. The partially transformed samples were thin sectioned and the ringwoodite rim thicknesses were plotted as a function of run duration. Both growth rates and activation enthalpy for growth were determined. Water appears to promote olivine to ringwoodite transformation by enhancing growth rates, decreasing activation enthalpy for growth, and facilitating the dissipation of transformation stress. Growth rates for the hydrated olivine are almost two orders of magnitude faster than for anhydrous olivine at 1100 °C and about four orders faster at 700°C. At 700°C, growth is fast enough to be observed on an experimental timescale; this is significant because kinetic parameters from previous studies suggest that, at 500°C, anhydrous olivine does not transform on a geologic timescale. The addition of water also enables ringwoodite growth rates to remain constant throughout transformation. The large volume decrease associated with olivine to ringwoodite transformation, if not accompanied by sufficient plastic deformation, causes transformation stress. As transformation progresses, this stress results in the build-up of strain energy, which slows growth by counteracting the driving force for transformation. Hydrolytic weakening of the olivine and/or ringwoodite facilitates the dissipation of transformation stress so that growth rates can remain constant throughout transformation. Using our kinetic parameters for hydrated olivine transformation, the addition of as little as 289 ppm by weight H2O effectively eliminates metastable olivine from the transition zone. Alternatively, if the presence of metastable olivine is confirmed, it must contain significantly less than 298 ppm H2O, placing a constraint on models of the deep earth water cycle. 29 �GOLD MINERALIZATION IN THE VIRGINIA HORN GREENSTONE TERRAIN, ST. LOUIS COUNTY, MINNESOTA: A PROSPECT RE-VISITED Elsenheimer, D.1, Frey, B. 2, and Hudak, J. 2, 1Minnesota Department of Natural Resources, 500 Lafayette Rd., St. Paul, MN 55155, 2Minnesota Department of Natural Resources, 1525 Third Ave. E, Hibbing, MN, 55746, donald.elsenheimer@dnr.state.mn.us Petrologic, geochemical and fluid inclusion data from archived drill core have been combined with a digital compilation of historic exploration data and new maps to reassess the gold mineralization potential of the Virginia Horn Greenstone Terrain, a fifty square kilometer exposure of Wawa subprovince rocks located in St. Louis County, Minnesota. The Virginia Horn Greenstone Terrain consists of a folded and faulted metamorphosed assemblage of Archean volcanic and sedimentary strata, intruded by a quartzofeldspathic porphyry (QFP) and overlain by a conglomerate sequence that was interpreted by Jirsa and Boerboom (2003) as a Timiskiming-type package correlative with parts of the Knife Lake and Shebadowan Groups (Figure 1). Metamorphic grade varies from prehnite-pumpellyite to middle greenschist facies. Visible gold within QFP outcrops were first identified in the 1930’s and associated with quartz veining, carbonate-sericite alteration and major shear zones. Historic mineral leasing and gold exploration activity involved the completion of more than fifty diamond drill core borings, mostly within or near the highly-altered and sheared QFP (Figure 2). Assay results within portions of drill core were significant, but too inconsistent to be considered economic. There has been no drilling within the Virginia Horn since 1991, and all mineral leases held on state-owned or managed lands (which cover more than 50% of the terrain) have been allowed to expire. Completion of subsequent Shebandowan 25km Moss Lake geologic mapping and analog modeling, Ardeen combined with a resurgence of priceSHEBANDOWAN Thunder sensitive gold exploration activity GREENSTONE BELTBay prompted the Minnesota Department of Mud Creek Shear Zone Natural Resources (MnDNR) to reassess Prospect Knife Lake the potential for economic gold deposits Timiskaming sequences Ely within the Virginia Horn. A digital Plutonic rocks Virginia Horn compilation was created of historic Greenstone Greenstone Belt mineral lease records and subsequent Gold Deposit After Jirsa and Boerboom (2003) Gold Occurrence petrologic and geochemical analyses on drill core housed in the MnDNR’s drill core repository. This compilation includes the conversion of Manitoba hard-copy sample location reports and laboratory reports into GIS-friendly maps, shape files and data tables. Drill core Ontario samples were reviewed to fill identified data gaps and better correlate drill core logs prepared by different companies, and new petrologic, geochemical and fluid inclusion samples were Minnesota collected to better constrain the conditions and timing of gold mineralization. Figure 1 30 �Historic exploration efforts focused on a lode gold mineralization model for the QFP. The depth and density of drill cores completed using this model were not sufficient to rule out the possibility of economic lode gold deposits within the QFP. That said, reinterpretation of the Virginia Horn as a Timiskiming-type greenstone terrain would support new exploration efforts beyond the QFP, and into both the overlying basal conglomerate and associated country rock, particularly where major shearing and alteration is found. Figure 2 # Virginia ### # # # # # # ## # ### ### # # # # # ## # #### ###### # ## Paleoproterozoic Animiki Group # # # ## Eveleth # Exploratory Drill Hole Virginia Fm Biwabik Iron Formation Pokegama Quartzite Neo-Archean Giants Range # Batholith Monzodiorite Tonalite Conglomerate Archean Greenstone Quartz Feldspar Porphyry Argillite/Greywacke Mafic intrusives Mafic volcanics 0 miles 5 Support for new drilling within relatively unexplored portions of the Virginia Horn comes from a new visible gold occurrence (Figure 3), discovered during re-logging and examination of drill core Figure 3 DML-3. This bore hole intersected a major fault and was completed within a meta-argillite unit located approximately 8,000 feet from the QFP. The visible gold occurred in a quartz-calcite vein with local minor pyrite. The quartz vein, at 207.5 to 209 feet, had a broken ribbon texture parallel to the vein margins. The vein was oriented about 5 degrees to the core axis. Other quartz-calcite (and Field of view = 1mm minor pyrite) veins were more numerous, but the internal texture was one of crackled quartz with infilling calcite. Prior orientation of the vein during this stage of deformation may have created the different styles. Gold grain counts and pathfinder element concentrations in glacial till collected from locations down ice from both the QFP and this new visible gold occurrence will be used to further constrain competing gold mineralization models. 31 �Textural Stratigraphy of Nipigon Diabase Sills: A Tool for Correlation and Petrologic Interpretations FORSHA, Clinton J. and ZIEG, Michael J., Department of Geography, Geology, and the Environment, Slippery Rock University, 1 Morrow Way, Slippery Rock, PA 16057, USA, cjf8856@sru.edu The relationship between texture and position in an igneous intrusion can be used to interpret the injection dynamics of a sill. In addition, because texture is a strong function of cooling history, textures can be used to determine position in the sill, when the upper and lower contacts are not exposed. The primary purpose of this study is to examine the textures variations in sections through a diabase sill at multiple locations in the Nipigon area, in an attempt to correlate the profiles and construct a composite section. Previous investigations of the Nipigon sills, 1.1-Ga olivine tholeiites, have demonstrated, on the basis of mineralogical variations (e.g., Sutcliffe, 1989), and textural anomalies (Zieg and Forsha, this volume), that the sills were filled via a series of injection pulses rather than in a single, instantaneous injection. This episodic filling history impacted the modal mineralogy and textural variations in the lower contact zone, and is clearly discernible using crystal size distribution (CSD) variations. In order to test the use of textural variations for stratigraphic correlation in igneous intrusions, three sample profiles, from different heights within the sill, were investigated. The Kama Point section includes the lowermost 35 m of the sill, and exhibits strong internal discontinuities related to reinjection events. The Moseau section, which has a total thickness of ~45 m, but does not include an upper or lower contact, is significantly coarser-grained than the Kama Point section and contains no significant textural anomalies. The Palisades section, which has a thickness of ~40 m but does not include an upper or lower contact, does not follow same coarsening-upwards trend as the Kama Point and Moseau sections. The rocks in this section display a slight coarsening toward the center of the section, and appear to become slightly finer-grained in the upper part. With the coarse nature of the Palisades section and its gradual change from coarsening to fining, it is believed that this section is located in the center of the sill. As an application of this method, we combine the different sections into a composite profile through the lower half of the sill. This technique is useful in areas such as Nipigon, where total relief is less than the thickness of individual sills. Used together with mineralogical and chemical constraints, textural stratigraphy is a powerful tool for comparing and interpreting the histories of igneous intrusions. References Sutcliffe, R.H. 1989. Mineral variation in Proterozoic diabase sills and dykes at Lake Nipigon, Ontario. Canadian Mineralogist, 27: 67-79. 32 �PGE-rich mineralization at the Marathon Deposit, Coldwell Alkaline Complex, Ontario David Good and Phillip Walford, Marathon PGM Inc. Mineralization at the Marathon deposit occurs within the Two Duck Lake gabbro, a late-stage gabbroic subunit of the Eastern Gabbro located along the eastern margin of the Coldwell alkaline complex. To date, 70 million tonnes of in-pit resource including 750,000 tonnes of PGE-rich ore have been outlined. PGE-rich ore occurs within a mineralized envelope that is stratigraphically above relatively Curich ore in the Malachite and Southern Resource areas and has been intersected in 64 drill holes over a strike length of 1 km. The highest grade intersection is 106 ppm Pt+Pd+Au and 0.02% Cu over 2m and the highest value intersection is 55 ppm Pt+Pd+Au and 0.59% Cu over 8m. The 64 drill hole intersections were subdivided into a subset of 28 intersections based on a cut-off value of 3ppm Pt+Pd and a minimum thickness of 4 m (2 samples). The average grade and thickness of this subset is 9.38 ppm Pt+Pd+Au and 0.21% Cu over 7.56m. The Pd/Pt ratio is 3.25+/-0.81. The host Two Duck Lake gabbro consists of coarse grained to pegmatitic gabbro with subhedral plagioclase and olivine with ophitic-textured clinopyroxene and magnetite, and minor apatite and biotite. Xenoliths of fine-to medium-grained gabbro are common. Alteration or secondary minerals such as chlorite, sericite, serpentine and calcite occur locally and typically make up less than 5% of the rock. Sulphide mineralization occurs as disseminated chalcopyrite and minor pyrrhotite within coarse grained to pegmatitic olivine gabbro. A detailed study of platinum-group minerals in samples from the Malachite zone determined that approximately 70% of the observed individual grains were closely associated with chalcopyrite or bornite and that less than 7% were associated with secondary minerals such as chlorite or serpentine (Liferovich, 2007). Previous examination of metal-metal distribution diagrams for the Main zone showed strong inter-element correlations between Pd-Pt, Pd-Rh, Pd-Cu and Pd-Au (Good and Crocket, 1994). A comparison of PGE-rich ore to that of the Main zone shows two interesting relationships. First, the trend for Pd-Pt and Pd-Rh shown in the Main zone is continuous with that for the high grade ore. Second, there is no correlation between Pd and Cu in the high grade ore. The Au data are less decisive and somewhat scattered but seem to show behaviour similar to that for Pd. A magmatic model for partitioning of PGE and Cu by sulphide segregation and variations in the R-factor as described for mineralization in the Partridge River Intrusion (Theriault et al, 2000) is somewhat consistent with the data. A simple calculation using a concentration factor of 1000 for the mineralized zones suggests a magma column that is at least 10,000 m thick is required to produce a 10 m intersection with 10 ppm Pd, but the host Two Duck Lake gabbro is only 200 to 400 m thick. 33 �Thériault, R. D., Barnes, S.-J. and Severson, M. J., 2000, Origin of Cu-Ni-PGE Sulfide Mineralization in the Partridge River Intrusion, Duluth Complex, Minnesota: Economic Geology, v. 95, p. 929-943 Liferovich, R., 2007, Mineralogy of PGE, Gold and Silver in the Malachite ore zone, Two Duck Lake deposit, Marathon, NW Ontario, Internal Company report. Good, D.J., and Crocket, J.H., 1994, Genesis of the Marathon Cu-platinum-group element deposit, Port Coldwell alkalic complex, Ontario: A mid-continent rift-related magmatic sulfide deposit: Economic Geology, v. 89, p.131 –149 34 �Sugarloaf – A State Scientific and Natural Area and a North Shore Non-Profit Association John C. Green Department of Geological Sciences, University of Minnesota Duluth, Duluth, MN 55812 Andrew Slade Sugarloaf: The North Shore Stewardship Association, 6008 London Road, Duluth, MN 55802 The 7-acre Sugarloaf Point Scientific and Natural Area (SNA) in southwesternmost Cook County, Minnesota, was designated in 1992 to preserve and protect the geological and ecological features of this bit of the North Shore, especially the basalt lava-flows and the dynamic beaches. Despite its history as a logging-era brownfield, it preserves excellent and accessible evidence for the character of both the 1.1 Ga Midcontinent Rift volcanism and Holocene postglacial processes, still active today. The bedrock consists of olivine tholeiite flows, part of the Schroeder basalts, near the top of the North Shore Volcanic Group (Miller et al., 2001; Miller et al., 2006). Features such as ophitic and amygdaloidal textures, columnar jointing, pipe amygdules, segregation veins, ropy surfaces, vesicle cylinders, and clastic dikes are well exposed (Green, 1989). Surficial features include an abandoned strandline and the overall tombolo form, with modern beaches linking the rocky point to the mainland. Beach clasts imply derivation from Rainy Lobe as well as Superior Lobe glacial deposits, along with local bedrock. The SNA, as well as 27 surrounding acres, is managed by a nonprofit group known as Sugarloaf, the North Shore Stewardship Association. The Association has built a log interpretive center, and has accomplished considerable environmental restoration on this heavily impacted site. Major restoration projects include excavation and replanting, with locally-grown native vegetation, of a rare coastal wetland between the berms of the tombolo. Sugarloaf hosts many popular interpretive programs, including day-long workshops on various aspects of North Shore natural history. It works with landowners and public land managers throughout the North Shore on both landscape restoration and interpretation. A major current initiative is the opening of the North Shore Interpretive Center at the University of Minnesota Duluth’s historic “Limno Lab” building at the Lester River in northeastern Duluth. Exhibits there will introduce visitors to North Shore natural and settlement history and UMD based research on the North Shore and Lake Superior. References Cited Green, J. C., 1989, Physical volcanology of mid-Proterozoic plateau lavas: The Keweenawan North Shore Volcanic Group, Minnesota: Geological Society of America Bulletin, v. 101, p. 486-500. Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., and Peterson, D.M., 2001, Geologic map of the Duluth Complex and related rocks, northeastern Minnesota: Minnesota Geological Survey Miscellaneous Map Series Map M-119, scale 1:200,000. Miller, J.D., Jr., Green, J.C., and Jerde, E.A., 2006, Bedrock geologic map of the Little Marais quadrangle, Lake and Cook County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series Map M-172, scale 1:24,000. 35 �Emplacement of the Nipigon Sill Complex and Mafic to Ultramafic Intrusions of the Nipigon Embayment T.R. Hart and C.A. MacDonald Precambrian Geoscience Section, Ontario Geological Survey (tom.hart@ontario.ca) (caroleanne.macdonald@ontario.ca) The Nipigon Embayment consists of Proterozoic sedimentary and intrusive rocks underlain by Archean rocks of the English River, Wabigoon, and Quetico subprovinces. Recent geological mapping suggests that the Nipigon Embayment is underlain by a series of north-, northwest- and northeast-trending faults (e.g. Hart 2005; MacDonald et al. 2005). The north-trending faults can be traced for over 150 km, and commonly have apparent sinuous traces resulting from an intersection with north- and northwest-trending faults. A number of the northwest-trending faults control the roughly circular-shape of diabase sills as in the Kopka River (Sutcliffe 1986). The east- to northeast-trending faults include deformation zones exposed in surrounding greenstone belts, as well as greenstone belt and subprovince boundaries. Some faults, and particularly the intersections of a number of faults, appear to have been the locus for magmatic events active at a number of geological periods. A number of 2697-2686 Ma late- to post-tectonic mafic to ultramafic intrusions (Heaman and Easton 2006) occur across the central Wabigoon Subprovince from Lac des Iles north to Awkward Lake in the Obonga Lake greenstone belt near major east- to northeast-trending structures. The ~1540 Ma English Bay Complex and ~1595 Ma Badwater Intrusion (Heaman and Easton 2006) are located near the junction of the east-trending Obonga Lake mylonite zone / Humboldt Bay high-strain zone and a number of north-trending faults traceable south to Black Sturgeon Lake. These intrusions may represent magmatism related to formation of the Nipigon Embayment. In the same area, the 1159±33 Ma Inspiration diabase sill and 1112 Ma ultramafic Jackfish sill (Heaman and Easton 2006) may represent early stages of the MRS. To the south, the 1109 Ma Disraeli, 1113 Ma Seagull, and 1107 Ma Hele ultramafic intrusions are located near the intersection of northand northwest-trending faults with the Quetico-Wabigoon subprovince boundary (QWSB), also represent early MRS magmatic events. The 1117 Ma mafic to ultramafic Kitto Intrusion (Heaman and Easton 2006) is located near the intersection of the QWSB, and the north-trending Nipigon River fault. Many of the diabase sills have morphologies controlled in part by these three faults directions, particularly the roughly circular sills exposed in the Kopka and Garden PROTEROZOIC ARCHEAN Volcanic rocks Osler Group (1109 Ma) Lake areas. Granitic rocks Nipigon Sills (1112 Ma) Franklin et al. (1980) suggested that Mafic and ultramafic Ultramafic intrusions Ultramafic intrusions intrusive rocks the Embayment represented a failed Sibley Group Metasedimentary and arm of the Midcontinent Rift System Metavolcanic rocks English Bay Complex Sanukitoid intrusions 36 �(MRS). Alternatively, the Embayment could represent a structurally controlled basin formed by subsidence following an anorogenic thermal upwelling event represented by the ~1540 Ma English Bay Complex (Hollings et al. 2004). Studies completed by Rogala et al. (2005) indicate that deposition of the lower portions of the Sibley Group sedimentary rocks (>1339 Ma; Franklin 1978) pre-dated the formation of the ~1100 Ma MRS by ~200 Ma. The presence of sills rather than dykes and apparent lack of a dyke swarm common in many failed rift arms (Ernst et al. 2006) suggest that the Embayment was not extensional during the MRS event. A uniform lithogeochemistry for the diabase sills suggests a single magma source (e.g. Hollings et al. in press). Limited anisotropy of magnetic susceptibility data (Middleton et al. 2004) indicates westerly flow directions in the western Embayment suggesting a source, or sources, near the north-trending faults in the Black Sturgeon Lake area. Manson and Halls (1997) proposed that the geometry of the MRS could be the result of the interaction between the MRS and a series of faults within the bounding Archean rocks. The faults in Nipigon Embayment could be part of this series of faults and partially explain the lack of a classic failed arm. Some of these faults could have been deep structures providing pathways for the emplacement of a variety of magmas at different periods. The timing and magnitude of displacement along these faults is difficult to determine, but a high degree of fracturing and hematite alteration in diamond drill core from the 1112 Ma Seagull Intrusion (Heaman and Easton 2006) suggests multiple periods of fault activity. Late stage activity may be related to a 1090+/-20 Ma uranium mineralization (Ruzicka and LeCheminant 1984) located along a north-trending structure east of Black Sturgeon Lake. This younger age is closer to many of the events in the late Keweenawan, including the 1091+/-4.5 Ma Blake Gabbro (Heaman and Easton 2006), and suggests fault activity in the Nipigon Embayment for a period of up to 20 Ma. REFERENCES Ernst, R.E., Buchan, K.L., Heaman, L.M., Hart, T.R., and Morgan, J. 2006. Multidisciplinary study of N to NNE trending dykes in the region west of the Nipigon Embayment: Lake Nipigon Region Geoscience Initiative; Ont. Geol. Sur., Misc. Release Data, MRD 194. Franklin, J.M. 1978. The Sibley Group, Ontario; in Rubidium-strontium isochron age studies, report 2; ed. R.K. Wanless and W.D. Loveridge; Geol. Sur. Canada, Paper 77-14: 31-34. Franklin, J.M., McIlwaine, W.H., Poulsen, K.H. and Wanless, R.K. 1980. Stratigraphy and depositional setting of the Sibley Group, Thunder Bay District, Ontario, Canada; Can. J. Earth Sci., 17: 633-651. Hart, T.R. 2005. Precambrian geology of the southern Black Sturgeon River and Seagull Lake area, Nipigon Embayment, northwestern Ontario; Ont. Geol. Sur., Open File Report 6165, 63p. Heaman, L.M. and Easton, R.M. 2006. Preliminary U/Pb Geochronology Results: Lake Nipigon Region Geoscience Inititative; Ont.Geol. Sur., Misc.Release—Data, MRD191 Hollings, P., Hart, T.R., Richardson, A., and MacDonald, C.A. in press. Geochemistry of the Mesoproterozoic Intrusive Rocks of the Nipigon Embayment, Northwestern Ontario: Evaluating the earliest phases of rift development; Can. J. Earth Sci. Hollings, P., Fralick, P. and Kissin, S. 2004. Geochemistry and geodynamic implications of the Mesoproterozoic English Bay granite-rhyolite complex, northwestern Ontario; Can. J. Earth Sci., 41: 1329-1338. MacDonald, C.A., Tremblay, E. and Easton, R.M. 2005. Precambrian geology of the west-central map area, Nipigon Embayment, northwestern Ontario: L.N.R.G.I.; Ont. Geol. Sur., Open File Report 6164, 49p. Manson, M.L. and Halls, H.C. 1997. Proterozoic reactivation of the southern Superior Province and its role in the evolution of the Midcontinent Rift. Can. J. of Earth Sci., 34: 562-575. Middleton, R. S., G. J. Borradaile, D. Baker, and K. Lucas, 2004. Proterozoic diabase sills of northern Ontario: Magnetic properties and history; J. of Geop. Res., 109, B02103, doi:10.1029/2003JB002581. Rogala, B., Fralick, P.W. and Metsaranta, R. 2005. Stratigraphy and Sedimentology of the Mesoproterozoic Sibley Group and Related Igneous Intrusions, Northwestern Ontario: Lake Nipigon Region Geoscience Initiative; Ont. Geol. Sur., Open File Report 6174, 87p. Ruzicka, V. and LeCheminant, G.M., 1984. Uranium deposit research, 1983; in Current Research, Part A, Geol. Sur. Canada, Paper 84-1a: 39-51. Sutcliffe, R.H. 1986. Proterozoic rift related igneous rocks at Lake Nipigon, Ontario; unpublished PhD thesis, University of Western Ontario, London, Ontario, 325p. 37 �Lacustrine Sedimentary Organic Matter Proxies of Recent Lake State Changes and Climatic Conditions in Christina and Morrison Lakes of Western Minnesota Amanda Hogan, Travis Jacobs, and Kevin Theissen Department of Geology, University of St. Thomas, 2115 Summit Ave., St. Paul, MN 55105 Organic matter in shallow lake sediments reveal past states of the lake, indicating dryer or wetter periods as well as periods which were dominated by algal or vascular plants. We have analyzed lake sediment cores dating approximately back to the year 1400 and have correlated stable isotopic and elemental data to support recent lake transitions. In the summer of 2006, we collected sediment cores from Lake Christina and Morrison Lake, shallow lakes (< 4 m max depth) in western Minnesota’s Prairie Pothole Region. We took samples from the cores at 1 cm intervals and prepared them to be sent to the Stanford University Stable Isotope laboratory where they were analyzed for stable isotopic (δ13C and δ15N) and elemental (C/N, TOC, TN) values. Finally, over the last two months, the data have been analyzed and correlated to a sedimentation-rate based time scale to determine changes in ecological status of the lakes as well as past changes of precipitation in the areas surrounding them. Here we report preliminary results for Lake Christina. C/N ratios (8 to10) and δ13C values (-24 to-19 per mil) show a history where the lake has been dominated by lacustrine algae (Figure 1). The range of these values is small, and while they fluctuate, the levels never reach those of a lake dominated by vascular plants (1). These small changes indicate slight changes in organic matter production and sourcing. Based on our preliminary age estimates for Lake Christina, the values for δ15N show a steady increase from the year 1680 to 1900 where they begin to decline sharply over the next 100 years (Figure 2). The low values in the years 1400-1700 might indicate a wetter climatic period where the lake level was higher and algal productivity in the lake was greater (1). Precipitation slightly decreased over the next 300 years causing upward shifts in the value of δ15N as well as a decrease in %CaCO3. This drier period was possibly windier, causing more sediment particles to be blown into the lake leading to more turbulent waters with large amounts of mixing (3). It is likely that land use changes had an impact on the overall carbon isotopic values of the sediments. Values of δ13C show a sharp decline around 1900, which corresponds to a time when most of this area was farmland rather than grassland. This might signal a change to a more algal dominated lake at this time. However, around the year 1940, δ15N values begin to decline again indicating a wetter period. In the years since 1938, more wet years are on record than in the years prior. The decline in δ15N could be attributed to the wetter climatic conditions in the area over the last 60 years (1). 38 �Lake Christina d13C vs. C:N Ratio -19.50 -20.00 -20.50 -21.00 -21.50 -22.00 -22.50 -23.00 -23.50 -24.00 7.50 8.00 8.50 9.00 9.50 10.00 C:N Ratio Figure 1. Lake Christina d15N vs. Time 2.5 2 1.5 1 0.5 0 -0.5 -1 -1.5 -2 -2.5 1400 1500 1600 1700 1800 1900 2000 2100 Time (years) Figure 2. References: (1) Meyers, P.A., and Lallier-Verges, E., 1999, Lacustrine sedimentary organic matter records of Late Quaternary Paleoclimates. Journal of Paleolimnology, 21, 345-372. (2) Pratt, L.M., Comer, J.B., and Brassell, S.C., 1992, Geochemistry of Organic Matter in Sediments and Sedimentary Rocks, 29-72. (3) Xu, H., Ai, L., Tan, L., An, Z., 2006, Stable isotopes in bulk carbonates and organic matter in recent sediments of Lake Qinghai and their climatic implications. Chemical Geology, 235, 262-275. 39 �Geochemistry of Midcontinent Rift-related mafic dykes and sills near Thunder Bay: New insights into geographic distribution and the geochemical affinities of Nipigon and Logan sills and Pigeon River and other dykes HOLLINGS, Pete, Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON, P7B 5E1, Canada, SMYK, Mark C., Ontario Geological Survey, Ministry of Northern Development and Mines, Suite B002, 435 James St. South, Thunder Bay, ON P7E 6S7 Canada, HART, Thomas, Precambrian Geoscience Section, Ontario Geological Survey, 933 Ramsey Lake Road, Sudbury, ON, P3E 6B5, Canada Recent geochronological studies (Heaman et al., 2007) have shown that the diabase sills and mafic/ultramafic intrusions in the vicinity of Lake Nipigon are among the oldest expression of igneous activity associated with the Midcontinent Rift (~1106 to 1115 Ma). Diabase sills, extending from the vicinity of Thunder Bay to east of Lake Nipigon, represent the northern expression of the Midcontinent Rift (MCR), and have previously been collectively referred to as the Logan sills (Stockwell et al. 1972). However recent work suggests a geochemical difference between the sills to the north and south of Thunder Bay (Hart 2003; Hart et al. 2005). It has been proposed that the term Logan Igneous Suite, which would fall within the Midcontinent Rift Intrusive Supersuite (Miller et al. 2002), should be applied to all the diabase sills in the area north of Lake Superior, with subdivision into more informal categories (i.e. Nipigon sills for the sills north of Thunder Bay, and Logan sills for those to the south) (Hollings et al., 2007). In 2006 a sampling traverse was undertaken through Thunder Bay in order to help determine the boundary between these two suites. The large geochemical data set for the Nipigon sills obtained as part of the Lake Nipigon Region Geoscience Initiative (Hollings et al., 2007) and a number of recently analyzed, east-northeast- to northeast-trending dykes south of Thunder Bay provide a means of comparing the sills near Thunder Bay. The sills and dykes sampled for this study are all characterized by LREE and enrichment and negative Nb anomalies typical of the majority of the local intrusive rocks associated with the MCR (Hollings et al., 2007). The majority of these sills fall are geochemically similar to the Nipigon sills. However, two ultramafic sills are geochemically similar to the Logan sill suite (i.e. with higher TiO2 and Gd/Ybn; Fig. 1). This suggests that the boundary between the Nipigon and Logan sills lies southwest of Thunder Bay. Data from other dykes south of Thunder Bay (L. Hulbert and R. Ernst, Geological Survey of Canada, pers. comm., 2006) show that the geochemistry of the Pigeon River dykes (Arrow River and Rita Bolduc occurrences) is similar to the range of data from other dykes of the Pigeon River swarm sampled on Lake Superior (Cloud Bay and Jarvis Point; Fig. 1). The Pigeon River dyke swarm geochemistry more closely resembles that of the sills of the Nipigon suite (Hollings et al., 2007) than that of the ultramafic intrusions or the Logan sills (Fig. 1). In contrast, the Mt. Mollie dyke appears to be transitional between Nipigon sills and Inspiration sills. Geochemical trends in individual dykes suggest fractionation or assimilation/contamination but data is currently limited. A sample taken from a Pigeon River dyke near its chilled contact with Rove Formation on Jarvis Point (JP-3) shows elevated Th and LREE compared to the dyke interior (JP-1, -2, -4; Fig. 2). A granophyric phase of the Mt. Mollie dyke on the southern end of Victoria Island (VI-9) displays similar, but elevated, REE compared to the main gabbroic part of the dyke. The granophyre also shows a marked depletion in Al and also in Ti and V, perhaps due to the relative lack of oxides in this late-stage differentiate. Additional isotopic and 40 �geochronological studies will be required in order to further investigate the relationships between these MCR-related intrusions. Figure 1: Major and trace element variation diagrams illustrating the geochemical affinities of the dykes and sills in around Thunder Bay. Data from Logan sills are from Hart (2002), Nipigon data are from Hollings et al. (2007). Date for dykes south of Thunder Bay are from and L. Hulbert and R. Ernst (Geological Survey of Canada, pers. comm.. 2006) from samples collected by M. Smyk and J. Scott (Ontario Geological Survey). Figure 2: Primitive mantle normalised diagram comparing geochemistry of dykes south of Thunder Bay. References Heaman, L.M., Easton, R.M., and Hart, T.R, 2006. Further Refinement to the Timing of Mesoproterozoic Magmatism, Lake Nipigon Region, Ontario. Canadian Journal of Earth Sciences, in press. Hart, T.R. 2003. Keweenawan mafic and ultramafic intrusive rocks of the Lake Nipigon and Crystal Lake areas, northwestern Ontario; ILSG, Proceedings Volume 49, Part 1-Programs and Abstracts: 21-22. Hart, T.R. 2005. South Black Sturgeon River–Seagull Lake Area, Nipigon Embayment, Northwest Ontario: Lithogeochemical, Assay and Compilation Data. Ontario Geological Survey, Miscellaneous Release of Data 147. Hollings, P., Hart, T.,Richardson, A. and MacDonald, C.A., 2007. Geochemistry of the Mesoproterozoic Intrusive Rocks of the Nipigon Embayment, Northwestern Ontario: Evaluating the Earliest Phases of Rift Development. Canadian Journal of Earth Sciences, in press. Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.M., and Wahl, T.E. 2002. Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geological Survey Report of Investigations 58. Stockwell, C.H., McGlynn, J.C., Emslie, R.F., Sanford, B.V., Norris, A.W., Donaldson, J.A., Fahrig, W.F., and Currie, K.L. 1972. Geology of the Canadian Shield. In Geology and economic minerals of Canada. Edited by R. Douglas. Geological Survey of Canada Economic Geology Report 1: 838. 41 �RECENT DEVELOPMENTS UNDERSTANDING THE VOLCANIC, MAGMATIC, TECTONIC AND METALLOGENIC EVOLUTION OF THE ELY GREENSTONE FORMATION, VERMILION DISTRICT, NE MINNESOTA HUDAK, G. J., Department of Geology, University of Wisconsin Oshkosh, Oshkosh, WI 54901, hudak@uwosh.edu HOFFMAN, A. T., Department of Geology, University of Minnesota – Duluth, Duluth, MN 55812 PETERSON, D.M., HEINE, J., Natural Resources Research Institute, University of Minnesota – Duluth, Duluth, MN 55811 The Ely Greenstone Formation comprises a steeply north- to southwest-dipping sequence of Neoarchean supracrustal and associated intrusive rocks associated with the Tower-Soudan anticline in the Vermilion District of northeastern Minnesota. Three distinctive units comprise this formation. The Lower Member of the Ely Greenstone (LMEG) is composed of calc-alkalic and tholeiitic basalt and basalt- andesite lava flows and tuffs with subordinate felsic lava flows, tuffs, epiclastic rocks and iron formations (Schulz, 1980; Southwick et al., 1998; Hudak et al., 2002, Hoffman, in press). The LMEG has been subdivided into the older Fivemile Lake Sequence (FLS) and the younger Central Basalt Sequence (CBS; Peterson and Patelke, 2003). The Soudan Iron Formation Member (SMEG) comprises Algoma-type interlayered cherty iron formation, basalt lava flows, epiclastic rocks and felsic tuffs (Peterson and Patelke, 2003; Hoffman, in press). The Upper Member of the Ely Greenstone Formation (UMEG) is composed of a monotonous sequence of poorly vesiculated tholeiitic basalt lava flows and localized Algoma-type iron formation lenses (Schulz, 1980; Southwick et al., 1998). The UMEG is commonly interstratified with the Lake Vermilion Formation (LVF:Schulz, 1980; Southwick et al., 1998), which is composed of greywacke, slate, conglomerate, and dacite tuff, as well as subaerial to submarine dacite to trachyandesite lava flows, tuffs, and associated intrusions (the informally named Gafvert Lake Sequence (GLS)); locally, however, LVF unconformably overlies LMEG and SMEG strata (Southwick et al., 1998; Peterson and Patelke, 2003). Schulz (1980) interpreted volcanological and sedimentary textures to indicate a change from a subaerial / shallow subaqueous setting to a deeper subaqueous environment during the temporal genesis of the Ely Greenstone Formation. Southwick et al. indicate a sharp transition from arc-associated volcanism (LMEG) to MORB-like volcanism (UMEG) occurs abruptly at the top of the SMEG. Recent volcanological reconstructions between the Soudan Mine and Armstrong Lake provide further evidence for the interpretations of Schulz (1980). The initial deepening of the submarine volcanic setting occurred during the development of the LMEG (Peterson et al., 2005; Hoffman, in press). Abundant primary mafic and felsic volcaniclastic strata, highly vesicular basalt to basalt-andesite pillow lavas and sheet flows, multiple selvege pillows, and epithermallike zinc stringer mineralization that characterize the FLS indicate a depositional setting in a shallow subaqueous environment (Hudak et al., 2002; Hoffman, in press). The overlying CBS comprises exceptionally well-preserved, single-selvege pillow lavas and associated sheet flows with sparse (generally <5%) vesicularity which are interstratified with subordinate banded iron formation and mafic tuff (resedimented basalt hyaloclastite) interpreted to have formed in a deep subaqueous setting (Peterson and Patelke, 2003; Peterson et al., 2005). The SMEG contains 42 �finely laminated chemical sediments and resedimented tuffs, as well as sparsely vesicular mafic lava flows suggestive of deposition within a deep subaqueous setting (Peterson and Patelke, 2003; Hoffman, in press). This deeper water setting appears to have persisted through the development of the UMEG, and at least locally during the development of the LVF. New major- and trace element data indicate the lithogeochemistry of volcanic rocks in the LMEG is more complicated than previously recognized. Arc-related basalts and basaltic andesites and FI- and FII-type rhyodacites and rhyolites characterize the FLS. In the CBS, arcassociated basalts and basaltic andesites transition up-section into E-MORB, OIB and back-arc basin-like basalts which are associated with FIII-type felsic volcanic rocks (Hudak et al., in prep. Hoffman, in press). The UMEG is characterized by MORB compositions that may also be the product of back-arc spreading (Southwick et al., 1998). A model encompassing initial arc development followed by back-arc development and rifting during the CBS, with subsequent development of the SMEG, appears to be most consistent with the observed volcanological and lithogeochemical characteristics in this part of the Vermilion District. Iron formations within the SMEG occur immediately up-section from the arc – back-arc transition, a stratigraphic position shown in many studies to have high prospectivity for hosting volcanogenic massive sulfide orebodies. References Hoffman, A. T., in press, Lithostratigraphy, Hydrothermal Alteration, and Lithogeochemistry of Neoarchean Rocks in the Lower and Soudan Members of the Ely Greenstone Formation, Vermilion District, NE Minnesota: Implications for Volcanogenic Massive Sulfide Deposits: Unpublished M. S. thesis, University of Minnesota – Duluth. Hudak, G. J., Heine, J., Newkirk, T., Odette, J., and Hauck, S., 2002, Comparative geology, stratigraphy, and lithogeochemistry of the Five Mile Lake, Quartz Hill, and Skeleton Lake VMS occurrences, Vermilion District, NE Minnesota: A report to the Minerals Coordinating Committee, DNR, Minerals Division, State of Minnesota: Natural Resources Research Institute Technical Report NRRI/TR-2002/03, 390 pages. Hudak, G. J., Heine, J., Newkirk, T. T., Hocker, S. M., and Hauck, S., in prep., Comparative Geology, Stratigraphy, and Lithogeochemistry of the Needleboy Lake – Six Mile Lake Area, Vermilion District, NE Minnesota: Natural Resources Research Institute Geological Report of Investigation. Peterson, D. M., Jirsa, M. A., and Hudak, G. J., 2005, Architecture of an Archean greenstone belt: stratigraphy, structure, and mineralization; in Robinson, L, ed., 2005, Field trip guidebook for selected geology in Minnesota and Wisconsin: Minnesota Geological Survey Guidebook 21, p. 154-180. Peterson, D. M., and Patelke, R. L., 2003, National Underground Science and Engineering Laboratory (NUSEL). Geological site investigation for the Soudan Mine, NE Minnesota: Natural Resources Research Institute Technical Report NRRI/TR-2003, 88p. Schulz, K. J., 1980, The magmatic evolution of the Vermilion Greenstone Belt, NE Minnesota: Precambrian Research, v. 11, p. 215-245. Southwick, D. L., Boerboom, T. J., and Jirsa, M. A., 1998, Geological setting and descriptive geochemistry of Archean supracrustal rocks and hypabyssal rocks, Soudan-Bigfork area, northern Minnesota: implications for metallic mineral exploration: Minnesota Geological Survey Report of Investigations 51, 69 p. 43 �EVALUATION OF MINERAL EXPLORATION DRILL CUTTINGS IN THE RICE RIVER AREA, EAST CENTERAL MINNESOTA Hudak, J. N. and Frey, B. A., Minnesota Department of Natural Resources, Lands and Minerals Division, 1525 3rd Ave. East, Hibbing, MN 55746 The Rice River area is located at the east end of the Cuyuna South Range, near the town of Aitkin. From 1949 to 1952, M.A. Hanna drilled 334 reverse circulation drill holes along approximately 12 km strike length of an iron formation related magnetic anomaly, to search for naturally enriched iron ore. Current DNR work in the Rice River area involves relogging of core and cuttings, the analysis of 325 samples for indications of base and precious metals, and the recovery of more accurate drill hole locations. The mid-Proterozoic basement rock does not crop out in the area, and drill cuttings and core are currently the only available bedrock samples. Recent replotting of geochemical lake sediment survey data revealed clustered metal anomalies in the Cuyuna South Range area. The Rice River area drill holes have been predominantly analyzed for iron and manganese and not fully evaluated for mineral potential. The main goal of the current evaluation is to see if there is any evidence of those anomalies in the Rice River area. Plausible ore deposit models for the area include SEDEX and iron-oxide-copper-gold. The magnetic iron anomaly is linear except for the large s-shaped fold, which is spatially associated with stratified sulfide occurrences and large quartz veins. Pyrite is often remobilized within shear zones of the graphitic argillite associated with this area. Veinlets with small amounts of bornite also occur scattered within the Rice River area. Currently, 60 of the 325 samples collected have been analyzed yielding anomalous levels of copper (2200 ppm) and antimony (6.75 ppm). A report of the work and findings are scheduled for release on June 30, 2007. 44 �PALEOENVIRONMENTAL INTERPRETATION OF A LOWER PALEOZOIC STROMATOLITE REEF, NORTHEASTERN WISCONSIN JASINEVICIUS, Renata R., GORDON, Elizabeth A., St. Norbert College Geology Department, De Pere, WI 54115, elizabeth.gordon@snc.edu INTRODUCTION Strata recently exposed in Schaal Quarry near Gillett, Wisconsin, provide an exceptional opportunity to examine the facies architecture of a lower Paleozoic stromatolite reef. The threedimensional nature of the reef, and its stratigraphy, are the focus of this study. The dolomitic rocks yield conodonts of early Ordovician age (Miller, 2006) and these strata are therefore assigned to the Oneota Formation. The Oneota dolomite overlies an unconformity, considered by many to be of regional extent. At this quarry, sandstones of uncertain Cambrian(?) age lie beneath the unconformity. Bedrock exposures in northeastern Wisconsin are extremely rare, due to widespread glacial deposits. This site therefore provides an important new opportunity to study Cambro-Ordovician boundary strata along the eastern margin of the Wisconsin Arch. RESULTS The Oneota Formation contains diverse stromatolite morphologies. At this locality, four stromatolite types occur in regular stratigraphic order: laterally linked hemispheroids, cabbageshapes, isolated small domes, and larger elliptical compound domes. In cross section, the larger stromatolites appear to form a mound- or ridge-shaped structure. The following paragraphs offer a summary and interpretation of the major sedimentary features associated with these stromatolites. The base of the Oneota in Schaal Quarry is dominated by an intraclastic conglomerate of variable thickness which overlies an unconformity. Desiccation cracks on the base of the conglomerate record subaerial exposure of the underlying units. Clast types include red shale, dolostone, sandy to silty dolomite pebbles and cobble to boulder-sized silcrete. The matrix is a dolomitic sandstone, and the unit grades upwards to sandy and silty dolomites. The silty dolomites contain discontinuous layers with microbial-like structures and laminated mats. These layers are interbedded with cross- and planar laminated, flaser- to wavy-bedded grainstones, and rare desiccation cracks. Grains types are primarily coated quartz sand and minor dolomite intraclasts. Upsection, discontinuous horizons of laterally linked hemispheroids (LLHS; Logan, 1964) mark the first visibly distinctive stromatolites, which predominate several horizons. In places they are vertically stacked, forming beds up to 20 cm thick. Individual hemispheres are round to elliptical with variable dimensions. LLHS are buried by ripple cross- and planar-laminated grainstones, grading to wackestones. Clay and glauconite locally contribute a greenish color to these rocks. 45 �Farther upsection, a fine-grained silty dolomite contains chert-lined vugs interpreted to be molds of anhydrite. These structures suggest hypersaline conditions. The overlying grainstones contain abundant crescent-shaped fossils of unknown origin, similar to “chitons” reported by Raasch (1952). Gastropod molds are present but rare. Conodonts include “Teridontus sp., Acanthodus uncinatus, Aloxoconus sp., Cordylodus lindstromi, and are assigned to the interval from the uppermost Cordylodus lindstromi Zone, the Iapetognathus Zone, and into the lower part of the Cordylodus angulatus Zone” (Miller, 2006), indicating an early (but not earliest) Ordovician age. Continuing upsection, there is a general upwards increase in bed thickness, accompanied by a decrease in coated quartz sand grains, and an increase in peloids and sparry cored ooids. Local grapestone intraclasts are also present. Within this interval cabbage-shaped stromatolites (CS) form discontinuous patches along one horizon. They are widely associated with chert. One wall reveals a curious relationship: adjacent stromatolite heads successively increase in size, suggesting a mound-like geometry of this cluster. CS are overlain by cross-bedded, oolitic peloidal grainstones, indicating burial by subaqueous dunes. Near the top of the quarry, thicker cross-bedded grainstones enclose nearly circular, isolated stromatolite domes. The domes appear to be concentrated in a mound or ridge. Higher still, the top surface of the quarry exposes large elliptical domes. A visible change in stromatolite synoptic relief indicates increasing water depth with time (Gerdes and Krumbein, 1994). The elliptical domes are clustered locally with subparallel long axes. The axes are aligned with paleocurrent azimuths measured from adjacent crossbeds and parting lineations. This association suggests stromatolite morphology was controlled in part by subaqueous currents. The predominant paleocurrent direction is approximately perpendicular to the inferred paleoshoreline. This implies subaqueous currents were tidal in origin and that stromatolite domes developed in subtidal channels. Collectively, these sedimentary features record evolution of a reef during marine transgressions. Interpreted paleoenvironments reflect tidal flat to subtidal environments under extreme conditions. REFERENCES Gerdes, G. and Krumbein, W., 1994, Peritidal stromatolites; in Phanerozoic Stromatolites II, ed. Bertrand-Sarfati, J. and Monty, C.: 114-17. Logan, B.W., Rezak, R., and Ginsburg, R.N.; 1964, Classification & environmental significance of stromatolites; Journal of Geology 72: 68-83. Miller, James F., 2006, Southwest Missouri State University; personal communication. Raasch, G. O., 1952, Oneota Formation, Stoddard Quadrangle, Wisconsin; Illinois Academy of Science Transactions, V. 15 p. 85-95. 46 �PROPOSED NEW BEDROCK GEOLOGIC MAP OF MINNESOTA JIRSA, Mark A. (jirsa001@umn.edu), BOERBOOM, T.J., CHANDLER, V.W., LIVELY, R.S., MILLER, J.D., Jr., MOSSLER, J.H., RUNKEL, A.C., SETTERHOLM, D.R., and WAHL, T.E, Minnesota Geological Survey (www.geo.umn.edu/mgs) Having conducted detailed geologic mapping in many areas of the state, and reprocessed state-wide geophysical data, the Minnesota Geological Survey (MGS) is well positioned to produce a new interpretation of bedrock geology. We are preparing to construct a state-wide bedrock geologic map— one that will be comprehensive, up to date, digital, and multi-layered. In the process, we also intend to upgrade digital bedrock topography and depth to bedrock maps and make them consistent with the new geologic framework (for preliminary imagery, see MGS website, Open-File 06-02). Together, these maps will provide the context necessary for various minerals and water-related applications in progress and under consideration. They will also establish the understanding of bedrock geology and crustal structure needed to address a number of impending, high-profile regional and national science initiatives, including EARTHSCOPE (www.earthscope.org). At this writing, the work of compiling hundreds of archival maps is underway. The existing state-wide bedrock map (MGS map S-20), completed in 2000, was reasonably detailed and accurate for its time; however, more than 50 new bedrock geologic maps have been created in the intervening 7 years since its production. Furthermore, the earlier map was compiled at a generalized 1:1,000,000 scale. As such, it portrays contacts that locally are inconsistent with high-resolution geophysical imagery, and it is 2-dimensional—in the sense that it depicts only the uppermost bedrock units. By contrast, the new map will be compiled at more detailed scales overall (compilation scale 1:100,000; publication scales 1:500,000 and 1:1,000,000), and geologic contacts will be reconciled with reprocessed geophysical data. It will also contain thematic layers, including trajectories of diabasic dikes, generalized metamorphic grades, and themes depicting Mesozoic, Paleozoic, Mesoproterozoic, Paleoproterozoic, and Archean geology. The Quaternary layer will be represented at this stage by revised depth to bedrock imagery. This layering will give users the opportunity to consider the third dimension by permitting removal of geologically younger layers to reveal older ones, to the extent that this can be done with some certainty. Perhaps more important than the printed maps and printable pdfs of archived maps, is the aim of producing a digital platform that will allow web-based user navigation from compilations to the most recent, accurate, and detailed mapping. Associated digital files will be searchable on attributes of lithology, age, and other themes. Mapping will be integrated with broader digital efforts using compilation formats consistent with those developed nationally (e.g., USGS) and globally (e.g., OneGeology.org). The project requires compilation of the best available maps—both digital and analog—together with new work designed to fill voids, augment existing map imagery, and evaluate the nature of buried geologic terranes. In addition, a small program of high-precision geochronology will acquire ages for a hand-full of critical samples. The results will be a seamless state-wide geologic interpretation, a webbased project that links this image to original mapping, and a digital cache of associated files. The poster presented here for general discussion depicts an initial effort to bring together the various digital map coverages. 47 �TRACE ELEMENT ANALYSES: AVOIDING DATA DISTORTION McSWIGGEN, Peter L., McSwiggen and Associates, P.A., 2855 Anthony Lane South, Suite B1, St. Anthony, MN (PMcS@McSwiggen.com) Electron microprobes have been used for trace element analyses for decades. This is because they can analyze minerals in situ, the size of the analytical area is very small, and the cost per analysis is very low. None-the-less, there is an ongoing push to measure trace elements in ever-lower concentrations. Over the years, improvements in both the electronics and the spectrometers of microprobes have made this possible. However, how the data is collected and how it is subsequently processed can mean the difference between meaningless numbers and real insights into the geologic question being investigated. Typically, people collecting trace element data want to know the minimum detection limit (MDL) of the method being used. Secondly, they want to know what should they do with the data that fall below the MDL. Should those values be set to zero? Should those values be set to the MDL? Should all of the data that fall below the MDL be discarded? If the set of data is going to be used to determine an average concentration for a rock body or a mineral type, all three options are wrong. In most cases involving trace element analyses, the calculated minimum detection limit for a single analysis should be ignored. Often, analysts will attempt to adjust their analytical conditions to get the lowest calculated minimum detection limit. However this can result in erroneous data. The MDL can be lowered using a number of strategies – for example, by counting for a longer period of time or by using a higher beam current to produce a higher count rate, both will lower the MDL. However both of these strategies can work against the overall objective. A higher beam current can damage the mineral being analyzed and therefore completely throw-off the results. Similarly, extremely long counting times can mask electronic instability in the instrument, thereby producing erroneous results. A far better strategy would be to collect a series of analyses, using more reasonable counting times and beam currents, and determine an average value from that set of data. This would minimize the effect of beam damage and would both monitor and correct for any electronic drift in the instrument; thus yielding a true detection limit that is much lower than that from the individual analyses. If one is using a set of analyses to determine an average composition, the calculated MDL becomes a meaningless value. Figure 1 shows a set of analyses run on a glass standard with a reported Ni content of 0.046 wt% using only a two second counting time per analysis. The average for this set of data was 0.053 wt%, a difference of only 0.007 wt% from the reported value. However, the calculated MDL for a single analysis was 0.25 wt%. Therefore, if all of the data that fell below the MDL was discarded, most of the data would have been thrown out, and those remaining values would have produced a completely erroneous composition of 0.275 wt% (Fig. 1). 48 �Not only should the data that fall below the MDL not be discarded, results that are below zero should also NOT be discarded. These negative values are just as important in determining the true average, as are the positive values. Each analysis has a certain error component associated with it. That error may be positive or negative. For trace element analyses, a negative error added to the real value, may result in a measured value less than zero. To get the true value, you need to average all of the data. By discarding only values on the negative side of the histogram, the data set becomes biased to a higher average value (Fig. 2). This is a big problem, because most commercial analytical instruments handle their data in this manner. Figure 1. Histogram showing multiple analyses of the NIST standard SRM-610. Two second counting times were used, resulting in a calculated MDL of 0.25 wt%, for a single analysis. The calculated average composition is 0.053 wt%, which agrees closely with the reported composition of 0.046 wt%. Figure 2. Shown is the same data set used in Figure 1, however all of the analyses that reported a negative concentration were set to zero. This is the typical procedure used by commercial analytical equipment. However, this method produces a distortion of the results. The average has been shifted from 0.053 wt% to 0.064 wt%, and this will distort any other statistics generated from the data. 49 �AN OCCURRENCE OF AGRELLITE IN THE WAUSAU ALKALINE IGNEOUS COMPLEX, MARATHON COUNTY, WISCONSIN MEDARIS, L. G. Jr and FOURNELLE, J. H., Dept. of Geology and Geophysics, Univ. of Wisconsin-Madison, Madison, WI 53706, medaris@geology.wisc.edu, johnf@geology.wisc.edu; GUGGENHEIM, S., Dept. of Earth and Environmental Sciences, Univ. of Illinois at Chicago, Chicago, IL 60607, xtal@uic.edu Agrellite is a rare, triclinic sodium-calcium fluorosilicate that occurs in alkaline (agpaitic) igneous complexes. Agrellite was first described from metamorphosed nepheline syenite in the Kipawa Complex, Quebec (Gittins et al., 1976), and its crystal structure was determined by Ghose and Wan (1979). To date, the only other reported occurrence is in the Khibina alkaline complex, Russia (Khomyakov, 1995). During construction of Interstate Highway 39 in northcentral Wisconsin, an agrellite-bearing dike was exposed in a roadcut, which was subsequently covered. The agrellite-bearing dike is located in the northern margin of Center 1 in the Wausau Complex (Fig. 1), where it cuts syenite and monzodiorite. White to light gray, tabular crystals of agrellite, ranging up to 15 mm in length, are concentrated along the center of the dike, where they are associated with quartz, perthitic alkali feldspar, plagioclase, aegirine-augite, miserite, apatite, fluorite, and locally eudialyte (Fig. 2). Fig. 1 Simplified map of igneous complexes near Wausau and the agrellite locality (modified from LaBerge and Myers, 1983) Fig. 2 Photomicrograph under crossed polarizers; a, agrellite; p, pyroxene (aegirine-augite); mf, finegrained miserite and feldspar 50 �Agrellite is biaxial, with 2VX = 55º, nX = 1.570(1), nY = 1.581(1), nZ = 1.584(1), and nZ - nX = 0.014. Cleavage is excellent on {110} and {110} and poor on {010}. The optic directions, Z and Y, are nearly parallel to the crystallographic axes, c and b, respectively, and the angle between the {110}and {110} cleavages is 40º (Fig. 3). Agrellite is triclinic, space group P1, and has the following cell parameters: a = 7.757(7) Å, b = 18.90(3) Å, c = 6.975(6) Å, α = 90.00(8)º, β = 116.81(6)º, γ = 94.4(1)º, V = 909.5(1) Å3, and Z = 4. Fig. 3 Sterographic projection of optical and crystallographic elements of agrellite The theoretical end-member formula for agrellite is NaCa2Si4O10F. Naturally occurring agrellite lies close to this composition, with the main deviations arising from the substitution of minor amounts of REE, Mn, and Sr for Ca (Table 1). Agrellite from the Wausau Complex is similar compositionally to that from the Kipawa Complex, except for having lower contents of REE and higher contents of MnO and SrO. The phase equilibrium conditions for agrellite are presently unknown, but the crystallization of agrellite, rather than wollastonite, in alkaline igneous complexes is likely promoted by high alkaline contents, i.e., (Na + K) / Al > 1, and high fluorine activity. The coexistence of agrellite with quartz in the Wausau Complex and with nepheline in the Kipawa Complex demonstrates that agrellite is stable in both silicasaturated and silica-undersaturated environments. Table 1 Composition of Agrellite SiO2 ZrO2 REE* MnO CaO SrO BaO Na2O K2O F -O=F Total 1 59.7 na 1.18 0.79 24.8 2.91 0.15 7.39 0.07 4.55 101.5 1.9 99.6 2 58.8 0.18 3.84 0.25 25.7 0.16 0.06 7.90 0.22 4.45 101.6 1.9 99.7 3 60.9 28.4 7.86 4.82 102.03 2.03 100.00 na, not analyzed 1 Wausau Complex, EMP analysis, UW-Madison 2 Kipawa Complex, Gittins et al., 1976 3 Theoretical end-member composition * Total of REE oxides References Ghose, S.G. and Wan, C (1979) Amer. Mineral. 64, 563-572. Gittins, J. et al. (1976) Canadian Mineral. 14, 120-126. Khomyakov, A.P. (1995) Mineralogy of Hyperagpaitic Alkaline Rocks: Oxford, 223 pp. LaBerge, G.L. and Myers, P.E. (1983) Wisc. Geol. Nat. Hist. Survey, Inf. Circ. 45, 88 pp. 51 �Metamorphosed, halite-dominated evaporites of the Lower Sibley Group Metsaranta, R.T.1, Fralick, P.W.2 and Bowdidge, C.1 1 RPT Uranium Corp. 537 Hilldale Rd., Thunder Bay, Ontario, P7B5N1, Canada 2 Department of Geology, Lakehead University, 955 Oliver Rd., Thunder Bay, Ontario, P7B 5E1, Canada. This paper describes newly discovered sedimentary halite deposits of Mesoproterozoic age from the Lower Sibley Group. The deposits were intersected in two closely spaced diamond drill holes, BSW-06-1a and BSW-06-4, drilled by RPT Uranium Corporation during the summer of 2006. The drill holes are located in the central portion of the preserved Sibley Basin, immediately west of the Black Sturgeon Fault, a major structure controlling the outcrop pattern of the Sibley Group. In the area, the Sibley Group is intruded by two thick Nipigon diabase sills that have produced significant contact metamophic effects. The evaporites occur in an approximately 50m thick section of Sibley Group rocks preserved between the lower diabase sill and Archean basement rocks. The basic stratigraphy beneath the lower diabase sill consists of from, bottom to top: a lower, well-cemented, sandstone dominated unit 14m thick, an interbedded halitemudstone unit 5m thick, an upper almost totally unconsolidated, partially halite cemented sandstone unit approximately 10-15m thick and an upper unit of interbedded calcareous mudstones and siltstones 20m thick. Lithostratigraphically, the lower sandstone unit can be correlated with the Pass Lake Formation while calcareous mudstones and siltstones in the upper portion of the section likely correlate with the lower Rossport Formation. Halite occurs in several forms within the section. First, it occurs as relatively large angular nodules within sandstones in the top few metres of the lower sandstone dominated interval (Fig. 1a). Second, it occurs as massive relatively pure beds 1-25cm in thickness within the interbedded halite-mudstone interval (Fig. 1b and c). Third, it occurs in irregular mixtures with clastic material, where clastic material is found as inclusions within halite and/or halite occurs in nodular form within the clastic sediment (Fig. 1.d). Finally, it occurs as cement and isolated nodular material in clastic sediments of the upper unit (Fig. 1e and f). Mineralogically, the composition of the evaporites is dominated by halite. Based on xray diffraction and reflectance spectroscopy gypsum is entirely absent from salt samples analyzed but there may be trace sylvite in some cases. Interbedded clastic materials commonly consist of mineral assemblages containing tremolite-actinolite, calcite, phlogopite, and serpentine group minerals consistent with contact metamorphism of carbonate-rich lithologies. Some samples contain clay minerals vermiculite, saponite and allophane which likely resulted from post metamorphic alteration of the contact metamorphic mineral assemblage. Sedimentologic and geochemical analysis of these unique deposits is currently underway. 52 �A B C D E F Figure 1. Various halite facies in the BSW-06-01a and BSW-06-4. A) Angular halite nodules within quartz-rich sandstone. B) Interbedded halite and mudstone, showing cyclically interbedded nature of halite beds (dark) with mudstone (light). C) Detail of a relatively pure, massive halite bed. D) Irregularly interbedded clastic material and halite. E) Very poorly consolidated halite-cemented sandstone. F) Photomicrograph of a halite cemented quartz sandstone. 53 �A CHEMICAL AND SR ISOTOPIC STUDY OF THE PIGEON POINT SILL, COOK COUNTY MINNESOTA M.G. Mudrey, Jr., 106 Ravine Road, Mount Horeb, WI 53572, mgmudrey@mhtc.net Isotopic and trace element by J.L. Wooden, then of LOCKHEED, NASA-Johnson Space Center, Houston, TX) In 1980, Wooden and Mudrey presented some isotopic and trace element data for the Keweenawan troctolite sill on Pigeon Point, Minnesota, the type locality of the mineral pigeonite. Wooden’s data were collected at the Johnson Space Center, Houston Texas, under exceedingly controlled laboratory conditions, comparable to lunar specimens at that time. The data have never been published, and interpretation has been limited because of other obligations of Mudrey and Wooden. Major elements were analyzed by a combination of colormetric, gravimetric and other classical techniques by S.S. Goldich and colleagues at Northern Illinois University and Pennsylvania State University. The Pigeon Point sill is an approximately 120 meter thick. differentiated troctolite to olivine gabbro intrusion of Keweenawan age situated on the Pigeon River between Minnesota and Ontario. The sill is of petrologic interest because of the association of three rock types troctonite/olivine gabbro, a ferro granodiorite, and a granophyre, whose genetic relationships have been debated for many years. Processes that have been invoked to explain the basaltic magma composition and significant abundance of granophyre (30 percent of sill thickness) on Pigeon Point include: Crystal Fractionation (Grout,1928); Magma Mixing Assimilation(Daly, 1917); Partial Melting (Bayley, 1893); Hydrothermal Alteration (Bastin, 1938); and Assimilation-Differentiation (Mudrey, 1973). The basal, chilled troctolite (PP-219-3) is a high alumina olivine tholeiite with a light enriched REE pattern (normalized La=19, Yb=6.8) having a positive Eu anomaly (Eu/*Eu=1.22), and for many years was viewed as a possibility as a parental magma for some parts of the Keweenawan. While this sample may approximate a parental liquid composition, an origin involving plagioclase and olivine accumulation is also possible. Rb-Sr isotopic data for 5 olivine gabbro and ferrogranodiorite samples form an approximate 1.429 Ga (initial 87Sr/86Sr = 0.70417). isochron. This "age" is similar to Rb-Sr ages determined for the RoveA Formation that the Pigeon Point sill intrudes. Compositional data for the sill suggests that selective contamination by alkalies and radiogenic Sr from the Rove Formation has occurred. The Sr isotopic data of the granophyre does not follow the pattern of the other samples, and its Rb/Sr ratio of 1.64 allow model ages of 1.11 to 1.15 Ga (probably representative of the crystal1ization age) to be calculated when the other sill samples are used for initial ratio control (I=0.7076 - 0.7046). (Best model age 1.132 Ga, 87Sr/86Sr = 0.70549). Petrologic, and trace element data argue for magma ascent from the upper mantle through Archean basement with low degrees of partial melting of potassic/alkaline constituents adjacent to the conduit followed by differentiation at an intermediate reservoir with ascent and emplacement at Pigeon Point at shallow depths (vaiolitic cavities, 1000 m). Local partial melting 54 �of overlying Rove Formation and stoping of quartzite within the Rove Formation account for the significant volume of granophyric rock. Bastin, E.S., 1938, Hydrothermal alteration in rocks of Pigeon Point, Minnesota, Journal of Geology, v. 46, p. 1058-1072. Bayley, W.S. 1893, The eruptive and s3edimentary rocks on Pigeon Point, Minnesota and their contact phenomena: U.S. Geological Survey Bulletin 109, 121 p. Daly, R.A., 1917, The geology of Pigeon Point, Minnesota: American Journal of Science, Series 4, v. 43, p. 423-448. Grout, F.F., 1928, Anorthosites and granite as differentials of a diabase sill on Pigeon Point, Minnesota, Geological Society of America Bulletin v. 39, p. 555-577. Mudrey, M.G., Jr., 1973, Structure and petrology of the sill on Pigeon Point, Cook County, Minnesota: University of Minnesota PhD dissertation, 310 p. Wooden, J.L., and Mudrey, M.G., 1980, A Chemical and Sr Isotopic Study of the Pigeon Point Sill, Cook County Minnesota (abs): EOS (Transactions AGU) v. 61, no 48 p. 1193. 55 �INVESTIGATIONS OF SULFIDE MINERALS LEACHED IN THE PRESENCE OF ALKALINE SOLIDS Nicholas, S.L. and Wirth, K.R., Geology Dept., Macalester College, St. Paul, MN 55105 Engstrom, J. and Lapakko, K.A., MN-DNR, Div. of Lands and Minerals, St. Paul, MN 55155 The oxidation of sulfide minerals present in mine waste exposed to the atmosphere can lead to the generation of acidic drainage. We used scanning electron microscopy and energy dispersive spectrometry (SEM-EDS) to examine unreacted rock and eight remnant solids. The solids were leached for periods of 117 to 899 weeks in laboratory experiments conducted by the Minnesota Department of Natural Resources (MN-DNR) on finely-ground (<0.149 mm) Duluth Complex rock (75 g) (Lapakko et al. 1997; 2000). This rock, which contains approximately four percent pyrrhotite (Fe1-xS) and lesser amounts of chalcopyrite-cubanite, pyrite, and pentlandite, was mixed with different amounts of rotary kiln (RK) fines or limestone. The rock samples were rinsed weekly with 200 mL of distilled deionized water, and the resultant drainage was analyzed regularly to assess acid-producing and acid-neutralizing reactions of each mixture and the controls. The effects of chemical reactions on water quality were documented and evaluated in detail, but little was known about the effects on the solidphase sulfide minerals. A working hypothesis was that oxidation of sulfide minerals occurred from the outside of the grain inward, leaving a sulfur-depleted rind around the grain. This oxidation rind might retard diffusion of sulfur from the center of the grain into solution. We identified the presence or absence of residual alkaline solids using a binocular microscope. SEM was used to describe the degree of sulfide oxidation, using the relative proportions of sulfur and oxygen determined by EDS. The extent of oxidation was described as the fraction of total analyzed sulfide area that was oxidized to a specific degree. Limestone grains persist in the limestone-loaded reactors, whereas no alkaline grains were detected in the reactors loaded with RK fines. Oxidation of sulfide grains begins along the edges, cracks, and partings of grains, eventually forming irregular rinds of sulfur-bearing iron oxide (Figures 1 and 2). Less of the pyrrhotite area was oxidized in samples with an alkaline addition than in the control (Figure 3), and reactors with limestone additions had smaller extents of oxidized areas than did the reactors loaded with RK fines. Comparison of the extent of pyrrhotite oxidation in two reactors with the same loading that were leached for different lengths of time (Figure 3, reactors loaded with 4.7g RK fines) indicates that the rate of pyrrhotite oxidation is greater under acidic conditions, a result that is consistent with leachate water chemistry. The four treated reactors that generated no acidic drainage were among the five least oxidized. References Lapakko, K.A., D.A. Antonson, and J. Wagner 1997. Mixing of limestone with finely-crushed acid-producing rock. Proc. 4th Int. Conf. on Acid Rock Drainage, Vol. 3, Vancouver, B.C., p. 1345-1360. Lapakko, K.A., D.A. Antonson, and J. Wagner 2000. Mixing of rotary-kiln fines with finegrained acid-producing rock. Proc. 5th Int. Conf. on Acid Rock Drainage. SME, Littleton, CO. p. 901-910. 56 �Figure 1: Backscattered electron (BSE) image of pyrrhotite from reactor (0.8 g RK fines addition) showing complex oxidation (darker regions) along grain edge and fractures (scale = 50 µm). Figure 2: BSE image of pyrrhotite with a rind of sulfur-bearing iron oxide from reactor with 7.9 g limestone addition. Shattering of grain occurred during sample preparation (scale = 50 µm). Figure 3: Extent of pyrrhotite oxidation as a function of reactor conditions and water quality data. The degree of oxidation increases from unoxidized pyrrhotite (unleached original) to sulfur-bearing iron oxide (>95% of control reactor). Weeks pH<6 is the number of weeks of leaching under acidic (pH<6) conditions. Alkaline Addition (g) is the total amount of Rotary Kiln Fines or limestone added to a reactor. Total S Leached (%) is the total sulfur measured in the reactor leachate expressed as a percentage of the initial sulfur present in the solid sample prior to leaching. 57 �DOCUMENTING UNDERGROUND MINE WORKINGS ON THE MESABI IRON RANGE IN GIS FORMAT ORESKOVICH, JULIE A., Natural Resources Research Institute, Duluth, MN 55811 (joreskov@nrri.umn.edu) Seventy years of mining natural iron ore by underground methods on the Mesabi Iron Range of northeastern Minnesota has resulted in an extensive network of subsurface voids along much of the Range. Subsidence resulting from these voids is manifested today as sinkhole depressions, pools, ponds, lakes and, most notably, road and structure damage. These voids impact the area hydrology and pose unknown risks to existing and potential surface developments, as well as to present-day mining operations. Because towns grew up adjacent to the early mines, today’s Range cities are often located, at least in part, on top of the Biwabik Iron Formation (BIF). Underground workings faded from the collective memory as generations passed and open pit mining became the norm, with an occasional jolt to the public consciousness caused by a dropped highway lane, a shifted building, or a newly opened hole in the ground. However, with prospects for several major developments on the Range, along with an expected population increase, planning groups such as the Central Iron Range Initiative (CIRI) have recognized the need to factor the underground mine workings into planning and design work in the region. The Central Iron Range Sanitary Sewer District (CIRSSD) has secured funding for the Minnesota Department of Natural Resources (MN DNR) to research the underground mines of the central Mesabi Range. CIRSSD’s study area extends from the Itasca county line on the west (T 57 N, R 21 W) to midway between the cities of Buhl and Mountain Iron on the east (T 58 N, R 19 W), following the trend of the Biwabik Iron Formation. Nearly 150 natural ore mines (underground, open pit, or both) operated in this area. Mining of natural iron ore on the Mesabi Range began with underground workings in the early 1890’s. The last of the underground mines, the Godfrey Mine, closed in 1963. The history of each mine in the study area can be traced through mining directories dating back to 1922. Data, including land parcels, operators, dates of operation, type of operations, fee interests, and lessees, is captured and entered into an Access database. Mine maps are collected from numerous sources, including mining companies, fee representatives, the MN Department of Revenue, the MN DNR map collection, and the archives of the Iron Range Resource Center (IRRC) located at Ironworld in Chisholm, MN. These maps are digitally scanned, and then rectified and digitized in ArcGIS. A unique USX aperture card collection housed at the IRRC contains annual level maps for Oliver Iron Mining Company operations dating back to 1903, enabling 3-D renderings of these mines. The end product of this research will be a GIS database that will be made accessible to counties, local communities, development organizations and mines for planning purposes. 58 �Rare Earth Element Patterns in Steep Rock Carbonates Noah Planavsky: Rosenstiel School of Marine and Atmospheric Sciences, 4600 Rickenbacker Causeway Miami FL 33149. nplanavsky@rsmas.miami.edu. Jennifer Murphy: Eastern Mineral Resources, USGS. Reston, VA 20194. jmurphy@usgs.gov One of the most dramatic changes in Earth’s history is the shift from a reducing to an oxidizing atmosphere. Multiple lines of evidence suggest the atmosphere first became oxygenated around 2.4 billion years ago. The evolution of oxygenic photosynthesis is ultimately responsible for the Earth’s oxygenation. The timing of the emergence of the earliest oxygenic photosynthetic organisms, likely cyanobacteria, is typically thought to predate the rise the atmospheric oxygen. Thus there exists the possibility for redox disequilibrium between ocean atmospheric systems; local areas of high photosynthetic productivity could induce local oxidizing conditions under an overall reducing atmosphere. Rare earth element (REE) trends can be used to explore paleoredox conditions. Modern oxygenated oceans display a strong negative cerium anomaly when normalized to shale composites, reprehensive of bulk crustal values, (Byrne and Sholkovitz, 1996) while anoxic or microaerophilic conditions display a positive or no cerium anomaly (shale normalized). The negative cerium anomaly is largely due to the oxidation of Ce3+ to Ce4+ and its preferential removal from the water column onto Mn-Fe oxide particles or as precipitate. In suboxic and anoxic waters Ce anomalies are smaller due to reductive dissolution and redox cycling of settling Mn- and Fe-rich particles within the suboxic and anoxic zones, respectively. Microbial induced carbonate precipitates and carbonate cements have been shown to faithfully record seawater REE values in modern reefs and can be used as a paleoredox proxy (Kamber and Webb, 2001). REE provide a unique tool at basin scale redox cycling in ancient environments. The approximately 2.9 Ga Steep Rock Group is one of the oldest extensive carbonate successions with a low metamorphic grade and a high state of preservation. The Steep Rock Group, therefore, is an ideal place to explore the redox state of mid-Archean marine environments. Stromatolites and abiogenic carbonate cements from various depositional settings do not contain negative cerium anomalies. The trace element systematics, traditional stable isotopes, and carbonate textures indicate exceptional preservation and rule out the possibility of a diagenetic overprint. Since Ce is preferentially released during anoxic diagenesis (Byrne and Sholkovitz, 1996), the cerium anomalies observed in the siliceous stromatolites could not have formed during early anoxic diagenesis. Thus despite apparently high biological productivity, noted by the abundance of stromatolites and organic matter, the Steep basin was anoxic or suboxic. The lack of negative cerium anomalies in previously investigated early Archean stromatolites has been used to argue for absence of cyanobacteria (e.g., Tice and Lowe, 2005). Basic modeling of reductant and oxidant fluxes at the basin scale, rather than the more commonly modeled global 59 �scale, provides atmospheric scenarios in which oxygenic photosynthesizing communities could exist in reducing conditions. References: Byrne R. H. and Sholkovitz E. R. 1996. Marine chemistry and geochemistry of the lanthanides. In Handbook on the Physics and Chemistry of Rare Earths (eds. K. A. Gschneidner, Jr. and L. Eyring), Vol. 23, pp. 497–593. Elsevier, Amsterdam. Kamber, B.S. and Webb, G.E. 2001. The geochemistry of late Archaean microbial carbonate: Implications for ocean chemistry and continental erosion history. Geochimica et Cosmochimica Acta. 65: 2509-2525. Tice, M.M. and Lowe, D.R. 2006. Hydrogen-based carbon fixation in the earliest known photosynthetic organisms: Geology, 34: 37-40. 60 �New Insights into the Metallogeny of the Eastern Portion of the Archean Uchi Domain, Superior Province, Ontario PUUMALA, Mark A., Ontario Geological Survey, Ministry of Northern Development and Mines, Suite B002, 435 James Street South, Thunder Bay, ON P7E 6S7 Canada In 2005, the Ontario Geological Survey began the Far North Geological Mapping Initiative (FNGMI). The goal of this program is to obtain a better understanding of the geological history and mineral resource potential of the portions of Ontario that are located north of 51ºN latitude. The Far North is a remote area that has a much smaller historical geological database than the southern portions of Ontario. Nevertheless, this region does have a long history of mineral exploration that has resulted in the collection of much valuable information about the nature of economic mineralization. These data are currently being compiled to evaluate metallogenic patterns that can in turn be used to better understand the regional tectonic framework and economic mineral distribution. Figure 1. Map illustrating Far North Mineral Deposit Compilation study area. To date, geological data have been compiled for mineral occurrences located within the eastern portions of the Archean Uchi Domain of the Sachigo Superterrane (Stott and Rainsford, 2006), of the Superior Province (Figure 1). The majority of the mineral deposits in this area are located within or immediately adjacent to supracrustal rocks of the Miminiska-Fort Hope, Pickle Lake, Lake St. Joseph, Meen-Dempster and Lang Lake greenstone belts of Stott and Corfu (1991). A wide variety of mineral deposit types have been identified in the study area, including: vein and replacement gold; polymetallic vein; mafic to ultramafic intrusion-hosted coppernickel-platinum group elements (PGE); rare-element-bearing pegmatite; volcanogenic massive sulphide (VMS) copper-zinc; intrusive porphyry-related copper-molybdenum-gold; and Algomatype banded iron formation. Although the individual greenstone belts listed above are useful geographic subdivisions, it is perhaps more useful to ascribe styles of mineralization to individual assemblages that were deposited during a discrete interval of time in a common depositional or tectonic setting (Stott 61 �and Rainsford 2006). The relationships between mineral occurrences and the individual tectonic assemblages have been used to obtain information regarding the possible locations and timing of significant metallogenic events in the study area. Preliminary observations suggest the following sequence of metallogenic events: 1. Mafic to ultramafic intrusion-hosted Cu-Ni-PGE mineralization in rocks of the >2860 Ma Pickle Crow assemblage in the Pickle Lake and Miminiska-Fort Hope belts; 2. VMS-type mineralization associated with rocks of the 2825-2842 Ma Kaminiskag (former Woman) Assemblage in the Pickle Lake and Meen-Dempster belts; 3. Later VMS-type mineralization associated with rocks of the <2744 Ma Confederation Assemblage in the Lang Lake, Meen-Dempster, and Pickle Lake greenstone belts, and with rocks of the <2723 Ma St. Joseph Assemblage in the Miminiska-Fort Hope Greenstone Belt; 4. Copper-nickel-PGE mineralization associated with late-tectonic mafic (commonly anorthositic) intrusions located near the southern boundary of the Miminiska-Fort Hope Greenstone Belt, and in the Lang Lake Greenstone Belt near the Bear Head Fault Zone; and 5. Structurally controlled gold mineralization that was likely to have been associated with the 2.72-2.70 Ma collision between the Uchi Domain and the Winnipeg River Terrane (Percival et al. 2006). In the Pickle Lake Greenstone Belt, this event post-dates Confederation Assemblage volcanism (>2739 Ma) and pre-dates the emplacement of the post-tectonic 2697-2716 Ma Hooker-Burkoski stock (Young et al. 2006). The metallogeny of other deposit types in the study area is currently more difficult to interpret due to the small number of known occurrences and/or a lack of geological data. However, as additional information is collected through FNGMI mapping projects, it is anticipated that the tectonic events associated with porphyry copper mineralization observed in the Lang Lake Belt and polymetallic veins found in the North Bamaji Lake area, for example, can be determined. References Percival, J.A., Sanborn-Barrie, M., Skulski, T., Stott, G.M., Helmstaedt, H., and White, D.J. 2006. Tectonic evolution of the western Superior Province from NATMAP and Lithoprobe Studies; Canadian Journal of Earth Sciences, Vol. 43, p. 1085-1117. Stott, G.M. and Corfu, F. 1991. Uchi Subprovince; in Geology of Ontario, Ontario Geological Survey, Special Volume 4, Part 1, p. 145-236. Stott, G.M. and Rainsford, D.R.B. 2006. The Precambrian geology underlying the James Bay and Hudson Bay lowlands as interpreted from aeromagnetic data and a revised terrane map for Northwestern Ontario; in Summary of Field Work and Other Activities 2006, Ontario Geological Survey, open File Report 6192, p. 13-1 to 13-10. Young, M.D., McNicoll, V., Helmstaedt, H., Skulski, T., and Percival, J.A. 2006. Pickle Lake revisited: New structural, geochronological and geochemical constraints on greenstone belt assembly, western Superior Province, Canada; Canadian Journal of Earth Sciences, Vol. 43, p. 821-857. 62 �MICHIGAN KIMBERLITES REVISITED: NEW MINERAL, CHEMICAL AND PETROGRAPHIC ANALYSES Quigley, P. O., Department of Geology and Geophysics, University of Minnesota, Minneapolis, MN 55414, quig0026@umn.edu Over twenty kimberlite pipes have been reported in the Western Upper Peninsula of Michigan (Jarvis, 1993). Most of the known activity in assessing these pipes took place during the 1980’s, and therefore did not include classification according to currently available mineral chemical and petrographic methods. Drill core samples held by the Michigan DNR from several pipes across the Michigan kimberlite field therefore are presently being analyzed in order to provide a further assessment of diamond grade prospects based on mineral chemistry, as well as an updated petrographic classification. Data collection consisting of electron microprobe analyses of garnet, ilmenite, and pyroxene from heavy mineral concentrates have been completed for three of the kimberlites, and have shown a high degree of variability. Two of the intrusions appear to have sampled a portion of the mantle within the diamond stability field, due to the presence of harzburgitic garnets, and other favorable compositions, according to the Grütter et al. (2004) classification scheme. Overall diamond grade is, however, inferred to likely be economically insignificant as the proportion of diamond indicator garnets to overall garnets is moderate to low. Analyses currently in progress will further examine degree of variability in mineral chemical analyses among these kimberlite intrusions, as well as examination of polished thin sections to permit use of textural features to categorize the intrusions as crater, diatreme, or hypabyssal facies kimberlite. REFERENCES Grütter, S. H., Gurney, J.J., Menzies, H.A., and Winter, F., 2004. An updated classification scheme for mantle-derived garnet, for use by diamond explorers. J. Barry Hawthorne Volume Proc. 8th Int. Kimb. Conf., pp 841-857. Jarvis, William, 1993. Michigan Kimberlites: An Update. Abstract at Prospectors and Developers Association of Canada 61st Annual Meeting (Paper M-10). 63 �Elemental and Isotopic Shallow Lake Proxies of Landscape Changes in the Prairie Pothole Region of Minnesota Tommy Rodengen1, Kevin Theissen1, and Shinya Sugita2 1 Department of Geology, University of St. Thomas, 2115 Summit Ave, St. Paul, MN 55105 2 Department of Ecology, Evolution, and Behavior, University of Minnesota, 1987 Upper Buford Circle, St. Paul, MN 55108 Investigations of the manipulation of the landscape around lakes have shown to have profound impact on their sedimentary records (1). We have explored lake sediment cores with records spanning the last two centuries and correlated changes in proxy data to large shifts in landscapes around lakes in Western Minnesota. In February 2006 we collected sediment cores from Lee, Bore, 8-Mile and Sweet lakes in the Prairie Pothole region of Minnesota. Cores were sampled for organic matter (SOM) at 1 cm intervals, dried, ground, weighed and treated with sulfurous acid (2). These samples were sent to the Stanford University Stable Isotope laboratory and analyzed for elemental (TOC, TN, C/N) and stable isotopic (δ13C and δ15N) values. Lastly, pollen was extracted from sediments using standard palynological methods (3). Over the past 6 months I have analyzed a set of aerial photographs spanning 1938-present for the region of interest. δ13C (-29 to -22 per mil) and C/N (5-17) values reflect a dominant algal source in the sediments but also show times of mixing and terrestrial sourcing across the lake core record (4). These values suggest changes in productivity and/or changes in sourcing organic matter. We used sharp increases in the abundance of ragweed (ambrosia) as a marker of the cultural horizon (~1850 A.D.) (5). We believe the sharp changes in N isotopes indicate land use change by humans and the addition of fertilizers. This is illustrated by a large δ15N spike between 39cm and 21cm depth (Figure 1), which if you assume a linear rate of sedimentation (.224 cm/yr) is between ~1832 A.D. and ~1912 A.D. According to our earliest records of aerial photography these years were a time of extreme drought and farming of fertile lake soil was common, directly introducing high levels of nitrogen to the sedimentological record. The importance of human manipulation of the landscape, specifically synthetic fertilizer, appears to have a clear effect on the sedimentological record found in these lakes. References: (1) Lamb, H.F., Damblon, R.W., Maxted, R.W., 1991, Human Impact on the Vegetation of the Middle Atlas, Morocco, During the Last 5000 Years. J. Biogeography, 18, 1-14. (2) Verardo, D.J., Froelich, P.N., McIntyre, A., 1990, Determination of organic carbon and nitrogen in marine sediments using Carlo-Erba NA1500. Deep-Sea Res., 37, 157165. (3) Faegri, K. Iversen, J., 1989. Textbook of Pollen Analysis. 4th ed. J. Wiley & Sons, Chichester. 328pp. 64 �(4) Meyers, P.A., and Lallier-Verges, E., 1999, Lacustrine sedimentary or Late Quaternanry paleoclimates. J. Paleolimnology, 21, 345-372. (5) Jacobson, G.L. and Grimm, E.C., 1986. A numerical analysis of Holocene forest and prairie vegetation in central Minnesota. Ecology, 67, 958-966. Figure 1: 8-Mile lake δ15N values 8-Mile Lake 0 10 Depth (cm) 20 30 40 50 0.00 60 1.00 2.00 3.00 4.00 5.00 6.00 δ N 15 65 7.00 8.00 9.00 10.00 �BEDROCK FRACTURES IN SOUTHEASTERN WISCONSIN: PALEOSTRESS ESTIMATES AND RELATIONSHIPS TO THE WAUKESHA FAULT RYMASZEWSKI, Jody A., FRIEDRICH, Jason L., and CZECK, Dyanna M. Department of Geosciences, University of Wisconsin – Milwaukee, P.O. Box 413, Milwaukee, WI 53201 jody@uwm.edu INTRODUCTION The Waukesha Fault is an enigmatic structure cutting through Silurian dolomite in southeastern Wisconsin. It is a normal fault, oriented ~N40E, 60SE with an apparent offset of 10 m (Mikulic and Mikulic, 1977; Sverdrup et al., 1997). It is only known to outcrop at the Waukesha Lime and Stone Co. quarry in Waukesha, Wisconsin. Detailed gravity surveys in the region reveal the lateral and vertical extent of the fault (Sverdrup et al., 1997; Skalbeck et al., 2006). The fault trace extends NE to the town of Port Washington, Wisconsin on the shore of Lake Michigan (Sverdrup et al., 1997) and can be traced to depths greater than 600m (Skalbeck et al., 2006), showing that it is a major feature of an otherwise undeformed region. The goals of our study are to 1) estimate the paleostress orientations that formed the Waukesha Fault, 2) measure the orientations of nearby small-scale fractures, 3) estimate the paleostress orientations that formed the small-scale fractures, and 4) compare the paleostress orientations for all the structures. METHODOLOGY The one known exposure of the Waukesha Fault is currently not available for active study. Therefore, we conducted paleostress orientation estimates on the Waukesha Fault, assuming its geometry matched that predicted by Sverdrup et al., 1997. We measured the orientations of 158 fractures in Silurian bedrock at the Lannon Stone Products quarry (located in Lannon, Wisconsin: ~11km NE of the known Waukesha Fault outcrop and ~3 km NW of the Waukesha Fault trace) during field seasons from 2004-2006. We also measured the orientations of 38 fractures in Devonian bedrock at the Harrington Beach State Park abandoned quarry and beach outcrops (located ~ 10km NE of Port Washington, and ~2.5 km NW of the Waukesha Fault trace). We used standard stereonet procedures to calculate the orientations of the paleostresses likely to have formed the Waukesha fault and the fractures at each of the field locations. RESULTS Based on the assumed geometry of the Waukesha Fault, σ1, the maximum principal stress was oriented vertically, σ2, the intermediate principal stress, was oriented (plunge/trend) 0°/040 σ3, the minimum principal stress, was oriented 0°/130. The Lannon quarry contains fractures with no discernable offset, and faults with small-scale (maximum ~4 cm, mostly normal sense) offsets. Some rare fractures have preserved ridge and groove lineations or plumose features, allowing classification as shear fractures or extensional fractures, respectively. The fractures at the top of the quarry have a random distribution. The small faults found at the low-mid levels of the quarry exhibit an approximate N32E/56SE 66 �orientation. Most of the fractures with no apparent offset at mid and lower levels have the approximate orientation N30E/65S; a second group has approximate orientation of N45W/ steepsubvertical. Cross-cutting relationships between the two sets are inconclusive. Two prominent fracture sets formed at Harrington Beach State Park. The approximate orientations of these two sets are 1) N74E/subvertical and 2) N25W/ subvertical. Unfortunately, the textures of most fracture surfaces are too weathered to determine whether the fractures are extensional or shear fractures. Thus, the paleostress estimates must be partly based on the relative geometries. The small faults at the Lannon quarry have estimated paleostress orientations: σ1 = vertical; σ2 = 0°/040, σ3 = 0°/130. Most of the fractures at the Lannon quarry have a similar orientation (but often with slightly steeper dips) to the small faults. Therefore, we interpret that most of these fractures are small extensional or shear fractures with the same paleostress orientations as the small faults. The second group of fractures at the Lannon quarry could either be extensional or shear fractures, but lack of any offset supports the likelihood that they are extensional fractures. If so, the paleostress orientations are σ1 = vertical; σ2 = 0°/315, σ3 = 0°/225. The fractures at Harrington Beach could either be two distinct sets of extensional fractures or a conjugate set of shear fractures. If they are extensional fractures, the paleostress orientations are 1) σ1 = vertical; σ2 = 0°/074, σ3 = 0°/164 and 2) σ1 = vertical; σ2 = 0°/335, σ3 = 0°/245. If they are shear fractures, the paleostress orientations are σ1 = 0°/114 σ2 = vertical, σ3 = 0°/024. CONCLUSIONS While the paleostresses on all fractures could not be conclusively determined, the paleostress orientations inferred from most of the geometries of the small-scale features in the Silurian bedrock at Lannon are consistent with those estimated for the Waukesha Fault. However, the paleostress orientations of the Devonian rocks at Harrington Beach are inconsistent with those estimated for the Waukesha Fault. Therefore, it seems likely that 1) the timing of the Waukesha Fault may be bracketed by the deposition of the Silurian rocks at Lannon and the Devonian rocks at Harrington Beach, or 2) the deformation associated with the Waukesha Fault encompassed a broader region in the south. Further study of fractures in the region is required to test these hypotheses. REFERENCES Mikulic, D.G., Mikulic, J.L., 1977. History of geologic work in the Silurian and Devonian of southeastern Wisconsin: Guidebook 41st annual tri-state field conference, A19-A27. Skalbeck, J.D., Couch, J.N., Helgesen, R.S., Swosinski, D.S., 2006. Coupled modeling of gravity and aeromagnetic data to estimate subsurface basement topography in southeastern Wisconsin. Geoscience Wisconsin 17, 53-64. Sverdrup, K.A., Kean, W.F., Herb, S., Brukardt, S.A., Friedel, R.J., 1997. Gravity signature of the Waukesha Fault, southeastern Wisconsin. Geoscience Wisconsin 16, 47-54. 67 �Precambrian Geology of the Opelt Quarry, Neillsville, Wisconsin SAXTON, Samantha and CORDUA, William, Department of Plant and Earth Science, University of Wisconsin- River Falls, River Falls, WI 54022, samantha.saxton@uwrf.edu, william.s.cordua@uwrf.edu Near Neillsville, Clark County, Wisconsin, a quarry owned and operated by Opelt Sand and Gravel exposes a complex group of rocks dominated by amphibolites but including biotite gneisses, granitic to dioritic plutons, intrusive breccias and chlorite-rich shear zones. Given the quarry's location south of the Eau Plaine shear zone, it gives an important fresh exposure of the rocks of the Marshfield Terrain in central Wisconsin. We did field reconnaissance at the quarry combined with thin section, geochemical and structural analyses to study the metamorphic history. Published research for the surrounding area suggests the Opelt rocks are associated with an accreting micro-continent, which formed the foundation for later deformation during the Penokean orogeny (1880 - 1830 mya) and perhaps younger metamorphic events (Holm and Schneider, 2007). Previous work performed by Maass (1983) on rocks within 10 miles of the quarry using U-Pb zircon dating gives ages in a range of 1875 to 1820 million years old. A study by Sims and Peterman (1980) using a Rb-Sr whole-rock technique places them at roughly 1885 +/- 65 million years, correlating with the Penokean Orogeny. Thin section analysis showed the common mineral assemblage in the gneisses and amphibolites to be a combination of plagioclase, biotite, hornblende, and clinozoisite, with several minor minerals such as calcite, chlorite, and epidote among others. Within the granitic samples, quartz, plagioclase, and clinozoisite become the dominant minerals, with the same range of minor minerals. Veins of quartz and calcite are found cross-cutting the granites. Replacement textures were evident from the thin sections. Amphibolite samples display replacement rims or coronas of titanite surrounding the opaque ilmenite (Figure 1). We also see replacement relationships between several sets of minerals: primarily clinozoisite, muscovite, epidote and/or calcite replacing plagioclase feldspar, and chlorite replacing biotite. These two suites of minerals most likely represent a retrograde metamorphic event. Figure 1: Amphibolite showing titanite replacing the opaque mineral, ilmenite, seen in the middle of the rings. Geochemical analysis using discrimination diagrams derived from Pearce et al. (1982) and Gomez-Pugnaire et al. (2003) proved to be helpful in defining the protolith of the assorted rocks. A discrimination diagram for granitic rocks, based on the ratio of Rb versus Y + Nb, allocates samples to three origins- anorogenic, oceanic ridge, and island arc. Our data points, two granites and a diorite, indicate within a reasonable margin of error, a calc-alkaline island arc origin for the plutonic rocks found in the quarry. In addition, 68 �several diagrams from Pearce (1982) show the igneous rocks that were protoliths for the amphibolites to be tholeiitic volcanic or island arc basalts. Maass (1983) describes several folding events within in nearby rocks: F-1- isoclinal folds, to F-3 - very open to tight folds, as well as a mineral lineation associated with the F-3 folding. He also proposes that the plutonic activity of the area seems to have started at the latter part of the F-1 folding and continued past the end of the F-3 event. (Maass, 1983) Many of these same features can be seen in the Opelt Quarry. Field, thin section, and geochemical data suggest the following sequence of events in the Opelt rocks: 1. Amphibolite protolith = tholeiitic island arc basalts 2. Penokean Orogeny ~ Metamorphic Event #1 - Amphibolite Facies (Hb, plag) Occurrences F-1 Foliation F-2 Veins and Granitic Intrusions ~ Metamorphic Event #2 - Greenschist Facies (Chl, epi) Occurrences F-3 Foliation ~ Formation of Shear Zones ~ Veins- quartz, calcite, epidote ~ Multiple series of faulting 3. Deposition of Younger Sediments/ Later Glacial Sediments The later folding and retrograde metamorphism events could be late stage Penokean orogeny, or could be related to the Yavapai (1800-1750 mya) and Mazatzal (1650 mya) events that took place post-Penokean Orogeny (Holm and Schneider, 2007). Many structures and features originally attributed to the Penokean Orogeny now may actually be an overprint from later Yavapai and Mazatzal events (Cannon and Schulz, 2007). Radiometric work and further geochemical analysis will help resolve these questions. Cannon, W. and Schulz, K., 2007, The Penokean Orogeny in the Lake Superior Region (abst.): The Geological Society of America Abstracts with Programs, 41st Annual Meeting, Lawrence, KS, v. 39, p. 76. Gomez-Pugnaire, M.T., Azor, A., Fernandez-Soler, J.M. and V. Lopez Sanchez- Vizcaino, 2003, The amphibolites from the Ossa-Morena/Central Iberian Variscan suture (Southwestern Iberian Massif): geochemistry and tectonic interpretation: Lithos, v. 68, p. 23-42. Holm, D. and Schneider, D., 2007, Metamorphic Record of the Yavapai and Mazatzal accretion in the upper Great Lakes region, U.S. and Canada (abst.): The Geological Society of America Abstracts with Programs, 41st Annual Meeting, Lawrence, KS, v. 39, p. 76-77. Maass, R.S., 1983, Early Proterozoic tectonic style in central Wisconsin: Geological Society of America Memoir, v. 160, p. 85-95. Pearce, J.A., 1982, Trace element characteristics of lavas from destructive plate boundaries, In: Thorpe, R.S. (ed.) Andesite, p. 525-548. Sims, P.K. and Peterman, Z.E., 1983, Evolution of Penokean foldbelt, Lake Superior region, and its tectonic environment: Geological Society of America Memoir, v. 160, p. 3-14. 69 �Revised Stratigraphy of the Biwabik Iron Formation, Mesabi Range, Minnesota – Developing the “Rosetta Stone” SEVERSON, Mark, J. and HEINE, John, J., Natural Resources Research Institute, 5013 Miller Trunk Highway, Duluth, MN 55811 The NRRI is currently evaluating the possibility of using specific waste rock products from taconite mining as aggregate materials. Paramount to defining specific horizons with good aggregate potential, a better understanding of the stratigraphy of the iron-formation has been accomplished through detailed logging of drill holes and detailed in-pit mapping (where core are unavailable). To date, 130 holes have been logged from seven areas that include: CliffsErie site (old Erie/LTV mine), Biwabik area (Mittal Steel’s planned “east reserve”), Laurentian Mine, United Taconite, Minntac, Hibtac, and the oxidized taconite of the Coleraine area. In each of these areas, the stratigraphy, as determined by bedding types in each of the holes, is compared to the stratigraphy as defined by each of the mining companies. Through this approach, a new picture of the iron-formation layering is evolving and for the first time it has become possible to correlate the mining units of one mine to those in adjacent mines - even though each of the mines uses a different stratigraphic terminology. In essence, this work has set forth the beginnings of a “Rosetta Stone” whereby ore and waste horizons can be better understood. Hopefully, the “Rosetta Stone” can eventually be used to aid in determining the depositional environments of internal units within the iron-formation. All descriptions of the Proterozoic iron-formations consistently attribute thin-bedded ironformations as reflecting deposition in a deep water environment, and granular, variably-bedded iron-formations as reflecting deposition in shallower water. Overall, the iron-formation has been divided into four members consisting of Lower and Upper Cherty members (regressive sequences), and Lower and Upper Slaty members (transgressive sequences). Using these simple concepts, coupled with other bedding types as seen in drill core, several unique relationships have been defined along the length of the Mesabi Range that include: • The Lower Cherty is the most consistent member of the iron-formation and consists of a single shallowing-upward parasequence consisting of thin-bedded iron-formation at the base grading upwards through sequences of regular-bedded, wavy-bedded, irregular-bedded, and thick-bedded granular iron-formation units. • The top of the Lower Cherty consists of an iron-poor, variably-bedded, granular chert that is currently being used as road aggregate, e.g., the “Mesabi Select” material from United Taconite’s Thunderbird North mine. This unit is inferred to have been deposited in shallow water near the edge of the shelf. • The base of the Lower Slaty is the most persistent marker bed in the iron-formation and at most locales consists of carbonaceous mudstone referred to as the “Intermediate Slate.” Tuffaceous units are reported to be present at the base of the Lower Slaty but have yet to be encountered in the drill holes looked at in this study. • For the most part, the Lower Slaty consists of a thick sequence of thin-bedded, ironcarbonate and iron-silicate iron-formation that was deposited in deep water. However, in the “east reserve” area, lenses of granular iron-formation (deposited in shallow water and similar to the “Mesabi Select” unit of the Lower Cherty) are commonly interbedded with the thin-bedded rocks of the Lower Slaty. Internal structures within these lenses 70 �• • suggest that they formed from slumpage of shelf material into the basin; possibly related to large storm events or earthquakes during periods of tectonic instability. The top of the Lower Slaty member is perhaps the most poorly defined as it is transitional into the overlying Upper Cherty member. This is most evident in the Virginia Horn area where both slaty and cherty iron-formation types are common and alternate at all scales. This has lead to stratigraphic inconsistencies wherein the actual division between the Upper Cherty and Lower Slaty members varies drastically from mine to mine. Recent stratigraphic correlations suggest that the upper portion of the Lower Slaty contains several laterally-restricted tongues/channels of Upper Cherty-type beds that have locally been referred to as “interbedded cherts” or IBCs. These IBCs appear to be related to several regressive/transgressive events and represent several small parasequences at the top of the Lower Slaty. The Upper Cherty member is profoundly different from the Lower Cherty in that it does not display a consistent stratigraphy nor is a shallowing-upward sequence evident. Furthermore, the Upper Chery is commonly “oolitic.” These relationships suggest that the Upper and Lower Cherty members were deposited in distinctly different depositional environments. While development of the “Rosetta Stone” adequately displays the various differences of the Biwabik Iron Formation along the entire length of the Mesabi Range, only continued logging of holes will aid in exhibiting the 3D nature of the iron-formation. When that point is reached, a more complete analysis of the actual depositional environments can be ascertained. 71 �Investigation of Ferromanganese Nodule Precipitation and Arsenic Uptake in Modern Lacustrine Biochemical Sediments Stevens, Larissa B. and Fralick, Philip, Department of Geology, Lakehead University, Thunder Bay, Ontario, Canada, (larissa.stevens@lakeheadu.ca) This study was conducted to evaluate the environmental setting, geochemistry and formational processes involved in the precipitation of lacustrine ferromanganese nodules, and to investigate the mechanisms involved in arsenic uptake in these deposits. The research was conducted on precipitates in Lake Charlotte, Nova Scotia. However, the findings are applicable to the Lake Superior region as ferromanganese deposits have been found in a number of temperate North American lakes and may be present in many more. Ferromanganese deposits in the form of nodules, coated sand and layers in the sediment are present in all the Great Lakes (Sly and Thomas, 1974; Callender, 1970). Mothersill and Shegelski (1973) studied iron- and manganese-rich layers in Lake Superior near Thunder Bay. Ferromanganese concretions have also been found in inland lakes of the Lake Superior Region including: Lake Schebandowan, Ontario (Carpenter et al., 1972), Trout Lake, Wisconsin (Twenhofel et al., 1945) and the Minnesota Lakes (Zumberge, 1952). The layering styles of ferromanganese nodules can be used to infer the approximate position of the redox boundary where precipitation is occurring. The concretions found in Lake Charlotte, NS, developed on the bottom in a shallow lacustrine setting. These deposits consist of 1 cm thick crusts divisible into 1) a lower zone of iron precipitates cementing siliciclastic sands; 2) thin laterally continuous laminae of manganiferous and ferric precipitates; 3) micro-stromatolites composed of manganiferous and ferric precipitates and 4) grumulous and clotted textured manganiferous and aluminous precipitates. These deposits formed by the interaction of oxygenated lake water with diffuse reduced groundwater seeps on the sandy bottom. The development of microbial mats and presence of stromatolites and clotted precipitates indicates biochemical processes played a role in the formation of the crusts. The ferromanganese nodules are accumulating arsenic, with concentrations ranging from ~ 670 to 1400 ppm. Elevated arsenic concentrations were also found in groundwater samples suggesting that the arsenic is entering the lake from the groundwater seeps. Microbiological experiments investigating bacterial involvement in arsenic coprecipitation have found that no arsenic oxidizers were present. However, iron oxidizers are. This suggests a possible mechanism whereby: the iron oxyhydroxide precipitates were formed, at least in part, by the iron oxidizers in the microbial mats on the bottom. As the iron oxyhydroxide precipitated from the diffuse groundwater flow entering the oxygenated lake it scavenged the arsenic that was also being transported in solution. 72 �References Sly, P.G. and Thomas, R.L., 1974. Review of geological research as it relates to an understanding of Great Lakes limnology. J. Fish. Res. Bd Can., 31: 795-825. Callender E., 1970. The economic potential of ferromanganese nodules in the Great Lakes. Proc. 6th Forum Geol. Ind. Miner. Michigan Geol. Surv. Misc., 1: 55-65. Mothersill, J.S. and Shegelski, R.J., 1973. The formation of iron and manganese rich layers in the Holocene sediments of Thunder Bay, Lake Superior. Can. J. Earth Sci., 10: 571-576. Carpenter, R., Johnson, H.P. and Twiss, E.S., 1972. Thermomagnetic behavior of manganese nodules. J. Geophys. Res., 77: 7163-7174. Twenhofel, W.H., McKelvey, V.E., and Feray, D.E., 1945. Sediments of Trout Lake, Wisconsin. Bull. Geol. Soc. Am., 56: 1099-1142. Zumberg, J.H., 1952, The lakes of Minnesota, their origin and classification. Bull. Minn. Geol. Surv., 35: 1-90. A C B D A) Picture of a typical ferromanganese nodule overgrowing a cobble. The small domes are stromatolites; B) Cross section of a nodule with a stromatolitic zone at the top; C) Reflected light photomicrograph of stromatolitic layering in a nodule. The round white areas are sand grains; D) Reflected light photomicrograph of the internal layering of a nodule. 73 �A REVISED TERRANE MAP FOR THE SUPERIOR PROVINCE AS INTERPRETED FROM AEROMAGNETIC DATA STOTT, Greg, Ontario Geological Survey, Sudbury, ON, P3E 6B5, greg.stott@ontario.ca CORKERY, Tim, Manitoba Geological Survey, Winnipeg, MB LECLAIR, Alain, Géologie Québec, Québec, QC BOILY, Michel, GÉON, Montréal, QC PERCIVAL, John, Geological Survey of Canada, Ottawa, ON The Superior Province has in the past been subdivided into “Subprovinces” characterized largely by contrasting lithology (e.g., Card and Ciesielski 1986). With an increasingly refined understanding of the tectonic assembly of the Superior Province through a progression of orogenies (Stott and Corfu 1988; Percival et al. 2006), supported by geochronological and SmNd isotopic data (e.g., Tomlinson et al. 2004; Rayner and Stott 2005), we can identify individual terranes and associated domains. Recent aeromagnetic interpretation of the Precambrian crust underlying the James Bay and Hudson Bay lowlands (Stott and Rainsford 2006), permits more confident terrane correlations across James Bay. Figure 1 illustrates a new map of the Superior Province in Canada with its principal tectonic subdivisions as currently used in Manitoba, Ontario and Quebec. Archean terranes are crustal blocks with geological histories substantially different from adjacent blocks, although parts of the late (Neoarchean) history of adjacent terranes typically overlap. They include one or more tectonic assemblages and plutonic suites, and at least parts of the terrane boundaries are clearly marked by major faults. Those regions with more complex Meso- to Neoarchean orogenic histories are superterranes thought to comprise at least two terranes. Domains are marginal parts of terranes or superterranes that are thought to have been added autochthonously or for which sparse evidence exists (e.g., Marmion domain) to treat them as separate terranes. Card, K.D. and Ciesielski, A. 1986. DNAG #1. Subdivisions of the Superior Province of the Canadian Shield; Geoscience Canada, 13, 5-13. Percival, J.A., Sanborn-Barrie, M., Skulski, T., Stott, G.M., Helmstaedt, H. and White, D.J. 2006. Tectonic evolution of the western Superior Province from NATMAP and Lithoprobe studies; Can. J. Earth Sci., 43, 10851117. Rayner, N. and Stott, G.M. 2005. Discrimination of Archean domains in the Sachigo Subprovince: a progress report on the geochronology; Ontario Geological Survey, Open File Report 6172, 10-1 to 10-21. Stott, G.M. and Corfu, F. 1988. Whither the Kenoran Orogeny?; Geological Association of Canada/Mineralogical Association of Canada Annual Meeting, Program with Abstracts, 13, A120. Stott, G.M. and Rainsford, D.R.B. 2006. The Precambrian geology underlying the James Bay and Hudson Bay lowlands as interpreted from aeromagnetic data and a revised terrane map for northwestern Ontario; Ontario Geological Survey, Open File Report 6192, 13-1 to 13-10. Tomlinson, K.Y., Stott, G.M., Percival, J.A. and Stone, D. 2004. Basement terrane correlations and crustal recycling in the western Superior Province: Nd isotopic character of granitoid and felsic volcanic rocks in the Wabigoon subprovince, N. Ontario, Canada; Precambrian Research, 132, 245-274. 74 �Figure 1 (next page). This map displays a composite of an aeromagnetic base and bedrock geology of the northern portion of the Archean Superior Province across Manitoba, Ontario and Quebec with terrane and domain boundaries. Names of superterranes, terranes and domains are as currently used in each of the provinces. Some features to note: - The older, Mesoarchean North Caribou terrane forms a core flanked to the north and south by outward additions of Meso- to Neoarchean crust across Uchi and Island Lake domains, which merge eastward where the North Caribou terrane pinches out. Uchi domain and Oxford-Stull domain merge under the James Bay Lowland; - A large indentor-like feature underlies the James Bay Lowland, flanked by faults; - English River and Quetico metasedimentary domains are separated east of the Wabigoon Subprovince by a ridge of felsic plutonic rocks, with high magnetic field intensity, extending westward from the Opatica gneisses of Québec. The Opatica terrane might extend to the Winnipeg River terrane where the latter is interpreted to underlie the northern part of the eastern Wabigoon Subprovince; - The Trans-Hudson Orogen underlies the northern half of the Hudson Bay Lowland; - Apparent reworked Archean Northern Superior superterrane crust under Hudson Bay Lowland within the Trans-Hudson Orogen; - Broad areas of pronounced aeromagnetic field intensity characterize large parts of the Northern Superior superterrane under the Hudson Bay Lowland in Ontario and correspondingly resemble the Pikwitonei gneisses, including the Assean and Split Lake blocks on strike in Manitoba. - Potential correlation of Northern Superior superterrane with the 3.8 Ga Nuvvuagittuq greenstone belt (David et al. 2002) in Tikkerutuk domain and the >3.5 Ga Assean gneisses (Böhm et al. 2000). - Areas interpreted to be part of the Paleoproterozoic Sutton Inlier are outlined where exposed. The Sutton “Inlier”, of shallow water platform sedimentary units and overlying gabbro sill, does not appear to be one large area but a set of inliers and should in future be referred to as the Sutton Inliers. The eastern Sutton Inliers with subsurface extensions are shallowly dipping, northward concave and crescent-shaped, based on aeromagnetic patterns, which correspond closely with the distribution of outcrops observed by Bostock (1970). They resemble “klippe” that might have been transported a short distance southwards. Folded strata related to the Sutton Inliers are shown aeromagnetically to extend discontinuously northwards towards the Hudson Bay coast upon apparently reworked Archean crust within the Trans-Hudson Orogen. The TransHudson Orogen, including areas of reworked Archean crust, appears to underlie the northern half of the Hudson Bay Lowland, based on interpretation of aeromagnetic images. 75 �76 Figure 1 Ibis map tJisiavs tha tectonic subdivision o!Lbe Archean Superior Province across Ntarutotm. Ontario and Quebec on a composite aeromaurtetic and hcdrocL aeolog\ hase Names arc as currently used or proposed In each of the pio inces- The northerriniost edge ol the Superior Ioeince in Maiinotxs and Ontario is locaIk o erprintcd hs the Paleoprocerozoic Trans-I [udson Orogeir �THERMAL EVOLUTION OF PROTEROZOIC (>1Ga) RHYOLITE MAGMA BASED ON ANALYSIS OF MELT INCLUSIONS AND TRACE ELEMENTS IN QUARTZ FROM THE KEWEENAW PENINSULA OF MICHIGAN STUDENT, James J., Central Michigan University, 314 Brooks Hall, Mount Pleasant, MI 48859, stude1jj@cmich.edu, WARK, D.A., Department of Earth and Environmental Sciences, Rensselaer Polytechnic Institute, Troy, NY 12180, MUTCHLER, S.R., Department of Geological Sciences, Virginia Tech, Blacksburg, VA 24061, and Bodnar, R.J., Department of Geological Sciences, Virginia Tech, Blacksburg, VA 24061. Melt inclusions (MI) form when small droplets (less than a few hundred microns in diameter) of magma become trapped inside growing crystals in magma. MI provide the best source of information concerning the chemical and physical evolution of a magma chamber. Glassbearing MI are common in quartz and zircon from porphyritic rhyolite occurrences in the Midcontinent Rift System, including rhyolite flows in the North Shore Volcanic Group, the Porcupine Volcanics, and the Portage Lake Volcanics on the Keweenaw Peninsula of Michigan. Porphyritic rhyolite cobbles are abundant in the Allouez conglomerate within the Portage Lake Volcanics, and the characteristics and geochemistry of MI and quartz phenocrysts contained in this conglomerate are described below. The MI have been categorized based on their phase assemblages and preservation style. Type 1 MI contain clear glass and a shrinkage bubble, Type 2 contain clear glass, a shrinkage bubble and 1 or more, coarser grained (> 3 um) crystals, and Type 3 MI are totally devitrified or otherwise breached. The MI range in size from 1 to over 200 um in diameter, and they typically have a negative hexagonal bi-pyramidal morphology. Cathodoluminescence (CL) observations of the quartz phenocrysts (n=42) reveal several periods of new quartz growth and dissolution on what we tentatively interpret to be nonmagmatic cores (Fig. 1A). In eight quartz phenocrysts from four texturally different porphyritic rhyolite cobbles, Ti (measured by EPMA) increases from about 50 to 220 ppm towards the rim in the magmatic growth zones, corresponding to a temperature increase from 675 to 850 °C, estimated using the TitaniQ method of Wark and Watson (2006) with a fixed TiO2 activity equal to 1. Zircon microphenocrysts and MI are distributed throughout the magmatic portions of the quartz phenocrysts. Type 1 and Type 2 MI were analyzed by EPMA and LA-ICPMS for major elements and 13 trace elements, including Zr. Zircon saturation temperatures calculated using the model described by Watson and Harrison (1983) based on Zr and major oxide concentrations in MI (n=11) from one cobble range from 670 to 866 °C and agree well with the Ti in quartz geothermometry from the same cobble (Fig. 1B). Petrographic observations and chemical analyses indicate that many (if not most) of the type 1 and 2 MI have remained chemically closed systems for over 1 Ga. Zoned calcite (imaged by CL) associated with native copper that is related to later non-magmatic mineralization has selectively replaced some of the feldspar in rhyolite cobbles from the Allouez conglomerate, and random fractures that are partially filled with calcite and secondary planes of fluid inclusions cut across the quartz phenocrysts and some Type 3 MI. The well-preserved growth zoning recorded by CL imaging of quartz is consistent with the preservation of other 77 �chemical characteristics in the MI. Based on both textural and chemical evidence, zircon and quartz remained saturated in the magma while temperatures increased by more than 175 °C sometime prior to eruption. Blundy et al., (2006) and Wark et al., (2007) recently presented evidence that the magma temperature increased during eruption at Mount Saint Helens WA and prior to the eruption of the Bishop Tuff at Long Valley CA. Quartz-bearing porphyritic rhyolite from the Keweenaw shows a similar thermal trend. The thermal and chemical evolution of the Midcontinent Rift System rhyolite magmas may be better constrained utilizing the chemical record preserved in MI, quartz, and zircon. Figure 1. A) CL image of a typical quartz phenocryst from porphyritic rhyolite cobbles of the Allouez conglomerate. Light and dark regions in the quartz correspond to high and low Ti concentrations, respectively. The dark region in the core has very low Ti concentrations (15 ppm) and may be “non-magmatic” based on TitaniQ temperature estimates (567 °C). B) Calculated zircon saturation temperatures from MI (left) and TitaniQ temperature estimates for quartz growth (right) for one Allouez conglomerate rhyolite cobble. References Blundy, J., Cashman, K., and Humphreys, M. (2006): Magma heating by decompressiondriven crystallization beneath andesite volcanoes. Nature, 442, 76-80. Wark, D.A, and Watson, E.B, (2006): TitaniQ: A Titanium-in-Quartz geothermometer. Contributions to Mineralogy and Petrology, 152, 743-754. Wark, D.A., Hildreth, W., Spear, F.S., Cherniak, D.J., And Watson, E.B. (2007): Preeruption recharge of the Bishop magma system. Geology, 34, 235-238. Watson, E.B, and Harrison, T.M., (1983): Zircon saturation revisited: temperature and composition effects in a variety of crustal magma types. Earth Planet. Sci. Lett., 64, 295-304. 78 �Pleistocene glaciation as a mechanism for emplacement of high-salinity groundwater at anomalously shallow depths in the Lake Superior basin MICHAEL L. TAYLOR and JOHN B. SWENSON Department of Geological Sciences, University of MinnesotaDuluth, Duluth, Minnesota Along the shores of Lake Superior, in northeast Minnesota and northern Wisconsin, highly saline groundwater exists at anomalously shallow depths of 30 meters or less, contaminating domestic wells and community water supplies. These saline waters, which produce significant hydrogeologic problems for determining locations for domestic water sources, appear geochemically similar to high salinity groundwater (Na-Ca-Cl brines) commonly found at depths greater than one kilometer on the Canadian Shield (Frape and Fritz, 1982). Currently there is little research that attempts to explain this phenomenon of anomalously shallow, highly saline groundwater from a hydrodynamic perspective. Most work completed thus far has been from a geochemical perspective. One recent hydrodynamic model suggests that glaciation acted as a mechanism for flushing deep brines closer to the surface: During the Pleistocene, the Superior Lobe of the Laurentide ice sheet repeatedly occupied the Lake Superior basin. The temperate (wet-based) lobe created a strong hydraulic-head gradient across the sedimentary and volcanic rock package beneath the ice. High hydraulic head existed down the axis of the lobe and decreased towards the margins. In response to this head gradient, meltwater at the base of the glacier may have been driven deep into the underlying rock package beneath the lobe axis, where it could mix with and flush brines to the basin margins. As the Superior Lobe retreated, the head gradient was removed and the high-salinity groundwater displaced to the lobe margins began to migrate (back-flush) to its original position deep within the basin. This study develops further this model and attempts to test it via a combination of mathematical modeling and geochemical fingerprinting. Mathematical modeling comprises the development of a relatively simple model of groundwater flow and solute transport to test basic physical plausibility of the ‘flushing’ model. The flushing model predicts that samples of groundwater should represent a combination of parent brine formed deep within the rock package underlying the Lake Superior basin, isotopically light, subglacial Pleistocene recharge, and modern meteoric recharge. Geochemically fingerprinting saline groundwater samples will test the model predictions which involved: (1) field work to sample wells in the Lake Superior basin, with emphasis on the south shore, where data are sparse, (2) stable isotope and major element analyses of the fluids, and (3) stable isotope and major-element analyses of the sedimentary rocks that comprise the Mid-continent Rift System. Reference: Frappe, S.K. and Fritz, P., 1982. The Chemistry and Isotopic Composition of Saline Groundwaters from the Sudbury Basin, Ontario. Canadian Journal of Earth Sciences, v. 19, p. 645-661. 79 �Possible influence of a buried fault on elevated indoor radon levels, south Washington County, Minnesota Stephanie A. Theriault and Dr. Thomas A. Hickson Geology Department, University of St. Thomas, St. Paul, Minnesota 55105 Correlation between high indoor radon levels and a local fault in south Washington County were found in a 2004-2005 study done by the Geology Department at the University of St. Thomas and the Washington County Public Health and Environment Department. The location of the fault was important to this study because radon characteristically can travel farther along leaky faults, which could explain the unusually high levels of radon in this specific area. However, the Minnesota Geological Survey revised their mapping shortly after the 2004-2005 study and did not include the specific fault on their new map. This research focuses on determining if there is evidence for the proposed fault. Stratigraphic offset, as seen in well logs, aid in determining the possible presence of the fault. Three cross sections were created across the proposed fault from well log data. Offset was found both in the Prairie du Chien and Jordan units across the proposed fault. Structure contour maps of the top of the Prairie du Chien and Jordan units also support the presence of this fault. In conclusion, evidence provided by the well logs from the study area suggests the possible presence of a fault and that it could be related to the unusually high indoor radon levels encountered in this area. 80 �POTENTIAL USE OF THE MIDCONTINENT RIFT FOR CO2 SEQUESTRATION Thorleifson, L. H., Minnesota Geological Survey, 2642 University Ave West Saint Paul, Minnesota 55114-1057 Increasing concern about climate change and reliance on fossil fuels necessitate the exploration of carbon capture and geologic CO2 sequestration as a technology to reduce CO2 emissions. Characterization of the Midcontinent Rift, a southwestward extension of the Lake Superior basin that presents the best sequestration opportunity in the immediate region, is needed to permit a full feasibility assessment of this technology in the region. Unlike better known rocks in oil or coal producing regions, we have little information on the Rift, yet geologic factors suggest that it may be a potential target. A recent meeting therefore was held to develop a shared understanding of current knowledge on the potential opportunities and tradeoffs for deep geologic sequestration of CO2 in Minnesota, Iowa, and western Wisconsin, and to facilitate collaboration between stakeholders. At the meeting, held on March 21, 2007, over forty representatives from upper Midwest governmental, community, academic, and industry groups met to discuss prospects for geologic CO2 sequestration in the region. At the meeting, Brad Crabtree, of the Great Plains Institute (GPI), described work done by the Coal Gasification Workgroup to build consensus on a long-term vision for low-carbon utilization of coal. In May 2007, GPI will release a 50-year regional energy roadmap including a chapter on the role of advanced coal technologies with geologic carbon capture and sequestration (CCS). Bill Grant of the Izaak Walton League outlined a bill currently before the Minnesota Legislature (H.F. 1666) that would provide funding for assessment of both geologic and terrestrial carbon sequestration in Minnesota. If enacted, this bill would provide funds to the Minnesota Geologic Survey and other agencies to begin geologic assessment of the Midcontinent Rift along with steps toward analysis of required policy and technology. Elizabeth Wilson of the University of Minnesota Hubert Humphrey Institute provided an overview of policy considerations for a carbon-managed energy system. Carbon Capture and Sequestration (CCS) is currently in commercial operation at several sites around the world, and is the focus of a major research effort by the U.S. Department of Energy (DOE). Technology alone, however, will not be enough, as deployment will need to be embedded in a wider societal dialogue. States will play a key role in property rights and liability issues, as well as in managing potential risks to public safety, health and the environment. Wilson drew the attention of meeting participants to the Intergovernmental Panel on Climate Change report on Carbon dioxide Capture and Storage as an important reference. Ed Steadman of the Energy & Environmental Research Center (EERC) at the University of North Dakota described the work of the Plains CO2 Reduction Partnership (PCOR), one of seven regional partnerships under the U.S. Department of Energy’s Regional Carbon Sequestration Partnership Program. PCORP has completed Phase I and Phase II research, including assessment and ranking of sequestration opportunities, outreach materials, and a PCOR Partnership Regional Atlas. PCOR is now preparing to conduct a number of CCS pilot projects 81 �in the Williston Basin under Phase III. Ed discussed the Midcontinent rift as part of his broad presentation, both in terms of its potential and the lack of available information on the topic. Ray Anderson of the Iowa Geologic Survey gave a presentation on the geology of the Midcontinent Rift. He described how the billion-year-old rift can be clearly seen across the region as a gravity and magnetic anomaly, while rocks of the rift are only exposed in the Lake Superior Area. The limited additional information available about rift formations comes from deep test borings done for petroleum exploration in Iowa and Wisconsin in the mid 1980s, along with a few shallower drillholes in Minnesota. He indicated that available data indicate that the rift is composed of a central volcanic block, flanked by sedimentary basins up to 5 miles in thickness. Smaller sedimentary basins sit atop the volcanic block in several areas. Geologic sequestration requires porous rock formations at a depth of at least 2,500 feet or roughly a km, overlain by impermeable rock formations to trap the carbon dioxide. Preliminary indications are that the Midcontinent Rift formations may have the necessary characteristics, but much more geologic assessment is required. A breakout on policy and technology recommended that we pursue coordinated policy and regulatory development among neighboring states to pursue early development of a carbon management infrastructure, linking jurisdictions, power plants, coal resources, enhanced oil recovery (EOR) options, along with other sequestration options. This is presently emerging among Illinois Basin states, but could be expanded to other neighbors and even beyond the Illinois Basin. Once initial geological results are available, we will need to conduct comprehensive analysis of different energy futures. This analysis will examine the policy, economic and environmental implications of a) importing electricity from out of state, b) producing electricity in-state but exporting produced CO2 through pipelines, and c) producing electricity and sequestering CO2 within the state. Such an analysis will allow for the advantages and tradeoffs of different energy futures to be considered. The geology breakout recommended that geologic assessment be conducted in phases. Phase One, at a cost of about $0.1M, would assemble existing information, and could include new modeling based on existing seismic data, in order to lay the groundwork for a more rigorous geologic assessment of the CCS potential in the Midcontinent Rift. Bill H.F. 1666, currently before the Minnesota Legislature, could potentially fund Phase One. An important purpose of a Phase I study would be to inform a decision on whether it is warranted to move on to a phase II study. A Phase Two would be a geologic assessment based on new geophysical surveys and about 3 to 6 boreholes, at a cost of perhaps $5M to $10M, perhaps through public funding or a public-private partnership. The objective of Phase Two would be to determine whether Geologic Sequestration potential exists in the Midcontinent Rift. If the results of Phase Two are positive, Phase Three would expand the geological assessment to estimate the geologic storage capacity of the Rift, and begin characterizing the most promising sites. References: • • http://www.geo.umn.edu/mgs/co2_seq.htm http://arch.rivm.nl/env/int/ipcc/pages_media/SRCCS-final/IPCCSpecialReportonCarbondioxideCaptureandStorage.htm 82 �TILL GEOCHEMICAL AND INDICATOR MINERAL RECONNAISSANCE OF MINNESOTA Thorleifson, L. H., K. L. Harris, H. C. Hobbs, C. E. Jennings, A. R. Knaeble, R. S. Lively, B. A. Lusardi, G. N. Meyer, Minnesota Geological Survey, 2642 University Ave West Saint Paul, Minnesota 55114-1057 As a cooperative project of the Minnesota Geological Survey and industry, the entire State of Minnesota and adjacent regions was sampled for till geochemistry and indicator minerals at a 30km spacing during summer 2004. Within target cells, each a quarter-degree latitude by a halfdegree longitude, till from between about 1 and 2 m depth was sampled by filling a 15 liter plastic pail. At a few sites, vertical profiles were collected. In addition, three transects to the north were sampled, to help identify sediments derived by long-distance glacial transport, to obtain reference samples from the Thompson nickel belt, and also to extend sampling to the limit of Hudson Bay-derived carbonate-bearing sediments, to permit comparison to Minnesota carbonate-bearing sediments. Three control samples anomalous in kimberlite indicator minerals from Kirkland Lake, Ontario, were also obtained. The resulting batch consisted of 250 samples covering Minnesota and adjacent areas, 20 samples from Canada, and the three standards. Upon completion of the sampling, the samples were randomized, given numeric laboratory identifications, and shipped to a processing lab, where four quarter-liter splits, two for fine fraction geochemistry, one for texture, and one for an archive were removed. The remaining 14 liters were disaggregated, screened at 2 mm, and the gravel was retained for lithological analysis. The <2 mm fraction was then processed for gold grains, a ferromagnetic heavy mineral concentrate, and a nonferromagnetic heavy mineral concentrate that supported subsequent analysis for precious metal, base metal, and gemstone indicator mineral counts, indicator mineral chemistry, bulk mineralogy counts, and heavy mineral geochemistry. The resulting data are now a significant new information resource with respect to environmental geochemistry topics such as understanding the distribution of deleterious elements in food and water, while providing insights into transport history and composition of the sediments that make up our soil parent materials. Many variables provide insights into regional geology, and reflect known mineral deposits. Some of the data seem to provide faint insights into what may be mineralization that was not previously recognized, such as various base-metal and precious-metal-related elements that show patterns of varying clarity over portions of the state. None of these patterns, however, are obvious discoveries of something that was previously unknown, at the current stage of interpretation. With respect to kimberlite indicator minerals, however, there are two noteworthy patterns, including a few Cr-pyrope garnets in an area from the Twin Cities to southwestern Minnesota (Figure 1), as well as Mg-ilmenites and high-chrome Cr-diopsides in the far northcentral part of the State. Sample spacing in the thin sediments of northeastern Minnesota was not adequate to fully test for the presence of sources such as potential single kimberlite pipes, although samples at a closer spacing are presently being processed by Natural Resources Research Institute to address this point. The current results are, however, faint but clear indications of kimberlite indicator minerals sources that are not unlike several of the patterns that have been found, for example, in Canada, where some of such patterns have eventually resulted in kimberlite discoveries. In the case of the Minnesota results, the data may indicate sources 83 �within the state, or quite possibly could be manifestations of long distance glacial sediment transport, possibly from known or unknown sources in neighboring states or in Canada. In summary, the results are a highly significant step forward in mapping our geochemical landscape, in clarifying mineral potential, in provision of reference data useful to environmental protection, public health, and exploration, and in supporting follow-up with respect to potential mineralization. Figure 1: Example of results from the statewide survey Reference: Thorleifson, L. H., K. L. Harris, H. C. Hobbs, C. E. Jennings, A. R. Knaeble, R. S. Lively, B. A. Lusardi, G. N. Meyer, 2007, Till geochemical and indicator mineral reconnaissance of Minnesota. Minnesota Geological Survey Open File Report OFR-07-01, 512 p., 15 pdf digital files, 5 digital images 84 �DEFINITION OF THE PROTEROZOIC TERRAIN UNDER THE PALEOZOIC-CENTRAL U.P., MICHIGAN T.D. WAGGONER 141 CHIPPEWA, NEGAUNEE, MI 49866 E-mail: thomaswaggonergeo@hotmail.com As soon as it was recognized in the 1880’s that the eastern Menominee range was covered by Paleozoic sediments east of Waucedah, exploration has sought to identify iron mineralization under cover in the central Upper Peninsula. For over 100 years work has progressed to identify the sources of the magnetic anomalies that are found west of the NW-SE rift hinge line and the western limit of the Paleozoic cover. Fig. 1 Magnetic highs under the Paleozoic cover of the central U.P. The amplitude of the airborne total intensity magnetic anomalies range from 3,000 to 23,000 gammas. The intensity is generally dependent on both the amount of magnetite and depth of burial. Airborne and ground magnetics along with ground gravity defined target areas that have since been drilled in the search for iron ore through the 1970’s. Most of the major anomalies are caused by steeply dipping banded iron formations (bifs) similar to the Proterozoic equivalents that outcrop west of the covered area. The primary iron oxide minerals are magnetite, specularite. These are accompanied by iron carbonate silicates, carbonate and chert. Supergene oxidation and enrichment is present in about half of these deposits. Paleozoic 85 �cover ranges from inches to at lease 1800 feet. Associated rocks include slates, quartzites and schists and in one case gneiss. Some of these rocks have been intruded by diabase and granite dikes. The east-west structural attitude of the anomalies and bifs are consistent with outcrop areas to the west. Metamorphic rank from chlorite through garnet is present as noted by the gangue mineralogy associated with the bifs. There have been geophysical data bases generated for diamond exploration that would be useful in further defining the buried Proterozoic terrain. Although economic deposits of bif are probably not present, there is a distinct possibility that IOCG type deposits, feeders for the water deposited bifs are present. Further geophysical surveys like airborne gravity could help provide better definition. 86 �Detrital Zircon Provenance and Structural Geology of the Hamilton Mounds Inlier, Central Wisconsin Jakob Wartman, John P. Craddock, Karl R. Wirth, Geology Department, Macalester College, St. Paul, MN 55105; e-mail: craddock@macalester.edu Gordon Medaris, Jr., Dept. of Geol. & Geophysics, UW-Madison, Madison, WI 53706 Jeff D. Vervoort, Dept. of Geology, Washington State Univ., Pullman, WA 99164 Cam Davidson, Geology Department, Carleton College, Northfield, MN 55057 Hamilton Mounds is one of a series of small inliers in Adams County, WI that are composed of Proterozoic quartzites (Oslander, 1931). Exposures in the Seven Sisters quarry at Hamilton Mounds reveal the presence of a lower, folded metasedimentary sequence, the Hamilton Mounds meta-arkose (HMM), that is intruded by granite (1762 ± 7 Ma [2σ], TIMS-zircons; Medaris et al., 2007). The overlying Seven Sisters orthoquartzite (SSQ) is also folded and contains detrital zircon and monazite that suggest correlation with Baraboo-equivalent quartzites (see Table 1). Finite strains in the Seven Sisters quartzite are also consistent with the regional, NW-SE shortening Mazatzal orogen strain pattern (Craddock and McKiernan, 2007). Both sedimentary units at Hamilton Mounds are crosscut by breccia zones associated with Wolf River batholith intrusion at 1470 Ma. The HMM and SSQ are overlain unconformably by Cambrian sandstones (see Brown and Greenberg, 1986). Detrital zircons are abundant in the Hamilton Mounds meta-arkose. We analyzed 120 grains of which 68 yielded nearly concordant (<10% discordant) results (Figure 1). The youngest zircon age we observed is 1750 ± 16 Ma (2σ), the approximate age of the crosscutting granite dike. The zircon population includes large numbers of post-Penokean (1775 Ma) to Penokean (1800-1875 Ma) grains, and lesser amounts of Archean (25003500 Ma) grains. These data suggest that the Hamilton Mound sediments were sourced Table 1: Summary of Geochronology for Proterozoic Quartzites n= Quartzite Barron 6* Sioux 9* 9* Flambeau McCaslin 2 Hamilton Mounds HMQ1 (SSQ) 69 HM-SSQ (m) 11* HM-Basal (m) 10* HM-Basal (z) 68 Baraboo Basal 7 1250 m up 7 BARQ2 47 BARQ1 100 Method SHRIMP SHRIMP SHRIMP TIMS Youngest Age 1751 Ma 1730 1714 1773 Range 1751-1180 Ma 1730-1850 1714-1880 1773-1775 Reference Holm et al., 1998 Holm et al., 1998 Holm et al., 1998 Van Wyck, 1995 LA-ICPMS Electron Probe Electron Probe LA-ICPMS 1772 1750 1720 1783 1772-3566 1750-2165 1720-2120 1783-3456 Van Wyck & Norman Medaris et al., 2007 Medaris et al., 2007 This Study TIMS TIMS LA-ICPMS LA-ICPMS 1691 1844 1716 1724 1691-1865 1844-2588 1716-3200 1724-3100 Medaris et al., 2003 Van Wyck, 1995 Van Wyck & Norman Van Wyck & Norman [Key: All ages are 207Pb/206 Pb. HM is Hamilton Mound, with upper Seven Sisters Quartzite (SSQ) and older, basal arkose (HM-Basal). Detrital monazite (m) and zircons (z) are indicated. Monazite ages (*) are from multiple analyses: Barron (6 grains, 28 spots); Sioux (9/19); Flambeau (9/16); HM-SSQ (11/83) and HM-Basal (10/154)]. Younger HMM monazite ages (<1760 Ma) are from non-detrital grains. 87 �from the north, including the Penokee Mountains, Archean granite-greenstone terranes, and the Marshfield (or Watersmeet) Archean gneiss terranes. This is broadly consistent with detrital zircon results for other contemporaneous units (e.g., Virginia, Rove and Michigamme Fms.; see Wirth et al., 2006) along the Penokean margin. The Hamilton Mound arkose was deformed in the Yavapai orogeny, then eroded and buried by quartz arenites of the Baraboo interval (1750-1630 Ma). 12 10 8 6 4 2 0 1500 2000 2500 3000 3500 Age (Ma) Figure 1. Histogram of detrital zircon ages from basal Hamilton Mound arkosic quartzite. References Cited Brown, B.A. and Greenberg, J.K., 1987, Proterozoic quartzite at Hamilton Mound, central Wisconsin: Geological Society America Centennial Field Guide—North Central Section (DNAG), p. 195-98. Craddock, J. P. and McKiernan, A.W., 2007, Finite strain gradient in Baraboo-interval quartzites, Wisconsin and Minnesota, USA: Prec. Res. Sp. Vol. Holm, D., Schneider, D., and Coath, C.D., 1998, Age and deformation of Early Proterozoic quartzites in the southern Lake Superior region: Implications for extent of foreland deformation during final assembly of Laurentia. Geology, v. 26, p. 907-910. Medaris, L.G., Singer, B.S., Dott, R.H., Naymark, A., Johnson, C.M., Schott, R.C., 2003. Late Paleoproterozoic climate and tectonics in the southern Lake Superior region and Proto-North America: Evidence from Baraboo interval quartzites: J. Geol., v. 111, p. 243-257. Medaris, L.G., Van Schmus, W.R., Loofboro, J., Stonier, P.J., Zhang, X., Holm, D.K., Singer, B.S., Dott, R.J., Jr., in press, Two Paleoproterozoic (Statherian) siliciclastic metasedimentary sequences in central Wisconsin: the end of the Penokean orogeny and cratonic stabilization of the southern Lake Superior region: Precambrian Research. Ostrander, A.R., 1931, Geology and structure of Hamilton Mounds, Adams County, Wisconsin: unpublished M.S. thesis, Univ. of Wisconsin, Madison, 27 p. Van Wyck, N., 1995, Major and trace element, common Pb, Sm-Nd, and zircon geochronology constraints on petrogenesis and tectonic setting of pre- and early Proterozoic rocks in Wisconsin. unpublished Ph.D. thesis, Univ. of Wisconsin, Madison, 47-280. Van Wyck, N. and Norman, M., 2004, Detrital zircon ages from early Proterozoic quartzites, Wisconsin, support rapid weathering and deposition of mature quartz arenites: J. Geol. 112, p. 305-315. Wirth, K.R., Vervoort, J., Craddock, J.P., Davidson, C., Finley-Blasi, L., Kerber, L., Lundquist, R., Vorhies, S., and Walker, E., 2006, Source rock ages and patterns of sedimentation in the Lake Superior region: results of preliminary U-Pb detrital zircon studies: 52nd Institute on Lake Superior Geology, p. 69-71. 88 �A Comparison of Textural Profiles in Diabase Sills from the Midcontinent and Transantarctic Rifts ZIEG, Michael J. and FORSHA, Clinton J., Department of Geography, Geology, and the Environment, Slippery Rock University, 1 Morrow Way, Slippery Rock, PA 16057, USA, michael.zieg@sru.edu The 176-Ma Ferrar Dolerite of the Transantarctic Rift (TAR) and the 1.1-Ga Nipigon Diabase of the Midcontinent Rift (MCR) represent magmatic activity during the early stages of continental rifting. Both of these systems include large sills intruded at or near the unconformity between basement rock and overlying sedimentary sequences (the Beacon Group and Sibley Group). Understanding the emplacement history of these sill complexes is critical to understanding the thermal and mechanical properties of the developing rifts. As in the Nipigon area, there are very few feeder dikes exposed in the Ferrar system. Thus, it has been suggested that a significant proportion of the magma transport in the TAR occurred within the sills themselves. Hart and Macdonald (2005) and Sutcliffe (1987, 1989) have suggested, and this work provides additional evidence, that the Nipigon sills experienced multiple injections of magma, which may be an indication that the sills acted as conduits for the distribution of magma through the system. Focused magma flow results in a very different thermal environment than episodic, discrete injection events, and therefore it is very important to understand the nature of the injection history when evaluating the tectonic development of a magmatically active region. In this study, we focus on the application of textural stratigraphy within mafic sills to the understanding of sill emplacement history. In particular, textural profiles from Nipigon sills are compared to profiles from Ferrar sills. These comparisons illustrate the textural signatures of singular, pulsed, and flow-differentiated injections. While the Nipigon sill from Kama Point exhibits a strong reinjection signature, the reinjection signatures in the Antarctic sills are less clear. The significance of different injection histories is that, while a network of sills and dikes emplaced via single injection events can efficiently distribute heat through a large crustal volume, reinjection and/or extended flow through a sill or dike can produce higher temperatures, though in a more localized volume. References Hart, T.R., and McDonald, C.A. 2005. Mesoproterozoic diabase sills of the Nipigon Embayment, northwest Ontario. In Institute on Lake Superior Geology Proceedings, 51st Annual Meeting, Nipigon, Ontario, Vol. 51, Part 1, pp. 22-23. Sutcliffe, R.H. 1987. Petrology of middle Proterozoic diabases and picrites from Lake Nipigon, Canada. Contributions to Mineralogy and Petrology, 96: 201-211. Sutcliffe, R.H. 1989. Mineral variation in Proterozoic diabase sills and dykes at Lake Nipigon, Ontario. Canadian Mineralogist, 27: 67-79. 89 � https://digitalcollections.lakeheadu.ca/files/original/22fb6182a89be392ecbc6d2c2bc1ab99.pdf 29c15d555e9c2109dc1f30c0cb447f7b PDF Text Text 53RD ANNUAL MEETING INSTITUTE ON LAKE SUPERIOR GEOLOGY LUTSEN, MINNESOTA MAY 8 – 13, 2007 PROCEEDINGS VOLUME 53 PART 2 – FIELD TRIP GUIDEBOOK �INSTITUTE ON LAKE SUPERIOR GEOLOGY LUTSEN, MINNESOTA MAY 8 – 13, 2007 HOSTED BY: LAUREL G. WOODRUFF, U.S. GEOLOGICAL SURVEY MEETING CHAIRPERSON JAMES D. MILLER, JR., MINNESOTA GEOLOGICAL SURVEY FIELD TRIP COORDINATOR PROCEEDINGS VOLUME 53 Part 2 Field Guidebook Compiled by James D. Miller, Jr. (MGS) and Dean M. Peterson (NRRI) -i- �- ii - �Table of Contents Proceedings Volume 53 Part 2 – Field Trips 1) IGNEOUS STRATIGRAPHY OF THE POPLAR LAKE INTRUSION 1 (FORMERLY NATHAN’S LAYERED SERIES) Leaders: J. Miller & E. Jerde 2) GEOLOGIC AND CULTURAL HISTORY OF THE GRAND PORTAGE NATIONAL MONUMENT 23 Leaders: B. Cannon, D. Cooper, & B. Phillips 3) MIDCONTINENT RIFT-RELATED MAFIC INTRUSIONS NORTH OF THE INTERNATIONAL BORDER 53 Leaders: M. Smyk & P. Hollings 4) GEOLOGY OF THE NICKEL LAKE MACRODIKE AND ITS ASSOCIATION WITH CU-NI-PGE MINERALIZATION IN THE NORTHERN SOUTH KAWISHIWI INTRUSION, DULUTH COMPLEX, NORTHEASTERN MINNESOTA 81 Leaders: D. Peterson & P. Albers 5) GEOLOGIC HIGHLIGHTS OF NEW MAPPING IN THE UPPER SOUTHWESTERN TO NORTHEASTERN SEQUENCE OF THE NORTH SHORE VOLCANIC GROUP AND BEAVER BAY COMPLEX 109 Leaders: T. Boerboom, J. Miller, & J. Green 6) GEOLOGY ALONG THE GUNFLINT TRAIL 143 Leaders: M. Jirsa & P. Weiblen Cover Illustrations (clockwise from upper left) • Canoeing on Poplar Lake (photo by Jim Miller) • Beach at Grand Portage (photo by Bill Cannon) • 3-D model of the South Kawishiwi and Bald Eagle intrusions (from Dean Peterson) • Stromatolitic Gunflint Iron-formation, Magnetic Rock Trail (photo by Jim Miller) • North shore view from Sugar Loaf Point (photo by Jim Miller) • Logan Sills in Rove Formation (photo by Mark Smyk) - iii - �- iv - �-v- �53rd Annual Institute on Lake Superior Geology FIELD TRIP 1 IGNEOUS STRATIGRAPHY OF THE POPLAR LAKE INTRUSION (formerly Nathan’s Layered Series) James D. Miller, Jr. Minnesota Geological Survey & Department of Geological Sciences University of Minnesota Duluth Duluth, Minnesota and Eric A. Jerde Department of Physical Sciences Morehead State University Morehead, Kentucky ILSG07 1 Trip 1 �Introduction The Duluth Complex is the largest exposed intrusive component of the Mesoproterozoic Midcontinent Rift, covering an area of over 5,000 km2 in northeastern Minnesota (Fig. 1-1). The Complex comprises a more or less continuous mass of mafic to felsic plutonic rocks that extends in an arcuate fashion from Duluth to nearly Grand Portage. These rocks were emplaced as sheet-like intrusions into the lower section of a comagmatic volcanic edifice (the North Shore Volcanic Group), which itself was erupted onto a peneplained surface composed of Paleoproterozoic sedimentary rocks and Neoarchean granite-greenstone assemblages. The Duluth Complex is generally subdivide into four major series on the basis of intrusive age, dominant lithology, internal structure, and structural position within the complex (Miller and others, 2002). In order of younging, these are: Felsic Series – massive granophyric granite and smaller amounts of intermediate rock that occurs as a semicontinous mass of intrusions strung along the eastern and central roof zone of the complex and was emplaced during early stage magmatism (~1108 Ma). Early Gabbro Series – layered sequences of dominantly gabbroic cumulates that occurs along the northeastern contact of the Duluth Complex and also was emplaced during early stage magmatism (~1108 Ma) Anorthositic Series – a structurally complex suite of foliated, but rarely layered, plagioclase-rich gabbroic cumulates that was emplaced throughout the complex during main stage magmatism (~ 1099 Ma). Layered Series – a suite of stratiform troctolitic to ferrogabboic cumulates that comprise at least 11 variably-differentiated mafic layered intrusions and occurs mostly along the base of the Duluth Complex. These intrusions were emplaced during main stage magmatism, but generally after the anorthositic series. The Poplar Lake Intrusion, which is the focus of this field trip, and the Crocodile Lake intrusion to the east comprise the only two known intrusions in the Early Gabbro Series. The early emplacement of these intrusions was first implied by cross-cutting field relations mapped by Nathan (1969), who recognized that the Poplar Lake intrusion units are cut on their west end by rocks now recognized as belonging to the Layered and Anorthositic Series. An early intrusive age was also implied by their reversed magnetic polarity (Beck, 1970), which is similar to the lowermost lavas of the North Shore Volcanic Group (Green, 1972). U-Pb ages of zircons from the base of the Poplar Lake intrusion confirm its early emplacement by yielding an age of 1106.9±0.6 Ma (Paces and Miller, 1993). This age is similar to lower reversed-polarity lavas of the Midcontinent Rift (1109-1107 Ma, Davis and Green, 1997) and to the reversed polarity Logan Sills (1108(+4/-2) Ma; Davis and Sutcliffe, 1985), which occur in the footwall of the Poplar Lake Intrusion (Fig. 1-1). ILSG07 2 Trip 1 �Figure 1-1. Generalized geology of northeastern Minnesota showing the locations of the Poplar Lake and Crocodile Lake intrusions, which comprise the Early Gabbro Series of the Duluth Complex. Previous Studies The geological survey of Minnesota (1872-1901), directed by N.H. Winchell and assisted by U.S. Grant, H.V. Winchell, and A. H. Elftman, visited the Gunflint Trail area many times over the latter years of the survey. A geologic map of Cook County (Fig. 1-2), as well as more detailed (1”= 2mi) plates of the Gunflint Lake and Rove Lake areas were published in volume 4 of the Final Report (Winchell, 1899). These maps do not distinguish the various types of gabbro, but they do accurately portray the basal contact of the gabbro with the Rove formation and gabbro-granophyre contact just south of Winchell Lake. Frank Grout and coworkers conducted reconnaissance field mapping in Cook County in a period between 1913 and 1953. This work was compiled soon after Grout’s death in 1958 in MGS Bulletin 39 (Grout, Sharp, and Schwartz, 1959). In township maps of the Poplar Lake area (Fig. 1-3), Grout distinguished gabbro, oxide-rich gabbro, anorthositic gabbro, intermediate ILSG07 3 Trip 1 �N Figure 1-2. Geologic map of Cook County by U.S. Grant from the Minnesota Geological Survey Final Report vol. 4 (Winchell, 1899). EWLAMA lION F.' IF I I I I..I! .i!! I!. I IF - if K U *S I •'I(iI,IIr.X'.- I,..XIV — (..,!III'Ii!nIK,F,l'I',iI.1,,r,r INIFrIlI. RIIIIKFIK'FFI II.PI.Iii.u,I.p*lt,\IIlIil,OtNI!rII,.IIInug,, I Figure 1-3. Reconnaissance township geologic maps of the Poplar Lake area from Grout, Sharp, and Schwartz (1959) ILSG07 4 Trip 1 �intrusive rocks and granophyre and noted the moderate southernly dip of layering and foliation. The oxide-rich intervals of the Poplar Lake intrusion were previously cited by Broderick (1917) and by Grout (1950) as potential titanium and iron ore resource. By far the most complete study of the Poplar Lake intrusion comes from the PhD dissertation work of Harold Nathan (1969). Nathan mapped the bedrock geology of the Duluth Complex in the Gunflint Lake, South Lake, and Hungry Jack Lake and followed this up with a very thorough petrographic study. Reconnaissance geologic maps of these quadrangles were published nearly a decade later (Morey and Nathan, 1977; Mathez, Nathan, and Morey, 1977; and Morey and Nathan, 1978). Phinney (1972) briefly summarized Nathan’s work for the MGS Centennial Volume in a section describing the northern prong of the Duluth Complex (Figure 1-4). In that same volume, Davidson (1972) first adopts the name “Nathan’s layered series”. Later, Weiblen and Morey (1980) referred to the intrusion as the “Layered Series of Nathan” and Weiblen (1982) used the term “Nathan’s Layered Series”. In the new geologic map compilation of the Duluth Complex (Miller and others, 2001) and the companion report (Miller and others, 2002), the informal name of Nathan’s Layered Series was replaced by the Poplar Lake intrusion to acknowledge the location where most of the lithologies recognized by Nathan (1969) are exposed and accessible. 9P00' 9045 9045 9O3O' C) SO!09y 07 fljfl7flfl7 LOIW, oorn Lose. EXPLANATION Hugry Jack Lake quadrangles r0m Nalhon. (969) Remainder (ran, 6.-our 010 orhrs (see ak I 2 3M!I,) ljt7IQlfljn aIte,otinn ci 71 by 4J J IA (mone ,hcei—liIc. nçI.niyn or Al 2 0 _5 IlifT fl Mac IJ F,GlkicIusicns ai P LIfT] °(::?rr iL±J cn4 HI Figure 1-4. Generalized geology of the Poplar Lake intrusion as portrayed by Phinney (1972) based on mapping by Nathan (1969) and Davidson (1972). Except for limited reconnaissance mapping and geochemical sampling by Jerde (2001), little new mapping or petrologic research has been devoted to this very interesting early gabbroic intrusion of the Duluth Complex. Therefore, the igneous stratigraphy and unit descriptions given for this field trip through the lower half of the intrusion will largely follow Nathan’s (1969) conventions. One notable exception is that whereas Nathan (1969) recommended naming rocks by listing the minerals present ILSG07 5 Trip 1 �in the rock in decreasing order of abundance (e.g., a plagioclase-augite-olivine rock), we have adopted more conventional modal rock name nomenclature (e.g., an olivine gabbro; see Fig. 1-5 for the modal classification scheme used here). Also, we will point out where our petrographic analysis of samples from certain field stop locations differ from Nathan’s descriptions. Mela - Pl < 60-50% Leuco - Pl > 75% Gabbroic Rocks Plagioclase = 30-80/85% OLIVINE TROCTOLITE 3:1 AUGITE TROCTOLITE OLIVINE GABBRONORITE OXIDE TROCTOLITE OXIDE-OLIVINE GABBRONORITE OXIDE-OLIVINE NORITE 1:1 OLIVINE GABBRO OLIVINE NORITE OXIDE-OLIVINE GABBRO 1:3 NORITE OXIDE GABBRONORITE FE-TI OXIDE OXIDE-RICH ROCKS 1:3 1:1 OXIDE GABBRO 3:1 3:1 GABBRONORITE LOW-CA PYROXENE GABBRO HIGH-CA PYROXENE Figure 1-5. Modal classification for gabbroic rocks (Pl 30-80/85%) used in this guide. Prefixes melaor leuco- (e.g. melatroctolite, olivine leucogabbro) are used when plagioclase abundance is <60 to 65% or >75%, respectively. Taken from Miller and others, 2002. Igneous Stratigraphy and Petrologic Interpretations of the Poplar Lake Intrusion Nathan (1969) distinguished 27 different units in his mapping of the three quadrangles that include the Poplar Lake intrusion (Fig. 1-4). He labeled these alphabetically from A through AA in order of decreasing age. Nathan interpreted the relative ages of the units on the basis of fine-grained margins, cross-cutting intrusive relationships, inclusions, and/or thermal effects. The various map units Nathan (1969) indentified within the Poplar Lake intrusion can be generally grouped into 6 major lithologies: 1) oxide-rich, gabbro to olivine gabbro: F, G, H, T, U, V 2) olivine gabbro to troctolite: P, Q 3) gabbronorite to olivine gabbronorite: A, B, M 4) anorthositic rock types: J, some M, some Q, S 5) intermediate to felsic rock types: K, O, AA, Maa 6) fine-grained mafic rocks (inclusions?): C, D, E, N, X, Y ILSG07 6 Trip 1 �Davidson conducted reconnaissance mapping in quadrangles to the south of the three that Nathan (1969) mapped and had some different interpretations of the upper units of the Poplar Lake Intrusion (Davidson, 1972, 1977a, 1977b; Davidson and Burnell, 1977). Davidson (1972) interpreted the extensive mass of granophyre at the top of the Poplar Lake intrusion, which Nathan (1969) designated as unit AA (Fig. 1-4), to be a separate intrusion that belongs to the felsic series of the Duluth Complex. Like Nathan (1969), he interpreted the massive granophyre to be one of the youngest intrusion of the Duluth Complex based on observations that granophyre dikes and irregular masses commonly cut most other rock types. Nathan’s unit Maa, which underlies the granophyre mass and is composed of a highly altered assemblage of intermediate rock types, is interpreted to be an altered and contaminated phase of unit M caused by the late intrusion of granophyre (unit AA) (Fig. 1-4). Davidson (1972, 1977a, 1977b; Davidson and Burnell, 1977) interpreted these intermediate rocks (unit grd) to be marginal phases of the granophyre. Another reinterpretation of Nathan’s (1969) mapping by Davidson (1972) and others (Morey and Nathan, 1978; Morey and others, 1981) is that Nathan’s unit R is actually part of the Tuscarora Intrusion and thus belongs to the layered series (or the troctolite-olivine gabbro series as Davidson (1972) called it). In the most recent map compilation of the Duluth Complex (M-119; Miller and others, 2001), several reinterpretations were made of both Nathan’s (1969) and Davidson’s (1972) map interpretations. One change is the suggestion that Nathan’s unit S, a gabbroic anorthosite that cross cuts lower units of the intrusion, is actually part of the younger anorthositic series. More significantly, whereas both workers considered the granophyre mass, now termed the Misquah Hills granophyre, to be one of the youngest intrusions is the Duluth Complex, it is now considered to be one of the oldest. Major evidence for this comes from U-Pb age dating, which indicates a crystallization age of 1106 ± 6 Ma (Vervoort and Wirth, 2004). Consequently, the intermediate rocks of Nathan’s unit Maa and Davidson’s unit grd, which underlie the granophyre, are now seen to represent hybridization between partially melted granophyre and underlying mafic magmas of the Poplar Lake intrusion. Miller and others (2002) suggested that the granophyre acted as a low-density trap, which caused the Poplar Lake intrusion to underplate and partially melt the felsic mass. Figure 1-6. Diagrammatic cross-section through the central part of the Poplar Lake intrusion. Figure from Phinney (1972, Fig. V-33) based on data from Nathan (1969). ILSG07 7 Trip 1 �Examining the stacking of Nathan’s (1969) units, it is immediately clear that he interpreted the emplacement of the Poplar Lake intrusion to be a complex multistage process (Figs. 1-6 and 1.7). He envisioned various sill-like intrusions to have been multiply emplaced at seemingly random levels in the intrusion. Nathan (1969) interpreted most intrusive units to have crystallized in place with little evidence of fractional crystallization. This rather unconventional interpretation of the igneous stratigraphy of a mafic layered intrusion compels some skepticism as to Nathan’s model. For example, the sulfide-mineralized heterogeneous basal unit (F), commonly the earliest formed rock type in other Duluth Complex layered intrusions, is thought to have been emplaced after considerable volumes of A and B were emplaced. Many of the fine-grained gabbro units have limited areal extent and appear similar to hornfels basalt inclusions commonly found throughout other Duluth Complex intrusions. Moreover, the anorthositic units are mostly seen as late, however, anorthositic rock are typically some of the earliest intrusive phases throughout most of the Duluth Complex. Nevertheless, until a rigorous follow-up field and petrologic of this intrusion is conducted, Nathan’s multiple emplacement hypothesis stands as the current model. F [11 kHIll Mesonroterozoic POPLAR LAKE INTRUSION '' (Units based on Nathan (1969)) prismatic 01; gradational into dp 01 gabbro- fine?, foliated, dp intergran, 0l:subpoik-subprism — dm dj dg F2 df dc db Gabbronorite- coarse,oph. intergran, commonly leucocratic Gabbronoritic Anorthosite - coarse, ophitic-subophitic Olivine oxide gabbro- crs, foliated, intergran,0l:spoik-sprism; dg'- oxide-rich (>10%) zone Olivine oxide gabbrovan-textured, locally sulfidic Mafic hornfels- fine, granoblastic; basalt inclusions? Gabbronorite- medium, fOliated, Opx:poikilitic-subpoikilitic rc1:i: Gabbronoritic Troctolite- — -.- 1-' Troctolite- medium, foliated, medium, fOliated, intergranular, Opx:poikilitic — —— LOGAN INTRUSIONS Diabase- fine-coarse, suboph- VJTLT.. + HI—I LrIrIai+t — 500 Ii iitrrm- i— -+ r-n.r4 - Fr4tI--I P 0 IHi-iH4:frHiLrT Hji500 1000 Meters H - d rri dt intergranular, locally PI-phyric Paleoproterozoic ROVE FORMATION 1 Argillite/Graywacke- thin-to thick-bedded; locally hornflsic Figure 1-7. Geologic map of the field trip area based on Morey and Nathan (1977). Unit labels after Nathan (1969), but standard modal rock names (see Fig. 1-5) interpreted from Morey and Nathan’s (1977) descriptions. Locations of more detailed maps of stop locations (Figs. 1-8, 1-9 and 1-14) are shown by inset boxes. ILSG07 8 Trip 1 �FIELD TRIP 1- DAY 1 Field Stop Descriptions Upon arriving at Rockwood Lodge and unpacking gear, we will set out to investigate roadcuts along a one-mile stretch of the Gunflint Trail for the rest of the morning. STOP 1-1. BASAL MARGIN UNITS – F, C, G Location: Roadcuts along Gunflint Trail between entrance and exit of Cook County Road 82. South Lake 7.5' quadrangle (T64N, R2W, Sec. 1). UTMs: start – 683540E 5326180N; end – 684950E 5325380N Description: Roadcuts along this section of the Gunflint Trail expose some of the rock types composing the marginal zone of Nathan's layered series (Fig. 1-8). Beginning from the junction of the Gunflint Trail with County Road 92, we will proceed east along the north side of the Trail back toward the Rockwood Lodge entrance. Mesoproterozoic POPLAR LAKE INTRUSION (Units based on Nathan (1969)) dp dg 01 gabbro- fine?, foliated, intergran, Ol:subpoik-subprism Olivine oxide gabbro- cr5, foliated, intergran,Ol:spoik-sprism; —-- df dc dg'- oxide-rich (>10%) zone Olivine oxide gabbrovan-textured, locally sulfidic Mafic hornfels- fine, granoblastic; basalt inclusions? LOGAN INTRUSIONS Diabase- fine-coarse, subophIi intergranular, locally PI-phynic Paleoproterozoic ROVE FORMATION 1 Argillite/Graywacke- thin- to thick-bedded; locally hornflsic Figure 1-8. Geology of the Gunflint Trail area northwest of the Rockwood Lodge (based on Morey and Nathan, 1977). Roadcuts A – E are described in the text for Stop 1-1. Roadcuts A – These exposures show a more homogeneous and sulfide poor phase of Unit F. The rock is a coarse-grained ophitic oxide olivine gabbro. Similar exposures occur in the roadcuts on the south side of the Trail, west Co Rd 92 junction. This is the location of geochronology sample NLS-5,which yielded a U-Pb zircon age of 1106.9±0.8 Ma (Paces and Miller, 1993). Roadcuts B – This series of a low, deeply weathered roadcuts and low pavements are more typical of unit F. About 150 m from the junction is of coarse-grained, decussate, ophitic, biotitic oxideolivine gabbro. This noncumulate, vari-textured (taxitic) gabbro forms the lowermost unit of ILSG07 9 Trip 1 �Nathan's (1969) layered series (his unit F). Locally, Cu-Fe sulfide is present and gives rise to the very rusty appearance of the outcrop. Sulfide mineralization in the lower contact zone is characteristic of many parts of the Duluth Complex (Bonnichsen, 1972; Miller and others, 2002). As is interpreted from the Cu-Ni-PGE sulfide deposits of the NW Duluth Complex, the probable source of the sulfur are the shale and graywacke of the Paleoproterozoic Rove Formation, which forms the footwall of the gabbro. The lack of significant sulfide mineralization here compared to the NW area may indicate a lower sulfur content to the sediments or depletion of sulfur by earlier intrusion of the Logan Sills. Roadcut C – At this prominent roadcut, a very fine grained gabbro is exposed. It has irregular areas of coarser gabbro scattered throughout and some pockets of sulfide. This is Nathan's unit C, which he suggested could be recrystallized inclusions of Keweenawan lavas, Logan sills, or more likely, the chilled contact of a precursor intrusion (his units A and B). In thin section, the rock is a strongly recrystallized, poikiloblastic gabbronorite with small (<5 mm) high-density oikocrysts of augite and hypersthene enclosing granular plagioclase. Because the texture of Logan sills remains unaffected as the contact with the layered series approached (Nathan, 1969), it is unlikely that this body is such an inclusion. Rather, the zones rich in ovoid clots of coarse feldspar are similar in appearance to what have been interpreted elsewhere in the complex as recrystallized amygdules (Bonnichsen, 1972), and this inclusion is most likely a basalt flow. The abundance of these types of inclusions and the relative lack of quartz- or cordierite-bearing inclusions of Rove Formation suggest that the focus of emplacement of Nathan's layered series was at the horizontal to shallow-dipping discontinuity between Early Proterozoic sedimentary rocks and Keweenawan lava flows. Roadcuts D and E About 700 m farther east past a bend in the road, another deeply weathered roadcut exposes a medium- to coarse-grained, moderately laminated oxide troctolite with ophitic augite. This belongs to Nathan's unit G which he characterized as "coarse-grained, olivine-plagioclase and augite-plagioclase rocks with strongly foliated plagioclase and abundant tironals (Fe-Ti oxides) . . . bioite is conspicuous . . . density graded layering is prominent" (Nathan, 1969, p. 68). Although they form structurally lowest cumulates in the layered series, Nathan interpreted this unit to be younger than structurally higher troctolitic to gabbronoritic units on the basis of a complex sequence of discordant relationships (Units A and B, Fig. 3.21). The validity of Nathan's intrusive stratigraphy clearly needs to be tested. --Return to Rockwood Lodge for lunch and prepare for afternoon canoe trip.-- WARNING: The ice just went out (we hope) on the lake and so the water is just above freezing. We will take some time over the lunch hour to instruct everyone on basic canoe safety and what to do in the event that someone dumps in the water. We have BWCAexperienced assistants on the trip and we will bring special equipment to guard against hypothermia. Despite these precautions, the best preventative is using extreme care and common sense. ILSG07 10 Trip 1 �it-' '} POPLAR LAKE INTRUSION Units based on Nathan (1969) dq d Troctolite medium, foliated, prismatic 01; gradational into dp 01 gabbro- fine?, foliated, intergran, 0l:subpoik-subprism Gabbronoritic Anorthosite - coarse, ophitic-subophitic dg Olivine oxide gabbro- crs, fOliated, intergran,0l:spoik-sprism; dgt oxide-rich (>10%) zone df dc —db - da Olivine oxide gabbro van-textured, locally sulfidic Mafic hornfels - fine, granoblastic; basalt inclusions? Gabbronorite- medium, fOliated, Opx:poikilitic-subpoikilitic Gabbronoritic Troctolite medium, fOliated, intengranular, Opx:poikilitic Figure 1-9. Geologic map of the central Poplar Lake area showing field stop locations for the afternoon of Day 1. Dashed line shows canoe route. Geology after Morey and Nathan (1977). STOP 1-2. UNIT G’ – OLIVINE OXIDE GABBRONORITE Location: Float by shoreline ledges on island about 300m due south of Rockwood Lodge. UTMs: 684800E 5325000N Description: These lichen-covered, grussy outcrop ledges are composed of medium-grained, wellfoliated, oxide gabbro typical of Unit G’, an oxide-rich (locally >20%) interval within Unit G. Paddling to the east, a prominent outcrop can be seen at the western point of a peninsula. The well-developed foliation gives rise to a sheet jointing that dips about 20° to the south. --Canoe around the east end of this island then south into a narrow channel. Head WSW to eastern point of another island which has a sizeable dock for disembarking. -- STOP 1-3. UNIT G – OXIDE OLIVINE GABBRONORITE Location: Private residence and dock at east point of island; please request permission from owners to view exposure. UTMs: 684905E 5325355N Description: The main rock type exposed over this outcrop point is a medium coarse-grained, foliated, intergranular, oxide olivine gabbronorite. This exposure is typical of Unit G. One curious feature is the occurrence of a 20- to 30-cm-thick, fine grained sill of a similar mineralogy ILSG07 11 Trip 1 �and texture exposed in the cliff face back from the boat dock. The lower contact is rather sharp but the upper contact is gradational (Fig. 1-10A). Throughout the sill and especially near the upper contact, coarse crystals and crystal clusters of pyroxene and plagioclase occur (Fig. 1-10B). Petrographic observations across this sill show that it differs from the coarser host in several subtle ways. Olivine in the fine-grained gabbro is commonly subpoikilitic compared to granular olivine in the host gabbro. Low-Ca pyroxene is less abundant in the fine-grained gabbro (5% compared to 20%) and is dominantly hypersthene rather than inverted pigeonite. Perhaps this represents a new pulse of magma into the chamber or it marks venting event from the magma chamber causing decompression quenching of a water-saturated evolved magma. The presence of up to 2% biotite and the generally similar mineralogy and texture of the fine-grained and medium-grained gabbros suggest the latter explanation is more likely. A. B. Figure 1-10. A) Fine-grained gabbro sill in medium-grained gabbro host; B) Close-up of piece of float from the sill show dispersed coarse plagioclase and pyroxenes in the fine matrix. --Canoe about 400m due south to northernmost point of eastern end of peninsula. Limited landing spots, so this stop is optional,-- STOP 1-4. UNIT G OR P?- LAYERED OLIVINE GABBRO (Optional) Location: Point of land in southwestern Poplar Lake near the far north central part of Sec. 12. UTMs: 684785E 5324469N Description: Nathan implies by his map and unit designation that this as a thin inclusion? of unit G enclosed between younger units P and J (Fig. 1-9). The exposure here is a medium- to medium fine-grained, moderately layered, well-foliated, intergranular oxide olivine gabbro. A thin section sample from here shows a medium fine-grained oxide olivine gabbro that has a very granular (granoblastic?) texture and is lacking in low-Ca pyroxene. This rock has similarities to Nathan’s Unit P by being fine, olivine-rich, and oxide poor. The lack of Opx makes it distinct from either Unit P (commonly contains some poikilitic hypersthene) and especially Unit G (typically contains significant amounts of inverted pigeonite). --Canoe about 550m to the east-southeast to island. Land canoes against outcrops on the southeast and southwest sides of the island.,-- ILSG07 12 Trip 1 �STOP 1-5. UNIT J – OLIVINE OXIDE LEUCOGABBRO Location: Island in southern part of Poplar Lake. UTMs: 685210E 5324200N Description: Exposures on the south side of the island are composed of coarse-grained, foliated, ophitic to intergranular olivine oxide leucogabbro, which Nathan classifies as Unit J. Plagioclase ranges from 70 to 85% of the rock and is the only consistently cumulus phase. Augite and Fe-Ti oxide are the dominant mafic minerals and vary from anhedral granular to poikilitic. Minor olivine (3-5%) is also variable in texture from subprismatic (Fig. 1-11A) to subpoikilitic. Layers and pods rich in granular Fe-Ti oxides and pyroxene are evident on the southwestern corner of the island (Fig. 1-11B). In poor exposures on the north side of the island (downsection), granular (cumulus) augite, olivine, and oxide are present and the rock seems less leucocratic. A. B. B Figure 1-11. A) Common texture of leucogabbro with granular mafic phases (Ol, Aug, FeOx). B) Massive Fe-Ti oxide layer in leucogabbro. -- Paddle ~175m to the southeast to small island with campsite. Land canoes against outcrops on the south side or in cove on east side of the island. Welcome to Happy Island!-- STOP 1-6. UNIT P – OLIVINE GABBRO Location: Island in southern part of Poplar Lake. UTMs: 685310E 5324050N Description: “Happy Island” is composed of two east-west oriented outcrop areas. Exposures on the south side of the island are composed of medium-grained, well-foliated, intergranular olivine gabbro with variable abundances of Fe-Ti oxide (1-7%). Olivine occurs as small (<1cm), subpoikilitic clots or as prisms 0.5 to 3 cm long (~1:10 aspect ratios; Fig. 1-12) that tend to be concentrated in thin layers. This rock type is consistent with Nathan's description of unit P. Morey and Nathan’s (1977) map shows a contact between unit P to the south and unit Q, a troctolite, to the north running through the middle of the island. However, outcrop exposures and petrographic observations of two samples from the northern end show the rock to be mostly an oxide olivine gabbro like Unit P. The only significant differences are a slightly coarser grain size, subtle textural and modal layering, and a slightly higher concentration in inverted pigeonite. Nothing on the north half looks like an augite-poor troctolite, which characterizes Unit Q (See Stop 1-9). ILSG07 13 Trip 1 �A. B. Figure 1-12. Textures of olivine found in Unit P: A) 5mm subpoikilitic clots (Stop 1-7); B) 1cm long prismatic olivine (Stop 1-6) -- Paddle ~200m to the east-southeast to the south side of an E-W island. Land canoes against outcrops on the south-central of the island. -- STOP 1-7. UNIT P – OLIVINE GABBRO Location: Island in southern part of Poplar Lake. UTMs: 685532E 5323990N Description: Island is composed of two outcrop areas on the south side of the island, between which access is difficult. Both exposure areas are composed of medium-grained, well-laminated, intergranular olivine gabbro (PAOf cumulate) as observed on the south side of the island at Stop 3-3. Locally, 2-5-cm clots of coarse pyroxene and oxide are present and elsewhere are pods (inclusions?) of leucogabbro (Unit J?). Again olivine occurs as small subpoikilitic clots or as prisms up to 3 cm long (Fig. 1-12). Because prismatic olivine tends to be concentrated in thin layers along which joints tend to develop, this texture is preferentially displayed on the outcrop face. Note that on the plane of layering, prismatic olivine is commonly oriented parallel to the strike of layering. This observation and the delicate prismatic habit of the olivine argues against crystal settling by density currents. -- Canoe ~250m to the southwest to steep cliffs lining the bay leading the portage to Lizz Lake. Float along the cliffs and then land canoes at the portage. -- STOP 1-8. UNIT J –LEUCOGABBRONORITE-GABBROIC ANORTHOSITE Location: Bay leading to portage (to Lizz Lake) at south shore of Poplar Lake. UTMs: 685460E 5323700N Description: Exposed in sheer cliffs along the west side of the bay leading to the portage to Lizz Lake is leucogabbroic rock typical of Unit J. Lichen and moss cover make it difficult to see much of the rock. A thin section of a sample collected here shows the rock to be a medium coarse-grained ophitic leucogabbronorite. In some places , pockmarked surfaces are evident and indicate the presence of large oikocrysts of poikilitic olivine. This texture is very well ILSG07 14 Trip 1 �displayed in the exposure on the west side of the portage trail (Fig. 1-13). Here the olivine oikocrysts are up to 5 cm across. Ophitic augite is also evident here in oikocrysts up to 10 cm across. Interestingly, this rock type is very similar to the dominant lithology of the Anorthositic Series of the Duluth Complex. It would worthwhile determining the magnetic polarity of this rock type to see if it has a typical Poplar Lake reversed polarity, or is if is normal like all known Anorthositic Series rocks. Figure 1-13. Mottled surface of ophitic olivine gabbroic anorthosite at Stop 1-8. Clots are formed from both olivine and augite oikocrysts. -- Canoe ~600m to the northeast past the eastern point of island where Stop 1-7 is located. Continue northeast to middle point of large island. Not sure how many canoes can land here so this is an optional stop.. -- STOP 1-9. UNIT Q –TROCTOLITE (OPTIONAL) Location: Middle point of west side of large island in southern Portage Lake. . UTMs: 685850E 5324150N Description: According to a thin section sample of this outcrop that shows a medium-grained, well foliated troctolite with small olivine grains, this site is more typical of Nathan’s Unit Q. -- Canoe northwest back to Rockwood Lodge (~1.6 kilometers). End of Day 1.— ILSG07 15 Trip 1 �FIELD TRIP 1- DAY 2 Depart from Rockwood Lodge and return to portage to Lizz Lake (Stop 1-8). Portage canoes to Lizz Lake landing then double back on portage trail to view first stop of the day. Mesoproterozoic POPLAR LAKE INTRUSION (Units based on Nathan (1969)) 01 gabbro- fine?, foliated, dp intergran, oI:subpoik-subprism F • Gabbronorite- coars%oph- - dni - - intergan, cornnoniy eucocratic Gabbronoritic Anorthosite dj — - db cia - coarse, ophitic-subophitic Gabbronorite- medium, bliated, Opx:pcikilitic-subpoikilitic Gabbroncritic Troctolitemedium, Ibliated, intergranular, Opx:poikiiitic Figure 1-14. Geologic map of the Lizz and Caribou Lakes area showing field stop locations for Day 2. Dashed line shows canoe route. Geology after Morey and Nathan (1977). STOP 1-10. UNITS J AND B AND THE LIZZ LAKE FAULT Location: Lizz Lake portage trail just 50m south of junction with Banadad trail. UTMs: 685500E 5323600N Description: Exposed on the east side of the trail is a medium coarse-grained ophitic oxide gabbronoritic anorthosite typical of leucocratic Unit J. About 10m to the west of the trail is a poorly exposed ledge of medium-grained, foliated, gabbronorite with poikilitic hypersthene. The rock contains no olivine or Fe-Ti oxide. This rock type is typical of Nathan’s Unit B. The juxtaposition of these two distinct rock types across this gap is the best evidence of right lateral offset across a fault that Morey and Nathan (1977) inferred through here and down the elongate trend of Lizz Lake (Fig. 1-14). The Lizz Lake fault is the only fault that Nathan (1969) identified in the interior of the Poplar Lake Intrusion. -- Canoe a short distance to the outcrop island about 100 m to the southeast of the portage-- ILSG07 16 Trip 1 �STOP 1-11. UNIT J? – GABBRONORITIC ANORTHOSITE Location: Island with abundant outcrop on north end of Lizz Lake. UTMs: 685685E 5323490N Description: Good exposures over most of this island show the rock to be a medium coarsegrained, poorly foliated, ophitic, gabbronoritic anorthosite similar to the Unit J outcrop at Stop 1-10. However, Nathan has this exposure mapped within Unit B. This may indicate that the Unit J-Unit A/B contact is farther south on the east side of the Lizz Lake fault. Alternatively, perhaps this is an inclusion in Unit B though this would violate Nathan’s interpretation of Unit B being an early phase of the Poplar Lake intrusion. More mapping in this area is needed to resolve this. -- Float along a series of cliff faces on the western shore of Lizz Lake to view Stop 12 exposures-- STOP 1-12. UNIT B – GABBRONORITE Location: Series of outcrop ledges along the northwestern shoreline of Lizz Lake. UTMs: 685715E 5323345N Description: Although largely obscured by lichen, the ledges along this shoreline display a homogeneous, medium-grained, well-foliated gabbronorite with Opx > Cpx. Orthopyroxene (hypersthene and inverted pigeonite) is typically subpoikilitic to poikilitic, whereas clinopyroxene (augite) is typically anhedral granular (Fig, 1-15A). In one well exposed location beneath a tree tip-up, a subtle modal layering was recognized (Fig. 1-15B). A. B. Figure 1-15. A) Texture of the gabbronorite typical of Unit B. B) Subtle modal layering common observed under a tip-up at Stop 1-12. -- Canoe to the south end of Lizz Lake to portage to Caribou Lake. From the portage landing, head west about 8m to a rock cliff -- ILSG07 17 Trip 1 �STOP 1-13. UNITS B AND X? – GABBRONORITE AND FINE GABBRO Location: Outcrop ledge southwestern corner of Lizz Lake near portage to Caribou Lake. UTMs: 686195E 5322605N Description: Most of this exposure is a very fine-grained mafic rock in sharp contact with a medium-grained gabbro. Thin sections of the fine-grained rock shows it to be a feltytextured, oxide gabbronorite and the medium-grained gabbro is a well foliated, intergranular gabbronorite with poikilitic olivine. Nathan mapped this area as largely unit B with an occurrence of Unit X, which he defines as a fine-grained Pl-porphyritic oxide gabbro. By its unit letter designation, Nathan interpreted this as young intrusive unit. The medium gabbronorite is similar to typical unit B, except for the poikilitic olivine. Perhaps it is a hybrid between Units B and A. The fine-grained rock being an intrusion into B would be consistent with its felty texture. One would expect the rock to display a granoblastic texture if it was a hornfels inclusion. We will investigate the exposure to ascertain whether the fine-grained rock is an intrusion or an inclusion. -- Take the portage to Caribou Lake. Canoe 180m south to a campsite fronted by a large outcrop pavement-- STOP 1-14. UNIT A – OLIVINE GABBRONORITE Location: Outcrop pavement at campsite on south shore of northeast arm of Caribou Lake. UTMs: 685960E 5322155N Description: Very homogeneous exposure of medium-grained, foliated, olivine gabbronorite with poikilitic inverted pigeonite. Some of the inverted pigeonite oikiocrysts can be up to 30 cm across. This exposure is identified as Unit A by Morey and Nathan (1977), which they describe as being a fine-grained troctolite with up to 9% inverted pigeonite and 3% augite. The thin section observed from this outcrop contains 8% augite and 12% inverted pigeonite as well as 20% olivine. Given that unit A occurs at many statigraphic levels in the upper part of the Poplar Lake intrusion, it is perhaps not surprising that it is variable in mode, -- Canoe ~1200 m to the southeast arm Caribou Lake to outcrop-fronted campsite on northern shore-- STOP 1-15. UNIT M – AUGITE NORITE Location: Outcrop pavement at campsite on north shore of southeastern arm of Caribou Lake. UTMs: 686347E 5321875N Description: Homogeneous outcrop of medium-grained, well-foliated, intergranular augite norite. Thin section show that 25% of rock is anhedral to subprismatic granular inverted pigeonite with anhedral granular augite comprising about 10% of the rock. The prismatic inverted pigeonite weathers brown which makes it appear similar to olivine in outcrop (Fig. 1-16). The rock is devoid of olivine and contains only trace Fe-Ti oxide. ILSG07 18 Trip 1 �Nathan and Morey (1977) describe unit M as a coarse-grained, poorly foliated, gabbronorite with average mafic mineral concentrations of up to 2% oxide, 13% inverted pigeonite, and 24% augite. Figure 1-16. Texture of the prismatic augite norite at Stop 1-15. -- Canoe ~400 m south to the portage to Horseshoe Lake -- STOP 1-16. UNIT M – GABBRONORITIC ANORTHOSITE Location: Small outcrop at portage landing to Horseshoe Lake in southeastern Caribou Lake. UTMs: 686375E 5321490N Description: A small outcrop at portage landing appears to be an example of what Nathan cites as the “plagioclase-rich facies” of Unit M, which typically contains coarsely poikilitic pyroxene (Morey and Nathan, 1977). A thin section of this rock shows it to be medium coarse-grained, poorly foliated, ophitic gabbronoritic anorthosite. -- Canoe ~1500 m to a campsite on a peninsular point in western Caribou Lake -- ILSG07 19 Trip 1 �STOP 1-17. UNIT M OR A? – OLIVINE GABBRONORITE Location: Outcrop-fronted campground on peninsula in western Caribou Lake. UTMs: 685030E 5322070N Description: Nathan includes this exposure within Unit M, but the rock is clearly olivine-bearing and is more similar to Unit A lithologies. A thin section from this location reveals a mediumgrained, foliated, ophitic olivine gabbronorite with about 17% granular olivine, 13% poikilitic inverted pigeonite, and 4% subophitic augite. Basically, it is similar to the olivine gabbronorite observed at Stop 1-14, which is mapped as Unit A. Morey and Nathan (1977) place the Unit M – Unit A contact just north of this peninsula (Fig. 1-14), suggesting that it should be farther south. -- Canoe north around island and then east; continue past small peninsula on north shore to campsite; total distance approximately 800m. -- STOP 1-18. UNIT M OR B – LAYERED GABBRONORITE Location: Outcrop pavement at BWCA campsite on the north shore of Caribou Lake. UTMs: 685575E 5322400N Description: Generally a medium coarse-grained, subophitic to intergranular leucogabbronorite (Fig. 1-17A) locally with cm-scale mesocratic to melanocratic layers (Fig. 1-17B). This exposure is situated right at a contact between Unit B and an irregular body of Unit M. Given its coarser grain size and generally leucocratic composition, it seems closer to Unit M. A. B. Figure 1-17. A) Texture of the leucogabbronorite at Stop 1-18. B) Cm-scale wavy modal layering displayed at Stop 1-18. -- Continue along north shore to Lizz Lake portage. Return to Rockwood Lodge. – END OF FIELD TRIP 1 ILSG07 20 Trip 1 �References Beck, M.E., 1970, Paleomagnetism of Keweenawan intrusive rocks, Minnesota. Journal of Geophysical Research, v. 75, p. 4985-4996. Bonnichsen, B., 1972, Southern part of the Duluth Complex. In: Sims, P.K. & Morey, G.B. (eds.) Geology of Minnesota - A centennial volume. Minnesota Geological Survey, p. 361-388 Broderick, T.M., 1917, The relationship of the titaniferous magnetites of northeastern Minnesota to the Duluth Gabbro. Economic Geology, v. 12, p. 663-696 Davidson, D.M., Jr., 1972. Eastern part of Duluth Complex. In: Sims, P.K. & Morey, G.B. (eds.) Geology of Minnesota - A Centennial Volume. Minnesota Geological Survey, p. 354-360 Davidson, D.M, Jr., 1977a, Reconnaissance geologic map of the Eagle Mountain quadrangle, Cook County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series, M-28, scale 1:24,000 Davidson, D.M, Jr., 1977b, Reconnaissance geologic map of the Lima Mountain quadrangle, Cook County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series, M-32, scale 1:24,000 Davidson, D.M, Jr., and Burnell, J.R., Jr., 1977, Reconnaissance geologic map of the Brule Lake quadrangle, Cook County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series, M-29, scale 1:24,000 Davis, D.W., and Green, J.C., 1997, Geochronology of the North American Midcontinent rift in western Lake Superior and implications for its geodynamic evolution: Canadian Journal of Earth Science, v. 34, p. 476-488 Davis, D.W., and Sutcliffe, R.H., 1985, U-Pb ages from the Nipigon plate and northern Lake Superior: Geological Society of America Bulletin 96, p. 1572-1579 Green, J.C., 1972, North Shore Volcanic Group, in Sims, P.K., and Morey, G.B., eds., Geology of Minnesota—A centennial volume. Minnesota Geological Survey, p. 294-332. Grout, F.F., 1950, The titaniferous magnetites of Minnesota: Department of Iron Range Resources and Rehabilitation, 117 p. Grout, F.F., Sharp, R.P., and Schwartz, G.M, 1959, The geology of Cook County Minnesota. Minnesota Geological Survey Bulletin, v. 39, 163 p. Jerde, E.A. , 2001, The early gabbroic series of the Midcontinent Rift system: Continued assessment magmatic origins. 47th Annual Institute on Lake Superior Geology, Madison, WI, p. 36-37. Mathez, E.A., Nathan, H.D., Morey, G.B., 1977, Geologic map of the Hungry Jack Lake quadrangle, Cook County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series, M-39, scale 1:24,000 Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., and Peterson, D.E., 2001, Geologic map of the Duluth Complex and related rocks, northeastern Minnesota. Miscellaneous Map Series, M-119, scale 1:200,000 Miller, J.D. Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.E., and Wahl, T.E., 2002, Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota. Minnesota Geological Survey Report of Investigations 58, 207p. w/ CD-ROM Morey, G.B., and Nathan, H.D., 1977, Geologic map of the South Lake quadrangle, Cook County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series, M-38, scale 1:24,000 ILSG07 21 Trip 1 �Morey, G.B. and Nathan, H.D., 1978, Geologic map of the Gunflint Lake quadrangle, Cook County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series, M-42, scale 1:24,000 Morey, G.B., Weiblen, P.W., Papike, J.J., and Anderson, D.H., 1981, Geologic map of the Long Island Lake quadrangle, Cook County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series, M-46, scale 1:24,000 Paces, J.B., and Miller, J.D., Jr., 1993, Precise U-Pb ages of Duluth Complex and related mafic intrusions, northeastern Minnesota: geochonological insights to physical, petrogenetic, paleomagnetic and tectono-magmatic processes associated with the 1.1 Ga Midcontinent rift system: Journal of Geophysical Research, v. 98, no.B8, p. 13,997-14,013. Phinney, W.C., 1972, Northern prong. In Sims, P.K., and Morey, G.B., eds., Geology of Minnesota— A centennial volume. Minnesota Geological Survey, p. 346-353 Vervoort, J.D. and Wirth, K.R., 2004, Origins of the rhyolites and granophyres of the Midcontinent Rift, northeast Minnesota. 50th Annual Institute on Lake Superior Geology, Duluth, MN, p. 158-159. Weiblen, P.W. and Morey, G.B., 1980, A summary of the stratigraphy, petrology, and structure of the Duluth Complex. American Journal of Science, v. 280-A, 88-133. Weiblen, P.W., 1982. Keweenawan intrusive igneous rocks. In: Wold, R.J. & Hinze, W.J. (eds.) Geology and tectonics of the Lake Superior Basin. Geological Society of America Memoir 156, 57-82. Winchell, N.H., 1899, The Geology of Minnesota. Geological and Natural History Survey of Minnesota, Final Report v. 4, 354p. w/ 100 plates. ILSG07 22 Trip 1 �53rd Annual Institute on Lake Superior Geology FIELD TRIP 2 GEOLOGIC AND CULTURAL HISTORY OF THE GRAND PORTAGE NATIONAL MONUMENT William F. Cannon U.S. Geological Survey David J. Cooper U.S. National Park Service, Grand Portage National Monument Brian A.M. Phillips Lakehead University, emeritus Engraving of Grand Portage area made from field sketch done in 1849 by survey party from U.S. General Land Office. Illustration is included in report by David Dale Owen in 1852. Engraving shows physiography of the area as well as geologic cross sections. ILSG07 23 Trip 2 �GEOLOGIC AND CULTURAL HISTORY OF THE GRAND PORTAGE NATIONAL MONUMENT William F. Cannon, U.S. Geological Survey Brian A.M. Phillips, Lakehead University, emeritus Including a section on cultural history excerpted from a report by Douglas Birk, Senior Archeologist emeritus, Institute for Minnesota Archeology Introduction The Grand Portage National Monument, maintained by the U.S. National Park Service, consists of a restoration of an 18th century fort and trading post on the shore of Lake Superior and the 8.5 mile portage trail extending northwestward from the lake shore to the Pigeon River. The unrestored Fort Charlotte lies at the northwest end of the trail, on the south bank of the Pigeon River, at the head of a long series of rapids and waterfalls that made this long portage necessary in order to continue westward canoe travel from Lake Superior. The portage trail was the principal highway to the west for 18th century explorers, settlers, and fur traders who crossed the portage from Lake Superior to the Pigeon River, then into the westward draining Rainy River system, and eventually deep into the northern Great Plains and Rocky Mountains. It was over this trail that the wealth of furs was brought on their way to Montreal and eventually to Europe. Those who crossed the trail were largely responsible for the exploration and development of the Canadian west and northern U. S. Great Plains. The early fur traders, the Voyageurs, made their annual trek over the portage in late summer and then began their long canoe journey to the west to trade with native peoples for furs over the ensuing winter. They returned over the portage the following summer to the trading post to deliver their bounty of furs to representatives of the North West Company (NWC) who annually traveled from Montreal to the Grand Portage trading post. The geology of the Monument and surrounding countryside consists of: 1) the Rove Formation, a Paleoproterozic clastic sequence of shale and sandstone, 2) mafic dikes and flows formed in the Mesoproterozoic Midcontinent Rift, and 3) glacial and post-glacial deposits produced by the last tongue of Pleistocene glaciers that occupied the western Lake Superior basin and a variety of lake shoreline features formed by a series of post-glacial lakes that inundated the area shortly after deglaciation. During this trip we will tour the restored fort and trading post, discuss the European and Native American history of the site, and examine bedrock and glacial to post-glacial geologic features of the area, particularly as they affected the human history through the unique set of physiographic features that they produced. Figure 2.1 is a map of the Monument showing the location of restored buildings on Grand Portage Bay, and three geologic field trip stops. ILSG07 24 Trip 2 �Figure 2.1. Map of the Grand Portage region showing the location of the National Monument and portage trail. Geologic field trip stops are also shown. Cultural history (This account of cultural history is excerpted and slightly modified from Douglas Birk, 2005, National Register of Historic Places, Grand Portage) Grand Portage derives its name and much of its historical significance from a formidable portage, more than eight miles long, which links the Great Lakes with an interconnected series of inland waterways to the northwest. Key to the development of the North American fur trade, the Grand Portage gave Canadian explorers access to the northern Plains and to travel routes extending to the Arctic and Pacific Oceans. In the late eighteenth century the portage became a hub of seasonal coalescence and transshipment activities that linked suppliers in far-off places like England, the West Indies, and Brazil with a network of trading posts set in mosaic Native hunting territories throughout western Canada. The setting of Grand Portage, on the west side of Lake Superior and near the mid-point of a 3,000-mile commercial canoe transportation system, enhanced its role in channeling, shaping, and controlling the northwestern Montreal fur trade. Inspired by these qualities, one historian has declared that Grand Portage was once “probably the single most important fur trade location in the world.” North American Indian peoples knew the use of canoes and portaging thousands of years before the arrival of Europeans, and they initially opened the Grand Portage route. French colonials traveling west from Montreal first arrived at Lake Superior early in the seventeenth century. Having learned of the Grand Portage from Indian informants by at least 1722, the French began crossing it in 1731 to advance their affairs and alliances in more remote areas to the Northwest. From then through the mid-1800s, the portage attracted a colorful succession of French, British, and American fur traders. Between 1780 and 1802, at the height of the commercial traffic, Grand Portage was a western headquarters and regional trade center of the North West Company. It was also the scene of an annual summer gathering that made it one of the busiest nodes of human activity in the midcontinent. By promoting the mercantile interests of France and Great Britain, the Grand Portage facilitated the spread of European colonialism and the development of the nations of North America. In the nineteenth century Grand Portage also played an important part in settling the disputed boundary between Minnesota and Ontario, which is to say between Canada and the contiguous United States in the region west of Lake Superior. ILSG07 25 Trip 2 �The most enduring human presence at Grand Portage is that of aboriginal North Americans, including, most recently, the Ojibwe, Algonkian-speaking peoples with Woodland cultural traditions. Details surrounding the emergence of the Ojibwe people or the Ojibwe identity are often debated. Some traditional accounts indicate that, in the seventeenth and eighteenth centuries, the Saulteur (Ojibwe) of the eastern Lake Superior region extended their territorial prerogatives westward with the advancing fur trade. A group since known as the Grand Portage Ojibwe established ties to Grand Portage by the 1730s. They were active in that region throughout the fur trade era, and they remain a vital community today. All landscapes, resources, sites, properties, themes, and contexts that are now part of the Grand Portage National Monument story are in some way linked to the Grand Portage Ojibwe. The National Monument was formed through their invitation and cooperation, and efforts to maintain and interpret historic properties and traditions at Grand Portage have long been assisted by that local band or by individual members of the band. French and later British traders commonly entered the Great Lakes-Northwest trade by traveling west from Montreal. The use of Indian guides, birchbark canoes, and a vast network of established Indian canoe routes and portages assisted their movements (Morse, 1969; Birk, 1994). From the beginning, the fur trade built on entrenched Indian exchange practices while catering to Indian preferences and needs (e.g., White, 1982, 1987). Over time, as the business of accruing and transporting goods grew more complex and exchange frontiers expanded westward, the systems of trade were institutionalized. Certain individuals, like voyageurs, became specialists in the trade hierarchy just as certain places gained wider distinction as nodes or corridors of commerce. The most influential settlements were those that played strategic roles in the flow of workers, provisions, merchandise, and information (e.g., Hirth, 1976). Grand Portage became such a place in the eighteenth century. Through regular use, the Grand Portage emerged as a hub for local trade and as a transshipment center--a gateway community that linked markets and linear transportation lanes in the east with branching trade routes and mosaic trade districts to the northwest (Birk, 1984). Between 1731 and 1804 tons of supplies and furs were shuttled over the portage; some, in and out of the warehouses eventually built at either end of the trail. After 1760 the portage became a general rendezvous. It was a veritable beehive of activity during summers, but in winters the outposts there were comparatively quiet and staffed only by small crews engaged in local trading and facilities management (Gilman, 1992). At the height of the trade, around 1800, Grand Portage was the western headquarters of the North West Company (NWC) and the rival XY Company (XYC), two of the largest commercial establishments in North America. When the NWC and XYC moved their operations north to Kaministikwia (later Fort William, Ontario) at the start of the nineteenth century, Grand Portage lost its identity and its status as trade center, abruptly becoming remote to the main channels of trade and communication and less important to the outside world. Grand Portage is a place of majestic scale and natural beauty with many prominent landmarks. The lakefront area faces Grand Portage Bay, a horseshoe-shaped inlet that forms the deepest natural indentation on the Minnesota coast of Lake Superior (Schwartz, 1928). Flanking the northeast side of the bay is the rolling ridge of Hat Point (Pointe au Chapeaux), backed farther inland by the promontory of Mount Josephine, the top of which towers nearly 750-feet above the surface of the lake. The opposite, southwest edge of the bay is framed by the sloping headland of Raspberry Point (Pointe à la Framboise). About a mile offshore, in the center of the bay and partly shielding the bay from the big lake, is Grand Portage Island (also known as Pete’s Island, Sheep Island, Isle aux Mouton, etc.) (Gates, 1965; Grout and others, 1959). Near the lakefront within the Grand Portage National Monument is an imposing hill known as Mount Rose. The rounded and rocky summit of the hill rises several hundred feet above the bay and overlooks all of the historic properties clustered along the shoreline there (Winchell, 1899; Thompson, 1969; Gilman, 1992), as well as the mouth of Grand Portage Creek, a stream channel that drains from the interior highlands and skirts the base of Mount Rose before entering Lake Superior. Mount Rose once ILSG07 26 Trip 2 �served as a vantage to watch for approaching watercraft, the earliest such documented use perhaps occurring in 1767 (Carver, 1956). The Grand Portage is the first leg of a remarkable inland canoe route that passes from Lake Superior through a chain of lakes and rivers along the present international boundary between Ontario and Minnesota. That mainline route, sometimes called the “Voyageur’s Highway,” links with a vast network of other branching canoe trails in the hinterlands beyond (Morse, 1969). Few viable inland water routes emanate from the west side of Lake Superior, and the Grand Portage-Pigeon River route early proved to be the most direct and efficient gateway to the “border lakes” region and the far Northwest (Gilman, 1992). The Pigeon River is a relatively short stream whose headwaters at the rim of the Superior basin are only about 50 miles west of Grand Portage Bay and enters Lake Superior about five miles to the northeast of Grand Portage. The river is navigable by canoe for much of its length above Fort Charlotte. For 20 miles below Fort Charlotte, however, the Pigeon River is impassable. That stretch follows an eastward course through a sinuated series of rapids, canyons, and falls before reaching the big lake. The terrain on the Canadian side of the lower Pigeon River is too rugged and the distance between the lake and navigable parts of the river too great for portaging to be practical there (Schwartz, 1928; Buck, 1931; Burpee, 1931; Morse, 1969, Birk, 1998). The ascent from Lake Superior through the Grand Portage and up the Pigeon River to the height-of-land separating the Laurentian and Hudsonian basins is the steepest part of the entire historic canoe route that once ran between Montreal and Athabasca (Morse, 1969; Birk, 1998). Prior to the time the NWC made improvements to the trail, heavily outfitted canoe brigades could take up to ten days to move from Lake Superior to the Pigeon River (Wallace, 1934). During the heyday of the fur trade, with the NWC installed at Grand Portage, the same carry might take from five to seven days (Gilman, 1992). Some men loaded with packs or kegs weighing 180 pounds or more were able to make a round trip on the portage within six hours (Buck, 1931). Because porters often moved at a trot and took short rests en route, their actual progress was about as expedient as that of an unencumbered walker, who, under optimal conditions and without rests, might cross the portage in 2.5 hours (Thompson, 1969). Early fur traders tried using horses and oxen on the portage but under normal conditions in warmer seasons of the year, the trail was best suited for human porters (Burpee, 1931; Thompson, 1969; Gilman, 1992). Animal-drawn carts or sleds were more commonly used during the “off season” (other than summers) or during the nineteenth century, when traffic on the portage was greatly reduced. Diminished forest cover along some parts of the portage in the 1800s opened extensive views of the surrounding terrain and perhaps changed the surface environment of the trail (e.g., Winchell, 1899). In 1858, for example, the trail was said to be dry and in good condition, making it passable for oxen teams (Hind in Dawson, 1968). The Fort Charlotte site complex involves the remains of fur trade facilities that once stood along the Pigeon River at the head of the portage on either side of the mouth of Snow Creek. Evidence suggests that the NWC conducted operations along the river north of the creek (between the creek and the whitewater rapids), while the XYC and perhaps other earlier firms occupied an area along the river south of the creek. To reach these various facilities the portage trail probably forked somewhere east of the Pigeon River so that one branch of the trail led to the south side of the creek and the other continued along the north side. The main trail, the one most used and over the greatest length of time, was that on the north side. Underwater archeological investigations conducted at the main Pigeon River landing suggest that, through about 1780 or so, the main portage intersected the river very close to the mouth of Snow Creek. With the formation and growth of the NWC, the portage landing migrated downstream, first to the place where the NWC built a canoe landing and quay or “dock” (Winchell, 1899), and later to where the trail meets the river today, at a point about midway between Snow Creek and the head of the whitewater rapids (Birk, 1975; Birk and Wheeler, 1976; Wheeler and others, 1975; Birk, 1979). Modern recreational use of the landing, along with floating ice, fluctuating water levels, river currents, and log drives on the ILSG07 27 Trip 2 �Pigeon River, may have damaged early landing structures or associated offshore archeological deposits at Fort Charlotte (Birk, 1975). French traders frequented the portage between 1731 and 1760 (Nute; 1944; Woolworth and Wooworth, 1982) and perhaps earlier (e.g., Burpee, 1931), yet suspected French-era deposits or structures at Grand Portage Bay have eluded archeological discovery. Surprisingly little information about French activities at Grand Portage is found in available written records either. Nevertheless, some scholars suggest that French traders erected buildings at Grand Portage before 1760 (e.g., Warren, 1957; Buck, 1931; Babcock, 1940; Nute, 1944; Woolworth, 1967; Woolworth and Woolworth, 1982), that they had facilities at both ends of the portage as early as 1732 (Woolworth and Woolworth, 1982), or that they maintained structures at each end of the portage from ca. 1732 through 1760 (Woolworth and Woolworth, 1982). An unsubstantiated “French post” at the Pigeon River landing even has an assigned cultural resource inventory number (CR-105) (Woolworth and Woolworth, 1982). Other researchers are skeptical of such varied interpretations (e.g., Burpee, 1931; Thompson, 1969; Gilman, 1992). The apparent lack of Frenchcolonial materials on the lakefront at Grand Portage Bay west of Grand Portage Creek hints that the greatest potential for such materials there may lie in areas east of the creek (Brown, 1937). To date, the only documented French colonial archeological loci are those identified during field investigations at the Pigeon River landing (Birk and Wheeler, 1976) and at the suspected Parting Trees pose on the Grand Portage trail (Douglas A. Birk, personal observation). Barring future archival discoveries, most questions regarding French presence at Grand Portage may only be answered through further archeological inquiry. Figure 2.2. Artist’s conception of North West Company Depot at Grand Portage The earliest known trading houses at Grand Portage Bay were those established by British traders in about 1768. The houses were built west of Grand Portage Creek at the place later developed for the NWC depot (Nute, 1940; 1944). Other trade facilities were later constructed along the lakefront and, by 1793, the NWC alone was said to have sixteen buildings within its fort. The NWC also claimed dominion over adjoining parcels at the bay and may have opened facilities at the Pigeon River at the time of the its formation (1783). Rival traders soon followed the NWC’s example by building a “hangard or store” on the Pigeon River in 1785 (Gates, 1965; Thompson, 1969). Being more abundant and pervasive, material ILSG07 28 Trip 2 �evidence of British presence at Grand Portage has proved easier to find, identify, and interpret than that relating to earlier French colonial operations. Some features noted or implied in early historical records have yet to be identified through archeology. For example, the NWC had horses, cattle, hogs, and sheep at Grand Portage (Thompson, 1969; Gilman, 1992), which likely necessitated the use of barns, stables, pens, or corrals. The central clearing at Grand Portage Bay could have been used for pasturage, and grazing livestock there may actually have given the NWC a reason and excuse to fence the perimeters of that opening. To overwinter livestock required labor-intensive preparations. Hay was said to be abundant at Grand Portage in the late 1700s, but it had to be cut and stacked by hand. At the same time, cold damp fog, along with high levels of ground moisture and lingering winters, often caused harvested hay to rot (e.g., Thompson, 1969). Gardening was similarly impaired by weather and soil conditions. Potatoes were the only food crop typically grown with any success (e.g., Thompson, 1969; Woolworth and Woolworth, 1982). The size, location, and arrangement of the NWC’s barns, corrals, pastures, and gardens are presently unknown. Grand Portage National Monument Grand Portage was designated a National Historic Site in 1951. In 1958 Grand Portage National Monument was established to commemorate and preserve a premier site and route of the 18th century fur trade that led to pioneering international commerce and exploration in North America as well as cultural contact between Ojibwe and other Native societies and the North West Company partners, clerks, and canoe-men. The Monument was also established to work with the Grand Portage Band in preserving and interpreting the heritage and lifeways of the Ojibwe people. Grand Portage National Monument is a homeplace of tribal and family history and cultural persistence. Grand Portage National Monument contains reconstructed buildings and well-preserved archeological remains of several fur trading posts instrumental in the exploration of the West and in the economic history of the United States and Canada. The national monument contains the entire length of the portage that marked the entrance into the interior of western Canada. The national monument is significant because of the fundamental interrelationship of Ojibwe heritage and fur trade history. Grand Portage National Monument protects, commemorates, and interprets a reconstructed fur depot of the North West Company, a rendezvous site for international commerce and canoe route for transcontinental exploration, Native heritage, natural scene, and history of cross cultural contact and accommodation between traders, Ojibwe, and other participants in the fur trade. Today nearly 90,000 visitors pass through the reconstructed stockade and related buildings each year. Three reconstructed historical buildings currently serve these visitors. Based on archeological excavations and research from 1938 through the present, the reconstructed buildings have been furnished in the period of the late 1700’s. Technology from the time can be viewed in the Ojibwe Village and Voyageur Encampment staffed by National Park Rangers. The Great Hall and kitchen buildings contain reproduction furnishings similar to those used by late 18th century North West Company business partners, clerks, voyageur guides and Ojibwe families. In a room of the Great Hall from June until September, Ojibwe artisans create and sell beaded designs and birchbark basketry popular to Indian people of the Lake Superior region. From late May until early September interpretive programs are presented around the historical site. Subjects include historic cooking and baking, Ojibwe craft demonstrations, historic black powder musket firings, lever fur press operation, historic gardens and conducted walking tours. An annual highlight is Rendezvous Days, held the second week of August, during which the annual meeting of company representatives from Montreal, fur traders and local native people is recreated. Geologic history The geology of Grand Portage is discussed in regard to Precambrian bedrock features, a relatively uncomplicated story based on both Paleoproterozoic and Mesoproterozoic rock units, and Pleistocene and ILSG07 29 Trip 2 �Holocene history representing a dynamic story of geologically rapid landscape change caused by postglacial changes in the levels of ancestral Lake Superior. These latter changes overlapped with early human occupation of the area and undoubtedly had a major influence on human activity. The most recent geologic investigations of the monument were done in 2001 and funded by the National Park Service. Results are in administrative reports to NPS: (1) the bedrock geology was studied by W.F. Cannon and Laurel G. Woodruff of the USGS; (2) glacial and post-glacial geology was surveyed by Brian A.M. Phillips of Lakehead University. Much of the following descriptions are taken from those reports. Bedrock geology The bedrock geology of extreme northeastern Minnesota, including the Grand Portage National Monument, consists of Precambrian rocks of two separate ages. The bedrock geology of the Grand Portage region is shown in Figure 2.3. Most of the area is underlain by sedimentary rocks of the Rove Formation, formed during Paleoproterozoic time, approximately 1,850 million years ago. These rocks have been slightly tilted toward the southeast but otherwise are remarkably little altered for rocks of such antiquity. A second period of rock formation occurred during Mesoproterozoic time, approximately 1,100 million years ago. At that time diabase dikes were emplaced into the Rove Formation. These dikes are of two distinct generations separated by a short time interval and exhibit contrasting magnetic polarity. The oldest are known as the Grand Portage dike swarm and are generally east-trending features no more than a few tens of feet thick. Slightly younger dikes of the Pigeon River swarm are much more voluminous and support the higher ridges of the region such as Mt. Rose and Mt. Josephine. The dikes form a roughly orthogonal array of NE-trending and NW-trending bodies. Figure 2.3. Bedrock geologic map of the Grand Portage area in northeastern Minnesota. Geology extracted from digital files by Miller and others, 2001 The essentials of the geologic relationships were established by the earliest geological reconnaissance of the region conducted by J.G Norwood in 1849 (Owen, 1852). An etching from Owen’s report (see title ILSG07 30 Trip 2 �page) illustrates in a schematic manner the geology of the Grand Portage area. It shows large diabase bodies that form both sills, as on Pigeon Point, and dikes, as on Hat Point. “Slates” of the Rove Formation in Grand Portage Bay are cut by thin diabase dikes. There are three principal references on the geology of the region. First, Grout and Schwartz (1933) studied the Rove Formation and diabase intrusions and presented the first detailed maps of the Grand Portage area. In addition to a compilation of the regional geology, they presented maps of individual townships showing the geology and the location of bedrock exposures observed during their studies. Later, Grout, Sharp, and Schwartz (1959) published a report on the geology of Cook County, which provided some updated information on the Grand Portage area. Finally, in 1969, Morey published a detailed examination of the Rove Formation in northeastern Minnesota and adjacent parts of Ontario. This publication provides the best description and interpretation of the geologic history of the Rove and includes some specific observations within the Monument, particularly at Mt. Rose. Rove Formation The report by Morey (1969) provides an excellent description of the Rove Formation. The abstract of that report is presented below as a summary of the regional character of the Rove and some of our specific observations within the Monument follow. According to Morey (1969, p. 1-2): “The Middle Precambrian Rove Formation, the upper part of the Animikie Group, is estimated to be at least 3,200 feet thick and is exposed between northwestern Cook County, Minnesota and the Thunder Bay district, Ontario. It is a sequence of graywacke, argillite, locally abundant intraformational conglomerate, quartzite, and carbonate rocks. The Formation was deposited sometime between 2.0 and 1.7 b.y. ago in a northeast-trending basin, the configuration of which was controlled by a pre-existing structural grain. Detailed mapping in the 7 ½ minute South Lake quadrangle combined with a field and laboratory study of approximately 150 other scattered stratigraphic sections provide a basis for the recognition of three informal lithic units. From oldest to youngest these are: (1) lower argillite, 400 feet thick; (2) transitional beds of interbedded argillite and greywacke, 70 to 100 feet thick: and (3) thin-bedded greywacke, as much as 2,700 feet thick. It is concluded that the argillite and associated greywacke-sandstone and graywacke-siltstone units were deposited in moderately deep, quiet water. Repeated greywacke sedimentation units indicate sediment transport and deposition by turbidity currents. A sedimentation unit reconstructed from composite sections consists of (1) a basal conglomeratic graywacke, (2) a structureless unit which grades indistinctly into (3) a graded graywacke that is overlain by (4) a laminated graywacke, which may be modified by (5) small-scale cross-bedding, or (6) contorted bedding. Any one or several of these may be absent, but the units are always overlain by (7) an argillite. Post-depositional soft-sediment structures such as load casts, flame structures, clastic dikes, bed pullaparts, overfolds, and micro-faults indicate rapid deposition of Rove sediments, active bottom currents, and post-depositional deformation, implying a significant paleoslope. A detailed analysis of paleocurrent directional indicators such as groove casts, flute casts, dendritic ridges, and cross-bedding shows that the turbidity currents had a southerly trend about perpendicular to the axis of the Rove basin. However, ripple marks, winnowed lag deposits at the tops of many greywacke beds, and possibly some festoon-type cross- bedding show that the turbidites were later modified by bottom currents that trended southwesterly or parallel to the axis of the basin. ILSG07 31 Trip 2 �The heavy minerals of the Rove are characterized by epidote-group minerals, apatite, sphene, and tourmaline, and are typical of older Precambrian igneous rocks now exposed north of the present Rove outcrop area. Thin-section and x-ray analyses of 200 samples show that the graywackes consist of angular, poorly sorted grains of clastic quartz and plagioclase (An10-An20) embedded in an argillaceous matrix that now consists of quartz, chlorite, and muscovite. The fine-grained fissile argillite and mudstone have the same mineralogy and microtextures as the greywacke. Erosion subsequent to pre-Keweenawan tilting removed an unknown amount of the formation prior to deposition of Lower Keweenawan sedimentary rocks. The intrusion of Middle Keweenawan mafic igneous rocks caused local metamorphism of the Rove Formation to a variety of mineral assemblages now assigned to the pyroxene- and hornblende-hornfels facies, but the remainder of the formation is essentially unmetamorphosed.” An additional study of the Rove has been reported recently (Maric and Fralick, 2005) and presents a somewhat different picture from Morey’s 1969 description. Based on examination of a series of drill holes near Thunder Bay, they defined an internal stratigraphy consisting of: 1) a 150 m-thick basal unit of carbonaceous shale with lesser siltstone and fine-grained sandstone beds, which grades upward into, 2) a unit about 350 m thick containing more than 100 individually coarsening upward beds of sandstone with interlayered shale. This sequence is overlain by the uppermost preserved unit of 3) black shale with thin rippled sandstone beds. They interpreted the lower carbon-rich sediments to have been deposited in a sediment-starved anoxic basin in which very slow sedimentation rates resulted in accumulation of only 100 m of black shale during tens of millions of years of sedimentation. The upper coarser-grained units were deposited by a southward prograding turbiditic to shelf system. Most, if not all, of the Rove Formation exposed in the Grand Portage Monument is part of the upper thinbedded greywacke unit described by Morey (1969). Most exposures consist of flaggy, fine- to mediumgrained greywacke, with minor interbeds of argillite. The unit tends to split into slabs 1-2 inches thick parallel to indistinct bedding. The extensive outcrops and roadcuts on the north side of Highway 61 where the portage trail crosses the highway (Fig. 2.1, Stop 2.3) are typical of many of the other exposures along the trail to the north. The exposures on Mt. Rose (Stop 2) differ in that bedding of the greywacke is somewhat thinner and more distinct. Features such as cross-bedding and contorted bedding are present, although rare. An additional significant difference is the abundance of units of fine-grained fissile argillite as much as several tens of feet thick. The Rove Formation immediately adjacent to the Mt. Rose diabase dike is strongly metamorphosed. Original sedimentary textures are largely obliterated and metamorphic minerals, including biotite and clots of a mineral preliminarily identified as cordierite, have formed in the original greywacke matrix. Over the past few years, the age of the Rove Formation has been clarified by radiometric dating and the results provide a more precise age estimate than the 300 m.y. span indicated by Morey’s study in the 1960’s. Thin volcanic layers near the base of the Rove Formation near Thunder Bay, Ontario have been dated by the U-Pb zircon technique and indicate that deposition began somewhat before 1835 Ma. The dated volcanic layer is about 6m above the base of the formation and has an age of 1836+/- 8 Ma (Addison and others, 2005). The base of the Rove in that same area lies on a layer of ejecta believed to have originated from the major 1850 Ma meteorite impact at Sudbury, Ontario (Addison and others, 2005), further helping to bracket the age of the basal beds of the formation between 1850 and 1836 Ma. An additional age constraint comes from radiometric dates on detrital zircons from a bed about 400 m above the base and near the top of the preserved part of the Rove about 15 km north of Grand Portage. Concordant ages of individual grains are as young as 1777 Ma, providing a maximum age for deposition of the middle part of the Rove Formation (figure 2.4) (Heamon and Easton, 2006). A minimum age is not well constrained at this time. ILSG07 32 Trip 2 �Figure 2.4. Age spectrum for detrital zircons in the Rove Formation presented by Heamon and Easton (2006, figure 33). Spectrum indicates that a majority of detrital zircons were derived from a source younger than rocks of the Penokean orogeny to the south, and have come from a terrane with an abundance of zircons with ages of 1850 to 1750 Ma. These new age data require a rethinking of traditional interpretations of the Rove, both in terms of stratigraphic correlations and tectonic setting of deposition. The Rove Formation has been long considered to be the stratigraphic equivalent of major turbidite sequences to the south, such as the Thomson Formation in east-central Minnesota and the Michigamme Formation in northern Michigan. Recent discovery of the Sudbury ejecta layer in Michigan near the base of the Michigamme Formation confirms that the onset of sedimentation was nearly synchronous across the region at 1850 Ma. However, the duration of sedimentation between Michigan and northern Minnestoa/Ontario appears to be drastically different. The 1777 Ma concordant detrital zircons from the Rove are from a bed about 400 m above the base of the formation. Although this is near the top of the preserved formation in the sample area, a total thickness nearly 1000 m was estimated for the Rove (Morey, 1969). Therefore, sedimentation may have lasted well beyond the 1777 Ma maximum age. In Michigan, it is well established that the Michigamme Formation was deposited, deeply buried by tectonic overthrusting, metamorphosed, and intruded by posttectonic granite by 1835 Ma (Schneider and others, 2002; Schneider and others, 2004), which is the approximate age of the volcanic layers near the base of the Rove. Thus, much of the thick clastic sequence of the Rove appears to be entirely younger than the Michigamme Formation. The Rove has commonly been considered to have been deposited in the foreland sedimentary basin north of the 18901835 Ma Penokean orogen, but it now appears that sedimentation was almost entirely after the Penokean deformation ceased in Michigan and Wisconsin. The detrital zircon suite in the Rove, as shown in figure 4, consists dominantly of grains in the 1850-1750 Ma age range rather than Archean zircons, indicating that a source area with rocks of that age was well established and contributing the majority of detritus to the basin during at least the younger part of Rove deposition. Detrital zircon analyses of the Thomson and Rove Formations in Minnesota yielded somewhat similar results (Kerber, 2006). Although no zircons younger than about 1800 Ma were found in either formation based on analyses of a single sample from ILSG07 33 Trip 2 �each, the great preponderance of zircons are in the range 2000-1800 Ma and Archean zircons are scarce). Thus, based on presently available data, it appears that the preponderance of detritus contained in the vast turbidite deposits of the Rove, Virginia, Thomson, Tyler, and Michigamme Formations across the western Lake Superior region was derived from a source terrane(s) with ages in the approximate range 2000-1750 Ma. A widely accepted model for this turbidite deposition has been a tectonic foreland in which subsidence was being driven by northward overthrusting of arc rocks of the Wisconsin magmatic terranes, now exposed in northern Wisconsin, and that these rocks provided much of the detritus of which the turbidites are composed. This model is still acceptable for the southern parts of the basin, but complications arise in applying it to our current understanding of the Rove Formation. Relationships in Michigan and Wisconsin indicate that tectonic loading by overthrusting onto the craton margin was completed by 1835 Ma (Schneider and others, 2002). However, deposition of the basal units of the Rove was just beginning at that time. Deposition of the Rove continued for at least 60 million years after that time, long after the end of the Penokean orogeny, and basin subsidence of about 1 km occurred during that period. What was the driving mechanism for that subsidence? An additional enigma is provided by a wealth of paleocurrent data from the Rove which indicates a consistent southerly sediment transport (Morey, 1969). This direction is inconsistent with a sediment source from the Wisconsin magmatic terranes. The preponderance of Paleoproterozoic detrital zircons suggests that much of the Rove detritus was derived from a Paleoproterozoic orogen north of the Lake Superior region, perhaps the Trans-Hudson orogen in part, which contains rock units as young as 1775 Ma. Thus the Rove Formation no longer fits well into the model of Penokean foreland sedimentation because the time of sedimentation and sediment source directions are inconsistent with that model. The nature and cause of the Rove basin is a newly emerging topic for further study. Because so much of the Rove sedimentation is significantly younger than the Penokean orogeny and paleocurrent data indicate that the Penokean orogen is not the source for the Paleoproterozoic-aged detritus that seems to make up most of the Rove, it is necessary to divorce the Rove, and perhaps at least part of the correlative Virginia Formation, from the suite of features ascribed to the Penokean orogeny, and seek a cause in younger orogenic events. It appears, based on the most recent data, that the basal parts of the Rove Formation composed of black shale deposited in a sediment-starved basin may be a product of Penokean foreland basin development. However, the remaining coarser clastic deposits of the upper part of the Rove reflect a reactivation of the basin during a time spanning the 1775 Ma maximum age of the middle part of the formation. A recent reinterpretation of the tectonic assembly of the north-central part of the US (NICE Working Group, in press) indicates that the mid-geon 17 Yavapai orogeny extends eastward from its originally defined extent in the southwestern US into the southern part of the Lake Superior region. A belt of gneiss domes, granite plutons, and metamorphic nodes also developed across central Minnesota and northern Michigan at that time (Schneider and others, 2004; Holm and others, 2005). Thus, it has been established recently that major mid-geon 17 tectonic activity occurred only about 200 km south of the Rove basin. Is this tectonic episode responsible for reactivation of the older Penokean foreland basin and deposition of the Rove Formation? Although these Yavapai-age events might in some poorly understood manner have caused basin subsidence and also provide a source of detritus of the same age as that established for the Rove, the enigma of southerly-directed sediment transport in the Rove of material of that age remains unresolved. Grand Portage diabase dikes A set of thin diabase dikes was described by Green and others (1987) that they named the Grand Portage dike swarm. These dikes are present throughout the Grand Portage region and intrude the Rove Formation as well as the Middle Proterozoic volcanic rocks exposed south of Grand Portage. More than 50 of these dikes have been identified, some of which are shown on maps in Green and others (1987) and ILSG07 34 Trip 2 �in the digital compilation by Miller and others (2001). The dikes have a relatively uniform trend from N65o – 90o E. The Grand Portage dikes tend to be quite strongly magnetized, but acquired their magnetic properties as they cooled during a period when the earth’s magnetic field was inverted. This reversed magnetic polarity interferes with the present earth’s magnetic field resulting in distinctive linear negative anomalies on magnetic maps. This characteristic makes them quite easy to identify, even in areas where they are not exposed. A combination of surface exposures and magnetic expression has allowed us to identify 11 new dikes of the Grand Portage swarm that cross the monument. The most prominent of these are a set of five dikes that are exposed along the east side of Mt Rose (one is just south of the Monument boundary, the other four are on U.S. Park Service property). Each dike stands as a low to prominent ridge as much as 10 m wide as a result of differential erosion between the relatively hard diabase and much softer Rove Formation. One of these dikes shown in Figure 2.5a is probably the dike that inspired the somewhat fanciful etching that appeared in Owen’s 1852 report (Figure 2.5b). Figure 2.5. a- Diabase dike of Grand Portage swarm at south edge of Monument property. b- Etching from Owen (1852) based on field sketch, probably of the dike in a. ILSG07 35 Trip 2 �The magnetic profile shown in Figure 2.6 illustrates the sharp negative magnetic anomalies caused by these dikes. Three other similar anomalies are shown on a magnetic profile along the Grand Portage trail. Although dikes are not exposed at the surface in these areas they are inferred with confidence to occur beneath the surficial cover material. Two other dikes were identified in outcrop just north of Highway 61 but their magnetic expression is less distinct. Figure 2.6. Magnetic profile measured at 100 foot spacing along roadway northward from south boundary of Monument. The four sharp negative anomalies are caused by the four Grand Portage dikes exposed along the roadway. Pigeon River diabase dikes The Pigeon River diabase dike swarm (Green and others, 1987) is compositionally similar to the Grand Portage dikes but is slightly younger and volumetrically much more prominent. The Pigeon River dikes trend mostly about N65oE and less commonly about N40oW. Because of their hard, massive nature they form the prominent, steep-sided ridges of the region, including Mt. Rose. Because they have normal magnetic polarity, these dikes tend to produce positive anomalies on magnetic maps, but they are apparently somewhat variably magnetized so cannot be recognized in magnetic data as readily as the Grand Portage dikes. Absolute ages of Grand Portage and Pigeon River dikes The Grand Portage and Pigeon River diabase dikes have not been dated directly, but their age can be closely approximated because of their relationships with other nearby units that have been dated. A regional correlation of rock units related to the Midcontinent Rift and a summary of radiometric ages was published by Nicholson and others (1997). The Grand Portage dikes are known to cut the Hovland and Grand Portage basalts south of the Monument. Those basalts were erupted about 1,110 to 1,105 million years ago. The shift of the earth’s magnetic field from reversed to normal polarity occurred about 1,100 years ago. The intrusion of the Grand Portage dikes, therefore, must have occurred after eruption of the Grand Portage basalts, but before the magnetic field reversal in the interval 1,105 to 1,100 million years ILSG07 36 Trip 2 �ago. The Pigeon River dikes, which have normal magnetic polarity, are younger than 1,100 million years. They may be related to the Lutsen basalts that were erupted about 1095 to 1090 million years ago. Glacial and post-glacial geology The effect of ice Because ice advances reached as far south as Illinois, Iowa, Nebraska, and Kansas numerous times during the Pleistocene it is safe to assume that the Grand Portage area has been affected by the passage of several ice lobes over the last 500,000 years. The general direction of ice movement was from the north and northeast, and present-day landscapes in the borderlands first were sculpted of sub-glacial processes of plucking, abrasion and the high pressure flow of sub-glacial meltwater. Next, the processes of deglaciation resulted in deposition of ablation tills and a variety of fluvioglacial landforms (eskers, kames, ice-contact deltas) and sediments (sorted and unsorted silts, sands and gravels). Meltwater, temporarily ponded between the waning ice margin and enclosing topography, formed short-lived pro-glacial lakes that changed in area and depth as the configuration of the ice margin changed. Many of these lakes existed long enough to deposit substantial layers of lacustrine clays, and many ended their existence by draining catastrophically down spillways exhumed by the melting ice. Inundation of the land recently revealed by ice melt was common as various lake bodies occupied the Lake Superior basin at different times and water levels, depending upon the rate of water supply and the loss through such outlets as were available. In the Grand Portage area, the present topography is the product of the effects of glacial ice. Rock types more resistant to glacial processes remain upstanding and less resistant rock materials have been more deeply eroded and removed. However, ice must be considered as a very viscous liquid, not only exerting its influence on the underlying topography, but also being influenced by that topography. Though ice crossed the area from generally north to south, the already upstanding diabase ridges that characterize the area will have deflected more recent ice events. As shown in R.P. Sharp’s map (Fig. 2.7), glacial striae, the grooves ground into solid rock by the scraping action of sub-glacial debris, are found sub-parallel to the ridges rather than orthogonal to them (as in R5E) (Sharp, 1953). The most effective ice erosion is performed early in ice advance. Later, under increasing thickness of ice, erosion of the underlying ground surface all but ceases. Shearing takes place between upper layers of ice sliding over lower layers of ice that are trapped within the topographic depressions. This trapped ice becomes largely stagnant and therefore ineffective at erosion. The topography of the Grand Portage area suggests that this scenario is quite likely to have taken place. ILSG07 37 Trip 2 �Figure 2.7. Part of R.P. Sharp’s map of Cook County, MN (Sharp, 1953) showing glacial striae directions (arrows). Sub-glacial water flow It is likely that the topographic differentiation between the hard intrusive rocks and the less resistant sedimentary rocks began in pre-glacial time by the time-honored agents of runoff, wind and chemical weathering. At this time too, the conspicuous breaks in the ridge features are likely to have first been formed. A number of major faults, along which detectable vertical movement has taken place, cut across the area (Fig. 2.3), but many unmapped smaller faults and shatter zones also are present. These linear weaknesses in the hard rock materials are the first to be exploited by the agents of denudation, and once initiated, processes such as running water become focused in these locations, exacerbating the erosional effects. The role of sub-glacial water flow was particularly important. Unrecognized by earlier workers studying glacial geomorphology, water flowing beneath and within ice is often under considerable hydraulic pressure. Unaffected by gravity, such flow can run counter to the slope or topographic grain of the underlying surface. The force of water, carrying subglacial debris and running at the interface of the ice and ground surface, is a highly effective erosive tool. Scouring of rock surfaces and the erosion of steep sided channels into the rock surface (called tunnel valleys) can take place. In post-glacial landscape glacial meltwater flow in these valleys might be seen to run uphill, cross divides or run contrary to the drainage pattern. Some are used by present drainage, but many remain dry or hold lakes and bogs. Associated with these sub-glacial features are eskers, which represent a final stage of deposition within the tunnels. These eskers might run along the valleys in places or climb over the margins onto ground alongside the main channel. ILSG07 38 Trip 2 �Isostatic uplift Although the mechanism for post-glacial isostatic uplift or rebound remains inadequately explained by geophysics, the effect of this mechanism can readily be observed and measured. The process began as ice margins began to wane and withdraw, and continues to the present day. Greater uplift occurs where greater depression was made by a greater weight of ice. As a result, models for post-glacial rebound see greater and longer continuing uplift towards the center of the former ice mass. Figure 2.8. Shoreline diagram for the Grand Portage area showing present elevations of former lake shores. Along the Minnesota north shore the axis of uplift is sub-parallel to the shore, and currently, uplift at the Pigeon River is about 250 feet greater than at Duluth. Consequently, an initially horizontal line, such as a lake shoreline, is now tilted such that its elevation rises over this range between Duluth and the Pigeon River. The sequence of lakes that occupied the Superior Basin as ice withdrew and isostatic uplift occurred resulted in a staircase of shorelines found along the north shore, diverging to the northeast (Fig. 2.8). Thus in Cook County, the vertical distance between these former shoreline features is greatest at Grand Portage. A more detailed account of pre-historic water levels can be found in Phillips (2001, pp.828). Evidence of former lake margins Typically, the presence of a lake margin, sustained for a period of time sufficient for waves to do work, is represented either by an erosional form such as a bluff, or by a depositional form such as a beach. In addition, rivers entering the lake body tend to deposit fluvial sediments in the form of deltas, or at least a series of river mouth bars that characteristically curve out of the river mouth and along the shore. Both features mark the position at which rivers formerly met with the shore of the lake. ILSG07 39 Trip 2 �Streams along the Minnesota north shore have had to respond to steady isostatic uplift and falling water levels. Typically, this has resulted in the lengthening of stream courses and the deep incision of their beds as they struggle to meet the lakeshore at grade. Not surprisingly, the north shore abounds in gorges, waterfalls and rapids as outcrops of more resistant rock inhibit equal downcutting along the stream course. Another characteristic feature that results from isostatic rebound is the building of deltas at successively lower elevations as the stream course extends, each delta in part built from the material eroded from an earlier one as a result of incision into the former delta surface. The Grand Portage Trail crosses a number of these features, each denoting a period of lake level stability lengthy enough for a defined landform to be formed (Figs. 2.9, 2.10). The features have significance for finding Paleo-Indian artifacts, since it is established that early people tended to favor shoreline and river mouth locations for their temporary habitation and activities. The succession of shorelines on the Grand Portage Trail As Marquette ice, the youngest glacial lobe in the western Superior basin, melted back and exhumed the basin successively from southwest to northeast, the highest post-Marquette shorelines are those of Lake Duluth, the earliest phase of which was restricted to the Duluth area. When ice still covered the Thunder Bay region, Lake Duluth first extended across the international border into the southern part of the Nor’wester mountains. At Grand Portage, the high Duluth level was at 1340 feet and although Mt. Maud was now part of the mainland, no part of the Grand Portage Trail was above water. Figure 2.9. Idealized former water levels at Grand Portage. The gradual withdrawal of ice uncovers recently glaciated ground. If, as in this case, the ground is lower than the ice marginal (pro-glacial) lake, it is immediately inundated and protected from exposure to weathering and erosion, even obtaining a modest cover of pro-glacial lake (glaciolacustrine) clays or silts if under water long enough for offshore sediment to accumulate. ILSG07 40 Trip 2 �Portions of the Grand Portage Trail first appeared above water during the later Low Lake Duluth phase (1290 feet). A good portion of the Trail north of Old Hwy 61 lay above water, including the uppermost part of the narrow ridge along which the Trail runs for a mile or more. This part of the Trail, having been inundated only briefly, is not overlain by lacustrine sediments. The thin till is wave washed, and bare rock is exposed in many places. A pattern can be seen emerging here. High areas, subject to inundation for a lesser period, are less likely to be covered by lacustrine sediments. Lower areas, subject to a long period under water may accumulate lacustrine sediments, though only if a source of such sediment is available. To a large extent, what happens is related to the degree of shelter or exposure to wave action. This will vary locally as the water level lowers. Sheltered areas may show little modification of the existing sediments, but exposed areas are likely to show winnowing of the glacial sediments, removal of finer particles and a lag of larger clasts left on the surface. Sufficient wave action may have time to sort and transport sediment, forming recognizable beach or bar features where longshore transport is consistent. Extreme wave action may have the energy to erode and form soft-sediment bluffs, or even undercut and fashion solid rock into wave-abraded forms. In summary, as the already complicated topography of the Grand Portage Trail area slowly emerged, an ever-changing pattern of wave refraction around the shoals, islands and peninsulas led to a localized sculpting of the topography. At the Lake Highbridge level (1200 feet), the present Grand Portage Trail was divided into two clearly different portions. The Trail above Old Hwy 61 was above water, while that below the highway was still inundated. This dual character is still very visible today. The character also occurs into the High Lake Washburn phase (1110 feet), with the steep section of the Trail immediately above Old Hwy 61 forming a fairly stable promontory, down which the water level fell in stages (Figure 2.10). Meanwhile, Mt. Josephine emerged, as did higher parts of the narrow diabase ridges, the links gradually joining as water level fell. During the High Lake Washburn phase (Fig. 2.10A), two ‘gaps’ were noticeably still inundated. The eastern one marks the line of the Grand Portage fault, and the Grand Portage Trail passes through the western one. At this time, the Poplar Creek basin had become significantly protected from wave action, though, as in present Lake Superior, water flow through the two gaps was likely intensified by seiche and meteorological events, leading to possible scouring and some transfer of sediment to one or both sides of the ‘channel’. The lakeward side of the emerging ridges probably experienced the full force of Lake Superior wave action, so the transfer was likely to have been mostly towards the Poplar Creek basin. Within the relatively calm waters of the basin, lacustrine sediment would have accumulated liberally. It is during the High Lake Manitou level (1020 feet) (Fig. 2.10B) that the ‘gaps’ became dry. On both sides of the gaps, some crossing feature such as a beach or a bar was likely, though if earlier scoured to bedrock, little sediment might have been available to form these. If soft sediments formed a shoreline feature in such an emerged gap, it would be named a ‘tombolo’, in that it joined mainland to a former island. This is a feature only occasionally seen on the shore of present Lake Superior. During the High Lake Manitou level, the basin of the Grand Portage Creek also became a more sheltered reentrant, and the upper part of the creek course probably flowed. At the subsequent High Lake Beaver Bay level (920 feet), a distinct Grand Portage Bay emerged and Grand Portage Creek was a sheltered river mouth extending far up valley. This character was less pronounced by the time of Mid Lake Beaver Bay (845 feet) (Fig. 2.10C), when the summit of Mt. Rose appeared as a small island, but the potential for river mouth habitation by Paleo-Indian people is good during the five Beaver Bay lake stages. As Mt. Rose became a larger island during the Lower Lake Beaver Bay stage (775 feet), it probably protected the river mouth and the potential for habitation was sustained. At this time, wave refraction around Mt. Rose would have been quite complex and the area immediately behind the steep island would have been very protected. Lake Minong, dated as about 9,500 BP, is notable as the first of the succession of lakes to occupy the whole of the ice-free Superior basin, and its shoreline can be traced around the present lake. Lake ILSG07 41 Trip 2 �Minong time is also notable in the Minnesota-Ontario borderlands as a period in which evidence of PaleoIndian activity became more pronounced than before. In the Grand Portage area, the Minong shoreline occurs at 715 feet. A defined bluff forms a deep ‘V’ shaped reentrant into the valley of the Grand Portage Creek, which the Grand Portage Trail now surmounts. At this time, Mt. Rose ceased to be an island and further contributed to the sheltered nature of Grand Portage Bay (Fig. 2.10D). The potential for PaleoIndian occupation around this sheltered river mouth is high. Figure 2.10. Reconstruction of former shorelines at five levels of post-glacial lakes in the Grand Portage region. ILSG07 42 Trip 2 �The Post-Minong period was a time when pro-glacial Lake Agassiz flowed through a series of eastern outlets into the Nipigon basin, and thence into Lake Superior and the lower Great Lakes. At least five episodes of catastrophic discharge have been recognized (Teller and Thorliefson, 1983). Each had the potential to briefly raise the level of the Post-Minong lakes, and erode the glacial sediments that swathed the St. Mary’s river outlet to Lake Huron. Ultimately, the outlet was eroded to bedrock and the Lake Houghton phase was established as the lowest of the succession of post-glacial lakes in the Superior basin. Water level declined to a low position of 582 feet in Grand Portage Bay, placing the shoreline adjacent to Grand Portage Island, which separated two shallow bays. Little if anything remains of this portion of history, because there followed a steady rise of water level in the Superior basin as the St. Mary’s outlet was isostatically raised. This transgression drowned the recently exposed Post-Minong surface and culminated in the Nipissing shoreline, which in places truncates and reoccupies older shoreline features. In the Grand Portage area the Nipissing rose to 636 feet, preserving the Minong and the upper PostMinong shorelines (Fig. 2.10E). Near Grand Marais, the Nipissing inherited the older Minong bluff, refreshing it, and further south the Minong was destroyed and drowned by the Nipissing transgression. Subsequently, water level fell to the Lake Algoma level (621 feet) and the Sault level (610 feet) before declining to present lake level (602 feet). Features formed by these more recent water levels run through the area of the Stockade, though more recent shore erosion has removed the lowest. In summary, the present Grand Portage Trail traverses a landscape that has been fashioned first by ancient tectonic activity that emplaced the pattern of dikes, which are responsible for the basic topographic character of the area. Secondly, multiple glaciations have ground across the ridges and basins, steepening and smoothing rock surfaces, and depositing varying depths of sandy, coarse tills, particularly in lower areas. Thirdly, a succession of post-glacial lakes has fully submerged the course of the Trail. As the water level gradually declined, the Trail topography progressively emerged. These lakes have left some evidence of their brief periods of stillstand in the form of wave-washed sediments, erosional bluffs and constructional beaches and bars. They have also deposited a variable thickness of lacustrine clays and silts, particularly within the basins and valleys of the bedrock topography. The geological underpinnings of human history of the Grand Portage region Early human activity in any region is strongly influenced by physical barriers to transit and by the easiest solutions to the impediments that those barriers represent. Geological events that occurred from as little as ten thousand years ago to as much as 1.1 billion years ago largely predetermined the course of European history in the Grand Portage region because they controlled the physiography and the unique physiographic characteristics of Grand Portage that made it the obvious location for the “highway to the west”. These events not only dictated that the portage was necessary, but determined the best route for the portage, and, on a broader scale, were responsible for the enormous advantage to westward travel inherent in making the portage. European exploration and settlement of the continental interior was strongly guided by the relatively easy water access provided by the Great Lakes as far west as western Lake Superior. Access farther west into the deep interior of the northern Great Plains, Canadian prairies, and beyond was facilitated by additional relatively easy water routes up the Pigeon River, over the continental divide, and through the Rainy River system westward to major north-flowing drainages and their tributaries. The shortest distance from Lake Superior to waters navigable westward by canoe is over the Grand Portage. This short route to the west was known and used by Native Americans well before the first Europeans arrived in the early 1600’s. ILSG07 43 Trip 2 �But the stretch of high ground from the lakeshore to the upper reaches of the Pigeon River was the most prominent physical barrier along the entire westward route from Montreal to the Rocky Mountains. In fact, the length and difficulty of the portage lead to a mutiny among the first westward-bound French exploration party that arrived at the site in 1731. The portage was necessary because of a long series of rapids and falls along the lower reaches of the Pigeon River which falls about 660 feet from the upper end of the portage down to the level of Lake Superior. But a remarkable feature of this area is that once past the 8.5 mile portage, the way west was through the relatively flat water of the upper Pigeon River and a series of interconnected lakes. Within only about 40 miles from the west end of the portage one leaves the Lake Superior drainage, crosses the continental divide, and enters the Rainy Lake watershed which drains ultimately to Hudson Bay. Thus, three regional features: 1) Lake Superior and the easy route that it provides for water-borne commerce from the east, particularly Montreal, 2) the steep gradient of the Pigeon River in its lower reaches that made canoe transit impossible, and 3) the very small size of the Lake Superior watershed which, once exited, provided essentially unlimited canoe access to the interior of North America, all played fundamental roles in making the Grand Portage the most favored and easiest route for westward travel. The present basin of Lake Superior is the exhumed sediment-filled central valley of an ancient rift that formed 1.1 billion years ago. At that time a great rift valley, known as the Midcontinent rift, began to split the North American continent. The rift was filled first with tens of kilometers of volcanic rocks, mostly basalt such as the Grand Portage basalts exposed just south of the Monument, and finally by many kilometers of sediments, now mostly red sandstones. Figure 11 shows the trace of the axis of the Midcontinent rift which faithfully follows the southward concave shape of the present lake. The sandstones in the central part of the rift presented an easily erodible substrate for glacial scouring in contrast to the much more durable igneous rocks along the rift flanks, which today stand as the prominent ridges and uplands of the region. As a result, repeated glaciations during the Pleistocene gouged the deep valley now occupied by Lake Superior. Thus, Lake Superior and its favorable transportation route owe their existence to these two ancient events of rifting and glaciation. But both of these events combined to create the unfavorable situation of the extensive series of rapids and waterfalls along the lower reaches of the Pigeon River, which required the arduous “grand portage” to circumvent them. The Pigeon, along with all other major rivers along the northwest shore of Lake Superior, reach the lake over a series of rapids and falls that extend nearly to the lake shore. This is in contrast to the more typical morphology of rivers which commonly have extended reaches of slack water upstream from their terminus and thus provide waterborne access well inland. It is again the repeated glacial scouring along the ancient rift axis that created this situation. Numerous lobes of ice flowed southwestward through the western Lake Superior basin and oversteepened the topographic gradient near the present lake shore. This steep gradient extends well below the present lake level into a bedrock trough whose bottom is more than 1500 feet below the lake and about 1000 feet below sea level. In the relatively short period since the last glaciation, about 10,000 years, the rivers have not had adequate time to modify this glacially induced gradient and thus plunge down this steep topography until they intersect the modern level of Lake Superior making upstream canoe travel from the river mouths impossible. A still more fundamental geologic phenomenon is responsible for the highly unusual situation of a remarkably small watershed surrounding Lake Superior thus allowing a short distance of transit from the lake to outward flowing drainages. Modern interpretations of the geodynamic causes of the Midcontinent rift suggest that the region was underlain by a newly developing plume of upwelling mantle which caused both intense basaltic volcanism along the rift and the active splitting of the continent (Hutchinson and others, 1990; Cannon and Hinze, 1992). As the new plume waned in intensity, welding of cooling mantle material to the base of the crust, a so called rift pillow, resulted in an unusually thick crust in the Lake Superior region. Interpretation of seismic refraction surveys shows that present day crust beneath the lake is as much as 55 km thick in contrast to a typical thickness of about 40 km in surrounding regions (Halls, ILSG07 44 Trip 2 �1982). The isostatic effect of this thick crust is to maintain the Lake Superior region as a subtle topographic high (Allen and others, 1992). Examination of the continental scale drainage patterns (Fig. 11) shows that the Lake Superior watershed is anomalously small in relation to the size of the lake, and that major river systems drain radially away from the lake as a result of this isostatic bulge. Thus Lake Superior is, in a sense, a hub from which numerous possible outward draining water routes are available once the Superior divide is crossed. The Grand Portage is the shortest of these available routes. On a smaller scale, geologic features also played a significant role in determining the course of the portage and both its upper and lower terminus. Grand Portage Bay is the most sheltered harbor anywhere along the northwest shore of Lake Superior and thus was a naturally favorable location for developing the trading post at the western terminus of the Great Lakes transportation route. The bay is underlain by relatively non-resistant sedimentary rocks of the Rove Formation which make up most of the low country in the region. The sheltering headlands are formed from resistant igneous rocks. To the northeast, Hat Point and the high ridge extending inland is a reflection of the Mt. Josephine dike of the Pigeon River swarm. To the southwest, basalt flows of the Grand Portage Volcanics support the topographic high that forms the southern shore of the harbor. These same volcanic rocks also underlie Grand Portage Island that lies across the mouth of the bay and provides additional shelter. In order to traverse westward from Grand Portage Bay a way needed to found through the imposing steep-sided northeast-trending ridges that are supported by the resistant diabase of the Pigeon River dikes. A gap through these ridges is now followed by Grand Portage Creek, which flows along a fault trace (see Fig. 2.3). Although this fault has only minor offset, it apparently created weakened rocks that were either eroded by pre-glacial streams or were plucked by glacial ice or subglacial streams. In any case, this topographic gap is readily seen from Lake Superior and likely attracted the earliest humans in the area as a relatively easy course to the west. At the western terminus of the portage, local geology also played a role in determining the most favorable location at which canoes could be easily embarked and disembarked. Fort Charlotte was established at a point where a diabase dike, probably of the Pigeon River swarm, intrudes the Rove Formation and trends across the Pigeon River. This resistant dike forms a small topographic rib which creates a small rapid on the river that could not be crossed by canoes and, perhaps more significantly, impounded an upstream reach of still and unusually deep water creating a small natural harbor. These features illustrate the importance of the geologic history of the region and its influence on present physiography. It was both local and regional physiography that dictated the course of human occupation and exploitation of the Grand Portage region and its enormous significance in the 18th century history of the northern part of the continent. This “geological predetermination” of human events serves as a prime example of the interaction between the physical challenges encountered by early human exploration and settlement of a region and the ingenuity and industry with which they adapted to those challenges and opportunities. ILSG07 45 Trip 2 �Figure 2.11. Map showing the Lake Superior watershed (stippled), the axis of the Midcontinent Rift (short dashed line), and regional drainage patterns, simplified from Allen and others (1992). Arrows indicate direction of drainage and illustrate the central position of Lake Superior relative to the outward radial continental drainage pattern. Reference circles of 250 km and 500 km radius from western Lake Superior, the presumed central point of the mantle plume responsible for the rift. ILSG07 46 Trip 2 �GEOLOGICAL STOPS The following are brief descriptions of specific geological features that can be observed in several different areas of the Grand Portage National Monument. Locations of stops are shown in Figure 2.1. Stop 2.1. Stockade to Visitor Center: Grand Portage diabase dikes, Rove Formation, and glacial lake features. Shoreline features of some of the lower post-glacial lakes are visible near the Stockade. A distinct feature that runs along the shore zone and through the Stockade is the gentle bluff of the Sault stage (610 feet). To the east of the Grand Portage Creek, the bluff curves lakeward on Premiers Point, and runs northeast through the picnic area, parallel to the shore until rising erosional bluffs of Lake Superior truncate and eliminate it. Offshore, a lake floor of large boulders that fringes the east side of the sandy creek channel indicates the former extension of Premiers Point. Just above the Sault shoreline lies another curving bluff, the Algoma (621 feet). To the west of Grand Portage Creek Creek, the Sault bluff runs through the Stockade enclosure. The Great Hall building is constructed on the bluff, such that the lakeward side lies at the foot and the inland side lies on the top. The bluff forms a marked step through the Ojibwe Village exhibit to the west of the Stockade and is then truncated again by rising erosional bluffs near the Visitor Center. Behind the Canoe Warehouse in the Ojibwe Village exhibit, a small section of Algoma bluff has survived the disturbance of road construction. As you walk the roadway between the stockade and visitor center (under construction in 2007) you will cross four diabase dikes of the Grand Portage swarm (Green and others, 1987). A fifth dike crops out just south of the Monument boundary and is especially prominent as a free-standing wall of diabase about 20 feet tall, a classic geomorphic dike form. Figure 5 compares a recent photograph of this dike with an etching from Owen’s 1852 General Land Office report. All of the dikes form linear ridges up to a few tens of feet wide as a result of being much more resistant to erosion than the Rove shale into which they are intruded. The topography created by the dikes was probably emphasized by wave erosion along the glacial Lake Nipissing shore when water level was a few tens of feet higher than present Lake Superior. Pocket beaches were probably formed on the Rove Formation between the resistant dikes although talus from the steep south flank of Mt. Rose now covers these areas. Individual dikes here are 10-20 feet thick, fine grained, massive, and commonly show chilled contacts against the Rove Formation. The dikes were formed during early stages of development of the Midcontinent rift. In addition to the Rove, they also intrude volcanic flows of the Grand Portage Volcanics to the southwest of the Monument. The dikes are rather stongly magnetized and have a reversed remanent polarity. This makes them easy to find and identify by magnetic surveying because they appear as linear negative anomalies. A magnetic profile measured along the road with a proton precession magnetometer in 2001 shows four distinct magnetic lows that correspond to the four dikes that cross the road (Fig. 2.6). A profile along the portage trail, also done in 2001, found several additional dikes that had not been know previously. The dikes can be dated rather precisely by a combination of geologic relationships and magnetic polarity. They cut the Grand Portage Volcanics that were extruded early in the history of the Midcontinent rift and acquired their remanent magnetization before the magnetic reversal, which is recognized widely in igneous rocks of the Midcontiinent rift, at about 1,100 Ma. Stop 2.2. Rove Formation and Pigeon River diabase on Mt. Rose ILSG07 47 Trip 2 �The Mt. Rose Trail provides a well exposed section of the Rove Formation. The summit is underlain by a diabase dike of the Pigeon River swarm and a contact metamorphic halo in the Rove near the diabase contact. The following descriptions are keyed to numbered placards along the trail. Much of the eastfacing flank of Mt. Rose, over which the trail climbs, consists of steep bluffs of bedrock exposures of the Rove Formation. The steepness is probably due to wave erosion during higher level stands of post-glacial lakes. The Nipissing transgression reoccupied the base of Mt. Rose around 636 feet, with wave action vigorously cutting a bluff seen from Stop 2.1 on the Mt. Rose Nature Trail. The base of the bluff is now mostly covered by more recent talus. Cut in shale, no abrasion notch is expected, but an angular undercut can be seen in places to the south of the Trail. The Trail follows a bench at approximately 678 feet between lookout Stops 5 and 6 that faces Lake Superior and which would have experienced strong wave action at the post-Minong water levels. Large accumulations of blocky talus below a sharp bluff between Mt. Rose Nature Trail Stops 7 and 10 represent the work of waves during the period of Lake Minong (715 feet). In several places, a convincing undercut is seen, though the base of the rock bluff is almost everywhere covered by more recent talus accumulation. At 775 feet, at the time of Lake Low Beaver Bay, Mt Rose was an island separated from the mainland by a shallow strait. On the Mt. Rose Nature Trail, there are one or two rock benches that could have been wave modified by this level of Beaver Bay, but the strata outcrops in nearly horizontal form and it is not possible to distinguish between wave formed features and structural control. The Rove Formation along the trail is mostly flaggy sandstone in which prominent bedding partings emphasize bedding at inch-scale. Cross bedding is seen rarely and a few beds show soft-sediment slump structures. The sandy units are separated by thinner units of finer-grained fissile argillite. Beds are nearly flat lying with dips of only a few degrees toward the southwest. Within 10 or 20 feet of the contact with the diabase, the Rove is distinctly baked in the contact aureole. Closest to the contact, bedding is largely obliterated and the sediments are recrystallized to a hornfels containing biotite and clots of a secondary mineral in the matrix that is tentatively identified in thin section as cordierite. The diabase is mostly medium-grained and massive. The nearly vertical southeastern contact can be seen near the top of steep bluff as well as in pavement outcrops near the summit. The rock seen here is typical of the Pigeon River diabase dikes in the region in that it forms dikes hundreds of feet thick, has normal magnetic polarity and forms the prominent high, steep-sided ridges that are the most distinctive elements of the local topography. Stop 2.3. Rove Formation and Grand Portage dikes along the Beaver Bay shoreline. At this stop, both bedrock and post-glacial lake features are displayed. On the north side of Highway 61 at the trail crossing there are extensive exposures of the Rove Formation and two diabase dikes of the Grand Portage swarm. The unusual amount of bedrock exposure is probably a result of wave washing along the coastline of Beaver Bay stage lakes. This shoreline erosion stripped the cover of glacial till or lake sediment that probably originally mantled the bedrock, as it does over most of the surrounding area. The Rove in this area is fine-grained gray sandstone that breaks into approximately inch-thick flaggy slabs. Bedding dips about 5 degrees to the southeast, typical of the Rove over most of the region. Vertical diabase dikes of the Grand Portage swarm can be traced across the outcrop area and into the roadcut exposures along Highway 61. Just north of the bedrock outcrop, the Grand Portage Trail follows a terrace along the 816-foot contour, the inner edge of which is bounded by an indistinct bluff at 828 feet. The surface of the Trail is ILSG07 48 Trip 2 �noticeably roughened with large boulders, suggesting scour and winnowing of the finer sediments. It seems likely that this remnant terrace represents the floor immediately within the river mouth as it was in Mid Beaver Bay time (Fig. 2.12). As it is traced up valley, the bluff rises to 831 feet and 834 feet as would be expected within the river mouth. The Trail drops down a bluff from the 816 foot level into a small boggy embayment. This represents the shore of the 4th level of Beaver Bay (810 feet, between Mid and Low) and is a strong feature. The Trail rises briefly again to the former terrace (now 822 feet) and then descends into a broad embayment, backed by the continuation of the same 4th level of Beaver Bay before rising again to the terrace, (now 834 feet). A tributary coming in from the east breaks the bluff. Figure 2.12. Sketch map of the area along Grand Portage Trail north of Highway 61 showing outcrop of the Rove Formation and the trace of the bluff of the Beaver Bay stage shoreline. References Addison, W.D., Brumpton, G.R., Vallini, D.A, McNaughton, N.J., Davis, D.W., Kissin, S. A., Fralick, P.W., and Hammond, A.L., 2005, Discovery of distal ejecta from the 1850 Ma Sudbury impact event: Geology, V., 33, p. 193-196. Allen, D.J., Hinze, W.J., and Cannon, W.F., 1992, Drainage, topographic, and gravity anomalies in the Lake Superior region: evidence for a 1110 Ma mantle plume: Geophysical Research Letters, v. 19, p 2199-2122. Babcock, Willoughby M. Jr. 1940, Re-building the Grand Portage Stockade: Some Problems in Historical Reconstruction: The Museum News, December 15, 1940, pages 6-8. Birk, Douglas A.,1975, Recent Underwater Recoveries at Fort Charlotte, Grand Portage National Monument, Minnesota: The International Journal of Nautical Archeology and Underwater Exploration 4(1):73-84. Birk, Douglas A., 1979, Whitewater Archeology. The Minnesota Volunteer 42(244):12-19 ILSG07 49 Trip 2 �Birk, Douglas A., 1984, John Sayer and the Fond du Lac Trade: The North West Company in Minnesota and Wisconsin Rendezvous: in Thomas C. Buckley, editor, Selected Papers of the Fourth North American Fur Trade Conference, 1981, pp. 51-61. St. Paul, North American Fur Trade Conference. Birk, Douglas A., 1994, When Rivers Were Roads. Deciphering the Role of Canoe Portages in the Western Lake Superior Fur Trade: The Fur Trade Revisited. Selected Papers of the Sixth North American Fur Trade Conference, Mackinac Island, Michigan, 1991, 359-76. Jennifer Brown, W. Eccles, and Donald Heldman, eds., East Lansing, Michigan State University Press. Birk, Douglas A.1998,The Hudson Bay Trail: A Study of Nineteenth Century Travel Routes Between Grand Portage, Minnesota, and Fort William, Ontario: Institute for Minnesota Archaeology Reports of Investigations, Number 466. Minneapolis. Birk, Douglas, 2005, National Register of Historic Places, Grand Portage. Birk, Douglas A., and Robert C. Wheeler, 1976, Fort Charlotte Underwater Archeology Project: Geographic Research Reports 1975, 791-99. Washington: National Geographic Society. National Brown, Ralph D., 1937, Recent Excavations at Grand Portage: Minnesota History, v, 18, p.456-458. Buck, Solon J., 1931, The Story of the Grand Portage: Minneapolis: Private printing. Burpee, Lawrence J., 1931, Grand Portage: Minnesota History, v. 12, p. 359-377. Cannon, W.F. and Hinze, W.J, 1992, Speculations on the origin of the North American Midcontinent rift: Teconophysics, v. 213, p. 49-55. Carver, Jonathan, 1956, Travels Through the Interior Parts of North America, in the Years 1766, 1767, and 1768: Reprint. Minneapolis: Ross and Haines. Dawson, Samuel J., 1968, Report on the Exploration of the Country Between Lake Superior and the Red River Settlement, and Between the Latter Place and the Assiniboine and Saskatchewan: Appendix No. 36. Reprint of 1859 edition. New York: Greenwood Press. Gates, Charles M. (editor), 1965, Five Fur Traders of the Northwest: St. Paul, Minnesota Historical Society. Gilman, Carolyn, 1992, The Grand Portage Story: St. Paul, Minnesota Historical Society Press. Green, J.C., Bornhorst, T.J., Chandler, V.W., Mudrey, M.G., Jr., Myers, P.E., Pesonen, L.J., and Wilband, J.T., 1987, Keweenawan dykes of the Lake Superior region: evidence for evolution of the Middle Proterozoic Midcontinent rift of North America: Geologic Association of Canada Special Paper 34, p. 289-302. Grout, F.F., and Schwartz, G.M., 1933, The geology of the Rove Formation and associated intrusives in northeastern Minnesota: Minnesota Geological Survey Bulletin 24, 103 p. Grout, Frank F., Robert P. Sharp, and George M. Schwartz, 1959, The Geology of Cook County Minnesota: Minnesota Geological Survey, Bulletin 39. Halls, H. C, 1982, Crustal thickness in the Lake Superior region: in Wold, R. J, and Hinze, W.J., eds., Geology and tectonics of the Lake Superior basin, Geological Society of America Memoir 156, p. 239-244. Heamon, L.M., and Easton, R.M., 2006, Preliminary U/Pb geochronology results: Lake Nipigon Region Geoscience Initiative: Ontario Geological Survey Miscellaneous Release-data 191, 78 p. Hirth, Kenneth G., 1976, Interregional Trade and the Formation of Prehistoric Gateway Communities: American Antiquity, v. 43, p. 35-45. Holm, D.K., VanSchmus, W.R., MacNeill, L.C., Boerboom, T.J., Schweitzer, D, and Schneider, D, 2005, U-Pb zircon geochronology of the Paleoproterozoic plutons from the northern mid-continent, USA: evidence for subduction flip and continued convergence after the geon 18 Penokean orogeny: Geological Society of America Bulletin, v. 117, p. 259-275. Hutchinson, D.R., White, R.S., Cannon, W.F., and Schulz, K.J., 1990, Keweenaw hotsopot: geophysical evidence for a 1.1 Ga mantle plume beneath the Midcontinent rift system: Journal of Geophysical Research, v. 95, p. 10869-10884. ILSG07 50 Trip 2 �Kerber, L., 2006, Minimum age and provenance of the correlated Thomson and Rove Formations of eastern Minnesota: 19th Annual Keck Symposium, p. 142-146. Maric, M., and Fralick, P., 2005, Sedimentology of the Rove and Virginia Formations and their tectonic significance: Proceedings of the 51st Institute on Lake Superior Geology, v. 51, part 1, Proceedings with Abstracts, p. 41-42. Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., and Peterson, D.M., 2001, Geologic map of the Duluth Complex and related rocks, northeastern Minnesota: Minnesota Geological Survey Miscellaneous Map Series M-119, digital or print on demand, nominal scale 1:200,000. Morey, G.B., 1969, The geology of the Middle Precambrian Rove Formation in northeastern Minnesota: Minnesota Geological Survey, Special Publication Series, SP-7, 62 P. Morse, Eric W., 1969, Fur Trade Canoe Routes of Canada/ Then and Now: Queen's Printer, Ottawa. NICE working group, in press, Reinterpretation of Paleoproterozoic accretionary boundaries of the north-central United States based on a new aeromagnetic-geologic compilation: Precambrian Research. Nicholson, S.W., Shirey, S.B., Schulz, K.J., and Green, John C., 1997, Rift-wide correlation of the 1.1 Ga Midcontinent rift sywtem basalts: implications for multiple mantle sources during rift development: Canadian Journal of Earth Science, v. 34, p. 504-520. Nute, Grace Lee (editor), 1940, A British Legal Case and Old Grand Portage: Minnesota History, v. 21, p. 117-148. Nute, Grace, 1944, Lake Superior: The American Lakes Series, Milo M. Quaife, editor, Indianapolis, Bobbs-Merrill Company Owen, D.D., 1852, Report of a geological survey of Wisconsin, Iowa, and Minnesota and incidentally of a portion of Nebraska Territory: Report to U.S. General Land Office, 623 p. Phillips, B.A.M., 2001, Water Level History and Shoreline Change – Grand Portage National Monument, MN: A Report for the U.S. National Park Service, Grand Portage National Monument, 88 pages. Schneider, D.A., Bickford, M.E., Cannon, W.F., Schulz, K.J., and Hamilton, M.A., 2002, Age of volcanic rocks and syndepositional iron formations, Marquette Range Supergroup: implications for the tectonic setting of Paleoproterozoic iron formations of the Lake Superior region: Canadian Journal of Earth Sciences, v. 39, p. 999-1012. Schneider, D.A., Holm, D.K., O’Boyle, C., Hamilton, M., and Jercinovic, M., 2004, Paleoproterozoic development of a gneiss dome corridor in the southern Lake Superior region, USA: in Whitney, D.L., and Siddoway, C.S., eds., Gneiss domes in orogeny, Geological Society of America Special Paper 380, p. 339-357. Schwartz, George M., 1928, The Topography and Geology of the Grand Portage: Minnesota History, v. 9, p.26-30. Sharp, R.P. 1953, Glacial features of Cook County, Minnesota: American Journal of Science, v. 251, p. 855-883. Teller, J.T., and Thorleifson, L.H., 1983, The Lake Agassiz-Lake Superior connection: in. Teller. J.T, and Clayton, L., eds., Glacial Lake Agassiz: Geological Association of Canada Special Paper 26. pp. 261-290. Thompson, Erwin N., 1969, Grand Portage. A History of the Sites, People, and Fur Trade: U. S. Department of the Interior. Washington: National Park Service. Wallace, W. Stewart (editor), 1934, Documents Relating to the North West Company: Reprint edition (1968). New York: Greenwood Press. Warren, William W.,1957, History of the Ojibway Nation: Minneapolis: Ross and Haines. Wheeler, Robert C., Kenyon, Walter A., Woolworth, Alan R., and. Birk, Douglas A.,1975, Voices From the Rapids. An Underwater Search for Fur Trade Artifacts, 1960-73: Minnesota Historical Archeology Series No. 3, St. Paul, Minnesota Historical Society. White, Bruce M., 1982, Give Us a Little Milk. The Social and Cultural Meanings of Gift Giving in the Lake Superior Fur Trade: Minnesota History, v. 48, p. 60-71. ILSG07 51 Trip 2 �White, Bruce M., 1987, A Skilled Game of Exchange. Ojibway Fur Trade Protocol: Minnesota History, v. 50, p. 229-240. Winchell, Newton H. (editor), 1899, The Geology of Minnesota: Geological and Natural History Survey of Minnesota, Volume 4 of the Final Report, St. Paul: Pioneer Press Company. Woolworth, Alan R., 1967, Archeological Excavations at Grand Portage: An 18th Century Fur Trade Metropolis: The Minnesota Archaeologist, v. 29, p.3-17. Woolworth, Alan R. and Nancy L. Woolworth, 1982, Grand Portage National Monument. An Historical Overview and An Inventory of Its Cultural Resources: Two Volumes. Unpublished, typewritten report prepared for the National Park Service. St. Paul: Minnesota Historical Society. ILSG07 52 Trip 2 �53rd Annual Institute on Lake Superior Geology FIELD TRIP 3 MIDCONTINENT RIFT-RELATED MAFIC INTRUSIONS NORTH OF THE INTERNATIONAL BORDER Mark Smyk Ontario Geological Survey, Ministry of Northern Development and Mines Suite B002, 435 James St. South Thunder Bay, ON P7E 6S7 Canada Peter Hollings Department of Geology, Lakehead University 955 Oliver Road Thunder Bay, ON P7B 5E1 Canada Frontispiece: View looking southwest from the top of ridge (Crystal Lake Gabbro) at the Great Lakes Nickel property. Mesas are underlain by Logan diabase sills which have intruded Rove Formation sedimentary rocks. ILSG07 53 Trip 3 �INTRODUCTION This field trip covers an area that has been the focus of much research and economic interest. The guidebook has benefited from recent geochronologic and geochemical studies conducted as part of the Lake Nipigon Region Geoscience Initiative. These studies have elucidated the nature of magmatism in the northern part of the Midcontinent Rift (MCR) and have been used to augment and refine our previous understanding of these magmatic events. This trip focuses on a variety of mafic intrusions associated with the MCR north of the International Border near Pigeon River. These intrusions represent changes in the nature of early to mid-stage MCR magmatism over a span of ~20 million years. They include Logan sills, Pigeon River and other dykes and the layered Crystal Lake Gabbro, host to the Great Lakes Nickel coppernickel deposit. Contacts with Paleoproterozoic Rove Formation sedimentary rocks are well-exposed in this area and illustrate some of the mechanisms of dyke/sill emplacement, as well as magma-wallrock interactions. These interactions may play an important role in local metallogenesis. This guide book builds upon those previously written and compiled by Franklin and Kustra (1972), Miller and Smyk (1995), Parker et al. (2001) and Miller et al. (2002). Bear in mind that when visiting exploration or private properties, permission must be granted by the property owner. Current ownership information can be obtained from the Resident Geologist’s Office, Ontario Geological Survey, in Thunder Bay. Please exercise caution along highway right-of-ways, cliffs and mine workings. REGIONAL GEOLOGY Situated within the Southern Province of the Canadian Shield, the field trip area is dominantly underlain by Paleoproterozoic Rove Formation clastic sedimentary rocks (Animikie Group) which have been intruded by MCR mafic intrusions (Figure 3-1). Previous mapping has been conducted by Tanton (1936a,b), Geul (1970, 1973) and Smith and Sutcliffe (1989). Detailed mapping, geophysical surveys and diamond drilling undertaken by exploration companies have provided additional detail and much-needed information about sub-surface geology. This area is a rugged, upland area of diabase-capped mesas and ridges that occupies a 70 km by 30 km, northeast-trending topographic feature between Thunder Bay and the Minnesota border, termed the “Logan Basin” by North (2000). Logan Sills underlie mesas that commonly rise 150 m above valleys underlain by deeply eroded, flat-lying, Rove Formation sedimentary rocks (see Frontispiece). Northwest of the Logan Basin, Archean granitoid rocks of the Superior Province form low, rolling hills. Southeast of the Logan Basin the topography is dominated by northeast-trending, linear ridges underlain by Pigeon River dykes. Animikie Group At approximately 2.45 Ga a rift to passive margin developed along the southern edge of the Superior Province when a land-mass to the south separated (Fralick and Miall 1989). Later closure of the resultant ocean led to the deposition of the Animikie and North Range Groups as a backarc basin developed (Pufahl and Fralick 1995, 2004: Hemming et al. 1995; Van Wyck and Johnson 1997; Pufahl et al. 2000), which collision later transformed into a foreland setting (Hoffman 1987; Southwick and Morey 1991; Hemming et al. 1995; Ojakangas et al. 2001; Maric and Fralick 2005). ILSG07 54 Trip 3 �Figure 3-1: General geology of the field trip area (after Smith and Sutcliffe 1987) ILSG07 55 Trip 3 �The Animikie Group in the study area is represented by the Rove Formation, which overlies the Gunflint formation and has an approximate thickness of 500 to 600 m south and east of Thunder Bay, thickening to the south. Rocks of the Rove Formation are flat-lying or dip gently to the southeast. Recent work by Amurawaiye (2001) and Maric and Fralick (2005) described a submarine ramp system in which the movement of coarse sediments into the deeper parts of the basin was mainly through the action of lowand high-density turbidity currents. Fair-weather and storm-generated currents dominated depositional activity at the edge of the basin. Mud and ash particles accumulated during tranquil periods (ibid). Amurawaiye (2001) has stated that approximately 70% of the Rove Formation locally consists of organic shale whose hydrocarbon contents have now been degraded. The lower 100 to 150 m of the Rove Formation and the correlative Virginia Formation in Minnesota consist of alternating shale-siltstone and black, pyritiferous shale successions, probably reflecting fluctuations in sea level (Maric and Fralick 2005). These successions, and especially the upper black shale, likely represent a major condensed interval deposited in water ~100 to 200 m deep. Lucente and Morey (1983) ascribed sedimentation of this interval to pelagic rainout of fine-grained sediment from dilute suspension or hemipelagic processes involving diffuse turbidity currents. The presence of abundant, sub-millimeter rip-up intraclasts also denotes the operation of sporadic bottom currents (Maric and Fralick 2005). Tidal deposits present in correlative rocks to the south of Lake Superior (Ojakangas et al. 2001) confirm open connection to the ocean. Above the upper, pure black shale interval, graded finegrained sandstones are organized into a coarsening-upward succession approximately 100 m thick that is transitional into 400 m of medium-grained, sandstone-dominated, stacked parasequences (Maric and Fralick 2005). This is overlain by lenticular to wavy bedded sandstones and shales with both wave and current ripples. The coarsening-upward to sandstone-dominated portion of the Virginia and Rove Formations has been interpreted as a submarine fan (Lucente and Morey 1983, Maric and Fralick 2005) with the uppermost ripple laminated succession representing progradation of distal distributary mouth bars of a delta (Maric and Fralick 2005). A sandstone sample from the submarine fan portion of the succession yielded a youngest U-Pb detrital zircon age of approximately 1780 Ma (Heaman and Easton 2006). The predominantly Paleoproterozoic zircon population and paleocurrents indicating sediment derivation from the north (Morey 1973), strongly suggest the Trans-Hudson Orogen was the source of the detritus. Keweenawan Supergroup Mesoproterozoic intrusive, volcanic and minor sedimentary rocks associated with the MCR collectively constitute the Keweenawan Supergroup. On the northern margin of the MCR, Keweenawan rocks include a variety of intrusive rocks and Osler Group volcanic rocks which represent some of the earliest magmatism in the MCR. As shown in Table 3-1, ages range from ca. 1140 Ma (Heaman and Easton 2006) to ages younger than the magnetic polarity reversal that occurred between 1105 and 1102 Ma (Davis and Green 1997). A tabulated synopsis is provided below; bolded units occur within the field trip area. Most mafic and ultramafic rocks in the Lake Nipigon and northern Lake Superior areas, including the Nipigon and Logan sills, appear to have been emplaced in a short, magnetically reversed, interval between ca. 1115 and 1100 Ma (Heaman and Easton 2006). Emplacement of alkalic intrusions, such as the 1108 Ma Coldwell Complex (Heaman and Machado 1992), and filling of much of the submerged part of the rift in Lake Superior, also occurred in this period. This was followed by a period of magnetically normal, waning mafic and felsic magmatism, between 1096 and 1085 Ma, that is preserved mainly along the Lake Superior shore by units such as the Crystal Lake (1099±1 Ma), Moss Lake (1095±2 Ma) and Blake (1095±2 Ma) gabbros, and a Pigeon River dyke near Arrow River (1093±3 Ma; Heaman and Easton 2006). ILSG07 56 Trip 3 �Table 3-1: Geochronology data of MCR-related rocks in Northwestern Ontario. Lithologic Unit St. Ignace Island Complex gabbro Arrow River Dyke Pigeon River Dyke Blake Gabbro Moss Lake Gabbro Crystal Lake Gabbro Pine Point – Mt. Mollie dyke Osler Group rhyolite (central suite) Osler Group rhyolite (lower suite) St. Ignace Island Complex Rhyolite Coldwell Complex Locality / Age (Ma) St. Ignace Island / 1089.2 +3.2 Arrow River / 1078 + 3 Rita Bolduc / 1141±20 Blake Township / 1091.0 + 4.5 Black Bay Peninsula / 1094.7 ±3.1 Great Lakes Nickel / 1099.6 + 1.2 n/a Agate Point / 1105±2 Reference(s) Smyk et al. (2006) Heaman and Easton (2006) Heaman and Easton (2007) Heaman and Easton (2006) Heaman and Easton (2006) Heaman and Easton (2006) Black Bay Peninsula / 1107.4 +4/-2 St. Ignace Island / 1107.2 ± 2.4 Davis and Sutcliffe (1985) Smyk et al. (2006) Coldwell Complex / 1108 + 1 Logan Sills Nipigon Sills Ultramafic Intrusions Pigeon River Inspiration Sill Marathon lamprophyre dykes Mt. McKay / 1115 + 1 Nipigon Embayment / 1114-1110 Nipigon Embayment / 1124-1113 Crooks Twp. / 1141 + 20 Lake Nipigon / 1141 + 20 McKellar Harbour / 1145 +15/-10 Heaman and Machado (1987) Heaman and Easton (2006) Heaman and Easton (2006) Heaman and Easton (2006) Heaman and Easton (2006) Heaman and Easton (2006) Queen et al. (1996) Davis and Green (1997) Hypabyssal Mafic Rocks Diabase sills, extending from the vicinity of Thunder Bay to east of Lake Nipigon, represent the northern remnants of the Midcontinent Rift, and have previously been referred to as the Logan sills (Stockwell et al. 1972), however recent work suggests a geochemical difference between the sills to the north and south of the City of Thunder Bay (Hart 2003; Hart et al. 2005b). Hollings et al. (2007) proposed that the term Logan Igneous Suite, which would fall within the Midcontinent Rift Intrusive Supersuite of Miller et al. (2002), should be applied to all the diabase sills in the area north of Lake Superior, with subdivision into the informal terms, Nipigon sills for the sills north of Thunder Bay, and Logan sills to the south. Logan sills generally consist of fine- to coarse-grained, ophitic to intergranular, quartz tholeiitic diabase/gabbro (Smith and Sutcliffe 1987; Geul 1970, 1973). Coarse-grained, intergranular gabbro, locally rich in granophyric mesostasis, is common in the interior of the thicker sills. The upper sections of the diabase sills are commonly plagioclase-porphyritic, containing as much as 60% phenocrysts. Chilled margin and bulk compositions are iron-rich, quartz-tholeiitic basalt. Compositional and textural variation in sills has been noted by North (2000) and Beskar (2001) in Blake Township, where varitextured, "taxitic" gabbro has been described. Logan sills are recognized by their reversed magnetic polarity and generally take the form of columnarjointed, thick sheets and sills whose geometry is strongly controlled by the subhorizontal bedding of the country rock. They form conspicuous erosional remnants that create mesa and cuesta topography. From the international boundary area to Thunder Bay as many as six diabase sheets were emplaced nearly conformably into Animikie sedimentary rocks (Weiblen et al., 1972; Smith and Sutcliffe 1987, 1989). Diamond drilling has also shown that stacked sills exist in the subsurface. For example, Dumont Nickel Inc. reported intersecting 14 gabbroic sills in a 705 m deep drill hole in central Pardee Township (Assessment Files, Thunder Bay South Resident Geologist’s District, Thunder Bay). North of the border, ILSG07 57 Trip 3 �Smith and Sutcliffe (1989) reported sills up to 50 m thick, whereas in Minnesota, Jones (1984) studied four sills ranging from 50 to 160 m in maximum thickness. Rare exposures of feeder dykes to sills and preserved sill terminations have been noted (e.g., Stop 3-3). The textural stratigraphy, which varies from a lower, ophitic zone to an upper pegmatitic zone, indicates that in most cases, the sills cooled as single units, probably over a period of 200 to 500 years (Smith and Sutcliffe 1989). Chilled contact zones are developed against sedimentary country rocks; sedimentary xenoliths are rare. Pigeon River dykes trend east-northeast to northeast and dip steeply to the southeast (Geul 1970, 1973; Smith and Sutcliffe 1989). Displacement and warping of the Rove Formation is evident along many of the dikes. Composite intrusions are noted in several dykes (add reference). Dyke widths average between 50 and 70 m, but may be as much as 150 m across in Ontario (Smith and Sutcliffe 1987) and 500 m in Minnesota (Green et al. 1987). Forming northeast-trending, linear ridges, dykes can be traced semicontinuously for 15 km along strike. As noted by many workers, some dykes clearly crosscut Logan sills. However, Geul (1973) and Smith and Sutcliffe (1987) noted that others display somewhat ambiguous crosscutting relationships. In these latter cases, dykes may appear to merge with sills, suggesting that they were contemporaneous or that sills impeded the upward migration of the dykes. The presence of multiple sets of horizontal columnar jointing suggests the development of multiple or composite dykes. The dykes typically consist of ophitic diabase that may be plagioclase-porphyritic. A typical, nonporphyritic olivine diabase consists of approximately 60% plagioclase (zoned labradorite; An 55-70), 20% augite + hypersthene, up to 15% olivine and up to 5% magnetite, ilmeno-magnetite and sulphides (Geul 1970, 1973). Average whole rock compositions of Pigeon River dikes are moderately evolved (mg# = 52) olivine tholeiitic basalt. A 15-km long, northwest-trending diabase dike, termed the Arrow River dyke by Smith and Sutcliffe (1989) crosscuts Pigeon River dikes in Ontario. This dike and two shorter, similarly oriented dikes are composed of intergranular, quartz diabase that is commonly plagioclase-phyric. They are collectively known as the Arrow River dykes, despite the fact that they occur well north of the Arrow River. The authors recommend that they be named Cloud River dykes.) Confusion has arisen because the term “Arrow River dyke” has also been used colloquially to denote a prominent Pigeon River dyke that occurs between the Arrow River and Pigeon River in southern Devon Township (i.e. Stop 3-1). The youngest intrusions in the area tend to be more irregularly shaped and internally zoned. One of these is the Crystal Lake gabbro, which is Y-shaped in plan view, with a west-northwest-striking limb 5km long and an east-northeast-striking, southern limb 2.75 km long (Figure 3-2). Internal layering and foliation suggest that the surface geometry of the northern limb may result from the tilting of a canoeshaped body, open on its western end (Smith and Sutcliffe 1987, 1989). The intrusion was subdivided into three major zones by Reeve (1969) and Geul (1970) and has been further subdivided by Smith and Sutcliffe (1987, 1989) and Cogulu (1990) into four major, roughly equivalent, lithologic zones: (1) an upper zone (60 to 80 m) of sulfide-barren troctolite, olivine gabbro and anorthositic gabbro; (2) a middle zone (30 to 42 m) of cyclic, layered anorthositic and olivine gabbro, Cr-spinel- bearing anorthosite, and olivine gabbro (Figures 3-3 and 3-4); (3) a lower, unlayered zone (50 m) of vari-textured gabbro and leucotroctolite, which hosts the bulk of the Cu-Ni-sulfide deposit; and (4) a basal zone (1 to 7 m) of fine-grained, chilled melagabbro and hornfelsed country-rock xenoliths. ILSG07 58 Trip 3 �Table 3-2: Geochemical analyses of select intrusive rocks from the Border Area Intrusive Unit: Source: Logan Sills Grand Portage dikes Mt Josephine Pigeon River dikes A B C D C Chilled Margin Bulk Comp Avg. of 17 dikes Composite dike Average of 14 dikes SiO2 49 50.1 52.5 47.9 48.9 TiO2 3.4 3.6 2.48 3.66 1.65 A12O3 13.1 13.1 13.4 11.8 16.3 FeOt 15.6 14.3 13.3 16.2 11.5 MnO 0.22 0.16 0.19 0.24 0.18 MgO 5.6 3.9 3.68 4.04 6.46 CaO 7.45 7.2 6.72 8.9 10.21 Na2O 2.52 3.4 3.17 2.63 2.42 K2O 1.16 1.5 1.79 1.29 0.52 P2O5 0.38 0.43 0.48 0.53 0.18 Volatiles 0.92 2.5 ----- ----- ----- Total 99.35 100.19 99.11 99.6 99.8 mg# 38.4 32.2 34.1 33 50.1 Cr 50 --- 44 50 100 Ni 80 --- 61 22 136 Rb --- --- 55 28 15.7 Sr 700 --- 420 359 280 Zr 250 --- 281 280 --- Hf --- --- 9 --- 2.94 Description: Trace Elements (ppm) Table from Miller and Smyk (1995); Sources: (A) Geul (1970), Table 3, Sample 2; (B) Jones (1984), Table A-I, Sill A; (C) Green et al. (1987), Table II; (D) Green (1986), Appendix E, Sample GP-60. ILSG07 59 Trip 3 �Figure 3-2: General geology of the Crystal Lake Gabbro after Smith and Sutcliffe (1987) ILSG07 60 Trip 3 �Figure 3-3: Polished slab of massive Crspinel layers in contact with medium-grained gabbro, Great Lakes Nickel property (Stop 3-4). White bar is 4 cm long. Figure 3-4: Igneous layering in the Crystal Lake Gabbro, exposed in the upper cliff section on the Great Lakes Nickel property (Stop 3-4). This is part of the middle zone of the intrusion, characterized by cyclic, layered anorthositic and olivine gabbro, Crspinel-bearing anorthosite, and olivine gabbro. ILSG07 61 Trip 3 �Projecting east of the Crystal Lake gabbro and arcing to the northeast to form a string of islands in Lake Superior is a major composite dike called the Pine River-Mount Mollie dyke (Geul 1973). Smith and Sutcliffe (1989) described this 35 km-long body as consisting of gabbroic, dioritic and granophyric rocks. Gabbroic rocks in the margins of the dike commonly have modal layering and sulphide mineralization similar to that in the Crystal Lake gabbro. Inward from the gabbro, quartz-bearing diorite cores much of the dike. In the central and western portions, granophyre occupies the core of the dike and shows contact relationships with diorite that indicates mixing of felsic and mafic liquids (Smith and Sutcliffe 1987, 1989). As described by Geul (1973), the Pine River-Mount Mollie Gabbro comprises two segments of an eaststriking intrusion which appears to postdate the Pigeon River dykes. The poorly exposed Pine River segment apparently connects to the eastern end of the Crystal Lake Gabbro. Magnetometer survey data and drilling indicate that the Pine River and Mount Mollie segments are connected beneath deep overburden. Field relationships and diamond drilling show that the dip of the gabbro contacts varies from near vertical to 35º north, indicating that the body has the shape of a tilted, truncated cone in crosssection. The exposed width ranges from approximately 60 m (true thickness) to 300 m (apparent thickness), depending upon the attitudes of the contacts. The gabbro has a distinct mottled appearance and is further distinguished from most other diabases in the area by larger grain size, leucogabbroic composition, and degree of mineralization. Diamond drilling logs summarized by Geul (1973) defined a crude zonation in the gabbro, based on variations in grain size, feldspar (i.e. anorthosite) and sulphide content. Thin section petrography by Geul (1973) indicated that the gabbro ranges in composition from anorthositic gabbro to olivine gabbro and quartz gabbro. Deuteric alteration is slight to intense and consists of secondary amphibole, biotite, saussurite, sericite, and hematite and appears to be closely related to the presence of an intergranular granophyric phase. Bodies of granophyre cut the gabbro in several locations and appear to represent the differentiated end product of the gabbroic magma. The gabbro is composed essentially of pyroxene and zoned labradorite. DISCUSSION OF GEOCHEMISTRY As part of the Lake Nipigon Region Geoscience Initiative whole rock analyses were performed on a number of the intrusions south of Thunder Bay; these data can be combined with published (Cogulu 1990), unpublished data for some of the dykes in the region (Larry Hulbert, Geological Survey of Canada, personal communication, 2006) and data from exploration projects (e.g., Rosatelli 2002), in order to assess variations in geochemistry. On a primitive mantle-normalized diagram, the samples of Crystal Lake gabbro and Pigeon River dykes show similar patterns (Figure 3-5). Pigeon River dykes from the Arrow River (Stop 3-1) and Rita Bolduc occurrence (Stop 3-2) are similar to the range of data from other dykes of the Pigeon River swarm sampled in Lake Superior (Victoria Island, Cloud Bay and Jarvis Point (Figure 3-6; Larry Hulbert, Geological Survey of Canada, personal communication, 2006). However, samples of the Pigeon River dyke swarm from Lake Superior are characterized by higher Th abundances than those from the study area, resulting in more pronounced negative Nb anomalies. The only published analysis for the Arrow Dyke swarm (Rosatelli 2002) is indistinguishable from the Pigeon River swarm. By contrast, samples from the Mount Mollie dyke are characterized by elevated La/Smn ratios, higher Th contents and more pronounced Nb anomalies (Figure 3-6). This may be a reflection of greater crustal contamination during emplacement or possibly a distinct mantle source. Regardless, this geochemistry provides another means for distinguishing these dykes. ILSG07 62 Trip 3 �When the data from the dyke swarms are compared to the regional data set generated for the sills and intrusions of the Lake Nipigon embayment (Hollings et al. 2007) it can be seen that Pigeon River dyke swarm geochemistry more closely resembles that of the sills of the Nipigon suite than that of the ultramafic intrusions or the Logan sills (Figure 3-6). By contrast, the Mt Mollie dyke geochemistry appears to be transitional between that of the Nipigon sills and Inspiration sills. Additional isotopic and geochronological studies will be required in order to further investigate the relationships between these MCR-related intrusions. Figure 3-5: Primitive mantle-normalized plots for mafic intrusive rocks in the field trip area. Fields from data contributed by L. Hulbert, Geological Survey of Canada, personal communication (2006). Normalizing values from Sun and McDonough (1989). CONTACT METAMORPHISM There is a remarkable range in the reported intensity and nature of contact metamorphic effects in Rove sedimentary rocks at diabase dyke and sill contacts, owing mainly to the subjectivity of the mapper and the exposures in question. Geul (1973) noted that sedimentary hornfelsic rocks are restricted to a narrow zone of baking between 2 to 10 cm wide at diabase dike contacts. Metamorphosed siltstone displays two stages: first: slight recrystallization of biotite aggregates in an incipient hornfelsic texture; and second, a more complete recrystallization of biotite, surrounded by pale sericitic aggregates, set in a quartzofeldspathic matrix. Franklin (1970) suggested that contact effects existed up to 8 m from sill contacts and possibly up to 23 m. They were manifested as microporphyroblasts of mica and chlorite (“spotted alteration”), graphite destruction and the conversion of pyrite to pyrrhotite. Geul (1973) noted that minute particles (< 0.01 mm) of oxide and sulphide minerals are locally abundant in the contact zone. Rove Formation sedimentary rocks may be deformed along dyke contacts. As noted by Geul (1973) beds appear to dip toward the dykes or are “up-dragged” along dyke contacts (e.g. Stop 3-2). Up-dragging of Rove Formation rocks was also noted along the southern contact of an east-striking Pigeon River dyke at Arrow Rapids by Geul (1970). Deformed and fractured sedimentary rocks have been noted near sill terminations (e.g. Stop 3-3). Narrow, parallel, tension gashes filled with quartzo-feldspathic leucosome/neosome occur in metatectic, deformed siltstones at Stop 3-2 (Figure 3-8). ILSG07 63 Trip 3 �Figure 3-6: Discrimination diagrams for mafic and ultramafic intrusions near Thunder Bay. Data are from Hollings et al. (2007) and Hart (2002). ILSG07 64 Trip 3 �MINERAL EXPLORATION Active prospecting was first carried out during the latter part of the nineteenth century, following the discovery of silver vein deposits along the shoreline and islands of Lake Superior (Island Silver Belt) and to the northwest (Mainland Silver Belt; Oja 1967). Small silver mining operations took place on Spar and Jarvis Islands and at the Prince Mine. Barite was produced from veins on McKellar Island and Jarvis Island. Silver-bearing veins are in or near crustal-scale, extensional listric faults that formed during MCR extension and are spatially associated with MCR-related mafic intrusions (Franklin et al. 1986). Copper-nickel-mineralized boulders were found in 1903 by J. A. McCuaig in Pardee Township (Geul 1970). This discovery led to a concerted exploration effort to find their source (Geul 1970, 1973). Local copper-nickel occurrences were soon described by Tanton (1935, 1937). United States Smelting, Refining and Mining Company conducted exploration in 1936, and were followed by Frobisher Exploration Company in 1942. In the summer of 1952, J.S. Brodie and T.W. Page examined a large outcrop of gabbroic rocks, 6 km northeast of the original float discovery on the property now held by the Great Lakes Nickel Corporation Limited. Following this discovery, several blocks of ground were staked in Crooks Township by Whitegate Mining Company Limited, J. S. Strickland, and others, in an effort to cover the interpreted eastward extension of the favourable host rock. In 1967, Canadian Exploration Limited investigated the Strickland-Whitegate claims in southwestern Crooks Township and Anaconda American Brass Limited prospected the Pine River-Mount Mollie area as part of a regional reconnaissance program. In 1968, Phelps Dodge Corporation of Canada Limited explored and drilled claims in the Pine RiverMount Mollie area of Crooks Township. In 1989, International Platinum Corp. drilled a 3592 m hole in central Crooks Township. A renewed interest in platinum group elements in the late 1990s led to a reevaluation of the area’s potential and a flurry of exploration and drilling by companies like BHP Billiton World Expl. Inc., Falconbridge Ltd., McVicar Minerals Ltd., North Atlantic Nickel Corp. and Dumont Nickel Corp. A Noril’sk-style model was employed, as had been earlier suggested by Lightfoot and Lavigne (1995) and others in promoting the area’s mineral potential. ILSG07 65 Trip 3 �Figure 3-7: Columnar joints in the Crystal Lake Gabbro, exposed above the upper adit on the Great Lakes Nickel property (Stop 3-4). The adit opening (lower left) is approximately 3m wide. Figure 3-8: Narrow, parallel tension gashed filled with quartzo-feldspathic leucosome/neosome in metatectic, deformed siltstone near contact with Pigeon River dyke, Stop 3-2 ILSG07 66 Trip 3 �FIELD TRIP STOPS: ROAD LOG AND DESCRIPTIONS STOP NAME Middle Falls Dyke Arrow River Dyke STOP NUMBER LANDMARK (0.0 km) DISTANCE (km) Reset Trip Odometer Visitor Centre @ Border 0.0 / travel north Junction with Hwy. 593 2.3 / turn left, travel west Middle Falls 4.5 Junction with Old Border Road 10.9 / turn left, travel west 500 m Junction with Logging Road 11.4 / turn right, travel northwest 3.8 km Arrow River bridge 14.4 Optional Reset Trip Odometer McCuaig Float Occurrence ILSG07 Junction of Hwy. 593 and Old Border Road 0.0/ left turn, travel north Junction with Hwy. 597 (Pardee Road) 1.7 (on right) 3.8 / 150 m north of hwy. (flagged trail) Optional Reset Trip Odometer Rita Bolduc Occurrence / Pigeon River Dyke 15.2 3-1 Junction of Hwy's. 593 and 61 NORTHING EASTING (NAD 83) 5321108 304872 5324134 296694 5325546 299009 0.0 / turn left, travel north 4.4 3-2 67 5324701 310563 Trip 3 �Mount Mollie Lookout Great Lakes Nickel Property 6.6 (gated, on left) Pine River Bridge 6.75 Junction with Memory Road 6.9 / turn right, travel east 3.1 km Trail to Mount Mollie lookout 10.0 / trail on east side of road Follow trail ~300 east to top of ridge Optional Reset Odometer Rove Formation Quarry Junction with Great Lakes Nickel Road Return to Highway 61; turn left, travel south Travel south 300 m to Great Lakes Nickel Road (gated, on right) Junction of Hwy. 61 with Great Lakes Nickel Road 0.0 / travel west 5325444 313843 3-3 4.7 (south side of road) 5328687 306401 3-4 7.5 (western end of ridge; cliff face) 5328392 303612 STOP (OPTIONAL): Middle Falls Location: South side of Highway 593, 4.5 km west of Visitor Centre at Border on Highway 61 (UTM 304872E / 5321108 N; NAD83) (N.B. On older maps, this location is referred to as Little Falls.) Description: This roadside pull-off offers a picturesque view of Middle Falls, an 18 m high waterfall underlain by a resistant, west-northwest-trending, diabase dyke cutting Rove Formation clastic sedimentary rocks. The dyke is exposed in a roadcut on the highway opposite the pull-off area. It is cut by a number of parallel, rusty carbonate veins. STOP 3-1: Pigeon River Dyke (a.k.a. the “Arrow River Dyke”) Location: Road cut, 15.2 km west of Visitor Centre at Border on Highway 61, via Old Border Road (@ 10.9 km) and logging road (@ 11.4 km) (UTM 296694E / 5324134N; NAD83) ILSG07 68 Trip 3 �Description: This stop is situated on top of a prominent northeast-trending ridge which extends for over 20 km and rises up to 150 m above the surrounding countryside. The ridge is underlain by a Pigeon River dyke which, owing to its location, has been colloquially referred to as “the Arrow River dyke”, although it is not part of the northwest-trending Arrow River dyke swarm of Smith and Sutcliffe (1989). The exposed dyke consists generally of massive fine- to medium-grained diabase. Conspicuous, pyroxene-phyric patches have diffuse to indistinct contacts with the finer-grained diabase, suggesting that they may represent fluid-rich pockets or cognate xenoliths. (Similar textures are noted at Stop 3-2 as well.) A very small amount (~50 grains) of tiny baddeleyite fragments and blades were recovered from a sample of diabase at this location (Heaman and Easton 2006). A larger number of 40–100 μm colourless resorbed zircons were also recovered, but these were interpreted to be of xenocrystic origin. The U/Pb results are from one small multi-grain baddeleyite fraction. This baddeleyite fraction has a low uranium content (188 ppm U) and a low Th/U ratio (0.022); the latter is typical for magmatic baddeleyite. This baddeleyite analysis displays slight reverse discordance (–1.4%) and has a 206Pb / 238U age of 1092.7±2.6 Ma, which is interpreted as a good estimate for the minimum age of the Arrow River diabase dike (ibid). STOP (OPTIONAL): McCuaig Float Occurrence Location: North side of Highway 593, 3.8 km west of junction of Highway 593 and Old Border Road / 1.7 km west of junction of Highways 593 and 597 (Pardee Road) (UTM 299009E / 5325546 N; NAD83) Description: This stop comprises a number of pits and trenches developed in a hummocky, boulderstrewn hillside. Copper-nickel-mineralized boulders were found in 1903 by J. A. McCuaig (Geul 1970). In 1936, a magnetometer survey was conducted by United States Refining and Mining Company. In 1942, Frobisher Exploration Company Limited completed a second magnetometer survey. In 1949-1950, J. A. McCuaig (original discoverer’s son) carried out some minor trenching and sampling and drilled a 200 foot subvertical hole which reportedly showed the mineralized rock was in the form of float boulders. A small bulk sample was tested by Falconbridge Nickel Mines Limited. In 1967, Hollinger Consolidated Gold Mines Limited carried out geophysical surveys. Hanna Gold Mines Limited completed a vertical hole nearby. In 1990, Fleck Resources Ltd. carried out magnetometer and geochemical surveys along with limited prospecting, trenching and sampling just north of this occurrence (Resident Geologist’s Files, Thunder Bay South District, Thunder Bay). Tanton (1935, 1937), McCuaig (1950) and Geul (1970) have described mineralized, vari-textured diabase, gabbroic anorthosite and norite. Interstitial to blebby pyrite, pyrrhotite, chalcopyrite and pentlandite characterize the sulphide mineralization. Assay results are shown below: Reference Cu (%) Ni (%) Co (ppm) Pd (ppb) Pt (ppb) Geul (1970) 0.23 .018 n/a Trace n/a Resident Geologist Files, OGS 1.76 0.72 190 n/a 610 Tanton (1935) 0.62 0.34 n/a n/a 5140 ILSG07 69 Trip 3 �STOP 3-2: Rita Bolduc Occurrence (Pigeon River Dyke) Location: Highway 61, 4.4 km north of the junction between Highways 593 and 61; (UTM 310563E / 5324701N; NAD83) Description: This highway roadcut (Figure 3-9) exposes the southern contact and an almost complete cross-section of a sulphide-mineralized Pigeon River dyke. The property was explored with a self potential survey and trenching in 1957 (Geul 1973). It was part of a property that was acquired and explored by Falconbridge Limited and McVicar Minerals. The main sulphide showing is situated along the south edge of a Pigeon River olivine diabase dike, carrying disseminated to massive coppernickel-sulphide mineralization in a fracture zone, up to 35 feet in width and extending for a distance of at least 100 feet along the dike (Geul 1973). A grab sample of massive to disseminated pentlandite, pyrrhotite with minor chalcopyrite returned 0.70% Cu and 0.35% Ni and trace amounts of Pd (ibid). Net-textured pyrrhotite and chalcopyrite blebs occur locally. Grab samples collected from sulphide-mineralized sections of the dyke on the west side of the highway yielded the following results: Sample 01-BRB-01 01-BRB-02 01-BRB-03 Au (ppb) Cu (ppm) Ni (ppm) Pd (ppb) Pt (ppb) 1442 707 28.03 21.84 14.29 880 505 17.55 13.90 25.02 6517 2664 23.16 n.d. (Resident Geologist's Files, Thunder Bay South District, Thunder Bay) Hornfelsed, rusty weathering, Rove Formation sedimentary rocks are exposed along the southern dyke margin. They are fissile and display an orthogonal joint pattern. Beds have been up-folded along the contact, suggesting either dip-slip fault motion and/or high magma flow rates. Similar “up-drag” features were noted on the southern contact of the Arrow River dyke at Arrow Rapids by Geul (1970), also in association with a copper-nickel sulphide-mineralized fault/fracture zone. Narrow, parallel tension gashed filled with quartzo-feldspathic leucosome/neosome occur in metatectic, deformed siltstone; infilled fractures may produce leucosome dykes (Figure 3-8). They are oriented parallel to the dyke contact and occur several metres away from the contact. Rare, small (< 1 cm diameter), round chloritic patches may represent incipient “spotted” alteration, similar to that described by Franklin (1970). Geochronologic sampling by Heaman and Easton (2006) at this location showed that the least magnetic fraction contained a large amount of colourless euhedral apatite, pyrite and a small amount of baddeleyite and zircon. A very small amount of tiny tan baddeleyite blades and fragments was recovered and a number of composite baddeleyite/zircon grains were identified. Although not that common, primary igneous baddeleyite overgrown by magmatic zircon has been reported previously and is consistent with the phase relationship for crystallization of these two minerals in a mafic magma where the silica activity is progressively increasing throughout crystallization. The four zircon analyses have varied uranium and thorium contents (163–1104 ppm and 34–237 ppm, respectively), but have high Th/U (1.17–1.81), consistent with primary magmatic zircon crystallizing from a mafic magma.The four zircon analyses are discordant (8.7–11.7%) and have 207Pb/206Pb ages that vary between 1050 and 1090 Ma. The baddeleyite fraction (#5) is least discordant (3.8%) and contains relatively low uranium (213 ppm) for baddeleyite. The 1133.8 Ma 207Pb/206Pb age obtained for this baddeleyite fraction is interpreted as a minimum estimate for the age of this dike. A Model 1 regression treatment of three zircon (#1, #2, #3) and the baddeleyite (#5) yields and upper intercept age of 1141±20 Ma, which is interpreted as the best estimate for the emplacement of the Pigeon River dike (Heaman and Easton 2006). ILSG07 70 Trip 3 �Figure 3-9: Sketch map of the road cut exposing a Pigeon River dyke on the eastern side of Highway 61 at Stop 3-2. Geology modified after Peck and Rosatelli (personal communication, 2001) in Parker et al.(2001). Geochemical sample locations are shown as numbered black dots. STOP (OPTIONAL): Mount Mollie Lookout Location: Approximately 300 m north along old road bed, 3.1 km east of Highway 61 on Memory Road. The junction of Memory Road and Highway 61 is 6.9 km north of the junction of Highways 593 and 61, just north of the Pine River bridge (UTM 313843E / 5325444N; NAD83) Please be advised that this stop is located on private land. Permission must be granted by the land owner. Contact the Resident Geologist’s Office in Thunder Bay for further information. Description: This stop, atop a prominent northeast-trending ridge, affords a panoramic view of Pine Bay, Lake Superior and the surrounding, ridge-dominated topography. This and other nearby ridges are underlain by ophitic Pigeon River diabase dykes. Pigeon Point, Minnesota (type locality for pigeonite) can be seen on the horizon to the south. Northwest-trending ridges, underlain by Arrow River dykes, extend inland from south of Big Trout Bay, just north of this location. A long (>1 km) and challenging trail extends north and east from this vantage point to copper-nickel-mineralized occurrences in the Pine River-Mount Mollie dyke (ca. UTM 314748E / 5325640N; NAD27), which is approximately 60 m wide at that location. Heslop (1968) noted blebs of pyrrhotite-pentlanditechalcopyrite and magnetite-ilmenite intergrowths at this occurrence. Supergene(?) native copper has also been noted there. ILSG07 71 Trip 3 �STOP 3-3: Rove Formation Quarry, Great Lakes Nickel Limited Road Location: 4.7 km west of Highway 61 on the unmarked Great Lakes Nickel Road (private access). The GLNR extends west from Highway 61 at a point just south of the Pine River bridge, 6.6 km north of the junction of Highways 61 and 593 (UTM 306401E, 5328687N; NAD83). Description: A dormant shale quarry on the south side of the Great Lakes Nickel Limited access road provides a representative section of nearly flat-lying Rove Formation turbidites, diabase dykes and a sill. The quarry face is capped by a 1.1 m thick wacke bed that is underlain by a 1.1 m thick, conformable diabase sill in the main part of the quarry. Approximately 12 m of thinly to thickly bedded wacke and shale are exposed below the sill. The wackes are typically massive, and display sharp, planar contacts with the recessively weathered shale units. Scours of underlying shale are locally developed. Some of the shale interbeds display sediment loading features and distinctive sole features and are locally flaser-bedded. Another, 22 cm wide diabase dyke intrudes the sedimentary rocks east of the sill/dyke exposure, strikes ~060º and dips ~80º east. The columnar-jointed diabase sill is fine- to medium-grained and has conspicuous, grey chilled margins. It pinches out on the eastern end of the quarry face where it apparently “steps down” through the sedimentary rocks and merges with a near-vertically dipping diabase (feeder?) dyke. This dyke is weakly magnetic, strikes 030º and has an exposed thickness of approximately 4m. A sample collected near the chilled margin returned the following analysis: Sample Al2O3 CaO Fe2O3 K2O LOI MgO MnO Na2O P2O5 SiO2 TiO2 TOTAL 01-BGL-01 14.51 9.15 13.92 0.78 1.08 5.31 0.2 2.69 0.15 51.78 1.52 101.1 (Resident Geologist’s Files, Thunder Bay South District, Thunder Bay) In contrast a sample of the upper sill was considerably more silica rich (GL-1; SiO2 = 72 wt%) and is characterized by elevated La/Smn ratios (3.2) and Th contents (7 ppm vs. <1.3 in other dykes and sills). This likely reflects localized assimilation of Rove wacke by the sill during emplacement. A thin veneer of the hornfelsed sedimentary host rocks is locally preserved against the chilled dyke margin. The diabase is plagioclase-phyric and contains small sub-rounded shale xenoliths. A tendril of diabase extends west from the dyke (below the sill) and forms a bulbous, pillow-like termination with radial cooling fractures. The adjacent shale is chaotically folded and crenulated, suggesting that shearing accompanied intrusion. Two metres further west, an isolated ovoid (45 cm by 75 cm) “blob” of diabase intrudes the shale. Similar fingering termination features were described at the periphery of sills by Pollard et al. (1975). At finger terminations, the host rock is wedged aside and compacted and along the edges of the fingers, buckling and shearing of strata are common. Pollard et al. (1975) ascribed finger formation to the instability of the advancing interface between a viscous magma and a more viscous host rock. Antonellini and Cambray (1992) also studied a possible structural control on local sill emplacement. A block of wacke was selected at this location for a detrital zircon study (Heaman and Easton 2006). The detrital zircon ages form two distinct age clusters, with the majority of grains having 207 Pb/206Pb ages in the 1780–1880 Ma range. A second population consists of Archean grains, most of which are in the 2600–2700 Ma range. The oldest grain has a 207Pb/206Pb age of 2914 Ma. The youngest grain, with a 206Pb/238U age of 1731 Ma places a maximum age on the depositional age of this wacke. This grain is 5% discordant, consequently, a better estimate for a maximum depositional age is provided by a 1777 Ma concordant detrital zircon grain. This interpretation is supported by the abundance of grains (n=10) between 1796 and 1777 Ma, and is consistent with U/Pb ash bed ages of 1836±5 and 1832±3 Ma from the basal Rove Formation (Addison et al. 2005). The section exposed in the quarry walls represents a portion of the middle of the almost kilometer-thick Rove Formation (P. Fralick, Lakehead University, personal communication, 2007). The lower, underlying Rove is dominated by approximately 120 m of carbonaceous shale. This gradationally coarsens and thickens upwards through 100 m by the addition of thin sandstone ILSG07 72 Trip 3 �turbidites, culminating in beds similar to the ones exposed in the quarry. Proceeding upwards, above the rocks exposed in this section, ripples become more abundant in the sandstones until the assemblage is dominated by lenticular to flaser-bedded, wave- and current-rippled units. The Rove Formation represents a period of sediment starvation followed by progradation of a prodeltaic turbidite apron and distal delta top distributary mouth bar complex. Sections in cored drillholes, equivalent to the exposed strata here, are composed of parasequences that coarsen and thicken upwards over meters to a few tens of meters by the addition of thicker sandstone beds. These represent prograding turbidite lobes that are rapidly abandoned as the subaqueous feeder channel switches position. A period of sediment starvation follows with mud accumulation. This is ended by the gradational outbuilding of another lobe as the feeder channel moves closer to this area again. The thicker shales overlying the sand-rich tops of the parasequences are carbonaceous and pyritic, compared to the thinner shales forming the tops of individual turbidites. This denotes that the clays forming the thicker shales separating lobe outbuilding periods were deposited slowly allowing the remains of microbes to become common in the sediment. During diagenesis the microbes degraded, forming strongly reducing conditions that led to formation of diagenetic pyrite in the sediment. In contrast, the more rapid deposition of clay between turbidity current events during lobe outbuilding produced lower concentrations of carbon and much less pyrite. The difference between the two types of shale can easily be seen due to staining of the pyrite-rich rock during oxidation. Complete Bouma Sequences are rarely exhibited by the turbidites as the rippled C-division is commonly not present. However, flute marks produced by scouring of the viscous turbidity current into the muddy bottom and subsequent filling with sand, are ubiquitous (Figure 3-12). Commonly, in other turbiditic formations, flute marks are randomly developed on the bedding surface, but here they are in places arranged geometrically in diagonally offset rows. They also take on unusual elongate shapes, more akin to groove marks produced by something dragging across the bottom. The cause of the abnormal patterns and shapes is not known. Figure 3-10: View, looking south, of the eastern end of the dormant quarry in Rove Formation sedimentary rocks on the Great Lakes Nickel road, Stop 3-3. A diabase feeder dyke (left-center db) branches out into two sills: a main one, near the top of the quarry face (top-center db) and midway up the face, where it fingers and terminates (white box corresponds to Figure 3-11). The white dashed line separates oxidized, pyritic shale below and relatively pyrite-free shale above. ILSG07 73 Trip 3 �Figure 3-11: Diabase (db) “finger and blob” in Rove Formation wacke, Stop 33. The diabase was injected from a feeder dyke to the left of the photograph. A propagation fracture, with attendant folding and shearing, is denoted by the dashed line. Figure 3-12: Casts of flute marks on the bottom of a sandstone layer, Stop 3-3. Some have the classic “V” shape with their deepest area at the apex of the V. The V opens in the down-current direction. Others are drawn out and appear to have migrated down-current as they cut, producing a groove rather than a V. ILSG07 74 Trip 3 �STOP 3-4: Crystal Lake Gabbro / Great Lakes Nickel Property Location: 7.5 km west of Highway 61 on the unmarked Great Lakes Nickel Road (private access). The GLNR extends west from Highway 61 at a point just south of the Pine River bridge, 6.6 km north of the junction of Highways 61 and 593 (UTM 303612E, 5328392N; NAD83). Description: A switch-back road extends from the base of a 140 m high ridge to the lower adit, excavated at the contact between Rove Formation country rocks and the base of the Crystal Lake gabbro. An overgrown trail continues upward to the upper adit in “taxitic”, varied-texture gabbro and talus boulders. The upper portions of the intrusion, including layered rocks, are visible from this vantage point but are inaccessible for safety reasons. A stockpile of mineralized gabbro is available for sampling on the road at the base of the ridge. A second dump, further up the road, consists mainly of Rove Formation shales and argillites with abundant zeolite and carbonate veins. The discovery of copper- and nickel-mineralized float boulders in this area early in this century led to a concerted exploration effort to find their source. United States Smelting, Refining and Mining Company conducted exploration in 1936, and were followed by Frobisher Exploration Company in 1942. In the summer of 1952, J.S. Brodie and T.W. Page examined a large outcrop of gabbroic rocks, 6 km northeast of the original float discovery. Prospecting soon indicated a Cu-NiPt-mineralized zone and the ground was staked for Mattawin Gold Mines. Due to lack of operating capital, the Mattawin property was optioned to Falconbridge Nickel Mines in late 1952. A trench was subsequently excavated to test the lower gabbro contact. Although trench samples returned anomalous copper and nickel values, they did not warrant further investigation and the option was allowed to lapse. Additional work by J.S. Brodie drew attention to the area north of the trench. The property was optioned to R. Barker and W. Dawidowich in 1954. Six diamond drill holes, totalling 1058 m, were completed. One 55-foot section returned 0.54% Cu and 0.18% Ni with some precious metal values. Mogul Mining Corporation Limited held the property from 1954 to 1957 and drilled seven holes, totalling 1693 m. Intersections of the mineralized zone averaged 9 to 12m and assayed 0.9% combined Cu and Ni. Late in 1964, Great Lakes Nickel Corporation Limited acquired the property and initiated their exploration program in June, 1965. From 1965 to 1970, 47 803 m of surface drilling was completed; 19 underground holes, totalling 392 m, were also drilled. Underground drilling was conducted from a newly constructed, 37 m long adit, driven into the base of the hillside. In addition, Thunder Bay Nickel Mining Corporation Limited drilled 16 holes, totalling 13 579 m, on the down-plunge extension of the deposit, 2.5 km east of the main workings. In 1972, access and development work was undertaken by Great Lakes Nickel to further test the deposit. This work included the excavation and driving of a 522 m development portal and drift and over 12 000 m of surface and underground drilling. Plant site surveys, bulk sampling, metallurgical and feasibility tests were also conducted, financed largely by the Swedish company, Boliden Aktiebolag. By 1974, plans were made to mine the deposit at an initial rate of 1.8 million tons per year (subsequently increased to 2.5 million tons per year). Up to that point, about $10 M (Canadian) had been spent on the property on 58 689 m of surface drilling, 26 182 m of underground drilling and the driving of the adit, which eventually reached a length of 1041 m. This work had outlined a deposit of 32.8 million tons grading 0.36% Cu and 0.20% Ni, with a further potential reserve of 40 million tons of about the same grade. However, cost escalations, high interest rates and uncertain metal prices forced suspension of mine development in October, 1974. Rising interest in platinum-group elements in the mid-1980’s prompted Fleck Resources Ltd. to re-evaluate the deposit whose reserves then stood at 45.6 million tons at a grade of 0.334% Cu and 0.183% Ni. Between September, 1986 and February, 1987, Fleck completed geological mapping and sampling, as well as the re-logging and assaying of more than 9144 m of drill core. Six holes were drilled to test the deposit for its PGE potential. Sampling by Fleck returned the following assays on a 3.7 million ton portion of the deposit: 0.006 oz./ton Pt, ILSG07 75 Trip 3 �0.57% Cu, 0.027 oz./ton Pd, 0.264% Ni, 0.003oz./ton Au, 0.016% Co, 0.04 oz./ton Ag. Based on February, 1987 metal prices, this material was valued at $34.29 (Can.) per ton (Fleck Resources Ltd., Annual Report, 1987). Great Lakes Nickel has undertaken a comprehensive, due-diligence review of the property (Hubacheck 2001) in order to generate a valuation opinion and summary for potential investors or joint-venture partners. The geology of the Crystal Lake gabbro has been described by Geul (1970), Reeve (1969), MacRae and Reeve (1968), Whittaker (1986) and Smith and Sutcliffe (1987, 1989). It intrudes Rove Formation argillaceous and arenaceous rocks. The intrusion was subdivided into three major zones by Reeve (1969) and Geul (1970) and has been further subdivided by Smith and Sutcliffe (1987, 1989) and Cogulu (1990) into four major, roughly equivalent, lithologic zones: (1) an upper zone (60 to 80 m) of sulphide-barren, troctolite, olivine gabbro and anorthositic gabbro; (2) a middle zone (30 to 42 m) of cyclic, layered anorthositic and olivine gabbro, Cr-spinel- bearing anorthosite and olivine gabbro (Figure 3-4); (3) a lower, unlayered zone (50 m) of vari-textured gabbro and leucotroctolite, which hosts the bulk of the Cu-Ni-sulphide deposit; and (4) a basal zone (1 to 7 m) of fine-grained, chilled melagabbro and hornfelsed country rock xenoliths. Orthocumulate rocks predominate in the Crystal Lake intrusion; adcumulates occur only in the cyclic, layered rocks, in association with Cr-spinel mineralization (Cogulu 1990). Plagioclase, with lesser olivine and Cr-spinel (Figure 3-3), is the main cumulus mineral, while clinopyroxene, magnetite and Cu-Ni-sulphides are intercumulus (Cogulu 1990; Smith and Sutcliffe 1987, 1989). Total REE, LREE/HREE ratios, and sulphide content decrease from the basal zone to the cyclic sequence, while the modal proportion of olivine increases (ibid). Smith and Sutcliffe (1987, 1989) have suggested the following crystallization sequence: Cr-spinel → olivine → plagioclase → clinopyroxene → magnetite → apatite. Biotite is epitaxial to olivine and augite, and it may also rim intercumulus sulfides and oxides (Cogulu 1990). Sulphides are disseminated, interstitial and included grains and droplets. Pyrrhotite, chalcopyrite, cubanite and pentlandite are the main sulphides. Accessory minerals include violarite, troilite, niccolite, maucherite, native bismuth, mackinawite, bornite, millerite, nickeloan pyrite, sphalerite and marcasite (Cogulu 1993a). Adcumulate and orthocumulate Cr-spinels have been subdivided by Cogulu (1993b) into compositionally and texturally distinct groups with complex re-equilibration histories. Heterogeneity between Cr-spinels was also noted by Whittaker (1986). Local sulphur sources exist in local sedimentary rocks, as well as in the Archean basement. Data from the Duluth Complex indicated similar sulphur-isotopic signatures exist between basement sedimentary rocks and Ni-Cu-sulphides (Mainwaring and Naldrett 1974). Similar S-isotopic signatures also exist between Crystal Lake gabbro and Rove Formation shales (J. Franklin, personal communication, 1999.) Franklin (1970; personal communication, 2001) has also noted contact effects in Rove shales below diabase sills which include the conversion of pyrite to pyrrhotite, the loss of carbon in the spotted contact aureole and the rearrangement of clay mineral structures. Eckstrand and Cogulu (1986) presented Se/S data which supported the contention that sulphur in the Ni-Cu-sulphide minerals at Crystal Lake was derived from the barren, Se-poor, sedimentary sulphides of the Rove Formation. Sulphide concentrates from the Rove Formation and the lower parts of the intrusion (which host the sulphide deposit) yielded lower Se/S ratios than those gleaned from the Crspinel-bearing, cyclic layered zone whose Se/S ratios correspond closely to chondritic values (~200 to 600 x 10-6), implying that the sulphur was likely mantle-derived. ILSG07 76 Trip 3 �Stratigraphic Unit Cyclic Layered Zone Lower Unlayered Zone Basal Contact Zone Rove Formation (Footwall) (Se/S) x 10-6 213 - 687 70 - 151 28 - 114 2 – 118 (Eckstrand and Cogulu 1986) A grab sample of non-magnetic, massive subhedral pyrite with minor, interstitial, fine-grained silicates taken from the lower adit area, near the basal contact of the Crystal Lake Gabbro, returned 116 ppb Au, 688 ppb Pd, 14 ppb Pt, 13494 ppm Cu, 4756 ppm Ni and 255 ppm Zn (Resident Geologist's Files, Thunder Bay). ACKNOWLEDGEMENTS The trip leaders wish to thank the following individuals and companies for their assistance in the preparation of this field trip guide book and the field trip. Phil Fralick (Lakehead University) contributed sections on the Animikie Group and wrote the Rove Formation description for Stop 3-3. John Scott (Resident Geologist’s Program, Ontario Geological Survey) provided logistical support. The authors have benefited from discussions with Tom Hart and Mike Easton (Precambrian Geoscience Section, OGS), Larry Heaman (University of Alberta) and Jim Miller (Minnesota Geological Survey / University of Minnesota Duluth). The ongoing cooperation of Great Lakes Nickel Limited and other exploration companies is also appreciated. REFERENCES Addison, W.D., Brumpton, G.R., Vallini, D.A., McNaughton, N.J., Davis, D.W., Kissin, S.A., Fralick, P.W. and Hammond, A.L. 2005. Discovery of distal ejecta from the 1850 Ma Sudbury impact event; Geology, v.33, p.193-196. Amurawaiye, O. 2001. The Paleoproterozoic Rove Formation of northwestern Ontario: A turbidite-dominated shelf sequence; unpublished H.B.Sc thesis, Lakehead University, Thunder Bay, Ontario, 44p. Antonellini, M.A. and Cambray, F.W. 1992. Relations between sill intrusions and bedding-parallel extensional shear zones in the Mid-continent Rift of the Lake Superior region: Tectonophysics, v. 212, p. 331-349. Beskar, S. 2001. The Blake gabbro: A taxitic-textured gabbro sill south of Thunder Bay, Ontario; 47th Institute on Lake Superior Geology, Annual Meeting, Madison, Wisconsin, May 9-12, 2001, Proceedings Volume 47, Part 1, p.1 Cogulu, E.H. 1990. Mineralogical and petrological studies of the Crystal Lake intrusion, Thunder Bay, Ontario: Geological Survey of Canada, Open File 2277, 15 p. plus figures and tables. -----. 1993a. Mineralogy and chemical variations of sulphides from the Crystal Lake intrusion, Thunder Bay, Ontario; Geological Survey of Canada, Open File 2749, 21 p. plus figures and tables. -----. 1993b. Factors controlling postcumulus compositional changes of chrome-spinels in the Crystal Lake intrusion, Thunder Bay, Ontario; Geological Survey of Canada, Open File 2748, 28 p. plus figures and tables. Davis, D.W. and Green, J.C. 1997. Geochronology of the North American Midcontinent rift in western Lake Superior and implications for its geodynamic evolution; Canadian Journal of Earth Sciences, v.34, p.476-488. Davis, D.W. and Sutcliffe, R.H. 1985. U-Pb ages from the Nipigon Plate and northern Lake Superior; Bulletin of the Geological Society of America, v. 96, p. 1572-1579. ILSG07 77 Trip 3 �Eckstrand, O.R. and Cogulu, E. 1986. Se/S evidence relating to genesis of sulphides in the Crystal Lake gabbro, Thunder Bay Ontario; Geological Association of Canada-Mineralogical Association of Canada-Canadian Geophysical Union, Joint Annual Meeting, Ottawa, Ontario, Program with Abstracts, p.66. Fralick, P.W. and Miall, A.D., 1989. Sedimentology of the Lower Huronian Supergroup (Early Proterozoic), Elliot Lake, Ontario, Canada. Sedimentary Geology, v. 63, p. 127-153. Franklin, J.M. 1970. Metallogeny of the Proterozoic rocks of the Thunder Bay District, Ontario; unpublished Ph.D. thesis, University of Western Ontario, London, 317p. Franklin, J.M. and Kustra, C.R. 1972. The Proterozoic rocks of the Lake Superior area, northwestern Ontario; in Field Excursion C34: The Precambrian rocks of the Atikokan-Thunder Bay-Marathon area, 24th International Geological Congress, Guidebook, p.20-46. Franklin, J.M., Kissin, S.A., Smyk, M.C., and Scott, S.D. 1986. Silver deposits associated with the Proterozoic rocks of the Thunder Bay District, Ontario; Canadian Journal of Earth Sciences, v.23, p.1576-1591. Geul, J.J.C. 1970. Geology of Devon and Pardee Townships and the Stuart Location; Ontario Department of Mines, Geological Report 87, 52 p. -----. 1973. Geology of Crooks Township, Jarvis and Prince Locations, and Offshore Islands, District of Thunder Bay; Ontario Department of Mines, Geological Report 102, 46 p. Green, J.C. 1986. Lithogeochemistry of Keweenawan igneous rocks; Minnesota Department of Natural Resources, Division of Minerals, St. Paul, Project 241-4, 94p. Green, J.C., Bornhorst, T.J., Chandler, V.W. et al. 1987. Keweenawan dikes of the Lake Superior region: evidence for evolution of the middle Proterozoic Midcontinent Rift of North America; in Mafic dike swarms, H.C. halls and W.F. Fahrig, eds., Geological Association of Canada, Special Paper 34, p.289-302. Hart, T.R. 2002. Proterozoic volcanic and intrusive whole rock geochemical data associated with the Keweenawan Midcontinent Rift, Lake Superior area, Ontario; Ontario Geological Survey, Miscellaneous Release of Data, MRD 114. -----. 2003. Keweenawan mafic and ultramafic intrusive rocks of the Lake Nipigon and Crystal Lake areas, northwestern Ontario; 49th Institute on Lake Superior Geology, Proceedings volume 49, Part 1, Programs and abstracts, p.21-22. Hart, T.R., MacDonald, C.A., Hollings, P., and Richardson, A., 2005. Proterozoic intrusive rocks of the Nipigon Embayment and Midcontinent Rift. In, T.O. Tormanen and T.T Alapieti, 10th International platinum Symposium Extended Abstracts, Geology Survey of Finland, 365-368. Heaman, L.M. and Easton, R.M. 2006. Preliminary U/Pb geochronology results: Lake Nipigon Region Geoscience Initiative. Ontario Geological Survey, Miscellaneous Release-Data 191, 79p. -----. 2007. Further refinement to the timing of Mesoproterozoic magmatism, Lake Nipigon Region, Ontario. Canadian Journal of Earth Sciences, in press. Heaman, L.M. and Machado, N. 1987. Isotope geochemistry of the Coldwell alkaline complex: 1. U-Pb studies on accessory minerals; Geological Association of Canada-Mineralogical Association of Canada, Joint Annual Meeting, Saskatoon, Saskatchewan, Program with abstracts, p.54. Hemming, S.R., McLennan, S.M. and Hanson, G.N., 1995. Geochemical and Nd/Pb isotopic evidence for the provenance of the Early Proterozoic Virginia Formation, Minnesota. Implications for tectonic setting of the Animikie Basin. Journal of Geology, v. 103, p. 147-168. Heslop, J.B. 1968. Mineralogy and textural relationships of the Mount Mollie sulphides, Pine Bay area, Thunder Bay District, Ontario. Unpublished B.Sc. thesis, University of Western Ontario, London, Ontario. Hoffman, P.F., 1987. Early Proterozoic foredeeps, foredeep magmatism and Superior-type iron-formations of the Canadian shield. In, ed. Kroner, A., Proterozoic Lithospheric Evolution, American Geophysical Union Series, v. 17, p. 85-98. Hollings, P., Hart, T., Richardson, A., and MacDonald, C.A. 2007. Geochemistry of the Midproterozoic intrusive rocks of the Nipigon Embayment. Canadian Journal of Earth Sciences, in press. ILSG07 78 Trip 3 �Hubacheck, P. 2001. Great Lakes Nickel copper-nickel-PGE project: Executive summary; unpublished report, Resident Geologist's Files, Thunder Bay, 7p. Jones, N.W. 1984. Petrology of some Logan diabase sills, Cook County, Minnesota; Minnesota Geological Survey, Report of Investigations 29, 40p. Lightfoot, P.C. and Lavigne, Jr., M.J. 1995. Nickel, copper, and platinum group element mineralization in Keweenawan intrusive rocks: new targets in the Keweenawan of the Thunder Bay region, northwestern Ontario; Ontario Geological Survey, Open File Report 5928, 32p. Lucente, M.E. and Morey, G.B., 1983. Stratigraphy and sedimentology of the lower Proterozoic Verginia Formation, northern Minnesota. Minnesota Geological Survey Report of Investigations 28, 28 p. MacRae, N.D. and Reeve, E.J. 1968. Differentiation sequence of the Great Lakes Nickel intrusion: 14th annual Institute on Lake Superior Geology, Superior, Wisconsin, Program with Abstracts, p.28. Mainwaring, P.R. and Naldrett, A.J. 1974. Genesis of Cu-Ni sulfides in the Duluth Complex; in Society of Economic Geologists, Annual Meeting, Economic Geology, v.69, no.7, p.1183-1184. Maric, M. and Fralick, P.W., 2005. Sedimentology of the Rove and Virginia Formations and their tectonic significance. Institute on Lake Superior Geology, v. 51, p. 41-42. McCuaig, J.A. 1950. A copper-nickel occurrence in Pardee Township, Thunder Bay District; unpublished M.Sc. thesis, McGill University, Montreal. Quebec, Miller, J.D. and Smyk, M.C. 1995. Gabbroic intrusions of the International Boundary area; in Field trip guidebook for the geology and ore deposits of the Midcontinent Rift in the Lake Superior region; International Geological Correlation Program Project 336, Minnesota Geological Survey, Guidebook 20, p.171-181. Miller, J.D., Smyk, M.C., Severson, M.J., Lavigne, M.J. and Middleton, R.S. 2002. PGE occurrences in mafic intrusions around western Lake Superior, USA and Canada; 9th International Platinum Symposium, Field Trip Guidebook, 135p. Morey, G.B. 1973. Stratigraphic framework of middle Proterozoic rocks in Minnesota. In, ed. G.M. Young, Huronian Stratigraphy and Sedimentation. Geological Association of Canada Special Paper 12, p. 211-249. North, J. 2000. Nature and distribution of Logan diabase sills and gabbro channels in the Keweenawan rift near Thunder Bay, Ontario: Brief comparison to Noril'sk; Abstract, 46th Institute on Lake Superior Geology, Annual Meeting, Thunder Bay, Ontario, Proceedings Volume 46, Part 1 (2000). Oja, R. 1967. Geochemical investigations of the Thunder Bay silver area; in Proceedings, Symposium on Geochemical Prospecting, Ottawa, 1966, Geological Survey of Canada, Paper 66-54, p.211-221. Ojakangas, R.W., Morey, G.B. and Southwick, D.L., 2001. Palaeoproterozoic basin development and sedimentation in the Lake Superior region, North America; Sedimentary Geology, v. 141, p. 319-341. Parker, D.P. (ed.), Middleton, B., Schnieders, B.R., Smyk, M.C. and Scott, J.F. 2001. Intrusions of the Nipigon Basin; Superior PGE 2001, Canadian Institute of Mining and Metallurgy, Geological Society Field Conference, Thunder Bay, September 16-19, 2001, Field Trip Guidebook, 43p. Pollard, D.D., Muller, O.H. and Dockstader, D.R. 1975. The form and growth of fingered sheet intrusions; Geological Society of America Bulletin, v.86, p.351-363. Pufahl, P.K. and Fralick, P.W., 1995. Paleogeographic reconstruction of the Gunflint-Mesabi-Cuyuna depositional system: a basin analysis approach; 41st Institute on Lake Superior Geology, v. 41, Proceedings with abstracts, p.59-60. Pufahl, P. and Fralick, P. 2000. Depositional environments of the Paleoproterozoic Gunflint Formation; 46th Institute on Lake Superior Geology, v.46, pt.2, Proceedings with abstracts. Pufahl, P.K. and Fralick, P.W., 2004. Depositional controls on Paleoproterozoic iron formation accumulation, Gogebic Range, Lake Superior region, USA.. Sedimentology, v. 51, p. 791-808. Pufahl, P.K., Fralick, P.W. and Scott, J., 2000. Depositional environments of the Palaeoproterozoic Gunflint Formation; 46th Institute on Lake Superior Geology, v.46, pt.2, Field Trip Guide, 46 p. ILSG07 79 Trip 3 �Queen, M., Heaman, L.M., Hanes, J.A., Archibald, D.A. and Farrar, E. 1996. 40Ar/39Ar phlogopite and U-Pb perovskite dating of lamprophyre dykes from the eastern Lake Superior region: Evidence for a 1.14 Ga magmatic precursor to Midcontinent Rift volcanism; Canadian Journal of Earth Sciences, v.33, p.958-965. Reeve, E.J. 1969. Petrology and mineralogy of a gabbroic intrusion in Pardee Township near Port Arthur, Ontario: unpublished M.Sc. thesis, University of Wisconsin, Milwaukee, 79 p. Rosatelli, M.P., 2002. Assessment report on the 2002 lithogeochemical rock sampling program, Pigeon River block. McVicar Minerals Ltd., BHP Billiton World Exploration Inc.. and Falconbridge Limited; Assessment Files, Thunder Bay South District, Thunder Bay, FN 2.24485, 36p. Smith, A.R. and Sutcliffe, R.H. 1987. Keweenawan intrusive rocks of the Thunder Bay area: in Summary of Field Work 1987, Ontario Geological Survey, Miscellaneous Paper 137, p. 248-255. -----. 1989. Precambrian geology of Keweenawan intrusive rocks in the Crystal Lake-Pigeon River area: Ontario Geological Survey, Map P.3139, scale 1:50 000. Smyk, M.C., Hollings P. and Heaman, L.M. 2006. Preliminary investigations of the petrology, geochemistry and geochronology of the St. Ignace Island Complex, Midcontinent Rift, northern Lake Superior, Ontario; Institute on Lake Superior Geology, 52nd Annual Meeting, Sault Ste. Marie, ON, Program with Abstracts, v. 52, p.6162. Southwick, D.L. and Morey, G.B., 1991. Tectonic imbrication and foredeep development in the Penokean Orogen, east-central Minnesota - an interpretation based on regional geophysics and the results of test-drilling. United States Geological Survey Bulletin 1904-C, 17 p. Stockwell, C.H., McGlynn, J.C., Emslie, R F., Sanford, B.V., Norris, A.W., Donaldson, J.A., Fahrig, W.F. and Currie K L. 1972. Geology of the Canadian Shield, in Geology and Economic Minerals of Canada, edited by R.J.W. Douglas, Geological Survey of Canada, Economic Geology Report 1, 838 p. Sun, S.S. and McDonough, W.F. 1989. Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes; in Saunders, A.D. and Norry, M.J. (Eds.), Magmatism in ocean basins, Geological Society of London Special Publication 42, p.313-345. Sutcliffe, R.H. 1989. Mineral variation in Proterozoic diabase sills and dykes at Lake Nipigon, Ontario; Canadian Mineralogist, v.27, p.67-79. Tanton, T.L. 1935. Copper-nickel mineral occurrences in Pigeon River area, Ontario; Geological Survey of Canada, Paper 35-1, 11p. -----. 1936a. Pigeon River area, Thunder Bay District; Geological Survey of Canada, Sheet 1, Map 354A, scale 1:63 360. -----. 1936b. Pigeon River area, Thunder Bay District; Geological Survey of Canada, Sheet 2, Map 355A, scale 1:63 360. -----. 1937. Copper-nickel occurrences in Pigeon River area, Ontario. The Precambrian, May, 1937, p.18-19. Van Wyck, N. and Johnson, C.M. 1997. Common lead, Sm-Nd, and U-Pb constraints on petrogenesis, crustal architecture and tectonic setting of the Penokean Orogen (Paleoproterozoic) in Wisconsin, U.S.A. Geological Society of America Bulletin, v. 109, p. 799-808. Weiblen, P.W., Mathez, E.A. and Morey, G.B. 1972. Logan intrusions; in Sims, P.K. and Morey, G.B. eds., Geology of Minnesota: A centennial volume; Minnesota Geological Survey, p.394-410. Whittaker, P.J. 1986. Chromite deposits in Ontario: Ontario Geological Survey, Study 55, 97 p. ILSG07 80 Trip 3 �53rd Annual Institute on Lake Superior Geology FIELD TRIP 4 GEOLOGY OF THE NICKEL LAKE MACRODIKE AND ITS ASSOCIATION WITH CU-NI-PGE MINERALIZATION IN THE NORTHERN SOUTH KAWISHIWI INTRUSION, DULUTH COMPLEX, NORTHEASTERN MINNESOTA Dean M. Peterson University of Minnesota Duluth, Natural Resources Research Institute Paul B. Albers Duluth Metals Limited, Ely, Minnesota “Imagination is more important than knowledge” Albert Einstein INTRODUCTION Mineral deposits are concentrations of specific ore-minerals that society utilizes in immeasurable ways. They have formed from Archean times up to the present, and vary greatly in commodity, mineralogy, alteration, trace element signature, geophysical properties, and grade-tonnage. Individual ore deposits are always unique, and this uniqueness arises from two main sources: 1) fundamental differences in geologic processes and environments; and 2) local, site-specific, geologic variations and associated bounding geometries. The seemingly limitless number of permutations of these (and other) features of mineral deposits defies imagination. Geologists employed in the search for ore deposits have, over the last century, developed the intellectual concept of Ore Deposit Models, which seek to organize many of ILSG07 81 Trip 4 �these variables, lump individual mineral deposits into classes, and establish criteria to aid in mineral exploration (Peterson, 2001a). Such models may be strictly empirical – a collection of observable facts associated with the occurrence of certain metals in economic proportions – or genetic – which attempts to describe the physical and chemical processes responsible for the development of an ore deposit and its related empirical features. Advances in our collective understanding of ore deposit types is nonlinear, rather it occurs in fits and starts. Take, for example, the great advance in knowledge – and thus mineral exploration models and exploration success – for porphyry copper deposits in the 1960s and 1970s (Lowell and Guilbert, 1970), volcanogenic massive sulfide deposits in the 1970s and 1980s (Franklin et al., 1981), and mesothermal lode gold deposits in the 1980s and 1990s (Hodgson, 1993). These advances were the product of many factors, which conceivably the most important was collaborative ore deposit research between the mineral industry, academia, government agencies, and research organizations. We are currently in the midst of a similar great advance in knowledge of magmatic Ni-Cu-PGE deposits (Arndt et al., 2006; Barnes and Lightfoot, 2006; Eckstrand and Hulbert, in prep.). Perhaps the most important impetus for this current revolution was the fall of the Iron Curtain, which opened the door in the early 1990s to some of the world’s greatest magmatic Ni-Cu-PGE deposits, i.e., the Noril’sk-Talnakh deposits in Russia and Jinchuan deposit in China, to economic geologists trained in open western societies (especially Canada). In addition, the 1993 discovery of the magmatic Ni-Cu deposit at Voisey’s Bay, Labrador, Canada, and its inferred origin as a magmatic feeder dike, has brought about a revolution in our collective understanding of the origin of magmatic Ni-Cu±PGE deposits. This revolution can simple be stated as “find the magmatic feeder dike and/or channelized magma flow zones” to the orebearing mafic/ultramafic intrusion(s). A magmatic conduit that experienced repeated influxes of magma appears to be the key to the formation of high-grade, world-class, Ni-Cu-PGE deposits (Naldrett, 1997). This field trip is truly the result of over two decades of dedicated Duluth Complex research, almost entirely funded by the State of Minnesota and the University of Minnesota, by geologists of the University of Minnesota Duluth’s (UMD), Natural Resources Research Institute (NRRI). Steve Hauck, director of the NRRI’s Economic Geology Group (EGG), is here thanked for his decades of dedicated research and seeing through that the EGG remains financially solvent through a never-ending series of budgetary crises. As well, the NRRI’s Mark Severson – who has logged well over 900,000 feet of Duluth Complex drill core – has without a doubt the more knowledge than anyone on the geology of the mineralized zones within the deposits of the Duluth Complex. The authors of this guidebook have simply built upon this intellectual capital in new ways, including geochemical and 3-D modeling of drill core data, and especially the most basic geologic endeavor, detailed field mapping. FIELD TRIP TENETS Many basic attributes of the Duluth Complex Cu-Ni-PGE sulfide deposits resemble those of deposits at Noril’sk, Russia, Jinchuan, China, and Voisey’s Bay, Canada that are associated with sulfide mineralization in intrusive feeder zones. Such attributes include shallow tholeiitic intrusions associated with plateau basalt volcanism, external sedimentary sources of sulfur, and openness to repeated magma influx and expulsion. However, the biggest difference between these world-class magmatic ore systems and the deposits of the Duluth Complex is the lack of significant nickel-rich (Ni>Cu) massive sulfide orebodies at Duluth. A critical attribute of the high-grade Noril’sk–Talnakh and Voisey’s Bay deposits, not previously positively identified in the Duluth Complex, is the location of a magmatic conduit, i.e., the feeder zone. Several geologists have previously identified possible conduits that may have fed the Partridge River (PRI) and South Kawishiwi (SKI) intrusions of the Duluth Complex. Severson and Zanko (unpub. data) suggest that the Grano fault might mark a possible feeder zone for the Local Boy ore zone at the northeast ILSG07 82 Trip 4 �end of the PRI. Thériault et al. (2000) postulated that a PRI conduit was present somewhere between the Wetlegs and Dunka Road deposits. Another possible PRI feeder zone may have been along the prolongation of the Siphon fault, which is a Paleoproterozoic growth fault (Graber, 1993) that may have been reactivated during emplacement of the Duluth Complex (Severson and Hauck, 1997). Several authors have suggested the presence of a sub-vertical magmatic feeder beneath the Bald Eagle intrusion (BEI) based on field relations (Weiblen and Morey, 1980) and geophysical attributes (Chandler, 1990). Peterson (2001b) interpreted the systematic variation in Cu-Ni-PGE mineralization in the Maturi deposit and its extension east to Maturi Extension, as indicative of magma input from the east-northeast via an arcing macrodike (herein first termed the Nickel Lake Macrodike (NLM)) that connects the deep-seated source of the BEI and the SKI. Subsequent mapping and research (Peterson, 2002a-f, 2006a, Peterson et al., 2004, Peterson and Hauck, 2005) has built on this model, which recently culminated with the publication of a new detailed bedrock geology map of the area (Peterson et al., 2006). If correct, this model predicts that a Voisey’s Bay-type Ni-Cu-PGE massive sulfide body may exist at depth in the area where the NLM meets the SKI, south-southeast of the Spruce Road deposit (see Figs. 4.2 and 4.3). This field trip will investigate evidence that the NLM is a feeder to the SKI, and thus is one of the principal conduits (only?) that brought Cu-Ni-PGE from the Earth’s mantle and/or lower crustal magmatic staging chambers into the Earth’s upper crust via the NLM into the SKI. This will be accomplished through scientific discussions (hopefully heated) on numerous outcrops (Day 1) as well as visualization of the geology in 3-D and displays of recently drilled cores by Duluth Metals Limited (Day 2). The fundamental tenet of this field trip is to convey to the participants the notion that science gives one the ability to imagine reality. Herein, science is geologic research of the Duluth Complex (geologic mapping, drill core logging, geochemical studies, and exploration drilling), and reality is new understanding how this magmatic system concentrated and enriched known and potential concentrations of Ni-Cu-PGE at the base of the NLM in the field trip locale as well as adjacent areas of the SKI. The field trip leaders ask the participants (and others who may use the guidebook in the future) to use their imaginations throughout the field trip (or subsequent field excursions) to think about a few basic known and possible realities of the Duluth Complex: 1) The Duluth Complex is perhaps the world’s largest untapped resource of Ni-Cu-PGE, with multibillion tons of geologic resources estimated to be worth >1 trillion dollars, (Peterson, 2006c); 2) The general geologic setting of the deposits in the Duluth Complex is similar to other world class Ni-Cu-PGE mining camps hosted by rocks in rift settings (Noril’sk-Talnakh, Jinchuan); 3) Overall, the ratio of Ni to Cu in Duluth Complex deposits average 1:3; 4) Worldwide, the ratio of Ni to Cu in similar deposits averages between 1:1 to 2:1; 5) By analogy, there seems to be an enormous mass of missing Ni-rich mineralization; 6) At depth, the Nickel Lake Macrodike may host a large percentage of this missing Ni in the SKI. MAGMATIC NI-CU-PGE ORE DEPOSIT MODEL The basic starting point to begin our quest to understand the significance (both geologic and economic) of the Nickel Lake Macrodike is a quick review of the magmatic Ni-Cu-PGE ore deposit model. There are literally hundreds of recent publications in the geological literature that deal with this important class of ore deposit, and readers interested in this topic may find that Barnes and Lightfoot (2006), Arndt et al. (2006), Eckstrand and Hulbert (in prep.), Naldrett, (1989, 1997, 1999), Naldrett et al. (2000), Li and Naldrett (1999), Lightfoot et al. (1994) and references therein are excellent reviews that describe magmatic Ni-Cu-PGE deposits in general, and major deposits in particular. ILSG07 83 Trip 4 �Magmatic Ni-Cu-PGE sulfide deposit occur as sulfide concentrations associated with a variety of mafic to ultramafic rocks in four major geological settings: 1) rifts and continental flood basalt settings (Noril’skTalnakh, Russia; Duluth Complex, Minnesota; Jinchuan, China); 2) meteorite impacts (Sudbury, Ontario, the only mining camp in this class); 3) komatiite lava flows and related intrusions (Thompson, Manitoba; Raglan, Québec; Kambalda, Australia; Pechenga, Russia); and 4) a variety of miscellaneous tholeiitic intrusions (Voisey’s Bay, Labrador; Lynn Lake, Manitoba). The ores are enriched in sulfur, iron, nickel, copper, cobalt, and the platinum group elements (Pt, Pd, Rh, Ru, Ir, and Os) and may contain minor Ag, As, Au, Bi, Hg, Pb, Sb, Se, Te, and Zn. Grade-tonnage diagrams for magmatic Ni-Cu-PGE sulfide deposits/camps are presented in Figure 4.1 which highlights the major camps/deposits listed above. Figure 4.1. Tonnage and Ni grades of magmatic Ni-Cu sulfide deposits; B. Tonnages and Cu grades of magmatic Ni-Cu sulfide deposits. Inclined contours show quantities of contained metals (tonnes) in each figure. Figure modified from Eckstrand and Hulbert, in prep. ILSG07 84 Trip 4 �The basic intrinsic geological features characteristic of a vast majority of magmatic Ni-Cu-PGE sulfide deposits include: (1) olivine-rich magmas; (2) proximity to a major crustal fault; (3) sulfide-bearing country rocks; (4) chalcophile element depletion in related intrusive or extrusive rocks; (5) field and/or geochemical evidence of interaction between the magma and the country rocks; and (6) presence of, or proximity to, a magma conduit (Naldrett, 1999). Fundamental geologic processes and constraints that together leads to the formation of these deposits include: 1) Deposits form as the result of segregation and concentration of droplets of liquid sulfide from mafic or ultramafic magmas; 2) Chalcophile elements from the silicate melt partition into the droplets as a result of turbulent magma flow; 3) An appropriate physical environment is required so that the sulfide liquid mixes with enough magma to become adequately enriched in chalcophile metals; 4) Sulfides must cluster in a restricted locality, generally due to the influence of gravity, so that the resulting metal concentration is of ore grade; 5) Massive sulfide concentrations form a high-temperature monosulfide solid solution (MSS); 6) As the MSS cools, it exsolves minerals and fractionates; a. Forms a solid cumulate mass of pyrrhotite-rich massive sulfide (enriched in Fe, Ni, Co, Ir, Ru, Rh). b. Forms a liquid residuum that is enriched in Cu, Pd, Pt, Au, and other minor elements, including As, Bi, Te, Sb (which will crystallize later into minerals as the system cools). 7) For some time, the Cu-Pd-Pt residual liquid can move and form high grade ore shoots/deposits a. i.e., footwall veins in Sudbury can be 30 wt. % Cu and multi-ounce/ton Pt + Pd. Possibly the greatest recent advance in understanding Ni-Cu-PGE sulfide deposits has been the appreciation of coherent and compelling scientific arguments that have shown that magma dynamics play a key role in the concentration and metal enrichment of sulfide minerals in these deposits (Naldrett, 1997). These arguments build on the long held notion that sulfide-rich orebodies achieve their concentrations mainly through the settling of sulfide droplets in magmas due to the effects of gravity. The new fundamental tenet has been the realization that decreases in the flow rate of magmas, principally due to the geometry and obstructions in magmatic conduits, is the major factor in settling entrained sulfide droplets and forming sulfide-rich orebodies (Eckstrand and Hulbert, in prep.). These geometries include, for example, widened parts of magmatic feeder dikes, the lowest zones of undulating basal zones of conduits, and the location where such dikes enter larger magma chambers. Geologists that are engaged in Ni-Cu-PGE exploration projects should use their imaginations to envision magma dynamics like a person fly-fishing along a stream, and search for deposits behind the large boulders and in the slow-moving pools below rapids where the big rainbow and brown trout are most likely to be found. REGIONAL GEOLOGIC SETTING, DULUTH COMPLEX The Duluth Complex and associated intrusions of Keweenawan age (~1.1 billion) in northeastern Minnesota constitute one of the largest mafic intrusive complexes in the world, second only to the Bushveld Complex of South Africa (Miller et al., 2002). These rocks cover a 5,700 square kilometer arcuate area associated with the two strongest gravity anomalies (+50 and +70 milligals) in North America, that imply intrusive roots more than 13 kilometers deep (Allen and others, 1997). The comagmatic flood basalts and intrusive rocks underlying most of northeastern Minnesota were emplaced during the development of the Mesoproterozoic Midcontinent rift, which can be traced geophysically from exposures in the Lake Superior region along a 2,000 kilometer-long, segmented, arcuate path to ILSG07 85 Trip 4 �Kansas and Lower Michigan. The Duluth Complex is defined as the more or less continuous mass of mafic to felsic plutonic rocks that extends for >275 kilometers in an arcuate fashion from Duluth nearly to Grand Portage (Fig. 4.2). It is bounded by a footwall of Paleoproterozoic sedimentary rocks and Archean granite-greenstone terranes (Peterson and Severson, 2002), and a hanging wall largely of comagmatic, rift related flood basalts and hypabyssal intrusions of the Beaver Bay Complex (Fig. 4.2). In genetic terms, the Duluth Complex is composed of multiple discrete intrusions of mafic to felsic tholeiitic magmas that were episodically emplaced into the base of a comagmatic volcanic edifice between 1108 and 1098 Ma. Figure 4.2. Generalized geologic map of northeastern Minnesota. Highlighted intrusions include the Bald Eagle (BEI) and South Kawishiwi (SKI) intrusions, as well as the linking Nickel Lake Macrodike (NLM) (modified from Miller et al., 2002). The geology of the Duluth Complex and adjacent areas has recently been described in two major publications by the Minnesota Geological Survey (MGS). These include a 1:200,000 scale regional bedrock geological map of northeastern Minnesota (Miller et al., 2001), and a comprehensive written description of the geology depicted on this map (Miller et al, 2002), commonly referred to as the “bible” by geologists working on Duluth Complex geology. Readers’ interested in more detailed descriptions of the geologic setting of the Duluth Complex should begin their quest for knowledge by downloading these publications from the MGS website (ftp://mgssun6.mngs.umn.edu/pub2/). Within the nearly continuous mass of intrusive igneous rock forming the Duluth Complex, four general rock series are distinguished on the basis of age, dominant lithology, internal structure, and structural position within the complex. ILSG07 86 Trip 4 �Felsic series—Massive granophyric granite and smaller amounts of intermediate rock that occur as a semicontinuous mass of intrusions strung along the eastern and central roof zone of the complex, emplaced during an early stage magmatism (~1108 Ma). Early gabbro series—Layered sequences of dominantly gabbroic cumulates that occur along the northeastern contact of the Duluth Complex, emplaced during early stage magmatism (~1108 Ma). Anorthositic series—A structurally complex suite of foliated, but rarely layered, plagioclase-rich gabbroic cumulates emplaced throughout the complex during main stage magmatism (~1099 Ma). Layered series—A suite of stratiform troctolitic intrusions that comprises at least 11 variably differentiated mafic layered intrusions that occur mostly along the base of the Duluth Complex. These intrusions were emplaced shortly after the Anorthositic series (~1099 Ma). This field trip will investigate rocks of the Layered Series – the SKI, NLM, and by implication the BEI – and Anorthositic series rocks in outcrops within and along the margins of the NLM. It is hoped that discussions on the outcrop (Day 1), coupled with examinations of selected sections of mineralized Duluth Metals Limited drill core from the SKI, and visualization of the geology in a 3-D presentation (Day 2) will bring new insight to the geology and mineral potential of the field trip area to the participants. Prior to these investigations, the quick descriptions of the local geology, basal contact-associated styles of CuNi-PGE mineralization, and calculated Cu-Ni grade-tonnage geologic resources within this area that follow will give the field trip participants a better appreciation of the significance of the NLM and its inferred potential for hosting great quantities of Ni-rich sulfide mineralization at depth. LOCAL GEOLOGIC SETTING Robust field and geophysical data suggest that the emplacements of the BEI and SKI may be closely linked (Weiblen and Morey, 1980). At the northern margin of the BEI, a macrodike of well-foliated troctolite (herein termed NLM) arcs northwest to southwest and merges with the middle of the northern SKI (Fig. 4.3). Green et al., (1966) mapped the macrodike as part of the SKI, but its composition is very similar to the troctolitic phase of the BEI. Peterson (2001b) proposed a model for the mineralization in the SKI whereby the initial emplacement of the intrusion was formed by sulfide-contaminated magmas that emerged from the NLM and flowed southwest between a footwall of Archean granite and a hanging wall of Anorthositic series rocks. Miller et al. (2002) present an important interpretive model for the emplacement of the BEI and SKI via a common feeder system, as well as depict the origin of sulfur saturation of basal SKI magmas (Fig. 4.4) due to contamination from Paleoproterozoic sedimentary rocks (Ripley, 1986). Peterson et al. (2006) completed detailed geological mapping along the western end of the NLM and adjacent SKI, and have defined distinct mappable units in both of these troctolitic bodies. BALD EAGLE INTRUSION The BEI is a large (4.5 to 16.5 km x 31 km) troctolitic to gabbroic body that was emplaced partially within Anorthositic series rocks, the SKI, and the Greenwood Lake Intrusion (Fig. 4.2). Weiblen (1965) mapped the well-exposed northern portion of the intrusion and showed that it consists of an outer zone of troctolite and an inner zone of olivine gabbro. In the poorly exposed southwestern portions of the intrusion, field mapping by Green et al., (1966) and Foose and Cooper (1978) showed the BEI and SKI in direct conformable contact. Steep foliation and modal layering (Weiblen, 1965; Green et al., 1966) integrated with a distinct gravity anomaly over the northern BEI imply that the northern part of this intrusion is funnel shaped and necks down to a steep feeder dike. Weiblen and Morey (1980) interpreted the limited cryptic variation (Weiblen, 1965), the steep dip of lamination and layering, and adcumulate nature of the BEI as indicative of its being an open conduit to higher intrusions and perhaps volcanic flows. ILSG07 87 Trip 4 �Figure 4.3. Simplified views of an integrated 3-D model of the BEI, NLM, and SKI of the Duluth Complex. A. plan view and B. view to the southwest. Model surfaces built from drill hole piercing points, detailed geological mapping, and interpretation of gravity and aeromagnetic data. ILSG07 88 Trip 4 �Petrologic observations and geophysical interpretations (Chandler, 1990; Chandler and Ferderer, 1989) suggest that the BEI and SKI were emplaced by successive overplating of magmas from a common feeder centered on the northern BEI and extending along the trace of the NLM that links the BEI and SKI. A model that depicts the origin of the BEI and SKI via a common dynamic feeder by Miller et al. (2002) is presented in Figure 4.4. In a related analogy, Cartwright and Møller-Hansen (2006) have shown that interconnected sill complexes transect the middle to upper crust over a vertical distance of 8-12 km offshore of Norway. The geometry of the gravity and magnetic anomalies of the BEI, as well as the overall Midcontinent Rift is very similar to the pattern of the seismic reflections profiles of active ridge systems (Vislova, 2003). In detail, the geophysical expressions of the BEI have the same shape and dimensions as the “bulls’ eye” pattern of low velocity seismic reflection anomalies along the East Pacific Rise. These anomalies are interpreted to define regions of melt concentrations, i.e., active magma chambers. These data suggest that the BEI could be a “frozen” dynamic magma chamber (Weiblen et al., 2005; Peterson and Hauck, 2005). The BEI has posed unresolved questions concerning its origin and magmatic significance since its discovery in 1961 (Weiblen, 1965, Weiblen and Morey, 1980, Miller et al., 2002). A number of its characteristics contrast markedly with those of the other mapped intrusions in the Midcontinent Rift: 1) it has a well-defined intrusive contact in Anorthositic series rocks around its northern perimeter; 2) there is a subtle, but recognizable metamorphic contact effect on these anorthositic gabbros; 3) a primary magmatic foliation is well defined by mineral orientation and discoid segregation of plagioclase from mafic phases; 4) foliation measurements define a steeply-dipping asymmetric funnel with the foliation paralleling the contact and grading from steep to horizontal inward; 5) the intrusion consists of two cumulus units, an outer troctolite and inner olivine gabbro; and 6) there is only minor (< a few %) intercumulus material in the cumulates, i.e., clinopyroxene and iron oxides (Weiblen et al., 2005). SOUTH KAWISHIWI INTRUSION The South Kawishiwi intrusion (SKI), together with the similar sized Partridge River intrusion (PRI) immediately to the south, are most renown for hosting the largest tonnage of Cu-Ni sulfide mineralization in the world (Naldrett, 1997). The realization that the SKI hosts vast quantities of Cu-Ni mineralization over 50 years ago has lead to the publication of numerous geologic maps, (Green et al., 1966; Bonnichsen, 1974; Foose and Cooper, 1974; Miller et al., 2001; Peterson, 2002e, f; Peterson et al., 2004; Peterson, 2006b; Peterson et al., 2006), articles (Bonnichsen et al., 1980; Weiblen and Morey, 1980; Ripley, 1986; Chandler and Ferderer, 1989; Lee and Ripley, 1996; Hauck et al., 1997; Peterson, 2001b) theses (Weiblen, 1965; Vislova, 2003; Marma, 2003), and reports (Phinney, 1969; Phinney, 1972; Listerude and Meineke, 1977, Morey and Cooper, 1977; Foose, 1984; Dahlberg, 1987; Dahlberg et al., 1989; Kuhns et al., 1990; Severson, 1994; Zanko et al., 1994; Hauck et al., 1997; Peterson, 1997; Peterson, 2001c; Miller et al., 2002; Peterson, 2002d; Patelke, 2003; Severson and Hauck, 2003). The SKI is shallow dipping (~20º to the east-southeast) sill-like intrusion dominantly composed of troctolitic cumulates that are exposed in an 8- x 32-kilometer arcuate band along the northwestern margin of the Duluth Complex (Fig. 4.2). Footwall rocks include the Paleoproterozoic Virginia Formation in the Serpentine and Dunka Pit deposits, the Paleoproterozoic Biwabik Iron Formation in the Dunka Pit and Birch Lake deposits, and the Archean Giants Range batholith from the northern Birch Lake deposit north to the Spruce Road deposit (see Fig. 4.3 for deposit locations). The presence of shallow-dipping Biwabik Iron Formation inclusions as far north as the Spruce Road deposit indicates that the majority of Paleoproterozoic units were assimilated and removed from the footwall during emplacement of the SKI, leaving the Giants Range batholith as the dominant footwall rock type. Alternately, the Virginia and Biwabik Iron Formations may simply have been largely eroded prior to the development of the Mid Continent Rift. Also present as inclusions in the SKI are mafic volcanic hornfels (North Shore Volcanic Group), quartz sandstone hornfels (either the Puckwunge or Nopeming sandstones), and anorthosite (of ILSG07 89 Trip 4 �the Anorthosite series). Anorthositic series rocks abut the SKI on the northeast – and enclose an interpreted SKI feeder dike (the NLM) that extends farther northeast – the PRI forms the southern sidewall of the SKI, and the BEI and Anorthositic series rocks overlie the SKI to the east (Fig. 4.2). Figure 4.4. Interpretive model which depicts the emplacement and mineralization mechanisms of the South Kawishiwi and Bald Eagle intrusions via a common feeder system. A) Intrusion of plagioclase crystal mushes into volcanic rocks to create Anorthositic series rocks; B) Intrusion of the SKI below Anorthositic series rocks, with the early basal units reaching sulfur saturation via contamination from Paleoproterozoic sedimentary rocks; and C) Intrusion of the BEI above Anorthositic series rocks. Modified from figure 6.15 of Miller et al. (2002). ILSG07 90 Trip 4 �On the regional Duluth Complex map of Miller et al. (2001), the SKI is subdivided into five major map units. These are, from the base upward, 1. Heterogeneous sulfide-bearing troctolite, gabbro, and norite with localized hornfels inclusions; 2. A thick unit of subophitic to ophitic augite troctolite; 3. Discontinuous and localized layers of poikilitic leucotroctolite; 4. A thick homogeneous sequence of ophitic troctolite; and 5. A thick uppermost sequence of homogeneous troctolite that contains numerous anorthositic layers. Severson (1994) and Zanko et al. (1994) further subdivided the SKI into 17 different lithostratigraphic units that are present in over 180 drill holes over a strike length of 31 kilometers. Sulfide mineralization is confined to the BH, BAN, UW, and U3 units near the base of the intrusion, and to a lesser extent the U1, U2, and PEG units. Major marker horizons that are correlated in drill holes include three horizons with abundant cyclic ultramafic layers (U1, U2, and U3 units) and a pegmatite-bearing unit (PEG unit) that was initially recognized by Foose (1984). The understanding of the significance of a large anorthositic inclusion (Fig. 4.3), originally intersected in six deep drill holes east of the Maturi deposit, and its role in magma dynamics of the SKI has been a key feature in the development of an exploration model for Duluth Metals Limited’s Maturi Extension deposit (Peterson, 2001c). Basal SKI Mineralization Basal mineralization within the South Kawishiwi intrusion has traditionally been divided into five distinct deposits: 1) Serpentine, 2) Dunka Pit, 3) Birch Lake, 4) Maturi, and 5) Spruce Road. Recent drilling by Duluth Metals Limited has confirmed the existence on an additional economically significant deposit east of Maturi first envisioned by Peterson (2001c), named the Maturi Extension deposit (Fig. 4.3). Although the style of mineralization in all of the deposits is dominated by disseminated Cu-Ni sulfides, differences occur between the deposits in igneous stratigraphy, sulfide mineralogy, Cu-Ni and PGE grade, mineralization thickness, and contained tonnes. In addition to mineralization spatially associated with the base of the intrusion, a distinct zone of Cu and PGE-enriched mineralization occurs thousands of feet above the base of the intrusion within linear zones in the South Filson Creek deposit (Kuhns et al., 1990). Compilation and analysis of drill hole assay data by Peterson (2001, 2002a-d) has led to new understanding of two distinct styles of mineralization associated with the base of the SKI. These distinctive styles of mineralization are spatially coherent, i.e., the boundaries between them are linear (Fig. 4.5), and Peterson (2001b, c) informally termed them open and confined, which are described in more detail below. "Open" - vertically extensive (can be > 450 meters) mineralization with low - high Cu-Ni grade and low Au+PGE grades. Cu-Ni grades commonly increase towards the basal contact although mineralized zones are typically erratic in their spatial extent and grade. Restricted zones of massive sulfide locally occur at, and/or immediately below, the basal contact. The erratic pattern of mineralization in part mirrors the lithologic heterogeneity of the basal units and may reflect repeated input of small pulses of barren and sulfur-contaminated magma. Examples of this "Open" style include the Spruce Road, Serpentine, and Dunka Pit deposits. The Serpentine deposit is unique within this group as it contains significant tonnage of pyrrhotite-rich massive sulfide at the basal contact that is associated with an immediate footwall sulfide source (Zanko et al., 1994). "Confined" - vertically restricted (< 150 meters) mineralization with moderate - high Cu-Ni grades and moderate to very high (locally) Au+PGE grades. Cu-Ni grades typically are the highest near the top of the mineralized zone (upper BH into U3) and gradually decrease with depth toward the basal contact. Only limited zones of massive sulfide occurring at, and/or immediately below, the basal contact have been identified. For example, the upper portion of the mineralized zone within the Maturi deposit (which averages ~150 feet thick) commonly exhibits copper values nearing 1.0% ILSG07 91 Trip 4 �that decrease to ~0.25% at the basal contact. The spatial continuity of both the igneous stratigraphy and Cu-Ni-PGE grades of this style of mineralization point toward larger sustained inputs of magma (and/or more turbulent input of magma, thus higher fractionation of base- and preciousmetals into the sulfide fraction) than the "Open" style. Examples of the "Confined" style include the Maturi, Maturi Extension, and the Birch Lake deposits. Figure 4.5. Location map of Open and Confined styles of mineralization in the South Kawishiwi Intrusion. Recently completed (Peterson, 2002a, 2006a, b, c) and ongoing research on the distribution of Cu-NiPGE in the SKI integrates detailed geological mapping, drill hole logging, assay compilation, gradetonnage calculation, recalculation of assay data to 100% sulfide, and 3-D visualization in order to imagine the coupled magmatic dynamics and mineralization history of the area. The most profound difference in the Open and Confined styles of Cu-Ni-(PGE) mineralization is perhaps best revealed in plots of assay data that have been recalculated to 100% sulfide compositions (Kerr, 2001, Naldrett et al., 2000). Such plots give one clues to the complicated history of immiscible sulfide droplets within SKI magmas, and could be used to calculate R-factors (herein the mass transfer of chalcophile elements between immiscible sulfide and silicate liquids in magmas) of the mineralization systems. As well, discrimination of different styles of mineralization and their internal metal budgets lets one come to grips with item 3 of the previously described Ni-Cu-PGE ore deposit model, that states, “An appropriate physical environment is required so that the sulfide liquid mixes with enough magma to become adequately enriched in chalcophile metals”. Geochemical plots of drill core assay data recalculated to 100% sulfide for the Open and Confined styles of mineralization are presented in Figure 4.6, and reveal profound differences in the metal budgets of the sulfide mineralization. Any genetic ore deposit model and/or mineral exploration model used to search for additional mineralization in the SKI must attempt to explain these differences in context with the overall NLM-SKI magmatic system, i.e. its timing, geometry, and magmatic plumbing. ILSG07 92 Trip 4 �Figure 4.6. Geochemical plots utilizing recalculation of assay data to 100% sulfide (Kerr, 2001) to discriminate variations of metals in sulfide minerals in the two major styles of basal contact associated mineralization in the SKI. A) Open-style PGE+Au vs. Cu plot, B) Open Style Cu/Pd ratio vs. Pd plot, C) Confined-style PGE+Au vs. Cu plot, D) Confined-style Cu/Pd ratio vs. Pd plot. Recent research in developing Cu and Ni grade maps (Peterson, 2002d) for deposits in the SKI, coupled with the realization that there are two distinct styles of basal-contact associated mineralization, has lead to the publication of inferred grade-tonnage estimates of Open and Confined styles of mineralization (Peterson, 2002c). Grade and tonnage data categorized into the Open and Confined styles of Cu-Ni mineralization within the South Kawishiwi intrusion are given in Table 4.1, which used all publicly available assay data in the year 2001. These mineral resource estimates have been calculated by the senior author (Peterson) following as much as possible the definitions and guidelines adopted by the Canadian Institute of Mining, Metallurgy, Petroleum (CIM "Standards on Mineral Resources and Reserves") in August 2000 (Postle et al.). Due to the fact that they rely on historic data from a variety of different sources, the level of data verification expected by the guidelines was not possible in all cases. Inherent uncertainties in the estimation and accuracy of these mineral resource estimates are a function of the quantity and quality of the available drill hole assay data and the quality of the methods used to determine them, which for the Spruce Road deposit are outlined in Peterson (2002d). It is hoped that the data presented so far in this field guide, once integrated together in one’s mind, leads to the conclusion that understanding the geologic history of the NLM may lead to profound advances in our understanding of Cu-Ni-PGE mineralization in the SKI. Such understanding may ultimately lead to the discovery of the “Missing Nickel” in the Duluth Complex Cu-Ni-PGE deposits, thus redefining the district as the Duluth Complex Ni-Cu-PGE deposits. ILSG07 93 Trip 4 �Table 4.1. Cumulative inferred mineral resource estimates for the Open and Confined styles of mineralization in the South Kawishiwi intrusion. Open style modeled to 1500 Ft. (457m.) and Confined style modeled to 500 Ft. (152m.) above base. Copper Cutoff 1.05 1.00 0.95 0.90 0.85 0.80 0.75 0.70 0.65 0.60 0.55 0.50 0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.01 Open Style Mineralization Cu % 1.043 1.029 0.969 0.933 0.924 0.844 0.818 0.758 0.685 0.644 0.597 0.547 0.502 0.461 0.421 0.374 0.329 0.286 0.242 0.201 0.156 0.107 Ni % 0.559 0.415 0.388 0.324 0.327 0.313 0.304 0.290 0.246 0.227 0.210 0.194 0.184 0.171 0.158 0.143 0.129 0.114 0.099 0.083 0.065 0.045 Tonnes 114,000 229,000 800,000 1,715,000 1,943,000 4,572,000 6,058,000 11,773,000 33,262,000 61,608,000 122,874,000 245,862,000 448,519,000 747,645,000 1,154,558,000 1,862,655,000 2,894,340,000 4,343,339,000 6,371,617,000 8,950,828,000 12,606,986,000 19,087,646,000 Confined Style Mineralization Cu % 1.035 1.010 0.989 0.962 0.936 0.893 0.852 0.802 0.748 0.693 0.641 0.590 0.553 0.517 0.482 0.446 0.408 0.369 0.329 0.283 0.219 0.131 Ni % 0.233 0.222 0.227 0.233 0.238 0.244 0.243 0.239 0.232 0.222 0.211 0.197 0.187 0.176 0.165 0.153 0.141 0.128 0.115 0.099 0.078 0.047 Tonnes 3,200,000 24,232,000 37,034,000 52,579,000 69,038,000 100,585,000 139,905,000 206,200,000 316,386,000 505,669,000 785,936,000 1,230,797,000 1,667,886,000 2,181,785,000 2,754,206,000 3,428,584,000 4,254,298,000 5,230,432,000 6,406,364,000 7,998,811,000 10,943,215,000 18,987,747,000 Note: 1) Cutoff grade intervals calculated from summation of modeled data (see Peterson, 2002d). For example, the 0.55 Cu cutoff includes all gridded data that falls between 0.575 and 0.525 wt. % Cu. 2) Cu%, Ni%, and tonnes represent cumulative addition of data from the Cu cutoff value and all data of higher grade. 3) Low Cu cutoff data dominantly represents barren rock within the mineralized zone of the Open style and barren rock above the mineralized zone within the Confined style. 4) Tonnes rounded to the nearest 1,000. 5) All intervals not assayed were assigned Cu and Ni values of 0.00 wt% and integrated into the model. NICKEL LAKE MACRODIKE Detailed geological mapping at a scale of 1:5,000 by the authors and Chris White, a Masters Candidate in the Department of Geological Sciences at the University of Minnesota Duluth, was completed in the late summer and fall of 2006, and published at a scale of 1:10,000 by the NRRI (Peterson et al., 2006). This map, available online at http://www.nrri.umn.edu/egg/REPORTS/MAP200604/MAP200604.html is the foundation upon which the field component of this trip will be based. During the course of this mapping, approximately 1,000 outcrops along nearly 100 kilometers of field traverses were examined to identify and confirm the internal lithologic variability, contact relationships, and structure of the western extent of the NLM, the adjacent SKI, and bounding rocks of the Anorthositic series. The authors wish to acknowledge Dr. Paul Weiblen (emeritus professor of geology at the University of Minnesota) for his keen insight on the geology of the area and Dr. George Hudak and undergraduate student Jeremiah ILSG07 94 Trip 4 �Gowey of the University of Wisconsin Oshkosh for assistance in mapping outcrops around and south of Omaday Lake. As well, a one day field excursion to Nickel Lake with Dave Peck (Anglo American), Harry Noyes (Encampment Resources), and Theodore DeMatties (consulting geologist) prior to the mapping campaign developed new insight on identifying dynamic magmatic systems in the field to the senior author (Peterson). Additional reconnaissance mapping in early November by Dean Peterson was completed to field check compiled outcrop locations depicted on the 1957 INCO map of the Spruce Road Deposit and the 1968 Hanna Mining map of the South Filson Creek deposit (both of which are publicly available in the DNR archive at Hibbing, Minnesota). The reconnaissance mapping confirmed the location of gossanous Cu-Ni bearing INCO outcrops and reconfirmed the outstanding field mapping of all types of Duluth Complex rocks by Hanna Mining Company geologists of the late 1960s. The NLM is a northwest to southwest-trending (Fig. 4.3), steeply dipping, asymmetric troctolitic and gabbroic intrusion interpreted to be a feeder dike for the northern portions of the SKI. The macrodike is interpreted to be located within a major rift-parallel normal fault (down to the southeast) now obscured by intrusion of NLM igneous rocks. Regional southward tilting (based on the deep level of erosion of the northern Bald Eagle Intrusion directly east of this area) leads to the interpretation that the southwest end of the NLM (near Omaday Lake) is structurally higher than the northeastern portion of the dike, and represents the location where magma flow changed from dike-like to sill-like, as it exited the dike – thus the magma velocity slowed – and entered the growing SKI magma chamber. Excellent potential exists for Ni-Cu rich massive sulfide at the basal contact where the dike enters the SKI (Section 31, T62N, R10W). The dike is composed of three main units: 1) inclusion-rich, locally sulfide-bearing, heterogeneous troctolite (unit N-Th); 2) layered troctolite, melatroctolite, and dunite (unit N-Tl); and 3) a late, crosscutting, coarse-grained to pegmatitic oxide-rich, olivine-gabbro to melagabbro (unit N-xG). Description of NLM Map Units The basis for all of the field aspects of this field trip is the aforementioned recently published bedrock geologic map of the NLM (Peterson et al., 2006). As well, the description of “Field Trip Stops” to follow differ from most geology field trip guidebooks in that we are simply going to take some walks in the bush, mostly along logging roads and snowmobile trails, look at numerous outcrops of the NML and adjacent rocks, and discuss the geology. We are NOT GOING TO SPECIFIC OUTCROPS to try to make a case for our interpretations; instead we urge you to use your imagination while we look at outcrops and add to the conversation. Instead of writing detailed descriptions of specific outcrops we’ll visit during the “stops” in the field trip, the authors have instead decided to simply copy verbatim the Description of Map Units from the map NRRI/MAP-2006-04 (Peterson et al., 2006) below, and use this as a reference during the field trip. The rocks of the NLM include, generally from youngest to oldest: Oxide Gabbro (N-xG) - Dark-grey, coarse-grained to pegmatitic, recessive weathered, oxide-rich (magnetite and ilmenite), olivine-gabbro to melagabbro. Contains small inclusions of anorthosite (unit N-Ai), basalt (unit N-Bi), and troctolite (unit N-Th). Interpreted to be the youngest phase of the dike based on inclusion types and cross-cutting relationships. Layered Troctolite to Dunite (N-Tl) - Grey to black, medium-grained, well-layered troctolite, melatroctolite, and dunite. Lamination of plagioclase and olivine parallel to modal layering is commonly observed as well as igneous scours and crossbedding. Small inclusions of anorthosite (unit N-Ai) and basalt (unit N-Bi) rare. Layering possibly developed as the up-welling magma streams through the dynamic (expanding) feeder dike. A constant temperature appropriate to plagioclase-olivine crystallization is maintained by a balance between the heat content of the incoming magma plus the heat of crystallization and the heat loss through the chamber walls. ILSG07 95 Trip 4 �Plagioclase and olivine are left behind and oriented/segregated on the walls of the expanding chamber (Weiblen, pers. comm.). Sulfide noted locally in outcrop in the SE corner of Section 31. Heterogeneous Troctolite (N-Th) - Light to dark grey, medium- to coarse-grained, inclusion-rich, heterogeneous troctolitic rocks with local igneous scour structures. Unit composed of intermixed troctolite, anorthositic-troctolite, melatroctolite, and gabbroic phases surrounding numerous local country rock (unit N-Ai) and exotic (units N-IF and N-Bi) inclusions that are elongate parallel to the macrodike, as well as hosts the sulfide-bearing unit N-Ts. Interpreted to be the initial highly dynamic magmatic phase of the dike that carried exotic inclusions from deep in the crust to their present level. Sulfide-Bearing Troctolite (N-Ts) - Rusty weathered, medium- to coarse-grained, sulfide-bearing, heterogeneous troctolitic and gabbroic rocks. Generally forms recessive weathering, Fe-stained, gossanous outcrops near, but not at, the northern margin of the macrodike. The current extent of this unit on the map is confined to those areas with outcrop, which in reality may be much more extensive as these outcrops generally end along linear swampy areas. Anorthosite Inclusion (N-Ai) - Light-grey, medium- to coarse-grained troctolitic-anorthosite, commonly with 1-2 cm poikilitic olivine pits. The large anorthosite inclusions at the southwest end of the dike (around Omaday Lake) are interpreted to represent a "logjam" of blocks that quit moving due to the decreased speed of the macrodike magmas as they entered the South Kawishiwi Intrusion magma chamber. Includes blocks within the SKI adjacent to the Nickel Lake macrodike. Basaltic Hornfels Inclusion (N-Bi) - Grey, fine-grained, steeply-dipping to vertical, granoblastic, locally magnetic, massive to amygdaloidal basaltic hornfels. Includes a highly magnetic block within the SKI adjacent to the Nickel Lake macrodike. Biwabik Iron Formation Inclusion (N-IF) - Well-bedded, steeply-dipping, recrystallized (layered magnetite and pyroxenite) iron-formation commonly with disseminated Cu-Ni sulfides. Forms an intense localized positive magnetic anomaly. ------------------------------------------------------------------------ Anorthositic Series - Subsuite of the Duluth Complex composed predominantly of plagioclase cumulates displaying complex internal structure and lacking obvious signs of in situ differentiation. Occurs throughout the Duluth Complex as anorthosite, troctolitic-anorthosite, and gabbroic-anorthosite, commonly poikilitic. Anorthositic Rocks Undivided (A-tA) - Mixed group of anorthositic cumulates occurring as large sill-like masses and as inclusions within troctolitic cumulates. Common rock types include troctoliticanorthosite, leucotroctolite, anorthosite, and olivine-bearing gabbroic-anorthosite. Olivine ranges from 2 to 15 percent in mode and from granular to poikilitic in texture, with oikocrysts ranging from 1 to 3 centimeters in diameter. Plagioclase mode ranges from 75 to 95 percent and varies from being non-foliated to well-foliated. Inclusions range in size from a few centimeters to elongate bodies hundreds of meters long that are parallel to foliation in the enclosing troctolite. DAY 1, FIELD TRIP STOPS Traverse #1 The location of the first field trip traverse (#1) is given in Figure 4.7. This walk in the bush begins within numerous outcrops of the bounding Anorthositic series rocks (map unit A-tA) on the northern margin of the NLM. The trail will take us southwest into inclusion-rich heterogeneous troctolitic rocks (map unit N-Th) and into southwestern most zone of known Cu-Ni-(PGE) mineralization in the NLM (map unit NTs), which in this location is associated with a large, sub-vertical inclusion of Biwabik Iron Formation (map unit N-IF). One must try to imagine from where such an inclusion came from (see Figure 4.4), how ILSG07 96 Trip 4 �it relates to the model that the NML is a feeder dike to the SKI, why it and other large mapped inclusions in the NML (Fig. 4.7) are concentrated at the southwestern end of the NML (magma velocity). Think back to the previously described fundamental tenet of Ni-Cu-PGE magmatic sulfide deposits, “… realization that decreases in the flow rate of magmas, principally due to the geometry of the conduit, is a major factor in settling entrained sulfide droplets and forming sulfide-rich ore bodies…”. Figure 4.7. Bedrock geology map of portions of the southwestern end of the Nickel Lake Macrodike in Sections 29 and 30, T62N, R10W. Dark lines are superimposed locations of field trip traverse #1, and traverse #2 along logging roads and snowmobile trails. Dashed lines represent short traverses through the bush to additional known outcrops. Traverse #2 The location of the second field trip traverse (#2) is presented in Figure 4.7. This walk to the southeast along a logging road/snowmobile trail begins in coarse-grained, oxide-rich olivine gabbro to melagabbro of map unit N-xG. Contact relationships between the N-xG and layered troctolitic to dunitic rocks (map unit N-Tl) can be complex, and we’ll investigate these relationships early on the traverse. The bulk of this traverse will be through the widest section (>650 m) of unit N-Tl that has been mapped in the NLM. Careful attention should be directed towards igneous textures in the N-Tl, and what they imply to magmatic processes (expanding dike with time, modal layering, pasting plagioclase and olivine phenocrysts on dike walls, solidification fronts, etc…). Optional traverses shown include a walk to ILSG07 97 Trip 4 �outcrops of the very large (~ 1.5 km long) basalt inclusion (map unit N-Bi), and to exceptional exposures of layered troctolite and dunite around a small beaver pond. Traverse #3 The location of the third field trip traverse (#3) is presented in Figure 4.8. This walk to the northnortheast of Nickel Lake will traverse through most of the main units identified in the NLM, including map units N-Th, N-xG, N-Tl, N-Ts, N-Bi, and N-Ai. An important walk through the bush (optional traverse on Figure 4.8) will visit several Cu-Ni-(PGE) mineralized outcrops near the northwestern margin of the NLM, and allow us to view some spectacular exposures, around the margin of a drained beaver pond, of the Anorthositic series rocks immediately northwest of the NLM. The authors cannot speak to strongly on the importance of finding these types of exposures (totally free of lichen, moss, trees, etc…), early in a mapping program, as they provide proxies for subsequent mapping of outcrops deep in the bush, where trees, shrubs, shade, forest litter, dirt, and black flies partially obscure exposures and/or ones willingness to observe them. Figure 4.8. Bedrock geology map of portions of the Nickel Lake Macrodike in the vicinity of Nickel Lake, Section 29, T62N, R10W. Dark line in the center of the image is the superimposed location of field trip traverse #3 along a logging road. Dashed line in the northwest quadrant represents a short traverse through the bush into the Cu-Ni-PGE mineralized northern zone of the NML. Note that beaver dams along the main traverse may cause very wet and/or impassable conditions. ILSG07 98 Trip 4 �Field relationships that offer evidence that a dike-like mafic to ultramafic intrusion is a conduit through which magma ascended upwards in the Earth’s crust all lead back to the fundamental tenets of the Ni-CuPGE ore deposit model. Such relationships provide evidence that the rocks formed in a dynamic, sulfidebearing magmatic system (once again, think like a person fly fishing a trout stream) that include: 1) early phases should be inclusion-rich (some of which should be from a deeper crustal level) and form as the igneous conduit breeches upwards into the Earth’s crust; 2) imbrication and/or elongation of entrained country rock inclusions parallel to igneous foliation; 3) igneous scour structures; 4) prominent steeplydipping igneous foliation and localized disruption due to magmatic injection; 5) cross-bedding of modal layering; 6) evidence of sulfide mineralization; and 7) evidence that that magma velocity varies. One of the authors’ goals of the field trip is to expand this list based on conversations on the outcrop with the participants. Outcrop photographs given in Figure 4.9 show a few of these lines of evidence for the dynamic nature of the NLM. Figure 4.9. Photographs of selected outcrops that give us clues to the dynamic nature of the magmatic processes which ultimately led to the formation of the NLM, and by interpretation, the northern portion of the SKI and its associated Cu-Ni-PGE mineralization. A) Scour structure in map unit N-Th. B) Disrupted igneous foliation along the southern margin of map unit N-Tl. C) Cross-bedding of troctolitic rocks in map unit NTl. D) Cu-Ni sulfide mineralization in map unit N-Ts. ILSG07 99 Trip 4 �DAY 2, 3-D VISUALIZATION AND DRILL CORE DISPLAYS Day two of this field trip will be spent at Duluth Metals Limited’s field office and drill core logging facility in Ely, Minnesota. The day will be split up into two principal activities: 1) 3-D visualization of subsurface geological features of the Nickel Lake macrodike and South Kawishiwi intrusion utilizing the computer program gOcad (geologic object computer aided design) and a Geowall (bring your camera because we’ll all be wearing those funny looking 3D glasses), and 2) examination of selected core intervals from a number of holes drilled by Duluth Metals Limited over the last year. 3-D GEOLOGICAL MODELING AND VISUALIZATION The science of geology uses a variety of tools to study the earth. However, the basis for every type of geologic study is fundamentally rooted in observations made of rocks in their natural habitat – “in the field”. Geologists that do not have an intimate appreciation of the power and fundamental nature of field geology cannot, in turn, appreciate coherent and compelling field-based scientific arguments from which all other geologic interpretations grow. This basic tenet may never be truer than for geologists engaged in mineral exploration. The advance in computer technology over the last twenty years has revolutionized all aspects of our lives. One such advance in geology has been the development of sophisticated 3-D geological software that, if used correctly, i.e., created and maintained by geologists who understand the rocks, can be an outstanding tool for letting geologists interpret data (field observations, drill hole data, etc...) into the subsurface where direct observation is impossible. The first author of this guidebook (Peterson) integrates many types of geological data into the computer program gOcad®, which is perhaps the most sophisticated 3-D geological modeling software available (see websites http://www.gocad.org/www/ and http://www.earthdecision.com/). 3-D models of active mines and/or advanced exploration projects benefit from continuous validation and upgrading of the underlying database, as well as the production of regional geological syntheses, integrating new geological, geophysical, geochemical, geotechnical, and geohydrological models into a single platform. The natural outgrowth of 3-D geological models is their extrapolation away from areas with large amounts of data to more remote areas with less and/or no data, and can assist in the definition of new exploration targets. For geologic teams working in a mineral exploration setting, the main advantages of using integrated 3-D geological models are to (from Fallara et al., 2006): 1) Share the information a. Conveys the data and their interpretation in an immersive format b. Avoids the loss of knowledge and interpretation which may be, i. Distributed within various locations within the company ii. Filed in disorganized ways iii. Never filed and existing only in a geologists memory 2) Be a catalyst in the development of geologic knowledge a. Easily integrates data in a common format b. Preserves data that is easily shared, seen, and analyzed 3) Hastens problem solving throughout the company a. Adaptable to team-driven resolution b. Easy access to geologists and management c. Allows for the shared comprehension of the data ILSG07 100 Trip 4 �4) Accelerates the process of data integration and interpretation a. Define potential exploration targets with reduced uncertainty b. Focuses work on interpretation c. Optimizes data subsets 5) Direct access to data and manipulation within the gOcad® software a. Integrated 3-D querying within the geological model b. Saves time and money, rapid validation of data, and uncertainty resolution A 3-D display of the first author’s gOcad modeling of geologic data from the Duluth Complex in general, and the SKI, NLM, and BEI in particular, will be presented in Duluth Metals Limited’s drill core logging facility at the start of Day 2 of this field trip. A simple screen dump out of gOcad of some of this data is presented in Figure 4.10. Figure 4.10. Print screen image of the SKI and NLM gOcad model (Peterson, unpublished data). DULUTH METALS LIMITED DRILL CORE DISPLAYS Recent drilling (Fig. 4.11) and core logging by Duluth Metals Limited east of the Maturi deposit in 2006 and 2007 has confirmed numerous stratigraphic units described by Severson (1994), especially the sulfide-bearing PEG, U3, BH, BAN, and upper GRB units. ILSG07 101 Trip 4 �Figure 11. Property map of Duluth Metals Limited. Included are historic (black) and recent (red) drill holes. The numerous correlative units of the SKI from historic and recent drill holes include the following: Main AGT – Thick zone of homogenous augite troctolite to olivine gabbro. AT&T – Thick zone of homogeneous medium-grained anorthositic troctolite with troctolites. AT(T) – Homogeneous anorthositic troctolite grading to troctolite and lesser amounts augite troctolite. T-AGT – Homogeneous medium-grained troctolite and augite troctolite. Pic – Thin horizon of medium-grained picrite that is rarely developed, and thus usually uncorrelative. Upper Gabbro – Heterogeneous, medium- to coarse-grained, oxide-rich gabbro and olivine gabbro. AN Group – Combination of coarse-grained homogenous anorthosite, troctolitic anorthosite, anorthositic troctolite, and medium-grained homogeneous gabbro, and olivine gabbro of the Anorthositic series of the Duluth Complex. Basalt Inclusion – Fine-grained, typically magnetic, basalt hornfels with sharp external contacts and locally preserved stretched amygdules. U2 – Ultramafic horizon of locally serpentinized, medium-grained picrite and troctolite with sharp contacts. PEG – Heterogeneous pegmatoidal and pegmatitic troctolite and anorthositic troctolite with minor amounts of augite troctolite, olivine gabbro, and troctolitic anorthosite. The PEG unit typically is weakly sulfide-bearing with sulfide-barren rocks above. ILSG07 102 Trip 4 �U3 – Ultramafic zone of locally serpentinized, sulfide-bearing, medium-grained dunite, picrite, and troctolite with sharp contacts. This unit is the PGE-dominate horizon at the Birch Lake deposit. BH – Basal heterogeneous zone that is the main sulfide-bearing unit of the SKI. Consists of fine- to medium- to coarse-grained and pegmatitic troctolite and to a lesser extent anorthositic troctolite and augite troctolite. BAN – Basal augite troctolite and norite which is the contaminated zone between the BH and underlying GRB. Consists of fine- to medium-grained, sulfide-bearing augite troctolite and norite. GRB – A variety of rocks from the footwall Archean Giants Range Batholith. Rock types include medium- to coarse-grained hornblendite, diorite, and porphyritic or nonporphyritic monzonite to quartz monzodiorite. Sulfide mineralization is usually restricted to the upper 20 feet of the GRB, but can exist in small concentrations over a hundred feet into the granitic footwall. Simplified drill hole logs portrayed as stratigraphic columns from two holes recently drilled in the eastern (MEX-07) and western (MEX-08) portions of the Maturi Extension deposit are presented in Figure 4.12. In addition, reported Cu-Ni-PGE grades from Duluth Metals Limited for these holes are given in Table 4.2. The stratigraphy observed in MEX-07 is typical of the eastern exploration area and the deep holes in this region (>3,000 feet) that are situated below a very large pillar/xenolith of Anorthosite series rocks (see Fig. 4.3). Channelized magma flow out of the NLM and under this Anorthosite block (think of eddies due to overlying friction as a fisherman would) has been interpreted by Peterson (2001b, c) as one of the driving mechanisms for enhanced turbulent flow of sulfide-bearing magmas, thus forming one of the elements of the Ni-Cu-PGE ore deposit model, “an appropriate physical environment is required so that the sulfide liquid mixes with enough magma to become adequately enriched in chalcophile metals”. The stratigraphy observed in MEX-08 correlates with many of the drill holes in the western exploration area (as well as Franconia Minerals’, Maturi deposit) immediately to the west of the Anorthosite series pillar/xenolith. Mineralization in both of these holes consist of chalcopyrite-dominated disseminated sulfides (typically 2-5% total sulfide) restricted to a confined zone directly above the footwall and contain moderate to high Cu, Ni, and PGE concentrations. If one would very simply (unlike the data depicted in Figure 4.6, which used the strict rules of Kerr, 2001) recalculate these reported grades (Table 4.2) to 100% sulfide, i.e. as a specific gravity concentrator would do as a first step at an active mine, then one would have to multiply the assay data by a factor between 20 to 50 (5% to 2% total sulfide minerals in the assayed drill hole intervals) to imagine the true reality that is the enrichment of Cu-Ni-PGE in the sulfide minerals of the Maturi Extension deposit. Such recalculations lead to grades (at 100% sulfide) of ~33% Cu, ~9% Ni, and ~11 – >82 g/t PGE+Au for the sulfide fraction of the Maturi Extension deposit, which point to these preliminary conclusions: 1) The extensive sulfide mineralization at the Maturi Extension deposit (as well as the Maturi, and Birch Lake deposits, i.e., the Confined mineralization of Peterson 2001, 2002a-d) is similar at a recalculated 100% sulfide grade to the metal budget of the fractionated residuum of a MSS; 2) One can imagine such mineralization (the Confined-style of Peterson 2002a) initially forming from incorporation of the Cu-PGE residuum of a fractionating Ni-rich MSS at the junction of the NLM and SKI into magmas streaming out of the NLM and into the SKI. 3) A seemingly robust geologic model of a dynamic magmatic system that incorporates the BEI, NLM, and the SKI exists herein that vector towards an exploration target for a Ni-rich massive sulfide body (the “missing Ni-rich MSS”) at the junction of the NLM and the SKI. ILSG07 103 Trip 4 �Table 2. Reported mineralized intervals for holes MEX-07 and MEX-08 drilled by Duluth Metals Limited on the Maturi Extension property in 2006. MEX-08 Overall including including From (Ft) 2032 2052 2202 To (Ft) 2236.5 2142 2236.5 Total (Ft) 204.5 90 34.5 Cu % 0.69 0.81 0.89 Ni % 0.22 0.24 0.28 Pd (g/t) 0.313 0.365 0.352 Pt (g/t) 0.121 0.147 0.12 Au (g/t) 0.108 0.056 0.096 MEX-07 Overall including including From (Ft) 2543 2608 2628 To (Ft) 2798 2673 2663 Total (Ft) 255 65 35 Cu % 0.429 0.824 0.898 Ni % 0.129 0.275 0.310 Pd (g/t) 0.370 0.906 1.004 Pt (g/t) 0.183 0.444 0.472 Au (g/t) 0.077 0.173 0.177 Figure 4.12. Stratigraphic sections of holes MEX-07 and MEX-08 drilled by Duluth Metals Limited on the Maturi Extension property in 2006. ILSG07 104 Trip 4 �REFERENCES Allen, D.J., Hinze, W.J., Dickas, A.B., and Mudrey, M.G., Jr., 1997, Integrated geophysical modeling of the North American Midcontinent rift system: New interpretations for western Lake Superior, northwestern Wisconsin, and eastern Minnesota, in Ojakangas, R.W., Dickas, A.B., and Green, J.C., eds.: Middle Proterozoic to Cambrian rifting, central North America: Geological Society of America, Special Paper 312, p. 47-72. Arndt, N.T., Lesher, C.M., and Czamanske, G.K., 2006, Mantle-derived magmas and magmatic Ni-Cu-(PGE) deposits, in Hedenquist, J.W., Thompson, J.F.H., Goldfard, R.J., and Richards, J.P., eds.: Economic Geology, One Hundredth Anniversary Volume 1905-2005, p. 5-24. 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Chandler, V.W., 1990, Geologic interpretation of gravity and magnetic data over the central part of the Duluth Complex, northeastern Minnesota: Economic Geology, v. 85, no. 4, p. 816-829. Chandler, V.W., and Ferderer, R.J., 1989, Copper-nickel mineralization of the Duluth Complex, Minnesota-A gravity and magnetic perspective: Economic Geology, v. 84, no. 6, p. 1690-1696. Dahlberg, E.H., 1987, Drill core evaluation for platinum group mineral potential of the basal zone of the Duluth Complex: Minnesota Department of Natural Resources, Division of Minerals, Report 255, 60 p. Dahlberg, E.H., Peterson, D.M., and Frey, B.A., 1989, Drill core repository projects (1988-1989): Minnesota Department of Natural Resources, Division of Minerals, Reports 255-1, 265, and 266, 316 p. Eckstrand, O.R., and Hulbert, L., in prep., Magmatic nickel-copper-platinum group element deposits, in Goodfellow, W.D., ed., Mineral Deposits of Canada, A synthesis of major deposit types, district metallogeny, the evolution of geological provinces, and exploration methods: Joint Geological Survey of Canada and Geological Association of Canada publication, 16 p. Fallara, F., Legault, M., and Rabeau, O., 2006, 3-D integrated geological modeling in the Abitibi subprovince (Québec, Canada): Techniques and applications: Exploration and Mining Geology, v. 15, no. 1-2, p. 27-42. Foose, M.P., 1984, Logs and correlation of drill holes within the South Kawishiwi intrusion, Duluth Complex, northeastern Minnesota: U.S. Geological Survey, Open-File Report 84-14, variously paged, 1 pl. Foose, M.P., and Cooper, R.W., 1978, Preliminary geologic report on the Harris Lake area, northeastern Minnesota: U.S. Geological Survey, Open-File Report 78-385, 24 p., 1 pl., scale 1:12,000. Franklin, J.M., Sangster, D.M., and Lydon, J.W., 1981, Volcanic associated massive sulfide deposits, in Skinner, B. J., ed.: Economic Geology 75th Anniversary Volume, p. 485-627. Graber, R.G., 1993, LTV Steel Mining Company: Institute on Lake Superior Geology, 39th Annual Meeting, Eveleth, Minn., Proceedings, v. 39, Fieldtrips, pt. 2, p. 39-42, 52-59. Green, J.C., Phinney, W.C., and Weiblen, P.W., 1966, Geologic map of Gabbro Lake quadrangle, Lake County, Minnesota: Minnesota Geological Survey, Miscellaneous Map Series, Map M-2, scale 1: 31,680. ILSG07 105 Trip 4 �Hauck, S., Severson, M., Ripley, E., Goldberg, S., and Alapieti, T., 1997, Geology and Cr-PGE mineralization of the Birch Lake area, South Kawishiwi intrusion, Duluth Complex: University of Minnesota Duluth, Natural Resources Research Institute, Technical Report, NRRI/TR-97/13, 32 p. Hauck, S.A., Severson, M.J., Zanko, L.M., Barnes, S.J., Morton, P., Alminas, H.V., Foord, E.E., and Dahlberg, E.H., 1997, An overview of the geology and oxide, sulfide, and platinum group element mineralization along the western and northern contacts of the Duluth Complex, in Ojakangas, R.W., Dickas, A.B., and Green, J.C., eds., Middle Proterozoic to Cambrian rifting, central North America: Geological Society of America, Special Paper 312, p. 137-185. Hodgson, C.J., 1993, Mesothermal lode-gold deposits, in Mineral Deposit Modeling, Kirkham, R.V., Sinclair, W.D., Thorpe, and R.I., Duke, J.M. eds.: Geological Survey of Canada, Special Paper 40, p. 635–678. Kerr, A., 2001, The calculation and use of sulfide metal contents in the study of magmatic ore deposits: A methodological analysis: Exploration and Mining Geology, v. 10, no. 4, p. 289-301. Kuhns, M.J., Hauck, S.A., and Barnes, R.J., 1990, Origin and occurrence of platinum group elements, gold and silver in the South Filson Creek copper-nickel mineral deposit, Lake County, Minnesota: University of Minnesota Duluth, Natural Resources Research Institute, Technical Report NRRI/GMIN-TR-89-15, 60 p., 3 pls. Lee, I., and Ripley, E.M., 1996, Mineralogic and oxygen isotopic studies of open system magmatic processes in the South Kawishiwi intrusion, Spruce Road area, Duluth Complex, Minnesota: Journal of Petrology, v. 37, no. 6, p. 1437-1461. Li, C. and Naldrett, A.J., 1999, Geology and petrology of the Voisey’s bay intrusion: reaction of olivine with sulfide and silicate liquids: Lithos, v. 47, p. 1-31. Lightfoot, P.C., Naldrett, A.J., Gorbachev, N.S., Fedorenko, V.A., Hawkesworth, C.J., Hergt, J., and Doherty, W., 1994, Chemostratigraphy of Siberian trap lavas, Noril’sk District: Implications for the source of flood basalt magmas and their associated Ni-Cu mineralization, in Lightfoot, P.C., and Naldrett, A.J., eds., Proceedings of the Sudbury–Noril’sk symposium: Ontario Geological Survey, Special Volume 5, p. 283312. Listerud, W.H., and Meineke, D.G., 1977, Mineral resources of a portion of the Duluth Complex and adjacent rocks in St. Louis and Lake Counties, northeastern Minnesota: Hibbing, Minnesota Department of Natural Resources, Division of Minerals, Report 93, 74 p. Lowell, J.D. and Guilbert, J.M., 1970, Lateral and vertical alteration-mineralization zoning in porphyry ore deposits: Economic Geology, v. 65, p. 373-408. Marma, J.C., 2003, Magmatic and hydrothermal PGE mineralization of the Birch Lake Cu-Ni-PGE deposit in the South Kawishiwi intrusion, Duluth Complex, northeastern Minnesota: Unpublished M.S. thesis, University of Wisconsin: condensed version, University of Minnesota Duluth, Natural Resources Research Institute, Technical Report, NRRI/TR-2003/39, 112 p. Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., and Peterson, D.M., 2001, Geologic map of the Duluth Complex and related rocks, northeastern Minnesota: Minnesota Geological Survey, Miscellaneous Map Series, Map M-119, scale 1:200,000. Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.M., and Wahl, T.E., 2002, Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geological Survey, Report of Investigations 58, 207 p. Morey, G.B., and Cooper, R.W., 1977, Bedrock geology of the Hoyt Lakes–Kawishiwi area, St. Louis and Lake Counties, northeastern Minnesota: Minnesota Geological Survey, Open-File Report, scale 1:48,000. Naldrett, A.J., 1989, Sulfide melts: Crystallization temperatures, solubilities in silicate melts, and Fe, Ni, and Cu partitioning between basaltic magmas and olivine, in Whitney, J.A., and Naldrett, A.J., eds., Ore deposition associated with magmas: Reviews in Economic Geology, v. 4, p. 5-20. Naldrett, A.J., 1997, Key factors in the genesis of Noril’sk, Sudbury, Jinchuan, Voisey’s Bay and other world-class Ni-Cu-PGE deposits: Implications for exploration: Australian Journal of Earth Sciences, v. 44, no. 3, p. 283-315. ILSG07 106 Trip 4 �Naldrett, A.J., 1999, World-class Ni-Cu-PGE deposits: key factors in their genesis: Mineralium Deposita, v. 34, Issue 3, p. 227-240. Naldrett, A.J., Asif, M., Krstic, S., and Li, C., 2000, The composition of mineralization at the Voisey’s Bay Ni-Cu sulfide deposit, with special reference to platinum-group elements: Economic Geology, v. 95, p. 845-865 Patelke, R.L., 2003, Exploration drill hole lithology, geologic unit, copper-nickel assay, and location database for the Keweenawan Duluth Complex, northeastern Minnesota: University of Minnesota Duluth, Natural Resources Research Institute, Technical Report, NRRI/TR-2003/21, 97 pages, 1 CD. Peterson, D.M., 1997, Ore deposit modeling of the footwall mineralization of the Duluth Complex: Minnesota Department of Natural Resources, Division of Minerals, Project 317, 55 p., 46 pls. Peterson, D.M., 2001a, Development of Archean lode-gold and massive sulfide deposit exploration models using geographic information system applications: Targeting mineral exploration in northeastern Minnesota from analysis of analog Canadian mining camps: Unpublished University of Minnesota Ph.D. thesis, 503 p., 12 plates, 1 CD. Peterson, D.M., 2001b, Development of a conceptual model of Cu-Ni-PGE mineralization in a portion of the South Kawishiwi intrusion, Duluth Complex, Minnesota: Society of Economic Geologists, Second Annual PGE Workshop, Sudbury, Ontario, 3 p. Peterson, D.M., 2001c, Copper-Nickel-PGE mineral potential of the eastward extension of the Maturi Cu-Ni deposit, Duluth Complex, Lake County, Minnesota: University of Minnesota Duluth, Natural Resources Research Institute, Confidential Report of Investigations NRRI/RI-2001-02, 29 pages, 15 plates, 1 CD. Peterson, D.M., 2002a, 3-Dimensional view through a mineralized system: the South Kawishiwi intrusion, Duluth Complex: Institute on Lake Superior Geology, 48th Annual Meeting, Thunder Bay, Ontario, v. 48. Peterson, D.M., 2002b, Cu-Ni-PGE mineralization in the South Kawishiwi intrusion, northeastern Minnesota; Variation due to magmatic processes: Institute on Lake Superior Geology, 48th Annual Meeting, Thunder Bay, Ontario, v. 48. Peterson, D.M., 2002c, Variation in the Cu-Ni-PGE mineralization in the South Kawishiwi intrusion, Duluth Complex, northeastern Minnesota: 9th International Platinum Symposium, Billings, Montana, USA, July 21-25. Peterson, D.M., 2002d, Copper-Nickel grade maps for the Spruce Road deposit, South Kawishiwi intrusion, Duluth Complex: University of Minnesota Duluth, Natural Resources Research Institute, Report of Investigations NRRI/RI-2002/03, 99 p. Peterson, D.M., 2002e, Shaded relief map of the basal contact of the South Kawishiwi intrusion, Duluth Complex, northeastern, Minnesota: University of Minnesota Duluth, Natural Resources Research Institute, Map Series NRRI/MAP-2002-01, scale 1:75,000. Peterson, D.M., 2002f, Bedrock geology, sample location, and property position maps of the west Birch Lake area, South Kawishiwi intrusion, Duluth Complex, northeastern, Minnesota: University of Minnesota Duluth, Natural Resources Research Institute, Map Series NRRI/MAP-2002-02, scale 1:10,000. Peterson, D.M. and Severson, M.J., 2002, Archean and Paleoproterozoic rocks that form the footwall to the Duluth Complex, in Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.M., and Wahl, T.E., 2002, Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geological Survey, Report of Investigations 58, p. 76-93. Peterson, D.M., 2006a, 3D Visualizations of mafic Intrusions in the Duluth Complex, northeastern Minnesota, Institute on Lake Superior Geology, 52nd Annual Meeting, Sault Ste Marie, Ontario, May 8-12, 2006, Minnesota, v. 52. Peterson, D.M., 2006b, Digital base for geological mapping within the northern South Kawishiwi intrusion: Lake and St. Louis Counties, northeastern Minnesota: University of Minnesota Duluth, Natural Resources Research Institute, Map Series NRRI/MAP-2006-01, scale 1:20,000. Peterson, D.M., 2006c, New ideas on mineralization in the Duluth Complex, Oral presentation and online pdf file to the Mesabi Range Geological Society, December 20, 66 pages. ILSG07 107 Trip 4 �Peterson, D.M., Albers, P.B., and White, C.R., 2006, Bedrock geology of the Nickel Lake macrodike and adjacent Areas, Lake County, northeastern Minnesota: University of Minnesota Duluth, Natural Resources Research Institute, Map Series NRRI/MAP-2006-04, scale 1:10,000. Peterson, D. M. and Hauck, S.A., 2005, Visualization of "Frozen" dynamic magma chambers in the Duluth Complex, northeastern Minnesota: Eos Trans. AGU 86(52), Fall Meet. Suppl., Abstract V23A-0680. Peterson, D.M., Patelke, R.L., and Severson, M.J., 2004, Bedrock geology map and Cu-Ni mineralization data for the basal contact of the Duluth Complex west of Birch Lake, St. Louis and Lake Counties, northeastern Minnesota: University of Minnesota Duluth, Natural Resources Research Institute, Map Series NRRI/MAP-2004-02, scale 1:10,000. Phinney, W.C., 1969, The Duluth Complex in the Gabbro Lake quadrangle, Minnesota: Minnesota Geological Survey, Report of Investigations 9, 20 p. Phinney, W.C., 1972, Northwestern part of Duluth Complex, in Sims, P.K., and Morey, G.B., eds., Geology of Minnesota: A centennial volume: Minnesota Geological Survey, p. 335-345. Postle, J., Bernie-Haystead, B., Clow, G., Hora, D., Vallée, M., and Jensen, M., 2000, CIM standards on mineral resources and reserves, definitions and guidelines: Canadian Institute of Mining, Metallurgy and Petroleum, 26 p. Ripley, E.M., 1986, Origin and concentration mechanisms of copper and nickel in Duluth Complex sulfide zones—a dilemma: Economic Geology, v. 81, no. 4, p. 974-978. Severson, M.J., 1994, Igneous stratigraphy of the South Kawishiwi intrusion, Duluth Complex, northeastern Minnesota: University of Minnesota Duluth, Natural Resources Research Institute, Technical Report NRRI/TR-93/34, 210 p., 15 pls. Severson, M.J., and Hauck, S.A., 1997, Igneous stratigraphy and mineralization in the basal portion of the Partridge River intrusion, Duluth Complex, Allen quadrangle, Minnesota: University of Minnesota Duluth, Natural Resources Research Institute, Technical Report NRRI/TR-97/19, 102 p., 4 pls. Severson, M.J., and Hauck, S.A., 2003, Platinum-group elements (PGEs) and platinum-group minerals (PGMs) in the Duluth Complex: University of Minnesota Duluth, Natural Resources Research Institute, Technical Report, NRRI/TR-2003/37, 296 p., 1 CD. Thériault, R.D., Barnes, S.J., and Severson, M.J., 2000, Origin of Cu-Ni-PGE sulfide mineralization in the Partridge River intrusion, Duluth Complex, Minnesota: Economic Geology, v. 95, p. 929-944. Vislova, T., 2003, Petrology of the Bald Eagle intrusion and associated rocks and its relevance to crystallization in dynamic magma chambers in the Midcontinent Rift: Unpublished Ph.D. Thesis, University of Minnesota. Weiblen, P. W., Morey, G. B., 1980, A summary of the stratigraphy, petrology, and structure of the Duluth Complex: American Journal of Science, vol. 280A, Part I, p 88-133. Weiblen, P., Peterson, D.M., and Vislova, T., 2005, Implications of Midcontinent Rift and oceanic ridges analogies and 3-D interpretations of the subsurface structure of the Bald Eagle intrusion in the Duluth Complex and the East Pacific Rise: Institute on Lake Superior Geology, 51st Annual Meeting, Sault Ste Marie, Ontario, v. 51, 3 p. Weiblen, Paul W., 1965, A funnel-shaped, gabbro-troctolite intrusion in the Duluth Complex, Lake County Minnesota: Unpublished Ph.D. Thesis, University of Minnesota, 161 p. Zanko, L.M., Severson, M.J., and Ripley, E.M., 1994, Geology and mineralization of the Serpentine copper-nickel deposit: University of Minnesota Duluth, Natural Resources Research Institute, Technical Report NRRI/TR-93/52, 90 p., 3 pls. ILSG07 108 Trip 4 �53rd Annual Institute on Lake Superior Geology FIELD TRIP 5 Geologic Highlights of New Mapping in the Upper Southwestern to Northeastern Sequence of the North Shore Volcanic Group and Beaver Bay Complex Terrence J. Boerboom Minnesota Geological Survey University of Minnnesota James D. Miller, Jr. Minnesota Geological Survey & Department of Geological Sciences University of Minnesota Duluth John C. Green Department of Geological Sciences University of Minnesota Duluth View northeast across Pork Bay on the North Shore ILSG07 109 Trip 5 �INTRODUCTION The Minnesota Geological Survey is continuing to map the Keweenawan bedrock geology of 1:24,000scale quadrangles near the North Shore of Lake Superior, in northeastern Minnesota. The first phase of mapping from 1985-1993, partially supported by the USGS COGEOMAP program, focused on the Beaver Bay Complex in the central part of the region (Fig. 5-1; Miller, 1988; Miller and others, 1989, 1993, 1994; Boerboom and Miller, 1994). More recent mapping, supported by the USGS STATEMAP program, has focused on quadrangles that intersect the shoreline, where the bedrock is predominantly volcanic rocks. This mapping phase started at the outskirts of Duluth in 2001, and to date has proceeded to the Lutsen quadrangle, having skipped past the COGEOMAP quadrangles. Ten quadrangle-scale geologic maps have been published to date from this work (Boerboom and others, 2002a, 2002b, 2003a, 2003b; 2006; 2007; Boerboom and Green, 2004; 2005, 2006; Miller and others, 2006), and at least two more are planned (Fig. 5-1). Figure 5-1. North Shore 7.5' quadrangles mapped by the MGS through the USGS-sponsored COGEOMAP and STATEMAP programs. M-numbers refer to MGS Miscellaneous Map Series. A trip similar to this, which covered the Upper Southwest sequence of the North Shore Volcanic Group (NSVG), was conducted for the 2004 Institute on Lake Superior Geology meeting held in Duluth. The trip for this year’s meeting will cross the transition from the uppermost Southwest limb to the uppermost Northeast limb, as well as the basal Schroeder-Lutsen sequence of the NSVG (Fig. 5-2). ILSG07 110 Trip 5 �Figure 5-2. Simplified geology of northeastern Minnesota showing the major sequences of the North Shore Volcanic Group (taken from Fig. 5.2 of Miller and others, 2002). VOLCANIC AND SEDIMENTARY ROCKS OF THE NORTH SHORE VOLCANIC GROUP This overview is a slightly modified excerpt from the MGS Report of Investigations 58, Chapter 5: "Volcanic and sedimentary rocks of the Keweenawan Supergroup in Northeastern Minnesota" by John Green (Miller and others, 2002). Magmatic activity related to the 1.1 Ga Midcontinent Rift produced a more than 10 kilometer thick edifice of lava flows and subvolcanic intrusions that are exposed along Minnesota's north shore of Lake Superior. The lava flows and minor sedimentary rocks are referred to as the North Shore Volcanic ILSG07 111 Trip 5 �Group (Goldich and others, 1961) and the intrusive rocks are variably assigned to the Duluth Complex, the Beaver Bay Complex, and miscellaneous intrusions of the Midcontinent Rift Intrusive Supersuite. Although previous publications have subdivided the North Shore Volcanic Group into informal volcanic suites and distinctive flows (Green, 1972, 1982), the 1:200,000-scale map (M-119) produced by Miller and others (2001) and accompanying report (Miller et al., 2002) represents the first time that the NSVG has been subdivided into coherent lithostratigraphic units on a geologic map. A brief description of the North Shore Volcanic Group and associated interflow sedimentary rocks of the Keweenawan Supergroup is given here. Rock classification, recognition, and textures As a coherent tholeiitic compositional suite, the volcanic rocks of the North Shore Volcanic Group can be described using only a few rock names (Fig. 5-3). The most primitive rocks are olivine tholeiites, which form an iron-enrichment trend with further evolution. They display ophitic textures and pahoehoe surfaces nearly everywhere. Most olivine tholeiites are aphyric, but those that are porphyritic contain dominantly plagioclase phenocrysts, less commonly olivine. Transitional basalts contain somewhat higher alkalies and other incompatible elements than the olivine tholeiites, but generally not enough to classify them as alkalic. Their texture is typically intergranular and fine- to medium-grained. Porphyritic varieties generally contain small phenocrysts of plagioclase, olivine, clinopyroxene, and magnetite. The reversed-polarity Hovland lavas, however, are characterized by transitional basalts (grading to basaltic andesites) that contain abundant, large, tabular, plagioclase phenocrysts. Transitional basalt flow surfaces are generally smooth (pahoehoe), although a few show breccia tops. The basaltic andesites and andesites (greater than 52 percent SiO2) are tholeiitic, rather than calcalkaline; they show iron enrichment and contain only anhydrous ferromagnesian minerals. These rocks are typically fine-grained and intergranular to felty or pilotaxitic, and many contain small phenocrysts of plagioclase, olivine, clinopyroxene, and magnetite. The andesites generally weather to a red-brown color, and have flow-brecciated (aa) tops but not bases. Many flows that contain 50 to 55 percent silica show millimeter-scale oxidation lamination (Green, 1989) parallel to the base. A few highly iron-enriched flows, separable only by chemical analysis, can be called ferroandesites. Figure 5-3. AFM compositional diagram for the lavas of the North Shore Volcanic Group (modified from Green, 1982). The boundary between tholeiitic and calcalkalic rocks is modified from Irvine and Baragar (1971). FeO* = FeO + 0.9Fe2O3. ILSG07 112 Trip 5 �Carmichael (1964) first used the name "icelandite" for rocks intermediate in character between andesites and rhyolites in the Tertiary lavas of eastern Iceland. They might be considered the tholeiitic equivalent of calc-alkaline dacite in orogenic suites. Other examples of these rocks have been described from the Galapagos (McBirney and Williams, 1969) and the Miocene of Nevada–Oregon (Wallace and others, 1980). Very large flows of similar composition in the Etendeka volcanics of Namibia have been referred to as quartz latites by Milner and others (1992). Icelandites in the North Shore Volcanic Group (Green and Fitz, 1993) are characterized chemically by SiO2 contents ranging from 60 to 68 percent, high FeO* (averaging 7 percent), K2O + Na2O values between 6.5 and 9 percent, a potassium/sodium atomic ratio of about 0.9, and an Mg number [Mg/(Mg + Fe) atomic] averaging 0.14. Petrographically, North Shore Volcanic Group icelandites grade continuously from the andesites to somewhat paler colors (brown or tan), but have a similar phenocryst assemblage. Quartz and alkali feldspar are common in the groundmass but never occur as phenocrysts. Flowtop features (crusty to coarsely brecciated) indicate that the icelandites were erupted as lavas. The rhyolites have higher silica and total alkali contents, and lower FeO than the icelandites. They are generally light gray, pink, or red. Several are very thick, extensive, and voluminous (up to several hundred cubic kilometers; Green and Fitz, 1993). Although most rhyolites are porphyritic (phenocrysts of quartz, alkali feldspar ± plagioclase, and altered fayalite ± ferroaugite), some lack quartz and alkali feldspar phenocrysts, and rare flows are aphyric. Groundmass textures are fine-grained holocrystalline, typically with a meshwork of platy quartz paramorphs after primary tridymite, which may show a flow structure. A "snowflake" texture is common, in which poikilitic quartz patches (coalesced ex-tridymite grains) enclose small, dusty alkali-feldspar grains. In quartz-phyric flows, these poikilitic quartz patches are in optical continuity with adjacent quartz phenocrysts (Green, 1990). Outcrop-scale flow structure, including folding, is common near flow tops and bases. In some flows, distinct fiamme, deformed to varying degrees, are recognizable. These imply an explosive eruption that produced a pyroclastic flow, which welded and remobilized to produce a rheoignimbrite. Structure and Lithostratigraphy As stated above, the Midcontinent Rift volcanic rocks and interbedded redbeds in northeastern Minnesota comprise the North Shore Volcanic Group (Goldich and others, 1961; Green, 1972, 1977, 1982; Basaltic Volcanism Study Project, 1981). In general, these rocks form an arcuate stack that is slightly tilted toward the southeast and forms the roof rocks into and under which the Duluth Complex and associated hypabyssal intrusions were emplaced. At the southwest end of the North Shore Volcanic Group near Duluth, the volcanic rocks strike north with a 10º to 20º easterly dip; at the northeast end at Grand Portage, the flows strike east–west with a 10º southerly dip (Fig. 5-2). Thus, traveling northeast along the Lake Superior shore northeast from Duluth, and southwest from Grand Portage, one encounters successively higher flows in the volcanic stratigraphy until the Tofte–Lutsen area in southern Cook County, where the highest exposed flows crop out. Exposure is generally excellent along the eroding lakeshore, and along the lower, high-gradient stretches of tributary streams, providing good control on the stratigraphy. A stack of volcanic rocks approximately 9.7-kilometers-thick has been measured in the “southwest limb” (Table 5-1), and another stack of volcanic rocks about 7.2-kilometers-thick has been measured in the “northeast limb” (Table 5-2, Miller and others, 2002). This implies nearly continuous subsidence during the rifting process. The difference in stratigraphic thickness between the two limbs reflects major complications in the central area, which appears not to have subsided at the same rate as in the limbs, and into which many of the subvolcanic intrusions were emplaced. Except for the capping Schroeder–Lutsen basalt sequence, no stratigraphic unit can be traced from one limb to the other; each limb has its own stratigraphic column. To aid in the correlation of intrusive and extrusive rock units throughout the Midcontinent Rift system, their paleomagnetic polarity has been used. Nearly all of the igneous and sedimentary rocks associated with the Midcontinent Rift were formed either during an earlier, reversed-polarity interval or a succeeding normal-polarity interval. Thus, in each limb of the North Shore Volcanic Group, the lower stratigraphic ILSG07 113 Trip 5 �units show reversed polarity, and the upper sequences show normal polarity. This polarity reversal forms the basis for distinguishing upper and lower sequences in the northeast and southwest limbs. U-Pb zircon dates demonstrate that the reversed-polarity magmatism occurred mainly in the time interval from 1108 to 1107 Ma; whereas, around the Lake Superior basin, normal-polarity magmatism occurred mainly in the interval from 1099 to 1094 Ma (for example Davis and Paces, 1990; Paces and Miller, 1993; Davis and Green, 1997). These two pulses were separated by a magmatically inactive time (at least in the upper crust), which appears to be expressed as a slight unconformity in the volcanic sequence on the north shore. However, because intrusions subsequently penetrated along this horizon in the North Shore Volcanic Group, this unconformity has not been recognized in outcrop. The Duluth Complex separates the upper and lower sequences of the southwestern limb (Fig. 5-2, Table 5-1). One other significant gap in the stratigraphic continuity of the North Shore Volcanic Group occurs near the stratigraphic top, where the Schroeder–Lutsen sequence overlies the upper units of the northeast and southwest sequences (Fig. 5-2, Tables 5-1 and 5-2). In the northeast limb, southwest of Grand Marais (Fig. 5-3), the Schroeder–Lutsen sequence unconformably overlies the Good Harbor Bay lavas, which are now recognized to include the Terrace Point basalt, as well as the Cut Face Creek Sandstone and the Good Harbor Bay andesites. The Terrace Point basalt contains a granite xenolith dated at 1096.7±0.9 Ma and thus must be younger than that. In the southwest limb, the Schroeder–Lutsen sequence overlies andesites and icelandites of the Onion River lavas (Boerboom and Green, 2006; Boerboom and others, 2006) as well as a sandstone and conglomerate unit (the Little Marais conglomerate) that in turn rests in sharp angular unconformity atop structurally disturbed flows of the Belmore Bay lavas near Little Marais. The Onion River lavas form a coherent package of dominantly intermediate-composition lavas that have only recently been mapped in detail, and their stratigraphic thickness is not taken into account in Table 5.1. They are located in approximately the same stratigraphic position as the Belmore Bay lavas, and may be continuous with them, but their relationship has been obscured by emplacement of the Beaver Bay Complex. Furthermore, the gently dipping Schroeder basalts have not been penetrated by the abundant hypabyssal intrusions of the Beaver Bay Complex that complicate the underlying volcanic sequence in this mid-shore area (Green, 1992; Miller and others, 1993). Attempts to date the Schroeder–Lutsen sequence have been unsuccessful to this point. The five major lithostratigraphic sequences comprising the two limbs of the North Shore Volcanic Group are further subdivided into informal lithostratigraphic units (Table 5-1 and 5-2). Some of these units are individual flows of distinctive lithology and/or substantial thickness and lateral extent. Most are suites of lava flows that have distinct lithologic characteristics or that are separated by intrusions. Some lava formations contain distinct flows or sedimentary rock units within an otherwise homogeneous package of lavas. Such units are given informal names (such as the Silver Beaver rhyolite within the Baptism River lavas, Manitou transitional basalt within the Schroeder basalts, and Indian Camp sandstone within the Lutsen basalts). See Chapter 1 of RI58 (Miller and others, 2002) for more details on stratigraphic nomenclature of Keweenwan rock in northeastern Minnesota. ILSG07 114 Trip 5 �Table 5-1. Generalized stratigraphy of the southwest limb of the North Shore Volcanic Group showing U/Pb ages (Davis and Green, 1997; Green and others, 2001). Positions of intrusions denote approximate stratigraphic level affected and not age of emplacement. Thickness(m) Lithostratigraphic units Lithologic character U/Pb age 9735 Total section basalt, andesite, and rhyolite flows 150 Carlton Quarry lavas (fault 1094.3±2.0 bounded) Schroeder–Lutsen sequence (normal polarity) 945 ophitic olivine tholeiite basalt flows; 900 Schroeder basalts <45 Little Marais conglomerate includes Manitou transitional basalt and Pork Bay breccia polymict volcanic conglomerate and sandstone angular unconformity 8275 Upper southwest sequence (normal polarity) 565 Bell Harbor lavas 100 Palisade Head rhyolite mostly quartz tholeiite basalt and basaltic andesite flows gray-pink, porphyritic rhyolite flow Beaver Bay Complex 700 Baptism River lavas 20 Silver Bay porphyritic basalt 730 Gooseberry River basalts 1096.6±1.7 ~1096 mixed lavas, mostly basalt; includes 165-meter-thick Silver Beaver rhyolite ophitic basalt flow with abundant large plagioclase phenocrysts mixed basalt flows, mostly ophitic Lafayette Bluff, Silver Creek diabase intrusions 315 Two Harbors basalts 550 Larsmont basalts mixed aphyric basalt flows; quartz tholeiite flows at base ophitic olivine tholeiite flows Stony Point–Knife Island diabase sheet 1500 Sucker River basalts mixed basalt flows, mostly ophitic 1350 Lakewood lavas mostly basalt flows; rhyolite, icelandite, and ferroandesite at base Lester River diabase sill 1285 mixed basalt, andesite, icelandite, and rhyolite flows Lakeside lavas 1098.4±1.9 Endion diabase sill 1160 Leif Erickson Park lavas ~1099 Duluth Complex 370 mixed basalts, andesites Lower southwest sequence (reversed polarity) 370 Ely's Peak basalts >8 Nopeming Sandstone porphyritic, diabasic, and ophitic basalts; pillowed and pyx-phyric basal flow white to tan quartzite and conglomerate angular unconformity Thomson Formation (Paleoproterozoic) ILSG07 115 Trip 5 �Table 5-2. Generalized stratigraphy of the northeast limb of the North Shore Volcanic Group showing U/Pb ages (Davis and Green, 1997; Green and others, 2001). Positions of intrusions denote approximate stratigraphic level affected and not age of emplacement. Thickness(m) ages 7359 325 Lithostratigraphic units Lithologic character U/Pb Total section Schroeder–Lutsen sequence (normal polarity) olivine tholeiite basalts, mostly Lutsen basalts ophitic; includes Indian Camp sandstone angular unconformity 3998 Upper northeast sequence (normal polarity) 130 Terrace Point basalt (within upper Good Harbor Bay andesites) 100 Cut Face Creek sandstone 131 Good Harbor Bay andesites ophitic, olivine tholeiite basalt younger than 1096.7±0.8 red, laminated, ripple-marked sandstone brown, porphyritic basaltic andesites Beaver Bay Complex – Beaver River diabase and Leveaux ferrodiorite brown, columnar-jointed basalt flow 122 Breakwater basalt 348 Grand Marais felsites pink to gray porphyritic rhyolite and felsite 335 Croftville basalts 250 70 Devil’s Track rhyolite Maple Hill rhyolite intergranular basalt and andesite flows, thick interflow sandstone aphyric, intergranular rhyolite flow porphyritic rhyolite flow 274 Red Cliff basalts 366 539 Kimball Creek rhyolite Marr Island lavas 198 235 900 Naniboujou basalts Devil’s Kettle rhyolite Brule River lavas ophitic olivine tholeiite flows, some plagioclase-phyric porphyritic rheoignimbrite mixed basalt, tholeiitic andesite, and icelandite flows intergranular basalt flows porphyritic lava flow interbedded basalt and rhyolite flows 1097.7±1.7 1100.2±1.9 Brule Lake-Hovland gabbro 3036 BASE 60 Lower northeast sequence (reversed polarity) 1932 Hovland lavas 67 92 945 Red Rock rhyolite Deronda Bay andesite Grand Portage lavas 60 Puckwunge Sandstone mostly plagioclase-phyric basalt flows, some rhyolite and andesite red/tan, porphyritic rhyolite tan/brown, porphyritic andesite basalt lava flows, pillowed at base 1107.7±1.9 1107.9±1.8 tan/white, cross-bedded quartz sandstone slight angular unconformity Rove Formation (Paleoproterozoic) ILSG07 116 Trip 5 �Physical Volcanology The volcanic rocks of the Midcontinent Rift, including the North Shore Volcanic Group, represent one of the world’s oldest and best-preserved examples of plateau lavas. However, they contain a greater thickness of flows and, in the North Shore Volcanic Group, a higher proportion of evolved compositions than typical plateau lavas. They are similar physically and chemically to the Tertiary lavas that make up eastern Iceland (Sigvaldason, 1974; Walker, 1974; Green, 1977; Wood, 1978), and they formed similarly over a plume at another major rift. The rocks also resemble the late Tertiary and Quaternary volcanic rocks of the southern Snake River Plain, Idaho and southeastern Oregon, because of their interbedded basalts and large rhyolites (for example Bonnichsen and Kauffman, 1987; Link and Hackett, 1988; Reidel and Hooper, 1989; Manley, 1996). The basalts range in character from typical flood flows as voluminous as tens of cubic kilometers to more modest, “plains-type” flows (Greeley, 1982) and thin flow units less than a meter thick. At Duluth in the southwest limb, and Grand Portage in the northeast limb, the lowest flows in the volcanic sequences are pillowed, and thus inferred to have erupted subaqueously; however, nearly all of the other flows were erupted subaerially. The flows show different physical characteristics, closely tied to their chemical compositions and viscosities (Green, 1977, 1989; Green and Fitz, 1993). Olivine tholeiites, which dominate the North Shore Volcanic Group, all have pahoehoe surfaces, with or without ropy structures. Other physical characteristics of the various rock types were previously discussed in the “Rock classification, recognition, and textures” section of this chapter. All of the flows ranging in composition from basalts to icelandites were erupted as lavas. The rhyolites are notable in their abundance relative to other plateau-lava sequences, their size (up to several hundred cubic kilometers), and extent (Green and Fitz, 1993). Several rhyolites show textural evidence of rheomorphic flow after eruption as ash-flow tuffs, though some were lavas. One of the largest, the Devil Track rhyolite in Cook County, which is as thick as 250 meters and can be traced for 40 kilometers along strike, has ambiguous features that make its mode of eruption difficult to discern; it may be a lava flow. Nearly all the icelandites and rhyolites show evidence of an unusually high temperature of eruption, such as magmatically crystallized groundmass tridymite. The evidently low viscosity of these large rhyolites is attributed to their high temperature, high iron and fluorine contents, and low oxidation state (Green and Fitz, 1993). Geochemistry and Chemostratigraphy The North Shore Volcanic Group constitutes a subalkalic, tholeiitic suite that ranges continuously from rather primitive olivine tholeiite to rhyolite, and shows a strong iron-enrichment trend (Fig. 5-3; also Basaltic Volcanism Study Project, 1981; Green, 1982; Brannon, 1984). However, relative abundances are strongly bimodal; basalts are greatly predominant, but rhyolites make up 10 to 25 percent of the section. The basalts show trace element and isotopic evidence of derivation mostly from a mantle plume (Nicholson and others, 1997), whereas most of the rhyolites include major contributions from partial melting of the Archean basement (Vervoort and Green, 1997). The most common basalt type, ophitic olivine tholeiite, is generally aluminum-rich (16 to 18 percent Al2O3). The most primitive flows have Mg numbers of about 0.65 to 0.68. The basal few flows in both limbs of the North Shore Volcanic Group have a unique geochemical and petrographic character. Typically they contain augite phenocrysts, are aluminum-poor, and are rich in both compatible (chromium and nickel) and incompatible elements (titanium, phosphorus, and lanthanum) with steep chondrite-normalized La/Yb ratios. This suggests derivation by relatively smallfraction melting of the initial plume head (Nicholson and others, 1991, 1997; Green, 1995). In general, there is little stratigraphic regularity of compositional change within the North Shore Volcanic Group, with the following exceptions. In the middle of the upper southwest sequence, there is a marked upward progression toward more primitive compositions through a 3.4-kilometer section from rhyolite east of the Lester River into a thick group of primitive olivine tholeiites in the Knife River–Two Harbors area (Brannon, 1984). This includes the Lakewood lavas, the Sucker River basalts, and the Larsmont basalts. In contrast, in the lower northeast sequence, the approximately 1-kilometer-thick basal ILSG07 117 Trip 5 �Grand Portage lavas progress upsection from basalt to increasingly evolved compositions, ending with Red Rock rhyolite (Green, 1995). As mentioned above, the Schroeder–Lutsen sequence, the youngest in the North Shore Volcanic Group, is composed almost entirely of olivine tholeiites. All of the North Shore Volcanic Group has been affected to some degree by hydrothermal/burial metamorphism. The more permeable (fractured, vesicular) tops and bases of the flows have undergone considerable mineralogical change (deposition of amygdule minerals, alteration of primary minerals), but in many cases the massive flow interiors are remarkably little-altered. Where alteration has approached equilibrium, mineral assemblages range from lower greenschist facies at the base of the North Shore Volcanic Group to zeolite facies at the top (Schmidt, 1993; Schmidt and Robinson, 1997). Interflow Sedimentary Rocks Clastic redbed strata occur at many horizons within the North Shore Volcanic Group (Jirsa, 1984). They are lenticular and range in thickness from a few centimeters to about 100 meters. As these rocks are relatively soft and erodable compared to the adjacent volcanic flows, they are mostly covered and are exposed only along actively eroding sites such as streambeds and the lakeshore. However several interflow sandstones have recently been recognized from water well cutting samples (Boerboom and others, 2007). Overall, the sedimentary rocks are predominantly red to brown, well sorted sandstone, with minor conglomerate, siltstone, and shale. Conglomerate beds are most abundant in the midshore area from Little Marais to Lutsen, either on or in close proximity to a prominent gravity low that is at a right angle to the North Shore. This gravity low is thought to reflect an uplifted crustal block (White, 1966), and intrusive rocks along this trend are known to contain large xenoliths of Archean crust (Boerboom, 1994). Although the relative timing and effects of uplift on the adjacent volcanic rocks are not fully understood, the association of interflow conglomeratic rocks with this structure implies that uplift may have been active during volcanism. Compositionally, these redbeds are mainly immature lithic arkose and feldspathic lithic arenite (see Fig. 5.3 in Miller and others, 2002). The angular to subrounded clasts are mainly plagioclase, mafic to felsic volcanic rock fragments, clinopyroxene, and Fe-Ti oxides; devitrified or replaced volcanic ash particles and shards are present in a few beds. Quartz is uncommon to absent. The framework grains have been variably cemented with hematite, calcite, prehnite, and a variety of zeolites, depending on the local hydrothermal/burial/contact metamorphic conditions. In some places hydrothermal minerals have replaced many or most of the clasts. A few of these redbed units have thicknesses in excess of 25 meters. These include a cross-bedded sandstone in Leif Erickson Park in Duluth (35 meters), which disconformably overlies an eroded basalt flow; the Little Marais conglomerate (and sandstone) exposed in the Manitou River area near Little Marais (as thick as 45 meters); the Indian Camp sandstone (68 meters) northeast of Lutsen; the Cut Face Creek sandstone southwest of Grand Marais (100 meters); and other newly recognized units to the north of and lower in the stratigraphy than the Cut Face Creek sandstone (See Boerboom – abstract volume for this meeting). The Cut Face Creek sandstone can be traced in outcrop for at least 4 kilometers along strike, and topographic and aeromagnetic data indicate the newly-recognized sandstones to the north extend for at least 10-15 kilometers along strike. Of these, the Little Marais conglomerate occurs at the base of the Schroeder–Lutsen basalt sequence. The Cut Face Creek sandstone underlies the Terrace Point basalt, which is now recognized to occur in the upper part of the Good Harbor Bay sequence (Boerboom and Green, 2007), and the sandstones intersected by well drilling to the north occur between basaltic lava flows tentatively grouped with the Croftville basalts. The sandstone in these units is typically planar- or cross-bedded, and some beds are ripple-marked or mud-cracked. The rocks are inferred to be dominantly fluvial, deposited by moderate-gradient, east- to southwest-flowing streams from sources nearly entirely within the subsiding Midcontinent rift basin (Jirsa, 1984). Many flow-top breccias of andesite and basalt with aa structure contain laminated red sandstone as a matrix because sand filtered down from the flow surface. Similarly, red, ILSG07 118 Trip 5 �laminated sandstone and siltstone form clastic dikes or crevice-fillings a few centimeters wide in the upper parts of some lava flows. GEOLOGY OF THE BEAVER BAY COMPLEX The Beaver Bay Complex (BBC) is a hypabyssal, multiple-intrusive igneous complex that was emplaced into the upper part of the NSVG over a 600-km2 area in northeastern Minnesota (Fig. 5-4). Much of this area was the focus of detailed bedrock mapping by the Minnesota Geological Survey between 1985 and 1994 (Miller, 1988; Miller and others, 1989, 1993, 1994; Boerboom and Miller, 1994). Recent and ongoing mapping in the eastern BBC (Albers, 2006; Boerboom and others, 2006) will better established the geology of this part of the complex. Three general areas of the BBC, southern, northern, and eastern, are distinguished on the basis of distinctive rock types and intrusion form (Miller and Chandler, 1997). The relationship of BBC intrusions to other subvolcanic intrusions within the NSVG (Fig. 5-4) is unclear, because of poor exposure to the southwest and insufficient mapping to the northeast. Within the mapped area of the BBC, thirteen intrusive units have been identified that represent at least six major intrusive events (Miller and Chandler, 1997). Most intrusive activity forming the BBC occurred around 1096 Ma based on U-Pb dates of 1095.8±1.2 Ma for a Silver Bay intrusion, the youngest unit of the BBC, and Figure 5-4: Geology of the southern and northern Beaver Bay Complex (after Miller and others, 2001). Units labels are: nsl - NSVG Schroeder basalts; nsb - NSVG basaltic volcanics; nsf - NSVG felsic volcanics; asa - anorthositic series of the Duluth Complex; wlg - Whitefish Lake granophyre; slid Shoepack Lake inclusion-rich diorite; ccp - Cabin Creek porphyritic diorite; hct - Houghtaling Creek trocolite; blg - Blesner Lake gabbro; llg - Lax Lake gabbro; fg - Finland granophyre and qtz ferromonzonite; slg - Sonju Lake troctolite and gabbro; lvp – Leveaux porphyritic ferrodiorite; brd Beaver River diabase; sbi - Silver Bay intrusions. See map M-119 for more details. Locations of field trip stops are shown by black dots with corresponding stop number in white. ILSG07 119 Trip 5 �1096.1±0.8 Ma for the Sonju Lake intrusion (Paces and Miller, 1993). Whether activity overlapped the main stage of Duluth Complex magmatism at 1099 Ma is unknown, because attempts to date the oldest component of the BBC were not successful (Paces and Miller, 1993). The boundary between the BBC and Duluth Complex is generally marked by a northeast-trending keel-shaped intrusion in the northern BBC (Houghtaling Creek troctolite – hct on Fig. 5-4) that separates largely dike and sill intrusions of the BBC to the southeast from massive granophyric granite and extensive areas of structurally complex gabbroic anorthosite to the northwest that are typical of the roof zone of the Duluth Complex. The range of BBC parent magma compositions is similar to the olivine tholeiite and transitional basalt compositions that dominate the NSVG (Fig. 5-5A). Moreover, like the NSVG, the sequence of intrusion of BBC magmas generally involved progressively more primitive compositions. Compositional variations within the various intrusive units developed as a result of in situ magmatic differentiation (Fig. 5-5B), assimilation of footwall rocks, and/or composite intrusions of evolved magma from deeper staging chambers (Fig. 5-5C). The tightly clustered trend of BBC parent magma compositions evident on an AFM diagram (Fig. 5-5A) and the systematic variation of other elemental abundances suggest that all mafic BBC magmas evolved from a common olivine tholeiitic primary magma type. Such a primary composition, which is approximated by the most primitive, high-Al olivine tholeiites of the NSVG, is thought to have given rise to most MCR magmas, especially in later stages of magmatism (Green, 1983; Figure 5-5: AFM diagrams of Beaver Bay Complex (BBC) intrusions. A) Plot of estimated parental magma compositions to various BBC intrusions (see Miller and Chandler, 1997 for unit descriptions and details) compared to major NSVG lava compositions: OT - olivine tholeiite, TB - transitional basalt, A - andesite, FA - ferroandesite, I - icelandite, R - rhyolite (after Green, 1983). NSVG-pot is a primitive NSVG olivine tholeiite composition. The composition of the Leveaux porphyry is indicated by the label – lp. B) Calculated liquid line of descent of the Sonju Lake intrusion through troctolitic (slt), gabbroic (slg-slmd) and monzodiorite (slmd) intervals of the layered sequence. Also plotted are whole rock compositions of Finland granite (frg) and quartz ferromonzodiorite (frpm). C) Whole rock composition plots of Beaver River diabase (brd; ophitic margins and coarse subophitic interiors distinguished), Silver Bay intrusions (sbi; coarse marginal facies, layered ferrogabbroic cumulate interiors, and granophyric compositions distinguished), and composite intrusions from the northern BBC (nbbc). Sonju Lake intrusion differentiation trend (dashed line) is also shown. ILSG07 120 Trip 5 �Miller and Weiblen, 1990; Klewin and Shirey, 1992). That even the most primitive of the BBC intrusions, the Beaver River diabase, is significantly evolved from a primitive olivine tholeiite composition (Fig. 5.5C) indicates that all BBC parent magmas were generated in turn by magmatic differentiation of such a primary composition in deeper staging chambers. Petrologic modeling of some BBC intrusions and other hypabyssal bodies that intruded the NSVG (Jerde, 1991) suggests that most magmas experienced multistage, polybaric fractionation between their extraction from the mantle and their subvolcanic emplacement. Although the available radiometric ages do not indicate an overlap of magmatic activity between the BBC and the Duluth Complex (Paces and Miller, 1993), additional dates and more detailed petrologic studies may show that some Duluth Complex intrusions acted as the final levels of staging and differentiation of some BBC-bound magmas. The focus of emplacement of BBC intrusions appears to have migrated toward the rift axis and toward higher stratigraphic levels with time, perhaps reflecting plate drift and thickening of the volcanic pile. Over the exposed extent of the BBC, intrusion shapes appear to have been controlled by a shallow crustal ridge which trends northwest across the BBC. The presence of this buried crustal ridge is indicated by a pronounced saddle in the gravity high over northeastern Minnesota (Chandler, 1990) and the presence of Archean granitic gneiss, biotite schist, metagraywacke and granodiorite inclusions in early intrusions over the gravity low (Boerboom, 1994). The broad network of dikes and sheets characterizing the southern BBC becomes tightly focused into a narrow zone of subparallel dikes in the northern BBC which is situated over the gravity minimum (Fig.5-4). The eastern BBC opens up again into thick sheet intrusions. Based on geologic, geochronologic, geophysical, and geochemical evidence, Miller and Chandler (1997) suggested that the BBC, particularly the youngest Beaver River diabase dike and sheet network, acted as a magma conduit and structural boundary to the formation and infilling of the western end of the Portage Lake Volcanic basin during the main to late stages of rift volcanism and graben formation. On the North Shore, these volcanic rocks are represented by the Schroeder-Lutsen basalts (Fig. 5-4). For more detailed information on these and other units of the Beaver Bay Complex see reports by Miller and Chandler (1997) and Miller and others (2002, Chapter 7) and the 1:24,000 bedrock geologic maps of the area (Miller, 1988; Miller and others, 1989, 1993a, 1994; Boerboom and Miller, 1994; Boerboom and others, 2006) This trip will have stops in the Beaver River diabase and the Leveaux porphyritic ferrodiorite. The Beaver River diabase typically forms prominent, lumpy topographic highs due in part to the large, resistant inclusions of anorthosite contained in it (e.g. Carlton Peak, stop 4B). It occurs as a series of interconnected set of dikes and sills composed predominantly of ophitic olivine gabbro. The northern and western limit is a dike that is sharply defined by outcrop and aeromagnetic data, and the area below this arc is a series of large, bifurcating dikes and thick, gently southeast-dipping sheets that are likely erosional remnants of a former semi-continuous sill (Figure 5-4). The upper and lower margins of the larger diabase sheets commonly contain inclusions of nearly pure anorthosite such as at Carlton Peak, and less commonly, large inclusions of granophyric granite. The Leveaux porphyritic diorite (LPD) is a semi-continuous, hypabyssal sill, at least 90 meters thick, that intruded into upper NSVG sequence rocks and was later intruded by the Beaver River diabase (Albers, 2006). The LPD makes up five prominent, cuesta-like ridges that trend parallel to Lake Superior over a 20 km distance and numerous smaller knob-like outcrops that occur as large inclusions within the younger Beaver River diabase. Examples of the Leveaux cuestas are Moose and Eagle Mountains, which can be seen uphill from the Lutsen Resort where this meeting is being held. Recent detailed bedrock geologic mapping by Albers (2006) and Boerboom and others (2006) has subdivided the LPD into four units: the Upper Contact Zone, the Porphyritic Zone, the Aphyric Zone, and the Sparsely Porphyritic Unit. The Upper Contact Zone is a thin (~10m), sparsely porphyritic (~5%), moderately vesicular, and locally amygdaloidal fine-grained ferromonzodiorite that forms the upper part of the LPD sill and is often not exposed due to erosion. The Porphyritic Zone is a thick (40-60m), fine- to ILSG07 121 Trip 5 �medium fine-grained ferromonzodiorite with 30-40% plagioclase megacrysts that composes the midsection of the LPD sill. The Aphyric Zone is a thin (4-19m), fine-grained ferromonzodiorite with rare plagioclase megacrysts (<3%) that occurs at the base of the sill. The Porphyritic-Aphyric Transition (PAT) is a gradational interval that separates the porphyritic and aphyric zones over a thickness of 10-40 cm. The Sparsely Porphyritic Ferrodiorite unit, which contains 5-20 % plagioclase megacrysts, occurs as outliers to the main LPD ridge. and may represent feeder dikes the LPD sill. The orientation of the PAT and sheet joints from the aphyric section indicate a gentle (10-20 degrees) southeast dip. Stops 6 and 9 will examine the porphyritic and upper contact zones of the Leveaux ferrodiorite. FIELD TRIP 5 STOP DESCRIPTIONS The general location of all stops are shown on Figure 5-6. A more detailed location map is included with each stop description, showing a portion of the appropriate 1:24,000 quadrangle map. Also listed are UTM coordinates (nad ’83), township-range-section, and the 7.5’ quadrangle name. Figure 5-6. Simple map showing relative locations of field trip stops, listed below. Stop 1. Kennedy Landing/Bell Harbor – conglomerate below SLB Stop 2 Caribou River - Pork Bay breccia and overlying basalt Stop 3. Sugar Loaf Cove – Schroeder-Lutsen sequence basalt Stop 4. Carlton Peak – Rhyolite and anorthosite Stop 5. Tofte Town Park – Schroeder-Lutsen sequence Stop 6. Springdale Hill – Porphyritic phases of Leveaux diorite Stop 7. Base of Schroeder-Lutsen sequence Stop 8. Onion River – Good Harbor Bay andesites Stop 9. Oberg Mountain -Leveaux porphyritic diorite and shoreline view Stop 10. Conglomerate Stop 11. Cut Face Creek – Terrace Point basalt, sandstone, and Good Harbor Bay andesite ILSG07 122 Trip 5 �---Leaving Lutsen Resort, head southwest on Hwy 61. Proceed approximately 26.5 miles to overpass over prominent creek. We will take the creek bed down to the shore and return to the road by private driveway. This is private property, so permission to access the shore is required. .--- Stop 5-1. Unconformity Conglomerate at Kennedy Landing/Bell Harbor. UTM coordinates (nad ’83) 640454E, 5274226N T.57N., R.6W., Sec.30, Finland quadrangle Highlights: ophitic basalt (Schroeder-type), polymict conglomerate, angular unconformity, felsic lithic tuff, agate-bearing quartz tholeiite flow, normal faults, interflow basalt clast conglomerates. U, 0 t z p.-, Stop 1 Description: From the cliff face at the west end of this bay, several volcanic and structural features are visible as noted in Figure 5-7. Exposed in the cliff face is an ophitic basalt which is the basal flow of the Schroeder-Lutsen sequence (OB). Underlying this basalt is a polymict volcanogenic conglomerate (PCg). Various types of basalt dominate this conglomerate, but it also contains clasts of felsite, granophyre, lithic tuff, and autolithic . Figure 5-7. Plan view of the geology exposed at Stop 5-1. Unit abbreviations are: OB – ophitic basalt, PCg – polymict conglomerate, B1-3 – basalt flows, FLT – felsic lithic tuff, BC1-2 – basalt clast conglomerate. Inset show fault relationships between OB and PCg units in western cove. ILSG07 123 Trip 5 �shale fragments. In the back wall of the cove between the ophitic basalt ledge and the point of conglomerate, two splayed faults are exposed (Fig. 5-7, inset). On the northeast side of the point of polymict conglomerate is in sharp angular contact with a thin sheet of basaltic andesite (B1) and and underlying a felsic lithic tuff (FLT) with imbricated clasts of porphyritic rhyolite. The contact between the polymict conglomerate and the basalt/tuff appears to be an unconformable erosional surface, but a fault contact cannot be discounted. . Continuing northeast, this basalt/lithic tuff sequence also abruptly give way across a small gap (fault?) to a basalt clast conglomerate (BC1). Near the outlet of a stream, the BC1 conglomerate grades downsection to a basalt breccia flow top of quartz tholeiite basalt flow (B2). This flow is colored a deep army green and contains abundant vugs and amygdules of agate and amethyst. Northeast of the point formed by flow B2, another basalt clast conglomerate (BC2) caps the upper part of the next underlying lava flow (B3). Discussion: The polymict conglomerate, which is termed the Little Marais conglomerate (Miller et al., 1993; Miller et al., 2006), is interpreted to occupy an angular unconformity that separates the SchroederLutsen sequence from the Upper Southwestern sequence of the NSVG. It can be traced at least 10 km to the northeast into the Little Marais quadrangle. Farther to the east, this unconformity develops a conformable relationship with the underlying volcanics and is marked by a thick sandstone unit (Cutface Creek Sandstone) (see Stop 5-11). ---Return northeast on Highway 61. Approximately 7.5 miles after crossing the Caribou River, turn left into parking area. Take the Superior Hiking trail path upstream about 300m to low-lying outcrops on the rivers edge .--- Stop 5-2. Pork Bay Breccia and Manitou River basalt. UTM coordinates (nad ’83) Area A-648250E, 5258850N; Area B-648480E, 5258950N T.58N., R.6W., Sec. 36, Little Marais quadrangle Highlights: Volcaniclastic breccia (Pork Bay breccia), thin olivine tholeiite flows, transitional basalt (Manitou River basalt) A Description: Location A- Exposed in low-lying outcrops on the south bank of the river is a brick red, polymict, matrix-supported, volcaniclastic breccia – the Pork Bay breccia. It is composed of subangular to subrounded volcanogenic clasts up to 40 cm across in an unbedded, sandy matrix (Fig. 5-8A). Clasts are composed dominantly of massive to scoriaceous and oxidized to fresh basalt and minor amounts (<5%) of other riftrelated rock types (e.g., gabbro, felsite, volcanic sandstone). The matrix consists of granule to medium sand-sized volcanic fragments cemented by calcite and zeolite. B Stop 2 Location B – Progressing upstream about 200 m to where the river make a sharp turn to the left (north), units overlying the Pork Bay Breccia are exposed (Fig. 5.8B). In the streambed are several thin (<1m) amygdaloidal olivine tholeiite flows oriented N50E/~20°SE. Exposures of volcaniclastic breccia occur ILSG07 124 Trip 5 �adjacent to the thin basalts, but contacts are not exposed. At the base of the cliff face on the southern shore are poor exposures of the breccia which presumably overlie the thin basalts. The overlying this breccia and exposed over most of the 8m-high cliff face is a massive, slightly Pl-porphyritic transitional basalt – the Manitou River basalt. At the basal contact, this flow is strongly chilled possibly irregular pockets of glass. We will not have time to continue upstream to Caribou Falls, however, a later visit is encouraged. The falls is interpreted to represent a major fault scarp that juxtaposes the Pork Bay breccia (downstream) with olivine tholeiitic basalts of the Schroeder-Lutsen sequence. Despite ample exposure in the falls area, a well-identified fault zone could not be identified leaving open the possibility that the contact may be steep lithologic contact rather than a structural one. MRB PBB OT Figure 5-8. A) Close-up photo of the polymict clasts in the Pork Bay breccia; whitish interstitial mineral is mostly calcite. B) Exposures in southern stream bank at Location B – OT – thin olivine tholeiite flows in streambed, PPB – breccia exposed in lower cliff face, MRB – massive, transitional basalt exposed in upper cliff face. Discussion: The origin of the Pork Bay Breccia is enigmatic. Various ideas put forth to explain its formation include that it is a lahar or some type of mass wasting deposit, that it represents a collapse breccia within a caldera, and that it represents deposits of an explosive cinder cone. None of these completely explain the salient attributes of the unit or its relationship with adjacent units. These attributes include: the wide variety of volcanic, intrusive and sedimentary clast types, the subrounded nature of the clasts, the porous sandy matrix, the lack of bedding, the thickness of the breccia (up to 60 m), the occurrence of variable thicknesses of tholeiitic basalts (2- 100m) between the breccia and the Manitou River basalt, and the occurrence of basalt flows within the upper parts of the breccia (as seen at location B). Given these attributes, we will discuss possible modes of origin on the outcrop. ---Continue northeast on Highway 61. Approximately 2.7 miles from the Caribou River, turn right into Sugar Loaf Cove Interpretive Center (look for a brown highway sign). Park in the lot and follow trails down to Lake Superior shore.--- ILSG07 125 Trip 5 �Stop 5-3. Olivine tholeiitic basalt flows at Sugarloaf Cove. UTM coordinates (nad ’83) 652044E, 5261120N T.58N., R.5W., Sec. 29, Little Marais quadrangle No hammers please! Note: This stop is on the property of the Sugarloaf Cove Interpretive Center Association and a State of Minnesota Scientific and Natural Area. It has been established to preserve the natural features of the beach and point for all to enjoy. Please leave your hammers behind and take only pictures, not samples. Highlights: Morphological features of olivine tholeiitic basalt flows. Description: The lava flows exposed at and near this stop are typical of the Schroeder-Lutsen basalts. Flows exhibit features characteristic of fluid, low-viscosity pahoehoe olivine tholeiites (Fig. 5-9). Columnar joints and sandstone-filled clastic dikes (sand-filled cracks) are common in the upper part of the massive flow interiors. Figure 5-9. Idealized internal structure of olivine tholeiite-composition lava flows. From Green, 1989. Six lava flows are exposed on the point (Fig 5-10A). The about 10 m of the upper part of flow 1, an ophitic basalt, is exposed on the southwest end of the point. Flows 2 and 3 are 1- to 2-meters thick and display bent pipe amygdules and oxidized ropey flow tops (Fig. 5-10B). Flow 4 is 2-3 meters thick and displays well developed columnar joints, ropey flow tops, and abundant clastic dikes at its eastern extent. Flows 5 and 6 define a trough structure atop flow 4 in the midsection of the point. At the northeast end, flow 5 occurs as two thin (~1m) flows and over 5 meters of ophitic basalt of represent the lower part of flow 6. ILSG07 126 Trip 5 �A. B. 4 3 35 2 Figure 5-10. A) Sketch of distribution and orientation of six flows on Sugarloaf Cove Point. B) Photo of thin lava flow 2-4 at the west end of the point. Discussion: These olivine tholeiite basalt flows are typical of the Schroeder-Lutsen sequence, the uppermost sequence of the North Shore Volcanic Group. In this area the Schroeder-Lutsen basalts unconformably overlie a thick sequence of mafic to felsic volcanic rocks, informally termed the Cross River lavas (Boerboom and Green, 2006), that strike approximately N55E and dip 10-20° SE, in contrast with the nearly shore-parallel strike of the Schroeder-Lutsen basalts (see Fig. 5-15, stop 7). The occurrence of these olivine tholeiite flows at the top of the NSVG highlights the fact that the NSVG generally becomes more primitive (less evolved) up section. ---Return to Highway 61 and drive NE approximately 7 miles to Temperance River. Turn left on small gravel road 0.8 miles NE of Temperance River across from National Forest Service Sign. Drive 0.95 miles up hill, over outcrops in road bed (Schroeder basalt) and park; Stop 4A outcrops just east of road.-- Stop 5-4A. Carlton Peak rhyolites. UTM coordinates (nad ’83) 660737E, 5271317N T.59N., R.4W., Sec. 20, Tofte quadrangle Highlights: Aphyric and porphyritic rhyolite flows. Description: There are several low outcrops in this area of rhyolite (at the south edge) and porphyritic rhyolite (at the north edge). There are also outcrops of andesite to the northeast of the rhyolite, and of ophitic basalt to the north of that. The porphyritic rhyolite is grayish-pink, with 58% phenocrysts of K-feldspar, quartz, oxidized iron silicate minerals, and magnetite. The aphyric rhyolite is light gray to purplish-red, with rare small phenocrysts of quartz, K-feldspar, altered mafic silicates, and magnetite. Small vuggy quartz-lined amygdules and tension gashes are common throughout. Discussion: The rhyolites here are part of the Carlton quarry sequence, which includes ophitic basalt at the top, ILSG07 127 Trip 5 �andesite, porphyritic rhyolite, and aphyric rhyolite at the base. This sequence has been intruded by the Beaver River diabase (stop 4B). This entire set of flows dips moderately northwest, and is presumably fault-bounded. The porphyritic rhyolite has an age of 1,094±2.0 Ma (Green and others, 2001), the youngest age obtained to date from Keweenawan rocks in Minnesota, and is comparable in age and stratigraphic position to rhyolite in the Porcupine Volcanics in Michigan and to the upper part of the Portage Lake Volcanics (Green, 2002 and references therein). The position of the Carlton Quarry lavas within the NSVG is somewhat enigmatic, given the age, and fortuitous gaps in outcrop preclude full understanding of the relationships of these lavas to the rest of the NSVG. ---Continue on up the gravel road to a large flat area below the main quarry face, or turn left on the way and wind your way up to the quarry wall. If you go up there, BE CAREFUL!--- Stop 5-4B. Carlton Peak anorthosite and Beaver River diabase. UTM coordinates (nad ’83) 661309E, 5272060N T.59N., R.4W., Sec. 20, Tofte quadrangle Highlights: Carlton Peak anorthosite, Beaver River diabase (BRD). Description: In the quarry staging area are multiple large blocks of pale green to locally purple, coarsegrained anorthosite, and blocks of weathered, dark green, Beaver River diabase. The anorthosite contains scattered 2-3 cm diameter dark clots of altered poikilitic olivine, but otherwise is composed of nearly 100% fresh, glassy plagioclase. The anorthosite is well exposed in natural outcrops at the top of Carlton Peak, which requires a moderately strenuous hike of approximately one mile on the Superior Hiking Trail, accessed by driving 2.8 miles north of Tofte on County Highway 2 (Caribou Trail) to the well-marked Superior Hiking Trail parking lot. An alternative short hike to the top of Britton Peak, located just east of and above the parking lot, leads to an inclusion of the same type of anorthosite, and a clear view of Carlton Peak. Other places on the shore where these anorthosite inclusions can be easily seen are at Split Rock Lighthouse State Park (see guidebooks by Miller and others, 1987, Stop 1 and Boerboom and others, 2004, Stop 2-8), and Tettegouche State Park. The easiest place of all is a newly-constructed road cut on Highway 61 in Silver Bay, which has clean exposures of anorthosite inclusions within the BRD. Discussion: Carlton Peak is a prominent topographic knob formed by the Carlton Peak anorthosite, which occurs as large inclusions in the BRD. Here and at many other places along the North Shore, anorthosite forms rounded, bare knobs because it is very massive, contains few fractures, and is more resistant to chemical weathering than the surrounding diabase (Fig. 5-11). Individual anorthosite inclusions at Carlton Peak range up to 1100 feet in diameter, and are among the largest of the known anorthosite inclusions in the BRD. The anorthosite inclusions throughout the BRD are round to angular in shape and vary in size from individual plagioclase crystals to large inclusions such as at Carlton Peak, and some are brecciated and recrystallized (Morrison and others, 1983). They are all composed of 90-99% course-grained bytownite to labradorite, and minor Mg-rich olivine, orthopyroxene, and less commonly clinopyroxene. The inclusions are most common near the bottoms of BRD sills, and in sharp contact with the BRD, but the BRD is not chilled against them. Anorthosite inclusions of this type are different from anorthositic-series rocks of the Duluth Complex, but there are similar inclusions of anorthosite within the anorthositic series. The highly disordered structural state of plagioclase and the absence of chilled margins in the BRD against the anorthosite indicate these inclusions were derived from a lower to mid-crustal source (Miller and Weiblen (1990). ILSG07 128 Trip 5 �Figure 5-11. Google earth oblique image of Carlton Peak anorthosite, looking northeast up the shore. From this perspective it appears to be a large slab-shaped inclusion. Isotopic and trace-element compositions of the anorthosite xenoliths imply that they may be preKeweenawan in age (Morrison and others, 1983), but the data are ambiguous. As an alternative, Miller and Weiblen (1990) suggest a crystal-mush theory in which significant proportions of Keweenawan anorthosite may have been generated by flotation of plagioclase under high pressure in the deep crust prior to Beaver Bay Complex magmatism at 1,096 Ma. Plagioclase cumulates formed under deep crustal conditions would probably be distinctive in character and composition, compared to anorthosite cumulates formed in the shallow crust. The ambiguous isotopic compositions of the inclusions may indicate that the anorthosites formed from Keweenawan magmas that were contaminated by older crust, rather than older anorthosites being contaminated by interaction with Keweenawan magmas. The Beaver River diabase (BRD) forms an extensive network of dikes and sills, and is one of the youngest intrusive phases of the Beaver Bay Complex. The BRD extends from Split Rock Point to just west of Grand Marais, in a large arc that is approximately 57 miles (90 km) long (Figure 5-4). Most of the diabase is fine-grained ophitic olivine gabbro, but the centers of some of the thicker sills (up to 150 meters) grade into coarse-grained subophitic to intergranular gabbro. Historic sidelight: In 1902, anorthosite from an inclusion similar to this, in present-day Split Rock Lighthouse State Park, was quarried by a group who mistakenly thought the anorthosite was corundum. In fact there is actually a high point of land on Lake Superior in the park, held up by anorthosite, known as Corundum Point. Another quarry site was near the Baptism River at Illgen City. Their plan was to sell the “corundum’ on the east coast, but they had to abandon that plan and turn to other ventures. This company went on to become the Minnesota Mining and Manufacturing Company, now simply 3M! ---Drive back to Highway 61 and head northeast towards Tofte for 0.8 miles, then veer right onto Tofte Park Road (24). Take first private drive to the right and head to shore OR continue to Tofte town park and boat access on right after about ¼ mile. From boat landing, head southwest about 1 km along semicontinuous outcrop of tholeiitic lava flows (~7 flows). Stop is 250m southwest of old boathouse.--- ILSG07 129 Trip 5 �Stop 5-5. Flow features and feeder dikes in Schroeder basalts UTM coordinates (nad ’83) 661800E, 5270200N T.59N., R.4W., Sec. 28, Tofte quadrangle Highlights: Ophitic olivine tholeiite basalt flows, columnar jointing, amygdaloidal feeder dikes. Description: Accessing the shore at the old boathouse (662000E, 52700350N), a smooth billowing flow contact dipping gently to the northeast is exposed at the boat ramp. Heading southwest about 30 meters, a north-trending 0.5m-wide feeder dike is exposed in the amygdaloidal flow. The dike is zoned in the degree of oxidation and in the concentration of vesicles and amygdules (Fig. 512A). Boat Landing Boat House Stop 5 Continuing about 130 meters to the southwest over the polygonally-jointed surface of an ophitic basalt flow, a domal flow contact with a thin flow is encountered. Passing across this dome back onto the ophitic basalt, one can see a feeder dike breaking through the amygdaloidal top of the underlying flow and feeding the ophitic basalt. The feeder dike is lined with pipe amygdules that are curved upward (Fig. 5-12B). A . B .. Figure 5-12. A) Feeder dike with zonation in oxidation and amygdule concentration. B) Feeder dike with upturned pipe amygdules. ILSG07 130 Trip 5 �---Continue northeast on Tofte Park Road to Highway 61. Turn right (northeast) and go three miles to Leveaux Ridge road on the left, across from the Chateau Leveaux. Go up Leveaux Ridge Road a short distance (~0.25 mi)., then turn right on Overlook Trail. Head uphill(~0.3 mi), park where road curves to west near driveway on right (21 Overlook Tr.). --- Stop 5-6. Porphyritic Phases of Leveaux Porphyritic Diorite (LPD) UTM coordinates (nad ’83) 665850E, 5274260N T.59N., R.4W., Sec. 14, Tofte quadrangle andesite Stop 6 - :i-çl " - H- ' -- Schroeder- Highlights: Amygdaloidal olivine tholeiite basalt of Schroeder Lutsen sequence, inferred fault zone, amygdaloidal, sparely porphyritic upper zone and porphyritic zones of the Leveaux porphyritic sill. Description: Exposures in a low ledge and the drainage ditch south of the driveway are moderately amygdaloidal intergranular basaltic andesite. Several mineralized northeast-trending shear zones cut the rock. On the opposite side of the driveway is a 5’ roadcut of sparsely plagioclase porphyritic and amygdaloidal, fine-grained ferrodiorite. Centimeter-sized plagioclase phenocrysts compose about 5% of the rock as does a similar abundance of quartz amygdules (Fig. 5-13A). This rock type continues along this roadcut to the next driveway (#33). Opposite the bridge across a substantial creek are pavement outcrops of strongly porphyritic (30-40%) ferrodiorite which are typical of the main Porphyritic Zone of the LPD sill (Fig. 5-13B) Figure 5-13. A) Outcrop appearance of sparsely porphyritic and amygdaloidal ferrodiorite typical of the Upper Zone of the Leveaux ferrodiorite. B) Outcrop appearance of strongly porphyritic ferrodiorite typical of the main Porphyritic Zone of the Leveaux ferrodiorite. Discussion: This exposure of the Leveaux porphyritic diorite sill is the only location where the upper contact zone is preserved. Along most of the cuesta mountain exposures, the upper part of the sill is ILSG07 131 Trip 5 �eroded into the porphyritic zone (see Stop 8). The vesicular/amygdaloidal character of the upper contact zone speaks to the shallow level of emplacement of the LPD into the volcanic edifice. The sparsely porphyritic nature of this zone is interpreted by Albers (2006) to have been caused by flow differentiation of phenocrysts out of the upper contact zone during emplacement and subsequent rapid crystallization. Also, this inferred fault is a major fault zone which can be traced for 10’s of kilometers subparallel to the shore. ---Go back to Highway 61 and continue NE 0.16 miles to roadcuts on uphill side of Highway. Park at edge of highway. WATCH OUT FOR HEAVY TRAFFIC!. --- Stop 5-7. Base of Schroeder-Lutsen Sequence and top of underlying andesites UTM coordinates (nad ’83) 666381E, 5274345N T.59N., R.4W., Sec. 13 Tofte quadrangle Highlights: Contact between base of Schroeder-Lutsen basalt sequence and underlying Good Harbor Bay andesites. This stop will be at a roadcut, but if permission can be obtained a better outcrop may be visited down on the lake shore. The description below is of the lakeshore outcrop, but most of the features in it can be see on the roadcut. Description: Upper part of andesites is composed of amygdaloidal andesite fragments up to 50 cm in size, surrounded by reddish-brown sandstone. The andesite clasts are angular, and vary in shape from equant to slab-like; the latter commonly oriented near vertical. Sand makes up 3040% of the volume of the upper 3 meters of the breccia, and the breccia is capped by a cross-bedded sandstone wedge that is cut off by erosion on a cliff corner (Fig. 5-14). Figure 5-14. Photographs of base of Schroeder basalt and underlying fragmental andesite, from outcrop on shoreline below stop 7. Hammer is approximately 45 cm long. ILSG07 132 Trip 5 �The base of the Schroeder basalt flow undulates slightly and contains pipe amygdules. In places, if the light is right, one can make out vague, bulbous lobe-like forms approximately 30 cm in diameter at the base of the flow. Discussion: There are two more exposures between here and Lutsen similar to this one, all characterized by large, boulder-sized and blocky amygdaloidal andesite clasts surrounded by sandstone, with a thin, discontinuous sandstone cap. These are also similar to andesite exposed beneath the Little Marais conglomerate, approximately 15 miles to the southwest (Fig. 5-15; Miller and others, 2006). The Little Marais Conglomerate is at least 60 feet thick, and composed mostly of sandstone, but contains some silt/shale beds, as well as large rounded boulders of basalt near the base and large angular blocks of basalt higher in the section that are ‘floating’ in sandstone. In all cases the underlying fragmental andesites are unusually thick, with substantially larger clasts than typical andesitic flow-top breccias. Despite the fact that the Schroeder-Lutsen sequence is clearly unconformable to the underlying rocks, as made evident by the discordant strike directions between the two (Fig. 5-15), the andesite flow tops below are all fragmental, at least in the available exposures. The upper surface of the Good Harbor Bay andesites may have been a slightly eroded surface made up of clinkery lava flows with an irregular topography, covered by sedimentary rocks of variable thickness. Local fault escarpments that formed prior to eruption of the Schroeder-Lutsen basalts may account for the unusually thick Little Marais conglomerate, with blocks of basalt fallen off the fault escarpment into the sedimentary pile. Figure 5-15. Simplified geologic map of a strip along Lake Superior from near Little Marais to Lutsen showing locations where unconformity beneath the SchroederLutsen basalts is exposed. ILSG07 133 Trip 5 �---DIRECTIONS: Continue northeast on Highway 61 about 0.7 miles. Just past the Onion River, pull into the Ray Berglund State Scenic Memorial Wayside. Park there and walk SW along highway to Onion River. An excerpt from MN Dept. of Transportation website: “The Onion River derived its name from a Paul Bunyan legend. The legend says that wild onions grew in such abundance in the area, that Paul Bunyan's loggers shed tears while cutting lumber”.--- Stop 5-8. Morphology of andesite lava flows in the Onion River valley. UTM coordinates (nad ’83) 667525E, 5275197N T.59N., R.4W., Sec. 12 Tofte quadrangle *The extent to which these flows can be seen is dependent upon the water level in the Onion River. If the river is high, some outcrops can be accessed on the highway and on the shoreline near the river mouth. Highlights: Andesite flow features, Terrace Point basalt, minor faults. Description: This stop will go through a multiple series of andesitic lava flows which exhibit classic flow features (Fig. 516). The andesite is porphyritic (plagioclase, clinopyroxene, some magnetite), with a matrix of fine felty plagioclase, interstitial K-feldspar, granular clinopyroxene, and blocky Fe-Ti oxides. A sand-filled flow-top breccia that caps the first flow in the river series is exposed on the roadcut just SW of the Onion River and on the shore at the river mouth. The base of the same flow, which exhibits classic andesite oxidation-lamination texture, from which the attitude of the flow can be reliably measured, is exposed in the river bed below the highway. A 3 meter wide brittle fault (attitude N45E, 85°N), with reverse sense of movement (north side down approximately 2 meters) is exposed about 200 feet north of the highway on the east bank. The fault zone contains brecciated andesite fragments surrounded by coarse white calcite and pinkish-orange laumontite. This fault projects back southwest across the river to a till-covered portion of the slope. Next to the fault, upstream, is another thick sand-filled flow-top breccia at the top of the next underlying flow. Proceed upstream past a total of five andesite flows. The sixth flow is the western end of the Terrace Point basalt flow (see also Stop 11 and Fig. 5-18). Although the top of the flow is fragmental similar to the andesite flows downstream, it is internally ophitic (very rare for an NSVG andesite), and contains rare fresh and glassy, blocky plagioclase phenocrysts. The next flow below this ophitic basalt is probably icelandite in composition, but lacking chemistry has been lumped with the andesites. After a series of andesite and icelandite flows, the river makes a sharp bend to the east where it intersects a prominent fault marked with slickenside surfaces that forms a high escarpment on the south side of the river. Above this is a narrow canyon that can be traversed in late summer when the water level is lowest, through a series of thin, irregular olivine tholeiite basalt flows. Traverse upstream until a snowmobile trail bridge is encountered; there is a small trail along the east side of the river that goes south from the snowmobile trail back to the parking area. Discussion: These flows were original termed the Onion River lavas by Boerboom and others (2006), with all the lava flows from Highway 61 to beyond the snowmobile bridge lumped together. Subsequent mapping to the east (Boerboom and others, 2007) has shown that the Onion River lavas are actually the western end of the Good Harbor Bay andesites. To the east the Terrace Point basalt is underlain by the Cut Face Creek sandstone, but that apparently pinches out and is no longer present here. ILSG07 134 Trip 5 �Figure 5-16. Idealized flow structure of quartz tholeiite to andesitecomposition lava flows. From Green, 1989. Figure 5-17. Geologic map of the Onion River lavas. ---Continue 0.4 miles northeast on Highway 61 to the Onion River Road. Take the Onion R. Rd 1.7 miles to parking area for the Superior Hiking Trail. Take Oberg Mtn trail about 0.6mi (~1000m) to the southwest overlook . --ILSG07 135 Trip 5 �Stop 5-9. Porphyritic Zone of Leveaux Porphyritic Diorite (optional stop) UTM coordinates (nad ’83) 666960E, 5276900N T.59N., R.4W., Sec. 1 Tofte and Honeymoon quadrangles Highlights: Porphyritic Zone of the LPD, vista of Leveaux Mountain and shoreline. Description: At the southwestern crest of Oberg Mtn, exposures of typical porphyritic ferrodiorite of the upper Porphyritic Zone of the LPD are exposed. The weathering of the outcrop accentuates the porphyritic texture of the white plagioclase against a brownish matrix. From this vantage point, the southeast-dipping sill can be seen in the topographic expression of Leveaux Mountain visible across the Onion River valley to the southwest. Farther southwest just off shore, Gull and Bear Islands can be seen (on a clear day) near the Taconite Harbor power plant, about 10 miles away. These islands are formed by outliers of LPD and represent the southernmost exposures of the Leveaux porphyritic diorite sill. ---DIRECTION: Continue east on Highway 61 for about 2.5 miles to the Poplar River. 600 feet past the Poplar River, veer right onto County Road 35 (gravel). Drive east on this road 1 mile and park at pull off where road joins back onto Highway 61. Walk straight down to lake through the woods to small cove.--- Stop 5-10. Interflow conglomerate. UTM coordinates (nad ’83) 673942E, 5279298N T.63N., R.3W., Sec. 24 Lutsen quadrangle Highlights: Interflow conglomerate, flow contact. Description: Small, 2 meter thick exposure of reddishbrown, poorly bedded, moderately well sorted and rounded, cobble conglomerate exposed below a thick flow of Lutsen basalt. The conglomerate is exposed as a small erosional window through the Lutsen basalts. Cobbles in the conglomerate are mostly less than 5 cm in diameter, in a matrix of coarse gritty sand and white calcite cement. In order of abundance, the clasts are composed of ambiguous brown, fine-grained mafic to intermediate volcanic rocks, flow-banded rhyolite, amygdaloidal basalt possibly similar to the adjacent Lutsen basalt, reworked sandstone, and rare agate. In thin section many of the clasts are composed entirely of a felty-matted, gray-birefringent mineral, presumably some type of secondary zeolite; others are replaced by ILSG07 136 Trip 5 �calcite in irregular alteration zones and clots. Less altered clasts exhibit relict flow banding typical of NSVG rhyolites, and relict textures typical of NSVG mafic to intermediate volcanic rocks. The contact with the overlying flow of dark green, ophitic Lutsen basalt is very sharp and straight, measured at N70E, 15° SE, typical for the NSVG. Discussion: Further northeast up the shore there are some reddish-brown, fine-grained, interflow sandstones contained entirely within the Lutsen basalts, as scattered 2-3 meter thick and one 75 meter thick bed. Although the conglomerate at this stop is likely not more than a few meters thick and may similarly be contained entirely within the Lutsen basalts, it does not outcrop in nearby stream exposures where anticipated if it were a conformable layer in the Lutsen basalts. More likely, this exposure is a window eroded through the base of the Schroeder-Lutsen sequence, and the conglomerate marks the unconformity between the Good Harbor Bay andesites and the Lutsen basalts (see Fig. 5-15, stop 7). The conglomerate contains few if any clasts derived from the texturally distinct Lutsen basalts, but rather is composed predominantly of felsic-intermediate composition volcanic rocks, basalt unlike Lutsen basalt, and some reworked sandstone. Thus the source for the conglomerate is apparently dominated by older volcanic and sedimentary rocks that are similar in composition to those observed inland from the Schroeder-Lutsen sequence. The conglomerate may represent local deposition along a minor fault escarpment, or a proximal-facies high-energy fluvial deposit filling topographic depressions on the andesites. ---Drive northeast on Highway 61 approximately 13 miles to Historical Marker/Geologic Marker at top of steep hill (Terrace Point) at curve in road. Park at wayside, stop is across Highway at large roadcut. BE CAREFUL CROSSING HIGHWAY.--- Stop 5-11A. Cut Face Creek Roadcut UTM coordinates (nad ’83) 691780E, 5289170N T.61N., R.1W., Sec. 34 Good Harbor Bay quadrangle Highlights: Thick interflow sandstone, deformation features in sandstone at base of flow, shale rip-up chips, dessication cracks, Terrace Point basalt flow, fragmental basalt intruded and overrun by basalt flow. Description: In high roadcut on north side of highway is an obvious contact between the Cut Face Creek Sandstone and the overlying Terrace Point basalt flow. The sandstone overlies the Good Harbor Bay andesites (stop 11b). Sandstone –Jirsa (1984) measured approximately 73 meters of sandstone and 3 meters of shale in this section of the Cut Face Creek sandstone, but reported overall that nearly 30 percent of it is composed of thinly bedded, graded layers of fine-grained sand, silt, and clay. He reports both symmetrical and asymmetrical ripple marks, and bimodal paleocurrent distribution, and concluded that the Cut Face Creek sandstone was deposited in a fluvial-lacustrine environment. Unlike nearly all the other exposed interflow sandstones within the NSVG, planar cross-bedding is predominant, but some trough cross beds are present near the top of the section. Other features that may be visible in ILSG07 137 Trip 5 �the sandstone are dessication cracks filled with sandstone or coarse pink zeolite minerals, and rip-up textures. The base of the sandstone is exposed to the north in the Cut Face Creek valley, where it overlies the Good Harbor Bay andesite (see stop 11B). Basalt – The Terrace Point basalt, which overlies the sandstone, is a major ridge-forming unit from here to the southwest, forming ‘sawtooth mountains’. It is a distinctive flow characterized by a dark green color, white thomsonite amygdules, uniform 3-4mm ophitic texture, and scattered small glassy plagioclase phenocrysts. In general the contact with the sandstone is sharp and straight, but in places the sandstone has been slightly deformed by the basalt flow. At the south end of the outcrop is a unit of scoriaceous, fragmental basalt that is intruded and by the Terrace Point flow. Similar rock types and relationships have been observed to the southwest, also near the flow base. The breccia contains 1-100 cm angular basalt fragments that are both massive and amygdaloidal, and scattered large blocks of massive basalt. At this locality and others, sub-volcanic dikes of Terrace Point basalt that intrude the fragmental basalt are slightly chilled, and contain small amygdules stretched parallel to, and columnar joints perpendicular to, curvilinear dike margins. The fragmental unit is interpreted as a cinder cone or lahar-type deposit that was subsequently intruded and overrun by the thick Terrace Point basalt flow. Figure 5-18. Simplified geologic map showing distribution of interflow sandstones and major volcanic units in the Lutsen to Terrace Point area. Discussion: The Terrace Point basalt flow has been considered (e.g., Green, 1972) to be the basal flow of the Schroeder-Lutsen sequence. However, recent detailed mapping to the southwest (e.g. Boerboom and Green, 2006, Boerboom and others, 2006 and 2007) has shown that the Terrace Point basalt is actually contained within the Good Harbor Bay lavas. On the Onion River continuous exposures show that the top of the Terrace Point basalt is fragmental and overlain by more andesite (Stop 8), and is clearly an ILSG07 138 Trip 5 �integral part of the flow sequence. Also, the base of the Schroeder-Lutsen sequence overlies both the Terrace Point basalt and andesites in the Good Harbor Bay lavas (Figure 5-15, stop 7). Thus, based on new mapping, the Terrace Point flow and the Good Harbor Bay andesites are now considered to be part of a continuous series termed the Good Harbor Bay lavas. The Cut Face Creek Sandstone was considered to represent clastic deposition during a hiatus in volcanism prior to eruption of the Terrace Point/Schroeder-Lutsen basalts. It was also considered to be somewhat unique in that it was one of the few thick sandstone units in the northeast limb of the North Shore Volcanic Group, along with the 68 meter thick Indian Camp sandstone (which is within the Schroeder-Lutsen basalts). However, recent remapping has shown that there are likely at least three more substantially thick sandstone units that occur within lower series of lava flows to the north of the Good Harbor Bay lavas (Fig. 5-18; Boerboom and others, 2007). Thus, it is now recognized that the Cutface Creek and Indian Camp sandstones, exposed because of more active erosion near the Lake Superior coast, are only one part of a larger set of thick sandstone units, some of which are located over 5 miles inland. The recognition of thick interflow sandstones throughout several different series of lava flows at different stratigraphic heights implies active sedimentation during a prolonged period of volcanism. Fragmental flow tops in the Good Harbor Bay Lavas contain abundant sandstone infillings, and thin, discontinuous, layers and crack fillings of sandstone are common in the Schroeder-Lutsen basalts. Elevated levels of clastic deposition during active volcanism may account for the relative abundance of sand at the tops of the lava flows, and the thick interflow sandstones may have formed during periods of volcanic quiescence, but continued basin subsidence. Available data seem to indicate that the thicker sandstones may vary in thickness along strike, consistent with deposition onto an irregular lava surface. ---Drive northeast on Highway 61 approximately ¼ of a mile to the Cut Face Creek wayside located to the right on the lake shore, just past Cut Face Creek. Park at wayside. Either cross highway to small roadcuts across from wayside (WATCH TRAFFIC), or if the creek is low walk back on beach to Cut Face Creek and proceed upstream in creek valley.--Stop 5-11B. Good Harbor Bay andesite UTM coordinates (nad ’83) 691953E, 5289526N T.61N., R.1W., Sec. 34 Good Harbor Bay quadrangle Highlights: Good Harbor Bay andesites, Cut Face Creek Sandstone Description: Small outcrop directly across highway from parking area and to northeast, and outcrops at the mouth of Cut Face Creek, are fine-grained, brownish-gray, sparsely porphyritic andesite. These are part of the Good Harbor Bay andesites, a series of andesitic flows that stretches from near Grand Marais to Tofte. The andesite here is overlain by the Cut Face Creek Sandstone, and the contact is exposed in numerous locations in the meandering Cut Face Creek valley. The basal contact appears to be sub-conformable with the andesite on a local scale, but on a regional scale the upper andesite surface probably undulates. Small pebbles of massive to amygdaloidal andesite are common in the lower 3 meters of the sandstone, typically in 3-25 cm thick planar cross-bedded pebbly beds. The upper part of the andesite typically contains stretched amygdules, and has cracks filled with sandstone (Fig. 5-19). Like the exposures in the Onion River (Stop 9), the andesites here have rubbly, sand-filled flow top breccias with fragments of stretched amygdaloidal andesite. Figure 5-19. Photograph of base of Cut Face Creek sandstone (white arrow), showing clastic dike (black arrow) filling a crack in underlying andesite. Hammer in circle is 45 cm long. ILSG07 139 Trip 5 �REFERENCES Albers, P.A., 2006, The geology and petrology of the Leveaux Porphyritic Diorite Intrusion: Investigating possible magmatic relationships to the anorthositic series of the Duluth Complex, Cook County, Minnesota. Unpublished MS thesis, University of Minnesota Duluth, 121 p. w/ map. Basaltic volcanism study project (BVSP), 1981, Pre-Tertiary continental flood basalts: in Basaltic Volcanism on the Terrestrial Planets, NY, Pergamon Press, p. 30-77. Boerboom, T. J., 1994, Archean crustal xenoliths in a Keweenawan hypabyssal sill, northeastern Minnesota. White was right!: Institute on Lake Superior Geology 40th Annual Meeting, Houghton, MI: Proceedings v. 40, pt. 1 Abstracts, p. 5-6. Boerboom, T. J., Green, J.C., and Albers, P., 2007, Bedrock geologic map of the Lutsen quadrangle, Cook County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series M-1XX, scale 1:24,000. (in press). Boerboom, T. J., and Green, J. C., 2006, Bedrock geology of the Schroeder quadrangle, Cook County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series M-170, scale 1:24,000. Boerboom, T. J., Green, J. C., Albers, P., and Miller, James D., Jr., 2006, Bedrock geology of the Tofte quadrangle, Cook County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series M-171, scale 1:24,000. Boerboom, T. J., and Green, J. C., 2005, Bedrock geology of the Two Harbors NE quadrangle, Lake County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series M-155, scale 1:24,000. Boerboom, T. J., and Green, J. C., 2004, Bedrock geology of the Split Rock Point quadrangle, Lake County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series M-147, scale 1:24,000. Boerboom, T.J., Green, J.C., and Jirsa, M.A., 2002a, Bedrock Geology of the French River and Lakewood 7.5’ quadrangles, St. Louis and Lake Counties, Minnesota: Minnesota Geological Survey Miscellaneous Map Series, Map M-128, scale 1:24,000. Boerboom, T.J., Green, J.C., and Jirsa, M.A., 2002b, Bedrock Geology of the Knife River 7.5’ quadrangle, Lake County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series, Map M-129, scale 1:24,000. Boerboom, T.J., Miller, J.D., Jr., and Green, J.C., 2004, Field trip 2: Geologic highlights of new mapping in the southwestern sequence of the North Shore Volcanic Group and in the Beaver Bay Complex: Institute on Lake Superior Geology, 50th Annual Meeting, Duluth, Minn., Proceedings, pt. 2, Field Trip Guidebook, p. 45-85. Boerboom, Terrence J., Miller, James D. Jr., and Green, John C., 2003a, Bedrock geology of the Two Harbors 7.5’ quadrangle, Minnesota Geological Survey Miscellaneous Map Series Map M-139, scale 1:24,000. Boerboom, Terrence J., Miller, James D. Jr., and Green, John C., 2003b, Bedrock geology of the Castle Danger 7.5’ quadrangle, Minnesota Geological Survey Miscellaneous Map Series Map M-140, scale 1:24,000. Boerboom, T.J., and Miller, J.D., 1994, Bedrock geologic map of the Silver Island Lake, Wilson Lake, and western Toohey Lake Quadrangles, Lake and Cook Counties, Minnesota: Minnesota Geological Survey Miscellaneous Map Series M-81, scale 1:24,000. Bonnichsen, B., and Kauffman, 1987, Physical features of rhyolite lava flows in the Snake River Plain Volcanic province, southwestern Idaho: in The Emplacement of Silicic Domes and Lava Flows, J.H. Fink, ed., p. 119145. Brannon, J.C., 1984, Geochemistry of successive lava flows of the Keweenawan North Shore Volcanic Group: unpublished PhD. Thesis, Washington University, St. Louis, 212 p. Carmichael, I.S.E., 1964, The petrology of Thingmuli, a Tertiary volcano in eastern Iceland” Journal of Petrology, v. 5, p. 435-460. Davis, D.W., and Green, J.C., 1997, Geochronology of the North American Midcontinent rift in western Lake Superior and implications for its geodynamic evolution: Canadian Journal of Earth Science, v. 34, p. 476-488. Davis, D.W., and Paces, J.B., 1990, Time resolution of geologic events on the Keweenaw Peninsula and implications for development of the midcontinent Rift system: Earth and Planetary Science Letters, v. 97, p. 5464. Goldich, S.S., Nier, A.O., Baadsgaard, H., Hoffman, J.H., and Krueger, H.W., 1961, The Precambrian geology and geochronology of Minnesota: Minnesota Geological Survey Bulletin 41, 193 p. Greeley, R., 1982, The Snake River Plain, Idaho: Representative of a new category of volcanism: Journal of Geophysical Research, v. 87, N. B4, P. 2705-2712. Green, J.C., 2002, Volcanic and sedimentary rocks of the Keweenawan Supergroup in northeastern Minnesota, in Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., and Wahl, T.E., Geology and ILSG07 140 Trip 5 �mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geological Survey Report of Investigations 58, p. 94-102. Green, J.C., 1995, Chemostratigraphy at the fringe of the Midcontinent Rift System: The northeast limb of the North Shore volcanic Group, Minnesota (ext. abs.): in Proceedings, Petrology and Metallogeny of Volcanic and Intrusive Rocks of the Midcontinent Rift System: 1995 IGCP Project 336 Field Conference and Symposium, p. 55-56. Green, J.C., 1992, Geologic map of the north shore of Lake Superior, Lake and Cook Counties, Minnesota: Part 1. Little Marais to Tofte: Minnesota Geological Survey Miscellaneous Map Series M-71, scale 1:24,000. Green, J.C., 1990, Primary tridymite crystallization and inferences from tridymite and quartz textures in high-T Keweenawan rhyolites and granophyres, Minnesota (abs.): Geological Society of America, Abstracts with Programs, v. 22, p. A289-290. Green, J. C., 1989, Physical volcanology of mid-Proterozoic plateau lavas: The Keweenawan North Shore Volcanic Group, Minnesota: Geological Society of America Bulletin, v. 101, p. 486-500, 21 figs., 2 tables. Green, J. C., 1983, Geologic and geochemical evidence for the nature and development of the middle Proterozoic (Keweenawan) Midcontinent Rift of North America. Tectonics, v. 94, p. 413-437. Green, J.C., 1982, Geologic map Atlas of Minnesota, Two Harbors Sheet: Minnesota Geological Survey, scale 1:250,000. Green, J.C., 1977, Keweenawan plateau volcanism in the Lake Superior region, in Baragar, W.R.A., Coleman, L. C., and Hall, J.M., eds., Geological Association of Canada Special Paper 16, p. 407-422. Green, J.C., 1972, North Shore Volcanic Group, in Sims, P.K., and Morey, G.B., eds., Geology of MinnesotaA centennial volume: Minnesota Geological Survey, p. 294-332. Green, J. C., Davis, D.W., and Schmitz, M.D., 2001, Three new zircon dates for the Midcontinent Rift, North Shore, Minnesota: More data, more questions: Institute on Lake Superior Geology, 47th Annual Meeting, Madison, Wis., Proceedings, pt. 1, Programs and Abstracts, p. 29. Green, J.C., and Fitz, T.J. III, 1993, Extensive felsic lavas and rheoignimbrites in the Keweenawan Midcontinent Rift plateau volcanics, Minnesota: petrographic and field recognition: Journal of Volcanology and Geothermal Research, v. 54, p. 177-196. Irvine, T.N., and Baragar, W.R.A., 1971, A guide to the chemical classifications of the common volcanic rocks: Canadian Journal of Earth Sciences, v. 8, no. 5, p. 523-548 Jerde, E.A., 1991, Geochemistry and petrology of hypabyssal rocks associated with the Midcontinent rift, northeastern Minnesota: unpublished Ph.D. Thesis, University of California, Loa Angeles, 305. pp. Jirsa, M.A., 1984, Interflow sedimentary rocks in the Keweenawan North Shore Volcanic Group, northeastern Minnesota: Minnesota Geological Survey Report of Investigations 30, 20 p. Klewin, K.W., and Shirey, S.B., 1992, The igneous petrology and magmatic evolution of the Midcontinent Rift System: Tectonophysics, v. 213, p. 33-40. Link, P. K. and Hackett, W. R., editors, 1988, Guidebook to the Geology of Central and Southern Idaho. Idaho Geological Survey, Bulletin 27, 319 p. Manley, C.R., 1996, Physical volcanology of a voluminous rhyolite lava flow: The Badlands lava, Owyhee Plateau, southwestern Idaho: Journal of Volcanology and Geothermal Research, v. 71, p. 129-153 McBirney, A.R., and Williams, H., 1969, Geology and petrology of the Galapagos Islands: Geological Society of America Memoir 118, 197 p. Miller, J.D., Jr., 1988, Geologic map of the Split Rock Point NE and Silver Bay quadrangles, Lake County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series M-68, scale 1:24,000. Miller, J.D., Jr., Boerboom, T.J., and Jerde, E.A., 1994, Bedrock geologic map of the Cabin Lake and Cramer 7.5’ quadrangles, Lake and Cook Counties, Minnesota: Minnesota Geological Survey Miscellaneous Map Series M82, scale 1:24,000. Miller, J.D., Jr., and Chandler, V.W., 1997, Geology, petrology, and tectonic significance of the Beaver Bay Complex, northeastern Minnesota: in Ojakangas, R.J., Dickas, A.B., Green, J.C., (eds.) Middle Proterozoic to Cambrian Rifting, Central North America”: geological Society of America Special Paper 312, p. 73-96. Miller, J.D., Jr., Green, J.C., Boerboom, T.J., and Chandler, V.W., 1993, Geology of the Doyle Lake and Finland quadrangles, Lake County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series Map M-72, scale 1:24,000. Miller, J.D., Jr., Green, J.C., and Boerboom, T.J., 1989, Geology of the Illgen City quadrangle, Lake County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series Map M-69, scale 1:24,000. ILSG07 141 Trip 5 �Miller, J. D., Jr., Green, J.C., and Jerde, E.A., 2006, Bedrock geology of the Little Marais quadrangle, Lake and Cook County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series Map M-172, scale 1:24,000. Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., and Peterson, D.E., 2001, Geologic map of the Duluth Complex and related rocks, northeastern Minnesota: Minnesota Geological Survey Miscellaneous Map Series Map M-119, scale 1:200,000. Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.M., ad Wahl, T.E., 2002, Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geological Survey Report of Investigations 58, 207 p. Miller, J. D., Jr., and Weiblen, P.W., 1990, Anorthositic rocks of the the Duluth Complex: Examples of rocks formed from plagioclase crystal mush: Journal of Petrology, v. 31, p. 295-339. Miller, J.D., Jr., Weiblen, P.W., and Green, J.C., 1987, Roadlog and stop descriptions for the Beaver Bay Complex, in Balaban, N.H., ed., Field trip guidebook for selected areas in Precambrian geology of northeastern Minnesota: Minnesota Geological Survey Guidebook 17, p. 55-60. Milner, S.C., Duncan, A.R., and Ewart, A., 1992, Quartz latite rheoignimbrites flows of the Etendeka Formation, northwestern Namibia: Bulletin of Volcanology, v. 54, p. 200-219. Morrison, D.A., Ashwal, L.D., Phinney, W.C., Shih, C., and Wooden, J.L., 1983, Pre-Keweenawan anorthosite inclusions in the Keweenawan Beaver Bay and Duluth Complexes, northeastern Minnesota: Geological Survey of America Bulletin, v. 94, no. 2, p. 206-221. Nicholson, S. W., Shirey, S. B., and Green, J. C., 1991, Regional Nd and Pb isotopic variations among the earliest Midcontinent Rift basalts in western Lake Superior (abstr). In Programs with Abstracts, GAC/MAC Annual Meeting, Toronto. Nicholson, S.W., Shirey, S.B., Schulz, K.J., and Green, J.C., 1997, Rift-wide correlation of 1.1 Ga Midcontinent rift system basalts: implications for multiple mantle sources during rift development Canadian Journal of Earth Science, v. 34, p. 504-520. Paces, J.B., and Miller, J.D., Jr., 1993, Precise U-Pb ages of Duluth Complex and related mafic intrusions, northeastern Minnesota: geochronological insights to physical, petrogenetic, paleomagnetic and tectonomagmatic processes associated with the 1.1 Ga Midcontinent rift system: Journal of Geophysical Research, v. 98, np. B8, p. 13,997-14,013. Reidel, S. R. and Hooper, P. R., 1989, Volcanism and Tectonism in the Columbia River Flood-basalt Province. Geological Society of America Special Paper 239, 386 p Sigvaldson, G.E., 1974, Basalts from the centre of the assumed Icelandic mantle plume: journal of Petrology, v. 15, p. 497-524. Schmidt, S. Th., 1993, Regional and local patterns of low-grade metamorphism in the North Shore Volcanic Group, Minnesota, USA: Journal of Metamorphic Geology, v. 11, p. 401-414. Schmidt, S. Th., and Robinson, D., 1997, Metamorphic grade and porosity and permeability controls on mafic phyllosilicate distributions in a regional zeolite to greenschist facies transition on the North Shore Volcanic Group, Minnesota” Geological Society of America Bulletin, v. 109, NO. 6, p. 683-697. Vervoort, J.D., and Green, J.C., 1997, Origin of evolved magmas in the Midcontinent Rift System, northeast Minnesota: Nd-isotope evidence for melting of Archean crust” Canadian Journal Earth Sciences, v. 34, p. 521535. Walker, G.P.L., 1974, The structure of eastern Iceland. In Kristiansson, L., ed., Geodynamics of Iceland and the North Atlantic Area, D. Reidel Publishing Co., Dordrecht, p. 177-188 Wallace, A.B., Drexter, J.W., Grant, N.K., and Noble, D.C., 1980, Icelandite and aenigmatite-bearing pantellerite from the McDermitt caldera complex, Nevada-Oregon: Geology, v. 8, p. 380-384. White, W.S., 1966, Tectonics of the Keweenawan basin, western Lake Superior region. U.S. Geological Survey Professional Paper 524-E, 23p. Wood, D.A., 1978, Major and trace element variations in the Tertiary lavas of eastern Iceland and their significance with respect to the Iceland geochemical anomaly: Journal of Petrology, v. 19, p. 393-436. ILSG07 142 Trip 5 �53rd Annual Institute on Lake Superior Geology FIELD TRIP 6 GEOLOGY ALONG THE GUNFLINT TRAIL Mark A. Jirsa Minnesota Geological Survey University of Minnesota and Paul W. Weiblen Department of Geology and Geophysics University of Minnesota ILSG07 143 Trip 6 �Figure 6.1. Regional geologic map of northeastern Minnesota showing the location of Gunflint Trail (modified from Jirsa and Miller, 2005). Inset box outlines Figure 6.4. Field trip area lies at the junction of Neoarchean, Paleoproterozoic, and Mesoproterozoic terranes. INTRODUCTION The Gunflint Trail provides access to the Boundary Waters Canoe Area Wilderness (BWCAW), which lies a short distance off either side of the road. The western end of the trail also transects a nearly complete geologic section of Archean, Paleoproterozoic, and Mesoproterozoic rocks (Fig. 6.1), and a diversity of well displayed contact relationships. In this area, the Archean greenstone-granite terrane of the Wawa subprovince of Superior Province is represented by a succession of metavolcanic rocks (ca 2700 Ma) intruded by the Saganaga Tonalite (ca 2689 Ma). The Archean rocks are unconformably overlain by Paleoproterozoic sedimentary strata of the Animikie Group (ca 1870-1830 Ma), which includes the Gunflint Iron Formation. Mesoproterozoic rifting is manifest in hypabyssal dikes and sills of the Logan intrusions (ca 1115 Ma), and several phases of the Duluth Complex (ca 1100 Ma), emplaced into both Archean and Paleoproterozoic rocks. Despite this exceptionally well exposed record of Precambrian history, little geologic mapping has been conducted in this region since the 1970's. As a result, this field guide relies heavily on prior mapping (Grout, 1929; Mathez and others, 1977; Morey and Nathan 1977; 1978; Morey and others 1981) and field trip guides (e.g., Weiblen and others, 1971; Miller ILSG07 144 Trip 6 �and others, 1987). Although the individual stops are different, much of this guide is modified only slightly from earlier publications, with permission from the authors. Mapping in this region by the Minnesota Geological Survey has resumed, after a 30+ year hiatus. The new mapping will benefit from geological techniques and insights acquired during the intervening decades, particularly those resulting from the work of Canadian geologists. Furthermore, exposures in some areas are more accessible due to road building and recent forest fires. Regarding the latter, many parts of the BWCAW and adjacent lands experienced a megastorm in 1999 that resulted in large tracts of downed and tangled trees. Controlled burns and wild fires in some of the devastated areas—including the 2006 Cavity Lake fire—laid bare many outcrops that once were deeply concealed in forest, lichen, and moss. GEOLOGIC SETTING Neoarchean The oldest rocks exposed in the region are Neoarchean metavolcanic and metasedimentary rocks— probably equivalent to, but not demonstrably continuous with, the Ely Greenstone. Although the supracrustal successions are dissected by faults and intrusions, some correlation can also be made with adjacent terrane in Ontario (Fig. 6.2). Most recent mapping in this terrane was conducted by Jirsa and Miller (2004) in the 1:100,000-scale, Ely-Basswood Lake map sheet that lies just west of the Gunflint Trail. A mapping thesis by Vervoort (1987) delineated geology in the JAP Lake area, which lies between that map sheet and the Gunflint Trail exposures. Rocks of the terrane are divided into a number of faultbounded segments, each having distinct geologic characteristics that cannot be easily correlated from place to place. The Gunflint Trail area exposes the eastern end of what Gruner (1941) referred to as the Gabimichigami segment. Supracrustal rocks in this segment include variably pillowed, hypabyssal mafic metavolcanic flows (Stop 6-1), and andesitic pyroclastic rocks. The supracrustal rocks are cut by hornblende-phyric intrusions that are similar in composition to the associated andesite, and on this basis, inferred to be more or less contemporaneous with them (Feirn, 1977). Based on stratigraphic facing directions established from pillowed metabasalt flows, the supracrustal sequence forms an east-trending and steeply south dipping and younging homocline. The Saganaga Tonalite was emplaced along the northern edge of the supracrustal succession. The metamorphic grade of greenstone along this contact is locally increased from greenschist facies that is typical of much of the belt, to amphibolite grade, and foliation is well-developed. The western edge of the Gabimichigami segment is terminated by a northnortheast-trending fault that juxtaposed greenstone against sedimentary and volcanic rocks of the Knife Lake Group. The Knife Lake Group includes the Ogishkemunce conglomerate that contains detrital clasts of the Saganaga Tonalite. This distinctive sequence of conglomerate, sandstone, and alkalic rocks is interpreted to have been deposited in a complex array of successor basins developed along earlyformed faults (Jirsa and Miller, 2005) at some time after emplacement of the Saganaga Tonalite. ILSG07 145 Trip 6 �Figure 6.2. Geologic setting of Archean bedrock in northeastern Minnesota and adjacent Ontario (modified from Jirsa and Miller, 2005). Inset map shows location relative to subprovinces of the Superior Province. Lacking detailed geochronologic data for this immediate area, much of the temporal distinction between various geological elements of the Neoarchean bedrock is based on the correlation of fabrics that resulted from three major phases of deformation, denoted D1, D2, and D3, with U-Pb zircon dates acquired elsewhere. All three deformation events are the result of N-S- to NW-SE-directed compression. The timing of D1 deformation is bracketed between deposition of the volcanic and clastic rocks of the Wawa subprovince at about 2722 Ma (Peterson and others, 2001), and emplacement of the Saganaga Tonalite at about 2689 Ma (Corfu and Stott, 1998). Folds in the Ely Greenstone and related rocks attributed to D1 deformation are truncated by faults associated with Knife Lake strata, indicating that the latter is synchronous with or post-dates deposition and early deformation of the Ely. As such, the Knife Lake Group is inferred to be a Timiskaming-type sequence temporally equivalent to the Shebandowan assemblage exposed in adjacent parts of Ontario (Corfu and Stott, 1998). D2 deformation effected all of the Archean supracrustal units and can be crudely bracketed by U-Pb dates of intrusions in the Giants Range batholith to the southwest that place the regional deformation and metamorphic event between about 2674 Ma and 2685 Ma (Boerboom and Zartman, 1993). D3 deformation produced faults in the low grade supracrustal and intrusive rocks of the Wawa subprovince, and folds and metamorphic fabric in granitic and migmatitic rocks of the Quetico subprovince to the north. The batholith generally referred to as the Saganaga Tonalite was emplaced into metavolcanic and gneissic granitoid country rocks. Igneous fabric is best developed in the border zones; but overall, the intrusion is moderately foliated and has a shallow eastward lineation. U-Pb age dates include 2689±1 Ma for the tonalite; whereas, 2 phases of the Northern Light Gneiss, which lies in contact with the Saganaga along its northeastern boundary in Ontario, are dated at 2707±2 Ma and 2750±2 Ma (Corfu and Stott, 1998). On the basis of geochemistry, Vervoort (1987) infers that the Saganaga batholith represents the magma source for dacitic lavas that overlie mafic and ultramafic rocks in the JAP Lake area. The Saganaga batholith was divided by Grout (1929) into a medium- to coarse-grained tonalitic phase that makes up the majority of the batholith (Stops 6-2 and 6-3), and a more mafic border phase (Stops 6-4 and 6-5). Grout (1929, p.568) defined this border zone approximately one mile in width along the south, east, and south sides of the batholith. In the Gunflint Trail area, the border phase is more irregular in ILSG07 146 Trip 6 �distribution and consists of strongly foliated, medium to coarse-grained, quartz-bearing hornblende diorite to granodiorite that generally lacks quartz eyes. The border phase is also marked by a wide variety of inclusions. Grout (1929) suggested that some of the inclusions in the Saganaga batholith are the products of igneous processes that ranged from assimilation to the mixing of mafic and felsic magmas. Alternatively, Hanson (1972) suggested that some of the included mafic material could be a product of assimilation of greenstone country rocks during pluton emplacement. Field evidence showing the presence of nearly all gradations of inclusions between mafic and felsic magmatic-looking components, and the presence of quartz phenocrysts in some mafic inclusions, supports the former interpretation for many of the inclusions. The results of analyses of two mafic inclusions acquired recently by the authors are summarized in Figures 6.3A, B, and C. These data indicate that the mafic inclusions (Fig. 6.3D) have the common chemical characteristics of the sanukitoid suite of Archean granitoid rocks. Thus, we infer that the inclusions studied are autoliths derived from a separate, primitive sanukitoid-magma. These data support Grout's earlier interpretations. This is not to say that country rock inclusions are not present—some fine-grained, dark-green, and variably cleaved inclusions do occur very near the contact with enclosing metavolcanic rocks (Stop 6-6a). To date there has been no systematic study of the mafic components of the Saganaga batholith. As pointed out by Didier and Barbarin (1991) in the conclusion of their multi-authored volume, further study of enclaves will contribute to the advancement of the overall petrogenesis of granites. The Saganaga batholith provides ample opportunity for such studies. Figure 6.3. Geochemical comparison of Saganaga batholith, mafic inclusions, and related rocks. A. SiO2-MgO plot. Abbreviations and data sources: TTG=Compositional field of the Archean TonaliteTrondhjemite-Granodiorite suite; S=Compositional field of primitive Archean sanukitoid samples from the southwestern Superior Province (Stern and others, 1989). Open circles: 1 Ely greenstone; 2 and 3 mafic inclusions at Stop 6-5; 4 border phase of the Saganaga Tonalite at Stop 6-5; 5, 6 Ely greenstone; 7-10 tonalite; 11 granodiorite. Analyses 1-4 were acquired for this study (on-file with Minnesota Geological Survey); analyses 5-11 are from Arth and Hanson, 1975, Table 2, p. 331, analyses 1-2, 19-22, and 24 respectively. Analyses 12-14 are from Stern and others, 1989, Table 1 p. 1695, samples 1-3. Crosses: analyses of the Roaring River Complex, Superior Province, Canada from Stern and Hanson (1991, Table 2, p. 220). They have demonstrated that incremental equilibrium crystallization of a melt ILSG07 147 Trip 6 �with the composition of analysis 15 (sample #7, op cit) can produce the three members of the TTG suite indicated. The two analyses of mafic inclusions at Stop 6-5 (#'s 2 and 3) plot in the primitive sanukitoid field, and their chemistry is similar to other chemical characteristics of the sanukitoid suite (Figs. 6.3B and C). These data imply that the mafic inclusions must either be deformed xenoliths of a sanukitoid body or segregations from a consanguineous melt. B. Chemical characteristics of selected sanukitoid suite rocks (data from Stern and others, 1989) and mafic enclaves in the Saganaga batholith (data from this study). Solid squares, dashed line = range of high-silica sanukitoid; Solid diamonds, dashed line = range of low-silica sanukitoid; Solid triangles = average of high- and low-silica sanukitoid; Pluses = high-silica mafic inclusion (#2 from Fig. 6.3A), in the Saganaga batholith; Crosses = low-silica mafic inclusion (#3) in the Saganaga batholith. The analyses are normalized to an average of high and low values for each chemical characteristic of several sanukitoid analyses. Data for the mafic inclusions lie well within the range of the sanukitoid values, except for Cr in mafic inclusion #3, which plots just above the high range of the sanukitoid rocks, and Ba and Sr for both mafic inclusions that plot slightly lower than the low range of the sanukitoid rocks. ILSG07 148 Trip 6 �C. REE analyses. Open squares = Saganaga mafic enclave from this study (#2 from Fig. 6.3A), Solid squares = Saganaga mafic enclave from this study (#3), Open diamonds = Saganaga tonalite this study (#4), Solid diamonds = Saganaga tonalite of Arth and Hanson, 1975, sample 19, Open circles = Saganaga tonalite of Arth and Hanson, 1975, sample 20. The REE data for the mafic enclaves in the Saganaga batholith are distinct from that of the tonalite samples, and lie roughly within the sanukitoid field (shaded) as defined by Stern and others (1989, Fig. 10, p.1702). These data support the conclusion from the chemical characteristics (Fig.6.3B), and the SiO2-MgO (Fig. 6.3A). D. Photographs of mafic inclusions analyzed for this study. Both inclusions occur in the strongly foliated, granodioritic border phase of Saganaga batholith at Stop 6-5. LEFT=inclusion #2, material for analyses shown on Fig. 6.3A-C was taken from circled (snow-filled) drill hole. RIGHT=Sample 3: material for analyses taken from bottom of sample below X. ILSG07 149 Trip 6 �Paleoproterozoic Animikie Group Sedimentary rocks of the Paleoproterozoic Animikie Group are exposed in an east-trending belt that extends from Thunder Bay on Lake Superior to a point 12 miles (19km) west of Gunflint Trail, where the belt is truncated by the Mesoproterozoic Duluth Complex (Fig. 6.4). The Animikie Group in this area consists of basal conglomerate and iron-bearing strata of the Gunflint Iron Formation, overlain by slate and siltstone of the Rove Formation. The sequence is broadly correlative with Animikie strata exposed to the southwest along the Mesabi Iron Range. The rocks form a homocline that dips gently southward, except where deformed by folding, faulting, and emplacement of Mesoproterozoic intrusions. For example, local folding, inferred to be associated with emplacement of the Logan intrusions, may be the product of magmatic shouldering into the relatively ductile sedimentary country rocks. Sequence "inflation" by the Logan sills may explain the observation that the dip of Paleoproterozoic rocks increases from 10 degrees away from the contact with Duluth Complex, to 60 degrees locally near it. Faults are present locally, but few have displacements greater than 50 feet. A notable exception is the block-shaped Lookout fault (Fig. 6.5) that lies adjacent to the Gunflint trail. As much as 200 feet of uplift on the west and south is speculated (Morey and others, 1981). In addition, the dip of Animikie strata on the west side of the fault is much steeper than that on the east, which explains in part the difference in the widths of map units apparent from the geologic map. Much of the complex-looking fold pattern east of the fault is likely an artifact of complicated surface topography and shallowly dipping units. The Lookout fault displaces both Animikie rocks and the Logan intrusions, but the timing of offset relative to emplacement of the Duluth Complex is unclear. Figure 6.4. Geologic map along the northwestern end of the Gunflint Trail, modified from published quadrangle maps (Morey and Nathan, 1977; 1978; and Morey and others, 1981), and mapping by Grout (1929). The locations of stops 6-2, 6-3, and 6-4 are shown—other field trip stops lie within the outline corresponding to Figure 6.5. Across the border in Canada, the Gunflint Iron Formation is essentially unmetamorphosed and consists of quartz, hematite, iron carbonates, greenalite, and magnetite. A thorough review and sedimentalogical interpretation of these rocks can be found in Pufahl and Fralick (2000), Fralick and others (2002), and references therein. In Minnesota, the original mineralogic character of the ironformation was modified by contact metamorphism adjacent to Mesoproterozoic intrusions, and many of the fine-scale textural and mineralogical attributes used to subdivide the iron-formation in Canada have ILSG07 150 Trip 6 �been destroyed. Consequently, a four-fold nomenclatural scheme that emphasizes various bedding aspects that are readily visible has been used for the Gunflint since the early work of Broderick (1920). The scheme was originally devised for the Biwabik Iron Formation on the Mesabi Iron Range the Mesabi Iron Range. It defines four informal members—Lower cherty, Lower slaty, Upper cherty, and Upper slaty. These terms emphasize the relative proportions of strata dominated by thickly bedded and granular (cherty) beds consisting of rounded sand-size intraclasts, vs. thinly bedded and very fine-grained (slaty) beds. The Lower and Upper members are grouped on the published geologic map that shows ironformation (Morey and others, 1981) and on Figure 6.5. The Lower cherty member ranges in thickness from 15 to 45 ft (4.5-13.5m). A conglomerate containing clasts of Archean granite and greenstone set in a matrix of feldspathic quartzite or stromatolite-bearing chert occurs locally (Stop 6-6b). The basal conglomerate is absent in most of the area, and a persistent thick-bedded to massive, magnetite-rich unit 5-15 ft (1.5-4.5m) thick (Stop 6-6c) composes the base of the Lower cherty member and unconformably overlies Archean rocks. The magnetite-rich layer is in turn overlain by a thick-bedded, chert-rich, magnetite-poor unit about 15 ft (4.5m) thick. The overlying Lower slaty member is 80-95 ft (24-29m) thick. The lowermost 10 ft (3m) is a black, thinly bedded, nearly magnetite-free argillite composed predominantly of volcanically derived material. This may equate with the Intermediate slate unit known on the Mesabi range. Massive and cherty beds that resemble the upper part of the Lower cherty member occur above the argillite, but they pass abruptly upward into a sequence of thick, chert- and silicate-rich beds with sparse magnetite intercalated with intervals of thinly laminated silicate-rich beds. The uppermost 50 ft (15m) of the Lower slaty member consists of thinly bedded to laminated slate containing various silicates and 20-35 percent magnetite (Stop 6-7a). The Upper cherty member is approximately 50 ft (15m) thick. The gradational contact between the Lower slaty and the Upper cherty members is marked by the appearance of irregularly bedded to lenticular chert-rich layers and by thin irregular layers consisting almost entirely of magnetite. The upper part of the Upper cherty member is characterized by several thick units of granular chert that contain microbial structures, intraformational conglomerate, and abundant magnetite. (Stop 6-7c). The Upper slaty member is approximately 150 ft (45m) thick. Thick lenticular beds of chert containing disseminated magnetite occur in the lower few meters, but most of the member consists of thin-bedded to laminated, silicate-rich units that are intercalated with intervals of thinly laminated graphitic argillite and centimeter-thick beds of relatively pure chert. The upper 10 ft (3m) are nearly magnetite-free and consist of limestone and chert interbedded with argillite (Stops 6-8a, 6-8b). The Rove Formation gradationally overlies the Gunflint Iron Formation and is at least 3,200 ft (970m) thick, although estimates of thickness are complicated by multiple Mesoproterozoic intrusions and likely fault repetition. It can be divided into a lower argillaceous unit, a transitional sequence, and an upper thin-bedded graywacke-rich unit (Morey, 1969). The lower argillite consists of intercalated argillaceous siltstone, silty argillite, and carbonaceous argillite (Stop 6-9). The transitional sequence separates dominantly argillaceous rocks below from an interval containing intercalated beds of coarseand fine-grained graywacke. The sandy beds contain many primary sedimentary structures indicative of deposition by turbidity currents that flowed dominantly southward. Contact metamorphic effects on the Gunflint Iron Formation were studied in detail by Floran and Papike (1975, 1978), and metamorphism of the Rove Formation was described by Labotka and others (1981). The sizes of the metamorphic aureoles adjacent to the Logan intrusions (as much as 33 ft (10m) wide) and their mineral assemblages are directly related to the thicknesses of the sills. Adjacent to the Duluth Complex, three metamorphic zones have been distinguished: 1) An outer zone of slightly metamorphosed iron-formation consisting of quartz, iron carbonate, minnesotaite, stilpnomelane, and hematite partially replaced by magnetite. 2) A 1.2 mile (2km) wide intermediate zone of moderately metamorphosed iron-formation containing grunerite-cummingtonite, hornblende, and actinolite, as well as quartz and finegrained magnetite. ILSG07 151 Trip 6 �3) A proximal zone of highly metamorphosed iron-formation occurring within 0.3 miles (0.5km) of the contact with the Duluth Complex and composed of quartz, magnetite, iron-rich pyroxenes, and fayalite. By contrast with the iron-formation, contact-related recrystallization of the Rove Formation is much less pronounced, even very near the Duluth Complex. The aureole is marked by a complex mixture of rock types suggestive of partial melting within a meter or so of the contact. In this contact zone, argillaceous rocks retain large-scale layering, but have a vague granoblastic texture. Individual layers contain cordierite and hypersthene locally, with minor biotite, ilmenite, plagioclase, K-feldspar, and olivine. At varying distances away from the contact, but generally no more than 100 ft (30m) away, biotite is the only well-developed metamorphic mineral in the pelitic rocks. Calcareous beds near the contact contain minor grossular garnet. A U-Pb age of 1878 ± 1 for the Gunflint Iron Formation was determined by Fralick and others (2002) from reworked volcaniclastic zircons in Ontario exposures. Subsequent geochronologic study of the Rove and equivalent Virginia Formations has considerably complicated and protracted the timeline of Animikie Group deposition. For example, dates by Addison and others (2005) of detrital zircons near the base of the Rove Formation yielded ages of 1827 ± 8 and 1836 ± 5 Ma, and 1832± 3 from a similar stratigraphic position in the Virginia Formation. All of these samples were acquired from several meters above what Addison and others infer is an ejecta layer derived from the Sudbury impact event (ca 1850). Recently acquired detrital zircon dates by Heaman and Easton (2005) indicate that upper parts of the Rove Formation are as young as 1777 Ma. Wirth and others (2006) report ca 1790 Ma detrital zircons in the Rove and equivalent Thomson Formations. Collectively, these data imply that deposition of strata assigned to the Animikie Group spanned some 100 million years, and hints at the likelihood of several major unconformities. The tectonic and depositional setting of the Gunflint and other semi-contemporaneous Lake Superior iron-formations remains somewhat contentious (summarized for all iron-formations in Clout and Simonson, 2005). Modern interpretations include deposition in one or more foredeep basins developed cratonward of crustal loading during Penokean orogenesis (Morey and Southwick, 1995). More recently, Pufahl and Fralick (2004) have inferred from sedimentalogical evidence that deposition occurred along a south-sloping continental margin undergoing subsidence in a back-arc extensional setting. Geochemical evidence for within-plate signatures of volcanic rocks associated with iron-formation in Michigan suggested to Schulz and Cannon (in press) a similar interpretation of back-arc spreading. From this and geochronologic work, they infer that deposition occurred in a broad foreland having local extensional basins hosting iron-formation. Following iron-formation deposition, turbidites of the Rove Formation and equivalent strata were deposited in a rapidly subsiding basin, with subsidence presumably driven by crustal loading by the northward migrating fold and thrust belt. A hiatal boundary between ironformation and overlying turbidites identified in Ontario and Minnesota (Addison and others, 2005), and Michigan (Schulz and Cannon, in press) roughly coincides with the ca 1850 Sudbury impact event. Mafic dikes Mafic dikes emplaced into the Saganaga Tonalite are prominent on aeromagnetic maps as two sets of positive linear anomalies trending northward and eastward. Exposures mapped along the north-trending magnetic trajectories (Fig. 6.4) show that the western of the two is diabasic, and the eastern consists of lamprophyre. The latter was the subject of work by O'Brien (1982) who mapped the 70 foot thick dike, reprocessed aeromagnetic data, and conducted petrologic work. He classifies the lamprophyre dike as camptonite, defined by its content of kaersutite (calcic amphibole), titanbiotite, and titanaugite phenocrysts in a plagioclase-rich groundmass. The precise age of these dikes is unclear, but a Paleoproterozoic age is likely. K-Ar dates of biotite by Goldich and others (1961) yielded an age of about 1750 Ma, which they attributed to late thermal resetting during alteration of the dikes. Petrographic and geochemical analyses by O'Brien indicate that much of the alteration was deuteric—having formed during cooling of the dike. On this basis, O'Brien speculated that the K-Ar date may be an accurate estimate for emplacement. A second set of aeromagnetic anomalies—trending east and slightly less pronounced than ILSG07 152 Trip 6 �the north-trending ones—have not been investigated in detail. The roadcut at Stop 6-4 exposes diabase inferred to lie along one of the anomalies. Mesoproterozoic Mesoproterozoic mafic intrusions comprise the remaining exposures in the Gunflint Trail area (Figures 6.4 and 6.5). The rocks represent the early magmatic stages of the Midcontinent Rift system. The apparently earliest of these are diabasic sills and dikes emplaced into the Animikie strata and collectively referred to as the Logan intrusions. A baddeleyite age of 1115 ± 1 Ma is reported from a sample of a Logan sill near Thunder Bay (Heaman and Easton, 2005). The sills are intruded, with slight angular discordance, by medium to coarse-grained gabbro and troctolite of the Poplar Lake and Tuscarora intrusions of the Duluth Complex. The Poplar Lake is part of the early gabbroic series of the complex. A basal gabbroic unit of the intrusion yielded a date of 1106.9 ±.8 (Miller and Severson, 2002). Field relationships indicate that the Poplar Lake is intruded by the Tuscarora intrusion, which is considered to be part of the layered series of Duluth Complex. Logan Intrusions The Logan intrusions are exposed along a series of prominent, east-trending ridges formed by the differential erosion of diabase sills and sedimentary rocks, particularly the Rove Formation. Individual sills are as much as 1100 ft (33m) thick, and can be traced along strike for several kilometers. Branching and merging of individual sills is common, and many sills thicken and thin down-dip. Some sills terminate against joints and inferred faults. Locally, fractures in the Rove Formation are occupied by thin dikes, which give a box-work configuration to the hypabyssal intrusions. Rock types include aphyric basalt, fine- to medium-grained diabase with ophitic clinopyroxene enclosing plagioclase, plagioclase cumulates, and granophyre (Jones, 1984). Chilled margins form sharp contacts with, and locally contain inclusions of, the country rocks (Stop 6-10). Diabase coarsens to medium-grained near the center of individual sills, and clinopyroxene is ophitic throughout. Minor differentiation is manifest in accumulations of plagioclase or granophyric intergrowths (quartz, sodic plagioclase, and orthoclase) in upper parts of sills. The configuration of Logan intrusions closely follows that of the gently folded Animikie Group strata, implying that minor ductile deformation occurred during the Mesoproterozoic. Duluth Complex The Duluth Complex is a sequence of generally discordant plutonic rocks consisting of many separate intrusions. Two of these intrusions, the Poplar Lake and Tuscarora, are exposed along the Gunflint Trail. The Poplar Lake intrusion, formerly referred to as Nathan's layered series, consists of interlayered gabbroic cumulates, with minor amounts of troctolitic and anorthositic cumulates. Rocks of the Poplar Lake have reversed magnetic polarity, and thus are broadly correlative with lower lavas of the North Shore Volcanic Group and with the Logan intrusions. As originally defined by Nathan (1969), the Poplar Lake intrusion is composed of at least 27 sheet-like units of mafic cumulates and intermediate to felsic rocks. More detail about the Poplar Lake intrusion can be found in this volume (Miller and Jerde, 2007; Field Trip #1). The Tuscarora intrusion irregularly cuts across the layered gabbro of the Poplar Lake intrusion (Morey and Nathan, 1978). The basal part of the Tuscarora intrusion consists of a fine-grained, augite-poikilitic, olivine gabbro (unit ttf on Fig. 6.5) (Stops 6-11 and 6-12). Within 0.3 mi (0.5km) of the basal contact, fine-grained troctolite coarsens to medium-grained (unit ttm). The troctolite units consist of 65-70 percent plagioclase and 10-15 percent olivine. Relative amounts of poikilitic augite and irontitanium oxides vary locally. Orthopyroxene mantles olivine and occurs in symplectic intergrowth with plagioclase. Biotite is locally present in association with iron-titanium oxides. Modal layering is well developed and generally concordant with unit boundaries that dip gently to the south—typically more shallowly dipping than the subjacent Animikie Group strata. The ttp and ttf units commonly contain chalcopyrite, pyrrhotite, and minor pentlandite interstitial to plagioclase and olivine. The sulfide concentrations are subeconomic, but locally form mappable zones (Stop 6-12). ILSG07 153 Trip 6 �FIELD TRIP 6 Stop Descriptions NOTES: 1) This group will not visit all of the stops described below. A number of stop descriptions are included in this guide to provide context and for future visits to the region. 2) Small maps showing location of individual stops are modified from USGS 7.5-minute (1:24,000scale) topographic maps, which are named below. 3) All UTM coordinates are given in NAD 83. Figure 6.5. Geologic map and schematic cross-section (A-A'), showing field trip stops 6-1 and 6-5 through 6-12 (geology modified from Morey and others, 1981). ILSG07 154 Trip 6 �STOP 6-1 Neoarchean pillowed basalt and basal Gunflint Iron Formation Location: T. 65 N., R. 4 W., sec. 28, NW, NE; Kekekabic Trail to Lookout (Fig. 6.5) Long Island Lake quadrangle 6-1a = So-called "Paulson Mine" at UTM 660,504E/5,328,673N (NAD83) 6-1b = Lookout at UTM 660,857E/5,328,364N [Requires 2 mile hike] Description: Follow the Kekekabic hiking trail from the parking lot, westward approximately 0.6 mi. to the junction with a north-bound trail that leads to the lookout. The main Kekekabic trail parallels the base of the Gunflint Iron Formation, and the north-facing slope immediately south of the trail contains exposures of the Lower slaty member. The iron-formation has been strongly metamorphosed in this area and now consists of various assemblages of quartz-grunerite-fayalite-magnetite and quartz-cummingtonitegrunerite-pyroxene-magnetite. 6-1a. Continuing westward about 800 ft from the trail junction, several test pits can be seen, including one that is fenced and labeled "Paulson Mine 1893," but probably isn't (the mine apparently lies some distance farther west). This and other test pits are developed in the Lower cherty member of ironformation, and waste piles contain abundant pyrrhotite, other sulfide minerals, and magnetite. 6-1b. Returning to the junction with the north-bound trail and following it up-slope, there are numerous exposures of Neoarchean metabasalt. Several small outliers of iron-formation containing iron-silicates and magnetite can be found along the route, implying that this south-facing surface is very near the unconformable contact between Archean and Paleoproterozoic rocks. About 120 feet north of the junction, a ledge of fine-grained diabase crosses the trail. Preliminary analyses (this study) indicate that it has a komatiitic composition. Intrusions of this composition are also found in the Newton Lake Formation, some 30 miles to the southwest, and high-Mg tholeiitic basalt flows and pyroxenitic to peridotitic sills were described by Vervoort (1987) in the JAP Lake area two miles to the west. At and near the lookout, metabasalt flows with pillow structures and autobreccia fabrics are exposed. Judged from pillows—which have been moderately flattened by regional D2 deformation—bedding trends eastnortheastward, is steeply south dipping and stratigraphic facing is southward. ILSG07 155 Trip 6 �STOP 6-2 Felsic phase of the Neoarchean Saganaga Tonalite cut by a small mafic dike. Location: T. 66 N., R. 4 W., sec. 31, NW, SW; Campsite #18 (Fig. 6.4) Munker Island quadrangle UTM: 656,333E/5,335,730N Description: Most of the outcrops in this area consist of gray, massive to trachytoid-foliated, medium to coarse-grained tonalite, having plagioclase in much greater abundance than microcline (Fig. 6.6). Large quartz phenocrysts, or eyes, as much as 1 cm in diameter are characteristic of this phase, which is typical of 90% of the batholith. Quartz eyes are polycrystalline aggregates, in which each crystal has a different optical orientation. Quartz also occurs as an interstitial mineral to subhedral plagioclase (An20-28). Small amounts of microcline occur as antipathetic exsolution in plagioclase, as rims on plagioclase, and as small interstitial grains. Hornblende is the dominant ferromagnesian mineral, together with minor amounts of augite, biotite, epidote, and chlorite. Figure 6.6. Texture typical of the Saganaga Tonalite, including lineated quartz "eyes" (medium gray). The small dike of aphanitic mafic rock in this exposure has not been sampled, but is inferred to be related to larger north-trending diabasic and lamprophyric dikes that form prominent northtrending anomalies on aeromagnetic maps. The pronounced foliation in the Saganaga Tonalite was inferred by Grout (1933) as a primary flow fabric (trachytoid). The tonalite is inferred to have been emplaced into Archean metavolcanic rocks shortly after early (D1) deformation, based a U-Pb date of 2689±1 in Canadian exposures (Corfu and Stott, 1998). As such, it experienced major regional metamorphism and transpression associated with D2 deformation. As with many large plutons in such terranes, the debate remains unresolved about whether the fabrics are wholly magmatic, wholly tectonic, or some hybrid of the two. ILSG07 156 Trip 6 �STOP 6-3 Neoarchean Saganaga Tonalite with rounded dioritic to granodioritic inclusions Location: T. 66 N., R. 4 W., sec. 31, SW, NW; Campsite #13 (Fig. 6.4) Munker Island quadrangle UTM: 656,219E/5,336,079N Description: Equigranular tonalite with characteristic quartz eyes, containing a wide variety of inclusions. The term "inclusion" has a tortured usage—we prefer to use the term to apply to material that has a contrasting composition or appearance from its host, regardless of origin, size, shape, degree of assimilation, or extent of equilibration with the enclosing host magma. Inclusions may represent xenolithic blocks of county rock incorporated into the Saganaga Tonalite, or cognate phases of the intrusion (autoliths). The inclusions found here have not been studied in detail. They present a unique opportunity for further petrologic and mineral chemistry research to explore their complex origin. The shape of inclusions at this stop is related to either magmatic or tectonic processes. In either case, one must keep in mind that 2dimensional shapes are dependent on the orientation of exposure-surface relative to that of the inclusions. This exposure, for example, provides only one surface—it is impossible to say from this whether the inclusions are discoid, sub-spherical, or egg-shaped. STOP 6-4 Granodioritic phase of Neoarchean Saganaga Tonalite with inclusions; cut by diabase dike Location: T. 66 N., R. 4 W., sec. 32, near center; Roadcut on both sides of Gunflint Trail (Fig. 6.4) Conners quadrangle UTM: 658,575E/5,335,837N Description: Exposures on the west side of the road consist of pinkish to gray hornblende granodiorite to granite inferred to be a border phase of the Saganaga Tonalite and containing inclusions that are both more felsic and more mafic than the enclosing rock (Fig. 6.7). For example, the large angular block shown in Fig. B ILSG07 157 Trip 6 �consists of quartz-eye-bearing tonalite, much like rock that is typical of the main phase of the intrusion. Foliation is poorly developed and likely magmatic in origin. This exposure demonstrates that the Saganaga is a composite batholith that, despite its apparent homogeneity, consists of quite varied magmatic phases, particularly near its border. Tonalite on the east side of the road is cut by a fine-grained diabasic dike several meters in width. Although this location lies some distance east of the prominent north-trending aeromagnetic trends associated with dikes that are described in the text above, the dikes presumably are related. Figure 6.7. Granodioritic phase of Saganaga batholith, containing many and varied inclusions. Large, lighter colored block in center of exposure is inferred to be an autolith of quartz-eye-bearing tonalite similar to the major phase of the batholith. STOP 6-5 Border phase Neoarchean Saganaga Tonalite with flattened inclusions and well-developed foliation in contact zone with Neoarchean metabasalt Location: T. 65 N., R. 4 W., sec. 22, SE, NE, SW; Roadcut on east side of Gunflint Trail (Fig. 6.5) Long Island Lake quadrangle UTM: 661,834E/5,329,257N Description: This location exposes the border phase of the Saganaga batholith, characterized by a granodioritic composition, general lack of quartz eyes, and an abundance of dioritic inclusions consisting of varied proportions of hornblende, pyroxene, biotite, plagioclase, and minor quartz (Fig. 6.8). The irregular ILSG07 158 Trip 6 �ovoid and discoid shape of inclusions is oriented subparallel to well developed, steeply dipping and easttrending foliation. Hornblende crystals and aggregates define a prominent lineation plunging shallowly to the east. Petrology indicates that much of this fabric appears magmatic, yet foliation may be a hybrid of approximately coaxial magmatic flow and regional tectonic deformation (D2). This is typical of the border zone of the intrusion against Archean metabasaltic country rocks, which presumably lie in the low ground just to the south. A preliminary comparative geochemical study, summarized in Fig. 6.3 above, indicates that the mafic inclusions are not partially assimilated, recrystallized, and tectonically deformed country rock volcanic xenoliths as implied by some earlier workers. They have the common chemical characteristics of the sanukitoid suite of Archean granitoid rocks. Thus, we infer that the inclusions studied are autoliths derived from a separate, primitive sanukitoid-magma. Figure 6.8. Granodioritic border phase of Saganaga Tonalite containing mafic inclusions (see discussion associated with Fig. 6.3). STOP 6-6 Contact zone of Neoarchean Saganaga Tonalite and metabasalt; unconformably overlain by Paleoproterozoic conglomerate of the Lower cherty member of Gunflint Iron Formation Location: 3 exposures, all in T. 65 N., R. 4 W., sec. 22, NW, SW, SE; Bush path off Gunflint Trail [individual UTM coordinates given below] (Fig. 6.5) Long Island Lake quadrangle ILSG07 159 Trip 6 �Description: 6-6a [UTM: 661,925E/5,329,065N] Archean metavolcanic rocks containing abundant granitic sheets and dikes, presumably related to border phases of Saganaga Tonalite (Fig. 6.9). The boundary between tonalite and metabasalt has been mapped in many places as a fault (Weiblen and others, 1971). These exposures do not preclude that possibility, but they imply that passive emplacement of the intrusion has also occurred, at least locally. Figure 6.9. Outcrop of border zone of Saganaga Tonalite. Dark gray material probably represents various phases of the intrusion; lighter gray, wedge-shaped area in the foreground is inferred to be metavolcanic country rock. 6-6b [UTM: 661,965E/5,329,062N] Conglomerate developed at the gently southward dipping unconformity between Neoarchean intrusive and metavolcanic rocks and the overlying basal part of the Paleoproterozoic Animikie Group. The unit, regionally known as the Kakabeka Conglomerate, is present only locally on the western end of the Gunflint. In most places, the Lower cherty member of ironformation lies directly on eroded Archean surfaces. This small outcrop is one of the few places where the conglomerate is exposed and accessible along the contact. The conglomerate is greenish gray, poorly bedded, and contains subangular to subrounded fragments of Saganaga Tonalite and related granitoid rocks, metabasalt, and quartz, in a granular siliceous matrix. 6-6c [UTM: 661,965E/5,329,037N] Walking southward from the basal conglomerate is a low "step-up" onto southward dipping strata of the Lower cherty member of Gunflint Iron Formation. ILSG07 160 Trip 6 �STOP 6-7 Stromatolitic cherty Gunflint Iron Formation Location: 3 exposures, all in T. 65 N., R. 4 W., sec. 22 [specific locations given below], Magnetic Trail (Fig. 6.5) Long Island Lake quadrangle CAUTION and ADVICE: This is a fairly long hike, approximately 1 mile round-trip; please be prepared with water and other field needs. Although this is not in the BWCAW, it does lie within Superior National Forest and is frequented by hikers. For this reason, and to preserve scientific value, please be respectful in matters of hammering and sampling. Description: 6-7a [Sec. 22 SW, SE; UTM: 662,034E/5,328,8895N] Thin-bedded, fine-grained, chert-amphibolemagnetite-bearing strata assigned to the upper part of the Lower slaty member of Gunflint Iron Formation. Beds strike ENE and dip generally less than 8 degrees southward. 6-7b [Sec. 22 SE, SE; UTM: 662,401E/5,329,093N] Near the edge of this large controlled burn area are several exposures of stromatolitic iron-formation—inferred to be stratigraphically near the base of the Upper cherty member of the Gunflint Iron Formation. Irregular domal and laminar forms are present. Some float blocks of basal conglomerate can be found here, presumably glacially transported southward from the basal unconformity (Fig. 6.10). Figure 6.10. Boulder of basal conglomeratic unit of Gunflint Iron Formation. ILSG07 161 Trip 6 �6-7c [Sec. 22 NE, SE; UTM: 662,583E/5,329,257N] Inferred to be in the Upper cherty member of Gunflint Iron Formation. Crest of ridge exposes abundant 3-dimensional views of stromatolites, intraformational conglomerate, and stromatolite "hash," all in a peloidal to ooidal, cherty grainstone matrix. Given the apparent mineralogic replacement and moderate metamorphic grade, little of the original carbon-based material is likely present. Despite this, examples of nearly all morphological forms of stromatolites can be found, including columnar, domal, and laminar (Fig. 6.11). Figure 6.11. Cherty, stromatolitic Gunflint Iron Formation. A=Oblique surface of small columnar stromatolites; B=Horizontal surface of irregular domal stromatolites; C=Vertical section of laminar stromatolites; D="Stromatolite hash." ILSG07 162 Trip 6 �STOP 6-8 Upper member Gunflint Iron Formation Location: 2 exposures in T. 65 N., R. 4 W., sec. 26 SW, NE; Gravel pit north of Gunflint Trail (Fig. 6.5) Long Island Lake quadrangle Description: 6-8a [UTM: 663,754E/5,328,212N] Exposed here is a dip-slope composed primarily of granular (cherty) and laminated (slaty) strata of the uppermost Gunflint Iron Formation. The slope defines the southern limb of a large, east-plunging anticline, outlined by interdigitated iron-formation and nearly concordant Logan sills (Fig. 6.5). The gentle dip of this limb illustrates the observation that low angle folding and moderate-relief topography are responsible for the complex map pattern here on the east side of the Lookout fault. The bedding surface is marked by what we tentatively infer to be syneresis cracks. The cracks, now filled with quartz, occur both concentrically and radially around a central, apparently raised core within a single granular layer of siliceous iron-formation (Fig. 6.12). Syneresis cracks are defined generally as shrinkage cracks formed by dewatering in a gel or colloidal suspension. They differ from septarian cracks that may develop in a similar way, in that the latter typically occur in concretions. Surprisingly diverse interpretations can be found in the literature about syneresis cracks (summarized in Pratt, 2001). There is, however, general agreement that they represent localized tensional failure during sediment dewatering. The explanation for localized semi-brittle response to what likely were formation-wide stresses—caused by compaction or vibration due to syn-sedimentary earthquakes—is more contentious. It has been ascribed variously to the localization of cements, locally increased pore pressure, or zones of granular sediment made coherent by "microbial glue." It is interesting to note that syneresis structures are more prevalent in Precambrian and Cambrian rocks than younger ones. This may be due in part to more uniform organic bonding of clays in younger strata, which reduced the occurrence of stress-localization. Figure 6.12. Syneresis cracks on eroded bedding surface of thinly bedded Gunflint Iron Formation. 6-8b [UTM: 663,637E/5,328,055N] Metamorphosed carbonate-rich iron-formation. This small exposure is likely an outcrop, based on the observation that carbonate strata are seen elsewhere in the uppermost part of the Gunflint, and it is properly positioned relative to stratigraphy as depicted on the geologic map (Morey and others, 1981). The rock appears to be a metamorphosed breccia containing fragments of thinly layered carbonate. Mineralogy is not well known, as the exposure was only recently discovered. Based on published studies (Floran and Papike, 1975) it may contain some combination of ferrohypersthene, fayalite (±quartz), grunerite-cummingtonite and perhaps garnet (Fig. 6.13). ILSG07 163 Trip 6 �Figure 6.13. Uppermost carbonate facies of Gunflint Iron Formation. Large, light-colored crystals may be grunerite-cummingtonite; dark spots appear to be garnet. STOP 6-9 Paleoproterozoic slate of the Rove Formation Location: T. 65 N., R. 4 W., sec. 25 NE, NE; Road cut on County Road #46 about 0.5 mi. N of Gunflint Trail (Fig. 6.5) Long Island Lake quadrangle UTM: 666,750E/5,328,753N Description: The small roadside outcrop exposes slate that is typical of the argillaceous parts of the Rove Formation. Bedding strikes eastward and dips gently to the south. ILSG07 164 Trip 6 �STOP 6-10 Mesoproterozoic Logan Intrusion Location: T. 65 N., R. 4 W., sec. 24 SE, SE; Bush trail off County Road #46 about 0.8 miles north of Gunflint Trail (Fig. 6.5) Long Island Lake quadrangle UTM: 665,674E/5,329,111N Description: Most of the Logan intrusions consist of medium-grained diabase, as seen along the trail leading to this outcrop. By contrast, this exposure—inferred to lie near the roof of the sill—consists of deformed and partially assimilated xenolithic fragments of Rove Formation slate and siltstone in white-weathering, feldspathic, locally porphyritic and granophyric rock (Fig. 6.14). Figure 6.14. Irregular border phase at upper part of one of the Logan Intrusions, containing irregular inclusions of Rove Formation slate country rock. ILSG07 165 Trip 6 �STOP 6-11 Mesoproterozoic Tuscarora intrusion of Duluth Complex—atypical border phase Location: T. 65 N., R. 4 W., sec. 27 SW, SE; Roadcut on Cross Lake road (CR#47) (Fig. 6.5) Long Island Lake quadrangle UTM: 662,074E/5,327,457N Description: A confusing exposure of the lower units (ttp, ttf) of the Tuscarora intrusion. The outcrop consists of intergranular to ophitic gabbro and augite troctolite, with pods and veinlets of coarse mafic pegmatite and shear bands containing sulfide mineralization. Spheroidal weathering produced "core stones" locally. STOP 6-12 Mesoproterozoic Tuscarora intrusion of Duluth Complex Location: T. 65 N., R. 3 W., sec. 30 SW, SE; roadcut on Gunflint Trail east of CR#50. (Fig. 6.5) Long Island Lake quadrangle UTM: 666,638E/5,327,433N Description: Just south of the parking pull-off is the rather poorly exposed intrusive contact between Rove Formation and units ttp and ttf of the Tuscorara Intrusion (Fig. 6.5 explanation; Morey and others, 1981). Unit ttf is a typical example of Cu-sulfide mineralized augite troctolite that is found at the base of the Duluth Complex here and in the Hoyt Lakes-Kawishiwi area to the southwest. It contains disseminated pyrite, pyrrhotite, and chalcopyrite. In the 1970's, International Nickel Company (INCO) drilled 7 holes in the basal Duluth Complex (Tuscarora and western Poplar Lake intrusions) to evaluate potential for Cu-Ni mining. All of these holes lie along the basal part of the intrusion within a few miles east and west of this stop. The archived drill cores were studied by Mogessie (1976) and Mogessie and others (1976). ILSG07 166 Trip 6 �REFERENCES Addison, W.D., Brumpton, G.R., Vallini, D.A., McNaughton, N. J., Davis, D.W., Kissin, S.A., Fralick, P.W., and Hammond, A.L., 2005, Discovery of distal ejecta from the 1850 Ma Sudbury impact event: Geology: 33:193196. Arth, J.G., and Hanson, G.N., 1975, Geochemistry and origin of the early Precambrian crust of northeastern Minnesota: Geochim. et Cosmo. Acta, 39:325-362, Table 2, p. 331-332. Boerboom, T.J., and Zartman, R.E., 1993, Geology, geochemistry, and geochronology of the central Giants Range batholith, northeastern Minnesota: Canadian Journal of Earth Sciences, 30:2510-2522 Broderick, T.M., 1920, Economic geology and stratigraphy in the Gunflint iron district, Minnesota: Economic Geology 15:422-452. Clout, J.M.F., and Simonson, B.M., 2005, Precambrian iron formation-hosted iron ore deposits: in Hedenquist, J.W., Thompson, J.F.H., Goldfarb, R.J., and Richards, J.P., Economic Geology 100th Anniversary Volume, p.643-679. Corfu, F., and Stott, G.M., 1998, Shebandowan greenstone belt, western Superior Province: U-Pb ages, tectonic implications, and correlations: Geological Society of America Bulletin 110:1467-1484 Didier, J., and Barbarin, B., 1991, Enclaves and granite petrology: Elsevier, 625p, reference to p. 549. Feirn, W.C., 1977, The geology of the early Precambrian rocks of the Jasper Lake area, Cook County, northeastern Minnesota: Unpublished MS thesis, University of Minnesota, Duluth, 146p. Floran, R.J., and Papike, J.J., 1975, Petrology of the low-grade rocks of the Gunflint iron-formation, OntarioMinnesota: GSA Bulletin 86:1169-1190. Floran, R.J., and Papike, J.J., 1978, Mineralogy and petrology of the Gunflint Iron Formation, Minnesota-Ontario: Journ. Petrology, 19:215-288. Fralick, P.W., Davis, D.W., and Kissin, S.A., 2002, The age of the Gunflint Formation, Ontario, Canada: single zircon U-Pb age determinations from reworked volcanic ask: Can. J. Earth Sci., 39:1085-1091. Goldich, S.S., Nier, A.O., Baadsgaard, H., Hoffman, J.H.M and Drueger, H.W., 1961, The Precambrian geology and geochronology of Minnesota: Minnesota Geological Survey Bulletin 41, p. 57. Grout, F.F., 1929, The Saganaga Granite of Minnesota-Ontario: Journal of Geology 37:562-591. Grout, F.F., 1933, Structural features of the Saganaga granite of Minnesota-Ontario: Report of XVI International Geological Congress, Washington, p. 255-270. Gruner, J.W., 1941, Structural geology of the Knife Lake area of northeastern Minnesota: Geological Society of America Bulletin 52:1577-1642. Hanson, G.N., 1972, Saganaga batholith: in Sims, P.K., and Morey, G.B., eds., Geology of Minnesota: A centennial volume: Minnesota Geological Survey, p. 102-107. Heaman, L.M., and Easton, R.M., 2005, Proterozoic history of the Lake Nipigon area, Ontario: Constraints from UPb zircon and baddeleyite dating: in Easton, M., and Hollings, P., eds., Institute on Lake Superior Geology Proceedings, 51st Annual Meeting, Nipigon, Ontario, Proceedings and Abstracts, v. 51, part 1, p. 24-25. Jirsa, M.A., and Miller, J.D., Jr., 2004, Bedrock geology of the Ely and Basswood Lake (U.S. portion) 30’ x 60’ quadrangles, northeastern Minnesota: Minnesota Geological Survey Miscellaneous Map Series M-148, scale 1:100,000. Jirsa, M.A., and Miller, J.D., Jr., 2005, Geologic implications of bedrock mapping in the Ely and Basswood Lake quadrangles, northeast Minnesota: in Easton, M., and Hollings, P. (eds.), Institute on Lake Superior Geology Proceedings, 51st Annual Meeting, Nipigon, Ontario, Proceedings and Abstracts, v. 51, Part 1, p. 28-29. Jirsa, M.A., and Miller, J.D., Jr., 2005, Classic Precambrian geology of northeast Minnesota: Field Trip 2 in Robinson, L., ed., Field trip guidebook for selected geology in Minnesota and Wisconsin: Minnesota Geological Survey Guidebook Series 21, p. 8-33. Jones, N.W., 1984, Petrology of some Logan sills, Cook County, Minnesota: Minnesota Geological Survey Report of Investigations 29, 40p. Lobatka, T.C., Papike, J.J., Vaniman, D.T., and Morey, G.B., 1981, Petrology of contact metamorphosed argillite from the Rove Formation, Gunflint Trail, Minnesota: Am. Mineralogist, 60:70-86. Mathez, E.A., Nathan, H.D., and Morey, G.B., 1977, Reconnaissance geology of the Hungry Jack Lake quadrangle, Cook County, Minnesota: Minnesota Geological Survey Misc. Map Series M-39, scale 1:24,000. Miller, J.D., Jr., and Jerde, E.A., 2007, Poplar Lake intrusion: this volume. ILSG07 167 Trip 6 �Miller, J.D., Jr., Morey, G.B., and Weiblen, P.W., 1987, Seagull Lake-Gunflint Lake area: A classical Precambrian stratigraphic sequence in northeastern Minnesota: in Biggs, D.L. ed., North-Central Section of the Geological Society of America Centennial Field Guide Volume 3, p.47-52. MillerJ.D., Jr., and Severson, M.J., 2002, Geology of the Duluth Complex: in Miller, J.D., Jr., Green, J.C., Severson, J.J., Chandler, V.W., Hauck, S.A., Peterson, D.M., and Wahl, T.E., Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geological Survey Report of Investigations 58, p. 106-143. Mogessie, A., 1976 Petrologic study of copper-nickel mineralization in the Tuscarora intrusion Duluth Complex, northeastern Minnesota: M.S. thesis, University of Minnesota, Twin Cities, 137p. Mogessie, A., Stumpfl, E.F., and Weiblen, P.W., 1976, The role of fluids in the formation of platinum-group minerals, Duluth Complex, Minnesota: Mineralogic, textural, and chemical evidence: Econ. Geol. 86:15061518. Morey, G.B., 1969, The geology of the Middle Precambrian Rove Formation in northeastern Minnesota: MGS Special Publication 7, 62p. Morey, G.B., 1972, Gunflint Range, in Sims, P.K., and Morey, G.B., eds., Geology of Minnesota: A centennial volume: Minnesota Geological Survey, p. 218-225. Morey, G.B., and Nathan, H.D., 1978, Geologic map of the Gunflint Lake quadrangle, Cook County, Minnesota: Minnesota Geological Survey Misc. Map M-42, scale 1:24,000. Morey, G.B., and Nathan, H.D., 1977, Reconnaissance geologic map of the South Lake quadrangle, Cook County, Minnesota: Minnesota Geological Survey Misc. Map M-38, scale 1:24,000. Morey, G.B., and Southwick, D.L., 1995, Allostratigraphic relationships of Early Proterozoic iron-formations in the Lake Superior region: Econ. Geol. 90:1983-1993. Morey, G.B., Weiblen, P.W., Papike, J.J., and Anderson, D.H., 1981, Geologic map of the Long Island Lake quadrangle, Cook County, Minnesota: Minnesota Geological Survey Misc. Map M-46, scale 1:24,000. O'Brien, H.E., 1982, Petrology, geochemistry, and magnetics of a lamprophyre dike intruded into the Saganaga Tonalite batholith, northeastern Minnesota: Senior Thesis, University of Minnesota, Twin Cities, 59p. Peterson, D.P., Gallup, C., Jirsa, M.A., and Davis, D.W., 2001, Correlation of Archean assemblages across the U.S.Canadian border: Phase I geochronology, (abstract): Institute on Lake Superior Geology Proceedings, 47th Annual Meeting, Madison, WI, 2001, Part 1, p.77-78. Pratt, B.R., 2001, Syneresis cracks: subaqueous shrinkage in argillaceous sediments caused by earthquake-induced dewatering: Sedimentary Geol., 117:1-10. Pufahl, P.K., and Fralick, P.W., 2000, Depositional environments of the Paleoproterozoic Gunflint Formation: in Institute on Lake Superior Geology Proceedings, 46th Annual Meeting, Thunder Bay, Ontario, Part 2 Field Trip Guidebook, v. 51. Pufahl, P.K., and Fralick, P.W., 2004, Depositional controls on Paleoproterozoic iron formation accumulation, Gogebic Range, Lake Superior region, USA: Sedimentology 51:791-808. Schulz, K.J., and Cannon, W.F., 2007 (in press), The Penokean Orogeny in the Lake Superior region: Precambrian Research. Stern, R.A., and Hanson, G.N., 1991, Archean high-Mg granodiorite: A derivative of light rare earth elementenriched monzodiorite of mantle origin: J. Petrol. 32:201-238, Table 2, p. 220. Stern, R.A., Hanson, G.N., and Shirey, S.B., 1989, Petrogenesis of mantle-derived, LILE-enriched Archean monzodiorites and trachyandesites (sanukitoids) in southwestern Superior Province: Can. J. Earth Sci., 26:1688-1712; Table 1 p. 1695. Vervoort, J.D., 1987, Petrology and geochemistry of the Archean of the JAP Lake area, northeastern Minnesota: M.S. Thesis, University of Minnesota-Duluth, 193 p. Weiblen, P.W., Morey, G.B., and Mudrey, M.G., 1971, Guide to the Precambrian rocks of northwestern Cook County as exposed along the Gunflint Trail: in Davidson, D.M., Darby, D.G., Green, J.C., and Grant, J.A., eds., Technical Sessions, Abstracts and Field Guides, 17th Annual Institute on Lake Superior Geology, Annual meeting, Duluth, Minnesota, p.97-127. Wirth, K.R., Vervoort, J., Craddock, J.P., Davidson, C., Finley-Blasi, L., Kerber, L., Lundquist, R., Vorhies, S., and Walker, E., 2006, Source rock ages and patterns of sedimentation in the Lake Superior Region: Results of preliminary U-Bp detrital zircon studies: in Wilson, A.C., (ed.), Proceedings and Abstracts, Institute on Lake Superior Geology Proceedings, 52nd Annual Meeting, Sault Ste Marie, Ontario, v. 51, Part 1, p. 69-71. ILSG07 168 Trip 6 � Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology: Proceedings, 2007 Description An account of the resource Institute on Lake Superior Geology. Lutsen, Minnesota. May 8-13, 2007. Creator An entity primarily responsible for making the resource Institute on Lake Superior Geology Date A point or period of time associated with an event in the lifecycle of the resource 2007 Format The file format, physical medium, or dimensions of the resource PDF Language A language of the resource English